Temperature-sensitive cellular mutant for expression of mRNA from murine retrovirus

Simple system for isolation of cellular and viral mutants for transformation by retrovirus

To investigate the cellular mechanism of transformation by retroviruses, we established a system for isolation of cellular and viral mutants for transformation of a rat cell line. Mutagenized untransformed cells of this line were infected with recombinant murine retrovirus containing the src gene of Rous sarcoma virus and the selective marker gene, neo. After reaching confluence, cells transformed by the src gene tend to overgrow and die. Utilizing this property of src transformed rat cells and the selective marker gene, we could easily select untransformed cell clones containing the retrovirus genome. Expression of the src gene product in the flat clones selected was examined by in vitro assay of src kinase activity. To determine whether the mutations of these flat clones were viral or cellular, the susceptibilities of the clones to transformation were examined after superinfection with the wild-type virus and also characterized the retroviruses recovered from these clones. With this system, two novel clones were isolated. One had a defect in viral information affecting the transformed phenotype, but still retained src kinase activity like fully transformed cells. The other showed low src kinase activity but retained wild-type transforming virus, suggesting that a cellular gene involved in viral gene expression was mutated.

Novel non-tumorigenic cell variants showing potentially different susceptibility to v-src-induced metastasis

Two non-tumorigenic variant cells were isolated from UV-irradiated Balb/c 3T3 cells on the basis of their different responsiveness in phorbol ester-induced morphological change (rounding formation). They showed marked differences of lung metastatic potentials after intravenous injections of their v-src transformants into nude mice; phorbol ester-resistant variant TR4 cells transformed by v-src were hypermetastatic, whereas v-src transformants of phorbol ester-sensitive variant TR5 cells were not metastatic at all. These different metastatic responses were not observed in v-K-ras-induced transformants of the variants. These non-tumorigenic variant cells may pre-acquire the genetic alteration of certain src-specific and metastasis-associated factors. This system may be useful for genetic analysis of the induction of metastasis.

A cell mutant that exhibits temperature-dependent sensitivity to transformation by various oncogenes

We previously isolated a Fisher rat fibroblast mutant, B812, that has the unique property of temperature-dependent transformation by various oncogenic retroviruses. At the permissive temperature (35 degrees C), this mutant was sensitive to oncogenic transformation and formed foci on a dish at the same frequency as did the parental fibroblast cell line. When Kirsten murine sarcoma virus (Ki-MSV) was applied to the cells, the frequency of focus formation decreased more than 25-fold at the nonpermissive temperature (39 degrees C), whereas the cells expressed nearly the same level of the ras transcript as well as the ras protein. The temperature-restricted focus formation was fully reversible and was completely suppressed upon fusion with the wild-type parent cell. In addition to ras, the mos, fos, src, and erbB-2 oncogenes transformed this mutant with the same temperature dependence as described above; polyomavirus middle T antigen, adenovirus type 12, and human papillomavirus 16-E67 also transformed, but without temperature dependence. These results suggest that ras, fos, mos, src, and erbB-2 use a common cellular pathway for transforming cells.

A cell mutant that exhibits temperature-dependent sensitivity to transformation by various oncogenes

We previously isolated a Fisher rat fibroblast mutant, B812, that has the unique property of temperature-dependent transformation by various oncogenic retroviruses. At the permissive temperature (35 degrees C), this mutant was sensitive to oncogenic transformation and formed foci on a dish at the same frequency as did the parental fibroblast cell line. When Kirsten murine sarcoma virus (Ki-MSV) was applied to the cells, the frequency of focus formation decreased more than 25-fold at the nonpermissive temperature (39 degrees C), whereas the cells expressed nearly the same level of the ras transcript as well as the ras protein. The temperature-restricted focus formation was fully reversible and was completely suppressed upon fusion with the wild-type parent cell. In addition to ras, the mos, fos, src, and erbB-2 oncogenes transformed this mutant with the same temperature dependence as described above; polyomavirus middle T antigen, adenovirus type 12, and human papillomavirus 16-E67 also transformed, but without temperature dependence. These results suggest that ras, fos, mos, src, and erbB-2 use a common cellular pathway for transforming cells.

A rat mutant cell clone showing temperature-dependent transformed phenotypes with functional expression of the src gene product

The cellular mutant B814 isolated from a Fischer rat cell line shows temperature-sensitivity of focus formation on infection with Moloney murine sarcoma virus (Mo-MSV) and Rous sarcoma virus (RSV). An RSV-transformed clone (S814-2) isolated from B814 cells shows temperature-sensitive transformed phenotypes for morphology, growth in soft agar, and glucose uptake. The expression, phosphorylation, and tyrosine kinase activity of pp60v-src in S814-2 were not affected at the nonpermissive temperature, and virus rescued from this clone had wild-type transforming ability, suggesting that a cellular factor altered in S814-2 is responsible for the cellular steps of transformation after the function of pp60v-src. In addition, the cellular 36K protein, a possible candidate as a target of pp60v-src, was phosphorylated at the nonpermissive temperature in S814-2, indicating that phosphorylation of the 36K protein is not correlated with transformed phenotypes.