Microcystin-LR Ameliorates Pulmonary Fibrosis via Modulating CD206 <sup>+</sup> M2-Like Macrophage Polarization

Abstract

Background: Idiopathic pulmonary fibrosis (IPF) refers to a group of chronic and untreatable fatal interstitial pulmonary diseases characterized by myofibroblast proliferation and extracellular matrix (ECM) deposition. The pathogenesis of IPF involves alternative activation of macrophages and excessive transforming growth factor β1 (TGF-β1) signaling. Microcystin-leucine arginine (microcystin-LR) is an environmental toxin produced by cyanobacteria. We previously observed a downregulated TGF-β1 expression in pulmonary tissues as a result of chronic exposure to microcystin-LR, which prompted us to assume a therapeutic role of this molecule on IPF. Methods: The therapeutic effect of microcystin-LR was evaluated using a rat model of bleomycin-induced pulmonary fibrosis and the confirmation test was performed with a fluorescein isothiocyanate-induced mouse pulmonary fibrosis model. An in vitro coculture model of monocytes/macrophages with alveolar epithelial cells or with fibroblasts was established to investigate the mechanisms of microcystin-LR-mediated anti-fibrotic effect. Findings: Treatment with microcystin-LR substantially reduced TGF-β1/Smad signaling and obviously alleviated the lesion of pulmonary fibrosis in animal models. Microcystin-LR was capable of blocking epithelialmesenchymal transition and fibroblast-myofibroblast transition through suppressing the differentiation of CD206+ macrophages. Furthermore, microcystin-LR could bind to glucose-regulated protein 78 kDa (GRP78) and suppress endoplasmic reticulum unfolded protein response signaling pathways. Moreover, no additional lesion related with the regimen of microcystin-LR was observed on the animal models in this study. Interpretation: We have provided evidences demonstrating that microcystin-LR could alleviate pulmonary fibrosis by antagonizing macrophage polarization to an M2-like phenotype. Our data revealed a novel mechanism that may account for therapeutic effect of microcystin-LR on IPF. Funding Statement: This work was supported by the National Natural Science Foundation of China (81270152, 81771504 and 81501977), Human Resource Summit Grant of Jiangsu Province (WSN-043) and The Young Talents Program of Jiangsu Cancer Hospital (QL201807). Declaration of Interests: All of the authors declared that no conflict of interest exists. Ethics Approval Statement: The animal care and the study procedures were approved by the Ethics Committee for Animal Research in Medical School of Nanjing University.