TRYPTOPHAN DEGRADATION AS AN ALTERNATIVE ENERGY SOURCE IN PANCREATIC CANCER

Introduction Pancreatic cancer (PDAC) is one of the most lethal diseases worldwide, still exhibiting almost equal mortality as incidence rates. In spite of strong efforts in clinical and basic research, the molecular mechanisms responsible for this unusual aggressive phenotype still remain not completely understood and insufficient diagnostic and therapeutic tools still substantiate its ranking as the 4th-leading cause of cancer-associated deaths in western countries. This devastating prognosis is partially due to frequent occurrence of intrinsic or acquired chemoresistance in PDAC specimens against the nucleoside analogue gemcitabine which is still the standard for chemotherapeutic treatment of locally advanced and metastatic PDAC. In principle, cancer cells are well known for their ability to divide uncontrollably and it was initially thought that high glucose consumption is the main energy-source for proliferating tumor cells, but a large body of evidence suggest that most of the cell mass that makes up new cells resulted from amino-acid metabolism. Aims & Methods The aim of this study was the identification of new biomarkers for chemoresistant pancreatic cancer. A secretome survey of chemoresistant PDAC cells was performed using SILAC-based mass-spectrometric analyses. Relative differences in protein-concentrations among samples were investigated and led to the identification of previously unknown proteins. The impact of RNAi-mediated knockdown of selected genes in proliferating PDAC cells were analyzed using MTT-viability assays. Gene- and protein-expression analyses were performed using Real-Time-PCR and immunohistochemistry using patient-derived PDAC samples. Results SILAC-based identification of the Tryptophan degrading enzyme Kynureninase (KYNU) in chemoresistant PDAC cells revealed an overexpressed and secreted form of the KYNU protein, compared to the chemosensitive counterpart. We further identified various stress-related external stimuli (Gemcitabine, IFNg, Hypoxia) as main inductors of KYNU expression/secretion. A RNAi-mediated KYNU knockdown approach was linked to substantially lower proliferation of chemoresistant and aggressive PDAC cells. Global expression analyses using a tissue-microarray of PDAC patient samples (n=368) revealed that high KYNU expression is significantly correlated with a worse outcome in PDAC patients. Conclusion The tryptophan degradation pathway member KYNU is overexpressed in a subset of PDAC patients and is linked to substantially increased cancer cell proliferation. Abundant KYNU expression in PDAC patients is linked with a worse clinical outcome. We found that KYNU is a new secreted biomarker of chemoresistant PDAC cells.