Abstract 4374: PP2A-B56α in nutrient scavenging: Tipping the balance from macropinocytosis to cell death in pancreatic cancer
Authors: Baral Garima , Pfeffer Claire M. , Doraivel Indiraa , Filippelli Sara N. , Allen-Petersen Brittany L. 
Published: March 2024

Abstract
Pancreatic Ductal Adenocarcinoma (PDAC) is the fourth leading cause of cancer related deaths in the US, with the lowest five-year survival rate of all major cancers. Nutrients in the PDAC microenvironment are commonly depleted, with the vital amino acid glutamine among the most deficient metabolites. To circumvent this deprivation, PDAC cells initiate KRAS dependent macropinocytosis, an actin-driven nutrient scavenging pathway. Macropinosomes fuse with lysosomes in a process mediated by the kinase PIKfyve. It allows cells to replenish the nutrients required for survival. As glutamine is essential for PDAC cell survival, therapeutic inhibition of macropinocytosis represents a novel strategy to suppress nutrient acquisition and drive cell death.
Protein phosphatase 2A (PP2A) is a heterotrimeric Serine/Threonine phosphatase known to inhibit downstream targets of the KRAS signaling cascade and is implicated in macropinocytosis regulation. PP2A holoenzyme comprises of A, B and C subunits, where the regulatory B subunit provides substrate specificity to the enzyme. Using high molecular weight TMR-Dextran, we demonstrate that overexpression of PP2A-B56α subunit or pharmacological activation by DT061 promotes macropinosome accumulation in cells. Macropinosome accumulation was caused by the inability of macropinosomes to fuse with lysosomes, leading to cell death. The cell death caused by DT061 was significantly rescued when treated with a Rac family inhibitor. To understand if B56α regulates the process of macropinosome-lysosome fusion, we performed co-immunoprecipitation and established the interaction of B56α with PIKfyve. Further, pharmacological PIKfyve inhibition with Apilimod phenocopies DT061 induced vesicle accumulation in cells, suggesting that B56α likely inhibits PIKfyve to promote aberrant macropinocytosis.
Prevention of lysosomal fusion will impact nutrient uptake through macropinocytosis. Our metabolomics study confirmed the deprivation of glutamine with activation of PP2A-B56α. This regulation can be therapeutically leveraged to metabolically stress cells with a combination of DT061 and glutamine transport inhibitor V-9302. Combination drug treatment was found to be synergistic in a panel of PDAC cells. Together, these findings indicate that activation of PP2A in late stage PDAC promotes aberrant macropinocytosis by preventing lysosomal fusion through PIKfyve regulation and establishes a novel role of PP2A-B56α in nutrient scavenging and cell death. These pathways can be further exploited to identify potential combination therapeutics in PDAC.
Citation Format: Garima Baral, Claire M. Pfeffer, Indiraa Doraivel, Sara N. Filippelli, Brittany L. Allen-Petersen. PP2A-B56α in nutrient scavenging: Tipping the balance from macropinocytosis to cell death in pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 4374.