Lung protective effects of dietary malate esters derivatives from Bletilla striata against SiO2 nanoparticles through activation of Nrf2 pathway.

Objective: To study the protective activities of the dietary malate esters derivatives of Bletilla striata against SiO2 nanoparticles-induced A549 cell lines and its mechanism action. Methods: The components were isolated and elucidated by spectroscopic methods such as 1D NMR and 2D NMR. And MTT assays was used to tested these components on the A549 cell survival rates and ROS or proteins levels were detected by Western blotting. Results: A new glucosyloxybenzyl 2-isobutylmalate (a malate ester derivative), along with 31 known compounds were isolated and identified from n-BuOH extract of EtOH extract of B. striata. Among them, compounds 3, 4, 11, 12 and 13 possessed noteworthy proliferative effects for damaged cells, with ED50 of 14.0, 13.1, 3.7, 11.6 and 11.5 µmol/L, respectively, compared to positive control resveratrol (ED50, 14.7 µmol/L). Militarine (8) prominently inhibited the intracellular ROS level, and increased the expression of Nrf2 and its downstream genes (HO-1 and γ-GCSc). Furthermore, Nrf2 activation mediates the interventional effects of compound 8 against SiO2 nanoparticles (nm SiO2)-induced lung injury. Moreover, treatment with compound 8 significantly reduced lung inflammation and oxidative stress in nm SiO2-instilled mice. Molecular docking experiment suggested that 8 bound stably to the HO-1 protein by hydrogen bond interactions. Conclusion: The dietary malate esters derivatives of B. striata could significantly increase the viability of nm SiO2-induced A549 cells and decrease the finer particles-induced cell damages. Militarine is especially promising compound for chemoprevention of lung cancer induced by nm SiO2 through activation of Nrf2 pathway.