Two forms of aryl hydrocarbon receptor type 2 in rainbow trout (Oncorhynchus mykiss). Evidence for differential expression and enhancer specificity

The Effects of Kynurenic Acid in Zebrafish Embryos and Adult Rainbow Trout

Kynurenic acid (KYNA) is a metabolite of tryptophan formed on the kynurenine pathway. Its pharmacological effects are relatively well characterized in mammals, whereas its role in fish is poorly understood. Therefore, the aim of the study was to expand the knowledge of KYNA's presence inside a fish's body and its impact on fish development and function. The study was performed on zebrafish larvae and adult rainbow trout. We provide evidence that KYNA is present in the embryo, larva and mature fish and that its distribution in organs varies considerably. A study of KYNA's effect on early larval development suggests that it can accelerate larval maturation, especially under conditions that are suboptimal for fish growth. Moreover, KYNA in concentrations over 1 mM caused morphological impairment and death of larvae. However, long-lasting exposure of larvae to subtoxic concentrations of KYNA does not affect the behavior of 5-day-old larvae kept under standard optimal conditions. We also show that ingestion of KYNA-supplemented feed can lead to KYNA accumulation, particularly in the pyloric caeca of mature trout. These results shed new light on the relevance of KYNA and provide new impulse for further research on the importance of the kynurenine pathway in fish.

Comparative toxicity of conventional and unconventional oils during rainbow trout (Oncorhynchus mykiss) embryonic development: From molecular to health consequences

Canadian freshwater ecosystems are vulnerable to oil spills from pipelines, which contain mostly diluted bitumen. This study aimed to compare the toxicity of a dilbit and a conventional oil on developing rainbow trout. A total of five exposure scenarios were performed, from 10 to 43 days, using water-accommodated fraction (WAF) with an initial loading of 1:9 oil to water ratio (w/v) in a range of dilutions from 0.32 to 32% WAF, respectively, with TPAH and VOC concentrations from 2.41 to 17.5 μg/L and 7.94-660.99 μg/L, and with or without a recovery period. Following the five exposures, several endpoints were examined, including survivorship, morphometrics, gene expression, and enzymatic activity. Significant mortality rates were measured for the highest WAF concentration of the dilbit in all five exposures (60-100% mortality at 32% WAF). In comparison, the highest WAF concentration of the conventional oil induced significant mortality in three out of the five exposure (from 35 to 100% mortality at 32% WAF). Hatching delays were noted in embryos exposed to both oils. Developmental delays were observed in dilbit-exposed embryos and are suspected to be an indicator of reduced survivorship after hatching. The induced expression of cyp1a remained a reliable biomarker of exposure and of fish malformations, though it did not always predict mortality. Using CYP1A activity in combination with cyp1a may bring more insights in studies of oil risk assessment. This study demonstrates that dilbits are more toxic to early life stages compared to conventional oils and highlights the need to consider the most sensitive stage of development when performing risk assessment studies on oils.

Xenobiotic metabolism and its physiological consequences in high-Antarctic Notothenioid fishes

The Antarctic ecosystem is progressively exposed to anthropogenic contaminants, such as polycyclic aromatic hydrocarbons (PAHs). So far, it is largely unknown if PAHs leave a mark in the physiology of high-Antarctic fish. We approached this issue via two avenues: first, we examined the functional response of the aryl hydrocarbon receptor (Ahr), which is a molecular initiating event of many toxic effects of PAHs in biota. Chionodraco hamatus and Trematomus loennbergii served as representatives for high-Antarctic Notothenioids, and Atlantic cod, Gadus morhua as non-polar reference species. We sequenced and cloned the Ahr ligand binding domain (LBD) of the Notothenioids and deployed a GAL4-based luciferase reporter gene assay expressing the Ahr LBD. Benzo[a]pyrene (BaP), beta-naphthoflavone and chrysene were used as ligands for the reporter gene assay. Second, we investigated the energetic costs of Ahr activation in isolated liver cells of the Notothenioids during acute, non-cytotoxic BaP exposure. In the reporter assay, the Ahr LBD of Atlantic cod and the Antarctic Notothenioids were activated by the ligands tested herein. In the in vitro assays with isolated liver cells of high-Antarctic Notothenioids, BaP exposure had no effect on overall respiration, but caused shifts in the respiration dedicated to protein synthesis. Thus, our study demonstrated that high-Antarctic fish possess a functional Ahr that can be ligand-activated in a concentration-dependent manner by environmental contaminants. This is associated with altered cost for cellular protein synthesis. Future studies have to show if the toxicant-induced activation of the Ahr pathway may lead to altered organism performance of Antarctic fish.

Molecular characterization of the aryl hydrocarbon receptor 2 gene in black rockfish, Sebastes schlegelii, and its expression patterns upon exposure to benzo[a]pyrene, 2,3,7,8-tetrachlorodibenzo-p-dioxin, and β-naphthoflavone

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates the toxicity of halogenated and polycyclic aromatic hydrocarbons in vertebrates. Thus, increased knowledge of AhR-mediated responses to xenobiotics is imperative. Sebastes schlegelii is increasingly being used as a model for studying environmental toxicology; hence, in this study, the presence of AhR2 was evaluated in S. schlegelii. The results showed that the predicted AhR2 amino acid sequence contained regions characteristic of other vertebrate AhRs, including the basic helix-loop-helix and PER-ARNT-SIM domains in the N-terminal half, but it had minor similarity with other vertebrate AhRs across the C-terminal half; it did not contain the distinct glutamine-rich domains found in mammalian AhR2. Phylogenetic analysis demonstrated that S. schlegelii AhR2 was clustered within the teleost AhR2 branch. Additionally, AhR2 mRNA was detectable in all 11 tissues tested, with the highest mRNA levels in the heart, pyloric ceca, and liver. Furthermore, exposure to the AhR agonists showed that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, 1 μg/g body weight) induced a significantly higher increases in AhR2 expression in the gills, liver, kidneys, and spleen in 48 h than benzo[a]pyrene (2 μg/g body weight), and β-naphthoflavone (50-μg/g body weight); AhR2 mRNA levels upon TCDD exposure were up-regulated by 16- and 10-fold in the gills and liver, respectively. These findings indicated that AhR was a highly sensitive receptor against TCDD. Thus, investigating AhR2 expression in the presence of other xenobiotics might offer further information for the elucidation of its crucial role in mediating toxicant metabolism in S. schlegelii.

Immunotoxicity of Xenobiotics in Fish: A Role for the Aryl Hydrocarbon Receptor (AhR)?

The impact of anthropogenic contaminants on the immune system of fishes is an issue of growing concern. An important xenobiotic receptor that mediates effects of chemicals, such as halogenated aromatic hydrocarbons (HAHs) and polyaromatic hydrocarbons (PAHs), is the aryl hydrocarbon receptor (AhR). Fish toxicological research has focused on the role of this receptor in xenobiotic biotransformation as well as in causing developmental, cardiac, and reproductive toxicity. However, biomedical research has unraveled an important physiological role of the AhR in the immune system, what suggests that this receptor could be involved in immunotoxic effects of environmental contaminants. The aims of the present review are to critically discuss the available knowledge on (i) the expression and possible function of the AhR in the immune systems of teleost fishes; and (ii) the impact of AhR-activating xenobiotics on the immune systems of fish at the levels of immune gene expression, immune cell proliferation and immune cell function, immune pathology, and resistance to infectious disease. The existing information indicates that the AhR is expressed in the fish immune system, but currently, we have little understanding of its physiological role. Exposure to AhR-activating contaminants results in the modulation of numerous immune structural and functional parameters of fish. Despite the diversity of fish species studied and the experimental conditions investigated, the published findings rather uniformly point to immunosuppressive actions of xenobiotic AhR ligands in fish. These effects are often associated with increased disease susceptibility. The fact that fish populations from HAH- and PAH-contaminated environments suffer immune disturbances and elevated disease susceptibility highlights that the immunotoxic effects of AhR-activating xenobiotics bear environmental relevance.

Aryl Hydrocarbon Receptor Signaling Is Functional in Immune Cells of Rainbow Trout (Oncorhynchus mykiss)

The arylhydrocarbon receptor (AhR) is an important signaling pathway in the immune system of mammals. In addition to its physiological functions, the receptor mediates the immunotoxic actions of a diverse range of environmental contaminants that bind to and activate the AhR, including planar halogenated aromatic hydrocarbons (PHAHs or dioxin-like compounds) and polynuclear aromatic hydrocarbons (PAHs). AhR-binding xenobiotics are immunotoxic not only to mammals but to teleost fish as well. To date, however, it is unknown if the AhR pathway is active in the immune system of fish and thus may act as molecular initiating event in the immunotoxicity of AhR-binding xenobiotics to fish. The present study aims to examine the presence of functional AhR signaling in immune cells of rainbow trout (Oncorhynchus mykiss). Focus is given to the toxicologically relevant AhR2 clade. By means of RT-qPCR and in situ hybdridization, we show that immune cells of rainbow trout express ahr 2α and ahr 2β mRNA; this applies for immune cells isolated from the head kidney and from the peripheral blood. Furthermore, we show that in vivo as well as in vitro exposure to the AhR ligand, benzo(a)pyrene (BaP), causes upregulation of the AhR-regulated gene, cytochrome p4501a, in rainbow trout immune cells, and that this induction is inhibited by co-treatment with an AhR antagonist. Taken together, these findings provide evidence that functional AhR signaling exists in the immune cells of the teleost species, rainbow trout.

Aryl hydrocarbon receptor nuclear translocators (ARNT1, ARNT2, and ARNT3) of white sturgeon (Acipenser transmontanus): Sequences, tissue-specific expressions, and response to β-naphthoflavone

Sturgeons (Acipenseridae) are ancient fishes that have tissue-specific profiles of transcriptional responses to dioxin-like compounds (DLCs) that are unique from those generally measured in teleost fishes. Because DLCs exert their critical toxicities through activation of the aryl hydrocarbon receptor (AHR), this transcription factor has been the subject of intensive study. However, less attention has focused on the aryl hydrocarbon receptor nuclear translocator (ARNT), which is the dimerization partner of the AHR and required for AHR-mediated transcription. The present study sequenced ARNT1, ARNT2, and ARNT3 in a representative species of sturgeon, the white sturgeon (Acipenser transmontanus), and quantified tissue-specific basal transcript abundance for each ARNT and the response following exposure to the model agonist of the AHR, β-naphthoflavone. In common with other proteins in sturgeons, the amino acid sequences of ARNTs are more similar to those of tetrapods than are ARNTs of other fishes. Transcripts of ARNT1, ARNT2, and ARNT3 were detected in all tissues investigated. Expression of ARNTs are tightly regulated in vertebrates, but β-naphthoflavone caused down-regulation in liver and up-regulation in gill, while an upward trend was measured in intestine. ARNTs are dimeric partners for multiple proteins, including the hypoxia inducible factor 1α (HIF1α), which mediates response to hypoxia. A downward trend in abundance of HIF1α transcript was measured in liver of white sturgeon exposed to β-naphthoflavone. Altered expression of ARNTs and HIF1α caused by activation of the AHR might affect the ability of certain tissues in sturgeons to respond to hypoxia when co-exposed to DLCs or other agonists.

Molecular and Functional Properties of the Atlantic Cod (Gadus morhua) Aryl Hydrocarbon Receptors Ahr1a and Ahr2a

The aryl hydrocarbon receptor (Ahr) is a ligand-activated transcription factor that mediates the toxicity of halogenated and polycyclic aromatic hydrocarbons in vertebrates. Atlantic cod (Gadus morhua) has recently emerged as a model organism in environmental toxicology studies, and increased knowledge of Ahr-mediated responses to xenobiotics is imperative. Genome mining and phylogenetic analyses revealed two Ahr-encoding genes in the Atlantic cod genome, gmahr1a and gmahr2a. In vitro binding assays showed that both gmAhr proteins bind to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), but stronger binding to gmAhr1a was observed. Transactivation studies with a reporter gene assay revealed that gmAhr1a is one order of magnitude more sensitive to TCDD than gmAhr2a, but the maximal responses of the receptors were similar. Other well-known Ahr agonists, such as β-naphthoflavone (BNF), 3,3',4,4',5-pentachlorobiphenyl (PCB126), and 6-formylindolo[3,2-b]carbazole (FICZ), also activated the gmAhr proteins, but gmAhr1a was, in general, the more sensitive receptor and produced the highest efficacies. The induction of cyp1a in exposed precision-cut cod liver slices confirmed the activation of the Ahr signaling pathway ex vivo. In conclusion, the differences in transcriptional activation by gmAhr's with various agonists, the distinct binding properties with TCDD and BNF, and the distinct tissue-specific expression profiles indicate different functional specializations of the Atlantic cod Ahr's.

Characterization of the Aryl Hydrocarbon Receptor (AhR) Pathway in Anabas testudineus and Mechanistic Exploration of the Reduced Sensitivity of AhR2a

Field investigations have revealed the ability of the climbing perch Anabas testudineus to survive in highly contaminated water bodies. The aryl hydrocarbon receptor (AhR) pathway is vital in mediating the toxicity of aromatic hydrocarbon contaminants, and genotypic variation in the AhR can confer resistance to these contaminants. Thus, we characterized the AhR pathway in A. testudineus in order to understand the mechanism(s) underlying the resistance of this species to contaminants and to broaden current knowledge on teleost AhR. In A. testudineus, four AhRs, two AhR nuclear translocators (ARNTs), and one AhR repressor (AhRR) were found. Transient transfection assays revealed that AhR1a, AhR1b, and AhR2b were functional, whereas AhR2a was poorly activated by the potent agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Two ARNTs (partner of AhR) and one AhRR (repressor of AhR) all were functional with each of the active AhR. As a major form, the insensitivity of AhR2a might serve as a potential mechanism for A. testudineus' reduced sensitivity to severe contamination. We explored the key residues that may account for AhR2a's insensitivity in silico and then functionally validated them in vitro. Two sites (VCS_322-324, M₃₇₀) in its ligand-binding domain (LBD) were proved critical for its sensitivity to TCDD. This systematic exploration of the AhR pathway showed that most members have maintained their traditional functions as expected, whereas a nonfunctionalization event has occurred for AhR2a.

Transcriptomic analysis of Anabas testudineus and its defensive mechanisms in response to persistent organic pollutants exposure

The freshwater climbing perch (Anabas testudineus) can tolerate water environments contaminated with persistent organic pollutants (POPs). The mechanisms underlying this tolerance are unknown. We used de novo transcriptomic analysis to investigate the defensive mechanisms of A. testudineus against POPs based on its genetic features and biological responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure. Our results revealed a specific expansion of cytochrome P450 (CYP) 3A subfamily, which may be involved in the elimination of certain POPs. In xenobiotic responses, the aryl-hydrocarbon receptor (AhR) pathway represents a critical signaling mechanism, and we characterized four AhR and two AhR nuclear translocator homologs and one AhR repressor (AhRR) gene in A. testudineus. TCDD-induced AhRR and CYP1A mRNA upregulation suggests that negative-feedback regulation of AhR signaling through AhRR helps avoid excessive xenobiotic responses. Furthermore, liver and gill transcriptomic profiles were markedly altered after TCDD exposure, with some of the altered genes being related to common defensive responses reported in other species. Based on the newly identified TCDD-altered genes, several A. testudineus-specific responses are proposed, such as enhanced fatty acid β-oxidation. The genetic features of CYP3A subfamily and AhR pathway and the TCDD-induced defensive biological processes elucidated here enhance our understanding of A. testudineus defensive responses against POPs.

Effects of sequential exposure to water accommodated fraction of crude oil and chlorpyrifos on molecular and biochemical biomarkers in rainbow trout

Fish can be simultaneously or sequentially exposed to various kinds of pollutants, resulting in combined effects. Polycyclic aromatic hydrocarbons induce cytochrome P450 monooxygenase 1A (CYP1A) expression, which catalyzes the conversion of the organophosphorus insecticide chlorpyrifos (CPF) into its most active derivative, CPF-oxon. CPF-oxon inhibits CYP1A and other enzymes, including carboxylesterases (CEs) and acetylcholinesterase (AChE). We studied the effects of an in vivo exposure to crude oil water accommodated fraction (WAF) followed by an ex vivo exposure of liver tissue to CPF on the expression of Cyp1a, AhR and ARNT mRNA, CYP1A protein and on the activity of biomarker enzymes in the rainbow trout (Oncorhynchus mykiss). Juvenile rainbow trout were exposed to WAF (62 μg L^-1 TPH) for 48 h. Then, liver was dissected out, sliced and exposed to 20 μg L^-1 CPF ex vivo for 1 h. Liver tissue was analyzed for mRNA and protein expression and for CEs, AChE, glutathione S-transferase (GST) and CYP1A (EROD) activity. WAF induced Cyp1a mRNA and CYP1A protein expression by 10-fold and 2.5-8.3-fold, respectively, with no effect of CPF. WAF induced AhR expression significantly (4-fold) in control but not in CPF treated liver tissue. ARNT mRNA expression was significantly lowered (5-fold) by WAF. CPF significantly reduced liver EROD activity, independently of WAF pre-treatment. CEs activity was significantly inhibited in an additive manner following in vivo exposure to WAF (42%) and ex vivo exposure to CPF (19%). CPF exposure inhibited AChE activity (37%) and increased GST activity (42%).

Expression of aryl hydrocarbon receptor-regulated genes and superoxide dismutase in the Antarctic eelpout Pachycara brachycephalum exposed to benzo[a]pyrene

The aryl hydrocarbon receptor (AhR) pathway mediates many, if not all, responses of fish to dioxin-like compounds. The Southern Ocean is progressively exposed to increasing concentrations of anthropogenic pollutants. Antarctic fish are known to accumulate those pollutants, yet nothing is known about their capability to induce chemical biotransformation via the AhR pathway. The objective of the present study was to investigate whether Antarctic eelpout, Pachycara brachycephalum, respond to anthropogenic pollutants by activation of the AhR and its target gene cytochrome P4501A (CYP1A), and of superoxide dismutase (SOD), which served as a representative for oxidative stress. We exposed P. brachycephalum to 10 and 100 mg benzo[a]pyrene (BaP)/kg body weight for 10 d and measured the expression of AhR, CYP1A, and SOD in liver tissue via quantitative polymerase chain reaction. We identified two distinct AhR isoforms in the liver of P. brachycephalum. Antarctic eelpout responded to both BaP exposures by an up-regulation of AhR and SOD, and by a particularly strong induction of CYP1A expression, which remained high until day 10 of the exposure time. Our data suggest that P. brachycephalum possesses the potential to up-regulate xenobiotic biotransformation pathways, at least at the gene expression level. The time course of the AhR and CYP1A response points to an efficient but slow xenobiotics metabolism. Moreover, BaP exposure could include adverse effects such as oxidative stress. Environ Toxicol Chem 2018;37:1487-1495. © 2018 SETAC.

Transcriptional response in rainbow trout (Oncorhynchus mykiss) B cells and thrombocytes following in vivo exposure to benzo[a]pyrene

Immune toxicity of polycyclic aromatic hydrocarbons (PAHs) in fishes has been frequently reported but the reasons for differential cell toxicity remains unclear. Rainbow trout were exposed in vivo with a single intraperitoneal injection of corn oil or 100mg/kg of the immunotoxic PAH benzo[a]pyrene (B[a]P) in corn oil. Leukocytes were harvested from head kidney, spleen and blood after 14days, the optimal time for B cell depletion found in a previous study. The mRNA expression of five cytochrome P450 (CYP) enzymes, the aryl hydrocarbon receptor (AhR), and an intrinsic pathway apoptosis checkpoint (p53) in B cells and thrombocytes were examined. Transcript levels were measured in immunomagnetically-isolated B cells and thrombocytes from those tissues as well as in liver as B cells had been previously shown to be responsive the BaP whereas thrombocytes were not. There was induction of CYP1A1 in liver, blood B cells, and blood and spleen thrombocytes; CYP1B1 in blood B cells, blood and spleen thrombocytes; CYP1A3 in liver, blood and spleen B cells, and blood thrombocytes; CYP1C1 in liver; and AhR in liver and spleen thrombocytes. There was no change in CYP1C2, or p53 mRNA levels across tissues or cell type. Induction in mRNA was observed 14 d after exposure, indicating a prolonged physiological effect of a single B[a]P injection. CYP1A1 and CYP1A3 were the most abundantly expressed CYP genes and CYP1B1 was generally least abundant. B[a]P-induced thrombocytes had a significantly different pattern of CYP expression than either liver or B cells. Given the importance of metabolites in the toxicity of PAHs, differences in CYP expression between tissues may explain differences in toxicity previously observed between B cells and thrombocytes.

Molecular cloning and characterization of the aryl hydrocarbon receptors and aryl hydrocarbon receptor nuclear translocators in the American alligator

Aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor, binds to a variety of chemical compounds including various environmental contaminants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin. This receptor regulates expression of target genes through dimerization with the AHR nuclear translocator (ARNT). Since AHR-ARNT signaling pathways differ among species, characterization of AHR and ARNT is important to assess the effects of environmental contamination and for understanding the molecular mechanism underlying the intrinsic function. In this study, we isolated the cDNAs encoding three types of AHR and two types of ARNT from a reptile, the American alligator (Alligator mississippiensis). In vitro reporter gene assays showed that all complexes of alligator AHR-ARNT were able to activate ligand-dependent transcription on a xenobiotic response element. We found that AHR-ARNT complexes had higher sensitivities to a ligand than AHR-ARNT2 complexes. Alligator AHR1B showed the highest sensitivity in transcriptional activation induced by indigo when compared with AHR1A and AHR2. Taken together, our data revealed that all three alligator AHRs and two ARNTs were functional in the AHR signaling pathway with ligand-dependent and isoform-specific transactivations in vitro.

Subfunctionalization of Paralogous Aryl Hydrocarbon Receptors from the Frog Xenopus Laevis: Distinct Target Genes and Differential Responses to Specific Agonists in a Single Cell Type

Gene duplication confers genetic redundancy that can facilitate subfunctionalization, the partitioning of ancestral functions between paralogs. We capitalize on a recent genome duplication in Xenopus laevis (African clawed frog) to interrogate possible functional differentiation between alloalleles of the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor that mediates toxicity of dioxin-like compounds and plays a role in the physiology and development of the cardiovascular, hepatic, and immune systems in vertebrates. X. laevis has 2 AHR genes, AHR1α and AHR1β To test the hypothesis that the encoded proteins exhibit different molecular functions, we used TALENs in XLK-WG cells, generating mutant lines lacking functional versions of each AHR and measuring the transcriptional responsiveness of several target genes to the toxic xenobiotic 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and the candidate endogenous ligand 6-formylindolo[3,2-b]carbazole (FICZ). Mutation of either AHR1α or AHR1β reduced TCDD induction of the canonical AHR target, Cytochrome P4501A6, by 75%, despite the much lower abundance of AHR1β in wild-type cells. More modestly induced target genes, encoding aryl hydrocarbon receptor repressor (AHRR), spectrin repeat-containing nuclear envelope protein 1 (SYNE-1), and gap junction protein gamma 1 (GJC1), were regulated solely by AHR1α. AHR1β was responsible for CYP1A6 induction by FICZ, while AHR1α mediated FICZ induction of AHRR We conclude that AHR1α and AHR1β have distinct transcriptional functions in response to specific agonists, even within a single cell type. Functional analysis of frog AHR paralogs advances the understanding of AHR evolution and as well as the use of frog models of developmental toxicology such as FETAX.

β-naphthoflavone interferes with cyp1c1, cox2 and IL-8 gene transcription and leukotriene B4 secretion in Atlantic cod (Gadus morhua) head kidney cells during inflammation

The objective of this study was to evaluate how β-naphthoflavone interacts with lipopolysaccharide (LPS) and polyinosinic acid: polycytidylic acid (poly I: C) induced innate immune parameters as well as phase I and phase II detoxification enzymes in head kidney cells isolated from Atlantic cod. β-naphthoflavone is a pure agonist of aryl hydrocarbon receptor (AhR) while LPS and poly I: C are not. β-naphthoflavone was added to head kidney leukocytes alone or together with LPS or poly I: C and the responses were evaluated in terms of protein and gene expression. The results showed that β-naphthoflavone (25 nM), with and without LPS, significantly induced cytochrome P450 (cyp1c) transcription in cod head kidney cells. β-naphthoflavone (100 nM) in the presence of the virus mimic, poly I: C, also increased cyp1c1transcription. LPS induced cyp1c1, cyclooxygenase 2 (cox2), interleukin 1β (IL-1β), interleukin 6 (IL-6) and interleukin 8 (IL-8) transcription, genes that were not affected by the tested β-naphthoflavone concentrations alone. However, β-naphthoflavone (25 and 50 nM) strengthened LPS induced cox2 and IL-8 transcription. Cod head kidney cells exposed to β-naphthoflavone concentrations ranging from 25 to 100 nM, with and without LPS or poly I: C, expressed AhR protein. LPS or β-naphthoflavone (5-50 nM) significantly induced leukotriene B4 (LTB4) secretion compared to control. In conclusion, this study suggests that β-naphthoflavone could interfere with LPS induced immune cell signaling in cod head kidney cells.

Auto-induction mechanism of aryl hydrocarbon receptor 2 (AHR2) gene by TCDD-activated AHR1 and AHR2 in the red seabream (Pagrus major)

The toxic effects of dioxins and related compounds (DRCs) are mediated by the aryl hydrocarbon receptor (AHR). Our previous study identified AHR1 and AHR2 genes from the red seabream (Pagrus major). Moreover, we found that AHR2 mRNA levels were notably elevated by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure in the early life stage of red seabream embryos, while AHR1 mRNA level was not altered. In this study, to investigate the regulatory mechanism of these AHR transcripts, we cloned and characterized 5'-flanking regions of AHR1 and AHR2 genes. Both of the 5'-flanking regions in these AHR genes contained three potential xenobiotic-responsive elements (XREs). To assess whether the 5'-flanking region is transactivated by rsAHR1 and rsAHR2 proteins, we measured the transactivation potency of the luciferase reporter plasmids containing the 5'-flanking regions by AHR1 and AHR2 proteins that were transiently co-expressed in COS-7. Only reporter plasmid (pGL4-rsAHR2-3XREs) that contained three putative XRE sites in the 5'-flanking region of AHR2 gene showed a clear TCDD dose-dependent transactivation by AHR1 and AHR2 proteins. TCDD-EC₅₀ values for the rsAHR2-derived XRE transactivation were 1.3 and 1.4 nM for AHR1 and AHR2, respectively. These results suggest that the putative XREs of AHR2 gene have a function for AHR1- and AHR2-mediated transactivation, supporting our in ovo observation of an induction of AHR2 mRNA levels by TCDD exposure. Mutations in XREs of AHR2 gene led to a decrease in luciferase induction. Electrophoretic mobility shift assay showed that XRE1, the closest XRE from the start codon in AHR2 gene, is mainly responsible for the binding with TCDD-activated AHR. This suggests that TCDD-activated AHR1 and AHR2 up-regulate the AHR2 mRNA levels and this auto-induced AHR2 may amplify the signal transduction of its downstream targets including CYP1A in the red seabream.

A Study of 2,3,7,8-Tetrachlorodibenzo-p-dioxin Induced Liver Injury in Jian Carp (Cyprinus carpio var. Jian) Using Precision-Cut Liver Slices

The aim of this study was to establish a model for the study of liver injury induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in Jian carp using precision-cut liver slices (PCLS). PCLS were treated with TCDD at concentrations of 0, 0.05, 0.1, 0.3, and 0.6 μg/L for 6 h, followed by collection of the culture supernatant and PCLS for analysis. Several biochemical indices were analyzed, including glutamic pyruvic transaminase (GPT), glutamic oxaloacetic transaminase (GOT), lactate dehydrogenase (LDH), superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and malondialdehyde (MDA). Expression of mRNA was also estimated for cytochrome P4501A (CYP1A), aryl hydrocarbon receptor2 (AhR2), and aryl hydrocarbon receptor nuclear translocator2 (ARNT2). Results showed that some significant effects (p < 0.05) in MDA, GSH-Px and PCLS viability were observed at a TCDD concentration as low as 0.05 µg/L, and the observed effects increased with exposure concentration. Following exposure to TCDD for 6 h at a concentration of 0.3 μg/L, significant increases (p < 0.01) in the content of GPT, GOT, MDA, and LDH were observed, while SOD activity, GSH-Px activity, and PCLS viability were decreased (p < 0.01 or p < 0.05). Exposure to 0.3 μg/L TCDD also resulted in increased expression of mRNA for CYP1A, AhR2, and ARNT2. Overall, these results provide evidence of TCDD-induced liver injury and oxidative stress in Jian carp. These results also support the use of PCLS as an in vitro model for the evaluation of hepatotoxicity in Jian carp.

Effects of pharmaceuticals present in aquatic environment on Phase I metabolism in fish

The fate of pharmaceuticals in aquatic environments is an issue of concern. Current evidence indicates that the risks to fish greatly depend on the nature and concentrations of the pharmaceuticals and might be species-specific. Assessment of risks associated with the presence of pharmaceuticals in water is hindered by an incomplete understanding of the metabolism of these pharmaceuticals in aquatic species. In mammals and fish, pharmaceuticals are primarily metabolized by cytochrome P450 enzymes (CYP450). Thus, CYP450 activity is a crucial factor determining the detoxification abilities of organisms. Massive numbers of toxicological studies have investigated the interactions of human pharmaceuticals with detoxification systems in various fish species. In this paper, we review the effects of pharmaceuticals found in aquatic environments on fish hepatic CYP450. Moreover, we discuss the roles of nuclear receptors in cellular regulation and the effects of various groups of chemicals on fish, presented in the recent literature.

Identification and expression of aryl hydrocarbon receptors (AhR1 and AhR2) provide insight in an evolutionary context regarding sensitivity of white sturgeon (Acipenser transmontanus) to dioxin-like compounds

Sturgeons are ancient fishes, which are endangered in many parts of the world. Due to their benthic nature and longevity, sturgeon are at great risk of exposure to bioaccumulative contaminants such as dioxin-like compounds (DLCs). Despite their endangered status, little research has been conducted to characterize the relative sensitivity of sturgeons to DLCs. Proper assessment of risk of DLCs posed to these fishes therefore, requires a better understanding of this sensitivity and the factors that are driving it. Adverse effects associated with exposure to DLCs are mediated by the aryl hydrocarbon receptor (AhR). This study identified and characterized two distinct AhRs, AhR1 and AhR2, in white sturgeon (Acipenser transmontanus) for the first time as a first step in studying the relative sensitivities of sturgeons to DLCs. Furthermore, tissue-specific expression of both AhRs under basal conditions and in response to exposure to the model DLC, β-naphthoflavone (βNF), was determined. The sequence of amino acids of AhR1 of white sturgeon had greater similarity to AhRs of tetrapods, including amphibians, birds, and mammals, than to AhR1s of other fishes. The sequence of amino acids in the ligand binding domain of the AhR1 had greater than 80% similarity to AhRs known to bind DLCs and was less similar to AhRs not known to bind DLCs. AhR2 of white sturgeon had greatest similarity to AhR2 of other fishes. Profiles of expression of AhR1 and AhR2 in white sturgeon were distinct from those known in other fishes and appear more similar to profiles observed in birds. Expressions of both AhR1 and AhR2 of white sturgeon were greatest in liver and heart, which are target organs for DLCs. Furthermore, abundances of transcripts of AhR1 and AhR2 in all tissues from white sturgeon were greater than controls (up to 35-fold) following exposure to βNF. Based upon both AhRs having similar abundances of transcript in target organs of DLC toxicity, both AhRs being up-regulated following exposure to βNF, and both AhRs having greatest similarity to AhRs known to bind DLCs, it is hypothesized that both AhR1 and AhR2 of white sturgeon might mediate effects of DLCs in this species. Since current risk assessments are based on data derived largely from highly divergent fishes within the Salmonidae, presence of two functional AhRs in white sturgeon, one of which has greatest similarity to AhRs of birds, might have significant implications for the sensitivity of sturgeons to DLCs compared to other fishes.

Predicting the sensitivity of fishes to dioxin-like compounds: possible role of the aryl hydrocarbon receptor (AhR) ligand binding domain

Dioxin-like compounds are chronically toxic to most vertebrates. However, dramatic differences in sensitivity to these chemicals exist both within and among vertebrate classes. A recent study found that in birds, critical amino acid residues in the aryl hydrocarbon receptor (AhR) ligand binding domain are predictive of sensitivity to dioxin-like compounds in a range of species. It is currently unclear whether similar predictive relationships exist for fishes, a group of animals at risk of exposure to dioxin-like compounds. Effects of dioxin-like compounds are mediated through the AhR in fishes and birds. However, AhR dynamics are more complex among fishes. Fishes possess AhRs that can be grouped within at least three distinct clades (AhR1, AhR2, AhR3) with each clade possibly containing multiple isoforms. AhR2 has been shown to be the active form in most teleosts, with AhR1 not binding dioxin-like compounds. The role of AhR3 in dioxin-like toxicity has not been established to date and this clade is only known to be expressed in some cartilaginous fishes. Furthermore, multiple mechanisms of sensitivity to dioxin-like compounds that are not relevant in birds could exist among fishes. Although, at this time, deficiencies exist for the development of such a predictive relationship for application to fishes, successfully establishing such relationships would offer a substantial improvement in assessment of risks of dioxin-like compounds for this class of vertebrates. Elucidation of such relationships would provide a mechanistic foundation for extrapolation among species to allow the identification of the most sensitive fishes, with the ultimate goal of the prediction of risk posed to endangered species that are not easily studied.

Estrogen receptor-hijacking by dioxin-like 3,3'4,4',5-pentachlorobiphenyl (PCB126) in salmon hepatocytes involves both receptor activation and receptor protein stability

Several hypotheses have been proposed explaining the interactions between estrogen receptor (ER) and aryl hydrocarbon receptor (AhR) signaling pathways in both fish and mammalian systems. In both piscine and mammalian systems, ligand-activated AhR may recruit basal ER (i.e. hijack) in the absence of ER ligand and bind to the estrogen responsive elements (ERE) to activate ER-responsive genes. We have evaluated, the roles of receptor activation and receptor-protein stability on dioxin-like [3,3'4,4',5-pentachlorobiphenyl: PCB 126] mediated ER-hijacking in a salmon in vitro system. Primary salmon hepatocytes were exposed to PCB126 (1, 10 and 50 nM) with or without an ER-antagonist (ICI), putative AhR inhibitor (3',4'-dimethoxyflavone; DMF) or protein synthesis inhibitor (cycloheximide; CHX). Hepatocytes were exposed for 6, 12 and 24h. The expression of genes and proteins involved in ER (ERα, ERβ and vitellogenin) and AhR (CYP1A1, AhR-repressor, AhR2-isotypes and cofactors) pathways were analysed using qPCR and immunochemical methods. PCB126 induced transcripts of ER and AhR signalling pathways that were variably influenced by protein synthesis and receptor inhibitors. CHX stimulated a coordinated recruitment of the proteasome complex, resulting in the ubiquitination and degradation of ER and AhR isoforms and downstream protein products. Interestingly, DMF produced differential effects on the AhR signalling pathway, in the presence or absence of PCB126. Overall, ER-hijacking by dioxin-like compounds and subsequent activation of ER responsive genes involves both receptor activation/deactivation and receptor-protein degradation/destabilization (stability). Given that the Per-AhR/Arnt-Sim homology sequence of transcription factors usually associate with each other to form heterodimers and bind the XRE or ERE sequences in the promoter regions of target genes to regulate their expression, the complete mechanism of interactions between dioxin-like and estrogenic compounds in vertebrate systems may require additional characterization.

Tissue specificity of aryl hydrocarbon receptor (AhR) mediated responses and relative sensitivity of white sturgeon (Acipenser transmontanus) to an AhR agonist

Sturgeons are endangered in some parts of the world. Due to their benthic nature and longevity sturgeon are at greater risk of exposure to bioaccumulative contaminants such as dioxin-like compounds that are associated with sediments. Despite their endangered status, little research has been conducted to characterize the relative responsiveness of sturgeon to dioxin-like compounds. In an attempt to study the biological effects and possible associated risks of exposure to dioxin-like compounds in sturgeon, the molecular and biochemical responses of white sturgeon (Acipenser transmontanus) to a model aryl hydrocarbon receptor (AhR) agonist, β-naphthoflavone (βNF) were investigated. White sturgeon were injected intraperitoneally with one of three doses of βNF (0, 50, or 500mg/kg, bw). Rainbow trout (Oncorhynchus mykiss) were used as a reference species since their responses have been well characterized in the past. Three days following injection with βNF, fish were euthanized and livers, gills, and intestines collected for biochemical and molecular analyses. White sturgeon exposed to βNF had significantly greater ethoxyresorufin O-deethylase (EROD) activity in liver (up to 37-fold), gill (up to 41-fold), and intestine (up to 36-fold) than did unexposed controls. Rainbow trout injected with βNF exhibited EROD activity that was significantly greater in liver (88-fold), than that of controls, but was undetectable in gills or intestine. Abundance of CYP1A transcript displayed a comparable pattern of tissue-specific induction with intestine (up to 189-fold), gills (up to 53-fold), and liver (up to 21-fold). Methoxyresorufin O-deethylase (MROD) and pentoxyresorufin O-deethylase (PROD) activities were undetectable in unexposed white sturgeon tissues while exposed tissues displayed MROD activity that was only moderately greater than the activity that could be detected. Differential inducibility among liver, gill, and intestine following exposure to an AhR agonist is likely associated with tissue-specific regulation of the AhR signalling pathway. Liver and gill of white sturgeon had significantly greater AhR transcript abundance than did the intestine, however following exposure to βNF, significantly greater induction in AhR transcript abundance was detected in intestine (up to 35-fold) compared to liver (up to 5-fold) or gills (up to 11-fold). It was shown that white sturgeon are responsive to AhR agonists in the liver, gill, and intestine and could be among the more sensitive fish species with regard to inducibility of CYP1A.

Reproductive and developmental toxicity of dioxin in fish

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD or dioxin) is a global environmental contaminant and the prototypical ligand for investigating aryl hydrocarbon receptor (AHR)-mediated toxicity. Environmental exposure to TCDD results in developmental and reproductive toxicity in fish, birds and mammals. To resolve the ecotoxicological relevance and human health risks posed by exposure to dioxin-like AHR agonists, a vertebrate model is needed that allows for toxicity studies at various levels of biological organization, assesses adverse reproductive and developmental effects and establishes appropriate integrative correlations between different levels of effects. Here we describe the reproductive and developmental toxicity of TCDD in feral fish species and summarize how using the zebrafish model to investigate TCDD toxicity has enabled us to characterize the AHR signaling in fish and to better understand how dioxin-like chemicals induce toxicity. We propose that such studies can be used to predict the risks that AHR ligands pose to feral fish populations and provide a platform for integrating risk assessments for both ecologically relevant organisms and humans.

A dominant negative zebrafish Ahr2 partially protects developing zebrafish from dioxin toxicity

The toxicity by 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD) is thought to be caused by activation of the aryl hydrocarbon receptor (AHR). However, our understanding of how AHR activation by TCDD leads to toxic effects is poor. Ideally we would like to manipulate AHR activity in specific tissues and at specific times. One route to this is expressing dominant negative AHRs (dnAHRs). This work describes the construction and characterization of dominant negative forms of the zebrafish Ahr2 in which the C-terminal transactivation domain was either removed, or replaced with the inhibitory domain from the Drosophila engrailed repressor protein. One of these dnAhr2s was selected for expression from the ubiquitously active e2fα promoter in transgenic zebrafish. We found that these transgenic zebrafish expressing dnAhr2 had reduced TCDD induction of the Ahr2 target gene cyp1a, as measured by 7-ethoxyresorufin-O-deethylase activity. Furthermore, the cardiotoxicity produced by TCDD, pericardial edema, heart malformation, and reduced blood flow, were all mitigated in the zebrafish expressing the dnAhr2. These results provide in vivo proof-of-principle results demonstrating the effectiveness of dnAHRs in manipulating AHR activity in vivo, and demonstrating that this approach can be a means for blocking TCDD toxicity.

Novel cDNA sequences of aryl hydrocarbon receptors and gene expression in turtles (Chrysemys picta and Pseudemys scripta) exposed to different environments

Reproductive changes have been observed in painted turtles from a site with known contamination located on Cape Cod, MA, USA. We hypothesize that these changes are caused by exposure to endocrine-disrupting compounds and that genes involved in reproduction are affected. The aryl hydrocarbon receptor (AHR) is an orphan receptor that is activated by environmental contaminants. AHR mRNA was measured in turtles exposed to soil collected from a contaminated site. Adult turtles were trapped from the study site (Moody Pond, MP) or a reference site and exposed to laboratory environments containing soil from either site. The red-eared slider was used to assess neonatal exposure to soil and water from the sites. The environmental exposures occurred over a 13-month period. Juveniles showed an age-dependent increase in brain AHR1. Juvenile turtles exposed to the MP environment had elevated gonadal AHR1. Adult turtles exposed to the MP environment showed significantly decreased brain AHR2. The painted turtle AHR is the first complete reptile AHR cDNA sequence. Phylogenetic analysis of the painted turtle AHR showed that it clusters with other AHR2s. Partial AHR1 and partial AHR2 cDNA sequences were cloned from the red-eared slider. MEME analysis identified 18 motifs in the turtle AHRs, showing high conservation between motifs that overlapped functional regions in both AHR isoforms.

Developmental effects related to angiogenesis and osteogenic differentiation in Salmon larvae continuously exposed to dioxin-like 3,3',4,4'-tetrachlorobiphenyl (congener 77)

We have studied the effects of dioxin-like 3,3',4,4'-tetrachlorobiphenyl (PCB-77) on developmental effects related to angiogenesis and osteogenesis during early life-stages of salmon. Larvae were kept at 6°C and continuously exposed to waterborne PCB-77 (1 or 10 ng/L) initiated at the egg stage or 416-day degrees (dd) and throughout yolk-sac stage (716 dd) and for a total duration of 50 days (or 300 dd). Gene transcription analysis was performed on whole larvae total RNA at 548, 632, 674 and 716 dd using real-time PCR. Bone morphogenetic protein (bmp2 and bmp4), transforming growth factor β (TGF-β), estrogen receptors (ERα and ERβ), runx2, sox9 and collagen type 2 alpha 1 (col2a1) and vascular endothelial growth factor (VEGF) genes were studied. Effect on VEGF gene transcription was related to observation of heart rate, arrhythmia and anemia, demonstrating effects on vascular system development. Alizarine-red staining and quantification of ossified bone structures showed that PCB-77 produced concentration-dependent increases in the rate of osteogenic tissue formation. PCB-77 produced increases in col2a1 and runx2 transcription with subsequent induction of chondrogenesis and osteogenesis, respectively. The transcription of TGF-β gene was associated with ERβ transcription. Transcripts of AhR gene battery were differentially modulated by PCB-77 and these effects were dependent on concentration and larval age. Evidence of vascular system disruption by PCB-77 was observed as cardiac edema, anemia and arrhythmia in exposed individuals and as decreased level of VEGF gene transcription at early age. In general, our data indicate that PCB-77 produced developmental effects related to angiogenesis and osteogenic differentiation and disruption of vascular system development.

Aroclor 1254 disrupts liver glycogen metabolism and enhances acute stressor-mediated glycogenolysis in rainbow trout

The objective of this study was to investigate the impact of short-term exposure to polychlorinated biphenyls on the acute stress response in rainbow trout. Fish were exposed to dietary Aroclor1254 (10mg kg(-1) body mass/day) for 3 days and then subjected to a 3-min handling disturbance and sampled over a 24h recovery after the stressor exposure. In the pre-stress fish, PCB exposure significantly elevated aryl hydrocarbon receptor (AhR) and cytochrome P4501A1 (Cyp1A1) mRNA abundance and Cyp1A protein expression confirming AhR activation. There was no significant effect of PCB on plasma cortisol and glucose levels, while plasma lactate levels were significantly elevated compared to the sham group. PCB exposure significantly elevated liver glycogen content and hexokinase activity, whereas lactate dehydrogenase activity was depressed. Short-term PCB exposure did not modify the acute stressor-induced plasma cortisol, glucose and lactate responses. Liver glycogen content dropped significantly after stressor exposure in the PCB group but not in the sham group. This was matched by a significantly higher liver LDH activity and a lower HK activity during recovery in the PCB group suggesting enhanced glycolytic capacity to fuel hepatic metabolism. Liver AhR, but not Cyp1A1, transcript levels were significantly reduced during recovery from handling stressor in the Aroclor fed fish. Collectively, this study demonstrates that short-term PCB exposure may impair the liver metabolic performance that is critical to cope with the enhanced energy demand associated with additional stressor exposure in rainbow trout.

Reduced cytochrome P4501A activity and recovery from oxidative stress during subchronic benzo[a]pyrene and benzo[e]pyrene treatment of rainbow trout

This study assessed the role of aryl hydrocarbon receptor (AHR) affinity, and cytochrome P4501A (CYP1A) protein and activity in polyaromatic hydrocarbon (PAH)-induced oxidative stress. In the 1-100nM concentration range benzo[a]pyrene (BaP) but not benzo[e]pyrene (BeP) competitively displaced 2nM [(3)H]2, 3, 7, 8-tetrachloro-dibenzo-p-dioxin from rainbow trout AHR2α. Based on appearance of fluorescent aromatic compounds in bile over 3, 7, 14, 28 or 50days of feeding 3μg of BaP or BeP/g fish/day, rainbow trout liver readily excreted these polyaromatic hydrocarbons (PAHs) and their metabolites at near steady state rates. CYP1A proteins catalyzed more than 98% of ethoxyresorufin-O-deethylase (EROD) activity in rainbow trout hepatic microsomes. EROD activity of hepatic microsomes initially increased and then decreased to control activities after 50days of feeding both PAHs. Immunohistochemistry of liver confirmed CYP1A protein increased in fish fed both PAHs after 3days and remained elevated for up to 28days. Neither BaP nor BeP increased hepatic DNA adduct concentrations at any time up to 50days of feeding these PAHs. Comet assays of blood cells demonstrated marked DNA damage after 14days of feeding both PAHs that was not significant after 50days. There was a strong positive correlation between hepatic EROD activity and DNA damage in blood cells over time for both PAHs. Neither CYP1A protein nor 3-nitrotyrosine (a biomarker for oxidative stress) immunostaining in trunk kidney were significantly altered by BaP or BeP after 3, 7, 14, or 28days. There was no clear association between AHR2α affinity and BaP and BeP-induced oxidative stress.

Molecular and functional characterization of aryl hydrocarbon receptor nuclear translocator 1 (ARNT1) and ARNT2 in chicken (Gallus gallus)

Our previous studies have provided evidence that birds have two isoforms of aryl hydrocarbon receptors (AHR1 and AHR2) and AHR nuclear translocators (ARNT1 and ARNT2) that potentially mediate toxic responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds. We have also shown that while both in vitro-expressed chicken AHR1 (ckAHR1) and AHR2 (ckAHR2) exhibit binding affinities to TCDD, only ckAHR1 but not ckAHR2 showed a TCDD-dose-dependent transactivation potency of chicken cytochrome P450 1A5 (ckCYP1A5) in in vitro reporter gene assays. To explore the molecular mechanism of functional difference in the two ckAHRs, the present study investigated the molecular characteristics and function of chicken ARNT (ckARNT) that is a potential dimerization partner for the activation of ckAHR. The full-length ckARNT1 and ckARNT2 cDNAs were isolated and their alternative splice variants were also identified. The ckARNT1 transcript was ubiquitously expressed in various tissues, but ckARNT2 showed restricted expressions in brain, kidney and eye, indicating a similar expression pattern to mammalian ARNTs. The expressions of tagged-ckARNT1 and -ckARNT2 were confirmed in a chicken hepatoma LMH cells by western blot analyses, and their interactions with each ckAHR and a specific recognition DNA element, xenobiotic response element (XRE), were examined by gel shift assays. The result showed that ckARNT1 and ckARNT2 dimerize with each ckAHR isoform and bind with the XRE in a TCDD-dependent manner. Hence, we conclude that functional loss on the dimerization with ckARNTs or the XRE binding is not the major cause of the deficient TCDD-dependency of ckAHR2 for the transactivation. Furthermore, in vitro reporter gene assays showed that transfected ckARNT1 failed to modulate the transcriptional induction of ckAHR-mediated ckCYP1A5 gene by TCDD in COS-7 and LMH cells, whereas ckARNT2 could potentiate the TCDD-dependent response in COS-7 but not in LMH cells. This suggests that ckARNT2 has a distinct role from ckARNT1 in AHR signaling pathway and in a cell-specific mode of action.

Molecular and functional characterization of Aryl hydrocarbon receptor repressor from the chicken (Gallus gallus): interspecies similarities and differences

The aryl hydrocarbon receptor (AHR) repressor (AHRR) has been recognized as a negative feedback modulator of AHR-mediated responses in fish and mammals. However, the repressive mechanism by the AHRR has not been investigated in other animals. To understand the molecular mechanism of dioxin toxicity and the evolutionary history of the AHR signaling pathway in avian species, the present study addresses chicken AHRR (ckAHRR). The complementary DNA sequence of ckAHRR encodes an 84-kDa protein sharing 29-52% identities with other AHRRs. High levels of ckAHRR messenger RNA were recorded in the kidney and intestine of nontreated chicks. In hepatoma LMH cells, the 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) 50% effective concentration value for ckAHRR induction (0.0016nM) was the same as that for chicken cytochrome P450 1A5 (ckCYP1A5), implying a shared transcriptional regulation of ckAHRR and ckCYP1A5 by chicken AHR (ckAHR). In ckAHRR transient transfection assays, ckAHRR repressed both ckAHR1- and ckAHR2-mediated transcriptional activities. Deletion and mutation assays revealed that basic helix-loop-helix/Per-ARNT-Sim A domains of ckAHRR, particularly 217-402 amino acid residues, are indispensable for the repression, but the AHR nuclear translocator sequestration by ckAHRR and SUMOylation of ckAHRR are not involved in its repressive mechanism. Additionally, subcellular localization assay of ckAHR1-enhanced green fluorescent protein fusion protein showed that ckAHRR did not affect nuclear translocation of the ckAHR1. Furthermore, ckAHRR inhibited the TCDD- and 17β estradiol-enhanced ckCYP1A5 transcription through AHR-estrogen receptor α (ERα) cross talk. Taken together, the function of AHRR is conserved in chicken in terms of the negative regulation of AHR and ERα activities, but its functional mechanism is likely distinct from those of the mammalian and fish homologues.

Induction patterns of new CYP1 genes in environmentally exposed rainbow trout

The cytochrome P4501 (CYP1) gene family comprises four subfamilies in fish: CYP1A, CYP1B, CYP1C, and CYP1D. Only two CYP1 genes, CYP1A1 and CYP1A3, are so far known in rainbow trout (Oncorhynchus mykiss). The present study aimed to identify other CYP1 subfamily genes in rainbow trout, to establish methods for quantitative mRNA expression analysis of these genes, and to determine their basal and induced mRNA expression in gills and liver. Another goal was to examine their mRNA expression in environmentally exposed fish. We cloned four new transcripts, denoted rbCYP1B1, rbCYP1C1, rbCYP1C2, and rbCYP1C3. Levels of these and the previously known rbCYP1A transcripts were determined by real-time PCR in unexposed fish, fish exposed to the potent aryl hydrocarbon receptor (AhR) agonist 3,3',4,4',5-pentachlorobiphenyl (PCB126), and fish caged in various waters in the Uppsala region (Sweden). The mRNA expression patterns observed in unexposed rainbow trout (basal levels) were markedly similar to those reported for orthologous genes in other species. All six transcripts were induced by PCB126 in gills and liver, suggesting all genes to be AhR regulated. The caged fish showed clear rbCYP1 induction in gills at all monitoring sites (up to 70-fold the basal level), whereas the liver responses were weak; induction (up to 5-fold) was recorded only at the Uppsala municipal sewage treatment plant outlet. Gill filament EROD activity was induced at all caging sites. Most interestingly, the rbCYP1 gene response patterns in gills differed among caging sites and among subfamilies. The EROD induction seemed to only reflect induction of rbCYP1A transcription. Response patterns of multiple CYP1 genes in gills and liver could provide an improved monitoring strategy. Such patterns could be used to characterize complex mixtures of AhR agonists and antagonists in aquatic environments.

Hepatic retention and toxicological responses during feeding and depuration periods in Atlantic salmon ( Salmo salar ) fed graded levels of the synthetic antioxidant, butylated hydroxytoluene

The human safety aspects of seafood production require the expansion of vital knowledge of both nutrients and possible contaminants along the entire production chain. Thus, production of safer seafood can be achieved by using feed materials that are low in contaminants, while maintaining balanced nutrition, in order to secure optimal fish and consumer health. Our understanding of primary responses of fish health and production related diseases, as well as biological processes that influence carry-over and lowering of contaminants in farmed fish, will contribute to a sustainable production of safer seafood products. Therefore, we have studied the liver deposition and toxicological effects in salmon fed graded levels of BHT during a 12-week feeding followed by a 2-week depuration period using chemical, molecular, and catalytic assays. In general, our data showed that BHT was significantly retained in the liver and selectively modulated toxicological responses in the xenobiotic biotransformation pathways during the feeding period. Specifically, BHT produced consistent dose- and time-specific gene expression patterns for AhR2alpha, AhR2beta, CYP1A1, CYP3A, UGT1, and GSTpi. The effect of BHT on the gene expression of biotransformation enzyme did not parallel enzyme activity levels, suggesting a possible inhibition by parent BHT or its metabolites. As a safety precaution, the production of farmed Atlantic salmon in Norway requires a mandatory 2-week depuration period prior to slaughtering and market delivery to ensure the elimination of veterinary medicaments, additives, and other undesirable components. Comparison of feeding and depuration periods showed that BHT was highly retained in fish liver, as only 8-13% of fed BHT was eliminated during the 2-week depuration period. This is just a part of the total concentration in the whole fish, since BHT may have been distributed and accumulated in other organs. Since BHT or its metabolites putatively inhibited biotransformation enzymes and affected metabolism of the compound, they may have potential for toxicological and adverse health effects for both fish and fish consumers through carry-over processes from the fish products.

AHR-dependent misregulation of Wnt signaling disrupts tissue regeneration

The origins of molecular toxicology can be traced to understanding the interactions between halogenated aromatic hydrocarbons and the aryl hydrocarbon receptor (AHR). The physiological consequences of activation of the aryl hydrocarbon receptor are diverse, and we are just beginning to understand the importance of the AHR signal transduction pathway in homeostasis and disease. The many downstream targets that mediate these biological responses remain undefined. Studies have exploited the power of the zebrafish model to elucidate the mechanisms by which AHR activation disrupts biological signaling. Recent genomic analysis performed in a zebrafish tissue regeneration model revealed functional cross talk between AHR and the well-established Wnt/beta-catenin signal transduction pathway. This review focuses on the development of the zebrafish model of AHR biology and the application of in vivo toxicogenomics to unravel molecular mechanisms.

Evidence for multiple mechanisms of toxicity in larval rainbow trout (Oncorhynchus mykiss) co-treated with retene and alpha-naphthoflavone

Alkylated polycyclic aromatic hydrocarbons, such as retene (7-isopropyl-1-methylphenanthrene), induce cytochrome P450 1A (CYP1A) enzymes and produce dioxin-like toxicity in the embryo-larval stages of fish characterized by the signs of blue sac disease (BSD). The signs of toxicity are well characterized; however, the mechanism is not well understood. To elucidate the role of CYP1A in retene toxicity, larval rainbow trout (Oncorhynchus mykiss) were co-treated with a range of concentrations of alpha-naphthoflavone (ANF), a known CYP1A inhibitor. The co-treatment produced synergistic toxicity at 3.2-100 microg/L ANF, after which toxicity at 180 microg/L ANF dropped to levels typical of retene-only. At 320 microg/L ANF, toxicity increased with or without retene, indicating that ANF alone was capable of inducing BSD. In addition, the additive toxicity of retene-only and 320 microg/L ANF-only approximately equalled that of the co-exposed larvae (100 microg/L retene+320 microg/L ANF), indicating response addition. Thus, two mechanisms of action occurred in co-exposed larvae at different concentrations of ANF. In trout larvae, there was a correlation between toxicity and CYP1A protein concentrations, and in juvenile trout, ANF produced a concentration-dependent inhibition of ethoxyresorufin-O-deethylase (EROD) activity without a measurable drop in CYP1A protein. Taken together, the mechanism underlying the synergistic toxicity is EROD-independent and may be AhR-dependent. This study demonstrated that multiple, exposure-dependent mechanisms can occur in mixture toxicity, suggesting that current risk assessment models may drastically underestimate toxicity, particularly of mixtures containing both CYP1A inducers and inhibitors.

Hepatic biotransformation responses in Atlantic salmon exposed to retinoic acids and 3,3',4,4'-tetrachlorobiphenyl (PCB congener 77)

Active derivatives of vitamin A are essential in physiological processes such as cell growth, differentiation, morphogenesis and development. The biological functions of vitamin A are mediated through the retinoid acid receptors (RARs) and retinoid X receptors (RXRs). Aryl hydrocarbon receptor (AhR) agonists such as planar halogenated compounds are known to interfere with vitamin A homeostasis in both field and laboratory studies. In this study, we have investigated the molecular interactions between vitamin A and AhR signalling pathways using juvenile Atlantic salmon and agonists for both receptor pathways. Groups of juvenile salmon were treated with all-trans- and 9-cis-retinoic acid mixture (7:3 ratio) dissolved in DMSO (dimethyl sulfoxide) at 0.1, 1 and 10 mg/kg fish weight. The mixture was force fed singly or in combination with 0.1 mg 3,3',4,4'-tetrachlorobiphenyl (co-planar congener 77)/kg fish weight dissolved in DMSO. Liver samples were collected 3 days after PCB-77 exposure. A separate group exposed to combined retinoic acid (1 mg/kg for 5 days) and PCB-77, was sampled at 3, 7 and 14 days after PCB-77 exposure. Liver samples collected from all exposure groups were analyzed for gene (RARalpha, AhR2alpha, AhR2beta, CYP1A1, UGT1 and GSTpi) expression using real-time PCR and activity (7-ethoxyresorufin O-deethylase (EROD), UGT and GST) using biochemical methods with specific substrates. Our data showed that exposure to RA alone did not produce a significant increase of RARalpha mRNA levels, and the presence of PCB-77 attenuated the expression of RARalpha in RA dose- and time-specific manner. In addition, RA produced a dose-dependent increase of CYP1A1 mRNA and activity (EROD) levels without concomitant increase in AhR2 isoforms. When administered alone, PCB-77 produced increased CYP1A1, UGT1 and GSTpi mRNA and enzyme levels. The PCB-77-induced CYP1A1, UGT1 and GSTpi (mRNA and activity) levels were modulated by RA, in a parameter and dose-specific manner. In general, our data show an interaction between vitamin A and AhR signalling that may affect retinoid homeostasis in fish.

Brain transcriptomics in response to beta-naphthoflavone treatment in rainbow trout: the role of aryl hydrocarbon receptor signaling

Polychlorinated biphenyls (PCBs) exposure disrupts steroid production in teleostean fishes. While this suppression of plasma steroid levels is thought to involve aryl hydrocarbon receptor (AhR) signaling, the target tissues impacted and the molecular mechanisms involved have rarely been addressed. We tested the hypothesis that AhR activation downregulates genes involved in neuroendocrine function, including the control of brain-pituitary-interrenal (BPI) and -gonadal (BPG) axes in rainbow trout. To elucidate receptor-specific signaling, we utilized a pharmacological approach using beta-naphthoflavone (BNF) and resveratrol (RVT) as AhR agonist and antagonist, respectively. The gene expression pattern in the brain was analysed using a low-density targeted trout cDNA array enriched with genes encoding proteins involved in endocrine signaling, stress response and metabolic adjustments. Upregulation of AhR and CYP1A1 gene expression with BNF and the inhibition of this response by RVT confirmed AhR-dependent signaling. RVT by itself impacted only a few genes, while BNF treatment significantly modulated the transcript level of 49 genes, many of which are involved in the neuroendocrine control of stress and reproduction. Of these, only 27% of the BNF-mediated transcriptional response was blocked by RVT, suggesting molecular regulation of neuroendocrine pathways that are also AhR-independent. Gene expression pattern for select genes seen with the microarray analysis was also confirmed using quantitative real-time PCR. Overall, our results reveal for the first time that BNF disrupts several key genes involved in the neuroendocrine control of stress and sex steroid biosynthesis, while the mode of action involves both AhR-dependent and -independent pathways in trout.

Tissue-specific expression of aryl hydrocarbon receptor and putative developmental regulatory modules in Baltic salmon yolk-sac fry

The aryl hydrocarbon receptor (AhR) is an ancient protein that is conserved in vertebrates and invertebrates, indicating its important function throughout evolution. AhR has been studied largely because of its role in toxicology-gene expression via AhR is induced by many aromatic hydrocarbons in mammals. Recently, however, it has become clear that AhR is involved in various aspects of development such as cell proliferation and differentiation, and cell motility and migration. The mechanisms by which AhR regulates these various functions remain poorly understood. Across-species comparative studies of AhR in invertebrates, non-mammalian vertebrates and mammals may help to reveal the multiple functions of AhR. Here, we have studied AhR during larval development of Baltic salmon (Salmon salar). Our results indicate that AhR protein is expressed in nervous system, liver and muscle tissues. We also present putative regulatory modules and module-matching genes, produced by chromatin immunoprecipitation (ChIP) cloning and in silico analysis, which may be associated with evolutionarily conserved functions of AhR during development. For example, the module NFKB-AHRR-CREB found from salmon ChIP sequences is present in human ULK3 (regulating formation of granule cell axons in mouse and axon outgrowth in Caernohabditis elegans) and SRGAP1 (GTPase-activating protein involved in the Slit/Robo pathway) promoters. We suggest that AhR may have an evolutionarily conserved role in neuronal development and nerve cell targeting, and in Wnt signaling pathway.

Functional properties of the four Atlantic salmon (Salmo salar) aryl hydrocarbon receptor type 2 (AHR2) isoforms

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor through which organochlorine contaminants including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and some polycyclic aromatic hydrocarbons induce toxicity and altered gene expression. Atlantic salmon has multiple AHR genes, of which two belong to the AHR1 clade and four belong to the AHR2 clade. The four AHR2 forms (alpha, beta, gamma, delta) are more highly expressed than the AHR1 (alpha, beta,) forms and all six AHRs are highly similar in pairs, likely originating from a whole-genome duplication in the salmonid ancestor. It has been speculated that having multiple AHRs contributes to the very high sensitivity of salmonid species to TCDD and related chemicals. To test the hypothesis that all four salmon AHR2 proteins are expressed and functional, we measured mRNA transcription for each AHR2 in several tissues, cloned the cDNAs and evaluated the functional properties of the expressed proteins. Analysis by real-time PCR revealed that the receptors showed differences in transcript levels among salmon tissues and that in general AHR2alpha was transcribed at higher levels than the other three AHR2s. Velocity sedimentation analysis showed that all four in vitro-expressed AHR2 proteins exhibit specific, high-affinity binding of [(3)H]TCDD. When expressed in COS-7 cells, all four AHR2 proteins were able to drive the expression of a reporter gene under control of murine CYP1A1 enhancer elements. From EC(50) values determined in TCDD concentration-response experiments, all four salmon AHR2s show similar sensitivity to TCDD. In summary, all four Atlantic salmon AHR2 appear to function in AHR-mediated signaling, suggesting that all four proteins are involved in TCDD-mediated toxicity.

Activation of estrogen receptor signaling by the dioxin-like aryl hydrocarbon receptor agonist, 3,3',4,4',5-pentachlorobiphenyl (PCB126) in salmon in vitro system

Available toxicological evidence indicates that environmental contaminants with strong affinity to the aryl hydrocarbon receptor (AhR) have anti-estrogenic properties in both mammalian and non-mammalian in vivo and in vitro studies. The primary objective of the present study was to investigate the interactions between the AhR and estrogen receptor (ER) in salmon in vitro system. Two separate experiments were performed and gene expression patterns were analyzed using real-time PCR, while protein analysis was done by immunoblotting. Firstly, salmon primary hepatocytes were exposed to the dioxin-like PCB126 at 1, 10 and 50 nM [corrected] and ER agonist nonylphenol (NP) at 5 and 10 microM, singly or in combination. Our data showed increased levels of ER-mediated gene expression (vitellogenin: Vtg, zona radiata protein: Zr-protein, ERalpha, ERbeta and vigilin) as well as increased cellular ERalpha protein levels after treatment with NP and PCB126, singly or in combination. PCB126 treatment alone produced, as expected, increased transcription of AhR nuclear translocator (Arnt), CYP1A1 and AhR repressor (AhRR) mRNA, and these responses were reduced in the presence of NP concentrations. PCB126 exposure alone did not produce significant effect on AhR2alpha mRNA but increased (at 1 and 50 pM) and decreased (at 10 pM) AhR2beta mRNA below control level. For AhR2delta and AhR2gamma isotypes, PCB126 (at 1 nM) [corrected] produced significant decreases (total inhibition for AhR2gamma) of mRNA levels but was indifferent at 10 and 50 pM, compared to control. NP exposure alone produced concentration-dependent significant decrease of AhR2beta mRNA. In contrast, while 5 microM NP produced an indifferent effect on AhR2delta and AhR2gamma, 10 microM NP produced significant decrease (total inhibition for AhR2gamma) and the presence of NP produced apparent PCB126 concentration-specific modulation of all AhR isotypes. A second experiment was performed to evaluate the involvement of ER isoforms in PCB126 mediated estrogenicity. Here, cells were treated with the different concentrations of PCB126, alone or in combination with ICI182,780 (ICI) and sampled at 12, 24 and 48 h post-exposure. Our data showed that PCB126 produced a time- and concentration-specific increase of ERalpha and Vtg expressions and these responses were decreased in the presence of ICI. In general, these responses show a direct PCB126 induced transcriptional activation of ERalpha and estrogenic responses in the absence of ER agonists. Although not conclusive, our findings represent the first study showing the activation of estrogenic responses by a dioxin-like PCB in fish in vitro system and resemble the "ER-hijacking" hypothesis that was recently proposed. Thus, the direct estrogenic actions of PCB126 observed in the present study add new insight on the mechanisms of ER-AhR cross-talk, prompting a new wave of discussion on whether AhR-mediated anti-estrogenicity is an exception rather than rule of action.

Aryl hydrocarbon receptor signaling in rainbow trout hepatocytes: role of hsp90 and the proteasome

The objective of this study was to investigate the role of heat shock protein 90 (hsp90) and the proteasome in regulating aryl hydrocarbon receptor (AhR) activation and cytochrome P450 1A (Cyp1A) protein expression in rainbow trout (Oncorhynchus mykiss). We exposed trout hepatocytes in primary culture to the AhR agonist beta-napthoflavone (betaNF; 10(-6) M) and examined AhR and Cyp1A expression. betaNF-induced a significant temporal accumulation of AhR and Cyp1A1 mRNA abundance in trout hepatocytes. This transcript response was followed by a significantly higher AhR and Cyp1A protein expression. Exposure to geldanamycin (GA; 1000 ng mL(-1)), a benzoquinone ansamycin antibiotic used to inhibit hsp90 function, significantly reduced ( approximately 70%) betaNF-induced Cyp1A protein expression. Also, exposure to the proteasomal inhibitor MG-132 (50 microM) completely abolished betaNF-induced Cyp1A protein expression in trout hepatocytes. In addition, MG-132 treatment further enhanced the GA-mediated suppression of the Cyp1A response. The effect of MG-132 on Cyp1A response corresponded with a significant inhibition of BNF-mediated AhR mRNA abundance, but not protein content. Altogether our results suggest a betaNF-mediated autoregulation of AhR content in trout hepatocytes. We propose a key role for hsp90 and the proteasome in this ligand-mediated AhR regulation and Cyp1A response.

Interactions between estrogen- and Ah-receptor signalling pathways in primary culture of salmon hepatocytes exposed to nonylphenol and 3,3',4,4'-tetrachlorobiphenyl (congener 77)

BACKGROUND

The estrogenic and xenobiotic biotransformation gene expressions are receptor-mediated processes that are ligand structure-dependent interactions with estrogen-receptor (ER) and aryl hydrocarbon receptor (AhR), probably involving all subtypes and other co-factors. The anti-estrogenic activities of AhR agonists have been reported. In teleost fish, exposure to AhR agonists has been associated with reduced Vtg synthesis or impaired gonadal development in both in vivo- and in vitro studies. Inhibitory AhR and ER cross-talk have also been demonstrated in breast cancer cells, rodent uterus and mammary tumors. Previous studies have shown that AhR-agonists potentiate xenoestrogen-induced responses in fish in vivo system. Recently, several studies have shown that AhR-agonists directly activate ER alpha and induce estrogenic responses in mammalian in vitro systems. In this study, two separate experiments were performed to study the molecular interactions between ER and AhR signalling pathways using different concentration of PCB-77 (an AhR-agonist) and time factor, respectively. Firstly, primary Atlantic salmon hepatocytes were exposed to nonylphenol (NP: 5 microM--an ER agonist) singly or in combination with 0.001, 0.01 and 1 microM PCB-77 and sampled at 48 h post-exposure. Secondly, hepatocytes were exposed to NP (5 microM) or PCB-77 (1 microM) singly or in combination for 12, 24, 48 and 72 h. Samples were analyzed using a validated real-time PCR for genes in the ER pathway or known to be NP-responsive and AhR pathway or known to be PCB-77 responsive.

RESULTS

Our data showed a reciprocal inhibitory interaction between NP and PCB-77. PCB-77 produced anti-NP-mediated effect by decreasing the mRNA expression of ER-responsive genes. NP produced anti-AhR mediated effect or as inhibitor of AhR alpha, AhRR, ARNT, CYP1A1 and UDPGT expression. A novel aspect of the present study is that low (0.001 microM) and medium (0.01 microM) PCB-77 concentrations increased ER alpha mRNA expression above control and NP exposed levels, and at 12 h post-exposure, PCB-77 exposure alone produced significant elevation of ER alpha, ER beta and Zr-protein expressions above control levels.

CONCLUSION

The findings in the present study demonstrate a complex mode of ER-AhR interactions that were dependent on time of exposure and concentration of individual chemicals (NP and PCB-77). This complex mode of interaction is further supported by the effect of PCB-77 on ER alpha and ER beta (shown as increase in transcription) with no concurrent activation of Vtg (but Zr-protein) response. These complex interactions between two different classes of ligand-activated receptors provide novel mechanistic insights on signalling pathways. Therefore, the degree of simultaneous interactions between the ER and AhR gene transcripts demonstrated in this study supports the concept of cross-talk between these signalling pathways.

Chemical Contamination in Aquatic Ecosystems

The 21st Century's Center of Excellence (COE) Program "Coastal Marine Environmental Research" in Ehime University, funded by the Ministry of Education, Culture, Sports, Science and Technology, Government of Japan, started its activities in October 2002. One of the core projects of the COE Program in Ehime University is "studies on environmental behavior of hazardous chemicals and their toxic effects on wildlife". This core project deals with studies of the local and global distribution of environmental contaminants in aquatic ecosystems, retrospective analysis of such chemicals, their toxicokinetics in humans and wildlife, molecular mechanisms to determine species-specific reactions, and sensitivity of chemically induced effects, and with the development of methodology for risk assessment for the conservation of ecological and species diversity. This presentation describes our recent achievements of this project, including research on contamination by arsenic and organohalogen pollutants in the Mekong River basin and molecular mechanisms of morphologic deformities in dioxin-exposed red seabream (Pagrus major) embryos. We established the Environmental Specimen Bank (es-BANK) in Ehime University in 2004, archiving approximately 100000 cryogenic samples containing tissues of wildlife and humans that have been collected for the past 40 years. The CMES homepage offers details of samples through online database retrieval. The es-BANK facility was in operation by the end of 2005.

Targeted salmon gene array (SalArray): a toxicogenomic tool for gene expression profiling of interactions between estrogen and aryl hydrocarbon receptor signalling pathways

In toxicogenomics, gene arrays are valuable tools in the identification of differentially expressed genes and potentially identify new gene biomarkers altered by exposure of organisms to xenobiotic compounds, either singly or as complex mixtures. In this study, we investigated the mechanisms of interaction between estrogen receptor (ER) and aryl hydrocarbon receptor (Ah receptor or AhR) signalling pathways using toxicogenomic approaches. First, we generated cDNA libraries using suppressive subtractive hybridization (SSH) of clones containing differentially expressed genes from Atlantic salmon (Salmo salar) separately exposed to ER and AhR agonists. Second, a targeted gene array (SalArray) was developed based on true-positive differentially expressed genes. In the experimental setup, primary cultures of salmon hepatocytes isolated by a two-step perfusion method were exposed for 48 h to nonylphenol (NP; 5 microM) and 3,3',4,4'-tetrachlorobiphenyl (TCB; 1 microM), singly and combined, in the absence or presence of antagonists. Using a targeted SalArray, we demonstrate that exposure of salmon to NP singly or in combination with TCB produced differential gene expression patterns in salmon liver. Array analysis showed that exposure of hepatocytes to NP mainly altered genes involved in the estrogenic pathway, including genes for steroid hormone synthesis and metabolism. The anti-estrogenic properties of TCB were demonstrated in the array analysis as genes induced by NP were decreased by TCB. To study the effects of TCB on ER-mediated transcription, hepatocytes were treated for 48 h with tamoxifen (Tam; 1 microM) and ICI182,780 (ICI; 1 microM). The effect of AhR on ER-mediated transcription was investigated by blocking AhR activity with alpha-naphthoflavone (ANF; 0.1 and 1 microM). Quantitative real-time polymerase chain reactions confirmed the changes in expression of ERalpha, ERbeta, vitellogenin (Vtg), zona radiata protein (Zr-protein), and vigilin for the ER pathway and AhRalpha, AhRbeta, AhRR, ARNT, CYP1A1, UDPGT, and a 20S proteasome beta-subunit for the AhR pathway. We found that exposure to NP and TCB both singly and in combination produced gene expression patterns that were negatively influenced by individual receptor antagonists. TCB caused decreased ER-mediated gene expression, and NP caused decreased AhR-mediated responses. Inhibition of AhR with ANF did not reverse the effect of TCB on ER-mediated transcription suggesting that AhRs do not have a direct role on TCB-mediated decreases of ER-mediated responses. In contrast, the inhibition of ER with Tam and ICI reversed the transcription of AhR-mediated responses (except AhRR). Taken together, the findings in the present study demonstrate a complex mode of ER-AhR interaction, possibly involving competition for common cofactors. This complex mode of interaction is further supported by the observation that the presence of ER antagonists potentiated the transcription of AhR isoforms and their mediated responses when TCB was given alone (more so for AhRbeta). Thus, the inhibitory ER-AhR interactions can be used to further investigate specific genes found to be affected in our targeted SalArray chip that are important for the reproductive effects of endocrine disruptors.

Toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in developing red seabream (Pagrus major) embryo: an association of morphological deformities with AHR1, AHR2 and CYP1A expressions

The toxicity of dioxins such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is mainly mediated by the aryl hydrocarbon receptor (AHR), which regulates the multiple target genes including cytochrome P4501A (CYP1A). In general, bony fishes, which possess at least two distinct AHRs are one of the most sensitive vertebrates to TCDD in early life stage. However, the physiological and toxicological roles of piscine multiple AHRs are not fully understood, especially in marine fish. To understand which AHR is responsible for TCDD toxicity in a marine fish species, we characterized the early life stage toxicity related to the expression of AHRs and CYP1A in red seabream (Pagrus major). The embryos at 10h post-fertilization (hpf) were treated with 0-100 microg/L TCDD for 80 min waterborne exposure. TCDD dose-dependently elicited developmental toxicities including mortality, yolk sac edema, retarded body growth, spinal deformity, reduced heart rate, shortened snout, underdeveloped fin, heart, and lower jaw. Intriguingly, hemorrhage and pericardium edema, typical TCDD developmental defects noticed in other fish species, were not found in red seabream until test termination. The EC(egg)50s for yolk sac edema, underdeveloped fin, and spinal deformity were 170, 240, and 340 pg/g, respectively. The LC(egg)50 was 360 pg/g embryo, indicating that this species is one of the most sensitive fishes to TCDD toxicity. The expression levels of rsAHR1, rsAHR2 and CYP1A mRNAs were also determined in different developmental stages. The rsAHR2 mRNA expression dose-dependently increased following TCDD exposure, while rsAHR1 mRNA level was not altered. Level of rsAHR2 mRNA measured by two-step real-time PCR was 30 times higher than rsAHR1 in embryos treated with the highest dose. Temporal patterns of rsAHR2 and CYP1A mRNAs were similar in TCDD-treated embryos, representing a significant positive correlation between rsAHR2 and CYP1A mRNA levels, but not between rsAHR1 and CYP1A. In comparison of temporal trends of TCDD-induced AHRs and CYP1A expression, and developmental toxicities, the highest expression of rsAHR2 and CYP1A mRNA were detected prior to the appearance of maximal incidence of TCDD toxic manifestations. These results suggest that rsAHR2 may be dominantly involved in the transcriptional regulation of CYP1A, and several TCDD defects are dependent on the alteration of rsAHR2 and/or rsAHR2-CYP1A signaling pathway that is controlled through their expression levels.

Unexpected diversity of aryl hydrocarbon receptors in non-mammalian vertebrates: insights from comparative genomics

Ligand-activated receptors are well-known targets of environmental chemicals that disrupt endocrine signaling. Genomic approaches are providing new opportunities to understand the comparative biology and molecular evolution of these receptors. One example of this is the aryl hydrocarbon receptor (AHR), a basic-helix-loop-helix (bHLH)-Per-Arnt-Sim (PAS) transcription factor through which planar aromatic hydrocarbons cause altered gene expression and toxicity. In contrast to humans and other mammals, which possess a single AHR, teleosts such as the Atlantic killifish (Fundulus heteroclitus) have at least two AHRs (AHR1 and AHR2). Analysis of sequenced genomes has revealed additional, unexpected AHR diversity in non-mammalian vertebrates, including the chicken Gallus gallus (three predicted AHR genes), bony fishes such as the pufferfish Takifugu (formerly Fugu) rubripes (five AHR genes) and zebrafish Danio rerio (three AHR genes), and cartilaginous fishes such as the spiny dogfish Squalus acanthias (three AHR genes). In contrast, invertebrates appear to possess single AHRs that do not bind typical ligands of vertebrate AHRs. We suggest that AHR diversity in vertebrates arose through both gene and whole-genome duplications combined with lineage-specific gene loss, and that sensitivity to the developmental toxicity of planar aromatic hydrocarbons may have had its origin in the evolution of the ligand-binding capacity of the AHR in the chordate lineage. Comparative molecular and genomic studies are providing new insights into AHR diversity and function in non-mammalian species, revealing additional complexity in mechanisms by which environmental chemicals interfere with receptor-dependent signaling.

Developmental toxicity of 4-ring polycyclic aromatic hydrocarbons in zebrafish is differentially dependent on AH receptor isoforms and hepatic cytochrome P4501A metabolism

Polycyclic aromatic hydrocarbons (PAHs) derived from fossil fuels are ubiquitous contaminants and occur in aquatic habitats as highly variable and complex mixtures of compounds containing 2 to 6 rings. For aquatic species, PAHs are generally accepted as acting through either of two modes of action: (1) "dioxin-like" toxicity mediated by activation of the aryl hydrocarbon receptor (AHR), which controls a battery of genes involved in PAH metabolism, such as cytochrome P4501A (CYP1A) and (2) "nonpolar narcosis", in which tissue uptake is dependent solely on hydrophobicity and toxicity is mediated through non-specific partitioning into lipid bilayers. As part of a systematic analysis of mechanisms of PAH developmental toxicity in zebrafish, we show here that three tetracyclic PAHs (pyrene, chrysene, and benz[a]anthracene) activate the AHR pathway tissue-specifically to induce distinct patterns of CYP1A expression. Using morpholino knockdown of ahr1a, ahr2, and cyp1a, we show that distinct embryolarval syndromes induced by exposure to two of these compounds are differentially dependent on tissue-specific activation of AHR isoforms or metabolism by CYP1A. Exposure of embryos with and without circulation (silent heart morphants) resulted in dramatically different patterns of CYP1A induction, with circulation required to deliver some compounds to internal tissues. Therefore, biological effects of PAHs cannot be predicted simply by quantitative measures of AHR activity or a compound's hydrophobicity. These results indicate that current models of PAH toxicity in fish are greatly oversimplified and that individual PAHs are pharmacologically active compounds with distinct and specific cellular targets.

The molecular basis for differential dioxin sensitivity in birds: role of the aryl hydrocarbon receptor

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related halogenated aromatic hydrocarbons (HAHs) are highly toxic to most vertebrate animals, but there are dramatic differences in sensitivity among species and strains. Aquatic birds including the common tern (Sterna hirundo) are highly exposed to HAHs in the environment, but are up to 250-fold less sensitive to these compounds than the typical avian model, the domestic chicken (Gallus gallus). The mechanism of HAH toxicity involves altered gene expression subsequent to activation of the aryl hydrocarbon receptor (AHR), a basic helix-loop-helix-PAS transcription factor. AHR polymorphisms underlie mouse strain differences in sensitivity to HAHs and polynuclear aromatic hydrocarbons, but the role of the AHR in species differences in HAH sensitivity is not well understood. Here, we show that although chicken and tern AHRs both exhibit specific binding of [3H]TCDD, the tern AHR has a lower binding affinity and exhibits a reduced ability to support TCDD-dependent transactivation as compared to AHRs from chicken or mouse. We further show through use of chimeric AHR proteins and site-directed mutagenesis that the difference between the chicken and tern AHRs resides in the ligand-binding domain and that two amino acids (Val-325 and Ala-381) are responsible for the reduced activity of the tern AHR. Other avian species with reduced sensitivity to HAHs also possess these residues. These studies provide a molecular understanding of species differences in sensitivity to dioxin-like compounds and suggest an approach to using the AHR as a marker of dioxin susceptibility in wildlife.