Objectives: Upregulation of acid extrusion has been assigned important roles in cancer progression, however, the mechanisms are poorly understood (1). Expression of an N-terminally truncated, constitutively active ErbB2 receptor (DNErbB2) is common in breast cancer and associated with poor prognosis. Here, we investigate the impact of two pH regulatory transporters, the Na⁺/H⁺ exchanger NHE1 and the Na⁺,HCO₃-cotransporter NBCn1, on survival and chemotherapy resistance in MCF-7 breast cancer cells, in the presence and absence of DNErbB2. Methods: mRNA and protein levels were assessed by qPCR and Western blotting, H⁺ extrusion as pHi recovery after an NH4Cl prepulse, and death mechanisms by viability-, chromatin condensation- , and cathepsin release assays. Results: Upon DNErbB2 expression, mRNA and protein levels of NBCn1, yet not of NHE1, increased several-fold. The pHi recovery rate increased by 50%, and siRNA knockdown and pharmacological inhibition confirmed the involvement of both NHE1 and NBCn1. Inhibition (EIPA)/knockdown of NHE1 sensitized the cells to cisplatin- and etoposide- induced cell death and increased cathepsin release. NHE1-dependent sensitization to cisplatin was augmented by DNErbB2 expression, and inhibited by the caspase inhibitor Z-VAD-FMK and the ROS scavenger NAC. In contrast, inhibition of NBCn1 reduced cathepsin release and did not induce sensitization to chemotherapeutic compounds. Conclusion: DNErbB2 expression in MCF-7 cells elicits robust NBCn1 upregulation and increases the rate of pHi regulation after an acid load. Inhibition/knockdown of NHE1, yet not of NBCn1, exacerbates cysteine cathepsin release and sensitizes DNErbB2-MCF-7 cells to cisplatin– and etoposide-induced PCD. This warrants studies of NHE1 and NBCn1 as potential targets in breast cancer and demonstrate that in spite of their similar transport functions, NHE1 and NBCn1 serve different functions in MCF-7 cells. (1) Gatenby, R.A., Gillies, R.J. Nat Rev Cancer 8: 56-61, 2008.