Regulation of Na/K-ATPase β₁-subunit gene expression by ouabain and other hypertrophic stimuli in neonatal rat cardiac myocytes

Partial inhibition of Na/K-ATPase by ouabain causes hypertrophic growth and regulates several early and late response genes, including that of Na/K-ATPase α₃ subunit, in cultured neonatal rat cardiac myocytes. The aim of this work was to determine whether ouabain and other hypertrophic stimuli affect Na/K-ATPase β₁ subunit gene expression. When myocytes were exposed to non-toxic concentrations of ouabain, ouabain increased β₁ subunit mRNA in a dose- and time-dependent manner. Like the α₃ gene, β₁ mRNA was also regulated by several other well-known hypertrophic stimuli including phenylephrine, a phorbol ester, endothelin-1, and insulin-like growth factor, suggesting involvement of growth signals in regulation of β₁ expression. Ouabain failed to increase β₁ subunit mRNA in the presence of actinomycin D. Using a luciferase reporter gene that is directed by the 5′-flanking region of the β₁ subunit gene, transient transfection assay showed that ouabain augmented the expression of luciferase. These data support the proposition that ouabain regulates the β₁ subunit through a transcriptional mechanism. The effect of ouabain on β₁ subunit induction, like that on α₃ repression, was dependent on extracellular Ca²⁺ and on calmodulin. Inhibitions of PKC, Ras, and MEK, however, had different quantitive effects on ouabain-induced regulations of β₁ and α₃ subunits. The findings show that partial inhibition of Na/K-ATPase activates multiple signaling pathways that regulate growth-related genes, including those of two subunit isoforms of Na/K-ATPase, in a gene-specific manner.