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

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.

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

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

Tolerogenic nanoparticles suppress central nervous system inflammation

Therapeutic approaches for the induction of immune tolerance remain an unmet clinical need for the treatment of autoimmune diseases, including multiple sclerosis (MS). Based on its role in the control of the immune response, the ligand-activated transcription factor aryl hydrocarbon receptor (AhR) is a candidate target for novel immunotherapies. Here, we report the development of AhR-activating nanoliposomes (NLPs) to induce antigen-specific tolerance. NLPs loaded with the AhR agonist ITE and a T cell epitope from myelin oligodendrocyte glycoprotein (MOG)35-55 induced tolerogenic dendritic cells and suppressed the development of experimental autoimmune encephalomyelitis (EAE), a preclinical model of MS, in preventive and therapeutic setups. EAE suppression was associated with the expansion of MOG35-55-specific FoxP3+ regulatory T cells (Treg cells) and type 1 regulatory T cells (Tr1 cells), concomitant with a reduction in central nervous system-infiltrating effector T cells (Teff cells). Notably, NLPs induced bystander suppression in the EAE model established in C57BL/6 × SJL F1 mice. Moreover, NLPs ameliorated chronic progressive EAE in nonobese diabetic mice, a model which resembles some aspects of secondary progressive MS. In summary, these studies describe a platform for the therapeutic induction of antigen-specific tolerance in autoimmune diseases.

Crosstalk between hydroxytyrosol, a major olive oil phenol, and HIF-1 in MCF-7 breast cancer cells

Olive oil intake has been linked with a lower incidence of breast cancer. Hypoxic microenvironment in solid tumors, such as breast cancer, is known to play a crucial role in cancer progression and in the failure of anticancer treatments. HIF-1 is the foremost effector in hypoxic response, and given that hydroxytyrosol (HT) is one of the main bioactive compounds in olive oil, in this study we deepen into its modulatory role on HIF-1. Our results in MCF-7 breast cancer cells demonstrate that HT decreases HIF-1α protein, probably by downregulating oxidative stress and by inhibiting the PI3K/Akt/mTOR pathway. Strikingly, the expression of HIF-1 target genes does not show a parallel decrease. Particularly, adrenomedullin and vascular endothelial growth factor are up-regulated by high concentrations of HT even in HIF-1α silenced cells, pointing to HIF-1-independent mechanisms of regulation. In fact, we show, by in silico modelling and transcriptional analysis, that high doses of HT may act as an agonist of the aryl hydrocarbon receptor favoring the induction of these angiogenic genes. In conclusion, we suggest that the effect of HT in a hypoxic environment is largely affected by its concentration and involves both HIF-1 dependent and independent mechanisms.

Risk for animal and human health related to the presence of dioxins and dioxin-like PCBs in feed and food

The European Commission asked EFSA for a scientific opinion on the risks for animal and human health related to the presence of dioxins (PCDD/Fs) and DL-PCBs in feed and food. The data from experimental animal and epidemiological studies were reviewed and it was decided to base the human risk assessment on effects observed in humans and to use animal data as supportive evidence. The critical effect was on semen quality, following pre- and postnatal exposure. The critical study showed a NOAEL of 7.0 pg WHO2005-TEQ/g fat in blood sampled at age 9 years based on PCDD/F-TEQs. No association was observed when including DL-PCB-TEQs. Using toxicokinetic modelling and taking into account the exposure from breastfeeding and a twofold higher intake during childhood, it was estimated that daily exposure in adolescents and adults should be below 0.25 pg TEQ/kg bw/day. The CONTAM Panel established a TWI of 2 pg TEQ/kg bw/week. With occurrence and consumption data from European countries, the mean and P95 intake of total TEQ by Adolescents, Adults, Elderly and Very Elderly varied between, respectively, 2.1 to 10.5, and 5.3 to 30.4 pg TEQ/kg bw/week, implying a considerable exceedance of the TWI. Toddlers and Other Children showed a higher exposure than older age groups, but this was accounted for when deriving the TWI. Exposure to PCDD/F-TEQ only was on average 2.4- and 2.7-fold lower for mean and P95 exposure than for total TEQ. PCDD/Fs and DL-PCBs are transferred to milk and eggs, and accumulate in fatty tissues and liver. Transfer rates and bioconcentration factors were identified for various species. The CONTAM Panel was not able to identify reference values in most farm and companion animals with the exception of NOAELs for mink, chicken and some fish species. The estimated exposure from feed for these species does not imply a risk.

Voriconazole-induced photocarcinogenesis is promoted by aryl hydrocarbon receptor-dependent COX-2 upregulation

Voriconazole (VRCZ) induces the development of UV-associated skin cancers. The mechanism underlying the VRCZ-induced carcinogenesis has been largely unknown. Here, we showed that VRCZ metabolites plus UVA generated reactive oxygen species and resultant DNA damage of the epidermis, but did not induce substantial apoptosis in human keratinocytes (KCs). Furthermore, VRCZ per se stimulates aryl hydrocarbon receptor (AhR) and upregulates COX-2, which is a pivotal enzyme for the promotion of UV-associated tumors, in an AhR-ARNT dependent manner of the classical (genomic) pathway. Our findings suggest that the phototoxic moieties of VRCZ metabolites may participate in the initiation phase of VRCZ skin cancer, while VRCZ per se promotes the tumor development. Therefore, during VRCZ therapy, sun exposure protection is essential to prevent photocarcinogenesis caused by VRCZ metabolites plus UV. Chemoprevention with selective COX-2 inhibitors may be helpful to repress the development of skin cancers derived from DNA-damaged KCs.

Detection of aryl hydrocarbon receptor agonists in human samples

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor with important functions in the immune response and cancer. AHR agonists are provided by the environment, the commensal flora and the metabolism. Considering AHR physiological functions, AHR agonists may have important effects on health and disease. Thus, the quantification of AHR agonists in biological samples is of scientific and clinical relevance. We compared different reporter systems for the detection of AHR agonists in serum samples of Multiple Sclerosis (MS) patients, and assessed the influence of transfection methods and cell lines in a reporter-based in vitro assay. While the use of stable or transient reporters did not influence the measurement of AHR agonistic activity, the species of the cell lines used in these reporter assays had important effects on the reporter readings. These observations suggest that cell-specific factors influence AHR activation and signaling. Thus, based on the reported species selectivity of AHR ligands and the cell species-of-origin effects that we describe in this manuscript, the use of human cell lines is encouraged for the analysis of AHR agonistic activity in human samples. These findings may be relevant for the analysis of AHR agonists in human samples in the context of inflammatory and neoplastic disorders.

Dynamic regulation of serum aryl hydrocarbon receptor agonists in MS

Objective:

Several factors influence the clinical course of autoimmune inflammatory diseases such as MS and inflammatory bowel disease. Only recently, the complex interaction between the gut microbiome, dietary factors, and metabolism has started to be appreciated with regard to its potential to modulate acute and chronic inflammation. One of the molecular sensors that mediates the effects of these environmental signals on the immune response is the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor with key functions in immune cells.

Methods:

In this study, we analyzed the levels of AHR agonists in serum samples from patients with MS and healthy controls in a case-control study.

Results:

We detected a global decrease of circulating AHR agonists in relapsing-remitting MS patients as compared to controls. However, during acute CNS inflammation in clinically isolated syndrome or active MS, we measured increased AHR agonistic activity. Moreover, AHR ligand levels in patients with benign MS with relatively mild clinical impairment despite longstanding disease were unaltered as compared to healthy controls.

Conclusions:

Collectively, these data suggest that AHR agonists in serum are dynamically modulated during the course of MS. These findings may guide the development of biomarkers to monitor disease activity as well as the design of novel therapeutic interventions for MS.

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.

Current basis for discovery and development of aryl hydrocarbon receptor antagonists for experimental and therapeutic use in atherosclerosis

The important role played by aryl hydrocarbon receptor activation in the pathophysiology of atherosclerosis induced by cigarette smoke exposure has spurred the clinical interest in the development of aryl hydrocarbon receptor antagonists with atheroprotective efficacy. A few aryl hydrocarbon receptor antagonists were developed but the lack of structural information regarding the receptor ligand binding domain resulted in several limitations in the pharmacological properties of these compounds including partial agonism, allosterism, non-selectivity, cytotoxicity and susceptibility to bioactivation. These limitations make the progress of preclinical and clinical assays with the available aryl hydrocarbon receptor antagonists difficult. There is a great interest in developing pure, competitive, selective, nontoxic and resistant to bioactivation aryl hydrocarbon receptor antagonists. Current technology permits the development of pharmacologically ideal antagonists based on the chemical features of the aryl hydrocarbon receptor ligand binding domain. According to these characteristics, chlorinated derivatives of trans-stilbene meta-substituted with electrophilic aromatic directing groups would be effective prototypes for pure, competitive, selective, nontoxic and resistant to bioactivation antagonists for such receptor.

Aryl hydrocarbon receptor activation leads to impairment of estrogen-driven chicken vitellogenin promoter activity in LMH cells

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that mediates most of the toxic effects of environmental contaminants. Among the multiple pleiotropic responses elicited by AHR agonists, the antiestrogenic and endocrine-disrupting action of the receptor activation is one of the most studied. It has been demonstrated that some AHR agonists disrupt estradiol-induced vitellogenin synthesis in the fish liver via a mechanism that involves crosstalk between the AHR and the estrogen receptor (ER). Chicken hepatocytes have become a model for the study of AHR action in birds and the induction of the signal and its effect in these cells are well established. However, the impact of AHR activation on estradiol-regulated responses in the chicken liver remains to be demonstrated. The aim of the present study was, therefore, to determine the effect of AHR action on ER-driven transcription in a convenient model of chicken liver cells. For this purpose, we designed a reporter construct bearing the 5' regulatory region of the chicken vitellogenin II gene and used it to transfect chicken hepatoma LMH cells. We found that β-naphthoflavone represses ER-driven vitellogenin promoter activity and that this action is mediated by the AHR. This inhibitory crosstalk between both pathways appears to be unidirectional, since estradiol did not alter the transcript levels of an AHR target gene. Besides, and highly relevant, we show that LMH cell line transfected with a reporter construct bearing the chicken vitellogenin promoter sequence is a useful and convenient model for the study of AHR-ER interaction in chicken liver-derived cells.

Common commercial and consumer products contain activators of the aryl hydrocarbon (dioxin) receptor

Activation of the Ah receptor (AhR) by halogenated aromatic hydrocarbons (HAHs), such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin), can produce a wide variety of toxic and biological effects. While recent studies have shown that the AhR can bind and be activated by structurally diverse chemicals, how widespread of these AhR agonists are in environmental, biological and synthetic materials remains to be determined. Using AhR-based assays, we demonstrate the presence of potent AhR agonists in a variety of common commercial and consumer items. Solvent extracts of paper, rubber and plastic products contain chemicals that can bind to and stimulate AhR DNA binding and/or AhR-dependent gene expression in hepatic cytosol, cultured cell lines, human epidermis and zebrafish embryos. In contrast to TCDD and other persistent dioxin-like HAHs, activation of AhR-dependent gene expression by these extracts was transient, suggesting that the agonists are metabolically labile. Solvent extracts of rubber products produce AhR-dependent developmental toxicity in zebrafish in vivo, and inhibition of expression of the metabolic enzyme CYP1A, significantly increased their toxic potency. Although the identity of the responsible AhR-active chemicals and their toxicological impact remain to be determined, our data demonstrate that AhR active chemicals are widely distributed in everyday products.

Development of novel CH223191-based antagonists of the aryl hydrocarbon receptor

Aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that regulates genes involved in drug/xenobiotic metabolism, cell cycle progression, cell fate determination, immune function, and inflammatory response. Increasing evidence that AHR plays a role in the pathophysiology of a number of human disease states is driving the need for improved pharmacological tools to be used for understanding the in vivo impact of AHR modulation. In this study, we have characterized and used structure-activity relationship analyses of a newly synthesized library of derivatives of the potent AHR antagonist 2-methyl-2H-pyrazole-3-carboxylic acid (2-methyl-4-o-tolylazo-phenyl)-amide (CH223191). Initial screening of these compounds revealed that those bearing groups with strong electronegativity at the R1 position (i.e., CHD-5, CHD-11, and CHD-12) versus those that are more electron-poor at this position (i.e., CHD-7 and CHD-8) elicited the most potent AHR antagonistic properties. The ability of these derivatives to inhibit agonist (2,3,7,8-tetrachlorodibenzo-p-dioxin) binding, nuclear translocation of AHR, and agonist-induced enzyme activity also were determined and support the initial findings. Furthermore, CH223191, but not CHD-5, CHD-11, or CHD-12, was found to exhibit AHR-independent proproliferative properties. These results contribute to our understanding of the structural requirements of potent AHR antagonists and the development of effective pharmacological tools to be used for studying the pathophysiological role of AHR.

New aryl hydrocarbon receptor homology model targeted to improve docking reliability

The aryl hydrocarbon receptor (AhR) is a ligand-dependent, basic helix-loop-helix Per-ARNT-Sim (PAS) containing transcription factor that can bind and be activated by structurally diverse chemicals, including the toxic environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). As no experimentally determined structures of the AhR ligand binding domain (LBD) are available and previous homology models were only derived from apo template structures, we developed a new model based on holo X-ray structures of the hypoxia-inducible factor 2α (HIF-2α) PAS B domain, targeted to improve the accuracy of the binding site for molecular docking applications. We experimentally confirmed the ability of two HIF-2α crystallographic ligands to bind to the mAhR with relatively high affinity and demonstrated that they are AhR agonists, thus justifying the use of the holo HIF-2α structures as templates. A specific modeling/docking approach was proposed to predict the binding modes of AhR ligands in the modeled LBD. It was validated by comparison of the calculated and the experimental binding affinities of active THS ligands and TCDD for the mAhR and by functional activity analysis using several mAhR mutants generated on the basis of the modeling results. Finally the ability of the proposed approach to reproduce the different affinities of TCDD for AhRs of different species was confirmed, and a first test of its reliability in virtual screening is carried out by analyzing the correlation between the calculated and experimental binding affinities of a set of 14 PCDDs.

Exactly the same but different: promiscuity and diversity in the molecular mechanisms of action of the aryl hydrocarbon (dioxin) receptor

The Ah receptor (AhR) is a ligand-dependent transcription factor that mediates a wide range of biological and toxicological effects that result from exposure to a structurally diverse variety of synthetic and naturally occurring chemicals. Although the overall mechanism of action of the AhR has been extensively studied and involves a classical nuclear receptor mechanism of action (i.e., ligand-dependent nuclear localization, protein heterodimerization, binding of liganded receptor as a protein complex to its specific DNA recognition sequence and activation of gene expression), details of the exact molecular events that result in most AhR-dependent biochemical, physiological, and toxicological effects are generally lacking. Ongoing research efforts continue to describe an ever-expanding list of ligand-, species-, and tissue-specific spectrum of AhR-dependent biological and toxicological effects that seemingly add even more complexity to the mechanism. However, at the same time, these studies are also identifying and characterizing new pathways and molecular mechanisms by which the AhR exerts its actions and plays key modulatory roles in both endogenous developmental and physiological pathways and response to exogenous chemicals. Here we provide an overview of the classical and nonclassical mechanisms that can contribute to the differential sensitivity and diversity in responses observed in humans and other species following ligand-dependent activation of the AhR signal transduction pathway.

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.

CH223191 is a ligand-selective antagonist of the Ah (Dioxin) receptor

The aryl hydrocarbon (dioxin) receptor (AhR) is a ligand-dependent transcription factor that produces a wide range of biological and toxic effects in many species and tissues. Whereas the best-characterized high-affinity ligands include structurally related halogenated aromatic hydrocarbons (HAHs) and polycyclic aromatic hydrocarbons (PAHs), the AhR is promiscuous and can also be activated by structurally diverse exogenous and endogenous chemicals. However, little is known about how these diverse ligands actually bind to and activate the AhR. Utilizing AhR ligand binding, DNA binding, and reporter gene expression assays, we have identified a novel ligand-selective antagonist (CH223191) that preferentially inhibits the ability of some classes of AhR agonists (2,3,7,8-tetrachlorodibenzo-p-dioxin and related HAHs), but not others (PAHs, flavonoids, or indirubin), to bind to and/or activate the AhR and AhR signal transduction. HAH-specific antagonism of AhR-dependent reporter gene expression by CH223191 was observed with mouse, rat, human, and guinea pig cell lines. Ligand- and species-selective antagonism was also observed with the AhR antagonists 3'-methoxy-4'-nitroflavone and 6,2',4',-trimethoxyflavone. Our results suggest that the differences in the binding by various ligands to the AhR contribute to the observed structural diversity of AhR ligands and could contribute in ligand-specific variation in AhR functionality and the toxic and biological effects of various classes of AhR agonists.

Amino acid substitutions in the aryl hydrocarbon receptor ligand binding domain reveal YH439 as an atypical AhR activator

The aryl hydrocarbon receptor (AhR) is traditionally defined as a transcription factor activated by exogenous polyaromatic and halogenated aromatic hydrocarbon (PAH/HAH) ligands. Active AhR induces genes involved in xenobiotic metabolism, including cytochrome P4501A1, which function to metabolize activating ligands. However, recent studies implicate AhR in biological events that are apparently unrelated to the xenobiotic response, implying that endogenous activation mechanisms exist. Three AhR genes in zebrafish (Danio rerio) encode proteins that demonstrate differential activation in response to PAH/HAHs, with the nonresponsive drAhR1a having some sequence divergence from the PAH/HAH-responsive AhRs in the ligand binding domain (LBD). We used these differences to guide the mutagenesis of mouse AhR (mAhR), aiming to generate variants that functionally discriminate between activation mechanisms. We found substitution of histidine 285 in the LBD with tyrosine gave a receptor that could be activated by isopropyl-2-(1,3-dithietane-2-ylidene)-2-[N-(4-methylthiazol-2-yl)carbamoyl]acetate (YH439), a potential AhR ligand chemically distinct from classic PAH/HAH-type ligands, but prevented activation by both exogenous PAH/HAH ligands and the endogenous activation mimics of suspension culture and application of shear-stressed serum. The differential response of H285Y mAhR to YH439 suggests that this activator has a novel mode of interaction that tolerates tyrosine at position 285 in the LBD and is distinct from the binding mode of the well characterized PAH/HAH ligands. In support of this, the PAH-type antagonist 3',4'-dimethoxyflavone blocked mAhR activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin but not YH439. Furthermore, the strict correlation between response to exogenous PAH/HAH ligands and mimics of endogenous activation suggests that a PAH-type ligand may underpin endogenous mechanisms of activation.

Detection of the TCDD binding-fingerprint within the Ah receptor ligand binding domain by structurally driven mutagenesis and functional analysis

The aryl hydrocarbon receptor (AhR) is a ligand-dependent, basic helix-loop-helix Per-Arnt-Sim (PAS)-containing transcription factor that can bind and be activated by structurally diverse chemicals, including the toxic environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Our previous three-dimensional homology model of the mouse AhR (mAhR) PAS B ligand binding domain allowed identification of the binding site and its experimental validation. We have extended this analysis by conducting comparative structural modeling studies of the ligand binding domains of six additional high-affinity mammalian AhRs. These results, coupled with site-directed mutagenesis and AhR functional analysis, have allowed detection of the "TCDD binding-fingerprint" of conserved residues within the ligand binding cavity necessary for high-affinity TCDD binding and TCDD-dependent AhR transformation DNA binding. The essential role of selected residues was further evaluated using molecular docking simulations of TCDD with both wild-type and mutant mAhRs. Taken together, our results dramatically improve our understanding of the molecular determinants of TCDD binding and provide a basis for future studies directed toward rationalizing the observed species differences in AhR sensitivity to TCDD and understanding the mechanistic basis for the dramatic diversity in AhR ligand structure.

Food flavonoid aryl hydrocarbon receptor-mediated agonistic/antagonistic/synergic activities in human and rat reporter gene assays

Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor mediating the adverse effects of dioxins and polycyclic aromatic hydrocarbons (PAHs). In this study, we investigated the genetic-, time-, dose-, species- and tissue-dependent AhR-mediated agonistic/antagonistic activities of three food flavonoids: quercetin, chrysin and genistein. To that end, four stably transfected cell lines were used in cell-based luciferase reporter gene assays: three lines were transformed with the ptKLuc vector harbouring four dioxin-responsive elements (DREs) upstream of the thymidine kinase promoter and the luciferase gene (HepG2-Luc, T-47D-Luc and H4IIE-ULg). The fourth is a patented cell line transformed with a different construct: H4IIE DR-CALUX((R)). Both H4IIE cells were compared for their genetic construction. Human hepatoma (HepG2-Luc) and human breast tumour (T-47D-Luc) cells were compared for tissue-dependent effects. Rat hepatoma (H4IIE-ULg) and human hepatoma (HepG2-Luc) cells were compared for species-dependent activities. We concluded that quercetin, chrysin and genistein act in a time-, dose-, species- and tissue-specific way. For example, genistein displayed agonistic activities when exposed to rat hepatoma cells during 6h but not after 24h. Flavonoids displayed agonistic/antagonistic activities in human breast tumour cells, depending on the exposure time, while in human hepatoma cells, only antagonistic activities of flavonoids were measured. In addition, we report, in all the cells, a synergy between an isoflavone and two food contaminants; the 2,3,7,8-tetrachlorodibenzo-p-dioxin and 3-methylcholanthrene, a PAH. In rat cells, this synergy occurred when cells were exposed to flavonoids and contaminant for 6h, while it was observed in human cells only after 24h.