Transforming growth factor alpha (TGF alpha), a polypeptide that binds to the epidermal growth factor (EGF) receptor, is expressed and secreted by human breast cancer cells and has been proposed as an autocrine growth factor and as a mediator of the mitogenic effect of estrogen. We investigated the potential importance of secreted TGF alpha in estrogen-responsive MCF-7 human breast cancer cells using monoclonal (528ab and 225ab) and polyclonal antibodies that block the EGF/TGF alpha receptor. Confirming other studies, these MCF-7 cells expressed TGF alpha with mRNA transcripts of 4.8 kilobases identified by Northern analysis, and they secreted TGF alpha activity measured by normal rat kidney colony-forming assay and an EGF RRA of conditioned medium. This activity was increased 3-fold by 1 nM 17 beta-estradiol and decreased by 1 microM tamoxifen. 528ab and 225ab bound to EGF receptors in MCF-7 cells with high affinity [dissociation constant (Kd) 0.1-0.5 nM] and blocked the binding of EGF/TGF alpha. These antibodies failed to inhibit baseline DNA synthesis or growth of MCF-7 cells although they were potent inhibitors of EGF/TGF alpha-induced growth of these cells. We hypothesized that if secreted TGF alpha mediates estrogen-induced growth, then EGF/TGF alpha receptor blockade should inhibit estrogen stimulation. MCF-7 cells were first treated with tamoxifen to inhibit growth and to reduce TGF alpha expression. Under these conditions, estrogen replenishment induced a marked dose-dependent rescue of TGF alpha secretion, DNA synthesis, and cell proliferation. Exogenous TGF alpha also partially restored growth of tamoxifen-inhibited cells. Although the simultaneous addition of 528ab or 225ab blocked TGF alpha-induced rescue of MCF-7 cells, it had no effect on rescue by estradiol.(ABSTRACT TRUNCATED AT 250 WORDS)