Leukotriene A4 metabolites are endogenous ligands for the Ah receptor

Targeting quorum sensing for manipulation of commensal microbiota

Bacteria communicate through the accumulation of autoinducer (AI) molecules that regulate gene expression at critical densities in a process called quorum sensing (QS). Extensive work using simple systems and single strains of bacteria have revealed a role for QS in the regulation of virulence factors and biofilm formation; however, less is known about QS dynamics among communities, especially in vivo. In this review, we summarize the diversity of QS signals as well as their ability to influence "non-target" behaviors among species that have receptors but not synthases for those signals. We highlight host-microbe interactions facilitated by QS and describe cross-talk between QS and the mammalian endocrine and immune systems, as well as host surveillance of QS. Further, we describe emerging evidence for the role of QS in non-infectious, chronic, microbially associated diseases including inflammatory bowel diseases and cancers. Finally, we describe potential therapeutic approaches that involve leveraging QS signals as well as quorum quenching approaches to block signaling in vivo to mitigate deleterious consequences to the host. Ultimately, QS offers a previously underexplored target that may be leveraged for precision modification of the microbiota without deleterious bactericidal consequences.

The Role of the Aryl Hydrocarbon Receptor (AhR) and Its Ligands in Breast Cancer

Breast cancer is a complex disease which is defined by numerous cellular and molecular markers that can be used to develop more targeted and successful therapies. The aryl hydrocarbon receptor (AhR) is overexpressed in many breast tumor sub-types, including estrogen receptor -positive (ER+) tumors; however, the prognostic value of the AhR for breast cancer patient survival is not consistent between studies. Moreover, the functional role of the AhR in various breast cancer cell lines is also variable and exhibits both tumor promoter- and tumor suppressor- like activity and the AhR is expressed in both ER-positive and ER-negative cells/tumors. There is strong evidence demonstrating inhibitory AhR-Rα crosstalk where various AhR ligands induce ER degradation. It has also been reported that different structural classes of AhR ligands, including halogenated aromatics, polynuclear aromatics, synthetic drugs and other pharmaceuticals, health promoting phytochemical-derived natural products and endogenous AhR-active compounds inhibit one or more of breast cancer cell proliferation, survival, migration/invasion, and metastasis. AhR-dependent mechanisms for the inhibition of breast cancer by AhR agonists are variable and include the downregulation of multiple genes/gene products such as CXCR4, MMPs, CXCL12, SOX4 and the modulation of microRNA levels. Some AhR ligands, such as aminoflavone, have been investigated in clinical trials for their anticancer activity against breast cancer. In contrast, several publications have reported that AhR agonists and antagonists enhance and inhibit mammary carcinogenesis, respectively, and differences between the anticancer activities of AhR agonists in breast cancer may be due in part to cell context and ligand structure. However, there are reports showing that the same AhR ligand in the same breast cancer cell line gives opposite results. These differences need to be resolved in order to further develop and take advantage of promising agents that inhibit mammary carcinogenesis by targeting the AhR.

Suhuang antitussive capsule ameliorates post-infectious cough in mice through AhR-Nrf2 pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Suhuang antitussive capsule (SH capsule), a typical traditional Chinese medicines (TCMs) compound, is widely used for the treatment of post-infectious cough (PIC) in the clinic. Our previous studies have demonstrated that SH capsule possesses significant ameliorative effects on cough variant asthma (CVA), sputum obstruction and airway remodeling.

AIM OF THE STUDY

This study is designed to investigate the ameliorative effects and potential mechanisms of SH capsule on PIC in mice.

MATERIALS AND METHODS

To establish the PIC model, ICR mice were induced by lipopolysaccharide (LPS) (3 mg/kg) once, followed by cigarettes smoke (CS) exposure for 30 min per day for 30 days. Mice were intragastrically (i.g.) administrated with SH capsule at the doses of 3.5, 7, 14 g/kg each day for 2 weeks since the 24th day. The number of coughs, coughs latencies, enzyme-linked immunosorbent assay (ELISA) and histological analysis were used to investigate the effects of SH capsule on PIC mice. Quantitative-polymerase chain reaction (Q-PCR) and western blotting were conducted to evaluate the levels of mRNA and proteins associated with Aryl hydrocarbon receptor (AhR)-NF-E2-related factor 2 (Nrf2) pathway. Superoxide dismutase (SOD), glutathione (GSH) and total antioxidant capacity (T-AOC) assays were performed to evaluate the oxidative stress. A549 cells were used to investigate the ameliorative effects of SH capsule in vitro.

RESULTS

SH capsule effectively diminished the number of coughs and extended coughs latencies in PIC mice. The airway inflammation was alleviated by decreasing the expression levels of inflammatory mediators including tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and interleukin-6 (IL-6). Moreover, SH capsule dose-dependently activated AhR-Nrf2 pathway and induced the nuclear translocation in vitro and in vivo. Besides, SH capsule significantly increased the levels of SOD, GSH and T-AOC in mice.

CONCLUSION

Our study demonstrates that SH capsule ameliorates airway inflammation-associated PIC in mice through activating AhR-Nrf2 pathway and consequently alleviating inflammatory responses and oxidative stress.

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.

How Ah Receptor Ligand Specificity Became Important in Understanding Its Physiological Function

Increasingly, the aryl hydrocarbon receptor (AHR) is being recognized as a sensor for endogenous and pseudo-endogenous metabolites, and in particular microbiota and host generated tryptophan metabolites. One proposed explanation for this is the role of the AHR in innate immune signaling within barrier tissues in response to the presence of microorganisms. A number of cytokine/chemokine genes exhibit a combinatorial increase in transcription upon toll-like receptors and AHR activation, supporting this concept. The AHR also plays a role in the enhanced differentiation of intestinal and dermal epithelium leading to improved barrier function. Importantly, from an evolutionary perspective many of these tryptophan metabolites exhibit greater activation potential for the human AHR when compared to the rodent AHR. These observations underscore the importance of the AHR in barrier tissues and may lead to pharmacologic therapeutic intervention.

Aryl Hydrocarbon Receptor (AHR) Ligands as Selective AHR Modulators (SAhRMs)

The aryl hydrocarbon receptor (AhR) was first identified as the intracellular protein that bound and mediated the toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) and dioxin-like compounds (DLCs). Subsequent studies show that the AhR plays an important role in maintaining cellular homeostasis and in pathophysiology, and there is increasing evidence that the AhR is an important drug target. The AhR binds structurally diverse compounds, including pharmaceuticals, phytochemicals and endogenous biochemicals, some of which may serve as endogenous ligands. Classification of DLCs and non-DLCs based on their persistence (metabolism), toxicities, binding to wild-type/mutant AhR and structural similarities have been reported. This review provides data suggesting that ligands for the AhR are selective AhR modulators (SAhRMs) that exhibit tissue/cell-specific AhR agonist and antagonist activities, and that their functional diversity is similar to selective receptor modulators that target steroid hormone and other nuclear receptors.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) alters hepatic polyunsaturated fatty acid metabolism and eicosanoid biosynthesis in female Sprague-Dawley rats

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent aryl hydrocarbon receptor (AhR) agonist that elicits a broad spectrum of dose-dependent hepatic effects including lipid accumulation, inflammation, and fibrosis. To determine the role of inflammatory lipid mediators in TCDD-mediated hepatotoxicity, eicosanoid metabolism was investigated. Female Sprague-Dawley (SD) rats were orally gavaged with sesame oil vehicle or 0.01-10 μg/kg TCDD every 4 days for 28 days. Hepatic RNA-Seq data was integrated with untargeted metabolomics of liver, serum, and urine, revealing dose-dependent changes in linoleic acid (LA) and arachidonic acid (AA) metabolism. TCDD also elicited dose-dependent differential gene expression associated with the cyclooxygenase, lipoxygenase, and cytochrome P450 epoxidation/hydroxylation pathways with corresponding changes in ω-6 (e.g. AA and LA) and ω-3 polyunsaturated fatty acids (PUFAs), as well as associated eicosanoid metabolites. Overall, TCDD increased the ratio of ω-6 to ω-3 PUFAs. Phospholipase A2 (Pla2g12a) was induced consistent with increased AA metabolism, while AA utilization by induced lipoxygenases Alox5 and Alox15 increased leukotrienes (LTs). More specifically, TCDD increased pro-inflammatory eicosanoids including leukotriene LTB4, and LTB3, known to recruit neutrophils to damaged tissue. Dose-response modeling suggests the cytochrome P450 hydroxylase/epoxygenase and lipoxygenase pathways are more sensitive to TCDD than the cyclooxygenase pathway. Hepatic AhR ChIP-Seq analysis found little enrichment within the regulatory regions of differentially expressed genes (DEGs) involved in eicosanoid biosynthesis, suggesting TCDD-elicited dysregulation of eicosanoid metabolism is a downstream effect of AhR activation. Overall, these results suggest alterations in eicosanoid metabolism may play a key role in TCDD-elicited hepatotoxicity associated with the progression of steatosis to steatohepatitis.

Phospholipase A2 products predict the hematopoietic support capacity of horse serum

Horse serum is commonly used as an additive to support the maintenance of hematopoietic progenitor cells in culture. However, the wide variability in the performance of different lots calls for parallel testing of multiple batches over extended periods of culture. Identification of the serum components that determine hematopoietic support would therefore save considerable time and effort and would help to standardize culture procedures. We report here that the ability of horse serum to support the self-renewal of multipotent murine hematopoietic progenitor FDCP-Mix cells is correlated to the concentration of specific fatty acid products of phospholipase A2 and more closely to the spectrum of eicosanoids generated by their further processing through cyclooxygenase and lipoxygenase pathways. Supportive sera have low levels of lysophosphatidylcholine and inflammatory eicosanoids. This links known markers of inflammation, infection and platelet activation to the ability of serum to maintain progenitor cells in an undifferentiated state, providing a means for prospective identification of suitable sera as well as quality control of the production process.

Conditional deletion of Ahr alters gene expression profiles in hematopoietic stem cells

The aryl hydrocarbon receptor (AHR) is a ligand activated bHLH transcription factor that belongs to the Per-Arnt-Sim (PAS) superfamily of proteins involved in mediating responses to cellular environment regulating normal physiological and developmental pathways. The AHR binds a broad range of naturally derived and synthetic compounds, and plays a major role in mediating effects of certain environmental chemicals. Although our understanding of the physiological roles of the AHR in the immune system is evolving, there is little known about its role in hematopoiesis and hematopoietic diseases. Prior studies demonstrated that AHR null (AHR-KO) mice have impaired hematopoietic stem cell (HSC) function; they develop myeloproliferative changes in peripheral blood cells, and alterations in hematopoietic stem and progenitor cell populations in the bone marrow. We hypothesized mice lacking AHR expression only within hematopoietic cells (AHRVav1 mice) would develop similar changes. However, we did not observe a complete phenocopy of AHR-KO and AHRVav1 animals at 2 or 18 months of age. To illuminate the signaling mechanisms underlying the alterations in hematopoiesis observed in these mice, we sorted a population of cells highly enriched for HSC function (LSK cells: CD34-CD48-CD150+) and performed microarray analyses. Ingenuity Pathway and Gene Set Enrichment Analyses revealed that that loss of AHR within HSCs alters several gene and signaling networks important for HSC function. Differences in gene expression networks among HSCs from AHR-KO and AHRVav1 mice suggest that AHR in bone marrow stromal cells also contributes to HSC function. In addition, numerous studies have suggested a role for AHR in both regulation of hematopoietic cells, and in the development of blood diseases. More work is needed to define what these signals are, and how they act upon HSCs.

A novel bioactive derivative of eicosapentaenoic acid (EPA) suppresses intestinal tumor development in ApcΔ14/+ mice

Familial adenomatous polyposis (FAP) is a genetic disorder characterized by the development of hundreds of polyps throughout the colon. Without prophylactic colectomy, most individuals with FAP develop colorectal cancer at an early age. Treatment with EPA in the free fatty acid form (EPA-FFA) has been shown to reduce polyp burden in FAP patients. Since high-purity EPA-FFA is subject to rapid oxidation, a stable form of EPA compound has been developed in the form of magnesium l-lysinate bis-eicosapentaenoate (TP-252). We assessed the chemopreventive efficacy of TP-252 on intestinal tumor formation using ApcΔ14/+ mice and compared it with EPA-FFA. TP-252 was supplemented in a modified AIN-93G diet at 1, 2 or 4% and EPA-FFA at 2.5% by weight and administered to mice for 11 weeks. We found that administration of TP-252 significantly reduced tumor number and size in the small intestine and colon in a dose-related manner and as effectively as EPA-FFA. To gain further insight into the cancer protection afforded to the colon, we performed a comprehensive lipidomic analysis of total fatty acid composition and eicosanoid metabolites. Treatment with TP-252 significantly decreased the levels of arachidonic acid (AA) and increased EPA concentrations within the colonic mucosa. Furthermore, a classification and regression tree (CART) analysis revealed that a subset of fatty acids, including EPA and docosahexaenoic acid (DHA), and their downstream metabolites, including PGE3 and 14-hydroxy-docosahexaenoic acid (HDoHE), were strongly associated with antineoplastic activity. These results indicate that TP-252 warrants further clinical development as a potential strategy for delaying colectomy in adolescent FAP patients.

Particulate matter containing environmentally persistent free radicals induces AhR-dependent cytokine and reactive oxygen species production in human bronchial epithelial cells

Particulate matter (PM) is emitted during the combustion of fuels and wastes. PM exposure exacerbates pulmonary diseases, and the mechanism may involve oxidative stress. At lower combustion temperatures such as occurs in the cool zone of a flame, aromatic compounds chemisorb to the surface of metal-oxide-containing PM, resulting in the formation of surface-stabilized environmentally persistent free radicals (EPFR). Prior studies showed that PM-containing EPFR redox cycle to produce reactive oxygen species (ROS), and after inhalation, EPFR induce pulmonary inflammation and oxidative stress. Our objective was to elucidate mechanisms linking EPFR-induced oxidant injury with increased cytokine production by pulmonary epithelial cells. We thus treated human bronchial epithelial cells with EPFR at sub-toxic doses and measured ROS and cytokine production. To assess aryl hydrocarbon receptor (AhR) activity, cells were transfected with a luciferase reporter for xenobiotic response element activation. To test whether cytokine production was dependent upon AhR activation or oxidative stress, some cells were co-treated with an antioxidant or an AhR antagonist. EPFR increased IL-6 release in an ROS and AhR- and oxidant-dependent manner. Moreover, EPFR induced an AhR activation that was dependent upon oxidant production, since antioxidant co-treatment blocked AhR activation. On the other hand, EPFR treatment increased a cellular ROS production that was at least partially attenuated by AhR knockdown using siRNA. While AhR activation was correlated with an increased expression of oxidant-producing enzymes like cytochrome P450 CYP1A1, it is possible that AhR activation is both a cause and effect of EPFR-induced ROS. Finally, lipid oxidation products also induced AhR activation. ROS-dependent AhR activation may be a mechanism for altered epithelial cell responses after EPFR exposure, potentially via formation of bioactive lipid or protein oxidation products.

5,6-DiHETE attenuates vascular hyperpermeability by inhibiting Ca2+ elevation in endothelial cells

Although more than 100 lipid metabolites have been identified, their bioactivities remain unknown. In a previous study, we discovered that the production of several lipid metabolites in the intestines dramatically changed in colitis. Of these metabolites, 5,6-dihydroxyeicosatetraenoic acid (DiHETE) possesses novel anti-inflammatory activity in the vasculature. In this study, we used mouse and human umbilical vein endothelial cell (HUVEC) models to elucidate the mechanisms underlying the vascular activity of lipid metabolites, particularly those related to the release of histamine, a major proinflammatory mediator that stimulates endothelial cells to produce NO, a mediator of vascular relaxation and hyperpermeability, by activating intracellular Ca²⁺ concentration-dependent signaling. In a mouse ear, the administration of 5,6-DiHETE did not induce inflammatory reactions, and pretreatment with 5,6-DiHETE inhibited histamine-induced inflammation, specifically vascular dilation and hyperpermeability. In an isolated mouse aorta, 5,6-DiHETE treatment did not influence vascular contraction but attenuated acetylcholine-induced vascular relaxation. In HUVECs, treatment with 5,6-DiHETE inhibited histamine-induced endothelial barrier disruption and inhibited the production of NO. Most notably, 5,6-DiHETE inhibited histamine-induced increases in intracellular Ca²⁺ concentrations in HUVECs. Our findings suggest that 5,6-DiHETE attenuates vascular hyperpermeability during inflammation by inhibiting endothelial Ca²⁺ elevation, which might lead to a novel pharmacological strategy against inflammatory diseases.

Ligand-mediated cytoplasmic retention of the Ah receptor inhibits macrophage-mediated acute inflammatory responses

The Ah receptor (AHR) has been shown to exhibit both inflammatory and anti-inflammatory activity in a context-specific manner. In vivo macrophage-driven acute inflammation models were utilized here to test whether the selective Ah receptor modulator 1-allyl-7-trifluoromethyl-1H-indazol-3-yl]-4-methoxyphenol (SGA360) would reduce inflammation. Exposure to SGA360 was capable of significantly inhibiting lipopolysaccharide (LPS)-mediated endotoxic shock in a mouse model, both in terms of lethality and attenuating inflammatory signaling in tissues. Topical exposure to SGA360 was also able to mitigate joint edema in a monosodium urate (MSU) crystal gout mouse model. Inhibition was dependent on the expression of the high-affinity allelic AHR variant in both acute inflammation models. Upon peritoneal MSU crystal exposure SGA360 pretreatment inhibited neutrophil and macrophage migration into the peritoneum. RNA-seq analysis revealed that SGA360 attenuated the expression of numerous inflammatory genes and genes known to be directly regulated by AHR in thioglycolate-elicited primary peritoneal macrophages treated with LPS. In addition, expression of the high-affinity allelic AHR variant in cultured macrophages was necessary for SGA360-mediated repression of inflammatory gene expression. Mechanistic studies revealed that SGA360 failed to induce nuclear translocation of the AHR and actually enhanced cytoplasmic localization. LPS treatment of macrophages enhanced the occupancy of the AHR and p65 to the Ptgs2 promoter, whereas SGA360 attenuated occupancy. AHR ligand activity was detected in peritoneal exudates isolated from MSU-treated mice, thus suggesting that the anti-inflammatory activity of SGA360 is mediated at least in part through AHR antagonism of endogenous agonist activity. These results underscore an important role of the AHR in participating in acute inflammatory signaling and warrants further investigations into possible clinical applications.

Allelic variants of the aryl hydrocarbon receptor differentially influence UVB-mediated skin inflammatory responses in SKH1 mice

The mouse strain SKH1 is widely used in skin research due to its hairless phenotype and intact immune system. Due to the complex nature of aryl hydrocarbon receptor (AHR) function in the skin, the development of additional in vivo models is necessary to study its role in cutaneous homeostasis and pathology. Variants of the Ah allele, exist among different mouse strains. The Ah^b-1 and Ah^d alleles express high and low affinity ligand binding forms of the AHR, respectively. The outbred SKH1 mice express the Ah^b-2 and/or Ah^d alleles. SKH1 mice were crossed with C57BL/6J mice, which harbor the Ah^b-1 allele, to create useful models for studying endogenous AHR function. SKH1 mice were bred to be homozygous for either the Ah^b-1 or Ah^d allele to establish strains for use in comparative studies of the effects of differential ligand-mediated activation through gene expression changes upon UVB exposure. Ah^b-1 or Ah^d allelic status was confirmed by DNA sequence analysis. We tested the hypothesis that SKH1-Ah^b-1 mice would display enhanced inflammatory signaling upon UVB exposure compared to SKH1-Ah^d mice. Differential basal AHR activation between the strains was determined by assessing Cyp1a1 expression levels in the small intestine, liver, and skin of the SKH1-Ah^b-1 mice compared to SKH1-Ah^d mice. To determine whether SKH1-Ah^b-1 mice are more prone to a pro-inflammatory phenotype in response to UVB, gene expression of inflammatory mediators was analyzed. SKH1-Ah^b-1 mice expressed enhanced gene expression of the chemotactic factors Cxcl5, Cxcl1, and Ccl20, as well as the inflammatory signaling factors S100a9 and Ptgs2, compared to SKH1-Ah^d mice in skin. These data supports a role for AHR activation and enhanced inflammatory signaling in skin.

The oral commensal Streptococcus mitis activates the aryl hydrocarbon receptor in human oral epithelial cells

Streptococcus mitis (S. mitis) is a pioneer commensal bacterial species colonizing many of the surfaces of the oral cavity in healthy individuals. Yet, not much information is available regarding its interaction with the host. We used examination of its transcriptional regulation in oral keratinocytes to elucidate some of its potential roles in the oral cavity. Transcription factor analysis of oral keratinocytes predicted S. mitis-mediated activation of aryl hydrocarbon receptor (AhR). Activation and functionality of AhR was confirmed through nuclear translocation determined by immunofluorescence microscopy and real-time polymerase chain reaction with reverse transcription analysis of CYP1A1, the hallmark gene for AhR activation. Addition of Streptococcus mutans or Streptococcus gordonii did not induce CYP1A1 transcription in the keratinocyte cultures. Introduction of an AhR-specific inhibitor revealed that S. mitis-mediated transcription of CXCL2 and CXCL8 was regulated by AhR. Elevated levels of prostaglandin E2 (enzyme-linked immunosorbent assay) in supernatants from S. mitis-treated oral epithelial cells were also attenuated by inhibition of AhR activity. The observed AhR-regulated activities point to a contribution of S. mitis in the regulation of inflammatory responses and thereby to wound healing in the oral cavity. The concept that the oral commensal microbiota can induce AhR activation is important, also in view of the role that AhR has in modulation of T-cell differentiation and as an anti-inflammatory factor in macrophages.

Cell and region specificity of Aryl hydrocarbon Receptor (AhR) system in the testis and the epididymis

Aryl hydrocarbon receptor (AhR) plays multiple important functions in adaptive responses. Exposure to AhR ligands may produce an altered metabolic activity controlled by the AhR pathways, and consequently affect drug/toxin responses, hormonal status and cellular homeostasis. This research revealed species-, cell- and region-specific pattern of the AhR system expression in the rat and human testis and epididymis, complementing the existing knowledge, especially within the epididymal segments. The study showed that AhR level in the rat and human epididymis is higher than in the testis. The downregulation of AhR expression after TCDD treatment was revealed in the spermatogenic cells at different stages and the epididymal epithelial cells, but not in the Sertoli and Leydig cells. Hence, this basic research provides information about the AhR function in the testis and epididymis, which may provide an insight into deleterious effects of drugs, hormones and environmental pollutants on male fertility.

Divergent Ah Receptor Ligand Selectivity during Hominin Evolution

We have identified a fixed nonsynonymous sequence difference between humans (Val381; derived variant) and Neandertals (Ala381; ancestral variant) in the ligand-binding domain of the aryl hydrocarbon receptor (AHR) gene. In an exome sequence analysis of four Neandertal and Denisovan individuals compared with nine modern humans, there are only 90 total nucleotide sites genome-wide for which archaic hominins are fixed for the ancestral nonsynonymous variant and the modern humans are fixed for the derived variant. Of those sites, only 27, including Val381 in the AHR, also have no reported variability in the human dbSNP database, further suggesting that this highly conserved functional variant is a rare event. Functional analysis of the amino acid variant Ala381 within the AHR carried by Neandertals and nonhuman primates indicate enhanced polycyclic aromatic hydrocarbon (PAH) binding, DNA binding capacity, and AHR mediated transcriptional activity compared with the human AHR. Also relative to human AHR, the Neandertal AHR exhibited 150-1000 times greater sensitivity to induction of Cyp1a1 and Cyp1b1 expression by PAHs (e.g., benzo(a)pyrene). The resulting CYP1A1/CYP1B1 enzymes are responsible for PAH first pass metabolism, which can result in the generation of toxic intermediates and perhaps AHR-associated toxicities. In contrast, the human AHR retains the ancestral sensitivity observed in primates to nontoxic endogenous AHR ligands (e.g., indole, indoxyl sulfate). Our findings reveal that a functionally significant change in the AHR occurred uniquely in humans, relative to other primates, that would attenuate the response to many environmental pollutants, including chemicals present in smoke from fire use during cooking.

Genetic and pharmacological analysis identifies a physiological role for the AHR in epidermal differentiation

Stimulation of the aryl hydrocarbon receptor (AHR) by xenobiotics is known to affect epidermal differentiation and skin barrier formation. The physiological role of endogenous AHR signaling in keratinocyte differentiation is not known. We used murine and human skin models to address the hypothesis that AHR activation is required for normal keratinocyte differentiation. Using transcriptome analysis of Ahr(-/-) and Ahr(+/+) murine keratinocytes, we found significant enrichment of differentially expressed genes linked to epidermal differentiation. Primary Ahr(-/-) keratinocytes showed a significant reduction in terminal differentiation gene and protein expression, similar to Ahr(+/+) keratinocytes treated with AHR antagonists GNF351 and CH223191, or the selective AHR modulator (SAhRM) SGA360. In vitro keratinocyte differentiation led to increased AHR levels and subsequent nuclear translocation, followed by induced CYP1A1 gene expression. Monolayer cultured primary human keratinocytes treated with AHR antagonists also showed an impaired terminal differentiation program. Inactivation of AHR activity during human skin equivalent development severely impaired epidermal stratification, terminal differentiation protein expression, and stratum corneum formation. As disturbed epidermal differentiation is a main feature of many skin diseases, pharmacological agents targeting AHR signaling or future identification of endogenous keratinocyte-derived AHR ligands should be considered as potential new drugs in dermatology.

Endogenous aryl hydrocarbon receptor promotes basal and inducible expression of tumor necrosis factor target genes in MCF-7 cancer cells

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that upon activation by the toxicant 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD) stimulates gene expression and toxicity. AHR is also important for normal mouse physiology and may play a role in cancer progression in the absence of environmental toxicants. The objective of this report was to identify AHR-dependent genes (ADGs) whose expression is regulated by AHR in the absence of toxicants. RNA-Seq analysis revealed that AHR regulated the expression of over 600 genes at an FDR<10% in MCF-7 breast cancer cells upon knockdown with short interfering RNA. Pathway analysis revealed that a significant number of ADGs were components of TCDD and tumor necrosis factor (TNF) pathways. We also demonstrated that siRNA knockdown of AHR modulated TNF induction of MNSOD and cytotoxicity in MCF-7 cells. Collectively, the major new findings of this report are: (1) endogenous AHR promotes the expression of xenobiotic metabolizing enzymes even in the absence of toxicants and drugs, (2) AHR by modulating the basal expression of a large fraction of TNF target genes may prime them for TNF stimulation and (3) AHR is required for TNF induction of MNSOD and the cellular response to cytotoxicity in MCF-7 cells. This latter result provides a potentially new role for AHR in MCF-7 cancer progression as a mediator of TNF and antioxidant responses.

Role of the aryl hydrocarbon receptor (AhR) in lung inflammation

Millions of individuals worldwide are afflicted with acute and chronic respiratory diseases, causing temporary and permanent disabilities and even death. Oftentimes, these diseases occur as a result of altered immune responses. The aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, acts as a regulator of mucosal barrier function and may influence immune responsiveness in the lungs through changes in gene expression, cell-cell adhesion, mucin production, and cytokine expression. This review updates the basic immunobiology of the AhR signaling pathway with regards to inflammatory lung diseases such as asthma, chronic obstructive pulmonary disease, and silicosis following data in rodent models and humans. Finally, we address the therapeutic potential of targeting the AhR in regulating inflammation during acute and chronic respiratory diseases.

Contributions of the three CYP1 monooxygenases to pro-inflammatory and inflammation-resolution lipid mediator pathways

All three cytochrome P450 1 (CYP1) monooxygenases are believed to participate in lipid mediator biosynthesis and/or their local inactivation; however, distinct metabolic steps are unknown. We used multiple-reaction monitoring and liquid chromatography-UV coupled with tandem mass spectrometry-based lipid-mediator metabololipidomics to identify and quantify three lipid-mediator metabolomes in basal peritoneal and zymosan-stimulated inflammatory exudates, comparing Cyp1a1/1a2/1b1(⁻/⁻) C57BL/6J-background triple-knockout mice with C57BL/6J wild-type mice. Significant differences between untreated triple-knockout and wild-type mice were not found for peritoneal cell number or type or for basal CYP1 activities involving 11 identified metabolic steps. Following zymosan-initiated inflammation, 18 lipid mediators were identified, including members of the eicosanoids and specialized proresolving mediators (i.e., resolvins and protectins). Compared with wild-type mice, Cyp1 triple-knockout mice exhibited increased neutrophil recruitment in zymosan-treated peritoneal exudates. Zymosan stimulation was associated with eight statistically significantly altered metabolic steps: increased arachidonic acid-derived leukotriene B₄ (LTB₄) and decreased 5S-hydroxyeicosatetraenoic acid; decreased docosahexaenoic acid-derived neuroprotectin D1/protectin D1, 17S-hydroxydocosahexaenoic acid, and 14S-hydroxydocosahexaenoic acid; and decreased eicosapentaenoic acid-derived 18R-hydroxyeicosapentaenoic acid (HEPE), 15S-HEPE, and 12S-HEPE. In neutrophils analyzed ex vivo, elevated LTB₄ levels were shown to parallel increased neutrophil numbers, and 20-hydroxy-LTB₄ formation was found to be deficient in Cyp1 triple-knockout mice. Together, these results demonstrate novel contributions of CYP1 enzymes to the local metabolite profile of lipid mediators that regulate neutrophilic inflammation.

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.

Role of the Ah receptor in homeostatic control of fatty acid synthesis in the liver

We have previously demonstrated a role for the aryl hydrocarbon receptor (AHR) in the attenuation of the cholesterol biosynthesis pathway. This regulation did not require that the AHR binds to its cognate response element. Based on these observations and other reports depicting a role for AHR in lipid metabolism, we chose to investigate the involvement of the receptor in the regulation of the fatty acid synthesis pathway in mice and humans. For this purpose, C57BL/6J, liver-specific transgenic DRE-binding mutant AhR (A78D-AhrTtr CreAlb Ahrfx/fx) and CreAlb Ahrfx/fx mice were treated with an AHR ligand, and hepatic mRNA expression levels of key fatty acid genes (e.g., Acaca, Fasn, Scd1) were measured. The basal levels of those genes were also compared between C57BL6/J and hepatic AHR-deficient mice, as well as between Ahb and Ahd congenic mice. To extend these results to humans, fatty acid gene expression in human cells were compared with AHR-silenced cells. In addition, primary human hepatocytes were treated with an AHR ligand to assess alterations in gene expression and fatty acid synthesis. These studies indicated that the AHR constitutively attenuates the expression of key fatty acid synthesis genes in the absence of binding to its cognate response element. In addition, activation of AHR led to further repression of the expression of these genes and a decrease in overall fatty acid synthesis and secretion in human hepatocytes. Based on our results, we can conclude that increased AHR activity represses fatty acid synthesis, suggesting it may be a future therapeutic target.

Aryl hydrocarbon receptor regulates the cholesterol biosynthetic pathway in a dioxin response element-independent manner

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor. Activation of AhR mediates the expression of target genes (e.g., CYP1A1) by binding to dioxin response element (DRE) sequences in their promoter region. To understand the multiple mechanisms of AhR-mediated gene regulation, a microarray analysis on liver isolated from ligand-treated transgenic mice expressing a wild-type (WT) Ahr or a DRE-binding mutant Ahr (A78D) on an ahr-null background was performed. Results revealed that AhR DRE binding is not required for the suppression of genes involved in cholesterol synthesis. Quantitative reverse-transcription polymerase chain reaction performed on both mouse liver and primary human hepatocyte RNA demonstrated a coordinated repression of genes involved in cholesterol biosynthesis, namely, HMGCR, FDFT1, SQLE, and LSS after receptor activation. An additional transgenic mouse line was established expressing a liver-specific Ahr-A78D on a Cre(Alb)/Ahr(flox/flox) background. These mice displayed a similar repression of cholesterol biosynthetic genes, compared to Ahr(flox/flox) mice, further indicating that the observed modulation is AhR specific and occurs in a DRE-independent manner. Elevated hepatic transcriptional levels of the genes of interest were noted in congenic C57BL/6J-Ah(d) allele mice, when compared to the WT C57BL/6J mice, which carry the Ah(b) allele. Down-regulation of AhR nuclear translocator levels using short interfering RNA in a human cell line revealed no effect on the expression of cholesterol biosynthetic genes. Finally, cholesterol secretion was shown to be significantly decreased in human cells after AhR activation.

CONCLUSION

These data firmly establish an endogenous role for AhR as a regulator of the cholesterol biosynthesis pathway independent of its DRE-binding ability, and suggest that AhR may be a previously unrecognized therapeutic target.

Dioxin toxicity, aryl hydrocarbon receptor signaling, and apoptosis-persistent pollutants affect programmed cell death

Exogenous ligands of the aryl hydrocarbon receptor (AhR) such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related substances are highly toxic pollutants ubiquitously present in the environment. They cause a variety of toxic effects to different organs and tissues. Among other effects, TCDD exposure to laboratory animals leads to thymus atrophy and immunosuppression on the one hand, and to tumor formation on the other. Apoptosis appears to be involved in both these toxic effects: AhR activation by TCDD was discussed to induce apoptosis of immune cells, leading to the depletion of thymocytes and ultimately immunosuppression. This mechanism could help to explain the highly immunotoxic actions of TCDD but it is nevertheless under debate whether this is the mode of action for immunosuppression by this class of chemical substances. In other cell types, especially liver cells, TCDD inhibits apoptosis induced by genotoxic treatment. In initiation-promotion studies, TCDD was shown to be a potent liver tumor promoter. Among other theories it was hypothesized that TCDD acts as a tumor promoter by preventing initiated cells from undergoing apoptosis. The exact mechanisms of apoptosis inhibition by TCDD are not fully understood, but both in vivo and in vitro studies consistently showed an involvement of the tumor suppressor p53 in this effect. Various strings of evidence have been established linking apoptosis to the detrimental effects of exogenous activation of the AhR. Within this article, studies elucidating the effects of TCDD and related substances on apoptosis signaling, be it inducing or repressing, is to be reviewed.

Ah receptor antagonism inhibits constitutive and cytokine inducible IL6 production in head and neck tumor cell lines

There is increasing evidence that the aryl hydrocarbon receptor (AHR) plays a role in tumor progression through numerous mechanisms. We have previously shown that, in certain cancer cell lines that are typically nonresponsive to cytokine-mediated IL6 induction, activation of the AHR with the agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin derepresses the IL6 promoter and allows for synergistic induction following IL1β treatment. The mechanism by which this occurs involves liganded AHR binding upstream from the transcription start site and dismissing HDAC-containing corepressor complexes, giving rise to a promoter structure that is more amenable to NF-κB activation. This fact, combined with observations of multiple endogenously produced chemicals activating the AHR, led us to study its role in basal expression among high cytokine-producing cancer cell lines. The current study provides evidence that several head and neck squamous cell carcinoma cell lines have a level of constitutively bound AHR at the IL6 promoter, allowing for higher basal and readily inducible IL6 transcription. Treatment of these cell lines with an AHR antagonist led to dismissal of the AHR from the IL6 promoter and recruitment of corepressor complexes, thus diminishing cytokine expression. Head and neck squamous cell carcinoma is typically a high cytokine-producing tumor type, with IL6 expression levels correlating with disease aggressiveness. For this reason, AHR antagonist treatment could represent a novel adjuvant therapy for patients, lowering pro-growth and antiapoptotic signaling with minimal systemic side effects.

The aryl hydrocarbon receptor interacts with c-Maf to promote the differentiation of type 1 regulatory T cells induced by IL-27

Type 1 regulatory T cells (Tr1 cells ) that produce interleukin 10 (IL-10) are instrumental in the prevention of tissue inflammation, autoimmunity and graft-versus-host disease. The transcription factor c-Maf is essential for the induction of IL-10 by Tr1 cells, but the molecular mechanisms that lead to the development of these cells remain unclear. Here we show that the ligand-activated transcription factor aryl hydrocarbon receptor (AhR), which was induced by IL-27, acted in synergy with c-Maf to promote the development of Tr1 cells. After T cell activation under Tr1-skewing conditions, the AhR bound to c-Maf and promoted transactivation of the Il10 and Il21 promoters, which resulted in the generation of Tr1 cells and the amelioration of experimental autoimmune encephalomyelitis. Manipulating AhR signaling could therefore be beneficial in the resolution of excessive inflammatory responses.

Estradiol regulates aryl hydrocarbon receptor expression in the rat uterus

Several authors have investigated the role of aryl hydrocarbon receptor (AHR) in the reproductive tract, but there are no data available whether 17beta-estradiol (E2) regulates expression of members of the AHR pathway in the uterus. We therefore examined the mRNA expression of Ahr as well as the genes of the AHR dimerization partners ARNT1 and ARNT2 and the AHR regulated genes Cyp1a1 and Gsta2 in the uterus of ovariectomized rats after administration of E2 at two different doses. The data show that Ahr mRNA expression is downregulated while AHR protein amounts increased in all uterine tissue compartments. In addition we observed a downregulation of Arnt1, Arnt2 and Cyp1a1 while Gsta2 mRNA expression is upregulated by E2 in a dose-dependent manner. These results show that members of the AHR pathway are regulated by E2 in the uterus. AHR may therefore play an important role in the mediation of uterine estrogenic effects.

Functions and transcriptional regulation of adult human hepatic UDP-glucuronosyl-transferases (UGTs): mechanisms responsible for interindividual variation of UGT levels

Ten out of 19 UDP-glucuronosyltransferases (UGTs) are substantially expressed in adult human liver (>1% of total UGTs); 5 UGT1 isoforms (UGT1A1, 1A3, 1A4, 1A6 and 1A9) and 5 UGT2 family members (UGT2B4, 2B7, 2B10, 2B15 and 2B17) (Izukawa et al. [11]). Surprisingly, UGT2B4 and UGT2B10 mRNA were found to be abundant in human liver suggesting an underestimated role of the liver in detoxification of their major substrates, bile acids and eicosanoids. Among factors responsible for high interindividual variation of hepatic UGT levels (genetic diversity including polymorphisms and splice variants, regulation by liver-enriched transcription factors such as HNF1 and HNF4, and ligand-activated transcription factors) nuclear receptors (PXR, CAR, PPARalpha, etc.), and the Ah receptor are discussed. Unraveling the mechanisms responsible for interindividual variation of UGT expression will be beneficial for drug therapy but still remains a major challenge.

Activation of PPARbeta/delta causes a psoriasis-like skin disease in vivo

Background

Psoriasis is one of the most frequent skin diseases world-wide. The disease impacts enormously on affected patients and poses a huge financial burden on health care providers. Several lines of evidence suggest that the nuclear hormone receptor peroxisome proliferator activator (PPAR) β/δ, known to regulate epithelial differentiation and wound healing, contributes to psoriasis pathogenesis. It is unclear, however, whether activation of PPARβ/δ is sufficient to trigger psoriasis-like changes in vivo.

Methodology/Principal Findings

Using immunohistochemistry, we define the distribution of PPARβ/δ in the skin lesions of psoriasis. By expression profiling, we confirm that PPARβ/δ is overexpressed in the vast majority of psoriasis patients. We further establish a transgenic model allowing inducible activation of PPARβ/δ in murine epidermis mimicking its distribution in psoriasis lesions. Upon activation of PPARβ/δ, transgenic mice sustain an inflammatory skin disease strikingly similar to psoriasis, featuring hyperproliferation of keratinocytes, dendritic cell accumulation, and endothelial activation. Development of this phenotype requires the activation of the Th17 subset of T cells, shown previously to be central to psoriasis. Moreover, gene dysregulation in the transgenic mice is highly similar to that in psoriasis. Key transcriptional programs activated in psoriasis, including IL1-related signalling and cholesterol biosynthesis, are replicated in the mouse model, suggesting that PPARβ/δ regulates these transcriptional changes in psoriasis. Finally, we identify phosphorylation of STAT3 as a novel pathway activated by PPARβ/δ and show that inhibition of STAT3 phosphorylation blocks disease development.

Conclusions

Activation of PPARβ/δ in the epidermis is sufficient to trigger inflammatory changes, immune activation, and signalling, and gene dysregulation characteristic of psoriasis.

The aryl hydrocarbon receptor: regulation of hematopoiesis and involvement in the progression of blood diseases

The aryl hydrocarbon receptor (AhR) is a basic helix-loop-helix protein that belongs to the superfamily of environment-sensing PAS (Per-ARNT-Sim) proteins. A large number of ligands have been described to bind AhR and promote its nuclear translocation. In the nucleus, the AhR and its dimerization partner the AhR nuclear translocator (ARNT) form a DNA-binding complex that acts as a transcriptional regulator. Animal and human data suggest that, beyond its mediating responses to xenobiotic and/or unknown endogenous ligands, the AhR has a role, although as yet undefined, in the regulation of cell cycle and inflammation. The AhR also appears to regulate the hematopoietic and immune systems during development and adult life in a cell-specific manner. While accidental exposure to xenobiotic AhR ligands has been associated with leukemia in humans, the specific mechanisms of AhR involvement are still not completely understood. However, recent data are consistent with a functional role of the AhR in the maintenance of hematopoietic stem and/or progenitor cells (HSCs/HPCs). Studies highlighting AhR regulation of HSCs/HPCs provide a rational framework to understand their biology, a role of the AhR in hematopoietic diseases, and a means to develop interventions for these diseases.

The mammalian aryl hydrocarbon (Ah) receptor: from mediator of dioxin toxicity toward physiological functions in skin and liver

The mammalian Ah receptor (AhR) is a ligand-activated transcription factor with multiple functions in adaptive metabolism, development and dioxin toxicity in a variety of organs and cell systems. Phenotypes observed following sustained activation by dioxin or in AhR-null mice suggest organ-dependent physiological functions. These functions are probably deregulated following exposure to dioxin. We focus on skin and liver to facilitate discussion of mechanisms linking phenotypes and AhR-modulated genotypes. After a brief summary of currently discussed AhR ligand candidates, two groups of direct AhR target genes/proteins and associated functions are highlighted: (i) xenobiotic-metabolizing enzymes which are also involved in homeostasis of endogenous ligands and (ii) proteins controlling cell proliferation/apoptosis, differentiation and inflammation. Homeostatic feedback loops might not only include CYP1A1 but also Phase II enzymes such as UGT1A1 which controls the antioxidant AhR ligand bilirubin. The AhR is involved in extensive crosstalk with other transcription factors and multiple signaling pathways. Efforts elucidating the pathway toward identification of physiological functions of the AhR remain challenging and promising.

Kynurenic acid is a potent endogenous aryl hydrocarbon receptor ligand that synergistically induces interleukin-6 in the presence of inflammatory signaling

Inflammatory signaling plays a key role in tumor progression, and the pleiotropic cytokine interleukin-6 (IL-6) is an important mediator of protumorigenic properties. Activation of the aryl hydrocarbon receptor (AHR) with exogenous ligands coupled with inflammatory signals can lead to synergistic induction of IL6 expression in tumor cells. Whether there are endogenous AHR ligands that can mediate IL6 production remains to be established. The indoleamine-2,3-dioxygenase pathway is a tryptophan oxidation pathway that is involved in controlling immune tolerance, which also aids in tumor escape. We screened the metabolites of this pathway for their ability to activate the AHR; results revealed that kynurenic acid (KA) is an efficient agonist for the human AHR. Structure-activity studies further indicate that the carboxylic acid group is required for significant agonist activity. KA is capable of inducing CYP1A1 messenger RNA levels in HepG2 cells and inducing CYP1A-mediated metabolism in primary human hepatocytes. In a human dioxin response element-driven stable reporter cell line, the EC(25) was observed to be 104nM, while in a mouse stable reporter cell line, the EC(25) was 10muM. AHR ligand competition binding assays revealed that KA is a ligand for the AHR. Treatment of MCF-7 cells with interleukin-1beta and a physiologically relevant concentration of KA (e.g., 100nM) leads to induction of IL6 expression that is largely dependent on AHR expression. Our findings have established that KA is a potent AHR endogenous ligand that can induce IL6 production and xenobiotic metabolism in cells at physiologically relevant concentrations.

Aryl hydrocarbon receptor-mediated induction of the cytosolic phospholipase A(2)alpha gene by 2,3,7,8-tetrachlorodibenzo-p-dioxin in mouse hepatoma Hepa-1c1c7 cells

Upon binding to ligands such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), an aryl hydrocarbon receptor (AhR) is activated to form a heterodimer with an aryl hydrocarbon receptor nuclear translocator (Arnt). This complex binds to DNA. It has been shown that the AhR bonds to a DNA sequence called the dioxin response element (DRE), which controls the expression of battery genes. It is reported that TCDD releases arachidonic acid from membrane phospholipids via activation of phospholipase A(2)s (PLA(2)s) in various cell types. Recently, we demonstrated that the TCDD-activated AhR binds to the second intron of the Pla2g4a gene, which encodes cytosolic phospholipase A(2)alpha (cPLA(2)alpha), in mouse hepatoma Hepa-1c1c7 cells. This result suggests that Pla2g4a appears to be a target gene of the AhR. In the present study, we investigated whether the transcriptional regulation of Pla2g4a is dependent on the AhR in Hepa-1c1c7 cells. Treatment of the cells with TCDD increased mRNA expression of Pla2g4a and enzymatic activity of PLA(2,) while this increased expression was not observed in AhR-defective c12 cells. After transient transfection of an Ahr gene-expressing plasmid into the c12 cells, expression of Pla2g4a was increased by TCDD. These results indicate that Pla2g4a may be a novel target gene of the AhR, and its transcriptional induction is mediated through binding of the AhR to the second intron of Pla2g4a, although this target site does not have a typical DRE sequence.

TCDD and a putative endogenous AhR ligand, ITE, elicit the same immediate changes in gene expression in mouse lung fibroblasts

The aryl hydrocarbon receptor (AhR), a ligand-dependent transcription factor, mediates toxicity of several classes of xenobiotics and also has important physiological roles in differentiation, reproduction, and immunity, although the endogenous ligand(s) mediating these functions is/are as yet unidentified. One candidate endogenous ligand, 2-(1'H-indolo-3'-carbonyl)-thiazole-4-carboxylic acid methyl ester (ITE), is a potent AhR agonist in vitro, activates the murine AhR in vivo, but does not induce toxicity. We hypothesized that ITE and the toxic ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), may modify transcription of different sets of genes to account for their different toxicity. To test this hypothesis, primary mouse lung fibroblasts were exposed to 0.5muM ITE, 0.2nM TCDD, or vehicle for 4 h, and total gene expression was evaluated using microarrays. After this short-term and low-dose treatment, several hundred genes were changed significantly, and the response to ITE and TCDD was remarkably similar, both qualitatively and quantitatively. Induced gene sets included the expected battery of AhR-dependent xenobiotic-metabolizing enzymes, as well as several sets that reflect the inflammatory role of lung fibroblasts. Real time quantitative RT-qPCR assay of several selected genes confirmed these microarray data and further suggested that there may be kinetic differences in expression between ligands. These data suggest that ITE and TCDD elicit an analogous change in AhR conformation such that the initial transcription response is the same. Furthermore, if the difference in toxicity between TCDD and ITE is mediated by differences in gene expression, then it is likely that secondary changes enabled by the persistent TCDD, but not by the shorter lived ITE, are responsible.

The aryl hydrocarbon receptor has a normal function in the regulation of hematopoietic and other stem/progenitor cell populations

The aryl hydrocarbon receptor (AhR) is known mainly as the mediator for the toxicity of certain xenobiotics. However, there is also much information to indicate that this transcription factor has important biological functions. Here we review the evidence that the AhR has a significant role in the regulation of hematopoietic stem cells (HSCs). Data to support this come from studies with xenobiotic AhR ligands, phenotypic analyses of mice lacking AhR, examining the presence and regulation of the AhR within HSCs, knowledge of genes and signaling pathways regulated by the AhR, and investigations of hematopoietic disorders. Based on this information, we hypothesize that AhR expression is necessary for the proper maintenance of quiescence in HSCs, and that AhR down-regulation is essential for "escape" from quiescence and subsequent proliferation of these cells. This implicates the AhR as a negative regulator of hematopoiesis with a function of curbing excessive or unnecessary proliferation. This provides an important advantage by preventing the premature exhaustion of HSCs and sensitivity to genetic alterations, thus preserving HSC function and long-term multi-lineage generation over the lifespan of the organism. This also implicates a role of the AhR in aging processes. AhR dysregulation may result in the altered ability of HSCs to sense appropriate signals in the bone marrow microenvironment leading to hematopoietic disease. It is also reasonable to hypothesize that this protein has an important function in the regulation of other tissue stem cell populations. Suggestive evidence is consistent with a role in skin and neural stem cells.

12(R)-Hydroxy-5(Z),8(Z),10(E),14(Z)-eicosatetraenoic acid [12(R)-HETE], an arachidonic acid derivative, is an activator of the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AHR) is a ligand-regulated transcription factor that can be activated by structurally diverse chemicals, ranging from environmental carcinogens to dietary metabolites. Evidence supporting a necessary role for the AHR in normal biology has been established; however, identification of key endogenous ligand/activator remains to be established. Here, we report the ability of 12(R)-hydroxy-5(Z),8(Z),10(E), 14(Z)-eicosatetraenoic acid [12(R)-HETE], an arachidonic acid metabolite produced by either a lipoxygenase or cytochrome P-450 pathway, to act as a potent indirect modulator of the AHR pathway. In contrast, structurally similar HETE isomers failed to demonstrate significant activation of the AHR. Electrophoretic mobility shift assays, together with ligand competition binding experiments, have demonstrated the inability of 12(R)-HETE to directly bind or directly activate the AHR to a DNA binding species in vitro. However, cell-based xenobiotic-responsive element-driven luciferase reporter assays indicate the ability of 12(R)-HETE to modulate AHR activity, and quantitation of induction of an AHR target gene confirmed 12(R)-HETE's ability to activate AHR-mediated transcription, even at high nanomolar concentrations in human hepatoma (HepG2)- and keratinocyte (HaCaT)-derived cell lines. One explanation for these results is that a metabolite of 12(R)-HETE is acting as a direct ligand for the AHR. However, several known metabolites failed to exhibit AHR activity. The ability of 12(R)-HETE to activate AHR target genes required receptor expression. These results indicate that 12(R)-HETE can serve as a potent activator of AHR activity and suggest that in normal and inflammatory disease conditions in skin, 12(R)-HETE is produced, perhaps leading to AHR activation.