Role of protein kinase C in the signal pathways that link Na+/K+-ATPase to ERK1/2

We have shown before that Na+/K+-ATPase acts as a signal transducer, through protein-protein interactions, in addition to being an ion pump. Interaction 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 determine the role of protein kinase C (PKC) in this pathway, neonatal rat cardiac myocytes were exposed to ouabain and assayed for translocation/activation 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 analysis of the alpha, delta, and epsilon isoforms of PKC. Dose-dependent stimulation of PKC translocation by ouabain (1-100 mum) was accompanied by no more than 50% inhibition of Na+/K+-ATPase and doubling of [Ca2+](i), changes that do not affect myocyte viability and are known to be associated with positive inotropic, but not toxic, effects of ouabain in rat cardiac ventricles. Ouabain-induced activation of ERK1/2 was blocked by PKC inhibitors calphostin C and chelerythrine. An inhibitor of phosphoinositide turnover in myocytes also antagonized ouabain-induced PKC translocation and ERK1/2 activation. These and previous findings indicate that ouabain-induced activation of PKC and Ras, each linked to Na+/K+-ATPase through Src/EGFR, are both required for the activation of ERK1/2. Ouabain-induced PKC translocation and ERK1/2 activation were dependent on the presence of Ca2+ in the medium, suggesting that the signal-transducing and ion-pumping functions of Na+/K+-ATPase cooperate in activation of these protein kinases and the resulting regulation of contractility and growth of the cardiac myocyte.