X-ray sensitization of chromatids with unifilarly and bifilary substituted DNA

Radiosensitization of murine normoblasts in vivo by bromodeoxyuridine to the genotoxicity and cytotoxicity of the bone-seeking radiopharmaceutical 153Sm-EDTMP

Abstract To establish a basis for a possible strategy for bone marrow ablation or therapy, we examined the effect of bromodeoxyuridine (BrdU) incorporation into DNA on the genotoxic and cytotoxic effects of samarium-153 ethylenediaminetetramethylene phosphonate ((153)Sm-EDTMP) in normoblasts in vivo. Cytotoxicity and genotoxicity were established by time-response curves of polychromatic erythrocyte (PCE) and micronucleated polychromatic erythrocyte (MN-PCE) frequencies, respectively, in mouse peripheral blood samples. The group treated with (153)Sm-EDTMP showed a clear induction of MN-PCEs; however, the group treated with BrdU plus (153)Sm-EDTMP paradoxically showed only a slight increase with respect to untreated controls. Treatment with (53)Sm-EDTMP caused a small reduction in PCE frequency, but exposure to BrdU or to BrdU plus (53)Sm-EDTMP reduced the PCE frequency significantly from 32 h to the end of the experiment. The PCE frequencies in the BrdU plus (53)Sm-EDTMP group were significantly lower than in the BrdU control group at the final time and were much lower than the group treated with only (53)Sm-EDTMP, which returned to basal values. The results suggest the radioinduction of a lethal lesion in BrdU-substituted DNA that cannot be repaired easily and does not permit cell division and micronucleus formation.

Enhancement of chromosome aberrations by the combination of DNA substitution with halogenated deoxyuridine and streptonigrin treatments

We treated CHO cells with streptonigrin (SN) alone, in combination with BrdUrd or IdUrd substitution, and with or without the addition of caffeine. The cells assessed for chromosome damage by SN were in the G2 period and the magnitude of the damage was expressed as monosubstituted chromatid breaks, bisubstituted chromatid breaks and boundary regions breaks (boundary regions indicate the point of exchange of mono- and bisubstituted chromatids). We found that the combination of BrdUrd or IdUrd substitution with SN treatments produced a remarkable increase in the frequency of breaks over the frequencies observed with the halogenated compound only. The effect was more evident with IdUrd than with BrdUrd, and more dramatic in bisubstituted than in monosubstituted chromatids. The frequency of boundary breaks in cells treated with BrdUrd plus SN was similar to the frequency of breaks in monosubstituted chromatids treated similarly. Conversely, the damage in boundary regions was almost similar to that in bisubstituted chromatids in cells challenged with IdUrd plus SN. The addition of caffeine to BrdUrd-substituted chromosomes gave rise to a marked enhancement of breakages with a gradient of chromatid damage that was: bisubstituted > monosubstituted > boundary regions. A further increase of chromatin breaks maintaining the gradient indicated above was obtained when the cells were treated with BrdUrd plus SN plus caffeine. We propose that BrdUrd and IdUrd substitution alone or in combination with caffeine treatments and with SN in its capacity to bind DNA, give rise to different chromatin structures capable of modulating the DNA damage induced along the chromatin fibril by the active oxygen species liberated by SN-DNA complexes.

Divergent patterns of incorporation of bromodeoxyuridine and iododeoxyuridine in human colorectal tumor cell lines

Using a panel of four human colorectal tumor (HCT) cell lines, we have quantitatively characterized the incorporation of bromodeoxyuridine (BrdUrd) and iododeoxyuridine (IdUrd) into DNA, both as individual agents and in combination with fluoropyrimidines. The intrinsic ability of these cell lines to incorporate BrdUrd, as reflected by the concentration required to achieve half-maximal incorporation, varied almost 4-fold across this panel, from 1.6 microM for HuTu80 cells to 6.1 microM for HT29 cells. Three of the four cell lines (HT29, SW480, SW620) responded to fluoropyrimidines as expected, displaying 100-150% increases in BrdUrd incorporation when combined with growth inhibitory concentrations of fluorouracil (FUra). In contrast, neither FUra nor fluorodeoxyuridine (FdUrd) was able to increase BrdUrd incorporation in HuTu80 cells by more than 25%, even in the presence of 100 microM leucovorin. IdUrd incorporation was modulated to a substantially higher degree in both HT29 and HuTu80 cell lines. Finally we demonstrate the feasibility of a technique for evaluating the net effect of fluoropyrimidine treatments on de novo thymidine nucleotide production in a single specimen, using a combination of normotopic and stable-isotope labeled BrdUrd. We propose that this approach may be useful in evaluating the response of an individual tumor to fluoropyrimidines in vivo.

Influence of treatment to sacrifice time and the presence of BrdUrd on chemically-induced aberration rates in mouse marrow cells

4 chemicals, with various modes of clastogenic action were used to evaluate induced chromosomal aberrations in mouse bone marrow at different times after intraperitoneal injection. Aberration frequencies induced by mitomycin C, cyclophosphamide and dimethylbenz[a]anthracene increased with increasing time between treatment and sampling until those time points (approximately 18 h) when significant proportions of second-division metaphases were among the cells being scored; this increase was not obvious following treatment with 4-nitroquinoline 1-oxide. When BrdUrd tablets were implanted prior to treatment and scoring was restricted to first-division metaphases, aberration rates continued to increase for as long as 24 h post-treatment. The presence of BrdUrd did not affect significantly the rate of aberration induction by the chemicals. Our data indicate that the sensitivity of the in vivo mouse marrow assay for clastogenic chemicals can be greatly increased by utilizing BrdUrd to insure the scoring of only first-division metaphases at post-treatment times of approx. 18 h.

Enhancement of radiation damage in cellular DNA following unifilar substitution with iododeoxyuridine

The exact mechanism of x-ray radiosensitization with iododeoxyuridine (IdUrd) is not known. Using log phase V79 cells, we compared radiosensitization following unifilar (9 hr exposure) and bifilar (17 hr exposure) substitution with 10(-5) M IdUrd. The % thymidine replacement was 8% and 16%, respectively. Significant radiosensitization was found with unifilar (Do = 1.35 Gy, n = 5.8) and bifilar substitution (Do = 1.2 Gy, n = 4.0) compared to controls (Do = 1.7 Gy, n = 8.5). Using filter elution techniques, the enhancement ratios (ER) for double strand breaks (SB) were 1.5 and 2.0 for unifilar and bifilar substitution, respectively, whereas the enhancement ratio for single strand breaks were both greater than or equal to 2.1. Comparing the enhancement ratios with unifilar substitution for single strand breaks in substituted (2.4) and unsubstituted complement strands (1.9) as well as in unsubstituted duplex DNA (1.4) in cells containing IdUrd substituted DNA, there was increased damage in unsubstituted strands compared to irradiated controls. These data suggest that in vitro radiosensitization following unifilar substitution with IdUrd results, in part, from damage to unsubstituted complementary strand and adjacent doubly unsubstituted DNA. The radiolysis of halogenated DNA produces mobile reactive intermediates which may cause intermolecular DNA damage such as single strand breaks in unsubstituted DNA and resultant double strand breaks and this damage is probably responsible for increased lethality after x-irradiation.

Response of human HT-29 colorectal tumor cells to extended exposure to bromodeoxyuridine

Effects of the extended exposure of a human colorectal tumor-cell line (HT-29) to bromodeoxyuridine (BrdUrd) were studied in anticipation of the clinical use of that agent to treat colorectal cancer, particularly as a regionally delivered radiosensitizer. We found that 72-h exposure to a concentration of BrdUrd that is estimated to be locally maintained in the liver (100 microM) was significantly cytotoxic with a 3-log reduction in survival. As measured by GC/MS-SIM method, incorporation of BrdUrd into DNA followed an unexpected time course in that continuous exposure to 10 microM BrdUrd resulted in maximal incorporation at 3 days, after which the extent of incorporated analog fell significantly (despite daily changes of the medium). This finding was apparently due to a greater rate of loss of BrdUrd from the medium at later time points. Flow cytometric analysis using an anti-BrdUrd antibody (IU-4) revealed that antibody binding also peaked and fell off with time. However, at exposure times of greater than 24 h, the timing and extent of this decline were significantly different than had been indicated by the GC/MS method. These results indicate that the quantitative relationship between antibody staining and BrdUrd incorporation changes as drug-exposure time increases and that quantitative studies of anti-BrdUrd antibody binding must be interpreted with caution, especially when extended drug-treatment protocols have been used.

Analysis of bromodeoxyuridine incorporation into DNA: comparison of gas chromatographic/mass spectrometric, CsCl gradient sedimentation, and specific radioactivity methods

A sensitive new method for the quantitation of 5-bromodeoxyuridine (BrdUrd) incorporated into DNA by GC/MS analysis of enzymatically released Thy and bromouracil (BrUra) is presented. The hydrolysis procedure was characterized and found to give uniform results when sample size was 1-10 micrograms DNA and incubation time for DNA digestion was between 40 min and 16 h. Samples of DNA containing 3H-labeled BrdUrd were analyzed in parallel by the GC/MS technique and by specific radioactivity and buoyant density measurements, in order to compare the three methods. The GC/MS procedure gave values for percentage replacement of Thy by BrUra which were higher than those obtained by specific activity and lower than those obtained by buoyant density. This GC/MS method can detect 1% replacement in a 1-microgram DNA sample, equivalent to approximately 10(5) cells or 0.1 mg tissue, and will permit sensitive and quantitative analysis of the presence of this chemotherapeutic agent/radiosensitizer in cellular DNA from biopsy samples of normal or tumor tissue.

The production of chromosome aberration in Chinese hamster fibroblasts exposed to 24 keV neutrons

A filtered reactor beam, consisting mainly of 24 keV neutrons, was used to study the induction of chromosome aberrations in the V79/4(AH1) Chinese hamster cell line. The yields of both dicentrics and acentrics were linear with dose and the value of relative biological effectiveness (RBE) for dicentrics at low doses was 6.5 +/- 1.4. This value was similar to that found previously for a neutron spectrum with mean energy 2.1 MeV, and suggests that the RBE of neutrons does not increase to very high values in the energy region below 100 keV. This result does not support the suggestions of Davy (1969) and Key (1971) that the neutron RBE rises to very high values in the intermediate energy range.

Incorporated bromodeoxyuridine enhances the sister-chromatid exchange and chromosomal aberration frequencies in an EMS-sensitive Chinese hamster cell line

The mutant Chinese hamster cell line, EM9, is characterized by a high baseline sister-chromatid exchange (SCE) frequency, increased sensitivity to cell killing, and a defect in DNA strand-break repair. The molecular basis for this pleotrophic phenotype is not known. We examined, at the chromosomal level, the increased sensitivity of this mutant to incorporated BrdUrd. By varying the amount of BrdUrd in template DNA and measuring the frequency of SCEs and chromosomal aberrations, we demonstrated the enhanced sensitivity of EM9 to BrdUrd present in the template strand of DNA. Our results show that a 6-fold increase in SCEs occurs due to DNA replication over a BrdUrd-substituted template relative to a dThd-substituted template. With regard to aberration production in EM9, there is a significant enhancement of aberrations and a specific bias toward damage for the chromatid with Brdurd in the template strand. While these cells share some phenotypic properties with cells from patients with Bloom's syndrome, the genotypic similarities have not yet been established.

The induction of chromosome exchange aberrations by carbon ultrasoft X-rays in V79 hamster cells

V79 hamster cells in plateau (extended G1) phase were irradiated with either 250 kV ('hard') X-rays or carbon K characteristic ultrasoft X-rays under conditions minimizing cell overlap. These cells were killed most effectively by the carbon X-rays, by a factor of about 3 relative to hard X-rays, in agreement with our previous findings with cells in exponential growth. Chromosome-type aberrations were measured at 3 fixation times within the first division cycle after irradiation, and an approximately uniform sensitivity to aberration induction was found for both radiations. The combined aberration data show that carbon X-rays are 2 or more times as effective as hard X-rays, depending on dose and/or data fit. Exchange aberrations require recombination between two separate chromosomes, but they are induced efficiently by carbon X-rays with a substantial linear component to the dose-response despite the very short electron tracks (approximately less than 7 nm) that they produce in the cell. This implies either that the participating DNA helices must be lying extremely close together at the time of radiation damage, so that one track can effectively damage both helices, or that only one radiation-damaged chromosome is needed to promote an exchange event.

Influence of incorporated bromodeoxyuridine on the induction of chromosomal alterations by ionizing radiation and long-wave UV in CHO cells

Incorporation of BrdUrd into nuclear DNA sensitizes CHO cells (1) to the induction of chromosomal aberrations by X-rays and 0.5 MeV neutrons and (2) to induction of chromosomal aberrations and SCEs by lw-UV. We have attempted to establish a correlation between induced chromosomal alterations and induced single- or double-strand breaks in DNA. The data show that while DSBs correlate very well with X-ray-induced aberrations, no clear correlation could be established between lw-UV induced SSBs (including alkali-labile sites) and chromosomal alterations. In addition the effect of 3-aminobenzamide (3AB) on the induction of chromosomal aberrations and SCEs induced by lw-UV has been determined. It is shown that 3AB is without any effect when lw-UV-irradiated cells are posttreated with this inhibitor. The significance of these results is discussed.

Effect of 5-bromodeoxyuridine substitution on sister chromatid exchange induction by chemicals

The fluorescence-plus-Giemsa (FPG) technique for analysis of sister chromatid exchange (SCE) is widely used as an assay for mutagenic carcinogens. There is very little information, however, on whether incorporation of the bromodeoxyuridine (BrdU) necessary for visualization of SCEs affects the sensitivity of the SCE test system to different chemical agents. We have investigated the effect of BrdU incorporation on SCE induction by labeling cells with BrdU for either the first cell cycle or the first and second cell cycles. The cells were then treated with bleomycin, which produces DNA strand breakage; proflavine, which intercalates into DNA; mitomycin C, which produces monoadducts and DNA crosslinks; or aphidicolin, which inhibits DNA polymerase alpha. Chemicals were added before BrdU exposure or during the first, second, or both cell cycles. Only mitomycin C, which induces long-lived lesions, elevated the SCE frequency when cells were treated before BrdU labeling. When bleomycin, proflavine, or mitomycin C was present concurrently with BrdU, the frequency of SCEs was increased independently of the BrdU labeling protocol. Aphidicolin, on the other hand, induced more SCEs when present for the second cell cycle, when DNA replicates on a template DNA strand containing BrdU. We also examined the induction of SCEs in the first cell cycle (twins) and in the second cell cycle (singles) after continuous treatment of cells with BrdU and the test chemicals. Only aphidicolin increased SCE frequency in the second cell cycle. These results indicate that aphidicolin, but not bleomycin, proflavine, or mitomycin C, affects BrdU-substituted DNA and unsubstituted DNA differently. This type of interaction should be taken into consideration when the SCE test is used as an assay system.

Kinetics of induction of sister-chromatid exchanges by X-rays through two cell cycles

V79 Chinese hamster cells were exposed to X-rays at various times through the two cell cycles required to obtain harlequin-stained chromosomes. A two-fold SCE enhancement was found between the first and the second G1 phase when BrdUrd was incorporated during the first S phase only. This BrdUrd effect was not found when MNNG was used. Furthermore, the kinetics of SCE and aberrations were different, suggesting two separate mechanisms for their formation: SCE activity takes place when DNA damage occurs before the DNA replication, and aberration activity when the DNA damage occurs chiefly after the DNA replication.