Flat revertants of EJ human bladder carcinoma cells show two different mechanisms of reversion

To investigate the way in which ras proteins cause transformation, we have isolated revertants from human tumour cell lines which contain transforming ras genes. Two types of revertant have been isolated from the human fibrosarcoma cell line, HT1080. One class has normal and mutant alleles in a ratio of 2:1, compared to 1:1 in the parental cells, showing that reversion can be a dosage phenomenon. The other class has lost the transforming allele. All the HT1080 revertants isolated can be re-transformed by transforming ras proteins. To test whether reversion is due to a change in the relative amounts of normal and mutant proteins, or to a reduction in the absolute amount of the transforming protein, mixtures of the purified proteins were microinjected into 208F (Rat-1) cells, chosen because they are less sensitive to transformation by p21ras. Normal H-ras p21 was unable to suppress the transforming effects of the mutant ras protein when co-injected at up to ninefold excess. Revertants of EJ human bladder carcinoma cells were of two types: one was sensitive to re-transformation by oncogenically activated ras proteins, the other was not. The EJ revertants that are resistant to re-transformation fall into two classes, since hybrids of one revertant with the parental EJ cells are non-transformed, whereas hybrids of another revertant with the parental cells are transformed.