A mutant p53 protein is required for maintenance of the transformed phenotype in cells transformed with p53 plus ras cDNAs.

Mutant p53 and activated ras cDNA clones cooperate to fully transform primary rat embryo fibroblasts in cell culture, whereas neither cDNA alone results in the full transformation of these cells. The mutant p53 protein may be required to initiate the transformation event with ras. Alternatively, mutant p53 gene expression may be required to maintain the properties of the transformed phenotype. To distinguish between these possibilities, primary rat embryo fibroblasts were transformed with mutant p53 plus ras cDNAs, where the expression of the p53 gene was regulated by an isopropyl beta-D-thiogalactoside-responsive promoter. When expression of the mutant p53 cDNA was inhibited and no detectable exogenous p53 protein was produced, both the growth rate and the morphology of the cells reverted to a normal phenotype. These results demonstrate that a mutant p53 protein is required for the maintenance of the transformed phenotype in cells transformed with p53 plus ras cDNAs.