Activation of the aryl hydrocarbon receptor dampens the severity of inflammatory skin conditions

The gut-skin axis: a bi-directional, microbiota-driven relationship with therapeutic potential

This review explores the emerging term "gut-skin axis" (GSA), describing the bidirectional signaling that occurs between the skin and the gastrointestinal tract under both homeostatic and disease conditions. Central to GSA communication are the gut and skin microbiota, the microbial communities that colonize these barrier surfaces. By influencing diverse host pathways, including innate immune, vitamin D receptor, and Aryl hydrocarbon receptor signaling, a balanced microbiota contributes to both tissue homeostasis and host defense. In contrast, microbiota imbalance, or dysbiosis at one site, can lead to local barrier dysfunction, resulting in the activation of signaling pathways that can disrupt tissue homeostasis at the other site, potentially leading to inflammatory skin conditions such as atopic dermatitis and psoriasis, or gut diseases like Inflammatory Bowel Disease. To date, most research on the GSA has examined the impact of the gut microbiota and diet on skin health, but recent studies show that exposing the skin to ultraviolet B-light can beneficially modulate both the gut microbiome and intestinal health. Thus, despite the traditional focus of clinicians and researchers on these organ systems as distinct, the GSA offers new opportunities to better understand the pathogenesis of cutaneous and gastrointestinal diseases and promote health at both sites.

The aryl hydrocarbon receptor: a rehabilitated target for therapeutic immune modulation

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor originally identified as the target mediating the toxic effects of environmental pollutants including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and dioxins. For years, AHR activation was actively avoided during drug development. However, the AHR was later identified as an important physiological regulator of the immune response. These findings triggered a paradigm shift that resulted in identification of the AHR as a regulator of both innate and adaptive immunity and outlined a pathway for its modulation by the diet, commensal flora and metabolism in the context of autoimmunity, cancer and infection. Moreover, the AHR was revealed as a candidate target for the therapeutic modulation of the immune response. Indeed, the first AHR-activating drug (tapinarof) was recently approved for the treatment of psoriasis. Clinical trials are underway to evaluate the effects of tapinarof and other AHR-targeting therapeutics in inflammatory diseases, cancer and infections. This Review outlines the molecular mechanism of AHR action, and describes how it regulates the immune response. We also discuss links to disease and AHR-targeting therapeutics that have been tested in past and ongoing clinical trials.

Efficacy of IDOR-1117-2520, a novel, orally available CCR6 antagonist in preclinical models of skin dermatitis

BACKGROUND AND PURPOSE

The chemokine receptor CCR6 guides pathogenic T17 cells, implicated in autoimmune diseases including psoriasis, to sites of inflammation via the chemokine CCL20. Therefor, pharmacological inhibition of CCR6+ immune cell migration provides a novel therapeutic approach. Translatability of such an intervention has not yet been assessed in detail. We evaluated the translatability of the Aldara® mouse model induced skin inflammation to psoriasis, with particular focus on immune cell trafficking and assessed the efficacy of IDOR-1117-2520, a highly selective, potent and orally available CCR6 small inhibitor.

EXPERIMENTAL APPROACH

Effects of IDOR-1117-2520 were investigated in the Aldara® and IL23 mouse models of skin inflammation using flow cytometry, RNA sequencing and transcriptome-based cell type deconvolution approaches to characterise immune cell migration patterns. These results were compared to human psoriasis transcriptomics data.

KEY RESULTS

IDOR-1117-2520 dose dependently reduced infiltration of CCR6+ immune cells into inflamed skin, and was equally efficacious as IL-17 and IL-23 inhibition in models of skin inflammation. Pathway analysis showed molecular similarities in the immune response between human psoriasis and the Aldara® mouse model. IL-17/IL-23 pathway genes were expressed in both human psoriasis and the mouse model. CCR6 inhibition modulated multiple pathways associated with inflammation beyond the proximal IL-17/IL-23 pathway. A chemokine-chemokine receptor interaction map implicated CCL20-CCR6 as the dominant axis in recruiting pathogenic T17 cells in both the model and in human psoriasis.

CONCLUSION AND IMPLICATIONS

IDOR-1117-2520 could provide a promising novel targeted approach to treating psoriasis and, potentially, other autoimmune diseases involving the CCR6/CCL20 axis and the IL-17/IL-23 pathway. IDOR-1117-2520 is currently being evaluated in a clinical phase 1 trial (ISRCTN28892128).

Emerging Contaminants: An Important But Ignored Risk Factor for Psoriasis

Industrialization and modernization have changed the environment. A group of emerging contaminants (ECs) has been defined recently. Psoriasis, whose incidence has increased in recent years, is a relapsing immune-mediated disease carrying a heavy disease burden. The erythematous scaly plaque is a typical symptom and occurs on several parts of the body. In addition, psoriasis has many comorbidities, such as psoriatic arthritis, diabetes, and depression, damaging the quality of life of patients. IL-17, IL-12, IL-23, and TNF-alpha are important related cytokines. ECs can influence psoriasis through the immune system and inflammatory responses. Specific mechanisms include increasing pro-inflammatory cytokines such as TNF-α and IL-17, and activating immune cells such as macrophages. And for psoriasis patients, it is suggested to reduce the exposure of most ECs. However, the complex mechanisms involved have not been discussed together and concluded. In this review, we summarize the relationship between ECs and psoriasis, focusing on the immune system, especially the immune cells and cytokines. These results can help guide clinical treatment and long-term management of psoriasis.

Topical Medications for Chronic Itch in Older Patients: Navigating a Pressing Need

Chronic itch in older patients is a common problem, with a significant impact on quality of life. Chronic itch in the older population may be attributable to several causes, such as age-related changes, skin conditions, systemic conditions, medications, and psychological conditions. Given the complexity of itch in this population, comorbidities, and polypharmacy in most geriatric patients, treating chronic itch can be challenging for healthcare providers. Therefore, optimized topical treatment regimens are paramount to help these patients and prevent side effects.

The AhR-Ovol1-Id1 regulatory axis in keratinocytes promotes epidermal and immune homeostasis in atopic dermatitis-like skin inflammation

The skin is our outer permeability and immune defense barrier against myriad external assaults. Aryl hydrocarbon receptor (AhR) senses environmental factors and regulates barrier robustness and immune homeostasis. AhR agonists have been approved by the FDA for psoriasis treatment and are in clinical trials for the treatment of atopic dermatitis (AD), but the underlying mechanism of action remains poorly defined. Here, we report that OVOL1/Ovol1 is a conserved and direct transcriptional target of AhR in epidermal keratinocytes. We show that OVOL1/Ovol1 influences AhR-mediated regulation of keratinocyte gene expression and that OVOL1/Ovol1 ablation in keratinocytes impairs the barrier-promoting function of AhR, exacerbating AD-like inflammation. Mechanistically, we have identified Ovol1's direct downstream targets genome-wide and provided in vivo evidence supporting the role of Id1 as a functional target in barrier maintenance, disease suppression, and neutrophil accumulation. Furthermore, our findings reveal that an IL-1/dermal γδT cell axis exacerbates type 2 and 3 immune responses downstream of barrier perturbation in Ovol1-deficient AD skin. Finally, we present data suggesting the clinical relevance of OVOL1 and ID1 functions in human AD skin. Our study highlights a keratinocyte-intrinsic AhR-Ovol1-Id1 regulatory axis that promotes both epidermal and immune homeostasis in the context of skin inflammation, identifying new therapeutic targets.

GWAS meta-analysis of psoriasis identifies new susceptibility alleles impacting disease mechanisms and therapeutic targets

Psoriasis is a common, debilitating immune-mediated skin disease. Genetic studies have identified biological mechanisms of psoriasis risk, including those targeted by effective therapies. However, the genetic liability to psoriasis is not fully explained by variation at robustly identified risk loci. To refine the genetic map of psoriasis susceptibility we meta-analysed 18 GWAS comprising 36,466 cases and 458,078 controls and identified 109 distinct psoriasis susceptibility loci, including 46 that have not been previously reported. These include susceptibility variants at loci in which the therapeutic targets IL17RA and AHR are encoded, and deleterious coding variants supporting potential new drug targets (including in STAP2, CPVL and POU2F3). We conducted a transcriptome-wide association study to identify regulatory effects of psoriasis susceptibility variants and cross-referenced these against single cell expression profiles in psoriasis-affected skin, highlighting roles for the transcriptional regulation of haematopoietic cell development and epigenetic modulation of interferon signalling in psoriasis pathobiology.

Crosstalk Between the Skin Environment and Microbial Community in Immune-Related Skin Diseases

The skin surface hosts diverse skin microbiota, including bacteria, fungi, and viruses. Intricate interactions between the skin microenvironment and microbial community are crucial for maintaining cutaneous homeostasis. This review explores the bidirectional relationship between the skin ecosystem and its microbiota. The skin microenvironment is shaped by a combination of intrinsic factors, dominated by sweat glands and pilosebaceous units, and external factors, such as UV radiation and personal care products, which create distinct niches that influence microbial colonization patterns across different skin regions. The skin microbiome, in turn, modulates the physical, chemical, immunological, and microbial barriers of the skin. We also discuss the alterations in this crosstalk in various immune-related skin conditions such as atopic dermatitis, psoriasis, rosacea, hidradenitis suppurativa, skin cancer, and aging. Understanding these interactions is vital for developing targeted microbiome-based therapies for various skin disorders. Further researches are needed to deepen insights into the microbial roles and their therapeutic potentials in skin health and disease.

The Aryl Hydrocarbon Receptor (AHR): Peacekeeper of the Skin

In the last decade, the aryl hydrocarbon receptor (AHR) has emerged as a critical peacekeeper for the maintenance of healthy skin. The evolutionary conservation of AHR implied physiological functions for this receptor, beyond the detoxification of man-made compounds, a notion further supported by the existence of physiological AHR ligands, notably derivates of tryptophan by the host and host microbiome. The UV light-derived ligand, 6-formylindolo[3,2-b]carbazole (FICZ), anticipated a role for AHR in skin, a UV light-exposed organ, where physiological AHR activation promotes a healthy skin barrier and constrains inflammation. The clinical development of tapinarof, the first topical AHR modulating drug for inflammatory skin disease, approved by the FDA for mild-to-moderate psoriasis and poised for approval in atopic dermatitis, supports the therapeutic targeting of the AHR pathway to harness its beneficial effect in skin inflammation. Here, we describe how a tightly controlled, physiological activation of the AHR pathway maintains skin homeostasis, and discuss how the pathway is dysregulated in psoriasis and atopic dermatitis, identifying areas offering opportunities for alternative therapeutic approaches, for further investigation.

Stilbene-enriched extract from the leaves of Cajanus cajan attenuates psoriasis in imiquimod-induced psoriatic mice by targeting aryl hydrocarbon receptor and chemokines

ETHNOPHARMACOLOGICAL RELEVANCE

The leaves of Cajanus cajan (L.) Millsp., an Asian traditional folkloric medicine, have been used to treat inflammatory conditions since ancient times. In Southern China, these leaves have been employed to alleviate the symptoms associated with various skin diseases. However, the therapeutic effects and the underlying mechanisms of Cajanus cajan leaves in the treatment of psoriasis remain poorly understood.

AIM OF THE STUDY

This study aims to investigate the efficacy of stilbene-enriched extract from C. cajan leaves (termed as "EXT") in treating imiquimod (IMQ)-induced psoriatic mice and to elucidate its possible underlying mechanism in psoriasis treatment.

MATERIALS AND METHODS

The coumpounds of EXT was analyzed through a UPLC-MS system, the MS survey scan was conducted across the mass range of m/z 100-1000 Da. The activation of aryl hydrocarbon receptor (AhR), a potential therapeutic target, by EXT in HaCaT cells was assessed using RT-qPCR and immunofluorescence. Subsequently, EXT was administrated to IMQ-induced psoriatic mice once daily for 10 days. The efficacy of EXT in treating psoriasis was evaluated through pathological analysis including change of weight, PASI score, Baker score, epidermal thickness, and H&E staining of lesion skin. Additionally, transcriptomic analysis of lesion skins was conducted to identify the potential therapeutic targets and possible mechanisms of EXT in psoriasis treatment.

RESULTS

It was identified that the primary stilbenes present in EXT were 3.10% pinosylvin monomethyl ether (PME), 12.32 % cajaninstilbene (CSA), 4.54 % ongistylin A (LGA) and 2.43 % longistylin C (LGC). In cellular tests, the addition of 2.5 μg/mL EXT to HaCaT cells enhanced the expression of AhR and its nuclear translocation. In vivo tests of EXT in IMQ-induced psoriasis mouse model, 50 mg 1.0 % EXT reduced PASI and Baker score of lesion skin to 2.67 and 4.5, respectively. In addition, the epidermis thickness of lesion skin induced by IMQ returned to normal following the application of 50 mg 1.0 % EXT in psoriatic mice. Transcriptomic profiling revealed significant downregulation of numerous chemokines (Ccl2, Ccl20, and Cxc5, etc.), pro-inflammatory cytokines (Il17a, Il19, Il22, and Il23, etc.), and genes associated with keratinocyte differentiation (Lce and Sprr family genes). Conversely, AhR and genes of the cytochrome P450 family were activated.

CONCLUSIONS

This study is the first to demonstrate that the ethyl acetate (EtOAc) extract enriched with stilbenes from Cajanus cajan leaves (EXT) effectively alleviates symptoms in IMQ-induced psoriatic mice. The mechanism involves the activation of the aryl hydrocarbon receptor (AhR) and a subsequent reduction in the production of various inflammatory chemokines and cytokines. These findings suggest that EXT holds significant potential as a plant-derived therapeutic agent for the treatment of psoriasis.

The Role of Aryl Hydrocarbon Receptor in Skin Homeostasis: Implications for Therapeutic Strategies in Skin Disorders

The aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, is extensively expressed in diverse human organs and plays a pivotal role in mediating the onset, progression, and severity of numerous diseases. Recent research has explored the substantial impact of AhR on skin homeostasis and related pathologies. As a multi-layered organ, the skin comprises multiple cell populations that express AhR. In this review, we introduce the role of AhR in various skin cells and its impact on skin barrier function. Furthermore, we explore the involvement of AhR in the development of various skin diseases, highlighting its potential as a therapeutic target for skin disorders. By targeting AhR, we may open new avenues for the development of novel and efficient skin disease treatments.

Bergapten Ameliorates Psoriatic Skin Lesions and IL-17A-Induced Activation of the NF-κB Signaling Pathway via the Downregulation of CYP1B1

Bergapten (BP) is a plant-derived furocoumarin that has a wide range of pharmacological effects. BP serves as a candidate amplifier in phototherapy against skin inflammation, such as psoriasis and atopic dermatitis. However, the anti-inflammatory role of BP remains elusive. We utilized IL-17A-stimulated keratinocyte line and imiquimod-challenged BALB/c mice to imitate psoriasis-like inflammation. Inflammatory phenotypes were determined by expressions of inflammatory genes and cytokines, histopathological changes and activities of nuclear factor-κB (NF-κB) pathway. An RNA-seq analysis of rodent skin was performed to explore possible mechanism lying behind. SiRNAs and antagonist (TMS) against cytochrome P450 family 1 subfamily B member 1 (CYP1B1) were subsequently used to determine the role of CYP1B1 in psoriasis pathogenesis in vitro and in vivo. Overexpression of CYP1B1 with lentivirus further validate therapeutic effect of BP. BP significantly suppressed activation of the NF-κB pathway by inhibiting p65 phosphorylation and improved the inflammatory phenotype both in vitro and in vivo. We revealed the key role of CYP1B1 in regulating the activation of the NF-κB signaling pathway. Knock-down with siRNAs significantly reduce the expression of inflammatory genes and cytokines. An intraperitoneal injection of TMS partially remediated IMQ-induced inflammation, mainly in terms of skin thickness. Overexpression of Cyp1b1 led to increased expression of the CYP1B1 protein and rescued the therapeutic effect of BP in vitro. This study revealed that BP suppressed expression of Cyp1b1 in keratinocytes and inhibited the activation of NF-κB signaling pathway by blocking the phosphorylation of p65.

PD-L1-CD80 interactions are required for intracellular signaling necessary for dendritic cell migration

Programmed cell death protein 1 (PD-1) and programmed death ligand 1 (PD-L1) interactions are targets for immunotherapies aimed to reinvigorate T cell function. Recently, it was documented that PD-L1 regulates dendritic cell (DC) migration through intracellular signaling events. In this study, we find that both preclinical murine and clinically available human PD-L1 antibodies limit DC migration. We show that cis interactions between PD-L1 and CD80 are critical for promoting migration and define specific regions within these proteins necessary for migration. Furthermore, we demonstrate that αPD-L1 significantly impedes DC migration in a B16 melanoma tumor model. Last, we outline how blocking cis PD-L1:CD80 interactions or mutation of the intracellular domain of PD-L1, in an imiquimod-induced murine model of psoriasis, limits DC migration to the lymph node, decreases interleukin-17 production by CD4+ T cells in the lymph node, and reduces epidermal thickening. Therefore, PD-L1 and CD80 interactions are important regulators of DC migration to the draining lymph node.

Indoleamine 2,3-Dioxygenase 1/Aryl Hydrocarbon Receptor Feedback Loop Mediates Anti-inflammation in lipopolysaccharide-stimulated Astrocytes to Dampen Inflammatory Neurotoxicity

ABSTRACT

Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that regulates cell immune responses in a cell type-specific and ligand-dependent manner. In the central nervous system, astrocytic AhR plays important roles in regulating neuroinflammation by mediating responses to endogenous ligands generated from the inflammation-induced indoleamine 2,3-dioxygenase 1 (IDO1)/kynurenine (KYN) pathway. We previously demonstrated that reduction of AhR expression decreases lipopolysaccharide (LPS)-induced pro-inflammatory responses in microglia. However, the role of AhR in the astrocytic immune responses and its subsequent effects on microglial activation and neurotoxicity remain unclear. In this study, we used LPS-induced neuroinflammation in rat cortical glia-neuron (GN) mix cultures, which increased the expression of tumor necrosis factor-α and interleukin-6 and microglial activation. These proinflammatory responses were attenuated by a specific AhR agonist 6-formylindolo [3,2-b] carbazole (FICZ), but not by the AhR antagonist CH223191. CH223191, which inhibits LPS- and FICZ-induced AhR activation, enhanced neurotoxicity induced by LPS-glutamate co-treatment in GN mix cultures. Furthermore, inhibition of AhR expression and activation enhanced LPS-induced proinflammatory responses, and LPS-induced AhR activation was abrogated by the inhibition of IDO1 expression in astrocytes. Notably, AhR knockdown inhibited the anti-inflammatory effects of KYN while enhancing LPS-induced IDO1 expression in astrocytes, suggesting that AhR mediates the anti-inflammatory effect of KYN and the negative feedback regulation of IDO1 expression. Finally, we examined the role of astrocytic AhR in inflammatory astrogliosis-induced neurotoxicity by treating primary cortical neurons with LPS-treated astrocyte-conditioned medium (ACM). The results revealed that ACM derived from siAhR-transfected astrocytes increased neurotoxicity. In conclusion, inflammation-activated AhR mediates the anti-inflammatory effects and negative feedback regulation of the IDO1/KYN pathway in astrocytes, thereby dampening inflammatory astrogliosis-induced neurotoxicity.

Activation of the aryl hydrocarbon receptor via indole derivatives is a common feature in skin bacterial isolates

AIMS

The aryl hydrocarbon receptor (AhR) is a ligand-activated receptor implicated in many inflammatory disorders. The skin microbiota plays a crucial role in maintaining epidermal barrier integrity and is thought to modulate skin homeostasis partly through the production of AhR ligands, including metabolites of microbial tryptophan metabolism such as indole derivatives. Here, we report the skin microbiota that activate AhR and their unique tryptophan metabolite profiles.

METHODS AND RESULTS

Of the bacteria isolated from healthy human skin and screened for the ability to metabolize tryptophan (18 species, five genera), 14 were positive. The tryptophan metabolites of 10 positive and two negative bacteria were then characterized using liquid chromatography-mass spectrometry. Whole genome sequencing confirmed the presence of key genes involved in the indole-3-pyruvic acid pathway within the genomes of indole-3-acetaldehyde, indole-3-acetic acid, and indole-3-aldehyde-producing organisms. A cell-based luciferase reporter gene assay identified functional agonist activity against human AhR in the culture supernatants of 12 of the 18 species tested. High indole derivative-producing organisms induced potent AhR activation.

CONCLUSIONS

These data demonstrate the relationship between skin microbiota, tryptophan metabolites, and AhR activation.

Indigo alleviates psoriasis through the AhR/NF-κB signaling pathway: an in vitro and in vivo study

Background

Psoriasis is a chronic inflammatory skin disease. A strong association between the AhR/ NFκB axis and the inflammatory response in psoriasis. Indigo (IDG) has demonstrated significant anti-inflammatory properties. This study aimed to assess the anti-psoriatic efficacy of IDG while investigating the underlying mechanisms involved.

Methods

In the in vitro experiments, cell viability was assessed using the CCK-8. qRT-PCR was employed to measure the mRNA levels of NF-κB, TNF-α, IL-1β, AhR, and CYP1A1. Western blotting was conducted to examine alterations in cytoplasmic and nuclear AhR protein levels. Additionally, an IDG nanoemulsion (NE) cream was prepared for the in vivo experiments. A psoriasis-like skin lesion mice model was induced using IMQ (62.5 mg/day for 7 days). The severity of psoriasis was evaluated using PASI, and skin lesions were scored while epidermal thickness was assessed via HE staining. The expression of inflammatory markers, including IL-6, IL-13, IL-17A, MCP-1, and TNF-α, was detected in skin lesions using Luminex. The levels of CYP1A1, p65, and p-p65 proteins were determined by Western blotting.

Results

LPS stimulation significantly elevated TNF-α, IL-6, and NF-κB mRNA levels, which were notably reduced by IDG treatment. Additionally, IDG significantly enhanced the expression of AhR and CYP1A1 mRNA. Further investigation revealed that IDG facilitated AhR translocation from the cytoplasm to the nucleus. In the IMQ-induced psoriasis-like mouse model, IDG NE substantially ameliorated the severity of skin lesions. Moreover, IDG NE treatment reduced the upregulation of inflammatory cytokines such as IL-6, IL-17A, MCP-1, and TNF-α in IMQ-induced skin lesions. It was also observed that IDG NE treatment increased CYP1A1 protein expression while inhibiting p65 and p-p65 protein expression.

Conclusion

IDG emerges as a promising treatment for psoriasis, demonstrating effective therapeutic outcomes. Its mechanism of action is likely linked to the modulation of the AhR/NFκB signaling pathway.

Punicalagin as a novel selective aryl hydrocarbon receptor (AhR) modulator upregulates AhR expression through the PDK1/p90RSK/AP-1 pathway to promote the anti-inflammatory response and bactericidal activity of macrophages

Aryl hydrocarbon receptor (AhR) plays an important role in inflammation and immunity as a new therapeutic target for infectious disease and sepsis. Punicalagin (PUN) is a Chinese herbal monomer extract of pomegranate peel that has beneficial anti-inflammatory, antioxidant and anti-infective effects. However, whether PUN is a ligand of AhR, its effect on AhR expression, and its signaling pathway remain poorly understood. In this study, we found that PUN was a unique polyphenolic compound that upregulated AhR expression at the transcriptional level, and regulated the AhR nongenomic pathway. AhR expression in lipopolysaccharide-induced macrophages was upregulated by PUN in vitro and in vivo in a time- and dose-dependent manner. Using specific inhibitors and siRNA, induction of AhR by PUN depended on sequential phosphorylation of 90-kDa ribosomal S6 kinase (p90RSK), which was activated by the mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) and phosphoinositide-dependent protein kinase (PDK)1 pathways. PUN promoted p90RSK-mediated activator protein-1 (AP-1) activation. AhR knockout or inhibitors reversed suppression of interleukin (IL)-6 and IL-1β expression by PUN. PUN decreased Listeria load and increased macrophage survival via AhR upregulation. In conclusion, we identified PUN as a novel selective AhR modulator involved in AhR expression via the MEK/ERK and PDK1 pathways targeting p90RSK/AP-1 in inflammatory macrophages, which inhibited macrophage inflammation and promoted bactericidal activity.

The influence of AHR on immune and tissue biology

The aryl hydrocarbon receptor is a ligand dependent transcription factor which functions as an environmental sensor. Originally discovered as the sensor for man made pollutants such as 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) it has recently gained prominence as an important mediator for environmental triggers via the diet or microbiota which influences many physiological functions in different cell types and tissues across the body. Notably AHR activity contributes to prevent excessive inflammation following tissue damage in barrier organs such as skin, lung or gut which has received wide attention in the past decade. In this review we will focus on emerging common AHR functions across cell types and tissues and discuss ongoing issues that confound the understanding of AHR physiology. Furthermore, we will discuss the need for deeper molecular understanding of the functional activity of AHR in different contexts with respect to development of potential therapeutic applications.

Molecular Insights into the Interaction of Tryptophan Metabolites with the Human Aryl Hydrocarbon Receptor in Silico: Tryptophan as Antagonist and no Direct Involvement of Kynurenine

BACKGROUND

A direct link between the tryptophan (Trp) metabolite kynurenine (Kyn) and the aryl hydrocarbon receptor (AhR) is not supported by metabolic considerations and by studies demonstrating the failure of Kyn concentrations of up to 100 μM to activate the receptor in cell culture systems using the proxy system of cytochrome P-450-dependent metabolism. The Kyn metabolite kynurenic acid (KA) activates the AhR and may mediate the Kyn link. Recent studies demonstrated down regulation and antagonism of activation of the AhR by Trp. We have addressed the link between Kyn and the AhR by looking at their direct molecular interaction in silico.

METHODS

Molecular docking of Kyn, KA, Trp and a range of Trp metabolites to the crystal structure of the human AhR was performed under appropriate docking conditions.

RESULTS

Trp and 30 of its metabolites docked to the AhR to various degrees, whereas Kyn and 3-hydroxykynurenine did not. The strongest docking was observed with the Trp metabolite and photooxidation product 6-Formylindolo[3,2-b]carbazole (FICZ), cinnabarinic acid, 5-hydroxytryptophan, N-acetyl serotonin and indol-3-yllactic acid. Strong docking was also observed with other 5-hydroxyindoles.

CONCLUSIONS

We propose that the Kyn-AhR link is mediated by KA. The strong docking of Trp and its recently reported down regulation of the receptor suggest that Trp is an AhR antagonist and may thus play important roles in body homeostasis beyond known properties or simply being the precursor of biologically active metabolites. Differences in AhR activation reported in the literature are discussed.

Promising strategies in natural products treatments of psoriasis-update

Psoriasis is a chronic, relapsing, inflammatory skin disease and has been increasing year by year. It is linked to other serious illnesses, such as psoriatic arthritis, cardiometabolic syndrome, and depression, resulting in a notable decrease in the quality of life for patients. Existing therapies merely alleviate symptoms, rather than providing a cure. An in-depth under-standing of the pathogenesis of psoriasis is helpful to discover new therapeutic targets and develop effective novel therapeutic agents, so it has important clinical significance. This article reviews the new progress in the study of pathogenesis and natural products of psoriasis in recent years. These natural products were summarized, mainly classified as terpenoids, polyphenols and alkaloids. However, the translation of experimental results to the clinic takes a long way to go.

The Aryl Hydrocarbon Receptor Regulates Epidermal Differentiation through Transient Activation of TFAP2A

The aryl hydrocarbon receptor (AHR) is an evolutionary conserved environmental sensor identified as an indispensable regulator of epithelial homeostasis and barrier organ function. Molecular signaling cascade and target genes upon AHR activation and their contribution to cell and tissue function are however not fully understood. Multiomics analyses using human skin keratinocytes revealed that upon ligand activation, AHR binds open chromatin to induce expression of transcription factors, for example, TFAP2A, as a swift response to environmental stimuli. The terminal differentiation program, including upregulation of barrier genes, FLG and keratins, was mediated by TFAP2A as a secondary response to AHR activation. The role of AHR-TFAP2A axis in controlling keratinocyte terminal differentiation for proper barrier formation was further confirmed using CRISPR/Cas9 in human epidermal equivalents. Overall, the study provides additional insights into the molecular mechanism behind AHR-mediated barrier function and identifies potential targets for the treatment of skin barrier diseases.

Psoriatic skin transcript phenotype: androgen/estrogen and cortisone/cortisol imbalance with increasing DNA damage response

BACKGROUND

Patients prone to psoriasis suffer after a breakdown of the epidermal barrier and develop poorly healing lesions with abnormal proliferation of keratinocytes. Strong inflammatory reactions with genotoxicity (short telomeres) suggest impaired immune defenses with DNA damage repair response (DDR) in patients with psoriasis. Recent evidence indicates the existence of crosstalk mechanisms linking the DDR machinery and hormonal signaling pathways that cooperate to influence both progressions of many diseases and responses to treatment. The aim of this study was to clarify whether steroid biosynthesis and genomic stability markers are altered in parallel during the formation of psoriatic skin. Understanding the interaction of the steroid pathway and DNA damage response is crucial to addressing underlying fundamental issues and managing resulting epidermal barrier disruption in psoriasis.

METHODS

Skin (Lesional, non-lesional) and blood samples from twenty psoriasis patients and fifteen healthy volunteers were collected. Real-Time-PCR study was performed to assess levels of known transcripts such as: estrogen (ESR1, ESR2), androgen (AR), glucocorticoid/mineralocorticoid receptors (NR3C1, NR3C2), HSD11B1/HSD11B2, and DNA damage sensors (SMC1A, TREX1, TREX2, SSBP3, RAD1, RAD18, EXO1, POLH, HUS1).

RESULTS

We found that ESR1, ESR2, HSD11B1, NR3C1, NR3C2, POLH, and SMC1A transcripts were significantly decreased and AR, TREX1, RAD1, and SSBP3 transcripts were increased dramatically in the lesional skin compared to skin samples of controls.

CONCLUSION

We found that the regulation of the steroidogenic pathway was disrupted in the lesional tissue of psoriasis patients and that a sufficient glucocorticoid and mineralocorticoid response did not form and the estrogen/androgen balance was altered in favour of androgens. We suggest that an increased androgen response in the presence of DDR increases the risk of developing psoriasis. Although this situation may be the cause or the consequence of a disruption of the epidermal barrier, our data suggest developing new therapeutic strategies.

NOCICEPTOR NEURONS CONTROL POLLUTION-MEDIATED NEUTROPHILIC ASTHMA

The immune and sensory nervous systems, having evolved together, use a shared language of receptors and transmitters to maintain homeostasis by responding to external and internal disruptions. Although beneficial in many cases, neurons can exacerbate inflammation during allergic reactions, such as asthma. Our research modeled asthma aggravated by pollution, exposing mice to ambient PM2.5 particles and ovalbumin. This exposure significantly increased bronchoalveolar lavage fluid neutrophils and γδ T cells compared to exposure to ovalbumin alone. We normalized airway inflammation and lung neutrophil levels by silencing nociceptor neurons at inflammation's peak using intranasal QX-314 or ablating TRPV1-expressing neurons. Additionally, we observed heightened sensitivity in chemical-sensing TRPA1 channels in neurons from pollution-exacerbated asthmatic mice. Elevated levels of artemin were detected in the bronchoalveolar lavage fluid from pollution-exposed mice, with artemin levels normalizing in mice with ablated nociceptor neurons. Upon exposure PM2.5 particles, alveolar macrophages expressing pollution-sensing aryl hydrocarbon receptors, were identified as the source of artemin. This molecule enhanced TRPA1 responsiveness and increased neutrophil influx, providing a novel mechanism by which lung-innervating neurons respond to air pollution and suggesting a potential therapeutic target for controlling neutrophilic airway inflammation in asthma, a clinically intractable condition.

Aryl hydrocarbon receptor: current perspectives on key signaling partners and immunoregulatory role in inflammatory diseases

The aryl hydrocarbon receptor (AhR) is a versatile environmental sensor and transcription factor found throughout the body, responding to a wide range of small molecules originating from the environment, our diets, host microbiomes, and internal metabolic processes. Increasing evidence highlights AhR's role as a critical regulator of numerous biological functions, such as cellular differentiation, immune response, metabolism, and even tumor formation. Typically located in the cytoplasm, AhR moves to the nucleus upon activation by an agonist where it partners with either the aryl hydrocarbon receptor nuclear translocator (ARNT) or hypoxia-inducible factor 1β (HIF-1β). This complex then interacts with xenobiotic response elements (XREs) to control the expression of key genes. AhR is notably present in various crucial immune cells, and recent research underscores its significant impact on both innate and adaptive immunity. This review delves into the latest insights on AhR's structure, activating ligands, and its multifaceted roles. We explore the sophisticated molecular pathways through which AhR influences immune and lymphoid cells, emphasizing its emerging importance in managing inflammatory diseases. Furthermore, we discuss the exciting potential of developing targeted therapies that modulate AhR activity, opening new avenues for medical intervention in immune-related conditions.

Dysregulated tryptophan metabolism and AhR pathway contributed to CXCL10 upregulation in stable non-segmental vitiligo

BACKGROUND

Tryptophan metabolism dysregulation has been observed in vitiligo. However, drawing a mechanistic linkage between this metabolic disturbance and vitiligo pathogenesis remains challenging.

OBJECTIVE

Aim to reveal the characterization of tryptophan metabolism in vitiligo and investigate the role of tryptophan metabolites in vitiligo pathophysiology.

METHODS

LC-MS/MS, dual-luciferase reporter assay, ELISA, qRT-PCR, small interfering RNA, western blotting, and immunohistochemistry were employed.

RESULTS

Kynurenine pathway activation and KYAT enzyme-associated deviation to kynurenic acid (KYNA) in the plasma of stable non-segmental vitiligo were determined. Using a public microarray dataset, we next validated the activation of kynurenine pathway was related with inflammatory-related genes expression in skin of vitiligo patients. Furthermore, we found that KYNA induced CXCL10 upregulation in keratinocytes via AhR activation. Moreover, the total activity of AhR agonist was increased while the AhR concentration per se was decreased in the plasma of vitiligo patients. Finally, higher KYAT, CXCL10, CYP1A1 and lower AhR expression in vitiligo lesional skin were observed by immunohistochemistry staining.

CONCLUSION

This study depicts the metabolic and genetic characterizations of tryptophan metabolism in vitiligo and proposes that KYNA, a tryptophan-derived AhR ligand, can enhance CXCL10 expression in keratinocytes.

Tapinarof validates the aryl hydrocarbon receptor as a therapeutic target: A clinical review

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that has wide-ranging roles, including regulation of inflammation and homeostasis. AhR is not a cell surface receptor; rather, it exists in a cytoplasmic complex that responds to a wide variety of structurally dissimilar endogenous, microbial, and environmental ligands. The ubiquitous expression of AhR, its ability to be activated by a wide range of ligands, and its capacity to act as a master regulator for gene expression and homeostasis make it a promising new therapeutic target. Clinical trials of tapinarof cream have now validated AhR agonism as a therapeutic approach that can deliver significant efficacy for treating inflammatory skin diseases, including psoriasis and atopic dermatitis. Tapinarof 1% cream is a first-in-class, nonsteroidal, topical, AhR agonist with a pharmacokinetic profile that results in localized exposure at sites of disease, avoiding systemic safety concerns, drug interactions, or off-target effects. Psoriasis and atopic dermatitis both involve epidermal inflammation, cellular immune responses, dysregulation of skin barrier protein expression, and oxidative stress. On the basis of the clinical effectiveness of tapinarof cream for treating inflammatory skin diseases, we review how targeting AhR may offer a significant opportunity in other conditions that share key aspects of pathogenesis, including asthma, inflammatory bowel disease, eosinophilic esophagitis, ophthalmic, and nervous system diseases.

Light Sensing beyond Vision: Focusing on a Possible Role for the FICZ/AhR Complex in Skin Optotransduction

Although our skin is not the primary visual organ in humans, it acts as a light sensor, playing a significant role in maintaining our health and overall well-being. Thanks to the presence of a complex and sophisticated optotransduction system, the skin interacts with the visible part of the electromagnetic spectrum and with ultraviolet (UV) radiation. Following a brief overview describing the main photosensitive molecules that detect specific electromagnetic radiation and their associated cell pathways, we analyze their impact on physiological functions such as melanogenesis, immune response, circadian rhythms, and mood regulation. In this paper, we focus on 6-formylindolo[3,2-b]carbazole (FICZ), a photo oxidation derivative of the essential amino acid tryptophan (Trp). This molecule is the best endogenous agonist of the Aryl hydrocarbon Receptor (AhR), an evolutionarily conserved transcription factor, traditionally recognized as a signal transducer of both exogenous and endogenous chemical signals. Increasing evidence indicates that AhR is also involved in light sensing within the skin, primarily due to its ligand FICZ, which acts as both a chromophore and a photosensitizer. The biochemical reactions triggered by their interaction impact diverse functions and convey crucial data to our body, thus adding a piece to the complex puzzle of pathways that allow us to decode and elaborate environmental stimuli.

Evaluation of OVOL1 and Filaggrin immunohistochemical expression and clinical relevance in psoriasis

BACKGROUND

Psoriasis is a disease of overactive immune system. OVOL1 and Filaggrin have been associated with many inflammatory skin lesions. To the best of our knowledge, the correlation between OVOL1 and Filaggrin in psoriasis was not previously investigated. This work aims to search the immunohistochemical expression and correlation between OVOL1 and Filaggrin in psoriasis.

MATERIALS AND METHODS

Slides cut from paraffin blocks of 30 psoriasis cases and 30 control subjects were stained with OVOL1 and Filaggrin. Clinicopathological data were correlated with the results of staining.

RESULTS

OVOL1 and Filaggrin expression in epidermis showed a significant gradual reduction from normal skin to peri-lesional and psoriasis biopsies (P < 0.001). In contrast, psoriasis dermis showed a significant overexpression of OVOL1 in inflammatory cells in relation to peri-lesional biopsies (P < 0.002). OVOL1 demonstrated a significant direct correlation with Filaggrin expression in psoriasis (r = 0.568, P < 0.004). OVOL1 and Filaggrin expression in psoriasis skin epidermis demonstrated a statistically significant negative correlation with PASI score.

CONCLUSION

OVOL1 and Filaggrin might be involved in psoriasis-associated inflammation and skin hyperproliferation. OVOL1 might have a protective barrier function in the skin and could be used to stratify progressive disease. Filaggrin may play a role in progression of psoriasis. OVOL1 inhibition could be considered in suppression of Filaggrin function. OVOL1 agonists may be beneficial in psoriasis treatment.

Targeting the aryl hydrocarbon receptor with FICZ regulates IL-2 and immune infiltration to alleviate Hashimoto's thyroiditis in mice

Hashimoto's thyroiditis (HT) is the most frequent autoimmune disorder. Growing work points to the involvement of aryl hydrocarbon receptor (AhR), a ligand-dependent transcription factor, in the regulation of immune homeostasis. However, the roles of AhR and its ligands in HT remains unclear. In this study, we leveraged public human database analyses to postulate that the AhR expression was predominantly in thyroid follicular cells, correlating significantly with the thyroid infiltration levels of multiple immune cells in HT patients. Using a thyroglobulin-induced HT mouse model and in vitro thyroid follicular epithelial cell cultures, we found a significant downregulation of AhR expression in thyrocytes both in vivo and in vitro. Conversely, activating AhR by FICZ, a natural AhR ligand, mitigated inflammation and apoptosis in thyrocytes in vitro and conferred protection against HT in mice. RNA sequencing (RNA-seq) of thyroid tissues indicated that AhR activation moderated HT-associated immune or inflammatory signatures. Further, immunoinfiltration analysis indicated that AhR activation regulated immune cell infiltration in the thyroid of HT mice, such as suppressing cytotoxic CD8+ T cell infiltration and promoting anti-inflammatory M2 macrophage polarization. Concomitantly, the expression levels of interleukin-2 (IL-2), a lymphokine that downregulates immune responses, were typically decreased in HT but restored upon AhR activation. In silico validation substantiated the binding interaction between AhR and IL-2. In conclusion, targeting the AhR with FICZ regulates IL-2 and immune infiltration to alleviate experimental HT, shedding new light on the therapeutic intervention of this prevalent disease.

Activation of the aryl hydrocarbon receptor improves allergen-specific immunotherapy of murine allergic airway inflammation: a novel adjuvant option?

Background

Allergen-specific immunotherapy (AIT) is able to restore immune tolerance to allergens in allergic patients. However, some patients do not or only poorly respond to current treatment protocols. Therefore, there is a need for deeper mechanistic insights and further improvement of treatment strategies. The relevance of the aryl hydrocarbon receptor (AhR), a ligand-dependent transcription factor, has been investigated in several inflammatory diseases, including allergic asthma. However, its potential role in AIT still needs to be addressed.

Methods

A murine model of AIT in ovalbumin-induced allergic airway inflammation was performed in AhR-deficient (AhR-/-) and wild-type mice. Furthermore, AIT was combined with the application of the high-affinity AhR agonist 10-chloro-7H-benzimidazo[2,1-a]benzo[de]iso-quinolin-7-one (10-Cl-BBQ) as an adjuvant to investigate the effects of AhR activation on therapeutic outcome.

Results

Although AhR-/- mice suffer stronger allergic responses than wild-type mice, experimental AIT is comparably effective in both. Nevertheless, combining AIT with the administration of 10-Cl-BBQ improved therapeutic effects by an AhR-dependent mechanism, resulting in decreased cell counts in the bronchoalveolar fluid, decreased pulmonary Th2 and Th17 cell levels, and lower sIgE levels.

Conclusion

This study demonstrates that the success of AIT is not dependent on the AhR. However, targeting the AhR during AIT can help to dampen inflammation and improve tolerogenic vaccination. Therefore, AhR ligands might represent promising candidates as immunomodulators to enhance the efficacy of AIT.

Natural Product-Derived Compounds Targeting Keratinocytes and Molecular Pathways in Psoriasis Therapeutics

Psoriasis is a chronic autoimmune inflammatory skin disorder that affects approximately 2-3% of the global population due to significant genetic predisposition. It is characterized by an uncontrolled growth and differentiation of keratinocytes, leading to the formation of scaly erythematous plaques. Psoriasis extends beyond dermatological manifestations to impact joints and nails and is often associated with systemic disorders. Although traditional treatments provide relief, their use is limited by potential side effects and the chronic nature of the disease. This review aims to discuss the therapeutic potential of keratinocyte-targeting natural products in psoriasis and highlight their efficacy and safety in comparison with conventional treatments. This review comprehensively examines psoriasis pathogenesis within keratinocytes and the various related signaling pathways (such as JAK-STAT and NF-κB) and cytokines. It presents molecular targets such as high-mobility group box-1 (HMGB1), dual-specificity phosphatase-1 (DUSP1), and the aryl hydrocarbon receptor (AhR) for treating psoriasis. It evaluates the ability of natural compounds such as luteolin, piperine, and glycyrrhizin to modulate psoriasis-related pathways. Finally, it offers insights into alternative and sustainable treatment options with fewer side effects.

The Role of Aryl Hydrocarbon Receptor in the Pathogenesis and Treatment of Psoriasis

The pathogenesis of psoriasis is complex. Aryl hydrocarbon receptor (AhR) is a transcription factor that can be bound and activated by structurally diverse ligands and plays an important role in a range of biological processes and in the pathogenesis of different diseases. Recently, the role of AhR in psoriasis has attracted attention. AhR has toxicological functions and physiological functions. The overexpression and activation of AhR induced by the environmental pollutant and exogenous AhR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) can drive the development of psoriasis. This TCDD-mediated toxicological response disrupts the physiological functions of AhR resulting in skin barrier disorders and the release of inflammatory cytokines, 2 of the pivotal factors of psoriasis. In addition, highly upregulated kynureninase in psoriasis decreases endogenous AhR agonists, thereby weakening the physiological functions of AhR. Activating AhR physiological signalling should be useful in the treatment of psoriasis. Studies have demonstrated that physiological activation of AhR can dampen the severity of psoriasis. The oldest and effective treatment for psoriasis coal tar works by activating AhR, and both new anti-psoriasis drugs tapinarof and benvitimod are formulations of AhR agonist, supporting that activation of AhR can be used as a new strategy for the treatment of psoriasis. Preclinical and preliminary clinical studies have revealed the anti-psoriasis effects of a number of AhR agonists, providing potential candidates for the development of new drugs for the treatment of psoriasis.

Small-molecule agents for treating skin diseases

Skin diseases are a class of common and frequently occurring diseases that significantly impact daily lives. Currently, the limited effective therapeutic drugs are far from meeting the clinical needs; most drugs typically only provide symptomatic relief rather than a cure. Developing small-molecule drugs with improved efficacy holds paramount importance for treating skin diseases. This review aimed to systematically introduce the pathogenesis of common skin diseases in daily life, list related drugs applied in the clinic, and summarize the clinical research status of candidate drugs and the latest research progress of candidate compounds in the drug discovery stage. Also, it statistically analyzed the number of publications and global attention trends for the involved skin diseases. This review might provide practical information for researchers engaged in dermatological drugs and further increase research attention to this disease area.

Dubosiella newyorkensis modulates immune tolerance in colitis via the L-lysine-activated AhR-IDO1-Kyn pathway

Commensal bacteria generate immensely diverse active metabolites to maintain gut homeostasis, however their fundamental role in establishing an immunotolerogenic microenvironment in the intestinal tract remains obscure. Here, we demonstrate that an understudied murine commensal bacterium, Dubosiella newyorkensis, and its human homologue Clostridium innocuum, have a probiotic immunomodulatory effect on dextran sulfate sodium-induced colitis using conventional, antibiotic-treated and germ-free mouse models. We identify an important role for the D. newyorkensis in rebalancing Treg/Th17 responses and ameliorating mucosal barrier injury by producing short-chain fatty acids, especially propionate and L-Lysine (Lys). We further show that Lys induces the immune tolerance ability of dendritic cells (DCs) by enhancing Trp catabolism towards the kynurenine (Kyn) pathway through activation of the metabolic enzyme indoleamine-2,3-dioxygenase 1 (IDO1) in an aryl hydrocarbon receptor (AhR)-dependent manner. This study identifies a previously unrecognized metabolic communication by which Lys-producing commensal bacteria exert their immunoregulatory capacity to establish a Treg-mediated immunosuppressive microenvironment by activating AhR-IDO1-Kyn metabolic circuitry in DCs. This metabolic circuit represents a potential therapeutic target for the treatment of inflammatory bowel diseases.

Aryl hydrocarbon receptor role in chronic inflammatory skin diseases: a narrative review

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that has gained increasing attention in the field of dermatology due to its multifaceted role in skin health and disease. This review provides a comprehensive overview of the current state of knowledge regarding the AHR and its implications in dermatological conditions. The AHR is well known for its involvement in xenobiotic metabolism, particularly in response to polycyclic aromatic hydrocarbons and dioxins. However, recent research has unveiled its pivotal role in the skin immune response, barrier function, and homeostasis. The AHR signalling pathway is intricately linked to various dermatological disorders, including psoriasis, atopic dermatitis, acne and hidradenitis suppurativa. In this review, we delve into the molecular mechanisms through which AHR activation influences skin physiology and highlight how dysregulation can lead to pathological conditions. Moreover, we discuss the emerging therapeutic potential of AHR modulators in the treatment of skin diseases. In conclusion, the AHR is a pivotal player in dermatology, with a multifaceted role in skin physiology and pathology. Understanding the intricacies of AHR signalling in the skin offers promising avenues for the development of novel therapies and preventive strategies for various dermatological conditions. Further research is warranted to elucidate the full scope of AHR's contributions to dermatology and its potential as a therapeutic target.

IL-23 induces CLEC5A+ IL-17A+ neutrophils and elicit skin inflammation associated with psoriatic arthritis

IL-23-activation of IL-17 producing T cells is involved in many rheumatic diseases. Herein, we investigate the role of IL-23 in the activation of myeloid cell subsets that contribute to skin inflammation in mice and man. IL-23 gene transfer in WT, IL-23RGFP reporter mice and subsequent analysis with spectral cytometry show that IL-23 regulates early innate immune events by inducing the expansion of a myeloid MDL1+CD11b+Ly6G+ population that dictates epidermal hyperplasia, acanthosis, and parakeratosis; hallmark pathologic features of psoriasis. Genetic ablation of MDL-1, a major PU.1 transcriptional target during myeloid differentiation exclusively expressed in myeloid cells, completely prevents IL-23-pathology. Moreover, we show that IL-23-induced myeloid subsets are also capable of producing IL-17A and IL-23R+MDL1+ cells are present in the involved skin of psoriasis patients and gene expression correlations between IL-23 and MDL-1 have been validated in multiple patient cohorts. Collectively, our data demonstrate a novel role of IL-23 in MDL-1-myelopoiesis that is responsible for skin inflammation and related pathologies. Our data open a new avenue of investigations regarding the role of IL-23 in the activation of myeloid immunoreceptors and their role in autoimmunity.

An AhR-Ovol1-Id1 regulatory axis in keratinocytes promotes skin homeostasis against atopic dermatitis

Skin is our outer permeability and immune defense barrier against myriad external assaults. Aryl hydrocarbon receptor (AhR) senses environmental factors and regulates barrier robustness and immune homeostasis. AhR agonist is in clinical trial for atopic dermatitis (AD) treatment, but the underlying mechanism of action remains ill-defined. Here we report OVOL1/Ovol1 as a conserved and direct transcriptional target of AhR in epidermal keratinocytes. We show that OVOL1/Ovol1 impacts AhR regulation of keratinocyte gene expression, and Ovol1 deletion in keratinocytes hampers AhR's barrier promotion function and worsens AD-like inflammation. Mechanistically, we identify Ovol1's direct downstream targets genome-wide, and provide in vivo evidence for Id1's critical role in barrier maintenance and disease suppression. Furthermore, our findings reveal an IL-1/dermal γδT cell axis exacerbating both type 2 and type 3 immune responses downstream of barrier perturbation in Ovol1 -deficient AD skin. Finally, we present data suggesting the clinical relevance of OVOL1 and ID1 function in human AD. Our study highlights a keratinocyte-intrinsic AhR-Ovol1-Id1 regulatory axis that promotes both epidermal and immune homeostasis against AD-like inflammation, implicating new therapeutic targets for AD.

Identification of gene signatures and molecular mechanisms underlying the mutual exclusion between psoriasis and leprosy

Leprosy and psoriasis rarely coexist, the specific molecular mechanisms underlying their mutual exclusion have not been extensively investigated. This study aimed to reveal the underlying mechanism responsible for the mutual exclusion between psoriasis and leprosy. We obtained leprosy and psoriasis data from ArrayExpress and GEO database. Differential expression analysis was conducted separately on the leprosy and psoriasis using DEseq2. Differentially expressed genes (DEGs) with opposite expression patterns in psoriasis and leprosy were identified, which could potentially involve in their mutual exclusion. Enrichment analysis was performed on these candidate mutually exclusive genes, and a protein-protein interaction (PPI) network was constructed to identify hub genes. The expression of these hub genes was further validated in an external dataset to obtain the critical mutually exclusive genes. Additionally, immune cell infiltration in psoriasis and leprosy was analyzed using single-sample gene set enrichment analysis (ssGSEA), and the correlation between critical mutually exclusive genes and immune cells was also examined. Finally, the expression pattern of critical mutually exclusive genes was evaluated in a single-cell transcriptome dataset. We identified 1098 DEGs in the leprosy dataset and 3839 DEGs in the psoriasis dataset. 48 candidate mutually exclusive genes were identified by taking the intersection. Enrichment analysis revealed that these genes were involved in cholesterol metabolism pathways. Through PPI network analysis, we identified APOE, CYP27A1, FADS1, and SOAT1 as hub genes. APOE, CYP27A1, and SOAT1 were subsequently validated as critical mutually exclusive genes on both internal and external datasets. Analysis of immune cell infiltration indicated higher abundance of 16 immune cell types in psoriasis and leprosy compared to normal controls. The abundance of 6 immune cell types in psoriasis and leprosy positively correlated with the expression levels of APOE and CYP27A1. Single-cell data analysis demonstrated that critical mutually exclusive genes were predominantly expressed in Schwann cells and fibroblasts. This study identified APOE, CYP27A1, and SOAT1 as critical mutually exclusive genes. Cholesterol metabolism pathway illustrated the possible mechanism of the inverse association of psoriasis and leprosy. The findings of this study provide a basis for identifying mechanisms and therapeutic targets for psoriasis.

Tryptophan, an important link in regulating the complex network of skin immunology response in atopic dermatitis

Atopic dermatitis (AD) is a common chronic relapsing inflammatory skin disease, of which the pathogenesis is a complex interplay between genetics and environment. Although the exact mechanisms of the disease pathogenesis remain unclear, the immune dysregulation primarily involving the Th2 inflammatory pathway and accompanied with an imbalance of multiple immune cells is considered as one of the critical etiologies of AD. Tryptophan metabolism has long been firmly established as a key regulator of immune cells and then affect the occurrence and development of many immune and inflammatory diseases. But the relationship between tryptophan metabolism and the pathogenesis of AD has not been profoundly discussed throughout the literatures. Therefore, this review is conducted to discuss the relationship between tryptophan metabolism and the complex network of skin inflammatory response in AD, which is important to elucidate its complex pathophysiological mechanisms, and then lead to the development of new therapeutic strategies and drugs for the treatment of this frequently relapsing disease.

Exosomes released by environmental pollutant-stimulated Keratinocytes/PBMCs can trigger psoriatic inflammation in recipient cells via the AhR signaling pathway

Introduction: Exosomes, pivotal in intercellular communication during skin disease pathogenesis, have garnered substantial attention. However, the impact of environmental pollutants, such as benzo[a]pyrene (BaP) and 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD), on exosome release amid inflammatory skin diseases remains unexplored. This study addresses this gap by examining the influence of BaP and TCDD on exosome function, specifically focusing on immune-related pathway alterations in normal recipient keratinocytes and peripheral blood mononuclear cells (PBMCs). Methods: HaCaT cells were treated with exosomes from BaP- or TCDD-treated keratinocytes. Proinflammatory cytokines and chemokines, including TNF-α, IL-1β, IL-6, IL-8, CXCL1, and CXCL5, were assessed. The involvement of the p65NF-κB/p38MAPK/ERK signaling pathway in recipient keratinocytes was investigated. Aryl hydrocarbon receptor (AhR) silencing was employed to elucidate its role in mediating the proinflammatory response induced by exosomes from BaP- or TCDD-treated keratinocytes. Results and discussion: Treatment with exosomes from BaP- or TCDD-treated keratinocytes induced a significant increase in proinflammatory cytokines and chemokines in HaCaT cells. The upregulation implicated the p65NF-κB/p38MAPK/ERK signaling pathway. AhR silencing attenuated this response, suggesting a role for AhR in mediating this response. In PBMCs from healthy controls, exosomes from BaP-stimulated PBMCs of psoriatic patients led to increased expression of proinflammatory cytokines and modulation of Th1/Th17 cell distribution via AhR activation. These findings unveil a novel dimension in the interplay between environmental xenobiotic agents (BaP and TCDD) and exosomal functions. The study establishes their influence on psoriatic inflammatory responses, shedding light on the underlying mechanisms mediated through the AhR signaling pathway in recipient keratinocytes and PBMCs.

The opposite effect of tapinarof between IMQ and IL-23 induced psoriasis mouse models

Tapinarof is an aryl hydrocarbon receptor (AHR) ligand which is used to treat plaque psoriasis in adults. However, the underlying mechanism is not yet fully understood. In this study, we applied two of the most studied psoriasis mouse models: topical application of imiquimod (IMQ) and subcutaneous injection of IL-23. Although both models successfully induced psoriasis-like lesions in mice, tapinarof had a completely opposite effect on the two models. Tapinarof decreased the expression of multiple essential cytokines involved in the pathological IL-23/IL-17/IL-22 axis and ameliorated IMQ-induced psoriatic dermatitis, inhibiting keratinocyte proliferation and abnormal differentiation. However, in the IL-23-injection-model, tapinarof instead aggravated the disease. Here, tapinarof increased epidermal thickness and differentiated epidermal dysplasia in mice. Our data suggest that tapinarof may have different effects on varied types of psoriasis.

Unraveling the gut-skin axis in atopic dermatitis: exploiting insights for therapeutic strategies

Gut microbiota exert functions of high importance in the intestine. Furthermore, there is increasing evidence for its role in immune regulation and maintenance of homeostasis in many physiological processes taking place in distant tissues. In particular, in this review, we explore the impact of metabolites produced by the gut microbiota on the development of atopic dermatitis (AD). Probiotics and prebiotics balance the microbiota and promote the generation of bacterial metabolites, such as short-chain fatty acids and tryptophan derivates, which promote the regulation of the exacerbated AD immune response through regulatory T cells and IL-10 and TGF-β cytokines. Metabolites also have a direct action on keratinocytes once they reach the bloodstream. Besides, probiotics decrease the levels of metabolites associated with AD onset, such as phenols. Understanding all these crosstalk processes between the gut and the skin reveals a number of possibilities, mainly through the manipulation of the gut microbiome, which may represent therapeutic strategies that can contribute to the standard treatments of AD patients to improve their quality of life.

AhR and CYP1B1 Control Oxygen Effects on Bone Marrow Progenitor Cells: The Enrichment of Multiple Olfactory Receptors as Potential Microbiome Sensors

Polycyclic aromatic hydrocarbon (PAH) pollutants and microbiome products converge on the aryl hydrocarbon receptor (AhR) to redirect selective rapid adherence of isolated bone marrow (BM) cells. In young adult mice, Cyp1b1-deficiency and AhR activation by PAH, particularly when prolonged by Cyp1a1 deletion, produce matching gene stimulations in these BM cells. Vascular expression of Cyp1b1 lowers reactive oxygen species (ROS), suppressing NF-κB/RelA signaling. PAH and allelic selectivity support a non-canonical AhR participation, possibly through RelA. Genes stimulated by Cyp1b1 deficiency were further resolved according to the effects of Cyp1b1 and Cyp1a1 dual deletions (DKO). The adherent BM cells show a cluster of novel stimulations, including select developmental markers; multiple re-purposed olfactory receptors (OLFR); and α-Defensin, a microbial disruptor. Each one connects to an enhanced specific expression of the catalytic RNA Pol2 A subunit, among 12 different subunits. Mesenchymal progenitor BMS2 cells retain these features. Cyp1b1-deficiency removes lymphocytes from adherent assemblies as BM-derived mesenchymal stromal cells (BM-MSC) expand. Cyp1b1 effects were cell-type specific. In vivo, BM-MSC Cyp1b1 expression mediated PAH suppression of lymphocyte progenitors. In vitro, OP9-MSC sustained these progenitors, while Csf1 induced monocyte progenitor expansion to macrophages. Targeted Cyp1b1 deletion (Cdh5-Cre; Cyp1b1fl/fl) established endothelium control of ROS that directs AhR-mediated suppression of B cell progenitors. Monocyte Cyp1b1 deletion (Lyz2-Cre; Cyp1b1fl/fl) selectively attenuated M1 polarization of expanded macrophages, but did not enhance effects on basal M2 polarization. Thus, specific sources of Cyp1b1 link to AhR and to an OLFR network to provide BM inflammatory modulation via diverse microbiome products.

Signaling pathways and targeted therapies for psoriasis

Psoriasis is a common, chronic, and inflammatory skin disease with a high burden on individuals, health systems, and society worldwide. With the immunological pathologies and pathogenesis of psoriasis becoming gradually revealed, the therapeutic approaches for this disease have gained revolutionary progress. Nevertheless, the mechanisms of less common forms of psoriasis remain elusive. Furthermore, severe adverse effects and the recurrence of disease upon treatment cessation should be noted and addressed during the treatment, which, however, has been rarely explored with the integration of preliminary findings. Therefore, it is crucial to have a comprehensive understanding of the mechanisms behind psoriasis pathogenesis, which might offer new insights for research and lead to more substantive progress in therapeutic approaches and expand clinical options for psoriasis treatment. In this review, we looked to briefly introduce the epidemiology, clinical subtypes, pathophysiology, and comorbidities of psoriasis and systematically discuss the signaling pathways involving extracellular cytokines and intracellular transmission, as well as the cross-talk between them. In the discussion, we also paid more attention to the potential metabolic and epigenetic mechanisms of psoriasis and the molecular mechanistic cascades related to its comorbidities. This review also outlined current treatment for psoriasis, especially targeted therapies and novel therapeutic strategies, as well as the potential mechanism of disease recurrence.

A tryptophan metabolite made by a gut microbiome eukaryote induces pro-inflammatory T cells

The large intestine harbors microorganisms playing unique roles in host physiology. The beneficial or detrimental outcome of host-microbiome coexistence depends largely on the balance between regulators and responder intestinal CD4+ T cells. We found that ulcerative colitis-like changes in the large intestine after infection with the protist Blastocystis ST7 in a mouse model are associated with reduction of anti-inflammatory Treg cells and simultaneous expansion of pro-inflammatory Th17 responders. These alterations in CD4+ T cells depended on the tryptophan metabolite indole-3-acetaldehyde (I3AA) produced by this single-cell eukaryote. I3AA reduced the Treg subset in vivo and iTreg development in vitro by modifying their sensing of TGFβ, concomitantly affecting recognition of self-flora antigens by conventional CD4+ T cells. Parasite-derived I3AA also induces over-exuberant TCR signaling, manifested by increased CD69 expression and downregulation of co-inhibitor PD-1. We have thus identified a new mechanism dictating CD4+ fate decisions. The findings thus shine a new light on the ability of the protist microbiome and tryptophan metabolites, derived from them or other sources, to modulate the adaptive immune compartment, particularly in the context of gut inflammatory disorders.

Management of Plaque Psoriasis in Adults: Clinical Utility of Tapinarof Cream

Topical medications represent the most commonly used drugs in the treatment of psoriasis. However, topical steroids are mainly limited to short-term or intermittent use, and traditional non-steroidal topicals such as vitamin D analogues, topical calcineurin inhibitors, and topical retinoids are limited by low efficacy and poor local skin tolerability. Tapinarof (GSK2894512, DMVT-505) is a novel, topical aryl hydrocarbon receptor (AHR) agonist, which was recently approved by the FDA for the treatment of plaque psoriasis in adults. Tapinarof acts to improve psoriasis through diminished IL-17A production by CD4+ T cells, increased barrier gene expression in keratinocytes, and reduced production of reactive oxygen species. Both short-term and long-term efficacy and safety have been evaluated in two Phase II and two Phase III (PSOARING 1 and 2) clinical trials in addition to a long-term extension study (PSOARING 3). Overall, the drug has shown beneficial effects in achieving clear skin in adults with moderate-to-severe psoriasis, good local tolerability, and also a long duration of effect even after discontinuation of the drug. Therefore, this therapy provides a new, highly effective and safe non-steroidal option to add to our psoriasis treatment toolbox for both initial clearance and long-term maintenance of disease.

Activation of the aryl hydrocarbon receptor in inflammatory bowel disease: insights from gut microbiota

Inflammatory bowel disease (IBD) is a chronic inflammatory intestinal disease that affects more than 3.5 million people, with rising prevalence. It deeply affects patients' daily life, increasing the burden on patients, families, and society. Presently, the etiology of IBD remains incompletely clarified, while emerging evidence has demonstrated that altered gut microbiota and decreased aryl hydrocarbon receptor (AHR) activity are closely associated with IBD. Furthermore, microbial metabolites are capable of AHR activation as AHR ligands, while the AHR, in turn, affects the microbiota through various pathways. In light of the complex connection among gut microbiota, the AHR, and IBD, it is urgent to review the latest research progress in this field. In this review, we describe the role of gut microbiota and AHR activation in IBD and discussed the crosstalk between gut microbiota and the AHR in the context of IBD. Taken as a whole, we propose new therapeutic strategies targeting the AHR-microbiota axis for IBD, even for other related diseases caused by AHR-microbiota dysbiosis.

Promoting New Approach Methodologies (NAMs) for research on skin color changes in response to environmental stress factors: tobacco and air pollution

Aging is one of the most dynamic biological processes in the human body and is known to carry significant impacts on individuals' self-esteem. Skin pigmentation is a highly heritable trait made possible by complex, strictly controlled cellular and molecular mechanisms. Genetic, environmental and endocrine factors contribute to the modulation of melanin's amount, type and distribution in the skin layers. One of the hallmarks of extrinsic skin aging induced by environmental stress factors is the alteration of the constitutive pigmentation pattern clinically defined as senile lentigines and/or melasma or other pigmentary dyschromias. The complexity of pollutants and tobacco smoke as environmental stress factors warrants a thorough understanding of the mechanisms by which they impact skin pigmentation through repeated and long-term exposure. Pre-clinical and clinical studies demonstrated that pollutants are known to induce reactive oxygen species (ROS) or inflammatory events that lead directly or indirectly to skin hyperpigmentation. Another mechanistic direction is provided by Aryl hydrocarbon Receptors (AhR) which were shown to mediate processes leading to skin hyperpigmentation in response to pollutants by regulation of melanogenic enzymes and transcription factors involved in melanin biosynthesis pathway. In this context, we will discuss a diverse range of New Approach Methodologies (NAMs) capable to provide mechanistic insights of the cellular and molecular pathways involved in the action of environmental stress factors on skin pigmentation and to support the design of raw ingredients and formulations intended to counter their impact and of any subsequently needed clinical studies.