Vacuolar-type H⁺-ATPases are functionally expressed in plasma membranes of human tumor cells

Mammalian cells generally regulate their intracellular pH (pH(i)) via collaboration between Na⁺-H⁺ exchanger and HCO₃/^- transport. In addition, a number of normal mammalian cells have been identified that express H⁺- adenosinetriphosphatases (ATPases) in their plasma membranes. Because tumor cells often maintain a high pH(i), we hypothesized that they might functionally express H⁺-ATPases in their plasma membranes. In the first phase of the present study, we screened 19 normal and tumorigenic human cell lines for the presence of plasmalemmal H⁺-ATPase activity using bafilomycin A₁ to inhibit V-type H⁺-ATPase and Sch-28080 to inhibit P-type H⁺-K⁺- ATPase. Bafilomycin A₁ decreased pH(i) in the six tumor cell lines with the highest resting pH(i) in the absence of HCO₃/^-. Sch-28080 did not affect pH(i) in any of the human cells. Simultaneous measurement of pH in the cytoplasm and in the endosomes/lysosomes localized the activity of bafilomycin to the plasma membrane in three cell lines. In the second phase of this study, these three cell lines were shown to recover from NH₄/⁺- induced acid loads in the absence of Na⁺. This recovery was inhibited by N- ethylmaleimide, bafilomycin A₁, and ATP depletion and was not significantly affected by vanadate, Sch-28080, or hexamethyl amiloride. These results indicate that a vacuolar type H⁺-ATPase is expressed in the plasma membrane of some tumor cells.