A human Cds1-related kinase that functions downstream of ATM protein in the cellular response to DNA damage

Minor Kinases with Major Roles in Cytokinesis Regulation

Cytokinesis, the conclusive act of cell division, allows cytoplasmic organelles and chromosomes to be faithfully partitioned between two daughter cells. In animal organisms, its accurate regulation is a fundamental task for normal development and for preventing aneuploidy. Cytokinesis failures produce genetically unstable tetraploid cells and ultimately result in chromosome instability, a hallmark of cancer cells. In animal cells, the assembly and constriction of an actomyosin ring drive cleavage furrow ingression, resulting in the formation of a cytoplasmic intercellular bridge, which is severed during abscission, the final event of cytokinesis. Kinase-mediated phosphorylation is a crucial process to orchestrate the spatio-temporal regulation of the different stages of cytokinesis. Several kinases have been described in the literature, such as cyclin-dependent kinase, polo-like kinase 1, and Aurora B, regulating both furrow ingression and/or abscission. However, others exist, with well-established roles in cell-cycle progression but whose specific role in cytokinesis has been poorly investigated, leading to considering these kinases as "minor" actors in this process. Yet, they deserve additional attention, as they might disclose unexpected routes of cell division regulation. Here, we summarize the role of multifunctional kinases in cytokinesis with a special focus on those with a still scarcely defined function during cell cleavage. Moreover, we discuss their implication in cancer.

Caspase-2 regulates S-phase cell cycle events to protect from DNA damage accumulation independent of apoptosis

In addition to its classical role in apoptosis, accumulating evidence suggests that caspase-2 has non-apoptotic functions, including regulation of cell division. Loss of caspase-2 is known to increase proliferation rates but how caspase-2 is regulating this process is currently unclear. We show that caspase-2 is activated in dividing cells in G1-phase of the cell cycle. In the absence of caspase-2, cells exhibit numerous S-phase defects including delayed exit from S-phase, defects in repair of chromosomal aberrations during S-phase, and increased DNA damage following S-phase arrest. In addition, caspase-2-deficient cells have a higher frequency of stalled replication forks, decreased DNA fiber length, and impeded progression of DNA replication tracts. This indicates that caspase-2 protects from replication stress and promotes replication fork protection to maintain genomic stability. These functions are independent of the pro-apoptotic function of caspase-2 because blocking caspase-2-induced cell death had no effect on cell division, DNA damage-induced cell cycle arrest, or DNA damage. Thus, our data supports a model where caspase-2 regulates cell cycle and DNA repair events to protect from the accumulation of DNA damage independently of its pro-apoptotic function.

Yeast as a Tool to Understand the Significance of Human Disease-Associated Gene Variants

At present, the great challenge in human genetics is to provide significance to the growing amount of human disease-associated gene variants identified by next generation DNA sequencing technologies. Increasing evidences suggest that model organisms are of pivotal importance to addressing this issue. Due to its genetic tractability, the yeast Saccharomyces cerevisiae represents a valuable model organism for understanding human genetic variability. In the present review, we show how S. cerevisiae has been used to study variants of genes involved in different diseases and in different pathways, highlighting the versatility of this model organism.

CHEK2 Germline Variants in Cancer Predisposition: Stalemate Rather than Checkmate

Germline alterations in many genes coding for proteins regulating DNA repair and DNA damage response (DDR) to DNA double-strand breaks (DDSB) have been recognized as pathogenic factors in hereditary cancer predisposition. The ATM-CHEK2-p53 axis has been documented as a backbone for DDR and hypothesized as a barrier against cancer initiation. However, although CHK2 kinase coded by the CHEK2 gene expedites the DDR signal, its function in activation of p53-dependent cell cycle arrest is dispensable. CHEK2 mutations rank among the most frequent germline alterations revealed by germline genetic testing for various hereditary cancer predispositions, but their interpretation is not trivial. From the perspective of interpretation of germline CHEK2 variants, we review the current knowledge related to the structure of the CHEK2 gene, the function of CHK2 kinase, and the clinical significance of CHEK2 germline mutations in patients with hereditary breast, prostate, kidney, thyroid, and colon cancers.

DNA damage response signaling pathways and targets for radiotherapy sensitization in cancer

Radiotherapy is one of the most common countermeasures for treating a wide range of tumors. However, the radioresistance of cancer cells is still a major limitation for radiotherapy applications. Efforts are continuously ongoing to explore sensitizing targets and develop radiosensitizers for improving the outcomes of radiotherapy. DNA double-strand breaks are the most lethal lesions induced by ionizing radiation and can trigger a series of cellular DNA damage responses (DDRs), including those helping cells recover from radiation injuries, such as the activation of DNA damage sensing and early transduction pathways, cell cycle arrest, and DNA repair. Obviously, these protective DDRs confer tumor radioresistance. Targeting DDR signaling pathways has become an attractive strategy for overcoming tumor radioresistance, and some important advances and breakthroughs have already been achieved in recent years. On the basis of comprehensively reviewing the DDR signal pathways, we provide an update on the novel and promising druggable targets emerging from DDR pathways that can be exploited for radiosensitization. We further discuss recent advances identified from preclinical studies, current clinical trials, and clinical application of chemical inhibitors targeting key DDR proteins, including DNA-PKcs (DNA-dependent protein kinase, catalytic subunit), ATM/ATR (ataxia–telangiectasia mutated and Rad3-related), the MRN (MRE11-RAD50-NBS1) complex, the PARP (poly[ADP-ribose] polymerase) family, MDC1, Wee1, LIG4 (ligase IV), CDK1, BRCA1 (BRCA1 C terminal), CHK1, and HIF-1 (hypoxia-inducible factor-1). Challenges for ionizing radiation-induced signal transduction and targeted therapy are also discussed based on recent achievements in the biological field of radiotherapy.

Patients with Cholangiocarcinoma Present Specific RNA Profiles in Serum and Urine Extracellular Vesicles Mirroring the Tumor Expression: Novel Liquid Biopsy Biomarkers for Disease Diagnosis

Cholangiocarcinoma (CCA) comprises a group of heterogeneous biliary cancers with dismal prognosis. The etiologies of most CCAs are unknown, but primary sclerosing cholangitis (PSC) is a risk factor. Non-invasive diagnosis of CCA is challenging and accurate biomarkers are lacking. We aimed to characterize the transcriptomic profile of serum and urine extracellular vesicles (EVs) from patients with CCA, PSC, ulcerative colitis (UC), and healthy individuals. Serum and urine EVs were isolated by serial ultracentrifugations and characterized by nanoparticle tracking analysis, transmission electron microscopy, and immunoblotting. EVs transcriptome was determined by Illumina gene expression array [messenger RNAs (mRNA) and non-coding RNAs (ncRNAs)]. Differential RNA profiles were found in serum and urine EVs from patients with CCA compared to control groups (disease and healthy), showing high diagnostic capacity. The comparison of the mRNA profiles of serum or urine EVs from patients with CCA with the transcriptome of tumor tissues from two cohorts of patients, CCA cells in vitro, and CCA cells-derived EVs, identified 105 and 39 commonly-altered transcripts, respectively. Gene ontology analysis indicated that most commonly-altered mRNAs participate in carcinogenic steps. Overall, patients with CCA present specific RNA profiles in EVs mirroring the tumor, and constituting novel promising liquid biopsy biomarkers.

DNA-PKcs and ATM epistatically suppress DNA end resection and hyperactivation of ATR-dependent G2-checkpoint in S-phase irradiated cells

We previously reported that cells exposed to low doses of ionizing radiation (IR) in the G₂-phase of the cell cycle activate a checkpoint that is epistatically regulated by ATM and ATR operating as an integrated module. In this module, ATR interphases exclusively with the cell cycle to implement the checkpoint, mainly using CHK1. The ATM/ATR module similarly regulates DNA end-resection at low IR-doses. Strikingly, at high IR-doses, the ATM/ATR coupling relaxes and each kinase exerts independent contributions to resection and the G₂-checkpoint. DNA-PKcs links to the ATM/ATR module and defects cause hyper-resection and hyperactivation of G₂-checkpoint at all doses examined. Surprisingly, our present report reveals that cells irradiated in S-phase utilize a different form of wiring between DNA-PKcs/ATM/ATR: The checkpoint activated in G₂-phase is regulated exclusively by ATR/CHK1; similarly at high and low IR-doses. DNA end-resection supports ATR-activation, but inhibition of ATR leaves resection unchanged. DNA-PKcs and ATM link now epistatically to resection and their inhibition causes hyper-resection and ATR-dependent G₂-checkpoint hyperactivation at all IR-doses. We propose that DNA-PKcs, ATM and ATR form a modular unit to regulate DSB processing with their crosstalk distinctly organized in S- and G₂- phase, with strong dependence on DSB load only in G₂-phase.

Radiation-dose-dependent functional synergisms between ATM, ATR and DNA-PKcs in checkpoint control and resection in G2-phase

Using data generated with cells exposed to ionizing-radiation (IR) in G₂-phase of the cell cycle, we describe dose-dependent interactions between ATM, ATR and DNA-PKcs revealing unknown mechanistic underpinnings for two key facets of the DNA damage response: DSB end-resection and G₂-checkpoint activation. At low IR-doses that induce low DSB-numbers in the genome, ATM and ATR regulate epistatically the G₂-checkpoint, with ATR at the output-node, interfacing with the cell-cycle predominantly through Chk1. Strikingly, at low IR-doses, ATM and ATR epistatically regulate also resection, and inhibition of either activity fully suppresses resection. At high IR-doses that induce high DSB-numbers in the genome, the tight ATM/ATR coupling relaxes and independent outputs to G₂-checkpoint and resection occur. Consequently, both kinases must be inhibited to fully suppress checkpoint activation and resection. DNA-PKcs integrates to the ATM/ATR module by regulating resection at all IR-doses, with defects in DNA-PKcs causing hyper-resection and G₂-checkpoint hyper-activation. Notably, hyper-resection is absent from other c-NHEJ mutants. Thus, DNA-PKcs specifically regulates resection and adjusts the activation of the ATM/ATR module. We propose that selected DSBs are shepherd by DNA-PKcs from c-NHEJ to resection-dependent pathways for processing under the regulatory supervision of the ATM/ATR module.

The dynamic and stress-adaptive signaling hub of 14-3-3: emerging mechanisms of regulation and context-dependent protein-protein interactions

14-3-3 proteins are a family of structurally similar phospho-binding proteins that regulate essentially every major cellular function. Decades of research on 14-3-3s have revealed a remarkable network of interacting proteins that demonstrate how 14-3-3s integrate and control multiple signaling pathways. In particular, these interactions place 14-3-3 at the center of the signaling hub that governs critical processes in cancer, including apoptosis, cell cycle progression, autophagy, glucose metabolism, and cell motility. Historically, the majority of 14-3-3 interactions have been identified and studied under nutrient-replete cell culture conditions, which has revealed important nutrient driven interactions. However, this underestimates the reach of 14-3-3s. Indeed, the loss of nutrients, growth factors, or changes in other environmental conditions (e.g., genotoxic stress) will not only lead to the loss of homeostatic 14-3-3 interactions, but also trigger new interactions, many of which are likely stress adaptive. This dynamic nature of the 14-3-3 interactome is beginning to come into focus as advancements in mass spectrometry are helping to probe deeper and identify context-dependent 14-3-3 interactions-providing a window into adaptive phosphorylation-driven cellular mechanisms that orchestrate the tumor cell's response to a variety of environmental conditions including hypoxia and chemotherapy. In this review, we discuss emerging 14-3-3 regulatory mechanisms with a focus on post-translational regulation of 14-3-3 and dynamic protein-protein interactions that illustrate 14-3-3's role as a stress-adaptive signaling hub in cancer.

ATM, ATR, and DNA-PK: The Trinity at the Heart of the DNA Damage Response

In vertebrate cells, the DNA damage response is controlled by three related kinases: ATM, ATR, and DNA-PK. It has been 20 years since the cloning of ATR, the last of the three to be identified. During this time, our understanding of how these kinases regulate DNA repair and associated events has grown profoundly, although major questions remain unanswered. Here, we provide a historical perspective of their discovery and discuss their established functions in sensing and responding to genotoxic stress. We also highlight what is known regarding their structural similarities and common mechanisms of regulation, as well as emerging non-canonical roles and how our knowledge of ATM, ATR, and DNA-PK is being translated to benefit human health.

Mutations of the CHEK2 gene in patients with cancer and their presence in the Latin American population

Background:CHEK2(Checkpoint Kinase 2) encodes CHK2, a serine/threonine kinase involved in maintaining the G1/S and G2/M checkpoints and repair of double-strand DNA breaks via homologous recombination. Functions of CHK2 include the prevention of damaged cells from going through the cell cycle or proliferating and the maintenance of chromosomal stability.CHEK2mutations have been reported in a variety of cancers including glioblastoma, ovarian, prostate, colorectal, gastric, thyroid, and lung cancer in studies performed mainly in White populations. The most studied mutation inCHEK2is c.1100delC, which was associated with increased risk of breast cancer. The objective of this study was to compile mutations inCHEK2identified in cancer genomics studies in different populations and especially in Latin American individuals.Methods:A revision of cancer genomics data repositories and a profound literature review of Latin American studies was performed.Results:Mutations with predicted high impact inCHEK2were reported in studies from Australia, Japan, United States, among other countries. The TCGA cancer types with most mutations inCHEK2were breast, colorectal, and non-small cell lung cancer. The most common mutation found was E321* in three patients with uterine cancer. In Latin American individuals nine mutations were found in melanoma, lymphoma, and head and neck cohorts from TCGA and ICGC. Latin American studies have been restricted to breast and colorectal cancer and only two mutations out of four that have been interrogated in this population were identified, namely c.1100delC and c.349A>G.Conclusions:This study presents a compilation of mutations inCHEK2with high impact in different cancer types in White, Hispanic and other populations. We also show the necessity of screeningCHEK2mutations in Latin American in cancer types different than breast and colorectal.

Low prevalence of CHEK2 gene mutations in multiethnic cohorts of breast cancer patients in Malaysia

CHEK2 is a protein kinase that is involved in cell-cycle checkpoint control after DNA damage. Germline mutations in CHEK2 gene have been associated with increase in breast cancer risk. The aim of this study is to identify the CHEK2 gene germline mutations among high-risk breast cancer patients and its contribution to the multiethnic population in Malaysia. We screened the entire coding region of CHEK2 gene on 59 high-risk breast cancer patients who tested negative for BRCA1/2 germline mutations from UKM Medical Centre (UKMMC), Hospital Kuala Lumpur (HKL) and Hospital Putrajaya (HPJ). Sequence variants identified were screened further in case-control cohorts consisting of 878 unselected invasive breast cancer patients (180 Malays, 526 Chinese and 172 Indian) and 270 healthy individuals (90 Malays, 90 Chinese and 90 Indian). By screening the entire coding region of the CHEK2 gene, two missense mutations, c.480A>G (p.I160M) and c.538C>T (p.R180C) were identified in two unrelated patients (3.4%). Further screening of these missense mutations on the case-control cohorts unveiled the variant p.I160M in 2/172 (1.1%) Indian cases and 1/90 (1.1%) Indian control, variant p.R180C in 2/526 (0.38%) Chinese cases and 0/90 Chinese control, and in 2/180 (1.1%) of Malay cases and 1/90 (1.1%) of Malay control. The results of this study suggest that CHEK2 mutations are rare among high-risk breast cancer patients and may play a minor contributing role in breast carcinogenesis among Malaysian population.

A novel recurrent CHEK2 Y390C mutation identified in high-risk Chinese breast cancer patients impairs its activity and is associated with increased breast cancer risk

Certain predisposition factors such as BRCA1/2 and CHEK2 mutations cause familial breast cancers that occur early. In China, breast cancers are diagnosed at relatively younger age, and higher percentage of patients are diagnosed before 40 years, than that in Caucasians. However, the prevalence for BRCA1/2 mutations and reported CHEK2 germline mutations is much lower or absent in Chinese population, arguing for the need to study other novel risk alleles among Chinese breast cancer patients. In this study, we searched for CHEK2 mutations in young, high-risk breast cancer patients in China and detected a missense variant Y390C (1169A > G) in 12 of 150 patients (8.0%) and 2 in 250 healthy controls (0.8%, P = 0.0002). Four of the Y390C carriers have family history of breast and/or ovarian cancer. In patients without family history, Y390C carriers tend to develop breast cancer early, before 35 years of age. The codon change at Y390, a highly conserved residue located in CHEK2's kinase domain, appeared to significantly impair CHEK2 activity. Functional analysis suggested that the CHEK2 Y390C mutation is deleterious as judged by the mutant protein's inability to inactivate CDC25A or to activate p53 after DNA damage. Cells expressing the CHEK2 Y390C variant showed impaired p21 and Puma expression after DNA damage, and the deregulated cell cycle checkpoint and apoptotic response may help conserve mutations and therefore contribute to tumorigeneisis. Taken together, our results not only identified a novel CHEK2 allele that is associated with cancer families and confers increased breast cancer risk, but also showed that this allele significantly impairs CHEK2 function during DNA damage response. Our results provide further insight on how the function of such an important cancer gene may be impaired by existing mutations to facilitate tumorigenesis. It also offers a new subject for breast cancer monitoring, prevention and management.

Protein kinase CK2 is required for the recruitment of 53BP1 to sites of DNA double-strand break induced by radiomimetic drugs

The ataxia telangiectasia mutated (ATM) signaling pathway responds rapidly to DNA double-strand breaks (DSBs) and it is characterized by recruitment of sensor, mediator, transducer and repair proteins to sites of DNA damage. Data suggest that CK2 is implicated in the early cellular response to DSBs. We demonstrate that CK2 binds constitutively the adaptor protein 53BP1 through the tandem Tudor domains and that the interaction is disrupted upon induction of DNA damage. Down-regulation of CK2 results in significant reduction of (i) 53BP1 foci formation, (ii) binding to dimethylated histone H4 and (iii) ATM autophosphorylation. Our data suggest that CK2 is required for 53BP1 accumulation at sites of DSBs which is a prerequisite for efficient activation of the ATM-mediated signaling pathway.

Recent progress in mouse models for tumor suppressor genes and its implications in human cancer

Gain-of-function mutations in oncogenes and loss-of-function mutations in tumor suppressor genes (TSG) lead to cancer. In most human cancers, these mutations occur in somatic tissues. However, hereditary forms of cancer exist for which individuals are heterozygous for a germline mutation in a TSG locus at birth. The second allele is frequently inactivated by gene deletion, point mutation, or promoter methylation in classical TSGs that meet Knudson's two-hit hypothesis. Conversely, the second allele remains as wild-type, even in tumors in which the gene is haplo-insufficient for tumor suppression. This article highlights the importance of PTEN, APC, and other tumor suppressors for counteracting aberrant PI3K, β-catenin, and other oncogenic signaling pathways. We discuss the use of gene-engineered mouse models (GEMM) of human cancer focusing on Pten and Apc knockout mice that recapitulate key genetic events involved in initiation and progression of human neoplasia. Finally, the therapeutic potential of targeting these tumor suppressor and oncogene signaling networks is discussed.

Carboxy-terminal phosphorylation sites in Cdc25 contribute to enforcement of the DNA damage and replication checkpoints in fission yeast

In fission yeast and vertebrate cells, Cdc25 phosphatase is the target of checkpoint-mediated response to DNA replication blocks, DNA damage, and extracellular stress. As such, it is a key regulator of cell cycle progress and genomic stability. In fission yeast, phosphorylation of Cdc25 by the checkpoint kinases Cds1 and Chk1 and also Srk1 during stress creates a binding site for the 14-3-3 homolog Rad24; the complex is then exported from the nucleus. Cdc25 contains 12 potential serine/threonine phosphorylation sites that are phosphorylated in vitro by Cds1; 9 reside in the amino terminal half of the protein with the remaining sites are located in the extreme C-terminus. We have previously shown that deletion of the nine amino terminal sites results in degradation of the mutant protein while the checkpoint is enforced by the Mik1 kinase acting on Cdc2 tyrosine-15. Here, we examine the influence of the three C-terminal sites on the negative regulation of Cdc25. These sites are conserved in vertebrates and have been shown to be phosphorylated following DNA damage and replication blocks. We show that these three sites have a role in the negative regulation of Cdc25 following replication arrest, but perhaps more importantly they appear to particularly contribute to regulating the duration, and thus the effectiveness of the arrested state.

Chk2 deficiency in Myc overexpressing lymphoma cells elicits a synergistic lethal response in combination with PARP inhibition

Myc is a transcription factor frequently found deregulated in human cancer. The Myc-mediated cellular transformation process is associated with fast proliferative cells and inherent genomic instability, giving rise to malignant, invasive neoplasms with poor prognosis for survival. Transcription-independent functions of Myc include stimulation of replication. Excessive Myc expression stimulates a replication-associated DNA damage response that signals via the phosphoinositide-3-kinase (PI3K)-related protein kinases (PIKKs) ATM and ATR. These, in turn, activate the DNA damage transducers Chk1 and Chk2. Here, we show that Myc can stimulate Chek2 transcript indirectly in vitro as well as in B cells of λ-Myc transgenic mice or in the intestine of Apc (Min) mice. However, Chk2 is dispensable for Myc's ability to transform cells in vitro and for the survival of established lymphoma cells from λ-Myc transgenic mice. Chk2 deficiency induces polyploidy and slow growth, but the cells are viable and protected against DNA damage. Furthermore, inhibition of both Chk1/Chk2 with AZD7762 induces cell death and significantly delays disease progression of transplanted lymphoma cells in vivo. DNA damage recruits PARP family members to sites of DNA breaks that, in turn, facilitate the induction of DNA repair. Strikingly, combining Chk2 and PARP inhibition elicits a synergistic lethal response in the context of Myc overexpression. Our data indicates that only certain types of chemotherapy would give rise to a synergistic lethal response in combination with specific Chk2 inhibitors, which will be important if Chk2 inhibitors enter the clinic.

Haploinsufficiency of DNA Damage Response Genes and their Potential Influence in Human Genomic Disorders

Genomic disorders are a clinically diverse group of conditions caused by gain, loss or re-orientation of a genomic region containing dosage-sensitive genes. One class of genomic disorder is caused by hemizygous deletions resulting in haploinsufficiency of a single or, more usually, several genes. For example, the heterozygous contiguous gene deletion on chromosome 22q11.2 causing DiGeorge syndrome involves at least 20-30 genes. Determining how the copy number variation (CNV) affects human variation and contributes to the aetiology and progression of various genomic disorders represents important questions for the future. Here, I will discuss the functional significance of one form of CNV, haploinsufficiency (i.e. loss of a gene copy), of DNA damage response components and its association with certain genomic disorders. There is increasing evidence that haploinsufficiency for certain genes encoding key players in the cells response to DNA damage, particularly those of the Ataxia Telangiectasia and Rad3-related (ATR)-pathway, has a functional impact. I will review this evidence and present examples of some well known clinically similar genomic disorders that have recently been shown to be defective in the ATR-dependent DNA damage response. Finally, I will discuss the potential implications of a haploinsufficiency-induced defective DNA damage response for the clinical management of certain human genomic disorders.

NPRL2 sensitizes human non-small cell lung cancer (NSCLC) cells to cisplatin treatment by regulating key components in the DNA repair pathway

NPRL2, one of the tumor suppressor genes residing in a 120-kb homozygous deletion region of human chromosome band 3p21.3, has a high degree of amino acid sequence homology with the nitrogen permease regulator 2 (NPR2) yeast gene, and mutations of NPRL2 in yeast cells are associated with resistance to cisplatin-mediated cell killing. Previously, we showed that restoration of NPRL2 in NPRL2-negative and cisplatin-resistant cells resensitize lung cancer cells to cisplatin treatment in vitro and in vivo. In this study, we show that sensitization of non-small cell lung cancer (NSCLC) cells to cisplatin by NPRL2 is accomplished through the regulation of key components in the DNA-damage checkpoint pathway. NPRL2 can phosphorylate ataxia telangiectasia mutated (ATM) kinase activated by cisplatin and promote downstream γ-H2AX formation in vitro and in vivo, which occurs during apoptosis concurrently with the initial appearance of high-molecular-weight DNA fragments. Moreover, this combination treatment results in higher Chk1 and Chk2 kinase activity than does treatment with cisplatin alone and can activate Chk2 in pleural metastases tumor xenograft in mice. Activated Chk1 and Chk2 increase the expression of cell cycle checkpoint proteins, including Cdc25A and Cdc25C, leading to higher levels of G2/M arrest in tumor cells treated with NPRL2 and cisplatin than in tumor cells treated with cisplatin only. Our results therefore suggest that ectopic expression of NPRL2 activates the DNA damage checkpoint pathway in cisplatin-resistant and NPRL2-negative cells; hence, the combination of NPRL2 and cisplatin can resensitize cisplatin nonresponders to cisplatin treatment through the activation of the DNA damage checkpoint pathway, leading to cell arrest in the G2/M phase and induction of apoptosis. The direct implication of this study is that combination treatment with NPRL2 and cisplatin may overcome cisplatin resistance and enhance therapeutic efficacy.

DNA damage-induced degradation of Cdc25A does not lead to inhibition of Cdk2 activity in mouse embryonic stem cells

Cyclin-dependent kinase two (Cdk2) is the major regulator of the G1/S transition and the target of an activated G1 checkpoint in somatic cells. In the presence of DNA damage, Cdk2 kinase activity is abrogated by a deficiency of Cdc25A phosphatase, which is marked by Chk1/Chk2 for proteasomal degradation. Embryonic stem cells (ESCs) lack a G1 checkpoint response. In this study, we analyzed the G1 checkpoint pathways in mouse ESCs (mESCs) in the presence of DNA double-strand breaks evoked by ionizing radiation (IR). We show that checkpoint pathways, which operate during G1 phase in somatic cells, are activated in mESCs after IR; however, Cdk2 activity is not abolished. We demonstrate that Cdc25A is degraded in mESCs, but this degradation is not regulated by Chk1 and Chk2 kinases because they are sequestered to the centrosome. Instead, Cdc25A degradation is governed by glycogen synthase kinase-3beta kinase. We hypothesize that Cdc25A degradation does not inhibit Cdk2 activity because a considerable proportion of Cdk2 molecules localize to the cytoplasm and centrosomes in mESCs, where they may be sheltered from regulation by nuclear Cdc25A. Finally, we show that a high Cdk2 activity, which is irresponsive to DNA damage, is the driving force of the rapid escape of mESCs from G1 phase after DNA damage.

Chk2 protects against radiation-induced genomic instability

The murine Chk2 kinase is activated after exposure to ionizing radiation and is necessary for p53-dependent apoptosis, but the role Chk2 plays in determining genomic stability is poorly understood. By analyzing the sensitivity of Chk2-deficient murine and human cells to a range of DNA-damaging agents, we show that Chk2 deficiency results in resistance to agents that generate double-strand breaks but not to other forms of damage. Surprisingly, the absence of Chk2 results in increased sensitivity to UV-radiation-induced DNA damage. Defective apoptosis after radiation-induced DNA damage may result in genomic instability; therefore, the consequences of Chk2 deficiency on genomic instability were assayed using an in vitro screen. Gene amplification was not detected in untreated Chk2(-/-) cells, but the rate of gene amplification after irradiation was elevated and was similar to that found in p53 compromised cells. A synergistic increase in genomic instability was seen after disruption of both Chk2 and p53 function, indicating that the two proteins have non-redundant roles in regulating genome stability after irradiation. The data demonstrate that Chk2 functions to maintain genome integrity after radiation-induced damage and has important implications for the use of Chk2 inhibitors as adjuvant cancer therapy.

Haplotypes of the I157T CHEK2 germline mutation in ethnically diverse populations

The CHEK2*I157T missense mutation, reported in ethnically diverse, high-risk families, moderately increases breast and colon cancer risk. The present study assessed whether this mutation represents a founder mutation. Participants identified in high risk clinics or from consecutive cancer patients in Israel, Poland, Latvia, and Finland, were either carriers of the CHEK2*I157T mutation or non-carrier family members. Multi-locus genotyping employed two intragenic markers and five CHEK2 gene flanking markers, spanning about 645 kb. Haplotyping was done when families were available for phasing. Overall, 101 individuals (83 I157T*CHEK2 mutation carriers) were genotyped: 16 Finnish individuals from 11 families (14 mutation carriers, two non-carrier family members), 50 Polish individuals (20 families) (35 carriers, 15 non-carriers), 28 unrelated Latvian mutation carriers, and seven Israeli participants (two families) (six mutation carriers, one non-carrier). Overall 36/83 mutation carriers (43%) were diagnosed with breast cancer, 15/83 (18%)-colon cancer, three-ovarian cancer, one-thyroid cancer, and the rest (n = 28) were asymptomatic. A common core haplotype was detected in all I157T*CHEK2 mutation carriers of Israeli, Polish, and Finnish origin between markers D22S275-D22S689 (approximately 258 kb), with a different allele pattern in Latvians. In conclusion, CHEK2*I157T missense mutation is a founder mutation in ethnically diverse populations, but may also be a mutational hotspot.

The Schizosaccharomyces pombe checkpoint kinases Chk1 and Cds1 are important for cell survival in response to cisplatin

BACKGROUND

DNA damage checkpoints insure that the integrity of genomic DNA is faithfully maintained throughout the eukaryotic cell cycle. In the presence of damaged DNA, checkpoints are triggered to delay cell cycle progression to allow for DNA repair. In fission yeast, the kinases Chk1 and Cds1 are major components of these DNA damage checkpoint pathways. Both Chk1 and Cds1 are important for viability in the presence of several DNA damaging agents. In this study we hypothesized that Chk1 and Cds1 play a vital role in fission yeast cells ability to survive exposure to the DNA damaging agent cisplatin. Cisplatin is a potent chemotherapeutic drug that interacts with DNA and causes both inter- and intra-strand DNA cross-links.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we demonstrated that treatment with cisplatin in fission yeast causes a Chk1-dependent DNA damage signal. chk1(-) cells were sensitive to cisplatin and Chk1 was phosphorylated in response to cisplatin treatment. We also showed that a Chk1-dependent DNA damage checkpoint pathway is activated in a dose-dependent fashion in cells challenged with cisplatin. Furthermore the Cds1 checkpoint kinase was also important for viability in cisplatin challenged cells. In cds1(-) cells, cisplatin treatment reduced cell viability and this phenotype was exacerbated in a chk1(-)/cds1(-) background.

CONCLUSIONS/SIGNIFICANCE

Thus, we conclude that the concerted effort of both major checkpoint kinases in fission yeast, Chk1 and Cds1, protect cells from cisplatin induced DNA damage. These observations are significant because they suggest that various classes of inter-strand crosslinking agents may generate slightly different lesions as work by others did not observe loss of viability in cds1(-) cells treated with other crosslinking agents like nitrogen mustard.

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.

Human papilloma virus type16 E6 deregulates CHK1 and sensitizes human fibroblasts to environmental carcinogens independently of its effect on p53

After treatment with ultraviolet radiation (UV), human fibroblasts that express the HPV type 16 E6 oncoprotein display defects in repair of cyclobutane pyrimidine dimers, hypersensitivity to inactivation of clonogenic survival and an inability to sustain DNA replication. To determine whether these effects are specific to depletion of p53 or inactivation of its function, fibroblast lines were constructed with ectopic expression of a dominant-negative p53 allele (p53-H179Q) to inactivate function or a short-hairpin RNA (p53-RNAi) to deplete expression of p53. Only the expression of HPV16E6 sensitized fibroblasts to UV or the chemical carcinogen, benzo[a]pyrene diolepoxide I (BPDE). Carcinogen-treated cells expressing p53-H179Q or p53-RNAi were resistant to inactivation of colony formation and did not suffer replication arrest. CHK1 is a key checkpoint kinase in the response to carcinogen-induced DNA damage. Control and p53-RNAi-expressing fibroblasts displayed phosphorylation of Ser345 on CHK1 45-120 min after carcinogen treatment with a return to near baseline phosphorylation by 6 h after treatment. HPV16E6-expressing fibroblasts displayed enhanced and sustained phosphorylation of CHK1. This was associated with enhanced phosphorylation of Thr68 on CHK2 and Ser139 on H2AX, both markers of severe replication stress and DNA double strand breaks. Incubation with the phosphatase inhibitor okadaic acid produced more phosphorylation of CHK1 in UV-treated HPV16E6-expressing cells than in p53-H179Q-expressing cells suggesting that HPV16E6 may interfere with the recovery of coupled DNA replication at replication forks that are stalled at [6-4]pyrimidine-pyrimidone photoproducts and BPDE-DNA adducts. The results indicate that HPV16E6 targets a protein or proteins other than p53 to deregulate the activity of CHK1 in carcinogen-damaged cells.

Inhibition of matrix metalloproteinase-2 enhances radiosensitivity by abrogating radiation-induced FoxM1-mediated G2/M arrest in A549 lung cancer cells

Matrix metalloproteinase-2 (MMP-2), is known to degrade the collagen IV, plays a role in radiation-induced lung injury. We therefore investigated the antitumor effects of combining MMP-2 inhibition using an adenovirus expressing siRNA against MMP-2 (Ad-MMP-2-Si) with radiation therapy (IR) on A549 lung cancer cells in vitro and in vivo. IR increased MMP-2 mRNA, protein and activity in lung cancer cells. MMP-2 inhibition along with IR enhanced radiosensitivity as determined by clonogenic assay, flow cytometry and TUNEL assay. We show that MMP-2 inhibition prior to irradiation reduced p53 phosphorylation, with a corresponding reduction in the expression of the p53 downstream target gene p21(Cip1/Waf1). Irradiated tumor cells induced the FoxM1-mediated DNA repair gene, XRCC1 and Checkpoint kinases 2/1, which were abrogated with combined treatment of Ad-MMP-2-Si and IR. Further, the combination of Ad-MMP-2-Si with radiotherapy significantly increased antitumor efficacy in vivo compared to either agent alone. Indeed, histological analysis of tumor sections collected from the combination group revealed more apoptotic cells. These studies suggest that MMP-2 inhibition in combination with radiotherapy abrogates G2 cell cycle arrest leading to apoptosis and provide evidence of the antitumor efficacy of combining MMP-2 inhibition with irradiation as a new therapeutic strategy for the effective treatment of NSCLC patients.

HIV-1 expression induces tubular cell G2/M arrest and apoptosis

Human renal biopsy studies suggest the presence of HIV-1 and associated signs of injury in renal tubular epithelial cells. Because renal epithelial cells lack conventional HIV-1 receptors, the modus operandi of HIV-1 in the induction of tubular cell injury remains a mystery. In the present study, we evaluated the role of HIV-1 gene expression in human proximal tubular cell apoptosis and cell cycle progression. HIV-1- or vector-transduced cells were assayed for cellular injury and cell cycle defect. HIV-1-transduced cells showed the progressive loss of viability in a time-dependent manner. Similarly, HIV-1-transduced cells showed greater apoptosis when compared with vector-transduced cells. A higher number of HIV-1 expressing cells showed cell cycle arrest at G2/M phase and enhanced tubular cell expression of phospho-p53(ser15), phospho-cdc-2(Tyr 15), and phospho-chk-2 (Thr 68). These findings suggest that in addition to the activation of apoptotic pathway, HIV-1-induced G2/M arrest may also contribute to tubular cell injury.

Chk2 oligomerization studied by phosphopeptide ligation: implications for regulation and phosphodependent interactions

Chk2/CHEK2/hCds1 is a modular serine-threonine kinase involved in transducing DNA damage signals. Phosphorylation by ataxia telangiectasia-mutated kinase (ATM) promotes Chk2 self-association, autophosphorylation, and activation. Here we use expressed protein ligation to generate a Chk2 N-terminal regulatory region encompassing a fork-head-associated (FHA) domain, a stoichiometrically phosphorylated Thr-68 motif and intervening linker. Hydrodynamic analysis reveals that Thr-68 phosphorylation stabilizes weak FHA-FHA interactions that occur in the unphosphorylated species to form a high affinity dimer. Although clearly a prerequisite for Chk2 activation in vivo, we show that dimerization modulates potential phosphodependent interactions with effector proteins and substrates through either the pThr-68 site, or the canonical FHA phosphobinding surface with which it is tightly associated. We further show that the dimer-occluded pThr-68 motif is released by intra-dimer autophosphorylation of the FHA domain at the highly conserved Ser-140 position, a major pThr contact in all FHA-phosphopeptide complex structures, revealing a mechanism of Chk2 dimer dissociation following kinase domain activation.

Never-in-mitosis related kinase 1 functions in DNA damage response and checkpoint control

Nek1, the first mammalian ortholog of the fungal protein kinase never in mitosis A, is involved early in the DNA damage sensing/repair pathway after ionizing radiation. Here we extend this finding by showing that Nek1 localizes to nuclear foci of DNA damage in response to many different types of damage in addition to IR. Untransformed cells established from kat2J/Nek1(-/-) mice fail to arrest properly at G(1)/S and M-phase checkpoints in response to DNA damage. G(1)-S-phase checkpoint control can be rescued by ectopically overexpressing wild-type Nek1. In Nek1(-/-) murine cells and in human cells with Nek1 expression silenced by siRNA, the checkpoint kinases Chk1 and Chk2 fail to be activated properly in response to ionizing or UV radiation. In cells without functional Nek1, DNA is not repaired properly, double-stranded DNA breaks persist long after low dose IR, and excessive numbers of chromosome breaks are observed. These data show that Nek1 is important for efficient DNA damage checkpoint control and for proper DNA damage repair.

Regulation of Chk2 ubiquitination and signaling through autophosphorylation of serine 379

The Chk2 protein kinase protects genome integrity by promoting cell cycle arrest or apoptosis in response to DNA double-strand breaks, and Chk2 mutations are found in both familial and sporadic cancers. Exposure of cells to ionizing radiation (IR) or radiomimetic drugs induces Chk2 phosphorylation by ATM, followed by Chk2 oligomerization, auto-/transphosphorylation, and activation. Here we demonstrate that Chk2 is ubiquitinated upon activation and that this requires Chk2 kinase activity. Serine 379 (S379) was identified as a novel IR-inducible autophosphorylation site required for ubiquitination of Chk2 by a Cullin 1-containing E3 ligase complex. Importantly, S379 was required for Chk2 to induce apoptosis in cells with DNA double-strand breaks. Thus, auto-/transphosphorylation of S379 is required for Chk2 ubiquitination and effector function.

Kinetics of histone H2AX phosphorylation and Chk2 activation in A549 cells treated with topotecan and mitoxantrone in relation to the cell cycle phase

The DNA topoisomerase I (topo1) inhibitor topotecan (TPT) and topo2 inhibitors doxorubicin, etoposide and mitoxantrone (MXT) are widely used antitumor drugs. They stabilize otherwise transient ("cleavable") complexes of topo1 or topo2 with DNA, respectively. Collisions of DNA replication forks (during replication) or progressing RNA polymerase molecules (during transcription) with these complexes convert them into double-strand DNA breaks (DSBs). Formation of DSBs triggers activation of ATM and phosphorylation of histone H2AX, the markers that have been used to correlate DNA damage with cell cycle phase or induction of apoptosis. In the present study we explored a relationship between H2AX phosphorylation and activation of checkpoint kinase 2 (Chk2) in human lung carcinoma A549 cells treated with TPT or with MXT. Activation of Chk2 was detected immunocytochemically using a phospho-specific (Thr68) Ab and measuring Chk2-Thr68(P)immunofluorescence (IF), concurrently with DNA content, by laser scanning cytometry. In the untreated cells, activated Chk2 was present predominantly in centrosomes. Upon treatment with TPT or MTX, the activated Chk2 presented itself in form of either minute or large IF foci in the cell's nucleoplasm. H2AX phosphorylation whether induced by TPT or MXT was rapid, with the maximal rate occurring during the initial 2 h and peaking at 2 h of treatment. TPT or MXT induced Chk2 activation occurred at a distinctly slower pace, peaking at 4 h. While TPT-induced H2AX phosphorylation and Chk2 activation were maximal in S-phase cells, Chk2 activation was also much pronounced in G(2)M cells; the least affected by TPT were G(1) cells. MTX-induced H2AX phosphorylation was maximal in G(1) cells while Chk2 activation was maximal in G(2)M and minimal in G(1) cells. The pattern of cell-cycle phase specific response to TPT or MXT by H2AX phosphorylation and Chk2 activation was different when measured either as integrated or maximal pixel of gammaH2AX or Chk2-Thr68(P) IF, the former reflecting total IF per nucleus the latter stressing the punctate (foci) character of expression of these phospho-modified proteins.

Autophosphorylated residues involved in the regulation of human chk2 in vitro

Human checkpoint kinase 2 is a major actor in checkpoint activation through phosphorylation by ataxia telangiectasia mutated in response to DNA double-strand breaks. In the absence of de novo DNA damage, its autoactivation, reported in the event of increased Cds1/checkpoint kinase 2 (Chk2) expression, has been attributed to oligomerization. Here we report a study performed on autoactivated recombinant Chk2 proteins that aims to correlate kinase activity and phosphorylation status. Using a fluorescence-based technique to assay human checkpoint kinase 2 catalytic activity, slight differences in the ability to phosphorylate Cdc25C were observed, depending on the recombinant system used. Using mass spectrometry, the phosphorylation sites were mapped to identify sites potentially involved in the kinase activity. Five phosphorylated positions, at Ser120, Ser260, Thr225, Ser379 and Ser435, were found to be common to bacteria and insect cells expression systems. They were present in addition to the six known phosphorylation sites induced by ionizing radiation (Thr68, Thr432, Thr387, Ser516, Ser33/35 and Ser19) detected by immunoblotting. After phosphatase treatment, Chk2 regained activity via autorephosphorylation. The determination of the five common sites and ionizing-radiation-inducible positions as rephosphorylated confirms that they are potential positive regulators of Chk2 kinase activity. For Escherichia coli's most highly phosphorylated 6His-Chk2, 13 additional phosphorylation sites were assigned, including 7 novel sites on top of recently reported phosphorylation sites.

DNA protein kinase-dependent G2 checkpoint revealed following knockdown of ataxia-telangiectasia mutated in human mammary epithelial cells

Members of the phosphatidylinositol 3-kinase-related kinase family, in particular the ataxia-telangiectasia mutated (ATM) kinase and the catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs), regulate cellular responses to DNA double-strand breaks. Increased sensitivity to ionizing radiation (IR) in DNA-PKcs- or ATM-deficient cells emphasizes their important roles in maintaining genome stability. Furthermore, combined knockout of both kinases is synthetically lethal, suggesting functional complementarity. In the current study, using human mammary epithelial cells with ATM levels stably knocked down by >90%, we observed an IR-induced G(2) checkpoint that was only slightly attenuated. In marked contrast, this G(2) checkpoint was significantly attenuated with either DNA-PK inhibitor treatment or RNA interference knockdown of DNA-PKcs, the catalytic subunit of DNA-PK, indicating that DNA-PK contributes to the G(2) checkpoint in these cells. Furthermore, in agreement with the checkpoint attenuation, DNA-PK inhibition in ATM-knockdown cells resulted in reduced signaling of the checkpoint kinase CHK1 as evidenced by reduced CHK1 phosphorylation. Taken together, these results show a DNA-PK-dependent component to the IR-induced G(2) checkpoint, in addition to the well-defined ATM-dependent component. This may have important implications for chemotherapeutic strategies for breast cancers.

CHK2 kinase: cancer susceptibility and cancer therapy - two sides of the same coin?

In the past decade, CHK2 has emerged as an important multifunctional player in the DNA-damage response signalling pathway. Parallel studies of the human CHEK2 gene have also highlighted its role as a candidate multiorgan tumour susceptibility gene rather than a highly penetrant predisposition gene for Li-Fraumeni syndrome. As discussed here, our current understanding of CHK2 function in tumour cells, in both a biological and genetic context, suggests that targeted modulation of the active kinase or exploitation of its loss in tumours could prove to be effective anti-cancer strategies.

DNA-damage response network at the crossroads of cell-cycle checkpoints, cellular senescence and apoptosis

Tissue homeostasis requires a carefully-orchestrated balance between cell proliferation, cellular senescence and cell death. Cells proliferate through a cell cycle that is tightly regulated by cyclin-dependent kinase activities. Cellular senescence is a safeguard program limiting the proliferative competence of cells in living organisms. Apoptosis eliminates unwanted cells by the coordinated activity of gene products that regulate and effect cell death. The intimate link between the cell cycle, cellular senescence, apoptosis regulation, cancer development and tumor responses to cancer treatment has become eminently apparent. Extensive research on tumor suppressor genes, oncogenes, the cell cycle and apoptosis regulatory genes has revealed how the DNA damage-sensing and -signaling pathways, referred to as the DNA-damage response network, are tied to cell proliferation, cell-cycle arrest, cellular senescence and apoptosis. DNA-damage responses are complex, involving "sensor" proteins that sense the damage, and transmit signals to "transducer" proteins, which, in turn, convey the signals to numerous "effector" proteins implicated in specific cellular pathways, including DNA repair mechanisms, cell-cycle checkpoints, cellular senescence and apoptosis. The Bcl-2 family of proteins stands among the most crucial regulators of apoptosis and performs vital functions in deciding whether a cell will live or die after cancer chemotherapy and irradiation. In addition, several studies have now revealed that members of the Bcl-2 family also interface with the cell cycle, DNA repair/recombination and cellular senescence, effects that are generally distinct from their function in apoptosis. In this review, we report progress in understanding the molecular networks that regulate cell-cycle checkpoints, cellular senescence and apoptosis after DNA damage, and discuss the influence of some Bcl-2 family members on cell-cycle checkpoint regulation.

Defective cell cycle checkpoint functions in melanoma are associated with altered patterns of gene expression

Defects in DNA damage responses may underlie genetic instability and malignant progression in melanoma. Cultures of normal human melanocytes (NHMs) and melanoma lines were analyzed to determine whether global patterns of gene expression could predict the efficacy of DNA damage cell cycle checkpoints that arrest growth and suppress genetic instability. NHMs displayed effective G1 and G2 checkpoint responses to ionizing radiation-induced DNA damage. A majority of melanoma cell lines (11/16) displayed significant quantitative defects in one or both checkpoints. Melanomas with B-RAF mutations as a class displayed a significant defect in DNA damage G2 checkpoint function. In contrast the epithelial-like subtype of melanomas with wild-type N-RAS and B-RAF alleles displayed an effective G2 checkpoint but a significant defect in G1 checkpoint function. RNA expression profiling revealed that melanoma lines with defects in the DNA damage G1 checkpoint displayed reduced expression of p53 transcriptional targets, such as CDKN1A and DDB2, and enhanced expression of proliferation-associated genes, such as CDC7 and GEMININ. A Bayesian analysis tool was more accurate than significance analysis of microarrays for predicting checkpoint function using a leave-one-out method. The results suggest that defects in DNA damage checkpoints may be recognized in melanomas through analysis of gene expression.

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.

Identification of protein phosphorylation sites within Ser/Thr-rich cluster domains using site-directed mutagenesis and hybrid linear quadrupole ion trap Fourier transform ion cyclotron resonance mass spectrometry

We describe a method for the analysis of multi-site phosphorylation in serine/threonine (Ser/Thr)-rich protein sequences. Site-specific mutagenesis was used to introduce tryptic cleavage sites in the serine glutamine/threonine glutamine cluster domain (SCD) of the human checkpoint protein kinase (Chk2). The mutant proteins were shown to autophosphorylate on residues that are inducibly phosphorylated when mammalian cells are exposed to ionizing radiation (serine 33/35, serine 516, threonine 68 and threonine 432). Five Ser/Thr clusters within the SCD were flanked by arginine or lysine residues to produce tryptic peptides for nanospray liquid chromatography (nanoLC)/linear quadrupole ion trap Fourier transform ion cyclotron resonance mass spectrometry. Phosphorylation sites were assigned using accurate-mass-driven analysis and interpretation of low-energy collision-induced dissociation spectra acquired in the ion trap. In addition to verifying known phosphorylation sites, seventeen novel sites were identified within the SCD of Chk2. The approach should be applicable to other O-linked post-translational modifications that occur in proteins with Ser/Thr-rich sequences.

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.

ATR, PML, and CHK2 Play a Role in Arsenic Trioxide-induced Apoptosis

Arsenic trioxide (ATO) is a potent anti-leukemic chemotherapeutic agent for acute promyelocytic leukemia (APL) that results from a t (15, 17) chromosomal translocation that produces PML-RARalpha, a fusion protein between a tumor suppressor PML and the retinoic acid receptor RARalpha. APL patients are initially treated with retinoic acid, but most develop resistance and relapse. In contrast, ATO induces prolonged remissions even in the relapsed cases. However, the molecular mechanisms by which ATO kills the leukemic cells are not fully understood. We find that ATO induces apoptosis, at least in part, by activating proapoptotic kinase Chk2. ATO does this by stimulating ATR (ataxia telangiectasia mutated and Rad3-related) kinase, a Chk2-activating kinase. In conjunction, ATO degrades PML-RARalpha, resulting in the restoration of PML, which is required for autophosphorylation and full activation of Chk2. As a result, the p53-dependent apoptosis pathway is activated. Based on this, we propose that a pathway composed of ATR, PML, Chk2, and p53 plays a role in ATO-mediated apoptosis, a notion that is consistent with the observation that Chk2 is genetically intact and mutations in the p53 gene are extremely rare in APL.

DDB1 maintains genome integrity through regulation of Cdt1

DDB1, a component of a Cul4A ubiquitin ligase complex, promotes nucleotide excision repair (NER) and regulates DNA replication. We have investigated the role of human DDB1 in maintaining genome stability. DDB1-depleted cells accumulate DNA double-strand breaks in widely dispersed regions throughout the genome and have activated ATM and ATR cell cycle checkpoints. Depletion of Cul4A yields similar phenotypes, indicating that an E3 ligase function of DDB1 is important for genome maintenance. In contrast, depletion of DDB2, XPA, or XPC does not cause activation of DNA damage checkpoints, indicating that defects in NER are not involved. One substrate of DDB1-Cul4A that is crucial for preventing genome instability is Cdt1. DDB1-depleted cells exhibit increased levels of Cdt1 protein and rereplication, despite containing other Cdt1 regulatory mechanisms. The rereplication, accumulation of DNA damage, and activation of checkpoint responses in DDB1-depleted cells require entry into S phase and are partially, but not completely, suppressed by codepletion of Cdt1. Therefore, DDB1 prevents DNA lesions from accumulating in replicating human cells, in part by regulating Cdt1 degradation.

DNA damage-induced cell cycle regulation and function of novel Chk2 phosphoresidues

Chk2 kinase is activated by DNA damage to regulate cell cycle arrest, DNA repair, and apoptosis. Phosphorylation of Chk2 in vivo by ataxia telangiectasia-mutated (ATM) on threonine 68 (T68) initiates a phosphorylation cascade that promotes the full activity of Chk2. We identified three serine residues (S19, S33, and S35) on Chk2 that became phosphorylated in vivo rapidly and exclusively in response to ionizing radiation (IR)-induced DNA double-strand breaks in an ATM- and Nbs1-dependent but ataxia telangiectasia- and Rad3-related-independent manner. Phosphorylation of these residues, restricted to the G(1) phase of the cell cycle, was induced by a higher dose of IR (>1 Gy) than that required for phosphorylation of T68 (0.25 Gy) and declined by 45 to 90 min, concomitant with a rise in Chk2 autophosphorylation. Compared to the wild-type form, Chk2 with alanine substitutions at S19, S33, and S35 (Chk2(S3A)) showed impaired dimerization, defective auto- and trans-phosphorylation activities, and reduced ability to promote degradation of Hdmx, a phosphorylation target of Chk2 and regulator of p53 activity. Besides, Chk2(S3A) failed to inhibit cell growth and, in response to IR, to arrest G(1)/S progression. These findings underscore the critical roles of S19, S33, and S35 and argue that these phosphoresidues may serve to fine-tune the ATM-dependent response of Chk2 to increasing amounts of DNA damage.

Intrinsic Kinase Activity and SQ/TQ Domain of Chk2 Kinase as Well as N-terminal Domain of Wip1 Phosphatase Are Required for Regulation of Chk2 by Wip1

The anti-oncogenic Chk2 kinase plays a crucial role in DNA damage-induced cell cycle checkpoint regulation. Recently, we have shown that Chk2 associates with the oncogenic Wip1 (PPM1D) phosphatase and that Wip1 acts as a negative regulator of Chk2 during DNA damage response by dephosphorylating phosphorylated Thr-68 in activated Chk2 (Fujimoto, H., Onishi, N., Kato, N., Takekawa, M., Xu, X. Z., Kosugi, A., Kondo, T., Imamura, M., Oishi, I., Yoda, A., and Minami, Y. (2006) Cell Death Differ. 13, 1170-1180). Here, we performed structure-function analyses of Chk2 and Wip1 by using a series of deletion or amino acid-substituted mutant proteins of Chk2 and Wip1. We show that nuclear localization of both Chk2 and Wip1 is required for their association in cultured cells and that the serine-glutamine (SQ)/threonine-glutamine (TQ) domain of Chk2, containing Thr-68, and the N-terminal domain of Wip1, comprising about 100 amino acids, are necessary and sufficient for the association of both molecules. However, it was found that an intrinsic kinase activity of Chk2, but not phosphatase activity of Wip1, is required for the association of fulllength Chk2 and Wip1. Interestingly, we also show that the mutant Wip1 proteins, bearing the N-terminal domain of Wip1 alone or lacking an intrinsic phosphatase activity, exhibit dominant negative effects on the functions of the wild-type Wip1, i.e. ectopic expression of either of these Wip1 mutants inhibits dephosphorylation of Thr-68 in Chk2 by Wip1 and anti-apoptotic function of Wip1. These results provide a molecular basis for developing novel anti-cancer drugs, targeting oncogenic Wip1 phosphatase.

Promyelocytic leukemia activates Chk2 by mediating Chk2 autophosphorylation

Chk2 is a kinase critical for DNA damage-induced apoptosis and is considered a tumor suppressor. Chk2 is essential for p53 transcriptional and apoptotic activities. Although mutations of p53 are present in more than half of all tumors, mutations of Chk2 in cancers are rare, suggesting that Chk2 may be inactivated by unknown alternative mechanisms. Here we elucidate one such alternative mechanism regulated by PML (promyelocytic leukemia) that is involved in acute promyelocytic leukemia (APL). Although p53-inactivating mutations are extremely rare in APL, t(15;17) chromosomal translocation which fuses retinoic acid receptor (RARalpha) to PML is almost always present in APL, while the other PML allele is intact. We demonstrate that PML interacts with Chk2 and activates Chk2 by mediating its autophosphorylation step, an essential step for Chk2 activity that occurs after phosphorylation by the upstream kinase ATM (ataxia telangiectasia-mutated). PML/RARalpha in APL suppresses Chk2 by dominantly inhibiting the auto-phosphorylation step, but inactivation of PML/RARalpha with alltrans retinoic acid (ATRA) restores Chk2 autophosphorylation and activity. Thus, by fusing PML with RARalpha, the APL cells appear to have achieved functional suppression of Chk2 compromising the Chk2-p53 apoptotic pathway.