Identification of cinnabarinic acid as a novel endogenous aryl hydrocarbon receptor ligand that drives IL-22 production

Immunosuppressive IDO in Cancer: Mechanisms of Action, Animal Models, and Targeting Strategies

Indoleamine 2, 3-dioxygenase 1 (IDO; IDO1; INDO) is a rate-limiting enzyme that metabolizes the essential amino acid, tryptophan, into downstream kynurenines. Canonically, the metabolic depletion of tryptophan and/or the accumulation of kynurenine is the mechanism that defines how immunosuppressive IDO inhibits immune cell effector functions and/or facilitates T cell death. Non-canonically, IDO also suppresses immunity through non-enzymic effects. Since IDO targeting compounds predominantly aim to inhibit metabolic activity as evidenced across the numerous clinical trials currently evaluating safety/efficacy in patients with cancer, in addition to the recent disappointment of IDO enzyme inhibitor therapy during the phase III ECHO-301 trial, the issue of IDO non-enzyme effects have come to the forefront of mechanistic and therapeutic consideration(s). Here, we review enzyme-dependent and -independent IDO-mediated immunosuppression as it primarily relates to glioblastoma (GBM); the most common and aggressive primary brain tumor in adults. Our group's recent discovery that IDO levels increase in the brain parenchyma during advanced age and regardless of whether GBM is present, highlights an immunosuppressive synergy between aging-increased IDO activity in cells of the central nervous system that reside outside of the brain tumor but collaborate with GBM cell IDO activity inside of the tumor. Because of their potential value for the in vivo study of IDO, we also review current transgenic animal modeling systems while highlighting three new constructs recently created by our group. This work converges on the central premise that maximal immunotherapeutic efficacy in subjects with advanced cancer requires both IDO enzyme- and non-enzyme-neutralization, which is not adequately addressed by available IDO-targeting pharmacologic approaches at this time.

The immunometabolic role of indoleamine 2,3-dioxygenase in atherosclerotic cardiovascular disease: immune homeostatic mechanisms in the artery wall

Coronary heart disease and stroke, the two most common cardiovascular diseases worldwide, are triggered by complications of atherosclerosis. Atherosclerotic plaques are initiated by a maladaptive immune response triggered by accumulation of lipids in the artery wall. Hence, disease is influenced by several non-modifiable and modifiable risk factors, including dyslipidaemia, hypertension, smoking, and diabetes. Indoleamine 2,3-dioxygenase (IDO), the rate-limiting enzyme in the kynurenine pathway of tryptophan (Trp) degradation, is modulated by inflammation and regarded as a key molecule driving immunotolerance and immunosuppressive mechanisms. A large body of evidence indicates that IDO-mediated Trp metabolism is involved directly or indirectly in atherogenesis. This review summarizes evidence from basic and clinical research showing that IDO is a major regulatory enzyme involved in the maintenance of immunohomeostasis in the vascular wall, as well as current knowledge about promising targets for the development of new anti-atherosclerotic drugs.

The Trace Kynurenine, Cinnabarinic Acid, Displays Potent Antipsychotic-Like Activity in Mice and Its Levels Are Reduced in the Prefrontal Cortex of Individuals Affected by Schizophrenia

Cinnabarinic acid (CA) is a kynurenine metabolite that activates mGlu4 metabotropic glutamate receptors. Using a highly sensitive ultra-performance liquid chromatography/tandem mass spectrometry (UPLC/MS-MS) method, we found that CA is present in trace amounts in human brain tissue. CA levels were largely reduced in the prefrontal cortex (PFC) of individuals affected by schizophrenia. This reduction did not correlate with age, sex, duration of the disease, and duration and type of antipsychotic medication and might, therefore, represent a trait of schizophrenia. Interestingly, systemic treatment with low doses of CA (<1 mg/kg, i.p.) showed robust efficacy in several behavioral tests useful to study antipsychotic-like activity in mice and rats and attenuated MK-801-evoked glutamate release. CA failed to display antipsychotic-like activity and inhibit excitatory synaptic transmission in mice lacking mGlu4 receptors. These findings suggest that CA is a potent endogenous antipsychotic-like molecule and reduced CA levels in the PFC might contribute to the pathophysiology of schizophrenia.

The kynurenine connection: how exercise shifts muscle tryptophan metabolism and affects energy homeostasis, the immune system, and the brain

Tryptophan catabolism through the kynurenine pathway generates a variety of bioactive metabolites. Physical exercise can modulate kynurenine pathway metabolism in skeletal muscle and thus change the concentrations of select compounds in peripheral tissues and in the central nervous system. Here we review recent advances in our understanding of how exercise alters tryptophan-kynurenine metabolism in muscle and its subsequent local and distal effects. We propose that the effects of kynurenine pathway metabolites on skeletal muscle, adipose tissue, immune system, and the brain suggest that some of these compounds could qualify as exercise-induced myokines. Indeed, some of the more recently discovered biological activities for kynurenines include many of the best-known benefits of exercise: improved energy homeostasis, promotion of an anti-inflammatory environment, and neuroprotection. Finally, by considering the tissue expression of the different membrane and cytosolic receptors for kynurenines, we discuss known and potential biological activities for these tryptophan metabolites.

The Ah Receptor: Adaptive Metabolism, Ligand Diversity, and the Xenokine Model

The Ah receptor (AHR) has been studied for almost five decades. Yet, we still have many important questions about its role in normal physiology and development. Moreover, we still do not fully understand how this protein mediates the adverse effects of a variety of environmental pollutants, such as the polycyclic aromatic hydrocarbons (PAHs), the chlorinated dibenzo-p-dioxins ("dioxins"), and many polyhalogenated biphenyls. To provide a platform for future research, we provide the historical underpinnings of our current state of knowledge about AHR signal transduction, identify a few areas of needed research, and then develop concepts such as adaptive metabolism, ligand structural diversity, and the importance of proligands in receptor activation. We finish with a discussion of the cognate physiological role of the AHR, our perspective on why this receptor is so highly conserved, and how we might think about its cognate ligands in the future.

The therapeutic potential of targeting tryptophan catabolism in cancer

Based on its effects on both tumour cell intrinsic malignant properties as well as anti-tumour immune responses, tryptophan catabolism has emerged as an important metabolic regulator of cancer progression. Three enzymes, indoleamine-2,3-dioxygenase 1 and 2 (IDO1/2) and tryptophan-2,3-dioxygenase (TDO2), catalyse the first step of the degradation of the essential amino acid tryptophan (Trp) to kynurenine (Kyn). The notion of inhibiting IDO1 using small-molecule inhibitors elicited high hopes of a positive impact in the field of immuno-oncology, by restoring anti-tumour immune responses and synergising with other immunotherapies such as immune checkpoint inhibition. However, clinical trials with IDO1 inhibitors have yielded disappointing results, hence raising many questions. This review will discuss strategies to target Trp-degrading enzymes and possible down-stream consequences of their inhibition. We aim to provide comprehensive background information on Trp catabolic enzymes as targets in immuno-oncology and their current state of development. Details of the clinical trials with IDO1 inhibitors, including patient stratification, possible effects of the inhibitors themselves, effects of pre-treatments and the therapies the inhibitors were combined with, are discussed and mechanisms proposed that might have compensated for IDO1 inhibition. Finally, alternative approaches are suggested to circumvent these problems.

Hypoxia Inducible Factor 1α Inhibits the Expression of Immunosuppressive Tryptophan-2,3-Dioxygenase in Glioblastoma

Abnormal circulation in solid tumors results in hypoxia, which modulates both tumor intrinsic malignant properties as well as anti-tumor immune responses. Given the importance of hypoxia in glioblastoma (GBM) biology and particularly in shaping anti-tumor immunity, we analyzed which immunomodulatory genes are differentially regulated in response to hypoxia in GBM cells. Gene expression analyses identified the immunosuppressive enzyme tryptophan-2,3-dioxygenase (TDO2) as the second most downregulated gene in GBM cells cultured under hypoxic conditions. TDO2 catalyses the oxidation of tryptophan to N-formyl kynurenine, which is the first and rate-limiting step of Trp degradation along the kynurenine pathway (KP). In multiple GBM cell lines hypoxia reduced TDO2 expression both at mRNA and protein levels. The downregulation of TDO2 through hypoxia was reversible as re-oxygenation rescued TDO2 expression. Computational modeling of tryptophan metabolism predicted reduced flux through the KP and lower intracellular concentrations of kynurenine and its downstream metabolite 3-hydroxyanthranilic acid under hypoxia. Metabolic measurements confirmed the predicted changes, thus demonstrating the ability of the mathematical model to infer intracellular tryptophan metabolite concentrations. Moreover, we identified hypoxia inducible factor 1α (HIF1α) to regulate TDO2 expression under hypoxic conditions, as the HIF1α-stabilizing agents dimethyloxalylglycine (DMOG) and cobalt chloride reduced TDO2 expression. Knockdown of HIF1α restored the expression of TDO2 upon cobalt chloride treatment, confirming that HIF1α controls TDO2 expression. To investigate the immunoregulatory effects of this novel mechanism of TDO2 regulation, we co-cultured isolated T cells with TDO2-expressing GBM cells under normoxic and hypoxic conditions. Under normoxia TDO2-expressing GBM cells suppressed T cell proliferation, while hypoxia restored the proliferation of the T cells, likely due to the reduction in kynurenine levels produced by the GBM cells. Taken together, our data suggest that the regulation of TDO2 expression by HIF1α may be involved in modulating anti-tumor immunity in GBM.

Engagement of Nuclear Coactivator 7 by 3-Hydroxyanthranilic Acid Enhances Activation of Aryl Hydrocarbon Receptor in Immunoregulatory Dendritic Cells

Indoleamine 2,3-dioxygenase 1 (IDO1) catalyzes the first step in the kynurenine pathway of tryptophan (Trp) degradation that produces several biologically active Trp metabolites. L-kynurenine (Kyn), the first byproduct by IDO1, promotes immunoregulatory effects via activation of the Aryl hydrocarbon Receptor (AhR) in dendritic cells (DCs) and T lymphocytes. We here identified the nuclear coactivator 7 (NCOA7) as a molecular target of 3-hydroxyanthranilic acid (3-HAA), a Trp metabolite produced downstream of Kyn along the kynurenine pathway. In cells overexpressing NCOA7 and AhR, the presence of 3-HAA increased the association of the two molecules and enhanced Kyn-driven, AhR-dependent gene transcription. Physiologically, conventional (cDCs) but not plasmacytoid DCs or other immune cells expressed high levels of NCOA7. In cocultures of CD4⁺ T cells with cDCs, the co-addition of Kyn and 3-HAA significantly increased the induction of Foxp3⁺ regulatory T cells and the production of immunosuppressive transforming growth factor β in an NCOA7-dependent fashion. Thus, the co-presence of NCOA7 and the Trp metabolite 3-HAA can selectively enhance the activation of ubiquitary AhR in cDCs and consequent immunoregulatory effects. Because NCOA7 is often overexpressed and/or mutated in tumor microenvironments, our current data may provide evidence for a new immune check-point mechanism based on Trp metabolism and AhR.

The Aryl Hydrocarbon Receptor: Connecting Immunity to the Microenvironment

The aryl hydrocarbon receptor (AhR) is a cytoplasmic receptor and transcription factor activated through cognate ligand binding. It is an important factor in immunity and tissue homeostasis, and structurally diverse compounds from the environment, diet, microbiome, and host metabolism can induce AhR activity. Emerging evidence suggests that AhR is a key sensor allowing immune cells to adapt to environmental conditions and changes in AhR activity have been associated with autoimmune disorders and cancer. Furthermore, AhR agonists or antagonists can impact immune disease outcomes identifying AhR as a potentially actionable target for immunotherapy. In this review, we describe known ligands stimulating AhR activity, downstream proinflammatory and suppressive mechanisms potentiated by AhR, and how this understanding is being applied to immunopathology to help control disease outcomes.

Tryptophan metabolites kynurenine and serotonin regulate fibroblast activation and fibrosis

Fibrosis is a pathological form of aberrant tissue repair, the complications of which account for nearly half of all deaths in the industrialized world. All tissues are susceptible to fibrosis under particular pathological sets of conditions. Though each type of fibrosis has characteristics and hallmarks specific to that particular condition, there appear to be common factors underlying fibrotic diseases. One of these ubiquitous factors is the paradigm of the activated myofibroblast in the promotion of fibrotic phenotypes. Recent research has implicated metabolic byproducts of the amino acid tryptophan, namely serotonin and kynurenines, in the pathology or potential pharmacologic therapy of fibrosis, in part through their effects on development of myofibroblast phenotypes. Here, we review literature underlying what is known mechanistically about the effects of these compounds and their respective pathways on fibrosis. Pharmacologic administration of kynurenine improves scarring outcomes in vivo likely not only through its well-characterized immunosuppressive properties but also via its demonstrated antagonism of fibroblast activation and of collagen deposition. In contrast, serotonin directly promotes activation of fibroblasts via activation of canonical TGF-β signaling, and overstimulation with serotonin leads to fibrotic outcomes in vivo. Recently discovered feedback inhibition between serotonin and kynurenine pathways also reveals more information about the cellular physiology of tryptophan metabolism and may also underlie possible paradigms for anti-fibrotic therapy. Together, understanding of the effects of tryptophan metabolism on modulation of fibrosis may lead to the development of new therapeutic avenues for treatment through exploitation of these effects.

Comparative In Vitro and In Silico Analysis of the Selectivity of Indirubin as a Human Ah Receptor Agonist

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that modulates gene expression following its binding and activation by structurally diverse chemicals. Species differences in AhR functionality have been observed, with the mouse AhR (mAhR) and human AhR (hAhR) exhibiting significant differences in ligand binding, coactivator recruitment, gene expression and response. While the AhR agonist indirubin (IR) is a more potent activator of hAhR-dependent gene expression than the prototypical ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), it is a significantly less potent activator of the mAhR. DNA binding analysis confirmed the greater potency/efficacy of IR in stimulating transformation/DNA binding of the hAhR in vitro and domain-swapping experiments demonstrated that the enhanced response to IR was primarily due to the hAhR ligand binding domain (LBD). Site-directed mutagenesis and functional analysis studies revealed that mutation of H326 and A349 in the mAhR LBD to the corresponding residues in the hAhR LBD significantly increased the potency of IR. Since these mutations had no significant effect on ligand binding, these residues likely contribute to an enhanced efficiency of transformation/DNA binding by IR-bound hAhR. Molecular docking to mAhR LBD homology models further elucidated the different roles of the A375V mutation in TCDD and IR binding, as revealed by [³H]TCDD competitive binding results. These results demonstrate the differential binding of structurally diverse ligands within the LBD of a given AhR and confirm that amino acid differences within the LBD of AhRs contribute to significant species differences in ligand response.

Assessing the response of morphea and limited scleroderma to tranilast: a small prospective study comparing topical corticosteroids to a combination of topical corticosteroids and tranilast

Background

Scleroderma is traditionally managed with immunomodulatory agents such as methotrexate, mycophenolate mofetil and corticosteroids. There are anecdotal reports for, and theoretical reasons why, the anti-fibrotic agent tranilast may provide an additional treatment modality.

Objective

The objective of the current study was to demonstrate if the addition of topical tranilast to an established regime resulted in an improvement in the Localized Scleroderma Assessment Tool (LoSCAT) and modified Rodnan score.

Patients and methods

A small double-blinded randomized prospective study of 11 pairs of treatment sites in four patients; three with morphea and one with limited scleroderma was performed. All patients continued with their prescribed treatment and applied 0.1% betamethasone valerate in PCCA PracaSil™ (B) to the control site with 0.1% betamethasone valerate and 1% tranilast (B/T) to the comparator site over a period of 3 months. Photographs and monthly LoSCAT scores were performed on the morphea patients and a modified Rodnan score on the limited scleroderma patient. Statistical analysis was via sign test.

Results

The mean baseline LoScat score at the B treated sites was 6.6 which improved to 4.3 (p= 0.16). The mean baseline LoScat score at the B/T treated sites was 5.75 which improved to 2.8 following treatment. (p=0.04)

Limitations

This was a small single center study. The ideal concentration of tranilast is unknown. As all patients continued with standard management the expected response may be less than would have been anticipated in a single agent trial.

Conclusion

The role of tranilast in the management in scleroderma warrants further investigation in larger trials.

A prospective evaluation of serum kynurenine metabolites and risk of pancreatic cancer

Background

Serum pyridoxal 5’-phosphate (PLP), the active form of vitamin B₆, is associated with reduced risk of pancreatic cancer. Data on functional measures of vitamin B₆ status and risk of pancreatic cancer is lacking.

Methods

A nested case-control study involving 187 incident cases of pancreatic cancer and 362 individually matched controls were conducted within two prospective cohorts to evaluate the associations between kynurenine metabolites in pre-diagnostic serum samples and risk of pancreatic cancer.

Results

Higher serum concentrations of 3-hydroxyanthranilic acid (HAA) and the HAA:3-hydroxykynurenine (HK) ratio (a measure for in vivo functional status of PLP) were significantly associated with reduced risk of pancreatic cancer. Compared with the lowest tertile, odds ratios (95% confidence intervals) of pancreatic cancer for the highest tertile was 0.62 (0.39, 1.01) for HAA, and 0.59 (0.35–0.98) for the HAA:HK ratio, after adjustment for potential confounders and serum PLP (both Ps for trend<0.05). The kynurenine:tryptophan ratio or neopterin was not significantly associated with pancreatic cancer risk.

Conclusions

The inverse association between HAA or the HAA:HK ratio and risk of pancreatic cancer supports the notion that functional status of PLP may be a more important measure than circulating PLP alone for the development of pancreatic cancer.

Adaptive changes in global gene expression profile of lung carcinoma A549 cells acutely exposed to distinct types of AhR ligands

Exposure to persistent ligands of aryl hydrocarbon receptor (AhR) has been found to cause lung cancer in experimental animals, and lung adenocarcinomas are often associated with enhanced AhR expression and aberrant AhR activation. In order to better understand the action of toxic AhR ligands in lung epithelial cells, we performed global gene expression profiling and analyze TCDD-induced changes in A549 transcriptome, both sensitive and non-sensitive to CH223191 co-treatment. Comparison of our data with results from previously reported microarray and ChIP-seq experiments enabled us to identify candidate genes, which expression status reflects exposure of lung cancer cells to TCDD, and to predict processes, pathways (e.g. ER stress, Wnt/β-cat, IFNɣ, EGFR/Erbb1), putative TFs (e.g. STAT, AP1, E2F1, TCF4), which may be implicated in adaptive response of lung cells to TCDD-induced AhR activation. Importantly, TCDD-like expression fingerprint of selected genes was observed also in A549 cells exposed acutely to both toxic (benzo[a]pyrene, benzo[k]fluoranthene) and endogenous AhR ligands (2-(1H-Indol-3-ylcarbonyl)-4-thiazolecarboxylic acid methyl ester and 6-formylindolo[3,2-b]carbazole). Overall, our results suggest novel cellular candidates, which could help to improve monitoring of AhR-dependent transcriptional activity during acute exposure of lung cells to distinct types of environmental pollutants.

Impact of the Gut Microbiota on Intestinal Immunity Mediated by Tryptophan Metabolism

The gut microbiota influences the health of the host, especially with regard to gut immune homeostasis and the intestinal immune response. In addition to serving as a nutrient enhancer, L-tryptophan (Trp) plays crucial roles in the balance between intestinal immune tolerance and gut microbiota maintenance. Recent discoveries have underscored that changes in the microbiota modulate the host immune system by modulating Trp metabolism. Moreover, Trp, endogenous Trp metabolites (kynurenines, serotonin, and melatonin), and bacterial Trp metabolites (indole, indolic acid, skatole, and tryptamine) have profound effects on gut microbial composition, microbial metabolism, the host's immune system, the host-microbiome interface, and host immune system-intestinal microbiota interactions. The aryl hydrocarbon receptor (AhR) mediates the regulation of intestinal immunity by Trp metabolites (as ligands of AhR), which is beneficial for immune homeostasis. Among Trp metabolites, AhR ligands consist of endogenous metabolites, including kynurenine, kynurenic acid, xanthurenic acid, and cinnabarinic acid, and bacterial metabolites, including indole, indole propionic acid, indole acetic acid, skatole, and tryptamine. Additional factors, such as aging, stress, probiotics, and diseases (spondyloarthritis, irritable bowel syndrome, inflammatory bowel disease, colorectal cancer), which are associated with variability in Trp metabolism, can influence Trp-microbiome-immune system interactions in the gut and also play roles in regulating gut immunity. This review clarifies how the gut microbiota regulates Trp metabolism and identifies the underlying molecular mechanisms of these interactions. Increased mechanistic insight into how the microbiota modulates the intestinal immune system through Trp metabolism may allow for the identification of innovative microbiota-based diagnostics, as well as appropriate nutritional supplementation of Trp to prevent or alleviate intestinal inflammation. Moreover, this review provides new insight regarding the influence of the gut microbiota on Trp metabolism. Additional comprehensive analyses of targeted Trp metabolites (including endogenous and bacterial metabolites) are essential for experimental preciseness, as the influence of the gut microbiota cannot be neglected, and may explain contradictory results in the literature.

The interplay between cytokines and the Kynurenine pathway in inflammation and atherosclerosis

The kynurenine pathway (KP) is the major metabolic route of tryptophan (Trp) metabolism. Indoleamine 2,3-dioxygenase (IDO1), the enzyme responsible for the first and rate-limiting step in the pathway, as well as other enzymes in the pathway, have been shown to be highly regulated by cytokines. Hence, the KP has been implicated in several pathologic conditions, including infectious diseases, psychiatric disorders, malignancies, and autoimmune and chronic inflammatory diseases. Additionally, recent studies have linked the KP with atherosclerosis, suggesting that Trp metabolism could play an essential role in the maintenance of immune homeostasis in the vascular wall. This review summarizes experimental and clinical evidence of the interplay between cytokines and the KP and the potential role of the KP in cardiovascular diseases.

Aryl hydrocarbon receptor (AHR) is a potential tumour suppressor in pituitary adenomas

Pituitary adenomas (PA) represent the largest group of intracranial neoplasms and yet the molecular mechanisms driving this disease remain largely unknown. The aim of this study was to use a high-throughput screening method to identify molecular pathways that may be playing a significant and consistent role in PA. RNA profiling using microarrays on eight local PAs identified the aryl hydrocarbon receptor (AHR) signalling pathway as a key canonical pathway downregulated in all PA types. This was confirmed by real-time PCR in 31 tumours. The AHR has been shown to regulate cell cycle progression in various cell types; however, its role in pituitary tissue has never been investigated. In order to validate the role of AHR in PA behaviour, further functional studies were undertaken. Over-expression of AHR in GH3 cells revealed a tumour suppressor potential independent of exogenous ligand activation by benzo α-pyrene (BαP). Cell cycle analysis and quantitative PCR of cell cycle regulator genes revealed that both unstimulated and BαP-stimulated AHR reduced E2F-driven transcription and altered expression of cell cycle regulator genes, thus increasing the percentage of cells in G₀/G₁ phase and slowing the proliferation rate of GH3 cells. Co-immunoprecipitation confirmed the interaction between AHR and retinoblastoma (Rb1) protein supporting this as a functional mechanism for the observed reduction. Endogenous Ahr reduction using silencing RNA confirmed the tumour suppressive function of the Ahr. These data support a mechanistic pathway for the putative tumour suppressive role of AHR specifically in PA, possibly through its role as a cell cycle co-regulator, even in the absence of exogenous ligands.

Epigenetic Regulation by Agonist-Specific Aryl Hydrocarbon Receptor Recruitment of Metastasis-Associated Protein 2 Selectively Induces Stanniocalcin 2 Expression

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that regulates a plethora of target genes. Historically, the AhR has been studied as a regulator of xenobiotic metabolizing enzyme genes, notably cytochrome P4501A1 encoded by CYP1A1, in response to the exogenous prototypical ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). AhR activity depends on its binding to the xenobiotic response element (XRE) in partnership with the AhR nuclear translocator (Arnt). Recent studies identified stanniocalcin 2 (Stc2) as a novel AhR target gene responsive to the endogenous AhR agonist cinnabarinic acid (CA). CA-dependent AhR-XRE-mediated Stc2 upregulation is responsible for cytoprotection against ectoplasmic reticulum/oxidative stress-induced apoptosis both in vitro and in vivo. Significantly, CA but not TCDD induces expression of Stc2 in hepatocytes. In contrast to TCDD, CA is unable to induce the CYP1A1 gene, thus revealing an AhR agonist-specific mutually exclusive dichotomous transcriptional response. Studies reported here provide a mechanistic explanation for this differential response by identifying an interaction between the AhR and the metastasis-associated protein 2 (MTA2). Moreover, the AhR-MTA2 interaction is CA-dependent and results in MTA2 recruitment to the Stc2 promoter, concomitant with agonist-specific epigenetic modifications targeting histone H4 lysine acetylation. The results demonstrate that histone H4 acetylation is absolutely dependent on CA-induced AhR and MTA2 recruitment to the Stc2 regulatory region and induced Stc2 gene expression, which in turn confers cytoprotection to liver cells exposed to chemical insults.

The Complex Biology of the Aryl Hydrocarbon Receptor and Its Role in the Pituitary Gland

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor best known for its ability to mediate the effects of environmental toxins such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD or dioxin), polycyclic aromatic hydrocarbons (PAHs), benzene, and polychlorinated biphenyls (PCBs) through the initiation of transcription of a number of metabolically active enzymes. Therefore, the AHR has been studied mostly in the context of xenobiotic signaling. However, several studies have shown that the AHR is constitutively active and plays an important role in general cell physiology, independently of its activity as a xenobiotic receptor and in the absence of exogenous ligands. Within the pituitary, activation of the AHR by environmental toxins has been implicated in disruption of gonadal development and fertility. Studies carried out predominantly in mouse models have revealed the detrimental influence of several environmental toxins on specific cell lineages of the pituitary tissue mediated by activation of AHR and its downstream effectors. Activation of AHR during fetal development adversely affected pituitary development while adult models exposed to AHR ligands demonstrated varying degrees of pituitary dysfunction. Such dysfunction may arise as a result of direct effects on pituitary cells or indirect effects on the hypothalamic-pituitary-gonadal axis. This review offers in-depth analysis of all aspects of AHR biology, with a particular focus on its role and activity within the adenohypophysis and specifically in pituitary tumorigenesis. A novel mechanism by which the AHR may play a direct role in pituitary cell proliferation and tumor formation is postulated. This review therefore attempts to cover all aspects of the AHR's role in the pituitary tissue, from fetal development to adult physiology and the pathophysiology underlying endocrine disruption and pituitary tumorigenesis.

Dissecting the enigma of scleroderma: possible involvement of the kynurenine pathway

The kynurenine pathway (KP) is the metabolic pathway via which L-tryptophan is converted to nicotinamide. It serves important immune-regulatory roles. This article will review the evidence for involvement of the KP in scleroderma and present a possible model of kynurenine regulation of the cytokine cascade.

Abnormal kynurenine pathway of tryptophan catabolism in cardiovascular diseases

Kynurenine pathway (KP) is the primary path of tryptophan (Trp) catabolism in most mammalian cells. The KP generates several bioactive catabolites, such as kynurenine (Kyn), kynurenic acid (KA), 3-hydroxykynurenine (3-HK), xanthurenic acid (XA), and 3-hydroxyanthranilic acid (3-HAA). Increased catabolite concentrations in serum are associated with several cardiovascular diseases (CVD), including heart disease, atherosclerosis, and endothelial dysfunction, as well as their risk factors, including hypertension, diabetes, obesity, and aging. The first catabolic step in KP is primarily controlled by indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO). Following this first step, the KP has two major branches, one branch is mediated by kynurenine 3-monooxygenase (KMO) and kynureninase (KYNU) and is responsible for the formation of 3-HK, 3-HAA, and quinolinic acid (QA); and another branch is controlled by kynurenine amino-transferase (KAT), which generates KA. Uncontrolled Trp catabolism has been demonstrated in distinct CVD, thus, understanding the underlying mechanisms by which regulates KP enzyme expression and activity is paramount. This review highlights the recent advances on the effect of KP enzyme expression and activity in different tissues on the pathological mechanisms of specific CVD, KP is an inflammatory sensor and modulator in the cardiovascular system, and KP catabolites act as the potential biomarkers for CVD initiation and progression. Moreover, the biochemical features of critical KP enzymes and principles of enzyme inhibitor development are briefly summarized, as well as the therapeutic potential of KP enzyme inhibitors against CVD is briefly discussed.

And Now for Something Completely Different: Diversity in Ligand-Dependent Activation of Ah Receptor Responses

Ligand-dependent activation of the Ah receptor (AhR) can result in an extremely diverse spectrum of biological and toxic effects that occur in a ligand-, species- and tissue-specific manner. While the classical mechanism of AhR-dependent signal transduction is directly related to its ability to modulate gene expression, the dramatic diversity in responses observed following AhR activation or inhibition is inconsistent with a single molecular mechanism of AhR action. Recent studies have revealed that key molecular events underlying the AhR signaling pathway are significantly more varied and complex than previously established, and the specificity and diversity in AhR response can be selectively modulated by a variety of factors. Here we describe new insights into the mechanistic diversity in AhR signal transduction that can contribute to ligand-, species- and tissue-specific differences in AhR reponse.

Ligand activation of the Ah receptor contributes to gastrointestinal homeostasis

The Ah receptor (AHR) is capable of binding a structurally diverse group of compounds that can be found in the diet, produced by bacteria in the gut and through endogenous metabolism. The gastrointestinal tract is a rich source of AHR ligands, which have been shown to protect the gut upon challenge with either pathogenic bacteria or toxic chemicals. The human AHR can be activated by a broader range of ligands compared to the mouse AHR, suggesting that studies in mice may underestimate the impact of AHR ligands in the human gut. The protective effect of AHR activation appears to be due to modulating the immune system within the gut. While several mechanisms have been established, due to the increasingly pleotropic nature of the AHR, other mechanisms of action likely exist that remain to be identified. The major contributors to AHR function in the gut and the most appropriate level of receptor activation that maintains intestinal homeostasis warrants further investigation.

Kynurenine pathway metabolism and neuroinflammatory disease

Immune-mediated activation of tryptophan (TRYP) catabolism via the kynurenine pathway (KP) is a consistent finding in all inflammatory disorders. Several studies by our group and others have examined the neurotoxic potential of neuroreactive TRYP metabolites, including quinolinic acid (QUIN) in neuroinflammatory neurological disorders, including Alzheimer's disease (AD), multiple sclerosis, amylotropic lateral sclerosis (ALS), and AIDS related dementia complex (ADC). Our current work aims to determine whether there is any benefit to the affected individuals in enhancing the catabolism of TRYP via the KP during an immune response. Under physiological conditions, QUIN is metabolized to the essential pyridine nucleotide, nicotinamide adenine dinucleotide (NAD⁺), which represents an important metabolic cofactor and electron transporter. NAD⁺ also serves as a substrate for the DNA ‘nick sensor’ and putative nuclear repair enzyme, poly(ADP-ribose) polymerase (PARP). Free radical initiated DNA damage, PARP activation and NAD⁺ depletion may contribute to brain dysfunction and cell death in neuroinflammatory disease.

The Aryl Hydrocarbon Receptor and Its Ligands Inhibit Myofibroblast Formation and Activation: Implications for Thyroid Eye Disease

Thyroid eye disease (TED) is a degenerative disease that manifests with detrimental tissue remodeling, myofibroblast accumulation, and scarring in the orbit of affected individuals. Currently, there are no effective therapies for TED that target or prevent the excessive tissue remodeling caused by myofibroblast formation and activation. The canonical cytokine that induces myofibroblast formation is transforming growth factor (TGF)-β. The TGF-β signaling pathway is influenced by aryl hydrocarbon receptor (AHR) signaling pathways. We hypothesized that AHR agonists can prevent myofibroblast formation in fibroblasts from patients with TED, and thus AHR ligands are potential therapeutics for the disease. Orbital fibroblasts explanted from patients with TED were treated with TGF-β to induce myofibroblast formation, contraction, and proliferation. We found that AHR ligands prevent TGF-β-dependent myofibroblast formation, and this ability is dependent on AHR expression. The AHR and AHR ligands block profibrotic Wnt signaling by inhibiting the phosphorylation of GSK3β to prevent myofibroblast formation. These results provide new insight into the molecular pathways underlying orbital scarring in TED. These novel studies highlight the potential of the AHR and AHR ligands as future therapeutic options for eye diseases and possibly also for other scarring conditions.

Aryl hydrocarbon receptor signaling modifies Toll-like receptor-regulated responses in human dendritic cells

Currently, it is not well understood how ligands of the aryl hydrocarbon receptor (AhR) modify inflammatory responses triggered by Toll-like receptor (TLR) agonists in human dendritic cells (DCs). Here, we show that AhR ligands 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the tryptophan derivatives 6-formylindolo[3,2-b] carbazole (FICZ), kynurenine (kyn), and the natural dietary compound indole-3-carbinol (I3C) differentially modify cytokine expression in human monocyte-derived DCs (MoDCs). The results show that TLR-activated MoDCs express higher levels of AhR and are more sensitive toward the effects of AhR ligands. Depending on the cytokine, treatment with AhR ligands led to a synergistic or antagonistic effect of the TLR-triggered response in MoDCs. Thus, activation of AhR increased the expression of interleukin (IL)-1β, but decreased the expression of IL-12A in TLR-activated MoDCs. Furthermore, TCDD and FICZ may have opposite effects on the expression of cytochrome P4501A1 (CYP1A1) in TLR8-activated MoDCs indicating that the effect of the specific AhR ligand may depend on the presence of the specific TLR agonist. Gene silencing showed that synergistic effects of AhR ligands on TLR-induced expression of IL-1β require a functional AhR and the expression of NF-κB RelB. On the other hand, repression of IL-12A by TCDD and FICZ involved the induction of the caudal type homeobox 2 (CDX2) transcription factor. Additionally, the levels of DC surface markers were decreased in MoDCs by TCDD, FICZ and I3C, but not by kyn. Overall, these data demonstrate that AhR modulates TLR-induced expression of cytokines and DC-specific surface markers in MoDCs involving NFκB RelB and the immune regulatory factor CDX2.

Interleukin-22 Signaling in the Regulation of Intestinal Health and Disease

Interleukin (IL)-22 is a member of the IL-10 family of cytokines that has been extensively studied since its discovery in 2000. This review article aims to describe the cellular sources and signaling pathways of this cytokine as well as the functions of IL-22 in the intestine. In addition, this article describes the roles of IL-22 in the pathogenesis of several gastrointestinal diseases, including inhibition of inflammation and barrier defense against pathogens within the intestine. Since many of the functions of IL-22 in the intestine are incompletely understood, this review is meant to assess our current understanding of the roles of IL-22 and provide new opportunities for inquiry to improve human intestinal health and disease.

Evidence for New Light-Independent Pathways for Generation of the Endogenous Aryl Hydrocarbon Receptor Agonist FICZ

Activation of the aryl hydrocarbon receptor (AhR), a conserved transcription factor best known as a target for highly toxic halogenated substances such as dioxin, under normal xenobiotic-free conditions is of considerable scientific interest. We have demonstrated previously that a photoproduct of tryptophan, 6-formylindolo[3,2-b]carbazole (FICZ), fulfills the criteria for an endogenous ligand for this receptor and proposed that this compound is the enigmatic mediator of the physiological functions of AhR. Here, we describe novel light-independent pathways by which FICZ can be formed. The oxidant H2O2 was shown to convert tryptophan to FICZ on its own in the absence of light. The enzymatic deamination of tryptamine yielded indole-3-acetaldehyde (I3A), which then rearranged to FICZ and its oxidation product, indolo[3,2-b]carbazole-6-carboxylic acid (CICZ). Indole-3-pyruvate (I3P) also produced I3A, FICZ, and CICZ. Malassezia yeast species, which constitute a part of the normal skin microbiota, produce a number of AhR activators from tryptophan. We identified both FICZ and CICZ among those products. Formation of FICZ from tryptophan or I3P produces a complex mixture of indole derivatives, some of which are CYP1A1 inhibitors. These can hinder the cellular clearance of FICZ and thereby increase its power as an AhR agonist. We present a general molecular mechanism involving dehydrogenations and oxidative coupling for the formation of FICZ in which I3A is the important precursor. In conclusion, our results suggest that FICZ is likely to be formed systemically.

Engagement of the Aryl Hydrocarbon Receptor in Mycobacterium tuberculosis-Infected Macrophages Has Pleiotropic Effects on Innate Immune Signaling

Understanding the mechanisms of host macrophage responses to Mycobacterium tuberculosis is essential for uncovering potential avenues of intervention to boost host resistance to infection. Macrophage transcriptome profiling revealed that M. tuberculosis infection strongly induced the expression of several enzymes controlling tryptophan catabolism. These included IDO1 and tryptophan 2,3-dioxygenase, which catalyze the rate-limiting step in the kynurenine pathway, producing ligands for the aryl hydrocarbon receptor (AHR). The AHR and heterodimeric partners AHR nuclear translocator and RELB are robustly expressed, and AHR and RELB levels increased further during infection. Infection enhanced AHR/AHR nuclear translocator and AHR/RELB DNA binding and stimulated the expression of AHR target genes, including that encoding the inflammatory cytokine IL-1β. AHR target gene expression was further enhanced by exogenous kynurenine, and exogenous tryptophan, kynurenine, or synthetic agonist indirubin reduced mycobacterial viability. Comparative expression profiling revealed that AHR ablation diminished the expression of numerous genes implicated in innate immune responses, including several cytokines. Notably, AHR depletion reduced the expression of IL23A and IL12B transcripts, which encode subunits of IL-23, a macrophage cytokine that stimulates production of IL-22 by innate lymphoid cells. AHR directly induced IL23A transcription in human and mouse macrophages through near-upstream enhancer regions. Taken together, these findings show that AHR signaling is strongly engaged in M. tuberculosis-infected macrophages and has widespread effects on innate immune responses. Moreover, they reveal a cascade of AHR-driven innate immune signaling, because IL-1β and IL-23 stimulate T cell subsets producing IL-22, another direct target of AHR transactivation.

The Aryl Hydrocarbon Receptor Antagonist StemRegenin1 Improves In Vitro Generation of Highly Functional Natural Killer Cells from CD34(+) Hematopoietic Stem and Progenitor Cells

Early natural killer (NK)-cell repopulation after allogeneic stem cell transplantation (allo-SCT) has been associated with reduced relapse rates without an increased risk of graft-versus-host disease, indicating that donor NK cells have specific antileukemic activity. Therefore, adoptive transfer of donor NK cells is an attractive strategy to reduce relapse rates after allo-SCT. Since NK cells of donor origin will not be rejected, multiple NK-cell infusions could be administered in this setting. However, isolation of high numbers of functional NK cells from transplant donors is challenging. Hence, we developed a cytokine-based ex vivo culture protocol to generate high numbers of functional NK cells from granulocyte colony-stimulating factor (G-CSF)-mobilized CD34(+) hematopoietic stem and progenitor cells (HSPCs). In this study, we demonstrate that addition of aryl hydrocarbon receptor antagonist StemRegenin1 (SR1) to our culture protocol potently enhances expansion of CD34(+) HSPCs and induces expression of NK-cell-associated transcription factors promoting NK-cell differentiation. As a result, high numbers of NK cells with an active phenotype can be generated using this culture protocol. These SR1-generated NK cells exert efficient cytolytic activity and interferon-γ production toward acute myeloid leukemia and multiple myeloma cells. Importantly, we observed that NK-cell proliferation and function are not inhibited by cyclosporin A, an immunosuppressive drug often used after allo-SCT. These findings demonstrate that SR1 can be exploited to generate high numbers of functional NK cells from G-CSF-mobilized CD34(+) HSPCs, providing great promise for effective NK-cell-based immunotherapy after allo-SCT.

The aryl hydrocarbon receptor in barrier organ physiology, immunology, and toxicology

The aryl hydrocarbon receptor (AhR) is an evolutionarily old transcription factor belonging to the Per-ARNT-Sim-basic helix-loop-helix protein family. AhR translocates into the nucleus upon binding of various small molecules into the pocket of its single-ligand binding domain. AhR binding to both xenobiotic and endogenous ligands results in highly cell-specific transcriptome changes and in changes in cellular functions. We discuss here the role of AhR for immune cells of the barrier organs: skin, gut, and lung. Both adaptive and innate immune cells require AhR signaling at critical checkpoints. We also discuss the current two prevailing views-namely, 1) AhR as a promiscuous sensor for small chemicals and 2) a role for AhR as a balancing factor for cell differentiation and function, which is controlled by levels of endogenous high-affinity ligands. AhR signaling is considered a promising drug and preventive target, particularly for cancer, inflammatory, and autoimmune diseases. Therefore, understanding its biology is of great importance.

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

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

Ah Receptor Pathway Intricacies; Signaling Through Diverse Protein Partners and DNA-Motifs

The Ah receptor is a transcription factor that modulates gene expression via interactions with multiple protein partners; these are reviewed, including the novel NC-XRE pathway involving KLF6.

Naturally occurring marine brominated indoles are aryl hydrocarbon receptor ligands/agonists

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that mediates the toxic and biological effects of structurally diverse chemicals, including the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). As part of a larger effort to identify the full spectrum of chemicals that can bind to and activate the AhR, we have examined the ability of several naturally occurring marine-derived brominated indoles and brominated (methylthio)indoles (collectively referred to as brominated indoles) to bind to the AhR and stimulate AhR-dependent gene expression. Incubation of mouse, rat, and guinea pig recombinant cell lines containing a stably transfected AhR-responsive luciferase reporter gene with eight brominated indoles revealed that all compounds stimulated luciferase reporter gene activity, although some species-specific differences were observed. All compounds induced significantly more luciferase activity when incubated with cells for 4 h as compared to 24 h, demonstrating that these compounds are transient activators of the AhR signaling pathway. Three of the brominated indoles induced CYP1A1 mRNA in human HepG2 cells in vitro and Cyp1a mRNA in zebrafish embryos in vivo. The identification of the brominated indoles as direct ligands and activators/agonists of the AhR was confirmed by their ability to compete with [(3)H]TCDD for binding to the AhR and to stimulate AhR transformation and DNA binding in vitro. Taken together, these results indicate that marine-derived brominated indoles are members of a new class of naturally occurring AhR agonists.

Fathoming the kynurenine pathway in migraine: why understanding the enzymatic cascades is still critically important

Kynurenine pathway, the quantitatively main branch of tryptophan metabolism, has been long been considered a source of nicotinamide adenine dinucleotide, although several of its products, the so-called kynurenines, are endowed with the capacity to activate glutamate receptors, thus potentially influencing a large group of functions in the central nervous system (CNS). Migraine, a largely unknown pathology, is strictly related to the glutamate system in the CNS pathologic terms. Despite the large number of studies conducted on migraine etio-pathology, the kynurenine pathway has been only recently linked to this disease. Nonetheless, some evidence suggests an intriguing role for some kynurenines, and an exploratory study on the serum kynurenine level might be helpful to better understand possible alterations of the kynurenine pathway in patients suffering from migraine.

Changing the face of kynurenines and neurotoxicity: therapeutic considerations

Kynurenines are the products of tryptophan metabolism. Among them, kynurenine and kynurenic acid are generally thought to have neuroprotective properties, while 3-hydroxykynurenine, 3-hydroxyanthranilic acid and quinolinic acid are considered neurotoxic. They participate in immunoregulation and inflammation and possess pro- or anti-excitotoxic properties, and their involvement in oxidative stress has also been suggested. Consequently, it is not surprising that kynurenines have been closely related to neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis and multiple sclerosis. More information about the less-known metabolites, picolinic and cinnabarinic acid, evaluation of new receptorial targets, such as aryl-hydrocarbon receptors, and intensive research on the field of the immunomodulatory function of kynurenines delineated the high importance of this pathway in general homeostasis. Emerging knowledge about the kynurenine pathway provides new target points for the development of therapeutical solutions against neurodegenerative diseases.

Aryl hydrocarbon receptor-dependent stanniocalcin 2 induction by cinnabarinic acid provides cytoprotection against endoplasmic reticulum and oxidative stress

The aryl hydrocarbon receptor (AhR) is a cytosolic ligand-activated transcription factor historically known for its role in xenobiotic metabolism. Although AhR activity has previously been shown to play a cytoprotective role against intrinsic apoptotic stimuli, the underlying mechanism by which AhR confers cytoprotection against apoptosis is largely unknown. Here, we demonstrate that activation of AhR by the tryptophan catabolite cinnabarinic acid (CA) directly upregulates expression of stanniocalcin 2 (Stc2) to elicit cytoprotection against apoptosis induced by endoplasmic reticulum stress and oxidative stress. Chromatin immunoprecipitation studies demonstrated that CA treatment induces direct AhR binding to a region of the Stc2 promoter containing multiple xenobiotic response elements. Using isolated primary hepatocytes from AhR wild-type (AhR floxed) and liver-specific AhR conditional knockout mice, we showed that pretreatment with CA conferred cytoprotection against hydrogen peroxide (H(2)O(2))-, thapsigargin-, and ethanol-induced apoptosis in an AhR-dependent manner. Furthermore, suppressing Stc2 expression using RNA interference confirmed that the cytoprotective properties of CA against H(2)O(2), thapsigargin, and ethanol injury were absolutely dependent on Stc2. Immunochemistry revealed the presence of Stc2 in the endoplasmic reticulum and on the cell surface, consistent with Stc2 secretion and autocrine and/or paracrine signaling. Finally, in vivo data using a mouse model of acute alcohol hepatotoxicity demonstrated that CA provided cytoprotection against ethanol-induced apoptosis, hepatic microvesicular steatosis, and liver injury. Collectively, our data uncovered a novel mechanism for AhR-mediated cytoprotection in the liver that is dependent on CA-induced Stc2 activity.

The environment of regulatory T cell biology: cytokines, metabolites, and the microbiome

Regulatory T cells (Tregs) are suppressive T cells that have an essential role in maintaining the balance between immune activation and tolerance. Their development, either in the thymus, periphery, or experimentally in vitro, and stability and function all depend on the right mix of environmental stimuli. This review focuses on the effects of cytokines, metabolites, and the microbiome on both human and mouse Treg biology. The role of cytokines secreted by innate and adaptive immune cells in directing Treg development and shaping their function is well established. New and emerging data suggest that metabolites, such as retinoic acid, and microbial products, such as short-chain fatty acids, also have a critical role in guiding the functional specialization of Tregs. Overall, the complex interaction between distinct environmental stimuli results in unique, and in some cases tissue-specific, tolerogenic environments. Understanding the conditions that favor Treg induction, accumulation, and function is critical to defining the pathophysiology of many immune-mediated diseases and to developing new therapeutic interventions.

Interleukin-22: immunobiology and pathology

Interleukin-22 (IL-22) is a recently described IL-10 family cytokine that is produced by T helper (Th) 17 cells, γδ T cells, NKT cells, and newly described innate lymphoid cells (ILCs). Knowledge of IL-22 biology has evolved rapidly since its discovery in 2000, and a role for IL-22 has been identified in numerous tissues, including the intestines, lung, liver, kidney, thymus, pancreas, and skin. IL-22 primarily targets nonhematopoietic epithelial and stromal cells, where it can promote proliferation and play a role in tissue regeneration. In addition, IL-22 regulates host defense at barrier surfaces. However, IL-22 has also been linked to several conditions involving inflammatory tissue pathology. In this review, we assess the current understanding of this cytokine, including its physiologic and pathologic effects on epithelial cell function.

The aryl hydrocarbon receptor: a review of its role in the physiology and pathology of the integument and its relationship to the tryptophan metabolism

The aryl hydrocarbon receptor (AHR) is a cytosolic receptor for low molecular weight molecules, of which the most widely recognized ligand is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and the most widely recognized effect, chloracne. Adverse effects of manipulation were most recently and graphically demonstrated by the poisoning of Viktor Yushchenko during the Ukrainian presidential elections of 2004. However, recent research has revealed a receptor with wide-ranging, and at times, paradoxical actions. It was arguably among the first biological receptors to be utilized by dermatologists, dating from the time of topical tar preparations as a therapeutic agent. I provide a review outlining the role AHR plays in the development, cellular oxidation/antioxidation, responses to ultraviolet light, melanogenesis, epidermal barrier function, and immune regulation and its relationship to tryptophan metabolism. Finally, I will review the role of AHR in diseases of the integument.

Cross-species transcriptomic analysis elucidates constitutive aryl hydrocarbon receptor activity

Background

Research on the aryl hydrocarbon receptor (AHR) has largely focused on variations in toxic outcomes resulting from its activation by halogenated aromatic hydrocarbons. But the AHR also plays key roles in regulating pathways critical for development, and after decades of research the mechanisms underlying physiological regulation by the AHR remain poorly characterized. Previous studies identified several core genes that respond to xenobiotic AHR ligands across a broad range of species and tissues. However, only limited inferences have been made regarding its role in regulating constitutive gene activity, i.e. in the absence of exogenous ligands. To address this, we profiled transcriptomic variations between AHR-active and AHR-less-active animals in the absence of an exogenous agonist across five tissues, three of which came from rats (hypothalamus, white adipose and liver) and two of which came from mice (kidney and liver). Because AHR status alone has been shown sufficient to alter transcriptomic responses, we reason that by contrasting profiles amongst AHR-variant animals, we may elucidate effects of the AHR on constitutive mRNA abundances.

Results

We found significantly more overlap in constitutive mRNA abundances amongst tissues within the same species than from tissues between species and identified 13 genes (Agt, Car3, Creg1, Ctsc, E2f6, Enpp1, Gatm, Gstm4, Kcnj8, Me1, Pdk1, Slc35a3, and Sqrdl) that are affected by AHR-status in four of five tissues. One gene, Creg1, was significantly up-regulated in all AHR-less-active animals. We also find greater overlap between tissues at the pathway level than at the gene level, suggesting coherency to the AHR signalling response within these processes. Analysis of regulatory motifs suggests that the AHR mostly mediates transcriptional regulation via direct binding to response elements.

Conclusions

These findings, though preliminary, present a platform for further evaluating the role of the AHR in regulation of constitutive mRNA levels and physiologic function.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1053) contains supplementary material, which is available to authorized users.

The cross-talk between opportunistic fungi and the mammalian host via microbiota's metabolism

An increased understanding of the importance of microbiota in shaping the host's immune and metabolic activities has rendered fungal interactions with their hosts more complex than previously appreciated. It is now clear that a three-way interaction between host, fungi, and microbiota dictates the types of host-fungus relationship. Indeed, microbial dysbiosis predisposes to a variety of chronic fungal infections and diseases at local and distant sites. By correlating changes in metabolite profiles with microbiota metagenomic composition, we have defined a functional node whereby certain bacteria species contribute to host-fungal symbiosis and mucosal homeostasis. A tryptophan catabolic pathway is exploited by commensal lactobacilli and the mammalian host to increase fitness in response to Candida albicans by inducing resistance and tolerance mechanisms of antifungal immunity. Much like lactobacilli in the gut, Firmicutes change significantly in the airways during aspergillosis. The aryl hydrocarbon receptor has a pivotal role in connecting tryptophan catabolism by microbial communities and the host's own pathway of tryptophan degradation through the enzyme indoleamine 2,3-dioxygenase 1. These data suggest that the study of the human microbiota in the trans-omics era, with a focus on metagenomics and metabonomics, is providing novel insights into the regulation of host immune responsiveness to fungi.

Development and survival of Th17 cells within the intestines: the influence of microbiome- and diet-derived signals

Th17 cells have emerged as important mediators of host defense and homeostasis at barrier sites, particularly the intestines, where the greatest number and diversity of the microbiota reside. A critical balance exists between protection of the host from its own microbiota and pathogens and the development of immune-mediated disease. Breaches of local innate immune defenses provide critical stimuli for the induction of Th17 cell development, and additional cues within these tissues promote Th17 cell survival and/or plasticity. Normally, this results in eradication of the microbial threat and restitution of homeostasis. When dysregulated, however, Th17 cells can cause a range of immune-mediated diseases, whether directed against Ags derived from the microbiota, such as in inflammatory bowel disease, or against self-Ags in a range of autoimmune diseases. This review highlights recent discoveries that provide new insights into ways in which environmental signals impact Th17 cell development and function in the intestines.