Lipoxin A4 activates alveolar epithelial sodium channel gamma via the microRNA-21/PTEN/AKT pathway in lipopolysaccharide-induced inflammatory lung injury

Lipoxin A(4) (LXA(4)), as an endogenously produced lipid mediator, promotes the resolution of inflammation. Previously, we demonstrated that LXA(4) stimulated alveolar fluid clearance through alveolar epithelial sodium channel gamma (ENaC-gamma). In this study, we sought to investigate the mechanisms of LXA(4) in modulation of ENaC-gamma in lipopolysaccharide (LPS)-induced inflammatory lung injury. miR-21 was upregulated during an LPS challenge and downregulated by LXA(4) administration in vivo and in vitro. Serum miR-21 concentration was also elevated in acute respiratory distress syndrome patients as compared with healthy volunteers. LPS increased miR-21 expression by activation of activator protein 1 (AP-1). In A549 cells, miR-21 upregulated phosphorylation of AKT activation via inhibition of phosphatase and tensin homolog (PTEN), and therefore reduced the expression of ENaC-gamma. In contrast, LXA(4) reversed LPS-inhibited ENaC-gamma expression through inhibition of AP-1 and activation of PTEN. In addition, an miR-21 inhibitor mimicked the effects of LXA(4); overexpression of miR-21 abolished the protective effects of LXA(4). Finally, both AKT and ERK inhibitors (LY294002 and UO126) blocked effects of LPS on the depression of ENaC-gamma. However, LXA(4) only inhibited LPS-induced phosphorylation of AKT. In summary, LXA(4) activates ENaC-gamma in part via the miR-21/PTEN/AKT signaling pathway.