The role of Protein Phosphatase 2A as a tumour suppressor in breast cancer

Breast cancer is a worldwide health issue, and while many advances have been made in recent years, continued understanding of the development and progression of breast cancer is required to produce novel therapies to improve patient survival. Breast cancer is characterised by disruption in signalling pathways that control key cellular processes such as growth, proliferation and survival. Protein Phosphatase 2A (PP2A) is a key cellular signalling molecule that regulates numerous signalling pathways involved in breast cancer. PP2A is a trimeric protein complex, consisting of a structural subunit (PP2A-A), to which a catalytic subunit (PP2A-C) and a regulatory B subunit bind. PP2A is a proposed tumour suppressor, yet the role of PP2A in breast cancer has not been examined in detail to date. This thesis firstly examines PP2A expression in breast cancer cell lines and human breast cancer tissue. Dramatic reductions in expression of the PP2A-A and also a number of regulatory B subunits were observed in a panel of breast cancer cell lines compared to normal human mammary epithelial cells. In addition, a significant reduction in PP2A-A expression was identified in human breast tumours compared to normal mammary tissue. These results suggest that PP2A is important for the development or progression of breast cancer. In order to determine the functional role of PP2A in breast cancer, PP2A subunit expression was altered in a mammary breast epithelial cell line, MCF10A. A number of MCF10A cell lines were generated by transduction of shRNA directed to the PP2A-A or regulatory B subunits, or by expression of cancer-associated PP2A-A mutant genes. Functional analyses showed that shRNA knockdown or PP2A-A mutant expression had very little effect on MCF10A cells when grown using traditional two-dimensional cell culture techniques. However, in a more physiologically relevant three-dimensional culture method that maintains cellular polarisation and signalling with the basement membrane, a number of phenotypes indicative of cellular transformation were observed. MCF10A cells with reduced expression of regulatory B subunits, or PP2A-A mutations unable to bind regulatory B subunits, demonstrated increased cellular proliferation, MCF10A PP2A-A mutants that cannot interact with either the catalytic or regulatory B subunits displayed invasive properties. The results presented in this thesis provide clear evidence that PP2A is involved in breast cancer and presents a number of avenues for future investigation and potential novel therapies.