Khan Shaheen, Liu Shengxi, Stoner Matthew, Safe Stephen
Department of Veterinary Physiology and Pharmacology, Texas A&M University, College Station, TX 77843-4466, USA.
Toxicol Appl Pharmacol. 2007 Aug 15;223(1):28-38. doi: 10.1016/j.taap.2007.05.010. Epub 2007 May 25.
The aryl hydrocarbon receptor (AhR) is expressed in estrogen receptor (ER)-positive ZR-75 breast cancer cells. Treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces CYP1A1 protein and mRNA levels and also activates inhibitory AhR-ERalpha crosstalk associated with hormone-induced reporter gene expression. In ZR-75 cells grown under hypoxia, induction of these AhR-mediated responses by TCDD was significantly inhibited. This was not accompanied by decreased nuclear AhR levels or decreased interaction of the AhR complex with the CYP1A1 gene promoter as determined in a chromatin immunoprecipitation assay. Hypoxia-induced loss of Ah-responsiveness was not associated with induction of hypoxia-inducible factor-1alpha or other factors that sequester the AhR nuclear translocation (Arnt) protein, and overexpression of Arnt under hypoxia did not restore Ah-responsiveness. The p65 subunit of NFkappaB which inhibits AhR-mediated transactivation was not induced by hypoxia and was primarily cytosolic in ZR-75 cells grown under hypoxic and normoxic conditions. In ZR-75 cells maintained under hypoxic conditions for 24 h, BRCA1 (an enhancer of AhR-mediated transactivation in breast cancer cells) was significantly decreased and this contributed to loss of Ah-responsiveness. In cells grown under hypoxia for 6 h, BRCA1 was not decreased, but induction of CYP1A1 by TCDD was significantly decreased. Cotreatment of ZR-75 cells with TCDD plus the protein synthesis inhibitor cycloheximide for 6 h enhanced CYP1A1 expression in cells grown under hypoxia and normoxia. These results suggest that hypoxia rapidly induces protein(s) that inhibit Ah-responsiveness and these may be similar to constitutively expressed inhibitors of Ah-responsiveness (under normoxia) that are also inhibited by cycloheximide.
芳烃受体（AhR）在雌激素受体（ER）阳性的ZR-75乳腺癌细胞中表达。用2,3,7,8-四氯二苯并对二恶英（TCDD）处理可诱导CYP1A1蛋白和mRNA水平升高，同时激活与激素诱导的报告基因表达相关的抑制性AhR-ERα相互作用。在缺氧条件下培养的ZR-75细胞中，TCDD对这些AhR介导反应的诱导作用显著受到抑制。如在染色质免疫沉淀试验中所测定的，这并非伴随着细胞核AhR水平的降低或AhR复合物与CYP1A1基因启动子相互作用的减弱。缺氧诱导的Ah反应性丧失与缺氧诱导因子-1α或其他隔离AhR核转运蛋白（Arnt）的因子的诱导无关，并且在缺氧条件下Arnt的过表达并未恢复Ah反应性。在缺氧和常氧条件下培养的ZR-75细胞中，抑制AhR介导的反式激活的NFκB的p65亚基未被缺氧诱导，且主要位于细胞质中。在缺氧条件下维持24小时的ZR-75细胞中，BRCA1（乳腺癌细胞中AhR介导的反式激活的增强子）显著减少，这导致了Ah反应性的丧失。在缺氧条件下培养6小时的细胞中，BRCA1未减少，但TCDD对CYP1A1的诱导作用显著降低。用TCDD与蛋白质合成抑制剂环己酰亚胺共同处理ZR-75细胞6小时，可增强缺氧和常氧条件下培养的细胞中CYP1A1的表达。这些结果表明，缺氧迅速诱导抑制Ah反应性的蛋白质，这些蛋白质可能类似于（在常氧条件下）组成性表达的Ah反应性抑制剂，它们也受到环己酰亚胺的抑制。
