DNA replication stress-induced phosphorylation of cyclic AMP response element-binding protein mediated by ATM

Multi-faceted regulation of CREB family transcription factors

cAMP response element-binding protein (CREB) is a ubiquitously expressed nuclear transcription factor, which can be constitutively activated regardless of external stimuli or be inducibly activated by external factors such as stressors, hormones, neurotransmitters, and growth factors. However, CREB controls diverse biological processes including cell growth, differentiation, proliferation, survival, apoptosis in a cell-type-specific manner. The diverse functions of CREB appear to be due to CREB-mediated differential gene expression that depends on cAMP response elements and multi-faceted regulation of CREB activity. Indeed, the transcriptional activity of CREB is controlled at several levels including alternative splicing, post-translational modification, dimerization, specific transcriptional co-activators, non-coding small RNAs, and epigenetic regulation. In this review, we present versatile regulatory modes of CREB family transcription factors and discuss their functional consequences.

Hypoxia-mediated alterations and their role in the HER-2/neuregulated CREB status and localization

The cAMP-responsive element-binding protein (CREB) is involved in the tumorigenicity of HER-2/neu-overexpressing murine and human tumor cells, but a link between the HER-2/neu-mediated CREB activation, its posttranslational modification and localization and changes in the cellular metabolism, due to an altered (tumor) microenvironment remains to be established. The present study demonstrated that shRNA-mediated silencing of CREB in HER-2/neu-transformed cells resulted in decreased tumor formation, which was associated with reduced angiogenesis, but increased necrotic and hypoxic areas in the tumor. Hypoxia induced pCREBSer133, but not pCREBSer121 expression in HER-2/neu-transformed cells. This was accompanied by upregulation of the hypoxia-inducible genes GLUT1 and VEGF, increased cell migration and matrix metalloproteinase-mediated invasion. Treatment of HER-2/neu+ cells with signal transduction inhibitors targeting in particular HER-2/neu was able to revert hypoxia-controlled CREB activation. In addition to changes in the phosphorylation, hypoxic response of HER-2/neu+ cells caused a transient ubiquitination and SUMOylation as well as a co-localization of nuclear CREB to the mitochondrial matrix. A mitochondrial localization of CREB was also demonstrated in hypoxic areas of HER-2/neu+ mammary carcinoma lesions. This was accompanied by an altered gene expression pattern, activity and metabolism of mitochondria leading to an increased respiratory rate, oxidative phosphorylation and mitochondrial membrane potential and consequently to an enhanced apoptosis and reduced cell viability. These data suggest that the HER-2/neu-mediated CREB activation caused by a hypoxic tumor microenvironment contributes to the neoplastic phenotype of HER-2/neu+ cells at various levels.

Control of CREB expression in tumors: from molecular mechanisms and signal transduction pathways to therapeutic target

The cyclic AMP response element binding (CREB) protein has pleiotropic activities in physiologic processes. Due to its central position downstream of many growth signaling pathways CREB has the ability to influence cell survival, growth and differentiation of normal, but also of tumor cells suggesting an oncogenic potential of CREB. Indeed, increased CREB expression and activation is associated with tumor progression, chemotherapy resistance and reduced patients' survival. We summarize here the different cellular functions of CREB in tumors of distinct histology as well as its use as potential prognostic marker. In addition, the underlying molecular mechanisms to achieve constitutive activation of CREB including structural alterations, such as gene amplification and chromosomal translocation, and deregulation, which could occur at the transcriptional, post-transcriptional and post-translational level, will be described. Since downregulation of CREB by different strategies resulted in inhibition of cell proliferation, invasion and induction of apoptosis, the role of CREB as a promising target for cancer therapy will be also discussed.

Regulation of Cancer Cell Responsiveness to Ionizing Radiation Treatment by Cyclic AMP Response Element Binding Nuclear Transcription Factor

Cyclic AMP response element binding (CREB) protein is a member of the CREB/activating transcription factor (ATF) family of transcription factors that play an important role in the cell response to different environmental stimuli leading to proliferation, differentiation, apoptosis, and survival. A number of studies highlight the involvement of CREB in the resistance to ionizing radiation (IR) therapy, demonstrating a relationship between IR-induced CREB family members' activation and cell survival. Consistent with these observations, we have recently demonstrated that CREB and ATF-1 are expressed in leukemia cell lines and that low-dose radiation treatment can trigger CREB activation, leading to survival of erythro-leukemia cells (K562). On the other hand, a number of evidences highlight a proapoptotic role of CREB following IR treatment of cancer cells. Since the development of multiple mechanisms of resistance is one key problem of most malignancies, including those of hematological origin, it is highly desirable to identify biological markers of responsiveness/unresponsiveness useful to follow-up the individual response and to adjust anticancer treatments. Taking into account all these considerations, this mini-review will be focused on the involvement of CREB/ATF family members in response to IR therapy, to deepen our knowledge of this topic, and to pave the way to translation into a therapeutic context.

Transcription Factors in the Cellular Response to Charged Particle Exposure

Charged particles, such as carbon ions, bear the promise of a more effective cancer therapy. In human spaceflight, exposure to charged particles represents an important risk factor for chronic and late effects such as cancer. Biological effects elicited by charged particle exposure depend on their characteristics, e.g., on linear energy transfer (LET). For diverse outcomes (cell death, mutation, transformation, and cell-cycle arrest), an LET dependency of the effect size was observed. These outcomes result from activation of a complex network of signaling pathways in the DNA damage response, which result in cell-protective (DNA repair and cell-cycle arrest) or cell-destructive (cell death) reactions. Triggering of these pathways converges among others in the activation of transcription factors, such as p53, nuclear factor κB (NF-κB), activated protein 1 (AP-1), nuclear erythroid-derived 2-related factor 2 (Nrf2), and cAMP responsive element binding protein (CREB). Depending on dose, radiation quality, and tissue, p53 induces apoptosis or cell-cycle arrest. In low LET radiation therapy, p53 mutations are often associated with therapy resistance, while the outcome of carbon ion therapy seems to be independent of the tumor's p53 status. NF-κB is a central transcription factor in the immune system and exhibits pro-survival effects. Both p53 and NF-κB are activated after ionizing radiation exposure in an ataxia telangiectasia mutated (ATM)-dependent manner. The NF-κB activation was shown to strongly depend on charged particles' LET, with a maximal activation in the LET range of 90-300 keV/μm. AP-1 controls proliferation, senescence, differentiation, and apoptosis. Nrf2 can induce cellular antioxidant defense systems, CREB might also be involved in survival responses. The extent of activation of these transcription factors by charged particles and their interaction in the cellular radiation response greatly influences the destiny of the irradiated and also neighboring cells in the bystander effect.

ATM signalling and cancer

ATM, the protein kinase mutated in the rare human disease ataxia telangiectasia (A-T), has been the focus of intense scrutiny over the past two decades. Initially this was because of the unusual radiosensitive phenotype of cells from A-T patients, and latterly because investigating ATM signalling has yielded valuable insights into the DNA damage response, redox signalling and cancer. With the recent explosion in genomic data, ATM alterations have been revealed both in the germline as a predisposing factor for cancer and as somatic changes in tumours themselves. Here we review these findings, as well as advances in the understanding of ATM signalling mechanisms in cancer and ATM inhibition as a strategy for cancer treatment.

TGF-β1-ROS-ATM-CREB signaling axis in macrophage mediated migration of human breast cancer MCF7 cells

Macrophages in the tumor microenvironment play an important role in tumor cell survival. They influence the tumor cell to proliferate, invade into surrounding normal tissues and metastasize to local and distant sites. In this study, we evaluated the effect of conditioned medium from monocytes and macrophages on growth and migration of breast cancer cells. Macrophage conditioned medium (MϕCM) containing elevated levels of cytokines TNF-α, IL-1β and IL-6 had a differential effect on non-invasive (MCF7) and highly invasive (MDA-MB-231) breast cancer cell lines. MϕCM induced the secretion of TGF-β1 in MCF7 cells. This was associated with apoptosis in a fraction of cells and generation of reactive oxygen and nitrogen species (ROS and RNS) and DNA damage in the remaining cells. This, in turn, increased expression of cAMP response element binding protein (CREB) and vimentin resulting in migration of cells. These effects were inhibited by neutralization of TNF-α, IL-1β and IL-6, inhibition of ROS and RNS, DNA damage and siRNA mediated knockdown of ATM. In contrast, MDA-MB-231 cells which had higher basal levels of pCREB were not affected by MϕCM. In summary, we have found that pro-inflammatory cytokines secreted by macrophages induce TGF-β1 in tumor cells, which activate pCREB signaling, epithelial-mesenchymal-transition (EMT) responses and enhanced migration.

Cyclin-dependent kinase 1-dependent phosphorylation of cAMP response element-binding protein decreases chromatin occupancy

The cyclic AMP response element-binding protein (CREB) initiates transcriptional responses to a wide variety of stimuli. CREB activation involves its phosphorylation on Ser-133, which promotes interaction between the CREB kinase-inducible domain (KID) and the KID-interacting domain of the transcriptional coactivator, CREB-binding protein (CBP). The KID also contains a highly conserved phosphorylation cluster, termed the ATM/CK cluster, which is processively phosphorylated in response to DNA damage by the coordinated actions of ataxia-telangiectasia-mutated (ATM) and casein kinases (CKs) 1 and 2. The ATM/CK cluster phosphorylation attenuates CBP binding and CREB transcriptional activity. Paradoxically, it was recently reported that DNA damage activates CREB through homeodomain-interacting protein kinase 2-dependent phosphorylation of Ser-271 near the CREB bZIP DNA binding domain. In this study we sought to further clarify DNA damage-dependent CREB phosphorylation as well as to explore the possibility that the ATM/CK cluster and Ser-271 synergistically or antagonistically modulate CREB activity. We show that, rather than being induced by DNA damage, Ser-270 and Ser-271 of CREB cophosphorylated in a CDK1-dependent manner during G2/M phase. Functionally, we show that phosphorylation of CREB on Ser-270/Ser-271 during mitosis correlated with reduced CREB chromatin occupancy. Furthermore, CDK1-dependent phosphorylation of CREB in vitro inhibited its DNA binding activity. The combined results suggest that CDK1-dependent phosphorylation of CREB on Ser-270/Ser-271 facilitates its dissociation from chromatin during mitosis by reducing its intrinsic DNA binding potential.

Metformin and the ATM DNA damage response (DDR): accelerating the onset of stress-induced senescence to boost protection against cancer

By activating the ataxia telangiectasia mutated (ATM)-mediated DNA Damage Response (DDR), the AMPK agonist metformin might sensitize cells against further damage, thus mimicking the precancerous stimulus that induces an intrinsic barrier against carcinogenesis. Herein, we present the new hypothesis that metformin might function as a tissue sweeper of pre-malignant cells before they gain stem cell/tumor initiating properties. Because enhanced glycolysis (the Warburg effect) plays a causal role in the gain of stem-like properties of tumor-initiating cells by protecting them from the pro-senescent effects of mitochondrial respiration-induced oxidative stress, metformin's ability to disrupt the glycolytic metabotype may generate a cellular phenotype that is metabolically protected against immortalization. The bioenergetic crisis imposed by metformin, which may involve enhanced mitochondrial biogenesis and oxidative stress, can lower the threshold for cellular senescence by pre-activating an ATM-dependent pseudo-DDR. This allows an accelerated onset of cellular senescence in response to additional oncogenic stresses. By pushing cancer cells to use oxidative phosphorylation instead of glycolysis, metformin can rescue cell surface major histocompatibility complex class I (MHC-I) expression that is downregulated by oncogenic transformation, a crucial adaptation of tumor cells to avoid the adaptive immune response by cytotoxic T-lymphocytes (CTLs). Aside from restoration of tumor immunosurveillance at the cell-autonomous level, metformin can activate a senescence-associated secretory phenotype (SASP) to reinforce senescence growth arrest, which might trigger an immune-mediated clearance of the senescent cells in a non-cell-autonomous manner. By diminishing the probability of escape from the senescence anti-tumor barrier, the net effect of metformin should be a significant decrease in the accumulation of dysfunctional, pre-malignant cells in tissues, including those with the ability to initiate tumors. As life-long or late-life removal of senescent cells has been shown to prevent or delay the onset or progression of age-related disorders, the tissue sweeper function of metformin may inhibit the malignant/metastatic progression of pre-malignant/senescent tumor cells and increase the human lifespan.

Evaluation of the insertion efficiencies of tapered silicon nanoneedles and invasiveness of diamond nanoneedles in manipulations of living single cells

We have been developing a low invasive cell manipulation technology based on inserting an ultra-thin needle--"nanoneedle"--into a living cell by using an atomic force microscope (AFM). The nanoneedle, made from a silicon AFM tip by focused-ion-beam etching, has a diameter of several hundred nanometers and a length of about 10 microns. Successful insertion of the nanoneedle into the cell can be confirmed by the appearance of a steep relaxation of repulsive force in the force-distance curve as monitored by the AFM system. This technology, termed "cell surgery", can be applied for the detection of intracellular proteins in a living cell or for highly efficient gene transfer. The present study shows that the durability of a tapered nanoneedle is superior to that of a cylindrical nanoneedle, and that a proper aspect ratio for the tapered nanoneedle must be chosen to maintain sufficient insertion efficiency for a particular target cell: tapered nanoneedles of an aspect ratio over 20 showed high insertion efficiency for various kinds of mammalian cells. We then used diamond for the material of the nanoneedle because its specific properties, such as high stiffness, heat conductivity, and electrical conductivity capacitated by boron doping, were deemed useful for the analysis and manipulation of intracellular phenomena. We compared the capability of the diamond nanoneedle in cell manipulation with that of the silicon nanoneedle. Evaluation of the effect of the former on transcription efficiency and localization analysis of p53 expression revealed the low invasiveness for cell manipulation as was also the case for the silicon nanoneedle. We also succeeded in achieving highly efficient plasmid DNA delivery into a mouse fibroblast C3H10T1/2 using the diamond nanoneedle. The diamond nanoneedle is expected to contribute to the versatility of "cell surgery" technology.

CREB: a multifaceted regulator of neuronal plasticity and protection

Since its initial characterization over 20 years ago, there has been intense and unwavering interest in understanding the role of the transcription factor cAMP-responsive element binding protein (CREB) in nervous system physiology. Through an array of experimental approaches and model systems, researchers have begun to unravel the complex and multifaceted role of this transcription factor in such diverse processes as neurodevelopment, synaptic plasticity, and neuroprotection. Here we discuss current insights into the molecular mechanisms by which CREB couples synaptic activity to long-term changes in neuronal plasticity, which is thought to underlie learning and memory. We also discuss work showing that CREB is a critical component of the neuroprotective transcriptional network, and data indicating that CREB dysregulation contributes to an array of neuropathological conditions.

Inhibition of ATM kinase activity does not phenocopy ATM protein disruption: implications for the clinical utility of ATM kinase inhibitors

Biallelic mutations in ataxia-telangiectasia mutated (ATM), which encodes for a protein kinase, cause ataxia telangiectasia (A-T). A-T is a pleiotropic disease, with a characteristic hypersensitivity to ionizing radiation (IR). A-T patients typically lack both detectable ATM protein and ATM kinase activity, and small molecule inhibitors of ATM kinase activity have been developed as strategies to improve radiotherapy for the treatment of cancers. As predicted, inhibition of ATM kinase activity is sufficient to radiosensitize cells. However, we recently showed that inhibition of ATM kinase activity disrupts DNA damage-induced sister chromatid exchange (SCE). This result was unanticipated since SCE is normal in A-T cells that lack detectable ATM protein. In these studies, we showed, for the first time, that the consequences of inhibition of ATM kinase activity and adaptation to ATM protein disruption are distinct. Here, we discuss the mechanistic implications of this finding for the function of ATM at the replication fork and the clinical utility of ATM kinase inhibitors.

Conserved and distinct modes of CREB/ATF transcription factor regulation by PP2A/B56gamma and genotoxic stress

Activating transcription factor 1 (ATF1) and the closely related proteins CREB (cyclic AMP resonse element binding protein) and CREM (cyclic AMP response element modulator) constitute a subfamily of bZIP transcription factors that play critical roles in the regulation of cellular growth, metabolism, and survival. Previous studies demonstrated that CREB is phosphorylated on a cluster of conserved Ser residues, including Ser-111 and Ser-121, in response to DNA damage through the coordinated actions of the ataxia-telangiectasia-mutated (ATM) protein kinase and casein kinases 1 and 2 (CK1/2). Here, we show that DNA damage-induced phosphorylation by ATM is a general feature of CREB and ATF1. ATF1 harbors a conserved ATM/CK cluster that is constitutively and stoichiometrically phosphorylated by CK1 and CK2 in asynchronously growing cells. Exposure to DNA damage further induced ATF1 phosphorylation on Ser-51 by ATM in a manner that required prior phosphorylation of the upstream CK residues. Hyperphosphorylated ATF1 showed a 4-fold reduced affinity for CREB-binding protein. We further show that PP2A, in conjunction with its targeting subunit B56gamma, antagonized ATM and CK1/2-dependent phosphorylation of CREB and ATF1 in cellulo. Finally, we show that CK sites in CREB are phosphorylated during cellular growth and that phosphorylation of these residues reduces the threshold of DNA damage required for ATM-dependent phosphorylation of the inhibitory Ser-121 residue. These studies define overlapping and distinct modes of CREB and ATF1 regulation by phosphorylation that may ensure concerted changes in gene expression mediated by these factors.

Transient ATM kinase inhibition disrupts DNA damage-induced sister chromatid exchange

Cells derived from ataxia telangiectasia (A-T) patients exhibit defective cell cycle checkpoints because of mutations in the gene encoding ATM (ataxia telangiectasia mutated). After exposure to ionizing radiation (IR), A-T cells exhibit sensitivity to IR-induced cellular damage that results in increased chromosome aberrations and cell death (radiosensitivity). ATM is a member of a family of kinases that become activated in response to DNA damage. We showed that even transient inhibition of ATM kinase for 1 hour, initiated 15 minutes after cellular irradiation, resulted in an accumulation of persistent chromosome aberrations and increased cell death. Using reversible inhibitors of DNA-PK (DNA-dependent protein kinase), another kinase involved in responding to DNA damage, and ATM, we showed that these two kinases acted through distinct DNA repair mechanisms: ATM resolved DNA damage through a mechanism involving sister chromatid exchange (SCE), whereas DNA-PK acted through nonhomologous end joining. Furthermore, because DNA damage-induced SCE occurred in A-T fibroblasts that lack functional ATM protein, and the inhibitors of ATM kinase had no effect on DNA damage-induced SCE in A-T fibroblasts, we showed that the consequences of short-term inhibition of the kinase activity of ATM and adaptation to ATM protein disruption were distinct. This suggests that A-T fibroblasts have adapted to the loss of ATM and have alternative mechanisms to initiate SCE.

Transcription-dependent activation of ataxia telangiectasia mutated prevents DNA-dependent protein kinase-mediated cell death in response to topoisomerase I poison

Camptothecin (CPT) is a topoisomerase I inhibitor, derivatives of which are being used for cancer chemotherapy. CPT-induced DNA double-strand breaks (DSBs) are considered a major cause of its tumoricidal activity, and it has been shown that CPT induces DNA damage signaling through the phosphatidylinositol 3-kinase-related kinases, including ATM (ataxia telangiectasia mutated), ATR (ATM and Rad3-related), and DNA-PK (DNA-dependent protein kinase). In addition, CPT causes DNA strand breaks mediated by transcription, although the downstream signaling events are less well characterized. In this study, we show that CPT-induced activation of ATM requires transcription. Mechanistically, transcription inhibition suppressed CPT-dependent activation of ATM and blocked recruitment of the DNA damage mediator p53-binding protein 1 (53BP1) to DNA damage sites, whereas ATM inhibition abrogated CPT-induced G(1)/S and S phase checkpoints. Functional inactivation of ATM resulted in DNA replication-dependent hyperactivation of DNA-PK in CPT-treated cells and dramatic CPT hypersensitivity. On the other hand, simultaneous inhibition of ATM and DNA-PK partially restored CPT resistance, suggesting that activation of DNA-PK is proapoptotic in the absence of ATM. Correspondingly, comet assay and cell cycle synchronization experiments suggested that transcription collapse occurring as the result of CPT treatment are converted to frank double-strand breaks when ATM-deficient cells bypass the G(1)/S checkpoint. Thus, ATM suppresses DNA-PK-dependent cell death in response to topoisomerase poisons, a finding with potential clinical implications.

Transcriptional regulation of tumor suppressor p53 by cAMP-responsive element-binding protein/AMP-activated protein kinase complex in response to glucose deprivation

Tumor suppressor p53 plays a pivotal role in the regulation of cell fate determination in response to a variety of cellular stress including carbon source depletion. In this study, we found that cAMP-responsive element-binding protein (CREB) collaborates with AMP-activated protein kinase alpha (AMPKalpha) to regulate the transcription of p53. Luciferase reporter assays showed that the genomic fragment spanning from -531 to -239 of human p53 gene is required for the transactivation of p53 in response to glucose deprivation. Within this region, we found out a putative CREB-binding site. siRNA-mediated knockdown of CREB resulted in a significant inhibition of the up-regulation of p53 and apoptosis under glucose deprivation. Consistent with these observations, glucose deprivation induced the transcription of p53 and CREB. Additionally, glucose deprivation led to an efficient recruitment of CREB onto the promoter region of p53 gene carrying the canonical CREB-binding site, indicating that CREB has an ability to bind to the promoter region of p53 gene and transactivate p53. Furthermore, the amounts of CREB/phospo-AMPKalpha complex increased in response to glucose deprivation. Taken together, our present findings suggest that p53 is transcriptionally regulated by CREB/phospho-AMPKalpha complex and thereby contributing to the induction of apoptosis under carbon source depletion.

Inhibitors of histone deacetylases suppress cisplatin-induced p53 activation and apoptosis in renal tubular cells

Inhibitors of histone deacetylases, including suberoylanilide hydroxamic acid (SAHA) and trichostatin A (TSA), are emerging anticancer agents. In the current study, we examined the cytoprotective effects of these agents. Cisplatin induced 40-50% apoptosis in rat kidney proximal tubular cells in 18 h, which was suppressed to 20-30% by 1-5 microM SAHA or 0.1 microM TSA. Consistently, SAHA partially prevented cisplatin-induced caspase activation. The cytoprotective effects of SAHA and TSA were associated with long-term cell survival. During cisplatin treatment, Bax translocated to mitochondria, leading to cytochrome c release. Both Bax translocation and cytochrome c release were ameliorated by SAHA. Mechanistically, SAHA inhibited and TSA delayed p53 phosphorylation, acetylation, and activation during cisplatin incubation. At the upstream signaling level, SAHA blocked cisplatin-induced phosphorylation of Chk2, a key DNA damage response kinase. Interestingly, in HCT116 colon cancer cells, SAHA suppressed cisplatin-induced p53 activation, but enhanced apoptosis. The results suggest that inhibitors of histone deacetylases can protect against cisplatin nephrotoxicity by attenuating DNA damage response and associated p53 activation.

Ionizing radiation induces apoptotic signal through protein kinase Cdelta (delta) and survival signal through Akt and cyclic-nucleotide response element-binding protein (CREB) in Jurkat T cells

Although ionizing radiation induces a loss of proliferative capacity as well as cell death by apoptosis and necrosis, cells can oppose the damaging effects by activating survival signal pathways. Here we report the effect of 1.5- and 6-Gy doses of ionizing radiation on apoptotic protein kinase Cdelta (PKCdelta) and survival cyclic-nucleotide response element-binding protein (CREB) signal in Jurkat T cells. Cell cycle analysis, performed by flow cytometry, showed a significant G2M arrest 24 h after exposure to 6 Gy. This arrest was accompanied by dead cells, which increased in number up to 7 days, when cell viability was further reduced. The response was apparently promoted by caspase-3-mediated PKCdelta activation, and thus apoptosis. Moreover, the presence of viable cells up to 7 days in samples exposed to 6 Gy is explained by Akt activation, which may influence the nuclear transcription factor CREB, leading to resistance to ionizing radiation. Thus, the knowledge of apoptotic and survival pathways activated in tumor cells may help in establishing specific therapies by combining selective inhibitors or stimulators of key signaling proteins with conventional chemotherapy, hormone therapy, and radiotherapy.

Association analyses of the interaction between the ADSS and ATM genes with schizophrenia in a Chinese population

BACKGROUND

The blood-derived RNA levels of the adenylosuccinate synthase (ADSS) and ataxia telangiectasia mutated (ATM) genes were found to be down- and up-regulated, respectively, in schizophrenics compared with controls, and ADSS and ATM were among eight biomarker genes to discriminate schizophrenics from normal controls. ADSS catalyzes the first committed step of AMP synthesis, while ATM kinase serves as a key signal transducer in the DNA double-strand breaks response pathway. It remains unclear whether these changes result from mutations or polymorphisms in the two genes.

METHODS

Six SNPs in the ADSS gene and three SNPs in the ATM gene in a Chinese population of 488 schizophrenics and 516 controls were genotyped to examine their association with schizophrenia (SZ). Genotyping was performed using the Sequenom platform.

RESULTS

There was no significant difference in the genotype, allele, or haplotype distributions of the nine SNPs between cases and controls. Using the Multifactor Dimensionality Reduction (MDR) method, we found that the interactions among rs3102460 in the ADSS gene and rs227061 and rs664143 in the ATM gene revealed a significant association with SZ. This model held a maximum testing accuracy of 60.4% and a maximum cross-validation consistency of 10 out of 10.

CONCLUSION

These findings suggest that the combined effects of the polymorphisms in the ADSS and ATM genes may confer susceptibility to the development of SZ in a Chinese population.

Induction of a pro-apoptotic ATM-NF-kappaB pathway and its repression by ATR in response to replication stress

The transcription factor NF-kappaB has critical functions in biologic responses to genotoxic stimuli. Activation of NF-kappaB in response to DNA double strand break (DSB) inducers can be mediated by ATM (ataxia telangiectasia mutated)-dependent phosphorylation of NEMO (NF-kappaB essential modulator). Here, we show that the replication stress inducers hydroxyurea (HU) and aphidicolin also activate this ATM-dependent signalling pathway. We further show that ATR (ATM- and Rad3-related) interacts with NEMO but surprisingly does not cause NEMO phosphorylation. Consequently, ATR represses NF-kappaB activation induced by replication stress. Reduction or increase of ATR expression by RNA interference or overexpression increased or reduced ATM-NEMO association and NF-kappaB activation induced by HU. Apoptosis gene expression and chromatin immunoprecipitation analyses indicated that HU and the DSB inducer etoposide caused complex patterns of NF-kappaB-dependent pro- and antiapoptotic gene expression with the overall outcome for the former being pro-apoptotic, whereas the latter antiapoptotic. Thus, replication stress and DSB inducers activate NF-kappaB through a conserved pathway with opposite biologic outcomes, and ATR antagonizes ATM function at least in part by competing for NEMO association.

RNF8-dependent and RNF8-independent regulation of 53BP1 in response to DNA damage

The DNA damage surveillance network orchestrates cellular responses to DNA damage through the recruitment of DNA damage-signaling molecules to DNA damage sites and the concomitant activation of protein phosphorylation cascades controlled by the ATM (ataxia-telangiectasia-mutated) and ATR (ATM-Rad3-related) kinases. Activation of ATM/ATR triggers cell cycle checkpoint activation and adaptive responses to DNA damage. Recent studies suggest that protein ubiquitylation or degradation plays an important role in the DNA damage response. In this study, we examined the potential role of the proteasome in checkpoint activation and ATM/ATR signaling in response to UV light-induced DNA damage. HeLa cells treated with the proteasome inhibitor MG-132 showed delayed phosphorylation of ATM substrates in response to UV light. UV light-induced phosphorylation of 53BP1, as well as its recruitment to DNA damage foci, was strongly suppressed by proteasome inhibition, whereas the recruitment of upstream regulators of 53BP1, including MDC1 and H2AX, was unaffected. The ubiquitin-protein isopeptide ligase RNF8 was critical for 53BP1 focus targeting and phosphorylation in ionizing radiation-damaged cells, whereas UV light-induced 53BP1 phosphorylation and targeting exhibited partial dependence on RNF8 and the ubiquitin-conjugating enzyme UBC13. Suppression of RNF8 or UBC13 also led to subtle defects in UV light-induced G2/M checkpoint activation. These findings are consistent with a model in which RNF8 ubiquitylation pathways are essential for 53BP1 regulation in response to ionizing radiation, whereas RNF8-independent pathways contribute to 53BP1 targeting and phosphorylation in response to UV light and potentially other forms of DNA replication stress.

Identification of carboxyl-terminal MCM3 phosphorylation sites using polyreactive phosphospecific antibodies

The functionally related ATM (ataxia telangiectasia-mutated) and ATR (ATM-Rad3-related) protein kinases are critical regulators of DNA damage responses in mammalian cells. ATM and ATR share highly overlapping substrate specificities and show a strong preference for the phosphorylation of Ser or Thr residues followed by Gln. In this report we used a polyreactive phosphospecific antibody (alpha-pDSQ) that recognizes a subset of phosphorylated Asp-Ser-Gln sequences to purify candidate ATM/ATR substrates. This led to the identification of phosphorylation sites in the carboxyl terminus of the minichromosome maintenance protein 3 (MCM3), a component of the hexameric MCM DNA helicase. We show that the alpha-DSQ antibody recognizes tandem DSQ phosphorylation sites (Ser-725 and Ser-732) in the carboxyl terminus of murine MCM3 (mMCM3) and that ATM phosphorylates both sites in vitro. ATM phosphorylated the carboxyl termini of mMCM3 and human MCM3 in vivo and the phosphorylated form of MCM3 retained association with the canonical MCM complex. Although DNA damage did not affect steady-state levels of chromatin-bound MCM3, the ATM-phosphorylated form of MCM3 was preferentially localized to the soluble, nucleoplasmic fraction. This finding suggests that the carboxyl terminus of chromatin-loaded MCM3 may be sequestered from ATM-dependent checkpoint signals. Finally, we show that ATM and ATR jointly contribute to UV light-induced MCM3 phosphorylation, but that ATM is the predominant UV-activated MCM3 kinase in vivo. The carboxyl-terminal ATM phosphorylation sites are conserved in vertebrate MCM3 orthologs suggesting that this motif may serve important regulatory functions in response to DNA damage. Our findings also suggest that DSQ motifs are common phosphoacceptor motifs for ATM family kinases.

Coregulated ataxia telangiectasia-mutated and casein kinase sites modulate cAMP-response element-binding protein-coactivator interactions in response to DNA damage

The cyclic AMP-response element-binding protein (CREB) is a bZIP family transcription factor implicated as an oncoprotein and neuron survival factor. CREB is activated in response to cellular stimuli, including cAMP and Ca(2+), via phosphorylation of Ser-133, which promotes interaction between the kinase-inducible domain (KID) of CREB and the KID-interacting domain of CREB-binding protein (CBP). We previously demonstrated that the interaction between CREB and CBP is inhibited by DNA-damaging stimuli through a mechanism whereby CREB is phosphorylated by the ataxia telangiectasia-mutated (ATM) protein kinase. We now show that the ATM phosphorylation sites in CREB are functionally intertwined with a cluster of coregulated casein kinase (CK) sites. We demonstrate that DNA damage-induced phosphorylation of CREB occurs in three steps. The initial event in the CREB phosphorylation cascade is the phosphorylation of Ser-111, which is carried out by CK1 and CK2 under basal conditions and by ATM in response to ionizing radiation. The phosphorylation of Ser-111 triggers the CK2-dependent phosphorylation of Ser-108 and the CK1-dependent phosphorylation of Ser-114 and Ser-117. The phosphorylation of Ser-114 and Ser-117 by CK1 then renders CREB permissive for ATM-dependent phosphorylation on Ser-121. Mutation of Ser-121 alone abrogates ionizing radiation-dependent repression of CREB-CBP complexes, which can be recapitulated using a CK1 inhibitor. Our findings outline a complex mechanism of CREB phosphorylation in which coregulated ATM and CK sites control CREB transactivation potential by modulating its CBP-binding affinity. The coregulated ATM and CK sites identified in CREB may constitute a signaling motif that is common to other DNA damage-regulated substrates.

ATR-dependent phosphorylation and activation of ATM in response to UV treatment or replication fork stalling

The phosphatidyl inositol 3-kinase-like kinases (PIKKs), ataxia-telangiectasia mutated (ATM) and ATM- and Rad3-related (ATR) regulate parallel damage response signalling pathways. ATM is reported to be activated by DNA double-strand breaks (DSBs), whereas ATR is recruited to single-stranded regions of DNA. Although the two pathways were considered to function independently, recent studies have demonstrated that ATM functions upstream of ATR following exposure to ionising radiation (IR) in S/G2. Here, we show that ATM phosphorylation at Ser1981, a characterised autophosphorylation site, is ATR-dependent and ATM-independent following replication fork stalling or UV treatment. In contrast to IR-induced ATM-S1981 phosphorylation, UV-induced ATM-S1981 phosphorylation does not require the Nbs1 C-terminus or Mre11. ATR-dependent phosphorylation of ATM activates ATM phosphorylation of Chk2, which has an overlapping function with Chk1 in regulating G2/M checkpoint arrest. Our findings provide insight into the interplay between the PIKK damage response pathways.

The ATM-mediated DNA-damage response: taking shape

Cellular responses to DNA damage are crucial for maintaining homeostasis and preventing the development of cancer. Our understanding of the DNA-damage response has evolved: whereas previously the focus was on DNA repair, we now appreciate that the response to DNA lesions involves a complex, highly branched signaling network. Defects in this response lead to severely debilitating, cancer-predisposing "genomic instability syndromes". Double strand breaks (DSBs) in DNA are potent triggers of the DNA-damage response, which is why they are used to study this pathway. The chief transducer of the DSB signal is the nuclear protein kinase ataxia-telangiectasia mutated (ATM). Genetic, biochemical and structural studies have recently provided insights into the ATM-mediated DSB response, reshaping our view of this signaling pathway while raising new questions.