Evidence against a requirement for Na+/H+ exchange activity for proliferation or differentiation of HL60 leukemic cells.

Although increased Na+/H+ exchange (antiport) activity has been shown to precede both granulocytic and monocytic differentiation of HL60 cells, its requirement for this process remains controversial. We show here that, in the absence of HCO3-, the Na+/H+ exchanger is the predominant mechanism involved in rapid recovery of pHi from cytoplasmic acidosis; however, in the presence of HCO3-, other mechanisms also contribute to pHi homeostasis. In the absence of HCO3-, dimethyl sulfoxide and 4 beta-phorbol 12-myristate 13 alpha-acetate result in an acute activation of antiport activity, whereas all-trans-retinoic acid, 1,25-dihydroxyvitamin D3, gamma-interferon, and tumor necrosis factor alpha do not. Furthermore, whether or not HCO3- is present, the amiloride analogue, 5-N,N-dimethylamiloride, a potent and more specific antiport inhibitor than the parent compound, failed to suppress HL60 cell proliferation or the decreased proliferation in response to dimethyl sulfoxide, all-trans-retinoic acid, 1,25-dihydroxyvitamin D3, gamma-interferon, or tumor necrosis factor alpha. In the absence of HCO3-, 5-N,N-dimethylamiloride also had little effect on the ability of these agents to induce functional maturation as assessed by acquisition of respiratory burst activity. Inhibition of antiport activity also did not prevent 4 beta-phorbol 12-myristate 13 alpha-acetate-induced expression of c-fos mRNA or cell adherence. The results suggest that, even under conditions in which the Na+/H+ exchanger is the predominant operable pHi regulator, antiport activity is not required for HL60 proliferation or differentiation.