ABSTRACT:The pregnane X receptor (PXR) is a transcriptional regulator of xenobiotic metabolizing enzymes, including cytochrome P450 3A (CYP3A), and transporters. Pretreatment of mice and rats with inducers of CYP3A increases acetaminophen (APAP) hepatotoxicity. In untreated mice, the amount of hepatic CYP3A11 mRNA is 4-fold greater in PXR(؊/؊) mice compared to wild-type mice (Guo et al., 2003), a finding anticipated to increase APAP hepatotoxicity in PXR(؊/؊) mice. We investigated APAP hepatotoxicity in wildtype and PXR(؊/؊) mice in a C57BL/6 background, with APAP administered by gavage. Despite a 2.5-fold higher level of total hepatic CYP3A protein and a 3.6-fold higher level of CYP3A activity compared to wild-type mice, PXR(؊/؊) mice were less sensitive to APAP hepatotoxicity. Hepatic levels of CYP2E1 were identical in the two mouse lines, but hepatic CYP1A2 levels were 3-fold greater in wild-type mice compared to PXR(؊/؊) mice. Caffeine, an inhibitor of CYP1A2 activity and an enhancer of CYP3A activity, decreased APAP hepatotoxicity in wild-type mice. APAP uptake was 1.5-fold greater in wild-type mice compared to PXR(؊/؊) mice. No significant differences in the formation of APAP glucuronide and sulfate-conjugated metabolites were observed between wild-type and PXR(؊/؊) mice. Glutathione levels were similar in the two mouse lines and were transiently decreased to similar amounts after APAP administration. Our finding that APAP hepatotoxicity was decreased in PXR(؊/؊) mice indicates that PXR is an important modulator of APAP hepatotoxicity, through positive modulation of constitutive CYP1A2 expression and possibly through increased APAP absorption.Acetaminophen (APAP) is an over-the-counter drug commonly used for its analgesic and antipyretic properties. It is also a component of numerous prescription medications. This compound is considered relatively safe at therapeutic doses; however, when taken in overdose, it has been shown to produce potentially fatal centrilobular hepatic necrosis (for review, see James et al., 2003).At pharmacological doses, APAP is mainly metabolized by phase II xenobiotic-metabolizing enzymes via sulfation and glucuronidation. A small amount of APAP is converted to N-acetyl-p-benzoquinone imine (NAPQI), a reactive metabolite, via cytochromes P450 (P450s) (for review, see James et al., 2003). NAPQI is detoxified by conjugation with glutathione (GSH); however, in APAP overdose, the capacities of phase II pathways are overwhelmed and GSH levels become depleted (for review, see James et al., 2003). The ensuing liver damage may be due to the binding of NAPQI to proteins, the generation of reactive oxygen species, or a combination of both (for review, see James et al