IDO1 and IDO2 are expressed in human tumors: levo- but not dextro-1-methyl tryptophan inhibits tryptophan catabolism

OBJECTIVES: Indoleamine-2,3-Dioxygenase (IDO) is an immunosuppressive molecule inducible in various cells. In addition to classic IDO (IDO1), a new variant, IDO2, has recently been described. When expressed in dendritic cells (DCs) or cancer cells, IDO was thought to suppress the immune response to tumors. A novel therapeutic approach in cancer envisages inhibition of IDO with 1-methyl-tryptophan (1MT). The levo-isoform (L-1MT) blocks IDO1, whereas dextro-1MT (D-1MT), which is used in clinical trials, inhibits IDO2. Here we analyze IDO2 expression in human cancer cells and the impact of both 1-MT isoforms on IDO activity.

METHODS: Surgically extirpated human primary tumors as well as human cancer cell lines were tested for IDO1 and IDO2 expression by RT-PCR. IDO1 activity of Hela cells was blocked by transfection with IDO1-specific siRNA and analysed for tryptophan degradation by RP-HPLC. The impact of D-1MT and L-1MT on IDO activity of Hela cells and protein isolates of human colon cancer were studied.

RESULTS: Human primary gastric, colon and renal cell carcinomas constitutively expressed both, IDO1 and IDO2 mRNA, whereas cancer cells lines had to be induced to by Interferon-gamma (IFN-gamma). Treatment of Hela cells with IDO1-specific siRNA resulted in complete abrogation of tryptophan degradation. Only L-1MT, and not D-1MT, was able to block IDO activity in IFN-gamma-treated Hela cells as well as in protein isolates of primary human colon cancer.

CONCLUSIONS: Although IDO2 is expressed in human tumors, tryptophan degradation is entirely provided by IDO1. Importantly, D-1MT does not inhibit the IDO activity of malignant cells. If ongoing clinical studies show a therapeutic effect of D-1MT, this cannot be attributed to inhibition of IDO in tumor cells.