Normal Breast Epithelial Cells Induce Apoptosis of MCF-7 Breast Cancer Cells through a p53-Mediated Pathway Robert-Alain Toillon,* Eric Adriaenssens,* Danie `le Wouters,† Severine Lottin,* Be ´noni Boilly,* Hubert Hondermarck,* and Xuefen Le Bourhis* *Laboratoire de Biologie du De ´veloppement, Equipe Facteurs de Croissance (UPRES 1033), Universite ´ des Sciences et Technologies de Lille, 59655 Villeneuve d’Ascq Cedex, France; and †INSERM U459, Faculte ´ de Me ´decine, 59000 Lille, France Received July 17, 2000 Cancer development depends not only on the nature of the tumor cells themselves but also on the regula- tory effects of various normal cells. The present study was performed to better understand the mechanism by which normal breast epithelial cells (NBEC) can control the growth of MCF-7 breast cancer cells. When MCF-7 cells were treated with NBEC conditioned me- dium, cell growth was inhibited in a concentration- dependent manner. This inhibition was due to an in- duction of apoptosis without any change in cell cycle progression. The induction of apoptosis was corre- lated with increased levels of p53, p21 waf1 and de- creased levels of bcl-2. Transient transfections of MCF-7 cells with two p53 cDNA constructs demon- strated the induction of apoptosis was mediated by endogenous p53. Taken together, our results indicate that NBEC inhibit the growth of MCF-7 breast cancer cells by inducing apoptosis in them via endogenous p53. © 2000 Academic Press Key Words: breast cancer; cell growth; apoptosis; normal breast epithelial cells; p53. The most prominent characteristic of tumors is the rapid increase of cell number, which is the result of an imbalance between cell proliferation and apopto- sis. These two processes are regulated by the tumor suppressor oncogene p53. The p53-dependent G1 ar- rest is mediated by direct transactivation of p21 waf1 which in turn encodes the inhibitor of cyclin- dependent kinases (1). Although the biological mech- anism by which p53 regulates the G2/M transition remains largely nuclear, it has been reported that its activation results in the down-regulation of Weel expression and the dephosphorylation of Cdc2; the dephosphorylated Cdc2 increases cyclin/Cdc2 kinase activity, thus suppressing the G2 cell cycle check point (2). p53 can induce apoptosis by modifying the expression of the bcl-2 family, as experimental over- expression of p53 is associated with a decreased ex- pression of bcl-2 and an increased expression of bax, an antagonist of bcl-2 (3, 4). Cancer development depends not only on the nature of the cancerous cells themselves, but also on the reg- ulatory effects of various normal cells such as fibro- blasts, endothelial cells and epithelial cells. This host regulation may be as great a determinant of a tumor cell’s behavior in vivo as the specific oncogenic or tumor suppressor alterations occurring within the malignant cell itself (5). While fibroblasts and endothelial cells are believed to favour tumor development (5), normal breast myoepithelial cells and epithelial cells have been shown to exert an inhibitory effect on breast can- cer cells (6 –10). In addition, improving knowledge of mammary gland development, especially during post- lactational involution, suggests that paracrine factors controlling mammary tissue homeostasis might be use- ful for breast cancer prevention and treatment (11). In a previous study we have demonstrated that normal breast epithelial cells (NBEC) inhibit the growth of breast cancer cells, especially that of hormono- sensitive cells such as MCF-7 and ZR75 (7). The MCF-7 cell line is the prototype for hormono-sensitive breast cancer cells, which are representative of early stage of breast cancer development. The fact that MCF-7 cells are strongly growth-inhibited by NBEC suggests that NBEC may exert an important negative control over the early stages of breast cancer development in vivo. It therefore becomes important to understand the mechanism by which NBEC reduce the growth of can- cer cells. The present work has demonstrated that NBEC inhibit the growth of MCF-7 breast cancer cells by inducing apoptosis. Moreover, this induction is me- diated by endogenous p53. Molecular Cell Biology Research Communications 3, 338 –344 (2000) doi:10.1006/mcbr.2000.0236, available online at http://www.idealibrary.com on 338 1522-4724/00 $35.00 Copyright © 2000 by Academic Press All rights of reproduction in any form reserved.