Ku, Artemis, and ataxia-telangiectasia-mutated: Signalling networks in DNA damage

Novel replisome-associated proteins at cellular replication forks in EBV-transformed B lymphocytes

Epstein-Barr virus (EBV) is an oncogenic herpesvirus and WHO class 1 carcinogen that resides in B lymphocytes of nearly all humans. While silent in most, EBV can cause endemic Burkitt lymphoma in children and post-transplant lymphoproliferative disorders/lymphomas in immunocompromised hosts. The pathogenesis of such lymphomas is multifactorial but to a large extent depends on EBV’s ability to aggressively drive cellular DNA replication and B cell proliferation despite cell-intrinsic barriers to replication. One such barrier is oncogenic replication stress which hinders the progression of DNA replication forks. To understand how EBV successfully overcomes replication stress, we examined cellular replication forks in EBV-transformed B cells using iPOND (isolation of Proteins on Nascent DNA)-mass spectrometry and identified several cellular proteins that had not previously been linked to DNA replication. Of eight candidate replisome-associated proteins that we validated at forks in EBV-transformed cells and Burkitt lymphoma-derived cells, three zinc finger proteins (ZFPs) were upregulated early in B cells newly-infected with EBV in culture as well as expressed at high levels in EBV-infected B blasts in the blood of immunocompromised transplant recipients. Expressed highly in S- and G2-phase cells, knockdown of each ZFP resulted in stalling of proliferating cells in the S-phase, cleavage of caspase 3, and cell death. These proteins, newly-identified at replication forks of EBV-transformed and Burkitt lymphoma cells therefore contribute to cell survival and cell cycle progression, and represent novel targets for intervention of EBV-lymphomas while simultaneously offering a window into how the replication machinery may be similarly modified in other cancers.

Cancer cells must overcome chronic replication stress, a central barrier to DNA replication. This is true also for cancers caused by Epstein-Barr virus (EBV). To understand how EBV overcomes this barrier to successfully drive cell proliferation, we isolated proteins associated with the cellular replication machinery in EBV-transformed B lymphocytes and identified several cellular proteins that had not previously been linked to DNA replication in cancer or healthy cells. Three of these were zinc finger proteins enriched at the replication machinery in EBV-transformed and EBV-positive Burkitt lymphoma-derived cells, upregulated in newly-infected B cells, and expressed at high levels in infected B cells from transplant recipients. These zinc finger proteins also contributed towards cell proliferation, survival, and cell cycle progression. While these proteins may also contribute to DNA replication in other cancers, they simultaneously represent potential targets in EBV-cancers, some of which are difficult to treat.

A review on role of ATM gene in hereditary transfer of colorectal cancer

Colorectal cancer found to be the most commonly occurring cancer worldwide which can be prevented by screening and its curable if diagnosed early. Lynch syndrome/HNPCC being an autosomal genetic disease and propensity in forming colorectal cancer is inherited wherein genomic instabilities and epigenetic changes are being the characteristic forms in hereditary cancers. It is very important to determine the polymorphism in several DNA repairing genes such as ATM, RAD51, XRCC2, XRCC3 and XRCC9 to study the risk exploring both the prognosis and the developing of colorectal cancer. The role of ATM gene has been studied which involves in the hereditary transfer of colorectal cancer associated with other related cancers such as stomach, lung and breast cancers. ATM found to be the mutation target and also a modifier gene with more risk of developing the disease by its polymorphism in variant of ATM D1853N. It was identified that ATM gene polymorphism did not drastically change HNPCC age of onset. ATM expression levels were studied and it has been concluded that the complete loss of ATM expression resulted in a propensity of worse survival and no better prognosis with increase in mortality rate. This ATM gene might be considered to be a predicted biomarker in colorectal cancer.

Docosahexaenoic Acid Induces Oxidative DNA Damage and Apoptosis, and Enhances the Chemosensitivity of Cancer Cells

The human diet contains low amounts of ω-3 polyunsaturated fatty acids (PUFAs) and high amounts of ω-6 PUFAs, which has been reported to contribute to the incidence of cancer. Epidemiological studies have shown that a high consumption of fish oil or ω-3 PUFAs reduced the risk of colon, pancreatic, and endometrial cancers. The ω-3 PUFA, docosahexaenoic acid (DHA), shows anticancer activity by inducing apoptosis of some human cancer cells without toxicity against normal cells. DHA induces oxidative stress and oxidative DNA adduct formation by depleting intracellular glutathione (GSH) and decreasing the mitochondrial function of cancer cells. Oxidative DNA damage and DNA strand breaks activate DNA damage responses to repair the damaged DNA. However, excessive DNA damage beyond the capacity of the DNA repair processes may initiate apoptotic signaling pathways and cell cycle arrest in cancer cells. DHA shows a variable inhibitory effect on cancer cell growth depending on the cells’ molecular properties and degree of malignancy. It has been shown to affect DNA repair processes including DNA-dependent protein kinases and mismatch repair in cancer cells. Moreover, DHA enhanced the efficacy of anticancer drugs by increasing drug uptake and suppressing survival pathways in cancer cells. In this review, DHA-induced oxidative DNA damage, apoptotic signaling, and enhancement of chemosensitivity in cancer cells will be discussed based on recent studies.

Cytotoxicity and Genotoxicity Assessment of Sandalwood Essential Oil in Human Breast Cell Lines MCF-7 and MCF-10A

Sandalwood essential oil (SEO) is extracted from Santalum trees. Although α-santalol, a main constituent of SEO, has been studied as a chemopreventive agent, the genotoxic activity of the whole oil in human breast cell lines is still unknown. The main objective of this study was to assess the cytotoxic and genotoxic effects of SEO in breast adenocarcinoma (MCF-7) and nontumorigenic breast epithelial (MCF-10A) cells. Proteins associated with SEO genotoxicity were identified using a proteomics approach. Commercially available, high-purity, GC/MS characterized SEO was used to perform the experiments. The main constituents reported in the oil were (Z)-α-santalol (25.34%), (Z)-nuciferol (18.34%), (E)-β-santalol (10.97%), and (E)-nuciferol (10.46%). Upon exposure to SEO (2-8 μg/mL) for 24 hours, cell proliferation was determined by the MTT assay. Alkaline and neutral comet assays were used to assess genotoxicity. SEO exposure induced single- and double-strand breaks selectively in the DNA of MCF-7 cells. Quantitative LC/MS-based proteomics allowed identification of candidate proteins involved in this response: Ku70 (p = 1.37E - 2), Ku80 (p = 5.8E - 3), EPHX1 (p = 3.3E - 3), and 14-3-3ζ (p = 4.0E - 4). These results provide the first evidence that SEO is genotoxic and capable of inducing DNA single- and double-strand breaks in MCF-7 cells.

Role of transgene regulation in ex vivo lentiviral correction of artemis deficiency

Artemis is a single-stranded endonuclease, deficiency of which results in a radiation-sensitive form of severe combined immunodeficiency (SCID-A) most effectively treated by allogeneic hematopoietic stem cell (HSC) transplantation and potentially treatable by administration of genetically corrected autologous HSCs. We previously reported cytotoxicity associated with Artemis overexpression and subsequently characterized the human Artemis promoter with the intention to provide Artemis expression that is nontoxic yet sufficient to support immunodevelopment. Here we compare the human Artemis promoter (APro) with the moderate-strength human phosphoglycerate kinase (PGK) promoter and the strong human elongation factor-1α (EF1α) promoter to regulate expression of Artemis after ex vivo lentiviral transduction of HSCs in a murine model of SCID-A. Recipient animals treated with the PGK-Artemis vector exhibited moderate repopulation of their immune compartment, yet demonstrated a defective proliferative T lymphocyte response to in vitro antigen stimulation. Animals treated with the EF1α-Artemis vector displayed high levels of T lymphocytes but an absence of B lymphocytes and deficient lymphocyte function. In contrast, ex vivo transduction with the APro-Artemis vector supported effective immune reconstitution to wild-type levels, resulting in fully functional T and B lymphocyte responses. These results demonstrate the importance of regulated Artemis expression in immune reconstitution of Artemis-deficient SCID.

Downregulated Ku70 and ATM associated to poor prognosis in colorectal cancer among Chinese patients

Background

Double-strand DNA breaks (DSBs) are a key factor in carcinogenesis. The necessary repair of DSBs is pivotal in maintaining normal cell division. To address the relationship between altered expression of DSB repair of proteins Ku70 and ataxia-telangiectasia mutated (ATM) in colorectal cancer (CRC), we examined the expression levels and patterns of Ku70 and ATM in CRC samples.

Methods

Expression and coexpression of Ku70 and ATM were investigated by using real-time quantitative polymerase chain reaction assays and confirmed further with fluorescent immunohistochemistry in CRC and pericancerous samples from 112 Chinese patients.

Results

Downexpression patterns for both Ku70 and ATM were found in the CRC samples and were significantly associated with advanced tumor node metastasis stage and decreased 5-year overall survival rate.

Conclusion

Downregulated Ku70 and ATM were associated with poor disease-free survival. Loss of Ku70 and ATM expression might act as a biomarker to predict poor prognosis in patients with CRC.

Oxidative DNA Damage Response in Helicobacter pylori-Infected Mongolian Gerbils

Helicobacter pylori (H. pylori) induced DNA damage which may be related to gastric cancer development. The DNA damage response coordinates DNA repair, cell-cycle transition, and apoptosis through activation of DNA damage response molecules. The damaged DNA is repaired through non-homologous end joining (NHEJ) or homologous recombination (HR). In the present study, we investigated the changes of HR DNA repair proteins (ataxia-telangiectasia-mutated; ATM, ATM and Rad3-related; ATR), NHEJ repair proteins (Ku70/80), cell cycle regulators (Chk1, Chk2), and apoptosis marker (p53/p-p53) were determined in H. pylori-infected Mongolian gerbils. In addition, the effect of an antioxidant N-acetylcysteine (NAC) on H. pylori-induced DNA damage response was determined to assess the involvement of oxidative stress on DNA damage of the animals infected with H. pylori. One week after intragastric inoculation with H. pylori, Mongolian gerbils were fed with basal diet with or without 3% NAC for 6 weeks. After 6 week, the expression levels of DNA repair proteins (Ku70/80, ATM, ATR), cell cycle regulators (Chk1, Chk2) and apoptosis marker (p-p53/p53) were increased in gastric mucosa of Mongolian gerbils, which was suppressed by NAC treatment. In conclusion, oxidative stress mediates H. pylori-induced DNA damage response including NHEJ and HR repairing processes, cell cycle arrest and apoptosis in gastric mucosa of Mongolian gerbils.

Regulation of ku70-bax complex in cells

Ku70, a DNA repair factor in the nucleus, also regulates cell death by binding to the apoptotic protein Bax in the cytoplasm. Acetylation of Ku70 triggers Bax release resulting in Bax dependent cell death. Thus dissociating Bax from Ku70, either by inhibiting histone deacetylase 6 (HDAC6) that deacetylates Ku70 or by increasing Ku70 acetylation induces cell death. Our results showed that in neuroblastoma cells, the depletion of Ku70 results in Bax-dependent cell death. This model provides a rationale for screening Ku70 acetylation modulators that can be tested in clinical trials, either alone or in combination with radiotherapy or DNA-damaging agents for the treatment of cancer.

Nuclear pore complexes in the maintenance of genome integrity

Maintaining genome integrity is crucial for successful organismal propagation and for cell and tissue homeostasis. Several processes contribute to safeguarding the genomic information of cells. These include accurate replication of genetic information, detection and repair of DNA damage, efficient segregation of chromosomes, protection of chromosome ends, and proper organization of genome architecture. Interestingly, recent evidence shows that nuclear pore complexes, the channels connecting the nucleus with the cytoplasm, play important roles in these processes suggesting that these multiprotein platforms are key regulators of genome integrity.

Effects of aging on apoptosis gene expression in oral mucosal tissues

Apoptotic processes are important for physiologic renewal of an intact epithelial barrier and contribute some antimicrobial resistance for bacteria and viruses, as well as anti-inflammatory effects that benefits the mucosa. The oral cavity presents a model of host-bacterial interactions at mucosal surfaces, in which a panoply of microorganisms colonizes various niches in the oral cavity and creates complex multispecies biofilms that challenge the gingival tissues. This report details gene expression in apoptotic pathways that occur in oral mucosal tissues across the lifespan, using a nonhuman primate model. Macaca mulatta primates from 2 to 23 years of age (n = 23) were used in a cross-sectional study to obtain clinical healthy gingival tissues specimens. Further, mRNA was prepared and evaluated using the Affymetrix Rhesus GeneChip and 88 apoptotic pathway genes were evaluated. The results identified significant positive correlations with age in 12 genes and negative correlations with an additional five genes. The gene effects were predicted to alter apoptosis receptor levels, extrinsic apoptotic pathways through caspases, cytokine effects on apoptotic events, Ca(+2)-induced death signaling, cell cycle checkpoints, and potential effects of survival factors. Both the positively and negatively correlated genes within the apoptotic pathways provided evidence that healthy tissues in aging animals exhibit decreased apoptotic potential compared to younger animals. The results suggested that decreased physiologic apoptotic process in the dynamic septic environment of the oral mucosal tissues could increase the risk of aging tissues to undergo destructive disease processes through dysregulated inflammatory responses to the oral microbial burden.

Resveratrol induces gastric cancer cell apoptosis via reactive oxygen species, but independent of sirtuin1

The currently available chemotherapeutic regimens against gastric cancer are not very effective, leading to high recurrence and poor survival. Resveratrol is a naturally occurring polyphenol with potent apoptosis-inducing activity. However, the mechanism underlying its actions remains unknown. In the present study, human gastric adenocarcinoma SGC7901 cells were treated with resveratrol (0, 25, 50, 100 and 200 μmol/L) for 48 h, and cellular apoptosis DNA damage were determined. In certain experiments, cells were incubated with superoxide dismutase (100 U/mL), catalase (300 U/mL) or sirtinol (10 μmol/L) to determine the role of reactive oxygen species (ROS) and sirtuin1 in resveratrol-induced cellular apoptosis. Treatment with resveratrol (50-200 μmol/L) for 48 h significantly induced apoptosis and DNA damage in human gastric cancer SGC7901 cells. This was due to the increased generation of ROS following resveratrol treatment because incubation of cells with superoxide dismutase (100 U/mL) or catalase (300 U/mL) attenuated resveratrol-induced cellular apoptosis. Interestingly, treatment with resveratrol (25-200 μmol/L) did not affect the level and activity of sirtuin1, whereas the sirtuin1 inhibitor sirtinol (10 μmol/L) significantly reduced sirtuin1 activity. Furthermore, treatment with sirtinol (10 μmol/L) did not have any effect on apoptosis induced by resveratrol. These data provide evidence that resveratrol induces apoptosis via ROS, but independent of sirtuin1, in the human gastric cancer cell line SGC7901.

Autocrine regulation of γ-irradiation-induced DNA damage response via extracellular nucleotides-mediated activation of P2Y6 and P2Y12 receptors

A key component of the response to DNA damage caused by ionizing radiation is DNA repair. Release of extracellular nucleotides, such as ATP, from cells plays a role in signaling via P2 receptors. We show here that release of ATP, followed by activation of P2Y receptors, is involved in the response to γ-irradiation-induced DNA damage. Formation of phosphorylated histone variant H2AX (γH2AX) foci, which are induced in nuclei by DNA damage and contribute to accumulation of DNA-repair factors, was increased at 1-3h after γ-ray irradiation (2.0Gy) of human lung cancer A549 cells. Focus formation was suppressed by pre-treatment with the ecto-nucleotidase apyrase. Pre-treatment with ecto-nucleotidase inhibitor ARL67156 or post-treatment with ATP or UTP facilitated induction of γH2AX, indicating that extracellular nucleotides play a role in induction of γH2AX foci. Next, we examined the effect of P2 receptor inhibitors on activation of ataxia telangiectasia mutated (ATM; a protein kinase) and accumulation of 53BP1 (a DNA repair factor), both of which are important for DNA repair, at DNA damage sites. P2Y6 receptor antagonist MRS2578, P2Y12 receptor antagonist clopidogrel, and P2X7 receptor antagonists A438079 and oxATP significantly inhibited these processes. Release of ATP was detected within 2.5min after irradiation, but was blocked by A438079. Activation of ATM and accumulation of 53BP1 were decreased in P2Y6 or P2Y12 receptor-knockdown cells. We conclude that autocrine/paracrine signaling through P2X7-dependent ATP release and activation of P2Y6 and P2Y12 receptors serves to amplify the cellular response to DNA damage caused by γ-irradiation.

The critically ill patient with ataxia-telangiectasia: a case series

OBJECTIVE

To describe the presentation, clinical course, and outcomes of critically ill patients with ataxia-telangiectasia.

DESIGN

Retrospective case series.

SETTING

Adult and pediatric intensive care units at an urban tertiary academic center.

PATIENTS

Seven consecutive patients with confirmed diagnosis of ataxia-telangiectasia had nine intensive care admissions between January 1995 and December 2009.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Mean age at time of admission 15.9 yrs (median, 13.9 yrs; range, 7.3-33.9 yrs). Mean duration of intensive care unit stay was 17 days (median, 9 days; range, 2-39 days). The most common admitting diagnosis was respiratory distress (six of seven patients). There was no difference in ventilator settings or duration of intensive care unit stay between survivors and nonsurvivors (p > .05). Forty-three percent (three of seven patients) survived to intensive care unit discharge with a 3-yr survival that was 14% (one of seven patients).

CONCLUSIONS

Critically ill patients with ataxia-telangiectasia have complex, multisystem diseases. In this case series, the most common intensive care unit admission diagnosis was respiratory failure. Suspected or confirmed bacterial infections were prevalent. Neuropathologic autopsy findings were similar to those previously reported. Special considerations for the critical care of patients with ataxia-telangiectasia are discussed.

Global gene expression changes in human urothelial cells exposed to low-level monomethylarsonous acid

Bladder cancer has been associated with chronic arsenic exposure. Monomethylarsonous acid [MMA(III)] is a metabolite of inorganic arsenic and has been shown to transform an immortalized urothelial cell line (UROtsa) at concentrations 20-fold less than arsenite. MMA(III) was used as a model arsenical to examine the mechanisms of arsenical-induced transformation of urothelium. A microarray analysis was performed to assess the transcriptional changes in UROtsa during the critical window of chronic 50nM MMA(III) exposure that leads to transformation at 3 months of exposure. The analysis revealed only minor changes in gene expression at 1 and 2 months of exposure, contrasting with substantial changes observed at 3 months of exposure. The gene expression changes at 3 months were analyzed showing distinct alterations in biological processes and pathways such as a response to oxidative stress, enhanced cell proliferation, anti-apoptosis, MAPK signaling, as well as inflammation. Twelve genes selected as markers of these particular biological processes were used to validate the microarray and these genes showed a time-dependent changes at 1 and 2 months of exposure, with the most substantial changes occurring at 3 months of exposure. These results indicate that there is a strong association between the acquired phenotypic changes that occur with chronic MMA(III) exposure and the observed gene expression patterns that are indicative of a malignant transformation. Although the substantial changes that occur at 3 months of exposure may be a consequence of transformation, there are common occurrences of altered biological processes between the first 2 months of exposure and the third, which may be pivotal in driving transformation.

Adeno-associated virus type 2 modulates the host DNA damage response induced by herpes simplex virus 1 during coinfection

Adeno-associated virus type 2 (AAV2) is a human parvovirus that relies on a helper virus for efficient replication. Herpes simplex virus 1 (HSV-1) supplies helper functions and changes the environment of the cell to promote AAV2 replication. In this study, we examined the accumulation of cellular replication and repair proteins at viral replication compartments (RCs) and the influence of replicating AAV2 on HSV-1-induced DNA damage responses (DDR). We observed that the ATM kinase was activated in cells coinfected with AAV2 and HSV-1. We also found that phosphorylated ATR kinase and its cofactor ATR-interacting protein were recruited into AAV2 RCs, but ATR signaling was not activated. DNA-PKcs, another main kinase in the DDR, was degraded during HSV-1 infection in an ICP0-dependent manner, and this degradation was markedly delayed during AAV2 coinfection. Furthermore, we detected phosphorylation of DNA-PKcs during AAV2 but not HSV-1 replication. The AAV2-mediated delay in DNA-PKcs degradation affected signaling through downstream substrates. Overall, our results demonstrate that coinfection with HSV-1 and AAV2 provokes a cellular DDR which is distinct from that induced by HSV-1 alone.

End-processing during non-homologous end-joining: a role for exonuclease 1

Non-homologous end-joining (NHEJ) is a critical error-prone pathway of double strand break repair. We recently showed that tyrosyl DNA phosphodiesterase 1 (Tdp1) regulates the accuracy of NHEJ repair junction formation in yeast. We assessed the role of other enzymes in the accuracy of junction formation using a plasmid repair assay. We found that exonuclease 1 (Exo1) is important in assuring accurate junction formation during NHEJ. Like tdp1Δ mutants, exo1Δ yeast cells repairing plasmids with 5′-extensions can produce repair junctions with templated insertions. We also found that exo1Δ mutants have a reduced median size of deletions when joining DNA with blunt ends. Surprisingly, exo1Δ pol4Δ mutants repair blunt ends with a very low frequency of deletions. This result suggests that there are multiple pathways that process blunt ends prior to end-joining. We propose that Exo1 acts at a late stage in end-processing during NHEJ. Exo1 can reverse nucleotide additions occurring due to polymerization, and may also be important for processing ends to expose microhomologies needed for NHEJ. We propose that accurate joining is controlled at two steps, a first step that blocks modification of DNA ends, which requires Tdp1, and a second step that occurs after synapsis that requires Exo1.

Evaluation and management of pulmonary disease in ataxia-telangiectasia

Ataxia-telangiectasia (A-T) is a rare autosomal recessive disorder caused by mutations in the ATM gene, resulting in faulty repair of breakages in double-stranded DNA. The clinical phenotype is complex and is characterized by neurologic abnormalities, immunodeficiencies, susceptibility to malignancies, recurrent sinopulmonary infections, and cutaneous abnormalities. Lung disease is common in patients with A-T and often progresses with age and neurological decline. Diseases of the respiratory system cause significant morbidity and are a frequent cause of death in the A-T population. Lung disease in this population is thought to exhibit features of one or more of the following phenotypes: recurrent sinopulmonary infections with bronchiectasis, interstitial lung disease, and lung disease associated with neurological abnormalities. Here, we review available evidence and present expert opinion on the diagnosis, evaluation, and management of lung disease in A-T, as discussed in a recent multidisciplinary workshop. Although more data are emerging on this unique population, many recommendations are made based on similarities to other more well-studied diseases. Gaps in current knowledge and areas for future research in the field of pulmonary disease in A-T are also outlined.

55K isoform of CDK9 associates with Ku70 and is involved in DNA repair

Positive elongation factor b (P-TEFb) is a cellular protein kinase that is required for RNA polymerase II (RNAP II) transcriptional elongation of protein coding genes. P-TEFb is a set of different molecular complexes, each containing CDK9 as the catalytic subunit. There are two isoforms of the CDK9 protein - the major 42KDa CDK9 isoform and the minor 55KDa isoform that is translated from an in-frame mRNA that arises from an upstream transcriptional start site. We found that shRNA depletion of the 55K CDK9 protein in HeLa cells induces apoptosis and double-strand DNA breaks (DSBs). The levels of apoptosis and DSBs induced by the depletion were reduced by expression of a 55K CDK9 protein variant resistant to the shRNA, indicating that these phenotypes are the consequence of depletion of the 55K protein and not off-target effects. We also found that the 55K CDK9 protein, but not the 42K CDK9 protein, specifically associates with Ku70, a protein involved in DSB repair. Our findings suggest that the 55K CDK9 protein may function in repair of DNA through an association with Ku70.

Ku86 is important for TrkA overexpression-induced breast cancer cell invasion

PURPOSE

We have recently shown that breast tumors express high levels of TrkA compared with normal breast tissues, with TrkA overexpression enhancing breast cancer cell invasion in vitro and metastasis in animal models. In this study, we tried to identify molecules involved in TrkA overexpression-mediated biological effects in breast cancer cells.

EXPERIMENTAL DESIGN

We used a proteomic-based approach to identify proteins involved in TrkA overexpression-stimulated invasion of MDA-MB-231 breast cancer cells. Proteins from control and TrkA overexpressing cells were separated using a cup-loading two-dimensional electrophoresis system before MALDI and LC-MS/MS mass spectrometry analysis.

RESULTS

Among several putative regulated proteins, Ku86 was found increased in TrkA overexpressing cells. Moreover, Ku86 was co-immunoprecipitated with TrkA, suggesting the interaction of these two proteins in TrkA overexpressing cells. Interestingly, inhibition with small-interfering RNA and neutralizing antibodies showed that Ku86 was required for TrkA-stimulated cell invasion.

CONCLUSIONS AND CLINICAL RELEVANCE

These data allowed the identification of Ku86 as a new player involved in metastasis in breast cancer cells. Our findings suggest that TrkA and its down stream signaling pathways should be regarded as potential new targets for the development of future breast cancer therapy.

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.

Cerebral expression of DNA repair protein, Ku70, and its association with cell proliferation following cerebral hypoxia-ischemia in neonatal rats

We hypothesized that increased Ku70 expression could be involved in recovery following cerebral hypoxia-ischemia. We investigated the progression of cerebral alterations in Ku70 expression at different time points (24 h, 72 h, 1 week, 4 weeks and 8 weeks) after hypoxia-ischemia (right carotid artery occlusion plus 1.5h of hypoxia) in neonatal rats. To determine whether in addition to its known role of DNA repair, Ku70 was associated with cell death or cell proliferation we performed double staining for Ku70 and DNA fragmentation or bromodeoxyuridine, respectively. The results show that Ku70 expression was increased in the infarct core and peri-infarct regions at 24h following hypoxia-ischemia. The increased Ku70 expression was transient in the infarct core with a loss of Ku70 positive cells over days. In contrast, in the peri-infarct region the expression of Ku70 remained increased at chronic times 8 weeks following the insult. Cells positive for DNA fragmentation were not co-localized with cells positive for Ku70 after an insult. However, most of the cells positive for bromodeoxyuridine indicative of cell proliferation were positive for Ku70 in the peri-infarct region at 8 weeks after the insult. Considering the roles of Ku70 in DNA repair or inhibiting apoptosis and its co-localization within cells that had undergone proliferation, Ku70 may be considered a potential novel target to enhance recovery following hypoxia-ischemia.