Breast cancer is the most common cancer in women and a leading cause of death. Dysregulation of cellular signalling pathways controlling proliferation, survival and migration, such as the PI3K/Akt and Ras/MAPK pathways, are key features of breast cancer. Protein phosphatase 2A (PP2A) negatively regulates many components of these pathways. PP2A is a family of trimeric serine/threonine phosphatases, each consisting of a structural, a catalytic and a regulatory subunit of which there are multiple isoforms. The addition of specific regulatory subunits provides subcellular targeting and substrate specificity to the enzyme. While PP2A is generally considered a tumor suppressor, a specific role for individual PP2A subunits in breast cancer has not been described. To address this, we first examined PP2A subunit expression in human breast tumors. Immunohistochemical analysis revealed significantly lower expression of the structural subunit, PP2A-A, and regulatory subunits PP2A-B55α and PP2A-B56α, in primary tumors and metastases, compared to adjacent normal mammary tissue. We further found an association of low PP2A-B55α with aggressive breast cancer subtypes, and with worse disease-free and overall survival. Functionally, shRNA-mediated knockdown of PP2A-B55α in normal mammary epithelial 3D cultures induced a tumorigenic phenotype, characterised by increased proliferation and enlarged multi-lobular acini. In contrast, overexpression of PP2A-B55α in breast cancer cells inhibited proliferation. Thus PP2A inactivation, in particular loss of B55α, is functionally important in breast tumorigenesis. PP2A-B55α complexes play an important role in DNA damage repair, and we found B55α knockdown impaired DNA damage repair. Thus low PP2A-B55α may contribute to genomic instability. To examine the functional role of PP2A-B55α in vivo, we have generated the first PP2A-B55α (Ppp2r2a) knockout mouse. Constitutive knockout of Ppp2r2a is embryonic lethal, with embryos dying during late development, post 14.5 days p.c. Heterozygous Pppr2ra mice (Ppp2r2a+/-) are viable, despite expressing only ~10% of PP2A-B55α protein levels compared to WT mice. Interestingly, Ppp2r2a+/- mice have significantly reduced branching in the developing mammary gland, similar to that observed in mice with mammary-specific loss of the breast tumor susceptibility gene, Brca1. Analysis of breast tumor formation in these mice, either alone or when crossed with MMTV-Neu animals, is underway. Finally, we show that pharmacological activation of PP2A, using FTY720, or the non-phosphorylatable analogue, AAL(S), inhibits tumor growth and metastases in an orthotopic xenograft model of aggressive, triple negative breast cancer (MDA-MB-231). Together this work demonstrates the importance of PP2A as a tumor suppressor in breast cancer, and suggests that targeting PP2A is a potential therapeutic strategy for poor outcome patients.

Citation Format: Nikita Panicker, Abdul Mannan, Lauren F. Watt, Ben Copeland, Matt D. Dun, Simon King, Megan Clarke, Kathryn Skelding, Severine Roselli, Nicole M. Verrills. Functional role of the tumor suppressor protein phosphatase, PP2A-B55α, in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2375. doi:10.1158/1538-7445.AM2017-2375