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 αENaC expression and activity are downregulated in alveolar epithelial cells by cycloheximide (Chx) and Pseudomonas aeruginosa. The regulatory mechanisms of αENaC mRNA expression by Chx and lipopolysaccharide (LPS) from P. aeruginosa were further studied in the present work. Both agents decreased αENaC mRNA expression to 50% of control values after 4 h. Chx repressed αENaC expression in a dose-dependent manner independently of protein synthesis. Although extracellular signal-regulated kinases 1 and 2 (ERK1/2) and p38 mitogen-activated 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 αENaC mRNA, whereas suppression of both signaling pathways was necessary to alleviate the outcome of LPS on αENaC mRNA. The molecular mechanisms involved in the decrease of αENaC expression were investigated in both conditions. LPS, but not Chx, significantly reduced αENaC promoter activity via the ERK1/2 and p38 MAPK pathways. These results suggest that LPS attenuates αENaC mRNA expression via diminution of transcription, whereas Chx could trigger some posttranscriptional mechanisms. Although LPS and Chx downregulate αENaC mRNA expression similarly and with similar signaling pathways, the mechanisms modulating ENaC expression are different depending on the nature of the cellular stress.

Acute respiratory distress syndrome (ARDS) features an exudative phase characterized by alveolar damage, lung edema and exacerbated inflammatory response. Given their anti‐inflammatory properties, the potential therapeutic effect of corticosteroids has been evaluated in ARDS clinical trials and experimental models of ALI. These studies produced contradictory results. Therefore, our aim was to investigate the effects of dexamethasone in an animal model of bleomycin‐induced acute lung injury and then to determine if the lack of response could be related to an impairment in repair ability of alveolar epithelial cells after injury. NMRI mice were challenged with bleomycin and then treated daily with dexamethasone or saline. Bronchoalveolar lavages (BAL) and lungs were collected for assessment of the inflammatory response and wet/dry ratio (lung edema) and for histological analyses. The effect of bleomycin and dexamethasone on wound repair was also evaluated in vitro on primary alveolar epithelial cell (ATII) cultures. Our data first showed that dexamethasone treatment did not reduce the weight loss or mortality rates induced by bleomycin. Although the TNF‐α level in BAL of bleomycin‐treated mice was reduced by dexamethasone, the neutrophil infiltration remained unchanged. Dexamethasone also failed to reduce lung edema and damage scores. Finally, bleomycin elicited a time‐ and dose‐dependent reduction in repair rates of ATII cell cultures. This inhibitory effect was further enhanced by dexamethasone, which also affected the expression of β3‐ and β6‐integrins, key proteins of alveolar repair. Altogether, our data indicate that the inability of dexamethasone to improve the resolution of ALI might be due to his deleterious effect on the alveolar epithelium repair.

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Active sodium transport by the epithelial Na+ channel (ENaC) is important for fluid movement across the alveolar epithelium in the lungs and for the resolution of pulmonary oedema in acute respiratory distress syndrome (ARDS). It was shown that cellular stress induced by lipopolysaccharides (LPS) or cycloheximide (Chx) downregulate αENaC mRNA via different molecular mechanisms. The purpose of this project was to investigate the importance of the αENaC 3′UTR in the mRNA modulation of ENaC. Alveolar epithelial cells were co‐transfected with 3′UTR mutants (V5/ENaC±3′UTR and Luc±3′UTR) inserted in pTRE‐tight vector along with the Tet‐Off vector. The cells were treated with Chx (1.0μM), LPS (15μg/mL) or Actinomycin D (5μg/mL) along with Dox (1μg/mL) to inhibit transcription. We found that 3′UTR deletion caused a ~60% decrease of luciferase activity and expression compared to the complete recombinant. The T1/2 of V5‐αENaC mRNA was 60 min and the deletion of 3′UTR reduced T1/2 to 37min. Chx reduced αENaC mRNA T1/2 to 33min whereas LPS had no effect. Actinomycin D, a transcription inhibitor, increased T1/2 >; 120min. These results suggest that the αENaC mRNA half‐life is shorter then previously estimated and that 3′UTR seems to be important for the stability of αENaC mRNA.Supported by FRSQ, CFC and CIHR.

The epithelial sodium channel (ENaC) plays an important role in the alveolar epithelium by mediating Na+ reabsorption, an essential process for resolution of pulmonary edema in acute respiratory distress syndrome (ARDS). Several proinflammatory conditions such as TNF‐α leads to a reduction of ENaC expression and activity that contributes to development of ARDS. To study how pro‐inflammatory conditions could modulate αENaC mRNA stability, we developped a Tet‐Off model that allows the specific inhibition of αENaC mRNA expression in presence of doxycycline. Using this model, we found that the half‐life (T1/2) for the transcript was about 1 hr. TNF‐α, a proinflammatory cytokine found in broncho‐alveolar lavage of ARDS patients, decreased T1/2 to 15 min, suggesting that destabilisation of αENaC transcript plays an important role in downregulation of ENaC expression in pro‐inflammatory conditions. The 3’ untranslated region (3’UTR) of a mRNA plays an important role in modulating mRNA stability. αENaC mRNA 3’UTR is very long (~30% of the transcript). Using a bioinformatic approach, we could define three domains (D1, D2, D3) able to form secondary structures that were highly conserved in different species. Alveolar epithelial cells were co‐transfected with 3’UTR deletion mutants inserted in pTRE‐tight vector along with the Tet‐Off vector. Deletion of the distal part of 3’UTR (D3) leaded to stabilization of the transcript with a T1/2 of 2.6 hrs. A longer deletion to remove also D2 showed a T1/2 of 4.6 hrs, while deletion of D1 decreased the half‐life to 0.5 hr. These results showed that the conserved domains have stabilizing (D1) and destabilizing (D2, D3) properties on αENaC mRNA expression. The hairpin structure of these domains could recruit RNA‐binding proteins responsible for modulation of αENaC mRNA stability. Altogether, our results suggest that modulation of αENaC mRNA stability via its 3’UTR could play an important role in regulation of ENaC expression during pro‐inflammatory conditions. Grant Funding Source: Supported by FRSQ, CFC and CIHR.