Low dose radiation-induced adaptive response preventing HPRT mutation is Fhit independent

Identification of a tumor-suppressive human-specific microRNA within the FHIT tumor-suppressor gene

Loss or attenuated expression of the tumor-suppressor gene FHIT is associated paradoxically with poor progression of human tumors. Fhit promotes apoptosis and regulates reactive oxygen species; however, the mechanism by which Fhit inhibits tumor growth in animals remains unclear. In this study, we used a multidisciplinary approach based on bioinformatics, small RNA library screening, human tissue analysis, and a xenograft mouse model to identify a novel member of the miR-548 family in the fourth intron of the human FHIT gene. Characterization of this human-specific microRNA illustrates the importance of this class of microRNAs in tumor suppression and may influence interpretation of Fhit action in human cancer.

Use of the γ-H2AX assay to investigate DNA repair dynamics following multiple radiation exposures

Radiation therapy is one of the most common and effective strategies used to treat cancer. The irradiation is usually performed with a fractionated scheme, where the dose required to kill tumour cells is given in several sessions, spaced by specific time intervals, to allow healthy tissue recovery. In this work, we examined the DNA repair dynamics of cells exposed to radiation delivered in fractions, by assessing the response of histone-2AX (H2AX) phosphorylation (γ-H2AX), a marker of DNA double strand breaks. γ-H2AX foci induction and disappearance were monitored following split dose irradiation experiments in which time interval between exposure and dose were varied. Experimental data have been coupled to an analytical theoretical model, in order to quantify key parameters involved in the foci induction process. Induction of γ-H2AX foci was found to be affected by the initial radiation exposure with a smaller number of foci induced by subsequent exposures. This was compared to chromatin relaxation and cell survival. The time needed for full recovery of γ-H2AX foci induction was quantified (12 hours) and the 1:1 relationship between radiation induced DNA double strand breaks and foci numbers was critically assessed in the multiple irradiation scenarios.

Use of the γ-H2AX assay to investigate DNA repair dynamics following multiple radiation exposures

Radiation therapy is one of the most common and effective strategies used to treat cancer. The irradiation is usually performed with a fractionated scheme, where the dose required to kill tumour cells is given in several sessions, spaced by specific time intervals, to allow healthy tissue recovery. In this work, we examined the DNA repair dynamics of cells exposed to radiation delivered in fractions, by assessing the response of histone-2AX (H2AX) phosphorylation (γ-H2AX), a marker of DNA double strand breaks. γ-H2AX foci induction and disappearance were monitored following split dose irradiation experiments in which time interval between exposure and dose were varied. Experimental data have been coupled to an analytical theoretical model, in order to quantify key parameters involved in the foci induction process. Induction of γ-H2AX foci was found to be affected by the initial radiation exposure with a smaller number of foci induced by subsequent exposures. This was compared to chromatin relaxation and cell survival. The time needed for full recovery of γ-H2AX foci induction was quantified (12 hours) and the 1:1 relationship between radiation induced DNA double strand breaks and foci numbers was critically assessed in the multiple irradiation scenarios.

Mutagenic adaptive response to high-LET radiation in human lymphoblastoid cells exposed to X-rays

The ability of cells to adapt low-dose or low-dose rate radiation is well known. High-LET radiation has unique characteristics, and the data concerning low doses effects and high-LET radiation remain fragmented. In this study, we assessed in vitro the ability of low doses of X-rays to induce an adaptive response (AR) to a subsequent challenging dose of heavy-ion radiation. Lymphoblastoid cells (TK6, AHH-1, NH32) were exposed to priming 0.02-0.1Gy X-rays, followed 6h later by challenging 1Gy heavy-ion radiation (carbon-ion: 20 and 40keV/μm, neon-ion: 150keV/μm). Pre-exposure of p53-competent cells resulted in decreased mutation frequencies at hypoxanthine-guanine phosphoribosyl transferase locus and different H2AX phosphorylation kinetics, as compared to cells exposed to challenging radiation alone. This phenomenon did not seem to be linked with cell cycle effects or radiation-induced apoptosis. Taken together, our results suggested the existence of an AR to mutagenic effects of heavy-ion radiation in lymphoblastoid cells and the involvement of double-strand break repair mechanisms.

The Ku-dependent non-homologous end-joining pathway contributes to low-dose radiation-stimulated cell survival

Low-dose (≤0.1 Gy) radiation-induced adaptive responses could protect cells from high-challenge dose radiation-induced killing. The protective role is believed to promote the repair of DNA double-strand breaks (DSBs) that are a severe threat to cell survival. However, it remains unclear which repair pathway, homologous recombination repair (HRR) or non-homologous end-joining (NHEJ), is promoted by low-dose radiation. To address this question, we examined the effects of low-dose (0.1 Gy) on high-challenge dose (2-4 Gy) induced killing in NHEJ- or HRR-deficient cell lines. We showed that 0.1 Gy reduced the high-dose radiation-induced killing for wild-type or HRR-deficient cells, but enhanced the killing for NHEJ-deficient cells. Interestingly, low-dose radiation also enhanced the killing for wild-type cells exposed to high-challenge dose radiation with high-linear energy transfer (LET). Because it is known that high-LET radiation induces an inefficient NHEJ, these results support that the low-dose radiation-stimulated protective role in reducing high-challenge dose (low-LET)-induced cell killing might depend on NHEJ. In addition, we showed that low-dose radiation activated the DNA-PK catalytic subunit (DNA-PKcs) and the inhibitor of DNA-PKcs destroyed the low-dose radiation-induced protective role. These results suggest that low-dose radiation might promote NHEJ through the stimulation of DNA-PKcs activity and; therefore, increase the resistance of cells to high-challenge dose radiation-induced killing.

Common fragile site tumor suppressor genes and corresponding mouse models of cancer

Chromosomal common fragile sites (CFSs) are specific mammalian genomic regions that show an increased frequency of gaps and breaks when cells are exposed to replication stress in vitro. CFSs are also consistently involved in chromosomal abnormalities in vivo related to cancer. Interestingly, several CFSs contain one or more tumor suppressor genes whose structure and function are often affected by chromosomal fragility. The two most active fragile sites in the human genome are FRA3B and FRA16D where the tumor suppressor genes FHIT and WWOX are located, respectively. The best approach to study tumorigenic effects of altered tumor suppressors located at CFSs in vivo is to generate mouse models in which these genes are inactivated. This paper summarizes our present knowledge on mouse models of cancer generated by knocking out tumor suppressors of CFS.

Repetitive exposures to low-dose X-rays attenuate testicular apoptotic cell death in streptozotocin-induced diabetes rats

To define whether repetitive exposures to low-dose radiation (LDR) can attenuate diabetes-induced testicular cell death, Type 1 diabetic rats were produced by single injection of streptozotocin (STZ). Once hyperglycemia was diagnosed, diabetic rats were treated with and without LDR (25 and 50 mGy X-rays) daily for 4 weeks. Eight and 12 weeks after diabetes onset, testicular apoptotic cell death was examined by flow cytometry with Annexin V/PI staining, Western blotting assay for caspase-3 cleavage, and TUNEL staining for localization of apoptotic cells. Diabetes induced a significant increase in testicular apoptotic cell death, which was able to be attenuated by repetitive exposures to LDR. Diabetes-induced testicular cell death was associated with increased mitochondrial dysfunction, shown by the decreased mitochondrial potential and increased expressions of Bax mRNA and protein. All these changes were significantly attenuated in certain extends by repetitive exposures to LDR. To investigate the mechanisms by which LDR attenuates diabetes-induced testicular apoptotic cell death, serum sex hormone (testosterone, luteinizing hormone and follicle stimulating hormone) levels, and both serum and testicular oxidative damage (lipid peroxides) and antioxidant contents (superoxide dismutase, catalase and glutathione) were measured. Serum sex hormones were significantly decreased in diabetic rats, but not significantly in diabetic rats with multiple exposures to LDR; serum and testicular oxidative damage was significantly increased along with significant decreases in serum and testicular antioxidants in diabetic rats; however, these changes were significantly prevented by repetitive exposures to LDR. Furthermore, diabetic effects on the testicular oxidative damage and cell death were all attenuated by antioxidant N-acetylcysteine. These results suggest that diabetes-induced testicular cell death is probably mediated by increased oxidative stress. LDR protection from diabetes-induced testicular cell death is most likely mediated by its preserving antioxidants.