Unrepaired cyclobutane pyrimidine dimers do not prevent proliferation of UV-B-irradiated cultured human fibroblasts

Mutagenic and carcinogenic UV-B radiation is known to damage DNA mostly through the formation of bipyrimidine photoproducts, including cyclobutane dimers (CPD) and (6-4) photoproducts ((6-4) PP). Using high-performance liquid chromatography coupled to tandem mass spectrometry, we investigated the formation and repair of thymine-thymine (TT) and thymine-cytosine (TC) CPD and (6-4) PP in the DNA of cultured human dermal fibroblasts. A major observation was that the rate of repair of the photoproducts did not depend on the identity of the modified pyrimidines. In addition, removal of CPD was found to significantly decrease with increasing applied UV-B dose, whereas (6-4) PP were efficiently repaired within less than 24 h, irrespective of the dose. As a result, a relatively large amount of CPD remained in the genome 48 h after the irradiation. Because the overall applied doses (<500 J m(-2)) were chosen to induce moderate cytotoxicity, fibroblasts could recover their proliferation capacities after transitory cell cycle arrest, as shown by 5-bromo-2'-deoxyuridine (BrdUrd) incorporation and flow cytometry analysis. It could thus be concluded that UV-B-irradiated cultured primary human fibroblasts normally proliferate 48 h after irradiation despite the presence of high levels of CPD in their genome. These observations emphasize the role of CPD in the mutagenic effects of UV-B.

Modelling the effect of incorporated halogenated pyrimidine on radiation-induced DNA strand breaks

PURPOSE

To estimate the enhancement of DNA strand breaks induced by low linear energy transfer (LET) radiation in the presence of halogenated pyrimidines and to examine complexity and clustering properties of damage that could provide a correlation between DNA damage and lethality.

MATERIALS AND METHODS

Monte Carlo track structure methods were used to model and estimate the induction of strand breakage by X-ray photons with and without the incorporated Br/I deoxyuridine in cell-mimetic conditions. The increase of DNA strand break induction was modelled by taking into account the direct energy deposition and the reactions of radicals. The yield and spectrum of strand breaks were calculated at various degrees of Br/IdU incorporation. The excess strand breaks due to Br/IdU incorporation was assumed to be induced by highly reactive uracilyl radicals. Four mechanisms were considered for the production of uracilyl radicals classified into three groups, by hydrated electrons, by direct energy deposition, and by both hydrated electrons and direct energy depositions. In total, nine different models were considered to test the excess strand breaks by incorporated Br/IdU assuming different pathways.

RESULTS

Model calculations show the following: the yield of strand breaks is enhanced by both the e(aq)(-) reaction and the direct energy deposition on base moiety; there is a significant contribution to the enhancement of yield of strand breaks due to energy transfer about four bases along the DNA to Br/IdU and DNA strand break complexity increases with degree of Br/IdU incorporation. Enhancement ratios of 1.8 and 2.5 for 40% Br/IdU substitution were obtained for single- and double-strand breaks, respectively.

CONCLUSIONS

The increase in the yield of strand breaks due to Br/IdU incorporation could be explained by the mechanism of uracilyl radical production by e(aq)(-) and direct energy deposition. The importance of energy transfer along the DNA is demonstrated. It is shown that the incorporation of Br/IdU causes a spectral shift towards a greater complexity of clustered DNA damage.

Introduction of a novel proliferation assay for pharmacological studies allowing the combination of BrdU detection and phenotyping

A recently developed technology for the non-enzymatic detection of the thymidine analog 5-bromo-2-deoxyuridine (BrdU) has been evaluated. In contrast to previous enzymatic approaches, Ultraviolet-Induced Detection (UVID) of halogenated pyrimidines allows for a mild detection procedure which enables the simultaneous detection of cellular markers and DNA-synthesis without enzyme-specific disadvantages. Superantigen-stimulated peripheral blood mononuclear cells (PBMNCs) have been treated with two different inhibitors of proliferation and the cell cycle of different lymphocyte subsets has been analysed. Both pentoxifylline (POF) and 2-methoxyestradiol (2ME2) exhibited strong antiproliferative activity, but led to distinctive changes in the cell cycle distribution. This study shows that the UVID technology is a simple and viable method which should find a wide range of applications in immunological and pharmacodynamic assays.

The effects of secondary structure and O2 on the formation of direct strand breaks upon UV irradiation of 5-bromodeoxyuridine-containing oligonucleotides

BACKGROUND

5-Bromodeoxyuridine is a radiosensitizing agent that is currently being evaluated in clinical trials as an adjuvant in the treatment of a variety of cancers. gamma-Radiolysis and UV irradiation of oligonucleotides containing 5-bromodeoxyuridine result in the formation of direct strand breaks at the 5'-adjacent nucleotide by oxidation of the respective deoxyribose. We investigated the effects of DNA secondary structure and O2 on the induction of direct strand breaks in 5-bromodeoxyuridine-containing oligonucleotides.

RESULTS

The efficiency of direct strand break formation in duplex DNA is dependent upon O2 and results in fragments containing 3'-phosphate and the labile 3'-ketodeoxyadenosine termini. The ratio of the 3'-termini is also dependent upon O2 and structure. Deuterium product isotope effects and tritium-transfer studies indicate that hydrogen-atom abstraction from the C1'- and C2'-positions occurs in an O2- and structure-dependent manner.

CONCLUSIONS

The reaction mechanisms by which DNA containing 5-bromodeoxyuridine is sensitized to damage by UV irradiation are dependent upon whether the substrate is hybridized and upon the presence or absence of O2. Oxygen reduces the efficiency of direct strand break formation in duplex DNA, but does not affect the overall strand damage. It is proposed that the sigma radical abstracts hydrogen atoms from the C1'- and C2'-positions of the 5'-adjacent deoxyribose moiety, whereas the nucleobase peroxyl radical selectively abstracts the C1'-hydrogen atom from this site. This is the second example of DNA damage amplification by a nucleobase peroxyl radical, and might be indicative of a general reaction pattern for this family of reactive intermediates.

Methods for registration of spontaneous DNA instability in mammalian cells

A phenomenon of spontaneous DNA instability displays itself as the low level of repair DNA synthesis that takes place during any cell cycle phases. However, there is a problem in detection of very low intensive repair DNA synthesis. This paper suggests two approaches to detect the spontaneous DNA instability. The first method involves a blockade of the DNA gaps sealing by a combination of inhibitors, hydroxyurea and arabinofuranosyl cytosine. An accumulation of single strand gaps leads to production of DNA double strand breaks and results to reproductive inactivation of cells. It was shown that registration of both these events by different methods (such as viscoelastometry of DNA, orthogonal pulse electrophoresis or comet assay for double strand breaks as well as effectiveness of colony growth for cell inactivation) may be used as suitable measure of the spontaneous DNA instability. The second approach bases on photolysis of bromodeoxyuridine incorporated into repair DNA patches during the spontaneous repair DNA synthesis. Long wave UV irradiation of cells containing bromodeoxyuridine labeled DNA stained with Hoechst 33342 causes their inactivation. Experimental results presented confirm that both methods actually detect the spontaneous DNA instability. It takes note of the spontaneous DNA instability varies for cells from different tissues and species and increases during aging.

Radiolytic products of bromodeoxyuridine in solids by 60Co gamma-rays and monoenergetic soft x-rays at the K-absorption edge of bromine

In order to investigate DNA damage due to Auger cascades in bromodeoxyuridine (BrdU), BrdU mixed with other nucleosides, as a model of DNA, was irradiated in solids by gamma-rays and monoenergetic x-rays at around the K-absorption edge of bromine (13.47 keV). The main products of BrdU were deoxyuridine produced through debromination, and bromouracil produced through the decomposition of a sugar group. The rates of the debromination and the nucleobases release of additives were markedly increased in the mixed sample. This observation indicated that the additives surrounding BrdU efficiently supplied protons and then decomposed. The major products by x-rays were the same as those by gamma-rays, indicating that Auger cascades in bromine atoms did not produce specific products. The production rates for all products from the mixed sample were about 2.5 times higher at 13.51 (above the K-absorption edge) keV than at 13.43-keV x-rays.

Photochemical production of double-strand breaks in cellular DNA

In a recent publication we described a novel route for the introduction of DNA double-strand breaks (DSBs) into cellular DNA. This involved the labelling of cellular DNA with bromodeoxyuridine (BrdU) and exposure to UVA light in the presence of Hoechst dye No. 33258. Here, we report an extension of that work to the use of iododeoxyuridine (IdU); cells substituted with known levels of IdU were subjected to a similar photolysis treatment and analyzed for strand breaks by elution assays. Results indicate that both single-strand breaks (SSBs) and DSBs depend linearly on the level of IdU substitution and fluence of UVA light. The yields of SSBs and DSBs were found to be 3.5 x 10(-5) and 9.5 x 10(-7)/IdU moiety/kJm-2, respectively. These results indicate that approximately 15-fold less SSBs and 5-fold less DSBs are produced per IdU than per BrdU moiety.

Biological effects of Auger processes of bromine on yeast cells induced by monochromatic synchrotron X-rays

The biological effects of inner-shell ionization in bromine atoms incorporated into DNA in the form of bromodeoxyuridine monophosphate (BrdUMP), induced by monochromatized synchrotron X-rays, were studied using a deoxythymidine monophosphate (dTMP)-permeable mutant of yeast, Saccharomyces cerevisiae. The BrdUMP-incorporated yeast cells were irradiated with monochromatic X-rays of 13.51 or 13.45 keV, between which the bromine K-absorption edge (13.47 keV) is located. The cells were 1.07 times more sensitive to irradiation by 13.51 keV X-rays than at 13.45 keV, while dTMP-incorporated cells did not show any difference in sensitivity. In the presence of a radioprotector during irradiation, BrdUMP-incorporated cells showed a larger enhancement (1.20). These enhancements observed in the bromine-incorporated cells cannot be explained only by an increase of the absorbed dose due to a substitution of CH3 group of thymine by bromine. It may be concluded that a major part of the enhancement was caused by inner-shell photoionization, followed by an Auger cascade of the bromine in the DNA. The quantum yield of lethality caused by the photoabsorption of bromine K-shell is not affected by the presence of cysteamine, suggesting the biological enhancement by the Auger processes may not be influenced by chemical protection.

Genetic analysis of resistance to total bromodeoxyuridine substitution in mammalian cell hybrids

Somatic cell hybrids derived from the fusion of Chinese hamster ovary cells (CHO) and mutant Syrian hamster melanoma cells (2E) were tested for their ability to grow with all of the thymidine (dThd) in their DNA replaced with 5-bromo-2'-deoxyuridine (BrdU), a phenotypic capability of the 2E cells but not of the CHO cells. Under these conditions, the 2E cells survived and grew, all of the hybrid clones survived and grew to varying degrees, and the CHO cells did not survive at all. When 2E cells were tested, they were also found to be resistant to the toxic effects of BrdU substitution and white light irradiation, relative to CHO cells. Thus, when the DNAs of 2E and CHO cells were equally (50%) substituted with BrdU, and the two cell lines irradiated with identical doses of white light, the survival of CHO cells was reduced to less than 1% of that of unirradiated cells, while 40% of the 2E cells survived. The 2E x CHO hybrid clones were found to survive at values from 10% to 40% under these identical conditions. Thus, the phenotypic characteristics of 2E cells involving total substitution and resistance to the toxic effects of BrdU substitution and white light irradiation appear to be expressed in a codominant fashion in somatic cell hybrids.

Ring opening photoreactions of 5-bromouracil and 5-bromo-2'-deoxyuridine with selected alkylamines

Several studies in the literature indicate that histones (lysine rich proteins found associated with DNA in eukaryotic chromatin), as well as poly-L-lysine, can be photocross-linked by ultraviolet (UV) light to DNA in which 5-bromo-2'-deoxyuridine has been substituted for thymidine. To gain some insight into the possible nature of this cross-linking, we have studied the photoreactions occurring in deoxygenated aqueous solutions containing 5-bromouracil (I) (BrUra) or 5-bromo-2'-deoxyuridine (III) (BrdUrd) and ethylamine, a lysine side chain analog. In the case of I this reaction produced the ring opened compound N-(N'-ethylcarbamoyl)-3-amino-2-bromoacrylamide (Ia). A small amount of N-(N'-ethylcarbamoyl)-3-ethylamino-2-bromoacrylamide (Ic) was also isolated. It was found that purified Ia, standing in the presence of ethylamine, was gradually converted to Ic in a dark reaction. The beta and alpha anomers of N-(N'-ethylcarbamoyl)-3-(2'deoxyribofuranos-1'-yl) amino-2-bromoacrylamide (IIIa and IIIb respectively) were isolated as products in the photoreaction of III with ethylamine; the alpha anomer was produced in a dark reaction from the beta anomer. The identity of these anomers was established by comparison of their proton NMR spectra with those of the four corresponding alpha and beta furanosyl and pyranosyl isomeric nucleosides of thymine, which are presented in the Appendix. A study was also made of the reaction of I with methylamine; a ring opened product analogous to Ia, viz. N-(N'-methylcarbamoyl)-3-amino-2-bromoacrylamide (IIa) was formed. A similar study with 5-bromo-1-methyluracil produced N-(N'-methylcarbamoyl)-3-methylamino-2-bromoacrylamide (IIc) as a product. Likewise, the reaction of 5-chlorouracil with ethylamine was studied and N-(N'-ethylcarbamoyl)-3-amino-2-chloroacrylamide (Ie), which is analogous in structure to Ia, was found to be produced. Structural identifications were made through use of UV spectroscopy, high resolution 1H-NMR spectroscopy, mass spectrometry and, in the case of Ia and IIa, 13C-NMR spectroscopy. In the BrUra and BrdUrd reaction systems, described above, dehalogenation reactions accounted for a major portion of the products. The yields of ring opened products, determined at pH 10, ranged from a high of 10.3% in the BrUra-ethylamine system to a low of 1.7% in the MeBrUra-methylamine system.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of UV irradiation on the fate of 5-bromodeoxyuridine-substituted bacteriophage T4 DNA

We have carried out a series of experiments designed to characterize the impact of UV irradiation (260 nm) on 5-bromodeoxyuridine-labeled (heavy) T4 bacteriophage, both before and after infection of Escherichia coli. In many respects, these effects differ greatly from those previously described for non-density-labeled (light) phage. Moreover, our results have led us to propose a model for a novel mechanism of host-mediated repair synthesis, in which excision of UV-damaged areas is followed by initiation of replication, strand displacement, and a considerable amount of DNA replication. UV irradiation of 5-bromodeoxyuridine-labeled phage results in single-stranded breaks in a linear, dose-dependent manner (1.3 to 1.5 breaks per genomic strand per lethal hit). This damage does not interfere with injection of the phage genome, but some of the UV-irradiated heavy phage DNA undergoes additional intracellular breakdown (also dose dependent). However, a minority (25%) of the injected parental DNA is protected, maintaining its preinjection size. This protected moiety is associated with a replicative complex of DNA and proteins, and is more efficiently replicated than is the parental DNA not so associated. Most of the progeny DNA is also found with the replicative complex. The 5-bromodeoxyuridine of heavy phage DNA is debrominated by UV irradiation, resulting in uracil which is removed by host uracil glycosylase. Unlike the simple gap-filling repair synthesis after infection with UV-irradiated light phage, the repair replication of UV-irradiated heavy phage is extensive as determined by density shift of the parental label in CsC1 gradients. The newly synthesized segments are covalently attached to the parental fragments. The repair replication takes place even in the presence of chloramphenicol, a protein synthesis inhibitor, suggesting it is host mediated. Furthermore, the extent of the repair replication is greater at higher doses of UV irradiation applied to the heavy phage. This abundant synthesis results ultimately in dispersion of the parental sequences as short stretches in the midst of long segments of newly synthesized progeny DNA. Together, the extensive replication and the resulting distribution pattern of parental sequences, without significant solubilization of parental label, are most consistent with a model of repair synthesis in which the leading strand displaces, rather than ligates to, the encountered 5' end.

DNA replication in Physarum polycephalum: UV photolysis of maturing 5-bromo-deoxyuridine substituted DNA

Combinations of 5-bromodeoxyuridine (BrdUrd) and 3H-deoxyadenosine (3H-DAdo) short pulses were given in the synchronous DNA-replication period of Physarum polycephalum. After a chase period, UV-photolysis products were analyzed on alkaline sucrose gradients. This strategy has allowed the following conclusions. a) at the time of master-initiation of DNA replication, points separated by 1.1-2.2x10(7) daltons of single strand DNA may initiate DNA synthesis. b) among these, only selected groups of replicons actually proceed in DNA replication at this time, while others appear to hold (later temporal sets of replicons). The origins of the ones that proceed in replication are separated from each other by a distance corresponding to 1.1-2.x10(7) daltons. c) regions in actual replication are separated from each other by increasing distances (up to 1.5x10(8) daltons single strand DNA) at later times in S.

Neoplastic transformation induced by a direct perturbation of DNA

Treatment of Syrian hamster embryo cells with 5-bromodeoxyuridine followed by near ultraviolet irradiation results in neoplastic transformation of these cells. This demonstrates that a direct perturbation of DNA is sufficient to initiate neoplastic transformation.

A C5'-centred deoxyribose radical in single crystals of 5-chloro-and 5-bromodeoxyuridine X-irradiated at low temperatures

X-irradiation at 77 K and subsequent warming to 150 K of single crystals of 5-chloro-and 5-bromodeoxyuridine produces a radical located at position C5' of the deoxyribose moiety. The radical exhibits identical spectral properties in both crystal systems. It is characterized by interaction of the unpaired electron with an alpha-proton(-17-0, -8-7, -33-7G), a beta-proton (18-6, 21-3, 17-5 G) as well as an OH-proton (4-8, 7-9, 12-8 G). The principal values of the g-tensor are 2-0034, 2-0049 and 2-0042. The spectral parameters are discussed in relation to those of the same or similar radicals in other nucleosides (-tides).

Repair replication in Escherichia coli as measured by the photolysis of bromodeoxyuridine

The ability to selectively photolyze bromouracil-(BrUra-)containing repaired regions in cellular DNA has allowed us to estimate the average size of repaired regions in ultraviolet (UV) light-irradiated Escherichia coli. Cells were labeled with thymidine-(3)H, irradiated at 254 nm, and incubated in nonradioactive bromodeoxyuridine (BrdUrd). After incubation the cells were exposed to 10(6) ergs.mm(-2) at 313 nm, lysed, and sedimented in alkaline sucrose gradients so as to measure the average molecular weight of single DNA strands. In strains that had excised approximately 45 cyclobutane pyrimidine dimers/10(8) daltons, the 313 nm treatment resulted in approximately 6 single-strand breaks/10(8) daltons. In an excisionless strain, the same treatment resulted in only 1.5 breaks/10(8) daltons. From the determination of the sensitivities of fully substituted DNAs to 313 nm light, we calculate that the repaired regions in excising strains of E. coli contain an average of 4-6 BrUra residues. Photoreactivation experiments indicate that the excision of pyrimidine dimers in the presence of BrdUrd is the primary source of repaired regions selectively photolyzed by 313 nm radiation.