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

We introduced the mutant p53 gene (codon 273(Arg-His)) into human fibr oblasts (SUSM-1 cells) previously immortalized with 4-nitroquinoline 1 -oxide (4NQO) and obtained 2 clonal cell lines (SUSM-1/p53-1 and SUSM- 1/p53-6) expressing the mutant p53. Since the genetic background of SU SM-1/p53 is the same as that of SUSM-1 except for the presence of the mutant p53, we expected to obtain more information on the mechanisms o f p53 functions without the influence of other genetic differences by comparing cellular characteristics of both cell lines. SUSM-1/p53 cell s became about twice as sensitive to the cytotoxic effects of X-rays a s their parent SUSM-1 cells. Mutation frequency was determined by the appearance of hypoxanthine guanine phosphoribosyl transferase deficien t (6-thioguanine resistant) cells. As a result, the mutation frequency of SUSM-1/p53 cells was about 5 times that of SUSM-1 cells transfecte d with or without the vector plasmid alone. Furthermore, when the SUSM -1/p53 cells were exposed to X-rays, the mutation frequency increased to about twice that of the non-irradiated SUSM-1/p53 cells. However, S USM-1/p53 cells showed neither anchorage-independent growth in soft ag ar nor tumorigenicity in nude mice. These results indicate that the mu tant p53 gene itself, which generally works in a dominant-negative way on cellular carcinogenesis, is not sufficient for neoplastic transfor mation of immortalized human cells, and that additional genetic change (s) may be necessary for transformation. (C) 1995 Wiley-Liss, Inc.