Abstract A101: Metformin prevents mammary tumors in p53-null mammary gland mice

Abstract

Abstract Background: Breast cancer prevention is now possible using antiestrogen drugs such as tamoxifen and raloxifene; however, antiestrogens are ineffective against estrogen receptor (ER)-negative breast cancers. Since ER-negative breast cancers are highly aggressive and particularly difficult to treat, preventive agents against these life-threatening cancers are urgently needed. Metformin, a first line treatment for adult-onset diabetes, recently was shown to be associated with reduced incidence of cancer (including breast cancer) in epidemiologic studies. Metformin activates AMP-dependent kinase (AMPK) and stimulation of AMPK by metformin reduces cell proliferation in both estrogen receptor alpha (ERa)-positive and -negative human breast cancer cell lines. We hypothesized that the metformin would effectively prevent the growth and development of ER-negative breast cancer. To test this hypothesis, we tested metformin's ability to prevent mammary tumors in two mouse models: the MMTV-erbB2 transgenic mouse model that develops ER-negative mammary tumors and the p53-null mammary gland mouse model that develops both ER-positive and ER-negative mammary tumors. Methods: MCF7, BT474, MDA MB 468 and MDA MB 453 cells were treated with10mM metformin and cell proliferation was measured using an MTS assay. Virgin female MMTV-erbB2 mice (Jackson Lab) were housed in the institutional animal facilities and fed a purified diet (AIN-76A; Harlan Teklad). Mice were separated into 2 treatment groups: 1) Vehicle control (PBS via gastric gavage daily), 2) Metformin (250mg/kg via gastric gavage daily; from 3 months to 12 months of age). P53-null Balb/C mammary glands from donor mice were transplanted into both cleared inguinal mammary fat pads of Balb/C p53 wild-type mice. Mice were observed daily for tumor formation and the percentage of tumor free mice were recorded. Tumor incidence and time to tumor formation was visualized using Kaplan Meier curves and analyzed using the generalized Wilcoxon test. Results: Metformin (at 10mM) significantly reduced proliferation of breast cancer cells irrespective to their ER, Her2 and p53 status. In the MMTV-erbB2 mice, metformin treatment did not prevent or delay mammary tumor formation. However, in p53-null mammary gland mice, metformin completely prevented mammary tumor formation. Conclusions: Metformin treatment totally prevented mammary tumor development in p53-null mammary gland mice. These results suggest that metformin will be most useful in preventing breast cancers with P53 mutations such as occurs in Li-Fraumeni syndrome individuals or individuals with ERa-negative or “triple-negative” breast cancer. These results support testing metformin in clinical trials for the prevention of ER-negative and p53-mutant breast cancers. This research was supported by the NIH/NCI R01 CA78480 grant (PHB). Citation Information: Cancer Prev Res 2011;4(10 Suppl):A101.