The aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor that is ubiquitously expressed across vertebrate species. Decades of research have associated AHR activation by pollutants to numerous harmful health effects in wildlife and humans. While the associations to these adversities are clear, the molecular cues dictating detrimental versus beneficial effects of AHR activation are still lacking. At the core of this project lies the challenge of uncovering molecular mechanisms related to AHR signalling, with the aim to understand how this receptor on the one side mediates chemical toxicity, and on the other, is critical for several physiological functions. We are focused on the consequences of AHR dysregulation in the intestine, where it regulates processes related to immune and epithelial physiology. Utilizing tailored mouse models and in vitro models, we demonstrated that the AHR-regulated cytochrome P4501 (Cyp1) enzymes are critical for controlling the duration of AHR signaling by mediating clearance of receptor ligands. Moreover, depleting AHR function systemically by either knocking out the Ahr itself or overexpressing the Cyp1a1 enzyme results in a distinct metabolome phenotype with e.g., dysregulation of cholesterol-derived metabolites and other lipids (unpublished data). The metabolome is similarly altered when restricting the Cyp1a1 overexpression specifically to intestinal epithelial cells (IECs), suggesting a critical role of intestinal AHR as regulator of systemic functions. The challenge now is to delineate the local versus systemic role of intestinal AHR function, and the role of the CYP1-mediated negative feedback in these. The overarching question addressed here is how an impaired intestinal AHR function can generate such a tangible and potentially adverse systemic response. To address this question, we utilized a set of mouse models including mice with i) constitutive Ahr knockout or Cyp1a1 overexpression restricted to IECs, ii) tamoxifen-inducible overexpression of Cyp1a1 restricted to IECs, or iii) constitutive systemic knockout of Ahr or overexpression of Cyp1a1. By combining these novel mouse models with a multi-OMICs approach, including parallel analysis of the serum metabolome, fecal metagenome, and colon- and liver transcriptomes, the major specific questions we aim to investigate are: • The consequences of a genetic versus functional depletion of intestinal AHR function • The importance of onset of AHR function depletion • Comparison of systemic vs. local depletion of AHR function on metabolome effects • Can the metabolome phenotype be explained by local changes in the intestine vs. the liver • Associations between microbial composition and function on local vs. systemic effects. Answering these questions will be of substantial importance not only for the basic understanding of this still enigmatic receptor, but also for preventing AHR-mediated chemical toxicity and for utilizing the AHR/CYP1 pathway as therapeutic target. Importantly, the different datasets were obtained from same mice, to reduce confounding factors at integration of the data.