Trophic factor withdrawal: p38 mitogen-activated protein kinase activates NHE1, which induces intracellular alkalinization

Trophic factor withdrawal induces cell death by mechanisms that are incompl etely understood. Previously we reported that withdrawal of interleukin-7 ( IL-7) or IL-3 produced a rapid intracellular alkalinization, disrupting mit ochondrial metabolism and activating the death protein Bax. We now observe that this novel alkalinization pathway is mediated by the pH regulator NHE1 , as shown by the requirement for sodium, blocking by pharmacological inhib itors or use of an NHE1-deficient cell line, and the altered phosphorylatio n of NHE1 Alkalinization also required the stress-activated p38 mitogen-act ivated protein kinase (MAPK). Inhibition of p38 MAPK activity with pharmaco logical inhibitors or expression of a dominant negative kinase prevented al kalinization. Activated p38 MAPK directly phosphorylated the C terminus of NHE1 within a 40-amino-acid region. Analysis by mass spectroscopy identifie d four phosphorylation sites on NHE1, Thr 717, Ser 722, Ser 725, and Ser 72 8. Thus, loss of trophic cytokine signaling induced the p38 MAPK pathway, w hich phosphorylated NHE1 at specific sites, inducing intracellular alkalini zation.