pH-dependent regulation of the α-subunit of H+-K+-ATPase (HKα2).

The H(+)-K(+)-ATPase α-subunit (HKα(2)) participates importantly in systemic acid-base homeostasis and defends against metabolic acidosis. We have previously shown that HKα(2) plasma membrane expression is regulated by PKA (Codina J, Liu J, Bleyer AJ, Penn RB, DuBose TD Jr. J Am Soc Nephrol 17: 1833-1840, 2006) and in a separate study demonstrated that genetic ablation of the proton-sensing G(s)-coupled receptor GPR4 results in spontaneous metabolic acidosis (Sun X, Yang LV, Tiegs BC, Arend LJ, McGraw DW, Penn RB, Petrovic S. J Am Soc Nephrol 21: 1745-1755, 2010). In the present study, we investigated the ability of chronic acidosis and GPR4 to regulate HKα(2) expression in HEK-293 cells. Chronic acidosis was modeled in vitro by using multiple methods: reducing media pH by adjusting bicarbonate concentration, adding HCl, or by increasing the ambient concentration of CO(2). PKA activity and HKα(2) protein were monitored by immunoblot analysis, and HKα(2) mRNA, by real-time PCR. Chronic acidosis did not alter the expression of HKα(2) mRNA; however, PKA activity and HKα(2) protein abundance increased when media pH decreased from 7.4 to 6.8. Furthermore, this increase was independent of the method used to create chronic acidosis. Heterologous expression of GPR4 was sufficient to increase both basal and acid-stimulated PKA activity and similarly increase basal and acid-stimulated HKα(2) expression. Collectively, these results suggest that chronic acidosis and GPR4 increase HKα(2) protein by increasing PKA activity without altering HKα(2) mRNA abundance, implicating a regulatory role of pH-activated GPR4 in homeostatic regulation of HKα(2) and acid-base balance.