Patterns of dioxin-altered mRNA expression in livers of dioxin-sensitive versus dioxin-resistant rats

The dioxin congener 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) causes a wide range of toxic effects in rodent species, all of which are mediated by a ligand-dependent transcription-factor, the aryl hydrocarbon receptor (AHR). The Han/Wistar (Kuopio) (H/W) strain shows exceptional resistance to many TCDD-induced toxicities; the LD50 of >9600 µg/kg for H/W rats is higher than for any other wild-type mammal known. We have previously shown that this resistance primarily results from H/W rats expressing a variant AHR isoform that has a substantial portion of the AHR transactivation domain deleted. Despite this large deletion, H/W rats are not entirely refractory to the effects of TCDD; the variant AHR in these animals remains fully competent to up-regulate well-known dioxin-inducible genes. TCDD-sensitive (Long-Evans, L-E) and resistant (H/W) rats were treated with either corn-oil (with or without feed-restriction) or 100 µg/kg TCDD for either four or ten days. Hepatic transcriptional profiling was done using microarrays, and was validated by RT-PCR analysis of 41 genes. . A core set of genes was altered in both strains at all time points tested, including CYP1A1, CYP1A2, CYP1B1, Nqo1, Aldh3a1, Tiparp, Exoc3, and Inmt. Outside this core, the strains differed significantly in the breadth of response: three-fold more genes were altered in L-E than H/W rats. At ten days almost all expressed genes were dysregulated in L-E rats, likely reflecting emerging toxic responses. Far fewer genes were affected by feed-restriction, suggesting that only a minority of the TCDD-induced changes are secondary to the wasting syndrome.二恶英同系物2,3,7,8-四氯二苯并对二恶英（2,3,7,8-tetrachlorodibenzo-p-dioxin, TCDD）可在啮齿类动物中引发多种毒性效应，所有此类毒性均由配体依赖型转录因子——芳基烃受体（aryl hydrocarbon receptor, AHR）介导。Han/Wistar（库奥皮奥）（H/W）品系大鼠对多种TCDD诱导的毒性表现出极强抗性；H/W大鼠的半数致死量（LD50）超过9600 μg/kg，高于目前已知的所有野生型哺乳动物。本团队此前已证实，这种抗性主要源于H/W大鼠表达的变异型AHR亚型，该亚型缺失了AHR转录激活结构域的大片段。尽管存在该大片段缺失，H/W大鼠并未完全对TCDD的毒性效应产生耐受：这些动物体内的变异型AHR仍可完全上调经典的二恶英诱导基因。本研究将对TCDD敏感的Long-Evans（L-E）大鼠与耐受型H/W大鼠分别予以玉米油（限食或不限食）处理，或予以100 μg/kg TCDD处理，处理时长分为4天与10天两组。采用基因芯片技术开展肝脏转录组谱分析，并通过针对41个基因的逆转录聚合酶链反应（RT-PCR）验证了芯片结果的可靠性。在所有检测时间点下，两个品系大鼠的肝脏中均出现了一组核心基因的表达异常，包括CYP1A1、CYP1A2、CYP1B1、Nqo1、Aldh3a1、Tiparp、Exoc3及Inmt。在核心基因集之外，两个品系的转录应答广度存在显著差异：L-E大鼠中发生表达异常的基因数量是H/W大鼠的3倍以上。处理10天时，L-E大鼠体内几乎所有表达的基因均出现表达失调，这大概率反映了其正在发生的毒性应答。限食处理仅对极少数基因的表达产生影响，这提示TCDD诱导的基因表达变化中仅有少数继发于恶病质综合征。