Activity of Escherichia coli DNA-glycosylases on DNA damaged by methylating and ethylating agents and influence of 3-substituted adenine derivatives

Bee venom genotoxicity on Saccharomyces cerevisiae cells - The role of mitochondria and YAP1 transcription factor

The present work aims to clarify the genotype differences of a model organism Saccharomyces cerevisiae in response to bee venom. The study evaluated various endpoints including cell survival, induction of physiologically active superoxide anions, mitotic gene conversion, mitotic crossing-over, reverse mutations, DNA double-strand breaks, and Ty1 retrotransposition. The role of the intact mitochondria and the YAP1 transcription factor was also evaluated. Our results indicate a genotype-specific response. The first experimental evidence has been provided that bee venom induces physiologically active superoxide anions and DNA double-strand breaks in S. cerevisiae. The lack of oxidative phosphorylation due to disrupted or missing mitochondrial DNA reduces but not diminishes the cytotoxicity of bee venom. The possible modes of action could be considered direct damage to membranes (cytotoxic effect) and indirect damage to DNA through oxidative stress (genotoxic effect). YAP1 transcription factor was not found to be directly involved in cell defense against bee venom treatment.

Biological toxicity risk assessment of two potential neutral carbon diesel fuel substitutes

We investigated the biological response of soluble organic fraction (SOF) and water-soluble fraction (WSF) extracted from particulate matter (PM) emitted by an automotive diesel engine operating in a representative urban driving condition. The engine was fueled with ultra-low sulfur diesel (ULSD), and its binary blends by volume with 13% of butanol (Bu13), and with hydrotreated vegetable oil (HVO) at 13% (HVO13) and 20% (HVO20). Cytotoxicity, genotoxicity, oxidative DNA damage and ecotoxicity tests were carried out, and 16 polycyclic aromatic hydrocarbons (PAH) expressed as tbenzo(a)pyrene total toxicity equivalent (BaP-TEQ) were also analyzed. The Hepatocarcinoma epithelial cell line (HepG2) was exposed to SOF for 24 h and analyzed using comet assay, with the inclusion of formamidopyrimidine DNA glycosylase (FPG) and endonuclease III (Endo III) to recognize oxidized DNA bases. The WSF was evaluated through acute ecotoxicity tests with the aquatic microcrustacean Daphnia pulex (D. Pulex). Results showed that there was no cytotoxic activity for all tested SOF concentrations. Genotoxic responses by all the SOF samples were at same level, except for the HVO13 which was weaker in the absence of the enzymes. The addition of the FPG and Endo III enzymes resulted in a significant increase in the comet tail, indicating that the DNA damage from SOF for all tested fuel blends involves oxidative damage including a higher level of oxidized purines for ULSD and Bu13 in comparison with HVO blends, but the oxidized pyrimidines for HVO blends were slightly higher compared to Bu13. The WSF did not show acute ecotoxicity for any of the fuels. Unlike other samples, Bu13-derived particles significantly increase the BaP-TEQ. The contribution to the genotoxic activity and oxidative DNA from SOF was not correlated to BaP-TEQ, which means that the biological activity of PM might be affected also by other toxic compounds present in particulate phase.

Ruthenium(IV) Complexes as Potential Inhibitors of Bacterial Biofilm Formation

With increasing antimicrobial resistance there is an urgent need for new strategies to control harmful biofilms. In this study, we have investigated the possibility of utilizing ruthenium(IV) complexes (H₃O)₂(HL1)₂[RuCl₆]·2Cl·2EtOH (1) and [RuCl₄(CH₃CN)₂](L³2)·H₂O (2) (where L1-2-hydroxymethylbenzimadazole, L³2-1,4-dihydroquinoxaline-2,3-dione) as effective inhibitors for biofilms formation. The biological activities of the compounds were explored using E. coli, S. aureus, P. aeruginosa PAO1, and P. aeruginosa LES B58. The new chloride ruthenium complexes were characterized by single-crystal X-ray diffraction analysis, Hirshfeld surface analysis, FT-IR, UV-Vis, magnetic and electrochemical (CV, DPV) measurements, and solution conductivity. In the obtained complexes, the ruthenium(IV) ions possess an octahedral environment. The intermolecular classical and rare weak hydrogen bonds, and π···π stacking interactions significantly contribute to structure stabilization, leading to the formation of a supramolecular assembly. The microbiological tests have shown complex 1 exhibited a slightly higher anti-biofilm activity than that of compound 2. Interestingly, electrochemical studies have allowed us to determine the relationship between the oxidizing properties of complexes and their biological activity. Probably the mechanism of action of 1 and 2 is associated with generating a cellular response similar to oxidative stress in bacterial cells.

Biological effects and bioaccumulation of gold in gilthead seabream (Sparus aurata) - Nano versus ionic form

The question of whether gold (Au) is more toxic as nanoparticles or in its ionic form remains unclear and controversial. The present work aimed to clarify the effects of 96 h exposure to 4, 80 and 1600 μg·L^-1 of 7 nm gold nanoparticles (AuNPs) - (citrate coated (cAuNPs) or polyvinylpyrrolidone coated (PVP-AuNPs)) - and ionic Au (iAu) on gilthead seabream (Sparus aurata). Effects at different levels of biological organization (behaviour, neurotransmission, biotransformation, oxidative stress/damage and genotoxicity) were assessed. cAuNPs induced oxidative stress and damage (lipid peroxidation increase), even at 4 μg·L^-1, and reduced the ability of S. aurata to swim against a water flow at 1600 μg·L^-1. Exposure to cAuNPs induced more adverse effects than exposure to PVP-AuNPs. All tested concentrations of Au (nano or ionic form) induced DNA breaks and cytogenetic damage in erythrocytes of S. aurata. Generally, iAu induced significantly more effects in fish than the nano form, probably associated with the significantly higher accumulation in the fish tissues. No fish mortality was observed following exposure to AuNPs, but mortality was observed in the group exposed to 1600 μg·L^-1 of iAu.

Inhibitors of DNA Glycosylases as Prospective Drugs

DNA glycosylases are enzymes that initiate the base excision repair pathway, a major biochemical process that protects the genomes of all living organisms from intrinsically and environmentally inflicted damage. Recently, base excision repair inhibition proved to be a viable strategy for the therapy of tumors that have lost alternative repair pathways, such as BRCA-deficient cancers sensitive to poly(ADP-ribose)polymerase inhibition. However, drugs targeting DNA glycosylases are still in development and so far have not advanced to clinical trials. In this review, we cover the attempts to validate DNA glycosylases as suitable targets for inhibition in the pharmacological treatment of cancer, neurodegenerative diseases, chronic inflammation, bacterial and viral infections. We discuss the glycosylase inhibitors described so far and survey the advances in the assays for DNA glycosylase reactions that may be used to screen pharmacological libraries for new active compounds.

Thiopurine Derivative-Induced Fpg/Nei DNA Glycosylase Inhibition: Structural, Dynamic and Functional Insights

DNA glycosylases are emerging as relevant pharmacological targets in inflammation, cancer and neurodegenerative diseases. Consequently, the search for inhibitors of these enzymes has become a very active research field. As a continuation of previous work that showed that 2-thioxanthine (2TX) is an irreversible inhibitor of zinc finger (ZnF)-containing Fpg/Nei DNA glycosylases, we designed and synthesized a mini-library of 2TX-derivatives (TXn) and evaluated their ability to inhibit Fpg/Nei enzymes. Among forty compounds, four TXn were better inhibitors than 2TX for Fpg. Unexpectedly, but very interestingly, two dithiolated derivatives more selectively and efficiently inhibit the zincless finger (ZnLF)-containing enzymes (human and mimivirus Neil1 DNA glycosylases hNeil1 and MvNei1, respectively). By combining chemistry, biochemistry, mass spectrometry, blind and flexible docking and X-ray structure analysis, we localized new TXn binding sites on Fpg/Nei enzymes. This endeavor allowed us to decipher at the atomic level the mode of action for the best TXn inhibitors on the ZnF-containing enzymes. We discovered an original inhibition mechanism for the ZnLF-containing Fpg/Nei DNA glycosylases by disulfide cyclic trimeric forms of dithiopurines. This work paves the way for the design and synthesis of a new structural class of inhibitors for selective pharmacological targeting of hNeil1 in cancer and neurodegenerative diseases.

Vitamin D Supplementation Reduces Both Oxidative DNA Damage and Insulin Resistance in the Elderly with Metabolic Disorders

Background: Research evidence indicates that vitamin D deficiency is involved in the pathogenesis of insulin resistance (IR) and associated metabolic disorders including hyperglycemia and dyslipidemia. It also suggested that vitamin D deficiency is associated with elevated levels of oxidative stress and its complications. Therefore, the aim of our study was to determine the effect of vitamin D supplementation on DNA damage and metabolic parameters in vitamin D deficient individuals aged >45 with metabolic disorders. Material and Methods: Of 98 initially screened participants, 92 subjects deficient in vitamin D were included in the study. They were randomly assigned to the following group: with vitamin D supplementation (intervention group, n = 48) and without supplementation (comparative group, n = 44). The patients from both groups were divided into two subgroups according to the presence or absence of type 2 diabetes (T2DM). The intervention group was treated with 2000 International Unit (IU) cholecalciferol/day between October and March for three months. At baseline and after three-month supplementation vitamin D concentration (25-OH)D3 and endogenous and oxidative DNA damage were determined. In addition, fast plasma glucose (FPG), fasting insulin, HbA1c and lipid fraction (total cholesterol (TC), low-density lipoprotein cholesterol (LDL), high-density lipoprotein cholesterol (HDL), triglyceride (TG)), as well as anthropometric measurements (weight, height) were gathered. The following IR-related parameters were calculated Homeostatic Model Assesment – Insulin Resistance (HOMA-IR) and TG/HDL ratio. Results: Three-month vitamin D supplementation increased the mean vitamin D concentration to generally accepted physiological level independently of T2DM presence. Importantly, vitamin D exposure decreased the level of oxidative DNA damage in lymphocytes of patients of intervention group. Among studied metabolic parameters, vitamin D markedly increased HDL level, decreased HOMA-IR, TG/HDL ratio. Furthermore, we found that HbA1c percentage diminished about 0.5% in T2DM patients supplemented with vitamin D. Conclusion: The current study demonstrated that daily 2000I U intake of vitamin D for three months decreased the level of oxidative DNA damage, a marker of oxidative stress, independently on T2DM presence. Furthermore, vitamin D reduced metabolic parameters connected with IR and improved glucose and lipid metabolism. Therefore, our results support the assertion that vitamin D, by reducing oxidative stress and improving of metabolic profile, may decrease IR and related diseases.

Histone tails decrease N7-methyl-2'-deoxyguanosine depurination and yield DNA-protein cross-links in nucleosome core particles and cells

Monofunctional alkylating agents preferentially react at the N7 position of 2'-deoxyguanosine in duplex DNA. Methylated DNA, such as that produced by methyl methanesulfonate (MMS) and temozolomide, exists for days in organisms. The predominant consequence of N7-methyl-2'-deoxyguanosine (MdG) is widely believed to be abasic site (AP) formation via hydrolysis, a process that is slow in free DNA. Examination of MdG reactivity within nucleosome core particles (NCPs) provided two general observations. MdG depurination rate constants are reduced in NCPs compared with when the identical DNA sequence is free in solution. The magnitude of the decrease correlates with proximity to the positively charged histone tails, and experiments in NCPs containing histone variants reveal that positively charged amino acids are responsible for the decreased rate of abasic site formation from MdG. In addition, the lysine-rich histone tails form DNA-protein cross-links (DPCs) with MdG. Cross-link formation is reversible and is ascribed to nucleophilic attack at the C8 position of MdG. DPC and retarded abasic site formation are observed in NCPs randomly damaged by MMS, indicating that these are general processes. Histone-MdG cross-links were also detected by mass spectrometry in chromatin isolated from V79 Chinese hamster lung cells treated with MMS. The formation of DPCs following damage by a monofunctional alkylating agent has not been reported previously. These observations reveal the possibility that such DPCs may contribute to the cytotoxicity of monofunctional alkylating agents, such as MMS, N-methyl-N-nitrosourea, and temozolomide.

Using the comet assay and lysis conditions to characterize DNA lesions from the acrylamide metabolite glycidamide

The alkaline comet assay and a cell-free system were used to characterise DNA lesions induced by treatment with glycidamide (GA), a metabolite of the food contaminant acrylamide. DNA lesions induced by GA were sensitively detected when the formamidopyrimidine-DNA-glycosylase (Fpg) enzyme was included in the comet assay. We used LC-MS to characterise modified bases from GA-treated naked DNA with and without subsequent Fpg treatment. N7-GA-Guanine and N3-GA-Adenine aglycons were detected in the supernatant showing some depurination of adducted bases; treatment of naked DNA with Fpg revealed no further increase in the adduct yield nor occurrence of other adducted nucleobases. We treated human lymphocytes with GA and found large differences in DNA lesion levels detected with Fpg, depending on the duration and the pH of the lysis step. These lysis-dependent variations in GA-induced Fpg sensitive sites paralleled those observed after treatment of cells with methyl methane sulfonate (MMS). On the other hand, oxidative lesions (8-oxoGuanine) induced by a photoactive compound (Ro 12-9786) plus light, and also DNA strand breaks induced by X-rays, were detected largely independently of the lysis conditions. The results suggest that the GA-induced lesions are predominantly N7-GA-dG adducts slowly undergoing imidazole ring opening at pH 10 as in the standard lysis procedure; such structures are substrate for Fpg leading to strand breaks. The data suggest that the characteristic alkaline lysis dependence of some DNA lesions may be used to study specific types of DNA modifications. The comet assay is increasingly used in regulatory testing of chemicals; in this context, lysis-dependent variations represent a novel approach to obtain insight in the molecular nature of a genotoxic insult.

N6-(2-Deoxy-d- erythro-pentofuranosyl)-2,6-diamino-3,4-dihydro-4-oxo-5- N-(2-hydroxy-3-buten-1-yl)-formamidopyrimidine Adducts of 1,3-Butadiene: Synthesis, Structural Identification, and Detection in Human Cells

1,3-Butadiene (BD) is an environmental and occupational toxicant classified as a human carcinogen. BD is metabolically activated by cytochrome P450 monooxygenases to 3,4-epoxy-1-butene (EB), which alkylates DNA to form a range of nucleobase adducts. Among these, the most abundant are the hydrolytically labile N7-guanine adducts such as N7-(2-hydroxy-3-buten-1-yl)-guanine (N7-EB-dG). We now report that N7-EB-dG can be converted to the corresponding ring open N⁶-(2-deoxy-d- erythro-pentofuranosyl)-2,6-diamino-3,4-dihydro-4-oxo-5- N-(2-hydroxy-3-buten-1-yl)-formamidopyrimidine (EB-Fapy-dG) adducts. EB-Fapy-dG lesions were detected in EB-treated calf thymus DNA and in EB-treated mammalian cells using quantitative isotope dilution nanoLC-ESI⁺-MS/MS. EB-Fapy-dG adduct formation in EB-treated calf thymus DNA was concentration dependent and was greatly accelerated at an increased pH. EB-FAPy-dG adduct amounts were 2-fold higher in base excision repair-deficient NEIL1^-/- mouse embryonic fibroblasts (MEF) as compared to isogenic controls (NEIL1^+/+), suggesting that this lesion may be a substrate for NEIL1. Furthermore, NEIL1^-/- cells were sensitized to EB treatment as compared to NEIL1^+/+ fibroblasts. Overall, our results indicate that ring-opened EB-FAPy-dG adducts form under physiological conditions, prompting future studies to determine their contributions to genotoxicity and mutagenicity of BD.

In-vivo assesment of the genotoxic and oxidative stress effects of particulate matter on Echinogammarus veneris

Seven types of atmospheric dusts (road dust, soil dust, brake dust, desert dust, pellet ash and coke and certified material NIST1648a - urban dust) have been tested for their genotoxicity on specimens of Echinogammarus veneris, a small aquatic amphipod. Experiments were carried out in vivo, by exposing the animals for 24 h to water containing 25 mg/L of dust. Each dust has been chemically analyzed for ions, elemental carbon, organic carbon and for the soluble and insoluble fractions of elements. Non-specific damages to DNA have been evaluated by the comet test, while oxidative damages have been estimated by coupling the comet test with formamido pyrimidine DNA glycosylase reaction. The animal tissues have been acid digested and analyzed for their elemental content to evaluate the bioaccumulation. All the considered dusts have caused a significant non-specific DNA damage, while the oxidative stress was shown only by dust types containing high concentration of elements. Furthermore, the oxidative damage has shown a positive correlation with the total bio-accumulated elemental concentration. For all the dust samples, the correlation with bio-accumulation in the tissues was more satisfactory for the insoluble fraction than for the soluble fraction of elements. Elements contained in solid particles seem then to be the main responsible bioaccumulation and for the oxidative stress.

Chemical Biology of N5-Substituted Formamidopyrimidine DNA Adducts

DNA nucleobases are the prime targets for chemical modifications by endogenous and exogenous electrophiles. Alkylation of the N7 position of guanine and adenine in DNA triggers base-catalyzed imidazole ring opening and the formation of N5-substituted formamidopyrimidine (N5-R-FAPy) lesions. Me-FAPy-dG adducts induced by exposure to methylating agents and AFB-FAPy-dG lesions formed by aflatoxin B1 have been shown to persist in cells and to contribute to toxicity and mutagenicity. In contrast, the biological outcomes of other N5-substituted FAPy lesions have not been fully elucidated. To enable their structural and biological evaluation, N5-R-FAPy adducts must be site-specifically incorporated into synthetic DNA strands using phosphoramidite building blocks, which can be complicated by their unusual structural complexity. N5-R-FAPy exist as a mixture of rotamers and can undergo isomerization between α, β anomers and furanose-pyranose forms. In this Perspective, we will discuss the main types of N5-R-FAPy adducts and summarize the strategies for their synthesis and structural elucidation. We will also summarize the chemical biology studies conducted with N5-R-FAPy-containing DNA to elucidate their effects on DNA replication and to identify the mechanisms of N5-R-FAPy repair.

Synthesis and characterization of oligonucleotides containing a nitrogen mustard formamidopyrimidine monoadduct of deoxyguanosine

N⁵-Substituted formamidopyrimidine adducts have been observed from the reaction of dGuo or DNA with aziridine containing electrophiles, including nitrogen mustards. However, the role of substituted Fapy-dGuo adducts in the biological response to nitrogen mustards and related species has not been extensively explored. We have developed chemistry for the site-specific synthesis of oligonucleotides containing an N⁵-nitrogen mustard Fapy-dGuo using the phosphoramidite approach. The lesion was found to be a good substrate for Escherichia coli endonuclease IV and formamidopyrimidine glycosylase.

Genotoxic effect of doxorubicin-transferrin conjugate on human leukemia cells

Doxorubicin (DOX) is an effective anthracycline antibiotic against a wide spectrum of tumors and hematological malignancies. It mainly interacts with DNA, but can also generate reactive oxygen species (ROS), which damage cell components. Unfortunately, numerous side effects, such as severe cardiotoxicity and bone-marrow suppression, limit its use. To reduce this obstacle and improve its pharmacokinetics, we conjugated DOX to transferrin (TRF), a human plasma protein. In our study, we compared the effect of DOX and the doxorubicin-transferrin conjugate (DOX-TRF) on human leukemic lymphoblasts (CCRF-CEM), and on normal peripheral blood mononuclear cells (PBMC). In parallel, experiments were carried out on two human chronic myeloid leukemia (CML) cell lines derived from K562 cells, of which one was sensitive and the other resistant to doxorubicin (K562/DOX). By use of the alkaline comet assay, the effect of the agents on the induction of DNA damage in normal human cells and human leukemia cells was determined. Oxidative and alkylating DNA damage were assayed by a slightly modified comet assay that included the use of the DNA-repair enzymes endonuclease III (Endo III) and formamidopyrimidine-DNA glycosylase (Fpg). To investigate whether DNA breaks are the result of apoptosis, we examined the induction of DNA fragmentation visualized as oligosomal ladders after simple agarose electrophoresis under neutral conditions. Modifications of the genome induced by the different drugs were analyzed following assessment of the cell-cycle phase. The DOX-TRF conjugate caused more DNA damage than the free drug, the degree of DNA fragmentation being dependent on the duration of treatment and the cell type analyzed. With neutral agarose electrophoresis we showed that the test compounds caused the formation of a characteristic DNA-ladder pattern. Furthermore, the DOX-TRF conjugate generated a higher percentage of apoptotic cells in the subG1 fraction and blocked more cells in the G2/M phase of the cell cycle than did free DOX. In summary, both agents induced DNA damage in cancer cells, but the DOX-TRF conjugate generated more genotoxic effects and apoptosis than the unconjugated drug.

Accelerated repair and reduced mutagenicity of DNA damage induced by cigarette smoke in human bronchial cells transfected with E.coli formamidopyrimidine DNA glycosylase

Cigarette smoke (CS) is associated to a number of pathologies including lung cancer. Its mutagenic and carcinogenic effects are partially linked to the presence of reactive oxygen species and polycyclic aromatic hydrocarbons (PAH) inducing DNA damage. The bacterial DNA repair enzyme formamidopyrimidine DNA glycosylase (FPG) repairs both oxidized bases and different types of bulky DNA adducts. We investigated in vitro whether FPG expression may enhance DNA repair of CS-damaged DNA and counteract the mutagenic effects of CS in human lung cells. NCI-H727 non small cell lung carcinoma cells were transfected with a plasmid vector expressing FPG fused to the Enhanced Green Fluorescent Protein (EGFP). Cells expressing the fusion protein EGFP-FPG displayed accelerated repair of adducts and DNA breaks induced by CS condensate. The mutant frequencies induced by low concentrations of CS condensate to the Na⁺K⁺-ATPase locus (oua^r) were significantly reduced in cells expressing EGFP-FPG. Hence, expression of the bacterial DNA repair protein FPG stably protects human lung cells from the mutagenic effects of CS by improving cells’ capacity to repair damaged DNA.

Alterations of GSH and MDA levels and their association with bee venom-induced DNA damage in human peripheral blood leukocytes

Bee venom (BV) has toxic effects in a variety of cell systems and oxidative stress has been proposed as a possible mechanism of its toxicity. This study investigated the in vitro effect of BV on glutathione (GSH) and malondialdehyde (MDA) levels, and their association with BV-induced DNA strand breaks and oxidative DNA damage in human peripheral blood leukocytes (HPBLs). Blood samples were treated with BV at concentrations ranging from 0.1 to 10 μg/ml over different lengths of time, and DNA damage in HPBLs was monitored with the alkaline and formamidopyrimidine glycoslyase (FPG)-modified comet assays, while GSH and MDA levels were determined in whole blood. Results showed a significant increase in overall DNA damage and FPG-sensitive sites in DNA of HPBLs exposed to BV compared with HPBLs from controls. An increase in DNA damage (assessed with both comet assays) was significantly associated with changes in MDA and GSH levels. When pretreated with N-acetyl-L-cysteine, a source of cysteine for the synthesis of the endogenous antioxidant GSH, a significant reduction of the DNA damaging effects of BV in HPBLs was noted. This suggests that oxidative stress is at least partly responsible for the DNA damaging effects of BV.

2-hydroxylethyl methacrylate (HEMA), a tooth restoration component, exerts its genotoxic effects in human gingival fibroblasts trough methacrylic acid, an immediate product of its degradation

HEMA (2-hydroxyethyl methacrylate), a methacrylate commonly used in dentistry, was reported to induce genotoxic effects, but their mechanism is not fully understood. HEMA may be degraded by the oral cavity esterases or through mechanical stress following the chewing process. Methacrylic acid (MAA) is the primary product of HEMA degradation. In the present work we compared cytotoxic and genotoxic effects induced by HEMA and MAA in human gingival fibroblasts (HGFs). A 6-h exposure to HEMA or MAA induced a weak decrease in the viability of HGFs. Neither HEMA nor MAA induced strand breaks in the isolated plasmid DNA, but both compounds evoked DNA damage in HGFs, as evaluated by the alkaline comet assay. Oxidative modifications to the DNA bases were monitored by the DNA repair enzymes Endo III and Fpg. DNA damage induced by HEMA and MAA was not persistent and was removed during a 120 min repair incubation. Results from the neutral comet assay indicated that both compounds induced DNA double strand breaks (DSBs) and they were confirmed by the γ-H2AX assay. Both compounds induced apoptosis and perturbed the cell cycle. Therefore, methacrylic acid, a product of HEMA degradation, may be involved in its cytotoxic and genotoxic action.

Genotoxicity of urethane dimethacrylate, a tooth restoration component

Urethane dimethacrylate (UDMA) is used in dental restorative materials in its polymeric form. However, the process of polymerization is usually incomplete and the monomers of UDMA can diffuse into the oral cavity and the pulp, reaching millimolar concentrations. In the present work we showed that UDMA at 0.1 and 1.0 mM decreased the viability of and induced DNA damage in lymphocytes in a concentration dependent manner, but it did not affect a plasmid DNA in vitro. UDMA at 1mM induced apoptosis in lymphocytes. The lymphocytes exposed to UDMA were able to repair their DNA within 60 min. Analysis with DNA repair enzymes Endo III and Fpg showed that UDMA induced mainly oxidative DNA lesions. Vitamin C and chitosan decreased genotoxic effect of UDMA. Our results show that monomers of UDMA may exert pronounced cyto- and genotoxic effects in human lymphocytes and chitosan can be considered as a protection against such effects.

DNA polymerase X from Deinococcus radiodurans implicated in bacterial tolerance to DNA damage is characterized as a short patch base excision repair polymerase

The Deinococcus radiodurans R1 genome encodes an X-family DNA repair polymerase homologous to eukaryotic DNA polymerase β. The recombinant deinococcal polymerase X (PolX) purified from transgenic Escherichia coli showed deoxynucleotidyltransferase activity. Unlike the Klenow fragment of E. coli, this enzyme showed short patch DNA synthesis activity on heteropolymeric DNA substrate. The recombinant enzyme showed 5′-deoxyribose phosphate (5′-dRP) lyase activity and base excision repair function in vitro, with the help of externally supplied glycosylase and AP endonuclease functions. A polX disruption mutant of D. radiodurans expressing 5′-dRP lyase and a truncated polymerase domain was comparatively less sensitive to γ-radiation than a polX deletion mutant. Both mutants showed higher sensitivity to hydrogen peroxide. Excision repair mutants of E. coli expressing this polymerase showed functional complementation of UV sensitivity. These results suggest the involvement of deinococcal polymerase X in DNA-damage tolerance of D. radiodurans, possibly by contributing to DNA double-strand break repair and base excision repair.

Cytotoxicity and genotoxicity of glycidyl methacrylate

Methacrylates are used in the polymer form as composite restorative materials in dentistry. However, the polymers can release monomers and co-monomers into the oral cavity and pulp, from where they can migrate into the bloodstream reaching virtually all organs. The local concentration of the released monomers can be in the millimolar range, high enough to induce adverse biological effects. Genotoxicity of methacrylate monomers is of a special significance due to potential serious phenotypic consequences, including cancer, and long latency period. In the present work, we investigated cytotoxicity and genotoxicity of glycidyl methacrylate (GMA) in the human peripheral blood lymphocytes and the CCR-CM human cancer cells. GMA at concentrations up to 5mM evoked a concentration-dependent decrease in the viability of the lymphocytes up to about 80%, as assessed by flow cytometry. This agent did not induce strand breaks in the isolated plasmid DNA, but evoked concentration-dependent DNA damage in the human lymphocytes evaluated by the alkaline and neutral comet assay. This damage included oxidative modifications to the DNA bases, as checked by DNA repair enzymes Endo III and Fpg as well as single and double DNA strand breaks. The lymphocytes exposed to GMA at 2.5 microM were able to remove about 90% of damage to their DNA in 120 min. The ability of GMA to induce DNA double-strand breaks was confirmed by pulsed field gel electrophoresis. The drug evoked apoptosis and induced an increase in the G2/M cell population, accompanied by a decrease in the S cell population and an increase in G0/G1 cell population. Due to broad spectrum of GMA genotoxicity, including DNA double-strand breaks, and a potential long-lasting exposure to this compound, its use should be accompanied by precautions, reducing the chance of its release into blood stream and the possibility to induce adverse biological effects.

Persistence of DNA damage in the freshwater mussel Unio pictorum upon exposure to ethyl methanesulphonate and hydrogen peroxide

An important endpoint in assessing pollution-related toxicity is genotoxicity. To obtain insight into the time-course of oxidative- and alkylation-induced DNA damage in the freshwater mussel, Unio pictorum, mussels were exposed for 24 hr to concentration gradients of pro-oxidant hydrogen peroxide (H(2)O(2)) and a mono-functional alkylating agent, ethyl methanesulfonate (EMS). DNA damage was assessed in haemocytes immediately upon exposure and over the recovery period of up to 72 days by means of comet and micronucleus assays. Following exposure to H(2)O(2), DNA damage as detected by the comet assay returned to control values after one day, except for the mussels exposed to the highest dose when damage was detectable for the next 3 days. In contrast, alkylation-induced DNA damage was detectable even after 72 days of recovery in de-chlorinated water, with a dose-response relationship observable throughout the whole recovery period. Micronucleus frequency was the highest on Day 3 after exposure to EMS; it decreased considerably by Day 7 and returned almost to the control levels 19 days after exposure, while no significant induction of micronuclei was observed in mussels exposed to H(2)O(2). Although the comet assay is considered a biomarker of recent genotoxic exposure, detecting DNA damage of shorter longevity than with the micronucleus assay, results presented here show that in the case of alkylation damage the comet assay reveals genotoxic exposure of U. pictorum in a dose-dependent manner even after 2 months.

Detection of excision repaired DNA damage in the comet assay by using Ara-C and hydroxyurea in three different cell types

Because of its characteristics, the comet assay has been used to evaluate the ability of virtually any type of eukaryotic cell to repair different kinds of DNA damage, including double and single strand breaks and base damage. The ability to detect excision repair sites using the alkaline version can be enhanced by the inclusion of repair inhibitors, DNA synthesis inhibitors, or chain terminators. In this sense, we evaluated the ability of hydroxyurea (HU) and cytosine arabinoside (Ara-C), for detecting lesions produced by the alkylating agents ethyl methanesulfonate (EMS) and methyl methanesulfonate (MMS) in three different cell systems. Two hundred cells for experimental point were analyzed in the alkaline version of the comet assay, and the results are evidences of the utility of the assay to detect alkylation of bases in the cells lines MRC-5 and TK-6, as the treatment with HU +Ara-C significantly increases both the basal and induced frequency of DNA damage. The use of whole blood, although it detected the effects of MMS, with and without repair inhibitors, failed to detect the effect of the selected dose of EMS and does not permit detection increases in the background level.

Protective action of melatonin against oxidative DNA damage: chemical inactivation versus base-excision repair

Melatonin is a hormone-like substance that has a variety of beneficial properties as regulator of the circadian rhythm and as anti-inflammatory and anti-cancer agent. The latter activity can be linked with the ability of melatonin to protect DNA against oxidative damage. It may exert such action either by scavenging reactive oxygen species or their primary sources, or by stimulating the repair of oxidative damage in DNA. Since such type of DNA damage is reflected in oxidative base modifications that are primarily repaired by base-excision repair (BER), we tried to investigate in the present work whether melatonin could influence this DNA-repair system. We also investigated the ability of melatonin to inactivate hydrogen peroxide, a potent source of reactive oxygen species. Melatonin at 50 microM and its direct metabolite N(1)-acetyl-N(2)-formyl-5-methoxykynuramine reduced DNA damage induced by hydrogen peroxide at approximately the same ratio. Melatonin stimulated the repair of DNA damage induced by hydrogen peroxide, as assessed by the alkaline comet assay. However, melatonin at 50 microM had no impact on the activity in vitro of three glycosylases playing a pivotal role in BER: Endo III, Fpg and ANPG 80. On the other hand, melatonin chemically inactivated hydrogen peroxide, reducing its potential to damage DNA. And finally, melatonin did not influence the repair of an a-basic (AP) site by cellular extracts, as was evaluated by a functional BER assay in vitro. In conclusion, melatonin can have a protective effect against oxidative DNA damage by chemical inactivation of a DNA-damaging agent as well as by stimulating DNA repair, but key factors in BER, viz. glycosylases and AP-endonucleases, do not seem to be affected by melatonin. Further study with other components of the BER machinery and studies aimed at other DNA-repair systems are needed to clarify the mechanism underlying the stimulation of DNA repair by melatonin.

Correlation between formamidopyrimidine DNA glycosylase (Fpg)-sensitive sites determined by a comet assay, increased MDA, and decreased glutathione during long exposure to thinner inhalation

Thinner inhalation causes toxic effects in a variety of organs, principally in the central nervous system. Some studies have shown oxidative stress effects of thinner inhalation, such as: activation of free radical processes, decrease of antioxidants, and oxidation products of proteins and lipids but not of DNA. The aim of this study is to investigate the effect of thinner inhalation on DNA. We used the comet assay in conjunction with the enzyme formamidopyrimidine glycoslyase (Fpg). Our results show a significant increase in Fpg-sensitive sites in DNA of lymphocytes from rats exposed to thinner fumes compared to lymphocytes from control rats (p < 0.05). Moreover, DNA damage detected with Fpg shows a high correlation with increased malondialdehyde (MDA) and decreased glutathione (GSH), two widely used biomarkers of oxidative stress. The most abundant base oxidation product found in DNA is 8-oxoguanine; it is the main substrate of Fpg and the most commonly used biomarker for oxidative DNA damage. This suggests that oxidative DNA damage is at least partly responsible for the DNA damage detected by Fpg. We propose the comet assay in combination with Fpg as a sensitive biomarker to monitor exposure to thinner inhalation. Limitations of this method are discussed.

Molecular networks perturbed in a developmental animal model of brain injury

Methylazoxymethanol (MAM) is widely used as a developmental neurotoxin and exposure to its glucoside (i.e., cycasin) is associated with the prototypical neurological disorder western Pacific ALS/PDC. However, the specific molecular targets that play a key role in MAM-induced brain injury remain unclear. To reveal potential molecular networks targeted by MAM in the developing nervous system, we examined characteristic phenotypic changes (DNA damage, cytoarchitecture) induced by MAM and their correlation with gene expression differences using microarray assays (27,648 genes). Three day-old postnatal C57BL/6 mice (PND3) received a single injection of MAM and the cerebellum and cerebral cortex of PND4, 8, 15, and 22 mice were analyzed. DNA damage was detected in both the cerebellum (N7-mGua, TUNEL labeling) and cerebral cortex (N7-mGua) of PND4 mice, but progressive disruption of the cytoarchitecture was restricted to the cerebellum. A majority (>75%) of the genes affected (cerebellum 636 genes, cortex 1080 genes) by MAM were developmentally regulated, with a predominant response early (PND4) in the cerebellum and delayed (PND8 and 15) in the cerebral cortex. The genes and pathways (e.g., proteasome) affected by MAM in the cerebellum are distinct from cortex. The genes perturbed in the cerebellum reflect critical cellular processes such as development (17%), cell cycle (7%), protein metabolism (12%), and transcriptional regulation (9%) that could contribute to the observed cytoarchitectural disruption of the cerebellum. This study demonstrates for the first time that specific genes and molecular networks are affected by MAM during CNS development. Further investigation of these targets will help to understand how disruption of these developmental programs could contribute to chronic brain injury or neurodegenerative disease.

Imatinib (STI571) induces DNA damage in BCR/ABL-expressing leukemic cells but not in normal lymphocytes

Imatinib (STI571) is a 2-phenylaminopyrimidine derivative used mostly in the treatment of chronic myeloid leukaemia. It targets the BCR/ABL oncogenic tyrosine kinase, inhibiting its activity. Using the alkaline comet assay we showed that STI571 at concentrations ranging from 0.2 to 2 microM induced DNA damage in human leukemic K562 and BV173 cells expressing the BCR/ABL oncogene, whereas it had no effect in normal human lymphocytes and leukemic CCRF-CEM cells without the expression of BCR/ABL. Imatinib did not induce DNA strand breaks in the direct interaction with DNA as examined by the circular plasmid relaxation assay. Because the extent of DNA damage observed in the neutral and pH 12.1 versions of the comet assay was much lesser than in the alkaline version, we concluded that the drug induced DNA alkali-labile sites rather than strand breaks. K562 cells were unable to repair H(2)O(2)-induced DNA damage during a 120-min incubation, if they had been preincubated with STI571, whereas normal lymphocytes did so within 60 min. Pre-treatment of K562 cells with Vitamins A, C and E reduced the extent of DNA damage evoked by STI571. Similar results brought experiments with the nitrone spin traps POBN and PBN, suggesting that free radicals may be involved in the formation of DNA lesions induced by STI571 in K562 cells. These cells exposed to imatinib and treated with endonuclease III, formamidopyrimidine-DNA glycosylase and 3-methyladenine-DNA glycosylase II, the enzymes recognizing oxidized and alkylated bases, displayed greater extent of DNA damage than those not treated with these enzymes. Therefore, the mechanism of the anti-leukemic action of STI571 may involve not only the inhibition of BCR/ABL, but also DNA damage in the cells expressing this fusion protein. DNA damage induced by STI571 may follow from oxidative and alkylative base modifications.

Basal, oxidative and alkylative DNA damage, DNA repair efficacy and mutagen sensitivity in breast cancer

Impaired DNA repair may fuel up malignant transformation of breast cells due to the accumulation of spontaneous mutations in target genes and increasing susceptibility to exogenous carcinogens. Moreover, the effectiveness of DNA repair may contribute to failure of chemotherapy and resistance of breast cancer cells to drugs and radiation. The breast cancer susceptibility genes BRCA1 and BRCA2 are involved in DNA repair. To evaluate further the role of DNA repair in breast cancer we determined: (1) the kinetics of removal of DNA damage induced by hydrogen peroxide and the anticancer drug doxorubicin, and (2) the level of basal, oxidative and alkylative DNA damage before and during/after chemotherapy in the peripheral blood lymphocytes of breast cancer patients and healthy individuals. The level of DNA damage and the kinetics of DNA repair were evaluated by alkaline single cell gel electrophoresis (comet assay). Oxidative and alkylative DNA damage were assayed with the use of DNA repair enzymes endonuclease III (Endo III) and formamidopyrimidine-DNA glycosylase (Fpg), recognizing oxidized DNA bases and 3-methyladenine-DNA glycosylase II (AlkA) recognizing alkylated bases. We observed slower kinetics of DNA repair after treatment with hydrogen peroxide and doxorubicin in lymphocytes of breast cancer patients compared to control individuals. The level of basal, oxidative and alkylative DNA damage was higher in breast cancer patients than in the control and the difference was more pronounced when patients after chemotherapy were engaged, but usually the level of DNA damage in these patients was too high to be measured with our system. Our results indicate that peripheral blood lymphocytes of breast cancer patients have more damaged DNA and display decreased DNA repair efficacy. Therefore, these features can be considered as risk markers for breast cancer, but the question whether they are the cause or a consequence of the illness remains open. Nevertheless, our results suggest that research on the mutagen sensitivity and efficacy of DNA repair could impact the development of new diagnostic and screening strategies as well as indicate new targets to prevent and cure cancer. Moreover, the comet assay may be applied to evaluate the suitability of a particular mode of chemotherapy to a particular cancer patient.

DNA damage and repair in type 2 diabetes mellitus

DNA damage may be associated with type 2 diabetes mellitus (T2DM) and its complications mainly through oxidative stress. Little is known about DNA repair disturbances potentially contributing to the overall extent of DNA damage in T2DM, which, in turn, may be linked with genomic instability resulting in cancer. To assess whether DNA repair may be perturbed in 2DM we determined: (1) the level of endogenous basal DNA damage, this means damage recognized in the alkaline comet assay (DNA strand breaks and alkali labile sites) as well as endogenous oxidative and alkylative DNA damage (2) the sensitivity to DNA-damaging agents hydrogen peroxide and doxorubicin and the efficacy of removing of DNA damage induced by these agents in peripheral blood lymphocytes of T2DM patients and healthy individuals. The level of DNA damage and the kinetics of DNA repair was evaluated by the alkaline single cell gel electrophoresis (comet assay). Oxidative and alkylative DNA damage were assayed with the use of DNA repair enzymes endonuclease III (Endo III) and formamidopyrimidine-DNA glycosylase (Fpg), recognizing oxidized DNA bases and 3-methyladenine-DNA glycosylase II (AlkA) recognizing alkylated bases. The levels of basal endogenous and oxidative DNA damage in diabetes patients were higher than in control subjects. There was no difference between the level of alkylative DNA in the patients and the controls. Diabetes patients displayed higher susceptibility to hydrogen peroxide and doxorubicin and decreased efficacy of repairing DNA damage induced by these agents than healthy controls. Our results suggest that type 2 diabetes mellitus may be associated not only with the elevated level of oxidative DNA damage but also with the increased susceptibility to mutagens and the decreased efficacy of DNA repair. These features may contribute to a link between diabetes and cancer and metrics of DNA damage and repair, measured by the comet assay, may be markers of risk of cancer in diabetes.

Oxidative DNA damage induced by a hydroperoxide derivative of cyclophosphamide

Interstrand DNA cross-linking has been considered to be the primary action mechanism of cyclophosphamide (CP) and its hydroperoxide derivative, 4-hydroperoxycyclophosphamide (4-HC). To clarify the mechanism of anti-tumor effects by 4-HC, we investigated DNA damage in a human leukemia cell line, HL-60, and its H(2)O(2)-resistant clone HP100. Apoptosis DNA ladder formation was detected in HL-60 cells treated with 4-HC, whereas it was not observed in HP100 cells. 4-HC significantly increased 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) formation, a marker of oxidative DNA damage, in HL-60 cells. On the other hand, CP did not significantly induce 8-oxodG formation and apoptosis in HL-60 cells under the same conditions as did 4-HC. Using (32)P-labeled DNA fragments from the human p53 tumor suppressor gene, 4-HC was found to cause Cu(II)-mediated oxidative DNA damage, but CP did not. Catalase inhibited 4-HC-induced DNA damage, including 8-oxodG formation, suggesting the involvement of H(2)O(2). The generation of H(2)O(2) during 4-HC degradation was ascertained by procedures using scopoletin and potassium iodide. We conclude that, in addition to DNA cross-linking, oxidative DNA damage through H(2)O(2) generation may participate in the anti-tumor effects of 4-HC.

Acclimation and repair of DNA damage in recombinant bioluminescent Escherichia coli

AIMS

The aim of this study is to understand different adaptive responses in bacteria caused by three different mutagens, namely, an intercalating agent, an alkylating agent and a hydroxylating agent, and the repair systems according to the type of DNA damage, that is, DNA cross-linking and delayed DNA synthesis, alkylation and hydroxylation of DNA. A recombinant bioluminescent Escherichia coli, DPD2794 with the recA promoter fused to luxCDABE originating from Vibrio fischeri, was used in this study.

METHODS AND RESULTS

The recombinant bioluminescent E. coli strain DPD2794, containing a recA promoter fused to luxCDABE from V. fischeri, was used to detect adaptive and repair responses to DNA damage caused by mitomycin C (MMC), and these responses were compared with those when the cells were induced with N-methyl-N-nitro-N-nitrosoguanidine (MNNG) and hydrogen peroxide (H2O2). The response ratio between the induced samples and that of the controls decreased suddenly when the induced culture was used in further inductions, indicating a possible adaptive response to DNA damage. DNA damage, or the proteins produced, because of MMC addition does not appear to be completely resolved until the seventh sub-culture after the initial induction, whereas simple damage, such as the base modification caused by MNNG and H2O2, appears to be repaired rapidly as evidenced by the quick recovery of sensitivity.

CONCLUSIONS

These results suggest that it takes more time to completely repair DNA damage caused by MMC, as compared with a simple repair such as that required for the damage caused by MNNG and H2O2. Therefore, repair of the damage caused by these three mutagens is controlled by different regulons, even though they all induced the recA promoter.

SIGNIFICANCE AND IMPACT OF THE STUDY

Using a bioluminescent E. coli harbouring a recA promoter-lux fusion, it was found that different adaptive responses and repair systems for DNA damage caused by several mutagens exists in E. coli.

Minimal methylated substrate and extended substrate range of Escherichia coli AlkB protein, a 1-methyladenine-DNA dioxygenase

The Escherichia coli AlkB protein, and two human homologs ABH2 and ABH3, directly demethylate 1-methyladenine and 3-methylcytosine in DNA. They couple Fe(II)-dependent oxidative demethylation of these damaged bases to decarboxylation of alpha-ketoglutarate. Here, we have determined the kinetic parameters for AlkB oxidation of 1-methyladenine in poly(dA), short oligodeoxyribonucleotides, nucleotides, and nucleoside triphosphates. Methylated poly(dA) was the preferred AlkB substrate of those tested. The oligonucleotide trimer d(Tp1meApT) and even 5'-phosphorylated 1-me-dAMP were relatively efficiently demethylated, and competed with methylated poly(dA) for AlkB activity. A polynucleotide structure was clearly not essential for AlkB to repair 1-methyladenine effectively, but a nucleotide 5' phosphate group was required. Consequently, 1-me-dAMP(5') was identified as the minimal effective AlkB substrate. The nucleoside triphosphate, 1-me-dATP, was inefficiently but actively demethylated by AlkB; a reaction with 1-me-ATP was even slower. E. coli DNA polymerase I Klenow fragment could employ 1-me-dATP as a precursor for DNA synthesis in vitro, suggesting that demethylation of alkylated deoxynucleoside triphosphates by AlkB could have biological significance. Although the human enzymes, ABH2 and ABH3, demethylated 1-methyladenine residues in poly(dA), they were inefficient with shorter substrates. Thus, ABH3 had very low activity on the trimer, d(Tp1meApT), whereas no activity was detected with ABH2. AlkB is known to repair methyl and ethyl adducts in DNA; to extend this substrate range, AlkB was shown to reduce the toxic effects of DNA damaging agents that generate hydroxyethyl, propyl, and hydroxypropyl adducts.

Free radical scavengers can differentially modulate the genotoxicity of amsacrine in normal and cancer cells

Amsacrine is an acridine derivative drug applied in haematological malignancies. It targets topoisomerase II enhancing the formation of a cleavable DNA-enzyme complex and leading to DNA fragmentation in dividing cancer cells. Little is known about other modes of the interaction of amsacrine with DNA, by which it could affect also normal cells. Using the alkaline comet assay, we showed that amsacrine at concentrations from the range 0.01 to 10 microM induced DNA damage in normal human lymphocytes, human promyelocytic leukemia HL-60 cells lacking the p53 gene and murine pro-B lymphoid cells BaF3 expressing BCR/ABL oncogene measured as the increase in percentage tail DNA. The effect was dose-dependent. Treated cells were able to recover within a 120-min incubation. Amifostine at 14 mM decreased the level of DNA damage in normal lymphocytes, had no effect on the HL-60 cells and potentiated the DNA-damaging effect of the drug in BCR/ABL-transformed cells. Vitamin C at 10 and 50 microM diminished the extent of DNA damage in normal lymphocytes, but had no effect in cancer cells. Pre-treatment of the cells with the nitrone spin trap, N-tert-butyl-alpha-phenylnitrone or ebselen, which mimics glutathione peroxidase, reduced the extent of DNA damage evoked by amsacrine in all types of cells. The cells exposed to amsacrine and treated with endonuclease III and 3-methyladenine-DNA glycosylase II, the enzymes recognizing oxidized and alkylated bases, respectively, displayed greater extent of DNA damage than those not treated with these enzymes. The results obtained suggest that free radicals may be involved in the formation of DNA lesions induced by amsacrine. The drug can also methylate DNA bases. Our results indicate that the induction of secondary malignancies should be taken into account as diverse side effects of amsacrine. Amifostine may potentate DNA-damage effect of amsacrine in cancer cells and decrease this effect in normal cells and Vitamin C can be considered as a protective agent against DNA damage in normal cells.

Reversal of DNA alkylation damage by two human dioxygenases

The Escherichia coli AlkB protein protects against the cytotoxicity of methylating agents by repair of the DNA lesions 1-methyladenine and 3-methylcytosine, which are generated in single-stranded stretches of DNA. AlkB is an alpha-ketoglutarate- and Fe(II)-dependent dioxygenase that oxidizes the relevant methyl groups and releases them as formaldehyde. Here, we identify two human AlkB homologs, ABH2 and ABH3, by sequence and fold similarity, functional assays, and complementation of the E. coli alkB mutant phenotype. The levels of their mRNAs do not appear to correlate with cell proliferation but tissue distributions are different. Both enzymes remove 1-methyladenine and 3-methylcytosine from methylated polynucleotides in an alpha-ketoglutarate-dependent reaction, and act by direct damage reversal with the regeneration of the unsubstituted bases. AlkB, ABH2, and ABH3 can also repair 1-ethyladenine residues in DNA with the release of acetaldehyde.

DNA repair: bioinformatics helps reverse methylation damage

Recent work has uncovered a novel DNA repair enzyme: the AlkB protein of Escherichia coli, which oxidises the methyl groups of 1-methyladenine and 3-methylcytosine to hydroxymethyl moieties; the oxidised groups are subsequently released as formaldehyde, regenerating the unmodified bases.

Genotoxicity of idarubicin and its modulation by vitamins C and E and amifostine

Idarubicin is an anthracycline anticancer drug used in haematological malignancies. The main side effect of idarubicin is free-radicals based cardiotoxicity. Using the comet assay we showed that the drug at concentrations from the range 0.001 to 10 microM induced DNA damage in normal human lymphocytes, measured as the increase in percentage of DNA in the tail (% tail DNA). The effect was dose-dependent. Treated cells were able to recover within a 120-min incubation. Recognised cell protector, amifostine at 14 mM decreased the mean % tail DNA of the cells exposed to idarubicin at all tested concentrations of the drug. So did vitamin C at 10 microM, but vitamin E (alpha-tocopherol) at 50 microM increased the % tail DNA. Lymphocytes exposed to idarubicin and treated with endonuclease III, formamidopyrimidine-DNA glycosylase and 3-methyladenine-DNA glycosylase II, enzymes recognizing oxidized and alkylated bases, displayed greater extent of DNA damage than those not treated with these enzymes. Pretreatment of lymphocytes with nitrone spin traps, N-tert-butyl-alpha-phenylnitrone and alpha-(4-pyridil-1-oxide)-N-tert-butylnitrone decreased the extent of DNA damage evoked by idarubicin. To discuss the influence of vitamins and amifostine in cancer cells we used also murine pro-B lymphoid BaF3 transformed with BCR/ABL oncogene. These cells can be treated as model cells of human acute myelogenous leukemia. The response of these cells to vitamin E was quantitatively the same as human lymphocytes. However, vitamin C did not exert any effect on DNA damage and amifostine, in spite to normal lymphocytes, potentiated this effect. The results obtained suggest that reactive oxygen species, including free radicals, may be involved in the formation of DNA lesions induced by idarubicin. The drug can also methylate DNA bases. Our results indicate that not only cardiotoxicity but also genotoxicity and in consequence induction of secondary malignancies should be taken into account as diverse side effects of idarubicin. Amifostine may potentate DNA-damage effect of idarubicin in cancer cells and decrease this effect in normal cells. Vitamin C can be considered as protective agents against DNA damage in normal cells in persons receiving idarubicin-based chemotherapy, but the use of vitamin E cannot be recommended and at least needs further research.

Mechanism of action of Escherichia coli formamidopyrimidine N-glycosylase: role of K155 in substrate binding and product release

Escherichia coli formamidopyrimidine N-glycosylase (fpg) is a DNA glycosylase with an associated beta,delta-lyase activity. We have recently shown that the highly conserved lysine residue K155 is important for base recognition. Incubation of a double-stranded DNA containing an abasic site with the wild-type fpg protein generated only beta,delta-product. However, incubation of a double-stranded DNA containing an abasic site opposite a small gap with fpg protein generated predominantly beta-product. These data suggested that the induction of a double-strand break by fpg led to the destabilization of the protein-DNA covalent intermediate, causing the fpg protein to prematurely dissociate from the DNA substrate. Furthermore, when a double-stranded DNA containing an abasic site opposite an A was used as a substrate, K155A mutant fpg protein yielded a mixture of beta- and beta,delta-products. These data suggested that K155 is essential for maintaining the stability of the intermediary protein-DNA covalent complex. Pre-steady-state burst kinetics showed that mutation in K155 led to the apparent disappearance of the initial burst, suggesting that the rate of product release from K155A is much greater than the rate of chemical reaction catalyzed by the mutant enzyme. This is consistent with the idea that K155A dissociates prematurely from the covalent complex, leading to a higher turnover number observed for K155A for DNA substrate containing an AP site.

Escherichia coli FPG and human OGG1 reduce DNA damage and cytotoxicity by BCNU in human lung cells

The pulmonary complications of 1,3-N,N'-bis(2-chloroethyl)-N-nitrosourea (BCNU) are among the most important dose-limiting factors of BCNU-containing cancer chemotherapeutic regimens. BCNU damages DNA of both cancer cells and normal cells. To increase the resistance of lung cells to BCNU, we employed gene transfer of Escherichia coli formamidopyrimidine-DNA glycosylase (FPG) and human 8-oxoguanine-DNA glycosylase (hOGG1) to A549 cells, a lung epithelial cell line, using a bicistronic retroviral vector, pSF91-RE, that encoded both FPG/hOGG1 and an enhanced green fluorescent protein. The transduced epithelial cells were sorted by flow cytometry, and expression of FPG/hOGG1 protein was determined by the level of FPG/hOGG1 RNA and enzyme activity. The single-cell gel electrophoresis (comet assay) measured DNA damage induced by BCNU. FPG/hOGG1-expressing A549 cells incubated with 40-500 microg/ml BCNU exhibited significantly less DNA damage than vector-transduced cells. In addition, FPG- and/or hOGG1-expressing cells incubated with 10-40 microg/ml BCNU showed at least a 25% increase in cell survival. Gene transfer of FPG/hOGG1 reduced BCNU-induced DNA damage and cytotoxicity of cultured lung cells and may suggest a new mechanism to reduce BCNU pulmonary toxicity.

N7-guanine adducts of the epoxy metabolites of 1,3-butadiene in mice lung

Epoxy metabolites of 1,3-butadiene are electrophilic and can bind to nucleophilic sites in DNA forming DNA adducts. In this study, guanine N7 adducts of epoxy butene and guanine N7 adducts of epoxy butanediol were measured in lung tissues of mice inhalation exposed to various concentrations of 1,3-butadiene. 32P-postlabeling of DNA adducts were used to demonstrate that the DNA adducts derived from epoxybutene and epoxybutanediol were formed in a dose dependent manner. More than 98% of all adducts detected were formed from epoxybutanediol. Enantiomeric distribution of the adducts formed in vivo differs from that of in vitro experiments demonstrated before. In the case of epoxybutene most of the adducts were formed to the terminal carbon of the S-epoxybutene enantiomer. Most of the adducts derived from epoxybutanediol were formed from the 2S-3R enantiomer. The data demonstrates that enzymatic processes involved with activation and/or detoxification of the metabolites are enantiospecific and/or DNA repair machinery repairs the damage with stereochemical considerations. These are the crucial factors if interspecies differences in tumor sensitiveness is concerned.

Interactions of the human, rat, Saccharomyces cerevisiae and Escherichia coli 3-methyladenine-DNA glycosylases with DNA containing dIMP residues

In DNA, the deamination of dAMP generates 2'-deoxy-inosine 5'-monophosphate (dIMP). Hypoxanthine (HX) residues are mutagenic since they give rise to A.T-->G.C transition. They are excised, although with different efficiencies, by an activity of the 3-methyl-adenine (3-meAde)-DNA glycosylases from Escherichia coli (AlkA protein), human cells (ANPG protein), rat cells (APDG protein) and yeast (MAG protein). Comparison of the kinetic constants for the excision of HX residues by the four enzymes shows that the E.coli and yeast enzymes are quite inefficient, whereas for the ANPG and the APDG proteins they repair the HX residues with an efficiency comparable to that of alkylated bases, which are believed to be the primary substrates of these DNA glycosylases. Since the use of various substrates to monitor the activity of HX-DNA glycosylases has generated conflicting results, the efficacy of the four 3-meAde-DNA glycosylases of different origin was compared using three different substrates. Moreover, using oligo-nucleotides containing a single dIMP residue, we investigated a putative sequence specificity of the enzymes involving the bases next to the HX residue. We found up to 2-5-fold difference in the rates of HX excision between the various sequences of the oligonucleotides studied. When the dIMP residue was placed opposite to each of the four bases, a preferential recognition of dI:T over dI:dG, dI:dC and dI:dA mismatches was observed for both human (ANPG) and E.coli (AlkA) proteins. At variance, the yeast MAG protein removed more efficiently HX from a dI:dG over dI:dC, dI:T and dI:dA mismatches.

3-Methyladenine-DNA glycosylase I from Escherichia coli-computer modeling and supporting experimental evidence

TagA (3-methyladenine-DNA glycosylase I) excises 3-methyadenine and 3-methylguanine from alkylated DNA. The structure of this enzyme has not yet been determined experimentally. We propose a three-dimensional model of the TagA protein based on the threading algorithm. The model shows that TagA is a mostly alpha-helical protein, in agreement with circular dichroism measurements. None of the eight cysteines present in the TagA sequence forms a disulfide bridge in the model structure, which has also been experimentally verified with the use of Ellman method.

Fapyadenine is a moderately efficient chain terminator for prokaryotic DNA polymerases

Hypoxanthine¿xanthine oxidase¿Fe3+¿ethylenediaminetetraacetate (EDTA) was used to modify ss M13 mp18 phage DNA. The dominant base modifications found by GC/IDMS-SIM were FapyGua, FapyAde, 8-hydroxyguanine, and thymine glycol. Analysis of in vitro DNA synthesis on oxidatively modified template by three DNA polymerases revealed that T7 DNA polymerase and Klenow fragment of polymerase I from Escherichia coli were blocked mainly by oxidized pyrimidines in the template whereas some purines that were easily bypassed by the prokaryotic polymerases constituted a block for DNA polymerase beta from calf thymus. DNA synthesis by T7 polymerase on poly(dA) template, where FapyAde content increased 16-fold on oxidation, yielded a final product with a discrete ladder of premature termination bands. When DNA synthesis was performed on template from which FapyAde, FapyGua, and 8OHGua were excised by the Fpg protein new chain terminations at adenine and guanine sites appeared or existing ones were enhanced. This suggests that FapyAde, when present in DNA, is a moderately toxic lesion. Its ability to arrest DNA synthesis depends on the sequence context and DNA polymerase. FapyGua might possess similar properties.

SOS-dependent A-->G transitions induced by hydroxyl radical generating system hypoxanthine/xanthine oxidase/Fe3+/EDTA are accompanied by the increase of Fapy-adenine content in M13 mp18 phage DNA

Gas chromatography/isotope dilution-mass spectrometry with selected ion monitoring (GC/IDMS-SIM) was used to measure oxidised bases in hypoxanthine/xanthine oxidase/Fe3+/EDTA modified ss M13 mp18 phage DNA. A dose-dependent increase of oxidised bases content in DNA was observed with the biggest augmentation of FapyGua, thymine glycol and FapyAde. The amount of 8-OH-Gua was relatively high both in non-oxidised and oxidised DNA, and increased to the same extent as FapyAde and ThyGly. DNA oxidation caused a dramatic decrease in phage survival after transfection to E. coli. Survival was improved 2.8-fold after induction of the SOS system by UV irradiation of bacteria and mutation frequency of the lacZ gene in SOS conditions increased 7-fold over that in non-irradiated bacteria. Spectrum of mutations was different from those reported previously and mutations were distributed rather randomly within M13 lacZ sequence, which was in contrast to previous findings, where with non-chelated metal ions other types of mutations were found in several clusters. Thus, conditions of DNA oxidation and accessibility of metal ions for DNA bases might be important factors for generating different DNA damages and mutations. Major base substitutions found both in SOS-induced and non-induced E. coli but with higher mutation frequency in SOS-induced cells were C-->A (approximately 20-fold increase in SOS-conditions), G-->A (9-fold increase) and G-->C (4.5-fold increase). Very few G-->T transitions were found. A particularly large group of A-->G transitions appeared only in SOS-induced bacteria and was accompanied by augmentation of FapyAde content in the phage DNA with undetectable 2-OH-Ade. It is then possible that imidazole ring-opened adenine mimics guanine during DNA replication and pairs with cytosine yielding A-->G transitions in SOS-induced bacteria.

3,N4-ethenocytosine, a highly mutagenic adduct, is a primary substrate for Escherichia coli double-stranded uracil-DNA glycosylase and human mismatch-specific thymine-DNA glycosylase

Exocyclic DNA adducts are generated in cellular DNA by various industrial pollutants such as the carcinogen vinyl chloride and by endogenous products of lipid peroxidation. The etheno derivatives of purine and pyrimidine bases 3,N4-ethenocytosine (epsilonC), 1, N6-ethenoadenine (epsilonA), N2,3-ethenoguanine, and 1, N2-ethenoguanine cause mutations. The epsilonA residues are excised by the human and the Escherichia coli 3-methyladenine-DNA glycosylases (ANPG and AlkA proteins, respectively), but the enzymes repairing epsilonC residues have not yet been described. We have identified two homologous proteins present in human cells and E. coli that remove epsilonC residues by a DNA glycosylase activity. The human enzyme is an activity of the mismatch-specific thymine-DNA glycosylase (hTDG). The bacterial enzyme is the double-stranded uracil-DNA glycosylase (dsUDG) that is the homologue of the hTDG. In addition to uracil and epsilonC-DNA glycosylase activity, the dsUDG protein repairs thymine in a G/T mismatch. The fact that epsilonC is recognized and efficiently excised by the E. coli dsUDG and hTDG proteins in vitro suggests that these enzymes may be responsible for the repair of this mutagenic lesion in vivo and be important contributors to genetic stability.