ANTIESTROGEN REGULATION OF CELL-CYCLE PROGRESSION AND CYCLIN D1 GENE-EXPRESSION IN MCF-7 HUMAN BREAST-CANCER CELLS

The molecular mechanisms by which antiestrogens inhibit breast cancer cell proliferation are not well understood. Using cultured breast canc er cell lines, we studied the effects of antiestrogens on proliferatio n and cell cycle progression and used this information to select candi date cell cycle regulatory genes that are potential targets for anties trogens. Under estrogen- and serum-free conditions antiestrogens inhib ited proliferation of MCF-7 cells stimulated with insulin. Cells were blocked at a point in G(1) phase. These effects are comparable with th ose in serum- and estrogen-containing medium and were also seen to a l esser degree in nude mice bearing MCF-7 tumors. Similar observations w ith other peptide mitogens suggest that the process inhibited by antie strogens is common to estrogen and growth factor activated pathways. O ther studies have identified G(1) cyclins as potential targets for gro wth factor and steroid hormone/steroid antagonist regulation of breast epithelial cell proliferation. In MCF-7 cells growing in the presence of fetal calf serum, cyclin D1 mRNA was rapidly down-regulated by ste roidal and nonsteroidal antiestrogens by an apparently estrogen recept or mediated mechanism. Cyclin D1 gene expression was maximally inhibit ed before effects on entry into S phase and inhibition was therefore n ot merely a consequence of changes in cell cycle progression. Together with data on the effects of antiestrogens in serum-free conditions [1 ], these results suggest down-regulation of cyclin D1 by antiestrogens may be a general phenomenon in estrogen receptor-positive breast canc er cells, independent of culture conditions and class of antiestrogen. These observations are compatible with the hypothesis that reductions in cyclin D1 levels may mediate in part the action of antiestrogens i n blocking entry of cells into S phase.