Gα13 stimulates Na+-H+ exchange through distinct Cdc42-dependent and RhoA-dependent pathways

Abstract

Activity of the ubiquitously expressed Na+-H+ exchanger subtype NHE1 is stimulated upon activation of receptor tyrosine kinases and G protein- coupled receptors. The intracellular signaling pathways mediating receptor regulation of the exchanger, however, are poorly understood. Using transient expression of dominant interfering and constitutively active alleles in CCL39 fibroblasts, we determined that the GTPases Ha-Ras and Gα13 stimulate NHE1 through distinct signaling cascades. Exchange activity stimulated by constitutively active RasV12 occurs through a Raf1- and mitogen-activated protein kinase kinase/extracellular signal-regulated kinase kinase (MEK)- dependent mechanism. Constitutively active Gα13QL, recently shown to stimulate the Jun kinase cascade, activates NHE1 through a Cdc42- and MEK kinase (MEKK1)-dependent mechanism that is independent of Rac1. Constitutively active Rac1V12 does stimulate NHE1 through a MEKK1-dependent mechanism, but dominant interfering Rac1N17 does not inhibit Gα13QL-mediated or constitutively active Cdc42V12-mediated stimulation of the exchanger. Conversely, Cdc42N17 does not inhibit Rac1V12 activation of NHE1, suggesting that Rac1 and Cdc42 independently regulate a MEKK1-dependent activation of the exchanger. Rapid (<10 min) stimulation of NHE1 with a Gα13/Gα(z) chimera also was inhibited by a kinase-inactive MEKK. Gα13QL, but not RasV12, also stimulates NHE1 through a RhoA-dependent pathway that is independent of MEKK, and microinjection of mutationally active Gα13 results in a Rho phenotype of increased stress fiber formation. These findings indicate a new target for Rho-like proteins: the regulation of H+ exchange and intracellular pH. Our findings also suggest that a MEKK cascade diverges to regulate effectors other than transcription factors.