6-Formylindolo[3,2-b]carbazole, a Potent Ligand for the Aryl Hydrocarbon Receptor Produced Both Endogenously and by Microorganisms, can Either Promote or Restrain Inflammatory Responses.

The aryl hydrocarbon receptor (AHR) binds major physiological modifiers of the immune system. The endogenous 6-formylindolo[3,2-b]carbazole (FICZ), which binds with higher affinity than any other compound yet tested, including TCDD, plays a well-documented role in maintaining the homeostasis of the intestines and skin. The effects of transient activation of AHR by FICZ differ from those associated with continuous stimulation and, depending on the dose, include either differentiation into T helper 17 cells that express proinflammatory cytokines or into regulatory T cells or macrophages with anti-inflammatory properties. Moreover, in experimental models of human diseases high doses stimulate the production of immunosuppressive cytokines and suppress pathogenic autoimmunity. In our earlier studies we characterized the formation of FICZ from tryptophan via the precursor molecules indole-3-pyruvate and indole-3-acetaldehyde. In the gut formation of these precursor molecules is catalyzed by microbial aromatic-amino-acid transaminase ArAT. Interestingly, tryptophan can also be converted into indole-3-pyruvate by the amino-acid catabolizing enzyme interleukin-4 induced gene 1 (IL4I1), which is secreted by host immune cells. By thus generating derivatives of tryptophan that activate AHR, IL4I1 may have a role to play in anti-inflammatory responses, as well as in a tumor escape mechanism that reduces survival in cancer patients. The realization that FICZ can be produced from tryptophan by sunlight, by enzymes expressed in our cells (IL4I1), and by microorganisms as well makes it highly likely that this compound is ubiquitous in humans. A diurnal oscillation in the level of FICZ that depends on the production by the fluctuating number of microbes might influence not only intestinal and dermal immunity locally, but also systemic immunity.

How the AHR Became Important in Intestinal Homeostasis-A Diurnal FICZ/AHR/CYP1A1 Feedback Controls Both Immunity and Immunopathology.

Ever since the 1970s, when profound immunosuppression caused by exogenous dioxin-like compounds was first observed, the involvement of the aryl hydrocarbon receptor (AHR) in immunomodulation has been the focus of considerable research interest. Today it is established that activation of this receptor by its high-affinity endogenous ligand, 6-formylindolo[3,2-b]carbazole (FICZ), plays important physiological roles in maintaining epithelial barriers. In the gut lumen, the small amounts of FICZ that are produced from L-tryptophan by microbes are normally degraded rapidly by the inducible cytochrome P4501A1 (CYP1A1) enzyme. This review describes how when the metabolic clearance of FICZ is attenuated by inhibition of CYP1A1, this compound passes through the intestinal epithelium to immune cells in the lamina propria. FICZ, the level of which is thus modulated by this autoregulatory loop involving FICZ itself, the AHR and CYP1A1, plays a central role in maintaining gut homeostasis by potently up-regulating the expression of interleukin 22 (IL-22) by group 3 innate lymphoid cells (ILC3s). IL-22 stimulates various epithelial cells to produce antimicrobial peptides and mucus, thereby both strengthening the epithelial barrier against pathogenic microbes and promoting colonization by beneficial bacteria. Dietary phytochemicals stimulate this process by inhibiting CYP1A1 and causing changes in the composition of the intestinal microbiota. The activity of CYP1A1 can be increased by other microbial products, including the short-chain fatty acids, thereby accelerating clearance of FICZ. In particular, butyrate enhances both the level of the AHR and CYP1A1 activity by stimulating histone acetylation, a process involved in the daily cycle of the FICZ/AHR/CYP1A1 feedback loop. It is now of key interest to examine the potential involvement of FICZ, a major physiological activator of the AHR, in inflammatory disorders and autoimmunity.

The tryptophan derivative 6-formylindolo[3,2-b]carbazole, FICZ, a dynamic mediator of endogenous aryl hydrocarbon receptor signaling, balances cell growth and differentiation.

The aryl hydrocarbon receptor (AHR) is not essential to survival, but does act as a key regulator of many normal physiological events. The role of this receptor in toxicological processes has been studied extensively, primarily employing the high-affinity ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). However, regulation of physiological responses by endogenous AHR ligands remains to be elucidated. Here, we review developments in this field, with a focus on 6-formylindolo[3,2-b]carbazole (FICZ), the endogenous ligand with the highest affinity to the receptor reported to date. The binding of FICZ to different isoforms of the AHR seems to be evolutionarily well conserved and there is a feedback loop that controls AHR activity through metabolic degradation of FICZ via the highly inducible cytochrome P450 1A1. Several investigations provide strong evidence that FICZ plays a critical role in normal physiological processes and can ameliorate immune diseases with remarkable efficiency. Low levels of FICZ are pro-inflammatory, providing resistance to pathogenic bacteria, stimulating the anti-tumor functions, and promoting the differentiation of cancer cells by repressing genes in cancer stem cells. In contrast, at high concentrations FICZ behaves in a manner similar to TCDD, exhibiting toxicity toward fish and bird embryos, immune suppression, and activation of cancer progression. The findings are indicative of a dual role for endogenously activated AHR in barrier tissues, aiding clearance of infections and suppressing immunity to terminate a vicious cycle that might otherwise lead to disease. There is not much support for the AHR ligand-specific immune responses proposed, the differences between FICZ and TCDD in this context appear to be explained by the rapid metabolism of FICZ.

Chemistry and Properties of Indolocarbazoles.

The indolocarbazoles are an important class of nitrogen heterocycles which has evolved significantly in recent years, with numerous studies focusing on their diverse biological effects, or targeting new materials with potential applications in organic electronics. This review aims at providing a broad survey of the chemistry and properties of indolocarbazoles from an interdisciplinary point of view, with particular emphasis on practical synthetic aspects, as well as certain topics which have not been previously accounted for in detail, such as the occurrence, formation, biological activities, and metabolism of indolo[3,2- b]carbazoles. The literature of the past decade forms the basis of the text, which is further supplemented with older key references.

Chromatin remodeling by curcumin alters endogenous aryl hydrocarbon receptor signaling.

The aim of this study was to gain more information about the mechanisms that regulate expression of the aryl hydrocarbon receptor (AHR) target gene CYP1A1. Human hepatoma cells (HepG2 and Huh7) and human immortalized keratinocytes (HaCaT) were treated with different concentrations of the dietary polyphenolic compound curcumin (CUR) alone or in combination with the natural AHR agonist 6-formylindolo[3,2-b]carbazole (FICZ). In an earlier study, we described that CUR can activate the AHR indirectly by inhibiting metabolic clearance of FICZ. Here, we measured cell viability, activation of AHR signaling, oxidative stress and histone modifying activities in response to CUR at concentrations ranging from 0.1 to 50 µM. We observed apparent non-linear responses on cell viability and activation of AHR signaling. The CYP1A1 expression and the CYP1A1 enzyme activity in the presence of CUR reflected the histone acetylation efficiency observed in nuclear extracts. At the lowest concentration, CUR significantly decreased histone deacetylase activity and increased the FICZ-induced CYP1A1 activity. In contrast, at the highest concentration, CUR increased the formation of reactive oxygen species, significantly inhibited histone acetylation, and temporally decreased FICZ-induced CYP1A1 activity. The results suggest that CUR can both increase and decrease the accessibility of DNA and thereby influence transcriptional responses to the ligand-activated AHR. This suggestion was supported by the fact that chromatin remodeling treatments with trichostatin A, p300, or 5-aza-dC increased CYP1A1 transcription. We conclude that the AHR-dependent transcriptional efficiency is modified by factors that influence the cellular redox status and the chromatin structure.

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.

NADPH Oxidase-Dependent Mechanism Explains How Arsenic and Other Oxidants Can Activate Aryl Hydrocarbon Receptor Signaling.

The mechanisms explaining arsenic toxicity are not well understood, but physiological consequences of stimulated aryl hydrocarbon receptor (AHR) signaling both directly and through cross-talk with other pathways have been indicated. The aim of this study was to establish how arsenic interacts with AHR-mediated transcription. The human hepatoma cell line (HepG2-XRE-Luc) carrying a luciferase reporter under the control of two AHR response elements (AHREs) and immortalized human keratinocytes (HaCaT) were exposed to sodium arsenite (NaAsO2; As(3+)), alone or in combination with the endogenous high affinity AHR ligand 6-formylindolo[3,2-b]carbazole (FICZ). Luciferase activity, cytochrome P4501A1 (CYP1A1) activity, oxidative stress-related responses, metabolic clearance of FICZ, and NADPH oxidase (NOX) activity as well as nuclear factor (erythroid-derived 2)-like 2 (Nrf2)-dependent gene expression were measured. Arsenic inhibited CYP1A1 enzyme activity and reduced the metabolic clearance of FICZ. Arsenic also led to activated CYP1A1 transcription but only in cells grown in medium containing trace amounts of the endogenous ligand FICZ, pointing to an indirect mechanism of activation. Initially, arsenic caused dose-dependent inhibition of FICZ-activated AHR signaling, disturbed intracellular GSH status, and increased expression of oxidative stress-related genes. Silencing of NOX4, addition of N-acetylcystein, or pretreatment with arsenic itself attenuated the initial dose-dependent inhibition of AHR signaling. Arsenic pretreatment led to elevated GSH levels and sensitized the cells to ligand-dependent AHR signaling, while silencing of Nrf2 significantly reduced arsenic-mediated activation of the AHR. In addition, influence of NOX on AHR activation was also observed in cells treated with the SH-reactive metals cadmium, mercury, and nickel. Together, the results suggest that SH-reactive agents via a new and possibly general NOX/H2O2-dependent mechanism can interfere with the endogenous regulation of the AHR.

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.

Arsenic, cadmium, mercury and nickel stimulate cell growth via NADPH oxidase activation.

Exposure to metals and metalloids including arsenic, cadmium, mercury, and nickel has been a worldwide health problem for several decades. The aim of this study was to learn how metal-induced oxidative stress triggers cell proliferation, a process of great significance for cancer. NADPH oxidase (NOX) activity and cell proliferation were measured as endpoints in both NOX-deficient and NOX-proficient cells. The X chromosome linked CGD (X-CGD) human promyelocytic leukemia PLB-985 cells lacking gp91phox and the X-CGD cells re-transfected with gp91phox (X-CGD-gp91(phox)) were used together with immortalized human keratinocyte cells (HaCaT). The cells were exposed to different concentrations of the metals alone or together with the NOX inhibitor, diphenyleneiodonium (DPI). We found that the studied metals increased NOX activity. They stimulated cell proliferation in HaCaT and X-CGD-gp91(phox) cells at concentrations below 1µM but not in the X-CGD cells that lack functional NOX. Addition of DPI attenuated the metal-induced cell proliferation. At concentrations above 1µM these metals inhibited cell proliferation. Based on these findings, we propose that many environmental pollutants, including metals and also endogenous NOX-activators such as oxidants and growth factors, interfere with cell growth kinetics by increasing the levels of the diffusible molecule H2O2. Here, we provide evidence that NOXs is central to the mechanism of metal-mediated reactive oxygen species production and stimulation of cell proliferation.

Quercetin, resveratrol, and curcumin are indirect activators of the aryl hydrocarbon receptor (AHR).

Several polyphenols have been shown to activate the aryl hydrocarbon receptor (AHR) in spite of the fact that they bind to the receptor with low affinity. The aim of this study was to investigate whether quercetin (QUE), resveratrol (RES), and curcumin (CUR) interfere with the metabolic degradation of the suggested endogenous AHR ligand 6-formylindolo[3,2-b]carbazole (FICZ) and thereby indirectly activate the AHR. Using recombinant human enzyme, we confirmed earlier reported inhibitory effects of the polyphenols on cytochrome P4501A1 (CYP1A1) activity, and inhibition of metabolic clearance of FICZ was documented in FICZ-treated immortalized human keratinocytes (HaCaT). CYP1A1 activity was induced in HaCaT cells by all three compounds, and when they were added together with FICZ, a prolonged activation was observed after a dose-dependent inhibition period. The same pattern of responses was seen at the transcriptional level as determined with a CYP1A1 reporter assay in human liver hepatoma (HepG2) cells. To test the ability of the polyphenols to activate the AHR in the absence of FICZ, the cells were treated in medium, which in contrast to commercial batches of medium did not contain background levels of FICZ. Importantly, AHR activation was only observed in the commercial medium. Taken together, these findings suggest that QUE, RES, and CUR induce CYP1A1 in an indirect manner by inhibiting the metabolic turnover of FICZ. Humans are exposed to these compounds through the diet and nutritional supplements, and we propose that altered systemic levels of FICZ caused by such compounds may have physiological consequences.

Inhibition of cytochrome P4501-dependent clearance of the endogenous agonist FICZ as a mechanism for activation of the aryl hydrocarbon receptor.

Altered systemic levels of 6-formylindolo[3,2-b]carbazole (FICZ), an enigmatic endogenous ligand for the aryl hydrocarbon receptor (AHR), may explain adverse physiological responses evoked by small natural and anthropogenic molecules as well as by oxidative stress and light. We demonstrate here that several different chemical compounds can inhibit the metabolism of FICZ, thereby disrupting the autoregulatory feedback control of cytochrome P4501 systems and other proteins whose expression is regulated by AHR. FICZ is both the most tightly bound endogenous agonist for the AHR and an ideal substrate for cytochrome CYP1A1/1A2 and 1B1, thereby also participating in an autoregulatory loop that keeps its own steady-state concentration low. At very low concentrations FICZ influences circadian rhythms, responses to UV light, homeostasis associated with pro- and anti-inflammatory processes, and genomic stability. Here, we demonstrate that, if its metabolic clearance is compromised, femtomolar background levels of this compound in cell-culture medium are sufficient to up-regulate CYP1A1 mRNA and enzyme activity. The oxidants UVB irradiation and hydrogen peroxide and the model AHR antagonist 3'-methoxy-4'-nitroflavone all inhibited induction of CYP1A1 enzyme activity by FICZ or 2,3,7,8-tetrachlorodibenzo-p-dioxin, thereby subsequently elevating intracellular levels of FICZ and activating AHR. Taken together, these findings support an indirect mechanism of AHR activation, indicating that AHR activation by molecules with low affinity actually may reflect inhibition of FICZ metabolism and raising questions about the reported promiscuity of the AHR. Accordingly, we propose that prolonged induction of AHR activity through inhibition of CYP1 disturbs feedback regulation of FICZ levels, with potential detrimental consequences.

The aryl hydrocarbon receptor mediates UVB radiation-induced skin tanning.

Melanogenesis is the vital response to protect skin cells against UVB-induced DNA damage. Melanin is produced by melanocytes, which transfer it to surrounding keratinocytes. Recently, we have shown that the aryl hydrocarbon receptor (AhR) is part of the UVB-stress response in epidermal keratinocytes. UVB triggers AhR signaling by generating the AhR ligand 6-formylindolo(3,2-b)carbazole from tryptophan. We show here that normal murine melanocytes express functional AhR. Using standard UVB tanning protocols, AhR-deficient mice were shown to tan significantly weaker than wild-type mice; in these mice, tyrosinase activity in the epidermis was lower as well. Tanning responses and tyrosinase activity, however, were normal in keratinocyte-specific conditional AhR knockout mice, indicating that release of melanogenic keratinocyte factors is unaffected by the UVB-AhR signaling pathway and that the diminished tanning response in AhR(-/-) mice is confined to the level of melanocytes. Accordingly, the number of dihydroxyphenylalanin-positive melanocytes increased significantly less on UVB irradiation in AhR(-/-) mice than in wild-type mice. This difference in melanocyte number was associated with a significantly reduced expression of stem cell factor-1 and c-kit in melanocytes of AhR(-/-) mice. Thus, the environmental signal sensor AhR links solar UVB radiation to skin pigmentation.

The aryl hydrocarbon receptor (AHR), a novel regulator of human melanogenesis.

Skin cancer, chloracne and hyperpigmentation have been associated with the exposure to environmental contaminants such as polychlorinated biphenyls, dioxins or polycyclic aromatic hydrocarbons. These compounds are xenobiotic high-affinity ligands for the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor with important physiological roles in, for example, the control of cell proliferation and inflammation. We show here that exposure of normal human melanocytes to the most potent dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), results in activation of the AHR signaling pathway and an AHR-dependent induction of tyrosinase activity, the key enzyme of the melanogenic pathway. In accordance with the upregulation of tyrosinase enzyme activity, total melanin content was also elevated in TCDD-exposed melanocytes. Neither the induction of tyrosinase enzyme activity or of total melanin could be attributed to enhanced cell proliferation, but was rather due to the induction of tyrosinase and tyrosinase-related protein 2 gene expression. Thus, the AHR is able to modulate melanogenesis by controlling the expression of melanogenic genes.

The GSTP1 Ile105 Val polymorphism modifies the metabolism of toluene di-isocyanate.

BACKGROUND: Toluene di-isocyanate (TDI) is widely used in the production of polyurethane foams and paints. As TDI causes respiratory disease in only a fraction of exposed workers, genetic factors may play a key role in disease susceptibility. Polymorphisms in TDI metabolising genes may affect elimination kinetics, resulting in differences in body retention, and in its turn differences in adverse effects. OBJECTIVES: To analyze how genotype modifies the associations between (i) TDI in air (2,4-TDI and 2,6-TDI) and its metabolites toluene diamine (TDA; 2,4-TDA and 2,6-TDA) in hydrolyzed urine; and (ii) 2,4-TDA and 2,6-TDA in hydrolyzed plasma and 2,4-TDA and 2,6-TDA in urine. METHODS: Workers exposed to TDI were analyzed for 2,4-TDI and 2,6-TDI in air (N=70), 2,4-TDA and 2,6-TDA in hydrolyzed urine (N=124) and in plasma (N=128), and genotype: CYP1A1*2A, CYP1A1*2B, GSTA1-52, GSTM1O, GSTM3B, GSTP1 I105V, GSTP1 A114V, GSTT1O, MPO-463, NAT1*3, *4, *10, *11, *14, *15, NAT2*5, *6, *7, and SULT1A1 R213H. RESULTS: GSTP1 105 strongly modified the relationship between 2,4-TDA in plasma and in urine: ValVal carriers had about twice as steep regression slope than IleIle carriers. A similar pattern was found for 2,6-TDA. CYP1A1*2A, GSTM1, GSTP1, GSTT1, and MPO possibly influenced the relationship between TDA in plasma and urine. CONCLUSION: Our results show, for the first time, genetic modification on the human TDI metabolism. The findings suggest that GSTP1 genotype should be considered when evaluating biomarkers of TDI exposure in urine and plasma. Moreover, the results support earlier findings of GSTP1 105 Val as protective against TDI-related asthma.

The aryl hydrocarbon receptor in immunity.

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

Short-term exposure to dioxin impairs bone tissue in male rats.

Chronic and sub-chronic studies in rats have previously shown that dioxin-like compounds impair the bone tissue homeostasis. In the present study, tibiae and serum were analyzed to study possible effects of short term dioxin exposure on rats. Two month old (ca. 200g) male rats were injected with 50microg 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) kg(-1) bw and tibiae were excised 5d following the exposure. Bone composition, dimensions and strength were analyzed by pQCT and three-point bending test on tibiae. In addition, detailed bone composition was analyzed by optical emission spectroscopy (ICP-OES) and Fourier transform infrared spectrometry (FTIR). Analysis of the serum bone biomarkers procollagen type-I N-terminal propeptide (PINP) and carboxyterminal cross linking teleopeptide (CTX) were also performed. pQCT-results showed alterations in the metaphysis, with a significant decrease in trabecular bone cross-sectional area (-19%, p<0.05) and a significant increase in total bone mineral density (+7%, p<0.05) in TCDD-exposed rats. Analyses of the bones by ICP-OES and FTIR showed that bones from exposed rats had a higher relative proportion of crystalline phosphate (+13% for a1080 and +11% for a1113, p<0.05) and lower acid phosphate content (-22% for a1145, p<0.05), resembling the composition of more mature bones. Serum analysis showed that the bone formation marker PINP was decreased (-37%, p<0.05) and that the bone resorption marker CTX was increased (+14%, p<0.05) indicating a net loss of bone tissue. In conclusion, 5d of exposure to TCDD was sufficient to negatively affect bone tissue in male rats.

The suggested physiologic aryl hydrocarbon receptor activator and cytochrome P4501 substrate 6-formylindolo[3,2-b]carbazole is present in humans.

Dioxins and other polycyclic aromatic compounds formed during the combustion of waste and fossil fuels represent a risk to human health, as well as to the well being of our environment. Compounds of this nature exert carcinogenic and endocrine-disrupting effects in experimental animals by binding to the orphan aryl hydrocarbon receptor (AhR). Understanding the mechanism of action of these pollutants, as well as the physiological role(s) of the AhR, requires identification of the endogenous ligand(s) of this receptor. We reported earlier that activation of AhR by ultraviolet radiation is mediated by the chromophoric amino acid tryptophan (Trp), and we suggested that a new class of compounds derived from Trp, in particular 6-formylindolo[3,2-b]carbazole (FICZ), acts as natural high affinity ligands for this receptor. Here we describe seven new FICZ-derived indolo[3,2-b]carbazole-6-carboxylic acid metabolites and two sulfoconjugates, and we demonstrate the following. (i) FICZ is formed efficiently by photolysis of Trp upon exposure to visible light. (ii) FICZ is an exceptionally good substrate for cytochromes P450 (CYP) 1A1, 1A2, and 1B1, and its hydroxylated metabolites are remarkably good substrates for the sulfotransferases (SULTs) 1A1, 1A2, 1B1, and 1E1. Finally, (iii) sulfoconjugates of phenolic metabolites of FICZ are present in human urine. Our findings indicate that formylindolo[3,2-b]carbazols are the most potent naturally occurring activators of the AhR signaling pathway and may be the key substrates of the CYP1 and SULT1 families of enzymes. These conclusions contradict the widespread view that xenobiotic compounds are the major AhR ligands and CYP1 substrates.

Influence of genetic factors on toluene diisocyanate-related symptoms: evidence from a cross-sectional study.

BACKGROUND: Toluene diisocyanate (TDI) is a highly reactive compound used in the production of, e.g., polyurethane foams and paints. TDI is known to cause respiratory symptoms and diseases. Because TDI causes symptoms in only a fraction of exposed workers, genetic factors may play a key role in disease susceptibility. METHODS: Workers (N = 132) exposed to TDI and a non-exposed group (N = 114) were analyzed for genotype (metabolising genes: CYP1A1*2A, CYP1A1*2B, GSTM1*O, GSTM3*B, GSTP1 I105V, GSTP1 A114V, GSTT1*O, MPO -463, NAT1*3, *4, *10, *11, *14, *15, NAT2*5, *6, *7, SULT1A1 R213H; immune-related genes: CCL5 -403, HLA-DQB1*05, TNF -308, TNF -863) and symptoms of the eyes, upper and lower airways (based on structured interviews). RESULTS: For three polymorphisms: CYP1A1*2A, CYP1A1*2B, and TNF -308 there was a pattern consistent with interaction between genotype and TDI exposure status for the majority of symptoms investigated, although it did reach statistical significance only for some symptoms: among TDI-exposed workers, the CYP1A1 variant carriers had increased risk (CYP1A1*2A and eye symptoms: variant carriers OR 2.0 95% CI 0.68-6.1, p-value for interaction 0.048; CYP1A1*2B and wheeze: IV carriers OR = 12, 1.4-110, p-value for interaction 0.057). TDI-exposed individuals with TNF-308 A were protected against the majority of symptoms, but it did not reach statistical significance. In the non-exposed group, however, TNF -308 A carriers showed higher risk of the majority of symptoms (eye symptoms: variant carriers OR = 2.8, 1.1-7.1, p-value for interaction 0.12; dry cough OR = 2.2, 0.69-7.2, p-value for interaction 0.036). Individuals with SULT1A1 213H had reduced risk both in the exposed and non-exposed groups. Other polymorphisms, showed associations to certain symptoms: among TDI-exposed,NAT1*10 carriers had a higher risk of eye symptoms and CCL5 -403 AG+AA as well as HLA-DQB1 *05 carriers displayed increased risk of symptoms of the lower airways. GSTM1, GSTM3 and GSTP1 only displayed effects on symptoms of the lower airways in the non-exposed group. CONCLUSION: Specific gene-TDI interactions for symptoms of the eyes and lower airways appear to exist. The results suggest different mechanisms for TDI- and non-TDI-related symptoms of the eyes and lower airways.

Lightening up the UV response by identification of the arylhydrocarbon receptor as a cytoplasmatic target for ultraviolet B radiation.

UVB radiation-induced signaling in mammalian cells involves two major pathways: one that is initiated through the generation of DNA photoproducts in the nucleus and a second one that occurs independently of DNA damage and is characterized by cell surface receptor activation. The chromophore for the latter one has been unknown. Here, we report that the UVB response involves tryptophan as a chromophore. We show that through the intracellular generation of photoproducts, such as the arylhydrocarbon receptor (AhR) ligand 6-formylindolo[3,2-b]carbazole, signaling events are initiated, which are transferred to the nucleus and the cell membrane via activation of the cytoplasmatic AhR. Specifically, AhR activation by UVB leads to (i) transcriptional induction of cytochrome P450 1A1 and (ii) EGF receptor internalization with activation of the EGF receptor downstream target ERK1/2 and subsequent induction of cyclooxygenase-2. The role of the AhR in the UVB stress response was confirmed in vivo by studies employing AhR KO mice.