To explore the effect and mechanism of isorhynchophylline (IRN) on bleomycin-induced pulmonary fibrosis in mice based on extracellular regulated protein kinases 1/2 (ERK1/2),p27^Kip1 signaling pathway.Forty-eight C57BL/6 mice were randomly divided into normal group,bleomycin (BLM) group,IRN low and high (10 and 20 mg/kg) groups,twelve mice in each group.The pulmonary fibrosis (PF) model was induced by intratracheal injection of BLM (5 000 U/kg).IRN were administered by intragastric every day after BLM treatment for three weeks.HE and Masson staining were used to observe the pathological changes and collagen deposition in lung tissue.The expression of alpha-smooth muscle actin (α-SMA) in lung tissue was detected by immunohistochemistry.Primary mouse lung fibroblasts cultured in vitro,cells were divided into control group,transforming growth factor-beta 1 (TGF-β1) group,and IRN (5,10 and 20 μmol/L) groups.Cell proliferation was measured by EdU marking and flow cytometry.The ability of cell migration was applied by Transwell assays.The mRNA levels of TGF-β1,collagen I and α-SMA were detected by RT-qPCR in lung tissue and cells.The proteins levels of TGF-β1,collagen I,α-SMA,p-ERK1/2,p27^Kip1,CDK2 and Cyclin E1 were detected by Western blot in lung tissue and cells.In vivo,compared with BLM group,IRN (10 and 20 mg/kg) reduced damaged lung structures,infiltrated inflammatory cells and accumulated areas of collagen deposition.Moreover,IRN reduced the mRNA and proteins expression of TGF-β1,collagen I and TGF-β1 in the lung tissues.Meanwhile,IRN decreased the level of ERR1/2 phosphorylation,up-regulation expression of p27Kip1 and down-regulation expression of CDK2 and Cyclin E1 in the lung tissues.In vitro,compared with TGF-β1 group,IRN (5,10 and 20 μmol/L) treatment significantly inhibited proliferation and migration of pulmonary fibroblasts,and obviously decreased the mRNA and proteins expression of collagen I and α-SMA induced by TGF-β1.Similarly,IRN (5,10 and 20 μmol/L) treatment reduced the level of ERR1/2 phosphorylation,up-regulation expression of p27^Kip1 and down-regulation expression of CDK2 and Cyclin E1 induced by TGF-β1.These results demonstrated that IRN could suppress lung fibroblast proliferation,migration and differentiation to myofibroblasts and alleviate bleomycin-induced pulmonary fibrosis in mice,possibly through suppressing the ERK1/2 signaling pathway and up-regulating p27^Kip1 expression.