Revertants of mouse cells transformed by murine sarcoma virus: I. Characterization of flat and transformed sublines without a rescuable murine sarcoma virus

Abstract

Murine sarcoma virus (MSV)-transformed mouse clonal cell lines produced variants with some properties of nontransformed cells. Such variant cells were epithelioid, contact inhibited, and grew to low density, and their low cloning efficiency in soft agar was similar to that of normal parental 3T3 cells. However, they contained murine leukemia (MuLV) group-specific antigen(s) without demonstrable virus production and reverse transcriptase activity. MSV could no longer be rescued from these flat variant cells by superinfection with MuLV, by cocultivation with normal 3T3 cells or by transspecies rescue into cat cells. An enhancement of sensitivity to MSV and MuLV infection was observed in all flat variant cultures. Flat variant clones spontaneously gave rise to retransformed cells during extended cultivation. Morphology, saturation density, and cloning efficiency in soft agar of cloned spontaneous retransformed cell lines were similar to the original MSV-transformed cells. However, they failed to demonstrate MuLV gs antigen(s), virus production, reverse transcriptase activity and a rescuable MSV genome. The spontaneously retransformed cells were susceptible to MSV and MuLV infection. After treatment with 5-iododeoxyuridine (IUrd), reverse transcriptase activity and virus particles were only rarely induced in flat variant or spontaneously retransformed clones. These particles were not infectious for the original host cells and were not induced in normal 3T3 cells or a majority of the variant clones. Chromosome studies of these variants suggested that the partial or complete loss of expression of transformation in variants might have been associated with an imbalance in the number of chromosomes mediating expression or suppression in these cells.