2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) Alters Hepatic Polyunsaturated Fatty Acid Metabolism and Eicosanoid Biosynthesis in Female Sprague-Dawley Rats [ChIP-Seq]

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent aryl hydrocarbon receptor (AhR) agonist that elicits a broad spectrum of dose-dependent effects in the liver, including hepatic lipid accumulation coupled with inflammation. To determine the role of inflammatory lipid mediators in TCDD-mediated hepatotoxicity, eicosanoid metabolism was investigated in female Sprague-Dawley (SD) rats. Rats were gavaged with sesame oil vehicle or 0.01-10 µg/kg TCDD every 4 days for 28 days. Hepatic RNA-Seq data from female SD rats was compared with data from female C57BL/6 mice and functionally annotated to determine key toxicogenomic differences between the two species regarding TCDD exposure. Hepatic RNA-Seq data from female SD rats integrated with untargeted metabolomics of liver, serum, and urine identified dose-dependent changes in linoleic acid (LA) and arachidonic acid (AA) metabolism. TCDD also elicited dose-dependent differential gene expression associated with cyclooxygenase, lipoxygenase, and cytochrome P450 epoxidation/ hydroxylation pathways with corresponding changes in omega-6 (e.g. AA and LA) and omega-3 polyunsaturated fatty acids (PUFAs) as well as their eicosanoid metabolites. Overall, total omega-6 PUFAs increased, while total omega-3 PUFAs decreased. Phospholipase A2 (Pla2g12a) was induced 6-fold consistent with increased AA metabolism, while AA utilization by lipoxygenases Alox5 (2-fold) and Alox15 (10-fold) increased leukotrienes (LTs), important mediators signaling an inflammatory response. More specifically, TCDD increased pro-inflammatory eicosanoids, including leukotriene (LT) B4 (3-fold), and LTB3 (5-fold), known signals for the recruitment of neutrophils to areas of tissue damage. Dose-response modeling of metabolite and gene expression changes suggests the cytochrome P450 hydroxylase/epoxygenase and the lipoxygenase pathways are the most sensitive to TCDD. While several differentially expressed genes (DEGs) associated with eicosanoid biosynthesis contained putative dioxin response elements (pDRE) within their regulatory region, ChIP-Seq analysis showed little AhR enrichment, suggesting TCDD-elicited induction of eicosanoid biosynthesis is not a direct effect of AhR activation.2,3,7,8-四氯二苯并对二噁英（TCDD）是一种强效芳基烃受体（AhR）激动剂，可在肝脏中引发一系列剂量依赖性效应，包括肝脂质蓄积伴发炎症反应。为明确炎症性脂质介质在TCDD介导的肝毒性中的作用，本研究对雌性斯普拉格-道利（SD）大鼠的类二十烷酸代谢进行了探究。大鼠每4天灌胃给予芝麻油赋形剂或0.01~10 μg/kg的TCDD，持续28天。将雌性SD大鼠的肝脏RNA测序（RNA-Seq）数据与雌性C57BL/6小鼠的对应数据进行比对，并进行功能注释，以明确两种物种在TCDD暴露下的关键毒物基因组学差异。将雌性SD大鼠的肝脏RNA-Seq数据与肝脏、血清及尿液的非靶向代谢组学数据整合后，可观察到亚油酸（LA）与花生四烯酸（AA）代谢出现剂量依赖性变化。TCDD还可引发与环氧合酶、脂氧合酶及细胞色素P450环氧化/羟化通路相关的剂量依赖性差异基因表达，同时伴随ω-6多不饱和脂肪酸（PUFAs，如AA与LA）、ω-3多不饱和脂肪酸及其类二十烷酸代谢物的水平变化。整体而言，总ω-6多不饱和脂肪酸水平升高，而总ω-3多不饱和脂肪酸水平降低。磷脂酶A2（Pla2g12a）的表达被诱导上调6倍，与AA代谢增强相一致；而脂氧合酶Alox5（上调2倍）与Alox15（上调10倍）对AA的利用增强，使得白三烯（LTs）——一类介导炎症反应的重要信号介质——的水平升高。更具体地说，TCDD可升高促炎类二十烷酸水平，包括白三烯B4（LTB4，上调3倍）与白三烯B3（LTB3，上调5倍），二者均为招募中性粒细胞至组织损伤部位的已知信号分子。对代谢物与基因表达变化进行剂量反应建模后发现，细胞色素P450羟化/环氧化通路与脂氧合酶通路对TCDD最为敏感。尽管若干与类二十烷酸生物合成相关的差异表达基因（DEGs）的调控区域内存在推定的二噁英应答元件（pDRE），但染色质免疫沉淀测序（ChIP-Seq）分析显示AhR富集程度极低，这表明TCDD诱导的类二十烷酸生物合成并非AhR激活的直接效应。