Extended life span and tumorigenicity of nonestablished mouse connective tissue cells transformed by the fos oncogene of FBR-MuSV.

We have analyzed the transforming potential of two fos oncogene products in nonestablished cultures of mouse connective tissue cells: p55fos of FBJ-MuSV and p75gag-fos of FBR-MuSV. Although both proteins induced morphological transformation and colony formation at low cell density in a G418 resistance selection assay, p75gag-fos exhibited more pronounced transforming potential than p55fos. In addition, p75gag-fos-transformed cells overcame crisis with a high probability and were tumorigenic in syngenic mice. These properties of the FBR-MuSV appear to be linked to structural alterations in the p75gag-fos oncogene product. Polyoma virus large T protein complemented the transforming potential of fos, in that it not only increased the probability of establishment of fos-transformed cells but also enhanced fos-induced morphological transformation. Our results suggest that different oncogenes affect morphological transformation, low cell density growth, establishment, and tumorigenicity to various degrees.