Takaharu Nagaoka

Spontaneous uterine adenocarcinomas in aged rats and their relation to endocrine imbalance.

SummaryIn addition to spontaneous uterine endometrial adenocarcinomas at a high incidence (35.1 %), development of endometrial hyperplasia/adenoma was also frequently detected in rats of the Donryu strain. The total yield of all observed proliferative endometrial lesions was very high (60.6%). The tumors arose commonly in the uterine horn of aged rats. Histologically, most demonstrated glandular structures, consisting of cuboidal or columnar cells with weak eosinophilic or basophilic cytoplasm ↭d large nuclei. In about half of the animals with adenocarcinomas, metastasis to remote organs such as the lung was observed. Histological examination of the ovary and vaginal epithelium revealed ovarian cysts, atrophy of the ovary and cornification of the vaginal epithelium more frequently in rats with endometrial carcinomas than in animals without tumors. These findings indicate that adenocarcinoma development in Donryu rats is associated with endocrine imbalance [increased serum estrogen: progesterone (E2∶P)ratios]. By comparative investigation of strain differences, it was confirmed that irregular estrous cycles began earlier with higher incidence in Donryu rats than in F344 rats, a low-incidence strain. Histological findings of the ovary and vaginal epithelium also suggested relatively increased estrogen levels in Donryu rats compared to F344 rats. Estimated plasma values of gonad steroids showed that the E2∶P ratio in Donryu rats at 12 months of age was about five times that in F344 rats. These results therefore indicate that hormone imbalance, particularly an increased E2∶P ratio, may play an important role in the spontaneous occurrence of endometrial adenocarcinoma in Donryu rats.

Sequential Observation of Spontaneous Endometrial Adenocarcinoma Development in Donryu Rats

Sequential observation of spontaneous endometrial adenocarcinoma development revealed a clear, hormone-dependent, histogenetic pathway in Donryu rats. The first histological changes of the uterine endometrium appeared in both the lining epithelium and uterine gland of the endometrium at 6 mo of age, along with the beginning of persistent estrus. These changes included areas of tall columnar epithelium and gland formation in the lining epithelium as well as metaplastic change in the uterine gland. At 8 mo of age, endometrial hyperplasias were found, with subsequent increase in both incidence and degree. At 8–10 mo of age, hyperplasias were all within the limit of grade ++. After 12 mo of age, however, severe hyperplasias (grade + + +) began to increase markedly, and adenocarcinomas developed at 15 mo of age. The findings thus suggest that uterine endometrial adenocarcinomas arise from hyperplastic lesions, which should therefore be regarded as preneoplastic, as in the human case. Sequential analysis of plasma gonad steroids also ascertained a link between the appearance of these lesions and an increased estrogen: progesterone ratio, suggesting that estrogen may play an important role in development of both hyperplastic and neoplastic lesions. In Fischer-344 rats used for comparative assessment of strain differences, neither advanced histological changes nor hormonal changes were evident.

Effects of Reproduction on Spontaneous Development of Endometrial Adenocarcinomas and Mammary Tumors in Donryu Rats

Effects of reproduction on spontaneous development of uterine endometrial adenocarcinomas and mammary tumors in Donryu rats were investigated. While the incidence of endometrial adenocarcinomas in Donryu rats was not influenced by a single reproductive experience (SRE), it showed a tendency to decrease in animals having three reproductive experiences (TRE), compared to the nulliparous case (NRE). In addition, both SRE and TRE animals showed delayed occurrence and decreased incidences and mean numbers of mammary tumors, along with reduced incidences of proliferative lesions in the pituitary gland and mucinous epithelium in the vagina. The appearancetime and incidences of persistent estrus in TRE rats were delayed and low, respectively, compared to the SRE and NRE values. The hormonal environment was altered in both groups, the prolactin level in TRE especially being decreased. These results suggest that suppression of the occurrence of endometrial adenocarcinomas and mammary tumors in rats experiencing reproduction is associated with change in the hormonal milieu.

Age-Related Lesions in the Cerebrum in Middle-Aged Female Cynomolgus Monkeys

Alzheimer’s disease (AD) in humans is a progressive neurogenic disease that can be linked with such characteristic pathological findings in the cerebrum as senile plaques (SPs), neurofibrillary tangles (NFTs), cerebral amyloid angiopathy (CAA), and neuronal loss. In the present study, the authors investigated the age-related morphological changes in 12 middle-aged and 12 young cynomolgus monkeys. Low numbers of neurons and astrocytes in the hippocampal region in cynomolgus monkeys accompanied ageing, and there was a high number of microglial cells; however, no clearly neurotoxic abnormalities due to β-amyloid were noted before the age of 20 years. The onset of SPs and CAA in the cerebrum in cynomolgus monkeys can occur before the age of 20 years. SPs were almost all categorized as diffuse plaques (DPs); they did not have amyloid cores and were unaccompanied by neuritic degeneration. In cynomolgus monkeys, SPs (DPs) occur before the appearance of CAA. From the above, it was concluded that cynomolgus monkeys showed pathological changes due to ageing similar to those related to Alzheimer’s disease in humans, even before they were 20 years old.

Two-year carcinogenicity study of 6-mercaptopurine in F344 rats

SummaryThe carcinogenicity of 6-mercaptopurine (6-MP), an anticancer drug, was examined in F344 rats of both sexes, administered the chemical at dietary levels of 0 (control), 25 ppm or 50 ppm for 2 years. Many tumors developed in all groups including the control group, the organ distribution and histological types being similar to those reported for spontaneous lesions. In males, there was no significant increase in the incidence of any tumor in the treated groups over that in the control group. In females, however, positive trends were noted in the occurrence of C-cell tumors, pheochromocytomas, uterine adenocarcinomas and gliomas, and the incidences of C-cell tumors and pheochromocytomas in the 50 ppm group were significantly higher than the values in the respective control group. In addition, the total numbers of malignant tumors increased significantly in the female 50 ppm group. However, most of the tumors demonstrating increase are frequently observed spontaneous lesions in this strain of rats, and their incidences in the present female control group were lower than in our historical data. In addition, there were no significant differences in the incidences of preneoplastic changes and induction times for the above-listed tumors between the female control and the 50 ppm groups. These results thus indicated that while the carcinogenic potential of 6-MP can not be precluded, it can be only very weak or marginal, after continuous administration in the diet at the 50 ppm level for 2 years. The leukemogenic action of 6-MP was negative under the present experimental conditions.

Carcinogenicity and organ specificity of N-trimethylsilylmethyl-N-nitrosourea (TMS-MNU), N-neopentyl-N-nitrosourea (neoPNU), and N-methyl-N-nitrosourea (MNU) in rats

SummaryThe carcinogenicity and organ specificity of TMS-MNU and neoPNU, a carbon-analogue of TMS-MNU, in rats were investigated and compared with those of MNU. Compounds were dissolved in olive oil and rats in the experimental groups received 20 weekly intragastric intubations of 10 mg/kg of MNU or equimolar amounts of TMS-MNU or neoPNU in the same manner. The experiment was terminated when the survivors were sacrificed at the 52nd week after the final adminstration. In the TMS-MNU and MNU groups, tumors of the forestomach were induced and the incidence was 100% in the groups of both sexes. In addition, tumors of the glandular stomach, nervous system, kidney, and lung were also observed in these groups. Neurogenic tumors were found more frequently in the MNU group than in the TMS-MNU group. The incidence of lung tumors, however, was higher in the TMS-MNU group than in the MNU group. On the other hand, in the control and neoPNU groups, no tumor was found in these organs except the lung, and all tumors observed in these two groups were histologically similar to spontaneous ones in this strain of rats. These results indicate that the carcinogenicity of N-alkyl-N-nitrosoureas is dependent on the chemical structure of their alkyl chain. The result of the present study coincides with the previous result that the species of TMS-MNU in the alkylating step is the same as that of MNU, but different from neoPNU. The difference in the organ specificity between TMS-MNU and MNU demonstrates that the organ specificity is dominantly dependent on the distribution of the chemicals, since TMS-MNU may possibly be distributed differently from MNU because of its different partition property.

Promoting effects of 6-mercaptopurine on carcinogenesis in various organs of F344 rats

Possible promoting effects of 6-mercaptopurine (6-MP) on carcinogenesis in various organs, including the hematopoietic system, were investigated in female F344 rats, using a 2-stage carcinogenesis model. 6-MP was given as a dietary supplement (50 ppm) for 35 weeks subsequent to wide-spectrum initiation with N-ethyl-N-nitrosourea (ENU). Various tumors were observed in the carcinogen-initiated groups. No significant influence of 6-MP on their development, including the occurrence of leukemia, was apparent. However, the incidences of some proliferative lesions in the lung, intestine and kidney were slightly higher in the ENU/6-MP group than the ENU group. Further studies may be needed on promoting effects of 6-MP, based on dose-effect relation using several 6-MP doses and/or other initiators.

A case report of a metastatic yolk sac carcinoma in the pulmonary artery of a young female Sprague-Dawley rat

Yolk sac carcinoma is an extremely rare tumor in rats and is usually found in the genital system of aged animals. We encountered a yolk sac carcinoma in the pulmonary artery of an 18-week-old female Sprague-Dawley rat. In a repeated dosing toxicity study (once weekly for 4 weeks, intraperitoneal), this rat was unexpectedly found dead on the 55th day after the final administration of the test article. At necropsy, grayish white nodules were found on the lung surface. Histopathologically, tumor emboli were observed in the trunk and branch of the pulmonary artery. Tumor cells with slightly basophilic vacuolated cytoplasm and large vesicular nuclei formed nests or clusters and were embedded in a homogenous eosinophilic and periodic acid-Schiff reaction positive matrix. The tumor cells and matrix were immunoreactive for laminin. The embolic tumor resembled yolk sac carcinoma showing a parietal pattern in rodents. Although the primary site was unknown, the tumor was considered to be a metastatic yolk sac carcinoma.