Fractionation Analysis of the Subcellular Distribution of GLUT-4 in 3T3-L1 Adipocytes

In 1980, two groups simultaneously provided evidence of the existence of an intracellular pool of glucose transporters in rat adipocytes (1 ,2 ). We now know that facilitative glucose uptake occurs through a family of highly related integral membrane proteins that share significant sequence similarity. Of the established glucose transporter isoforms, GLUT-4 is highly expressed in adipose tissue and striated muscle (3 ). In the basal state, GLUT-4 cycles slowly between the plasma membrane and one or more intracellular compartments, with the vast majority of the transporter residing in vesicular compartments within the cell interior (4 –6 ). Activation of the insulin receptor triggers a large increase in the rate of GLUT-4 vesicle exocytosis and a smaller decrease in the rate of internalization by endocytosis (7 –10 ). The stimulation of exocytosis by insulin is probably the major step for GLUT-4 translocation because complete inhibition of GLUT-4 endocytosis only modestly increases plasma membrane-associated GLUT-4 protein without affecting the extent of insulin-stimulated GLUT-4 translocation (11 –13 ). In contrast to GLUT-4, GLUT-1 is an intracellular and plasma membrane localized in the basal state and displays a modest insulin-stimulated redistribution to the plasma membrane. Thus, the overall insulin-dependent shift in the cellular dynamics of GLUT-4 vesicle trafficking results in a net increase of GLUT-4 on the cell surface, thereby increasing the rate of glucose uptake.