Down-regulation of the carcinogen-metabolizing enzyme cytochrome P450 1a1 by vanadium.
Vanadium (V(5+)), a heavy metal contaminant with important toxicological consequences, has received considerable attention as an anticancer agent, although the mechanisms remain unknown. As a first step to investigate these mechanisms, we examined the effect of V(5+) (as ammonium metavanadate, NH(4)VO(3)) on the expression of the aryl hydrocarbon receptor (AhR)-regulated gene: cytochrome P450 1a1 (Cyp1a1) at each step of the AhR signal transduction pathway, using Hepa 1c1c7 cells. Our results showed a significant reduction in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated induction of Cyp1a1 mRNA, protein and activity levels after V(5+) treatments in a dose-dependent manner. Investigation of the effect of coexposure to V(5+) and TCDD at transcriptional levels revealed that V(5+) significantly inhibited TCDD-mediated induction of AhR-dependent luciferase reporter gene expression. Furthermore, despite not affecting the direct activation of the cytosolic AhR by TCDD and subsequently transforming it to a DNA-binding form, V(5+) inhibited the nuclear accumulation of liganded AhR and subsequent formation of the AhR/aryl hydrocarbon nuclear translocator (Arnt)/xenobiotic responsive element (XRE) complex. Importantly, the V(5+)-mediated inhibition of AhR/Arnt/XRE complex formation coincided with a significant decrease in ecto-ATPase activity. Looking at the post-transcriptional and post-translational effects of V(5+) on existing Cyp1a1 mRNA and protein levels, we showed that V(5+) did not affect Cyp1a1 mRNA or protein stability, thus eliminating possible role of V(5+) in modifying Cyp1a1 gene expression through these mechanisms. This study provides the first evidence that V(5+) down-regulates the expression of Cyp1a1 at the transcriptional level through an ATP-dependent mechanism.
Faculty of Pharmacy and Pharmaceutical Sciences, 3126 Dentistry/Pharmacy Centre, University of Alberta, Edmonton, AB, Canada. aelkadi@pharmacy.ualberta.ca
Anwar
A
Anwar-Mohamed
Ayman O S
AO
El-Kadi
D001483:Base Sequence; D002273:Carcinogens / Q000378:metabolism*; D002384:Catalysis; D002460:Cell Line; D019363:Cytochrome P-450 CYP1A1 / Q000235:genetics / Q000378:metabolism*; D017931:DNA Primers; D015536:Down-Regulation / Q000187:drug effects*; D024202:Electrophoretic Mobility Shift Assay; D006418:Heme / Q000378:metabolism; D006419:Heme Oxygenase (Decyclizing) / Q000235:genetics; D006801:Humans; D000072317:Polychlorinated Dibenzodioxins / Q000494:pharmacology; D012323:RNA Processing, Post-Transcriptional; D012333:RNA, Messenger / Q000235:genetics; D014639:Vanadium / Q000494:pharmacology*
D002273:Carcinogens; D017931:DNA Primers; D000072317:Polychlorinated Dibenzodioxins; D012333:RNA, Messenger; D014639:Vanadium; D006418:Heme; D019363:Cytochrome P-450 CYP1A1; D006419:Heme Oxygenase (Decyclizing)