Co-operative effects of thoracic X-ray irradiation and N-nitrosobis(2-hydroxypropyl) amine administration on lung tumorigenesis in neonatal, juvenile and adult Wistar rats.

Assessment of risks associated with childhood exposure to ionizing radiation when combined with chemical carcinogens is of great importance. We studied the age-dependence of the effect of combined exposure to ionizing radiation (IR) and a chemical carcinogen on lung carcinogenesis. Female 1-, 5-, and 22-week-old Wistar rats were locally irradiated on the thorax with X-rays (3.18 Gy) and/or were injected intraperitoneally with N-nitrosobis(2-hydroxypropyl)amine (BHP) (1g/kg body weight) 1 week after X-ray exposure or at 23 weeks of age. Rats were terminated at 90 weeks of age. We found that: (i) the incidence of lung tumors (adenoma and adenocarcinoma) increased slightly as a function of age at X-ray exposure, although this was not statistically significant, while the incidence induced by BHP decreased with increasing age at administration; (ii) combined exposure to X-rays at 5 or 22 weeks with BHP 1 week later enhanced the tumor incidence, and the effect at early-life stage (5 weeks irradiation) was more effective than that at late-life stage (22 weeks irradiation); (iii) combined exposure preferentially enhanced malignant transformation; (iv) although a longer interval between the X-ray and BHP treatments reduced the combined effect, risks of early-life irradiation at 1 or 5 weeks of age lasted into adulthood; (v) adenomas and adenocarcinomas induced by X-ray and/or BHP originated from surfactant apoprotein A-positive alveolar type II cells; and (vi), extracellular signal-regulated kinase pathway activation was observed in half the adenocarcinomas, regardless of the exposure schedule. In conclusion, combined exposure may enhance lung tumorigenesis more synergistically at early-life stage (5 weeks of age) than later-life stage.

Immunohistochemical detection of possible cellular origin of hepatic histiocytic sarcoma in mice.

Histiocytic sarcoma (HS) spontaneously arises in the liver in mice ; however, the cellular origins of hepatic HS have not been fully clarified. In this study, we immunohistochemically analyzed 18 cases of hepatic HS from the archives of our previous experiments. In all cases, the tumor cells showed positive reactions for the macrophage-specific markers F4/80 and CD68. The cells were negative for mesenchymal cell and lymphoid cell markers, suggesting that germ cell tumor or lymphoma components do not coexist in the neoplasm. We detected scattered Ly6C(+)F4/80(-) macrophage precursors in the extramedullary hematopoietic foci and liver tissue around the HS lesions. We also showed that certain populations of HS cells express the Ly-6C antigen. These findings suggest that Ly-6C(+) macrophage progenitor cells are a possible cellular origin of murine hepatic HS. Our study identified a novel phenotype of murine HS in two of 18 cases. These cases showed the nodular accumulations of tumor cells with cohesive cytoplasm mimicking the features of epithelioid granuloma. In agreement with the expression of CD204 in epithelioid cells in granulomatous diseases, these HS cells hardly expressed CD204, although the common type HS cells were strongly positive for this antigen. These data suggest that hepatic HS may stem from Ly-6C(+) macrophage precursors. Furthermore, a subset of hepatic HS cases can possibly differentiate into epithelioid cell-like phenotypes.

Transcription factor-recognition sequences potentially involved in modulation of gene expression after exposure to low-dose-rate γ-rays in the mouse liver.

In vivo modulation of gene expression profiles after low-dose and low-dose-rate irradiation has been observed in a variety of experimental systems. However, few studies actually investigated the underlying mechanisms for these genetic responses. In this study, we used pre-existing microarray data and searched for gene modulations in response to long-term, low-dose-rate irradiation. Nucleotide sequences in the neighboring region of the up-regulated, down-regulated, and unaffected genes were retrieved from the Entrez Gene database, and recognition sequences for transcription factors (TFs) were searched using the TFSEARCH database. As a result, we suggested 21 potential TF-binding sites with significantly different incidence between the three gene groups (up-regulated, down-regulated and unaffected gene groups). The binding sites for sterol regulatory element-binding protein 1 (SREBP-1), aryl hydrocarbon receptor (AhR/Ar) and olfactory 1 (Olf-1) were suggested to be involved in up-regulation, while the binding sites for glucocorticoid receptor (GR(GGTACAANNT GTYCTK) ) and hepatocyte nuclear factor 1 (HNF-1) were suggested to be involved in down-regulation of the genes. In addition, the binding sites for activating enhancer-binding protein 4 (AP-4), nuclear factor-κB (NFκB), GR (NNNNNNCNNTNTGTNCTNN) and early growth response 3 (Egr-3) were correlated with modulation of gene expression regardless of the direction of modulation. Our results suggest that these TF-binding sites are involved in gene modulations after long-term continuous irradiation with low-dose-rate γ rays. GR and/or SREBP-1 might be associated with the altered metabolic process observed in liver after exposure to low-dose-rate irradiation.

Enhanced transplantability of a cell line from a murine ovary granulosa cell tumour in syngeneic B6C3F(1) mice continuously irradiated with low dose-rate gamma-rays.

PURPOSE: To understand the mechanisms of life-shortening due to early neoplastic death caused by chronic low dose-rate (LDR; 20 mGy/22 h/day) radiation which accumulates to a high dose (HD; 8 Gy) (LDR/HD) as reported previously. MATERIALS AND METHODS: Female B6C3F(1) mice were continuously exposed to LDR/HD gamma-rays under specific-pathogen-free (SPF) conditions for 400 days. OV3121 cells, which were derived from an ovarian granulosa cell tumour that arose in irradiated B6C3F(1) mice, were inoculated into LDR/HD irradiated and age-matched non-irradiated control mice. The transplantability of tumour cells as well as T cell subsets and the proliferative activities of T cells were compared between irradiated and non-irradiated mice. RESULTS: We found that tumour formation of subcutaneously inoculated tumour cells occurred earlier in irradiated mice than in non-irradiated mice. Proliferative activity of draining lymph node lymphocytes against transplanted tumour cells as well as allogeneic mixed lymphocyte reactions were significantly reduced in irradiated mice compared to non-irradiated mice. CONCLUSIONS: These results suggest that decreased tumour-specific immune response due to LDR/HD irradiation may enhance tumorigenesis resulting in life-shortening of mice after chronic LDR/HD irradiation.

Gene expression profiles in mouse liver after long-term low-dose-rate irradiation with gamma rays.

Changes in gene expression profiles in mouse liver induced by long-term low-dose-rate γ irradiation were examined by microarray analysis. Three groups of male C57BL/6J mice were exposed to whole-body radiation at dose rates of 17-20 mGy/day, 0.86-1.0 mGy/day or 0.042-0.050 mGy/day for 401-485 days with cumulative doses of approximately 8 Gy, 0.4 Gy or 0.02 Gy, respectively. The gene expression levels in the livers of six animals from each exposure group were compared individually with that of pooled sham-irradiated animals. Some genes revealed a large variation in expression levels among individuals within each group, and the number of genes showing common changes in individuals from each group was limited: 20 and 11 genes showed more than 1.5-fold modulation with 17-20 mGy/day and 0.86-1.0 mGy/day, respectively. Three genes showed more than 1.5-fold modulation even at the lowest dose-rate of 0.04-0.05 mGy/day. Most of these genes were down-regulated. RT-PCR analysis confirmed the expression profiles of the majority of these genes. The results indicate that a few genes are modulated in response to very low-dose-rate irradiation. The functional analysis suggests that these genes may influence many processes, including obesity and tumorigenesis.

The uneven irradiation of a target cell and its dynamic movement can mathematically explain incubation period for the induction of cancer by internally deposited radionuclides.

Irradiation from internally deposited radionuclides induces malignant tumors. Ingested radionuclides accumulate in specific organs, which are irradiated over a lifelong period. Our aim is to elucidate why the development of malignant tumors requires long-term internal exposure, on the order of decades, despite the fact that irradiation is continuous over this period. Three major factors are considered to be responsible for the long incubation time in carcinogenesis caused by internally deposited alpha-emitters: uneven distribution of radionuclides, limited range of irradiation, and dynamic movement of tumor precursor cells. We hypothesized that target cells susceptible to malignant transformation may undergo one event by alpha particles and may then migrate outside of the range of alpha particles, thereby avoiding immediate induction of successive additional events that would lead to cell death or neoplastic changes. Based on this hypothesis, we further proposed a mathematical model to predict the relationship between dose rate and incubation period of tumors induced by internally deposited alpha-emitters. The function was non-linear and included terms of both direct and indirect radiation effects. It well fitted both human Th-ICC cases and rat Pu-induced lung cancer, suggesting that indirect radiation effects are independent from dose rate. The significance of parameters of the model is discussed.

Implication of p16 inactivation in tumorigenic activity of respiratory epithelial cell lines and adenocarcinoma cell line established from plutonium-induced lung tumor in rat.

To investigate whether p16 inactivation is involved in the development of rat pulmonary tumors, we compared the p16 status and tumorigenicity of cell lines which indicated different p16 status. The tumor cell line (PuD2) was established from lung adenocarcinoma induced in plutonium dioxide-inhaled rat in this study. The virus-immortalized SV40T2 cells, benzo[a]pyrene-induced BP cells, BP-derived BP(P)Tu cells, and gamma ray-transformed RTiv3 cells were utilized as the respiratory epithelial cell lines. A tumorigenicity assay-inoculating cells into nude mice revealed that PuD2, BP, and BP(P)Tu cells were tumorigenic, but SV40T2 and RTiv3 cells were not. Methylation-specific PCR of the p16 promoter region revealed that SV40T2 cells were unmethylated, BP cells displayed heterogeneous methylation, and BP(P)Tu and RTiv3 cells were completely methylated. Methylation-specific PCR and PCR of genomic DNA in the p16 region did not amplify product in PuD2 cells, indicating deletion of p16. Banded karyotypes prepared from PuD2 cells exhibited trisomy of chromosome 4, inversion in chromosome 11, and partial deletion of chromosomes 4 and 5. The demethylating agent 5Aza2dC partially demethylated the p16 promoter region of BP(P)Tu, BP and RTiv3 cells, increasing expression of the p16 transcript and decreasing growth of the cells. These results indicate that hypermethylation of the p16 promoter region occurs early in neoplastic transformation before acquisition of tumorigenicity in rat respiratory epithelium. Loss of genes located on chromosomes 4 and 5 may be important for tumor progression and acquisition of high tumorigenic activity in the Pu-induced rat lung tumor.

Radiation dose-rate effect on mutation induction in spleen and liver of gpt delta mice.

The effect of dose rate on radiation-induced mutations in two somatic tissues, the spleen and liver, was examined in transgenic gpt delta mice. These mice can be used for the detection of deletion-type mutations, and these are the major type of mutation induced by radiation. The dose rates examined were 920 mGy/min, 1 mGy/min and 12.5 microGy/min. In both tissues, the number of mutations increased with increasing dose at each of the three dose rates examined. The mutation induction rate was dependent on the dose rate. The mutation induction rate was higher in the spleen than in the liver at the medium dose rate but was similar in the two tissues at the high and low dose rates. The mutation induction rate in the liver did not show much change between the medium and low dose rates. Analysis of the molecular nature of the mutations indicated that 2- to 1,000-bp deletion mutations were specifically induced by radiation in both tissues after high- and low-dose-rate irradiation. The occurrence of deletion mutation without any sequence homology at the break point was elevated in spleen after high-dose-rate irradiation. The results indicate that the mutagenic effects of radiation in somatic tissues are dependent on dose rate and that there is some variability between tissues.

Microarray analysis of differentially expressed genes in the kidneys and testes of mice after long-term irradiation with low-dose-rate gamma-rays.

Measuring global gene expression using cDNA or oligonucleotide microarrays is an effective approach to understanding the complex mechanisms of the effects of radiation. However, few studies have been carried out that investigate gene expression in vivo after prolonged exposure to low-dose-rate radiation. In this study, C57BL/6J mice were continuously irradiated with gamma-rays for 485 days at dose-rates of 0.032-13 microGy/min. Gene expression profiles in the kidney and testis from irradiated and unirradiated mice were analyzed, and differentially expressed genes were identified. A combination of pathway analysis and hierarchical clustering of differentially expressed genes revealed that expression of genes involved in mitochondrial oxidative phosphorylation was elevated in the kidney after irradiation at the dose-rates of 0.65 microGy/min and 13 microGy/min. Expression of cell cycle-associated genes was not profoundly modulated in the kidney, in contrast to the response to acute irradiation, suggesting a threshold in the dose-rate for modulation of the expression of cell cycle-related genes in vivo following exposure to radiation. We demonstrated that changes to the gene expression profile in the testis were largely different from those in the kidney. The Gene Ontology categories "DNA metabolism", "response to DNA damage" and "DNA replication" overlapped significantly with the clusters of genes whose expression decreased with an increase in the dose-rate to the testis. These observations provide a fundamental insight into the organ-specific responses to low-dose-rate radiation.

Dose-rate effectiveness for unstable-type chromosome aberrations detected in mice after continuous irradiation with low-dose-rate gamma rays.

Chronological changes in the chromosome aberration rates of splenocytes from specific-pathogen-free (SPF) mice after continuous and long-term exposure to low-dose-rate gamma rays were studied. Incidences of dicentrics plus centric rings (Dic+Rc), detected by conventional Giemsa staining, and dicentric chromosomes, detected by fluorescence in situ hybridization (Dic by FISH) using a centromere probe, showed an essentially linear increase up to a total accumulated dose of 8000 mGy after irradiation for about 400 days at a low dose rate of 20 mGy/day. For comparison, acute high-dose-rate and medium-dose-rate irradiation were performed. The values of the alpha coefficients in the linear regression lines for these unstable-type aberrations decreased as the dose rates were lowered from medium dose rates (200 and 400 mGy/day) to low dose rates (1 and 20 mGy/day). The dose and dose-rate effectiveness factor (DDREF), estimated by the ratio of calculated incidences using the best-fit regression lines at a high dose rate (890 mGy/min) and low dose rate (20 mGy/day), was 4.5 for Dic by FISH and 5.2 for Dic+Rc, respectively, at the same dose of 100 mGy, while different DDREFs were obtained for different accumulated doses. This is the first study to provide information regarding the effects of long-term exposure to low-dose-rate radiation on chromosomes.

Induction of rhodanese, a detoxification enzyme, in livers from mice after long-term irradiation with low-dose-rate gamma-rays.

The health effects of low-dose radiation exposure are of public concern. Although molecular events in the cellular response to high-dose-rate radiation exposure have been fully investigated, effects of long-term exposure to extremely low-dose-rate radiation remain unclear. Protein expression was analyzed by two-dimensional electrophoresis in livers from mice irradiated for 485 days (22 hr/day) at low-dose-rates of 0.032 microGy/min, 0.65 microGy/min and 13 microGy/min (total doses of 21 mGy, 420 mGy and 8000 mGy, respectively). One of the proteins that showed marked changes in expression was identified as rhodanese (thiosulfate sulfurtransferase). Rhodanese expression was increased after irradiation at 0.65 microGy/min and 13 microGy/min, while its expression was not changed at 0.032 microGy/min. Rhodanese is a detoxification enzyme, probably related to the regulation of antioxidative function. However, antioxidative proteins, such as superoxide dismutase (SOD)1 (also known as Cu,Zn-SOD) and SOD2 (also known as Mn-SOD), which can be induced by high-dose-rate radiation, were not induced at any low-dose-rates tested. These findings indicate that rhodanese is a novel protein induced by low-dose-rate radiation, and further analysis could provide insight into the effects of extremely low-dose-rate radiation exposure.

Chromosome aberration frequencies and chromosome instability in mice after long-term exposure to low-dose-rate gamma-irradiation.

Chronological changes of chromosome aberration rates related to accumulated doses in chronically exposed humans and animals at a low-dose-rate have not been well studied. C3H female specific pathogen-free mice (8 weeks of age) were chronically irradiated. Chromosome aberration rate in mouse splenocytes after long-term exposure to low-dose-rate (LDR) gamma-rays was serially determined by conventional Giemsa method. Incidence of dicentrics and centric rings increased almost linearly up to 8000 mGy following irradiation for about 400 days at a LDR of 20 mGy/day. Clear dose-rate effects were observed in the chromosome aberration frequencies between dose rates of 20 mGy/day and 200 Gy/day. Furthermore, the frequencies of complex aberrations increased as accumulated doses increased in LDR irradiation. This trend was also observed for the incidences of micronuclei and trisomies of chromosomes 5, 13 and 18 in splenocytes, detected by micronucleus assay and metaphase fluorescence in situ hybridization (FISH) method, respectively. Incidences of 2-4 micronuclei and trisomy increased in mouse splenocytes after irradiation of 8000 mGy at a LDR of 20 mGy/day. These complex chromosome aberrations and numerical chromosome aberrations seem to be induced indirectly after radiation exposure and thus the results indicate that continuous gamma-ray irradiation for 400 days at LDR of 20 mGy/day induced chromosomal instability in mice. These results are important to evaluate the biological effects of long-term exposure to LDR radiation in humans.

Microarray-based global mapping of integration sites for the retrotransposon, intracisternal A-particle, in the mouse genome.

Mammalian genomes contain numerous evolutionary harbored mobile elements, a part of which are still active and may cause genomic instability. Their movement and positional diversity occasionally result in phenotypic changes and variation by causing altered expression or disruption of neighboring host genes. Here, we describe a novel microarray-based method by which dispersed genomic locations of a type of retrotransposon in a mammalian genome can be identified. Using this method, we mapped the DNA elements for a mouse retrotransposon, intracisternal A-particle (IAP), within genomes of C3H/He and C57BL/6J inbred mouse strains; consequently we detected hundreds of probable IAP cDNA-integrated genomic regions, in which a considerable number of strain-specific putative insertions were included. In addition, by comparing genomic DNAs from radiation-induced myeloid leukemia cells and its reference normal tissue, we detected three genomic regions around which an IAP element was integrated. These results demonstrate the first successful genome-wide mapping of a retrotransposon type in a mammalian genome.

Upregulation of c-myc gene accompanied by PU.1 deficiency in radiation-induced acute myeloid leukemia in mice.

OBJECTIVE: High-dose radiation exposure induces acute myeloid leukemia (AML) in C3H mice, most of which have a frequent hemizygous deletion around the D2Mit15 marker on chromosome 2. This region includes PU.1, a critical candidate gene for initiation of leukemogenesis. To identify novel cooperative genes with PU.1, relevant to radiation-induced leukemogenesis, we analyzed the copy number alterations of tumor-related gene loci by array CGH, and their expressions in primary and transplanted AMLs. MATERIALS AND METHODS: For the induction of AMLs, C3H/He Nrs mice were exposed to 3 Gy of x-rays or gamma-rays. The genomic alterations of 35 primary AMLs and 34 transplanted AMLs obtained from the recipient mice transplanted the primary AMLs were analyzed by array CGH. According to the genomic alterations and mutations of the 235th arginine of PU.1 allele, we classified the radiogenic AMLs into three types such as Chr2(del) PU.1(del/R235-) AML, Chr2(del) PU.1(del/R235+) AML and Chr2(intact) PU.1(R235+/R235+) AML, to compare the expression levels of 8 tumor-related genes quantitatively by real-time polymerase chain reaction and cell-surface antigen expression. Results. In addition to well-known loss of PU.1 with hemizygous deletion of chromosome 2, novel genomic alterations such as partial gain of chromosome 6 were recurrently detected in AMLs. In this study, we found similarity between cell-surface antigen expressions of bone marrows and those of spleens in AML mice and significantly higher expressions of c-myc and PU.1 expression, especially in the PU.1-deficient (Chr2(del) PU.1(del/R235-)) AML and Chr2(del) PU.1(del/R235+) compared to Chr2(intact) PU.1(R235+/R235+) AMLs. CONCLUSION: The new finding on upregulation of c-myc and PU.1 in both and hemizygous PU.1-deficient AMLs and different genomic alterations detected by array CGH suggests that the molecular mechanism for development of radiation-induced AML should be different among three types of AML.

Analysis of changes in DNA copy number in radiation-induced thymic lymphomas of susceptible C57BL/6, resistant C3H and hybrid F1 Mice.

Radiation-induced thymic lymphoma in mice is a useful model for studying both the mechanism of radiation carcinogenesis and genetic susceptibility to tumor development. Using array-comparative genomic hybridization, we analyzed genome-wide changes in DNA copy numbers in radiation-induced thymic lymphomas that had developed in susceptible C57BL/6 and resistant C3H mice and their hybrids, C3B6F1 and B6C3F1 mice. Besides aberrations at known relevant genetic loci including Ikaros and Bcl11b and trisomy of chromosome 15, we identified strain-associated genomic imbalances on chromosomes 5, 10 and 16 and strain-unassociated trisomy of chromosome 14 as frequent aberrations. In addition, biallelic rearrangements at Tcrb were detected more frequently in tumors from C57BL/6 mice than in those from C3H mice, suggesting aberrant V(D)J recombination and a possible link with tumor susceptibility. The frequency and spectrum of these copy-number changes in lymphomas from C3B6F1 and B6C3F1 mice were similar to those in C57BL/6 mice. Furthermore, the loss of heterozygosity analyses of tumors in F(1) mice indicated that allelic losses at Ikaros and Bcl11b were caused primarily by multilocus deletions, whereas those at the Cdkn2a/Cdkn2b and Pten loci were due mainly to uniparental disomy. These findings provide important clues to both the mechanisms for accumulation of aberrations during radiation-induced lymphomagenesis and the different susceptibilities of C57BL/6 and C3H mice.

Occurrence of mutations in the epidermal growth factor receptor gene in X-ray-induced rat lung tumors.

Epidermal growth factor receptor (EGFR) gene alterations have been found in human lung cancers. However, there is no information on the factors inducing EGFR mutations. In rodents, K-ras mutations are frequently found in many lung carcinogenesis models, but hitherto, Egfr mutations have not been reported. Their presence was therefore investigated in representative lung carcinogenesis models with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), N-nitrosobis(2-hydroxypropyl)amine (BHP), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) and ethyl carbamate (urethane), as well as X-ray irradiation. With the chemical carcinogenesis models, no mutations were detected in Egfr, which is in clear contrast to the high rates observed in either codon 12 or 61 of K-ras (21/23 of the lung tumors induced with NNK, 4/5 with MeIQx, 1/4 with urethane and 7/18 with BHP). However, in the X-ray-induced lung tumors, Egfr mutations with amino acid substitution were observed in exons 18 and 21 (4/12, 33%), but no activating mutation of K-ras was detected. In addition, one and four silent mutations were identified in K-ras (exon 1) and Egfr (exons 18, 20 and 21), respectively. Most mutations in both Egfr and K-ras were G/C-->A/T transitions (7/8, 88% and 31/34, 91%, respectively). Although, the mutational patterns in equivalent human lesions were not completely coincident, this first report of Egfr mutations in an experimental lung tumor model suggests that X-rays or other factors producing oxygen radicals could cause EGFR mutations in some proportion of lung cancers in humans.

Inverse dose-rate-effects on the expressions of extra-cellular matrix-related genes in low-dose-rate gamma-ray irradiated murine cells.

Based on the results of previous microarray analyses of murine NIH3T3/PG13Luc cells irradiated with continuous low-dose-rate (LDR) gamma-ray or end-high-dose-rate-irradiations (end-HDR) at the end of the LDR-irradiation period, the inverse dose-rate-effects on gene expression levels were observed. To compare differences of the effects between LDR-irradiation and HDR-irradiation, HDR-irradiations at 2 different times, one (ini-HDR) at the same time at the start of LDR-irradiation and the other (end-HDR), were performed. The up-regulated genes were classified into two types, in which one was up-regulated in LDR-, ini-HDR-, and end-HDR irradiation such as Cdkn1a and Ccng1, which were reported as p53-dependent genes, and the other was up-regulated in LDR- and ini-HDR irradiations such as pro-collagen TypeIa2/Col1a2, TenascinC/Tnc, and Fibulin5/Fbln5, which were reported as extra-cellular matrix-related (ECM) genes. The time dependent gene expression patterns in LDR-irradiation were also classified into two types, in which one was an early response such as in Cdkn1a and Ccng1 and the other was a delayed response such as the ECM genes which have no linearity to total dose. The protein expression pattern of Cdkn1a increased dose dependently in LDR- and end-HDR-irradiations, but those of p53Ser15/18 and MDM2 in LDR-irradiations were different from end-HDR-irradiations. Furthermore, the gene expression levels of the ECM genes in embryonic fibroblasts from p53-deficient mice were not increased by LDR- and end-HDR-irradiation, so the delayed expressions of the ECM genes seem to be regulated by p53. Consequently, the inverse dose-rate-effects on the expression levels of the ECM genes in LDR- and end-HDR-irradiations may be explained from different time responses by p53 status.

Array-CGH analyses of murine malignant lymphomas: genomic clues to understanding the effects of chronic exposure to low-dose-rate gamma rays on lymphomagenesis.

We previously reported that mice chronically irradiated with low-dose-rate gamma rays had significantly shorter mean life spans than nonirradiated controls. This life shortening appeared to be due primarily to earlier death due to malignant lymphomas in the irradiated groups (Tanaka et al., Radiat. Res. 160, 376-379, 2003). To elucidate the molecular pathogenesis of murine lymphomas after low-dose-rate irradiation, chromosomal aberrations in 82 malignant lymphomas from mice irradiated at a dose rate of 21 mGy/day and from nonirradiated mice were compared precisely by microarray-based comparative genomic hybridization (array-CGH) analysis. The array carried 667 BAC clones densely selected for the genomic regions not only of lymphoma-related loci but also of surface antigen receptors, enabling immunogenotyping. Frequent detection of the apparent loss of the Igh region on chromosome 12 suggested that most lymphomas in both groups were of B-cell origin. Array-CGH profiles showed a frequent gain of whole chromosome 15 in lymphomas predominantly from the irradiated group. The profiles also demonstrated copy-number imbalances of partial chromosomal regions. Partial gains on chromosomes 12, 14 and X were found in tumors from nonirradiated mice, whereas losses on chromosomes 4 and 14 were significantly associated with the irradiated group. These findings suggest that lymphomagenesis under the effects of continuous low-dose-rate irradiation is accelerated by a mechanism different from spontaneous lymphomagenesis that is characterized by the unique spectrum of chromosomal aberrations.

Comparative study on Tp53 gene mutations in lung tumors from rats exposed to 239Pu, 237Np and 222Rn.

The tumor suppressor gene Tp53 was analyzed by polymerase chain reaction-amplification of genomic DNA extracted from paraffin-embedded tissue sections of rat lung tumors to compare mutations that occurred after inhalation exposures to plutonium dioxide, neptunium dioxide, or radon and radon progenies. Exons 5 to 8 of the gene were amplified in 16 plutonium-, 23 neptunium- and 15 radon-induced lung tumors, and their polymerase chain reaction products were examined for mutations by single strand conformational polymorphism analysis and direct sequencing method. Two point mutations were detected in the plutonium-induced tumors, i.e., a guanine to adenine transition at codon 219 of exon 6 and a cytosine to thymine transition at codon 266 of exon 8. Although only one point mutation was found at codon 175 of exon 5 (cytosine to thymine transition) from neptunium-induced tumors, no mutations were detectable from radon-induced tumors. These results indicate that the abnormalities of the Tp53 gene might not be so critical for the pulmonary carcinogenesis after the inhalation of different alpha emitters, even though the presence and frequencies of the Tp53 gene mutations were different.

Comparisons of pulmonary carcinogenesis in rats following inhalation exposure to plutonium dioxide or X-ray irradiation.

Radiation-induced pulmonary carcinogenesis was compared in female Wistar rats following either inhalation exposure to alpha-emitting (239)PuO(2) aerosols, whole-body or thoracic X-ray irradiation. Dose-dependent survival reduction was correlated with increased malignant lung tumors at doses over 0.45 Gy, reaching the maximum incidence of 90% at 6.6-8.5 Gy in (239)Pu-exposed rats. While the differential dose responses for each histopathological type of tumors were noted, almost 70-80% were carcinomas among all of the primary tumors from (239)Pu-exposed rats. As the dose response curves for lung carcinomas were compared, the slope of the fit linear equation and the calculated relative effectiveness for 50% incidence of lung carcinomas were approximately 11-times as high in (239)Pu-exposure as those of thoracic X-irradiation. The numbers of tumor lesions distributed in the lung per tumor-bearing animal were about 2-fold more in (239)Pu-exposed rats, while the proportions of their histopathological types were similar between (239)Pu-exposure and X-irradiation. These results indicate that the magnitudes of the relative effectiveness or risk for pulmonary carcinogenesis are greater in (239)Pu-exposure than X-irradiation, and that radiation-induced lung tumors appear to originate mostly from the same target epithelial cells.