Functional characterization of DNA-binding domains of the subunits of the heterodimeric aryl hydrocarbon receptor complex imputing novel and canonical basic helix-loop-helix protein-DNA interactions

Dubosiella newyorkensis modulates immune tolerance in colitis via the L-lysine-activated AhR-IDO1-Kyn pathway

Commensal bacteria generate immensely diverse active metabolites to maintain gut homeostasis, however their fundamental role in establishing an immunotolerogenic microenvironment in the intestinal tract remains obscure. Here, we demonstrate that an understudied murine commensal bacterium, Dubosiella newyorkensis, and its human homologue Clostridium innocuum, have a probiotic immunomodulatory effect on dextran sulfate sodium-induced colitis using conventional, antibiotic-treated and germ-free mouse models. We identify an important role for the D. newyorkensis in rebalancing Treg/Th17 responses and ameliorating mucosal barrier injury by producing short-chain fatty acids, especially propionate and L-Lysine (Lys). We further show that Lys induces the immune tolerance ability of dendritic cells (DCs) by enhancing Trp catabolism towards the kynurenine (Kyn) pathway through activation of the metabolic enzyme indoleamine-2,3-dioxygenase 1 (IDO1) in an aryl hydrocarbon receptor (AhR)-dependent manner. This study identifies a previously unrecognized metabolic communication by which Lys-producing commensal bacteria exert their immunoregulatory capacity to establish a Treg-mediated immunosuppressive microenvironment by activating AhR-IDO1-Kyn metabolic circuitry in DCs. This metabolic circuit represents a potential therapeutic target for the treatment of inflammatory bowel diseases.

AhR and Wnt/β-Catenin Signaling Pathways and Their Interplay

As evolutionarily conserved signaling cascades, AhR and Wnt signaling pathways play a critical role in the control over numerous vital embryonic and somatic processes. AhR performs many endogenous functions by integrating its signaling pathway into organ homeostasis and into the maintenance of crucial cellular functions and biological processes. The Wnt signaling pathway regulates cell proliferation, differentiation, and many other phenomena, and this regulation is important for embryonic development and the dynamic balance of adult tissues. AhR and Wnt are the main signaling pathways participating in the control of cell fate and function. They occupy a central position in a variety of processes linked with development and various pathological conditions. Given the importance of these two signaling cascades, it would be interesting to elucidate the biological implications of their interaction. Functional connections between AhR and Wnt signals take place in cases of crosstalk or interplay, about which quite a lot of information has been accumulated in recent years. This review is focused on recent studies about the mutual interactions of key mediators of AhR and Wnt/β-catenin signaling pathways and on the assessment of the complexity of the crosstalk between the AhR signaling cascade and the canonical Wnt pathway.

The intestine: A highly dynamic microenvironment for IgA plasma cells

To achieve longevity, IgA plasma cells require a sophisticated anatomical microenvironment that provides cytokines, cell-cell contacts, and nutrients as well as metabolites. The intestinal epithelium harbors cells with distinct functions and represents an important defense line. Anti-microbial peptide-producing paneth cells, mucus-secreting goblet cells and antigen-transporting microfold (M) cells cooperate to build a protective barrier against pathogens. In addition, intestinal epithelial cells are instrumental in the transcytosis of IgA to the gut lumen, and support plasma cell survival by producing the cytokines APRIL and BAFF. Moreover, nutrients are sensed through specialized receptors such as the aryl hydrocarbon receptor (AhR) by both, intestinal epithelial cells and immune cells. However, the intestinal epithelium is highly dynamic with a high cellular turn-over rate and exposure to changing microbiota and nutritional factors. In this review, we discuss the spatial interplay of the intestinal epithelium with plasma cells and its potential contribution to IgA plasma cell generation, homing, and longevity. Moreover, we describe the impact of nutritional AhR ligands on intestinal epithelial cell-IgA plasma cell interaction. Finally, we introduce spatial transcriptomics as a new technology to address open questions in intestinal IgA plasma cell biology.

Transcriptional Regulation of Math1 by Aryl Hydrocarbon Receptor: Effect on Math1+ Progenitor Cells in Mouse Small Intestine

The physiological roles of aryl hydrocarbon receptor (AhR) in the small intestine have been revealed as immunomodulatory and barrier functions. However, its contributions to cell fate regulation are incompletely understood. The Notch-activated signaling cascade is a central component of intestinal cell fate determinations. The lateral inhibitory mechanism governed by Notch directs cell fates toward distinct cell lineages (i.e., absorptive and secretory cell lineages) through its downstream effector, mouse atonal homolog 1 (MATH1). An investigation employing cell lines and intestinal crypt cells revealed that AhR regulates Math1 expression in a xenobiotic response element (XRE)-dependent manner. The AhR-Math1 axis was further addressed using intestinal organoids, where AhR-Math1 and HES1-Math1 axes appeared to coexist within the underlying Math1 transcriptional machinery. When the HES1-Math1 axis was pharmacologically suppressed, β-naphthoflavone-mediated AhR activation increased the number of goblet and Math1+ progenitor cells in the organoids. The same pharmacological dissection of the AhR-Math1 axis was applied in vivo, demonstrating an enhanced number of Math1+ progenitor cells in the small intestine following AhR activation. We report here that AhR-Math1 is a direct transcriptional axis with effects on Math1+ progenitor cells in the small intestine, highlighting a novel molecular basis for fine-tuning Notch-mediated cell fate regulation.

Indoleamine 2,3-dioxygenase 1 activation in mature cDC1 promotes tolerogenic education of inflammatory cDC2 via metabolic communication

Conventional dendritic cells (cDCs), cDC1 and cDC2, act both to initiate immunity and maintain self-tolerance. The tryptophan metabolic enzyme indoleamine 2,3-dioxygenase 1 (IDO1) is used by cDCs in maintaining tolerance, but its role in different subsets remains unclear. At homeostasis, only mature CCR7⁺ cDC1 expressed IDO1 that was dependent on IRF8. Lipopolysaccharide treatment induced maturation and IDO1-dependent tolerogenic activity in isolated immature cDC1, but not isolated cDC2. However, both human and mouse cDC2 could induce IDO1 and acquire tolerogenic function when co-cultured with mature cDC1 through the action of cDC1-derived l-kynurenine. Accordingly, cDC1-specific inactivation of IDO1 in vivo exacerbated disease in experimental autoimmune encephalomyelitis. This study identifies a previously unrecognized metabolic communication in which IDO1-expressing cDC1 cells extend their immunoregulatory capacity to the cDC2 subset through their production of tryptophan metabolite l-kynurenine. This metabolic axis represents a potential therapeutic target in treating autoimmune demyelinating diseases.

Structural Characterization of the Interaction of Hypoxia Inducible Factor-1 with Its Hypoxia Responsive Element at the -964G > A Variation Site of the HLA-G Promoter Region

Human Antigen Leukocyte-G (HLA-G) gene encodes an immune checkpoint molecule that has restricted tissue expression in physiological conditions; however, the gene may be induced in hypoxic conditions by the interaction with the hypoxia inducible factor-1 (HIF1). Hypoxia regulatory elements (HRE) located at the HLA-G promoter region and at exon 2 are the major HIF1 target sites. Since the G allele of the −964G > A transversion induces higher HLA-G expression when compared to the A allele in hypoxic conditions, here we analyzed HIF1-HRE complex interaction at the pair-atom level considering both −964G > A polymorphism alleles. Mouse HIF2 dimer crystal (Protein Data Bank ID: 4ZPK) was used as template to perform homology modelling of human HIF1 quaternary structure using MODELLER v9.14. Two 3D DNA structures were built from 5′GCRTG’3 HRE sequence containing the −964G/A alleles using x3DNA. Protein-DNA docking was performed using the HADDOCK v2.4 server, and non-covalent bonds were computed by DNAproDB server. Molecular dynamic simulation was carried out per 200 ns, using Gromacs v.2019. HIF1 binding in the HRE containing −964G allele results in more hydrogen bonds and van der Waals contact formation than HRE with −964A allele. Protein-DNA complex trajectory analysis revealed that HIF1-HRE-964G complex is more stable. In conclusion, HIF1 binds in a more stable and specific manner at the HRE with G allele.

Aryl Hydrocarbon Receptor and Its Diverse Ligands and Functions: An Exposome Receptor

The aryl hydrocarbon receptor (AhR) is a transcriptional factor that regulates multiple functions following its activation by a variety of ligands, including xenobiotics, natural products, microbiome metabolites, and endogenous molecules. Because of this diversity, the AhR constitutes an exposome receptor. One of its main functions is to regulate several lines of defense against chemical insults and bacterial infections. Indeed, in addition to its well-established detoxication function, it has several functions at physiological barriers, and it plays a critical role in immunomodulation. The AhR is also involved in the development of several organs and their homeostatic maintenance. Its activity depends on the type of ligand and on the time frame of the receptor activation, which can be either sustained or transient, leading in some cases to opposite modes of regulations as illustrated in the regulation of different cancer pathways. The development of selective modulators and their pharmacological characterization are important areas of research. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

AhR and Cancer: From Gene Profiling to Targeted Therapy

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that has been shown to be an essential regulator of a broad spectrum of biological activities required for maintaining the body’s vital functions. AhR also plays a critical role in tumorigenesis. Its role in cancer is complex, encompassing both pro- and anti-tumorigenic activities. Its level of expression and activity are specific to each tumor and patient, increasing the difficulty of understanding the activating or inhibiting roles of AhR ligands. We explored the role of AhR in tumor cell lines and patients using genomic data sets and discuss the extent to which AhR can be considered as a therapeutic target.

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.

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.

The DNA target determines the dimerization partner selected by bHLHZ-like hybrid proteins AhRJun and ArntFos

The molecular basis of protein–partner selection and DNA binding of the basic helix–loop–helix (bHLH) and basic region-leucine zipper (bZIP) superfamilies of dimeric transcription factors is fundamental toward understanding gene regulation.

Cloning, characterization, hypoxia and heat shock response of hypoxia inducible factor-1 (HIF-1) from the small abalone Haliotis diversicolor

In this study, hypoxia inducible factor-1α (HIF-1α) and hypoxia inducible factor-1β (HIF-1β) from small abalone Haliotis diversicolor were cloned. The cDNA of H. diversicolor HIF-1α (HdHIF-1α) is 2,833 bp encoding a protein of 711aa and H. diversicolor HIF-1β (HdHIF-1β) is 1919 bp encoding a protein of 590aa. Similar to other species' HIF-1, HdHIF-1 has one basic helix-loop-helix (bHLH) domain and two Per-Arnt-Sim (PAS) domains, and HdHIF-1α has a oxygen-dependent degradation domain (ODDD) with two proline hydroxylation motifs and a C-terminal transactivation domain (C-TAD) with an asparagine hydroxylation motif. Under normoxic conditions, HdHIF-1α and HdHIF-1β mRNAs were constitutively present in all examined tissues. Under hypoxia (2.0mg/L DO at 25°C) stress, HdHIF-1α expression was up-regulated in gills at 4h, 24h and 96 h, and in hemocytes at 24h and 96 h, while HdHIF-1β remained relatively constant. Under thermal stress (31°C), HdHIF-1α expression was significantly increased in gills at 4h, and hemocytes at 0 h and 4 h, while HdHIF-1β expression still remained relatively constant. These results suggested that HIF-1α may play an important role in adaption to poor environment in H. diversicolor.

Inhibition of apolipoprotein A-I gene by the aryl hydrocarbon receptor: a potential mechanism for smoking-associated hypoalphalipoproteinemia

AIMS

Smokers have lower plasma concentrations of high-density lipoprotein (HDL) cholesterol and apolipoprotein A-I (apo A-I) compared with nonsmokers. To determine the molecular basis of this observation, the effect of activation of the aryl hydrocarbon receptor (AhR) on apo A-I gene expression was examined.

MAIN METHODS

HepG2 cells were treated with AhR receptor agonists benzo(a)pyrene (BaP) and CAY10465, and AhR receptor antagonist CAY10464 and apo A-I protein, mRNA levels and promoter activity were measured. The effect of nicotine on apo A-I protein secretion was also tested. Using a series or apo A-I gene promoter deletion constructs, a xenobiotic response element (XRE) was identified.

KEY FINDINGS

Treatment of HepG2 cells with the AhR receptor agonists BaP and CAY10465, inhibited apo A-I protein synthesis while nicotine, which does not bind AhR had no effect. Benzo(a)pyrene treatment also suppressed apo A-I mRNA and gene promoter activity. Treatment of HepG2 cells with the AhR receptor antagonist CAY10464 reversed the suppressive effect of BaP on apo A-I gene expression. A putative xenobiotic response element (XRE) was identified between nucleotides -325 and -186 (relative to the transcriptional start site, +1).

SIGNIFICANCE

These results suggest that the cigarette smoking related environmental contaminant BaP promotes hypoalphalipoproteinemia in part through activation of the hepatic AhR.

Dioxins, the aryl hydrocarbon receptor and the central regulation of energy balance

Dioxins are ubiquitous environmental contaminants that have attracted toxicological interest not only for the potential risk they pose to human health but also because of their unique mechanism of action. This mechanism involves a specific, phylogenetically old intracellular receptor (the aryl hydrocarbon receptor, AHR) which has recently proven to have an integral regulatory role in a number of physiological processes, but whose endogenous ligand is still elusive. A major acute impact of dioxins in laboratory animals is the wasting syndrome, which represents a puzzling and dramatic perturbation of the regulatory systems for energy balance. A single dose of the most potent dioxin, TCDD, can permanently readjust the defended body weight set-point level thus providing a potentially useful tool and model for physiological research. Recent evidence of response-selective modulation of AHR action by alternative ligands suggests further that even therapeutic implications might be possible in the future.

Toxicology mechanism of the persistent organic pollutants (POPs) in fish through AhR pathway

With the development of industry and agriculture, the cases of cancer and tumor have been increasing gradually in the last 30 years, and quite a few cases are caused by persistent organic pollutants (POPs), some of them belonging to environmental endocrine disruptors, and they have become ubiquitous in the environment, especially in the aquatic ecosystem; so this issue has aroused the extensive attention of the world. The mechanism of POPs toxicology is very complicated, but it is mainly mediated by the aryl hydrocarbon receptor (AhR) pathway in fish. In order to gain a comprehensive understanding of the AhR pathway, the present paper focuses on reviewing it from four major steps, including formation of cytosolic complex, translocation of AhR, heterodimerization of AhR, and induction of CYP1A. This study summarized the isoform numbers of AhR pathway genes and the expression patterns in the regulation process of POPs toxicology in zebrafish.

Molecular phylogenies and evolutionary behavior of AhR (aryl hydrocarbon receptor) pathway genes in aquatic animals: implications for the toxicology mechanism of some persistent organic pollutants (POPs)

Phylogenetic analysis of AhR pathway genes and their evolutionary rate variations were studied on aquatic animals. The gene sequences for the proteins involved in this pathway were obtained from four major phylogenetic groups, including bivalvia, amphibian, teleostei and mammalia. These genes were distributed under four major steps of toxicology regulation: formation of cytosolic complex, translocation of AhR, heterodimerization of AhR and induction of CYP1A. The NJ, MP, and ML algorithm were used on protein coding DNA sequences to deduce the evolutionary relationship for the respective AhR pathway gene among different aquatic animals. The rate of non-synonymous nucleotide substitutions per non-synonymous site (d(N)) and synonymous nucleotide substitutions per synonymous site (d(S)) were calculated for different clade of the respective phylogenetic tree for each AhR pathway gene. The phylogenetic analysis suggests that evolutionary pattern of AhR pathway genes in aquatic animals is characterized mainly through gene duplication events or alterative splicing. The d(N) values indicate that all AhR pathway genes are well conserved in aquatic animals, except for CYP1A gene. Furthermore, compare with other aquatic animals, the d(N) value indicates that AhR pathway genes of fish are less conserved, and these genes likely go through an adaptive evolution within aquatic animals.

Abnormal liver development and resistance to 2,3,7,8-tetrachlorodibenzo-p-dioxin toxicity in mice carrying a mutation in the DNA-binding domain of the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AHR) is known for its role in the adaptive and toxic responses to a large number of environmental contaminants, as well as its role in hepatovascular development. The classical AHR pathway involves ligand binding, nuclear translocation, heterodimerization with the AHR nuclear translocator (ARNT), and binding of the heterodimer to dioxin response elements (DREs), thereby modulating the transcription of an array of genes. The AHR has also been implicated in signaling events independent of nuclear localization and DNA binding, and it has been suggested that such pathways may play important roles in the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Here, we report the generation of a mouse model that expresses an AHR protein capable of ligand binding, interactions with chaperone proteins, functional heterodimerization with ARNT, and nuclear translocation, but is unable to bind DREs. Using this model, we provide evidence that DNA binding is required AHR-mediated liver development, as Ahr^dbd/dbd mice exhibit a patent ductus venosus, similar to what is seen in Ahr^−/− mice. Furthermore, Ahr^dbd/dbd mice are resistant to TCDD-induced toxicity for all endpoints tested. These data suggest that DNA binding is necessary for AHR-mediated developmental and toxic signaling.

Repression of aryl hydrocarbon receptor (AHR) signaling by AHR repressor: role of DNA binding and competition for AHR nuclear translocator

Activation of the aryl hydrocarbon receptor (AHR) by 2,3,7,8-tetrachlorodibenzo-p-dioxin causes altered gene expression and toxicity. The AHR repressor (AHRR) inhibits AHR signaling through a proposed mechanism involving competition with AHR for dimerization with AHR nuclear translocator (ARNT) and binding to AHR-responsive enhancer elements (AHREs). We sought to delineate the relative roles of competition for ARNT and AHREs in the mechanism of repression. In transient transfections in which AHR2-dependent transactivation was repressed by AHRR1 or AHRR2, increasing ARNT expression failed to reverse the repression, suggesting that AHRR inhibition of AHR signaling does not occur through sequestration of ARNT. An AHRR1 point mutant (AHRR1-Y9F) that could not bind to AHREs but that retained its nuclear localization was only slightly reduced in its ability to repress AHR2, demonstrating that AHRR repression does not occur solely through competition for AHREs. When both proposed mechanisms were blocked (AHRR1-Y9F plus excess ARNT), AHRR remained functional. AHRR1 neither blocked AHR nuclear translocation nor reduced the levels of AHR2 protein. Experiments using AHRR1 C-terminal deletion mutants showed that amino acids 270 to 550 are dispensable for repression. These results demonstrate that repression of AHR transactivation by AHRR involves the N-terminal portion of AHRR; does not involve competition for ARNT; and does not require binding to AHREs, although AHRE binding can contribute to the repression. We propose a mechanism of AHRR action involving "transrepression" of AHR signaling through protein-protein interactions rather than by inhibition of the formation or DNA binding of the AHR-ARNT complex.

The aryl hydrocarbon receptor sans xenobiotics: endogenous function in genetic model systems

For more than 30 years, the aryl hydrocarbon receptor [Ah receptor (AHR)] has been extensively scrutinized as the cellular receptor for numerous environmental contaminants, including polychlorinated dioxins, dibenzofurans, and biphenyls. Recent evidence argues that this description is incomplete and perhaps myopic. Ah receptor orthologs have been demonstrated to mediate diverse endogenous functions in our close vertebrate relatives as well as our distant invertebrate ancestors. Moreover, these endogenous functions suggest that xenobiotic toxicity may be best understood in the context of intrinsic AHR physiology. In this literature review, we survey the emerging picture of endogenous AHR biology from work in the vertebrate and invertebrate model systems Mus musculus, Caenorhabditis elegans, and Drosophila melanogaster.

Novel DNA Binding by a Basic Helix-Loop-Helix Protein

Central issues surrounding the basic helix-loop-helix (bHLH) superfamily of dimeric transcription factors concern how specificity of partner selection and DNA binding are achieved. bHLH proteins bind DNA through the basic sequence that is contiguous with a helix-loop-helix dimerization domain. For the two subgroups within the family, dimerization is further regulated by an adjacent Per-Arnt-Sim homology (PAS) or leucine zipper (LZ) domain. We provide evidence that for the bHLH.PAS transcription factors Dioxin Receptor (DR) and Arnt, the DR PAS A domain has a unique interaction with the bHLH region that underpins both dimerization strength and affinity for an atypical E-box DNA sequence. A PAS swap heterodimer, where the DR bHLH domain was fused to Arnt PAS A and the Arnt bHLH fused to DR PAS A, gave strong DNA binding, but dimerization was only effective with the native arrangement, suggesting the PAS A domain is critical for each process via distinct mechanisms. LZ domains, which regulate heterodimerization for the bHLH.LZ family members Myc and Max, could not replace the PAS domains for either dimerization or DNA binding in the DR/Arnt heterodimer. In vitro footprinting revealed that the PAS domains influence the conformation of target DNA in a manner consistent with DNA bending. These results provide the first insights for understanding mechanisms of selective dimerization and DNA interaction that distinguish bHLH.PAS proteins from the broader bHLH superfamily.

Molecular characterization of the aryl hydrocarbon receptors (AHR1 and AHR2) from red seabream (Pagrus major)

The aryl hydrocarbon receptor (AHR) mediates the toxic effects of planar halogenated aromatic hydrocarbons (PHAHs). Bony fishes exposed to PHAHs exhibit a wide range of developmental defects. However, functional roles of fish AHR are not yet fully understood, compared with those of mammalian AHRs. To investigate the potential sensitivity to PHAHs toxic effects, an AHR cDNA was initially cloned and sequenced from red seabream (Pagrus major), an important fishery resource in Japan. The present study succeeded in identifying two highly divergent red seabream AHR cDNA clones, which shared only 32% identity in full-length amino acid sequence. The phylogenetic analysis revealed that one belonged to AHR1 clade (rsAHR1) and another to AHR2 clade (rsAHR2). The rsAHR1 encoded a 846-residue protein with a predicted molecular mass of 93.2 kDa, and 990 amino acids and 108.9 kDa encoded rsAHR2. In the N-terminal half, both rsAHR genes included bHLH and PAS domains, which participate in ligand binding, AHR/ARNT dimerization and DNA binding. The C-terminal half, which is responsible for transactivation, was poorly conserved between rsAHRs. Quantitative analyses of both rsAHRs mRNAs revealed that their tissue expression profiles were isoform-specific; rsAHR1 mRNA expressed primarily in brain, heart, ovary and spleen, while rsAHR2 mRNA was observed in all tissues examined, indicating distinct roles of each rsAHR. Furthermore, there appeared to be species-differences in the tissue expression profiles of AHR isoforms between red seabream and other fish. These results suggest that there are isoform- and species-specific functions in piscine AHRs.

Role of arginine residues 14 and 15 in dictating DNA binding stability and transactivation of the aryl hydrocarbon receptor/aryl hydrocarbon receptor nuclear translocator heterodimer

The aryl hydrocarbon receptor (AHR) and its DNA binding partner, the aryl hydrocarbon receptor nuclear translocator (ARNT) are basic helix-loop-helix/PAS proteins. The goal of the current study was to determine the extent to which residues R14 and R15 contained within the basic region of the AHR contribute to the DNA binding affinity and stability of the AHR/ARNT heterodimer. Towards this end, we first performed equilibrium binding and dissociation rate analyses using a single dioxin response element (DRE-1). While the K(D) and Bmax values obtained from the equilibrium binding analysis were similar for the wild-type AHR (wt AHR) and that containing the substitutions of R14 and R15 with Q residues (Q14Q15 AHR), dissociation rate analyses revealed that the stability of the Q14Q15 AHR DNA binding complex was approximately 10-fold less. Using a two-site DNA binding model, we also found that AHR/ARNT heterodimer does not participate in cooperative binding, as binding of the second dimer appears to be prohibited by occupation of the first. This property was similar regardless of the composition of the amino acids at positions 14 and 15. Finally, reporter assays revealed that the Q14Q15 substitutions severely compromised the ability of the AHR to activate gene expression despite appropriate nuclear localization. The present results revealed that DNA binding stability of the AHR/ARNT heterodimer is an important requirement for its transactivation capabilities and that this stability is governed, in part, by residues R14 and R15 that lie within the basic region of the AHR.

Multiple mechanisms are involved in Ah receptor-mediated cell cycle arrest

The liver is the only solid organ that can respond to major tissue loss or damage by regeneration to restore liver biomass. The aryl hydrocarbon receptor (AhR) agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) can disrupt the regenerative process, as evidenced by suppression of DNA synthesis in rat primary hepatocytes in culture and in vivo liver regeneration after partial hepatectomy. Independent observations demonstrated that AhR-mediated G(1) phase cell cycle arrest depends on an interaction with the retinoblastoma tumor suppressor protein (pRb), but differences exist regarding proposed mechanisms of action. Two distinct models have been proposed, one supporting the AhR-pRb interaction functioning in corepression of E2F activity and the other favoring an AhR-pRb interaction participating in transcriptional coactivation of genes encoding G(1) phase regulatory proteins. In the present study, experiments in rat hepatoma cells using dominant-negative DNA-binding-defective AhR and Ah receptor nuclear translocator (Arnt) mutants provided evidence that TCDD-induced AhR-mediated G(1) arrest is only partially regulated by direct AhR transcriptional activity, suggesting that both coactivation and corepression are involved. Studies using a small interfering RNA to down-regulate Arnt protein expression revealed that TCDD-induced G(1) arrest is absolutely dependent on the Arnt protein.

The mammalian basic helix-loop-helix/PAS family of transcriptional regulators

Basic helix-loop-helix (bHLH)/PAS proteins are critical regulators of gene expression networks underlying many essential physiological and developmental processes. These include transcriptional responses to environmental pollutants and low oxygen tension, mediated by the aryl hydrocarbon (Dioxin) receptor and hypoxia inducible factors (HIF), respectively, and controlling aspects of neural development, mediated by the single minded (SIM) proteins. bHLH proteins must dimerise to form functional DNA binding complexes and bHLH/PAS proteins are distinguished from other members of the broader bHLH superfamily by the dimerisation specificity conferred by their PAS homology domains. bHLH/PAS proteins tend to be ubiquitous, latent signal-regulated transcription factors that often recognise variant forms of the classic E-box enhancer sequence bound by other bHLH proteins. Two closely related forms of each of the hypoxia inducible factors alpha and single minded proteins and the general partner protein, aryl hydrocarbon receptor nuclear translocator (ARNT), are present in many cell types. Despite high sequence conservation within their DNA binding and dimerisation domains, and having very similar DNA recognition specificities, the homologues are functionally non-redundant and biologically essential. While the mechanisms controlling partner choice and target gene activation that determine this functional specificity are poorly understood, interactions mediated by the PAS domains are essential. Information on structures and protein/protein interactions for members of the steroid hormone/nuclear receptor superfamily has contributed to our understanding of the way these receptors function and assisted the development of highly specific agonists and antagonists. Similarly, it is anticipated that developing a detailed mechanistic and structural understanding of bHLH/PAS proteins will ultimately facilitate drug design.

Phosphorylation of nuclear localization signal inhibits the ligand-dependent nuclear import of aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor which plays a role as an intracellular mediator of the xenobiotic signaling pathway. We previously identified the minimum nuclear localization signal (NLS) of AhR(13-39): it is composed of two basic amino acid segments, AhR(13-16:RKRR) and AhR(37-39:KRH). In this study, we showed that the two protein kinase C (PKC) sites of Ser-12 and Ser-36 are located one amino acid upstream from each of the two segments, and that a ligand-dependent nuclear import of AhR is inhibited by substitution of aspartic acid for Ser-12 (S12D) or Ser-36 (S36D), which mimics the negative charge of phosphorylation. This observation was supported by microinjection analysis, an in vitro nuclear transport assay, and a luciferase reporter assay, suggesting a two-step mechanism in the ligand-dependent nuclear translocation of AhR.

The aryl hydrocarbon receptor (AhR) tyrosine 9, a residue that is essential for AhR DNA binding activity, is not a phosphoresidue but augments AhR phosphorylation

We delineate a mechanism by which dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin or TCDD)-mediated formation of the aryl hydrocarbon receptor (AhR) DNA binding complex is disrupted by a single mutation at the conserved AhR tyrosine 9. Replacement of tyrosine 9 with the structurally conservative phenylalanine (AhRY9F) abolished binding to dioxin response element (DRE) D, E, and A and abrogated DRE-driven gene induction mediated by the AhR with no effect on TCDD binding, TCDD-induced nuclear localization, or ARNT heterodimerization. The speculated role for phosphorylation at tyrosine 9 was also examined. Anti-phosphotyrosine immunoblotting could not detect a major difference between the AhRY9F mutant and wild-type AhR, but a basic isoelectric point shift was detected by two-dimensional gel electrophoresis of AhRY9F. However, an antibody raised to recognize only phosphorylated tyrosine 9 (anti-AhRpY9) confirmed that AhR tyrosine 9 is not a phosphorylated residue required for DRE binding. Kinase assays using synthetic peptides corresponding to the wild-type and mutant AhR residues 1-23 demonstrated that a tyrosine at position 9 is important for substrate recognition at serine(s)/threonine(s) within this sequence by purified protein kinase C (PKC). Also, compared with AhRY9F, immunopurified full-length wild-type receptor was more rapidly phosphorylated by PKC. Furthermore, co-treatment of AhR-deficient cells that expressed AhRY9F and a DRE-driven luciferase construct with phorbol 12-myristate 13-acetate and TCDD resulted in a 30% increase in luciferase activity compared with AhRY9F treated with TCDD alone. Overall, AhR tyrosine 9, which is not a phosphorylated residue itself but is required for DNA binding, appears to play a crucial role in AhR activity by permitting proper phosphorylation of the AhR.

Contribution of the Per/Arnt/Sim (PAS) Domains to DNA Binding by the Basic Helix-Loop-Helix PAS Transcriptional Regulators

The basic helix-loop-helix (bHLH) PAS transcriptional regulators control critical developmental and metabolic processes, including transcriptional responses to stimuli such as hypoxia and environmental pollutants, mediated respectively by hypoxia inducible factors (HIF-alpha) and the dioxin (aryl hydrocarbon) receptor (DR). The bHLH proteins contain a basic DNA binding sequence adjacent to a helix-loop-helix dimerization domain. Dimerization among bHLH.PAS proteins is additionally regulated by the PAS region, which controls the specificity of partner choice such that HIF-alpha and DR must dimerize with the aryl hydrocarbon nuclear translocator (Arnt) to form functional DNA binding complexes. Here, we have analyzed purified bacterially expressed proteins encompassing the N-terminal bHLH and bHLH.PAS regions of Arnt, DR, and HIF-1alpha and evaluated the contribution of the PAS domains to DNA binding in vitro. Recovery of functional DNA binding proteins from bacteria was dramatically enhanced by coexpression of the bHLH.PAS regions of DR or HIF-1alpha with the corresponding region of Arnt. Formation of stable protein-DNA complexes by DR/Arnt and HIF-1alpha/Arnt heterodimers with their cognate DNA sequences required the PAS A domains and exhibited KD values of 0.4 nM and approximately 50 nM, respectively. In contrast, the presence of the PAS domains of Arnt had little effect on DNA binding by Arnt homodimers, and these bound DNA with a KD of 45 nM. In the case of the DR, both high affinity DNA binding and dimer stability were specific to its native PAS domain, since a chimera in which the PAS A domain was substituted with the equivalent domain of Arnt generated a destabilized protein that bound DNA poorly.

Phylogenetic analysis of plant basic helix-loop-helix proteins

The basic helix-loop-helix (bHLH) family of proteins is a group of functionally diverse transcription factors found in both plants and animals. These proteins evolved early in eukaryotic cells before the split of animals and plants, but appear to function in 'plant-specific' or 'animal-specific' processes. In animals bHLH proteins are involved in regulation of a wide variety of essential developmental processes. On the contrary, bHLH proteins have not been extensively studied in plants. Those that have been characterized function in anthocyanin biosynthesis, phytochrome signaling, globulin expression, fruit dehiscence, carpel and epidermal development. We have identified 118 different bHLH genes in the completely sequenced Arabidopsis thaliana genome and 131 bHLH genes in the rice genome. Here we report a phylogenetic analysis of these genes, including 46 genes from other plant species and a classification of these proteins into 15 distinct plant clades. Results imply a polyphyletic origin for the plant bHLH proteins related only by their bHLH DNA binding motif. We suggest that plant bHLH proteins are under weaker selective constraints than their animal counterparts and that lineage specific expansions and subfunctionalization have fashioned regulatory proteins for plant specific functions.

Resistance to 2,3,7,8-tetrachlorodibenzo-p-dioxin toxicity and abnormal liver development in mice carrying a mutation in the nuclear localization sequence of the aryl hydrocarbon receptor

The Ah receptor (AHR) mediates the metabolic adaptation to a number of planar aromatic chemicals. Essential steps in this adaptive mechanism include AHR binding of ligand in the cytosol, translocation of the receptor to the nucleus, dimerization with the Ah receptor nuclear translocator, and binding of this heterodimeric transcription factor to dioxin-responsive elements (DREs) upstream of promoters that regulate the expression of genes involved in xenobiotic metabolism. The AHR is also involved in other aspects of mammalian biology, such as the toxicity of molecules like 2,3,7,8-tetrachlorodibenzo-p-dioxin as well as regulation of normal liver development. In an effort to test whether these additional AHR-mediated processes require a nuclear event, such as DRE binding, we used homologous recombination to generate mice with a mutation in the AHR nuclear localization/DRE binding domain. These Ahr(nls) mice were found to be resistant to all 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced toxic responses that we examined, including hepatomegaly, thymic involution, and cleft palate formation. Moreover, aberrations in liver development observed in these mice were identical to that observed in mice harboring a null allele at the Ahr locus. Taken in sum, these data support a model where most, if not all, of AHR-regulated biology requires nuclear localization.

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.

Agonist but not antagonist ligands induce conformational change in the mouse aryl hydrocarbon receptor as detected by partial proteolysis

The cytosolic transcription factor known as the aryl hydrocarbon receptor (AhR) undergoes transformation to a DNA-binding form by a series of processes initiated by binding of ligand. Subsequent steps include dissociation of several proteins that are complexed with the inactive receptor, nuclear translocation, and dimerization with Arnt. We have used limited proteolysis of the in vitro-translated mouse AhR to determine whether this technique can detect conformational change(s) associated with AhR transformation and whether the effect of agonist and antagonist ligands can be distinguished by this assay. Limited digestion of [(35)S]AhR/AhR nuclear translocator (Arnt) by trypsin produced a peptide of approximately 40 kDa that was more resistant to proteolysis in the presence of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) than vehicle and was also Arnt-dependent. This trypsin-resistant peptide was also elicited in the presence of other agonist ligands, but not with antagonist ligands that do not form the DNA-binding AhR/Arnt complex. Immunoblot of trypsin-treated AhR/Arnt +/- TCDD indicated that the trypsin-resistant peptide did not include the N-terminal portion of the AhR against which the antibody was made. Truncated AhRs were also subjected to limited trypsinization. From AhR(1-399), a TCDD-dependent peptide of approximately 35 kDa was observed; from the constitutively active AhR(1-348), a band of approximately 30 kDa was produced from vehicle- and TCDD-treated protein. From these observations, we hypothesize that the trypsin-resistant peptide from full-length AhR spans approximately from amino acid 80 to 440. We conclude that agonist ligands initiate structural alteration in AhR that is Arnt-dependent and at least partially involves the ligand-binding/Per-Arnt-Sim domain.

DNA binding and protein interactions of the AHR/ARNT heterodimer that facilitate gene activation

Gene activation by the aryl hydrocarbon receptor (AHR) and its DNA binding partner, the aryl hydrocarbon receptor nuclear translocator (ARNT) requires a number of sequential steps that occur following the binding of ligand and entry of the AHR into the nuclear compartment. This includes heterodimerization of the AHR and ARNT, formation of the appropriate amino acid/nucleotide contacts at the GCGTG recognition site and interactions between either the AHR or ARNT with proteins that facilitate changes in chromatin structure. The majority of these steps are likely modulated by changes in both phosphorylation and oxidation status of the AHR, ARNT and associated proteins. Studies of both the basic helix-loop-helix transcription factors and the nuclear hormone receptor family can provide significant insights into how this unique signaling pathway activates its target genes.

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.

The basic helix-loop-helix domain of the aryl hydrocarbon receptor nuclear transporter (ARNT) can oligomerize and bind E-box DNA specifically

The aryl hydrocarbon receptor nuclear transporter (ARNT) is a basic helix-loop-helix (bHLH) protein that contains a Per-Arnt-Sim (PAS) domain. ARNT heterodimerizes in vivo with other bHLH PAS proteins to regulate a number of cellular activities, but a physiological role for ARNT homodimers has not yet been established. Moreover, no rigorous studies have been done to characterize the biochemical properties of the bHLH domain of ARNT that would address this issue. To begin this characterization, we chemically synthesized a 56-residue peptide encompassing the bHLH domain of ARNT (residues 90-145). In the absence of DNA, the ARNT-bHLH peptide can form homodimers in lower ionic strength, as evidenced by dynamic light scattering analysis, and can bind E-box DNA (CACGTG) with high specificity and affinity, as determined by fluorescence anisotropy. Dimers and tetramers of ARNT-bHLH are observed bound to DNA in equilibrium sedimentation and dynamic light scattering experiments. The homodimeric peptide also undergoes a coil-to-helix transition upon E-box DNA binding. Peptide oligomerization and DNA affinity are strongly influenced by ionic strength. These biochemical and biophysical studies on the ARNT-bHLH reveal its inherent ability to form homodimers at concentrations supporting a physiological function and underscore the significant biochemical differences among the bHLH superfamily.

Regulation of subcellular localization of the aryl hydrocarbon receptor (AhR)

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates the toxicity of dioxin and other xenobiotics. In the absence of exogenous ligand, AhR is cytosolic. We investigated how AhR is retained in the cytosol and how dioxin induces AhR to move to the nucleus. Disruption of nuclear export of AhR by the nuclear export inhibitor leptomycin B (LMB) or by mutation of the AhR nuclear export signal resulted in nuclear accumulation of AhR in the absence of exogenous ligand. Mutation of the AhR nuclear localization signal resulted in defects in nuclear import of AhR in both the presence and the absence of exogenous ligand. Dioxin treatment caused a more rapid accumulation of AhR in the nucleus than LMB treatment. In the presence of both dioxin and LMB, nuclear accumulation of AhR was more rapid than in the presence of dioxin alone. Our results show that AhR shuttles between the nucleus and the cytosol in the absence of exogenous ligand. Binding of ligand induces an increase in the rate of nuclear import of AhR but does not eliminate nuclear export of AhR.

Induction of cytochrome P4501A1

Cytochrome P4501A1 is a substrate-inducible microsomal enzyme that oxygenates polycyclic aromatic hydrocarbons, such as the carcinogen benzo(a)pyrene, as the initial step in their metabolic processing to water-soluble derivatives. Enzyme induction reflects increased transcription of the cognate CYP1A1 gene. The environmental toxicant 2,3,7,8-tetrachlorodibenzo-p-dioxin is the most potent known cytochrome P4501A1 inducer. Two regulatory proteins, the aromatic (aryl) hydrocarbon receptor (AhR) and the AhR nuclear translocator (Arnt), mediate induction. AhR and Arnt are prototypical members of the basic helix-loop-helix/Per-Arnt-Sim class of transcription factors. Mechanistic analyses of cytochrome P4501A1 induction provide insights into ligand-dependent mammalian gene expression, basic helix-loop-helix/Per-Arnt-Sim protein function, and dioxin action; such studies also impact public health issues concerned with molecular epidemiology, carcinogenesis, and risk assessment.

Regulation of DNA binding activity of the ligand-activated aryl hydrocarbon receptor by tyrosine phosphorylation

Aryl hydrocarbon receptor (AhR), a member of the bHLH-PAS family, is a ligand-activated transcription factor which plays an important role in normal liver development and in mediating the toxicity of polycyclic and halogenated aromatic hydrocarbon pollutants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin. Phosphorylation is known to regulate the transformation process of unliganded AhR into functionally active AhR/ARNT heterodimer that has high affinity for dioxin-responsive elements (DRE) and transactivation activity. Here, we report that DRE binding activity of the AhR is regulated by phosphorylation on the AhR/ARNT complex itself. Studies with specific protein phosphatases indicated that tyrosine phosphorylation is involved in this modulation. In addition, the AhR is phosphorylated at tyrosine residue(s) as determined by anti-phosphotyrosine immunoblot analysis. These results suggest that tyrosine phosphorylation on the AhR is required for its DNA binding activity and may provide mammalian cells with another layer of control mechanism that allows cell type specific and developmental stage specific induction of the AhR target genes.

Analysis of the complex relationship between nuclear export and aryl hydrocarbon receptor-mediated gene regulation

The aryl hydrocarbon receptor (AHR) contains signals for both nuclear import and nuclear export (NES). The purpose of the studies in this report was to determine the relationship between the nuclear export of the AHR and AHR-mediated gene regulation. Blockage of nuclear export in HepG2 cells with leptomycin B (LMB) resulted in increased levels of AHR-AHR nuclear translocator (ARNT) complex in the nucleus and correlative reductions in agonist-stimulated AHR degradation. However, LMB exposure inhibited agonist-mediated induction of numerous AHR-responsive reporter genes by 75 to 89% and also inhibited induction of endogenous CYP1A1. LMB did not transform the AHR to a ligand binding species or affect activation by TCDD (2, 3,7,8-tetrachlorodibenzo-p-dioxin). Mutagenesis of leucines 66 and 71 of the putative AHR NES resulted in a protein with reduced function in dimerization to ARNT and binding to DNA, while alanine substitution at leucine 69 (AHR(A69)) resulted in an AHR that bound with ARNT and associated with DNA. AHR(A69) protein injected directly into the nuclei of E36 cells remained nuclear following 6 h of agonist stimulation. In transient-transfection assays, AHR(A69) accumulated within the nucleus was not degraded efficiently following agonist exposure. Finally, AHR(A69) supported induction of AHR-responsive reporter genes in an agonist-dependent manner. These findings show that it is possible to generate an AHR protein defective in nuclear export that is functional in agonist-mediated gene induction. This implies that the negative effect of LMB on agonist-mediated gene induction is independent of the nuclear export of the AHR.

An aryl hydrocarbon receptor conformation acts as the functional core of nuclear dioxin signaling

DNA-complexed heterodimers of the aryl hydrocarbon receptor (AhR) with the Ah receptor nuclear translocator (Arnt) are the molecular switches for nuclear signaling of 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD). AhR-Arnt heterodimers regulate genes involved in the metabolism of xenobiotics or fatty acids and various genes important for growth and differentiation. In this report several potent methods, such as the limited protease digestion, gel shift and gel shift clipping assays, allowed the investigation of ligand-stabilized conformations of AhR monomers in comparison to that of AhR-Arnt heterodimers. Interestingly, the ligand sensitivity of monomeric AhR was found to be very low at 25 nM, whereas DNA-dependent methods consistently provided EC(50) values between 0.12 and 0.6 nM for AhR in a heterodimeric complex, i. e. an approximate 100-fold higher ligand sensitivity. This indicates that complex formation of AhR with Arnt on DNA is an important and critical step in transforming AhR into a high affinity receptor for TCDD. A comparison of wild-type AhR with different C-terminal receptor truncations suggests that the PAS-B subregion of its PAS domain is of central importance for stabilization of a functional, i. e. ligand-sensitive, AhR-Arnt conformation, whereas the PAS-A subregion appears to be critical for dimerization of AhR and Arnt. In conclusion, the results of this study provide important information on the ligand sensitivity of AhR and AhR-Arnt heterodimer conformations.

Aryl hydrocarbon receptor imported into the nucleus following ligand binding is rapidly degraded via the cytosplasmic proteasome following nuclear export

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that dimerizes with the AHR nuclear translocator protein to mediate gene regulation. However, the AHR protein is rapidly depleted in vitro and in vivo following exposure to ligands. The purpose of the studies in this report was to characterize the mechanism of AHR degradation and determine the consequence of blocking the degradation process. Western blot and immunological analysis of rat smooth muscle (A7), murine Hepa-1, and human HepG2 cells show that ligand-induced degradation of AHR is blocked when the proteasome is inhibited by MG-132. AHR degradation is also blocked in Hepa-1 and HepG2 cells when nuclear export is inhibited with leptomycin B. Mutation of a putative nuclear export signal present in the AHR results in the accumulation of AHR in the nucleus and reduced levels of degradation following ligand exposure. In addition, inhibition of AHR degradation results in an increase in the concentration of AHR.AHR nuclear translocator complexes associated with DNA and extends the duration that the complex resides in the nucleus. These findings show that nuclear export and degradation of the AHR protein are two additional steps in the AHR-mediated signal transduction pathway and suggest novel areas for regulatory control.

Divergent mechanisms for loss of Ah-responsiveness in benzo[a]pyrene- and adriamycinR-resistant MCF-7 cells

The intracellular aryl hydrocarbon receptor (AhR) mediates signal transduction by environmental pollutants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and benzo[a]pyrene by functioning as a ligand-activated transcription factor. We have investigated AhR signaling in sublines of the human breast cancer cell line MCF-7 selected for resistance to AdriamycinR (AdrR) and benzo[a]pyrene (BP(R)). Previously we reported that AdrR cells have a loss of estrogen receptor (ER) expression and are Ah-nonresponsive. Here we show that AhR mRNA and protein are expressed at normal levels in AdrR cells, and the activated AhR complex is functionally capable of binding a xenobiotic responsive element. In MCF-7 cells AhR was depleted to 15% of normal levels after 4 hr TCDD treatment; however, 45% of AhR remained in AdrR cells during this time course. In BP(R) cells AhR mRNA levels were found to be decreased relative to wild-type cells, which led to decreased AhR protein levels and DNA-binding activity. Cellular ER content has been shown to correlate with Ah-responsiveness in human breast cancer cell lines. BP(R) cells were found to be ER-positive, although chronic (BP(R) cells) and acute (24 hr) exposure to benzo[a]pyrene led to significantly lower ER protein levels in MCF-7 cells. We conclude that loss of Ah-responsiveness occurs by different mechanisms in xenobiotic-resistant MCF-7 sublines: AhR mRNA is down-regulated in BP(R) cells, whereas AdrR cells are deficient in AhR signaling by a mechanism unrelated to AhR expression and activity.

Functional analysis of activation and repression domains of the rainbow trout aryl hydrocarbon receptor nuclear translocator (rtARNT) protein isoforms

The aryl hydrocarbon receptor nuclear translocator (ARNT) protein is involved in many signaling pathways. Rainbow trout express isoforms of ARNT protein that are divergent in their C-terminal domains due to alternative RNA splicing. Rainbow trout ARNT(b) (rtARNT(b)) contains a C-terminal domain rich in glutamine and asparagine (QN), whereas the C-terminal domain of rtARNT(a) is rich in proline, serine, and threonine (PST). rtARNT(b) functions positively in AH receptor-mediated signaling, whereas rtARNT(a) functions negatively. Studies were performed to understand how changes in the C-terminal domains of the two rtARNT isoforms affect function. Deletion of the QN-rich C-terminal domain of rtARNT(b) did not affect function in aryl hydrocarbon receptor (AHR)-mediated signaling, whereas deletion of the PST-rich domain of rtARNT(a) restored function. Expression of the PST-rich domain on truncated rtARNT(b) or mouse ARNT (mARNT) reduced function of this protein by 50-80%. Gel shift assays revealed that the PST-rich domain affected AHR-mediated signaling by inhibiting DNA binding of the AHR*ARNT heterodimer. Gal4 transactivation assays revealed a potent transactivation domain in the QN-rich domain of rtARNT(b). In contrast, Gal4 proteins containing the PST-rich domain of rtARNT(a) did not transactivate because the proteins did not bind to DNA. Secondary structure analysis of the PST-rich domain revealed hydrophilic and hydrophobic regions. Truncation of the hydrophobic domain that spanned the final 20-40 amino acids of the rtARNT(a) restored function to the protein, suggesting that repressor function was related to protein misfolding or masking of the basic DNA binding domain. Functional diversity within the C-terminal domain is consistent with other negatively acting transcription factors and illustrates a common biological theme.

Regulation of cytochrome P4501A metabolism by glutathione

Gene expression of cytochrome P4501A (CYP1A) in the rainbow trout Oncorhynchus mykiss is dependent on aromatic hydrocarbon receptor signal transduction, and is markedly sensitive to tissue thiol status. Tissue glutathione (GSH) status was manipulated by exogenous GSH, L-buthionine-[S,R]-sulfoximine (BSO), lipoate or 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). Tissue GSH contents were significantly elevated in GSH- and lipoate-supplemented trout. Hepatic, renal and plasma GSH levels were markedly arrested in BSO-treated trout. Oxidized glutathione (oxidized GSH) levels were significantly elevated in the BCNU-supplemented group. Both BCNU treatment and BSO-induced GSH deficiency increased steady-state levels of hepatic CYPIA mRNA. Additional exposure to 0.1 mg/kg 3,3',4,4'-tetrachlorobiphenyl marginally suppressed the tetrachlorobiphenyl-dependent CYP1A induction in BSO-treated livers compared with the respective thiol treatment groups. Tetrachlorobiphenyl exposures altered efficiencies of thiol treatments and increased oxidized GSH content in all but the BSO-treated groups. However, exposure to 5 mg/kg tetrachlorobiphenyl altered effects of thiol treatments on CYP1A mRNA to a small extent, but catalytic activity of CYP1A was many times suppressed in BSO-treated and lipoate-supplemented fish. These results suggest that thiol status interferes with CYPIA metabolism in a two-way mode of action and provide further evidence for a cross-talk between cytochrome P4501A and glutathione.

Functional diversity of vertebrate ARNT proteins: identification of ARNT2 as the predominant form of ARNT in the marine teleost, Fundulus heteroclitus

The aryl hydrocarbon receptor nuclear translocator (ARNT) is a member of the bHLH/PAS protein superfamily. ARNT dimerizes with several PAS superfamily members, including the ligand-activated aryl hydrocarbon receptor (AHR), forming a complex that alters transcription by binding specific elements within the promoters of target genes. Two genes encode different forms of the protein in rodents: ARNT1, which is widely expressed, and ARNT2, which is limited to the brain and kidneys of adults and specific neural and branchial tissues of embryos. In an effort to characterize aryl hydrocarbon signaling mechanisms in Fundulus heteroclitus, a marine teleost that can develop heritable xenobiotic resistance, we have isolated a liver cDNA encoding an ARNT homolog. The protein exhibits AHR-dependent DNA binding capability typical of other vertebrate ARNTs. Unexpectedly, phylogenetic analysis reveals that the cDNA encodes an ARNT2. This is the only detectable ARNT sequence in Fundulus liver, gill, ovary, and brain, suggesting that ARNT2 is the predominant form of ARNT in this species. Also surprising is the relative lack of sequence identity with another fish ARNT protein, rainbow trout ARNTb, which we show forms a distinct branch outside the ARNT1 and ARNT2 clades in phylogenetic analyses. Functional diversity of ARNT proteins in fish may have important implications for the assessment of aryl hydrocarbon effects on natural populations. The increasing use of fish models in developmental and toxicological studies underscores the importance of identifying taxon-specific roles of ARNT proteins and their potential dimeric partners in the PAS superfamily.

Mechanisms of ligand-induced aryl hydrocarbon receptor-mediated biochemical and toxic responses

The ubiquitous environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) is a member of a broad group of halogenated aromatic hydrocarbons (HAHs) that is known to induce a wide range of toxic and biochemical responses in laboratory animals and humans. The effects of HAH exposure are mediated by binding to the cytosolic aryl hydrocarbon receptor (AhR), which is expressed in a tissue- and cell type-specific manner. The AhR is a ligand-activated transcription factor belonging to the basic helix-loop-helix/Per-AhR-Arnt-Sim (bHLH/PAS) superfamily of proteins. The mechanism of induction of gene transcription by TCDD involves ligand recognition and binding by the AhR, nuclear translocation, and dimerization with the AhR cofactor, AhR nuclear translocator (Arnt). The nuclear heterodimer interacts with cognate xenobiotic responsive elements (XREs) in promoter/enhancer regions of multiple Ah-responsive genes. Subsequent changes in chromatin structure and/or interaction of the AhR complex with the basal transcriptional machinery play a significant role in AhR-mediated gene expression. Although Arnt is a necessary component of a functional nuclear AhR complex, this protein also forms transcriptionally active heterodimers with other bHLH/PAS factors, including those involved in the transcriptional response to hypoxia. Arnt is ubiquitously expressed in mammalian systems, and results from transgenic mouse studies suggest that this protein plays a vital role in early mammalian embryonic development. Similar experiments suggest that the AhR may be involved in development of various organ systems. Thus, molecular mechanistic studies of TCDD action have contributed significantly to an improved understanding of the role of at least 2 bHLH/PAS proteins, as well as organ- and tissue-specific biochemical and toxic responses to this class of environmental toxins.

Caenorhabditis elegans orthologs of the aryl hydrocarbon receptor and its heterodimerization partner the aryl hydrocarbon receptor nuclear translocator

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor, until now described only in vertebrates, that mediates many of the carcinogenic and teratogenic effects of certain environmental pollutants. Here, we describe orthologs of AHR and its dimerization partner AHR nuclear translocator (ARNT) in the nematode Caenorhabditis elegans, encoded by the genes ahr-1 and aha-1, respectively. The corresponding proteins, AHR-1 and AHA-1, share biochemical properties with their mammalian cognates. Specifically, AHR-1 forms a tight association with HSP90, and AHR-1 and AHA-1 interact to bind DNA fragments containing the mammalian xenobiotic response element with sequence specificity. Yeast expression studies indicate that C. elegans AHR-1, like vertebrate AHR, requires some form of post-translational activation. Moreover, this requirement depends on the presence of the domains predicted to mediate binding of HSP90 and ligand. Preliminary experiments suggest that if AHR-1 is ligand-activated, its spectrum of ligands is different from that of the mammalian receptor: C. elegans AHR-1 is not photoaffinity labeled by a dioxin analog, and it is not activated by beta-naphthoflavone in the yeast system. The discovery of these genes in a simple, genetically tractable invertebrate should allow elucidation of AHR-1 function and identification of its endogenous regulators.

Nuclear localization and export signals of the human aryl hydrocarbon receptor

The aryl hydrocarbon receptor (Ahr) is a ligand-activated transcription factor that binds DNA in the form of a heterodimer with the Ahr nuclear translocator (hypoxia-inducible factor 1beta). We found in this study that Ahr contains both nuclear localization and export signals in the NH2-terminal region. A fusion protein composed of beta-galactosidase and full-length Ahr translocates from the cytoplasm to the nucleus in a ligand-dependent manner. However, a fusion protein lacking the PAS (Per-Ahr nuclear translocator-Sim homology) domain of the Ahr showed strong nuclear localization activity irrespective of the presence or absence of ligand. A minimum bipartite Ahr nuclear localization signal (NLS) consisting of amino acid residues 13-39 was identified by microinjection of fused proteins with glutathione S-transferase-green fluorescent protein. A NLS having mutations in bipartite basic amino acids lost nuclear translocation activity completely, which may explain the reduced binding activity to the NLS receptor, PTAC58. A 21-amino acid peptide (residues 55-75) containing the Ahr nuclear export signal is sufficient to direct nuclear export of a microinjected complex of glutathione S-transferase-Ahr-green fluorescent protein. These findings strongly suggest that Ahr act as a ligand- and signal-dependent nucleocytoplasmic shuttling protein.

A mutation in the aryl hydrocarbon receptor (AHR) in a cultured mammalian cell line identifies a novel region of AHR that affects DNA binding

Introduction of a retroviral expression vector for the aryl hydrocarbon receptor (AHR) restores CYP1A1 inducibility to a mutant derivative of the Hepa-1 cell line that is defective in induction of CYP1A1 by ligands for the receptor. An AHR protein with normal ligand binding activity is expressed in the mutant but ligand treatment of mutant cell extract fails to induce binding of the AHR. ARNT (aryl hydrocarbon receptor nuclear translocator) dimer to the xenobiotic responsive element (XRE). AHR cDNAs derived from the mutant encode a protein that is unimpaired in ligand-dependent dimerization with ARNT, but the AHR.ARNT dimer so formed is severely impaired in XRE binding activity. The mutant cDNAs contain a C to G mutation at base 648, causing a cysteine to tryptophan alteration at amino acid 216, located between the PER-ARNT-SIM homology region (PAS) A and PAS B repeats. Introduction of the same mutation in the wild-type AHR sequence by site-directed mutagenesis similarity impaired XRE binding activity. Substitution with the conservative amino acid, serine, had no effect on XRE binding. The tryptophan mutation, but not the wild-type allele, was detectable in genomic DNA of the mutant. The implication that an amino acid within the PAS region may be involved in DNA binding indicates that the DNA binding behavior of AHR may be more anomalous than previously suspected.

Cloning and characterization of the 5'-flanking region of the mouse diastrophic dysplasia sulfate transporter gene

Dyastrophic dysplasia sulfate transporter (DTDST) plays an important role in proteoglycan synthesis in the extracellular matrix of bone and cartilage. Recently, we found that the mouse DTDST gene was induced in pluripotent C3H10T1/2 cells during differentiation by bone morphogenetic protein-2 (BMP-2). To clarify the transcriptional regulation of the DTDST gene, we have cloned the 5'-flanking region of the mouse DTDST gene by the PCR based gene walking method. Sequence analysis revealed the presence of the TATA box followed by GC rich sequences containing two Sp-1 binding sites and a CBFA1 binding site. Transient transfection assays demonstrated that the basal transcriptional activity in osteoblastic MC3T3-E1 cells was mainly present between -309 and -275 bp upstream of the transcription start site (Segment -309/-275) which contained the consensus sequence for the xenobiotic-responsible element (XRE). Nuclear proteins from MC3T3-E1 cells and C3H10T1/2 cells could bind to this short segment in vitro. BMP-2 increased the promoter activity as well as the nuclear protein binding to the sequence in C3H10T1/2 cells. The present data suggest that the DTDST gene expression in osteoblasts and differentiating precursor cells to osteoblast/chondrocyte lineage would be mainly regulated by undetermined XRE binding transcription factors.

A nuclear localization signal of human aryl hydrocarbon receptor nuclear translocator/hypoxia-inducible factor 1beta is a novel bipartite type recognized by the two components of nuclear pore-targeting complex

Aryl hydrocarbon receptor nuclear translocator (ARNT) is a component of the transcription factors, aryl hydrocarbon receptor (AhR) and hypoxia-inducible factor 1, which transactivate their target genes, such as CYP1A1 and erythropoietin, in response to xenobiotic aromatic hydrocarbons and to low O2 concentration, respectively. Since ARNT was isolated as a factor required for the nuclear translocation of AhR from the cytoplasm in response to xenobiotics, the subcellular localization of ARNT has been of great interest. In this investigation, we analyzed the subcellular distribution of ARNT using transient expression of a fusion gene with beta-galactosidase and microinjection of recombinant proteins containing various fragments of ARNT in the linker region of glutathione S-transferase/green fluorescent protein. We found a clear nuclear localization of ARNT in the absence of exogenous ligands to AhR, and identified the nuclear localization signal (NLS) of amino acid residues 39-61. The characterized NLS consists of 23 amino acids, and can be classified as a novel variant of the bipartite type on the basis of having two separate regions responsible for efficient nuclear translocation activity, but considerable deviation of the sequence from the consensus of the classical bipartite type NLSs. Like the well characterized NLS of the SV40 T-antigen, this variant bipartite type of ARNT NLS was also mediated by the two components of nuclear pore targeting complex, PTAC58 and PTAC97, to target to the nuclear rim in an in vitro nuclear transport assay.