Unknown,Transcriptomics,Genomics,Proteomics

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TCDD-induced hepatic transcriptomic responses in transgenic AHR-variant mice

ABSTRACT: The aryl hydrocarbon receptor (AHR) mediates most of the toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). However, TCDD toxicity phenotypes vary widely between species, strains and even between sexes within a strain. While the exact reasons for this variation remain unclear, it is thought to be related to differences in the structure of the AHR. Previous studies comparing the downstream effects of TCDD exposure between animals with different AHR isoforms have been confounded by the genetic differences between these model systems. To address this issue conclusively, we evaluated three transgenic mouse lines, each of which express a different rat AHR isoform (rWT, DEL, and INS) from two strains of rat with highly divergent TCDD-susceptibilities, within identical genetic backgrounds. Here we profile hepatic transcriptomic responses following exposure to TCDD, and use these to identify transcripts associated with toxicity. We have confirmed that the variation in toxicity is inherent to the AHR isoform. Additionally, we note the enhanced activity of the modified transactivation domain of the DEL isoform, relative to the INS isoform, and provide further evidence that the INS isoform is responsible for the high resistance to TCDD observed in H/W rats. We also uncover several candidate genes that were consistently differentially expressed in TCDD-sensitive mice and rats. Adult male transgenic mice were treated by gavage with 0, 125, 250, 500, or 1000 Âµg/kg TCDD dissolved in corn oil vehicle. Mice were euthanized 4 days following treatment and liver tissue was harvested for analysis. RNA was isolated and the transcriptome for each animal assayed on separate microarrays.

ORGANISM(S): Mus musculus

SUBMITTER: Stephenie Prokopec

PROVIDER: E-GEOD-72270 | biostudies-arrayexpress |

REPOSITORIES: biostudies-arrayexpress

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Project description: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. Rats from sensitive (Long-Evans, LE) and resistant (Han/Wistar, HW) strains were treated with 100 ug/kg TCDD or corn oil vehicle and sacrificed either 4 or 10 days after treatment. LE control rats were either fed normally or feed-restricted to control for the wasting effects of TCDD treatment. Each treatment group contains four or five animals (biological replicates), each of which was assayed on an individual microarray.

Project description:2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is an environmental contaminant that produces myriad toxicities across various experimental models. In rodents alone, there is huge divergence in the toxicological response across species, between strains within a species and even between sexes within a strain. This difference in sensitivities between sexes has been characterized in multiple rodent models, however with contrasting results: the severity of toxicity is greater in female rats than in males while male mice and guinea pigs are more sensitive to the effects of TCDD than females. While the presence of estrogens or androgens has a known impact on response, the specific transcriptional events that cause this difference remain unclear. We sought to characterize the transcriptional environment of male and female C57BL/6 mice treated with 500 M-NM-<g/kg TCDD and followed across a time-course. This dose was chosen as it is greater than the LD50 for male mice yet produces minimal effects in females. Additionally, evaluation along a time-course allows for the detection of specific changes that occur throughout the development of phenotypic toxicities. The transcriptional profile across the time-course was highly divergent between the sexes. Female TCDD-treated mice demonstrated a large number of altered transcripts as early as 6 hours following treatment, suggesting a large primary response, possibly indicative of the activation of numerous defense mechanisms. Conversely, male animals showed the greatest TCDD-mediated response 144 hours following exposure, potentially implicating significant secondary responses for the increased appearance of TCDD toxicities. Nr1i3 is statistically significantly altered at all time-points in the sensitive male animals. This mRNA encodes a transcription factor (constitutive androstane receptor; CAR) involved in the regulation of xenobiotic metabolism, as well as lipid metabolism, cell cycle and apoptosis. In particular, increased levels of CAR may result in induction of the anti-apoptotic gene Gadd45b and carboxylesterase Ces1d, both of which we identify as transcriptionally altered and may be responsible for phenotypic hepatic abnormalities, such as steatohepatitis and tumor formation. Further evaluation across studies of mice and rats into the role of Nr1i3 and associated genes are crucial to enhance our understanding of various TCDD-induced toxicities. Adult male and female C57BL/6 mice were treated by gavage with either 500 ug/kg TCDD in corn oil or corn oil vehicle alone. Animals were euthanized at either 6, 24, 72 or 144 hours after treatment and tissues were harvested. RNA was isolated from hepatic tissue and the transcriptome for each animal assayed on an individual microarray. Please note that 7 samples (out of total 65 samples) were identified as outliers and therefore the data were processed without the outliers as well. The normalized data without outliers were provided in the 'normalized_data_without_outliers.txt' file.

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TCDD-induced hepatic transcriptomic responses in transgenic AHR-variant mice

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