Interaction of the regulatory domains of the murine Cyp1a1 gene with two DNA-binding proteins in addition to the Ah receptor and the Ah receptor nuclear translocator (ARNT)

The aromatic hydrocarbon (Ah) receptor complex is a ligand-activated transcriptional activator consisting of at least two protein components. The ligand-binding component is the AhR protein, a cytosolic receptor encoded by the Ahr gene, which, upon ligand binding, translocates to the nucleus in a heterodimeric complex with the ARNT (Ah receptor nuclear translocator) component. The complex binds to several discrete DNA domains containing aromatic hydrocarbon responsive elements (AhRE) present in the regulatory region of the murine cytochrome P(1)450 Cyp1a1 gene and of the other genes in the [Ah] gene battery. As a consequence of binding, a transcriptional complex is formed that activates the expression of these genes by as yet unidentified mechanisms. We have analyzed DNA-protein interactions in four of these domains, specifically, the AhREs located between -1085 and -482 (sites A, C, E, and D) of the upstream regulatory region of the murine Cyp1a1 gene. We found that two DNA-binding proteins, present in cytosolic and nuclear extracts of mouse Hepa-1 cells, showed overlapping DNA-binding specificities to those of the Ah receptor. One of these proteins had an apparent molecular mass of 35-40 kDa, bound only to AhRE3 (site D), and has been identified tentatively as a member of the C/EBP family of transcription factors. The second protein, purified by DNA-affinity chromatography, had an apparent molecular mass of 95 kDa and bound to a larger DNA motif that included the AhRE sequence, in AhRE3 and AhRE5 (sites D and A), but not in AhRE1 or AhRE2 (sites C and E). This protein was not AhR nor was it ARNT, since it was found in receptorless (Ahr-) and in nuclear translocation-defective (Arnt-) cells, as well as in cells that had not been exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; dioxin), a potent inducer of Cyp1a1 expression. Evidence from in vivo methylation protection indicated that two G residues flanking AhRE3, one of which is required for binding of the 95-kDa protein, may be protected from methylation in uninduced cells and become exposed upon dioxin treatment, suggesting that the 95-kDa protein may be constitutively bound to AhRE3, and be displaced by binding of the Ah receptor complex. These results lend support to the concept that the transcriptional regulation of the [Ah] battery genes could be modulated by combinatorial interactions of the Ah receptor complex with other transcription factors.

Baculovirus expression of the Ah receptor and Ah receptor nuclear translocater. Evidence for additional dioxin responsive element-binding species and factors required for signaling

In an effort to facilitate the structural and biochemical analyses of the Ah receptor (AHR) and the Ah receptor nuclear translocator (ARNT), a baculovirus system was developed to express microgram-milligram quantities of the human version of these proteins. To simplify purification, a polyhistidine tag was cloned at their C termini so that the recombinant proteins could be specifically adsorbed to nickel-nitriloacetic acid-Sepharose. Expression studies revealed that approximately 23% of the overexpressed AHR was recovered in cell extracts with the remaining 77% forming insoluble aggregates. ARNT was found to be more soluble, with 90% recovery from cell extracts and only 10% aggregation. Photoaffinity labeling and gel shift assays demonstrated that the recombinant proteins bound ligand, heterodimerized, and recognized their cognate "dioxin response element" (DRE) in a manner similar to their native counterparts. Coexpression of the AHR and ARNT in Sf9 cells resulted in the in vivo generation of heterodimers that bound the DRE in the absence of ligand. Studies with the nickel-nitriloacetic acid-purified recombinant proteins demonstrated that the AHR and ARNT could bind DRE only when reconstituted with a heat-sensitive factor(s) present in soluble extracts from a variety of cell types. Use of these proteins also demonstrated the existence of at least three AHR-dependent DRE-binding species, suggesting that the AHR can bind to DRE in at least three distinct conformations.

Regulation of [Ah] gene battery enzymes and glutathione levels by 5,10-dihydroindeno[1,2-b]indole in mouse hepatoma cell lines

The murine aromatic hydrocarbon ([Ah]) gene battery consists of at least six genes that code for two functionalizing (Phase I) enzymes and four non-functionalizing (Phase II) enzymes. These enzymes are induced by compounds such as aromatic hydrocarbons and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) that bind to the cytosolic Ah receptor protein. Studies in rodents indicate that certain enzymes of this battery, namely cytochrome P4501A1 (CYP1A1), UDP-glucuronosyltransferase (UGT1*06) and NAD(P)H: quinone acceptor oxidoreductase (NMO1) are induced by the synthetic antioxidant 5,10-dihydroindeno[1,2-b]indole (DHII). The induction of [Ah] gene battery enzymes and the levels of reduced glutathione (GSH) were examined in mouse Hepa-1c1c7 hepatoma wild-type cells (wt), a CYP1A1 metabolism-deficient mutant (c37) and an Ah receptor nuclear translocation-defective mutant (c4). DHII and TCDD increased the activities of ethoxyresorufin O-deethylase, an indicator of CYP1A1 activity, as well as NMO1, UGT1*06, cytosolic aldehyde dehydrogenase class 3 and glutathione S-transferase form A1 in wt cells, but had little or no induction effect in c37 or c4 cells. DHII and TCDD differed in their effects on GSH levels; while DHII increased GSH levels 3-fold in wt, but not at all in c37 or c4 cells, TCDD had no effect on GSH levels in any cell type. However, GSH levels were enhanced in both wt and c4 cells by tert-butyl hydroquinone (TBHQ). L-Buthionine S,R-sulfoximine, an inhibitor of gamma-glutamylcysteine synthetase, prevented DHII-induced increases in wt cell GSH. The increase in GSH levels occurred after 8 h, while the induction of enzymes occurred within 4 h. The induction of the higher GSH levels in wt cells by DHII and TBHQ correlated with increases in intracellular levels of the GSH precursor thiol cysteine, as well as with increased activities of gamma-glutamylcysteine synthetase, the rate-limiting enzyme of GSH synthesis. However, TBHQ-mediated GSH increases in c4 cells were accompanied by increased gamma-glutamylcysteine synthetase activity with no change in intracellular cysteine concentration. The results suggest that DHII induction of [Ah] gene battery enzymes requires a functional Ah receptor, but not the functional gene product CYP1A1. Furthermore, metabolism, possibly via CYP1A1, appears to be required for DHII to enhance intracellular levels of cysteine and GCS activity that result in higher GSH levels.

Physicochemical and immunocytochemical analysis of the aryl hydrocarbon receptor nuclear translocator: characterization of two monoclonal antibodies to the aryl hydrocarbon receptor nuclear translocator

The aryl hydrocarbon receptor nuclear translocator (Arnt) is a basic helix-loop-helix transcription factor that heterodimerizes with the aryl hydrocarbon receptor to mediate signal transduction pathways inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin and other planar aromatic hydrocarbons. Monoclonal antibodies (MAbs) have been raised against a carboxyl-terminal 19-amino acid peptide hapten (MAb 2B10) and against a carboxyl-terminal 378-amino acid polypeptide-staphylococcal Protein A fusion protein (MAb 4G9) of Arnt and their characterization is described. Western blot experiments show that both MAbs specifically cross-react with an approximately 85-kDa band in cytosol prepared from COS-7 cells transfected with the full length human Arnt cDNA pBMSNeo-D24-1 and in Hepa 1c1c7 cytosol but not in Arnt-deficient Hepa 1-C4 mutant cytosol. Velocity sedimentation of Hepa 1c1c7 cytosol on sucrose gradients and Superose 6 gel permeation chromatography were used to estimate the sedimentation coefficient. Stokes radius, and relative molecular mass of Arnt as approximately 3.6-4.1 S, 6.8 nm, and 101-115 kDa, respectively. These results indicate that Arnt probably exists in monomeric form in Hepa 1c1c7 cytosolic extracts. Laser scanning confocal microscopy and indirect immunofluorescence microscopy revealed Arnt to be distributed throughout the non-nucleolar portion of the nucleus of Hepa 1c1c7, VT(2) (Hepa 1-C4T mutant cell line deficient in Arnt function and stably transfected with pBMSNeo D24-1, expressing the full length human Arnt cDNA), and HeLa cells. The establishment of the nuclear localization of Arnt in human and murine cell lines shown here indicates that its nuclear localization may be conserved across species. Immunofluorescence analysis of Arnt in three cell lines using two MAbs (to distinct epitopes) provides evidence that suggests that the aryl hydrocarbon receptor heterodimerizes with Arnt in the nucleus.

Identification of functional domains of the aryl hydrocarbon receptor nuclear translocator protein (ARNT)

The activated aryl hydrocarbon receptor (AHR) and the AHR nuclear translocator (ARNT) bind DNA as a heterodimer. Both proteins represent a novel class of basic helix-loop-helix (bHLH)-containing transcription factors in that (i) activation of AHR requires the binding of ligand (e.g., 2,3,7,8-tetrachlorodibenzo-p-dioxin [TCDD]), (ii) the xenobiotic responsive element (XRE) recognized by the AHR/ARNT heterodimer differs from the recognition sequence for nearly all other bHLH proteins, and (iii) both proteins contain a PAS homology region, which in the Drosophila PER and SIM proteins functions as a dimerization domain. A cDNA for mouse ARNT has been cloned, and potential functional domains of ARNT were investigated by deletion analysis. A mutant lacking all regions of ARNT other than the bHLH and PAS regions is unimpaired in TCDD-dependent dimerization and subsequent XRE binding and only modestly reduced in ability to complement an ARNT-deficient mutant cell line, c4, in vivo. Both the first and second alpha helices of the bHLH region are required for dimerization. The basic region is required for XRE binding but not for dimerization. Deletion of either the A or B segments of the PAS region slightly affects TCDD-induced heterodimerization, while deletion of the complete PAS region severely affects (but does not eliminate) dimerization. Thus, ARNT possesses multiple domains required for maximal heterodimerization. Mutants deleted for PAS A, PAS B, and the complete PAS region all retain some degree of XRE binding, yet none can rescue the c4 mutant. Therefore, both the PAS A and PAS B segments, besides contributing to dimerization, apparently fulfill additional, unknown functions required for biological activity of ARNT.

Tissue specific expression of the rat Ah-receptor and ARNT mRNAs

The Ah-receptor (AHR) is a ligand activated transcription factor that mediates the biological effects of agonists such as 2,3,7,8-tetrachlorodibenzo-p-dioxin. Upon binding agonists, the AHR dimerizes with a structurally related protein known as ARNT and this heterodimer then binds cognate enhancer elements and activates the expression of target genes. In this report we describe the cloning of the rat AHR cDNA and a fragment of the rat ARNT cDNA for use as probes in ribonuclease protection analysis. Ribonuclease protection analysis indicated that the rat AHR mRNA is expressed at the highest levels in the lung > thymus > kidney > liver while lower levels were expressed in heart and spleen. The rat AHR and ARNT mRNAs were expressed in a largely coordinate manner across the eight tissues examined with the exception of the placenta where AHR levels were relatively low compared to ARNT. In these experiments, a rare splice variant of the AHR was cloned that encoded a protein with a deletion in the ligand binding domain. In vitro expression studies demonstrated that in contrast to the full length AHR, the splice variant did not bind ligand nor did it bind to a cognate enhancer element in the presence of ARNT.

Agonistic and antagonistic effects of alpha-naphthoflavone on dioxin receptor function. Role of the basic region helix-loop-helix dioxin receptor partner factor Arnt

The dioxin receptor is a ligand-dependent transcription factor that binds to target DNA sequences (xenobiotic responsive elements, XREs) following ligand-dependent dimerization with its partner factor, Arnt (aryl hydrocarbon receptor nuclear translocator). Both factors contain an N-terminal basic region helix-loop-helix motif mediating dimerization and subsequent DNA binding. In this study we investigate the possible role of Arnt in agonistic and antagonistic effects of the dioxin receptor ligand alpha-naphthoflavone (ANF). Using specific antisera for the ligand binding dioxin receptor and Arnt, respectively, we show that exposure of the dioxin receptor to ANF in vitro induced recruitment of Arnt, thus stimulating binding of the heteromeric complex to XRE. In transient transfection assays, ANF at high concentrations stimulated expression of an XRE-driven reporter gene. This agonistic effect of ANF is, therefore, most likely attributable to ANF stimulation of dioxin receptor-Arnt heterodimerization and subsequent binding of the complex to XRE. Using a minimal XRE-driven reporter gene construct, we could further confirm earlier studies showing that ANF antagonizes the effect of a dioxin receptor agonist, 2,3,7,8-tetrachlorodibenzo-p-dioxin. Next we employed chimeric receptor constructs containing amino acids 1-500 of the human glucocorticoid receptor fused to dioxin receptor fragments lacking the very N-terminal basic region helix-loop-helix dimerization and DNA binding motif. These chimeric receptor constructs show dioxin responsiveness upon transient transfection into mutant Arnt-deficient hepatoma cells and are, thus, functionally uncoupled from Arnt. Importantly, dioxin-dependent activation of the chimeric receptors was inhibited in the presence of ANF, demonstrating that dimerization of dioxin receptor with Arnt was not necessary for manifestation of the antagonistic effect of ANF. Rather, dioxin receptor sequences, which confer dioxin regulation upon a heterologous DNA binding and transactivating domain, also mediated the antagonistic effects of ANF.

Interindividual difference in expression of human Ah receptor and related P450 genes

The genomic clones of human aryl hydrocarbon receptor (Ahr) and Ahr nuclear translocator (Arnt) were isolated, and the structures of exon-intron junctions of these genes were partially determined. Based on the sequence information, a quantitative RT-PCR analysis was developed, and the expression of these genes was studied in various human tissues. mRNAs for Ahr and Arnt were widely expressed in human tissues and abundantly in lung. Individual difference in expression levels of Ahr and Arnt mRNA was observed in liver, lung and blood. In order to examine whether expression levels of Ahr and Arnt were associated with those of CYP1A1, we studied the expression of these mRNAs in blood among 20 healthy subjects, taking account of individuals' cigarette smoking habits. We found that the expression levels of CYP1A1 appeared to associate with those of Ahr and Arnt mRNAs (P < 0.06), and also that the expression of Ahr and Arnt was influenced by cigarette smoking. The expression of human Ahr and Arnt is reported here for the first time, providing a quantitative RT-PCR analysis as a useful tool for further studies.

A cellular factor stimulates ligand-dependent release of hsp90 from the basic helix-loop-helix dioxin receptor

In response to dioxin, the nuclear basic helix-loop-helix (bHLH) dioxin receptor forms a complex with the bHLH partner factor Arnt that regulates target gene transcription by binding to dioxin-responsive sequence motifs. Previously, we have demonstrated that the latent form of dioxin receptor present in extracts from untreated cells is stably associated with molecular chaperone protein hsp90, and Arnt is not a component of this complex. Here, we used a coimmunoprecipitation assay to demonstrate that the in vitro-translated dioxin receptor, but not Arnt, is stably associated with hsp90. Although it showed ligand-binding activity, the in vitro-translated dioxin receptor failed to dissociate from hsp90 upon exposure to ligand. Addition of a specific fraction from wild-type hepatoma cells, however, to the in vitro-expressed receptor promoted dioxin-dependent release of hsp90. This stimulatory effect was mediated via the bHLH dimerization and DNA-binding motif of the receptor. Moreover, ligand-dependent release of hsp90 from the receptor was not promoted by fractionated cytosolic extracts from mutant hepatoma cells which are deficient in the function of bHLH dioxin receptor partner factor Arnt. Thus, our results provide a novel model for regulation of bHLH factor activity and suggest that derepression of the dioxin receptor by ligand-induced release of hsp90 may require bHLH-mediated concomitant recruitment of an additional cellular factor, possibly the structurally related bHLH dimerization partner factor Arnt. In support of this model, addition of in vitro-expressed wild-type Arnt, but not a mutated form of Arnt lacking the bHLH motif, promoted release of hsp90 from the dioxin receptor in the presence of dioxin.

The aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator protein show distinct subcellular localizations in Hepa 1c1c7 cells by immunofluorescence microscopy

The aryl hydrocarbon receptor (AhR) and AhR nuclear translocator (Arnt) protein were evaluated in the Hepa 1c1c7 (Hepa-1) cell line by indirect immunofluorescence microscopy and Western blot analysis. Wild-type (WT) Hepa-1 cells stained for AhR show intense cytoplasmic fluorescence with minimal nuclear reactivity. WT cells treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) show a time-dependent decrease in cytoplasmic AhR staining and a concomitant increase in nuclear fluorescence. WT cells stained for Arnt show nuclear fluorescence with minimal cytoplasmic reactivity, a pattern unchanged after TCDD treatment. Hepa-1 type II variants express normal levels of AhR but are defective in TCDD-mediated induction of cytochrome P4501A1. Type II variants stained for Arnt show reduced nuclear fluorescence, compared with WT cells, and express minimal levels of Arnt protein, as determined by Western blot analysis. Type II variants stained for the AhR show intense cytoplasmic fluorescence that becomes nuclear after TCDD treatment. Detailed evaluation by immunoelectron microscopy of the AhR and Arnt present in the nuclear compartment of WT cells shows that both proteins are uniformly distributed and do not appear to be associated with nuclear pores, membranes, or nucleoli. Western blot analysis of nuclei isolated from WT Hepa-1 cells fractionated with Nonidet P-40 shows that minimal levels of AhR or Arnt are retained in the nuclear fraction after TCDD treatment. Collectively, these results indicate that the unliganded AhR resides in the cytoplasm, Arnt is localized to the nucleus, and Hepa-1 cells defective in Arnt expression exhibit TCDD-mediated nuclear accumulation of the AhR.

Purification of the DNA binding form of dioxin receptor. Role of the Arnt cofactor in regulation of dioxin receptor function

The basic region/helix-loop-helix dioxin receptor mediates signal transduction by dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin). Upon ligand binding the dioxin receptor is converted from a latent, non-DNA binding form to a form that directly interacts with target genes by binding to dioxin-responsive transcriptional control elements. We have purified by conventional and DNA affinity chromatographic procedures the ligand-activated, DNA binding form of dioxin receptor to examine its architecture and functional properties. We observed that the DNA binding activity of the receptor was labile. Most notably, this activity was lost following DNA affinity purification. In complementation experiments we have identified an auxiliary factor(s) that exhibited very poor, if any, intrinsic affinity for the DNA target sequence in vitro but strongly increased the DNA binding activity of the purified receptor-containing material identified by immunoblot analysis. In a similar fashion the in vitro expressed basic region/helix-loop-helix factor Arnt (that has been postulated to modulate the nuclear translocation function of the receptor) reconstituted the DNA binding function of the purified receptor, and the purified auxiliary factor reconstituted receptor activity upon addition to an extract from mutant, Arnt-deficient hepatoma cells. Conversely, purified dioxin receptor reconstituted DNA binding activity in extracts from receptor-deficient hepatoma cells which express bona fide levels of Arnt. Interestingly, UV cross-linking studies using a BrdU-substituted DNA target sequence indicated that primarily the receptor protein was bound to DNA. Moreover, we demonstrate that purified receptor or Arnt exhibited virtually no detectable affinity for the target sequence individually but, in the presence of one another, showed a strong synergy in DNA binding activity in vitro. Importantly, simultaneous expression of the receptor and Arnt resulted in synergistic induction of gene expression in vivo. These data demonstrate that Arnt plays a central role in control of dioxin receptor function by cooperatively modulating the DNA binding activity of the receptor in vitro and dioxin-dependent transactivation in vivo.

Definition of a novel ligand binding domain of a nuclear bHLH receptor: co-localization of ligand and hsp90 binding activities within the regulable inactivation domain of the dioxin receptor

The dioxin receptor mediates signal transduction by dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) and binds to DNA target sequences as a heterodimer of the approximately 100 kDa ligand binding receptor and the approximately 85 kDa auxiliary factor, Arnt. Both of these factors encompass an N-terminal basic helix-loop-helix (bHLH) motif required for DNA binding and dimerization. In this study we describe the construction of glucocorticoid/dioxin receptor fusion proteins which allow the regulation of glucocorticoid receptor activity by dioxin in transient transfections of CHO and hepatoma cells. Thus, in the absence of dioxin, chimeric receptor constructs which contain large 500-720 amino acid C-terminal dioxin receptor fragments, but lack the N-terminal bHLH motif, confer repression upon the transcriptional activity of a glucocorticoid receptor derivative, tau DBD, containing its N-terminal strong transactivating signal (tau) and its DNA binding domain (DBD). In the presence of dioxin, this repression is reversed. Importantly, these chimeric receptors did not require the bHLH Arnt co-factor for function. A considerably smaller region of the dioxin receptor, located between amino acids 230 and 421, showed specific dioxin binding activity in vitro. Moreover, dioxin binding in vitro correlated with the ability of receptor fragments to form stable complexes in vitro with the molecular chaperone hsp90. These findings support the notion that hsp90 may be important for folding of a dioxin binding configuration of the receptor. Finally, tau DBD activity was constitutively repressed in a dioxin non-responsive manner by dioxin receptor fragments which failed to bind ligand but also failed to bind hsp90 in vitro, indicating that alternative mechanisms in addition to hsp90 binding may contribute to the inactivation function. In summary, the dioxin receptor system provides a novel and complex model of regulation of bHLH factors that may also give important insights into the mechanism of action of ligand-activated nuclear receptors.

A factor binding to the xenobiotic responsive element (XRE) of P-4501A1 gene consists of at least two helix-loop-helix proteins, Ah receptor and Arnt

Xenobiotic responsive element (XRE) is an inducible enhancer element that drives inducible expression of P-4501A1 gene in response to xenobiotic inducers. The XRE-binding factor appears in the nuclei of Hepa-1 cells treated with 3-methylcholanthrene (3-MC). Association of the Ah receptor and Arnt (Ah receptor nuclear translocator) in an XRE-binding complex was examined by anti-Ah receptor and Arnt antibodies. Both antibodies inhibited the sequence-specific XRE-binding activity of nuclear extracts from 3-MC-treated Hepa-1 cells and of the cytosolic fraction which was prepared from the nontreated cells and treated in vitro with 3-MC. These results indicate that Ah receptor and Arnt proteins are components of the XRE-binding factor and suggest that Arnt as well as the Ah receptor are localized in the cytosol of nontreated cells. The Ah receptor present in C4 cells, a mutant of Hepa-1 cells defective in the Arnt function, showed an inducer-dependent association with Arnt synthesized in an in vitro translation system. Co-transfection of the expression plasmids of the Ah receptor and Arnt exhibited synergistically more activated transcription from a reporter gene pMC6.3k consisting of the P-4501A1 gene promoter and enhancer than transfection with either of the two plasmids alone. These findings indicate that the Ah receptor and Arnt proteins form a complex that activates transcription in an inducer-dependent manner.

The AH-receptor: genetics, structure and function

The AH-receptor is a ligand-activated transcription factor that regulates a number of biological responses to planar aromatic hydrocarbons. Interest in this receptor is related to its role in the toxic action of a variety of environmental chemicals, the simplicity and elegance of the murine genetics that led to its characterization and the distinctive mechanism by which this receptor activates gene expression. Recent cloning experiments have demonstrated that the AH-receptor is structurally related to the Per, ARNT and Sim proteins. Members of this newly described gene family are characterized by two N-terminal domains, the most characteristic of which is a motif referred to as a PAS domain. In the AH-receptor, this domain harbours sequences involved in the formation of a hydrophobic pocket that bind receptor agonists. Adjacent to the PAS domain in the AH-receptor, ARNT and Sim proteins is a basic/helix-loop-helix (bHLH) domain that appears to mediate heterodimerization and sequence specific DNA binding properties. The observation that the bHLH domain is present in the AH-receptor and the ARNT protein, a factor required for proper AH-receptor function, suggests that these proteins are heterodimeric partners that activate gene expression in a manner similar to Myc/Max and MyoD/E2A. The objectives of this review are to describe recent experimental results in this field and to use this information to develop a molecular model of AH-receptor mediated signal transduction.

The Ah receptor nuclear translocator gene (ARNT) is located on q21 of human chromosome 1 and on mouse chromosome 3 near Cf-3

We have mapped the Ah receptor nuclear translocator (ARNT) gene to a conserved linkage group located on mouse chromosome 3 and human chromosome 1. EcoRI-digested DNA from a panel of 17 human x mouse somatic cell hybrids was probed with a cDNA fragment of the human ARNT gene. Six of the 17 independent mouse x human hybrids were positive for human bands. Human chromosome 1 showed complete cosegregation with the gene, whereas discordant segregation was observed for all other human chromosomes. The human gene was localized to 1q21 by using DNA from mouse x human hybrid clones that retain translocations involving human chromosome 1, by segregation analysis in nine informative CEPH families, and by in situ hybridization. The mouse homologue was mapped to mouse chromosome 3 using a panel of 16 hamster x mouse somatic cell hybrids. Six of 16 mouse x hamster hybrids were positive for mouse bands, showing complete concordance with mouse chromosome 3. The mouse Arnt gene was regionally mapped on chromosome 3, using linkage analysis in an interspecific backcross. The results indicate that the mouse gene resides about 40 cM from the centromere and about 10 cM proximal to Cf-3, the gene for tissue factor.

Role of the aryl hydrocarbon receptor nuclear translocator protein in aryl hydrocarbon (dioxin) receptor action

Immunoprecipitation experiments performed on cytosolic extracts of the mouse hepatoma cell line Hepa-1c1c7 (Hepa-1) confirm that the 9-S, unliganded, cytosolic aryl hydrocarbon (Ah) receptor complex contains the 90-kDa heat shock protein and the Ah receptor protein but reveal that it does not contain the Ah receptor nuclear translocator (ARNT) protein. These experiments confirm that the 6-S liganded form of the receptor identified in nuclear extracts of cells treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) contains the Ah receptor protein and ARNT but not the 90-kDa heat shock protein. The 6-S liganded Ah receptor complex activates transcription of the CYP1A1 gene via its binding to upstream xenobiotic-responsive elements (XREs). Treatment of cytosolic extracts of Hepa-1 cells with TCDD in vitro transforms the Ah receptor complex to the XRE-binding state. No such transformation occurs in a C- mutant deficient in ARNT activity. When in vitro synthesized ARNT was added concomitantly with TCDD to C- cytosolic extracts, it associated with the Ah receptor and restored Ah receptor-dependent XRE-binding activity to the extracts. Covalent cross-linking experiments in nuclear extracts of Hepa-1 and human LS180 cells treated with TCDD in vivo demonstrate that both ARNT and the Ah receptor bind directly to the XRE core sequence.

Ligand-dependent recruitment of the Arnt coregulator determines DNA recognition by the dioxin receptor

The intracellular basic region/helix-loop-helix (bHLH) dioxin receptor mediates signal transduction by dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) and functions as a ligand-activated DNA binding protein directly interacting with target genes by binding to dioxin response elements. Here we show that the partially purified, ligand-bound receptor alone could not bind target DNA. In contrast, DNA binding by the receptor could be induced by addition of a cytosolic auxiliary activity which functionally and biochemically corresponded to the bHLH factor Arnt. While Arnt exhibited no detectable affinity for the dioxin response element in the absence of the dioxin receptor, it strongly promoted the DNA binding function of the ligand-activated but not the ligand-free receptor forms. Arnt also functionally reconstituted in vitro the DNA binding activity of a mutant, nuclear translocation-deficient dioxin receptor phenotype in cytosolic extracts from a dioxin-resistant hepatoma cell line. Importantly, coimmunoprecipitation experiments showed that Arnt physically interacted in solution with the ligand-activated dioxin receptor but failed to heterodimerize with the ligand-free, hsp90-associated receptor form. Mutational analysis suggested that the functional interaction between these two factors occurred via the bHLH motif of Arnt. These data suggest that dioxin receptor activity is governed by a complex pattern of combinatorial regulation involving repression by hsp90 and then by ligand-dependent recruitment of the positive coregulator Arnt. The dioxin receptor system also provides the first example of signal-controlled dimerization of bHLH factors.

Investigation on the potential role of the Ah receptor nuclear translocator protein in vitamin D receptor action

The Ah receptor nuclear translocator protein (ARNT) is required for binding of the Ah (dioxin) receptor to the xenobiotic responsive element (XRE), and is a structural component of the XRE-binding form of the Ah receptor. The vitamin D receptor requires an accessory protein for binding to the vitamin D responsive element (VDRE) in the osteocalcin gene. Since the vitamin D receptor has similarities to the Ah receptor, we investigated whether ARNT is also required for vitamin D receptor activity. Two lines of evidence demonstrate that ARNT is not required for vitamin D receptor activity, and therefore does not correspond to the vitamin D receptor accessory protein: i) Antibodies to ARNT have no effect on binding of the vitamin D receptor to the VDRE. ii) c4, a mutant of Hepa-1 cells that is defective in ARNT activity, and in which binding of the Ah receptor to the XRE does not occur, possesses a vitamin D receptor with full activity for binding the VDRE.

Cross-coupling of signal transduction pathways: the dioxin receptor mediates induction of cytochrome P-450IA1 expression via a protein kinase C-dependent mechanism

Signal transduction by dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) is mediated by the intracellular dioxin receptor which, in its dioxin-activated state, regulates transcription of target genes encoding drug-metabolizing enzymes, such as cytochrome P-450IA1 and glutathione S-transferase Ya. Exposure of the dioxin receptor to dioxin leads to an apparent translocation of the receptor to the nucleus in vivo and to a rapid conversion of the receptor from a latent, non-DNA-binding form to a species that binds to dioxin-responsive positive control elements in vitro. This DNA-binding form of receptor appears to be a heterodimeric complex with the helix-loop-helix factor Arnt. In this study, we show that activation of the cytochrome P-450IA1 gene and minimal dioxin-responsive reporter constructs by the dioxin receptor was inhibited following prolonged treatment of human keratinocytes with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate. Inhibition of the receptor-mediated activation response was also achieved by treatment of the cells with a number of protein kinase inhibitors, one of which, calphostin C, shows selectivity for protein kinase C. Taken together, these data suggest that protein kinase C-dependent phosphorylation may play an essential role in the dioxin signaling pathway. This hypothesis is supported by the observation that pretreatment of the cells with 12-O-tetradecanoylphorbol-13-acetate inhibited the DNA-binding activity of the dioxin receptor in vivo. In vivo, the dioxin receptor was found to be a phosphoprotein. In vitro, dephosphorylation of the ligand-activated, heteromeric dioxin receptor form or dephosphorylation of the individual ligand-binding and Arnt receptor subunits inhibited the xenobiotic response element-binding activity. Moreover, dephosphorylation experiments with the individual receptor subunits prior to assembly of the xenobiotic response element-binding receptor form indicated that phosphorylation seemed to be important for the DNA-binding activity per se of the receptor, whereas Arnt appeared to require phosphorylation to interact with the receptor. Finally, a protein kinase C inhibitor-sensitive cytosolic catalytic activity that could restore the DNA-binding activity of the dephosphorylated dioxin receptor form was identified.

Identification of the Ah receptor nuclear translocator protein (Arnt) as a component of the DNA binding form of the Ah receptor

The Ah (dioxin) receptor binds a number of widely disseminated environmental pollutants, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and polycyclic aromatic hydrocarbons, and mediates their carcinogenic effects. The ligand-bound receptor activates Cyp 1a1 gene transcription through interaction with specific DNA sequences, termed xenobiotic responsive elements (XREs). The Ah receptor nuclear translocator protein (Arnt) is required for Ah receptor function. Arnt is now shown to be a structural component of the XRE binding form of the Ah receptor. Furthermore, Arnt and the ligand-binding subunit of the receptor were extracted as a complex from the nuclei of cells treated with ligand. Arnt contains a basic helix-loop-helix motif, which may be responsible for interacting with both the XRE and the ligand-binding subunit.

Cloning of a factor required for activity of the Ah (dioxin) receptor

The aryl hydrocarbon (Ah) receptor binds various environmental pollutants, such as polycyclic aromatic hydrocarbons, heterocyclic amines, and polychlorinated aromatic compounds (dioxins, dibenzofurans, and biphenyls), and mediates the carcinogenic effects of these agents. The complementary DNA and part of the gene for an 87-kilodalton human protein that is necessary for Ah receptor function have been cloned. The protein is not the ligand-binding subunit of the receptor but is a factor that is required for the ligand-binding subunit to translocate from the cytosol to the nucleus after binding ligand. The requirement for this factor distinguishes the Ah receptor from the glucocorticoid receptor, to which the Ah receptor has been presumed to be similar. Two portions of the 87-kilodalton protein share sequence similarities with two Drosophila proteins, Per and Sim. Another segment of the protein shows conformity to the consensus sequence for the basic helix-loop-helix motif found in proteins that bind DNA as homodimers or heterodimers.