Cortisol metabolism in hypertension

Best Practice & Research Clinical Endocrinology & Metabolism Vol. 20, No. 3, pp. 337e353, 2006 doi:10.1016/j.beem.2006.07.001 available online at http://www.sciencedirect.com

1 Cortisol metabolism in hypertension Fabian Hammer

MD

Clinical Lecturer

Paul M. Stewart*

MD, FRCP, FMedSci

Professor of Medicine Division of Medical Sciences, University of Birmingham, Institute of Biomedical Research, Birmingham B15 2TT, UK

Corticosteroids are critically involved in blood pressure regulation. Lack of adrenal steroids in Addison’s disease causes life-threatening hypotension, whereas glucocorticoid excess in Cushing’s syndrome invariably results in high blood pressure. At a pre-receptor level, glucocorticoid action is modulated by 11b-hydroxysteroid dehydrogenases (11b-HSDs). 11b-HSD1 activates cortisone to cortisol to facilitate glucocorticoid receptor (GR)-mediated action. By contrast, 11b-HSD2 plays a pivotal role in aldosterone target tissues where it catalyses the opposite reaction (i.e. inactivation of cortisol to cortisone) to prevent activation of the mineralocorticoid receptor (MR) by cortisol. Mutations in the 11b-HSD2 gene cause a rare form of inherited hypertension, the syndrome of apparent mineralocorticoid excess (AME), in which cortisol activates the MR resulting in severe hypertension and hypokalemia. Ingestion of competitive inhibitors of 11b-HSD2 such as liquorice and carbenoxolone result in a similar but milder clinical phenotype. Epidemiological data suggests that polymorphic variability in the HSD11B2 gene determines salt sensitivity in the general population, which is a key predisposing factor to adult onset hypertension in some patients. Extrarenal sites of glucocorticoid action and metabolism that might impact on blood pressure include the vasculature and the central nervous system. Intriguingly, increased exposure to glucocorticoids during fetal life promotes high blood pressure in adulthood suggesting an early programming effect. Thus, metabolism and action in many peripheral tissues might contribute to the pathophysiology of human hypertension. Key words: hypertension; cortisol; aldosterone; corticosteroid; 11b-hydroxysteroid dehydrogenase (11b-HSD); apparent mineralocorticoid excess (AME); Liquorice; Cushing’s syndrome.

* Corresponding author. Tel.: þ44 121 415 8708; Fax: þ44 121 415 8712. E-mail address: [email protected] (P.M. Stewart). 1521-690X/$ - see front matter ª 2006 Elsevier Ltd. All rights reserved.

338 F. Hammer and P. M. Stewart

Hypertension has a prevalence of 20% in the general population and is a major risk factor for cardiovascular and renal morbidity and mortality. In the vast majority of cases no underlying causes for the raised blood pressure are found and, therefore, they are referred to as ‘essential’ hypertension. In contrast, ‘secondary’ hypertension is defined by an identifiable cause and adrenal corticosteroids are now recognised as playing a major role in this group. Cortisol is the main glucocorticoid secreted by the zona fasciculata of the human adrenal gland, whereas aldosterone is the principal mineralocorticoid derived from the zona glomerulosa. The implications of corticosteroid action on blood pressure regulation are exemplified in pathological conditions of hormone deficiency and excess. Inadequate secretion of corticosteroids in Addison’s disease causes lifethreatening hypotension, whereas states of mineralocorticoid and glucocorticoid excess in primary hyperaldosteronism and Cushing’s syndrome, respectively, result in high blood pressure. Mineralocorticoid-based hypertension refers to a distinct entity of secondary hypertension defined by increased sodium and water retention by the kidneys resulting in an expansion of the extracellular fluid compartment and suppressed plasma renin activity.1 Without a doubt primary hyperaldosteronism (see Chapters 3 and 4) is now recognised as the most prevalent cause of mineralocorticoid hypertension2, but rarer acquired and inherited mineralocorticoid excess states unveil important pathophysiological mechanisms that are involved in the much more prevalent form of essential hypertension. This review focuses on the pre-receptor metabolism and potential sites of action of glucocorticoids in blood pressure regulation. CORTISOL METABOLISM Circulating cortisol levels are tightly regulated by the activity of the hypothalamoe pituitaryeadrenal (HPA) axis in order to maintain cortisol homeostasis according to physiological demands. At a cellular level, cortisol availability is modulated by two isoenzymes of 11b-hydroxysteroid dehydrogenase, 11b-HSD1 and 11b-HSD2.3,4 Although both enzymes control the interconversion of biologically active 11hydroxy-glucocorticoids (cortisol in humans and corticosterone in rodents) to their inactive 11-keto forms (cortisone in humans and 11-deyhydrocortisone in rodents), they only share a 21% homology and have entirely different functions, substrate affinities and tissue distribution (Table 1). 11b-HSD1 is widely distributed but is most abundantly expressed in liver and fat tissue.5 The enzyme preferentially utilises NADP(H) as a co-factor and is anchored in the membrane of the endoplasmic reticulum with its catalytic domain protruding into the lumen.6 Principally the enzyme has bi-directional activities, capable of carrying out both 11-oxoreductase (cortisone to cortisol) and dehydrogenase reactions (cortisol to cortisone) (Figure 1). However in vivo, the enzyme predominantly functions as an oxoreductase and consequently facilitates glucocorticoid receptor (GR)-mediated hormone action. Very recently it was demonstrated that oxoreductase activity critically relies on high NADPH concentrations as ablation of the hexose-6-phosphate dehydrogenase (H6PDH) gene, which regenerates NADPH from NADP within the endoplasmic reticulum, results in a complete loss of oxoreductase activity and reversal of its enzymatic activity from an oxoreductase to a dehydrogenase.7 11b-HSD2 is a unidirectional, NAD-dependent dehydrogenase inactivating cortisol to cortisone (Table 1). It is also located in the endoplasmic reticulum but, in contrast

Cortisol metabolism in hypertension 339

Table 1. Comparison of 11b-hydroxysteroid dehydrogenase (11bHSD) isoenzymes. 11b-HSD1

11b-HSD2

1 30 kb (6 exons)

16 6.2 kb (5 exons)

292 aa, 34 kDa NADPþ/NADPH In vivo: oxidoreductase (in vitro: bidirectional) mM (low substrate affinity)

405 aa, 44 kDa NADþ/NADH Dehydrogenase nM (high substrate affinity)

Expression pattern

Liver, adipose, lung, gonads, brain

Kidney, colon, salivary gland, placenta

Function

Tissue specific modulation of cortisol concentrations

Protection of MR from glucocorticoids to ensure aldosterone selectivity

Gene Chromosome Gene size Enzyme Size Co-factor Kinetics Km

aa, amino acids; kDa, kilo Dalton; kb, kilo bases; MR, mineralocorticoid receptor; NAD, nicotinamide adenine dinucleotide; NADH, reduced nicotinamide adenine dinucleotide; NADP, nicotinamide adenine dinucleotide phosphate; NADPH, reduced nicotinamide adenine dinucleotide phosphate.

to 11b-HSD1, its catalytic domain faces the cytoplasm and has a high affinity for its endogenous substrate cortisol.8 11b-HSD2 exhibits a distinct tissue-specific expression in classical mineralocorticoid target tissues, such as epithelial cells from the distal nephron, colon and salivary glands, where it serves to protect the mineralocorticoid receptor (MR) from cortisol (Figure 2). In addition, 11b-HSD2 is abundantly expressed in the placenta where it protects the fetus from active maternal glucocorticoids.9 Circulating concentrations of cortisol are 100e1000-fold higher than those of aldosterone but, paradoxically, the MR has similar affinities for its natural ligand aldosterone and cortisol in vitro.10 Thus, in vivo MR selectivity to aldosterone relies on the

11 -HSD1

O H3C

O H3C

OH OH

HO H3C

H H

OH OH

H H

H

H

O

O Cortisone (“inactive”)

O H3C

11 -HSD2

Cortisol (“active”)

Figure 1. Biochemical properties of 11b-hydroxysteroid dehydrogenases (11b-HSDs). The type 1 enzyme exhibits both oxoreductase (cortisone to cortisol) and dehydrogenase activities (cortisol to cortisone) in vitro, but in vivo it mainly functions as an oxoreductase. The type 2 enzyme exhibits only dehydrogenase activity (cortisol to cortisone).

340 F. Hammer and P. M. Stewart

F

F

F

F F

A

F

F

Na+ K+ ATPase

Basal F

F

A F

F

11

E

Aldosterone target genes

D2 HS

A MR

A

MR MR

E

E E

E

E

Luminal

ENaC Na+

Figure 2. Schematic depiction of aldosterone action in a mineralocorticoid target cell of the renal cortical collecting duct. The mineralocorticoid receptor (MR) binds cortisol (F) and aldosterone (A) with equal affinity, but plasma F concentrations exceed those of A by 100e1000-fold. Selective binding and adequate activation of the MR by A is ensured by the action of the enzyme 11b-hydroxysteroid dehydrogenase type 2 (11b-HSD2), which converts hormonally active F to its hormonally inactive counterpart cortisone (E). Upon binding of A to the MR, the MR-A complex translocates from the cytoplasm to the nucleus where it binds as a dimer to regulatory DNA elements of aldosterone target genes. These events induce upregulation of the apical epithelial sodium channel (ENaC) and the basolateral Naþ-Kþ ATPase. Impaired 11bHSD2 activity results in insufficient inactivation of F and consequently F illicitly binds to the MR to increase transcription of MR target genes resulting in increased sodium reabsorption from, and potassium excretion into, the urine.

inactivation of cortisol to cortisone by 11b-HSD2 at the pre-receptor level. Aldosterone, in contrast to cortisol, is not metabolised by 11b-HSD2 because it forms a C11-C18 hemi-ketal group in aqueous solution. In vivo, the global activity of both enzymatic reactions can conveniently be assayed through an assessment of urinary steroid metabolites. Following interconversion of cortisol and cortisone, both steroids undergo A-ring reduction by 5a- and 5breductases and 3a-hydroxysteroid dehydrogenase (3a-HSD) to yield 5b-tetrahydrocortisol (THF), 5a-tetrahydrocortisol (allo-THF) and 5b-tetrahydrocortisone (THE) (Figure 3A). Overall 11b-HSD activity in the body is reflected by the ratio of total urinary cortisol and cortisone metabolites: [THF þ allo-THF]/THE.

Cortisol metabolism in hypertension 341

A

11 -HSD-2 (-1)

Cortisol 5 -reductase

Cortisone

5 -reductase

5 -dihydrocortisol

5 -dihydrocortisol

5 -reductase

5 -dihydrocortisone

3 -HSD

3 -HSD allo-tetrahydrocortisol (allo-THF)

B

11 -HSD-1

tetrahydrocortisol (THF)

3 -HSD tetrahydrocortisone (THE)

normal Cortisol

11 HSD2

AME

Cortisone

THF + allo-THF

THE

ratio: 1

11 HSD2

Cortisol

Cortisone

THF + allo-THF

THE

ratio: 8-80

Figure 3. A, Schematic depiction of the enzymatic activity involved in glucocorticoid metabolism. Cortisol and cortisone are interconverted by 11b-hydroxysteroid dehydrogenases (11b-HSDs). In the liver, 5a- and 5b-reductases and 3a-hydroxysteroid dehydrogenases (3a-HSDs) convert cortisol to 5a-tetrahydrocortisol (allo-THF) and 5b-tetrahydrocortisol (THF) and convert cortisone to tetrahydrocortisone (THE). B, In normal subjects urinary excretion of cortisol metabolites (THF and allo-THF) compared to the cortisone metabolite (THE) is equivalent, resulting in a [THF þ allo-THF]/THE ratio of 1. Mutations that inactivate 11b-HSD2 in apparent mineralocorticoid excess (AME) patients result in a grossly increased urinary excretion of THF and allo-THF compounds, whereas THE is dramatically reduced, resulting in a high [THF þ allo-THF]/THE ratio.

THE SYNDROME OF APPARENT MINERALOCORTICOID EXCESS Clinical features Apparent mineralocorticoid excess (AME) is a rare form of hypertension; so far fewer than 100 cases have been described worldwide.11,12 AME is inherited in an autosomal recessive fashion and several families with affected siblings have been reported. Presentation is usually during neonatal life or childhood with low birth weight, failure to thrive, short stature, severe hypertension and hypokalemic metabolic alkalosis. Hypokalemia may cause arrhythmia, nephrogenic diabetes insipidus and rhabdomyolysis. In addition, renal cysts and nephrocalcinosis have been reported and may lead to renal insufficiency. Not surprisingly, profound hypertension, if untreated, results in damage to end organs such as the kidney, cardiovascular system, retina and central nervous system.13

342 F. Hammer and P. M. Stewart

Laboratory findings Biochemically, blood test abnormalities comprise hypokalemia, suppressed renin and undetectable aldosterone levels, hence the term ‘apparent mineralocorticoid excess’ was coined. The diagnosis is based on urinary cortisol metabolites, which show a greatly increased [THF þ allo-THF]/THE ratio of 8e80 (reference range 0.7e1.3) with very low or absent levels of THE clearly suggesting a defect in 11b-HSD2 activity (Figure 3).12,14 Interestingly, the excretion of 5a-cortisol metabolites exceeds that of 5b-cortisol metabolites, resulting in a high urinary allo-THF/THF ratio, which suggests an additional defect in 5b-reductase activity.15,16 However, unless grossly abnormal, the [THF þ allo-THF]/THE ratio provides only an overall index of 11b-HSD activity within the body, the main contributors being 11b-HSD1 in the liver and 11b-HSD2 in the kidney. Thus it has been suggested that the urinary free cortisol to cortisone (UFF/UFE) ratio may provide a more accurate measure as it only reflects 11b-HSD2 activity in the kidney but not 11b-HSD1 activity in the liver.17 In AME the urinary UFF/UFE ratio is greatly elevated with virtually undetectable levels of UFE.17 Furthermore, deficiency of 11b-HSD2 in AME results in prolonged cortisol half-life as a consequence of impaired conversion to cortisone. Although cortisol metabolism in AME is deranged, patients do not exhibit any cushingoid features and have normal circulating cortisol levels as a consequence of an intact negative feedback mechanism. AME is caused by mutations in the HSD11B2 gene that lead to partial or complete inactivation of the enzyme 11b-HSD2. Lack of 11b-HSD2 activity results in illicit activation of the MR by cortisol in aldosterone target tissues rendering cortisol a potent mineralocorticoid in this condition. A milder variant of AME (named type II AME) has been described in several patients.18 These patients present later in life, typically in late adolescence or early adulthood, with a milder clinical phenotype of hypertension and hypokalemia. While the urinary [THF þ allo-THF]/THE may only be mildly elevated in comparison to type I AME, the UFF/UFE ratio and cortisol half-life is markedly elevated in keeping with a defect in 11b-HSD2 activity.19,20 Therapeutic options The principal aim of treatment in type I and II AME is to control hypertension and correct life-threatening hypokalemia. Dexamethasone has been used therapeutically as it does not exhibit any mineralocorticoid activity by itself, yet effectively suppresses endogenous cortisol secretion. However, dose titration is important, as over-treatment may lead to iatrogenic Cushing’s syndrome. High doses of amiloride and triamterene, both inhibitors of the mineralocorticoid regulated epithelial sodium channel (ENaC) in the kidney (Figure 2), may be an alternative option or can be used in combination with dexamethasone. Furthermore, the MR antagonist spironolactone has been used successfully, although high concentrations are needed to efficiently block cortisol binding. However, other antihypertensive drugs may be required in order to achieve therapeutic targets. ‘Cure’ of AME was reported in one patient following kidney transplantation for end stage renal disease.21 Molecular basis At a molecular level the clinical and biochemical puzzle of AME was only resolved by the discovery and cloning of the type 2 11b-HSD isoenzyme.22 The HSD11B2 gene is

Cortisol metabolism in hypertension 343

6.2 kilo bases (kb) in length, located on chromosome 16q22 and contains 5 exons.23 At present, over 30 different mutations have been defined within the HSD11B2 gene in approximately 60 kindreds to cause type I and type II AME (Figure 4).11,24 In keeping with its autosomal recessive mode of inheritance most mutations causing AME are explained by consanguinity, endogamy or a founder effect and, hence, are found in a homozygote state.12,25e28 For example, three Zoroastrian kindreds from India and Iran are all homozygous for the same mutation (R337D3nt;DY338). Interestingly, six kindreds harbouring the L250P/L251P, R208C and E356,D1nt mutations are of Native American origin and it has been speculated as to whether a heterozygote state may warrant a selection advantage. Such individuals may have an increased ability to hold onto sodium under conditions of extreme salt deprivation.12 Not surprisingly, only a few patients with compound heterozygote mutations have been described so far, suggesting a low prevalence of AME mutations in the general population.13,29,30 Missense mutations have emerged as the most frequent genetic alteration to cause AME. Such mutations result in single amino acid exchanges and impair catalytic activity to variable degrees, whereas nonsense, frameshift and splice-site mutations typically introduce premature stop-codons leading to a truncated protein with virtually no enzymatic activity. Regression analysis has established a close correlation between the clinical/biochemical phenotype and underlying genotype: the greater the enzymatic impairment in vitro the lower the patient’s birth weight, the earlier the clinical presentation in life, the lower the serum potassium level and the higher the urinary [THF þ allo-THF]/THE ratio.31,32 Hence, AME should not be classified into the more severe type I or the milder type II form but rather be perceived as a disease with a spectrum of clinical severity that is determined by the degree of 11b-HSD2 impairment.24 Heterozygotes appear clinically normal, although experimental studies have suggested that heteromeric 11b-HSD2 formation may compromise overall 11b-HSD2

R74G P75, 1nt

1

D224N Y226N P227L Y232L, 9nt Y232C A237V F246,+1nt L250R L250P L251S C771G

L179R S180F R186C R208C R208H R213C A221V L114, 6nt

2

R279C N286, 1nt 299 G305, 11nt V322,+9nt A328V R337H, 3nt; Y338 R356, 1nt R359W R374X L376P

A-G nt1 C-T nt14

3

C1393T

4

5

Figure 4. Schematic depiction of the HSD11B2 gene encoding for 11b-hydroxysteroid dehydrogenase type 2 (11b-HSD2) and localisation of identified mutations giving rise to the syndrome of apparent mineralocorticoid excess (AME). Mutations identified in a compound heterozygote state are labelled with , =, -, 5, :, , . Squares represent exons, grey indicates coding region, white indicates 50 and 30 untranslated region (UTR).

344 F. Hammer and P. M. Stewart

activity. Indeed, in one kindred, both heterozygote parents had evidence of mineralocorticoid-based hypertension, while in a second kindred the father of an affected child was hypertensive and displayed an abnormal urinary [THF þ allo-THF]/THE ratio.32 INHIBITION OF 11b-HSD2 BY CARBENOXOLONE AND LIQUORICE For hundreds of years extracts of the liquorice plant (Glycyrrhiza glabra) have been used both as a sweetener in confectionery and as a herbal remedy. Isolation of the active component, glycyrrhetinic acid (GA) and the observation that liquorice promotes healing of peptic ulcers ultimately led to the development of carbenoxolone, an 18bhemisuccinate derivative of GA, which was used successfully in the treatment of both duodenal and gastric ulcerations. However, both liquorice and carbenoxolone induce undesirable mineralocorticoid-like side effects including oedema, hypertension and hypokalemia in up to 50% of patients.33 In the era of proton pump inhibitors (PPI), carbenoxolone is now rarely prescribed for the treatment of peptic ulcers but liquorice flavouring is still used in sweets in Europe and in chewing tobacco in the United States and sufficient quantities may lead to significant medical problems.34,35 Although GE has a very low affinity for the MR it is a very potent competitive inhibitor of 11bHSD2 with a Ki in the low nanomolar range.8 Consequently, administration of liquorice to healthy subjects leads to decreased levels of plasma cortisone, prolongation of plasma cortisol half-life and an increase in the urinary [THF þ allo-THF]/THE ratio in vivo.36,37 Beyond doubt, liquorice intoxication is now well accepted as the acquired counterpart to the inherited syndrome of AME. GLYCYRRHETINIC ACID-LIKE FACTORS Interestingly, endogenous compounds exhibiting similar features to glycyrrhetinic acid have been identified in partially purified urine extracts from normotensive men and non-pregnant women and have therefore been named glycyrrhetinic acid-like factors (GALFs).38 In pregnancy, especially during the second and third trimester, these compounds are elevated and, thus, it has been speculated that they may be progesteronederived metabolites. Indeed, several GALFs possess characteristics similar to those of neutral steroids and steroid glucuronides.39 However, attempts to correlate indices of MR activation or blood pressure levels with GALF excretion have been negative.40 Following extraction and chromatographic separation, GALFs selectively inhibiting 11b-HSD1 or 11b-HSD2 have been found. Nevertheless, the exact nature of these compounds and their biological significance remains nebulous. ECTOPIC ADRENOCORTICOTROPIC HORMONE SYNDROME Patients with Cushing’s syndrome secondary to ectopic adrenocorticotropic hormone (ACTH) secretion almost invariably (95e100%) develop a state of mineralocorticoid excess defined by hypertension and hypokalaemic alkalosis. In contrast to AME, these patients exhibit both high levels of plasma and urinary free cortisone, indicating that 11b-HSD2 function is intact but saturated by the high substrate levels of circulating cortisol resulting in cortisol spill over to illicitly act on the MR. Nevertheless, UFF/UFE and [THF þ allo-THF]/THE ratios are dramatically elevated, reflecting the ‘relative’ incapacity of 11b-HSD2 in the presence of high substrate levels. Although hypertension is

Cortisol metabolism in hypertension 345

a common feature in other causes of Cushing’s syndrome, UFF/UFE and [THF þ alloTHF]/THE ratios are only mildly elevated and the underlying pathophysiological mechanisms of hypertension in these patients remains poorly understood.41 The subject of Cushing’s syndrome and hypertension is discussed in greater detail in Chapter 9. ESSENTIAL HYPERTENSION Inactivating mutations in 11b-HSD2, as well as competitive inhibition of the enzyme by liquorice or carbenoxolone, lead to inherited and acquired hypertension in humans. Both forms are defined by a reduced activity of 11b-HSD2 resulting in a state of mineralocorticoid excess caused by insufficient protection of the MR in the kidney. Although patients with essential hypertension do not exhibit overt signs of mineralocorticoid excess, blood pressure levels have been positively correlated with plasma sodium and negatively correlated with plasma potassium levels, suggesting that adrenal corticosteroids may play a crucial role in its pathogenesis.42 Thus, the HSD11B2 gene represents a plausible candidate locus for more common forms of hypertension and a number of studies have addressed the question of whether mild deficiencies in 11b-HSD2 activity caused by frequent polymorphisms may play a contributing role in the pathogenesis of hypertension in the general population, particularly in the subgroup of salt-sensitive hypertensive patients with low-renin levels. In a small caseecontrol study from Scotland an increased half-life for 11-3Hcortisol was found in hypertensive subjects, suggesting reduced 11b-HSD2 activity, but no obvious differences in the urinary ratio of cortisol to cortisone metabolites were detected.43 By contrast, a study from Italy found an elevated urinary [THF þ allo-THF]/THE ratio in patients with essential hypertension44, but the high prevalence of partially inactivating mutations of 11b-HSD2 in that community might be a confounding factor.45 In a more selected hypertensive cohort, a higher ratio of urinary cortisol to cortisone metabolites was only found in young men who had a family history of hypertension, but not if parental blood pressure was normal.46 However, when looking at the urinary free cortisol to cortisone (UFF/UFE) ratio, which has been suggested to be a better indicator of renal 11b-HSD2 activity, a higher UFF/UFE ratio has been reported in unselected patients with essential hypertension compared to controls.47 More recently, genetic studies have been performed with a view to find potential molecular explanations for impaired 11b-HSD2 activity in essential hypertension. The first study reported an association with a microsatellite marker (D6S496) close to the HSD11B2 gene in African-Americans with end stage renal disease.48 In addition, another polymorphism in exon2 (Thr156/Thr(C468A)) of the HSD11B2 gene was found to be associated with high blood pressure47, whereas both association and linkage studies of a polymorphism in exon 3 (Glu178/Glu(G534A)) have turned out negative in essential hypertension compared to normotensive controls.47,49e51 However, given that the reduction of 11b-HSD2 activity in AME is a paradigm for a salt-sensitive form of hypertension, stratification of patients with essential hypertension according to their salt status might be an important factor to consider when investigating HSD11B2 loci. Indeed, a microsatellite of CA repeats in exon 1 of the HSD11B2 gene has been associated with salt sensitivity in both normotensive52 and hypertensive53,54 subjects, with shorter alleles being more common in salt-sensitive compared to salt-resistant subjects. In keeping with a higher prevalence of salt-sensitive,

346 F. Hammer and P. M. Stewart

low-renin hypertension in Blacks compared to Caucasians, the same marker was associated with hypertension in this cohort but no linkage of hypertension to this locus could be demonstrated.55 In summary these studies suggest that 11b-HSD2 plays a contributing role in the pathogenesis of essential hypertension. It appears that some of the identified polymorphisms are associated with essential hypertension per se, whereas others seem to correlate with sensitivity to salt irrespective of blood pressure levels. Undoubtedly, larger genetic and molecular studies are needed to better understand phenotypeegenotype correlations and their clinical significance in the pathogenesis of essential hypertension. GLUCOCORTICOIDS AND THE CARDIOVASCULAR SYSTEM Vascular tone In addition to their effects on fluid and salt homeostasis in the kidney, corticosteroids critically impact on vascular tone, an important determinant of blood pressure. This is exemplified in Addisonian crisis, where a complete lack of adrenal steroids results in vascular collapse aggravating the fluid depleted state and thereby contributing to life-threatening hypotension. In animal models, intravenous administration of hydrocortisone was shown to enhance vascular resistance in response to epinephrine, suggesting a permissive role of glucocorticoids for catecholamine action.56,57 In keeping with these findings, hydrocortisone administration in septic shock, an inflammatory state where relative adrenal insufficiency might be a contributing factor, reduced the time to cessation of vasopressor therapy in humans.58,59 Vascular tone is dynamically modulated by vascular smooth muscle cells (VSMCs) according to their state of contraction. In vitro, glucocorticoids have been shown to augment aortic contraction in response to catecholamines and angiotensin II by the upregulation of a1B-adrenergic and type I angiotensin II (AT1) receptors in VSMCs.60e62 Both GRs and MRs have been identified by means of ligand binding and expression studies in VSMCs.63 The action of glucocorticoids in VSMCs, however, is modulated by 11bHSDs at a pre-receptor level. Both 11b-HSD type I and II are expressed and functionally active in VSMCs.64,65 If the conversion of 11-dehydrocorticosterone to corticosterone is blocked by carbenoxolone or 11b-HSD1-antisense RNA, aortic rings demonstrate an attenuated contractile response to phenylephrine.66,67 Conversely, incubation with corticosterone and 11b-HSD2-antisense RNA compared to corticosterone alone augmented contractile response to phenylephrine.67 The role of the GR and MR in mediating these effects is unknown. In another study, angiotensin II binding was shown to be increased when VSMCs were incubated with 11b-HSD2-antisense RNA and cortisol compared to cortisol alone. Only combined GR and MR blockade reversed these effects completely, suggesting that both receptors are involved. Incubation of VSMCs with both corticosterone and 11-dehydrocorticosterone increased expression of the Naþ/Kþ-ATPase, an important membrane protein regulating intracellular sodium and pH in VSMCs, whereas carbenoxolone blunted the effects of corticosterone.68 More recently, vascular endothelial cells (ECs) have been recognised as another glucocorticoid target for modulating vascular tone. ECs release nitric oxide (NO), which is a potent vasodilator relaxing juxtaposed VSMCs. Glucocorticoids inhibit NO release by ECs through various mechanisms and thereby increase vascular

Cortisol metabolism in hypertension 347

tone.69 Like VSMCs, both 11b-HSD isoenzymes are expressed and functionally active in EC.69,70 In rats, inhibition of 11b-HSD2 by carbenoxolone results in hypertension and impaired endothelial-dependent aortic relaxation. Both effects could be reversed by concomitant administration of MR antagonists, indicating that glucocorticoidinduced endothelial dysfunction is mediated through the MR.71 These findings are supported by studies in 11b-HSD knockout mice. While 11b-HSD2 knockout mice exhibit endothelial dysfunction and enhanced norepinephrine-mediated aortic contraction compared to wild-type mice, 11b-HSD1 knockout mice possess a normal aortic contractory response.72 These studies suggest that pre-receptor modulation of glucocorticoids in both ECs and VSMCs plays an important role in the regulation of vascular tone. Heart The heart is a corticosteroid target organ expressing GR, MR and 11b-HSD1.73 With regard to blood pressure regulation no cardiac-specific glucocorticoid-mediated effects have been demonstrated but corticosteroids are critically involved in cardiac remodelling. Inappropriate MR activation by aldosterone causes cardiac hypertrophy and fibrosis. In keeping with these deleterious effects, conversely, MR blockade in patients with acute or chronic left ventricular failure drastically reduces hospitalisation and mortality.74,75 Interestingly, in contrast to classical aldosterone target tissues, 11bHSD2 is not expressed in the heart and therefore the MR is permanently occupied by cortisol and aldosterone accessibility to the MR remains a conundrum.76 However, it appears that in the heart, unlike the kidney, glucocorticoid occupancy of the MR antagonises rather than mimics the effects of aldosterone. This is strongly supported by a transgenic mouse model in which overexpression of 11b-HSD2 in cardiomyocytes results in cardiac hypertrophy, fibrosis and heart failure, presumably through augmented MR activation by aldosterone as a consequence of reduced occupancy by cortisol.77 The physiological role of 11b-HSD1 in the heart remains unclear but no obvious cardiac phenotype was reported in 11b-HSD1 knockout mice.78 BLOOD PRESSURE REGULATION BY GLUCOCORTICOID ACTION IN THE BRAIN The central nervous system is another potential site of blood pressure regulation by corticosteroids. Selective activation of MRs in the brain by intracerebroventricular (ICV) infusions of aldosterone increases salt appetite and central sympathetic nervous system drive to the periphery, thereby producing hypertension through multiple mechanisms.79 By contrast, ICV infusion of corticosterone alone does not alter blood pressure but reverses aldosterone-induced hypertension, suggesting an antagonistic effect of glucocorticoids.80 GR, MR and 11b-HSD1 are widely expressed in the brain, whereas 11b-HSD2 is expression is much more restricted.81 Thus the majority of MRs in the brain seem to be permanently occupied by glucocorticoids and only spatially protected from glucocorticoids by 11b-HSD2.82 However, cerebral 11b-HSD2 activity is important for blood pressure regulation, as inhibition of cerebral 11b-HSD2 activity by ICV infusion or paraventricular injection of carbenoxolone produces hypertension and increased sympathetic drive, which is blunted by co-administration of an MR antagonist.83,84 MR protection from corticosterone binding by 11b-HSD2 has recently been demonstrated by MR translocation studies in nucleus tractus solitarius neurons,

348 F. Hammer and P. M. Stewart

a brain region involved in salt appetite.85 In keeping with these findings, ICV infusion of 11b-HSD2 inhibitors increases salt appetite, presumably by glucocorticoidinduced MR activation.86 These studies show that the brain is critically involved in corticosteroid-mediated blood pressure regulation and suggest an important role for 11b-HSD2 in the regulation of salt appetite and sympathetic nervous activity. GLUCOCORTICOIDS AND INTRAUTERINE GROWTH RETARDATION One of the most striking observations concerning essential hypertension comes from epidemiological studies that showed blood pressure levels in adult life correlated inversely with birth weight at term.87,88 These studies suggest that a suboptimal intrauterine environment secondary to nutrient and oxygen supply limitations during critical periods of foetal life may lead to permanent alterations or ‘programming’ of the developing organism which, later in life, increases the risk of developing high blood pressure.89 Animal studies suggest that foetal glucocorticoid excess leads to both intrauterine growth retardation and high blood pressure levels in later life.90 During normal pregnancy, the foetus is protected from the much higher maternal glucocorticoid levels by placental 11b-HSD2, which efficiently inactivates cortisol to cortisone in humans and corticosterone to 11-dehydrocorticosterone in rodents. In keeping with these findings, treatment of pregnant rats with the 11b-HSD2 inhibitor carbenoxolone, reduces birth weight and results in high blood pressure of the adult offspring.91e93 A common mechanism may underlie foetal programming through maternal undernutrition and glucocorticoid exposure. Dietary protein restriction during rat pregnancy selectively attenuates 11b-HSD2, but not other placental enzymes.94 In humans, 11b-HSD2 deficiency in AME results in severe growth retardation of the foetus and very low birth weight of the newborn, presumably by trespassing of maternal cortisol onto the foetus. In pregnancies complicated by intrauterine growth restriction, activity and expression levels of 11b-HSD2 in the placenta correlate with birth weight.95,96 These studies suggest that attenuated 11b-HSD2 activity in the placenta results in increased foetal exposure to glucocorticoids and higher blood pressure in adulthood. The principal underlying mechanisms leading to foetal ‘programming’ of high blood pressure remain unknown but animal studies have revealed permanent changes in adult organs, such as brain, kidney, heart and vasculature, following foetal glucocorticoid excess. These effects comprise increased hypothalamoepituitaryeadrenal drive, alterations of GR, angiotensinogen and AT1-receptor expression in the hippocampus, reduced nephron numbers and attenuation of vascular responses to vasoconstrictors.90,97

Practice points
In low-renin low-aldosterone hypertension, consider congenital or acquired 11b-hydroxysteroid dehydrogenase 2 (11b-HSD2) deficiency
Consider treatment with spironolactone in patients with low-renin essential hypertension or hypertension resistant to treatment

Cortisol metabolism in hypertension 349

Research agenda
To establish the significance of HSD11B2 gene polymorphisms in the pathogenesis of salt-sensitive essential hypertension
To isolate and determine glycyrrhetinic acid-like factors (GALFs) and their role in essential and pregnancy-induced hypertension
To establish glucocorticoid metabolism in the vasculature and brain in essential hypertension

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