Puga A, Barnes S J, Chang C, Zhu H, Nephew K P, Khan S A, Shertzer H G
Department of Environmental Health, Center for Environmental Genetics, University of Cincinnati Medical Center, Cincinnati, OH, USA.
Biochem Pharmacol. 2000 Apr 15;59(8):997-1005. doi: 10.1016/s0006-2952(99)00406-2.
2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD; dioxin), the prototype agonist of the aromatic hydrocarbon (Ah) receptor, is a potent tumor promoter as well as a complete liver carcinogen that produces an oxidative stress response in rodents and in cultured cell lines. It has been proposed that TCDD promotes neoplastic transformation through oxidative signal transduction pathways, which results in activation of immediate-early response transcription factors. To set the stage for a test of this hypothesis, we evaluated the effect of TCDD treatment on the activation of several transcription factors, including those in the nuclear factor-kappaB (NF-kappaB) and activator protein-1 (AP-1) families, which are activated by changes in the redox state of cells. In an extension of prior results, we found that TCDD treatment produced a sustained overexpression of AP-1 for at least 72 hr in wild-type mouse hepatoma Hepa-1 cells, but not in the Ah receptor-deficient derivative c35 or in cytochrome P450-1A1 (CYP1A1)-negative c37 cells. In addition, TCDD treatment caused a significant increase in the DNA binding activity of NF-kappaB, but not in the activities of the other transcription factors tested. AP-1 and NF-kappaB activation were blocked by the thiol antioxidant N-acetylcysteine and by nordihydroguaiaretic acid, an antioxidant and lipooxygenase inhibitor and an inhibitor of the epoxygenase activity of CYP1A1, and did not take place in c35, c37, or in Ah nuclear translator-deficient c4 cells. Hence, sustained activation of these two transcription factors by TCDD is likely to result from a CYP1A1-dependent and Ah receptor complex-dependent oxidative signal. Electrophoretic mobility supershift analyses with specific antibodies showed that most of the increase in NF-kappaB binding activity could be accounted for by increases in p50/p50 complexes. Since these complexes are known to repress NF-kappaB-dependent gene transcription, our results delineate a second molecular mechanism, in addition to the recently found block of tumor necrosis factor-alpha-mediated p50/p65 activation, that may be responsible for the immunosuppresive effects of TCDD.
2,3,7,8-四氯二苯并对二恶英（TCDD；二恶英）是芳烃（Ah）受体的原型激动剂，是一种强效肿瘤促进剂以及完全性肝致癌物，可在啮齿动物和培养细胞系中产生氧化应激反应。有人提出，TCDD通过氧化信号转导途径促进肿瘤转化，这会导致立即早期反应转录因子的激活。为了为验证这一假设奠定基础，我们评估了TCDD处理对几种转录因子激活的影响，包括核因子-κB（NF-κB）和激活蛋白-1（AP-1）家族中的转录因子，这些转录因子会因细胞氧化还原状态的变化而被激活。在先前结果的扩展中，我们发现TCDD处理在野生型小鼠肝癌Hepa-1细胞中导致AP-1持续过表达至少72小时，但在Ah受体缺陷衍生物c35或细胞色素P450-1A1（CYP1A1）阴性的c37细胞中则没有。此外，TCDD处理导致NF-κB的DNA结合活性显著增加，但其他测试的转录因子活性没有增加。AP-1和NF-κB的激活被硫醇抗氧化剂N-乙酰半胱氨酸以及去甲二氢愈创木酸阻断，去甲二氢愈创木酸是一种抗氧化剂和脂氧合酶抑制剂，也是CYP1A1环氧合酶活性的抑制剂，并且在c35、c37或Ah核转运体缺陷的c4细胞中不会发生。因此，TCDD对这两种转录因子的持续激活可能是由CYP1A1依赖性和Ah受体复合物依赖性氧化信号导致的。用特异性抗体进行的电泳迁移率超迁移分析表明，NF-κB结合活性的大部分增加可归因于p50/p50复合物的增加。由于已知这些复合物会抑制NF-κB依赖性基因转录，我们的结果除了最近发现的肿瘤坏死因子-α介导的p50/p65激活受阻之外，还描绘了第二种分子机制，这可能是TCDD免疫抑制作用的原因。
