Altered radiation responses of breast cancer cells resistant to hormonal therapy.

Endocrine therapy agents (the selective estrogen receptor (ER) modulators such as tamoxifen or the selective ER down-regulators such as ICI 182,780) are key treatment regimens for hormone receptor-positive breast cancers. While these drugs are very effective in controlling ER-positive breast cancer, many tumors that initially respond well to treatment often acquire drug resistance, which is a major clinical problem. In clinical practice, hormonal therapy agents are commonly used in combination or sequence with radiation therapy. Tamoxifen treatment and radiotherapy improve both local tumor control and patient survival. However, tamoxifen treatment may render cancer cells less responsive to radiation therapy. Only a handful of data exist on the effects of radiation on cells resistant to hormonal therapy agents. These scarce data show that cells that were resistant to tamoxifen were also resistant to radiation. Yet, the existence and mechanisms of cross-resistance to endocrine therapy and radiation therapy need to be established. Here, we for the first time examined and compared radiation responses of MCF-7 breast adenocarcinoma cells (MCF-7/S0.5) and two antiestrogen resistant cell lines derived from MCF-7/S0.5: the tamoxifen resistant MCF-7/TAMR-1 and ICI 182,780 resistant MCF-7/182R-6 cell lines. Specifically, we analyzed the radiation-induced changes in the expression of genes involved in DNA damage, apoptosis, and cell cycle regulation. We found that the tamoxifen-resistant cell line in contrast to the parental and ICI 182,780-resistant cell lines displayed a significantly less radiation-induced decrease in the expression of genes involved in DNA repair. Furthermore, we show that MCF-7/TAMR-1 and MCF-7/182R-6 cells were less susceptible to radiation-induced apoptosis as compared to the parental line. These data indicate that tamoxifen-resistant breast cancer cells have a reduced sensitivity to radiation treatment. The current study may therefore serve as a roadmap to the future analysis of the mechanisms of cross-resistance between hormonal therapy and radiation.

Micronuclei in genotoxicity assessment: from genetics to epigenetics and beyond.

Micronuclei (MN) are extra-nuclear bodies that contain damaged chromosome fragments and/or whole chromosomes that were not incorporated into the nucleus after cell division. MN can be induced by defects in the cell repair machinery and accumulation of DNA damages and chromosomal aberrations. A variety of genotoxic agents may induce MN formation leading to cell death, genomic instability, or cancer development. In this review, the genetic and epigenetic mechanisms of MN formation after various clastogenic and aneugenic effects on cell division and cell cycle are described. The knowledge accumulated in literature on cytotoxicity of various genotoxins is precisely reflected and individual sensitivity to MN formation due to single gene polymorphisms is discussed. The importance of rapid MN scoring with respect to the cytokinesis-block micronucleus assay is also evaluated.

Sex-specific microRNAome deregulation in the shielded bystander spleen of cranially exposed mice.

The bystander effect is a phenomenon that occurs when exposed cells signal distress to their naïve, unexposed neighbors. It is now accepted as a ubiquitous consequence of radiation exposure. It is well documented to occur in cultured cells, 3D tissue models, and in organs and organisms. Notwithstanding, the exact mechanisms of the bystander effect remain unclear. Recent studies hinted that bystander effects may, in part, be distinct in males and females, and possibly mediated via short non-coding RNAs, specifically, microRNAs. MicroRNAs are small, abundant and capable of regulating the expression of a wide variety of targets. Yet, their roles in bystander effects have not been analyzed in detail. The mechanisms behind sex differences observed in in vivo bystander effects also remain to be uncovered. We hypothesized that the radiation-induced expression of microRNAs in exposed and bystander tissue may be distinct in males and females. Using a well-establish bystander mouse model when the animal's head is exposed, while the body is completely protected by a medical-grade shield, we have for the first time shown that radiation exposure triggers a significant and sex-specific deregulation of the microRNAome in the non-exposed bystander spleen. The altered miRNA levels were paralleled by sex-specific changes in the levels of the miRNA processing enzyme Dicer and components of the RNA-induced silencing complex (RISC). Sterilization of animals resulted in drastic microRNAome alterations and significantly affected radiation and bystander miRNA responses. Our data may provide a roadmap for further analysis of the role of microRNAome in genotoxic stress responses and may help us explain sex specificity of radiation-induced carcinogenesis.

Hyperoxia results in transient oxidative stress and an adaptive response by antioxidant enzymes in goldfish tissues.

The effects of hyperoxia on the status of antioxidant defenses and markers of oxidative damage were evaluated in goldfish tissues. The levels of lipid peroxides, thiobarbituric acid reactive substances, carbonyl proteins and the activities of some antioxidant enzymes were measured in brain, liver, kidney and skeletal muscle of goldfish, Carassius auratus L., over a time course of 3-12 h of hyperoxia exposure followed by 12 or 36 h of normoxic recovery. Exposure to high oxygen resulted in an accumulation of protein carbonyls in tissues throughout hyperoxia and recovery whereas lipid peroxides and thiobarbituric acid reactive substances accumulated transiently under short-term hyperoxia stress (3-6 h) but were then strongly reduced. This suggests that hyperoxia stimulated an enhancement of defenses against lipid peroxidation or mechanisms for enhancing the catabolism of peroxidation products. The activities of principal antioxidant enzymes, superoxide dismutase and catalase, were not altered under hyperoxia but catalase increased during normoxic recovery; activities may rise in anticipation of further hyperoxic excursions. In most tissues, the activities of glutathione-utilizing enzymes (glutathione peroxidase, glutathione-S-transferase, glutathione reductase) as well as glucose-6-phosphate dehydrogenase, were not affected under hyperoxia but increased sharply during normoxic recovery. Correlations between some enzyme activities and oxidative stress markers were found, for example, an inverse correlation was seen between levels of thiobarbituric acid reactive substances and glutathione-S-transferase activity in liver and catalase and glucose-6-phosphate dehydrogenase in kidney. The results suggest that liver glutathione-S-transferase plays an important role in detoxifying end products of lipid peroxidation accumulated under hyperoxia stress.