Effect of SMRT knock-down on growth of ERα-positive ZR-75–1 cells and ERα-negative MDA-MB-231 cells. A, ZR-75–1 cells were transfected with nontargeting control (siCtrl) or SMRT-specific (siSMRT) siRNA and 24 hours later treated with vehicle, 1 nM E2, 100 nM 4HT, or 100 nM ICI. Cell growth was determined after 5 days of culture. Cell numbers are normalized to E2-treated siCtrl cells (n = 3). B, Representative Western blot of SMRT (top) expression for ZR-75–1 cells treated with siRNA for control (-) and SMRT (+). Actin is shown as a loading control (bottom). C, Representative Western blot of SMRT knock-down in MDA-MB-231 cells (left) and Western blot comparing levels of SMRT across cell lines grown in media supplemented with 10% FBS indicates there are variable levels of total SMRT and SMRT isoforms in the cell lines examined (right). D, MDA-MB-231 cells were transfected with control (siCtrl) or SMRT (siSMRT) siRNA, and 5 days thereafter, cell numbers were assessed. Cell numbers are normalized to siCtrl cells (n = 7). Numerical data are presented as the average ± SEM. Statistical analyses were by Student's t test; #, P < .05 versus vehicle treatment within the same siRNA group and *, P < .05 versus the corresponding ligand treatment in the siCtrl group.