Role of c-SRC and ERK in acid-induced activation of NHE3

Background. In the renal proximal tubule, chronic acidosis causes increases in apical membrane NHE3 activity, which serve to increase transepithelial H+ secretion and return systemic pH to normal levels. Incubation of cultured renal epithelial cells in acid media activates c-Src. Methods. OKP cells were incubated in control (pH 7.4) or acid (7.0) media, and NHE3 activity measured as cytoplasmic pH (pHi) recovery from an acid load using BCECF. c-Src, ERK. and JNK kinase activities were measured by immune complex kinase assays with enolase. MBP, and GST-c-Jun, respectively, as substrates in the in vitro assays. To determine the role of c-Src in acid-induced NHE3 activation, cells were transfected with vector alone or a dominant negative c-Src (c-Src(K295M)). Results. Expression of dominant negative c-src(K295M) in OKP cells prevented acid-induced activation of NHE3. Incubation of OKP cells in acid media increased ERK activity and c-fos expression. but did not increase JNK activity. Acidosis in vivo also activated renal cortical c-Src and ERK kinases. whereas incubation of 3T3 cells in acid media activated c-Src but not ERK kinase. Expression of c-src(K295M) did not affect ERK or c-fos activation by acid incubation. Inhibition of MEK with Pb98059 inhibited activation of NHE3 by acid incubation. Conclusions. These studies suggest that acidosis activates c-Src and MEK/ERK/c-fos. While both pathways are necessary for activation of NHE3, they are activated independently.