Constitutive TDO2 expression promotes liver cancer progression by an autocrine IL-6 signaling pathway

BACKGROUND: Increased tryptophan (Trp) metabolism by indoleamine 2,3-dioxygenase (IDO)/tryptophan 2,3-dioxygenase (TDO) represents one of the most studied pathways for immunosuppression in tumor tissues. However, the pro-tumor effects induced by Trp metabolism remain controversial. METHODS: The paraffin sections of tumor tissues were obtained from patients with liver cancer and examined by immunohistochemical staining to investigate the role of Trp metabolic enzymes. To further confirm the pro-tumor effects induced by TDO2, we established TDO2 overexpression SMC-7721 and HepG2 liver cancer cell lines, and western blotting, cell proliferation, and colony formation were evaluated. Meanwhile, liver cancer subcutaneous mice models were established, and the tumorigenic rates of SMC-7721 cells, tumor volume and survival of bearing mice were calculated. In addition, the survival data of liver cancer patients from The Cancer Genome Atlas (TCGA) database were downloaded to analyze the effect of TDO2 expression on the survival of patients with liver cancer. RESULTS: Here, we showed that constitutive TDO2 expression gave rise to liver cancer through upregulation of Trp metabolism. And the TDO2 expression was positively correlated with the poor prognosis in liver cancer patients. TDO2 expression in tumor cells accounted for the release of kynurenine (Kyn), which activated aryl hydrocarbon receptor (AhR) to promote liver cancer cells proliferation. Mechanistically, we found that AhR expression contributed to the secretion of Interleukin-6 (IL-6), thereby promoting tumor cells proliferation through the STAT3 and NF-kB/TIM4 signals. Interrupt of AhR signals by PDM2 revealed improved outcomes in subcutaneous tumor-bearing mice. CONCLUSIONS: Together, our study showed that the TDO2/Kyn/AhR/IL-6 signaling pathway was a novel mechanism underlying the malignancy of liver cancer, and suggested that AhR signals might be a valuable therapeutic target for tumor therapy.