Abstract 3722: Exploring the role of eIF4E in cancer cells with targeted protein degradation
Authors: Sharp Swee Y. , Martella Marianna , Milton Christopher I. , Ward George , Richardson Caroline , Woodhead Andrew , Clarke Paul A. 
Published: April 2023

Abstract
Initiation of translation is considered the main rate-limiting step of protein synthesis and requires the recognition of 5’ m7G-cap on mature mRNAs and the formation of eukaryotic translation initiation factor 4F (eIF4F) multi-protein mRNA cap-binding complex. Formation of this complex requires the interaction of eIF4E and the scaffold protein eIF4G, and RNA helicase eIF4A. This eIF4F complex along with eIF3 mediate the recruitment of the 40S ribosomal particle to the 5′ cap of mRNA. Activation of eIF4E is a regulatory hub of many major oncogenic pathways, thus, targeting eIF4E has emerged as a potential therapeutic strategy in cancer.
Here we have used a targeted protein degradation approach coupled with genetic rescue to explore the molecular and cellular dependency of cancer cells on eIF4E. Stable H1299 human NSCLC clones expressing FKBP12F36V-tagged eIF4E but lacking endogenous eIF4E were established. Treatment of multiple N- or C-tagged-eIF4E clones with dTAGv-1, an FKBP12F36V selective heterobifunctional molecule that recruits VHL, induced rapid degradation of eIF4E to undetectable levels by 6hr exposure. This also resulted in reduced expression of MCL1, a previously reported biomarker of eIF4E activity. Longer exposures to dTAGv-1 resulted in a cytostasis that was not associated with cell death. A diastereomer negative control of dTAGv-1 that cannot recruit VHL did not elicit loss of eIF4E or the downstream events associated with its loss. Global analysis of protein synthesis initiation by RIBOseq and proteome profiling following dTAGv-1 treatment out to 32hr exposure demonstrated surprisingly few alterations in protein expression despite the significant effect on cancer cell growth.
We also expressed wild-type or eIF4E mutants predicted to disrupt key functions and determined their ability to rescue molecular or cellular phenotype associate with eIF4E-loss following dTAGv-1 treatment. Expression of wild-type eIF4E completely rescued cell growth and MCL1 expression. A W56A mutant predicted to disrupt mRNA-cap binding was unable to rescue eIF4E loss. In contrast, expression of W73F or S290A mutants (predicted to disrupt eIF4G binding or exhibit reduced eIF4E activity, respectively) were able to rescue the loss of eIF4E.
In summary, our rescue experiments show that mRNA-cap binding by eIF4E is required and that eIF4E:eIF4G interaction in cells may be more complex than predicted. This may also explain the challenges associated with developing selective and cellularly potent inhibitors of the eIF4E:eIF4G interaction and that targeting mRNA-cap binding may be a more effective strategy. We predicted that removing eIF4E would impact on the global synthesis of many proteins. However, our data demonstrate that targeting eIF4E leads to limited effects on protein synthesis that remain sufficient to inhibit cancer cell growth. Further experiments are underway to understand which proteins drive this dependency on eIF4E.
Citation Format: Swee Y. Sharp, Marianna Martella, Christopher I. Milton, George Ward, Caroline Richardson, Andrew Woodhead, Paul A. Clarke. Exploring the role of eIF4E in cancer cells with targeted protein degradation. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3722.