Transfected mutant p53 gene increases X‐ray‐induced cell killing and mutation in human fibroblasts immortalized with 4‐nitroquinoline 1‐oxide but does not induce neoplastic transformation of the cells

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Abstract
We introduced the mutant p53 gene (codon 273Arg‐His) into human fibroblasts (SUSM‐I cells) previously immortalized with 4‐nitroquinoline I‐oxide (4NQO) and obtained 2 clonal cell lines (SUSM‐I/p53‐I and SUSM‐I /p53‐6) expressing the mutant p53. Since the genetic background of SUSM‐I /p53 is the same as that of SUSM‐I except for the presence of the mutant p53, we expected to obtain more information on the mechanisms of p53 functions without the influence of other genetic differences by comparing cellular characteristics of both cell lines. SUSM‐I /p53 cells became about twice as sensitive to the cytotoxic effects of X‐rays as their parent SUSM‐I cells. Mutation frequency was determined by the appearance of hypoxanthine guanine phosphoribosyl transferase deficient (6‐thioguanine resistant) cells. As a result, the mutation frequency of SUSM‐I /p53 cells was about 5 times that of SUSM‐I cells transfected with or without the vector plasmid alone. Furthermore, when the SUSM‐I/p53 cells were exposed to X‐rays, the mutation frequency increased to about twice that of the non‐irradiated SUSM‐I /p53 cells. However, SUSM‐I /p53 cells showed neither anchorage‐independent growth in soft agar nor tumorigenicity in nude mice. These results indicate that the mutant p53 gene itself, which generally works in a dominant‐negative way on cellular carcinogenesis, is not sufficient for neoplastic transformation of immortalized human cells, and that additional genetic change(s) may be necessary for transformation. © 1995 Wiley‐Liss, Inc.