[Aging and immortalization of human cells].
In vitro models of malignant transformation of human cells may provide considerable insight into the mechanisms of multistep carcinogenesis. It is well established that normal human cells must be immortalized before they are malignantly transformed, but normal human cells are extremely refractory to immortalization, making malignant transformation very difficult. Thus immortalization is a rate-limiting step in the malignant transformation of human cells. In order to immortalize normal human cells with a chemical carcinogen, 4-nitroquinoline 1-oxide, or with 60Co gamma rays, the cells had to be repeatedly treated with these agents. This indicates that the immortalization process involves multiple mutational events and is itself a multi-step process. Then we found that there were mutations in p53 in all our immortalized cell lines. This encouraged us to attempt to immortalize human cells with the mutant p53. We introduced the mutant p53 (codon 273Arg-His) into normal human fibroblasts to learn whether the cells could be immortalized. The p53 extended the life-span of the cells but could not immortalize them. However, when we treated the p53-introduced cells with 4NQO, the cells were immortalized. The 4NQO treatment alone did not make the cells immortal. These results indicate that the p53 gene play a role in immortalization of human cells rather than in their malignant transformation.