Growth Hormone

C H A P T E R

4 Growth Hormone Vivien S. Bonert and Shlomo Melmed

INTRODUCTION

trophoblasts [3]. GH1 codes for a 22-kDa protein consisting of 191 amino acids (Fig. 4.1). Approximately 10% of pituitary GH circulates as a 20-kDa spliced variant lacking amino acid residues 32 46, as well as less abundant shorter variants [4]. GH2 is expressed by placental syncytiotrophoblasts during the second and third trimesters of gestation and encodes a 22-kDa protein secreted form detected in maternal circulation from midpregnancy [5,6]. The role of placental GH2 is unknown, however, the rise in maternal serum

Growth hormone (GH), secreted by anterior pituitary somatotroph cells, binds to hepatic GH receptors, initiating several intracellular signaling pathways resulting in the generation of insulin-like growth factor (IGF)-1, cytoskeletal changes, alterations in glucose metabolism, and modulation of cell proliferation genes. IGF-1, synthesized primarily in the liver, mediates most of the growth-promoting actions of GH. Metabolic actions of GH also affect carbohydrate, protein, and lipid metabolism and alter body composition. GH and IGF-1 influence body composition, cardiovascular function, muscle strength, and exercise performance. Clinical evaluation of GH excess or deficiency requires estimation of GH responses to inhibitory or stimulatory factors during provocative testing. GH replacement therapy has beneficial effects in GH-deficient children with short stature and adults with GH deficiency. Shortand long-term actions of GH have been evaluated for potential beneficial effects in the aging population and for enhanced athletic performance by athletes, with lack of proven efficacy.

GROWTH HORMONE GENE STRUCTURE The human growth hormone (GH) genomic locus spans approximately 66 kb and contains a cluster of five highly homologous genes located on the long arm of human chromosome 17 at bands q22 24. The 5’ to 3’ arrangement includes GH1, chorionic somatomammotropin hormone (CSH) 1, CSH2, GH2, and CSH4 [1], all of which have the same basic structure, comprising five exons separated by four introns [1,2]. GH1 gene is transcribed in anterior pituitary somatotrophs, while CSH1 and CSH2 are expressed in placental

The Pituitary. DOI: http://dx.doi.org/10.1016/B978-0-12-804169-7.00004-0

FIGURE 4.1 Amino acid structure of human GH. GH is a 191amino-acid single-chain 21.5-kDa polypeptide with two intramolecular disulfide bonds. Fifteen percent of GH is deleted from amino acid (32 46) and is secreted as a 20-kDa protein.

85

© 2017 Elsevier Inc. All rights reserved.

86

4. GROWTH HORMONE

correlates with a fall in GH1 concentrations, suggesting the possibility of a feedback loop on the maternal hypothalamic pituitary axis [5]. Postpartum, GH2 levels drop rapidly and are undetectable after 1 hour [5]. GH may also be expressed at low abundance in peripheral tissues, including breast and colon [7].

Somatotroph Development and Differentiation Somatotrophs comprise up to 45% of pituitary cells and are located predominantly in the lateral wings of the anterior pituitary gland, which contains 5 15 mg of GH. Acidophil cells are the progenitors for both GH-producing somatotrophs and prolactin (PRL)producing lactotrophs. Expression of the α-subunit transcript in the hypophyseal placode within pharyngeal ectoderm prior to formation of Rathke’s pouch defines the onset of pituitary organogenesis [8,9]. Evidence from transgenic mice studies suggests that most PRL-expressing cells arise from GH-producing cells [10]. Specifically, GH expression in somatotroph cells develops in a time-and-space-dependent manner. The anterior pituitary develops from Rathke’s pouch during the early stages of embryonic development [11], and specific cytodifferentiation pathways lead to differentiated hormone-producing cells. Ablation of somatotrophs by expression of GHdiphtheria toxin and GH-thymidine kinase fusion genes inserted into the germ line of transgenic mice also results in elimination of the majority of lactotrophs; however a small percentage of lactotrophs escape destruction [10,12]. This suggests that the majority of PRL-producing cells arise from postmitotic somatotrophs. The transcription factors Prophet of Pit1 (PROP-1) and POUIFI determine somatotroph and lactotroph growth, differentiation, and commitment to expressing the GH or PRL gene product [8] (Fig. 4.2). The Pit-1 gene transcript and POUIF1 protein are expressed in somatotrophs, lactotrophs, and thyrotrophs [8]. Pit-1 actions are complemented by other factors required to achieve physiologic patterns of cell-specific gene activation [8]. Inherited syndromes of GH deficiency or GH action may be attributed to several cellular defects, including mutations of transcription factors, the GH-1 gene, the growth hormone releasing hormone (GHRH) receptor, GH receptor (GHR) signaling, or very rarely IGF-related molecules. PROP-1, a paired homeobox protein, is required for initial commitment of Pit-1 cell lineages [14]. PROP-1 represses Rpx expression, and missense and spliced mutations of PROP-1 leading to loss of DNA-binding or transactivation leads to pituitary failure with short stature and varying degrees of thyroid failure,

hypogonadism, and adrenocorticotrophic hormone (ACTH) deficiency [15]. POUIFI [16] mutations may also lead to pituitary failure. Patients with combined pituitary hormone deficiency have predominantly GH and PRL deficiency, with variable degrees of hypothyroidism [16]. In the absence of Pit-1 the promoter region is inactive, and binding of Pit-1 facilitates interactions with other ubiquitous activators to enhance GH transcription. This model of cooperative interaction likely contributes to tissue-specific expression of the hGH gene by a single cell type-specific activator.

GH SYNTHESIS Somatotrophs comprise up to 45% of pituitary cells and are located predominantly in the lateral wings of the anterior pituitary gland which contains 5 15 mg of GH. The GH molecule, a single-chain polypeptide hormone consisting of 191 amino acids (Fig. 4.1), is synthesized, stored, and secreted by somatotroph cells. The 217-amino-acid GH precursor is synthesized and transported to the endoplasmic reticulum lumen. The 1 26-amino-acid peptide is cleaved and the mature GH molecule transported to the golgi for packaging into secretory vesicles mediated by zinc ions [17]. The crystal structure of human GH reveals four α-helixes; and several structural features confer functional characteristics which determine GH signaling. These included the third α-helix with amphiphilic domains, and a large helical loop [18,19]. Circulating GH molecules comprise at least three monomeric forms and several oligomers. The monomeric moieties include a 22- and 20-kDa form, acetylated 22 K, and two desamido GHs. The 22-kDa peptide is the major physiologic GH component. The 20-kDa GH has a slower metabolic clearance than the 22-kDa form. which accounts for the plasma 20:22 ratio being higher than in the pituitary gland. The 22-kDa and 20-kDa peptides have similar growth-promoting activity. Monomeric GH forms found in the plasma of acromegaly patients are qualitatively similar to those found in normal plasma [20]. Circulating GH is first detectable in fetal serum at the end of the first trimester, peaks at a concentration of 100 150 ng/mL at 20 weeks of gestation, and subsequently falls to 30 ng/mL at birth. GH levels continue to fall during infancy. During childhood, levels are similar to those in adulthood, until puberty, when circulating levels are elevated. GH levels decline after adolescent growth and remain stable until midadulthood, when they decline progressively through old age [21].

I. HYPOTHALAMIC PITUITARY FUNCTION

GH SYNTHESIS

87

FIGURE 4.2 Hypothalamic pituitary control of GH secretion. Control of the secretion of GH is achieved by hypothalamic GHRH and somatostatin, which traverse the portal vein, somatotroph-specific transcription factors, and negative feedback control of IGF-1. Accurate measurement of pulsatile secretion of GH requires ultrasensitive assays. POU1F1, POU domain class 1 transcription factor 1; Prop-1, Prophet of Pit-1; GHS, growth hormone secretagogues; SRIF, somatostatin. Source: From Melmed [13].

Neuroendocrine Control of GH Central neurogenic control of GH is complex. Neuropeptides, neurotransmitters, and opiates impinge on the hypothalamus and modulate GHRH and SRIF release. The net effect of these complex influences determines the final secretory pattern of GH. Small synthetic molecules termed growth hormone secretagogues (GHS) [22] stimulate and amplify

pulsatile pituitary GH release, via a separate pathway distinct from GHRH/SRIF. GHS, administered alone or in combination with GHRH, are potent and reproducible GH releasers and are useful tools for the diagnosis of GH deficiency [23]. The GHS receptor (GHSR), a heterotrimeric GTPbinding protein (G-protein)-coupled protein [24], comprises seven α-helical membrane-spanning domains and three intracellular and extracellular loops. The

I. HYPOTHALAMIC PITUITARY FUNCTION

88

4. GROWTH HORMONE

GHSR is expressed in pituitary somatotroph cells and in both hypothalamic and nonhypothalamic brain regions. The endogenous ligand of GHSR is a 28-aminoacid peptide, ghrelin, isolated from the gastrointestinal tract, and is n-octanoylated at the serine 3 residue [25]. Ghrelin releases GH both in vivo and in vitro, and noctanoylation is essential for GH releasing activity. Ghrelin is expressed in the arcuate nucleus of the hypothalamus, and also in the pituitary gland [25,26]. Ghrelin modulates GH secretion at both a hypothalamic and pituitary level [27], and signals through the GHSR to induce GH release [25]. In vitro, GHRH and ghrelin have additive effects on GH release, whereas in vivo administration of GHRH with GHS/ghrelin is synergistic [28]. GH secretagogues and GHRH act via different mechanisms [28]. Furthermore, GHRP-6 which activates the GHSR, does not elicit GH release following hypothalamic pituitary disconnection [29]. Ghrelin amplifies the GH secretory pattern [30] and enhances GH responsiveness to GHRH [28,31,32]. These observations may be explained by findings that GHRH acts as an allosteric coagonist for the GHSR [33]. Ghrelin thus appears to act with GHRH to regulate GH secretion and energy balance [34]. Functional GHSR is detected in the human pituitary by the fifth week of gestation [35], and transgenic mice with decreased GHSR mRNA expression exhibit reduced GH and insulin-like growth factor (IGF)-1 levels [36], while GHSR knockout mice have lower IGF1 levels and decreased body weight [37]. However, ghrelin-null mice do not develop dwarfism [38]. Missense mutations in the GHSR (TRP2X, Arg237Trp, Ala204Glu), with attenuated ghrelin binding or possibly GHSR constitutive signaling, result in partial isolated GH deficiency [29,39], with incomplete penetrance and a range of phenotypes. Healthy volunteers demonstrate synchronicity between ghrelin and GH pulsatility, suggesting stimulation of GH by ghrelin or possibly coregulation of both by other neuroendocrine factors [40]. As GH secretagogues elicit a synergy with GHRH on GH release, which is minimally altered by age, sex, or adiposity and is devoid of potential side effects (unlike insulin-induced hypoglycemia), this combined test is a useful and safe diagnostic tool in the diagnosis of adult growth hormone deficiency (GHD). Thyrotrophin-releasing hormone (TRH) does not stimulate GH secretion in normal subjects, but induces GH secretion in about 70% of patients with acromegaly, and also in patients with liver disease, renal disease, ectopic GHRH-releasing carcinoid tumors [41], anorexia nervosa [42], and depression. A group of reproductive kisspeptin neuropeptides encoded by the KISS-1 gene, stimulate hypothalamic GnRH neurons, and also induce GH release in peripubertal rats [43]. The physiological relevance in humans has not been elucidated.

Leptin, a 167-amino-acid cytokine product of the ob gene, plays a key role in regulating body fat mass [44], food intake, and energy expenditure. As GH secretion is markedly impaired in obese subjects, leptin may act as a metabolic signal to regulate GH secretion. In the fasting state, leptin levels decrease rapidly, prior to and out of proportion to changes in fat mass [45], triggering a neuroendocrine adaptive response to acute energy deprivation, including decreased reproductive and thyroid hormone levels, increased GH levels that mobilize energy, and reduced IGF-1 levels that may slow growth-related processes [46]. Interactions between leptin and the GH and adrenal axes may be less important in humans than in animal models, as patients with congenital leptin deficiency have normal linear growth and adrenal function [46]. Dopamine, a precursor of epinephrine and norepinephrine, influences GH secretion. GH-deficient children exhibit increased growth velocity after 6 months of L-dopa treatment, while adults increase their serum GH levels from 0 to 5 20 ng/mL within 60 90 minutes after oral L-dopa administration. Similarly, the central dopamine receptor agonist apomorphine, stimulates GH secretion. Norepinephrine increases GH secretion via α-adrenergic pathways and inhibits GH release via β-adrenergic pathways. Insulin-induced hypoglycemia increases GH secretion via an α2-adrenergic pathway, whereas clonidine acts on αl-adrenergic receptors to increase GH secretion. Arginine administration, exercise, L-dopa, and antidiuretic hormone (ADH) facilitate GH secretion by α-adrenergic effects [47]. β-Adrenergic blockade increases GHRH-induced GH release, possibly due to a β-adrenergic effect at the pituitary level or via decreased hypothalamic somatostatin release. β-Adrenergic blockade also enhances GH release elicited by insulin-induced hypoglycemia, ADH, glucagon, and L-dopa [47]. Epinephrine may regulate GH secretion by decreasing somatostatin release. Cholinergic and serotoninergic neurons have been implicated in the etiology of sleep-induced GH secretion. The lateral hypothalamus expresses orexin-A (hypocretin-1) and orexin-B (hypocretin-2) [48], which primarily regulate food intake and modulate the sleep wake cycle and arousal, and also play a role in control of several endocrine axes. Orexin-A is expressed mostly in lactotrophs and to a lesser extent in thyrotrophs, somatotrophs, and gonadotrophs, but is not expressed in corticotrophs. Orexin-B is expressed in most pituitary corticotroph cells [49]. Orexin-A markedly reduces spontaneous GH secretion and GH pulsatility as well as GH response to ghrelin in rats [50] mediated via somatostatinergic neurons [51]. Furthermore, loss of orexin function in knockout mice results in narcolepsy [52], implicating orexins in the

I. HYPOTHALAMIC PITUITARY FUNCTION

HYPOTHALAMIC HORMONES

regulation of arousal and the sleep wake cycle. As GH secretion is linked to the sleep wake cycle and feeding state in humans, pituitary-derived orexins may play a role in coordinating sleep and energy homeostasis. Several gastrointestinal neuropeptides stimulate GH secretion in animal models, including substance P, neurotensin, vasoactive intestinal polypeptide, peptide histidine isoleucine (PHI) amide, motilin, galanin, cholecystokinin, and glucagon [53].

89

Age-associated decrease in GH output is due to decreased GH pulse amplitude, with no appreciable changes in GH pulse frequency or nadir levels, implicating reduced GHRH pulse amplitude in the somatopause [65]. Sexually dimorphic GH secretion may also be attributed to GHRH. In males, GH is secreted predominantly at night, with low daytime baseline secretion, whereas females exhibit more daytime GH secretory pulses with higher basal GH levels [66], mirroring increased nocturnal male GHRH secretion, with elevated daytime GHRH levels in females [66].

HYPOTHALAMIC HORMONES Growth Hormone-Releasing Hormone

Somatostatin

Hypothalamic GHRH was characterized from ectopic pancreatic GHRH-secreting tumors causing acromegaly [41,54]. Analysis of one tumor revealed a 44-amino-acid GHRH residue; the other contained 37-, 40-, and 44-amino-acid forms [55]. GHRH (1 40) and GHRH (1 44) are both found in extracts derived from the human hypothalamus. GHRH is secreted from neurons in the hypothalamic arcuate nucleus and premammillary area, with axons that project to the median eminence. The hGHRH gene encodes a 108amino-acid preprohormone for GHRH-44 [56,57], which has a free amino terminal and amidated carboxy terminal residue. The amino terminal appears to bestow biologic activity on the GHRH molecule. There is considerable structural homology between GHRH and several gut peptides, the highest between GHRH and PHI, which have 12 amino acids in common in equivalent positions [58]. Varying degrees of homology exist between GHRH and VIP, glucagon, secretin, and GIP. All of these peptides stimulate GH secretion in various physiologic systems, but with lower potency than GHRH. GHRH binds to a specific receptor on the somatotroph membrane, resulting in increased intracellular 3’, 5’cAMP levels [59]. The GHRH receptor encodes a 47kDa protein of 423 amino acids [60]. GHRH has a selective action on GH synthesis as well as secretion, and stimulates GH gene transcription. GHRH stimulates GH release from both stored and newly synthesized intracellular GH pools. Somatostatin suppresses both basal and GHRH-stimulated GH release, but does not affect GH biosynthesis [61]. GHRH administered to normal adults elicits a prompt increase in serum GH levels, with higher levels occurring in female subjects [62]. Furthermore, GHRH facilitates GH responses to several pharmacological stimuli including levodopa, arginine, clonidine, insulin hypoglycemia, pyridostigmine, and GHRP-6 [63]. GHRH is the principal regulator of pulsatile GH secretion, and age-related decline in GH secretion (somatopause) is likely GHRH-mediated [64].

Somatostatin (SRIF), a cyclic tetradecapeptide, includes quantitatively predominant, but less bioactive SRIF-14, and more bioactive SRIF-28 [67]. The SRIF precursor is a 116-amino-acid prohormone consisting of a 24-amino-acid signal peptide, a 64-amino-acid connecting region, followed by SRIF-28 [68] which incorporates SRIF-14. Prosomatostatin is synthesized in the anterior hypothalamic periventricular nuclei, and is transported by axonal flow to nerve terminals terminating at the hypophyseal portal vessels. SRIF is also expressed in pancreatic islets, gastrointestinal, neural and epithelial cells, and extrahypothalamic central nervous system neurons. SRIF has a short plasma half-life of 2 3 minutes [67] and inhibits GH, ACTH, and TSH release, TRH stimulation of TSH but not PRL, and pancreatic secretion of insulin and glucagon [69]. SRIF-28 binds to pituitary receptors with a threefold greater affinity than SRIF-14. Both SRIF-14 and SRIF-28 block GHRH effects on GH, as well as secretory responses to insulin-induced hypoglycemia, exercise, arginine, morphine, levodopa, and sleep-related GH release. Somatostatin exerts biologic effects through specific membrane-bound high-affinity receptors. Five somatostatin receptor (SSTR) subtypes, termed SSTR1 5 [70] are coupled to guanine nucleotide protein (G), and comprise seven-transmembrane domains. There is 42% to 60% amino acid homology among the five SSTR subtypes. SSTRs mediate responses via cellular effectors including adenylyl cyclase, protein phosphatases, Na1-H1 exchanger, cyclic GMP-dependent protein kinases, phospholipase C, potassium and calcium channels [70]. The human pituitary gland expresses predominantly SSTR1, 2, and 5 [71], whereas human pituitary adenomas express SSTR1, 2, 3, and 5 [71 73]. Somatostatin analogues, used to control GH hypersecretion in acromegaly, bind with varying affinity to SSTR2 and SSTR5 [74,75]. SRIF receptors may also signal constitutively in the absence of ligand, to regulate basal pituitary hormone release [70].

I. HYPOTHALAMIC PITUITARY FUNCTION

90

4. GROWTH HORMONE

FIGURE 4.3 Central and peripheral components that regulate the GH axis. NPY, neuropeptide Y; FFA, free fatty acids; GH, growth hormone; IGF1, insulin-like growth factor 1; GHRH, GHreleasing hormone; SRIF, somatotrophin release inhibitory factor. Source: From Murray PG, Higham CE, Clayton PE. 60 YEARS OF NEUROENDOCRINOLOGY: the hypothalamo-GH axis: the past 60 years. J Endocrinol 2015;226(2):T123 40. [78].

GHRH and SRIF Interaction in Regulating GH Secretion SRIF and GHRH secreted in independent pulses from the hypothalamus interact to generate pulsatile GH release. SRIF inhibits GH secretion, while GHRH stimulates GH synthesis and secretion. GH secretion is further regulated by its target growth factor, IGF-1, which participates in a hypothalamic pituitary peripheral regulatory feedback system [76,77] (Fig. 4.3). GH stimulates the liver and other peripheral tissues to produce IGF-1, which exerts a feedback effect on the hypothalamus and pituitary. IGF-1 also induces hypothalamic SRIF release. Specific antibodies directed against GHRH or SRIF have been used to dissect the respective contributions of these two peptides in the generation of GH pulsatility in rats. Anti-SRIF

administration results in elevated baseline GH levels, with intact intervening GH pulses [79]. These studies imply that hypothalamic SRIF secretion generates GH troughs. Anti-GHRH antibodies eliminate spontaneous GH surges and GH pulsatility persists when GHRH is tonically elevated due to ectopic GHRH production by a tumor or during GHRH infusion [80], suggesting that hypothalamine SRIF is also largely required for GH pulsatility. The rat hypothalamus releases GHRH and SRIF 180 out of phase every 3 4 hours, resulting in pulsatile GH levels [79]. GHRH and SRIF also act synergistically, in that pre-exposure to SRIF enhances subsequent somatotroph sensitivity to GHRH stimulation [81]. Hence, during a GH trough period, high SRIF levels likely prime the somatotroph to respond maximally to subsequent GHRH pulses, thus optimizing GH release. In addition, SRIF exerts a central

I. HYPOTHALAMIC PITUITARY FUNCTION

91

HYPOTHALAMIC HORMONES

inhibitory effect on GHRH release via direct synaptic connections between SRIF-containing axons and GHRH-containing perikarya in the arcuate nucleus.

TABLE 4.1 Adult GH Secretion Interval 24-h secretion (μg/24 h)

GH Autoregulation Chronic GHRH stimulation downregulates GH release in humans [82] due to somatotroph desensitization. However, loss of GH sensitivity to administered GHRH does not occur in acromegaly [83] or in somatotroph adenomas in vitro, possibly reflecting larger intracellular pools of GH or abnormal signaling. GHRH pretreatment in vitro also leads to a 50% decrease in somatotroph GHRH-binding sites [84]. Feedback loops exist between GH and IGF-1 and the release of SRIF and GHRH (Fig. 4.3). GH stimulates hypothalamic SRIF release in vitro [85], and in vivo, GH administration decreases GH responses to GHRH [86], most likely by increasing hypothalamic SRIF release [87]. GHRH and SRIF also autoregulate their own secretion. GHRH inhibits its own secretion but stimulates SRIF secretion in vitro, while SRIF inhibits its own secretion in vitro [88].

Physiologic Factors Affecting GH Secretion GH secretion from the anterior pituitary gland is pulsatile, and almost undetectable basal levels between bursts [89]. The number of GH pulses detected depends on the frequency of blood sampling. Integrated GH levels are higher in women than in men, and are also enhanced in postmenopausal women following estrogen replacement [89]. In healthy individuals, discrete pulses account for the majority (.85%) of GH release over the 24-hour period [89]. Pathophysiological regulation of GH output is achieved by altering GH secretory-burst size rather than by modulating pulse frequency [53,90]. Burst size is increased by GHRH, decreased by somatostatin, and synergistically augmented in vivo by combined stimulation with GHRH and GHreleasing peptide (GHRP, also known as GH secretagogue), including ghrelin [36,91,92]. GHRH, SRIF, and ghrelin regulate GH secretion, with contribution from multiple secondary regulators, including gender, gonadal sex steroids, visceral fat, pregnancy, puberty, aging, exercise, sleep, amino and fatty acids, glucose, fasting, insulin, IGF-1, and GH feedback [53,90,93] (Table 4.1). Thus, acute hour-by-hour regulators, such as GH pulse-stimulating effects of exercise and GH pulsesuppressing effects of glucose, are superimposed on individual day-to-day baseline GH secretion determined by age, degree of adiposity, nutritional status, physical fitness, and insulin and sex-steroid levels [95].

Young adult 540 6 44

Obesity

Middle age

2171 6 333 77 6 20

196 6 65

Fasting

Secretory bursts (number in 24 h)

12 6 1

32 6 2

3 6 0.5

10 6 1

GH burst (μg)

45 6 4

64 6 9

24 6 5

10 6 6

From Melmed [94].

Aging Circulating GH levels decrease significantly with aging, and are reduced by 15% to 70% in men and in women older than 60 years [96], with 24-hour integrated GH concentrations in elderly individuals comparable to those observed in young GH-deficient patients. Aging is associated with decreasing GH responses to most single secretagogues, with the exception of insulin-induced hypoglycemia [53], and is likely facilitated by excessive somatostatin release, diminished GHRH secretion, ghrelin deficiency, and/or a relative failure of feedback inhibition of pulsatile GH secretion independent of IGF-1 concentrations [53]. Gender Women have higher mean GH levels throughout the day due to higher incremental and maximal GH peak amplitudes, but show no significant difference in GH half-life, interpulse times, or pulse frequency [97], and manifest less orderly patterns of pulsatile GH release. Higher basal GH levels may underlie higher nadir GH levels seen in normal women after GH suppression with oral glucose [98]. Sexual differences in expression of mouse pituitary somatostatin and SSTR subtypes likely cause differences in the physiological regulation of GH release [99]. Sleep Sleep stimulates GH secretion. Approximately 60 70% of daily GH secretion occurs during early sleep, in association with slow-wave sleep [100], with a major GH secretory pulse occurring shortly after the onset of sleep and coinciding with the first episode of slow-wave sleep. Rapid-eye movement sleep is reduced by approximately 50% after age 50, with significant sleep fragmentation. Increased GHRH, decreased somatostatin, and increased ghrelin levels may mediate nocturnal GH surges, but the mechanism is unknown [101]. “Jet lag” transiently increases the height of GH peaks during the day and night, resulting in a transient increase of 24-hour GH secretion. Jet

I. HYPOTHALAMIC PITUITARY FUNCTION

92

4. GROWTH HORMONE

lag also shifts the major GH secretory spike from early to late sleep [102]. Exercise Exercise is a potent inducer of GH secretion and increases GH secretion, probably mediated by cholinergic mechanisms [103]. Intensity and duration of exercise, fitness, gender, and age all influence GH response to exercise [104]. GH excursions during exercise are influenced mainly by muscle exercise power (voluntary work/unit time), estradiol, and insulin concentrations [95]. Stress GH release is stimulated by physical stress, including trauma with hypovolemic shock and sepsis. However, chronic debilitating diseases, including cancer, are not associated with increased GH levels. Increased GHRH release, mediated by adrenergic pathways, is thought to mediate stress-induced GH secretion. Emotional deprivation is associated with suppressed GH secretion, and subnormal GH responses to provocative stimuli have been described in endogenous depression. Nutritional and Metabolic Regulation Nutritional and metabolic factors profoundly influence GH secretion. Chronic malnutrition and voluntary 5-day fasting [105] are associated with elevated GH levels, most likely as a result of direct somatotroph stimulation by decreasing IGF-1 levels relieving negative-feedback inhibition [106]. Both pulse frequency and amplitude of GH secretory peaks increase with fasting. In contrast, obesity decreases both basal and stimulated GH secretion, and the degree of GH attenuation correlates with the amount of total and visceral body fat [107]. Obese subjects demonstrate decreased somatotroph response to GHRH and GH [92], suggesting increased SRIF activity or a direct pituitary suppressive effect of free fatty acids (FFAs). Insulin-induced hypoglycemia stimulates GH release 30 45 minutes after the glucose trough, whereas acute hyperglycemia inhibits GH secretion for 1 3 hours [108], followed by a GH increase 3 5 hours after oral glucose administration. Insulin-induced hypoglycemia forms the basis of the insulin tolerance test (ITT), which is a gold-standard GH provocative test. Diabetic patients with chronic hyperglycemia, however, do not have suppressed GH levels, and many poorly controlled diabetic patients have increased basal and exercise-induced GH levels. Central nervous system glucoreceptors appear to sense fluctuations, rather than absolute glucose levels. Glucose homeostasis is thus not the major determinant of GH secretion, but is overridden by effects of sleep, exercise, stress, and random GH bursts.

A high-protein meal and single amino acids (including arginine and leucine) administered intravenously stimulate GH secretion. Arginine may suppress endogenous somatostatin secretion and thereby stimulate GH secretion [109]. Decreased serum FFA levels cause acute GH release and increased serum FFA blunts the effects of various stimuli on GHRH-stimulated GH release, including arginine infusion, sleep, levodopa, and exercise [110]. In acromegaly patients, dexamethasone suppresses GH secretion [111], and supraphysiologic serum glucocorticoid concentrations retard growth. In Cushing disease, due to an ACTH-secreting adenoma, growth retardation, decreased serum GH [112], and decreased pituitary GH content in tissue surrounding the adenoma are seen [113]. In normal subjects, glucocorticoid administration suppresses GHRH-induced GH release and produces a dose-dependent inhibition of GHRH-stimulated GH secretion, similar to that seen in Cushing’s syndrome, but acute administration induces GH levels [114]. Thus glucocorticoids exhibit short-term stimulatory effects and delayed inhibitory effects on GH secretion. GH-Binding Proteins Circulating GH is attached to two specific GHbinding proteins (GHBPs), one of high affinity and one of low affinity. The 60-kDa high-affinity BP corresponds to the extracellular domain of the hepatic GHR, produced by proteolytic cleavage with receptor ectodomain shedding. Under basal conditions, half of the circulating 22-kDa GH is bound to the high-affinity BP when GH levels are up to 10 15 ng/mL [115], while 20-kDa GH binds preferentially to the low-affinity BP. Binding to plasma GHBP prolongs GH plasma half-life by decreasing GH metabolic clearance rate [116]. The high-affinity BP also inhibits GH binding to surface receptors by competing for the ligand. Thus, GHBP may serve to dampen acute oscillations in serum GH levels caused by pulsatile pituitary GH secretion. High-affinity BP levels are low in the fetus and neonate, rise most rapidly in the first 1 2 years after birth, and are constant throughout adult life, with similar levels found in males and females. Circulating GHBP levels correlate with fat mass, as well as with circulating leptin levels. GHBP levels increase gradually during pregnancy, and peak in the second trimester, declining to normal levels before term. Placental GH levels correlate inversely with serum GHBP levels [117]. GH resistance, demonstrated in Laron dwarfism [118] and in African pygmies, is characterized by decreased plasma levels of high-affinity BP. Other syndromes of growth retardation with low GHBP levels include the “Pygmies” of the Democratic Republic of Congo and “Little Women” of Loja.

I. HYPOTHALAMIC PITUITARY FUNCTION

HYPOTHALAMIC HORMONES

In adult patients with GH deficiency and changes in body composition and increased fat mass, GHBP levels are either normal or increased. GH replacement in adult GHD patients is not associated with changes in GHBP levels. In acromegaly, measuring GHBP levels offers no diagnostic utility.

Peripheral GH Action GH Receptor GH binds to its peripheral receptor and induces intracellular signaling by a phosphorylation cascade involving the Janus Kinase (JAK)/signal transducing activators of transcription (STAT) pathway [19]. GH also acts indirectly by inducing synthesis of IGF-1, the potent growth and differentiation factor. The GHR is a 70-kDA protein member of the class I cytokine/hematopoietin receptor superfamily [119,120]. GHR consists of an extracellular ligand-binding domain, a single membrane-spanning domain, and a cytoplasmic signaling component. The GH ligand complexes with a preformed dimer of two GHR components leading to internal receptor rotation critical for subsequent GH signaling [120]. The activated receptor dimer induces separation of JAK2 sites, and GHR rotation is followed by rapid activation of JAK2 tyrosine kinase, leading to phosphorylation of cytoplasmic signaling molecules, including the GHR itself, and STAT proteins, critical signaling components for GH action [121]. Phosphorylated cytoplasmic proteins are translocated to the cell nucleus where they elicit GH-specific target gene expression by binding to nuclear DNA [122]. STAT1 and STAT5 may also interact directly with the GHR molecule [123]. Other target actions induced by GH include c-fos induction, IRS-1 phosphorylation, and insulin synthesis, cell proliferation, and cytoskeletal changes. As a differentiating and growth factor, IGF-1 is a critical protein induced by GH, and likely responsible for most of the growth-promoting activities of GH [124]. GH-activated STAT5B directly induces IGF-1 transcription [125]. Thus, STAT5B mediates GHinduced somatic growth [126]. IGF-1 itself may also directly regulate GH in a negative feedback loop [124] and GHR trafficking [127]. The liver contains abundant GHRs, and several peripheral tissues, including muscle and fat, also express modest amounts of receptor. STAT5B is required for GH-mediated postnatal growth, adipocyte functions, and sexual dimorphism of GH hepatic actions [19]. Transgenic mice with inactivated STAT5B exhibit impaired growth, with low IGF-1 levels, and are insensitive to injected GH [128]. GHR mutations are associated with partial or complete GH insensitivity and growth failure. These syndromes are associated with normal or high circulating

93

GH levels, decreased circulating GHBP levels and low levels of circulating IGF-1. Multiple homozygous or heterozygous exonic and intronic GHR mutations have been described, most of which occur in the extracellular receptor ligand-binding domain. Tissue responses to GH signaling may be determined by the pattern of GH secretion, rather than the absolute amount of circulating hormone. Thus, linear growth patterns, liver enzyme induction, and STAT5B activity may be phenotypically distinct for male animals due to higher rates of GH pulse frequency [129]. STAT5B is sensitive to repeated pulses of injected GH [130], unlike other GH-induced patterns which are desensitized by repeated GH pulsing. Mice harboring a disrupted STAT5B transgene exhibit impaired male pattern body growth [128] with IGF-1 and testosterone levels normally seen in female mice. Thus, sexual dimorphic patterns of GH secretion and GH tissue targeting appear to be determined by STAT5B, as does the requirement for appropriate GH pulsatility to determine body growth [131]. In humans, STAT mutations result in short stature and relative GH insensitivity [132]. Intracellular GH signaling is also abrogated by suppressor of cytokine signaling proteins, which disrupt the JAK/STAT pathway and thus exert a further level of control over the action of GH [133] (Fig. 4.4). Insulin-Like Growth Factors (IGFs) IGF-1 and IGF-2 are single-chain polypeptide molecules with three intrachain sulfide bridges (Fig. 4.5). IGF-1, composed of 70 amino acids, and IGF-2, consisting of 67 amino acids, have a sequence homology of 62%. The IGFs consist of B and A peptide domains (structurally homologous with the insulin B and A chains), a C domain analogous to the connecting (C) peptide of proinsulin, and a D domain. IGF-1 and IGF-2 are single distinct gene loci, localized on chromosome 12 (12q22-q24.1) and chromosome 11 (11p15), respectively. The IGF-1 gene primary transcript can be alternately spliced to different products resulting in IGF-1a (exons 1, 2, 3, 5) or IGF-1b (exons 1, 2, 3, 4). Several IGF-1 mRNA species have been isolated from adult and fetal tissues, and the liver is the main source of circulating IGF-1 levels. The IGF-1 gene is expressed in human fetal connective tissues and cells of mesenchymal origin [135]. This ubiquitous localization of IGFs favors a paracrine/ autocrine function as well as an endocrine function of IGF-1. GH is the major regulator of IGF-1 gene expression in adult liver, heart, lung, and pancreas [136] and acts at the level of IGF-1 transcription. Fetal IGF-1 production is GH-independent, and platelet-derived growth factor and fibroblast growth factor also

I. HYPOTHALAMIC PITUITARY FUNCTION

94

4. GROWTH HORMONE

FIGURE 4.4 GH action. GH binds to the GHR dimer, which undergoes internal rotation, resulting in JAK2 phosphorylation (P) and subsequent signal transduction. GH signaling is mediated by JAK2 phosphorylation of depicted signaling molecules or by JAK2-independent signaling including Src/ERK pathways (S42). Ligand binding to a preformed GHR dimer results in internal rotation and subsequent phosphorylation cascades. GH targets include IGF-1, c-fos, cell proliferation genes, glucose metabolism, and cytoskeletal proteins. GHR internalization and translocation (dotted lines) induce nuclear proproliferation genes via importin α/β (Impα/Impβ) coactivator (CoAA) signaling. IGF-1 may also block GHR internalization, acting in a feedback loop. The GHR antagonist, pegvisomant, blocks GHR signaling; SRLs also attenuate GH binding and signaling (not shown). Source: From Melmed S. Acromegaly pathogenesis and treatment. J Clin Invest 2009;119 (11):3189 202.

FIGURE 4.5 Amino acid sequence of human IGF-1. The black amino acids are identical to those in human insulin. The numbering corresponds to the numbering of residues in the proinsulin molecule. IGF-1 consists of a 70-amino-acid single-peptide chain with A, B, C, and D domains. A and B domains are structurally homologous to the A and B chains in the insulin molecule, and the C domain is equivalent to the connecting (C) peptide in proinsulin. Source: From Humbel [134].

stimulate IGF-1 production from human fibroblasts in vitro [137]. ACTH, TSH, LH, and FSH stimulate paracrine production of IGF-1 in their respective target tissues. In addition to GH, nutritional status is an

important regulator of IGF-1 production at all ages [138]. IGFs are found in lymph, breast milk, saliva, and amniotic fluid. IGF-1 levels are low before birth, rise during childhood to high levels during puberty, and decline with age [139]. Multiple cellular actions of IGF-1 are mediated via the IGF-1 receptor, a transmembrane tyrosine kinase cell surface receptor, with high homology to the insulin receptor. IGFs are expressed widely throughout most tissues in the body, are not stored in cellular secretory granules, and are secreted associated with high-affinity circulating IGF-binding proteins (IGFBPs). IGFs play an important role in regulating somatic growth and ensure that growth and development proceed appropriate to nutritional supply. IGF-Binding Proteins IGF-1 and IGF-2 are bound to carrier proteins in the serum. IGF-1 and IGF-2 are complexed to six specific binding proteins in biological fluids [140] (Table 4.2). These proteins are regulated by signals derived from nutritional status, as well as by hormone action [142]. IGFBPs are cysteine-rich proteins, with similar amino acid sequences, and a unique ability to bind IGFs with high affinity (Fig. 4.6). Actions of the IGFBPs include

I. HYPOTHALAMIC PITUITARY FUNCTION

95

GH ACTION

TABLE 4.2 Human IGFBPs No. of amino acids

Core molecular mass (kDa)

Chromosomal localization

IGF Modulation of affinity IGF action

IGFBP-I

234

25.3

7

I 5 II

Inhibition and/ Amniotic fluid, serum, placenta, endometrium, milk, urine, or potentiation synovial fluid, interstitial fluid and seminal fluid

IGFBP-II

289

31.4

2

II . I

Inhibition

IGFBP-III 264

28.7

7

I 5 II

Inhibition and/ Serum, follicular fluid, milk, urine, CSF, amniotic fluid, or potentiation synovial fluid, interstitial fluid, and seminal fluid

IGFBP-IV 237

25.9

17

I 5 II

Inhibition

Serum follicular fluid, seminal fluid, interstitial fluid, and synovial fluid

IGFBP-V

252

28.5

5

II . I

Potention

Serum and CSF

IGFBP-VI 216

22.8

12

II . I

Inhibition

CSF, serum, and amniotic fluid

Source in biological fluids

CSF, serum, milk, urine, synovial fluid, interstitial fluid, lymph follicular fluid, seminal fluid, and amniotic fluid

From Rajaram [141].

FIGURE 4.6 Structural features of IGFBPs. N- and C-terminal sequences of six members of the IGFBP family contain regions important for IGF binding, and form a high-affinity binding site. The mid-region of each protein, the linker region, diverges in sequence among the family members, allowing structural modifications enabled by glycosylation, proteolytic cleavage, and phosphorylation. Unique regions are also important for nuclear localization and cellsurface protein interaction. Source: From Clemmons DR. Role of IGF binding proteins in regulating metabolism. Trends Endocrinol Metab 2016;27(6):375 91.

modulation of IGF action and storage of IGFs in extracellular matrices. IGFBP-3, the most common form of binding protein in human circulation, associates the IGF molecule with an 80-kDa acid-labile subunit (ALS) to form a 150 200-kDa complex [140]. Approximately 75% of IGF-1 and IGF-2 circulates in this IGF IGFBP ALS ternary complex, which is stabilized by IGF binding [143]. Complexed IGFs do not readily leave the vascular compartment, and have prolonged half-lives [144] compared to the half-life of free IGF-1, which is less than 10 minutes [145]. A circulating protease acting specifically on IGFBP-3 results in limited cleavage of IGFBP-3, with subsequent decreased binding affinity of IGF-1. There is little detectable IGFBP-3 protease activity in normal serum due to the presence of inhibitors that protect IGFBP-3 from proteolysis. A pregnancy-associated plasma protein A system cleaves IGFBP-4 [146]. Plasma concentrations of IGFBPs are hormonally regulated. IGFBP-I levels are high at birth and decline until puberty [147], and diurnal variation with a

nocturnal peak in serum IGFBP-1 levels occurs [148]. Serum IGFBP-3 levels correlate with IGF-1 and -2 levels, increase in patients with acromegaly, and are lower in hypopituitarism [149]. In contrast, IGFBP-1 levels are elevated in hypopituitarism, [150] decreased in acromegaly, and increased in acromegaly patients receiving octreotide [151]. Malnutrition, insulin-dependent diabetes mellitus, and cirrhosis are associated with suppressed IGFBP-3 levels [152]. IGFBP-1 levels are regulated by insulin. Increased IGFBP-1 levels associated with insulin-dependent diabetes mellitus [140] are normalized by insulin, insulinoma is associated with suppressed IGFBP-1 levels, and the fall in IGFBP-1 levels after glucose ingestion in normal subjects correlates inversely with rising insulin levels [153]. Insulin also increases IGFBP-2 levels [152] and hypophysectomy is associated with elevated rat IGFBP-2 levels, which fall with GH administration [152]. Overall, IGFBPs serve to determine the availability of free IGF-1, which ultimately binds to the IGF receptor.

GH ACTION GH regulates several biologic functions, including intermediary metabolism and homeostasis, and plays a pivotal role in normal postnatal growth and development. Most GH actions are mediated by IGF-1 induced by GH in target tissues, mainly in the liver [154], but also has direct actions [155] (Fig. 4.7). IGF-1 is synthesized independent of GH, under the control of other regulatory factors [156], and may act synergistically with GH, as illustrated by their bone-growthpromoting properties, or antagonistically, as in hepatic glucose metabolism [156].

I. HYPOTHALAMIC PITUITARY FUNCTION

96

4. GROWTH HORMONE

FIGURE 4.7

Physiological actions of GH and IGF-1. Actions that involve more direct GH action are depicted on the right, whereas those involving induced IGF-1 are on the left. The GH IGF-1 system is regulated by a negative-feedback loop between hepatic IGF-1 production and pituitary growth hormone secretion. Source: From Brooks AJ, Waters MJ. The growth hormone receptor: mechanism of activation and clinical implications. Nat Rev Endocrinol 2010; 6(9):515 25.

Cell GH actions are mediated by the dimeric GHR, a member of the cytokine receptor superfamily [157], expressed on the target cell plasma membrane [158]. After GH binding to the GHR, intracellular signaling pathways are activated, including JAK2/STAT5 and ERK1/2 [159]. Similarly, IGF-1 acts through the IGF-1 receptor (IGF-1R), a tyrosine kinase receptor, and binding of IGF-1 activates canonical intracellular signaling cascades, including ERK1/2 and PI3K/AKT.

TABLE 4.3 Effects of GH on Bone Function

Effects

GROWTH PLATE Chondrocyte replication

mm

Endochondral bone formation

mm

BONE REMODELING UNIT Osteoblastogenesis

m

Osteoblast proliferation

m

Role of GH/IGF-1 in Growth and Development Throughout the Lifespan

Function of mature osteoblasts

2m

Osteoprotegerin production

m

Longitudinal bone growth is initiated in the epiphyseal growth plate of long bones, mediated by endochondral ossification [160]. When growth occurs, progenitor chondrocytes in the resting zone of long bone epiphyseal growth plates proliferate and replicate rapidly as clonal populations are arranged in columns. Subsequent differentiation of hypertrophic chondrocytes occurs, and extracellular matrix is secreted, resulting in new cartilage formation (chondrogenesis), leading to bone formation. Chondrocytes in the skeletal growth plate express GHRs, which are downregulated by local and systemic IGF-1 and upregulated when IGF-1 binds to IGFBPs [161]. In the presence of GH, mesenchymal precursors favor chondrogenesis and osteoblastogenesis over adipogenesis [162]. IGF-1 regulation of chondrocyte differentiation in IGF-1 null mutants is abrogated as evidenced by impaired chondrocyte maturation and shortened femoral length [163]. Genetic, hormonal, and nutritional factors influence invasion of newly formed cartilage by blood vessels and bone cell precursors, which facilitates calcification into bone trabeculae, i.e., endochondral ossification [164]. As an adjunct to longitudinal bone growth, bone tissue also undergoes remodeling and modeling. Bone

RANK-L production

2

Phosphate retention

m

2, no effect; m, minor stimulating effect; mm, major stimulating effect. From Giustina [169].

modeling occurs mostly during growth, whereas bone remodeling is a process of coordinated bone resorption and formation occurring in multicellular units throughout the lifespan [165]. During remodeling, multinucleated osteoclasts are attracted to specific sites to resorb bone, and osteoblasts are attracted to fill the cavity with newly synthesized matrix. GH stimulates proliferation of osteoblast cells and IGF-1 is required for selected anabolic effects of GH in osteoblasts [166]. GH also stimulates expression of bone morphogenetic proteins, which are important for osteoblast differentiation and bone formation [167]. In addition, GH stimulates mature osteoblast function, either directly or indirectly through IGF-1, and also stimulates carboxylation of osteocalcin, a marker of osteoblastic function [168] (Table 4.3, Fig. 4.8).

I. HYPOTHALAMIC PITUITARY FUNCTION

GH ACTION

97

FIGURE 4.8 Effects of GH and IGF-1 on bone growth. Skeletal effects of GH and IGF-1 are modulated by interactions between circulating and locally produced IGF-1 and IGFBPs. IGF-1 and IGF-2 are most abundant in skeletal tissue and are regulated by GH and PTH. GH may also exert direct effects on skeletal cells and induce IGF-1 action in bone. FGF, fibroblast growth factor; E2, estradiol; OPG, osteoprotegerin. Dotted line, no consistently demonstrated effect; thin solid line, minor stimulating effect; thick solid line, major stimulating effect. Source: From Giustina A, Mazziotti G, Canalis E. Growth hormone, insulin-like growth factors, and the skeleton. Endocr Rev 2008;29(5):535 59.

Low serum IGF-1 levels in GHR-mutated mice result in small growth plates, osteopenia, and reduced cortical bone with normal trabecular bone [170], suggesting a more pronounced effect of systemic IGF-1 on cortical bone than on trabecular bone. In contrast, mice with osteoblast-specific knockout of the IGF receptor gene exhibit decreased osteoblast number and function, causing reduced bone formation and trabecular volume [171], indicating a more significant role for skeletal IGF-1 in maintenance of trabecular bone. GH and IGF-1 influence bone metabolism throughout the lifespan [169]. During embryonic development, IGF-1 and IGF-2 are key determinants of bone growth, acting independent of GH, such that GH deficiency or insensitivity caused by GHR mutations or defects in GH signaling pathways, markedly impairs postnatal, but not prenatal growth [172]. Thus, GH plays a minor role in determining fetal growth. However, postnatally and during puberty, both GH and IGF-1 are critical in determining longitudinal skeletal growth [173] as well as skeletal maturation and acquisition of bone mass in the prepubertal period. Children with GH deficiency

manifest short stature, while GH excess in childhood causes gigantism. In contrast, an IGF-1 gene mutation causing IGF-1 deficiency [174] and IGF-1 resistance due to IGF receptor gene mutations [175] are associated with both prenatal and postnatal growth deficits. Anabolic effects of systemic and local skeletal GH and IGF-1 are important in the acquisition of bone mass and maintenance of skeletal architecture, particularly in the late adolescence and adulthood stages that are critical for peak bone mass achievement [169].

Bone Acquisition GH facilitates longitudinal bone growth and attainment of peak BMD during postnatal and pubertal growth, but is not required for intrauterine growth [154]. Childhood-onset GH deficiency is associated with increased fracture risk in adulthood and decreased BMD and is reversed by GH treatment [176]. Many of these GH effects are age-dependent; GH deficiency leads to a fourfold greater reduction in BMD

I. HYPOTHALAMIC PITUITARY FUNCTION

98

4. GROWTH HORMONE

during postpubertal growth than during prepubertal growth [177], possibly due to lack of GH activation of hepatic gene transcription prior to puberty [178]. IGF-1 mediates several GH effects on bone, as evidenced by GH insensitivity syndromes. Patients with Laron syndrome caused by GHR mutations, manifest with low/absent IGF-1 or IGFBP-3, decreased longitudinal bone growth; short body length at birth, and short stature [179], all of which are associated with reduced cortical bone parameters and endosteal and periosteal bone formation. In GH-insensitive patients, treatment with IGF-1 improves longitudinal bone growth, although it cannot fully restore all absent GH effects [179 181]. GH and IGF osteoblast activity are synergistic, and contribute to the stimulation of growth increase [182]. IGF-1 is required for growth throughout all stages of development [183], facilitating bone formation rates, and contributing to adequate BMD and femoral length via a combination of GH-dependent and GH-independent mechanisms [177,184]. IGF-1 effects in regulating bone formation vary in cortical versus trabecular bone and in appendicular versus axial skeletal regions, but both its independent effects, as well as its synergistic effects with GH, are essential for attainment of peak bone mass. IGF1 deficiency is associated with decreased BMD and growth retardation. GH thus acts independently as well as in synergy with IGF-1 to attain peak bone mass.

Bone Loss GH also increases bone resorption and turnover [185], independent of IGF-1, and patients with excess GH show an increased fracture risk [186]. Furthermore, the GH/IGF system may contribute to bone loss due to decline of GH secretion with increasing age, which contributes to age-associated bone loss. Serum IGF-1 and IGFBP levels [187] also decrease with age, and are associated with reduced stimulation of osteoprogenitor cells by both circulating serum and local bone IGF-1 [188]. However, the extent to which these age-related declines in the GH/IGF system contribute to age-related bone loss are unclear. Adults with GHD manifest low bone turnover, osteoporosis, and increased fracture risk, with decreased osteoid and mineralizing surfaces and a reduced rate of bone formation. Decreased osteocalcin and bone resorption markers reflect low bone turnover. Cortical loss is greater than trabecular bone loss [189], and bone loss is proportional to age of onset of GHD and duration and severity of the disease. Childhood-onset GHD is associated with more severely reduced vertebral bone mineral density than adult-onset patients, possibly due to failure to attain peak bone mass [190]. Nonvertebral fracture risk is increased threefold in

TABLE 4.4 Metabolic Effects of Growth Hormone EFFECTS ON CARBOHYDRATE METABOLISM Antagonism of insulin action EFFECTS ON LIPID METABOLISM Adipose tissue: increase lipolysis-increase free fatty acids Muscle/liver: increase lipoprotein lipase expression-triglyceride uptake EFFECTS ON PROTEIN METABOLISM Increase protein synthesis

untreated GHD patients, with fractures frequently localized to the radius [191].

GH AND METABOLISM GH continues to be secreted in adulthood after growth cessation, implying important metabolic functions in adult life. Metabolic actions may be acute and insulin-like or chronically antagonistic to insulin action, and may be direct or indirectly mediated by IGF-1. GH effects on carbohydrate metabolism are dominantly anti-insulin, with a net anabolic effect on protein metabolism (Table 4.4). GH-deficient children are mildly obese, with a decreased number of larger fat cells that have increased lipid content. GH replacement therapy leads to decreased body fat and, eventually, decreased size and lipid content of subcutaneous adipocytes. GHdeficient adults have altered body composition, with increased fat mass and decreased lean body mass (LBM). Initial acute effects of GH on lipid metabolism are antilipolytic (insulin-like) and subsequently, GH exerts a chronic lipolytic (anti-insulin) effect.

Lipids GH predominately stimulates adipocyte lipolysis, with increased circulating FFAs, and increased muscle and liver lipoprotein lipase expression with enhanced triglyceride uptake. GH increases lipolysis largely in visceral adipose tissue, and somewhat in subcutaneous adipose tissue, with release of circulatory FFAs [192]. GH activates hormone-sensitive lipase via enhanced agonist-induced stimulation of β-adrenergic receptors [193], resulting in increased hydrolysis of triglycerides to FFAs and glycerol. GH also facilitates differentiation of small preadipocytes into large, mature adipocytes, with increased capacity to store triglycerides and a higher lipolytic potential. Activation of STAT5 and possibly subsequent association with PPAR-γ may be associated with GH-induced adipogenesis [194].

I. HYPOTHALAMIC PITUITARY FUNCTION

99

GH AND REPRODUCTION

In the liver, however, effect of GH is opposite to that observed in adipose tissue. GH induces triglyceride uptake by increasing lipoprotein lipase and/or hepatic lipase expression and GH treatment increases hepatic triglyceride storage. In skeletal muscle, GH promotes lipid utilization by increasing lipoprotein lipase expression, which stimulates triglyceride uptake and subsequent storage as intramyocellular triglyceride, or stimulates lipid oxidation with energy release. GH-deficient adults have elevated total cholesterol, low-density lipoprotein cholesterol (LDL), triglycerides, and apolipoprotein B (ApoB) [195], with decreased highdensity lipoprotein (HDL). This lipid profile is associated with premature atherosclerosis and cardiovascular disease. GH replacement decreases total cholesterol [196], LDL cholesterol, and ApoB, and also increases HDL levels, although long-term surveillance is required to determine whether GH replacement therapy reverses premature atherosclerosis and reduces cardiovascular morbidity and mortality in GH-deficient adults. Lowdose GH replacement decreases total and visceral adipose tissue and reduces elevated levels of inflammatory markers, including highly sensitive c-reactive protein and interleukin-6 (IL-6) in women with hypopituitarism, with a relatively modest increase in IGF-1 levels and without worsening insulin resistance [197].

Body Composition Anabolic, lipolytic, and antinatriuretic GH actions impact body composition, affecting fat mass, LBM, and fluid volume in GH-deficient adults. LBM is reduced, and fat mass is increased in GH-deficient adults compared to predicted values for age-, sex-, and heightmatched normal controls. With GH deficiency, excess fat accumulates mostly in the visceral compartment in a central, mainly abdominal distribution and total body water is reduced. GH replacement therapy reverses these effects on body composition by increasing LBM. GH replacement also reduces fat mass by 4 6 kg in GH-deficient adults, with the most significant reduction in visceral fat. GH therapy increases total body water, especially extracellular water, within 3 5 days. Total blood volume increases after 3 months of treatment. GH and IGF-1 stimulate sodium reabsorption via epithelial sodium channels in the rat distal nephron [198], contributing to the antinatriuretic action of GH.

Carbohydrate Metabolism GH decreases glucose uptake in adipose tissue, and regulates glucose transporter-I in adipose tissuederived cell lines [199]. GH may antagonize adipocyte

insulin and lower serum leptin levels, but effects on adiponectin are unclear. In the liver, GH increases glycogenolysis, thereby increasing hepatic glucose production, possibly as a result of insulin antagonism. GH-deficient children have decreased fasting glucose levels [200], decreased insulin secretion [200], contradictory impairment of glucose tolerance [201], and increased insulin sensitivity due to increased glucose utilization and blunted hepatic glucose release. GH replacement increases fasting glucose levels [201], insulin levels [201], and hepatic glucose production. Endogenous GH secretion antagonizes insulin action. GH secretion increases 3 5 hours after oral glucose ingestion, and hyperinsulinemia occurs 2 hours after GH levels peak. Both intravenous and oral glucose tolerance tests are impaired if performed during periods of increased GH secretion, such as sleep onset. GH-deficient adults have elevated fasting insulin levels that correlate with fat mass and waist girth, suggesting the presence of insulin resistance. GH replacement increases insulin resistance in the first 1 6 weeks of therapy, but studies suggest unchanged insulin sensitivity over the long term [202].

Protein Metabolism Both insulin and IGF-1 have been implicated in the anabolic effects of GH on protein metabolism. GH causes urinary nitrogen retention and decreases plasma urea levels when administered to both normal and GH-deficient children.

Muscle Strength and Exercise Performance GH deficiency is associated with reduced muscle strength, due to altered body composition. Reduction in muscle cross-sectional area, as well as lack of conditioning and training, may contribute to weakness. Prolonged GH replacement therapy may increase muscle mass, but it may not result in improved strength.

GH AND REPRODUCTION The male and female reproductive systems are targets of GH action and also sites of GH synthesis, suggesting both autocrine and paracrine actions of GH within the reproductive system. The somatotrophic and gonadotrophic axes interact to signal the onset of puberty, sexual maturation, and accelerated pubertal growth. The GnRH pulse generator initiates puberty, while growth and nutritional factors influence timing and pace of puberty [203].

I. HYPOTHALAMIC PITUITARY FUNCTION

100

4. GROWTH HORMONE

In males, GH is expressed by the testis and accessory organs, and autocrine and paracrine actions promote seminiferous tubule differentiation and normal testicular growth [204], possibly mediated by IGF-1. GH stimulates androgen and/or estradiol production by Leydig cells in vitro [205] and promotes gametogenesis. Fertile GH-deficient males [206] receiving chronic GH therapy exhibit improved chorionic gonadotrophininduced testosterone production. In females, GH modulates steroidogenesis, folliculogenesis, and oocyte maturation, thus optimizing multiple processes that culminate in a viable embryo. Indeed, women with GHD have decreased fertility, and GH replacement improves spontaneous pregnancy rates in previously infertile GHD women [207]. While pituitary gonadotrophins are the prime regulators of ovarian steroidogenesis, GH also modulates progesterone and estradiol release [208] as well as gametogenesis, and may establish optimal conditions for nuclear maturation, perhaps by promoting follicular development [209]. GH is also proliferative and antiapoptotic in the corpus luteum [208]. The multiple actions of GH on the reproductive axis have led to the use of GH as an adjunct to assisted reproductive technology in women who are poor ovarian responders, but optimal use of this strategy remains somewhat unclear [210].

GH/IGF-1 AND CARDIOVASCULAR FUNCTION Data regarding blood pressure and peripheral resistance in untreated GH-deficient patients are unclear. Increased prevalence of hypertension [211], no change in blood pressure [212], and reduced blood pressure have been reported [213] in adult-onset GH-deficient patients. A stimulatory effect of GH on nitric oxide production [214] could explain reduced peripheral vascular resistance and diastolic blood pressure reported in some trials of GH replacement. Alternatively, GH replacement may reduce blood pressure in subgroups of GH-deficient adults with high baseline diastolic blood pressure, such as elderly patients or those with previous Cushing disease [215]. Atherosclerosis may be more prevalent in adults with GHD [212], who manifest increased carotid artery wall thickness, but early carotid artery atherosclerotic changes may be reversible with GH replacement [216]. In acromegaly, advanced cardiomyopathy is characterized by cardiomegaly, ventricular hypertrophy, replacement fibrosis, and cardiomyocyte degeneration [217]. Three stages of cardiovascular disease—early, intermediate, and late—have been identified in acromegaly [218]. Patients with acromegaly for a relatively short

duration display a “hyperkinetic” cardiovascular system with increased cardiac output and decreased total peripheral resistance. In contrast, untreated acromegaly and progression to more advanced stages is commonly associated with hypertension. The prevalence of hypertension is high in acromegaly patients (20 50%), due to expanded plasma volume, stimulation of smooth muscle cell growth leading to increased vascular resistance, and increased insulin resistance as a potential facilitator of increased blood pressure [219]. However, these patients do not have increased prevalence of coronary artery disease, carotid atherosclerosis, or carotid internal media thickness compared to normal subjects [220], possibly due to lower high-sensitive C-reactive protein [221].

Effects of GH and IGF-1 on Cardiac Structure and Function IGF-1 increases cardiomyocyte size [222] and protein synthesis [223]; there may also be a direct effect of GH, independent of IGF-1 [224]. IGF-1 promotes fibroblast collagen synthesis and GH increases cardiac collagen deposition [225], but collagen volume fraction is normal [226]. GH and IGF-1 may also modulate myocardial structure by preventing cardiomyocyte loss through apoptosis. IGF-1 acts as an inhibitor of ongoing apoptosis in the normal heart [227], and the antiapoptotic effect of GH/IGF-1 may confer myocardial protection during ischemic injury [228]. Cultured rat cardiac myocytes express GH and IGF-1 receptors, and IGF-1 induces cultured rat myocytes and delays apoptosis. At the same time, IGF-1 also sensitizes cultured rat myofilaments to calcium, thereby enhancing myocardial contractility. Moreover, locally produced IGF-1 promotes arterial cell growth, and paracrine IGF-1 contributes to inflammatory angiogenesis during atherosclerosis [229]. Untreated GH-deficient adults exhibit reduced left ventricular (LV) mass and cardiac output [230] and decreased exercise capacity [218], with more severe cardiac dysfunction in childhood-onset GH-deficient patients [229]. GH replacement therapy in adult GH-deficient patients has an anabolic effect on cardiac structure, resulting in improved diastolic and systolic function [231]. Increased cardiovascular morbidity and mortality are associated with both GH deficiency and GH excess [232]. Epidemiological studies suggest that lowernormal range IGF-1 levels in the general population may increase the risk of ischemic heart disease [233] and cardiac failure [234]. GH-deficient adults have several cardiovascular risk factors, including increased abdominal adiposity, insulin resistance, hypercoagulability, high total and LDL

I. HYPOTHALAMIC PITUITARY FUNCTION

GH EFFECTS ON RENAL FUNCTION

cholesterol, low HDL cholesterol, and decreased exercise performance and pulmonary capacity, all of which are negative cardiovascular risk factors for coronary artery disease [235]. Furthermore, GHD adults manifest less aortic distensibility and endothelial dysfunction, with higher fibrinogen, tissue plasminogen activator antigen and plasminogen activator inhibitor activity, and increased blood vessel intima-medial thickness. LV posterior wall and interventricular septal thickness is reduced, resulting in decreased LV mass index, and decreased LV internal diameter in adolescents with childhood-onset GHD. GH replacement improves peak exercise cardiac performance, and reduces carotid artery intima-medial thickness. GH replacement also has beneficial effects on lean body and fat mass, total and LDL cholesterol levels, and diastolic blood pressure [196], and may reduce the risk of premature cardiovascular mortality [195,236]. GH treatment has a beneficial effect on cardiovascular risk factors, including homocysteine and C-reactive protein [195], and may improve these cardiovascular risk factors and markers in hypopituitarism [237]. Body composition deteriorates in hypopituitary adults with increased body fat and decreased LBM. As extracellular water is decreased, which may result in reduced cardiac preload (Starling effect), decreased sweating, impaired thermoregulation, and increased risk for developing hyperthermia during exercise in hot environments [238] have been observed in GHdeficient adult patients. GH replacement therapy normalizes most cardiovascular risk factors observed in hypopituitary patients [237], and increased mortality in hypopituitary adults not treated with GH replacement is attributed to cardiovascular causes [239]. However, there are few data regarding the effects of long-term GH replacement therapy on cardiovascular morbidity and mortality.

GH Therapy in Congestive Heart Failure A meta-analysis suggests that GH treatment improves LV geometry, ejection fraction, and exercise parameters, and the improvement correlates with an increase in serum IGF-1 levels [240]. Discrepant data on IGF-1 levels in heart failure, suggest that low, normal, or even high IGF-1 levels might be attributable to variability in IGF-1 assays or inclusion of heterogeneous heart failure patients [241,242]. Using a GH provocative test to enroll only GH-deficient patients, heart failure patients treated with GH for 6 months in a randomized, single-blind study showed improved quality of life score, increased peak oxygen uptake, exercise duration, and flow-mediated vasodilation [243], with increased LV ejection fraction and reduced circulating

101

N-terminal natriuretic peptide. These results suggest a potential therapeutic role for GH in patients with congestive heart failure.

GH as a Biomarker of CVD As GH levels and CV risk factors and outcomes have been linked [244], GH could be considered a biomarker for CVD [245]. GH and IGF-1 receptors are expressed in the heart and blood vessels, and the GH IGF-1 somatotrophic axis influences cardiac structure, function, and peripheral resistance via effects on vascular tone and central sympathetic outflow [244]. Ghrelin may also influence cardiovascular remodeling [246], thereby linking the pituitary gastric axis to cardiometabolic disease. Acromegaly patients have an increased risk of CVD and increased CVD-related and all-cause mortality [244], suggesting a link between excess GH and CVD and supporting the role of GH level as a CVD biomarker. A large Swedish study [244] found that increased fasting GH levels was associated with increased risk of coronary artery disease, stroke, heart failure, CVD-related mortality, and all-cause mortality, such that each standard deviation (SD) increment in GH levels increased the odds of early fatal myocardial infarction by 54%. Nevertheless, an effect on estimated 10-year CVD-related mortality was modest as assessed by multivariate analysis.

GH EFFECTS ON RENAL FUNCTION GH and IGF-1 regulate renal development, glomerular function, and tubular handling of sodium, calcium, phosphate, and glucose. Renal GHRs localize to epithelial cells in the proximal and distal tubules, mesangial cells, and podocytes in the proximal straight tubule and the medullary thick ascending limb of the loop of Henle, and, in both mice and humans, in the collecting duct and distal nephron [247]. Renal IGF-1 originates from circulating IGF-1, which is mainly synthesized in the liver, and acts in an endocrine manner on target tissues, as well as from IGF-1 synthesized locally in kidney, which acts as an autocrine/paracrine regulatory factor for renal cell metabolism [247]. Higher IGF-1 levels in renal venous blood compared to renal arterial blood also suggest renal IGF-1 biosynthesis [248], although relative growth contributions of circulating and locally produced GHdriven IGF-1 are poorly understood. The GH/IGF-1 axis acts on all the component cells of the glomerulus. GH and especially IGF-1 stimulate mesangial cell proliferation and migration and inhibit

I. HYPOTHALAMIC PITUITARY FUNCTION

102

4. GROWTH HORMONE

podocyte function, with increased permeability of the filtration barrier [249]. The GH-IGF-1 system is a significant hormonal modulator of renal tubular sodium and water reabsorption. GH stimulates sodium and water reabsorption in the kidney tubule, with GH and IGF-1 acting together to induce epithelial sodium channel (ENaC)dependent transepithelial sodium transport in the distal nephron [250]. GH and IGF-1 also facilitate increased phosphate requirements during growth, likely via increased renal tubular phosphate retention. GH and IGF-1 effects on renal structure and function are apparent in patients with acromegaly and GH deficiency. The sodium-retaining GH and IGF-1 action in the distal tubule, with enhanced ENaC-dependent sodium transport, is associated with extracellular volume expansion and contributes to soft-tissue swelling and arterial hypertension in acromegaly. Chronic renal exposure to the growth-promoting effects of GH/IGF-1 in acromegaly results in renal hypertrophy [251], increased glomerular filtration, and renal plasma flow [252]. GH enhances glomerular filtration through IGF-1-mediated decreased renal vascular resistance, leading in turn to increased glomerular perfusion [253]. This effect rapidly reverses if acromegaly treatment is undertaken prior to development of structural renal changes.

producing a combined aliquot in which the IC-GH concentration is measured. The 24-hour IC-GH reflects the average GH concentration over a 24-hour period, eliminating peak or trough levels that might otherwise be obtained by single random sampling of GH.

Evaluation of GH Hypersecretion Increased serum IGF-1 levels are a consistent finding in acromegaly [254] and IC-GH levels show a log (dose) response correlation with serum IGF-1 levels [255]. The currently accepted diagnostic test of GH hypersecretion is failure of GH levels to be suppressed to less than 1 ng/mL within 2 hours following a 75-g oral glucose load using a two site immuno-radiometric assay or chemiluminescent assay [256]. In normal subjects receiving oral glucose loading, serum GH levels initially are suppressed and then subsequently increase as plasma glucose declines. However, in acromegaly, oral glucose fails to suppress GH to the normal range. GH levels may paradoxically increase in response to an oral glucose load, remain unchanged, or fall. As basal GH secretion is tonically elevated with minor bursts, a random GH value of less than 0.4 ng/mL invariably excludes the diagnosis of acromegaly [256].

Evaluation of GH Deficiency TESTS OF GH SECRETION Because of the pulsatile nature of pituitary GH secretion, a single random blood sample for GH measurement is not helpful in the diagnosis of GH hypersecretory or deficiency states, or GH neurosecretory disorders. Nonphysiologic provocative or suppression tests, or measurement of spontaneous GH secretion by 24-hour integrated serum GH concentration (IC-GH), are therefore employed to assess GH secretion.

Integrated 24-Hour GH Concentrations Pituitary GH secretion occurs episodically during waking hours, as well as during sleep, necessitating measurement over 24 hours [53] to accurately assess integrated GH secretion. Constant blood collection over a 24-hour period allows determination of a true mean or IC-GH, requiring a nonthrombogenic continuous withdrawal pump or patent indwelling catheter from subjects whose food intake and physical activity are not limited. Sampling intervals of 20 minutes are most widely used, but 5-minute and 30-second sampling frequencies detect significantly more pulses per hour. Samples from collection periods may be pooled,

Single GH and IGF-1 Measurements Single GH measurements are not helpful for diagnosis of GH deficiency, as GH secretion is pulsatile and daytime levels are often low in normal subjects and also suppressed after meals. Low IGF-1 levels are suggestive of GH deficiency, but are also encountered in malnutrition, acute illness, celiac disease, poorly controlled diabetes mellitus, liver disease, and estrogen ingestion. Fifteen percent of children diagnosed as GHdeficient by stimulation tests may have normal IGF-1 levels [257]. IGF-1 levels are normally very low before 3 years of age and highest in adolescence. Normal and GH-deficient children may have IGF-1 levels that overlap with those observed in infancy [258]. Furthermore, both normal and low IGF-1 levels are encountered in children with growth delay and genetic short stature [259]. IGF-1 levels do not always correlate with GH levels after provocative GH stimulation and low IGFBP-3 levels are also encountered in children with GH deficiency. Importantly, normal IGF-1 levels occur in about 20% of patients with proven GHD. Provocative Tests Provocative testing for GHD should only be undertaken in the clinical context of probable GHD (childhood history of GHD or a clinical context predisposing

I. HYPOTHALAMIC PITUITARY FUNCTION

TESTS OF GH SECRETION

to adult GHD). Dynamic testing of GH reserve involves stimulation of somatotrophs to elicit GH in response to a pharmacologic stimulus. Several GH stimulatory agents have been utilized, including insulin, clonidine, arginine, L-dopa, GHRH, propranolol, and glucagon. Diagnosis of adult GHD is established by demonstrating a subnormal rise in peak serum GH levels elicited in response to one or more dynamic stimulation tests. Insulin-Induced Hypoglycemia (Insulin Tolerance Test) This reliable stimulus for GH secretion is the historical standard provocative test [260]. Regular insulin 0.1 IU/kg is administered intravenously to decrease basal glucose levels by 50% to a value below 40 mg/dL. Maximal GH secretion peaks at 30 60 minutes. Patients may experience symptoms of hypoglycemia, including light-headedness, anxiety, tremulousness, sweating, tachycardia, seizures and, rarely, unconsciousness. Insulin-induced hypoglycemia is contraindicated in patients with a history of seizure disorder, coronary artery disease, or age over 55 years. The test should be performed under close supervision, and intravenous glucose (50%) should be readily at hand for rapid administration. The risk of inducing profound hypoglycemia is greater in GH-deficient patients because of increased insulin sensitivity. A potential advantage of the ITT is the ability to simultaneously assess the hypothalamic pituitary adrenal axis for adrenal insufficiency. Clonidine This α-adrenergic agonist stimulates GH release via a central action. Clonidine (0.15 mg/m2) is administered orally, with a maximum GH secretory peak occurring after 60 90 minutes. Patients may experience drowsiness, with decreased systolic blood pressure in sodium-depleted GH-deficient adults at doses required to release GH (0.25 0.30 mg orally). Clonidine, although used as a stimulus for GH release in children, is not reliable to assess GHD in adults. L-Dopa/Propranolol L-Dopa induces GH release by stimulating hypothalamic dopaminergic receptors. Adrenergic blockade using propranolol enhances GH response to L-dopa. Ldopa is administered orally according to the patient’s weight (125 mg if weighing ,30 kg; 250 mg if 10 30 kg; and 500 mg if .30 kg) together with propranolol 0.75 mg/kg (maximum dose 40 mg) after an overnight fast. Maximum GH secretion is elicited after 60 90 minutes. L-dopa is effective in stimulating GH release and rarely results in adverse effects.

103

Arginine/GHRH Arginine potentiates maximal somatotroph responsiveness to GHRH by inhibiting release of somatostatin from the hypothalamus. GHRH directly elicits GH secretion from pituitary somatotroph cells, potentiated by arginine. After an overnight fast, GHRH (1 μg/kg) is administered as an intravenous bolus at 0 minutes with arginine (30 g) in 100 mL infused from 0 to 30 minutes, with subsequent blood sampling for GH performed every 15 minutes for 90 minutes. Combined arginine/GHRH responses are age-independent. This highly reproducible GH provocative test [261] is at least as sensitive as insulin-induced hypoglycemia [262,263]. The arginine/GHRH test has been validated as a reliable alternative test when the ITT is contraindicated or impractical [262], and is endorsed by the Endocrine Society [260], the American Association of Clinical Endocrinologists [264], and the GH Research Society [265]. Ghrelin Mimetics GHRPs are synthetic secretagogues that elicit dosedependent and specific GH release by binding to GH5R, for which ghrelin has been shown to be the natural ligand [25]. GHRPs can be administered alone or in combination with GHRH. Combined administration of GHRP-6 and GHRH is the most potent stimulus to GH release, with excellent reproducibility and no serious side effects [23]. GHRH/GHRP-6 is highly specific, but is less sensitive than ITT. It is a viable alternative to the ITT in patients with organic pituitary disease, but overlap has been reported between GH levels attained in the control group and severely GH-deficient patients. Since GHRH and GHRP act directly on the pituitary, coadministration restores GH secretion in patients with hypothalamic disease [266]. GHRP-2 administration has different diagnostic cut-off points in adult GHD compared to ITT, and is highly reproducible [267]. A multicenter study comparing the oral GH secretagogue macimorelin with arginine/GHRH found it to be safe, convenient, and of comparable efficacy (82% sensitivity, 92% specificity, and 87% accuracy in diagnosing adult GHD), with a GH cut-off point of 6.8 μg/L for patients with a body mass index (BMI) ,30 kg/m2 and 2.7 μg/L for patients with a BMI .30 kg/m2 [268]. GHRPs are not currently commercially available in the United States. Glucagon Glucagon elicits GH secretory potency similar to or only slightly lower than the ITT for differentiating GHdeficient patients from normal subjects [269]. The

I. HYPOTHALAMIC PITUITARY FUNCTION

104

4. GROWTH HORMONE

mechanism of glucagon-induced GH release is not fully understood. After fasting for at least 8 hours, 1 mg glucagon (1.5 mg if patient weight .90 kg) is administered intramuscularly, with serum GH and capillary blood glucose levels measured every 30 minutes for 4 hours. Glucagon stimulation is contraindicated in malnourished patients and in patients who are fasting for 48 hours. Side effects include nausea and late hypoglycemia. A normal response is defined as a GH peak above 3 μg/L; in adults with GHD, GH levels do not rise above 3 μg/L. With the unavailability of GHRH in the United States, glucagon stimulation has been increasingly used as an alternative to the ITT because of its availability, reproducibility, safety, and lack of influence by gender and hypothalamic cause of GHD [270].

Approach to Provocative GH Testing As GH reserve testing is expensive and also fraught with high rates of false-positive results, patients should fulfill at least one of these rigorous preexisting criteria [271]: (1) young adults transitioning from adolescence who required GH therapy for short stature and had demonstrated anatomic genetic or acquired cause of short stature; (2) evidence for a pituitary lesion or damage including surgery or irradiation; (3) MRI evidence for a sellar lesion, pituitary hypoplasia, hypophysitis, or infiltration; (4) history of significant head trauma. Multiple sampling of GH levels most accurately reflects GH secretion. However, it is not practical in clinical practice as GH secretion is influenced by age, nutritional status, exercise, and BMI [262]. Provocative tests of GH secretion are employed when patients suspected of having GHD require confirmation of the diagnosis [260]. Serum IGF-1 levels below the age-adjusted normal range, in the absence of liver dysfunction and catabolic disorders, usually indicate GH deficiency [265]. However, the finding of normal IGF-1 levels does not exclude the diagnosis of GHD [262] and GH provocative testing is still required for diagnosis in the appropriate clinical setting [260]. A single GH stimulation test is sufficient to confirm the diagnosis of adult GHD [265], and provocative GH testing is not required for hypopituitary patients, those with serum IGF-1 levels below the reference range, and those exhibiting three or more other pituitary hormones deficits, as these patients have a .97% chance of having GHD [272]. Historically, the ITT has been the “gold standard” GH provocative test, but is contraindicated in patients with seizure disorders or cardiovascular disease and requires intensive monitoring. Combined arginine/

GHRH testing is considered a reliable alternative, with 95% sensitivity and 91% specificity at a GH cutoff of 4.1 ng/mL, compared to 96% sensitivity and 92% specificity for the ITT with an optimal GH cutoff of 5.1 ng/mL [262]. A caveat for the arginine/GHRH test is the falsely normal GH response in patients with GHD due to hypothalamic disease, in whom GHRH directly stimulates the pituitary gland [273]. The relative performance of ITT and arginine/GHRH stimulation is comparable; however arginine alone, clonidine, levodopa, and the combination of arginine plus levodopa are less robust tests for the diagnosis of adult GHD [262]. The GH cutoff for diagnosis of GHD varies with the test used. A peak GH response of ,3 ng/mL during ITT and glucagon test confirms the diagnosis of GHD. Relative adiposity in the abdominal region blunts GH responses to stimulation [274], and thus cutoffs for arginine/GHRH testing have been validated by BMI: validated GH cutoff levels are defined as peak GH ,11 ng/mL for patients with BMI ,25 kg/m2, peak GH ,8 ng/mL for BMI 25 30 kg/m2, and peak GH ,4 ng/mL for BMI .30 kg/m2 [275]. Lack of age- and gender-adjusted normative data, as well as assay variability influence definitions of GH cutoff diagnostic criteria. GH levels between 3 and 5 ng/mL were previously defined using polyclonal radioimmunoassays. Cutoff values for newer, more sensitive, two-site assays have not been rigorously defined [265]. However, GH values of 5.1 ng/mL and 4.1 ng/mL have been proposed using ITT and arginine/GHRH respectively using immunochemiluminescent two-site assays [262]. Pediatric patients with idiopathic GHD (either isolated or with one additional hormone deficit) should be retested for GHD after completion of puberty [276] after discontinuing GH treatment for at least a month [260]. Patients with a high likelihood of permanent GHD and who may not require retesting after puberty include those with radiologically confirmed sellar/ suprasellar abnormality, a transcription factor mutation, or acquired hypothalamic pituitary disease, as well as those who have had hypothalamic pituitary surgery or who have undergone hypothalamic pituitary radiation. Lack of international assay standardization further hinders the definition of GH cutoff values. Analytic methods used in individual assays influence GH results and ideally assay-specific cutoff values should be defined for each provocative test [265]. The calibrant used in the assay, GH isoform detected, as well as the presence or absence of GHBP all influence GH assay results. Adoption of a universal GH calibration standard would be valuable in the international harmonization and standardization of GH provocative test results.

I. HYPOTHALAMIC PITUITARY FUNCTION

TESTS OF GH SECRETION

Variability of GH Assays The comparison of results of various GH assays obtained in different laboratories is difficult because of differences between several aspects of the immunoassays. Older assay methods employed polyclonal competitive techniques and were relatively insensitive. Newer sensitive noncompetitive sandwich-type GH immunoassays employ antibodies directed against different epitopes on the surface of the GH molecule. One antibody captures the GH molecules, whereas the second labeled antibody generates a signal proportional to the amount of GH in the sample. Also, older radioimmunoassays used radiolabelled GH, whereas newer nonradioactive sandwich-type assays employ various labels, including enzyme-linked, fluorescence, and chemiluminescence. Different circulating forms of GH are not all recognized in GH assays. Because monomeric 22k is the only GH form available as a standard in sufficient purity and quantity, and because monomeric 22k is also the most abundant circulating form, it is used as the basis for GH measurement. Other GH forms are recognized to varying and largely unknown degrees. Thus, different antibodies or assay protocols yield different results. Polyclonal antibodies used in the early radioimmunoassays recognized several molecular forms of GH, thus inducing higher estimates of GH compared to newer immunometric assays employing highly specific monoclonal antibodies. GH standards also affect comparison of GH values in different laboratories. In 1994, the first international standard for somatotrophin, IRP 88/624, was prepared by the World Health Organization (WHO) using recombinant technology in contrast to the previous standards prepared from pituitary extracts. Use of an international standard enables uniformity of calibration between different GH kits, and provides an opportunity for international use of a single calibrant for GH assays. The recent recombinant International

105

Standard (IS) preparation, WHO IS 98/574, is a recombinant 22-kDa GH of .95% purity. GHBPs may influence GH estimates by interfering in some GH assays, as approximately 50% of circulating GH is complexed to GHBP. GHBP present in high concentrations in serum samples can block epitope accessibility of respective antibodies used in GH assays, and lead to underestimation of GH concentrations. However, as GHBP has a greater affinity for the 20-kDa GH molecule, it presumably does not interfere in GH estimates in the new GH assays, specific for the 22-kDa GH molecule [277]. GH immunoassay heterogeneity thus poses a major challenge in the definition of standards for the diagnosis of GHD. Different conversion factors are used to report GH assay results in mass units, a further cause for assay variability. In one series, a borderline GH value obtained from a patient with suspected acromegaly, was sent to 104 laboratories for analysis [278] (Fig. 4.9). The median GH was 2.6 mU/L (range 1.04 3.5 mU/L). When a conversion factor of 3.0 (1 μg/L 5 3 mU/L) was used, 11% of result values were consistent with acromegaly; with a conversion factor of 2.6, 55% diagnosed acromegaly, whereas using a conversion factor of 2.0, 86% diagnosed acromegaly. Reliable and harmonious GH assays with robust reference standards still need to be developed [279].

Variability of IGF Assays Serum IGF-1 levels are regulated by GH, as well as nutrient intake, estrogen, thyroid, cortisol levels, and IGFBPs. Testosterone, age, gender, ethnicity, and BMI also influence IGF-1 levels [280]. IGF-1 levels increase until puberty and then decline (Fig. 4.10), necessitating adequate age-adjusted ranges with large numbers of healthy male and female control subjects within each age range [281].

FIGURE 4.9 The impact of conversion factors (CFs) on GH results. 1, Immunotech IRMA; 2, Wallac DELFIA; 3, NETRIA IRMA; 4, Nichols Allegro IRMA; 5, Tosoh AIA; 6, Nichols Advantage; 7, Nichols ICMA; 8, Beckman Access; 9, DSL ELISA; 10, DPC Immulite; 11, DPC Immulite 2000; 12, in-house ELISA; 13, DiaSorin IRMA; 14, in-house IRMA. The lines present the Cortina Consensus cutoff value transformed into mU/L using various CFs: solid line, CF 3, 11% of results consistent with active acromegaly; wide dashed line, CF 2.6, 55% acromegaly; narrow dashed line, CF 2, 86% active acromegaly. Source: From Pokrajac A, Wark G, Ellis AR, Wear J, Wieringa GE, Trainer PJ. Variation in GH and IGF-I assays limits the applicability of international consensus criteria to local practice. Clin Endocrinol (Oxf) 2007;67(1):65 70.

I. HYPOTHALAMIC PITUITARY FUNCTION

106

4. GROWTH HORMONE

FIGURE 4.10 Left, 24-h integrated GH levels in 173 nonobese subjects, aged 7 65 years, stratified by age decades. From Zadik [96]. Right, age- and gender-specific IGF-1 levels in 3900 healthy subjects. Serum IGF-1 levels measured by Nichols Advantage Assay. Source: From Brabant G, von zur MA, Wuster C, et al. Serum insulin-like growth factor I reference values for an automated chemiluminescence immunoassay system: results from a multicenter study. Horm Res 2003;60(2):53 60.

Commonly used commercial IGF-1 assays are mostly calibrated against the older standard preparation WHO 87/518, and have similar sensitivities and coefficients of variation, but exhibit marked nonlinear differences in comparative studies [282]. Newer assays are calibrated against the 02/254 standard [283]. Reliable assays require validation of recovery of exogenous IGF-1, crossreactivity with IGF-2, and assay reproducibility, as well as comparison of sample types, in order to ensure accurate data. Validation of reported results should be published in the kit inserts of commercial assays [284]. Meaningful interpretation of IGF-1 concentrations requires rigorous assay- and age-specific reference ranges. High-quality, method-specific reference ranges and a high degree of IGF-1 assay methodological consistency are essential for reliable comparison of results across studies and for long-term monitoring acromegaly therapy and GH replacement therapy in GHdeficient patients. IGF-1 immunoassays, in which antibodies competitively or noncompetitively bind to IGF-1, are commonly used to measure circulating IGF-1 levels. No universally accepted assay has clearly emerged. Available immunoassays employ different methodologies, laboratory protocols, and reference ranges [283,284], resulting in a well-documented lack of consistency in IGF-1 measurement. Wide variations in IGF-1 measurement are encountered when submitting the same patient samples to multiple laboratories using the same assay [278]. Furthermore, lot-to-lot variation with current immunoassays resulted in a nearly twofold range of results, observed in two laboratories using the same reagents, over a 5-year period [285].

Several factors influence the results of IGF-1 immunoassays [279]. Under physiologic conditions, approximately 99% of IGF-1 is bound to IGFBPs, predominantly IGFBP-3 [286], and improper extraction techniques dissociating the IGF-1/IGFBP complex may undermine assay performance [279]. Antibody specificity for IGF-1 also varies between assays, which affects measurement of IGF-1 levels. Reference ranges for many of the currently used immunoassays were developed based on a small number of samples and/or a short age span, so reported “normal” and “abnormal” values may not be reflective of all adult patients. Comparing results obtained from IGF-1 assays calibrated to the older WHO 87/518 standard against those from assays calibrated to the new 02/254 standard can be challenging [279,284,287]. The chemiluminescent IGF-1 immunoassay IDSiSYS (Immunodiagnostic Systems; Tyne & Wear, United Kingdom) was developed in an attempt to avoid pitfalls of commonly used immunoassays [283]. IDS-iSYS immunoassay reference ranges are robust, ensuring that they are representative of the general population. The immunoassay was calibrated to the new 02/254 standard, and specificity of the two mouse monoclonal antibodies used was validated against recombinant human IGF-1, exhibiting adequate dissociation of IGF-1/IGFBP complexes and preventing IGFBP interference [283]. Liquid chromatography mass spectrometry (LC-MS) has emerged as an alternative to immunoassay. Unlike immunoassays, which introduce antibodies to bind and separate target antigens, LC-MS selects the target analyte based on mass, and quantifies it based on a unique mass: charge ratio after passing through an electric ion

I. HYPOTHALAMIC PITUITARY FUNCTION

CLINICAL USE OF GH

field [288 290]. The narrow-mass-extraction, highresolution LC-MS (Quest Diagnostics, San Juan Capistrano, California, USA) selects IGF-1 from the serum sample using a one-step extraction for dissociation of IGF-1/IGFBP complexes, and does not employ antibodies to isolate serum IGF-1 molecules, mitigating concerns about antibody specificity or interference at IGF-1/IGFBP binding sites [291]. This assay has a robust set of sex- and age-specific reference ranges, and is calibrated to the 02/254 WHO IGF-1 standard. Although mass spectrometry technology is not limited by the pitfalls of immunoassay technology, it is susceptible to interlaboratory variability [292]. In the absence of a standardized kit, there is concern for both technical and human laboratory error, and discrepancies between laboratories using the same technology have been reported [289].

107

TABLE 4.5 Indications for GH Therapy APPROVED USE Adults GH deficiency AIDS-associated muscle wasting Children GH deficiency Idiopathic short stature Turner syndrome Born small for gestational age Chronic renal insufficiency SHOX gene deficiency INVESTIGATIONAL Frailty

CLINICAL USE OF GH

Osteoporosis Catabolic states, cachexia

GH Therapy in Childhood

Burns

Recombinant hGH is administered to promote linear growth in short children. In the United States, the FDA has approved GH treatment for the following conditions: GH deficiency, idiopathic short stature (ISS), chronic kidney disease, Turner syndrome, Prader Willi syndrome, SHOX gene haploinsufficiency, Noonan syndrome, and small for gestational age (SGA) age infants (Table 4.5). The efficacy of GH treatment in children with non-GHD growth disorders is also well-established. However, individual responses are variable, and prediction of adult height is guarded. Long-term follow-up studies do not confirm a higher incidence of neoplasia in children or adults who received GH therapy in childhood. However, in light of high GH doses employed, careful monitoring of IGF-1 and IGFBP-3 is recommended. Isolated GH deficiency may be due to congenital or acquired causes and is most commonly idiopathic. About 10% of patients with sporadic GH deficiency exhibit identifiable mutations [293], while up to 30% exhibit familial patterns of inheritance. Hereditary GH deficiency (Table 4.6) may be due to mutations occurring at each level of the hypothalamic pituitary/ GH-IGF-1 axis. Mutations of the GHRH receptor, transcription factors determining GH synthesis, the GH molecule itself, or peripheral GHR may all lead to short stature [294,295]. True familial isolated GHD may occur as four distinct syndromes (Table 4.7). Childhood GH deficiency ranges from complete absence of GH associated with severe growth retardation to partial GH deficiency resulting in short stature. Diagnosis is based on decreased height ( .2.5 SDs below the mean for age-matched normal children),

Postoperative recovery Wound healing Parenteral nutrition Ovulation induction Immune deficiency

poor growth velocity (,25th percentile), delayed bone age, and a predicted adult height below mean parental height [296]. GHD is usually confirmed by inadequate pituitary GH responses to standard provocative stimuli. Combined clinical evaluation and provocative testing are used in assessment and concomitant endocrine deficiencies, especially hypothyroidism, should be corrected to maximize growth-promoting benefits of hGH. GH replacement should be started as early as possible before height drops below the third percentile, as total height gain is inversely proportional to the pretreatment chronologic and bone age, as well as severity of GH deficiency. The most pronounced acceleration in linear growth rate occurs during the first 2 years of treatment. Dose and frequency of administration of hGH both influence height velocity.

Idiopathic Short Stature ISS describes otherwise normal children who are at or below the 5th percentile for height, with normal GH responses to provocative stimuli. This group of

I. HYPOTHALAMIC PITUITARY FUNCTION

108

4. GROWTH HORMONE

TABLE 4.6 Etiology of Inherited GH Deficiency STRUCTURAL Pituitary aplasia Pituitary hypoplasia CNS masses RECEPTOR MUTATION GHRH mutation-GH deficit TRANSCRIPTION FACTOR MUTATION Gene

Chromosome Deficiency

Pituitary

Associated malformations

Inheritance

POU1F1 3p11

GH, PRL, 6 TSH

Normal or hypoplastic

Recessive, dominant

PROP1

5q35

GH, PRL, TSH, LH, FSH

Normal, hypoplastic, hyperplastic, or cystic

Recessive

HESX1

3p21

GH, PRL, TSH, LH, FSH, ACTH; posterior defects

Hypoplastic or hyperplastic; normal or ectopic posterior

PITX2

4q25

GH, PRL, TSH, FSH, LH

LHX3

9q34

GH, PRL, TSH, LH, FSH

Hypoplastic or hyperplastic

LHX4

1q25

GH, TSH, ACTH

Hypoplastic

GH, PRL, TSH, LH, FSH, ACTH

Hypoplastic

Eye malformations Anophthalmia, esophageal atresia

OTX2 SOX2

3q26

GH, FSH, LH

Hypoplasia, mid-brain defects

SOX3

Xq27

GH, TSH, ACTH, FSH, LH

Hypoplasia, ectopic posterior pituitary

IGSF1

Xq25

GH, PRL, TSH

Septo-optic dysplasia

Recessive

Rieger syndrome

Dominant

Stubby neck with rigid cervical spine

Recessive Dominant Dominant/ negative

X-linked recessive Testicular enlargement

X-linked recessive

HORMONE MUTATION GH1-GH deficiency Bioinactive GH-GH deficiency From Kaiser U, Ho KKY. Pituitary physiology and diagnostic evaluation. In: Melmed S, Polonsky KS, Larsen PR, Kronenberg HM, editors. Williams Textbook of Endocrinology, 13th edition. Elsevier, Philadelphia; 2016. p. 176 231.

TABLE 4.7 Genetic Forms of Isolated GH Deficiency Type Inheritance

Phenotype

Gene

Mutations

IA

Autosomal recessive

Severe short stature; serum GH undetectable; anti-GH antibodies on treatment

GH1

Deletions, frameshift, nonsense

IB

Autosomal recessive

Less severe short stature; serum GH low but detectable; no anti-GH antibodies on treatment

GH1 Splice site, frameshift, GHRHR missense, nonsense

II

Autosomal dominant

Variable height (severe short stature to normal); normal or hypoplastic anterior pituitary; other pituitary hormone deficiencies

GH1

III

X-linked

GH deficiency with agammaglobulinemia with or without mental retardation; ectopic posterior pituitary on MRI

SOX3 Deletions, expansions, Others? others

Splice site, missense, splice enhancer; intronic deletions

From Alatzoglou KS, Dattani MT. Genetic causes and treatment of isolated growth hormone deficiency-an update. Nat Rev Endocrinol 2010;6(10):562 76.

I. HYPOTHALAMIC PITUITARY FUNCTION

109

GH THERAPY IN ADULTS

short-stature children may also harbor as yet unidentified mutations. Children with ISS are normal size at birth, but grow slowly during early childhood so that the average height falls below 2.0 SD by school-age, maintaining a height velocity within the lower normal range, growing below but parallel to the normal centile channels. Untreated adult height is below the normal range and below mid-parental height by about 1 SD. Increasing numbers of genetic defects in genes associated with GH/IGF-1 secretion/growth, resulting in short stature, and previously labeled as ISS, have been described, including pituitary gene defects (GHSR and GH1 locus), defects in the GHR and intracellular signaling, and defects in GHR extracellular domain (Sta5b and SH2) and growth plate (SHOX transcription factor deficiency) [297]. GH administered to children with ISS induces an adult height gain of between 3 and 7 cm, depending on the duration of treatment [181]. Rates of adverse events associated with GH therapy in children with ISS are lower compared to side-effect profiles observed in other GH-treated disorders, as these children are generally otherwise healthy.

Turner Syndrome Patients with Turner syndrome manifest dysmorphic body features, ovarian failure, and reduced growth rate, starting during intrauterine life and continuing through childhood and puberty, resulting in reduced final adult height. GH therapy in girls with Turner syndrome increases predicted height, with a greater increase in height when treatment is started early and when estrogen replacement is postponed until at least age 14 years. In a randomized controlled study, mean adult height was 7 cm greater than the untreated group after 6 years [298]. However, GHtreated Turner syndrome patients, may manifest increased incidence of type 2 diabetes mellitus.

Children Born Small for Gestational Age Children with a birth length at least 2 SD below the mean are defined as SGA. Poor fetal growth may be idiopathic, or due to maternal toxins or associated with defined syndromes. Most SGA infants experience catchup growth within the first or second year of life; the remaining 10 15% increase adult height by approximately 1.0 1.4 SD with long-term GH treatment [299].

malnutrition, acid base disturbances, hyperparathyroidism, or GH insensitivity manifest by elevated GH levels and reduced IGF-1:IGFBP ratios with decreased free IGF-1 concentrations. Decreased renal GH clearance is also present, with consequent high basal and elicited GH levels. Approximately one-third of children with chronic renal insufficiency have heights below the third centile [300]. Growth patterns vary depending on age of onset of renal insufficiency. Despite high GH levels, children with renal failure are short. Following renal transplantation, return to normal growth is variable. A meta-analysis of randomized controlled trials concluded that catch-up growth occurred in the first year of treatment and continued GH treatment likely prevents progressive growth failure [301]. The indication for GH treatment in chronic renal insufficiency is growth failure (subnormal height velocity) rather than short stature. GH treatment elicits a doubling of pretreatment height velocity in the first year of treatment [302]. Children with chronic renal insufficiency receiving GH treatment should be carefully monitored for impaired glucose tolerance, as they have relative glucose intolerance, even in the absence of GH treatment.

SHOX Gene Deficiency The SHOX gene, at the distal ends of the X and Y chromosomes, encodes a homeodomain transcription factor responsible for a significant proportion of long bone growth. Deficiency leads to atypical proliferation and differentiation of chondrocytes, with delayed bone growth in intrauterine and postnatal growth. The SHOX gene plays a role in the short stature of Turner syndrome, Leri Weill syndrome, and some cases of ISS. A recent study showed that 57% of patients with SHOX gene deficiency and 32% of Turner syndrome patients treated with GH for a mean of 6 7 years reached a final height greater than 2 SD, with no effect in pubertal maturation [303].

GH THERAPY IN ADULTS In adults, GH is indicated for GH deficiency, muscle wasting due to HIV/AIDS, and short bowel syndrome.

Adult GHD Syndrome Etiology

Chronic Renal Insufficiency Chronic renal insufficiency is frequently associated with growth failure, which may be due to protein-calorie

The diagnosis of adult GHD (AGHD) should be suspected in patients with hypothalamic or pituitary disease, or in those with a history of having received

I. HYPOTHALAMIC PITUITARY FUNCTION

110

4. GROWTH HORMONE

cranial irradiation or pituitary adenoma treatment, or prior traumatic brain injury or subarachnoid hemorrhage. AGHD may be isolated or can occur in association with several other pituitary hormone deficiencies (panhypopituitarism). Childhood-onset GHD is most commonly idiopathic, but may be genetic, or associated with congenital anatomical malformations in the brain or sella turcica region (Table 4.8). AGHD may follow childhood-onset GHD, which persists into adulthood or can be acquired in adulthood secondary to structural sella and parasellar lesions or may be secondary to head trauma. In the United States, the incidence of AGHD is about 6000 new cases annually, with a population prevalence of about 50,000 cases [304]. In recent years, with the availability of recombinant GH, the pattern of AGHD diagnosis has changed, with an increase in idiopathic etiologies accounting for about 17% of patients in the HypoCSS surveillance database [305]. Pitfalls of diagnosing isolated adult GDH include inappropriate testing and less rigorous diagnostic criteria [271]. The most common cause of AGHD is a pituitary macroadenoma (30 60% of which are associated with single or multiple pituitary hormone deficiencies), or pituitary adenoma treatment (surgery or radiotherapy). Up to 20% of patients who sustain traumatic brain injury subsequently develop GHD with varying degrees of concomitant hypopituitarism [306]. GHD is usually the first hormone deficiency to develop when pituitary damage occurs; thus in patients diagnosed with multiple pituitary hormone deficits, the likelihood of GHD is high. The incidence of hypopituitarism associated with pituitary irradiation increases over time, with 50% of patients diagnosed with varying degrees of hypopituitarism 10 years after having received conventional radiotherapy. Diagnosis GHD adults have altered body composition with increased fat mass and decreased muscle volume and strength, decreased bone mineral density, altered glucose and lipid metabolism, lower psychosocial achievement, and possibly increased mortality due to cardiovascular disease (Table 4.9). These patients have a lower employment rate, are more often on sick leave or disability, and either live alone or with parents [308]. IGF-1 is a robust screening test in lean, younger patients (,40 years) suspected of having GHD. However, at any age, screening IGF-1 levels in hypopituitary adults may be normal in the presence of severe GHD. Other causes of low IGF-1 levels include liver disease and malnutrition. Single GH serum measurements are not informative. The diagnosis of AGHD is confirmed by

TABLE 4.8 Causes of GH Deficiency PRESENTING IN CHILDHOOD Congenital Idiopathic Embryologic defects (structural) Agenesis of corpus callosum Hydrocephalus Septo-optic dysplasia Arachnoid cyst Empty sella syndrome Genetic Transcription factor defect GHRH receptor defect GH gene defect GH receptor/postreceptor defect GH resistance Laron dwarfism Pygmy Neurosecretory defects Radiation for brain tumors, leukemia Head trauma Perinatal birth injury Child abuse Accidental Inflammatory diseases Viral encephalitis Meningitis, bacterial, fungal, tuberculosis ACQUIRED IN ADULTHOOD Pituitary/hypothalamic/tumors Pituitary adenoma Craniopharyngioma Rathke’s cleft cyst Metastasis Parasellar tumors Germinoma Astrocytoma Postpituitary surgery Head trauma Hemochromatosis Sickle cell disease Thalassemia Lymphocytic hypophysitis Cranial irradiation Infiltrative/granulomatous/infectious disease Histiocytosis Sarcoidosis Idiopathic Tuberculosis Syphilis Vascular Acromegaly treatment

provocative testing of GH secretion after other hormonal deficits have been adequately replaced (Table 4.10). A single stimulation test is adequate for the diagnosis of AGHD, but not all patients suspected of having GHD require a GH stimulation test for diagnosis. Adult patients with three or four pituitary hormone deficits and a low IGF-1 level, do not require GH stimulation testing to establish the diagnosis [272].

I. HYPOTHALAMIC PITUITARY FUNCTION

111

GH THERAPY IN ADULTS

The ITT remains the test of reference despite concerns about reproducibility, safety, and specificity. ITT is contraindicated in adults with ischemic heart disease and seizure disorders, and is a potential risk in elderly patients as occult vascular disease increases with age. ITT requires close clinical monitoring to attain adequate hypoglycemia, with prompt reversal of severe insulininduced hypoglycemia to avoid neuroglycopenia. Sensitive and reliable alternative GH stimulants have been evaluated. The GHRH-arginine test, with 95% sensitivity, and 91% specificity, at a GH cutoff of 4.1 μg/L compares very favorably to the ITT, with a GH cutoff of 5.1 μg/L (96% sensitivity and 92% specificity) [262]. Arginine alone, clonidine, levodopa, and arginine plus levodopa are not reliable alternatives to the ITT. GHRH-arginine is well-tolerated and requires less monitoring than the ITT. However, two caveats should be considered when interpreting results of GHRHarginine stimulation testing: the impact of increased BMI on GH secretion, and whether the GHD is due to hypothalamic or pituitary damage. TABLE 4.9 Physical Findings in the Adult Growth Hormone Deficiency Syndrome Truncal adiposity Increased waist/hip ratio Thin, dry, cool skin Reduced exercise performance Reduced muscle strength Reduced bone mineral density Depressed mood Psychosocial impairment From Carroll [307].

TABLE 4.10

Obese subjects have reduced spontaneous and stimulated GH secretion; negatively associated with BMI [274]. Diagnostic GH cutoff values have been evaluated for lean (BMI , 25 kg/m2), overweight (BMI .25 but ,30 kg/m2), and obese (BMI .30 kg/m2) subjects, with high sensitivity and specificity for GH deficiency. In lean subjects, a peak GH cutoff point of 11.5 μg/L had the highest sensitivity and specificity using receiver operator characteristics (ROC) curve analysis; in the overweight and obese population lower cutoff points were determined, at 8 4.2 μg/L [275]. To avoid false-positive responses in overweight and obese subjects, and false-negative results in lean subjects, BMI must be considered in the interpretation of GH responses to GHRH-arginine provocative stimulation, and approximate GH cutoff points must be considered. GHRH stimulates the pituitary directly, and thus falsely “normal” responses can be elicited in patients with hypothalamic GHD, because exogenously administered GHRH directly stimulates pituitary somatotroph cells. Therefore, in patients with suspected hypothalamic damage (e.g., after cranial irradiation), the peak GH response to GHRH and arginine may be normal, whereas the ITT may reveal an abnormal response [273]. GHRH-arginine is accepted as a reliable alternative to the ITT [260,265], however, since 2008, GHRH has been difficult to obtain in the United States. Glucagon stimulation testing is a well-tolerated alternative GH provocative test to GHRH-arginine. Glucagon is relatively inexpensive and widely available for treating hypoglycemia in patients with diabetes mellitus. Glucagon is contraindicated in patients who have fasted for more than 48 hours. Side-effects may include nausea and late hypoglycemia, which can be prevented by eating small frequent meals after completing

Diagnostic Tests for Adult GHD

Test

Procedure

Interpretation/expected normal response

Insulin tolerance

• Administer insulin 0.05 0.15 U/kg IV • Sample blood at 30, 0, 30, 60, and 120 min for GH and glucose

• Glucose should drop ,40 mg/dL (2.2 mmol/L) • GH should be .3 5 μg/L • Cutoffs for GH response are BMI-related

GHRHarginine

• Administer GHRH 1 μg/kg (max 100 μg) IV followed by arginine infusions 0.5 g/kg (max 35 g) over 30 min • Sample blood at 0, 30, 45, 60, 75, 90, 105, and 120 min for GH

• Can give false-normal GH response if GHD is due to hypothalamic damage (e.g., following radiation) • Cutoffs for GH response should be correlated to BMI (obesity may blunt GH response to stimulation)

Glucagon

• Administer glucagon 1 mg (1.5 mg if weight .90 kg) IM • Sample blood at 0, 30, 60, 90, 120, 150, 180, 210, and 240 min for GH and glucose

From Molitch ME, Clemmons DR, Malozowski S, Merriam GR, Vance ML, Endocrine S. Evaluation and treatment of adult growth hormone deficiency: an endocrine society clinical practice guideline. J Clin Endocrinol Metab 2011;96(6):1587 609.

I. HYPOTHALAMIC PITUITARY FUNCTION

112

4. GROWTH HORMONE

the test. Glucagon is administered intramuscularly and GH is measured half-hourly for 4 hours. In adults with GHD, GH levels do not rise above 3 μg/L. Using ROC analysis, a GH cutoff value of 3 μg/L provides the best pair of sensitivity (100% and 97%, respectively) and specificity (100% and 88%, respectively). Unlike the GHRH-arginine test, there is no inverse correlation between BMI and peak GH response to glucagon [270].

TABLE 4.11

Effects of GH Replacement in Adults

Clinical consequence

Effect of GH replacement

BODY COMPOSITION General and central adiposity

Decrease

Reduced lean mass

Increase

Reduced bone mass

Increase

FUNCTION

GH Replacement Therapy

Reduced exercise capacity

Improve

GH secretion continues into adulthood, and GH influences many metabolic systems other than growth. The goal of GH replacement therapy in adulthood is to correct the metabolic, functional and psychological deficiencies associated with rigorously diagnosed adult GHD (Table 4.11). GH replacement may also reduce mortality associated with pituitary failure from a standardized mortality ratio (SMR) of 2.4 (95% CI, 1.46 3.34) to 1.99 (95% CI, 1.21 2.76). This effect is more pronounced in men [309]. GH replacement in GH-deficient adults is associated with increased energy levels, improved mood, vitality and emotional reactions, and less feeling of social isolation. GH replacement therapy is associated with significantly improved quality of life scores [310]. GH-deficient adults demonstrate reduced VO2 max (maximum capacity to take in and use oxygen) with impaired exercise capacity. A meta-analysis of 268 GHD patients treated with 3.3 15.7 mg/week GH for 6 18 months in 11 randomized placebo-controlled studies, demonstrated significant improvement in exercise capacity evaluated by maximally increased work rate and VO2 max [311]. Lipolytic effects of GH increase availability of circulating FFAs to muscle during prolonged exercise [312], with potential conservation of glycogen stores. GH-enhanced increase in the cardiac LV ejection fraction also may contribute to improved oxygen delivery to exercising muscle [313]. GHD adults manifest reduced skeletal muscle mass, with reduced isometric muscle strength and possibly reduced isokinetic strength. GH and IGF-1 exert anabolic effects on skeletal muscle [314], with increased protein synthesis and reduced protein oxidation, an effect which is enhanced with concurrent administration of testosterone [315]. GH replacement in GHD adults rescues isometric and isokinetic strength, especially in those patients with the most compromised baseline muscle strength; an effect which is sustained for 5 years [316]. GH replacement in GHD adults also improves body composition and thermoregulation, with increased sweat secretion rates during heat exposure and exercise in GHD adults.

Muscle weakness

Improve

Impaired cardiac function

Improve

Hypohydrosis

Improve

QUALITY OF LIFE Low mood

Improve

Fatigue

Improve

Low motivation

Improve

Reduced satisfaction

Improve

CARDIOVASCULAR RISK PROFILE Abnormal lipid profile

Improve

Insulin resistance

Improves in long term

Inflammatory markers

Decrease

Intimal media thickening

Decrease

Cardiovascular and cerebrovascular events

Unknown

LABORATORY Blunted peak GH to stimulation Low IGF-1

Increase

Hyperinsulinemia

Increase

High LDL and low HDL cholesterol

Improve

Longevity

Unknown

From Melmed [271].

Within a year of GH replacement, visceral adipose tissue mass decreases by 9% [197], while LBM improves by up to 7% [317]. GH antagonizes insulin action and lipoprotein profiles improve, with reduced total and LDL cholesterol, and increased HDL cholesterol, triglycerides, and ApoB 100 levels [196]. Although LBM, cardiac stroke volume, and LV mass are increased [196], reports of cardiovascular risk profile improvement have been inconsistent [318]. Effects of GH replacement on bone mineral density are more beneficial in GH-deficient men [319] and bone fracture development is slowed in patients with no prior history of osteoporosis [320]. In postmenopausal women

I. HYPOTHALAMIC PITUITARY FUNCTION

GH THERAPY IN ADULTS

113

FIGURE 4.11 Algorithm for management of adult GH deficiency.

followed for 10 years, GH treatment improved fracture outcomes [321]. GH doses in adulthood are adjusted to individual needs (Fig. 4.11). As more GH is secreted in younger, lean individuals and in females, elderly, male, or obese individuals require lower GH replacement doses. The use of oral estrogen replacement affects the GH replacement dose. Premenopausal women or postmenopausal women using transdermal estrogen replacement require lower GH doses than postmenopausal women receiving oral estrogen replacement [322]. Oral, but not transdermal estrogens antagonize GH actions and reduce IGF-1 levels (Fig. 4.12). Historically, GH treatment regimens were weight-based, resulting in a higher incidence of side-effects as well as higher maintenance doses than in currently used individualized dose-titration GH replacement regimens. Current dosing recommendations suggest a starting GH dose of 0.2 0.4 mg/day in young patients and 0.1 0.2 mg/ day in those over age 60 years [323]. GH is selfadministered as a single subcutaneous evening injection in an attempt to recapitulate normal physiological nocturnal GH secretion. Daily doses are titrated by 100 200 μg/day every 6 weeks according to clinical responses, side-effects, and IGF-1 levels [260]. After maintenance doses have been established, patients can be monitored at 6 12-monthly intervals, for clinical evaluation, side-effects, and serum IGF-1 levels

FIGURE 4.12 Time course of GH dose and serum IGF-1 concentration in a representative patient (38-year-old woman) who was switched from oral to transdermal estrogen therapy during the course of GH replacement. Source: From Cook DM, Ludlam WH, Cook MB. Route of estrogen administration helps to determine growth hormone (GH) replacement dose in GH-deficient adults. J Clin Endocrinol Metab 1999;84(11):3956 60.

(Table 4.12). GH doses are adjusted until IGF-1 levels reach mid-normal range for age and sex (see Fig. 4.10). Lipid profile and fasting blood glucose levels should be evaluated annually. If the pretreatment bone DEXA scan is abnormal, follow-up DEXA scan is evaluated at 1 2-year intervals. Hypopituitary patients may require

I. HYPOTHALAMIC PITUITARY FUNCTION

114

4. GROWTH HORMONE

TABLE 4.12 Patient Monitoring After Initiating GH Replacement in Adults

TABLE 4.13 Replacement

1 Assess body weight, blood pressure, waist circumference, and BMI at initiation and every 6 months

Patient age and gender

2 Measure IGF-1 6 weeks after initiating GH replacement, after dose escalations, and every 6 months thereafter 3 Assess thyroid and adrenal function, and replace or adjust replacement doses as indicated 4 Assess metabolic profile including blood sugar and lipids every 6 months 5 Assess BMD by DEXA annually

Factors Determining Side Effects of GH

Enhanced IGF-1 response Greater body weight and body mass index Adult onset versus childhood onset of GH deficiency

TABLE 4.14

Side Effects of GH Replacement Therapy

ADULTS

6 Periodically assess residual pituitary mass with a pituitary MRI 7 Assess quality of life

Salt and fluid retention; peripheral edema Glucose homeostasis

From Fleseriu [324].

Unmasking of thyroid dysfunction Arthralgias and myalgias

evaluation of thyroid and adrenal axes after initiation of GH therapy [323]. Duration of GH therapy depends on benefits of treatment. Discontinuing GH therapy may be appropriate, if objective benefits are not apparent after at least 1 year of treatment; however, if objective clinical benefits are obtained from GH replacement, treatment is continued. Side-effects of GH replacement are reported in about 20% of patients (Tables 4.13 and 4.14). They are usually transient, and include arthralgias, edema, and carpal tunnel syndrome due to fluid retention; dose reduction may be required to alleviate these effects. As GH may reduce insulin sensitivity, glycemic control should be monitored, especially in patients receiving higher GH doses. In a placebo-controlled study, GH therapy induced impaired glucose tolerance in 13% of patients, and diabetes mellitus in 4%, with a significant number of patients developing worsening of glucose tolerance in the GH-treated group [317]. New-onset diabetes mellitus occurs in ,5% of patients. GH replacement for up to 9.6 years in males with nonfunctioning adenomas did not increase all-cause mortality [325]. Associated pituitary dysfunctions were likely independent causes of increased mortality. GH replacement therapy is contraindicated in patients with an active malignancy, benign intracranial hypertension, and proliferative diabetic retinopathy. There is concern that the growth-promoting and mitogenic effects of GH and IGF-1 could potentially increase cancer risk and promote tumor regrowth. However, significant increases in the occurrence of intrasellar, intracranial, or extracranial tumors has not been reported in adult GHD patient on long-term GH replacement therapy [326,327]. Increased rates of regrowth of postoperative craniopharyngioma [328], other childhood brain tumors [329], or adult pituitary macroadenomas have

Carpal tunnel syndrome Sleep apnea Headache Iatrogenic acromegaly CHILDREN Slipped capital femoral epiphysis Benign intracranial hypertension

not been reported following GH replacement. GH administered to patients after pituitary tumor resection does not induce adenoma regrowth [330]. In healthy men, serum IGF-1 levels in the upper normal range may be of predictive value for risk of developing prostate cancer [331], breast cancer in healthy premenopausal females [332], and colorectal cancer [333]. Furthermore, cancer risk was inversely correlated with serum IGFBP-3 levels. These studies which have not been uniformly reproduced, provide a strong rationale for maintaining IGF-1 in the mid-normal age-adjusted range, in AGHD patients on GH replacement therapy. Effects of GH Replacement Therapy HYPOPITUITARISM

Complex hormonal interactions occur between GH and other pituitary hormones, which impact diagnosis as well as optimal hormone replacement therapy. Thyroid, adrenal, and sex steroid replacement must be optimized for at least 3 months prior to testing for GHD. In addition, GH replacement may unmask incipient adrenal and thyroid insufficiency, necessitating monitoring of these hormonal interactions in order to achieve optimal hormone replacement [324].

I. HYPOTHALAMIC PITUITARY FUNCTION

GH THERAPY IN ADULTS

THYROID HORMONE

GH replacement increases conversion of T4 to T3 and decreases conversion of T4 to reverse T3 [334]. Initiation of GH replacement therapy in eurthyroid patients may therefore be associated with a fall in serum T4 levels, unmasking pre-existing central hypothyroidism [334]. GH stimulation performed in untreated central hypothyroidism may lead to an inaccurate GHD diagnosis. Careful monitoring of thyroid function is important in patients taking thyroid replacement, who are initiated on GH replacement therapy, as thyroid hormone dose increases may be required to maintain normal T4 [324]. GONADAL STEROIDS

Female patients taking oral estrogens require at least twofold higher doses of GH replacement [322], as estrogen administered orally impairs GH action. Therefore, to reduce GH requirements, a nonoral route (such as a transdermal patch) for estrogen replacement in hypopituitary women should be considered. Recommendations for sex steroid replacement in hypopituitary patients after menopause should follow guidelines for the general population. If adjustments are made in the dose of oral estrogens in hypopituitary female patients, the GH replacement dose should be reevaluated, as it may need to be changed. Changes in dose or route of androgen replacement therapy do not require reevaluation of GH dosage. GH replacement during pregnancy is not recommended as there is as yet no clear-cut evidence for efficacy or safety. Furthermore, the placenta is an abundant source of GH [335]. Nevertheless, an observational study of 201 pregnant women reported that, in more than 50% who continued GH therapy, pregnancy outcomes were unchanged [335]. GLUCOCORTICOIDS

GH or IGF-1 decrease 11 β-hydroxysteroid dehydrogenase type I activity, resulting in reduced conversion of inactive cortisone to active cortisol, and thus may unmask secondary hypoadrenalism. The hypothalamic pituitary adrenal axis should therefore be reevaluated during initiation of GH therapy as increased glucocorticoid replacement therapy or initiation of steroid replacement therapy may be required [336]. GH Replacement in Acromegaly In cured acromegaly patients, GHD may sometimes be documented [337]. Determinants of postoperative GHD, seen in about 10% of patients, include immediate (72 hours) postoperative GH levels as well as bilaterality of intrasellar tumor [338]. A retrospective study found that cardiovascular mortality was higher

115

in patients with documented GHD associated with treated acromegaly as compared to mortality occurring after resection of nonfunctioning pituitary adenomas (SMR 3.03, p , 0.02) [339]. In 42 acromegaly patients receiving GH replacement, quality of life was improved and body composition and lipid profiles were controlled without development of glucose intolerance, a hallmark of acromegaly [340].

GH in the Healthy Elderly In animal models, mutations resulting in suppressed GH/IGF-1 axis with reduced GH/IGF-1 signaling actually increase life span [341]. Snell and Ames dwarf mice with defects in anterior pituitary function due to Pit-1 and PROP-1 mutations, respectively, exhibit severely reduced insulin, IGF-1, glucose, and thyroid hormone levels, female infertility, and increased longevity [342]. Lit/Lit drwarf mice with mutations in the extracellular domain of the GHRH receptor had reduced serum IGF-1 levels, increased adiposity, and B25% increased longevity. Heterozygous IGF-1 receptor gene disrupted mice have a 50% reduction in receptor levels, and a 33% increased life-span in females, who are not dwarf [343]. Caloric restriction, another mechanism of decreasing circulating IGF-1 levels, also prolongs the life-span in several species [344]. Serum GH and IGF-1 levels decline progressively with age, a phenomenon referred to as “somatopause.” Increased adiposity and decreased LBM observed in the adult GHD syndrome, also occur with aging. The purported rationale for GH use as an antiaging therapy is the potential for improvement in body composition, bone density, and cholesterol levels observed in GH-deficient adults treated with GH replacement therapy. GH is not approved for use as an antiaging hormone by the FDA, but abuse of GH for this purpose continues to escalate [345]. Randomized controlled studies evaluating safety and efficacy of GH in the healthy elderly are limited [345]. The scant data suggest small but clinically nonsignificant improvements in body composition with adverse events including impaired fasting glucose, onset of diabetes mellitus, carpal tunnel syndrome, edema, arthralgias, and no beneficial effect on strength or physical function. Thus, available evidence does not validate physiological benefits from augmenting the declining GH levels in the normal aging process [346]. Sarcopenia (loss of muscle mass) increases with age and contributes to frailty. Clinical trials using GH in healthy elderly have not been proven to enhance muscle strength or quality of life. Ghrelin may prove to be beneficial in catabolic states, and increases appetite and LBM in healthy older men [347].

I. HYPOTHALAMIC PITUITARY FUNCTION

116

4. GROWTH HORMONE

A study evaluating the contribution of physiological supplementation with GH and testosterone for 16 weeks in elderly community-dwelling males showed improved LBM, muscle strength, and performance with reduced total body and truncal fat [348]. However, as there are no prospective controlled studies, and as inappropriate GH replacement may cause adverse events, GH should not be administered to elderly adults with low IGF-1 and no history of hypothalamic pituitary disease [260,324,346].

endurance. Maximum oxygen consumption was also unchanged [356]. In a single study of a highly select group of abstinent dependent users of anabolic androgenic steroids, hGH (19 μg/kg per day) for 1 week improved strength, peak power output, and IGF-1 levels [357]. Given the published evidence, GH administered to enhance athletic performance has unsubstantiated efficacy and scientific or ethical justification, and the practice is illegal [324,346]. Testing for GH Doping in Athletes

GH Abuse by Athletes Exercise is a potent stimulus of GH secretion and GH levels increase within 10 20 minutes of the onset of exercise, and are sustained for up to 2 hours following exercise. Furthermore, healthy subjects who exercise regularly demonstrate increased 24-hour GH secretion rates. Age, gender, BMI, physical fitness, and duration and intensity of exercise influence the magnitude of the GH response to exercise [349]. Beneficial effects of GH replacement therapy on exercise capacity in truly GHD adults have encouraged unapproved use of GH by athletes [350], and inappropriate use of rhGH by athletes (“doping”) increased from 6% in 2001 to 24% in 2006 [351]. Supraphysiological GH doses to pituitary-replete athletes increases FFA availability, with no effect on fat oxidation [352]. Oxidative protein loss at rest, during, and following exercise is reduced [353] and LBM is increased [354]. However, effects on performance are limited to anaerobic exercise capacity [354]. A systematic review [355] of 56 studies reported on 303 young recreational athletes, average age 27 years, who had received GH for an average of 20 days, many of whom only received one hGH injection. The average GH dose was 36 μg/kg per day (approximately 5 10-fold the replacement dose used in GHdeficient adults). LBM increased in the treatment group, compared to those not treated, with a statistically insignificant decrease in fat mass. There was no improvement in muscle strength after 24 and 84 days of GH administration in two studies. This analysis revealed little beneficial effect of GH in recreational athletes and failed to document improved performance. In a double-blind, randomized, placebo-controlled trial of 96 recreationally trained healthy athletes [356], GH supplementation (2 mg/day, or about 30 μg/kg for a 70-kg person) significantly reduced fat mass and increased LBM, and the addition of testosterone enhanced these effects. The only measure of enhanced physical performance was sprint capacity; no changes were seen in measures of strength, power, or

GH is not FDA-approved for enhancement of athletic performance, and the International Olympic Committee has prohibited GH doping. The GH-2000 project, comprising endocrinologists from four European countries, proposed a test based on the measurement of two GH-sensitive markers, IGF-1 and type III procollagen, which was first used at the Olympic Games in Athens in 2004 [358,359]. Pituitary GH contains several different GH isoforms, while recombinant human GH comprises only the 22-kDa isoform [360,361]. Administration of rhGH suppresses endogenous GH secretion, with increased 22-kDa GH to total GH ratios. To detect the 22-kDa GH isoform, the test must be performed within 24 hours of GH administration; discontinuation of GH the day before the test will result in a false-negative result [361]. The GH-2004 project demonstrated minor ethnic differences in IGF-1 and procollagen III peptide levels in athletes, which did not affect test performance [362]. However, a large cross-sectional study in over 1000 athletes from 12 countries, representing four major ethnic groups and 10 major sport types showed that age, gender, BMI, ethnicity, and sport type contributed to 56% of the variability of IGF-1 axis markers (IGF-1, IGFBP-3, and ALS) and collagen markers (type I procollagen, cterminal telopeptide of type I collagen, and N-terminal propeptide of type III procollagen) [363]. Thus, demographic factors should be taken into account in interpretation of tests using IGF-1 and collagen markers to detect GH doping. Gene expression analysis of peripheral blood leukocytes was attempted as a method to detect GH doping. However, this approach was not found to be clinically valuable for widespread screening [364].

Complications of GH Treatment Adverse reactions to adult GH replacement include peripheral edema, glucose intolerance, arthralgias, myalgias, backache, parasthesias, carpal tunnel syndrome, headache, hypertension, and rhinitis. These are frequent, often transient or disappear with

I. HYPOTHALAMIC PITUITARY FUNCTION

REFERENCES

lowering of the GH dose, and are more common in adult-onset than childhood-onset GH deficiency. Side-effects of GH treatment have been described in GH-deficient patients inappropriately treated with GH (see Table 4.14). Non-GH-deficient adult patients receiving excess GH doses can be likened to acromegaly patients, with increased GH levels. In a metaanalysis of seven studies with 22,654 patients, there was no association of GH replacement therapy with pituitary tumor recurrence (RR 0.87; 95% CI, 0.56 1.33) or the risk of secondary malignancies (RR 1.24; 95% CI, 0.65 2.33) [365]. Prospective surveillance for a mean of 2.3 years of 1988 GH-replaced adult GHD patients compared to controls showed no significant differences in mortality, cancer, diabetes, pituitary tumor growth, or cardiovascular events [366]. Insomnia and sleep apnea are observed to occur at higher frequencies. Elevated (but still within normal range) endogenous IGF-1 concentrations have been epidemiologically correlated with prostate, breast, colon, and lung cancer risk [331,332].

Decreased IGF-1 Levels Protein Calorie Malnutrition, Starvation, Anorexia Nervosa Short-term fasting and protein calorie malnutrition result in elevated basal GH levels and low IGF-1 levels [367], reflecting an uncoupling of the IGF-1 feedback regulation of GH secretion. In patients with anorexia nervosa, basal GH levels are also elevated [368]. Diabetes Mellitus Poorly controlled diabetes mellitus is associated with elevated basal GH levels and increased GH response to exercise. Elevated GH levels return to normal with improved diabetic control after insulin administration. IGF-1 levels are low in children with poorly controlled insulin-dependent diabetes, including those entering puberty, suggesting insulin resistance exacerbated by GH. Laron Syndrome Laron syndrome, an autosomal recessive disorder, is a condition of peripheral unresponsiveness to GH due to inactivating mutations of the GHR [369]. Serum GH levels are normal or elevated, circulating IGF-1 is absent, and there is no IGF-1 response to exogenously administered GH [118]. IGF-1 therapy increases height velocity, with improved body composition as evidenced by loss of fat mass [370].

117

References [1] Miller WL, Eberhardt NL. Structure and evolution of the growth hormone gene family. Endocr Rev 1983;4(2):97 130. [2] Fleetwood MR, Ho Y, Cooke NE, Liebhaber SA. DNase I hypersensitive site II of the human growth hormone locus control region mediates an essential and distinct long-range enhancer function. J. Biol. Chem. 2012;287(30):25454 65. [3] Chen EY, Liao YC, Smith DH, Barrera-Saldana HA, Gelinas RE, Seeburg PH. The human growth hormone locus: nucleotide sequence, biology, and evolution. Genomics 1989;4 (4):479 97. [4] Ryther RC, Flynt AS, Harris BD, Phillips III JA, Patton JG. GH1 splicing is regulated by multiple enhancers whose mutation produces a dominant-negative GH isoform that can be degraded by allele-specific small interfering RNA (siRNA). Endocrinology 2004;145(6):2988 96. [5] Frankenne F, Closset J, Gomez F, Scippo ML, Smal J, Hennen G. The physiology of growth hormones (GHs) in pregnant women and partial characterization of the placental GH variant. J Clin Endocrinol Metab 1988;66(6):1171 80. [6] Boguszewski CL, Svensson PA, Jansson T, Clark R, Carlsson LM, Carlsson B. Cloning of two novel growth hormone transcripts expressed in human placenta. J Clin Endocrinol Metab 1998;83(8):2878 85. [7] Chesnokova V, Zonis S, Zhou C, et al. Growth hormone is permissive for neoplastic colon growth. Proc Natl Acad Sci USA 2016;113(23):E3250 9. [8] Zhu X, Lin CR, Prefontaine GG, Tollkuhn J, Rosenfeld MG. Genetic control of pituitary development and hypopituitarism. Curr Opin Genet Dev 2005;15(3):332 40. [9] Pulichino AM, Vallette-Kasic S, Couture C, Brue T, Drouin J. Tpit mutations reveal a new model of pituitary differentiation and account for isolated ACTH deficiency. Med Sci (Paris) 2004;20(11):1009 13. [10] Behringer RR, Mathews LS, Palmiter RD, Brinster RL. Dwarf mice produced by genetic ablation of growth hormoneexpressing cells. Genes Dev 1988;2(4):453 61. [11] Watanabe YG, Daikoku S. An immunohistochemical study on the cytogenesis of adenohypophysial cells in fetal rats. Dev Biol 1979;68(2):557 67. [12] Borrelli E, Heyman RA, Arias C, Sawchenko PE, Evans RM. Transgenic mice with inducible dwarfism. Nature 1989;339 (6225):538 41. [13] Melmed S. Medical progress: acromegaly. N Engl J Med 2006;355(24):2558 73. [14] Sornson MW, Wu W, Dasen JS, et al. Pituitary lineage determination by the Prophet of Pit-1 homeodomain factor defective in Ames dwarfism. Nature 1996;384(6607):327 33. [15] Wu W, Cogan JD, Pfaffle RW, et al. Mutations in PROP1 cause familial combined pituitary hormone deficiency. Nat Genet 1998;18(2):147 9. [16] Romero C, Nesi-Franc¸a S, Radovick S. The molecular basis of hypopituitarism. Trends Endocrinol Metab 2009. [17] Mullis PE, Deladoey J, Dannies PS. Molecular and cellular basis of isolated dominant-negative growth hormone deficiency, IGHD type II: insights on the secretory pathway of peptide hormones. Horm Res 2002;58(2):53 66. [18] Chen XZ, Shafer AW, Yun JS, Li YS, Wagner TE, Kopchick JJ. Conversion of bovine growth hormone cysteine residues to serine affects secretion by cultured cells and growth rates in transgenic mice. Mol Endocrinol 1992;6(4):598 606. [19] Carter-Su C, Schwartz J, Argetsinger LS. Growth hormone signaling pathways. Growth Horm IGF Res 2016;28:11 15. [20] Baumann G, MacCart JG, Amburn K. The molecular nature of circulating growth hormone in normal and acromegalic man:

I. HYPOTHALAMIC PITUITARY FUNCTION

118

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34] [35]

[36]

[37]

[38] [39]

4. GROWTH HORMONE

evidence for a principal and minor monomeric forms. J Clin Endocrinol Metab 1983;56(5):946 52. Rudman D, Kutner MH, Rogers CM, Lubin MF, Fleming GA, Bain RP. Impaired growth hormone secretion in the adult population: relation to age and adiposity. J Clin Invest 1981; 67(5):1361 9. Smith RG, Van der Ploeg LH, Howard AD, et al. Peptidomimetic regulation of growth hormone secretion. Endocr Rev 1997;18(5):621 45. Casanueva FF, Dieguez C. Growth hormone secretagogues: physiological role and clinical utility. Trends Endocrinol Metab 1999;10(1):30 8. Howard AD, Feighner SD, Cully DF, et al. A receptor in pituitary and hypothalamus that functions in growth hormone release. Science 1996;273(5277):974 7. Kojima M, Hosoda H, Date Y, Nakazato M, Matsuo H, Kangawa K. Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature 1999;402(6762):656 60. Gnanapavan S, Kola B, Bustin SA, et al. The tissue distribution of the mRNA of ghrelin and subtypes of its receptor, GHS-R, in humans. J Clin Endocrinol Metab 2002;87(6):2988. van der Lely AJ, Tschop M, Heiman ML, Ghigo E. Biological, physiological, pathophysiological, and pharmacological aspects of ghrelin. Endocr Rev 2004;25(3):426 57. Hataya Y, Akamizu T, Takaya K, et al. A low dose of ghrelin stimulates growth hormone (GH) release synergistically with GH-releasing hormone in humans. J Clin Endocrinol Metab 2001;86(9):4552. Popovic V, Miljic D, Micic D, et al. Ghrelin main action on the regulation of growth hormone release is exerted at hypothalamic level. J Clin Endocrinol Metab 2003;88(7):3450 3. Zizzari P, Halem H, Taylor J, et al. Endogenous ghrelin regulates episodic growth hormone (GH) secretion by amplifying GH Pulse amplitude: evidence from antagonism of the GH secretagogue-R1a receptor. Endocrinology 2005;146(9):3836 42. Tannenbaum GS, Epelbaum J, Bowers CY. Interrelationship between the novel peptide ghrelin and somatostatin/growth hormone-releasing hormone in regulation of pulsatile growth hormone secretion. Endocrinology 2003;144(3):967 74. Kamegai J, Tamura H, Shimizu T, et al. The role of pituitary ghrelin in growth hormone (GH) secretion: GH-releasing hormone-dependent regulation of pituitary ghrelin gene expression and peptide content. Endocrinology 2004;145(8):3731 8. Casanueva FF, Camina JP, Carreira MC, Pazos Y, Varga JL, Schally AV. Growth hormone-releasing hormone as an agonist of the ghrelin receptor GHS-R1a. Proc Natl Acad Sci USA 2008;105(51):20452 7. Melmed S. Acromegaly pathogenesis and treatment. J Clin Invest 2009;119(11):3189 202. Shimon I, Yan X, Melmed S. Human fetal pituitary expresses functional growth hormone-releasing peptide receptors. J Clin Endocrinol Metab 1998;83(1):174 8. Shuto Y, Shibasaki T, Otagiri A, et al. Hypothalamic growth hormone secretagogue receptor regulates growth hormone secretion, feeding, and adiposity. J Clin Invest 2002;109 (11):1429 36. Sun Y, Wang P, Zheng H, Smith RG. Ghrelin stimulation of growth hormone release and appetite is mediated through the growth hormone secretagogue receptor. Proc Natl Acad Sci USA 2004;101(13):4679 84. Sun Y, Ahmed S, Smith RG. Deletion of ghrelin impairs neither growth nor appetite. Mol Cell Biol 2003;23(22):7973 81. Pantel J, Legendre M, Cabrol S, et al. Loss of constitutive activity of the growth hormone secretagogue receptor in familial short stature. J Clin Invest 2006;116(3):760 8.

[40] Koutkia P, Canavan B, Breu J, Johnson ML, Grinspoon SK. Nocturnal ghrelin pulsatility and response to growth hormone secretagogues in healthy men. Am J Physiol Endocrinol Metab 2004;287(3):E506 12. [41] Thorner MO, Perryman RL, Cronin MJ, et al. Somatotroph hyperplasia. Successful treatment of acromegaly by removal of a pancreatic islet tumor secreting a growth hormone-releasing factor. J Clin Invest 1982;70(5):965 77. [42] Maeda K, Kato Y, Yamaguchi N, Chihara K, Ohgo S. Growth hormone release following thyrotrophin-releasing hormone injection into patients with anorexia nervosa. Acta Endocrinol (Copenh) 1976;81(1):1 8. [43] Gutierrez-Pascual E, Martinez-Fuentes AJ, Pinilla L, TenaSempere M, Malagon MM, Castano JP. Direct pituitary effects of kisspeptin: activation of gonadotrophs and somatotrophs and stimulation of luteinising hormone and growth hormone secretion. J Neuroendocrinol 2007;19(7):521 30. [44] Kelesidis T, Kelesidis I, Chou S, Mantzoros CS. Narrative review: the role of leptin in human physiology: emerging clinical applications. Ann Intern Med 2010;152(2):93 100. [45] Chan JL, Heist K, DePaoli AM, Veldhuis JD, Mantzoros CS. The role of falling leptin levels in the neuroendocrine and metabolic adaptation to short-term starvation in healthy men. J Clin Invest 2003;111(9):1409 21. [46] Chan JL, Williams CJ, Raciti P, et al. Leptin does not mediate short-term fasting-induced changes in growth hormone pulsatility but increases IGF-I in leptin deficiency states. J Clin Endocrinol Metab 2008;93(7):2819 27. [47] Martin JB. Neural regulation of growth hormone secretion. N Engl J Med 1973;288(26):1384 93. [48] de Lecea L, Kilduff TS, Peyron C, et al. The hypocretins: hypothalamus-specific peptides with neuroexcitatory activity. Proc Natl Acad Sci USA 1998;95(1):322 7. [49] Blanco M, Gallego R, Garcia-Caballero T, Dieguez C, Beiras A. Cellular localization of orexins in human anterior pituitary. Histochem Cell Biol 2003;120(4):259 64. [50] Hagan JJ, Leslie RA, Patel S, et al. Orexin A activates locus coeruleus cell firing and increases arousal in the rat. Proc Natl Acad Sci USA 1999;96(19):10911 16. [51] Kojima M, Kangawa K. Ghrelin: structure and function. Physiol Rev 2005;85(2):495 522. [52] Chemelli RM, Willie JT, Sinton CM, et al. Narcolepsy in orexin knockout mice: molecular genetics of sleep regulation. Cell 1999;98(4):437 51. [53] Giustina A, Veldhuis JD. Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human. Endocr Rev 1998;19(6):717 97. [54] Guillemin R, Brazeau P, Bohlen P, Esch F, Ling N, Wehrenberg WB. Growth hormone-releasing factor from a human pancreatic tumor that caused acromegaly. Science 1982;218(4572):585 7. [55] Rivier J, Spiess J, Thorner M, Vale W. Characterization of a growth hormone-releasing factor from a human pancreatic islet tumour. Nature 1982;300(5889):276 8. [56] Gubler U, Monahan JJ, Lomedico PT, et al. Cloning and sequence analysis of cDNA for the precursor of human growth hormone-releasing factor, somatocrinin. Proc Natl Acad Sci USA 1983;80(14):4311 14. [57] Mayo KE, Vale W, Rivier J, Rosenfeld MG, Evans RM. Expression-cloning and sequence of a cDNA encoding human growth hormone-releasing factor. Nature 1983;306(5938):86 8. [58] Tatemoto K, Mutt V. Isolation and characterization of the intestinal peptide porcine PHI (PHI-27), a new member of the glucagon-secretin family. Proc Natl Acad Sci USA 1981;78(11):6603 7. [59] Bilezikjian LM, Vale WW. Stimulation of adenosine 3’,5’-monophosphate production by growth hormone-releasing factor and

I. HYPOTHALAMIC PITUITARY FUNCTION

REFERENCES

[60]

[61]

[62]

[63]

[64]

[65]

[66]

[67] [68] [69]

[70] [71]

[72]

[73]

[74]

[75] [76]

[77]

[78]

its inhibition by somatostatin in anterior pituitary cells in vitro. Endocrinology 1983;113(5):1726 31. Mayo KE. Molecular cloning and expression of a pituitaryspecific receptor for growth hormone-releasing hormone. Mol Endocrinol 1992;6(10):1734 44. Barinaga M, Bilezikjian LM, Vale WW, Rosenfeld MG, Evans RM. Independent effects of growth hormone releasing factor on growth hormone release and gene transcription. Nature 1985;314(6008):279 81. Gelato MC, Pescovitz O, Cassorla F, Loriaux DL, Merriam GR. Effects of a growth hormone releasing factor in man. J Clin Endocrinol Metab 1983;57(3):674 6. Pandya N, DeMott-Friberg R, Bowers CY, Barkan AL, Jaffe CA. Growth hormone (GH)-releasing peptide-6 requires endogenous hypothalamic GH-releasing hormone for maximal GH stimulation. J Clin Endocrinol Metab 1998;83(4):1186 9. Russell-Aulet M, Jaffe CA, Demott-Friberg R, Barkan AL. In vivo semiquantification of hypothalamic growth hormone-releasing hormone (GHRH) output in humans: evidence for relative GHRH deficiency in aging. J Clin Endocrinol Metab 1999;84(10):3490 7. Russell-Aulet M, Dimaraki EV, Jaffe CA, DeMott-Friberg R, Barkan AL. Aging-related growth hormone (GH) decrease is a selective hypothalamic GH-releasing hormone pulse amplitude mediated phenomenon. J Gerontol A Biol Sci Med Sci 2001;56(2):M124 9. Jessup SK, Dimaraki EV, Symons KV, Barkan AL. Sexual dimorphism of growth hormone (GH) regulation in humans: endogenous GH-releasing hormone maintains basal GH in women but not in men. J Clin Endocrinol Metab 2003;88(10):4776 80. Reichlin S. Somatostatin. N Engl J Med 1983;309(24):1495 501. Shen LP, Pictet RL, Rutter WJ. Human somatostatin I: sequence of the cDNA. Proc Natl Acad Sci USA 1982;79(15):4575 9. Koerker DJ, Ruch W, Chideckel E, et al. Somatostatin: hypothalamic inhibitor of the endocrine pancreas. Science 1974; 184(135):482 4. Ben-Shlomo A, Melmed S. Pituitary somatostatin receptor signaling. Trends Endocrinol Metab 2010;21(3):123 33. Miller GM, Alexander JM, Bikkal HA, Katznelson L, Zervas NT, Klibanski A. Somatostatin receptor subtype gene expression in pituitary adenomas. J Clin Endocrinol Metab 1995; 80(4):1386 92. Greenman Y, Melmed S. Heterogeneous expression of two somatostatin receptor subtypes in pituitary tumors. J Clin Endocrinol Metab 1994;78(2):398 403. Greenman Y, Melmed S. Expression of three somatostatin receptor subtypes in pituitary adenomas: evidence for preferential SSTR5 expression in the mammosomatotroph lineage. J Clin Endocrinol Metab 1994;79(3):724 9. Shimon I, Yan X, Taylor JE, Weiss MH, Culler MD, Melmed S. Somatostatin receptor (SSTR) subtype-selective analogues differentially suppress in vitro growth hormone and prolactin in human pituitary adenomas. Novel potential therapy for functional pituitary tumors. J Clin Invest 1997;100(9):2386 92. Melmed S. New therapeutic agents for acromegaly. Nat Rev Endocrinol 2016;12(2):90 8. Berelowitz M, Szabo M, Frohman LA, Firestone S, Chu L, Hintz RL. Somatomedin-C mediates growth hormone negative feedback by effects on both the hypothalamus and the pituitary. Science 1981;212(4500):1279 81. Yamashita S, Melmed S. Insulin-like growth factor I action on rat anterior pituitary cells: suppression of growth hormone secretion and messenger ribonucleic acid levels. Endocrinology 1986;118 (1):176 82. Murray PG, Higham CE, Clayton PE. 60 YEARS OF NEUROENDOCRINOLOGY: the hypothalamo-GH axis: the past 60 years. J Endocrinol 2015;226(2):T123 40.

119

[79] Tannenbaum GS, Ling N. The interrelationship of growth hormone (GH)-releasing factor and somatostatin in generation of the ultradian rhythm of GH secretion. Endocrinology 1984; 115(5):1952 7. [80] Vance ML, Kaiser DL, Martha Jr. PM, et al. Lack of in vivo somatotroph desensitization or depletion after 14 days of continuous growth hormone (GH)-releasing hormone administration in normal men and a GH-deficient boy. J Clin Endocrinol Metab 1989;68(1):22 8. [81] Tannenbaum GS, Painson JC, Lengyel AM, Brazeau P. Paradoxical enhancement of pituitary growth hormone (GH) responsiveness to GH-releasing factor in the face of high somatostatin tone. Endocrinology 1989;124(3):1380 8. [82] Davis JR, Sheppard MC, Shakespear RA, Lynch SS, Clayton RN. Does growth hormone releasing factor desensitize the somatotroph? Interpretation of responses of growth hormone during and after 10-hour infusion of GRF 1-29 amide in man. Clin Endocrinol (Oxf) 1986;24(2):135 40. [83] Losa M, Chiodini PG, Liuzzi A, et al. Growth hormonereleasing hormone infusion in patients with active acromegaly. J Clin Endocrinol Metab 1986;63(1):88 93. [84] Bilezikjian LM, Seifert H, Vale W. Desensitization to growth hormone-releasing factor (GRF) is associated with downregulation of GRF-binding sites. Endocrinology 1986;118 (5):2045 52. [85] Sheppard MC, Kronheim S, Pimstone BL. Stimulation by growth hormone of somatostatin release from the rat hypothalamus in vitro. Clin Endocrinol (Oxf) 1978;9(6):583 6. [86] Rosenthal SM, Hulse JA, Kaplan SL, Grumbach MM. Exogenous growth hormone inhibits growth hormone-releasing factor-induced growth hormone secretion in normal men. J Clin Invest 1986;77(1):176 80. [87] Ross RJ, Borges F, Grossman A, et al. Growth hormone pretreatment in man blocks the response to growth hormonereleasing hormone; evidence for a direct effect of growth hormone. Clin Endocrinol (Oxf) 1987;26(1):117 23. [88] Peterfreund RA, Vale WW. Somatostatin analogs inhibit somatostatin secretion from cultured hypothalamus cells. Neuroendocrinology 1984;39(5):397 402. [89] Goldenberg N, Barkan A. Factors regulating growth hormone secretion in humans. Endocrinol Metab Clin North Am 2007;36 (1):37 55. [90] Veldhuis JD, Roemmich JN, Richmond EJ, Bowers CY. Somatotropic and gonadotropic axes linkages in infancy, childhood, and the puberty-adult transition. Endocr Rev 2006;27 (2):101 40. [91] Di Vito L, Broglio F, Benso A, et al. The GH-releasing effect of ghrelin, a natural GH secretagogue, is only blunted by the infusion of exogenous somatostatin in humans. Clin Endocrinol (Oxf) 2002;56(5):643 8. [92] Alvarez-Castro P, Isidro ML, Garcia-Buela J, et al. Marked GH secretion after ghrelin alone or combined with GH-releasing hormone (GHRH) in obese patients. Clin Endocrinol (Oxf) 2004;61(2):250 5. [93] Arvat E, Maccario M, Di Vito L, et al. Endocrine activities of ghrelin, a natural growth hormone secretagogue (GHS), in humans: comparison and interactions with hexarelin, a nonnatural peptidyl GHS, and GH-releasing hormone. J Clin Endocrinol Metab 2001;86(3):1169 74. [94] Melmed S, Kleinberg DL. Anterior pituitary. In: Larsen P, Kronenberg HM, Melmed S, Polonsky KS, editors. Williams textbook of endocrinology. Philadelphia: Saunders; 2003. p. 177 279. [95] Veldhuis JD, Olson TP, Takahashi PY, et al. Multipathway modulation of exercise and glucose stress effects upon GH secretion in healthy men. Metabolism 2015;64(9):1022 30.

I. HYPOTHALAMIC PITUITARY FUNCTION

120

4. GROWTH HORMONE

[96] Zadik Z, Chalew SA, McCarter Jr. RJ, Meistas M, Kowarski AA. The influence of age on the 24-hour integrated concentration of growth hormone in normal individuals. J Clin Endocrinol Metab 1985;60(3):513 16. [97] Veldhuis JD. Neuroendocrine control of pulsatile growth hormone release in the human: relationship with gender. Growth Horm IGF Res 1998;8(Suppl. B):49 59. [98] Chapman IM, Hartman ML, Straume M, Johnson ML, Veldhuis JD, Thorner MO. Enhanced sensitivity growth hormone (GH) chemiluminescence assay reveals lower postglucose nadir GH concentrations in men than women. J Clin Endocrinol Metab 1994;78(6):1312 19. [99] Cordoba-Chacon J, Gahete MD, Castano JP, Kineman RD, Luque RM. Somatostatin and its receptors contribute in a tissue-specific manner to the sex-dependent metabolic (fed/ fasting) control of growth hormone axis in mice. Am J Physiol Endocrinol Metab 2011;300(1):E46 54. [100] Van Cauter E, Leproult R, Plat L. Age-related changes in slow wave sleep and REM sleep and relationship with growth hormone and cortisol levels in healthy men. Jama 2000;284(7):861 8. [101] Yildiz BO, Suchard MA, Wong ML, McCann SM, Licinio J. Alterations in the dynamics of circulating ghrelin, adiponectin, and leptin in human obesity. Proc Natl Acad Sci USA 2004;101 (28):10434 9. [102] Golstein J, Van Cauter E, Desir D, et al. Effects of “jet lag” on hormonal patterns. IV. Time shifts increase growth hormone release. J Clin Endocrinol Metab 1983;56(3):433 40. [103] Casanueva FF, Villanueva L, Cabranes JA, Cabezas-Cerrato J, Fernandez-Cruz A. Cholinergic mediation of growth hormone secretion elicited by arginine, clonidine, and physical exercise in man. J Clin Endocrinol Metab 1984;59(3):526 30. [104] Pritzlaff-Roy CJ, Widemen L, Weltman JY, et al. Gender governs the relationship between exercise intensity and growth hormone release in young adults. J Appl Physiol (1985) 2002;92(5):2053 60. [105] Moller N, Jorgensen JO. Effects of growth hormone on glucose, lipid, and protein metabolism in human subjects. Endocr Rev 2009;30(2):152 77. [106] Katz LE, DeLeon DD, Zhao H, Jawad AF. Free and total insulin-like growth factor (IGF)-I levels decline during fasting: relationships with insulin and IGF-binding protein-1. J Clin Endocrinol Metab 2002;87(6):2978 83. [107] Clasey JL, Weltman A, Patrie J, et al. Abdominal visceral fat and fasting insulin are important predictors of 24-hour GH release independent of age, gender, and other physiological factors. J Clin Endocrinol Metab 2001;86(8):3845 52. [108] Roth J, Glick SM, Yalow RS, Bersonsa. Hypoglycemia: a potent stimulus to secretion of growth hormone. Science 1963;140:987 8. [109] Alba-Roth J, Muller OA, Schopohl J, von Werder K. Arginine stimulates growth hormone secretion by suppressing endogenous somatostatin secretion. J Clin Endocrinol Metab 1988;67 (6):1186 9. [110] Casanueva FF, Villanueva L, Dieguez C, et al. Free fatty acids block growth hormone (GH) releasing hormone-stimulated GH secretion in man directly at the pituitary. J Clin Endocrinol Metab 1987;65(4):634 42. [111] Nakagawa K, Akikawa K, Matsubara M, Kubo M. Effect of dexamethasone on growth hormone (GH) response to growth hormone releasing hormone in acromegaly. J Clin Endocrinol Metab 1985;60(2):306 10. [112] Tyrrell JB, Wiener-Kronish J, Lorenzi M, Brooks RM, Forsham PH. Cushing’s disease: growth hormone response to hypoglycemia after correction of hypercortisolism. J Clin Endocrinol Metab 1977;44(1):218 21.

[113] Suda T, Demura H, Demura R, Jibiki K, Tozawa F, Shizume K. Anterior pituitary hormones in plasma and pituitaries from patients with Cushing’s disease. J Clin Endocrinol Metab 1980;51(5):1048 53. [114] Casanueva FF, Burguera B, Muruais C, Dieguez C. Acute administration of corticoids: a new and peculiar stimulus of growth hormone secretion in man. J Clin Endocrinol Metab 1990;70(1):234 7. [115] Schilbach K, Bidlingmaier M. Growth hormone binding protein - physiological and analytical aspects. Best Pract Res Clin Endocrinol Metab 2015;29(5):671 83. [116] Baumann G, Shaw MA. Plasma transport of the 20,000-dalton variant of human growth hormone (20K): evidence for a 20Kspecific binding site. J Clin Endocrinol Metab 1990;71 (5):1339 43. [117] McIntyre HD, Serek R, Crane DI, et al. Placental growth hormone (GH), GH-binding protein, and insulin-like growth factor axis in normal, growth-retarded, and diabetic pregnancies: correlations with fetal growth. J Clin Endocrinol Metab 2000;85(3):1143 50. [118] Laron Z. Laron syndrome (primary growth hormone resistance or insensitivity): the personal experience 1958 2003. J Clin Endocrinol Metab 2004;89(3):1031 44. [119] Brown RJ, Adams JJ, Pelekanos RA, et al. Model for growth hormone receptor activation based on subunit rotation within a receptor dimer. Nat Struct Mol Biol 2005;12(9):814 21. [120] Brooks AJ, Waters MJ. The growth hormone receptor: mechanism of activation and clinical implications. Nat Rev Endocrinol 2010;6(9):515 25. [121] Brooks AJ, Dai W, O’Mara ML, et al. Mechanism of activation of protein kinase JAK2 by the growth hormone receptor. Science 2014;344(6185):1249783. [122] Cui TX, Lin G, LaPensee CR, et al. C/EBPbeta mediates growth hormone-regulated expression of multiple target genes. Mol Endocrinol 2011;25(4):681 93. [123] Xu BC, Wang X, Darus CJ, Kopchick JJ. Growth hormone promotes the association of transcription factor STAT5 with the growth hormone receptor. J Biol Chem 1996;271(33):19768 73. [124] Melmed S, Yamashita S, Yamasaki H, et al. IGF-I receptor signalling: lessons from the somatotroph. Recent Prog Horm Res 1996;51:189 215 discussion -6 [125] Rotwein P. Mapping the growth hormone--Stat5b--IGF-I transcriptional circuit. Trends Endocrinol Metab 2012;23 (4):186 93. [126] Varco-Merth B, Rotwein P. Differential effects of STAT proteins on growth hormone-mediated IGF-I gene expression. Am J Physiol Endocrinol Metab 2014;307(9):E847 55. [127] Leung KC, Waters MJ, Markus I, Baumbach WR, Ho KK. Insulin and insulin-like growth factor-I acutely inhibit surface translocation of growth hormone receptors in osteoblasts: a novel mechanism of growth hormone receptor regulation. Proc Natl Acad Sci USA 1997;94(21):11381 6. [128] Chia DJ, Ono M, Woelfle J, Schlesinger-Massart M, Jiang H, Rotwein P. Characterization of distinct Stat5b binding sites that mediate growth hormone-stimulated IGF-I gene transcription. J Biol Chem 2006;281(6):3190 7. [129] Udy GB, Towers RP, Snell RG, et al. Requirement of STAT5b for sexual dimorphism of body growth rates and liver gene expression. Proc Natl Acad Sci USA 1997;94 (14):7239 44. [130] Ram PA, Park SH, Choi HK, Waxman DJ. Growth hormone activation of Stat 1, Stat 3, and Stat 5 in rat liver. Differential kinetics of hormone desensitization and growth hormone stimulation of both tyrosine phosphorylation and serine/threonine phosphorylation. J Biol Chem 1996;271(10):5929 40.

I. HYPOTHALAMIC PITUITARY FUNCTION

121

REFERENCES

[131] Davey HW, Park SH, Grattan DR, McLachlan MJ, Waxman DJ. STAT5b-deficient mice are growth hormone pulseresistant. Role of STAT5b in sex-specific liver p450 expression. J Biol Chem 1999;274(50):35331 6. [132] Kofoed EM, Hwa V, Little B, et al. Growth hormone insensitivity associated with a STAT5b mutation. N Engl J Med 2003;349 (12):1139 47. [133] Greenhalgh CJ, Rico-Bautista E, Lorentzon M, et al. SOCS2 negatively regulates growth hormone action in vitro and in vivo. J Clin Invest 2005;115(2):397 406. [134] Humbel RE. Insulin-like growth factors I and II. Eur J Biochem 1990;190(3):445 62. [135] Han VK, D’Ercole AJ, Lund PK. Cellular localization of somatomedin (insulin-like growth factor) messenger RNA in the human fetus. Science 1987;236(4798):193 7. [136] Mathews LS, Norstedt G, Palmiter RD. Regulation of insulinlike growth factor I gene expression by growth hormone. Proc Natl Acad Sci USA 1986;83(24):9343 7. [137] Clemmons DR, Shaw DS. Variables controlling somatomedin production by cultured human fibroblasts. J Cell Physiol 1983;115(2):137 42. [138] Thissen JP, Ketelslegers JM, Underwood LE. Nutritional regulation of the insulin-like growth factors. Endocr Rev 1994;15 (1):80 101. [139] Brabant G, von zur MA, Wuster C, et al. Serum insulin-like growth factor I reference values for an automated chemiluminescence immunoassay system: results from a multicenter study. Horm Res 2003;60(2):53 60. [140] Clemmons DR. Value of insulin-like growth factor system markers in the assessment of growth hormone status. Endocrinol Metab Clin North Am 2007;36(1):109 29. [141] Rajaram S, Baylink DJ, Mohan S. Insulin-like growth factorbinding proteins in serum and other biological fluids: regulation and functions. Endocr Rev 1997;18(6):801 31. [142] Clemmons DR. Role of IGF binding proteins in regulating metabolism. Trends Endocrinol Metab 2016;27(6):375 91. [143] Baxter RC, Martin JL. Structure of the Mr 140,000 growth hormone-dependent insulin-like growth factor binding protein complex: determination by reconstitution and affinity-labeling. Proc Natl Acad Sci USA 1989;86(18):6898 902. [144] Guler HP, Zapf J, Schmid C, Froesch ER. Insulin-like growth factors I and II in healthy man. Estimations of half-lives and production rates. Acta Endocrinol (Copenh) 1989;121(6):753 8. [145] Hodgkinson SC, Davis SR, Moore LG, Henderson HV, Gluckman PD. Metabolic clearance of insulin-like growth factor-II in sheep. J Endocrinol 1989;123(3):461 8. [146] Holly J, Perks C. The role of insulin-like growth factor binding proteins. Neuroendocrinology 2006;83(3 4):154 60. [147] Drop SL, Kortleve DJ, Guyda HJ, Posner BI. Immunoassay of a somatomedin-binding protein from human amniotic fluid: levels in fetal, neonatal, and adult sera. J Clin Endocrinol Metab 1984;59(5):908 15. [148] Degerblad M, Povoa G, Thoren M, Wivall IL, Hall K. Lack of diurnal rhythm of low molecular weight insulin-like growth factor binding protein in patients with Cushing’s disease. Acta Endocrinol (Copenh) 1989;120(2):195 200. [149] Baxter RC, Martin JL. Radioimmunoassay of growth hormonedependent insulinlike growth factor binding protein in human plasma. J Clin Invest 1986;78(6):1504 12. [150] Busby WH, Snyder DK, Clemmons DR. Radioimmunoassay of a 26,000-dalton plasma insulin-like growth factor-binding protein: control by nutritional variables. J Clin Endocrinol Metab 1988;67(6):1225 30. [151] Ezzat S, Ren SG, Braunstein GD, Melmed S. Octreotide stimulates insulin-like growth factor binding protein-1 (IGFBP-1)

[152]

[153]

[154] [155]

[156]

[157]

[158]

[159]

[160] [161]

[162]

[163]

[164] [165] [166]

[167]

[168]

[169]

[170]

[171]

levels in acromegaly. J Clin Endocrinol Metab 1991;73 (2):441 3. Zapf J, Hauri C, Waldvogel M, et al. Recombinant human insulin-like growth factor I induces its own specific carrier protein in hypophysectomized and diabetic rats. Proc Natl Acad Sci USA 1989;86(10):3813 17. Holly JM, Biddlecombe RA, Dunger DB, et al. Circadian variation of GH-independent IGF-binding protein in diabetes mellitus and its relationship to insulin. A new role for insulin?. Clin Endocrinol (Oxf) 1988;29(6):667 75. LeRoith D, Bondy C, Yakar S, Liu J, Butler A. The somatomedin hypothesis: 2001. Endocr Rev 2001;22:53 74. Isaksson OG, Jansson JO, Gause IA. Growth hormone stimulates longitudinal bone growth directly. Science 1982;216 (4551):1237 9. Clemmons DR. Modifying IGF1 activity: an approach to treat endocrine disorders, atherosclerosis and cancer. Nat Rev Drug Discov 2007;6(10):821 33. Kelly PA, Djiane J, Postel-Vinay MC, Edery M. The prolactin/ growth hormone receptor family. Endocr Rev 1991;12 (3):235 51. Leung DW, Spencer SA, Cachianes G, et al. Growth hormone receptor and serum binding protein: purification, cloning and expression. Nature 1987;330(6148):537 43. Pilecka I, Whatmore A, Hooft van Huijsduijnen R, Destenaves B, Clayton P. Growth hormone signalling: sprouting links between pathways, human genetics and therapeutic options. Trends Endocrinol Metab 2007;18(1):12 18. Lui JC, Garrison P, Baron J. Regulation of body growth. Curr Opin Pediatr 2015;27(4):502 10. Slootweg MC, Ohlsson C, Salles JP, de Vries CP, Netelenbos JC. Insulin-like growth factor binding proteins-2 and -3 stimulate growth hormone receptor binding and mitogenesis in rat osteosarcoma cells. Endocrinology 1995;136(10):4210 17. Gevers EF, Loveridge N, Robinson IC. Bone marrow adipocytes: a neglected target tissue for growth hormone. Endocrinology 2002;143(10):4065 73. Wang J, Zhou J, Bondy CA. Igf1 promotes longitudinal bone growth by insulin-like actions augmenting chondrocyte hypertrophy. FASEB J 1999;13(14):1985 90. Nilsson O, Marino R, De Luca F, Phillip M, Baron J. Endocrine regulation of the growth plate. Horm Res 2005;64(4):157 65. Canalis E. The fate of circulating osteoblasts. N Engl J Med 2005;352(19):2014 16. DiGirolamo DJ, Mukherjee A, Fulzele K, et al. Mode of growth hormone action in osteoblasts. J Biol Chem 2007;282 (43):31666 74. Canalis E, Economides AN, Gazzerro E. Bone morphogenetic proteins, their antagonists, and the skeleton. Endocr Rev 2003;24(2):218 35. Hubina E, Lakatos P, Kovacs L, et al. Effects of 24 months of growth hormone (GH) treatment on serum carboxylated and undercarboxylated osteocalcin levels in GH-deficient adults. Calcif Tissue Int 2004;74(1):55 9. Giustina A, Mazziotti G, Canalis E. Growth hormone, insulinlike growth factors, and the skeleton. Endocr Rev 2008;29 (5):535 59. Sims NA, Clement-Lacroix P, Da Ponte F, et al. Bone homeostasis in growth hormone receptor-null mice is restored by IGF-I but independent of Stat5. J Clin Invest 2000;106 (9):1095 103. Zhang M, Xuan S, Bouxsein ML, et al. Osteoblast-specific knockout of the insulin-like growth factor (IGF) receptor gene reveals an essential role of IGF signaling in bone matrix mineralization. J Biol Chem 2002;277(46):44005 12.

I. HYPOTHALAMIC PITUITARY FUNCTION

122

4. GROWTH HORMONE

[172] David A, Hwa V, Metherell LA, et al. Evidence for a continuum of genetic, phenotypic, and biochemical abnormalities in children with growth hormone insensitivity. Endocr Rev 2011;32(4):472 97. [173] van der Eerden BC, Karperien M, Wit JM. Systemic and local regulation of the growth plate. Endocr Rev 2003;24 (6):782 801. [174] Woods KA, Camacho-Hubner C, Savage MO, Clark AJ. Intrauterine growth retardation and postnatal growth failure associated with deletion of the insulin-like growth factor I gene. N Engl J Med 1996;335(18):1363 7. [175] Abuzzahab MJ, Schneider A, Goddard A, et al. IGF-I receptor mutations resulting in intrauterine and postnatal growth retardation. N Engl J Med 2003;349(23):2211 22. [176] Bouillon R, Prodonova A. Growth and hormone deficiency and peak bone mass. J Pediatr Endocrinol Metab 2000;13 (Suppl. 6):1327 36. [177] Mohan S, Richman C, Guo R, et al. Insulin-like growth factor regulates peak bone mineral density in mice by both growth hormone-dependent and -independent mechanisms. Endocrinology 2003;144(3):929 36. [178] Choi HK, Waxman DJ. Plasma growth hormone pulse activation of hepatic JAK-STAT5 signaling: developmental regulation and role in male-specific liver gene expression. Endocrinology 2000;141(9):3245 55. [179] Laron Z, Kauli R. Fifty seven years of follow-up of the Israeli cohort of Laron Syndrome patients-From discovery to treatment. Growth Horm IGF Res 2016;28:53 6. [180] Collett-Solberg PF, Misra M, Drug, therapeutics committee of the Lawson Wilkins Pediatric Endocrine S. The role of recombinant human insulin-like growth factor-I in treating children with short stature. J Clin Endocrinol Metab 2008;93(1):10 18. [181] Pfaffle R. Hormone replacement therapy in children: the use of growth hormone and IGF-I. Best Pract Res Clin Endocrinol Metab 2015;29(3):339 52. [182] Hazel SJ, Gillespie CM, Moore RJ, Clark RG, Jureidini KF, Martin AA. Enhanced body growth in uremic rats treated with IGF-I and growth hormone in combination. Kidney Int 1994;46(1):58 68. [183] Baker J, Liu JP, Robertson EJ, Efstratiadis A. Role of insulinlike growth factors in embryonic and postnatal growth. Cell 1993;75(1):73 82. [184] Xing W, Govoni KE, Donahue LR, et al. Genetic evidence that thyroid hormone is indispensable for prepubertal insulin-like growth factor-I expression and bone acquisition in mice. J Bone Miner Res 2012;27(5):1067 79. [185] Thomas JD, Monson JP. Adult GH deficiency throughout lifetime. Eur J Endocrinol 2009;161(Suppl. 1):S97 106. [186] Mazziotti G, Bianchi A, Porcelli T, et al. Vertebral fractures in patients with acromegaly: a 3-year prospective study. J Clin Endocrinol Metab 2013;98(8):3402 10. [187] Karasik D, Rosen CJ, Hannan MT, et al. Insulin-like growth factor binding proteins 4 and 5 and bone mineral density in elderly men and women. Calcif Tissue Int 2002;71(4):323 8. [188] Tanaka H, Liang CT. Mitogenic activity but not phenotype expression of rat osteoprogenitor cells in response to IGF-I is impaired in aged rats. Mech Ageing Dev 1996;92(1):1 10. [189] Murray RD, Adams JE, Shalet SM. A densitometric and morphometric analysis of the skeleton in adults with varying degrees of growth hormone deficiency. J Clin Endocrinol Metab 2006;91(2):432 8. [190] Attanasio AF, Lamberts SW, Matranga AM, et al. Adult growth hormone (GH)-deficient patients demonstrate heterogeneity between childhood onset and adult onset before and during human GH treatment. Adult Growth Hormone

[191]

[192]

[193]

[194]

[195]

[196]

[197]

[198]

[199]

[200]

[201]

[202]

[203] [204] [205]

[206]

[207]

[208] [209]

Deficiency Study Group. J Clin Endocrinol Metab 1997;82 (1):82 8. Bouillon R, Koledova E, Bezlepkina O, et al. Bone status and fracture prevalence in Russian adults with childhood-onset growth hormone deficiency. J Clin Endocrinol Metab 2004;89 (10):4993 8. Pasarica M, Zachwieja JJ, Dejonge L, Redman S, Smith SR. Effect of growth hormone on body composition and visceral adiposity in middle-aged men with visceral obesity. J Clin Endocrinol Metab 2007;92(11):4265 70. Yang S, Mulder H, Holm C, Eden S. Effects of growth hormone on the function of beta-adrenoceptor subtypes in rat adipocytes. Obes Res 2004;12(2):330 9. Kawai M, Namba N, Mushiake S, et al. Growth hormone stimulates adipogenesis of 3T3-L1 cells through activation of the Stat5A/5B-PPARgamma pathway. J Mol Endocrinol 2007;38 (1 2):19 34. Gola M, Bonadonna S, Doga M, Giustina A. Clinical review: growth hormone and cardiovascular risk factors. J Clin Endocrinol Metab 2005;90(3):1864 70. Maison P, Griffin S, Nicoue-Beglah M, Haddad N, Balkau B, Chanson P. Impact of growth hormone (GH) treatment on cardiovascular risk factors in GH-deficient adults: a Metaanalysis of Blinded, Randomized, Placebo-Controlled Trials. J Clin Endocrinol Metab 2004;89(5):2192 9. Beauregard C, Utz AL, Schaub AE, et al. Growth hormone decreases visceral fat and improves cardiovascular risk markers in women with hypopituitarism: a randomized, placebocontrolled study. J Clin Endocrinol Metab 2008;93(6):2063 71. Kamenicky P, Viengchareun S, Blanchard A, et al. Epithelial sodium channel is a key mediator of growth hormone-induced sodium retention in acromegaly. Endocrinology 2008;149 (7):3294 305. Tai PK, Liao JF, Chen EH, Dietz J, Schwartz J, Carter-Su C. Differential regulation of two glucose transporters by chronic growth hormone treatment of cultured 3T3-F442A adipose cells. J Biol Chem 1990;265(35):21828 34. Bell J, Parker KL, Swinford RD, Hoffman AR, Maneatis T, Lippe B. Long-term safety of recombinant human growth hormone in children. J Clin Endocrinol Metab 2010;95(1):167 77. Merimee TJ, Felig P, Marliss E, Fineberg SE, Cahill Jr. GG. Glucose and lipid homeostasis in the absence of human growth hormone. J Clin Invest 1971;50(3):574 82. Svensson J, Fowelin J, Landin K, Bengtsson BA, Johansson JO. Effects of seven years of GH-replacement therapy on insulin sensitivity in GH-deficient adults. J Clin Endocrinol Metab 2002;87(5):2121 7. Veldhuis JD. Neuroendocrine facets of human puberty. Neurobiol Aging 2003;24(Suppl. 1):S93 119 discussion S21-2 Hull KL, Harvey S. Growth hormone: roles in male reproduction. Endocrine 2000;13(3):243 50. Sirotkin AV. Control of reproductive processes by growth hormone: extra- and intracellular mechanisms. Vet J 2005;170 (3):307 17. Carani C, Granata AR, De Rosa M, et al. The effect of chronic treatment with GH on gonadal function in men with isolated GH deficiency. Eur J Endocrinol 1999;140(3):224 30. Giampietro A, Milardi D, Bianchi A, et al. The effect of treatment with growth hormone on fertility outcome in eugonadal women with growth hormone deficiency: report of four cases and review of the literature. Fertil Steril 2009;91(3):930.e7 930.e11. Hull KL, Harvey S. Growth hormone: roles in female reproduction. J Endocrinol 2001;168(1):1 23. Magalhaes DM, Duarte AB, Araujo VR, et al. In vitro production of a caprine embryo from a preantral follicle cultured in

I. HYPOTHALAMIC PITUITARY FUNCTION

REFERENCES

[210]

[211]

[212]

[213]

[214]

[215]

[216]

[217]

[218] [219]

[220]

[221]

[222]

[223]

[224]

[225]

[226]

media supplemented with growth hormone. Theriogenology 2011;75(1):182 8. Duffy JM, Ahmad G, Mohiyiddeen L, Nardo LG, Watson A. Growth hormone for in vitro fertilization. Cochrane Database Syst Rev 2010;(1):CD000099. Rosen T, Eden S, Larson G, Wilhelmsen L, Bengtsson BA. Cardiovascular risk factors in adult patients with growth hormone deficiency. Acta Endocrinol (Copenh) 1993;129 (3):195 200. Markussis V, Beshyah SA, Fisher C, Sharp P, Nicolaides AN, Johnston DG. Detection of premature atherosclerosis by highresolution ultrasonography in symptom-free hypopituitary adults. Lancet 1992;340(8829):1188 92. Amato G, Carella C, Fazio S, et al. Body composition, bone metabolism, and heart structure and function in growth hormone (GH)- deficient adults before and after GH replacement therapy at low doses. J Clin Endocrinol Metab 1993;77:1671 6. Boger RH, Skamira C, Bode-Boger SM, Brabant G, zur Muhlen A, Frolich JC. Nitric oxide may mediate the hemodynamic effects of recombinant growth hormone in patients with acquired growth hormone deficiency. A double-blind, placebo-controlled study. J Clin Invest 1996;98:2706 13. Johannsson G, Sunnerhagen KS, Svensson J. Baseline characteristics and the effects of two years of growth hormone replacement therapy in adults with growth hormone deficiency previously treated for Cushing’s disease. Clin Endocrinol (Oxf) 2004;60(5):550 9. Borson-Chazot F, Serusclat A, Kalfallah Y, et al. Decrease in carotid intima-media thickness after one year growth hormone (GH) treatment in adults with GH deficiency. J Clin Endocrinol Metab 1999;84(4):1329 33. Sacca L, Napoli R, Cittadini A. Growth hormone, acromegaly, and heart failure: an intricate triangulation. Clin Endocrinol (Oxf) 2003;59(6):660 71. Sacca L, Cittadini A, Fazio S. Growth hormone and the heart. Endocr Rev 1994;15(5):555 73. Mosca S, Paolillo S, Colao A, et al. Cardiovascular involvement in patients affected by acromegaly: an appraisal. Int J Cardiol 2013;167(5):1712 18. Bogazzi F, Battolla L, Spinelli C, et al. Risk factors for development of coronary heart disease in patients with acromegaly: a five-year prospective study. J Clin Endocrinol Metab 2007;92 (11):4271 7. Verhelst J, Velkeniers B, Maiter D, et al. Active acromegaly is associated with decreased hs-CRP and NT-proBNP serum levels: insights from the Belgian registry of acromegaly. Eur J Endocrinol 2013;168(2):177 84. Ito H, Hiroe M, Hirata Y, et al. Insulin-like growth factor-I induces hypertrophy with enhanced expression of muscle specific genes in cultured rat cardiomyocytes. Circulation 1993;87 (5):1715 21. Donath MY, Zapf J, Eppenberger-Eberhardt M, Froesch ER, Eppenberger HM. Insulin-like growth factor I stimulates myofibril development and decreases smooth muscle alpha-actin of adult cardiomyocytes. Proc Natl Acad Sci USA 1994;91 (5):1686 90. Schnabel P, Mies F, Nohr T, Geisler M, Bohm M. Differential regulation of phospholipase C-beta isozymes in cardiomyocyte hypertrophy. Biochem Biophys Res Commun 2000;275(1):1 6. Bruel A, Oxlund H. Biosynthetic growth hormone increases the collagen deposition rate in rat aorta and heart. Eur J Endocrinol 1995;132(2):195 9. Bruel A, Oxlund H, Nyengaard JR. Growth hormone increases the total number of myocyte nuclei in the left ventricle of adult rats. Growth Horm IGF Res 2002;12(2):106 15.

123

[227] Creemers EE, Cleutjens JP, Smits JF, Daemen MJ. Matrix metalloproteinase inhibition after myocardial infarction: a new approach to prevent heart failure? Circ Res 2001;89(3):201 10. [228] Li Q, Li B, Wang X, et al. Overexpression of insulin-like growth factor-1 in mice protects from myocyte death after infarction, attenuating ventricular dilation, wall stress, and cardiac hypertrophy. J Clin Invest 1997;100(8):1991 9. [229] Bayes-Genis A, Conover CA, Schwartz RS. The insulin-like growth factor axis: a review of atherosclerosis and restenosis. Circ Res 2000;86(2):125 30. [230] Cittadini A, Cuocolo A, Merola B, et al. Impaired cardiac performance in GH-deficient adults and its improvement after GH replacement. Am J Physiol 1994;267(2 Pt 1):E219 25. [231] Cuneo RC, Salomon F, Wiles CM, Hesp R, Sonksen PH. Growth hormone treatment of growth hormone deficient adults. II. Effects on exercise performance. J Appl Physiol 1991;70:695 700. [232] Colao A, Marzullo P, Di Somma C, Lombardi G. Growth hormone and the heart. Clin Endocrinol (Oxf) 2001;54(2):137 54. [233] Laughlin GA, Barrett-Connor E, Criqui MH, Kritz-Silverstein D. The prospective association of serum insulin-like growth factor I (IGF-I) and IGF-binding protein-1 levels with all cause and cardiovascular disease mortality in older adults: the Rancho Bernardo Study. J Clin Endocrinol Metab 2004;89 (1):114 20. [234] Vasan RS, Sullivan LM, D’Agostino RB, et al. Serum insulinlike growth factor I and risk for heart failure in elderly individuals without a previous myocardial infarction: the Framingham Heart Study. Ann Intern Med 2003;139(8):642 8. [235] Colao A, Di Somma C, Savanelli MC, De Leo M, Lombardi G. Beginning to end: cardiovascular implications of growth hormone (GH) deficiency and GH therapy. Growth Horm IGF Res 2006;16(Suppl. A):S41 8. [236] Abs R, Feldt-Rasmussen U, Mattsson AF, et al. Determinants of cardiovascular risk in 2589 hypopituitary GH-deficient adults a KIMS database analysis. Eur J Endocrinol 2006;155 (1):79 90. [237] Gazzaruso C, Gola M, Karamouzis I, Giubbini R, Giustina A. Cardiovascular risk in adult patients with growth hormone (GH) deficiency and following substitution with GH--an update. J Clin Endocrinol Metab 2014;99(1):18 29. [238] Juul A, Hjortskov N, Jepsen LT, et al. Growth hormone deficiency and hyperthermia during exercise: a controlled study of sixteen GH-deficient patients. J Clin Endocrinol Metab 1995;80(11):3335 40. [239] Tomlinson JW, Holden N, Hills RK, et al. Association between premature mortality and hypopituitarism. West Midlands Prospective Hypopituitary Study Group. Lancet 2001;357 (9254):425 31. [240] Le Corvoisier P, Hittinger L, Chanson P, Montagne O, Macquin-Mavier I, Maison P. Cardiac effects of growth hormone treatment in chronic heart failure: a meta-analysis. J Clin Endocrinol Metab 2007;92(1):180 5. [241] Kontoleon PE, Anastasiou-Nana MI, Papapetrou PD, et al. Hormonal profile in patients with congestive heart failure. Int J Cardiol 2003;87(2 3):179 83. [242] Watanabe S, Tamura T, Ono K, et al. Insulin-like growth factor axis (insulin-like growth factor-I/insulin-like growth factorbinding protein-3) as a prognostic predictor of heart failure: association with adiponectin. Eur J Heart Fail 2010;12 (11):1214 22. [243] Cittadini A, Saldamarco L, Marra AM, et al. Growth hormone deficiency in patients with chronic heart failure and beneficial effects of its correction. J Clin Endocrinol Metab 2009;94 (9):3329 36.

I. HYPOTHALAMIC PITUITARY FUNCTION

124

4. GROWTH HORMONE

[244] Isgaard J, Arcopinto M, Karason K, Cittadini A. GH and the cardiovascular system: an update on a topic at heart. Endocrine 2015;48(1):25 35. [245] Hallengren E, Almgren P, Engstrom G, et al. Fasting levels of high-sensitivity growth hormone predict cardiovascular morbidity and mortality: the Malmo Diet and Cancer study. J Am Coll Cardiol 2014;64(14):1452 60. [246] Yuan MJ, Huang H, Huang CX. Potential new role of the GHSR-1a-mediated signaling pathway in cardiac remodeling after myocardial infarction (Review). Oncol Lett 2014;8 (3):969 71. [247] Kamenicky P, Mazziotti G, Lombes M, Giustina A, Chanson P. Growth hormone, insulin-like growth factor-1, and the kidney: pathophysiological and clinical implications. Endocr Rev 2014;35(2):234 81. [248] Schimpff RM, Donnadieu M, Duval M. Serum somatomedin activity measured as sulphation factor in peripheral, hepatic and renal veins in normal mongrel dogs: early effects of intravenous injection of growth hormone. Acta Endocrinol (Copenh) 1980;93(2):155 61. [249] Reddy GR, Pushpanathan MJ, Ransom RF, et al. Identification of the glomerular podocyte as a target for growth hormone action. Endocrinology 2007;148(5):2045 55. [250] Viengchareun S, Kamenicky P, Teixeira M, et al. Osmotic stress regulates mineralocorticoid receptor expression in a novel aldosterone-sensitive cortical collecting duct cell line. Mol Endocrinol 2009;23(12):1948 62. [251] Auriemma RS, Galdiero M, De Martino MC, et al. The kidney in acromegaly: renal structure and function in patients with acromegaly during active disease and 1 year after disease remission. Eur J Endocrinol 2010;162(6):1035 42. [252] Kamenicky P, Blanchard A, Gauci C, et al. Pathophysiology of renal calcium handling in acromegaly: what lies behind hypercalciuria? J Clin Endocrinol Metab 2012;97 (6):2124 33. [253] Guler HP, Schmid C, Zapf J, Froesch ER. Effects of recombinant insulin-like growth factor I on insulin secretion and renal function in normal human subjects. Proc Natl Acad Sci USA 1989;86(8):2868 72. [254] Clemmons DR, Van Wyk JJ, Ridgway EC, Kliman B, Kjellberg RN, Underwood LE. Evaluation of acromegaly by radioimmunoassay of somatomedin-C. N Engl J Med 1979;301 (21):1138 42. [255] Barkan AL, Beitins IZ, Kelch RP. Plasma insulin-like growth factor-I/somatomedin-C in acromegaly: correlation with the degree of growth hormone hypersecretion. J Clin Endocrinol Metab 1988;67(1):69 73. [256] Melmed S, Colao A, Barkan A, et al. Guidelines for acromegaly management: an update. J Clin Endocrinol Metab 2009;94 (5):1509 17. [257] Dean HJ, Kellett JG, Bala RM, et al. The effect of growth hormone treatment on somatomedin levels in growth hormonedeficient children. J Clin Endocrinol Metab 1982;55 (6):1167 73. [258] Bala RM, Lopatka J, Leung A, McCoy E, McArthur RG. Serum immunoreactive somatomedin levels in normal adults, pregnant women at term, children at various ages, and children with constitutionally delayed growth. J Clin Endocrinol Metab 1981;52(3):508 12. [259] Rubin KR, Lichtenfels JM, Ratzan SK, Ozonoff M, Rowe DW, Carey DE. Relationship of somatomedin-C concentration to bone age in boys with constitutional delay of growth. Am J Dis Child 1986;140(6):555 8. [260] Molitch ME, Clemmons DR, Malozowski S, Merriam GR, Vance ML, Endocrine S. Evaluation and treatment of adult

[261]

[262]

[263]

[264]

[265]

[266] [267]

[268]

[269]

[270]

[271] [272]

[273]

[274]

growth hormone deficiency: an endocrine society clinical practice guideline. J Clin Endocrinol Metab 2011;96(6):1587 609. Ghigo E, Bellone J, Mazza E, et al. Arginine potentiates the GHRH- but not the pyridostigmine-induced GH secretion in normal short children. Further evidence for a somatostatin suppressing effect of arginine. Clin Endocrinol (Oxf) 1990;32 (6):763 7. Biller BM, Samuels MH, Zagar A, et al. Sensitivity and specificity of six tests for the diagnosis of adult GH deficiency. J Clin Endocrinol Metab 2002;87(5):2067 79. Chanson P, Cailleux-Bounacer A, Kuhn JM, et al. Comparative validation of the growth hormone-releasing hormone and arginine test for the diagnosis of adult growth hormone deficiency using a growth hormone assay conforming to recent international recommendations. J Clin Endocrinol Metab 2010;95 (8):3684 92. Cook DM, Yuen KC, Biller BM, Kemp SF, Vance ML, American Association of Clinical E. American Association of Clinical Endocrinologists medical guidelines for clinical practice for growth hormone use in growth hormone-deficient adults and transition patients 2009 update. Endocr Pract 2009;15(Suppl. 2):1 29. Ho KK. Consensus guidelines for the diagnosis and treatment of adults with GH deficiency II: a statement of the GH Research Society in association with the European Society for Pediatric Endocrinology, Lawson Wilkins Society, European Society of Endocrinology, Japan Endocrine Society, and Endocrine Society of Australia. Eur J Endocrinol 2007;157 (6):695 700. Ho KK. Diagnosis of adult GH deficiency. Lancet 2000;356 (9236):1125 6. Chihara K, Shimatsu A, Hizuka N, Tanaka T, Seino Y, Katofor Y. A simple diagnostic test using GH-releasing peptide-2 in adult GH deficiency. Eur J Endocrinol 2007;157(1):19 27. Garcia JM, Swerdloff R, Wang C, et al. Macimorelin (AEZS130)-stimulated growth hormone (GH) test: validation of a novel oral stimulation test for the diagnosis of adult GH deficiency. J Clin Endocrinol Metab 2013;98(6):2422 9. Hamrahian AH, Yuen KC, Gordon MB, Pulaski-Liebert KJ, Bena J, Biller BM. Revised GH and cortisol cut-points for the glucagon stimulation test in the evaluation of GH and hypothalamic-pituitary-adrenal axes in adults: results from a prospective randomized multicenter study. Pituitary 2016;19 (3):332 41. Yuen K, Biller B, Molitch M, Cook D. Clinical review: is lack of recombinant growth hormone (GH)-releasing hormone in the United States a setback or time to consider glucagon testing for adult GH deficiency? J Clin Endocrinol Metab 2009;94 (8):2702 7. Melmed S. Idiopathic adult growth hormone deficiency. J Clin Endocrinol Metab 2013;98(6):2187 97. Hartman ML, Crowe BJ, Biller BM, Ho KK, Clemmons DR, Chipman JJ. Which patients do not require a GH stimulation test for the diagnosis of adult GH deficiency? J Clin Endocrinol Metab 2002;87(2):477 85. Darzy KH, Aimaretti G, Wieringa G, Gattamaneni HR, Ghigo E, Shalet SM. The usefulness of the combined growth hormone (GH)-releasing hormone and arginine stimulation test in the diagnosis of radiation-induced GH deficiency is dependent on the post-irradiation time interval. J Clin Endocrinol Metab 2003;88(1):95 102. Bonert VS, Elashoff JD, Barnett P, Melmed S. Body mass index determines evoked growth hormone (GH) responsiveness in normal healthy male subjects: diagnostic caveat for adult GH deficiency. J Clin Endocrinol Metab 2004;89(7):3397 401.

I. HYPOTHALAMIC PITUITARY FUNCTION

REFERENCES

[275] Corneli G, Di Somma C, Baldelli R, et al. The cut-off limits of the GH response to GH-releasing hormone-arginine test related to body mass index. Eur J Endocrinol 2005;153 (2):257 64. [276] Tauber M, Moulin P, Pienkowski C, Jouret B, Rochiccioli P. Growth hormone (GH) retesting and auxological data in 131 GH-deficient patients after completion of treatment. J Clin Endocrinol Metab 1997;82(2):352 6. [277] Bidlingmaier M, Freda PU. Measurement of human growth hormone by immunoassays: current status, unsolved problems and clinical consequences. Growth Horm IGF Res 2010;20 (1):19 25. [278] Pokrajac A, Wark G, Ellis AR, Wear J, Wieringa GE, Trainer PJ. Variation in GH and IGF-I assays limits the applicability of international consensus criteria to local practice. Clin Endocrinol (Oxf) 2007;67(1):65 70. [279] Clemmons DR. Consensus statement on the standardization and evaluation of growth hormone and insulin-like growth factor assays. Clin Chem 2011;57(4):555 9. [280] Clemmons DR. IGF-I assays: current assay methodologies and their limitations. Pituitary 2007;10(2):121 8. [281] Massart C, Poirier JY. Serum insulin-like growth factor-I measurement in the follow-up of treated acromegaly: comparison of four immunoassays. Clin Chim Acta 2006;373 (1 2):176 9. [282] Krebs A, Wallaschofski H, Spilcke-Liss E, et al. Five commercially available insulin-like growth factor I (IGF-I) assays in comparison to the former Nichols Advantage IGF-I in a growth hormone treated population. Clin Chem Lab Med 2008;46(12):1776 83. [283] Bidlingmaier M, Friedrich N, Emeny RT, et al. Reference intervals for insulin-like growth factor-1 (igf-i) from birth to senescence: results from a multicenter study using a new automated chemiluminescence IGF-I immunoassay conforming to recent international recommendations. J Clin Endocrinol Metab 2014;99(5):1712 21. [284] Frystyk J, Freda P, Clemmons DR. The current status of IGF-I assays--a 2009 update. Growth Horm IGF Res 2010;20(1):8 18. [285] Algeciras-Schimnich A, Bruns DE, Boyd JC, Bryant SC, La Fortune KA, Grebe SK. Failure of current laboratory protocols to detect lot-to-lot reagent differences: findings and possible solutions. Clin Chem 2013;59(8):1187 94. [286] Clemmons DR. Clinical utility of measurements of insulin-like growth factor 1. Nat Clin Pract Endocrinol Metab 2006;2 (8):436 46. [287] Burns C, Rigsby P, Moore M, Rafferty B. The first international standard for insulin-like growth factor-1 (IGF-1) for immunoassay: preparation and calibration in an international collaborative study. Growth Horm IGF Res 2009;19(5):457 62. [288] Arsene CG, Kratzsch J, Henrion A. Mass spectrometry an alternative in growth hormone measurement. Bioanalysis 2014;6(18):2391 402. [289] Cox HD, Lopes F, Woldemariam GA, et al. Interlaboratory agreement of insulin-like growth factor 1 concentrations measured by mass spectrometry. Clin Chem 2014;60(3):541 8. [290] Monaghan PJ, Keevil BG, Trainer PJ. The use of mass spectrometry to improve the diagnosis and the management of the HPA axis. Rev Endocr Metab Disord 2013;14(2):143 57. [291] Bystrom C, Sheng S, Zhang K, Caulfield M, Clarke NJ, Reitz R. Clinical utility of insulin-like growth factor 1 and 2; determination by high resolution mass spectrometry. PLoS One 2012;7(9):e43457. [292] Vogeser M, Seger C. Pitfalls associated with the use of liquid chromatography-tandem mass spectrometry in the clinical laboratory. Clin Chem 2010;56(8):1234 44.

125

[293] Alatzoglou KS, Turton JP, Kelberman D, et al. Expanding the spectrum of mutations in GH1 and GHRHR: genetic screening in a large cohort of patients with congenital isolated growth hormone deficiency. J Clin Endocrinol Metab 2009;94 (9):3191 9. [294] Alatzoglou KS, Dattani MT. Genetic causes and treatment of isolated growth hormone deficiency-an update. Nat Rev Endocrinol 2010;6(10):562 76. [295] Alatzoglou KS, Webb EA, Le Tissier P, Dattani MT. Isolated growth hormone deficiency (GHD) in childhood and adolescence: recent advances. Endocr Rev 2014;35(3):376 432. [296] Vance ML, Mauras N. Growth hormone therapy in adults and children. N Engl J Med 1999;341(16):1206 16. [297] Quigley CA. Growth hormone treatment of non-growth hormone-deficient growth disorders. Endocrinol Metab Clin North Am 2007;36(1):131 86. [298] Stephure DK. Impact of growth hormone supplementation on adult height in turner syndrome: results of the Canadian randomized controlled trial. J Clin Endocrinol Metab 2005;90 (6):3360 6. [299] de Zegher F, Hokken-Koelega A. Growth hormone therapy for children born small for gestational age: height gain is less dose dependent over the long term than over the short term. Pediatrics 2005;115(4):e458 62. [300] Salevic P, Radovic P, Milic N, et al. Growth in children with chronic kidney disease: 13 years follow up study. J Nephrol 2014;27(5):537 44. [301] Hodson EM, Willis NS, Craig JC. Growth hormone for children with chronic kidney disease. Cochrane Database Syst Rev 2012;(2):CD003264. [302] Vimalachandra D, Craig JC, Cowell C, Knight JF. Growth hormone for children with chronic renal failure. Cochrane Database Syst Rev 2001;(4):CD003264. [303] Blum WF, Ross JL, Zimmermann AG, et al. GH treatment to final height produces similar height gains in patients with SHOX deficiency and Turner syndrome: results of a multicenter trial. J Clin Endocrinol Metab 2013;98(8): E1383 92. [304] Brod M, Hojbjerre L, Adalsteinsson JE, Rasmussen MH. Assessing the impact of growth hormone deficiency and treatment in adults: development of a new disease-specific measure. J Clin Endocrinol Metab 2014;99(4):1204 12. [305] Webb SM, Strasburger CJ, Mo D, et al. Changing patterns of the adult growth hormone deficiency diagnosis documented in a decade-long global surveillance database. J Clin Endocrinol Metab 2009;94(2):392 9. [306] Tanriverdi F, Schneider HJ, Aimaretti G, Masel BE, Casanueva FF, Kelestimur F. Pituitary dysfunction after traumatic brain injury: a clinical and pathophysiological approach. Endocr Rev 2015;36(3):305 42. [307] Carroll PV, Christ ER, Bengtsson BA, et al. Growth hormone deficiency in adulthood and the effects of growth hormone replacement: a review. Growth Hormone Research Society Scientific Committee. J Clin Endocrinol Metab 1998;83 (2):382 95. [308] Holmer H, Svensson J, Rylander L, et al. Psychosocial health and levels of employment in 851 hypopituitary Swedish patients on long-term GH therapy. Psychoneuroendocrinology 2013;38(6):842 52. [309] Pappachan JM, Raskauskiene D, Kutty VR, Clayton RN. Excess mortality associated with hypopituitarism in adults: a meta-analysis of observational studies. J Clin Endocrinol Metab 2015;100(4):1405 11. [310] Gardner CJ, Mattsson AF, Daousi C, Korbonits M, KoltowskaHaggstrom M, Cuthbertson DJ. GH deficiency after traumatic

I. HYPOTHALAMIC PITUITARY FUNCTION

126

[311]

[312]

[313]

[314]

[315]

[316]

[317]

[318]

[319]

[320]

[321]

[322]

[323]

[324]

[325] [326]

4. GROWTH HORMONE

brain injury: improvement in quality of life with GH therapy: analysis of the KIMS database. Eur J Endocrinol 2015;172 (4):371 81. Widdowson WM, Gibney J. The effect of growth hormone replacement on exercise capacity in patients with GH deficiency: a metaanalysis. J Clin Endocrinol Metab 2008;93 (11):4413 17. Gibney J, Healy ML, Stolinski M, et al. Effect of growth hormone (GH) on glycerol and free fatty acid metabolism during exhaustive exercise in GH-deficient adults. J Clin Endocrinol Metab 2003;88(4):1792 7. Colao A, Di Somma C, Cuocolo A, et al. The severity of growth hormone deficiency correlates with the severity of cardiac impairment in 100 adult patients with hypopituitarism: an observational, case-control study. J Clin Endocrinol Metab 2004;89(12):5998 6004. Woodhouse LJ, Mukherjee A, Shalet SM, Ezzat S. The influence of growth hormone status on physical impairments, functional limitations, and health-related quality of life in adults. Endocr Rev 2006;27(3):287 317. Gibney J, Wolthers T, Johannsson G, Umpleby AM, Ho KK. Growth hormone and testosterone interact positively to enhance protein and energy metabolism in hypopituitary men. Am J Physiol Endocrinol Metab 2005;289(2):E266 71. Svensson J, Sunnerhagen KS, Johannsson G. Five years of growth hormone replacement therapy in adults: age- and gender-related changes in isometric and isokinetic muscle strength. J Clin Endocrinol Metab 2003;88(5):2061 9. Hoffman AR, Kuntze JE, Baptista J, et al. Growth hormone (GH) replacement therapy in adult-onset gh deficiency: effects on body composition in men and women in a double-blind, randomized, placebo-controlled trial. J Clin Endocrinol Metab 2004;89(5):2048 56. Sesmilo G, Biller BM, Llevadot J, et al. Effects of growth hormone administration on inflammatory and other cardiovascular risk markers in men with growth hormone deficiency. A randomized, controlled clinical trial. Ann Intern Med 2000;133(2):111 22. Drake WM, Rodriguez-Arnao J, Weaver JU, et al. The influence of gender on the short and long-term effects of growth hormone replacement on bone metabolism and bone mineral density in hypopituitary adults: a 5-year study. Clin Endocrinol (Oxf) 2001;54(4):525 32. Mo D, Fleseriu M, Qi R, et al. Fracture risk in adult patients treated with growth hormone replacement therapy for growth hormone deficiency: a prospective observational cohort study. Lancet Diabetes Endocrinol 2015;3(5):331 8. Krantz E, Trimpou P, Landin-Wilhelmsen K. Effect of growth hormone treatment on fractures and quality of life in postmenopausal osteoporosis: a 10-year follow-up study. J Clin Endocrinol Metab 2015;100(9):3251 9. Cook DM, Ludlam WH, Cook MB. Route of estrogen administration helps to determine growth hormone (GH) replacement dose in GH-deficient adults. J Clin Endocrinol Metab 1999;84 (11):3956 60. Kargi AY, Merriam GR. Diagnosis and treatment of growth hormone deficiency in adults. Nat Rev Endocrinol 2013;9 (6):335 45. Fleseriu M, Hashim IA, Karavitaki N, et al. Hormonal replacement in hypopituitarism in adults: an endocrine society clinical practice guideline. J. Clin. Endocrinol. Metab. 2016;101 (11):3888 921. Erfurth EM. Update in mortality in GH-treated patients. J Clin Endocrinol Metab 2013;98(11):4219 26. Frajese G, Drake WM, Loureiro RA, et al. Hypothalamopituitary surveillance imaging in hypopituitary patients

[327]

[328]

[329]

[330]

[331]

[332]

[333]

[334]

[335]

[336]

[337]

[338]

[339]

[340]

[341]

[342]

[343]

receiving long-term GH replacement therapy. J Clin Endocrinol Metab 2001;86(11):5172 5. Child CJ, Conroy D, Zimmermann AG, Woodmansee WW, Erfurth EM, Robison LL. Incidence of primary cancers and intracranial tumour recurrences in GH-treated and untreated adult hypopituitary patients: analyses from the Hypopituitary Control and Complications Study. Eur J Endocrinol 2015;172 (6):779 90. Karavitaki N, Warner JT, Marland A, et al. GH replacement does not increase the risk of recurrence in patients with craniopharyngioma. Clin Endocrinol (Oxf) 2006;64(5):556 60. Jostel A, Mukherjee A, Hulse PA, Shalet SM. Adult growth hormone replacement therapy and neuroimaging surveillance in brain tumour survivors. Clin Endocrinol (Oxf) 2005;62 (6):698 705. van Varsseveld NC, van Bunderen CC, Franken AA, Koppeschaar HP, van der Lely AJ, Drent ML. Tumor recurrence or regrowth in adults with nonfunctioning pituitary adenomas using GH replacement therapy. J Clin Endocrinol Metab 2015;100(8):3132 9. Chan JM, Stampfer MJ, Giovannucci E, et al. Plasma insulinlike growth factor-I and prostate cancer risk: a prospective study. Science 1998;279(5350):563 6. Hankinson SE, Willett WC, Colditz GA, et al. Circulating concentrations of insulin-like growth factor-I and risk of breast cancer. Lancet 1998;351(9113):1393 6. Ma J, Pollak MN, Giovannucci E, et al. Prospective study of colorectal cancer risk in men and plasma levels of insulin-like growth factor (IGF)-I and IGF-binding protein-3. J Natl Cancer Inst 1999;91(7):620 5. Losa M, Scavini M, Gatti E, et al. Long-term effects of growth hormone replacement therapy on thyroid function in adults with growth hormone deficiency. Thyroid 2008;18 (12):1249 54. Vila G, Akerblad AC, Mattsson AF, et al. Pregnancy outcomes in women with growth hormone deficiency. Fertil Steril 2015;104(5) 1210-7 e1 Mazziotti G, Giustina A. Glucocorticoids and the regulation of growth hormone secretion. Nat Rev Endocrinol 2013;9 (5):265 76. Mazziotti G, Marzullo P, Doga M, Aimaretti G, Giustina A. Growth hormone deficiency in treated acromegaly. Trends Endocrinol Metab 2015;26(1):11 21. Ku CR, Hong JW, Kim EH, Kim SH, Lee EJ. Clinical predictors of GH deficiency in surgically cured acromegalic patients. Eur J Endocrinol 2014;171(3):379 87. Tritos NA, Johannsson G, Korbonits M, et al. Effects of long-term growth hormone replacement in adults with growth hormone deficiency following cure of acromegaly: a KIMS analysis. J Clin Endocrinol Metab 2014;99(6):2018 29. Giavoli C, Profka E, Verrua E, et al. GH replacement improves quality of life and metabolic parameters in cured acromegalic patients with growth hormone deficiency. J Clin Endocrinol Metab 2012;97(11):3983 8. Berryman DE, Christiansen JS, Johannsson G, Thorner MO, Kopchick JJ. Role of the GH/IGF-1 axis in lifespan and healthspan: lessons from animal models. Growth Horm IGF Res 2008;18(6):455 71. Bartke A, Wright JC, Mattison JA, Ingram DK, Miller RA, Roth GS. Extending the lifespan of long-lived mice. Nature 2001;414 (6862):412. Flurkey K, Papaconstantinou J, Miller RA, Harrison DE. Lifespan extension and delayed immune and collagen aging in mutant mice with defects in growth hormone production. Proc Natl Acad Sci USA 2001;98(12):6736 41.

I. HYPOTHALAMIC PITUITARY FUNCTION

REFERENCES

[344] Barzilai N, Bartke A. Biological approaches to mechanistically understand the healthy life span extension achieved by calorie restriction and modulation of hormones. J Gerontol A Biol Sci Med Sci 2009;64(2):187 91. [345] Liu H, Bravata DM, Olkin I, et al. Systematic review: the safety and efficacy of growth hormone in the healthy elderly. Ann Intern Med 2007;146(2):104 15. [346] Clemmons DR, Molitch M, Hoffman AR, et al. Growth hormone should be used only for approved indications. J Clin Endocrinol Metab 2014;99(2):409 11. [347] Nass R, Pezzoli SS, Oliveri MC, et al. Effects of an oral ghrelin mimetic on body composition and clinical outcomes in healthy older adults: a randomized trial. Ann Intern Med 2008;149 (9):601 11. [348] Sattler FR, Castaneda-Sceppa C, Binder EF, et al. Testosterone and growth hormone improve body composition and muscle performance in older men. J Clin Endocrinol Metab 2009;94 (6):1991 2001. [349] Holt RI, Webb E, Pentecost C, Sonksen PH. Aging and physical fitness are more important than obesity in determining exercise-induced generation of GH. J Clin Endocrinol Metab 2001;86(12):5715 20. [350] Holt RI, Erotokritou-Mulligan I, Sonksen PH. The history of doping and growth hormone abuse in sport. Growth Horm IGF Res 2009;19(4):320 6. [351] Baker JS, Graham MR, Davies B. Steroid and prescription medicine abuse in the health and fitness community: a regional study. Eur J Intern Med 2006;17(7):479 84. [352] Irving BA, Patrie JT, Anderson SM, et al. The effects of time following acute growth hormone administration on metabolic and power output measures during acute exercise. J Clin Endocrinol Metab 2004;89(9):4298 305. [353] Healy ML, Gibney J, Russell-Jones DL, et al. High dose growth hormone exerts an anabolic effect at rest and during exercise in endurance-trained athletes. J Clin Endocrinol Metab 2003;88 (11):5221 6. [354] Birzniece V, Nelson AE, Ho KK. Growth hormone and physical performance. Trends Endocrinol Metab 2011;22(5):171 8. [355] Liu H, Bravata DM, Olkin I, et al. Systematic review: the effects of growth hormone on athletic performance. Ann Intern Med 2008;148(10):747 58. [356] Meinhardt U, Nelson AE, Hansen JL, et al. The effects of growth hormone on body composition and physical performance in recreational athletes: a randomized trial. Ann Intern Med 2010;152(9):568 77. [357] Graham MR, Baker JS, Evans P, et al. Physical effects of shortterm recombinant human growth hormone administration in abstinent steroid dependency. Horm Res 2008;69(6):343 54.

127

[358] Powrie JK, Bassett EE, Rosen T, et al. Detection of growth hormone abuse in sport. Growth Horm IGF Res 2007;17 (3):220 6. [359] Erotokritou-Mulligan I, Guha N, Stow M, et al. The development of decision limits for the implementation of the GH2000 detection methodology using current commercial insulin-like growth factor-I and amino-terminal pro-peptide of type III collagen assays. Growth Horm IGF Res 2012;22 (2):53 8. [360] Wallace JD, Cuneo RC, Bidlingmaier M, et al. Changes in non-22-kilodalton (kDa) isoforms of growth hormone (GH) after administration of 22-kDa recombinant human GH in trained adult males. J Clin Endocrinol Metab 2001;86 (4):1731 7. [361] Baumann GP. Growth hormone doping in sports: a critical review of use and detection strategies. Endocr Rev 2012;33 (2):155 86. [362] Erotokritou-Mulligan I, Bassett EE, Cowan DA, et al. Influence of ethnicity on IGF-I and procollagen III peptide (P-III-P) in elite athletes and its effect on the ability to detect GH abuse. Clin Endocrinol (Oxf) 2009;70(1):161 8. [363] Nelson AE, Howe CJ, Nguyen TV, et al. Influence of demographic factors and sport type on growth hormone-responsive markers in elite athletes. J Clin Endocrinol Metab 2006;91 (11):4424 32. [364] Mitchell CJ, Nelson AE, Cowley MJ, et al. Detection of growth hormone doping by gene expression profiling of peripheral blood. J Clin Endocrinol Metab 2009;94(12):4703 9. [365] Jasim S, Alahdab F, Ahmed AT, et al. Mortality in adults with hypopituitarism: a systematic review and meta-analysis. Endocrine 2016; [Epub ahead of print] [366] Hartman ML, Xu R, Crowe BJ, et al. Prospective safety surveillance of GH-deficient adults: comparison of GH-treated vs untreated patients. J Clin Endocrinol Metab 2013;98(3):980 8. [367] Cahill Jr. GF, Herrera MG, Morgan AP, et al. Hormone-fuel interrelationships during fasting. J Clin Invest 1966;45 (11):1751 69. [368] Isley WL, Underwood LE, Clemmons DR. Dietary components that regulate serum somatomedin-C concentrations in humans. J Clin Invest 1983;71(2):175 82. [369] Backeljauw PF, Underwood LE. Therapy for 6.5-7.5 years with recombinant insulin-like growth factor I in children with growth hormone insensitivity syndrome: a clinical research center study. J Clin Endocrinol Metab 2001;86 (4):1504 10. [370] Laron Z, Klinger B. Comparison of the growth-promoting effects of insulin-like growth factor I and growth hormone in the early years of life. Acta Paediatr 2000;89(1):38 41.

I. HYPOTHALAMIC PITUITARY FUNCTION