Abstract B064: The role of PP2A-B56α in KRAS-mediated pancreatic tumorigenesis
Authors: Tinsley Samantha L , Shelley Rebecca A. , Mall Gagan K. , Chianis Ella Rose D. , Thoma Mary C. , di Magliano Marina Pasca , Narla Goutham , Sears Rosalie C. , Allen-Petersen Brittany L. 
Published: November 2022

Abstract
Protein phosphatase 2A (PP2A) is a major serine-threonine phosphatase that regulates many cellular pathways including KRAS, whose oncogenic mutation is prevalent in 95% of patients with Pancreatic Ductal Adenocarcinoma (PDAC). Previous research has identified a decrease in global PP2A activity and an increase in the expression of PP2A inhibitors in PDAC cell lines, suggesting that suppression of PP2A activity may be pertinent in PDAC maintenance. Importantly, PP2A has low mutation rates in PDAC, making it a viable target for therapeutic reactivation. While PP2A has been shown to have global tumor suppressive capabilities, the regulation of specific pathways by PP2A can be altered based on PP2A holoenzyme composition. Therefore, there is a critical need to understand the mechanisms by which oncogenic KRAS can affect PP2A function and differential substrate targeting in PDAC. The PP2A holoenzyme consists of 3 subunits: the scaffolding subunit (A), the catalytic subunit (C), and the regulatory subunit (B). There are 16 different B subunits that can be incorporated into the PP2A holoenzyme that are responsible for substrate specificity. The B56α subunit of PP2A has been shown to negatively regulate cellular transformation. Our research aims to investigate the mechanisms by which PP2A-B56α is regulated through oncogenic KRAS and how suppression of B56α impacts the initiation and progression of PDAC. To determine how oncogenic KRAS alters the dynamics of PP2A-B56α and overall PP2A activity we utilized tet-inducible KRASG12D cell lines to allow direct manipulation of KRAS mutational activation. Using this system, we have identified time dependent alterations in cancerous inhibitor of PP2A (CIP2A) following induction of KRASG12D expression, indicating that PP2A suppression may be an early event in PDAC initiation. Consistent with this hypothesis, we characterized changes in the acceleration of PDAC formation in vivo using the Ptf1a-Cre; LSL- KRASG12D (KC) genetic mouse model combined with a B56α hypomorph model (KCBhm/hm). Our data show that the loss of B56α accelerates PDAC initiation, with an increase in pancreatic precursor lesion (PanIN) number and a decrease in healthy acinar area. In response to B56α loss, similar acceleration of acinar to ductal metaplasia (ADM) kinetics were observed in a 3D-cultured ADM Assay. Furthermore, when 3D-cultured acinar cells were treated with a small molecule activator of PP2A (SMAP), SMAP treatment resulted in smaller and fewer ductal structures, preventing the ADM process. Collectively, these data suggest that PP2A-B56α plays a regulatory role in cellular plasticity and loss contributes to PDAC initiation. Future studies will investigate how mutant KRAS-mediated CIP2A expression effects overall PP2A phosphatase activity and how subsequent sequestration of B56α contributes to development of PDAC. Together, these studies identify PP2A as a critical regulator of KRAS-induced cellular plasticity and support reactivation of PP2A as a novel therapeutic strategy in PDAC patients.
Citation Format: Samantha L Tinsley, Rebecca A. Shelley, Gagan K. Mall, Ella Rose D. Chianis, Mary C. Thoma, Marina Pasca di Magliano, Goutham Narla, Rosalie C. Sears, Brittany L. Allen-Petersen. The role of PP2A-B56α in KRAS-mediated pancreatic tumorigenesis [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer; 2022 Sep 13-16; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2022;82(22 Suppl):Abstract nr B064.