Cortistatin-17 and -14 exert the same endocrine activities as somatostatin in humans

Cortistatin (CST) is a neuropeptide, which binds with high affinity all somatostatin (SS) receptor subtypes and shows high structural homology with SS itself. A receptor specific for CST only, i.e., not recognized by SS, has been recently described in agreement with data reporting that not all CST actions are shared by SS. Interestingly, CST but not SS also binds ghrelin receptor (GHS-R1a) in vitro, suggesting a potential interplay between CST and ghrelin system. The aim of this study was to investigate in humans the endocrine and metabolic activities of human CST-17 in comparison with rat CST-14 that has previously been shown to exert the same endocrine actions of SS in healthy volunteers. To this aim, in six healthy male volunteers (age [median, 3rd-97th centiles]: 28.5; 23.6-34.3 years; Body Mass Index: 23.5; 21.0-25.1 kg/m2), we studied the effects of human CST-17 (2.0 μg/kg/h iv over 120 min), rat CST-14 (2.0 μg/kg/h iv over 120 min) and SS-14 (2.0 μg/kg/h iv over 120 min) on: (a) spontaneous GH, ACTH, PRL, cortisol, insulin and glucose levels; (b) the GH responses to GHRH (1.0 μg/kg iv at 0 min); (c) the GH, PRL, ACTH, cortisol, insulin and glucose responses to ghrelin (1.0 μg/kg iv at 0 min). CST-17 inhibited (p<0.01) basal GH secretion to the same extent of CST-14 and SS-14. Spontaneous PRL, ACTH and cortisol secretion were not significantly modified by CST-17, CST-14 or SS-14. CST-17 as well as CST-14 and SS-14 also inhibited (p<0.05) spontaneous insulin secretion to a similar extent. None of these peptides modified glucose levels. The GH response to GHRH was inhibited to the same extent by CST-17 (p<0.01), CST-14 (p<0.01) and SS-14 (p<0.05). The ghrelin-induced GH response was higher than that elicited by GHRH (p<0.01) and inhibited by CST-17 (p<0.05) as well as by CST-14 (p<0.05) and SS-14 (p<0.01). The PRL, ACTH and cortisol responses to ghrelin were unaffected by CST-17, CST-14 or SS-14. On the other hand, the inhibitory effect of ghrelin on insulin levels was abolished by CST-17, CST-14 or SS-14 (p<0.05) that, in turn, did not modify the ghrelin-induced increase in glucose levels. In conclusion, this study demonstrates that human CST-17 and rat CST-14 exert the same endocrine activities of SS in humans. The endocrine actions of human and rat CST therefore are likely to reflect activation of classical SS receptors

Metabolic aspects of the ghrelin system: Role of acylated and unacylated ghrelin in glucose homeostasis

In the last decade the discovery of ghrelin, a gut peptide discovered in 1999 by Kojima and colleagues (1), has led to the identification of a complex system that introduced new perspectives in neuroendocrine and metabolic research. Ghrelin is a peptide-hormone of 28 amino acids, predominantly produced by the stomach and detected in a lower amount in other central and peripheral tissues (1-11). The ghrelin peptide has a biological peculiarity, which is the esterification of a fatty (mostly n-octanoic) acid at its third serine residue (1). This modification is necessary for binding and activation of the growth hormone secretagogue receptor type 1a (GHS-R1a), the only cloned ghrelin receptor so far (1, 12, 13). Before the discovery of ghrelin, the GHS-R1a was an orphan G-protein coupled receptor specific for a family of synthetic molecules exerting a strong GH-releasing activity and therefore named Growth Hormone Secretagogues (GHS). The acyl-modified forms of ghrelin (AG), as well as some of the synthetic GHSs, have pleiotropic activities, including modulation of insulin secretion and glucose homeostasis. Besides the acylated form of ghrelin (AG), an unacylated ghrelin molecule (unacylated ghrelin, UAG) is also present in circulation. The absence of the acyl modification makes UAG unable to bind or activate the GHS-R1a (1). Moreover, although a specific UAG receptor has not been isolated to date, its existence has been strongly suggested. UAG shares with AG a variety of biological actions, but it also exerts AG-independent activities (11). Recently, a third molecule has been identified as a ghrelin-associated peptide and named obestatin (14). Obestatin is encoded by the same ghrelin gene and is a 23-amino acid product of the pro-ghrelin peptide. However, it does not bind the GHS-R1a (14). The growing body of literature over the last few years profiled the complex identities and interactions of these newly discovered molecules and their known and unknown receptor(s), which constitute the ghrelin system. The line of research and the studies included in this thesis focus on the involvement of the ghrelin system in the regulation of glucose metabolism, with particular emphasis on AG, UAG and their receptor(s)

Bolus administration of obestatin does not change glucose and insulin levels neither in the systemic nor in the portal circulation of the rat

Obestatin is a second peptide derived from the preproghrelin polypeptide. it was originally thought to have anorexigenic effects, thereby functioning as an antagonist of ghrelin. However, this has been a subject of debate ever since. Since acylated ghrelin strongly induces insulin resistance, it could be hypothesized that obestatin plays a role in glucose homeostasis as well. In the present study we evaluated the effect of obestatin on glucose and insulin metabolism in the systemic and portal circulation. Obestatin 200 nmol/kg was administered systemically as a single intravenous bolus injection to fasted pentobarbital anesthetized adult male Wistar rats. Up to 50 min after administration, blood samples were taken to measure glucose and insulin concentrations, both in the portal and in the systemic circulation. The effect of obestatin was evaluated in fasted and in glucose-stimulated conditions (IVGTT) and compared to control groups treated with saline or IVGTT, respectively. Intravenous administration of obestatin did not have any effect on glucose and insulin concentrations, neither systemic nor portal, when compared to the control groups. Only the glucose peak 1 min after administration of IVGTT was slightly higher in the obestatin treated rats: 605.8 +/- 106.3% vs. 522.2 +/- 47.1% in the portal circulation, respectively (NS), and 800.7 +/- 78.7% vs. 549.6 +/- 37.0% in the systemic circulation, respectively (P < 0.02), but it can be debated whether this has any clinical relevance. In the present study, we demonstrated that intravenously administered obestatin does not influence glucose and insulin concentrations, neither in the portal nor in the systemic circulation. (c) 2008 Elsevier Inc. All rights reserved