Low-dose extrapolation and dose-related transitions are paramount in the ongoing debate regarding the quantification of
cancer risks for nongenotoxic
carcinogens.
Phenobarbital (PB) is a prototypical nongenotoxic
carcinogen that activates the
constitutive androstane receptor (CAR) resulting in rodent liver
tumors. In this study, male and female CD-1 mice administered dietary PB at 0, 0.15, 1.5, 15, 75, or 150 mg/kg-day for 2 or 7 days to characterize multiple apical and molecular endpoints below, at (∼75 mg/kg-day), and above the carcinogenic dose level of PB and examine these responses using benchmark dose modeling. Linear toxicokinetics were observed for all doses. Increased liver weight, hepatocellular
hypertrophy, and mitotic figures were seen at 75 and 150 mg/kg-day. CAR activation, based on Cyp2b qPCR and
pentoxyresorufin dealkylase activity, occurred at doses ≥ 1.5 mg/kg-day. The no-observable transcriptional effect level for global gene expression was 15 mg/kg-day. At 2 days, several
xenobiotic metabolism and cell protective pathways were activated at lower doses and to a greater degree in females. However, hepatocellular proliferation, quantified by
bromodeoxyuridine immunohistochemistry, was the most sensitive
indicator of PB exposure with female mice more sensitive than males, contrary to sex-specific differences in sensitivity to hepatocarcinogenesis. Taken together, the identification of low-dose cellular and molecular transitions in the subtumorigenic dose range
aids the understanding of early key events in CAR-mediated hepatocarcinogenesis.