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 α-, β-, and γ-subunits, and the α-subunit is indispensable for ENaC function in the lung. In human lungs, the α-subunit is expressed as various splice variants. Among them, α1- and α2-subunits are two major variants with different upstream regulatory sequences that possess similar channel characteristics when tested in Xenopus oocytes. Despite the importance of α-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 α-ENaC expression, oxidative stress, and pulmonary edema. However, it was not clear how oxidative stress affects expression of α-ENaC variants. In this study, we examined relative expression levels of α-subunit variants in four human lung epithelial cell lines. We also tested the hypothesis that oxidative stress inhibits α-ENaC expression. Our results show that both α1- and α2-ENaC variants are expressed in the cells we tested, but relative abundance varies. In the two monolayer-forming cell lines, H441 and Calu-3, α2-ENaC is the predominant variant. We also show that H2O2 specifically suppresses α1- and α2-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 α2-subunit variant and suggest that glucocorticoids and antioxidants may be useful in correcting infection/inflammation-induced lung fluid imbalance.