Secretion of gastric inhibitory polypeptide (GIP), a duodenal hormone, is primarily induced by absorption of ingested fat. Here we describe a novel pathway of obesity promotion via GIP. Wild-type mice fed a high-fat diet exhibited both hypersecretion of GIP and extreme visceral and subcutaneous fat deposition with insulin resistance. In contrast, mice lacking the GIP receptor (Gipr(-/-)) fed a high-fat diet were clearly protected from both the obesity and the insulin resistance. Moreover, double-homozygous mice (Gipr(-/-), Lep(ob)/Lep(ob)) generated by crossbreeding Gipr(-/-) and obese ob/ob (Lep(ob)/Lep(ob)) mice gained less weight and had lower adiposity than Lep(ob)/Lep(ob) mice. The Gipr(-/-) mice had a lower respiratory quotient and used fat as the preferred energy substrate, and were thus resistant to obesity. Therefore, GIP directly links overnutrition to obesity and it is a potential target for anti-obesity drugs.

Reactive oxygen species are involved in a diversity of biological phenomena such as inflammation, carcinogenesis, aging, and atherosclerosis. We and other investigators have shown that the level of 8-hydroxy-2'-deoxyguanosine (8-OHdG), a marker for oxidative stress, is increased in either the urine or the mononuclear cells of the blood of type 2 diabetic patients. However, the association between type 2 diabetes and oxidative stress in the pancreatic beta-cells has not been previously described. We measured the levels of 8-OHdG and 4-hydroxy-2-nonenal (HNE)-modified proteins in the pancreatic beta-cells of GK rats, a model of nonobese type 2 diabetes. Quantitative immunohistochemical analyses with specific antibodies revealed higher levels of 8-OHdG and HNE-modified proteins in the pancreatic beta-cells of GK rats than in the control Wistar rats, with the levels increasing proportionally with age and fibrosis of the pancreatic islets. We further investigated whether the levels of 8-OHdG and HNE-modified proteins would be modified in the pancreatic beta-cells of GK rats fed with 30% sucrose solution or 50 ppm of voglibose (alpha-glucosidase inhibitor). In the GK rats, the levels of 8-OHdG and HNE-modified proteins, as well as islet fibrosis, were increased by sucrose treatment but reduced by voglibose treatment. These results indicate that the pancreatic beta-cells of GK rats are oxidatively stressed, and that chronic hyperglycemia might be responsible for the oxidative stress observed in the pancreatic beta-cells.

Mice with a targeted mutation of the gastric inhibitory polypeptide (GIP) receptor gene (GIPR) were generated to determine the role of GIP as a mediator of signals from the gut to pancreatic ␤ cells. GIPR؊͞؊ mice have higher blood glucose levels with impaired initial insulin response after oral glucose load. Although blood glucose levels after meal ingestion are not increased by high-fat diet in GIPR؉͞؉ mice because of compensatory higher insulin secretion, they are significantly increased in GIPR؊͞؊ mice because of the lack of such enhancement. Accordingly, early insulin secretion mediated by GIP determines glucose tolerance after oral glucose load in vivo, and because GIP plays an important role in the compensatory enhancement of insulin secretion produced by a high insulin demand, a defect in this entero-insular axis may contribute to the pathogenesis of diabetes.

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We previously reported the cloning of two distinct somatostatin receptor (SSTR) subtypes, SSTR1 and SSTR2. Although both SSTR1 and SSTR2 bound somatostatin specifically and with high affinity, neither was coupled to adenylyl cyclase, a major cellular effector of somatostatin's actions. Here we report the cloning and functional characterization of a third member of the SSTR family. Human SSTR3 is a protein of 418 amino acids and has 45% and 46% identity with human SSTR1 and SSTR2, respectively. RNA blotting studies showed that SSTR3 mRNA could be readily detected in brain and pancreatic islets. The pharmacological properties of human SSTR3 were characterized by transiently expressing the human SSTR3 gene in COS-1 cells. Membranes from cells expressing human SSTR3 bound the somatostatin agonist [125I]CGP 23996 specifically and with high affinity, with a rank order of potency of somatostatin-28 = CGP 23996 > somatostatin-14 > SMS-201-995. Studies using cells transiently coexpressing the human dopamine D1 receptor and human SSTR3 showed that somatostatin was able to inhibit dopamine-stimulated cAMP formation in a dose-dependent manner, indicating that SSTR3 was functionally coupled to adenylyl cyclase. These results indicate that the diverse biological effects of somatostatin are mediated by a family of receptor with distinct, but overlapping, tissue distributions, unique pharmacological properties, and potentially different functions.

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We have shown recently that oxidative stress by chronic hyperglycemia damages the pancreatic L L-cells of GK rats, a model of non-obese type 2 diabetes, which may worsen diabetic condition and suggested the administration of antioxidants as a supportive therapy. To determine if natural antioxidant K K-tocopherol (vitamin E) has beneficial effects on the glycemic control of type 2 diabetes, GK rats were fed a diet containing 0, 20 or 500 mg/kg diet K K-tocopherol. Intraperitoneal glucose tolerance test revealed a significant increment of insulin secretion at 30 min and a significant decrement of blood glucose levels at 30 and 120 min after glucose loading in the GK rats fed with high K K-tocopherol diet. The levels of glycated hemoglobin A1c, an indicator of glycemic control, were also reduced. Vitamin E supplementation clearly ameliorated diabetic control of GK rats, suggesting the importance of not only dietary supplementation of natural antioxidants but also other antioxidative intervention as a supportive therapy of type 2 diabetic patients.z 2000 Federation of European Biochemical Societies.

Mutations in the hepatocyte nuclear factor (HNF)-1␣ gene have been linked to subtype 3 of maturity-onset diabetes of the young (MODY), a disease characterized by a primary defect in insulin secretion. Here we show that the human GLUT2 gene is closely regulated by HNF-1␣ via sequences downstream of the transcriptional start site by interaction with transcriptional co-activator p300. The promoter region of the human GLUT2 gene was subcloned into luciferase expression plasmids that were transfected together with HNF-1␣ expression plasmid into a pancreatic ␤-cell line, HIT-T15, to evaluate transcriptional activities. HNF-1␣ enhanced human GLUT2 promoter activity sixfold. Sitedirect mutagenesis and footprint analyses showed that the HNF-1␣ binding site (؉200 to ؉218) is critical in human GLUT2 gene expression. Furthermore, mammalian two-hybrid and immunoprecipitation studies revealed the transactivation domain of HNF-1␣ (amino acids 391-540) to interact with both the NH 2 -terminal region (amino acids 180 -662) and the COOH-terminal region (amino acids 1,818 -2,079) of p300. These findings demonstrated that HNF-1␣ binds to the 5-untranslated region of GLUT2 and that p300 acts as a transcriptional co-activator for HNF-1␣. In addition, these results provided new insight into the regulatory function of HNF-1␣ by suggesting a molecular basis for human GLUT2 gene expression.