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Query: UMLS:C0004135 (ATM)
 13,001 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Ionizing radiation-induced stabilization and the resultant transient accumulation of the p53 tumor suppressor protein is impaired in cells from ataxia telangiectasia (AT) patients, indicating a key role for ATM, the gene mutated in AT, upstream in the radiation-responsive p53 signaling pathway. Activation of this pathway is generally assumed to be triggered by DNA strand breaks produced directly following genotoxic stress or indirectly during excision repair of DNA lesions. The aim of this study was to identify the triggering signal for induction of p53 in diploid human dermal fibroblasts treated with 4-nitroquinoline 1-oxide (4NQO), a model environmental carcinogen that produces both DNA strand breaks (like ionizing radiation) and alkali-stable bulky DNA lesions (like UV light). 4NQO treatment of fibroblasts cultured from normal and AT donors and those from patients with the UV-hypersensitivity disorder xeroderma pigmentosum (XP, complementation groups A, E and G) resulted in up-regulation of p53 protein. In normal fibroblasts, there was no temporal relationship between the incidence of DNA strand breaks and levels of p53 protein; >90% of strand breaks and alkali-labile sites were repaired over 2 h following treatment with 1 microM 4NQO, whereas approximately 3 h of post-treatment incubation was required to demonstrate a significant rise in p53 protein. In contrast, exposure of normal fibroblasts to gamma-rays resulted in a rapid up-regulation of p53 and the level peaked at 2 h post-irradiation. XP cells with a severe deficiency in the nucleotide excision repair pathway showed abnormally high levels of p53 protein in response to 4NQO treatment, indicating that lesions other than incision-associated DNA strand breaks trigger p53 up-regulation. We observed a consistent, inverse correlation between the ability of the various fibroblast cultures to induce p53 following 4NQO treatment and their DNA repair efficiencies. Treatment with 0.12 microM 4NQO, for example, caused a >2-fold up-regulation of p53 in excision repair-deficient (AT, XPA and XPG) strains without eliciting any effect on p53 levels in repair-proficient (normal and XPE) strains. We conclude that up-regulation of p53 by 4NQO is mediated solely by an ATM-independent mechanism and that the p53 response is primarily triggered by persistent alkali-stable 4NQO-DNA adducts.
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PMID:Inverse correlation between p53 protein levels and DNA repair efficiency in human fibroblast strains treated with 4-nitroquinoline 1-oxide: evidence that lesions other than DNA strand breaks trigger the p53 response. 1035 71

The contribution of DNA strand breaks accumulating in the course of nucleotide excision repair to upregulation of the p53 tumor suppressor protein was investigated in human dermal fibroblast strains after treatment with 254 nm ultraviolet (UV) light. For this purpose, fibroblast cultures were exposed to UV and incubated for 3 h in the presence or absence of l-beta-D-arabinofuranosylcytosine (araC) and/or hydroxyurea (HU), and then assayed for DNA strand breakage and p53 protein levels. As expected from previous studies, incubation of normal and ataxia telangiectasia (AT) fibroblasts with araC and HU after UV irradiation resulted in an accumulation of DNA strand breaks. Such araC/HU-accumulated strand breaks (reflecting nonligated repair-incision events) following UV irradiation were not detected in xeroderma pigmentosum (XP) fibroblast strains belonging to complementation groups A and G. Western blot analysis revealed that normal fibroblasts exhibited little upregulation of p53 (approximately 1.2-fold) when incubated without araC after 5 J/m2 irradiation, but showed significant (three-fold) upregulation of p53 when incubated with araC after irradiation. AraC is known to inhibit nucleotide excision repair at both the damage removal and repair resynthesis steps. Therefore, the potentiation of UV-induced upregulation of p53 evoked by araC in normal cells may be a consequence of either persistent bulky DNA lesions or persistent incision-associated DNA strand breaks. To distinguish between these two possibilities, we determined p53 induction in AT fibroblasts (which do not upregulate p53 in response to DNA strand breakage) and in XP fibroblasts (which do not exhibit incision-associated breaks after UV irradiation). The p53 response after treatment with 5 J/m2 UV and incubation with araC was similar in AT, XPA, XPG and normal fibroblasts. In addition, exposure of XPA and XPG fibroblasts to UV (5, 10 or 20 J/m2) followed by incubation without araC resulted in a strong upregulation of p53. We further demonstrated that HU, an inhibitor of replicative DNA synthesis (but not of nucleotide excision repair), had no significant impact on p53 protein levels in UV irradiated and unirradiated human fibroblasts. We conclude that upregulation of p53 at early times after exposure of diploid human fibroblasts to UV light is triggered by persistent bulky DNA lesions, and that incision-associated DNA strand breaks accumulating in the course of nucleotide excision repair and breaks arising as a result of inhibition of DNA replication contribute little (if anything) to upregulation of p53.
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PMID:Lack of correlation between DNA strand breakage and p53 protein levels in human fibroblast strains exposed to ultraviolet lights. 1104 30

The heterotrimeric DNA-binding protein, replication protein A (RPA), consists of 70-, 34-, and 14-kDa subunits and is involved in maintaining genomic stability by playing key roles in DNA replication, repair, and recombination. RPA participates in these processes through its interaction with other proteins and its strong affinity for single-stranded DNA (ssDNA). RPA-p34 is phosphorylated in a cell-cycle-dependent fashion primarily at Ser-29 and Ser-23, which are consensus sites for Cdc2 cyclin-dependent kinase. By systematically examining RPA-p34 phosphorylation throughout the cell cycle, we have found there are distinct phosphorylated forms of RPA-p34 in different cell-cycle stages. We have isolated and purified a unique phosphorylated form of RPA that is specifically associated with the mitotic phase of the cell cycle. The mitotic form of RPA (m-hRPA) shows no difference in ssDNA binding activity as compared with recombinant RPA (r-hRPA), yet binds less efficiently to double-stranded DNA (dsDNA). These data suggest that mitotic phosphorylation of RPA-p34 inhibits the destabilization of dsDNA by RPA complex, thereby decreasing the binding affinity for dsDNA. The m-hRPA also exhibits altered interactions with certain DNA replication and repair proteins. Using highly purified proteins, m-hRPA exhibited decreased binding to ATM, DNA pol alpha, and DNA-PK as compared to unphosphorylated recombinant RPA (r-hRPA). Dephosphorylation of m-hRPA was able to restore the interaction with each of these proteins. Interestingly, the interaction of RPA with XPA was not altered by RPA phosphorylation. These data suggest that phosphorylation of RPA-p34 plays an important role in regulating RPA functions in DNA metabolism by altering specific protein-protein interactions.
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PMID:RPA phosphorylation in mitosis alters DNA binding and protein-protein interactions. 1264 57

The nucleolus is the site of ribosomal gene transcription, processing of rRNA transcripts and maturation of preribosomal particles. Recent studies have shown that nucleoli are also involved in processes as diverse as aging, proliferation control, stress response and mitotic regulation. The proliferation-dependent nucleolar antigen pKi-67 is a sensitive marker of both proliferative activity and nucleolar integrity. We show that staining for the nucleolar-associated antigen pKi-67 is lost from nucleoli during growth arrest following UV irradiation. Surprisingly, before cells enter growth arrest, Ki-67 staining translocates from nucleolar to nucleoplasmic sites within 4-6 h of irradiation. Ki-67 redistribution is accompanied by segregation of nucleolar components. The timing of p53 response correlates well with pKi-67 translocation, growth arrest and restoration of proliferation. However, nucleolar segregation and pKi-67 translocation occur in the absence of functional p53 and other components of damage response pathways (DNA-PK, CSA, CSB, XPA, XPC, ATM ATR, p38(MAPK) and MEK1). Neither gamma-irradiation nor H(2)O(2) treatment causes pKi-67 translocation or loss of nucleolar integrity. In marked contrast, treatment of cells with UV-mimetic 4-NQO does induce nucleolar disruption and relocalisation of pKi-67, suggesting that bulky adduct formation in rDNA rather than strand breaks is sufficient to cause nucleolar segregation. Our data reveal a previously unrecognized cellular response to genotoxic stress and may reveal novel pathways leading to growth arrest.
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PMID:A p53-independent pathway regulates nucleolar segregation and antigen translocation in response to DNA damage induced by UV irradiation. 1472 May 17

As many as 5% of human cancers appear to be of hereditable etiology. Of the more than 50 characterized familial cancer syndromes, most involve disease affecting multiple organs and many can be traced to one or more abnormalities in specific genes. Studying these syndromes in humans is a difficult task, especially when it comes to genes that may manifest themselves early in gestation. It has been made somewhat easier with the development of genetically engineered mice (GEM) that phenotypically mimic many of these inheritable human cancers. The past 15 years has seen the establishment of mouse lines heterozygous or homozygous null for genes known or suspected of being involved in human cancer syndromes, including APC, ATM, BLM, BRCA1, BRCA2, LKB1, MEN1, MLH, MSH, NF1, TP53, PTEN, RB1, TSC1, TSC2, VHL, and XPA. These lines not only provide models for clinical disease and pathology, but also provide avenues to investigate molecular pathology, gene-gene and protein-tissue interaction, and, ultimately, therapeutic intervention. Possibly of even greater importance, they provide a means of looking at placental and fetal tissues, where genetic abnormalities are often first detected and where they may be most easily corrected. We will review these mouse models, examine their usefulness in medical research, and furnish sources of animals and references.
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PMID:Mouse models of human familial cancer syndromes. 1520 8

An essential component of the ATR (ataxia telangiectasia-mutated and Rad3-related)-activating structure is single-stranded DNA. It has been suggested that nucleotide excision repair (NER) can lead to activation of ATR by generating such a signal, and in yeast, DNA damage processing through the NER pathway is necessary for checkpoint activation during G1. We show here that ultraviolet (UV) radiation-induced ATR signaling is compromised in XPA-deficient human cells during S phase, as shown by defects in ATRIP (ATR-interacting protein) translocation to sites of UV damage, UV-induced phosphorylation of Chk1 and UV-induced replication protein A phosphorylation and chromatin binding. However, ATR signaling was not compromised in XPC-, CSB-, XPF- and XPG-deficient cells. These results indicate that damage processing is not necessary for ATR-mediated S-phase checkpoint activation and that the lesion recognition function of XPA may be sufficient. In contrast, XP-V cells deficient in the UV bypass polymerase eta exhibited enhanced ATR signaling. Taken together, these results suggest that lesion bypass and not lesion repair may raise the level of UV damage that can be tolerated before checkpoint activation, and that XPA plays a critical role in this activation.
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PMID:Opposing effects of the UV lesion repair protein XPA and UV bypass polymerase eta on ATR checkpoint signaling. 1667 50

DNA damage can lead to either DNA repair with cell survival or to apoptotic cell death. Although the biochemical processes underlying DNA repair and apoptosis have been extensively studied, the mechanisms by which cells determine whether the damage will be repaired or the apoptotic pathway will be activated is largely unknown. We have studied the role of nucleotide excision repair (NER) in cisplatin DNA damage-induced apoptotic cell death using both normal human fibroblasts and NER-defective xeroderma pigmentosum (XP) XPA and XPG cells. The caspase-3 activation experiment demonstrated a greatly increased casapse-3 activation in the NER-defective cells following cisplatin treatment. The flow cytometry experiment revealed an altered cell cycle arrest pattern of the NER-defective cells following cisplatin treatment. The results obtained from the Western blot experiment showed that NER defects resulted in enhanced CHK1 phosphorylation and p21 induction after cisplatin treatment. The cisplatin treatment-induced ATM phosphorylation, however, was attenuated in NER-defective cells. The results obtained from our immunoprecipitation experiment further demonstrated that the ATM protein interacted with the TFIIH basal transcription factor and the XPG protein of the NER pathway. It also showed that a functional XPC protein was required for the association of the ATM protein to genomic DNA. These results suggest that the NER process may prevent the cisplatin treatment-induced apoptosis by activating the ATM protein, and that the presence of the XPC protein is essential for recruiting the ATM protein to the DNA template.
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PMID:The involvement of ataxia-telangiectasia mutated protein activation in nucleotide excision repair-facilitated cell survival with cisplatin treatment. 1684 32

In response to DNA damage, mammalian cells activate various DNA repair pathways to remove DNA lesions and, meanwhile, halt cell cycle progressions to allow sufficient time for repair. The nucleotide excision repair (NER) and the ATR-dependent cell cycle checkpoint activation are two major cellular responses to DNA damage induced by UV irradiation. However, how these two processes are coordinated in the response is poorly understood. Here we showed that the essential NER factor XPA (xeroderma pigmentosum group A) underwent nuclear accumulation upon UV irradiation, and strikingly, such an event occurred in an ATR (Ataxia-Telangiectasia mutated and RAD3-related)-dependent manner. Either treatment of cells with ATR kinase inhibitors or transfection of cells with small interfering RNA targeting ATR compromised the UV-induced XPA nuclear translocation. Consistently, the ATR-deficient cells displayed no substantial XPA nuclear translocation while the translocation remained intact in ATM (Ataxia-Telangiectasia mutated)-deficient cells in response to UV irradiation. Moreover, we found that ATR is required for the UV-induced nuclear focus formation of XPA. Taken together, our results suggested that the ATR checkpoint pathway may modulate NER activity through the regulation of XPA redistribution in human cells upon UV irradiation.
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PMID:ATR-dependent checkpoint modulates XPA nuclear import in response to UV irradiation. 1686 73

DDB1, a component of a Cul4A ubiquitin ligase complex, promotes nucleotide excision repair (NER) and regulates DNA replication. We have investigated the role of human DDB1 in maintaining genome stability. DDB1-depleted cells accumulate DNA double-strand breaks in widely dispersed regions throughout the genome and have activated ATM and ATR cell cycle checkpoints. Depletion of Cul4A yields similar phenotypes, indicating that an E3 ligase function of DDB1 is important for genome maintenance. In contrast, depletion of DDB2, XPA, or XPC does not cause activation of DNA damage checkpoints, indicating that defects in NER are not involved. One substrate of DDB1-Cul4A that is crucial for preventing genome instability is Cdt1. DDB1-depleted cells exhibit increased levels of Cdt1 protein and rereplication, despite containing other Cdt1 regulatory mechanisms. The rereplication, accumulation of DNA damage, and activation of checkpoint responses in DDB1-depleted cells require entry into S phase and are partially, but not completely, suppressed by codepletion of Cdt1. Therefore, DDB1 prevents DNA lesions from accumulating in replicating human cells, in part by regulating Cdt1 degradation.
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PMID:DDB1 maintains genome integrity through regulation of Cdt1. 1694 Jan 74

Much effort has long been devoted to unraveling the coordinated cellular response to genotoxic insults. In view of the difficulty of obtaining human biological samples of homogeneous origin, I have established a set of stable human clones where one DNA repair gene has been stably silenced by means of RNA interference. I used pEBVsiRNA plasmids that greatly enhance long-term gene silencing in human cells. My older clones reached >500 days in culture. Knock-down HeLa clones maintained a gene silencing phenotype for an extended period in culture, demonstrating that I was able to mimic cells from cancer-prone syndromes. I have silenced >20 genes acting as sensors/transducers (ATM, ATR, Rad50, NBS1, MRE11, PARG and KIN17), or of different DNA repair pathways. In HeLa cells, I have switched off the expression of genes involved in nucleotide excision repair (XPA, XPC, hHR23A, hHR23B, CSA and CSB), nonhomologous end-joining (DNA-PKcs, XRCC4 and Ligase IV), homologous recombination repair (Rad51 and Rad54), or base excision repair (Ogg1 and Ligase III). These cells displayed the expected DNA repair phenotype. We could envisage untangling the complex network between the different DNA repair pathways. In this study, no viral vehicles, with their attendant ethical and safety concerns, were used.
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PMID:Untangling the relationships between DNA repair pathways by silencing more than 20 DNA repair genes in human stable clones. 1748 20


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