Tamoxifen but not 4-hydroxytamoxifen initiates apoptosis in p53(-) normal human mammary epithelial cells by inducing mitochondrial depolarization

Despite the widespread clinical use of tamoxifen as a breast cancer prevent ion agent, the molecular mechanism of tamoxifen chemoprevention is poorly u nderstood. Abnormal expression of p53 is felt to be an early event in mamma ry carcinogenesis. We developed an in vitro model of early breast cancer pr evention to investigate how tamoxifen and 4-hydroxytamoxifen may act in nor mal human mammary epithelial cells (HMECs) that have acutely lost p53 funct ion. p53 function was suppressed by retrovirally mediated expression of the human papillomavirus type 16 E6 protein. Tamoxifen, but not 4-hydroxytamox ifen, rapidly induced apoptosis in p53(-) HMEC-E6 cells as evidenced by cha racteristic morphologic changes, annexin V binding, and DNA fragmentation. We observed that a decrease in mitochondrial membrane potential, mitochondr ial condensation, and caspase activation preceded the morphologic appearanc e of apoptosis in tamoxifen-treated early passage p53(-) HMEC-E6 cells, p53 (-) HMEC-E6 cells rapidly developed resistance to tamoxifen-mediated apopto sis within 10 passages in vitro. Resistance to tamoxifen in late passage p5 3(-) HMEC-E6 cells correlated with an increase in mitochondrial mass and a lack of mitochondrial depolarization and caspase activation following tamox ifen treatment. We hypothesize that an early event in the induction of apop tosis by tamoxifen involves mitochondrial depolarization and caspase activa tion, and this may be important for effective chemoprevention.