Abstract A50: IMPACT inhibits metastatic outgrowth in pancreatic cancer by restraining GCN1-ATF4 signaling
Authors: Sinha Surajit , Ayabe Reed I , Ranjan Alok , Wach Michael , Ruff Samantha , Remmert Kirsten , Beek Elke Van , Alexander Imani , McDonald James D. , Gupta Shreya , Li Eileen , Hernandez Jonathan 
Published: March 2020

Abstract
Introduction: Although liver metastasis is the major cause of pancreatic cancer mortality, the mechanisms by which KRAS-driven pancreatic tumor cells initiate metastatic outgrowth in the liver are unknown. High-throughput next-generation sequencing studies have revealed that driver mutations and copy number alterations are highly concordant between matched primary tumors and metastases in pancreatic cancer. This suggests that the mechanisms leading to the metastatic outgrowth and colonization involve the cooperation of many other cellular factors in addition to driver mutations. We hypothesize that in the absence of metastasis-specific driver mutations, the initial adaptation and outgrowth steps of metastasis are driven by metabolic reprogramming, which is induced or activated in vivo under physiologic conditions of the target organ like nutrient deprivation.
Method: To identify the metabolic drivers, a retroviral cDNA library was generated from a highly metastatic pancreatic cell line (M-4964Liv) derived from a KPC mouse that forms macroscopic liver lesions upon splenic injection in mice. The cDNA library was then transduced into dormant pancreatic KPC cells (D-4964Liv) cells that do not give liver lesions upon splenic injection in mice. This methodology will allow all the cDNAs from the metastatic cells to express in the dormant cells, but only those cDNAs that can trigger the dormant cells to outgrow will give rise to a metastatic liver lesion.
Results: The screening strategy led to the identification of the translational regulator general control of amino-acid synthesis 1-like 1 (GCN1). We show that activation of GCN1 at physiologic glutamine activates the master regulator of metabolic homeostasis ATF4, oncogenic mTORC1 signaling, and pluripotency factor SOX2. Consistent with this, knockdown of GCN1 not only inhibits ATF4, mTORC1, and expression of Sox2 but dramatically abrogates liver metastasis in immunocompetent mice upon intrasplenic injection, suggesting that GCN1-ATF4 signaling is indispensable for metastatic outgrowth. Mechanistic insights reveal that the GCN1-ATF4 signaling pathway is governed by a competitive inhibitor, IMPACT, which is induced at physiologic glutamine and inhibits GCN1 function. Surprisingly, the expression of IMPACT is lost in highly metastatic pancreatic cell lines, leading to activation of the GCN1-ATF4 pathway and mTORC1 at physiologic glutamine, while overexpression of IMPACT effectively inhibits the pathway and reduces metastatic outgrowth of pancreatic cancer cells in the liver in immunocompetent mice.
Conclusions: The loss of expression of IMPACT leading to GCN1-dependent activation of ATF4 and mTORC1 at physiologic levels of glutamine provides metabolic plasticity during the initial steps of metastatic outgrowth. Thus, inhibitors targeting the GCN1-ATF4 pathway or mTORC1 will greatly benefit pancreatic adenocarcinoma patients by inhibiting the outgrowth of their metastatic disease.
Citation Format: Surajit Sinha, Reed I Ayabe, Alok Ranjan, Michael Wach, Samantha Ruff, Kirsten Remmert, Elke Van Beek, Imani Alexander, James D. McDonald, Shreya Gupta, Eileen Li, Jonathan Hernandez. IMPACT inhibits metastatic outgrowth in pancreatic cancer by restraining GCN1-ATF4 signaling [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 A50.