The synergistic action of estrogens and insulin-like growth factors (IGFs) promotes the growth of many human breast cancer cell lines. This synergistic effect involves estrogen-dependent induction of the IGF system, i.e., estrogens augment the number of IGF-I receptors, stimulate the secretion of IGF-II, and promote the synthesis of certain IGF-binding proteins. On the other hand, the sustained activation of the IGF-I receptor (IGF-IR) by the overexpression of IGF-II has been found to contribute to the development of the estrogen-independent phenotype in breast cancer cells. In this study, we have investigated whether the amplification of the IGF-IR intracellular signaling in MCF-7 cells can abolish or reduce estrogen requirements for growth and transformation. To this end we developed several MCF-7 clones that overexpressed insulin receptor substrate 1 (IRS-1), one of the principal substrates of the IGF-IR. We report here that in MCF-7 cells overexpressing IRS-1, estrogen requirements for growth in monolayer culture as well as in soft agar were reduced. The decreased estrogen requirements depended on the level of the overexpressed IRS-1 protein, and in cells which contained several-fold more functional IRS-1 than the parental cells, we observed total loss of estrogen dependence for growth. In addition, the importance of IRS-1 in proliferation of MCF-7 cells has been confirmed through the use of antisense strategies.