Antidiabetic Drug Metformin Induces Apoptosis in Human MCF Breast Cancer via Targeting ERK Signaling

Metformin is the most widely used antidiabetic drug for type II diabetes in the world. Recent studies provide clues that the use of metformin may be associated with reduced incidence and improved prognosis of certain cancers and there is increasing evidence of a potential efficacy of this agent as an anticancer drug. This observation led us to hypothesize that metformin might inhibit human breast cancer cells (MCF-7) growth. Here, we report that metformin induced apoptosis in human breast carcinoma cell lines MCF-7 cells via novel signaling pathway. Metformin induced apoptosis by arresting cells in G₁ phase and reducing cyclin D level and increasing the expression of p21 and cyclin E. Molecular and cellular studies indicated that metformin significantly elevated p53 and Bax levels and reduced STAT3 and Bcl-2. Inhibitors of signaling proteins were used to study the mechanism(s) of metformin function. Receptor inhibitor studies indicated that p53 activation was mediated through insulin receptor (IR), not insulin-like growth factor-1 receptor (IGF-1R). Furthermore, MEK inhibitor significantly suppressed metformin-induced p53 and Bax elevation while ERK inhibitor generated a slight reduction in p53 levels. In contrast, PI3K inhibitor did not produce any effect on the metformin-elevated p53 levels. Finally, SAPK/JNK, known to be involved in apoptosis, was activated in cells treated with metformin and the activation appeared to occur downstream of ERK. All these results suggested that metformin activated p53, Bax, and induced tumor cell apoptosis through the ERK signaling pathway. This pathway has not been previously described for IR, p53, Bax activation, or apoptosis. Metformin, a novel inducer of apoptosis, and its analogs may offer a novel strategy for the treatment of cancer cells.