Differentially expressed genes in TNBC cells grown in attached vs. forced suspension conditions (S1); Increasing intracellular and secreted metabolites by unsupervised metabolomic analysis (S2); Increased expression of kynurenine pathway enzymes is unique to TNBC cell lines and facilitates endogenous activation of AhR (S3); AhR pathway components are differentially expressed in attached versus suspended conditions (S4); Analysis of TDO2 Inhibitor GI50 in TNBC cell lines (S5); Knockdown of TDO2 and AhR decrease proliferation of TNBC cells (S6); IDO1 expression in breast cancer clinical samples (S7).
ARTICLE ABSTRACT
The ability of a cancer cell to develop resistance to anoikis, a programmed cell death process triggered by substratum detachment, is a critical step in the metastatic cascade. Triple-negative breast cancers (TNBC) exhibit higher rates of metastasis after diagnosis, relative to estrogen-positive breast cancers, but while TNBC cells are relatively more resistant to anoikis, the mechanisms involved are unclear. Through gene expression and metabolomic profiling of TNBC cells in forced suspension culture, we identified a molecular pathway critical for anchorage-independent cell survival. TNBC cells in suspension upregulated multiple genes in the kynurenine pathway of tryptophan catabolism, including the enzyme tryptophan 2,3-dioxygenase (TDO2), in an NF-κB–dependent manner. Kynurenine production mediated by TDO2 in TNBC cells was sufficient to activate aryl hydrocarbon receptor (AhR), an endogenous kynurenine receptor. Notably, pharmacologic inhibition or genetic attenuation of TDO2 or AhR increased cellular sensitivity to anoikis, and also reduced proliferation, migration, and invasion of TNBC cells. In vivo, TDO2 inhibitor–treated TNBC cells inhibited colonization of the lung, suggesting that TDO2 enhanced metastatic capacity. In clinical specimens of TNBC, elevated expression of TDO2 was associated with increased disease grade, estrogen receptor–negative status, and shorter overall survival. Our results define an NF-κB–regulated signaling axis that promotes anoikis resistance, suggest functional connections with inflammatory modulation by the kynurenine pathway, and highlight TDO2 as an attractive target for treatment of this aggressive breast cancer subtype. Cancer Res; 75(21); 4651–64. ©2015 AACR.
