The interacting pathways for prevention and repair of oxidative DNA damage

Mitochondrial DNA repair and association with aging--an update

Mitochondrial DNA is constantly exposed to oxidative injury. Due to its location close to the main site of reactive oxygen species, the inner mitochondrial membrane, mtDNA is more susceptible than nuclear DNA to oxidative damage. The accumulation of DNA damage is thought to play a critical role in the aging process and to be particularly deleterious in post-mitotic cells. Thus, DNA repair is an important mechanism for maintenance of genomic integrity. Despite the importance of mitochondria in the aging process, it was thought for many years that mitochondria lacked an enzymatic DNA repair system comparable to that in the nuclear compartment. However, it is now well established that DNA repair actively takes place in mitochondria. Oxidative DNA damage processing, base excision repair mechanisms were the first to be described in these organelles, and consequently the best understood. However, new proteins and novel DNA repair pathways, thought to be exclusively present in the nucleus, have recently been described also to be present in mitochondria. Here we review the main mitochondrial DNA repair pathways and their association with the aging process.

Nutrient release from extracted activated sludge cells using the microwave enhanced advanced oxidation process

This study investigates the effectiveness of the microwave enhanced advanced oxidation process (MW/H(2)O(2)-AOP) for nutrient release and cell destruction of the extracted activated sludge cells that are EPS-free. The concentrations of ammonia and soluble chemical oxygen demand increased with an increase of microwave temperature and hydrogen peroxide dosage. Orthophosphate could be released from these extracted cells at lower microwave temperatures and lower H(2)O(2) dosages compared to our previous studies using activated sludge. Higher concentrations of carbohydrate were released into the solution with an increase of microwave temperature. For the same microwave temperatures, carbohydrate release was first increased with the addition of H(2)O(2), and then decreased as the H(2)O(2) dosages increased further. The amount of DNA in solution was a good indicator of the extent of cell damage; the high concentration of DNA released into solution after treatment indicated significant cell damage.

Global gene expression analysis reveals differences in cellular responses to hydroxyl- and superoxide anion radical-induced oxidative stress in caco-2 cells

Reactive oxygen species-induced oxidative stress in the colon is involved in inflammatory bowel diseases and suggested to be associated with colorectal cancer risk. However, our insight in molecular responses to different oxygen radicals is still fragmentary. Therefore, we studied global gene expression by an extensive time series (0.08, 0.25, 0.5, 1, 2, 4, 8, 16, or 24 h) analyses in human colon cancer (caco-2) cells after exposure to H(2)O(2) or the superoxide anion donor menadione. Differences in gene expression were investigated by hybridization on two-color microarrays against nonexposed time-matched control cells. Next to gene expression, correlations with related phenotypic markers (8-oxodG levels and cell cycle arrest) were investigated. Gene expression analysis resulted in 1404 differentially expressed genes upon H(2)O(2) challenge and 979 genes after menadione treatment. Further analysis of gene expression data revealed how these oxidant responses can be discriminated. Time-dependent coregulated genes immediately showed a pulse-like response to H(2)O(2), while the menadione-induced expression is not restored over 24 h. Pathway analyses demonstrated that H(2)O(2) immediately influences pathways involved in the immune function, while menadione constantly regulated cell cycle-related pathways Altogether, this study offers a novel and detailed insight in the similarities and differences of the time-dependent oxidative stress responses induced by the oxidants H(2)O(2) and menadione and show that these can be discriminated regarding their modulation of particular colon carcinogenesis-related mechanisms.

The effect of Msh2 knockdown on methylating agent induced toxicity in DNA glycosylase deficient cells

The DNA structure recognition protein MSH2 is an important protein in DNA mismatch repair due to its role in initiating the repair process. To examine the potential interactions between mismatch repair and base excision repair (BER) we have examined the effect of MSH2 knockdown on 6-thioguanine (6-TG), temozolomide (TMZ) and methylmethane sulphonate (MMS) induced toxicity in BER proficient and deficient cell lines. An shRNA expression vector containing Msh2 target sequences was designed and used to transfect mouse embryonic fibroblasts lacking either alkylpurine DNA N-glycosylase (Mpg) or endonuclease III homologue (Nth1). Significant knockdown of Msh2 gene expression was achieved with three different target sequences, with the highest level being shown by Msh2(283). Clonal selection resulted in differing levels of knockdown in Mpg(-/-) cells: (69.0+/-12.1% from 5 cell clones). Transfection of the Msh2(283) sequence in Mpg+/+, Nth1+/+ and Nth1(-/-) cells resulted in average knockdowns of 45.1+/-40.5% (3 clones), 58.0+/-21.4% (5 clones) and 74.9+/-14.8% (3 clones), respectively. Msh2 knockdown resulted in increased resistance to 6-TG in BER (MPG and NTH1) proficient and deficient cell lines with similar levels of knockdown (84+/-4%) but increased resistance to TMZ only in Mpg+/+ and Nth1(-/-) cell lines and not Mpg(-/-) or Nth1+/+ cells as assessed by an MTT assay. Msh2 knockdown had no effect on sensitivity to MMS induced toxicity. In a clonogenic assay, Msh2 silenced Mpg+/+, Mpg(-/-), Nth1+/+ and Nth1(-/-) cells were more resistant to TMZ. These results confirm previous studies showing that MSH2 is a key protein in influencing 6-TG and O(6)-methylguanine induced toxicity but also suggest that the effect of this protein depends upon the presence of other proteins in different DNA repair pathways.

Chk2 protects against radiation-induced genomic instability

The murine Chk2 kinase is activated after exposure to ionizing radiation and is necessary for p53-dependent apoptosis, but the role Chk2 plays in determining genomic stability is poorly understood. By analyzing the sensitivity of Chk2-deficient murine and human cells to a range of DNA-damaging agents, we show that Chk2 deficiency results in resistance to agents that generate double-strand breaks but not to other forms of damage. Surprisingly, the absence of Chk2 results in increased sensitivity to UV-radiation-induced DNA damage. Defective apoptosis after radiation-induced DNA damage may result in genomic instability; therefore, the consequences of Chk2 deficiency on genomic instability were assayed using an in vitro screen. Gene amplification was not detected in untreated Chk2(-/-) cells, but the rate of gene amplification after irradiation was elevated and was similar to that found in p53 compromised cells. A synergistic increase in genomic instability was seen after disruption of both Chk2 and p53 function, indicating that the two proteins have non-redundant roles in regulating genome stability after irradiation. The data demonstrate that Chk2 functions to maintain genome integrity after radiation-induced damage and has important implications for the use of Chk2 inhibitors as adjuvant cancer therapy.

Modulatory role of alizarin from Rubia cordifolia L. against genotoxicity of mutagens

Rubia cordifolia L. (Rubiaceae) is an important medicinal plant used in the Ayurvedic medicinal system. Its use as a traditional therapeutic has been related to the treatment of skin disorders and cancer. Besides its medicinal value, anthraquinones from this plant are used as natural food colourants and as natural hair dyes. Dyes derived from natural sources have emerged as important alternatives to synthetic dyes. Alizarin (1,2-dihydroxyanthraquinone) was isolated and characterized from R. cordifolia L. and evaluated for its antigenotoxic potential against a battery of mutagens viz. 4-nitro-o-phenylenediamine (NPD) and 2-aminofluorene (2-AF) in Ames assay using TA98 tester strain of Salmonella typhimurium; hydrogen peroxide (H(2)O(2)) and 4-nitroquinoline-1-oxide (4NQO) in SOS chromotest using PQ37 strain of Escherichia coli and in Comet assay using human blood lymphocytes. Our results showed that alizarin possessed significant modulatory role against the genotoxicity of mutagens.

Fanconi anemia proteins and endogenous stresses

Each of the thirteen identified Fanconi anemia (FA) genes is required for resistance to DNA interstrand crosslinking agents, such as mitomycin C, cisplatin, and melphalan. While these agents are excellent tools for understanding the function of FA proteins in DNA repair, it is uncertain whether a defect in the removal of DNA interstrand crosslinks (ICLs) is the basis for the pathophysiology of FA. For example, DNA interstrand crosslinking agents induce other types of DNA damage, in addition to ICLs. Further, other DNA-damaging agents, such as ionizing or ultraviolet radiation, activate the FA pathway, leading to monoubiquitination of FANCD2 and FANCI. Also, FA patients display congenital abnormalities, hematologic deficiencies, and a predisposition to cancer in the absence of an environmental source of ICLs that is external to cells. Here we consider potential sources of endogenous DNA damage, or endogenous stresses, to which FA proteins may respond. These include ICLs formed by products of lipid peroxidation, and other forms of oxidative DNA damage. FA proteins may also potentially respond to telomere shortening or replication stress. Defining these endogenous sources of DNA damage or stresses is critical for understanding the pathogenesis of deficiencies for FA proteins.We propose that FA proteins are centrally involved in the response to replication stress, including replication stress arising from oxidative DNA damage.

Something old, something new, something borrowed; how the thermoacidophilic archaeon Sulfolobus solfataricus responds to oxidative stress

To avoid molecular damage of biomolecules due to oxidation, all cells have evolved constitutive and responsive systems to mitigate and repair chemical modifications. Archaea have adapted to some of the most extreme environments known to support life, including highly oxidizing conditions. However, in comparison to bacteria and eukaryotes, relatively little is known about the biology and biochemistry of archaea in response to changing conditions and repair of oxidative damage. In this study transcriptome, proteome, and chemical reactivity analyses of hydrogen peroxide (H(2)O(2)) induced oxidative stress in Sulfolobus solfataricus (P2) were conducted. Microarray analysis of mRNA expression showed that 102 transcripts were regulated by at least 1.5 fold, 30 minutes after exposure to 30 microM H(2)O(2). Parallel proteomic analyses using two-dimensional differential gel electrophoresis (2D-DIGE), monitored more than 800 proteins 30 and 105 minutes after exposure and found that 18 had significant changes in abundance. A recently characterized ferritin-like antioxidant protein, DPSL, was the most highly regulated species of mRNA and protein, in addition to being post-translationally modified. As expected, a number of antioxidant related mRNAs and proteins were differentially regulated. Three of these, DPSL, superoxide dismutase, and peroxiredoxin were shown to interact and likely form a novel supramolecular complex for mitigating oxidative damage. A scheme for the ability of this complex to perform multi-step reactions is presented. Despite the central role played by DPSL, cells maintained a lower level of protection after disruption of the dpsl gene, indicating a level of redundancy in the oxidative stress pathways of S. solfataricus. This work provides the first "omics" scale assessment of the oxidative stress response for an archeal organism and together with a network analysis using data from previous studies on bacteria and eukaryotes reveals evolutionarily conserved pathways where complex and overlapping defense mechanisms protect against oxygen toxicity.

Opposite-base dependent excision of 5-formyluracil from DNA by hSMUG1

PURPOSE

The aim of this study was to determine the excision efficiency of hSMUG1 (human single-strand-selective monofunctional uracil-DNA glycosylase) for 5-formyluracil (fU), a major thymine lesion formed by ionizing radiation, opposite all normal bases in DNA, to possibly explain mutation induction by fU in the DNA of mammalian cells.

MATERIALS AND METHODS

An enzymatically [(32)P]labelled fU-containing 36 nucleotide DNA sequence plus its complementary sequence (with an A, C, G or T residue inserted opposite fU) was subjected to hSMUG1 in a pH 7.5-buffer, followed by NaOH-mediated cleavage of the resultant abasic sites. Cleaved and uncleaved DNA were separated by denaturing electrophoresis and quantified by autoradiography.

RESULTS

The hSMUG1 excised fU from DNA opposite all normal bases with the highest activity when opposite non-cognate C or T followed by G and cognate A.

CONCLUSIONS

The predominant T --> G and T --> A transversions induced by fU in mammalian cells may be explained by replicative incorporation of C and T, respectively, opposite the lesion and subsequent SMUG1-initiated repair of fU.

Functional role for senataxin, defective in ataxia oculomotor apraxia type 2, in transcriptional regulation

Ataxia oculomotor apraxia type 2 (AOA2) is an autosomal recessive neurodegenerative disorder characterized by cerebellar ataxia and oculomotor apraxia. The gene mutated in AOA2, SETX, encodes senataxin, a putative DNA/RNA helicase which shares high homology to the yeast Sen1p protein and has been shown to play a role in the response to oxidative stress. To investigate further the function of senataxin, we identified novel senataxin-interacting proteins, the majority of which are involved in transcription and RNA processing, including RNA polymerase II. Binding of RNA polymerase II to candidate genes was significantly reduced in senataxin deficient cells and this was accompanied by decreased transcription of these genes, suggesting a role for senataxin in the regulation/modulation of transcription. RNA polymerase II-dependent transcription termination was defective in cells depleted of senataxin in keeping with the observed interaction of senataxin with poly(A) binding proteins 1 and 2. Splicing efficiency of specific mRNAs and alternate splice-site selection of both endogenous genes and artificial minigenes were altered in senataxin depleted cells. These data suggest that senataxin, similar to its yeast homolog Sen1p, plays a role in coordinating transcriptional events, in addition to its role in DNA repair.

Oxidative stress alters base excision repair pathway and increases apoptotic response in apurinic/apyrimidinic endonuclease 1/redox factor-1 haploinsufficient mice

Apurinic/apyrimidinic endonuclease 1/redox factor-1 (APE1/Ref-1) is the redox regulator of multiple stress-inducible transcription factors, such as NF-kappaB, and the major 5'-endonuclease in base excision repair (BER). We utilized mice containing a heterozygous gene-targeted deletion of APE1/Ref-1 (Apex(+/-)) to determine the impact of APE1/Ref-1 haploinsufficiency on the processing of oxidative DNA damage induced by 2-nitropropane (2-NP) in the liver tissue of mice. APE1/Ref-1 haploinsufficiency results in a significant decline in NF-kappaB DNA-binding activity in response to oxidative stress in liver. In addition, loss of APE1/Ref-1 increases the apoptotic response to oxidative stress, in which significant increases in GADD45g expression, p53 protein stability, and caspase activity are observed. Oxidative stress displays a differential impact on monofunctional (UNG) and bifunctional (OGG1) DNA glycosylase-initiated BER in the liver of Apex(+/-) mice. APE1/Ref-1 haploinsufficiency results in a significant decline in the repair of oxidized bases (e.g., 8-OHdG), whereas removal of uracil is increased in liver nuclear extracts of mice using an in vitro BER assay. Apex(+/-) mice exposed to 2-NP displayed a significant decline in 3'-OH-containing single-strand breaks and an increase in aldehydic lesions in their liver DNA, suggesting an accumulation of repair intermediates of failed bifunctional DNA glycosylase-initiated BER.

Physical and functional interactions between Escherichia coli MutL and the Vsr repair endonuclease

DNA mismatch repair (MMR) and very-short patch (VSP) repair are two pathways involved in the repair of T:G mismatches. To learn about competition and cooperation between these two repair pathways, we analyzed the physical and functional interaction between MutL and Vsr using biophysical and biochemical methods. Analytical ultracentrifugation reveals a nucleotide-dependent interaction between Vsr and the N-terminal domain of MutL. Using chemical crosslinking, we mapped the interaction site of MutL for Vsr to a region between the N-terminal domains similar to that described before for the interaction between MutL and the strand discrimination endonuclease MutH of the MMR system. Competition between MutH and Vsr for binding to MutL resulted in inhibition of the mismatch-provoked MutS- and MutL-dependent activation of MutH, which explains the mutagenic effect of Vsr overexpression. Cooperation between MMR and VSP repair was demonstrated by the stimulation of the Vsr endonuclease in a MutS-, MutL- and ATP-hydrolysis-dependent manner, in agreement with the enhancement of VSP repair by MutS and MutL in vivo. These data suggest a mobile MutS–MutL complex in MMR signalling, that leaves the DNA mismatch prior to, or at the time of, activation of downstream effector molecules such as Vsr or MutH.

Oxidative DNA damage in retinopathy of prematurity

PURPOSE

This study examines the levels of oxidative damage in patients with retinopathy of prematurity (ROP).

METHODS

Fifty patients were recruited with a birthweight below 1500 g or gestational age below 32 weeks. The cases were classified into those who developed ROP (n=25) and those without ROP (n=25). The authors obtained blood and urine samples from each infant, for measuring 8-hydroxy 2-deoxyguanosine (8-OHdG) and malondialdehyde (MDA) levels, at the time of the first examination at 4-6 postnatal weeks.

RESULTS

A significant difference was observed in leukocyte and urine 8-OHdG levels in patients with ROP compared to those without ROP (p<0.001 for both). Similarly, a significant difference was observed in plasma and urine MDA levels in patients with ROP compared to those without ROP (p<0.001 for both). In addition, significant correlations were found between levels of 8-OHdG in leukocyte DNA and plasma MDA (r=0.859, p<0.001), and between levels of urine 8-OHdG excretion and urine MDA (r=0.563, p<0.001).

CONCLUSIONS

8-OHdG in leukocyte DNA and urine levels in premature infants can be useful as an indicator for ROP screening.

Catalytically impaired hMYH and NEIL1 mutant proteins identified in patients with primary sclerosing cholangitis and cholangiocarcinoma

The human hMYH and NEIL1 genes encode DNA glycosylases involved in repair of oxidative base damage and mutations in these genes are associated with certain cancers. Primary sclerosing cholangitis (PSC), a chronic cholestatic liver disease characterized by inflammatory destruction of the biliary tree, is often complicated by the development of cholangiocarcinoma (CCA). Here, we aimed to investigate the influence of genetic variations in the hMYH and NEIL1 genes on risk of CCA in PSC patients. The hMYH and NEIL1 gene loci in addition to the DNA repair genes hOGG1, NTHL1 and NUDT1 were analyzed in 66 PSC patients (37 with CCA and 29 without cancer) by complete genomic sequencing of exons and adjacent intronic regions. Several single-nucleotide polymorphisms and mutations were identified and severe impairment of protein function was observed for three non-synonymous variants. The NEIL1 G83D mutant was dysfunctional for the major oxidation products 7,8-dihydro-8-oxoguanine (8oxoG), thymine glycol and dihydrothymine in duplex DNA, and the ability to perform δ-elimination at abasic sites was significantly reduced. The hMYH R260Q mutant had severe defect in adenine DNA glycosylase activity, whereas hMYH H434D could excise adenines from A:8oxoG pairs but not from A:G mispairs. We found no overall associations between the 18 identified variants and susceptibility to CCA in PSC patients; however, the impaired variants may be of significance for carcinogenesis in general. Our findings demonstrate the importance of complete resequencing of selected candidate genes in order to identify rare genetic variants and their possible contribution to individual susceptibility to cancer development.

Uracil in DNA and its processing by different DNA glycosylases

Uracil in DNA may result from incorporation of dUMP during replication and from spontaneous or enzymatic deamination of cytosine, resulting in U:A pairs or U:G mismatches, respectively. Uracil generated by activation-induced cytosine deaminase (AID) in B cells is a normal intermediate in adaptive immunity. Five mammalian uracil-DNA glycosylases have been identified; these are mitochondrial UNG1 and nuclear UNG2, both encoded by the UNG gene, and the nuclear proteins SMUG1, TDG and MBD4. Nuclear UNG2 is apparently the sole contributor to the post-replicative repair of U:A lesions and to the removal of uracil from U:G contexts in immunoglobulin genes as part of somatic hypermutation and class-switch recombination processes in adaptive immunity. All uracil-DNA glycosylases apparently contribute to U:G repair in other cells, but they are likely to have different relative significance in proliferating and non-proliferating cells, and in different phases of the cell cycle. There are also some indications that there may be species differences in the function of the uracil-DNA glycosylases.

The current knowledge on radiosensitivity of ovarian follicle development stages

BACKGROUND

The aim of this paper is to review the available information on ovarian radiation sensitivity and the genetic hazard of ionizing radiation in female mammals including humans.

METHODS

The literature present in the author's laboratories (international papers from the 1970s) was complemented by a Medline literature search using the keywords 'ionizing radiation genetic effects', 'oocyte radiosensitivity' and 'oocyte DNA repair' (1990-2008). Further articles were acquired from citations in the research papers and reports.

RESULTS

Animal data show that oocyte radiosensitivity varies widely according to the follicle/oocyte stage and the species. Oocytes near ovulation show the highest susceptibility to radiation induction of mutational events. Congenital anomalies have been observed after exposure to high doses (1-5 Gy), but extrapolation of these data to humans requires caution. In humans, the dose required to induce permanent ovarian failure would vary from 20.3 Gy at birth to 14.3 Gy at 30 years. Most epidemiological studies found little evidence of genetic diseases at the doses at which medical, occupational or accidental exposure occurred.

CONCLUSIONS

The fact that genetic effects were observed in irradiated animals suggests that these could also occur in humans. The probability of such events remains low compared with the 'spontaneous' risks of genetic effects.

The Epstein-Barr virus nuclear antigen-1 promotes genomic instability via induction of reactive oxygen species

The Epstein-Barr virus (EBV) nuclear antigen (EBNA)-1 is the only viral protein expressed in all EBV-carrying malignancies, but its contribution to oncogenesis has remained enigmatic. We show that EBNA-1 induces chromosomal aberrations, DNA double-strand breaks, and engagement of the DNA damage response (DDR). These signs of genomic instability are associated with the production of reactive oxygen species (ROS) and are reversed by antioxidants. The catalytic subunit of the leukocyte NADPH oxidase, NOX2/gp91(phox), is transcriptionally activated in EBNA-1-expressing cells, whereas inactivation of the enzyme by chemical inhibitors or RNAi halts ROS production and DDR. These findings highlight a novel function of EBNA-1 and a possible mechanism by which expression of this viral protein could contribute to malignant transformation and tumor progression.

Pancreatic islets are very poor in rectifying oxidative DNA damage

OBJECTIVE

Free radicals that escape scavenging by antioxidant defense damage lipids, proteins, and DNA. Damage to DNA can be repaired. Therefore, both cells' antioxidant defense and their ability to repair oxidatively damaged DNA decide its fate to survive oxidative stress. Pancreatic islets cells with poor antioxidant defense were checked for their ability to remove oxidative damage form DNA.

METHODS

For ex vivo DNA repair, assay-cultured pancreatic islets and liver slices were treated with 1 and 10 mM H2O2, respectively, for 30 minutes. After incubation for different time intervals, 8-hydroxy-2'-deoxyguanosine (8-OHdG) in DNA of these cells was estimated using monoclonal antibody raised against 8-OHdG by competitive enzyme-linked immunosorbent assay. For in vitro DNA repair assay, oxidatively damaged pBR322 was incubated with nuclear extracts of islet and liver cells, and 8-OHdG retained in the plasmid was quantitated.

RESULTS

Oxidative damage induced by H2O2 was removed quickly and efficiently from DNA by liver cells compared with islet cells. The repair of oxidatively damaged plasmid DNA in vitro was also performed more efficiently (P < 0.05) by nuclear extracts from liver cells compared with islet cell.

CONCLUSIONS

We clearly demonstrate that in addition to their low antioxidant defense, islets are very poor in rectifying the oxidative DNA damage.

Mitochondrial DNA oxidative damage and mutagenesis in Saccharomyces cerevisiae

Mutation of human mitochondrial DNA (mtDNA) has been linked to maternally inherited neuromuscular disorders and is implicated in more common diseases such as cancer, diabetes, and Parkinson's disease. Mutations in mtDNA also accumulate with age and are therefore believed to contribute to aging and age-related pathology. Housed within the mitochondrial matrix, mtDNA encodes several of the proteins involved in the production of ATP via the process of oxidative phosphorylation, which involves the flow of high-energy electrons through the electron transport chain (ETC). Because of its proximity to the ETC, mtDNA is highly vulnerable to oxidative damage mediated by reactive oxygen species (ROS) such as hydrogen peroxide, superoxide, and hydroxyl radicals that are constantly produced by this system. Therefore, it is important to be able to measure oxidative mtDNA damage under normal physiologic conditions and during environmental or disease-associated stress. The budding yeast, Saccharomyces cerevisiae, is a facile and informative model system in which to study such mtDNA oxidative damage because it is a unicellular eukaryotic facultative anaerobe that is conditionally dependent on mitochondrial oxidative phosphorylation for viability. Here, we describe methods for quantifying oxidative mtDNA damage and mutagenesis in S. cerevisiae, several of which could be applied to the development of similar assays in mammalian cells and tissues. These methods include measuring the number of point mutations that occur in mtDNA with the erythromycin resistance assay, quantifying the amount of oxidative DNA damage utilizing a modified Southern blot assay, and measuring mtDNA integrity with the "petite induction" assay.

Regulation of reactive oxygen species homeostasis by peroxiredoxins and c-Myc

Peroxiredoxins (Prxs) are highly conserved proteins found in most organisms, where they function primarily to scavenge reactive oxygen species (ROS). Loss of the most ubiquitous member of the family, Prx1, is associated with the accumulation of oxidatively damaged DNA and a tumor-prone phenotype. Prx1 interacts with the transcriptional regulatory domain of the c-Myc oncoprotein and suppresses its transforming activity. The DNA damage in tissues of prx1-/- mice is associated in some cases with only modest increases in total ROS levels. However, these cells show dramatic increases in nuclear ROS and reduced levels of cytoplasmic ROS, which explains their mutational susceptibility. In the current work, we have investigated whether changes in other ROS scavengers might account for the observed ROS redistribution pattern in prx1-/- cells. We show approximately 5-fold increases in Prx5 levels in prx1-/- embryo fibroblasts relative to prx1+/+ cells. Moreover, Prx5 levels normalize when Prx1 expression is restored. Prx5 levels also appear to be highly dependent on c-Myc, and chromatin immunoprecipitation experiments showed differential occupancy of c-Myc and Prx1 complexes at E-box elements in the prx5 gene proximal promoter. This study represents a heretofore unreported mechanism for the c-Myc-dependent regulation of one Prx family member by another and identifies a novel means by which cells reestablish ROS homeostasis when one of these family members is compromised.

A high-throughput RNA interference screen for DNA repair determinants of PARP inhibitor sensitivity

Synthetic lethality is an attractive strategy for the design of novel therapies for cancer. Using this approach we have previously demonstrated that inhibition of the DNA repair protein, PARP1, is synthetically lethal with deficiency of either of the breast cancer susceptibility proteins, BRCA1 and BRCA2. This observation is most likely explained by the inability of BRCA deficient cells to repair DNA damage by homologous recombination (HR) and has led to the clinical trials of potent PARP inhibitors for the treatment of BRCA mutation-associated cancer. To identify further determinants of PARP inhibitor response, we took a high-throughput genetic approach. We tested each of the genes recognised as having a role in DNA repair using short-interfering RNA (siRNA) and assessed the sensitivity of siRNA transfected cells to a potent PARP inhibitor, KU0058948. The validity of this approach was confirmed by the identification of known genetic determinants of PARP inhibitor sensitivity, including genes involved in HR. Novel determinants of PARP inhibitor response were also identified, including the transcription coupled DNA repair (TCR) proteins DDB1 and XAB2. These results suggest that DNA repair pathways other than HR may determine sensitivity to PARP inhibitors and highlight the likelihood that ostensibly distinct DNA repair pathways cooperate to maintain genomic stability and cellular viability. Furthermore, the identification of these novel determinants may eventually guide the optimal use of PARP inhibitors in the clinic.

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.

Synergetic pretreatment of sewage sludge by microwave irradiation in presence of H2O2 for enhanced anaerobic digestion

A microwave-enhanced advanced hydrogen peroxide oxidation process (MW/H(2)O(2)-AOP) was studied in order to investigate the synergetic effects of MW irradiation on H(2)O(2) treated waste activated sludges (WAS) in terms of mineralization (permanent stabilization), sludge disintegration/solubilization, and subsequent anaerobic biodegradation as well as dewaterability after digestion. Thickened WAS sample pretreated with 1gH(2)O(2)/g total solids (TS) lost 11-34% of its TS, total chemical oxygen demand (COD) and total biopolymers (humic acids, proteins and sugars) via advanced oxidation. In a temperature range of 60-120 degrees C, elevated MW temperatures (>80 degrees C) further increased the decomposition of H(2)O(2) into OH* radicals and enhanced both oxidation of COD and solubilization of particulate COD (>0.45 micron) of WAS indicating that a synergetic effect was observed when both H(2)O(2) and MW treatments were combined. However, at all temperatures tested, MW/H(2)O(2) treated samples had lower first-order mesophilic (33+/-2 degrees C) biodegradation rate constants and ultimate (after 32 days of digestion) methane yields (mL per gram sample) compared to control and MW irradiated WAS samples, indicating that synergistically (MW/H(2)O(2)-AOP) generated soluble organics were slower to biodegrade or more refractory than those generated during MW irradiation.

Chk2-dependent phosphorylation of XRCC1 in the DNA damage response promotes base excision repair

The DNA damage response (DDR) has an essential function in maintaining genomic stability. Ataxia telangiectasia-mutated (ATM)-checkpoint kinase 2 (Chk2) and ATM- and Rad3-related (ATR)-Chk1, triggered, respectively, by DNA double-strand breaks and blocked replication forks, are two major DDRs processing structurally complicated DNA damage. In contrast, damage repaired by base excision repair (BER) is structurally simple, but whether, and how, the DDR is involved in repairing this damage is unclear. Here, we demonstrated that ATM-Chk2 was activated in the early response to oxidative and alkylation damage, known to be repaired by BER. Furthermore, Chk2 formed a complex with XRCC1, the BER scaffold protein, and phosphorylated XRCC1 in vivo and in vitro at Thr(284). A mutated XRCC1 lacking Thr(284) phosphorylation was linked to increased accumulation of unrepaired BER intermediate, reduced DNA repair capacity, and higher sensitivity to alkylation damage. In addition, a phosphorylation-mimic form of XRCC1 showed increased interaction with glycosylases, but not other BER proteins. Our results are consistent with the phosphorylation of XRCC1 by ATM-Chk2 facilitating recruitment of downstream BER proteins to the initial damage recognition/excision step to promote BER.

The injured nervous system: a Darwinian perspective

Much of the permanent damage that occurs in response to nervous system damage (trauma, infection, ischemia, etc.) is mediated by endogenous secondary processes that can contribute to cell death and tissue damage (excitotoxicity, oxidative damage and inflammation). For humans to evolve mechanisms to minimize secondary pathophysiological events following CNS injuries, selection must occur for individuals who survive such insults. Two major factors limit the selection for beneficial responses to CNS insults: for many CNS disease states the principal risk factor is advanced, post-reproductive age and virtually all severe CNS traumas are fatal in the absence of modern medical intervention. An alternative hypothesis for the persistence of apparently maladaptive responses to CNS damage is that the secondary exacerbation of damage is the result of unavoidable evolutionary constraints. That is, the nervous system could not function under normal conditions if the mechanisms that caused secondary damage (e.g., excitotoxicity) in response to injury were decreased or eliminated. However, some vertebrate species normally inhabit environments (e.g., hypoxia in underground burrows) that could potentially damage their nervous systems. Yet, neuroprotective mechanisms have evolved in these animals indicating that natural selection can occur for traits that protect animals from nervous system damage. Many of the secondary processes and regeneration-inhibitory factors that exacerbate injuries likely persist because they have been adaptive over evolutionary time in the healthy nervous system. Therefore, it remains important that researchers consider the role of the processes in the healthy or developing nervous system to understand how they become dysregulated following injury.

Expression of an adenovirus encoded reporter gene and its reactivation following UVC and oxidative damage in cultured fish cells

PURPOSE

Recombinant human adenovirus, AdCA35lacZ, was used to examine expression of a reporter gene and its reactivation following UVC (200-280 nm) and oxidative damage in fish cells.

MATERIALS AND METHODS

AdCA35lacZ is a recombinant nonreplicating human adenovirus, which expresses the beta-galactosidase (beta-gal) reporter gene. UVC light produces DNA damage repaired by nucleotide excision repair (NER). In contrast, methylene blue plus visible light (MB+VL) produces oxidative DNA damage, mainly 8-oxoguanine, that is repaired by base excision repair (BER). We examined expression of the reporter gene and host cell reactivation (HCR) of the UVC-treated and MB+VL-treated reporter gene in fish cells.

RESULTS

AdCA35lacZ infection of Chinook salmon cells (CHSE-214), eel cells (PBLE) and four rainbow trout cell lines (RTG-2, RT-Gill, RTS-34st and RTS-pBk), but not zebrafish (ZEB) or carp (EPC) cells resulted in expression of beta-gal. HCR of UVC-treated AdCA35lacZ in fish cells varied from that obtained in NER-deficient xeroderma pigmentosum group A fibroblasts to greater than that for NER-proficient normal human fibroblasts. HCR of UVC-treated AdCA35lacZ correlated with beta-gal expression levels for untreated AdCA35lacZ. Exposure of cells to fluorescent light (400-700 nm) increased expression of the undamaged reporter gene in normal human fibroblasts and in all fish cells except PBLE and increased HCR of the UVC-damaged reporter gene in fish cells but not in human fibroblasts. HCR of the MB + VL-treated reporter gene was similar to that in human cells for PBLE, CHSE-214, RTG-2 and RTS-pBk, but was reduced in RT-Gill and RTS-34st cells.

CONCLUSIONS

These results indicate the detection of functional photoreactivation (PR) of UVC-induced DNA damage in fish cells but not in normal human fibroblasts and a link between NER and transcription of the reporter gene in the fish cells in the absence of PR. We show also efficient BER of the reporter gene in several fish cell lines.

Novel evidences for a tumor suppressor role of Rev3, the catalytic subunit of Pol zeta

Cell cycle checkpoints and DNA repair act in concert to ensure DNA integrity during perturbation of normal replication or in response to genotoxic agents. Deficiencies in these protective mechanisms can lead to cellular transformation and ultimately tumorigenesis. Here we focused on Rev3, the catalytic subunit of the low-fidelity DNA repair polymerase zeta. Rev3 is believed to play a role in double-strand break (DSB)-induced DNA repair by homologous recombination. In line with this hypothesis, we show the accumulation of chromatin-bound Rev3 protein in late S-G2 of untreated cells and in response to clastogenic DNA damage as well as an gamma-H2AX accumulation in Rev3-depleted cells. Moreover, serine 995 of Rev3 is in vitro phosphorylated by the DSB-inducible checkpoint kinase, Chk2. Our data also disclose a significant reduction of rev3 gene expression in 74 colon carcinomas when compared to the normal adjacent tissues. This reduced expression is independent of the carcinoma stages, suggesting that the downregulation of rev3 might have occurred early during tumorigenesis.

In vitro antioxidant and antiproliferative activities of selenium-containing phycocyanin from selenium-enriched Spirulina platensis

Both selenium and phycocyanin have been reported to show potent cancer chemopreventive activities. In this study, we investigated the in vitro antioxidant and antiproliferative activities of selenium-containing phycocyanin (Se-PC) purified from selenium-enriched Spirulina platensis. The antioxidant activity of Se-PC was evaluated by using four different free radical scavenging assays, namely, the 2,2'-azinobis-3-ethylbenzothiazolin-6-sulfonic acid (ABTS) assay, 1,1-diphenyl-2-picryhydrazyl (DPPH) assay, superoxide anion scavenging assay, and erythrocyte hemolysis assay. The results indicated that Se-PC exhibited stronger antioxidant activity than phycocyanin by scavenging ABTS, DPPH, superoxide anion, and 2,2'-azobis-(2-amidinopropane)dihydrochloride free radicals. Se-PC also showed dose-dependent protective effects on erythrocytes against H 2O 2-induced oxidative DNA damage as evaluated by the Comet assay. Moreover, Se-PC was identified as a potent antiproliferative agent against human melanoma A375 cells and human breast adenocarcinoma MCF-7 cells. Induction of apoptosis in both A375 and MCF-7 cells by Se-PC was evidenced by accumulation of sub-G1 cell populations, DNA fragmentation, and nuclear condensation. Further investigation on intracellular mechanisms indicated that depletion of mitochondrial membrane potential (DeltaPsi m) was involved in Se-PC-induced cell apoptosis. Our findings suggest that Se-PC is a promising organic Se species with potential applications in cancer chemoprevention.

Inhibition of the 5' to 3' exonuclease activity of hEXO1 by 8-oxoguanine

The mismatch repair pathway is responsible for maintaining genomic stability by correcting base-base mismatches and insertion/deletion loops that arise mainly via replication errors. Additionally, the mismatch repair pathway performs a central role in the cellular response to both alkylation and reactive oxygen species induced DNA damage. An important step in mismatch processing is the recruitment of hEXO1, a 5' to 3' exonuclease, by hMSH2-hMSH6 to remove the nascent DNA strand. However, very little is currently known about the capacity of hEXO1 to exonucleolytically process damaged DNA bases. Therefore, we examined whether hEXO1 can degrade double-stranded DNA substrates containing alkylated or oxidized nucleotides. Our results demonstrated that hEXO1 is capable of degrading duplex DNA containing an O6-methylguanine (O6-meG) adduct paired with either a C or a T. Additionally, the hMSH2-hMSH6 complex stimulated hEXO1 exonuclease activity on the O6-meG/T and O6-meG/C DNA substrates. In contrast, hEXO1 exonuclease activity was significantly blocked by the presence of an 8-oxoguanine adduct in both single and double stranded DNA substrates. Further, hMSH2-hMSH6 was not able to alleviate the nucleolytic block caused by the 8-oxoguanine adduct in heteroduplex DNA.

E2F1 regulates the base excision repair gene XRCC1 and promotes DNA repair

The E2F1 transcription factor activates S-phase-promoting genes, mediates apoptosis, and stimulates DNA repair through incompletely understood mechanisms. XRCC1 (x-ray repair cross-complementing group 1) protein is important for efficient single strand break/base excision repair. Although both damage and proliferative signals increase XRCC1 levels, the mechanisms regulating XRCC1 transcription remain unclear. To study these upstream mechanisms, the XRCC1 promoter was cloned into a luciferase reporter. Ectopic expression of wild-type E2F1, but not an inactive mutant E2F1(132E), activated the XRCC1 promoter-luciferase reporter, and deletion of predicted E2F1 binding sites in the promoter attenuated E2F1-induced activation. Endogenous XRCC1 expression increased in cells conditionally expressing wild-type, but not mutant E2F1, and methyl methanesulfonate-induced DNA damage stimulated XRCC1 expression in E2F1(+/+) but not E2F1(-/-) mouse embryo fibroblasts (MEFs). Additionally, E2F1(-/-) MEFs displayed attenuated DNA repair after methyl methanesulfonate-induced damage compared with E2F1(+/+) MEFs. Moreover, Chinese hamster ovary cells with mutant XRCC1 (EM9) were more sensitive to E2F1-induced apoptosis compared with Chinese hamster ovary cells with wild-type XRCC1 (AA8). These results provide new mechanistic insight into the role of the E2F pathway in maintaining genomic stability.

Rad50 is not essential for the Mre11-dependent repair of DNA double-strand breaks in Halobacterium sp. strain NRC-1

The genome of the halophilic archaeon Halobacterium sp. strain NRC-1 encodes homologs of the eukaryotic Mre11 and Rad50 proteins, which are involved in the recognition and end processing of DNA double-strand breaks in the homologous recombination repair pathway. We have analyzed the phenotype of Halobacterium deletion mutants lacking mre11 and/or rad50 after exposure to UV-C radiation, an alkylating agent (N-methyl-N'-nitro-N-nitrosoguanidine), and gamma radiation, none of which resulted in a decrease in survival of the mutant strains compared to that of the background strain. However, a decreased rate of repair of DNA double-strand breaks in strains lacking the mre11 gene was observed using pulsed-field gel electrophoresis. These observations led to the hypothesis that Mre11 is essential for the repair of DNA double-strand breaks in Halobacterium, whereas Rad50 is dispensable. This is the first identification of a Rad50-independent function for the Mre11 protein, and it represents a shift in the Archaea away from the eukaryotic model of homologous recombination repair of DNA double-strand breaks.

Oxidative stress causes telomere damage in Fanconi anaemia cells - a possible predisposition for malignant transformation

Fanconi anaemia (FA) is an autosomal recessive and X-linked disease characterized by severe genetic instability and increased incidence of cancer. One explanation for this instability may be the cellular hypersensitivity to oxidative stress leading to chromosomal breaks. This study explored the possible oxidative damage to telomeres of FA lymphocyte cell line, HSC536/N, and its possible effect on telomere function. We postulated that combination of oxidative damage with overexpression of telomerase may provide a possible model for malignant transformation in FA. The cells were grown in the presence of telomerase inhibitor and exposed for 1 month to H(2)O(2) combined with various antioxidants. This exposure caused shortening of telomere length and damage to the telomere single stranded overhang, which was prevented by several oxidants. This shortening was associated with development of severe telomere dysfunction. Control cells did not exhibit this sensitivity to H(2)O(2). Telomere dysfunction did not evoke damage response in FA cells, in contrast to normal P53 upregulation in control cells. Reconstitution of telomerase activity protected FA telomeres from further oxidative damage. These results suggest a scenario in which oxidative stress causes telomere shortening and ensuing telomere dysfunction may form the basis for malignant transformation in FA cells. Upregulation of telomerase activity in sporadic FA cells may perpetuate that process, thus explaining the malignant character of FA cells in vivo.

Broad overexpression of ribonucleotide reductase genes in mice specifically induces lung neoplasms

Ribonucleotide reductase (RNR) catalyzes the rate-limiting step in nucleotide biosynthesis and plays a central role in genome maintenance. Although a number of regulatory mechanisms govern RNR activity, the physiologic effect of RNR deregulation had not previously been examined in an animal model. We show here that overexpression of the small RNR subunit potently and selectively induces lung neoplasms in transgenic mice and is mutagenic in cultured cells. Combining RNR deregulation with defects in DNA mismatch repair, the cellular mutation correction system, synergistically increased RNR-induced mutagenesis and carcinogenesis. Moreover, the proto-oncogene K-ras was identified as a frequent mutational target in RNR-induced lung neoplasms. Together, these results show that RNR deregulation promotes lung carcinogenesis through a mutagenic mechanism and establish a new oncogenic activity for a key regulator of nucleotide metabolism. Importantly, RNR-induced lung neoplasms histopathologically resemble human papillary adenocarcinomas and arise stochastically via a mutagenic mechanism, making RNR transgenic mice a valuable model for lung cancer.

Mismatch repair in Trypanosoma brucei: heterologous expression of MSH2 from Trypanosoma cruzi provides new insights into the response to oxidative damage

Trypanosomes are unicellular eukaryotes that cause disease in humans and other mammals. Trypanosoma cruzi and Trypanosoma brucei are the causative agents, respectively, of Chagas disease in the Americas and sleeping sickness in sub-Saharan Africa. To better comprehend the interaction of these parasites with their hosts, understanding the mechanisms involved in the generation of genetic variability is critical. One such mechanism is mismatch repair (MMR), which has a crucial, evolutionarily conserved role in maintaining the fidelity of DNA replication, as well as acting in other cellular processes, such as DNA recombination. Here we have attempted to complement T. brucei MMR through the expression of MSH2 from T. cruzi. Our results show that T. brucei MSH2-null mutants are more sensitive to hydrogen peroxide (H2O2) than wild type cells, suggesting the involvement of MSH2 in the response to oxidative stress in this parasite. This phenotype is reverted by the expression of either the T. cruzi or the T. brucei MSH2 protein in the MSH2-null mutants. In contrast, MMR complementation, as assessed by resistance to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and microsatellite instability, was not achieved by the heterologous expression of T. cruzi MSH2. This finding, associated to the demonstration that mutation of MLH1, another component of the MMR system, did not affect sensitivity of T. brucei cells to H2O2, suggests an additional role of MSH2 in dealing with oxidative damage in these parasites, which may occur independently of MMR.

Deficiencies of double-strand break repair factors and effects on mutagenesis in directly gamma-irradiated and medium-mediated bystander human lymphoblastoid cells

Using RNA interference techniques to knock down key proteins in two major double-strand break (DSB) repair pathways (DNA-PKcs for nonhomologous end joining, NHEJ, and Rad54 for homologous recombination, HR), we investigated the influence of DSB repair factors on radiation mutagenesis at the autosomal thymidine kinase (TK) locus both in directly irradiated cells and in unirradiated bystander cells. We also examined the role of p53 (TP53) in these processes by using cells of three human lymphoblastoid cell lines from the same donor but with differing p53 status (TK6 is p53 wild-type, NH32 is p53 null, and WTK1 is p53 mutant). Our results indicated that p53 status did not affect either the production of radiation bystander mutagenic signals or the response to these signals. In directly irradiated cells, knockdown of DNA-PKcs led to an increased mutant fraction in WTK1 cells and decreased mutant fractions in TK6 and NH32 cells. In contrast, knockdown of DNA-PKcs led to increased mutagenesis in bystander cells regardless of p53 status. In directly irradiated cells, knockdown of Rad54 led to increased induced mutant fractions in WTK1 and NH32 cells, but the knockdown did not affect mutagenesis in p53 wild-type TK6 cells. In all cell lines, Rad54 knockdown had no effect on the magnitude of bystander mutagenesis. Studies with extracellular catalase confirmed the involvement of H2O2 in bystander signaling. Our results demonstrate that DSB repair factors have different roles in mediating mutagenesis in irradiated and bystander cells.

Mitochondrial base excision repair of uracil and AP sites takes place by single-nucleotide insertion and long-patch DNA synthesis

Base excision repair (BER) corrects a variety of small base lesions in DNA. The UNG gene encodes both the nuclear (UNG2) and the mitochondrial (UNG1) forms of the human uracil-DNA glycosylase (UDG). We prepared mitochondrial extracts free of nuclear BER proteins from human cell lines. Using these extracts we show that UNG is the only detectable UDG in mitochondria, and mitochondrial BER (mtBER) of uracil and AP sites occur by both single-nucleotide insertion and long-patch repair DNA synthesis. Importantly, extracts of mitochondria carry out repair of modified AP sites which in nuclei occurs through long-patch BER. Such lesions may be rather prevalent in mitochondrial DNA because of its proximity to the electron transport chain, the primary site of production of reactive oxygen species. Furthermore, mitochondrial extracts remove 5' protruding flaps from DNA which can be formed during long-patch BER, by a "flap endonuclease like" activity, although flap endonuclease (FEN1) is not present in mitochondria. In conclusion, combined short- and long-patch BER activities enable mitochondria to repair a broader range of lesions in mtDNA than previously known.