Limitations to the expression of parvoviral nonstructural proteins may determine the extent of sensitization of EJ-ras-transformed rat cells to minute virus of mice.

The FR3T3 and NRK rat cell lines and their human EJ Ha-ras-1 oncogene-transformed derivatives, termed FREJ and NREJ, were compared for their susceptibility to the parvovirus MVMp. For a similar production of p21ras protein, FREJ clones are markedly sensitized to killing by MVMp, whereas the NREJ cells are not. Such a contrasting effect of ras transformation on the sensitivity of cells of different origins to MVMp can be traced back to their respective abilities to support the parvoviral life cycle. The FR3T3 line produces a substantial amount of viral DNA whose expression in the form of the nonstructural protein NS-1 is stimulated in its transformed derivatives. Conversely, NRK cells offer an early block to parvoviral DNA replication and expression that appears to persist in the ras-transformed clones. Thus, at least two intracellular restrictions can protect normal rat cells against MVMp infection, and transformation by ras relieves one of them at the level of parvoviral gene expression. A fair correlation was also found between the degree of sensitivity of the various lines to MVMp-induced killing and their capacity to synthesize the nonstructural viral proteins, suggesting a possible role of parvoviral nonstructural proteins in cytotoxicity.