Overexpression of normal human p53 in established fibroblasts leads to their tumorigenic conversion

Some, but not all, mouse p53 genes are able to cooperate with an activated ras oncogene to transform primary cells. Overexpression of what is presumed to be wild-type murine p53 is sufficient to confer a tumorigenic phenotype on established cell lines. We have investigated the effect of overexpression of normal human p53 genes on the growth and morphology of both primary and established mouse and rat cells. When plasmids containing functional human p53 genes under the control of strong viral promoter/enhancer elements were transfected into NIH3T3 cells or Rat-1 cells, no gross alterations in cell shape or morphology were observed. When stable NIH3T3 transfectants were established by co-transfection of the p53 plasmids with pSV2neo and subsequent selection in medium containing G418, many of the lines generated exhibited altered growth characteristics. While, again, the cells did not form foci above the monolayer and were not capable of growing in soft agar, they showed a reduced dependence on serum for growth, were able to grow to higher saturation densities, and displayed markedly enhanced tumorigenicity when inoculated into nude mice. The expression of human p53 in the transfectants was assessed by immunoblotting with a monoclonal antibody, PAb1801, which is reactive to human but not mouse p53. There was a clear correlation between the extent of p53 overexpression and acquisition of the tumorigenic phenotype. None of nine human p53 constructs was capable of cooperating with an activated ras oncogene to transform primary cells under conditions where a mouse clone, pLTRp53cG, could do so efficiently. None of the human p53 constructs was capable of rescuing primary rat cells from senescence. Taken together, these data show that overproduction of normal human p53 can confer an enhanced tumorigenic phenotype on established fibroblasts and support the idea that mutational activation may be necessary for p53 to express its full oncogenic potential.