Constitutively Active Aryl Hydrocarbon Receptor Expressed Specifically in T-Lineage Cells Causes Thymus Involution and Suppresses the Immunization-Induced Increase in Splenocytes

Aire drives steroid hormone biosynthesis by medullary thymic epithelial cells

Thymic epithelial cells (TECs) produce glucocorticoids, which antagonize negative selection of autoreactive thymocytes and promote a competent T cell antigen-specific repertoire. To characterize their source, we generated a knock-in reporter mouse in which endogenous Cyp11b1, the final enzyme in de novo production of active glucocorticoids, was fluorescently tagged with mScarlet. Here, we find that Cyp11b1 is expressed in medullary TECs (mTECs) but not cortical TECs or other cells in the thymus. A distinct characteristic of mTECs is the presence of Aire, a transcription factor that drives expression of tissue-restricted antigens (TRAs) important for establishing immune tolerance. Cyp11b1 expression was highest in Aire+ mTECs, lower in post-Aire mTECs, and absent in mTECs of Aire-deficient mice. Transcriptomic analyses found that multiple enzymatic biosynthetic pathways are expressed specifically in mTECs and are also Aire dependent. In particular, we found that the thymus expresses messenger RNA for enzymes that catalyze production of many bioactive steroids and that glucocorticoids and sex steroids were secreted by cultured thymi. Expression of the transcripts for these genes and production of their final steroid products were markedly reduced in the absence of Aire. Thus, in addition to its well-established role in inducing TRAs that promote negative selection, Aire has an additional and contrary function of inducing glucocorticoids that antagonize negative selection, which together may expand and enhance the TCR repertoire. Furthermore, because Aire drives expression of multiple enzymes responsible for production of other non-gene-encoded bioactive molecules, it might have yet other roles in thymus development and function.

Modulating AHR function offers exciting therapeutic potential in gut immunity and inflammation

Aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a classical exogenous synthetic ligand of AHR that has significant immunotoxic effects. Activation of AHR has beneficial effects on intestinal immune responses, but inactivation or overactivation of AHR can lead to intestinal immune dysregulation and even intestinal diseases. Sustained potent activation of AHR by TCDD results in impairment of the intestinal epithelial barrier. However, currently, AHR research has been more focused on elucidating physiologic AHR function than on dioxin toxicity. The appropriate level of AHR activation plays a role in maintaining gut health and protecting against intestinal inflammation. Therefore, AHR offers a crucial target to modulate intestinal immunity and inflammation. Herein, we summarize our current understanding of the relationship between AHR and intestinal immunity, the ways in which AHR affects intestinal immunity and inflammation, the effects of AHR activity on intestinal immunity and inflammation, and the effect of dietary habits on intestinal health through AHR. Finally, we discuss the therapeutic role of AHR in maintaining gut homeostasis and relieving inflammation.

Rodent genetic models of Ah receptor signaling

The aryl hydrocarbon receptor (AHR) is a ligand activated transcription factor that is a member of the PER-ARNT-SIM superfamily of environmental sensors. This receptor has been a molecule of interest for many years in the field of toxicology, as it was originally discovered to mediate the toxic effects of certain environmental pollutants like benzo(a)pyrene and 2,3,7,8-tetrachlorodibenzo-p-dioxin. While all animals express this protein, there is naturally occurring variability in receptor size and responsiveness to ligand. This naturally occurring variation, particularly in mice, has been an essential tool in the discovery and early characterization of the AHR. Genetic models including congenic mice and induced mutations at the Ahr locus have proven invaluable in further understanding the role of the AHR in adaptive metabolism and TCDD-induced toxicity. The creation and examination of Ahr null mice revealed an important physiological role for the AHR in vascular and hepatic development and mediation of the immune system. In this review, we attempt to provide an overview to many of the AHR models that have aided in the understanding of AHR biology thus far. We describe the naturally occurring polymorphisms, congenic models, induced mutations at the Ahr locus and at the binding partner Ah Receptor Nuclear Translocator and chaperone, Ah receptor associated 9 loci in mice, with a brief description of naturally occurring and induced mutations in rats.

Discovery and Mechanistic Characterization of a Select Modulator of AhR-regulated Transcription (SMAhRT) with Anti-cancer Effects

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor and a member of the bHLH/PAS (basic Helix-Loop-Helix/Per-Arnt-Sim) family of proteins. The AhR was cloned and characterized for its role in mediating the toxicity of dioxins. Subsequent research has identified the role of AhR in suppression of cancer cell growth. We hypothesized that the AhR is a molecular target for therapeutic intervention in cancer, and that activation of the AhR by unique AhR ligands in cancer cells could have anti-cancer effects including induction of cell death. This study describes the discovery and characterization of a new class of anti-cancer agents targeting the AhR, that we designate as Select Modulators of AhR-regulated Transcription (SMAhRTs). We employed two independent small molecule screening approaches to identify potential SMAhRTs. We report the identification of CGS-15943 that activates AhR signaling and induces apoptosis in an AhR-dependent manner in liver and breast cancer cells. Investigation of the downstream signaling pathway of this newly identified SMAhRT revealed upregulation of Fas-ligand (FasL), which is required for AhR-mediated apoptosis. Our results provide a basis for further development of a new class of anti-cancer therapeutics targeting an underappreciated molecular target, the AhR.

Aryl hydrocarbon receptor connects dysregulated immune cells to atherosclerosis

As a chronic inflammatory disease with autoimmune components, atherosclerosis is the major cause of cardiovascular morbidity and mortality. Recent studies have revealed that the development of atherosclerosis is strongly linked to the functional activities of aryl hydrocarbon receptor (AHR), a chemical sensor that is also important for the development, maintenance, and function of a variety of immune cells. In this review, we focus on the impact of AHR signaling on the different cell types that are closely related to the atherogenesis, including T cells, B cells, dendritic cells, macrophages, foam cells, and hematopoietic stem cells in the arterial walls, and summarize the latest development on the interplay between this environmental sensor and immune cells in the context of atherosclerosis. Hopefully, elucidation of the role of AHR in atherosclerosis will facilitate the understanding of case variation in disease prevalence and may aid in the development of novel therapies.

The Aryl Hydrocarbon Receptor Preferentially Marks and Promotes Gut Regulatory T Cells

The local environment may affect the development and function of tissue-resident T regulatory cells (Tregs), which are crucial for controlling inflammation. Although the aryl hydrocarbon receptor (Ahr), an environmental sensor, is expressed by Tregs, its role in Treg cell development and/or function remains elusive. Here, we generated mouse genetic models to ablate or activate Ahr expression specifically in Tregs. We showed that Ahr was expressed more abundantly by peripherally induced Tregs (pTregs) in the gut and that its expression was independent of microbiota. Ahr was important for Treg gut homing and function. Ahr inhibited pro-inflammatory cytokines produced by Tregs but was dispensable for Treg stability. Furthermore, Ahr-expressing Tregs had enhanced in vivo suppressive activity compared with Tregs lacking Ahr expression in a T cell transfer model of colitis. Our data suggest that Ahr signaling in Tregs may be important for gut immune homeostasis.

Tetrandrine, an agonist of aryl hydrocarbon receptor, reciprocally modulates the activities of STAT3 and STAT5 to suppress Th17 cell differentiation

Tetrandrine, a bisbenzylisoquinoline alkaloid constituent of the root of Stephania tetrandra S. Moore, was previously shown to suppress the differentiation of T helper 17 (Th17) cells and consequently ameliorate the collagen‐induced arthritis (CIA) in mice by activating the aryl hydrocarbon receptor (AhR), but its underlying mechanism is incompletely understood. Here, we investigated how tetrandrine suppressed Th17 cell differentiation through the AhR pathway. The naïve CD4⁺ T cells were stimulated with anti‐CD3/CD28 for 72 hrs in the presence or absence of tetrandrine under the Th17‐polarizing condition. Tetrandrine inhibited the phosphorylation of signal transducer and activator of transcription‐3 (STAT3) and boosted the phosphorylation of STAT5, while it did not alter the expression levels of phospho‐Janus kinase‐1 (p‐JAK1), p‐JAK2, p‐JAK3, and suppressor of cytokine signalling‐3 (SOCS3). The tetrandrine‐mediated inhibition of the Th17 cell differentiation could be diminished by the activator of STAT3 and the inhibitor of STAT5. Meanwhile, the effect of tetrandrine on the either STAT3 or STAT5 phosphorylation was almost completely reversed by the AhR antagonist CH223191 and the AhR knockdown. In CIA mice, tetrandrine decreased p‐STAT3 levels and increased p‐STAT5 levels, which could also be reversed by the AhR antagonist resveratrol administration. Furthermore, tetrandrine promoted the AhR binding to the STAT5, but not to the STAT3. The tetrandrine‐induced inhibition of the STAT3 phosphorylation was diminished by the inhibitor of STAT5. Taken together, tetrandrine suppressed Th17 cell differentiation by reciprocally modulating the activities of STAT3 and STAT5 in an AhR‐dependent manner.

AHR Function in Lymphocytes: Emerging Concepts

The aryl hydrocarbon receptor (AHR) is an important regulator of the development and function of both innate and adaptive immune cells through roles associated with AHR's ability to respond to cellular and dietary ligands. Recent findings have revealed tissue and context-specific functions for AHR in both homeostasis and in during an immune response. I review these findings here, and integrate them into the current understanding of the mechanisms that regulate AHR transcription and function. I propose a conceptual framework in which AHR function is determined by three factors: the amount of AHR in any given cell, the abundance and potency of AHR ligands within certain tissues, and the tissue microenvironment wherein AHR(+) cells reside. This complexity emphasizes the necessity cell-type specific genetic approaches towards the study of AHR function.

MFG-E8 Is Critical for Embryonic Stem Cell-Mediated T Cell Immunomodulation

The molecules and mechanisms pertinent to the low immunogenicity of undifferentiated embryonic stem cells (ESCs) remain poorly understood. Here, we provide evidence that milk fat globule epidermal growth factor 8 (MFG-E8) is a vital mediator in this phenomenon and directly suppresses T cell immune responses. MFG-E8 is enriched in undifferentiated ESCs but diminished in differentiated ESCs. Upregulation of MFG-E8 in ESCs increases the successful engraftment of both undifferentiated and differentiated ESCs across major histocompatibility complex barriers. MFG-E8 suppresses T cell activation/proliferation and inhibits Th1, Th2, and Th17 subpopulations while increasing regulatory T cell subsets. Neutralizing MFG-E8 substantially abrogates these effects, whereas addition of recombinant MFG-E8 to differentiated ESCs restores immunosuppression. Furthermore, we provide the evidence that MFG-E8 suppresses T cell activation and regulates T cell polarization by inhibiting PKCθ phosphorylation through the α3/5βV integrin receptor. Our findings offer an approach to facilitate transplantation acceptance.

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MFG-E8 is enriched in undifferentiated but diminished in differentiated ESCs

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MFG-E8 promotes allogeneic engraftment of ESC-derived tissues across the MHC barrier

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ESC-produced MFG-E8 inhibits Th1/Th2/Th17 while promoting regulatory T cells

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MFG-E8 modulates T cell polarization via inhibiting PKCθ phosphorylation

Aryl Hydrocarbon Receptor Activation Down-Regulates IL-7 and Reduces Inflammation in a Mouse Model of DSS-Induced Colitis

BACKGROUND AND AIMS

The pathogenesis of inflammatory bowel disease (IBD) is associated with dysregulation of intestinal immune system. Aryl hydrocarbon receptor (AHR) is believed to control the chronic inflammation in the gut. Besides, interleukin-7 (IL-7) is proved to be an important cytokine that activates mucosal inflammation in IBD. Moreover, intraepithelial lymphocytes (IELs) are one of the key immunological compartments involved in regulating intestinal inflammation. In this study, we investigated the function of 6-formylindolo (3,2-b) carbazole (Ficz), a ligand of AHR, on IL-7, colitis, and IEL phenotypes.

METHODS

Colitis was induced by administration of dextran sulfate sodium (DSS) to wild-type C57BL/6J mice for 7 days. Mice were weighted, colon tissues were collected and measured, and histology analyses were performed. IELs were isolated from colon, and the phenotype and activation of IELs were examined using flow cytometry detection. The expression of AHR and IL-7 was measured by immunofluorescence, Western blot, and RT-PCR.

RESULTS

Ficz down-regulated epithelial-derived IL-7 expression in mice with DSS-induced colitis and ameliorated DSS-induced colitis. Ficz also decreased CD8αβ(+) and CD8(+) IEL subpopulations, enhanced TCRγδ(+) IEL subpopulation, and reduced the percentage of activated CD4(+) and CD8(+) subpopulations.

CONCLUSIONS

Ficz could down-regulate epithelial-derived IL-7 expression in mice with DSS-induced colitis and inhibit inflammation in the gastrointestinal tract of mice. AHR-related compounds might be the new and promising therapeutic medicaments for the treatment of patients with IBD.

Genome-wide analysis of DNA methylation changes induced by gestational arsenic exposure in liver tumors

Inorganic arsenic is known to be a human carcinogen. Previous studies have reported that DNA methylation changes are involved in arsenic-induced carcinogenesis, therefore, DNA methylation changes that are specific to arsenic-induced tumors would be useful to distinguish tumors induced by arsenic from tumors caused by other factors and to dissect arsenic carcinogenesis. Previous studies have shown that gestational arsenic exposure of C3H mice, which tend to spontaneously develop liver tumors, increases the incidence of tumors in male offspring. In this study we used the same experimental protocol as in those previous studies and searched for DNA regions where methylation status was specifically altered in the liver tumors of arsenic-exposed offspring by using methylated DNA immunoprecipitation-CpG island microarrays. The methylation levels of the DNA regions selected were measured by quantitative methylation-specific PCR and bisulfite sequencing. The results of this study clarified a number of regions where DNA methylation status was altered in the liver tumors in the C3H mice compared to normal liver tissues. Among such regions, we showed that a gene body region of the oncogene Fosb underwent alteration in DNA methylation by gestational arsenic exposure. We also showed that Fosb expression significantly increased corresponding to the DNA methylation level of the gene body in the arsenic-exposed group. These findings suggest that the DNA methylation status can be used to identify tumors increased by gestational arsenic exposure.

Phenylhydroquinone induces loss of thymocytes through cell cycle arrest and apoptosis elevation in p53-dependent pathway

ortho-Phenylphenol has been employed in post-harvest treatment of citrus fruits. Although o-phenylphenol has been reported to cause carcinomas in the urinary tract in rats, toxicity to the immune organs is still unknown. Herein, we report that administration of o-phenylphenol induces thymic atrophy and loss of thymocytes in female BALB/c mice. The influence seems to result from inhibition of the thymocyte development, because increased and decreased populations of the CD4⁻ CD8⁻ double-negative and CD4⁺ CD8⁺ double-positive thymocytes were observed in the o-phenylphenol-administered mice, respectively. ortho-Phenylphenol is metabolized to phenylhydroquinone by cytochrome P450 monooxygenases. Phenylhydroquinone made cell cycle of thymocytes to be arrested through reduced expression of the genes associated with G₂/M phase and through phosphorylation of p53 at Ser15. Phosphorylation of p53 at Ser15 was upregulated by activation of not only ATR but also Erk1/2 and p38, leading to increase of apoptosis. Gene expression of cytochrome P450 1A1 (CYP1A1) was promoted in thymocytes from the o-phenylphenol-administered mice. Overall, our results suggest that o-phenylphenol induces CYP1A1 expression and is metabolized into phenylhydroquinone by the expressed CYP1A1 in thymocytes. The produced phenylhydroquinone in turn induces inhibition of thymocyte development through cell cycle arrest and apoptosis in the p53-dependent pathway.

Inorganic arsenic exposure induces E2F-dependent G0/G1 arrest via an increase in retinoblastoma family protein p130 in B-cell lymphoma A20 cells

Inorganic arsenic exerts toxic effect on multiple systems including the immune system. We previously showed in a study on mouse thymocytes and B-cell lymphoma A20 cells that arsenite induces cell cycle arrest at G0/G1 by suppressing expression of E2F-target genes. In this study, we furthermore investigated the involvement of retinoblastoma (RB) family proteins in E2F-dependent cell cycle arrest by arsenite. Arsenite exposure of A20 cells was showed to increase the protein level of p130, a RB family member, without changing the mRNA level. Suppression of arsenite-induced p130 by siRNA reduced the G0/G1 phase, indicating that p130 accumulation is responsible for arsenite-induced G0/G1 arrest. The accumulated p130 was shown to increase the p130 complex with E2F4, a transcription-suppressing E2F. Comparison by Western blotting of arsenite-induced p130 and p130 accumulated by a proteasome inhibitor suggested that arsenite-induced p130 is hypophosphorylated and hypoubiquitinated and refractory to proteasome-dependent degradation. We also showed that arsenite increases mRNA and protein of p16(INK4a), an inhibitor of CDK4/6 that phosphorylates p130. Down-regulation of arsenite-induced p16(INK4a) by siRNA suppressed the p130 accumulation. We propose a novel mechanism in which arsenite inhibits phosphorylation/ubiquitin-dependent proteasome degradation of p130 by inducing p16(INK4a) and the accumulated p130 causes cell cycle arrest with E2F4.

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 and food allergy

The immune system is important for protection against pathogens and malignant cells. However, malfunction of the immune system can also result in detrimental auto-immune diseases, inflammatory diseases, cancers and allergies. The aryl hydrocarbon receptor (AhR), present in numerous tissues and cell subsets, including cells of the immune system, plays an important role in the functioning of the immune system. Activation of the AhR is for example associated with various effects on dendritic cells (DCs), regulatory T cells and the Th1/Th2 cell balance. These cells play a major role in the development of food allergy. Food allergy is an increasing health problem in both humans and animals. Despite the knowledge in risk factors and cellular mechanisms for food allergy, no approved treatments are available yet. Recently, it has been shown that activation of the AhR by dioxin-like compounds suppresses allergic sensitization by suppressing the absolute number of precursor and effector T cells, by preserving CD4(+)CD25(+)Foxp3(+) Treg cells and by affecting DCs and their interaction with effector T cells. Future research should elucidate whether and how AhR activation can be used to interfere in food allergic responses in humans and in animals. This may lead to new prevention strategies and therapeutic possibilities for food allergy.

The aryl hydrocarbon receptor: a novel target for immunomodulation in organ transplantation

The aryl hydrocarbon receptor (AHR), which has been central to studies in toxicology for years as the receptor for the toxicant dioxin, is rapidly gaining interest in immunology based on its ability to influence T-cell differentiation. Multiple studies have documented that binding of this receptor with certain ligands favors T-cell differentiation toward regulatory T cells, and paradoxically, binding of this same receptor with different ligands enhances Th17 effector cell differentiation. This finding has been confirmed in both in vitro and in vivo models, where different ligands are able to either ameliorate or conversely aggravate autoimmunity in experimental autoimmune encephalomyelitis. The AHR has both an endogenous role that is important in development and normal physiology and an exogenous role as a receptor for manmade toxicants, with their binding leading to transcription of cytochrome P450 enzymes that metabolize these same ligands. Based on recent reports that will be summarized in this overview, we will consider the role that the AHR might play as a sensor to the outside environment, leading to alteration of the acquired immune system that might have relevance in transplantation or other medical conditions. In addition to describing the data in normal physiology and T-cell differentiation, we will present examples of the importance of this receptor in preclinical models of disease and highlight specific ligands that target the AHR and will have efficacy in treating transplant rejection and in tolerance protocols.

Activation of the aryl hydrocarbon receptor reduces the number of precursor and effector T cells, but preserves thymic CD4+CD25+Foxp3+ regulatory T cells

Aryl hydrocarbon receptor (AhR) activation suppresses immune responses, including allergic sensitization, by increasing the percentage of regulatory (Treg) cells. Furthermore, AhR activation is known to affect thymic precursor T cells. However, the effect of AhR activation on intrathymic CD4+CD25+Foxp3+ Treg cells is unknown. Therefore, we investigated the effect of AhR activation on the percentage and number of CD4+CD25+Foxp3+ Treg cells during allergic sensitization in relevant immunological organs. C3H/HeOuJ mice were treated on day 0 with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and subsequently sensitized to peanut. On day 8, mice were sacrificed and thymus, spleen and mesenteric lymph nodes (MLN) were isolated. TCDD treatment decreased the number of CD4-CD8-, CD4+CD8+, CD4+CD8- and CD4-CD8+ precursor T cells, but not the number of thymic CD4+CD25+Foxp3+ Treg cells. TCDD treatment increased the number of splenic CD4+CD25+Foxp3+ Treg cells and decreased Th1, Th2 and cytotoxic T cells in the spleen. This appeared to be independent of allergic sensitization. In MLN, TCDD treatment suppressed the increase of the number of CD4+CD25+Foxp3+ Treg cells, Th1, Th2 and cytotoxic T cells induced by peanut sensitization. Together, TCDD treatment preserves thymic CD4+CD25+Foxp3+ Treg cells and decreases peripheral T helper and cytotoxic T cells. This effect of TCDD may contribute to the increased influence of CD4+CD25+Foxp3+ Treg cells on immune mediated responses and to the understanding of how AhR activation modulates immune mediated diseases, including food allergy.

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.

Expression of constitutively-active aryl hydrocarbon receptor in T-cells enhances the down-regulation of CD62L, but does not alter expression of CD25 or suppress the allogeneic CTL response

Activation of aryl hydrocarbon receptor (AhR) by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in T-cells is required for TCDD-induced suppression of the allogeneic CTL response and for induction of CD25(hi)CD62L(low) adaptive regulatory T-cells. Here, the ability of a constitutively-active AhR (CA-AhR) expressed in T-cells alone to replicate the effects of TCDD was examined. The response of CA-AhR-expressing B6 donor T-cells in B6xD2F1 mice was compared to the response of wild-type B6 donor T-cells in B6xD2F1 mice given a single dose of TCDD. Expression of CA-AhR in donor T-cells enhanced the down-regulation of CD62L on Day 2 after injection, similar to a single oral dose of TCDD, but did not induce up-regulation of CD25 on Day 2 or affect CTL activity on Day 10. This suggests that activation of AhR in T-cells alone may not be sufficient to alter T-cell responses in this acute graft-versus-host (GvH) model. Since host APC are responsible for activating the donor T-cells, we examined the influence of the F1 host's AhR on donor T-cell responses by creating an AhR(-/-) B6xD2F1 host that had a greatly diminished AhR response to TCDD compared to wild-type F1 mice. As in AhR(+/+) B6xD2F1 mice, the CTL response in AhR(-/-) B6xD2F1 mice was completely suppressed by TCDD. This suggests that either CA-AhR dose not fully replicate the function of TCDD-activated AhR in suppression of the CTL response, or that minimal activation of AhR in host cells is required to combine with activation of AhR in T-cells to elicit the immunosuppressive effects of TCDD.

The effects of vitamin E on NK cell activity and lymphocyte proliferation in treated mice by 2,3,7,8-tetrachlorodibenzo-p-dioxin

The study was conducted to investigate the effects of vitamin E on NK cell activity and lymphocyte proliferation in tr eated female mice by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In chronic TCDD trial, 45 mice were divided into 5 groups, and the levels of TCDD and vitamin E were 0 and 0, 100 and 0, 100 and 20, 100 and 100, and 100 ng/kg/d and 500 mg/kg/d, respectively. In acute TCDD trial, 24 mice were divided into three groups, and the levels of TCDD and vitamin E were 0 and 0, 30 and 0, and 30 microg/kg and 100 mg/kg, respectively. The results showed chronic TCDD-treatment caused decrease tendencies of spleen NK cell activity and lymphocyte proliferation, and vitamin E 100mg/kg alleviated the decreases tendencies caused by chronic TCDD-treatment, and the lymphocyte proliferation in the group given vitamin E 100mg/kg was significantly higher than that of the chronic TCDD-treated group. Acute TCDD-treatment suppressed the NK cell activity and lymphocyte proliferation, and vitamin E 100mg/kg significantly alleviated the decrease caused by acute TCDD-treatment. These results suggested that TCDD resulted in immunotoxicity, and the toxicity of acute TCDD-treatment was severe compared with chronic TCDD, while vitamin E alleviated the immunotoxicity from TCDD.

Effects of chronic exposure to DDT and TCDD on disease activity in murine systemic lupus erythematosus

Oestrogens contribute to the female preponderance of autoimmune diseases such as systemic lupus erythematosus (SLE). Environmental xenoestrogens superimposed upon endogenous pituitary-gonadal axis may affect the development of autoimmunity. This study examined the effects of chronic exposure to xenoestrogens -- o,p'-dichlorodiphenyltrichloroethane (DDT) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on disease activity in the New Zealand Black/New Zealand White F1 hybrid (B/W) mouse model of SLE. Intact female mice had repeatedly received injections of DDT, TCDD or control vehicle since 6 weeks of age. Weight change, albuminuria, mortality, relevant immunological and histological parameters were assessed. DDT exposure markedly increased the incidence of albuminuria and reduced uterine weight but had no measured effects on immunity or mortality in this study. TCDD-exposed mice had significantly lower incidence of albuminuria, serum anti-DNA antibody and total IgG levels, and mortality compared to controls. Also, TCDD group had significantly lower thymic and splenic weights, decreased percentages of CD4(+)CD8(+) thymocytes and splenic CD4(+) T cells, increased percentage of splenic B220(+)sIgM(+) B cells and higher serum interferon gamma concentration. Taken together, DDT exposure appeared to accelerate the development of albuminuria in lupus-prone mice. TCDD was immunosuppressive to murine SLE. Xenoestrogens may have compound- and tissue-specific effects that require further elucidation in future work.

Identification of AhR-regulated genes involved in PAH-induced immunotoxicity using a highly-sensitive DNA chip, 3D-Gene Human Immunity and Metabolic Syndrome 9k

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants with various toxic effects including immune suppression. However, the molecular mechanism of their toxicity has not been fully clarified. The purpose of this study was to identify novel aryl hydrocarbon receptor (AhR)-regulated genes involved in PAH-induced immunotoxicity using a highly-sensitive DNA chip, 3D-Gene(TM) Human Immunity & Metabolic Syndrome 9k. Leucine-rich repeat-containing protein 25, glucosaminyl (N-acetyl) transferase 3 (GCNT3), thyroxine-binding globulin, aldehyde dehydrogenase 8A1, diacylglycerol O-acyltransferase homolog 2 (DGAT2), haptoglobin, neuron navigator 2 isoform 1, hemopexin and bile acid receptor were found to be up- or down-regulated by PAHs via AhR. Among these genes, GCTN3 and DGAT2 were responsible for immune responses. Therefore, disruption of the expression of these genes via AhR may be one of the causes of the immunotoxicity of PAHs.

Possible involvement of arylhydrocarbon receptor variants in TCDD-induced thymic atrophy and XRE-dependent transcriptional activity in Wistar Hannover GALAS rats

Wistar Hannover Global Alliance for Laboratory Animal Standardization (WH GALAS) rats have been distributed for international standardization of preclinical and toxicological research. Han/Wistar (Kuopio) rats are exceptionally resistant to acute toxicities caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and mediated by the aryl hydrocarbon receptor (AhR), and they have a mutated AhR, named AhR(hw/hw). We found that the WH GALAS rat has either of the three AhR allele, AhR(wt/wt), AhRwt/hw and AhRhw/hw. We administered TCDD (0, 5 and 10 microg/kg) to Long-Evans (L-E) rats having AhR(wt/wt) and two WH GALAS rat strains having either AhR(wt/wt) or AhR(hw/hw), and examined the weights of their body, liver and thymus 168 hr post-administration. WH GALAS AhR(hw/hw) strain was more resistant to TCDD-induced effects on thymus weight than L-E and WH GALAS AhR(wt/wt) strains. In order to study differences in susceptibility of thymic atrophy among the strains, we examined CYP1A1 mRNA and AhR protein levels between L-E and WH GALAS strains. However, no significant difference was observed in the amount of AhR protein or CYP1A1 mRNA in the thymus. Next, we carried out in vitro assays to examine the transactivation activities of AhR variants and found that the AhR deletion variant (AhRdv) transcribed from AhR(hw/hw) significantly enhanced transactivation activity of the synthesized xenobiotic response element. All AhR variants similarly suppressed the growth of Jurkat T cells upon TCDD exposure. This study suggests that WH GALAS rat having different AhR alleles is an interesting experimental animal model but should be utilized with caution for preclinical research on chemicals having AhR agonistic activities.

Neonatal exposure to low-dose 2,3,7,8-tetrachlorodibenzo-p-dioxin causes autoimmunity due to the disruption of T cell tolerance

Although 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been shown to influence immune responses, the effects of low-dose TCDD on the development of autoimmunity are unclear. In this study, using NFS/sld mice as a model for human Sjögren's syndrome, in which the lesions are induced by the thymectomy on day 3 after birth, the autoimmune lesions in the salivary glands, and in later phase, inflammatory cell infiltrations in the other organs were developed by neonatal exposure to nonapoptotic dosage of TCDD without thymectomy on day 3 after birth. We found disruption of thymic selection, but not thymic atrophy, in TCDD-administered mice. The endogenous expression of aryl hydrocarbon receptor in the neonatal thymus was significantly higher than that in the adult thymus, suggesting that the neonatal thymus may be much more sensitive to TCDD compared with the adult thymus. In addition, the production of T(H)1 cytokines such as IL-2 and IFN-gamma from splenic CD4(+) T cells and the autoantibodies relevant for Sjögren's syndrome in the sera from TCDD-exposed mice were significantly increased compared with those in control mice. These results suggest that TCDD/aryl hydrocarbon receptor signaling in the neonatal thymus plays an important role in the early thymic differentiation related to autoimmunity.

Constitutively active aryl hydrocarbon receptor expressed in T cells increases immunization-induced IFN-gamma production in mice but does not suppress T(h)2-cytokine production or antibody production

The ligand-dependent transcription factor aryl hydrocarbon receptor (AhR) has been implicated in various immune functions. Our previous studies have shown that AhR activation by exposure of ovalbumin (OVA)-immunized mice to the potent ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) increases immunization-induced IFN-gamma production in the spleen and suppresses the production of T(h)2 cytokines and OVA-specific antibodies. In the present study, we used transgenic (Tg) mice that express a constitutively active mutant of aryl hydrocarbon receptor (CA-AhR) specifically in T-lineage cells to clarify the role of AhR activation in T cells in these reactions. The results of this study clearly demonstrated that AhR activation only in the T cells augments IFN-gamma production upon OVA immunization. By contrast, production of T(h)2 cytokines and antibodies were not significantly suppressed by CA-AhR in the T cells. These results suggest that suppression of T(h)2 cytokines and antibodies production require AhR activation not only in T cells but also in other cell types as caused by TCDD exposure. Alternatively, these results may indicate that IFN-gamma augmentation and T(h)2 cytokines and antibodies suppression depend on different ways of functions of AhR in the T cells and that CA-AhR does not replicate the suppressive effect of TCDD-activated AhR on T(h)2 cytokines and antibodies. Expression of CA-AhR in the T cells was also shown to increase the percentage of CD25(+) cells among CD4(+) cells in the thymus and spleen. Thus, studies using T-cell-specific CA-AhR Tg mice provide a way to dissect the role of AhR in individual cell types and how the AhR functions.

Blockade of the aryl hydrocarbon receptor pathway triggered by dioxin, polycyclic aromatic hydrocarbons and cigarette smoke by Phellinus linteus

Environmental pollutants including halogenated and polycyclic aromatic hydrocarbons activate the aryl hydrocarbon receptor (AhR) and thereby cause a wide range of pathological changes. Development of AhR antagonists will be useful for prevention and treatment of diseases related to AhR activation. Towards this end, we aimed in the present study at seeking for potential inhibitors of the AhR pathway in mycelial extracts using the dioxin responsive element-based sensing via secreted alkaline phosphatase (DRESSA). Through the screening of 13 mycelia, extracts prepared from Phellinus linteus, Cordyceps militaris and Hericium erinaceum inhibited activation of AhR by 2,3,7,8-tetrachlorodibenzo-p-dioxin, benzo[a]pyrene or 3-methylcholanthrene. Subsequent studies revealed that only Phellinus linteus suppressed activation of AhR and AhR-dependent gene expression triggered by all of these agonists. Cigarette smoke is known to contain a number of halogenated and polycyclic aromatic hydrocarbons. We found that Phellinus linteus has the potential to block activation of AhR and AhR-dependent gene expression triggered by cigarette smoke. Furthermore, the inhibitory effect of Phellinus linteus on the AhR pathway was independent of; 1) depression of AhR or AhR nuclear translocator, and 2) induction of AhR repressor. We conclude that Phellinus linteus contains potent inhibitor(s) of AhR activation and may be useful for prevention of pathologies associated with aberrant activation of AhR.

Direct, continuous monitoring of air pollution by transgenic sensor mice responsive to halogenated and polycyclic aromatic hydrocarbons

BACKGROUND

The aryl hydrocarbon receptor (AhR, also called the dioxin receptor) plays crucial roles in toxicologic responses of animals to environmental pollutants, especially to halogenated and polycyclic aromatic hydrocarbons. To achieve direct, continuous risk assessment of air pollution using biological systems, we generated transgenic sensor mice that produce secreted alkaline phosphatase (SEAP) under the control of AhR.

METHODS

To characterize responses of the mice to AhR agonists, sensor mice were orally administered 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 3-methylcholanthrene (3MC), benzo[a]pyrene (B[a]P), or beta-naphthoflavone (BNF), and serum levels of SEAP were evaluated. To monitor air pollution caused by cigarette smoke, we placed the mice each day in an experimental smoking room, and evaluated activity of serum SEAP for up to 4 days. Activation of AhR in individual organs was also examined by reverse transcription-polymerase chain reaction (RT-PCR) analysis of SEAP.

RESULTS

In response to oral exposure to TCDD, sensor mice exhibited dramatic and sustained activation of AhR. The mice also responded sensitively to 3MC, B[a]P, and BNF. Activation of AhR was dose dependent, and the liver was identified as the main responding organ. After exposure to the smoking environment, sensor mice consistently exhibited transient, reversible activation of AhR. RT-PCR analysis of SEAP revealed that activation of AhR occurred predominantly in the lung.

CONCLUSION

We are the first laboratory to demonstrate successfully direct, comprehensive monitoring of air pollution using genetically engineered mammals. The established system would be useful for real risk assessment of halogenated and polycyclic aromatic hydrocarbons in the air, especially in smoking environments.

Cigarette smoke as a trigger for the dioxin receptor-mediated signaling pathway

Dioxins and dioxin-like chemicals cause a wide range of pathologies including carcinogenesis, immune dysfunction, and developmental/reproductive abnormalities. Most of these toxic effects are mediated by aryl hydrocarbon receptor (AhR; also called the dioxin receptor), a ligand-activated transcription factor. Constitutive activation of AhR via genetic manipulation causes development of cancers, inflammation and immune abnormality in mice even without exposure to xenobiotic ligands. Recent investigation disclosed that cigarette smoke contains high levels of agonists for AhR and strongly activates the dioxin signaling pathway. In this review, we describe and discuss possible roles of AhR activation in cigarette smoke-related pathologies, especially focusing on carcinogenesis, inflammation, atherosclerosis, immune dysfunction and teratogenesis.

Cytochrome P450 gene regulation and physiological functions mediated by the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that functions as an intracellular mediator in the xenobiotic signaling pathway. Although a number of studies have examined AhR-mediated CYP1A1 induction in detail, recent studies of AhR-null mice have revealed that AhR plays important regulatory roles in the normal homeostasis and development of animals. In this short review, we summarize the present state of knowledge about the molecular mechanisms of AhR-mediated CYP1 induction, and we also focus on recent advances in the study of the physiological functions of AhR.

The aryl hydrocarbon receptor, more than a xenobiotic-interacting protein

The aryl hydrocarbon (dioxin) receptor (AhR) has been studied for several decades largely because of its critical role in xenobiotic-induced toxicity and carcinogenesis. Albeit this is a major issue in basic and clinical research, an increasing number of investigators are turning their efforts to try to understand the physiology of the AhR under normal cellular conditions. This is an exciting area that covers cell proliferation and differentiation, endogenous mechanisms of activation, gene regulation, tumor development and cell motility and migration, among others. In this review, we will attempt to summarize the studies supporting the implication of the AhR in those endogenous cellular processes.

Ah receptor: Dioxin-mediated toxic responses as hints to deregulated physiologic functions

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor and member of the bHLH/PAS (basic Helix-Loop-Helix/Per-Arnt-Sim) family of chemosensors and developmental regulators. It represents a multifunctional molecular switch regulating endo- and xenobiotic metabolism as well as cell proliferation and differentiation. Physiologic functions of the AhR are beginning to be understood, including functions in vascular development, and in detoxification of endo- and xenobiotics. The AhR is also recognized as the culprit for most toxic responses observed after exposure to dioxins and related compounds such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The non-metabolizable AhR agonist TCDD has to be distinguished from the myriad of metabolizable agonists present as dietary contaminants and plant constituents as well as endogenous toxins. The hypothesis is emerging that the diverse tissue-specific, TCDD-mediated toxicities are due to sustained and inappropriate AhR activation leading to deregulated physiologic functions. In support of this hypothesis recent observations in the context of some TCDD-mediated toxic responses are discussed, such as chloracne, cleft palate, thymus involution and in particular carcinogenesis. Major open questions are addressed, such as ligand-independent AhR activation by phosphorylation and the large differences in species-dependent susceptibility to toxic responses. Though important issues remain unresolved, the commentary is intended to stimulate efforts to understand dioxin-mediated toxic responses with emphasis on carcinogenesis in comparison with AhR-mediated physiologic functions.

The constitutively active Ah receptor (CA-AhR) mouse as a potential model for dioxin exposure—Effects in vital organs

The dioxin/aryl hydrocarbon receptor (AhR) mediates most, if not all, toxic effects of dioxins and functions as a ligand-activated transcription factor regulating transcription of a battery of genes. In order to study the mechanisms behind the toxicity of ligands of the Ah receptor we have created a transgenic mouse model expressing a constitutively active Ah receptor (CA-AhR). The mutant Ah receptor is expressed and functionally active in all organs studied. The purpose of the present study was to characterize histopathologically, the phenotype of the CA-AhR with regard to the liver, kidney, lung, heart, spleen and thymus of male and female transgenic CA-AhR mice. Moreover, cell-specific activity of the CA-AhR using up-regulation of the AhR target gene CYP1A1 as a marker, was also examined. The relative weight of liver, kidney and heart were increased while relative thymus weight was decreased. Furthermore, slight morphological lesions of the liver, kidney and spleen was seen. Expression of CYP1A1 was found in cells corresponding to endothelial cells in all of the organs studied. In some tissues additional cell types, such as hepatocytes, renal tubuli cell and Clara cells expressed CYP1A1. Both the effects on organ weights and the cellular expression of CYP1A1 in CA-AhR mice correspond well to observations in TCDD-exposed mice. In conclusion, this characterization further support that the CA-AhR mouse is a useful model for life-long continuous low-level activity of the AhR, i.e. the dioxin exposure situation of humans of the general population.

The immune system as a target for environmental chemicals: Xenoestrogens and other compounds

The immune system in higher organisms is under integrated control and has the capacity to rapidly respond to the environment. Recently, there has been a significant increase in the prevalence of allergic diseases. Environmental factors likely play a major role in the explosion of allergy. Although the "hygiene hypothesis" may explain the increase in allergic diseases which are prone to T helper 2 (Th2) immune responses, recent findings highlight the possible involvement of environmental xenobiotic chemicals which can modulate normal immune function. Interestingly, several reports suggest that the prevalence of systemic lupus erythematosus, a Th2-type autoimmune disease, is also increasing, although the development of high-sensitivity immunological tests may be a possible cause. The increased prevalence of autoimmune disease in women, the sexual dimorphism of the immune response, and the immunomodulatory effects of sex steroids, have focused attention on the role of chemicals which influence sex steroids in the development of immune diseases. Moreover, recent reports indicate that some environmental chemicals can work on nuclear hormone receptors, other than sex hormone receptors, and modulate immune reactions. This review focuses on the impact of environmental chemicals on immune system function and pathogenesis of immune diseases, including allergy and autoimmune diseases.

Comparison of the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced CYP1A1 gene expression profile in lymphocytes from mice, rats, and humans: most potent induction in humans

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) exerts its toxicity by binding a transcription factor, the aryl hydrocarbon receptor (AhR). C57BL/6 (C57) mice express AhRs that have high affinity for TCDD, and they strongly express target genes and develop severe toxic effects upon TCDD exposure. By contrast, DBA/2 (DBA) mice have a low-affinity form of AhR, weakly express target genes, and are resistant to TCDD. Although humans express low-affinity AhRs and have been assumed to be refractory to TCDD, their sensitivity to TCDD has yet to be determined. In this study we compared the TCDD-induced CYP1A1 gene expression profiles in lymphocytes from humans, C57 mice, DBA mice, and SD rats to obtain data as a basis for estimating human sensitivity to TCDD. Lymphocyte fractions prepared from the blood of individual humans and animals were cultured with TCDD. Their mRNAs for CYP1A1 and housekeeping genes were measured by RT-PCR or real-time PCR with primers designed for regions that are 100% homologous among each of the genes of all species/strains tested to obtain similar PCR efficiency. TCDD-induced CYP1A1 expression peaked at 2h in DBA mice and SD rats and at 6h in C57 mice and humans. At the peak times human lymphocytes showed the most potent CYP1A1 mRNA induction of the four species/strains tested. These results suggest that human lymphocytes are more sensitive to TCDD than the lymphocytes of mice and rats. Since the AhR-dependent gene expression did not reflect the AhR affinity for TCDD, these results also suggest that AhR-dependent gene expression in lymphocytes is modulated by an as yet unidentified mechanism in addition to the AhR affinity.

Xenobiotic-induced alterations in thymocyte development

The thymus is a very sensitive target for environmental pollutants, which can affect this organ as well as thymocyte differentiation. A failure in thymocyte development can be due to the exacerbation of apoptosis, arrest of thymocyte maturation, generation of autoreactive T cells, and inhibition or stimulation of the output of recent thymic emigrants to the periphery. Recent data demonstrate that the immune system has the potential to maintain homeostasis under conditions of elevated risk, and the thymus plays a crucial role in this process. Environmental xenobiotics can exert their effects through receptor-mediated interactions or independently on receptor involvement. Under natural conditions organisms are exposed to a variety of xenobiotics. The final effect of such exposure is not related to the action of a single chemical, but to the action of a mixture of chemicals. The toxic effect of environmental xenobiotics on the generation and functions of immune cells may result in suppression or stimulation of the immune response. The most intensive studies have been done on halogenated aromatic hydrocarbons, heavy metals and various chemicals acting as endocrine disrupters. Recently, special interest has focused on the action of air particulate matter.

The dioxin receptor is silenced by promoter hypermethylation in human acute lymphoblastic leukemia through inhibition of Sp1 binding

The transcription factor aryl hydrocarbon receptor (AhR) has relevant functions in cell proliferation. Interestingly, the AhR can either promote or inhibit proliferation depending on the cell phenotype. Although recent data reveal potential pathways for AhR signaling in cell proliferation, the mechanisms that regulate its activity in tumor cells remain unknown. Here, we have analyzed promoter hypermethylation as a potential mechanism controlling AhR expression in human tumor cells. AhR promoter CpG methylation was sporadic in a panel of 19 tumor cell lines except for the chronic myeloid leukemia (CML) K562 and the acute lymphoblastic leukemia (ALL) REH. When compared with normal lymphocytes, REH had very low constitutive AhR expression that could be attributed to promoter hypermethylation since treatment with the DNA demethylating agent 5-aza-2'-deoxycitidine (AZA) significantly increased AhR mRNA and protein. These results in leukemia-derived cell lines were further confirmed in primary ALL, where 33% of the patients (7/21) had AhR promoter hypermethylation. Chromatin immunoprecipitation (ChIP) showed that methylation impaired binding of the transcription factor Sp1 to the AhR promoter, thus providing a mechanism for AhR downregulation in REH cells. Therefore, promoter hypermethylation represents a novel epigenetic mechanism downregulating AhR activity in hematological malignancies such as ALL.

Oral benzo[a]pyrene in Cyp1 knockout mouse lines: CYP1A1 important in detoxication, CYP1B1 metabolism required for immune damage independent of total-body burden and clearance rate

CYP1A1 and CYP1B1 metabolically activate many polycyclic aromatic hydrocarbons (PAHs), including benzo[a]pyrene, to reactive intermediates associated with toxicity, mutagenesis, and carcinogenesis. Paradoxically, however, Cyp1a1-/- knockout mice are more sensitive to oral benzo[a]pyrene exposure, compared with wild-type Cyp1a1+/+ mice (Mol Pharmacol 65:1225, 2004). To further investigate the mechanism for this enhanced sensitivity, Cyp1a1-/-, Cyp1a2-/-, and Cyp1b1-/- single-knockout, Cyp1a1/1b1-/- and Cyp1a2/1b1-/- double-knockout, and Cyp1+/+ wild-type mice were analyzed. After administration of oral benzo[a]pyrene (125 mg/kg/day) for 18 days, Cyp1a1-/- mice showed marked wasting, immunosuppression, and bone marrow hypocellularity, whereas the other five genotypes did not. After 5 days of feeding, steady-state blood levels of benzo[a]pyrene were approximately 25 and approximately 75 times higher in Cyp1a1-/- and Cyp1a1/1b1-/- mice, respectively, than in wild-type mice. Benzo[a]pyrene-DNA adduct levels were highest in liver, spleen, and marrow of Cyp1a1-/- and Cyp1a1/1b1-/- mice. Many lines of convergent data obtained with oral benzo[a]pyrene dosing suggest that: 1) inducible CYP1A1, probably in both intestine and liver, is most important in detoxication; 2) CYP1B1 in spleen and marrow is responsible for metabolic activation of benzo[a]pyrene, which results in immune damage in the absence of CYP1A1; 3) both thymus atrophy and hepatocyte hypertrophy are independent of CYP1B1 metabolism but rather may reflect long-term activation of the aryl hydrocarbon receptor; and 4) the magnitude of immune damage in Cyp1a1-/- and Cyp1a1/1b1-/- mice is independent of plasma benzo[a]pyrene and total-body burden and clearance. Thus, a balance between tissue-specific expression of the CYP1A1 and CYP1B1 enzymes governs sensitivity of benzo[a]pyrene toxicity and, possibly, carcinogenicity.

Constitutive expression of aryl hydrocarbon receptor in keratinocytes causes inflammatory skin lesions

Occupational and environmental exposure to polycyclic aromatic hydrocarbons (PAHs) has been suggested to provoke inflammatory and/or allergic disorders, including asthma, rhinitis, and dermatitis. The molecular mechanisms of this PAH-mediated inflammation remain to be clarified. Previous studies implied the involvement of PAHs as irritants and allergens, with the reactive oxygen species generated from the oxygenated PAHs believed to be an exacerbating factor. It is also possible that PAHs contribute to the pathogenesis through activation of aryl-hydrocarbon receptor (AhR)-mediated transcription, since PAHs are potent inducers of the AhR. To address this point, we generated transgenic mouse lines expressing the constitutive active form of the AhR in keratinocytes. In these lines of mice, the AhR activity was constitutively enhanced in the absence of ligands, so that any other direct effects of PAHs and their metabolites could be ignored. At birth, these transgenic mice were normal, but severe skin lesions with itching developed postnatally. The skin lesions were accompanied by inflammation and immunological imbalance and resembled typical atopic dermatitis. We demonstrate that constitutive activation of the AhR pathway causes inflammatory skin lesions and suggests a new mechanism for the exacerbation of inflammatory diseases after exposure to occupational and environmental xenobiotics.

Constitutive activation of the aryl hydrocarbon receptor in T-lineage cells induces thymus involution independently of the Fas/Fas ligand signaling pathway

Thymus involution is one of the most prominent consequences of exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The characteristic features of TCDD-induced thymic changes include reductions in the number of the thymocytes and in the ratio of CD4 to CD8 T cells in the thymus. While these changes have been shown to be caused by activation of a transcription factor, the aryl hydrocarbon receptor (AhR), the down-stream biological events that induce the thymic changes have not been determined. In the present study, we examined the involvement of Fas/Fas ligand (FasL)-dependent apoptosis, a likely mechanism suggested by previous studies, in the thymocyte loss by AhR activation of thymocytes. We recently generated transgenic (Tg) mice expressing a constitutively active AhR (CA-AhR) mutant specifically in T-lineage cells. These Tg mice reproduced the thymus involution caused by TCDD at relatively high doses. In this study, we crossed the T-cell-specific CA-AhR Tg mice with Faslpr mice, which have the homozygous defective fas (lpr) gene, or with FasLgld mice, which have the homozygous mutated fas ligand (gld) gene, to generate mice that are defective in Fas/FasL signaling and express the CA-AhR in T lineage cells. Faslpr and FasLgld CA-AhR Tg mice showed the same extent of thymocyte reduction as Faswt and FasLwt CA-AhR Tg mice. The ratio of CD4 to CD8 T cells in thymocytes was also not affected by the absence of Fas or FasL in the CA-AhR Tg mice. These results show that strong activation of the AhR in thymocytes induces thymus involution independently of Fas/FasL signaling.