Inhibition of phosphatidylinositol 3‐kinase facilitates proton‐induced apoptosis of B lymphoma cells

Our previous studies have shown that phosphatidylinositol 3‐kinase (PI3K) promotes B lymphoma cell cycle progression by stimulating an intermediate‐conductance Ca 2+ ‐activated K + channel. To determine whether PI3K also participates in a pathway that accounts for defective apoptosis in B lymphoma cells, we used flow cytometry and confocal microscopy methods to evaluate apoptotic cells with the propidium iodide/annexin V double‐labeling technique. The data showed that B lymphoma cells were very resistant to acidic conditions. Significant intracellular acidification and apoptosis occurred only when extracellular pH was lower than 5. Inhibition of PI3K with LY‐294002 facilitated proton‐induced intracellular acidification and apoptosis. The effects of LY‐294002 on proton‐induced apoptosis and intracellular acidification were mimicked by bafilomycin, a potent inhibitor of vacuolar H + ‐ATPase. Our results suggest that inhibition of PI3K facilitates proton‐induced apoptosis in B lymphoma cells probably by reducing H + ‐ATPase‐mediated proton efflux. However, whether H + ‐ATPase is mislocated in the plasma membrane and whether the mislocation is caused by elevated PI3K activity in B lymphoma cells remain to be further determined. This work is supported by Natural Science Foundation of Heilongjiang Province D2004‐03 to Li‐Hua Wang and Department of Health and Human Services, National Institutes of Health (NIH) Grant R01‐DK‐067110 to He‐Ping Ma