Direct Interaction of a Ligand for the erb B2 Oncogene Product with the EGF Receptor and p185 erb B2

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Abstract
The erb B2 oncogene encodes a 185-kilodalton transmembrane protein whose sequence is similar to the epidermal growth factor receptor (EGFR). A 30-kilodalton factor (gp30) secreted from MDA-MB-231 human breast cancer cells was shown to be a ligand for p185 erb B2 . An antibody to EGFR abolished the tyrosine phosphorylation induced by EGF and transforming growth factor-α (TGF-α) but only partially blocked that produced by gp30 in SK-BR-3 breast cancer cells. In two cell lines that overexpress erb B2 but do not express EGFR (MDA-MB-453 breast cancer cells and a Chinese hamster ovary cell line that had been transfected with erb B2), phosphorylation of p185 erb B2 was induced only by gp30. The gp30 specifically inhibited the growth of cells that overexpressed p185 erb B2 . An antibody to EGFR had no effect on the inhibition of SK-BR-3 cell colony formation obtained with gp30. Thus, it appeared that gp30 interacted directly with the EGFR and erb B2. Direct binding of gp30 to p185 erb B2 was confirmed by binding competition experiments, where gp30 was found to displace the p185 erb B2 binding of a specific antibody to p185 erb B2 . The evidence described here suggests that gp30 is a ligand for p185 erb B2 .