The AH receptor of the most dioxin-sensitive species, guinea pig, is highly homologous to the human AH receptor

Indirubin mediates adverse intestinal reactions in guinea pigs by downregulating the expression of AchE through AhR

Indirubin is the main component of the traditional Chinese medicine Indigo naturalis (IN), a potent agonist of aryl hydrocarbon receptors (AhRs). In China, IN is used to treat psoriasis and ulcerative colitis, and indirubin is used for the treatment of chronic myelogenous leukaemia. However, IN and indirubin have adverse reactions, such as abdominal pain, diarrhoea, and intussusception, and their specific mechanism is unclear.The purpose of our research was to determine the specific mechanism underlying the adverse effects of IN and indirubin. By tracking the modifications in guinea pigs after the intragastric administration of indirubin for 28 days.The results demonstrate that indirubin could accelerate bowel movements and decrease intestinal acetylcholinesterase (AchE) expression. Experiments with NCM460 cells revealed that indirubin significantly reduced the expression of AchE, and the AchE levels were increased after the silencing of AhR and re-exposure to indirubin.This study showed that the inhibition of AchE expression by indirubin plays a key role in the occurrence of adverse reactions to indirubin and that the underlying mechanism is related to AhR-mediated AchE downregulation.

Tumor-Suppressive Functions of the Aryl Hydrocarbon Receptor (AhR) and AhR as a Therapeutic Target in Cancer

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor involved in regulating a wide range of biological responses. A diverse array of xenobiotics and endogenous small molecules bind to the receptor and drive unique phenotypic responses. Due in part to its role in mediating toxic responses to environmental pollutants, AhR activation has not been traditionally viewed as a viable therapeutic approach. Nonetheless, the expression and activation of AhR can inhibit the proliferation, migration, and survival of cancer cells, and many clinically approved drugs transcriptionally activate AhR. Identification of novel select modulators of AhR-regulated transcription that promote tumor suppression is an active area of investigation. The development of AhR-targeted anticancer agents requires a thorough understanding of the molecular mechanisms driving tumor suppression. Here, we summarized the tumor-suppressive mechanisms regulated by AhR with an emphasis on the endogenous functions of the receptor in opposing carcinogenesis. In multiple different cancer models, the deletion of AhR promotes increased tumorigenesis, but a precise understanding of the molecular cues and the genetic targets of AhR involved in this process is lacking. The intent of this review was to synthesize the evidence supporting AhR-dependent tumor suppression and distill insights for development of AhR-targeted cancer therapeutics.

Adverse outcome pathway from activation of the AhR to breast cancer-related death

Adverse outcome pathways (AOPs) are formalized and structured linear concepts that connect one molecular initiating event (MIE) to an adverse outcome (AO) via different key events (KE) through key event relationships (KER). They are mainly used in eco-toxicology toxicology, and regulatory health issues. AOPs must respond to specific guidelines from the Organization for Economic Co-operation and Development (OECD) to weight the evidence between each KE. Breast cancer is the deadliest cancer in women with a poor prognosis in case of metastatic breast cancer. The role of the environments in the formation of metastasis has been suggested. We hypothesized that activation of the AhR (MIE), a xenobiotic receptor, could lead to breast cancer related death (AO), through different KEs, constituting a new AOP. An artificial intelligence tool (AOP-helpfinder), which screens the available literature, was used to collect all existing scientific abstracts to build a novel AOP, using a list of key words. Four hundred and seven abstracts were found containing at least a word from our MIE list and either one word from our AO or KE list. A manual curation retained 113 pertinent articles, which were also screened using PubTator. From these analyses, an AOP was created linking the activation of the AhR to breast cancer related death through decreased apoptosis, inflammation, endothelial cell migration, angiogenesis, and invasion. These KEs promote an increased tumor growth, angiogenesis and migration which leads to breast cancer metastasis and breast cancer related death. The evidence of the proposed AOP was weighted using the tailored Bradford Hill criteria and the OECD guidelines. The confidence in our AOP was considered strong. An in vitro validation must be carried out, but our review proposes a strong relationship between AhR activation and breast cancer-related death with an innovative use of an artificial intelligence literature search.

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

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

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

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

Gene-environment interactions in male reproductive health: special reference to the aryl hydrocarbon receptor signaling pathway

Over the last few decades, there have been numerous reports of adverse effects on the reproductive health of wildlife and laboratory animals caused by exposure to endocrine disrupting chemicals (EDCs). The increasing trends in human male reproductive disorders and the mounting evidence for causative environmental factors have therefore sparked growing interest in the health threat posed to humans by EDCs, which are substances in our food, environment and consumer items that interfere with hormone action, biosynthesis or metabolism, resulting in disrupted tissue homeostasis or reproductive function. The mechanisms of EDCs involve a wide array of actions and pathways. Examples include the estrogenic, androgenic, thyroid and retinoid pathways, in which the EDCs may act directly as agonists or antagonists, or indirectly via other nuclear receptors. Dioxins and dioxin-like EDCs exert their biological and toxicological actions through activation of the aryl hydrocarbon-receptor, which besides inducing transcription of detoxifying enzymes also regulates transcriptional activity of other nuclear receptors. There is increasing evidence that genetic predispositions may modify the susceptibility to adverse effects of toxic chemicals. In this review, potential consequences of hereditary predisposition and EDCs are discussed, with a special focus on the currently available publications on interactions between dioxin and androgen signaling.

Correlation between TCDD acute toxicity and aryl hydrocarbon receptor structure for different mammals

The 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) toxicity has large species differences, and TCDD exerts its toxicity by binding into aryl hydrocarbon receptor (AHR). In this study, we applied bioinformatics approaches to quantitatively analyze the correlation between TCDD acute toxicity and AHRs. Seven mammalian AHRs were chosen as target receptors. Low conserved functional domains of AHRs were identified and quantitatively characterized. Linear regression was applied to determine the relationships of different mammalian AHRs and TCDD LD(50) values. The results indicated that ligand binding domain and glutamine-rich domain of mammalian AHRs showed a low degree of conservation. Based on previous literatures, the number of glutamine residues (NOQ) and binding free energy with TCDD were applied to quantitatively represent the differences of glutamine-rich domain and ligand binding domain, respectively. Then, regression equations between studied mammalian AHR structures and TCDD LD(50) were constructed, and high linear correlation was found (R(2)=0.986). This study indicated that mammalian differences of TCDD acute toxicity might be partly determined by the differences of glutamine-rich domain and ligand binding domain of AHR, which provides a potential insight to analyze the species differences of TCDD toxicity.

A critique of the European Commission document, "State of the Art Assessment of Endocrine Disrupters"

In this commentary, we critique a recently finalized document titled "State of the Art Assessment of Endocrine Disrupters" (SOA Assessment). The SOA Assessment was commissioned by the European Union Directorate-General for the Environment to provide a basis for developing scientific criteria for identifying endocrine disruptors and reviewing and possibly revising the European Community Strategy on Endocrine Disrupters. In our view, the SOA Assessment takes an anecdotal approach rather than attempting a comprehensive assessment of the state of the art or synthesis of current knowledge. To do the latter, the document would have had to (i) distinguish between apparent associations of outcomes with exposure and the inference of an endocrine-disruption (ED) basis for those outcomes; (ii) constitute a complete and unbiased survey of new literature since 2002 (when the WHO/IPCS document, "Global Assessment of the State-of-the-Science of Endocrine Disruptors" was published); (iii) consider strengths and weaknesses and issues in interpretation of the cited literature; (iv) follow a weight-of-evidence methodology to evaluate evidence of ED; (v) document the evidence for its conclusions or the reasoning behind them; and (vi) present the evidence for or reasoning behind why conclusions that differ from those drawn in the 2002 WHO/IPCS document need to be changed. In its present form, the SOA Assessment fails to provide a balanced and critical assessment or synthesis of literature relevant to ED. We urge further evidence-based evaluations to develop the needed scientific basis to support future policy decisions.

Hormones and endocrine-disrupting chemicals: low-dose effects and nonmonotonic dose responses

For decades, studies of endocrine-disrupting chemicals (EDCs) have challenged traditional concepts in toxicology, in particular the dogma of "the dose makes the poison," because EDCs can have effects at low doses that are not predicted by effects at higher doses. Here, we review two major concepts in EDC studies: low dose and nonmonotonicity. Low-dose effects were defined by the National Toxicology Program as those that occur in the range of human exposures or effects observed at doses below those used for traditional toxicological studies. We review the mechanistic data for low-dose effects and use a weight-of-evidence approach to analyze five examples from the EDC literature. Additionally, we explore nonmonotonic dose-response curves, defined as a nonlinear relationship between dose and effect where the slope of the curve changes sign somewhere within the range of doses examined. We provide a detailed discussion of the mechanisms responsible for generating these phenomena, plus hundreds of examples from the cell culture, animal, and epidemiology literature. We illustrate that nonmonotonic responses and low-dose effects are remarkably common in studies of natural hormones and EDCs. Whether low doses of EDCs influence certain human disorders is no longer conjecture, because epidemiological studies show that environmental exposures to EDCs are associated with human diseases and disabilities. We conclude that when nonmonotonic dose-response curves occur, the effects of low doses cannot be predicted by the effects observed at high doses. Thus, fundamental changes in chemical testing and safety determination are needed to protect human health.

Is the fear of dioxin cancer more harmful than dioxin?

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a proven animal carcinogen. Occupational cohorts with the highest exposures imply that there is a small risk of all cancers combined, but it is difficult to pinpoint the confounding effect of the main chemicals. Studies after major accidents do not unequivocally confirm this risk. The risks to populations at the current dioxin levels seem trivial if present at all. There is increasing evidence that the aryl hydrocarbon receptor (AhR), i.e. the so called "dioxin receptor", is a physiological transcription factor exerting important functions in the body. Consequently a certain level of AhR activation may be beneficial rather than harmful. This challenges the wisdom of excessive regulation of dioxin levels in certain foods and nutrients. This could pose indirect nutritional risks, in fact being more harmful than even the worst case predictions of the putative cancer risks attributable to dioxins.

Transgenic Humanized AHR Mouse Reveals Differences between Human and Mouse AHR Ligand Selectivity

The Aryl-hydrocarbon receptor (AHR) is a ligand activated transcription factor involved in xenobiotic metabolism. Most of the toxic effects of halogenated and non-halogenated polycyclic aromatic hydrocarbons (HAHs and PAHs respectively) are mediated by the AHR. For the AHR, a number of intra and interspecies differences exist in terms of responsiveness to the prototypical AHR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Interspecies differences in AHR ligand binding affinity has been shown to be linked to contrasting TCDD tolerance between species and among inbred strains of mice expressing different AHR alleles. Compared to the human AHR (hAHR), the mouse AHR(b) (mAHR(b)) has a ~10 fold higher affinity for typical AHR ligands. Using a transgenic humanized mouse model that expresses hAHR protein specifically in the liver, we have discovered that for certain ligands, such as indirubin, the hAHR exhibits higher relative ligand binding affinity and responsiveness compared to the mAHR(b). These findings may potentially influence the ongoing search for endogenous hAHR ligands and expand our understanding of the unique physiological role of the hAHR.

Ligand selectivity and gene regulation by the human aryl hydrocarbon receptor in transgenic mice

The aryl hydrocarbon receptor (AHR) is a ligand-inducible transcription factor that displays interspecies differences with the human and mouse AHR C-terminal region sequences sharing only 58% amino acid sequence identity. Compared with the mouse AHR (mAHR), the human AHR (hAHR) displays approximately 10-fold lower relative affinity for prototypical AHR ligands such as 2,3,7,8-tetrachlorodibenzo-p-dioxin, which has been attributed to the amino acid residue valine 381 (alanine 375 in the mAHR) in the ligand binding domain of the hAHR. We investigated whether the 10-fold difference in ligand-binding affinity between the mAHR and hAHR would be observed with a diverse range of AHR ligands. To test this hypothesis, ligand binding assays were performed using the photo-affinity ligand 2-azido-3-[(125)I]iodo-7,8-dibromodibenzo-p-dioxin and liver cytosol isolated from hepatocyte-specific transgenic hAHR mice and C57BL/6J mice. It is noteworthy that competitive ligand-binding assays revealed that, compared with the mAHR, the hAHR has a higher relative affinity for certain compounds, including indirubin [(2Z)-2,3-biindole-2,3 (1'H,1'H)-dione and quercetin (2-(3,4dihydroxyphenyl)-3,5,7-trihydroxy-4H-chromen-4-one]. Electrophoretic mobility shift assays revealed that indirubin was more efficient at transforming the hAHR compared with the mAHR. Indirubin was also a more potent inducer of Cyp1a1 expression in transgenic hAHR mouse hepatocytes compared with C57BL/6J mouse hepatocytes. These observations suggest that indirubin is a potent hAHR ligand that is able to selectively bind to and activate the hAHR. These discoveries imply that there may be a significant degree of structural divergence between mAHR and hAHR ligands and highlights the importance of the hAHR transgenic mouse as a model to study the hAHR in vivo.

Aryl hydrocarbon receptor (AhR)-mediated reporter gene expression systems in transgenic tobacco plants

In mammals, the aryl hydrocarbon receptor (AhR) mediates expression of certain genes, including CYP1A1, in response to exposure to dioxins and related compounds. We have constructed a mouse AhR-mediated gene expression systems for a beta-glucuronidase (GUS) reporter gene consisting of an AhR, an AhR nuclear translocator (Arnt), and a xenobiotic response element (XRE)-driven promoter in transgenic tobacco plants. On treatment with the AhR ligands 3-methylcholanthrene (MC), beta-naphthoflavone (betaNF), and indigo, the transgenic tobacco plants exhibited enhanced GUS activity, presumably by inducible expression of the reporter gene. The recombinant AhR (AhRV), with the activation domain replaced by that of the Herpes simplex virus protein VP16, induced GUS activity much more than the wild-type AhR in the transgenic tobacco plants. Plants carrying AhRV expressed the GUS reporter gene in a dose- and time-dependent manner when treated with MC; GUS activity was detected at 5 nM MC on solid medium and at 12 h after soaking in 25 microM MC. Histochemical GUS staining showed that this system was active mainly in leaf and stem. These results suggest that the AhR-mediated reporter gene expression system has potential for the bioassay of dioxins in the environment and as a novel gene expression system in plants.

Aryl hydrocarbon receptor splice variants in the dioxin-resistant rat: tissue expression and transactivational activity

The AHR locus encodes the aryl hydrocarbon receptor (AHR), a transcriptional regulator of multiple drug-metabolizing enzymes and mediator of toxicity of dioxin-like chemicals. The Han/Wistar (Kuopio) rat strain (H/W) is remarkably resistant to lethal effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) because of a point mutation in the exon/intron 10 boundary in AHR genomic structure that leads to use of 3 alternative cryptic splice sites, potentially creating 3 alternative transcripts and 2 protein products. The deletion variant (DV), which lacks 43 amino acids in the transactivation domain, has the highest intrinsic transactivation activity in vitro; amino acids 766 to 783 suppress transactivation function. However, DV expression levels in H/W rats in vivo are low in liver, lung, thymus, kidney, and testis; insertion variant mRNAs (IVs) are the dominant mRNA forms in H/W rats in which wild-type AHR mRNA is undetectable. In dioxin-sensitive rat strains and lines that are homozygous for wild-type AHR alleles, wild-type AHR mRNA is the most abundant transcript but some IV transcripts are detectable. TCDD treatment in vivo increases transcript levels for both the DV and IVs in H/W rats and increases wild-type transcript levels in dioxin-sensitive rats but does not alter which transcript forms are expressed. In silico modeling indicates that the DV mRNA has lost considerable secondary structure, whereas at the protein level, the transactivation domain of the IV in the dioxin-resistant H/W rat has greater alpha-helical content and a more hydrophobic terminus than wild-type AHR, which may produce a protein conformation that is less amenable to interaction with other regulatory proteins.

The comparative toxicogenomics database: a cross-species resource for building chemical-gene interaction networks

Chemicals in the environment play a critical role in the etiology of many human diseases. Despite their prevalence, the molecular mechanisms of action and the effects of chemicals on susceptibility to disease are not well understood. To promote understanding of these mechanisms, the Comparative Toxicogenomics Database (CTD; http://ctd.mdibl.org/) presents scientifically reviewed and curated information on chemicals, relevant genes and proteins, and their interactions in vertebrates and invertebrates. CTD integrates sequence, reference, species, microarray, and general toxicology information to provide a unique centralized resource for toxicogenomic research. The database also provides visualization capabilities that enable cross-species comparisons of gene and protein sequences. These comparisons will facilitate understanding of structure-function correlations and the genetic basis of susceptibility. Manual curation and integration of cross-species chemical-gene and chemical-protein interactions from the literature are now underway. These data will provide information for building complex interaction networks. New CTD features include (1) cross-species gene, rather than sequence, query and visualization capabilities; (2) integrated cross-links to microarray data from chemicals, genes, and sequences in CTD; (3) a reference set related to chemical-gene and protein interactions identified by an information retrieval system; and (4) a "Chemicals in the News" initiative that provides links from CTD chemicals to environmental health articles from the popular press. Here we describe these new features and our novel cross-species curation of chemical-gene and chemical-protein interactions.

Human response to dioxin: aryl hydrocarbon receptor (AhR) molecular structure, function, and dose-response data for enzyme induction indicate an impaired human AhR

The aryl hydrocarbon receptor (AhR) mediates nearly all studied adverse effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and many related compounds. Binding of TCDD or related ligands to AhR is the key initiating event in downstream biochemical responses. The binding affinity of AhR for TCDD is specific to species and strain, and studies of human AhR demonstrate binding affinities approximately an order of magnitude or more lower than those observed in the most sensitive laboratory strains and species. Molecular genetic studies confirmed that human AhR shares key mutations with the DBA mouse strain that result in an "impaired" AhR (with respect to TCDD binding and responsiveness). Despite a number of polymorphisms in human AhR, the key "DBA-type" mutations appear to be a constant feature of the human AhR, and no polymorphisms have been identified that compensate for the impaired binding function conferred by these mutations. Consistent with the impaired binding status of the human AhR, human cells have consistently required approximately 10-fold higher concentrations of TCDD in vitro than rodent cells to respond with enzyme induction. Recent studies of in vivo enzyme induction-related endpoints in human populations with moderately and highly increased TCDD body burdens detected no relationship between these endpoints and TCDD body burdens at body-burden levels up to 250 ng TEQ/kg body weight, or approximately 25 times above the upper range of current general population background body burdens, while marked elevations in enzyme activity were observed in persons with body burdens above 750 ng TEQ/kg. In contrast, the more sensitive laboratory rodent strains and species exposed to TCDD exhibit significant enzyme induction at body burdens below 50 ng/kg. These interspecies data on the most sensitive and best understood response to binding of TCDD and related compounds to the AhR are consistent with the binding affinity and molecular structure data and support the hypothesis that the human AhR is less functional than the AhR of the more sensitive laboratory animals at a molecular level. Quantitative risk assessments involving interspecies extrapolation from sensitive laboratory species and strains should take these fundamental differences into account when margins of exposure and safety factors are considered.

Molecular cloning of the guinea pig CYP1A2 gene 5'-flanking region: identification of functional aromatic hydrocarbon response element and characterization of CYP1A2 expression in GPC16 cells

Aromatic hydrocarbon (AH) effects are mediated by binding of the AH receptor and its heterodimeric partner aromatic hydrocarbon nuclear translocator to specific response elements on DNA (AHREs). CYP1A2 expression is induced by AHs, yet AHREs have been identified in CYP1A2 genes of only two species and their functional role assessed only in the human gene. There have been few analyses of CYP1A2 gene regulation in nonhepatic cells. To gain further insight into CYP1A2 regulation, we cloned the initial 1.2 kilobases (kb) of the guinea pig CYP1A2 gene 5'-flanking region and characterized CYP1A2 expression in guinea pig colon adenocarcinoma cells (GPC16). Two putative AHRE sites were identified (-830 and -575 bp). They are considerably more proximal than the functional AHRE found in the human CYP1A2 gene (-2.5 kb). GPC16 cells expressed CYP1A2 after treatment with AH, enabling characterization of the putative AHRE sites in a homologous cell line. Double-stranded oligonucleotide probes, corresponding to each putative AHRE, bound in an AH-induced and specific manner to nuclear proteins prepared from GPC16 cells. In transfection analyses, only the distal site mediated AH-induced reporter gene activity. Mutation of this site suppressed AH-induced activity, supporting the concept that it is involved in AH-mediated induction of CYP1A2. However, the low level of AH-induction by the wild type suggests that other factors modulate AH-response by the CYP1A2 gene.

A new southwestern chemistry-based ELISA for detection of aryl hydrocarbon receptor transformation: application to the screening of its receptor agonists and antagonists

Halogenated aromatic hydrocarbons (HAHs), such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), produce a wide variety of biological and toxic effects mainly through the aryl hydrocarbon receptor (AhR)-dependent mechanism. After the binding of HAHs, the AhR subsequently transforms its form in order to interact with a specific DNA sequence, the dioxin responsive element (DRE). Thus, detection of the transformed AhR is a target for estimation of the biological and toxic potency of ligands. In this study, we have developed a simple method for quantitative assessment of the transformation state of AhR based on an enzyme-linked immunosorbent assay (ELISA) combined with southwestern chemistry technique (SW-ELISA) that detects the complex of transformed AhR:fluorescein isothiocyanate (FITC)-labeled DRE probe. SW-ELISA has shown the response to HAHs including TCDD and other known agonists in a dose-dependent manner. In the case of TCDD, SW-ELISA has revealed a minimum detection limit (MDL) of 2 pM (0.026 pg/assay), a median effective concentration (EC(50)) value of 0.125 nM (1.6 pg/assay), and a maximum response at 10 nM (129 pg/assay). Furthermore, SW-ELISA provides the confirmation that flavonoids, the potent antagonists for AhR as reported previously, show the inhibitory effects on TCDD-induced AhR transformation. These results indicate that SW-ELISA is a new and straightforward method for the detection of AhR transformation and will be useful in screening of agonists or antagonists for AhR.

Construction of reporter yeasts for mouse aryl hydrocarbon receptor ligand activity

Aryl hydrocarbons such as dioxins, polychlorinated biphenyls and polyaromatic hydrocarbons bind to the cellular aryl hydrocarbon receptor (AhR) in the initial step of their metabolism. The activation of intracellular signaling subsequent to the AhR binding is highly correlated with the toxicity and carcinogenicity of these chemicals. We produced Saccharomyces cerevisiae coexpressing mouse AhR and aryl hydrocarbon receptor nuclear translocator (Arnt) protein in accordance with Miller III's method for constructing yeasts with human Ahr and Arnt [Toxicol. Appl. Pharmacol. 160 (1998) 297]. Ligand treatment induced a dose-dependent increase in beta-galactosidase activity from a reporter plasmid in the yeast. Then, we compared activities of several ligands in yeast having the mouse Ahr/Arnt genes with those in yeast having the human genes, both of which have the same genetic background. There was no significant difference in the EC50 values of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), benzo[a]pyrene, 3-methylcholanthrene and beta-naphthoflavone between the mouse and human genes. However, indirubin, which was recently found in human urine as a potent AhR ligand [J. Biol. Chem. 276 (2001) 31475], had a 35-140 times higher EC50 value in the yeast with human genes than mouse genes. This difference might reflect species-specificity between mouse and human AhR/Arnt.

Distinct response to dioxin in an arylhydrocarbon receptor (AHR)-humanized mouse

There are large inter- and intraspecies differences in susceptibility to dioxin-induced toxicities. A critical question in risk assessment of dioxin and related compounds is whether humans are sensitive or resistant to their toxicities. The diverse responses of mammals to dioxin are strongly influenced by functional polymorphisms of the arylhydrocarbon receptor (AHR). To characterize responses mediated by the human AHR (hAHR), we generated a mouse possessing hAHR instead of mouse AHR. Responses of these mice to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 3-methylcholanthrene were compared with the responses of naturally sensitive (C57BL6J) and resistant (DBA2) mice. Mice homozygous for hAHR exhibited weaker induction of AHR target genes such as cyp1a1 and cyp1a2 than did C57BL6J (Ahr(b-1/b-1)) mice. DBA2 (Ahr(d/d)) mice were less responsive to induction of cyp genes than C57BL6J mice. hAHR and DBA2 AHR exhibit similar ligand-binding affinities and homozygous hAHR and Ahr(d/d) mice displayed comparable induction of AHR target genes by 3-methylcholanthrene. However, when TCDD was administered, a greatly diminished response was observed in homozygous hAHR mice compared with Ahr(d/d) mice, indicating that hAHR expressed in mice is functionally less responsive to TCDD than DBA2 AHR. After maternal exposure to TCDD, homozygous hAHR fetuses developed embryonic hydronephrosis, but not cleft palate, whereas fetuses possessing Ahr(b-1) or Ahr(d) developed both anomalies. These results suggest that hAHR may define the specificity of the responses to various AHR ligands. Thus, the hAHR knock-in mouse is a humanized model mouse that may better predict the biological effects of bioaccumulative environmental toxicants like TCDD in humans.

Species-specific transcriptional activity of synthetic flavonoids in guinea pig and mouse cells as a result of differential activation of the aryl hydrocarbon receptor to interact with dioxin-responsive elements

To investigate possible species-specificity of aryl hydrocarbon receptor (AhR)-mediated signal transduction pathways, activities of 2,3,7,8-tetrochlorodibenzo-p-dioxin (TCDD) and six synthetic flavonoids were evaluated in mouse hepatoma and guinea pig adenocarcinoma cells transfected with an AhR-responsive luciferase reporter. Rank order potency in these two cell lines was similar for the ability of these flavonoids to antagonize TCDD-induced reporter gene expression. However, in the presence of flavone alone, a species-specific difference in agonist activity was observed. In guinea pig cells, several flavonoids demonstrated agonist activity up to 50% of the maximum TCDD response. In mouse cells, however, no significant agonist activity was observed at the same concentrations based on luciferase enzyme activity, protein expression, and mRNA analysis. Moreover, competitive ligand-binding assays, using [(3)H]TCDD in cytosolic fractions, demonstrated that 3'-methoxy-4'-nitroflavone had a similar IC(50) in both recombinant cell lines, suggesting that the flavone has similar binding affinity to receptors from both species. However, electrophoretic mobility shift assay using the cytosolic fractions demonstrated that this flavone elicited binding to the DRE by guinea pig but not mouse AhR complex. The dependence of the AhR in this differential interaction was further demonstrated using in vitro synthesized guinea pig and mouse Ah receptors and mouse Arnt. Together, these data suggest that the differential agonist/antagonist activity of these flavone derivatives is caused by the efficacy of these flavonoids in eliciting an AhR conformation that recognizes regulatory response elements in a species-specific manner.

Mutational analysis of the mouse aryl hydrocarbon receptor tyrosine residues necessary for recognition of dioxin response elements

Tyrosine phosphorylation of the aryl hydrocarbon receptor (AhR), a member of the basic helix-loop-helix/PER-ARNT-SIM transcription factor family, has been shown to regulate its dioxin response elements (DRE) binding ability, although no specific residues have been directly demonstrated to be phosphorylated. Of the 23 tyrosines in the mouse AhR, 19 are conserved across all mammalian species sequenced thus far. The studies presented here were conducted to examine tyrosine residue(s) that are both likely candidates of phosphorylation and necessary for DNA binding and/or transcriptional activity of the AhR. Two-dimensional gel electrophoresis of phosphatase-treated AhR indicated that the receptor is phosphorylated on serine/threonine and tyrosine residues. Computational analysis predicted several highly conserved tyrosine residues to be phosphorylated. Both the N terminus (amino acids 1-399) and the C terminus (amino acids 399-805) of the mouse receptor synthesized in vitro using a rabbit reticulocyte lysate system are tyrosine phosphorylated as detected by antiphosphotyrosine antibodies. Furthermore, the N-terminal AhR bound DRE in a ligand-dependent manner similar to that by the full-length receptor, suggesting that phosphorylated tyrosines involved in DNA binding are likely located in the region between residues 1 and 399. Mouse AhR tyrosine (Y) residues were evaluated by phenylalanine (F) mutational analysis for both DNA binding (electrophoretic mobility shift assays; EMSAs) and ability to induce a DRE-driven reporter gene in transiently transfected AhR-deficient cells. Of the 12 tyrosine residues in the N-terminal AhR, only a tyrosine 9 mutant (AhRY9F) significantly decreased DRE binding as determined by EMSA. Similarly, only the AhRY9F mutant decreased the DRE-driven luciferase expression in AhR-deficient cells. Overall, these data strongly suggest that the putative posttranslational modification at, or mediated by, tyrosine 9, and not any other individual mouse AhR tyrosine residue, is necessary for AhR DRE binding and transcriptional activity.

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.

Polymorphisms in the human AH receptor

The AH receptor (AHR) mediates toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) as well as induction of three cytochrome P450 enzymes and certain Phase II enzymes. In laboratory animals, genetic variations in the AHR lead to substantial differences in sensitivity to biochemical and toxic effects of TCDD and related compounds. Relatively few polymorphisms have been discovered in the human AHR gene; these occur predominantly in exon 10, a region that encodes a major portion of the transactivation domain of the receptor that is responsible for regulating expression of other genes. In human populations there is a wide range of variation in responses regulated by the AHR for example, induction of CYP1A1. Some variation in human responsiveness likely is due to genetically based variations in AHR structure. Thus far, however, only one pair of polymorphisms, those at codons 517 and 570, has been shown to have a clear cut and strong effect on the phenotype of an AHR-mediated response. The search continues for polymorphisms that alter AHR function because this receptor is a central factor in determining responses to important environmental contaminants and also plays a physiologic role in early development in mammals.

cDNA cloning of an aryl hydrocarbon receptor from Baikal seals (Phoca sibirica)

Species differences in sensitivity to related planar halogenated aromatic hydrocarbons (PHAH) add significant uncertainty in assessing the ecological risk to aquatic mammals. To investigate mechanisms of PHAH sensitivity in aquatic mammals, we cloned and sequenced the cDNA of Baikal seal aryl hydrocarbon receptor (AHR), an intracellular protein that initiates PHAH-mediated effects. The Baikal seal AHR cDNA has an open reading frame of 843 amino acid residues with a predicted molecular mass of 94.6 kDa. Comparison of AHR amino acid sequences indicated a high degree of sequence conservation (98%) between Baikal and harbor seals. The high conservation of AHRs between Baikal and harbor seals indicates that these seals express AHR proteins closely related structurally. In our previous report (Kim & Hahn, 2002), the dioxin-binding affinity of the harbor seal AHR was at least as high as that of the AHR from a dioxin-sensitive strain of mice, suggesting that this seal species may be sensitive to PHAH effects. This implies that Baikal seal may also be sensitive to dioxin effects.

Identification of a critical amino acid in the aryl hydrocarbon receptor

Two aryl hydrocarbon receptors (rtAHR2alpha and rtAHR2beta) have been identified in the rainbow trout (Oncorhynchus mykiss). These receptors share 98% amino acid identity, yet their functional properties differ. Both rtAHR2alpha and rtAHR2beta bind 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), dimerize with rainbow trout ARNTb (rtARNTb), and recognize dioxin response elements in vitro. However, in a transient transfection assay the two proteins show differential ability to recognize enhancers, produce transactivation, and respond to TCDD. To identify the sequence differences that confer the functional differences between rtAHR2alpha and rtAHR2beta, we constructed chimeric rtAHRs, in which segments of one receptor form was replaced with the corresponding part from the other isoform. This approach progressively narrowed the region being examined to a single residue, corresponding to position 111 in rtAHR2beta. Altering this residue in rtAHR2beta from the lysine to glutamate found in rtAHR2alpha produced an rtAHR2beta with the properties of rtAHR2alpha. All other known AHRs resemble rtAHR2alpha and carry glutamate at this position, located at the N terminus of the PAS-A domain. We tested the effect of altering this glutamate in the human and zebrafish AHRs to lysine. This lysine substitution produced AHRs with transactivation properties that were similar to rtAHR2beta. These results identify a critical residue in AHR proteins that has an important impact on transactivation, enhancer site recognition, and regulation by ligand.