INTERACTIONS OF PROTEIN-KINASE-C WITH INSULIN SIGNALING - INFLUENCE ON GAP AND SOS ACTIVITIES

In this study, we investigated the influence of the protein kinase C ( PKC)-dependent system upon the ability of insulin to stimulate p21(ras )-GTP loading in 3T3-L1 adipocytes, Activation of PRC by 12-0-tetradec anoyl-phorbol-13-acetate (TPA) did not affect the basal amount of p21( ras)-GTP GTP but significantly reduced insulin-induced increases in p2 1(ras)-GTP. This reduction was due to inhibition of the insulin's abil ity to stimulate guanine nucleotide exchange activity of Sos in cells incubated with 100 nw TPA for either 30 min or 3 h, TPA had no effect on basal activity of Sos. Depletion of PKC by an 18-h incubation with TPA or inhibition by bisindolylmaleimide resulted in profound inhibiti on of the insulin-induced p21(ras)-GTP loading. In contrast to PRC act ivation, removal of PK did not influence Sos activity but resulted in a 2-fold stimulation of GTPase activating protein (GAP), This effect o f PB;C depletion is unique to 3T3-L1 adipocytes and was not observed i n either 3T3-L1 fibroblasts or Rat-1 fibroblasts. Thus, it appears tha t in 3T3-L1 adipocytes, PKC has a constitutive inhibitory effect on GA P that permits insulin to activate Sos and p21(ras). Removal of this i nhibitory influence activates GAP and reduces insulin-stimulated p21(r as)-GTP loading.