Comparison of three recombinant murine leukemia viruses carrying the v-src oncogene of avian sarcoma virus: differences in in vitro transformation and in vivo pathogenicity.

We previously described a recombinant Moloney murine leukemia virus (Mo-MuLV) carrying the v-src oncogene, Mo-MuLV(src). Mo-MuLV(src) encodes a gag-src fusion protein, transforms cells in culture, and induces fibrosarcomas in vivo. To compare transforming properties of the gag-src fusion protein to pp60src encoded by Rous sarcoma virus, we constructed a new recombinant virus, Mo-MuLV(+ src). Mo-MuLV(+ src) encodes pp60src in the context of Mo-MuLV. Cells transformed by Mo-MuLV(+ src) were round and formed colonies in soft agar, whereas Mo-MuLV(src)-infected cells were fusiform and did not grow in suspension. Thus, the extent of transformation induced by Mo-MuLV(+ src) was greater than that induced by Mo-MuLV(src). Subcutaneous inoculation of either virus into neonatal NIH Swiss mice resulted in fibrosarcomas at the site of injection. Further studies indicated that tumors induced by Mo-MuLV(+ src) grew rapidly but rarely metastasized. In contrast, tumors induced by Mo-MuLV(src) grew somewhat more slowly but metastasized with a high frequency (60%). These viruses may provide a useful model system for tumor metastasis. Another src-containing virus was also studied, MRSV (constructed by Anderson and Scolnick). MRSV also encodes pp60src but in the context of amphotropic MuLV. When injected intravenously into six-week-old mice, MRSV induced splenomegaly and spleen foci but no solid tumors, as reported previously. In contrast, Mo-MuLV(src)-induced fibrosarcomas mostly in the spleen under the same inoculation protocol. These results suggest that the v-src oncogene was the major pathogenic determinant in neonatal mice for all three src-containing viruses; however, variations in the nature of the transforming protein modulated the behavior of the induced tumors. In adult mice, greater differences in pathogenicity were observed.