CAMP COUNTER-REGULATES INSULIN-MEDIATED PROTEIN PHOSPHATASE-2A INACTIVATION IN RAT SKELETAL-MUSCLE CELLS

In this study, we examined the mechanism of recently reported inactiva tion of protein phosphatase-2A (PP-2A) by insulin (Srinivasan, IM., an d Begum, N, (1994) J. Biol. Chem, 269, 12514-12520) and its counter-re gulation by cAMP agonists, Exposure of L6 myotubes to insulin resulted in a rapid inhibition of PP-W that was accompanied by a 3-fold increa se in the phosphotyrosine content of the immunoprecipitated PP-2A cata lytic subunit, Pretreatment with (S-p) cAMP, a cAMP agonist, completel y blocked insulin-mediated inhibition of PP-2A activity and decreased the tyrosine phosphorylation of PP-2A catalytic subunit to control lev els. To understand the mechanism of counter regulation of PP-2A by (S- p)-cAMP, cells were pretreated with sodium orthovanadate, an inhibitor of phosphotyrosine phosphatases. Vanadate prevented the effect of (S- p)-cAMP on PP-2A activity and increased the phosphorylation status of PP-2A catalytic subunit to the level observed with insulin, Wortmannin , a phosphatidylinositol 8-kinase inhibitor, and rapamycin, an inhibit or of 70-kDa S6 kinase activation, prevented insulin-mediated inactiva tion of PP-2A, suggesting that these pathways may participate in insul in mediated phosphorylation and inactivation of PP-2A These results sh ow that insulin signaling results in a rapid inactivation of PP-PA by increased tyrosine phosphorylation and cAMP agonists counter-regulate insulin's effect on PP-2A by decreasing phosphorylation, presumably vi a an activated phosphatase.