Apelin-36 protects against lipopolysaccharide-induced acute lung injury by inhibiting the ASK1/MAPK signaling pathway

Apelin-36 is able to mediate a range of effects on various diseases, and is upregulated in lipopolysaccharide (LPS)-induced acute lung injury (ALI). However, to the best of our knowledge, whether apelin-36 is able to regulate LPS-induced ALI has yet to be investigated. The present study aimed to investigate the role of apelin-36 in LPS-induced ALI, and the putative underlying mechanisms. Rats were assigned to one of four treatment groups: The Control group, apelin-36 group, LPS group and LPS + apelin-36 group. At 4 h after intratracheal instillation of LPS (5 mg/kg), rats were intraperitoneally treated with 10 nmol/kg apelin-36. Subsequently, pathological manifestations and the extent of inflammation and apoptosis of the lung tissues were assessed. Untransfected and apoptosis signal-regulating kinase 1 (ASK1)-overexpressing Beas-2B cells were treated with LPS in the absence or presence of apelin-36, and subsequently the levels of inflammation and apoptosis were assessed. The results obtained showed that the level of apelin-36 was increased in the bronchoalveolar lavage fluid (BALF) of LPS-treated rats. Co-treatment with apelin-36 alleviated LPS-induced lung injury and pulmonary edema, reduced the levels of pro-inflammatory cytokines, including interleukin-6, monocyte chemoattractant protein-1 and tumor necrosis factor-α, in BALF, and inhibited apoptosis in the lung tissues. The presence of apelin-36 also blocked the activation of LPS-induced ASK1, p38, c-Jun N-terminal kinase and extracellular signal-regulated kinase in lung tissues. In vitro studies performed with Beas-2B cells showed that the addition of apelin-36 led to an increase in the cell viability of LPS-induced Beas-2B cells in a concentration-dependent manner. Additionally, co-treatment with 1 µM apelin-36 prevented LPS-induced inflammation and apoptosis. However, overexpression of ASK1 significantly reversed the inhibitory effects of apelin-36 on LPS-induced inflammation and apoptosis. Taken together, the results of the present study demonstrated that apelin-36 was able to protect against LPS-induced lung injury both in vivo and in vitro, and these actions may be dependent on inhibition of the ASK1/mitogen-activated protein kinase signaling pathway.