Restoring Translational Control with Novel Small Molecule Activators of PP2A in Cancer Cells

Hyperactivation of eIF4E-dependent translation in tumor cells is required for robust expression of oncogenic proteins and maintenance of the transformed phenotype. eIF4E binds to the 7-methylguanylate structure at the 5’ end of cellular mRNAs and nucleates the formation of the translation initiation complex, eIF4F, which recruits the preinitiation complex and resolves secondary structure in the mRNA. The ability of eIF4E to promote eIF4F assembly is negatively regulated by the translational repressor, 4E-BP1, and tumorigenesis depends on functional inactivation of 4E-BP1. A major mechanism of 4E-BP1 inactivation involves inhibitory phosphorylation by mTOR which prevents eIF4E binding. Functional inactivation can also occur by overwhelming the binding capacity of 4E-BP1 through upregulation of eIF4E or suppression of 4E-BP1 expression. Previous studies have determined that 4E-BP1 can be activated by PP2A. PP2A is heterotrimeric phosphatase consisting of a catalytic subunit, a scaffolding subunit, and one of numerous regulatory subunits which dictate substrate specificity. Novel Small Molecule Activators of PP2A (SMAPs) selectively activate PP2A heterotrimers containing B56α, B56β or B56ε regulatory subunits. Use of SMAPs led to the discovery that B56-PP2A activation not only induces dephosphorylation of 4E-BP1 but also increases its expression by transcriptional induction of the EIF4EBP1 gene. SMAP-treatment led to transcriptional upregulation of ATF4, which was required for B56-PP2A-mediated 4E-BP1 upregulation. Upstream, B56PP2A activation induced dephosphorylation and nuclear translocation of TFEB and TFE3 iii to induce ATF4 transcription. Together, these studies identified a novel B56-PP2ATFEB/TFE3ATF44E-BP1 signaling axis that negatively regulates eIF4Edependent translation initiation. Analysis of 4E-BP1 modulators also determined that B56PP2A induces proteasomal degradation of Snail, a master regulator of the epithelial-tomesenchymal transition (EMT) and metastasis. PP2A-induced Snail degradation did not involve the GSK3β- or PKD1-mediated canonical mechanisms for proteasomal targeting of Snail and thus represents a previously unknown regulatory pathway. SMAPs have promising pharmacological properties and display potent antitumor activity in animal models. The identification of B56-PP2A mediated effects on eIF4E-dependent translation and Snail expression supports further clinical development of these compounds.