Insulin induces a change in Rab5 subcellular localization in adipocytes independently of phosphatidylinositol 3-kinase activation.
We investigated whether Rab5, a small guanosine triphosphatase that regulates early endocytic transport in different cell types is involved in the insulin-regulated endocytic pathways in adipocytes. Rab5 was detected in freshly isolated adipocytes and 3T3-L1 adipocytes, but its expression level was not markedly increased with adipocyte differentiation. After subcellular fractionation of adipocytes incubated in the absence of insulin, Rab5 was found to be abundant in plasma membrane and cytosol, but was also present in high and low density microsomes. This subcellular distribution was compatible with a role in early endocytosis. When cells were incubated with insulin, the concentration of Rab5 decreased by about 50% in the internal compartments. In contrast to Rab4, which also leaves the low density microsomes in response to insulin, Rab5 was not found in Glut4-containing vesicles purified by immunoadsorption on antibodies to Glut4. When adipocytes were treated with wortmannin, an inhibitor of phosphatidylinositol 3-kinase, the effect of insulin on Rab5 movement was not affected, whereas the insulin-induced movements of Rab4 and Glut4 were abolished. In parallel, wortmannin inhibited the increase in horseradish peroxidase uptake induced by insulin, an index of fluid phase endocytosis, but did not prevent the endocytosis of the glucose transporters. As a whole, our results suggest that Rab5 is not involved in insulin-stimulated Glut4 exocytosis. These results are compatible with the postulated role of Rab5 in the endocytotic pathway, at a step that does not require phosphatidyl-inositol 3-kinase activation.
Cormont
M
M
INSERM U-145, Faculty of Medicine, Nice, France.
Van Obberghen
E
E
Zerial
M
M
Le Marchand-Brustel
Y
Y
D016475:3T3 Cells; D017667:Adipocytes / Q000166:cytology / Q000378:metabolism*; D000730:Androstadienes / Q000494:pharmacology; D000818:Animals; D002454:Cell Differentiation; D002469:Cell Separation; D004705:Endocytosis / Q000187:drug effects; D004789:Enzyme Activation; D020558:GTP Phosphohydrolases / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism*; D051275:Glucose Transporter Type 4; D007328:Insulin / Q000494:pharmacology*; D051379:Mice; D009004:Monosaccharide Transport Proteins / Q000378:metabolism; D009124:Muscle Proteins; D019869:Phosphatidylinositol 3-Kinases; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000037:antagonists & inhibitors / Q000378:metabolism*; D013347:Subcellular Fractions / Q000378:metabolism*; D000077191:Wortmannin; D020696:rab5 GTP-Binding Proteins
D000730:Androstadienes; D051275:Glucose Transporter Type 4; D007328:Insulin; D009004:Monosaccharide Transport Proteins; D009124:Muscle Proteins; C495693:Slc2a4 protein, mouse; D019869:Phosphatidylinositol 3-Kinases; D017853:Phosphotransferases (Alcohol Group Acceptor); D020558:GTP Phosphohydrolases; D019204:GTP-Binding Proteins; D020696:rab5 GTP-Binding Proteins; D000077191:Wortmannin