Oxymatrine (OM) is an alkaloid extracted from the Chinese herb Sophora flavescens Ait. with a variety of pharmacological activities. The aim of this study was to investigate the preventive effects of OM on bleomycin (BLM)-induced pulmonary fibrosis (PF) and to further explore the underlying mechanisms. C57BL/6 mice were randomly assigned to five groups: the saline sham group; the BLM group, in which mice were endotracheally instilled with BLM (3.0 mg/kg); and the BLM plus OM groups, in which OM was given to mice daily (10, 20 or 40 mg/kg) one day after BLM instillation for 21 days. The bronchoalveolar lavage fluid (BALF) and lung tissues were collected at 15 and 22 days post BLM administration, respectively. Lung tissues were stained with hematoxylin and eosin (H&E) for histological evaluation. Levels of tumor necrosis factor (TNF)-α, interleukin-6 (IL-6) and nitric oxide (NO) in mouse BALF were measured, as well as myeloperoxidase (MPO) activity and malondialdehyde (MDA) content in lung homogenates. The inducible nitric oxide synthase (iNOS) expression in the lung tissues was determined by immunohistochemical staining, quantitative real-time PCR and western blot analysis. Moreover, the expression of transforming growth factor (TGF)-β1, Smad2, Smad3, p-Smad2 and p-Smad3 were also detected. We found that OM improved BLM-induced lung pathological changes, inhibited MPO activity and reduced MDA levels in a dose-dependent manner. OM also dose-dependently inhibited the release of TNF-α and IL-6, and decreased the expression of iNOS in lung tissues and thus prevented NO release in response to BLM challenge. In addition, OM decreased the expression of TGF-β1, p-Smad2 and p-Smad3, which are all important members of the TGF-β/Smad signaling pathway. Our study provides evidence that OM significantly ameliorated BLM-induced PF in mice via the inhibition of iNOS expression and the TGF-β/Smad pathway.

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