We developed and analyzed two types of transgenic mice: rat insulin II promoter-ghrelin transgenic (RIP-G Tg) and rat glucagon promoter-ghrelin transgenic mice (RGP-G Tg). The pancreatic tissue ghrelin concentration measured by C-terminal radioimmunoassay (RIA) and plasma desacyl ghrelin concentration of RIP-G Tg were about 1000 and 3.4 times higher than those of nontransgenic littermates, respectively. The pancreatic tissue n-octanoylated ghrelin concentration measured by N-terminal RIA and plasma n-octanoylated ghrelin concentration of RIP-G Tg were not distinguishable from those of nontransgenic littermates. RIP-G Tg showed suppression of glucose-stimulated insulin secretion. Arginine-stimulated insulin secretion, pancreatic insulin mRNA and peptide levels, ␤ cell mass, islet architecture, and GLUT2 and PDX-1 immunoreactivity in RIP-G Tg pancreas were not significantly different from those of nontransgenic littermates. Islet batch incubation study did not show suppression of insulin secretion of RIP-G Tg in vitro. The insulin tolerance test showed lower tendency of blood glucose levels in RIP-G Tg. Taking lower tendency of triglyceride level of RIP-G Tg into consideration, these results may indicate that the suppression of insulin secretion is likely due to the effect of desacyl ghrelin on insulin sensitivity. RGP-G Tg, in which the pancreatic tissue ghrelin concentration measured by C-RIA was about 50 times higher than that of nontransgenic littermates, showed no significant changes in insulin secretion, glucose metabolism, islet mass, and islet architecture. The present study raises the possibility that desacyl ghrelin may have influence on glucose metabolism.Ghrelin is a 28-amino acid peptide with unique modification of acylation, which is essential for its biological action (1). Ghrelin was originally identified in rat stomach as an endogenous ligand for an orphan receptor, which has been so far called growth hormone secretagogue receptor (GHS-R) 1 (1). Ghrelin expression is detected in the stomach, intestine, hypothalamus, pituitary gland, kidney, placenta, and testis (2-6). Ghrelin is involved in a wide variety of the functions, including the regulation of growth hormone release, food intake, gastric acid secretion, gastric motility, blood pressure, and cardiac output (7-19).Recently Date et al. (20) reported that ghrelin is present in ␣ cells of normal human and rat pancreatic islets. Volante et al. (20,31,32). Salehi et al. have reported ghrelin has both inhibitory and stimulatory effects depending on its concentration (33). Therefore, there is still a lot of controversy about the localization of ghrelin in the pancreas and the effects of ghrelin on the insulin secretion. As for the effects of desacyl ghrelin on insulin secretion, Broglio et al. (34) have reported that acute desacyl ghrelin administration has no effect on insulin secretion in human but that it counteracts the inhibitory effect of n-octanoylated ghrelin on insulin secretion when co-administrated with n-octanoylated ghrelin (35).Here...

To determine the comprehensive G protein-coupled receptor (GPCR) expression profile in ghrelin-producing cells and to elucidate the role of GPCR-mediated signaling in the regulation of ghrelin secretion, we determined GPCR expression profiles by RNA sequencing in the ghrelin-producing cell line MGN3-1 and analyzed the effects of ligands for highly expressed receptors on intracellular signaling and ghrelin secretion. Expression of selected GPCRs was confirmed in fluorescence-activated cell-sorted fluorescently tagged ghrelin-producing cells from ghrelin-promoter CreERT2/Rosa-CAG-LSL-ZsGreen1 mice. Expression levels of GPCRs previously suggested to regulate ghrelin secretion including adrenergic-β1 receptor, GPR81, oxytocin receptor, GPR120, and somatostatin receptor 2 were high in MGN3-1 cells. Consistent with previous reports, isoproterenol and oxytocin stimulated the Gs and Gq pathways, respectively, whereas lactate, palmitate, and somatostatin stimulated the Gi pathway, confirming the reliability of current assays. Among other highly expressed GPCRs, prostaglandin E receptor 4 agonist prostaglandin E2 significantly stimulated the Gs pathway and ghrelin secretion. Muscarine, the canonical agonist of cholinergic receptor muscarinic 4, stimulated both the Gq and Gi pathways. Although muscarine treatment alone did not affect ghrelin secretion, it did suppress forskolin-induced ghrelin secretion, suggesting that the cholinergic pathway may play a role in counterbalancing the stimulation of ghrelin by Gs (eg, by adrenaline). In addition, GPR142 ligand tryptophan stimulated ghrelin secretion. In conclusion, we determined the comprehensive expression profile of GPCRs in ghrelin-producing cells and identified two novel ghrelin regulators, prostaglandin E2 and tryptophan. These results will lead to a greater understanding of the physiology of ghrelin and facilitate the development of ghrelin-modulating drugs.

Ghrelin is a 28-amino acid peptide that regulates GH release together with GHRH and somatostatin. The expression of ghrelin has been detected in the stomach, small intestine, hypothalamus, pituitary gland, kidney, placenta, and testis. Recently it was reported that ghrelin is present in pancreatic alpha-cells and that it stimulates insulin secretion. In this study, we examined the ghrelin expression in two cases of glucagonoma and two cases of insulinoma by Northern blot analysis and immunohistochemistry. Ghrelin expression was identified in a case of glucagonoma associated with multiple endocrine neoplasm type I both by Northern blot analysis using total RNA and by immunohistochemistry, although the plasma ghrelin level was not elevated. This is the first case of tumor in which ghrelin gene expression was detected by Northern blot analysis using total RNA.

Recent studies have shown that sodium–glucose cotransporter 2 inhibitors decrease the risk of heart failure in patients with type 2 diabetes. However, the precise mechanisms of action of these drugs are not well understood. In the present study, we evaluated the effect of treatment with tofogliflozin for 6 months on cardiac and vascular endothelial function in 26 patients with type 2 diabetes and heart diseases. Tofogliflozin treatment significantly decreased left ventricular end‐diastolic dimensions and significantly increased flow‐mediated vasodilation. Although E/e′ did not significantly change after treatment, the decrease observed in the E/e′ ratio was significantly correlated with the increase in acetoacetic acid and 3‐hydroxybutyrate levels. These results suggest that sodium–glucose cotransporter 2 inhibitor might improve left ventricular dilatation and vascular endothelial function in patients with type 2 diabetes. Furthermore, it is suggested that the elevation of ketone bodies induced by sodium–glucose cotransporter 2 inhibitors might contribute to a protective effect in left ventricular diastolic dysfunction.