Quercetin Attenuates Quinocetone-Induced Cell Apoptosis In Vitro by Activating the P38/Nrf2/HO-1 Pathway and Inhibiting the ROS/Mitochondrial Apoptotic Pathway

Quinocetone (QCT), a member of the quinoxaline 1,4-di-N-oxides (QdNOs) family, can cause genotoxicity and hepatotoxicity, however, the precise molecular mechanisms of QCT are unclear. This present study investigated the protective effect of quercetin on QCT-induced cytotoxicity and the underlying molecular mechanisms in human L02 and HepG2 cells. The results showed that quercetin treatment (at 7.5–30 μM) significantly improved QCT-induced cytotoxicity and oxidative damage in human L02 and HepG2 cells. Meanwhile, quercetin treatment at 30 μM significantly inhibited QCT-induced loss of mitochondrial membrane potential, an increase in the expression of the CytC protein and the Bax/Bcl-2 ratio, and an increase in caspases-9 and -3 activity, and finally improved cell apoptosis. Quercetin pretreatment promoted the expression of the phosphorylation of p38, Nrf2, and HO-1 proteins. Pharmacological inhibition of p38 significantly inhibited quercetin-mediated activation of the Nrf2/HO-1 pathway. Consistently, pharmacological inhibitions of the Nrf2 or p38 pathways both promoted QCT-induced cytotoxicity and partly abolished the protective effects of quercetin. In conclusion, for the first time, our results reveal that quercetin could improve QCT-induced cytotoxicity and apoptosis by activating the p38/Nrf2/HO-1 pathway and inhibiting the ROS/mitochondrial apoptotic pathway. Our study highlights that quercetin may be a promising candidate for preventing QdNOs-induced cytotoxicity in humans or animals.