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 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.
Maianu
L
L
Department of Medicine, Medical University of South Carolina and Ralph H. Johnson Veterans Affairs Medical Center, Charleston, South Carolina 29425, USA.
Keller
S R
SR
Garvey
W T
WT
D017667:Adipocytes / Q000378:metabolism* / Q000648:ultrastructure; D000328:Adult; D000626:Aminopeptidases / Q000378:metabolism*; D015153:Blotting, Western; D010122:Cystinyl Aminopeptidase; D003924:Diabetes Mellitus, Type 2 / Q000378:metabolism*; D005260:Female; D005947:Glucose / Q000378:metabolism; D051275:Glucose Transporter Type 4; D006801:Humans; D007150:Immunohistochemistry; D008297:Male; D008861:Microsomes / Q000378:metabolism / Q000648:ultrastructure; D009004:Monosaccharide Transport Proteins / Q000378:metabolism*; D009124:Muscle Proteins; D013347:Subcellular Fractions / Q000378:metabolism* / Q000648:ultrastructure
D051275:Glucose Transporter Type 4; D009004:Monosaccharide Transport Proteins; D009124:Muscle Proteins; C495692:SLC2A4 protein, human; D000626:Aminopeptidases; D010122:Cystinyl Aminopeptidase; C105964:leucyl-cystinyl aminopeptidase; D005947:Glucose