Fig. 4. TGFA-induced ER-a downregulation resulted in acquired tamoxifen resistance in BT474 breast cancer cells. (A, B) After 24 h serum starvation, we pretreated cells with or without 50 ng/ml TGFA for 24 h and cultured with or without 10 mM 4-OHT for 24 h. (A) Cell viability was analyzed by MTT assays. (B) Levels of PARP-1 and procaspase-3 protein were detected by western blots. (C) Cells were pretreated with or without 50 ng/ml TGFA for 24 h, trypsinized, and reseeded with or without 10 mM 4-OHT in agarose plates for colony-forming assays. Results are representative of three independent experiments. Values are mean ± SEM. * P<0.05, ** P<0.01 vs. (-) control.