Exogenous and endogenous ghrelin counteracts GLP-1 action to stimulate cAMP signaling and insulin secretion in islet ß-cells.

We studied interactive effects of insulinotropic GLP-1 and insulinostatic ghrelin on rat pancreatic islets. GLP-1 potentiated glucose-induced insulin release and cAMP production in isolated islets and [Ca(2+)](i) increases in single ß-cells, and these potentiations were attenuated by ghrelin. Ghrelin suppressed [Ca(2+)](i) responses to an adenylate cyclase activator forskolin. Moreover, GLP-1-induced insulin release and cAMP production were markedly enhanced by [D-lys(3)]-GHRP-6, a ghrelin receptor antagonist, in isolated islets. These results indicate that both exogenous and endogenous islet-derived ghrelin counteracts glucose-dependent GLP-1 action to increase cAMP production, [Ca(2+)](i) and insulin release in islet ß-cells, positioning ghrelin as a modulator of insulinotropic GLP-1.

Ghrelin attenuates cAMP-PKA signaling to evoke insulinostatic cascade in islet ß-cells.

OBJECTIVE: Ghrelin reportedly restricts insulin release in islet ß-cells via the Gα(i2) subtype of G-proteins and thereby regulates glucose homeostasis. This study explored whether ghrelin regulates cAMP signaling and whether this regulation induces insulinostatic cascade in islet ß-cells. RESEARCH DESIGN AND METHODS: Insulin release was measured in rat perfused pancreas and isolated islets and cAMP production in isolated islets. Cytosolic cAMP concentrations ([cAMP](i)) were monitored in mouse MIN6 cells using evanescent-wave fluorescence imaging. In rat single ß-cells, cytosolic protein kinase-A activity ([PKA](i)) and Ca(2+) concentration ([Ca(2+)](i)) were measured by DR-II and fura-2 microfluorometry, respectively, and whole cell currents by patch-clamp technique. RESULTS: Ghrelin suppressed glucose (8.3 mmol/L)-induced insulin release in rat perfused pancreas and isolated islets, and these effects of ghrelin were blunted in the presence of cAMP analogs or adenylate cyclase inhibitor. Glucose-induced cAMP production in isolated islets was attenuated by ghrelin and enhanced by ghrelin receptor antagonist and anti-ghrelin antiserum, which counteract endogenous islet-derived ghrelin. Ghrelin inhibited the glucose-induced [cAMP](i) elevation and [PKA](i) activation in MIN6 and rat ß-cells, respectively. Furthermore, ghrelin potentiated voltage-dependent K(+) (Kv) channel currents without altering Ca(2+) channel currents and attenuated glucose-induced [Ca(2+)](i) increases in rat ß-cells in a PKA-dependent manner. CONCLUSIONS: Ghrelin directly interacts with islet ß-cells to attenuate glucose-induced cAMP production and PKA activation, which lead to activation of Kv channels and suppression of glucose-induced [Ca(2+)](i) increase and insulin release.

Lack of TRPM2 impaired insulin secretion and glucose metabolisms in mice.

OBJECTIVE: TRPM2 is a Ca²(+)-permeable nonselective cation channel activated by adenosine dinucleotides. We previously demonstrated that TRPM2 is activated by coapplication of heat and intracellular cyclic adenosine 5'-diphosphoribose, which has been suggested to be involved in intracellular Ca²(+) increase in immunocytes and pancreatic ß-cells. To clarify the involvement of TRPM2 in insulin secretion, we analyzed TRPM2 knockout (TRPM2-KO) mice. RESEARCH DESIGN AND METHODS: Oral and intraperitoneal glucose tolerance tests (OGTT and IPGTT) were performed in TRPM2-KO and wild-type mice. We also measured cytosolic free Ca²(+) in single pancreatic cells using fura-2 microfluorometry and insulin secretion from pancreatic islets. RESULTS: Basal blood glucose levels were higher in TRPM2-KO mice than in wild-type mice without any difference in plasma insulin levels. The OGTT and IPGTT demonstrated that blood glucose levels in TRPM2-KO mice were higher than those in wild-type mice, which was associated with an impairment in insulin secretion. In isolated ß-cells, smaller intracellular Ca²(+) increase was observed in response to high concentrations of glucose and incretin hormone in TRPM2-KO cells than in wild-type cells. Moreover, insulin secretion from the islets of TRPM2-KO mice in response to glucose and incretin hormone treatment was impaired, whereas the response to tolbutamide, an ATP-sensitive potassium channel inhibitor, was not different between the two groups. CONCLUSIONS: These results indicate that TRPM2 is involved in insulin secretion stimulated by glucose and that further potentiated by incretins. Thus, TRPM2 may be a new target for diabetes therapy.

Stressor-responsive central nesfatin-1 activates corticotropin-releasing hormone, noradrenaline and serotonin neurons and evokes hypothalamic-pituitary-adrenal axis.

A recently discovered satiety molecule, nesfatin-1, is localized in neurons of the hypothalamus and brain stem and colocalized with stress-related substances, corticotropin-releasing hormone (CRH), oxytocin, proopiomelanocortin, noradrenaline (NA) and 5-hydroxytryptamine (5-HT). Intracerebroventricular (icv) administration of nesfatin-1 produces fear-related behaviors and potentiates stressor-induced increases in plasma adrenocorticotropic hormone (ACTH) and corticosterone levels in rats. These findings suggest a link between nesfatin-1 and stress. In the present study, we aimed to further clarify the neuronal network by which nesfatin-1 could induce stress responses in rats. Restraint stress induced c-Fos expressions in nesfatin-1-immunoreactive neurons in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus, and in the nucleus of solitary tract (NTS), locus coeruleus (LC) and dorsal raphe nucleus (DR) in the brain stem, without altering plasma nesfatin-1 levels. Icv nesfatin-1 induced c-Fos expressions in the PVN, SON, NTS, LC, DR and median raphe nucleus, including PVN-CRH, NTS-NA, LC-NA and DR-5-HT neurons. Nesfatin-1 increased cytosolic Ca2+ concentration in the CRH-immunoreactive neurons isolated from PVN. Icv nesfatin-1 increased plasma ACTH and corticosterone levels. These results indicate that the central nesfatin-1 system is stimulated by stress and activates CRH, NA and 5-HT neurons and hypothalamic-pituitary-adrenal axis, evoking both central and peripheral stress responses.

Reconstruction-dependent recovery from anorexia and time-related recovery of regulatory ghrelin system in gastrectomized rats.

Gastrectomy reduces food intake and body weight (BW) hampering recovery of physical conditions. It also reduces plasma levels of stomach-derived orexigenic ghrelin. This study explored changes in orexigenic ghrelin system in rats receiving total gastrectomy with Billroth II (B-II) or Roux-en-Y (R-Y) method. Feeding and BW were reduced by gastrectomy and subsequently recovered to a greater extent with R-Y than B-II while plasma ghrelin decreased similarly. At postoperative 12th week, ghrelin contents increased in the duodenum and pancreas, plasma ghrelin levels increased upon fasting, and ghrelin injection promoted feeding but not in earlier periods. In summary, gastrectomized rats partially recover feeding and BW, in a reconstruction-dependent manner. At 12th week, ghrelin is upregulated in extra-stomach tissues, plasma ghrelin levels are physiologically regulated, and orexigenic effect of exogenous ghrelin is restored. This time-related recovery of ghrelin system may provide a strategy for promoting feeding, BW, and thereby physical conditions in gastrectomized patients.

Ghrelin regulates insulin release and glycemia: physiological role and therapeutic potential.

Insulin release from pancreatic islet beta-cells is stimulated by glucose. Glucose-induced insulin release is potentiated or suppressed by hormones and neural substances. Ghrelin, a novel acylated 28-amino acid peptide isolated from stomach, is the endogenous ligand for the growth hormone (GH) secretagogue-receptor (GHS-R). Circulating ghrelin is produced predominantly in stomach. Ghrelin is a potent stimulator of GH release and feeding as well as exhibiting positive cardiovascular effects. In relation to the glucose metabolism, initial studies indicated that low plasma ghrelin levels are associated with elevated fasting insulin levels, insulin resistance, and obesity. It has recently been demonstrated that ghrelin suppresses glucose-induced insulin release via G alpha(i2) subtype of GTP-binding proteins and delayed outward K(+) (Kv) channels, representing a novel signaling mechanism, and that the ghrelin originating from islets regulates insulin release and thereby glycemia. Furthermore, elimination of ghrelin enhances insulin release to prevent or ameliorate glucose intolerance in high-fat diet fed mice and ob/ob mice. This review focuses on the physiological roles of ghrelin in regulating insulin release and glycemia, the insulinostatic mechanisms of ghrelin in islet beta-cells, and the potential of ghrelin-GHS-R system as the therapeutic target to treat type 2 diabetes.