Ayako Yumine

Atopic dermatitis (AD) is characterized by pruritus and epidermal barrier dysfunction. Tapinarof, a therapeutic aryl hydrocarbon receptor (AHR)-modulating agent, is clinically effective in AD, but its mechanisms in allergen-driven models remain unclear. We evaluated tapinarof in a Dermatophagoides farinae (Df)-induced murine model of AD, focusing on AHR-mediated barrier restoration. AD-like dermatitis was induced in female NC/Nga mice, followed by daily topical tapinarof or betamethasone for four days. Disease severity, transepidermal water loss (TEWL), histopathology, and gene/protein expression were assessed. Tapinarof significantly reduced dermatitis scores and TEWL and, unlike betamethasone, preserved epidermal architecture without suppressing ear swelling. Tapinarof upregulated the AHR-responsive genes Cyp1a1 and Nqo1 and induced Ovol1, an epidermal differentiation-associated transcription factor, with increased CYP1A1, NRF2, NQO1, and OVOL1 protein expression. Tapinarof markedly increased filaggrin (FLG)- and loricrin (LOR)-positive areas in lesional skin compared with vehicle (FLG: 11.3% vs. 3.1%; LOR: 18.9% vs. 3.9%; both p < 0.05), restoring FLG to a level comparable to the no-treatment control, while LOR significantly exceeded it. These findings indicate that tapinarof may restore skin barrier function in allergen-induced AD via AHR-NRF2 and AHR-OVOL1 signaling, supporting its potential as a steroid-sparing topical therapy for long-term AD management.

Atopic dermatitis (AD) is a chronic inflammatory dermatitis underpinned by Type 2 inflammation driven by cytokines such as IL‐4 and IL‐13. It is characterized by skin barrier dysfunction, Th2 immune deviation, and pruritus. While biologics and oral Janus kinase (JAK) inhibitors demonstrate high therapeutic efficacy by targeting cytokines that exert their effects via the JAK, specifically IL‐4, IL‐13, and IL‐31, comprehensive disease control requires additional approaches that modulate JAK‐independent pathways. Factors such as TNF‐α, IL‐25, IL‐33, microbial antigens, and physical stimuli activate mitogen‐activated protein kinase and nuclear factor‐κB signaling in a JAK‐independent manner, sustaining the disease activity. Consequently, in some cases, a strategy incorporating topical treatment that inhibits JAK‐independent pathways is indispensable alongside systemic treatment. One potential strategy of this kind involves the aryl hydrocarbon receptor (AhR), ligand‐activated transcription factor. Under Th2‐polarized conditions, the expression of indoleamine 2,3‐dioxygenase 1 (IDO1) is downregulated, limiting the availability of tryptophan‐derived metabolites. This scarcity of endogenous AhR ligands subsequently compromises physiological AhR signaling. Tapinarof, a therapeutic AhR‐modulating agent (TAMA), activates AhR without generating excessive reactive oxygen species. This exerts multiple effects: enhancing antioxidant defenses via nuclear factor erythroid 2‐related factor 2 (NRF2) activation, restoring barrier dysfunction, suppressing Th2 inflammation, and correcting abnormalities associated with pruritus‐related molecules. Furthermore, clinical trials of tapinarof have demonstrated improvement of the disease activity and pruritus, with efficacy intensifying over long‐term treatment. Although adverse events such as folliculitis, acne, contact dermatitis, and headache occur, they are generally manageable by adjusting the frequency of application based on their reversibility and time of onset. Moreover, considering potential antagonism by IL‐24 induced by tapinarof, combining tapinarof with systemic agents, topical therapies, or phototherapy may serve to optimize the therapeutic effects. In conclusion, AhR functions as a molecular hub integrating AD pathology, and the TAMA tapinarof expands therapeutic strategies in the treatment of AD.

Difamilast, a phosphodiesterase 4 (PDE4) inhibitor, has been shown to be effective in the treatment of atopic dermatitis (AD), although the mechanism involved remains unclear. Since IL-33 plays an important role in the pathogenesis of AD, we investigated the effect of difamilast on IL-33 activity. Since an in vitro model of cultured normal human epidermal keratinocytes (NHEKs) has been utilized to evaluate the pharmacological potential of adjunctive treatment of AD, we treated NHEKs with difamilast and analyzed the expression of the suppression of tumorigenicity 2 protein (ST2), an IL-33 receptor with transmembrane (ST2L) and soluble (sST2) isoforms. Difamilast treatment increased mRNA and protein levels of sST2, a decoy receptor suppressing IL-33 signal transduction, without affecting ST2L expression. Furthermore, supernatants from difamilast-treated NHEKs inhibited IL-33-induced upregulation of TNF-α, IL-5, and IL-13 in KU812 cells, a basophil cell line sensitive to IL-33. We also found that difamilast activated the aryl hydrocarbon receptor (AHR)–nuclear factor erythroid 2-related factor 2 (NRF2) axis. Additionally, the knockdown of AHR or NRF2 abolished the difamilast-induced sST2 production. These results indicate that difamilast treatment produces sST2 via the AHR–NRF2 axis, contributing to improving AD symptoms by inhibiting IL-33 activity.

Semaphorin 3A (SEMA3A), a nerve-repellent factor produced by keratinocytes, has an inhibitory effect on nerve extension to the epidermis. Epidermal innervation is involved in pruritus in inflammatory skin diseases such as atopic dermatitis (AD) and dry skin. We previously reported that tapinarof, a stilbene molecule, upregulates SEMA3A in human keratinocytes. We also showed that this mechanism is mediated via the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor, and the nuclear factor erythroid 2-related factor 2 (NRF2) axis. Since some stilbenes activate AHR and NRF2, we attempted to identify other stilbenes that upregulate SEMA3A. We analyzed normal human epidermal keratinocytes (NHEKs) treated with 11 types of stilbenes and examined SEMA3A expression. We found that resveratrol and pinostilbene, antioxidant polyphenols, upregulated SEMA3A and increased nuclear AHR and NRF2 expression. In addition, AHR knockdown by small interfering RNA (siRNA) transfection abolished the NRF2 nuclear expression. Furthermore, AHR and NRF2 knockdown by siRNA transfection abrogated resveratrol- and pinostilbene-induced SEMA3A upregulation. Finally, we confirmed that resveratrol and pinostilbene increased SEMA3A promoter activity through NRF2 binding using ChIP-qPCR analysis. These results suggest that resveratrol and pinostilbene upregulate SEMA3A via the AHR–NRF2 axis in human keratinocytes.

Neurofibromatosis type 1 is an autosomal dominant genetic disorder caused by mutation in the neurofibromin 1 (NF1) gene. Its hallmarks are cutaneous findings including neurofibromas, benign peripheral nerve sheath tumors. We analyzed the collagen and matrix metalloproteinase 1 (MMP1) expression in Neurofibromatosis 1 cutaneous neurofibroma and found excessive expression of collagen and reduced expression of MMP1. To identify new therapeutic drugs for neurofibroma, we analyzed phosphorylation of components of the Ras pathway, which underlies NF1 regulation, and applied treatments to block this pathway (PD184352, U0126, and rapamycin) and lysosomal processes (chloroquine (CQ), hydroxychloroquine (HCQ), and bafilomycin A (BafA)) in cultured Neurofibromatosis 1 fibroblasts. We found that downregulation of the MMP1 protein was a key abnormal feature in the neurofibromatosis 1 fibroblasts and that the decreased MMP1 was restored by the lysosomal blockers CQ and HCQ, but not by the blockers of the Ras pathway. Moreover, the MMP1-upregulating activity of those lysosomal blockers was dependent on aryl hydrocarbon receptor (AHR) activation and ERK phosphorylation. Our findings suggest that lysosomal blockers are potential candidates for the treatment of Neurofibromatosis 1 neurofibroma.