Modulation of alpha-ENaC and alpha(1)-Na+-K+-ATPase by cAMP and dexamethasone in alveolar epithelial cells

cAMP and dexamethasone are known to modulate Na+ transport in epithelial ce lls. We investigated whether dibutyryl cAMP (DBcAMP) and dexamethasone modu late the mRNA expression of two key elements of the Nac transport system in isolated rat alveolar epithelial cells: alpha-, beta-, and gamma -subunits of the epithelial Na+ channel (ENaC) and the aland beta (1)-subunits of Na +-K+-ATPase. The cells were treated for up to 48 h with DBcAMP or dexametha sone to assess their long-term impact on the steady-state level of ENaC and Na+-K+-ATPase mRNA. DBcAMP induced a twofold transient increase of alpha - ENaC and alpha (1)-Na+-K+-ATPase mRNA that peaked after 8 h of treatment. I t also upregulated beta- and gamma -ENaC mRNA but not beta (1)-Na+-K+-ATPas e mRNA. Dexamethasone augmented alpha -ENaC mRNA expression 4.4-fold in cel ls treated for 24 h and also upregulated beta- and gamma -ENaC mRNA. There was a 1.6-fold increase at 8 h of beta (1)-Na+-K+-ATPase mRNA but no signif icant modulation of alpha (1)-Na+-K+-ATPase mRNA expression. Because DBcAMP and dexamethasone did not increase the stability of alpha -ENaC mRNA, we c loned 3.2 kb of the 5' sequences flanking the mouse alpha -ENaC gene to stu dy the impact of DBcAMP and dexamethasone on alpha -ENaC promoter activity. The promoter was able to drive basal expression of the chloramphenicol ace tyltransferase (CAT) reporter gene in A549 cells. Dexamethasone increased t he activity of the promoter by a factor of 5.9. To complete the study, the physiological effects of DBcAMP and dexamethasone were investigated by meas uring transepithelial current in treated and control cells. DBcAMP and dexa methasone modulated transepithelial current with a time course reminiscent of the profile observed for alpha -ENaC mRNA expression. DBcAMP had a great er impact on transepithelial current (2.5-fold increase at 8 h) than dexame thasone (1.8-fold increase at 24 h). These results suggest that modulation of alpha -ENaC and Na+-K+-ATPase gene expression is one of the mechanisms t hat regulates Na+ transport in alveolar epithelial cells.