Beyond the Aryl Hydrocarbon Receptor: Pathway Interactions in the Hepatotoxicity of 2,3,7,8-Tetrachlorodibenzo-p-dioxin and Related Compounds

How single-cell transcriptomics provides insight on hepatic responses to TCDD

The prototypical aryl hydrocarbon receptor (AHR) ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), has been a valuable model for investigating toxicant-associated fatty liver disease (TAFLD). TCDD induces dose-dependent hepatic lipid accumulation, followed by the development of inflammatory foci and eventual progression to fibrosis in mice. Previously, bulk approaches and in vitro examination of different cell types were relied upon to study the mechanisms underlying TCDD-induced liver pathologies. However, the advent of single-cell transcriptomic technologies, such as single-nuclei RNA sequencing (snRNAseq) and spatial transcriptomics (STx), has provided new insights into the responses of hepatic cell types to TCDD exposure. This review explores the application of these single-cell transcriptomic technologies and highlights their contributions towards unraveling the cell-specific mechanisms mediating the hepatic responses to TCDD.

Jdp2 is a spatiotemporal transcriptional activator of the AhR via the Nrf2 gene battery

BACKGROUND

Crosstalk between the aryl hydrocarbon receptor (AhR) and nuclear factor (erythroid-derived 2)-like 2 (Nrf2) signaling is called the "AhR-Nrf2 gene battery", which works synergistically in detoxification to support cell survival. Nrf2-dependent phase II gene promoters are controlled by coordinated recruitment of the AhR to adjacent dioxin responsive element (DRE) and Nrf2 recruitment to the antioxidative response element (ARE). The molecular interaction between AhR and Nrf2 members, and the regulation of each target, including phase I and II gene complexes, and their mediators are poorly understood.

METHODS

Knockdown and forced expression of AhR-Nrf2 battery members were used to examine the molecular interactions between the AhR-Nrf2 axis and AhR promoter activation. Sequential immunoprecipitation, chromatin immunoprecipitation, and histology were used to identify each protein complex recruited to their respective cis-elements in the AhR promoter. Actin fiber distribution, cell spreading, and invasion were examined to identify functional differences in the AhR-Jdp2 axis between wild-type and Jdp2 knockout cells. The possible tumorigenic role of Jdp2 in the AhR-Nrf2 axis was examined in mutant Kras-Trp53-driven pancreatic tumors.

RESULTS

Crosstalk between AhR and Nrf2 was evident at the transcriptional level. The AhR promoter was activated by phase I ligands such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) through the AhR-Jdp2-Nrf2 axis in a time- and spatial transcription-dependent manner. Jdp2 was a bifunctional activator of DRE- and ARE-mediated transcription in response to TCDD. After TCDD exposure, Jdp2 activated the AhR promoter at the DRE and then moved to the ARE where it activated the promoter to increase reactive oxygen species (ROS)-mediated functions such as cell spreading and invasion in normal cells, and cancer regression in mutant Kras-Trp53-driven pancreatic tumor cells.

CONCLUSIONS

Jdp2 plays a critical role in AhR promoter activation through the AhR-Jdp2-Nrf2 axis in a spatiotemporal manner. The AhR functions to maintain ROS balance and cell spreading, invasion, and cancer regression in a mouse model of mutant Kras-Trp53 pancreatic cancer. These findings provide new insights into the roles of Jdp2 in the homeostatic regulation of oxidative stress and in the antioxidation response in detoxification, inflammation, and cancer progression.

TCDD dysregulation of lncRNA expression, liver zonation and intercellular communication across the liver lobule

The persistent environmental aryl hydrocarbon receptor agonist and hepatotoxin TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) induces hepatic lipid accumulation (steatosis), inflammation (steatohepatitis) and fibrosis. Thousands of liver-expressed, nuclear-localized lncRNAs with regulatory potential have been identified; however, their roles in TCDD-induced hepatoxicity and liver disease are unknown. We analyzed single nucleus (sn)RNA-seq data from control and subchronic (4 wk) TCDD-exposed mouse liver to determine liver cell-type specificity, zonation and differential expression profiles for thousands of lncRNAs. TCDD dysregulated >4000 of these lncRNAs in one or more liver cell types, including 684 lncRNAs specifically dysregulated in liver non-parenchymal cells. Trajectory inference analysis revealed major disruption by TCDD of hepatocyte zonation, affecting >800 genes, including 121 lncRNAs, with strong enrichment for lipid metabolism genes. TCDD also dysregulated expression of >200 transcription factors, including 19 Nuclear Receptors, most notably in hepatocytes and Kupffer cells. TCDD-induced changes in cell-cell communication patterns included marked decreases in EGF signaling from hepatocytes to non-parenchymal cells and increases in extracellular matrix-receptor interactions central to liver fibrosis. Gene regulatory networks constructed from the snRNA-seq data identified TCDD-exposed liver network-essential lncRNA regulators linked to functions such as fatty acid metabolic process, peroxisome and xenobiotic metabolism. Networks were validated by the striking enrichments that predicted regulatory lncRNAs showed for specific biological pathways. These findings highlight the power of snRNA-seq to discover functional roles for many xenobiotic-responsive lncRNAs in both hepatocytes and liver non-parenchymal cells and to elucidate novel aspects of foreign chemical-induced hepatotoxicity and liver disease, including dysregulation of intercellular communication within the liver lobule.

TCDD dysregulation of lncRNA expression, liver zonation and intercellular communication across the liver lobule

The persistent environmental aryl hydrocarbon receptor agonist and hepatotoxin TCDD (2,3,7,8-tetrachlorodibenzo- p -dioxin) induces hepatic lipid accumulation (steatosis), inflammation (steatohepatitis) and fibrosis. Thousands of liver-expressed, nuclear-localized lncRNAs with regulatory potential have been identified; however, their roles in TCDD-induced hepatoxicity and liver disease are unknown. We analyzed single nucleus (sn)RNA-seq data from control and chronic TCDD-exposed mouse liver to determine liver cell-type specificity, zonation and differential expression profiles for thousands of IncRNAs. TCDD dysregulated >4,000 of these lncRNAs in one or more liver cell types, including 684 lncRNAs specifically dysregulated in liver non-parenchymal cells. Trajectory inference analysis revealed major disruption by TCDD of hepatocyte zonation, affecting >800 genes, including 121 IncRNAs, with strong enrichment for lipid metabolism genes. TCDD also dysregulated expression of >200 transcription factors, including 19 Nuclear Receptors, most notably in hepatocytes and Kupffer cells. TCDD-induced changes in cellâ€"cell communication patterns included marked decreases in EGF signaling from hepatocytes to non-parenchymal cells and increases in extracellular matrix-receptor interactions central to liver fibrosis. Gene regulatory networks constructed from the snRNA-seq data identified TCDD-exposed liver network-essential lncRNA regulators linked to functions such as fatty acid metabolic process, peroxisome and xenobiotic metabolic. Networks were validated by the striking enrichments that predicted regulatory IncRNAs showed for specific biological pathways. These findings highlight the power of snRNA-seq to discover functional roles for many xenobiotic-responsive lncRNAs in both hepatocytes and liver non-parenchymal cells and to elucidate novel aspects of foreign chemical-induced hepatotoxicity and liver disease, including dysregulation of intercellular communication within the liver lobule.

Integrated metabolomic and transcriptomic analysis identifies benzo[a]pyrene-induced characteristic metabolic reprogramming during accumulation of lipids and reactive oxygen species in macrophages

Polycyclic aromatic hydrocarbon exposure is a major risk factor for cardiovascular diseases. Macrophage lipid accumulation is a characteristic molecular event in the pathophysiology of cardiovascular diseases. Metabolic reprogramming is an intervention target for diseases and toxic effects of environmental pollutants. However, comprehensive metabolic reprogramming related to BaP-induced macrophage lipid accumulation is currently unexplored. Therefore, metabolomics and transcriptomics were conducted to unveil relevant metabolic reprogramming in BaP-exposed macrophages, and to discover potential intervention targets. Metabolomics revealed that most amino acids, nucleotides, monosaccharides, and organic acids were significantly decreased, while most fatty acids and steroids accumulated in BaP-exposed macrophages. Transcriptomics showed that fatty acid synthesis and oxidation, and steroid synthesis and export were decreased, while import of fatty acids and steroids was increased, indicating potential roles of lipid transport in macrophage lipid accumulation following BaP exposure. Meanwhile, alanine, aspartate and glutamate metabolism, branched-chain amino acid degradation, nucleotide synthesis, monosaccharide import, pentose phosphate pathway, citrate synthesis, and glycolysis were decreased, while nucleotide degradation was increased, thus inducing decreases in most amino acids, nucleotides, monosaccharides, and organic acids in BaP-exposed macrophages. Additionally, increases in oxidative stress and the activation of antioxidant systems were observed in BaP-exposed macrophages, which was evinced by increases in reactive oxygen species, and the activation of Fenton reaction, Vdac2/3, Sod2, and Nrf2. Moreover, BaP-induced accumulation of reactive oxygen species and lipids in macrophages could be abolished by epigallocatechin-3-gallate. Quantitative PCR showed that BaP exposure activated aryl hydrocarbon receptor signaling and promoted the proinflammatory phenotype in macrophages, and these effects were inhibited or even abolished by the separate treatment with epigallocatechin-3-gallate or CH-223191, suggesting the regulatory role of aryl hydrocarbon receptor signaling in BaP-induced toxic effects. This study provides novel insights into the toxic effects of polycyclic aromatic hydrocarbons on macrophage metabolism and potential intervention targets.

Transcriptomic analysis in zebrafish larvae identifies iron-dependent mitochondrial dysfunction as a possible key event of NAFLD progression induced by benzo[a]pyrene/ethanol co-exposure

Non-alcoholic fatty liver disease (NAFLD) is a worldwide epidemic for which environmental contaminants are increasingly recognized as important etiological factors. Among them, the combination of benzo[a]pyrene (B[a]P), a potent environmental carcinogen, with ethanol, was shown to induce the transition of steatosis toward steatohepatitis. However, the underlying mechanisms involved remain to be deciphered. In this context, we used high-fat diet fed zebrafish model, in which we previously observed progression of steatosis to a steatohepatitis-like state following a 7-day-co-exposure to 43 mM ethanol and 25 nM B[a]P. Transcriptomic analysis highlighted the potent role of mitochondrial dysfunction, alterations in heme and iron homeostasis, involvement of aryl hydrocarbon receptor (AhR) signaling, and oxidative stress. Most of these mRNA dysregulations were validated by RT-qPCR. Moreover, similar changes were observed using a human in vitro hepatocyte model, HepaRG cells. The mitochondria structural and functional alterations were confirmed by transmission electronic microscopy and Seahorse technology, respectively. Involvement of AhR signaling was evidenced by using in vivo an AhR antagonist, CH223191, and in vitro in AhR-knock-out HepaRG cells. Furthermore, as co-exposure was found to increase the levels of both heme and hemin, we investigated if mitochondrial iron could induce oxidative stress. We found that mitochondrial labile iron content was raised in toxicant-exposed larvae. This increase was prevented by the iron chelator, deferoxamine, which also inhibited liver co-exposure toxicity. Overall, these results suggest that the increase in mitochondrial iron content induced by B[a]P/ethanol co-exposure causes mitochondrial dysfunction that contributes to the pathological progression of NAFLD.

Aryl hydrocarbon receptor mediates benzo[a]pyrene-induced metabolic reprogramming in human lung epithelial BEAS-2B cells

Polycyclic aromatic hydrocarbon exposure accelerates the initiation and progression of lung cancer through aryl hydrocarbon receptor (AHR) signaling. Metabolic reprogramming is a hallmark of cancer. However, how AHR reprograms metabolism related to the malignant transformation in of benzo[a]pyrene (BaP)-exposed lung cells remains unclear. After confirming that BaP exposure activated AHR signaling and relevant downstream factors and then promoted epithelial-mesenchymal transition, an untargeted metabolomics approach was employed to discover AHR-mediated metabolic reprogramming and potential therapeutic targets in BaP-exposed BEAS-2B cells. We found that 52 metabolites were significantly altered in BaP-exposed BEAS-2B cells and responsive to resveratrol (RSV) intervention. Pathway analysis revealed that 28 and 30 metabolic pathways were significantly altered in response to BaP exposure and RSV intervention, respectively. Notably, levels of most amino acids were significantly decreased, while those of most fatty acids were significantly increased in BaP-exposed BEAS-2B cells, and above changes were abolished by RSV intervention. Besides, levels of amino acids and fatty acids were highly correlated with those of many metabolites and AHR signaling upon BaP exposure and RSV intervention (the absolute values of Pearson correlation coefficients above 0.8). We further discovered a decrease in peroxisome proliferator-activated receptor (PPAR) A/G signaling and an increase in fatty acid import by the transporter FATP1 in BaP-exposed BEAS-2B cells. Furthermore, inhibition of AHR signaling by CH-223191 abolished BaP-induced repression of PPARA/G signaling and activation of FATP1 in BEAS-2B cells, demonstrating the regulatory role of AHR signaling in fatty acid accumulation via mediating PPARA/G-FATP1 signaling. These data suggested amino acid and fatty acid metabolism, AHR and PPAR-FATP1 signaling as potential therapeutic targets for intervening BaP-induced toxicity and related diseases. As far as we known, fatty acid accumulation and high correlations of AHR signaling with amino acid and fatty acid metabolism are novel phenomena discovered in BaP-exposed lung epithelial cells.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) alters hepatic polyunsaturated fatty acid metabolism and eicosanoid biosynthesis in female Sprague-Dawley rats

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent aryl hydrocarbon receptor (AhR) agonist that elicits a broad spectrum of dose-dependent hepatic effects including lipid accumulation, inflammation, and fibrosis. To determine the role of inflammatory lipid mediators in TCDD-mediated hepatotoxicity, eicosanoid metabolism was investigated. Female Sprague-Dawley (SD) rats were orally gavaged with sesame oil vehicle or 0.01-10 μg/kg TCDD every 4 days for 28 days. Hepatic RNA-Seq data was integrated with untargeted metabolomics of liver, serum, and urine, revealing dose-dependent changes in linoleic acid (LA) and arachidonic acid (AA) metabolism. TCDD also elicited dose-dependent differential gene expression associated with the cyclooxygenase, lipoxygenase, and cytochrome P450 epoxidation/hydroxylation pathways with corresponding changes in ω-6 (e.g. AA and LA) and ω-3 polyunsaturated fatty acids (PUFAs), as well as associated eicosanoid metabolites. Overall, TCDD increased the ratio of ω-6 to ω-3 PUFAs. Phospholipase A2 (Pla2g12a) was induced consistent with increased AA metabolism, while AA utilization by induced lipoxygenases Alox5 and Alox15 increased leukotrienes (LTs). More specifically, TCDD increased pro-inflammatory eicosanoids including leukotriene LTB4, and LTB3, known to recruit neutrophils to damaged tissue. Dose-response modeling suggests the cytochrome P450 hydroxylase/epoxygenase and lipoxygenase pathways are more sensitive to TCDD than the cyclooxygenase pathway. Hepatic AhR ChIP-Seq analysis found little enrichment within the regulatory regions of differentially expressed genes (DEGs) involved in eicosanoid biosynthesis, suggesting TCDD-elicited dysregulation of eicosanoid metabolism is a downstream effect of AhR activation. Overall, these results suggest alterations in eicosanoid metabolism may play a key role in TCDD-elicited hepatotoxicity associated with the progression of steatosis to steatohepatitis.

Polybrominated dibenzo-p-dioxins (PBDDs) and - dibenzofurans (PBDFs) in cod (Gadus morhua) liver-derived products from 1972 to 2017

Literature data on the occurrence and prevalence of polybrominated dibenzo-p-dioxins (PBDDs) and polybrominated dibenzofurans (PBDFs) in foods including seafood are scarce. In this study, a number of cod-derived products including medicinal grade cod liver oils sourced from Northern Atlantic waters (Iceland, Norway) and the Baltic Sea (Poland) during 1972-2001 and canned cod liver sourced from the Baltic Sea in 2017, showed detectable levels of PBDFs: such as 2,3,8-TrBDF at 0.57 to 5.249 pg g^-1 fat and 1,2,3,4,6,7,8-HpBDF at <0.018 to 0.302 pg g^-1 fat. PBDDs were not detected in the cod liver oils. Canned cod liver products showed low levels of 2,3,7,8-TeBDD in the range <0.017 to 0.022 pg g^-1 whole weight and 1,2,3,7,8-PeBDD at <0.03 to 0.039 pg g^-1 whole weight. These concentrations were computed to yield upper bound toxic equivalences (TEQs) of 0.14 to 0.17 pg g^-1 for the oils and 0.12 to 0.25 pg g^-1 for the canned products (0.08 pg g^-1 ww for both products). The resulting supplementary and dietary intakes are low (0.02 to 0.11 pg kg^-1 bm day^-1 for the oils and 0.07 to 0.17 pg kg^-1 bm week^-1 for the canned livers) in comparison to the recently expressed tolerable weekly intake of 2 pg kg^-1 bm week^-1. However, the intakes are underestimates, as due to a lack of analytical standards not all PBDD/F TEQ contributing congeners could be included. The PBDD/F TEQ contributes to the cumulative toxicity arising from other contaminants such as chlorinated dioxins and polychlorinated biphenyls.

Prediction Model of Aryl Hydrocarbon Receptor Activation by a Novel QSAR Approach, DeepSnap-Deep Learning

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that senses environmental exogenous and endogenous ligands or xenobiotic chemicals. In particular, exposure of the liver to environmental metabolism-disrupting chemicals contributes to the development and propagation of steatosis and hepatotoxicity. However, the mechanisms for AhR-induced hepatotoxicity and tumor propagation in the liver remain to be revealed, due to the wide variety of AhR ligands. Recently, quantitative structure–activity relationship (QSAR) analysis using deep neural network (DNN) has shown superior performance for the prediction of chemical compounds. Therefore, this study proposes a novel QSAR analysis using deep learning (DL), called the DeepSnap–DL method, to construct prediction models of chemical activation of AhR. Compared with conventional machine learning (ML) techniques, such as the random forest, XGBoost, LightGBM, and CatBoost, the proposed method achieves high-performance prediction of AhR activation. Thus, the DeepSnap–DL method may be considered a useful tool for achieving high-throughput in silico evaluation of AhR-induced hepatotoxicity.

Mechanisms of Environmental Contributions to Fatty Liver Disease

PURPOSE

Fatty liver disease (FLD) affects over 25% of the global population and may lead to liver-related mortality due to cirrhosis and liver cancer. FLD caused by occupational and environmental chemical exposures is termed "toxicant-associated steatohepatitis" (TASH). The current review addresses the scientific progress made in the mechanistic understanding of TASH since its initial description in 2010.

RECENT FINDINGS

Recently discovered modes of actions for volatile organic compounds and persistent organic pollutants include the following: (i) the endocrine-, metabolism-, and signaling-disrupting chemical hypotheses; (ii) chemical-nutrient interactions and the "two-hit" hypothesis. These key hypotheses were then reviewed in the context of the steatosis adverse outcome pathway (AOP) proposed by the US Environmental Protection Agency. The conceptual understanding of the contribution of environmental exposures to FLD has progressed significantly. However, because this is a new research area, more studies including mechanistic human data are required to address current knowledge gaps.

Computational prediction and in vitro validation of VEGFR1 as a novel protein target for 2,3,7,8-tetrachlorodibenzo-p-dioxin

The toxic manifestations of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), an environmental contaminant, primarily depend on its ability to activate aryl hydrocarbon receptor (AhR), which is a ligand-dependent transcription factor belonging to the superfamily of basic-helix-loop-helix DNA-binding proteins. In the present study, we aimed to identify novel protein receptor targets for TCDD using computational and in vitro validation experiments. Interestingly, results from computational methods predicted that Vascular Endothelial Growth Factor Receptor 1 (VEGFR1) could be one of the potential targets for TCDD in both mouse and humans. Results from molecular docking studies showed that human VEGFR1 (hVEGFR1) has less affinity towards TCDD compared to the mouse VEGFR1 (mVEGFR1). In vitro validation results showed that TCDD can bind and phosphorylate hVEGFR1. Further, results from molecular dynamic simulation studies showed that hVEGFR1 interaction with TCDD is stable throughout the simulation time. Overall, the present study has identified VEGFR1 as a novel target for TCDD, which provides the basis for further elucidating the role of TCDD in angiogenesis.

Effects of cross-fostering and developmental exposure to mixtures of environmental contaminants on hepatic gene expression in prepubertal 21 days old and adult male Sprague-Dawley rats

The notion that adverse health effects produced by exposure to environmental contaminants (EC) may be modulated by the presence of non-chemical stressors is gaining attention. Previously, our lab demonstrated that cross-fostering (adoption of a litter at birth) acted as a non-chemical stressor that amplified the influence of developmental exposure to EC on the glucocorticoid stress-response in adult rats. Using liver from the same rats, the aim of the current study was to investigate whether cross-fostering might also modulate EC-induced alterations in hepatic gene expression profiles. During pregnancy and nursing, Sprague-Dawley dams were fed cookies laced with corn oil (control, C) or a chemical mixture (M) composed of polychlorinated biphenyls (PCB), organochlorine pesticides (OCP), and methylmercury (MeHg), at 1 mg/kg/day. This mixture simulated the contaminant profile reported in maternal human blood. At birth, some control and M treated litters were cross-fostered to form two additional groups with different biological/nursing mothers (CC and MM). The hepatic transcriptome was analyzed by DNA microarray in male offspring at postnatal days 21 and 78-86. Mixture exposure altered the expression of detoxification and energy metabolism genes in both age groups, but with different sets of genes affected at day 21 and 78-86. Cross-fostering modulated the effects of M on gene expression pattern (MM vs M), as well as expression of energy metabolism genes between control groups (CC vs C). In conclusion, while describing short and long-term effects of developmental exposure to EC on hepatic transcriptomes, these cross-fostering results further support the consideration of non-chemical stressors in EC risk assessments.

Structure-Activity Relationships of the Main Bioactive Constituents of Euodia rutaecarpa on Aryl Hydrocarbon Receptor Activation and Associated Bile Acid Homeostasis

Rutaecarpine (RUT), evodiamine (EOD), and dehydroevodiamine (DHED) are the three main bioactive indoloquinazoline alkaloids isolated from Euodia rutaecarpa, a widely prescribed traditional Chinese medicine. Here, the structure-activity relationships of these analogs for aryl hydrocarbon receptor (AHR) activation were explored by use of Ahr-deficient (Ahr^-/-) mice, primary hepatocyte cultures, luciferase reporter gene assays, in silico ligand-docking studies, and metabolomics. In vitro, both mRNA analysis of AHR target genes in mouse primary hepatocytes and luciferase reporter assays in hepatocarcinoma cell lines demonstrated that RUT, EOD, and DHED significantly activated AHR, with an efficacy order of RUT > DHED > EOD. Ligand-docking analysis predicted that the methyl substitute at the N-14 atom was a key factor affecting AHR activation. In vivo, EOD was poorly orally absorbed and failed to activate AHR, whereas RUT and DHED markedly upregulated expression of the hepatic AHR gene battery in wild-type mice, but not in Ahr^-/- mice. Furthermore, RUT, EOD, and DHED were not hepatotoxic at the doses used; however, RUT and DHED disrupted bile acid homeostasis in an AHR-dependent manner. These findings revealed that the methyl group at the N-14 atom of these analogs and their pharmacokinetic behaviors were the main determinants for AHR activation, and suggest that attention should be given to monitoring bile acid metabolism in the clinical use of E. rutaecarpa.