Project description: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.

Project description:Wild-type and AHR-KO rats harboring a 29-bp deletion mutation in exon 2 of the AHR gene were used in this study. Wild-type and AHR-KO rats were generated from heterozygous breeding stocks generated on a Sprague-Dawley outbred background and maintained at the Hamner Institutes for Health Sciences, Research Triangle Park, North Carolina Persistent activation of the aryl hydrocarbon receptor (AHR) is believed to play a key role in the mode-of-action for TCDD induced rat liver tumorigenesis. It has been hypothesized that the cellular responses of hepatocytes to AHR activation may differ across regions of the liver lobule and that zone-specific effects of AHR agonists may play a role in the pathogenesis of rat liver tumorigenesis. Dose-dependent changes in gene expression were observed in both populations of hepatocytes collected from WT rats which were consistent with activation of AHR signaling. No significant or dose-dependent changes in gene expression were observed in samples from AHR-KO rats. In addition, evidence of inflammatory signaling pathway activation was observed only in centrilobular hepatocytes. Evidence of cell adhesion pathway enrichment was observed only in periportial hepatocytes. Benchmark dose analysis demonstrated that dose-dependent changes in gene expression occurred at lower doses in centrilobular as compared to periportal hepatocytes. These results indicate zone-specific differences in the sensitivity and response of hepatocytes to persistent AHR activation. AHR-KO and WT rats (n = 10 rats/dose/genotype) were dosed by oral gavage (4-5 doses / week, 19 total doses) with varying concentrations of TCDD in corn oil (0, 3, 22, 100, 300, 1000 ng/kg/day). Cryopreserved liver slices collected at necropsy were thawed, cryosectioned (12 Âµm thickness), fixed with ethanol and âquick-stainedâ with hematoxylin. Centrilobular and periportal hepatocytes were collected using laser capture microdissection. Total RNA was extracted, amplified and labeled using low-yield techniques and global transcriptomic profiles were obtained using Affymetrix Gene-Titan peg arrays.

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Toxicogenomic Analysis of Gender, Chemical, and Dose Effects in Livers of TCDD- or Aroclor 1254-Exposed Rats

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