Endogenous oncornaviruses in chemically induced transformation: I. Transformation independent of virus production

Abstract

The C3H/10T1/2 cell line, established from mouse embryo fibroblasts, is highly sensitive to postconfluence inhibition of division and is susceptible to malignant transformation by chemical carcinogens in culture. Normal, chemically transformed and spontaneously transformed clones of C3H/10T1/2 cells were examined for the production of infectious murine leukemia virus (MuLV) and MuLV and mouse mammary tumor virus (MTV) antigens. Criteria for the expression of oncornaviruses included the production of: (1) MuLV and MTV gs-antigens, (2) MuLV G L and G T cell surface antigens, (3) DNA polymerase-containing particles, and (4) infectious virions. Normal C3H/10T1/2 cells were free of oncornaviruses by all four of these criteria. Transformation of these cells to malignancy by chemical carcinogens did not result in an increased production of MuLV or MTV products; hence, these cells demonstrated the same virus-free phenotype as the parental cell line. Attempts to rescue transforming viral information from transformed C3H cell lines by superinfection with leukemia viruses were unsuccessful. Repressed endogenous MuLV (but not MTV) could be induced from all cell lines by treatment with 5-iododeoxyuridine (IUdR). Induction of MuLV in normal cells occurred in three phases: I, an initial transient production of low-titered virus; II, a 3–4-week interval where virus production fell to a lower level and then gradually increased. This was accompanied by the recruitment of virus-free cells in the culture to produce MuLV gs-antigens and DNA polymerase-containing particles; and III, the constitutive production of high-titered virus. Transformed cells were more sensitive to treatment with IUdR than normal C3H/10T1/2 cells. These cells underwent a more rapid induction of MuLV, contained a larger fraction of cells that expressed MuLV functions, continued the production of MuLV throughout the second phase of induction, and had an earlier onset of the third phase of high-titered MuLV production. Oncornaviruses induced from IUdR-treated cells showed diversity in their host range and in their ability to produce XC plaques; none of these viruses, however, were capable of transforming normal C3H/10T1/2 fibroblasts in culture.