Metformin inhibits proliferation of cultured breast cancer cells. A. MCF7 cells were treated with metformin at the indicated concentrations for one day and then cell number was determined using a hemocytometer. The mean cell number from 3 independent cultures is shown. Error bars represent standard deviation. Using a t test, all treatments were significantly different from the control with P values of less than 0.01. B. Six different breast cancer cell lines were treated with 8 mM metformin. At the indicated times viable cell number was determined using a hemocytometer. The reported status of estrogen receptor (ER) expression, HER2 amplification, and p53 expression (Wild-type, WT, or mutant, mt) are indicated for each cell line. HER2+ indicates HER2 amplification while HER2- indicates normal HER2 expression. The data points represent the mean cell number from 3 independent cultures and error bars represent standard deviation.