2,3,7,8-Tetrachlorodibenzo-p-Dioxin Alters Lipid Metabolism and Depletes Immune Cell Populations in the Jejunum of C57BL/6 Mice

Toxicogenomics of the C-C chemokine receptor type 5 (CCR5): Exploring the potential impacts of chemical-CCR5 interactions on inflammation and human health

This article explores the impact of environmental chemicals on CCR5 expression and related inflammatory responses based on curated data from the Comparative Toxicogenomics Database (CTD). A total of 143 CCR5-interacting chemicals was found, with 229 chemical interactions. Of note, 67 (29.3%) out of 229 interactions resulted in "increased expression" of CCR5 mRNA or CCR5 protein, and 42 (18.3%) chemical interactions resulted in "decreased expression". The top-5 CCR5-interacting chemicals were "Tetrachlorodibenzodioxin", "Lipopolysaccharides", "Benzo(a)pyrene", "Drugs, Chinese Herbal", and "Ethinyl Estradiol". Based on the number of interactions and importance as environmental contaminant, we then focused our analysis on Tetrachlorodibenzodioxin and Benzo(a)pyrene. There is some consistency in the data supporting an increase in CCR5 expression triggered by Tetrachlorodibenzodioxin; although data concerning CCR5-Benzo(a)pyrene interactions is limited. Considering the high linkage disequilibrium between CCR5 and CCR2 genes, we also search for chemicals that interact with both genes, which resulted in 72 interacting chemicals, representing 50.3% of the 143 CCR5-interacting chemicals and 37.5% of the 192 CCR2-interacting chemicals. In conclusion, CTD data showed that environmental contaminants indeed affect CCR5 expression, with a tendency towards increased expression. The interaction of environmental contaminants with other chemokine receptor genes may potentialize their toxic effects on the chemokine system, favoring inflammation.

Boron Compounds Mitigate 2,3,7,8-Tetrachlorodibenzo-p-dioxin-Induced Toxicity in Human Peripheral Blood Mononuclear Cells

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) stands as one of the most potent halogenated polycyclic hydrocarbons, known to inflict substantial cytotoxic effects on both animal and human tissues. Its widespread presence and recalcitrance make it an environmental and health concern. Efforts are being intensively channeled to uncover strategies that could mitigate the adverse health outcomes associated with TCDD exposure. In the realm of counteractive agents, boron compounds are emerging as potential candidates. These compounds, which have found applications in a spectrum of industries ranging from agriculture to pharmaceutical and cosmetic manufacturing, are known to modulate several cellular processes and enzymatic pathways. However, the dose-response relationships and protective potentials of commercially prevalent boron compounds, such as boric acid (BA), ulexite (UX), and borax (BX), have not been comprehensively studied. In our detailed investigation, when peripheral blood mononuclear cells (PBMCs) were subjected to TCDD exposure, they manifested significant cellular disruptions. This was evidenced by compromised membrane integrity, a marked reduction in antioxidant defense mechanisms, and a surge in the malondialdehyde (MDA) levels, a recognized marker for oxidative stress. On the genomic front, increased 8-OH-dG levels and chromosomal aberration (CA) frequency suggested that TCDD had the potential to cause DNA damage. Notably, our experiments have revealed that boron compounds could act as protective agents against these disruptions. They exhibited a pronounced ability to diminish the cytotoxic, genotoxic, and oxidative stress outcomes instigated by TCDD. Thus, our findings shed light on the promising role of boron compounds. In specific dosages, they may not only counteract the detrimental effects of TCDD but also serve as potential chemopreventive agents, safeguarding the cellular and genomic integrity of PBMCs.

AhR-STAT3-HO-1/COX-2 signalling pathway may restrict ferroptosis and improve hMSC accumulation and efficacy in mouse liver

BACKGROUND AND PURPOSE

The low efficacy of mesenchymal stem cells (MSCs) has restricted their application in the treatment of liver disease. Emerging evidence suggested that ferroptosis may provoke hepatocyte dysfunction and exacerbate damage to the liver microenvironment. Here, we have investigated the contribution of liver ferroptosis to the elimination and effectiveness of human MSC (hMSC). Furthermore, potential links between liver ferroptosis and aryl hydrocarbon receptors (AhR) were explored.

EXPERIMENTAL APPROACH

Two mouse models, iron supplement-induced hepatic ferroptosis and hepatic ischaemia/reperfusion (I/R) injury, were used to identify effects of ferroptosis on hMSC pharmacokinetics (PK)/pharmacodynamics (PD).

KEY RESULTS

AhR inhibition attenuated hepatic ferroptosis and improved survival of hMSCs. hMSC viability was decreased by iron supplementation or serum from I/R mice. The AhR antagonist CH223191 reversed iron overload and oxidative stress induced by ferroptosis and increased hMSC concentration and efficacy in mouse models. Effects of CH223191 were greater than those of deferoxamine, a conventional ferroptosis inhibitor. Transcriptomic results suggested that the AhR-signal transducer and activator of transcription 3 (STAT3)-haem oxygenase 1/COX-2 signalling pathway is critical to this process. These results were confirmed in a mouse model of hepatic I/R injury. In mice pre-treated with CH223191, hMSC exhibited more potent protective effects, linked to decreased hepatic ferroptosis.

CONCLUSION AND IMPLICATIONS

Our findings showed that ferroptosis was a critical factor in determining the fate of hMSCs. Inhibition of AhR decreased hepatic ferroptosis, thereby increasing survival and therapeutic effects of hMSCs in mouse models of liver disease.

Hepatic cholesterol biosynthesis and dioxin-induced dysregulation: A multiscale computational approach

Humans are constantly exposed to lipophilic persistent organic pollutants (POPs) that accumulate in fatty foods. Among the numerous POPs, dioxins, in particular 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), can impact several organ systems. While the hazard is clearly recognized, it is still difficult to develop a comprehensive understanding of the overall health impacts of dioxins. As chemical toxicity testing is steadily adopting new approach methodologies (NAMs), it becomes imperative to develop computational models that can bridge the data gaps between in vitro testing and in vivo outcomes. As an effort to address this challenge, we propose a multiscale computational approach using a "template-and-anchor" (T&A) structure. A template is a high-level umbrella model that permits the integration of information from various, detailed anchor models. In the present study, we use this T&A approach to describe the effect of TCDD on cholesterol dynamics. Specifically, we represent hepatic cholesterol biosynthesis as an anchor model that is perturbed by TCDD, leading to steatosis, along with alterations of plasma cholesterol. In the future, incorporating pertinent information from all anchor models into the template model will allow the characterization of the global effects of dioxin, which can subsequently be translated into overall - and ultimately personalized - human health risk assessment.

Cystine/Glutamate Xc- Antiporter Induction Compensates for Transsulfuration Pathway Repression by 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) to Ensure Cysteine for Hepatic Glutathione Biosynthesis

Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been associated with the induction of oxidative stress and the progression of steatosis to steatohepatitis with fibrosis. It also disrupts metabolic pathways including one-carbon metabolism (OCM) and the transsulfuration pathway with possible consequences on glutathione (GSH) levels. In this study, complementary RNAseq and metabolomics data were integrated to examine the hepatic transsulfuration pathway and glutathione biosynthesis in mice following treatment with TCDD every 4 days for 28 days. TCDD dose-dependently repressed hepatic cystathionine β-synthase (CBS) and cystathionine γ-lyase (CTH) mRNA and protein levels. Reduced CBS and CTH levels are also correlated with dose-dependent decreases in hepatic extract hydrogen sulfide (H2S). In contrast, cysteine levels increased consistent with the induction of Slc7a11, which encodes for the cystine/glutamate Xc- antiporter. Cotreatment of primary hepatocytes with sulfasalazine, a cystine/glutamate Xc- antiporter inhibitor, decreased labeled cysteine incorporation into GSH with a corresponding increase in TCDD cytotoxicity. Although reduced and oxidized GSH levels were unchanged following treatment due to the induction of GSH/GSSG efflux transporter by TCDD, the GSH:GSSG ratio decreased and global protein S-glutathionylation levels in liver extracts increased in response to oxidative stress along with the induction of glutamate-cysteine ligase catalytic subunit (Gclc), glutathione synthetase (Gss), glutathione disulfide reductase (Gsr), and glutathione transferase π (Gstp). Furthermore, levels of ophthalmic acid, a biomarker of oxidative stress indicating GSH consumption, were also increased. Collectively, the data suggest that increased cystine transport due to cystine/glutamate Xc- antiporter induction compensated for decreased cysteine production following repression of the transsulfuration pathway to support GSH synthesis in response to TCDD-induced oxidative stress.

2,3,7,8-Tetrachlorodibenzo-p-dioxin induces liver lipid metabolism disorder via the ROS/AMPK/CD36 signaling pathway

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is widely considered as the most toxic and common carcinogen in the world. Exposure to TCDD causes liver lipid metabolism disorder and steatosis. However, the molecular mechanism of TCDD-induced liver lipid accumulation is not completely clear. Here, we found that a 5 μg/kg TCDD exposure for 3 weeks induced hepatocyte lipid deposition, increased CD36 expression, and promoted AMP-activated protein kinase (AMPK) ɑ phosphorylation in the liver of C57BL/6J mice. Furthermore, sulfo-N-succinimidyl oleate, a CD36 inhibiter, blunted TCDD-induced lipid deposition in Huh7 cells, confirming the critical role of CD36 in TCDD-induced hepatic steatosis. In terms of molecular mechanisms, we found that TCDD exposure increased reactive oxygen species (ROS) levels in Huh7 cells, which activated AMPK. Moreover, the activated AMPK upregulated CD36 expression. Therefore, we can see that the increase in CD36 expression induced by TCDD was regulated by ROS/AMPK/CD36 signaling pathway. Our results help to clarify the molecular mechanism of TCDD-induced hepatic steatosis.

Exposure to dechlorane 602 induces perturbation of gut immunity and microbiota in female mice

The homeostasis of gut immunity and microbiota are associated with the health of the gut. Dechlorane 602 (Dec 602) with food web magnification potential has been detected in daily food. People who were orally exposed to Dec 602 may encounter increased risk of health problems in the gut. In order to reveal the influence of short-term exposure of Dec 602 on gut immunity and microbiota, adult female C57BL/6 mice were administered orally with Dec 602 (low/high doses: 1.0/10.0 μg/kg body weight per day) for 7 days. Lymphocytes were examined by flow cytometry. Gut microbiota was measured by 16S rRNA gene sequencing. Results showed that fecal IgA was upregulated after exposure to the high dose of Dec 602, suggesting that there might be inflammation in the gut. Then, changes of immune cells in mesenteric lymph nodes and colonic lamina propria were examined. We found that exposure to the high dose of Dec 602 decreased the percentages of the anti-inflammatory T regulatory cells in mesenteric lymph nodes. In colonic lamina propria, the production of gut protective cytokine interleukin-22 by CD4⁺ T cells was decreased, and a decreased trend of interleukin-22 production was also observed in type 3 innate lymphoid cells in the high dose group. Furthermore, an altered microbiota composition toward inflammation in the gut was observed after exposure to Dec 602. Additionally, the altered microbiota correlated with changes of immune parameters, suggesting that there were interactions between influenced microbiota and immune parameters after exposure to Dec 602. Taken together, short-term exposure to Dec 602 induced gut immunity and microbiota perturbations, and this might be the mechanisms for Dec 602 to elicit inflammation in the gut.

Raising the Alarm: Environmental Factors in the Onset and Maintenance of Chronic (Low-Grade) Inflammation in the Gastrointestinal Tract

Chronic inflammatory disease of the gastrointestinal (GI) tract is defined by several pathophysiological characteristics, such as dysbiosis of the microbiota, epithelial barrier hyperpermeability, systemic dissemination of endotoxins and chronic inflammation. In addition to well-reported environmental factors in non-communicable disease, such as smoking, diet, and exercise, humans are frequently exposed to myriads more environmental factors, from pesticides to food additives. Such factors are ubiquitous across both our diet and indoor/outdoor environments. A major route of human exposure to these factors is ingestion, which frequently occurs due to their intentional addition (intentional food additives) and/or unintentional contamination (unintentional food contaminants) of food products-often linked to environmental pollution. Understanding how this persistent, diverse exposure impacts GI health is of paramount importance, as deterioration of the GI barrier is proposed to be the first step towards systemic inflammation and chronic disease. Therefore, we aim to evaluate the impact of ingestion of environmental factors on inflammatory processes in the GI tract. In this review, we highlight human exposure to intentional food additives (e.g. emulsifiers, bulking agents) and unintentional food contaminants (e.g. persistent organic pollutants, pesticides, microplastics), then present evidence for their association with chronic disease, modification of the GI microbiota, increased permeability of the GI barrier, systemic dissemination of endotoxins, local (and distal) pro-inflammatory signalling, and induction of oxidative stress and/or endoplasmic reticulum stress. We also propose a link to NLRP3-inflammasome activation. These findings highlight the contribution of common environmental factors towards deterioration of GI health and the induction of pathophysiology associated with onset and maintenance of chronic inflammation in the GI tract.

Multi-walled carbon nanotubes inhibit potential detoxification of dioxin-mediated toxicity by blocking the nuclear translocation of aryl hydrocarbon receptor

Despite numerous studies on effects of environmental accumulation of nano-pollutants, the influence of nanoparticles on the biological perturbations of coexisting pollutants in the environment remained unknown. The present study aimed at elucidating the perturbations of six environmental nanoparticles on detoxification of dioxin-induced toxicity at cellular level. We discovered that there was no remarkable difference in the cell uptake and intracellular distributions of these six nanoparticles. However, they have different effects on the detoxification of 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD). Multi-walled carbon nanotubes (MWCNTs) inhibited the translocation of aryl hydrocarbon receptor (AhR) from cytosol to the nucleus, leading to the downregulation of cytochrome P450 family 1 subfamily A member 1 (CYP1A1) and inhibition of detoxification function. These findings demonstrate that MWCNTs can impact the potential detoxification of dioxin-induced toxicity through modulating AhR signaling pathway. Co-exposures to MWCNTs and dioxin may cause even more toxicity than single exposure to dioxin or MWCNTs alone.

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.

Aryl Hydrocarbon Receptor (AhR) Activation by 2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) Dose-Dependently Shifts the Gut Microbiome Consistent with the Progression of Non-Alcoholic Fatty Liver Disease

Gut dysbiosis with disrupted enterohepatic bile acid metabolism is commonly associated with non-alcoholic fatty liver disease (NAFLD) and recapitulated in a NAFLD-phenotype elicited by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in mice. TCDD induces hepatic fat accumulation and increases levels of secondary bile acids, including taurolithocholic acid and deoxycholic acid (microbial modified bile acids involved in host bile acid regulation signaling pathways). To investigate the effects of TCDD on the gut microbiota, the cecum contents of male C57BL/6 mice orally gavaged with sesame oil vehicle or 0.3, 3, or 30 µg/kg TCDD were examined using shotgun metagenomic sequencing. Taxonomic analysis identified dose-dependent increases in Lactobacillus species (i.e., Lactobacillus reuteri). Increased species were also associated with dose-dependent increases in bile salt hydrolase sequences, responsible for deconjugation reactions in secondary bile acid metabolism. Increased L. reuteri levels were further associated with mevalonate-dependent isopentenyl diphosphate (IPP) biosynthesis and o-succinylbenzoate synthase, a menaquinone biosynthesis associated gene. Analysis of the gut microbiomes from cirrhosis patients identified an increased abundance of genes from the mevalonate-dependent IPP biosynthesis as well as several other menaquinone biosynthesis genes, including o-succinylbenzoate synthase. These results extend the association of lactobacilli with the AhR/intestinal axis in NAFLD progression and highlight the similarities between TCDD-elicited phenotypes in mice to human NAFLD.

Adaptive Membrane Fluidity Modulation: A Feedback Regulated Homeostatic System Hiding in Plain Sight

The structure of the plasma membrane affects its function. Changes in membrane fluidity with concomitant effects on membrane protein activities and cellular communication often accompany the transition from a healthy to a diseased state. Although deliberate modulation of membrane fluidity with drugs has not been exploited to date, the latest data suggest the "druggability" of the membrane. Azelaic acid esters (azelates) modulate plasma membrane fluidity and exhibit a broad range of immunomodulatory effects in vitro and in vivo. Azelates represent a new class of drugs, membrane active immunomodulators (MAIMs), which use the entire plasma membrane as the target, altering the dynamics of an innate feedback regulated homeostatic system, adaptive membrane fluidity modulation (AMFM). A review of the literature data spanning >200 years supports the notion that molecules in the MAIMs category including known drugs do exert immunomodulatory effects that have been either neglected or dismissed as off-target effects.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-induced suppression of immunity in THP-1-derived macrophages and the possible mechanisms

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a well-known immunotoxic environmental pollutant. However, most immunotoxicology studies of TCDD were based on the animal models and the inner mechanisms have just focused on a few genes/proteins. In this study, the immune functions of THP-1-derived macrophages was measured with in-vitro bioassays after 24-h exposure of TCDD including environmentally relevant concentrations. RNA-seq and Weighted Gene Co-expression Network Analysis were used to characterize the immunotoxicity molecular mechanisms. Our study is the first report on the TCDD-induced effects of cell adhesion, morphology, and multiple cytokines/chemokines production on THP-1 macrophages. After TCDD treatment, we observed an inhibited cell adherence, probably attributed to the suppressed mRNA levels of adhesion molecules ICAM-1, VCAM-1 and CD11b, and a decrease in cell pseudopodia and expression of F-actin. The inflammatory cytokines TNF-α, IL-10 and other 8 cytokines/chemokines regulating granulocytes/T cells and angiogenesis were disrupted by TCDD. Alternative splicing event was found to be a sensitive target for TCDD. Using WGCNA, we identified 10 hub genes (TNF, SRC, FGF2, PTGS2, CDH2, GNG11, BDNF, WNT5A, CXCR5 and RUNX2) highly relevant to these observed phenotypes, suggesting AhR less important in the effects TCDD have on THP-1 macrophages than in other cells. Our findings broaden the understanding of TCDD immunotoxicity on macrophages and provide new potential targets for clarifying the molecular mechanisms.

Effects of environmental contaminants in water resources on nonalcoholic fatty liver disease

The global prevalence of nonalcoholic fatty liver disease (NAFLD) has been increasing rapidly in recent years, which is now estimated to be over 25%. NAFLD is one of the most common chronic liver diseases in the world. At present, with the rapid development of economy and industrialization, many chemicals are released into the environment. These chemical contaminants in the environment might cause harm to human health and result in lipid metabolism disorder during long-term exposure. Moreover, the incentive of many NAFLD cases is unknown, and the environmental risk factors of NAFLD need to be urgently identified. Hence, we focus on the impacts of several popular environmental contaminants in water environment on the development and progression of NAFLD. These contaminants mainly include microcystins (MCs), disinfection by-products (DBPs), heavy metals (HMs), dioxins and polychlorinated biphenyls (PCBs). Through analyzing a great many epidemiological and toxicological studies, we have found positive associations between NAFLD and chronic exposure to these contaminants at the environmental levels. This review may enhance the understanding of liver damage caused by environmental pollutants, which are considered as tangible environmental risk factors for NAFLD.

Proteomic Analysis Revealed the Characteristics of Key Proteins Involved in the Regulation of Inflammatory Response, Leukocyte Transendothelial Migration, Phagocytosis, and Immune Process during Early Lung Blast Injury

Previous studies found that blast injury caused a significant increased expression of interleukin-1, IL-6, and tumor necrosis factor, a significant decrease in the expression of IL-10, an increase in Evans blue leakage, and a significant increase in inflammatory cell infiltration in the lungs. However, the molecular characteristics of lung injury at different time points after blast exposure have not yet been reported. Therefore, in this study, tandem mass spectrometry (TMT) quantitative proteomics and bioinformatics analysis were used for the first time to gain a deeper understanding of the molecular mechanism of lung blast injury at different time points. Forty-eight male C57BL/6 mice were randomly divided into six groups: control, 12 h, 24 h, 48 h, 72 h, and 1 w after low-intensity blast exposure. TMT quantitative proteomics and bioinformatics analysis were performed to analyze protein expression profiling in the lungs from control and blast-exposed mice, and differential protein expression was verified by Western blotting. The results demonstrated that blast exposure induced severe lung injury, leukocyte infiltration, and the production of inflammatory factors in mice. After analyzing the expression changes in global proteins and inflammation-related proteomes after blast exposure, the results showed that a total of 6861 global proteins and 608 differentially expressed proteins were identified, of which 215, 128, 187, 232, and 65 proteins were identified at 12 h, 24 h, 48 h, 72 h, and 1 week after blast exposure, respectively. Moreover, blast exposure-induced 177 differentially expressed proteins were associated with inflammatory responses, which were enriched in the inflammatory response regulation, leukocyte transendothelial migration, phagocytosis, and immune response. Therefore, blast exposure may induce early inflammatory response of lung tissue by regulating the expression of key proteins in the inflammatory process, suggesting that early inflammatory response may be the initiating factor of lung blast injury. These data can provide potential therapeutic candidates or approaches for the development of future treatment of lung blast injury.

AhR Activation Leads to Alterations in the Gut Microbiome with Consequent Effect on Induction of Myeloid Derived Suppressor Cells in a CXCR2-Dependent Manner

Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a potent ligand for AhR and a known carcinogen. While AhR activation by TCDD leads to significant immunosuppression, how this translates into carcinogenic signal is unclear. Recently, we demonstrated that activation of AhR by TCDD in naïve C57BL6 mice leads to massive induction of myeloid derived-suppressor cells (MDSCs). In the current study, we investigated the role of the gut microbiota in TCDD-mediated MDSC induction. TCDD caused significant alterations in the gut microbiome, such as increases in Prevotella and Lactobacillus, while decreasing Sutterella and Bacteroides. Fecal transplants from TCDD-treated donor mice into antibiotic-treated mice induced MDSCs and increased regulatory T-cells (Tregs). Injecting TCDD directly into antibiotic-treated mice also induced MDSCs, although to a lesser extent. These data suggested that TCDD-induced dysbiosis plays a critical role in MDSC induction. Interestingly, treatment with TCDD led to induction of MDSCs in the colon and undetectable levels of cysteine. MDSCs suppressed T cell proliferation while reconstitution with cysteine restored this response. Lastly, blocking CXC chemokine receptor 2 (CXCR2) impeded TCDD-mediated MDSC induction. Our data demonstrate that AhR activation by TCDD triggers dysbiosis which, in turn, regulates, at least in part, induction of MDSCs.

Metabolic Signatures of the Exposome-Quantifying the Impact of Exposure to Environmental Chemicals on Human Health

Human health and well-being are intricately linked to environmental quality. Environmental exposures can have lifelong consequences. In particular, exposures during the vulnerable fetal or early development period can affect structure, physiology and metabolism, causing potential adverse, often permanent, health effects at any point in life. External exposures, such as the “chemical exposome” (exposures to environmental chemicals), affect the host’s metabolism and immune system, which, in turn, mediate the risk of various diseases. Linking such exposures to adverse outcomes, via intermediate phenotypes such as the metabolome, is one of the central themes of exposome research. Much progress has been made in this line of research, including addressing some key challenges such as analytical coverage of the exposome and metabolome, as well as the integration of heterogeneous, multi-omics data. There is strong evidence that chemical exposures have a marked impact on the metabolome, associating with specific disease risks. Herein, we review recent progress in the field of exposome research as related to human health as well as selected metabolic and autoimmune diseases, with specific emphasis on the impacts of chemical exposures on the host metabolome.

Xanthohumol Effect on 2,3,7,8-Tetrachlorodibenzo-p-dioxin-Treated Japanese Quails in Terms of Serum Lipids, Liver Enzymes, Estradiol, and Thyroid Hormones

Dioxins are compounds classified as persistent organic pollutants, from which 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most toxic to living organisms. TCDD is considered a carcinogen and has proinflammatory influence on animals and humans, promoting free radicals' formation, and binding with the aryl hydrocarbon receptor (AhR) leads to expression of cytochrome p-450 genes that in turn predisposes to mutations. Natural flavonoids, in this case xanthohumol (XN), have been reported to attenuate TCDD toxicity through inhibition of the transformation of the AhR. Moreover, XN shows antioxidant properties. The aim of the study was to compare the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin and XN on lipid, liver enzyme, estradiol, and thyroid enzyme levels in the serum of Japanese quails. Adult, six-month-old, Japanese quails were divided into eight groups according to treatment procedures. Serum levels of total cholesterol (TCh), high-density lipoproteins (HDLs), triglycerides (TGs), estradiol, triiodothyronine, and thyroxine, and activities of alkaline phosphatase, aspartate aminotransferase, and alanine aminotransferase were measured. In comparison with the control group, 2,3,7,8-tetrachlorodibenzo-p-dioxin significantly decreased concentrations of serum HDLs and thyroid hormones and significantly increased the serum TCh level. Levels of serum TGs, liver enzymes, and estradiol were not changed after 2,3,7,8-tetrachlorodibenzo-p-dioxin treatment. Based on our data, XN treatment may also increase the levels of thyroid hormones. Moreover, the tested dioxin disrupts the liver function, especially changing lipids' metabolism. Therefore, more studies are needed for better understanding the mechanism of toxic influence of 2,3,7,8-tetrachlorodibenzo-p-dioxin on key metabolic pathways and organs in living organisms.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) dysregulates hepatic one carbon metabolism during the progression of steatosis to steatohepatitis with fibrosis in mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a persistent environmental contaminant, induces steatosis that can progress to steatohepatitis with fibrosis, pathologies that parallel stages in the development of non-alcoholic fatty liver disease (NAFLD). Coincidently, one carbon metabolism (OCM) gene expression and metabolites are often altered during NAFLD progression. In this study, the time- and dose-dependent effects of TCDD were examined on hepatic OCM in mice. Despite AhR ChIP-seq enrichment at 2 h, OCM gene expression was not changed within 72 h following a bolus dose of TCDD. Dose-dependent repression of methionine adenosyltransferase 1A (Mat1a), adenosylhomocysteinase (Achy) and betaine-homocysteine S-methyltransferase (Bhmt) mRNA and protein levels following repeated treatments were greater at 28 days compared to 8 days. Accordingly, levels of methionine, betaine, and homocysteic acid were dose-dependently increased, while S-adenosylmethionine, S-adenosylhomocysteine, and cystathionine exhibited non-monotonic dose-dependent responses consistent with regulation by OCM intermediates and repression of glycine N-methyltransferase (Gnmt). However, the dose-dependent effects on SAM-dependent metabolism of polyamines and creatine could not be directly attributed to alterations in SAM levels. Collectively, these results demonstrate persistent AhR activation disrupts hepatic OCM metabolism at the transcript, protein and metabolite levels within context of TCDD-elicited progression of steatosis to steatohepatitis with fibrosis.

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.

Gene co-regulation and co-expression in the aryl hydrocarbon receptor-mediated transcriptional regulatory network in the mouse liver

Four decades after its discovery, the aryl hydrocarbon receptor (AHR), a ligand-inducible transcription factor (TF) activated by the persistent environmental contaminant 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), remains an enigmatic molecule with a controversial endogenous role. Here, we have assembled a global map of the AHR gene regulatory network in female C57BL/6 mice orally gavaged with 30 µg/kg of TCDD from a combination of previously published gene expression and genome-wide TF-binding data sets. Using Kohonen self-organizing maps and subspace clustering, we show that genes co-regulated by common upstream TFs in the AHR network exhibit a pattern of co-expression. Directly bound, indirectly bound, and non-genomic AHR target genes exhibit distinct expression patterns, with the directly bound targets associated with highest median expression. Interestingly, among the directly bound AHR target genes, the expression level increases with the number of AHR-binding sites in the proximal promoter regions. Finally, we show that co-regulated genes in the AHR network activate distinct groups of downstream biological processes. Although the specific findings described here are restricted to hepatic effects under short-term TCDD exposure, this work describes a generalizable approach to the reconstruction and analysis of transcriptional regulatory cascades underlying cellular stress response, revealing network hierarchy and the nature of information flow from the initial signaling events to phenotypic outcomes. Such reconstructed networks can form the basis of a new generation of quantitative adverse outcome pathways.

Targeted metabolome analysis of the dog brain exposed to PCBs suggests inhibition of oxidative phosphorylation by hydroxylated PCBs

Canis lupus familiaris (domestic dog) possess a high capacity to metabolize higher-chlorinated polychlorinated biphenyls (PCBs) to thyroid hormone (TH)-like hydroxylated PCB metabolites (OH-PCBs). As a result, the brain could be at high risk of toxicity caused by OH-PCBs. To evaluate the effect of OH-PCBs on dog brain, we analyzed OH-PCB levels in the brain and the metabolome of the frontal cortex following exposure to a mixture of PCBs (CB18, 28, 70, 77, 99, 101, 118, 138, 153, 180, 187, and 202). 4-OH-CB202 and 4-OH-CB107 were major OH-PCBs in the brain of PCB-exposed dogs. These OH-PCBs were associated with metabolites involved in urea cycle, proline-related compounds, and purine, pyrimidine, glutathione, and amino-acid metabolism in dog brain. Moreover, adenosine triphosphate levels in the PCBs exposure group were significantly lower than in the control group. These results suggest that OH-PCB exposure is associated with a disruption in TH homeostasis, generation of reactive oxygen species, and/or disruption of oxidative phosphorylation (OXPHOS) in brain cells. Among them, OXPHOS disturbance could be associated with both disruptions in cellular amino-acid metabolism and urea cycle. Therefore, an OXPHOS activity assay was performed to evaluate the disruption of OXPHOS by OH-PCBs. The results indicated that 4-OH-CB107 inhibits the function of Complexes III, IV, and V of the electron transport chain, suggesting that 4-OH-CB107 inhibit these complexes in OXPHOS. The neurotoxic effects of PCB exposure may be mediated through mitochondrial toxicity of OH-PCBs in the brain.

Effects of single and combined toxic exposures on the gut microbiome: Current knowledge and future directions

Human populations are chronically exposed to mixtures of toxic chemicals. Predicting the health effects of these mixtures require a large amount of information on the mode of action of their components. Xenobiotic metabolism by bacteria inhabiting the gastrointestinal tract has a major influence on human health. Our review aims to explore the literature for studies looking to characterize the different modes of action and outcomes of major chemical pollutants, and some components of cosmetics and food additives, on gut microbial communities in order to facilitate an estimation of their potential mixture effects. We identified good evidence that exposure to heavy metals, pesticides, nanoparticles, polycyclic aromatic hydrocarbons, dioxins, furans, polychlorinated biphenyls, and non-caloric artificial sweeteners affect the gut microbiome and which is associated with the development of metabolic, malignant, inflammatory, or immune diseases. Answering the question 'Who is there?' is not sufficient to define the mode of action of a toxicant in predictive modeling of mixture effects. Therefore, we recommend that new studies focus to simulate real-life exposure to diverse chemicals (toxicants, cosmetic/food additives), including as mixtures, and which combine metagenomics, metatranscriptomics and metabolomic analytical methods achieving in that way a comprehensive evaluation of effects on human health.

2,3,7,8-Tetrachlorodibenzo-p-dioxin abolishes circadian regulation of hepatic metabolic activity in mice

Aryl hydrocarbon receptor (AhR) activation is reported to alter the hepatic expression of circadian clock regulators, however the impact on clock-controlled metabolism has not been thoroughly investigated. This study examines the effects of AhR activation on hepatic transcriptome and metabolome rhythmicity in male C57BL/6 mice orally gavaged with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) every 4 days for 28 days. TCDD diminished the rhythmicity of several core clock regulators (e.g. Arntl, Clock, Nr1d1, Per1, Cry1, Nfil3) in a dose-dependent manner, involving either a ≥ 3.3-fold suppression in amplitude or complete loss of oscillation. Accordingly, protein levels (ARNTL, REV-ERBα, NFIL3) and genomic binding (ARNTL) of select regulators were reduced and arrhythmic following treatment. As a result, the oscillating expression of 99.6% of 5,636 clock-controlled hepatic genes was abolished including genes associated with the metabolism of lipids, glucose/glycogen, and heme. For example, TCDD flattened expression of the rate-limiting enzymes in both gluconeogenesis (Pck1) and glycogenesis (Gys2), consistent with the depletion and loss of rhythmicity in hepatic glycogen levels. Examination of polar hepatic extracts by untargeted mass spectrometry revealed that virtually all oscillating metabolites lost rhythmicity following treatment. Collectively, these results suggest TCDD disrupted circadian regulation of hepatic metabolism, altering metabolic efficiency and energy storage.

Type 3 innate lymphoid cells are altered in colons of C57BL/6 mice with dioxin exposure

Type 3 innate lymphoid cells (ILC3s) are distributed in the gut and regulate inflammation by secreting cytokines, including interferon (IFN)-γ and interleukin (IL)-17. The maintenance and function of ILC3s involve the activity of aryl hydrocarbon receptor (AhR), a potent ligand of which is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), one of the most toxic dioxin congeners. Thus, TCDD exposure might affect ILC3s. To obtain in vivo evidence supporting this notion, we exposed female C57BL/6 mice orally to TCDD (low/high doses: 0.1/10 μg/kg body weight) during pregnancy and lactation periods, and after the exposure, evaluated the mothers and offspring for alterations in ILC3 differentiation and function in the colon. ILC3 frequency among colonic lamina propria lymphocytes was preferentially diminished in the offspring, and, in parallel, the median fluorescence intensity (MFI) of retinoic acid receptor-related orphan receptor (ROR)γt, which is associated with ILC3 differentiation, was also decreased in ILC3s. Conversely, the percentages of two subsets of the cells, one positive for natural cytotoxicity receptor NKp46 and the other for IL-17a, were increased in TCDD-exposed mothers and offspring. Moreover, the percentage of IFN-γ⁺ ILC3s was increased specifically in the mothers, but this was in conjunction with a significant decrease in the MFI of IFN-γ, which suggests that the IFN-γ⁺ ILC3 subset was functionally altered. In conclusion, maternal exposure to TCDD suppresses ILC3 differentiation in the offspring and influences ILC3 function in distinct manners in the mother and offspring. Our study provides new insights into the intergenerational interference of dioxins in colonic ILC3s.

Hepatic metabolite profiling of polychlorinated biphenyl (PCB)-resistant and sensitive populations of Atlantic killifish (Fundulus heteroclitus)

Atlantic killifish inhabiting polluted sites along the east coast of the U.S. have evolved resistance to toxic effects of contaminants. One such contaminated site is the Acushnet River estuary, near New Bedford Harbor (NBH), Massachusetts, which is characterized by very high PCB concentrations in the sediments and in the tissues of resident killifish. Though killifish at this site appear to be thriving, the metabolic costs of survival in a highly contaminated environment are not well understood. In this study we compared the hepatic metabolite profiles of resistant (NBH) and sensitive populations (Scorton Creek (SC), Sandwich, MA) using a targeted metabolomics approach in which polar metabolites were extracted from adult fish livers and quantified. Our results revealed differences in the levels of several metabolites between fish from the two sites. The majority of these metabolites are associated with one-carbon metabolism, an important pathway that supports multiple physiological processes including DNA and protein methylation, nucleic acid biosynthesis and amino acid metabolism. We measured the gene expression of DNA methylation (DNA methyltransferase 1, dnmt1) and demethylation genes (Ten-Eleven Translocation (TET) genes) in the two populations, and observed lower levels of dnmt1 and higher levels of TET gene expression in the NBH livers, suggesting possible differences in DNA methylation profiles. Consistent with this, the two populations differed significantly in the levels of 5-methylcytosine and 5-hydroxymethylcytosine nucleotides. Overall, our results suggest that the unique hepatic metabolite signatures observed in NBH and SC reflect the adaptive mechanisms for survival in their respective habitats.

Risk for animal and human health related to the presence of dioxins and dioxin-like PCBs in feed and food

The European Commission asked EFSA for a scientific opinion on the risks for animal and human health related to the presence of dioxins (PCDD/Fs) and DL-PCBs in feed and food. The data from experimental animal and epidemiological studies were reviewed and it was decided to base the human risk assessment on effects observed in humans and to use animal data as supportive evidence. The critical effect was on semen quality, following pre- and postnatal exposure. The critical study showed a NOAEL of 7.0 pg WHO2005-TEQ/g fat in blood sampled at age 9 years based on PCDD/F-TEQs. No association was observed when including DL-PCB-TEQs. Using toxicokinetic modelling and taking into account the exposure from breastfeeding and a twofold higher intake during childhood, it was estimated that daily exposure in adolescents and adults should be below 0.25 pg TEQ/kg bw/day. The CONTAM Panel established a TWI of 2 pg TEQ/kg bw/week. With occurrence and consumption data from European countries, the mean and P95 intake of total TEQ by Adolescents, Adults, Elderly and Very Elderly varied between, respectively, 2.1 to 10.5, and 5.3 to 30.4 pg TEQ/kg bw/week, implying a considerable exceedance of the TWI. Toddlers and Other Children showed a higher exposure than older age groups, but this was accounted for when deriving the TWI. Exposure to PCDD/F-TEQ only was on average 2.4- and 2.7-fold lower for mean and P95 exposure than for total TEQ. PCDD/Fs and DL-PCBs are transferred to milk and eggs, and accumulate in fatty tissues and liver. Transfer rates and bioconcentration factors were identified for various species. The CONTAM Panel was not able to identify reference values in most farm and companion animals with the exception of NOAELs for mink, chicken and some fish species. The estimated exposure from feed for these species does not imply a risk.

Flux, Impact, and Fate of Halogenated Xenobiotic Compounds in the Gut

Humans and their associated microbiomes are exposed to numerous xenobiotics through drugs, dietary components, personal care products as well as environmental chemicals. Most of the reciprocal interactions between the microbiota and xenobiotics, such as halogenated compounds, occur within the human gut harboring diverse and dense microbial communities. Here, we provide an overview of the flux of halogenated compounds in the environment, and diverse exposure routes of human microbiota to these compounds. Subsequently, we review the impact of halogenated compounds in perturbing the structure and function of gut microbiota and host cells. In turn, cultivation-dependent and metagenomic surveys of dehalogenating genes revealed the potential of the gut microbiota to chemically alter halogenated xenobiotics and impact their fate. Finally, we provide an outlook for future research to draw attention and attract interest to study the bidirectional impact of halogenated and other xenobiotic compounds and the gut microbiota.

2,3,7,8-Tetrachlorodibenzo-p-dioxin dose-dependently increases bone mass and decreases marrow adiposity in juvenile mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and other aryl hydrocarbon receptor (AhR) agonists have been shown to regulate bone development and remodeling in a species-, ligand-, and age-specific manner, however the underlying mechanisms remain poorly understood. In this study, we characterized the effect of 0.01-30 μg/kg TCDD on the femoral morphology of male and female juvenile mice orally gavaged every 4 days for 28 days and used RNA-Seq to investigate gene expression changes associated with the resultant phenotype. Micro-computed tomography revealed that TCDD dose-dependently increased trabecular bone volume fraction (BVF) 2.9- and 3.3-fold in male and female femurs, respectively. Decreased serum tartrate-resistant acid phosphatase (TRAP) levels, combined with a reduced osteoclast surface to bone surface ratio and repression of femoral proteases (cathepsin K, matrix metallopeptidase 13), suggests that TCDD impaired bone resorption. Increased osteoblast counts at the trabecular bone surface were consistent with a reciprocal reduction in the number of bone marrow adipocytes, suggesting AhR activation may direct mesenchymal stem cell differentiation towards osteoblasts rather than adipocytes. Notably, femoral expression of transmembrane glycoprotein NMB (Gpnmb; osteoactivin), a positive regulator of osteoblast differentiation and mineralization, was dose-dependently induced up to 18.8-fold by TCDD. Moreover, increased serum levels of 1,25-dihydroxyvitamin D3 were in accordance with the renal induction of 1α-hydroxylase Cyp27b1 and may contribute to impaired bone resorption. Collectively, the data suggest AhR activation tipped the bone remodeling balance towards bone formation, resulting in increased bone mass with reduced marrow adiposity.

Persistent Endocrine-Disrupting Chemicals and Fatty Liver Disease

PURPOSE OF REVIEW

Non-alcoholic fatty liver disease (NAFLD) is the most prominent chronic liver disease in Western countries, affecting approximately 25% of the population worldwide. Sex-specific differences in the development of NAFLD are apparent. While obesity and insulin resistance are major contributors to the increasing prevalence of NAFLD, a growing body of literature suggests that exposure to persistent endocrine-disrupting chemicals (pEDCs) may also play a role. This review summarizes recent (2011 and later) scientific literature investigating exposures to pEDCs, specifically persistent organic pollutants (POPs), and NAFLD, with a focus on sex-specific associations.

RECENT FINDINGS

The overwhelming majority of studies were conducted in single-sex animal models and provide biological evidence that exposures to 2,3,7,8-tetrachlorodibenzo-p-dioxin polychlorinated biphenyls, and other POPs or POP mixtures are negatively associated with liver health. There were four cross-sectional epidemiological studies in humans that reported associations for several POPs, including polychlorinated biphenyls and perfluorinated chemicals, with elevated liver enzymes. Only one of these studies, using a sample of gastric bypass surgery patients, examined sex-specific associations of POPs and liver enzymes, finding adverse associations among women only. The noticeable lack of studies investigating how differences (i.e., biochemical, physiological, and behavioral) between men and women may influence associations of pEDCs and NAFLD represents a large research gap in environmental health. Sexual dimorphism in metabolic processes throughout the body, including the liver, is established but often overlooked in the designs and analyses of studies. Other factors identified in this review that may also act to modulate associations of environmental chemicals and NAFLD are reproductive status and dietary nutrient intakes, which also remain understudied in the literature. Despite knowledge of sexual dimorphism in the actions of pEDCs, as well as in metabolic processes related to NAFLD development, few experimental or epidemiological studies have investigated sex-dependent associations. Future studies, especially those in humans, should be designed to address this research need. Consideration of other factors, such as reproductive status, dietary intakes, and mixtures of chemicals with varying endocrine-disrupting capabilities, should be explored.

Short-term treatment with metformin reduces hepatic lipid accumulation but induces liver inflammation in obese mice

The study aimed to evaluate the metabolic and inflammatory effects of short-term treatments (10 days) with metformin (MET) on the NAFLD caused by a high-fat diet (HFD) in C57BL/6 mice. After the treatment, histological liver slices were obtained, hepatocytes and macrophages were extracted and cultured with phosphate buffered saline, LPS (2.5 µg/mL) and MET (1 µM) for 24 h. Cytokine levels were determined by ELISA. NAFLD caused by the HFD was partially reduced by MET. The lipid accumulation induced by the HFD was not associated with liver inflammation; however, MET seemed to promote pro-inflammatory effects in liver, since it increased hepatic concentration of IL-1β, TNF-α, IL-6, MCP-1 and IFN-γ. Similarly, MET increased the concentration of IL-1β, IL-6 in hepatocyte cultures. However, in macrophages culture, MET lowered levels of IL-1β, IL-6 and TNF-α stimulated by LPS. Overall, MET reduced liver NAFLD but promoted hepatocyte increase in pro-inflammatory cytokines, thus, leading to liver inflammation.

Modulatory Influence of Segmented Filamentous Bacteria on Transcriptomic Response of Gnotobiotic Mice Exposed to TCDD

Environmental toxicants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), an aryl hydrocarbon receptor (AhR), are known to induce host toxicity and structural shifts in the gut microbiota. Key bacterial populations with similar or opposing functional responses to AhR ligand exposure may potentially help regulate expression of genes associated with immune dysfunction. To examine this question and the mechanisms for AhR ligand-induced bacterial shifts, C57BL/6 gnotobiotic mice were colonized with and without segmented filamentous bacteria (SFB) – an immune activator. Mice were also colonized with polysaccharide A producing Bacteroides fragilis – an immune suppressor to serve as a commensal background. Following colonization, mice were administered TCDD (30 μg/kg) every 4 days for 28 days by oral gavage. Quantified with the nCounter^® mouse immunology panel, opposing responses in ileal gene expression (e.g., genes associated with T-cell differentiation via the class II major histocompatibility complex) as a result of TCDD dosing and SFB colonization were observed. Genes that responded to TCDD in the presence of SFB did not show a significant response in the absence of SFB, and vice versa. Regulatory T-cells examined in the mesenteric lymph-nodes, spleen, and blood were also less impacted by TCDD in mice colonized with SFB. TCDD-induced shifts in abundance of SFB and B. fragilis compared with previous studies in mice with a traditional gut microbiome. With regard to the mouse model colonized with individual populations, results indicate that TCDD-induced host response was significantly modulated by the presence of SFB in the gut microbiome, providing insight into therapeutic potential between AhR ligands and key commensals.

TCDD administered on activated carbon eliminates bioavailability and subsequent shifts to a key murine gut commensal

Activated carbon (AC) is an increasingly attractive remediation alternative for the sequestration of dioxins at contaminated sites globally. However, the potential for AC to reduce the bioavailability of dioxins in mammals and the residing gut microbiota has received less attention. This question was partially answered in a recent study examining 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced hallmark toxic responses in mice administered with TCDD sequestered by AC or freely available in corn oil by oral gavage. Results from that study support the use of AC to significantly reduce the bioavailability of TCDD to the host. Herein, we examined the bioavailability of TCDD sequestered to AC on a key murine gut commensal and the influence of AC on the community structure of the gut microbiota. The analysis included qPCR to quantify the expression of segmented filamentous bacteria (SFB) in the mouse ileum, which has responded to TCDD-induced host toxicity in previous studies and community structure via sequencing the 16S ribosomal RNA (rRNA) gene. The expression of SFB 16S rRNA gene and functional genes significantly increased with TCDD administered with corn oil vehicle. Such a response was absent when TCDD was sequestered by AC. In addition, AC appeared to have a minimal influence on murine gut community structure and diversity, affecting only the relative abundance of Lactobacillaceae and two other groups. Results of this study further support the remedial use of AC for eliminating bioavailability of TCDD to host and subsequent influence on the gut microbiome.

Chronic exposure of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces an obesogenic effect in C57BL/6J mice fed a high fat diet

Contaminant involvement in the pathophysiology of obesity is widely recognized. It has been shown that low dose and chronic exposure to endocrine disruptor compounds (EDCs) potentiated diet- induced obesity. High and acute exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a persistent organic pollutant (POP) and an EDC with anti-estrogenic property, causes wasting syndrome . However at lower doses, the TCDD metabolic effects remain poorly understood. We investigated the obesogenic effect during chronic exposure of TCDD at 1μg/kg body weight (bw)/week in adult C57BL/6J mice fed with a high fat diet (HFD) and exposed from 10 to 42 weeks old to TCDD or equal volume of vehicle by intragastric gavage. Under these conditions, TCDD was obesogenic in adult mice (7% in males and 8% in females), which was linked to fat mass. A sex effect was observed in the fat mass distribution in adipose tissue and in the hepatic triglyceride content evolution. In visceral fat pad weight, we observed a decrease (11%) in males and an increase (14%) in females. The hepatic triglyceride content increase (41%) in females only. TCDD failed to induce any change in plasma parameters regarding glucose and lipid homeostasis. Messenger ribonucleic acid (mRNA) levels involved in adipose tissue and hepatic metabolism, inflammation, xenobiotic metabolism and endocrine disruption were differently regulated between males and females. In conclusion, these results provide new evidence that dioxin, a POP and EDC can be obesogenic for adult mice with multi-organ effects.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-elicited effects on bile acid homeostasis: Alterations in biosynthesis, enterohepatic circulation, and microbial metabolism

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a persistent environmental contaminant which elicits hepatotoxicity through activation of the aryl hydrocarbon receptor (AhR). Male C57BL/6 mice orally gavaged with TCDD (0.01–30 µg/kg) every 4 days for 28 days exhibited bile duct proliferation and pericholangitis. Mass spectrometry analysis detected a 4.6-fold increase in total hepatic bile acid levels, despite the coordinated repression of genes involved in cholesterol and primary bile acid biosynthesis including Cyp7a1. Specifically, TCDD elicited a >200-fold increase in taurolithocholic acid (TLCA), a potent G protein-coupled bile acid receptor 1 (GPBAR1) agonist associated with bile duct proliferation. Increased levels of microbial bile acid metabolism loci (bsh, baiCD) are consistent with accumulation of TLCA and other secondary bile acids. Fecal bile acids decreased 2.8-fold, suggesting enhanced intestinal reabsorption due to induction of ileal transporters (Slc10a2, Slc51a) and increases in whole gut transit time and intestinal permeability. Moreover, serum bile acids were increased 45.4-fold, consistent with blood-to-hepatocyte transporter repression (Slco1a1, Slc10a1, Slco2b1, Slco1b2, Slco1a4) and hepatocyte-to-blood transporter induction (Abcc4, Abcc3). These results suggest that systemic alterations in enterohepatic circulation, as well as host and microbiota bile acid metabolism, favor bile acid accumulation that contributes to AhR-mediated hepatotoxicity.

TCDD influences reservoir of antibiotic resistance genes in murine gut microbiome

Dysbiosis of the gut microbiome via antibiotics, changes in diet and infection can select for bacterial groups that more frequently harbor antimicrobial resistance genes (ARGs) and mobile genetic elements (MGEs). However, the impact of environmental toxicants on the reservoir of ARGs in the gut microbiome has received less attention. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent aryl hydrocarbon receptor (AhR) agonist with multiple toxic health effects including immune dysfunction. The selective pressure of TCDD on the abundance of ARG and MGE-harboring gut populations was examined using C57BL/6 mice exposed to 0-30 μg/kg TCDD for 28 and 92 days with the latter having a 30-day recovery period. DNA extracted from temporally collected fecal pellets was characterized using a qPCR array with 384 assays targeting ARGs and MGEs. Fourteen genes, typically observed in Enterobacteriaceae, increased significantly within 8 days of initial dosing, persisted throughout the treatment period, and remained induced 30 days post dosing. A qPCR primer set targeting Enterobacteriaceae also showed 10- to 100-fold higher abundance in TCDD-treated groups, which was further verified using metagenomics. Results show a bloom of ARG-harboring bacterial groups in the gut due to a xenobiotic compound that is not a metal, biocide or antimicrobial.

Lipidomic Evaluation of Aryl Hydrocarbon Receptor-Mediated Hepatic Steatosis in Male and Female Mice Elicited by 2,3,7,8-Tetrachlorodibenzo-p-dioxin

The environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces hepatic steatosis mediated by the aryl hydrocarbon receptor. To further characterize TCDD-elicited hepatic lipid accumulation, mice were gavaged with TCDD every 4 days for 28 days. Liver samples were examined using untargeted lipidomics with structural confirmation of lipid species by targeted high-resolution MS/MS, and data were integrated with complementary RNA-Seq analyses. Approximately 936 unique spectral features were detected, of which 379 were confirmed as unique lipid species. Both male and female samples exhibited similar qualitative changes (lipid species) but differed in quantitative changes. A shift to higher mass lipid species was observed, indicative of increased free fatty acid (FFA) packaging. For example, of the 13 lipid classes examined, triglycerides increased from 46 to 48% of total lipids to 68-83% in TCDD treated animals. Hepatic cholesterol esters increased 11.3-fold in male mice with moieties consisting largely of dietary fatty acids (FAs) (i.e., linolenate, palmitate, and oleate). Phosphatidylserines, phosphatidylethanolamines, phosphatidic acids, and cardiolipins decreased 4.1-, 5.0-, 5.4- and 7.4-fold, respectively, while ceramides increased 6.6-fold. Accordingly, the integration of lipidomic data with differential gene expression associated with lipid metabolism suggests that in addition to the repression of de novo fatty acid synthesis and β-oxidation, TCDD also increased hepatic uptake and packaging of lipids, while inhibiting VLDL secretion, consistent with hepatic fat accumulation and the progression to steatohepatitis with fibrosis.

Convergence of hepcidin deficiency, systemic iron overloading, heme accumulation, and REV-ERBα/β activation in aryl hydrocarbon receptor-elicited hepatotoxicity

Persistent aryl hydrocarbon receptor (AhR) agonists elicit dose-dependent hepatic lipid accumulation, oxidative stress, inflammation, and fibrosis in mice. Iron (Fe) promotes AhR-mediated oxidative stress by catalyzing reactive oxygen species (ROS) production. To further characterize the role of Fe in AhR-mediated hepatotoxicity, male C57BL/6 mice were orally gavaged with sesame oil vehicle or 0.01-30μg/kg 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) every 4days for 28days. Duodenal epithelial and hepatic RNA-Seq data were integrated with hepatic AhR ChIP-Seq, capillary electrophoresis protein measurements, and clinical chemistry analyses. TCDD dose-dependently repressed hepatic expression of hepcidin (Hamp and Hamp2), the master regulator of systemic Fe homeostasis, resulting in a 2.6-fold increase in serum Fe with accumulating Fe spilling into urine. Total hepatic Fe levels were negligibly increased while transferrin saturation remained unchanged. Furthermore, TCDD elicited dose-dependent gene expression changes in heme biosynthesis including the induction of aminolevulinic acid synthase 1 (Alas1) and repression of uroporphyrinogen decarboxylase (Urod), leading to a 50% increase in hepatic hemin and a 13.2-fold increase in total urinary porphyrins. Consistent with this heme accumulation, differential gene expression suggests that heme activated BACH1 and REV-ERBα/β, causing induction of heme oxygenase 1 (Hmox1) and repression of fatty acid biosynthesis, respectively. Collectively, these results suggest that Hamp repression, Fe accumulation, and increased heme levels converge to promote oxidative stress and the progression of TCDD-elicited hepatotoxicity.

The aryl hydrocarbon receptor as a moderator of host-microbiota communication

The aryl hydrocarbon receptor (AHR) is an important component of the host-microbiota communication network. Comparisons of wild-type and Ahr-null mice as well as from exposure studies with potent AHR ligands (e.g., 2,3,7,8-tetrachlorodibenzo-p-dioxin) have provided compelling evidence that the AHR may be a master regulator of the host-microbiota interaction thus helping to shape the immune system and impact host metabolism. Metabolomics and sequenced-based microbial community profiling, two recent technological advances, have helped to solidify this host-microbiota signaling concept and identified not only how specific ligands generated by the host and by the microbiota can activate the AHR, but also how activation/disruption of the AHR can influence and shape the microbiota. We are just beginning to understand how the temporal nature and tissue- and microbiota-specific generation of AHR ligands contribute to many AHR-dependent processes. In this review, we focus on several recent advances where metabolomics and characterization of the microbiota structure and function have generated new perspectives by which to evaluate AHR activity.

From the Cover: Coagulation-Driven Hepatic Fibrosis Requires Protease Activated Receptor-1 (PAR-1) in a Mouse Model of TCDD-Elicited Steatohepatitis

Emerging evidence supports a role for environmental chemical exposure in the pathology of non-alcoholic fatty liver disease (NAFLD), a disease process tightly linked to increased activity of the blood coagulation cascade. Exposure of C57BL/6 mice to the persistent environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) recapitulates features of the NAFLD spectrum, including steatosis, hepatic injury, inflammation, and fibrosis. We assessed coagulation cascade activation, and determined the role of the thrombin receptor protease activated receptor-1 (PAR-1) in experimental TCDD-elicited NAFLD. Chronic exposure to TCDD (30 µg/kg every 4 days for 28 days) was associated with intrahepatic coagulation, indicated by increased plasma thrombin-antithrombin levels and hepatic fibrin(ogen) deposition. PAR-1 deficiency diminished TCDD-elicited body weight loss and relative liver weight was reduced in TCDD-exposed PAR-1^-/- mice compared with TCDD-exposed wild-type mice. PAR-1 deficiency did not affect TCDD-induced hepatic steatosis or hepatocellular injury, as indicated by serum alanine aminotransferase activity. Despite a lack of effect on these 2 features of NAFLD pathology, PAR-1 deficiency was associated with a reduction in hepatic inflammation evident in liver histopathology, and reflected by a reduction in serum levels of the proinflammatory cytokine interleukin-6. Moreover, TCDD-driven hepatic collagen deposition was markedly reduced in PAR-1-deficient mice. These results indicate that experimental TCDD-elicited steatohepatitis is associated with coagulation cascade activation and PAR-1-driven hepatic inflammation and fibrosis.

Dose-Dependent Metabolic Reprogramming and Differential Gene Expression in TCDD-Elicited Hepatic Fibrosis

We have previously shown that in response to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-elicited NAFLD progression, central carbon, glutaminolysis, and serine/folate metabolism are reprogrammed to support NADPH production and ROS defenses. To further investigate underlying dose-dependent responses associated with TCDD-induced fibrosis, female C57BL/6 mice were gavaged with TCDD every 4 days (d) for 28 d or 92 d. RNA-Seq, ChIP-Seq (2 h), and 28 d metabolomic (urine, serum, and hepatic extract) analyses were conducted with complementary serum marker assessments at 92 d. Additional vehicle and 30 µg/kg treatment groups were allowed to recover for 36 d following the 92-d treatment regimen to examine recovery from TCDD-elicited fibrosis. Histopathology revealed dose-dependent increases in hepatic fat accumulation, inflammation, and periportal collagen deposition at 92 days, with increased fibrotic severity in the recovery group. Serum proinflammatory and profibrotic interleukins-1β, -2, -4, -6, and -10, as well as TNF-α and IFN-γ, exhibited dose-dependent induction. An increase in glucose tolerance was observed with a concomitant 3.0-fold decrease in hepatic glycogen linked to increased ascorbic acid biosynthesis and proline metabolism, consistent with increased fibrosis. RNA-Seq identified differential expression of numerous matrisome genes including an 8.8-fold increase in Tgfb2 indicating myofibroblast activation. Further analysis suggests reprogramming of glycogen, ascorbic acid, and amino acid metabolism in support of collagen deposition and the use of proline as a substrate for ATP production via the proline cycle. In summary, we demonstrate that glycogen, ascorbic acid, and amino acid metabolism are also reorganized to support remodeling of the extracellular matrix, progressing to hepatic fibrosis in response to chronic injury from TCDD.

Novel roles for AhR and ARNT in the regulation of alcohol dehydrogenases in human hepatic cells

The mechanisms by which pollutants participate in the development of diverse pathologies are not completely understood. The pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) activates the AhR (aryl hydrocarbon receptor) signaling pathway. We previously showed that TCDD (25 nM, 30 h) decreased the expression of several alcohol metabolism enzymes (cytochrome P450 2E1, alcohol dehydrogenases ADH1, 4 and 6) in differentiated human hepatic cells (HepaRG). Here, we show that, as rapidly as 8 h after treatment (25 nM TCDD) ADH expression decreased 40 % (p < 0.05). ADH1 and 4 protein levels decreased 40 and 27 %, respectively (p < 0.05), after 72 h (25 nM TCDD). The protein half-lives were not modified by TCDD which suggests transcriptional regulation of expression. The AhR antagonist CH-223191 or AhR siRNA reduced the inhibitory effect of 25 nM TCDD on ADH1A, 4 and 6 expression 50-100 % (p < 0.05). The genomic pathway (via the AhR/ARNT complex) and not the non-genomic pathway involving c-SRC mediated these effects. Other AhR ligands (3-methylcholanthrene and PCB 126) decreased ADH1B, 4 and 6 mRNAs by more than 78 and 55 %, respectively (p < 0.01). TCDD also regulated the expression of ADH4 in the HepG2 human hepatic cell line, in primary human hepatocytes and in C57BL/6J mouse liver. In conclusion, activation of the AhR/ARNT signaling pathway by AhR ligands represents a novel mechanism for regulating the expression of ADHs. These effects may be implicated in the toxicity of AhR ligands as well as in the alteration of ethanol or retinol metabolism and may be associated further with higher risk of liver diseases or/and alcohol abuse disorders.