Pregnane X Receptor Protects HepG2 Cells from BaP-Induced DNA Damage

Abstract

Pregnane X receptor (PXR) is a nuclear receptor that coordinately regulates transcriptional expression of both phase I and phase II metabolizing enzymes. PXR plays an important role in the pharmacokinetics of a broad spectrum of endogenous and xenobiotic compounds and appears to have evolved in part to protect organisms from toxic xenobiotics. Metabolism of benzo[a]pyrene (BaP), a well-established carcinogen and ubiquitous environmental contaminant, can result in either detoxification or bioactivation to its genotoxic forms. Therefore, PXR could modulate the genotoxicity of BaP by changing the balance of the metabolic pathways in favor of BaP detoxification. To examine the role of PXR in BaP genotoxicity, BaP-DNA adduct formation was measured by 32P-postlabeling in BaP-treated parental HepG2 cells and human PXR-transfected HepG2 cells. The presence of transfected PXR significantly reduced the level of adducts relative to parental cells by 50-65% (p < 0.001), demonstrating that PXR protects liver cells from genotoxicity induced by exposure to BaP. To analyze potential PXR-regulated detoxification pathways in liver cells, a panel of genes involved in phase I and phase II metabolism and excretion was surveyed with real-time quantitative reverse transcription PCR. The messenger RNA levels of CYP1A2, GSTA1, GSTA2, GSTM1, UGT1A6, and BCRP (ABCG2) were significantly higher in cells overexpressing PXR, independent of exposure to BaP. In addition, the total GST enzymatic activity, which favors the metabolic detoxification of BaP, was significantly increased by the presence of PXR (p < 0.001), independent of BaP exposure. Taken together, these results suggest that PXR plays an important role in protection against DNA damage by polycyclic aromatic hydrocarbons (PAHs) such as BaP, and that these protective effects may be through a coordinated regulation of genes involved in xenobiotic metabolism.

Mitotane induces CYP3A4 expression via activation of the steroid and xenobiotic receptor

“…1, mitotane, as well as RFP, increased transcription of the CYP3A4 gene in a dose-dependent manner, although the magnitude of the activation by mitotane was less than that of RFP. Co-transfection of the human SXR plasmid further enhanced transcriptional activation, consistent with previous reports of the relative low expression of endogenous SXR in this cell line (Zucchini et al 2005, Naspinski et al 2008. This reporter assay suggests that mitotane binds to SXR as a ligand and then stimulates CYP3A4 transcription.…”

Cytoprotective Effect of 20(S)-Rg3 on Benzo[a]pyrene-Induced DNA Damage

“…7). Activated PXR can regulate the expression of metabolic enzymes and transporters to protect cells from toxic xenobiotics (Naspinski et al, 2008). As mentioned previously, 20(S)-Rg3 and 20(R)-Rg3 are stereoisomers that differ in the position of the hydroxyl group carbon-20.…”

“…Because PXR is barely expressed in HDFs (Supplemental Fig. 3) Naspinski et al, 2008), we used human hepatoma HepG2 cells, which express a high level of PXR, as a cellular model to study the possible involvement of PXR in 20(S)-Rg3-induced protection against BaP. Here we examine the effects of 20(S)-Rg3 on PXR.…”

“…It was demonstrated that PXR protects HepG2 cells from BaP-induced DNA damage. In PXR-overexpressing liver cells, the mRNA levels of glutathione transferase (GST) M1, GSTA1, GSTA2, UDP-glucuronyltransferase (UGT) 1A6, and breast cancer resistance protein are up-regulated on challenge with BaP (Naspinski et al, 2008). PXR can be activated by different herbal compounds such as Ginkgo biloba (Yeung et al, 2008) and St. John's wort (Moore et al, 2000).…”

Pregnane X receptor suppresses proliferation and tumourigenicity of colon cancer cells

“…For example, our recent results suggest that PXR is important in preventing carcinogen-induced DNA damages (Naspinski et al, 2008). We and others have found cross talk between PXR-regulated pathway and NF-kB-regulated pathways (Gu et al, 2006;Zhou et al, 2006).…”