Purpose: To investigate the influence of chromatin organization and scavenging capacity in relation to irradiation temperature on the induction of double-strand breaks (DSB) in structures derived from human diploid fibroblasts. Materials and methods: Agarose plugs with different chromatin structures (intact cells±wortmannin, permeabilized cells with condensed chromatin, nucleoids and DNA) were prepared and irradiated with X-rays at 2 or 37°C and lysed using two different lysis protocols (new ice-cold lysis or standard lysis at 37°C). Induction of DSB was determined by constant-field gel electrophoresis. Results: The dose-modifying factor (DMFtemp) for irradiation at 37 compared with 2°C was 0.92 in intact cells (i.e. more DSB induced at 2°C), but gradually increased to 1.5 in permeabilized cells, 2.2 in nucleoids and 2.6 in naked DNA, suggesting a role of chromatin organization for temperature modulation of DNA damage. In addition, DMFtemp was influenced by the presence of 0.1 M DMSO or 30 mM glutathione, but not by post-irradiation temperature. Conclusion: The protective effect of low temperature was correlated to the indirect effects of ionizing radiation and was not dependent on post-irradiation temperature. Reasons for a dose modifying factor <1 in intact cells are discussed.

It is assumed that the efficient antitumor activity of calicheamicin γ1 is mediated by its ability to introduce DNA double-strand breaks in cellular DNA. To test this assumption we have compared calicheamicin γ1-mediated cleavage of cellular DNA and purified plasmid DNA. Cleavage of purified plasmid DNA was not inhibited by excess tRNA or protein indicating that calicheamicin γ1 specifically targets DNA. Cleavage of plasmid DNA was not affected by incubation temperature. In contrast, cleavage of cellular DNA was 45-fold less efficient at 0°C as compared to 37° due to poor cell permeability at low temperatures. The ratio of DNA double-strand breaks (DSB) to single-stranded breaks (SSB) in cellular DNA was 1:3, close to the 1:2 ratio observed when calicheamicin γ1 cleaved purified plasmid DNA. DNA strand breaks introduced by calicheamicin γ1 were evenly distributed in the cell population as measured by the comet assay. Calicheamicin γ1-induced DSBs were repaired slowly but completely and resulted in high levels of H2AX phosphorylation and efficient cell cycle arrest. In addition, the DSB-repair deficient cell line Mo59J was hyper sensitive to calicheamicin γ. The data indicate that DSBs is the crucial damage after calicheamicin γ1 and that calicheamicin γ1-induced DSBs are recognized normally. The high DSB:SSB ratio, specificity for DNA and the even damage distribution makes calicheamicin γ1 a superior drug for studies of the DSB-response and emphasizes its usefulness in treatment of malignant disease.

Cell suspensions enriched in cells at various stages of apoptosis were obtained by separation of irradiated human peripheral blood lymphocytes on density gradients at different post-irradiation times. The state of DNA fragmentation in the cells was determined by comet assay and pulsed field gel electrophoresis. The morphologically distinguishable features of apoptosis such as chromatin condensation and cell shrinkage correlated with discrete stages of DNA fragmentation. It was found that greater than or equal to50kbp fragmentation of DNA occurs already in cells of normal density whereas the subsequent DNA fragmentation onto fragments <50 kbp occurs in parallel with cell shrinkage and simultaneous increase in cell density. The observed stages of DNA fragmentation seem to be separated in time that could allow in case of abortive apoptosis formation of chromosomal aberrations.

CHO-K1 cells were synchronized at the G(1)/S border by mitotic shake-off and aphidicolin incubation. Pulse-labeling with tritium was done at 30 min, 2 or 5 h into the S-phase, with chase incubations for different times in non-radioactive medium. The cells were subjected to neutral microelectrophoresis to extend the DNA into "comets," after which the label was visualized through autoradiography. At zero chase time, all label was positioned in the head. The displacement of label into the tails increased with time, reaching a maximum at about 5 h after the pulse. A lag phase of 2 - 3 It was observed for the early-labeled cells before the displacement started. Also, more label was released after overnight serum starvation, but this was reversed through a 3-h incubation at normal growth conditions. It was found that late-replicating chromatin is organized in larger domains than early-replicating chromatin, and DNA polymerase seems to be an important organizer. Early-replicating chromatin has other important attachments to the nuclear matrix, dependent on metabolic activity.

Recently, we described a new in vivo pathway in the metabolism of benzo[a]pyrene (BP) that involves an opening of the aromatic ring system. One of the products of this pathway, isolated from rat urine, was the anhydride of 7-oxo-benz[d]anthracene-3,4-dicarboxylic acid (ABADA). We have now investigated the effect of ABADA on several cellular targets, known to be important in tumor formation. ABADA was as efficient as BP-7,8-diol-9,10-epoxide in inducing direct strand breaks but not alkali labile sites in DNA in HT-29 cells and exhibited weak mutagenic activity in Salmonella typhimurium strain TA 102. The cytotoxicity of ABADA to HCT 116 cells appeared to be due to apoptosis, as caspase-3 activity and poly-ADP-ribose polymerase (PARP) cleavage was observed. COX-2 promoter activity was induced by ABADA in HCT 116 cells. In conclusion, this novel metabolic pathway may also be contributing to the carcinogenicity of BP.

The metabolism of benzo[a]pyrene (BP) is known to lead to a large number of oxygenated compounds, some of which can bind covalently to DNA. We have studied the integrated metabolism of BP in vivo in germ-free rats given C-14-labeled BP. Urinary metabolites were separated into groups according to acidity using lipophilic ion exchangers. The groups were analyzed by mass spectrometry and were further fractionated by high-performance liquid chromatography. The fraction of urinary metabolites previously shown to contain N-acetylcysteine and glucuronic acid conjugates was found to contain derivatives of 7-oxo-benz[d]anthracene-3,4-dicarboxylic acid as major components. These compounds, which were identified by mass spectrometry and NMR, accounted for about 30% of the total metabolites in urine, demonstrating that, surprisingly, ring opening is a major pathway for metabolism of BP in the germ-free rat. The dicarboxylic acid may be excreted in urine as an ester glucuronide. By using the single cell gel electrophoresis or COMET assay, we were able to demonstrate that the anhydride of 7-oxo-benz[d]anthracene-3,4-dicarboxylic acid was an efficient inducer of DNA damage. Taken together, these results indicate that the novel ring opening metabolic pathway may provide alternative mechanisms for the toxicity of BP.

Purpose: The repair of double-strand breaks (dsb) in mammalian cells is characterized by a rapid phase with a half-life of less than half an hour and a slower phase that lasts for many hours. The proportion of slow repair increase with LET and it has been suggested that the slow repair component consists of more complex damage and is more deleterious to the cells. To see if removal of OH radicals could remove part of the damage in complex dsb and make them easier to repair, human cells were irradiated in the presence of dimethyl sulphoxide (DMSO). Methods: Induction and repair of dsb were studied by neutral elution in human VH10 cells exposed to γ-rays, helium ions (mean LET 40 keV/μm) and 80 and 125 keV/μm monoenergetic nitrogen ions in the presence and absence of 2 M DMSO. Results: Incubation of cells exposed to γ-rays, 40 keV/μm helium and 80 keV/μm N ions demonstrated that scavenging of OH radicals by DMSO removed most of the rapid repair component. The response to DMSO was less marked after 125 keV/μm nitrogen ions, where about half of the repair was resistant to DMSO. Conclusions: It is unlikely that the complexity of dsb is responsible for the slow repair because the removal of OH radicals did not make the breaks easier to repair. Instead, it is suggested that rapid and slow repair can be explained on the basis of how different parts of the chromatin are accessible to repair enzymes.

The effects of ethidium bromide (EtBr) on human lymphocytes were studied by the method of anomalous viscosity time dependence (AVTD) and by the comet assay. EtBr at low concentrations increased the maximum viscosity and time of radial migration as measured with AVTD at neutral conditions of lysis. A pronounced relaxation of DNA loops was observed with the neutral comet assay. The maximal comet length corresponded to 2 Mb DNA loops. At high concentrations of EtBr, 2. mg/ml, significant reduction in AVTD below control level was seen that suggested hypercondensation of chromatin. The hypercondensation was directly observed with the neutral comet assay. EtBr did not induce DNA strand breaks as measured by the alkaline comet assay. The hypercondensed nuclei could be decondensed by irradiation with gamma-rays or exposure to light. The data provide evidence that EtBr at high concentrations resulted in hypercondensation of chromatin below control level. The comet assay confirmed that the increase in AVTD peaks deals with relaxation of loops and AVTD decrease is caused by chromatin condensation. The prediction of the AVTD theory for a correlation between time of radial migration and condensation of chromatin was verified. Further, the data show that the comet assay at neutral conditions of lysis is rather sensitive to DNA loop relaxation in the absence of DNA damage. Finally, donor specificity was found for the hypercondensation.

The influence of hypothermia (2 degrees, 15 degrees and 28 degrees C) ripen the effect of X-irradiation on chromatin from human diploid fibroblast cells (AG1518) was studied using the fluorescent halo assay. Rewinding of supercoils was inhibited in a dose-dependent manner when cells were irradiated with 4, 8 or 16 Gy. This inhibition of rewinding was reduced when cells were irradiated at subnormal temperatures compared with cells irradiated at 37 degrees C. One hour's preincubation at low temperature did nor influence rewinding. When AG1518 cells were irradiated at 37 degrees C in the presence of the radical scavenger DMSO (0.5 M), the radiation-induced damage was reduced. No additional protection of DMSO in hypothermic cells (2 degrees C) was found, possibly indicating that OH-radical-mediated effects al-e more temperature dependent. These results are similar to those recently found for the malignant MCF-7 cell line.