| Research Abstract |
Immortalization is an essential step for in vitro neoplastic transformation of human cells. This study address the question of whether mutant p53 contributes to the immortalization process of human cells.
The mutant p53 gene (mp53 ; codon 275Arg-His) was introduced into normal human fibroblasts (OUMS-24), and a G418-resistant clone, OUMS-24/p6, was obtained. This clone expressed mp53 and showed an extended life span, but it senesced at 79th population-doubling level (PDL). When these cells were treated with 2 Gy of x-rays or 4-nitroquinoline 1 oxide, they became immortalized. Although the immortalized cells showed chromosome abnormalities, they were not tumorigenic. On the other hand, normal OUMS-24 cells into which mp53 had not been introduced were not immortalized by the same treatment.
Expression of p21/cip1/waf1/sdi1, which is located downstream of p53, was remarkably reduced in the immortalized cell lines, resulting in an increase in cdk2 and cyclin A kinase activity. These findings indicate that the p53-p21 cascade may play some role in the immortalization of human cells. On the other hand, there was no significant different expression of proteins such as Rb, p16, cdk4, cdk6, cyclin A and cyclinD between the normal and immortalized human fibroblasts.
Taken togehter, our results indicate that mutation of the 53 gene alone was not sufficient for immortalization and some unknown genes other than p53 may be involved in the immortalization process of human cells.
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