2,3,7,8-Tetrachlorodibenzo-p-dioxin increases the expression of genes in the human epidermal differentiation complex and accelerates epidermal barrier formation

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.

Allelic variants of the aryl hydrocarbon receptor differentially influence UVB-mediated skin inflammatory responses in SKH1 mice

The mouse strain SKH1 is widely used in skin research due to its hairless phenotype and intact immune system. Due to the complex nature of aryl hydrocarbon receptor (AHR) function in the skin, the development of additional in vivo models is necessary to study its role in cutaneous homeostasis and pathology. Variants of the Ah allele, exist among different mouse strains. The Ah^b-1 and Ah^d alleles express high and low affinity ligand binding forms of the AHR, respectively. The outbred SKH1 mice express the Ah^b-2 and/or Ah^d alleles. SKH1 mice were crossed with C57BL/6J mice, which harbor the Ah^b-1 allele, to create useful models for studying endogenous AHR function. SKH1 mice were bred to be homozygous for either the Ah^b-1 or Ah^d allele to establish strains for use in comparative studies of the effects of differential ligand-mediated activation through gene expression changes upon UVB exposure. Ah^b-1 or Ah^d allelic status was confirmed by DNA sequence analysis. We tested the hypothesis that SKH1-Ah^b-1 mice would display enhanced inflammatory signaling upon UVB exposure compared to SKH1-Ah^d mice. Differential basal AHR activation between the strains was determined by assessing Cyp1a1 expression levels in the small intestine, liver, and skin of the SKH1-Ah^b-1 mice compared to SKH1-Ah^d mice. To determine whether SKH1-Ah^b-1 mice are more prone to a pro-inflammatory phenotype in response to UVB, gene expression of inflammatory mediators was analyzed. SKH1-Ah^b-1 mice expressed enhanced gene expression of the chemotactic factors Cxcl5, Cxcl1, and Ccl20, as well as the inflammatory signaling factors S100a9 and Ptgs2, compared to SKH1-Ah^d mice in skin. These data supports a role for AHR activation and enhanced inflammatory signaling in skin.

Editor's Highlight: Ah Receptor Activation Potentiates Neutrophil Chemoattractant (C-X-C Motif) Ligand 5 Expression in Keratinocytes and Skin

Chemokines are components of the skin microenvironment, which enable immune cell chemotaxis. Traditionally, transcription factors involved in inflammatory signaling (eg, NFκB) are important mediators of chemokine expression. To what extent xenobiotics and their associated receptors control chemokine expression is poorly understood. The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor known to mediate physiological responses in the skin through the regulation of genes involved in xenobiotic metabolism, epidermal differentiation, and immunity. Here, we demonstrate that AHR activation within primary mouse keratinocytes regulates the expression of a neutrophil directing chemokine (C-X-C motif) ligand 5 (Cxcl5). AHR-mediated regulation of Cxcl5 is because of direct transcriptional activity upon treatment with AHR agonists such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Additionally, AHR mediates enhanced induction of Cxcl5 upon exposure to an agonist and the inflammatory cytokine interleukin 1 beta. This synergy is confined primarily to keratinocytes, as dermal fibroblasts did not achieve the same level of combinatorial induction. AHR-specific antagonists were able to reduce basal and induced levels of Cxcl5, demonstrating the potential for pharmacological intervention. Exposure of C57BL/6 J mice to ultraviolet (UV) light followed by topical treatment with the AHR agonist formylindolo(3,2-b)carbazole (FICZ) significantly induced Cxcl5 expression in skin compared with UV alone, and this response was absent in Ahr-/- mice. These results establish AHR as an important mediator of Cxcl5, with implications for the treatment of inflammatory skin diseases.

Antioxidant Artemisia princeps Extract Enhances the Expression of Filaggrin and Loricrin via the AHR/OVOL1 Pathway

The Japanese mugwort, Artemisia princeps (yomogi in Japanese), has anti-inflammatory and antioxidant effects. Skin care products containing Artemisia princeps extract (APE) are known to improve dry skin symptoms in atopic dermatitis. Atopic dry skin is associated with a marked reduction of skin barrier proteins, such as filaggrin (FLG) and loricrin (LOR). Recently, aryl hydrocarbon receptor (AHR), and its downstream transcription factor OVO-like 1 (OVOL1), have been shown to regulate the gene expression of FLG and LOR. The focus of this paper is to evaluate the effects of APE on the AHR/OVOL1/FLG or LOR pathway since they have remained unknown to this point. We first demonstrated that non-cytotoxic concentrations of APE significantly upregulated antioxidant enzymes, NAD(P)H dehydrogenase quinone 1 and heme oxygenase 1, in human keratinocytes. Even at these low concentrations, APE induced nuclear translocation of AHR and significantly upregulated CYP1A1 (a specific target gene for AHR activation), FLG, and LOR expression. AHR knockdown downregulated OVOL1 expression. The APE-induced upregulation of FLG and LOR was canceled in keratinocytes with AHR or OVOL1 knockdown. In conclusion, antioxidant APE is a potent phytoextract that upregulates FLG and LOR expression in an AHR/OVOL1-dependent manner and this may underpin the barrier-repairing effects of APE in treating atopic dry skin.

Endocrine-disrupting chemicals and skin manifestations

Endocrine-disrupting chemicals (EDCs) are exogenous compounds that have the ability to disrupt the production and actions of hormones through direct or indirect interaction with hormone receptors, thus acting as agonists or antagonists. Human health is affected after either individual occupation or dietary and environmental exposure to EDCs. On the other hand, skin is one of the largest organs of the body and its main function is protection from noxious substances. EDCs perturb the endocrine system, and they are also carcinogenic, immunotoxic, and hepatotoxic to human skin. In addition, their effects on keratinocytes, melanocytes, sebocytes, inflammatory and immunological cells, and skin stem cells produce inflammatory and allergic skin diseases, chloracne, disorders of skin pigmentation, skin cancer, and skin aging. Mechanisms, which EDCs use to induce these skin disorders are complicated, and involve the interference of endogenous hormones and most importantly the activation of the aryl hydrocarbon receptor signal pathway. Further studies on EDCs and skin diseases are necessary to elucidate these mechanisms.

New Trends in Aryl Hydrocarbon Receptor Biology

Traditionally considered as a critical intermediate in the toxic and carcinogenic response to dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin, TCDD), the Aryl hydrocarbon/Dioxin receptor (AhR) has proven to be also an important regulator of cell physiology and organ homeostasis. AhR has become an interesting and actual area of research mainly boosted by a significant number of recent studies analyzing its contribution to the proper functioning of the immune, hepatic, cardiovascular, vascular and reproductive systems. At the cellular level, AhR establishes functional interactions with signaling pathways governing cell proliferation and cell cycle, cell morphology, cell adhesion and cell migration. Two exciting new aspects in AhR biology deal with its implication in the control of cell differentiation and its more than likely involvement in cell pluripotency and stemness. In fact, it is possible that AhR could help modulate the balance between differentiation and pluripotency in normal and transformed tumor cells. At the molecular level, AhR regulates an increasingly large array of physiologically relevant genes either by traditional transcription-dependent mechanisms or by unforeseen processes involving genomic insulators, chromatin dynamics and the transcription of mobile genetic elements. AhR is also closely related to epigenetics, not only from the point of view of target gene expression but also with respect to its own regulation by promoter methylation. It is reasonable to consider that deregulation of these many functions could have a causative role, or at least contribute to, human disease. Consequently, several laboratories have proposed that AhR could be a valuable tool as diagnostic marker and/or therapeutic target in human pathologies. An additional point of interest is the possibility of regulating AhR activity by endogenous non-toxic low weight molecules agonist or antagonist molecules that could be present or included in the diet. In this review, we will address these molecular and functional features of AhR biology within physiological and pathological contexts.

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.

Gene regulation of filaggrin and other skin barrier proteins via aryl hydrocarbon receptor

Aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor that binds to structurally diverse chemicals including dioxins, coal tar, flavonoids and tryptophan photoproducts. Upon ligation, cytoplasmic AHR translocates to the nucleus, heterodimerizes with aryl hydrocarbon receptor nuclear translocator and mediates numerous biological effects by inducing the transcription of various AHR-responsive genes such as epidermal barrier proteins. The activation of AHR usually generates oxidative stress. However, AHR also mediates antioxidant signaling by a plethora of ligands via nuclear factor-erythroid 2-related factor-2. Both oxidative and antioxidant ligands upregulate the expression of the filaggrin gene. We review the role of AHR signaling in the gene regulation of epidermal barrier proteins.

The adverse outcome pathway for rodent liver tumor promotion by sustained activation of the aryl hydrocarbon receptor

An Adverse Outcome Pathway (AOP) represents the existing knowledge of a biological pathway leading from initial molecular interactions of a toxicant and progressing through a series of key events (KEs), culminating with an apical adverse outcome (AO) that has to be of regulatory relevance. An AOP based on the mode of action (MOA) of rodent liver tumor promotion by dioxin-like compounds (DLCs) has been developed and the weight of evidence (WoE) of key event relationships (KERs) evaluated using evolved Bradford Hill considerations. Dioxins and DLCs are potent aryl hydrocarbon receptor (AHR) ligands that cause a range of species-specific adverse outcomes. The occurrence of KEs is necessary for inducing downstream biological responses and KEs may occur at the molecular, cellular, tissue and organ levels. The common convention is that an AOP begins with the toxicant interaction with a biological response element; for this AOP, this initial event is binding of a DLC ligand to the AHR. Data from mechanistic studies, lifetime bioassays and approximately thirty initiation-promotion studies have established dioxin and DLCs as rat liver tumor promoters. Such studies clearly show that sustained AHR activation, weeks or months in duration, is necessary to induce rodent liver tumor promotion--hence, sustained AHR activation is deemed the molecular initiating event (MIE). After this MIE, subsequent KEs are 1) changes in cellular growth homeostasis likely associated with expression changes in a number of genes and observed as development of hepatic foci and decreases in apoptosis within foci; 2) extensive liver toxicity observed as the constellation of effects called toxic hepatopathy; 3) cellular proliferation and hyperplasia in several hepatic cell types. This progression of KEs culminates in the AO, the development of hepatocellular adenomas and carcinomas and cholangiolar carcinomas. A rich data set provides both qualitative and quantitative knowledge of the progression of this AOP through KEs and the KERs. Thus, the WoE for this AOP is judged to be strong. Species-specific effects of dioxins and DLCs are well known--humans are less responsive than rodents and rodent species differ in sensitivity between strains. Consequently, application of this AOP to evaluate potential human health risks must take these differences into account.

Genetic and pharmacological analysis identifies a physiological role for the AHR in epidermal differentiation

Stimulation of the aryl hydrocarbon receptor (AHR) by xenobiotics is known to affect epidermal differentiation and skin barrier formation. The physiological role of endogenous AHR signaling in keratinocyte differentiation is not known. We used murine and human skin models to address the hypothesis that AHR activation is required for normal keratinocyte differentiation. Using transcriptome analysis of Ahr(-/-) and Ahr(+/+) murine keratinocytes, we found significant enrichment of differentially expressed genes linked to epidermal differentiation. Primary Ahr(-/-) keratinocytes showed a significant reduction in terminal differentiation gene and protein expression, similar to Ahr(+/+) keratinocytes treated with AHR antagonists GNF351 and CH223191, or the selective AHR modulator (SAhRM) SGA360. In vitro keratinocyte differentiation led to increased AHR levels and subsequent nuclear translocation, followed by induced CYP1A1 gene expression. Monolayer cultured primary human keratinocytes treated with AHR antagonists also showed an impaired terminal differentiation program. Inactivation of AHR activity during human skin equivalent development severely impaired epidermal stratification, terminal differentiation protein expression, and stratum corneum formation. As disturbed epidermal differentiation is a main feature of many skin diseases, pharmacological agents targeting AHR signaling or future identification of endogenous keratinocyte-derived AHR ligands should be considered as potential new drugs in dermatology.

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.

The aryl hydrocarbon receptor: multitasking in the immune system

The aryl hydrocarbon receptor (AhR), for many years almost exclusively studied by the pharmacology/toxicology field for its role in mediating the toxicity of xenobiotics such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), has more recently attracted the attention of immunologists. The evolutionary conservation of this transcription factor and its widespread expression in the immune system point to important physiological functions that are slowly being unraveled. In particular, the emphasis is now shifting from the role of AhR in the xenobiotic pathway toward its mode of action in response to physiological ligands. In this article, we review the current understanding of the molecular interactions and functions of AhR in the immune system in steady state and in the presence of infection and inflammation, with a focus on barrier organs such as the skin, the gut, and the lung.

Effects of in utero exposure of C57BL/6J mice to 2,3,7,8-tetrachlorodibenzo-p-dioxin on epidermal permeability barrier development and function

BACKGROUND

Development of the epidermal permeability barrier (EPB) is essential for neonatal life. Defects in this barrier are found in many skin diseases such as atopic dermatitis.

OBJECTIVE

We investigated the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the development and function of the EPB.

METHODS

Timed-pregnant C57BL/6J mice were gavaged with corn oil or TCDD (10 μg/kg body weight) on gestation day 12. Embryos were harvested on embryonic day (E) 15, E16, E17, and postnatal day (PND) 1.

RESULTS

A skin permeability assay showed that TCDD accelerated the development of the EPB, beginning at E15. This was accompanied by a significant decrease in transepidermal water loss (TEWL), enhanced stratification, and formation of the stratum corneum (SC). The levels of several ceramides were significantly increased at E15 and E16. PND1 histology revealed TCDD-induced acanthosis and epidermal hyperkeratosis. This was accompanied by disrupted epidermal tight junction (TJ) function, with increased dye leakage at the terminal claudin-1-staining TJs of the stratum granulosum. Because the animals did not have enhanced rates of TEWL, a commonly observed phenotype in animals with TJ defects, we performed tape-stripping. Removal of most of the SC resulted in a significant increase in TEWL in TCDD-exposed PND1 pups compared with their control group.

CONCLUSIONS

These findings demonstrate that in utero exposure to TCDD accelerates the formation of an abnormal EPB with leaky TJs, warranting further study of environmental exposures, epithelial TJ integrity, and atopic disease.

Role of AhR/ARNT system in skin homeostasis

Aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that binds to structurally diverse synthetic and naturally occurring chemicals including dioxins, flavonoids, tryptophan photoproducts, and Malassezia metabolites. Upon binding to its ligands, cytoplasmic AhR translocates to the nucleus, heterodimerizes with aryl hydrocarbon receptor nuclear translocator (ARNT), and mediates numerous biological and toxicological effects by inducing the transcription of various AhR-responsive genes. AhR ligation controls oxidation/antioxidation, epidermal barrier function, photo-induced response, melanogenesis, and innate immunity. This review summarizes recent advances in the understanding of the regulatory mechanisms of skin homeostasis mediated by the AhR/ARNT system.

The emerging role of Nrf2 in dermatotoxicology

The nuclear factor erythroid 2-related factor 2 (Nrf2) is best known for its role in resistance to oxidant stress. In this issue of EMBO Molecular Medicine, Nrf2-prolonged genetic activation is shown with devastating effects on skin homeostasis. The study provides novel molecular insights into poison-induced chloracne and metabolizing acquired dioxin-induced skin hamartomas or MADISH.

Activation of Nrf2 in keratinocytes causes chloracne (MADISH)-like skin disease in mice

The transcription factor Nrf2 is a key regulator of the cellular stress response, and pharmacological Nrf2 activation is a promising strategy for skin protection and cancer prevention. We show here that prolonged Nrf2 activation in keratinocytes causes sebaceous gland enlargement and seborrhea in mice due to upregulation of the growth factor epigen, which we identified as a novel Nrf2 target. This was accompanied by thickening and hyperkeratosis of hair follicle infundibula. These abnormalities caused dilatation of infundibula, hair loss, and cyst development upon aging. Upregulation of epigen, secretory leukocyte peptidase inhibitor (Slpi), and small proline-rich protein 2d (Sprr2d) in hair follicles was identified as the likely cause of infundibular acanthosis, hyperkeratosis, and cyst formation. These alterations were highly reminiscent to the phenotype of chloracne/“metabolizing acquired dioxin-induced skin hamartomas” (MADISH) patients. Indeed, SLPI, SPRR2, and epigen were strongly expressed in cysts of MADISH patients and upregulated by dioxin in human keratinocytes in an NRF2-dependent manner. These results identify novel Nrf2 activities in the pilosebaceous unit and point to a role of NRF2 in MADISH pathogenesis.

Proteomic analysis of human keratinocyte response to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure

Chronic exposure to 2,3,7,8-tetrachlorodibeno-p-dioxin (TCDD) and related polyhalogenated organic pollutants occurs as a consequence of modern life. Exploring the cellular basis for their action is anticipated to help understand the risk they pose and improve the foundation for their regulation. A basis for the striking change in human keratinocyte colony morphology due to TCDD exposure has been investigated by shotgun proteomics. Concentrating on changes in protein levels among three cell strains has revealed significant decreases in the differentiation markers filaggrin, keratin 1, and keratin 10. EGF treatment in concert with TCDD enhanced the changes in these markers and several other proteins while reducing the levels of certain other proteins. The only protein stimulated by TCDD in all three strains and reversed by EGF in them was vimentin, not previously observed to be in the aryl hydrocarbon receptor response domain. Although TCDD is often proposed to enhance keratinocyte differentiation, proteomic analysis reveals it uncouples the differentiation program and suggests that reduced levels of differentiation marker proteins contribute to the observed excessive stratification it induces.

Induction of a chloracne phenotype in an epidermal equivalent model by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is dependent on aryl hydrocarbon receptor activation and is not reproduced by aryl hydrocarbon receptor knock down

BACKGROUND

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent activator of the aryl hydrocarbon receptor (AhR) and causes chloracne in humans. The pathogenesis and role of AhR in chloracne remains incompletely understood.

OBJECTIVE

To elucidate the mechanisms contributing to the development of the chloracne-like phenotype in a human epidermal equivalent model and identify potential biomarkers.

METHODS

Using primary normal human epidermal keratinocytes (NHEK), we studied AhR activation by XRE-luciferase, AhR degradation and CYP1A1 induction. We treated epidermal equivalents with high affinity TCDD or two non-chloracnegens: β-naphthoflavone (β-NF) and 2-(1'H-indole-3'-carbonyl)-thiazole-4-carboxylic acid methyl ester (ITE). Using Western blotting and immunochemistry for filaggrin (FLG), involucrin (INV) and transglutaminase-1 (TGM-1), we compared the effects of the ligands on keratinocyte differentiation and development of the chloracne-like phenotype by H&E.

RESULTS

In NHEKs, activation of an XRE-luciferase and CYP1A1 protein induction correlated with ligand binding affinity: TCDD>β-NF>ITE. AhR degradation was induced by all ligands. In epidermal equivalents, TCDD induced a chloracne-like phenotype, whereas β-NF or ITE did not. All three ligands induced involucrin and TGM-1 protein expression in epidermal equivalents whereas FLG protein expression decreased following treatment with TCDD and β-NF. Inhibition of AhR by α-NF blocked TCDD-induced AhR activation in NHEKs and blocked phenotypic changes in epidermal equivalents; however, AhR knock down did not reproduce the phenotype.

CONCLUSION

Ligand-induced CYP1A1 and AhR degradation did not correlate with their chloracnegenic potential, indicating that neither CYP1A1 nor AhR are suitable biomarkers. Mechanistic studies showed that the TCDD-induced chloracne-like phenotype depends on AhR activation whereas AhR knock down did not appear sufficient to induce the phenotype.

Functions of the aryl hydrocarbon receptor in the skin

Among other functions, the skin serves as the barrier against the environment and provides vital protection from physical or chemical harm and from infection. Skin cells express the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor and sensor of environmental chemicals; at the same time, AHR ligands are abundant in skin from exogenous or endogenous sources. For example, solar radiation, in particular ultraviolet (UV) B, generates AHR ligands from tryptophan in the skin. Recent evidence has shown that AHR is involved in the (patho)physiology of skin including the regulation of skin pigmentation, photocarcinogenesis, and skin inflammation. We here provide a state-of-the-art summary of work which relates to the role of the AHR in (1) adaptive responses against environmental challenges such as UVB or topical chemicals and (2) intrinsic developmental roles for homeostasis of skin cells and (3) skin immunity. We also discuss the existing evidence that AHR antagonists or AHR ligands may be used for the prevention and/or treatment of skin disease.

Defects of filaggrin-like proteins in both lesional and nonlesional atopic skin

BACKGROUND

Atopic dermatitis (AD) is a chronic inflammatory skin disease characterized by a disturbed epidermal barrier. In a subset of patients, this is explained by nonsense mutations in the gene encoding filaggrin (FLG).

OBJECTIVES

We sought to evaluate the respective role of FLG mutations and proinflammatory cytokines and to assess the expression of FLG, hornerin (HRNR), and FLG2, 2 FLG-like proteins, which are involved in epidermal barrier functions, in normal skin and both lesional and nonlesional skin of patients with AD.

METHODS

An FLG-genotyped cohort of 73 adults with AD and 73 aged-matched control subjects was analyzed by using immunohistochemistry and immunoblotting. Normal primary human keratinocytes were differentiated in either the absence or presence of IL-4, IL-13, and IL-25.

RESULTS

Compared with control subjects, FLG, HRNR, and FLG2 were detected at significantly lower levels in the skin of patients with AD, irrespective of their FLG genotype. The reduction was greater in lesional compared with nonlesional skin. In addition, the proFLG/FLG ratio was found to be higher in the skin of wild-type patients than in control subjects. Cytokine treatment of keratinocytes induced a dramatic reduction in FLG, FLG2, and HRNR expression both at the mRNA and protein levels.

CONCLUSION

The stratum corneum of lesional but also clinically unaffected skin of adults with AD is abnormal, with reduced expression of FLG and FLG-like proteins. In addition to nonsense mutations, proinflammatory cytokines and some defects in the proFLG processing can contribute to the FLG downregulation. Our study suggests that skin inflammation reduces the expression of FLG-like proteins, contributing to the AD-related epidermal barrier dysfunction.

Coal tar induces AHR-dependent skin barrier repair in atopic dermatitis

Topical application of coal tar is one of the oldest therapies for atopic dermatitis (AD), a T helper 2 (Th2) lymphocyte-mediated skin disease associated with loss-of-function mutations in the skin barrier gene, filaggrin (FLG). Despite its longstanding clinical use and efficacy, the molecular mechanism of coal tar therapy is unknown. Using organotypic skin models with primary keratinocytes from AD patients and controls, we found that coal tar activated the aryl hydrocarbon receptor (AHR), resulting in induction of epidermal differentiation. AHR knockdown by siRNA completely abrogated this effect. Coal tar restored filaggrin expression in FLG-haploinsufficient keratinocytes to wild-type levels, and counteracted Th2 cytokine-mediated downregulation of skin barrier proteins. In AD patients, coal tar completely restored expression of major skin barrier proteins, including filaggrin. Using organotypic skin models stimulated with Th2 cytokines IL-4 and IL-13, we found coal tar to diminish spongiosis, apoptosis, and CCL26 expression, all AD hallmarks. Coal tar interfered with Th2 cytokine signaling via dephosphorylation of STAT6, most likely due to AHR-regulated activation of the NRF2 antioxidative stress pathway. The therapeutic effect of AHR activation herein described opens a new avenue to reconsider AHR as a pharmacological target and could lead to the development of mechanism-based drugs for AD.

2,3,7,8-Tetrachlorodibenzo-p-dioxin-mediated production of reactive oxygen species is an essential step in the mechanism of action to accelerate human keratinocyte differentiation

Chloracne is commonly observed in humans exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD); yet, the mechanism of toxicity is not well understood. Using normal human epidermal keratinocytes, we investigated the mechanism of TCDD-mediated enhancement of epidermal differentiation by integrating functional genomic, metabolomic, and biochemical analyses. TCDD increased the expression of 40% of the genes of the epidermal differentiation complex found on chromosome 1q21 and 75% of the genes required for de novo ceramide biosynthesis. Lipid analysis demonstrated that eight of the nine classes of ceramides were increased by TCDD, altering the ratio of ceramides to free fatty acids. TCDD decreased the expression of the glucose transporter, SLC2A1, and most of the glycolytic transcripts, followed by decreases in glycolytic intermediates, including pyruvate. NADH and Krebs cycle intermediates were decreased, whereas NAD(+) was increased. Mitochondrial glutathione (GSH) reductase activity and the GSH/glutathione disulfide ratio were decreased by TCDD, ultimately leading to mitochondrial dysfunction, characterized by decreased inner mitochondrial membrane potential and ATP production, and increased production of the reactive oxygen species (ROS), hydrogen peroxide. Aryl hydrocarbon receptor (AHR) antagonists blocked the response of many transcripts to TCDD, and the endpoints of decreased ATP production and differentiation, suggesting regulation by the AHR. Cotreatment of cells with chemical antioxidants or the enzyme catalase blocked the TCDD-mediated acceleration of keratinocyte cornified envelope formation, an endpoint of terminal differentiation. Thus, TCDD-mediated ROS production is a critical step in the mechanism of this chemical to accelerate keratinocyte differentiation.