Small Molecule Activators of B56-PP2A Restore 4E-BP Expression and Function to Suppress Cap-dependent Translation in Cancer Cells

Abstract

Dysregulation of cap-dependent translation is a hallmark of cancer, with key roles in supporting the transformed phenotype. The eIF4E binding proteins (4E-BP1, 2, 3) are major negative regulators of cap-dependent translation that are inactivated in tumors through inhibitory phosphorylation by oncogenic kinases (e.g., mTOR) or by downregulation. Previous studies from our group and others have linked tumor suppressive PP2A family serine/threonine phosphatases to activation of 4E-BP1. Here, we leveraged a novel small molecule activator of PP2A (SMAP), DT-061, to explore the role of a subset of B56-PP2As in regulation of 4E-BP activity and to evaluate the potential of B56-PP2A reactivation for restoring translation control in tumor cells. We show that DT-061 promotes PP2A-dependent hypophosphorylation of 4E-BP1/4EBP2 in the presence of active upstream inhibitory kinases (mTOR, ERK, AKT), supporting a role for B56-PP2As as 4E-BP phosphatases. Unexpectedly, DT-061 also led to robust PP2A-dependent upregulation of 4E-BP1, but not 4E-BP2 or 4E-BP3. Cap-binding assays and eIF4E immunoprecipitation showed that SMAP/B56-PP2A blocks the formation of the eIF4F translation initiation complex. Bicistronic reporter assays that directly measure cap-dependent translation activity confirmed the translational consequences of these effects. siRNA knockdown pointed to B56α-PP2A as a mediator of SMAP effects on 4E-BPs, although B56-β and/or B56ϵ-PP2A may also play a role. 4E-BP1 upregulation involved ATF4-dependent transcription of the 4E-BP1 gene (EIF4EBP1) and the effect was partially dependent on TFE3/TFEB transcription factors. Thus, B56-PP2A orchestrates a translation repressive program involving transcriptional induction and hyposphorylation of 4E-BP1, highlighting the potential of PP2A-based therapeutic strategies for restoration of translation control in cancer cells.