Retinoic acid-mediated G(1)-S-phase arrest of normal human mammary epithelial cells is independent of the level of p53 protein expression

Retinoids mediate the normal growth of a variety of epithelial cells and ma y play an important role in the chemoprevention of breast cancer. Despite t he widespread clinical use of retinoids, specific target genes that are reg ulated by retinoids are relatively poorly characterized. We reported previo usly that all-trans-retinoic acid (ATRA) mediates G(1)-S-phase arrest in no rmal human mammary epithelial cells (HMECs). The tumor suppressor gene p53 is thought to be a critical regulator of G(1)-S-phase arrest mediated by DN A-damaging agents such as chemotherapy and radiation. The role of p53 prote in expression in G(1)-S-phase arrest mediated by the differentiating agent ATRA is unknown. Increased expression of p53 protein is observed in ATRA-tr eated HMECs at 72 h; however, initiation of G(1)-S-phase arrest starts at 2 4 h, suggesting that this observed induction of p53 is a secondary event. U sing retroviral-mediated gene transfer, we expressed the E6 protein of the human papillomavirus strain 16 (HPV-16) in HMECs. The HPV-16 E6 protein bin ds to p53 and targets it for degradation. Western analysis confirmed that H PV-16 EG-transduced HMECs had markedly decreased levels of p53 protein expr ession. Suppression of cellular p53 levels in HMECs did not alter the sensi tivity of HMECs to ATRA-mediated growth arrest. Our studies suggest that AT RA-mediated G(1)-S-phase arrest is independent of the level of p53 protein expression. We also tested the ability of estrogen and antiestrogens to ind uce growth arrest in HMECs lacking p53 expression and found no decrease in the sensitivity of these cells to these agents. Our results emphasize the c hemotherapeutic potential of ATRA and antiestrogens, particularly for suppr essing the growth of tumors lacking functional p53.