2,3,7,8 Tetrachlorodibenzo-p-dioxin (TCDD) is a ubiquitous environmental contaminant that causes a wide array of species-specific adverse biochemical and physiological responses, including increased tumor promotion, lethality and hepatotoxicity. Most, if not all of the effects elicited by TCDD are due to inappropriate changes in gene expression that are mediated through activation of the aryl hydrocarbon receptor (AhR). Although the mechanism of AhR gene regulation is well known, the full spectrum of targeted genes leading to the subsequent toxicological responses remains poorly understood. The objective of this research was to integrate disparate and complementary toxicogenomic approaches to identify putative biomarkers of TCDD-induced hepatotoxicity that would aide in reducing the uncertainties involved in cross-species and cross-model extrapolations.In vitro microarray investigation of a mouse hepatoma cell line treated with TCDD identified complex temporal and dose-dependent gene expression responses. Comparative analysis with in vivo hepatic gene expression responses in mice identified a small subset of conserved genes with biological functions related to xenobiotic metabolism, consistent with the known responses observed in vivo. Furthermore, in vitro cross-species comparison using human, mouse, and rat hepatoma cell lines identified relatively few species-conserved gene expression and is corroborates prior reports of species-specific TCDD-induced toxicities. Genome-wide computational identification and characterization of dioxin response elements (DREs) using a position weight matrix identified species-specific regulons in the promoter regions of targeted genes that may account for the observed species-divergent and -specific responses. In order to better understand the molecular mechanisms responsible for regulating the transcriptional responses and downstream hepatotoxicity, ChIP-chip analysis was performed to globally identify TCDD-induced AhR/DNA interactions in mouse hepatic tissue. Interestingly, integration of the DRE, ChIP-chip and gene expression analyses found that only ~32% of all TCDD-elicited hepatic gene expression responses are mediated by a DRE-dependent mechanism. These direct targets of AhR regulation have biological functions related to xenobiotic and lipid metabolism, which correspond with the physiological responses observed in vivo. The remaining transcriptional responses that are mediated through a DRE-independent mechanism illustrate the diverse regulatory role of the AhR. Collectively, these results have expanded our knowledge of the hepatic AhR regulatory network and provide insight into the species-conserved responses elicited by TCDD.
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