A characteristic property of the tumor environment is that extracellular pH is more acidic, and intracellular pH more alkaline, than that of normal cells. As transformation is associated with greater dependence on glycolysis and hence H⁺ production, increased activity of H⁺ extruding transport proteins is likely to be important for tumor growth¹. Two such transporters are the Na⁺/H⁺ exchanger NHE1 and the Na⁺,HCO₃^- cotransporter NBCn1. Expression of a truncated, constitutively active ErbB2 receptor (dNErbB2) is common in human breast cancer and associated with increased malignancy and poor prognosis. Here, we investigate the effect of dNErbB2 expression in MCF-7 breast cancer cells on the expression and activity of NHE1 and NBCn1, and we start to investigate the roles of pH regulatory ion transport in chemotherapy resistance of MCF-7 cells in the absence and presence of dNErbB2. MCF-7 cells expressing dNErbB2 under control of a tetracycline (tet)-off system (dNErbB2 cells)² were grown without tet for 48 h to induce dNErbB2 expression. Uninduced dNErbB2 cells and MCF-7 cells expressing the corresponding vector without dNErbB2, grown without tet for 48 h, were employed as controls. 48 h after dNErbB2 expression, increases of about 15 and 50%, respectively, in NHE1 and NBCn1 expression were determined by Western blotting, and immunofluorescence analysis confirmed that this reflected increased expression of the transporters in the plasma membrane. Accordingly, the rate of pH_i recovery after an acute acid load in the presence of HCO₃^- was increased by about 25%, and the fraction of the recovery inhibitable by the NHE1 inhibitor EIPA was reduced. In accordance with previous reports, control MCF-7 cells were highly resistant to the chemotherapeutic agent cisplatin. As evaluated by MTT viability assays, exposure to 100 mM cisplatin for 18 h induced only about 60% loss of viability. Cisplatin did however induce Asp-Glu-Val-Asp (DEVD)ase activity already at <= 10 mM, presumably reflecting caspase-7 activity, as MCF-7 cells do not express caspase-3. Cisplatin resistance was modestly reduced by dNErbB2 expression. In the absence of cisplatin, NHE1 inhibition by EIPA had no detectable effect on viability of control cells, yet reduced viability of dNErbB2 cells by 20%. Notably, the additional presence of EIPA reduced viability after exposure to only 5 mM cisplatin from 80% to 35% in dNErbB2 cells, while this value was reduced from about 100% to 80% by EIPA in control cells. In conclusion, NHE1 and NBCn1 expression was upregulated, and the rate of recovery from acid loading increased, by dNErbB2 expression. In conjunction with the finding that inhibition of NHE1 strongly potentiated cisplatin-induced death especially in dNErbB2 expressing cells, this warrants investigation of whether NHE1 and NBCn1 may be interesting potential targets in breast cancer treatment.

1. Cardone et al. 2005. 5: 786–795; 2. Egeblad M et al. 2001. 94:185–91