Interleukin-9 promotes EMT-mediated PM2.5-induced pulmonary fibrosis by activating the STAT3 pathway

This study investigated the impact of PM_2.5 on promoting EMT in PM_2.5-induced pulmonary fibrosis (PF) development and explored molecular mechanisms of the IL-9/STAT3/Snail/TWIST1 signaling pathway in PF owing to PM_2.5. Four groups of male SD rats were formed: control (0 mg/kg.bw), low (1 mg/kg.bw), medium (5 mg/kg.bw), and high-dose (25 mg/kg.bw) PM_2.5 groups. Experimental rats were subjected to PM_2.5 exposure via intratracheal instillation, given once weekly for 16 weeks. 24 h after the final exposure, blood, BALF, and lung tissues were collected. Pulmonary epithelial cells underwent cultivation and exposure to varying PM_2.5 concentrations with/without inhibitors for 24 h, after which total protein was extracted for relevant protein assays. The findings demonstrated that PM_2.5 damaged lung tissue to different degrees and led to PF in rats. Rats subjected to PM_2.5 exposure exhibited elevated concentrations of IL-9 protein in both serum and BALF, and elevated levels of IL-9 and its receptor, IL-9R, in lung tissues, compared to control counterparts. Furthermore, PM_2.5-exposed groups demonstrated significantly augmented protein levels of p-STAT3, Snail, TWIST1, Vimentin, COL-I, and α-SMA, while displaying notably diminished levels of E-Cadherin compared to control group. The same findings were observed in PM_2.5-treated cells. In BEAS-2B cells co-treated with Stattic (STAT3 inhibitor) and PM_2.5, the opposite results occurred. Similar results were obtained for cells co-treated with IL-9-neutralizing antibody and PM_2.5. Our findings suggest PM_2.5 mediates PF development by promoting IL-9 expression, leading to STAT3 phosphorylation and upregulation of Snail and TWIST1 expression, triggering EMT occurrence and progression in lung epithelial cells.