ENaC α-subunit variants are expressed in lung epithelial cells and are suppressed by oxidative stress

Abstract

Amiloride-sensitive epithelial sodium channel (ENaC) is a major sodium channel in the lung facilitating fluid absorption. ENaC is composed of alpha-, beta-, and gamma-subunits, and the alpha-subunit is indispensable for ENaC function in the lung. In human lungs, the alpha-subunit is expressed as various splice variants. Among them, alpha(1)- and alpha(2)-subunits are two major variants with different upstream regulatory sequences that possess similar channel characteristics when tested in Xenopus oocytes. Despite the importance of alpha-ENaC, little was known about the relative abundance of its variants in lung epithelial cells. Furthermore, lung infection and inflammation are often accompanied by reduced alpha-ENaC expression, oxidative stress, and pulmonary edema. However, it was not clear how oxidative stress affects expression of alpha-ENaC variants. In this study, we examined relative expression levels of alpha-subunit variants in four human lung epithelial cell lines. We also tested the hypothesis that oxidative stress inhibits alpha-ENaC expression. Our results show that both alpha(1)- and alpha(2)-ENaC variants are expressed in the cells we tested, but relative abundance varies. In the two monolayer-forming cell lines, H441 and Calu-3, alpha(2)-ENaC is the predominant variant. We also show that H(2)O(2) specifically suppresses alpha(1)- and alpha(2)-ENaC variant expression in H441 and Calu-3 cells in a dose-dependent fashion. This suppression is achieved by inhibition of their promoters and is attenuated by dexamethasone. These data demonstrate the importance of the alpha(2)-subunit variant and suggest that glucocorticoids and antioxidants may be useful in correcting infection/inflammation-induced lung fluid imbalance.

TGF-β directs trafficking of the epithelial sodium channel ENaC which has implications for ion and fluid transport in acute lung injury

“…ROS, NO, and H 2 O 2 are second messengers (33), and both ROS and NO regulate ENaC activity (33). Additionally, reports exist that indicate that oxidative stress, including exogenously applied H 2 O 2 , can both negatively (64) and positively (65,66) regulate ENaC expression and activity, demonstrating that the regulation of ENaC by oxidizing agents is complex, and that the source and compartmentalization of the oxidative agent likely influence the outcome of oxidative reactions on ENaC function. We demonstrate here that NOX4-generated ROS (measured as H 2 O 2 ) regulate ENaC internalization in response to TGF-β.…”

Pro-survival and anti-apoptotic properties of androgen receptor signaling by oxidative stress promote treatment resistance in prostate cancer

“…In addition to AR mutations, several AR splice variants that exhibit transcriptional activity even in the absence of androgen and contribute to the promotion of CRPC have recently been identified (Dehm et al 2008, Guo et al 2009, Hu et al 2009, Sun et al 2010, Watson et al 2010. Although a relationship between AR splice variants and oxidative stress has not been reported to date, it is possible that oxidative stress may be implicated in the expression of AR splice variants as it is for splice variants of other genes (Xu & Chu 2007, Soliman et al 2009, Takeo et al 2009. Therefore, future studies should examine the effects of oxidative stress-induced mutations of the AR gene or the expression of AR splice variants.…”

Oxidized glutathione (GSSG) inhibits epithelial sodium channel activity in primary alveolar epithelial cells

“…Previous studies have suggested that S-glutathionylation occurs during oxidative stress to protect Cys residues against irreversible oxidation and can be reversed when environmental conditions permit (10). Additionally, ENaC subunit expression is repressed by oxidative stress (50), which also serves to counteract ROS-mediated increases in channel activity. The present study suggests that an additional role of S-glutathionylation may serve to protect the lung against ROS-mediated excess fluid clearance, leading to drying of the lung, thus indicating the important role of the GSH E h in ENaC regulation.…”