PROJECT SUMMARY The long-term goal of this project is to understand the role of the aryl hydrocarbon receptor (AHR) in the regulation of the function and homeostasis of the skin epidermal permeability barrier. In this project we will expand on our discovery that the AHR regulates keratinocyte differentiation by controlling metabolic reprogramming and test the hypothesis that differences in the ligand-directed metabolomes moderate the balance between toxic and therapeutic AHR ligands. Past project periods of this application have focused on understanding the mechanisms of action that result in the skin toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and other environmental pollutants that work through the AHR. In 2022, tapinarof (TAP) was approved as the first therapeutic AHR ligand for treatment of psoriasis. This application proposes to leverage the differences in the actions of these AHR ligands to explore their distinct programs of metabolic reprogramming, contributing to their respective metabolomes and their actions on the skin. In preliminary studies we identified lipid biosynthesis and metabolism as major components of AHR-regulated metabolic reprogramming. Using lipidomic analyses, we found differences in key bioactive lipids of the lipidomes of TCDD and TAP that differ in pro- and anti-inflammatory activities. To test our hypothesis that differences in the ligand-directed metabolomes moderate the balance between disease-promoting (“Toxic”) and health-promoting (“Therapeutic”) AHR ligands, we propose three aims. 1) Define the ligand-directed metabolomes of TCDD and TAP, cataloging common and divergent lipidomes and their bioactive lipids; 2) Determine the modulatory effects of TCDD and TAP on human keratinocyte models of psoriasis; 3) Identify the key bioactive lipid pathways of TCDD and TAP in mouse epidermis and determine their roles in ligand modulation of imiquimod- induced psoriasiform lesions. Findings from these studies will advance the fundamental understanding of the molecular responses to ligand activation of the keratinocyte AHR and the physiological processes of the epidermis that are affected in a ligand-selective manner.