Mutations of the p53 tumor suppressor gene constitute one of the most frequent molecular changes in a wide variety of human cancers and mice deficient in p53 have recently attracted attention for their potential to identify chemical genotoxins. In this article, we review the data on the susceptibility of p53 nullizygote (-/-), heterozygote (+/-), and wild type (+/+) mice to various carcinogens. Induction of esophageal and tongue squamous cell carcinomas (SCCs) by methyl-n-amylnitrosamine was shown to be increased in p53 (+/-) mice, in addition to the high sensitivity shown by p53 (-/-) littermates. N,N-dibutylnitrosamine (DBN) treatment also caused more tumor development in p53 (+/-) than wild-type mice, as with N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) in the urinary bladder. In addition, p53 (+/-) heterozygotes proved more sensitive than wild type littermates to the induction of stromal cell tumors like hemangiomas/hemangiosarcomas by N-bis(2-hydroxypropyl)nitrosamine (BHP) or lymphomas and fibrosarcomas with other carcinogens. Analysis of exons 5-8 of the p53 gene demonstrated mutations in approximately one half of the lesions. With N-methyl-N-nitrosourea, preneoplastic lesions of the stomach, pepsinogen altered pyloric glands (PAPG), and a gastric adenocarcinoma, were found after only 15 weeks in p53 (-/-) mice, although there was no significant difference in the incidence of gastric tumors between p53 (+/+) and (+/-) mice in the longer-term. Regarding colon carcinogenicity, adenocarcinomas were observed limited to 1, 2-dimethylhydrazine treated p53 (-/-) mice in the short term, but again, no significant difference was evident between the p53 (+/+) and (+/-) cases at the end of the study. Furthermore, diethylnitrosamine or aminophenylnorharman treated p53 (+/-) mice did not demonstrate elevated susceptibility to tumors in the liver. With BHP, which induces tumors in multiple organs, p53 (+/-) mice were not more statistically sensitive with regard to lung tumor development than p53 (+/+). Of the malignant tumors examined in p53 (+/+) and (+/-) mice, as many as 10% demonstrated mutations in the p53 gene. These results suggest that p53 may not be a direct target for mouse adenomas/adenocarcinomas, but rather plays an important role as a gatekeeper in their genesis. In contrast p53 itself is frequently mutated in squamous, urothelial, or stromal tumors with a clear order of susceptibility: p53 (-/-) > p53 (+/-) > p53 (+/+) mice. p53 (-/-) mice are versatile animals for carcinogenicity testing, despite their disadvantage of a high background of spontaneous tumor development, and tissue dependence must be taken into account when exposing p53 (+/-) mice to chemical carcinogens.

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Bromodeoxyuridine (BrdU) labeling of S-phase cells, which are DNA- synthesizing cells, was demonstrated in 30 canine and 6 feline neoplasms. Simultaneously, the optimization of the BrdU-labeling method, including tissue fixation, administration dose of BrdU, and the immunohistochemical procedure, was examined. BrdU-incorporated cells in a neoplasm of a dog administered 3 mg/kg of BrdU could be detected. Although BrdU-positive cells could be visualized in 70% ethanol fixed samples, we could satisfactorily demonstrate BrdU-immunoreactivities in samples fixed in 10% buffered formalin and subjected to enzymatic treatment and acid hydrolysis. The BrdU-labeling index (LI) of malignant neoplasms tended to be higher than that of benign neoplasms. The mean BrdU LI of malignant neoplasms was significantly higher than that of benign neoplasms (P<0.05). The present study demonstrated that the BrdU-labeling method may be useful for detecting S-phase cells in canine and feline tissues.

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Flutamide, p,p'-dichlorodiphenyldichloroethylene, vinclozolin, diethylstilbestrol, ethynylestradiol and tamoxifen were administered by gavage to pregnant Sprague-Dawley rats on gestational days 14-17 or 18-21, and to male offspring on postnatal days 1-5, 17-21 or 35-39. The influence on the sexual maturation was assessed by preputial separation. Cleft phallus with hypospadias was induced by prenatal exposure to 10 mg/kg flutamide on gestational days 14-17 and 18-21, or administration of 100 mg/kg vinclozolin on gestational days 14-17 to the dams. The day of preputial separation in these offspring could not be determined, because complete separation did not occur. Prenatal exposure of males to other chemicals did not affect the preputial separation. Postnatal exposure of 10 and 30 mg/kg flutamide and 30 mg/kg vinclozolin led to delays of preputial separation. A marked delay was observed in males exposed to 100 μg/kg of ethynylestradiol or 3 mg/kg of tamoxifen on postnatal days 1-5. Diethylstilbestrol, 300 μg/kg, administration on postnatal days 1-5 and 35-39 caused a delay in preputial separation. These results indicate that observing preputial separation is useful for evaluating anti-androgen treatment in the prepubertal period, and estrogen-related chemical treatment from the neonatal period.

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In our previous study in which diets containing 0, 1.5 or 3% Kojic acid (KA) were administered to heterozygous p53-deficient mice of the CBA strain [p53 (+/-) mice] and their wild-type littermates [p53 (+/+) mice] for 26 weeks, the incidences of hepatocellular adenomas as well as altered hepatocellular foci were increased in p53 (+/-) and p53 (+/+) mice of the KA treated groups without initiation, as compared to those in the untreated control mice. In order to confirm the reproducibility of the induction of hepatocellular proliferative lesions in p53 (+/+) mice given KA, male CBA mice were given dietary administration of 0, 0.5, 1 or 2% KA for 26 weeks. Body weight gain in the 2% KA group was depressed after week 20 as compared to the corresponding control group. Significant increases of absolute and relative liver weights were observed in the groups treated with 1% and 2% KA. The incidences of hepatocellular adenomas in the control and 0.5, 1 and 2% KA groups were 5, 17, 10 and 21%, respectively, and those of hepatocellular foci in these groups were 15, 39, 45 and 47%, respectively, the difference between the control and 2% KA groups being statistically significant. The mean values for multiplicity of hepatocellular adenomas in the control and 0.5, 1 and 2% KA groups were 0.05, 0.2, 0.2 and 0.2, respectively, and those for hepatocellular foci were 0.2, 0.6, 0.8 and 0.6, respectively. The results of the present study suggest that KA has a hepatocarcinogenic potential in CBA mice.

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The possibility of establishing a diabetes and renal tumor induction model in Göttingen minipigs by administration of streptozotocin (STZ) was investigated in a 60-month observation. Nine 5-week-old male Göttingen minipigs received single intravenous doses of STZ (300 mg/kg; n = 6), or saline (n = 3). The results of blood glucose levels and glucose tolerance tests showed that STZ-administered animals were in a chronic diabetic state. In the animals necropsied at 60 months after STZ administration, nodules were observed in the kidneys in 2 out of 4 animals. The nodules were histopathologically diagnosed as renal cell carcinoma in one case and as renal cell adenoma with multiplicity in the other. The high prevalence of rare tumors, coincidence of benign and malignant tumors of the same origin, occurrence of tumors with a multi-centric nature, and consistency of tumor character and target organ with other STZ-treated species led us to the conclusion that not only a stable diabetes animal model but also a chemical-induced tumor model had been successfully developed with a single administration of STZ to Göttingen minipigs.

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