Abstract IA13: An mTOR/eIF4E-independent translation mechanism promotes breast cancer metastasis
Authors: de la Parra Columba , Ernlund Amanda , Alard Amandine , Ruggles Kelly , Ueberheide Beatrix , Katsara Olga , Schneider Robert J. 
Published: December 2020

Abstract
While translation initiation of most mammalian mRNAs is mediated by a 5' cap structure that binds eIF4E, inactivation/silencing of eIF4E, or quantitative sequestration of eIF4E by 4E-BPs through mTORC1 inhibition, does not impair translation of many capped mRNAs. This suggests an alternate mechanism may exist for cap-dependent but eIF4E/mTORC1-independent translation. DAP5 (NAT1, eIF4G2, p97) is an eIF4G family member that lacks the N-terminal binding domains for eIF4E and PABP. DAP5 was shown to mediate translation initiation through internal ribosome entry sites (IRES elements). We recently showed that DAP5 utilizes the cap binding activity of eIF3d to facilitate cap-dependent translation of a large number of capped mRNAs, making it a previous unknown major mechanism of cap-dependent but eIF4E/mTORC1-independent mRNA translation. Genome-wide transcriptomic and translatomic analyses indicate that DAP5 is required for translation of many transcription factor and cell receptor capped mRNAs and their mRNA targets involved in cell survival, motility, DNA repair, oncogenesis, translation initiation, and cancer cell metastasis. Up to 20% of cellular mRNAs utilize DAP5-eIF3d for cap-dependent mRNA translation. Mass spectrometry and in vivo cross-linking studies found that eIF3d is a direct binding partner of DAP5 but does not bind strongly to eIF4GI or II family members that bind eIF4E. In vitro translation and ribosome complex studies demonstrate that DAP5 and eIF3d are both essential for eIF4E and mTORC1-independent capped-mRNA translation. Given the prevalence of oncogenic and survival mRNAs directed in translation by DAP5-eIF3d, we analyzed the NCI human tissue genome cancer atlas (TCGA) and found that high expression of DAP5 mRNA is strongly associated with estrogen receptor-negative (ER-) breast cancer metastasis and poor survival. We therefore investigated the role of DAP5 in preclinical mouse models of highly metastatic ER- breast cancer. Reduction of DAP5 expression by ~70% almost eliminated metastasis to the lung and significantly increased survival with no impact on primary tumor growth. These and other studies indicate that the DAP5-eIF3d complex is a significant alternate mechanism for cap-dependent, eIF4E/mTORC1-independent mRNA translation despite lacking eIF4E and PABP binding, that DAP5 directs translation of many capped mRNAs that are involved in cell stress and survival pathways, is essential for promoting breast cancer metastasis, and that most mRNAs translated by DAP5 do not utilize an IRES.
Citation Format: Columba de la Parra, Amanda Ernlund, Amandine Alard, Kelly Ruggles, Beatrix Ueberheide, Olga Katsara, Robert J. Schneider. An mTOR/eIF4E-independent translation mechanism promotes breast cancer metastasis [abstract]. In: Proceedings of the AACR Special Conference on Targeting PI3K/mTOR Signaling; 2018 Nov 30-Dec 8; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(10_Suppl):Abstract nr IA13.