Abstract B31: A protein synthesis switch underlies initial survival of EGFR-mutant lung cancer to EGFR inhibitors
Authors: Song Kyung-A , Lochmann Timothy L. , Patel Neha U. , Ham Jungoh , Windle Brad E. , Harada Hisashi , Leverson Joel D. , Souers Andrew J. , Hata Aaron N. , Ebi Hiromichi , Faber Anthony C. 
Published: February 2019

Abstract
EGFR inhibitors (EGFRi) are effective at inducing transient tumor shrinkage in EGFR-mutant lung cancers. The efficacy of these drugs however is mitigated by the outgrowth of resistant cells: this is most often manifested by a secondary mutation in EGFR, T790M, which leads to reactivation of key intracellular signaling despite continued drug treatment. We recently demonstrated that T790M can occur both at low frequencies prior to initiation of EGFR inhibitor therapy, or alternatively arise de novo during treatment (Hata et al., Nat Med 2016). Since some cancers form T790M mutations de novo, one potential therapeutic strategy to thwart resistance is to identify the cells surviving initial therapy (referred to as persister cells or drug-tolerant cells [DTCs]) that eventually acquire the T790M mutation, and eliminate them prior to T790M acquisition. To this end, we hypothesized that some cells were refractory to EGFR inhibitor-induced apoptosis, surviving initial therapy and forming a reservoir of cells that could then eventually acquire T790M. We demonstrate that Western blots of lysates from EGFR-mutant lung cancers surviving initial therapy to the EGFR inhibitor gefitinib detect quick (&lt;10 days) and selective upregulation of the anti-apoptotic protein MCL-1. We performed experiments with fluorescent-tagged exogenous MCL-1-expressing cells and demonstrate enrichment of MCL-1-expressing cells following gefitinib treatment by high-content imaging; these cells persisted as early survivors. Accordingly, exogenous expression of MCL-1 prevented gefitinib from shrinking EGFR-mutant lung cancer tumors, and the emergence of DTCs could be largely thwarted by co-incubation with MCL-1 specific inhibitors A-1210477 and S63845. Mechanistically, we report DTCs undergo a “translational switch” that manifests in increased cap-dependent mRNA translation of MCL-1, which corresponds to cellular upregulation of mTOR/eIF4 and downregulation of ribosomal proteins. These data reveal a novel mechanism in which lung cancer cells adapt to short-term pressures of apoptosis-inducing kinase inhibitors by shifting protein biosynthesis through cap-dependent translation of MCL-1 protein. Moreover, in EGFR-mutant lung cancer, MCL-1 is a key molecule governing the emergence of early DTCs to EGFR inhibitors and can be effectively co-targeted with clinically-emerging MCL-1 inhibitors, which may delay the acquisition of secondary mutations including T790M mutations, therefore prolonging therapy efficacy.
Citation Format: Kyung-A Song, Timothy L. Lochmann, Neha U. Patel, Jungoh Ham, Brad E. Windle, Hisashi Harada, Joel D. Leverson, Andrew J. Souers, Aaron N. Hata, Hiromichi Ebi, Anthony C. Faber. A protein synthesis switch underlies initial survival of EGFR-mutant lung cancer to EGFR inhibitors [abstract]. In: Proceedings of the Fifth AACR-IASLC International Joint Conference: Lung Cancer Translational Science from the Bench to the Clinic; Jan 8-11, 2018; San Diego, CA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(17_Suppl):Abstract nr B31.