Species-specific binding of transformed Ah receptor to a dioxin responsive transcriptional enhancer

Implications of xenobiotic-response element(s) and aryl hydrocarbon receptor in health and diseases

The effect of air pollution on public health is severely detrimental. In humans; the physiological response against pollutants is mainly elicited via the activation of aryl hydrocarbon receptor (AhR). It acts as a prime sensor of xenobiotic chemicals, also functioning as a transcription factor regulating a variety of gene expressions. Along with AhR, another pivotal element of the pollution stress pathway is Xenobiotic Response Elements (XREs). XRE, as studied are some conserved sequences in the DNA, responsible for the physiological response against pollutants. XRE is present at the upstream of the inducible target genes of AhR and it regulates the function of the AhR. XRE(s) are highly conserved in species as it has only eight specific sequences found so far in humans, mice, and rats. Inhalation of toxicants like dioxins, gaseous industrial effluents, and smoke from burning fuel and tobacco leads to predominant damage to the lungs. However, scientists are exploring the involvement of AhR in chronic diseases for example chronic obstructive pulmonary disease (COPD) and also other lethal diseases like lung cancer. In this review, we summarise what is known at this time about the roles played by the XRE and AhR in our molecular systems that have a defined control in the normal maintenance of homeostasis as well as dysfunctions.

Aromatic hydrocarbon receptors in mitochondrial biogenesis and function

Mitochondria are ubiquitous membrane-bound organelles that not only play a key role in maintaining cellular energy homeostasis and metabolism but also in signaling and apoptosis. Aryl hydrocarbons receptors (AhRs) are ligand-activated transcription factors that recognize a wide variety of xenobiotics, including polyaromatic hydrocarbons and dioxins, and activate diverse detoxification pathways. These receptors are also activated by natural dietary compounds and endogenous metabolites. In addition, AhRs can modulate the expression of a diverse array of genes related to mitochondrial biogenesis and function. The aim of the present review is to analyze scientific data available on the AhR signaling pathway and its interaction with the intracellular signaling pathways involved in mitochondrial functions, especially those related to cell cycle progression and apoptosis. Various evidence have reported the crosstalk between the AhR signaling pathway and the nuclear factor κB (NF-κB), tyrosine kinase receptor signaling and mitogen-activated protein kinases (MAPKs). The AhR signaling pathway seems to promote cell cycle progression in the absence of exogenous ligands, whereas the presence of exogenous ligands induces cell cycle arrest. However, its effects on apoptosis are controversial since activation or overexpression of AhR has been observed to induce or inhibit apoptosis depending on the cell type. Regarding the mitochondria, although activation by endogenous ligands is related to mitochondrial dysfunction, the effects of endogenous ligands are not well understood but point towards antiapoptotic effects and inducers of mitochondrial biogenesis.

Evolution of Hominin Detoxification: Neanderthal and Modern Human Ah Receptor Respond Similarly to TCDD

In studies of hominin adaptations to fire use, the role of the aryl hydrocarbon receptor (AHR) in the evolution of detoxification has been highlighted, including statements that the modern human AHR confers a significantly better capacity to deal with toxic smoke components than the Neanderthal AHR. To evaluate this, we compared the AHR-controlled induction of cytochrome P4501A1 (CYP1A1) mRNA in HeLa human cervix epithelial adenocarcinoma cells transfected with an Altai-Neanderthal or a modern human reference AHR expression construct, and exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We compared the complete AHR mRNA sequences including the untranslated regions (UTRs), maintaining the original codon usage. We observe no significant difference in CYP1A1 induction by TCDD between Neanderthal and modern human AHR, whereas a 150–1,000 times difference was previously reported in a study of the AHR coding region optimized for mammalian codon usage and expressed in rat cells. Our study exemplifies that expression in a homologous cellular background is of major importance to determine (ancient) protein activity. The Neanderthal and modern human dose–response curves almost coincide, except for a slightly higher extrapolated maximum for the Neanderthal AHR, possibly caused by a 5′-UTR G-variant known from modern humans (rs7796976). Our results are strongly at odds with a major role of the modern human AHR in the evolution of hominin detoxification of smoke components and consistent with our previous study based on 18 relevant genes in addition to AHR, which concluded that efficient detoxification alleles are more dominant in ancient hominins, chimpanzees, and gorillas than in modern humans.

The Ah Receptor: Adaptive Metabolism, Ligand Diversity, and the Xenokine Model

The Ah receptor (AHR) has been studied for almost five decades. Yet, we still have many important questions about its role in normal physiology and development. Moreover, we still do not fully understand how this protein mediates the adverse effects of a variety of environmental pollutants, such as the polycyclic aromatic hydrocarbons (PAHs), the chlorinated dibenzo-p-dioxins ("dioxins"), and many polyhalogenated biphenyls. To provide a platform for future research, we provide the historical underpinnings of our current state of knowledge about AHR signal transduction, identify a few areas of needed research, and then develop concepts such as adaptive metabolism, ligand structural diversity, and the importance of proligands in receptor activation. We finish with a discussion of the cognate physiological role of the AHR, our perspective on why this receptor is so highly conserved, and how we might think about its cognate ligands in the future.

A mouse strain less responsive to dioxin-induced prostaglandin E2 synthesis is resistant to the onset of neonatal hydronephrosis

Dioxin is a ubiquitous environmental pollutant that induces toxicity when bound to the aryl hydrocarbon receptor (AhR). Significant differences in susceptibility of mouse strains to dioxin toxicity are largely accounted for by the dissociation constant of binding to dioxins of AhR subtypes encoded by different alleles. We showed that cyclooxygenase-2 (COX-2) and microsomal prostaglandin E synthase-1 (mPGES-1), components of a prostanoid synthesis pathway, play essential roles in the onset of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induced hydronephrosis of neonatal mice. Although C57BL/6J and BALB/cA mice harbor AhR receptors highly responsive to TCDD, they were found by chance to differ significantly in the incidence of TCDD-induced hydronephrosis. Therefore, the goal of the present study was to determine the molecular basis of this difference in susceptibility to TCDD toxicity. For this purpose, we administered C57BL/6J and BALB/cA dams' TCDD at an oral dose of 15 or 80 μg/kg on postnatal day (PND) 1 to expose pups to TCDD via lactation, and the pups' kidneys were collected on PND 7. The incidence of hydronephrosis in C57BL/6J pups (64%) was greater than in BALB/cA pups (0%, p < 0.05), despite similarly increased levels of COX-2 mRNA. The incidence of hydronephrosis in these mouse strains paralleled the levels of renal mPGES-1 mRNA and early growth response 1 (Egr-1) that modulates mPGES-1 gene expression, as well as PGE2 concentrations in urine. Although these mouse strains possess AhR alleles tightly bound to TCDD, their difference in incidence and severity of hydronephrosis can be explained, in part, by differences in the expression of mPGES-1 and Egr-1.

Functional analysis of the dioxin response elements (DREs) of the murine CYP1A1 gene promoter: beyond the core DRE sequence

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that mediates the biological and toxicological effects of halogenated aromatic hydrocarbons, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). When activated by dioxin, the cytosolic AhR protein complex translocates into the nucleus and dimerizes with the ARNT (Ah receptor nuclear translocator) protein. The heteromeric ligand:AhR/Arnt complex then recognizes and binds to its specific DNA recognition site, the dioxin response element (DRE). DREs are located upstream of cytochrome P4501A1 (CYP1A1) and other AhR-responsive genes, and binding of the AhR complex stimulates their transcription. Although CYP1A1 expression has been used as the model system to define the biochemical and molecular mechanism of AhR action, there is still limited knowledge about the roles of each of the seven DREs located in the CYP1A1 promoter. These seven DREs are conserved in mouse, human and rat. Deletion analysis showed that a single DRE at −488 was enough to activate the transcription. Truncation analysis demonstrated that the DRE at site −981 has the highest transcriptional efficiency in response to TCDD. This result was verified by mutation analysis, suggesting that the conserved DRE at site −981 could represent a significant and universal AhR regulatory element for CYP1A1. The reversed substituted intolerant core sequence (5′-GCGTG-3′ or 5′-CACGC-3′) of seven DREs reduced the transcriptional efficiency, which illustrated that the adjacent sequences of DRE played a vital role in activating transcription. The core DRE sequence (5′-TNGCGTG-3′) tends to show a higher transcriptional level than that of the core DRE sequence (5′-CACGCNA-3′) triggered by TCDD. Furthermore, in the core DRE (5′-TNGCGTG-3′) sequence, when “N” is thymine or cytosine (T or C), the transcription efficiency was stronger compared with that of the other nucleotides. The effects of DRE orientation, DRE adjacent sequences and the nucleotide “N” in the core DRE (5′-TNGCGTG-3′) sequence on the AhR-regulated CYP1A1 transcription in response to TCDD were studied systematically, and our study laid a good foundation for further investigation into the AhR-dependent transcriptional regulation triggered by dioxin and dioxin-like compounds.

Nucleotide specificity of DNA binding of the aryl hydrocarbon receptor:ARNT complex is unaffected by ligand structure

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates the toxic and biological effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) and a wide variety of structurally diverse ligands through its ability to translocate into the nucleus and bind to a specific DNA recognition site (the dioxin-responsive element [DRE]) adjacent to responsive genes. Although the sequence of the DRE is well defined, several reports suggested that the nucleotide specificity of AhR DNA binding may vary depending on the structure of its bound ligand. Given the potential toxicological significance of this hypothesis, an unbiased DNA-selection-and-PCR-amplification approach was utilized to directly determine whether binding and activation of the AhR by structurally diverse agonists alter its nucleotide specificity of DNA binding. Guinea pig hepatic cytosolic AhR activated in vitro by equipotent concentrations of TCDD, 3-methylcholanthrene, β-naphthoflavone, indirubin, L-kynurenine, or YH439 was incubated with a pool of DNA oligonucleotides containing a 15-base pair variable region consisting of all possible nucleotides. The AhR-bound oligonucleotides isolated by immunoprecipitation were PCR amplified and used in subsequent rounds of selection. Sequence analysis of a total of 196 isolated oligonucleotides revealed that each ligand-activated AhR:ARNT complex only bound to DRE-containing DNA oligonucleotides; no non-DRE-containing DNA oligonucleotides were identified. These results demonstrate that the binding and activation of the AhR by structurally diverse agonists do not appear to alter its nucleotide specificity of DNA binding and suggest that stimulation of gene expression mediated by direct DNA binding of ligand-activated AhR:ARNT complexes is DRE dependent.

Lack of ligand-selective binding of the aryl hydrocarbon receptor to putative DNA binding sites regulating expression of Bax and paraoxonase 1 genes

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that mediates the biological and toxicological effects of structurally diverse chemicals through its ability to bind specific DNA recognition sites (dioxin responsive elements (DREs)), and activate transcription of adjacent genes. While the DRE has a highly conserved consensus sequence, it has been suggested that the nucleotide specificity of AhR DNA binding may be ligand-dependent. The upstream regulatory regions of the murine Bax and human paraoxonase 1 (PON1) genes reportedly contain unique DRE-like sequences that respond to AhRs activated by some ligands but not others. Given the significant implications of this observation to understanding the diversity in AhR responses and that of other ligand-dependent nuclear receptors, a combination of DNA binding, nuclear translocation and gene expression analysis was used to investigate the molecular mechanisms underlying these ligand-selective responses. Although known AhR agonists stimulated AhR nuclear translocation, DRE binding and gene expression, the ligand-selective DRE-like DNA elements identified in the Bax and PON1 upstream regulatory regions failed to bind ligand-activated AhR or confer AhR-responsiveness upon a reporter gene. These results argue against the reported ligand-selectivity of AhR DNA binding and suggest DNA binding by ligand activated AhR involves DRE-containing DNA.

Comparison of TCDD-elicited genome-wide hepatic gene expression in Sprague-Dawley rats and C57BL/6 mice

Although the structure and function of the AhR are conserved, emerging evidence suggests that downstream effects are species-specific. In this study, rat hepatic gene expression data from the DrugMatrix database (National Toxicology Program) were compared to mouse hepatic whole-genome gene expression data following treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). For the DrugMatrix study, male Sprague-Dawley rats were gavaged daily with 20μg/kg TCDD for 1, 3 and 5days, while female C57BL/6 ovariectomized mice were examined 1, 3 and 7days after a single oral gavage of 30μg/kg TCDD. A total of 649 rat and 1386 mouse genes (|fold change|≥1.5, P1(t)≥0.99) were differentially expressed following treatment. HomoloGene identified 11,708 orthologs represented across the rat Affymetrix 230 2.0 GeneChip (12,310 total orthologs), and the mouse 4×44K v.1 Agilent oligonucleotide array (17,578 total orthologs). Comparative analysis found 563 and 922 orthologs differentially expressed in response to TCDD in the rat and mouse, respectively, with 70 responses associated with immune function and lipid metabolism in common to both. Moreover, QRTPCR analysis of Ceacam1, showed divergent expression (induced in rat; repressed in mouse) functionally consistent with TCDD-elicited hepatic steatosis in the mouse but not the rat. Functional analysis identified orthologs involved in nucleotide binding and acetyltransferase activity in rat, while mouse-specific responses were associated with steroid, phospholipid, fatty acid, and carbohydrate metabolism. These results provide further evidence that TCDD elicits species-specific regulation of distinct gene networks, and outlines considerations for future comparisons of publicly available microarray datasets.

Exactly the same but different: promiscuity and diversity in the molecular mechanisms of action of the aryl hydrocarbon (dioxin) receptor

The Ah receptor (AhR) is a ligand-dependent transcription factor that mediates a wide range of biological and toxicological effects that result from exposure to a structurally diverse variety of synthetic and naturally occurring chemicals. Although the overall mechanism of action of the AhR has been extensively studied and involves a classical nuclear receptor mechanism of action (i.e., ligand-dependent nuclear localization, protein heterodimerization, binding of liganded receptor as a protein complex to its specific DNA recognition sequence and activation of gene expression), details of the exact molecular events that result in most AhR-dependent biochemical, physiological, and toxicological effects are generally lacking. Ongoing research efforts continue to describe an ever-expanding list of ligand-, species-, and tissue-specific spectrum of AhR-dependent biological and toxicological effects that seemingly add even more complexity to the mechanism. However, at the same time, these studies are also identifying and characterizing new pathways and molecular mechanisms by which the AhR exerts its actions and plays key modulatory roles in both endogenous developmental and physiological pathways and response to exogenous chemicals. Here we provide an overview of the classical and nonclassical mechanisms that can contribute to the differential sensitivity and diversity in responses observed in humans and other species following ligand-dependent activation of the AhR signal transduction pathway.

Aryl hydrocarbon receptor-mediated induction of Stearoyl-CoA desaturase 1 alters hepatic fatty acid composition in TCDD-elicited steatosis

2,3,7,8-tetrachlorodibenzo-ρ-dioxin (TCDD) induces hepatic dyslipidemia mediated by the aryl hydrocarbon receptor (AhR). Stearoyl-CoA desaturase 1 (Scd1) performs the rate-limiting step in monounsaturated fatty acid (MUFA) synthesis, desaturating 16:0 and 18:0 into 16:1n7 and 18:1n9, respectively. To further examine the role of Scd1 in TCDD-induced hepatotoxicity, comparative studies were performed in Scd1(+/+) and Scd1(-/-) mice treated with 30 μg/kg TCDD. TCDD induced Scd1 activity, protein, and messenger RNA (mRNA) levels approximately twofold. In Scd1(+/+) mice, hepatic effects were marked by increased vacuolization and inflammation and a 3.5-fold increase in serum alanine aminotransferase (ALT) levels. Hepatic triglycerides (TRGs) were induced 3.9-fold and lipid profiling by gas chromatography-mass spectroscopy measured a 1.9-fold increase in fatty acid (FA) levels, consistent with the induction of lipid transport genes. Induction of Scd1 altered FA composition by decreasing saturated fatty acid (SFA) molar ratios 8% and increasing MUFA molar ratios 9%. Furthermore, ChIP-chip analysis revealed AhR enrichment (up to 5.7-fold), and computational analysis identified 16 putative functional dioxin response elements (DREs) within Scd1 genomic loci. Band shift assays confirmed AhR binding with select DREs. In Scd1(-/-) mice, TCDD induced minimal hepatic vacuolization and inflammation, while serum ALT levels remained unchanged. Although Scd1 deficiency attenuated TCDD-induced TRG accumulation, overall FA levels remained unchanged compared with Scd1(+/+) mice. In Scd1(-/-) mice, TCDD induced SFA ratios 8%, reduced MUFA ratios 13%, and induced polyunsaturated fatty acid ratios 5% relative to treated Scd1(+/+) mice. Collectively, these results suggest that AhR regulation of Scd1 not only alters lipid composition but also contributes to the hepatotoxicity of TCDD.

Third-generation Ah receptor-responsive luciferase reporter plasmids: amplification of dioxin-responsive elements dramatically increases CALUX bioassay sensitivity and responsiveness

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD, dioxin) and related dioxin-like chemicals are widespread and persistent environmental contaminants that produce diverse toxic and biological effects through their ability to bind to and activate the Ah receptor (AhR) and AhR-dependent gene expression. The chemically activated luciferase expression (CALUX) system is an AhR-responsive recombinant luciferase reporter gene-based cell bioassay that has been used in combination with chemical extraction and cleanup methods for the relatively rapid and inexpensive detection and relative quantitation of dioxin and dioxin-like chemicals in a wide variety of sample matrices. Although the CALUX bioassay has been validated and used extensively for screening purposes, it has some limitations when screening samples with very low levels of dioxin-like chemicals or when there is only a small amount of sample matrix for analysis. Here, we describe the development of third-generation (G3) CALUX plasmids with increased numbers of dioxin-responsive elements, and stable transfection of these new plasmids into mouse hepatoma (Hepa1c1c7) cells has produced novel amplified G3 CALUX cell bioassays that respond to TCDD with a dramatically increased magnitude of luciferase induction and significantly lower minimal detection limit than existing CALUX-type cell lines. The new G3 CALUX cell lines provide a highly responsive and sensitive bioassay system for the detection and relative quantitation of very low levels of dioxin-like chemicals in sample extracts.

Developing a qPCR method to quantify AhR-PCP-DNA complex for detection of environmental trace-level PCP

Pentachlorophenol (PCP), a widely-used aseptic or biocide, is known as an environmental toxicant involved in endocrine disruption even at a trace level. In order to reliably and efficiently quantify environmental trace-quantity PCP, this study developed a novel PCP detection method using the aryl hydrocarbon receptor (AhR) and fluorescence quantitative PCR (qPCR). DNA probe with AhR binding sites was synthesized by PCR before added into AhR-PCP complex. After AhR-PCP-DNA complex was digested with exonuclease, copy number of DNA probe was determined using fluorescence qPCR. To calculate PCP concentration in samples, a standard curve (PCP concentration versus Ct value) was constructed and the detection range was 10(-13) to 10(-9) M. PCP detection limit was 0.0089 ppt for the AhR-PCP-DNA complex assay and 8.8780 ppm for high performance liquid chromatography, demonstrating that the method developed in this study is more sensitive. These results suggest that AhR-PCP-DNA complex method may be successfully applicable in detection and quantification of environmental trace-level PCP.

Persistent binding of ligands to the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates many of the biological and toxic effects of halogenated aromatic hydrocarbons (HAHs), polycyclic aromatic hydrocarbons (PAHs), and other structurally diverse ligands. While HAHs are several orders of magnitude more potent in producing AhR-dependent biochemical effects than PAHs or other AhR agonists, only the HAHs have been observed to produce AhR-dependent toxicity in vivo. Here we have characterized the dissociation of a prototypical HAH ligand ([(3)H] 2,3,7,8-tetrachlorodibenzo-p-dioxin [TCDD]) and PAH-like ligand ([(3)H] beta-naphthoflavone [betaNF]) from the guinea pig, hamster, mouse, and rat hepatic cytosolic AhR in order to elucidate the relationship between the apparent ligand-binding affinities and the divergent potency of these chemicals. Both compounds dissociated very slowly from the AhR with the amount of specific binding remaining at 96 h ranging from 53% to 70% for [(3)H]TCDD and 26% to 85% for [(3)H] betaNF, depending upon the species examined. The rate of ligand dissociation was unaffected by protein concentration or incubation temperature. Preincubation of cytosol with 2,3,7,8-tetrachlorodibenzofuran, carbaryl, or primaquine, prior to the addition of [(3)H]TCDD, shifted the apparent IC(50) of these compounds as competitive AhR ligands by approximately 10- to 50-fold. Our results support the need for reassessment of previous AhR ligand-binding affinity calculations and competitive binding analysis since these measurements are not carried out at equilibrium binding conditions. Our studies suggest that AhR binding affinity/occupancy has little effect on the observed differences in the persistence of gene expression by HAHs and PAHs.

Ontogenetic development, sexual differentiation, and effects of Aroclor 1254 exposure on expression of the arylhydrocarbon receptor and of the arylhydrocarbon receptor nuclear translocator in the rat hypothalamus

Interaction of polychlorinated biphenyls (PCBs) with the aryl hydrocarbon receptor (AhR)/nuclear translocator (ARNT) system might interfere with the mechanisms controlling the sexual differentiation of the developing hypothalamus. The aim of this study was to evaluate the presence of AhR/ARNT in brain cells and the developmental profile of their expression in the hypothalamus of male and female rats during the perinatal period. Brain accumulation of the main PCB congeners after prenatal exposure to Aroclor 1254 and its influence on hypothalamic expression of AhR/ARNT was also assessed. The results show that: (a) AhR and ARNT are expressed both in neurons and in glia; (b) AhR expression progressively increases in the developing hypothalamus particularly in males, while ARNT is relatively constant in both sexes; (c) the prenatal administration of Aroclor to dams produces a differential accumulation of PCBs, depending on the chlorine atom number, and stimulates AhR expression only in the male hypothalamus. In conclusion, the developing male hypothalamus might be more sensitive to disrupting potential of PCBs.

Aryl hydrocarbon receptors: diversity and evolution

Animals have evolved inducible enzymatic defenses to facilitate the biotransformation and elimination of toxic compounds encountered in the environment. The sensory component of this system consists of soluble receptors that regulate the expression of certain isoforms of cytochrome P450, other enzymes, and transporters in response to environmental chemicals. These receptors include several members of the steroid/nuclear receptor superfamily as well as the aryl hydrocarbon receptor (AHR), a member of the bHLH-PAS gene superfamily. In addition to its adaptive functions, the AHR serves poorly understood physiological roles; interference with those roles by dioxins and related chemicals causes toxicity. One approach to understanding the physiological significance of the AHR is to characterize its structure, function, and regulation in diverse species, including mammals, birds, fish, and invertebrates. These animal groups include model species with unique features that can be exploited to broaden our understanding of AHR function. Studies carried out in diverse species also provide phylogenetic information that allows inferences about the evolutionary history of the AHR. This review summarizes the current understanding of AHR diversity among animal species and the evolution of the AHR signaling pathway, as inferred from molecular studies in vertebrate and invertebrate animals. The AHR gene has undergone duplication and diversification in vertebrate animals, resulting in at least three members of an AHR gene family: AHR1, AHR2, and AHR repressor. The inability of invertebrate AHR homologs to bind dioxins and related chemicals, along with other evidence, suggests that the adaptive role of the AHR as a regulator of xenobiotic metabolizing enzymes may have been a vertebrate innovation. The physiological functions of the AHR during development appear to be ancestral to the adaptive functions. Sensitivity to the developmental toxicity of dioxins and related chemicals may have had its origin in the evolution of dioxin-binding capacity of the AHR in the vertebrate lineage.

The aryl hydrocarbon receptor: a comparative perspective

The aryl hydrocarbon receptor (Ah receptor or AHR) is a ligand-activated transcription factor involved in the regulation of several genes, including those for xenobiotic-metabolizing enzymes such as cytochrome P450 1A and 1B forms. Ligands for the AHR include a variety of aromatic hydrocarbons, including the chlorinated dioxins and related halogenated aromatic hydrocarbons whose toxicity occurs through activation of the AHR. The AHR and its dimerization partner ARNT are members of the emerging bHLH-PAS family of transcriptional regulatory proteins. In this review, our current understanding of the AHR signal transduction pathway in non-mammalian and other non-traditional species is summarized, with an emphasis on similarities and differences in comparison to the AHR pathway in rodents and humans. Evidence and prospects for the presence of a functional AHR in early vertebrates and invertebrates are also examined. An overview of the bHLH-PAS family is presented in relation to the diversity of bHLH-PAS proteins and the functional and evolutionary relationships of the AHR and ARNT to the other members of this family. Finally, some of the most promising directions for future research on the comparative biochemistry and molecular biology of the AHR and ARNT are discussed.

Brevetoxin-6 (PbTx-6), a nonaromatic marine neurotoxin, is a ligand of the aryl hydrocarbon receptor

Brevetoxins (PbTx) are a family of marine polyether toxins that exert their toxic action by activating voltage-sensitive sodium channels. Two forms of brevetoxin, PbTx-2 and -3, induce hepatic cytochrome P4501A1, measured as ethoxyresorufin O-deethylase (EROD) activity, in redfish and striped bass. P4501A1 induction is transcriptionally regulated through the binding of a ligand, typically a planar aromatic compound, to the aryl hydrocarbon receptor (AhR) and subsequent complex formation with the dioxin response element (DRE), an upstream regulatory region of the CYP1A1 gene. To determine if PbTx, a nonaromatic compound, induced EROD by this mechanism, two sets of experiments were performed. Initially, saturation binding assays with PbTx-2, -3, and -6 were carried out to determine if PbTx-2, -3, or -6 was an AhR ligand. Results showed that PbTx-6 inhibited specific binding of dioxin to the AhR, whereas PbTx-2 and -3 had no effect. Subsequently, gel retardation assays showed that PbTx-6 caused a concentration-dependent increase in AhR-DRE complex formation. The most abundant and neurotoxic forms of brevetoxin, PbTx-2 and -3, did not appear to be involved in this process. However, PbTx-6, the epoxide which is a likely biotransformation product, is at least one of the forms of PbTx involved in EROD induction.

Overlapping but unique DNA binding specificities of the Ah receptor and constitutive dioxin-responsive element binding proteins from human keratinocytes

To understand the relationships between the protein architecture assembled on dioxin-responsive elements (DRE) and transcriptional regulation by dioxin in human keratinocytes, the nuclear DRE-binding proteins from human keratinocytes were identified and characterized. In addition to the aryl hydrocarbon receptor (AHR) complex inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), nuclear extracts from 3 human keratinocyte cell lines also contained one or more proteins that bound specifically to the DRE but whose levels were unaffected by TCDD or by anti-AHR antibody pretreatment. Alteration of a conserved T, within the core DRE sequence needed for transcriptional activation by the AHR complex, did not affect AHR binding but severely affected the ability of the constitutive proteins to bind. These data suggest that the nonidentical interplay of the AHR and constitutive DRE-binding proteins on the DRE is important in the regulation of genes whose expression is controlled by DRE.

Low hepatic 7-ethoxyresorufin-O-deethylase (EROD) activity and minor alterations in retinoid and thyroid hormone levels in flounder (Platichthys flesus) exposed to the polychlorinated biphenyl (PCB) mixture, Clophen A50

The effect of the polychlorinated biphenyl (PCB) mixture Clophen A50 on hepatic cytochrome P4501A1 dependent EROD (7-ethoxyresorufin-O-deethylase) activity, plasma thyroid hormone levels and plasma, kidney and liver retinoid concentrations of the euryhaline flatfish flounder (Platichthys flesus) was determined 2 and 10 days after i.p. (intraperitoneal) injection with 20, 100 and 500 mg Clophen A50/kg body weight. No effect of Clophen A50 on total cytochrome P450 content in flounder liver was observed at both time points. A six-fold, dose-dependent, significant increase in EROD activity was found at exposure day 10 in flounder receiving 100 or 500 mg Clophen A50/kg body weight. Plasma retinol concentrations were not altered at both time points after Clophen A50 administration, whereas renal retinol levels showed a minor dose-related increase at day 2 and day 10 of exposure. Significant alterations in hepatic retinoid concentrations were observed, which were not dependent on the dose of PCB administered. In addition Clophen A50 administration did not result in a dose-related alteration of total T4 concentrations in plasma. Total T3 concentrations in plasma were only significantly increased at day 2 after exposure, whereas free T4 concentrations were increased at both time points after Clophen A50 administration. These data indicate that with regard to the parameters investigated and in contrast to other fish species studied, the flounder is not a sensitive species to PCB exposure.

Species-specific antagonism of Ah receptor action by 2,2',5,5'-tetrachloro- and 2,2',3,3'4,4'-hexachlorobiphenyl

Using recombinant cell lines showing Ah receptor-controlled expression of a luciferase reporter gene, the interaction of di-ortho-substitute polychlorinated biphenyls (PCBs) with Ah receptor agonists was studied. In the recombinant Hepa1c1c7 mouse hepatoma (H1L1.1c7) cells strong antagonistic interaction of 2,2',5,5'-tetrachlorobiphenyl (PCB52) with luciferase expression induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or 3,3',4,4'-tetrachlorobiphenyl (PCB77) was observed, and similarly, between 2,2',3,3',4,4'-hexachlorobiphenyl (PCB128) and PCB77. Accordingly, PCB52 was found to inhibit ethoxyresorufin-O-deethylase (EROD) induction by PCB77 in wild-type Hepa1c1c7 cells. In contrast, the antagonistic effect of PCB52 on TCDD-induced luciferase expression was only minor in recombinant guinea pig GPC16 colon adenocarcinoma (G16L1.1c8) and human HepG2 hepatoma (HG2L1.1c3) cells, and intermediate in recombinant H4IIE rat hepatoma (H4L1.1c4) cells. Gel retardation studies using a 32 P-labelled dioxin responsive element (DRE)-containing oligonucleotide, and ligand binding studies using [3H]TCDD, demonstrated that the species-specific antagonistic activity of PCB52 on Ah receptor-controlled luciferase expression is due to inhibition of Ah receptor ligand and DNA binding. We conclude, that Ah-mediated luciferase expression provides a useful tool to study the species specificity of Ah receptor (ant)agonists.