title: Transcriptional and posttranslational mechanisms modulating the expression of the cytochrome P450 1A1 gene by lead in HepG2 cells: a role of heme oxygenase.
abstract: Co-contamination with complex mixtures of heavy metals, such as lead (Pb(2+)) and halogenated aromatic hydrocarbons (HAHs), such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) may disrupt the coordinated regulation of the carcinogen activating enzyme cytochrome P450 1A1 (CYP1A1). Therefore, in this study we examined the effects of co-exposure to Pb(2+) and TCDD on the expression of CYP1A1 in human hepatoma HepG2 cells and explored the involvement of transcriptional and posttranscriptional mechanisms. Our results showed that Pb(2+) significantly decreased TCDD-induced CYP1A1 mRNA, protein, and catalytic activity levels in a concentration-dependent manner. Importantly, this inhibition is specific to CYP1A1 and not to other aryl hydrocarbon receptor (AhR)-regulated gene, as Pb(2+) induced NAD(P)H:Quinone oxidoreductase 1 mRNA. Mechanistically, the Pb(2+)-mediated inhibition of CYP1A1 was associated with a significant decrease in the xenobiotic responsive element (XRE)-dependent luciferase activity without affecting the level of AhR protein, suggesting a transcriptional mechanism. On the other hand, the inhibitory effect of Pb(2+) on the induction of CYP1A1 coincided with an increase in heme oxygenase-1 (HO-1) mRNA level and reactive oxygen species production at the posttranslational level. Furthermore, the inhibition of HO-1 activity, by tin mesoporphyrin, or supplementing heme, using hemin, caused a partial restoration of Pb(2+)-mediated inhibition of CYP1A1 induction by TCDD. In addition, transfection of HepG2 cells with siRNA targeting the human HO-1 gene restored the Pb(2+)-mediated inhibition of TCDD-induced CYP1A1. In conclusion, this study demonstrated that Pb(2+) down-regulates the expression of CYP1A1 through transcriptional and posttranslational mechanisms and confirms the role of HO-1 in a Pb(2+)-mediated effect.
authors: Department of Pharmacology and Toxicology, College of Pharmacy, King Saud University, PO Box 2457, Riyadh 11451, Saudi Arabia.
authors: Hesham M
authors: HM
authors: Korashy
authors: Ayman O S
authors: AO
authors: El-Kadi
mesh_terms: D015153:Blotting, Western; D045744:Cell Line, Tumor; D002470:Cell Survival / Q000187:drug effects; D019363:Cytochrome P-450 CYP1A1 / Q000096:biosynthesis* / Q000235:genetics*; D018076:DNA, Complementary / Q000096:biosynthesis / Q000235:genetics; D004785:Environmental Pollutants / Q000633:toxicity; D004790:Enzyme Induction / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects*; D006419:Heme Oxygenase (Decyclizing) / Q000037:antagonists & inhibitors / Q000096:biosynthesis / Q000378:metabolism*; D006427:Hemin / Q000494:pharmacology; D006801:Humans; D007854:Lead / Q000633:toxicity*; D000072317:Polychlorinated Dibenzodioxins / Q000494:pharmacology; D011499:Protein Processing, Post-Translational / Q000187:drug effects*; D012333:RNA, Messenger / Q000096:biosynthesis / Q000302:isolation & purification; D034741:RNA, Small Interfering / Q000235:genetics / Q000494:pharmacology; D017382:Reactive Oxygen Species / Q000378:metabolism; D060888:Real-Time Polymerase Chain Reaction; D018336:Receptors, Aryl Hydrocarbon / Q000096:biosynthesis / Q000235:genetics; D014158:Transcription, Genetic / Q000187:drug effects*
chemical_list: D018076:DNA, Complementary; D004785:Environmental Pollutants; D004791:Enzyme Inhibitors; D000072317:Polychlorinated Dibenzodioxins; D012333:RNA, Messenger; D034741:RNA, Small Interfering; D017382:Reactive Oxygen Species; D018336:Receptors, Aryl Hydrocarbon; D007854:Lead; D006427:Hemin; D019363:Cytochrome P-450 CYP1A1; D006419:Heme Oxygenase (Decyclizing)