Transforming growth factor α (TGFα), 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 TGFα in estrogen- responsive MCF-7 human breast cancer cells using monoclonal (528ab and 225ab) and polyclonal antibodies that block the EGF/TGFα receptor. Confirming other studies, these MCF-7 cells expressed TGFα with mRNA transcripts of 4.8 kilobases identified by Northern analysis, and they secreted TGFa activity measured by normal rat kidney colony-forming assay and an EGF RRA of conditioned medium. This activity was increased 3-fold by 1 nwi 17β- estradiol and decreased by 1 μm 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/TGFα. These antibodies failed to inhibit baseline DNA synthesis or growth of MCF-7 cells although they were potent inhibitors of EGF/TGFα-induced growth of these cells. We hypothesized that if secreted TGFa mediates estrogen-induced growth, then EGF/TGFα receptor blockade should inhibit estrogen stimulation. MCF-7 cells were first treated with tamoxifen to inhibit growth and to reduce TGFα expression. Under these conditions, estrogen replenishment induced a marked dose-dependent rescue of TGFα secretion, DNA synthesis, and cell proliferation. Exogenous TGFα also partially restored growth of tamoxifen- inhibited cells. Although the simultaneous addition of 528ab or 225ab blocked TGFα-induced rescue of MCF-7 cells, it had no effect on rescue by estradiol. Similar results were observed with a polyclonal anti- EGF receptor antibody, and with two other estrogen- responsive breast cancer cell lines. In summary, blockade of the EGF/TGFα receptor in hormone- dependent human breast cancer cells does not alter estrogen-regulated growth suggesting that secreted TGFα is not a primary mediator of the growth effects of estrogen.
ASJC Scopus subject areas
- Molecular Biology