MG-132 inhibits the TCDD-mediated induction of Cyp1a1 at the catalytic activity but not the mRNA or protein levels in Hepa 1c1c7 cells.
Previous studies have shown that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced degradation of aryl hydrocarbon receptor (AhR) is inhibited by MG-132, a potent inhibitor of the 26S proteasome. Therefore, the current study aims to address the effect of MG-132 on the AhR-regulated gene, cytochrome P450 1a1 (Cyp1a1), using murine hepatoma Hepa 1c1c7 cells. Our results showed that MG-132 at the highest concentration tested, 10 microM significantly increased the Cyp1a1 at mRNA, protein and catalytic activity levels through a transcriptional mechanism. On the other hand, MG-132 further potentiated the TCDD-mediated induction of Cyp1a1 at mRNA but not at protein level. In contrast, MG-132 significantly inhibited the TCDD-mediated induction of Cyp1a1 catalytic activity. In addition, we showed that the decrease in Cyp1a1 catalytic activity is not Cyp specific, as MG-132 significantly inhibited Cyp2b1 and total cytochrome P450 catalytic activities. These results prompted us to examine the effect of MG-132 on total cellular heme content and heme oxygenase-1 (HO-1) mRNA, a rate limiting enzyme of heme degradation. Our results showed that MG-132 significantly induced HO-1 mRNA in a concentration-dependent manner. Furthermore, MG-132 potentiated the induction of HO-1 mRNA by TCDD in a concentration-dependent manner. The induction of HO-1 mRNA level coincided with a decrease in total cellular heme content. In conclusion, the present study demonstrates for the first time that MG-132, despite of increasing Cyp1a1 mRNA expression, it decreases its activity probably through decreasing its heme content.
Faculty of Pharmacy and Pharmaceutical Sciences, 3126 Dentistry/Pharmacy Centre, University of Alberta, Edmonton, Alberta, Canada T6G 2N8.
Anwar
A
Anwar-Mohamed
Reem H
RH
Elbekai
Ayman O S
AO
El-Kadi
D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology*; D015153:Blotting, Western; D002384:Catalysis; D045744:Cell Line, Tumor; D019363:Cytochrome P-450 CYP1A1 / Q000096:biosynthesis* / Q000378:metabolism*; D019362:Cytochrome P-450 CYP2B1 / Q000096:biosynthesis / Q000235:genetics; D004785:Environmental Pollutants / Q000037:antagonists & inhibitors* / Q000494:pharmacology*; D006418:Heme / Q000378:metabolism; D007976:Leupeptins / Q000494:pharmacology*; D008156:Luciferases / Q000096:biosynthesis; D051379:Mice; D000072317:Polychlorinated Dibenzodioxins / Q000037:antagonists & inhibitors* / Q000494:pharmacology*; D012333:RNA, Messenger / Q000096:biosynthesis*; D014158:Transcription, Genetic / Q000187:drug effects; D014162:Transfection
D000970:Antineoplastic Agents; D004785:Environmental Pollutants; D007976:Leupeptins; D000072317:Polychlorinated Dibenzodioxins; D012333:RNA, Messenger; D006418:Heme; D008156:Luciferases; D019363:Cytochrome P-450 CYP1A1; D019362:Cytochrome P-450 CYP2B1; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde