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Inhibition of epidermal growth factor/transforming growth factor‐α–stimulated cell growth by a synthetic peptide

Estrogen‐stimulated growth of the human mammary adenocarcinoma cell line MCF‐7 is significantly inhibited by monoclonal antibodies to the epidermal growth factor (EGF) receptor that act as antagonists of EGF's mitogenic events by competing for high‐affinity EGF receptor binding sites. These antibodies likewise inhibit the EGF or transforming growth factor‐α (TGF‐α)‐stimulated growth of these MCF‐7 cells. An analogous pattern of specific EGF or TGF‐α growth inhibitory activity was obtained using a synthetic peptide analog encompassing the third disulfide loop region of TGF‐α, but containing additional modifications designed for increased membrane affinity ([Ac‐D‐hArg(Et)  312 , Gly32,33]HuTGF‐α(31‐43)NH2). The growth factor antagonism by this synthetic peptide was specific in that it inhibited EGF, TGF‐α, or estrogen‐stimulated growth of MCF‐7 cells but did not inhibit insulin‐like growth factor‐1 (IGF‐1)‐stimulated cell growth. Altogether, these results suggest that a significant portion of the estrogen‐stimulated growth of these MCF‐7 cells is mediated in an autocrine/paracrine manner by release of EGF or TGF‐α‐like growth factors. The TGF‐α peptide likewise inhibited EGF‐ but not fibroblast growth factor (FGF)‐ or platelet‐derived growth factor (PDGF) ‐stimulated growth of NIH‐3T3 cells in completely defined media; but had no effect on growth or DNA synthesis of G0‐arrested cells, nor did it effect growth of NR‐6 cells, which are nonresponsive to EGF. Although this synthetic peptide did not directly compete with EGF for cell surface receptor binding, it exhibited binding to a cell surface component (followed by internalization), which likewise was not competed by EGF. The peptide did not directly inhibit EGF‐stimulated phosphorylation of the EGF receptor, nor did it inhibit phosphorylation of an exogenous substrate, angiotensin II, by activated EGF receptor. The TGF‐α peptide did, however, affect the structure of laminin as manifested by laminin self‐aggregation; this affect on laminin may, in turn, have a modulatory effect on EGF‐mediated cell growth.