Insulin resistance is central to the pathogenesis of Type II diabetes (NIDDM), and is largely due to decreased activity of the glucose transport system in target tissue. However, the mechanisms responsible for reduced transport activity are unknown. In pre- liminary studies, we found both cellular depletion and impaired function of glucose transport proteins causing insulin resistance in adipocytes isolated from these patients. Therefore, in the current proposal, we will extensively examine the role of glucose transporters in the cellular insulin resistance of NIDDM, by measuring their number, structure, function, and regulation in target tissue. In adipocytes from control and NIDDM subjects, we will use both cytochalasin B binding and immunologic methods to measure the number and cellular distribution of transporters under basal conditions as well as insulin's ability to recruit intracellular carriers to the cell-surface. Function will be assessed by correlating numbers of cell-surface transporters with glucose transport rates in intact cells, and by correlating glucose transporter number and functional activity of transporters isolated from various subcellular and membrane fractions and then reconstituted into synthetic phospholipid vesicles. The Km and Vmax of substrate uptake will also be measured in whole cells since defects in these functional parameters have mechanistic implications. To test for structural abnormalities, we will combine SDS-PAGE and isoelectric focusing to determine the MW, charge heterogeneity, and extent of post-translational glycosylation of photolabeled transporters, as well as measure the non-enzymatic glycosylation of transporters using an antibody specific for glycosyl-lysine adducts. In other studies, we will try to delineate those regulatory events which impair transporter number and function in NIDDM. To determine whether transporter number is regulated at the transcriptional level, we will measure the amount of transporter mRNA in adipocytes and muscle on Northern blots using a hybridizing cDNA probe. We will also test the hypothesis that high concentrations of glucose and insulin regulate transporter number, structure, function, and mRNA levels in vitro using our primary culture method for adipocytes, and in vivo before and after a period of euglycemic therapy. Finally, we will pursue a collaborative project to raise site-specific antibodies against the transporter. Hopefully, these studies will delineate the role of glucose transporters in the mechanism and pathogenesis of insulin resistance in NIDDM, and suggest improved therapeutic strategies.
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