Cooperation of oncogenes in cell transformation and sensitization to killing by the parvovirus minute virus of mice.

The established line of normal Fisher rat fibroblasts (FR3T3) is naturally resistant to the parvovirus minute virus of mice (MVM), and was used as a model system to study the influence of stepwise transformation on the susceptibility of cells to this virus. When transformed with genes encoding the class I nuclear oncoproteins large T antigen of polyomavirus (PyLT) or v-myc, cells retained a normal appearance, but acquired some ability to form colonies in soft agar. On the other hand, the class II transforming oncogenes encoding the middle T antigen of polyomavirus (PyMT) and c-Ha-ras-1 induced both morphological alterations and a high capacity for anchorage-independent growth in transfected cells. The concomitant expression of oncogenes from both classes (PyLT-(+)PyMT; v-myc+c-Ha-ras-1) induced a supertransformed phenotype characterized by the piling-up of cells into poorly adherent foci, even in low density cultures. The progressive transformation of this cellular system was found to coincide with a gradual increase in its susceptibility to MVMp (MVM prototype strain) infection. Compared to parental cells, class I, class II and double transformants proved to be sensitized to killing by MVMp to a low, moderate and large extent, respectively. Thus, oncogenes from different functional classes appeared to cooperate in the responsiveness of cells to parvovirus attack. Interestingly, this cooperation exacerbated both the killing of infected cells and their capacity to produce viral non-structural (NS) proteins, in agreement with the reported cytotoxic activity of NS polypeptides. Therefore, in this system, parameters of the parvovirus life cycle may serve as indications of the overall progression of the transformation process.