The expressions of p53 and proliferating cell nuclear antigen (PCNA) were studied immunohistochemically from paraffin sections of 7 cases (9 lesions) of radiation-induced colon cancer and 42 cases of spontaneous colon cancer. Age distribution of radiation-induced and spontaneous colon cancer were 68.1 years (range, 56 to 77 years) and 67.4 years (range, 31 to 85 years), respectively. Among the radiation-induced colon cancers, there were 3 lesions of mucinous carcinoma (33%), a much higher than found for spontaneous mucinous cancer. Immunohistochemically, p53 protein expression was detected in 7/9 (78%) of radiation-induced cancers and in 23/42 (55%) of spontaneous colon cancers. χ² analysis found no significant differences between radiation-induced and spontaneous colon cancers in age distribution or p53-positive staining for frequency, histopathology, or Dukes'' classification. In radiation colitis around the cancers including aberrant crypts, spotted p53 staining and abnormal and scattered PCNA-positive staining were observed. In histologically normal cells, p53 staining was almost absent and PCNA-positive staining was regularly observed in the lower half of the crypt. In radiation colitis including aberrant glands, cellular proliferation increased and spotted p53 expression was observed. This study suggests that radiation colitis and aberrant glands might possess malignant potential and deeply associate with carcinogenesis of radiation-induced colon cancer.

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We labeled proliferating cells of the cerebellum of 6-day-old mice with 5-bromo-2''-deoxyuridine (BrdU) followed by a single exposure to 0.5, 1 or 2 Gy of X-rays. We then studied the effects of low-dose irradiation on the migration and survival of granule neurons in the mouse cerebellum. The animals were killed at 4 days, or at 2, 4 or 6 weeks after irradiation. Brains were fixed and BrdU-labeled cells in the cerebella were immunohistochemically analyzed. BrdU was predominantly distributed in the superficial layer of the external granular layer soon after injection. Four days after irradiation with 0.5 or 1 Gy, labeled cells were mainly seen in the inner granular layer, which was also the case in non-irradiated mice. However, following 2 Gy irradiation BrdU was found not only in the inner granular layer, but also in the Purkinje cell layer. This distribution was also seen at 2 and 4-6 weeks after irradiation. In animals irradiated with 1 Gy 4-6 weeks after irradiation, the proportion of labeled cells present in the inner granular layer decreased, while labeled cells in the Purkinje cell layer increased. On the other hand, 0.5 Gy irradiation did not change the distribution of labeled cells, except that the proportion of labeled cells in the inner granular layer decreased at 2 weeks after irradiation. The number of labeled cells in the cerebellar cortex per unit area decreased with time and dose. These results suggest that 2 Gy irradiation induces a migratory delay, abnormal distribution, and cell death of the granule neurons of the mouse cerebellum.

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3-Amino-1, 4-dimethyl-5H-pyrido[4, 3-b]indole (Trp-P-1) is known to be a mutagen and carcinogen isolated from the charred parts of cooked foods. We found previously that Trp-P-1 enhanced UV-induced lethality and mutation frequency in Escherichia coli by inhibiting the repair of UV-induced DNA damage. In the present study, we investigated whether Trp-P-1 also potentiated UV-induced lethality by inhibiting the repair of UV-induced DNA damage in cultured mammalian cells. As a result, Trp-P-1 enhanced UV-induced lethality in a concentration-dependent manner in human and Chinese hamster cells. However, Trp-P-1 was unable to inhibit the repair of the two major photolesions (cyclobutane pyrimidine dimers and (6-4)photoproducts) from the genomic DNA, as determined using monoclonal antibodies specific for each type of lesion. On the other hand, Trp-P-1, with or without UV irradiation, efficiently suppressed DNA synthesis and arrested cells in S phase in concentration and time-dependent manners, as measured by pulse-labelling with ³H-thymidine and flow cytometry. Thus, the present results suggest that Trp-P-1 potentiates UV-induced lethality in cultured mammalian cells by causing the S-phase arrest, not by inhibiting the repair of UV-induced DNA damage as observed in Escherichia coli.

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We examined whether the shortening of telomere structure is related to in vitro cellular aging after multiple low-dose irradiation. We used three strains of HE cells (HE23, HE31, and HE40) exhibiting different levels of telomerase activity and irradiated these cells twice a week with a dose of 2 cGy or 4 cGy of X-rays until they senesced. The cells were in total exposed to doses of 52-208 cGy of X-rays. Only the HE31 cells, which had no telomerase activity, experienced an increase in the number of cell divisions, reaching a maximum of 120-124% of the non-irradiated controls. However, in two strains which did exhibit telomerase activity in an early passage in culture, no extension of cell life span was found. Telomerase-positive cells completely lost all telomerase activity when the cells were subcultured several times without irradiation. In the HE31 cells where the life span was extended, the ratio of cell having a long telomere was higher than those of the other two cells (HE23 and HE40). Cytogenetic analysis revealed that the life span extension due to multiple low-dose irradiation which was observed in HE31 cells did not correlate with specific chromosome alterations.
Our results suggest that the telomerase activity remaining in the cells at an early passage does not correlate with the extension of life span in vitro by X-irradiation. The factor other than telomerase activity may play an important role in the regulation of telomere length and the extension of life span.

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Mutagenesis assays using transgenic mice have been recently developed and applied to the studies on the mutagenesis. The present study was undertaken to clarify whether the mutagenesis assay with transgenic mice could detect the mutations induced by the internal β-ray irradiation from ⁸⁹Sr or the external γ-ray irradiation. The transgenic mice used were Muta^TM mouse strain, which carries 80 copies of the bacterial lacZ gene per cell as a target of mutagenesis. Female animals were given intraperitoneal injections of ethylnitrosourea (50 mg/kg per day) for five days, a single intravenous injection of ⁸⁹Sr (7.4 and 74 MBq/kg), or daily irradiation with 1.5 Gy γ-rays for five days. The liver, spleen, and bone marrow were collected 15 days after the treatment with each agent. After the genomic DNA was extracted from each tissue, mutation analysis at lacZ locus was carried out. The spontaneous lacZ mutant frequencies were 2-4 × 10^-5. The frequencies of mutants induced by ethylnitrosourea in the liver, spleen and bone marrow were 68, 55, and 11 × 10^-5, respectively. In contrast, the mutant frequencies detected after the treatment with γ-rays were not so high in all three tissues as those treated with ethylnitrosourea. The injection of ⁸⁹Sr at a dose of 74 MBq/kg induced mutation at significantly higher frequencies in the bone marrow, but not in the liver and spleen. The results clearly showed that the mutation assay system used here could detect mutagenic effects of the local irradiation from ⁸⁹Sr, but was relatively insensitive to the β and γ-ray irradiation compared with the chemical mutagens such as ethylnitrosourea.

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Dose responses were compared among primary lung tumors and their histological types induced by a single inhalation exposure of female Wistar strain rats to submicron-size and polydispersed aerosols of plutonium dioxide (²³⁹PuO₂). While the primary lung tumors were found only in 2.3% of the unexposed control animals, the frequency of all the primary lung tumors in the exposed animals was 44% at the mean lung dose of 0.71 Gy, and increased sharply at the doses of 1.5 Gy or more, reaching the maximum of 97% at 5.4 Gy, and the dose responses around at 1.0 Gy were different between benign and malignant lung tumors. Almost all the pulmonary tumors in the exposed animals were classified into epithelial types such as adenomas, adenocarcinomas, adenosquamous carcinomas, and squamous cell carcinomas. The dose responses were different between these tumor types as shown by the peak incidence of adenomas at 0.71 Gy, adenocarcinomas at 2.9 Gy, adenosquamous and squamous cell carcinomas at 5.4-8.5 Gy, respectively. As the magnitudes of neoplastic lesions in pulmonary carcinomas were expressed by histological scores, metaplasias and adenomatous lesions most frequently appeared at doses of 1.5 Gy, while the appearance and increase of carcinomatous lesions differed in the dose ranges as shown by the peak incidence of adenocarcinomatous lesions at 2.9 Gy, and adenosquamous or squamous lesions at 5.4-6.6 Gy. These results indicate a differential dose response of pulmonary carcinogenesis in which metaplasias and benign adenomas were induced at lower doses (< 1.0 Gy), whereas malignant carcinomas were induced at relatively higher doses (> 1.5 Gy). Together with the increase of carcinomatous lesions at higher doses, the intranuclear p53 protein accumulation was detectable, but only in a few percentages of malignant carcinomas.

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We established and characterized a systematic ultraviolet light-induced carcinogenesis model using KSN nude mice. We prepared five groups of KSN mice and exposed them six times a week to five levels of daily ultraviolet B (UVB) doses; 1340, 670, 320, 160 and 0 J/m²/day. In 670, 320 and 160 J/m²/day, the latency period tended to become shorter in proportion to the daily doses and prevalence data fitted well to log-normal distribution. In the log-log plot of days till 50% prevalence versus daily dose, we saw a linear relationship for 1mm tumor diameter. From this analysis, we determined that days necessary to reach 50% prevalence is in proportion to the -0.49 power of daily dose. The average number of tumors per survivor correlated with prevalence data. Direct measured rates of tumor growth were independent of daily UVB-dose. Therefore we speculated that UV-irradiation did not affect tumor growth after its appearance. Most UVB-induced tumors were squamous cell carcinoma, the rest were spindle cell carcinoma, papilloma and mixed type. We concluded that our experimental data with nude mice was in accordance with data with hairless mice in nature.

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