Surface proteins and fibrinolytic activity of cultured mammalian cells

Surface proteins and fibrinolysis were investigated in a variety of cell types. A large external transformation-sensitive protein was demonstrated by lactoperoxidase-catalyzed iodination. Electron microscope autoradiography confirmed that the technique labeled surface material only. The protein was present in explants of normal tissues was well as in nontumor-producing cultured cell lines. It was not lost during long-term culture. Neoplastic transformation in vitro, whether spontaneous or induced by a chemical carcinogen or virus, led to the loss of the material in most but not all cases. The protein was also absent from many but not all spontaneous and induced tumors of the different types tested. Elevated fibrinolytic activity was demonstrated in a number of normal tissues and non-tumor-producing cell lines. It was also present in most sarcomas but absent from most carcinomas that we have examined, except for a cell line from a well-differentiated bladder tumor. The correlation between absence of large external transformation-sensitive protein and fibrinolysis was examined, and it was found that activation of plasminogen was not sufficient to cause the absence of this surface protein.