EVIDENCE FOR DEFECTS IN THE TRAFFICKING AND TRANSLOCATION OF GLUT4 GLUCOSE TRANSPORTERS IN SKELETAL-MUSCLE AS A CAUSE OF HUMAN INSULIN-RESISTANCE

Insulin resistance is instrumental in the pathogenesis of type 2 diabe tes mellitus, and the Insulin Resistance Syndrome. While insulin resis tance involves decreased glucose transport activity in skeletal muscle , its molecular basis is unknown. Since muscle GLUT4 glucose transport er levels are normal in type 2 diabetes, we have tested the hypothesis that insulin resistance is due to impaired translocation of intracell ular GLUT4 to sarcolemma. Both insulin-sensitive and insulin-resistant nondiabetic subgroups were studied, in addition to type 2 diabetic pa tients. Biopsies were obtained from basal and insulin-stimulated muscl e, and membranes were subfractionated on discontinuous sucrose density gradients to equilibrium or under nonequilibrium conditions after a s hortened centrifugation time. In equilibrium fractions from basal musc le, GLUT4 was decreased by 25-29% in both 25 and 28% sucrose density f ractions and increased twofold in both the 32% sucrose fraction and bo ttom pellet in diabetics compared with insulin-sensitive controls, wit hout any differences in membrane markers (phospholemman, phosphalamban , dihydropyridine-binding complex alpha-1 subunit). Thus, insulin resi stance was associated with redistribution of GLUT4 to denser membrane vesicles, Na effects of insulin stimulation on GLUT4 localization were observed. In non-equilibrium fractions, insulin led to small GLUT4 de crements in the 25 and 28% sucrose fractions and increased GLUT4 in th e 32% sucrose fraction by 2.8-fold over basal in insulin-sensitive hut only by 1.5-fold in both insulin-resistant and diabetic subgroups. Th e GLUT4 increments in the 32% sucrose fraction were correlated with ma ximal in vivo glucose disposal rates (r = +0.51, P = 0.026), and, ther efore, represented GLUT4 recruitment to sarcolemma or a quantitative m arker for this process. Similar to GLUT4, the insulin-regulated aminop eptidase (vp165) was redistributed to a dense membrane compartment and did not translocate in response to insulin in insulin-resistant subgr oups. In conclusion, insulin alters the subcellular localization of GL UT4 vesicles in human muscle, and this effect is impaired equally in i nsulin-resistant subjects with and without diabetes. This translocatio n defect is associated with abnormal accumulation of GLUT4 in a dense membrane compartment demonstrable in basal muscle, We have previously observed a similar pattern of defects causing insulin resistance in hu man adipocytes, Based on these data, we propose that human insulin res istance involves a defect in GLUT traffic and targeting leading to acc umulation in a dense membrane compartment from which insulin is unable to recruit GLUT4 to the cell surface.