The liver responses to repeated doses of CCl₄ (0.5 mg/kg b.w., twice weekly) were compared between Wistar and Mini rats, in which the expression of growth hormone (GH) gene is suppressed by the presence of an antisense gene, at 2, 4, and 6 weeks after treatment (WAT). Fibrosis started earlier and its degree was severer in Mini rats than in Wistar rats, and hepatocyte damage was also severer in Mini rats than in Wistar rats. This corresponded well with the changes in serum AST and ALP levels. Oval cell proliferation in the fibrous septa of the liver of CCl₄-treated Mini rats may also have some relations to such increase in ALP activity and fibrogenesis in Mini rats. The increase in the level of TGF-β1 mRNA was more prominent in Mini rats than in Wistar rats at 4 and 6 WAT, and this corresponded to the strain difference in the degree of liver fibrosis. The present results indicate that the responsibility of the liver to repeated doses of CCl₄ was different between Mini rats and Wistar rats. Mini rats seem to be useful as a new tool in the investigation of regulatory mechanisms of GH on liver injury and regeneration.

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Histological features and genetic changes induced by X-rays in intestinal metaplasia (IM) in rats were assessed by histochemistry and immunohistochemistry. A time course study and a PCR-SSCP analysis were performed. The IMs in rats were classified into two major types according to the cells forming the metaplastic glands. The first (the GI type) had both gastric and intestinal type cells forming the metaplastic glands. The second (the I-sol type) had solely intestinal cells forming the metaplastic glands. This characterization is similar to that used to define human IMs. The occurrence of IMs of the I-sol and GI types in rats gradually increased with time after X-ray irradiation. The number of IMs of the GI type was relatively high at 2 and 4 weeks after X-ray irradiation, and was low thereafter. On the other hand, the number of IMs of the I-sol type was extremely low at 2 weeks after treatment, then increased with time, and reached a maximum at 77 weeks after treatment. In the PCR-SSCP analysis, there were no alterations of the H-ras, K-ras, and p53 genes in the IM glands of rats treated with X-ray irradiation 8 weeks earlier. These observations suggest that the phenotypic change from IMs of the GI type to the I-sol type occurred without ras and p53 gene alterations.

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The effect of allyl alcohol (AA) administration on the induction of glutathione S-transferase placental form (GST-P) positive foci in medium term liver bioassay was investigated for the detection of initiation activities. In experiment I, the cell kinetics of rat liver after AA administration (0.1 ml/kg, i.g.) was analyzed by the 5-bromo-2'-deoxyuridine (BrdU) labeling method. Cytochrome P450 (CYP) 2E1 protein, which is involved in the bioactivation of 1,2-dimethylhydrazine (DMH), was quantificated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and the immunohistochemical localization of CYP2E1 was also examined. In experiment II, the induction of GST-P positive foci by DMH was evaluated in a modified in vivo five-weeks initiation assay model using AA treatment to induce cell proliferation. AA induced periportal necrosis and led to regenerative proliferation. High BrdU labeling indices were observed from 24 h to 48 h (5-6%). CYP2E1 protein contents decreased transiently to approximately 40% of control after AA administration, but subsequently recovered to the control level. Immunohistochemically, CYP2E1 was localized in the centrilobular area each time, and the staining pattern was constant. The numbers and areas of GST-P positive foci were highly induced in the animals given DMH at 12 h after AA administration, compared to before the high BrdU labeling term. There was a correlation between the kinetics of cell proliferation and the induction of GST-P positive foci. Although CYP2E1 decreased after AA treatment, it is likely that the enzyme remained enough to metabolize DMH. These results suggest that AA-induced cell proliferation is an effective proliferative stimulus in medium-term initiation assay.

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The relationship between endocrine imbalance and vaginal epithelial features in rats was examined in aging Donryu and F344 rats, pregnant and lactated Donryu rats, and antipsychotic agent-treated Wistar rats. In addition, the influence of sex steroid hormones and prolactin secretion due to haloperidol administration and pituitary implantation was also investigated in ovariectomized Donryu females. Vaginal cornification and mucification were found to be linked with relatively high 17 β-estradiol (E₂) levels and the relatively high progesterone (P) value under higher E₂ concentration, respectively. Under ovariectomized conditions, the degree and incidence of vaginal mucification were emphasized by P and E₂, being dose-dependency of E₂. Mucification of the vaginal epithelium by antipsychotic agents might be induced by high P value through the effect of corpus stimulation by prolactin. These results indicated that the vaginal epithelium of rats is very sensitive to hormonal imbalance, and its morphological features are very useful for the assessment of individual hormone milieu.

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The tumorigenic potential of clofibrate, a peroxisome proliferator, was evaluated in a 26-week bioassay using CB6F1-TgHras2 (rasH2) mice carrying a human prototype c-Ha-ras gene. Clofibrate was administered to rasH2 mice orally by gavage at doses of 0 (vehicle control), 125, 250 or 500 mg/kg/day for 26 weeks. Single i.p. injection of 75 mg/kg methyl nitrosourea (MNU) was given to additional rasH2 group as positive control. CB6F1 non-Tg-rasH2 (non-Tg) mice were given either vehicle or 500 mg/kg of clofibrate. In rasH2 mice receiving clofibrate, hepatocellular adenoma was observed in males; 0 (1/15), 125 (0/15), 250 (3/15), and 500 mg/kg (3/15), but not in females. Although the trend test was positive (p<0.01), there was no dose-response and no statistical significance for hepatocellular adenoma. There was no hepatocellular adenoma in non-Tg mice. Increased incidence of neoplastic lesions, e.g. thymic lymphomas, forestomach/skin tumors were found in MNU treated mice as expected. In conclusion, the tumorigenic potential of clofibrate was equivocal in this model, as there was a weak response in the male 250 and 500 mg/kg dose groups but no dose-response. These results indicated further validation of this model as an alternative to the 2-year rodent carcinogenicity bioassay for human carcinogenic safety assessment will be needed.

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Fenoldopam, a dopaminergic (DA₁) agonist, has been known to induce arteritis in the splanchnic arteries in rats by intravenous infusion. The arteritis was characterized by medial necrosis, hemorrhage, and endothelial hyperplasia in the acute phase and subsequent inflammatory responses in the adventitia. In the present study, the hypertrophic endothelial cells were examined immunohistochemically and electronmicroscopically. Immunohistochemical examination revealed marked positive reactions of von Wilbrand factor (vWF) and factor VIII in the endothelial cells. Electronmicroscopically, the number of rough endoplasmic reticulum (rER) and the size of the nucleolus were increased. These results suggest that endothelial hypertrophy in the acute phase of fenoldopam-induced arteritis is associated with increased protein synthesis of vWF and factor VIII in response to medial necrosis and hemorrhage.

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Cancer evolves through a sequential process from normal cells in many tissues of humans and animals. The natural history of tumor development can be seen histologically and by biochemical and molecular changes. There are two common basic pathways for the formation of malignant epithelial tumors; through preneoplastic foci and benign tumors (carcinoma developing in an adenoma) in parenchymal tissue or progression from intraepithelial neoplasia (IN) (atypical hyperplasia, noninvasive carcinoma, carcinoma in situ), a lesion in flat or lining epithelium. In epithelial-lining tissues of humans and rodents (e.g. cervix, mammary gland, prostate, skin), these lesions have been described as IN. In solid epithelial organs (liver, kidney, endocrine tissues) focal hyperplasia leads to adenomas. Adenomas develop foci of carcinoma, a process that is more common in rodents than in humans. These precancerous lesions in many rodent tissues often have multiple biochemical and molecular lesions which can be similar or different from those found in malignant tumors. The rodent molecular lesions include mutations in oncogenes (K-ras, H-ras) and tumor suppressor genes (p53, β-catenin, apc) or loss of heterozygosity (LOH) in tumor suppressor genes of mutant mouse models. This manuscript will review specific sequential morphologic and molecular lesions in the histopathogenesis of cancer in several rodent tissues. The significance of molecular lesions for diagnosis of rodent lesions will be discussed.

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As precancerous lesions for large bowel cancers, early appearing lesions like aberrant crypt foci (ACF) are recognized. Recently, we identified β-cetenin accumulated crypts (BCAC) in the colonic mucosa of rats given a colon carcinogen. BCAC have frequent β-catenin gene mutations and their pathological features are different from those of ACF. Further comparative studies on the molecular pathology and biology of both lesions, gave rise to evidences that BCAC are probably the direct precursor for the large intestinal cancers in rodents. The newly identified lesions are suggested to be a reliable biomarker for the risk assessment of environmental chemicals and for the screening of cancer preventing agents.

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Investigations of the clonal growth of gastric carcinomas clearly suggest that individual cancers are derived from single cells with multi-potential activities and that cellular differentiation of gastric cancer cells occurs secondarily. By mucin histochemistry, gastric cancer cells of each histological group could be clearly classified into a gastric type and an intestinal type. The present results suggest the independent induction of intestinal metaplasia and gastric cancers and the occurrence of change of phenotypic expression of tumor cells from the gastric type to the intestinal type during growth of tumors. The Pepsinogen altered pyloric gland (PAPG) detected immunohistochemically may be considered to be a preneoplastic change. Rare mutations of p53 and ras genes in rat and mouse stomach cancers were found and p53 knockout mice (-/-) are more susceptible than (+/-) or (+/+) mice to N-methyl-N-nitrosourea stomach carcinogenesis.

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