Ionizing radiation-induced phosphorylation of RPA p34 is deficient in ataxia telangiectasia and reduced in aged normal fibroblasts

Interplay of DNA damage and cell cycle signaling at the level of human replication protein A

Replication protein A (RPA) is the main human single-stranded DNA (ssDNA)-binding protein. It is essential for cellular DNA metabolism and has important functions in human cell cycle and DNA damage signaling. RPA is indispensable for accurate homologous recombination (HR)-based DNA double-strand break (DSB) repair and its activity is regulated by phosphorylation and other post-translational modifications. HR occurs only during S and G2 phases of the cell cycle. All three subunits of RPA contain phosphorylation sites but the exact set of HR-relevant phosphorylation sites on RPA is unknown. In this study, a high resolution capillary isoelectric focusing immunoassay, used under native conditions, revealed the isoforms of the RPA heterotrimer in control and damaged cell lysates in G2. Moreover, the phosphorylation sites of chromatin-bound and cytosolic RPA in S and G2 phases were identified by western and IEF analysis with all available phosphospecific antibodies for RPA2. Strikingly, most of the RPA heterotrimers in control G2 cells are phosphorylated with 5 isoforms containing up to 7 phosphates. These isoforms include RPA2 pSer23 and pSer33. DNA damaged cells in G2 had 9 isoforms with up to 14 phosphates. DNA damage isoforms contained pSer4/8, pSer12, pThr21, pSer23, and pSer33 on RPA2 and up to 8 unidentified phosphorylation sites.

DNA-PK-dependent RPA2 hyperphosphorylation facilitates DNA repair and suppresses sister chromatid exchange

Hyperphosphorylation of RPA2 at serine 4 and serine 8 (S4, S8) has been used as a marker for activation of the DNA damage response. What types of DNA lesions cause RPA2 hyperphosphorylation, which kinase(s) are responsible for them, and what is the biological outcome of these phosphorylations, however, have not been fully investigated. In this study we demonstrate that RPA2 hyperphosphorylation occurs primarily in response to genotoxic stresses that cause high levels of DNA double-strand breaks (DSBs) and that the DNA-dependent protein kinase complex (DNA-PK) is responsible for the modifications in vivo. Alteration of S4, S8 of RPA2 to alanines, which prevent phosphorylations at these sites, caused increased mitotic entry with concomitant increases in RAD51 foci and homologous recombination. Taken together, our results demonstrate that RPA2 hyperphosphorylation by DNA-PK in response to DSBs blocks unscheduled homologous recombination and delays mitotic entry. This pathway thus permits cells to repair DNA damage properly and increase cell viability.

Ionizing radiation-dependent and independent phosphorylation of the 32-kDa subunit of replication protein A during mitosis

The human single-stranded DNA-binding protein, replication protein A (RPA), is regulated by the N-terminal phosphorylation of its 32-kDa subunit, RPA2. RPA2 is hyperphosphorylated in response to various DNA-damaging agents and also phosphorylated in a cell-cycle-dependent manner during S- and M-phase, primarily at two CDK consensus sites, S23 and S29. Here we generated two monoclonal phospho-specific antibodies directed against these CDK sites. These phospho-specific RPA2-(P)-S23 and RPA2-(P)-S29 antibodies recognized mitotically phosphorylated RPA2 with high specificity. In addition, the RPA2-(P)-S23 antibody recognized the S-phase-specific phosphorylation of RPA2, suggesting that during S-phase only S23 is phosphorylated, whereas during M-phase both CDK sites, S23 and S29, are phosphorylated. Immunofluorescence microscopy revealed that the mitotic phosphorylation of RPA2 starts at the onset of mitosis, and dephosphorylation occurs during late cytokinesis. In mitotic cells treated with ionizing radiation (IR), we observed a rapid hyperphosphorylation of RPA2 in addition to its mitotic phosphorylation at S23 and S29, associated with a significant change in the subcellular localization of RPA. Our data also indicate that the RPA2 hyperphosphorylation in response to IR is facilitated by the activity of both ATM and DNA-PK, and is associated with activation of the Chk2 pathway.

Candidate protein biodosimeters of human exposure to ionizing radiation

PURPOSE

To conduct a literature review of candidate protein biomarkers for individual radiation biodosimetry of exposure to ionizing radiation.

MATERIALS AND METHODS

Reviewed approximately 300 publications (1973 - April 2006) that reported protein effects in mammalian systems after either in vivo or in vitro radiation exposure.

RESULTS

We found 261 radiation-responsive proteins including 173 human proteins. Most of the studies used high doses of ionizing radiation (>4 Gy) and had no information on dose- or time-responses. The majority of the proteins showed increased amounts or changes in phosphorylation states within 24 h after exposure (range: 1.5- to 10-fold). Of the 47 proteins that are responsive at doses of 1 Gy and below, 6 showed phosphorylation changes at doses below 10 cGy. Proteins were assigned to 9 groups based on consistency of response across species, dose- and time-response information and known role in the radiation damage response.

CONCLUSIONS

ATM (Ataxia telengiectasia mutated), H2AX (histone 2AX), CDKN1A (Cyclin-dependent kinase inhibitor 1A), and TP53 (tumor protein 53) are top candidate radiation protein biomarkers. Furthermore, we recommend a panel of protein biomarkers, each with different dose and time optima, to improve individual radiation biodosimetry for discriminating between low-, moderate-, and high-dose exposures. Our findings have applications for early triage and follow-up medical assessments.

N-methyl-N'-nitro-N-nitrosoguanidine activates multiple cell death mechanisms in human fibroblasts

Response to genotoxic stress may trigger the activation of distinct mechanisms that serve to promote cell death, including apoptosis and necrosis. In this study we examined the response of human fibroblasts, either proficient or deficient for the damage-activated protein kinase ataxia telangiectasia-mutated (ATM), to the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Analysis of both long- and short-term viability shows that both ATM-proficient YZ-5 and ATM-deficient EBS-7 fibroblasts display a cytotoxic response to MNNG. Consistent with activation of apoptosis in response to MNNG, we observed increased caspase-3 cleavage and activity, appearance of fragmented nuclei, and increased staining with annexin V in both ATM-proficient and -deficient fibroblasts. Flow cytometry demonstrated that these cell lines also display a nonapoptotic cell death in response to MNNG. This form of cell death is associated with activation of poly-ADP ribose polymerase (PARP), and analysis of PARP activity indicated increased protein poly(ADP-ribosylation) in YZ-5 when compared to EBS-7. This PARP activity was accompanied by apoptosis-inducing factor release and translocation from the mitochondria to the nucleus. Finally, the PARP inhibitor 3,4-dihydro-5-[4-(1-piperidinyl)butoxy]-1(2H)-isoquinolinone (DPQ) or the caspase-3 inhibitor benzyloxycarbonyl-VAD-fluoromethyl ketone dramatically diminished the cytotoxic response to MNNG, reinforcing the roles for apoptotic and nonapoptotic cell death in human fibroblasts treated with MNNG. From these findings, we conclude that MNNG induces a heterogeneous death response in human fibroblasts.

RPA mediates recombination repair during replication stress and is displaced from DNA by checkpoint signalling in human cells

The replication protein A (RPA) is involved in most, if not all, nuclear metabolism involving single-stranded DNA. Here, we show that RPA is involved in genome maintenance at stalled replication forks by the homologous recombination repair system in humans. Depletion of the RPA protein inhibited the formation of RAD51 nuclear foci after hydroxyurea-induced replication stalling leading to persistent unrepaired DNA double-strand breaks (DSBs). We demonstrate a direct role of RPA in homology directed recombination repair. We find that RPA is dispensable for checkpoint kinase 1 (Chk1) activation and that RPA directly binds RAD52 upon replication stress, suggesting a direct role in recombination repair. In addition we show that inhibition of Chk1 with UCN-01 decreases dissociation of RPA from the chromatin and inhibits association of RAD51 and RAD52 with DNA. Altogether, our data suggest a direct role of RPA in homologous recombination in assembly of the RAD51 and RAD52 proteins. Furthermore, our data suggest that replacement of RPA with the RAD51 and RAD52 proteins is affected by checkpoint signalling.

Single-stranded DNA mimicry in the p53 transactivation domain interaction with replication protein A

One of many protein-protein interactions modulated upon DNA damage is that of the single-stranded DNA-binding protein, replication protein A (RPA), with the p53 tumor suppressor. Here we report the crystal structure of RPA residues 1-120 (RPA70N) bound to the N-terminal transactivation domain of p53 (residues 37-57; p53N) and, by using NMR spectroscopy, characterize two mechanisms by which the RPA/p53 interaction can be modulated. RPA70N forms an oligonucleotide/oligosaccharide-binding fold, similar to that previously observed for the ssDNA-binding domains of RPA. In contrast, the N-terminal p53 transactivation domain is largely disordered in solution, but residues 37-57 fold into two amphipathic helices, H1 and H2, upon binding with RPA70N. The H2 helix of p53 structurally mimics the binding of ssDNA to the oligonucleotide/oligosaccharide-binding fold. NMR experiments confirmed that both ssDNA and an acidic peptide mimicking a phosphorylated form of RPA32N can independently compete the acidic p53N out of the binding site. Taken together, our data suggest a mechanism for DNA damage signaling that can explain a threshold response to DNA damage.

DNA damage induced hyperphosphorylation of replication protein A. 1. Identification of novel sites of phosphorylation in response to DNA damage

Replication protein A (RPA) is the predominant eukaryotic single-stranded DNA binding protein composed of 70, 34, and 14 kDa subunits. RPA plays central roles in the processes of DNA replication, repair, and recombination, and the p34 subunit of RPA is phosphorylated in a cell-cycle-dependent fashion and is hyperphosphorylated in response to DNA damage. We have developed an in vitro procedure for the preparation of hyperphosphorylated RPA and characterized a series of novel sites of phosphorylation using a combination of in gel tryptic digestion, SDS-PAGE and HPLC, MALDI-TOF MS analysis, 2D gel electrophoresis, and phosphospecific antibodies. We have mapped five phosphorylation sites on the RPA p34 subunit and five sites of phosphorylation on the RPA p70 subunit. No modification of the 14 kDa subunit was observed. Using the procedures developed with in vitro phosphorylated RPA, we confirmed a series of phosphorylation events on RPA from HeLa cells that was hyperphosphorylated in vivo in response to the DNA damaging agents, aphidicolin and hydroxyurea.

Complex H2AX phosphorylation patterns by multiple kinases including ATM and DNA-PK in human cells exposed to ionizing radiation and treated with kinase inhibitors

In eukaryotic cells, DNA double strand breaks (DSBs) cause the prompt phosphorylation of serine 139 at the carboxy terminus of histone H2AX to generate gamma-H2AX, detectable by Western blotting or immunofluorescence. The consensus sequence at the phosphorylation site implicates the phosphatidylinositol 3-like family of protein kinases in H2AX phosphorylation. It remains open whether ATM (ataxia telangiectasia mutated) is the major H2AX kinase, or whether other members of the family, such as DNA-PK (DNA dependent protein kinase) or ATR (ATM and Rad3 related), contribute in a functionally complementary manner. To address this question, we measured global H2AX phosphorylation in cell lysates and foci formation in individual cells of either wild type or mutant (ATM or DNA-PK) genetic background. Normal global phosphorylation kinetics is observed after irradiation in cells defective either in ATM or DNA-PK alone, suggesting a complementary contribution to H2AX phosphorylation. This is further supported by the observation that initial H2AX phosphorylation is delayed when both kinases are inhibited by wortmannin, as well as when ATM is inhibited by caffeine in DNA-PK deficient cells. However, robust residual global phosphorylation is detectable under all conditions of genetic or chemical inhibition suggesting the function of additional kinases, such as ATR. Treatment with wortmannin, caffeine, or UCN-01 produces a strong DNA-PK dependent late global hyperphosphorylation of H2AX, uncoupled from DNA DSB rejoining and compatible with an inhibition of late steps in DNA DSB processing. Evaluation of gamma-H2AX foci formation confirms the major conclusions made on the basis of global H2AX phosphorylation, but also points to differences particularly several hours after exposure to IR. The results in aggregate implicate DNA-PK, ATM and possibly other kinases in H2AX phosphorylation. The functional significance and the mechanisms of coordination in space and time of these multiple inputs require further investigation.

Phosphatidyl inositol 3-kinase-like serine/threonine protein kinases (PIKKs) are required for DNA damage-induced phosphorylation of the 32 kDa subunit of replication protein A at threonine 21

Replication protein A (RPA) is a single-stranded DNA (ssDNA) binding protein involved in various processes, including nucleotide excision repair and DNA replication. The 32 kDa subunit of RPA (RPA32) is phosphorylated in response to various DNA-damaging agents, and two protein kinases, ataxia-telangiectasia mutated (ATM) and the DNA-dependent protein kinase (DNA-PK) have been implicated in DNA damage-induced phosphorylation of RPA32. However, the relative roles of ATM and DNA-PK in the site-specific DNA damage-induced phosphorylation of RPA32 have not been reported. Here we generated a phosphospecific antibody that recognizes Thr21-phosphorylated RPA32. We show that both DNA-PK and ATM phosphorylate RPA32 on Thr21 in vitro. Ionizing radiation (IR)-induced phosphorylation of RPA32 on Thr21 was defective in ATM-deficient cells, while camptothecin (CPT)-induced phosphorylation of RPA32 on Thr21 was defective in cells lacking functional DNA-PK. Neither ATM nor DNA-PK was required for etoposide (ETOP)-induced RPA32 Thr21 phosphorylation. However, two inhibitors of the ATM- and Rad3-related (ATR) protein kinase activity prevented ETOP-induced Thr21 phosphorylation. Inhibition of DNA replication prevented both the IR- and CPT-induced phosphorylation of Thr21, whereas ETOP-induced Thr21 phosphorylation did not require active DNA replication. Thus, the regulation of RPA32 Thr21 phosphorylation by multiple DNA damage response protein kinases suggests that Thr21 phosphorylation of RPA32 is a crucial step within the DNA damage response.

RPA phosphorylation in mitosis alters DNA binding and protein-protein interactions

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

Genetic background as a possible determinant of clinical and biological features of Epstein-Barr virus infection--a hypothetical view

Epstein-Barr virus (EBV) is involved in various clinical disorders and many of the disease entities are lymphoid and epithelial malignancies. The exact mechanisms that determine the exact form of EBV-related disorder are not clear at present. Many of the clinical manifestations of these diseases are based on the biological characteristics of the target cells for EBV infection and the expression and function of EBV gene, which also perturb host immune functions. In this monograph, I propose a hypothesis regarding the mechanism involved in shaping the manifestation of EBV infection that genomic instability of EBV-infected cells and how a defective immune surveillance system against such cells plays a critical role in determining the clinical manifestation of EBV infection. Using EBV-infected B-cells from patients and carriers with ataxia telangiectasia as an example of EBV infection, I present and discuss evidence in support of the proposed hypothesis.

Replication protein A is required for meiotic recombination in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, meiotic recombination is initiated by transient DNA double-stranded breaks (DSBs). These DSBs undergo a 5' --> 3' resection to produce 3' single-stranded DNA ends that serve to channel DSBs into the RAD52 recombinational repair pathway. In vitro studies strongly suggest that several proteins of this pathway--Rad51, Rad52, Rad54, Rad55, Rad57, and replication protein A (RPA)--play a role in the strand exchange reaction. Here, we report a study of the meiotic phenotypes conferred by two missense mutations affecting the largest subunit of RPA, which are localized in the protein interaction domain (rfa1-t11) and in the DNA-binding domain (rfa1-t48). We find that both mutant diploids exhibit reduced sporulation efficiency, very poor spore viability, and a 10- to 100-fold decrease in meiotic recombination. Physical analyses indicate that both mutants form normal levels of meiosis-specific DSBs and that the broken ends are processed into 3'-OH single-stranded tails, indicating that the RPA complex present in these rfa1 mutants is functional in the initial steps of meiotic recombination. However, the 5' ends of the broken fragments undergo extensive resection, similar to what is observed in rad51, rad52, rad55, and rad57 mutants, indicating that these RPA mutants are defective in the repair of the Spo11-dependent DSBs that initiate homologous recombination during meiosis.

DNA-PK and ATM are required for radiation-enhanced integration

Ionizing radiation is known to improve transfection of exogenous DNA, a process we have termed radiation-enhanced integration. Previous observations have demonstrated that Ku proteins are critical for radiation-enhanced integration. Since Ku proteins form the DNA-binding domain of DNA-PK and since DNA-PK is important in nonhomologous DNA end joining, it was hypothesized that DNA-PK function might be important for radiation-enhanced integration. The ATM protein has been shown to be important in the recognition of a variety of types of DNA damage and to associate with DNA-PK under certain conditions. It was thus hypothesized that ATM might also play a role in radiation-enhanced integration. To test these hypotheses, radiation-enhanced integration was measured in hamster cells that are defective in the catalytic subunit of DNA-PK and in human cells containing mutant ATM. Radiation-enhanced integration was not detected in any of the cell lines with mutant PRKDC (also known as DNA-PKcs), but it was present in cells of the same lineage with wild-type PRKDC. Radiation-enhanced integration was defective in cells lacking kinase activation. ATM-deficient cell lines also showed defective radiation-enhanced integration. These data demonstrate that DNA-PK and ATM must both be active for radiation-enhanced integration to be observed.

UV-induced hyperphosphorylation of replication protein a depends on DNA replication and expression of ATM protein

Exposure to DNA-damaging agents triggers signal transduction pathways that are thought to play a role in maintenance of genomic stability. A key protein in the cellular processes of nucleotide excision repair, DNA recombination, and DNA double-strand break repair is the single-stranded DNA binding protein, RPA. We showed previously that the p34 subunit of RPA becomes hyperphosphorylated as a delayed response (4-8 h) to UV radiation (10-30 J/m(2)). Here we show that UV-induced RPA-p34 hyperphosphorylation depends on expression of ATM, the product of the gene mutated in the human genetic disorder ataxia telangiectasia (A-T). UV-induced RPA-p34 hyperphosphorylation was not observed in A-T cells, but this response was restored by ATM expression. Furthermore, purified ATM kinase phosphorylates the p34 subunit of RPA complex in vitro at many of the same sites that are phosphorylated in vivo after UV radiation. Induction of this DNA damage response was also dependent on DNA replication; inhibition of DNA replication by aphidicolin prevented induction of RPA-p34 hyperphosphorylation by UV radiation. We postulate that this pathway is triggered by the accumulation of aberrant DNA replication intermediates, resulting from DNA replication fork blockage by UV photoproducts. Further, we suggest that RPA-p34 is hyperphosphorylated as a participant in the recombinational postreplication repair of these replication products. Successful resolution of these replication intermediates reduces the accumulation of chromosomal aberrations that would otherwise occur as a consequence of UV radiation.

Regulation of dna replication after heat shock by replication protein a-nucleolin interactions

Heat shock inhibits replicative DNA synthesis, but the underlying mechanism remains unknown. We investigated mechanistic aspects of this regulation in melanoma cells using a simian virus 40 (SV40)-based in vitro DNA replication assay. Heat shock (44 degrees C) caused a monotonic inhibition of cellular DNA replication following exposures for 5-90 min. SV40 DNA replication activity in extracts of similarly heated cells also decreased after 5-30 min of exposure, but returned to near control levels after 60-90 min of exposure. This transient inhibition of SV40 DNA replication was eliminated by recombinant replication protein A (rRPA), suggesting a regulatory process targeting this key DNA replication factor. SV40 DNA replication inhibition was associated with a transient increase in the interaction between nucleolin and RPA that peaked at 20-30 min. Because binding to nucleolin compromises the ability of RPA to support SV40 DNA replication, we suggest that the observed interaction reflects a mechanism whereby DNA replication is regulated after heat shock. The relevance of this interaction to the regulation of cellular DNA replication is indicated by the transient translocation in heated cells of nucleolin from the nucleolus into the nucleoplasm with kinetics very similar to those of SV40 DNA replication inhibition and of RPA-nucleolin interaction. Because the targeting of RPA by nucleolin in heated cells occurs in an environment that preserves the activity of several essential DNA replication factors, active processes may contribute to DNA replication inhibition to a larger degree than presently thought. RPA-nucleolin interactions may reflect an early step in the regulation of DNA replication, as nucleolin relocalized into the nucleolus 1-2 h after heat exposure but cellular DNA replication remained inhibited for up to 8 h. We propose that the nucleolus functions as a heat sensor that uses nucleolin as a signaling molecule to initiate inhibitory responses equivalent to a checkpoint.

The cellular response to DNA damage induced by the enediynes C-1027 and neocarzinostatin includes hyperphosphorylation and increased nuclear retention of replication protein a (RPA) and trans inhibition of DNA replication

This study examined the cellular response to DNA damage induced by antitumor enediynes C-1027 and neocarzinostatin. Treatment of cells with either agent induced hyperphosphorylation of RPA32, the middle subunit of replication protein A, and increased nuclear retention of RPA. Nearly all of the RPA32 that was not readily extractable from the nucleus was hyperphosphorylated, compared to < or =50% of the soluble RPA. Enediyne concentrations that induced RPA32 hyperphosphorylation also decreased cell-free SV40 DNA replication competence in extracts of treated cells. This decrease did not result from damage to the DNA template, indicating trans-acting inhibition of DNA replication. Enediyne-induced RPA hyperphosphorylation was unaffected by the replication elongation inhibitor aphidicolin, suggesting that the cellular response to enediyne DNA damage was not dependent on elongation of replicating DNA. Neither recovery of replication competence nor reversal of RPA effects occurred when treated cells were further incubated in the absence of drug. C-1027 and neocarzinostatin doses that caused similar levels of DNA damage resulted in equivalent increases in RPA32 hyperphosphorylation and RPA nuclear retention and decreases in replication activity, suggesting a common response to enediyne-induced DNA damage. By contrast, DNA damage induced by C-1027 was at least 5-fold more cytotoxic than that induced by neocarzinostatin.

cDNA expression array analysis of DNA repair genes in human glioma cells that lack or express DNA-PK

M059J cells provide the only example of DNA-PKcs (now known as PRKDC) deficiency in a human cell line. M059K cells, derived from the same tumor specimen, express PRKDC protein and activity and, together with M059J, provide a useful model in which to study the role of DNA-PK in cellular responses to DNA-damaging agents. Because these cells are of tumor origin, we used Atlas human cancer cDNA expression arrays to investigate possible differential expression of other DNA repair genes in control and irradiated samples. cDNA array results indicated differential expression of 14 genes. Northern blotting confirmed relatively greater expression of replication factor C 37-kDa subunit mRNA in M059J cells compared to M059K cells and reduced expression of DNA ligase IV compared to ligase III in both cell lines independent of irradiation. These results suggest that other DNA repair proteins are altered in these cell lines and that repair mechanisms predicted from the study of normal tissues may be fundamentally altered in human cancer cells.

The majority of human replication protein A remains complexed throughout the cell cycle

Replication Protein A (RPA), the replicative single-strand DNA binding protein from eukaryotic cells, is a stable heterotrimeric complex consisting of three polypeptides. Cytological studies have investigated the subcellular distribution and association characteristics of the three RPA subunits during different stages of the cell cycle with varying results. In this study, various HeLa cell fractions were subjected to separation by either immunoprecipitation or velocity sedimentation. These separations were evaluated by immunoblotting for specific RPA subunits to determine whether the RPA in these fractions retains its heterotrimeric association. Immunoprecipitation of either the large (RPA70) or middle-sized (RPA32) subunit of RPA followed by immunoblotting for the other subunits demonstrate that RPA remains complexed throughout the G(1), S and G(2) phases of the cell cycle. Immunoprecipitation and sedimentation separations of both the nucleosolic and chromatin-bound RPA populations from both cycling and nocodazole-blocked cells showed that the majority of RPA remains complexed under all conditions examined. Consistent with previous reports, hypotonic extracts from 293 cells were shown to contain some RPA32 not complexed with RPA70. These results indicate that in some cell types, extracts may contain small amounts of RPA32 free of RPA70; however, in HeLa cells the majority of RPA clearly remains complexed as a heterotrimer throughout the cell cycle.

The Werner syndrome. A model for the study of human aging

Human aging is a complex process that leads to the gradual deterioration of body functions with time. Various models to approach the study of aging have been launched over the years such as the genetic analysis of life span in the yeast S. cerevisiae, the worm C. elegans, the fruitfly, and mouse, among others. In human models, there have been extensive efforts using replicative senescence, the study of centenerians, comparisons of young versus old at the organismal, cellular, and molecular levels, and the study of premature aging syndromes to understand the mechanisms leading to aging. One good model for studying human aging is a rare autosomal recessive disorder known as the Werner syndrome (WS), which is characterized by accelerated aging in vivo and in vitro. A genetic defect implicated in WS was mapped to the WRN locus. Mutations in this gene are believed to be associated, early in adulthood, with clinical symptoms normally found in old individuals. WRN functions as a DNA helicase, and recent evidence, summarized in this review, suggests specific biochemical roles for this multifaceted protein. The interaction of WRN protein with RPA (replication protein A) and p53 will undoubtedly direct efforts to further dissect the genetic pathway(s) in which WRN protein functions in DNA metabolism and will help to unravel its contribution to the human aging process.

DNA replication but not nucleotide excision repair is required for UVC-induced replication protein A phosphorylation in mammalian cells

Exposure of mammalian cells to short-wavelength light (UVC) triggers a global response which can either counteract the deleterious effect of DNA damage by enabling DNA repair or lead to apoptosis. Several stress-activated protein kinases participate in this response, making phosphorylation a strong candidate for being involved in regulating the cellular damage response. One factor that is phosphorylated in a UVC-dependent manner is the 32-kDa subunit of the single-stranded DNA-binding replication protein A (RPA32). RPA is required for major cellular processes like DNA replication, and removal of DNA damage by nucleotide excision repair (NER). In this study we examined the signal which triggers RPA32 hyperphosphorylation following UVC irradiation in human cells. Hyperphosphorylation of RPA was observed in cells from patients with either NER or transcription-coupled repair (TCR) deficiency (A, C, and G complementation groups of xeroderma pigmentosum and A and B groups of Cockayne syndrome, respectively). This exclude both NER intermediates and TCR as essential signals for RPA hyperphosphorylation. However, we have observed that UV-sensitive cells deficient in NER and TCR require lower doses of UV irradiation to induce RPA32 hyperphosphorylation than normal cells, indicating that persistent unrepaired lesions contribute to RPA phosphorylation. Finally, the results of UVC irradiation experiments on nonreplicating cells and S-phase-synchronized cells emphasize a major role for DNA replication arrest in the presence of UVC lesions in RPA UVC-induced hyperphosphorylation in mammalian cells.

Adozelesin triggers DNA damage response pathways and arrests SV40 DNA replication through replication protein A inactivation

The cyclopropylpyrroloindole anti-cancer drug, adozelesin, binds to and alkylates DNA. Treatment of human cells with low levels of adozelesin results in potent inhibition of both cellular and simian virus 40 (SV40) DNA replication. Extracts were prepared from adozelesin-treated cells and shown to be deficient in their ability to support SV40 DNA replication in vitro. This effect on in vitro DNA replication was dependent on both the concentration of adozelesin used and the time of treatment but was not due to the presence of adozelesin in the in vitro assay. Adozelesin treatment of cells was shown to result in the following: induction of p53 protein levels, hyperphosphorylation of replication protein A (RPA), and disruption of the p53-RPA complex (but not disruption of the RPA-cdc2 complex), indicating that adozelesin treatment triggers cellular DNA damage response pathways. Interestingly, in vitro DNA replication could be rescued in extracts from adozelesin-treated cells by the addition of exogenous RPA. Therefore, whereas adozelesin and other anti-cancer therapeutics trigger common DNA damage response markers, adozelesin causes DNA replication arrest through a unique mechanism. The S phase checkpoint response triggered by adozelesin acts by inactivating RPA in some function essential for SV40 DNA replication.

Involvement of DNA-dependent protein kinase in UV-induced replication arrest

Cells exposed to UV irradiation are predominantly arrested at S-phase as well as at the G(1)/S boundary while repair occurs. It is not known how UV irradiation induces S-phase arrest and yet permits DNA repair; however, UV-induced inhibition of replication is efficiently reversed by the addition of replication protein A (RPA), suggesting a role for RPA in this regulatory event. Here, we show evidence that DNA-dependent protein kinase (DNA-PK), plays a role in UV-induced replication arrest. DNA synthesis of M059K (DNA-PK catalytic subunit-positive (DNA-PKcs(+))), as measured by [(3)H]thymidine incorporation, was significantly arrested by 4 h following UV irradiation, whereas M059J (DNA-PKcs(-)) cells were much less affected. Similar results were obtained with the in vitro replication reactions where immediate replication arrest occurred in DNA-PKcs(+) cells following UV irradiation, and only a gradual decrease in replication activity was observed in DNA-PKcs(-) cells. Reversal of replication arrest was observed at 8 h following UV irradiation in DNA-PKcs(+) cells but not in DNA-PKcs(-) cells. Reversal of UV-induced replication arrest was also observed in vitro by the addition of a DNA-PK inhibitor, wortmannin, or by immunodepletion of DNA-PKcs, supporting a positive role for DNA-PK in damage-induced replication arrest. The RPA-containing fraction from UV-irradiated DNA-PKcs(+) cells poorly supported DNA replication, whereas the replication activity of the RPA-containing fraction from DNA-PKcs(-) cells was not affected by UV, suggesting that DNA-PKcs may be involved in UV-induced replication arrest through modulation of RPA activity. Together, our results strongly suggest a role for DNA-PK in S-phase (replication) arrest in response to UV irradiation.

ATM: a mediator of multiple responses to genotoxic stress

The ATM protein kinase is the product of the gene responsible for the pleiotropic recessive disorder ataxia-telangiectasia. ATM-deficient cells show enhanced sensitivity and greatly reduced responses to genotoxic agents that generate DNA double strand breaks (DSBs), such as ionizing radiation and radiomimetic chemicals, but exhibit normal responses to DNA adducts and base modifications induced by other agents. Therefore, DSBs are most likely the predominant signal for the activation of ATM-mediated pathways. Identification of the ATM gene triggered extensive research aimed at elucidating the numerous functions of its large multifaceted protein product. While ATM has both nuclear and cytoplasmic functions, this review will focus on its roles in the nucleus where it plays a central role in the very early stages of damage detection and serves as a master controller of cellular responses to DSBs. By activating key regulators of multiple signal transduction pathways, ATM mediates the efficient induction of a signaling network responsible for repair of the damage, and for cellular recovery and survival.

Ataxia-telangiectasia, cancer and the pathobiology of the ATM gene

Ataxia-telangiectasia (A-T) is a pleiotropic inherited disease characterized by neurodegeneration, cancer, immunodeficiencies, radiation sensitivity, and genetic instability. Although A-T homozygotes are rare, the A-T gene may play a role in sporadic breast cancer and leukemia. ATM, the gene responsible for A-T, is homologous to several cell cycle checkpoint genes from other organisms. ATM is thought to play a crucial role in a signal transduction network that modulates cell cycle checkpoints, genetic recombination, apoptosis, and other cellular responses to DNA damage. New insights into the pathobiology of A-T have been provided by the creation of Atm-/- mice and by in vitro studies of ATM function. Analyses of ATM mutations in A-T patients and in sporadic tumors suggest the existence of two classes of ATM mutation: null mutations that lead to A-T and dominant negative missense mutations that may predispose to cancer in the heterozygous state.

Replication-mediated DNA damage by camptothecin induces phosphorylation of RPA by DNA-dependent protein kinase and dissociates RPA:DNA-PK complexes

Replication protein A (RPA) is a DNA single-strand binding protein essential for DNA replication, recombination and repair. In human cells treated with the topoisomerase inhibitors camptothecin or etoposide (VP-16), we find that RPA2, the middle-sized subunit of RPA, becomes rapidly phosphorylated. This response appears to be due to DNA-dependent protein kinase (DNA-PK) and to be independent of p53 or the ataxia telangiectasia mutated (ATM) protein. RPA2 phosphorylation in response to camptothecin required ongoing DNA replication. Camptothecin itself partially inhibited DNA synthesis, and this inhibition followed the same kinetics as DNA-PK activation and RPA2 phosphorylation. DNA-PK activation and RPA2 phosphorylation were prevented by the cell-cycle checkpoint abrogator 7-hydroxystaurosporine (UCN-01), which markedly potentiates camptothecin cytotoxicity. The DNA-PK catalytic subunit (DNA-PKcs) was found to bind RPA which was replaced by the Ku autoantigen upon camptothecin treatment. DNA-PKcs interacted directly with RPA1 in vitro. We propose that the encounter of a replication fork with a topoisomerase-DNA cleavage complex could lead to a juxtaposition of replication fork-associated RPA and DNA double-strand end-associated DNA-PK, leading to RPA2 phosphorylation which may signal the presence of DNA damage to an S-phase checkpoint mechanism.

KEYWORDS

camptothecin/DNA damage/DNA-dependent protein kinase/RPA2 phosphorylation

[Radiotherapy of the elderly patient. Radiotherapy tolerance and results in older patients]

BACKGROUND

Despite a growing number of elderly patients receiving radiation therapy little is known about side effects and outcome of irradiation in this section of the population.

METHODS

In a review article epidemiologic data, aspects of radiation-biology as well as side effects and outcome of radiation therapy of elderly patients are discussed.

RESULTS

Cancer incidence rises with age (Figure 1) and is exceeding 3.5% for males older than 85 years. With a life expectancy of more than 4 years, curative therapy is indicated even at this age. Furthermore several retrospective studies indicate that local control and disease-Specific survival after radiation therapy of elderly patients is comparable with that of younger persons (Tables 3 and 4). The exception contains elderly patients with Grade-III to IV gliomas or with rectal carcinoma who show a reduced survival which is perhaps caused by less aggressive combined treatment (tumor resection). Although some biological and molecular data indicate a rise in radiation sensitivity with growing age like the reduction of the capacity of some DNA-repair enzymes, there is no convincing evidence in animal studies or in retrospective clinical studies that radiation therapy is generally less well tolerated by older individuals (Tables 1 and 2). Some age-depending differences in organ toxicities are described in 3 large studies, which evaluate the data of patients who were enrolled in different EORTC-trials: Older patients suffer more of functional mucositis in case of radiation therapy to the head and neck, they have an increased weight loss and a higher frequency of late esophageal damage when irradiated in the thorax, and they show a higher prevalence of sexual dysfunction when treated with radiation therapy to the pelvis. On the other hand younger patients suffer more from acute toxicity like skin damage, nausea, and deterioration of the performance status during pelvic radiotherapy. When discussing the dose intensity of radiation therapy concomitant disease which leads to extensive atherosclerotic vessel damage should be kept in mind. Old patients should be monitored closely during therapy, since the loss of electrolytes or fluid is often not very well tolerated.

CONCLUSION

The indication to radiation therapy of elderly cancer patients should take into account their performance status as well as the extent and the severity of comorbidity. Age per se is seldom a contraindication for radiation therapy. Regarding the available data in literature there is no indication for a dose reduction in radiation therapy only because of age, especially in the curative setting.

The DNA replication fork in eukaryotic cells

Replication of the two template strands at eukaryotic cell DNA replication forks is a highly coordinated process that ensures accurate and efficient genome duplication. Biochemical studies, principally of plasmid DNAs containing the Simian Virus 40 origin of DNA replication, and yeast genetic studies have uncovered the fundamental mechanisms of replication fork progression. At least two different DNA polymerases, a single-stranded DNA-binding protein, a clamp-loading complex, and a polymerase clamp combine to replicate DNA. Okazaki fragment synthesis involves a DNA polymerase-switching mechanism, and maturation occurs by the recruitment of specific nucleases, a helicase, and a ligase. The process of DNA replication is also coupled to cell-cycle progression and to DNA repair to maintain genome integrity.

Ataxia-telangiectasia and the Nijmegen breakage syndrome: related disorders but genes apart

Gene mutations provide valuable clues to cellular metabolism. In humans such insights come mainly from genetic disorders. Ataxia-telangiectasia (A-T) and Nijmegen breakage syndrome (NBS) are two distinct but closely related, single gene disorders that highlight a complex junction of several signal transduction pathways. These pathways appear to control defense mechanisms against specific types of damage to cellular macromolecules, and probably regulate the processing of certain types of DNA damage or normal intermediates of DNA metabolism. A-T is characterized primarily by cerebellar degeneration, immunodeficiency, genome instability, clinical radiosensitivity, and cancer predisposition. NBS shares all these features except cerebellar deterioration. The cellular phenotypes of A-T and NBS are almost indistinguishable, however, and include chromosomal instability, radiosensitivity, and defects in cell cycle checkpoints normally induced by ionizing radiation. The recent identification of the gene responsible for A-T, ATM, has revealed its product to be a large, constitutively expressed phosphoprotein with a carboxy-terminal region similar to the catalytic domain of phosphatidylinositol 3-kinases (PI 3-kinases). ATM is a member of a family of proteins identified in various organisms, which share the PI 3-kinase domain and are involved in regulation of cell cycle progression and response to genotoxic agents. Some of these proteins, most notably the DNA-dependent protein kinase, have an associated protein kinase activity, and preliminary data indicate this activity in ATM as well. Mutations in A-T patients are null alleles that truncate or destabilize the ATM protein. Atm-deficient mice recapitulate the human phenotype with slower nervous-system degeneration. Two ATM interactors, c-Abl and p53, underscore its role in cellular responses to genotoxic stress. The complexity of ATM's structure and mode of action make it a paradigm of multifaceted signal transduction proteins involved in many physiological pathways via multiple protein-protein interactions. The as yet unknown NBS protein may be a component in an ATM-based complex, with a key role in sensing and processing specific DNA damage or intermediates and signaling their presence to the cell cycle machinery.

Ataxia-telangiectasia: is ATM a sensor of oxidative damage and stress?

Ataxia-telangiectasia (A-T) is a pleiotropic recessive disorder characterized by cerebellar ataxia, immunodeficiency, specific developmental defects, profound predisposition to cancer and acute radiosensitivity. Functional inactivation of a single gene product, ATM, accounts for this compound phenotype. We suggest that ATM acts as a sensor of reactive oxygen species and/or oxidative damage of cellular macromolecules, including DNA. In turn, ATM induces signalling through multiple pathways, thereby coordinating acute phase stress responses with cell cycle checkpoint control and repair of oxidative damage. Absence of ATM is proposed to limit the repair of insidious oxidative damage that can occur under normal physiological conditions, ultimately leading to apoptosis of particularly sensitive cells, such as neurons and thymocytes.