Synergistic Proliferative Action of Insulin-like Growth Factor I and 17 β-Estradiol in MCF-7S Breast Tumor Cells

We have analyzed the mechanism by which the combination of insulin-like growth factor I (IGF-I) and 17 β-estradiol (E2) induces cell cycle progression in MCF-7S cells. This cell line differs from many other breast cancer-derived cell lines in that E2 (1 nM) does not induce cell cycle progression, whereas the combination of submitogenic concentrations of IGF-I (2 ng/ml) and E2 does. We find that addition of IGF-I to MCF-7S cells leads to a dose-dependent activation of the IGF type I receptor and of the MAP kinase and PI3-kinase signaling pathways. No synergy of IGF-I and E2 was detected in the activation of these signaling cascades. In terms of cell cycle-related molecules, we find that IGF-I dose-dependently raises cyclin D1 levels in serum-starved cells. Subsequent activation of cyclin E/CDK2, hyperphosphorylation of pRb, and DNA synthesis are only induced by mitogenic concentrations of IGF-I (≥20 ng/ml). Treatment of the cells with E2 also results in the induction of cyclin D1, but in the absence of IGF-I the cells remain arrested in G1 phase. We conclude that in MCF-7S cells, the synergistic action of E2 and IGF-I derives from the ability of both hormones to induce cyclin D1 expression. The action of IGF-I is required in these cells to induce activity of the cyclin D1/CDK4 complex, which triggers progression through the cell cycle.