No abstract

Mutations of the p53 tumor suppressor gene constitute one of the most frequent molecular changes in a wide variety of human cancers, including those in the esophagus. Mice deficient in p53 have recently attracted attention for their potential to identify chemical genotoxins. In this study we investigated the susceptibility of p53 nullizygous (-/-), heterozygous (+/-) and wild-type (+/+) mice to methyl-N-amylnitrosamine (MNAN), which specifically induces esophageal tumors in mice and rats. The p53 (+/-) and (+/+) mice were treated with 5 or 15 p.p.m. MNAN in their drinking water for 8 weeks then maintained without further treatment for an additional 7 or 17 weeks, being killed at experimental weeks 15 or 25. An additional group of p53 (-/-) mice were given 5 p.p.m. MNAN for 8 weeks and killed at week 15. At 15 weeks in the 5 p.p.m. groups, squamous cell carcinomas (SCCs) were observed in 10/12 (83.3%) p53 (-/-) and 1/15 (6.7%) p53 (+/-) mice, but in none of the p53 (+/+) mice. In the animals receiving 15 p.p.m., 2/14 (14.3%) p53 (+/-) and 1/11 (9.1%) p53 (+/+) mice developed SCCs. At 25 weeks, the incidence of SCCs was 7/16 (43.8%) and 8/14 (57.1%) in p53 (+/-) mice and 1/13 (7.7%) and 2/10 (20.0%) in p53 (+/+) mice at 5 and 15 p.p.m., respectively. Of the SCCs examined by PCR-single strand conformation polymorphism analysis, 61% (14/23) from p53 (+/-) and 50% (6/12) from p53 (+/+) mice demonstrated mutations in the p53 gene (exons 5-8). These results indicate the order of susceptibility to MNAN-induced esophageal tumorigenesis to be as follows: nullizygotes (-/-) > heterozygotes (+/-) > wild-type (+/+), and provide strong evidence of involvement of p53 mutations in the development of esophageal SCCs.

Differential effects of partial hepatectomy (PH) and carbon tetrachloride (CCl 4 ) administration on induction of glutathione S-transferase placental form (GST-P)-positive foci were investigated in a model for detection of initiation activity. Firstly, we surveyed cell proliferation kinetics and fluctuation in cytochrome P450 (CYP) mRNA levels by means of relative-quantitative real-time reverse transcriptase-polymerase chain reaction (RT-PCR) and CYP 2E1 apoprotein amount by immunoblotting (experiment I) after PH or CCl 4 administration. Next, to assess the interrelationships among cell proliferation, fluctuation of CYPs after PH or CCl 4 administration and induction of liver cell foci, the non-hepatocarcinogen, 1,2-dimethylhydrazine (DMH) was administered to 7-week-old male F344 rats and initiated populations were selected using the resistant hepatocyte model (experiment II). In experiment I, the values of all CYP isozyme mRNAs after PH or CCl 4 administration were drastically decreased at the 12-h time point. From 72 h, mRNAs for all CYP isozymes began increasing, with complete recovery after 7 days. The CYP 2E1 apoprotein content in the PH group fluctuated weakly, whereas in the CCl 4 group it had decreased rapidly after 12 h and was still low at the 48 h point. In experiment II, induction of GST-P-positive foci was related to cell kinetics in the PH group, with about a 6-h time lag between time for carcinogen administration giving greatest induction of GST-P-positive foci and peaks in bromodeoxyuridine (BrdU) labeling, presumably due to the necessity for bioactivation of DMH. With CCl 4 administration, induction of foci appeared dependent on the recovery of CYP 2E1. In conclusion, PH was able to induce cell proliferation with maintenance of CYP 2E1, therefore being advantageous for induction of liver cell foci in models to detect initiation activity.

These results suggest that p53 might not be a direct target of DMH but complete loss of p53 might elevate susceptibility to DMH-induced colorectal carcinogenesis.