Alterations of the p53 tumor-suppressor gene in transformed mouse liver cells.

Mutational inactivation of p53, a potential tumor-suppressor gene, has been found in many tumors of humans as well as rodents. The p53 status in normal and transformed mouse liver cell lines has, however, not been investigated. We examined possible point mutations and compared mRNA and protein expression of the p53 gene in normal vs. transformed mouse liver cells. The transformed cells studied included lines spontaneously transformed by sub-culture, virally transformed by simian virus 40 (SV40), and chemically transformed by N-methyl-N-nitro-N-nitrosoguanidine (MNNG) or methylcholanthrene epoxide (MC). A heterozygous G-->A point mutation at codon 241, position 1, of p53 was detected in MNNG-transformed cells after screening of 5 evolutionarily conserved regions where mutation hot-spots are clustered. The mutation causes a gly-->arg substitution. No mutations were found in normal or other transformed cells. The steady-state levels of p53 mRNA were decreased in chemically transformed (both MNNG- and MC-transformed) cells. Elevated levels of p53 protein were found in spontaneously transformed and SV40-transformed cells, an observation that may reflect a longer half-life of the protein, as has been shown in other transformed lines. The low level of the p53 protein in MC-transformed cells may result from transcriptional depression of the p53 gene. We conclude from these data that abnormal p53 status, such as point mutation or altered expression, may play a role during the malignant transformation of mouse liver cells.