Toxaphene‐induced mouse liver tumorigenesis is mediated by the constitutive androstane receptor

Toxaphene was shown to increase liver tumor incidence in B6C3F1 mice following chronic dietary exposure. Preliminary evidence supported a role for the constitutive androstane receptor (CAR) in the mode of action of toxaphene‐induced mouse liver tumors. However, these results could not rule out a role for the pregnane X receptor (PXR) in liver tumor formation. To define further the nuclear receptors involved in this study, we utilized CAR, PXR and PXR/CAR knockout mice (CAR−/−, PXR−/− and PXR−/−/CAR−/−) along with the wild‐type C57BL/6. In this study CAR‐responsive genes Cyp3a11 and Cyp2b10 were induced in the liver of C57BL/6 (wild‐type) mice by toxaphene (30–570‐fold) (at the carcinogenic dose 320 ppm) and phenobarbital (positive control) (16–420‐fold) following 14 days' dietary treatment. In contrast, in CAR−/− mice, no induction of these genes was seen following treatment with either chemical. Cyp3a11 and Cyp2b10 were also induced in PXR−/− mice with toxaphene and phenobarbital but were not changed in treated PXR−/−/CAR−/− mice. Similarly, induction of liver pentoxyresorufin‐O‐deethylase (CAR activation) activity by toxaphene and phenobarbital was absent in CAR−/− and PXR−/−/CAR−/− mice treated with phenobarbital or toxaphene. Ethoxyresorufin‐O‐deethylase (EROD, represents aryl hydrocarbon receptor activation) activity in CAR−/− mice treated with toxaphene or phenobarbital was increased compared with untreated control, but lower overall in activity in comparison to the wild‐type mouse. Liver EROD activity was also induced by both phenobarbital and toxaphene in the PXR−/− mice but not in the PXR−/−/CAR−/− mice. Toxaphene treatment increased 7‐benzyloxyquinoline activity (a marker for PXR activation) in a similar pattern to that seen with pentoxyresorufin‐O‐deethylase. These observations indicate that EROD and PXR activation are evidence, as expected, of secondary overlap to primary CAR receptor activation. Together, these results definitively show that activation of the CAR nuclear receptor is the mode of action of toxaphene‐induced mouse liver tumors. Copyright © 2017 John Wiley & Sons, Ltd.