부산대학교 도서관

Modulation of [Alpha]-ENaC and [[Alpha].sub.1]-[Na.sup.+]-[K.sup.+]-ATPase by cAMP and dexamethasone in alveolar epithelial cells. Am J Physiol Lung Cell Mol Physiol 281: L217-L230, 2001.--cAMP and dexamethasone are known to modulate [Na.sup.+] transport in epithelial cells. We investigated whether dibutyryl cAMP (DBcAMP) and dexamethasone modulate the mRNA expression of two key elements of the [Na.sup.+] transport system in isolated rat alveolar epithelial cells: [Alpha]-, [Beta]-, and [Gamma]-subunits of the epithelial [Na.sup.+] channel (ENaC) and the [[Alpha].sub.1]- and [[Beta].sub.1]-subunits of [Na.sup.+]-[K.sup.+]-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.sup.+]-[K.sup.+]-ATPase mRNA. DBcAMP induced a twofold transient increase of [Gamma]-ENaC and [[Alpha].sub.1]-[Na.sup.+]-[K.sup.+]-ATPase mRNA that peaked after 8 h of treatment. It also upregulated [Beta]- and [Gamma]-ENaC mRNA but not [[Beta].sub.1]-[Na.sup.+]-[K.sup.+]-ATPase mRNA. Dexamethasone augmented [Alpha]-ENaC mRNA expression 4.4-fold in cells treated for 24 h and also upregulated [Beta]- and [Gamma]-ENaC mRNA. There was a 1.6-fold increase at 8 h of [[Beta].sub.1]-[Na.sup.+]-[K.sup.+]-ATPase mRNA but no significant modulation of [[Alpha].sub.1]-[Na.sup.+]-[K.sup.+]-ATPase mRNA expression. Because DBcAMP and dexamethasone did not increase the stability of [Alpha]-ENaC mRNA, we cloned 3.2 kb of the 5' sequences flanking the mouse [Alpha]-ENaC gene to study the impact of DBcAMP and dexamethasone on [Alpha]-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 [Alpha]-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 [Alpha]-ENaC and [Na.sup.+]-[K.sup.+]-ATPase gene expression is one of the mechanisms that regulates [Na.sup.+] transport in alveolar epithelial cells. sodium channel; adenosine 3',5'-cyclic monophosphate; steroid; sodium-potassium-adenosinetriphosphatase; transepithelial current; epithelial sodium channel