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

The aryl hydrocarbon receptor as a model PAS sensor

Proteins containing PER-ARNT-SIM (PAS) domains are commonly associated with environmental adaptation in a variety of organisms. The PAS domain is found in proteins throughout Archaea, Bacteria, and Eukarya and often binds small-molecules, supports protein-protein interactions, and transduces input signals to mediate an adaptive physiological response. Signaling events mediated by PAS sensors can occur through induced phosphorelays or genomic events that are often dependent upon PAS domain interactions. In this perspective, we briefly discuss the diversity of PAS domain containing proteins, with particular emphasis on the prototype member, the aryl hydrocarbon receptor (AHR). This ligand-activated transcription factor acts as a sensor of the chemical environment in humans and many chordates. We conclude with the idea that since mammalian PAS proteins often act through PAS-PAS dimers, undocumented interactions of this type may link biological processes that we currently think of as independent. To support this idea, we present a framework to guide future experiments aimed at fully elucidating the spectrum of PAS-PAS interactions with an eye towards understanding how they might influence environmental sensing in human and wildlife populations.

Rodent genetic models of Ah receptor signaling

The aryl hydrocarbon receptor (AHR) is a ligand activated transcription factor that is a member of the PER-ARNT-SIM superfamily of environmental sensors. This receptor has been a molecule of interest for many years in the field of toxicology, as it was originally discovered to mediate the toxic effects of certain environmental pollutants like benzo(a)pyrene and 2,3,7,8-tetrachlorodibenzo-p-dioxin. While all animals express this protein, there is naturally occurring variability in receptor size and responsiveness to ligand. This naturally occurring variation, particularly in mice, has been an essential tool in the discovery and early characterization of the AHR. Genetic models including congenic mice and induced mutations at the Ahr locus have proven invaluable in further understanding the role of the AHR in adaptive metabolism and TCDD-induced toxicity. The creation and examination of Ahr null mice revealed an important physiological role for the AHR in vascular and hepatic development and mediation of the immune system. In this review, we attempt to provide an overview to many of the AHR models that have aided in the understanding of AHR biology thus far. We describe the naturally occurring polymorphisms, congenic models, induced mutations at the Ahr locus and at the binding partner Ah Receptor Nuclear Translocator and chaperone, Ah receptor associated 9 loci in mice, with a brief description of naturally occurring and induced mutations in rats.

Evolution of Hominin Detoxification: Neanderthal and Modern Human Ah Receptor Respond Similarly to TCDD

In studies of hominin adaptations to fire use, the role of the aryl hydrocarbon receptor (AHR) in the evolution of detoxification has been highlighted, including statements that the modern human AHR confers a significantly better capacity to deal with toxic smoke components than the Neanderthal AHR. To evaluate this, we compared the AHR-controlled induction of cytochrome P4501A1 (CYP1A1) mRNA in HeLa human cervix epithelial adenocarcinoma cells transfected with an Altai-Neanderthal or a modern human reference AHR expression construct, and exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We compared the complete AHR mRNA sequences including the untranslated regions (UTRs), maintaining the original codon usage. We observe no significant difference in CYP1A1 induction by TCDD between Neanderthal and modern human AHR, whereas a 150–1,000 times difference was previously reported in a study of the AHR coding region optimized for mammalian codon usage and expressed in rat cells. Our study exemplifies that expression in a homologous cellular background is of major importance to determine (ancient) protein activity. The Neanderthal and modern human dose–response curves almost coincide, except for a slightly higher extrapolated maximum for the Neanderthal AHR, possibly caused by a 5′-UTR G-variant known from modern humans (rs7796976). Our results are strongly at odds with a major role of the modern human AHR in the evolution of hominin detoxification of smoke components and consistent with our previous study based on 18 relevant genes in addition to AHR, which concluded that efficient detoxification alleles are more dominant in ancient hominins, chimpanzees, and gorillas than in modern humans.

Role of aryl hydrocarbon receptor in central nervous system tumors: Biological and therapeutic implications

Aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor, whose canonical pathway mainly regulates the genes involved in xenobiotic metabolism. However, it can also regulate several responses in a non-canonical manner, such as proliferation, differentiation, cell death and cell adhesion. AhR plays an important role in central nervous system tumors, as it can regulate several cellular responses via different pathways. The polymorphisms of the AHR gene have been associated with the development of gliomas. In addition, the metabolism of tumor cells promotes tumor growth, particularly in tryptophan synthesis, where some metabolites, such as kynurenine, can activate the AhR pathway, triggering cell proliferation in astrocytomas, medulloblastomas and glioblastomas. Furthermore, as part of the changes in neuroblastomas, AHR is able to downregulate the expression of proto-oncogene c-Myc, induce differentiation in tumor cells, and cause cell cycle arrest and apoptosis. Collectively, these data suggested that the modulation of the AhR pathway may downregulate tumor growth, providing a novel strategy for applications for the treatment of certain tumors through the control of the AhR pathway.

Involvement of Ahr Pathway in Toxicity of Aflatoxins and Other Mycotoxins

The purpose of this review is to present information about the role of activation of aflatoxins and other mycotoxins, of the aryl hydrocarbon receptor (AhR) pathway. Aflatoxins and other mycotoxins are a diverse group of secondary metabolites that can be contaminants in a broad range of agricultural products and feeds. Some species of Aspergillus, Alternaria, Penicilium, and Fusarium are major producers of mycotoxins, some of which are toxic and carcinogenic. Several aflatoxins are planar molecules that can activate the AhR. AhR participates in the detoxification of several xenobiotic substances and activates phase I and phase II detoxification pathways. But it is important to recognize that AhR activation also affects differentiation, cell adhesion, proliferation, and immune response among others. Any examination of the effects of aflatoxins and other toxins that act as activators to AhR must consider the potential of the disruption of several cellular functions in order to extend the perception thus far about the toxic and carcinogenic effects of these toxins. There have been no Reviews of existing data between the relation of AhR and aflatoxins and this one attempts to give information precisely about this dichotomy.

Development and characterization of monoclonal antibodies against human aryl hydrocarbon receptor

Aryl hydrocarbon receptor (AhR), a ligand-dependent nuclear receptor, is involved in a diverse spectrum of biological and toxicological effects. Due to the lack of three dimensional (3D) crystal or nuclear magnetic resonance structure, the mechanisms of these complex effects of AhR remain to be unclear. Also, commercial monoclonal antibodies (mAbs) against human AhR protein (hAhR), as alternative immunological tools, are very limited. Thus, in order to provide more tools for further studies on hAhR, we prepared two mAbs (1D6 and 4A6) against hAhR. The two newly generated mAbs specifically bound to amino acids 484-508 (located in transcription activation domain) and amino acids 201-215 (located in Per-ARNT-Sim domain) of hAhR, respectively. These epitopes were new as compared with those of commercial mAbs. The mAbs were also characterized by enzyme-linked immunosorbent assay, western blot, immunoprecipitation and indirect immunofluorescence assay in different cell lines. The results showed that the two mAbs could recognize the linearized AhRs in six different human cell lines and a rat hepatoma cell line, as well as the hAhR with native conformations. We concluded that the newly generated mAbs could be employed in AhR-based bioassays for analysis of environmental contaminants, and held great potential for further revealing the spatial structure of AhR and its biological functions in future studies.

Stability of the aryl hydrocarbon receptor and its regulated genes in the low activity variant of Hepa-1 cell line

We examined the expression kinetics of some of the aryl hydrocarbon receptor (AhR)-regulated genes in LA1 variant cells compared to wild type (WT) Hepa-1 mouse hepatoma cell lines, and we investigated the stability of AhR protein as a key step in the function of this receptor. Treatment of both cell types with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) resulted in increased CYP1A1 and CYP1B1 mRNA with a subsequent down regulation of AhR. We show here that co-treatment with transcription inhibitor actinomycin D (ActD) has reversed the TCDD-induced depletion of AhR protein in WT. However, the proteolytic degradation of AhR in absence of TCDD was significantly higher in LA1 cells than in WT, and ActD treatment reduced this loss. Induction of CYP1A1 and CYP1B1 mRNA by TCDD in WT cells each exhibited bursts of activity in the initial hour which were about 3-fold greater than in LAI cells. The induced mRNA levels in LA1 exhibited a slow and sustained increase approximating the WT levels by 20 h. The induction of two other AhR-regulated genes also showed comparable turnover differences between the two cell types. Thus, altered regulation of the AhR responsive genes in LA1 may result from a difference in AhR stability.

In vitro re-expression of the aryl hydrocarbon receptor (Ahr) in cultured Ahr-deficient mouse antral follicles partially restores the phenotype to that of cultured wild-type mouse follicles

BACKGROUND

The aryl hydrocarbon receptor (AHR) mediates the toxic effects of various endocrine disrupting chemicals. In female mice, global deletion of the Ahr (AhrKO) results in slow growth of ovarian antral follicles. No studies, however, have examined whether injection of the Ahr restores the phenotypes of cultured AhrKO ovarian antral follicles to wild-type levels.

METHODS

We developed a system to construct a recombinant adenovirus containing the Ahr to re-express the Ahr in AhrKO granulosa cells and whole antral follicles. We then compared follicle growth and levels of factors in the AHR signaling pathway (Ahr, Ahrr, Cyp1a1, and Cyp1b1) in wild-type, AhrKO, and Ahr re-expressed follicles. Further, we compared the response to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in wild-type, AhrKO, and Ahr re-expressed follicles.

RESULTS

Ahr injection into AhrKO follicles partially restored their growth pattern to wild-type levels. Further, Ahr re-expressed follicles had significantly higher levels of Ahr, Ahrr, Cyp1a1, and Cyp1b1 compared to wild-type follicles. Upon TCDD treatment, only Cyp1a1 levels were significantly higher in Ahr re-expressed follicles compared to the levels in wild-type follicles.

CONCLUSION

Our system of re-expression of the Ahr partially restores follicle growth and transcript levels of factors in the AHR signaling pathway to wild-type levels.

The aryl hydrocarbon receptor interacts with nuclear factor erythroid 2-related factor 2 to mediate induction of NAD(P)H:quinoneoxidoreductase 1 by 2,3,7,8-tetrachlorodibenzo-p-dioxin

NAD(P)H:quinoneoxidoreductase 1 (NQO1) belongs to a group of the aryl hydrocarbon receptor (AhR) battery of drug-metabolizing enzymes that are characteristically induced by both AhR agonists and nuclear factor erythroid 2-related factor 2 (Nrf2) activators. We have previously reported that induction of Nqo1 by the AhR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in hepa1c1c7 cells involves Nrf2 (Ma et al., Biochem J 377, 205-213, 2004). Here we analyzed the molecular mechanism of induction. Induction required AhR and its DNA-binding partner Arnt because induction was not observed in AhR or Arnt-defective cells, but induction was restored upon reconstitution of the variant cells with functional AhR or Arnt. Induction also required Nrf2, as induction by benzo[a]pyrene was lost in the liver of Nrf2 knockout mice similarly to induction by butyl hydroxyanisol, demonstrating a cross-interaction between the AhR and Nrf2 pathways for induction in vivo. TCDD increased the protein level and induced the nuclear accumulation of Nrf2 with a delayed kinetics compared with activation of AhR. Chromatin immunoprecipitation revealed that TCDD recruited both AhR and Nrf2 to the Nqo1 promoter enhancer region containing a DRE and an ARE in time-dependent manners. Co-immunoprecipitation experiments revealed that, in addition to AhR-Arnt binding, TCDD induced an interaction between AhR and Nrf2 as well as Keap1. The findings reveal that TCDD induces multi protein complexes to mediate cross-interaction between the AhR and Nrf2 pathways, uncovering a novel mechanistic aspect of gene regulation by environmental chemicals through AhR and Nrf2.

Differential gene regulation by the human and mouse aryl hydrocarbon receptor

The human aryl hydrocarbon receptor (hAHR) and mouse aryl hydrocarbon receptor (mAHR(b)) share limited (58%) transactivation domain (TAD) sequence identity. Compared to the mAHR(b) allele, the hAHR displays 10-fold lower relative affinity for prototypical ligands, such as 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD). However, in previous studies, we have demonstrated that the hAHR can display a higher relative ligand-binding affinity than the mAHR(b) for specific AHR ligands, such as indirubin. Each receptor has also been shown to differentially recruit LXXLL coactivator motif proteins and to utilize different TAD subdomains in gene transactivation. Using hepatocytes isolated from C57BL/6J mice (Ahr(b/b)) and AHR(Ttr) transgenic mice, which express hAHR protein specifically in hepatocytes, we investigated whether the hAHR and mAHR(b) differentially regulate genes. DNA microarray and quantitative PCR analysis of Ahr(b/b) and AHR(Ttr) primary mouse hepatocytes treated with 10nM TCDD revealed that a number of established AHR target genes such as Cyp1a1 and Cyp1b1 are significantly induced by both receptors. Remarkably, of the 1752 genes induced by mAHR(b) and 1186 genes induced by hAHR, only 265 genes (approximately 18%) were significantly activated by both receptors in response to TCDD. Conversely, of the 1100 and 779 genes significantly repressed in mAHR(b) and hAHR hepatocytes, respectively, only 462 (approximately 49%) genes were significantly repressed by both receptors in response to TCDD treatment. Genes identified as differentially expressed are known to be involved in a number of biological pathways, including cell proliferation and inflammatory response, which suggest that compared to the mAHR(b), the hAHR may play contrasting roles in TCDD-induced toxicity and endogenous AHR-mediated gene regulation.

Differential gene regulation by the human and mouse aryl hydrocarbon receptor

The human aryl hydrocarbon receptor (hAHR) and mouse aryl hydrocarbon receptor (mAHR(b)) share limited (58%) transactivation domain (TAD) sequence identity. Compared to the mAHR(b) allele, the hAHR displays 10-fold lower relative affinity for prototypical ligands, such as 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD). However, in previous studies, we have demonstrated that the hAHR can display a higher relative ligand-binding affinity than the mAHR(b) for specific AHR ligands, such as indirubin. Each receptor has also been shown to differentially recruit LXXLL coactivator motif proteins and to utilize different TAD subdomains in gene transactivation. Using hepatocytes isolated from C57BL/6J mice (Ahr(b/b)) and AHR(Ttr) transgenic mice, which express hAHR protein specifically in hepatocytes, we investigated whether the hAHR and mAHR(b) differentially regulate genes. DNA microarray and quantitative PCR analysis of Ahr(b/b) and AHR(Ttr) primary mouse hepatocytes treated with 10nM TCDD revealed that a number of established AHR target genes such as Cyp1a1 and Cyp1b1 are significantly induced by both receptors. Remarkably, of the 1752 genes induced by mAHR(b) and 1186 genes induced by hAHR, only 265 genes (approximately 18%) were significantly activated by both receptors in response to TCDD. Conversely, of the 1100 and 779 genes significantly repressed in mAHR(b) and hAHR hepatocytes, respectively, only 462 (approximately 49%) genes were significantly repressed by both receptors in response to TCDD treatment. Genes identified as differentially expressed are known to be involved in a number of biological pathways, including cell proliferation and inflammatory response, which suggest that compared to the mAHR(b), the hAHR may play contrasting roles in TCDD-induced toxicity and endogenous AHR-mediated gene regulation.

Transgenic Humanized AHR Mouse Reveals Differences between Human and Mouse AHR Ligand Selectivity

The Aryl-hydrocarbon receptor (AHR) is a ligand activated transcription factor involved in xenobiotic metabolism. Most of the toxic effects of halogenated and non-halogenated polycyclic aromatic hydrocarbons (HAHs and PAHs respectively) are mediated by the AHR. For the AHR, a number of intra and interspecies differences exist in terms of responsiveness to the prototypical AHR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Interspecies differences in AHR ligand binding affinity has been shown to be linked to contrasting TCDD tolerance between species and among inbred strains of mice expressing different AHR alleles. Compared to the human AHR (hAHR), the mouse AHR(b) (mAHR(b)) has a ~10 fold higher affinity for typical AHR ligands. Using a transgenic humanized mouse model that expresses hAHR protein specifically in the liver, we have discovered that for certain ligands, such as indirubin, the hAHR exhibits higher relative ligand binding affinity and responsiveness compared to the mAHR(b). These findings may potentially influence the ongoing search for endogenous hAHR ligands and expand our understanding of the unique physiological role of the hAHR.

Ontogenic expression of hepatic Ahr mRNA is associated with histone H3K4 di-methylation during mouse liver development

The aryl hydrocarbon receptor (Ahr) is a xenobiotic sensor that regulates the expression of a battery of drug-metabolizing genes. However, Ahr is also important for normal liver development. The purpose of the present study was to examine the ontogeny of Ahr mRNA in mouse liver, and determine the epigenetic mechanisms regulating Ahr gene transcription during postnatal liver development. There was a 224% increase in hepatic Ahr mRNA from 2 days before birth to 45 days after birth. ChIP-on-chip analysis demonstrated that DNA methylation and histone H3K27 tri-methylation (H3K27Me3), two epigenetic marks for suppression of gene transcription, were consistently low around the Ahr gene locus. In contrast, enrichment of histone H3K4 di-methylation (H3K4Me2), a hallmark for gene activation, increased 182% from prenatal to young adult period around the Ahr gene locus. Regression analysis revealed a strong correlation between enrichment of H3K4Me2 and Ahr mRNA (r=0.91). In conclusion, postnatal H3K4Me2 enrichment positively associates with Ahr mRNA in developing mouse liver, providing a permissive chromatin state allowing Ahr gene transactivation in postnatal liver development.

Molecular basis of coiled coil coactivator recruitment by the aryl hydrocarbon receptor nuclear translocator (ARNT)

The aryl hydrocarbon receptor nuclear translocator (ARNT) serves as the obligate heterodimeric partner for bHLH-PAS proteins involved in sensing and coordinating transcriptional responses to xenobiotics, hypoxia, and developmental pathways. Although its C-terminal transactivation domain is dispensable for transcriptional activation in vivo, ARNT has recently been shown to use its N-terminal bHLH and/or PAS domains to interact with several transcriptional coactivators that are required for transcriptional initiation after xenobiotic or hypoxic cues. Here we show that ARNT uses a single PAS domain to interact with two coiled coil coactivators, TRIP230 and CoCoA. Both coactivators interact with the same interface on the ARNT PAS-B domain, located on the opposite side of the domain used to associate with the analogous PAS domain on its heterodimeric bHLH-PAS partner HIF-2alpha. Using NMR and biochemical studies, we identified the ARNT-interacting motif of one coactivator, TRIP230 as an LXXLL-like nuclear receptor box. Mutation of this motif and proximal sequences disrupts the interaction with ARNT PAS-B. Identification of this ARNT-coactivator interface illustrates how ARNT PAS-B is used to form critical interactions with both bHLH-PAS partners and coactivators that are required for transcriptional responses.

The aryl hydrocarbon receptor complex and the control of gene expression

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that controls the expression of a diverse set of genes. The toxicity of the potent AhR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin is almost exclusively mediated through this receptor. However, the key alterations in gene expression that mediate toxicity are poorly understood. It has been established through characterization of AhR-null mice that the AhR has a required physiological function, yet how endogenous mediators regulate this orphan receptor remains to be established. A picture as to how the AhR/ARNT heterodimer actually mediates gene transcription is starting to emerge. The AhR/ARNT complex can alter transcription both by binding to its cognate response element and through tethering to other transcription factors. In addition, many of the coregulatory proteins necessary for AhR-mediated transcription have been identified. Cross talk between the estrogen receptor and the AhR at the promoter of target genes appears to be an important mode of regulation. Inflammatory signaling pathways and the AhR also appear to be another important site of cross talk at the level of transcription. A major focus of this review is to highlight experimental efforts to characterize nonclassical mechanisms of AhR-mediated modulation of gene transcription.

CYP1A Induction and Human Risk Assessment: An Evolving Tale of in Vitro and in Vivo Studies: TABLE 1

CYP1A1 and 1A2 play critical roles in the metabolic activation of carcinogenic polycyclic aromatic hydrocarbons (PAHs) and heterocyclic aromatic amines/amides (HAAs), respectively, to electrophilic reactive intermediates, leading to toxicity and cancer. CYP1As are highly inducible by PAHs and halogenated aromatic hydrocarbons via aryl hydrocarbon receptor-mediated gene transcription. The impact of CYP1A induction on the carcinogenic and toxic potentials of environmental, occupational, dietary, and therapeutic chemicals has been a central focus of human risk evaluation and has broadly influenced the fields of cancer research, toxicology, pharmacology, and risk assessment over the past half-century. From the early discovery of CYP1A induction and its role in protection against chemical carcinogenesis in intact animals, to the establishment of CYP1A enzymes as the principal cytochromes P450 for bioactivation of PAHs and HAAs in in vitro assays, to the recent realization of an essential protective role of CYP1A in benzo[a]pyrene-induced lethality and carcinogenesis with CYP1A knockout mice, the understanding of the interrelation between CYP1A induction and chemical safety has followed a full circle. This unique path of CYP1A research underscores the importance of whole animal and human studies in chemical safety evaluation.

Regulatory mechanisms modulating the expression of cytochrome P450 1A1 gene by heavy metals

We recently demonstrated that heavy metals, Hg2+, Pb2+, and Cu2+ induced Cyp1a1 gene expression, yet the mechanisms involved remain unknown. To explore the molecular mechanisms involved in the modulation of Cyp1a1 by heavy metals, Hepa 1c1c7 cells were treated with the metals in the presence and absence of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a potent Cyp1a1 inducer. Time-dependent effect study showed that all metals significantly induced the basal Cyp1a1 mRNA. This was apparent 3 h after treatment, and levels remained elevated for at least 24 h. At the inducible level, Hg2+ and Pb2+ further increased, while Cu2+ decreased, the TCDD-mediated induction of Cyp1a1 mRNA. The RNA synthesis inhibitor, actinomycin D, completely blocked the Cyp1a1 induction by heavy metals. The protein synthesis inhibitor, cycloheximide, and 26S proteasome inhibitor, carbobenzoxy-L-leucyl-L-leucyl-leucinal (MG-132), super-induced the metal-mediated induction of Cyp1a1 mRNA. In addition, all three metals induced aryl hydrocarbon receptor/xenobiotic-responsive element (AhR/XRE) binding, suggesting an AhR-dependent mechanism. Cyp1a1 mRNA and protein decay experiments showed that the three metals did not significantly affect the half-life of mRNA; however, they significantly decreased the degradation rate of its protein, implying a posttranslational regulation of the Cyp1a1 by the heavy metals. A significant decrease in TCDD-mediated induction of Cyp1a1 activity associated with an increase in HO-1 mRNA and a decrease in cellular heme content was observed after all metals treatment. This suggests that heme degradation plays a role in reducing Cyp1a1 activity. This is the first demonstration that heavy metals can directly induce Cyp1a1 gene expression in an AhR-dependent manner through transcriptional and posttranslational mechanisms.

The Aryl Hydrocarbon Receptor Displaces p300 from E2F-dependent Promoters and Represses S Phase-specific Gene Expression

The environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) causes a wide range of toxic, teratogenic, and carcinogenic effects. TCDD is a ligand for the aromatic hydrocarbon receptor (AHR), a ligand-activated transcription factor believed to be the primary mediator of these effects. Activation of the AHR by TCDD also elicits a variety of effects on cell cycle progression, ranging from proliferation to arrest. In this report, we have characterized further the role of the activated AHR in cell cycle regulation. In human mammary carcinoma MCF-7 and mouse hepatoma Hepa-1 cells, TCDD treatment decreased the number of cells in S phase and caused the accumulation of cells in G(1). In Hepa-1 cells, this effect correlated with the transcriptional repression of several E2F-regulated genes required for S phase progression. AHR-mediated gene repression was dependent on its interaction with retinoblastoma protein but was independent of its transactivation function because AHR mutants lacking DNA binding or transactivation domains repressed E2F-dependent expression as effectively as wild type AHR. Overexpression of p300 suppressed retinoblastoma protein-dependent gene repression, and this effect was reversed by TCDD. Chromatin immunoprecipitation assays showed that TCDD treatment caused the recruitment of AHR to E2F-dependent promoters and the concurrent displacement of p300. These results delineate a novel mechanism whereby the AHR, a known transcriptional activator, also mediates gene repression by pathways involving combinatorial interactions at E2F-responsive promoters, leading to the repression of E2F-dependent, S phase-specific genes. The AHR seems to act as an environmental checkpoint that senses exposure to environmental toxicants and responds by signaling cell cycle inhibition.

Polymorphisms in the human AH receptor

The AH receptor (AHR) mediates toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) as well as induction of three cytochrome P450 enzymes and certain Phase II enzymes. In laboratory animals, genetic variations in the AHR lead to substantial differences in sensitivity to biochemical and toxic effects of TCDD and related compounds. Relatively few polymorphisms have been discovered in the human AHR gene; these occur predominantly in exon 10, a region that encodes a major portion of the transactivation domain of the receptor that is responsible for regulating expression of other genes. In human populations there is a wide range of variation in responses regulated by the AHR for example, induction of CYP1A1. Some variation in human responsiveness likely is due to genetically based variations in AHR structure. Thus far, however, only one pair of polymorphisms, those at codons 517 and 570, has been shown to have a clear cut and strong effect on the phenotype of an AHR-mediated response. The search continues for polymorphisms that alter AHR function because this receptor is a central factor in determining responses to important environmental contaminants and also plays a physiologic role in early development in mammals.

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.

The silencing mediator of retinoic acid and thyroid hormone receptors can interact with the aryl hydrocarbon (Ah) receptor but fails to repress Ah receptor-dependent gene expression

Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related chemicals causes a variety of tissue- and species-specific biological and toxicological effects, most of which are mediated by the aryl hydrocarbon receptor (AhR). The AhR complex is a ligand-dependent transcription factor that binds to its specific DNA recognition site as a dimer with the AhR nuclear translocator (ARNT) and activates gene transcription. Here, we have examined the ability of a nuclear corepressor, the silencing mediator of retinoic acid and thyroid hormone receptors (SMRT), to interact with and modulate AhR-dependent gene expression. Using glutathione S-transferase (GST) "pull-down" binding assays, we have mapped a major interaction between these factors to the silencing domain of SMRT and the PAS B ligand binding domain of AhR, and this interaction is unaffected by the addition of an AhR ligand. Association of SMRT with the AhR:ARNT:DNA complex was not detected by GST pull-down or gel retardation assays. Transient cotransfections of mammalian cells (Hepa1c1c7, MCF-7, and BG-1) with SMRT and a TCDD-inducible luciferase reporter containing the dioxin-responsive domain from the mouse CYP1A1 regulatory region revealed that SMRT does not repress, but enhances, AhR signaling. However, when a reporter containing a human CYP1A1 upstream region was cotransfected with SMRT into human MCF-7 cells, AhR-driven reporter activity was decreased by half, suggesting that SMRT acts on the human CYP1A1 promoter via a factor other than the AhR in MCF-7 cells. Furthermore, the interaction between SMRT and the AhR may have implications in pathways other than the AhR signaling pathway.

Identification of a critical amino acid in the aryl hydrocarbon receptor

Two aryl hydrocarbon receptors (rtAHR2alpha and rtAHR2beta) have been identified in the rainbow trout (Oncorhynchus mykiss). These receptors share 98% amino acid identity, yet their functional properties differ. Both rtAHR2alpha and rtAHR2beta bind 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), dimerize with rainbow trout ARNTb (rtARNTb), and recognize dioxin response elements in vitro. However, in a transient transfection assay the two proteins show differential ability to recognize enhancers, produce transactivation, and respond to TCDD. To identify the sequence differences that confer the functional differences between rtAHR2alpha and rtAHR2beta, we constructed chimeric rtAHRs, in which segments of one receptor form was replaced with the corresponding part from the other isoform. This approach progressively narrowed the region being examined to a single residue, corresponding to position 111 in rtAHR2beta. Altering this residue in rtAHR2beta from the lysine to glutamate found in rtAHR2alpha produced an rtAHR2beta with the properties of rtAHR2alpha. All other known AHRs resemble rtAHR2alpha and carry glutamate at this position, located at the N terminus of the PAS-A domain. We tested the effect of altering this glutamate in the human and zebrafish AHRs to lysine. This lysine substitution produced AHRs with transactivation properties that were similar to rtAHR2beta. These results identify a critical residue in AHR proteins that has an important impact on transactivation, enhancer site recognition, and regulation by ligand.

A cycloheximide-sensitive factor regulates TCDD-induced degradation of the aryl hydrocarbon receptor

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a prototype of environmental halogenated aromatic hydrocarbons, induces a rapid reduction in steady state aryl hydrocarbon receptor (AhR). Here, we analyzed the biochemical pathway and function of the downregulation. Our results reveal that TCDD downregulates the AhR protein by shortening the halflife of AhR. The TCDD-induced degradation of AhR is inhibited by MG132, a potent inhibitor of the 26S proteasome, indicating the ubiquitin-26S proteasome mediated proteolysis as a mechanism for the degradation of AhR. Furthermore, inhibition of protein synthesis by cycloheximide blocks the degradation of AhR by TCDD, suggesting a labile factor in controlling the stability of ligand-activated AhR (hence, designated as AhR degradation promoting factor, or ADPF). Analyses of nuclear AhR demonstrated that cycloheximide increases nuclear AhR protein and functional AhR/Arnt DNA-binding complex, resulting in superinduction of CYP1A1. Lastly, genetic analyses by using AhR- or Arnt-defective variant cells demonstrate that superinduction by cycloheximide requires the transcription activation (TA) domain of AhR, implicating the TA domain in the control of AhR turnover by ADPF. These findings provide new insights into the mechanism by which TCDD-activated AhR is regulated in nucleus through the 26S proteasome protein degradation pathway.

Definition of a dioxin receptor mutant that is a constitutive activator of transcription: delineation of overlapping repression and ligand binding functions within the PAS domain

The intracellular dioxin (aryl hydrocarbon) receptor is a ligand-activated transcription factor that mediates the adaptive and toxic responses to environmental pollutants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin and structurally related congeners. Whereas the ligand-free receptor is characterized by its association with the molecular chaperone hsp90, exposure to ligand initiates a multistep activation process involving nuclear translocation, dissociation from the hsp90 complex, and dimerization with its partner protein Arnt. In this study, we have characterized a dioxin receptor deletion mutant lacking the minimal ligand-binding domain of the receptor. This mutant did not bind ligand and localized constitutively to the nucleus. However, this protein was functionally inert since it failed to dimerize with Arnt and to bind DNA. In contrast, a dioxin receptor deletion mutant lacking the minimal PAS B motif but maintaining the N-terminal half of the ligand-binding domain showed constitutive dimerization with Arnt, bound DNA, and activated transcription in a ligand-independent manner. Interestingly, this mutant showed a more potent functional activity than the dioxin-activated wild-type receptor in several different cell lines. In conclusion, the constitutively active dioxin receptor may provide an important mechanistic tool to investigate receptor-mediated regulatory pathways in closer detail.

The AH receptor of the most dioxin-sensitive species, guinea pig, is highly homologous to the human AH receptor

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) brings about a wide spectrum of toxic and biochemical changes, most of which are mediated by the AH receptor (AHR). Recent cloning of the AHR from the two most TCDD-resistant laboratory animals, Han/Wistar (Kuopio) rats and hamsters, suggested a critical role for the C-terminal transactivation domain structure in TCDD sensitivity. Here we cloned the AHR from the most TCDD-susceptible species, guinea pig. The N-terminus of its AHR was highly similar to that in the resistant animals. However, the C-terminal Q-rich subdomain was only about half the size of this subunit in the hamster AHR. There was a distinct correlation across published mammalian species between the number of glutamine residues in the Q-rich subdomain and sensitivity to the acute lethality of TCDD. The closest homolog of the Guinea pig receptor turned out to be the human AHR, which may be relevant for dioxin risk assessment.

Dioxin-inducible transactivation in a chromosomal setting. Analysis of the acidic domain of the Ah receptor

We analyzed the transactivation function of the acidic segment of the Ah receptor (amino acids 515-583) by reconstituting AhR-defective mouse hepatoma cells with mutants. Our data reveal that both hydrophobic and acidic residues are important for transactivation and that these residues are clustered in two regions of the acidic segment of AhR. Both regions are crucial for function, because disruption of either one substantially impairs transactivation of the chromosomal CYP1A1 target gene. Neither region contains an amino acid motif that resembles those reported for other acidic activation domains. Furthermore, proline substitutions in both regions do not impair transactivation in vivo, a finding that implies that alpha-helix formation is not required for function.

Induction of cytochrome P4501A1

Cytochrome P4501A1 is a substrate-inducible microsomal enzyme that oxygenates polycyclic aromatic hydrocarbons, such as the carcinogen benzo(a)pyrene, as the initial step in their metabolic processing to water-soluble derivatives. Enzyme induction reflects increased transcription of the cognate CYP1A1 gene. The environmental toxicant 2,3,7,8-tetrachlorodibenzo-p-dioxin is the most potent known cytochrome P4501A1 inducer. Two regulatory proteins, the aromatic (aryl) hydrocarbon receptor (AhR) and the AhR nuclear translocator (Arnt), mediate induction. AhR and Arnt are prototypical members of the basic helix-loop-helix/Per-Arnt-Sim class of transcription factors. Mechanistic analyses of cytochrome P4501A1 induction provide insights into ligand-dependent mammalian gene expression, basic helix-loop-helix/Per-Arnt-Sim protein function, and dioxin action; such studies also impact public health issues concerned with molecular epidemiology, carcinogenesis, and risk assessment.

Proteasome inhibition induces nuclear translocation and transcriptional activation of the dioxin receptor in mouse embryo primary fibroblasts in the absence of xenobiotics

The aryl hydrocarbon receptor (AHR) is a transcription factor that is highly conserved during evolution and shares important structural features with the Drosophila developmental regulators Sim and Per. Although much is known about the mechanism of AHR activation by xenobiotics, little information is available regarding its activation by endogenous stimuli in the absence of exogenous ligand. In this study, using embryonic primary fibroblasts, we have analyzed the role of proteasome inhibition on AHR transcriptional activation in the absence of xenobiotics. Proteasome inhibition markedly reduced cytosolic AHR without affecting its total cellular content. Cytosolic AHR depletion was the result of receptor translocation into the nuclear compartment, as shown by transient transfection of a green fluorescent protein-tagged AHR and by immunoblot analysis of nuclear extracts. Gel retardation experiments showed that proteasome inhibition induced transcriptionally active AHR-ARNT heterodimers able to bind to a consensus xenobiotic-responsive element. Furthermore, nuclear AHR was transcriptionally active in vivo, as shown by the induction of the endogenous target gene CYP1A2. Synchronized to AHR activation, proteasome inhibition also induced a transient increase in AHR nuclear translocator (ARNT) at the protein and mRNA levels. Since nuclear levels of AHR and ARNT are relevant for AHR transcriptional activation, our data suggest that proteasome inhibition, through a transient increase in ARNT expression, could promote AHR stabilization and accumulation into the nuclear compartment. An elevated content of nuclear AHR could favor AHR-ARNT heterodimers able to bind to xenobiotic-responsive elements and to induce gene transcription in the absence of xenobiotics. Thus, depending on the cellular context, physiologically regulated proteasome activity could participate in the control of endogenous AHR functions.

The PAS superfamily: sensors of environmental and developmental signals

Over the past decade, PAS domains have been identified in dozens of signal transduction molecules and various forms have been found in animals, plants, and prokaryotes. In this review, we summarize this rapidly expanding research area by providing a detailed description of three signal transduction pathways that utilize PAS protein heterodimers to drive their transcriptional output. It is hoped that these model pathways can provide a framework for use in understanding the biology of the less well-understood members of this emerging superfamily, as well as of those to be characterized in the days to come. We use this review to develop the idea that most eukaryotic PAS proteins can be classified by functional similarities, as well as by predicted phylogenetic relationships. We focus on the alpha-class proteins, which often act as sensors of environmental signals, and the beta-class proteins, which typically act as broad-spectrum partners that target these heterodimers to their genomic targets.

Bilirubin and bile acids may modulate their own metabolism via regulating uridine diphosphate-glucuronosyltransferase expression in the rat

BACKGROUND AND AIMS

Uridine diphosphate (UDP)-glucuronosyltransferase (UGT) is a critical enzyme in the elimination of bilirubin and it also plays a role in the metabolism of bile acids. The aim of this study was to determine whether bilirubin and bile acids could modulate their own metabolism by regulating UGT levels in cultured rat hepatocytes.

METHODS AND RESULTS

Incubation of hepatocytes with bilirubin (48 micromol/L) for 24 h significantly increased the mRNA expression of UGT1A1 and UGT1A5, two UGT isoforms responsible for the conjugation of bilirubin. The induction of UGT1A1 and UGT1A5 by bilirubin was concentration and time dependent. Treatment with chenodeoxycholic acid, cholic acid, deoxycholic acid, hyodeoxycholic acid and lithocholic acid at a concentration of 100 micromol/L for 48 h significantly enhanced the mRNA expression of UGT2B1, a UGT isoform responsible for the glucuronidation of bile acids. The UGT2B3 mRNA level was also increased by hyodeoxycholic acid. The regulation of UGT2B1 mRNA by chenodeoxycholic acid and hyodeoxycholic acid was dose and time dependent.

CONCLUSION

Our results suggest that bilirubin and bile acids can induce UGT expression and as a result, these compounds may modulate their own metabolism. Such regulation could play a compensatory role in the pathological increased concentrations of these compounds in some hepatobiliary diseases.

Restructured transactivation domain in hamster AH receptor

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

An aryl hydrocarbon receptor conformation acts as the functional core of nuclear dioxin signaling

DNA-complexed heterodimers of the aryl hydrocarbon receptor (AhR) with the Ah receptor nuclear translocator (Arnt) are the molecular switches for nuclear signaling of 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD). AhR-Arnt heterodimers regulate genes involved in the metabolism of xenobiotics or fatty acids and various genes important for growth and differentiation. In this report several potent methods, such as the limited protease digestion, gel shift and gel shift clipping assays, allowed the investigation of ligand-stabilized conformations of AhR monomers in comparison to that of AhR-Arnt heterodimers. Interestingly, the ligand sensitivity of monomeric AhR was found to be very low at 25 nM, whereas DNA-dependent methods consistently provided EC(50) values between 0.12 and 0.6 nM for AhR in a heterodimeric complex, i. e. an approximate 100-fold higher ligand sensitivity. This indicates that complex formation of AhR with Arnt on DNA is an important and critical step in transforming AhR into a high affinity receptor for TCDD. A comparison of wild-type AhR with different C-terminal receptor truncations suggests that the PAS-B subregion of its PAS domain is of central importance for stabilization of a functional, i. e. ligand-sensitive, AhR-Arnt conformation, whereas the PAS-A subregion appears to be critical for dimerization of AhR and Arnt. In conclusion, the results of this study provide important information on the ligand sensitivity of AhR and AhR-Arnt heterodimer conformations.

Dioxin-induced perturbations in tryptophan homeostasis in laboratory animals

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

Superinduction of CYP1A1 gene expression. Regulation of 2,3,7, 8-tetrachlorodibenzo-p-dioxin-induced degradation of Ah receptor by cycloheximide

Cycloheximide superinduces the transcription of CYP1A1 in the presence of an agonist for the Ah receptor (AhR). To investigate the molecular target for "superinduction," we analyzed the agonist-induced degradation of AhR. Whereas 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD), a potent agonist of AhR, induces a rapid reduction of the AhR protein, cycloheximide blocks the down-regulation of steady state AhR. Analyses of the turnover of AhR reveal that cycloheximide blocks the shortening of the half-life of AhR by TCDD. Blocking of the TCDD-induced AhR degradation requires inhibition of protein synthesis, because (a) cycloheximide inhibits protein synthesis at the concentration at which it causes superinduction and inhibition of AhR degradation; and (b) puromycin, an inhibitor of protein synthesis by mimicking aminoacyl-tRNA, also blocks the TCDD-induced AhR degradation. The blocking of the TCDD-induced AhR degradation correlates with the superinduction of CYP1A1 gene expression in a time- and dose-dependent manner. Furthermore, cycloheximide is shown to increase the accumulation of the TCDD-activated AhR and the functional AhR x Arnt complex in nucleus. Collectively, our results reveal a mechanism of superinduction by cycloheximide by enhancing the stability of agonist-activated AhR. The finding that inhibition of protein synthesis blocks the TCDD-induced AhR turnover implicates a cycloheximide-sensitive, labile factor (designated as AhR degradation promoting factor, or ADPF) in controlling the removal of agonist-activated AhR in nucleus.

2,3,7,8-tetrachlorodibenzo-p-dioxin-induced degradation of aryl hydrocarbon receptor (AhR) by the ubiquitin-proteasome pathway. Role of the transcription activaton and DNA binding of AhR

Activation of the aryl hydrocarbon receptor (AhR) by 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD), a potent agonist of AhR, induces a marked reduction in steady state AhR. To analyze the mechanism of regulation of ligand-activated AhR, we examined the biochemical pathway and function of the down-regulation of the receptor by TCDD. Pulse-chase experiments reveal that TCDD shortens the half-life (t1/2) of AhR from 28 to 3 h in mouse hepatoma cells. Inhibitors of the 26 S proteasome, lactacystin and MG132, block the TCDD-induced turnover of AhR. The TCDD-induced degradation of AhR involves ubiquitination of the AhR protein, because (a) TCDD induces formation of high molecular weight, ubiquitinated AhR and (b) degradation of AhR is inhibited in ts20 cells, which bear a temperature-sensitive mutation in the ubiquitin-activating enzyme E1, at a nonpermissive temperature. Inhibition of proteasomal degradation of AhR increases the amount of the nuclear AhR.Arnt complex and "superinduces" the expression of endogenous CYP1A1 gene by TCDD, indicating that the proteasomal degradation of AhR serves as a mechanism for controlling the activity of the activated receptor. We also show that deletion of the transcription activation domain of AhR abolishes the degradation, whereas a mutation in the DNA-binding region of AhR or Arnt reduces the degradation; these data implicate the transcription activation domain and DNA binding in AhR degradation. Our findings provide new insights into the regulation of TCDD-activated AhR through ubiquitin-mediated protein degradation.

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

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

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

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

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

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

Multiple roles of ligand in transforming the dioxin receptor to an active basic helix-loop-helix/PAS transcription factor complex with the nuclear protein Arnt

The dioxin receptor is a ligand-activated transcription factor belonging to an emerging class of basic helix-loop-helix/PAS proteins which show interaction with the molecular chaperone hsp90 in their latent states and require heterodimerization with a general cofactor, Arnt, to form active DNA binding complexes. Upon binding of polycyclic aromatic hydrocarbons typified by dioxin, the dioxin receptor translocates from the cytoplasm to the nucleus to allow interaction with Arnt. Here we have bypassed the nuclear translocation step by creating a cell line which expresses a constitutively nuclear dioxin receptor, which we find remains in a latent form, demonstrating that ligand has functional roles beyond initiating nuclear import of the receptor. Treatment of the nuclear receptor with dioxin induces dimerization with Arnt to form an active transcription factor complex, while in stark contrast, treatment with the hsp90 ligand geldanamycin results in rapid degradation of the receptor. Inhibition of degradation by a proteasome inhibitor allowed geldanamycin to transform the nuclear dioxin receptor to a heterodimer with Arnt (DR-Arnt). Our results indicate that unchaperoned dioxin receptor is extremely labile and is consistent with a concerted nuclear mechanism for receptor activation whereby hsp90 is released from the ligand-bound dioxin receptor concomitant with Arnt dimerization. Strikingly, artificial transformation of the receptor by geldanamycin provided a DR-Arnt complex capable of binding DNA but incapable of stimulating transcription. Limited proteolysis of DR-Arnt heterodimers indicated different conformations for dioxin versus geldanamycin-transformed receptors. Our studies of intracellular dioxin receptor transformation indicate that ligand plays multiple mechanistic roles during receptor activation, being important for nuclear translocation, transformation to an Arnt heterodimer, and maintenance of a structural integrity key for transcriptional activation.

Two forms of aryl hydrocarbon receptor type 2 in rainbow trout (Oncorhynchus mykiss). Evidence for differential expression and enhancer specificity

Two aryl hydrocarbon receptors (AhRs), rtAhR2alpha and rtAhR2beta, were cloned from rainbow trout (rt) cDNA libraries. The distribution of sequence differences, genomic Southern blot analysis, and the presence of both transcripts in all individual rainbow trout examined suggest that the two forms of rtAhR2 are derived from separate genes. The two rtAhR2s have significant sequence similarity with AhRs cloned from mammalian species, especially in the basic helix-loop-helix and PAS functional domains located in the amino-terminal 400 amino acids of the protein. In contrast, the Gln-rich transactivation domain found in the carboxyl-terminal half of mammalian AhRs is absent from both rtAhR2s. Both clones were expressed by in vitro transcription/translation and proteins of approximately 125 kDa were produced. These proteins bind 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) and are able to bind dioxin response elements in gel shift assays. rtAhR2alpha and rtAhR2beta are expressed in a tissue-specific manner with the highest expression of rtAhR2beta in the heart. Expression of rtAhR2alpha and rtAhR2beta mRNAs is positively regulated by TCDD. Both rtAhR2alpha and rtAhR2beta produced TCDD-dependent activation of a reporter gene driven by dioxin response elements. Surprisingly, the two receptors showed distinct preferences for different enhancer sequences. These results suggest that the two receptor forms may regulate different sets of genes, and may play different roles in the toxic responses produced by AhR agonists such as TCDD.

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.

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

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

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

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

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

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

Protein kinase C activity is required for aryl hydrocarbon receptor pathway-mediated signal transduction

The role of protein kinase C (PKC) in the human aryl hydrocarbon receptor (hAhR) signal transduction pathway was examined in cell lines stably transfected with pGUDLUC6.1, in which luc+ is solely controlled by four dioxin-responsive elements (DREs). These cell lines, P5A11 and HG40/6, were derived from HeLa and HepG2 cells respectively. Simultaneous treatment of these cells with 2,3,7,8, -tetrachlorodibenzo-p-dioxin (TCDD) and phorbol-12-myristate-13-acetate (PMA) enhanced trans-activation of the reporter construct several-fold relative to cells treated with TCDD alone. PKC inhibitors block the PMA effect and hAhR-mediated signal transduction, demonstrating these processes require PKC activity. Examination of other independently generated, HeLa-derived cell lines stably transfected with pGUDLUC6.1 demonstrates the PMA effect in P5A11 cells is not a clonal artifact. Transient transfections indicate the PMA effect is not due to a luciferase message/gene product stabilization mechanism or stimulation of the basal transcription machinery. Examination of cytosolic preparations demonstrates PKC stimulation or inhibition does not alter hAhR and hAhR nuclear translocator protein levels or TCDD-induced down-regulation of hAhR levels. Similarly, examination of nuclear extracts indicated PKC stimulation or inhibition does not alter nuclear AhR levels or hAhR/hAhR nuclear translocator protein heterodimer DRE-binding activity as assessed by electrophoretic mobility shift assay. These results demonstrate a PKC-mediated event is required for the hAhR to form a functional transcriptional complex that leads to trans-activation and that the DRE is the minimal DNA element required for PMA to enhance AhR-mediated trans-activation.

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

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