Blockade of the Epidermal Growth Factor Receptor Inhibits Transforming Growth Factor α-Induced but Not Estrogen-Induced Growth of Hormone-Dependent Human Breast Cancer

Open Access
Publisher Website
Google Scholar
Abstract
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 estrogenresponsive 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 TGFα 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 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 TGFα 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 tamoxifeninhibited 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 estrogenresponsive breast cancer cell lines. In summary, blockade of the EGF/TGFα receptor in hormonedependent 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.