Abstract 4727: RAS and PP2A activities converge on phosphoregulation of epigenetic complexes in cancer

Abstract RAS-mediated human cell transformation requires inhibition of the tumor suppressor Protein Phosphatase 2A (PP2A). Both RAS and PP2A mediate their effects by phosphoregulation, but phosphoprotein targets in which RAS and PP2A activities converge in human cancers have not been systematically analyzed. Here, based on mass spectrometry phosphoproteome data, we discover that phosphosites co-regulated by RAS and PP2A are enriched on proteins involved in epigenetic gene regulation. As examples, RAS and PP2A co-regulate the same phosphorylation sites on HDAC1/2, KDM1A, MTA1/2, RNF168 and TP53BP1. Mechanistically, we validate co-regulation of NuRD chromatin repressor complex by RAS and PP2A. Consistent with their known synergistic effects in cancer, RAS activation and PP2A inhibition resulted in epigenetic reporter de-repression and activation of oncogenic transcription. Notably, transcriptional de-repression by PP2A inhibition was associated with increased euchromatin and decrease in global DNA methylation. In RAS-driven cancers, PP2A activity is inhibited by nuclear PP2A inhibitor proteins PME-1 and SET. We further demonstrate that these PP2A inhibitors drive oncogenic transcription, but that their transcriptional targets are highly diversified. Whereas PME-1 controls RAS and MYC-driven transcription, SET controls TP53 targets and G2/M checkpoint genes. We further provide evidence that these nuclear PP2A inhibitors significantly differ in their roles in regulating DNA methylation. Collectively the results indicate that epigenetic protein complexes involved in oncogenic gene expression constitute a significant point of convergence for RAS hyperactivity and PP2A inhibition in cancer. Further, the results provide a rich source for future understanding of phosphorylation as a previously unappreciated layer of regulation of epigenetic gene regulation in cancer, and in other RAS/PP2A-regulated cellular processes. Citation Format: Mukund Sharma, Anna Aakula, Francesco Tabaro, Henrik Honkanen, Jesse Kamila, Matthieu Schapira, Cheryl Arrowsmith, Matti Nykter, Jukka K. Westermarck. RAS and PP2A activities converge on phosphoregulation of epigenetic complexes in cancer. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4727.