TMIC-26. EXPLORATION OF TRYPTOPHAN METABOLISM VIA THE KYNURENINE PATHWAY IN HUMAN GLIOMAS

Abstract

The kynurenine pathway (KP) is responsible for a majority of the catabolism of the essential amino acid tryptophan, and recently was implicated in the pathophysiology of human gliomas. The KP generates various active metabolites, is the main source of the essential co-factor nicotinamide adenine dinucleotide (NAD+), and plays a pivotal role in promoting anti-tumor immune suppression through depleting local tryptophan and causing T-cell growth arrest. Kynurenine, a KP metabolite, was recently discovered to be an endogenous ligand of the aryl hydrocarbon receptor (AHR), a transcription factor associated with carcinogenesis. We hypothesized that higher-grade gliomas will have a more dysregulated KP, and therefore sought to examine the potential differences in KP expression in low- versus high-grade gliomas. Active tumor tissue was acquired immediately following microsurgical resection, and the tumor was dissociated into a single-cell suspension for performance of in vitro experiments. Immunohistochemical staining of Grade II, III, and IV glioma (glioblastoma: GBM) patient tumor sections with antibodies specific for: human indoleamine 2,3-dioxygenase (IDO) 1; IDO2; tryptophan 2,3-dioxgyenase (TDO2); kynureninase; kynurenine 3-monooxygenase (KMO); and AHR. Tissue sections were scored from 0-3 based on staining intensity. Patient-derived tumor cell cultures were treated with commercially available small molecule inhibitors for IDO1 (epacadostat), IDO2 (chloroquine), TDO2 (680C91), KMO (Ro61-8048), and AHR (CH-223191) for 48-hours. GBM tissues show significantly (p<0.001, ANOVA) higher levels of KP enzymes and AHR compared to Grade II and Grade III tissues. In vitro drug treatment showed markedly diminished cell proliferation via MTT assay in response to chloroquine and CH-223191. The increased staining of KP enzymes, as well as the diminished proliferation with the inhibitors, suggests that the KP plays a role in malignant gliomas, and may offer novel therapeutic targets for a patient population with highly limited treatment options.