Abstract C50: Dissecting vulnerabilities of pancreatic tumors’ silent fraction unravels secrets of an alternative cell state
Authors: Sela Yogev , Li Jinyang , Doepner Miriam , Maheswaran Shivahamy , Mesaros Clementina , Blair Ian , Shalem Ophir , Stanger Ben 
Published: March 2020

Abstract
Tumors contain a heterogeneous mixture of live cells that are either undergoing proliferation or reside in a nonproliferative state. Cells that do not divide are often associated with increased chemoresistance, challenging therapies that target rapidly proliferating cells. In pancreatic cancer, paucity of blood vessels frequently generates hypoperfused, “arid” regions that constrain tumor cell metabolism and proliferation. Nonetheless, experimental conditions for studying tumor cells in vitro typically mimic well-nourished, “fertile” environments optimized for supporting proliferating cells rather than the more prevalent quiescent state. Characterization of the poorly vascularized KPC model confirmed that the quiescent phenotype is common and is strongly correlated to reduced tissue perfusion. To formulate culture conditions that will emulate the arid microenvironment associated with the nonproliferative state in vitro, we systematically deprived various nutrients from tumor cells and established the necessary components for attaining cell quiescence. Deprivation of amino acids on top of glucose, oxygen, and serum levels was essential for obtaining a complete and reversible cell cycle arrest without loss of viability. Cell cycle arrest was accompanied by a number of physiologic adaptations. Metabolomic analysis indicated reduction in the levels of TCA cycle and mevalonate pathway intermediates indicative of increased reliance on nonglycolytic metabolism. In addition, free amino acid pools were diminished and tumor cells exhibited a significant increase in autophagic flux and micropinocytosis, suggesting that amino acid availability is limited under arid conditions. Concomitant with reduced proliferation and metabolic rewiring, tumor cells under arid conditions exhibited remarkable resistance to gemcitabine. To explore alternative strategies to target these nonproliferating pancreatic tumor cells, we performed a comprehensive genome-wide CRISPR/Cas9-based genetic screen and compared vulnerabilities under ”fertile” and “arid” conditions. Functional annotation of sgRNAs depleted under arid conditions indicated a striking dependence on oxidative phosphorylation for survival. Remarkably, our analysis also revealed that a number of targetable genetic and epigenetic programs driving cell cycle progression were not essential and, in some instances, even hazardous for cells under arid conditions. These results suggest that pancreatic tumor cells that cease proliferation in a nutrient-depleted environment are physiologically rewired and may exhibit a distinct drug response profile from rapidly proliferating cells. The experimental model we developed allows investigation of this dominant yet understudied cell population and may serve as a platform for identification of agents targeting the quiescent fraction of tumors.
Citation Format: Yogev Sela, Jinyang Li, Miriam Doepner, Shivahamy Maheswaran, Clementina Mesaros, Ian Blair, Ophir Shalem, Ben Stanger. Dissecting vulnerabilities of pancreatic tumors’ silent fraction unravels secrets of an alternative cell state [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2019 Sept 6-9; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2019;79(24 Suppl):Abstract nr C50.