Endothelin-1 Inhibits the Epithelial Na⁺ Channel through βPix/14-3-3/Nedd4-2

Abstract

Epithelial Na⁺ channels (ENaCs) mediate sodium reabsorption in the cortical collecting duct (CCD), but the regulatory pathways that modulate the activity of these channels are incompletely understood. Here, we observed that endothelin-1 (ET-1) attenuates ENaC activity acutely by reducing the channel's open probability and chronically by decreasing the number of channels in the plasma membrane. To investigate whether β₁Pix, a signaling protein activated by ET-1, mediates ENaC activity, we reconstituted ENaC in CHO cells with or without coexpressed β₁Pix and found that β₁Pix negatively regulates ENaC. Knockdown of βPix in native principal cells abolished the ET-1-induced decrease in ENaC channel number. Furthermore, we found that βPix does not decrease ENaC activity through its guanine nucleotide exchange factor (GEF) activity for Rac1 and Cdc42. Instead, coexpression of β₁Pix mutant constructs revealed that β₁Pix affects ENaC activity through binding 14-3-3 proteins. Coimmunoprecipitation experiments supported a physical interaction between β₁Pix and 14-3-3β in cultured principal cells. Coexpression of 14-3-3β increased ENaC activity in CHO cells, but concomitant expression of β₁Pix attenuated this increase. Recruitment of 14-3-3β by β₁Pix impaired the interaction of 14-3-3β with the ubiquitin ligase Nedd4-2, thereby promoting ubiquitination and degradation of ENaC. Taken together, these results suggest that the inhibitory effects of chronic ET-1 on ENaC result from βPix interacting with the 14-3-3/Nedd4-2 pathway.