The expression of the γ subunit of Na-K-ATPase is regulated by osmolality via C-terminal Jun kinase and phosphatidylinositol 3-kinase-dependent mechanisms

The α and β subunits of Na-K-ATPase are up-regulated by hypertonicity in inner-medullary collecting duct cells adapted to survive in hypertonic conditions. We examined the regulation of the γ subunit by hypertonicity. Although cultured inner-medullary collecting duct cells lacked the γ subunits, both variants γ(a) and γ(b) were expressed in cells adapted to 600 and 900 mosmol/KgH(2)O. This expression was reversible with a half-time of 17.2 ± 0.5 h. The message of the γ subunit was absent in isotonic conditions and increased with higher tonicity in adapted cells. In acute experiments the appearance of the γ subunit was found to be both time-dependent (≥24 h) and osmolality-dependent (≥500 mosmol/KgH(2)O). No induction was noted with urea and only minimal induction with mannitol. Increasing concentrations of the phosphatidylinositol 3-kinase inhibitor LY294002 resulted in a dose-dependent decrement in the expression of the γ subunit with total abolition at 10 μM. This was associated with a decrease in cell viability as <20% survived the treatment with 10 μM of LY294002. Neither inhibition of extracellular response kinase nor p38 mitogen-activated protein kinase inhibited osmotic induction of the γ subunit. In contrast, cells transfected with a dominant negative c-Jun N-terminal kinase 2-APF construct displayed complete inhibition of the γ subunit. Such cells have accelerated loss of viability in hypertonic conditions. This study describes the regulation of the γ subunit of Na-K-ATPase by hypertonicity. This regulation is transcriptionally regulated and involves signaling mediated by phosphatidylinositol 3-kinase and c-Jun N-terminal kinase 2 pathways.