Activation of the Na⁺/H⁺ antiport is not required for epidermal growth factor-dependent gene expression, growth inhibition or proliferation in human breast cancer cells

Mitogen interaction with specific receptors in many cell types leads to activation of the Na⁺/H⁺ antiport and a resultant cytoplasmic alkalinization. Since amiloride inhibits both Na⁺/H⁺ exchange and cell proliferation, it has been hypothesized that activation of the antiport is an obligatory requirement for mitogenesis. However, concentrations of amiloride which inhibit the antiport also inhibit other cellular processes, including protein synthesis and phosphorylation. We have used an epidermal growth factor (EGF) receptor gene-amplified human breast cancer cell line, the growth of which is inhibited by high levels of EGF in culture (MDA-468) and a variant, the growth of which is stimulated by EGF (MDA-468-S4), along with two potent amiloride analogues to examine whether activation of the Na⁺/H⁺ antiport and cytoplasmic alkalinization is necessary for both EGF-dependent effects to occur. At concentrations of the amiloride analogues which block Na⁺/H⁺ exchange in both cell types by 76-98%, the EGF-dependent alterations in [³H]thymidine incorporation or induction in c-myc or c-fos gene transcription were unaltered. These results were confirmed by a lack of effect of the amiloride analogues on both the growth-stimulatory and growth-inhibitory effects on EGF in an anchorage-independent growth assay. Similarly, in pH-altered media that prevented normal cytoplasmic alkalinization, the response of both MDA-468 and MDA-468-S4 to EGF activation was unaltered. In addition, activation of the Na⁺/H⁺ antiport alone was not sufficient to induce c-myc and c-fos transcription in either cell type. Taken together, these data suggest that neither the Na⁺/H⁺ antiport nor cytoplasmic alkalinization are necessary or sufficient for either EGF-dependent growth stimulation or growth inhibition in MDA-468 human breast cancer cells.