Evidence for an aryl hydrocarbon receptor-mediated cytochrome p450 autoregulatory pathway

Aryl hydrocarbon receptor activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin impairs human B lymphopoiesis

The homeostasis of peripheral B cell compartment requires lifelong B lymphopoiesis from hematopoietic stem cells (HSC). As a result, the B cell repertoire is susceptible to disruptions of hematopoiesis. Increasing evidence, primarily from rodent models, shows that the aryl hydrocarbon receptor (AHR) regulates hematopoiesis. To study the effects of persistent AHR activation on human B cell development, a potent AHR agonist and known environmental contaminant, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was utilized. An in vitro B cell development model system was established by co-culturing human cord blood-derived HSCs with irradiated human primary bone marrow stromal cells. Using this in vitro model, we found that TCDD significantly suppressed the total number of hematopoietic stem and progenitor cells (HSPC) in a concentration-dependent manner. Cell death analysis demonstrated that the decrease in cell number was not due to cytotoxicity by TCDD. In addition, TCDD markedly decreased CD34 expression on HSPCs. Structure-activity relationship studies using dioxin congeners demonstrated a correlation between the relative AHR binding affinity and the magnitude of decrease in the number of HSPCs and CD34 expression, suggesting that AHR mediates the observed TCDD-elicited changes in HSPCs. Moreover, a significant reduction in lineage committed B cell-derived from HSCs was observed in the presence of TCDD, indicating impairment of human B cell development. Similar effects of TCDD were observed regardless of the use of stromal cells in cultures indicating a direct effect of TCDD on HSCs. Collectively, we demonstrate that AHR activation by TCDD on human HSCs impairs early stages of human B lymphopoiesis.

In vitro and in silico evaluation of transactivation potencies of avian AHR1 and AHR2 by endogenous ligands: Implications for the physiological role of avian AHR2

Aryl hydrocarbon receptor (AHR) is well conserved from invertebrates to vertebrates, and it mediates the toxic effects of exogenous ligands, including dioxins. Recent studies reported that AHRs activated by endogenous ligands play critical roles in mammalian physiological homeostasis. Avian species possess at least two AHR isoforms (AHR1 and AHR2), which exhibit species- and isoform-specific transactivation potencies to exogenous ligands, whereas mammals possess a single AHR. To delineate the profiles and roles of endogenous ligands for avian AHR isoforms, we investigated in vitro transactivation potencies of avian AHRs (AHR1 and AHR2 from the jungle crow, Corvus macrorhynchos; common cormorant, Phalacrocorax carbo; and black-footed albatross, Phoebastria nigripes) treated with the endogenous tryptophan metabolites 6-formylindolo [3,2-b] carbazole (FICZ), l-kynurenine (l-Kyn), kynurenic acid (KYNA), and indoxyl sulfate (IS). Furthermore, we analyzed the binding mode of these ligands to each avian AHR isoform by in silico docking simulations. The EC50 of FICZ (0.009-0.032nM) was similar regardless of the species or isoform of AHR. The estimated in silico binding mode of FICZ to AHRs was well conserved in both isoforms. The transactivation potencies of avian AHRs to other tryptophan metabolites were 10(5)-10(7) fold lower than those for FICZ, and EC50 values varied in a species- and isoform-specific manner. This was consistent with poor conservation of the binding mode of l-Kyn, KYNA, and IS predicted in in silico docking simulations. Our results suggest that in avian species, FICZ is the most potent endogenous AHR ligand, and that AHR1 and AHR2 are physiologically functional.

Cyp1b1-mediated suppression of lymphoid progenitors in bone marrow by polycyclic aromatic hydrocarbons coordinately impacts spleen and thymus: a selective role for the Ah Receptor

Bone marrow (BM) hematopoietic stem cells differentiate to common lymphoid progenitors (CLP) that emigrate to the thymus to form T cells or differentiate into immature B cells that then migrate to the spleen for maturation. Rapid in vivo suppression of BM progenitor cells by a single oral or intraperitoneal dose of 7,12-dimethylbenz(a)anthracene (DMBA) subsequently decreased mature lymphoid populations in BM, spleen, and thymus. These suppressions depended on BM CYP1B1, but not on aryl hydrocarbon receptor (AhR) activity. Suppression of pre-B colony formation at 6 h, correlated with subsequent decreases in mature BM, spleen, and thymus populations (48-168 h). Thymus T-cell ratios were unaffected, suggesting low local toxicity. DMBA treatment suppressed progenitor cells 24-h post treatment in wild type (WT), AhRb mice, but not in Cyp1b1-ko mice. The stem cell populations were sustained. Benzo(a)pyrene (BP) mediated a similar progenitor suppression up to 6 h, but reversal rapidly ensued. This recovery was absent in mice with a polycyclic aromatic hydrocarbon (PAH)-resistant, AhRd genotype. This AhR-dependent progenitor recovery with BP induction accounts for the absence of suppression of B220+ BM and spleen populations at 48-168 h. However, DMBA and BP produced similar profiles for thymus cell suppression, independent of AhR genotype. Thus, lymphoid progenitors may exit the BM to the thymus prior to the BP reversal. This progenitor recovery is associated with elevated chemokines and cytokines that depend on AhR-mediated induction of CYP1A1. This response increased constitutively in Cyp1b1-ko BM, demonstrating that CYP1B1 metabolizes local stimulants that impact a basal progenitor protection process.

Hepatic Aryl Hydrocarbon Receptor Attenuates Fibroblast Growth Factor 21 Expression

The Aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor involved in many physiological processes. Several studies indicate that AHR is also involved in energy homeostasis. Fibroblast growth factor 21 (FGF21) is an important regulator of the fasting and feeding responses. When administered to various genetic and diet-induced mouse models of obesity, FGF21 can attenuate obesity-associated morbidities. Here, we explore the role of AHR in hepatic Fgf21 expression through the use of a conditional, hepatocyte-targeted AHR knock-out mouse model (Cre(Alb)Ahr(Fx/Fx)). Compared with the congenic parental strain (Ahr(Fx/Fx)), non-fasted Cre(Alb)Ahr(Fx/Fx) mice exhibit a 4-fold increase in hepatic Fgf21 expression, as well as elevated expression of the FGF21-target gene Igfbp1 Furthermore, in vivo agonist activation of AHR reduces hepatic Fgf21 expression during a fast. The Fgf21 promoter contains several putative dioxin response elements (DREs). Using EMSA, we demonstrate that the AHR-ARNT heterodimer binds to a specific DRE that overlaps binding sequences for peroxisome proliferator-activated receptor α (PPARα), carbohydrate response element-binding protein (ChREBP), and cAMP response element-binding protein, hepatocyte specific (CREBH). In addition, we reveal that agonist-activated AHR impairs PPARα-, ChREBP-, and CREBH-mediated promoter activity in Hepa-1 cells. Accordingly, agonist treatment in Hepa-1 cells ablates potent ER stress-driven Fgf21 expression, and pre-treatment with AHR antagonist blocks this effect. Finally, we show that pre-treatment of primary human hepatocytes with AHR agonist diminishes PPARα-, glucose-, and ER stress-driven induction of FGF21 expression, indicating the effect is not mouse-specific. Together, our data show that AHR contributes to hepatic energy homeostasis, partly through the regulation of FGF21 expression and signaling.

Biological effects of 6-formylindolo[3,2-b]carbazole (FICZ) in vivo are enhanced by loss of CYP1A function in an Ahr2-dependent manner

6-Formylindolo[3,2-b]carbazole (FICZ) is a potent aryl hydrocarbon receptor (AHR) agonist that is efficiently metabolized by AHR-regulated cytochrome P4501 enzymes. FICZ is a proposed physiological AHR ligand that induces its own degradation as part of a regulatory negative feedback loop. In vitro studies in cells show that CYP1 inhibition in the presence of FICZ results in enhanced AHR activation, suggesting that FICZ accumulates in the cell when its metabolism is blocked. We used zebrafish (Danio rerio) embryos to investigate the in vivo effects of FICZ when CYP1A is knocked down or inhibited. Embryos were injected with morpholino antisense oligonucleotides targeting CYP1A (CYP1A-MO), Ahr2, or a combination of both. FICZ exposure of non-injected embryos or embryos injected with control morpholino had little effect. In CYP1A-MO-injected embryos, however, FICZ dramatically increased mortality, incidence and severity of pericardial edema and circulation failure, reduced hatching frequency, blocked swim bladder inflation, and strongly potentiated expression of Ahr2-regulated genes. These effects were substantially reduced in embryos with a combined knockdown of Ahr2 and CYP1A, indicating that the toxicity was mediated at least partly by Ahr2. Co-exposure to the CYP1 inhibitor alpha-naphthoflavone (αNF) and FICZ had similar effects as the combination of CYP1A-MO and FICZ. HPLC analysis of FICZ-exposed embryos showed increased levels of FICZ after concomitant CYP1A-MO injection or αNF co-exposure. Together, these results show that a functioning CYP1/AHR feedback loop is crucial for regulation of AHR signaling by a potential physiological ligand in vivo and further highlights the role of CYP1 enzymes in regulating biological effects of FICZ.

Identification of leads for antiproliferative activity on MDA-MB-435 human breast cancer cells through pharmacophore and CYP1A1-mediated metabolism

CYP1A1 is a potential target for anticancer drug development due to its overexpression in certain cancer cells and role in cancer progression. To identify new leads for CYP1A1 mediated anticancer action, we attempted ligand based pharmacophore mapping, virtual screening of databases, molecular docking, MetaSite based filtering, and molecular dynamics simulations. Initial computational and in vitro screening identified 11 compounds from which we identified two lead compounds, ZINC33468944 and ZINC32101539, showed potential antitumor activity on MDA-MB-435 cell lines (GI50 < 0.1 μM) and CYP1A1 inhibition of 0.13 and 0.3 μM, respectively. Furthermore, the lead compounds were evaluated for CYP1A1 mediated metabolism, showing N-hydroxylated metabolites, which have potential of DNA adduct formation and cause cancerous cell death. Analysis of molecular dynamics simulations provided important guidelines for the further modification of the lead compounds. Hence, we claim the lead molecules for further development in anticancer drug discovery.

Indole and Tryptophan Metabolism: Endogenous and Dietary Routes to Ah Receptor Activation

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor recognized for its role in xenobiotic metabolism. The physiologic function of AHR has expanded to include roles in immune regulation, organogenesis, mucosal barrier function, and the cell cycle. These functions are likely dependent upon ligand-mediated activation of the receptor. High-affinity ligands of AHR have been classically defined as xenobiotics, such as polychlorinated biphenyls and dioxins. Identification of endogenous AHR ligands is key to understanding the physiologic functions of this enigmatic receptor. Metabolic pathways targeting the amino acid tryptophan and indole can lead to a myriad of metabolites, some of which are AHR ligands. Many of these ligands exhibit species selective preferential binding to AHR. The discovery of specific tryptophan metabolites as AHR ligands may provide insight concerning where AHR is activated in an organism, such as at the site of inflammation and within the intestinal tract.

In-depth genomic data analyses revealed complex transcriptional and epigenetic dysregulations of BRAFV600E in melanoma

BACKGROUND

The recurrent BRAF driver mutation V600E (BRAF (V600E)) is currently one of the most clinically relevant mutations in melanoma. However, the genome-wide transcriptional and epigenetic dysregulations induced by BRAF (V600E) are still unclear. The investigation of this driver mutation's functional consequences is critical to the understanding of tumorigenesis and the development of therapeutic strategies.

METHODS AND RESULTS

We performed an integrative analysis of transcriptomic and epigenomic changes disturbed by BRAF (V600E) by comparing the gene expression and methylation profiles of 34 primary cutaneous melanoma tumors harboring BRAF (V600E) with those of 27 BRAF (WT) samples available from The Cancer Genome Atlas (TCGA). A total of 711 significantly differentially expressed genes were identified as putative BRAF (V600E) target genes. Functional enrichment analyses revealed the transcription factor MITF (p < 3.6 × 10(-16)) and growth factor TGFB1 (p < 3.1 × 10(-9)) were the most significantly enriched up-regulators, with MITF being significantly up-regulated, whereas TGFB1 was significantly down-regulated in BRAF (V600E), suggesting that they may mediate tumorigenesis driven by BRAF (V600E). Further investigation using the MITF ChIP-Seq data confirmed that BRAF (V600E) led to an overall increased level of gene expression for the MITF targets. Furthermore, DNA methylation analysis revealed a global DNA methylation loss in BRAF (V600E) relative to BRAF (WT). This might be due to BRAF dysregulation of DNMT3A, which was identified as a potential target with significant down-regulation in BRAF (V600E). Finally, we demonstrated that BRAF (V600E) targets may play essential functional roles in cell growth and proliferation, measured by their effects on melanoma tumor growth using a short hairpin RNA silencing experimental dataset.

CONCLUSIONS

Our integrative analysis identified a set of BRAF (V600E) target genes. Further analyses suggested a complex mechanism driven by mutation BRAF (V600E) on melanoma tumorigenesis that disturbs specific cancer-related genes, pathways, and methylation modifications.

Snowball: resampling combined with distance-based regression to discover transcriptional consequences of a driver mutation

MOTIVATION

Large-scale cancer genomic studies, such as The Cancer Genome Atlas (TCGA), have profiled multidimensional genomic data, including mutation and expression profiles on a variety of cancer cell types, to uncover the molecular mechanism of cancerogenesis. More than a hundred driver mutations have been characterized that confer the advantage of cell growth. However, how driver mutations regulate the transcriptome to affect cellular functions remains largely unexplored. Differential analysis of gene expression relative to a driver mutation on patient samples could provide us with new insights in understanding driver mutation dysregulation in tumor genome and developing personalized treatment strategies.

RESULTS

Here, we introduce the Snowball approach as a highly sensitive statistical analysis method to identify transcriptional signatures that are affected by a recurrent driver mutation. Snowball utilizes a resampling-based approach and combines a distance-based regression framework to assign a robust ranking index of genes based on their aggregated association with the presence of the mutation, and further selects the top significant genes for downstream data analyses or experiments. In our application of the Snowball approach to both synthesized and TCGA data, we demonstrated that it outperforms the standard methods and provides more accurate inferences to the functional effects and transcriptional dysregulation of driver mutations.

AVAILABILITY AND IMPLEMENTATION

R package and source code are available from CRAN at http://cran.r-project.org/web/packages/DESnowball, and also available at http://bioinfo.mc.vanderbilt.edu/DESnowball/.

Dioxin and estrogen signaling in lung adenocarcinoma cells with different aryl hydrocarbon receptor/estrogen receptor α phenotypes

Evidence suggests that estrogen affects the pulmonary response to carcinogenic pollutants, such as dioxins. In this study, we examined dioxin and estrogen signaling cross-talk in lung adenocarcinoma cell lines that were engineered to exhibit different aryl hydrocarbon receptor (AhR)/estrogen receptor (ER) α phenotypes. Data showed that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) weakly antagonized estrogen-activated ERα activity in cells expressing abundant ERα, but little AhR. Increase of AhR expression or presence of a dioxin-responsive element in proximity silenced the antiestrogenic effect of TCDD. AhR was bound to dioxin-responsive element and transcriptionally active in both TCDD-untreated and -treated lung adenocarcinoma cells. 17β-estradiol (E2) reduced basal and TCDD-induced AhR activity only in ERα-positive cells. AhR and ERα exhibited a protein-protein interaction in the presence of E2. Cotreatment with TCDD moderated this protein interaction. Colocalization of ERα and AhR at the estrogen-responsive site under E2 and TCDD/E2 treatments implied that E2 ∣ ERα might hijack AhR away from the dioxin-responsive site. Increasing the relative expression of AhR to ERα counteracted inhibition of AhR activity by E2 ∣ ERα. When AhR and ERα were both highly expressed, TCDD and E2 up-regulated expression of dual-responsive genes cytochrome P450 (CYP) 1A1 and CYP1B1 in a cumulative manner, increasing the danger of metabolic activation of carcinogens. Whereas TCDD ∣ AhR and E2 ∣ ERα appeared to regulate CYP1B1 separately through their binding sites, E2 ∣ ERα increased the TCDD responsiveness and mRNA expression of CYP1A1 in a noncanonical way. In conclusion, AhR/ERα expression pattern, estrogen level, and promoter context determine the genomic action of dioxin in lung adenocarcinoma cells.

Aryl hydrocarbon receptor control of adaptive immunity

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that belongs to the family of basic helix-loop-helix transcription factors. Although the AhR was initially recognized as the receptor mediating the pathologic effects of dioxins and other pollutants, the activation of AhR by endogenous and environmental factors has important physiologic effects, including the regulation of the immune response. Thus, the AhR provides a molecular pathway through which environmental factors modulate the immune response in health and disease. In this review, we discuss the role of AhR in the regulation of the immune response, the source and chemical nature of AhR ligands, factors controlling production and degradation of AhR ligands, and the potential to target the AhR for therapeutic immunomodulation.

The AhR twist: ligand-dependent AhR signaling and pharmaco-toxicological implications

The aryl hydrocarbon receptor (AhR) is a transcription factor which is activated by diverse compounds and regulates the expression of xenobiotic metabolism genes. Recent studies have unraveled unsuspected physiological roles and novel alternative ligand-specific pathways for this receptor. In this review, we discuss these novel aspects and focus on the different responses elicited by the diverse endogenous and/or exogenous AhR ligands. In addition to challenging the relevance of the 'agonist/antagonist' classification of ligands, we introduce the concept of AhR plasticity as a primordial factor in the generation of these pathways. Finally, we suggest several promising perspectives for the pharmacological modulation of these responses.

TLR2 controls intestinal carcinogen detoxication by CYP1A1

Intestinal cytochrome P450 subclass 1A1 (CYP1A1) contributes to a metabolic “shield” protecting the host from ingested carcinogens such as polycyclic aromatic hydrocarbons (PAH). The expression of CYP1 (including CYP1A2 and CYP1B1) is considered to depend solely on a heterodimeric transcription factor consisting of the arylhydrocarbon receptor (AHR) and the AHR nuclear translocator (ARNT). So far, no interference has been noted between the regulation of CYP1 and the activation of Toll-like receptor 2 (TLR2), which modulates the inflammatory response to bacterial cell wall components in immune cells and enterocytes. Here we report that intestinal CYP1A1 is silenced in TLR2-deficient mice, even when under exposure to the carcinogenic PAH benzo[a]pyrene (BaP). In contrast, hepatic CYP1A1 was moderately induced in TLR2-deficient mice without restoring their ability to clear BaP from systemic circulation, as present in wild-type animals. After feeding of BaP for 21 days, only TLR2^−/− mice, but not their wild type littermates developed polyps in the colon. Gene expressions and protein concentrations of AHR and ARNT in the intestine did not differ between the genotypes. In conclusion, the presence of ligands for TLR2 of bacterial origin seems to be crucial for detoxication of luminal carcinogens by CYP1A1 in the intestine. This unprecedented finding indicates a complex interplay between the immune system of the host and intestinal bacteria with detoxication mechanisms. This highlights the relevance of intestinal microbiota when trying to unravel pathways present in mammals and opens new perspectives for research in human health.

Inhibition of cytochrome P4501-dependent clearance of the endogenous agonist FICZ as a mechanism for activation of the aryl hydrocarbon receptor

Altered systemic levels of 6-formylindolo[3,2-b]carbazole (FICZ), an enigmatic endogenous ligand for the aryl hydrocarbon receptor (AHR), may explain adverse physiological responses evoked by small natural and anthropogenic molecules as well as by oxidative stress and light. We demonstrate here that several different chemical compounds can inhibit the metabolism of FICZ, thereby disrupting the autoregulatory feedback control of cytochrome P4501 systems and other proteins whose expression is regulated by AHR. FICZ is both the most tightly bound endogenous agonist for the AHR and an ideal substrate for cytochrome CYP1A1/1A2 and 1B1, thereby also participating in an autoregulatory loop that keeps its own steady-state concentration low. At very low concentrations FICZ influences circadian rhythms, responses to UV light, homeostasis associated with pro- and anti-inflammatory processes, and genomic stability. Here, we demonstrate that, if its metabolic clearance is compromised, femtomolar background levels of this compound in cell-culture medium are sufficient to up-regulate CYP1A1 mRNA and enzyme activity. The oxidants UVB irradiation and hydrogen peroxide and the model AHR antagonist 3'-methoxy-4'-nitroflavone all inhibited induction of CYP1A1 enzyme activity by FICZ or 2,3,7,8-tetrachlorodibenzo-p-dioxin, thereby subsequently elevating intracellular levels of FICZ and activating AHR. Taken together, these findings support an indirect mechanism of AHR activation, indicating that AHR activation by molecules with low affinity actually may reflect inhibition of FICZ metabolism and raising questions about the reported promiscuity of the AHR. Accordingly, we propose that prolonged induction of AHR activity through inhibition of CYP1 disturbs feedback regulation of FICZ levels, with potential detrimental consequences.

Cytochrome p450 1 genes in birds: evolutionary relationships and transcription profiles in chicken and Japanese quail embryos

Background

Cytochrome P450 1 (CYP1) genes are biomarkers for aryl hydrocarbon receptor (AHR) agonists and may be involved in some of their toxic effects. CYP1s other than the CYP1As are poorly studied in birds. Here we characterize avian CYP1B and CYP1C genes and the expression of the identified CYP1 genes and AHR1, comparing basal and induced levels in chicken and quail embryos.

Methodology/Principal Findings

We cloned cDNAs of chicken CYP1C1 and quail CYP1B1 and AHR1. CYP1Cs occur in several bird genomes, but we found no CYP1C gene in quail. The CYP1C genomic region is highly conserved among vertebrates. This region also shares some synteny with the CYP1B region, consistent with CYP1B and CYP1C genes deriving from duplication of a common ancestor gene. Real-time RT-PCR analyses revealed similar tissue distribution patterns for CYP1A4, CYP1A5, CYP1B1, and AHR1 mRNA in chicken and quail embryos, with the highest basal expression of the CYP1As in liver, and of CYP1B1 in eye, brain, and heart. Chicken CYP1C1 mRNA levels were appreciable in eye and heart but relatively low in other organs. Basal transcript levels of the CYP1As were higher in quail than in chicken, while CYP1B1 levels were similar in the two species. 3,3′,4,5,5′-Pentachlorobiphenyl induced all CYP1s in chicken; in quail a 1000-fold higher dose induced the CYP1As, but not CYP1B1.

Conclusions/Significance

The apparent absence of CYP1C1 in quail, and weak expression and induction of CYP1C1 in chicken suggest that CYP1Cs have diminishing roles in tetrapods; similar tissue expression suggests that such roles may be met by CYP1B1. Tissue distribution of CYP1B and CYP1C transcripts in birds resembles that previously found in zebrafish, suggesting that these genes serve similar functions in diverse vertebrates. Determining CYP1 catalytic functions in different species should indicate the evolving roles of these duplicated genes in physiological and toxicological processes.

QSAR models of cytochrome P450 enzyme 1A2 inhibitors using CoMFA, CoMSIA and HQSAR

Quantitative structure-activity relationship (QSAR) studies were conducted on an in-house database of cytochrome P450 enzyme 1A2 inhibitors using the comparative molecular field analysis (CoMFA), comparative molecular similarity analysis (CoMSIA) and hologram QSAR (HQSAR) approaches. The database consisted of 36 active molecules featuring varied core structures. The model based on the naphthalene substructure alignment incorporating 19 molecules yielded the best model with a CoMFA cross validation value q(2) of 0.667 and a Pearson correlation coefficient r(2) of 0.976; a CoMSIA q(2) value of 0.616 and r(2) value of 0.985; and a HQSAR q(2) value of 0.652 and r(2) value of 0.917. A second model incorporating 34 molecules aligned using the benzene substructure yielded an acceptable CoMFA model with q(2) value of 0.5 and r(2) value of 0.991. Depending on the core structure of the molecule under consideration, new CYP1A2 inhibitors will be designed based on the results from these models.

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.

Cytochrome P450 1A, 1B, and 1C mRNA induction patterns in three-spined stickleback exposed to a transient and a persistent inducer

Cytochrome P450 1 (CYP1) mRNA induction patterns in three-spined stickleback (Gasterosteus aculeatus) were explored for use in environmental monitoring of aryl hydrocarbon receptor (AHR) agonists. The cDNAs of stickleback CYP1A, CYP1B1, CYP1C1, and CYP1C2 were cloned and their basal and induced expression patterns were determined in the brain, gill, liver and kidney. Also, their induction time courses were compared after waterborne exposure to a transient (indigo) or a persistent (3,3',4,4',5-pentacholorbiphenyl PCB 126) AHR agonist. The cloned stickleback CYP1s exhibited a high amino acid sequence identity compared with their zebrafish orthologs and their constitutive tissue distribution patterns largely agreed with those reported in other species. PCB 126 (100 nM) induced different CYP1 expression patterns in the four tissues, suggesting tissue-specific regulation. Both indigo (1 nM) and PCB 126 (10 nM) induced a strong CYP1 expression in gills. However, while PCB 126 gave rise to a high and persistent induction in gills and liver, induction by indigo was transient in both organs. The number of putative dioxin response elements found in each CYP1 gene promoter roughly reflected the induction levels of the genes. The high responsiveness of CYP1A, CYP1B1, and CYP1C1 observed in several organs suggests that three-spined stickleback is suitable for monitoring of pollution with AHR agonists.

Cytochrome P450-mediated pulmonary metabolism of carcinogens: regulation and cross-talk in lung carcinogenesis

Lung cancer is strongly associated with exogenous risk factors, in particular tobacco smoking and asbestos exposure. New research data are accumulating about the regulation of the metabolism of tobacco carcinogens and the metabolic response to oxidative stress. These data provide mechanistic details about why well known risk factors cause lung cancer. The purpose of this review is to evaluate the present knowledge of the role of cytochrome P450 (CYP) enzymes in the metabolism of tobacco carcinogens and associations with tobacco and asbestos carcinogenesis. Major emphasis is placed on human data and regulatory pathways involved in CYP regulation and lung carcinogenesis. The most exciting new research findings concern cross-talk of the CYP-regulating aryl hydrocarbon receptor with other transcription factors, such as nuclear factor-erythroid 2-related factor 2, involved in the regulation of xenobiotic metabolism and antioxidant enzymes. This cross-talk between transcription factors may provide mechanistic evidence for clinically relevant issues, such as differences in lung cancers between men and women and the synergism between tobacco and asbestos as lung carcinogens.

Augmented oxygen-mediated transcriptional activation of cytochrome P450 (CYP)1A expression and increased susceptibilities to hyperoxic lung injury in transgenic mice carrying the human CYP1A1 or mouse 1A2 promoter in vivo

Supplemental oxygen administration is frequently administered to pre-term and term infants having pulmonary insufficiency. However, hyperoxia contributes to the development of bronchopulmonary dysplasia (BPD) in premature infants. Cytochrome P450 (CYP)A enzymes have been implicated in hyperoxic lung injury. In this study, we tested the hypothesis that hyperoxia induces CYP1A1 and 1A2 enzymes by transcriptional activation of the corresponding promoters in vivo, and transgenic mice expressing the human CYP1A1 or the mouse 1A2 promoter would be more susceptible to hyperoxic lung injury than wild type (WT) mice. Adult WT (CD-1) (12week-old) mice, transgenic mice carrying a 10kb human CYP1A1 promoter and the luciferase (luc) reporter gene (CYP1A1-luc), or mice expressing the mouse CYP1A2 promoter (CYP1A2-luc) were maintained in room air or exposed to hyperoxia for 24-72h. Hyperoxia exposure of CYP1A1-luc mice for 24 and 48h resulted in 2.5- and 1.25-fold increases, respectively, in signal intensities, compared to room air controls. By 72h, the induction had declined to control levels. CYP1A2-luc mice also showed enhanced luc expression after 24-48h, albeit to a lesser extent than those expressing the CYP1A1 promoter. Also, these mice showed decreased levels of endogenous CYP1A1 and 1A2 expression after prolonged hyperoxia, and were also more susceptible to lung injury than similarly exposed WT mice, with CYP1A2-luc mice showing the greatest injury. Our results support the hypothesis that hyperoxia induces CYP1A enzymes by transcriptional activation of its corresponding promoters, and that decreased endogenous expression of these enzymes contribute to the increased susceptibilities to hyperoxic lung injury in the transgenic animals. In summary, this is the first report providing direct evidence of hyperoxia-mediated induction of CYP1A1 and CYP1A2 expression in vivo by mechanisms entailing transcriptional activation of the corresponding promoters, a phenomenon that has implications for hyperoxic lung injury, as well as other pathologies caused by oxidative stress.

Mechanistic insights into the events that lead to synergistic induction of interleukin 6 transcription upon activation of the aryl hydrocarbon receptor and inflammatory signaling

The aryl hydrocarbon receptor (AHR) is the ligand-activated transcription factor responsible for mediating the toxicological effects of dioxin and xenobiotic metabolism. However, recent evidence has implicated the AHR in additional, nonmetabolic physiological processes, including immune regulation. Certain tumor cells are largely nonresponsive to cytokine-mediated induction of the pro-survival cytokine interleukin (IL) 6. We have demonstrated that multiple nonresponsive tumor lines are able to undergo synergistic induction of IL6 following combinatorial treatment with IL1beta and the AHR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin. Such data implicate the AHR in tumor expansion, although the mechanistic basis for the AHR-dependent synergistic induction of IL6 has not been determined. Here, we demonstrate that ligand-activated AHR is involved in priming the IL6 promoter through binding to nonconsensus dioxin response elements located upstream of the IL6 start site. Such binding appears to render the promoter more permissive to IL1beta-induced binding of NF-kappaB components. The nature of the AHR-dependent increases in IL6 promoter transcriptional potential has been shown to involve a reorganization of repressive complexes as exemplified by the presence of HDAC1 and HDAC3. Dismissal of these HDACs correlates with post-translational modifications of promoter-bound NF-kappaB components in a time-dependent manner. Thus the AHR plays a role in derepressing the IL6 promoter, leading to synergistic IL6 expression in the presence of inflammatory signals. These observations may explain the association between enhanced expression of AHR and tumor aggressiveness. It is likely that AHR-mediated priming is not restricted to the IL6 promoter and may contribute to the expression of a variety of genes, which do not have consensus dioxin response elements.

The pleiotropy of dioxin toxicity--xenobiotic misappropriation of the aryl hydrocarbon receptor's alternative physiological roles

The aryl hydrocarbon receptor is a signal regulated transcription factor that has best been characterised as regulating the xenobiotic response to a variety of planar aromatic hydrocarbons. There is compelling evidence that it mediates most, if not all, of the toxic effects of dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin). Dioxin exposure results in a wide variety of toxic outcomes including severe wasting syndrome, chloracne, thymic involution, severe immune suppression, reduced fertility, hepatotoxicity, teratogenicity, tumour promotion and death. The pleiotropy of toxic outcomes implies the disruption of a wide range of normal physiological functions. The aryl hydrocarbon receptor has developmentally restricted expression as well as developmental defects in gene-targeted mice. It has a wide range of target genes that do not fit into the classical xenobiotic metabolising gene battery and has recently been shown to interact with NF-kappa B and the estrogen receptor. There is also evidence for its activation in the absence of exogenous ligand, all of which point to various roles outside xenobiotic metabolism. Ligands so far identified display differential activation potential with respect to receptor activity. This article addresses activities of the aryl hydrocarbon receptor that are outside the xenobiotic response. Known physiological roles are discussed as well as how their disruption contributes to the pleiotropic toxicity of TCDD.

Cytochrome P450 CYP1A1: wider roles in cancer progression and prevention

CYP1A1 is one of the main cytochrome P450 enzymes, examined extensively for its capacity to activate compounds with carcinogenic properties. Continuous exposure to inhalation chemicals and environmental carcinogens is thought to increase the level of CYP1A1 expression in extrahepatic tissues, through the aryl hydrocarbon receptor (AhR). Although the latter has long been recognized as a ligand-induced transcription factor, which is responsible for the xenobiotic activating pathway of several phase I and phase II metabolizing enzymes, recent evidence suggests that the AhR is involved in various cell signaling pathways critical to cell cycle regulation and normal homeostasis. Disregulation of these pathways is implicated in tumor progression. In addition, it is becoming increasingly evident that CYP1A1 plays an important role in the detoxication of environmental carcinogens, as well as in the metabolic activation of dietary compounds with cancer preventative activity. Ultimately the contribution of CYP1A1 to cancer progression or prevention may depend on the balance of procarcinogen activation/detoxication and dietary natural product extrahepatic metabolism.

Aryl hydrocarbon receptor polymorphism modulates DMBA-induced inflammation and carcinogenesis in phenotypically selected mice

We tested the role of aryl hydrocarbon receptor (Ahr) gene polymorphism in the inflammatory response and in skin and lung tumorigenesis in 2 lines of mice phenotypically selected for maximum or minimum acute inflammatory reaction (AIRmax and AIRmin, respectively). Following 7,12-dimethylbenz[a]anthracene (DMBA) treatment, AIRmin but not AIRmax mice showed early skin reactions and eventually developed malignant skin tumors and lung adenocarcinomas. In skin tissue, transcript levels of IL1beta, Tnf, Il6, Tgfbeta1 and Cyp1b1 genes were upregulated in AIRmin but not AIRmax mice, consistent with the inflammatory responses to the carcinogen. These findings appeared to be related to the homozygosity status of the Ahr functional A375V polymorphism, which influences the binding capability of the receptor for DMBA: the 375A allele, encoding the high-affinity ligand-binding receptor (Ahr(b1)), segregated in AIRmin mice, whereas AIRmax mice carried the 375V, corresponding to the low-affinity binding receptor (Ahr(d)), to DMBA. The differential segregation of Ahr functional Ahr(d)versus Ahr(b1) alleles in AIRmax and AIRmin suggests a role for the Ahr gene in the control of inflammatory responsiveness and tumor development of these mouse lines.

Expression of CYP1A1 and CYP1B1 in human endothelial cells: regulation by fluid shear stress

AIMS

CYP1A1 and CYP1B1, members of the cytochrome P450 protein family, are regulated by fluid shear stress. This study describes the effects of duration, magnitude and pattern of shear stress on CYP1A1 and CYP1B1 expressions in human endothelial cells, towards the goal of understanding the role(s) of these genes in pro-atherogenic or anti-atherogenic endothelial cell functions.

METHODS AND RESULTS

We investigated CYP1A1 and CYP1B1 expressions under different durations, levels, and patterns of shear stress. CYP1A1 and CYP1B1 mRNA, protein, and enzymatic activity were maximally up-regulated at > or =24 h of arterial levels of shear stress (15-25 dynes/cm2). Expression of both genes was significantly attenuated by reversing shear stress when compared with 15 dynes/cm2 steady shear stress. Small interfering RNA knockdown of CYP1A1 resulted in significantly reduced CYP1B1 and thrombospondin-1 expression, genes regulated by the aryl hydrocarbon receptor (AhR). Immunostaining of human coronary arteries showed constitutive CYP1A1 and CYP1B1 protein expressions in endothelial cells. Immunostaining of mouse aorta showed nuclear localization of AhR and increased expression of CYP1A1 in the descending thoracic aorta, whereas reduced nuclear localization of AhR and attenuated CYP1A1 expression were observed in the lesser curvature of the aortic arch.

CONCLUSION

CYP1A1 and CYP1B1 gene and protein expressions vary with time, magnitude, and pattern of shear stress. Increased CYP1A1 gene expression modulates AhR-regulated genes. Based on our in vitro reversing flow data and in vivo immunostained mouse aorta, we suggest that increased expression of both genes reflects an anti-atherogenic endothelial cell phenotype.

Leukotriene A4 metabolites are endogenous ligands for the Ah receptor

In addition to orchestrating an adaptive metabolic response to xenobiotic compounds, the aryl hydrocarbon receptor (AHR) also plays a necessary role in the normal physiology of mice. The AHR is activated by a structurally diverse group of chemicals ranging from carcinogenic environmental pollutants to dietary metabolites and a number of endogenous molecules. Leukotriene A 4 (5,6-LTA 4) metabolites were identified in DRE-driven luciferase reporter assays as activators of AHR signaling. Various LTA 4 metabolites, including several 5,6- and 5,12-DiHETE products, were screened for AHR activity with 6- trans-LTB 4, 6- trans-12- epi-LTB 4, 5( S),6( S)-DiHETE, and 5( S),6( R)-DiHETE eliciting a significant level of AHR transcriptional activity. However, electrophoretic mobility shift assays (EMSAs) revealed that only 5,6-DiHETE isomers were capable of directly binding and activating the AHR to a DNA-binding species in vitro. Furthermore, ligand competition binding experiments confirm the ability of these compounds to directly bind to the AHR. Interestingly, "aged" preparations of 5,6-DiHETE isomers produced an enhanced level of AHR activation while demonstrating an increase in binding affinity for the receptor. Although the reason for this has not been fully determined, the formation of geometric isomers in the conjugated triene region of these molecules may play a role in the observed increase in AHR-mediated transcriptional activity. This work suggests a connection between AHR activation and inflammatory signaling molecules produced by the 5-lipoxygenase pathway.

Phenotype of the Cyp1a1/1a2/1b1-/- triple-knockout mouse

Crossing the Cyp1a1/1a2(-/-) double-knockout mouse with the Cyp1b1(-/-) single-knockout mouse, we generated the Cyp1a1/1a2/1b1(-/-) triple-knockout mouse. In this triple-knockout mouse, statistically significant phenotypes (with incomplete penetrance) included slower weight gain and greater risk of embryolethality before gestational day 11, hydrocephalus, hermaphroditism, and cystic ovaries. Oral benzo[a]pyrene (BaP) daily for 18 days in the Cyp1a1/1a2(-/-) produced the same degree of marked immunosuppression as seen in the Cyp1a1(-/-) mouse; we believe this reflects the absence of intestinal CYP1A1. Oral BaP-treated Cyp1a1/1a2/1b1(-/-) mice showed the same "rescued" response as that seen in the Cyp1a1/1b1(-/-) mouse; we believe this reflects the absence of CYP1B1 in immune tissues. Urinary metabolite profiles were dramatically different between untreated triple-knockout and wild-type; principal components analysis showed that the shifts in urinary metabolite patterns in oral BaP-treated triple-knockout and wild-type mice were also strikingly different. Liver microarray cDNA differential expression (comparing triple-knockout with wild-type) revealed at least 89 genes up- and 62 genes down-regulated (P-value < or = 0.00086). Gene Ontology "classes of genes" most perturbed in the untreated triple-knockout (compared with wild-type) include lipid, steroid, and cholesterol biosynthesis and metabolism; nucleosome and chromatin assembly; carboxylic and organic acid metabolism; metal-ion binding; and ion homeostasis. In the triple-knockout compared with the wild-type mice, response to zymosan-induced peritonitis was strikingly exaggerated, which may well reflect down-regulation of Socs2 expression. If a single common molecular pathway is responsible for all of these phenotypes, we suggest that functional effects of the loss of all three Cyp1 genes could be explained by perturbations in CYP1-mediated eicosanoid production, catabolism and activities.

Crosstalk between the aryl hydrocarbon receptor and hypoxia on the constitutive expression of cytochrome P4501A1 mRNA

Endothelial cells are a target of halogenated aromatic hydrocarbon toxicity following aryl hydrocarbon receptor (AHR) activation. Further, evidence suggests that AHR has a physiological function in endothelial cells in the absence of exogenous ligands. Understanding these "normal" functions of AHR may help to reveal the mechanisms that contribute to the toxicity of xenobiotic ligands. Thus, this study focused on the crosstalk between hypoxia and AHR in the absence of exogenous ligands. Constitutive CYP1A1 mRNA was measured by real time PCR in human pulmonary microvascular endothelial cells exposed to hypoxia (1 or 2.5% O2), 25 nM AHR siRNA, 25 nM hypoxia-inducible factor (HIF)-2alpha siRNA, or their combinations. Hypoxia significantly induced known hypoxia-regulated genes, and this induction was highly attenuated by HIF-2alpha siRNA, suggesting that HIF-2alpha is a primary mediator of hypoxic responses in these cells. Hypoxia also significantly reduced CYP1A1 mRNA and this reduction was also attenuated by HIF-2alpha siRNA. As expected, AHR siRNA significantly reduced constitutive CYP1A1 mRNA. While the combination of hypoxia plus AHR siRNA reduced CYP1A1 mRNA more than either treatment alone, the reduction was less than additive, suggesting that hypoxia and AHR deficiency may share a common pathway in reducing CYP1A1 expression. Finally, hypoxia significantly reduced AHR mRNA and this reduction was completely prevented by HIF-2alpha siRNA. In conclusion, constitutive CYP1A1 mRNA expression is dependent on AHR and is reduced by hypoxia via a HIF-2alpha-dependent mechanism, which may be mediated by a HIF-2alpha-dependent reduction of AHR expression.