Abstract 3403: Drugging the undruggable: Lessons learned from protein phosphatase 2A
Authors: Taylor Derek , Narla Goutham 
Published: September 2020

Abstract
Protein phosphatase 2A (PP2A) is a key tumor suppressor responsible for the dephosphorylation of many oncogenic signaling pathways. The PP2A holoenzyme is comprised of a scaffolding subunit (A), which serves as the structural platform for the catalytic subunit (C) and for an array of regulatory subunits (B) to assemble. Impairment of PP2A is essential for the pathogenesis of many diseases including cancer. In cancer, PP2A is inactivated through a variety of mechanisms including somatic mutation of the Aαsubunit. Our studies show that the most recurrent Aαmutation, P179R, results in an altered protein conformation which prevents the catalytic subunit from binding. Additionally, correcting this mutation, by expressing wild type PP2A Aαin cell lines harboring the P179R mutation, causes a reduction in tumor growth and metastasis. Given its central role in human disease pathogenesis, many strategies have been developed to therapeutically target PP2A.Our lab developed a series of small molecules activators of protein phosphatase 2A. One of our more advanced analogs in this series, DT-061, drives dephosphorylation and degradation of select pathogenic substrates of PP2A such as c-MYC in cellular and in vivo systems. Additionally, we have demonstrated the phosphomimetics of MYC that prevent PP2A mediated dephosphorylation and degradation markedly reduce the anti-tumorigenic activity of this series of PP2A activators further validating the target-substrate specificity of this approach. Specific mutations in the site of drug interaction or overexpression of the DNA tumor virus small T antigen which has been shown to specifically bind to and inactivate PP2A abrogate the in vivo activity of this small molecule series further validating the PP2A specificity of this approach. Importantly, treatment with DT-061 results in tumor growth inhibition in an array of in vivocancer models and marked regressions in combination with MEKi and PARPi.To further define the mechanism of action of this small molecule series, we have used cryo-electron microscopy (cryo-EM) to visualize directly theinteraction between DT-061 and a PP2A heterotrimeric complex. We have identified molecular interactions between DT-061 and all three PP2A subunits that prevent dissociation of the active enzyme through the marked prolongation of the kOFF of the native complex. Furthermore, we demonstrate that DT-061 specifically stabilizes the B56α-PP2A holoenzyme in a fully assembled, active state to dephosphorylate oncogenic targets such as c-MYC in both cellular and in vivo systems. This 3.6 Å structure identifies dynamic molecular interactions between the three distinct PP2A subunits and highlight the inherent mechanisms of PP2A complex assembly and disassembly in both cell free and cellular systems. Thus, our findings provide fundamental insights into PP2A complex assembly and regulation, identify a unique interfacial stabilizing mode of action for the therapeutic targeting of previously undruggable proteins, and aid in the development of phosphatase-based therapeutics tailored against disease specific phosphor-protein targets. The marriage of multidisciplinary scientific practices has allowed us to present here a previously unrecognized therapeutic strategy of complex stabilization for the activation of endogenous disease combating enzymes.
Citation Format: Derek Taylor, Goutham Narla. Drugging the undruggable: Lessons learned from protein phosphatase 2A [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3403.