Characteristics of Tumors Induced in Mammals, Especially Rodents, by Viruses of the Avian Leukosis Sarcoma Group

Construction and isolation of a transforming murine retrovirus containing the src gene of Rous sarcoma virus

Recombinant murine retroviruses containing the src gene of the avian retrovirus Rous sarcoma virus were isolated. Such viruses were isolated from cells after transfection with DNAs in which the src gene was inserted into the genome of the amphotropic murine retrovirus 4070A. The isolated viruses had functional gag and pol genes, but they were all env defective since the src gene was inserted in the middle of the env gene coding region. Infectious transforming virus could be isolated only from cells transfected with DNA constructions in which the src gene was in the same polarity as that of a long terminal repeat of the amphotropic viral genome. These recombinant viruses encoded a pp60src protein with a molecular weight similar to that of the Schmidt-Ruppin strain of Rous sarcoma virus. In addition, the src protein(s) of these recombinant viruses was as active as protein kinases in the immune complex protein kinase assay. Intravenous injection of helper-independent Moloney and Friend murine leukemia virus pseudotypes of the src recombinant viruses into 6-week-old NIH Swiss mice resulted in the appearance of splenic foci within 2 weeks, splenomegaly and, later after infection (8 to 10 weeks), anemia. Infectious transforming virus could be recovered from the spleens of diseased animals. Such viruses encoded pp60src but not p21ras or mink cell focus-forming virus-related glycoproteins.

Changes in polypeptide pattern in ASV-transformed rat cells are correlated with the degree of morphological transformation

Normal rat kidney cells (NRK) have been transformed with an avian sarcoma virus (ASV) mutant that induces a temperature-sensitive transformed phenotype. When compared in vitro different ASV-transformed clones showed different degrees of morphological transformation at the permissive temperature, while they all reverted to normal at 39 degrees C. Comparisons of the polypeptide patterns of the different cells showed two things. First, a group of about 30 polypeptides, amounting to 5% of all the polypeptides analyzed in normal cells, were affected in all ASV-transformed clones and also in two papovavirus-transformed clones and also in two papovavirus-transformed NRK clones. Second, the patterns were found to differ between the clones in that varying numbers of polypeptides were found to be affected depending on the clone; the number of polypeptide changes showed a good correlation with the degree of morphological transformation. At the nonpermissive temperature the polypeptide patterns of ts ASV-transformed cells were indistinguishable from that of the untransformed cells, which is in agreement with the morphological data.

Immunofluorescence on avian sarcoma virus-transformed cells: localization of the src gene product

The localization of the avian sarcoma virus src gene product (termed p60src) was examined by indirect immunofluorescence in cells transformed by the Schmidt-Ruppin strain of Rous sarcoma virus, subgroup D (SR-RSV-D). Antiserum to p60src was obtained from rabbits bearing SR-RSV-D-induced tumors, and immunofluorescence was performed on chicken embryo fibroblasts (CEF) transformed with SR-RSV-D, as well as normal rat kidney (NRK) cells transformed by the same virus (termed SR-RK cells). Both acetone and formaldehyde fixation were used for the immunofluorescence tests. The specificity of the anti-tumor serum was first demonstrated in both cell systems by gel electrophoresis of immunoprecipitates prepared from 35S--methionine-labeled cells. Anti-tumor serum precipitated p60src from SR-RSV-D-transformed CEF but not from CEF infected with a transformation-defective mutant of SR-RSV-D. All viral structural proteins and precursors contained in these immunoprecipitates could be eliminated by competition with unlabeled virus. Similar experiments on SR-RK cells indicated that no viral proteins other than p60src were expressed in these cells, and this observation was supported by immunofluorescence tests using antiserum to whole virus. For immunofluorescence localization of p60src, reactions with viral structural proteins were blocked with unlabeled virus. This presaturation step, obligatory for p60src detection in the SR-RSV-D-transformed CEF, was unnecessary when antitumor serum was tested on SR-RK cells, since p60src was the only viral protein detectable in these cells. With acetone-fixed cells, p60src-specific immunofluorescence revealed a characteristic fluorescence pattern which was similar in both cell systems. The principal pattern was diffuse and situated in the cytoplasm. A clear nuclear fluorescence was never observed. Immunofluorescence on formaldehyde-fixed cells also indicated the cytoplasmic location of p60src and revealed a specific subcytoplasmic concentration of the fluorescence. With both fixation methods, an additional fluorescence pattern was seen between cells in contact, and was also found in both SR-RK cells and SR-RSV-D-transformed CEF. Immunofluorescence on viable cells suggested that p60src was not on the surface of these transformed cells. The fluorescence patterns were specific for avian sarcoma virus-transformed cells and were not found in uninfected cells, cells infected with a transformation-defective mutant of SR-RSV-D or cells transformed by an antigenically unrelated murine sarcoma virus. Furthermore, anti-tumor serum did not contain antibodies to proteins of the microtubules or intermediate filaments.

Properties of mammalian cells transformed by temperature-sensitive mutants of avian sarcoma virus

Fibroblasts from European field vole (Microtus agrestis) and from normal rat kidney (NRK) have been infected by avian sarcoma virus mutants which are temperature-sensitive for the maintenance of transformation. These cells are transformed at 33 degrees C, but show normal cell characteristics in morphology, colony formation in agar, saturation density, sugar uptake and membrane proteins at 39 degrees C and 40 degrees C, the nonpermissive temperatures. Ts mutant virus was rescued from most of the ts transformed cell lines. NRK cells infected by avian sarcoma virus ts mutants and kept at the nonpermissive temperature can be transformed by wild-type avian sarcoma virus. The susceptibility of the temperature-sensitive NRK lines to this transformation is higher than the susceptibility of uninfected NRK at either permissive or nonpermissive temperature.

Expression of viral proteins in mammalian cells transformed by avian sarcoma viruses

The expression of viral proteins in nine lines of hamster and rat cells transformed by avian sarcoma viruses (ASV) was studied by indirect immunofluorescence with monospecific antisera to purified gp85 and p27 of AMV-B and a polyvalent antiserum to all the p proteins of this same virus. The lines of ASV-transformed cells were either low virus producers (VP) or inducible or non-inducible non producers (NP). Cytoplasmic expression of p proteins was observed in all the cell lines except the least inducible NP cell line, and cytoplasmic expression of gp85 in all the cell lines. The degree of expression varied widely with the lines and was not related to the class of permissiveness or inducibility. However, in the inducible NP class, the expression of p proteins and gp85 was higher in the most inducible cell lines. The data also suggest that the expression of the p proteins must be uncoordinate in at least some cell lines and must also be uncoordinate with the expression of gp85. In the VP cell lines and the most inducible NP lines, g85 and some p proteins other than p27 were also expressed on the cell membrane. The membrane expression of gp85 and the p proteins which were expressed appeared to be coordinate and to parallel the degree of cytoplasmic expression. In contrast, no, or a negligible expression of viral proteins was observed on the membrane of the least inducible and the non-inducible cell lines. These results suggest that there may exist translational and/or post-translational controls of the expression of viral proteins in the ASV-transformed mammalian cells and that the permissiveness and the inducibility of the cells may depend on the insertion of viral proteins in the cell membrane. The failure of p27 to insert in the cell membrane could account for the low permissiveness or the non-permissiveness of the cells.