Overexpression of csk inhibits acid-induced activation of NHE-3

Opossum kidney OKP cells express an apical membrane Na⁺/H⁺ antiporter that is encoded by NHE-3 (for Na⁺/H⁺ exchanger 3) and is similar in many respects to the renal proximal tubule apical membrane Na⁺/H⁺ antiporter. Chronic incubation of OKP cells in acid medium for 24 hr increases Na⁺/H⁺- antiporter activity and NHE-3 mRNA abundance. The increase in Na⁺/H⁺- antiporter activity was not prevented by H7, a protein kinase C/protein kinase A inhibitor, but was prevented by herbimycin A, a tyrosine kinase inhibitor. Incubation of cells in acid medium increased c-src activity, and this was inhibited by herbimycin A. To determine the role of the src family of nonreceptor protein-tyrosine kinases, Csk for carboxyl-terminal src kinase), a physiologic inhibitor of these kinases, was overexpressed in OKP cells. In three clones overexpressing csk, acid-induced increases in Na⁺/H⁺-antiporter activity and NHE-3 mRNA abundance were inhibited. In these clones, inhibition of acid activation of Na⁺/H⁺-antiporter activity paralleled inhibition of acid activation of c-src. Neither herbimycin A nor overexpression of csk inhibited dexamethasone-induced increases in Na⁺/H⁺- antiporter activity. These studies show that decreases in pH activate c-src and that the src family nonreceptor protein-tyrosine kinases play a key role in acid activation of NHE-3.