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Tryptophanemia is controlled by a tryptophan-sensing mechanism ubiquitinating tryptophan 2,3-dioxygenase.

ABSTRACT: Maintaining stable tryptophan levels is required to control neuronal and immune activity. We report that tryptophan homeostasis is largely controlled by the stability of tryptophan 2,3-dioxygenase (TDO), the hepatic enzyme responsible for tryptophan catabolism. High tryptophan levels stabilize the active tetrameric conformation of TDO through binding noncatalytic exosites, resulting in rapid catabolism of tryptophan. In low tryptophan, the lack of tryptophan binding in the exosites destabilizes the tetramer into inactive monomers and dimers and unmasks a four-amino acid degron that triggers TDO polyubiquitination by SKP1-CUL1-F-box complexes, resulting in proteasome-mediated degradation of TDO and rapid interruption of tryptophan catabolism. The nonmetabolizable analog alpha-methyl-tryptophan stabilizes tetrameric TDO and thereby stably reduces tryptophanemia. Our results uncover a mechanism allowing a rapid adaptation of tryptophan catabolism to ensure quick degradation of excess tryptophan while preventing further catabolism below physiological levels. This ensures a tight control of tryptophanemia as required for both neurological and immune homeostasis.

SUBMITTER: Klaessens S

PROVIDER: S-EPMC8201853 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Kynurenine (Kyn), a metabolite of tryptophan (Trp), is a key regulator of mammal immune responses such as cancer immune tolerance. Indoleamine-2,3-dioxygenase (IDO) and tryptophan-2,3-dioxygenase (TDO) are main enzymes regulating the first and rate-limiting step of the Kyn pathway. To identify new small molecule inhibitors of TDO, we selected A172 glioblastoma cell line constitutively expressed TDO. Characterization of this cell line using kinase inhibitor library resulted in identification of MEK/ERK pathway-dependent TDO expression. After knowing the properties for TDO expression, we further proceeded to screen chemical library for TDO inhibitors. We previously determined that S-benzylisothiourea derivatives are enzymatic inhibitors of indoleamine 2,3-dioxygenase 1 (IDO1) and suggested that the isothiourea moiety could be an important pharmacophore for binding to heme. Based on this premise, we screened an in-house library composed of various isothiourea derivatives and identified a bisisothiourea derivative, PVZB3001, as an inhibitor of TDO. Interestingly, PVZB3001 also inhibited the enzymatic activity of IDO1 in both cell-based and cell-free assays but did not inhibit other heme enzymes. Molecular docking studies suggested the importance of isothiourea moieties at the ortho position of the phenyl ring for the inhibition of catalytic activity. PVZB3001 showed competitive inhibition against TDO, and this was supported by the docking simulation. PVZB3001 recovered natural killer (NK) cell viability and functions by inhibiting Kyn accumulation in conditioned medium of both IDO1- and TDO-expressing cells. Furthermore, oral administration of IDO1-overexpressing tumor-bearing mice with PVZB3001 significantly inhibited tumor growth. Thus, we identified a novel selective dual inhibitor of IDO1 and TDO using the Kyn production assay with a glioblastoma cell line. This inhibitor could be a useful pharmacological tool for modulating the Kyn pathway in a variety of experimental systems.

Project description:Tryptophan 2,3-dioxygenase [L-tryptophan--oxygen 2,3-oxidoreductase (decyclizing), EC 1.13.11.11] has been reported to act solely on the L-isomer of tryptophan. However, by using a sensitive assay method with D- and L-[ring-2-14C]tryptophan and improved assay conditions, we were able to demonstrate that both the D- and L-stereoisomers of tryptophan were cleaved by the supernatant fraction (30000 g, 30 min) of liver homogenates of several species of mammals, including rat, mouse, rabbit and human. The ratio of activities toward D- and L-tryptophan was species variable, the highest (0.67) in ox liver and the lowest (0.07) in rat liver, the latter being hitherto exclusively used for the study of hepatic tryptophan 2,3-dioxygenase. In the supernatant fraction from mouse liver, the ratio was 0.23 but the specific activity with D-tryptophan was by far the highest of all the species tested. To identify the D-tryptophan cleaving enzyme activity, the enzyme was purified from mouse liver to apparent homogeneity. The specific activities toward D- and L-tryptophan showed a parallel rise with each purification step. The electrophoretically homogeneous protein had specific activities of 0.55 and 2.13 mumol/min per mg of protein at 25 degrees C toward D- and L-tryptophan, respectively. Additional evidence from heat treatment, inhibition and kinetic studies indicated that the same active site of a single enzyme was responsible for both activities. The molecular weight (150000), subunit structure (alpha 2 beta 2) and haem content (1.95 mol/mol) of the purified enzyme from mouse liver were similar to those of rat liver tryptophan 2,3-dioxygenase. The assay conditions employed in the previous studies on the stereospecificity of hepatic tryptophan 2,3-dioxygenase were apparently inadequate for determination of the D-tryptophan cleaving activity. Under the assay conditions in the present study, the purified enzyme from rat liver also acted on D-tryptophan, whereas the pseudomonad enzyme was strictly specific for the L-isomer.

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Tryptophanemia is controlled by a tryptophan-sensing mechanism ubiquitinating tryptophan 2,3-dioxygenase.

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