Novel tyrosine phosphorylations accompany the activation of pp60c-src during chemical carcinogenesis

Treatment of normal Syrian hamster embryo (SHE) cells in vitro with various chemical carcinogens results in transformed preneoplastic cell lines. Continued passage of these preneoplastic cells gives rise to rare variant cells with enhanced capacity for tumorigenic growth. We have previously shown that tumor-derived SHE cell lines contain an activated proto-oncogene product, pp60c-src. Here we demonstrate that tumor-derived SHE cell lines contain several novel tyrosine phosphoproteins in addition to those found in preneoplastic parent cell lines. A correlation was observed between the activation of endogenous pp60c-src tyrosine kinase specific activity and the presence of new phosphotyrosine-containing proteins. Tyrosine phosphoproteins of approximate Mr 81 kilodaltons (kDa), 55 kDa, and 39 kDa were noted in different tumor-derived cell lines. The 81 kDa and 55 kDa proteins were membrane-associated phosphoproteins, whereas the 39 kDa protein was predominantly cytosolic. Additional signature tyrosine phosphoproteins in individual tumor-derived cell lines apparently were unique to the particular inducing carcinogen or target cell. These studies indicate that during chemical carcinogenesis, activation of the tyrosine kinase proto-oncogene protein pp60c-src coincides with the appearance of novel tyrosine phosphorylations.