BIPHASIC ALTERATION OF GLUCOSE-TRANSPORT IN 3T3-L1 CELLS DURING DIFFERENTIATION TO THE ADIPOCYTE-LIKE PHENOTYPE

Glucose transport activity and subcellular distribution of glucose transporters GLUT1 and GLUT4 were studied in non-confluent (NCF), confluent (CF), and differentiated 3T3-L1 cells (A). During growth of the fibroblasts to confluence, basal transport activity decreased to 20 % of that in non-confluent cells. Corresponding with the reduction in transport activity, the abundance of GLUT1 in plasma membranes as normalized per cell decreased by 75 % during growth of the cells to confluence. This effect was mainly due to a reduction of total cellular GLUT1. In addition, the portion of GLUT1 located in intracellular vesicles (low-density microsomes) was moderately increased in confluent cells, and was further increased in cells differentiated to the adipocyte-like phenotype (in NCF 11 %, in CF 24.5 %, and in A 60 % of the total GLUT1). GLUT4, in contrast, was approximately 10-times more abundant in low-density microsomes than in the plasma membranes of the differentiated cells. Insulin failed to stimulate glucose transport activity in non-confluent cells but produced an approximately 2-fold stimulation in confluent cells, probably through translocation of the GLUT1 from the intracellular compartment to the plasma membrane. In the differentiated adipocytes, insulin stimulated a 10-fold increase in glucose transport activity, the maximum levels approaching basal transport rates of non-confluent cells: both GLUT1 and GLUT4 were translocated in response to insulin. These data indicate that insulin sensitivity in 3T3-L1 cells develops in a biphasic pattern. In confluent fibroblasts, a moderate effect of insulin conferred exclusively by GLUT1 is detectable, probably reflecting the small intracellular compartment of GLUT1. In differentiated cells, the effect of insulin reflects translocation of both GLUT1 and GLUT4, but appears to be mainly conferred by the large intracellular compartment of the GLUT4.