Mapping the protein/DNA contact sites of the Ah receptor and Ah receptor nuclear translocator

The aryl hydrocarbon receptor cell intrinsically promotes resident memory CD8+ T cell differentiation and function

The Aryl hydrocarbon receptor (Ahr) regulates the differentiation and function of CD4+ T cells; however, its cell-intrinsic role in CD8+ T cells remains elusive. Herein we show that Ahr acts as a promoter of resident memory CD8+ T cell (TRM) differentiation and function. Genetic ablation of Ahr in mouse CD8+ T cells leads to increased CD127-KLRG1+ short-lived effector cells and CD44+CD62L+ T central memory cells but reduced granzyme-B-producing CD69+CD103+ TRM cells. Genome-wide analyses reveal that Ahr suppresses the circulating while promoting the resident memory core gene program. A tumor resident polyfunctional CD8+ T cell population, revealed by single-cell RNA-seq, is diminished upon Ahr deletion, compromising anti-tumor immunity. Human intestinal intraepithelial CD8+ T cells also highly express AHR that regulates in vitro TRM differentiation and granzyme B production. Collectively, these data suggest that Ahr is an important cell-intrinsic factor for CD8+ T cell immunity.

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

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

Rodent genetic models of Ah receptor signaling

The aryl hydrocarbon receptor (AHR) is a ligand activated transcription factor that is a member of the PER-ARNT-SIM superfamily of environmental sensors. This receptor has been a molecule of interest for many years in the field of toxicology, as it was originally discovered to mediate the toxic effects of certain environmental pollutants like benzo(a)pyrene and 2,3,7,8-tetrachlorodibenzo-p-dioxin. While all animals express this protein, there is naturally occurring variability in receptor size and responsiveness to ligand. This naturally occurring variation, particularly in mice, has been an essential tool in the discovery and early characterization of the AHR. Genetic models including congenic mice and induced mutations at the Ahr locus have proven invaluable in further understanding the role of the AHR in adaptive metabolism and TCDD-induced toxicity. The creation and examination of Ahr null mice revealed an important physiological role for the AHR in vascular and hepatic development and mediation of the immune system. In this review, we attempt to provide an overview to many of the AHR models that have aided in the understanding of AHR biology thus far. We describe the naturally occurring polymorphisms, congenic models, induced mutations at the Ahr locus and at the binding partner Ah Receptor Nuclear Translocator and chaperone, Ah receptor associated 9 loci in mice, with a brief description of naturally occurring and induced mutations in rats.

Ahr-Foxp3-RORγt axis controls gut homing of CD4+ T cells by regulating GPR15

The orphan chemoattractant receptor GPR15 is important for homing T lymphocytes to the large intestine, thereby maintaining intestinal immune homeostasis. However, the molecular mechanisms underlying the regulation of GPR15 expression remain elusive. Here, we show a central role of the aryl hydrocarbon receptor (Ahr) in promoting GPR15 expression in both mice and human, thus gut homing of T lymphocytes. Mechanistically, Ahr directly binds to open chromatin regions of the Gpr15 locus to enhance its expression. Ahr transcriptional activity in directing GPR15 expression was modulated by two transcription factors, Foxp3 and RORγt, both of which are expressed preferentially by gut regulatory T cells (T_regs) in vivo. Specifically, Foxp3 interacted with Ahr and enhanced Ahr DNA binding at the Gpr15 locus, thereby promoting GPR15 expression. In contrast, RORγt plays an inhibitory role, at least in part, by competing with Ahr binding to the Gpr15 locus. Our findings thus demonstrate a key role for Ahr in regulating T_reg intestinal homing under the steady state and during inflammation and the importance of Ahr-RORγt-Foxp3 axis in regulating gut homing receptor GPR15 expression by lymphocytes.

Meta-Analysis of Transcriptome Data Detected New Potential Players in Response to Dioxin Exposure in Humans

Dioxins are one of the most potent anthropogenic poisons, causing systemic disorders in embryonic development and pathologies in adults. The mechanism of dioxin action requires an aryl hydrocarbon receptor (AhR), but the downstream mechanisms are not yet precisely clear. Here, we performed a meta-analysis of all available transcriptome datasets taken from human cell cultures exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Differentially expressed genes from different experiments overlapped partially, but there were a number of those genes that were systematically affected by TCDD. Some of them have been linked to toxic dioxin effects, but we also identified other attractive targets. Among the genes that were affected by TCDD, there are functionally related gene groups that suggest an interplay between retinoic acid, AhR, and Wnt signaling pathways. Next, we analyzed the upstream regions of differentially expressed genes and identified potential transcription factor (TF) binding sites overrepresented in the genes responding to TCDD. Intriguingly, the dioxin-responsive element (DRE), the binding site of AhR, was not overrepresented as much as other cis-elements were. Bioinformatics analysis of the AhR binding profile unveils potential cooperation of AhR with E2F2, CTCFL, and ZBT14 TFs in the dioxin response. We discuss the potential implication of these predictions for further dioxin studies.

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.

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.

Gene cloning and expression analysis of AhR and CYP4 from Pinctada martensii after exposed to pyrene

Pyrene, a typical polycyclic aromatic hydrocarbon, is a common pollutant in the marine environment. Polycyclic aromatic hydrocarbons initiate cellular detoxification in an exposed organism via the activation of the aryl hydrocarbon receptor (AhR). Subsequent metabolism of these xenobiotics is mainly by the cytochrome P450 enzymes of the phase I detoxification system. Full-length complementary DNA sequences from the pearl oyster Pinctada martensii (pm) encoding AhR and cytochrome P4 were cloned. The P. martensii AhR complementary DNA sequence constitutes an open reading frame that encodes for 848 amino acids. Sequence analysis indicated PmAhR showed high similarity with its homologues of other bivalve species. The cytochrome P(CYP)4 complementary DNA sequence of P. martensii constitutes an open reading frame that encodes for 489 amino acids. Quantitative real-time analysis detected both PmAhR and PmCYP4 messenger RNA expressions in the mantle, gill, hepatapancreas and adductor muscle of P. martensii exposed to pyrene. The highest transcript-band intensities of PmAhR and PmCYP4 were observed in the gill. Temporal expression of PmAhR and PmCYP4 messenger RNAs induction was observed in gills and increased between 3 and 5 days post exposure; then returned to control level. These results suggest that messenger RNAs of PmAhR and PmCYP4 in pearl oysters might be useful parameters for monitoring marine environment pyrene pollution.

Differential suppression of the aryl hydrocarbon receptor nuclear translocator-dependent function by an aryl hydrocarbon receptor PAS-A-derived inhibitory molecule

The aryl hydrocarbon receptor (AhR) heterodimerizes with the aryl hydrocarbon receptor nuclear translocator (Arnt) for transcriptional regulation. We generated three N-terminal deletion constructs of the human AhR of 12-24 kDa in size--namely D1, D2, and D3--to suppress the Arnt function. We observed that all three deletions interact with the human Arnt with similar affinities. D2, which contains part of the AhR PAS-A domain and interacts with the PAS-A domain of Arnt, inhibits the formation of the AhR gel shift complex. D2 suppresses the 3-methylcholanthrene-induced, dioxin response element (DRE)-driven luciferase activity in Hep3B cells and exogenous Arnt reverses this D2 suppression. D2 suppresses the induction of CYP1A1 at both the message and protein levels in Hep3B cells; however, the CYP1B1 induction is not affected. D2 suppresses the recruitment of Arnt to the cyp1a1 promoter but not to the cyp1b1 promoter, partly because the AhR/Arnt heterodimer binds better to the cyp1b1 DRE than to the cyp1a1 DRE. Interestingly, D2 has no effect on the cobalt chloride-induced, hypoxia inducible factor-1 (HIF-1)-dependent expression of vegf, aldolase c, and ldh-a messages. Our data reveal that the flanking sequences of the DRE contribute to the binding affinity of the AhR/Arnt heterodimer to its endogenous enhancers and the function of AhR and HIF-1 can be differentially suppressed by the D2 inhibitory molecule.

Tryptamine serves as a proligand of the AhR transcriptional pathway whose activation is dependent of monoamine oxidases

The function of the aryl hydrocarbon receptor (AhR) in mediating the biological effect to environmental pollutants is well established. However, accumulated evidence indicates a wide range of physiological and pathological functions mediated by the AhR, suggesting the existence of endogenous AhR ligand(s). The nature of an AhR ligand remain elusive; however, it is known that the AhR is activated by several compounds, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin or the tryptophan photoproduct 6-formylindolo[3,2-b]carbazole. In this study, we show that physiological concentrations of tryptamine (TA) lead to induction of cytochrome P4501A1 transcription through an AhR-dependent mechanism. In addition, we show that activation of the AhR by TA requires a functional monoamino oxidase system, suggesting that TA acts as an AhR proligand possibly by converting to a high-affinity AhR ligand. Taken together, we show a possible mechanism, through which AhR signaling is activated by endogenous conversion of TA involving monoamine oxidases.

Molecular cloning and sequence analysis and the response of a aryl hydrocarbon receptor homologue gene in the clam Ruditapes philippinarum exposed to benzo(a)pyrene

The aryl hydrocarbon receptor (AhR) is a member of the basic helix-loop-helix/Per-Arnt-Sim (bHLH/PAS) family of transcription factors. In the present study, a cDNA encoding an AhR homologue was initially cloned and sequenced from the clam, Ruditapes philippinarum. The predicted amino acid sequences contain regions characteristic of AhRs from other species including basic helix-loop-helix (bHLH) and Per-ARNT-Sim (PAS) domains, but it does not contain distinct Q-rich domain as found in the mammalian AhRs. The clam AhR homologue cDNA coded for a 765 amino acids protein and phylogenetic analysis demonstrated that it was clustered within the invertebrate AhRs branch. The clam AhR homologue mRNA expression was detected in all the adult tissues tested (gill, digestive gland, adductor muscle and mantle) and highest transcription level were observed in gill compared to other tissues. Quantitative real-time RT-PCR analysis revealed that the clam AhR homologue mRNA expression levels in gill and digestive gland of R. philippinarum were induced by benzo(a)pyrene (BaP) and the absolute expression levels of the gene showed temporal and dose-dependent response.

Lack of the aryl hydrocarbon receptor leads to impaired activation of AKT/protein kinase B and enhanced sensitivity to apoptosis induced via the intrinsic pathway

The aryl hydrocarbon receptor (AHR) is a ligand activated transcription factor that is best known for its role in mediating the toxicity of many environmental contaminants such as 2,3,7,8 tetrachlorodibenzo-p-dioxin. However, the endogenous role of AHR, especially with respect to the apoptotic process, is largely unknown and contradictory. In this report, we have used a mouse hepatoma cell line (Hepa1c1c7) and its AHR-deficient derivative (LA1) to examine the effect of differing AHR levels on apoptosis susceptibility, in particular, apoptosis regulated by the intrinsic pathway. Toward this end, the cells were subjected to UV irradiation, hydrogen peroxide, and serum starvation. Analyses of a number of different endpoints of apoptosis revealed that the LA1 cells were more sensitive to these stresses than the wild-type cells, indicating that the AHR plays a cytoprotective role in the face of stimuli that initiate the intrinsic apoptotic pathway. A direct role of the AHR in mediating this effect was confirmed using both pharmacological and molecular approaches. Further analyses imply that lack of the AHR leads to an impaired survival response mediated by phosphatidylinositol 3'-OH kinase-Akt/protein kinase B and, to a lesser degree, epidermal growth factor receptor activation. These findings indicate that exploring the use of the AHR antagonist as agents that enhance the proapoptotic actions of cancer therapies may be a valid approach.

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.

Associations of the UCP2 Gene Locus With Asymptomatic Carotid Atherosclerosis in Middle-Aged Women

Objective— Reactive oxygen species (ROS) contribute to atherogenesis. Uncoupling protein 2 (UCP2) reduces mitochondrial ROS generation and protects against the disease in animal models. A common −866G/A promoter polymorphism that has been associated with obesity and β-cell function may also affect UCP2 gene expression in cells of the arterial wall.

Methods and Results— Genotype distributions of the −866G/A and of a 45nt-del/ins polymorphism in the 3′-untranslated region of the UCP2 gene were determined in 1334 participants of the Salzburg Atherosclerosis Prevention Program in Subjects at High Individual Risk (SAPHIR). We observed a modest association of the −866G/A promoter polymorphism and 2-loci haplotypes with asymptomatic carotid atherosclerosis in female study participants. Functional studies revealed increased expression of the −866G wild-type allele in human umbilical vein endothelial cells and differentiated THP-1 cells. Electrophoretic mobility shift assay studies and antibody-interference assays performed with nuclear extracts of various cell lines showed binding of cell-type specific protein complexes to the region encompassing the −866 site and suggested involvement of hypoxia inducible factor 1α in the regulation of UCP2 gene expression in endothelial cells and macrophages.

Conclusions— Our results suggest a role of UCP2 in atherogenesis as originally proposed from studies in animal and cell culture models.

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.

Dioxin-induced immortalization of normal human keratinocytes and silencing of p53 and p16INK4a

Dioxin, a potent tumor promoter, activates the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor belonging to the basic helix-loop-helix-PAS family, to enhance tumorigenesis via unknown mechanisms. We undertook this study to determine the mechanisms underlying the impact of dioxin on cell fate, in particular senescence that occurs in normal human cells and is considered to play important tumor suppressive function. We have previously shown that in primary human keratinocytes, dioxin attenuates senescence while retaining the proliferative capacity and represses expression of the tumor suppressors, p16(INK4a) and p53. Here, we show that repression of p16(INK4a) and p53 transcriptional activity by dioxin absolutely requires the AHR and is accompanied by promoter methylation. Furthermore, dioxin alone is sufficient to immortalize normal human keratinocytes. Our data introduce a previously unrecognized regulatory pathway, that of the AHR, that impacts senescence. More importantly, this is the first report of a tumor promoter capable of inhibiting senescence in a receptor mediated manner and introduces a novel mechanism by which this carcinogen may contribute to human malignancies.

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.

Molecular cloning and expression analysis of the aryl hydrocarbon receptor of Xenopus laevis

The aryl hydrocarbon receptor (AHR) is a member of the basic helix-loop-helix/Per-Arnt-Sim (bHLH/PAS) family of transcription factors. Although this receptor has been known to mediate the toxic effects of environmental pollutants, its physiological functions remain elusive. Here, we describe the isolation and expression pattern of the Xenopus AHR gene. The predicted amino acid sequence contained regions characteristic of other vertebrate AHRs. However, in line with previously described fish AHR genes, no distinct Q-rich domain was found. Phylogenetic analysis demonstrated that Xenopus AHR was clustered within the AHR1 clade. As in the case of mammalian AHR genes, the Xenopus AHR gene was expressed in all the adult tissues tested. Xenopus AHR was also expressed during early development, in parallel with expression of the CYP1A7 gene, which is thought to be regulated by AHR. These results suggest that while frogs are relatively tolerant to TCDD toxicity, the AHR of frogs has characteristics similar to those of other vertebrate AHRs.

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.

Characterization of two distinct aryl hydrocarbon receptor (AhR2) genes in Atlantic salmon (Salmo salar) and evidence for multiple AhR2 gene lineages in salmonid fish

The aryl hydrocarbon receptor (AhR) mediates the toxicity of several environmental contaminants, e.g. 2,3,7,8-tetrachlorodibenzo-p-dioxin, and other halogenated hydrocarbons in vertebrates. This receptor initiates the transcription of several biotransformation enzymes, which in turn are responsible for causing severe harm to biological tissue. Here we describe the isolation and complete characterization of the first two AhR genes from the teleost fish Atlantic salmon (Salmo salar). The predicted amino acid sequences contain regions characteristic of other vertebrate AhRs including basic helix-loop-helix (bHLH) and PER-ARNT-SIM (PAS) domains but show little similarity to other vertebrate AhRs across the C-terminal half. Furthermore, they do not contain distinct Q-rich domains as found in the mammalian AhR, which is in line with previously described fish AhR genes. The salmon cDNAs encode 1106 and 1107 putative residues, respectively, approximately 50 amino acids longer than previously characterized AhR genes. Phylogenetic analyses demonstrated that the two salmon AhR sequences cluster within the AhR subfamily of the bHLH-PAS family, in a clade containing fish AhR2 genes. Although the two AhR2 forms are 92% identical at the amino acid level, the distribution of sequence differences and the presence of both forms in 30 tested individuals suggest that they are not allelic but derived from separate loci. Interestingly, they are not orthologs of the rainbow trout (Oncorhynchus mykiss) AhR2 alpha and beta genes and the new salmon loci are therefore here designated AhR2 gamma and AhR2 delta. In line with this, PCR with DNA from rainbow trout revealed a new trout AhR locus that was more similar to the two salmon genes than to the trout AhR2 alpha and beta genes, suggesting that the rainbow trout possesses at least three distinct AhR2 genes. The presence of multiple AhR genes in these species is probably a consequence of the genome duplications that occurred in the early evolution of fish and later also specifically in the salmonid lineage. Reverse transcription-PCR analyses revealed that both AhR2 gamma and AhR2 delta are transcribed in the liver, spleen and muscles of adult salmon.

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.

The zebrafish (Danio rerio) aryl hydrocarbon receptor type 1 is a novel vertebrate receptor

Fish are known to have two distinct classes of aryl hydrocarbon receptors, and their roles in mediating xenobiotic toxicity remain unclear. In this study, we have identified and characterized a cDNA tentatively named zebrafish AHR1 (zfAHR1). Analysis of the deduced amino acid sequence reveals that the protein is distinct from zfAHR2 and is more closely related to the mammalian aryl hydrocarbon receptor (AHR). zfAHR1 and zfAHR2 share 40% amino acid identity overall and 58% in the N-terminal half. The zfAHR1 gene maps to linkage group 16 in a region that shares conserved synteny with human chromosome 7 containing the human AHR, suggesting that the zfAHR1 is the ortholog of the human AHR. zfAHR2 maps to a separate linkage group (LG22). Both zfAHR mRNAs are expressed in early development, but they are differentially expressed in adult tissues. zfAHR2 can dimerize with zfARNT2b and binds with specificity to dioxin-responsive elements (DREs). Under identical conditions, zfAHR1/zfARNT2b/DRE complexes are formed; however, the interactions are considerably weaker. In COS-7 cells expressing zfARNT2b and zfAHR2, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure leads to a significant induction of dioxin-responsive reporter genes. In identical experiments, TCDD exposure fails to induce the reporter gene in zfAHR1-expressing cells. Ligand-binding experiments suggested that the differential zfAHR activities are attributable to differences in TCDD binding because only zfAHR2 exhibits high-affinity binding to [(3)H]TCDD or beta-naphthoflavone. Finally, using chimeric zfAHR1/zfAHR2 constructs, the lack of TCDD-mediated transcriptional activity was localized to the ligand-binding and C-terminal domains of zfAHR1.

cDNA cloning and characterization of an aryl hydrocarbon receptor from the harbor seal (Phoca vitulina): a biomarker of dioxin susceptibility?

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related planar halogenated aromatic hydrocarbons (PHAHs) are found at high concentrations in some marine mammals. Species differences in sensitivity to TCDD and PHAHs are a major limitation in assessing the ecological risk to these animals. Harbor seals accumulate high levels of PHAHs and are thought to be highly sensitive to the toxic effects of these compounds. To investigate the mechanistic basis for PHAH toxicity in harbor seals (Phoca vitulina), we sought to characterize the aryl hydrocarbon receptor (AHR), an intracellular protein that is responsible for PHAH effects. Here we report the cDNA cloning and characterization of a harbor seal AHR. The harbor seal AHR cDNA has an open reading frame of 2529 nucleotides that encodes a protein of 843 amino acids with a predicted molecular mass of 94.6 kDa. The harbor seal AHR protein possesses basic helix-loop-helix (bHLH) and Per-ARNT-Sim (PAS) domains. It is most closely related to the beluga AHR (82%) and human AHR (79%) in overall amino acid identity, indicating a high degree of conservation of AHR structure between terrestrial and some marine mammals. The ligand binding properties of the harbor seal AHR were determined using protein synthesized by in vitro transcription and translation from the cloned cDNA. Velocity sedimentation analysis on sucrose gradients showed that the harbor seal AHR exhibits specific binding of [(3)H]TCDD. The [(3)H]TCDD-binding affinity of the harbor seal AHR was compared with that of the AHR from a dioxin-sensitive mouse strain (C57BL/6) using a hydroxylapatite assay. The equilibrium dissociation constants of seal and mouse AHRs were 0.93+/-0.19 and 1.70+/-0.26 nM, respectively. Thus, the harbor seal AHR bound TCDD with an affinity that was at least as high as that of the mouse AHR, suggesting that this seal species may be sensitive to PHAH effects. The characteristics of the AHR potentially can be used as a biomarker of susceptibility to dioxin-like compounds, contributing to the assessment of the risk of these compounds to marine mammals and other protected animals.

Characterization of the aryl hydrocarbon receptor repressor gene and association of its Pro185Ala polymorphism with micropenis

BACKGROUND

Genetic background of a fetus contributes to the abnormal development after teratogen exposure. In rodents, in utero exposure to dioxins affects male external genital development. The effects of dioxins are mediated via the aryl hydrocarbon receptor (AHR) and its binding protein, aryl hydrocarbon receptor nuclear translocator (ARNT). In mice, aryl hydrocarbon receptor repressor (AHRR), which binds to ARNT in competition with AHR, plays a critical negative regulatory role in AHR signaling. We attempt to characterize the human AHRR gene and investigate the relationship between AHRR polymorphisms and the incidence of micropenis, a phenotype of undermasculinization.

METHODS

We identified and characterized the human homolog of mouse AHRR, taking advantage of the publicly available draft version of the human genome sequence. After detecting an AHRR protein polymorphism by the direct sequencing of pooled human genomic DNA, we evaluated the association between the polymorphism and the presence or absence of micropenis (< -2.5 SD) in patients with micropenis and control subjects.

RESULTS

The deduced sequence for human AHRR (715 residues) and the mouse AHRR protein exhibited 81% sequence homology to each other. The Pro185Ala polymorphism was identified between the PAS-A region and the highly conserved arginine/cysteine-rich RCFRCRL/VRC region. Forty-six percent (27/59) of patients with micropenis and 27% (22/80) of the controls were homozygous for 185Pro; this difference in frequencies was significant (P = 0.03).

CONCLUSIONS

Homozygosity for the 185Pro allele of AHRR may increase the susceptibility of a fetus to the undermasculinizing effects of dioxin exposure in utero, presumably through the diminished inhibition of AHR-mediated signaling.

An aryl hydrocarbon receptor (AHR) homologue from the soft-shell clam, Mya arenaria: evidence that invertebrate AHR homologues lack 2,3,7,8-tetrachlorodibenzo-p-dioxin and beta-naphthoflavone binding

The aryl hydrocarbon receptor (AHR) mediates numerous toxic effects following exposure of vertebrate animals to certain aromatic environmental contaminants, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). To investigate possible effects of TCDD on invertebrates, a cDNA encoding an AHR homologue was cloned from the soft-shell clam, Mya arenaria. The predicted amino acid sequence contains regions characteristic of vertebrate AHRs: basic helix-loop-helix (bHLH) and PER-ARNT-SIM (PAS) domains and a glutamine-rich region. Phylogenetic analysis shows that the clam AHR sequence groups within the AHR subfamily of the bHLH-PAS family, in a clade containing AHR homologues from Drosophila melanogaster and Caenorhabditis elegans. AHR mRNA expression was detected in all tissue types tested: adductor muscle, digestive gland, foot, gill, gonad, mantle, and siphon. The in vitro-expressed clam AHR exhibited sequence-specific interactions with a mammalian xenobiotic response element (XRE). Velocity sedimentation analysis using either in vitro-expressed clam AHR or clam cytosolic proteins showed that this AHR homologue binds neither [(3)H]TCDD nor [(3)H]beta-naphthoflavone (BNF). Similarly, in vitro-expressed D. melanogaster and C. elegans AHR homologues lacked specific binding of these compounds. Thus, the absence of specific, high-affinity binding of the prototypical AHR ligands TCDD and BNF, is a property shared by known invertebrate AHR homologues, distinguishing them from vertebrate AHRs. Comparative studies of phylogenetically diverse organisms may help identify an endogenous ligand(s) and the physiological role(s) for this protein.

The PAS superfamily: sensors of environmental and developmental signals

Over the past decade, PAS domains have been identified in dozens of signal transduction molecules and various forms have been found in animals, plants, and prokaryotes. In this review, we summarize this rapidly expanding research area by providing a detailed description of three signal transduction pathways that utilize PAS protein heterodimers to drive their transcriptional output. It is hoped that these model pathways can provide a framework for use in understanding the biology of the less well-understood members of this emerging superfamily, as well as of those to be characterized in the days to come. We use this review to develop the idea that most eukaryotic PAS proteins can be classified by functional similarities, as well as by predicted phylogenetic relationships. We focus on the alpha-class proteins, which often act as sensors of environmental signals, and the beta-class proteins, which typically act as broad-spectrum partners that target these heterodimers to their genomic targets.

Molecular characterization and developmental expression of the aryl hydrocarbon receptor from the chick embryo

The aryl hydrocarbon receptor (AhR) was cloned from the chick embryo and its function and developmental expression characterized. Chicken AhR cDNA coded for 858 amino acid protein and 396 bp of 3' UTR. The basic helix loop helix domain exhibited 87-100% amino acid identity to avian, mammalian, and amphibian AhR, and 69-74% to piscine AhR. The PAS (Per-ARNT-Sim) region was slightly less well conserved with (a) 97% identity to other avian sequences, (b) 81-86% to amphibian and mammalian AhR, and (c) 64-69% with piscine AhR. The carboxy terminus diverged the most among species with less than 53% amino acid identity between chicken and any available mammalian and piscine AhR sequences. The chicken AhR RNA and protein were 6.1 kb and 103 kDa, respectively. Chicken AhR dimerized with human AhR nuclear translocator and bound the mammalian dioxin-response element in a ligand-dependent manner. AhR protein was detected in neural ganglia; smooth, cardiac, and skeletal muscle; and epithelium involved in epithelial-to-mesenchymal transformations, such as pituitary, gastrointestinal tract, limb apical-ectodermal ridge, and kidney collecting ducts. AhR mRNA was detected in all tissues expressing protein, except myocardium. Cytochrome P4501A4 mRNA was highly induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in a subset of tissues expressing AhR, including small intestine, liver, kidney, blood vessels, and outflow tract myocardium. In conclusion, the AhR sequence and function is highly conserved between birds and mammals, and although many tissues express AhR during chick embryo development, only a subset are responsive to TCDD induction of CYP1A4.

Identification and functional characterization of two highly divergent aryl hydrocarbon receptors (AHR1 and AHR2) in the teleost Fundulus heteroclitus. Evidence for a novel subfamily of ligand-binding basic helix loop helix-Per-ARNT-Sim (bHLH-PAS) factors

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor through which 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds cause altered gene expression and toxicity. The AHR belongs to an emerging multigene family of transcription factors possessing basic helix loop helix (bHLH) and Per-ARNT-Sim (PAS) domains. Most bHLH-PAS proteins occur as duplicates or "paralog groups" in mammals, but only a single mammalian AHR has been identified. Here we report the cDNA cloning of two distinct AHRs, designated FhAHR1 and FhAHR2, from a single vertebrate species, the teleost Fundulus heteroclitus (Atlantic killifish). Both Fundulus AHR proteins possess bHLH and PAS domains that are closely related to those of the mammalian AHR. FhAHR1 and FhAHR2 are highly divergent (40% overall amino acid identity; 61% identity in the N-terminal half), suggesting that they arose from a gene duplication predating the divergence of mammals and fish. Photoaffinity labeling with 2-azido-3-[(125)I]iodo-7, 8-dibromodibenzo-p-dioxin and velocity sedimentation analysis using 2,3,7,8-[1,6-(3)H]TCDD showed that both FhAHR1 and FhAHR2 exhibit specific, high-affinity binding of dioxins. Both AHRs also showed specific, TCDD- and ARNT-dependent interactions with a mammalian xenobiotic response element. The two Fundulus AHR genes displayed different tissue-specific patterns of expression; FhAHR1 transcripts were primarily expressed in brain, heart, ovary, and testis, while FhAHR2 transcripts were equally abundant in many tissues. Phylogenetic analysis demonstrated that Fundulus AHR1 is an ortholog of mammalian AHRs, while AHR2 forms in Fundulus and other fish are paralogous to Fundulus AHR1 and the mammalian AHRs and thus represent a novel vertebrate subfamily of ligand-binding AHRs.

Specificity of DNA binding of the c-Myc/Max and ARNT/ARNT dimers at the CACGTG recognition site

Basic helix-loop-helix proteins that interact with the DNA recognition site CACGTG include the c-Myc/Max heterodimer and the ARNT (Ahreceptornucleartranslocator) homodimer. We have utilized a PCR-based protocol to identify high affinity binding sites of either the c-Myc/Max or ARNT/ARNT dimers and analyzed the ability of these dimers to interact with their derived consensus sequences and activate genes. chi(2)analysis of the selected DNA recognition sites revealed that DNA binding of the ARNT homodimer is symmetric, resulting in the consensus sequence RTCACGTGAY. Gel shift analysis demonstrated that the flanking nucleotides play an important role in dictating DNA binding affinity of the ARNT homodimer. These flanking sequences also regulate the ability of ARNT to competitively displace the c-Myc/Max heterodimer from a CACGTG-containing sequence. However, transient transfection analyses in CV-1 cells revealed that ARNT and c-Myc/Max exhibited similar abilities to activate transcription through each other's consensus sequences. Taken together, these results indicate that although binding affinity of these dimers for the CACGTG core sequences may be differentially influenced by flanking nucleotides, transcriptional activity may also be determined by other factors, such as cellular concentrations of these proteins and their co-activators.

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.

The same xenobiotic response element is required for constitutive and inducible expression of the mammalian aldehyde dehydrogenase-3 gene

The mammalian aldehyde dehydrogenase-3 gene (ALDH3) exhibits several aspects of tissue-specific expression. Certain normal tissues, such as the cornea, constitutively express ALDH3 at very high levels. Other tissues, such as normal liver, do not express ALDH3. In liver, ALDH3 is inducible by polycyclic aromatic hydrocarbon xenobiotics by an Ah-receptor (AhR)-mediated pathway in which a liganded AhR complexes with nuclear ARNT protein, and the complex binds to a xenobiotic response element (XRE) sequence located near -3.0 kb in the ALDH3 5' flanking region and initiates transcription. We used our recently developed rat corneal epithelium culture model (Boesch et al., J. Biol. Chem. 271, 5150-5157, 1996) to study the molecular basis of constitutive ALDH3 expression. Transient transfection assays of corneal epithelium using a battery of ALDH3 5' flanking region-CAT reporter gene constructs indicate that high constitutive ALDH3 expression involves the same cis-acting elements as xenobiotic-induced ALDH3 expression in liver. These elements include a strong basal promoter region and the XRE located near -3.0 kb. Western analysis confirms the presence of AhR and ARNT proteins in 3-methylcholanthrene-treated rat liver, as well as ARNT protein in rat corneal epithelium. No AhR protein is found in rat cornea. The -3.0-kb ALDH3 XRE region contains multiple overlapping transcription factor binding sequences, including consensus sites for AhR, ARNT, HNF1, HNF4, and C/ebp. Electrophoretic mobility shift assays (EMSAs) indicate that constitutive expression of ALDH3 in cornea involves binding of ARNT, HNF1, and HNF4 to the ALDH3-XRE in an Ah-receptor-independent, ARNT-requiring manner. Transient transfection of ALDH3-CAT reporter gene constructs possessing a mutation in either the ARNT- or HNF4-DNA binding sites of the XRE confirms the functional importance of these sequence motifs in constitutive ALDH3 expression.

The aryl hydrocarbon receptor: a comparative perspective

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

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.

The human ICAM-2 promoter is endothelial cell-specific in vitro and in vivo and contains critical Sp1 and GATA binding sites

The expression of intercellular adhesion molecule 2 (ICAM-2) in adult tissues is restricted to vascular endothelial cells and megakaryocytes. We have previously shown that the endothelial-specific in vivo activity of the human ICAM-2 promoter is contained within a small (0.33-kilobase (kbp)) 5'-flanking region of the gene. Here we describe the in vitro characterization of this region. The ICAM-2 promoter is TATA-less, and transcription in endothelial cells initiates at four sites. Reporter gene expression directed by the promoter was 125-fold greater than vector alone in bovine aortic endothelial cells but less than 2-fold vector alone in non-endothelial (COS) cells, confirming that specificity in vivo was paralleled in vitro. The addition of 2.7 kbp of 5'-flanking region to the 0.33-kbp fragment had no effect on promoter activity or specificity. The mutation of an Sp1 motif centered at base pair -194 or an eight-base pair palindrome at -268 each reduced promoter activity by 70%. Mutation of GATA motifs at -145 and -53 reduced promoter activity by 78 and 61%, respectively. Specific binding of bovine aortic endothelial cells nuclear proteins to the Sp1 and GATA sites was demonstrated by gel shift analysis. Promoter activity in COS cells was transactivated 3-4-fold by overexpression of GATA-2. The results presented here suggest that transcription from the ICAM-2 promoter in endothelial cells is regulated by the interplay of several positive-acting factors and provide the basis for further analysis of endothelial-specific gene expression.

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.

Molecular evolution of two vertebrate aryl hydrocarbon (dioxin) receptors (AHR1 and AHR2) and the PAS family

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor through which halogenated aromatic hydrocarbons such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) cause altered gene expression and toxicity. The AHR belongs to the basic helix-loop-helix/Per-ARNT-Sim (bHLH-PAS) family of transcriptional regulatory proteins, whose members play key roles in development, circadian rhythmicity, and environmental homeostasis; however, the normal cellular function of the AHR is not yet known. As part of a phylogenetic approach to understanding the function and evolutionary origin of the AHR, we sequenced the PAS homology domain of AHRs from several species of early vertebrates and performed phylogenetic analyses of these AHR amino acid sequences in relation to mammalian AHRs and 24 other members of the PAS family. AHR sequences were identified in a teleost (the killifish Fundulus heteroclitus), two elasmobranch species (the skate Raja erinacea and the dogfish Mustelus canis), and a jawless fish (the lamprey Petromyzon marinus). Two putative AHR genes, designated AHR1 and AHR2, were found both in Fundulus and Mustelus. Phylogenetic analyses indicate that the AHR2 genes in these two species are orthologous, suggesting that an AHR gene duplication occurred early in vertebrate evolution and that multiple AHR genes may be present in other vertebrates. Database searches and phylogenetic analyses identified four putative PAS proteins in the nematode Caenorhabditis elegans, including possible AHR and ARNT homologs. Phylogenetic analysis of the PAS gene family reveals distinct clades containing both invertebrate and vertebrate PAS family members; the latter include paralogous sequences that we propose have arisen by gene duplication early in vertebrate evolution. Overall, our analyses indicate that the AHR is a phylogenetically ancient protein present in all living vertebrate groups (with a possible invertebrate homolog), thus providing an evolutionary perspective to the study of dioxin toxicity and AHR function.

Water channels and urea transporters

1. It now appears that when water crosses an endothelium which is not fenestrated, or an epithelium with tight junctions, it does so rapidly, and with low energy cost, only if the cell membrane contains an adequate number of specific water channels, encoded by one of at least six different genes. 2. The water channel genes so far cloned encode a series of integral membrane proteins called aquaporins, all of approximately 30 kDa (265-282 amino acids), in the unglycosylated state. All but one (AQP3) are specific water channels and all but one (AQP4) are inactivated by mercurial compounds. 3. Aquaporin 0 is the major (60%) intrinsic protein (MIP) of lens fibre cells of the eye. Mutations in this gene are associated with cataract formation in mice. 4. Aquaporin 1, also called CHIP-28, exists in the membrane as a homotetramer, and is present in red blood cells, the choroid plexus, the proximal tubule and descending limb of the loop of Henle in the kidney as well as in many other sites. Surprisingly, no pathological consequence is known in patients lacking a functional AQP1 gene. 5. Aquaporin 2, also called WCH-CD, is the water channel of the principal cell of the cortical and medullary collecting duct, and is located in cytoplasmic vesicles unless arginine vasopressin is acting, when it is translocated to the apical membrane by synaptobrevins or vesicle associated membrane protein 2 (VAMP2). Lack of a functional AQP2 gene leads to a rare form of nephrogenic diabetes insipidus. 6. Aquaporins 3, 4, and 5 are located in many tissues-AQP3 and AQP4 being in the basolateral membrane of the renal cortical and medullary principal cell, as well as in the gastrointestinal tract (AQP3) and the brain (AQP4). 7. Four sequences are known for urea transporters HUT11-the urea transporter of the human red cell membrane, and HUT2, rUT2, rbUT2-the arginine vasopressin inducible urea transporters of the human, rat and rabbit kidney. They are specifically permeable to urea, not to water, and are claimed to be inhibited by phloretin. 8. The water channel proteins contain six membrane-spanning regions, whilst the urea transporters are thought to contain at least 10 membrane spanning segments. 9. Very little work has examined the ontogeny of these proteins, except in the rat, and virtually nothing is known of the expression of these genes in pregnancy or in any disorder of fluid balance in the mother or foetus.