Metformin has been shown to be active against ovarian cancer cells in vitro and in vivo. We demonstrate here that metformin acts on ovarian cancer stem cells, reducing the percentage of ALDH(+) CSC in vitro and in vivo, and inhibiting the growth of ovarian tumor spheres. Metformin was active against primary human CSC in vitro and, metformin therapy alone slows the growth of ovarian CSC in vivo.
Our results are most consistent with metformin having anti-proliferative rather than cytotoxic effects. This is consistent with the results of others suggesting that metformin can induce cell cycle arrest at G1 [17
] and restrict cellular proliferation in AMPK dependent and independent manners [18
]. While we did not observe significant cytotoxicity with metformin, metformin has been reported to induce apoptosis in ovarian cancer cells [10
]. Apoptosis was enhanced in the presence of cisplatin therapy.
Our results suggesting that metformin reduces tumor microvascular density are also consistent with previous studies which reported reduced VEGF levels and reduced microvascular density with metformin therapy [11
]. Interestingly, CSC and tumor vasculature appear to be mutually dependent on one another [19
]. While angiogenesis is essential for tumor growth and tumor endothelial cells provide important growth factors for CSC [20
], CSC are in turn highly angiogenic [4
]. Our studies raise the possibility that the anti-angiogenic effects of metformin are due to the actions of metformin on CSC and that the reduction in CSC subsequently leads to a reduction in angiogenesis.
Our data adds to the growing number of laboratory studies (i) indicating anti-CSC effects of metformin and (ii) supporting metformin therapy for ovarian cancer. Metformin has been demonstrated to have anti-CSC activity on breast, prostate, and lung cancer cell lines [5
]. One of the earlier studies indicating a role for metformin in cancer therapy demonstrated that metformin acted primarily on tumors with p53 mutations [22
]. Importantly, sequencing of approximately 500 ovarian tumors as part of the Tumor Genome Atlas Project recently revealed that greater than 95% of ovarian cancers carry p53 mutations [23
]. Thus if the anti-neoplastic effects of metformin are dependent on p53 mutations, ovarian cancer represents an ideal target for use of metformin.
As noted above, epidemiologic data also supports the thesis that metformin is active against ovarian cancer. We recently demonstrated that type 2 diabetes has a negative impact on the survival of ovarian cancer patients [24
]. Interestingly, diabetic patients were more likely to have a poorly differentiated tumor histology [24
]. Poor tumor differentiation could be consistent with increased tumor “stemness.” However, ovarian cancer patients with type 2 diabetes treated with metformin have better outcomes than those treated with other anti-glycemic agents, supporting a role for metformin as a therapeutic in ovarian cancer. Furthermore, recent studies found that the long-term use of metformin, but not of sulfonylureas, was associated with a decreased risk of ovarian cancer [25
]. Thus metformin use may actually prevent or delay ovarian cancer development.
Taken together, these data strongly indicate that metformin is an active therapeutic in ovarian cancer which impacts ovarian CSC growth. These important results shed light on how metformin works and provide critical preclinical rationale for the use of metformin in phase 2 clinical trials for ovarian cancer patients. We believe that metformin co-treatment with chemotherapy may prevent ovarian cancer recurrence and improve long-term survival.