TDO2 and tryptophan metabolites promote kynurenine/AhR signals to facilitate glioma progression and immunosuppression.

Tumor cells exhibit enhanced uptake and processing of nutrients to fulfill the demands of rapid growth of tumor tissues. Tryptophan metabolizing dioxygenases are frequently up-regulated in several tumor types, which has been recognized as a crucial determinant in accelerated tumor progression. In our study, we explored the specific role of tryptophan 2,3-dioxygenase 2 (TDO2) in glioma progression. Analysis of mRNA profiles in 325 glioma patients based on the rich set of CCGA database was performed, which revealed that high TDO2 expression was tightly correlated with poor prognosis in glioma patients. TDO2 increased intracellular levels of tryptophan metabolism in the kynurenine (Kyn) pathway in vitro and in vivo, resulting in sustained glioma cell proliferation. Mechanistically, overexpression of TDO2 promoted the secretion of Kyn, which in turn stimulated the activation of the aryl hydrocarbon receptor (AhR)/AKT signaling pathway, resulting in heightened proliferative properties and tumorigenic potential in glioma cells. Meanwhile, Kyn produced by tumor cells further suppressed the proliferation of functional T cells, thereby resulting in immunosuppression and enhanced tumor growth in glioma. Our study showed that TDO2-induced increase in tryptophan metabolite Kyn played a pivotal role in glioma development via the AhR/AKT pro-survival signals and immunosuppressive effects, suggesting that the use of TDO2 inhibitors in combination with chemotherapy may be a novel strategy to effectively and synergistically eliminate glioma cells.