Kynurenine Is the Main Metabolite of Tryptophan Degradation by Tryptophan 2,3-Dioxygenase in HepaRG Tumor Cells

Abstract

There are two main enzymes that convert tryptophan (Trp) to kynurenine (Kyn): tryptophan-2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO). Kyn accumulation can promote immunosuppression in certain cancers. In this study, we investigated Trp degradation to Kyn by IDO and TDO in primary human hepatocytes (PHH) and tumoral HepG2 cells. To quantify Trp-degradation and Kyn-accumulation, using reversed-phase high-pressure liquid chromatography, the levels of Trp and Kyn were determined in the culture media of PHH and HepG2 cells. The role of IDO in Trp metabolism was investigated by activating IDO with IFN-γ and inhibiting IDO with 1-methyl-tryptophan (1-DL-MT). The role of TDO was investigated using one of two TDO inhibitors: 680C91 or LM10. Real-time PCR was used to measure TDO and IDO expression. Trp was degraded in both PHH and HepG2 cells, but degradation was higher in PHH cells. However, Kyn accumulation was higher in the supernatants of HepG2 cells. Stimulating IDO with IFN-γ did not significantly affect Trp degradation and Kyn accumulation, even though it strongly upregulated IDO expression. Inhibiting IDO with 1-DL-MT also had no effect on Trp degradation. In contrast, inhibiting TDO with 680C91 or LM10 significantly reduced Trp degradation. The expression of TDO but not of IDO correlated positively with Kyn accumulation in the HepG2 cell culture media. Furthermore, TDO degraded L-Trp but not D-Trp in HepG2 cells. Kyn is the main metabolite of Trp degradation by TDO in HepG2 cells. The accumulation of Kyn in HepG2 cells could be a key mechanism for tumor immune resistance. Two TDO inhibitors, 680C91 and LM10, could be useful in immunotherapy for liver cancers.

Contribution of Circulating Host and Microbial Tryptophan Metabolites Toward Ah Receptor Activation

“…An estimated 90% of dietary Trp enters the kynurenine pathway, playing an important role in regulating homeostatic Trp concentrations, while also producing compounds capable of activating the AHR, including kynurenine, kynurenic acid, and xanthurenic acid. [9][10][11] A much smaller proportion of dietary Trp (~2%) enters the serotonin pathway, although these compounds are not known to directly interact with AHR and do not represent a current significant area of research. 12 Bacteria in the gut microbiome use an estimated 5% of Trp to produce indole-based derivatives.…”

COVID‐19‐associated liver injury: Adding fuel to the flame

“…161 TDO has been revealed to be responsible for tryptophan degradation and kynurenine accumulation in liver cancer-associated hepatocytes. 162 Noteworthy, tryptophan metabolic reprogramming caused by CLD may aggravate the severity of COVID-19 patients. However, the participation of TDO and IDO in tryptophan metabolism during viral infection needs further investigation.…”

“…In the MHV‐A95 infection mouse model, both TDO and IDO were significantly increased 161 . TDO has been revealed to be responsible for tryptophan degradation and kynurenine accumulation in liver cancer‐associated hepatocytes 162 . Noteworthy, tryptophan metabolic reprogramming caused by CLD may aggravate the severity of COVID‐19 patients.…”

The kynurenine pathway presents multi-faceted metabolic vulnerabilities in cancer

“…The utility of these newer generation TDO2 inhibitors for anti-cancer treatment remains to be determined. Studies using HepG2 liver cancer cells demonstrated that TDO2-inhibitors 680C91 or LM10 significantly reduced Trp degradation and that TDO2, but not IDO1, was the primary source of Kyn production in these cells ( 144 ). 680C91 has also been explored in other cancer types including melanoma ( 145 – 147 ), colon ( 147 ), leiomyomas ( 148 ), and gliomas ( 149 ).…”