Rb Dephosphorylation and Suppression of E2F Activity in Human Breast Tumor Cells Exposed to a Pharmacological Concentration of Estradiol

This report characterizes the influence of a pharmacological concentration of estradiol on growth arrest and cell death in MCF-7 breast tumor cells, with a focus on elements of the Rb-E2F cell-cycle regulatory pathway. Continuous exposure of MCF-7 breast tumor cells to 100 μM estradiol produces a marked reduction in the G1 and S phase populations and a corresponding increase in the G2/M population within 24 h; after 48 h, accumulation of cells in G1 becomes evident while after 72 h the cells appear to be equally distributed between the G1 and G2/M phases. The accumulation of cells in G1 is temporally associated with dephosphorylation of the Rb protein and suppression of E2F activity. Estradiol also produces an initial burst of cell death with loss of approximately 40% of the tumor cell population within 24 h; however, there is no tangible evidence for the occurrence of apoptosis based on terminal transferase end-labeling of DNA, DNA fragmentation analysis by alkaline unwinding, cell-cycle analysis or cell morphology. In addition to the lack of caspase-3 in MCF-7 cells, the absence of apoptosis could be related, at least in part, to the fact that estradiol promotes a rapid reduction in levels of the E2F-1 and Myc proteins. Overall, these studies are consistent with the concept that alterations in the levels and/or activity of the E2F family of proteins as well as proteins interacting with the E2F family may influence the nature of the antiproliferative and cytotoxic responses of the breast tumor cell.