Blocking tryptophan catabolism reduces triple-negative breast cancer invasive capacity

Anchorage-independent triple-negative breast cancer (TNBC) cells exhibit elevated levels of the tryptophan (TRP) catabolizing enzyme tryptophan 2,3-dioxygenase 2 (TDO2) compared to the same cells grown in two-dimensional culture. Tracing of 13C11-TRP demonstrated that anchorage-independent culture and/or inflammatory cytokines that activate nuclear factor kappa-light-chain-enhancer of activated B (NFκB) increase TRP catabolism and production of downstream catabolites such as kynurenine (KYN), 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 non-homologous 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 protein, while exogenous KYN increased invasion through AhR-mediated ZEB1 expression. Thus, dual inhibition of TDO2/IDO1 may prove efficacious against TNBC progression.