Abstract
Staurosporine, a widely used protein kinase C inhibitor, completely inhibited both phorbol 12-myristate 13-acetate (PMA)- and insulin-stimulated glucose transport activity in isolated rat adipocytes. The inhibition was non-competitive and was attributed to a blockade of the PMA- and insulin-induced translocation of both GLUT1 and GLUT4 glucose transporters. The PMA-stimulated glucose transport activity was more sensitive to inhibition by staurosporine than was insulin-stimulated transport activity (PMA, IC50 = 1.1 +/- 0.1 microM; insulin, IC50 = 6.4 +/- 0.7 microM; P < 0.05, n = 3). At 1 microM staurosporine the insulin-sensitivity was decreased, i.e. EC50 increased from 0.12 nM to 5.4 nM, but the maximum response to insulin and the time course for stimulation were unaffected. At 6 microM staurosporine the insulin-sensitivity was further decreased, the maximal stimulation was decreased by 25%, and the apparent half-time for stimulation was extended from 2.5 min in control cells to 9.4 min. Staurosporine (30 microM) was able to block insulin's ability to stimulate glucose transport, whether added before or after insulin, by a mechanism that did not alter the rate of GLUT4 internalization. In intact adipose cells, staurosporine (30 microM) induced a slight (30%) decrease in the maximal insulin-induced receptor autophosphorylation and a similar decrease in the tyrosine phosphorylation of pp60 and pp160 (insulin-receptor substrate-1: 'IRS-1'), but was without effect on insulin binding to its receptor. Conversely, staurosporine induced a concentration-dependent inhibition of the constitutively tyrosine-phosphorylated (pp120) protein and of an insulin-stimulated protein pp53 in the cytosol. The locus of staurosporine's action appears to be distal from the initial insulin-receptor signalling, at a step that regulates the specific translocation of the glucose transporters to the plasma membranes.