Contrasting ability of antiestrogens to inhibit MCF-7 growth stimulated by estradiol or epidermal growth factor.

A potential mechanism is described by which a growth factor may prevent the action of antiestrogens or reactive the growth of hormone-responsive breast carcinoma in patients undergoing tamoxifen (TAM) treatment. Epidermal growth factor (EGF)-stimulated growth (10(-8) M EGF) was assayed in the MCF-7 breast cancer cell line in the presence of various concentrations (10(-10) to 10(-6) M) of three antiestrogens, 4-hydroxytamoxifen (OH TAM), TAM and ICI 164384. In each case, the EGF-stimulated increases in DNA were not inhibited by the antiestrogen. OH TAM and ICI 164384 inhibited estradiol (E2) stimulated cell proliferation in a dose-related fashion. However, in the presence of both E2 and EGF, these two antiestrogens inhibited E2 effects only; EGF promotion of growth was unaffected. Pretreatment of MCF-7 cells for 2 days with either OH TAM or ICI 164384 did not inhibit EGF-induced increases in cell proliferation. We propose that eventual antiestrogen therapeutic failure may be caused by the paracrine influences of growth factors from neighboring cells.