REGULATION OF PROTEIN-KINASE-B AND GLYCOGEN-SYNTHASE KINASE-3 BY INSULIN AND BETA-ADRENERGIC AGONISTS IN RAT EPIDIDYMAL FAT-CELLS - ACTIVATION OF PROTEIN-KINASE-B BY WORTMANNIN-SENSITIVE AND WORTMANNIN-INSENSITTVE MECHANISMS

Previous studies using L6 myotubes have suggested that glycogen syntha se kinase-3 (GSK-3) is phosphoryl ated and inactivated in response to insulin by protein kinase B (PKB, also known as Akt or RAG) (Cross, D, A, E., Alessi, D, R., Cohen, P., Andjelkovic, M., and Hemmings, B, A. (1995) Nature 378, 785-789), In the present study, marked increases i n the activity of PKB have been shown to occur in insulin-treated rat epididymal fat cells with a time course compatible with the observed d ecrease in GSK-3 activity, Isoproterenol, acting primarily through bet a(3)-adrenoreceptors, was found to decrease GSK-3 activity to a simila r extent (approximately 50%) to insulin, However, unlike the effect of insulin, the inhibition of GSK by isoproterenol was not found to be s ensitive to inhibition by the phosphatidylinositol 3'-kinase inhibitor s, wortmannin or LY 294002, The change in GSK-3 activity brought about by isoproterenol could not be mimicked by the addition of permeant cy clic AMP analogues or forskolin to the cells, although at the concentr ations used, these agents were able to stimulate lipolysis. Isoprotere nol, but again not the cyclic AMP analogues, was found to increase the activity of PKB, although to a lesser extent than insulin. While wort mannin abolished the stimulation of PKB activity by insulin, it was wi thout effect on the activation seen in response to isoproterenol, The activation of PKB by isoproterenol was not accompanied by any detectab le change in the electrophoretic mobility of the protein on SDS-polyac rylamide gel electrophoresis. It would therefore appear that distinct mechanisms exist for the stimulation of PKB by insulin and isoproteren ol in rat fat cells.