Anwar-Mohamed Anwar, Elbekai Reem H, El-Kadi Ayman O S
Faculty of Pharmacy and Pharmaceutical Sciences, 3126 Dentistry/Pharmacy Centre, University of Alberta, Edmonton, Alberta, Canada T6G 2N8.
Toxicol Lett. 2008 Nov 10;182(1-3):121-6. doi: 10.1016/j.toxlet.2008.09.007. Epub 2008 Sep 18.
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.
先前的研究表明，2,3,7,8-四氯二苯并对二恶英（TCDD）诱导的芳烃受体（AhR）降解受到MG-132（一种26S蛋白酶体的有效抑制剂）的抑制。因此，本研究旨在利用小鼠肝癌Hepa 1c1c7细胞探讨MG-132对AhR调控基因细胞色素P450 1a1（Cyp1a1）的影响。我们的结果表明，在测试的最高浓度10微摩尔的MG-132通过转录机制显著增加了Cyp1a1的mRNA、蛋白质和催化活性水平。另一方面，MG-132进一步增强了TCDD介导的Cyp1a1在mRNA水平而非蛋白质水平的诱导。相反，MG-132显著抑制了TCDD介导的Cyp1a1催化活性。此外，我们发现Cyp1a1催化活性的降低并非Cyp特异性的，因为MG-132显著抑制了Cyp2b1和总细胞色素P450的催化活性。这些结果促使我们研究MG-132对总细胞血红素含量和血红素加氧酶-1（HO-1）mRNA（血红素降解的限速酶）的影响。我们的结果表明，MG-132以浓度依赖性方式显著诱导HO-1 mRNA。此外，MG-132以浓度依赖性方式增强了TCDD对HO-1 mRNA的诱导。HO-1 mRNA水平的诱导与总细胞血红素含量的降低相一致。总之，本研究首次证明，MG-132尽管增加了Cyp1a1 mRNA表达，但可能通过降低其血红素含量来降低其活性。
