Overproduction of p53 antigen makes established cells highly tumorigenic

The p53 cellular tumour antigen, long known to be overproduced in a variety of neoplastically transformed cells1,2, was recently shown to be directly involved in transformation. Thus, p53 can complement activated Ha-ras in transforming secondary rat embryo fibroblasts into grossly altered, tumorigenic cells3,4. Moreover, p53 can also be shown to possess immortalizing activity5. Our previous results indicated, however, that the contribution of p53 to the transformation was not synonymous with immortalization, suggesting that the two activities of the protein are probably separable3. We demonstrate here that this is indeed the case, as overproduction of p53 in an established cell line, while not causing gross morphological changes, endows these cells with an overt tumorigenic potential. Furthermore, the tumorigenic efficiency of such cell lines may be correlated with the extent of p53 overproduction.