Investigating the potential of metformin as an anti-cancer therapeutic in a model of breast disease

Metformin has been one of the most widely prescribed oral medications for type II diabetes for over six decades. It has recently received considerable attention because there is now evidence to show that metformin has a potential role in reducing the risk of cancer development and progression. However, the mechanisms behind the growth-inhibitory effect of metformin on breast cancer cells are not fully understood with little consensus on which tumour subtypes and/or patient populations will benefit from metformin treatment. Furthermore, it should be noted that much of the in vitro work published to date has used drug concentrations greatly exceeding the recommended clinical dose and most preclinical studies have given little attention to the cellular pharmacology of metformin uptake including the expression of metformin transporter molecules and intratumoral accumulation. As a result these studies may not translate directly into clinical practice. This project therefore tests the hypothesis that the anti-tumour effect of clinically relevant doses (0.03-0.3 mM) of metformin depends on breast cancer subtype and the presence of metformin transporters on breast cancer cells. Using immunohistochemistry on patient-derived tissues and various in vitro cell-based assays in a panel of increasingly transformed breast cell lines representing an in vitro model of breast disease progression, the expression of metformin transporters and the potential anti-proliferative effects of the clinical (0.03-0.3 mM) and potential tissue accumulation (1-5 mM) doses of metformin were evaluated. In parallel, global proteomic profiling was performed on three metastatic breast cancer cell lines to identify new potential molecular targets for metformin treatment. The data in this thesis show that metformin transporters are present on breast epithelial cells, pre-neoplastic, pre-invasive, invasive and metastatic breast cancer cells and that metformin has a cytostatic effect on the proliferation of these cells, causing cell cycle arrest, but not apoptosis at clinically relevant doses. The proteomic data suggest that metformin inhibits the expression of proteins within key cellular pathways in both triple negative breast cancer and the bone and lung-homed variants, with the lung-homed cells showing a greater response to metformin treatment. Taken together these data provide important novel insight into the useful role of metformin in breast cancer treatment, but further research is certainly required to identify biomarkers of response and mechanisms of action in breast cancer before metformin can be recommended in clinical practice.