Computational identification of dioxin response elements in human, mouse, and rat genomes

Dioxin response elements were computationally identified and matrix similarity scores estimated as previously described (PMIDs: 15328365, 21762485, and 26582802

AHR ChIP-Seq of male mouse liver following 2 hour TCDD exposure

Aryl hydrocarbon receptor ChIP-Seq performed in livers of male mice gavaged with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) for 2hr

Genome-Wide ChIPseq Analysis of AhR, COUP-TF, and HNF4 Enrichment in TCDD-Treated Mouse Liver

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor known for mediating the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds. Although the canonical mechanism of AhR activation involves heterodimerization with the aryl hydrocarbon receptor nuclear translocator, other transcriptional regulators that interact with AhR have been identified. Enrichment analysis of motifs in AhR-bound genomic regions implicated co-operation with COUP transcription factor (COUP-TF) and hepatocyte nuclear factor 4 (HNF4). The present study investigated AhR, HNF4α and COUP-TFII genomic binding and effects on gene expression associated with liver-specific function and cell differentiation in response to TCDD. Hepatic ChIPseq data from male C57BL/6 mice at 2 h after oral gavage with 30 µg/kg TCDD were integrated with bulk RNA-sequencing (RNAseq) time-course (2–72 h) and dose–response (0.01–30 µg/kg) datasets to assess putative AhR, HNF4α and COUP-TFII interactions associated with differential gene expression. Functional enrichment analysis of differentially expressed genes (DEGs) identified differential binding enrichment for AhR, COUP-TFII, and HNF4α to regions within liver-specific genes, suggesting intersections associated with the loss of liver-specific functions and hepatocyte differentiation. Analysis found that the repression of liver-specific, HNF4α target and hepatocyte differentiation genes, involved increased AhR and HNF4α binding with decreased COUP-TFII binding. Collectively, these results suggested TCDD-elicited loss of liver-specific functions and markers of hepatocyte differentiation involved interactions between AhR, COUP-TFII and HNF4α

A Physiologically Based Pharmacokinetic (PBPK) Modeling Framework for Mixtures of Dioxin-like Compounds

Humans are exposed to persistent organic pollutants, such as dioxin-like compounds (DLCs), as mixtures. Understanding and predicting the toxicokinetics and thus internal burden of major constituents of a DLC mixture is important for assessing their contributions to health risks. PBPK models, including dioxin models, traditionally focus on one or a small number of compounds; developing new or extending existing models for mixtures often requires tedious, error-prone coding work. This lack of efficiency to scale up for multi-compound exposures is a major technical barrier toward large-scale mixture PBPK simulations. Congeners in the DLC family, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), share similar albeit quantitatively different toxicokinetic and toxicodynamic properties. Taking advantage of these similarities, here we reported the development of a human PBPK modeling framework for DLC mixtures that can flexibly accommodate an arbitrary number of congeners. Adapted from existing TCDD models, our mixture model contains the blood and three diffusion-limited compartments—liver, fat, and rest of the body. Depending on the number of congeners in a mixture, varying-length vectors of ordinary differential equations (ODEs) are automatically generated to track the tissue concentrations of the congeners. Shared ODEs are used to account for common variables, including the aryl hydrocarbon receptor (AHR) and CYP1A2, to which the congeners compete for binding. Binary and multi-congener mixture simulations showed that the AHR-mediated cross-induction of CYP1A2 accelerates the sequestration and metabolism of DLC congeners, resulting in consistently lower tissue burdens than in single exposure, except for the liver. Using dietary intake data to simulate lifetime exposures to DLC mixtures, the model demonstrated that the relative contributions of individual congeners to blood or tissue toxic equivalency (TEQ) values are markedly different than those to intake TEQ. In summary, we developed a mixture PBPK modeling framework for DLCs that may be utilized upon further improvement as a quantitative tool to estimate tissue dosimetry and health risks of DLC mixtures

Use of Xenopus Laevis as a Model for Investigating in Vitro and in Vivo Endocrine Disruption in Amphibians

The estrogenic activity of 17β-estradiol (E2), α-zearalenol (α-ZEA), genistein (GEN), and 4-t-octylphenol (4-t-OP) was investigated using Xenopus laevis-based assays. All test compounds competed with [3H]E2 for binding to a recombinant Xenopus estrogen receptor (xER) with the following relative affinities: E2 \u3e α-ZEA \u3e 4-t-OP \u3e GEN. the ability of these compounds to induce xER-mediated reporter gene expression was then assessed in MCF-7 human breast cancer cells cotransfected with a Gal4-xERdef chimeric estrogen receptor and a Gal4-regulated luciferase reporter gene. Luciferase activity was increased 30- to 50-fold by 10 nM E2 relative to that in solvent control. Maximal reporter gene activity induced by 10 nM α-ZEA was 54% of that induced by E2; however, the activity did not increase following doses of up to 10 μM. a dose of 1 μM 4-t-OP induced 23% of the maximal reporter gene activity induced by E2, whereas 10 μM GEN induced activity to the same level as E2. a dose-dependent increase in vitellogenin (VTG) mRNA expression was observed in Xenopus treated intraperitoneally with E2 at 0.05 to 5 mg/kg/d for three consecutive days, with the maximal induction observed in the group receiving 1 mg/kg/d. the α-ZEA, GEN, and 4-t-OP also significantly induced VTG mRNA expression, although at higher doses. These results demonstrate the utility of X. laevis as an amphibian model to assess the estrogenic activity of endocrine disruptor