Modulation of the aryl hydrocarbon receptor by endogenous and exogenous ligands
Parks, Ashley Joan
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This year it is estimated that over 34,000 American women will be diagnosed with triple-negative breast cancer (TNBC), an aggressive disease resistant to current targeted therapies. Consequently, development of new therapeutics that can be used to combat TNBC is an area of intense medical research. The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that mediates many biological processes. Dysregulation of normal AhR expression and function is observed in breast cancers and promotes tumor growth. It is likely that AhR activation is involved in many steps of mammary tumor progression; however, the endogenous AhR ligand(s) driving these functions in mammary tissue remains a mystery. Here, we surveyed tryptophan metabolites to identify endogenous AhR agonists in mammary epithelial cells. Several metabolites, including 6-formylindolo[3,2-b]carbazole (FICZ), xanthurenic acid, indoxyl sulfate and kynurenic acid were found to activate the AhR in a mammary epithelial cell line and to bind directly to human AhR. Untargeted metabolomic studies identified indoxyl sulfate in cell extracts and strongly suggested that serum added to cultures is its source. The common presence of an efficacious AhR agonist in human sera and in serum used in our studies suggests the intriguing possibility that AhR activity in vivo is controlled, at least in part, by production of this bacteria-derived metabolite, potentially linking the microbiome to AhR-mediated mammalian cell function. Given the potential role of the AhR in mammary tumorigenesis, we sought to develop a new method for high throughput identification of novel AhR modulators for therapeutic applications. By screening of structure-guided chemical libraries, we identified CB7993113 as a pure, competitive AhR antagonist. In vivo, CB7993113 inhibited the acute toxicity associated with exposure to 7,12-dimethylbenz[α]anthracene, a prototypic AhR agonist. Additionally, CB7950998 was discovered to be a non-toxic AhR agonist that is able to decrease cell proliferation in human ER- breast cancer cells. These results are the first steps in the preclinical investigation for these AhR modulators. These studies advance the understanding of the endogenous agonists that drive endogenous AhR activation in mammary cells, and present two novel exogenous AhR ligands to be investigated for their therapeutic potential through modulation of AhR activity.
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