A Multi-stage Carcinogenesis Model to Investigate Caloric Restriction as a Potential Tool for Post-irradiation Mitigation of Cancer Risk.

The risk of radiation-induced cancer adds to anxiety in low-dose exposed populations. Safe and effective lifestyle changes which can help mitigate excess cancer risk might provide exposed individuals the opportunity to pro-actively reduce their cancer risk, and improve mental health and well-being. Here, we applied a mathematical multi-stage carcinogenesis model to the mouse lifespan data using adult-onset caloric restriction following irradiation in early life. We re-evaluated autopsy records with a veterinary pathologist to determine which tumors were the probable causes of death in order to calculate age-specific mortality. The model revealed that in both irradiated and unirradiated mice, caloric restriction reduced the age-specific mortality of all solid tumors and hepatocellular carcinomas across most of the lifespan, with the mortality rate dependent more on age owing to an increase in the number of predicted rate-limiting steps. Conversely, irradiation did not significantly alter the number of steps, but did increase the overall transition rate between the steps. We show that the extent of the protective effect of caloric restriction is independent of the induction of cancer from radiation exposure, and discuss future avenues of research to explore the utility of caloric restriction as an example of a potential post-irradiation mitigation strategy.

Ionizing radiation, inflammation, and their interactions in colon carcinogenesis in Mlh1-deficient mice.

Genetic, physiological and environmental factors are implicated in colorectal carcinogenesis. Mutations in the mutL homolog 1 (MLH1) gene, one of the DNA mismatch repair genes, are a main cause of hereditary colon cancer syndromes such as Lynch syndrome. Long-term chronic inflammation is also a key risk factor, responsible for colitis-associated colorectal cancer; radiation exposure is also known to increase colorectal cancer risk. Here, we studied the effects of radiation exposure on inflammation-induced colon carcinogenesis in DNA mismatch repair-proficient and repair-deficient mice. Male and female Mlh1(-/-) and Mlh1(+/+) mice were irradiated with 2 Gy X-rays when aged 2 weeks or 7 weeks and/or were treated with 1% dextran sodium sulfate (DSS) in drinking water for 7 days at 10 weeks old to induce mild inflammatory colitis. No colon tumors developed after X-rays and/or DSS treatment in Mlh1(+/+) mice. Colon tumors developed after DSS treatment alone in Mlh1(-/-) mice, and exposure to radiation prior to DSS treatment increased the number of tumors. Histologically, colon tumors in the mice resembled the subtype of well-to-moderately differentiated adenocarcinomas with tumor-infiltrating lymphocytes of human Lynch syndrome. Immunohistochemistry revealed that expression of both p53 and ß-catenin and loss of p21 and adenomatosis polyposis coli proteins were observed at the later stages of carcinogenesis, suggesting a course of molecular pathogenesis distinct from typical sporadic or colitis-associated colon cancer in humans. In conclusion, radiation exposure could further increase the risk of colorectal carcinogenesis induced by inflammation under the conditions of Mlh1 deficiency.

Cancer prevention by adult-onset calorie restriction after infant exposure to ionizing radiation in B6C3F1 male mice.

Children are especially sensitive to ionizing radiation and chemical carcinogens, and limiting their cancer risk is of great public concern. Calorie restriction (CR) is a potent intervention for suppressing cancer. However, CR is generally not appropriate for children. This study, therefore, examined to see if adult-onset CR influences the lifetime cancer risk in mice after early-life exposure to ionizing radiation. Infant male mice (1-week-old) were exposed to 3.8 Gy X-rays, fed a control 95 kcal/week or CR 65 kcal/week diet from 7 weeks of age (adult stage), and their lifespan and tumor development were assessed. Irrespective of CR, X-rays shortened lifespan by 38%, and irrespective of irradiation CR extended lifespan by 20%. Thymic lymphoma (TL) and early-occurring non-TL were induced by radiation. The liver and Harderian gland were more susceptible to radiation-induced tumors than the lungs and non-thymic lymphoid tissues (late occurring). CR reduced the risk of hepatocellular carcinoma, late-occurring non-TL, lung tumor, Harderian tumor, and hemangioma but had less impact on TL and early-occurring non-TL. Most notably, the effects of X-rays on induction of lung tumors, late-occurring non-TL and hemangioma were essentially canceled by CR. The ability of CR to prevent late-occurring tumors was the same for non-irradiated and irradiated mice, indicating that the mechanism by which CR influences cancer is independent of irradiation. Our results indicate that adult-onset CR significantly inhibits late-occurring tumors in a tissue-dependent manner regardless of infant radiation exposure.

Molecular characterization of cancer reveals interactions between ionizing radiation and chemicals on rat mammary carcinogenesis.

Although various mechanisms have been inferred for combinatorial actions of multiple carcinogens, these mechanisms have not been well demonstrated in experimental carcinogenesis models. We evaluated mammary carcinogenesis initiated by combined exposure to various doses of radiation and chemical carcinogens. Female rats at 7 weeks of age were γ-irradiated (0.2-2 Gy) and/or exposed to 1-methyl-1-nitrosourea (MNU) (20 or 40 mg/kg, single intraperitoneal injection) or 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) (40 mg/kg/day by gavage for 10 days) and were observed until 50 weeks of age. The incidence of mammary carcinoma increased steadily as a function of radiation dose in the absence of chemicals; mathematical analysis supported an additive increase when radiation was combined with a chemical carcinogen, irrespective of the chemical species and its dose. Hras mutations were characteristic of carcinomas that developed after chemical carcinogen treatments and were overrepresented in carcinomas induced by the combination of radiation and MNU (but not PhIP), indicating an interaction of radiation and MNU at the level of initiation. The expression profiles of seven classifier genes, previously shown to distinguish two classes of rat mammary carcinomas, categorized almost all examined carcinomas that developed after individual or combined treatments with radiation (1 Gy) and chemicals as belonging to a single class; more comprehensive screening using microarrays and a separate test sample set failed to identify differences in gene expression profiles among these carcinomas. These results suggest that a complex, multilevel interaction underlies the combinatorial action of radiation and chemical carcinogens in the experimental model.

Loss of the BRCA1-interacting helicase BRIP1 results in abnormal mammary acinar morphogenesis.

BRIP1 is a DNA helicase that directly interacts with the C-terminal BRCT repeat of the breast cancer susceptibility protein BRCA1 and plays an important role in BRCA1-dependent DNA repair and DNA damage-induced checkpoint control. Recent studies implicate BRIP1 as a moderate/low-penetrance breast cancer susceptibility gene. However, the phenotypic effects of BRIP1 dysfunction and its role in breast cancer tumorigenesis remain unclear. To explore the function of BRIP1 in acinar morphogenesis of mammary epithelial cells, we generated BRIP1-knockdown MCF-10A cells by short hairpin RNA (shRNA)-mediated RNA interference and examined its effect in a three-dimensional culture model. Genome-wide gene expression profiling by microarray and quantitative RT-PCR were performed to identify alterations in gene expression in BRIP1-knockdown cells compared with control cells. The microarray data were further investigated using the pathway analysis and Gene Set Enrichment Analysis (GSEA) for pathway identification. BRIP1 knockdown in non-malignant MCF-10A mammary epithelial cells by RNA interference induced neoplastic-like changes such as abnormal cell adhesion, increased cell proliferation, large and irregular-shaped acini, invasive growth, and defective lumen formation. Differentially expressed genes, including MCAM, COL8A1, WIPF1, RICH2, PCSK5, GAS1, SATB1, and ELF3, in BRIP1-knockdown cells compared with control cells were categorized into several functional groups, such as cell adhesion, polarity, growth, signal transduction, and developmental process. Signaling-pathway analyses showed dysregulation of multiple cellular signaling pathways, involving LPA receptor, Myc, Wnt, PI3K, PTEN as well as DNA damage response, in BRIP1-knockdown cells. Loss of BRIP1 thus disrupts normal mammary morphogenesis and causes neoplastic-like changes, possibly via dysregulating multiple cellular signaling pathways functioning in the normal development of mammary glands.

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.

Development of a new E. coli strain to detect oxidative mutation and its application to the fungicide o-phenylphenol and its metabolites.

Oxidative mutation is mainly induced by reactive oxygen species (ROS), such as the superoxide anion radical (O(2)(-)) and hydrogen peroxide (H(2)O(2)). However, in Escherichia coli (E. coli), ROS are eliminated by enzymes such as superoxide dismutase and catalase, which are coded by sodAB and katEG genes. In this study, to detect mutagens that induce oxidative mutation, a mutant (WP2katEGsodAB) with katEG and sodAB deleted was constructed by gene manipulation of E. coli WP2. H(2)O(2) and menadione sodium bisulfite generated mutation in WP2katEGsodAB but not in WP2. o-Phenylphenol (OPP) and its metabolites (phenylhydroquinone (PHQ) and phenyl-1,4-benzoquinone (PBQ)), which had been shown to be negative in the Ames test but reported to be carcinogenic, induced mutation in WP2katEGsodAB but not in WP2. These results suggest that the new assay may be useful for the detection of oxidative mutagens.