Cetuximab augments cytotoxicity with poly (adp-ribose) polymerase inhibition in head and neck cancer

Overexpression of the epidermal growth factor receptor (EGFR) is a hallmark of head and neck cancers and confers increased resistance and inferior survival rates. Despite targeted agents against EGFR, such as cetuximab (C225), almost half of treated patients fail this therapy, necessitating novel therapeutic strategies. Poly (ADP-Ribose) polymerase (PARP) inhibitors (PARPi) have gained recent attention due to their unique selectivity in killing tumors with defective DNA repair. In this study, we demonstrate that C225 enhances cytotoxicity with the PARPi ABT-888 in UM-SCC1, UM-SCC6, and FaDu head and neck cancer cells. The mechanism of increased susceptibility to C225 and PARPi involves C225-mediated reduction of non-homologous end-joining (NHEJ)- and homologous recombination (HR)-mediated DNA double strand break (DSB) repair, the subsequent persistence of DNA damage, and activation of the intrinsic apoptotic pathway. By generating a DSB repair deficiency, C225 can render head and neck tumor cells susceptible to PARP inhibition. The combination of C225 and the PARPi ABT-888 can thus be an innovative treatment strategy to potentially improve outcomes in head and neck cancer patients. Furthermore, this strategy may also be feasible for other EGFR overexpressing tumors, including lung and brain cancers.     

DNA ligase III acts as a DNA strand break sensor in the cellular orchestration of DNA strand break repair

In the current model of DNA SSBR, PARP1 is regarded as the sensor of single-strand breaks (SSBs). However, biochemical studies have implicated LIG3 as another possible SSB sensor. Using a laser micro-irradiation protocol that predominantly generates SSBs, we were able to demonstrate that PARP1 is dispensable for the accumulation of different single-strand break repair (SSBR) proteins at sites of DNA damage in live cells. Furthermore, we show in live cells for the first time that LIG3 plays a role in mediating the accumulation of the SSBR proteins XRCC1 and PNKP at sites of DNA damage. Importantly, the accumulation of LIG3 at sites of DNA damage did not require the BRCT domain-mediated interaction with XRCC1. We were able to show that the N-terminal ZnF domain of LIG3 plays a key role in the enzyme''s SSB sensing function. Finally, we provide cellular evidence that LIG3 and not PARP1 acts as the sensor for DNA damage caused by the topoisomerase I inhibitor, irinotecan. Our results support the existence of a second damage-sensing mechanism in SSBR involving the detection of nicks in the genome by LIG3.     

Ferulic acid (FA) abrogates γ-radiation induced oxidative stress and DNA damage by up-regulating nuclear translocation of Nrf2 and activation of NHEJ pathway

The present study was aimed to evaluate the radioprotective effect of ferulic acid (FA), a naturally occurring plant flavonoid in terms of DNA damage and damage related alterations of repair pathways by gamma radiation. FA was administered at a dose of 50?mg/kg body weight for five consecutive days prior to exposing the swiss albino mice to a single dose of 10?Gy gamma radiation. Ionising radiation induces oxidative damage manifested by decreased expression of Cu, Zn-SOD (SOD stands for super oxide dismutase), Mn-SOD and catalase. Gamma radiation promulgated reactive oxygen species (ROS) mediated DNA damage and modified repair pathways. ROS enhanced nuclear translocation of p53, activated ATM (ataxia telangiectasia-mutated protein), increased expression of GADD45a (growth arrest and DNA-damage-inducible protein) gene and inactivated Non homologous end joining (NHEJ) repair pathway. The comet formation in irradiated mice peripheral blood mononuclear cells (PBMC) reiterated the DNA damage in IR exposed groups. FA pretreatment significantly prevented the comet formation and regulated the nuclear translocation of p53, inhibited ATM activation and expression of GADD45a gene. FA promoted the nuclear translocation of nuclear factor (erythroid-derived 2)-like 2 (Nrf2) and activated NHEJ repair pathway to overcome ROS mediated oxidative stress and DNA damage. Therefore, the current study stated that FA can challenge the oxidative stress by (i) inducing nuclear translocation of Nrf2, (ii) scavenging ROS, and (iii) activating NHEJ DNA repair process.     

3-氨基苯酰胺对氧化应激诱导的HeLa细胞端粒DNA链断裂重连接作用的影响

端粒是位于真核细胞染色体末端的DNA-蛋白质复合体,在维持染色体稳定上起着重要的作用,并且与细胞的衰老和凋亡有着密切的关系.在各种DNA损伤中,单链断裂(single-strand breaks, SSBs)是最常见的类型之一,既可直接通过内源活性氧或离子化辐射产生,也可间接地在DNA代谢或碱基切除修复期间产生.已知多聚(ADP-核糖)聚合酶［poly(ADPribose) polymerase, PARP］在SSBs修复中起着极为重要的作用.本实验观察了PARP抑制剂3-氨基苯酰胺(3-aminobenzamide, 3-AB)对氧化应激诱导的HeLa细胞端粒DNA链断裂重连接的效应以及对过氧化氢(H2O2)抑制HeLa细胞增殖的影响.结果表明3-AB能够显著地抑制氧化应激诱导的HeLa细胞端粒DNA链断裂后的重连接作用,并能增强H2O2对HeLa细胞增殖的抑制作用,提示PARP参与了端粒DNA链断裂损伤的修复过程.     

HMGB3 promotes PARP inhibitor resistance through interacting with PARP1 in ovarian cancer

Poly (ADP-ribose) polymerase (PARP) inhibitor (PARPi) resistance remains a therapeutic challenge in ovarian cancer. High-mobility group box 3 (HMGB3) plays significant roles in the development of drug resistance of many cancers. However, the function of HMGB3 in PARPi resistance is poorly understood. In the current study, we clarified that HMGB3 was aberrantly overexpressed in high-grade serous ovarian carcinoma (HGSOC) tissues, and high HMGB3 levels indicated shorter overall survival and drug resistance in HGSOC. The overexpression of HMGB3 increased the insensitivity of ovarian cancer to PARPi, whereas HMGB3 knockdown reduced PARPi resistance. Mechanistically, PARP1 was identified as a novel interaction partner of HMGB3, which could be blocked using olaparib and was enhanced upon DNA damage conditions. We further showed that loss of HMGB3 induced PARP1 trapping at DNA lesions and inhibited the PARylation activity of PARP1, resulting in an increased DNA damage response and cell apoptosis. The PARPi-resistant role of HMGB3 was also verified in a xenograft mouse model. In conclusion, HMGB3 promoted PARPi resistance via interacting with PARP1, and the targeted inhibition of HMGB3 might overcome PARPi resistance in ovarian cancer therapy.Subject terms: Chemotherapy, Ovarian cancer, Ovarian cancer, Cancer therapeutic resistance     

Glycogen synthase kinase 3β inhibition synergizes with PARP inhibitors through the induction of homologous recombination deficiency in colorectal cancer

Monotherapy with poly ADP-ribose polymerase (PARP) inhibitors results in a limited objective response rate (≤60% in most cases) in patients with homologous recombination repair (HRR)-deficient cancer, which suggests a high rate of resistance in this subset of patients to PARP inhibitors (PARPi). To overcome resistance to PARPi and to broaden their clinical use, we performed high-throughput screening of 99 anticancer drugs in combination with PARPi to identify potential therapeutic combinations. Here, we found that GSK3 inhibitors (GSK3i) exhibited a strong synergistic effect with PARPi in a panel of colorectal cancer (CRC) cell lines with diverse genetic backgrounds. The combination of GSK3β and PARP inhibition causes replication stress and DNA double-strand breaks, resulting in increased anaphase bridges and abnormal spindles. Mechanistically, inhibition or genetic depletion of GSK3β was found to impair the HRR of DNA and reduce the mRNA and protein level of BRCA1. Finally, we demonstrated that inhibition or depletion of GSK3β could enhance the in vivo sensitivity to simmiparib without toxicity. Our results provide a mechanistic understanding of the combination of PARP and GSK3 inhibition, and support the clinical development of this combination therapy for CRC patients.Subject terms: Cancer therapeutic resistance, Targeted therapies     

PARP-1 inhibition induces a late increase in the level of reactive oxygen species in cells after ionizing radiation

Poly(ADP-ribose) polymerase 1 (PARP1), an enzyme activated by DNA strand breaks, synthesizes polymers of poly(ADP-ribose) (PAR) that modify chromatin and other proteins and play a role in DNA repair. Inhibition of PARP1 activity is considered a potentially important strategy in clinical practice, especially to sensitize tumor cells to chemo- and radio-therapy. Here we examined the influence of inhibition of PARP1 on formation of reactive oxygen species (ROS) and on DNA repair in cells exposed to ionizing radiation (IR). K562 (human myelogenous leukaemia) cells were grown and exposed to 4 or 12Gy of ionizing radiation in presence or absence of the PARP inhibitor NU1025 (100μM). Intracellular ROS were assayed using the probe 2,7-dichlorofluorescein with detection by flow cytometry and the rejoining of DNA strand breaks were followed by alkaline single cell gel electrophoresis (comet) assays. In untreated cells a significant increase in PAR formation occurred during the first 5min after IR, followed by a gradual decrease up to 30min. Addition of a PARP inhibitor arrested the production of PAR almost completely and decreased the rate of rejoining of DNA strand breaks significantly; however, 3h after irradiation we observed no difference in the amount of DNA strand breaks between PARP inhibitor-treated and untreated cells. Twelve to 48h after irradiation, an increase of ROS concentration was observed in irradiated cells and ROS levels in PARP inhibitor-treated cells were significantly higher than in cells without inhibitor. Irradiated cells grown in the presence or absence of PARP inhibitor did not differ in the frequencies of apoptotic and necrotic cells or in the activity of caspases at 24, 48 and 72h after irradiation. Poly(ADP-ribosylation) and inhibition of PARP1 appeared to modulate DNA strand break rejoining and influence the concentration of ROS in irradiated cells.     

聚腺苷二磷酸-核糖聚合酶1功能与其抑制剂的作用及耐药机制

聚腺苷二磷酸-核糖聚合酶1(poly ADP-ribose polymerase-1,PARP1)是细胞中重要的修饰酶,其最广为人知的作用是通过自身PAR修饰,募集以XRCC1为首的多种DNA损伤修复效应蛋白质,参与DNA单、双链损伤修复。PARP1还能通过促进复制叉停滞与核小体解聚,为DNA损伤修复提供有利条件,维持基因组稳定性。近年来,除DNA损伤修复方面的作用,还发现PARP1能影响细胞凋亡、自噬与炎症通路,与神经退行性疾病的发生发展密切相关。而PARP抑制剂(PARP inhibitor,PARPi)是一种靶向PARP1,与细胞同源重组(homologous recombination,HR)缺陷表型共同作用,产生合成致死效应的抗肿瘤药物。该药物可捕获PARP1并抑制其活性,一方面直接干扰PARP1参与的DNA损伤修复通路,另一方面也抑制了PARP1介导的DNA损伤修复通路选择和复制叉停滞,使细胞基因组不稳定。然而,在临床治疗中常发现肿瘤细胞对PARPi不敏感。肿瘤细胞对PARPi耐药与自身基因突变高度相关,这些基因分别作用于细胞HR修复途径、PARP1循环途径、复制叉稳定性和药物主动外排等方面,在耐药肿瘤患者中确定具体的突变位点,将为临床治疗提供帮助。本文旨在对PARP1的功能作一综述,并重点介绍PARPi的作用机制和与肿瘤耐药相关的突变基因及其耐药机制,以期加深对细胞中PARP1介导的DNA损伤修复通路的认识,并为将来的临床治疗提供新思路。     

Inhibition of DNA damage repair by artificial activation of PARP with siDNA

One of the major early steps of repair is the recruitment of repair proteins at the damage site, and this is coordinated by a cascade of modifications controlled by phosphatidylinositol 3-kinase-related kinases and/or poly (ADP-ribose) polymerase (PARP). We used short interfering DNA molecules mimicking double-strand breaks (called Dbait) or single-strand breaks (called Pbait) to promote DNA-dependent protein kinase (DNA-PK) and PARP activation. Dbait bound and induced both PARP and DNA-PK activities, whereas Pbait acts only on PARP. Therefore, comparative study of the two molecules allows analysis of the respective roles of the two signaling pathways: both recruit proteins involved in single-strand break repair (PARP, XRCC1 and PCNA) and prevent their recruitment at chromosomal damage. Dbait, but not Pbait, also inhibits recruitment of proteins involved in double-strand break repair (53BP1, NBS1, RAD51 and DNA-PK). By these ways, Pbait and Dbait disorganize DNA repair, thereby sensitizing cells to various treatments. Single-strand breaks repair inhibition depends on direct trapping of the main proteins on both molecules. Double-strand breaks repair inhibition may be indirect, resulting from the phosphorylation of double-strand breaks repair proteins and chromatin targets by activated DNA-PK. The DNA repair inhibition by both molecules is confirmed by their synthetic lethality with BRCA mutations.     

TDP1 facilitates repair of ionizing radiation-induced DNA single-strand breaks

Tyrosyl DNA phosphodiesterase-1 (TDP1) is the gene product mutated in spinocerebellar ataxia with axonal neuropathy1 (SCAN1). SCAN1 is a hereditary ataxia that lacks extra-neurological phenotype, pointing to a critical role for TDP1 in the nervous system. Recently, we showed that TDP1 is associated with the DNA single-strand break (SSBR) repair machinery through an interaction with DNA ligase 3alpha (Lig3alpha) and that SCAN1 cells are defective in the repair of chromosomal DNA single-strand breaks (SSBs) arising from abortive Topoisomerase 1 (Top1)-DNA intermediates. Here we demonstrate that TDP1 is also required for the repair of SSBs induced by ionizing radiation (IR), though not measurably for IR-induced DNA double-strand breaks (DSBs). In addition, we provide evidence that abortive Top1 cleavage complexes are processed by the proteasome prior to the action of TDP1 in vivo, and we exploit this observation to show that the SSBR defect in SCAN1 following IR reflects, in part at least, the presence of IR-induced protein-DNA cross-links. Finally we show that TDP1 activity at abortive Top1-SSBs is stimulated by XRCC1/Lig3alpha in vitro. These data expand the type of SSBs processed by TDP1 to include those induced by ionizing radiation, and raise the possibility that TDP1 inhibitors may improve radiotherapy.     

Adenosine receptor A2b confers ovarian cancer survival and PARP inhibitor resistance through IL-6-STAT3 signalling

Ovarian cancer is the deadliest gynecologic cancer worldwide, and the therapeutic options are limited. PARP inhibitor (PARPi) represents an effective therapeutic strategy and has been approved for maintenance therapy. However, the intrinsic or acquired resistance to PARPi becomes a big challenge. To investigate the mechanisms for PARPi resistance, we analysed public databases and established Olaparib-resistant ovarian cancer cells for exploration. Our results showed that the inflammatory pathway and adenosine receptor A2b (Adora2b/A_2B) expression were significantly increased in Olaparib-resistant cells. A_2B was highly expressed in recurrent ovarian tumours and negatively correlated with the clinical outcomes in cancer patients. Olaparib treatment enhanced A_2B expression through NF-κB activation. The elevated A_2B contributed to Olaparib resistance by sensing adenosine signal and promoting tumour cell survival, growth and migration via IL-6-STAT3 signalling. Therefore, inhibition of A_2B-IL-6-STAT3 axis could overcome Olaparib resistance and synergize with Olaparib to reduce cancer cell growth and lead to cell death. Our findings reveal a critical role of A_2B signalling in mediating PARPi resistance independent of DNA damage repair, providing insights into developing novel therapies in ovarian cancers.     

beta-sitosterol decreases irradiation-induced thymocyte early damage by regulation of the intracellular redox balance and maintenance of mitochondrial membrane stability

Both radiation injury and oxidation toxicity occur when cells are exposed to ion irradiation (IR), ultimately leading to apoptosis. This study was designed to determine the effect of beta-sitosterol (BSS) on early cellular damage in irradiated thymocytes and a possible mechanism of effect on irradiation-mediated activation of the apoptotic pathways. Thymocytes were irradiated (6 Gy) with or without BSS. Cell apoptosis and apoptosis-related proteins were evaluated. BSS decreased irradiation-induced cell death and nuclear DNA strand breaks while attenuating intracellular reactive oxygen species (ROS) and increasing the activities of antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx). BSS decreased the release of cytochrome c from mitochondria to the cytosol and the mitochondrio-nuclear translocation of apoptosis-inducing factor (AIF). Furthermore, BSS partially inhibited the radiation-induced increase of cleaved caspase 3 and cleaved PARP, and attenuated the activation of JNK and AP-1. In addition, evidence suggests that ROS generated by irradiation are involved in this course of cell damage. The results indicate that BSS confers a radioprotective effect on thymocytes by regulation of the intracellular redox balance which is carried out via the scavenging of ROS and maintenance of mitochondrial membrane stability.     

PARP inhibition versus PARP-1 silencing: different outcomes in terms of single-strand break repair and radiation susceptibility

The consequences of PARP-1 disruption or inhibition on DNA single-strand break repair (SSBR) and radio-induced lethality were determined in synchronized, isogenic HeLa cells stably silenced or not for poly(ADP-ribose) polymerase-1 (PARP-1) (PARP-1(KD)) or XRCC1 (XRCC1(KD)). PARP-1 inhibition prevented XRCC1-YFP recruitment at sites of 405 nm laser micro irradiation, slowed SSBR 10-fold and triggered the accumulation of large persistent foci of GFP-PARP-1 and GFP-PCNA at photo damaged sites. These aggregates are presumed to hinder the recruitment of other effectors of the base excision repair (BER) pathway. PARP-1 silencing also prevented XRCC1-YFP recruitment but did not lengthen the lifetime of GFP-PCNA foci. Moreover, PARP-1(KD) and XRCC1(KD) cells in S phase completed SSBR as rapidly as controls, while SSBR was delayed in G1. Taken together, the data demonstrate that a PARP-1- and XRCC1-independent SSBR pathway operates when the short patch repair branch of the BER is deficient. Long patch repair is the likely mechanism, as GFP-PCNA recruitment at photo-damaged sites was normal in PARP-1(KD) cells. PARP-1 silencing elicited hyper-radiosensitivity, while radiosensitization by a PARP inhibitor reportedly occurs only in those cells treated in S phase. PARP-1 inhibition and deletion thus have different outcomes in terms of SSBR and radiosensitivity.     

Poly(ADP-ribosyl)ation accelerates DNA repair in a pathway dependent on Cockayne syndrome B protein

Activation of poly(ADP-ribose)polymerases 1 and 2 (PARP-1 and PARP-2) is one of the earliest responses of mammalian cells to DNA damage by numerous genotoxic agents. We have analysed the influence of PARP inhibition, either achieved by over-expression of the DNA binding domain of PARP-1 or by treatment with 3,4-dihydro-5-[4-(1-piperidinyl)butoxyl]-1(2H)-isoquinolinone, on the repair of single-strand breaks (SSB), pyrimidine dimers and oxidative base modifications sensitive to Fpg protein (mostly 8-hydroxyguanine) in mammalian cells at very low, non-cytotoxic levels of DNA damage. The data show that the repair rates of all three types of DNA damage are significantly lower in PARP-inhibited cells. Importantly, the retardation of the repair of base modifications is not associated with accumulation of intermediates such as SSB or abasic sites. Moreover, the influence of the PARP inhibition is not observed in cells deficient in Cockayne syndrome B protein (Csb). The results indicate that PARP activation and Csb are both involved in a novel mechanism that accelerates the global repair of various types of DNA modifications.     

XRCC1 is required for DNA single-strand break repair in human cells

The X-ray repair cross complementing 1 (XRCC1) protein is required for viability and efficient repair of DNA single-strand breaks (SSBs) in rodents. XRCC1-deficient mouse or hamster cells are hypersensitive to DNA damaging agents generating SSBs and display genetic instability after such DNA damage. The presence of certain polymorphisms in the human XRCC1 gene has been associated with altered cancer risk, but the role of XRCC1 in SSB repair (SSBR) in human cells is poorly defined. To elucidate this role, we used RNA interference to modulate XRCC1 protein levels in human cell lines. A reduction in XRCC1 protein levels resulted in decreased SSBR capacity as measured by the comet assay and intracellular NAD(P)H levels, hypersensitivity to the cell killing effects of the DNA damaging agents methyl methanesulfonate (MMS), hydrogen peroxide and ionizing radiation and enhanced formation of micronuclei following exposure to MMS. Lowered XRCC1 protein levels were also associated with a significant delay in S-phase progression after exposure to MMS. These data clearly demonstrate that XRCC1 is required for efficient SSBR and genomic stability in human cells.     

XRCC1 protein; Form and function

The human gene that encodes XRCC1 was cloned nearly thirty years ago but experimental analysis of this fascinating protein is still unveiling new insights into the DNA damage response. XRCC1 is a molecular scaffold protein that interacts with multiple enzymatic components of DNA single-strand break repair (SSBR) including DNA kinase, DNA phosphatase, DNA polymerase, DNA deadenylase, and DNA ligase activities that collectively are capable of accelerating the repair of a broad range of DNA single-strand breaks (SSBs). Arguably the most exciting aspect of XRCC1 function that has emerged in the last few years is its intimate relationship with PARP1 activity and critical role in preventing hereditary neurodegenerative disease. Here, I provide an update on our current understanding of XRCC1, and on the impact of hereditary mutations in this protein and its protein partners on human disease.     

Poly(ADP-ribose) polymerases in double-strand break repair: Focus on PARP1, PARP2 and PARP3

Poly(ADP-ribosyl)ation (PARylation) is a post-translational modification of proteins catalysed by Poly(ADP-ribose) polymerases (PARP). A wealth of recent advances in the biochemical and functional characterization of the DNA-dependent PARP family members have highlighted their key contribution in the DNA damage response network, the best characterized being the role of PARP1 and PARP2 in the resolution of single-strand breaks as part of the BER/SSBR process. How PARylation contributes to the repair of double-strand breaks is less well defined but has become recently the subject of significant research in the field. The aim of this review is to provide an overview of the current knowledge concerning the role of the DNA-activated PARP1, PARP2 and PARP3 in cellular response to double-strand breaks (DSB). In addition, we outline the biological significance of these properties in response to programmed DNA lesions formed during physiological processes such as antibody repertoire assembly and diversification.     

A Single-Molecule Atomic Force Microscopy Study of PARP1 and PARP2 Recognition of Base Excision Repair DNA Intermediates

Nuclear poly(ADP-ribose) polymerases 1 and 2 (PARP1 and PARP2) catalyze the synthesis of poly(ADP-ribose) (PAR) and use NAD⁺ as a substrate for the polymer synthesis. Both PARP1 and PARP2 are involved in DNA damage response pathways and function as sensors of DNA breaks, including temporary single-strand breaks formed during DNA repair. Consistently, with a role in DNA repair, PARP activation requires its binding to a damaged DNA site, which initiates PAR synthesis. Here we use atomic force microscopy to characterize at the single-molecule level the interaction of PARP1 and PARP2 with long DNA substrates containing a single damage site and representing intermediates of the short-patch base excision repair (BER) pathway. We demonstrated that PARP1 has higher affinity for early intermediates of BER than PARP2, whereas both PARPs efficiently interact with the nick and may contribute to regulation of the final ligation step. The binding of a DNA repair intermediate by PARPs involved a PARP monomer or dimer depending on the type of DNA damage. PARP dimerization influences the affinity of these proteins to DNA and affects their enzymatic activity: the dimeric form is more effective in PAR synthesis in the case of PARP2 but is less effective in the case of PARP1. PARP2 suppresses PAR synthesis catalyzed by PARP1 after single-strand breaks formation. Our study suggests that the functions of PARP1 and PARP2 overlap in BER after a site cleavage and provides evidence for a role of PARP2 in the regulation of PARP1 activity.     

Asbestos-induced pulmonary toxicity: role of DNA damage and apoptosis

Asbestos causes asbestosis and various malignancies by mechanisms that are not clearly defined. Here, we review the accumulating evidence showing that asbestos is directly genotoxic by inducing DNA strand breaks (DNA-SB) and apoptosis in relevant lung target cells. Although the exact mechanisms by which asbestos causes DNA damage and apoptosis are not firmly established, some of the implicated mechanisms include the generation of iron-derived reactive oxygen species (ROS) as well as reactive nitrogen species (RNS), alteration in the mitochondrial function, and activation of the death receptor pathway. We focus on the accumulating evidence implicating ROS. DNA repair mechanisms have a key role in limiting the extent of DNA damage. Recent studies show that asbestos activates DNA repair enzymes such as apurinic/apyrimidinic endonuclease (APE) and poly (ADP-ribose) polymerase (PARP). Asbestos-induced neoplastic transformation may result in the setting where DNA damage overwhelms DNA repair in the face of a persistent proliferative signal. Strategies aimed at limiting asbestos-induced oxidative stress may reduce DNA damage and, as such, prevent malignant transformation.     

Inhibition of poly (ADP-ribose) polymerase activates ATM which is required for subsequent homologous recombination repair

Poly (ADP-ribose) polymerase (PARP-1), ATM and DNA-dependent protein kinase (DNA-PK) are all involved in responding to DNA damage to activate pathways responsible for cellular survival. Here, we demonstrate that PARP-1^−/− cells are sensitive to the ATM inhibitor KU55933 and conversely that AT cells are sensitive to the PARP inhibitor 4-amino-1,8-napthalamide. In addition, PARP-1^−/− cells are shown to be sensitive to the DNA-PK inhibitor NU7026 and DNA-PKcs or Ku80 defective cells shown to be sensitive to PARP inhibitors. We believe PARP inhibition results in an increase in unresolved spontaneous DNA single-strand breaks (SSBs), which collapse replication forks and trigger homologous recombination repair (HRR). We show that ATM is activated following inhibition of PARP. Furthermore, PARP inhibitor-induced HRR is abolished in ATM, but not DNA-PK, inhibited cells. ATM and DNA-PK inhibition together give the same sensitivity to PARP inhibitors as ATM alone, indicating that ATM functions in the same pathways as DNA-PK for survival at collapsed forks, likely in non-homologous end joining (NHEJ). Altogether, we suggest that ATM is activated by PARP inhibitor-induced collapsed replication forks and may function upstream of HRR in the repair of certain types of double-strand breaks (DSBs).