Species-specific transcriptional activity of synthetic flavonoids in guinea pig and mouse cells as a result of differential activation of the aryl hydrocarbon receptor to interact with dioxin-responsive elements

Species-Specific Differences in Aryl Hydrocarbon Receptor Responses: How and Why?

The aryl hydrocarbon receptor (AhR) is a transcription factor that regulates a wide range of biological and toxicological effects by binding to specific ligands. AhR ligands exist in various internal and external ecological systems, such as in a wide variety of hydrophobic environmental contaminants and naturally occurring chemicals. Most of these ligands have shown differential responses among different species. Understanding the differences and their mechanisms helps in designing better experimental animal models, improves our understanding of the environmental toxicants related to AhR, and helps to screen and develop new drugs. This review systematically discusses the species differences in AhR activation effects and their modes of action. We focus on the species differences following AhR activation from two aspects: (1) the molecular configuration and activation of AhR and (2) the contrast of cis-acting elements corresponding to AhR. The variations in the responses seen in humans and other species following the activation of the AhR signaling pathway can be attributed to both factors.

Genome-Wide Transcriptional Analysis Reveals Novel AhR Targets That Regulate Dendritic Cell Function during Influenza A Virus Infection

Activation of the ligand inducible aryl hydrocarbon receptor (AhR) during primary influenza A virus infection diminishes host responses by negatively regulating the ability of dendritic cells (DC) to prime naive CD8⁺ T cells, which reduces the generation of CTL. However, AhR-regulated genes and signaling pathways in DCs are not fully known. In this study, we used unbiased gene expression profiling to identify differentially expressed genes and signaling pathways in DCs that are modulated by AhR activation in vivo. Using the prototype AhR agonist TCDD, we identified the lectin receptor Cd209a (DC-SIGN) and chemokine Ccl17 as novel AhR target genes. We further show the percentage of DCs expressing CD209a on their surface was significantly decreased by AhR activation during infection. Whereas influenza A virus infection increased CCL17 protein levels in the lung and lung-draining lymph nodes, this was significantly reduced following AhR activation. Targeted excision of AhR in the hematopoietic compartment confirmed AhR is required for downregulation of CCL17 and CD209a. Loss of AhR's functional DNA-binding domain demonstrates that AhR activation alone is necessary but not sufficient to drive downregulation. AhR activation induced similar changes in gene expression in human monocyte-derived DCs. Analysis of the murine and human upstream regulatory regions of Cd209a and Ccl17 revealed a suite of potential transcription factor partners for AhR, which may coregulate these genes in vivo. This study highlights the breadth of AhR-regulated pathways within DCs, and that AhR likely interacts with other transcription factors to modulate DC functions during infection.

[Aryl Hydrocarbon Receptor Ligand Activity of Extracts from 62 Herbal Medicines and Effect on Cytochrome P450 Activity]

Aryl hydrocarbon receptor (AhR) ligand activity of the extracts of 62 herbal medicines was examined using yeast reporter assay. Fifty-eight herbal extracts exhibited AhR ligand activity. The highest activity was observed with Ogon (Scutellariae Radix), followed by Oren (Coptidis Rhizoma), Kujin (Sophorae Radix) and Shoma (Cimicifiigae Rhizoma). When these extracts were treated with hesperinase, a hydrolase for sugar conjugates, the aglycones showed higher activity than the parent extracts. Among the constituents of Ogon extract, baicalein and wogonin showed AhR ligand activity, while the sugar conjugate of baicalein, baicalin, was inactive. Among the flavonoid components of these herbal medicines, flavone and chrysin exhibited high ligand activity for AhR. Ethoxyresorufin O-dealkylase (EROD) activity due to CYP1A1 in HepG2 cells was enhanced by the addition of baicalein. Baicalein also decreased the 3-methylcholanthrene-induced increase of EROD activity, but this effect was not statistically significant. When wogonin or baicalein was orally administered at the dose of 100 mg/kg to mice, EROD activity in liver was only slightly changed. Furthermore, when Ogon extract was co-administered with 3-methylcholanthrene, the EROD and methoxyresorufin O-dealkylase activities were not significantly changed. These results indicate that many herbal extracts have AhR ligand activity, and their inducing effect on CYP1A1/2 can be evaluated in HepG2 cells.

In vitro dioxin-like potencies of HO- and MeO-PBDEs and inter-species sensitivity variation in birds

Due to their bioaccumulative properties, hydroxylated and methoxylated polybrominated diphenyl ethers (HO-/MeO-PBDEs) may pose ecological risks to wild life, including birds. However, their toxicity potencies in avian species are largely unknown. In the present study, an avian AHR1 luciferase reporter gene (LRG) assay with luciferase probes from chicken, pheasant and quail was used to test activations of avian aryl hydrocarbon receptor (AHR)-mediated pathways by 19 HO- or MeO-PBDEs in different avian species. Species-specific relative potencies (RePs) of HO-/MeO-PBDEs to tetrachlorodibenzo-p-dioxin (TCDD) and relative sensitivities of various species to each chemical were estimated. The results indicated that the ReP of the most potent HO-/MeO-PBDEs, 5-Cl-6-HO-BDE-47, was 7.8×10(-4) for chicken, 1.1×10(-2) for pheasant, and 1.7×10(-1) for quail comparing to TCDD. In addition, it was found that avian species with the greatest sensitivity to TCDD did not always have the greatest sensitivity to HO-/MeO-PBDEs and vice versa. This study contributed to filling in the knowledge gap regarding the dioxin-like activity of HO-/MeO-PBDEs in birds, and provided beneficial information for the prioritization of HO-/MeO-PBDEs for further research.

CAPSULE ABSTRACT

HO-/MeO-PBDEs activate avian AHR-mediated pathways in a congener- and species- specific manner. 5-Cl-6-HO-BDE-47 was the most potent among the nineteen HO-/MeO-PBDEs tested.

The Aryl Hydrocarbon Receptor: A Key Bridging Molecule of External and Internal Chemical Signals

The aryl hydrocarbon receptor (AhR) is a highly evolutionary conserved, ligand-activated transcription factor that is best known to mediate the toxicities of dioxins and dioxin-like compounds. Phenotype of AhR-null mice, together with the recent discovery of a variety of endogenous and plant-derived ligands, point to the integral roles of AhR in normal cell physiology, in addition to its roles in sensing the environmental chemicals. Here, we summarize the current knowledge about AhR signaling pathways, its ligands and AhR-mediated effects on cell specialization, host defense and detoxification. AhR-mediated health effects particularly in liver, immune, and nervous systems, as well as in tumorgenesis are discussed. Dioxin-initiated embryotoxicity and immunosuppressive effects in fish and birds are reviewed. Recent data demonstrate that AhR is a convergence point of multiple signaling pathways that inform the cell of its external and internal environments. As such, AhR pathway is a promising potential target for therapeutics targeting nervous, liver, and autoimmune diseases through AhR ligand-mediated interventions and other perturbations of AhR signaling. Additionally, using available laboratory data obtained on animal models, AhR-centered adverse outcome pathway analysis is useful in reexamining known and potential adverse outcomes of specific or mixed compounds on wildlife.

Naturally occurring marine brominated indoles are aryl hydrocarbon receptor ligands/agonists

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that mediates the toxic and biological effects of structurally diverse chemicals, including the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). As part of a larger effort to identify the full spectrum of chemicals that can bind to and activate the AhR, we have examined the ability of several naturally occurring marine-derived brominated indoles and brominated (methylthio)indoles (collectively referred to as brominated indoles) to bind to the AhR and stimulate AhR-dependent gene expression. Incubation of mouse, rat, and guinea pig recombinant cell lines containing a stably transfected AhR-responsive luciferase reporter gene with eight brominated indoles revealed that all compounds stimulated luciferase reporter gene activity, although some species-specific differences were observed. All compounds induced significantly more luciferase activity when incubated with cells for 4 h as compared to 24 h, demonstrating that these compounds are transient activators of the AhR signaling pathway. Three of the brominated indoles induced CYP1A1 mRNA in human HepG2 cells in vitro and Cyp1a mRNA in zebrafish embryos in vivo. The identification of the brominated indoles as direct ligands and activators/agonists of the AhR was confirmed by their ability to compete with [(3)H]TCDD for binding to the AhR and to stimulate AhR transformation and DNA binding in vitro. Taken together, these results indicate that marine-derived brominated indoles are members of a new class of naturally occurring AhR agonists.

Activation of avian aryl hydrocarbon receptor and inter-species sensitivity variations by polychlorinated diphenylsulfides

It was hypothesized that polychlorinated diphenyl sulfides (PCDPSs) can potentially interact with an aryl hydrocarbon receptor (AHR) and thereby cause adverse effects in wildlife like birds. A recently developed avian AHR1-luciferase report gene (LRG) assay was used to assess the interaction between avian AHR1 and 18 PCDPSs and to compare the interspecies sensitivity among chicken, ring-necked pheasant, and Japanese quail by PCDPSs. Most of the tested PCDPSs could activate the AHR1-mediated pathways in avian species, and the relative potency (ReP) of the PCDPSs increased with the increasing number of substituted Cl atoms. The rank orders of PCDPSs potency were generally similar among birds, although the ReP varied. In addition, not all the sensitivity rank orders of avian AHR1 constructs for PCDPSs were consistent with that of TCDD. ReP values of PCDPSs suggested that some PCDPSs like 2,3,3',4,5,6-hexa-CDPS and 2,2',3,3',4,5,6-hepta-CDPS are higher than the avian WHO-TEFs of OctaCDD, OctaCDF, and most of the coplanar PCBs. Our results report for the first time the activation of an AHR1-mediated molecular toxicological mechanism by PCDPSs, and provide the ranking of ReP and relative sensitivity values of different congeners, which could guide the further toxicity test of this group of potential high priority environmental pollutants.

Impacts of different exposure scenarios on transcript abundances in Danio rerio embryos when investigating the toxicological burden of riverine sediments

Purpose

Recently, a proof-of-concept study revealed the suitability of transcriptome analyses to obtain and assess changes in the abundance of transcripts in zebrafish (Danio rerio) embryos after exposure to organic sediment extracts. The present study investigated changes in the transcript abundance in zebrafish embryos exposed to whole sediment samples and corresponding organic extracts in order to identify the impact of different exposure pathways on sediment toxicity.

Materials and Methods

Danio rerio embryos were exposed to sublethal concentrations of three sediment samples from the Danube River, Germany. The sediment samples were investigated both as freeze-dried samples and as organic extracts. Silica dust and a process control of the extraction procedure were used as references. After exposure, mRNA was isolated and changes in profiles of gene expression levels were examined by an oligonucleotide microarray. The microarray results were compared with bioassays, chemical analysis of the sediments and profiles of gene expression levels induced by several single substances.

Results and Discussion

The microarray approach elucidated significant changes in the abundance of transcripts in exposed zebrafish embryos compared to the references. Generally, results could be related to Ah-receptor-mediated effects as confirmed by bioassays and chemical analysis of dioxin-like contaminants, as well as to exposure to stress-inducing compounds. Furthermore, the results indicated that mixtures of chemicals, as present in sediment and extract samples, result in complex changes of gene expression level profiles difficult to compare with profiles induced by single chemical substances. Specifically, patterns of transcript abundances were less influenced by the chemical composition at the sampling site compared t the method of exposure (sediment/extract). This effect might be related to different bioavailability of chemicals.

Conclusions

The apparent difference between the exposure scenarios is an important aspect that needs to be addressed when conducting analyses of alterations in the expression level of mRNA.

Cytochrome P4501A induction in avian hepatocyte cultures exposed to polychlorinated biphenyls: comparisons with AHR1-mediated reporter gene activity and in ovo toxicity

Avian-specific toxic equivalency factors (TEFs) were developed by the World Health Organization to simplify environmental risk assessments of dioxin-like compounds (DLCs), but TEFs do not account for differences in the toxic and biochemical potencies of DLCs among species of birds. Such variability may be due to differences in species sensitivity to individual DLCs. The sensitivity of avian species to DLCs was recently associated with the identity of amino acids 324 and 380 in the aryl hydrocarbon receptor 1 (AHR1) ligand binding domain. A luciferase reporter gene (LRG) assay, measuring AHR1-mediated induction of a cytochrome P450 1A5 (CYP1A5) reporter gene, in combination with a species' AHR1 ligand binding domain sequence, were also shown to predict avian species sensitivity to polychlorinated biphenyls (PCBs) and PCB relative potency in a given species. The goals of the present study were to (1) characterize the concentration-dependent effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin and PCBs 126, 77, 105 and 118 on induction of ethoxyresorufin O-deethylase (EROD) activity and CYP1A4/5 mRNA in chicken, ring-necked pheasant and Japanese quail embryo hepatocytes and (2) compare these in vitro results to those previously generated by the LRG assay and in ovo toxicity studies. EROD activity and CYP1A4/5 mRNA expression data support and complement the findings of the LRG assay. CYP1A enzyme activity and mRNA expression were significantly correlated both with luciferase activity and in ovo toxicity induced by PCBs. Relative potency values were generally similar between the LRG and EROD assays and indicate that the relative potency of some PCBs may differ among species.

Transcriptional and posttranslational inhibition of dioxin-mediated induction of CYP1A1 by harmine and harmol

Dioxins are widespread environmental contaminants that induce the carcinogen-activating enzyme, cytochrome P450 1A1 (CYP1A1) through an aryl hydrocarbon receptor (AhR)-dependent mechanism. We previously demonstrated that harmine inhibits the dioxin-mediated induction of Cyp1a1 activity in murine hepatoma cells. Therefore, the aim of this study is to determine the effect of harmine and its main metabolite, harmol, on the dioxin-mediated induction of CYP1A1 in human HepG2 and murine Hepa 1c1c7 hepatoma cells. Our results showed that harmine and harmol significantly inhibited the dioxin-mediated induction of CYP1A1 at mRNA, protein, and activity levels in a concentration-dependent manner in human and murine hepatoma cells. Moreover, harmine and harmol inhibited the AhR-dependent luciferase activity and the activation and transformation of AhR using the electrophoretic mobility shift assay. In addition, harmine and harmol displaced [(3)H]TCDD in the competitive ligand binding assay. At posttranslational level, both harmine and harmol decreased the protein stability of CYP1A1, suggesting that posttranslational mechanism is involved. Furthermore, we demonstrated that the underlying mechanisms of the posttranslational modifications of both compounds involve ubiquitin-proteasomal pathway and direct inhibitory effects of CYP1A1 enzyme. We concluded that harmine and its metabolite, harmol, are new inhibitors of dioxin-mediated effects.

Identification of benzothiazole derivatives and polycyclic aromatic hydrocarbons as aryl hydrocarbon receptor agonists present in tire extracts

Leachate from rubber tire material contains a complex mixture of chemicals previously shown to produce toxic and biological effects in aquatic organisms. The ability of these leachates to induce Ah receptor (AhR)-dependent cytochrome P4501A1 expression in fish indicated the presence of AhR active chemicals, but the responsible chemicals and their direct interaction with the AhR signaling pathway were not examined. Using a combination of AhR-based bioassays, we have demonstrated the ability of tire extract to stimulate both AhR DNA binding and AhR-dependent gene expression and confirmed that the responsible chemicals were metabolically labile. The application of CALUX (chemical-activated luciferase gene expression) cell bioassay-driven toxicant identification evaluation not only revealed that tire extract contained a variety of known AhR-active polycyclic aromatic hydrocarbons but also identified 2-methylthiobenzothiazole and 2-mercaptobenzothiazole as AhR agonists. Analysis of a structurally diverse series of benzothiazoles identified many that could directly stimulate AhR DNA binding and transiently activate the AhR signaling pathway and identified benzothiazoles as a new class of AhR agonists. In addition to these compounds, the relatively high AhR agonist activity of a large number of fractions strongly suggests that tire extract contains a large number of physiochemically diverse AhR agonists whose identities and toxicological/biological significances are unknown.

Dioxins, the aryl hydrocarbon receptor and the central regulation of energy balance

Dioxins are ubiquitous environmental contaminants that have attracted toxicological interest not only for the potential risk they pose to human health but also because of their unique mechanism of action. This mechanism involves a specific, phylogenetically old intracellular receptor (the aryl hydrocarbon receptor, AHR) which has recently proven to have an integral regulatory role in a number of physiological processes, but whose endogenous ligand is still elusive. A major acute impact of dioxins in laboratory animals is the wasting syndrome, which represents a puzzling and dramatic perturbation of the regulatory systems for energy balance. A single dose of the most potent dioxin, TCDD, can permanently readjust the defended body weight set-point level thus providing a potentially useful tool and model for physiological research. Recent evidence of response-selective modulation of AHR action by alternative ligands suggests further that even therapeutic implications might be possible in the future.

Antagonism of aryl hydrocarbon receptor signaling by 6,2',4'-trimethoxyflavone

The aryl hydrocarbon receptor (AHR) is regarded as an important homeostatic transcriptional regulator within physiological and pathophysiological processes, including xenobiotic metabolism, endocrine function, immunity, and cancer. Agonist activation of the AHR is considered deleterious based on toxicological evidence obtained with environmental pollutants, which mediate toxic effects through AHR. However, a multitude of plant-derived constituents, e.g., polyphenols that exhibit beneficial properties, have also been described as ligands for the AHR. It is conceivable that some of the positive aspects of such compounds can be attributed to suppression of AHR activity through antagonism. Therefore, we conducted a dioxin response element reporter-based screen to assess the AHR activity associated with a range of flavonoid compounds. Our screen identified two flavonoids (5-methoxyflavone and 7,4'-dimethoxyisoflavone) with previously unidentified AHR agonist potential. In addition, we have identified and characterized 6,2',4'-trimethoxyflavone (TMF) as an AHR ligand that possesses the characteristics of an antagonist having the capacity to compete with agonists, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo[a]pyrene, thus effectively inhibiting AHR-mediated transactivation of a heterologous reporter and endogenous targets, e.g., CYP1A1, independent of cell lineage or species. Furthermore, TMF displays superior action by virtue of having no partial agonist activity, in contrast to other documented antagonists, e.g., alpha-napthoflavone, which are partial weak agonists. TMF also exhibits no species or promoter dependence with regard to AHR antagonism. TMF therefore represents an improved tool allowing for more precise dissection of AHR function in the absence of any conflicting agonist activity.

The aryl hydrocarbon receptor attenuates tobacco smoke-induced cyclooxygenase-2 and prostaglandin production in lung fibroblasts through regulation of the NF-kappaB family member RelB

Diseases such as chronic obstructive pulmonary disease and lung cancer caused by cigarette smoke affect millions of people worldwide. The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that influences responses to certain environmental pollutants such as tobacco smoke. However, the physiological function(s) of the AhR is unknown. Herein we propose that the physiologic role of the AhR is to limit inflammation. We show that lung fibroblasts from AhR(-/-) mice produce a heightened inflammatory response to cigarette smoke, typified by increased levels of cyclooxygenase-2 (COX-2) and prostaglandins (PGs), when compared with wild type (AhR(+/+)) fibroblasts. This response was dependent on AhR expression as transient transfection of an AhR expression plasmid into AhR(-/-) fibroblasts significantly attenuated the smoke-induced COX-2 and PG production, confirming the anti-inflammatory role of the AhR. The AhR can interact with NF-kappaB. However, the heightened inflammatory response observed in AhR(-/-) fibroblasts was not the result of NF-kappaB (p50/p65) activation. Instead it was coupled with a loss of the NF-kappaB family member RelB in AhR(-/-) fibroblasts. Taken together, these studies provide compelling evidence that AhR expression limits proinflammatory COX-2 and PG production by maintaining RelB expression. The association between RelB and AhR may represent a new therapeutic and more selective target with which to combat inflammation-associated diseases.

Changes in toxicity and Ah receptor agonist activity of suspended particulate matter during flood events at the rivers Neckar and Rhine - a mass balance approach using in vitro methods and chemical analysis

BACKGROUND, AIM, AND SCOPE

As a consequence of flood events, runoff and remobilized sediments may cause an increase of ecotoxicologically relevant effects from contaminant reservoirs. Aquatic and terrestrial organisms as well as cattle and areas of settlement are exposed to dislocated contaminants during and after flood events. In this study, the impacts of two flood events triggered by intense rain at the rivers Neckar and Rhine (Southern Germany) were studied. Effects in correlation to flood flow were assessed at the river Neckar using samples collected at frequent intervals. River Rhine suspended particulate matter (SPM) was sampled over a longer period at normal flow and during a flood event. Three cell lines (H4L1.1c4, GPC.2D.Luc, RTL-W1) were used to compare Ah receptor agonist activity in different biotest systems. Multilayer fractionation was performed to identify causative compounds, focusing on persistent organic contaminants.

MATERIALS AND METHODS

Native water and SPM of flood events were collected at the river Neckar and at the monitoring station (Rheinguetestation, Worms, Germany) of the river Rhine. Water samples were XAD-extracted. SPM were freeze-dried and Soxhlet-extracted using acetone and finally dissolved in dimethyl sulfoxide. Resulting crude extracts were analyzed for cytotoxicity with the neutral red assay. Aryl hydrocarbon receptor (AhR) agonist activity was measured in a set of biological test systems (DR-CALUX, GPC.2D, and ethoxyresorufin-O-deethylase (EROD) assay) and different cell lines. In addition, crude extracts were fractionated using a combined method of multilayer (sequence of acidified silica layers) and carbon fractionation. Fractions from the multilayer fractionation contained persistent organic compounds (polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), polychlorinated biphenyls (PCBs), and some polycyclic aromatic hydrocarbon (PAHs)); fractions from the carbon fractionation were separated into a PCDD/F and a PCB fraction. Dioxin-like activity of multilayer and carbon fractions was determined in the EROD assay and expressed as biological toxicity equivalency concentrations of 2,3,7,8-tetrachlorodibenzo-p-dioxin (bio-TEQs). The calculation of chemical equivalency concentrations (chem-TEQs) and comparison to bio-TEQ values allowed the determination of the contribution of the analyzed persistent compounds to the total biological effects measured.

RESULTS

Soluble compounds in native and extracted water samples resulted in no or minor activity in the toxicity tests, respectively. Filter residues of native water caused increased AhR-mediated activity at the peak of the flood. Activities of SPM of the river Neckar correlated well with the flow rate indicating a flood-dependent increase of toxicity culminating at the peak of flow. River Rhine SPM showed a decrease of activity regarding an SPM sample of the flood event compared to a long-term sample. Excellent correlations with AhR agonistic activity were determined for DR-CALUX and EROD assay, while the GPC.2D assay did not correlate with both other biotests. The activity of persistent dioxin-like acting compounds in multilayer and carbon fractionated PCDD/F and PCB fractions was low if compared to corresponding crude extracts. The congener pattern of PCDD/F revealed that the contaminations mainly originated from products and productions of the chlorine and organochlorine industries.

DISCUSSION

Native and extracted water samples could be shown to contain little or no cytotoxic or AhR agonistic compounds. In contrast, particle-bound compounds were shown to be the relevant effect-causing fraction, as indicated by the activities of filter residues of native water and SPM. Compounds other than fractionated persistent PCBs and PCDD/Fs were more relevant to explain AhR-mediated activities of crude flood SPM at both rivers assessed. Biologically detected activities could at least in part be traced back to chemically analyzed and quantified compounds.

CONCLUSIONS

The calculation of the portion of persistent PCBs and PCDD/Fs in multilayer fractions causing the high inductions in the EROD assay in combination with chemical analysis provides a suitable tool to assess dioxin-like activity of persistent compounds in SPM sampled over the course of flood events. Depending on the catchment area and annual course of flood events, end points may either indicate an increase or a decrease of activity. In order to determine the ecological hazard potential of mobilized contaminants during flood events, the focus should be set on particle-bound pollutants. Furthermore, PCDD/Fs and PCBs, commonly expected to be the most relevant pollutants in river systems, could be shown to contribute only to a minor portion of the overall AhR-mediated activity. However, they might be most relevant for human exposure when considering persistence and bioaccumulation-biomagnification in the food chain.

RECOMMENDATIONS AND PERSPECTIVES

As a consequence of climate change, flood events will increase in frequency and intensity at least in some regions such as Central Europe. Thus, it is crucial to identify the potential hazard of (re-)mobilized contaminants from reservoirs dislocated via floods and threatening especially aquatic organisms and cattle grazing in flood plains. Since other less persistent compounds seem to be more relevant to explain AhR-mediated activities in flood SPM, nonconventional PAHs and more polar compounds also need to be considered for risk assessment. Effect-directed analysis using broad-range fractionation methods taking into account compounds from polar to nonpolar should be applied for identification of pollutants causing biological effects, thus integrating biological and chemical parameters.

Molecular determinants of species-specific agonist and antagonist activity of a substituted flavone towards the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AhR) mediates the toxicity of dioxins and related xenobiotics. Other chemicals also bind the AhR to elicit either agonist or antagonist responses. Here we used site-directed mutagenesis within the ligand binding domain of murine AhR to probe for specific residues that might interact differentially with the antagonist 3'-methoxy-4'-nitroflavone (MNF) compared with the prototypical agonist TCDD. Reduced (3)H-TCDD binding, dioxin-response element (DRE) binding, and transcriptional activity were observed for several point mutants. One mutation, R355I, changed the response to MNF from antagonist to agonist. Notably, Ile is the residue found in the guinea pig AhR, towards which MNF has partial agonist activity in contrast to its strong antagonist activity in mouse. A similar reversal of response to MNF was observed in chimeric AhRs in which the C-terminal region of mAhR was replaced with the guinea pig C-terminal region. These data demonstrate that different amino acids can be important in binding of different AhR ligands and can mediate distinct responses. The ultimate response of the AhR also depends on how other portions of the receptor protein are functionally coupled to the initial ligand binding event.

Activities and identification of aryl hydrocarbon receptor agonists in sediments from the Danube river

This study is a consequence of a distinct fish decline in the Danube river since the beginning of the 1990s. In contrast to the decline of fish population, former studies have repeatedly documented that the water quality along the Danube river is improving. However, the conclusion of a pilot study in 2002 was that a high hazard potential is associated with local sediments. The present study documents that sediment samples from the Danube river showed comparatively high aryl hydrocarbon receptor mediated activity in biotests, using the cell lines GPC.2D.Luc, H4IIE (DR-CALUX) and RTL-W1. The combination of chemical analysis, fractionation techniques and different in vitro tests revealed that priority pollutants could not explain the main induction, even though the concentrations of priority polycyclic aromatic hydrocarbons (PAHs) were very high (maximum in the tributary Schwarzach, sum of 16 EPA PAHs 26 mug/g). In conclusion, this investigation shows that nonpriority pollutants mainly mediate the high induction rates. Nevertheless, owing to the effects of PAHs towards fish and the connection between dioxin-like activity and carcinogenicity, the link between contamination and the fish population decline cannot be ruled out.

Environmental toxicants may modulate osteoblast differentiation by a mechanism involving the aryl hydrocarbon receptor

The AHR mediates many of the toxicological effects of aromatic hydrocarbons. We show that AHR expression in osteoblasts parallels the induction of early bone-specific genes involved in maturation. The AHR may not only mediate the effects of toxicants, but with an as yet unidentified ligand, be involved in the differentiation pathways of osteoblasts.

INTRODUCTION

Metabolic bone diseases arise as a result of an imbalance in bone cell activities. Recent evidence suggests that environmental toxicants may be contributing factors altering these activities. One candidate molecule implicated in mediating the toxic effects of exogenous compounds is the aryl hydrocarbon receptor (AHR).

MATERIALS AND METHODS

Osteoblasts isolated from neonatal rat calvaria were analyzed for AHR expression by quantitative PCR, Western blot, and immunohistochemistry. In addition, AHR activation was evaluated by electromobility gel shift assay and fluorescence microscopy.

RESULTS

Our findings showed AHR expression in mature osteoblasts in vivo. The pattern of AHR expression peaks after alkaline phosphatase and before induction of osteocalcin. We first show that AHR functions as a transactivating receptor in osteoblasts, as evidenced by its ligand-dependent migration to the nucleus and its association with known dioxin response elements. AHR activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) mediated the induction of cytochrome p450 1A1 and cycloxygenase-2 protein levels. This effect could be inhibited by the potent AHR antagonist, 3'4 methoxynitroflavone. Furthermore, lead treatment of osteoblasts upregulates the expression of AHR mRNA and protein levels, supporting a novel mechanism whereby lead in the skeleton may increase the sensitivity of bone cells to toxicant exposure.

CONCLUSIONS

These data imply that the AHR mediates the effects of aromatic toxicants on bone and that AHR expression is regulated during osteoblast differentiation.

Modulation of aryl hydrocarbon receptor transactivation by carbaryl, a nonconventional ligand

Carbaryl (1-naphthyl-N-methylcarbamate), a widely used carbamate insecticide, induces cytochrome P450 1A gene expression in mammalian cells. This activity is usually mediated by the interaction of the compound with the aryl hydrocarbon receptor. However, it has been proposed that this mechanism does not apply to carbaryl because its structure differs from that of typical aryl hydrocarbon receptor ligands. We show here that carbaryl promotes activation of target genes in a yeast-based bioassay expressing both aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator. By contrast, carbaryl acted as a competitive inhibitor, rather than as an agonist, in a simplified yeast system, in which aryl hydrocarbon receptor nuclear translocator function is bypassed by fusing aryl hydrocarbon receptor to a heterologous DNA binding domain. This dual action of carbaryl, agonist and partial antagonist, was also observed by comparing carbaryl response in two vertebrate cell lines. A yeast two-hybrid assay showed that the mammalian coactivator cAMP response element-binding protein readily interacts with aryl hydrocarbon receptor bound to its canonical ligand beta-naphthoflavone, but not with the carbaryl-aryl hydrocarbon receptor complex. We propose that carbaryl interacts with aryl hydrocarbon receptor, but that its peculiar structure imposes a substandard configuration on the aryl hydrocarbon receptor ligand-binding domain that prevents interaction with key coactivators and activates transcription without the need for aryl hydrocarbon receptor nuclear translocator. This effect may be relevant in explaining its physiological effects in exposed animals, and may help to predict its effects, and that of similar compounds, in humans. Our data also identify the aryl hydrocarbon receptor/cAMP response element-binding protein interaction as a molecular target for the identification and development of new aryl hydrocarbon receptor antagonists.

β-Naphthoflavone and 3′-methoxy-4′-nitroflavone exert ambiguous effects on Ah receptor-dependent cell proliferation and gene expression in rat liver ‘stem-like’ cells

Both natural and synthetic flavonoids are known to interact with the aryl hydrocarbon receptor (AhR); however, their agonist/antagonist properties in vitro have been so far studied mostly in the context of cytochrome P450 1A1 gene (Cyp1a1) regulation. We investigated effects of two synthetic flavones known either as AhR agonist (beta-naphthoflavone; BNF) or antagonist (3'-methoxy-4'-nitroflavone; 3M4NF), using an in vitro model of liver 'stem-like' cells, on expression of various AhR target genes and AhR-dependent cell proliferation. We found that the presumed antagonist 3M4NF induces a partial nuclear translocation and activation of AhR. Although inhibiting the 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced Cyp1a1 expression, 3M4NF alone induced a minor increase of CYP1A1 mRNA and protein. However, 3M4NF did not induce AhR binding to synthetic dioxin response elements (DRE). In contrast to Cyp1a1, 3M4NF induced a marked expression of other AhR-regulated genes, such as Cyp1b1 and Nqo1, as well as transcriptional repression of Cdh13 gene, confirming that its effects may be promoter-context specific. Like BNF, 3M4NF induced AhR-dependent cell proliferation of contact-inhibited rat liver 'stem-like' WB-F344 cells, associated with a marked upregulation of Cyclin A, as well as the downregulation of proteins involved in formation of cell-cell contacts. Based on these experimental findings, we conclude that partial agonists/antagonists of AhR can increase cell proliferation rate and AhR-dependent genes expression in both cell type- and gene-specific manner. The specificity of effects of flavones on diverse AhR targets should be taken into account, when studying AhR signaling using presumed AhR antagonists.

Using nutrition for intervention and prevention against environmental chemical toxicity and associated diseases

BACKGROUND

Nutrition and lifestyle are well-defined modulators of chronic diseases. Poor dietary habits (such as high intake of processed foods rich in fat and low intake of fruits and vegetables), as well as a sedentary lifestyle clearly contribute to today's compromised quality of life in the United States. It is becoming increasingly clear that nutrition can modulate the toxicity of environmental pollutants.

OBJECTIVES

Our goal in this commentary is to discuss the recommendation that nutrition should be considered a necessary variable in the study of human disease associated with exposure to environmental pollutants.

DISCUSSION

Certain diets can contribute to compromised health by being a source of exposure to environmental toxic pollutants. Many of these pollutants are fat soluble, and thus fatty foods often contain higher levels of persistent organics than does vegetable matter. Nutrition can dictate the lipid milieu, oxidative stress, and antioxidant status within cells. The modulation of these parameters by an individual's nutritional status may have profound affects on biological processes, and in turn influence the effects of environmental pollutants to cause disease or dysfunction. For example, potential adverse health effects associated with exposure to polychlorinated biphenyls may increase as a result of ingestion of certain dietary fats, whereas ingestion of fruits and vegetables, rich in antioxidant and anti-inflammatory nutrients or bioactive compounds, may provide protection.

CONCLUSIONS

We recommend that future directions in environmental health research explore this nutritional paradigm that incorporates a consideration of the relationships between nutrition and lifestyle, exposure to environmental toxicants, and disease. Nutritional interventions may provide the most sensible means to develop primary prevention strategies of diseases associated with many environmental toxic insults.

A potential endogenous ligand for the aryl hydrocarbon receptor has potent agonist activity in vitro and in vivo

The aryl hydrocarbon receptor (AhR) is best known as a mediator of toxicity of a diverse family of xenobiotic chemicals such as dioxins and PCBs. However, many naturally occurring compounds also activate AhR. One such compound, 2-(1'H-indole-3'-carbonyl)-thiazole-4-carboxylic acid methyl ester (ITE), was isolated from tissue and found to be potent in preliminary tests [J. Song, M. Clagett-Dame, R.E. Peterson, M.E. Hahn, W.M. Westler, R.R. Sicinski, H.F. DeLuca, Proc. Natl. Acad. Sci. USA 99 (2002) 14694-14699]. We have synthesized ITE and [(3)H]ITE and further evaluated its AhR activity in several in vitro and in vivo assays in comparison with the toxic ligand, TCDD. AhR in Hepa1c1c7 cell cytosol bound [(3)H]ITE with high affinity and the AhR.ITE complex formed in vitro bound dioxin response element (DRE) oligonucleotide as potently as TCDD.AhR. In cells treated with ITE, nuclear translocation of AhR, and induction of CYP1A1 protein and of a DRE-dependent luciferase reporter gene were observed. ITE administered to pregnant DRE-LacZ transgenic mice activated fetal AhR, observed as X-gal staining in the same sites as in TCDD-treated mice. However, unlike TCDD, ITE did not induce cleft palate or hydronephrosis. TCDD but not ITE induced thymic atrophy in young adult mice, but both ITE and TCDD caused similar loss of cells and alterations of cell profiles in cultured fetal thymi. These data demonstrate that ITE is a potent AhR agonist in cell extracts, cultured cells, and intact animals, but does not cause the toxicity associated with the more stable xenobiotic ligand, TCDD.

Effects of aryl hydrocarbon receptor signaling on the modulation of TH1/TH2 balance

An orally active antiallergic agent, M50367, skews the Th1/Th2 balance toward Th1 dominance by suppressing naive Th cell differentiation into Th2 cells in vitro. Administration results in the suppression of IgE synthesis and peritoneal eosinophilia in vivo. In this report, we determined that M50354 (an active metabolite of M50367) was a ligand for the aryl hydrocarbon receptor (AhR); the immunological effects of this compound on in vitro Th1/Th2 differentiation from naive Th cells and Th1/Th2 balance in vivo were manifested through binding to AhR. These effects were completely abolished in AhR-deficient mice. AhR expression in the naive Th cell was significantly up-regulated by costimulation of TCR and CD28. Suppression of naive Th cell differentiation into Th2 cells via binding of M50354 to AhR was associated with inhibition of GATA-3 expression in Th cells. In addition, forced expression of a constitutively active form of AhR or activation of AhR by the addition of representative ligands suppressed naive Th cell differentiation into Th2 cells. Based on these results, we conclude that AhR functions as a modulator of the in vivo Th1/Th2 balance through activation in naive Th cells.

The aryl hydrocarbon receptor is a regulator of cigarette smoke induction of the cyclooxygenase and prostaglandin pathways in human lung fibroblasts

Cigarette smoking can lead to chronic lung inflammation and lung cancer. Chronic inflammation, associated with expression of cyclooxygenase-2 (COX-2) and prostaglandins, predisposes to malignancy. We recently demonstrated that human lung fibroblasts are activated by cigarette smoke to express COX-2 and prostaglandin E(2) (PGE(2)). Little is known about the mechanism whereby smoke activates human lung fibroblasts to produce proinflammatory mediators. Herein, we report the central role of the aryl hydrocarbon receptor (AHR) in cigarette smoke extract (CSE)-induced COX-2, microsomal PGE(2) synthase (mPGES), and PGE(2) production in human lung fibroblasts. Western blot analysis revealed that primary strains of human lung fibroblasts express AHR and aryl hydrocarbon nuclear translocator protein, supporting the possibility that smoke activates lung fibroblasts through this pathway. Experiments were subsequently performed to determine whether the AHR was activated by CSE. Immunocytochemistry and EMSA analysis revealed that CSE induced nuclear translocation of the AHR in human lung fibroblasts. CSE decreased protein levels of the AHR, consistent with AHR ligand-induced proteosome-mediated degradation. CSE also induced mPGES-1 and COX-2 protein and increased PGE(2) production. Treatment of human fibroblasts with AHR antagonists in the presence of CSE inhibited AHR nuclear translocation as well as COX-2, mPGES-1, and PGE(2) production. These data indicate that the AHR pathway plays an important role in cigarette smoke-mediated COX-2 and PG production in human lung fibroblasts and may contribute to tobacco-associated inflammation and lung disease.

A new southwestern chemistry-based ELISA for detection of aryl hydrocarbon receptor transformation: application to the screening of its receptor agonists and antagonists

Halogenated aromatic hydrocarbons (HAHs), such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), produce a wide variety of biological and toxic effects mainly through the aryl hydrocarbon receptor (AhR)-dependent mechanism. After the binding of HAHs, the AhR subsequently transforms its form in order to interact with a specific DNA sequence, the dioxin responsive element (DRE). Thus, detection of the transformed AhR is a target for estimation of the biological and toxic potency of ligands. In this study, we have developed a simple method for quantitative assessment of the transformation state of AhR based on an enzyme-linked immunosorbent assay (ELISA) combined with southwestern chemistry technique (SW-ELISA) that detects the complex of transformed AhR:fluorescein isothiocyanate (FITC)-labeled DRE probe. SW-ELISA has shown the response to HAHs including TCDD and other known agonists in a dose-dependent manner. In the case of TCDD, SW-ELISA has revealed a minimum detection limit (MDL) of 2 pM (0.026 pg/assay), a median effective concentration (EC(50)) value of 0.125 nM (1.6 pg/assay), and a maximum response at 10 nM (129 pg/assay). Furthermore, SW-ELISA provides the confirmation that flavonoids, the potent antagonists for AhR as reported previously, show the inhibitory effects on TCDD-induced AhR transformation. These results indicate that SW-ELISA is a new and straightforward method for the detection of AhR transformation and will be useful in screening of agonists or antagonists for AhR.

THE JUN N-TERMINAL KINASE INHIBITOR SP600125 IS A LIGAND AND ANTAGONIST OF THE ARYL HYDROCARBON RECEPTOR

Exposure of the immortalized human breast epithelial cell line MCF10A to the Jun N-terminal kinase (JNK) inhibitor anthra[1,9-cd]pyrazol-6(2H)-one (SP600125) suppressed, in a concentration-dependent manner (IC50 is approximately 2 microM), the induction of CYP1A1 by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Cotreatment with SP600125 also suppressed the accumulation of TCDD-induced nuclear aryl hydrocarbon receptor (AhR)-DNA complexes, as assessed by electrophoretic mobility shift assays. Concentrations of SP600125 < or = 50 microM did not transform the AhR into a DNA-binding species when added to rat liver cytosol. However, addition of SP600125 to cytosol just before TCDD addition completely suppressed AhR transformation and DNA binding (IC50 approximately 7 microM). Sucrose gradient analyses using rat liver and murine hepatoma 1c1c7 extracts demonstrated that SP600125 competed with TCDD for binding to the AhR. These results suggest that SP600125 is an AhR ligand and functions as an AhR antagonist at concentrations used to pharmacologically inhibit JNK.

3'-methoxy-4'-nitroflavone, a reported aryl hydrocarbon receptor antagonist, enhances Cyp1a1 transcription by a dioxin responsive element-dependent mechanism

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor, regulating expression of a group of specific genes including cytochrome P450 1A1 (Cyp1a1). Stably transfected luciferase with dioxin responsive elements (DRE) in its promoter region has been commonly used as a reporter gene to study the mechanism of AhR signaling and compare potencies of TCDD and related compounds. However, how these two genes might respond to structurally diverse AhR ligands was unknown. This study investigates their expression in the same cells in response to TCDD, the most potent agonist, and 3'M4'NF, a reported potent antagonist. Our data suggest that these two compounds appear to play different roles in regulating these genes. While TCDD enhanced transcription of both genes, 3'M4'NF induced the endogenous Cyp1a1, but not the reporter gene. Mechanistic studies indicated that the increase in induction of CYP1A1 protein by 3'M4'NF was mediated by AhR-dependent transcriptional activation. Further analysis of the Cyp1a1 promoter sequence did not reveal any 3'M4'NF-specific responsive elements other than DREs. Rather, the interaction between the 3'M4'NF-bound receptor complex and DREs was confirmed by the observation that a single nucleotide mutation in DRE core sequences obliterated AhR enhancer activity in response to both TCDD and 3'M4'NF. Together these data suggest that 3'M4'NF, a weak AhR agonist, activates the AhR to recognize and interact with the same DREs as TCDD. However, depending on its concentration as well as the promoter context of a particular gene, the ability of 3'M4'NF to act as an AhR antagonist or agonist may appear different for various genes.