Suppression of Fpr2 expression protects against endotoxin-induced acute lung injury by interacting with Nrf2-regulated TAK1 activation

Acute lung injury (ALI) is caused by severe infection, and urgently needs effective treatments or validated pharmacological targets. Formyl peptide receptor 2 (Fpr2) plays essential roles in immune responses and inflammatory diseases. In the present study, Fpr2 expression was markedly increased in lung tissues of lipopolysaccharide (LPS)-challenged mice, and these effects were confirmed in LPS-stimulated macrophages. Then, the in vitro analysis suggested that Fpr2 knockdown significantly decreased LPS-induced inflammatory response in macrophages. Notably, the in vivo experiments indicated that Fpr2 deficiency alleviated ALI in LPS-treated mice, as evidenced by the improved histological changes in lung, reduced protein concentrations in bronchoalveolar lavage fluid (BALF) and decreased neutrophil infiltration. In addition, LPS-induced pulmonary inflammation was ameliorated by Fpr2 knockout, which was partly through blocking nuclear factor-kappa B (NF-kappa B) and mitogen-activated protein kinases (MAPKs) signaling pathways. Furthermore, oxidative stress stimulated by LPS was also attenuated by Fpr2 knockout, as indicated by the reduced malondialdehyde (MDA) levels and reactive oxygen species (ROS) production, accompanied with the elevated glutathione (GSH), superoxide dismutase (SOD), heme oxygenase-1 (HO-1) and NAD (P) H: quinone oxidoreductase (NQO1) levels. These antioxidative processes were mainly via the activation of Nrf2. Importantly, the in vitro results showed that Fpr2 over-expression markedly accelerated the inflammation and ROS production in LPS-incubated macrophages, which could be reversed by restoring the Nrf2 activation, demonstrating that Nrf2 was partially involved in Fpr2-regulated inflammatory response and oxidative stress during ALI progression. Then, we found that Fpr2 inhibition markedly reduced the activation of transforming growth factor beta-activated kinase 1 (TAK1) induced by LPS. What's more important, immunoprecipitation results demonstrated that Fpr2 directly interacted with the kinase TAK1. Taken together, findings in the present study illustrated that Fpr2 could directly interact with TAK1 to promote ALI through enhancing inflammation and oxidative stress associated with the activation of Nrf2, providing a novel therapeutic target to develop effective treatment against ALI progression.