The glucose transporter in 3T3-L1 adipocytes is phosphorylated in response to phorbol ester but not in response to insulin.

At maximally active concentrations with 20-min exposure, insulin and phorbol myristate acetate (PMA) stimulated hexose transport in 3T3-L1 adipocytes by 11- and 2-fold, respectively. The potential role of phosphorylation of the glucose transporter (GT) in these stimulations was investigated by the isolation of GT through immunoprecipitation from ortho[32P]phosphate-labeled 3T3-L1 adipocytes. It was found that there was no significant 32P incorporation into GT from basal adipocytes after 2- or 18 h-labeling in the presence of 0.5 mCi of 32Pi/ml. Furthermore, under these labeling conditions, insulin treatment for 1, 4, or 30 min failed to stimulate the phosphorylation of GT. Also, there was no detectable phosphate incorporation into GT upon reversal of insulin-stimulated hexose transport by the removal of insulin (half-time for reversal approximately 8 min). In contrast to these results, exposure of adipocytes to PMA (1 microM) for 20 min elicited a phosphorylation of GT to the extent of about 0.1 phosphate/GT molecule. Exposure of cells to both insulin and PMA resulted in a 3-fold increase in the level of phosphate in GT compared to that seen with PMA alone. Possibly this increase is due to the translocation of GT to the plasma membrane where it is a better substrate for activated protein kinase C. Stimulation of hexose transport was the same with the combined treatment of insulin and PMA compared to that seen with insulin alone. These results indicate that neither a change in the phosphorylation state of the GT nor activation of protein kinase C is involved in the mechanism by which the insulin receptor stimulates glucose transport.