Verity E. Mick, Omar A. Itani, Randy W. Loftus, Russell F. Husted, Thomas J. Schmidt, Christie P. Thomas

Aldosterone stimulates Na(+) reabsorption in the collecting ducts by increasing the activity of the epithelial sodium channel, ENaC. Systemic administration of aldosterone increases alpha ENaC mRNA expression in mammalian kidney, suggesting that the alpha ENaC gene is a target for aldosterone action in the distal nephron. To determine whether aldosterone increases alpha ENaC gene transcription, a portion of the alpha ENaC 5'- flanking region coupled to luciferase was transfected into MDCK-C7 cells, a collecting duct cell line with aldosterone-stimulated Na(+) transport. Both dexamethasone and aldosterone stimulated alpha ENaC-coupled reporter gene activity via the glucocorticoid receptor (GR), and this response correlated with the effect of these hormones on endogenous alpha ENaC expression. The aldosterone-stimulated alpha ENaC expression was blocked by actinomycin D, and aldosterone had no effect on alpha ENaC mRNA decay, confirming a transcriptional effect. In HT-29 cells, a GR/mineralocorticoid receptor (MR)-deficient colonic cell line with constitutive alpha ENaC expression, cotransfection with GR or MR restored aldosterone-stimulated alpha ENaC gene transcription, although aldosterone had a functional preference for MR. Analysis of deletion constructs confirmed that a single imperfect glucocorticoid response element (GRE) is necessary and sufficient to confer the aldosterone responsiveness to the alpha ENaC gene promoter in MDCK-C7 and HT-29 cells. These results confirm that alpha ENaC is an aldosterone-induced transcript in the collecting duct and delineates the molecular mechanism for this effect.