cAMP counter-regulates insulin-mediated protein phosphatase-2A inactivation in rat skeletal muscle cells.

In this study, we examined the mechanism of recently reported inactivation of protein phosphatase-2A (PP-2A) by insulin (Srinivasan, M., and Begum, N. (1994) J. Biol. Chem. 269, 12514-12520) and its counter-regulation by cAMP agonists. Exposure of L6 myotubes to insulin resulted in a rapid inhibition of PP-2A that was accompanied by a 3-fold increase in the phosphotyrosine content of the immunoprecipitated PP-2A catalytic subunit. Pretreatment with (Sp)-cAMP, a cAMP agonist, completely blocked insulin-mediated inhibition of PP-2A activity and decreased the tyrosine phosphorylation of PP-2A catalytic subunit to control levels. To understand the mechanism of counter-regulation of PP-2A by (Sp)-cAMP, cells were pretreated with sodium orthovanadate, an inhibitor of phosphotyrosine phosphatases. Vanadate prevented the effect of (Sp)-cAMP on PP-2A activity and increased the phosphorylation status of PP-2A catalytic subunit to the level observed with insulin. Wortmannin, a phosphatidylinositol 3-kinase inhibitor, and rapamycin, an inhibitor of 70-kDa S6 kinase activation, prevented insulin-mediated inactivation of PP-2A, suggesting that these pathways may participate in insulin-mediated phosphorylation and inactivation of PP-2A. These results show that insulin signaling results in a rapid inactivation of PP-2A by increased tyrosine phosphorylation and cAMP agonists counter-regulate insulin's effect on PP-2A by decreasing phosphorylation, presumably via an activated phosphatase.