Regulation of epithelial sodium channels (ENaC) by small G-proteins

Small G proteins are recognized to regulate diverse cellular processes including signal transduction, cell proliferation and differentiation. We investigated regulation of the epithelial Na⁺ channel (ENaC) by Ras and Rho GTPases. K-Ras, RhoA and Rac1 markedly increased activity of ENaC reconstituted in CHO cells. These GTPases activate ENaC in a GTP-dependent manner. However, the mechanisms of activation and signaling pathways differ between these two small G-proteins: Ras GTPases increase channel open probability; whereas, Rho GTPases, increase channel membrane levels. Inhibition of PI3-K abolished K-Ras actions on ENaC. Inhibition of other K-Ras effector cascades, including the MAPK and Ral/Rac/Rho cascades, did not affect K-Ras actions on ENaC. In contrast to K-Ras, RhoA-dependent activation of ENaC was markedly attenuated by inhibition of Rho-kinase signaling, but not the RhoA effector PI3-K and MAPK cascades. Similar to K-Ras and RhoA, overexpression of PI3-kinase, Rho-kinase and PI(4)P 5-K increased ENaC activity. We conclude that K-Ras enhances ENaC activity via PI3-K signaling by increasing channel open probability and Rho A activates ENaC via Rho-kinase and subsequent activation of PI(4)P 5-K with concomitant increases in PI(4, 5)P₂ levels promoting channel insertion into the plasma membrane.