Cycloheximide and lipopolysaccharide downregulate αENaC mRNA via different mechanisms in alveolar epithelial cells.

Active Na+ transport mediated by epithelial Na+ channel (ENaC) is vital for fetal lung fluid reabsorption at birth and pulmonary edema resolution. Previously, we demonstrated that ENaC expression and activity are downregulated in alveolar epithelial cells by cycloheximide (Chx) and Pseudomonas aeruginosa. The regulatory mechanisms of ENaC mRNA expression by Chx and lipopolysaccharide (LPS) from P. aeruginosa were further studied in the present work. Both agents decreased ENaC mRNA expression to 50% of control values after 4 h. Chx repressed ENaC expression in a dose-dependent manner independently of protein synthesis. Although extracellular signal-regulated kinases 1 and 2 (ERK1/2) and p38 mitogenactivated protein kinase (MAPK) pathways were activated by the two treatments, their mechanisms of ENaC mRNA modulation were different. First, activation of the signaling pathways was sustained by Chx but only transiently by LPS. Second, ERK1/2 or p38 MAPK inhibition attenuated the effects of Chx on ENaC mRNA, whereas suppression of both signaling pathways was necessary to alleviate the outcome of LPS on ENaC mRNA. The molecular mechanisms involved in the decrease of ENaC expression were investigated in both conditions. LPS, but not Chx, significantly reduced ENaC promoter activity via the ERK1/2 and p38 MAPK pathways. These results suggest that LPS attenuates ENaC mRNA expression via diminution of transcription, whereas Chx could trigger some posttranscriptional mechanisms. Although LPS and Chx downregulate ENaC mRNA expression similarly and with similar signaling pathways, the mechanisms modulating ENaC expression are different depending on the nature of the cellular stress. [ABSTRACT FROM AUTHOR]