Toosendanin (TSN) is the main active compound in Toosendan Fructus and Meliae Cortex,two commonly used traditional Chinese medicines.TSN has been reported to induce hepatotoxicity,but its mechanism remains unclear.In this study,we demonstrated the critical role of nuclear factor erythroid 2-related factor 2 (Nrf2) in protecting against TSN-induced hepatotoxicity in mice and human normal liver L-02 cells.In mice,administration of TSN (10 mg/kg)-induced acute liver injury evidenced by increased serum alanine/aspartate aminotransferase (ALT/AST) and alkaline phosphatase (ALP) activities,and total bilirubin (TBiL) content as well as the histological changes.Furthermore,TSN markedly increased liver reactive oxygen species (ROS) and malondialdehyde (MDA) levels,and decreased liver glutathione (GSH) content and Nrf2 expression.In L-02 cells,TSN(2 μM) time-dependently reduced glutamatecysteine ligase (GCL) activity and cellular expression of the catalytic/modify subunit of GCL (GCLC/GCLM).Moreover,TSN reduced cellular GSH content and the increased ROS formation,and time-dependently decreased Nrf2 expression and increased the expression of the Nrf2 inhibitor protein kelch-like ECH-associated protein-1 (Keap1).Pre-administration of quercetin (40,80 mg/kg)effectively inhibited TSN-induced liver oxidative injury and reversed the decreased expression of Nrf2 and GCLC/GCLM in vivo and in vitro.In addition,the quercetin-provided protection against TSN-induced hepatotoxicity was diminished in Nrf2 knock-out mice.In conclusion,TSN decreases cellular GSH content by reducing Nrf2-mediated GCLC/GCLM expression via decreasing Nrf2 expression.Quercetin attenuates TSN-induced hepatotoxicity by inducing the Nrf2/GCL/GSH antioxidant signaling pathway.This study implies that inducing Nrf2 activation may be an effective strategy to prevent TSN-induced hepatotoxicity.