Obestatin (rat)

Obestatin, a newly discovered peptide encoded by the ghrelin gene, induces the expression of genes regulating pancreatic beta-cell differentiation, insulin biosynthesis, and glucose metabolism. It also activates antiapoptotic signaling pathways such as phosphoinositide 3-kinase (PI3K) and ERK1/2 in pancreatic beta-cells and human islets. Since these kinases have been shown to protect against myocardial injury, we sought to investigate whether obestatin would exert cardioprotective effects. Both isolated perfused rat heart and cultured cardiomyocyte models of ischemia-reperfusion (I/R) were used to measure infarct size and cell apoptosis as end points of injury. The presence of specific obestatin receptors on cardiac cells as well as the signaling pathways underlying the obestatin effect were also studied. In the isolated heart, the addition of rat obestatin-(1-23) before ischemia reduced infarct size and contractile dysfunction in a concentration-dependent manner, whereas obestatin-(23-1), a synthetic analog with an inverse aminoacid sequence, was ineffective. The cardioprotective effect of obestatin-(1-23) was observed at concentrations of 10-50 nmol/l and was abolished by inhibiting PI3K or PKC by the addition of wortmannin (100 nmol/l) or chelerythrine, (5 micromol/l), respectively. In rat H9c2 cardiac cells or isolated ventricular myocytes subjected to I/R, 50 nmol/l obestatin-(1-23) reduced cardiomyocyte apoptosis and reduced caspase-3 activation; the antiapoptotic effect was blocked by the inhibition of PKC, PI3K, or ERK1/2 pathways. In keeping with these functional findings, radioreceptor binding results revealed the presence of specific high-affinity obestatin-binding sites, mainly localized on membranes of the ventricular myocardium and cardiomyocytes. Our data suggest that, by acting on specific receptors, obestatin-(1-23) activates PI3K, PKC-epsilon, PKC-delta, and ERK1/2 signaling and protects cardiac cells against myocardial injury and apoptosis induced by I/R.

The functional antagonism between obestatin and ghrelin in the testis is under investigation. We investigated the ability of obestatin to counteract the inhibitory effect of ghrelin on basal and stimulated testosterone (T) secretion in vitro. Testicular strips from adult rats were incubated with 10 ng/ml and 100 ng/ml of obestatin alone, ghrelin alone and obestatin + ghrelin. Obestatin modulation of stimulated T secretion was evaluated by incubation of testicular samples with 10 ng/ml and 100 ng/ml obestatin, ghrelin and obestatin + ghrelin in the absence and presence of 10 IU of human chorionic gonadotrophin (hCG). T concentrations in the hCG treated groups were significantly (P<0.0001) higher than those in the control groups. Obestatin caused a significant increase in basal T secretion in a dose-dependent manner; however, obestatin at the both 10 ng/ml and 100 ng/ml significantly (P<0.0001) increased hCG-stimulated T secretion. In contrast, ghrelin in a dose-dependent manner significantly (P<0.001) decreased both basal and hCG-induced T secretion by testicular slices. Obestatin opposed the inhibitory effect of ghrelin on T secretion under both basal and hCG-stimulated conditions at all doses tested. In conclusions, administration of obestatin was able to antagonize the inhibitory effect of ghrelin on testosterone secretion in vitro.

Obestatin is a 23-amino acid peptide encoded by the ghrelin gene, which regulates food intake, body weight and insulin sensitivity. Obestatin influences glucose and lipid metabolism in mature adipocytes in rodents. However, the role of this peptide in rat preadipocytes remains to be fully understood. The current study characterized the effects of obestatin on lipid accumulation, preadipocyte differentiation, lipolysis and leptin secretion in rat primary preadipocytes. Obestatin enhanced lipid accumulation in rat preadipocytes and increased the expression of surrogate markers of preadipocyte differentiation. At the early stage of differentiation, obestatin suppressed lipolysis. By contrast, lipolysis was stimulated at the late stage of adipogenesis. Furthermore, obestatin stimulated the release of leptin, a key satiety hormone. Overall, the results indicated that obestatin promotes preadipocyte differentiation. Obestatin increased leptin release in preadipocytes, while the modulation of lipolysis appears to depend upon the stage of differentiation.