Characterization of the aryl hydrocarbon receptor complex in human B lymphocytes: evidence for a distinct nuclear DNA-binding form

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) suppresses B lymphocyte proliferation and immunoglobulin production. We previously reported that the aryl hydrocarbon receptor (AhR) complex, composed of the AhR ligand binding subunit and the Ah receptor nuclear translocator (ARNT), was constitutively present in nuclear extracts from two human B lymphocyte cell lines (Biochem. Biophys. Res. Commun. 212, 27-34, 1995). The present study compared the AhR complex in the IM-9 and PJS-91 human B lymphocyte and HepG2 human hepatoma cell lines. AhR mRNA levels in the two lymphocyte cell lines were substantially lower than those in HepG2 cells, as was immunoreactive AhR protein. In contrast, ARNT mRNA and protein were expressed at a high level in all three cell lines. TCDD induction of cytochrome P450 1A1 mRNA and protein was detected in only the PJS-91 lymphocyte cell line, and at a markedly lower level than that in HepG2 cells. In gel shift assays, the cytosolic DNA-binding AhR complex in IM-9 and PJS-91 cells was indistinguishable from that in HepG2 cells. In contrast, the nuclear DNA-binding AhR complex in IM-9 and PJS-91 cells consisted of several closely migrating species, one being recognized by an AhR antibody, while an ARNT antibody reacted with all species. Protein:DNA cross-linking analysis revealed the presence of a novel Mr 100,000 DNA-binding protein in nuclear extracts from IM-9 and PJS-91, but not HepG2, cells that was not recognized by either AhR or ARNT antibodies. These results show that IM-9 and PJS-91 human B cells constitutively express a distinct nuclear DNA-binding form of the AhR complex that may result from the presence of an additional protein or a structural variant of the AhR.

Expression of aryl hydrocarbon receptor (AhR) and aryl hydrocarbon receptor nuclear translocator (Arnt) in adult rabbits known to be non-responsive to cytochrome P-450 1A1 (CYP1A1) inducers

Induction of aryl hydrocarbon hydroxylase by aryl hydrocarbons occurs only in neonatal rabbits and not in adult rabbits [Kahl, G. F., Friederich, D. E., Bigelow, S. W., Okey, A. B. & Nebert, D. W. (1980) Dev. Pharmacol. Ther. 1,137-162]. In the present study, we isolated cDNA clones encoding aryl hydrocarbon receptor (AhR) and aryl hydrocarbon receptor nuclear translocator (Arnt) from adult rabbits. The deduced amino acid sequences of rabbit AhR and Arnt showed 80% and 94% identities with those of human AhR and Arnt, respectively. Rabbit AhR mRNA was predominantly expressed in the lung and liver. In contrast, rabbit Arnt mRNA was expressed at almost the same level in all tissues except for the heart, liver, and small intestine. Gel shift analysis showed that the AhR. Arnt complex could bind to the consensus xenobiotic-responsive element, which indicates that AhR expressed in adult rabbit liyers possessed binding activity to the consensus xenobiotic-responsive element in vitro, although aryl hydrocarbons did not induce the activity of AHH in adult rabbits. We propose that the incapability of adult rabbits to induce cytochrome P-450 1A1 (CYP1A1) is caused by factors other than AhR and Arnt.

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

The Ah receptor (AHR) and its DNA binding partner, the Ah receptor nuclear translocator (ARNT), are basic helix-loop-helix proteins distinguished by their PER, AHR, ARNT, and SIM (PAS) homology regions. To identify the amino acids of the AHR.ARNT heterodimer that contact the TNGCGTG recognition sequence, we have performed deletion mapping and amino acid substitutions within the N termini of both the AHR and ARNT. The ability of the variant AHR and ARNT proteins to bind DNA and activate gene transcription was determined by the gel shift analysis and transient transfection assays. We have found that the amino acids of ARNT that contact DNA are similar to those of other basic/helix-loop-helix proteins and include glutamic acid residue 83 and arginine residues 86 and 87. Although our initial experiments indicated that DNA binding of the AHR may involve two regions that are bordered by amino acids 9-17 and amino acids 34-42, further analysis demonstrated that only amino acids 34-39 are critical for the AHR.TNGC interaction. These experiments indicate that while the structural features of the ARNT.GTG complex may closely resemble that deduced for proteins such as Max, E47, and USF, the AHR.TNGC complex may represent a unique DNA binding form of basic/helix-loop-helix proteins.

Basis for the loss of aryl hydrocarbon receptor gene expression in clones of a mouse hepatoma cell line

Rare benzo[a]pyrene-resistant clones were previously isolated from the mouse hepatoma cell line, Hepa-1 (Hepa1c1c7), and shown to be deficient in induction of CYP1A1 mRNA by ligands for the aryl hydrocarbon receptor (AHR). Clones belonging to complementation group B were shown to have reduced levels of ligand binding to AHR. It is shown here that all 15 independently derived B clones analyzed had much reduced levels of AHR mRNA, but in each case, the mRNA was normal in size. Infection of B clones with a retroviral expression vector for AHR restores CYP1A1 inducibility (although viral AHR expression is progressively silenced and CYP1A1 expression progressively diminishes as the cells are maintained in culture). Treatment of the B clones with the histone deacetylase inhibitors sodium butyrate or trichostatin A restores AHR expression and also restores CYP1A1 inducibility to nearly 100% of the cells in the treated cultures. Fusion of a representative B clone with a rat hepatoma cell line restores expression to the mouse AHR gene encoded by the B clone's genome. These results demonstrate that the loss of CYP1A1 inducibility in B clones is probably totally ascribable to their reduced levels of AHR and that the clones are most probably not mutated in the AHR gene but are deficient in its expression. The evidence suggests that the reduction in expression of mRNA encoded by the endogenous AHR gene in the B clones is not due to an epigenetic alteration in chromatin structure but that the clones are probably defective either in a transcription factor for the AHR gene or in a protein required for generating an open chromatin configuration over the gene.

Isolation and expression of cDNAs from rainbow trout (Oncorhynchus mykiss) that encode two novel basic helix-loop-Helix/PER-ARNT-SIM (bHLH/PAS) proteins with distinct functions in the presence of the aryl hydrocarbon receptor. Evidence for alternative mRNA splicing and dominant negative activity in the bHLH/PAS family

cDNAs encoding two distinct basic helix-loop-helix/PER-ARNT-SIM (bHLH/PAS) proteins with similarity to the mammalian aryl hydrocarbon nuclear translocator (ARNT) protein were isolated from RTG-2 rainbow trout gonad cells. The deduced proteins, termed rtARNTa and rtARNTb, are identical over the first 533 amino acids and contain a basic helix-loop-helix domain that is 100% identical to human ARNT. rtARNTa and rtARNTb differ in their COOH-terminal domains due to the presence of an additional 373 base pairs of sequence that have the characteristics of an alternatively spliced exon. The presence of the 373-base pair region causes a shift in the reading frame. rtARNTa lacks the sequence and has a COOH-terminal domain of 104 residues rich in proline, serine, and threonine. rtARNTb contains the sequence and has a COOH-terminal domain of 190 residues rich in glutamine and asparagine. mRNAs for both rtARNT splice variants were detected in RTG-2 gonad cells, trout liver, and gonad tissue. rtARNTa and rtARNb protein were identified in cell lysates from RTG-2 cells. Transfection of rtARNT expression vectors into murine Hepa-1 cells that are defective in ARNT function (type II) result in rtARNT protein expression localized to the nucleus. Treatment of these cells with 2,3,7,8-tetrachlorodibenzo-p-dioxin results in a 20-fold greater induction of endogenous P4501A1 protein in cells expressing rtARNTb when compared with rtARNTa, even though both proteins effectively dimerize with the aryl hydrocarbon receptor. The decreased function of rtARNTa appears to be due to inefficient binding of rtARNTa.AHR complexes to DNA. In addition, the presence of rtARNTa can reduce the aryl hydrocarbon receptor-dependent function of rtARNTb in vivo and in vitro.

Retinoic acid affects the expression rate of the differentiation-related genes aryl hydrocarbon receptor, ARNT and keratin 4 in proliferative keratinocytes only

The environmental contaminant dioxin exerts most of its effects by activating the aryl hydrocarbon receptor (AhR). The AhR is considered to play not only a role in the regulation of xenobiotic metabolism, but also for development, growth, and differentiation. The transcript levels of the AhR and its associated translocator protein (ARNT) were found to increase with ongoing differentiation in the human keratinocyte cell line HaCaT. Correspondingly, in situ hybridization studies in normal human skin revealed an absence of AhR-expression in proliferating basal cells and increasing transcript levels in upper cell layers, in dependence of keratinocyte differentiation. AhR expression in differentiation-deficient hyperproliferative psoriatic skin was markedly decreased. When keratinocytes were continuously treated with 1 microM retinoic acid (RA), the upregulation of AhR- and ARNT-mRNA levels was inhibited as was keratin 4-expression, a marker of HaCaT-keratinocyte differentiation. In contrast, treatment of already differentiated cells with RA did not down-regulate these transcript levels. The mRNA levels of the prevalent retinoic acid receptors in keratinocytes, RAR gamma and RXR alpha, were not influenced by the process of differentiation or by addition of RA. Our data suggest that the regulation of AhR-, ARNT- and keratin 4-expression by RA is indirect and mediated by a yet to be identified factor.

Absolute requirement of aryl hydrocarbon receptor nuclear translocator protein for gene activation by hypoxia

Hypoxia-inducible factor 1 (HIF-1) is a DNA-binding heterodimeric protein complex originally described in the transcriptional activation of the erythropoietin gene by hypoxia. This protein complex is composed of two subunits, HIF-1alpha and -1beta (aryl hydrocarbon receptor nuclear translocator, ARNT). In this study, we used ARNT-deficient cells, derived from the mouse hepatoma cell line Hepa1c1c7, to further characterize HIF-1 complex formation and its relationship with gene activation by hypoxia and desferrioxamine (Df). Gel shift assays revealed that ARNT is absolutely required for the formation of the HIF-1 DNA-binding complex. Results from RNase protection assays and Northern blots showed that the lack of functional HIF-1 complex completely abrogated the response to hypoxia of vascular endothelial growth factor (VEGF) and the glycolytic enzymes aldolase A (ALDA) and phosphoglycerate kinase 1 (PGK-1), genes known to be upregulated by low oxygen tension. Desferrioxamine induction of VEGF and PGK-1 genes was reduced in the ARNT-deficient cells, but at difference with hypoxia, it was not completely suppressed. These results suggest that Df is able to activate gene transcription through HIF-1-independent mechanisms. Exposure to hypoxia or Df did not induce any changes in HIF-1alpha and -1beta mRNA levels, suggesting that posttranscriptional mechanisms are involved in HIF-1 complex activation.

Functional interference between hypoxia and dioxin signal transduction pathways: competition for recruitment of the Arnt transcription factor

Hypoxia-inducible factor 1 alpha (HIF-1 alpha) and the intracellular dioxin receptor mediate hypoxia and dioxin signalling, respectively. Both proteins are conditionally regulated basic helix-loop-helix (bHLH) transcription factors that, in addition to the bHLH motif, share a Per-Arnt-Sim (PAS) region of homology and form heterodimeric complexes with the common bHLH/PAS partner factor Arnt. Here we demonstrate that HIF-1 alpha required Arnt for DNA binding in vitro and functional activity in vivo. Both the bHLH and PAS motifs of Arnt were critical for dimerization with HIF-1 alpha. Strikingly, HIF-1 alpha exhibited very high affinity for Arnt in coimmunoprecipitation assays in vitro, resulting in competition with the ligand-activated dioxin receptor for recruitment of Arnt. Consistent with these observations, activation of HIF-1 alpha function in vivo or overexpression of HIF-1 alpha inhibited ligand-dependent induction of DNA binding activity by the dioxin receptor and dioxin receptor function on minimal reporter gene constructs. However, HIF-1 alpha- and dioxin receptor-mediated signalling pathways were not mutually exclusive, since activation of dioxin receptor function did not impair HIF-1 alpha-dependent induction of target gene expression. Both HIF-1 alpha and Arnt mRNAs were expressed constitutively in a large number of human tissues and cell lines, and these steady-state expression levels were not affected by exposure to hypoxia. Thus, HIF-1 alpha may be conditionally regulated by a mechanism that is distinct from induced expression levels, the prevalent model of activation of HIF-1 alpha function. Interestingly, we observed that HIF-1 alpha was associated with the molecular chaperone hsp90. Given the critical role of hsp90 for ligand binding activity and activation of the dioxin receptor, it is therefore possible that HIF-1 alpha is regulated by a similar mechanism, possibly by binding an as yet unknown class of ligands.

Identification of functional aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator in murine splenocytes

The objective of the present studies was to determine whether the aryl hydrocarbon receptor (AhR) and AhR nuclear translocator (ARNT) protein are present and functional in B6C3F1 (C57BL/6 x C3H) mouse splenocytes. Northern analysis of poly(A) RNA isolated from splenocytes revealed transcripts of approximately 6.6 kb which hybridized to the AhR complementary DNA (cDNA) probe. Anti-AhR antibodies identified two major cytosolic forms of the AhR in splenocytes, approximately 95 and 104 kDa, corresponding to the codominately expressed Ahrb alleles in the B6C3F1 mice. Northern analysis utilizing an oligomer probe for ARNT identified three messenger RNA (mRNA) transcripts, approximately 5.6, 2.0, and 1.1 kb, in spleen which was consistent with the banding pattern observed in the B6C3F1 mouse liver. Western blotting confirmed the presence of the approximately 87 kDa ARNT protein in splenocytes. Protein quantitation by slot blot analysis demonstrated approximately 2.0-fold more AhR in liver than in splenocytes. Interestingly, ARNT was approximately 2.4-fold more abundant in splenocytes than in liver. Consistent with these results, comparison by quantitative reverse transcriptase-polymerase chain reaction analysis of AhR and ARNT transcripts in liver and splenocytes demonstrated approximately 2.3-fold more AhR transcripts in liver than in splenocytes and approximately 3.2-fold more ARNT transcripts in splenocytes than in liver. In addition, comparisons between AhR and ARNT transcripts isolated from the liver and splenocytes indicated a greater number of ARNT transcripts as compared with AhR in both preparations. TCDD treatment of splenocytes induced binding of the AhR nuclear complex to the dioxin-responsive enhancer (DRE) as detected by the electrophoretic mobility shift assay. These findings confirm that the AhR and ARNT are present in mouse splenocytes and are capable of binding to the DRE.

Trans-activation by the human aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator proteins: direct interactions with basal transcription factors

The aryl hydrocarbon (or dioxin) receptor (AhR) is a ligand-activated basic helix-loop-helix (bHLH) protein that heterodimerizes with the bHLH protein AhR nuclear translocator (ARNT) to form a complex that binds to xenobiotic regulatory elements in the enhancers of target genes. We used a series of fusion proteins, with a heterologous DNA-binding domain, to study independently the trans-activating function of the human AhR and ARNT proteins in yeast. The results confirm that both the human AhR and ARNT contain carboxyl-terminal trans-activation domains. The AhR has a complex trans-activation domain that is composed of multiple segments that function independently and exhibit varying levels of activation. Furthermore, these regions within the AhR cooperate when linked together, resulting in a synergistic activation of transcription. Fusion proteins of the AhR and ARNT trans-activation domains with the LexA DNA-binding domain, expressed in bacteria and purified to near-homogeneity, stimulated transcription of a minimal promoter in vitro in yeast nuclear extracts. Using this in vitro transcription assay, it was also possible to demonstrate that the AhR and ARNT trans-activation domains, in the absence of a DNA-binding domain, inhibited activated and basal transcription. Furthermore, in vitro the receptor bound selectively to the basal transcription factors, the TATA-binding protein and TFIIF, whereas ARNT bound preferentially to TFIIF. Taken together, these results suggest that AhR and ARNT activate target gene expression, at least in part, through direct interactions with basal transcription factors.

Induction of phosphoglycerate kinase 1 gene expression by hypoxia. Roles of Arnt and HIF1alpha

To identify new dimerization partners for the aromatic hydrocarbon receptor nuclear translocator (Arnt), we used its N-terminal region (amino acids 1-470) as a target in a two-hybrid screening procedure, and we cloned the murine form of hypoxia-inducible factor 1alpha (HIF1alpha). Sequence comparisons reveal substantial identity between mouse and human HIF1alpha. Hypoxia induces a 10-fold accumulation of phosphoglycerate kinase 1 mRNA in wild type mouse hepatoma (Hepa 1c1c7) cells; the induction mechanism is Arnt dependent because induction does not occur in Arnt-defective cells. Furthermore, induction of phosphoglycerate kinase 1 mRNA requires Arnt's N-terminal region, which mediates DNA binding and heterodimerization; in contrast, induction does not require Arnt's C-terminal region, which mediates transactivation. We also show that a GAL4-HIF1alpha fusion protein transactivates a GAL4-dependent gene in the absence of Arnt, that HIF1alpha's transactivation capability is inducible by hypoxia, and that both hypoxia responsiveness and transactivation capability reside within the C-terminal 83 amino acids of HIF1alpha. Our findings generate new insights into the mechanism by which Arnt and HIF1alpha induce transcription in response to hypoxia.

Cloning and selective expression in brain and kidney of ARNT2 homologous to the Ah receptor nuclear translocator (ARNT)

Arnt2, a new member of the basic-helix-loop-helix transcription factor family, was cloned from rat brain cDNAs. Its deduced 712 amino acid sequence displays 63% identity with that of the aryl hydrocarbon receptor nuclear translocator (Arnt1) that was completely established. Whereas Arnt2 gene expression, established by Northern blotting and in situ hybridization histochemistry, occurred selectively in brain and kidney, that of Arnt1 was ubiquitous, suggesting that the two proteins play distinct roles, presumably via dimerization and DNA binding with different partners.

In vivo expression of mRNAs encoding hypoxia-inducible factor 1

Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric basic helix-loop-helix transcription factor that regulates genes whose products play key roles in maintaining O2 homeostasis. We have previously demonstrated that HIF-1 mRNA, protein, and DNA-binding activity are induced when mammalian tissue culture cells are subjected to hypoxia. In this paper, we report our analysis of HIF-1 mRNA expression in vivo. We demonstrate expression of HIF-1 alpha and HIF-1 beta (ARNT) mRNA in all human, rat, and mouse organs assayed and show for the first time that HIF-1 mRNA expression was induced in brain, kidney, and lung when rats or mice were exposed to reduced ambient O2 concentrations for 30 to 60 min. The ubiquitous in vivo expression of HIF-1 alpha and HIF-1 beta (ARNT) mRNA is consistent with the proposed role of HIF-1 in coordinating adaptive transcriptional responses to hypoxia.

Identification of a cell-specific transcription activation domain within the human Ah receptor nuclear translocator

In the presence of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related chemicals, the Ah receptor nuclear translocator (Arnt) forms a heterodimeric complex with the ligand-bound Ah receptor, leading to recognition of dioxin-responsive elements within the enhancer of the CYP1A1 gene and transcription activation by an unknown mechanism. To understand the role of Arnt in transcription activation by the Ah receptor-Arnt heterodimer, we performed a deletion analysis of Arnt to locate domains that are directly involved in transcription activation. We showed that the C-terminal 34 amino acids of Arnt encode a transcription activation domain (TAD) that functions independently of other sequences in the Ah receptor complex when attached to the heterologous Gal4 DNA binding domain. Deletion of the C-terminal acidic-rich 14 amino acids completely abolishes activity. Sequences important in Arnt TAD function were independent of the glutamine-rich region which is an important structural feature in the TAD of other transcription factors. The strength of the Arnt TAD when compared with the strong TAD from the herpes simplex virus VP16 protein was cell-type specific. Both the Arnt and VP16 TAD were equally strong in COS-1 cells, but the Arnt TAD had weak activity in an Arnt-deficient mouse hepatoma cell line and was not needed for restoration of CYP1A1 activation. These results imply that for CYP1A1 activation the Ah receptor provides the dominant activation function for the heterodimer in hepatoma cells. The potential of the Arnt TAD to contribute to activation by the Ah receptor complex is likely determined by availability or activity of cell-specific factors with which the TAD interacts.

The role of the aryl hydrocarbon receptor nuclear translocator (ARNT) in hypoxic induction of gene expression. Studies in ARNT-deficient cells

Hypoxia-inducible factor-1 (HIF-1), a DNA-binding complex implicated in the regulation of gene expression by oxygen, has been shown to consist of a heterodimer of two basic helix-loop-helix Per-AHR-ARNT-Sim (PAS) proteins, HIF-1alpha, and HIF-1beta. One partner, HIF-1beta, had been recognized previously as the aryl hydrocarbon receptor nuclear translocator (ARNT), an essential component of the xenobiotic response. In the present work, ARNT-deficient mutant cells, originally derived from the mouse hepatoma line Hepa1c1c7, have been used to analyze the role of ARNT/HIF-1beta in oxygen-regulated gene expression. Two stimuli were examined: hypoxia itself and desferrioxamine, an iron-chelating agent that also activates HIF-1. Induction of the DNA binding and transcriptional activity of HIF-1 was absent in the mutant cells, indicating an essential role for ARNT/HIF-1beta. Analysis of deleted ARNT/HIF-1beta genes indicated that the basic, helix-loop-helix, and PAS domains, but not the amino or carboxyl termini, were necessary for function in the response to hypoxia. Comparison of gene expression in wild type and mutant cells demonstrated the critical importance of ARNT/HIF-1beta in the hypoxic induction of a wide variety of genes. Nevertheless, for some genes a reduced response to hypoxia and desferrioxamine persisted in these mutant cells, clearly distinguishing ARNT/HIF-1beta-dependent and ARNT/HIF-1beta-independent mechanisms of gene activation by both these stimuli.

Mapping the 90 kDa heat shock protein binding region of the Ah receptor

Expression of a series of Ah receptor (AhR) deletion mutants in an in vitro translation system has been previously used to map several functional domains of the murine AhR (Dolwick et al. (1993) Proc. Natl. Acad. Sci. USA 90, 8566-8570). In this report, quantitative immunoprecipitation of 90-kDa heat shock protein (hsp90) from reticulocyte lysate allowed us to measure the level of the AhR and AhR deletion mutants complexed with hsp90. After translation of a series of deletion mutants it was determined that there are two distinct domains important in forming a stable AhR/hsp90 complex, corresponding to amino acid sequences 1-166 and 289-347 of the AhR. Neither ARNT, nor Per were able to stably interact with hsp90. Thus, the AhR appears to be a unique member of the PAS domain family of proteins that binds a known ligand and stably interacts with hsp90.

Cooperative interaction between AhR.Arnt and Sp1 for the drug-inducible expression of CYP1A1 gene

Expression of CYP1A1 gene is regulated in a substrate-inducible manner through at least two kinds of regulatory DNA elements in addition to the TATA sequence, XRE (xenobiotic responsive element), and BTE (basic transcription element), a GC box sequence. The trans-acting factor on the XRE is a heterodimer consisting of arylhydrocarbon receptor (AhR) and AhR nuclear translocator (Arnt), while Sp1 acts as a regulatory factor on the BTE. We have investigated how these factors interact with one another to induce expression of the CYP1A1 gene. Both in vivo transfection assays using Drosophila Schneider line 2 (SL2) cells, which is devoid of endogenous Sp1, AhR, and Arnt, and in vitro transcription assays using baculovirus-expressed AhR, Arnt, and Sp1 proteins revealed that these factors enhanced synergistically expression of the reporter genes driven by a model CYP1A1 promoter, consisting of four repeated XRE sequences and a BTE sequence, in agreement with previous observation (Yanagida, A., Sogawa, K., Yasumoto, K., and Fujii-Kuriyama, Y. (1990) Mol. Cell. Biol. 10, 1470-1475). We have proved by coimmunoprecipitation assays and DNase I footprinting that both AhR and Arnt interact with the zinc finger domain of Sp1 via their basic HLH/PAS domains. When either the AhR.Arnt heterodimer of Sp1 was bound to its cognate DNA element, DNA binding of the second factor was facilitated. Survey of DNA sequences in the promoter region shows that the XRE and GC box elements are commonly found in the genes whose expressions are induced by polycyclic aromatic hydrocarbons, suggesting that the two regulatory DNA elements and their cognate trans-acting factors constitute a common mechanism for induction of a group of drug-metabolizing enzymes.

Characterization of the activated form of the aryl hydrocarbon receptor in the nucleus of HeLa cells in the absence of exogenous ligand

The aryl hydrocarbon receptor (AhR) is known to mediate 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced toxic effects. Immunocytochemical studies revealed that AhR in HeLa cells is localized throughout the cell. Upon TCDD treatment most of the cytoplasmic receptor is translocated into the nucleus in a time-dependent manner. A significant amount of AhR was found to be tightly associated with the nuclear fraction of untreated HeLa cells. The level of receptor in the nuclear fraction was approximately 16% of the total cellular receptor pool. Further characterization of AhR heterocomplex from the HeLa nuclear fraction by sucrose density gradient analysis revealed that the AhR was present in the 6 S form, and that the nuclear AhR could be coimmunoprecipitated using anti-Arnt mAb. The ability of the AhR to specifically interact with dioxin-responsive elements (DRE) was demonstrated utilizing wild-type and two mutant DREs in gel shift assays. These results would suggest that, in HeLa cells, the AhR-Arnt heterodimer is associated with the nuclear fraction under normal culture conditions. Therefore, HeLa cells can be used as a model system to study the biochemical and molecular function of the Ah receptor and the process that leads to activation of the AhR in the absence of exogenous ligand.

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

The aryl hydrocarbon receptor (AHR) and the aryl hydrocarbon receptor nuclear translocator (ARNT) belong to a novel subclass of basic helix-loop-helix transcription factors. The AHR.ARNT heterodimer binds to the xenobiotic responsive element (XRE). Substitution of each of four amino acids in the basic region of ARNT with alanine severely diminishes or abolishes XRE binding, intimating that these amino acids contact DNA bases. Three of these amino acids are conserved among basic helix-loop-helix proteins, and the corresponding amino acids of Max and USF are known to contact DNA bases. Alanine scanning mutagenesis of the basic domain of AHR and substitution with conservative amino acids at particular positions in this domain and in a more amino-proximal AHR segment previously shown to be required for XRE binding (Fukunaga, B. N., and Hankinson, O. (1996) J. Biol. Chem. 271, 3743-3749) demonstrate that the most carboxyl-proximal amino acid position of the basic domain and a position within the amino-proximal segment are intolerant to amino acid substitution with regard to XRE binding, suggesting that these two amino acids make base contacts. Amino acid positions in these AHR regions and in the ARNT basic region less adversely affected by substitution are also identified. The amino acids at these positions may contact the phosphodiester backbone. The apparent bipartite nature of the DNA binding region of AHR and the identity of those of its amino acids that apparently make DNA contacts impute a novel protein-DNA binding behavior for AHR.

DNA binding by the heterodimeric Ah receptor. Relationship to dioxin-induced CYP1A1 transcription in vivo

The environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin induces the microsomal enzyme cytochrome P4501A1 by increasing the transcription rate of the CYP1A1 gene. Induction requires two basic helix-loop-helix proteins, the ligand-binding aromatic hydrocarbon receptor (AhR) and its heterodimerization partner, the AhR nuclear translocator (Arnt). The AhR/Arnt heterodimer induces transcription by binding to dioxin-responsive elements (DREs) within an enhancer upstream of the CYP1A1 gene. The basic regions of AhR and Arnt are crucial for DRE binding. We have mutated these regions in order to analyze the relationship between DRE binding (determined in vitro using an electrophoretic mobility shift assay) and induction of CYP1A1 transcription (determined in vivo by genetic complementation of AhR-defective and Arnt-defective mouse hepatoma cells, using an RNase protection assay to measure mRNA accumulation). Our findings reveal the amino acids in the basic regions of AhR/Arnt that are important for both DRE binding and induction of transcription. This information provides biological background for the interpretation of structural (e.g. crystallographic) studies of the interactions between AhR/Arnt and the DRE. Our findings also indicate that the in vitro behavior of the mutants does not consistently predict their functional activity in vivo. Thus, genetic complementation constitutes an important and stringent test for analyzing the effects of mutations on AhR/Arnt function.

cDNA cloning and tissue-specific expression of a novel basic helix-loop-helix/PAS factor (Arnt2) with close sequence similarity to the aryl hydrocarbon receptor nuclear translocator (Arnt)

We isolated mouse cDNA clones (Arnt2) that are highly similar to but distinct from the aryl hydrocarbon receptor (AhR) nuclear translocator (Arnt). The composite cDNA covered a 2,443-bp sequence consisting of a putative 2,136-bp open reading frame encoding a polypeptide of 712 amino acids. The predicted Arnt2 polypeptide carries a characteristic basic helix-loop-helix (bHLH)/PAS motif in its N-terminal region with close similarity (81% identity) to that of mouse Arnt and has an overall sequence identity of 57% with Arnt. Biochemical properties and interaction of Arnt2 with other bHLH/PAS proteins were investigated by coimmunoprecipitation assays, gel mobility shift assays, and the yeast two-hybrid system. Arnt2 interacted with AhR and mouse Sim as efficiently as Arnt, and the Arnt2-AhR complex recognized and bound specifically the xenobiotic responsive element (XRE) sequence. Expression of Arnt2 successfully rescued XRE-driven reporter gene activity in the Arnt-defective c4 mutant of Hepa-1 cells. RNA blot analysis revealed that expression of Arnt2 mRNA was restricted to the brains and kidneys of adult mice, while Arnt mRNA was expressed ubiquitously. In addition, whole-mount in situ hybridization of 9.5-day mouse embryos showed that Arnt2 mRNA was expressed in the dorsal neural tube and branchial arch 1, while Arnt transcripts were detected broadly in various tissues of mesodermal and endodermal origins. These results suggest that Arnt2 may play different roles from Arnt both in adult mice and in developing embryos. Finally, sequence comparison of the currently known bHLH/PAS proteins indicates a division into two phylogenetic groups: the Arnt group, containing Arnt, Arnt2, and Per, and the AhR group, consisting of AhR, Sim, and Hif-1alpha.

TCDD-inducible plasminogen activator inhibitor type 2 (PAI-2) in human hepatocytes, HepG2 and monocytic U937 cells

Induction of PAI-2 by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been studied in human primary hepatocytes, hepatoma HepG2 cells and monocytic U937 cells, extending recent findings in human keratinocytes. PAI-2 represents a serpine-type protease inhibitor with wide-ranging implications in fibrinolysis, extracellular matrix proteolysis, growth factor activation and carcinogenesis. PAI-2 was induced by >10(-9) M TCDD in hepatocytes and HepG2 cells and by >10(-10) M TCDD in U937 cells. In the latter cell line, PAI-2 induction by TCDD and by 12-O-tetradecanoyl phorbol-13-acetate (TPA) has been compared. TCDD appeared to be less efficient than TPA as an inducer of PAI-2. In contrast to induction by TPA, PAI-2 induction by TCDD was found to be biphasic, with an early peak of mRNA at 1-3 h and a late peak at 12-24 h. A biphasic response was also seen at the protein level although production of PAI-2 protein lagged behind the corresponding mRNA. PAI-2 is known to contain AP-1 sites, i.e. Jun/Fos protein-binding sites, in its promotor region. Hence, PAI-2 induction by TCDD has originally been conceived to be due to an indirect response, secondary to the induction of Jun/Fos proteins. Therefore, expression of jun/fos genes and their AP-1 activity were studied at the early phase of PAI-2 induction by TCDD. TCDD did not increase mRNA of c-fos, c-jun, junB or junD (in contrast to TPA which markedly increased the expression of c-fos and junB), nor did TCDD increase AP-1 activity. In conclusion, the findings suggest that PAI-2 induction by TCDD is not restricted to human keratinocytes but includes liver cells and monocytic U937 cells. The induction mechanism is complex but the early phase does not appear to involve Jun/Fos proteins.

The aryl-hydrocarbon receptor, but not the aryl-hydrocarbon receptor nuclear translocator protein, is rapidly depleted in hepatic and nonhepatic culture cells exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Western blot analysis and indirect immunofluorescence microscopy were used to evaluate the fate of the aryl-hydrocarbon receptor (AhR) and aryl-hydrocarbon receptor nuclear translocator (Arnt) protein in culture cell models exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In wild-type (WT) murine Hepa-1c1c7 cells, AhR protein was depleted by 85% after 4 hr of TCDD treatment as measured in total cell lysates. In contrast, the concentration of Arnt protein was unaffected by TCDD treatment in WT cells. Analysis of the AhR with immunofluorescence microscopy revealed that nuclear translocation of the liganded AhR preceded its depletion from cells. AhR protein was depleted from Hepa-1 type I variants (that contain a concentration of AhR that is 10% of WT) with a similar time course and to the same maximal level observed in WT cells (85%). The EC50 for AhR depletion in Hepa-1 cells was 39 pm TCDD and correspond to the EC50 for induction of P4501A1 protein. Murine embryonic fibroblasts (NIH-3T3), rat aortic smooth muscle cells (A7), and murine skeletal muscle cells (C2C12) all exhibited >90% depletion of the AhR after 2-4 hr of TCDD treatment. Arnt concentration was not affected by TCDD in these cell lines. These results indicate that the liganded AhR is rapidly depleted within the nuclear compartment of hepatic and nonhepatic cells in a manner independent of the Arnt protein.

Autoregulation of human CYP1A1 gene promotor activity in HepG2 and MCF-7 cells

Cytochrome CYP1A1 gene expression, induced by polycyclic aromatic hydrocarbons and dioxins, eg. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), is regulated mainly at the level of transcription. Inducible activation of the CYP1A1 promotor is mediated by a ligand-dependent transcription factor dimer complex including the aryl hydrocarbon receptor (AHR) and the AHR nuclear translocator (ARNT) proteins. Additional factors seem to be involved in tissue- and cell-specific modification of the induction process. In the present study HepG2 and MCF-7 cell lines were used to examine a possible cell-specific autoregulation of CYP1A1 promotor function. Chimeric CYP1A1-CAT reporter constructs and a human CYP1A1 cDNA expression plasmid were used in transient co-expression experiments. In HepG2 cells co-expression of increasing amounts of CYP1A1 cDNA significantly down-regulated constitutive as well as the TCDD-induced CYP1A1 promotor driven CAT activity. In contrast, co-transfection of MCF-7 cells with a 3-fold molar excess of CYP1A1 cDNA relative to the CYP1A1-CAT reporter construct caused an approximately 2-fold increase in the TCDD-induced CAT activity, whereas no effect was observed on constitutive promotor activity. This autoregulatory mechanism(s) of the human CYP1A1 gene product was independent of specific 5' flanking promotor segments tested. RT-PCR analyses did not indicate any changes in mRNA level of AHR and ARNT in the co-transfection studies. Thus these studies show that the human CYP1A1 gene is exposed to cell-specific autoregulation, probably achieved via different functions of trans-acting factors.

Identification of a novel domain in the aryl hydrocarbon receptor required for DNA binding

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 protein (ARNT). Both proteins possess basic helix-loop-helix motifs. ARNT binds to the side of the xenobiotic responsive element (XRE) that resembles an E-box (the sequence recognized by the majority of other basic helix-loop-helix proteins), whereas AHR binds to the side of the XRE that does not conform to the E-box sequence. The basic region of ARNT closely resembles those of other E-box-binding proteins, whereas the "nominal basic region" of AHR (amino acids 27 39), although required for XRE binding, deviates from this consensus. By extensive mutational analysis it is shown here that an additional block of amino acids of AHR (from tyrosine 9 to lysine 20) that contains a highly basic segment is required for XRE binding and transcriptional activation. Deletion of the first nine amino acids negates XRE binding. Substitution of either tyrosine 9 or arginine 14 with alanine eliminates XRE binding, whereas alanine substitutions at certain other sites within the block reduce but do not eliminate binding. The reported absence of the first nine amino acids in the purified protein may therefore be artifactual. These results suggest that the amino acids of AHR involved in binding to the XRE constitute a novel DNA-binding domain, comprising amino acids located within and amino-terminal to the nominal basic region.

cDNA cloning of a murine homologue of Drosophila single-minded, its mRNA expression in mouse development, and chromosome localization

A combination of the RT-PCR method and subsequent screening of the cDNA library of mouse skeletal muscle with the cDNA isolated by RT-PCR used as a probe led to isolation of cDNAs encoding a polypeptide (mSim) with bHLH and PAS domains which show high similarity to the corresponding regions of Drosophila Sim, a master regulator in neurogenesis. Experiments using a GST-fusion protein demonstrated that mSim heterodimerizes with Arnt (Ah receptor nuclear translocator), even more efficiently than AhR (Ah receptor) does with Arnt. RNA blot analysis using RNAs from various tissues of mice indicated that mSim transcript is expressed in several limited tissues such as muscle, kidney and lung of adult animals. Distribution of mSim mRNA was always accompanied with that of Arnt. All the results suggest a regulatory role of mSim in partnership with Arnt. Chromosomal location of the mSim gene was determined by fluorescent in situ hybridization to be localized on the C3.3-C4 band of mouse chromosome 16 which is syntenic with the human chromosome 21q22 carrying the Down syndrome critical region, where a gene highly homologous to the Drosophila sim was localized. Whole mount in situ hybridization using a unique part of mSim cDNA sequence showed that mSim mRNA was expressed in the ventral diencephalon, branchial arches and limbs. These findings will provide an approach to the cause of the Down syndrome as well as the elucidation of the functional roles of mSim in animal development.

Dioxin-induced CYP1A1 transcription in vivo: the aromatic hydrocarbon receptor mediates transactivation, enhancer-promoter communication, and changes in chromatin structure

We have analyzed the dioxin-inducible transcriptional control mechanism for the mouse CYP1A1 gene in its native chromosomal context. Our genetic and biochemical studies indicate that a C-terminal segment of the aromatic hydrocarbon receptor (AhR) contains latent transactivation capability and communicates the induction signal from enhancer to promoter. Thus, transactivation and enhancer-promoter communication may be congruent functions of AhR. Both functions require heterodimerization between AhR and the AhR nuclear translocator (Arnt). Our findings also indicate that heterodimerization activates AhR's latent transactivation function and silences that of Arnt. Furthermore, removal of Arnt's transactivation domain does not affect dioxin-induced CYP1A1 transcription in vivo. In addition, our studies demonstrate that dioxin-induced changes in chromatin structure occur by different mechanisms at the CYP1A1 enhancer and promoter and that events at an enhancer can be experimentally dissociated from events at the cognate promoter during mechanistic analyses of mammalian transcription in vivo.

The basic helix-loop-helix/PAS factor Sim is associated with hsp90. Implications for regulation by interaction with partner factors

Sim is a Drosophila developmental basic helix-loop-helix (bHLH) transcription factor containing a Per-Arnt-Sim (PAS) region of homology. Here we demonstrate that Sim, in analogy to the structurally related bHLH/PAS dioxin receptor, was stably associated with the molecular chaperone hsp90. In the case of the dioxin receptor, release of hsp90 and derepression of receptor function appear to be regulated by ligand binding and dimerization with Arnt, a non-hsp90-associated bHLH/PAS factor. Dimerization with Arnt very efficiently disrupted Sim-hsp90 interaction, a process that required both the bHLH and PAS dimerization motifs of Arnt. Moreover, hsp90 was also released upon dimerization of Sim with the Drosophila PAS factor Per, whereas the hsp90-associated dioxin receptor failed to interact with Sim. These results indicate that hsp90 may play a role in conditional regulation of Sim function, and that Per and possibly bHLH/PAS partner factors may activate Sim by inducing release of hsp90 during the dimerization process.

Identification of functional domains of the aryl hydrocarbon receptor

Functional domains of the mouse aryl hydrocarbon receptor (Ahr) were investigated by deletion analysis. Ligand binding was localized to a region encompassing the PAS B repeat. The ligand-mediated dissociation of Ahr from the 90-kDa heat shock protein (HSP90) does not require the aryl hydrocarbon receptor nuclear translocator (Arnt), but it is slightly enhanced by this protein. One HSP90 molecule appears to bind within the PAS region. The other molecule of HSP90 appears to require interaction at two sites: one over the basic helix-loop-helix region, and the other located within the PAS region. Each mutant was analyzed for dimerization with full-length mouse Arnt and subsequent binding of the dimer to the xenobiotic responsive element (XRE). In order to minimize any artificial steric hindrances to dimerization and XRE binding, each Ahr mutant was also tested with an equivalently deleted Arnt mutant. The basic region of Ahr is required for XRE binding but not for dimerization. Both the first and second helices of the basic helix-loop-helix motif and the PAS region are required for dimerization. These last results are analogous to those previously obtained for Arnt (Reisz-Porszasz, S., Probst, M.R., Fukunaga, B. N., and Hankinson, O. (1994) Mol. Cell. Biol. 14, 6075-6086) compatible with the notion that equivalent regions of Ahr and Arnt associate with each other. Deletion of the carboxyl-terminal half of Ahr does not affect dimerization or XRE binding but, in contrast to an equivalent deletion of Arnt, eliminates biological activity as assessed by an in vivo transcriptional activation assay, suggesting that this region of Ahr plays a more prominent role in transcriptional activation of the cyp1a1 gene than the corresponding region of Arnt.

DNA binding specificities and pairing rules of the Ah receptor, ARNT, and SIM proteins

The Ah receptor (AHR), the Ah receptor nuclear translocator protein (ARNT), and single-minded protein (SIM) are members of the basic helix-loop-helix-PAS (bHLH-PAS) family of regulatory proteins. In this study, we examine the DNA half-site recognition and pairing rules for these proteins using oligonucleotide selection-amplification and coprecipitation protocols. Oligonucleotide selection-amplification revealed that a variety of bHLH-PAS protein combinations could interact, with each generating a unique DNA binding specificity. To validate the selection-amplification protocol, we demonstrated the preference of the AHR.ARNT complex for the sequence commonly found in dioxin-responsive enhancers in vivo (TNGCGTG). We then demonstrated that the ARNT protein is capable of forming a homodimer with a binding preference for the palindromic E-box sequence, CACGTG. Further examination indicated that ARNT may have a relaxed partner specificity, since it was also capable of forming a heterodimer with SIM and recognizing the sequence GT(G/A)CGTG. Coprecipitation experiments using various PAS proteins and ARNT were consistent with the idea that the ARNT protein has a broad range of interactions among the bHLH-PAS proteins, while the other members appear more restricted in their interactions. Comparison of this in vitro data with sites known to be bound in vivo suggests that the high affinity half-site recognition sequences for the AHR, SIM, and ARNT are T(C/T)GC, GT(G/A)C (5'-half-sites), and GTG (3'-half-sites), respectively.

Functional analysis of aryl hydrocarbon receptor nuclear translocator interactions with aryl hydrocarbon receptor in the yeast two-hybrid system

The aryl hydrocarbon receptor (AHR) mediates dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin)-induced transcriptional activation of a battery of genes by interaction with a cofactor, called aryl hydrocarbon receptor nuclear translocator (ARNT) protein. Both AHR and ARNT belong to a family of proteins that includes the Drosophila circadian-rhythm protein and "single-minded" protein. These proteins share a domain called the PAS domain. In addition to the PAS domain, both AHR and ARNT contain basic helix-loop-helix (bHLH) and glutamine (Q)-rich domains. The roles of these domains in the receptor-mediated transcriptional activation are not understood completely. By using the yeast two-hybrid system with the N-terminal half of AHR as a probe, which contains the bHLH and PAS regions, to screen cDNA libraries prepared from human lymphocytes and C57BL mouse liver for clones encoding proteins capable of binding to these regions, we isolated a partial ARNT cDNA clone. These results demonstrated that the N-terminal half of AHR is capable of interacting with ARNT in yeast (probably through the bHLH motif). A fusion protein containing the GAL4 DNA binding domain (DB) linked to the full-length AHR was not capable of activating expression of a reporter gene containing the GAL4 DNA binding site, suggesting that ligand-free AHR alone has no transactivating properties in yeast. However, the C-terminal portion (amino acid residues 580-797) of the AHR, including the Q-rich domain, could confer transactivation of the reporter gene expression in the same system, suggesting that the N-terminal portion of the AHR contains transcription repression properties. In contrast, GAL4(DB)-ARNT fusion protein was able to activate expression of the same reporter gene. Deletion analysis of ARNT revealed that the C-terminal 75 amino acids, including the Q-rich domain, exhibited full transactivation function in yeast and mammalian cells. These results revealed different structural organizations for the transactivation properties between AHR and ARNT, although both contained transactivation domains at the C-termini.

Developmental expression of two members of a new class of transcription factors: II. Expression of aryl hydrocarbon receptor nuclear translocator in the C57BL/6N mouse embryo

The aryl hydrocarbon receptor (AhR) and the AhR nuclear translocator protein (ARNT) are basic-helix-loop-helix (bHLH) proteins involved in transcriptional regulation. The AhR is a ligand-activated partner of the ARNT protein. Both proteins are required to transcriptionally regulate gene expression. ARNT must be complexed to AhR to permit binding to the regulatory DNA sequence. The AhR-ligand complex is known to mediate a range of biological responses, such as developmental toxicity, induction of cleft palate, and hydronephrosis. AhR and ARNT are expressed in human embryonic palatal cells and AhR was recently shown to have a specific developmental pattern of expression in the mouse embryo. In the present study, expression of ARNT is characterized in C57Bl/6N mouse embryos from gestation day (GD) 10-16 using immunohistochemistry and in situ hybridization. Af affinity purified antibody against human ARNT (1.1 micrograms/ml) was detected with an avidin-biotin-peroxidase complex. ARNT mRNA was localized with a 35S-RNA probe from pBM5/NEO-M1-1. Specific spatial and temporal patterns of ARNT expression emerged and mRNA and protein expression correlated. The GD 10-11 embryos showed highest levels of ARNT in neuroepithelial cells of the neural tube, visceral arches, otic and optic placodes, and preganglionic complexes. The heart also had significant expression of ARNT with strong nuclear localization. After GD11, expression in heart and brain declined. In GD 12-13 embryos expression was highest in the liver where expression increased from GD 12-16. At GD 15-16 the highest levels of ARNT occurred in adrenal gland and liver, although ARNT was also detected in submandibular gland, ectoderm, tongue, bone, and muscle. In all of these tissues ARNT was cytoplasmic as well as nuclear, except in some of the cortical adrenal cells in which ARNT was strongly cytoplasmic with little or no nuclear localization. These specific patterns of ARNT expression, which differ in certain tissues from the expression of AhR, suggest that ARNT may have additional roles in normal embryonic development.

Transcriptional activation domains of the Ah receptor and Ah receptor nuclear translocator

The Ah receptor (AhR) and Ah receptor nuclear translocator (Arnt) heterodimer bind the xenobiotic-responsive element (XRE) sequence in the upstream region of the genes for some drug-metabolizing enzymes, such as P4501A1 and glutathione S-transferase Ya, to activate their transcription. This paper describes transcriptional activation domains of the AhR and Arnt as examined in vivo by DNA transfection experiments using GAL4-AhR or GAL4-Arnt chimeric plasmids and a reporter plasmid containing five GAL4 DNA binding sites. The major activation domain of Arnt was localized in a short segment of the C-terminal 34 amino acids, while the glutamine-rich domain of Arnt showed no transcriptional activity. This activation domain of Arnt could be further divided into two subdomains with some sequence similarity. Point mutation analysis of one of the subdomains revealed that bulky hydrophobic amino acids and neighboring acidic amino acids were necessary for the transcription-enhancing activity of Arnt. The C-terminal half of the AhR showed a strong transcription-stimulating activity, apparently five times as strong as that of Arnt. Further analysis of the activity revealed that the C-terminal transcriptional activity was distributed in several activation domains, one of which is rich in glutamine residues. These results indicate that the glutamine-rich domains of the AhR and Arnt function differently in the heterodimer regulatory complex. Previously, we showed that the enhancer activity of XRE was repressed by E1A proteins, especially the 12S form of E1A. Cotransfection experiments using an E1A12S expression plasmid and a GAL4-AhR or GAL4-Arnt expression plasmid demonstrated that E1A protein rather predominantly inhibited the transcriptional activity of Arnt.

Protein-protein interaction via PAS domains: role of the PAS domain in positive and negative regulation of the bHLH/PAS dioxin receptor-Arnt transcription factor complex

Gene regulation by dioxins is mediated by the dioxin receptor-Arnt heterodimer, a ligand generated complex of two basic helix-loop-helix (bHLH)/Per-Arnt-Sim (PAS) transcription factors. By using dioxin receptor chimeras where the dimerization and DNA binding bHLH motif has been replaced by a heterologous DNA binding domain, we have detected an ability of Arnt to interact with the dioxin receptor via the PAS domain in a mammalian 'hybrid interaction' system. By coimmunoprecipitation assays, we have confirmed the ability of PAS domains of the dioxin receptor and Arnt to mediate independent heterodimerization in vitro. Selectivity for PAS dimerization was noted in our hybrid interaction system, as dioxin receptor or Arnt PAS-mediated homodimers were not detected. Surprisingly, however, the PAS domain of Per could dimerize with both the dioxin receptor and Arnt subunits in vitro, and disrupt the ability of these subunits to form a DNA binding heterodimer. Moreover, ectopic expression of Per blocked dioxin signalling in mammalian cells. The PAS domains of the dioxin receptor and Arnt are therefore novel dimerizing regions critical in formation of a functional dioxin receptor-Arnt complex, while the PerPAS domain is a potential negative regulator of bHLH/PAS factor function.

Identification of a 120-kDa protein associated with aromatic hydrocarbon receptor nuclear translocator

The aromatic hydrocarbon receptor nuclear translocator (ARNT) is a basic helix-loop-helix-PAS protein which forms a heterodimer with aromatic hydrocarbon receptor (AHR), this heterodimer mediating the signal transduction in response to the various xenobiotics such as 2,3,7,8-tetrachlorodibenzo-p-dioxin and directly interacting with target genes by binding to xenobiotic responsive elements. An anti-ARNT antibody was raised in rabbits against the bacterially expressed ARNT of amino acids 21-328 from the N-terminal. Using this antibody, besides ARNT itself, we detected at least one protein, 120 kDa, in the immunoprecipitate of anti-ARNT antibodies in HepG2 cells as well as in Hepa-1 cells. However, this protein is not present in the immunoprecipitate of the anti-AHR antisera nor in that of the preimmune sera of the rabbits used for the immunization.

Dioxin induces localized, graded changes in chromatin structure: implications for Cyp1A1 gene transcription

In mouse hepatoma cells, the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, or dioxin) induces Cyp1A1 gene transcription, a process that requires two basic helix-loop-helix regulatory proteins, the aromatic hydrocarbon receptor (AhR) and the aromatic hydrocarbon receptor nuclear translocator (Arnt). We have used a ligation-mediated PCR technique to analyze dioxin-induced changes in protein-DNA interactions and chromatin structure of the Cyp1A1 enhancer-promoter in its native chromosomal setting. Dioxin-induced binding of the AhR/Arnt heteromer to enhancer chromatin is associated with a localized (about 200 bp) alteration in chromatin structure that is manifested by increased accessibility of the DNA; these changes probably reflect direct disruption of a nucleosome by AhR/Arnt. Dioxin induces analogous AhR/Arnt-dependent changes in chromatin structure and accessibility at the Cyp1A1 promoter. However, the changes at the promoter must occur by a different, more indirect mechanism, because they are induced from a distance and do not reflect a local effect of AhR/Arnt binding. Dose-response experiments indicate that the changes in chromatin structure at the enhancer and promoter are graded and mirror the graded induction of Cyp1A1 transcription by dioxin. We discuss these results in terms of a TCDD-induced shift in an equilibrium between nucleosomal and nonnucleosomal chromatin configurations.

Constitutive function of the basic helix-loop-helix/PAS factor Arnt. Regulation of target promoters via the E box motif

Arnt is a nuclear basic helix-loop-helix (bHLH) transcription factor that, contiguous with the bHLH motif, contains a region of homology (PAS) with the Drosophila factors Per and Sim. Arnt dimerizes in a ligand-dependent manner with the bHLH dioxin receptor, a process that enables the dioxin-(2,3,7,8-tetrachlorodibenzo-p-dioxin)-activated Arnt-dioxin receptor complex to recognize dioxin response elements of target promoters. In the absence of dioxin, Arnt does not bind to this target sequence motif. The constitutive function of Arnt is presently not understood. Here we demonstrate that Arnt constitutively bound the E box motif CACGTG that is also recognized by a number of distinct bHLH factors, including USF and Max. Importantly, amino acids that have been identified to be critical for E box recognition by Max and USF are conserved in Arnt. Consistent with these observations, full-length Arnt, but not an Arnt deletion mutant lacking its potent C-terminal transactivation domain, constitutively activated CACGTG E box-driven reporter genes in vivo. These results indicate a role of Arnt in regulation of a network of target genes that is distinct from that regulated by the Arnt-dioxin receptor complex in dioxin-stimulated cells.

Expression of Ah receptor (TCDD receptor) during human monocytic differentiation

We have previously found a high expression of human Ah receptor (TCDD receptor) mRNA in peripheral blood cells of individuals. In this paper, the expression of this gene in blood cells was first investigated in fractions of nucleated cells, revealing predominant expression of the Ah receptor gene in the monocyte fraction. Then the expression levels of AhR mRNA in various hematopoietic cell lines were examined together with those of Arnt and P450IA1. AhR was expressed at high levels in monocytoid U937, THP1, and HEL/S cells, and at moderate levels in promyelocytic HL60 cells and erythroblastic HEL cells. However, it was not detected in lymphoid cells MOLT4 (T cell) and BALL1 (B cell), nor in K562 erythroblasts. Furthermore, a specific induction of AhR during monocytic differentiation was investigated in HL60 and HEL cells. HL60 cells were induced to differentiate toward monocytes-macrophages by incubation with phorbol ester, showing a 5- to 2-fold increase of AhR mRNA. The incubation with transforming growth factor beta 1 and 1 alpha,25-dihydroxyvitamin D3 resulted in a 5- to 7-fold increase of AhR mRNA. The HEL cells also exhibited a similar elevation of AhR mRNA level, when they had differentiated toward monocyte-macrophage cells by these combined inducers, but little change in the mRNA level was observed when the cells were induced to differentiate into other cell types. Treatment of the differentiated HL60 cells with 3-methylcholanthrene, a ligand of AhR, induced the expression of the P450IA1 gene. These results indicated that expression of AhR mRNA was significantly induced during monocytic differentiation and that the differentiated cells were responsive to xenobiotics. Our results suggest that AhR may play an important role in the function of monocytes and also in the eventual activation of environmental carcinogens.

Transcriptional activation by the mouse Ah receptor. Interplay between multiple stimulatory and inhibitory functions

The aromatic hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that mediates cellular responses to the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We cloned AhR cDNA from C57BL/6 mouse liver and verified by transfection that it encodes a functional protein. Analyses of deletion mutants indicate that the carboxyl half of AhR contains several types of transactivation domain, which function independently of domains that mediate TCDD recognition, DNA binding, and heterodimerization with the Ah receptor nuclear translocator (Arnt) protein. The transactivation domains function independently of each other, display different levels of activity, and act synergistically when linked. In addition, AhR contains an 82-amino acid domain that inhibits transactivation. The inhibitory domain displays specificity, in that it blocks the transactivating functions of AhR and Arnt, but not that of the herpes simplex protein VP16. The inhibitory activity depends upon the cell type in which AhR is expressed, implying that a cell-specific protein mediates the effect.

Heat shock protein hsp90 regulates dioxin receptor function in vivo

The dioxin (aryl hydrocarbon) receptor is a ligand-dependent basic helix-loop-helix (bHLH) factor that binds to xenobiotic response elements of target promoters upon heterodimerization with the bHLH partner factor Arnt. Here we have replaced the bHLH motif of the dioxin receptor with a heterologous DNA-binding domain to create fusion proteins that mediate ligand-dependent transcriptional enhancement in yeast (Saccharomyces cerevisiae). Previously, our experiments indicated that the ligand-free dioxin receptor is stably associated with the 90-kDa heat shock protein, hsp90. To investigate the role of hsp90 in dioxin signaling we have studied receptor function in a yeast strain where hsp90 expression can be down-regulated to about 5% relative to wild-type levels. At low levels of hsp90, ligand-dependent activation of the chimeric dioxin receptor construct was almost completely inhibited, whereas the activity of a similar chimeric construct containing the structurally related Arnt factor was not affected. Moreover, a chimeric dioxin receptor construct lacking the central ligand- and hsp90-binding region of the receptor showed constitutive transcriptional activity in yeast that was not impaired upon down-regulation of hsp90 expression levels. Thus, these data suggest that hsp90 is a critical determinant of conditional regulation of dioxin receptor function in vivo via the ligand-binding domain.

Possible function of Ah receptor nuclear translocator (Arnt) homodimer in transcriptional regulation

Arnt (Ah receptor nuclear translocator) is a member of a transcription factor family having characteristic motifs designated bHLH (basic helix-loop-helix) and PAS and was originally found as a factor forming a complex with Ah receptor (AhR) to bind the specific xenobiotic responsive element (XRE) sequence for induction of drug-metabolizing P4501A1. We have examined interaction of Arnt with other PAS proteins--Drosophila Per, Sim, and AhR--by the coimmunoprecipitation method. Arnt formed a homodimer with itself as well as heterodimers with the others by means of the PAS and HLH domains in a cooperative way. The Arnt homodimer binds the sequence of adenovirus major late promoter (MLP) with the E box core sequence CACGTG, suggesting that the CAC half of the XRE, CACGCN(A/T), recognized by the AhR-Arnt heterodimer is a target for Arnt. Cotransfection experiments using CV-1 cells with an Arnt expression plasmid and a MLP chloramphenicol acetyltransferase (CAT) reporter plasmid revealed that Arnt markedly activated CAT expression, indicative of a newly discovered regulatory role of Arnt.

Orientation of the heterodimeric aryl hydrocarbon (dioxin) receptor complex on its asymmetric DNA recognition sequence

The 2,3,7,8-tetrachlorodibenzo-p-dioxin-transformed aryl hydrocarbon receptor (AHR) complex binds to xenobiotic-responsive element (XRE) sequences in the 5' flanking region of the CYP1A1 gene, resulting in initiation of transcription. Both components of the transformed AHR complex [the ligand-binding AHR monomer and the AHR nuclear translocator (ARNT)] directly contact the XRE. These proteins belong to a novel subclass of basic helix-loop-helix transcription factors. The binding sites of AHR and ARNT on the asymmetric XRE were determined using nuclear extracts of 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated Hepa-1c1c7 cells and a panel of double-stranded oligonucleotides containing XRE1 of the CYP1A1 gene (5'-TTGCGTGAGAA-3'), in which all combinations of three, two, or one of the thymines indicated were substituted by the photoreactive thymine analog 5-bromodeoxyuracil. Covalent cross-linking analysis and immunoprecipitation with antibodies specific for AHR or ARNT demonstrated that ARNT directly contacts the 3'-most thymine position, that AHR directly contacts the second thymine position, and that neither protein contacts the 5'-most thymine position. The thymine position contacted by ARNT lies within a three-nucleotide sequence (5'-GTG-3') identical to a half-site of an E-box element (5'-CACGTG-3') that is recognized by a number of other basic helix-loop-helix transcription factors. AHR binds to a portion of the XRE that does not resemble an E-box. Additional experiments demonstrated that neither protein loops over to contact residues located beyond the other's binding site.

Comparison of expression of aldehyde dehydrogenase 3 and CYP1A1 in dominant and recessive aryl hydrocarbon hydroxylase-deficient mutant mouse hepatoma cells

The mouse hepatoma cell line Hepa-1 is inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) for both CYP1A1 (aryl hydrocarbon hydroxylase, AHH) and class 3 aldehyde dehydrogenase (ALDH3) enzymes. To test the hypothesis of a common regulatory mechanism, several AHH deficient mutants of Hepa-1 were studied for their ALDH3 activities and specific mRNA levels before and after TCDD treatment. The recessive (with respect to the wild-type Hepa-1) mutants have defects in Cypla-1 structural gene (mutant c1) or in the Ah (aryl hydrocarbon) receptor (mutants c2 and c6 with decreased levels of Ah receptor; mutant c4 defective in the DNA binding of the Ah receptor). The results with these mutants suggested that Ah receptor nuclear translocator protein, ARNT, is needed for ALDH3 expression. Two dominant mutants, one of which is characterized by preventing the binding of the Ah receptor complex to DNA, were also studied. Surprisingly, these mutants possessed elevated levels of ALDH3 mRNA and enzyme activities which were also inducible by TCDD. The binding of Ah receptor-ligand complex to DNA was thus not needed for the expression of ALDH3. A dominant repressor for Cypla-1 gene transcription did not prevent the derepression or induction of ALDH3. The results thus suggest that Aldh-3 gene is regulated by a mechanism independent of the Ah receptor.

Suspension-mediated induction of Hepa 1c1c7 Cyp1a-1 expression is dependent on the Ah receptor signal transduction pathway

We have recently demonstrated that release of normal human epithelial cells from cell-substratum and/or cell-cell adhesion generates cellular signals that induce the expression of CYP1A1 in the absence of xenobiotic polycyclic aromatic hydrocarbons (Sadek, C. M., and Allen-Hoffmann, B. L. (1994) J. Biol. Chem. 169, 16067-16074). To directly test the involvement of the Ah receptor signal transduction pathway in CYP1A1 induction following suspension of epithelial cells, we analyzed wild-type Hepa 1c1c7 cells, a subclone of the Hepa-1c1 mouse hepatoma line, and two mutant Hepa 1c1c7 lines, Class I and Class II. Suspension of wild-type Hepa 1c1c7 cells for 4 h led to an induction of steady state levels of CYP1A1 mRNA, similar to that obtained following treatment of adherent cells with 10(-9) M 2,3,7,8-tetrachlorodibenzo-p-dioxin. Mutants of the Hepa 1c1c7 cells defective in different aspects of the Ah receptor signal transduction pathway exhibited negligible (Class I) or no (Class II) suspension-mediated induction of CYP1A1 mRNA. Gel mobility shift analysis of nuclear extracts from suspended or 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated wild-type cells showed that both treatments produced identical shifts in the mobility of an XRE-containing probe. Antibody supershift experiments confirmed that the Ah receptor was a component of the DNA-protein complex from suspended wild-type Hepa 1c1c7 cells. These data directly demonstrate that suspension of wild-type Hepa 1c1c7 cells leads to nuclear localization and activation of the Ah receptor to a DNA-binding form.

Potent transactivation domains of the Ah receptor and the Ah receptor nuclear translocator map to their carboxyl termini

The Ah receptor (AHR) is a ligand-activated transcription factor that is structurally related to its dimerization partner, the Ah receptor nuclear translocator (ARNT), and two Drosophila proteins, SIM and PER. All four proteins contain a region of homology now referred to as a PAS homology domain. In addition, the AHR, ARNT, and SIM harbor a basic region helix-loop-helix motif in their N termini, whereas PER does not. Previous mapping studies of the AHR have demonstrated that the PAS domain contains sequences required for ligand recognition, dimerization, and interaction with the 90-kDa heat shock protein. They also have confirmed that the basic region helix-loop-helix domain plays a role in both dimerization and sequence-specific DNA binding. To identify domains involved in transactivation of target genes, we generated chimeras of AHR/ARNT deletion mutants with the DNA binding region of the yeast Gal4 protein, transiently expressed these in COS-1 cells, and monitored their capacity to activate the chloramphenicol acetyltransferase reporter gene under the control of a minimal promoter driven by enhancer elements recognized by Gal4. Extensive analysis of these fusions revealed that the AHR and ARNT harbor potent transactivation domains within their C termini. Importantly, the amino-terminal halves of both the AHR and ARNT were found to be devoid of transactivation activity.

Development of an in vitro model for investigating the formation of the nuclear Ah receptor complex in mouse Hepa 1c1c7 cells

An in vitro assay for investigating factors which modulate formation of the nuclear aryl hydrocarbon (Ah) receptor complex was developed using 9S cytosolic Ah receptor isolated from wild-type Hepa 1c1c7 cells treated with 2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin (TCDD) at 4 degrees C and nuclei from Ah-responsive and -nonresponsive wild-type and mutant Hepa 1c1c7 cells. Incubation of the radiolabeled ([3H]TCDD) 9S cytosolic Ah receptor with nuclei from untreated wild-type Ah-responsive mouse Hepa 1c1c7 cells resulted in a time- and temperature-dependent formation of the nuclear Ah receptor complex as determined by either velocity sedimentation analysis or gel mobility shift assays using a consensus 32P-labeled dioxin-responsive element. Maximal levels of the nuclear Ah receptor formed within 30 min at 37 degrees C and significantly lower levels were observed after incubation at 4, 15, or 25 degrees C. Complementation studies using nuclei from untreated wild-type and Ah-nonresponsive class II mutant (translocation-deficient) cells and radiolabeled 9S cytosolic receptor (bound with [3H]TCDD) from both wild-type and mutant cell lines were also carried out. The results indicated that nuclear translocation was primarily inhibited using cytosol from mutant cells confirming the requirement for the aryl hydrocarbon receptor nuclear translocator protein for formation of the nuclear Ah receptor complex. The effects of a series of Ah receptor antagonists, ATP, vanadate apyrase, phosphatases, and lectin WGA, on formation of the nuclear Ah receptor complex were also investigated in the in vitro model using radiolabeled cytosolic Ah receptor and nuclei from untreated wild-type Hepa 1c1c7 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of transactivation and repression functions of the dioxin receptor and its basic helix-loop-helix/PAS partner factor Arnt: inducible versus constitutive modes of regulation

Gene regulation by dioxins is mediated via the dioxin receptor, a ligand-dependent basic helix-loop-helix (bHLH)/PAS transcription factor. The latent dioxin receptor responds to dioxin signalling by forming an activated heterodimeric complex with a specific bHLH partner, Arnt, an essential process for target DNA recognition. We have analyzed the transactivating potential within this heterodimeric complex by dissecting it into individual subunits, replacing the dimerization and DNA-binding bHLH motifs with heterologous zinc finger DNA-binding domains. The uncoupled Arnt chimera, maintaining 84% of Arnt residues, forms a potent and constitutive transcription factor. Chimeric proteins show that the dioxin receptor also harbors a strong transactivation domain in the C terminus, although this activity was silenced by inclusion of 82 amino acids from the central ligand-binding portion of the dioxin receptor. This central repression region conferred binding of the molecular chaperone hsp90 upon otherwise constitutive chimeras in vitro, indicating that hsp90 has the ability to mediate a cis-repressive function on distant transactivation domains. Importantly, when the ligand-binding domain of the dioxin receptor remained intact, the ability of this hsp90-binding activity to confer repression became conditional rather than irreversible. Our data are consistent with a model in which crucial activities of the dioxin receptor, such as dimerization with Arnt and transactivation, are conditionally repressed by the central ligand- and-hsp90-binding region of the receptor. In contrast, the Arnt protein appears to be free from any repressive activity. Moreover, within the context of the dioxin response element (xenobiotic response element), the C terminus of Arnt conferred a potent, dominating transactivation function onto the native bHLH heterodimeric complex. Finally, the relative transactivation potencies of the individual dioxin receptor and Arnt chimeras varied with cell type and promoter architecture, indicating that the mechanisms for transcriptional activation may differ between these two subunits and that in the native complex the transactivation pathway may be dependent upon cell-specific and promoter contexts.

Transcriptional activation function of the mouse Ah receptor nuclear translocator

We cloned from mouse hepatoma cells a cDNA which encodes the Ah receptor nuclear translocator (Arnt). Sequence comparisons reveal 89% nucleotide and 92% amino acid identity between mouse and human Arnt. Transfection of the cDNA into Arnt-defective mouse hepatoma cells fully restores their responsiveness to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), indicating that the cDNA encodes a functional Arnt protein. Transfection of the cDNA into wild type mouse hepatoma cells increases the magnitude, but not the sensitivity, of the transcriptional response to TCDD. Analyses of mutants indicate that Arnt has a modular organization. The unit that mediates both heterodimerization with the liganded Ah receptor and DNA recognition is functionally distinct from the unit that mediates transcriptional activation. A 96-amino acid, C-terminal domain of Arnt, which includes a glutamine-rich region, confers transcriptional activation capability upon the protein.

Immunohistochemical double-staining for Ah receptor and ARNT in human embryonic palatal shelves

The aryl hydrocarbon receptor (AhR) and the AhR nuclear translocator protein (ARNT) are basic-helix-loop-helix-PAS (HLH) proteins involved in transcriptional regulation. Polycyclic aromatic halogenated chemicals, of which 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most potent, bind to the AhR. In the cellular cytoplasm, the AhR exists as a complex with the heat shock protein HSP90 and other small peptides. This complex dissociates following ligand binding and then the ligand-bound AhR binds ARNT. The ligand-AhR-ARNT complex interacts with a specific, nuclear DNA sequence, the dioxin response element (DRE), altering transcription of a regulated gene. Studies in hepatoma cell lines indicate that both proteins are required for regulation of transcription. In this study, AhR and ARNT were localized immunohistochemically in human embryonic palatal cells and specific patterns of expression were seen for each protein. A double-staining protocol revealed that epithelial cells expressed both AhR and ARNT, but in mesenchyme and nasal spine cartilage individual cells were identified which expressed either AhR or ARNT. This heterogeneous pattern may be a means of suppressing transcriptional regulation and also suggests the existence of other, unidentified basic-helix-loop-helix partner(s). The heterogeneous expression pattern may also reflect a complex role for these HLH proteins as transcriptional regulators of embryonic development.