Effects of hypoxia and intracellular iron chelation on hypoxia-inducible factor-1? and -1? in the rat carotid body and glomus cells

The present investigation provides for the first time, unambiguous information on the occurrence of hypoxia-inducible factors (HIF-1alpha and HIF-1beta proteins) in normoxia (Nx) and their interaction with hypoxia (Hx) and intracellular Fe(2+) chelation in the rat carotid body (CB) glomus cells. HIF-1alpha bound to HIF-1beta translocated into the nucleus is identified on the basis of immunohistochemistry and immunofluorescence. In Nx, a weak expression of HIF-1alpha was observed in CB glomus cells. However, exposure of CB and glomus cells to Hx (Po(2) approximately 7 Torr) and Nx with ciclopirox olamine (CPX, 5 microM) for 1 h showed a significant ( P<0.001) increase in HIF-1alpha protein. The CBs and glomus cells exposed to Nx, Hx, and Nx with CPX showed a constant level of HIF-1beta protein expression. HIF-1alpha subunit is continuously synthesized and degraded under normoxic conditions, while it accumulates rapidly following exposure to low oxygen tensions. Hydroxylation of HIF-1alpha by prolyl hydroxylase for proteasomal degradation was dependent on iron, 2-oxoglutarate, and oxygen concentration. The intracellular iron that acts as a cofactor for prolyl hydroxylase activity belongs to the labile iron pool and can be easily chelated. Thus, chelation of intracellular labile iron by CPX in Nx significantly increased HIF-1alpha in CB glomus cells. Thus, the results are consistent with the hypothesis that HIF-1alpha which is present in the glomus cells translocates to the nucleus during exposure to Hx and to CPX in Nx.

2,3,7,8-tetrachlorodibenzo-p-dioxin inhibits zebrafish caudal fin regeneration

Adult zebrafish completely regenerate their caudal fins following partial amputation. Fin regrowth can easily be monitored in vivo and regenerating tissues can be used to study this dynamic developmental process. In this study we determined that fin regeneration is significantly affected by exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Zebrafish caudal fins were partially amputated, and the fish received intraperitoneal (ip) injection of 2.8, 14, or 70 ng/g weight TCDD or vehicle control. By 7 days postamputation, fish exposed to the highest concentration of TCDD regenerated 15% of their fin compared to 65% regrowth in control fish. To determine if this effect was stage specific, zebrafish were exposed to 70 ng/g TCDD on 1, 2, 3, or 4 days postamputation. Fin regeneration was significantly inhibited at all time points following TCDD exposure. TCDD exposure also induced hyperpigmentation in de novo tissue. Zebrafish were dosed with BrdU, following fin amputation and TCDD exposure, to study changes in cell proliferation. By 4 days postamputation, cell proliferation rates were significantly lower in TCDD-exposed fish. TCDD toxicity is mediated through the aryl hydrocarbon receptor (AHR), and RT-PCR experiments confirmed AHR2, ARNT2b, and TCDD-dependent CYP1A expression in the regenerating tissue. These results demonstrate that zebrafish caudal fin regeneration is a unique model to investigate molecular mechanism(s) of TCDD toxicity.

A molecular mechanism regulating circadian expression of vascular endothelial growth factor in tumor cells

Because angiogenesis is essential for tumor growth and metastasis, inhibition of angiogenesis has emerged as a new therapy to treat cancers. Hypoxia-induced expression of vascular endothelial growth factor (VEGF) plays a central role in tumor-induced angiogenesis. In this study, we found that expression of VEGF in hypoxic tumor cells was affected by the circadian organization of molecular clockwork. The core circadian oscillator is composed of an autoregulatory transcription-translation feedback loop in which CLOCK and BMAL1 are positive regulators, and Period and Cryptochrome genes act as negative ones. The levels of VEGF mRNA in tumor cells implanted in mice rose substantially in response to hypoxia, but the levels fluctuated rhythmically in a circadian fashion. Luciferase reporter gene analysis revealed that Period2 and Cryptochrome1, whose expression in the implanted tumor cells showed a circadian oscillation, inhibited the hypoxia-induced VEGF promoter activity. These results suggest that the negative limbs of the molecular loop periodically inhibit the hypoxic induction of VEGF transcription, resulting in the circadian fluctuation of its mRNA expression. We also showed that the antitumor efficacy of antiangiogenic agents could be enhanced by administering the drugs at the time when VEGF production increased. These findings support the notion that monitoring of the circadian rhythm in VEGF production is useful for choosing the most appropriate time of day for administration of antiangiogenic agents.

Insulin and hypoxia share common target genes but not the hypoxia-inducible factor-1alpha

Both hypoxia and insulin induce common target genes, including vascular endothelial growth factors and several glycolytic enzymes. However, these two signals eventually trigger quite different metabolic pathways. Hypoxia induces glycolysis, resulting in anaerobic ATP production, while insulin increases glycolysis for energy storage. Hypoxia-induced gene expression is mediated by the hypoxia-inducible factor-1 (HIF-1) that consists of HIF-1alpha and the aromatic hydrocarbon nuclear translocator (Arnt). Hypoxia-induced gene expression is initiated by the stabilization of the HIF-1alpha subunit. Here we investigated whether insulin-induced gene expression also requires stabilization of HIF-1alpha. Our results indicate that hypoxia but not insulin stabilizes HIF-1alpha protein levels, whereas both insulin- and hypoxia-induced gene expression require the presence of the Arnt protein. Insulin treatment fails to inactivate proline hydroxylation of HIF-1alpha, which triggers recruitment of the von Hippel-Lindau protein and oxygen-dependent degradation of HIF-1alpha. Insulin-induced gene expression is inhibited by the presence of the phosphoinositide (PI) 3-kinase inhibitor LY294002 and the dominant negative mutant of the p85 subunit of PI 3-kinase, whereas hypoxia-induced gene expression is not. Pyrrolidine dithiocarbamate, a scavenger of H2O2, reduces insulin-induced gene expression but not hypoxia-induced gene expression. Although both hypoxia and insulin induce the expression of common target genes through a hypoxia-responsive element- and Arnt-dependent mechanism, insulin cannot stabilize the HIF-1alpha protein. We believe that insulin activates other putative partner proteins for Arnt in PI 3-kinase- and H2O2-dependent pathways.

Construction of reporter yeasts for mouse aryl hydrocarbon receptor ligand activity

Aryl hydrocarbons such as dioxins, polychlorinated biphenyls and polyaromatic hydrocarbons bind to the cellular aryl hydrocarbon receptor (AhR) in the initial step of their metabolism. The activation of intracellular signaling subsequent to the AhR binding is highly correlated with the toxicity and carcinogenicity of these chemicals. We produced Saccharomyces cerevisiae coexpressing mouse AhR and aryl hydrocarbon receptor nuclear translocator (Arnt) protein in accordance with Miller III's method for constructing yeasts with human Ahr and Arnt [Toxicol. Appl. Pharmacol. 160 (1998) 297]. Ligand treatment induced a dose-dependent increase in beta-galactosidase activity from a reporter plasmid in the yeast. Then, we compared activities of several ligands in yeast having the mouse Ahr/Arnt genes with those in yeast having the human genes, both of which have the same genetic background. There was no significant difference in the EC50 values of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), benzo[a]pyrene, 3-methylcholanthrene and beta-naphthoflavone between the mouse and human genes. However, indirubin, which was recently found in human urine as a potent AhR ligand [J. Biol. Chem. 276 (2001) 31475], had a 35-140 times higher EC50 value in the yeast with human genes than mouse genes. This difference might reflect species-specificity between mouse and human AhR/Arnt.

Estrogen increases messenger RNA and immunoreactivity of aryl-hydrocarbon receptor nuclear translocator 2 in the rat mediobasal hypothalamus

We examined the effect of estrogen on the expression of aryl-hydrocarbon receptor (AhR) and two types of AhR nuclear translocator (Arnt1 and Arnt2) mRNAs in the hypothalamus of ovariectomized rats. Northern blotting demonstrated that, in the mediobasal hypothalamus, a subcutaneous injection of 20 microg estradiol benzoate (E(2)) significantly increased the expression of Arnt2 mRNA, but induced no significant changes in the expression of AhR and Arnt1 mRNAs. The expression of Arnt2 mRNA was significantly increased at 4, 24, and 72h after the injection. Immunocytochemical study revealed that the number of Arnt2 immunoreactive cells was also significantly increased at 72h after the injection. Conversely, in the preoptic area, injection of E(2) did not cause significant changes in the expression of any of the three mRNAs. These observations suggest that estrogen regulates Arnt2 expression in the mediobasal hypothalamus and modulates the toxic action of dioxins in rats.

Xenopus single-minded (xSim) is a nuclear factor allowing nuclear translocation of its cytoplasmic partner xArnt

Transcription factors belonging to the basic helix-loop-helix Per-Arnt-Sim (bHLH/PAS) family control a wide variety of biological processes in mammalian and/or Drosophila. We have previously isolated bHLH/PAS Xenopus amphibian homologs of Single-minded (xSim) and aryl receptor nuclear translocator (xArnt) and characterized their expression pattern during embryogenesis. We show in this paper that xSim protein is a functional homolog of Drosophila or mammalian Sim(s). Biochemical analysis indicates that xSim forms a heterodimer with xArnt. Subcellular localization analysis of bHLH/PAS chimeric fluorescent versions in Xenopus or mammalian cell lines shows that xSim is constitutively localized in the nuclear compartment. On the opposite, xArnt appears to be predominantly expressed in the cytoplasm. In addition, we demonstrate that xArnt nuclear localization depends on the presence of xSim. Thus xSim appears to be an essential factor in the nuclear translocation of the xSim/xArnt complex. In perfect agreement, we show that the C-terminal half of xSim contains the information for this nuclear localization.

Epithelial cells in the oviduct and vagina and steroid-synthesizing cells in the rabbit ovary express AhR and ARNT

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor. Beside exogenous ligands, an increasing list of endogenous ligands has recently been described. The AhR and its dimerization partner AhR nuclear translocator (ARNT) were found in embryos, fetuses and in genital tract tissue. Its role in reproduction and fertility is not known. In the current study, AhR and ARNT expression and co-localization were studied immunohistochemicaly during the pre-implantation period in various genital tract organs (ovary, oviduct, vagina) of the rabbit. In the ovary, the AhR was expressed in the steroid-secreting interstitial cells, in follicular and granulosa cells, and in lutein cells. The receptor was localized in the cytoplasm. A cytoplasmic localization was also found in the oviduct epithelium with a diffuse cytoplasmic staining in the ampulla and a localized cytoplasmic localization in the isthmus. In the vagina, AhR localization changed from cytoplasmic in the non-pregnant animal to nuclear staining in the basal layer of the vaginal epithelium on day 6 of pregnancy. The ARNT protein was found in all AhR expressing cells except for oocytes within primordial follicles. Its localization was in the cytoplasm and/or nuclei. Therefore, the full complement for AhR/ARNT transcriptional activity was found in the studied organs. AhR expression showed stage-specific changes in both the uterus (Hasan and Fischer 2001) and the vagina during the pre-implantation period, while the ARNT protein exhibited no change in expression during this period. In summary, these findings indicate a functional AhR/ARNT complex in the rabbit genital tract epithelia. Its precise role and activation mechanism in reproductive tissue is not clear. It is supposed, however, that this complex may be involved in both hormone production and hormone-induced cellular changes during early pregnancy of the rabbit.

Functional activation of arylhydrocarbon receptor (AhR) in primary T cells by 2,3,7,8-tetrachlorodibenzo-p-dioxin

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) exerts diverse adverse health effects by activating the transcription factor arylhydrocarbon receptor (AhR). The activated AhR induces the expression of various genes having xenobiotic responsive elements (XREs) in their enhancer regions, such as the gene for cytochrome P-450 1A1 (CYP1A1). The immune system is sensitively affected by TCDD, while the precise mechanism of how TCDD acts in each immune cell type remains to be determined. The results of previous studies on AhR activation in B cell lines, T cell clones, and thymocytes, which mainly consist of immature T cells, suggested that AhR in mature T cells is inactive, whereas that in B cells and immature T cells act functionally. In the present study, we investigated whether or not TCDD induces the CYP1A1 gene by functionally activating AhR in primary mature T cells in mice. When the splenocytes that contain mature T and B lymphocytes as their predominant cell types or the thymocytes were cultured in the presence of TCDD, each of them showed a similar magnitude of CYP1A1 induction with a peak induction at 4 h. Both mature T cells and B cells that had been separated from total splenocytes also showed CYP1A1 induction at the same magnitude with a peak induction at 4 h. Gene expression of CYP1A1 was observed at 0.1 nM or greater concentrations of TCDD in splenocytes and separated T cells. The induction of CYP1A1 in T cells was confirmed in mice exposed to TCDD. These results indicate that TCDD induces the functional activation of AhR in primary mature T cells in mice.

Heterodimers of bHLH-PAS protein fragments derived from AhR, AhRR, and Arnt prepared by co-expression in Escherichia coli: characterization of their DNA binding activity and preparation of a DNA complex

AhR (aryl hydrocarbon receptor), AhRR (AhR repressor), and Arnt (AhR nuclear translocator) are members of the bHLH (basic-helix-loop-helix)-PAS (Per-AhR/Arnt-Sim homology sequence) transcription factor superfamily. They associate with each other to form heterodimers, AhR/Arnt or AhRR/Arnt, and bind the XRE (xenobiotic responsive element) sequences in the promoter regions of the target genes to regulate their expression. Their basic regions and HLH motifs mediate DNA binding activity and protein dimerization, respectively. The PAS domain includes two incomplete repeats, PAS-A and PAS-B, and is considered to determine the specificity on protein dimerization. However, the three-dimensional structures of PAS folds reported so far are all monomeric, therefore, little is known about the structural basis of protein interaction through PAS domains. In the present study, we prepared heterodimeric bHLH-PAS domains derived from AhR and Arnt, and AhRR and Arnt by co-expressing each pair in E. coli, and showed that the heterodimers formed exhibited full DNA-binding activity, which was not apparently affected by deletion of the highly basic amino acid cluster most N-terminal as to the HLH region of AhR or AhRR. Methylation of the two CpG sites in the XRE core sequence reduced the binding affinity to heterodimeric proteins, with 5-methylcytosine in the AhR recognition site exhibiting a greater inhibitory effect than that in the Arnt recognition site.

Aryl hydrocarbon receptor gene silencing with small inhibitory RNA differentially modulates Ah-responsiveness in MCF-7 and HepG2 cancer cells

Sequence-specific small interfering RNA (siRNA) duplexes can be used for gene silencing in mammalian cells and as mechanistic probes for determining gene function. Transfection of siRNAs for the aryl hydrocarbon receptor (AhR) and AhR nuclear translocator (ARNT) mRNAs in MCF-7 breast cancer cells resulted in a 60 to 80% decrease in levels of AhR and ARNT proteins in whole-cell extracts and decreased binding of nuclear extracts to 32P-labeled dioxin-responsive element. siRNA for the AhR also decreased 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced CYP1A1 protein, CYP1A1-dependent activity, and luciferase activity in cells transfected with an Ah-responsive construct. 17beta-estradiol (E2) induces proliferation of MCF-7 cells through enhanced G0/G1 --> S phase progression, and this response is inhibited in cells cotreated with E2 plus TCDD. The effects of TCDD on E2-induced cell-cycle progress were partially blocked in MCF-7 cells transfected with siRNA for AhR. The results also indicated that siRNA-dependent decreases in AhR protein in MCF-7 cells were accompanied by increased G0/G1 --> S phase progression, suggesting a growth-inhibitory role for the "endogenous" AhR. Surprisingly, TCDD alone induced G0/G1 --> S phase progression and exhibited estrogenic activity in MCF-7 cells transfected with siRNA for the AhR. In contrast, degradation of the AhR in HepG2 liver cancer cells resulted in decreased G0/G1 --> S phase progression, and this was accompanied by decreased expression of cyclin D1, cyclin E, cyclin-dependent kinase 2 (cdk2), and cdk4. In the absence of ligand, the AhR exhibits growth-inhibitory (MCF-7) and growth-promoting (HepG2) activity that is cell context-dependent.

Modulation of oestrogen receptor signalling by association with the activated dioxin receptor

Environmental contaminants affect a wide variety of biological events in many species. Dioxins are typical environmental contaminants that exert adverse oestrogen-related effects. Although their anti-oestrogenic actions are well described, dioxins can also induce endometriosis and oestrogen-dependent tumours, implying possible oestrogenic effects. However, the molecular mechanism underlying oestrogen-related actions of dioxins remains largely unknown. A heterodimer of the dioxin receptor (AhR) and Arnt, which are basic helix-loop-helix/PAS-family transcription factors, mediates most of the toxic effects of dioxins. Here we show that the agonist-activated AhR/Arnt heterodimer directly associates with oestrogen receptors ER-alpha and ER-beta. This association results in the recruitment of unliganded ER and the co-activator p300 to oestrogen-responsive gene promoters, leading to activation of transcription and oestrogenic effects. The function of liganded ER is attenuated. Oestrogenic actions of AhR agonists were detected in wild-type ovariectomized mouse uteri, but were absent in AhR-/- or ER-alpha-/- ovariectomized mice. Our findings suggest a novel mechanism by which ER-mediated oestrogen signalling is modulated by a co-regulatory-like function of activated AhR/Arnt, giving rise to adverse oestrogen-related actions of dioxin-type environmental contaminants.

The basic helix-loop-helix-PAS protein ARNT functions as a potent coactivator of estrogen receptor-dependent transcription

The biological effects of estrogens are mediated by the estrogen receptors ERalpha and ERbeta. These receptors regulate gene expression through binding to DNA enhancer elements and subsequently recruiting factors such as coactivators that modulate their transcriptional activity. Here we show that ARNT (aryl hydrocarbon receptor nuclear translocator), the obligatory heterodimerization partner for the aryl hydrocarbon receptor and hypoxia inducible factor 1alpha, functions as a potent coactivator of ERalpha- and ERbeta- dependent transcription. The coactivating effect of ARNT depends on physical interaction with the ERs and involves the C-terminal domain of ARNT and not the structurally conserved basic helix-loop-helix and PAS (Per-ARNT-Sim) motifs. Moreover, we show that ARNT/ER interaction requires the E2-activated ligand binding domain of ERalpha or ERbeta. These observations, together with the previous role of ARNT as an obligatory partner protein for conditionally regulated basic helix-loop-helix-PAS proteins like the aryl hydrocarbon receptor or hypoxia inducible factor 1alpha, expand the cellular functions of ARNT to include regulation of ERalpha and ERbeta transcriptional activity. ARNT was furthermore recruited to a natural ER target gene promoter in a estrogen-dependent manner, supporting a physiological role for ARNT as an ER coactivator.

The role of the Ah receptor and p38 in benzo[a]pyrene-7,8-dihydrodiol and benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide-induced apoptosis

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous contaminants in the environment. Benzo[a]pyrene (B[a]P), a prototypical member of this class of chemicals, affects cellular signal transduction pathways and induces apoptosis. In this study, the proximate carcinogen of B[a]P metabolism, trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (B[a]P-7,8-dihydrodiol) and the ultimate carcinogen, B[a]P-r-7,t-8-dihydrodiol-t-9,10-epoxide(+/-) (BPDE-2) were found to induce apoptosis in human HepG2 cells. Apoptosis initiated by B[a]P-7,8-dihydrodiol was linked to activation of the Ah receptor and induction of CYP1A1, an event that can lead to the formation of BPDE-2. With both B[a]P-7,8-dihydrodiol and BPDE-2 treatment, changes in anti- and pro-apoptotic events in the Bcl-2 family of proteins correlated with the release of mitochondrial cytochrome c and caspase activation. The onset of apoptosis as monitored by caspase activation was linked to mitogen-activated protein (MAP) kinases. Utilizing mouse hepa1c1c7 cells and the Arnt-deficient BPRc1 cells, activation of MAP kinase p38 by B[a]P-7,8-dihydrodiol was shown to be Ah receptor-dependent, indicating that metabolic activation by CYP1A1 was required. This was in contrast to p38 activation by BPDE-2, an event that was independent of Ah receptor function. Confirmation that MAP kinases play a critical role in BPDE-2-induced apoptosis was shown by inhibiting caspase activation of poly(ADP-ribose)polymerase 1 (PARP-1) by chemical inhibitors of p38 and ERK1/2. Furthermore, mouse embryo p38-/- fibroblasts were shown to be resistant to the actions of BPDE-2-induced apoptosis as determined by annexin V analysis, cytochrome c release, and cleavage of PARP-1. These results confirm that the Ah receptor plays a critical role in B[a]P-7,8-dihydrodiol-induced apoptosis while p38 MAP kinase links the actions of an electrophilic metabolite like BPDE-2 to the regulation of programmed cell death.

Quantitative analysis of the Ah receptor/cytochrome P450 CYP1B1/CYP1A1 signalling pathway

Cytochrome P450 (CYP) drug metabolising enzymes CYP1A1 and CYP1B1 are regulated through the ligand-activated aryl hydrocarbon (Ah) receptor. Differential expression of CYP1A1 and CYP1B1 mRNA and protein has previously been reported in human tissues with the presence of the message often extrapolated to indicate the presence of protein. The aim of this study was to clarify these potentially misleading findings, by analysing components of the Ah receptor pathway (CYP1B1, CYP1A1, Ah receptor and ARNT) using a combination of quantitative real-time RT-PCR and immunoblotting. Three human cell lines (MOG-G-CCM, MCF7 and HEPG2) known to differentially express CYP1A1 and CYP1B1 mRNA and protein were exposed to the Ah receptor agonist 3-MC, and basal and inducible levels of CYP1A1, CYP1B1, Ah receptor and ARNT were determined. The key finding of this study was the demonstration of equivalent levels of CYP1B1 mRNA in both the treated and untreated MOG-G-CCM cell lines, with expression of the corresponding CYP1B1 protein only after exposure to an Ah receptor agonist. This finding suggests that a post-transcriptional mechanism is involved in the regulation of CYP1B1. In addition, the expression pattern of CYP1B1 mRNA and protein in the MOG-G-CCM cells highlights this cell line as a potential model for studying CYP1B1 expression in human tissue.

Critical enhancer region to which AhR/ARNT and Sp1 bind in the human CYP1B1 gene

Cytochrome P450 (CYP) 1B1 is known to be induced by polycyclic aromatic hydrocarbons including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The constitutive and TCDD-inducible transcriptional expression of human CYP1B1 is known to be cell-specific. In order to identify the cis-elements that cell-specifically regulate the constitutive and TCDD-inducible transcription of CYP1B1, we constructed luciferase reporter plasmids containing a series of deletions of the XRE core sequence in the 5'-flanking region of the human CYP1B1 gene. Luciferase assays were performed with MCF-7 (breast carcinoma), HepG2 (hepatocellular carcinoma), LS-180 (colon carcinoma), and OMC-3 (ovarian carcinoma) cells. Although there were large differences in the relative luciferase activity and inducibility between these four cell lines, the contribution of each reporter construct was similar. Constitutive expression increased with the regulatory elements that are present at -910 to -852 and -1652 to -1243. Potential enhancer elements for TCDD-induction were located from -1022 to -852 including three XREs, XRE3 at -853, XRE4 at -940, and XRE5 at -989. Gel shift analyses revealed binding of the AhR/ARNT heterodimer to XRE2 at -834, XRE3 at -853, XRE6 at -1024, and XRE7 at -1490. In addition, the binding of a nuclear transcriptional factor, Sp1, near XRE2 and XRE8 was observed. It was suggested that mutual interaction of XRE2 and XRE3 is important for transcriptional regulation, and that the Sp1 binding to the Sp1-like motif (-824) enhances both the constitutive and inducible transcriptional activities of the human CYP1B1 gene.

Identification of novel splice variants of ARNT and ARNT2 in the rat

Most of the biochemical and toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) are mediated by the bHLH/PAS protein AH receptor (AHR). For regulation of gene activities, AHR dimerizes with another member of the bHLH/PAS protein family, AHR nuclear translocator (ARNT). A substrain of Wistar rats, Han/Wistar (Kuopio) (H/W), is about 1000-fold more resistant to the acute lethality of TCDD than other strains, exemplified by Long-Evans (Turku/AB) (L-E); the LD50 values for these two strains are >9600 and 10-20 microg/kg, respectively. Previous studies have demonstrated that the major reason for the exceptional TCDD resistance of H/W rats lies in their AHR, which is remodeled at its C-terminal transactivation domain, but there appears to be another contributing gene product. The present study set out to compare the primary structure of ARNT and the closely related ARNT2 proteins in H/W and L-E rats by cDNA cloning. To our surprise, we found several isoforms of these proteins only one of which has previously been reported in rats. All of the isoforms appeared to arise from alternative splicing. For ARNT, isoforms with deletions at exon 5, 3(') end of exon 6 or 5(') end of exon 11, or with an insertion at 5(') end of exon 20 were discovered. There was also interindividual variation in the number of glutamine-encoding codons at 5(') end of exon 16. The most exciting new variant was revealed for ARNT2, because the insertion found at 5(') end of exon 19 disrupts the functionally critical transactivation domain in the protein, implying a dominant negative role for this isoform. The relative expression levels of the variants did not differ in the two rat strains, nor did TCDD modify the ratios, suggesting that the variants do not contribute to TCDD resistance. However, the regulation of ARNT and ARNT2 activities may be more intricate than previously assumed.

TCDD-mediated alterations in the AhR-dependent pathway in Seveso, Italy, 20 years after the accident

Approximately 20 years after the Seveso, Italy, accident we conducted a population-based study to evaluate the impact of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure on cancer using mechanistically based biomarkers of dioxin response in humans. TCDD toxic effects are mediated by the aryl hydrocarbon receptor (AhR). We studied the AhR-dependent pathway in lymphocytes from 62 subjects randomly sampled from the highest exposed zones and 59 subjects from the surrounding non-contaminated area, frequency matched for age, gender and smoking. To our knowledge, this is the most comprehensive investigation to date designed to evaluate the key genes in the pathway, including AhR, aryl hydrocarbon receptor nuclear translocator, CYP1A1 and CYP1B1 transcripts and CYP1A1-associated 7-ethoxyresorufin O-deethylase (EROD) activity in a population heavily exposed to dioxin. Current lipid-adjusted plasma TCDD concentrations in these subjects ranged from 3.5 to 90 ng/kg (or p.p.t.) and were negatively associated with AhR mRNA in unstimulated peripheral blood mononuclear cells (P = 0.03). When mitogen-induced lymphocytes were cultured with 10 nM TCDD, all AhR-dependent genes were induced 1.2- to 13-fold. In these cells, plasma TCDD was associated with decreased EROD activity. In addition, there was a strong positive correlation between AhR and CYP1A1 expression (P = 0.001) and between AhR and CYP1B1 expression (P = 0.006). CYP1A1 expression was also strongly correlated with EROD activity (P = 0.001). The analysis of the expression of dioxin-inducible genes involved in carcinogenesis may help in determining dose-response relationships for human exposure to dioxin in vivo and in assessing the variability of human response, which may indicate the presence of subjects more susceptible to disease as a result of such exposures.

P23 enhances the formation of the aryl hydrocarbon receptor-DNA complex

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that requires heterodimerization with its partner, the Ah receptor nuclear translocator (Arnt), for activation of transcription. The heterodimer specifically recognizes the dioxin response element (DRE), which contains a core sequence (5'-TNGCGTG-3'). This AhR/Arnt/DRE complex has been well characterized and can be observed readily by the gel shift assay. Human AhR and Arnt with a C-terminal histidine tag have been expressed functionally using a baculovirus expression system. However, after purification of these proteins using the metal resin, they are not able to bind the response element in a ligand-dependent manner unless crude extracts, such as the rabbit reticulocyte lysate (RRL), are reconstituted with these proteins. Proteins in the RRL are responsible for this restoration of the gel shift complex because the activity is sensitive to both heat and proteolytic treatments. We have examined whether hsp90 and p23 are among the protein factors in the RRL that are responsible for this activity. By performing fractionation studies using filtration devices and immunodepletion studies, we have selectively fractionated these proteins. Among all the fractions, the centricon-10 retentate, which contains 100% of p23 but no hsp90, possessed the most enriched activity. Purified bacterial-expressed p23 restored the gel shift complex; the mechanism was mediated at the heterodimerization step and was hsp90-dependent.

Expression of aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator messenger ribonucleic acids and proteins in rat and human testis

Dioxins, e.g. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), use the aryl hydrocarbon receptor (AHR)/aryl hydrocarbon receptor nuclear translocator (ARNT) receptor complex to mediate their toxic actions. In addition to interaction with environmental pollutants, several transcription factors, steroid receptors, and growth factors are capable interacting with the AHR/ARNT complex, which suggests a constitutive role for the receptor complex. The testis has been reported to be among the most sensitive organs to TCDD exposure. Our experiments revealed a complex distribution of AHR and ARNT mRNAs and proteins in rat and human testis. AHR and ARNT immunoreactivities could be detected in the nuclei of interstitial and tubular cells. The incubation of seminiferous tubules in a serum-free culture medium resulted in up-regulation of AHR mRNA, which could be depressed by adding FSH to the culture medium. Furthermore, the incubation of tubular segments with a solution of 1 or 100 nM TCDD resulted in a 2- to 3-fold increase in apoptotic cells. Thus, up-regulation of AHR in cultured tubular segments and consecutive depression by FSH suggest a role for AHR in controlled cell death during spermatogenesis. We suggest that AHR and ARNT mediate effects by direct action on testicular cells in the rat and human testis.

Regulation of the multidrug resistance transporter P-glycoprotein in multicellular tumor spheroids by hypoxia-inducible factor (HIF-1) and reactive oxygen species

Hypoxia in tumors is generally associated with chemoresistance and radioresistance. However, the correlation between the heterodimeric hypoxia-inducible factor-1 (HIF-1) and the multidrug resistance transporter P-glycoprotein (P-gp) has not been investigated. Herein, we demonstrate that with increasing size of DU-145 prostate multicellular tumor spheroids the pericellular oxygen pressure and the generation of reactive oxygen species decreased, whereas the alpha-subunit of HIF-1 (HIF-1alpha) and P-gp were up-regulated. Furthermore, P-gp was up-regulated under experimental physiological hypoxia and chemical hypoxia induced by either cobalt chloride or desferrioxamine. The pro-oxidants H2O2 and buthionine sulfoximine down-regulated HIF-1alpha and P-gp, whereas up-regulation was achieved with the radical scavengers dehydroascorbate, N-acetylcysteine, and vitamin E. The correlation of HIF-1alpha and P-gp expression was validated by the use of hepatoma tumor spheroids that were either wild type (Hepa1) or mutant (Hepa1C4) for aryl hydrocarbon receptor nuclear translocator (ARNT), i.e., HIF-1beta. Chemical hypoxia robustly increased HIF-1alpha as well as P-gp expression in Hepa1 tumor spheroids, whereas no changes were observed in Hepa1C4 spheroids. Hence, our data demonstrate that expression of P-gp in multicellular tumor spheroids is under the control of HIF-1.

Functional role of AhR in the expression of toxic effects by TCDD

Cytochrome P450 1A1 (CYP1A1) is one of the xenobiotic metabolizing enzymes (XMEs), which is induced by polycyclic aromatic hydrocarbons (PAHs). The most potent inducer of CYP1A1 is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In addition, TCDD induces a broad spectrum of biochemical and toxic effects, such as teratogenesis, immunosuppression and tumor promotion. Most, if not all, of the effects caused by TCDD and other PAHs are known to be mediated by AhR (aryl hydrocarbon receptor or dioxin receptor) which has a high binding affinity to TCDD. The liganded AhR translocates from cytoplasm to nuclei where it switches its partner molecule from Hsp90 to Arnt. Thus formed AhR/Arnt heterodimer binds a specific DNA sequence designated XRE in the promoter region of the target genes including CYP1A1, UDP-glucuronosyl transferase and others to enhance their expression. Although it remains to be studied how AhR is involved in the other TCDD-induced biological effects such as teratogenesis and immunosuppression than induction of XMEs, it is believed that these adverse TCDD effects are caused by untimely activation of gene expression by ligand-activated AhR in the biological process. We summarize the present knowledge about functional role of AhR in TCDD-induced biological effects.

Possible involvement of aryl hydrocarbon receptor (AhR) in adult T-cell leukemia (ATL) leukemogenesis: constitutive activation of AhR in ATL

Human T-cell leukemia virus type 1 is the etiologic agent of adult T-cell leukemia (ATL), although the precise mechanism involved in the transformation process has not yet been defined. The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that can influence cell proliferation and differentiation. We investigated the expression and activation of AhR in ATL. RT-PCR and Western blot analyses showed high expression levels of AhR in ATL cell lines. The elevated expression of AhR was in part attributable to the action of the viral transactivator protein, Tax. Interestingly, activation of the AhR was found in ATL cell lines in the absence of apparent exogenous ligands. Importantly, the increased expression and activation of AhR were also observed in some primary ATL cells. To our best knowledge, this is the first report to show the lymphoid malignancy having constitutive activation of AhR. A possible link between increased AhR expression and leukemogenesis in ATL is discussed.

Correlation between gene expression of aryl hydrocarbon receptor (AhR), hydrocarbon receptor nuclear translocator (Arnt), cytochromes P4501A1 (CYP1A1) and 1B1 (CYP1B1), and inducibility of CYP1A1 and CYP1B1 in human lymphocytes

The relationships between gene expression of aryl hydrocarbon receptor (AhR), aryl hydrocarbon receptor nuclear translocator (Arnt), cytochromes P4501A1 (CYP1A1), 1B1 (CYP1B1), CYP1A1, and the inducibility of CYP1A1 and CYP1B1 were determined in 32 cultivated human lymphocytes. Cytochrome P450 induction was performed by incubating lymphocytes with benzanthracene. The relative gene expression levels were determined by quantitative real-time RT-PCR assay. We found that gender is an important confounding factor for gene expression in cultivated lymphocytes. AhR, CYP1A1 and CYP1B1 levels in noninduced lymphocytes were significantly higher in female nonsmokers than in male nonsmokers (p < 0.05). Nevertheless, CYP1A1 and CYP1B1 inducibility was lower in female nonsmokers. CYP1A1 inducibility was higher in male smokers than in male nonsmokers (p < 0.05). After controlling for gender and cigarette smoking, AhR levels positively correlated with CYP1B1 levels and CYP1A1 inducibility (p < 0.01 and p = 0.03, respectively). Arnt levels also correlated with CYP1B1 levels in induced lymphocytes (p < 0.01). However, AhR levels were negatively correlated with CYP1B1 inducibility. These data indicate that AhR expression associates with individual variation of CYP1A1 inducibility and CYP1B1 expression in cultivated lymphocytes. Furthermore, gender and cigarette smoking are important confounding factors for gene expression levels in cultivated lymphocytes.

Synthesis of 32P-labelled protein probes using a modified thioredoxin fusion protein expression system in Escherichia coli

The thioredoxin fusion protein expression system from invitrogen was modified so that 32P-labelled recombinant proteins can be easily obtained in large quantities for functional studies. Proteins that are prone to form the inclusion bodies can be functionally expressed as thioredoxin fusion proteins in Escherichia coli. After expression, the recombinant proteins can be easily phosphorylated with 32P-gamma ATP and the 32P-labelled protein can be obtained functionally via a mild proteolytic digestion to cleave off the thioredoxin moiety. A deletion construct of the Ah receptor nuclear translocator protein was used as an example to illustrate how this protein expression system works.

Hypoxia and High Altitude

Increased erythropoietin plasma levels and the consequent augmented production of red blood cells is the best known systemic adaptation to reduced oxygen partial pressure (pO2). Intensive research during the last years revealed that the molecular mechanism behind the regulation of erythropoietin is ubiquitous and has far more implications than first thought. Erythropoietin regulation results from the activation of the hypoxia-inducible factor-1 (HIF-1) pathway under hypoxic conditions. HIF-1 is a heterodimer consisting of an oxygen sensitive--HIF-1--and an oxygen-independent subunit--HIF-1beta (also known as the aryl hydrocarbon receptor nuclear translocator--ARNT). In addition to erythropoietin, more than 30 genes are now known to be up-regulated by HIF-1. Recently, the critical involvement of HIF-1alpha post-translational modifications in the cellular oxygen sensing mechanism was discovered. In this review we will focus on the regulation of the HIF-1 pathway and the cellular oxygen sensor and discuss their implications in high altitude hypoxia.

Mechanism of rat UDP-glucuronosyltransferase 1A6 induction by oltipraz: evidence for a contribution of the Aryl hydrocarbon receptor pathway

The utility of oltipraz as a cancer chemopreventive agent is thought to depend on the induction of enzymes involved in phase 2 xenobiotic detoxification. Although studies of some enzymes induced by oltipraz implicate a novel transcriptional activating pathway involving Nrf2 and antioxidant-response elements (AREs), the mechanism of phenol UGT induction has remained unclear. Previous work showed that UGT1A6 is transcribed from two promoters, P1 and P2, that are both induced by oltipraz in rat liver. The effect also occurs in rat hepatocytes treated with oltipraz (concentrations >3 microM). To investigate the mechanism, luciferase reporter plasmids under the control of P1 [p(-1078/+27)1A6P1-luc] or P2 [p(-1354/+65)1A6P2-luc] were transfected into rat hepatocytes and tested for inducibility. P1, but not P2, showed responsiveness to oltipraz (2- to 5-fold increase) and 3-methylcholanthrene (10- to 30-fold increase). Because P1 contained no visible AREs, the role of a xenobiotic response element (XRE) centered between bases -134 and -129 was evaluated. Mutation of the XRE core reduced the effects of both oltipraz and 3-methylcholanthrene on the P1 reporter. The 1A6 XRE conferred oltipraz responsiveness on the simian virus 40 promoter of pGL3-Promoter. Comparative effects of oltipraz and 3-methylcholanthrene on transfected cytochrome P4501A1 reporters support the general but relatively weak XRE-stimulating activity of oltipraz. The involvement of the aryl hydrocarbon receptor (AHR) and aryl hydrocarbon nuclear translocator (ARNT) in mediating the effects of oltipraz on the XRE is supported by electrophoretic mobility supershift data and AHR/ARNT overexpression studies. These data raise questions about the contribution of AHR and other secondary induction pathways in the mechanism of oltipraz.

Expression of Arnt and Arnt2 mRNA in developing murine tissues

The basic helix-loop-helix (bHLH-PAS) proteins aryl hydrocarbon receptor nuclear translocator (Arnt) and Arnt2 are transcriptional regulators that function as dimerizing partners for several bHLH-PAS proteins and also some nonrelated partners. They are involved in various biological functions, including regulation of developmental genes. In earlier studies, the developmental expression of Arnt was reported to be almost ubiquitous, whereas Arnt2 expression has been shown to be more limited, comprising neuronal tissues as the main site of expression. Here we provide a detailed description of the expression of Arnt and Arnt2 mRNA in mouse tissues during embryonic and early postnatal development. Arnt and also Arnt2 transcripts, in contrast to earlier reports, are shown to be expressed more widely during development yet show a temporally and spatially specific pattern.

Identification of functional hypoxia response elements in the promoter region of the DEC1 and DEC2 genes

Adaptation to hypoxia is a crucial process both physiologically (i.e. in chondrocytes) and pathologically (i.e. in tumor cells). Previous studies have shown that DEC1, a basic helix-loop-helix transcription factor, is induced by hypoxia in glioma cells (Ivanova, A. V., Ivanov, S. V., Danilkovitch-Miagkova, A., and Lerman, M. I. (2001) J. Biol. Chem. 276, 15306-15315). In the present study, we found that hypoxia or CoCl(2) enhanced the mRNA expression of DEC2, as well as DEC1, within 24 h in chondrogenic ATDC5, 293T, and HeLa cells. In luciferase assays, the regions between -524 and -401 in the DEC1 promoter, and between -863 and -258 in the DEC2 promoter, were responsible for the hypoxia- or hypoxia-inducible factor-1alpha (HIF-1alpha)-induced transcription. In these regions, we identified functional hypoxia response elements (HREs) that bound to HIF-1alpha and HIF-1beta. In addition to an HIF-1 binding site consensus sequence, the DEC1 HRE had cAMP response element-like and CACAG sequences, which were also involved in the transcription activation in response to HIF-1alpha. Although the DEC2 HRE did not have a cAMP response element-like or CACAG sequence, it showed a higher affinity for HIF-1 than did the DEC1 HRE. Because DEC1 and DEC2 are directly inducible by HIF-1, these transcription factors may be crucial for the adaptation to hypoxia.

Aromatic hydrocarbon receptor expression and function in liver of hypophysectomized male rats

The aromatic hydrocarbon receptor (AHR) acts as a ligand-activated transcription factor that mediates many of the biological responses to aromatic hydrocarbons, such as 3-methylcholanthrene (MC) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Some toxic effects are thought to be the result of AHR-mediated changes in the expression of endocrine-related genes, such as the estrogen receptor and genes involved in cell growth and differentiation. Since little is known about endocrine factors that regulate AHR expression and function, we evaluated the effect of hypophysectomy (hypx) on these parameters in the liver of male rats. Cytosolic AHR immunoreactive protein was reduced in hypx rats to 61% of levels in sham-operated rats. Sucrose density gradient analysis of cytosolic AHR radioligand binding using a saturating concentration of [3H]TCDD showed that ligand binding was reduced in hypx rats to 31% of sham levels. TCDD showed similar potency for transforming cytosolic AHR from hypx and sham rats to a DNA-binding form; however, maximal AHR DNA-binding was reduced in hypx rats to 57% of sham levels, although this did not achieve statistical significance. These changes observed at the protein level were not accompanied by a corresponding decrease in hypx rats of mRNA for AHR or the AHR nuclear translocator. Despite this change in AHR protein level, hypx rats were not compromised in hepatic cytochrome P450 1A1 induction following in vivo administration of MC. These data indicate that the AHR signaling pathway is intact in hypx rats and that a pituitary factor regulates hepatic AHR expression at the protein level.

The aryl hydrocarbon receptor nuclear transporter is modulated by the SUMO-1 conjugation system

The aryl hydrocarbon receptor nuclear transporter (ARNT) is a member of the basic helix-loop-helix/PAS (Per-ARNT-Sim) family of transcription factors, which are important for cell regulation in response to environmental conditions. ARNT is an indispensable partner of the aryl hydrocarbon receptor (AHR) or hypoxia-inducible factor-1alpha. This protein is also able to form homodimers such as ARNT/ARNT. However, the molecular mechanism that regulates the transcriptional activity of ARNT remains to be elucidated. Here, we report that ARNT is modified by SUMO-1 chiefly at Lys(245) within the PAS domain of this protein, both in vivo and in vitro. Substitution of the target lysine with alanine enhanced the transcriptional potential of ARNT per se. Furthermore, green fluorescent protein-fused ARNT tended to form nuclear foci in approximately 20% of the transfected cells, and the foci partly colocalized with PML nuclear bodies. PML, one of the well known substrates for sumoylation, was found to augment the transcriptional activities of ARNT. ARNT bound AHR or PML, whereas the sumoylated form of ARNT associated with AHR, but not with PML, resulting in a reduced effect of PML on transactivation by ARNT. Our data suggest that the sumoylation of ARNT modulates its transcriptional role through affecting the ability of ARNT to interact with cooperative molecules such as PML. This exemplifies a crucial role of protein sumoylation in modulating protein-protein interactions.

Okadaic acid increases ARNT homodimer transactivation potential

The human aryl hydrocarbon nuclear translocator (hARNT) protein belongs to the family of basic helix-loop-helix (bHLH) PAS transcription factors and regulates a range of cellular processes by either homodimerizing or heterodimerizing with other bHLH-PAS proteins. hARNT has been shown to be almost exclusively phosphorylated on serine residues. However, regulation of hARNT with respect to phosphorylation remains poorly understood. The phosphatase inhibitor okadaic acid was used to explore whether a change in hARNT phosphorylation status could influence hARNT homodimer activity. The hARNT homodimer has been shown to bind to E boxes and E-box binding factors are believed to be important in the regulation of cell differentiation and proliferation. Okadaic acid significantly increased hARNT-mediated class B, E-box-driven reporter activity in COS-1 cells, transiently expressing hARNT without affecting hARNT protein levels. This alteration in hARNT-mediated class B, E-box-driven reporter activity correlates with an observed increase in [32P]orthophosphate incorporation into hARNT. Treatment with okadaic acid resulted in a 12-fold increase in [32P]orthophosphate incorporation into hARNT that was transiently expressed in COS-1 cells; an increase in the number of tryptic phosphopeptides generated from hARNT digests on two-dimensional phosphopeptide maps was also observed. Despite the significant increase in [32P]orthophosphate incorporation into hARNT, serine remained the predominantly phosphorylated residue. Clearly, increased serine phosphorylation does not appear to negatively regulate hARNT homodimerization or transactivation potential. These results demonstrate that increased hARNT homodimer signaling in COS-1 cells may result from a direct change in hARNT phosphorylation status.

Hypoxia-inducible factor 1 transactivates the human leptin gene promoter

Increased placental leptin has been demonstrated in preeclampsia, a pregnancy disorder associated with placental hypoxia. This suggests that leptin gene expression is enhanced in response to oxygen deficiency in this organ. In support of this hypothesis, we have previously shown that hypoxia activates the leptin promoter in trophoblast-derived BeWo cells. Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric HIF-1alpha/HIF-1beta complex that regulates the transcription of hypoxia-responsive genes. To test whether this factor is involved in hypoxia-induced leptin promoter activation, BeWo cells were transiently transfected with a HIF-1alpha expression vector. Exogenous HIF-1alpha markedly increased luciferase reporter activity driven by the leptin promoter when HIF-1beta was co-expressed in the same cells. This effect was similar to that elicited by CoCl2, an agent known to stabilize endogenous HIF-1alpha. These data suggest that HIF-1alpha/HIF-1beta dimers are involved in the effect of CoCl2 to activate the leptin promoter. To confirm the implication of HIF-1, the cells were transfected with a dominant negative form of HIF-1alpha producing transcriptionally inactive HIF-1beta/HIF-1alpha dimers. This mutant HIF-1alpha protein abolished CoCl2 activation of the leptin promoter, providing direct evidence that the effect of CoCl2 is mediated by endogenous HIF-1alpha. Deletion analysis and site-specific mutagenesis demonstrated that a HIF-1 consensus binding site (HRE) spanning -120 to -116 bp relative to the start site was required for CoCl2 and exogenous HIF-1alpha induction of leptin promoter activity. Electrophoretic mobility shift assays performed with in vitro-translated HIF-1alpha and HIF-1beta proteins demonstrated binding to this HRE and not to mutated sequences only when both subunits were used together. These data demonstrate that leptin is a new hypoxia-inducible gene, which is stimulated in a placental cell line through HIF-1 interaction with a consensus HRE site located at -116 in the proximal promoter.

The arylhydrocarbon receptor (AhR), but not the AhR-nuclear translocator (ARNT), is increased in hearts of patients with cardiomyopathy

The objective of this study was to investigate the expression of the arylhydrocarbon receptor (AhR) and its partner AhR-nuclear translocator (ARNT) in left ventricle specimens from explanted hearts from patients with cardiomyopathy (CMP). Explanted hearts from 16 patients with ischemic (n=9, age 63+/-12 years) and dilative (n=7, age 54+/-12 years) CMP, undergoing heart transplantation were examined. Healthy donor hearts from five accident victims served as controls. As these donors were of younger age (32+/-11 years), additionally, donor hearts from three older accident victims (age 48+/-15 years) without clinical symptoms but with signs of ventricular hyperthrophy (n=1) or atherosclerotic lesions (n=2) were included ("pathological controls"). Expression of AhR and ARNT was analyzed using semi-quantitative immunohistochemistry, and in selected samples, Western blot- and reverse-transcription polymerase chain reaction analysis were performed to confirm AhR and ARNT expression. Immunohistological analysis revealed weak to intermediate staining of anti-AhR in control, but weak to intense staining in CMP- and "pathologic control" specimens, indicating significantly increased AhR levels in the diseased heart. Moreover, in CMP specimens, the percentage of AhR-positive cells was strongly increased. Higher anti-AhR staining was also seen in two atherosclerotic "pathologic control" specimens. In all groups, the intensity of anti-ARNT staining was more pronounced than AhR staining, but significant differences or any age-related alterations were not observed. In conclusion, the increased cellular content of AhR in left ventricular specimens from CMP patients suggests a role for AhR in heart disease.

Ligand-dependent and independent modulation of aryl hydrocarbon receptor localization, degradation, and gene regulation

Changes in the concentration or subcellular location of the key proteins involved in signal transduction pathways have been shown to impact gene regulation. Studies were designed to evaluate the relationship between aryl hydrocarbon receptor (AHR) localization, stability, and gene regulation in a defined system where the endogenous AHR protein could be evaluated. The findings indicate that treatment of cells with geldanamycin (GA) or MG-132 (an inhibitor of the 26S proteasome) results in nuclear translocation of the endogenous AHR in both human HepG2 and murine Hepa-1 cells without induction of endogenous CYP1A1 protein. Exposure to GA resulted in the degradation of AHR by >90% in the nucleus via the 26S proteasome. Importantly, the reduced level of AHR resulted in a 50% reduction in the maximal level of CYP1A1 induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In all treatments the concentration of the AHR nuclear translocator (ARNT) protein was unchanged and had no impact on the localization of the AHR. Thus, ligand-independent translocation of the AHR to the nucleus was not sufficient to induce CYP1A1 in the absence of ligand, but reductions in the level of the endogenous AHR protein pool shifted the dose-response curve for TCDD to the right.

Control of the hypoxic response in Drosophila melanogaster by the basic helix-loop-helix PAS protein similar

In mammalian systems, the heterodimeric basic helix-loop-helix (bHLH)-PAS transcription hypoxia-inducible factor (HIF) has emerged as the key regulator of responses to hypoxia. Here we define a homologous system in Drosophila melanogaster, and we characterize its activity in vivo during development. By using transcriptional reporters in developing transgenic flies, we show that hypoxia-inducible activity rises to a peak in late embryogenesis and is most pronounced in tracheal cells. We show that the bHLH-PAS proteins Similar (Sima) and Tango (Tgo) function as HIF-alpha and HIF-beta homologues, respectively, and demonstrate a conserved mode of regulation for Sima by oxygen. Sima protein, but not its mRNA, was upregulated in hypoxia. Time course experiments following pulsed ectopic expression demonstrated that Sima is stabilized in hypoxia and that degradation relies on a central domain encompassing amino acids 692 to 863. Continuous ectopic expression overrode Sima degradation, which remained cytoplasmic in normoxia, and translocated to the nucleus only in hypoxia, revealing a second oxygen-regulated activation step. Abrogation of the Drosophila Egl-9 prolyl hydroxylase homologue, CG1114, caused both stabilization and nuclear localization of Sima, indicating a central involvement in both processes. Tight conservation of the HIF/prolyl hydroxylase system in Drosophila provides a new focus for understanding oxygen homeostasis in intact multicellular organisms.

Curcumin activates the aryl hydrocarbon receptor yet significantly inhibits (-)-benzo(a)pyrene-7R-trans-7,8-dihydrodiol bioactivation in oral squamous cell carcinoma cells and oral mucosa

The development of oral squamous cell carcinoma (SCC) shows a positive correlation with the carcinogen exposure that occurs during tobacco and alcohol use. The purpose of this study was to investigate whether the naturally occurring chemopreventive agent, curcumin, modulates expression and function of carcinogen- metabolizing enzymes in human keratinocytes isolated from oral SCC tumors. Dose-response studies demonstrated that curcumin concentrations of >or=25 micro M were cytotoxic for oral SCC cells. Curcumin increased both expression (reverse transcription-PCR analyses) and function (high-performance liquid chromatography determination of ethoxyresorufin metabolism) of cytochrome P-450 (CYP) 1A1 and/or CYP1B1. The aryl hydrocarbon receptor (AhR), which up-regulates a battery of genes associated with carcinogen metabolism, is activated by polycyclic aromatic hydrocarbons such as the tobacco-associated carcinogen benzo(a)pyrene. Electromobility shift assays demonstrated that similar to the established AhR ligand 2,3,7,8,-tetrachlorodibenzo-p-dioxin, curcumin inclusion resulted in AhR nuclear translocation and formation of the transcriptionally active AhR-aryl hydrocarbon receptor nuclear translocator complex. Cellular capacity to bioactivate the tobacco-associated carcinogen (-)-benzo(a)pyrene-7R-trans-7,8-dihydrodiodiol was determined by evaluating conversion of the carcinogenic metabolite diol epoxide to stable tetrols via high-performance liquid chromatography. Results of our metabolism studies showed that curcumin significantly inhibited CYP1A1-mediated benzo(a)pyrene diol bioactivation in both oral SCC cells and intact oral mucosa. Because CYP1A1 is one of the primary carcinogen-activating enzymes in oral mucosa, the use of curcumin as an oral cavity chemopreventive agent could have significant clinical impact via its ability to inhibit carcinogen bioactivation.

DNA binding and protein interactions of the AHR/ARNT heterodimer that facilitate gene activation

Gene activation by the aryl hydrocarbon receptor (AHR) and its DNA binding partner, the aryl hydrocarbon receptor nuclear translocator (ARNT) requires a number of sequential steps that occur following the binding of ligand and entry of the AHR into the nuclear compartment. This includes heterodimerization of the AHR and ARNT, formation of the appropriate amino acid/nucleotide contacts at the GCGTG recognition site and interactions between either the AHR or ARNT with proteins that facilitate changes in chromatin structure. The majority of these steps are likely modulated by changes in both phosphorylation and oxidation status of the AHR, ARNT and associated proteins. Studies of both the basic helix-loop-helix transcription factors and the nuclear hormone receptor family can provide significant insights into how this unique signaling pathway activates its target genes.

2,3,7,8-tetrachlorodibenzo-p-dioxin causes alterations in lymphocyte development and thymic atrophy in hemopoietic chimeras generated from mice deficient in ARNT2

It is well established that dioxins cause a variety of toxic effects and syndromes including alterations of lymphocyte development. Exposure to the prototypical dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) leads to severe thymic atrophy in all species studied. It has been shown that most of this toxicity is due to TCDD binding to and activating the aryl hydrocarbon receptor (AHR). Upon activation, the AHR enters the nucleus, dimerizes with the AHR nuclear translocator (ARNT), and this heterodimer modulates a number of genes that mediate toxicity. The AHR and ARNT are members of the basic-helix-loop-helix-Per, ARNT, and Sim homology (bHLH-PAS) family of transcription factors. In this study, we wanted to determine if another bHLH-PAS transcription factor, ARNT2, which has high amino acid sequence identity to ARNT and has been shown to dimerize with the TCDD-activated AHR, is involved in mediating TCDD's effect on lymphocyte development. We determined by RT-PCR that ARNT2 is expressed at a low level in whole thymus, thymocytes, and bone marrow lymphocytes. We created hemopoietic chimeras by lethally irradiating C57BL/6 mice and reconstituting them with fetal liver stem cells that either have or are deficient in a portion of chromosome 7 that contains ARNT2. Regardless of whether chimeras possessed or lacked this chromosome fragment, equal sensitivity to TCDD-induced thymic atrophy was observed despite expression of ARNT2 in the thymus. Furthermore, the absence of ARNT2 (or any other genes found on this portion of chromosome 7) did not confer any protection against TCDD-induced alterations in bone marrow B-cell subsets. These data indicate that in this model system the effects of TCDD-induced thymic atrophy and alterations in B-cell maturation are not dependent on an AHR-ARNT2 heterodimer.

Increased expression of vascular endothelial growth factor in human skeletal muscle in response to short-term one-legged exercise training

The effects of short-term exercise training on vascular endothelial growth factor (VEGF) and one of its regulatory transcription factors, the hypoxia inducible factor 1 (HIF-1) subunit, were studied in eight healthy males. Muscle and blood samples were obtained before the 1st, and 24 h after the 7th training session. VEGF and HIF-1 mRNA were analysed using RT-PCR, VEGF mRNA localization with in situ hybridization and VEGF protein with ELISA. Concurrent increases in VEGF mRNA and protein levels were observed in skeletal muscle, and the mRNA was expressed within the skeletal muscle fibres and in cells in the interstitium. These data support the idea of a pretranslational regulation of exercise-induced changes in VEGF mRNA, and indicate that increased VEGF protein expression is an early event in skeletal muscle adaptation to training. Furthermore, different cell types may act as sources for the production of angiogenic factors in response to exercise. The levels of HIF-1 mRNA subunits did not change, suggesting no change in HIF-1 mRNA transcript levels in the regulation of training-induced VEGF expression. In contrast to increased tissue VEGF expression, the arterial and femoral venous plasma levels of VEGF were decreased by training, which may indicate an exercise-induced enhancement of the peripheral uptake of VEGF.

3-Methylcholanthrene, which binds to the arylhydrocarbon receptor, inhibits proliferation and differentiation of osteoblasts in vitro and ossification in vivo

3-Methylcholanthrene (3MC) is a ligand for arylhydrocarbon receptor (AhR), which binds dioxin. We examined the effects of 3MC on the proliferation and differentiation of osteoblasts using cultures of rat calvarial osteoblast-like cells (ROB cells) and mouse calvarial clonal preosteoblastic cells (MC3T3-E1 cells). Analysis by RT-PCR revealed that the mRNAs for AhR and AhR nuclear translocators were expressed in both ROB and MC3T3-E1 cells. Cell proliferation and the synthesis of DNA by ROB cells and MC3T3-E1 cells were markedly inhibited on exposure of cells to 3MC. Furthermore, 3MC reduced the activity of alkaline phosphatase and the rate of deposition of calcium by cells. The level of expression of mRNA for osteocalcin, which is a marker of osteoblastic differentiation, was also depressed by 3MC. Moreover, when 3MC (1 mg/kg body weight) was administered sc to pregnant mice at 10.5, 12.5, and 14.5 d post coitus, fetuses examined subsequently at 15.5 or 17.5 d post coitus revealed evidence of inhibition of appropriate calcification of bones. The treated metacarpals showed no subperiosteal bone matrix histologically. Our findings indicate that 3MC might have critical effects on the formation of bone both in vivo and in vitro.

The redox protein thioredoxin-1 (Trx-1) increases hypoxia-inducible factor 1alpha protein expression: Trx-1 overexpression results in increased vascular endothelial growth factor production and enhanced tumor angiogenesis

Hypoxia-inducible factor 1 (HIF-1), a heterodimer of HIF-1alpha and HIF-1beta subunits, is a transcriptional activator central to the cellular response to low oxygen that includes metabolic adaptation, angiogenesis, metastasis, and inhibited apoptosis. Thioredoxin-1 (Trx-1) is a small redox protein overexpressed in a number of human primary tumors. We have examined the effects of Trx-1 on HIF activity and the activation of downstream genes. Stable transfection of human breast carcinoma MCF-7 cells with human Trx-1 caused a significant increase in HIF-1alpha protein levels under both normoxic (20% oxygen) and hypoxic (1% oxygen) conditions. Trx-1 increased hypoxia-induced HIF-1 transactivation activity measured using a luciferase reporter under the control of the hypoxia response element. Changes in HIF-1alpha mRNA levels did not account for the changes observed at the protein level, and HIF-1beta protein levels did not change. Trx-1 transfection also caused a significant increase in the protein products of hypoxia-responsive genes, including vascular endothelial growth factor (VEGF) and nitric oxide synthase 2 in a number of different cell lines (MCF-7 human breast and HT29 human colon carcinomas and WEHI7.2 mouse lymphoma cells) under both normoxic and hypoxic conditions. The pattern of expression of the different isoforms of VEGF was not changed by Trx-1. Transfection of a redox-inactive Trx-1 (C32S/C35S) markedly decreased levels of HIF-1alpha protein, HIF-1 transactivating activity, and VEGF protein in MCF-7 cells compared with empty vector controls. In vivo studies using WEHI7.2 cells transfected with Trx-1 showed significantly increased tumor VEGF and angiogenesis. The results suggest that Trx-1 increases HIF-1alpha protein levels in cancer cells and increases VEGF production and tumor angiogenesis.

Induction and superinduction of 2,3,7,8-tetrachlorodibenzo-rho-dioxin-inducible poly(ADP-ribose) polymerase: role of the aryl hydrocarbon receptor/aryl hydrocarbon receptor nuclear translocator transcription activation domains and a labile transcription repressor

The environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces a novel poly(ADP-ribose) polymerase (TiPARP). In this study, the signaling pathway of the induction was analyzed. Induction of TiPARP by TCDD occurs in both hepa1c1c7 cells and C57 mouse liver. Induction is concentration and time dependent. Genetic analyses reveal that induction is abolished in aromatic hydrocarbon receptor (AhR)- or aromatic hydrocarbon receptor nuclear translocator (Arnt)-defective variants but restored upon reconstitution of the variant cells with cDNAs expressing functional AhR or Arnt. Moreover, induction is largely reduced in cells expressing a deletion mutant of AhR or Arnt lacking the transcription activation (TA) domain, thus implicating the TA activities of both AhR and Arnt in the induction. Inhibition of protein synthesis by cycloheximide enhances the induction of TiPARP in the presence of an AhR agonist. The superinduction is transcriptional and does not require pretreatment with TCDD. Finally, inhibition of the 26S proteasomes by MG132 superinduces TiPARP. These findings establish that induction of TiPARP by TCDD is mediated through an AhR and Arnt transcription activation-dependent signal transduction that is repressed by a labile factor through the ubiquitin-26S proteasome-mediated protein degradation.

Oct2 transcription factor of a teleost fish: activation domains and function from an enhancer

Oct2 transcription factors of the catfish (Ictalurus punctatus) are expressed as alternatively spliced alpha and beta isoforms. Functional analysis revealed an N-terminal glutamine (Q)-rich transactivation domain common to both isoforms of catfish Oct2. A C-terminal proline, serine, threonine (PST)-rich activation domain was identified exclusively in the beta isoform. Activation domains of fish and mammalian Oct2 showed cell type- and species-specific activity correlated with their biochemical composition (Q-rich vs PST-rich). In contrast the activation domains of the aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator of fish and mammals showed no correlation of activity with biochemical composition or species of origin. Although isolated catfish Oct2 activation domains were unable to drive transcription from a site 1.9kb distal to the promoter, Oct2beta activated transcription from both an IgH enhancer and an array of octamer motifs at this distal position. The properties of catfish Oct2 activation domains differ depending on whether they are studied in isolation or as components of the intact transcription factor.

Tissue-specific expression of AHR2, ARNT2, and CYP1A in zebrafish embryos and larvae: effects of developmental stage and 2,3,7,8-tetrachlorodibenzo-p-dioxin exposure

To better understand the role of the aryl hydrocarbon receptor (AHR) signaling pathway in causing tissue-specific signs of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) toxicity in zebrafish, the temporal and spatial expression of the zebrafish aryl hydrocarbon receptor 2 (zfAHR2), aryl hydrocarbon receptor nuclear translocator 2 (zfARNT2), and an AHR regulated gene, cytochrome P4501A (zfCYP1A), were assessed in larvae exposed to vehicle or TCDD (1.55 nM) from 3-4 h postfertilization (hpf). Coexpression of a transcriptionally active AHR pathway was apparent by the expression of zfCYP1A mRNA and protein in certain larval tissues. zfCYP1A protein was first detected in the skin and vasculature of TCDD-exposed larvae at 36 hpf. Vascular-specific zfCYP1A protein expression continued from 36 to120 hpf at which time it was also detected in the heart, kidney, and liver. zfCYP1A mRNA was observed in TCDD treated larvae as early as 24 hpf in the developing vascular system. Vascular specific zfCYP1A mRNA expression in the head, trunk, and tail by 36 hpf in TCDD-exposed larvae, confirmed immunohistochemical localization. The expression of zfAHR2 and zfARNT2 mRNAs was generally similar in control and TCDD-exposed larvae. Coexpression of zfAHR2, zfARNT2, and zfCYP1A mRNAs was evident in TCDD-exposed larvae by 36 hpf and in the vasculature, heart, and trunk kidney by 48 hpf, well before the first signs of overt developmental toxicity are observed. In addition to their function in response to AHR agonists, zfAHR2 and zfARNT2 may be involved in development and function of the nervous system. zfAHR2 and zfARNT2 were detected in the brain, spinal cord, and sensory organs. However, TCDD-induced zfCYP1A expression was not detected in these tissues. Taken together, these results are consistent with the notion that the cardiovascular system is a primary target of TCDD developmental toxicity in zebrafish.

Aryl hydrocarbon receptor nuclear translocator 2 (ARNT2): structure, gene mapping, polymorphisms, and candidate evaluation for human orofacial clefts

BACKGROUND

Nonsyndromic orofacial clefts have an estimated incidence of 1/1000 live births. Population genetic and embryologic studies suggest that cleft palate only (CPO) may be a distinct clinical entity from cleft lip with or without cleft palate (CL/P). Both CPO and CL/P are thought to be multifactorial in etiology, with evidence indicating that genetic, environmental, and developmental determinants may all play a role. The ARNT2 gene localizes to a conserved linkage group on mouse chromosome 7 that is syntenic with human chromosome 15q23-25. This chromosomal region was previously identified as a teratogen-induced clefting susceptibility locus in a genome-wide scan of AXB and BXA recombinant inbred mice. Arnt2 is expressed in the first branchial arch in mice. The teratogen 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) acts through the aryl hydrocarbon receptor (Ahr) pathway to produce dose-dependent CPO and thymic wasting in mice exposed in utero. Arnt2 and Ahr proteins dimerize in vitro. TCDD exposure is also associated with orofacial clefting in children of parents involved in agricultural work.

METHODS

To determine whether ARNT2 influences human craniofacial development, we identified the human ARNT2 gene and conducted genomic structural analysis. Mutational screening was performed in infants with nonsyndromic CPO or CL/P who were identified by the Iowa Birth Defects Registry.

RESULTS

A common amino acid polymorphism was detected but, no obvious disease-causing mutations were detected by SSCP analysis. The microsatellite marker, GATA89D04 (D15S823) was identified within intron 11 of the human ARNT2 gene, and linkage disequilibrium of nonsyndromic CPO and CL/P parent-infant trios was conducted.

CONCLUSIONS

No association was demonstrated with CPO (n = 45) and CL/P (n = 37). Teratology 66:85-90, 2002.

Cellular adaptation to hypoxia: O2-sensing protein hydroxylases, hypoxia-inducible transcription factors, and O2-regulated gene expression

Although it was known for a long time that oxygen deprivation leads to the transcriptional induction of the gene encoding erythropoietin, the molecular mechanisms behind this process remained enigmatic. The cloning of the hypoxia-inducible factors (HIFs), the finding that HIF-1 regulates the expression of many more genes apart from erythropoietin, and the elucidation of the oxygen-dependent mechanisms degrading the HIF alpha subunits recently led to the spectacular discovery of the molecular principles of oxygen sensing. This review aims to summarize our current knowledge of oxygen-regulated gene expression..

Variability of the human aryl hydrocarbon receptor nuclear translocator (ARNT) gene

The aryl hydrocarbon receptor nuclear translocator (ARNT) plays an essential role in vertebrate transcriptional regulation as the common subunit of transcriptionally active complexes like the aryl hydrocarbon receptor (AHR)/ARNT heterodimer and hypoxia-inducible factor 1, mediating cellular responses to certain xenobiotics and to hypoxia, respectively. A cohort of healthy Caucasian volunteers was screened for genetic variations of ARNT. Six polymorphic sites could be identified, a variation in a G-stretch upstream of the ATG translation start site, a frequent silent mutation (G567C), two polymorphic sites in intron 9, and two single nucleotide substitutions leading to amino acid exchanges, G1531A (D511N) and T1551G (D517E). The frequencies were 0.005 for the Asn-coding allele and for the Glu-coding allele, respectively, with no linkage between these two mutations. Although no significant correlation with activities of CYP1A2, which is under regulatory control of the AHR/ARNT transcription complex, could be established, metabolic or pathological phenotypes may be associated with these variations.