Sex Difference of Radiation Response in Occupational and Accidental Exposure.

Ionizing radiation is a well-established cause of deleterious effects on human health. Understanding the risks of radiation exposure is important for the development of protective measures and guidelines. Demographic factors such as age, sex, genetic susceptibility, comorbidities, and various other lifestyle factors influence the radiosensitivity of different subpopulations. Amongst these factors, the influence of sex differences on radiation sensitivity has been given very less attention. In fact, the International Commission on Radiological Protection (ICRP) has based its recommendations on a population average, rather than the data on the radiosensitivity of distinct subpopulations. In this study, we reviewed major human studies on the health risks of radiation exposure and showed that sex-related factors may potentially influence the long-term response to radiation exposure. Available data suggest that long-term radiosensitivity in women is higher than that in men who receive a comparable dose of radiation. The report on the biological effects of ionizing radiation (BEIR VII) published in 2006 by the National Academy of Sciences, United States emphasized that women may be at significantly greater risk of suffering and dying from radiation-induced cancer than men exposed to the same dose of radiation. We show that radiation effects are sex-specific, and long-term radiosensitivity in females is higher than that in males. We also discuss the radiation effects as a function of age. In the future, more systematic studies are needed to elucidate the sex differences in radiation responses across the life continuum - from preconception through childhood, adulthood, and old age - to ensure that boys and girls and men and women are equally protected across ages.

Mammary gland and radiation: Knowns and unknowns.

Effective breast cancer management and decreasing breast cancer fatalities is contingent upon reliable diagnostic procedures and treatment modalities, including those based on ionizing radiation. On the one hand, ionizing radiation is widely used for cancer diagnostics and therapy, on the other hand it is genotoxic cancer-causing agent. Here we discuss recent studies on the effects of low (diagnostic) and high (treatment) doses of ionizing radiation on healthy breast cells, breast cancer cells, and cancer cells resistant to common drug therapies.

High and low dose radiation effects on mammary adenocarcinoma cells - an epigenetic connection.

The successful treatment of cancer, including breast cancer, depends largely on radiation therapy and proper diagnostics. The effect of ionizing radiation on cells and tissues depends on the radiation dose and energy level, but there is insufficient evidence concerning how tumor cells respond to the low and high doses of radiation that are often used in medical diagnostic and treatment modalities. The purpose of this study was to investigate radiation-induced gene expression changes in the MCF-7 breast adenocarcinoma cell line. Using microarray technology tools, we were able to screen the differential gene expressions profiles between various radiation doses applied to MCF-7 cells. Here, we report the substantial alteration in the expression level of genes after high-dose treatment. In contrast, no dramatic gene expression alterations were noticed after the application of low and medium doses of radiation. In response to a high radiation dose, MCF-7 cells exhibited down-regulation of biological pathways such as cell cycle, DNA replication, and DNA repair and activation of the p53 pathway. Similar dose-dependent responses were seen on the epigenetic level, which was tested by a microRNA expression analysis. MicroRNA analysis showed dose-dependent radiation-induced microRNA expression alterations that were associated with cell cycle arrest and cell death. An increased rate of apoptosis was determined by an Annexin V assay. The results of this study showed that high doses of radiation affect gene expression genetically and epigenetically, leading to alterations in cell cycle, DNA replication, and apoptosis.

Gene expression and epigenetic profiles of mammary gland tissue: insight into the differential predisposition of four rat strains to mammary gland cancer.

Rats are excellent experimental models for studying breast cancer, but rat strains differ in susceptibility. Among the four strains used in this study, Fischer rats are less susceptible to spontaneous breast cancer, yet they are highly prone to extremely severe metastatic and drug-resistant tumors, in those case where they actually develop the disease. In contrast, Sprague Dawley rats are the most susceptible to spontaneous breast cancer among the strains. ACI rats are highly prone to estrogen-induced cancer. Long-Evans rats are commonly used in mammary gland carcinogenesis studies. The molecular mechanisms of differential breast cancer susceptibility among rat strains are not well understood. Here, gene expression analysis was conducted in the mammary gland tissue of four rat strains--August × Copenhagen Irish (ACI), Long Evans, Fischer-344 and Sprague Dawley--to evaluate possible explanations for the differing breast cancer predispositions. According to the DAVID functional annotation analysis, there were at least eleven, five, and one significantly different pathways, respectively, in Fischer-344, Long-Evans and Sprague Dawley rats, in comparison to ACI rats. Two strains, Fischer-344 and Long-Evans, displayed differential expression in the complement and coagulation cascades, chemokine signaling, PPAR signaling, renin-angiotensin system, ECM-receptor interaction, focal adhesion and glutathione metabolism pathways. The only pathway that was significantly different between the Sprague Dawley and the ACI rats was the ribosome pathway. Our data indicate that general cancer susceptibility and predisposition to the development of aggressive and metastatic cancer are independent genetic conditions. Moreover, we have identified several important differences in the basal epigenetic profile of four rat strains with varying degrees of susceptibility to spontaneous and induced mammary carcinogenesis.

Mobilization of LINE-1 in irradiated mammary gland tissue may potentially contribute to low dose radiation-induced genomic instability.

It is known that cellular stresses such as ionizing radiation activate LINE-1 (long interspersed nuclear element type 1, L1), but the molecular mechanisms of LINE-1 activation have not been fully elucidated. There is a possibility that DNA methylation changes induced by genotoxic stresses might contribute to LINE-1 activation in mammalian cells. L1 insertions usually cause major genomic rearrangements, such as deletions, transductions, the intrachromosomal homologous recombination between L1s, and the generation of pseudogenes, which could lead to genomic instability. The purpose of this study was to evaluate the effects of low and high doses of ionizing radiation on the DNA methylation status of LINE-1 transposable elements in rat mammary glands. Here we describe radiation-induced hypomethylation and activation of LINE-1 ORF1 in rat mammary gland tissues. We show that radiation exposure has also led to the translation of the LINE-1 element, whereby the 148 kDa LINE-1 protein level was increased 96 hours after treatment with a low dose and low energy level radiation and remained elevated for 24 weeks after treatment. The mobilization of LINE-1 in irradiated tissue may potentially contribute to genomic instability. The observed activation of mobile elements in response to radiation exposure is consistently discussed as a plausible mechanism of cancer etiology and development.

Low dose irradiation profoundly affects transcriptome and microRNAme in rat mammary gland tissues.

Ionizing radiation has been successfully used in medical tests and treatment therapies for a variety of medical conditions. However, patients and health-care workers are greatly concerned about overexposure to medical ionizing radiation and possible cancer induction due to frequent mammographies and/or CT scans. Diagnostic imaging involves the use of low doses of ionizing radiation, and its potential carcinogenic role creates a cancer risk concern for exposed individuals. In this study, the effects of X-ray exposure of different doses on the gene expression patterns and the micro-RNA expression patterns in normal breast tissue were investigated in rats. Our results revealed the activation of immune response pathways upon low dose of radiation exposure. These included natural killer mediated cytotoxicity pathways, antigen processing and presentation pathways, chemokine signaling pathways, and T- and B-cell receptor signaling pathways. Both high and low doses of radiation led to miRNA expression alterations. Increased expression of miR-34a may be linked to cell cycle arrest and apoptosis. Up-regulation of miR-34a was correlated with down-regulation of its target E2F3 and up-regulation of p53. This data suggests that ionizing radiation at specific high and low doses leads to cell cycle arrest and a possible initiation of apoptosis.

Molecular mechanisms of radiation resistance in doxorubicin-resistant breast adenocarcinoma cells.

A positive response to breast cancer treatment is largely dependent on the successful combination of anticancer treatment modalities, such as chemotherapy and radiation therapy. Unfortunately, chemotherapy resistance occurs frequently. Furthermore, drug­resistant tumors can become unresponsive to other antitumor therapies, and they often fail to respond to radiation therapy. The molecular structures underlying the radiation responses of chemoresistant cells and tumors are not well understood. We analyzed the effect of ionizing radiation on MCF-7 human breast adenocarcinoma cells and their doxorubicin­resistant variant, MCF-7/DOX. The results demonstrated that drug­resistant MCF-7/DOX cells were less susceptible to radiation-induced DNA damage and apoptosis. This was proven through gene expression profiling, lower levels of γH2AX foci upon irradiation, and altered levels of DNA repair proteins, including pATM, KU70 and RAD51. Additionally, MCF-7/DOX drug­resistant cells harbored DNA polymerases with significantly low fidelity. In summary, our study revealed that drug-resistant MCF-7/DOX cells have high DNA repair potential and low-fidelity DNA polymerases, seemingly sacrificing specificity and efficiency to gain higher survival potential. In the long run, this may lead to an increased probability of mutation accumulation and further the development of an even more pronounced resistance phenotype. Therefore, this study provides a roadmap for the analysis of the roles of the DNA repair function and effectiveness, and apoptosis in response to radiation, chemotherapy and combinations of both treatment modalities.

Modulation of DNA methylation levels sensitizes doxorubicin-resistant breast adenocarcinoma cells to radiation-induced apoptosis.

Chemoresistant tumors often fail to respond to other cytotoxic treatments such as radiation therapy. The mechanisms of chemo- and radiotherapy cross resistance are not fully understood and are believed to be epigenetic in nature. We hypothesize that MCF-7 cells and their doxorubicin-resistant variant MCF-7/DOX cells may exhibit different responses to ionizing radiation due to their dissimilar epigenetic status. Similar to previous studies, we found that MCF-7/DOX cells harbor much lower levels of global DNA methylation than MCF-7 cells. Furthermore, we found that MCF-7/DOX cells had lower background apoptosis levels and were less responsive to radiation than MCF-7 cells. Decreased radiation responsiveness correlated to significant global DNA hypomethylation in MCF-7/DOX cells. Here, for the first time, we show that the radiation resistance of MCF-7/DOX cells can be reversed by an epigenetic treatment--the application of methyl-donor SAM. SAM-mediated reversal of DNA methylation led to elevated radiation sensitivity in MCF-7/DOX cells. Contrarily, application of SAM on the radiation sensitive and higher methylated MCF-7 cells resulted in a decrease in their radiation responsiveness. This data suggests that a fine balance of DNA methylation is needed to insure proper radiation and drug responsiveness.

Oxidative stress and antioxidant defenses in goldfish liver in response to short-term exposure to arsenite.

Arsenic is an environmental pollutant capable of causing oxidative stress, disturbance of metabolism, and cancer development. The present study was undertaken to investigate the effects of exposure to sodium arsenite on the glutathione pool, lipid peroxidation, protein carbonyl levels, global DNA methylation, and activities of six antioxidant enzymes in goldfish liver. In a preliminary experiment, 7-day exposure to 200 microM sodium arsenite, but not 10 or 100 microM, disturbed the glutathione status. A detailed investigation of oxidative stress development and antioxidant responses was further examined during different periods of exposure to 200 microM sodium arsenite. This treatment increased lipid peroxide levels after 1 and 4 days of exposure but did not affect thiobarbituric acid reactive substances and protein carbonyls. Oxidized glutathione and the oxidative stress index rose after 4 days, but de novo glutathione synthesis decreased both parameters after 7 days. Activities of the main antioxidant enzymes-superoxide dismutase, catalase, and glutathione peroxidase, were elevated after longer periods of exposure, indicating an enhanced antioxidant response. Arsenite exposure led to DNA hypomethylation, which is an early marker of disturbed epigenetic regulations. The findings suggest that goldfish livers cope with arsenic-induced oxidative stress mainly through adaptive changes in the glutathione pool and antioxidant enzymes.