The contribution of deficient DNA repair to chromosomal radiosensitivity of CHO cells after G2 irradiation

DNA repair deficiency as a susceptibility marker for spontaneous lymphoma in golden retriever dogs: a case-control study

There is accumulating evidence that an individual's inability to accurately repair DNA damage in a timely fashion may in part dictate a predisposition to cancer. Dogs spontaneously develop lymphoproliferative diseases such as lymphoma, with the golden retriever (GR) breed being at especially high risk. Mechanisms underlying such breed susceptibility are largely unknown; however, studies of heritable cancer predisposition in dogs may be much more straightforward than similar studies in humans, owing to a high degree of inbreeding and more limited genetic heterogeneity. Here, we conducted a pilot study with 21 GR with lymphoma, 20 age-matched healthy GR and 20 age-matched healthy mixed-breed dogs (MBD) to evaluate DNA repair capability following exposure to either ionizing radiation (IR) or the chemical mutagen bleomycin. Inter-individual variation in DNA repair capacity was evaluated in stimulated canine lymphoctyes exposed in vitro utilizing the G2 chromosomal radiosensitivity assay to quantify clastogen-induced chromatid-type aberrations (gaps and breaks). Golden retrievers with lymphoma demonstrated elevated sensitivity to induction of chromosome damage following either challenge compared to either healthy GR or MBD at multiple doses and time points. Using the 75(th) percentile of chromatid breaks per 1,000 chromosomes in the MBD population at 4 hours post 1.0 Gy IR exposure as a benchmark to compare cases and controls, GR with lymphoma were more likely than healthy GR to be classified as "sensitive" (odds ratio = 21.2, 95% confidence interval 2.3-195.8). Furthermore, our preliminary findings imply individual (rather than breed) susceptibility, and suggest that deficiencies in heritable factors related to DNA repair capabilities may be involved in the development of canine lymphoma. These studies set the stage for larger confirmatory studies, as well as candidate-based approaches to probe specific genetic susceptibility factors.

Tp53 codon-72 polymorphisms identify different radiation sensitivities to g2-chromosome breakage in human lymphoblast cells

Both the G2 chromosomal radiosensitivity assay and allelic differences in TP53 codon-72 have been associated with cancer predisposition. The relationship between the two endpoints was determined in 56 human EBV-transformed lymphoblastoid cell lines. Although there were overlapping distributions of sensitivity for the different genotypes, cell lines that were homozygous for the proline coding allele were more likely to be resistant to chromatid break formation than those containing two arginine coding alleles, whereas cell lines expressing both the proline and arginine codon were either resistant like proline-proline lines or sensitive like arginine-arginine lines. The results support an important role of the TP53 codon-72 polymorphism in modifying G2-chromosome radiosensitivity. Distinguishing the effect of TP53 codon-72 variations from other modifiers of G2-chromosome radiosensitivity might aid in identifying new markers of cancer risk.

Gold nanoparticle sensitize radiotherapy of prostate cancer cells by regulation of the cell cycle

Glucose-capped gold nanoparticles (Glu-GNPs) have been used to improve cellular targeting and radio-sensitization. In this study, we explored the mechanism of Glu-GNP enhanced radiation sensitivity in radiation-resistant human prostate cancer cells. Cell survival and proliferation were measured using MTT and clonogenic assay. Flow cytometry with staining by propidium iodide (PI) was performed to study the cell cycle changes induced by Glu-GNPs, and western blotting was used to determine the expression of p53 and cyclin proteins that correlated to cell cycle regulation. With 2 Gy of ortho-voltage irradiation, Glu-GNP showed a 1.5-2.0 fold enhancement in growth inhibition when compared to x-rays alone. Comparing the cell cycle change, Glu-GNPs induced acceleration in the G0/G1 phase and accumulation of cells in the G2/M phase at 29.8% versus 18.4% for controls at 24 h. G2/M arrest was accompanied by decreased expression of p53 and cyclin A, and increased expression of cyclin B1 and cyclin E. In conclusion, Glu-GNPs trigger activation of the CDK kinases leading to cell cycle acceleration in the G0/G1 phase and accumulation in the G2/M phase. This activation is accompanied by a striking sensitization to ionizing radiation, which may have clinical implications.

Tachpyridine, a metal chelator, induces G2 cell-cycle arrest, activates checkpoint kinases, and sensitizes cells to ionizing radiation

Iron is critical for cell growth and proliferation. Iron chelators are being explored for a number of clinical applications, including the treatment of neurodegenerative disorders, heart disease, and cancer. To uncover mechanisms of action of tachpyridine, a chelator currently undergoing preclinical evaluation as an anticancer agent, cell-cycle analysis was performed. Tachpyridine arrested cells at G2, a radiosensitive phase of the cell cycle, and enhanced the sensitivity of cancer cells but not nontransformed cells to ionizing radiation. G2 arrest was p53 independent and was accompanied by activation of the checkpoint kinases CHK1 and CHK2. G2 arrest was blocked by UCN-01, a CHK1 inhibitor, but proceeded in CHK2 knock-out cells, indicating a critical role for CHK1 in G2 arrest. Tachpyridine-induced cell-cycle arrest was abrogated in cells treated with caffeine, an inhibitor of the ataxia-telangiectasia mutated/ataxia-telangiectasia-mutated and Rad3-related (ATM/ATR) kinases. Further, G2 arrest proceeded in ATM-deficient cells but was blocked in ATR-deficient cells, implicating ATR as the proximal kinase in tachpyridine-mediated G2 arrest. Collectively, our results suggest that iron chelators may function as antitumor and radioenhancing agents and uncover a previously unexplored activity of iron chelators in activation of ATR and checkpoint kinases.

Radiosensitizing properties of novel hydroxydicarboxylatoplatinum(II) complexes with high or low reactivity with thiols: two modes of action

We examined radiosensitizing properties of two novel platinum complexes (ethylenediamine(L-malato)platinum(II)), Pt1 and bis(1-ethylimidazole(L-malato)platinum(II)), Pt4. Initial double strand break (DSB) level and DSB rejoining were measured, using pulse field gel electrophoresis (PFGE) in human G1 phase lymphocytes subjected to Pt complex treatment alone and in combination with 10Gy of X-rays. Effects of Pt complex pre-treatment followed by X-irradiation were examined on survival (clonogenic ability) and growth (48 h growth tests) in Chinese hamster ovary (CHO-K1), xrs6 and L5178Y (LY) cells (LY-R and LY-S sublines). Cell cycle distributions of CHO cells after drug treatment were determined with the use of flow cytometry. Pt1 slowed down rejoining of X-ray induced DSB. It exerted a more than additive lethal effect on CHO-K1 cells but not on L5178Y cells subjected to combined Pt complex treatment and X-irradiation. In xrs6 cells the effect of combined Pt1+X treatment was additive. We conclude that, as earlier proposed for other Pt complexes, the radiosensitizing effect of Pt1 is connected with converting repairable DNA damage into irrepairable one (mode (i) of action). The requirements for this mode of sensitization are functional DNA repair systems (nucleotide excision repair (NER) and non-homologous end-joining (NHEJ)). Pt4 does not slow down DSB rejoining. It shows a considerable ability to arrest cells in G2 phase. We assume that Pt4 pre-treatment arrests cells in G2 phase and thus sensitizes to X-rays these cells that have a radiosensitive G2 phase (mode (ii) of action).

Radiation-induced chromatid breaks and deficient DNA repair in cancer predisposition

Deficient repair of DNA double-strand breaks, resulting in an abnormally high frequency of chromatid breaks after G(2) exposure of cells to radiation, appears to be associated with cancer predisposition. Unrepaired DNA strand breaks contribute to genomic instability. Unrepaired chromatid breaks representing DNA strand breaks can result in chromosome deletions, translocations and gene amplifications seen in human cancers. This cytogenetic response of cells to radiation may be useful as a marker of cancer susceptibility and in identifying individuals at risk of developing cancer in cancer families.

Increased G2 chromosomal radiosensitivity in cancer patients: the role of cdk1/cyclin-B activity level in the mechanisms involved

PURPOSE

To test the hypothesis that deficient DNA repair as measured by increased G2 chromosomal radiosensitivity results from up-regulation of cdk1/cyclinB and cell cycle control mechanisms during the G2 to M transition.

MATERIALS AND METHODS

A total of 185 cancer patients and 25 normal individuals were tested for G2 chromosomal radiosensitivity. The chromatid breaks were analysed in metaphase using the G2 assay or directly in G0 and G2 phase using premature chromosome condensation (PCC). The activity of cdk1/cyclinB, a key regulator of the G2 to M-phase transition, was measured by histone H1 kinase activity and correlated with the development of chromatid breaks after irradiation of cell lines in vitro.

RESULTS

Based on the G2 assay, cancer patients on average showed increased chromosomal radiosensitivity above controls. When the analysis was carried out directly in G0 or G2 lymphocytes using PCC, no differences in the induction of chromosomal damage and its repair were observed between G2 assay-sensitive and G2-normal donors. Using the G2 assay to test G2 radiosensitivity in various cell lines, it was found that the higher the cdk1/cyclinB activity level of the cell line tested, the higher the yield of chromatid breaks scored. Furthermore, when mitotic cells from these cell lines were used for PCC induction in irradiated G2 lymphocytes it was observed that the higher the cdk1/cyclinB activity level of mitotic cells used, the higher was the induced yield of chromatid breaks.

CONCLUSION

The cdk1/cyclin-B activity levels during the G2 to M transition impair DNA repair processes and play a major role in the yield of chromatid breaks induced after G2-irradiation. Regulation of cdk1/cyclinB complex activity rather than deficient repair enzymes of DNA damage may underlie the mechanisms of G2 radiosensitivity.

Enhancement of radiation effects in vitro by the estrogen metabolite 2-methoxyestradiol

2-Methoxyestradiol. 2-Methoxyestradiol (2-ME) is an endogenous estradiol metabolite that disrupts microtubule function, suppresses murine tumors, and inhibits angiogenesis. Since some microtubule inhibitors have been shown to alter radiosensitivity, we have evaluated 2-ME as a radiation enhancer in vitro. H460 human lung cancer cells were plated, treated with 2-ME for 24 h, and irradiated; then colony-forming ability was assessed. The radiation dose enhancement ratios (DERs) using this protocol were 1.3, 1.8 and 2.1 for 1, 1.5 and 2 microM 2-ME, respectively. Using a single-cell plating protocol, the respective DERs were 1.2, 1.5 and 1.8. The parent compound of 2-ME, beta-estradiol, did not enhance radiation effects at equally cytotoxic doses. Isobologram analysis showed that 1 microM 2-ME was additive with radiation, but that 1.5 and 2 microM were synergistic. Cell cycle analysis showed a dose-dependent increase in the percentage of cells in the radiosensitive G(2)/M phase after a 24-h treatment with 2-ME; a threefold increase in the percentage of cells in G(2)/M phase was observed using 2 microM 2-ME. Treatment with 2 microM 2-ME almost completely inhibited repair of sublethal damage (SLD) as shown using split-dose recovery. Radiosensitive, repair-deficient murine SCID (severe combined immunodeficient) cells did not show enhancement of radiation effects with 2 microM 2-ME, but enhancement was observed in the wild-type parental cells (CB-17). SCID cells complemented with human DNA-dependent protein kinase restored radioenhancement by 2-ME. In addition, MCF-7 breast cancer cells were also radiosensitized by 2 microM 2-ME (DER = 2.1). These data suggest that 2-ME is a potential radiation sensitizer, in addition to its previously reported antitumor and antiangiogenic properties. We have verified the antiangiogenic activity of 2-ME in vitro using human endothelial cells. Based on these results, we hypothesize that the mechanism of radiation enhancement may involve redistribution of cells into G(2)/M phase by 2-ME, and that the resulting population of cells is repair-deficient and thus radiosensitive.