ERα inactivation and cell sensitivity to rapamycin. (a) The comparative analysis of the transcriptional activity of the estrogen receptor in MCF-7 cells and MCF-7/DCC cells, pretreated in the steroid-depleted medium for 3 days. The ERE luciferase and β-galactosidase activities were determined as described in Materials and Methods. Data represent the mean value ± SD of three independent experiments. (b,c) The sensitivity of MCF-7/DCC cells and ERα-negative MDA-MB-231 cells to metformin and rapamycin. The cells were treated without or with 1 µM rapamycin in the presence or absence of 2 mM metformin for 3 days, and the amount of the viable cells was counted by the MTT-test. Data represent the mean value ± SD of three independent experiments. p < 0.05: $ versus MCF-7 cells, * versus control, # versus metformin-treated cells.