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Received for publication June 23, 2005.
Revised December 12, 2005.
Accepted for publication December 14, 2005.
Activation of the Pregnane X receptor (PXR) mediates the induction of several drug transporters and metabolizing enzymes. In vitro studies have reported that several of these genes are induced after exposure to the hepatocarcinogen, 2-acetylaminofluorene (2-AAF). Thus, we hypothesized that PXR may play a role in the in vivo induction of gene expression by 2-AAF. We examined the expression of the drug metabolizing enzymes CYP1A2 and CYP3A11 and the drug transporters BCRP, MRP2 and OATP2. Wild-type (PXR+/+) and PXR-null (PXR-/-) C57BL/6 mice were injected daily for 7 days with 150 or 300 mg/kg of 2-AAF suspended in corn oil (i.p.), while control group received corn oil vehicle. Levels of mRNA isolated from liver were measured by RT-PCR and normalized to 
-actin. Treatment of PXR+/+ mice resulted in a dose-dependent 2- to 4-fold induction (p<0.001) of MRP2, OATP2, BCRP, CYP3A11 and CYP1A2, but no induction was observed in PXR-/- mice. Induction of PXR mRNA was observed in the 2-AAF-treated PXR+/+ mice. Furthermore, a dose-dependent increase in CYP3A4 promoter construct activity was observed in HepG2 cells co-transfected with human or rat PXR, indicating that 2-AAF does indeed activate PXR. These results suggest that murine PXR is responsible for 2-AAF-mediated induction of drug efflux transporters and biotransformation enzymes in the liver. Moreover, novel findings demonstrate that PXR plays a role in regulation of the drug efflux transporter, BCRP, in mice.
Key words:
ABC transporters, chemical carcinogenesis, CYP induction, cytochrome P450 regulation, hepatotoxicity, PXR, transporters
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