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

Uremic toxin indoxyl 3-sulfate regulates the differentiation of Th2 but not of Th1 cells to lessen allergic asthma

Immune system dysfunctions including the increased Th1/Th2 ratio are common in chronic kidney disease (CKD) patients, and a wide variety of skin diseases including Th1-mediated uremic pruritis are associated with CKD. Although there are more than 90 uremic toxins reported, it is yet to be known which uremic solute is associated with the unbalanced Th1/Th2 ratio and how it works. Indoxyl 3-sulfate (I3S), one of uremic toxins and a potent aryl hydrocarbon receptor (AhR) ligand, accumulates in blood and tissues, increasing up to 81.04 μM in CKD patients, compared with 1.03 μM in healthy subjects. I3S activates NF-κB and AhR. Thus, we investigated roles of I3S in the differentiation of Th1 and Th2 cells. I3S inhibited Th2 differentiation but showed little or no effect on Th1 differentiation. I3S suppressed Th2-mediated ovalbumin-induced allergic asthma in mice and decreased the frequency of IL-4 producing CD4 T cells in the lungs. I3S inhibited phosphorylation of STAT5 and STAT6, transcription factors associated with Th2 differentiation. Effects of I3S on Th2 differentiation were suppressed by α-naphtoflavone, an AhR antagonist, indicating that I3S regulates Th2 differentiation AhR-dependently.

Functions of the aryl hydrocarbon receptor in the skin

Among other functions, the skin serves as the barrier against the environment and provides vital protection from physical or chemical harm and from infection. Skin cells express the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor and sensor of environmental chemicals; at the same time, AHR ligands are abundant in skin from exogenous or endogenous sources. For example, solar radiation, in particular ultraviolet (UV) B, generates AHR ligands from tryptophan in the skin. Recent evidence has shown that AHR is involved in the (patho)physiology of skin including the regulation of skin pigmentation, photocarcinogenesis, and skin inflammation. We here provide a state-of-the-art summary of work which relates to the role of the AHR in (1) adaptive responses against environmental challenges such as UVB or topical chemicals and (2) intrinsic developmental roles for homeostasis of skin cells and (3) skin immunity. We also discuss the existing evidence that AHR antagonists or AHR ligands may be used for the prevention and/or treatment of skin disease.

The role of the aryl hydrocarbon receptor in normal and malignant B cell development

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor historically studied for its role in environmental chemical-mediated toxicity and carcinogenicity. In the last 5 years, however, it has become clear that the AhR, presumably activated by endogenous ligand(s), plays an important role in immune system development and function. Other articles in this edition summarize AhR function during T cell and antigen-presenting cell development and function, including the effects of AhR activation on dendritic cell function, T cell skewing, inflammation, and autoimmune disease. Here, we focus on AhR expression and function during B cell differentiation. Studies exploiting immunosuppressive environmental chemicals to probe the role of the AhR in humoral immunity are also reviewed to illustrate the multiple levels at which a “nominally activated” AhR could control B cell differentiation from the hematopoietic stem cell through the pro-B cell, mature B cell, and antibody-secreting plasma cell stages. Finally, a putative role for the AhR in the basic biology of B cell malignancies, many of which have been associated with exposure to environmental AhR ligands, is discussed.

The emerging roles of AhR in physiology and immunity

The aryl hydrocarbon receptor (AhR) is traditionally defined as a transcriptional regulator involved in adaptive xenobiotic response, however, emerging evidence supports physiological functions of AhR in normal cell development and immune response. The role of AhR in immunomodulation is multi-dimensional. On the one hand, activation of AhR by TCDD and other ligands leads to profound immunosuppression, potentially via skewed Th1/Th2 cell balance toward Th1 dominance, and boosted Treg cell differentiation. On the other hand, activation of AhR can also induce Th17 cell polarization and increase the severity of autoimmune disease. In addition to T lymphocytes, the AhR also appears to play a vital role in B cell maturation, and regulates the activity of macrophages, dendritic cells and neutrophils following lipopolysaccharide challenge or influenza virus infection. In these scenarios, activation of AhR is associated with decreased host response and reduced survival. Furthermore, gene knock out studies suggest that AhR is indispensable for the postnatal maintenance of intestinal intraepithelial lymphocytes and skin-resident dendritic epidermal gamma delta T cells, providing a potential link between AhR and gut immunity and wound healing. It is well accepted that the magnitude and the type of immune response is dependent on the local cytokine milieu and the AhR appears to be one of the key factors involved in the fine turning of this cytokine balance.

The aryl hydrocarbon receptor directs hematopoietic progenitor cell expansion and differentiation

The evolutionarily conserved aryl hydrocarbon receptor (AhR) has been studied for its role in environmental chemical-induced toxicity. However, recent studies have demonstrated that the AhR may regulate the hematopoietic and immune systems during development in a cell-specific manner. These results, together with the absence of an in vitro model system enabling production of large numbers of primary human hematopoietic progenitor cells (HPs) capable of differentiating into megakaryocyte- and erythroid-lineage cells, motivated us to determine if AhR modulation could facilitate both progenitor cell expansion and megakaryocyte and erythroid cell differentiation. Using a novel, pluripotent stem cell-based, chemically-defined, serum and feeder cell-free culture system, we show that the AhR is expressed in HPs and that, remarkably, AhR activation drives an unprecedented expansion of HPs, megakaryocyte-lineage cells, and erythroid-lineage cells. Further AhR modulation within rapidly expanding progenitor cell populations directs cell fate, with chronic AhR agonism permissive to erythroid differentiation and acute antagonism favoring megakaryocyte specification. These results highlight the development of a new Good Manufacturing Practice-compliant platform for generating virtually unlimited numbers of human HPs with which to scrutinize red blood cell and platelet development, including the assessment of the role of the AhR critical cell fate decisions during hematopoiesis.

3',4'-Dimethoxyflavone and valproic acid promotes the proliferation of human hematopoietic stem cells

INTRODUCTION

Human hematopoietic stem cells (HSCs) have been clinically used for transplantation and gene and cellular therapy for more than 4 decades. However, this use is limited because of the challenges in the ex vivo culturing of HSCs. The major hurdle is to amplify these cells without losing their self-renewing property.

METHODS

In our study, we tested 3',4'-dimethoxyflavone (3'4'-DMF) and valproic acid (VPA) on the ex vivo expansion of HSCs under both normoxic (20% O2) and hypoxic (1% O2) conditions. 3'4'-DMF is a widely used anticancer drug that acts as a competitive antagonist of the aryl hydrocarbon receptor. VPA is a potent inhibitor of histone deacetylase and is used in the treatment of neurologic disorders.

RESULTS

Culturing HSCs (from mobilized peripheral blood) under normoxia, with 3'4'-DMF and VPA, highly preserved the CD34 positivity (3'4'-DMF, 22.1%, VPA, 20.3%) after 1 week and strongly enhanced the CD34(+) cells (3'4'-DMF, 27.8 fold; VPA, 34.1 fold) compared with the control cultures (11.6% and 14.4 fold). Addition of 3'4'-DMF and VPA also resulted in more primary colonies and replating efficiency compared with control cultures. Although no significant effect was observed on the enhancement of CD34(+) cells under hypoxia, the number of primary colonies was significantly higher than the control cultures.

CONCLUSIONS

Based on these findings, this study presents, for the first time, in vitro evidence for a new and relevant effect of 3'4'-DMF on human HSCs. In addition, the results suggest a potential clinical use of 3'4'-DMF and VPA in HSC therapy.

The evolving role of the aryl hydrocarbon receptor (AHR) in the normophysiology of hematopoiesis

In addition to its role as a toxicological signal mediator, the Aryl Hydrocarbon Receptor (AHR) is also a transcription factor known to regulate cellular responses to oxidative stress and inflammation through transcriptional regulation of molecules involved in the signaling of nucear factor-erythroid 2-related factor-2 (Nrf2), p53 (TRP53), retinoblastoma (RB1), and NFκB. Recent research suggests that AHR activation of these signaling pathways may provide the molecular basis for understanding AHR's evolving role in endogenous developmental functions during hematopoietic stem-cell maintenance and differentiation. Recent developments into the hematopoietic roles for AHR are reviewed, aiming to reconcile divergent findings as to the endogenous function of AHR in hematopoiesis. Potential mechanistic explanations for AHR's involvement in hematopoietic differentiation are discussed, focusing on its known role as a cell cycle mediator and its interactions with Hypoxia-inducible transcription factor-1 alpha (HIF1-α). Understanding the physiological mechanisms of AHR activation and signaling have far reaching implications ranging from explaining the action of various toxicological agents to providing novel ways to expand stem cell populations ex vivo for use in transplant therapies.

FICZ, a tryptophan photoproduct, suppresses pulmonary eosinophilia and Th2-type cytokine production in a mouse model of ovalbumin-induced allergic asthma

Most studies about functions of aryl hydrocarbon receptor (AhR) in the pathogenesis of asthma have been carried out with non-physiological industrial by-products such as 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo(a)pyrene. In the present study, effects of 6-formylindolo[3,2-b]carbazole (FICZ), a tryptophan photoproduct postulated as a candidate physiological ligand of AhR, on the pathogenesis of asthma were examined and then underlying mechanisms of its immumodulatory effects were investigated. FICZ significantly reduced pulmonary eosinophilia and Th2 cytokine expression in the lungs. Flow cytometric analysis of mediastinal lymph nodes showed that IL-4 producing cells decreased in FICZ-treated mice compared with PBS control. Next, effects of FICZ on in vitro Th2 differentiation and expression of the Th2 transcription factor GATA-3 were examined. CD4+ T cells were isolated from the spleen and incubated under the Th2 differentiation conditions. FICZ inhibited both Th2 differentiation and the expression of GATA-3. Finally, activation of STAT6, which is necessary for Th2 differentiation, was inhibited by FICZ.

Current understanding of the mechanism of benzene-induced leukemia in humans: implications for risk assessment

Benzene causes acute myeloid leukemia and probably other hematological malignancies. As benzene also causes hematotoxicity even in workers exposed to levels below the US permissible occupational exposure limit of 1 part per million, further assessment of the health risks associated with its exposure, particularly at low levels, is needed. Here, we describe the probable mechanism by which benzene induces leukemia involving the targeting of critical genes and pathways through the induction of genetic, chromosomal or epigenetic abnormalities and genomic instability, in a hematopoietic stem cell (HSC); stromal cell dysregulation; apoptosis of HSCs and stromal cells and altered proliferation and differentiation of HSCs. These effects modulated by benzene-induced oxidative stress, aryl hydrocarbon receptor dysregulation and reduced immunosurveillance, lead to the generation of leukemic stem cells and subsequent clonal evolution to leukemia. A mode of action (MOA) approach to the risk assessment of benzene was recently proposed. This approach is limited, however, by the challenges of defining a simple stochastic MOA of benzene-induced leukemogenesis and of identifying relevant and quantifiable parameters associated with potential key events. An alternative risk assessment approach is the application of toxicogenomics and systems biology in human populations, animals and in vitro models of the HSC stem cell niche, exposed to a range of levels of benzene. These approaches will inform our understanding of the mechanisms of benzene toxicity and identify additional biomarkers of exposure, early effect and susceptibility useful for risk assessment.

AHR2 mutant reveals functional diversity of aryl hydrocarbon receptors in zebrafish

The aryl hydrocarbon receptor (AHR) is well known for mediating the toxic effects of TCDD and has been a subject of intense research for over 30 years. Current investigations continue to uncover its endogenous and regulatory roles in a wide variety of cellular and molecular signaling processes. A zebrafish line with a mutation in ahr2 (ahr2(hu3335)), encoding the AHR paralogue responsible for mediating TCDD toxicity in zebrafish, was developed via Targeting Induced Local Lesions IN Genomes (TILLING) and predicted to express a non-functional AHR2 protein. We characterized AHR activity in the mutant line using TCDD and leflunomide as toxicological probes to investigate function, ligand binding and CYP1A induction patterns of paralogues AHR2, AHR1A and AHR1B. By evaluating TCDD-induced developmental toxicity, mRNA expression changes and CYP1A protein in the AHR2 mutant line, we determined that ahr2(hu3335) zebrafish are functionally null. In silico modeling predicted differential binding of TCDD and leflunomide to the AHR paralogues. AHR1A is considered a non-functional pseudogene as it does not bind TCCD or mediate in vivo TCDD toxicity. Homology modeling, however, predicted a ligand binding conformation of AHR1A with leflunomide. AHR1A-dependent CYP1A immunohistochemical expression in the liver provided in vivo confirmation of the in silico docking studies. The ahr2(hu3335) functional knockout line expands the experimental power of zebrafish to unravel the role of the AHR during development, as well as highlights potential activity of the other AHR paralogues in ligand-specific toxicological responses.

Mechanisms controlling Th17 cytokine expression and host defense

Th17 cells contribute to mucosal immunity by stimulating epithelial cells to induce antimicrobial peptides, granulopoiesis, neutrophil recruitment, and tissue repair. Recent studies have identified important roles for commensal microbiota and Ahr ligands in stabilizing Th17 gene expression in vivo, linking environmental cues to CD4 T cell polarization. Epigenetic changes that occur during the transition from naïve to effector Th17 cells increase the accessibility of il17a, il17f, and il22 loci to transcription factors. In addition, Th17 cells maintain the potential for expressing T-bet, Foxp3, or GATA-binding protein-3, explaining their plastic nature under various cytokine microenvironments. Although CD4 T cells are major sources of IL-17 and IL-22, innate cell populations, including γδ T cells, NK cells, and lymphoid tissue-inducer cells, are early sources of these cytokines during IL-23-driven responses. Epithelial cells and fibroblasts are important cellular targets for IL-17 in vivo; however, recent data suggest that macrophages and B cells are also stimulated directly by IL-17. Thus, Th17 cells interact with multiple populations to facilitate protection against intracellular and extracellular pathogens.

Acute disruption of bone marrow hematopoiesis by benzo(a)pyrene is selectively reversed by aryl hydrocarbon receptor-mediated processes

Bone marrow (BM) hematopoietic cells are selectively sensitive to polycyclic aromatic hydrocarbons (PAH) in vivo. 7,12-Dimethylbenz(a)anthracene (DMBA), but not benzo(a)pyrene (BP), depletes BM hematopoietic cells in C57BL/6 mice. This difference is due to a BP-selective aryl hydrocarbon receptor (AhR)-mediated recovery. Colony-forming unit assays show suppression of lymphoid progenitors by each PAH within 6 h but a subsequent recovery, exclusively after BP treatment. Suppression of myeloid progenitors (6 h) occurs only for DMBA. Each progenitor responded equally to DMBA and BP in congenic mice expressing the PAH-resistant AhR (AhR(d)). AhR, therefore, mediates this BP recovery in each progenitor type. These PAH suppressions depend on Cyp1b1-mediated metabolism. Paradoxically, few genes responded to DMBA, whereas 12 times more responded to BP. Progenitor suppression by DMBA, therefore, occurs with minimal effects on the general BM population. Standard AhR-mediated stimulations (Cyp1a1, Cyp1b1, Ahrr) were similar for each PAH and for the specific agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin but were absent in AhR(d) mice. A group of 12 such AhR responses was sustained from 6 to 24 h. A second, larger set of BP responses (chemokines, cytokines, cyclooxygenase 2) differed in two respects; DMBA responses were low and BP responses declined extensively from 6 to 24 h. A third cluster exhibited BP-induced increases in protective genes (Nqo1, GST-mu) that appeared only after 12 h. Conversion of BP to quinones contributes oxidative signaling not seen with DMBA. We propose that genes in this second cluster, which share oxidative signaling and AhR activation, provide the AhR-dependent protection of hematopoietic progenitors seen for BP.

Suppression of cytokine-mediated complement factor gene expression through selective activation of the Ah receptor with 3',4'-dimethoxy-α-naphthoflavone

We have characterized previously a class of aryl hydrocarbon receptor (AHR) ligand termed selective AHR modulators (SAhRMs). SAhRMs exhibit anti-inflammatory properties, including suppression of cytokine-mediated acute phase genes (e.g., Saa1), through dissociation of non-dioxin-response element (DRE) AHR activity from DRE-dependent xenobiotic gene expression. The partial AHR agonist α-naphthoflavone (αNF) mediates the suppressive, non-DRE dependent effects on SAA1 expression and partial DRE-mediated CYP1A1 induction. These observations suggest that αNF may be structurally modified to a derivative exhibiting only SAhRM activity. A screen of αNF derivatives identifies 3',4'-dimethoxy-αNF (DiMNF) as a candidate SAhRM. Competitive ligand binding validates DiMNF as an AHR ligand, and DRE-dependent reporter assays with quantitative mRNA analysis of AHR target genes reveal minimal agonist activity associated with AHR binding. Consistent with loss of agonist activity, DiMNF fails to promote AHR binding to DRE probes as determined through electromobility shift assay. Importantly, mRNA analysis indicates that DiMNF retains the suppressive capacity of αNF regarding cytokine-mediated SAA1 expression in Huh7 cells. Interestingly, predictive docking modeling suggests that DiMNF adopts a unique orientation within the AHR ligand binding pocket relative to αNF and may facilitate the rational design of additional SAhRMs. Microarray studies with a non-DRE binding but otherwise functional AHR mutant identified complement factor C3 as a potential SAhRM target. We confirmed this observation in Huh7 cells using 10 μM DiMNF, which significantly repressed C3 mRNA and protein. These data expand the classes of AHR ligands exerting DRE-independent anti-inflammatory SAhRM activity, suggesting SAhRMs may have application in the amelioration of inflammatory disorders.

Epigenetics: prenatal exposure to genistein leaves a permanent signature on the hematopoietic lineage

Recent studies demonstrate that maternal diet during pregnancy results in long-lasting effects on the progeny. Supplementation of maternal diet with genistein, a phytoestrogen ubiquitous in the daily diet, altered coat color of agouti mice due to epigenetic changes. We studied hematopoiesis of mice prenatally exposed to genistein (270 mg/kg feed) compared with that of mice prenatally exposed to phytoestrogen-poor feed and observed a significant increase in granulopoiesis, erythropoiesis, and mild macrocytosis at the adult age of 12 wk. Genistein exposure was associated with hypermethylation of certain repetitive elements, which coincided with a significant down-regulation of estrogen-responsive genes and genes involved in hematopoiesis in bone marrow cells of genistein-exposed mice, as assessed by microarray technology. Although genistein exposure did not affect global methylation in fetal liver of fetuses at embryonic day 14.5, it accelerated the switch from primitive to definitive erythroid lineage. Taken together, our data demonstrate that prenatal exposure to genistein affects fetal erythropoiesis and exerts lifelong alterations in gene expression and DNA methylation of hematopoietic cells.

The long winding road toward understanding the molecular mechanisms for B-cell suppression by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Suppression of humoral immune responses by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was first reported in the mid-1970s. Since this initial observation, much effort has been devoted by many laboratories toward elucidation of the cellular and molecular mechanisms responsible for the profound impairment of humoral immune responses by TCDD, which is characterized by decreased B cell to plasma cell differentiation and suppression of immunoglobulin production. These efforts have led to a significant body of research demonstrating a direct effect of TCDD on B-cell maturation and function as well as a requisite but as yet undefined role of the aryl hydrocarbon receptor (AhR) in these effects. Likewise, a number of molecular targets putatively involved in mediating B-cell dysfunction by TCDD, and other AhR ligands, have been identified. However, our current understanding has primarily relied on findings from mouse models, and the translation of this knowledge to effects on human B cells and humoral immunity in humans is less clear. Therefore, a current challenge is to determine how TCDD and the AhR affect human B cells. Efforts have been made in this direction but continued progress in developing adequate human models is needed. An in-depth discussion of these advances and limitations in elucidating the cellular and molecular mechanisms putatively involved in the suppression of B-cell function by TCDD as well as the implications on human diseases associated in epidemiological studies with exposure to TCDD and dioxin-like compounds is the primary focus of this review.

Aryl hydrocarbon receptor antagonists promote the expansion of human hematopoietic stem cells

Although practiced clinically for more than 40 years, the use of hematopoietic stem cell (HSC) transplants remains limited by the ability to expand these cells ex vivo. An unbiased screen with primary human HSCs identified a purine derivative, StemRegenin 1 (SR1), that promotes the ex vivo expansion of CD34+ cells. Culture of HSCs with SR1 led to a 50-fold increase in cells expressing CD34 and a 17-fold increase in cells that retain the ability to engraft immunodeficient mice. Mechanistic studies show that SR1 acts by antagonizing the aryl hydrocarbon receptor (AHR). The identification of SR1 and AHR modulation as a means to induce ex vivo HSC expansion should facilitate the clinical use of HSC therapy.

Aryl hydrocarbon receptor activation reduces dendritic cell function during influenza virus infection

It has long been known that activation of the aryl hydrocarbon receptor (AhR) by ligands such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) suppresses T cell-dependent immune responses; however, the underlying cellular targets and mechanism remain unclear. We have previously shown that AhR activation by TCDD reduces the proliferation and differentiation of influenza virus-specific CD8(+) T cells through an indirect mechanism; suggesting that accessory cells are critical AhR targets during infection. Respiratory dendritic cells (DCs) capture antigen, migrate to lymph nodes, and play a key role in activating naive CD8(+) T cells during respiratory virus infection. Herein, we report an examination of how AhR activation alters DCs in the lung and affects their trafficking to and function in the mediastinal lymph nodes (MLN) during infection with influenza virus. We show that AhR activation impairs lung DC migration and reduces the ability of DCs isolated from the MLN to activate naive CD8(+) T cells. Using novel AhR mutant mice, in which the AhR protein lacks its DNA-binding domain, we show that the suppressive effects of TCDD require that the activated AhR complex binds to DNA. These new findings suggest that AhR activation by chemicals from our environment impacts DC function to stimulate naive CD8(+) T cells and that immunoregulatory genes within DCs are critical targets of AhR. Moreover, our results reinforce the idea that environmental signals and AhR ligands may contribute to differential susceptibilities and responses to respiratory infection.

Effects of TCDD on the fate of naive dendritic cells

The environmental contaminant, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), causes immune suppression via activation of the aryl hydrocarbon receptor. Dendritic cells (DCs), the professional antigen-presenting cells in the immune system, are adversely affected by TCDD. We hypothesized that TCDD alters DC homeostasis, resulting in a loss of DCs in naive mice. To test this hypothesis, C57Bl/6 mice were gavaged with either vehicle or an immunosuppressive dose of TCDD (15 microg/kg). TCDD exposure decreased the frequency and number of splenic CD11c(high) DCs on day 7 when compared with vehicle-treated controls. TCDD increased the expression of CD86 and CD54, while decreasing the frequency of splenic CD11c(high) DCs expressing CD11a and major histocompatibility complex (MHC) class II. Moreover, TCDD selectively decreased the CD11c(high)CD8alpha(-)33D1(+) splenic DCs specialized at activating CD4(+) T cells but did not affect the regulatory CD11c(high)CD8alpha(+)DEC205(+) splenic DCs. TCDD did not alter the number or frequency of CD11c(low) splenic DCs but decreased their MHC class II and CD11a expression. Loss of splenic CD11c(high) DCs was independent of Fas-mediated apoptosis and was not due to alterations in the numbers of common DC precursors in the bone marrow or their ability to generate steady-state DCs in vitro. Instead, increased CCR7 expression on CD11c(high) DCs suggested involvement of a migratory event. Popliteal and brachial lymph node CD11c(+) cells showed elevated levels of MHC class II and CD40 following TCDD exposure. Collectively, this study shows the presence of a TCDD-sensitive splenic DC subpopulation in naive mice, suggesting that TCDD may induce suppression of T-cell-mediated immunity by disrupting DC homeostasis.

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.

Advances in understanding benzene health effects and susceptibility

Benzene is a ubiquitous chemical in our environment that causes acute leukemia and probably other hematological cancers. Evidence for an association with childhood leukemia is growing. Exposure to benzene can lead to multiple alterations that contribute to the leukemogenic process, indicating a multimodal mechanism of action. Research is needed to elucidate the different roles of multiple metabolites in benzene toxicity and the pathways that lead to their formation. Studies to date have identified a number of polymorphisms in candidate genes that confer susceptibility to benzene hematotoxicity. However, a genome-wide study is needed to truly assess the role of genetic variation in susceptibility. Benzene affects the blood-forming system at low levels of occupational exposure, and there is no evidence of a threshold. There is probably no safe level of exposure to benzene, and all exposures constitute some risk in a linear, if not supralinear, and additive fashion.

An overview of the effects of dioxins and dioxin-like compounds on vertebrates, as documented in human and ecological epidemiology

Dioxins and dioxin-like compounds are primary examples of persistent organic pollutants that induce toxicity in both wildlife and humans. Over the past 200 years these compounds have been almost exclusively generated by human activity and have left a string of disasters in the wake of their accidental release. Most recently, the contamination of the Irish pork supply with dioxins resulted in an international recall of all Irish pork products. Epidemiologic data on human and ecological dioxin exposures have revealed a common pattern of biological response among vertebrate species, which is mediated through activation of the Aryl hydrocarbon Receptor (AhR). These AhR-mediated effects include profound consequences on the vertebrate individual exposed in early life with respect to myriad developmental endpoints including neurologic, immunologic, and reproductive parameters. Humans appear to be susceptible to these effects in a manner similar to that of the laboratory and wildlife species, which have demonstrated such outcomes. Furthermore, epidemiologic data suggest that there is little or no margin of exposure for humans with respect to these developmental effects. Given these concerns, prudent public health policy should include the continued reduction of exposures.

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.

The aryl hydrocarbon receptor in immunity

Low-molecular-weight chemicals or xenobiotics might contribute to the increasing prevalence of allergies and autoimmunity. Certain chemicals can alter immune responses via their action on the cytosolic transcription factor aryl hydrocarbon receptor (AhR). AhR recognizes numerous small xenobiotic and natural molecules, such as dioxin and the tryptophan photoproduct 6-formylindolo[3,2-b]carbazole. Although AhR is best known for mediating dioxin toxicity, knockout studies have indicated that AhR also plays a role in normal physiology, including certain immune responses. In particular, Th17 cells and dendritic cells express high levels of AhR. We review here current evidence for the physiological role of AhR in the immune system, focussing in particular on T-cell biology.