Mitogen-stimulated activation of the Na+/H+ antiporter does not regulate S6 phosphorylation or protein synthesis in murine thymocytes or Swiss 3T3 fibroblasts.

The activation of protein synthesis by mitogens in quiescent (G0) mammalian cells is obligatory for progression from G0 through G1 to DNA synthesis in S phase. When the activation of the Na+/H+ antiporter which occurs in mitogen-stimulated Swiss 3T3 fibroblasts or murine fibroblasts is completely blocked by dimethylamiloride, there is little or no effect on the phosphorylation of the ribosomal protein S6 or the activation of protein synthesis assayed by [35S]methionine incorporation. Furthermore, the accumulation of the protein product of the activated c-myc gene is unaffected by dimethylamiloride in 3T3 fibroblasts. The data show that there is no requirement for activation of the Na+/H+ antiporter for the activation of S6 phosphorylation or protein synthesis by mitogens but do not preclude the possibility that activation of the antiporter is necessary for some other response(s) obligatory for DNA synthesis. These data are contrasted with previous reports for Chinese hamster lung fibroblasts that the increase in intracellular pH which results from activation of the Na+/H+ antiporter in bicarbonate-free media is necessary for S6 phosphorylation, protein synthesis, and hence, for subsequent DNA synthesis (Pouyssegur, J., Chambard, J. C., Franchi, A., Paris, S., and Van Obberghen-Schilling, E. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 3935-3939; Chambard, J.C., and Pouyssegur, J. (1986) Exp. Cell Res. 164, 282-294).