Abstract 5991: p53-induced FBXL20 suppresses the PP2A regulatory subunit PR55α

Abstract Background: Protein phosphatase 2A (PP2A) is a major serine/threonine phosphatase that regulates cell survival and proliferation. Each PP2A holoenzyme consists of a scaffolding subunit, a catalytic subunit, and a regulatory subunit, with the regulatory subunit determining substrate specificity and cellular localization. Work from our group and others has shown that the PR55α regulatory subunit promotes oncogenic signaling through YAP, c-Myc, β-catenin, and ERK pathways. Despite its established role in supporting tumorigenesis and progression, the upstream mechanisms controlling PR55α expression and activity remain poorly understood. Pancreatic cancer (PC), one of the most lethal human cancers, is characterized by frequent loss of p53 function, with mutations occurring in more than 75% of cases. We recently discovered that p53 induces the F-box protein FBXL20, which in turn reduces PR55α stability and expression. This finding reveals a previously unrecognized mechanism whereby p53 restrains PR55α through SCF-mediated ubiquitination and proteasomal degradation. Defining this regulatory interaction is essential for understanding how p53 loss enhances PR55α-driven oncogenic signaling in PC. Methods: To identify the FBXL20 interaction site on PR55α, we generated Myc-tagged PR55α truncation constructs and performed co-immunoprecipitation assays. Additional immunoprecipitations are underway to determine whether FBXL20 engages PR55α within an assembled SCF complex by assessing SKP1 and CUL1-4 association. To evaluate functional consequences, we are generating PR55α point mutations that disrupt FBXL20 binding and will examine their biological effects on PR55α stability and downstream signaling pathways. Results: Mapping studies indicate that FBXL20 binds to a region between WD40 domains 5-6 of PR55α, potentially involving an α-helical linker. FBXL20 also associates with the heterotrimeric PR55α-PP2A complex, suggesting that PR55α engagement with PP2A does not block its recognition by the SCF ligase. Ongoing experiments are testing whether core SCF components co-precipitate with PR55α, which will determine whether a functional ubiquitin ligase complex forms. Parallel studies using PR55α point-mutation constructs are examining how loss of p53/FBXL20 interaction alters PR55α stability and downstream signaling. Conclusions: Our findings establish a novel mechanism by which p53 constrains PR55α abundance through FBXL20-mediated ubiquitination and proteasomal degradation. Identifying the FBXL20 recognition site on PR55α provides a foundation for defining the phosphodegron required for its turnover and significance on PR55α stability and oncogenic activity. Understanding this regulatory pathway offers insight into how p53 loss through mutations enables unchecked PR55α/PP2A-driven oncogenic signaling in PC and other malignancies and may uncover new PR55α-based therapeutic strategies. Citation Format: Alison L. Camero, Michel Ouellette, Keith Johnson, Ying Yan, . p53-induced FBXL20 suppresses the PP2A regulatory subunit PR55α [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 5991.