Schisandrin B elicits the Keap1-Nrf2 defense system via carbene reactive metabolite which is less harmful to mice liver.

BACKGROUND: Schisandrin B (Sch B) a main active component of Schisandra chinensis, has been shown to act as a liver protectant via activation of the Nrf2 pathway. Nevertheless, it remains unclear whether its reactive metabolite is responsible for Nrf2 activation; also, the effects of its reactive metabolite on liver function are still unknown. METHODS: The present study determined and identifed the carbene reactive metabolite of Sch B in human and mice liver microsomes. Its roles in activating Nrf2 pathway and modifying macromolecules were further explored in human liver microsomes. Moreover the potential cytotoxicity and hepatoxicity of carbene on HepG-2 and mice were also investigated. RESULTS: In the present study, cytochromes P450 (CYP450s) metabolized Sch B to carbene reactive metabolite, which, with the potential to modify peptides, were identifed and observed in human and mice liver microsomes. Moreover, the relevance of carbene in Nrf2 activation was verifed by co-incubation in the presence of CYP450 inhibitors in HepG-2 cells, as well as by molecular docking study of carbene and Keap1. Additionally, the cytotoxicity of Sch B on HepG-2 cells was signifcantly aggravated by CYP450 inducer (with LD50 decreasing from 63 to 21 µM) and signifcantly alleviated by CYP450 inhibitor and glutathione (with LD50 increasing from 63 µM to 200 µM). Besides, after oral administration of mice with Sch B (25-100 mg/kg) for 21 days, only the highest dose induced mild hepatotoxicity, which was accompanied by increasing the aminotransferase activity and centrilobular hepatocellular infltration of lymphocytes. In addition, upregulation of CYP450 activity; Nrf2, NQO-1, and GST expression; and glutathione level was observed in Sch B treatment groups. CONCLUSION: The present study revealed that CYP450s mediate the conversion of Sch B to carbene, which subsequently binds to Keap1 and elicits Nrf2 pathway, which could further increase the elimination of carbene and thus exhibit a less harmful effect on mice liver.

[Effects of Schisandrae Chinensis Fructus induced CYPs and Nrf2 activation on acute liver injury induced by acetaminophen].

Schisandra chinensis is a commonly used hepatoprotective medicine in clinic. Previous studies have showed that Schisandrae Chinensis Fructus has dual effects on the activity of CYPs. Short-term administration of Schisandrae Chinensis Fructus may inhibit CYP450s activity, while long-term administration may up-regulate CYP activity. High CYP450s activity level may increase the frequency of reactive metabolites-induced liver injury. It remains unclear how long-term administration of Schisandrae Chinensis Fructus may affect acetaminophen-induced acute hepatotoxicity. After oral administration of Schisandrae Chinensis Fructus extract (0.5-2.0 g·kg⁻¹) for 21 d, the activity of CYPs, Nrf2, HO-1, GST expressions, SOD and GST activity as well as glutathione level of SD rats were up-regulated. Besides, Schisandrae Chinensis Fructus extract ameliorated APAP (500 mg·kg⁻¹)-induced acute hepatotoxicity in a dose-dependent manner, as evidenced by decrease in ALT, AST, and MDA level and increase in GSH level (P<0.05). What's more, the liver histopathology was alleviated, and cleaved caspase-3 expression was decreased. Besides, the increase of N-acetyl-p-benzoquinoneimine-GSH (reactive metabolite of acetaminophen) formation was observed in Schisandrae Chinensis Fructus extract groups. In conclusion, the present study indicated that the effects of Schisandrae Chinensis Fructuson acetaminophen-induced hepatotoxicity may rely on the Nrf2 signal pathway activation, and less depends on the increase in CYP450s activity.