Human papillomavirus type 16 E6 inactivation of p53 in normal human mammary epithelial cells promotes tamoxifen-mediated apoptosis

Aberrant p53 expression is frequently observed in mammary epithelial cells obtained from women at high risk for developing breast cancer and is a predictor for the subsequent development of malignancy, Tamoxifen has recently been shown to reduce the incidence of noninvasive breast cancer in high-risk women, but the molecular mechanism of tamoxifen chemoprevention in mammary epithelial tissue that does not overexpress the estrogen receptor is poorly understood, We suppressed p53 expression by retroviral-mediated expression of human papillomavirus type-16 E6 protein (HPV-16 E6) in human mammary epithelial cells (HMECs) to develop an in vitro model of tamoxifen chemoprevention in the context of p53 loss. Early passage p53(-) HMEC-E6-transduced cells treated with 1.0 muM tamoxifen rapidly underwent apoptosis, In contrast, early passage p53(+) HMEC-LXSN vector controls treated with 1.0 muM tamoxifen underwent G(1)-G(0)-phase arrest but did not undergo apoptosis, p53(-) HMEC-E6 cells rapidly acquired resistance to tamoxifen-mediated apoptosis after 10 passages in culture (in the absence of tamoxifen), Both p53(+) and p53(-) HMECs exhibited a low level of estrogen receptor staining and minimal estrogen binding, characteristic of proliferating normal luminal mammary epithelial cells, Tamoxifen-mediated apoptosis in p53(-) HMEC-E6 cells was not blocked by inhibitors of transcription and protein synthesis, These data suggest that the acute loss of p53 function in HMECs by expression of HPV-16 E6 results in marked sensitivity to tamoxifen-mediated apoptosis but that resistance to apoptosis rapidly develops within 10 passages in vitro, Observations in our model system predict a critical role for the early institution of tamoxifen chemoprevention.