Biochemical evidence for Ku-independent backup pathways of NHEJ

Cells of higher eukaryotes process within minutes double strand breaks (DSBs) in their genome using a non-homologous end joining (NHEJ) apparatus that engages DNA-PKcs, Ku, DNA ligase IV, XRCC4 and other as of yet unidentified factors. Although chemical inhibition, or mutation, in any of these factors delays processing, cells ultimately remove the majority of DNA DSBs using an alternative pathway operating with an order of magnitude slower kinetics. This alternative pathway is active in mutants deficient in genes of the RAD52 epistasis group and frequently joins incorrect ends. We proposed, therefore, that it reflects an alternative form of NHEJ that operates as a backup (B-NHEJ) to the DNA-PK-dependent (D-NHEJ) pathway, rather than homology directed repair of DSBs. The present study investigates the role of Ku in the coordination of these pathways using as a model end joining of restriction endonuclease linearized plasmid DNA in whole cell extracts. Efficient, error-free, end joining observed in such in vitro reactions is strongly inhibited by anti-Ku antibodies. The inhibition requires DNA-PKcs, despite the fact that Ku efficiently binds DNA ends in the presence of antibodies, or in the absence of DNA-PKcs. Strong inhibition of DNA end joining is also mediated by wortmannin, an inhibitor of DNA-PKcs, in the presence but not in the absence of Ku, and this inhibition can be rescued by pre-incubating the reaction with double stranded oligonucleotides. The results are compatible with a role of Ku in directing end joining to a DNA-PK dependent pathway, mediated by efficient end binding and productive interactions with DNA-PKcs. On the other hand, efficient end joining is observed in extracts of cells lacking DNA-PKcs, as well as in Ku-depleted extracts in line with the operation of alternative pathways. Extracts depleted of Ku and DNA-PKcs rejoin blunt ends, as well as homologous ends with 3' or 5' protruding single strands with similar efficiency, but addition of Ku suppresses joining of blunt ends and homologous ends with 3' overhangs. We propose that the affinity of Ku for DNA ends, particularly when cooperating with DNA-PKcs, suppresses B-NHEJ by quickly and efficiently binding DNA ends and directing them to D-NHEJ for rapid joining. A chromatin-based model of DNA DSB rejoining accommodating biochemical and genetic results is presented and deviations between in vitro and in vivo results discussed.

Rad52 and Ku bind to different DNA structures produced early in double-strand break repair

DNA double-strand breaks are repaired by one of two main pathways, non-homologous end joining or homologous recombination. A competition for binding to DNA ends by Ku and Rad52, proteins required for non-homologous end joining and homologous recombination, respectively, has been proposed to determine the choice of repair pathway. In order to test this idea directly, we compared Ku and human Rad52 binding to different DNA substrates. How ever, we found no evidence that these proteins would compete for binding to the same broken DNA ends. Ku bound preferentially to DNA with free ends. Under the same conditions, Rad52 did not bind preferentially to DNA ends. Using a series of defined substrates we showed that it is single-stranded DNA and not DNA ends that were preferentially bound by Rad52. In addition, Rad52 aggregated DNA, bringing different single-stranded DNAs in close proximity. This activity was independent of the presence of DNA ends and of the ability of the single-stranded sequences to form extensive base pairs. Based on these DNA binding characteristics it is unlikely that Rad52 and Ku compete as 'gatekeepers' of different DNA double-strand break repair pathways. Rather, they interact with different DNA substrates produced early in DNA double-strand break repair.

Dissection of human papillomavirus type 33 L2 domains involved in nuclear domains (ND) 10 homing and reorganization

We have recently shown that the minor capsid protein L2 of human papillomavirus type 33 (HPV33) recruits the transcriptional repressor Daxx into nuclear domains (ND) 10 and causes the loss of the transcriptional activator Sp100 from these subnuclear structures. In order to dissect L2 domains involved in nuclear translocation, ND10 homing, loss of Sp100, and recruitment of Daxx, a detailed deletion mutagenesis of L2 was performed. Using immunofluorescence and green fluorescent protein fusions, we have identified two nuclear localization signals (NLS) in the central and C-terminal part of L2, respectively, homologous to previously identified NLS in HPV6B L2 (Sun et al., 1995). We mapped the ND10 localization domain to within a 30 amino acid peptide in the C-terminal half of L2. L2-induced attraction of Daxx into ND10, coimmunoprecipitation of L2 and Daxx, as well as induction of the loss of Sp100 from ND10 require an intact ND10 localization domain. This domain contains conserved PXXP motives characteristic of some protein/protein interacting domains. Our data also suggest that the Daxx/L2 interaction may be the driving force for L2 accumulation in ND10.

Multiple molecular mechanisms contribute to radiation sensitivity in mantle cell lymphoma

Mantle cell lymphomas (MCL) are characterized by their aggressive behavior and poor response to chemotherapy regimens. We report here evidence of increased in vitro radiation sensitivity in two cell lines that we have generated from two MCL patients (UPN1 and UPN2). However, despite their increased radiation sensitivity, UPN2 cells were totally resistant to apoptotic cell death, whereas UPN1 cells underwent massive apoptosis 6 h after irradiation. The frequency of induced chromosomal abnormalities was higher in UPN1 as compared to UPN2. Distinct mechanisms have been found to contribute to this phenotype: a major telomere shortening (UPN1 and UPN2), deletion of one ATM allele and a point mutation in the remaining allele in UPN2, mutation of p53 gene (UPN1 and UPN2) with absence of functional p53 as revealed by functional yeast assays. After irradiation, Ku70 levels in UPN1 increased and decreased in UPN2, whereas in the same conditions, DNA-PKcs protein levels decreased in UPN1 and remained unchanged in UPN2. Thus, irradiation-induced apoptotic cell death can occur despite the nonfunctional status of p53 (UPN1), suggesting activation of a unique pathway in MCL cells for the induction of this event. Overall, our study demonstrates that MCL cells show increased radiation sensitivity, which can be the result of distinct molecular events. These findings could clinically be exploited to increase the dismal response rates of MCL patients to the current chemotherapy regimens.

Activation of the DNA-dependent protein kinase by drug-induced and radiation-induced DNA strand breaks

The DNA-dependent protein kinase (DNA-PK) is a DNA-end activated protein kinase that is required for efficient repair of DNA double-strand breaks (DSBs) and for normal resistance to ionizing radiation. DNA-PK is composed of a DNA-binding subunit, Ku, and a catalytic subunit, DNA-PKcs (PRKDC). We have previously shown that PRKDC is activated when the enzyme interacts with the terminal nucleotides of a DSB. These nucleotides are often damaged when DSBs are introduced by anticancer agents and could therefore prevent recognition by DNA-PK. To determine whether DNA-PK could recognize DNA strand breaks generated by agents used in the treatment of cancer, we damaged plasmid DNA with anticancer drugs and ionizing radiation. The DNA breaks were tested for the ability to activate purified DNA-PK. The data indicate that DSBs produced by bleomycin, calicheamicin and two types of ionizing radiation ((137)Cs gamma rays and N(7+) ions: high and low linear energy transfer, respectively) activate DNA-PK to levels matching the kinase activation obtained with simple restriction endonuclease-induced DSBs. In contrast, the protein-linked DSBs produced by etoposide and topoisomerase II failed to bind and activate DNA-PK. Our findings indicate that DNA-PK recognizes DSBs regardless of chemical complexity but cannot recognize the protein-linked DSBs produced by etoposide and topoisomerase II.

Fission yeast Rhp51 is required for the maintenance of telomere structure in the absence of the Ku heterodimer

The Schizosaccharomyces pombe Ku70-Ku80 heterodimer is required for telomere length regulation. Lack of pku70+ results in telomere shortening and striking rearrangements of telomere-associated sequences. We found that the rearrangements of telomere-associated sequences in pku80+ mutants are Rhp51 dependent, but not Rad50 dependent. Rhp51 bound to telomere ends when the Ku heterodimer was not present at telomere ends. We also found that the single-stranded G-rich tails increased in S phase in wild-type strains, while deletion of pku70+ increased the single-stranded overhang in both G2 and S phase. Based on these observations, we propose that Rhp51 binds to the G-rich overhang and promotes homologous pairing between two different telomere ends in the absence of Ku heterodimer. Moreover, pku80 rhp51 double mutants showed a significantly reduced telomere hybridization signal. Our results suggest that, although Ku heterodimer sequesters Rhp51 from telomere ends to inhibit homologous recombination activity, Rhp51 plays important roles for the maintenance of telomere ends in the absence of the Ku heterodimer.

Differential modulation of Ku70/80 DNA-binding activity in a patient with multiple basal cell carcinomas

Ku70/80 nonhomologous end-joining activity is essential for resolving random DNA double-strand breaks, and the Ku70/80 protein complex has been proposed as "caretaker" of genomic stability. We studied the Ku70/80 heterodimer activity in a patient affected by multiple basal cell carcinomas with a personal history of moderate exposure to ionizing radiation. The Ku70/80 DNA-binding activity was analyzed, by electrophoretic mobility shift assay, in five tumor biopsies from different sites and at distinct clinical stages, and in three matched normal skin samples from the same patient. As control normal tissues from healthy individuals were also tested. The five basal cell carcinomas were classified as "non aggressive" and "aggressive" on the basis of morphologic parameters and expression of the molecular markers bcl-2, Ki67/MIB1, and p53. A 62% increase in the Ku70/80 DNA-binding activity was found in normal skin from the patient, compared to unexposed individuals (p<0.0001). The nuclear activity of the heterodimer was further increased in nonaggressive basal cell carcinomas compared to both matched normal skin from the patient (31%, p=0.0001) and tissues from healthy controls (73%, p=0.0001). Strikingly, the two aggressive basal cell carcinomas tested showed very low Ku70/80 DNA-binding activity with a reduction of 87% compared to normal skin from the patient (p<0.0001) and 64% compared to controls (p=0.001). Although these results are limited to only one patient, together with other recent studies they support the hypothesis that downregulation of the nonhomologous end-joining pathway may be associated with tumor progression.

Interaction of Aurora-A and centrosomin at the microtubule-nucleating site in Drosophila and mammalian cells

A mitosis-specific Aurora-A kinase has been implicated in microtubule organization and spindle assembly in diverse organisms. However, exactly how Aurora-A controls the microtubule nucleation onto centrosomes is unknown. Here, we show that Aurora-A specifically binds to the COOH-terminal domain of a Drosophila centrosomal protein, centrosomin (CNN), which has been shown to be important for assembly of mitotic spindles and spindle poles. Aurora-A and CNN are mutually dependent for localization at spindle poles, which is required for proper targeting of gamma-tubulin and other centrosomal components to the centrosome. The NH2-terminal half of CNN interacts with gamma-tubulin, and induces cytoplasmic foci that can initiate microtubule nucleation in vivo and in vitro in both Drosophila and mammalian cells. These results suggest that Aurora-A regulates centrosome assembly by controlling the CNN's ability to targeting and/or anchoring gamma-tubulin to the centrosome and organizing microtubule-nucleating sites via its interaction with the COOH-terminal sequence of CNN.

The non-homologous end-joining pathway is not involved in the radiosensitization of mammalian cells by heat shock

A synergistic increase in cell killing is observed when a heat-shock is administered prior to, during, or immediately after exposure to ionizing radiation (IR). This phenomenon, known as heat-radiosensitization, is believed to be mediated by inhibition of repair of radiation-induced double strand breaks (DSB) when cells are exposed to temperatures above 42 degrees C. However, the mechanism by which heat inhibits DSB repair is unclear. The bulk of radiation-induced DSBs are repaired via the non-homologous end-joining pathway (NHEJ). Several reports indicate that the Ku70 and Ku80 subunits of the mammalian DNA-dependent protein kinase (DNA-PK), a complex involved in NHEJ, appear to be susceptible to a heat-induced loss of DNA-binding activity, with Ku80 representing the heat-sensitive component. Since the heat-induced loss and subsequent recovery of Ku-DNA binding activity correlates well with heat-radiosensitization, a role for Ku80 and NHEJ in heat-radiosensitization has been proposed. However, direct evidence implicating Ku80 (and NHEJ) in heat-radiosensitization has been indeterminate. In this study, we demonstrate that equitoxic heat treatments at 42.5-45.5 degrees C induce a similar amount of aggregation of Ku80 in human U-1 melanoma cells. These data suggest that the time-temperature-dependent relationship between heat lethality and Ku80 aggregation are similar. However, the aggregation/disaggregation of Ku80 and its transient or permanent inactivation is unrelated to heat-radiosensitization. When survival curves were obtained for irradiated or irradiated and heated Ku80(-/-) mouse embryo fibroblasts (MEFs) and compared with survival curves obtained for wild-type (WT) cells, we found that heat-radiosensitization was not reduced in the Ku80(-/-) cells, but actually increased. Thus, our findings indicate that Ku80 is not essential for heat-radiosensitization. Non-involvement of Ku-dependent or Ku-independent NHEJ pathways in heat-radiosensitization was confirmed by comparing clonogenic survival between DNA ligase IV-defective and WT human cells. Our data therefore implicate homologous recombination in inhibition of repair of radiation-induced DSBs and as a target for heat-radiosensitization.

Paradoxical decrease in mutant frequencies and chromosomal rearrangements in a transgenic lacZ reporter gene in Ku80 null mice deficient in DNA double strand break repair

Repair of DNA double strand breaks (DSB), either by homologous recombination (HR) or nonhomologous end-joining (NHEJ), is essential to maintain genomic stability. To examine the impact of NHEJ deficiency on genomic integrity in Ku80 null (Ku-) mice, the chromosomally integrated shuttle vector pUR288, which includes a lacZ reporter gene, was used to measure mutations in vivo. Unexpectedly, a significant decrease was found in mutant frequencies of Ku- liver (5.04x10(-5)) and brain (4.55x10(-5)) compared to tissues obtained from normal (Ku+) littermates (7.92x10(-5)and 7.30x10(-5), respectively). No significant difference was found in mutant frequencies in spleen from Ku- (7.21x10(-5)) and Ku+ mice (8.16x10(-5)). The determination of the mutant spectrum in lacZ revealed the almost complete absence of chromosomal rearrangements (R) in Ku- tissues (0.5%, 3/616), a notable distinction from Ku+ controls (16.7%, 104/621). These findings suggest that accurate repair of DSB by HR and elimination of cells with unrepaired DNA damage by apoptosis are capable of maintaining genomic stability of the lacZ reporter in Ku- mice.

Molecular cloning, genomic characterization and over-expression of a novel gene, XRRA1, identified from human colorectal cancer cell HCT116Clone2_XRR and macaque testis

BACKGROUND

As part of our investigation into the genetic basis of tumor cell radioresponse, we have isolated several clones with a wide range of responses to X-radiation (XR) from an unirradiated human colorectal tumor cell line, HCT116. Using human cDNA microarrays, we recently identified a novel gene that was down-regulated by two-fold in an XR-resistant cell clone, HCT116Clone2_XRR. We have named this gene as X-ray radiation resistance associated 1 (XRRA1) (GenBank BK000541). Here, we present the first report on the molecular cloning, genomic characterization and over-expression of the XRRA1 gene.

RESULTS

We found that XRRA1 was expressed predominantly in testis of both human and macaque. cDNA microarray analysis showed three-fold higher expression of XRRA1 in macaque testis relative to other tissues. We further cloned the macaque XRRA1 cDNA (GenBank AB072776) and a human XRRA1 splice variant from HCT116Clone2_XRR (GenBank AY163836). In silico analysis revealed the full-length human XRRA1, mouse, rat and bovine Xrra1 cDNAs. The XRRA1 gene comprises 11 exons and spans 64 kb on chromosome 11q13.3. Human and macaque cDNAs share 96% homology. Human XRRA1 cDNA is 1987 nt long and encodes a protein of 559 aa. XRRA1 protein is highly conserved in human, macaque, mouse, rat, pig, and bovine. GFP-XRRA1 fusion protein was detected in both the nucleus and cytoplasm of HCT116 clones and COS-7 cells. Interestingly, we found evidence that COS-7 cells which over-expressed XRRA1 lacked Ku86 (Ku80, XRCC5), a non-homologous end joining (NHEJ) DNA repair molecule, in the nucleus. RT-PCR analysis showed differential expression of XRRA1 after XR in HCT116 clones manifesting significantly different XR responses. Further, we found that XRRA1 was expressed in most tumor cell types. Surprisingly, mouse Xrra1 was detected in mouse embryonic stem cells R1.

CONCLUSIONS

Both XRRA1 cDNA and protein are highly conserved among mammals, suggesting that XRRA1 may have similar functions. Our results also suggest that the genetic modulation of XRRA1 may affect the XR responses of HCT116 clones and that XRRA1 may have a role in the response of human tumor and normal cells to XR. XRRA1 might be correlated with cancer development and might also be an early expressed gene.

Expression of Ku80 in cervical cancer correlates with response to radiotherapy and survival

To reveal the genes relevant for prediction of cervical cancer after radiotherapy, we previously carried out cDNA microarray experiments on primary cervical cancer comparing patients with a complete response (CR) and those with no change (NC). Some of these genes had already been associated with the radiation response, such as x-ray repair cross-complementing 5 (XRCC5), which was found more in radioresistant tumors than in radiosensitive ones. The aim of this study was to confirm the possible roles of XRCC5 mRNA levels by a real-time polymerase chain reaction method in 20 cervical cancers, and Ku80 protein, which is the gene product of XRCC5, using a histopathologic method of formalin-fixed sections of tumor biopsies in determining tumor response to radiotherapy and survival in 89 patients with cervical cancer. The levels of XRCC5 mRNA were 10(4.82) +/- 10(0.2) copies/microg total RNA in tumor tissues in the CR group (mean +/- standard deviation) and 10(4.95) +/- 10(0.32) copies/microg total RNA in those in the NC group. The levels of XRCC5 mRNA were not significantly different between the CR and NC groups. Histopathologic methods revealed 29.2% (26 of 89) of the patients to be Ku80-negative, with Ku80-positive findings in 70.8% (63 of 89). Of the Ku80-negative patients, 19 had CR, 3 had a partial response (PR), and 4 had NC. Of the Ku80-positive patients, 25 had CR, 22 had PR, and 16 had NC. Ku80-negative tumors showed significantly better responses than Ku80-positive ones, comparing CR and PR/NC responses (p = 0.01). In addition, overall survival was significantly better in the Ku80-negative patients as compared with those who were Ku80-positive (p = 0.04). The results of this study suggest that a low expression of Ku80 protein leads to radiosensitivity in cervical cancer and that Ku80 might play a role in treatment outcome.

Autophosphorylation of the catalytic subunit of the DNA-dependent protein kinase is required for efficient end processing during DNA double-strand break repair

The DNA-dependent protein kinase (DNA-PK) plays an essential role in nonhomologous DNA end joining (NHEJ) by initially recognizing and binding to DNA breaks. We have shown that in vitro, purified DNA-PK undergoes autophosphorylation, resulting in loss of activity and disassembly of the kinase complex. Thus, we have suggested that autophosphorylation of the DNA-PK catalytic subunit (DNA-PKcs) may be critical for subsequent steps in DNA repair. Recently, we defined seven autophosphorylation sites within DNA-PKcs. Six of these are tightly clustered within 38 residues of the 4,127-residue protein. Here, we show that while phosphorylation at any single site within the major cluster is not critical for DNA-PK's function in vivo, mutation of several sites abolishes the ability of DNA-PK to function in NHEJ. This is not due to general defects in DNA-PK activity, as studies of the mutant protein indicate that its kinase activity and ability to form a complex with DNA-bound Ku remain largely unchanged. However, analysis of rare coding joints and ends demonstrates that nucleolytic end processing is dramatically reduced in joints mediated by the mutant DNA-PKcs. We therefore suggest that autophosphorylation within the major cluster mediates a conformational change in the DNA-PK complex that is critical for DNA end processing. However, autophosphorylation at these sites may not be sufficient for kinase disassembly.

The degradation of promyelocytic leukemia and Sp100 proteins by herpes simplex virus 1 is mediated by the ubiquitin-conjugating enzyme UbcH5a

Infected cell protein 0 (ICP0) of herpes simplex virus 1 expresses two E3 ubiquitin (Ub) ligase activities mapping in the domains encoded by exons 2 and 3, respectively. Site 1 (exon 3) is responsible for the degradation of the E2 Ub-conjugating enzyme cdc34 whereas site 2 (exon 2) is associated with a ring finger and has been shown to mediate the degradation of promyelocytic leukemia (PML) and Sp100 proteins and the dispersal of nuclear domain 10 (ND10). In in vitro assays site 2 polyubiquitylates the E2 enzymes UbcH5a and UbcH6 but not other (e.g., UbcH7) enzymes. In this article, we show that ectopic expression of dominant negative UbcH5a carrying the substitution C85A delayed or blocked the degradation of PML and Sp100 and dispersal of ND10 whereas ectopic expression of wild-type UbcH5a or dominant negative UbcH6 and UbcH7 carrying the substitutions C131A and C86A, respectively, had no effect. These results link the degradation of PML and Sp100 and the dispersal of ND10 to the E3 activities of ICP0 associated with the UbcH5a E2 enzyme.

Dentate granule cell neurogenesis after seizures induced by pentylenetrazol in rats

Epileptic seizures originating from the limbic system have been shown to stimulate the proliferation rate of granule cell precursors in the adult brain, but it is not clear if other type(s) of seizures have the similar effects. This study examined the effects of pentylenetrazol (PTZ)-induced generalized clonic seizures on dentate granule cell neurogenesis in adult rats. Using systemic bromodeoxyuridine (BrdU) to label dividing cells, we studied the proliferation rate of neural precursor cells in the dentate gyrus at various time points after PTZ-induced seizures. The double-label immunofluorescence with confocal microscopy was used to determine the newborn cell phenotypes. Quantitative analysis of BrdU labeling revealed a significant increase in the proliferation rate of neural precursor cells in the dentate gyrus 3, 7, and 14 days after seizures. The number of BrdU-labeled cells in the dentate gyrus returned to baseline levels by 28 days after the initial seizures. Most of newborn cells migrated into the granule cell layer from the subgranular zone, displayed the neuronal phenotype, and developed morphological characteristics of differentiated dentate granule cells. These results indicated that neuron proliferation in the dentate gyrus was enhanced during a time window (3-14 days) after PTZ-induced seizures. Its underlying mechanism is discussed.

Selective inhibition of class switching to IgG and IgE by recruitment of the HoxC4 and Oct-1 homeodomain proteins and Ku70/Ku86 to newly identified ATTT cis-elements

Immunoglobulin (Ig) class switching is central to the maturation of the antibody response as IgG, IgA, and IgE are endowed with more diverse biological effector functions than IgM. It is induced upon engagement of CD40 on B lymphocytes by CD40L expressed by activated CD4+ T cells and exposure of B cells to T cell-secreted cytokines including interleukin-4 and transforming growth factor-beta. It begins with germ line IH-CH transcription and unfolds through class switch DNA recombination (CSR). We show here that the HoxC4 and Oct-1 homeodomain proteins together with the Ku70/Ku86 heterodimer bind as a complex to newly identified switch (S) regulatory ATTT elements (SREs) in the Igamma and Iepsilon promoters and downstream regions to dampen basal germ line Igamma-Cgamma and Iepsilon-Cepsilon transcriptions and repress CSR to Cgamma and Cepsilon. This mechanism is inactive in the Calpha1/Calpha2 loci because of the lack of SREs in the Ialpha1/Ialpha2 promoters. Accordingly, in resting human IgM+IgD+ B cells, HoxC4, Oct-1, and Ku70/Ku86 can be readily identified as bound to the Igamma and Iepsilon promoters but not the Ialpha1/Ialpha2 promoters. CD40 signaling dissociates the HoxC4.Oct-1. Ku complex from the Igamma and Iepsilon promoter SREs, thereby relieving the IH-CH transcriptional repression and allowing CSR to unfold. Dissociation of HoxC4.Oct-1. Ku from DNA is hampered by CD153 engagement, a CD40-signaling inhibitor. Thus, these findings outline a HoxC4.Oct-1. Ku-dependent mechanism of selective regulation of class switching to IgG and IgE and further suggest distinct co-evolution and shared CSR activation pathways in the Cgamma and Cepsilon as opposed to the Calpha1/Calpha2 loci.

Ku stimulation of DNA ligase IV-dependent ligation requires inward movement along the DNA molecule

The DNA ligase IV.XRCC4 complex (LX) functions in DNA non-homologous-end joining, the main pathway for double-strand break repair in mammalian cells. We show that, in contrast to ligation by T4 ligase, the efficiency of LX ligation of double-stranded (ds) ends is critically dependent upon the length of the DNA substrate. The effect is specific for ds ligation, and LX/DNA binding is not influenced by the substrate length. Ku stimulates LX ligation at concentrations resulting in 1-2 Ku molecules bound per substrate, whereas multiply Ku-bound DNA molecules inhibit ds ligation. The combined footprint of DNA with Ku and LX bound is the sum of each individual footprint suggesting that the two complexes are located in tandem at the DNA end. Inhibition of Ku translocation by the presence of cis-platinum adducts on the DNA substrate severely inhibits ligation by LX. Fluorescence resonance energy transfer analysis using fluorophore-labeled Ku and DNA molecules showed that, as expected, Ku makes close contact with the DNA end and that addition of LX can disrupt this close contact. Finally, we show that recruitment of LX by Ku is impaired in an adenylation-defective mutant providing further evidence that LX interacts directly with the DNA end, possibly via the 5'-phosphate as shown for prokaryotic ligases. Taken together, our results suggest that, when LX binds to a Ku-bound DNA molecule, it causes inward translocation of Ku and that freedom to move inward on the DNA is essential to Ku stimulation of LX activity.

Continuous loss of oocytes throughout meiotic prophase in the normal mouse ovary

The number of germ cells reaches the maximum just prior to entry into meiosis, yet decreases dramatically by a few days after birth in the female mouse, rat, and human. Previous studies have reported a major loss at the pachytene stage of meiotic prophase during fetal development, leading to the hypothesis that chromosomal pairing abnormalities may be a signal for oocyte death. However, the identification as well as the quantification of germ cells in these studies have been questioned. A recent study using Mouse Vasa Homologue (MVH) as a germ cell marker reached a contradictory conclusion claiming that oocyte loss occurs in the mouse only after birth. In the present study, we established a new method to quantify murine germ cells by using Germ Cell Nuclear Antigen-1 (GCNA-1) as a germ cell marker. Comparison of GCNA-1 and MVH immunolabeling revealed that the two markers identify the same population of germ cells. However, nuclear labeling of GCNA-1 was better suited for counting germ cells in histological sections as well as for double labeling with the antibody against synaptonemal complex (SC) proteins in chromosome spreading preparations. The latter experiment demonstrated that the majority of GCNA-1-labeled cells entered and progressed through meiotic prophase during fetal development. The number of GCNA-1-positive cells in the ovary was estimated by counting the labeled cells retained in chromosome spreading preparations and also in histological sections by using the ratio estimation method. Both methods demonstrated a continuous decline in the number of GCNA-1-labeled cells during fetal development when the oocytes progress through meiotic prophase. These observations suggest that multiple causes are responsible for oocyte elimination.

Novel localization of the DNA-PK complex in lipid rafts: a putative role in the signal transduction pathway of the ionizing radiation response

Increased sensitivity to ionizing radiation (IR) has been shown to be due to defects in DNA double-strand break repair machinery. The major pathway in mammalian cells dedicated to the repair of DNA double-strand breaks is by the nonhomologous end-joining machinery. Six components function in this pathway, of which three (Ku70, Ku86, and DNA-PKcs) constitute a protein complex known as DNA-dependent protein kinase (DNA-PK). However, it is now recognized that the cellular radiation response is complex, and radiosensitivity may be also regulated at different levels in the radiation signal transduction pathway. In addition to DNA damage, exposure to IR triggers intracellular signaling cascades that overlap with pathways initiated by ligand engagement to a receptor. In this study, we provide evidence for the novel localization of the DNA-PK complex in lipid rafts. We also show this property is not a generalized characteristic of all DNA repair proteins. Furthermore, we have detected Ku86 in yeast lipid rafts. Our results suggest that the components of this complex might be recruited separately to the plasma membrane by tethering with raft-resident proteins. In addition, we found an irradiation-induced differential protein phosphorylation pattern dependent upon DNA-PKcs in lipid rafts. Thus, we speculate that another role for the DNA-PKcs subunit and perhaps for the holoenzyme is in the signal transduction of IR response.

Continuing primordial germ cell differentiation in the mouse embryo is a cell-intrinsic program sensitive to DNA methylation

The initial cohort of mammalian gametes is established by the proliferation of primordial germ cells in the early embryo. Primordial germ cells first appear in extraembyronic tissues and subsequently migrate to the developing gonad. Soon after they arrive in the gonad, the germ cells cease dividing and undertake sexually dimorphic patterns of development. Male germ cells arrest mitotically, while female germ cells directly enter meiotic prophase I. These sex-specific differentiation events are imposed upon a group of sex-common differentiation events that are shared by XX and XY germ cells. We have studied the appearance of GCNA1, a postmigratory sex-common germ cell marker, in cultures of premigratory germ cells to investigate how this differentiation program is regulated. Cultures in which proliferation was either inhibited or stimulated displayed a similar extent of differentiation as controls, suggesting that some differentiation events are the result of a cell-intrinsic program and are independent of cell proliferation. We also found that GCNA1 expression was accelerated by agents which promote DNA demethylation or histone acetylation. These results suggest that genomic demethylation of proliferative phase primordial germ cells is a mechanism by which germ cell maturation is coordinated.

Involvement of DNA-dependent protein kinase in regulation of the mitochondrial heat shock proteins

Since DNA-dependent protein kinase (DNA-PK) has been known to play a protective role against drug-induced apoptosis, the role of DNA-PK in the regulation of mitochondrial heat shock proteins by anticancer drugs was examined. The levels of basal and drug-induced mitochondrial heat shock proteins of drug-sensitive parental cells were higher than those of multidrug-resistant (MDR) cells. We also demonstrated that the development of MDR might be correlated with the increased expression of Ku-subunit of DNA-PK and concurrent down-regulation of mitochondrial heat shock proteins. The basal mtHsp70 and Hsp60 levels of Ku70(-/-) cells, which were known to be sensitive to anticancer drugs, were higher than those of parental MEF cells, but conversely these mitochondrial heat shock proteins of R7080-6 cells over-expressing both Ku70 and Ku80 were lower than those of parental Rat-1 cells. Also, the mtHsp70 and Hsp60 levels of DNA-PKcs-deficient SCID cells were higher than those of parental CB-17 cells. Our results suggest the possibility that mitochondrial heat shock protein may be one of determinants of drug sensitivity and could be regulated by DNA-PK activity.

Constitutively active mutant D816VKit induces megakayocyte and mast cell differentiation of early haemopoietic cells from murine foetal liver

Mutations of Kit at position D816 have been implicated in mastocytosis, acute myeloid leukaemia and germ cell tumours. Expression of this mutant Kit in cell lines results in factor-independent growth, differentiation and increased survival in vitro and tumourigenicity in vivo. Mutant D816VKit and wild-type Kit were expressed in murine primary haemopoietic cells and grown in stem cell factor (SCF) or the absence of factors. Expression of D816VKit did not lead to transformation as assessed by a colony assay, but resulted in enhanced differentiation of cells when compared to control cells. D816VKit induced an increase in the number of cells differentiating along the megakaryocyte lineage in the absence of factors. SCF had an added effect with an increase in differentiation of mast cells. Expression of wild-type Kit in the presence of SCF also failed to cause transformation and induced differentiation of mast cells and megakaryocytes. We conclude that constitutive expression of D816VKit in primary haemopoietic cells is not a sufficient transforming stimulus but leads to the survival and maturation of cells whose phenotype is influenced by the presence of SCF.

Genetic analysis identifies putative tumor suppressor sites at 2q35-q36.1 and 2q36.3-q37.1 involved in cervical cancer progression

We performed comparative genomic hybridization (CGH) and high-resolution deletion mapping of the long arm of chromosome 2 (2q) in invasive cervical carcinoma (CC). The CGH analyses on 52 CCs identified genetic losses at 2q33-q36, gain of 3q26-q29, and frequent chromosomal amplifications. Characterization of 2q deletions by loss of heterozygosity (LOH) in 60 primary tumors identified two sites of minimal deleted regions at 2q35-q36.1 and 2q36.3-q37.1. To delineate the stage at which these genetic alterations occur in CC progression, we analysed 33 cervical intraepithelial neoplasia (CIN) for LOH. We found that 89% of high-grade (CINII and CINIII) and 40% of low-grade (CINI) CINs exhibited LOH at 2q. To identify the target tumor suppressor gene (TSG), we performed an extensive genetic and epigenetic analyses of a number of candidate genes mapped to the deleted regions. We did not find inactivating mutations in CASP10, BARD1, XRCC5, or PPP1R7 genes mapped to the deleted regions. However, we did find evidence of downregulated gene expression in CFLAR, CASP10 and PPP1R7 in CC cell lines. We also found reactivated gene expression in CC cell lines in vitro after exposure to demethylating and histone deacetylase (HDAC) inhibiting agents. Thus, these data identify frequent chromosomal amplifications in CC, and sites of TSGs at 2q35-q36.1 and 2q36.3-q37.1 that are critical in CC development.

Enhancement of radiation response by roscovitine in human breast carcinoma in vitro and in vivo

Frequent deregulation of cyclin-dependent kinase (CDK) activation associated with loss of cell cycle control was found in most of human cancers. A recent development of a new class of antineoplasic agents targeting the cell cycle emerged as a small molecule CDK inhibitor, roscovitine, which presents potential antiproliferative and antitumoral effects in human tumors. Additional studies reported that roscovitine combined with cytotoxic agents can cooperate with DNA damage to activate p53 protein. However, little is known about the biological effect of roscovitine combined with ionizing radiation (IR) in human carcinoma, and no studies were reported thus far in p53 mutated carcinoma. In the breast cancer cell line MDA-MB 231, which lacks a functional p53 protein, we found a strong radiosensitization effect of roscovitine in vitro by clonogenic survival assay and in vivo in MDA-MB 231 xenograft model. Using Pulse Field Gel Electrophoresis, a strong impairment in DNA-double-strand break rejoining was observed after roscovitine and IR treatment as compared with IR alone. Cell cycle analysis showed a G(2) delay and no increase in radiation-induced apoptosis in the cells treated with IR or roscovitine and IR. On the other hand, we found a significant induction in micronuclei frequency after roscovitine and IR treatment as compared with IR alone. This effect was also observed in BALB murine cells in contrast to SCID murine cells, which are deficient in DNA-PKcs, suggesting a possible DNA-double-strand break repair defect in the nonhomologous end joining pathways. In MDA-MB 231 cells, the radiosensitization effect of roscovitine was associated with an inhibition of the DNA-dependent protein kinase activity caused by a marked decrease in Ku-DNA binding by using the electrophoretic mobility shift assay. In conclusion, we found a novel effect on DNA repair of the CDK inhibitor roscovitine, which acts as a radiosensitizer in vitro and in vivo in breast cancer cells lacking a functional p53.

Role of Pin2/TRF1 in telomere maintenance and cell cycle control

Telomeres are specialized structures found at the extreme ends of chromosomes, which have many functions, including preserving genomic stability, maintaining cell proliferative capacity, and blocking the activation of DNA-damage cell cycle checkpoints. Deregulation of telomere length has been implicated in cancer and ageing. Telomere maintenance is tightly regulated by telomerase and many other telomere-associated proteins and is also closely linked to cell cycle control, especially mitotic regulation. However, little is known about the identity and function of the signaling molecules connecting telomere maintenance and cell cycle control. Pin2/TRF1 was originally identified as a protein bound to telomeric DNA (TRF1) and as a protein involved in mitotic regulation (Pin2). Pin2/TRF1 negatively regulates telomere length and importantly, its function is tightly regulated during the cell cycle, acting as an important regulator of mitosis. Recent identification of many Pin2/TRF1 upstream regulators and downstream targets has provided important clues to understanding the dual roles of Pin2/TRF1 in telomere maintenance and cell cycle control. These results have led us to propose that Pin2/TRF1 functions as a key molecule in connecting telomere maintenance and cell cycle control.