Abstract
NAD(P)H:quinoneoxidoreductase 1 (NQO1) belongs to a group of the aryl hydrocarbon receptor (AhR) battery of drug-metabolizing enzymes that are characteristically induced by both AhR agonists and nuclear factor erythroid 2-related factor 2 (Nrf2) activators. We have previously reported that induction of Nqo1 by the AhR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in hepa1c1c7 cells involves Nrf2 (Ma et al., Biochem J 377, 205-213, 2004). Here we analyzed the molecular mechanism of induction. Induction required AhR and its DNA-binding partner Arnt because induction was not observed in AhR or Arnt-defective cells, but induction was restored upon reconstitution of the variant cells with functional AhR or Arnt. Induction also required Nrf2, as induction by benzo[a]pyrene was lost in the liver of Nrf2 knockout mice similarly to induction by butyl hydroxyanisol, demonstrating a cross-interaction between the AhR and Nrf2 pathways for induction in vivo. TCDD increased the protein level and induced the nuclear accumulation of Nrf2 with a delayed kinetics compared with activation of AhR. Chromatin immunoprecipitation revealed that TCDD recruited both AhR and Nrf2 to the Nqo1 promoter enhancer region containing a DRE and an ARE in time-dependent manners. Co-immunoprecipitation experiments revealed that, in addition to AhR-Arnt binding, TCDD induced an interaction between AhR and Nrf2 as well as Keap1. The findings reveal that TCDD induces multi protein complexes to mediate cross-interaction between the AhR and Nrf2 pathways, uncovering a novel mechanistic aspect of gene regulation by environmental chemicals through AhR and Nrf2.