Blocking Tryptophan Catabolism Reduces Triple-Negative Breast Cancer Invasive Capacity

Abstract Anchorage-independent triple-negative breast cancer (TNBC) cells exhibit elevated levels of the tryptophan (TRP)-catabolizing enzyme tryptophan 2,3-dioxygenase 2 (TDO2) compared with the same cells grown under two-dimensional culture conditions. Tracing of 13C11-TRP demonstrated that anchorage-independent culture and/or inflammatory cytokines that activate NF-κB increase TRP catabolism and production of downstream catabolites such as kynurenine, which activate the aryl hydrocarbon receptor (AhR). TDO2 expression is heterogeneous within TNBC cell lines. To determine the function of TDO2, both pharmacologic inhibition and genetic manipulation were conducted. TDO2 knockdown revealed a compensatory increase in indoleamine 2,3-dioxygenase 1 (IDO1), a nonhomologous TRP-catabolizing enzyme, indicating that dual inhibition of these two enzymes is necessary to reliably block TRP catabolism. Thus, we tested a newly developed TDO2/IDO1 dual inhibitor, AT-0174, and found that it effectively inhibits TNBC TRP catabolism. Furthermore, AT-0174 treatment or AhR inhibitor significantly decreased TNBC anchorage-independent survival, invasive capacity, and expression of mesenchymal genes and proteins, whereas exogenous kynurenine increased invasion through AhR-mediated zinc finger E-box–binding homeobox 1 (ZEB1) expression. Thus, dual inhibition of TDO2/IDO1 may prove efficacious against TNBC progression. Significance: TDO2 is more highly expressed than the nonhomologous TRP-catabolizing enzyme IDO1 in TNBC. We find that TDO2 knockdown can lead to a compensatory increase in IDO1. Therefore, we tested a newly developed TDO2/IDO1 dual inhibitor and found that it decreases TRP catabolism, anchorage-independent survival, and invasive capacity.