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.
Husted
Russell F
RF
Department of Internal Medicine, 200 Hawkins Drive, University of Iowa, Iowa City, IA 52246, USA.
Volk
Kenneth A
KA
Sigmund
Rita D
RD
Stokes
John B
JB
D000450:Aldosterone / Q000494:pharmacology*; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects / Q000502:physiology; D002460:Cell Line; D003907:Dexamethasone / Q000494:pharmacology*; D053503:Epithelial Sodium Channels / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D005786:Gene Expression Regulation / Q000187:drug effects*; D006801:Humans; D021122:Protein Subunits / Q000235:genetics* / Q000378:metabolism*; D051381:Rats; D012964:Sodium / Q000378:metabolism
D053503:Epithelial Sodium Channels; D021122:Protein Subunits; D000450:Aldosterone; D003907:Dexamethasone; D012964:Sodium