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 alpha and beta 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 gamma subunit by hypertonicity. Although cultured inner-medullary collecting duct cells lacked the gamma subunits, both variants gamma (a) and gamma (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 gamma subunit was absent in isotonic conditions and increased with higher tonicity in adapted cells. In acute experiments the appearance of the y subunit was found to be both time-dependent (greater than or equal to 24 h) and osmolality-dependent (greater than or equal to 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 gamma subunit with total abolition at 10 muM. This was associated with a decrease in cell viability as < 20% survived the treatment with 10 muM of LY294002. Neither inhibition of extracellular response kinase nor p38 mitogen-activated protein kinase inhibited osmotic induction of the gamma subunit. In contrast, cells transfected with a dominant negative c-Jun N-terminal kinase 2-APF construct displayed complete inhibition of the gamma subunit. Such cells have accelerated loss of viability in hypertonic conditions. This study describes the regulation of the gamma 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.