Fibroblast Surface Antigen (SF): the External Glycoprotein Lost in Proteolytic Stimulation and Maligant Transfromation

It was previously shown that the fibroblast surface antigen (SF antigen, SFA) is composed of polypeptides of high molecular weight 210,000 (SF210) and 145,000 (SF145) and that both of these decrease in quantity after transformation of the fibroblasts by Rous sarcoma virus (RSV). The present experiments show that SF210 is a glycoprotein. It is accessible to surface labelling by lactoperoxidase catalyzed iodination. The SF210 molecule is highly susceptible to trypsin on cell surface. Anti-SFA antibodies specifically precipitated the surface labelled polypeptide. The lactoperoxidase iodinated SF210 polypeptide was greatly reduced in cells transformed by RSV. It is concluded from these studies that the large external transformation sensitive (LETS) protein detected by other workers is the same molecule as SF210. Part of the label of surface iodinated fibroblasts did not enter the polyacrylamide gels. This high molecular weight material is also susceptible to trypsin treatment and decreases in quantity after transformation by RSV. The data suggest that it may be antigenically related to SF protein. Treatment of surface of 35S-methionine-labelled cultures with trypsin in concentrations able to initiate proliferation of density-inhibited cells rapidly released SF210 from fibroblast surface. A single high molecular weight polypeptide (mol. wt about 200,000, SF200) was detected in the culture medium. SF210 may thus be a major target molecule of trypsin action. Treatment of cultures with insulin that also stimulated the fibroblasts to initiate proliferation did not result in any detectable alteration in the external glycoprotein SF210. It is concluded that although release of SF210 may be a sufficient trigger to stimulate proliferation in stationary cells, this molecule appears not to be directly involved in initiation of fibroblast proliferation from the G1 (or G0) phase of the cell cycle.