The antioxidant signaling pathway regulated by quercetin. Environmental factors increase the production of reactive oxygen species (ROS). The mitochondrial electron transport chain (mito ETC) is another robust source of intracellular ROS generation. Quercetin can regulate the enzyme-mediated antioxidant defense system and the non-enzyme-dependent antioxidant defense system. It can also regulate signal pathways such as NRFB, AMPK, and MAPK caused by ROS to promote the antioxidant defense system and maintain oxidative balance. ROS in turn enhance the production of APE1/Ref1 and the activation of several signaling events including p53-mediated apoptotic events, MAPK pathways, the NF-E2-related factor (NRF2)-mediated activation of genes containing antioxidant response element (ARE), and NF-κB [22,23,24,25,26].