Abstract 919: Regulation of PS externalization on tumor cells by TMEM16F and P2X7 receptors

Abstract Immune evasion in the tumor microenvironment (TME) is critical for tumor progression, and many therapeutic strategies are under development to activate host anti-tumor immunity. Tumors avoid immune detection and clearance through several mechanisms, one of which is the chronic externalization of the phospholipid phosphatidylserine (PS) to the outer leaflet of the plasma membrane. Externalized PS on tumor cells alters the tumor microenvironment to an immunosuppressive phenotype. Immune cells expressing PS-binding TAM (Tyro3, Axl, MerTK) receptors interact with the tumor-exposed PS and release immunosuppressive cytokines, leading to the polarization of the TME to pro-tumorigenic. While it is understood how PS is externalized by calcium-induced scramblase TMEM16F in non-pathological conditions and caspase-cleaved scramblase Xkr8 in apoptotic conditions, the mechanism through which the constitutive externalization of PS on tumor cells occurs is still unknown. We hypothesize that the increased concentration of calcium in tumor cells activates TMEM16F to externalize PS, and the increased levels of extracellular ATP bind P2X7 receptors (P2X7R), further leading to PS externalization in the TME. To better understand the mechanisms of PS exposure under oncogenic stress, we have developed both TMEM16F and P2X7R CRISPR knockout EO771 breast cancer cell lines. These cells are unable to externalize PS when subjected to their respective triggers (calcium or ATP). They have additionally been shown to be unable to upregulate immune-suppressive MerTK signaling upon exposure to their PS-externalizing triggers, whereas wild-type EO771 cells show large upregulation in MerTK binding upon treatment. Orthotopic injection into the mammary fat pad of C57/Bl6 mice yielded a reduction in tumor size in TMEM16F knockout tumors compared to the wild-type EO771 tumors (P2X7R knockout tumor experiments are ongoing). Based on these studies, we show that both calcium-induced TMEM16F and ATP-binding P2X7R contribute to the externalization of PS and immune suppression in the TME, resulting in tumor growth and progression. Our studies add new insights to how PS acts as an immunosuppressive signal in the tumor microenvironment. Citation Format: Rachael Pulica, Varsha Gadiyar, David C. Calianese, Mariana De Lorenzo, Raymond B. Birge. Regulation of PS externalization on tumor cells by TMEM16F and P2X7 receptors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 919.