Pregnane X receptor PXR is a nuclear receptor

Pregnane X receptor (PXR) is a nuclear receptor that plays an important role in regulation of metabolism and disposition of structurally diverse xenobiotics as well as endogenous steroids and bile acids (Bertilsson et al., 1998, Kliewer et al., 1998, Lehmann et al., 1998). PXR regulates the metabolism of these compounds through coordinated regulation of genes encoding phase I and II metabolizing enzymes and transport proteins (Guengerich, 1999, Handschin and Meyer, 2003, Zhou et al., 2009). PXR has been found to interact with other nuclear receptors such as the constitutively-active androstane receptor (CAR) (Xie et al., 2000a, Xie et al., 2000b) and farnesoid X receptor (FXR) (Jung et al., 2006) forming the basis for the pathway cross-talks. The crosstalk between pathways with overlapping ligand specificity constitute a protective network against xenobiotic insults on one hand and on the other hand may cause adverse drug–drug interactions. Therefore, it is important to investigate the interactions between the receptor-mediated xenobiotic/drug metabolic pathways. The interactions between the receptor-regulated pathways are complex and may take place at different levels. For example, the metabolic alteration of structures by one receptor/enzyme system may impart new ligand binding specificity for activation of different receptor/enzyme system (Luckert et al., 1996). One the other hand, the interactions may be through interactions between receptors such as the PXR-CAR interaction (Xie et al., 2000a, Xie et al., 2000b) or AhR-PXR interaction, which is the focus of our current study. Previously, we found that expression of PXR significantly reduced BaP-induced DNA damages in HepG2 Z-LEHD-FMK (Naspinski et al., 2008). One mechanism for the reduced DNA damages is through PXR-regulated metabolic detoxification of BaP. As AhR is the pivotal factor for bioactivation of BaP, one additional possible mechanism for PXR-regulated reduction of the DNA damages is through inhibition of the AhR-regulated BaP bioactivation. We investigated this potential mechanism using in vitro cell culture model as well as in vivo animal model and found that PXR directly interacted with AhR to inhibit its transcriptional activity. We showed that PXR activation inhibited expression of genes encoding enzymes for BaP bioactivation both in cultured cells and in mice thus uncovering a novel mechanism by which PXR prevents liver cells from BaP-induced DNA damage through transcripitional repression of AhR activity.
Experimental procedures
Results
Discussion BaP is a prototypic PAHs and a known carcinogen. BaP is metabolized to its ultimate carcinogen BaP-7,8-diol-9,10-epoxide (BPDE) by the actions of CYPs (mainly cyp1a1, cyp1b1 and cyp1a2), which are mainly regulated by the AhR, and epoxide hydrolase (EH) (Levin et al., 1982). In an earlier study, we found that PXR expression reduced BaP-induced DNA adduct formation (Naspinski et al., 2008). Because BaP is a prototypic AhR agonist, we reasoned that the effects of PXR on BaP-induced DNA damages may be through regulating AhR activity. We and others have shown that AhR transcriptional activity can be regulated by several other transcription factors. For example, AhR interacts with estrogen receptor (ER) α and β. While AhR activation by TCDD causes well-characterized antiestrogenic effects through suppression of the transcriptional activity of ER, effects of ER on AhR activity appear to be ligand, receptor and promoter specific (Beischlag and Perdew, 2005). The AhR-ER interaction appears to affect the cyclical recruitment patterns for AhR thereby modulating AhR-dependent transcription of cyp1a1 and cyp1b1 (Wihlen et al., 2009). AhR can also directly interact with the NF-κB RelA subunit (Tian et al., 1999). Such an interaction can cause the suppression of AhR activity in liver cells (Tian, 2009). To investigate the putative interaction between PXR and AhR, we determined the effects of PXR on the activity of an AhR-responsive promoter. Our results revealed that PXR repressed the transcriptional activity of AhR in a ligand- and receptor-dependent manner in both human and mouse cells (Fig. 4). Consistent with these results, PXR expression inhibited the EROD activity (Fig. 3E).