2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) alters the mRNA expression of critical genes associated with cholesterol metabolism, bile acid biosynthesis, and bile transport in rat liver: a microarray study

AhR, PXR and CAR: From Xenobiotic Receptors to Metabolic Sensors

Traditionally, xenobiotic receptors are known for their role in chemical sensing and detoxification, as receptor activation regulates the expression of various key enzymes and receptors. However, recent studies have highlighted that xenobiotic receptors also play a key role in the regulation of lipid metabolism and therefore function also as metabolic sensors. Since dyslipidemia is a major risk factor for various cardiometabolic diseases, like atherosclerosis and non-alcoholic fatty liver disease, it is of major importance to understand the molecular mechanisms that are regulated by xenobiotic receptors. In this review, three major xenobiotic receptors will be discussed, being the aryl hydrocarbon receptor (AhR), pregnane X receptor (PXR) and the constitutive androstane receptor (CAR). Specifically, this review will focus on recent insights into the metabolic functions of these receptors, especially in the field of lipid metabolism and the associated dyslipidemia.

A review of food contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin and its toxicity associated with metabolic disorders

Dioxins are a group of chemicals not only regarded as highly toxic trace environmental contaminants, but also considered typical contaminants in food. Dioxins spread across the ecosystem after factory manufacture, contaminate the soil and vegetation before either directly or indirectly entering the food chain through meat products, dairy products, and aquatic products. The compound in question poses a challenge for metabolic processes within the human body, due to its intricate mechanism for inducing diseases. Therefore, it presents a significant risk and is largely undisclosed. Dioxins are mainly exposed to humans by water, food, and air, as well as inducing organ failure and metabolic disorders through but not limited to the activation of aryl hydrocarbon receptors (AhR). As a notorious compound in the family of dioxins, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exhibits long-term toxic effects on diverse organs, which induces continuous metabolic disorders. This review discussed the mechanisms of TCDD-associated metabolic syndrome. The expression of the cytochrome P450 subfamily transfers TCDD into liver, promotes its accumulation in fat tissue, and affects cholesterol metabolism. This process also alters the glucose tolerance of the human organism, disrupting glucose metabolism. It can also elicit cardiovascular pathogenesis, exacerbate liver fibrosis and neuronal death. The long-term metabolic impact of this effect is found to be sex-related. This review summarized the toxicity of TCDD on the human metabolism system and discussed the plausible correlation between TCDD and five metabolic disorders, which helped offer novel insights for future research and therapeutic interventions for these ailments.

Persistent organic pollutants promote aggressiveness in prostate cancer

Increasing evidence points towards a causal link between exposure to persistent organic pollutants (POPs) with increased incidence and aggressivity of various cancers. Among these POPs, dioxin and PCB-153 are widely found in our environment and represent a significant source of contamination. Dioxin exposure has already been linked to cancer such as non-Hodgkin's lymphoma, but remains to be more extensively investigated in other cancers. Potential implications of dioxin and PCB-153 in prostate cancer progression spurred us to challenge both ex vivo and in vivo models with low doses of these POPs. We found that dioxin or PCB-153 exposure increased hallmarks of growth and metastasis of prostate cancer cells ex vivo and in grafted NOD-SCID mice. Exposure induced histopathological carcinoma-like patterns in the Ptenpc-/- mice. We identified up-regulation of Acetyl-CoA Acetyltransferase-1 (ACAT1) involved in ketone bodies pathway as a potential target. Mechanistically, genetic inhibition confirmed that ACAT1 mediated dioxin effect on cell migration. Using public prostate cancer datasets, we confirmed the deregulation of ACAT1 and associated gene encoded ketone bodies pathway enzymes such as OXCT1, BDH1 and HMGCL in advanced prostate cancer. To further explore this link between dioxin and ACAT1 deregulation, we analyzed a unique prostate-tumour tissue collection from the USA veterans exposed to agent orange, known to be highly contaminated by dioxin because of industrial production. We found that ACAT1 histoscore is significantly increased in exposed patients. Our studies reveal the implication of dioxin and PCB-153 to induce a prometastatic programme in prostate tumours and identify ACAT1 deregulation as a key event in this process.

Sex, Nutrition, and NAFLD: Relevance of Environmental Pollution

Non-alcoholic fatty liver disease (NAFLD) is the most common form of chronic liver disease and represents an increasing public health issue given the limited treatment options and its association with several other metabolic and inflammatory disorders. The epidemic, still growing prevalence of NAFLD worldwide cannot be merely explained by changes in diet and lifestyle that occurred in the last few decades, nor from their association with genetic and epigenetic risk factors. It is conceivable that environmental pollutants, which act as endocrine and metabolic disruptors, may contribute to the spreading of this pathology due to their ability to enter the food chain and be ingested through contaminated food and water. Given the strict interplay between nutrients and the regulation of hepatic metabolism and reproductive functions in females, pollutant-induced metabolic dysfunctions may be of particular relevance for the female liver, dampening sex differences in NAFLD prevalence. Dietary intake of environmental pollutants can be particularly detrimental during gestation, when endocrine-disrupting chemicals may interfere with the programming of liver metabolism, accounting for the developmental origin of NAFLD in offspring. This review summarizes cause-effect evidence between environmental pollutants and increased incidence of NAFLD and emphasizes the need for further studies in this field.

Atypical functions of xenobiotic receptors in lipid and glucose metabolism

Xenobiotic receptors are traditionally defined as xenobiotic chemical-sensing receptors, the activation of which transcriptionally regulates the expression of enzymes and transporters involved in the metabolism and disposition of xenobiotics. Emerging evidence suggests that "xenobiotic receptors" also have diverse endobiotic functions, including their effects on lipid metabolism and energy metabolism. Dyslipidemia is a major risk factor for cardiovascular disease, diabetes, obesity, metabolic syndrome, stroke, nonalcoholic fatty liver disease (NAFLD), and nonalcoholic steatohepatitis (NASH). Understanding the molecular mechanism by which transcriptional factors, including the xenobiotic receptors, regulate lipid homeostasis will help to develop preventive and therapeutic approaches. This review describes recent advances in our understanding the atypical roles of three xenobiotic receptors: aryl hydrocarbon receptor (AhR), pregnane X receptor (PXR), and constitutive androstane receptor (CAR), in metabolic disorders, with a particular focus on their effects on lipid and glucose metabolism. Collectively, the literatures suggest the potential values of AhR, PXR and CAR as therapeutic targets for the treatment of NAFLD, NASH, obesity and diabetes, and cardiovascular diseases.

Sexual Dimorphism in Adipose-Hypothalamic Crosstalk and the Contribution of Aryl Hydrocarbon Receptor to Regulate Energy Homeostasis

There are fundamental sex differences in the regulation of energy homeostasis. Better understanding of the underlying mechanisms of energy balance that account for this asymmetry will assist in developing sex-specific therapies for sexually dimorphic diseases such as obesity. Multiple organs, including the hypothalamus and adipose tissue, play vital roles in the regulation of energy homeostasis, which are regulated differently in males and females. Various neuronal populations, particularly within the hypothalamus, such as arcuate nucleus (ARC), can sense nutrient content of the body by the help of peripheral hormones such leptin, derived from adipocytes, to regulate energy homeostasis. This review summarizes how adipose tissue crosstalk with homeostatic network control systems in the brain, which includes energy regulatory regions and the hypothalamic–pituitary axis, contribute to energy regulation in a sex-specific manner. Moreover, development of obesity is contingent upon diet and environmental factors. Substances from diet and environmental contaminants can exert insidious effects on energy metabolism, acting peripherally through the aryl hydrocarbon receptor (AhR). Developmental AhR activation can impart permanent alterations of neuronal development that can manifest a number of sex-specific physiological changes, which sometimes become evident only in adulthood. AhR is currently being investigated as a potential target for treating obesity. The consensus is that impaired function of the receptor protects from obesity in mice. AhR also modulates sex steroid receptors, and hence, one of the objectives of this review is to explain why investigating sex differences while examining this receptor is crucial. Overall, this review summarizes sex differences in the regulation of energy homeostasis imparted by the adipose–hypothalamic axis and examines how this axis can be affected by xenobiotics that signal through AhR.

Comparative Toxicotranscriptomics of Single Cell RNA-Seq and Conventional RNA-Seq in TCDD-Exposed Testicular Tissue

In this report, we compare the outcomes and limitations of two methods of transcriptomic inquiry on adult zebrafish testes exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) during sexual differentiation: conventional or bulk RNA-seq (bulk-seq) and single cell RNA sequencing (scRNA-seq) data. scRNA-seq has emerged as a valuable tool for uncovering cell type-specific transcriptome dynamics which exist in heterogeneous tissue. Our lab previously showed the toxicological value of the scRNA-seq pipeline to characterize the sequelae of TCDD exposure in testes, demonstrating that loss of spermatids and spermatozoa, but not other cell types, contributed to the pathology of infertility in adult male zebrafish exposed during sexual differentiation. To investigate the potential for technical artifacts in scRNA-seq such as cell dissociation effects and reduced transcriptome coverage, we compared bulk-sequenced and scRNA-seq-paired samples from control and TCDD-exposed samples to understand what is gained and lost in scRNA-seq vs bulk-seq, both transcriptomically and toxicologically. We hypothesized that the testes may be sensitive to tissue disruption as they contain multiple cell types under constant division and/or maturation, and that TCDD exposure may mediate the extent of sensitivity. Thus, we sought to understand the extent to which this dissociation impacts the toxicological value of data returned from scRNA-seq. We confirm that the required dissociation of individual cells from intact tissue has a significant impact on gene expression, affecting gene pathways with the potential to confound toxicogenomics studies on exposures if findings are not well-controlled and well-situated in context. Additionally, a common scRNA-seq method using cDNA amplified from the 3’ end of mRNA under-detects low-expressing transcripts including transcription factors. We confirm this, and show TCDD-related genes may be overlooked by scRNA-seq, however, this under-detection effect is not mediated by TCDD exposure. Even so, scRNA-seq generally extracted toxicologically relevant information better than the bulk-seq method in the present study. This report aims to inform future experimental design for transcriptomic investigation in the growing field of toxicogenomics by demonstrating the differential information extracted from sequencing cells—despite being from the same tissue and exposure scheme—is influenced by the specific protocol used, with implications for the interpretation of exposure-induced risk.

Environmental Chemical Contribution to the Modulation of Bile Acid Homeostasis and Farnesoid X Receptor Signaling

Maintaining bile acid (BA) homeostasis is important and regulated by BA activated receptors and signaling pathways. Farnesoid X receptor (FXR) and its regulated target networks in both the liver and the intestines are critical in suppressing BA synthesis and promoting BA transport and enterohepatic circulation. In addition, FXR is critical in regulating lipid metabolism and reducing inflammation, processes critical in the development of cholestasis and fatty liver diseases. BAs are modulated by, but also control, gut microflora. Environmental chemical exposure could affect liver disease development. However, the effects and the mechanisms by which environmental chemicals interact with FXR to affect BA homeostasis are only emerging. In this minireview, our focus is to provide evidence from reports that determine the effects of environmental or therapeutic exposure on altering homeostasis and functions of BAs and FXR. Understanding these effects will help to determine liver disease pathogenesis and provide better prevention and treatment in the future. SIGNIFICANCE STATEMENT: Environmental chemical exposure significantly contributes to the development of cholestasis and nonalcoholic steatohepatitis (NASH). The impact of exposures on bile acid (BA) signaling and Farnesoid X receptor-mediated gut-liver crosstalk is emerging. However, there is still a huge gap in understanding how these chemicals contribute to the dysregulation of BA homeostasis and how this dysregulation may promote NASH development.

Environmental Factors as Diabetic Mediators: A Mechanistic Approach

Despite substantial investment in research and treatment options, diabetes mellitus remains a pressing public health concern with potential epidemic proportions globally. There are reports that by the end of 2040, 642 million people will be suffering from diabetes. Also, according to an estimation, 1.6 million deaths were caused directly by diabetes in 2016. Diabetes is a metabolic disorder characterized by impaired glucose regulation in the body due to the destruction of pancreatic β-cells or insulin resistance. Genetic propensity, unhealthy and imbalanced diet, obesity and increasing urbanization are the common risk factors for diabetes. Besides this, it has been reported that environmental pollutants like organic pesticides, heavy metals, and air pollutants act as strong predisposing factors for diabetes owing to their highly bio-accumulative nature. These pollutants disturb glucose homeostasis either by up-regulating or down-regulating the expression of diabetic marker genes like insulin (INS) and glucokinase (GCK). Unfortunately, the molecular mechanism of the role of pollutants in causing diabetes is not very clear. This mechanistic review provides evidence of different environmental determinants, including persistent organic pollutants (POPs), air pollutants, toxic metals, etc., in inducing diabetes and proposes a framework for the possible mechanisms involved. It also illuminates the current status and future challenges, which will not only broaden our understanding but can also be a reasonable platform for further investigation.

Species-Specific Differences in Aryl Hydrocarbon Receptor Responses: How and Why?

The aryl hydrocarbon receptor (AhR) is a transcription factor that regulates a wide range of biological and toxicological effects by binding to specific ligands. AhR ligands exist in various internal and external ecological systems, such as in a wide variety of hydrophobic environmental contaminants and naturally occurring chemicals. Most of these ligands have shown differential responses among different species. Understanding the differences and their mechanisms helps in designing better experimental animal models, improves our understanding of the environmental toxicants related to AhR, and helps to screen and develop new drugs. This review systematically discusses the species differences in AhR activation effects and their modes of action. We focus on the species differences following AhR activation from two aspects: (1) the molecular configuration and activation of AhR and (2) the contrast of cis-acting elements corresponding to AhR. The variations in the responses seen in humans and other species following the activation of the AhR signaling pathway can be attributed to both factors.

Bone toxicity induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and the retinoid system: A causality analysis anchored in osteoblast gene expression and mouse data

Dioxin exposures impact on bone quality and osteoblast differentiation, as well as retinoic acid metabolism and signaling. In this study we analyzed associations between increased circulating retinol concentrations and altered bone mineral density in a mouse model following oral exposure to 2,3,7,8-tetrachlordibenzo-p-dioxin (TCDD). Additionally, effects of TCDD on differentiation marker genes and genes involved with retinoic acid metabolism were analysed in an osteoblast cell model followed by benchmark dose-response analyses of the gene expression data. Study results show that the increased trabecular and decreased cortical bone mineral density in the mouse model following TCDD exposure are associated with increased circulating retinol concentrations. Also, TCDD disrupted the expression of genes involved in osteoblast differentiation and retinoic acid synthesis, degradation, and nuclear translocation in directions compatible with increasing cellular retinoic acid levels. Further evaluation of the obtained results in relation to previously published data by the use of mode-of-action and weight-of-evidence inspired analytical approaches strengthened the evidence that TCDD-induced bone and retinoid system changes are causally related and compatible with an endocrine disruption mode of action.

Liver-specific decrease in Tff3 gene expression in infant mice perinatally exposed to 2,3,7,8-tetrabromodibenzofuran or 2,3,7,8-tetrachlorodibenzo-p-dioxin

Polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/DFs) are byproducts of brominated flame retardants and can cause adverse health effects. Although exposure to polychlorinated (PC) DD/DFs induces toxic effects, including liver injury and neurobehavioral disorder, little is known about toxicities associated with PBDD/DF exposure. Thus, we examined effects of perinatal exposure to brominated congener on the infant mouse. Gene expression in several organs, such as the liver and brain, was analyzed in mouse offspring born to dams administered 2,3,7,8-tetrabromodibenzofuran (TBDF; 9 or 45 μg/kg body weight) or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 3 μg/kg body weight) on gestational day 12.5. An increase in liver size was observed in TBDF- or TCDD-exposed offspring in infancy. Gene microarray analysis revealed that 163 and 36 genes were markedly upregulated and downregulated, respectively, in the liver of TBDF-exposed mice compared with those in vehicle-treated mice on postnatal day (PND) 5. Significant increases in Cyp1a1, Cyp1a2, Fmo3, and Pnliprp1 and decreases in Tff3, Ocstamp, Kcnk16, and Lgals2 mRNA levels in TBDF-exposed offspring on PNDs 5 and 12 were confirmed by quantitative PCR. In particular, a significant reduction in Tff3 mRNA in the liver, but not in the brain, small intestine, colon, and kidney, was observed in offspring perinatally exposed to TBDF or TCDD. Ultrasonic calls of TBDF- or TCDD-exposed offspring on PNDs 3-5 were impaired. Taken together, perinatal exposure to polyhalogenated dioxin/furan congeners disrupts gene expression patterns in the liver and ultrasonic calling during infancy. These results suggest that liver injury may contribute to neurobehavioral disorder.

Polycyclic Aromatic Hydrocarbons Alter the Hepatic Expression of Genes Involved in Sanderling (Calidris alba) Pre-migratory Fueling

Polycyclic aromatic hydrocarbons (PAHs) impaired pre-migratory fueling in 49 orally dosed Sanderling (Calidris alba). In the present study, 8 genes related to fat deposition and PAH exposure were measured in liver subsamples from these same shorebirds. At the highest dose (1260 µg total PAH [tPAH]/kg body wt/day), PAH exposure decreased liver basic fatty acid binding protein 1 (Lbfabp) and hepatic lipase (Lipc) expression. The present study reveals candidate molecular-level pathways for observed avian pre-migratory refueling impairment. Environ Toxicol Chem 2021;40:1983-1991. © 2021 SETAC.

Role of aryl hydrocarbon receptor (AHR) in overall retinoid metabolism: Response comparisons to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure between wild-type and AHR knockout mice

Young adult wild-type and aryl hydrocarbon receptor knockout (AHRKO) mice of both sexes and the C57BL/6J background were exposed to 10 weekly oral doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; total dose of 200 μg/kg bw) to further characterize the observed impacts of AHR as well as TCDD on the retinoid system. Unexposed AHRKO mice harboured heavier kidneys, lighter livers and lower serum all-trans retinoic acid (ATRA) and retinol (REOH) concentrations than wild-type mice. Results from the present study also point to a role for the murine AHR in the control of circulating REOH and ATRA concentrations. In wild-type mice, TCDD elevated liver weight and reduced thymus weight, and drastically reduced the hepatic concentrations of 9-cis-4-oxo-13,14-dihydro-retinoic acid (CORA) and retinyl palmitate (REPA). In female wild-type mice, TCDD increased the hepatic concentration of ATRA as well as the renal and circulating REOH concentrations. Renal CORA concentrations were substantially diminished in wild-type male mice exclusively following TCDD-exposure, with a similar tendency in serum. In contrast, TCDD did not affect any of these toxicity or retinoid system parameters in AHRKO mice. Finally, a distinct sex difference occurred in kidney concentrations of all the analysed retinoid forms. Together, these results strengthen the evidence of a mandatory role of AHR in TCDD-induced retinoid disruption, and suggest that the previously reported accumulation of several retinoid forms in the liver of AHRKO mice is a line-specific phenomenon. Our data further support participation of AHR in the control of liver and kidney development in mice.

Transporter Gene Regulation in Sandwich Cultured Human Hepatocytes Through the Activation of Constitutive Androstane Receptor (CAR) or Aryl Hydrocarbon Receptor (AhR)

The induction potentials of ligand-activated nuclear receptors on metabolizing enzyme genes are routinely tested for new chemical entities. However, regulations of drug transporter genes by the nuclear receptor ligands are underappreciated, especially in differentiated human hepatocyte cultures. In this study, gene induction by the ligands of constitutive androstane receptor (CAR) and aryl hydrocarbon receptor (AhR) was characterized in sandwich-cultured human hepatocytes (SCHH) from multiple donors. The cells were treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), omeprazole (OP), 6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime (CITCO) and phenobarbital (PB) for three days. RNA samples were analyzed by qRT-PCR method. As expected, CITCO, the direct activator, and PB, the indirect activator of CAR, induced CYP3A4 (31 and 40-fold), CYP2B6 (24 and 28-fold) and UGT1A1 (2.9 and 4.2-fold), respectively. Conversely, TCDD and OP, the activators of AhR, induced CYP1A1 (38 and 37-fold), and UGT1A1 (4.3 and 5.0-fold), respectively. In addition, OP but not TCDD induced CY3A4 by about 61-fold. Twenty-four hepatic drug transporter genes were characterized, and of those, SLC51B was induced the most by PB and OP by about 3.3 and 6.5 fold, respectively. Marginal inductions (about 2-fold) of SLC47A1 and SLCO4C1 genes by PB, and ABCG2 gene by TCDD were observed. In contrast, SLC10A1 gene was suppressed about 2-fold by TCDD and CITCO. While clinical relevance of SLC51B gene induction or SLC10A1 gene suppression warrants further investigation, the results verified that the assessment of transporter gene inductions are not required for new drug entities, when a drug does not remarkably induce metabolizing enzyme genes by CAR and AhR activation.

The Aryl Hydrocarbon Receptor in Energy Balance: The Road from Dioxin-Induced Wasting Syndrome to Combating Obesity with Ahr Ligands

The aryl hydrocarbon receptor (AHR) has been studied for over 40 years, yet our understanding of this ligand-activated transcription factor remains incomplete. Each year, novel findings continually force us to rethink the role of the AHR in mammalian biology. The AHR has historically been studied within the context of potent activation via AHR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), with a focus on how the AHR mediates TCDD toxicity. Research has subsequently revealed that the AHR is actively involved in distinct physiological processes ranging from the development of the liver and reproductive organs, to immune system function and wound healing. More recently, the AHR was implicated in the regulation of energy metabolism and is currently being investigated as a potential therapeutic target for obesity. In this review, we re-trace the steps through which the early toxicological studies of TCDD led to the conceptual framework for the AHR as a potential therapeutic target in metabolic disease. We additionally discuss the key discoveries that have been made concerning the role of the AHR in energy metabolism, as well as the current and future directions of the field.

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.

Renal changes and apoptosis caused by subacute exposure to Aroclor 1254 in selenium-deficient and selenium-supplemented rats

Aroclor 1254 (A1254), a mixture of polychlorinated biphenyls, exerts hepatic, renal, and reproductive toxicity in rodents. This study aimed to determine a protective role of selenium on histopathological changes, oxidative stress, and apoptosis caused by A1254 in rat kidney. It included a control group, which received regular diet containing 0.15 mg/kg Se (C), a Se-supplemented group (SeS) receiving 1 mg/kg Se, a Se-deficient group (SeD) receiving Se-deficient diet of ≤0.05 mg/kg Se, an A1254-treated group (A) receiving 10 mg/kg of Aroclor 1254 and regular diet, an A1254-treated group receiving Se-supplementation (ASeS), and an A1254-treated group receiving Se-deficient diet (ASeD). Treatments lasted 15 days. After 24 h of the last dose of A1254, the animals were decapitated under anaesthesia and their renal antioxidant enzyme activities, lipid peroxidation (LP), glutathione, protein oxidation, and total antioxidant capacity levels measured. Histopathological changes were evaluated by light and electron microscopy. Apoptosis was detected with the TUNEL assay. Kidney weights, CAT activities, and GSH levels decreased significantly in all A1254-treated groups. Renal atrophic changes and higher apoptotic cell counts were observed in the A and ASeD groups. Both groups also showed a significant drop in GPx1 activities (A – 34.92 % and ASeD – 86.46 %) and rise in LP (A – 30.45 % and ASeD – 20.44 %) vs control. In contrast, LP levels and apoptotic cell counts were significantly lower in the ASeS group vs the A group. Histopathological changes and renal apoptosis were particularly visible in the ASeD group. Our findings suggest that selenium supplementation provides partial protection against renal toxicity of Aroclor 1254.

Transgenerational epigenetic and transcriptomic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin exposure in rat

In rats, direct exposure to TCDD causes myriad toxicities. Exposed rats experience hepatotoxicity, wasting syndrome and immune suppression, amongst others. "Inherited exposure", as occurs in the F3 generation of directly exposed F0 animals, has also been shown to cause toxicity: both male and female F3 rats demonstrate an increased incidence of adult onset disease, females also display reproductive abnormalities and increased incidence of ovarian diseases while males show increased incidence of kidney disease and an altered sperm epigenome. Here, we explore the hepatic transcriptomic profile of male and female F3 Sprague-Dawley rats bred through the paternal germ line from F0 dams exposed to a single dose of TCDD (0, 30, 100, 300 or 1000 ng/kg body weight) by oral gavage. We hypothesize that RNA transcripts with altered abundance in livers of unexposed F3 progeny of treated F0 Sprague-Dawley rats may result from epigenetic modifications to the genome. We further survey patterns of differential methylation within male F3 rat testis. Female F3 rats demonstrated more TCDD-mediated hepatic transcriptomic changes than males, with differences primarily in the lowest dose group. In testis from male F3 rats, multiple olfactory receptors displayed patterns of differential methylation. Hypermethylation of Egfr and Mc5r among testes from TCDD lineage rats was observed, but without corresponding changes in hepatic mRNA abundance. Further studies examining these differences in other tissue types are warranted.

Mixture toxicity effects and uptake of titanium dioxide (TiO2) nanoparticles and 3,3',4,4'-tetrachlorobiphenyl (PCB77) in juvenile brown trout following co-exposure via the diet

Titanium dioxide nanoparticles (n-TiO₂) are among the man-made nanomaterials that are predicted to be found at high concentrations in the aquatic environment. There, they likely co-exist with other chemical pollutants. Thus, n-TiO₂ and other chemical pollutants can be taken up together or accumulate independently from each other in prey organisms of fish. This can lead to dietary exposure of fish to n-TiO₂-chemical pollutant mixtures. In this study, we examine if simultaneous dietary exposure to n-TiO₂ and 3,3',4,4'-Tetrachlorobiphenyl (PCB77) -used as a model compound for persistent organic pollutants with dioxin-like properties- can influence the uptake and toxicological response elicited by the respective other substance. Juvenile brown trout (Salmo trutta) were fed custom-made food pellets containing n-TiO_2, PCB77 or n-TiO₂+PCB77 mixtures for 15 days. Ti and PCB77 concentrations in the liver were measured by ICP-MS and GC-MS, respectively. Besides, n-TiO₂ uptake was assessed using TEM. Combination effects on endpoints specific for PCB77 (i.e., cytochrome P450 1A (CYP1A) induction) and endpoints shared by both PCB77 and n-TiO₂ (i.e., oxidative stress-related parameters) were measured in intestine and liver using RT-qPCR and enzyme activity assays. The results show that genes encoding for proteins/enzymes essential for tight junction function (zo-1) and ROS elimination (sod-1) were significantly upregulated in the intestine of fish exposed to n-TiO₂ and PCB77 mixtures, but not in the single-substance treatments. Besides, n-TiO₂ had a potentiating effect on PCB77-induced CYP1A and glutathione reductase (GR) expression/enzyme activity in the liver. This study shows that simultaneous dietary exposure to nanomaterials and traditional environmental pollutants might result in effects that are larger than observed for the substances alone, but that understanding the mechanistic basis of such effects remains challenging.

Low-dose PCB126 compromises circadian rhythms associated with disordered glucose and lipid metabolism in mice

It has been documented that 3, 3', 4, 4', 5-pentachlorobiphenyl (PCB126) elicits diverse detrimental effects on human health including metabolic syndrome and non-alcoholic fatty-liver disease (NAFLD), through a wide array of non-carcinogenic mechanisms, which require further detailed investigations. The circadian clock system consists of central clock machinery (located in the suprachiasmatic nucleus in the hypothalamus) and the peripheral clocks (located in nearly all peripheral tissues). Peripheral clocks in the liver play fundamental roles in maintaining liver homeostasis, including the regulation of energy metabolism and the expression of enzymes that fine-tune the absorption and metabolism of xenobiotics. However, the molecular basis of whether PCB126 disrupts liver homeostasis (e.g., glucose and lipid metabolism) by dysregulating the circadian clock system is still unknown. Thus, we performed a set of comprehensive analyses of glucose and lipid metabolism in the liver tissues from low-dose PCB126-treated mice. Our results demonstrated that PCB126 diminished glucose and cholesterol levels in serum and elevated glucose and cholesterol levels in the liver. Moreover, PCB126 compromised PGC1α and PDHE1α, which are the driving force for mitochondrial biogenesis and entry of pyruvate into the tricarboxylic acid (TCA) cycle, respectively, and resulted in the accumulation of glucose, glycogen and pyruvate in the liver after PCB126 exposure. Additionally, PCB126 blocked hepatic cholesterol metabolism and export pathways, leading to an elevated localization of hepatic cholesterol. Mechanistic investigations illustrated that PCB126 greatly altered the expression profile of core clock genes and their target rhythm genes involved in orchestrating glucose and cholesterol metabolism. Together, our results demonstrated that a close correlation between PCB126-disturbed glucose and lipid metabolism and disordered physiological oscillation of circadian genes.

Establishment of a human embryonic stem cell-based liver differentiation model for hepatotoxicity evaluations

The liver is one of the major targets of hormones, including thyroid hormones (THs), and many industrial chemicals, such as endocrine-disrupting chemicals. Those compounds may permeate the placenta barrier and pose a risk for embryonic development. Therefore, it is necessary to assess the toxic effects of those kind of industrial chemicals during liver development. In this study, to mimic liver specification in vitro, we differentiated human embryonic stem cells (ESCs) into functional hepatocyte-like cells. We performed this differentiation process in presence of two THs, triiodothyronine (T3) and thyroxine (T4), with the purpose of identifying biomarkers for toxicity screening. TH exposure (3, 30 and 300 nM) yielded to hepatocytes with impaired glycogen storage ability and abnormal lipid droplets' accumulation. Global gene expression analysis by RNA-seq identified a number of genes responsible for hepatic differentiation and function which were affected by 30 nM T3 and T4. Those differentially expressed genes were used to assess the potential developmental liver toxicity of two famous environmental pollutants, 2, 2, 4, 4-tetrabromodiphenyl ether (BDE-47) and decabromodiphenyl ether (BDE-209), at 10 nM to 1 μM treatments. Our findings demonstrate that BDE-47 and BDE-209, dysregulated pathways such as "chemical carcinogenesis", "steroid hormone biosynthesis" and "drug metabolism-cytochrome P450". Moreover, we were able to identify a set of 17 biomarkers, very useful to predict the potential developmental hepatotoxicity of industrial chemicals.

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.

Polycyclic aromatic hydrocarbon exposure impairs pre-migratory fuelling in captively-dosed Sanderling (Calidris alba)

Efficient fuelling is essential for migratory birds because fuel loads and fuelling rates affect individual fitness and survival during migration. Many migrant shorebirds are exposed to oil pollution and its toxic constituents, polycyclic aromatic hydrocarbons (PAHs), at migratory staging sites, which has the potential to interfere with avian refuelling physiology. In this study, we orally dosed shorebirds with environmentally-relevant PAH mixtures to simulate dietary exposure during staging. Forty-nine wild-caught Sanderling (Calidris alba) were exposed to 0 (control), 12.6 (low), 126 (medium), or 1260 (high) μg total PAH/kg body weight/day. Birds were dosed during a 21-day period of autumn pre-migratory fuelling to mimic the typical staging duration of Sanderling. We measured daily changes in mass and fat loads, as well as ethoxyresorufin-O-deethylase (EROD) activity, serum biochemical profiles, and liver mass and lipid content following dosing. All dose groups gained fat and increased in mass (size-corrected) during the study period, with females having a higher average body mass than males. However, mass gain was 3.9, 5.4, and 3.8 times lower in the low, medium, and high dose groups, respectively, relative to controls, and body mass in the medium and high dose groups significantly declined near the end of the experiment. EROD activity showed a dose-dependent increase and was significantly elevated in the high dose group relative to controls. Higher individual EROD activity was associated with reduced serum bile acid and elevated serum creatine kinase concentrations in both sexes, and with elevated serum lipase concentrations in females. These results suggest that PAH exposure in Sanderling can interfere with mechanisms of lipid transport and metabolism, can cause muscle damage, and can lead to reduced overall fat loads that are critical to staging duration, departure decisions, migratory speed, and flight range. Given that many shorebirds migrate thousands of kilometers between the breeding and wintering grounds and frequently aggregate at key staging sites that are subject to contamination, PAH exposure likely represents a significant threat to shorebird migratory success.

Aryl hydrocarbon receptor (AhR) mediated short-term effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on bile acid homeostasis in mice

The effects of the most potent aryl hydrocarbon receptor (AhR) agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on bile acid (BA) homeostasis was examined in male and female wild-type and AhR-null mice shortly after 4-day exposure, rather than at a later time when secondary non-AhR dependent effects are more likely to occur. TCDD had similar effects on BA homeostasis in male and female mice. TCDD decreased the concentration of total-(Σ) BAs in liver by approximately 50% (all major BA categories except for the non-6,12-OH BAs), without decreasing the expression of the rate limiting BA synthetic enzyme (Cyp7a1) or altering the major BA regulatory pathways (FXR) in liver and intestine. Even though the Σ-BAs in liver were markedly decreased, the Σ-BAs excreted into bile were not altered. TCDD decreased the relative amount of 12-OH BAs (TCA, TDCA, CA, DCA) in bile and increased the biliary excretion of TCDCA and its metabolites (TαMCA, TUDCA); this was likely due to the decreased Cyp8b1 (12α-hydroxylase) in liver. The concentration of Σ-BAs in serum was not altered by TCDD, indicating that serum BAs do not reflect BA status in liver. However, proportions of individual BAs in serum reflected the decreased expression of Cyp8b1. All these TCDD-induced changes in BA homeostasis were absent in AhR-null mice. In summary, through the AhR, TCDD markedly decreases BA concentrations in liver and reduces the 12α-hydroxylation of BAs without altering Cyp7a1 and FXR signaling. The TCDD-induced decrease in Σ-BAs in liver did not result in a decrease in biliary excretion or serum concentrations of Σ-BAs.

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.

Toxicological characterisation of two novel selective aryl hydrocarbon receptor modulators in Sprague-Dawley rats

The aryl hydrocarbon receptor (AHR) mediates the toxicity of dioxins, but also plays important physiological roles. Selective AHR modulators, which elicit some effects imparted by this receptor without causing the marked toxicity of dioxins, are presently under intense scrutiny. Two novel such compounds are IMA-08401 (N-acetyl-N-phenyl-4-acetoxy-5-chloro-1,2-dihydro-1-methyl-2-oxo-quinoline-3-carboxamide) and IMA-07101 (N-acetyl-N-(4-trifluoromethylphenyl)-4-acetoxy-1,2-dihydro-5-methoxy-1-methyl-2-oxo-quinoline-3-carboxamide). They represent, as diacetyl prodrugs, AHR-active metabolites of the drug compounds laquinimod and tasquinimod, respectively, which are intended for the treatment of autoimmune diseases and cancer. Here, we toxicologically assessed the novel compounds in Sprague-Dawley rats, after a single dose (8.75-92.5mg/kg) and 5-day repeated dosing at the highest doses achievable (IMA-08401: 100mg/kg/day; and IMA-07101: 75mg/kg/day). There were no overt clinical signs of toxicity, but body weight gain was marginally retarded, and the treatments induced minimal hepatic extramedullary haematopoiesis. Further, both the absolute and relative weights of the thymus were significantly decreased. Cyp1a1 gene expression was substantially increased in all tissues examined. The hepatic induction profile of other AHR battery genes was distinct from that caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The only marked alterations in serum clinical chemistry variables were a reduction in triglycerides and an increase in 3-hydroxybutyrate. Liver and kidney retinol and retinyl palmitate concentrations were affected largely in the same manner as reported for TCDD. In vitro, the novel compounds activated CYP1A1 effectively in H4IIE cells. Altogether, these novel compounds appear to act as potent activators of the AHR, but lack some major characteristic toxicities of dioxins. They therefore represent promising new selective AHR modulators.

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.

Induction of expression of aryl hydrocarbon receptor-dependent genes in human HepaRG cell line modified by shRNA and treated with β-naphthoflavone

The aryl hydrocarbon receptor (AhR) mediates a variety of biological responses to ubiquitous environmental pollutants. In this study, the effects of administration of β-naphthoflavone (BNF), a potent AhR ligand, on the expression of AhR-dependent genes were examined by microarray and qPCR analysis in both, differentiated and undifferentiated HepaRG cell lines. To prove that BNF-induced changes of investigated genes were indeed AhR-dependent, we knock down the expression of AhR by stable transfection of HepaRG cells with shRNA. Regardless of genetical identity, our results clearly demonstrate different expression profiles of AhR-dependent genes between differentiated and undifferentiated HepaRG cells. Genes involved in metabolism of xenobiotics constitute only minute fraction of all genes regulated by AhR in HepaRG cells. Participation of AhR in induction of expression of genes associated with regulation of apoptosis or involved in cell proliferation as well as AhR-dependent inhibition of genes connected to cell adhesion could support suggestion of involvement of AhR not only in initiation but also in progression of carcinogenesis. Among the AhR-dependent genes known to be involved in metabolism of xenobiotics, cytochromes P4501A1 and 1B1 belong to the most inducible by BNF. On the contrary, expression of GSTA1 and GSTA2 was significantly inhibited after BNF treatment of HepaRG cells. Among the AhR-dependent genes that are not involved in metabolism of xenobiotics SERPINB2, STC2, ARL4C, and TIPARP belong to the most inducible by BNF. Our results imply involvement of Ah receptor in regulation of CYP19A1, the gene-encoding aromatase, and an enzyme responsible for a key step in the biosynthesis of estrogens.

Differential cellular metabolite alterations in HaCaT cells caused by exposure to the aryl hydrocarbon receptor-binding polycyclic aromatic hydrocarbons chrysene, benzo[a]pyrene and dibenzo[a,l]pyrene

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the human environment. Since they are present in crude oilfractions used for the production of rubber and plastics, consumers may come into direct dermal contacts with these compounds (e.g., via tool handles) on a daily basis. Some individual PAHs are identified as genotoxic mutagens thereby prompting particular toxicological and environmental concern. Among this group, benzo[a]pyrene (BAP) constitutes a model carcinogen which is also used as reference compound for risk assessment purposes. It acts as a strong agonist of the aryl hydrocarbon receptor (AHR) and becomes metabolically activated toward mutagenic and carcinogenic intermediates by cytochrome P450-dependent monooxygenases (CYPs). While BAP has been exhaustively characterized with regard to its toxicological properties, there is much less information available for other PAHs. We treated an AHR-proficient immortal human keratinocyte cell line (i.e., HaCaT) with three selected PAHs: BAP, chrysene (CRY) and dibenzo[a,l]pyrene (DALP). Compound-mediated alterations of endogenous metabolites were investigated by an LC–MS/MS-based targeted approach. To examine AHR-dependent changes of the measured metabolites, AHR-deficient HaCaT knockdown cells (AHR-KD) were used for comparison. Our results reveal that 24 metabolites are sufficient to separate the PAH-exposed cells from untreated controls by application of a multivariate model. Alterations in the metabolomics profiles caused by each PAH show influences on the energy and lipid metabolism of the cells indicating reduced tricarboxylic acid (TCA) cycle activity and β-oxidation. Up-regulation of sphingomyelin levels after exposure to BAP and DALP point to pro-apoptotic processes caused by these two potent PAHs. Our results suggest that in vitro metabolomics can serve as tool to develop bioassays for application in hazard assessment.

Dioxin-Produced Alteration in the Profiles of Fecal and Urinary Metabolomes: A Change in Bile Acids and Its Relevance to Toxicity

This study investigated dioxin-induced changes in metabolomes in pubertal rat excrement. The administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or restricting dietary intake (pair-fed group) markedly altered the metabolomic profile including lipids, hormones, and vitamins in the urine and feces. TCDD caused an increase in the fecal chenodeoxycholic acid and taurocholic acid content and in urinary adrenaline and 17β-estradiol, while the urinary melatonin level was reduced by TCDD. These changes were not observed in the pair-fed group. In accordance with the elevated level of fecal bile acids, TCDD reduced the intestinal expression of the apical sodium-dependent bile salt transporter, which plays a role in resorbing bile acids from the bile duct. In addition, CYP7A1, a rate-limiting enzyme for bile acid biosynthesis, was attenuated by TCDD treatment, although TCDD induced hepatic CYP8B1, an enzyme essential for cholic acid synthesis. Supplying cholic acid or chenodeoxycholic acid to TCDD-exposed rats tended to restore the TCDD-produced reduction in serum triglycerides, whereas no similar trend was observed in wasting syndrome and lipid accumulation in the liver. These results suggest that: 1) TCDD alters the circulating levels of bile acids and hormones via a mechanism distinct from an attenuation in dietary intake, although the majority of TCDD-induced changes in nutrient contents in the excrement is due to a reduction in food intake; and 2) TCDD facilitates the excretion of bile acids and disrupts their biosynthesis, resulting in the disturbance of lipid homeostasis.

Time-dependent transcriptomic and biochemical responses of 6-formylindolo[3,2-b]carbazole (FICZ) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) are explained by AHR activation time

6-Formylindolo[3,2-b]carbazole (FICZ) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) are ligands of the aryl hydrocarbon receptor (AHR) and bind to the AHR with high affinity. Until recently, TCDD was considered to be the most potent AHR agonist, but several recent studies indicate that FICZ binds with greater affinity to the AHR than TCDD. To advance our understanding of the similarities and differences of the effects of FICZ and TCDD exposure in chicken embryo hepatocyte (CEH) cultures, we compared relative expression changes of 27 dioxin-responsive genes by the use of a chicken PCR array, porphyrin accumulation and ethoxyresorufin-O-deethylase (EROD) activity at different time points. In addition, an egg injection study was performed to assess the effects of FICZ on the developing chicken embryo. The results of the current study showed: (1) mean EROD-derived relative potency values for FICZ compared to TCDD changed as a function of time (i.e. 9, 0.004, 0.0008 and 0.00008 at 3, 8, 24, and 48h, respectively) in CEH cultures; (2) FICZ exposure did not result in porphyrin accumulation in CEH cultures; (3) concordance between gene expression profiles for FICZ and TCDD was time- and concentration-dependent, and (4) no mortality or morphological abnormalities were observed in chicken embryos injected with 0.87ng FICZ/g egg into the air cell. The results presented herein suggest that while FICZ and TCDD share similar molecular targets, transient versus sustained AHR activation by FICZ and TCDD result in differential transcriptomic responses. Moreover, rapid metabolism of FICZ in hepatocytes resulted in a significant decrease in the induction of EROD activity.

2,3,7,8 Tetrachlorodibenzo-p-dioxin-induced RNA abundance changes identify Ackr3, Col18a1, Cyb5a and Glud1 as candidate mediators of toxicity

2,3,7,8 Tetrachlorodibenzo-p-dioxin (TCDD) is an aromatic, long-lived environmental contaminant. While the pathogenesis of TCDD-induced toxicity is poorly understood, it has been shown that the aryl hydrocarbon receptor (AHR) is required. However, the specific transcriptomic changes that lead to toxic outcomes have not yet been identified. We previously identified a panel of 33 genes that respond to TCDD treatment in two TCDD-sensitive rodent species. To identify genes involved in the onset of hepatic toxicity, we explored 25 of these in-depth using liver from two rat strains: the TCDD-resistant Han/Wistar (H/W) and the TCDD-sensitive Long–Evans (L–E). Time course and dose–response analyses of mRNA abundance following TCDD insult indicate that eight genes are similarly regulated in livers of both strains of rat, suggesting that they are not central to the severe L–E-specific TCDD-induced toxicities. The remaining 17 genes exhibited various divergent mRNA abundances between L–E and H/W strains after TCDD treatment. Several genes displayed a biphasic response where the initial response to TCDD treatment was followed by a secondary response, usually of larger magnitude in L–E liver. This secondary response was most often an exaggeration of the original TCDD-induced response. Only cytochrome b5 type A (microsomal) (Cyb5a) had equivalent TCDD sensitivity to the prototypic AHR-responsive cytochrome P450, family 1, subfamily a, polypeptide 1 (Cyp1a1), while six genes were less sensitive. Four genes showed an early inter-strain difference that was sustained throughout most of the time course (atypical chemokine receptor 3 (Ackr3), collagen, type XVIII, alpha 1 (Col18a1), Cyb5a and glutamate dehydrogenase 1 (Glud1)), and of those genes examined in this study, are most likely to represent genes involved in the pathogenesis of TCDD-induced hepatotoxicity in L–E rats.

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.

Retene causes multifunctional transcriptomic changes in the heart of rainbow trout (Oncorhynchus mykiss) embryos

Fish are particularly sensitive to aryl hydrocarbon receptor (AhR)-mediated developmental toxicity. The molecular mechanisms behind these adverse effects have remained largely unresolved in salmonids, and for AhR-agonistic polycyclic aromatic hydrocarbons (PAHs). This study explored the cardiac transcriptome of rainbow trout (Oncorhynchus mykiss) eleuteroembryos exposed to retene, an AhR-agonistic PAH. The embryos were exposed to retene (nominal concentration 32 μg/L) and control, their hearts were collected before, at and after the onset of the visible signs of developmental toxicity, and transcriptomic changes were studied by microarray analysis. Retene up- or down-regulated 122 genes. The largest Gene Ontology groups were signal transduction, transcription, apoptosis, cell growth, cytoskeleton, cell adhesion/mobility, cardiovascular development, xenobiotic metabolism, protein metabolism, lipid metabolism and transport, and amino acid metabolism. Together these findings suggest that retene affects multiple signaling cascades in the heart of rainbow trout embryos, and potentially disturbs processes related to cardiovascular development and function.

Retinoid-xenobiotic interactions: the Ying and the Yang

The literature provides compelling evidence pointing to tight metabolic interactions between retinoids and xenobiotics. These are extensive and important for understanding xenobiotic actions in the body. Within the body, retinoids affect xenobiotic metabolism and actions and conversely, xenobiotics affect retinoid metabolism and actions. This article summarizes data that establish the importance of retinoid-dependent metabolic pathways for sustaining the body's responses to xenobiotic exposure, including the roles of all-trans- and 9-cis-retinoic acid for protecting mammals from harmful xenobiotic effects and for ensuring xenobiotic elimination from the body. This review will also consider molecular mechanisms underlying xenobiotic toxicity focusing on how this may contribute to retinoid deficiency and disruption of normal retinoid homeostasis. Special attention is paid to xenobiotic molecular targets (nuclear receptors, regulatory proteins, enzymes, and transporters) which affect retinoid metabolism and signaling.

Metabolomics Reveals that Aryl Hydrocarbon Receptor Activation by Environmental Chemicals Induces Systemic Metabolic Dysfunction in Mice

Environmental exposure to dioxins and dioxin-like compounds poses a significant health risk for human health. Developing a better understanding of the mechanisms of toxicity through activation of the aryl hydrocarbon receptor (AHR) is likely to improve the reliability of risk assessment. In this study, the AHR-dependent metabolic response of mice exposed to 2,3,7,8-tetrachlorodibenzofuran (TCDF) was assessed using global (1)H nuclear magnetic resonance (NMR)-based metabolomics and targeted metabolite profiling of extracts obtained from serum and liver. (1)H NMR analyses revealed that TCDF exposure suppressed gluconeogenesis and glycogenolysis, stimulated lipogenesis, and triggered inflammatory gene expression in an Ahr-dependent manner. Targeted analyses using gas chromatography coupled with mass spectrometry showed TCDF treatment altered the ratio of unsaturated/saturated fatty acids. Consistent with this observation, an increase in hepatic expression of stearoyl coenzyme A desaturase 1 was observed. In addition, TCDF exposure resulted in inhibition of de novo fatty acid biosynthesis manifested by down-regulation of acetyl-CoA, malonyl-CoA, and palmitoyl-CoA metabolites and related mRNA levels. In contrast, no significant changes in the levels of glucose and lipid were observed in serum and liver obtained from Ahr-null mice following TCDF treatment, thus strongly supporting the important role of the AHR in mediating the metabolic effects seen following TCDF exposure.

Two persistent organic pollutants which act through different xenosensors (alpha-endosulfan and 2,3,7,8 tetrachlorodibenzo-p-dioxin) interact in a mixture and downregulate multiple genes involved in human hepatocyte lipid and glucose metabolism

Individuals, typically, are exposed to mixtures of environmental xenobiotics affecting multiple organs and acting through different xenosensors and pathways in species and cell-type specific manners. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and α-endosulfan are Persistent Organic Pollutants (POPs) and endocrine disruptors which act through different xenosensors and accumulate in the liver. Our objective in this HEALS study was to investigate the effects of the mixture of these POPs on gene expression in a human-derived hepatocyte cell line, HepaRG. We found that, in spite of having largely uncorrelated effects, TCDD and α-endosulfan, when mixed, alter the expression of genes. The combined effects of the mixture of the POPs significantly altered the expression of 100 genes (42 up- and 58 down-regulated) whereas the same concentration of either POP alone did not alter significantly the expression of these genes. For 32 other genes, selective inhibitory crosstalk between TCDD and α-endosulfan was observed. One of the POPs inhibited the effect, on gene expression, of the other in the mixture although, when used alone, that POP did not affect expression. The expression of another 82 genes was significantly altered (up- or down-regulated) by a single POP. The addition of the second POP either increased, in the same direction, the effect on gene expression or had no further effect. At low concentrations (0.2 nM TCDD and 1 μM α-endosulfan), the POPs still had significant effects and the levels of expression of the corresponding proteins were found to be affected for some genes. Particularly striking was the 80-90% inhibition, by the mixture, of the expression of a number of genes of several hepatic intermediary metabolic pathways (glycerolipid metabolism, FXR/RXR activation, glycolysis/gluconeogenesis, retinoid and bile acid biosynthesis), whereas each pollutant alone had only a moderate effect.

The adverse outcome pathway for rodent liver tumor promotion by sustained activation of the aryl hydrocarbon receptor

An Adverse Outcome Pathway (AOP) represents the existing knowledge of a biological pathway leading from initial molecular interactions of a toxicant and progressing through a series of key events (KEs), culminating with an apical adverse outcome (AO) that has to be of regulatory relevance. An AOP based on the mode of action (MOA) of rodent liver tumor promotion by dioxin-like compounds (DLCs) has been developed and the weight of evidence (WoE) of key event relationships (KERs) evaluated using evolved Bradford Hill considerations. Dioxins and DLCs are potent aryl hydrocarbon receptor (AHR) ligands that cause a range of species-specific adverse outcomes. The occurrence of KEs is necessary for inducing downstream biological responses and KEs may occur at the molecular, cellular, tissue and organ levels. The common convention is that an AOP begins with the toxicant interaction with a biological response element; for this AOP, this initial event is binding of a DLC ligand to the AHR. Data from mechanistic studies, lifetime bioassays and approximately thirty initiation-promotion studies have established dioxin and DLCs as rat liver tumor promoters. Such studies clearly show that sustained AHR activation, weeks or months in duration, is necessary to induce rodent liver tumor promotion--hence, sustained AHR activation is deemed the molecular initiating event (MIE). After this MIE, subsequent KEs are 1) changes in cellular growth homeostasis likely associated with expression changes in a number of genes and observed as development of hepatic foci and decreases in apoptosis within foci; 2) extensive liver toxicity observed as the constellation of effects called toxic hepatopathy; 3) cellular proliferation and hyperplasia in several hepatic cell types. This progression of KEs culminates in the AO, the development of hepatocellular adenomas and carcinomas and cholangiolar carcinomas. A rich data set provides both qualitative and quantitative knowledge of the progression of this AOP through KEs and the KERs. Thus, the WoE for this AOP is judged to be strong. Species-specific effects of dioxins and DLCs are well known--humans are less responsive than rodents and rodent species differ in sensitivity between strains. Consequently, application of this AOP to evaluate potential human health risks must take these differences into account.

The Transcriptional Effects of PCB118 and PCB153 on the Liver, Adipose Tissue, Muscle and Colon of Mice: Highlighting of Glut4 and Lipin1 as Main Target Genes for PCB Induced Metabolic Disorders

Epidemiological studies have associated environmental exposure to polychlorinated biphenyls (PCBs) with an increased risk of type 2 diabetes; however, little is known about the underlying mechanisms involved in the metabolic side-effects of PCB. Our study evaluated the transcriptional effects of a subchronic exposure (gavage at Day 0 and Day 15 with 10 or 100 μmol/Kg bw) to PCB118 (dioxin-like PCB), PCB153 (non-dioxin-like PCB), or an equimolar mixture of PCB118 and PCB153 on various tissues (liver, visceral adipose tissue, muscle, and colon) in mice. Our results showed that a short-term exposure to PCB118 and/or PCB153 enhanced circulating triglyceride levels but did not affect glycemia. Among the studied tissues, we did not observe any modification of the expression of inflammation-related genes, such as cytokines or chemokines. The main transcriptional effects were observed in visceral adipose and liver tissues. We found a downregulation of lipin1 and glut4 expression in these two target organs. In adipose tissue, we also showed a downregulation of Agpat2, Slc25a1, and Fasn. All of these genes are involved in lipid metabolism and insulin resistance. In muscles, we observed an induction of CnR1 and Foxo3 expression, which may be partly involved in PCB metabolic effects. In summary, our results suggest that lipin1 and glut4, notably in adipose tissue, are the main targeted genes in PCB-induced metabolic disorders, however, further studies are required to fully elucidate the mechanisms involved.

Transcriptional and physiological response of fathead minnows (Pimephales promelas) exposed to urban waters entering into wildlife protected areas

The mission of protected areas is to conserve biodiversity and improve human welfare. To assess the effect of urban waters entering into protected areas, we performed 48-h whole-effluent exposures with fathead minnows, analyzing changes in steady state levels of mRNAs in the livers of exposed fish. Raw wastewater, treated city wastewater, and treated wastewater from a university were collected for exposures. All exposed fish showed altered mRNA levels of DNA damage-repair genes. Fish exposed to raw and treated wastewaters showed down-regulation of transcripts for key intermediates of cholesterol biosynthesis and elevated plasma cholesterol. The type of wastewater treatment influenced the response of gene transcription. Because of the relevance of some of the altered cellular pathways, we suggest that these effluents may cause deleterious effects on fish inside protected areas that receive these waters. Inclusion of research and mitigation efforts for this type of threat in protected areas management is advised.

Proposing a scientific confidence framework to help support the application of adverse outcome pathways for regulatory purposes

An adverse outcome pathway (AOP) describes the causal linkage between initial molecular events and an adverse outcome at individual or population levels. Whilst there has been considerable momentum in AOP development, far less attention has been paid to how AOPs might be practically applied for different regulatory purposes. This paper proposes a scientific confidence framework (SCF) for evaluating and applying a given AOP for different regulatory purposes ranging from prioritizing chemicals for further evaluation, to hazard prediction, and ultimately, risk assessment. The framework is illustrated using three different AOPs for several typical regulatory applications. The AOPs chosen are ones that have been recently developed and/or published, namely those for estrogenic effects, skin sensitisation, and rodent liver tumor promotion. The examples confirm how critical the data-richness of an AOP is for driving its practical application. In terms of performing risk assessment, human dosimetry methods are necessary to inform meaningful comparisons with human exposures; dosimetry is applied to effect levels based on non-testing approaches and in vitro data. Such a comparison is presented in the form of an exposure:activity ratio (EAR) to interpret biological activity in the context of exposure and to provide a basis for product stewardship and regulatory decision making.

Cross-species transcriptomic analysis elucidates constitutive aryl hydrocarbon receptor activity

Background

Research on the aryl hydrocarbon receptor (AHR) has largely focused on variations in toxic outcomes resulting from its activation by halogenated aromatic hydrocarbons. But the AHR also plays key roles in regulating pathways critical for development, and after decades of research the mechanisms underlying physiological regulation by the AHR remain poorly characterized. Previous studies identified several core genes that respond to xenobiotic AHR ligands across a broad range of species and tissues. However, only limited inferences have been made regarding its role in regulating constitutive gene activity, i.e. in the absence of exogenous ligands. To address this, we profiled transcriptomic variations between AHR-active and AHR-less-active animals in the absence of an exogenous agonist across five tissues, three of which came from rats (hypothalamus, white adipose and liver) and two of which came from mice (kidney and liver). Because AHR status alone has been shown sufficient to alter transcriptomic responses, we reason that by contrasting profiles amongst AHR-variant animals, we may elucidate effects of the AHR on constitutive mRNA abundances.

Results

We found significantly more overlap in constitutive mRNA abundances amongst tissues within the same species than from tissues between species and identified 13 genes (Agt, Car3, Creg1, Ctsc, E2f6, Enpp1, Gatm, Gstm4, Kcnj8, Me1, Pdk1, Slc35a3, and Sqrdl) that are affected by AHR-status in four of five tissues. One gene, Creg1, was significantly up-regulated in all AHR-less-active animals. We also find greater overlap between tissues at the pathway level than at the gene level, suggesting coherency to the AHR signalling response within these processes. Analysis of regulatory motifs suggests that the AHR mostly mediates transcriptional regulation via direct binding to response elements.

Conclusions

These findings, though preliminary, present a platform for further evaluating the role of the AHR in regulation of constitutive mRNA levels and physiologic function.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1053) contains supplementary material, which is available to authorized users.

The transcriptional response to oxidative stress during vertebrate development: effects of tert-butylhydroquinone and 2,3,7,8-tetrachlorodibenzo-p-dioxin

Oxidative stress is an important mechanism of chemical toxicity, contributing to teratogenesis and to cardiovascular and neurodegenerative diseases. Developing animals may be especially sensitive to chemicals causing oxidative stress. The developmental expression and inducibility of anti-oxidant defenses through activation of NF-E2-related factor 2 (NRF2) affect susceptibility to oxidants, but the embryonic response to oxidants is not well understood. To assess the response to chemically mediated oxidative stress and how it may vary during development, zebrafish embryos, eleutheroembryos, or larvae at 1, 2, 3, 4, 5, and 6 days post fertilization (dpf) were exposed to DMSO (0.1%), tert-butylhydroquinone (tBHQ; 10 µM) or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 2 nM) for 6 hr. Transcript abundance was assessed by real-time qRT-PCR and microarray. qRT-PCR showed strong (4- to 5-fold) induction of gstp1 by tBHQ as early as 1 dpf. tBHQ also induced gclc (2 dpf), but not sod1, nqo1, or cyp1a. TCDD induced cyp1a but none of the other genes. Microarray analysis showed that 1477 probes were significantly different among the DMSO-, tBHQ-, and TCDD-treated eleutheroembryos at 4 dpf. There was substantial overlap between genes induced in developing zebrafish and a set of marker genes induced by oxidative stress in mammals. Genes induced by tBHQ in 4-dpf zebrafish included those involved in glutathione synthesis and utilization, signal transduction, and DNA damage/stress response. The strong induction of hsp70 determined by microarray was confirmed by qRT-PCR and by use of transgenic zebrafish expressing enhanced green fluorescent protein (EGFP) under control of the hsp70 promoter. Genes strongly down-regulated by tBHQ included mitfa, providing a molecular explanation for the loss of pigmentation in tBHQ-exposed embryos. These data show that zebrafish embryos are responsive to oxidative stress as early as 1 dpf, that responsiveness varies with development in a gene-specific manner, and that the oxidative stress response is substantially conserved in vertebrate animals.

Pathway and time-resolved benzo[a]pyrene toxicity on Hepa1c1c7 cells at toxic and subtoxic exposure

Benzo[a]pyrene (B[a]P) is an environmental contaminant mainly studied for its toxic/carcinogenic effects. For a comprehensive and pathway orientated mechanistic understanding of the effects directly triggered by a toxic (5 μM) or a subtoxic (50 nM) concentration of B[a]P or indirectly by its metabolites, we conducted time series experiments for up to 24 h to study the effects in murine hepatocytes. These cells rapidly take up and actively metabolize B[a]P, which was followed by quantitative analysis of the concentration of intracellular B[a]P and seven representative degradation products. Exposure with 5 μM B[a]P led to a maximal intracellular concentration of 1604 pmol/5 × 10(4) cells, leveling at 55 pmol/5 × 10(4) cells by the end of the time course. Changes in the global proteome (>1000 protein profiles) and metabolome (163 metabolites) were assessed in combination with B[a]P degradation. Abundance profiles of 236 (both concentrations), 190 (only 5 μM), and 150 (only 50 nM) proteins were found to be regulated in response to B[a]P in a time-dependent manner. At the endogenous metabolite level amino acids, acylcarnitines and glycerophospholipids were particularly affected by B[a]P. The comprehensive chemical, proteome and metabolomic data enabled the identification of effects on the pathway level in a time-resolved manner. So in addition to known alterations, also protein synthesis, lipid metabolism, and membrane dysfunction were identified as B[a]P specific effects.

2,3,4,7,8-Pentachlorodibenzofuran is far less potent than 2,3,7,8-tetrachlorodibenzo-p-dioxin in disrupting the pituitary-gonad axis of the rat fetus

The effect of 2,3,4,7,8-pentachlorodibenzofuran (PnCDF) on the fetal pituitary-gonad axis was compared with that produced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in Wistar rats. Maternal treatment at gestational day (GD) 15 with PnCDF and TCDD reduced the fetal expression at GD20 of pituitary luteinizing hormone (LH) and the testicular proteins necessary for steroidogenesis. The relative potencies of PnCDF ranged from 1/42nd to 1/63rd of the TCDD effect. While PnCDF, at a dose sufficient to cause a reduction in fetal LH, provoked defects in sexual behavior at adulthood, a dose less than the ED50 failed to produce any abnormality. There was a loss of fetal body weight following in utero exposure to PnCDF, and the effect of PnCDF was also much less than that of TCDD. The disturbance in fetal growth was suggested to be due to a reduction in the level of fetal growth hormone (GH) by dioxins. The disorder caused by PnCDF/TCDD in the fetal pituitary-gonad axis occurred at doses less than those needed to cause wasting syndrome in pubertal rats. The harmful effect of PnCDF relative to TCDD was more pronounced in fetal rats than in pubertal rats. These lines of evidence suggest that: 1) PnCDF as well as TCDD imprints defects in sexual behavior by disrupting the fetal pituitary-gonad axis; 2) these dioxins hinder fetal growth by reducing the expression of fetal GH; and 3) the fetal effects of PnCDF/TCDD are more sensitive than sub-acute toxicity during puberty, and the relative effect of PnCDF varies markedly depending on the indices used.

Toxicological effect of single contaminants and contaminant mixtures associated with plant ingredients in novel salmon feeds

Increasing use of plant feed ingredients may introduce contaminants not previously associated with farming of salmonids, such as pesticides and PAHs from environmental sources or from thermal processing of oil seeds. To screen for interaction effects of contaminants newly introduced in salmon feeds, Atlantic salmon primary hepatocytes were used. The xCELLigence cytotoxicity system was used to select optimal dosages of the PAHs benzo(a)pyrene and phenanthrene, the pesticides chlorpyrifos and endosulfan, and combinations of these. NMR and MS metabolic profiling and microarray transcriptomic profiling was used to identify novel biomarkers. Lipidomic and transcriptomic profiling suggested perturbation of lipid metabolism, as well as endocrine disruption. The pesticides gave the strongest responses, despite having less effect on cell viability than the PAHs. Only weak molecular responses were detected in PAH-exposed hepatocytes. Chlorpyrifos suppressed the synthesis of unsaturated fatty acids. Endosulfan affected steroid hormone synthesis, while benzo(a)pyrene disturbed vitamin D3 metabolism. The primary mixture effect was additive, although at high concentrations the pesticides acted in a synergistic fashion to decrease cell viability and down-regulate CYP3A and FABP4 transcription. This work highlights the usefulness of 'omics techniques and multivariate data analysis to investigate interactions within mixtures of contaminants with different modes of action.