NBS1 regulates a novel apoptotic pathway through Bax activation

Identification of Ku70 Domain-Specific Interactors Using BioID2

Since its inception, proximity-dependent biotin identification (BioID), an in vivo biochemical screening method to identify proximal protein interactors, has seen extensive developments. Improvements and variants of the original BioID technique are being reported regularly, each expanding upon the existing potential of the original technique. While this is advancing our capabilities to study protein interactions under different contexts, we have yet to explore the full potential of the existing BioID variants already at our disposal. Here, we used BioID2 in an innovative manner to identify and map domain-specific protein interactions for the human Ku70 protein. Four HEK293 cell lines were created, each stably expressing various BioID2-tagged Ku70 segments designed to collectively identify factors that interact with different regions of Ku70. Historically, although many interactions have been mapped to the C-terminus of the Ku70 protein, few have been mapped to the N-terminal von Willebrand A-like domain, a canonical protein-binding domain ideally situated as a site for protein interaction. Using this segmented approach, we were able to identify domain-specific interactors as well as evaluate advantages and drawbacks of the BioID2 technique. Our study identifies several potential new Ku70 interactors and validates RNF113A and Spindly as proteins that contact or co-localize with Ku in a Ku70 vWA domain-specific manner.

Bax-induced apoptosis shortens the life span of DNA repair defect Ku70-knockout mice by inducing emphysema

Cells with DNA damage undergo apoptosis or cellular senescence if the damage cannot be repaired. Recent studies highlight that cellular senescence plays a major role in aging. However, age-associated diseases, including emphysema and neurodegenerative disorders, are caused by apoptosis of lung alveolar epithelial cells and neurons, respectively. Therefore, enhanced apoptosis also promotes aging and shortens the life span depending on the cell type. Recently, we reported that ku70(-) (/) (-)bax(-) (/) (-) and ku70(-) (/) (-)bax(+/) (-) mice showed significantly extended life span in comparison with ku70(-) (/) (-)bax(+/+) mice. Ku70 is essential for non-homologous end joining pathway for DNA double strand break repair, and Bax plays an important role in apoptosis. Our study suggests that Bax-induced apoptosis has a significant impact on shortening the life span of ku70(-) (/) (-) mice, which are defective in one of DNA repair pathways. The lung alveolar space gradually enlarges during aging, both in mouse and human, and this age-dependent change results in the decrease of respiration capacity during aging that can lead to emphysema in more severe cases. We found that emphysema occurred in ku70(-) (/) (-) mice at the age of three-months old, and that Bax deficiency was able to suppress it. These results suggest that Bax-mediated apoptosis induces emphysema in ku70(-) (/) (-) mice. We also found that the number of cells, including bronchiolar epithelial cells and type 2 alveolar epithelial cells, shows a higher DNA double strand break damage response in ku70 KO mouse lung than in wild type. Recent studies suggest that non-homologous end joining activity decreases with increased age in mouse and rat model. Together, we hypothesize that the decline of Ku70-dependent DNA repair activity in lung alveolar epithelial cells is one of the causes of age-dependent decline of lung function resulting from excess Bax-mediated apoptosis of lung alveolar epithelial cells (and their progenitor cells).

The ATM kinase inhibitor KU-55933 provides neuroprotection against hydrogen peroxide-induced cell damage via a γH2AX/p-p53/caspase-3-independent mechanism: Inhibition of calpain and cathepsin D

The role of the kinase ataxia-telangiectasia mutated (ATM), a well-known protein engaged in DNA damage repair, in the regulation of neuronal responses to oxidative stress remains unexplored. Thus, the neuroprotective efficacy of KU-55933, a potent inhibitor of ATM, against cell damage evoked by oxidative stress (hydrogen peroxide, H₂O₂) has been studied in human neuroblastoma SH-SY5Y cells and compared with the efficacy of this agent in models of doxorubicin (Dox)- and staurosporine (St)-evoked cell death. KU-55933 inhibited the cell death induced by H₂O₂ or Dox but not by St in undifferentiated (UN-) and retinoic acid-differentiated (RA)-SH-SY5Y cells, with a more pronounced effect in the latter cell phenotype. Furthermore, this ATM inhibitor attenuated the Dox- but not H₂O₂-induced caspase-3 activity in both UN- and RA-SH-SY5Y cells. Although KU-55933 inhibited the H₂O₂- and Dox-induced activation of ATM, it attenuated the toxin-induced phosphorylation of the proteins H2AX and p53 only in the latter model of cell damage. Moreover, the ATM inhibitor prevented the H₂O₂-evoked increases in calpain and cathepsin D activity and attenuated cell damage to a similar degree as inhibitors of calpain (MDL28170) and cathepsin D (pepstatin A). Finally, we confirmed the neuroprotective potential of KU-55933 against the H₂O₂- and Dox-evoked cell damage in primary mouse cerebellar granule cells and in the mouse hippocampal HT-22 cell line. Altogether, our results extend the neuroprotective portfolio of KU-55933 to a model of oxidative stress, with this effect not involving inhibition of the γH2AX/p-p53/caspase-3 pathway and instead associated with the attenuation of calpain and cathepsin D activity.

A new phosphorylated form of Ku70 identified in resistant leukemic cells confers fast but unfaithful DNA repair in cancer cell lines

Ku70-dependent canonical nonhomologous end-joining (c-NHEJ) DNA repair system is fundamental to the genome maintenance and B-cell lineage. c-NHEJ is upregulated and error-prone in incurable forms of chronic lymphocytic leukemia which also displays telomere dysfunction, multiple chromosomal aberrations and the resistance to DNA damage-induced apoptosis. We identify in these cells a novel DNA damage inducible form of phospho-Ku70. In vitro in different cancer cell lines, Ku70 phosphorylation occurs in a heterodimer Ku70/Ku80 complex within minutes of genotoxic stress, necessitating its interaction with DNA damage-induced kinase pS2056-DNA-PKcs and/or pS1981-ATM. The mutagenic effects of phospho-Ku70 are documented by a defective S/G2 checkpoint, accelerated disappearance of γ-H2AX foci and kinetics of DNA repair resulting in an increased level of genotoxic stress-induced chromosomal aberrations. Together, these data unveil an involvement of phospho-Ku70 in fast but inaccurate DNA repair; a new paradigm linked to both the deregulation of c-NHEJ and the resistance of malignant cells.

Bax deficiency extends the survival of Ku70 knockout mice that develop lung and heart diseases

Ku70 (Lupus Ku autoantigen p70) is essential in nonhomologous end joining DNA double-strand break repair, and ku70^−/− mice age prematurely because of increased genomic instability and DNA damage responses. Previously, we found that Ku70 also inhibits Bax, a key mediator of apoptosis. We hypothesized that Bax-mediated apoptosis would be enhanced in the absence of Ku70 and contribute to premature death observed in ku70^−/− mice. Here, we show that ku70^−/−bax^+/− and ku70^−/−bax^−/− mice have better survival, especially in females, than ku70^−/− mice, even though Bax deficiency did not decrease the incidence of lymphoma observed in a Ku70-null background. Moreover, we found that ku70^−/− mice develop lung diseases, like emphysema and pulmonary arterial (PA) occlusion, by 3 months of age. These lung abnormalities can trigger secondary health problems such as heart failure that may account for the poor survival of ku70^−/− mice. Importantly, Bax deficiency appeared to delay the development of emphysema. This study suggests that enhanced Bax activity exacerbates the negative impact of Ku70 deletion. Furthermore, the underlying mechanisms of emphysema and pulmonary hypertension due to PA occlusion are not well understood, and therefore ku70^−/− and Bax-deficient ku70^−/− mice may be useful models to study these diseases.

Increased oxidative stress in AOA3 cells disturbs ATM-dependent DNA damage responses

Ataxia telangiectasia (AT) is caused by a mutation in the ataxia-telangiectasia-mutated (ATM) gene; the condition is associated with hyper-radiosensitivity, abnormal cell-cycle checkpoints, and genomic instability. AT patients also show cerebellar ataxia, possibly due to reactive oxygen species (ROS) sensitivity in neural cells. The ATM protein is a key regulator of the DNA damage response. Recently, several AT-like disorders have been reported. The genes responsible for them are predicted to encode proteins that interact with ATM in the DNA-damage response. Ataxia with oculomotor apraxia types 1-3 (AOA1, 2, and 3) result in a neurodegenerative and cellular phenotype similar to AT; however, the basis of this phenotypic similarity is unclear. Here, we show that the cells of AOA3 patients display aberrant ATM-dependent phosphorylation and apoptosis following γ-irradiation. The ATM-dependent response to H2O2 treatment was abrogated in AOA3 cells. Furthermore, AOA3 cells had reduced ATM activity. Our results suggest that the attenuated ATM-related response is caused by an increase in endogenous ROS in AOA3 cells. Pretreatment of cells with pyocyanin, which induces endogenous ROS production, abolished the ATM-dependent response. Moreover, AOA3 cells had decreased homologous recombination (HR) activity, and pyocyanin pretreatment reduced HR activity in HeLa cells. These results indicate that excess endogenous ROS represses the ATM-dependent cellular response and HR repair in AOA3 cells. Since the ATM-dependent cell-cycle checkpoint is an important block to carcinogenesis, such inactivation of ATM may lead to tumorigenesis as well as neurodegeneration.

The Ku heterodimer: function in DNA repair and beyond

Ku is an abundant, highly conserved DNA binding protein found in both prokaryotes and eukaryotes that plays essential roles in the maintenance of genome integrity. In eukaryotes, Ku is a heterodimer comprised of two subunits, Ku70 and Ku80, that is best characterized for its central role as the initial DNA end binding factor in the "classical" non-homologous end joining (C-NHEJ) pathway, the main DNA double-strand break (DSB) repair pathway in mammals. Ku binds double-stranded DNA ends with high affinity in a sequence-independent manner through a central ring formed by the intertwined strands of the Ku70 and Ku80 subunits. At the break, Ku directly and indirectly interacts with several C-NHEJ factors and processing enzymes, serving as the scaffold for the entire DNA repair complex. There is also evidence that Ku is involved in signaling to the DNA damage response (DDR) machinery to modulate the activation of cell cycle checkpoints and the activation of apoptosis. Interestingly, Ku is also associated with telomeres, where, paradoxically to its DNA end-joining functions, it protects the telomere ends from being recognized as DSBs, thereby preventing their recombination and degradation. Ku, together with the silent information regulator (Sir) complex is also required for transcriptional silencing through telomere position effect (TPE). How Ku associates with telomeres, whether it is through direct DNA binding, or through protein-protein interactions with other telomere bound factors remains to be determined. Ku is central to the protection of organisms through its participation in C-NHEJ to repair DSBs generated during V(D)J recombination, a process that is indispensable for the establishment of the immune response. Ku also functions to prevent tumorigenesis and senescence since Ku-deficient mice show increased cancer incidence and early onset of aging. Overall, Ku function is critical to the maintenance of genomic integrity and to proper cellular and organismal development.

Mutations in the FHA-domain of ectopically expressed NBS1 lead to radiosensitization and to no increase in somatic mutation rates via a partial suppression of homologous recombination

Ionizing radiation induces DNA double-strand breaks (DSBs). Mammalian cells repair DSBs through multiple pathways, and the repair pathway that is utilized may affect cellular radiation sensitivity. In this study, we examined effects on cellular radiosensitivity resulting from functional alterations in homologous recombination (HR). HR was inhibited by overexpression of the forkhead-associated (FHA) domain-mutated NBS1 (G27D/R28D: FHA-2D) protein in HeLa cells or in hamster cells carrying a human X-chromosome. Cells expressing FHA-2D presented partially (but significantly) HR-deficient phenotypes, which were assayed by the reduction of gene conversion frequencies measured with a reporter assay, a decrease in radiation-induced Mre11 foci formation, and hypersensitivity to camptothecin treatments. Interestingly, ectopic expression of FHA-2D did not increase the frequency of radiation-induced somatic mutations at the HPRT locus, suggesting that a partial reduction of HR efficiency has only a slight effect on genomic stability. The expression of FHA-2D rendered the exponentially growing cell population slightly (but significantly) more sensitive to ionizing radiation. This radiosensitization effect due to the expression of FHA-2D was enhanced when the cells were irradiated with split doses delivered at 24-h intervals. Furthermore, enhancement of radiation sensitivity by split dose irradiation was not seen in contact-inhibited G0/G1 populations, even though the cells expressed FHA-2D. These results suggest that the FHA domain of NBS1 might be an effective molecular target that can be used to induce radiosensitization using low molecular weight chemicals, and that partial inhibition of HR might improve the effectiveness of cancer radiotherapy.

Productive replication of human papillomavirus 31 requires DNA repair factor Nbs1

Activation of the ATM (ataxia telangiectasia-mutated kinase)-dependent DNA damage response (DDR) is necessary for productive replication of human papillomavirus 31 (HPV31). We previously found that DNA repair and homologous recombination (HR) factors localize to sites of HPV replication, suggesting that ATM activity is required to recruit factors to viral genomes that can productively replicate viral DNA in a recombination-dependent manner. The Mre11-Rad50-Nbs1 (MRN) complex is an essential component of the DDR that is necessary for ATM-mediated HR repair and localizes to HPV DNA foci. In this study, we demonstrate that the HPV E7 protein is sufficient to increase levels of the MRN complex and also interacts with MRN components. We have found that Nbs1 depletion blocks productive viral replication and results in decreased localization of Mre11, Rad50, and the principal HR factor Rad51 to HPV DNA foci upon differentiation. Nbs1 contributes to the DDR by acting as an upstream activator of ATM in response to double-strand DNA breaks (DSBs) and as a downstream effector of ATM activity in the intra-S-phase checkpoint. We have found that phosphorylation of ATM and its downstream target Chk2, as well as SMC1 (structural maintenance of chromosome 1), is maintained upon Nbs1 knockdown in differentiating cells. Given that ATM and Chk2 are required for productive replication, our results suggest that Nbs1 contributes to viral replication outside its role as an ATM activator, potentially through ensuring localization of DNA repair factors to viral genomes that are necessary for efficient productive replication.

IMPORTANCE

The mechanisms that regulate human papillomavirus (HPV) replication during the viral life cycle are not well understood. Our finding that Nbs1 is necessary for productive replication even in the presence of ATM (ataxia telangiectasia-mutated kinase) and Chk2 phosphorylation offers evidence that Nbs1 contributes to viral replication downstream of facilitating ATM activation. Nbs1 is required for the recruitment of Mre11 and Rad50 to viral genomes, suggesting that the MRN complex plays a direct role in facilitating productive viral replication, potentially through the processing of substrates that are recognized by the key homologous recombination (HR) factor Rad51. The discovery that E7 increases levels of MRN components, and MRN complex formation, identifies a novel role for E7 in facilitating productive replication. Our study not only identifies DNA repair factors necessary for HPV replication but also provides a deeper understanding of how HPV utilizes the DNA damage response to regulate viral replication.

SIRT1/PARP1 crosstalk: connecting DNA damage and metabolism

An intricate network regulates the activities of SIRT1 and PARP1 proteins and continues to be uncovered. Both SIRT1 and PARP1 share a common co-factor nicotinamide adenine dinucleotide (NAD+) and several common substrates, including regulators of DNA damage response and circadian rhythms. We review this complex network using an interactive Molecular Interaction Map (MIM) to explore the interplay between these two proteins. Here we discuss how NAD + competition and post-transcriptional/translational feedback mechanisms create a regulatory network sensitive to environmental cues, such as genotoxic stress and metabolic states, and examine the role of those interactions in DNA repair and ultimately, cell fate decisions.

Mutation inactivation of Nijmegen breakage syndrome gene (NBS1) in hepatocellular carcinoma and intrahepatic cholangiocarcinoma

Nijmegen breakage syndrome (NBS) with NBS1 germ-line mutation is a human autosomal recessive disease characterized by genomic instability and enhanced cancer predisposition. The NBS1 gene codes for a protein, Nbs1(p95/Nibrin), involved in the processing/repair of DNA double-strand breaks. Hepatocellular carcinoma (HCC) is a complex and heterogeneous tumor with several genomic alterations. Recent studies have shown that heterozygous NBS1 mice exhibited a higher incidence of HCC than did wild-type mice. The objective of the present study is to assess whether NBS1 mutations play a role in the pathogenesis of human primary liver cancer, including HBV-associated HCC and intrahepatic cholangiocarcinoma (ICC). Eight missense NBS1 mutations were identified in six of 64 (9.4%) HCCs and two of 18 (11.1%) ICCs, whereas only one synonymous mutation was found in 89 control cases of cirrhosis and chronic hepatitis B. Analysis of the functional consequences of the identified NBS1 mutations in Mre11-binding domain showed loss of nuclear localization of Nbs1 partner Mre11, one of the hallmarks for Nbs1 deficiency, in one HCC and two ICCs with NBS1 mutations. Moreover, seven of the eight tumors with NBS1 mutations had at least one genetic alteration in the TP53 pathway, including TP53 mutation, MDM2 amplification, p14ARF homozygous deletion and promoter methylation, implying a synergistic effect of Nbs1 disruption and p53 inactivation. Our findings provide novel insight on the molecular pathogenesis of primary liver cancer characterized by mutation inactivation of NBS1, a DNA repair associated gene.

Proteomic analysis of mismatch repair-mediated alkylating agent-induced DNA damage response

Background

Mediating DNA damage-induced apoptosis is an important genome-maintenance function of the mismatch repair (MMR) system. Defects in MMR not only cause carcinogenesis, but also render cancer cells highly resistant to chemotherapeutics, including alkylating agents. To understand the mechanisms of MMR-mediated apoptosis and MMR-deficiency-caused drug resistance, we analyze a model alkylating agent (N-methyl-N’-nitro-N-nitrosoguanidine, MNNG)-induced changes in protein phosphorylation and abundance in two cell lines, the MMR-proficient TK6 and its derivative MMR-deficient MT1.

Results

Under an experimental condition that MNNG-induced apoptosis was only observed in MutSα-proficient (TK6), but not in MutSα-deficient (MT1) cells, quantitative analysis of the proteomic data revealed differential expression and phosphorylation of numerous individual proteins and clusters of protein kinase substrates, as well differential activation of response pathways/networks in MNNG-treated TK6 and MT1 cells. Many alterations in TK6 cells are in favor of turning on the apoptotic machinery, while many of those in MT1 cells are to promote cell proliferation and anti-apoptosis.

Conclusions

Our work provides novel molecular insights into the mechanism of MMR-mediated DNA damage-induced apoptosis.

New insights into the link between DNA damage and apoptosis

SIGNIFICANCE

When lesions are unrepaired or there are defects in the DNA repair system, DNA damage is often correlated to apoptosis. However, different kinds of lesions and different degrees of lesion severity can trigger numerous signaling responses.

RECENT ADVANCES

DNA repair proteins involved in specific DNA repair pathways can modulate the function or activity of some apoptotic factors, further emphasizing the crosstalk between DNA damage and cell death.

CRITICAL ISSUES

Here, we discuss the signaling networks that link DNA damage to apoptosis, and we focus on post-translational modifications, leading to crucial changes in protein behavior, following various kinds of DNA damage. Moreover, we analyze the existence of apoptosis-related functions of typical repair proteins, leading to diverse, often-overlapping, DNA damage responses.

FUTURE DIRECTIONS

The better understanding of the regulation and the functionality of key DNA repair proteins, also involved in apoptosis regulation, has the potential of modulating the cell outcomes on DNA damage, particularly in the context of cancer treatment.

Time course changes of anti- and pro-apoptotic proteins in apigenin-induced genotoxicity

Background

Apigenin (4′,5,7-trihydroxyflavone, AP), an active component of many medicinal Chinese herbs, exhibits anticancer properties in vitro and in vivo. This study aims to investigate the genotoxic, cytostatic, and cytotoxic effects of AP and time course changes in the levels of anti- and pro-apoptotic proteins involved in the DNA damage response in HepG2 cells.

Methods

The genotoxic potential of AP was determined by sister chromatid exchanges (SCEs) and chromosomal aberrations (CAs) analysis. The levels of cytostaticity and cytotoxicity were evaluated by the proliferation rate and mitotic indices, respectively. MTT was used to study cytotoxicity, while the induction of apoptosis and the expression of apoptosis-related proteins were determined by ELISA.

Results

At concentrations greater than 10 μM, AP decreased cell survival in a dose- (48 h: 10 vs. 20 μΜ, P < 0.001 and 20 vs. 50 μΜ, P = 0.005; 72 h: 10 vs. 20 μΜ, P < 0.001 and 20 vs. 50 μΜ, P = 0.001) and time-dependent manner (20 μΜ: 24 vs. 48 h, P < 0.001 and 48 vs. 72 h, P = 0.003; 50 μΜ: 24 vs. 48 h, P < 0.001 and 48 vs. 72 h, P < 0.001; 100 μΜ: 24 vs. 48 h, P < 0.001 and 48 vs. 72 h, P < 0.001). SCEs rates, cell proliferation, and mitotic divisions were also affected in a dose-dependent manner (P < 0.001). There was no change in the frequency of aberrant cells (1 μΜ ΑP: P = 0.554; 10 μM AP: P = 0.337; 20 μΜ AP: P = 0.239). Bcl-2 levels were reduced 3 h after AP administration (P = 0.003) and remained reduced throughout the 48 h observation period (6 h, P = 0.044; 12 h, P = 0.001; 24 h, P = 0.042; 48 h, P = 0.012). Bax and soluble Fas exhibited a transient upregulation 24 h after AP treatment. The Bax/Bcl-2 ratio was also increased at 12 h and remained increased throughout the 48 h observation period.

Conclusion

AP exhibited dose-dependent genotoxic potential in HepG2 cells. The protein levels of sFas, Bcl-2, and Bax were affected by AP to promote cell survival and cell death, respectively.

Decoupling of DNA damage response signaling from DNA damages underlies temozolomide resistance in glioblastoma cells

Glioblastoma multiforme (GBM) is the most aggressive primary brain tumor in adults. Current therapy includes surgery, radiation and chemotherapy with temozolomide (TMZ). Major determinants of clinical response to TMZ include methylation status of the O6-methylguanine-DNA methyltransferase (MGMT) promoter and mismatch repair (MMR) status. Though the MGMT promoter is methylated in 45% of cases, for the first nine months of follow-up, TMZ does not change survival outcome. Furthermore, MMR deficiency makes little contribution to clinical resistance, suggesting that there exist unrecognized mechanisms of resistance. We generated paired GBM cell lines whose resistance was attributed to neither MGMT nor MMR. We show that, responding to TMZ, these cells exhibit a decoupling of DNA damage response (DDR) from ongoing DNA damages. They display methylation-resistant synthesis in which ongoing DNA synthesis is not inhibited. They are also defective in the activation of the S and G2 phase checkpoint. DDR proteins ATM, Chk2, MDC1, NBS1 and gammaH2AX also fail to form discrete foci. These results demonstrate that failure of DDR may play an active role in chemoresistance to TMZ. DNA damages by TMZ are repaired by MMR proteins in a futile, reiterative process, which activates DDR signaling network that ultimately leads to the onset of cell death. GBM cells may survive genetic insults in the absence of DDR. We anticipate that our findings will lead to more studies that seek to further define the role of DDR in ultimately determining the fate of a tumor cell in response to TMZ and other DNA methylators.

A functional polymorphism at microRNA-629-binding site in the 3'-untranslated region of NBS1 gene confers an increased risk of lung cancer in Southern and Eastern Chinese population

The genetic variations in NBS1 gene have been reported to be associated with cancer risk. The polymorphisms in 3'-untranslated region (3'-UTR) of NBS1 might affect gene's function and thus contribute to cancer susceptibility. We hypothesized that these polymorphisms of NBS1 are associated with the lung cancer risk. In two independent case-control studies conducted in Southern and Eastern Chinese, we genotyped three tagSNPs (rs14448, rs13312986 and rs2735383) in Southern Chinese and then validated the discovered association in Eastern Chinese. No significant association was observed for rs13312986 and rs14448; we only found that the rs2735383CC genotype had a significantly increased risk of lung cancer under a recessive genetic model in the total 1559 cases versus 1679 controls (odds ratio = 1.40, 95% confidence interval = 1.18-1.66, P = 0.0001) when compared with GG or GC genotypes; the rs2735383CC genotype carriers had lower messenger RNA and protein expression levels in tumor tissues than those of other genotypes as quantitative polymerase chain reaction and western blot shown. Luciferase assay revealed that the rs2735383C allele had a lower transcription activity than G allele, and the hsa-miR-629 but not hsa-miR-499-5P had effect on modulation of NBS1 gene in vitro. We further observed that the X-ray radiation induced more chromatid breaks in lymphocyte cells from the carriers of rs2735383CC homozygote than those from the subjects with other genotypes (P = 0.0008). Our data suggested that the rs2735383G>C variation contributes to an increased risk of lung cancer by diminishing gene's expression through binding of microRNA-629 to the polymorphic site in the 3'-UTR of NBS1 gene.

A novel defect in mitochondrial p53 accumulation following DNA damage confers apoptosis resistance in Ataxia Telangiectasia and Nijmegen Breakage Syndrome T-cells

We have previously shown that whereas T-cells from normal individuals undergo accumulation of p53 and apoptosis when treated with the genotoxic agent Actinomycin D (ActD), those from Ataxia Telangiectasia (AT) and Nijmegen Breakage Syndrome (NBS) patients resist ActD-induced apoptosis [1]. We have now found similar resistance by the p53-null Jurkat T-cell line and by siRNA p53-knockdown normal T-cells. This evidence that ActD initiates a p53-dependent apoptotic responce prompted us to look for defective p53 accumulation by AT and NBS T-cells. Surprisingly the total p53 level was only slightly reduced compared to normal T cells but its intracellular localization was highly defective: p53 was poorly accumulated in the cytosol and nearly undetectable in mitochondria. In accordance with the dependence of ActD-induced apoptosis on a mitochondrial p53 function, in control T-cells specific inhibition of mitochondrial p53 translocation with μ pifithrin reduced apoptosis by 86%, whereas treatment with α pifithrin, which blocks p53-mediated transcription, had no effect. We also showed that nuclear export is not required for mitochondrial p53 translocation. Observation of an altered p53 ubiquitination pattern and Mdm2 accumulation in ActD-treated AT and NBS T-cells provided a mechanistic link to their defective extranuclear p53 localization. Our results disclose an undescribed defect in mitochondrial p53 accumulation in AT and NBS T-cells that makes them resistant to apoptosis following unrepairable DNA damage.

Involvement of DNA-PK and ATM in radiation- and heat-induced DNA damage recognition and apoptotic cell death

Exposure to ionizing radiation and hyperthermia results in important biological consequences, e.g. cell death, chromosomal aberrations, mutations, and DNA strand breaks. There is good evidence that the nucleus, specifically cellular DNA, is the principal target for radiation-induced cell lethality. DNA double-strand breaks (DSBs) are considered to be the most serious type of DNA damage induced by ionizing radiation. On the other hand, verifiable mechanisms which can lead to heat-induced cell death are damage to the plasma membrane and/or inactivation of heat-labile proteins caused by protein denaturation and subsequent aggregation. Recently, several reports have suggested that DSBs can be induced after hyperthermia because heat-induced phosphorylated histone H2AX (γ-H2AX) foci formation can be observed in several mammalian cell lines. In mammalian cells, DSBs are repaired primarily through two distinct and complementary mechanisms: non-homologous end joining (NHEJ), and homologous recombination (HR) or homology-directed repair (HDR). DNA-dependent protein kinase (DNA-PK) and ataxia-telangiectasia mutated (ATM) are key players in the initiation of DSB repair and phosphorylate and/or activate many substrates, including themselves. These phosphorylated substrates have important roles in the functioning of cell cycle checkpoints and in cell death, as well as in DSB repair. Apoptotic cell death is a crucial cell suicide mechanism during development and in the defense of homeostasis. If DSBs are unrepaired or misrepaired, apoptosis is a very important system which can protect an organism against carcinogenesis. This paper reviews recently obtained results and current topics concerning the role of DNA-PK and ATM in heat- or radiation-induced apoptotic cell death.

MRN and the race to the break

In all living cells, DNA is constantly threatened by both endogenous and exogenous agents. In order to protect genetic information, all cells have developed a sophisticated network of proteins, which constantly monitor genomic integrity. This network, termed the DNA damage response, senses and signals the presence of DNA damage to effect numerous biological responses, including DNA repair, transient cell cycle arrests ("checkpoints") and apoptosis. The MRN complex (MRX in yeast), composed of Mre11, Rad50 and Nbs1 (Xrs2), is a key component of the immediate early response to DNA damage, involved in a cross-talk between the repair and checkpoint machinery. Using its ability to bind DNA ends, it is ideally placed to sense and signal the presence of double strand breaks and plays an important role in DNA repair and cellular survival. Here, we summarise recent observation on MRN structure, function, regulation and emerging mechanisms by which the MRN nano-machinery protects genomic integrity. Finally, we discuss the biological significance of the unique MRN structure and summarise the emerging sequence of early events of the response to double strand breaks orchestrated by the MRN complex.

Heterozygous germ-line mutations in the NBN gene predispose to medulloblastoma in pediatric patients

The NBN (NBS1) gene belongs to a group of double-strand break repair genes. Mutations in any of these genes cause genome instability syndromes and contribute to carcinogenesis. NBN gene mutations cause increased tumor risk in Nijmegen breakage syndrome (NBS) homozygotes as well as in NBN heterozygotes. NBS patients develop different types of malignancies; among solid tumors, medulloblastoma (MB), an embryonal tumor of the cerebellum, has been reported most frequently. The majority of medulloblastomas occur sporadically, some of them manifest within familial cancer syndromes. Several signaling pathways are known to be engaged in hereditary and sporadic MB. The aim of our study was to identify mutations in selected exons of the NBN gene and to determine the frequency of the most common NBN gene mutations in pediatric patients with different types of medulloblastoma. We screened a total of 104 patients with MB and identified 7 heterozygous carriers (6.7%) of two different germ-line mutations of NBN gene; all of them had classic MB. Our results indicate that heterozygous carriers of the germ-line NBN gene mutations (c.511A>G and c.657_661del5) may exhibit increased susceptibility to developing MB. The risk of medulloblastoma is estimated to be 3.0 (for c.511A>G) and 4.86 (for c.657_661del5) times higher than in the general Polish population (p<0.05). These results suggest that heterozygous NBN germ-line mutations may contribute to the etiology of medulloblastoma.

Hdm2 is a ubiquitin ligase of Ku70-Akt promotes cell survival by inhibiting Hdm2-dependent Ku70 destabilization

Earlier, we have reported that 70 kDa subunit of Ku protein heterodimer (Ku70) binds and inhibits Bax activity in the cytosol and that ubiquitin (Ub)-dependent proteolysis of cytosolic Ku70 facilitates Bax-mediated apoptosis. We found that Hdm2 (human homolog of murine double minute) has an ability to ubiquitinate Ku70 and that Hdm2 overexpression in cultured cells causes a decrease in Ku70 expression levels. An interaction between Ku70 and Hdm2 was shown by means of immunoprecipitation, whereas none could be shown between 80 kDa subunit of Ku protein heterodimer and Hdm2. Vascular endothelial growth factor (VEGF) is known to inhibit endothelial cell (EC) apoptosis through an Akt-mediated survival kinase signal; however, the mechanism underlying this inhibition of apoptosis has not been fully elucidated. We found that VEGF inhibited cytosolic Ku70 degradation induced by apoptotic stress. It is known that Akt-dependent phosphorylation of Hdm2 causes nuclear translocation of Hdm2 followed by Hdm2-mediated inactivation of p53. We found that VEGF stimulated nuclear translocation of Hdm2 in EC and efficiently inhibited Ku70 degradation. We also found that constitutively active Akt, but not kinase-dead Akt, inhibited Ku70 degradation in the cytosol. Furthermore, Ku70 knockdown diminished antiapoptotic activity of Akt. Taken together, we propose that Hdm2 is a Ku70 Ub ligase and that Akt inhibits Bax-mediated apoptosis, at least in part, by maintaining Ku70 levels through the promotion of Hdm2 nuclear translocation.