The life and death of DNA-PK

New tricks for old drugs: the anticarcinogenic potential of DNA repair inhibitors

Defective or abortive repair of DNA lesions has been associated with carcinogenesis. Therefore it is imperative for a cell to accurately repair its DNA after damage if it is to return to a normal cellular phenotype. In certain circumstances, if DNA damage cannot be repaired completely and with high fidelity, it is more advantageous for an organism to have some of its more severely damaged cells die rather than survive as neoplastic transformants. A number of DNA repair inhibitors have the potential to act as anticarcinogenic compounds. These drugs are capable of modulating DNA repair, thus promoting cell death rather than repair of potentially carcinogenic DNA damage mediated by error-prone DNA repair processes. In theory, exposure to a DNA repair inhibitor during, or immediately after, carcinogenic exposure should decrease or prevent tumorigenesis. However, the ability of DNA repair inhibitors to prevent cancer development is difficult to interpret depending upon the system used and the type of genotoxic stress. Inhibitors may act on multiple aspects of DNA repair as well as the cellular signaling pathways activated in response to the initial damage. In this review, we summarize basic DNA repair mechanisms and explore the effects of a number of DNA repair inhibitors that not only potentiate DNA-damaging agents but also decrease carcinogenicity. In particular, we focus on a novel anti-tumor agent, beta-lapachone, and its potential to block transformation by modulating poly(ADP-ribose) polymerase-1.

Interplay between Ku, Artemis, and the DNA-dependent protein kinase catalytic subunit at DNA ends

Repair of DNA double strand breaks (DSB) by the nonhomologous end-joining pathway in mammals requires at least seven proteins involved in a simplified two-step process: (i) recognition and synapsis of the DNA ends dependent on the DNA-dependent protein kinase (DNA-PK) formed by the Ku70/Ku80 heterodimer and the catalytic subunit DNA-PKcs in association with Artemis; (ii) ligation dependent on the DNA ligase IV.XRCC4.Cernunnos-XLF complex. The Artemis protein exhibits exonuclease and endonuclease activities that are believed to be involved in the processing of a subclass of DSB. Here, we have analyzed the interactions of Artemis and nonhomologous end-joining pathway proteins both in a context of human nuclear cell extracts and in cells. DSB-inducing agents specifically elicit the mobilization of Artemis to damaged chromatin together with DNA-PK and XRCC4/ligase IV proteins. DNA-PKcs is necessary for the loading of Artemis on damaged DNA and is the main kinase that phosphorylates Artemis in cells damaged with highly efficient DSB producers. Under kinase-preventive conditions, both in vitro and in cells, Ku-mediated assembly of DNA-PK on DNA ends is responsible for a dissociation of the DNA-PKcs. Artemis complex. Conversely, DNA-PKcs kinase activity prevents Artemis dissociation from the DNA-PK.DNA complex. Altogether, our data allow us to propose a model in which a DNA-PKcs-mediated phosphorylation is necessary both to activate Artemis endonuclease activity and to maintain its association with the DNA end site. This tight functional coupling between the activation of both DNA-PKcs and Artemis may avoid improper processing of DNA.

Preclinical evaluation of a potent novel DNA-dependent protein kinase inhibitor NU7441

DNA double-strand breaks (DSB) are the most cytotoxic lesions induced by ionizing radiation and topoisomerase II poisons, such as etoposide and doxorubicin. A major pathway for the repair of DSB is nonhomologous end joining, which requires DNA-dependent protein kinase (DNA-PK) activity. We investigated the therapeutic use of a potent, specific DNA-PK inhibitor (NU7441) in models of human cancer. We measured chemosensitization by NU7441 of topoisomerase II poisons and radiosensitization in cells deficient and proficient in DNA-PK(CS) (V3 and V3-YAC) and p53 wild type (LoVo) and p53 mutant (SW620) human colon cancer cell lines by clonogenic survival assay. Effects of NU7441 on DSB repair and cell cycle arrest were measured by gammaH2AX foci and flow cytometry. Tissue distribution of NU7441 and potentiation of etoposide activity were determined in mice bearing SW620 tumors. NU7441 increased the cytotoxicity of ionizing radiation and etoposide in SW620, LoVo, and V3-YAC cells but not in V3 cells, confirming that potentiation was due to DNA-PK inhibition. NU7441 substantially retarded the repair of ionizing radiation-induced and etoposide-induced DSB. NU7441 appreciably increased G(2)-M accumulation induced by ionizing radiation, etoposide, and doxorubicin in both SW620 and LoVo cells. In mice bearing SW620 xenografts, NU7441 concentrations in the tumor necessary for chemopotentiation in vitro were maintained for at least 4 hours at nontoxic doses. NU7441 increased etoposide-induced tumor growth delay 2-fold without exacerbating etoposide toxicity to unacceptable levels. In conclusion, NU7441 shows sufficient proof of principle through in vitro and in vivo chemosensitization and radiosensitization to justify further development of DNA-PK inhibitors for clinical use.

YC-1 Induces S Cell Cycle Arrest and Apoptosis by Activating Checkpoint Kinases

Hypoxia-inducible factor-1alpha (HIF-1alpha) seems central to tumor growth and progression because it up-regulates genes essential for angiogenesis and the hypoxic adaptation of cancer cells, which is why HIF-1alpha inhibition is viewed as a cancer therapy strategy. Paradoxically, HIF-1alpha also leads to cell cycle arrest or the apoptosis of cancer cells. Thus, the possibility cannot be ruled out that HIF-1alpha inhibitors unlock cell cycle arrest under hypoxic conditions and prevent cell death, which would limit the anticancer effect of HIF-1alpha inhibitors. Previously, we reported on the development of YC-1 as an anticancer agent that inhibits HIF-1alpha. In the present study, we evaluated the effects of YC-1 on hypoxia-induced cell cycle arrest and cell death. It was found that YC-1 does not reverse the antiproliferative effect of hypoxia, but rather that it induces S-phase arrest and apoptosis at therapeutic concentrations that inhibit HIF-1alpha and tumor growth; however, YC-1 did not stimulate cyclic guanosine 3',5'-monophosphate production in this concentration range. It was also found that YC-1 activates the checkpoint kinase-mediated intra-S-phase checkpoint, independently of ataxia-telangiectasia mutated kinase or ataxia-telangiectasia mutated and Rad3-related kinase. These results imply that YC-1 does not promote the regrowth of hypoxic tumors because of its cell cycle arrest effect. Furthermore, YC-1 may induce the combined anticancer effects of HIF-1alpha inhibition and cell growth inhibition.

Following the path of the virus: the exploitation of host DNA repair mechanisms by retroviruses

Numerous host cellular cofactors are involved in the life cycle of retroviruses. Importantly, DNA repair machinery of infected cells is activated by retroviruses and retroviral vectors during the process of integration and host cell DNA repair proteins are employed to create a fully integrated provirus. The full delineation of these repair mechanisms that are triggered by retroviruses also has implications outside of the field of retrovirology. It will undoubtedly be of interest to developers of gene therapy and will also further facilitate our understanding of DNA repair and cancer. This review gives a brief summary of the accomplishments in the field of DNA repair and retroviral integration and the opportunities that this area of science provides with regards to the elucidation of repair mechanisms, in the context of retroviral infection.

Emerging cancer therapeutic opportunities target DNA-repair systems

DNA-damaging agents have a central role in non-surgical cancer treatment. The balance between DNA damage and repair determines the final therapeutic consequences. An elevated DNA-repair capacity in tumor cells leads to drug or radiation resistance and severely limits the efficacy of these agents. Interference with DNA repair has emerged as an important approach in combination therapy against cancer. Anticancer targets in DNA-repair systems have emerged, against which several small-molecule compounds are currently undergoing clinical trials.

Differential involvement of phosphatidylinositol 3-kinase-related protein kinases in hyperphosphorylation of replication protein A2 in response to replication-mediated DNA double-strand breaks

Replication protein A2 (RPA2), a component of the RPA heterotrimer, is hyperphosphorylated and forms nuclear foci in response to camptothecin (CPT) that directly induces replication-mediated DNA double-strand breaks (DSBs). Ataxia-telangiectasia mutated and Rad3-related kinase (ATR) and DNA-dependent protein kinase (DNA-PK) are activated by CPT, and RPA2 is hyperphosphorylated in a DNA-PK-dependent manner. To distinguish the roles of phosphatidylinositol 3-kinase-related protein kinases including DNA-PK, ataxia-telangiectasia mutated (ATM), and ATR, in the response to replication-mediated DSBs, we analyzed RPA2 focus formation and hyperphosphorylation during exposure to CPT. ATR knock-down with siRNA suppressed CPT-induced RPA2 hyperphosphorylation and focus formation. CPT-induced RPA2 focus formation was normally observed in DNA-PK- or ATM-deficient cells. Comparison between CPT and hydroxyurea (HU) indirectly inducing DSBs showed that RPA2 hyperphosphorylation is DNA-PK-dependent in CPT-treated cells and DNA-PK-independent in HU-treated cells. Although RPA2 foci rapidly formed in response to HU and CPT, the RPA2 hyperphosphorylation in HU-treated cells occurred later than in the CPT-treated cells, indicating that the DNA-PK dependency of RPA2 hyperphosphorylation is likely to be related to the mode of DSB induction. These results suggest that DNA-PK is responsible for the RPA2 hyperphosphorylation following ATR-dependent RPA2 focus formation in response to replication-mediated DSBs directly induced by CPT.

The DNA repair complex DNA-PK, a pharmacological target in cancer chemotherapy and radiotherapy

A line of investigation in the search for sensitizing tumor cells to chemotherapy or radiotherapy relies on the selection of DNA repair inhibitors. In the area of DNA repair mechanisms, DNA-dependent protein kinase (DNA-PK) represents a key complex. Indeed DNA-PK is involved in the non-homologous end joining (NHEJ) process that corresponds to the major activity responsible for cell survival after ionizing radiation or chemotherapeutic treatment producing DNA double strand breaks. DNA-PK belongs to the PI3-K related kinase family and specific inhibitors have been recently selected and evaluated as radio- and chemo-sensitizers. These drugs, along with other ways to inhibit the DSBs repair process, are presented and discussed.

In vitro functional correction of the mutation responsible for murine severe combined immune deficiency by small fragment homologous replacement

A homologous recombination (HR) approach for site-specific correction of mutations would be highly desirable for the treatment of genetic disorders if recombination efficiencies were sufficiently high as to permit a biological effect. Using a T cell thymoma line derived from severe combined immunodeficient (SCID) mice with a point mutation in the gene encoding the DNA-dependent protein kinase catalytic subunit (DNA-PKcs), we have shown that short DNA fragments (SDFs; 621 bases) can provide genotypic and functional correction of these cells. Double-stranded SDFs (dsSDFs) or single-stranded SDFs (ssSDFs) were designed to span the wild-type sequence of exon 85 in the DNA-PKcs gene and part of the 3' and 5' flanking intron regions. SCID cells were nucleofected with both single- and double-stranded wild-type SDF sequences. Corrected cells were selected on the basis of protection from radiation hypersensitivity that occurs as a consequence of the SCID mutation. Correction was mediated by both SDF forms (double and single stranded). These results indicate that SDFs can correct point mutations by HR with the possibility of harnessing ionizing radiation (IR) as a selection method to eliminate noncorrected cells and enrich for corrected SCID radioresistant cells.

The role of NBS1 in the modulation of PIKK family proteins ATM and ATR in the cellular response to DNA damage

Ataxia telangiectasia mutated (ATM) and ataxia telangiectasia and Rad3-related (ATR) kinases have been considered the primary activators of the cellular response to DNA damage. They belong to the protein kinase family, phosphoinositide 3-kinase-related kinase (PIKKs). In human beings, deficiency of these kinases leads to hereditary diseases, namely ataxia telangiectasia (AT) with ATM deficiency and ATR-Seckel with ATR deficiency. NBS1, a component of MRE11/RAD50/NBS1 (MRN) complex, is another important player in DNA damage response (DDR). Mutations of NBS1 are responsible for Nijmegen breakage syndrome (NBS), a human hereditary disease with the characteristics that almost encompassed those of AT and ATR-Seckel. NBS1 has been conventionally thought to be a downstream substrate of ATM and ATR in DDR; however, recent studies suggest that NBS1/MRN functions upstream of both ATM and ATR by recruiting them to the proximity of DNA damage sites and activating their functions. In this mini-review, we would emphasize the requirement of NBS1 as an upstream mediator for the modulation of PIKK family proteins ATM and ATR.

Molecular pathogenesis of Fanconi anemia: recent progress

A rare genetic disease, Fanconi anemia (FA), now attracts broader attention from cancer biologists and basic researchers in the DNA repair and ubiquitin biology fields as well as from hematologists. FA is a chromosome instability syndrome characterized by childhood-onset aplastic anemia, cancer or leukemia susceptibility, and cellular hypersensitivity to DNA crosslinking agents. Identification of 11 genes for FA has led to progress in the molecular understanding of this disease. FA proteins, including a ubiquitin ligase (FANCL), a monoubiquitinated protein (FANCD2), a helicase (FANCJ/BACH1/BRIP1), and a breast/ovarian cancer susceptibility protein (FANCD1/BRCA2), appear to cooperate in a pathway leading to the recognition and repair of damaged DNA. Molecular interactions among FA proteins and responsible proteins for other chromosome instability syndromes (BLM, NBS1, MRE11, ATM, and ATR) have also been found. Furthermore, inactivation of FA genes has been observed in a wide variety of human cancers in the general population. These findings have broad implications for predicting the sensitivity and resistance of tumors to widely used anticancer DNA crosslinking agents (cisplatin, mitomycin C, and melphalan). Here, we summarize recent progress in the molecular biology of FA and discuss roles of the FA proteins in DNA repair and cancer biology.

Terminal DNA structure and ATP influence binding parameters of the DNA-dependent protein kinase at an early step prior to DNA synapsis

The DNA-dependent protein kinase catalytic subunit (DNA-PKcs) regulates the non-homologous end-joining pathway of DNA double-strand break repair in mammalian cells. The ability of DNA-PKcs to sense and respond to different terminal DNA structures is postulated to be important for its regulatory function. It is unclear whether discrimination occurs at the time of formation of the initial protein–DNA complex or later, at the time of formation of a paired, or synaptic complex between opposing DNA ends. To gain further insight into the mechanism of regulation, we characterized the binding of DNA-PKcs to immobilized DNA fragments that cannot undergo synapsis. Results showed that DNA-PKcs strongly discriminates between different terminal structures at the time of initial complex formation. Although Ku protein stabilizes DNA-PKcs binding overall, it is not required for discrimination between terminal structures. Base mispairing, temperature and the presence of an interstrand linkage influence the stability of the initial complex in a manner that suggests a requirement for DNA unwinding, reminiscent of the ‘open complex’ model of RNA polymerase–promoter DNA interaction. ATP and a nonhydrolyzable ATP analog also influence the stability of the DNA-PKcs•DNA complex, apparently by an allosteric mechanism that does not require DNA-PKcs autophosphorylation.

ATM regulates ATR chromatin loading in response to DNA double-strand breaks

DNA double-strand breaks (DSBs) are among the most deleterious lesions that can challenge genomic integrity. Concomitant to the repair of the breaks, a rapid signaling cascade must be coordinated at the lesion site that leads to the activation of cell cycle checkpoints and/or apoptosis. In this context, ataxia telangiectasia mutated (ATM) and ATM and Rad-3-related (ATR) protein kinases are the earliest signaling molecules that are known to initiate the transduction cascade at damage sites. The current model places ATM and ATR in separate molecular routes that orchestrate distinct pathways of the checkpoint responses. Whereas ATM signals DSBs arising from ionizing radiation (IR) through a Chk2-dependent pathway, ATR is activated in a variety of replication-linked DSBs and leads to activation of the checkpoints in a Chk1 kinase-dependent manner. However, activation of the G2/M checkpoint in response to IR escapes this accepted paradigm because it is dependent on both ATM and ATR but independent of Chk2. Our data provides an explanation for this observation and places ATM activity upstream of ATR recruitment to IR-damaged chromatin. These data provide experimental evidence of an active cross talk between ATM and ATR signaling pathways in response to DNA damage.

MYC-containing double minutes in hematologic malignancies: evidence in favor of the episome model and exclusion of MYC as the target gene

Double minutes (dmin)-circular, extra-chromosomal amplifications of specific acentric DNA fragments-are relatively frequent in malignant disorders, particularly in solid tumors. In acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS), dmin are observed in approximately 1% of the cases. Most of them consist of an amplified segment from chromosome band 8q24, always including the MYC gene. Besides this information, little is known about their internal structure. We have characterized in detail the genomic organization of 32 AML and two MDS cases with MYC-containing dmin. The minimally amplified region was shown to be 4.26 Mb in size, harboring five known genes, with the proximal and the distal amplicon breakpoints clustering in two regions of approximately 500 and 600 kb, respectively. Interestingly, in 23 (68%) of the studied cases, the amplified region was deleted in one of the chromosome 8 homologs at 8q24, suggesting excision of a DNA segment from the original chromosomal location according to the 'episome model'. In one case, sequencing of both the dmin and del(8q) junctions was achieved and provided definitive evidence in favor of the episome model for the formation of dmin. Expression status of the TRIB1 and MYC genes, encompassed by the minimally amplified region, was assessed by northern blot analysis. The TRIB1 gene was found over-expressed in only a subset of the AML/MDS cases, whereas MYC, contrary to expectations, was always silent. The present study, therefore, strongly suggests that MYC is not the target gene of the 8q24 amplifications.

Repair of mitochondrial DNA in aging and carcinogenesis

Mitochondria are responsible for the generation of energy in the form of adenosine triphosphate. These organelles contain their own genetic material, mitochondrial (mt) DNA. This mtDNA has been hypothesized to play a role in the processes of aging and carcinogenesis. Initial reports have shown that there is no repair of cyclobutylpyrimidine dimers (CPD). More recent reports indicate however, that the mitochondrion contains several defence mechanisms against endogenous or exogenous damaging agents such as ultraviolet radiation or oxidative damage. The role of these defence mechanisms in the removal of mitochondrial DNA damage and the link to aging and carcinogenesis-associated processes are discussed in this review.

Inactivating intracellular antiviral responses during adenovirus infection

DNA viruses promote cell cycle progression, stimulate unscheduled DNA synthesis, and present the cell with an extraordinary amount of exogenous DNA. These insults elicit vigorous responses mediated by cellular factors that govern cellular homeostasis. To ensure productive infection, adenovirus has developed means to inactivate these intracellular antiviral responses. Among the challenges to the host cell is the viral DNA genome, which is viewed as DNA damage and elicits a cellular response to inhibit replication. Adenovirus therefore encodes proteins that dismantle the cellular DNA damage machinery. Studying virus-host interactions has yielded insights into the molecular functioning of fundamental cellular mechanisms. In addition, it has suggested ways that viral cytotoxicity can be exploited to offer a selective means of restricted growth in tumor cells as a therapy against cancer. In this review, we discuss aspects of the intracellular response that are unique to adenovirus infection and how adenoviral proteins produced from the early region E4 act to neutralize antiviral defenses, with a particular focus on DNA damage signaling.

The perichromosomal layer

In addition to genetic information, mitotic chromosomes transmit essential components for nuclear assembly and function in a new cell cycle. A specialized chromosome domain, called the perichromosomal layer, perichromosomal sheath, chromosomal coat, or chromosome surface domain, contains proteins required for a variety of cellular processes, including the synthesis of messenger RNA, assembly of ribosomes, repair of DNA double-strand breaks, telomere maintenance, and apoptosis regulation. The layer also contains many proteins of unknown function and is a major target in autoimmune disease. Perichromosomal proteins are found along the entire length of chromosomes, excluding centromeres, where sister chromatids are paired and spindle microtubules attach. Targeting of proteins to the perichromosomal layer occurs primarily during prophase, and they generally remain associated until telophase. During interphase, perichromosomal proteins localize to nucleoli, the nuclear envelope, nucleoplasm, heterochromatin, centromeres, telomeres, and/or the cytoplasm. It has been suggested that the perichromosomal layer may contribute to chromosome structure, as several of the associated proteins have functions in chromatin remodeling during interphase. We review the identified proteins associated with this chromosome domain and briefly discuss their known functions during interphase and mitosis.

Acetylation and deacetylation of non-histone proteins

Since the first report of p53 as a non-histone target of a histone acetyltransferase (HAT), there has been a rapid proliferation in the description of new non-histone targets of HATs. Of these, transcription factors comprise the largest class of new targets. The substrates for HATs extend to cytoskeletal proteins, molecular chaperones and nuclear import factors. Deacetylation of these non-histone proteins by histone deacetylases (HDACs) opens yet another exciting new field of discovery in the role of the dynamic acetylation and deacetylation on cellular function. This review will focus on these non-histone targets of HATs and HDACs and the consequences of their modification.

A proteomic snapshot of the human heat shock protein 90 interactome

Heat shock protein 90 (Hsp90) is a molecular chaperone which modulates several signalling pathways within a cell. By applying co-immunoprecipitation with endogeneous Hsp90, we were able to identify 39 novel protein interaction partners of this chaperone in human embryonic kidney cells (HEK293). Interestingly, levels of DNA-activated protein kinase catalytic subunit, an Hsp90 interaction partner found in this study, were found to be sensitive to Hsp90 inhibitor treatment only in HeLa cells but not in HEK293 cells referring to the tumorgenicity of this chaperone.

PDK2: the missing piece in the receptor tyrosine kinase signaling pathway puzzle

Activation of members of the protein kinase AGC (cAMP dependent, cGMP dependent, and protein kinase C) family is regulated primarily by phosphorylation at two sites: a conserved threonine residue in the activation loop and a serine/threonine residue in a hydrophobic motif (HM) near the COOH terminus. Although phosphorylation of these kinases in the activation loop has been found to be mediated by phosphoinositide-dependent protein kinase-1 (PDK1), the kinase(s) that catalyzes AGC kinase phosphorylation in the HM remains uncharacterized. So far, at least 10 kinases have been suggested to function as an HM kinase or the so-called "PDK2," including mitogen-activated protein (MAP) kinase-activated protein kinase-2 (MK2), integrin-linked kinase (ILK), p38 MAP kinase, protein kinase Calpha (PKCalpha), PKCbeta, the NIMA-related kinase-6 (NEK6), the mammalian target of rapamycin (mTOR), the double-stranded DNA-dependent protein kinase (DNK-PK), and the ataxia telangiectasia mutated (ATM) gene product. However, whether any or all of these kinases act as a physiological HM kinase remains to be established. Nonetheless, available data suggest that multiple systems may be used in cells to regulate the activation of the AGC family kinases. It is possible that, unlike activation loop phosphorylation, phosphorylation of the HM site in the different AGC family kinases is mediated by distinct kinases. In addition, phosphorylation of the AGC family kinase at the HM site could be cell type, signaling pathway, and substrate specific. Identification and characterization of the bonafide HM kinase(s) will be essential to verify these hypotheses.

Hypoxia down-regulates DNA double strand break repair gene expression in prostate cancer cells

BACKGROUND AND PURPOSE

Intratumoral hypoxia has been correlated with poor clinical outcome in prostate cancer. Prostate cancer cells can be genetically unstable and have altered DNA repair. We, therefore, hypothesized that the expression of DNA double-strand break (DNA-dsb) repair genes in normal and malignant prostate cultures can be altered under hypoxic conditions.

METHODS AND MATERIALS

The expression of homologous recombination (HR) and non-homologous recombination (NHEJ) genes following gas hypoxia (0.2%) or exposure to HIF1alpha-inducing agent, CoCl2 (100 microM), was determined for normal diploid fibroblasts (GM05757) and the pre-malignant and malignant prostate cell lines, BPH-1, 22RV-1, DU145 and PC3. RNA and protein levels were determined using RT-PCR and Western blotting. Additionally, p53 genotype and function, the level of hypoxia-induced apoptosis, and cell cycle distribution, were determined to correlate to changes in DNA-dsb gene expression.

RESULTS

Induction of hypoxia was confirmed using HIF1alpha and VEGF expression in gas- and CoCl2-treated cultures. Hypoxia (48-72 h of 0.2% O2) decreased RNA expression of a number of HR-related genes (e.g. Rad51, Rad52, Rad54, BRCA1, BRCA2) in both normal and malignant cultures. Similar decreases in RNA pertaining to the NHEJ-related genes (e.g. Ku70, DNA-PKcs, DNA Ligase IV, Xrcc4) were observed. In selected cases, hypoxia-mediated decreases in RNA expression led to decreased DNA-dsb protein expression. CoCl2-treated cultures did not show decreased DNA-dsb protein expression. The ability of hypoxia to down-regulate Rad51 and other HR-associated genes under hypoxia was not correlated to c-Abl or c-Myc gene expression, p53 genotype or function, propensity for hypoxia-mediated apoptosis, or specific changes in cell cycle distribution.

CONCLUSIONS

Hypoxia can down-regulate expression of DNA-dsb repair genes in both normal and cancer cells. If associated with a functional decrease in DNA-dsb repair, this observation could provide a potential basis for the observed genetic instability within tumor cells exposed to hypoxia.

Ku86 preserves chromatin integrity in cells adapted to high NaCl

Cells adapted to high NaCl have many DNA breaks both in cell culture and in the renal inner medulla in vivo; yet they survive, function, and even proliferate. Here, we show that Ku86 is important for maintaining chromosomal integrity despite the continued presence of DNA breaks. The Ku heterodimer is part of DNA-dependent PK (DNA-PK), a complex that contributes by nonhomologous end joining to repair of double-strand breaks. We demonstrate that cells deficient in Ku86, but not cells deficient in DNA-PKcs (the catalytic subunit of DNA-PK), are hypersensitive to high NaCl as manifested by profound inhibition of proliferation, aberrant mitosis, and increased chromosomal fragmentation. Lower eukaryotes, including the soil nematode Caenorhabditis elegans, lack a DNA-PKcs homologue but are able to adapt to high NaCl. We show that cells of C. elegans adapted to high NaCl have many DNA breaks, similar to the mammalian cells adapted to high NaCl. Ku86 mutant C. elegans as well as C. elegans fed with cku86 dsRNA also display hypersensitivity to high NaCl, characterized by a reduced number of progeny and prolonged generation time in high NaCl. We propose that Ku86 ameliorates the effects of high NaCl-induced DNA breaks in adapted cells by supporting alignment of the broken ends of the DNA and thus maintaining integrity of the fragmented chromatin.

DNA damage response and Ku80 function in the vertebrate embryo

Cellular responses to DNA damage reflect the dynamic integration of cell cycle control, cell–cell interactions and tissue-specific patterns of gene regulation that occurs in vivo but is not recapitulated in cell culture models. Here we describe use of the zebrafish embryo as a model system to identify determinants of the in vivo response to ionizing radiation-induced DNA damage. To demonstrate the utility of the model we cloned and characterized the embryonic function of the XRCC5 gene, which encodes Ku80, an essential component of the nonhomologous end joining pathway of DNA repair. After the onset of zygotic transcription, Ku80 mRNA accumulates in a tissue-specific pattern, which includes proliferative zones of the retina and central nervous system. In the absence of genotoxic stress, zebrafish embryos with reduced Ku80 function develop normally. However, low dose irradiation of these embryos during gastrulation leads to marked apoptosis throughout the developing central nervous system. Apoptosis is p53 dependent, indicating that it is a downstream consequence of unrepaired DNA damage. Results suggest that nonhomologous end joining components mediate DNA repair to promote survival of irradiated cells during embryogenesis.

Non-homologous end joining, but not homologous recombination, enables survival for cells exposed to a histone deacetylase inhibitor

on-homologous end joining (NHEJ) and homologous recombination (HR) are pathways that repair DNA double-strand breaks (DSBs). In Saccharomyces cerevisiae, the repair of these breaks is influenced by histone acetylation. Therefore, we tested mammalian cells deleted for NHEJ (Ku80 or DNA Ligase IV) or altered for HR (breast cancer associated gene, Brca2, or Bloom's syndrome, Blm) for sensitivity to trichostatin A (TSA), a histone deacetylase inhibitor that is being investigated as an anti-cancer therapeutic. We show that cells mutated for Ku80 (ku80-/-) or DNA Ligase IV (lig 4-/-), but not cells mutated for Brca2 (brca2lex1/lex2) or Blm (blm(tm3Brd/tm4Brd)), are hypersensitive to TSA in a dose-dependent manner. TSA-induced toxicity stimulates apoptosis and cell cycle checkpoint responses independent of p53, but does not increase phosphorylated histone H2AX (-H2AX) as compared with a clastogenic agent, camptothecin, indicating that the quantity of DSBs is not the primary cause of TSA-induced cell death. In addition, we show that potential anti-cancer drugs (LY-294002 and vanillin) that inhibit the family of phosphatidylinositol 3 kinases that include the NHEJ protein, DNA-PKCS act in synergy with TSA to reduce the viability of HeLa cells in tissue culture presenting the possibility of using the two drugs in combination to treat cancer.