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 additio n, a number of normal mammalian cells have been identified that expres s H+-adenosinetriphosphatases (ATPases) in their plasma membranes. Bec ause tumor cells often maintain a high pH(i), we hypothesized that the y might functionally express H+-ATPases in their plasma membranes. In the first phase of the present study, we screened 19 normal and tumori genic human cell lines for the presence of plasmalemmal H+-ATPase acti vity 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 si x tumor cell lines with the highest resting pH(i) in the absence of HC O3-. Sch-28080 did not affect pH(i) in any of the human cells. Simulta neous measurement of pH in the cytoplasm and in the endosomes/lysosome s localized the activity of bafilomycin to the plasma membrane in thre e cell lines. In the second phase of this study, these three cell line s 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 vacuola r type H+-ATPase is expressed in the plasma membrane of some tumor cel ls.