Expression of Calcineurin B Homologous Protein 2 Protects Serum Deprivation-induced Cell Death by Serum-independent Activation of Na+/H+ Exchanger (2002)

The calcineurin B homologous protein (designated CHP1) has been shown to be a common essential cofactor for the plasma membrane Na+/H+ exchangers (NHEs) (Pang, T., Su, X., Wakabayashi, S., and Shigekawa, M. (2001)J. Biol. Chem. 276, 17367–17372). In this study, we characterized the function of another isoform of CHP (designated CHP2) that has a 61% amino acid identity with CHP1. CHP2, like CHP1, conferred the ability to NHEs 1–3 to express a high exchange activity by binding to the juxtamembrane region of the cytoplasmic domain of the exchanger, but it interacts more strongly (∼5-fold) with NHE1 than does CHP1. Although CHP1 is expressed ubiquitously at relatively high levels, CHP2 expression was extremely low in most human tissues but was higher in tumor cells. We produced stable cell clones overexpressing either CHP1 or CHP2 in which one of them is predominantly bound to NHE1. Serum (10%) induced a significant cytoplasmic alkalinization (0.1–0.2 pH unit) in cells co-expressing CHP1 and NHE1 but not in cells co-expressing CHP2 and NHE1. In the latter, pHi was high (7.4–7.5) even in the absence of serum, suggesting that NHE1 was already activated. Surprisingly, most (>80%) of CHP2/NHE1 cells unlike CHP1/NHE1 cells were viable even after long serum starvation (>7 days). Thus, the expression of CHP2 appears to protect cells serum by of CHP2/NHE1 cells to of cells. We that of NHE1 by bound CHP2 is one of the for the of high pHi and the to serum cell in cells. The calcineurin B homologous protein (designated CHP1) has been shown to be a common essential cofactor for the plasma membrane Na+/H+ exchangers (NHEs) (Pang, T., Su, X., Wakabayashi, S., and Shigekawa, M. (2001)J. Biol. Chem. 276, 17367–17372). In this study, we characterized the function of another isoform of CHP (designated CHP2) that has a 61% amino acid identity with CHP1. CHP2, like CHP1, conferred the ability to NHEs 1–3 to express a high exchange activity by binding to the juxtamembrane region of the cytoplasmic domain of the exchanger, but it interacts more strongly (∼5-fold) with NHE1 than does CHP1. Although CHP1 is expressed ubiquitously at relatively high levels, CHP2 expression was extremely low in most human tissues but was higher in tumor cells. We produced stable cell clones overexpressing either CHP1 or CHP2 in which one of them is predominantly bound to NHE1. Serum (10%) induced a significant cytoplasmic alkalinization (0.1–0.2 pH unit) in cells co-expressing CHP1 and NHE1 but not in cells co-expressing CHP2 and NHE1. In the latter, pHi was high (7.4–7.5) even in the absence of serum, suggesting that NHE1 was already activated. Surprisingly, most (>80%) of CHP2/NHE1 cells unlike CHP1/NHE1 cells were viable even after long serum starvation (>7 days). Thus, the expression of CHP2 appears to protect cells serum by of CHP2/NHE1 cells to of cells. We that of NHE1 by bound CHP2 is one of the for the of high pHi and the to serum cell in cells. The Na+/H+ Na+/H+ calcineurin B homologous Na+/H+ calcineurin B homologous that to the The of at that in or M. Biol. Chem. Biol. Chem. The NHE1 isoform a in pH and cell M. Biol. Chem. Biol. Chem. and has been in of and of NHE1 is that it is in to and cell in cytoplasmic alkalinization in the absence of M. Biol. Chem. Biol. Chem. In the NHE1 by the of a of B homologous protein T., Su, M. Biol. Chem. M. Biol. Chem. M. Biol. Chem. M. Biol. Chem. low and Biol. Chem. Biol. Chem. M. Biol. Chem. protein Biol. Chem. Biol. Chem. protein Biol. Chem. Biol. Chem. and Biol. has been the the of to the NHE1 not been has been that the pHi by NHE1 a or for cell and M. Biol. Chem. Biol. Chem. Biol. Chem. Biol. Chem. the pHi to NHE1 activity has been shown to in or cell M. the of NHE1 has been with M. Biol. Biol. Chem. Biol. Chem. S., M. M. M. cells M. Biol. Biol. Chem. Biol. Chem. or been shown to high pHi in the absence of serum with high activity of which be one of in cell or cell we that calcineurin B homologous protein essential which is for at NHEs to express high of exchange activity CHP1 in of activity T., Su, M. Biol. Chem. it is not CHP1 is in the of NHEs in to the is another human CHP isoform that was in human with of CHP2 protein high with CHP1 amino acid to the CHP to the of CHP in the NHE1 this study, we that cells co-expressing NHE1 and CHP2 but not cells co-expressing NHE1 and CHP1 high pHi the of NHE1 even in the absence of In cells viable even after serum starvation We that the of NHE1 with CHP2 but not with CHP1 to of which is a in CHP2 protein has a homologous to of human CHP1 protein 61% CHP1 protein and CHP2 protein to CHP1, CHP2 binding of which not of the of We the of CHP1 and CHP2 by a human and for CHP1 were in of the human tissues for CHP2 were not in tissues even after of with we to CHP2 in human tissues and by in which were with CHP2 high not In to the CHP2 CHP1 in cells The CHP2 expression in cells was by that CHP2 is expressed in cells expressed in tissues or that CHP1 to juxtamembrane in of of the cytoplasmic of NHEs T., Su, M. Biol. Chem. CHP1 is to to a region in of the high of the We CHP2 to in NHEs 1–3 by of In the CHP2 was in the not In in cells or a of was in the plasma membrane plasma membrane of was not in cells exchangers or which not CHP1 T., Su, M. Biol. Chem. that CHP2 to in NHEs of CHP2 with NHE1 was by a protein the cytoplasmic domain of NHE1 a we that CHP2 does not to a protein the cytoplasmic domain of that CHP2 interacts with the juxtamembrane in the plasma the binding of CHP1 CHP2, we in the of a of CHP2 and of CHP1. shown in the of CHP2 protein by with of CHP1 by for the common binding in NHE1. of CHP1 and of CHP2 were of CHP1 and were suggesting that CHP2 to NHE1 more strongly (∼5-fold) than does for and of protein was with of with of or and for were were with and by of protein and of protein were with of with of for and by were shown for and the expression to the of CHP2 in the exchange shown in the of for NHE1 or exchange activity in of for CHP1 or CHP2 with for NHE1 or the exchange The that CHP1 and CHP2 the ability to exchange of NHE1 and to a after of was in the or absence of of the CHP1 and CHP2, we produced stable cell clones overexpressing CHP1 or that cells express a relatively high of CHP1 and a low of CHP2 with the in the cells NHE1 protein cells co-expressing CHP1/NHE1 but not cells co-expressing NHE1 protein cells co-expressing CHP2/NHE1 but not cells co-expressing CHP1/NHE1 of NHE1 protein NHE1 not CHP1 or CHP2 with the that cells express CHP1 and The NHE1 protein was not in the with cells not NHE1 CHP2) or cells a NHE1 that CHP1 or CHP2 CHP bound to NHE1 of and NHE1 expressed in cells shown the shown the of In and of were by In the for the cell were by In the for cell were to with and by the of serum exchange activity in cells co-expressing CHP1/NHE1 or CHP2/NHE1 at pHi of cells activity at low pHi of serum In CHP1/NHE1 exchange activity at a pHi was with serum than serum In with or serum not exchange activity in CHP2/NHE1 cells at pHi with the pHi in CHP2/NHE1 cells was (7.4–7.5) in the absence or of of with or serum, pHi was in CHP1/NHE1 cells to serum Thus, NHE1 was in cells long serum pHi in CHP2 not NHE1 or in cells co-expressing the NHE1 and CHP2 not was in cells not suggesting that relatively long serum of serum cells co-expressing CHP1/NHE1 or CHP2/NHE1 in were for The pHi of or cells were at or by cells in the and the of and was of serum the The pHi of cells CHP2, or which were for in the or absence of serum, was in the absence or of by the of serum pHi of cells that been for serum Serum induced a relatively cytoplasmic alkalinization in CHP1/NHE1 cells by or acid alkalinization in NHE1 cells or in CHP1/NHE1 cells In serum a alkalinization in CHP2/NHE1 cells. was not in CHP2 not NHE1 or in CHP2 and that NHE1 was already in CHP2/NHE1 in the absence of pH has been to cell and cell M. M. Biol. Chem. cell in the of serum expression of NHE1 of the of CHP isoform cells relatively this of CHP1 and CHP2 in cell the ability to cell serum starvation was cells and We cell by the of cells to serum cells and were to of CHP2/NHE1 were more to serum starvation than cells CHP1/NHE1 days). Surprisingly, of cells CHP2/NHE1 were viable after serum starvation CHP1/NHE1 cells The high of CHP2/NHE1 cells NHE1 activity but not the expression of CHP2 CHP2 or cells were to serum starvation and the of in CHP2/NHE1 cells that the high of CHP2/NHE1 cells be the of high pHi by high Na+/H+ activity in of expression of CHP1 or CHP2 cell and cell serum of cells was in the of the cells were and by B and cell serum The of cells were and serum was in the or We the of cells that in We the cell in the and them in the were for this study, we characterized the function of CHP2 that a high with CHP1. We that CHP1 essential the activity of plasma Na+/H+ exchangers T., Su, M. Biol. Chem. CHP2 CHP2 with NHEs 1–3 but not with or in the plasma CHP2 was bound to a protein the NHE1 cytoplasmic domain but not the protein cytoplasmic CHP2 exchange of NHE1 and with them in The of CHP2/NHE1 was with that of cells a low exchange activity with CHP1/NHE1 and CHP2/NHE1 cells to serum a high exchange activity in In the absence of serum, CHP2/NHE1 a higher pHi and higher exchange activity in the pHi with Thus, NHE1 with bound CHP2 of We that of pHi in CHP1/NHE1 and CHP2/NHE1 cells for in or were not by the or absence of In the of the of CHP1/NHE1 cells to a with The pHi of CHP1/NHE1 was in to to serum in the absence of but this not in the of not of pHi be of of the it is not not CHP1/NHE1 and CHP2/NHE1 cells to serum that CHP2/NHE1 cells viable even a long serum CHP1 was to be in cell of calcineurin activity Biol. Chem. of Biol. Chem. with M. Biol. and with a protein protein M. M. Although we not CHP2 is in not be to the of CHP2 the of CHP1/NHE1 and CHP2/NHE1 were in this we a that to CHP2 be in the CHP2 it is that high of CHP2/NHE1 cells of high pHi of cells overexpressing CHP2 were to serum starvation NHE1 was not expressed or was in the and to cell which cell in cell cells cells human B and human cells M. M. Biol. Chem. that pHi is for serum cell that relatively high of CHP2 were in of cells but not in tissues or cells. is with that CHP2 expression was in cells and tumor cells has been that cells high protein M. Biol. and stable of Biol. Chem. Biol. Chem. or of in a in the pHi in cells the of NHE1. high pHi of of NHE1 has been in human human and human cells S., M. Biol. Chem. in this S., M. Biol. Chem. pHi was shown to be higher than of that NHE1 in the for a The of NHE1 in cells to of CHP2/NHE1 CHP2 appears to be expressed in cells. In NHE1 interacts more strongly with CHP2 than with CHP1. we that the of NHE1 by bound CHP2 be a for the of high pHi in we not NHE1 is by that serum the of CHP1. Although it is not of CHP1 is a in the of NHE1 in the in the be one to the in the of of NHE1 by CHP1 or that CHP1 and for protein or protein and in CHP1 not in CHP with that serum to with or and of the of to the in the of the CHP1 and the that the of NHE1 with CHP2 to of which in in the of cells serum S., M. M. M. has that NHE1 the of tumor in that CHP2 be a for it appears to be expressed in cells. The Na+/H+ Na+/H+ calcineurin B homologous Na+/H+ calcineurin B homologous that to the The of at that in or M. Biol. Chem. Biol. Chem. The NHE1 isoform a in pH and cell M. Biol. Chem. Biol. Chem. and has been in of and of NHE1 is that it is in to and cell in cytoplasmic alkalinization in the absence of M. Biol. Chem. Biol. Chem. In the NHE1 by the of a of B homologous protein T., Su, M. Biol. Chem. M. Biol. Chem. M. Biol. Chem. M. Biol. Chem. low and Biol. Chem. Biol. Chem. M. Biol. Chem. protein Biol. Chem. Biol. Chem. protein Biol. Chem. Biol. Chem. and Biol. has been the the of to the NHE1 not been has been that the pHi by NHE1 a or for cell and M. Biol. Chem. Biol. Chem. Biol. Chem. Biol. Chem. the pHi to NHE1 activity has been shown to in or cell M. the of NHE1 has been with M. Biol. Biol. Chem. Biol. Chem. S., M. M. M. cells M. Biol. Biol. Chem. Biol. Chem. or been shown to high pHi in the absence of serum with high activity of which be one of in cell or cell we that calcineurin B homologous protein essential which is for at NHEs to express high of exchange activity CHP1 in of activity T., Su, M. Biol. Chem. it is not CHP1 is in the of NHEs in to the is another human CHP isoform that was in human with of CHP2 protein high with CHP1 amino acid to the CHP to the of CHP in the NHE1 In this study, we that cells co-expressing NHE1 and CHP2 but not cells co-expressing NHE1 and CHP1 high pHi the of NHE1 even in the absence of In cells viable even after serum starvation We that the of NHE1 with CHP2 but not with CHP1 to of which is a in cells. CHP2 protein has a homologous to of human CHP1 protein 61% CHP1 protein and CHP2 protein to CHP1, CHP2 binding of which not of the of We the of CHP1 and CHP2 by a human and for CHP1 were in of the human tissues for CHP2 were not in tissues even after of with we to CHP2 in human tissues and by in which were with CHP2 high not In to the CHP2 CHP1 in cells The CHP2 expression in cells was by that CHP2 is expressed in cells expressed in tissues or that CHP1 to juxtamembrane in of of the cytoplasmic of NHEs T., Su, M. Biol. Chem. CHP1 is to to a region in of the high of the We CHP2 to in NHEs 1–3 by of In the CHP2 was in the not In in cells or a of was in the plasma membrane plasma membrane of was not in cells exchangers or which not CHP1 T., Su, M. Biol. Chem. that CHP2 to in NHEs of CHP2 with NHE1 was by a protein the cytoplasmic domain of NHE1 a we that CHP2 does not to a protein the cytoplasmic domain of that CHP2 interacts with the juxtamembrane in the plasma the binding of CHP1 CHP2, we in the of a of CHP2 and of CHP1. shown in the of CHP2 protein by with of CHP1 by for the common binding in NHE1. of CHP1 and of CHP2 were of CHP1 and were suggesting that CHP2 to NHE1 more strongly (∼5-fold) than does the expression to the of CHP2 in the exchange shown in the of for NHE1 or exchange activity in of for CHP1 or CHP2 with for NHE1 or the exchange The that CHP1 and CHP2 the ability to exchange of NHE1 and to a after of was in the or absence of of the CHP1 and CHP2, we produced stable cell clones overexpressing CHP1 or that cells express a relatively high of CHP1 and a low of CHP2 with the in the cells NHE1 protein cells co-expressing CHP1/NHE1 but not cells co-expressing NHE1 protein cells co-expressing CHP2/NHE1 but not cells co-expressing CHP1/NHE1 of NHE1 protein NHE1 not CHP1 or CHP2 with the that cells express CHP1 and The NHE1 protein was not in the with cells not NHE1 CHP2) or cells a NHE1 that CHP1 or CHP2 CHP bound to NHE1 of and NHE1 expressed in cells shown the shown the of In and of were by In the for the cell were by In the for cell were to with and by the of serum exchange activity in cells co-expressing CHP1/NHE1 or CHP2/NHE1 at pHi of cells activity at low pHi of serum In CHP1/NHE1 exchange activity at a pHi was with serum than serum In with or serum not exchange activity in CHP2/NHE1 cells at pHi with the pHi in CHP2/NHE1 cells was (7.4–7.5) in the absence or of of with or serum, pHi was in CHP1/NHE1 cells to serum Thus, NHE1 was in cells long serum pHi in CHP2 not NHE1 or in cells co-expressing the NHE1 and CHP2 not was in cells not suggesting that relatively long serum of serum cells co-expressing CHP1/NHE1 or CHP2/NHE1 in were for The pHi of or cells were at or by cells in the and the of and was of serum the The pHi of cells CHP2, or which were for in the or absence of serum, was in the absence or of by the of serum pHi of cells that been for serum Serum induced a relatively cytoplasmic alkalinization in CHP1/NHE1 cells by or acid alkalinization in NHE1 cells or in CHP1/NHE1 cells In serum a alkalinization in CHP2/NHE1 cells. was not in CHP2 not NHE1 or in CHP2 and that NHE1 was already in CHP2/NHE1 in the absence of pH has been to cell and cell M. M. Biol. Chem. cell in the of serum expression of NHE1 of the of CHP isoform cells relatively this of CHP1 and CHP2 in cell the ability to cell serum starvation was cells and We cell by the of cells to serum cells and were to of CHP2/NHE1 were more to serum starvation than cells CHP1/NHE1 days). Surprisingly, of cells CHP2/NHE1 were viable after serum starvation CHP1/NHE1 cells The high of CHP2/NHE1 cells NHE1 activity but not the expression of CHP2 CHP2 or cells were to serum starvation and the of in CHP2/NHE1 cells that the high of CHP2/NHE1 cells be the of high pHi by high Na+/H+ activity in of expression of CHP1 or CHP2 cell and cell serum of cells was in the of the cells were and by B and cell serum The of cells were and serum was in the or We the of cells that in We the cell in the and them in the were for CHP2 protein has a homologous to of human CHP1 protein 61% CHP1 protein and CHP2 protein to CHP1, CHP2 binding of which not of the of We the of CHP1 and CHP2 by a human and for CHP1 were in of the human tissues for CHP2 were not in tissues even after of with we to CHP2 in human tissues and by in which were with CHP2 high not In to the CHP2 CHP1 in cells The CHP2 expression in cells was by that CHP2 is expressed in cells expressed in tissues or cells. We that CHP1 to juxtamembrane in of of the cytoplasmic of NHEs T., Su, M. Biol. Chem. CHP1 is to to a region in of the high of the We CHP2 to in NHEs 1–3 by of In the CHP2 was in the not In in cells or a of was in the plasma membrane plasma membrane of was not in cells exchangers or which not CHP1 T., Su, M. Biol. Chem. that CHP2 to in NHEs of CHP2 with NHE1 was by a protein the cytoplasmic domain of NHE1 a we that CHP2 does not to a protein the cytoplasmic domain of that CHP2 interacts with the juxtamembrane in the plasma the binding of CHP1 CHP2, we in the of a of CHP2 and of CHP1. shown in the of CHP2 protein by with of CHP1 by for the common binding in NHE1. of CHP1 and of CHP2 were of CHP1 and were suggesting that CHP2 to NHE1 more strongly (∼5-fold) than does CHP1. We the expression to the of CHP2 in the exchange shown in the of for NHE1 or exchange activity in of for CHP1 or CHP2 with for NHE1 or the exchange The that CHP1 and CHP2 the ability to exchange of NHE1 and to a the CHP1 and CHP2, we produced stable cell clones overexpressing CHP1 or that cells express a relatively high of CHP1 and a low of CHP2 with the in the cells NHE1 protein cells co-expressing CHP1/NHE1 but not cells co-expressing NHE1 protein cells co-expressing CHP2/NHE1 but not cells co-expressing CHP1/NHE1 of NHE1 protein NHE1 not CHP1 or CHP2 with the that cells express CHP1 and The NHE1 protein was not in the with cells not NHE1 CHP2) or cells a NHE1 that CHP1 or CHP2 CHP bound to NHE1 We the of serum exchange activity in cells co-expressing CHP1/NHE1 or CHP2/NHE1 at pHi of cells activity at low pHi of serum In CHP1/NHE1 exchange activity at a pHi was with serum than serum In with or serum not exchange activity in CHP2/NHE1 cells at pHi with the pHi in CHP2/NHE1 cells was (7.4–7.5) in the absence or of of with or serum, pHi was in CHP1/NHE1 cells to serum Thus, NHE1 was in cells long serum pHi in CHP2 not NHE1 or in cells co-expressing the NHE1 and CHP2 not was in cells not suggesting that relatively long serum We the of serum pHi of cells that been for serum Serum induced a relatively cytoplasmic alkalinization in CHP1/NHE1 cells by or acid alkalinization in NHE1 cells or in CHP1/NHE1 cells In serum a alkalinization in CHP2/NHE1 cells. was not in CHP2 not NHE1 or in CHP2 and that NHE1 was already in CHP2/NHE1 in the absence of pH has been to cell and cell M. M. Biol. Chem. cell in the of serum expression of NHE1 of the of CHP isoform cells relatively this of CHP1 and CHP2 in cell the ability to cell serum starvation was cells and We cell by the of cells to serum cells and were to of CHP2/NHE1 were more to serum starvation than cells CHP1/NHE1 days). Surprisingly, of cells CHP2/NHE1 were viable after serum starvation CHP1/NHE1 cells The high of CHP2/NHE1 cells NHE1 activity but not the expression of CHP2 CHP2 or cells were to serum starvation and the of in CHP2/NHE1 cells that the high of CHP2/NHE1 cells be the of high pHi by high Na+/H+ activity in cells. this study, we characterized the function of CHP2 that a high with CHP1. We that CHP1 essential the activity of plasma Na+/H+ exchangers T., Su, M. Biol. Chem. CHP2 CHP2 with NHEs 1–3 but not with or in the plasma CHP2 was bound to a protein the NHE1 cytoplasmic domain but not the protein cytoplasmic CHP2 exchange of NHE1 and with them in The of CHP2/NHE1 was with that of cells a low exchange activity with CHP1/NHE1 and CHP2/NHE1 cells to serum a high exchange activity in In the absence of serum, CHP2/NHE1 a higher pHi and higher exchange activity in the pHi with Thus, NHE1 with bound CHP2 of We that of pHi in CHP1/NHE1 and CHP2/NHE1 cells for in or were not by the or absence of In the of the of CHP1/NHE1 cells to a with The pHi of CHP1/NHE1 was in to to serum in the absence of but this not in the of not of pHi be of of the it is not not CHP1/NHE1 and CHP2/NHE1 cells to serum that CHP2/NHE1 cells viable even a long serum CHP1 was to be in cell of calcineurin activity Biol. Chem. of Biol. Chem. with M. Biol. and with a protein protein M. M. Although we not CHP2 is in not be to the of CHP2 the of CHP1/NHE1 and CHP2/NHE1 were in this we a that to CHP2 be in the CHP2 it is that high of CHP2/NHE1 cells of high pHi of cells overexpressing CHP2 were to serum starvation NHE1 was not expressed or was in the and to cell which cell in cell cells cells human B and human cells M. M. Biol. Chem. that pHi is for serum cell that relatively high of CHP2 were in of cells but not in tissues or cells. is with that CHP2 expression was in cells and tumor cells has been that cells high protein M. Biol. and stable of Biol. Chem. Biol. Chem. or of in a in the pHi in cells the of NHE1. high pHi of of NHE1 has been in human human and human cells S., M. Biol. Chem. in this S., M. Biol. Chem. pHi was shown to be higher than of that NHE1 in the for a The of NHE1 in cells to of CHP2/NHE1 CHP2 appears to be expressed in cells. In NHE1 interacts more strongly with CHP2 than with CHP1. we that the of NHE1 by bound CHP2 be a for the of high pHi in we not NHE1 is by that serum the of CHP1. Although it is not of CHP1 is a in the of NHE1 in the in the be one to the in the of of NHE1 by CHP1 or that CHP1 and for protein or protein and in CHP1 not in CHP with that serum to with or and of the of to the in the of the CHP1 and the that the of NHE1 with CHP2 to of which in in the of cells serum S., M. M. M. has that NHE1 the of tumor in that CHP2 be a for it appears to be expressed in cells. In this study, we characterized the function of CHP2 that a high with CHP1. We that CHP1 essential the activity of plasma Na+/H+ exchangers T., Su, M. Biol. Chem. CHP2 CHP2 with NHEs 1–3 but not with or in the plasma CHP2 was bound to a protein the NHE1 cytoplasmic domain but not the protein cytoplasmic CHP2 exchange of NHE1 and with them in The of CHP2/NHE1 was with that of cells a low exchange activity with CHP1/NHE1 and CHP2/NHE1 cells to serum a high exchange activity in In the absence of serum, CHP2/NHE1 a higher pHi and higher exchange activity in the pHi with Thus, NHE1 with bound CHP2 of We that of pHi in CHP1/NHE1 and CHP2/NHE1 cells for in or were not by the or absence of In the of the of CHP1/NHE1 cells to a with The pHi of CHP1/NHE1 was in to to serum in the absence of but this not in the of not of pHi be of of the it is not not CHP1/NHE1 and CHP2/NHE1 cells to serum We that CHP2/NHE1 cells viable even a long serum CHP1 was to be in cell of calcineurin activity Biol. Chem. of Biol. Chem. with M. Biol. and with a protein protein M. M. Although we not CHP2 is in not be to the of CHP2 the of CHP1/NHE1 and CHP2/NHE1 were in this we a that to CHP2 be in the CHP2 it is that high of CHP2/NHE1 cells of high pHi of cells overexpressing CHP2 were to serum starvation NHE1 was not expressed or was in the and to cell which cell in cell cells cells human B and human cells M. M. Biol. Chem. that pHi is for serum cell We that relatively high of CHP2 were in of cells but not in tissues or cells. is with that CHP2 expression was in cells and tumor cells has been that cells high protein M. Biol. and stable of Biol. Chem. Biol. Chem. or of in a in the pHi in cells the of NHE1. high pHi of of NHE1 has been in human human and human cells S., M. Biol. Chem. in this S., M. Biol. Chem. pHi was shown to be higher than of that NHE1 in the for a The of NHE1 in cells to of CHP2/NHE1 CHP2 appears to be expressed in cells. In NHE1 interacts more strongly with CHP2 than with CHP1. we that the of NHE1 by bound CHP2 be a for the of high pHi in cells. we not NHE1 is by that serum the of CHP1. Although it is not of CHP1 is a in the of NHE1 in the in the be one to the in the of of NHE1 by CHP1 or that CHP1 and for protein or protein and in CHP1 not in CHP with that serum to with or and of the of to the in the of the CHP1 and In the that the of NHE1 with CHP2 to of which in in the of cells serum S., M. M. M. has that NHE1 the of tumor in that CHP2 be a for it appears to be expressed in cells.