Modulation of ghrelin o-acyltransferase expression in pancreatic islets

Background: Ghrelin, the only identified circulating orexigenic signal, is unique in structure in which a specific acyl-modification of its third serine occurs. This acylation is necessary for ghrelin to bind to its receptor and to exert its biologic activity, which is catalyzed by ghrelin O-acyltransferase (GOAT). Although ghrelin is mainly secreted from gastric X/A like endocrine cells, it is also expressed in pancreatic islet cells and regulates insulin secretion. In this study, we examined the expression and regulation of GOAT in pancreas. Methods: GOAT mRNA and immunoreactivity were examined in pancreatic islets and INS-1 cells by RT-PCR and immunofluorescent staining or Western blotting. Results: Insulin inhibits the expression of GOAT mRNA and GOAT promoter activity in a dose and time-dependent manner. The mammalian target of rapamycin (mTOR) is activated by insulin. Blocking mTOR signaling by either rapamycin or overexpression of its negative regulator tuberous sclerosis complex 1 (TSC1) or TSC2 attenuates the inhibitory effect of insulin on the transcription and translation of GOAT. Conclusion: Our study suggests that GOAT is present in pancreatic islet cells and that insulin inhibits the expression of GOAT via the mediation of mTOR signaling.