Transformation-sensitive Protein with Molecular Weight of 45,000 Secreted by Mouse Fibroblasts

The [³⁵S]methionine-labeled proteins released in the medium conditioned by normal and transformed mouse fibroblasts have been analyzed by sodium dodecyl sulfate-polyacrylamide electrophoresis. Three major proteins, fibronectin, procollagens, and a protein with a molecular weight of 45,000 (45K protein) have been identified. The 45K protein, which has not yet been described, accounts for about 30% of the proteins released by control 3T3 fibroblasts or mouse embryo cultures. Quantitation of the radioactivity incorporated by the 45K protein indicated a 10- to 15-fold decrease in 3T3 fibroblasts transformed by Kirsten, Abelson, or Rous sarcoma viruses. The amounts of fibronectin and procollagens released in the medium by transformed cells were also reduced by factors of 3- and 5-fold, respectively. Pulse chase experiments have shown that the decreased level of the 45K protein in the medium of transformed cells cannot be explained by a reduced rate of secretion or by extracellular proteolytic degradation. It is not known, however, whether decreased synthesis is responsible for this alteration. The “tumor promoter” phobol myristate acetate, which is known to induce many of the alterations associated with neoplastic cells, also induced 3T3 fibroblasts to release the 45K protein in amounts comparable to that of transformed cells. Thus, this protein represents a new molecular marker of oncoviral transformation.