Modulation of α-ENaC and α1-Na+-K+-ATPase by cAMP and dexamethasone in alveolar epithelial cells

Abstract

cAMP and dexamethasone are known to modulate Na+ transport in epithelial cells. We investigated whether dibutyryl cAMP (DBcAMP) and dexamethasone modulate the mRNA expression of two key elements of the Na+ transport system in isolated rat alveolar epithelial cells: α-, β-, and γ-subunits of the epithelial Na+channel (ENaC) and the α1- and β1-subunits of Na+-K+-ATPase. The cells were treated for up to 48 h with DBcAMP or dexamethasone 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 α-ENaC and α1-Na+-K+-ATPase mRNA that peaked after 8 h of treatment. It also upregulated β- and γ-ENaC mRNA but not β1-Na+-K+-ATPase mRNA. Dexamethasone augmented α-ENaC mRNA expression 4.4-fold in cells treated for 24 h and also upregulated β- and γ-ENaC mRNA. There was a 1.6-fold increase at 8 h of β1-Na+-K+-ATPase mRNA but no significant modulation of α1-Na+-K+-ATPase mRNA expression. Because DBcAMP and dexamethasone did not increase the stability of α-ENaC mRNA, we cloned 3.2 kb of the 5′ sequences flanking the mouse α-ENaC gene to study the impact of DBcAMP and dexamethasone on α-ENaC promoter activity. The promoter was able to drive basal expression of the chloramphenicol acetyltransferase (CAT) reporter gene in A549 cells. Dexamethasone increased the activity of the promoter by a factor of 5.9. To complete the study, the physiological effects of DBcAMP and dexamethasone were investigated by measuring transepithelial current in treated and control cells. DBcAMP and dexamethasone modulated transepithelial current with a time course reminiscent of the profile observed for α-ENaC mRNA expression. DBcAMP had a greater impact on transepithelial current (2.5-fold increase at 8 h) than dexamethasone (1.8-fold increase at 24 h). These results suggest that modulation of α-ENaC and Na+-K+-ATPase gene expression is one of the mechanisms that regulates Na+ transport in alveolar epithelial cells.