Molecular Toxicology Program, Dept of Pathology and Laboratory Medicine, Jonsson Comprehensive Cancer Center, University of California, Los Angeles, CA 90095, USA.
Toxicol Lett. 2010 Apr 15;194(1-2):26-33. doi: 10.1016/j.toxlet.2010.01.019. Epub 2010 Jan 29.
The xenobiotic metabolizing enzymes Cyp1a1 and Cyp1b1 can be induced by the environmental contaminant 2,3,7,8-tetrachlorodibenzo-rho-dioxin (dioxin) via the aryl hydrocarbon receptor (AhR). These genes are differentially induced by dioxin in different mouse cell lines. In the mouse hepatoma cell line Hepa1c1c7 (Hepa-1), the Cyp1a1 gene is induced to very high levels by dioxin, but the levels of Cyp1b1 mRNA are extremely low and are not inducible by dioxin. The reverse is the case for the mouse embryonic fibroblast cell line C3H10T1/2, in which Cyp1b1 is induced to very high levels by dioxin, but the levels of Cyp1a1 mRNA are extremely low and not inducible by dioxin. However, dioxin treatment leads to the recruitment of AhR to the enhancer regions of both genes in both cell lines. Somatic cell hybrid clones generated between the two cell lines display high levels of induction of both genes in response to dioxin. Strong reactivation of the Cyp1a1 gene was also observed in C3H10T1/2 cell line after treatment with the DNA methyl transferase inhibitor, 5-aza-2'-deoxycytidine (5-AzadC) and the histone deacetylase inhibitor, trichostatin-A (TSA). However, only modest reactivation of Cyp1b1 was observed in Hepa-1 cells after 5-AzadC or TSA treatment. These data demonstrate that the presence or absence of binding of AhR to regulatory regions is not responsible for determining the differences in levels of induction of the two genes in these cell lines and indicate that DNA methylation plays a major role in silencing of Cyp1a1 gene expression in C3H10T1/2 cells, but appears to play only a minor role in silencing Cyp1b1 gene expression in Hepa-1 cells, which likely occurs principally because Hepa-1 cells lack a factor required for high levels of induction of this gene.
异生素代谢酶 Cyp1a1 和 Cyp1b1 可被环境污染物 2,3,7,8-四氯二苯并对二恶英（二恶英）通过芳香烃受体（AhR）诱导。这些基因在不同的小鼠细胞系中被二恶英差异诱导。在小鼠肝癌细胞系 Hepa1c1c7（Hepa-1）中，Cyp1a1 基因被二恶英诱导至非常高的水平，但 Cyp1b1mRNA 的水平极低，且不能被二恶英诱导。相反，在小鼠胚胎成纤维细胞系 C3H10T1/2 中则是 Cyp1b1 被二恶英诱导至非常高的水平，而 Cyp1a1mRNA 的水平极低，且不能被二恶英诱导。然而，二恶英处理导致 AhR 募集到两个细胞系中两个基因的增强子区域。在这两个细胞系之间生成的体细胞核杂交克隆显示出对二恶英的强烈诱导，两个基因的诱导水平都很高。在用 DNA 甲基转移酶抑制剂 5-氮杂-2'-脱氧胞苷（5-AzadC）和组蛋白去乙酰化酶抑制剂曲古抑菌素 A（TSA）处理 C3H10T1/2 细胞系后，也观察到 Cyp1a1 基因的强烈重新激活。然而，在用 5-AzadC 或 TSA 处理 Hepa-1 细胞后，仅观察到 Cyp1b1 的适度重新激活。这些数据表明，AhR 与调节区的结合的存在或不存在，对于决定这两个基因在这些细胞系中的诱导水平差异不是决定性的，并且表明 DNA 甲基化在 C3H10T1/2 细胞中 Cyp1a1 基因表达的沉默中起主要作用，但在 Hepa-1 细胞中沉默 Cyp1b1 基因表达似乎只起次要作用，这很可能主要是因为 Hepa-1 细胞缺乏该基因高诱导水平所必需的一个因子。
