Role of protein kinase C in the signal pathways that link Na+/K+-ATPase toERK1/2

We have shown before that Na+/K+-ATPase acts as a signal transducer, throug h protein-protein interactions, in addition to being an ion pump. Interacti on of ouabain with the enzyme of the intact cells causes activation of Src, transactivation of EGFR, and activation of the Ras/ERK1/2 cascade. To dete rmine the role of protein kinase C (PKC) in this pathway, neonatal rat card iac myocytes were exposed to ouabain and assayed for translocation/activati on of PKC from cytosolic to particulate fractions. Ouabain caused rapid and sustained stimulation of this translocation, evidenced by the assay of Ca2 +-dependent and Ca2+-independent PKC activities and by the immunoblot analy sis of the alpha, delta, and epsilon isoforms of PKC. Dose-dependent stimul ation of PKC translocation by ouabain (1-100 mum) was accompanied by no mor e than 50% inhibition of Na+/K+-ATPase and doubling of [Ca2+](i), changes t hat do not affect myocyte viability and are known to be associated with pos itive inotropic, but not toxic, effects of ouabain in rat cardiac ventricle s. Ouabain-induced activation of ERK1/2 was blocked by PKC inhibitors calph ostin C and chelerythrine. An inhibitor of phosphoinositide turnover in myo cytes also antagonized ouabain-induced PKC translocation and ERK1/2 activat ion. These and previous findings indicate that ouabain-induced activation o f PKC and Ras, each linked to Na+/K+-ATPase through Src/EGFR, are both requ ired for the activation of ERK1/2. Ouabain-induced PKC translocation and ER K1/2 activation were dependent on the presence of Ca2+ in the medium, sugge sting that the signal-transducing and ion-pumping functions of Na+/K+-ATPas e cooperate in activation of these protein kinases and the resulting regula tion of contractility and growth of the cardiac myocyte.