Tetrachlorobenzoquinone Activates Nrf2 Signaling by Keap1 Cross-Linking and Ubiquitin Translocation but Not Keap1-Cullin3 Complex Dissociation

Tetrachlorobenzoquinone (TCBQ), a metabolite of industrial herbicide pentachlorophenol, showed hepatotoxicity and genotoxicity through reactive oxygen species (ROS) mechanism in vivo and in vitro models. Nuclear factor erythroid-derived 2-like 2 (Nrf2) is a cellular sensor of oxidative or electrophilic stress, which controls the expression of detoxifying enzymes and antioxidant proteins. Using the human hepatoma HepG2 cell line as an in vitro model, we demonstrated a significant induction of Nrf2 but not its negative regulator Kelch-like ECH-associated protein 1 (Keap1), following exposure to TCBQ. Also, our results clearly demonstrated the translocation of cytosolic Nrf2 into the nucleus. After translocation, Nrf2 subsequently binds to the antioxidant response element (ARE), up-regulated heme oxygenase-1 (HO-1), and NADH quinone oxidoreductase subunit 1 (NQO1), which may be considered as an antioxidative response to TCBQ-intoxication. The luciferase reporter assay confirmed the formation of the Nrf2-ARE complex. Furthermore, mechanism studies proposed that TCBQ promoted the formation of the Keap1 cross-linking dimer, a ubiquitination switch from Nrf2 to Keap1 but not the dissociation of the Keap1-Cullin3 (Cul3) complex.