Identification of aryl hydrocarbon receptor as a functional target that enhances astrocytic ApoE secretion

We report the discovery of a chemical series that enhances ApoE secretion from human astrocytes through mechanisms independent of LXR agonism. Target deconvolution of hits from a phenotypic screen in astrocytoma cells employed chemoproteomics, photoaffinity probes, in vitro KINOMEscan analysis, and targeted siRNA knockdown experiments. Photoaffinity labeling coupled with quantitative chemical proteomics identified aryl hydrocarbon receptor (AhR), a transcription factor not previously associated with ApoE secretion, as the primary target. A diverse panel of AhR agonists and antagonists together with genetic knockdown confirmed that ApoE secretion increases when AhR activity is reduced. Using a luciferase reporter assay, we demonstrated that active series analogs exhibit AhR antagonism while inactive compounds do not. Since deletion of AhR has severe peripheral effects, chronic inhibition of AhR is not an attractive therapeutic approach for Alzheimer's disease; nevertheless, these results position AhR as a modulator of ApoE secretion and a biological pathway worth exploring.