α7nAChR Agonist Activates the Nrf2/HO-1 Signaling Pathway to Reduce Acute Lung Injury Induced by Endotoxic Shock

Abstract Objective: Alpha 7 nicotinic acetylcholine receptors (α7nAChRs) can inhibit the activation of macrophages and the production of pro-inflammatory cytokines and exert inhibitory effects on systemic and local inflammatory responses. The objective of this study was to observe the protective effect of α7nAChR agonist against acute lung injury (ALI) caused by endotoxic shock, and to explore the regulatory mechanism of the nuclear factor erythroid 2-related factor 2 (Nrf2)/heme oxygenase-1 (HO-1) signaling pathway. Methods: A total of 40 Sprague-Dawley rats were randomly divided into sham operation group (Sham group, n=10), endotoxic shock-induced ALI model group (ALI group, n=10), ALI + α7nAChR agonist (PHA568487, 3134, Tocris Bioscience, USA) group (PHA group, n=10) and ALI + α7nAChR agonist + Nrf2 inhibitor (ML385, HY-100523, MCE, USA) group (ML group, n=10). The rats received a tail vein injection of LPS to initiate ALI. Six hrs after injection, arterial blood was analyzed for blood gases and lung wet weight/dry weight (W/D) was determined. Lung histopathology was determined by H&E staining and apoptosis quantified by TUNEL. Levels of intercellular adhesion molecule-1 (ICAM-1), TNF-α, IL-1β, malondialdehyde (MDA), myeloperoxidase (MPO), superoxide dismutase (SOD), choline acetyltransferase (ChAT) and acetylcholine esterase (AchE) in bronchoalveolar lavage fluid were measured via ELISA. Western blotting revealed levels of nuclear factor kappa-B (NF-κB), B-cell lymphoma 2 (Bcl-2), Bcl-2-associated X protein (Bax), phosphatidylin-ositol-3-kinase (PI3K), protein kinase B (Akt), phosphorylated Akt (p-Akt), HO-1, Nrf2, thioredoxin reductase 1 (Trx-1) and glyceraldehyde 3-phosphate dehydrogenase (GAPDH). Results: It was found that α7nAChR agonist increased the partial pressure of oxygen (PaO2) and pH, reduced the partial pressure of carbon dioxide (PaCO2) and W/D ratio, alleviated pulmonary edema and oxidative stress injury, and suppressed inflammatory responses. At the same time, it activated PI3K to phosphorylate Akt, inhibited cell apoptosis, and protected the lung tissues of ALI rats. Moreover, α7nAChR agonist facilitated nuclear translocation of Nrf-2 and up-regulated HO-1 and Trx-1 expression. Nrf-2 activity was required for the protective effect of α7nAChR . Conclusion: α7nAChR agonist can improve endotoxic shock-induced ALI by activating the cholinergic anti-inflammatory pathway and the Nrf2/HO-1 signaling pathway.