Ghrelin is an acyl-peptide gastric hormone acting on the pituitary and hypothalamus to stimulate growth hormone (GH) release, adiposity, and appetite. Ghrelin endocrine activities are entirely dependent on its acylation and are mediated by GH secretagogue (GHS) receptor (GHSR)-1a, a G protein–coupled receptor mostly expressed in the pituitary and hypothalamus, previously identified as the receptor for a group of synthetic molecules featuring GH secretagogue (GHS) activity. Des-acyl ghrelin, which is far more abundant than ghrelin, does not bind GHSR-1a, is devoid of any endocrine activity, and its function is currently unknown. Ghrelin, which is expressed in heart, albeit at a much lower level than in the stomach, also exerts a cardio protective effect through an unknown mechanism, independent of GH release. Here we show that both ghrelin and des-acyl ghrelin inhibit apoptosis of primary adult and H9c2 cardiomyocytes and endothelial cells in vitro through activation of extracellular signal–regulated kinase-1/2 and Akt serine kinases. In addition, ghrelin and des-acyl ghrelin recognize common high affinity binding sites on H9c2 cardiomyocytes, which do not express GHSR-1a. Finally, both MK-0677 and hexarelin, a nonpeptidyl and a peptidyl synthetic GHS, respectively, recognize the common ghrelin and des-acyl ghrelin binding sites, inhibit cell death, and activate MAPK and Akt.These findings provide the first evidence that, independent of its acylation, ghrelin gene product may act as a survival factor directly on the cardiovascular system through binding to a novel, yet to be identified receptor, which is distinct from GHSR-1a.

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Ghrelin possesses strong GH-releasing activity but also other endocrine activities including stimulation of PRL and ACTH secretion, modulation of insulin secretion and glucose metabolism. It is assumed that the GH secretagogue (GHS) receptor (GHS-R) 1a mediates ghrelin actins provided its acylation in Serine 3; in fact, acylated ghrelin only is able to exert endocrine activities. Acylated ghrelin (AG) is present in serum at a 2.5 fold lower concentration than unacylated ghrelin (UAG). UAG, however, is not biologically inactive; it shares with AG some non-endocrine actions like cardiovascular effects, modulation of cell proliferation and even some influence on adipogenesis. Thus, these actions are likely to be mediated by GHS-R subtypes able to bind ghrelin independently of its acylation. In order to further clarify whether UAG is really devoid of any endocrine action, we studied the interaction of the combined administration of AG and UAG (1.0 microg/kg i.v.) in 6 normal young volunteers (age [mean +/- SE]: 25.4 +/- 1.2 yr; BMI: 22.3 +/- 1.0 kg/m2). As expected, AG induced marked increase (p < 0.01) in circulating GH, PRL, ACTH and cortisol levels. AG administration was also followed by a decrease in insulin levels (-285.4 +/- 64.8 mU*min/l; p < 0.05) and an increase in plasma glucose levels (1068.4 +/- 390.4 mg*min/dl; p < 0.01). UAG alone did not induce any change in these parameters. UAG also failed to modify the GH, PRL, ACTH and cortisol responses to AG. However, when UAG was co-administered together with AG, no significant change in insulin (-0.5 +/- 40.9 mU*min/l) and glucose levels (455.9 +/- 88.3 mg*min/dl) was recorded anymore, indicating that the insulin and glucose response to AG has been abolished by UAG. In conclusion, non-acylated ghrelin does not affect the GH, PRL, and ACTH response to acylated ghrelin but is able to antagonize the effects of acylated ghrelin on insulin secretion and glucose levels. These findings indicate that unacylated ghrelin is metabolically active and is likely to counterbalance the influence of acylated ghrelin on insulin secretion and glucose metabolism. As GHS-R1a is not bound by unacylated ghrelin, these findings suggest that GHS receptor subtypes mediate the metabolic actions of both acylated and unacylated ghrelin.

OBJECTIVE-Obestatin is a newly discovered peptide encoded by the ghrelin gene whose biological functions are poorly understood. We investigated obestatin effect on survival of ␤-cells and human pancreatic islets and the underlying signaling pathways. RESEARCH DESIGN AND METHODS-␤-Cells and humanislets were used to assess obestatin effect on cell proliferation, survival, apoptosis, intracellular signaling, and gene expression. RESULTS-Obestatinshowed specific binding on HIT-T15 and INS-1E ␤-cells, bound to glucagon-like peptide-1 receptor (GLP-1R), and recognized ghrelin binding sites. Obestatin exerted proliferative, survival, and antiapoptotic effects under serumdeprived conditions and interferon-␥/tumor necrosis factor-␣/ interleukin-1␤ treatment, particularly at pharmacological concentrations. Ghrelin receptor antagonist [D-Lys 3 ]-growth hormone releasing peptide-6 and anti-ghrelin antibody prevented obestatin-induced survival in ␤-cells and human islets. ␤-Cells and islet cells released obestatin, and addition of anti-obestatin antibody reduced their viability. Obestatin increased ␤-cell cAMP and activated extracellular signal-related kinase 1/2 (ERK1/2) and phosphatidylinositol 3-kinase (PI 3-kinase)/Akt; its antiapoptotic effect was blocked by inhibition of adenylyl cyclase/cAMP/ protein kinase A (PKA), PI 3-kinase/Akt, and ERK1/2 signaling. Moreover, obestatin upregulated GLP-1R mRNA and insulin receptor substrate-2 (IRS-2) expression and phosphorylation. The GLP-1R antagonist exendin-(9-39) reduced obestatin effect on ␤-cell survival. In human islets, obestatin, whose immunoreactivity colocalized with that of ghrelin, promoted cell survival and blocked cytokine-induced apoptosis through cAMP increase and involvement of adenylyl cyclase/cAMP/PKA signaling. Moreover, obestatin 1) induced PI 3-kinase/Akt, ERK1/2, and also cAMP response element-binding protein phosphorylation; 2) stimulated insulin secretion and gene expression; and 3) upregulated GLP-1R, IRS-2, pancreatic and duodenal homeobox-1, and glucokinase mRNA. O bestatin is a 23-amino acid amidated peptide, recently identified as a product of the ghrelin gene (1). It was originally reported to be the ligand for the orphan receptor G-protein-coupled receptor 39 (GPR39); however, several groups were unable to confirm that obestatin has agonist properties on GPR39 or activates specific GPR39 signaling (2-6). Therefore, to date, the receptor for obestatin remains unknown. CONCLUSIONS-TheseObestatin has been reported to reduce food intake, body weight gain, gastric emptying, and jejunal motility (1,7,8). Moreover, it was found to counteract ghrelin stimulatory effects on these end points (1,9) and to inhibit ghrelininduced growth hormone secretion in vivo (9) but not in vitro (10), suggesting that it would serve as a physiological opponent of ghrelin. However, a number of studies failed to confirm obestatin anorexigenic effects (11-14), and besides not being the cognate ligand for GPR39, its biological actions seem to be a controversial issue.Obestatin ...

Among its pleiotropic actions, ghrelin modulates insulin secretion and glucose metabolism. Herein we investigated the role of ghrelin in pancreatic beta-cell proliferation and apoptosis induced by serum starvation or interferon (IFN)-gamma/TNF-alpha, whose synergism is a major cause for beta-cell destruction in type I diabetes. HIT-T15 beta-cells expressed ghrelin but not ghrelin receptor (GRLN-R), which binds acylated ghrelin (AG) only. However, both unacylated ghrelin (UAG) and AG recognized common high-affinity binding sites on these cells. Either AG or UAG stimulated cell proliferation through Galpha(s) protein and prevented serum starvation- and IFN-gamma/TNF-alpha-induced apoptosis. Antighrelin antibody enhanced apoptosis in either the presence or absence of serum but not cytokines. AG and UAG even up-regulated intracellular cAMP. Blockade of adenylyl cyclase/cAMP/protein kinase A signaling prevented the ghrelin cytoprotective effect. AG and UAG also activated phosphatidyl inositol 3-kinase (PI3K)/Akt and ERK1/2, whereas PI3K and MAPK inhibitors counteracted the ghrelin antiapoptotic effect. Furthermore, AG and UAG stimulated insulin secretion from HIT-T15 cells. In INS-1E beta-cells, which express GRLN-R, AG and UAG caused proliferation and protection against apoptosis through identical signaling pathways. Noteworthy, both peptides inhibited cytokine-induced NO increase in either HIT-T15 or INS-1E cells. Finally, they induced cell survival and protection against apoptosis in human islets of Langerhans. These expressed GRLN-R but showed also UAG and AG binding sites. Our data demonstrate that AG and UAG promote survival of both beta-cells and human islets. These effects are independent of GRLN-R, are likely mediated by AG/UAG binding sites, and involve cAMP/PKA, ERK1/2, and PI3K/Akt.

Ghrelin, a 28 amino acid gastric hormone is a natural ligand of the GH Secretagogue (GHS) receptor (GHS-R) and strongly stimulates GH secretion though, like synthetic GHS, it shows other endocrine and non-endocrine activities. Aim of the present study was to clarify whether ghrelin administration influences insulin and glucose levels in humans. To this goal, we compared the effects of ghrelin, hexarelin, a synthetic GHS, or placebo on insulin and glucose as well as on GH levels in 11 normal young volunteers (age [mean +/- SEM]: 28.5 +/- 3.1 yr; BMI: 22.2 +/- 0.9 Kg/m(2)). Ghrelin induced very marked increase in GH secretion (DeltaAUC(0-180): 5777.1 +/- 812.6 microg/l/h; p < 0.01) which was not modified by placebo. Placebo administration did not modify insulin and glucose levels. On the other hand, ghrelin administration induced a prompt increase in glucose levels (DeltaAUC(0-180): 1343.1 +/- 443.5 mg/dl/h; p < 0.01 vs. saline). Absolute glucose levels at +15' were already higher than those at baseline (93.9 +/- 7.1 mg/dl; p < 0.01) and persisted elevated up to 165' (90.3 +/- 5.8 mg/dl; p < 0.01 vs. 0'). Ghrelin administration was also followed by a decrease in serum insulin levels (DeltaAUC(0-180): -207.1 +/- 70.5 mU/l/h; p < 0.05 vs. saline). Absolute insulin levels were significantly reduced from 30' (11.4 +/- 0.9 mU/l, p < 0.1 vs. 0'), showed the nadir at +45' (10.0 +/- 0.6 mU/l, p < 0.01 vs. 0') and then persisted lower (p < 0.01) than baseline up to +105'. Hexarelin administration did not modify glucose and insulin levels despite its marked GH-releasing effect (DeltaAUC(0-180): 4156.8 +/- 1180.3 microg/l/h; p < 0.01 vs. saline) that was slightly lower (p < 0.05) than that of ghrelin. In conclusion, these findings show that, besides stimulating GH secretion, ghrelin is a gastric hormone possessing metabolic actions such as hyperglycemic effect and lowering effect on insulin secretion in humans, at least after acute administration.

An endogenous ligand for the GH secretagogue-receptor (GHS-R) has been recently purified from rat and human stomach and named Ghrelin. It has been demonstrated that Ghrelin specifically stimulates GH secretion from rat pituitary cells in culture as well as in rats in vivo. In this preliminary study, in 4 normal adults [age (mean+/-SE): 28.6+/-3.5 yr; body mass index (BMI): 22.3+/-2.1 kg/m2] we administered 1.0 microg/kg Ghrelin or GHRH-29 to compare their GH-releasing activities in humans. In all subjects Ghrelin induced a prompt, marked and long-lasting increase in circulating GH levels (peak: 107.9+/-26.1 microg/l; AUC: 6503.1+/-1632.7 microg/l/h). The GH response to Ghrelin was clearly higher (p<0.05) than that after GHRH (peak: 22.3+/-4.5 microg/l; AUC: 1517.5+/-338.4 microg/l/h). In conclusion, this preliminary study shows that Ghrelin exerts a strong stimulatory effect on GH secretion in humans releasing more GH than GHRH.

SummaryGhrelin, an acylated peptide produced predominantly by the stomach, has been discovered to be a natural ligand of the growth hormone secretagogue receptor type 1a (GHS-R1a). Ghrelin has recently attracted considerable interest as a new orexigenic factor. However, ghrelin exerts several other neuroendocrine, metabolic and also nonendocrine actions that are explained by the widespread distribution of ghrelin and GHS-R expression. The likely existence of GHS-R subtypes and evidence that the neuroendocrine actions, but not all the other actions, of ghrelin depend on its acylation in serine-3 revealed a system whose complexity had not been completely explored by studying synthetic GHS. Ghrelin secretion is mainly regulated by metabolic signals and, in turn, the modulatory action of ghrelin on the control of food intake and energy metabolism seems to be among its most important biological actions. However, according to a recent study, ghrelin-null mice are neither anorectics nor dwarfs and this evidence clearly depicts a remarkable difference from leptin null mice. Nevertheless, the original and fascinating story of ghrelin, as well as its potential pathophysiological implications in endocrinology and internal medicine, is not definitively cancelled by these data as GHS-R1a null aged mice show significant alterations in body composition and growth, in glucose metabolism, cardiac function and contextual memory. Besides potential clinical implications for natural or synthetic ghrelin analogues acting as agonists or antagonists, there are several open questions awaiting an answer. How many ghrelin receptor subtypes exist? Is ghrelin 'the' or just 'a' GHS-R ligand? That is, are there other natural GHS-R ligands? Is there a functional balance between acylated and unacylated ghrelin forms, potentially with different actions? Within the next few years suitable answers to these questions will probably be found, making it possible to gain a better knowledge of ghrelin's potential clinical perspectives.