Inflammation-induced DNA damage,mutations and cancer

The relationships between inflammation and cancer are varied and complex. An important connection linking inflammation to cancer development is DNA damage. During inflammation reactive oxygen and nitrogen species (RONS) are created to combat pathogens and to stimulate tissue repair and regeneration, but these chemicals can also damage DNA, which in turn can promote mutations that initiate and promote cancer. DNA repair pathways are essential for preventing DNA damage from causing mutations and cytotoxicity, but RONS can interfere with repair mechanisms, reducing their efficacy. Further, cellular responses to DNA damage, such as damage signaling and cytotoxicity, can promote inflammation, creating a positive feedback loop. Despite coordination of DNA repair and oxidative stress responses, there are nevertheless examples whereby inflammation has been shown to promote mutagenesis, tissue damage, and ultimately carcinogenesis. Here, we discuss the DNA damage-mediated associations between inflammation, mutagenesis and cancer.     

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 μM and its direct metabolite N¹-acetyl-N²-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 μM 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.     

Repair Responses to DNA Damage: Enzymatic Pathways in E coli and Human Cells

Bacteria and eukaryotic cells employ a variety of enzymatic pathways to remove damage from DNA or to lessen its impact upon cellular functions. Most of these processes were discovered in Escherichia coli and have been most extensively analyzed in this organism because suitable mutants have been isolated and characterized. Analogous pathways have been inferred to exist in mammalian cells from the presence of enzyme activities similar to those known to be involved in repair in bacteria, from the analysis of events in cells treated with DNA damaging agents, and from the analysis of the few naturally occurring mutant cell types. Excision repair of pyrimidine dimers produced by UV in E coli is initiated by an incision event catalyzed by a complex composed of uvrA, uvrB, and uvrC gene products. Multiple exonuclease and polymerase activities are available for the subsequent excision and resynthesis steps. In addition to the constitutive pathway, which produces short patches of 20–30 nucleotides, an inducible excision repair process exists that produces much longer patches. This long patch pathway is controlled by the recA-lexA regulatory circuit and also requires the recF gene. It is apparently not responsible for UV-induced mutagenesis. However, the ability to perform inducible long patch repair correlates with enhanced bacterial survival and with a major component of the Weigle reactivation of bacteriophage with double-strand DNA genomes. Mammalian cells possess an excision repair pathway similar to the constitutive pathway in E coli. Although not as well understood, the incision event is at least as complex, and repair resynthesis produces patches of about the same size as the constitutive short patches. In mammalian cells, no patches comparable in size to those produced by the inducible pathway of E coli are observed. Repair in mammalian cells may be more complicated than in bacteria because of the structure of chromatin, which can affect both the distribution of DNA damage and its accessibility to repair enzymes. A coordinated alteration and reassembly of chromatin at sites of repair may be required. We have observed that the sensitivity of digestion by staphylococcal nuclease (SN) of newly synthesized repair patches resulting from excision of furocoumarin adducts changes with time in the same way as that of patches resulting from excision of pyrimidine dimers. Since furocoumarin adducts are formed only in the SN-sensitive linker DNA between nucleosome cores, this suggests that after repair resynthesis is completed, the nucleosome cores in the region of the repair event do not return exactly to their original positions. We have also studied excision repair of UV and chemical damage in the highly repeated 172 base pair α DNA sequence in African green monkey cells. In UV irradiated cells, the rate and extent of repair resynthesis in this sequence is similar to that in bulk DNA. However, in cells containing furocoumarin adducts, repair resynthesis in α DNA is only about 30% of that in bulk DNA. Since the frequency of adducts does not seem to be reduced in α DNA, it appears that certain adducts in this unique DNA may be less accessible to repair. Endonuclease V of bacteriophage T4 incises DNA at pyrimidine dimers by cleaving first the glycosylic bond between deoxyribose and the 5′ pyrimidine of the dimer and then the phosphodiester bond between the two pyrimidines. We have cloned the gene (denV) that codes for this enzyme and have demonstrated its expression in uvrA recA and uvrB recA cells of E coli. Because T4 endonuclease V can alleviate the excision repair deficiency of xeroderma pigmentosum when added to permeabilized cells or to isolated nuclei after UV irradiation, the cloned denV gene may ultimately be of value for analyzing DNA repair pathways in cultured human cells.     

How does a cell repair damaged DNA?

DNA in living cells is constantly subjected to different chemical and physical factors of the environment and to cell metabolites. Some changes altering DNA structure occur spontaneously. This raises the potential danger of harmful mutations that could be transmitted to offspring. To avoid the danger of mutations and changing genetic information, a cell is capable to switch on multiple mechanisms of DNA repair that remove damage and restore native structure. In many cases, removal of the same damage may involve several alternative pathways; this is very important for DNA repair under the most unfavorable conditions. This review summarizes data about all known mechanisms of eukaryotic DNA repair including excision repair (base excision repair and nucleotide excision repair), mismatch repair, repair of double-strand breaks, and cross-link repair. Special attention is given to the regulation of excision repair by different proteins—proliferating cell nuclear antigen (PCNA), p53, and proteasome. The review also highlights problem of bypassing irremovable lesions in DNA.Translated from Biokhimiya, Vol. 70, No. 3, 2005, pp. 341–359.Original Russian Text Copyright © 2005 by Sharova.     

Simulated microgravity decreases DNA repair capacity and induces DNA damage in human lymphocytes

The effect of simulated microgravity on DNA damage and apoptosis is still controversial. The objective of this study was to test whether simulated microgravity conditions affect the expression of genes for DNA repair and apoptosis. To achieve this objective, human lymphocyte cells were grown in a NASA‐developed rotating wall vessel (RWV) bioreactor that simulates microgravity. The same cell line was grown in parallel under normal gravitational conditions in culture flasks. The effect of microgravity on the expression of genes was measured by quantitative real‐time PCR while DNA damage was examined by comet assay. The result of this study revealed that exposure to simulated microgravity condition decreases the expression of DNA repair genes. Mismatch repair (MMR) class of DNA repair pathway were more susceptible to microgravity condition‐induced gene expression changes than base excision repair (BER) and nucleotide excision repair (NER) class of DNA repair genes. Downregulation of genes involved in cell proliferation (CyclinD1 and PCNA) and apoptosis (Bax) was also observed. Microgravity‐induced changes in the expression of some of these genes were further verified at the protein level by Western blot analysis. The findings of this study suggest that microgravity may induce alterations in the expression of these DNA repair genes resulting in accumulation of DNA damage. Reduced expression of cell‐cycle genes suggests that microgravity may cause a reduction in cell growth. Downregulation of pro‐apoptotic genes further suggests that extended exposure to microgravity may result in a reduction in the cells' ability to undergo apoptosis. Any resistance to apoptosis seen in cells with damaged DNA may eventually lead to malignant transformation of those cells. J. Cell. Biochem. 107: 723–731, 2009. © 2009 Wiley‐Liss, Inc.     

Unraveling DNA Repair in Human: Molecular Mechanisms and Consequences of Repair Defect

Cellular genomes are vulnerable to an array of DNA-damaging agents, of both endogenous and environmental origin. Such damage occurs at a frequency too high to be compatible with life. As a result cell death and tissue degeneration, aging and cancer are caused. To avoid this and in order for the genome to be reproduced, these damages must be corrected efficiently by DNA repair mechanisms. Eukaryotic cells have multiple mechanisms for the repair of damaged DNA. These repair systems in humans protect the genome by repairing modified bases, DNA adducts, crosslinks and double-strand breaks. The lesions in DNA are eliminated by mechanisms such as direct reversal, base excision and nucleotide excision. The base excision repair eliminates single damaged-base residues by the action of specialized DNA glycosylases and AP endonucleases. Nucleotide excision repair excises damage within oligomers that are 25 to 32 nucleotides long. This repair utilizes many proteins to remove the major UV-induced photoproducts from DNA, as well as other types of modified nucleotides. Different DNA polymerases and ligases are utilized to complete the separate pathways. The double-strand breaks in DNA are repaired by mechanisms that involve DNA protein kinase and recombination proteins. The defect in one of the repair protein results in three rare recessive syndromes: xeroderma pigmentosum, Cockayne syndrome, and trichothiodystrophy. This review describes the biochemistry of various repair processes and summarizes the clinical features and molecular mechanisms underlying these disorders.     

DNA瞬态损伤的快速修复阻断癌变最早事件的发展

DNA瞬态损伤(DNA自由基)引起DNA碱基对断裂和双螺旋结构下沉,导致基因失稳,可能是构成癌变的最早事件。基因失稳是癌的共同特征标志之一,而DNA修复可通过维持基因完整性来降低癌的发生。修复DNA的能力低下也是癌的共同特征标志之一。近年新发现了一种独特的修复机制,称为快速修复,专一地修复DNA瞬态损伤,只需几个微秒就能完成,比已知的酶性修复快十亿倍,修复速率常数达到109L.mol-1.s-1。还发现凡是具有快速修复能力的植物多酚类几乎都能不同程度地削弱癌的特征标志,包括使癌细胞形态正常化、抑制癌细胞增殖、促进癌细胞分化或凋亡、降低端粒酶活性、减少植瘤率、阻止细胞迁移和扩散、抑制血管生成、延长存活时间、降低患癌风险等。     

DNA损伤修复过程表观遗传学改变

多种化学、物理及生物因素可诱发细胞DNA损伤,损伤后DNA损伤位点被相关损伤感受器识别,激活相应的修复通路进行DNA修复。越来越多的证据表明DNA甲基化状态、蛋白翻译后修饰、染色质重塑、miRNA等修饰方式参与了DNA的损伤修复。文章通过不同损伤修复通路中这些修饰的特点,阐述表观遗传学改变在DNA损伤修复发展过程中的作用机制。     

DNA Damage Checkpoint, Damage Repair, and Genome Stability

Genomic DNA is under constant attack from both endogenous and exogenous sources of DNA damaging agents. Without proper care, the ensuing DNA damages would lead to alteration of genomic structure thus affecting the faithful transmission of genetic information. During the process of evolution, organisms have acquired a series of mechanisms responding to and repairing DNA damage, thus assuring the maintenance of genome stability and faithful transmission of genetic information. DNA damage checkpoint is one such important mechanism by which, in the face of DNA damage, a cell can respond to amplified damage signals, either by actively halting the cell cycle until it ensures that critical processes such as DNA replication or mitosis are complete or by initiating apoptosis as a last resort. Over the last decade, complex hierarchical interactions between the key components like ATM/ATR in the checkpoint pathway and various other mediators, effectors including DNA damage repair proteins have begun to emerge. In the meantime, an intimate relationship between mechanisms of damage checkpoint pathway, DNA damage repair, and genome stability was also uncovered. Reviewed hereinare the recent findings on both the mechanisms of activation of checkpoint pathways and their coordination with DNA damage repair machinery as well as their effect on genomic integrity.     

调控DNA损伤修复基因的微小RNA

随着对DNA损伤修复基因研究的深入,其信号转导路径及调控网络也进一步明了,调控DNA损伤修复基因的微小RNA(miRNA)也越来越多地被认识和发现。简要综述了DNA损伤途径中调控主要的损伤修复基因的miRNA,有助于深入阐明DNA损伤修复机制,为开发抗辐射药物和临床上DNA损伤修复异常相关肿瘤的基因治疗提供新的靶点。     

DNA损伤检验点与损伤修复及基因组稳定性

生物有机体基因组DNA经常会受到内源或外源因素的影响而导致结构发生变化,产生损伤;在长期进化过程中,有机体也相应形成了一系列应对与修复损伤DNA,并维持染色体基因组正常结构功能的机制。其中DNA损伤检验点（DNA damage checkpoint）就是在感应DNA损伤的基础上,对损伤感应信号进行转导,或引起细胞周期的暂停,从而使细胞有足够的时间对损伤DNA进行修复,或最终导致细胞发生凋亡。DNA损伤检验点信号转导途径是一个高度保守的信号感应过程,整个途径大致可以分为损伤感应、信号传递及信号效应3个组成部分。其中3-磷脂酰肌醇激酶家族类成员ATM（ataxia-telangiectasia mutated）和ATR（ataxia-telangiectasia and Rad3-related）活性的增加构成整个途径活化的第一步。它们通过激活下游的效应激酶,Chk2／Chk1,通过协同作用许多其他调控细胞周期、DNA复制、DNA损伤修复及细胞凋亡等过程的蛋白质因子来实现细胞对DNA损伤的高度协调反应。近十几年,随着此领域研究的不断深入,人们逐步揭示了DNA损伤检验点途径发生过程中,各种核心组分通过与不同调节因子、效应因子及DNA损伤修复蛋白间的复杂相互作用,以实现监测感应异常DNA结构并实施相应反应的机制;其中,检验点衔接因子（mediators）及染色质结构,尤其是核小体组蛋白的共价修饰在调控ATM／ATR活性,促进ATM／ATR与底物间的相互作用以及介导DNA损伤位点周围染色质区域上多蛋白复合物在时间与空间上的动态形成发挥着重要的作用。同时,人们也开始发现DNA损伤检验点途径与DNA损伤修复、基因组稳定性以及肿瘤发生等过程之间某些内在的联系。该反应途径在通过协调细胞针对DNA损伤做出各种反应的基础上,直接或间接地参与或调控DNA损伤修复过程,并与DNA损伤修复途径协同作用最终保证染色体基凶组结构的完整性,而检验点途径的改变,则会引起基因组不稳定的发生,包括从突变频率的提高到大范围的染色体重排,以及染色体数量的畸变。如：突变发生在肿瘤形成早期,会大大增加肿瘤发生的几率。文章将对DNA损伤检验点途径机制及其对DNA损伤修复、基因组稳定性影响的最新进展进行综述。     

Sirtuin家族与DNA损伤修复

DNA损伤的发生与积累是造成细胞功能紊乱的根本原因,也是引起衰老与肿瘤等疾病发生的关键事件。为维持机体自身遗传物质的完整性与稳定性,生物体内拥有多种针对不同类型DNA损伤的修复方式。Sirtuin蛋白是一组NAD+依赖的、高度保守的组蛋白去乙酰化酶,可通过去乙酰化作用调节众多底物蛋白质的表达、活性与稳定性。 近来的研究显示,DNA损伤修复途径的多个关键蛋白质是Sirtuin的下游底物。Sirtuin蛋白通过调节同源重组修复、非同源末端修复、核苷酸切除修复等途径中的核心蛋白质参与修复包括双链断裂（double stranded breakes, DSBs）在内的多种DNA损伤类型,从而在维持基因组稳定性、寿命以及细胞能量代谢调节等一系列生物学作用中发挥至关重要的作用。本综述将介绍近年来Sirtuin与DNA损伤修复的研究进展。     

Crosstalk between autophagy and DNA repair systems

Autophagy and DNA repair are two essential biological mechanisms that maintain cellular homeostasis. Impairment of these mechanisms was associated with several pathologies such as premature aging, neurodegenerative diseases, and cancer. Intrinsic or extrinsic stress stimuli (e.g., reactive oxygen species or ionizing radiation) cause DNA damage. As a biological stress response, autophagy is activated following insults that threaten DNA integrity. Hence, in collaboration with DNA damage repair and response mechanisms, autophagy contributes to the maintenance of genomic stability and integrity. Yet, connections and interactions between these two systems are not fully understood. In this review article, current status of the associations and crosstalk between autophagy and DNA repair systems is documented and discussed.     

Studies on DNA Damage and Repair in the Mammalian Brain

Methods for studying breaks in DNA strands and their repair, originally developed for prokaryotes and cultured cell lines, have been applied to preparations from rat brain. The relative sensitivities of these methods, which include alkaline sucrose density gradient sedimentation, nucleoid sedimentation, and ADP-ribosyltransferase assay, are compared.     

A new perspective on oxidation of DNA repair proteins and cancer

Reactive oxygen and nitrogen species (RONS) are formed as byproducts of many endogenous cellular processes, in response to infections, and upon exposure to various environmental factors. An increase in RONS can saturate the antioxidation system and leads to oxidative stress. Consequently, macromolecules are targeted for oxidative modifications, including DNA and protein. The oxidation of DNA, which leads to base modification and formation of abasic sites along with single and double strand breaks, has been extensively investigated. Protein oxidation is often neglected and is only recently being recognized as an important regulatory mechanism of various DNA repair proteins. This is a review of the current state of research on the regulation of DNA repair by protein oxidation with emphasis on the correlation between inflammation and cancer.     

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.     

A novel DNA repair inhibitor,diallyl disulfide (DADS), impairs DNA resection during DNA double-strand break repair by reducing Sae2 and Exo1 levels

Combining natural products with chemotherapy and/or radiotherapy may increase the efficacy of cancer treatment. It has been hypothesized that natural products may inhibit DNA repair and sensitize cancer cells to DNA damage-based cancer therapy. However, the molecular mechanisms underlying these activities remain unclear. In this study, we found that diallyl disulfide (DADS), an organosulfur compound, increased the sensitivity of yeast cells to DNA damage and has potential for development as an adjuvant drug for DNA damage-based cancer therapy. We induced HO endonuclease to generate a specific DNA double-strand break (DSB) by adding galactose to yeast and used this system to study how DADS affects DNA repair. In this study, we found that DADS inhibited DNA repair in single-strand annealing (SSA) system and sensitized SSA cells to a single DSB. DADS impaired DNA repair by inhibiting the protein levels of the DNA resection-related proteins Sae2 and Exo1. We also found that the recruitment of MRX and the Mec1-Ddc2 complex to a DSB was prevented by DADS. This result suggests that DADS counteracts G2/M DNA damage checkpoint activation in a Mec1 (ATR)- and Tel1 (ATM)-dependent manner. Only by elucidating the molecular mechanisms by which DADS influences DNA repair will we be able to discover new adjuvant drugs to improve chemotherapy and/or radiotherapy.     

Cadmium: cellular effects, modifications of biomolecules, modulation of DNA repair and genotoxic consequences (a review)

Cadmium is an important toxic environmental heavy metal. Occupational and environmental pollution with cadmium results mainly from mining, metallurgy industry and manufactures of nickel-cadmium batteries, pigments and plastic stabilizers. Important sources of human intoxication are cigarette smoke as well as food, water and air contaminations. In humans, cadmium exposures have been associated with cancers of the prostate, lungs and testes. Acute exposures are responsible for damage to these organs. Chronic intoxication is associated with obstructive airway disease, emphysema, irreversible renal failure, bone disorders and immuno-suppression. At the cellular level, cadmium affects proliferation, differentiation and causes apoptosis. It has been classified as a carcinogen by the International Agency for Research on Cancer (IARC). However, it is weakly genotoxic. Indirect effects of cadmium provoke generation of reactive oxygen species (ROS) and DNA damage. Cadmium modulates also gene expression and signal transduction, reduces activities of proteins involved in antioxidant defenses. Several studies have shown that it interferes with DNA repair. The present review focuses on the effects of cadmium in mammalian cells with special emphasis on the induction of damage to DNA, membranes and proteins, the inhibition of different types of DNA repair and the induction of apoptosis. Current data and hypotheses on the mechanisms involved in cadmium genotoxicity and carcinogenesis are outlined.