n-BuOH extract of Bletilla striata exerts chemopreventive effects on lung against SiO2 nanoparticles through activation of Nrf2 pathway

BackgroundSiO₂ nanoparticles (nm SiO₂) are ubiquitous in daily life and are acknowledged to be detrimental to human health. Bletilla striata is a traditional medicine used for generations in China and its polysaccharide has the anti-pulmonary fibrosis effect.PurposeTo investigate the lung protective effect of the small molecules (n-BuOH extract) of B. striata and clarify the underlying mechanism.Study design and methodsC57BL/6 mice were subjected to intratracheal instillation with nm SiO₂ nanoparticle suspension (7 mg/kg) to construct the in vivo model of nm SiO₂-induced lung injury. The chemical profile of the n-BuOH extract of B. striata was investigated by HPLC analysis using authentic samples isolated from B. striata. Gymnoside II with the most potent chemoprotective capacity in the n-BuOH extract was used to clarify the potential bio-active molecular basis of the n-BuOH extract using in vitro experiments. The cytotoxicity, apoptosis, oxidative stress, and the Nrf2 signaling pathway were examined in SiO₂-induced A549 cells. ML385 was adopted to down-regulate the Nrf2 expression.ResultsThe n-BuOH extract of B. striata (40 mg/kg) could alleviate the SiO₂-induced lung injury by increasing Nrf2 expression and thereby suppressing Bax/Bcl-2 pathway in the nm SiO₂-induced mice model. The chemical profile study showed that militarine, gymnoside II, and 4-allyl-2, 6-dimethoxyphenol glucoside were the main constituents of n-BuOH extract. Studies on gymnoside II revealed that it could partially restore the SiO₂-induced decline in cell viability while did not affect the growth of normal A549 cells within the concentration range of 1-50 μM, suggesting a protective effect against nm SiO₂ in lung A549 cells. The hoechst 33258 staining, flow cytometry, and western blot experiments demonstrated that gymnoside II (25 μM) could partially reverse the SiO₂-induced cell apoptosis and ROS production by enhancing Nrf2, HO-1, and γ-GCSc expressions and Nrf2 silencing by ML385 abrogated the effects of gymnoside II (25 μM) on apoptosis and ROS production in A549 cells.ConclusionThe present study suggests that in addition to the polysaccharide, small molecules (n-BuOH extract) of B. striata can also elicit a protective effect on lung injuries through the Nrf2-dependent mechanism and gymnoside II is one of the main bio-active constituents contributing to the n-BuOH extract-elicited lung protective effect against nm SiO₂.