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Peptide Hormones as Tumor Markers in Clinical Practice
CHAPTER THREE
Peptide Hormones as Tumor Markers in Clinical Practice Qian Sun, Zhen Zhao1 National Institutes of Health, Bethesda, MD, United States 1 Corresponding author: e-mail address: [email protected]
Contents 1. Introduction 2. Selected Peptide Hormones as Tumor Markers 2.1 Calcitonin 2.2 Alpha- and Beta-hCG 2.3 Insulin and C-Peptide 2.4 PTHrp 2.5 Gastrin 2.6 VIP 2.7 Glucagon 2.8 ACTH 3. Conclusion Acknowledgement References
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Abstract Peptide hormones represent a major class of hormones that are made from amino acids by specialized endocrine glands. The maturation of bioactive hormones take place in the rough endoplasmic reticulum and Golgi apparatus, where preprohormones are proteolytically cleaved into prohormones, and subsequently into mature peptide hormones. Once the bioactive hormones are released into the circulation, they interact with receptors located on the plasma membrane of target cells, and initiate intracellular signaling pathways to regulate physiological processes including energy metabolism, growth, stress, and reproduction. However, excessive amount of circulating peptide hormones often associates with the presence of tumors. Section 2 discusses 10 peptide hormones as tumor markers and their clinical application in aiding the diagnosis of tumors as well as monitoring the disease process.
The Enzymes, Volume 42 ISSN 1874-6047 https://doi.org/10.1016/bs.enz.2017.09.001
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2017 Elsevier Inc. All rights reserved.
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1. INTRODUCTION Peptide hormones represent a major class of hormones that are made from amino acids by specialized endocrine glands [1]. A peptide hormone is first synthesized as part of a large preprohormone in the rough endoplasmic reticulum (ER) (Fig. 1). Shortly after synthesis, the cleavage of the N-terminal signal sequence of the preprohormone results in prohormone [2]. To release the bioactive hormone, the prohormone matures during the intracellular transport from the rough ER to Golgi apparatus and finally to secretory granules [3]. During this process, enzymes play a number of important roles in the modification of preprohormones and prohormones. As mentioned earlier, the first enzymatic cleavage occurs in the rough ER through removing the N-terminal hydrophobic signal sequence to generate prohormones.
Fig. 1 Synthesis, maturation and release of peptide hormones. A peptide hormone is first synthesized as part of a large preprohormone in the rough endoplasmic reticulum. Preprohormone then matures into prohormone through the cleavage of the N-terminal signal sequence, and further into the bioactive hormone as a result of enzymatic processing. The maturation process takes place during the intracellular transport from the rough ER to Golgi apparatus and finally to secretory granules. After the release of mature peptide hormone into circulation from tumor cells, it interacts with receptors located on the plasma membrane of target cells to trigger a cascade of intracellular signaling.
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At this point, the prohormones still contain domains that are necessary for folding, stability, and sorting of the hormone molecules [4]. The subsequent cleavage by endopeptidases and exopeptidases removes these domains in the rough ER and Golgi apparatus, most often at sites characterized by the presence of consecutive basic residues (Arg-Arg, Arg-Lys, Lys-Arg, or Lys-Lys). [3–5]. In addition to proteolysis, enzymes are also involved in the maturation process of amino acid derivatizations including acetylation, glycosylation, methylation, sulfation, and others [3]. Once the mature peptide hormone is released into the circulation, it interacts with receptors located on the plasma membrane of cells, which are also called transmembrane receptors [1,2]. Two major types of transmembrane hormone receptors are the guanine nucleotide-binding regulatory protein (G protein)-coupled receptors and the enzyme-linked receptors. The activation of these receptors triggers a cascade of intracellular signaling through secondary messengers, such as cyclic AMP (cAMP), cyclic GMP (cGMP), and inositol phospholipids [6].
2. SELECTED PEPTIDE HORMONES AS TUMOR MARKERS Peptide hormones have essential roles in regulating physiological processes including energy metabolism, growth, stress, and reproduction [1]. However, excessive amount of circulating peptide hormones often associates with the presence of tumors. A number of peptide hormones are listed here as important tumor markers (Table 1).
2.1 Calcitonin Calcitonin is a linear 32 amino acid peptide that is released from the parafollicular or C cells of the thyroid gland in response to increased circulating calcium [7]. The physiologic role of endogenous calcitonin is not entirely clear, but it has been suggested to regulate calcium homeostasis [8]. Clinically, serum calcitonin is an important cytologic marker in patients with medullary thyroid carcinoma, a malignant tumor of the parafollicular C cells [8]. 2.1.1 Biosynthesis From Prohormone Calcitonin synthesis begins with the proteolytic cleavage of preprocalcitonin by endopeptidase to procalcitonin, which is a 116 amino acid precursor of calcitonin [23]. The procalcitonin is then cleaved at its N- and C-terminal ends to generate the bioactive peptide calcitonin [23]. The mature calcitonin is released from C cells into the circulation after the formation of its
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Table 1 Selected Peptide Hormones as Tumor Markers Peptide Size of Active Hormone Hormone Role as Tumor Marker
References
Calcitonin 32 Amino acids
Monitoring medullary thyroid cancer
[9]
AlphahCG
Aid the diagnosis of pituitary tumors
[10]
92 Amino acids
Beta-hCG 145 Amino acids
[11–13] Serum-Aid the diagnosis of choriocarcinoma; Monitoring testicular germ cell tumors CSF-Aid in the diagnosis of intracerebral germ cell tumors
PTHrp
139, 141, 173 Monitoring hypercalcemia of malignancy [7,14] Amino acids
Gastrin
17, 34 Amino Diagnosis of gastrinoma acids
[15,16]
VIP
28 Amino acids
Diagnosis of VIP-producing tumors (VIPomas)
[15,17]
Insulin
51 Amino acids
Diagnosis of insulinoma
[18]
Glucagon 29 Amino acids
Diagnosis of glucagonoma
[19]
ACTH
Monitoring pituitary ACTH-producing tumors (Cushing’s disease)
[1,20,21]
Aid the diagnosis of insulinoma
[22]
39 Amino acids
C-peptide 31 Amino acids
ACTH: adrenocorticotropic hormone; CSF: cerebrospinal fluid; GRP: gastrin-releasing peptide; hCG: human chorionic gonadotropin; PP: pancreatic polypeptide; PTHrp: parathyroid hormone-related peptide; VIP: vasoactive intestinal polypeptide.
secondary and tertiary structures, including an N-terminal disulfide bond and a C-terminal proline-amide [7]. Both of these structures are essential for the interaction of the peptide hormone with its receptor, a specific G protein-coupled receptor found on osteoclasts [7,23]. 2.1.2 Clinical Significance as Tumor Marker The increased production of calcitonin is a characteristic feature of medullary thyroid cancer, which accounts for 1%–2% of thyroid malignancies in
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the United States [24]. It has been suggested that serum calcitonin concentrations correlate well with tumor size in patients with medullary thyroid cancer, so clinically it serves as a tumor marker to monitor patients’ treatment response [9]. However, serum calcitonin is not recommended for use as a diagnostic tool for medullary thyroid cancer because increased calcitonin concentrations also associate with other conditions, including acute and chronic renal failure, hypercalcemia, pulmonary disease, and other malignancies [7,25]. Of note, although being the precursor of calcitonin, procalcitonin has very different clinical applications. Rather than a tumor marker, this 116 amino acid peptide is useful for diagnosing severe systemic inflammation and guiding antibiotic therapy [26,27]. It was suggested that the procalcitonin released during inflammation was produced by neuroendocrine cells in the lungs and intestines and does not undergo enzymatic cleavage as in C cells [23,28].
2.2 Alpha- and Beta-hCG Human chorionic gonadotropin (hCG) is one of the most important hormones during pregnancy. It promotes the production of progesterone, which protects the growing fetus by preventing menstrual bleeding and maintaining uterine lining [29]. hCG is mainly produced by trophoblast cells of the placenta, but a minute amount is also synthesized in the pituitary of nonpregnant women and men [30]. Clinically, laboratory tests for serum hCG are not only useful for pregnancy, but also serve as markers for monitoring choriocarcinoma, testicular tumors, and ovarian germ cell tumors (GCTs) [11]. 2.2.1 Processing of hCG hCG is a heterodimer composed of a 145 amino acid beta(β)-subunit and a 92 amino acid alpha(α)-subunit [30]. The hCGα subunit is encoded by a gene located on chromosome 6, the same gene that also encodes the α subunit of three other pituitary hormones, namely, luteinizing hormone (LH), follicle-stimulating hormone (FSH), and thyroid-stimulating hormone (TSH) [30]. The β subunit, on the other hand, is unique to hCG and encoded by a separate gene on chromosome 16 [31]. The two subunits associate with each other in the rough ER to form the mature hCG, and when they disassociate following enzymatic nicking, the hormone activity is lost [30,32].
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2.2.2 Clinical Applications as Tumor Markers Alpha subunit is identical in four glycoprotein hormones, including hCG, LH, FSH, and TSH. The increased α subunit level is most commonly associated with thyrotroph- or gonadotroph-derived pituitary tumors [33]. It aids the diagnosis of TSH-secreting pituitary adenomas, as increased concentrations are seen in 50%–85% of patients [10]. The clinical utility of α subunit in diagnosing gonadotroph adenomas, on the other hand, is limited because only 35% of those patients secrete enough intact α subunit to raise its serum levels [34]. Laboratory testings for serum hCG and β-hCG are widely applied in the screening and monitoring of patients with molar pregnancy who are at risk of developing choriocarcinoma (gestational trophoblastic neoplasia), with specificity and sensitivity of 99% [11]. In addition, β-hCG is an important tumor marker for the management of patients with testicular GCTs, along with two other markers alpha fetoprotein and lactate dehydrogenase [11,12]. Measurement of hCG before treatment for tumors is also essential for prognosis and monitoring of patients’ response to therapy [12]. Intracranial GCTs commonly occurs in children under the age of 15 years and they represent around 3% of primary pediatric central nervous systems tumors [13]. Based on the histologic components, intracranial GCTs are divided into germinomas and nongerminomatous GCTs (NGGCTs) by the World Health Organization (WHO) [35]. Cerebrospinal fluid (CSF) measurement of β-hCG is an essential tumor marker to help distinguish between germinomas and NGGCTs [13]. This distinction is important because NGGCTs carry a much poorer prognosis compared with pure germinomas [36].
2.3 Insulin and C-Peptide Insulin is a 51 amino acid peptide hormone produced by β-cells of pancreatic islets. It regulates the metabolism of carbohydrates, fats, and proteins by promoting the uptake of glucose from the blood into fat and skeletal muscle cells, simulating synthesis of glycogen and inhibiting glucose production by the liver [37]. In addition to managing patients with diabetes mellitus, serum insulin testing is useful for diagnosing insulinoma, in conjunction with proinsulin and connecting peptide (C-peptide) measurements. 2.3.1 Biosynthesis From Prohormone Insulin is first synthesized as a polypeptide preproinsulin in the rough ER of the pancreatic β cells [37]. After the signal peptide in preprohormone gets
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removed by cleaving enzymes, an 86 amino acid peptide proinsulin is formed. The proinsulin then is transported to the Golgi complex, where it matures into active insulin and a 31 amino acid C-peptide [22]. This maturation process is catalyzed by two endopeptidases, namely, prohormone convertases 1 and 2 (PC1 and PC2), as well as a third enzyme carboxypeptidase-H [22,37]. Although equimolar amounts of insulin and C-peptide are secreted into the circulation, C-peptide concentration in serum is generally higher because of its longer half-life [37]. 2.3.2 Clinical Significance as Tumor Marker Insulinoma is a rare form of neuroendocrine tumor that occurs in 1–4 people per million persons every year [38]. The disease is characterized by high levels of insulin secretion from beta cells of the pancreas in the presence of hypoglycemia [39]. Measurement of plasma glucose, insulin, C-peptide, and proinsulin during a prolonged 72-h fasting test can detect up to 99% of insulinomas, and therefore is the gold standard for biochemical diagnosis of the disease [18,38].
2.4 PTHrp Parathyroid hormone-related protein (PTHrp) is a 139 to 173 amino acid peptide hormone [7]. Although it was initially isolated from patients with humoral hypercalcemia of malignancy (HHM), later it was found to be produced by virtually all tissues in low concentrations [40]. As an autocrine and paracrine hormone, PTHrp functions to regulate bone resorption, smooth muscle relaxation, and lactation [7,40]. Clinically, PTHrp testing is often used as a tumor marker in prognosis, selection of therapy, and monitoring of patients with HHM. 2.4.1 Biosynthesis From Prohormone PTHrp is a monomeric peptide that exists in several isoforms, ranging from 139 to 173 amino acids in size, which are created by differential messenger RNA splicing and posttranslational processing [7]. PTHrp originates from its preprohormone, which is composed by an N-terminal signal peptide and a prohormone. After the signal peptide is removed, pro-PTHrp undergoes further intracellular conversion to mature peptides by a number of endoproteases including prohormone convertase furin [41,42]. The three peptide isoforms differ at the C-terminus, but their N-terminal region is identical, which also shows close homology to parathyroid hormone (PTH) [41].
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2.4.2 Clinical Significance as Tumor Marker Hypercalcemia occurs in 20%–30% of patients with solid tumors and hematologic malignancies [43]. Among these patients 75%–80% suffer from HHM, which is caused by secretion of peptide hormone PTHrp from the tumors [14]. PTHrp causes hypercalcemia in these patients by uncoupling bone resorption and formation, which leads to a large flux of calcium from bone into the circulation; additionally, it decreases ability of the kidney to clear calcium [44,45]. HHM is most often observed in patients with advanced squamous (lung, head, and neck), renal, bladder, breast, and ovarian carcinomas [7]. In these patients who also developed a recent onset of hypercalcemia, an elevated serum concentration of PTHrp is useful to confirm the diagnosis of HHM [14]. In addition, PTHrp is often used as a marker to assess the prognosis of patients, as well as their response to treatment [7].
2.5 Gastrin Gastrin is a polypeptide hormone produced by G cells of the gastric antrum in response to antral distention from food and the presence of partially digested protein [15,46]. Once released, it stimulates the secretion of gastric acid by parietal cells of the stomach to increase gastric motility [15]. The secretion of gastrin is greatly suppressed at low gastric pH except in gastrinoma patients [16]. Therefore, its level is used for the diagnosis of gastrinoma in the presence of gastric acid hypersecretion. 2.5.1 Biosynthesis From Prohormone The peptide hormone gastrin originates from the cleavage of a 101 amino acid preprogastrin [15]. After removal of the signal peptide from the preprohormone, progastrin is generated, which undergoes further proteolysis by trypsin-like endopeptidases and carboxypeptidase B-like enzymes [47]. The major form of mature gastrin in the circulation is gastrin-34, and it can be further cleaved into a shorter peptide gastrin-17 by trypsin [15]. 2.5.2 Clinical Significance as Tumor Marker Gastrinomas are neuroendocrine tumors located in the pancreas or duodenum that secrete excessive gastrin [48]. As a result of high circulating gastrin concentrations, gastric acid secretion is persistently stimulated which eventually causes peptic ulcers [49]. The presence of gastrinoma, hypergastrinemia, and severe ulcer disease is referred to as the Zollinger–Ellison syndrome (ZES), which was first described in 1955 by Zollinger and Ellison [50].
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ZES or gastrinoma is diagnosed if a serum gastrin value is greater than 10 times the upper limit of the reference range in the presence of a gastric pH below 2 [15,16].
2.6 VIP Vasoactive intestinal peptide (VIP) is a 28 amino acid polypeptide that can serve both as a neurotransmitter and hormone [51]. As a neurotransmitter, it causes vasodilation and relaxation of the circulatory system and the gut; as a peptide hormone, it functions to stimulate glycogenolysis, and inhibit gastric acid secretion [15]. VIP is produced in many tissues including the nervous system and the gut, but its circulating concentration remains low due to the short half-life, which is only around 1 min [15,52]. However, in patients with VIP-producing tumors (VIPomas), its level can be significantly increased [53]. 2.6.1 Biosynthesis From Prohormone VIP is derived from a 170 amino acid precursor prepro-VIP encoded by a gene located on chromosome 6 [51]. Enzymatic cleavage of the precursor generates three biologically active peptides, namely, VIP, peptide histidine isoleucine (PHI), and peptide histidine valine (PHV) [54]. Although these peptides share high levels of structural similarities, PHI/PHV and VIP can activate separate sets of receptors, suggesting they could have different physiological functions [54]. 2.6.2 Clinical Significance as Tumor Marker VIPomas are rare pancreatic neuroendocrine tumors that are detected in 1 in 10 million people per year [55]. The tumors are characterized by uncontrolled secretion of VIP, which is a strong muscle relaxant and causes watery diarrhea [15]. In patients with chronic diarrheal diseases, a persistently elevated serum VIP concentration with fasting is virtually the diagnosis for VIPomas [17].
2.7 Glucagon Glucagon is a polypeptide hormone of 29 amino acids produced by α cells of the pancreas [37]. The peptide hormone is secreted in response to hypoglycemia, stress, and exercise, with resultant increases in blood glucose concentration by stimulating gluconeogenesis and glycogenolysis of the liver [37,56]. Glucagon secretion is inhibited by high plasma glucose and insulin, a peptide hormone that decreases glucagon gene expression [57].
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Therefore in type I diabetes patients, elevated glucagon concentrations due to insulin deficiency are often observed [37]. In addition, excessive production of glucagon from pancreatic islets is the hallmark of glucagonoma, so serum glucagon is a useful tumor marker to diagnose this tumor [19]. 2.7.1 Biosynthesis From Prohormone Glucagon is derived from proglucagon, a polypeptide that contains not only glucagon but also two other bioactive peptides glucagon-like peptide-1 and -2 (GLP-1 and GLP-2) [58]. Proglucagon undergoes posttranslational processing in a tissue-specific manner, resulting in the release of different peptide hormones. Enzyme PC2 is responsible for the proteolytic processing of proglucagon into glucagon in pancreatic alpha cells, whereas in the enteroendocrine L cells, GLP-1 and GLP-2 are released upon the cleavage of proglucagon by PC1/3 [37,59]. Two major target organs for glucagon are the liver and the kidney [60]. Once glucagon binds to its receptor in these organs, which is a specific G protein-coupled receptor, a cascade of intracellular signaling is triggered to activate enzymes in gluconeogenesis and glycogenolysis pathways [60]. 2.7.2 Clinical Significance as Tumor Marker Glucagonomas are rare pancreatic neuroendocrine tumors that produce glucagon [61]. The patients’ clinical features include weight loss, glycose intolerance, and necrolytic migratory erythema, which is a skin rash involving the face, perineum, and extremities [62]. The diagnosis of glucagonoma is established with a fasting plasma glucagon level greater than 10 times the upper limit of the reference range (>1000 pg/mL) [19]. However, it has been suggested that up to 70% of the glucagon detected by immunoassays in these patients are biologically inactive [63].
2.8 ACTH Adrenocorticotropic hormone (ACTH, also called corticotropin) is a 39 amino acid peptide hormone produced by the anterior pituitary gland [1]. In response to stress and changes in circadian rhythm, the release of corticotropin-releasing hormone and arginine vasopressin from hypothalamus drives the secretion of ACTH, which in turn stimulates the production of three major hormones from adrenal gland, namely, aldosterone, cortisol, and dehydroepiandrosterone [20]. When excessive cortisol production is caused by a pituitary ACTH-secreting tumor, the condition is called Cushing’s disease. Since a variety of conditions can cause cortisol overproduction,
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ACTH testings are useful in determining the location of the abnormality and cause of hypercortisolism [1,64]. 2.8.1 Biosynthesis From Prohormone ACTH is derived from its 266 amino acid precursor proopiomelanocortin (POMC) [65]. Proteolytic cleavage of pro-POMC by endopeptidase PC1/3 first generates pro-ACTH, which is then further processed to mature ACTH by PC1/3 [20]. Once ACTH is released into the circulation, it binds to G protein-coupled melanocortin-2 receptor on adrenocortical cells to stimulate the growth of adrenocortical cells and synthesis of cortisol [66]. 2.8.2 Clinical Significance as Tumor Marker As mentioned earlier, Cushing’s disease is a benign ACTH-producing pituitary adenoma that produces excessive amount of cortisol. The disease is responsible for 70% of endogenous Cushing’s syndrome, which is characterized by central obesity with thin arms and legs, rounded face, high blood pressure, severe fatigue, weekend bones and muscles [67]. In addition to Cushing’s disease, a number of conditions can cause Cushing’s syndrome, some of which include use of glucocorticoid medication, adrenal gland tumors, and nonpituitary ACTH-producing tumors (also called ectopic ACTH syndrome) [1,68]. For this reason, ACTH testing is essential in determining the cause of Cushing’s syndrome as ACTH-dependent or independent (a common source is adrenal tumor) [20]. Moreover, ACTH levels greater than four times the upper limit of reference range are suggestive of an ectopic source, such as malignant tumors [1,21]. With the recent advancement of immunoassays, an intraoperative measurement of ACTH during removal of ACTH-secreting tumors is emerging to be a useful tool in guiding the extent of surgical resection [69].
3. CONCLUSION With the increase in specificity and sensitivity of immunoassays, laboratory testing for peptide hormones in body fluids has become available. When patients presented symptoms that lead to the suspicion of peptidesecreting tumors, determination of peptide concentrations is useful in aiding the diagnosis, later monitoring patients’ response to treatment, and assessing their prognosis. However, clinical application of these peptide hormones as tumor markers should be done with caution because most peptides are not specific to one single tumor, and most tumors produce multiple peptides.
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ACKNOWLEDGEMENT We thank Dr. Bo Ning at the Arizona State University in Arizona for his valuable assistance with the figure.
REFERENCES [1] G.D. Clines, Laurence, General endocrinology, in: L.A. Kaplan, A.J. Pesce, S.C. Kazmierczak (Eds.), Clinical Chemistry: Theory, Analysis, Correlation, Mosby, St. Louis., 2003. [2] S. Nussey, S. Whitehead, Principles of endocrinology, in: Endocrinology: An Integrated Approach, BIOS Scientific Publishers, Ltd, Oxford, England, 2001. [3] J.F. Rehfeld, et al., Peptide-hormone expression and precursor processing, Acta Oncol. 28 (3) (1989) 315–318. [4] J.W.M. Creemers, R.S. Jackson, J.C. Hutton, Molecular and cellular regulation of prohormone processing, Semin. Cell Dev. Biol. 9 (1) (1998) 3–10. [5] J. Martinez, P. Potier, Peptide-hormones as prohormones, Trends Pharmacol. Sci. 7 (4) (1986) 139–147. [6] L.M. Luttrell, Transmembrane signaling by G protein-coupled receptors, Methods Mol. Biol. 332 (2006) 3–49. [7] W. Fraser, Bone and mineral metabolism, in: C.A. Burtis et al., (Eds.), Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, Elsevier, St. Louis, MO, 2016, p. xviii. 2238 p. [8] N. Baregamian, C. Solo´rzano, The physiology of serum calcitonin and carcinoembryonic antigen in medullary thyroid cancer, in: T.S. Wang, D.B. Evans (Eds.), Medullary Thyroid Cancer, Springer, Cham, Switzerland, 2016, p. XI. 205 p. 40 illus., 28 illus. in color. [9] R. Cohen, et al., Preoperative calcitonin levels are predictive of tumor size and postoperative calcitonin normalization in medullary thyroid carcinoma, J. Clin. Endocrinol. Metab. 85 (2) (2000) 919–922. [10] H.V. Socin, et al., The changing spectrum of TSH-secreting pituitary adenomas: diagnosis and management in 43 patients, Eur. J. Endocrinol. 148 (4) (2003) 433–442. [11] C. Sturgenon, Tumor markers, in: C.A. Burtis et al., (Eds.), Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, Elsevier, St. Louis, MO, 2016, p. xviii. 2238 p. [12] T.D. Gilligan, et al., American Society of Clinical Oncology clinical practice guideline on uses of serum tumor markers in adult males with germ cell tumors, J. Clin. Oncol. 28 (20) (2010) 3388–3404. [13] M.E. Echevarria, J. Fangusaro, S. Goldman, Pediatric central nervous system germ cell tumors: a review, Oncologist 13 (6) (2008) 690–699. [14] W.A. Ratcliffe, et al., Role of assays for parathyroid-hormone-related protein in investigation of hypercalcaemia, Lancet 339 (8786) (1992) 164–167. [15] R.A. Sherwood, N.E. Walsham, I. Bjarnason, Gastric, pancreatic, and intestinal function, in: C.A. Burtis et al., (Eds.), Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, Elsevier, St. Louis, MO, 2016, p. xviii. 2238 p. [16] M.J. Berna, et al., Serum gastrin in Zollinger-Ellison syndrome—I. Prospective study of fasting serum gastrin in 309 patients from National Institutes of Health and comparison with 2229 cases from the literature, Medicine 85 (6) (2006) 295–330. [17] R. Yu, Chapter 150—Neuroendocrine tumor syndromes, in: J.L. Jameson, L.J. DeGroot (Eds.), Endocrinology: Adult and Pediatric, seventh ed., Elsevier Saunders, Philadelphia, 2016, pp. 2606–2614.e4. [18] F.J. Service, N. Natt, The prolonged fast, J. Clin. Endocrinol. Metab. 85 (11) (2000) 3973–3974.
Peptide Hormones as Tumor Markers in Clinical Practice
77
[19] R.A. Wermers, et al., The glucagonoma syndrome. Clinical and pathologic features in 21 patients, Medicine (Baltimore) 75 (2) (1996) 53–63. [20] R.L. Bertholf, M. Cooper, W.E. Winter, Adrenal cortex, in: C.A. Burtis et al., (Eds.), Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, Elsevier, St. Louis, MO, 2016, p. xviii. 2238 p. [21] T.A. Howlett, et al., Diagnosis and management of ACTH-dependent Cushing’s syndrome: comparison of the features in ectopic and pituitary ACTH production, Clin. Endocrinol. (Oxf ) 24 (6) (1986) 699–713. [22] C.J. Rhodes, S. Shoelson, P.A. Halban, Insulin biosynthesis, processing, and chemistry, in: E.P. Joslin, C.R. Kahn (Eds.), Joslin’s Diabetes Mellitus, Lippincott Williams & Willkins, Philadelphia, PA, 2005, p. xi. 1209 p. [23] P. Maruna, K. Nedelnikova, R. Gurlich, Physiology and genetics of procalcitonin, Physiol. Res. 49 (2000) S57–S61. [24] S.A. Wells, et al., Revised american thyroid association guidelines for the management of medullary thyroid carcinoma, Thyroid 25 (6) (2015) 567–610. [25] G. Costante, S. Filetti, Early diagnosis of medullary thyroid carcinoma: is systematic calcitonin screening appropriate in patients with nodular thyroid disease? Oncologist 16 (1) (2011) 49–52. [26] C. Wacker, et al., Procalcitonin as a diagnostic marker for sepsis: a systematic review and meta-analysis, Lancet Infect. Dis. 13 (5) (2013) 426–435. [27] P. Schuetz, W. Albrich, B. Mueller, Procalcitonin for diagnosis of infection and guide to antibiotic decisions: past, present and future, BMC Med. 9 (2011) 107. [28] S. Russwurm, et al., Molecular aspects and natural source of procalcitonin, Clin. Chem. Lab. Med. 37 (8) (1999) 789–797. [29] L.A. Cole, Biological functions of hCG and hCG-related molecules, Reprod. Biol. Endocrinol. 8 (2010) 102. [30] M.L. Yarbrough, M. Stout, A.M. Gronowski, Pregnancy and its disorders, in: C.A. Burtis et al., (Eds.), Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, Elsevier, St. Louis, MO, 2016, p. xviii. 2238 p. [31] P. Policastro, et al., The Beta-subunit of human chorionic-gonadotropin is encoded by multiple genes, J. Biol. Chem. 258 (19) (1983) 1492–1499. [32] L.A. Cole, et al., The deactivation of Hcg by nicking and dissociation, J. Clin. Endocrinol. Metab. 76 (3) (1993) 704–710. [33] P. BeckPeccoz, et al., Thyrotropin-secreting pituitary tumors, Endocr. Rev. 17 (6) (1996) 610–638. [34] D.M. Ho, et al., The clinicopathological characteristics of gonadotroph cell adenoma: a study of 118 cases, Hum. Pathol. 28 (8) (1997) 905–911. [35] D.N. Louis, et al., The 2007 WHO classification of tumours of the central nervous system, Acta Neuropathol. 114 (2) (2007) 97–109. [36] M. Matsutani, et al., Primary intracranial germ cell tumors: a clinical analysis of 153 histologically verified cases, J. Neurosurg. 86 (3) (1997) 446–455. [37] D. Sacks, Diabetes mellitus, in: C.A. Burtis et al., (Eds.), Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, Elsevier, St. Louis, MO, 2016, p. xviii. 2238 p. [38] T. Okabayashi, et al., Diagnosis and management of insulinoma, World J. Gastroenterol. 19 (6) (2013) 829–837. [39] A.H. Minn, et al., Insulinomas and expression of an insulin splice variant, Lancet 363 (9406) (2004) 363–367. [40] W.J. Burtis, Parathyroid hormone-related protein—structure, function, and measurement, Clin. Chem. 38 (11) (1992) 2171–2183. [41] B. Liu, D. Goltzman, S.A. Rabbani, Processing of pro-PTHrp by the prohormone convertase, Furin—effect on biological-activity, Am. J. Phys. Endocrinol. Metab. 268 (5) (1995) E832–E838.
78
Qian Sun and Zhen Zhao
[42] T. Nakajima, et al., Prohormone convertase furin has a role in gastric cancer cell proliferation with parathyroid hormone-related peptide in a reciprocal manner, Dig. Dis. Sci. 47 (12) (2002) 2729–2737. [43] A.F. Stewart, p. Clinical, Hypercalcemia associated with cancer, N. Engl. J. Med. 352 (4) (2005) 373–379. [44] R. Rizzoli, et al., Actions of parathyroid hormone and parathyroid hormone-related protein, J. Endocrinol. Invest. 15 (9 Suppl. 6) (1992) 51–56. [45] M.A. Syed, et al., Parathyroid hormone-related protein-(1–36) stimulates renal tubular calcium reabsorption in normal human volunteers: implications for the pathogenesis of humoral hypercalcemia of malignancy, J. Clin. Endocrinol. Metab. 86 (4) (2001) 1525–1531. [46] G.J. Dockray, et al., The gastrins: their production and biological activities, Annu. Rev. Physiol. 63 (2001) 119–139. [47] S. Jensen, et al., Progastrin processing during antral G-cell hypersecretion in humans, Gastroenterology 96 (4) (1989) 1063–1070. [48] J.A. Norton, Neuroendocrine tumors of the pancreas and duodenum, Curr. Probl. Surg. 31 (2) (1994) 82–156. [49] E.C. Ellison, J.A. Johnson, The Zollinger-Ellison syndrome: a comprehensive review of historical, scientific, and clinical considerations, Curr. Probl. Surg. 46 (1) (2009) 13–106. [50] R.M. Zollinger, E.H. Ellison, Primary peptic ulcerations of the jejunum associated with islet cell tumors of the pancreas, Ann. Surg. 142 (4) (1955) 709–728. [51] J. Fahrenkrug, VIP and PACAP, in: J.F. Rehfeld, J.R. Bundgaard, SpringerLink (Online service) (Eds.), Cellular Peptide Hormone Synthesis and Secretory Pathways, Springer-Verlag Berlin Heidelberg, Berlin, Heidelberg, 2010. [52] T. Juhasz, et al., PACAP and VIP signaling in chondrogenesis and osteogenesis, Peptides 66 (2015) 51–57. [53] A.A. Ghaferi, et al., Pancreatic VIPomas: subject review and one institutional experience, J. Gastrointest. Surg. 12 (2) (2008) 382–393. [54] D.L. Tse, et al., Identification of a potential receptor for both peptide histidine isoleucine and peptide histidine valine, Endocrinology 143 (4) (2002) 1327–1336. [55] S.L. Smith, et al., Vasoactive intestinal polypeptide secreting islet cell tumors: a 15-year experience and review of the literature, Surgery 124 (6) (1998) 1050–1055. [56] G. Jiang, B.B. Zhang, Glucagon and regulation of glucose metabolism, Am. J. Physiol. Endocrinol. Metab. 284 (4) (2003) E671–8. [57] P.E. Cryer, Hypoglycemia risk reduction in type 1 diabetes, Exp. Clin. Endocrinol. Diabetes 109 (Suppl. 2) (2001) S412–23. [58] Y. Rouille, et al., Proglucagon is processed to glucagon by prohormone convertase Pc2 in alpha-Tc1-6 cells, Proc. Natl. Acad. Sci. U. S. A. 91 (8) (1994) 3242–3246. [59] N.M. Whalley, et al., Processing of proglucagon to GLP-1 in pancreatic alpha-cells: is this a paracrine mechanism enabling GLP-1 to act on beta-cells? J. Endocrinol. 211 (1) (2011) 99–106. [60] D.A. Sandoval, D.A. D’Alessio, Physiology of Proglucagon peptides: role of glucagon and Glp-1 in health and disease, Physiol. Rev. 95 (2) (2015) 513–548. [61] A.P. van Beek, et al., The glucagonoma syndrome and necrolytic migratory erythema: a clinical review, Eur. J. Endocrinol. 151 (5) (2004) 531–537. [62] D.S. Wilkinson, Necrolytic migratory erythema with carcinoma of the pancreas, Trans. St. Johns Hosp. Dermatol. Soc. 59 (2) (1973) 244–250. [63] H. Kindmark, et al., Endocrine pancreatic tumors with glucagon hypersecretion: a retrospective study of 23 cases during 20 years, Med. Oncol. 24 (3) (2007) 330–337. [64] C. Invitti, et al., Diagnosis and management of Cushing’s syndrome: results of an Italian multicentre study. Study Group of the Italian Society of endocrinology on the
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[65] [66] [67] [68] [69]
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pathophysiology of the hypothalamic-pituitary-adrenal Axis, J. Clin. Endocrinol. Metab. 84 (2) (1999) 440–448. H. Imura, et al., Biosynthesis and distribution of ACTH and related peptides, Acta Physiol. Pol. 32 (Suppl. 23) (1981) 15–35. L.L. Elias, A.J. Clark, The expression of the ACTH receptor, Braz. J. Med. Biol. Res. 33 (10) (2000) 1245–1248. L.K. Nieman, et al., The diagnosis of Cushing’s syndrome: an Endocrine Society clinical practice guideline, J. Clin. Endocrinol. Metab. 93 (5) (2008) 1526–1540. D.E. Schteingart, et al., Cushing’s syndrome secondary to ectopic corticotropinreleasing hormone-adrenocorticotropin secretion, J. Clin. Endocrinol. Metab. 63 (3) (1986) 770–775. H. Raff, et al., Intraoperative measurement of Adrenocorticotropin (Acth) during removal of Acth-secreting bronchial carcinoid-tumors, J. Clin. Endocrinol. Metab. 80 (3) (1995) 1036–1039.
Peptide Hormones as Tumor Markers in Clinical Practice Qian Sun, Zhen Zhao1 National Institutes of Health, Bethesda, MD, United States 1 Corresponding author: e-mail address: [email protected]
Contents 1. Introduction 2. Selected Peptide Hormones as Tumor Markers 2.1 Calcitonin 2.2 Alpha- and Beta-hCG 2.3 Insulin and C-Peptide 2.4 PTHrp 2.5 Gastrin 2.6 VIP 2.7 Glucagon 2.8 ACTH 3. Conclusion Acknowledgement References
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Abstract Peptide hormones represent a major class of hormones that are made from amino acids by specialized endocrine glands. The maturation of bioactive hormones take place in the rough endoplasmic reticulum and Golgi apparatus, where preprohormones are proteolytically cleaved into prohormones, and subsequently into mature peptide hormones. Once the bioactive hormones are released into the circulation, they interact with receptors located on the plasma membrane of target cells, and initiate intracellular signaling pathways to regulate physiological processes including energy metabolism, growth, stress, and reproduction. However, excessive amount of circulating peptide hormones often associates with the presence of tumors. Section 2 discusses 10 peptide hormones as tumor markers and their clinical application in aiding the diagnosis of tumors as well as monitoring the disease process.
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1. INTRODUCTION Peptide hormones represent a major class of hormones that are made from amino acids by specialized endocrine glands [1]. A peptide hormone is first synthesized as part of a large preprohormone in the rough endoplasmic reticulum (ER) (Fig. 1). Shortly after synthesis, the cleavage of the N-terminal signal sequence of the preprohormone results in prohormone [2]. To release the bioactive hormone, the prohormone matures during the intracellular transport from the rough ER to Golgi apparatus and finally to secretory granules [3]. During this process, enzymes play a number of important roles in the modification of preprohormones and prohormones. As mentioned earlier, the first enzymatic cleavage occurs in the rough ER through removing the N-terminal hydrophobic signal sequence to generate prohormones.
Fig. 1 Synthesis, maturation and release of peptide hormones. A peptide hormone is first synthesized as part of a large preprohormone in the rough endoplasmic reticulum. Preprohormone then matures into prohormone through the cleavage of the N-terminal signal sequence, and further into the bioactive hormone as a result of enzymatic processing. The maturation process takes place during the intracellular transport from the rough ER to Golgi apparatus and finally to secretory granules. After the release of mature peptide hormone into circulation from tumor cells, it interacts with receptors located on the plasma membrane of target cells to trigger a cascade of intracellular signaling.
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At this point, the prohormones still contain domains that are necessary for folding, stability, and sorting of the hormone molecules [4]. The subsequent cleavage by endopeptidases and exopeptidases removes these domains in the rough ER and Golgi apparatus, most often at sites characterized by the presence of consecutive basic residues (Arg-Arg, Arg-Lys, Lys-Arg, or Lys-Lys). [3–5]. In addition to proteolysis, enzymes are also involved in the maturation process of amino acid derivatizations including acetylation, glycosylation, methylation, sulfation, and others [3]. Once the mature peptide hormone is released into the circulation, it interacts with receptors located on the plasma membrane of cells, which are also called transmembrane receptors [1,2]. Two major types of transmembrane hormone receptors are the guanine nucleotide-binding regulatory protein (G protein)-coupled receptors and the enzyme-linked receptors. The activation of these receptors triggers a cascade of intracellular signaling through secondary messengers, such as cyclic AMP (cAMP), cyclic GMP (cGMP), and inositol phospholipids [6].
2. SELECTED PEPTIDE HORMONES AS TUMOR MARKERS Peptide hormones have essential roles in regulating physiological processes including energy metabolism, growth, stress, and reproduction [1]. However, excessive amount of circulating peptide hormones often associates with the presence of tumors. A number of peptide hormones are listed here as important tumor markers (Table 1).
2.1 Calcitonin Calcitonin is a linear 32 amino acid peptide that is released from the parafollicular or C cells of the thyroid gland in response to increased circulating calcium [7]. The physiologic role of endogenous calcitonin is not entirely clear, but it has been suggested to regulate calcium homeostasis [8]. Clinically, serum calcitonin is an important cytologic marker in patients with medullary thyroid carcinoma, a malignant tumor of the parafollicular C cells [8]. 2.1.1 Biosynthesis From Prohormone Calcitonin synthesis begins with the proteolytic cleavage of preprocalcitonin by endopeptidase to procalcitonin, which is a 116 amino acid precursor of calcitonin [23]. The procalcitonin is then cleaved at its N- and C-terminal ends to generate the bioactive peptide calcitonin [23]. The mature calcitonin is released from C cells into the circulation after the formation of its
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Table 1 Selected Peptide Hormones as Tumor Markers Peptide Size of Active Hormone Hormone Role as Tumor Marker
References
Calcitonin 32 Amino acids
Monitoring medullary thyroid cancer
[9]
AlphahCG
Aid the diagnosis of pituitary tumors
[10]
92 Amino acids
Beta-hCG 145 Amino acids
[11–13] Serum-Aid the diagnosis of choriocarcinoma; Monitoring testicular germ cell tumors CSF-Aid in the diagnosis of intracerebral germ cell tumors
PTHrp
139, 141, 173 Monitoring hypercalcemia of malignancy [7,14] Amino acids
Gastrin
17, 34 Amino Diagnosis of gastrinoma acids
[15,16]
VIP
28 Amino acids
Diagnosis of VIP-producing tumors (VIPomas)
[15,17]
Insulin
51 Amino acids
Diagnosis of insulinoma
[18]
Glucagon 29 Amino acids
Diagnosis of glucagonoma
[19]
ACTH
Monitoring pituitary ACTH-producing tumors (Cushing’s disease)
[1,20,21]
Aid the diagnosis of insulinoma
[22]
39 Amino acids
C-peptide 31 Amino acids
ACTH: adrenocorticotropic hormone; CSF: cerebrospinal fluid; GRP: gastrin-releasing peptide; hCG: human chorionic gonadotropin; PP: pancreatic polypeptide; PTHrp: parathyroid hormone-related peptide; VIP: vasoactive intestinal polypeptide.
secondary and tertiary structures, including an N-terminal disulfide bond and a C-terminal proline-amide [7]. Both of these structures are essential for the interaction of the peptide hormone with its receptor, a specific G protein-coupled receptor found on osteoclasts [7,23]. 2.1.2 Clinical Significance as Tumor Marker The increased production of calcitonin is a characteristic feature of medullary thyroid cancer, which accounts for 1%–2% of thyroid malignancies in
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the United States [24]. It has been suggested that serum calcitonin concentrations correlate well with tumor size in patients with medullary thyroid cancer, so clinically it serves as a tumor marker to monitor patients’ treatment response [9]. However, serum calcitonin is not recommended for use as a diagnostic tool for medullary thyroid cancer because increased calcitonin concentrations also associate with other conditions, including acute and chronic renal failure, hypercalcemia, pulmonary disease, and other malignancies [7,25]. Of note, although being the precursor of calcitonin, procalcitonin has very different clinical applications. Rather than a tumor marker, this 116 amino acid peptide is useful for diagnosing severe systemic inflammation and guiding antibiotic therapy [26,27]. It was suggested that the procalcitonin released during inflammation was produced by neuroendocrine cells in the lungs and intestines and does not undergo enzymatic cleavage as in C cells [23,28].
2.2 Alpha- and Beta-hCG Human chorionic gonadotropin (hCG) is one of the most important hormones during pregnancy. It promotes the production of progesterone, which protects the growing fetus by preventing menstrual bleeding and maintaining uterine lining [29]. hCG is mainly produced by trophoblast cells of the placenta, but a minute amount is also synthesized in the pituitary of nonpregnant women and men [30]. Clinically, laboratory tests for serum hCG are not only useful for pregnancy, but also serve as markers for monitoring choriocarcinoma, testicular tumors, and ovarian germ cell tumors (GCTs) [11]. 2.2.1 Processing of hCG hCG is a heterodimer composed of a 145 amino acid beta(β)-subunit and a 92 amino acid alpha(α)-subunit [30]. The hCGα subunit is encoded by a gene located on chromosome 6, the same gene that also encodes the α subunit of three other pituitary hormones, namely, luteinizing hormone (LH), follicle-stimulating hormone (FSH), and thyroid-stimulating hormone (TSH) [30]. The β subunit, on the other hand, is unique to hCG and encoded by a separate gene on chromosome 16 [31]. The two subunits associate with each other in the rough ER to form the mature hCG, and when they disassociate following enzymatic nicking, the hormone activity is lost [30,32].
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2.2.2 Clinical Applications as Tumor Markers Alpha subunit is identical in four glycoprotein hormones, including hCG, LH, FSH, and TSH. The increased α subunit level is most commonly associated with thyrotroph- or gonadotroph-derived pituitary tumors [33]. It aids the diagnosis of TSH-secreting pituitary adenomas, as increased concentrations are seen in 50%–85% of patients [10]. The clinical utility of α subunit in diagnosing gonadotroph adenomas, on the other hand, is limited because only 35% of those patients secrete enough intact α subunit to raise its serum levels [34]. Laboratory testings for serum hCG and β-hCG are widely applied in the screening and monitoring of patients with molar pregnancy who are at risk of developing choriocarcinoma (gestational trophoblastic neoplasia), with specificity and sensitivity of 99% [11]. In addition, β-hCG is an important tumor marker for the management of patients with testicular GCTs, along with two other markers alpha fetoprotein and lactate dehydrogenase [11,12]. Measurement of hCG before treatment for tumors is also essential for prognosis and monitoring of patients’ response to therapy [12]. Intracranial GCTs commonly occurs in children under the age of 15 years and they represent around 3% of primary pediatric central nervous systems tumors [13]. Based on the histologic components, intracranial GCTs are divided into germinomas and nongerminomatous GCTs (NGGCTs) by the World Health Organization (WHO) [35]. Cerebrospinal fluid (CSF) measurement of β-hCG is an essential tumor marker to help distinguish between germinomas and NGGCTs [13]. This distinction is important because NGGCTs carry a much poorer prognosis compared with pure germinomas [36].
2.3 Insulin and C-Peptide Insulin is a 51 amino acid peptide hormone produced by β-cells of pancreatic islets. It regulates the metabolism of carbohydrates, fats, and proteins by promoting the uptake of glucose from the blood into fat and skeletal muscle cells, simulating synthesis of glycogen and inhibiting glucose production by the liver [37]. In addition to managing patients with diabetes mellitus, serum insulin testing is useful for diagnosing insulinoma, in conjunction with proinsulin and connecting peptide (C-peptide) measurements. 2.3.1 Biosynthesis From Prohormone Insulin is first synthesized as a polypeptide preproinsulin in the rough ER of the pancreatic β cells [37]. After the signal peptide in preprohormone gets
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removed by cleaving enzymes, an 86 amino acid peptide proinsulin is formed. The proinsulin then is transported to the Golgi complex, where it matures into active insulin and a 31 amino acid C-peptide [22]. This maturation process is catalyzed by two endopeptidases, namely, prohormone convertases 1 and 2 (PC1 and PC2), as well as a third enzyme carboxypeptidase-H [22,37]. Although equimolar amounts of insulin and C-peptide are secreted into the circulation, C-peptide concentration in serum is generally higher because of its longer half-life [37]. 2.3.2 Clinical Significance as Tumor Marker Insulinoma is a rare form of neuroendocrine tumor that occurs in 1–4 people per million persons every year [38]. The disease is characterized by high levels of insulin secretion from beta cells of the pancreas in the presence of hypoglycemia [39]. Measurement of plasma glucose, insulin, C-peptide, and proinsulin during a prolonged 72-h fasting test can detect up to 99% of insulinomas, and therefore is the gold standard for biochemical diagnosis of the disease [18,38].
2.4 PTHrp Parathyroid hormone-related protein (PTHrp) is a 139 to 173 amino acid peptide hormone [7]. Although it was initially isolated from patients with humoral hypercalcemia of malignancy (HHM), later it was found to be produced by virtually all tissues in low concentrations [40]. As an autocrine and paracrine hormone, PTHrp functions to regulate bone resorption, smooth muscle relaxation, and lactation [7,40]. Clinically, PTHrp testing is often used as a tumor marker in prognosis, selection of therapy, and monitoring of patients with HHM. 2.4.1 Biosynthesis From Prohormone PTHrp is a monomeric peptide that exists in several isoforms, ranging from 139 to 173 amino acids in size, which are created by differential messenger RNA splicing and posttranslational processing [7]. PTHrp originates from its preprohormone, which is composed by an N-terminal signal peptide and a prohormone. After the signal peptide is removed, pro-PTHrp undergoes further intracellular conversion to mature peptides by a number of endoproteases including prohormone convertase furin [41,42]. The three peptide isoforms differ at the C-terminus, but their N-terminal region is identical, which also shows close homology to parathyroid hormone (PTH) [41].
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2.4.2 Clinical Significance as Tumor Marker Hypercalcemia occurs in 20%–30% of patients with solid tumors and hematologic malignancies [43]. Among these patients 75%–80% suffer from HHM, which is caused by secretion of peptide hormone PTHrp from the tumors [14]. PTHrp causes hypercalcemia in these patients by uncoupling bone resorption and formation, which leads to a large flux of calcium from bone into the circulation; additionally, it decreases ability of the kidney to clear calcium [44,45]. HHM is most often observed in patients with advanced squamous (lung, head, and neck), renal, bladder, breast, and ovarian carcinomas [7]. In these patients who also developed a recent onset of hypercalcemia, an elevated serum concentration of PTHrp is useful to confirm the diagnosis of HHM [14]. In addition, PTHrp is often used as a marker to assess the prognosis of patients, as well as their response to treatment [7].
2.5 Gastrin Gastrin is a polypeptide hormone produced by G cells of the gastric antrum in response to antral distention from food and the presence of partially digested protein [15,46]. Once released, it stimulates the secretion of gastric acid by parietal cells of the stomach to increase gastric motility [15]. The secretion of gastrin is greatly suppressed at low gastric pH except in gastrinoma patients [16]. Therefore, its level is used for the diagnosis of gastrinoma in the presence of gastric acid hypersecretion. 2.5.1 Biosynthesis From Prohormone The peptide hormone gastrin originates from the cleavage of a 101 amino acid preprogastrin [15]. After removal of the signal peptide from the preprohormone, progastrin is generated, which undergoes further proteolysis by trypsin-like endopeptidases and carboxypeptidase B-like enzymes [47]. The major form of mature gastrin in the circulation is gastrin-34, and it can be further cleaved into a shorter peptide gastrin-17 by trypsin [15]. 2.5.2 Clinical Significance as Tumor Marker Gastrinomas are neuroendocrine tumors located in the pancreas or duodenum that secrete excessive gastrin [48]. As a result of high circulating gastrin concentrations, gastric acid secretion is persistently stimulated which eventually causes peptic ulcers [49]. The presence of gastrinoma, hypergastrinemia, and severe ulcer disease is referred to as the Zollinger–Ellison syndrome (ZES), which was first described in 1955 by Zollinger and Ellison [50].
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ZES or gastrinoma is diagnosed if a serum gastrin value is greater than 10 times the upper limit of the reference range in the presence of a gastric pH below 2 [15,16].
2.6 VIP Vasoactive intestinal peptide (VIP) is a 28 amino acid polypeptide that can serve both as a neurotransmitter and hormone [51]. As a neurotransmitter, it causes vasodilation and relaxation of the circulatory system and the gut; as a peptide hormone, it functions to stimulate glycogenolysis, and inhibit gastric acid secretion [15]. VIP is produced in many tissues including the nervous system and the gut, but its circulating concentration remains low due to the short half-life, which is only around 1 min [15,52]. However, in patients with VIP-producing tumors (VIPomas), its level can be significantly increased [53]. 2.6.1 Biosynthesis From Prohormone VIP is derived from a 170 amino acid precursor prepro-VIP encoded by a gene located on chromosome 6 [51]. Enzymatic cleavage of the precursor generates three biologically active peptides, namely, VIP, peptide histidine isoleucine (PHI), and peptide histidine valine (PHV) [54]. Although these peptides share high levels of structural similarities, PHI/PHV and VIP can activate separate sets of receptors, suggesting they could have different physiological functions [54]. 2.6.2 Clinical Significance as Tumor Marker VIPomas are rare pancreatic neuroendocrine tumors that are detected in 1 in 10 million people per year [55]. The tumors are characterized by uncontrolled secretion of VIP, which is a strong muscle relaxant and causes watery diarrhea [15]. In patients with chronic diarrheal diseases, a persistently elevated serum VIP concentration with fasting is virtually the diagnosis for VIPomas [17].
2.7 Glucagon Glucagon is a polypeptide hormone of 29 amino acids produced by α cells of the pancreas [37]. The peptide hormone is secreted in response to hypoglycemia, stress, and exercise, with resultant increases in blood glucose concentration by stimulating gluconeogenesis and glycogenolysis of the liver [37,56]. Glucagon secretion is inhibited by high plasma glucose and insulin, a peptide hormone that decreases glucagon gene expression [57].
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Therefore in type I diabetes patients, elevated glucagon concentrations due to insulin deficiency are often observed [37]. In addition, excessive production of glucagon from pancreatic islets is the hallmark of glucagonoma, so serum glucagon is a useful tumor marker to diagnose this tumor [19]. 2.7.1 Biosynthesis From Prohormone Glucagon is derived from proglucagon, a polypeptide that contains not only glucagon but also two other bioactive peptides glucagon-like peptide-1 and -2 (GLP-1 and GLP-2) [58]. Proglucagon undergoes posttranslational processing in a tissue-specific manner, resulting in the release of different peptide hormones. Enzyme PC2 is responsible for the proteolytic processing of proglucagon into glucagon in pancreatic alpha cells, whereas in the enteroendocrine L cells, GLP-1 and GLP-2 are released upon the cleavage of proglucagon by PC1/3 [37,59]. Two major target organs for glucagon are the liver and the kidney [60]. Once glucagon binds to its receptor in these organs, which is a specific G protein-coupled receptor, a cascade of intracellular signaling is triggered to activate enzymes in gluconeogenesis and glycogenolysis pathways [60]. 2.7.2 Clinical Significance as Tumor Marker Glucagonomas are rare pancreatic neuroendocrine tumors that produce glucagon [61]. The patients’ clinical features include weight loss, glycose intolerance, and necrolytic migratory erythema, which is a skin rash involving the face, perineum, and extremities [62]. The diagnosis of glucagonoma is established with a fasting plasma glucagon level greater than 10 times the upper limit of the reference range (>1000 pg/mL) [19]. However, it has been suggested that up to 70% of the glucagon detected by immunoassays in these patients are biologically inactive [63].
2.8 ACTH Adrenocorticotropic hormone (ACTH, also called corticotropin) is a 39 amino acid peptide hormone produced by the anterior pituitary gland [1]. In response to stress and changes in circadian rhythm, the release of corticotropin-releasing hormone and arginine vasopressin from hypothalamus drives the secretion of ACTH, which in turn stimulates the production of three major hormones from adrenal gland, namely, aldosterone, cortisol, and dehydroepiandrosterone [20]. When excessive cortisol production is caused by a pituitary ACTH-secreting tumor, the condition is called Cushing’s disease. Since a variety of conditions can cause cortisol overproduction,
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ACTH testings are useful in determining the location of the abnormality and cause of hypercortisolism [1,64]. 2.8.1 Biosynthesis From Prohormone ACTH is derived from its 266 amino acid precursor proopiomelanocortin (POMC) [65]. Proteolytic cleavage of pro-POMC by endopeptidase PC1/3 first generates pro-ACTH, which is then further processed to mature ACTH by PC1/3 [20]. Once ACTH is released into the circulation, it binds to G protein-coupled melanocortin-2 receptor on adrenocortical cells to stimulate the growth of adrenocortical cells and synthesis of cortisol [66]. 2.8.2 Clinical Significance as Tumor Marker As mentioned earlier, Cushing’s disease is a benign ACTH-producing pituitary adenoma that produces excessive amount of cortisol. The disease is responsible for 70% of endogenous Cushing’s syndrome, which is characterized by central obesity with thin arms and legs, rounded face, high blood pressure, severe fatigue, weekend bones and muscles [67]. In addition to Cushing’s disease, a number of conditions can cause Cushing’s syndrome, some of which include use of glucocorticoid medication, adrenal gland tumors, and nonpituitary ACTH-producing tumors (also called ectopic ACTH syndrome) [1,68]. For this reason, ACTH testing is essential in determining the cause of Cushing’s syndrome as ACTH-dependent or independent (a common source is adrenal tumor) [20]. Moreover, ACTH levels greater than four times the upper limit of reference range are suggestive of an ectopic source, such as malignant tumors [1,21]. With the recent advancement of immunoassays, an intraoperative measurement of ACTH during removal of ACTH-secreting tumors is emerging to be a useful tool in guiding the extent of surgical resection [69].
3. CONCLUSION With the increase in specificity and sensitivity of immunoassays, laboratory testing for peptide hormones in body fluids has become available. When patients presented symptoms that lead to the suspicion of peptidesecreting tumors, determination of peptide concentrations is useful in aiding the diagnosis, later monitoring patients’ response to treatment, and assessing their prognosis. However, clinical application of these peptide hormones as tumor markers should be done with caution because most peptides are not specific to one single tumor, and most tumors produce multiple peptides.
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ACKNOWLEDGEMENT We thank Dr. Bo Ning at the Arizona State University in Arizona for his valuable assistance with the figure.
REFERENCES [1] G.D. Clines, Laurence, General endocrinology, in: L.A. Kaplan, A.J. Pesce, S.C. Kazmierczak (Eds.), Clinical Chemistry: Theory, Analysis, Correlation, Mosby, St. Louis., 2003. [2] S. Nussey, S. Whitehead, Principles of endocrinology, in: Endocrinology: An Integrated Approach, BIOS Scientific Publishers, Ltd, Oxford, England, 2001. [3] J.F. Rehfeld, et al., Peptide-hormone expression and precursor processing, Acta Oncol. 28 (3) (1989) 315–318. [4] J.W.M. Creemers, R.S. Jackson, J.C. Hutton, Molecular and cellular regulation of prohormone processing, Semin. Cell Dev. Biol. 9 (1) (1998) 3–10. [5] J. Martinez, P. Potier, Peptide-hormones as prohormones, Trends Pharmacol. Sci. 7 (4) (1986) 139–147. [6] L.M. Luttrell, Transmembrane signaling by G protein-coupled receptors, Methods Mol. Biol. 332 (2006) 3–49. [7] W. Fraser, Bone and mineral metabolism, in: C.A. Burtis et al., (Eds.), Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, Elsevier, St. Louis, MO, 2016, p. xviii. 2238 p. [8] N. Baregamian, C. Solo´rzano, The physiology of serum calcitonin and carcinoembryonic antigen in medullary thyroid cancer, in: T.S. Wang, D.B. Evans (Eds.), Medullary Thyroid Cancer, Springer, Cham, Switzerland, 2016, p. XI. 205 p. 40 illus., 28 illus. in color. [9] R. Cohen, et al., Preoperative calcitonin levels are predictive of tumor size and postoperative calcitonin normalization in medullary thyroid carcinoma, J. Clin. Endocrinol. Metab. 85 (2) (2000) 919–922. [10] H.V. Socin, et al., The changing spectrum of TSH-secreting pituitary adenomas: diagnosis and management in 43 patients, Eur. J. Endocrinol. 148 (4) (2003) 433–442. [11] C. Sturgenon, Tumor markers, in: C.A. Burtis et al., (Eds.), Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, Elsevier, St. Louis, MO, 2016, p. xviii. 2238 p. [12] T.D. Gilligan, et al., American Society of Clinical Oncology clinical practice guideline on uses of serum tumor markers in adult males with germ cell tumors, J. Clin. Oncol. 28 (20) (2010) 3388–3404. [13] M.E. Echevarria, J. Fangusaro, S. Goldman, Pediatric central nervous system germ cell tumors: a review, Oncologist 13 (6) (2008) 690–699. [14] W.A. Ratcliffe, et al., Role of assays for parathyroid-hormone-related protein in investigation of hypercalcaemia, Lancet 339 (8786) (1992) 164–167. [15] R.A. Sherwood, N.E. Walsham, I. Bjarnason, Gastric, pancreatic, and intestinal function, in: C.A. Burtis et al., (Eds.), Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, Elsevier, St. Louis, MO, 2016, p. xviii. 2238 p. [16] M.J. Berna, et al., Serum gastrin in Zollinger-Ellison syndrome—I. Prospective study of fasting serum gastrin in 309 patients from National Institutes of Health and comparison with 2229 cases from the literature, Medicine 85 (6) (2006) 295–330. [17] R. Yu, Chapter 150—Neuroendocrine tumor syndromes, in: J.L. Jameson, L.J. DeGroot (Eds.), Endocrinology: Adult and Pediatric, seventh ed., Elsevier Saunders, Philadelphia, 2016, pp. 2606–2614.e4. [18] F.J. Service, N. Natt, The prolonged fast, J. Clin. Endocrinol. Metab. 85 (11) (2000) 3973–3974.
Peptide Hormones as Tumor Markers in Clinical Practice
77
[19] R.A. Wermers, et al., The glucagonoma syndrome. Clinical and pathologic features in 21 patients, Medicine (Baltimore) 75 (2) (1996) 53–63. [20] R.L. Bertholf, M. Cooper, W.E. Winter, Adrenal cortex, in: C.A. Burtis et al., (Eds.), Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, Elsevier, St. Louis, MO, 2016, p. xviii. 2238 p. [21] T.A. Howlett, et al., Diagnosis and management of ACTH-dependent Cushing’s syndrome: comparison of the features in ectopic and pituitary ACTH production, Clin. Endocrinol. (Oxf ) 24 (6) (1986) 699–713. [22] C.J. Rhodes, S. Shoelson, P.A. Halban, Insulin biosynthesis, processing, and chemistry, in: E.P. Joslin, C.R. Kahn (Eds.), Joslin’s Diabetes Mellitus, Lippincott Williams & Willkins, Philadelphia, PA, 2005, p. xi. 1209 p. [23] P. Maruna, K. Nedelnikova, R. Gurlich, Physiology and genetics of procalcitonin, Physiol. Res. 49 (2000) S57–S61. [24] S.A. Wells, et al., Revised american thyroid association guidelines for the management of medullary thyroid carcinoma, Thyroid 25 (6) (2015) 567–610. [25] G. Costante, S. Filetti, Early diagnosis of medullary thyroid carcinoma: is systematic calcitonin screening appropriate in patients with nodular thyroid disease? Oncologist 16 (1) (2011) 49–52. [26] C. Wacker, et al., Procalcitonin as a diagnostic marker for sepsis: a systematic review and meta-analysis, Lancet Infect. Dis. 13 (5) (2013) 426–435. [27] P. Schuetz, W. Albrich, B. Mueller, Procalcitonin for diagnosis of infection and guide to antibiotic decisions: past, present and future, BMC Med. 9 (2011) 107. [28] S. Russwurm, et al., Molecular aspects and natural source of procalcitonin, Clin. Chem. Lab. Med. 37 (8) (1999) 789–797. [29] L.A. Cole, Biological functions of hCG and hCG-related molecules, Reprod. Biol. Endocrinol. 8 (2010) 102. [30] M.L. Yarbrough, M. Stout, A.M. Gronowski, Pregnancy and its disorders, in: C.A. Burtis et al., (Eds.), Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, Elsevier, St. Louis, MO, 2016, p. xviii. 2238 p. [31] P. Policastro, et al., The Beta-subunit of human chorionic-gonadotropin is encoded by multiple genes, J. Biol. Chem. 258 (19) (1983) 1492–1499. [32] L.A. Cole, et al., The deactivation of Hcg by nicking and dissociation, J. Clin. Endocrinol. Metab. 76 (3) (1993) 704–710. [33] P. BeckPeccoz, et al., Thyrotropin-secreting pituitary tumors, Endocr. Rev. 17 (6) (1996) 610–638. [34] D.M. Ho, et al., The clinicopathological characteristics of gonadotroph cell adenoma: a study of 118 cases, Hum. Pathol. 28 (8) (1997) 905–911. [35] D.N. Louis, et al., The 2007 WHO classification of tumours of the central nervous system, Acta Neuropathol. 114 (2) (2007) 97–109. [36] M. Matsutani, et al., Primary intracranial germ cell tumors: a clinical analysis of 153 histologically verified cases, J. Neurosurg. 86 (3) (1997) 446–455. [37] D. Sacks, Diabetes mellitus, in: C.A. Burtis et al., (Eds.), Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, Elsevier, St. Louis, MO, 2016, p. xviii. 2238 p. [38] T. Okabayashi, et al., Diagnosis and management of insulinoma, World J. Gastroenterol. 19 (6) (2013) 829–837. [39] A.H. Minn, et al., Insulinomas and expression of an insulin splice variant, Lancet 363 (9406) (2004) 363–367. [40] W.J. Burtis, Parathyroid hormone-related protein—structure, function, and measurement, Clin. Chem. 38 (11) (1992) 2171–2183. [41] B. Liu, D. Goltzman, S.A. Rabbani, Processing of pro-PTHrp by the prohormone convertase, Furin—effect on biological-activity, Am. J. Phys. Endocrinol. Metab. 268 (5) (1995) E832–E838.
78
Qian Sun and Zhen Zhao
[42] T. Nakajima, et al., Prohormone convertase furin has a role in gastric cancer cell proliferation with parathyroid hormone-related peptide in a reciprocal manner, Dig. Dis. Sci. 47 (12) (2002) 2729–2737. [43] A.F. Stewart, p. Clinical, Hypercalcemia associated with cancer, N. Engl. J. Med. 352 (4) (2005) 373–379. [44] R. Rizzoli, et al., Actions of parathyroid hormone and parathyroid hormone-related protein, J. Endocrinol. Invest. 15 (9 Suppl. 6) (1992) 51–56. [45] M.A. Syed, et al., Parathyroid hormone-related protein-(1–36) stimulates renal tubular calcium reabsorption in normal human volunteers: implications for the pathogenesis of humoral hypercalcemia of malignancy, J. Clin. Endocrinol. Metab. 86 (4) (2001) 1525–1531. [46] G.J. Dockray, et al., The gastrins: their production and biological activities, Annu. Rev. Physiol. 63 (2001) 119–139. [47] S. Jensen, et al., Progastrin processing during antral G-cell hypersecretion in humans, Gastroenterology 96 (4) (1989) 1063–1070. [48] J.A. Norton, Neuroendocrine tumors of the pancreas and duodenum, Curr. Probl. Surg. 31 (2) (1994) 82–156. [49] E.C. Ellison, J.A. Johnson, The Zollinger-Ellison syndrome: a comprehensive review of historical, scientific, and clinical considerations, Curr. Probl. Surg. 46 (1) (2009) 13–106. [50] R.M. Zollinger, E.H. Ellison, Primary peptic ulcerations of the jejunum associated with islet cell tumors of the pancreas, Ann. Surg. 142 (4) (1955) 709–728. [51] J. Fahrenkrug, VIP and PACAP, in: J.F. Rehfeld, J.R. Bundgaard, SpringerLink (Online service) (Eds.), Cellular Peptide Hormone Synthesis and Secretory Pathways, Springer-Verlag Berlin Heidelberg, Berlin, Heidelberg, 2010. [52] T. Juhasz, et al., PACAP and VIP signaling in chondrogenesis and osteogenesis, Peptides 66 (2015) 51–57. [53] A.A. Ghaferi, et al., Pancreatic VIPomas: subject review and one institutional experience, J. Gastrointest. Surg. 12 (2) (2008) 382–393. [54] D.L. Tse, et al., Identification of a potential receptor for both peptide histidine isoleucine and peptide histidine valine, Endocrinology 143 (4) (2002) 1327–1336. [55] S.L. Smith, et al., Vasoactive intestinal polypeptide secreting islet cell tumors: a 15-year experience and review of the literature, Surgery 124 (6) (1998) 1050–1055. [56] G. Jiang, B.B. Zhang, Glucagon and regulation of glucose metabolism, Am. J. Physiol. Endocrinol. Metab. 284 (4) (2003) E671–8. [57] P.E. Cryer, Hypoglycemia risk reduction in type 1 diabetes, Exp. Clin. Endocrinol. Diabetes 109 (Suppl. 2) (2001) S412–23. [58] Y. Rouille, et al., Proglucagon is processed to glucagon by prohormone convertase Pc2 in alpha-Tc1-6 cells, Proc. Natl. Acad. Sci. U. S. A. 91 (8) (1994) 3242–3246. [59] N.M. Whalley, et al., Processing of proglucagon to GLP-1 in pancreatic alpha-cells: is this a paracrine mechanism enabling GLP-1 to act on beta-cells? J. Endocrinol. 211 (1) (2011) 99–106. [60] D.A. Sandoval, D.A. D’Alessio, Physiology of Proglucagon peptides: role of glucagon and Glp-1 in health and disease, Physiol. Rev. 95 (2) (2015) 513–548. [61] A.P. van Beek, et al., The glucagonoma syndrome and necrolytic migratory erythema: a clinical review, Eur. J. Endocrinol. 151 (5) (2004) 531–537. [62] D.S. Wilkinson, Necrolytic migratory erythema with carcinoma of the pancreas, Trans. St. Johns Hosp. Dermatol. Soc. 59 (2) (1973) 244–250. [63] H. Kindmark, et al., Endocrine pancreatic tumors with glucagon hypersecretion: a retrospective study of 23 cases during 20 years, Med. Oncol. 24 (3) (2007) 330–337. [64] C. Invitti, et al., Diagnosis and management of Cushing’s syndrome: results of an Italian multicentre study. Study Group of the Italian Society of endocrinology on the
Peptide Hormones as Tumor Markers in Clinical Practice
[65] [66] [67] [68] [69]
79
pathophysiology of the hypothalamic-pituitary-adrenal Axis, J. Clin. Endocrinol. Metab. 84 (2) (1999) 440–448. H. Imura, et al., Biosynthesis and distribution of ACTH and related peptides, Acta Physiol. Pol. 32 (Suppl. 23) (1981) 15–35. L.L. Elias, A.J. Clark, The expression of the ACTH receptor, Braz. J. Med. Biol. Res. 33 (10) (2000) 1245–1248. L.K. Nieman, et al., The diagnosis of Cushing’s syndrome: an Endocrine Society clinical practice guideline, J. Clin. Endocrinol. Metab. 93 (5) (2008) 1526–1540. D.E. Schteingart, et al., Cushing’s syndrome secondary to ectopic corticotropinreleasing hormone-adrenocorticotropin secretion, J. Clin. Endocrinol. Metab. 63 (3) (1986) 770–775. H. Raff, et al., Intraoperative measurement of Adrenocorticotropin (Acth) during removal of Acth-secreting bronchial carcinoid-tumors, J. Clin. Endocrinol. Metab. 80 (3) (1995) 1036–1039.