Targeting Translational Control in Cancer Using Small Molecule Activators of PP2A

Abstract

BackgroundCancer cells rely on hyperactive protein synthesis to reprogram the proteome to sustain the transformed phenotype. Thus, aberrant protein translation has emerged as a promising therapeutic target. Cap‐dependent translation is an exquisitely regulated process that requires the assembly of the eIF4F translation initiation complex. The rate limiting component of the eIF4F complex is eIF4E, which binds to the 5’ cap structure of mRNAs to initiate translation. Cap‐dependent translation is kept in check by the translation repressor 4E‐BP1; in its active hypophosphorylated form, 4E‐BP1 sequesters eIF4E to prevent translation initiation. Cancer cells use several mechanisms to bypass 4E‐BP1 translational suppres­sion, including hyperactivation of mTORC1 for hyperphos­phorylation and inactivation of 4E‐BP1; inactivation of the 4E‐BP1 phosphatase, PP2A; and profound suppression of 4E‐BP1 expression. We have found that the expression and activity of 4E‐BP1 can be restored in tumor cells using novel small molecule acti­vators of PP2A (SMAPs), a class of compounds that activate selected PP2A tumor suppressors and are being ac­tively developed as anticancer agents (Leonard et al. Cell, 2020). The objective of this study was to investigate the mechanism(s) by which SMAPs lead to restoration of the expression and activity of 4E‐BP1 in cancer cells for inhibition of hyperactive protein translation and tumor suppression.HypothesisBased on our finding that SMAPs promote PP2A‐dependent transcriptional activation of the 4E‐BP1 gene, we hypothesize that PP2A activation modulates the levels and/or activity of key transcription factors that regulate 4E‐BP1 gene expression.Results/DataWe have found that SMAPs (e.g., lead compound DT‐061) upregulate 4E‐BP1 mRNA and protein in tumor cells by inducing transcription of the 4E‐BP1 gene. Importantly, SMAPs promote the accumulation of active 4E‐BP1, capable of suppressing cap‐dependent translation in cancer cells. SMAPs promote PP2A‐dependent loss of Snail/Slug and upregulation of ATF4, transcription factors that have been implicated in negative and posi­tive control of 4E‐BP1 gene ex­pression, respectively. Snail downregulation was prevented by proteasome inhibition, indicating that Snail loss is due to proteasomal degradation. Use of GSK‐3β and PKD1 inhibitors excluded canonical mechanisms of Snail degradation in the effects of SMAPs. siRNA knockdown experiments are underway to explore the role of changes in Snail and ATF4 in SMAP‐induced upregulation of 4E‐BP1. Ongoing studies are also examining the mechanisms of SMAP/PP2A regulation of Snail/Slug and ATF4 in tumor cells.ConclusionsSMAPs promote PP2A‐dependent proteasomal degradation of Snail/Slug protein via a non‐canonical mechanism, as well as upregulation of ATF4. We propose that these effects relieve transcriptional repression of 4E‐BP1 by Snail/Slug while promoting 4E‐BP1 transcription by ATF4, leading to a robust increase in 4E‐BP1 levels. 4E‐BP1 translation repressive activity is ensured by PP2A‐induced 4E‐BP1 dephosphorylation. Thus, SMAPs offer a powerful tool for therapeutic targeting of aberrant protein translation in cancer.