Characterization of Tryptophan 2,3-dioxygenase in human melanoma cell lines and endothelial cells as a new therapeutic target

Indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase (TDO) are heme-enzymes involved in the degradation of tryptophan (Trp) into kynurenine (Kyn) within the kynurenine pathway (KP). While the role of IDO1 in cancer immune escape has been extensively studied, recent evidence suggests that TDO may also contribute to tumor progression, particularly in melanoma. This study aimed to characterize TDO expression and function in human melanoma cell lines and the tumor microenvironment, assessing its potential as a therapeutic target. The findings demonstrate that TDO is expressed in melanoma cells with different characteristics (sex, donor age, metastatic site, and aggressiveness) and regulates key tumor functions such as proliferation, cell cycle progression, and clonogenic potential. Notably, TDO expression increased following IDO1 inhibition with epacadostat, suggesting a compensatory mechanism that may sustain immune evasion. Moreover, TDO expression is regulated by glucocorticoids (GCs), particularly dexamethasone (Dex), which enhances melanoma cell proliferation, migration, and colony formation through TDO activation. The study also explored TDO’s role in melanoma stemness, showing that Dex promotes the formation of melanospheres and upregulates cancer stem cell markers, an effect counteracted by TDO inhibition. In addition, the research investigated TDO’s involvement in tumor angiogenesis and melanoma-endothelial interactions. TDO, but not IDO1, was found to mediate VEGF-A-induced proliferation and tube formation in endothelial cells, as well as the pro-angiogenic effects of melanoma cells. Finally, melanoma bone metastasis was studied using an in vitro model with human bone fragments. While melanoma cells did not exhibit significant bone-directed migration, bone fragments stimulated cancer cell proliferation. Notably, TDO inhibition reduced this effect, suggesting a potential role for TDO in melanoma bone metastasis. Overall, these findings establish TDO as a key player in melanoma progression, influencing both tumor-intrinsic mechanisms and interactions within the microenvironment. Given its role in immune escape, angiogenesis, and response to glucocorticoids, TDO represents a promising therapeutic target, particularly in the context of IDO1 inhibition.