title: IDO1 and IDO2 are expressed in human tumors: levo- but not dextro-1-methyl tryptophan inhibits tryptophan catabolism.
abstract: OBJECTIVE
  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.
authors.affiliation: Department of General, Visceral and Transplant Surgery, University Hospital of Tübingen, Tübingen, Germany. stefan.loeb@med.uni-tuebingen.de
authors.forename: Stefan
authors.initials: S
authors.lastname: Löb
authors.forename: Alfred
authors.initials: A
authors.lastname: Königsrainer
authors.forename: Derek
authors.initials: D
authors.lastname: Zieker
authors.forename: Björn L D M
authors.initials: BL
authors.lastname: Brücher
authors.forename: Hans-Georg
authors.initials: HG
authors.lastname: Rammensee
authors.forename: Gerhard
authors.initials: G
authors.lastname: Opelz
authors.forename: Peter
authors.initials: P
authors.lastname: Terness
mesh_terms: D045744:Cell Line, Tumor; D002851:Chromatography, High Pressure Liquid; D006367:HeLa Cells; D006801:Humans; D050503:Indoleamine-Pyrrole 2,3,-Dioxygenase / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D009369:Neoplasms / Q000378:metabolism*; D012333:RNA, Messenger / Q000096:biosynthesis; D034741:RNA, Small Interfering / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013237:Stereoisomerism; D014364:Tryptophan / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism* / Q000494:pharmacology
chemical_list: D050503:Indoleamine-Pyrrole 2,3,-Dioxygenase; D012333:RNA, Messenger; D034741:RNA, Small Interfering; D014364:Tryptophan