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 pHi regulate the timing of G2/M entry and transition. Prior to G2/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 G2/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 G2/M entry and transition. These data indicate that a transient increase in pH i induced by NHE1 promotes the timing of G2/M, and they suggest that increases in pH i at the completion of S phase may constitute a previously unrecognized checkpoint for progression to G2 and mitosis. 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 pHi regulate the timing of G2/M entry and transition. Prior to G2/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 G2/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 G2/M entry and transition. These data indicate that a transient increase in pH i induced by NHE1 promotes the timing of G2/M, and they suggest that increases in pH i at the completion of S phase may constitute a previously unrecognized checkpoint for progression to G2 and mitosis. The ubiquitously expressed plasma membrane Na-H exchanger NHE1 1The abbreviations used are: NHE1, Na-H exchanger isoform 1; pH i , intracellular pH; BCECF, 2,7-biscarboxyethyl-5(6)-carboxyfluorescein; Cdc2, cyclin-dependent kinase 2; EIPA, ethylisopropylamiloride; FCS, fetal calf serum; HA, hemagglutinin.1The abbreviations used are: NHE1, Na-H exchanger isoform 1; pH i , intracellular pH; BCECF, 2,7-biscarboxyethyl-5(6)-carboxyfluorescein; Cdc2, cyclin-dependent kinase 2; EIPA, ethylisopropylamiloride; FCS, fetal calf serum; HA, hemagglutinin. regulates intracellular pH (pH i) homeostasis and has a permissive effect in promoting cell proliferation. Activation of NHE1 and increased pH i are early and universal responses to mitogenic stimulation (1Putney L.K. Denker S.P. Barber D.L. Annu. Rev. Pharmacol. Toxicol. 2002; 42: 527-552Crossref PubMed Scopus (422) Google Scholar). Growth factor-dependent cell proliferation is attenuated in NHE1-deficient cells (2Pouyssegur J. Sardet C. Franchi A. L'Allemain G. Paris S. Proc. Natl. Acad. Sci. U. S. A. 1984; PubMed Scopus Google A. S. S. J. J. PubMed Google in cells of NHE1 C. G. A. A. J. PubMed Google J. A. J. PubMed Google S. A. J. Google and in cells expressing a mutant NHE1 that is in ion translocation S.P. J. Barber D.L. PubMed Scopus Google Scholar). increase in pHi i , and pH i to proliferation and a A. S. J. PubMed Scopus Google Scholar). Despite established role for NHE1 in cell the NHE1 activity and increased pH i promote cell proliferation are well We cell cycle progression in fibroblasts expressing a mutant NHE1 that lacks ion translocation activity and is to regulate pH i and fibroblasts J. of A. J. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). cells expressing and by of NHE1 of previously S.P. J. Barber D.L. PubMed Scopus Google Scholar). in and activity previously S. Barber D.L. J. PubMed Scopus Google in cells the of the NHE1 activity by the of pH i i by the of the of the pH i at of The pH i the of i and the intracellular and expressed previously Barber D.L. J. PubMed Google Scholar). pH by in the of to the of ion S.P. J. Barber D.L. PubMed Scopus Google Scholar). the of to cell by of cells and data by by a of previously S. J. PubMed Scopus Google Scholar). In to the cells and the for in by and cells for in and for the previously J. J. PubMed Scopus Google and and of NHE1 previously S.P. J. Barber D.L. PubMed Scopus Google of Cdc2, cells in in and of for for and for by for Cdc2 kinase activity by of a in in kinase Cdc2 previously A. PubMed Scopus Google Scholar). Cdc2, and of by and to the and by the The expression by and the of by in cells the The and for Wee1 kinase and and the to of in of and a of the and The and the cycle cycle that the cycle for is used to the of in the in for by and the of the for the for at the in for by the for in the mutant cells and the of the for the previously 2002; PubMed Scopus Google the expression that in to in ion translocation and pH i NHE1 a to (1Putney L.K. Denker S.P. Barber D.L. Annu. Rev. Pharmacol. Toxicol. 2002; 42: 527-552Crossref PubMed Scopus (422) Google 2002; PubMed Scopus Google and a plasma membrane for the S.P. J. Barber D.L. PubMed Scopus Google S.P. Barber D.L. 2002; PubMed Scopus Google S.P. Barber D.L. J. 2002; PubMed Scopus Google Scholar). the ion translocation of NHE1, ion NHE1 for In cells expressing the and of NHE1 are but ion translocation is S.P. J. Barber D.L. PubMed Scopus Google S.P. Barber D.L. 2002; PubMed Scopus Google S.P. Barber D.L. J. 2002; PubMed Scopus Google Scholar). the the effect of ion translocation by NHE1 cell proliferation by fibroblasts A. J. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google to NHE1 a or of NHE1 in cells is that in cells and the increased of a in J. PubMed Scopus Google Scholar). cells are cells expressing A. J. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google and cells have a that is in cells expression of NHE1 in but in this in and cells in and the of and to the of pH i and the of cell proliferation that in and These are of A. S. S. J. J. PubMed Google S.P. J. Barber D.L. PubMed Scopus Google that NHE1 activity a permissive for cell of in cells that G2/M progression in cells cells by cells in S phase a and by at the the of of cells and of cells in S phase cell of but of G2/M of cells and of cells in G2/M, and of cells and of cells in S in the absence of NHE1 activity there is a in S phase and entry In cell there that cells increased or activity promotes G2/M of and cells at the a by the of and cells in S phase and G2/M at the a the of cell the transition S phase to G2/M a increase in pH i and NHE1 in the of increased a a of at pH i and to and for to the increase in pH i in cells at a transient increase in NHE1 activity by the of pH i i in a In the absence of NHE1 activity in cells there a increase in pH i that of NHE1 activity, The pH i of in however, that in cells These data indicate that the transition S phase to G2/M is increase in NHE1 activity, promotes a and transient increase in pH In the absence of NHE1 activity, the pH i increase is attenuated, S phase is delayed, and entry G2/M is phase completion is transient increases in pH i and NHE1 pH in and cells at the a the of cell NHE1 activity in cells at the a by the pH i of pHi are expressed the of NHE1 activity at pH i in cell cells G2/M transition and the activity and expression of G2/M In the of cells that of cells In entry is increased activity of the cyclin-dependent kinase Cdc2 PubMed Scopus Google Scholar). and Cdc2 kinase activity in cells by of in in kinase Cdc2 Cdc2 kinase activity increased in but at and and activity by cells The of Cdc2 in and however, at Cdc2 activity is in by the phase cyclin to at the of S phase J. PubMed Scopus Google Scholar). In and cells the expression of cyclin B1 increased at and however, the of cyclin B1 at in cells In the of cyclin cyclin and cyclin in cell at Cdc2 kinase activity is by J. PubMed Scopus Google S. PubMed Scopus Google is by the of PubMed Scopus Google PubMed Scopus Google Scholar). a that of Cdc2 in cells cells is decreased Cdc2 kinase In of cells a increase in Wee1 kinase expression and in by that Wee1 in cells in cells cell data suggest that ion translocation by NHE1 the activity and expression of phase and promotes activity regulates the activity and expression of G2/M Cdc2 kinase activity at the a by of in in kinase Cdc2 Cdc2 and Cdc2 at the a by of in Cdc2 kinase in and cells at the a the of cell of and in and cells at the a by for and data are of cell of Wee1 kinase in and by the of cell pH i increased NHE1 activity promote G2/M entry and or increased pH i G2/M entry and and S phase is in increases in pH i are In of pH i the timing of G2/M, pH i in the absence of NHE1 activity sufficient to the entry G2/M and the activity and expression of phase that G2/M entry and transition to the of NHE1 activity in but of NHE1, the effects of pH i in cells the NHE1 activity J. Pharmacol. Google Scholar). the of a cells or and in the of and the pH i of cells and NHE1 activity increase pH i in the absence of NHE1 activity, cells in at at the of in to a pH i of pH i in cells increased of cells in G2/M at the of cells in G2/M for for cells EIPA, and for cells but in at a in the of cells in G2/M at in cells expressing a mutant NHE1 ion translocation and in cells cells NHE1 ion NHE1 activity in cells decreased Cdc2 kinase activity by by Cdc2 and increased of Cdc2 and decreased cyclin B1 expression that Cdc2 activity and cyclin B1 expression a of of NHE1 activity in cells but of NHE1 pH i in the absence of NHE1 activity sufficient to increase Cdc2 activity and of Cdc2 and cyclin B1 expression pH i in the absence of NHE1 activity regulates G2/M timing and pH in cells at a the of cells and in at EIPA, and in at or and in at the of cell the of cells in G2/M, and at and a by to in and data the of and Cdc2 kinase activity, by of in the to and of Cdc2, by at the of and in cells and in cells of and cyclin B1 in cells and in cells at a suggest that increases in pH i promote the timing of G2/M entry and transition and that timing a pH i of is by NHE1 NHE1 activity of the exchanger to in pH the transient increase in NHE1 activity a to increased and in S the increase in pH i a for progression S phase to the kinase of Cdc2 and Wee1 are is undetermined. It remains to a specific G2/M is the of increased pH i , cyclin B1 is a B1 increases at the of S phase J. PubMed Scopus Google and cyclin B1 or by pH cyclin B1 are by S. J. PubMed Scopus Google and decreased pH i of a to cyclin B1 expression is sufficient to the timing of G2/M transition in the absence of increased NHE1 activity and pH is the that transient increases in pH i at the completion of S phase constitute a previously unrecognized of a checkpoint The ubiquitously expressed plasma membrane Na-H exchanger NHE1 1The abbreviations used are: NHE1, Na-H exchanger isoform 1; pH i , intracellular pH; BCECF, 2,7-biscarboxyethyl-5(6)-carboxyfluorescein; Cdc2, cyclin-dependent kinase 2; EIPA, ethylisopropylamiloride; FCS, fetal calf serum; HA, hemagglutinin.1The abbreviations used are: NHE1, Na-H exchanger isoform 1; pH i , intracellular pH; BCECF, 2,7-biscarboxyethyl-5(6)-carboxyfluorescein; Cdc2, cyclin-dependent kinase 2; EIPA, ethylisopropylamiloride; FCS, fetal calf serum; HA, hemagglutinin. regulates intracellular pH (pH i) homeostasis and has a permissive effect in promoting cell proliferation. Activation of NHE1 and increased pH i are early and universal responses to mitogenic stimulation (1Putney L.K. Denker S.P. Barber D.L. Annu. Rev. Pharmacol. Toxicol. 2002; 42: 527-552Crossref PubMed Scopus (422) Google Scholar). Growth factor-dependent cell proliferation is attenuated in NHE1-deficient cells (2Pouyssegur J. Sardet C. Franchi A. L'Allemain G. Paris S. Proc. Natl. Acad. Sci. U. S. A. 1984; PubMed Scopus Google A. S. S. J. J. PubMed Google in cells of NHE1 C. G. A. A. J. PubMed Google J. A. J. PubMed Google S. A. J. Google and in cells expressing a mutant NHE1 that is in ion translocation S.P. J. Barber D.L. PubMed Scopus Google Scholar). increase in pHi i , and pH i to proliferation and a A. S. J. PubMed Scopus Google Scholar). Despite established role for NHE1 in cell the NHE1 activity and increased pH i promote cell proliferation are well We cell cycle progression in fibroblasts expressing a mutant NHE1 that lacks ion translocation activity and is to regulate pH i and fibroblasts J. of A. J. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). cells expressing and by of NHE1 of previously S.P. J. Barber D.L. PubMed Scopus Google Scholar). in and activity previously S. Barber D.L. J. PubMed Scopus Google in cells the of the NHE1 activity by the of pH i i by the of the of the pH i at of The pH i the of i and the intracellular and expressed previously Barber D.L. J. PubMed Google Scholar). pH by in the of to the of ion S.P. J. Barber D.L. PubMed Scopus Google Scholar). the of to cell by of cells and data by by a of previously S. J. PubMed Scopus Google Scholar). In to the cells and the for in by and cells for in and for the previously J. J. PubMed Scopus Google and and of NHE1 previously S.P. J. Barber D.L. PubMed Scopus Google of Cdc2, cells in in and of for for and for by for Cdc2 kinase activity by of a in in kinase Cdc2 previously A. PubMed Scopus Google Scholar). Cdc2, and of by and to the and by the The expression by and the of by in cells the The and for Wee1 kinase and and the to of in of and a of the and The and the cycle cycle that the cycle for is used to the of in the in for by and the of the for the for at the in for by the for in the mutant cells and the of the for the previously 2002; PubMed Scopus Google the expression and fibroblasts J. of A. J. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). cells expressing and by of NHE1 of previously S.P. J. Barber D.L. PubMed Scopus Google Scholar). in NHE1 and activity previously S. Barber D.L. J. PubMed Scopus Google in cells the of the NHE1 activity by the of pH i i by the of the of the pH i at of The pH i the of i and the intracellular and expressed previously Barber D.L. J. PubMed Google Scholar). pH by in the of to the of ion S.P. J. Barber D.L. PubMed Scopus Google Scholar). the of to cell by of cells and data by by a of previously S. J. PubMed Scopus Google Scholar). In to the cells and the for in by and cells for in and for the previously J. J. PubMed Scopus Google Scholar). and and of NHE1 previously S.P. J. Barber D.L. PubMed Scopus Google of Cdc2, cells in in and of for for and for by for Cdc2 kinase activity by of a in in kinase Cdc2 previously A. PubMed Scopus Google Scholar). Cdc2, and of by and to the and by the The expression by and the of by in cells the The and for Wee1 kinase and and the to of in of and a of the and The and the cycle cycle that the cycle for is used to the of in the in for by and the of the for the for at the in for by the for in the mutant cells and the of the for the previously 2002; PubMed Scopus Google the expression that in to in ion translocation and pH i NHE1 a to (1Putney L.K. Denker S.P. Barber D.L. Annu. Rev. Pharmacol. Toxicol. 2002; 42: 527-552Crossref PubMed Scopus (422) Google 2002; PubMed Scopus Google and a plasma membrane for the S.P. J. Barber D.L. PubMed Scopus Google S.P. Barber D.L. 2002; PubMed Scopus Google S.P. Barber D.L. J. 2002; PubMed Scopus Google Scholar). the ion translocation of NHE1, ion NHE1 for In cells expressing the and of NHE1 are but ion translocation is S.P. J. Barber D.L. PubMed Scopus Google S.P. Barber D.L. 2002; PubMed Scopus Google S.P. Barber D.L. J. 2002; PubMed Scopus Google Scholar). the the effect of ion translocation by NHE1 cell proliferation by fibroblasts A. J. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google to NHE1 a or of NHE1 in cells is that in cells and the increased of a in J. PubMed Scopus Google Scholar). cells are cells expressing A. J. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google and cells have a that is in cells expression of NHE1 in but in this in and cells in and the of and to the of pH i and the of cell proliferation that in and These are of A. S. S. J. J. PubMed Google S.P. J. Barber D.L. PubMed Scopus Google that NHE1 activity a permissive for cell of in cells that G2/M progression in cells cells by cells in S phase a and by at the the of of cells and of cells in S phase cell of but of G2/M of cells and of cells in G2/M, and of cells and of cells in S in the absence of NHE1 activity there is a in S phase and entry In cell there that cells increased or the transition S phase to G2/M a increase in pH i and NHE1 in the of increased a a of at pH i and to and for to the increase in pH i in cells at a transient increase in NHE1 activity by the of pH i i in a In the absence of NHE1 activity in cells there a increase in pH i that of NHE1 activity, The pH i of in however, that in cells These data indicate that the transition S phase to G2/M is increase in NHE1 activity, promotes a and transient increase in pH In the absence of NHE1 activity, the pH i increase is attenuated, S phase is delayed, and entry G2/M is phase completion is transient increases in pH i and NHE1 pH in and cells at the a the of cell NHE1 activity in cells at the a by the pH i of pHi are expressed the of NHE1 activity at pH i in cell cells G2/M transition and the activity and expression of G2/M In the of cells that of cells In entry is increased activity of the cyclin-dependent kinase Cdc2 PubMed Scopus Google Scholar). and Cdc2 kinase activity in cells by of in in kinase Cdc2 Cdc2 kinase activity increased in but at and and activity by cells The of Cdc2 in and however, at Cdc2 activity is in by the phase cyclin to at the of S phase J. PubMed Scopus Google Scholar). In and cells the expression of cyclin B1 increased at and however, the of cyclin B1 at in cells In the of cyclin cyclin and cyclin in cell at Cdc2 kinase activity is by J. PubMed Scopus Google S. PubMed Scopus Google is by the of PubMed Scopus Google PubMed Scopus Google Scholar). a that of Cdc2 in cells cells is decreased Cdc2 kinase In of cells a increase in Wee1 kinase expression and in by that Wee1 in cells in cells cell data suggest that ion translocation by NHE1 the activity and expression of phase and promotes activity regulates the activity and expression of G2/M Cdc2 kinase activity at the a by of in in kinase Cdc2 Cdc2 and Cdc2 at the a by of in Cdc2 kinase in and cells at the a the of cell of and in and cells at the a by for and data are of cell of Wee1 kinase in and by the of cell pH i increased NHE1 activity promote G2/M entry and or increased pH i G2/M entry and and S phase is in increases in pH i are In of pH i the timing of G2/M, pH i in the absence of NHE1 activity sufficient to the entry G2/M and the activity and expression of phase that G2/M entry and transition to the of NHE1 activity in but of NHE1, the effects of pH i in cells the NHE1 activity J. Pharmacol. Google Scholar). the of a cells or and in the of and the pH i of cells and NHE1 activity increase pH i in the absence of NHE1 activity, cells in at at the of in to a pH i of pH i in cells increased of cells in G2/M at the of cells in G2/M for for cells EIPA, and for cells but in at a in the of cells in G2/M at in cells expressing a mutant NHE1 ion translocation and in cells cells NHE1 ion NHE1 activity in cells decreased Cdc2 kinase activity by by Cdc2 and increased of Cdc2 and decreased cyclin B1 expression that Cdc2 activity and cyclin B1 expression a of of NHE1 activity in cells but of NHE1 pH i in the absence of NHE1 activity sufficient to increase Cdc2 activity and of Cdc2 and cyclin B1 expression pH i in the absence of NHE1 activity regulates G2/M timing and pH in cells at a the of cells and in at EIPA, and in at or and in at the of cell the of cells in G2/M, and at and a by to in and data the of and Cdc2 kinase activity, by of in the to and of Cdc2, by at the of and in cells and in cells of and cyclin B1 in cells and in cells at a suggest that increases in pH i promote the timing of G2/M entry and transition and that timing a pH i of is by NHE1 NHE1 activity of the exchanger to in pH the transient increase in NHE1 activity a to increased and in S the increase in pH i a for progression S phase to the kinase of Cdc2 and Wee1 are is undetermined. It remains to a specific G2/M is the of increased pH i , cyclin B1 is a B1 increases at the of S phase J. PubMed Scopus Google and cyclin B1 or by pH cyclin B1 are by S. J. PubMed Scopus Google and decreased pH i of a to cyclin B1 expression is sufficient to the timing of G2/M transition in the absence of increased NHE1 activity and pH is the that transient increases in pH i at the completion of S phase constitute a previously unrecognized of a checkpoint that in to in ion translocation and pH i NHE1 a to (1Putney L.K. Denker S.P. Barber D.L. Annu. Rev. Pharmacol. Toxicol. 2002; 42: 527-552Crossref PubMed Scopus (422) Google 2002; PubMed Scopus Google and a plasma membrane for the S.P. J. Barber D.L. PubMed Scopus Google S.P. Barber D.L. 2002; PubMed Scopus Google S.P. Barber D.L. J. 2002; PubMed Scopus Google Scholar). the ion translocation of NHE1, ion NHE1 for In cells expressing the and of NHE1 are but ion translocation is S.P. J. Barber D.L. PubMed Scopus Google S.P. Barber D.L. 2002; PubMed Scopus Google S.P. Barber D.L. J. 2002; PubMed Scopus Google Scholar). the the effect of ion translocation by NHE1 cell proliferation by fibroblasts A. J. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google to NHE1 a or of NHE1 in cells is that in cells and the increased of a in J. PubMed Scopus Google Scholar). cells are cells expressing A. J. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google and cells have a that is in cells expression of NHE1 in but in this in and cells in and the of and to the of pH i and the of cell proliferation that in and These are of A. S. S. J. J. PubMed Google S.P. J. Barber D.L. PubMed Scopus Google that NHE1 activity a permissive for cell proliferation. of in cells that G2/M progression in cells cells by cells in S phase a and by at the the of of cells and of cells in S phase cell of but of G2/M of cells and of cells in G2/M, and of cells and of cells in S in the absence of NHE1 activity there is a in S phase and entry In cell there that cells increased or In the transition S phase to G2/M a increase in pH i and NHE1 in the of increased a a of at pH i and to and for to the increase in pH i in cells at a transient increase in NHE1 activity by the of pH i i in a In the absence of NHE1 activity in cells there a increase in pH i that of NHE1 activity, The pH i of in however, that in cells These data indicate that the transition S phase to G2/M is increase in NHE1 activity, promotes a and transient increase in pH In the absence of NHE1 activity, the pH i increase is attenuated, S phase is delayed, and entry G2/M is impaired. In cells G2/M transition and the activity and expression of G2/M In the of cells that of cells In entry is increased activity of the cyclin-dependent kinase Cdc2 PubMed Scopus Google Scholar). and Cdc2 kinase activity in cells by of in in kinase Cdc2 Cdc2 kinase activity increased in but at and and activity by cells The of Cdc2 in and however, at Cdc2 activity is in by the phase cyclin to at the of S phase J. PubMed Scopus Google Scholar). In and cells the expression of cyclin B1 increased at and however, the of cyclin B1 at in cells In the of cyclin cyclin and cyclin in cell at Cdc2 kinase activity is by J. PubMed Scopus Google S. PubMed Scopus Google is by the of PubMed Scopus Google PubMed Scopus Google Scholar). a that of Cdc2 in cells cells is decreased Cdc2 kinase In of cells a increase in Wee1 kinase expression and in by that Wee1 in cells in cells cell data suggest that ion translocation by NHE1 the activity and expression of phase and promotes pH i increased NHE1 activity promote G2/M entry and or increased pH i G2/M entry and and S phase is in increases in pH i are In of pH i the timing of G2/M, pH i in the absence of NHE1 activity sufficient to the entry G2/M and the activity and expression of phase that G2/M entry and transition to the of NHE1 activity in but of NHE1, the effects of pH i in cells the NHE1 activity J. Pharmacol. Google Scholar). the of a cells or and in the of and the pH i of cells and NHE1 activity increase pH i in the absence of NHE1 activity, cells in at at the of in to a pH i of pH i in cells increased of cells in G2/M at the of cells in G2/M for for cells EIPA, and for cells but in at a in the of cells in G2/M at in cells expressing a mutant NHE1 ion translocation and in cells cells NHE1 ion NHE1 activity in cells decreased Cdc2 kinase activity by by Cdc2 and increased of Cdc2 and decreased cyclin B1 expression that Cdc2 activity and cyclin B1 expression a of of NHE1 activity in cells but of NHE1 pH i in the absence of NHE1 activity sufficient to increase Cdc2 activity and of Cdc2 and cyclin B1 expression suggest that increases in pH i promote the timing of G2/M entry and transition and that timing a pH i of is by NHE1 NHE1 activity of the exchanger to in pH the transient increase in NHE1 activity a to increased and in S the increase in pH i a for progression S phase to the kinase of Cdc2 and Wee1 are is undetermined. It remains to a specific G2/M is the of increased pH i , cyclin B1 is a B1 increases at the of S phase J. PubMed Scopus Google and cyclin B1 or by pH cyclin B1 are by S. J. PubMed Scopus Google and decreased pH i of a to cyclin B1 expression is sufficient to the timing of G2/M transition in the absence of increased NHE1 activity and pH is the that transient increases in pH i at the completion of S phase constitute a previously unrecognized of a checkpoint We and for