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

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.

Rainbow trout cytochrome P450s: purification, molecular aspects, metabolic activity, induction and role in environmental monitoring

Cytochromes P450 (P450s or CYPs) constitute a superfamily of heme-thiolate proteins that play important roles in oxidative metabolism of endogenous and exogenous compounds. This review provides some limited history but addresses mainly the research progress on the cytochrome P450s in rainbow trout (Oncorhynchus mykiss), their purification, structures at the primary level, role in metabolism, responses to chemicals and environmental pollutants, application to biomonitoring and the effect of various factors on their expression or activities. Information obtained to date suggests that the rainbow trout P450 systems are as complex as those seen in mammals. Fourteen P450s have been purified from liver or trunk kidney to relatively high specific content. cDNAs belonging to seven different P450 families have been documented from trout liver, kidney and ovary. Two CYP1A genes, nine cDNAs containing open reading frames, and a cDNA fragment were entered into GenBank. Among them, CYP2K1, CYP2K3, CYP2K4, CYP2M1, CYP3A27 and CYP4T1 are the most recently described forms. CYP2K1, CYP2M1 and CYP4T1 represent newly identified P450 subfamilies first described in the rainbow trout. In many cases, the cloned rainbow trout P450s have subsequently been expressed in heterologous expressions systems such as COS-7 cells, yeast and baculovirus infected insect cells. Some of the overexpressed P450 isoforms have been partially characterized. Potential future research directions are discussed.

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.

Insulin induces transcription of target genes through the hypoxia-inducible factor HIF-1alpha/ARNT

Hypoxic stress induces the expression of genes associated with increased energy flux, including the glucose transporters Glut1 and Glut3, several glycolytic enzymes, nitric oxide synthase, tyrosine hydroxylase, erythropoietin and vascular endothelial growth factor (VEGF). Induction of these genes is mediated by a common basic helix-loop-helix-PAS transcription complex, the hypoxia-inducible factor-1alpha (HIF-1alpha)/aryl hydrocarbon nuclear translocator (ARNT). Insulin also induces some of these genes; however, the underlying mechanism is unestablished. We report here that insulin shares with hypoxia the ability to induce the HIF-1alpha/ARNT transcription complex in various cell types. This induction was demonstrated by electrophoretic mobility shift of the hypoxia response element (HRE), and abolished by specific antisera to HIF-1alpha and ARNT, and by transcription activation of HRE reporter vectors. Furthermore, basal and insulin-induced expression of Glut1, Glut3, aldolase A, phosphoglycerate kinase and VEGF was reduced in cells having a defective ARNT. Similarly, the insulin-induced activation of HRE reporter vectors and VEGF was impaired in these cells and was rescued by re-introduction of ARNT. Finally, insulin-like growth factor-I (IGF-I) also induced the HIF-1alpha/ARNT transcription complex. These observations establish a novel signal transduction pathway of insulin and IGF-I and broaden considerably the scope of activity of HIF-1alpha/ARNT.

A direct interaction between the aryl hydrocarbon receptor and retinoblastoma protein. Linking dioxin signaling to the cell cycle

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor in eukaryotic cells that alters gene expression in response to the environmental contaminant 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD). In 5L hepatoma cells, TCDD induces a G1 cell cycle arrest through a mechanism that involves the AhR. The retinoblastoma tumor suppressor protein (pRb) controls cell cycle progression through G1 in addition to promoting differentiation. We examined whether the human AhR or its dimerization partner, the AhR nuclear translocator, interacts with pRb as a basis of the TCDD-induced cell cycle arrest. In vivo and in vitro assays reveal a direct interaction between pRb and the AhR but not the AhR nuclear translocator protein. Binding between the AhR and pRb occurs through two distinct regions in the AhR. A high affinity site lies within the N-terminal 364 amino acids of the AhR, whereas a lower affinity binding region colocalizes with the glutamine-rich transactivation domain of the receptor. AhR ligand binding is not required for the pRb interaction per se, although immunoprecipitation experiments in 5L cells reveal that pRb associates preferentially with the liganded AhR, consistent with a requirement for ligand-induced nuclear translocation. These observations provide a mechanistic insight into AhR-mediated cell cycle arrest and a new perspective on TCDD-induced toxicity.

Aryl hydrocarbon receptor function in early vertebrates: inducibility of cytochrome P450 1A in agnathan and elasmobranch fish

The mammalian aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that controls the expression of cytochrome P450 1A (CYP1A) genes in response to halogenated aromatic hydrocarbons such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The natural ligand and normal physiologic function of this protein are as yet unknown. One approach to understanding AHR function and significance is to determine the evolutionary history of this receptor and of processes such as CYP1A induction that are controlled by the AHR in mammals. In these studies, AHR function was evaluated in representative cartilaginous fish (little skate, Raja erinacea) and jawless fish (sea lamprey, Petromyzon marinus and Atlantic hagfish, Myxine glutinosa), using CYP1A induction as a model AHR-dependent response. Treatment of skate with beta-naphthoflavone (BNF) caused an 8-fold increase in hepatic ethoxyresorufin O-deethylase (EROD) activity as well as a 37-fold increase in the content of immunodetectable CYP1A protein. Evidence of CYP1A inducibility was also obtained for another cartilaginous fish, the smooth dogfish Mustelus canis. In contrast, hepatic EROD activity was not detected in untreated lamprey nor in lamprey treated with 3,3'4,4'-tetrachlorobiphenyl (TCB), a potent AHR agonist in teleosts. A possible CYP1A homolog was detected in lamprey hepatic microsomes by one of three antibodies to teleost CYP1A, but expression of this protein was not altered by TCB treatment. CYP1A protein and catalytic activity were measurable in hagfish, but neither was induced after treatment with TCB. These results suggest that the AHR-CYP1A signal transduction pathway is highly conserved in gnathostomes, but that there may be fundamental differences in AHR signaling or AHR-CYP1A coupling in agnathan fish. Agnathan fish such as hagfish and lamprey may be interesting model species for examining possible ancestral AHR functions not related to CYP1A 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.

Altered gene expression and genetic damage in North American fish populations

Populations of marine, estuarine, and freshwater fish from highly urban and industrialized sites in North America often exhibit elevated prevalences of neoplastic, preneoplastic, and nonneoplastic hepatic lesions, and sometimes epidermal neoplasms compared to conspecifics from more pristine reference locales. Positive statistical associations with environmental concentrations of PAHs and other xenobiotics and experimental laboratory studies suggest a chemical etiology to these epizootics. Studies have investigated the expression of carcinogenically relevant genes, the extent of overall DNA damage, somatic cell mutations, germ line polymorphisms, and overall levels of genetic diversity in fish from these populations and other polluted sites. In general, elevated levels of cytochrome P4501A expression have been found in fish from contaminated locales; however, inhibition of gene induction has been seen in hepatic lesions and in normal tissue in fish from the most contaminated sites, perhaps due to genetic adaptation or physiological acclimation. Levels of bulky hepatic DNA adducts, as detected by 32P-postlabeling, are almost always elevated in fish from populations that are exposed to highly contaminated environments. However, levels of DNA adducts were not always predictive of the vulnerability to neoplasia of populations and species from polluted sites. Elevated levels of oxygen radical-induced DNA damage have been observed in hepatic tumors, preneoplastic lesions, and normal livers in a single species of flatfish from contaminated sites; however, the prevalences of these alterations in other species and at other polluted sites has yet to be evaluated. Frequent alterations in the K-ras oncogene have been reported in hepatic neoplasms in several species from highly contaminated sites and also in embryos that were experimentally exposed to oil-contaminated sediments. Studies also suggest that heritable germ line polymorphisms, altered allelic frequencies, and reductions in overall genetic diversity may have occurred in some highly impacted populations; however, the origin and functional significance of altered allelic frequencies have largely yet to be evaluated. In summary, feral fish appear particularly sensitive to DNA alterations from xenobiotics, perhaps due to their unusually high levels of exposure, relatively inefficient DNA repair, and the high frequency of polyploidy in some taxa and provide excellent models to explore the relationships between xenobiotic exposure and altered gene structure and expression.

The Drosophila tango gene encodes a bHLH-PAS protein that is orthologous to mammalian Arnt and controls CNS midline and tracheal development

The Drosophila single-minded and trachealess bHLH-PAS genes control transcription and development of the CNS midline cell lineage and tracheal tubules, respectively. We show that Single-minded and Trachealess activate transcription by forming dimers with the Drosophila Tango protein that is an orthologue of the mammalian Arnt protein. Both cell culture and in vivo studies show that a DNA enhancer element acts as a binding site for both Single-minded::Tango and Trachealess::Tango heterodimers and functions in controlling CNS midline and tracheal transcription. Isolation and analysis of tango mutants reveal CNS midline and tracheal defects, and gene dosage studies demonstrate in vivo interactions between single-minded::tango and trachealess::tango. These experiments support the existence of an evolutionarily conserved, functionally diverse bHLH-PAS protein regulatory system.

Characterization of the aromatic hydrocarbon receptor gene and its expression in Atlantic tomcod

Cytochrome P4501A1 (CYP1A1) mRNA is not inducible in Atlantic tomcod from the Hudson River that are treated with halogenated aromatic hydrocarbons (HAHs). In contrast, CYP1A1 mRNA is inducible in Hudson River tomcod that are treated with polycyclic aromatic hydrocarbons (PAHs) and in tomcod that are collected from cleaner rivers and treated with HAHs or PAHs. We hypothesize that CYP1A1 transcription is inhibited in Hudson River tomcod because of down-regulation of the aromatic hydrocarbon receptor (AhR) pathway and that separate molecular pathways modulate CYP1A1 transcription in fish treated with HAHs and PAHs. We initially evaluated levels of hepatic nuclear protein binding at enhancer elements (DREs) in the regulatory region of tomcod CYP1A1. No difference in levels of protein binding was observed between tomcod from the Hudson and Miramichi (cleaner) rivers that were untreated or were treated with benzo[a]pyrene. In contrast, levels of protein binding were lower in tomcod from the Hudson River that were treated with TCB than in similarly treated fish from the Miramichi River, suggesting differences between the populations in the structure or expression of AhR pathway molecules. To address this possibility, AhR DNA sequences were characterized from tomcod cDNA and genomic DNA libraries. In tomcod and mammals, AhR is represented by 11 exons, overall peptide sizes are similar, and amino acid sequences at basic, helix-loop-helix, PAAS A, and PAAS B domains are highly conserved. In contrast, little similarity was observed between tomcod and mammals in the sizes or sequences of AhR exons 10 and 11, including the absence in tomcod of glutamine-rich domains. No differences in levels of hepatic AhR mRNA were observed between the two populations or treatment groups when tomcod were untreated or were treated with aromatic hydrocarbons. In contrast, variation in levels of AhR mRNA expression was observed among tomcod tissues; however, no relationship was observed between levels of AhR mRNAs and CYP1A1 mRNAs in tissues from chemically or vehicle control-treated fish. RFLP analysis revealed extensive variation in exons 10 and 11 of AhR cDNA among tomcod from different rivers. Our results suggest that variation between tomcod populations in CYP1A1 mRNA inducibility is reflected by differences in levels of inducible hepatic protein binding to DREs. However, levels of hepatic AhR mRNA are not down-regulated in the Hudson River population, are not affected by AH treatments, and levels of AhR mRNA expression are not responsible for the differential inducibility of CYP1A1 transcription.

Determination of aryl hydrocarbon receptor nuclear translocator protein concentration and subcellular localization in hepatic and nonhepatic cell culture lines: development of quantitative Western blotting protocols for calculation of aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator protein in total cell lysates

Western blot analysis was used to determine the concentration of the aryl hydrocarbon receptor nuclear translocator (ARNT) protein and aryl hydrocarbon receptor (AHR) in 11 mammalian cell culture lines derived from hepatic and nonhepatic tissues. The strategy was to first use Western blot analysis to determine the expression of ARNT or AHR in each cell line relative to its concentration in murine wild-type Hepa-1c1c7 (Hepa-1) cells. Actual ARNT and AHR concentrations in known amounts of total cell lysates were then determined by generating a standard curve with defined amounts of a highly purified ARNT or AHR protein and performing regression analysis. The results show that the level of ARNT expression in each of the cell lines is similar and represents approximately 0.001-0.002% of total cellular protein. The range of expression was only approximately 3-fold with wild-type Hepa-1 cells expressing the highest level of ARNT (33,000/cell) and canine kidney cells (MDCK line) expressing 14,000 ARNT molecules/cell. In contrast, the concentration of AHR varied by 65-fold over the different cell lines with the wild-type Hepa-1 expressing 323,000 AHR/cell and rat hepatoma cells (H4IIE) expressing 4700. The ratio of AHR to ARNT ranged from 0.3 in H4IIE cells to 10 in the Hepa-1 line with the majority of cells expressing 1-5 times more AHR than ARNT protein. Immunocytochemical staining of each cell line showed that ARNT was exclusively localized to the nuclear compartment and that a conserved nuclear localization signal mapped to the NH-terminal portion of the protein.

Expression of the aryl hydrocarbon receptor (AhR) and AhR nuclear translocator during chick cardiogenesis is consistent with 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced heart defects

We examined cardiotoxicity induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the chick embryo and the cardiac expression of transcription factors, the aryl hydrocarbon receptor (AhR) which binds TCDD, and its dimer partner, the AhR nuclear translocator (Arnt). Chicken eggs were injected with control (triolein) or 1.0 pmol TCDD/g egg prior to incubation and collected on Day 10 when cardiomorphogenesis is complete. Relative to controls, TCDD increased heart wet weight (27.2 +/- 0.5 versus 36.6 +/- 1.3 mg, p < 0.001) and dry weight (2.7 +/- 0.1 versus 3.1 +/- 0.1 mg, p < 0.01), and tended to increase heart myosin content (3.5 +/- 0.6 versus 6.3 +/- 2.5 microg, p < 0.07), suggesting an increase in cardiac muscle mass and edema. Histologic and morphometric analyses revealed that 10/13 TCDD-exposed hearts exhibited enlarged right and left ventricles, thickened ventricular septum, and a thinner left ventricular wall with increased trabeculation, and 4/13 exhibited ventricular septal defects compared to controls (0/23). To evaluate AhR and Arnt expression, untreated chick embryos were collected on Days 2.2, 3, 4, 5, and 8 of incubation, preserved in Bouin's fixative, sectioned, and stained with AhR and Arnt antibodies. The AhR was expressed ubiquitously in cardiac myocytes, while Arnt expression was restricted to myocytes overlying developing septa: atrioventricular canal, outflow tract, and atrial and ventricular septa. Both proteins were absent from endocardium and endocardial-derived mesenchyme. In addition, cardiac expression of an AhR/Arnt target, cytochrome P4501A1, was restricted to myocardium coexpressing AhR and Arnt. Thus, the spatial and temporal expression of AhR and Arnt suggests that the developing myocardium and cardiac septa are potential targets of TCDD-induced teratogenicity, and such targets are also consistent with cardiac hypertrophy and septal defects observed following TCDD exposure.