Abstract
The erbB-2 oncogene encodes a 185-kDa transmembrane protein that has been suggested to be a growth factor receptor. We have previously identified and purified a 30-kDa growth factor (gp30) that is a ligand for the p185erbB-2 protein that at high concentrations induces growth inhibition of cells with erbB-2 amplification. We now report the purification and characterization of a protein from SKBr-3 human breast cancer cells with a molecular mass of 75 kDa (p75) that is a p185erbB-2 ligand. An affinity column coupled to the extracellular domain of p185erbB-2 was used for the purification. We found that p75 induced tyrosine phosphorylation of the erbB-2 oncoprotein, as determined by in vivo and in vitro phosphorylation and phosphoamino acid analysis. p75, as well as gp30, stimulated cell proliferation and colony formation of cells overexpressing erbB-2. The specificity of this effect was confirmed by showing that the antiproliferative effects of soluble erbB-2 extracellular domain were reversed by either p75 or gp30. p75 did not show binding to the epidermal growth factor receptor and had no growth effects on cells overexpressing epidermal growth factor receptor. These data show that SKBR-3 cells, which exhibit erbB-2 amplification and overexpression, secrete a growth factor that binds and activates p185erbB-2 specifically.