Sunlight Generates Multiple Tryptophan Photoproducts Eliciting High Efficacy CYP1A Induction in Chick Hepatocytes and In Vivo

Rosmarinus officinalis L. Leaf Extracts and Their Metabolites Inhibit the Aryl Hydrocarbon Receptor (AhR) Activation In Vitro and in Human Keratinocytes: Potential Impact on Inflammatory Skin Diseases and Skin Cancer

Aryl hydrocarbon receptor (AhR) activation by environmental agents and microbial metabolites is potentially implicated in a series of skin diseases. Hence, it would be very important to identify natural compounds that could inhibit the AhR activation by ligands of microbial origin as 6-formylindolo[3,2-b]carbazole (FICZ), indirubin (IND) and pityriazepin (PZ) or the prototype ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Five different dry Rosmarinus officinalis L. extracts (ROEs) were assayed for their activities as antagonists of AhR ligand binding with guinea pig cytosol in the presence of [³H]TCDD. The methanolic ROE was further assayed towards CYP1A1 mRNA induction using RT-PCR in human keratinocytes against TCDD, FICZ, PZ, and IND. The isolated metabolites, carnosic acid, carnosol, 7-O-methyl-epi-rosmanol, 4′,7-O-dimethylapigenin, and betulinic acid, were assayed for their agonist and antagonist activity in the presence and absence of TCDD using the gel retardation assay (GRA). All assayed ROE extracts showed similar dose-dependent activities with almost complete inhibition of AhR activation by TCDD at 100 ppm. The methanol ROE at 10 ppm showed 99%, 50%, 90%, and 85% inhibition against TCDD, FICZ, IND, and PZ, respectively, in human keratinocytes. Most assayed metabolites exhibited dose-dependent antagonist activity. ROEs inhibit AhR activation by TCDD and by the Malassezia metabolites FICZ, PZ, and IND. Hence, ROE could be useful for the prevention or treatment of skin diseases mediated by activation of AhR.

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.

Roles of the ubiquitin ligase CUL4B and ADP-ribosyltransferase TiPARP in TCDD-induced nuclear export and proteasomal degradation of the transcription factor AHR

The aryl hydrocarbon receptor (AHR) is a transcription factor activated by exogenous halogenated polycyclic aromatic hydrocarbon compounds, including the environmental toxin TCDD, 2,3,7,8-tetrachlorodibenzo-p-dioxin, and naturally occurring dietary and endogenous compounds. The activated AHR enhances transcription of specific genes including phase I and phase II metabolism enzymes and other targets genes such as the TCDD-inducible poly(ADP-ribose) polymerase (TiPARP). The regulation of AHR activation is a dynamic process: immediately after transcriptional activation of the AHR by TCDD, the AHR is exported from the nucleus to the cytoplasm where it is subjected to proteasomal degradation. However, the mechanisms regulating AHR degradation are not well understood. Here, we studied the role of two enzymes reported to enhance AHR breakdown: the cullin 4B (CUL4B)^AHR complex, an E3 ubiquitin ligase that targets the AHR and other proteins for ubiquitination, and TiPARP, which targets proteins for ADP-ribosylation, a posttranslational modification that can increase susceptibility to degradation. Using a WT mouse embryonic fibroblast (MEF) cell line and an MEF cell line in which CUL4B has been deleted (MEF^Cul4b-null), we discovered that loss of CUL4B partially prevented AHR degradation after TCDD exposure, while knocking down TiPARP in MEF^Cul4b-null cells completely abolished AHR degradation upon TCDD treatment. Increased TCDD-activated AHR protein levels in MEF^Cul4b-null and MEF^Cul4b-null cells in which TiPARP was knocked down led to enhanced AHR transcriptional activity, indicating that CUL4B and TiPARP restrain AHR action. This study reveals a novel function of TiPARP in controlling TCDD-activated AHR nuclear export and subsequent proteasomal degradation.

Organ-specific effects on glycolysis by the dioxin-activated aryl hydrocarbon receptor

Activation of the aryl hydrocarbon receptor (AHR) by the environmental toxin dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin, TCDD) causes diverse toxicities, including thymus atrophy and hepatosteatosis. The mechanisms by which AHR activation by TCDD leads to these toxicities are not fully understood. Here we studied the effects of TCDD on a major energy pathway, glycolysis, using the chick embryo close to hatching, a well-established model for studying dioxin toxicity. We showed that 24 hr of TCDD treatment causes changes in glycolysis in both thymus and liver. In thymus glands, TCDD decreased mRNAs for glycolytic genes and glucose transporters, glycolytic indices and levels of IL7 mRNA, phosphorylated AKT (pAKT) and HIF1A, stimulators of glycolysis and promoters of survival and proliferation of thymic lymphocytes. In contrast, in liver, TCDD increased mRNA levels for glycolytic genes and glucose transporters, glycolytic endpoints and pAKT levels. Similarly, increases by TCDD in mRNA levels for glycolytic genes and glucose transporters in human primary hepatocytes showed that effects in chick embryo liver pertain also to human cells. Treatment with the glycolytic inhibitor 2-deoxy-d-glucose exacerbated the effects on thymus atrophy by TCDD, supporting a role for decreased glycolysis in thymus atrophy by TCDD, but did not prevent hepatosteatosis. NAD⁺ precursors abolished TCDD effects on glycolytic endpoints in both thymus and liver. In summary, we report here that dioxin disrupts glycolysis mediated energy metabolism in both thymus and liver, and that it does so in opposite ways, decreasing it in the thymus and increasing it in the liver. Further, the findings support NAD⁺ boosting as a strategy against metabolic effects of environmental pollutants such as dioxins.

Reactivity and degradation products of tryptophan in solution and proteins

Tryptophan is one of the essential mammalian amino acids and is thus a required component in human nutrition, animal feeds, and cell culture media. However, this aromatic amino acid is highly susceptible to oxidation and is known to degrade into multiple products during manufacturing, storage, and processing. Many physical and chemical processes contribute to the degradation of this compound, primarily via oxidation or cleavage of the highly reactive indole ring. The central contributing factors are reactive oxygen species, such as singlet oxygen, hydrogen peroxide, and hydroxyl radicals; light and photosensitizers; metals; and heat. In a multi-component mixture, tryptophan also commonly reacts with carbonyl-containing compounds, leading to a wide variety of products. The purpose of this review is to summarize the current state of knowledge regarding the degradation and interaction products of tryptophan in complex liquid solutions and in proteins. For the purposes of context, a brief summary of the key pathways in tryptophan metabolism will be included, along with common methods and issues in tryptophan manufacturing. The review will focus on the conditions that lead to tryptophan degradation, the products generated in these processes, their known biological effects, and methods which may be applied to stabilize the amino acid.

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.

Toxicity and DNA repair in normal human keratinocytes co-exposed to benzo[a]pyrene and sunlight

Skin has the potential to be exposed to both solar UV radiation and polycyclic aromatic hydrocarbons, especially in occupational environments. In the present work, we investigated how benzo[a]pyrene (B[a]P) modulates cellular phototoxicity and impacts formation and repair of pyrimidine dimers induced by simulated sunlight (SSL) in normal human keratinocytes (NHK). We were especially interested in determining whether the aryl hydrocarbon receptor (AhR) was involved since it was recently shown to negatively impact repair. Addition of 1 μM B[a]P after exposure to 2 minimal erythemal doses of SSL had little impact on NHK. The inverse protocol involving incubation with B[a]P followed by irradiation led to a strong increase in phototoxicity. Repair of DNA photoproducts was drastically impaired. Using agonists and antagonists of AhR allowed us to conclude that this factor was not involved in these results. Observation of a strong increase in the level of the oxidative marker 8-oxo-7,8-dihydroguanine in the protocol involving B[a]P treatment followed by exposure to SSL strongly suggested that a photosensitized oxidative stress was responsible for cell death and inhibition of DNA repair. Accordingly, both adverse effects were diminished with a lower concentration of B[a]P and a lower SSL dose, leading to less oxidative stress.

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.

6-Formylindolo[3,2-b]carbazole (FICZ) is a Very Minor Photoproduct of Tryptophan at Biologically Relevant Doses of UVB and Simulated Sunlight

Exposure to solar UV is at the origin of numerous photodegradation pathways in biomolecules. Tryptophan is readily modified by UVB radiation into ring-opened and oxidized photoproducts. One of them, 6-formylindolo[3,2-b]carbazole (FICZ), has been extensively studied in the recent years because it very efficiently binds to AhR, a major factor in numerous biologic processes, such as metabolism of xenobiotics. Unfortunately, little information is available on the actual yield of FICZ upon exposure to low and biologically relevant doses of UV radiation. In the present work, we used a sensitive and specific HPLC-tandem mass spectrometry assay to quantify a series of photoproducts induced by UVB and simulated sunlight (SSL) in solutions of tryptophan. FICZ represented only a minute amount of the photoproducts (0.02 and 0.03%, respectively). Experiments were repeated in culture medium where the yield of FICZ was also found to be very low, even when Trp was added. Last, no FICZ could be detected in cytosolic fractions of cultured cells exposed to SSL. Altogether, the present results show that FICZ is a very minor photoproduct and that it cannot be considered the only endogenous photoproduct responsible for the induction of AhR-dependent responses in UV-irradiated cells.

Mechanisms and Mitochondrial Redox Signaling in Photobiomodulation

Photobiomodulation (PBM) involves the use of red or near-infrared light at low power densities to produce a beneficial effect on cells or tissues. PBM therapy is used to reduce pain, inflammation, edema, and to regenerate damaged tissues such as wounds, bones, and tendons. The primary site of light absorption in mammalian cells has been identified as the mitochondria and, more specifically, cytochrome c oxidase (CCO). It is hypothesized that inhibitory nitric oxide can be dissociated from CCO, thus restoring electron transport and increasing mitochondrial membrane potential. Another mechanism involves activation of light or heat-gated ion channels. This review will cover the redox signaling that occurs in PBM and examine the difference between healthy and stressed cells, where PBM can have apparently opposite effects. PBM has a marked effect on stem cells, and this is proposed to operate via mitochondrial redox signaling. PBM can act as a preconditioning regimen and can interact with exercise on muscles.

Aryl hydrocarbon receptor signaling modulates antiviral immune responses: ligand metabolism rather than chemical source is the stronger predictor of outcome

The aryl hydrocarbon receptor (AHR) offers a compelling target to modulate the immune system. AHR agonists alter adaptive immune responses, but the consequences differ across studies. We report here the comparison of four agents representing different sources of AHR ligands in mice infected with influenza A virus (IAV): TCDD, prototype exogenous AHR agonist; PCB126, pollutant with documented human exposure; ITE, novel pharmaceutical; and FICZ, degradation product of tryptophan. All four compounds diminished virus-specific IgM levels and increased the proportion of regulatory T cells. TCDD, PCB126 and ITE, but not FICZ, reduced virus-specific IgG levels and CD8⁺ T cell responses. Similarly, ITE, PCB126, and TCDD reduced Th1 and Tfh cells, whereas FICZ increased their frequency. In Cyp1a1-deficient mice, all compounds, including FICZ, reduced the response to IAV. Conditional Ahr knockout mice revealed that all four compounds require AHR within hematopoietic cells. Thus, differences in the immune response to IAV likely reflect variances in quality, magnitude, and duration of AHR signaling. This indicates that binding affinity and metabolism may be stronger predictors of immune effects than a compound’s source of origin, and that harnessing AHR will require finding a balance between dampening immune-mediated pathologies and maintaining sufficient host defenses against infection.