Skin-targeted AhR activation by microbial and synthetic indoles: Insights from the AhaRaCaT reporter cell line.

Disruption of the epidermal barrier contributes to skin disorders such as atopic dermatitis and psoriasis. The aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, plays a key role in skin homeostasis and immune regulation. While traditionally associated with toxicity, AhR has emerged as a promising therapeutic target, particularly via tryptophan-derived indoles. To support AhR research in a dermatological context, we developed AhaRaCaT, a stable luciferase-based reporter cell line derived from human keratinocytes (HaCaT), enabling the assessment of AhR transcriptional activity in a skin-relevant model. We characterized the inducibility of AhaRaCaT in response to model AhR ligands (TCDD, BaP, FICZ) in dose- and time-dependent assays. Antagonist profiling with MNF, CH223191, GNF, carvone, and jasmone yielded IC50 values over 4- and 24-hour exposures. A panel of indoles previously studied in other models was evaluated for AhR activation, revealing a robust luciferase response at 4 h that declined at 24 h, consistent with trends observed in other cell types. Selected indoles also induced CYP1A1 mRNA expression and reversed cytokine-induced downregulation of filaggrin in HaCaT cells, highlighting their potential in mitigating inflammation-associated skin barrier defects. In summary, the AhaRaCaT cell line offers a sensitive and physiologically relevant tool for studying AhR signaling in skin, with broad applications in toxicology, dermatological research, and the development of AhR-targeted therapies for inflammatory skin diseases.