Transformation of Rat Liver Cell Line by Rous Sarcoma Virus Causes Loss of Cell Surface Fibronectin, Accompanied with Secretion of Metallo-Proteinase That Preferentially Digests the Fibronectin

An epithelial cell line derived from the liver of a normal Buffalo rat (BRL) was transformed by Rous sarcoma virus (RSV). The RSV-transformed cells were separated into five clones (RSV-BRL1 through 5), which were morphologically different. RSV-BRL cells exhibited the following characteristics distinct from those of BRL cells: tumorigenicity, irregular cell arrangement, loose intercellular junction, growth in soft agar (anchorage-independent growth) except for RSV-BRL3 and 5, and loss of cell surface fibronectin. When BRL cells were cultured in the standard medium supplemented with the serum-free conditioned medium of RSV-BRL cells, the amount of the cell surface fibronectin decreased significantly.It was found that RSV-BRL cells secreted a proteinase capable of hydrolyzing the fibronectin, whereas BRL cells secreted hardly any of this proteinase. The fibro-nectin-hydrolyzing proteinase (FNase) could also hydrolyze plasma fibronectin added as an exogenous substrate. The hydrolysis of plasma fibronectin was inhibited by ethylenediamine tetraacetate, but stimulated by p-chloromercuribenzoate and calcium ion. This indicates that FNase is a metallo-enzyme, but not a serine or thiol enzyme. In addition to the proteinase, RSV-BRL cells secreted plasminogen activator and a proteinase inhibitor which inhibited the activity of plasmin but not FNase.