Insulin resistance in type 2 diabetes is due to impaired stimulation of the glucose transport system in muscle and fat. Different defects are operative in these two target tissues because glucose transporter 4 (GLUT 4) expression is normal in muscle but markedly reduced in fat. In muscle, GLUT 4 is redistributed to a dense membrane compartment, and insulin-mediated translocation to plasma membrane (PM) is impaired. Whether similar trafficking defects are operative in human fat is unknown. Therefore, we studied subcellular localization of GLUT4 and insulin-regulated aminopeptidase (IRAP; also referred to as vp165 or gp160), which is a constituent of GLUT4 vesicles and also translocates to PM in response to insulin. Subcutaneous fat was obtained from eight normoglycemic control subjects (body mass index, 29 ± 2 kg/m2) and eight type 2 diabetic patients (body mass index, 30 ± 1 kg/m2; fasting glucose, 14 ± 1 mm). In adipocytes isolated from diabetics, the basal 3-O-methylglucose transport rate was decreased by 50% compared with controls (7.1 ± 2.9 vs. 14.1 ± 3.7 mmol/mm2 surface area/min), and there was no increase in response to maximal insulin (7.9 ± 2.7 vs. 44.5 ± 9.2 in controls). In membrane subfractions from controls, insulin led to a marked increase of IRAP in the PM from 0.103 ± 0.04 to 1.00± 0.33 relative units/mg protein, concomitant with an 18% decrease in low-density microsomes and no change in high-density microsomes (HDM). In type 2 diabetes, IRAP overall expression in adipocytes was similar to that in controls; however, two abnormalities were observed. First, in basal cells, IRAP was redistributed away from low-density microsomes, and more IRAP was recovered in HDM (1.2-fold) and PM (4.4-fold) from diabetics compared with controls. Second, IRAP recruitment to PM by maximal insulin was markedly impaired. GLUT4 was depleted in all membrane subfractions (43–67%) in diabetes, and there was no increase in PM GLUT4 in response to insulin. Type 2 diabetes did not affect the fractionation of marker enzymes. We conclude that in human adipocytes: 1) IRAP is expressed and translocates to PM in response to insulin; 2) GLUT4 depletion involves all membrane subfractions in type 2 diabetes, although cellular levels of IRAP are normal; and 3) in type 2 diabetes, IRAP accumulates in membrane vesicles cofractionating with HDM and PM under basal conditions, and insulin-mediated recruitment to PM is impaired. Therefore, in type 2 diabetes, adipocytes express defects in trafficking of GLUT4/IRAP-containing vesicles similar to those causing insulin resistance in skeletal muscle.
Insulin resistance in Type 2 (non-insulin-dependent) diabetic patients and their relatives is not associated with a defect in the expression of the insulin-responsive glucose transporter (GLUT-4) gene in human skeletal muscle