Na⁺/H⁺ exchange and cytoplasmic pH in the action of growth factors in human fibroblasts

Abstract

The mechanisms by which growth factors stimulate metabolism and cell proliferation are largely unknown. Recent evidence suggests that mitogens rapidly activate a Na⁺/H⁺ exchange mechanism in the plasma membrane of their target cells^1–3, implicating cytoplasmic pH (pH_i) as a potential ‘messenger’. Indeed, growth stimulation of quiescent fibroblasts leads to intracellular alkalinization at ∼1h after mitogen addition, as measured by weak-acid distribution methods^4,5. We have used an internalized fluorescent pH_i indicator⁶ to examine the pH_i regulating mechanisms in diploid human fibroblasts and to obtain the first continuous pH_i recordings of the response to growth factors. We report here that (1) pH_i in human fibroblasts is controlled by a membrane-bound Na⁺/H⁺ exchanger, which rapidly restores pH_i after an acute cytoplasmic acidification, and (2) epidermal growth factor (EGF) and serum factors induce a rapid and persistent elevation of pH_i by modifying the pH_i sensitivity of the Na⁺/H⁺ exchanger. We conclude that in addition to having a basic role in pH_i regulation, Na⁺/H⁺ exchange may function as a transmembrane signal transducer in the action of growth factors.