The effect of aging on the DNA damage and repair capacity in 2BS cells undergoing oxidative stress

Aging is associated with a reduction in the DNA repair capacity under oxidative stress. However, whether the DNA damage and repair capacity can be a biomarker of aging remains controversial. In this study, we demonstrated two cause-and-effect relationships, the one is between the DNA damage and repair capacity and the cellular age, another is between DNA damage and repair capacity and the level of oxidative stress in human embryonic lung fibroblasts (2BS) exposed to different doses of hydrogen peroxide (H₂O₂). To clarify the mechanisms of the age-related reduction in DNA damage and repair capacity, we preliminarily evaluated the expressions of six kinds of pivotal enzymes involved in the two classical DNA repair pathways. The DNA repair capacity was observed in human fibroblasts cells using the comet assay; the age-related DNA repair enzymes were selected by RT-PCR and then verified by Western blot in vitro. Results showed that the DNA repair capacity was negatively and linearly correlated with (i) cumulative population doubling (PD) levels only in the group of low concentration of hydrogen peroxide treatment, (ii) with the level of oxidative stress only in the group of young PD cells. The mRNA expression of DNA polymerase δ1 decreased substantially in senescent cells and showed negative linear-correlation with PD levels; the protein expression level was well consistent with the mRNA level. Taken together, DNA damage and repair capacity can be a biomarker of aging. Reduced expression of DNA polymerase δ1 may be responsible for the decrease of DNA repair capacity in senescent cells.     

Deep senescent human fibroblasts show diminished DNA damage foci but retain checkpoint capacity to oxidative stress

Critically shortened telomeres trigger a DNA damage response in replicatively senescent cells. Here we report that while DNA damage foci can be detected in newly senescent cells, these foci eventually diminished in deep senescent cells. However, DNA checkpoint signalling and repair machinery in response to oxidative stress remain uncompromised in these deep senescent cells. Activation of p53 by oxidative stress is unaffected despite a marked decrease in expression of platelet-derived growth factor alpha-receptor. These findings suggest that cellular senescence is not a static process hence care must be taken in the selection of biomarkers of senescence in studies of ageing.     

Detection of oxidative DNA damage in lymphocytes of patients with Alzheimer's disease.

Oxidative damage to DNA may play an important role in both normal ageing and in neurodegenerative diseases. The deleterious consequences of excessive oxidations and the pathophysiological role of reactive oxygen species have been intensively studied in Alzheimer's disease. Although the role of oxidative stress in the aetiology of Alzheimer's disease is still not clear, the detection of an increased damage status in the cells of patients could have important therapeutic implications. The levels of oxidative damage in peripheral lymphocytes of 24 Alzheimer's disease patients and of 21 age-matched controls were determined by comet assay applied to freshly isolated blood samples with oxidative lesion-specific DNA repair endonucleases (endonuclease III for oxidized pyrimidines, formamidopyrimidine glycosylase for oxidized purines). It was demonstrated that Alzheimer's disease is associated with elevated levels of oxidized pyrimidines and purines (p<0.0001) as compared with age-matched control subjects. It was also demonstrated that the comet assay is useful as a biomarker of oxidative DNA damage when used with oxidative lesion-specific enzymes.     

Regulation of a novel cell differentiation-associated gene,JWA during oxidative damage in K562 and MCF-7 cells

Oxidative stress, or the production of oxygen-centered free radicals, has been hypothesized as the major source of DNA damage that can lead to a variety of diseases including cancer. It is known that 8-hydroxy-deoxyguanosine (8-oxo-dG) is a useful biomarker of oxidative DNA damage. Our recent data showed that JWA, initially being cloned as a novel cell differentiation-associated gene, was also actively responsive to environmental stressors, such as heat-shock, oxidative stress and so on. In the present study, we have applied a modified comet assay and bacterial repair endonucleases system (endonuclease III and formamidopyrimidine glycosylase) to investigate if JWA is involved in hydrogen peroxide (H₂O₂)-induced DNA damage and repair in K562 and MCF-7 cells, and to demonstrate if the damage is associated with 8-oxo-dG. The results from the comet assay have shown that the average tail length and the percentage of the cells with DNA tails are greatly induced by H₂O₂ treatment and further significantly enhanced by the post-treatment of repair endonucleases. The H₂O₂-induced 8-oxo-dG formation in K562 and MCF-7 cells is dose-dependent. In addition, the data have clearly demonstrated that JWA gene expression is actively induced by H₂O₂ treatment in K562 and MCF-7 cells. The results suggest that JWA can be regulated by oxidative stress and is actively involved in the signal pathways of oxidative stress in the cells.     

DNA damage and repair in Fuchs endothelial corneal dystrophy

Fuchs endothelial corneal dystrophy (FECD) is a slowly progressive eye disease leading to blindness, mostly affecting people above 40 years old. The only known method of curing FECD is corneal transplantation. The disease is characterized by the presence of extracellular deposits called “cornea guttata”, apoptosis of corneal endothelial cells, dysfunction of Descement’s membrane and corneal edema. Oxidative stress is suggested to play a role in FECD pathogenesis. Reactive oxygen species produced during the stress may damage biomolecules, including DNA. In the present study we evaluated the extent of endogenous DNA damage, including oxidatively modified DNA bases, and damage induced by hydrogen peroxide as well as the kinetics of DNA repair in peripheral blood mononuclear cells of 50 patients with FECD and 43 age-matched controls without visual disturbances. To quantify DNA damage and repair we used the alkaline comet assay technique with the enzymes recognizing oxidative DNA damage, hOGG1 and EndoIII. We did not observe differences in the extent of endogenous and hydrogen peroxide-induced DNA damage between FECD patients and controls. However, we found a lower efficacy of DNA repair in FECD patients as compared with control individuals. The results obtained suggest that the lowering of the DNA repair capacity may be one of the mechanisms underlying the role of oxidative stress in the FECD pathology.     

人衰老成纤维细胞经紫外线损伤后的DNA修复和细胞周期调控

 以体外培养的不同代龄的人胚肺二倍体成纤维细胞（２ Ｂ Ｓ）为对象,紫外线诱导 Ｄ Ｎ Ａ 损伤后,观察细胞形态、增殖特性、细胞周期、 Ｄ Ｎ Ａ 修复变化等细胞应答以及 ｇａｄｄ１５３、ｐ２１ Ｗ Ａ Ｆ１／ Ｃ Ｉ Ｐ１／ Ｓ Ｄ Ｉ１、ｐ５３ 等基因的转录水平的表达变化．结果显示：紫外线诱导 Ｄ Ｎ Ａ 损伤后,衰老（＞ ５５ 代）２ Ｂ Ｓ细胞形态及增殖能力的改变不如年轻细胞（＜ ３０ 代）显著;不同代龄的细胞损伤后均出现 Ｇ１ 期阻滞现象,年轻细胞 Ｇ１ 期阻滞率明显高于衰老细胞（ Ｐ＜ ００５）;衰老细胞总的修复能力较年轻细胞明显下降（ Ｐ＜ ００１）;同时,ｇａｄｄ１５３、ｐ２１、ｐ５３ 等的可诱导性均低于年轻 ２ Ｂ Ｓ细胞．由此,分别在细胞水平与基因水平反映了衰老细胞经紫外线照射损伤后的细胞应答变化与修复机能减退的关系．     

Oxidative DNA damage in peripheral blood cells in type 2 diabetes mellitus: higher vulnerability of polymorphonuclear leukocytes

Since oxidative stress is thought to play an important role in the pathogenesis and complications of diabetes, we used the comet assay (single cell alkaline gel electrophoresis) to evaluate DNA strand breaks and DNA base oxidation, measured as FPG (formamidopyrimidine DNA glycosylase)-sensitive sites, in peripheral blood cells (PBC) from type 2 diabetes patients and healthy controls. Oxidative DNA damage in leukocytes was increased in diabetic compared to normal subjects. However, no differences in the levels of DNA damage in isolated lymphocytes were found between the two groups. These data indicate a higher vulnerability to oxidative damage of polymorphonuclear as compared to mononuclear leukocytes in type 2 diabetes. Thus, the measurement of oxidative DNA damage in leukocytes by means of the comet assay is a suitable marker for the evaluation of systemic oxidative stress in diabetic patients.     

Repair and tolerance of oxidative DNA damage in plants

DNA damage caused by exposure to reactive oxygen species is one of the primary causes of DNA decay in most organisms. In plants, endogenous reactive oxygen species (ROS) are generated not only by respiration and photosynthesis, but also by active responses to certain environmental challenges, such as pathogen attack. Significant extracellular sources of activated oxygen include air pollutants such as ozone and oxidative effects of UV light and low-level ionizing radiation. Plants are well equipped to cope with oxidative damage to cellular macromolecules, including DNA. Oxidative attack on DNA generates both altered bases and damaged sugar residues that undergo fragmentation and lead to strand breaks. Recent advances in the study of DNA repair in higher plants show that they use mechanisms similar to those present in other eukaryotes to remove and/or tolerate oxidized bases and other oxidative DNA lesions. Therefore, plants represent a valuable model system for the study of DNA oxidative repair processes in eukaryotic cells.     

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.     

The role of DNA repair in brain related disease pathology

Oxidative DNA damage is implicated in brain aging, neurodegeneration and neurological diseases. Damage can be created by normal cellular metabolism, which accumulates with age, or by acute cellular stress conditions which create bursts of oxidative damage. Brain cells have a particularly high basal level of metabolic activity and use distinct oxidative damage repair mechanisms to remove oxidative damage from DNA and dNTP pools. Accumulation of this damage in the background of a functional DNA repair response is associated with normal aging, but defective repair in brain cells can contribute to neurological dysfunction. Emerging research strongly associates three common neurodegenerative conditions, Alzheimer's, Parkinson's and stroke, with defects in the ability to repair chronic or acute oxidative damage in neurons. This review explores the current knowledge of the role of oxidative damage repair in preserving brain function and highlights the emerging models and methods being used to advance our knowledge of the pathology of neurodegenerative disease.     

Heterogeneity in premature senescence by oxidative stress correlates with differential DNA damage during the cell cycle

The development of cellular senescence both by replication and by oxidative stress is not homogenous in cultured primary human fibroblasts. To investigate whether this is due to the heterogeneity in the susceptibility of DNA in different phases of the cell cycle, we subjected synchronised cells to oxidative stress and examined the extent of DNA damage and its long-term effects on the induction of cellular senescence. Here, we first show marked heterogeneity in DNA damage as detected by markers of double strand breaks caused by oxidative stress in an asynchronous human fibroblast culture. Cell cycle synchronization followed by oxidative stress demonstrated that DNA in S-phase is most susceptible to oxidative stress whereas DNA in the quiescent phase is most resistant. DNA repair is an ongoing process after sensing DNA damage; reparable DNA damage is repaired even in cells that contain persistent DNA damage. The extent of persistent DNA damage is tightly correlated with permanent cessation of DNA replication and SA-beta-gal activity. Oxidative stress encountered by cells in S-phase resulted in more persistent DNA damage, more permanent cell cycle arrest and the induction of premature senescence.     

Dynamic Compartmentalization of Base Excision Repair Proteins in Response to Nuclear and Mitochondrial Oxidative Stress

DNAs harbored in both nuclei and mitochondria of eukaryotic cells are subject to continuous oxidative damage resulting from normal metabolic activities or environmental insults. Oxidative DNA damage is primarily reversed by the base excision repair (BER) pathway, initiated by N-glycosylase apurinic/apyrimidinic (AP) lyase proteins. To execute an appropriate repair response, BER components must be distributed to accommodate levels of genotoxic stress that may vary considerably between nuclei and mitochondria, depending on the growth state and stress environment of the cell. Numerous examples exist where cells respond to signals, resulting in relocalization of proteins involved in key biological transactions. To address whether such dynamic localization contributes to efficient organelle-specific DNA repair, we determined the intracellular localization of the Saccharomyces cerevisiae N-glycosylase/AP lyases, Ntg1 and Ntg2, in response to nuclear and mitochondrial oxidative stress. Fluorescence microscopy revealed that Ntg1 is differentially localized to nuclei and mitochondria, likely in response to the oxidative DNA damage status of the organelle. Sumoylation is associated with targeting of Ntg1 to nuclei containing oxidative DNA damage. These studies demonstrate that trafficking of DNA repair proteins to organelles containing high levels of oxidative DNA damage may be a central point for regulating BER in response to oxidative stress.     

Specific checkpoints regulate plant cell cycle progression in response to oxidative stress

The effects of oxidative stress on plant cell cycle progression were studied both in cell suspensions and in planta . Oxidative stress of variable severity was imposed by the addition of different concentrations of the methyl-quinone, menadione, into the growth media. In cell suspensions, flow cytometry analyses demonstrated that low concentrations (20–50 μM) of menadione impaired the G1/S transition, slowed DNA replication, and delayed the entry into mitosis. Furthermore, cells in G1 were more sensitive to menadione-mediated oxidative stress than cells in S phase. Cell cycle arrest was associated with an inhibition of the activity of cyclin-dependent kinases, cell cycle gene expression, and a concomitant activation of stress genes. Menadione-mediated oxidative stress was shown to have very similar effects on tobacco plants, suggesting that a general regulation mechanism takes place in plants. These results define an oxidative stress checkpoint pathway that modulates both the expression of the core cell cycle genes and oxidative defence genes. Redox sensing could be of key importance in controlling cell cycle progression in environmental stress conditions.     

Overexpression of plant uncoupling mitochondrial protein in transgenic tobacco increases tolerance to oxidative stress

An Arabidopsis thaliana cDNA clone encoding a plant uncoupling mitochondrial protein (AtPUMP1) was overexpressed in transgenic tobacco plants. Analysis of the AtPUMP1 mRNA content in the transgenic lines, determined by Northernblot, revealed variable levels of transgene expression. Antibody probing ofWestern blots of mitochondrial proteins from three independent transgenic lines showed significant accumulation of AtPUMP1 in this organelle. Overproduction of AtPUMP1 in transgenic tobacco plants led to a significantincrease in tolerance to oxidative stress promoted by exogenous hydrogen peroxide as compared to wild-type control plants. These results provide thefirst biological evidence for a role of PUMP in protection of plant cells against oxidative stress damage.     

Overexpression of human copper/zinc-superoxide dismutase in transgenic animals attenuates the reduction of apurinic/apyrimidinic endonuclease expression in neurons after in vitro ischemia and after transient global cerebral ischemia

Oxidative stress after ischemia/reperfusion has been shown to induce DNA damage and subsequent DNA repair activity. Apurinic/apyrimidinic endonuclease (APE) is a multifunctional protein in the DNA base excision repair pathway which repairs apurinic/apyrimidinic sites in DNA. We investigated the involvement of oxidative stress and expression of APE in neurons after oxygen-glucose deprivation and after global cerebral ischemia. Our results suggest that overexpression of human copper/zinc-superoxide dismutase reduced oxidative stress with a subsequent decrease in APE expression. Production of oxygen free radicals and inhibition of the base excision repair pathway may play pivotal roles in the cell death pathway after ischemia.     

Oxidative stress in environmental-induced carcinogenesis

Reactive oxygen species (ROS) are the more abundant free radicals in nature and have been related with a number of tissue/organ injuries induced by xenobiotics, ischemia, activation of leucocytes, UV exposition, etc. Oxidative stress is caused by an imbalance between ROS production and a biological system's ability to readily detoxify these reactive intermediates or easily repair the resulting damage. Thus, oxidative stress is accepted as a critical pathophysiological mechanism in different frequent human pathologies, including cancer. In fact ROS can cause protein, lipid, and DNA damage, and malignant tumors often show increased levels of DNA base oxidation and mutations. Different lifestyle- and environmental-related factors (including, e.g., tobacco smoking, diet, alcohol, ionizing radiations, biocides, pesticides, viral infections) and other health-related factors (e.g. obesity or the aging process) may be procarcinogenic. In all these cases oxidative stress acts as a critical pathophysiological mechanism. Nevertheless it is important to remark that, in agreement with present knowledge, oxidative/nitrosative/metabolic stress, inflammation, senescence, and cancer are linked concepts that must be discussed in a coordinated manner.     

Chloroplast‐targeted bacterial RecA proteins confer tolerance to chloroplast DNA damage by methyl viologen or UV‐C radiation in tobacco (Nicotiana tabacum) plants

The nature and importance of the DNA repair system in the chloroplasts of higher plants under oxidative stress or UV radiation‐induced genotoxicity was investigated via gain‐of‐functional approaches exploiting bacterial RecAs. For this purpose, transgenic tobacco (Nicotiana tabacum) plants and cell suspensions overexpressing Escherichia coli or Pseudomonas aeruginosa RecA fused to a chloroplast‐targeting transit peptide were first produced. The transgenic tobacco plants maintained higher amounts of chloroplast DNA compared with wild‐type (WT) upon treatments with methyl viologen (MV), a herbicide that generates reactive oxygen species (ROS) in chloroplasts. Consistent with these results, the transgenic tobacco leaves showed less bleaching than WT following MV exposure. Similarly, the MV‐treated transgenic Arabidopsis plants overexpressing the chloroplast RecA homologue RECA1 showed weak bleaching, while the recA1 mutant showed opposite results upon MV treatment. In addition, when exposed to UV‐C radiation, the dark‐grown E. coli RecA‐overexpressing transgenic tobacco cell suspensions, but not their WT counterparts, resumed growth and greening after the recovery period under light conditions. Measurements of UV radiation‐induced chloroplast DNA damage using DraI assays (Harlow et al. 1994) with the chloroplast rbcL DNA probe and quantitative PCR analyses showed that the transgenic cell suspensions better repaired their UV‐C radiation‐induced chloroplast DNA lesions compared with WT. Taken all together, it was concluded that RecA‐overexpressing transgenic plants are endowed with an increased chloroplast DNA maintenance capacity and enhanced repair activities, and consequently have a higher survival tolerance to genotoxic stresses. These observations are made possible by the functional compatibility of the bacterial RecAs in chloroplasts.     

Enhanced mitochondrial DNA repair and cellular survival after oxidative stress by targeting the human 8-oxoguanine glycosylase repair enzyme to mitochondria

Oxidative damage to mitochondrial DNA (mtDNA) has been implicated as a causative factor in many disease processes and in aging. We have recently discovered that different cell types vary in their capacity to repair this damage, and this variability correlates with their ability to withstand oxidative stress. To explore strategies to enhance repair of oxidative lesions in mtDNA, we have constructed a vector containing a mitochondrial transport sequence upstream of the sequence for human 8-oxoguanine DNA glycosylase. This enzyme is the glycosylase/AP lyase that participates in repair of purine lesions, such as 8-oxoguanine. Western blot analysis confirmed that this recombinant protein was targeted to mitochondria. Enzyme activity assays showed that mitochondrial extracts from cells transfected with the construct had increased enzyme activity compared with cells transfected with vector only, whereas nuclear enzyme activity was not changed. Repair assays showed that there was enhanced repair of oxidative lesions in mtDNA. Additional studies revealed that this augmented repair led to enhanced cellular viability as determined by reduction of the tetrazolium compound to formazan, trypan blue dye exclusion, and clonogenic assays. Therefore, targeting of DNA repair enzymes to mitochondria may be a viable approach for the protection of cells against some of the deleterious effects of oxidative stress.     

DNA repair in plant mitochondria – a complete base excision repair pathway in potato tuber mitochondria

Mitochondria are one of the major sites of reactive oxygen species (ROS) production in the plant cell. ROS can damage DNA, and this damage is in many organisms mainly repaired by the base excision repair (BER) pathway. We know very little about DNA repair in plants especially in the mitochondria. Combining proteomics, bioinformatics, western blot and enzyme assays, we here demonstrate that the complete BER pathway is found in mitochondria isolated from potato (Solanum tuberosum) tubers. The enzyme activities of three DNA glycosylases and an apurinic/apyrimidinic (AP) endonuclease (APE) were characterized with respect to Mg²⁺ dependence and, in the case of the APE, temperature sensitivity. Evidence for the presence of the DNA polymerase and the DNA ligase, which complete the repair pathway by replacing the excised base and closing the gap, was also obtained. We tested the effect of oxidative stress on the mitochondrial BER pathway by incubating potato tubers under hypoxia. Protein carbonylation increased significantly in hypoxic tuber mitochondria indicative of increased oxidative stress. The activity of two BER enzymes increased significantly in response to this oxidative stress consistent with the role of the BER pathway in the repair of oxidative damage to mitochondrial DNA.