Adipocytes exhibit abnormal subcellular distribution and translocation of vesicles containing glucose transporter 4 and insulin-regulated aminopeptidase in type 2 diabetes mellitus: Implications regarding defects in vesicle trafficking

Insulin resistance in type 2 diabetes is due to impaired stimulation of the glucose transport system in muscle and fat. Different defects are operativ e in these two target tissues because glucose transporter 4 (GLUT 4) expres sion is normal in muscle but markedly reduced in fat. In muscle, GLUT 4 is redistributed to a dense membrane compartment, and insulin-mediated translo cation to plasma membrane (PM) is impaired. Whether similar trafficking def ects are operative in human fat is unknown. Therefore, we studied subcellul ar localization of GLUT4 and insulin-regulated aminopeptidase (IRAP; also r eferred to as vp165 or gp160), which is a constituent of GLUT4 vesicles and also translocates to PM in response to insulin. Subcutaneous fat was obtai ned from eight normoglycemic control subjects (body mass index, 29 +/- 2 kg /m(2)) and eight type 2 diabetic patients (body mass index, 30 +/- 1 kg/m(2 ); fasting glucose, 14 +/- 1 mm). In adipocytes isolated from diabetics, th e basal 3-O-methylglucose transport rate was decreased by 50% compared with controls (7.1 +/- 2.9 vs. 14.1 +/- 3.7 mmol/mm(2) surface area/min), and t here 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 r elative units/mg protein, concomitant with an 18% decrease in low-density m icrosomes and no change in high-density microsomes (HDM). In type 2 diabete s, IRAP overall expression in adipocytes was similar to that in controls; h owever, two abnormalities were observed. First, in basal cells, IRAP was re distributed away from low-density microsomes, and more IRAP was recovered i n HDM (1.2-fold) and PM (4.4-fold) from diabetics compared with controls. S econd, IRAP recruitment to PM by maximal insulin was markedly impaired. GLU T4 was depleted in all membrane subfractions (43-67%) in diabetes, and ther e was no increase in PM GLUT4 in response to insulin. Type 2 diabetes did n ot affect the fractionation of marker enzymes. We conclude that in human ad ipocytes: 1) IRAP is expressed and translocates to PM in response to insuli n; 2) GLUT4 depletion involves all membrane subfractions in type 2 diabetes , although cellular levels of IRAP are normal; and 3) in type 2 diabetes, I RAP accumulates in membrane vesicles cofractionating with HDM and PM under basal conditions, and insulin-mediated recruitment to PM is impaired. There fore, in type 2 diabetes, adipocytes express defects in trafficking of GLUT 4/IRAP-containing vesicles similar to those causing insulin resistance in s keletal muscle.