Lentinan Alleviated PM2.5 Exposure-Induced Epithelial–Mesenchymal Transition in Pulmonary Epithelial Cells by Inhibiting the GARP/TGF-β/Smad Pathway

PM2.5 (fine particulate matter) is an air pollutant widely present in urban and industrial areas, which has emerged as a significant threat to human health. Specifically, long-term exposure to PM2.5 could lead to various lung diseases, including pulmonary fibrosis and Chronic Obstructive Pulmonary Disease (COPD). The Glycoprotein A Repetitions Predominant (GARP) protein, a key receptor and regulator for TGF-β1, has recently emerged as a vital cytokine in PM2.5-induced pulmonary pathological changes. As a membrane glycoprotein, GARP binds to TGF-β, keeping it in an active state. Herein, PM2.5 treatment upregulated GARP and promoted Epithelial–Mesenchymal Transition (EMT) via TGF-β/SMAD signaling pathway activation. Conversely, lentinan (a shiitake mushroom-derived polysaccharide) effectively reversed the PM2.5-induced GARP upregulation, alleviating EMT. This study elucidates the role of GARP in PM2.5-induced EMT through the TGF-β/SMAD pathway in pulmonary epithelial cells and discusses the therapeutic potential of lentinan.