Gene regulation of ENaC subunits by serum- and glucocorticoid-inducible kinase-1

Aldosterone is a key regulator of epithelial Na+ channels (ENaC) in renal cortical collecting ducts (CCD). The goal of this study was to examine whether serum- and glucocorticoid-inducible kinase-1 (SGK1), an aldosterone-induced gene, is vital to the delayed effect of aldosterone by increasing the gene expression of ENaC subunits. To test this hypothesis, we compared the levels of ENaC mRNA in mouse CCD cells that stably express either full-length (FL)-SGK1 or a kinase-dead dominant negative (K127M)-SGK1. Our results revealed that SGK1 regulates gene expression of ENaC, whether cells are maintained in steroid-free media or in the presence of corticosteroids (CS) and/or other growth factors. Under all conditions, the loss of function of SGK1 caused a significant decrease in the expression of alpha- and beta-ENaC, but not gamma-ENaC. Compared with cells expressing FL-SGK1, K127M-SGK1 decreased the expression of alpha- and beta-subunit mRNA by approximately 45 and approximately 90%, respectively. Next, to determine whether SGK1 is one of the proteins mediating the induction of alpha-ENaC mRNA by CS, we compared steroid induction of alpha-ENaC in cells expressing K127M-SGK1 vs. FL-SGK1. The maximum level of alpha-ENaC mRNA levels following CS was significantly (approximately 45%) higher in FL-SGK1- vs. K127M-SGK1-expressing cells, although the fold-induction by CS was similar in both FL-SGK1- and K127M-SGK1-expressing cells. In summary, we report for the first time that SGK1 regulates transcription of ENaC subunits. We propose that the effect of SGK1 on ENaC transcription is mediated by the activation of unidentified transcription factors.