Direct interaction of a ligand for the erbB2 oncogene product with the EGF receptor and p185^erbB2

The erbB2 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^erbB2. 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 erbB2 but do not express EGFR (MDA-MB-453 breast cancer cells and a Chinese hamster ovary cell line that had been transfected with erbB2), phosphorylation of p185 ^erbB2 was induced only by gp30. The gp30 specifically inhibited the growth of cells that overexpressed p185^erbB2. 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 erbB2. Direct binding of gp30 to p185^erbB2 was confirmed by binding competition experiments, where gp30 was found to displace the p185 ^erbB2 binding of a specific antibody to p185^erbB2. The evidence described here suggests that gp30 is a ligand for p185^erbB2.