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Genotoxic Polycyclic Aromatic Hydrocarbon ortho-Quinones Generated by Aldo-Keto Reductases Induce CYP1A1 via Nuclear Translocation of the Aryl Hydrocarbon Receptor¹

Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6084

Procarcinogenic polycyclic aromatic hydrocarbons (PAHs) induce their own metabolism and activation by binding to the cytosolic aryl hydrocarbon receptor (AhR), which then translocates to the nucleus and activates CYP1A1 gene transcription via xenobiotic response elements (XREs). Although the AhR demonstrates a strict specificity for planar aromatics, nonplanar (±)-trans-7,8-dihydroxy-7,8-dihydrobenzo(a)pyrene also induced CYP1A1 expression in HepG2 cells over a delayed timecourse (6–12 h), suggesting a requirement for (±)-trans-7,8-dihydroxy-7,8-dihydrobenzo(a)pyrene metabolism. Aldo-keto reductase (AKR) inhibitors blocked this effect, suggesting that benzo(a)pyrene-7,8-dione (BPQ), a planar PAH o-quinone generated by AKRs, was the downstream inducer. BPQ was found to be a potent and rapid inducer of CYP1A1, with an EC₅₀ value in HepG2 cells identical to that of the parent benzo(a)pyrene. BPQ was a more potent inducer of CYP1A1 when compared with the 1,6-, 3,6-, and 6,12-benzo(a)pyrene-diones. Multiple PAH o-quinones caused induction of CYP1A1, demonstrating that this was a general property of AKR-generated PAH o-quinones. HepG2-101L cells stably transfected with a XRE-luciferase construct showed that BPQ activated CYP1A1 transcription via a XRE-dependent mechanism. BPQ failed to induce CYP1A1 in AhR-deficient and AhR nuclear translocator-deficient murine hepatoma cell lines and confirmed that induction of CYP1A1 was AhR and AhR nuclear translocator-dependent. Electrophoretic mobility shift assays demonstrated the specific appearance of BPQ-activated AhR in the nucleus, and immunofluorescence studies confirmed that BPQ mediated nuclear translocation of the AhR. Classical bifunctional inducers elevate CYP1A1 expression via a XRE and are subsequently converted by CYP1A1 to electrophiles that induce phase II enzymes via an electrophilic response element/antioxidant response element. PAH o-quinones represent a novel class of bifunctional inducer because they are electrophiles produced by phase II enzymes that simultaneously induce phase I enzymes via a XRE and phase II enzymes via a electrophilic response element/antioxidant response element (see also M. E. Burczynski et al., Cancer Res., 59: 607–614, 1999). This study shows that the AhR provides the only known mechanism by which genotoxic PAH o-quinones generated in the cytosol can be targeted to the nucleus with specificity.

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