| Non-insulin dependent diabetes mellitus (NIDDM) arises from metabolic defects associated with insulin resistance. In order to identify the molecular defects responsible for insulin resistance, we have investigated three different model systems: the ability of sulfonylurea receptor (SUR) ligands to modify insulin regulated events in cultured rodent adipocytes; the ability of agouti and/or agouti related polypeptide (AGRP) to modify insulin-regulated events; and the regulation of the activity and amount of the HNF-1α transcription factor, mutations of which result in a subset of NIDDM, by variations in glucose concentration.; Diazoxide and glibenclamide are both SUR ligands and anti-diabetic drugs. Our results suggest that diazoxide has no direct effect on insulin signaling and glucose transport. In contrast, glibenclamide exhibited dual effects on glucose uptake. Acute exposure to glibenclamide resulted in inhibition of glucose uptake, while long-term incubation with glibenclamide enhanced glucose uptake independent of insulin action. Diazoxide was unable to antagonize either effect of glibenclamide, suggesting that the effects of glibenclamide were not mediated by the sulfonylurea receptor.; Ectopic expression of the Agouti gene results in late-onset obesity and type II diabetes. We have investigated the effects of agouti and AGRP on insulin signaling and its biological responses in cultured 3T3-L1 adipocytes. We observed no effects of either agouti or AGRP on glucose uptake or fatty acid synthase (FAS) activity. In contrast, insulin stimulates glucose uptake in a dose-dependent manner and induces FAS activity. In addition, the insulin-stimulated protein tyrosine phosphorylation and activation of downstream kinases were not affected by either agouti or AGRP.; Mutations in HNF-1 genes result in MODY3 (a subtype of NIDDM). Examination of hepatonuclear extracts prepared from the obese Zucker rats revealed that HNF-1 binding activity was reduced. However, the reduction is not due to diminished HNF-1 protein levels. Incubating Fao rat hepatoma cells with high concentrations of glucose resulted in a similar decrease in HNF-1 binding activity, with no changes in HNF-1 protein levels. The results with the hepatoma cells reflect the observation made with the obese Zucker rats and point to nutritional regulation of HNF-1 activity in the liver. |
