Genotoxic polycyclic aromatic hydrocarbon ortho-quinones generated by aldo-keto reductases induce CYP1A1 via nuclear translocation of the aryl hydrocarbon receptor

Procarcinogenic polycyclic aromatic hydrocarbons (PAHs) induce their own me tabolism and activation by binding to the cytosolic aryl hydrocarbon recept or (AhR), which then translocates to the nucleus and activates CYP1A1 gene transcription via xenobiotic response elements (XREs), Although the AhR dem onstrates a strict specificity for planar aromatics, nonplanar (+/-)trans-7 ,8-dihy- droxy-7,8-dihydrobenzo(a)pyrene also induced CYP1A1 expression in HepG2 cells over a delayed timecourse (similar to 6-12 h), suggesting a req uirement for (+/-)-trans-7,8-dihydroxy-7,8-dihydrobenzo metabolism. Aldo-ke to reductase (AKR) inhibitors blocked this effect, suggesting that benzo(a) pyrene-7,8-dione (BPQ), a planar PAW o-quinone generated by AKRs, was the d ownstream 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 ind uction of CYP1A1, demonstrating that this was a general property of AKR-gen erated PAH o-quinones, HepG2-101L, cells stably transfected with a XRE-luci ferase construct: showed that BPQ activated CYP1A1 transcription via a XRE- dependent mechanism. BPQ Failed to induce CYP1A1 in AhR-deficient and AhR n uclear translocator-deficient murine hepatoma cell lines and confirmed that induction of CYP1A1 was AhR and AhR nuclear translocator-dependent, Electr ophoretic mobility shift assays demonstrated the specific appearance of BPQ -activated AhR in the nucleus, and immunonufluorescence studies confirmed t hat BPQ mediated nuclear translocation of the AhR, Classical bifunctional i nducers elevate CYP1A1 expression via a XRE and are subsequently converted by CYP1A1 to electrophiles that induce phase II enzymes via an electrophili c response element/antioxidant response element. PAH o-quinonas: represent a novel class of bifunctional inducer because they are electrophiles produc ed by phase IT enzymes that simultaneously induce phase I enzymes via a XRE and phase II enzymes via a electrophilic response element/antioxidant resp onse 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 w hich genotoxic PAH o-quinones generated in the cytosol can be targeted to t he nucleus with specificity.