Effects of X-irradiation on N-methyl-N-nitrosourea-induced multi-organ carcinogenesis in rats

Combined exposure to X-irradiation followed by N-ethyl-N-nitrosourea treatment alters the frequency and spectrum of Ikaros point mutations in murine T-cell lymphoma

Ionizing radiation is a well-known carcinogen, but its potency may be influenced by other environmental carcinogens, which is of practical importance in the assessment of risk. Data are scarce, however, on the combined effect of radiation with other environmental carcinogens and the underlying mechanisms involved. We studied the mode and mechanism of the carcinogenic effect of radiation in combination with N-ethyl-N-nitrosourea (ENU) using doses approximately equal to the corresponding thresholds. B6C3F1 mice exposed to fractionated X-irradiation (Kaplan's method) followed by ENU developed T-cell lymphomas in a dose-dependent manner. Radiation doses above an apparent threshold acted synergistically with ENU to promote lymphoma development, whereas radiation doses below that threshold antagonized lymphoma development. Ikaros, which regulates the commitment and differentiation of lymphoid lineage cells, is a critical tumor suppressor gene frequently altered in both human and mouse lymphomas and shows distinct mutation spectra between X-ray- and ENU-induced lymphomas. In the synergistically induced lymphomas, we observed a low frequency of LOH and an inordinate increase of Ikaros base substitutions characteristic of ENU-induced point mutations, G:C to A:T at non-CpG, A:T to G:C, G:C to T:A and A:T to T:A. This suggests that radiation doses above an apparent threshold activate the ENU mutagenic pathway. This is the first report on the carcinogenic mechanism elicited by combined exposure to carcinogens below and above threshold doses based on the mutation spectrum of the causative gene. These findings constitute a basis for assessing human cancer risk following exposure to multiple carcinogens.

Differential effects of low- and high-dose X-rays on N-ethyl-N-nitrosourea-induced mutagenesis in thymocytes of B6C3F1 gpt-delta mice

Carcinogenesis in humans is thought to result from exposure to numerous environmental factors. Little is known, however, about how these different factors work in combination to cause cancer. Because thymic lymphoma is a good model of research for combined exposure, we examined the occurrence of mutations in thymic DNA following exposure of B6C3F1 gpt-delta mice to both ionizing radiation and N-ethyl-N-nitrosourea (ENU). Mice were exposed weekly to whole body X-irradiation (0.2 or 1.0 Gy), ENU (200 ppm) in the drinking water, or X-irradiation followed by ENU treatment. Thereafter, genomic DNA was prepared from the thymus and the number and types of mutations in the reporter transgene gpt was determined. ENU exposure alone increased mutant frequency by 10-fold compared to untreated controls and over 80% of mutants had expanded clonally. X-irradiation alone, at either low or high dose, unexpectedly, reduced mutant frequency. Combined exposure to 0.2 Gy X-rays with ENU dramatically decreased mutant frequency, specifically G:C to A:T and A:T to T:A mutations, compared to ENU treatment alone. In contrast, 1.0 Gy X-rays enhanced mutant frequency by about 30-fold and appeared to accelerate clonal expansion of mutated cells. In conclusion, repeated irradiation with 0.2 Gy X-rays not only reduced background mutation levels, but also suppressed ENU-induced mutations and clonal expansion. In contrast, 1.0 Gy irradiation in combination with ENU accelerated clonal expansion of mutated cells. These results indicate that the mode of the combined mutagenic effect is dose dependent.

Comparison of dose-dependent enhancing effects of gamma-ray irradiation on urethan-induced lung tumorigenesis in athymic nude (nu/nu) mice ac (nu/+) littermates

The role of immunological surveillance in carcinogenesis is still controversial. In our previous experiments, urethan-induced lung tumorigenesis in athymic (nu/nu) mice and euthymic (nu/+) littermates was examined, and it was concluded that immunosurveillance mediated by T cells could not be demonstrated. However, the reported enhancement of development of various tumors following ionizing radiation might be achieved through modulating the host immunological conditions. In the present experiment, nu/nu and littermate nu/+ mice were treated with 1-4 Gy gamma-rays alone at 6 weeks of age or treated with urethan at 0.5 mg/g body weight when aged 14 days followed by 1-4 GY gamma-rays 4 weeks later. Lung tumors were assessed at 6.5 months of age. Ionizing radiation itself caused a very low incidence of these lesions. On the other hand, multiplicities and incidences of lung tumors after urethan treatment at 0.5 mg/g body weight were similar between the two phenotypically different groups of mice (1.66 and 1.84 tumors/mouse, 73% and 80% incidences, for nu/nu and nu/+ cases respectively). This urethan-induced lung tumorigenesis was significantly enhanced by gamma-rays in both nu/nu and nu/+ mice, and the magnitude of tumor enhancement was somewhat higher in nu/+ mice than in nu/nu mice, especially with a 2-Gy dose. In conclusion, it may be said that lung tumorigenicity of gamma-ray irradiation itself and the enhancing effect of radiation on urethan-induced tumorigenesis are scarcely influenced by immunosurveillance mechanisms mediated by T cells.

Synergistic effect of radiation on colon carcinogenesis induced by methylazoxymethanol acetate in ACI/N rats

The effect on colon and liver carcinogenicity in rats of a single X-irradiation exposure given either before or after methylazoxymethanol (MAM) acetate was studied in ACI/N rats of both sexes. A single dose of X-irradiation (3 Gy) was administered either 3 months before or after three weekly s.c. injections of MAM acetate (25 mg/kg body weight). At 365 days after the start, the incidence and multiplicity of MAM acetate-induced intestinal tumors were enhanced by X-irradiation either prior to or after the MAM acetate treatment. In addition, X-irradiation before MAM acetate increased the incidence of hepatocellular foci in either sex. In females, X-irradiation either before or after MAM acetate exposure decreased intestinal tumorigenesis. These findings suggest an apparent synergism of these agents in intestinal carcinogenesis of male rats.