O01 Molecular investigations of the AHR pathway in atopic dermatitis reveals a functional dysregulation impacting its homeostatic role in skin

Abstract Introduction and aims Atopic dermatitis (AD) is a chronic inflammatory skin disease characterized by epidermal barrier disruption and type-2 immune dysregulation. Clinical response to targeted biologics varies, highlighting the need for additional therapeutic strategies. The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor maintaining skin homeostasis by reducing inflammation. Activated AHR induces CYP1A1 expression, a biomarker of pathway activation, critical for AHR ligand metabolism. Tapinarof, a topical AHR modulator, is approved for psoriasis and is effective in phase III clinical trials in AD, highlighting the importance of this pathway in inflammatory skin disease. Here, we perform a global investigation of the AHR pathway to uncover at which level it is dysregulated and not exerting its anti-inflammatory effect. Methods Skin biopsies were obtained from patients with AD not receiving any systemic treatment (n = 16) and age-, sex- and site-matched healthy volunteers (n = 17 biopsies, n = 3 keratinocytes) to measure: (i) AHR and CYP1A1 expression by quantitative reverse-transcription polymerase chain reaction (qRT-PCR), with spatial expression assessed by RNAscope; (ii) AHR expression in interleukin (IL)-4- and IL-13-stimulated keratinocytes by qRT-PCR; and (iii) CYP1 enzymatic activity in healthy and AD epidermis by ethoxyresorufin-O-deethylase assay. Results AHR is significantly upregulated in lesional AD (n = 6) vs. healthy skin (n = 9, P < 0.05) while CYP1A1 expression shows a downward trend in lesional AD vs. healthy skin (P = 0.0625). AHR localizes more in the basal epidermal layer in AD (n = 7) vs. healthy skin (n = 8). IL-4 and IL-13 significantly upregulated AHR (n = 3 donors, P < 0.05) in keratinocytes. Finally, CYP1 enzymes are normally active in AD epidermis (n = 10, Q < 0.05) with no difference in basal or inducible activity vs. healthy controls (n = 8). Conclusions We have uncovered a dysfunctional AHR pathway in AD, with increased AHR expression but reduced pathway activation. Further work investigating AHR nuclear translocation and ligand availability will provide a full assessment of the pathway, aiming at maximizing its potential as a therapeutic target in inflammatory skin disease.