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

Dimerization Rules of Mammalian PAS Proteins

The PAS (PER, ARNT, SIM) protein family plays a vital role in mammalian biology and human disease. This analysis arose from an interest in the signaling mechanics by the Ah receptor (AHR) and the Ah receptor nuclear translocator (ARNT). After more than fifty years by studying this and related mammalian sensor systems, describing the role of PAS domains in signal transduction is still challenging. In this perspective, we attempt to interpret recent studies of mammalian PAS protein structure and consider how this new insight might explain how these domains are employed in human signal transduction with an eye towards developing strategies to target and engineer these molecules for a new generation of therapeutics. Our approach is to integrate our understanding of PAS protein history, cell biology, and molecular biology with recent structural discoveries to help explain the mechanics of mammalian PAS protein signaling. As a learning set, we focus on sequences and crystal structures of mammalian PAS protein dimers that can be visualized using readily available software.

Role of Ubiquitination and Epigenetics in the Regulation of AhR Signaling in Carcinogenesis and Metastasis: "Albatross around the Neck" or "Blessing in Disguise"

The molecular mechanisms and signal transduction cascades evoked by the activation of aryl hydrocarbon receptor (AhR) are becoming increasingly understandable. AhR is a ligand-activated transcriptional factor that integrates environmental, dietary and metabolic cues for the pleiotropic regulation of a wide variety of mechanisms. AhR mediates transcriptional programming in a ligand-specific, context-specific and cell-type-specific manner. Pioneering cutting-edge research works have provided fascinating new insights into the mechanistic role of AhR-driven downstream signaling in a wide variety of cancers. AhR ligands derived from food, environmental contaminants and intestinal microbiota strategically activated AhR signaling and regulated multiple stages of cancer. Although AhR has classically been viewed and characterized as a ligand-regulated transcriptional factor, its role as a ubiquitin ligase is fascinating. Accordingly, recent evidence has paradigmatically shifted our understanding and urged researchers to drill down deep into these novel and clinically valuable facets of AhR biology. Our rapidly increasing realization related to AhR-mediated regulation of the ubiquitination and proteasomal degradation of different proteins has started to scratch the surface of intriguing mechanisms. Furthermore, AhR and epigenome dynamics have shown previously unprecedented complexity during multiple stages of cancer progression. AhR not only transcriptionally regulated epigenetic-associated molecules, but also worked with epigenetic-modifying enzymes during cancer progression. In this review, we have summarized the findings obtained not only from cell-culture studies, but also from animal models. Different clinical trials are currently being conducted using AhR inhibitors and PD-1 inhibitors (Pembrolizumab and nivolumab), which confirm the linchpin role of AhR-related mechanistic details in cancer progression. Therefore, further studies are required to develop a better comprehension of the many-sided and "diametrically opposed" roles of AhR in the regulation of carcinogenesis and metastatic spread of cancer cells to the secondary organs.

Biochemical properties of human full-length aryl hydrocarbon receptor (AhR)

The aryl hydrocarbon receptor (AhR) is a very unstable protein. AhR binds to the molecular chaperone complex (HSP90-p23-XAP2) to maintain a stable structure in the cytoplasm. After binding to ligands, such as dioxin, AhR translocates from the cytoplasm to the nucleus with a molecular chaperone complex. The protein forms a heterodimer with Arnt after nuclear transfer, functions as a transcription factor by binding to a xenobiotic responsive element (XRE), and induces the cytochrome P450 1A1 (CYP1A1). Because of the unstable protein, expression of the full-length AhR in the E. coli expression system is very difficult. Many studies investigated AhR using AhR domains in vitro. We expressed and purified the human full-length AhR in E. coli expression system. Furthermore, specific antibodies were prepared. Purified full-length AhR could bind to ligand. In the presence of ligand, α-helix and random coil of AhR increased and β-sheet decreased on CD spectrum. Full-length AhR could bind to HSP90, XAP2 and p23 in the presence or absence of ligand. We now show the biochemical properties of full-length AhR.

p23 protects the human aryl hydrocarbon receptor from degradation via a heat shock protein 90-independent mechanism

The aryl hydrocarbon receptor (AHR) is a ligand-activated signaling molecule which is involved in diverse biological functions ranging from cancer metastasis to immune regulation. This receptor forms a cytoplasmic complex with Hsp90, p23, and XAP2. We have previously reported that down-regulation of p23 triggers degradation of the AHR protein, uncovering a potentially dynamic event which controls the cellular AHR levels without ligand treatment. Here we investigate the underlying mechanisms for this p23 effect using wild-type HeLa and the p23 knockdown HeLa cells. Reduction of the Hsp90 and XAP2 contents, however, did not affect the AHR protein levels, implying that this p23 effect on AHR is more than just alteration of the cytoplasmic complex dynamics. Association of p23 with Hsp90 is not important for the modulation of the AHR levels since exogenous expression of p23 mutants with modest Hsp90-binding affinity effectively restored the AHR message and protein levels. The protein folding property of p23 which resides at the terminal 50-amino acid region is not involved for this p23 effect. Results from our interaction study using the affinity purified thioredoxin fusion proteins and GST fusion proteins showed that p23 directly interacts with AHR and the interaction surface lies within AHR amino acid 1-216 and p23 amino acid 1-110. Down-regulation of the p23 protein content promotes the ubiquitination of AHR, indicating that p23 protects AHR from the ubiquitin-meditated protein degradation.

Structural hierarchy controlling dimerization and target DNA recognition in the AHR transcriptional complex

The aryl hydrocarbon receptor (AHR) belongs to the PAS (PER-ARNT-SIM) family transcription factors and mediates broad responses to numerous environmental pollutants and cellular metabolites, modulating diverse biological processes from adaptive metabolism, acute toxicity, to normal physiology of vascular and immune systems. The AHR forms a transcriptionally active heterodimer with ARNT (AHR nuclear translocator), which recognizes the dioxin response element (DRE) in the promoter of downstream genes. We determined the crystal structure of the mammalian AHR-ARNT heterodimer in complex with the DRE, in which ARNT curls around AHR into a highly intertwined asymmetric architecture, with extensive heterodimerization interfaces and AHR interdomain interactions. Specific recognition of the DRE is determined locally by the DNA-binding residues, which discriminates it from the closely related hypoxia response element (HRE), and is globally affected by the dimerization interfaces and interdomain interactions. Changes at the interdomain interactions caused either AHR constitutive nuclear localization or failure to translocate to nucleus, underlying an allosteric structural pathway for mediating ligand-induced exposure of nuclear localization signal. These observations, together with the global higher flexibility of the AHR PAS-A and its loosely packed structural elements, suggest a dynamic structural hierarchy for complex scenarios of AHR activation induced by its diverse ligands.

Binding studies using Pichia pastoris expressed human aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator proteins

The aryl hydrocarbon receptor (AHR) is a transcription factor which activates gene transcription by binding to its corresponding enhancer as the heterodimer, which is consisted of AHR and the aryl hydrocarbon receptor nuclear translocator (ARNT). Human AHR can be rather difficult to study, when compared among the AHR of other species, since it is relatively unstable and less sensitive to some ligands in vitro. Overexpression of human AHR has been limited to the baculovirus expression, which is costly and tedious due to the need of repetitive baculovirus production. Here we explored whether we could generate abundant amounts of human AHR and ARNT in a better overexpression system for functional study. We observed that human AHR and ARNT can be expressed in Pichia pastoris with yields that are comparable to the baculovirus system only if their cDNAs are optimized for Pichia expression. Fusion with a c-myc tag at their C-termini seems to increase the expression yield. These Pichia expressed proteins can effectively heterodimerize and form the ternary AHR/ARNT/enhancer complex in the presence of β-naphthoflavone or kynurenine. Limited proteolysis using thermolysin can be used to study the heterodimerization of these human AHR and ARNT proteins.

Differential suppression of the aryl hydrocarbon receptor nuclear translocator-dependent function by an aryl hydrocarbon receptor PAS-A-derived inhibitory molecule

The aryl hydrocarbon receptor (AhR) heterodimerizes with the aryl hydrocarbon receptor nuclear translocator (Arnt) for transcriptional regulation. We generated three N-terminal deletion constructs of the human AhR of 12-24 kDa in size--namely D1, D2, and D3--to suppress the Arnt function. We observed that all three deletions interact with the human Arnt with similar affinities. D2, which contains part of the AhR PAS-A domain and interacts with the PAS-A domain of Arnt, inhibits the formation of the AhR gel shift complex. D2 suppresses the 3-methylcholanthrene-induced, dioxin response element (DRE)-driven luciferase activity in Hep3B cells and exogenous Arnt reverses this D2 suppression. D2 suppresses the induction of CYP1A1 at both the message and protein levels in Hep3B cells; however, the CYP1B1 induction is not affected. D2 suppresses the recruitment of Arnt to the cyp1a1 promoter but not to the cyp1b1 promoter, partly because the AhR/Arnt heterodimer binds better to the cyp1b1 DRE than to the cyp1a1 DRE. Interestingly, D2 has no effect on the cobalt chloride-induced, hypoxia inducible factor-1 (HIF-1)-dependent expression of vegf, aldolase c, and ldh-a messages. Our data reveal that the flanking sequences of the DRE contribute to the binding affinity of the AhR/Arnt heterodimer to its endogenous enhancers and the function of AhR and HIF-1 can be differentially suppressed by the D2 inhibitory molecule.

Identification of the Ah-receptor structural determinants for ligand preferences

The aryl hydrocarbon receptor (AHR) is a transcription factor that responds to diverse ligands and plays a critical role in toxicology, immune function, and cardiovascular physiology. The structural basis of the AHR for ligand promiscuity and preferences is critical for understanding AHR function. Based on the structure of a closely related protein HIF2α, we modeled the AHR ligand binding domain (LBD) bound to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and benzo(a)pyrene (BaP) and identified residues that control ligand preferences by shape and H-bond potential. Mutations to these residues, particularly Q377 and G298, resulted in robust and opposite changes in the potency of TCDD and BaP and up to a 20-fold change in the ratio of TCDD/BaP efficacy. The model also revealed a flexible "belt" structure; molecular dynamic (MD) simulation suggested that the "belt" and several other structural elements in the AHR-LBD are more flexible than HIF2α and likely contribute to ligand promiscuity. Molecular docking of TCDD congeners to a model of human AHR-LBD ranks their binding affinity similar to experimental ranking of their toxicity. Our study reveals key structural basis for prediction of toxicity and understanding the AHR signaling through diverse ligands.

Suppression of the hypoxia inducible factor-1 function by redistributing the aryl hydrocarbon receptor nuclear translocator from nucleus to cytoplasm

The aryl hydrocarbon receptor nuclear translocator (ARNT) heterodimerizes with hypoxia inducible factor-1α (HIF-1α), followed by upregulation of genes that are essential for carcinogenesis. We utilized a novel peptide (Ainp1) to address whether the HIF-1α signaling could be suppressed by an ARNT-mediated mechanism. Ainp1 suppresses the HIF-1α-dependent luciferase expression in Hep3B cells and this suppression can be reversed by ARNT. Ainp1 reduces the interaction between ARNT and HIF-1α, suppresses the formation of the HIF-1 gel shift complex, and suppresses the ARNT recruitment to the vegf promoter. These effects are partly mediated by redistribution of the nuclear ARNT contents to the cytoplasm.

Beta tubulin affects the aryl hydrocarbon receptor function via an Arnt-mediated mechanism

We have been studying the requirement for the aryl hydrocarbon receptor nuclear translocator (Arnt)-dependent DNA complex formation, which precedes the activation of gene transcription. Using DEAE chromatography, we have obtained a Sf9 insect fraction F5 that is highly enriched with beta-tubulin. F5 inhibits the formation of the AhR gel shift complex and this inhibition is sensitive to protease, suggesting that proteins that are present in this F5 fraction are responsible for the inhibition. Additional experiments have revealed that this inhibition is less pronounced in the presence of anti-beta-tubulin IgG and beta-tubulin enriched fraction from pig brain also inhibits the AhR gel shift formation. Sf9 beta-tubulin interacts with Arnt and suppresses the binding of the AhR/Arnt heterodimer to its corresponding enhancer. Human beta4-tubulin, which shares high sequence identity with Sf9 beta-tubulin, suppresses the AhR-dependent luciferase expression by reducing the nuclear Arnt content and retaining Arnt in the cytoplasm. Fluorescence studies using the GFP fusion of human beta4-tubulin have revealed that beta4-tubulin prevents the localization of Arnt in Sf9 cells. Here we have provided evidence suggesting that beta-tubulin may regulate the physiological content of Arnt.

Regulation of aryl hydrocarbon receptor function by selective estrogen receptor modulators

Selective estrogen receptor modulators (SERMs), such as tamoxifen (TAM), have been used extensively for the treatment and prevention of breast cancer and other pathologies associated with aberrant estrogen receptor (ER) signaling. These compounds exhibit cell-selective agonist/antagonist activities as a consequence of their ability to induce different conformational changes in ER, thereby enabling it to recruit functionally distinct transcriptional coregulators. However, the observation that SERMs can also regulate aspects of calcium signaling and apoptosis in an ER-independent manner in some systems suggests that some of the activity of drugs within this class may also arise as a consequence of their ability to interact with targets other than ER. In this study, we demonstrate that 4-hydroxy-TAM (4OHT), an active metabolite of TAM, directly binds to and modulates the transcriptional activity of the aryl hydrocarbon receptor (AHR). Of specific interest was the observation, that in the absence of ER, 4OHT can induce the expression of AHR target genes involved in estradiol metabolism, cellular proliferation, and metastasis in cellular models of breast cancer. The potential role for AHR in SERM pharmacology was further underscored by the ability of 4OHT to suppress osteoclast differentiation in vitro in part through AHR. Cumulatively, these findings provide evidence that it is necessary to reevaluate the relative roles of ER and AHR in manifesting the pharmacological actions and therapeutic efficacy of TAM and other SERMs.

Role of aryl hydrocarbon receptor nuclear translocator in KATP channel-mediated insulin secretion in INS-1 insulinoma cells

Aryl hydrocarbon receptor nuclear translocator (ARNT) has been known to participate in cellular responses to xenobiotic and hypoxic stresses, as a common partner of aryl hydrocarbon receptor and hypoxia inducible factor-1/2alpha. Recently, it was reported that ARNT is essential for adequate insulin secretion in response to glucose input and that its expression is downregulated in the pancreatic islets of diabetic patients. In the present study, the authors addressed the mechanism by which ARNT regulates insulin secretion in the INS-1 insulinoma cell line. In ARNT knock-down cells, basal insulin release was elevated, but insulin secretion was not further stimulated by a high-glucose challenge. Electrophysiological analyses revealed that glucose-dependent membrane depolarization was impaired in these cells. Furthermore, K(ATP) channel activity and expression were reduced. Of two K(ATP) channel subunits, Kir6.2 was found to be positively regulated by ARNT at the mRNA and protein levels. Based on these results, the authors suggest that ARNT expresses K(ATP) channel and by so doing regulates glucose-dependent insulin secretion.

Recombinant expression of aryl hydrocarbon receptor for quantitative ligand-binding analysis

Recombinant expression of the aryl hydrocarbon receptor (AhR) yields small amounts of ligand-binding-competent AhR. Therefore, Spodoptera frugiperda (Sf9) cells and baculovirus have been evaluated for high-level and functional expression of AhR. Rat and human AhR were expressed as soluble protein in significant amounts. Expression of ligand-binding-competent AhR was sensitive to the protein concentration of Sf9 extract, and coexpression of the chaperone p23 failed to affect the yield of functional ligand-binding AhR. The expression system yielded high levels of functional protein, with the ligand-binding capacity (Bmax) typically 20-fold higher than that obtained with rat liver cytosol. Quantitative estimates of the ligand-binding affinity of human and rat AhR were obtained; the Kd for recombinant rat AhR was indistinguishable from that of native rat AhR, thereby validating the expression system as a faithful model for native AhR. The human AhR bound TCDD with significantly lower affinity than the rat AhR. These findings demonstrate high-level expression of ligand-binding-competent AhR, and sufficient AhR for quantitative analysis of ligand binding.

Cyclophilin-40 has a cellular role in the aryl hydrocarbon receptor signaling

Cyclophilin-40 (CyP40) promotes the formation of the gel shift complex that contains the aryl hydrocarbon receptor (AhR), AhR nuclear translocator (Arnt) and dioxin response element (DRE) using baculovirus expressed proteins. Here we reported that CyP40 plays a role in the AhR signaling. When the CyP40 content in MCF-7 cells is reduced, up-regulation of cyp1a1 and cyp1b1 by 3-methylchloranthrene (3MC) is also reduced, suggesting that CyP40 is essential for maximal AhR function. The CyP40 region containing amino acids 186-215, but not the peptidyl-prolyl cis-trans isomerase and tetratricopeptide repeat domains, is essential for forming the AhR/Arnt/DRE complex. CyP40 is found in the cell nucleus after 3MC treatment and appears to promote the DRE binding form of the AhR/Arnt heterodimer.

A novel Arnt-interacting protein Ainp2 enhances the aryl hydrocarbon receptor signalling

In an effort to better understand the Ah receptor nuclear translocator (Arnt)-dependent signaling mechanisms, we employed a phage display system to identify Arnt-interacting peptides. Human liver cDNA library was utilized to screen for Arnt-interacting peptides using an Arnt construct fused to thioredoxin (TH-ArntCDelta418). Two clones, namely Ainp1 and Ainp2 (Arnt-interacting peptide), were identified and subsequently Ainp2 was further characterized. Ainp2 interacts with TH-ArntCDelta418 in the GST pull-down and mammalian two-hybrid assays. Northern blot results revealed that Ainp2 is predominantly expressed in human liver. The putative full-length Ainp2 cDNA sequence was subsequently cloned using RACE PCR. Endogenous expression of Ainp2 was found in Jurkat cells at the mRNA and protein levels. Results from the transient transfection studies using a DRE-driven reporter plasmid and the real-time QPCR experiments examining the endogenous CYP1A1 expression showed that Ainp2 enhances the 3-methylchloranthrene-induced activity in HepG2 cells, suggesting that Ainp2 plays a role in the Arnt-dependent function

CyP40, but not Hsp70, in rabbit reticulocyte lysate causes the aryl hydrocarbon receptor-DNA complex formation

Upon ligand binding, the aryl hydrocarbon receptor (AhR) translocates into the nucleus and dimerizes with its partner aryl hydrocarbon receptor nuclear translocator (Arnt). The AhR-Arnt heterodimer binds to the dioxin response element (DRE) to regulate target gene expression. Using baculovirus expressed human AhR and Arnt, we showed that the formation of the ligand-dependent AhR-Arnt-DRE complex requires protein factors in vitro. Recently, we provided evidence that p23, an Hsp90-associated protein, is involved in the complex formation. The aim of this study was to determine whether two other Hsp90-associated proteins present in rabbit reticulocyte lysate (RRL), namely CyP40 and Hsp70, play any role in forming the AhR-Arnt-DRE complex. Fractionation and immunodepletion experiments revealed that Hsp70 is not necessary for the formation of this complex. In contrast, CyP40 is involved in forming the complex since (1) immunodepletion of CyP40 from a RRL fraction reduces the intensity of the AhR-Arnt-DRE complex by 48% and (2) recombinant human CyP40 alone causes the formation of this complex. In addition, CyP40-interacting proteins appear to be essential for the full CyP40 effect on the AhR gel shift complex.

A maternal Ahr null genotype sensitizes embryos to chemical teratogenesis

The aryl hydrocarbon receptor (encoded by the Ahr locus) is a ligand-activated transcription factor that mediates the toxicology and teratology of 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin). In an effort to understand the role of the maternal compartment in dioxin teratology, we designed a breeding strategy that allowed us to compare the teratogenic response in embryos from Ahr(-/-) (null) and Ahr(+/+) (wild-type) dams. Using this strategy, we demonstrate that embryos from the Ahr(-/-) dams are 5-fold more sensitive to dioxin-induced cleft palate and hydronephrosis as compared with embryos from an Ahr(+/+) dam. Moreover, this increased teratogenic sensitivity extends beyond dioxin, because embryos from Ahr(-/-) dams exhibited a 9-fold increase in their sensitivity to the fetotoxic effects of the glucocorticoid, dexamethasone. In searching for an explanation for this increased sensitivity, we found that more dioxin and dexamethasone reached the embryos from Ahr(-/-) dams as compared with embryos from Ahr(+/+) dams. We propose that increased deposition of teratogens/fetotoxicants to the embryonic compartment is the result of porto-systemic shunting and/or blocked P4501A induction in Ahr(-/-) dams. In addition to demonstrating the importance of maternal AHR in teratogenesis, these data may have implications that reach beyond the mechanism of action of dioxin. In this regard, the Ahr(-/-) mouse may provide a system that allows pharmacological agents and toxicants to be more easily studied in a model where first pass clearance is a significant obstacle.

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.

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.

The aryl hydrocarbon receptor interacts with estrogen receptor alpha and orphan receptors COUP-TFI and ERRalpha1

The molecular mechanisms underlying the apparent "cross-talk" between estrogen receptor (ER)- and arylhydrocarbon receptor (AHR)-mediated activities are unknown. To determine how AHR ligand 2, 3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) may inhibit ER action and, conversely, to examine how 17-beta-estradiol (E(2)) affects AHR activity, we examined discrete activities of each receptor, i.e., protein-protein interactions, DNA binding, and transcriptional activation. We report that AHR interacts directly with ERalpha, COUP-TF, and ERRalpha1, in a ligand-specific manner in vitro. Unoccupied or beta-napthoflavone (beta-NF)-occupied AHR showed stronger interaction with ERalpha, COUP-TF, and ERRalpha1 than when AHR was occupied by the partial antagonist alpha-naphthoflavone (alpha-NF), indicating a role for ligand in AHR interaction with these proteins. We also report that AHR interacts with COUP-TF in transfected CV-1 cells. In contrast, the AHR nuclear translocator protein (ARNT) did not interact with COUP-TF, ERRalpha1, or ERalpha. We next examined the interaction of either ERalpha or COUP-TF with a consensus xenobiotic response element (XRE). Purified ERalpha did not bind the consensus XRE, but COUP-TFI bound the consensus XRE, suggesting a role for COUP-TF as a AHR/ARNT competitor for XRE binding. In transiently transfected MCF-7 human breast cancer cells, overexpression of COUP-TFI inhibited TCDD-activated reporter gene activity from the CYP1A1 promoter. TCDD inhibited estradiol (E(2))-activated reporter gene activity from a consensus ERE and from the EREs in the pS2 and Fos genes, and COUP-TFI did not block the antiestrogenic activity of TCDD. The specific interaction of COUP-TF with XREs and AHR together with the inhibition of TCDD-induced gene expression by COUP-TF suggests that COUP-TF may regulate AHR action both by direct DNA binding competition and through protein-protein interactions.

The aryl hydrocarbon receptor (AHR)/AHR nuclear translocator (ARNT) heterodimer interacts with naturally occurring estrogen response elements

To determine the molecular mechanisms underlying the "cross talk" between the activity of 2,3,7,8-tetra-chlorodibenzo-p-dioxin (TCDD), which binds to arylhydrocarbon receptor (AHR) and estradiol (E2)-liganded estrogen receptor (ER), we first examined the initial step of estrogen action, ligand binding to ER. None of the AHR ligands tested, i.e. TCDD, benzo[a]pyrene, 3,3',4,4',5-pentachlorobiphenyl, beta-naphthoflavone, or alpha-naphthoflavone, bound to ER alpha. We report the first examination of TCDD interaction with ER beta: TCDD did not displace E2 from ER beta. We then examined a second possible mechanism, i.e. direct inhibition of ER alpha binding to estrogen response elements (EREs) by the AHR/AHR nuclear translocator (ARNT) complex. The AHR/ARNT heterodimer did not bind either a full or half-site ERE. However, AHR/ARNT bound specifically to oligomers containing naturally occurring EREs derived from the human c-fos, pS2, and progesterone receptor (PR) gene promoters that include xenobiotic response element (XRE)-like sequences. In contrast, neither purified E2-liganded-ER from calf uterus or recombinant human ER alpha bound a consensus XRE. TCDD inhibited E2-activated reporter gene activity from a consensus ERE and from EREs in the pS2, PR, and Fos genes in transiently transfected MCF-7 human breast cancer cells. However, this inhibition was not reciprocal since E2 did not inhibit TCDD-stimulated luciferase activity from the CYP1A1 promoter in transiently transfected MCF-7 or human endometrial carcinoma HEC-1A cells. We propose that at least part of the mechanism by which the AHR/ARNT complex inhibits estrogen action is by competitively inhibiting ER alpha binding to imperfect ERE sites, adjacent to or overlapping XREs.

Specificity of DNA binding of the c-Myc/Max and ARNT/ARNT dimers at the CACGTG recognition site

Basic helix-loop-helix proteins that interact with the DNA recognition site CACGTG include the c-Myc/Max heterodimer and the ARNT (Ahreceptornucleartranslocator) homodimer. We have utilized a PCR-based protocol to identify high affinity binding sites of either the c-Myc/Max or ARNT/ARNT dimers and analyzed the ability of these dimers to interact with their derived consensus sequences and activate genes. chi(2)analysis of the selected DNA recognition sites revealed that DNA binding of the ARNT homodimer is symmetric, resulting in the consensus sequence RTCACGTGAY. Gel shift analysis demonstrated that the flanking nucleotides play an important role in dictating DNA binding affinity of the ARNT homodimer. These flanking sequences also regulate the ability of ARNT to competitively displace the c-Myc/Max heterodimer from a CACGTG-containing sequence. However, transient transfection analyses in CV-1 cells revealed that ARNT and c-Myc/Max exhibited similar abilities to activate transcription through each other's consensus sequences. Taken together, these results indicate that although binding affinity of these dimers for the CACGTG core sequences may be differentially influenced by flanking nucleotides, transcriptional activity may also be determined by other factors, such as cellular concentrations of these proteins and their co-activators.

Effects of diesel organic extracts on chemokine production by peripheral blood mononuclear cells

BACKGROUND

Polyaromatic hydrocarbons (PAHs) associated with diesel exhaust particles (DEPs) are found in the atmospheric urban pollution. Such compounds have been shown to favor IgE production, bronchial hyperresponsiveness, and airway inflammation. Chemokines are a group of chemotactic cytokines involved in the recruitment of inflammatory cells.

OBJECTIVE

We investigated the effect of DEP-PAHs on the release and mRNA expression of IL-8, MCP-1, and RANTES by PBMCs obtained from healthy subjects.

METHODS

Protein production in supernatants was assessed by ELISA, and mRNA expression was evaluated by semiquantitative RT-PCR.

RESULTS

Secretion of IL-8 and RANTES increased in a dose-dependent manner with increasing concentrations of DEP-PAHs (range, 0.5 ng to 50 ng/mL). On the contrary, the release of MCP-1 was significantly inhibited, also in a dose-dependent manner. Messenger RNA production coding for IL-8, RANTES, and MCP-1 showed parallel variations to the production of the correspondent proteins. Effects of DEP-PAHs became significant at 7 hours and up to 48 hours time culture for MCP-1, and up to 24 hours time culture for IL-8 and RANTES. Moreover, supernatants from DEP-PAH-activated cells, compared with those of controls, exhibited a significantly enhanced chemotactic activity for neutrophils and eosinophils, which was significantly inhibited by pretreatment with anti-IL-8 and anti-RANTES neutralizing antibodies, respectively.

CONCLUSION

These findings suggest that the chemokine pathways are modulated by DEP-PAHs at the transcriptional level, reinforcing the idea that the development of inflammatory reactions might be affected by diesel exhaust emission.

Cross-talk between the aryl hydrocarbon receptor and hypoxia inducible factor signaling pathways. Demonstration of competition and compensation

The aryl hydrocarbon receptor (AHR) and the alpha-class hypoxia inducible factors (HIF1alpha, HIF2alpha, and HIF3alpha) are basic helix-loop-helix PAS (bHLH-PAS) proteins that heterodimerize with ARNT. In response to 2,3,7,8-tetrachlorodibenzo-p-dioxin, the AHR. ARNT complex binds to "dioxin responsive enhancers" (DREs) and activates genes involved in the metabolism of xenobiotics, e.g. cytochrome P4501A1 (Cyp1a1). The HIF1alpha.ARNT complex binds to "hypoxia responsive enhancers" and activates the transcription of genes that regulate adaptation to low oxygen, e.g. erythropoietin (Epo). We postulated that activation of one pathway would inhibit the other due to competition for ARNT or other limiting cellular factors. Using pathway specific reporters in transient transfection assays, we observed that DRE driven transcription was markedly inhibited by hypoxia and that hypoxia responsive enhancer driven transcription was inhibited by AHR agonists. When we attempted to support this cross-talk model using endogenous loci, we observed that activation of the hypoxia pathway inhibited Cyp1a1 up-regulation, but that activation of the AHR actually enhanced the induction of Epo by hypoxia. To explain this unexpected additivity, we examined the Epo gene and found that its promoter harbors DREs immediately upstream of its transcriptional start site. These experiments outline conditions where inhibitory and additive cross-talk occur between the hypoxia and dioxin signal transduction pathways and identify Epo as an AHR-regulated gene.

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

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

Caenorhabditis elegans orthologs of the aryl hydrocarbon receptor and its heterodimerization partner the aryl hydrocarbon receptor nuclear translocator

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor, until now described only in vertebrates, that mediates many of the carcinogenic and teratogenic effects of certain environmental pollutants. Here, we describe orthologs of AHR and its dimerization partner AHR nuclear translocator (ARNT) in the nematode Caenorhabditis elegans, encoded by the genes ahr-1 and aha-1, respectively. The corresponding proteins, AHR-1 and AHA-1, share biochemical properties with their mammalian cognates. Specifically, AHR-1 forms a tight association with HSP90, and AHR-1 and AHA-1 interact to bind DNA fragments containing the mammalian xenobiotic response element with sequence specificity. Yeast expression studies indicate that C. elegans AHR-1, like vertebrate AHR, requires some form of post-translational activation. Moreover, this requirement depends on the presence of the domains predicted to mediate binding of HSP90 and ligand. Preliminary experiments suggest that if AHR-1 is ligand-activated, its spectrum of ligands is different from that of the mammalian receptor: C. elegans AHR-1 is not photoaffinity labeled by a dioxin analog, and it is not activated by beta-naphthoflavone in the yeast system. The discovery of these genes in a simple, genetically tractable invertebrate should allow elucidation of AHR-1 function and identification of its endogenous regulators.

A nuclear localization signal of human aryl hydrocarbon receptor nuclear translocator/hypoxia-inducible factor 1beta is a novel bipartite type recognized by the two components of nuclear pore-targeting complex

Aryl hydrocarbon receptor nuclear translocator (ARNT) is a component of the transcription factors, aryl hydrocarbon receptor (AhR) and hypoxia-inducible factor 1, which transactivate their target genes, such as CYP1A1 and erythropoietin, in response to xenobiotic aromatic hydrocarbons and to low O2 concentration, respectively. Since ARNT was isolated as a factor required for the nuclear translocation of AhR from the cytoplasm in response to xenobiotics, the subcellular localization of ARNT has been of great interest. In this investigation, we analyzed the subcellular distribution of ARNT using transient expression of a fusion gene with beta-galactosidase and microinjection of recombinant proteins containing various fragments of ARNT in the linker region of glutathione S-transferase/green fluorescent protein. We found a clear nuclear localization of ARNT in the absence of exogenous ligands to AhR, and identified the nuclear localization signal (NLS) of amino acid residues 39-61. The characterized NLS consists of 23 amino acids, and can be classified as a novel variant of the bipartite type on the basis of having two separate regions responsible for efficient nuclear translocation activity, but considerable deviation of the sequence from the consensus of the classical bipartite type NLSs. Like the well characterized NLS of the SV40 T-antigen, this variant bipartite type of ARNT NLS was also mediated by the two components of nuclear pore targeting complex, PTAC58 and PTAC97, to target to the nuclear rim in an in vitro nuclear transport assay.

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

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

DNA binding and transcriptional enhancement by purified TCDD.Ah receptor complex

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcriptional enhancer which mediates the biochemical and toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and structurally related chemicals and which may have a role in the normal development of some tissues. We have previously reported the purification of the transformed TCDD.receptor complex from rat liver cytosol based on binding to its dioxin-responsive enhancer sequence (DRE) and that it comprises the AhR ligand-binding monomer and its dimerization partner, ARNT. The present studies were designed to compare the DRE-binding characteristics of the purified receptor with the cruder preparations that are commonly used and ultimately to determine whether the purified receptor complex itself (in the absence of additional cytosolic or nuclear factors) is capable of enhancing transcription in an in vitro system. The purified AhR retained in vitro DRE binding activity in the presence of carrier protein and dithiothreitol, and its affinity for the DRE oligonucleotide was equivalent to that of the other receptor preparations (crude and partially purified cytosolic and crude nuclear). When the ligand.receptor complex was bound to a DRE oligonucleotide containing BrdU and then UV-irradiated, two proteins in each of the receptor preparations were found to crosslink to BrdU-DRE, and we concluded that they are the AhR monomer and ARNT protein. All receptor preparations also gave a similar footprint of interaction with G-residues within the DRE consensus sequence, as assessed by methylation interference. Furthermore, purified and partially purified receptors were able to stimulate transcription from a DRE-containing template in a cell-free system in the presence of HeLa cell nuclear extract. Transcriptional enhancement was receptor dose-dependent, TCDD-dependent, and specific for the DRE sequence upstream of the promotor in our template construct. These data document for the first time that a purified TCDD.Ah receptor complex retains both specific DNA binding and transcriptional activities. This observation constitutes an important step toward understanding the mechanism of gene regulation by TCDD since it implies that the transformed receptor.ligand complex itself is competent as a transcriptional enhancer without a requirement for other factors.

Aryl hydrocarbon receptor-mediated signal transduction

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

Mapping the protein/DNA contact sites of the Ah receptor and Ah receptor nuclear translocator

The Ah receptor (AHR) and its DNA binding partner, the Ah receptor nuclear translocator (ARNT), are basic helix-loop-helix proteins distinguished by their PER, AHR, ARNT, and SIM (PAS) homology regions. To identify the amino acids of the AHR.ARNT heterodimer that contact the TNGCGTG recognition sequence, we have performed deletion mapping and amino acid substitutions within the N termini of both the AHR and ARNT. The ability of the variant AHR and ARNT proteins to bind DNA and activate gene transcription was determined by the gel shift analysis and transient transfection assays. We have found that the amino acids of ARNT that contact DNA are similar to those of other basic/helix-loop-helix proteins and include glutamic acid residue 83 and arginine residues 86 and 87. Although our initial experiments indicated that DNA binding of the AHR may involve two regions that are bordered by amino acids 9-17 and amino acids 34-42, further analysis demonstrated that only amino acids 34-39 are critical for the AHR.TNGC interaction. These experiments indicate that while the structural features of the ARNT.GTG complex may closely resemble that deduced for proteins such as Max, E47, and USF, the AHR.TNGC complex may represent a unique DNA binding form of basic/helix-loop-helix proteins.

DNA binding specificities and pairing rules of the Ah receptor, ARNT, and SIM proteins

The Ah receptor (AHR), the Ah receptor nuclear translocator protein (ARNT), and single-minded protein (SIM) are members of the basic helix-loop-helix-PAS (bHLH-PAS) family of regulatory proteins. In this study, we examine the DNA half-site recognition and pairing rules for these proteins using oligonucleotide selection-amplification and coprecipitation protocols. Oligonucleotide selection-amplification revealed that a variety of bHLH-PAS protein combinations could interact, with each generating a unique DNA binding specificity. To validate the selection-amplification protocol, we demonstrated the preference of the AHR.ARNT complex for the sequence commonly found in dioxin-responsive enhancers in vivo (TNGCGTG). We then demonstrated that the ARNT protein is capable of forming a homodimer with a binding preference for the palindromic E-box sequence, CACGTG. Further examination indicated that ARNT may have a relaxed partner specificity, since it was also capable of forming a heterodimer with SIM and recognizing the sequence GT(G/A)CGTG. Coprecipitation experiments using various PAS proteins and ARNT were consistent with the idea that the ARNT protein has a broad range of interactions among the bHLH-PAS proteins, while the other members appear more restricted in their interactions. Comparison of this in vitro data with sites known to be bound in vivo suggests that the high affinity half-site recognition sequences for the AHR, SIM, and ARNT are T(C/T)GC, GT(G/A)C (5'-half-sites), and GTG (3'-half-sites), respectively.

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

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