Title: The Physiology and Potential Clinical Applications of Ghrelin, a Novel Peptide Hormone
Abstract: Ghrelin, a peptide hormone originally identified as the endogenous ligand of the growth hormone secretagogue receptor, is secreted primarily from the stomach and secondarily from the small intestine and colon. Ghrelin may also be expressed in the pancreatic islets, hypothalamus, pituitary, and several tissues in the periphery. The growth hormone secretagogue receptor is widely expressed, suggesting diverse physiologic roles for ghrelin. A growing body of evidence suggests that, in addition to its predictable effect on growth hormone secretion, ghrelin has an important role in the short-term regulation of appetite and the long-term regulation of energy balance and glucose homeostasis. Recent studies have implicated ghrelin in the regulation of gastrointestinal, cardiovascular, and immune function and have suggested a role for ghrelin in bone physiology. The identification of obestatin, a novel peptide hormone derived from the same gene as ghrelin, has recently added further complexity to ghrelin physiology. Obestatin appears to have actions opposite of ghrelin on energy homeostasis and gastrointestinal function. Despite the rapid progress, many questions remain unanswered, including the regulation of ghrelin and obestatin secretion, the downstream pathways that mediate their effects, and their precise physiologic endocrine and paracrine roles. This review presents data on ghrelin structure, expression, and function, with emphasis placed on human studies, highlighting areas that require future investigation and providing speculation about potential clinical applications of ghrelin agonists or antagonists. Ghrelin, a peptide hormone originally identified as the endogenous ligand of the growth hormone secretagogue receptor, is secreted primarily from the stomach and secondarily from the small intestine and colon. Ghrelin may also be expressed in the pancreatic islets, hypothalamus, pituitary, and several tissues in the periphery. The growth hormone secretagogue receptor is widely expressed, suggesting diverse physiologic roles for ghrelin. A growing body of evidence suggests that, in addition to its predictable effect on growth hormone secretion, ghrelin has an important role in the short-term regulation of appetite and the long-term regulation of energy balance and glucose homeostasis. Recent studies have implicated ghrelin in the regulation of gastrointestinal, cardiovascular, and immune function and have suggested a role for ghrelin in bone physiology. The identification of obestatin, a novel peptide hormone derived from the same gene as ghrelin, has recently added further complexity to ghrelin physiology. Obestatin appears to have actions opposite of ghrelin on energy homeostasis and gastrointestinal function. Despite the rapid progress, many questions remain unanswered, including the regulation of ghrelin and obestatin secretion, the downstream pathways that mediate their effects, and their precise physiologic endocrine and paracrine roles. This review presents data on ghrelin structure, expression, and function, with emphasis placed on human studies, highlighting areas that require future investigation and providing speculation about potential clinical applications of ghrelin agonists or antagonists. The discovery of ghrelin has broadened our understanding of the interplay between the stomach and the brain and has shed new light on multiple physiologic processes, including the regulation of pituitary hormone secretion and energy homeostasis, gastrointestinal activity, and cardiovascular function, among others.1Kojima M Hosoda H Date Y Nakazato M Matsuo H Kangawa K Ghrelin is a growth-hormone-releasing acylated peptide from stomach.Nature. 1999; 402: 656-660Crossref PubMed Scopus (4313) Google Scholar, 2Kojima M Hosoda H Kangawa K Clinical endocrinology and metabolism: ghrelin, a novel growth-hormone-releasing and appetite-stimulating peptide from stomach.Best Pract Res Clin Endocrinol Metab. 2004; 18: 517-530Abstract Full Text Full Text PDF Scopus (33) Google Scholar, 3Kojima M Hosoda H Matsuo H Kangawa K Ghrelin: discovery of the natural endogenous ligand for the growth hormone secretagogue receptor.Trends Endocrinol Metab. 2001; 12: 118-122Abstract Full Text Full Text PDF PubMed Google Scholar The road that led to the discovery of ghrelin began close to 3 decades ago with the identification of several synthetic compounds, including both peptides and nonpeptides, with growth hormone (GH)-releasing activity from pituitary somatotrophs in vitro and in vivo, followed by the cloning of the receptor that mediates this response, termed the growth hormone secretagogue (GHS) receptor.4Bowers CY Momany F Reynolds GA Chang D Hong A Chang K Structure-activity relationships of a synthetic pentapeptide that specifically releases growth hormone in vitro.Endocrinology. 1980; 106: 663-667Crossref Google Scholar, 5Bowers CY Momany FA Reynolds GA Hong A On the in vitro and in vivo activity of a new synthetic hexapeptide that acts on the pituitary to specifically release growth hormone.Endocrinology. 1984; 114: 1537-1545Crossref PubMed Google Scholar, 6Smith RG Cheng K Schoen WR et al.A nonpeptidyl growth hormone secretagogue.Science. 1993; 260: 1640-1643Crossref PubMed Google Scholar, 7Cheng K Chan WW Butler B et al.Stimulation of growth hormone release from rat primary pituitary cells by L-692,429, a novel non-peptidyl GH secretagogue.Endocrinology. 1993; 132: 2729-2731Crossref Scopus (58) Google Scholar, 8Howard AD Feighner SD Cully DF et al.A receptor in pituitary and hypothalamus that functions in growth hormone release.Science. 1996; 273: 974-977Crossref PubMed Google Scholar Efforts to identify the endogenous ligand of the GHS receptor culminated in 1999 with the isolation of ghrelin from a stomach extract.1Kojima M Hosoda H Date Y Nakazato M Matsuo H Kangawa K Ghrelin is a growth-hormone-releasing acylated peptide from stomach.Nature. 1999; 402: 656-660Crossref PubMed Scopus (4313) Google Scholar, 3Kojima M Hosoda H Matsuo H Kangawa K Ghrelin: discovery of the natural endogenous ligand for the growth hormone secretagogue receptor.Trends Endocrinol Metab. 2001; 12: 118-122Abstract Full Text Full Text PDF PubMed Google Scholar In addition to its effect on GH secretion, ghrelin was subsequently found to have pleiotropic effects on appetite regulation and gastrointestinal and cardiovascular function. Obestatin, a second peptide derived from proteolytic cleavage of the ghrelin prohormone, was recently identified and shown to have effects opposite of ghrelin on energy homeostasis and gastrointestinal function.9Zhang JV Ren PG Avsian-Kretchmer O et al.Obestatin, a peptide encoded by the ghrelin gene, opposes ghrelin's effects on food intake.Science. 2005; 310: 996-999Crossref PubMed Scopus (570) Google Scholar Obestatin binds to a previously orphan receptor (GPR39), which shares similarities with the GHS receptor.10McKee KK Tan CP Palyha OC et al.Cloning and characterization of two human G protein-coupled receptor genes (GPR38 and GPR39) related to the growth hormone secretagogue and neurotensin receptors.Genomics. 1997; 46: 426-434Crossref PubMed Scopus (171) Google Scholar In this review, data are presented on ghrelin structure, expression, and function, with emphasis placed on human studies, highlighting areas that require future investigation and providing speculation about potential clinical applications of ghrelin agonists or antagonists. To compile this review article, we searched the literature by performing computerized MEDLINE searches (1976-present) using the following keywords: ghrelin, obestatin, and growth hormone secretagogue (receptor). Articles were selected for inclusion based on our best judgment. Ghrelin is a 28-amino acid peptide with a unique posttranslational modification of the Ser3 residue to which an octanoyl moiety is esterified.1Kojima M Hosoda H Date Y Nakazato M Matsuo H Kangawa K Ghrelin is a growth-hormone-releasing acylated peptide from stomach.Nature. 1999; 402: 656-660Crossref PubMed Scopus (4313) Google Scholar The presence of a large hydrophobic group at Ser3 is essential for GHS receptor activation.11Matsumoto M Hosoda H Kitajima Y et al.Structure-activity relationship of ghrelin: pharmacological study of ghrelin peptides.Biochem Biophys Res Commun. 2001; 287: 142-146Crossref Scopus (130) Google Scholar, 12Bednarek MA Feighner SD Pong SS et al.Structure-function studies on the new growth hormone-releasing peptide, ghrelin: minimal sequence of ghrelin necessary for activation of growth hormone secretagogue receptor 1a.J Med Chem. 2000; 43: 4370-4376Crossref PubMed Scopus (330) Google Scholar There is also a nonacylated ghrelin form (des-acyl ghrelin) present in excess in plasma, whose physiologic role is not clear.13Akamizu T Shinomiya T Irako T et al.Separate measurement of plasma levels of acylated and desacyl ghrelin in healthy subjects using a new direct ELISA assay.J Clin Endocrinol Metab. 2005; 90: 6-9Crossref Scopus (77) Google Scholar, 14Hosoda H Kojima M Matsuo H Kangawa K Ghrelin and des-acyl ghrelin: two major forms of rat ghrelin peptide in gastrointestinal tissue.Biochem Biophys Res Commun. 2000; 279: 909-913Crossref PubMed Scopus (514) Google Scholar, 15Date Y Kojima M Hosoda H et al.Ghrelin, a novel growth hormone-releasing acylated peptide, is synthesized in a distinct endocrine cell type in the gastrointestinal tracts of rats and humans.Endocrinology. 2000; 141: 4255-4261Crossref PubMed Google Scholar After collection of blood specimens, EDTA and aprotinin should be added and the plasma fraction separated by centrifugation and immediately acidified before freezing at -70°C to ensure stability of acylated ghrelin during storage.14Hosoda H Kojima M Matsuo H Kangawa K Ghrelin and des-acyl ghrelin: two major forms of rat ghrelin peptide in gastrointestinal tissue.Biochem Biophys Res Commun. 2000; 279: 909-913Crossref PubMed Scopus (514) Google Scholar Ghrelin is highly conserved among mammals and has even been detected in chickens, fish, and bullfrogs, suggesting an important evolutionary role.1Kojima M Hosoda H Date Y Nakazato M Matsuo H Kangawa K Ghrelin is a growth-hormone-releasing acylated peptide from stomach.Nature. 1999; 402: 656-660Crossref PubMed Scopus (4313) Google Scholar, 2Kojima M Hosoda H Kangawa K Clinical endocrinology and metabolism: ghrelin, a novel growth-hormone-releasing and appetite-stimulating peptide from stomach.Best Pract Res Clin Endocrinol Metab. 2004; 18: 517-530Abstract Full Text Full Text PDF Scopus (33) Google Scholar, 16Kojima M Kangawa K Ghrelin: structure and function.Physiol Rev. 2005; 85: 495-522Crossref PubMed Scopus (676) Google Scholar Significant homology exists between ghrelin and motilin.17Folwaczny C Chang JK Tschop M Ghrelin and motilin: two sides of one coin?.Eur J Endocrinol. 2001; 144: R1-R3Crossref Google Scholar The human preproghrelin gene is located on chromosome 3p25-26 and consists of 5 exons with 4 introns.18Kanamoto N Akamizu T Tagami T et al.Genomic structure and characterization of the 5′-flanking region of the human ghrelin gene.Endocrinology. 2004; 145: 4144-4153Crossref Scopus (43) Google Scholar Spliced ghrelin messenger RNA is translated to a 117-amino acid preproghrelin precursor, which is subsequently cleaved to yield ghrelin, some of which is further modified with the addition of the octanoyl moiety at Ser3.19Hosoda H Kojima M Mizushima T Shimizu S Kangawa K Structural divergence of human ghrelin: identification of multiple ghrelin-derived molecules produced by post-translational processing.J Biol Chem. 2003; 278: 64-70Crossref PubMed Scopus (194) Google Scholar However, the precise enzymatic mechanisms that lead to ghrelin acylation have not been established. In addition, obestatin, a 23-amino acid peptide, has recently been identified in silico as a putative proteolytic fragment of the preproghrelin precursor and purified from rat stomach extracts.9Zhang JV Ren PG Avsian-Kretchmer O et al.Obestatin, a peptide encoded by the ghrelin gene, opposes ghrelin's effects on food intake.Science. 2005; 310: 996-999Crossref PubMed Scopus (570) Google Scholar Obestatin is further modified by C terminal amidation and circulates in the rat plasma.9Zhang JV Ren PG Avsian-Kretchmer O et al.Obestatin, a peptide encoded by the ghrelin gene, opposes ghrelin's effects on food intake.Science. 2005; 310: 996-999Crossref PubMed Scopus (570) Google Scholar However, it is currently not known whether obestatin is secreted from the human stomach as well. Ghrelin is most abundantly expressed in specialized cells in the oxyntic glands of the gastric epithelium, originally termed X/A-like cells.15Date Y Kojima M Hosoda H et al.Ghrelin, a novel growth hormone-releasing acylated peptide, is synthesized in a distinct endocrine cell type in the gastrointestinal tracts of rats and humans.Endocrinology. 2000; 141: 4255-4261Crossref PubMed Google Scholar Approximately 60% to 70% of circulating ghrelin is secreted by the stomach, and most of the remainder originates in the small intestine.15Date Y Kojima M Hosoda H et al.Ghrelin, a novel growth hormone-releasing acylated peptide, is synthesized in a distinct endocrine cell type in the gastrointestinal tracts of rats and humans.Endocrinology. 2000; 141: 4255-4261Crossref PubMed Google Scholar It has been suggested that low-level ghrelin expression also occurs in several tissues outside the gut, including ɛ-pancreatic islet cells, hypothalamus (arcuate nucleus and paraventricular neuron groups), pituitary, lung, adrenal cortex, kidney, and bone.1Kojima M Hosoda H Date Y Nakazato M Matsuo H Kangawa K Ghrelin is a growth-hormone-releasing acylated peptide from stomach.Nature. 1999; 402: 656-660Crossref PubMed Scopus (4313) Google Scholar, 20Prado CL Pugh-Bernard AE Elghazi L Sosa-Pineda B Sussel L Ghrelin cells replace insulin-producing beta cells in two mouse models of pancreas development.Proc Natl Acad Sci U S A. 2004; 101: 2924-2929Crossref PubMed Scopus (272) Google Scholar, 21Wierup N Yang S McEvilly RJ Mulder H Sundler F Ghrelin is expressed in a novel endocrine cell type in developing rat islets and inhibits insulin secretion from INS-1 (832/13) cells.J Histochem Cytochem. 2004; 52: 301-310Crossref Google Scholar, 22Fukushima N Hanada R Teranishi H et al.Ghrelin directly regulates bone formation.J Bone Miner Res. 2005; 20: 790-798Crossref Scopus (132) Google Scholar, 23Cowley MA Smith RG Diano S et al.The distribution and mechanism of action of ghrelin in the CNS demonstrates a novel hypothalamic circuit regulating energy homeostasis.Neuron. 2003; 37: 649-661Abstract Full Text Full Text PDF PubMed Scopus (733) Google Scholar, 24Lu S Guan JL Wang QP et al.Immunocytochemical observation of ghrelin-containing neurons in the rat arcuate nucleus.Neurosci Lett. 2002; 321: 157-160Crossref PubMed Scopus (154) Google Scholar, 25Gnanapavan S Kola B Bustin SA et al.The tissue distribution of the mRNA of ghrelin and subtypes of its receptor, GHS-R, in humans.J Clin Endocrinol Metab. 2002; 87: 2988Crossref PubMed Scopus (762) Google Scholar, 26Mozid AM Tringali G Forsling ML et al.Ghrelin is released from rat hypothalamic explants and stimulates corticotrophin-releasing hormone and arginine-vasopressin.Horm Metab Res. 2003; 35: 455-459Crossref Scopus (81) Google Scholar This is still a matter of some controversy because tissue expression in these locations has generally been demonstrated using sensitive reverse transcriptase-polymerase chain reaction methods. Ghrelin immunoreactivity has also been found in the testis, including both Sertoli and Leydig cells, and the placenta (syncytiotrophoblast and cytotrophoblast).27Barreiro ML Gaytan F Caminos JE et al.Cellular location and hormonal regulation of ghrelin expression in rat testis.Biol Reprod. 2002; 67: 1768-1776Crossref PubMed Scopus (104) Google Scholar, 28Tena-Sempere M Barreiro ML Gonzalez LC et al.Novel expression and functional role of ghrelin in rat testis.Endocrinology. 2002; 143: 717-725Crossref PubMed Scopus (251) Google Scholar, 29Gualillo O Caminos J Blanco M et al.Ghrelin, a novel placental-derived hormone.Endocrinology. 2001; 142: 788-794Crossref PubMed Scopus (283) Google Scholar Low ghrelin levels have been measured in the cerebrospinal fluid by immunoassay, although the origin of this activity (brain vs peripheral tissues) is uncertain.30Tritos NA Kokkinos A Lampadariou E Alexiou E Katsilambros N Maratos-Flier E Cerebrospinal fluid ghrelin is negatively associated with body mass index.J Clin Endocrinol Metab. 2003; 88: 2943-2946Crossref PubMed Google Scholar Plasma ghrelin levels exhibit a pronounced diurnal variation, are increased by fasting and before meals and at night, and are rapidly (within <1 hour) suppressed by food intake, particularly by high-calorie or high-carbohydrate meals.31Tschop M Wawarta R Riepl RL et al.Post-prandial decrease of circulating human ghrelin levels.J Endocrinol Invest. 2001; 24: RC19-RC21PubMed Google Scholar, 32Overduin J Frayo RS Grill HJ Kaplan JM Cummings DE Role of the duodenum and macronutrient type in ghrelin regulation.Endocrinology. 2005; 146: 845-850Crossref PubMed Scopus (89) Google Scholar, 33Cummings DE Purnell JQ Frayo RS Schmidova K Wisse BE Weigle DS A preprandial rise in plasma ghrelin levels suggests a role in meal initiation in humans.Diabetes. 2001; 50: 1714-1719Crossref PubMed Google Scholar, 34Cummings DE Weigle DS Frayo RS et al.Plasma ghrelin levels after diet-induced weight loss or gastric bypass surgery.N Engl J Med. 2002; 346: 1623-1630Crossref PubMed Scopus (1247) Google Scholar, 35Weigle DS Cummings DE Newby PD et al.Roles of leptin and ghrelin in the loss of body weight caused by a low fat, high carbohydrate diet.J Clin Endocrinol Metab. 2003; 88: 1577-1586Crossref PubMed Scopus (130) Google Scholar In contrast, systemic obestatin levels do not change with fasting, at least in experimental animals.9Zhang JV Ren PG Avsian-Kretchmer O et al.Obestatin, a peptide encoded by the ghrelin gene, opposes ghrelin's effects on food intake.Science. 2005; 310: 996-999Crossref PubMed Scopus (570) Google Scholar The underlying mechanisms that mediate suppression of systemic ghrelin secretion by food are not known. Post-gastric or postabsorptive mechanisms have been implicated in studies performed in experimental animals and humans.32Overduin J Frayo RS Grill HJ Kaplan JM Cummings DE Role of the duodenum and macronutrient type in ghrelin regulation.Endocrinology. 2005; 146: 845-850Crossref PubMed Scopus (89) Google Scholar, 36Blom WA Lluch A Vinoy S et al.Effects of gastric emptying on the postprandial ghrelin response.Am J Physiol Endocrinol Metab. 2006; 290: E389-E395Crossref PubMed Scopus (26) Google Scholar In addition, both fasting plasma and cerebrospinal fluid ghrelin levels are negatively associated with body adiposity, supporting a role of ghrelin in the long-term regulation of energy homeostasis (see subsequent discussion).30Tritos NA Kokkinos A Lampadariou E Alexiou E Katsilambros N Maratos-Flier E Cerebrospinal fluid ghrelin is negatively associated with body mass index.J Clin Endocrinol Metab. 2003; 88: 2943-2946Crossref PubMed Google Scholar, 37Tschop M Weyer C Tataranni PA Devanarayan V Ravussin E Heiman ML Circulating ghrelin levels are decreased in human obesity.Diabetes. 2001; 50: 707-709Crossref PubMed Google Scholar, 38Cummings DE Shannon MH Roles for ghrelin in the regulation of appetite and body weight.Arch Surg. 2003; 138: 389-396Crossref PubMed Scopus (150) Google Scholar, 39Shiiya T Nakazato M Mizuta M et al.Plasma ghrelin levels in lean and obese humans and the effect of glucose on ghrelin secretion.J Clin Endocrinol Metab. 2002; 87: 240-244Crossref PubMed Scopus (812) Google Scholar Cholinergic stimulation leads to increased plasma ghrelin levels.40Broglio F Gottero C Van Koetsveld P et al.Acetylcholine regulates ghrelin secretion in humans.J Clin Endocrinol Metab. 2004; 89: 2429-2433Crossref PubMed Scopus (49) Google Scholar Combined growth hormone-releasing hormone (GHRH)-arginine administration similarly leads to increased plasma ghrelin levels.41Koutkia P Canavan B Breu J Johnson ML Grinspoon SK Nocturnal ghrelin pulsatility and response to growth hormone secretagogues in healthy men.Am J Physiol Endocrinol Metab. 2004; 287: E506-E512Crossref Scopus (31) Google Scholar Estrogen and recombinant human insulin-like growth factor I increase systemic ghrelin levels in patients with anorexia nervosa.42Grinspoon S Miller KK Herzog DB Grieco KA Klibanski A Effects of estrogen and recombinant human insulin-like growth factor-I on ghrelin secretion in severe undernutrition.J Clin Endocrinol Metab. 2004; 89: 3988-3993Crossref PubMed Scopus (27) Google Scholar In contrast, oral or intravenous glucose, insulin, glucagon, GH, and somatostatin suppress systemic ghrelin levels.39Shiiya T Nakazato M Mizuta M et al.Plasma ghrelin levels in lean and obese humans and the effect of glucose on ghrelin secretion.J Clin Endocrinol Metab. 2002; 87: 240-244Crossref PubMed Scopus (812) Google Scholar, 43Anderwald C Brabant G Bernroider E et al.Insulin-dependent modulation of plasma ghrelin and leptin concentrations is less pronounced in type 2 diabetic patients.Diabetes. 2003; 52: 1792-1798Crossref Google Scholar, 44Mohlig M Spranger J Otto B Ristow M Tschop M Pfeiffer AF Euglycemic hyperinsulinemia, but not lipid infusion, decreases circulating ghrelin levels in humans.J Endocrinol Invest. 2002; 25: RC36-RC38Google Scholar, 45Arafat MA Otto B Rochlitz H et al.Glucagon inhibits ghrelin secretion in humans.Eur J Endocrinol. 2005; 153: 397-402Crossref Scopus (21) Google Scholar, 46Qi X Reed J Englander EW Chandrashekar V Bartke A Greeley Jr, GH Evidence that growth hormone exerts a feedback effect on stomach ghrelin production and secretion.Exp Biol Med (Maywood). 2003; 228: 1028-1032Google Scholar, 47Norrelund H Hansen TK Orskov H et al.Ghrelin immunoreactivity in human plasma is suppressed by somatostatin.Clin Endocrinol (Oxf). 2002; 57: 539-546Crossref Scopus (108) Google Scholar, 48Barkan AL Dimaraki EV Jessup SK Symons KV Ermolenko M Jaffe CA Ghrelin secretion in humans is sexually dimorphic, suppressed by somatostatin, and not affected by the ambient growth hormone levels.J Clin Endocrinol Metab. 2003; 88: 2180-2184Crossref PubMed Scopus (150) Google Scholar, 49Broglio F van Koetsveld P Benso A et al.Ghrelin secretion is inhibited by either somatostatin or cortistatin in humans.J Clin Endocrinol Metab. 2002; 87: 4829-4832Crossref Scopus (124) Google Scholar Taken together with the stimulatory effect of ghrelin on GH secretion, the suppressive effect of GH on ghrelin expression in the stomach and systemic ghrelin levels may suggest the presence of a negative feedback loop mechanism between the stomach and the pituitary. Alternatively, it is possible that common factors may regulate both ghrelin and GH secretion in a reciprocal fashion.50Koutkia P Schurgin S Berry J et al.Reciprocal changes in endogenous ghrelin and growth hormone during fasting in healthy women.Am J Physiol Endocrinol Metab. 2005; 289: E814-E822Crossref Scopus (17) Google Scholar, 51Misra M Miller KK Herzog DB et al.Growth hormone and ghrelin responses to an oral glucose load in adolescent girls with anorexia nervosa and controls.J Clin Endocrinol Metab. 2004; 89: 1605-1612Crossref PubMed Scopus (56) Google Scholar, 52Misra M Miller KK Kuo K et al.Secretory dynamics of ghrelin in adolescent girls with anorexia nervosa and healthy adolescents.Am J Physiol Endocrinol Metab. 2005; 289: E347-E356Crossref PubMed Scopus (74) Google Scholar Short-term infusions of peptide YY, oxyntomodulin, and urocortin, all putative appetite-suppressing peptides, lead to a decrease in plasma ghrelin levels.53Cohen MA Ellis SM Le Roux CW et al.Oxyntomodulin suppresses appetite and reduces food intake in humans.J Clin Endocrinol Metab. 2003; 88: 4696-4701Crossref PubMed Scopus (216) Google Scholar, 54Davis ME Pemberton CJ Yandle TG et al.Urocortin-1 infusion in normal humans.J Clin Endocrinol Metab. 2004; 89: 1402-1409Crossref Scopus (55) Google Scholar, 55Batterham RL Cohen MA Ellis SM et al.Inhibition of food intake in obese subjects by peptide YY3-36.N Engl J Med. 2003; 349: 941-948Crossref PubMed Scopus (801) Google Scholar In contrast, leptin administration does not appear to have an effect on plasma ghrelin.56Chan JL Bullen J Lee JH Yiannakouris N Mantzoros CS Ghrelin levels are not regulated by recombinant leptin administration and/or three days of fasting in healthy subjects.J Clin Endocrinol Metab. 2004; 89: 335-343Crossref PubMed Scopus (83) Google Scholar Low systemic ghrelin levels have been reported in untreated hyperthyroidism, in male hypogonadism, in the polycystic ovary syndrome, in the presence of Helicobacter pylori-induced gastritis, or after total gastrectomy.57Riis AL Hansen TK Moller N Weeke J Jorgensen JO Hyperthyroidism is associated with suppressed circulating ghrelin levels.J Clin Endocrinol Metab. 2003; 88: 853-857Crossref Scopus (74) Google Scholar, 58Rojdmark S Calissendorff J Danielsson O Brismar K Hunger-satiety signals in patients with Graves' thyrotoxicosis before, during, and after long-term pharmacological treatment.Endocrine. 2005; 27: 55-61Crossref Google Scholar, 59Pagotto U Gambineri A Pelusi C et al.Testosterone replacement therapy restores normal ghrelin in hypogonadal men.J Clin Endocrinol Metab. 2003; 88: 4139-4143Crossref PubMed Scopus (70) Google Scholar, 60Pagotto U Gambineri A Vicennati V Heiman ML Tschop M Pasquali R Plasma ghrelin, obesity, and the polycystic ovary syndrome: correlation with insulin resistance and androgen levels.J Clin Endocrinol Metab. 2002; 87: 5625-5629Crossref PubMed Scopus (131) Google Scholar, 61Cummings DE Helicobacter pylori and ghrelin: interrelated players in body-weight regulation?.Am J Med. 2004; 117: 436-439Abstract Full Text Full Text PDF PubMed Scopus (19) Google Scholar, 62Isomoto H Ueno H Saenko VA et al.Impact of Helicobacter pylori infection on gastric and plasma ghrelin dynamics in humans.Am J Gastroenterol. 2005; 100: 1711-1720Crossref PubMed Scopus (57) Google Scholar, 63Takachi K Doki Y Ishikawa O et al.Postoperative ghrelin levels and delayed recovery from body weight loss after distal or total gastrectomy.J Surg Res. 2006; 130: 1-7Abstract Full Text Full Text PDF PubMed Scopus (38) Google Scholar However, the pathophysiologic implications of these findings have not been clearly elucidated. The factors involved in the regulation of systemic ghrelin levels and the underlying mechanisms have not been fully characterized. Clearly, several areas of controversy exist. Insulin appears to suppress ghrelin secretion at high concentrations in humans, but the physiologic relevance of these findings is less certain.43Anderwald C Brabant G Bernroider E et al.Insulin-dependent modulation of plasma ghrelin and leptin concentrations is less pronounced in type 2 diabetic patients.Diabetes. 2003; 52: 1792-1798Crossref Google Scholar, 44Mohlig M Spranger J Otto B Ristow M Tschop M Pfeiffer AF Euglycemic hyperinsulinemia, but not lipid infusion, decreases circulating ghrelin levels in humans.J Endocrinol Invest. 2002; 25: RC36-RC38Google Scholar Insulin may be required for maximal postprandial suppression of serum ghrelin levels, as suggested by some but not all studies.64Murdolo G Lucidi P Di Loreto C et al.Insulin is required for prandial ghrelin suppression in humans.Diabetes. 2003; 52: 2923-2927Crossref PubMed Scopus (132) Google Scholar, 65Gelling RW Overduin J Morrison CD et al.Effect of uncontrolled diabetes on plasma ghrelin concentrations and ghrelin-induced feeding.Endocrinology. 2004; 145: 4575-4582Crossref Scopus (42) Google Scholar, 66Spranger J Ristow M Otto B et al.Post-prandial decrease of human plasma ghrelin in the absence of insulin.J Endocrinol Invest. 2003; 26: RC19-RC22Google Scholar Data on regulation of systemic ghrelin levels are given in Table 1.TABLE 1States and Factors Associated With Changes in Systemic Ghrelin LevelsIncreased systemic ghrelinDecreased systemic ghrelinPreprandial or fasting statePostprandial stateWeight loss achieved through diet and/or exerciseWeight gainPrader-Willi syndromeGastric bypass surgeryAnorexia or eating disordersTotal gastrectomyCachexia (cardiac or malignancy associated)Helicobacter pylori—induced gastritisGhrelin-secreting neoplasmHyperthyroidismCholinergic stimulationMale hypogonadism; polycystic ovary syndromeGrowth hormone–releasing hormone–arginineGlucoseEstrogen (in anorexia nervosa)InsulinInsulin-like growth factor I (in anorexia nervosa)GlucagonGrowth hormoneSomatostatinPeptide YYOxyntomodulinUrocortin Open table in a new tab The ghrelin and obestatin receptors share significant homology and belong to the ghrelin-motilin receptor subfamily of 7 transmembrane G protein-coupled receptors.8Howard AD Feighner SD Cully DF et al.A receptor in pituitary and hypothalamus that functions in growth hormone release.Science. 1996; 273: 974-977Crossref PubMed Google Scholar, 10McKee KK Tan CP Palyha OC et al.Cloning and characterization of two human G protein-coupled receptor genes (GPR38 and GPR39) related to the growth hormone secretagogue and neurotensin receptors.Genomics. 1997; 46: 426-434Crossref PubMed Scopus (171) Google Scholar The GHS receptor has an essential role in the transduction of the effects of acylated ghrelin on GH secretion and energy homeostasis.8Howard AD Feighner SD Cully DF et al.A receptor in pituitary and hypothalamus that functions in growth hormone release.Science. 1996; 273: 974-977Crossref PubMed Google Scholar, 12Bednarek MA Feighner SD Pong SS et al.Structure-function studies on the new growth hormone-releasing peptide, ghrelin: minimal sequence of ghrelin necessary for activation of gro