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 ma ny respects to the renal proximal tubule apical membrane Na+/H+ antipo rter, Chronic incubation of OKP cells in acid medium for 24 hr increas es Na+/H+-antiporter activity and NHE-3 mRNA abundance, The increase i n Na+/H+-antiporter activity was not prevented by H7, a protein kinase C/protein kinase A inhibitor, but was prevented by herbimycin A, a ty rosine kinase inhibitor, Incubation of cells in acid medium increased c-src activity, and this was inhibited by herbimycin A. To determine t he 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 overexpressin g csk, acid-induced increases in Na+/H+-antiporter activity and NHE-3 mRNA abundance were inhibited, In these clones, inhibition of acid act ivation of Na+-/H+-antiporter activity paralleled inhibition of acid a ctivation of c-src, Neither herbimycin A nor overexpression of csk inh ibited dexamethasone-induced increases in Na+/H+-antiporter activity. These studies show that decreases in pH activate c-src and that the sr c family nonreceptor protein-tyrosine kinases play a key role in acid activation of NHE-3.