Requirement of the Na⁺/H⁺ Exchanger for Tumor Growth¹

The Na⁺/H⁺ exchanger is involved in a variety of cellular processes, including regulation of intracellular pH and possibly the control of cell growth and proliferation. To study the role of the Na⁺/H⁺ exchanger in tumor growth, human sodium proton exchanger-deficient (HSPD) mutants were derived from the human bladder carcinoma cell line MGH-U1 (EJ) by the proton suicide selection technique (J. Pouyssegur et al., Proc. Natl. Acad. Sci. USA, 81: 4833–4837, 1984). The HSPD cells were ∼40% larger and contained ∼70% more DNA than the parental cells. They were unable to grow in vitro in the absence of bicarbonate at pH <7.0, whereas the parental cells grew well at pH ≥6.6. This difference in acid sensitivity was abolished in the presence of bicarbonate. In contract to the parental MGH-U1 cells, the Na⁺/H⁺-deficient HSPD cells either failed to grow tumors, or showed severely retarded tumor growth when implanted into immune-deprived mice. This difference in tumor growth was not attributed to differences in cell size and DNA content, because Na⁺/H⁺ exchange-competent large cells (HLC), derived during the same proton suicide selection process as the HSPD cells, grew tumors at a rate close to that of the parental cells. Cells derived from the few tumors which grew after implantation of HSPD mutant cells were revertants which had regained Na⁺/H⁺ activity. HSPD cells also failed to form spheroids in culture, and the only spheroid formed consisted of revertant cells which had regained both Na⁺/H⁺ exchange activity and tumorigenic capacity. These results suggest that the Na⁺/H⁺ exchanger is important for tumor growth.