The role of environmental and endogenous AHR ligands in estrogen receptor, progesterone receptor, human epidermal growth factor receptor 2-negative (ER-/PR-/HER2-) breast cancer progression
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Recent studies indicate that endogenously ligand-activated Aryl Hydrocarbon Receptor (AHR) plays an important role in normal and pathological processes, including the induction and progression of breast cancer. As the known number of AHR-mediated processes grows, so too does the importance of identifying endogenous AHR ligands that influence breast cancer progression. The following studies focus on two tryptophan metabolism pathway branches, the kynurenine (KYN) branch and the indoxyl sulfate (IS) branch, to determine if ER-/PR-/HER2- breast cancer cells can produce, or are exposed to, AHR ligands from these branches, how these ligands affect cell migration and, if produced, how their production is controlled. It is hypothesized that: 1) malignant cells produce, or derive from their microenvironment, AHR ligands through the KYN and/or IS pathways, 2) these metabolites drive AHR-dependent breast cancer migration, 3) environmental AHR ligands mimic the effects of endogenous ligands, 4) rate-limiting kynurenine pathway enzymes are responsible for endogenous AHR ligand production and their downstream effects, and 5) the AHR controls expression of a rate-limiting kynurenine pathway enzyme(s) in a positive feedback loop. ER-/PR-/HER2- mammary epithelial cells were assayed for production of AHR-activating tryptophan metabolites and for the kynurenine pathway enzymes tryptophan 2,3-dioxygenase (TDO) and indolamine 2,3-dioxygenase (IDO). The relationship between kynurenine and IS pathways, AHR activity, cell migration, and aldehyde dehydrogenase 1 (ALDH1), a cancer stem cell marker associated with poor prognosis, was investigated using tryptophan metabolites, enzyme-specific gene knockdown or over-expression, qPCR, cell migration assays and ALDH1 activity assay. ER-/PR-/HER2- tumor cells produce KYN and its downstream metabolite xanthurenic acid (XA), at levels sufficient to activate the AHR. KYN, XA, and IS significantly accelerate migration in an AHR-dependent fashion, and at physiological doses, while environmental AHR ligands 2,3,7,8-tetrachlorodibenzodioxin (TCDD) and benzo[a]pyrene (B[a]P) mimic this effect. IS induces ALDH1 activity. AHR knockdown or inhibition significantly reduces Tdo2 expression. Finally, gene expression dataset analyses reveal high Tdo2 levels in primary breast tumors, with the highest levels in ER-/PR-/Her2- and stage 4 tumors. These studies identify three tryptophan-derived AHR ligands that contribute to breast cancer progression and demonstrate a positive feedback loop, where AHR activity up-regulates Tdo2, which drives endogenous AHR ligand production.