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

Cell biology of cytochrome P-450 in the liver

Cytochromes P-450 (P-450) are members of a multigene superfamily of hemoproteins consisting the microsomal monooxygenase system with NADPH P-450 reductase (reductase) and/or reducing equivalents. Expression of many P-450 isoforms in hepatocytes is shown to be regulated at the level of transcription through interaction between cis-acting elements in the genes and DNA-binding (transacting) factors. Some isoforms of the CYP1A, 2B, 2E, and 3A subfamilies are regulated at the posttranscriptional level. For the topology of P-450 and reductase molecules in ER membrane of hepatocytes, models from stopped flow analysis and electron spin resonance are proposed. The densities of total P-450 and reductase molecules are revealed to be high enough to support the cluster model, suggesting that about ten P-450 molecules form an aggregate and surround one reductase molecule, and therefore the two enzymes form large micelles. ER proliferation after PB administration, which had been correlated with increase in P-450 level, is shown to be probably independent of the increase in P-450 level. There are considerable discrepancies among results reported on sublobular expression of various P-450 isoforms. Causes of the discrepancies are likely to be differences in experimental conditions of histochemical detection carried out and/or in species, strain, and/or sex.

Restructured transactivation domain in hamster AH receptor

Hamsters and Han/Wistar (Kuopio; H/W) rats show peculiarly selective responsiveness to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). They are extremely resistant to its acute lethality but sensitive to, e.g. , enzyme induction. The biological effects of TCDD are mediated by the AH receptor (AHR). Recent studies on H/W rat AHR discovered a remodelled transactivation domain which appears to be critical for the TCDD resistance of these animals. Here, molecular cloning and sequencing of hamster AHR reveals another type of restructured transactivation domain. In hamsters, the functionally pivotal Q-rich region is substantially expanded and enriched in glutamine compared with all other AHRs cloned to date. By contrast, the amino-terminal end is highly conserved, which is in agreement with the H/W rat AHR. Because of the additional material in the transactivation domain, hamster AHR protein is larger than that in rats or mice, but the pattern of AHR mRNA expression in tissues is similar.

Inhibition of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-stimulated Cyp1a1 promoter activity by hypoxic agents

Since hypoxia-inducible factor-1alpha (HIF-1alpha) and the arylhydrocarbon receptor (AhR) shared the AhR nuclear translocator (Arnt) for hypoxia- and AhR-mediated signaling, respectively, it was possible to establish the hypothesis that hypoxia could regulate cytochrome P450 1a1 (Cyp1a1) expression. In order to test this hypothesis, we undertook to examine the effect of hypoxia on Cyp1a1 transcription in Hepa-I cells. Mouse Cyp1a1 5'-flanking DNA, 1.6 kb was cloned into pGL3 expression vector in order to construct pmCyp1a1-Luc. Hepa-I cells were transfected with pmCyp1a1-Luc and treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the presence or absence of various hypoxic agents such as 1-100 microM cobalt chloride, 1-100 microM picolinic acid, and 1-100 microM desferrioxamine. Luciferase activity of the reporter gene was measured from pmCyp1a1-Luc-transfected Hepa-I cell lysate which contains 2 microgram total protein using luciferin as a substrate. Hypoxic agents such as cobalt chloride, picolinic acid, and desferrioxamine showed inhibition of luciferase activity that was induced by 1-nM TCDD treatment in a dose-and time-dependent manner. Concomitant treatment of 150 microM ferrous sulfate with 1-100 microM desferrioxamine or 1-100 microM picolinic acid recovered luciferase activity from that inhibited by hypoxic agents or induced by TCDD. These data demonstrated that iron-chelating and hypoxic agents inhibited dioxin-induced Cyp1a1 transcription in Hepa-I cells. Thus, we might suggest that hypoxia inhibits TCDD-induced Cyp1a1 expression due to the competition between HIF-1alpha and the AhR for the Arnt in Hepa-I cells.

Dioxin-induced perturbations in tryptophan homeostasis in laboratory animals

Polychlorinated dioxins (PCDD) are widespread environmental contaminants. The most potent and the general model compound for dioxins is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Our laboratory has developed a new model for studies of dioxin toxicity based on totally disparate sensitivity to the lethal action of TCDD between Long-Evans (L-E, Turku AB; LD50 ca. 10 micrograms/kg) and Han/Wistar (H/W, Kuopio; LD50 over 10,000 micrograms/kg) rat strains. We have shown that body weight regulation is differentially regulated by TCDD in these rat strains: body weight gain is permanently reduced in the sensitive L-E but not in the resistant H/W strain. In concert with reduced body weight, TCDD increased brain TRP concentration, 5-HT synthesis and its metabolism to 5-HIAA at lethal doses in TCDD-susceptible L-E rats, and almost not at all in resistant H/W rats in which lethal dose levels were not reached. Further studies showed that TCDD indirectly increases free TRP concentration in the circulation in TCDD-susceptible L-E rats. Blood free fatty acids seem to be involved in the latter phenomenon. It is not likely that the enhanced serotonergic tone in the CNS is a causative factor in TCDD-induced anorexia. However, the present results may open up an interesting avenue to better understand physiology of TRP and the complex regulation of energy balance.

Angiogenesis: how a tumor adapts to hypoxia

The growth of new blood vessels from the preexisting vascular tree, also known as angiogenesis, occurs in situations such as wound and fracture healing, arthritis, cardiovascular and cerebral ischemia, and nearly every type of cancer known. Vascular endothelial growth factor (VEGF) has been shown to play a crucial role in these events. Hypoxia-dependent VEGF induction is mediated by hypoxia-inducible factor-1 (HIF-1). HIF-1 is a heterodimeric transcription factor tightly regulated by oxygen concentration. In this short review, we summarize recent data concerning the control of HIF-1 activity and notably the regulation of HIF-1alpha subunit by phosphorylation and the ubiquitin proteasomal degradation system. A complete knowledge of this mechanism could, by the design of new antiangiogenic strategies, have a strong impact in clinical oncology.

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

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

Lack of an absolute requirement for the native aryl hydrocarbon receptor (AhR) and AhR nuclear translocator transactivation domains in protein kinase C-mediated modulation of the AhR pathway

Protein kinase C (PKC)-mediated modulation of the aryl hydrocarbon receptor (AhR) pathway was examined in CHOK1-derived L10.I cells stably transfected with the pGUDLUC6.1 reporter; pGUDLUC6.1 is solely controlled by four dioxin-responsive enhancer elements. Co treatment of L10.I cells with 10 nM 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) and 81 nM phorbol 12-myristate 13-acetate (PMA), an activator of sn-1,2-diacylglyerol binding PKCs, enhanced transactivation of the reporter construct several-fold relative to cells treated with a saturating 10 nM TCDD dose alone; this effect was dubbed the "PMA effect." A domain swapping and deletional analysis of the native AhR and AhR nuclear translocator (ARNT) protein transactivation domains (TADs) was performed to determine if these domains are absolutely required for the AhR x ARNT dimer-mediated PMA effect in the L10.I model system; controls demonstrate the suitability of the L10.I model for these analyses and that endogenous AhR and ARNT levels are extremely low in this model. Transient coexpression of the AhR and ARNT-474-FLAG, an ARNT protein lacking the native ARNT TAD, in L10.I cells reveals the native ARNT TAD is not absolutely required for the AhR x ARNT-474-FLAG dimer to mediate the PMA effect. Transient coexpression of AhRDeltaCVP, a chimeric AhR protein in which the native AhR TAD has been replaced with the VP16 (herpes simplex virus protein 16) TAD (which control experiments demonstrate is unaffected by PMA), and ARNT in L10.I cells indicates that the native AhR TAD is not absolutely required for this AhRDeltaCVP x ARNT dimer to mediate the PMA effect. These observations strongly suggest that PKC-mediated modulation of the AhR pathway is not absolutely dependent on coactivators recruited to the AhR. ARNT dimer by the native TADs of the AhR and its heterodimerization partner ARNT.

p42/p44 mitogen-activated protein kinases phosphorylate hypoxia-inducible factor 1alpha (HIF-1alpha) and enhance the transcriptional activity of HIF-1

Hypoxia-inducible factor-1 (HIF-1) controls the expression of a number of genes such as vascular endothelial growth factor and erythropoietin in low oxygen conditions. However, the molecular mechanisms that underlie the activation of the limiting subunit, HIF-1alpha, are still poorly resolved. Results showing that endogenous HIF-1alpha migrated 12 kDa higher than in vitro translated protein led us to evaluate the possible role of phosphorylation on this phenomenon. We report here that HIF-1alpha is strongly phosphorylated in vivo and that phosphorylation is responsible for the marked differences in the migration pattern of HIF-1alpha. In vitro, HIF-1alpha is phosphorylated by p42 and p44 mitogen-activated protein kinases (MAPKs) and not by p38 MAPK or c-Jun N-terminal kinase. Interestingly, p42/p44 MAPK stoichiometrically phosphorylate HIF-1alpha in vitro, as judged by a complete upper shift of HIF-1alpha. More importantly, we demonstrate that activation of the p42/p44 MAPK pathway in quiescent cells induced the phosphorylation and shift of HIF-1alpha, which was abrogated in presence of the MEK inhibitor, PD 98059. Finally, we found that in a vascular endothelial growth factor promoter mutated at sites previously shown to be MAPK-sensitive (SP1/AP2-88-66 site), p42/p44 MAPK activation is sufficient to promote the transcriptional activity of HIF-1. This interaction between HIF-1alpha and p42/p44 MAPK suggests a cooperation between hypoxic and growth factor signals that ultimately leads to the increase in HIF-1-mediated gene expression.

Generation of a dominant-negative mutant of endothelial PAS domain protein 1 by deletion of a potent C-terminal transactivation domain

Endothelial PAS domain protein 1 (EPAS1) is a basic helix-loop-helix/PAS domain transcription factor that is preferentially expressed in vascular endothelial cells. EPAS1 shares high homology with hypoxia-inducible factor-1alpha (HIF-1alpha) and, like HIF-1alpha, has been shown to bind to the HIF-1-binding site and to activate its downstream genes such as vascular endothelial growth factor (VEGF) and erythropoietin. In this report, we show that EPAS1 increased VEGF gene expression through the HIF-1-binding site. This transactivation was enhanced further by cotransfection of an aryl hydrocarbon receptor nuclear translocator expression plasmid. Deletion analysis of EPAS1 revealed a potent activation domain (amino acids 486-639) essential for EPAS1 to transactivate the VEGF promoter. We confirmed the ability of this domain to activate transcription using a Gal4 fusion protein system. Because a truncated EPAS1 protein lacking the transactivation domain at amino acids 486-639 eliminated induction of the VEGF promoter by wild-type EPAS1, the truncated protein functions as a dominant-negative mutant. Most important, infection of the cells with an adenoviral construct expressing this mutant inhibited the induction of VEGF mRNA under conditions that mimic hypoxia. Our results suggest that EPAS1 is an important regulator of VEGF gene expression. Since VEGF plays a crucial role in angiogenesis, the ability of dominant-negative EPAS1 to inhibit VEGF promoter activity raises the possibility of a novel approach to inhibiting pathological angiogenesis.

Differential recruitment of coactivator RIP140 by Ah and estrogen receptors. Absence of a role for LXXLL motifs

The Ah receptor (AhR), a soluble cytosolic protein, mediates most of the toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related environmental contaminants. The mechanism of ligand-mediated AhR activation has been, in part, elucidated. The sequence of events following the binding of the AhR/AhR nuclear translocator protein (ARNT) heterodimer to dioxin response elements has yet to be completely understood. The role of coactivator, RIP140, in the modulation of transcriptional activity of AhR/ARNT heterodimer was examined. RIP140 enhanced TCDD-mediated, dioxin response element-driven reporter gene activity in three cell lines. Co-immunoprecipitation and co-localization assays revealed that RIP140 interacted with AhR, but not with ARNT, both in vitro and in cells. Mapping of the interaction sites revealed that RIP140 was recruited by the AhR transactivation domain via the Q-rich subdomain. The RIP140 domain that interacts with the AhR was mapped to a location between amino acid residues 154 and 350, which is distinct from those involved in estrogen receptor binding. The signature motif, LXXLL, which is responsible for binding of several coactivators to nuclear receptors, is not required for RIP140 binding to AhR. These results demonstrate that the AhR recruits coactivators that are capable of enhancing transcription and, thus, the AhR may compete with steroid receptors for a common coactivator pool. In addition, the data suggest that there are distinct motif(s) for the recruitment of RIP140 to AhR and possibly other non-steroid receptors/transcription factors.

Role of estradiol receptor-alpha in differential expression of 2,3,7, 8-tetrachlorodibenzo-p-dioxin-inducible genes in the RL95-2 and KLE human endometrial cancer cell lines

The present study was conducted to investigate the mechanism of the response of human uterine endometrial carcinoma cells, RL95-2 and KLE, to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). RL95-2 cells were highly responsive to TCDD in terms of cytochrome P4501A1 (CYP1A1), cytochrome P4501B1 (CYP1B1), and plasminogen activator inhibitor-2 (PAI-2), whereas KLE cells showed little stimulatory effects only at high doses. Neither showed any growth inhibition upon exposure to TCDD. KLE cells expressed higher levels of aryl hydrocarbon receptor (AhR) than RL95-2 and gel mobility shift assay also identified more liganded AhR-ARNT complex bound to xenobiotic response elements (XRE). TCDD had no downregulatory effects on the expression of either AhR or the estradiol receptor (ER). Though both cell types expressed ER-alpha almost equally, immunofluorescence demonstrated a defect in its nuclear translocation in KLE cells where ER-alpha was mainly cytoplasmic and estradiol-17beta (E(2)) was unable to translocate it to the nucleus. However, both cells were nonresponsive to E(2) in terms of transcriptional activation and transient expression of normal ER-alpha restored the E(2) responsiveness. Transient expression of ER-alpha in KLE cells also restored its responsiveness to TCDD on transcriptional activation. Collectively, these results indicate that ER-alpha acts as a positive modulator in regulation of the TCDD-inducible genes.

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

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

Physicochemical differences in the AH receptors of the most TCDD-susceptible and the most TCDD-resistant rat strains

Long-Evans rats (strain Turku AB; L-E) are at least 1000-fold more sensitive (LD50 about 10 microg/kg) to the acute lethal effects of 2, 3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) than are Han/Wistar (Kuopio; H/W) rats (LD50 > 9600 microg/kg). The AH receptor (AHR) is believed to mediate the toxic effects of TCDD and related halogenated aromatic hydrocarbons. We compared the AHRs of L-E and H/W rats to determine if there were any structural or functional receptor differences that might be related to the dramatic difference in the sensitivity of these two strains to the lethal effects of TCDD. Cytosols from liver and lung of the sensitive L-E rats contained about twofold higher levels of specific binding sites for [3H]TCDD than occurred in H/W rats; the Kd for binding of [3H]TCDD to AHR in hepatic cytosols was similar between the two strains. Addition of the oxyanions, molybdate or tungstate (20 mM), had little effect upon ligand binding to AHR in hepatic cytosols from L-E rats whereas in cytosols from H/W rats these agents substantially diminished or totally abolished TCDD binding. The AHR in H/W cytosols also lost ligand-binding function when NaCl (20 to 400 mM) was added to the buffer whereas, in cytosols from L-E rats, the addition of 400 mM NaCl caused the receptor complex to shift from 9S to 6S during velocity sedimentation but did not destroy ligand binding function. AHR from hepatic cytosol of both the L-E and H/W rats could be transformed to the DNA-binding state in the presence of TCDD or other dioxin congeners as assessed by gel mobility shift assays. The most dramatic difference in AHR properties between L-E and H/W rats is molecular mass. Immunoblotting of cytosolic proteins revealed that the AHR in L-E rats has an apparent mass of approximately 106 kDa, similar to the mass of the receptor previously reported in several other common laboratory rat strains. In contrast, the mass of the AHR in H/W rats is approximately 98 kDa, significantly smaller than the mass of receptor reported in any other rat strains. F1 offspring of a cross between L-E and H/W rats expressed both the 106- and the 98-kDa protein. There was no apparent difference in the mass of the AHR nuclear translocator protein (ARNT) between the two strains, but the hepatic concentration of ARNT was about three times as high in L-E as in H/W rats. It will be interesting to find out how the altered structure of the AHR in H/W rats is related to their remarkable resistance to the lethal effects of TCDD.

The AH receptor and a novel gene determine acute toxic responses to TCDD: segregation of the resistant alleles to different rat lines

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD),12 the most toxic congener of dioxins, exhibits wide sensitivity differences between a sensitive Long-Evans (L-E) rat and a resistant Han/Wistar (H/W) rat. The sensitivity is determined probably by two autosomal genes and it is highly end point dependent. The difference is more than 1000-fold for acute toxicity and negligible for CYP1A1 induction. The rat strains were recently shown to have differences in the size of AH receptor (AHR), which mediates most effects of TCDD. In the present study, the rat strains were crossed and the resistant alleles of genes determining TCDD sensitivity were segregated to new rat lines. Selection was based on AHR phenotype determined by Western blot and resistance to TCDD lethality. Two genes determining resistance were found: the Ahr and a novel gene designated "B." In homozygous rats, the H/W type Ahrhw allele prevented TCDD lethality up to 2000 microg/kg or more, and the H/W type "Bhw" allele also increased resistance to TCDD lethality but to a lesser extent. Heterozygous rats were only slightly more resistant to acute lethality than the respective sensitive homozygous rats. CYP1A1 induction was similar irrespective of the Ahr and "B" genotypes, but a substantial increase in serum bilirubin seen after low doses in sensitive rats occurred only after large doses in "Bhw/hw" and not at all in Ahrhw/hw rats. In conclusion, the Ahrhw allele is a major determinant of the exceptional resistance of H/W rats to TCDD lethality. There is also an additional gene, whose function remains to be characterized, conferring limited resistance to TCDD toxicity. These two H/W rat-derived alleles are separately expressed in the new rat lines created.

Oxygen-regulated and transactivating domains in endothelial PAS protein 1: comparison with hypoxia-inducible factor-1alpha

Endothelial PAS protein 1 (EPAS1) is a basic helix-loop-helix Per-AHR-ARNT-Sim transcription factor related to hypoxia-inducible factor-1alpha (HIF-1alpha). To analyze EPAS1 domains responsible for transactivation and oxygen-regulated function, we constructed chimeric fusions of EPAS1 with a GAL4 DNA binding domain, plus or minus the VP16 activation domain. Two transactivation domains were defined in EPAS1; a C-terminal domain (amino acids 828-870), and a larger internal domain (amino acids 517-682). These activation domains were interspersed by functionally repressive sequences, several of which independently conveyed oxygen-regulated activity. Two types of activity were defined. Sequences lying N-terminal to and overlapping the internal transactivation domain conferred regulated repression on the VP16 transactivator. Sequences lying C-terminal to this internal domain conveyed repression and oxygen-regulated activity on the native EPAS1 C-terminal activation domain, but not the Gal/VP16 fusion. Fusions containing internal but not C-terminal regulatory domains manifested regulation of fusion protein level. Comparison of EPAS1 with HIF-1alpha demonstrated a similar organization for both proteins, and for the C terminus defined a conserved RLL motif critical for inducibility. Overall, EPAS1 sequences were less inducible than those of HIF-1alpha, and inducibility was strikingly reduced as their expression level was increased. Despite these quantitative differences, EPAS1 regulation appeared similar to HIF-1alpha, conforming to a model involving the modulation of both protein level and activity, through distinct internal and C-terminal domains.

Cloning and characterization of the zebrafish (Danio rerio) aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AhR) mediates the toxicity of 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds in vertebrates. To further establish zebrafish as a vertebrate model to study the molecular mechanism of TCDD toxicity, we have isolated and characterized the cDNA encoding the zebrafish aryl hydrocarbon receptor (zfAhR2). Analysis of the deduced protein sequence revealed the 1027 amino acid protein is approximately 200 amino acids longer than previously isolated receptors. zfAhR2 is homologous to previously cloned PAS proteins within the basic helix-loop-helix and PAS domains. The C-terminal domain of zfAhR2 diverges from the mammalian AhR at position 420, and does not contain a Q-rich domain. zfAhR2 mRNA is first detected by Northern blot analysis at 24 h post fertilization, and expression increases throughout early development. Treatment of zebrafish embryos and zebrafish liver cells with graded doses of TCDD results in a dose-dependent increase in zfAhR2 mRNA. The time course for zfAhR2 and cytochrome P4501A mRNA induction by TCDD are similar. In vitro produced zfAhR2 protein dimerizes with the rainbow trout aryl hydrocarbon receptor nuclear translocator (rtARNTb) and binds dioxin response elements derived from the rainbow trout CYP1A gene. Finally, transient coexpression of zfAhR2 and rtARNTb in COS-7 cells results in a TCDD dose-related increase in transcription driven by the rainbow trout CYP1A promoter and enhancer.

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

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

The aryl hydrocarbon receptor: a comparative perspective

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

DNA binding activity of the aryl hydrocarbon receptor is sensitive to redox changes in intact cells

The potential involvement of vicinal dithiols in the transformation of the aryl hydrocarbon (Ah) receptor from its ligand binding to DNA binding form in Hepa-1 cells was explored through the use of diamide and phenylarsine oxide (PAO), which have been shown to specifically form a stable ring complex with vicinal sulfhydryl groups in selected proteins. Pretreatment with diamide and PAO rapidly prevented the inducer-dependent formation of the Ah receptor/xenobiotic response element complex detected by electrophoretic mobility shift assays and suppressed Ah receptor-mediated transcription. Diamide and PAO also inhibited DNA binding activity of the nuclear Ah receptor subsequent to its translocation to the nucleus but to a lesser extent than that observed with pretreatment conditions. The Ah receptor exhibited much higher sensitivity to cellular redox changes than Sp1, a transcription factor previously shown to be very sensitive to redox regulation. Diamide added to nuclear extracts inhibited Ah receptor DNA binding more than when it was added in intact cells. In contrast, Ah receptor DNA binding activity was more sensitive to PAO when it was added to intact cells than when it was added to nuclear extracts. Finally, dithiol 2,3-dimercaptopropanol was over 100 times more effective than monothiol 2-mercaptoethanol in reversing the PAO-dependent inhibition of Ah receptor DNA binding activity. This suggests that vicinal sulfhydryl residues may be involved in DNA binding of the Ah receptor.

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

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

Suppression of CYP1A1 transcription by H2O2 is mediated by xenobiotic-response element

We have previously demonstrated that H2O2 downregulates CYP1A1 and CYP1A2 transcription in isolated rat hepatocytes (C. W. Barker, et al., 1994, J. Biol. Chem. 269, 3985-3990). In the present study, induction of chloramphenicol acetyltransferase (CAT) expression driven by 3.1 kb of rat CYP1A1 upstream regulatory sequences was suppressed by 56% in Hepa-1 cells treated with H2O2. Similarly, H2O2 inhibited CAT expression from vectors containing two copies of either xenobiotic-response element (XRE) 1 or XRE2. H2O2 did not inhibit basal CAT expression in cells that were not treated with the inducer beta-napthoflavone. Electrophoretic mobility shift assays demonstrated that the suppression of XRE-dependent transcription by H2O2 was not due to changes in nuclear aryl hydrocarbon (Ah) receptor DNA binding activity. Several types of experiments indicated that modulation of XRE enhancer strength by various means could modify H2O2-dependent suppression of CAT expression. Conditions that increased the transactivation potential of the Ah receptor (increase in XRE copy number or shortening of the distance between XREs and the minimal CYP1A1 promoter) attenuated the action of H2O2, while conditions that reduced XRE-mediated transactivation potential (decrease in XRE copy number, increase of the distance between the XRE and the promoter, or reduction of the number of bound Ah receptors by lowering the concentration of inducer) potentiated the inhibitory action of H2O2.

Point mutation in intron sequence causes altered carboxyl-terminal structure in the aryl hydrocarbon receptor of the most 2,3,7,8-tetrachlorodibenzo-p-dioxin-resistant rat strain

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is the most potent dioxin. There are exceptionally wide inter- and intraspecies differences in sensitivity to TCDD toxicity with Han/Wistar (H/W) (Kuopio) rats being the most resistant mammals tested. A peculiar feature of H/W rats is that despite their unresponsiveness to the acute lethality of TCDD, their sensitivity to other biological impacts of TCDD (e.g., CYP1A1 induction) is preserved. The biological effects of TCDD are mediated by the aryl hydrocarbon receptor (AhR). We recently found that the AhR of H/W rats (about 98 kDa) is smaller than the receptor in other rat strains (106 kDa). In the present study, molecular cloning and sequencing of the H/W rat AhR revealed that the reason for its smaller size is a deletion/insertion-type change at the 3' end of exon 10 in the receptor cDNA. This change emanates from a single point mutation at the first nucleotide of intron 10, resulting in altered mRNA splicing. At the protein level, the mutation leads to a total loss of either 43 or 38 amino acids (with altered sequence for the last seven amino acids in the latter case) toward the carboxyl-terminal end in the trans-activation domain of the AhR. H/W rats also harbor a point mutation in exon 10 that will cause a Val-to-Ala substitution in codon 497, but this occurs in a variable region of the AhR. These findings suggest that there is a relatively small region in the AhR trans-activation domain that may be capable of providing selectivity to its function.

A novel response to dioxin. Induction of ecto-ATPase gene expression

We used differential display to discover a new gene that the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) regulates in mouse hepatoma cells. Its predicted amino acid sequence suggests that the gene encodes an ecto-ATPase that contains multiple glycosylation sites, conserved cysteine residues, and apyrase conserved regions. cDNA expression experiments in mouse hepatoma cells confirm that the new gene encodes an ecto-ATPase. Wild-type mouse hepatoma cells contain both constitutive and TCDD-inducible ecto-ATPase activity. Induction of ecto-ATPase gene expression by TCDD is direct and occurs at the transcriptional level. Studies in mutant hepatoma cells indicate that induction requires both the aromatic hydrocarbon receptor (AhR) and the AhR nuclear translocator (Arnt). Furthermore, induction requires AhR's transactivation domain, but not that of Arnt. Our findings reveal new aspects of dioxin's biological effects and TCDD-dependent gene regulation.

A mutant Drosophila homolog of mammalian Clock disrupts circadian rhythms and transcription of period and timeless

We report the identification, characterization, and cloning of a novel Drosophila circadian rhythm gene, dClock. The mutant, initially called Jrk, manifests dominant effects: heterozygous flies have a period alteration and half are arrhythmic, while homozygous flies are uniformly arrhythmic. Furthermore, these flies express low levels of the two clock proteins, PERIOD (PER) and TIMELESS (TIM), due to low per and tim transcription. Mapping and cloning of the Jrk gene indicates that it encodes the Drosophila homolog of mouse Clock. The mutant phenotype results from a premature stop codon that eliminates much of the putative activation domain of this bHLH-PAS transcription factor, thus explaining the dominant features of Jrk. The remarkable sequence conservation strongly supports common clock components present in the common ancestor of Drosophila and mammals.

Transcriptional activation of cytochrome P450 genes by different classes of chemical inducers

1. We review here the molecular mechanisms underlying the xenobiotic induction of genes encoding cytochrome P450 (CYP) enzymes in the liver and other tissues. We will focus on four major families of CYP genes. 2. Members of the CYP1 gene family are induced by polycyclic aromatic hydrocarbons and this process is mediated by the basic helix-loop-helix proteins: the Ah receptor and its heterodimeric partner Arnt. Considerable progress has been made in elucidating the molecular details of this induction process. 3. CYP4 genes are activated by peroxisomal proliferators, a group of structurally diverse chemicals that also induce peroxisome proliferation. The transcriptional response is dependent on the peroxisome proliferator-activated receptor and its partner RXR, both members of the nuclear receptor superfamily; their role in the induction process has been well characterized at the molecular level. 4. In contrast, the mechanism of gene induction of CYP2 genes by phenobarbital and other structurally diverse inducers is not well understood and a specific phenobarbital-responsive receptor has not been identified. 5. Induction of the CYP3 gene family by the glucocorticoid dexamethasone appears to involve the glucocorticoid receptor, but this receptor is not apparently required for induction by metapyrone and a complete molecular understanding of the induction processes is lacking at present.

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

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

Aryl hydrocarbon (Ah) nonresponsiveness in estrogen receptor-negative MDA-MB-231 cells is associated with expression of a variant arnt protein

Several studies have reported a correlation between expression of the estrogen receptor (ER) and aryl hydrocarbon (Ah) responsiveness in human breast cancer cell lines. MDA-MB-231 cells are ER-negative and Ah-nonresponsive; however, initial studies showed that 2,3,7,8-tetrachlorodibenzo-p-dioxin induced CYP1A1 mRNA levels (5.8-fold) and chloramphenicol acetyltransferase activity (2.6-fold) in high passage (Hp, >50 passages) cells transiently transfected with an Ah-responsive plasmid. In contrast, no induction responses were observed in low passage (Lp, <20 passages) cells. The Ah responsiveness of Hp compared to Lp MDA-MB-231 cells was associated with a >2-fold increased expression of the Ah receptor in Hp cells. Further analysis revealed that the apparent molecular weight of the Ah receptor mRNA transcript and immunoreactive protein were comparable in Lp MDA-MB-231 and Ah-responsive human HepG2 cells. In contrast, RT-PCR analysis of the Ah receptor nuclear translocator (Arnt) protein showed that HepG2 cells expressed the expected 2.6-kb transcript, whereas a 1.3-kb transcript was the major product in MDA-MB-231 cells. Western blot analysis confirmed that HepG2 cells primarily expressed a 97-kDa wild-type form of Arnt, whereas a dominant 36-kDa variant was expressed in MDA-MB-231 cells. Complete sequence analysis of the variant form of Arnt revealed a major deletion of the C-terminal region of the protein (aa 330 to 789). Like HepG2 cells, the wild-type 2.6-kb transcript was detected in ER-positive (Ah-responsive) MCF-7 cells, whereas the low-molecular-weight variant Arnt was dominant in ER-negative MDA-MB-231, MDA-MB-435, and Adriamycin-resistant MCF-7 cells. These results suggest that expression of this protein may be useful as a prognostic factor in breast cancer.

Rtg3p, a basic helix-loop-helix/leucine zipper protein that functions in mitochondrial-induced changes in gene expression, contains independent activation domains

Rtg3p and Rtg1p are basic helix-loop-helix/leucine zipper protein transcription factors in yeast that interact and bind to sites in an upstream activation sequence element in the 5'-flanking region of CIT2, a gene encoding a peroxisomal isoform of citrate synthase. These factors are required both for basal expression of CIT2 and its elevated expression in cells with dysfunctional mitochondria, such as in respiratory-deficient petite cells lacking mitochondrial DNA (rho degrees ). This elevated expression of CIT2 is called the retrograde response. Here we show that fusion constructs between the Gal4p DNA binding domain and Rtg3p transactivate the expression of a LacZ reporter gene under the control of a GAL1 promoter element. We have identified two activation domains in Rtg3p: a strong carboxyl-terminal domain from amino acids 375-486, and a weaker amino-terminal domain from amino acids 1-175; neither of these activation domains contain the bHLH/Zip motif. We have also identified a serine/threonine-rich domain of Rtg3p within amino acids 176-282 that is inhibitory to transactivation. In addition, the transcriptional activity of the Gal4-Rtg3p fusion proteins does not require either Rtg1p or Rtg2p; the latter is a protein containing an hsp70-like ATP binding domain that is also necessary for CIT2 expression. In contrast, transcriptional activation by Gal4-Rtg1p fusion proteins requires the Rtg1p basic helix-loop-helix/leucine zipper protein domain, as well as Rtg3p and Rtg2p. These data suggest that transcriptional activation by the Rtg1p-Rtg3p complex is largely the function of Rtg3p. Experiments are also presented suggesting that Rtg3p is limiting for gene expression in respiratory-competent (rho+) cells.

Characterization of a subset of the basic-helix-loop-helix-PAS superfamily that interacts with components of the dioxin signaling pathway

In an effort to better understand the mechanism of toxicity of 2,3,7, 8-tetrachlorodibenzo-p-dioxin, we employed an iterative search of human expressed sequence tags to identify novel basic-helix-loop-helix-PAS (bHLH-PAS) proteins that interact with either the Ah receptor (AHR) or the Ah receptor nuclear translocator (ARNT). We characterized five new "members of the PAS superfamily," or MOPs 1-5, that are similar in size and structural organization to the AHR and ARNT. MOPs 1-4 have N-terminal bHLH and PAS domains and C-terminal variable regions. MOP5 contained the characteristic PAS domain and a variable C terminus; it is possible that the cDNA contains a bHLH domain, but the entire open reading frame has yet to be completed. Coimmunoprecipitation studies, yeast two-hybrid analysis, and transient transfection experiments demonstrated that MOP1 and MOP2 dimerize with ARNT and that these complexes are transcriptionally active at defined DNA enhancer sequences in vivo. MOP3 was found to associate with the AHR in vitro but not in vivo. This observation, coupled with the fact that MOP3 formed tighter associations with the 90-kDa heat shock protein than the human AHR, suggests that MOP3 may be a conditionally active bHLH-PAS protein that requires activation by an unknown ligand. The expression profiles of the AHR, MOP1, and MOP2 mRNAs, coupled with the observation that they all share ARNT as a common dimeric partner, suggests that the cellular pathways mediated by MOP1 and MOP2 may influence or respond to the dioxin signaling pathway.

Aryl hydrocarbon receptor-mediated signal transduction

The aryl hydrocarbon (or dioxin) receptor (AhR) is a ligand-activated basic helix-loop-helix (bHLH) protein that heterodimerizes with the bHLH protein ARNT (aryl hydrocarbon nuclear translocator) forming a complex that binds to xenobiotic regulatory elements in target gene enhancers. Genetic, biochemical, and molecular biology studies have revealed that the AhR mediates the toxic and biological effects of environmentally persistent dioxins and related compounds. Cloning of the receptor and its DNA-binding partner, ARNT, has facilitated detailed efforts to understand the mechanisms of AhR-mediated signal transduction. These studies have determined that this unique receptor consists of several functional domains and belongs to a subfamily of bHLH proteins that share a conserved motif termed the PAS domain. In addition, recent genetic studies have revealed that expression of the AhR is a requirement for proper embryonal development, which appears to be a common function shared by many other bHLH proteins. This review is a summary of recent molecular studies of AhR-mediated gene regulation.

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.

Ah receptor signaling pathways

The aryl hydrocarbon (Ah) receptor has occupied the attention of toxicologists for over two decades. Interest arose from the early observation that this soluble protein played key roles in the adaptive metabolic response to polycyclic aromatic hydrocarbons and in the toxic mechanism of halogenated dioxins and dibenzofurans. More recent investigations have provided a fairly clear picture of the primary adaptive signaling pathway, from agonist binding to the transcriptional activation of genes involved in the metabolism of xenobiotics. Structure-activity studies have provided an understanding of the pharmacology of this receptor; recombinant DNA approaches have identified the enhancer sequences through which this factor regulates gene expression; and functional analysis of cloned cDNAs has allowed the characterization of the major signaling components in this pathway. Our objective is to review the Ah receptor's role in regulation of xenobiotic metabolism and use this model as a framework for understanding the less well-characterized mechanism of dioxin toxicity. In addition, it is hoped that this information can serve as a model for future efforts to understand an emerging superfamily of related signaling pathways that control biological responses to an array of environmental stimuli.

Protein kinase C modulates regulation of the CYP1A1 gene by the aryl hydrocarbon receptor

Transcriptional activation of the human CYP1A1 gene by halogenated and polycyclic aromatic hydrocarbons is mediated by the aryl hydrocarbon receptor (AhR) complex, a ligand-dependent transcription factor. A competent AhR comprises at least two components following nuclear translocation and DNA binding, the AhR and the AhR nuclear translocator (Arnt) protein, whose combined action on human CYP1A1 gene transcription is shown to be dependent upon functional protein kinase C (PKC). In the present study, we examined the effects of phorbol 12-myristate 13-acetate, a potent PKC activator, on the ligand-induced transcriptional activation of the CYP1A1 gene and cellular function of the AhR in human HepG2 101L cells. The 101L cells carry a stable transgene consisting of 1800 bases of 5'-flanking DNA and the promoter of the human CYP1A1 gene linked to the firefly luciferase structural gene (Postlind, H., Vu, T. P., Tukey, R. H. & Quattrochi, L. C. (1993) Toxicol. Appl. Pharmacol. 118, 255-262). Pretreatment of cells with 12-myristate 13-acetate enhanced ligand-induced CYP1A1 gene expression 2-3-fold. Inhibition of PKC activity blocked directly the transcriptional activation and the transactivation of the CYP1A1 gene, indicating a role for PKC in the AhR-mediated transcriptional activation process. However, the DNA binding activities of the in vitro activated and the induced nuclear AhR as measured by electrophoretic mobility shift analysis were not affected when CYP1A1 transcription was inhibited, indicating the actions of PKC to be a nuclear event that works in concert with or precedes AhR binding to the gene. These results illustrate that PKC is absolutely essential for the cellular and molecular events that control induction of CYP1A1 gene transcription.

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.

Dimerization, DNA binding, and transactivation properties of hypoxia-inducible factor 1

Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric basic helix-loop-helix transcription factor that regulates hypoxia-inducible genes including the human erythropoietin (EPO) gene. In this study, we report structural features of the HIF-1alpha subunit that are required for heterodimerization, DNA binding, and transactivation. The HIF-1alpha and HIF-1beta (ARNT; aryl hydrocarbon receptor nuclear translocator) subunits were coimmunoprecipitated from nuclear extracts, indicating that these proteins heterodimerize in the absence of DNA. In vitro-translated HIF-1alpha and HIF-1beta generated a HIF-1/DNA complex with similar electrophoretic mobility and sequence specificity as HIF-1 present in nuclear extracts from hypoxic cells. Compared to 826-amino acid, full-length HIF-1alpha, amino acids 1-166 mediated heterodimerization with HIF-1beta (ARNT), but amino acids 1-390 were required for optimal DNA binding. A deletion involving the basic domain of HIF-1alpha eliminated DNA binding without affecting heterodimerization. In cotransfection assays, forced expression of recombinant HIF-1alpha and HIF-1beta (ARNT) activated transcription of reporter genes containing EPO enhancer sequences with intact, but not mutant, HIF-1 binding sites. Deletion of the carboxy terminus of HIF-1alpha (amino acids 391-826) markedly decreased the ability of recombinant HIF-1 to activate transcription. Overexpression of a HIF-1alpha construct with deletions of the basic domain and carboxy terminus blocked reporter gene activation by endogenous HIF-1 in hypoxic cells.

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.

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.

Definition of a minimal domain of the dioxin receptor that is associated with Hsp90 and maintains wild type ligand binding affinity and specificity

The dioxin receptor is a cytoplasmic basic helix-loop-helix/Per-Arnt-Sim homology (bHLH/PAS) protein known to bind planar polycyclic ligands including polycyclic aromatic hydrocarbons, benzoflavones, heterocyclic amines, and halogenated aromatic hydrocarbons, e.g. dioxins. Ligand-induced activation of the dioxin receptor initiates a process whereby the receptor is transformed into a nuclear transcription factor complex with a specific bHLH/PAS partner protein, Arnt. In analogy to the glucocorticoid receptor, the latent dioxin receptor is found associated with the molecular chaperone hsp90. We have defined and isolated a minimal ligand binding domain of the dioxin receptor from the central PAS region, comprising of amino acids 230 to 421, and found this domain to interact with hsp90 in vitro. Expression of the minimal ligand binding domain in wheat germ lysates or bacteria, systems which harbor hsp90 homologs unable to interact with the glucocorticoid or dioxin receptors, resulted in non-ligand binding forms of this minimal 230 to 421 fragment. Importantly, affinity of the minimal ligand binding domain for dioxin was similar to the affinity inherent in the full-length dioxin receptor, and a profile of ligand structures which specifically bound the minimal ligand binding domain was found to be conserved between this domain and the native receptor. These experiments show that the minimal ligand binding domain maintains the quantitative and qualitative aspects of ligand binding exhibited by the full-length receptor, implying that the central ligand binding pocket may exist to accommodate all classes of specific dioxin receptor ligands, and that this pocket is critically dependent upon hsp90 for its ligand binding conformation.

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.

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.

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.

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.

Immune system impairment and hepatic fibrosis in mice lacking the dioxin-binding Ah receptor

The aryl hydrocarbon (Ah) receptor (AHR) mediates many carcinogenic and teratogenic effects of environmentally toxic chemicals such as dioxin. An AHR-deficient (Ahr-/-) mouse line was constructed by homologous recombination in embryonic stem cells. Almost half of the mice died shortly after birth, whereas survivors reached maturity and were fertile. The Ahr-/- mice showed decreased accumulation of lymphocytes in the spleen and lymph nodes, but not in the thymus. The livers of Ahr-/- mice were reduced in size by 50 percent and showed bile duct fibrosis Ahr-/- mice were also nonresponsive with regard to dioxin-mediated induction of genes encoding enzymes that catalyze the metabolism of foreign compounds. Thus, the AHR plays an important role in the development of the liver and the immune system.

Human dioxin receptor chimera transactivation in a yeast model system and studies on receptor agonists and antagonists

A yeast dioxin receptor chimera model has been developed to study ligand binding and transactivation properties of the human dioxin receptor. Using this new yeast model, the human dioxin receptor chimera was found to possess a constitutive transactivity on a LacZ reporter gene, however, the transactivation by the chimera was enhanced by the addition of several polycyclic aromatic hydrocarbons to the culture medium. The order of best polycyclic aromatic hydrocarbon inducer to worst correlated well with the known in vitro dioxin receptor binding affinities for these polycyclic aromatic hydrocarbons. 7,8-Benzoflavone, a weak dioxin receptor agonist and strong antagonist of the mammalian dioxin receptor also behaved as a weak agonist and strong antagonist of the human dioxin receptor chimera expressed in yeast. The implications for these findings as well as the utility of this new yeast human dioxin receptor chimera model are discussed.

Modulation of gene expression and endocrine response pathways by 2,3,7,8-tetrachlorodibenzo-p-dioxin and related compounds

The aryl hydrocarbon (Ah) receptor binds several different structural classes of chemicals, including halogenated aromatics, typified by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polynuclear aromatic and heteropolynuclear aromatic hydrocarbons. TCDD induces expression of several genes including CYP1A1, and molecular biology studies show that the Ah receptor acts as a nuclear ligand-induced transcription factor that interacts with xenobiotic or dioxin responsive elements located in 5'-flanking regions of responsive genes. TCDD also elicits diverse toxic effects, modulates endocrine pathways and inhibits a broad spectrum of estrogen (17 beta-estradiol)-induced responses in rodents and human breast cancer cell lines. Molecular biology studies show that TCDD inhibited 17 beta-estradiol-induced cathepsin D gene expression by targeted interaction of the nuclear Ah receptor with imperfect dioxin responsive elements strategically located within the estrogen receptor-Sp1 enhancer sequence of this gene.