Infrequent loss of heterozygosity and mutation of the p53 gene in immortal and transformed mouse embryo fibroblasts

p53 gene mutations in neoplastic transformation of C3H 10T1/2 and severe combined immunodeficiency fibroblasts

The relevance of p53 mutations to the neoplastic malignant transformation of rodent fibroblasts by genotoxic physical and chemical agents is not clear. In the present study, we investigated p53 mutations (in exons 5-8) in non-transformed and neoplastically transformed C3H 10T1/2 and severe combined immunodeficiency (SCID) cells. No p53 mutations were detected in 15 neoplastically transformed (two spontaneous, one 3-methylcholanthrene-induced, seven gamma-ray-induced and five 'hot particle'-induced) and two non-transformed 10T1/2 cells. Wild-type p53 gene was also detected in all non-transformed (immortalized) SCID cell lines analyzed (four lines) whereas all three neoplastically transformed (two spontaneous, one gamma-ray-induced) cell lines displayed missense mutations in the p53 gene. These mutations were all transitions: A > G in codon 123, G > A in codon 152, and C > T in codon 238. We conclude that mutation in the p53 gene appears to be an infrequent event in 10T1/2 cells regardless of the transforming agent, but a frequent event in the neoplastic transformation of immortalized SCID cells. Non-transformed SCID cells are deficient in repair of DNA double-strand breaks, and neoplastically transformed cells are assumed to be deficient as well.

Inactivation of tumor suppressor p53 by mot-2, a hsp70 family member

The mortalin genes, mot-1 and mot-2, are hsp70 family members that were originally cloned from normal and immortal murine cells, respectively. Their proteins differ by only two amino acid residues but exhibit different subcellular localizations, arise from two distinct genes, and have contrasting biological activities. We report here that the two proteins also differ in their interactions with the tumor suppressor protein p53. The pancytosolic mot-1 protein in normal cells did not show colocalization with p53; in contrast, nonpancytosolic mot-2 and p53 overlapped significantly in immortal cells. Transfection of mot-2 but not mot-1 resulted in the repression of p53-mediated transactivation in p53-responsive reporter assays. Inactivation of p53 by mot-2 was supported by the down-regulation of p53-responsive genes p21(WAF-1) and mdm-2 in mot-2-transfected cells only. Furthermore, NIH 3T3 cells transfected with expression plasmid encoding green fluorescent protein-tagged mot-2 but not mot-1 showed an abrogation of nuclear translocation of wild-type p53. These results demonstrate a novel mechanism of p53 inactivation by mot-2 protein.

Induction of invasive squamous cell carcinomas in the forestomach of (C3H x MSM)F1, MSM, and C3H mice by N-methyl-N-nitrosourea and mutational analysis of the H-ras and p53 genes

Genetic analysis of tumors developing in F1 hybrids between genetically separate strains of mice makes it possible to search for loss of heterozygosity (LOH), information on which provides clues to finding tumor-suppressor genes. For this purpose, however, reproducible carcinogenic conditions for the organ of interest need to be first determined. In the present study, a forestomach model of squamous cell carcinomas (SCCs), induced in (C3H x MSM)F1 mice by N-methyl-N-nitrosourea (MNU), was established and mutational changes in the H-ras and p53 genes were examined in tumors. Male (C3H x MSM)F1, MSM and C3H mice were given MNU by i.g. intubation once a week at a dose of 0.03 mg/g body weight for 10 weeks, then kept without further treatment. At experimental weeks 38-46, markedly invasive SCCs were observed in the forestomach at incidences of 9/14 (64.3%), 9/16 (56.3%), and 2/10 (20.0%), respectively. In the three strains of mice, DNA analysis of SCCs by PCR-SSCP analysis followed by direct DNA sequencing revealed low incidences of point mutations in the H-ras (4/20, 20%) and p53 (3/20, 15%) genes. The results demonstrate the usefulness of the present animal experimental protocol for induction of high grade SCC in the forestomach of (C3H x MSM)F1 mice, and suggest the possibility that point mutations in the H-ras or p53 genes may play some role in pathways leading to the development of such lesions.

Establishment of spontaneously immortalized rat type 1 astroglial cell lines: the role of p53 in astroglial carcinogenesis

We established five spontaneously immortalized cell lines using purified rat type 1 astroglia on a rigid transfer schedule. All the cell lines maintained their polygonal shape, regular pavement growth, low saturation density, positive glial fibrillary acidic protein expression, and serum requirements, while none were tumorigenic in nude mice. We then obtained a spontaneously transformed cell line by maintaining the cells for 6 months at a high cell density. Since alterations of the tumor suppressor p53 gene have been reported in the immortalization of some cell lines and in transformation of others, we characterized p53 in immortalized, spontaneously transformed, and 5 Nethyl-N-nitrosourea (ENU)-transformed cell lines. While each of the ENU-induced or the spontaneously transformed cell lines exhibited p53 gene mutations that resulted in amino acid alterations, no alterations in the p53 gene were observed in any of the immortalized cell lines. Thus, alterations of the p53 protein correlate more strongly with transformation than with immortalization of type 1 astroglia. Immortalization may be regulated by gene(s) other than p53. Spontaneously immortalized type 1 astroglial cell lines may provide a new tool to investigate an initial step of astroglial carcinogenesis.

Role of poly(ADP-ribose) polymerase in cell-cycle checkpoint mechanisms following gamma-irradiation

A nuclear poly(ADP-ribose) polymerase (PARP) is activated by gamma-irradiation and consequently synthesizes poly(ADP-ribose) by binding to DNA strand-breaks. This property suggests that PARP is a DNA strand-break-signal generator. Meanwhile, the cell-cycle arrest occurs in G1 and G2 phases following gamma-irradiation. We found that PARP inhibitors including 3-aminobenzamide (3-AB) suppressed G1 arrest and enhanced G2 arrest following gamma-irradiation. These observations suggested that PARP is critical for the induction of G1 arrest and is also involved in the regulation of G2 arrest. Furthermore, the effects of 3-AB on the G1-arrest signal-transduction pathway were also studied. We found that p53 stabilization following gamma-irradiation was not inhibited but the p53-responsive transient increases of WAF1/CIP1/p21 and MDM-2 mRNA were suppressed by 3-AB. Therefore, it is suggested that PARP participates in G1-arrest signal-transduction pathway through the modulation of WAF1/CIP1/p21 and MDM-2 mRNA expression.

Mutation, loss of heterozygosity, and recombination of the p53 gene in mouse forestomach tumors induced by 2-amino-3,4-dimethylimidazo[4,5-f]quinoline

2-Amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ), a food mutagen, induces forestomach tumors in CDF1 mice. We established a polymerase chain reaction (PCR)-single-strand conformation polymorphism (SSCP) analysis system to detect mutations in the mouse p53 gene exons 2-10, which encompass all five regions conserved among species, and a system to examine loss of heterozygosity (LOH) that uses newly identified polymorphisms between BALB/c and DBA mice, the parental strains of CDF1 mice. Four original forestomach tumors (one papilloma, two carcinomas, and one lymph-node metastasis) and four cell lines derived from four independent forestomach tumors were examined with the PCR-SSCP system and by polymorphism analysis. Of the four original tumors, the papilloma had a G-->A transition at the second position of codon 171, and one carcinoma had a G-->T transversion at the second position of codon 113 with loss of the wild-type allele, whereas the other two carcinomas had no detectable mutations. Of the four cell lines, two had a base substitution and LOH, and the other two had double mutations (a base substitution and a deletion). By amplification of the double mutations in a fragment, the two cell lines were shown to have four kinds of alleles, indicating induction of recombination within the p53 gene. Our results show that our PCR-SSCP analysis system is efficient for detecting p53 mutations in mouse genomic DNA and that alteration of the p53 gene plays a significant role in MeIQ-induced mouse forestomach carcinogenesis.

Suppression of G1 arrest and enhancement of G2 arrest by inhibitors of poly(ADP-ribose) polymerase: possible involvement of poly(ADP-ribosyl)ation in cell cycle arrest following gamma-irradiation

Low-dose gamma-irradiation of mouse embryonic fibroblast C3D2F1 3T3-a cells caused G1 arrest along with G2 arrest and inhibition of replicative DNA synthesis. When the cells were cultured in the presence of inhibitors of poly(ADP-ribose) polymerase [EC 2.4.2.30], such as 3-aminobenzamide, benzamide and luminol, G1 arrest of C3D2F1 3T3-a cells was suppressed and enhancement of G2 arrest was observed. In contrast, 3-aminobenzoic acid, a non-inhibitory analog of 3-aminobenzamide, did not suppress G1 arrest following gamma-irradiation. These results suggest that the poly(ADP-ribosyl)ation reaction is critical for the pathway of G1 arrest and is also involved in the pathway of G2 arrest.