The gastric hormone ghrelin and its receptor, growth hormone secretagogue receptor (GHSR), are expressed in pancreas. Here, we report that ghrelin is released from pancreatic islets to regulate glucose-induced insulin release. Plasma concentrations of ghrelin, as well as insulin, were higher in pancreatic veins than in arteries. GHSR antagonist and immunoneutralization of endogenous ghrelin enhanced glucose-induced insulin release from perfused pancreas, whereas exogenous ghrelin suppressed it. GHSR antagonist increased plasma insulin levels in gastrectomized and normal rats to a similar extent. Ghrelin knockout mice displayed enhanced glucose-induced insulin release from isolated islets, whereas islet density, size, insulin content, and insulin mRNA levels were unaltered. Glucose tolerance tests (GTTs) in ghrelin knockout mice showed increased insulin and decreased glucose responses. Treatment with high-fat diet produced glucose intolerance in GTTs in wild-type mice. In ghrelin knockout mice, the high-fat diet-induced glucose intolerance was largely prevented, whereas insulin responses to GTTs were markedly enhanced. These findings demonstrate that ghrelin originating from pancreatic islets is a physiological regulator of glucose-induced insulin release. Antagonism of the ghrelin function can enhance insulin release to meet increased demand for insulin in high-fat diet-induced obesity and thereby normalize glycemic control, which may provide a potential therapeutic application to counteract the progression of type 2 diabetes. Diabetes 55: 3486 -3493, 2006

Aims/hypothesis: During the pathogenesis of type 2 diabetes insulin resistance causes compensatory proliferation of beta cells. As beta cells have a limited replication potential, this compensatory proliferation might accelerate cellular senescence and lead to diabetes. We examined the cellular senescence of beta cells after proliferation during lipoglucotoxicity. Methods: Senescence-associated markers in beta cells were examined in nutrient-induced diabetic C57BL/6J mice that were fed a high-fat diet. After 4 and 12 months of the high-fat diet, intraperitoneal glucose tolerance tests (IPGTTs) and histochemical analyses of Ki-67, p38, senescence-associated beta-galactosidase, and beta cell mass were performed. Results: At 4 months, the AUC for plasma insulin levels during the IPGTT (AUC insulin ) was higher, beta cell mass was 3.1-fold greater, and the proliferation of beta cells was 2.2-fold higher than in the control group. However, at 12 months, AUC insulin declined, the frequency of Ki-67-positive beta cells decreased to one-third that of the control group, and the senescence-associated, beta-galactosidasepositive area increased to 4.7-fold that of the control group. Moreover, small amounts of p38, which is induced by oxidative stress and mediates cellular senescence, were found in beta cells from the high-fat diet group, but not in beta cells from the control group. Furthermore, the senescence-associated, beta-galactosidase-positive area in the high-fat diet group had a highly significant negative correlation with AUC insulin (r=−0.852, p<0.01). Conclusions/interpretation: Beta cell senescence occurred in diet-induced type 2 diabetes and led to insufficient insulin release. These findings suggest that cellular senescence contributes to the pathogenesis of diet-induced diabetes.

Yeast flocculation is a phenomenon which is believed to result from an interaction between a lectin-like protein and a mannose chain located on the yeast cell surface. The FLO1 gene, which encodes a cell wall protein, is considered to play an important role in yeast flocculation, which is inhibited by mannose but not by glucose (mannose-specific flocculation). A new homologue ofFLO1, named Lg-FLO1, was isolated from a flocculent bottom-fermenting yeast strain in which flocculation is inhibited by both mannose and glucose (mannose/glucose-specific flocculation). In order to confirm that both FLO1 and Lg-FLO1 are involved in the yeast flocculation phenomenon, the FLO1 gene in the mannose-specific flocculation strain was replaced by the Lg-FLO1 gene. The transformant in which the Lg-FLO1 gene was incorporated showed the same flocculation phenotype as the mannose/glucose-specific flocculation strain, suggesting that the FLO1 and Lg-FLO1genes encode mannose-specific and mannose/glucose-specific lectin-like proteins, respectively. Moreover, the sugar recognition sites for these sugars were identified by expressing chimeric FLO1 and Lg-FLO1 genes. It was found that the region from amino acid 196 to amino acid 240 of both gene products is important for flocculation phenotypes. Further mutational analysis of this region suggested that Thr-202 in the Lg-Flo1 protein and Trp-228 in the Flo1 protein are involved in sugar recognition.

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