Na-H exchange-dependent increase in intracellular pH times G2/M entry and transition

It is well established that activation of the Na-H exchanger NHE1 and increases in intracellular pH (pH(i)) are early and universal responses to mitogens and have permissive effects in promoting cell proliferation. Despite this evidence, a specific role for NHE1 or pH(i) in cell cycle progression remains undetermined. We now show that NHE1 activity and pH(i) regulate the timing of G(2)/M entry and transition. Prior to G(2)/M entry there is a rapid and transient increase in NHE1 activity and pH(i), but in fibroblasts expressing a mutant NHE1 that lacks ion translocation activity, this increase in pH(i) is attenuated, S phase is delayed, and G(2)/M transition is impaired. In the absence of ion translocation by NHE1, expression of cyclin B1 and the kinase activity of Cdc2 are decreased and Wee1 kinase expression increases. Increasing pH(i) in the absence of NHE1 activity, however, is sufficient to restore Cdc2 activity and cyclin B1 expression and to promote G(2)/M entry and transition. These data indicate that a transient increase in pH(i) induced by NHE1 promotes the timing of G(2)/M, and they suggest that increases in pH(i) at the completion of S phase may constitute a previously unrecognized checkpoint for progression to G(2) and mitosis.