Repair of chromosome and DNA breaks versus cell survival in Chinese hamster cells

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.

Differential responses of Chinese hamster mutagen sensitive cell lines to low and high concentrations of calicheamicin and neocarzinostatin

To shed light on the mechanism underlying the cellular response to the radiomimetic agents calicheamicin Y(1)(1) (CAL) and neocarzinostatin (NCS), several hamster cell mutants defective in different DNA repair pathways were used. Two X-ray sensitive Chinese hamster V79 mutant cell lines, XR-V9B and V-E5 were studied for their response to the induction of cell killing, micronuclei, and G2-chromosomal aberrations relative to that of parental wild-type cells. In addition, effects of CAL and NCS on bleomycin sensitive BL-V40 cells and on UV sensitive V-H1 cells were analyzed. In general, the radiosensitive cell lines showed the highest sensitivities to CAL and NCS, but also the other mutants demonstrated differences in their responses compared to wild-type cells. With respect to cell killing, expressed as D(10)-value, enhanced sensitivities of mutants with factors up to 4.4 were recorded. For the induction of micronuclei (MN) and chromosomal aberrations (CA) all cell lines, including the parental cells, show a steep increase in the frequencies at the lowest tested doses and a leveling off at higher concentrations. Probably toxic effects at the higher exposure levels are responsible for these biphasic dose effect curves. Enhanced sensitivities of the various mutants were primarily observed at the higher exposure levels. With respect to the induction of MN increased sensitivities up to a factor of 18.1 were observed for the radiosensitive mutants, whereas for CA the mutant cell lines showed a variation from resistance (0.3) of VH-1 cells up to a 3.8-fold higher sensitivity to the radiomimetic agents. However, at the lowest tested concentrations for both MN and CA, the differences between the sensitive mutants and wild-type clearly diminished, suggesting the existence of residual and/or alternative DNA repair pathways in these mutants.

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.

Radiosensitivity of human normal and tumoral thyroid cells using fluorescence in situ hybridization and clonogenic survival assay

PURPOSE

By using cell survival as a reference, we evaluated the radiosensitivity of human normal and tumoral thyroid cells using of radiation-induced translocations.

METHODS AND MATERIALS

Tissue samples were obtained from patients undergoing thyroidectomy. Cell cultures were established, irradiated with 60Co, and metaphases painted using commercial whole-chromosome 4 hybridization probe and pancentromeric probe. The clonogenic survival was assessed by conventional colony forming assay.

RESULTS

After irradiation, normal cultured thyroid cells yielded a higher number of translocations than cultures derived from adenomas or thyroid carcinoma. The colony forming assay demonstrated, by way of the mean inactivation dose, a higher survival of thyroid carcinoma and adenoma cells than of normal thyroid cells. This difference between tumoral and nontumoral cells is significant in each method (p = 0.0001), and cannot be explained by apoptosis in irradiated malignant cells. Correlation of the results obtained by both methods is shown by comparing the survival fraction at 2 Gy (SF2) and the percentage of chromosome 4 translocations at 2 Gy.

CONCLUSION

These results indicate that the yield of radiation-induced translocations serves as a good and rapid prediction of the intrinsic radiosensitivity of thyroid cells, and that this test could be applied to other tumors.

Differential proliferation dependence of alpha and beta damage in X-irradiated Chinese hamster cells

PURPOSE

To determine quantitatively the influence of altering proliferation rates on clonal survival of asynchronously growing Chinese hamster (CHO) cells after X-irradiation and to evaluate the related contribution of alpha and beta damage.

MATERIAL AND METHODS

Cell cycle distributions at the time of X-irradiation of CHO cells were assessed by flow cytometry. Clonal radiation survival was established by colony forming assay. Survival data were fitted to the linear-quadratic model and analyzed on the basis of the mean inactivation dose, D.

RESULTS

Increased S-phases were associated with increased resistance to X-rays. Radiosensitivity as expressed by D differed by a factor of 1.6 between the most sensitive and the most resistant populations. Separately analyzing the alpha and beta coefficients of survival curves revealed that the proliferation dependent effect was correlated only with beta. The major determinant of D was alpha, but its substantial interexperimental variations were independent of the cell cycle.

CONCLUSIONS

Due to altering cell cycle distributions, considerable changes of radiosensitivity can occur. They can in part be understood as a consequence of S-phase dependent alterations of DNA damage repair. Reasons for the changes of a damage dependent lethality remain to be discovered by further research.