G-ALPHA-13 STIMULATES NA-H+ EXCHANGE THROUGH DISTINCT CDC42-DEPENDENTAND RHOA-DEPENDENT PATHWAYS()

Activity of the ubiquitously expressed Na+-H+ exchanger subtype NHE1 i s stimulated upon activation of receptor tyrosine kinases and G protei n-coupled receptors. The intracellular signaling pathways mediating re ceptor regulation of the exchanger, however, are poorly understood. Us ing transient expression of dominant interfering and constitutively ac tive alleles in CCL39 fibroblasts, we determined that the GTPases Ha-R as and G alpha 13 stimulate NHE1 through distinct signaling cascades. Exchange activity stimulated by constitutively active RasV12 occurs th rough a Raf1- and mitogen-activated protein kinase kinase/extracellula r signal-regulated kinase kinase (MEK)-dependent mechanism. Constituti vely active G alpha 13QL, recently shown to stimulate the Jun kinase c ascade, activates NHE1 through a Cdc42- and MEK kinase (MEKK1)-depende nt mechanism that is independent of Rad. Constitutively active Rac1V12 does stimulate NHE1 through a MEKK1-dependent mechanism, but dominant interfering Rac1N17 does not inhibit G alpha 13QL-mediated or constit utively active Cdc42V12-mediated stimulation of the exchanger. Convers ely, Cdc42N17 does not inhibit Rac1V17 activation of NHE1, suggesting that Rad and Cdc42 independently regulate a MEKK1-dependent activation of the exchanger. Rapid (<10 min) stimulation of NHE1 with a G alpha 13/G alpha(z) chimera also was inhibited by a kinase-inactive MEKK. G alpha 13QL, but not RasV12, also stimulates NHE1 through a RhoA-depend ent pathway that is independent of MEKK, and microinjection of mutatio nally active G alpha 13 results in a Rho phenotype of increased stress fiber formation. These findings indicate a new target for Rho-like pr oteins: the regulation of H+ exchange and intracellular pH. Our findin gs also suggest that a MEKK cascade diverges to regulate effecters oth er than transcription factors.