Discordant effects of corticosteroids and expression of subunits on ENaC activity

In renal distal nephron and airway epithelial cells, adrenocortical steroids increase epithelial Na+ channel (ENaC) activity and also markedly increase the expression of the alpha-subunit. The present experiments were designed to reconstitute this steroid effect in ENaC-expressing cells by overexpressing the subunits whose expression is enhanced by corticosteroids. In renal collecting duct monolayers, corticosteroids increased ENaC activity 5- to 8-fold, endogenous alpha-ENaC mRNA and protein approximately 10-fold, and beta-ENaC protein and mRNA 1.2- to 2-fold. gamma-ENaC expression was unchanged. To determine whether this increase in expression was sufficient to increase ENaC activity, we used a regulated adenovirus system to increase expression of each subunit alone and in combination. Unexpectedly, increased expression of the alpha- and/or beta-subunit had no effect on ENaC activity in collecting duct cells or lung epithelial cells. In contrast, a small increase in gamma-ENaC expression increased ENaC activity about threefold. This increase in activity was additive to the effect of steroids. Thus, even though corticosteroids strongly increase alpha-ENaC expression and moderately increase beta-ENaC expression, these effects are not, by themselves, sufficient to increase ENaC activity. Knockdown experiments are consistent with the idea that the increased expression of alpha-ENaC is necessary for the full steroid effect on ENaC. Increased expression of gamma-ENaC and corticosteroid treatment enhances ENaC activity by parallel, noninteracting pathways. These results underscore the importance of other actions of steroid hormones for long-term enhancement of ENaC activity and raise new possibilities for regulation of ENaC activity by gamma-ENaC expression.