The translocation of the glucose transporter sub-types GLUT1 and GLUT4 in isolated fat cells is differently regulated by phorbol esters.

Insulin stimulates glucose transport in isolated fat cells by activation of glucose transporters in the plasma membranes and through translocation of the glucose transporter sub-types GLUT4 (insulin-regulatable) and GLUT1 (HepG2 transporter). The protein kinase C-stimulating phorbol ester phorbol 12-myristate 13-acetate (PMA) is able to mimic partially the effect of insulin on glucose transport, apparently through stimulation of carrier translocation. In order to ascertain whether protein kinase C is involved in the translocation signal to both carrier sub-types, we determined the effect of PMA on the subcellular distribution of GLUT1 and GLUT4 by immunoblotting with specific antibodies directed against these transporters. Isolated rat fat cells (4 x 10(6) cells/ml) were stimulated for 20 min with insulin (6 nM) or PMA (1 nM). 3-O-Methylglucose transport was determined and plasma membranes and low-density microsomes were prepared for Western blotting. 3-O-Methylglucose transport was stimulated 8-9-fold by insulin, and 3-4-fold by PMA (basal, 5.6 +/- 2.3%; insulin, 43.6 +/- 7.3%; PMA, 18.4 +/- 4.9%, n = 9). PMA was able to increase the amount of GLUT4 in the plasma membrane fraction by 2.5(+/- 0.9)-fold (n = 6) whereas insulin stimulation was 4.4(+/- 1.7)-fold (n = 6), paralleled by a corresponding decrease of transport in the low-density microsomes (insulin, 50 +/- 5% of basal; PMA, 63 +/- 11% of basal, n = 6). Although PMA regulates the translocation of GLUT4, it has no effect on GLUT1 in the same cell fractions (increase in plasma membranes: insulin, 1.7 +/- 0.5-fold; PMA, 0.91 +/- 0.1-fold, n = 4; decrease in low-density microsomes: insulin, 53 +/- 11% of basal; PMA, 101 +/- 5% of basal, n = 4). These data are in favour of a role for protein kinase C in signal transduction to GLUT4 but not to GLUT1 in fat cells.