2,3,7,8-Tetrachlorodibenzo-p-dioxin receptors regulate transcription of the cytochrome P1-450 gene

Differential Modulation of Dendritic Cell Biology by Endogenous and Exogenous Aryl Hydrocarbon Receptor Ligands

The aryl hydrocarbon receptor (AhR) is a decisive regulatory ligand-dependent transcription factor. It binds highly diverse ligands, which can be categorized as either endogenous or exogenous. Ligand binding activates AhR, which can adjust inflammatory responses by modulating immune cells such as dendritic cells (DCs). However, how different AhR ligand classes impact the phenotype and function of human monocyte-derived DCs (hMoDCs) has not been extensively studied in a comparative manner. We, therefore, tested the effect of the representative compounds Benzo(a)pyrene (BP), 6-formylindolo[3,2-b]carbazole (FICZ), and Indoxyl 3-sulfate (I3S) on DC biology. Thereby, we reveal that BP significantly induces a tolerogenic response in lipopolysaccharide-matured DCs, which is not apparent to the same extent when using FICZ or I3S. While all three ligand classes activate AhR-dependent pathways, BP especially induces the expression of negative immune regulators, and subsequently strongly subverts the T cell stimulatory capacity of DCs. Using the CRISPR/Cas9 strategy we also prove that the regulatory effect of BP is strictly AhR-dependent. These findings imply that AhR ligands contribute differently to DC responses and incite further studies to uncover the mechanisms and molecules which are involved in the induction of different phenotypes and functions in DCs upon AhR activation.

2,3,7,8-Tetrachlorodibenzo-p-dioxin induces multigenerational alterations in the expression of microRNA in the thymus through epigenetic modifications

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a potent AhR ligand, is an environmental contaminant that is known for mediating toxicity across generations. However, whether TCDD can induce multigenerational changes in the expression of microRNAs (miRs) has not been previously studied. In the current study, we investigated the effect of administration of TCDD in pregnant mice (F0) on gestational day 14, on the expression of miRs in the thymus of F0 and subsequent generations (F1 and F2). Of the 3200 miRs screened, 160 miRs were dysregulated similarly in F0, F1, and F2 generations, while 46 miRs were differentially altered in F0 to F2 generations. Pathway analysis revealed that the changes in miR signature profile mediated by TCDD affected the genes that regulate cell signaling, apoptosis, thymic atrophy, cancer, immunosuppression, and other physiological pathways. A significant number of miRs that showed altered expression exhibited dioxin response elements (DRE) on their promoters. Focusing on one such miR, namely miR-203 that expressed DREs and was induced across F0 to F2 by TCDD, promoter analysis showed that one of the DREs expressed by miR-203 was functional to TCDD-mediated upregulation. Also, the histone methylation status of H3K4me3 in the miR-203 promoter was significantly increased near the transcriptional start site in TCDD-treated thymocytes across F0 to F2 generations. Genome-wide chromatin immunoprecipitation sequencing study suggested that TCDD may cause alterations in histone methylation in certain genes across the three generations. Together, the current study demonstrates that gestational exposure to TCDD can alter the expression of miRs in F0 through direct activation of DREs as well as across F0, F1, and F2 generations through epigenetic pathways.

Aryl hydrocarbon receptor: The master regulator of immune responses in allergic diseases

The aryl hydrocarbon receptor (AhR) is a widely studied ligand-activated cytosolic transcriptional factor that has been associated with the initiation and progression of various diseases, including autoimmune diseases, cancers, metabolic syndromes, and allergies. Generally, AhR responds and binds to environmental toxins/ligands, dietary ligands, and allergens to regulate toxicological, biological, cellular responses. In a canonical signaling manner, activation of AhR is responsible for the increase in cytochrome P450 enzymes which help individuals to degrade and metabolize these environmental toxins and ligands. However, canonical signaling cannot be applied to all the effects mediated by AhR. Recent findings indicate that activation of AhR signaling also interacts with some non-canonical factors like Kruppel-like-factor-6 (KLF6) or estrogen-receptor-alpha (Erα) to affect the expression of downstream genes. Meanwhile, enormous research has been conducted to evaluate the effect of AhR signaling on innate and adaptive immunity. It has been shown that AhR exerts numerous effects on mast cells, B cells, macrophages, antigen-presenting cells (APCs), Th1/Th2 cell balance, Th17, and regulatory T cells, thus, playing a significant role in allergens-induced diseases. This review discussed how AhR mediates immune responses in allergic diseases. Meanwhile, we believe that understanding the role of AhR in immune responses will enhance our knowledge of AhR-mediated immune regulation in allergic diseases. Also, it will help researchers to understand the role of AhR in regulating immune responses in autoimmune diseases, cancers, metabolic syndromes, and infectious diseases.

EDC-induced mechanisms of immunotoxicity: a systematic review

Endocrine-disrupting chemicals (EDCs) refer to a group of chemicals that cause adverse effects in human health, impairing hormone production and regulation, resulting in alteration of homeostasis, reproductive, and developmental, and immune system impairments. The immunotoxicity of EDCs involves many mechanisms altering gene expression that depend on the activation of nuclear receptors such as the aryl hydrocarbon receptor (AHR), the estrogen receptor (ER), and the peroxisome proliferator-activated receptor (PPAR), which also results in skin and intestinal disorders, microbiota alterations and inflammatory diseases. This systematic review aims to review different mechanisms of immunotoxicity and immunomodulation of T cells, focusing on T regulatory (Treg) and Th17 subsets, B cells, and dendritic cells (DCs) caused by specific EDCs such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), bisphenols (BPs) and polyfluoroalkyl substances (PFASs). To achieve this objective, a systematic study was conducted searching various databases including PubMed and Scopus to find in-vitro, in-vivo, and biomonitoring studies that examine EDC-dependent mechanisms of immunotoxicity. While doing the systematic review, we found species- and cell-specific outcomes and a translational gap between in-vitro and in-vivo experiments. Finally, an adverse outcome pathway (AOP) framework is proposed, which explains mechanistically toxicity endpoints emerging from different EDCs having similar key events and can help to improve our understanding of EDCs mechanisms of immunotoxicity. In conclusion, this review provides insights into the mechanisms of immunotoxicity mediated by EDCs and will help to improve human health risk assessment.

The aryl hydrocarbon receptor as a mediator of host-microbiota interplay

Increasing evidence suggests a significant role for microbiota dependent metabolites and co-metabolites, acting as aryl hydrocarbon receptor (AHR) ligands, to facilitate bidirectional communication between the host and the microbiota and thus modulate physiology. Such communication is particularly evident within the gastrointestinal tract. Through binding to or activating the AHR, these metabolites play fundamental roles in various physiological processes and likely contribute to the maintenance of intestinal homeostasis. In recent years, tryptophan metabolites were screened to identify physiologically relevant AHR ligands or activators. The discovery of specific microbiota-derived indole-based metabolites as AHR ligands may provide insight concerning how these metabolites affect interactions between gut microbiota and host intestinal homeostasis and how this relates to chronic GI disease and overall health. A greater understanding of the mechanisms that modulate the production of such metabolites and associated AHR activity may be utilized to effectively treat inflammatory diseases and promote human health. Here, we review microbiota-derived AHR ligands generated from tryptophan that modulate host-gut microbiota interactions and discuss possible intervention strategies for potential therapies in the future.

Urolithin A Is a Dietary Microbiota-Derived Human Aryl Hydrocarbon Receptor Antagonist

Urolithins (e.g., UroA and B) are gut microbiota-derived metabolites of the natural polyphenol ellagic acid. Urolithins are associated with various health benefits, including attenuation of inflammatory signaling, anti-cancer effects and repression of lipid accumulation. The molecular mechanisms underlying the beneficial effects of urolithins remain unclear. We hypothesize that some of the human health benefits of urolithins are mediated through the aryl hydrocarbon receptor (AHR). Utilizing a cell-based reporter system, we tested urolithins for the capacity to modulate AHR activity. Cytochrome P450 1A1 (CYP1A1) mRNA levels were assessed by real-time quantitative polymerase chain reaction. Competitive ligand binding assays were performed to determine whether UroA is a direct ligand for the AHR. Subcellular AHR protein levels were examined utilizing immunoblotting analysis. AHR expression was repressed in Caco-2 cells by siRNA transfection to investigate AHR-dependency. UroA and B were able to antagonize 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced AHR-mediated transcriptional activity. Furthermore, UroA and B attenuated TCDD-mediated stimulation of CYP1A1 mRNA levels. In addition, competitive ligand binding assays characterized UroA as a direct AHR ligand. Consistent with other AHR antagonists, UroA failed to induce AHR retention in the nucleus. AHR is necessary for UroA-mediated attenuation of cytokine-induced interleukin 6 (IL6) and prostaglandin-endoperoxide synthase 2 (PTGS2) expression in Caco-2 cells. Here we identified UroA as the first dietary-derived human selective AHR antagonist produced by the gut microbiota through multi-step metabolism. Furthermore, previously reported anti-inflammatory activity of UroA may at least in part be mediated through AHR.

Regulation of the Immune Response by the Aryl Hydrocarbon Receptor

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that is activated by small molecules provided by the diet, microorganisms, metabolism, and pollutants. AhR is expressed by a number of immune cells, and thus AhR signaling provides a molecular pathway that integrates the effects of the environment and metabolism on the immune response. Studies have shown that AhR signaling plays important roles in the immune system in health and disease. As its activity is regulated by small molecules, AhR also constitutes a potential target for therapeutic immunomodulation. In this review we discuss the role of AhR in the regulation of the immune response in the context of autoimmunity, infection, and cancer, as well as the potential opportunities and challenges of developing AhR-targeted therapeutics.

Dietary Indoles Suppress Delayed-Type Hypersensitivity by Inducing a Switch from Proinflammatory Th17 Cells to Anti-Inflammatory Regulatory T Cells through Regulation of MicroRNA

Aryl hydrocarbon receptor (AhR) has been shown to have profound influence on T cell differentiation, and use of distinct AhR ligands has shown that whereas some ligands induce regulatory T cells (Tregs), others induce Th17 cells. In the present study, we tested the ability of dietary AhR ligands (indole-3-carbinol [I3C] and 3,3'-diindolylmethane [DIM]) and an endogenous AhR ligand, 6-formylindolo(3,2-b)carbazole (FICZ), on the differentiation and functions of Tregs and Th17 cells. Treatment of C57BL/6 mice with indoles (I3C or DIM) attenuated delayed-type hypersensitivity (DTH) response to methylated BSA and generation of Th17 cells while promoting Tregs. In contrast, FICZ exacerbated the DTH response and promoted Th17 cells. Indoles decreased the induction of IL-17 but promoted IL-10 and Foxp3 expression. Also, indoles caused reciprocal induction of Tregs and Th17 cells only in wild-type (AhR(+/+)) but not in AhR knockout (AhR(-/-)) mice. Upon analysis of microRNA (miR) profile in draining lymph nodes of mice with DTH, treatment with I3C and DIM decreased the expression of several miRs (miR-31, miR-219, and miR-490) that targeted Foxp3, whereas it increased the expression of miR-495 and miR-1192 that were specific to IL-17. Interestingly, treatment with FICZ had precisely the opposite effects on these miRs. Transfection studies using mature miR mimics of miR-490 and miR-1192 that target Foxp3 and IL-17, respectively, or scrambled miR (mock) or inhibitors confirmed that these miRs specifically targeted Foxp3 and IL-17 genes. Our studies demonstrate, to our knowledge for the first time, that the ability of AhR ligands to regulate the differentiation of Tregs versus Th17 cells may depend on miR signature profile.

Prenatal exposure to TCDD triggers significant modulation of microRNA expression profile in the thymus that affects consequent gene expression

Background

MicroRNAs (miRs) are a class of small RNAs that regulate gene expression. There are over 700 miRs encoded in the mouse genome and modulate most of the cellular pathways and functions by controlling gene expression. However, there is not much known about the pathophysiological role of miRs. TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin), an environmental contaminant is well known to induce severe toxicity (acute and chronic) with long-term effects. Also, in utero exposure of fetus to TCDD has been shown to cause thymic atrophy and alterations in T cell differentiation. It is also relevant to understand “the fetal basis of adult disease” hypothesis, which proposes that prenatal exposure to certain forms of nutritional and environmental stress can cause increased susceptibility to clinical disorders later in life. In the current study, therefore, we investigated the effects of prenatal exposure to TCDD on miR profile in fetal thymocytes and searched for their possible role in causing thymic atrophy and alterations in the expression of apoptotic genes.

Methodology/Principal Findings

miR arrays of fetal thymocytes post exposure to TCDD and vehicle were performed. Of the 608 mouse miRs screened, 78 miRs were altered more than 1.5 fold and 28 miRs were changed more than 2 fold in fetal thymocytes post-TCDD exposure when compared to vehicle controls. We validated the expression of several of the miRs using RT-PCR. Furthermore, several of the miRs that were downregulated contained highly complementary sequence to the 3′-UTR region of AhR, CYP1A1, Fas and FasL. Also, the Ingenuity Pathway Analysis software and database was used to analyze the 78 miRs that exhibited significant expression changes and revealed that as many as 15 pathways may be affected.

Conclusions/Significance

These studies revealed that TCDD-mediated alterations in miR expression may be involved in the regulation of its toxicity including cancer, hepatic injury, apoptosis, and cellular development.

Dibenzofuran induces oxidative stress, disruption of trans-mitochondrial membrane potential (ΔΨm) and G1 arrest in human hepatoma cell line

Dioxins are a class of extremely toxic environmentally persistent pollutant, comprised of halogenated dibenzo-p-dioxins, dibenzofurans and biphenyls. Despite significant human exposure via multiple routes, very little is known about toxicity induced by dibenzofuran (DF). Current study shed lights on the potential toxicity mechanism of DF using human hepatoma cell line (HepG2). It was observed that the exposure to DF potentiate oxidative stress, apoptosis and necrosis at 10μM within 8h in HepG2 cells. Interestingly, when we pre-incubated the cells with α-NF (1nM) for 12h, an aromatic hydrocarbon receptor antagonist, the IC(50) of DF increased by 14 folds indicating the cytoprotective ability of α-NF from DF induced toxicity. Furthermore, three additional metabolites were observed while studying the metabolic profile of DF in HepG2 cells with and without pre-incubation with α-NF using chromatography-mass spectroscopy (GC-MS). Of these, two metabolites were characterized as dihydroxylated derivative of DF and third metabolite was characterized as quinone derivative of DF. By flow cytometry and confocal laser microscopy analysis we followed the ROS formation after DF (10μM) exposure for 3h. Significantly low ROS was generated in cells which were pre-incubated with α-NF than cells which were not pre-incubated with α-NF underlining the importance of metabolism in DF toxicity. The same pattern of protection was consistent while measuring mitochondrial membrane potential (MMP), i.e., less MMP dip was observed in 'with α-NF pre-incubated and DF (10μM) exposed cells' than 'without α-NF pre-incubated but DF exposed cells'. In cell cycle studies, it was confirmed that cell population of HepG2 at G1 stage progressively increased in number (∼74%) within 24h. Thus, DF and its metabolites induce significantly higher cytotoxicity after metabolism in HepG2 cells than its parent compound (DF) by ROS formation, MMP dip and impaired cell cycle.

SOCS2-induced proteasome-dependent TRAF6 degradation: a common anti-inflammatory pathway for control of innate immune responses

Pattern recognition receptors and receptors for pro-inflammatory cytokines provide critical signals to drive the development of protective immunity to infection. Therefore, counter-regulatory pathways are required to ensure that overwhelming inflammation harm host tissues. Previously, we showed that lipoxins modulate immune response during infection, restraining inflammation during infectious diseases in an Aryl hydrocarbon receptor (AhR)/suppressors of cytokine signaling (SOCS)2-dependent-manner. Recently, Indoleamine-pyrrole 2,3- dioxygenase (IDO)-derived tryptophan metabolites, including L-kynurenine, were also shown to be involved in several counter-regulatory mechanisms. Herein, we addressed whether the intracellular molecular events induced by lipoxins mediating control of innate immune signaling are part of a common regulatory pathway also shared by L-kynurenine exposure. We demonstrate that Tumor necrosis factor receptor-associated factor (TRAF)6 – member of a family of adapter molecules that couple the TNF receptor and interleukin-1 receptor/Toll-like receptor families to intracellular signaling events essential for the development of immune responses – is targeted by both lipoxins and L-kynurenine via an AhR/SOCS2-dependent pathway. Furthermore, we show that LXA₄- and L-kynurenine-induced AhR activation, its subsequent nuclear translocation, leading SOCS2 expression and TRAF6 Lys47-linked poly-ubiquitination and proteosome-mediated degradation of the adapter proteins. The in vitro consequences of such molecular interactions included inhibition of TLR- and cytokine receptor-driven signal transduction and cytokine production. Subsequently, in vivo proteosome inhibition led to unresponsiveness to lipoxins, as well as to uncontrolled pro-inflammatory reactions and elevated mortality during toxoplasmosis. In summary, our results establish proteasome degradation of TRAF6 as a key molecular target for the anti-inflammatory pathway triggered by lipoxins and L-kynurenine, critical counter-regulatory mediators in the innate and adaptive immune systems.

The aryl hydrocarbon receptor complex and the control of gene expression

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that controls the expression of a diverse set of genes. The toxicity of the potent AhR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin is almost exclusively mediated through this receptor. However, the key alterations in gene expression that mediate toxicity are poorly understood. It has been established through characterization of AhR-null mice that the AhR has a required physiological function, yet how endogenous mediators regulate this orphan receptor remains to be established. A picture as to how the AhR/ARNT heterodimer actually mediates gene transcription is starting to emerge. The AhR/ARNT complex can alter transcription both by binding to its cognate response element and through tethering to other transcription factors. In addition, many of the coregulatory proteins necessary for AhR-mediated transcription have been identified. Cross talk between the estrogen receptor and the AhR at the promoter of target genes appears to be an important mode of regulation. Inflammatory signaling pathways and the AhR also appear to be another important site of cross talk at the level of transcription. A major focus of this review is to highlight experimental efforts to characterize nonclassical mechanisms of AhR-mediated modulation of gene transcription.

The toxicokinetics and metabolism of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) and their relevance for toxicity

This article reviews the present state of the art regarding the toxicokinetics and metabolism of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs). The absorption, body distribution, and metabolism can vary greatly between species and also may depend on the congener and dose. In biota, the 2,3,7,8-substituted PCDDs and PCDFs are almost exclusively retained in all tissue types, preferably liver and fat. This selective tissue retention and bioaccumulation are caused by a reduced rate of biotransformation and subsequent elimination of congeners with chlorine substitution at the 2,3,7, and 8 positions. 2,3,7,8-Substituted PCDDs and PCDFs also have the greatest toxic and biological activity and affinity for the cytosolic arylhydrocarbon (Ah)-receptor protein. The parent compound is the causal agent for Ah-receptor-mediated toxic and biological effects, with metabolism and subsequent elimination of 2,3,7,8- substituted congeners representing a detoxification process. Congener-specific affinity of PCDDs and PCDFs for the Ah-receptor, the genetic events following receptor binding, and toxicokinetics are factors that contribute to the relative in vivo potency of an individual PCDD or PCDF in a given species. Limited human data indicate that marked species differences exist in the toxicokinetics of these compounds. Thus, human risk assessment for PCDDs and PCDFs needs to consider species-, congener-, and dose-specific toxicokinetic data. In addition, exposure to complex mixtures, including PCBs, has the potential to alter the toxicokinetics of individual compounds. These alterations in toxicokinetics may be involved in some of the nonadditive toxic or biological effects that are observed after exposure to mixtures of PCDDs or PCDFs with PCBs.

In vitro analysis of Ah receptor domains involved in ligand-activated DNA recognition

The Ah receptor (AHR) is a basic helix-loop-helix protein that mediates the effects of 2,3,7,8-tetrachloro-dibenzo-p-dioxin. In this report, we describe a rabbit reticulocyte system that allows functional expression of both the AHR and its dimeric partner, the AHR nuclear translocator protein (ARNT). By using this in vitro system, we were able to reconstitute agonist binding to the AHR and agonist-induced AHR-ARNT recognition of a cognate DNA enhancer sequence. Expression of AHR deletion mutants revealed the location of N-terminal domains responsible for ligand and DNA recognition and C-terminal domains that play roles in agonist-induced DNA recognition.

Stimulatory effect of the CYP1A1 inducer 2,3,7,8-tetrachlorodibenzo-p-dioxin on the reproduction of HIV-1 in human lymphoid cell culture

1. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) was an inducer of microsomal benzo[alpha]pyrene hydroxylase (AHH) and 7-ethoxyresorufin O-deethylase (EROD) in MT-4 human lymphoid cell culture. 2. The monoclonal antibody (Mab), 1-7-1 and the immunodepleted polyclonal antibody (Pab), anti-CYP1A1(-A2), inhibit AHH and EROD activities pre-induced by 10 nM TCDD in MT-4 cells. Hence, the specific monooxygenase isoform induced in the lymphoid cells by TCDD appears to be CYP1A1 the expression of which is mediated by the Ah receptor. 3. Incubation of MT-4 cells with TCDD at 10, 50 and 150 nM for 1.5 and 48 h followed by infection of the cells with human immunodeficiency virus 1 (HIV-1) was accompanied by a 3-6-fold increase in the activity of viral RNA-dependent DNA-polymerase. The most marked effect on reverse transcriptase activity occurred with 10 nM TCDD 5-9 days after HIV-1 infection. 4. In the same period there was accumulation of viral protein, determined by ELISA, with a 4-8-fold increase in production of viral protein. The above effects of TCDD have been observed even when MT-4 cells were washed 1.5 h after beginning the incubation with TCDD.

2,3,7,8-Tetrachlorodibenzo-P-dioxin as a possible activator of HIV infection

The effects of 10-150 nM 2,3,7,8-TCDD on the HIV infection in the various lymphoid cell cultures have been studied. The level of reproduction of HIV was examined by determination of virus antigen as well as by measurement of reverse trascriptase activity. 2,3,7,8-TCDD does not exert any toxic influence in this concentration on the MT-4 cells and causes the induction of cytochrome P-450-containing monooxygenase. Furthermore, in MT-4 cell culture infected by HIV-1 virus an increase of virus production after a treatment of cells with 2,3,7,8-TCDD has been revealed.

TCDD resistance is inherited as an autosomal dominant trait in the rat

In the present study, genetic crossings were performed between the most TCDD-susceptible (Long-Evans) and the most TCDD-resistant (Han/Wistar) rat strains. The F1 offspring were as resistant to TCDD as the Han/Wistar rats irrespective of the sex of their Han/Wistar parents. In test-cross and F2 progeny the distribution of resistant and susceptible phenotypes was consistent with inheritance regulated by 2 (possibly 3) autosomal genes displaying complete dominance, independent segregation, and an additive co-effect. These data show that, in contrast to earlier findings in mice, TCDD resistance seems to be the dominant trait in the rat. Moreover, the results challenge the current view that the Ah-locus is the exclusive determinant of TCDD sensitivity.

Comparative aspects of the mammalian cytochrome P450 IV gene family

1. The structures of mammalian cytochrome P-450 isoenzymes have been compared with respect to micro-sequence heterogeneity and their haem-binding cysteinyl peptides. 2. Mechanisms of induction of several P450 gene families are described including transcriptional activation and mRNA stabilization in cytochrome P450 I, II and IV families. 3. The tissue expression and substrate specificity of the cytochrome P450 IV family in liver, kidney and lung have been discussed. 4. The role of hepatic cytochrome P450 IVA1 induction in peroxisome proliferation is presented, and emphasis placed on the identification of susceptible and non-susceptible species.

In vivo and in vitro metabolism of benzo(a)pyrene by the larva of the newt, Pleurodeles waltl

1. The larva of the amphibian species, Pleurodeles waltl was shown to metabolize benzo(a)pyrene in vivo into a variety of oxidized products. 2. In vitro, BaP hydroxylase (AHH) activity was found in hepatic microsomes and postmitochondrial fractions from both larvae and adults of the pleurodele. 3. The clastogenic effect of BaP formation of micronuclei in the erythrocytes was shown to be related to the presence of BaP quinones in the tissues of the newt.