O02 A molecular investigation of the AHR pathway reveals a multilevel dysregulation that abrogates its anti-inflammatory activity in psoriasis

Abstract Introduction and aims The aryl hydrocarbon receptor (AHR) is a ligand-activated anti-inflammatory transcription factor and the first-in-class AHR-modulating topical drug, tapinarof, is approved for mild-to-moderate psoriasis. The AHR pathway is tightly regulated, including through the cytochrome P450 1 (CYP1) enzymes, which degrade AHR ligands providing negative feedback. Here we investigate the AHR pathway in depth to uncover the molecular mechanisms preventing its anti-inflammatory effect in psoriasis. Methods Three independent cohorts of patients (n = 47) with moderate-to-severe psoriasis not receiving systemic therapy were recruited, alongside four cohorts of age-, sex- and ethnicity-matched healthy volunteers (n = 60). We measured: (i) AHR and CYP1A1 bulk and spatial mRNA and protein in skin, (ii) in vitro modulation of AHR and CYP1A1 expression in normal human epidermal keratinocytes (NHEKs) and skin epidermal sheet by psoriasis-relevant cytokines, and (iii) CYP1 enzymatic activity in psoriasis and healthy epidermal sheets. Results AHR expression was decreased (n = 21, P < 0.01; n = 18, P < 0.01; n = 8, P < 0.05) in all three cohorts of psoriasis lesional skin (PsL) vs. healthy skin (n = 38, n = 9, n = 8), while CYP1A1 expression was marginally reduced (P = 0.0546; n = 16). AHR mRNA predominantly localized in the basal layer in PsL (n = 5), with reduced frequency of AHR + CYP1A1 + PsL basal keratinocytes (n = 5) vs. healthy controls (n = 9, P < 0.01). In vitro culture of epidermal sheets with an AHR ligand-induced CYP1 activity in healthy skin and psoriasis nonlesional skin but not in PsL, while upregulating CYP1A1 expression in all tissue types (n = 7, Q < 0.05). Culturing normal epidermal sheets with a proinflammatory cocktail containing tumour necrosis factor reduced CYP1 activity (n = 3, P < 0.05). Finally, AHR ligation downregulated its own mRNA expression in the presence of proinflammatory cytokines (n = 3, P < 0.001), mirroring what is observed in PsL. Conclusions We have identified a multilevel dysregulation of the AHR pathway in psoriasis, abrogating its anti-inflammatory effect. These findings have important implications for novel therapeutic interventions aimed at restoring the beneficial effect of AHR in skin.