Determination of substrate motifs for human Chk1 and hCds1/Chk2 by the oriented peptide library approach

Chemo-phosphoproteomic profiling with ATR inhibitors berzosertib and gartisertib uncovers new biomarkers and DNA damage response regulators

The ATR kinase protects cells against DNA damage and replication stress and represents a promising anti-cancer drug target. The ATR inhibitors (ATRi) berzosertib and gartisertib are both in clinical trials for the treatment of advanced solid tumours as monotherapy or in combination with genotoxic agents. We carried out quantitative phospho-proteomic screening for ATR biomarkers that are highly sensitive to berzosertib and gartisertib, using an optimized mass spectrometry pipeline. Screening identified a range of novel ATR-dependent phosphorylation events, which were grouped into three broad classes: i) targets whose phosphorylation is highly sensitive to ATRi and which could be the next generation of ATR biomarkers; ii) proteins with known genome maintenance roles not previously known to be regulated by ATR; iii) novel targets whose cellular roles are unclear. Class iii targets represent candidate DNA damage response proteins and, with this in mind, proteins in this class were subjected to secondary screening for recruitment to DNA damage sites. We show that one of the proteins recruited, SCAF1, interacts with RNAPII in a phospho-dependent manner and recruitment requires PARP activity and interaction with RNAPII. We also show that SCAF1 deficiency partly rescues RAD51 loading in cells lacking the BRCA1 tumour suppressor. Taken together these data reveal potential new ATR biomarkers and new genome maintenance factors.

ELK3 destabilization by speckle-type POZ protein suppresses prostate cancer progression and docetaxel resistance

Accumulating evidence demonstrates that the activity regulation of ELK3, a member of the E26 transformation-specific oncogene family, is critical to regulating cell proliferation, migration, and survival in human cancers. However, the molecular mechanisms of how ELK3 induces chemoresistance in prostate cancer (PCa) have not been elucidated. In this study, we found that SPOP and ELK3 are an interacting partner. The interaction between SPOP and ELK3 resulted in increased ELK3 ubiquitination and destruction, assisted by checkpoint kinase-mediated ELK3 phosphorylation. Notably, the modulation of SPOP-mediated ELK3 protein stability affected the c-Fos-induced cell proliferation and invasion of PCa cells. The clinical involvement of the SPOP-ELK3 axis in PCa development was confirmed by an immunohistochemical assay on 123 PCa tissues, with an inverse correlation between increased ELK3 and decreased SPOP being present in ~80% of the specimens. This observation was supported by immunohistochemistry analysis using a SPOP-mutant PCa specimen. Finally, docetaxel treatment induced cell death by activating checkpoint kinase- and SPOP-mediated ELK3 degradation, while SPOP-depleted or SPOP-mutated PCa cells showed cell death resistance. Notably, this observation was correlated with the protein levels of ELK3. Taken together, our study reveals the precise mechanism of SPOP-mediated degradation of ELK3 and provides evidence that SPOP mutations contribute to docetaxel resistance in PCa.

Recruitment of Polo-like kinase couples synapsis to meiotic progression via inactivation of CHK-2

Meiotic chromosome segregation relies on synapsis and crossover (CO) recombination between homologous chromosomes. These processes require multiple steps that are coordinated by the meiotic cell cycle and monitored by surveillance mechanisms. In diverse species, failures in chromosome synapsis can trigger a cell cycle delay and/or lead to apoptosis. How this key step in 'homolog engagement' is sensed and transduced by meiotic cells is unknown. Here we report that in C. elegans, recruitment of the Polo-like kinase PLK-2 to the synaptonemal complex triggers phosphorylation and inactivation of CHK-2, an early meiotic kinase required for pairing, synapsis, and double-strand break (DSB) induction. Inactivation of CHK-2 terminates DSB formation and enables CO designation and cell cycle progression. These findings illuminate how meiotic cells ensure CO formation and accurate chromosome segregation.

Enhanced access to the human phosphoproteome with genetically encoded phosphothreonine

Protein phosphorylation is a ubiquitous post-translational modification used to regulate cellular processes and proteome architecture by modulating protein-protein interactions. The identification of phosphorylation events through proteomic surveillance has dramatically outpaced our capacity for functional assignment using traditional strategies, which often require knowledge of the upstream kinase a priori. The development of phospho-amino-acid-specific orthogonal translation systems, evolutionarily divergent aminoacyl-tRNA synthetase and tRNA pairs that enable co-translational insertion of a phospho-amino acids, has rapidly improved our ability to assess the physiological function of phosphorylation by providing kinase-independent methods of phosphoprotein production. Despite this utility, broad deployment has been hindered by technical limitations and an inability to reconstruct complex phopho-regulatory networks. Here, we address these challenges by optimizing genetically encoded phosphothreonine translation to characterize phospho-dependent kinase activation mechanisms and, subsequently, develop a multi-level protein interaction platform to directly assess the overlap of kinase and phospho-binding protein substrate networks with phosphosite-level resolution.

Switchable Control of Scaffold Protein Activity via Engineered Phosphoregulated Autoinhibition

Scaffold proteins operate as organizing hubs to enable high-fidelity signaling, fulfilling crucial roles in the regulation of cellular processes. Bottom-up construction of controllable scaffolding platforms is attractive for the implementation of regulatory processes in synthetic biology. Here, we present a modular and switchable synthetic scaffolding system, integrating scaffold-mediated signaling with switchable kinase/phosphatase input control. Phosphorylation-responsive inhibitory peptide motifs were fused to 14-3-3 proteins to generate dimeric protein scaffolds with appended regulatory peptide motifs. The availability of the scaffold for intermolecular partner protein binding could be lowered up to 35-fold upon phosphorylation of the autoinhibition motifs, as demonstrated using three different kinases. In addition, a hetero-bivalent autoinhibitory platform design allowed for dual-kinase input regulation of scaffold activity. Reversibility of the regulatory platform was illustrated through phosphatase-controlled abrogation of autoinhibition, resulting in full recovery of 14-3-3 scaffold activity.

The Chk2-PKM2 axis promotes metabolic control of vasculogenic mimicry formation in p53-mutated triple-negative breast cancer

Vasculogenic mimicry (VM) formation, which participates in the process of neovascularization, is highly activated in p53-mutated triple-negative breast cancer (TNBC). Here, we show that Chk2 is negatively correlated with VM formation in p53-mutated TNBC. Its activation by DNA-damaging agents such as cisplatin, etoposide, and DPT reduces VM formation. Mechanistically, the Chk2-PKM2 axis plays an important role in the inhibition of VM formation at the level of metabolic regulation. Chk2 promotes the Chk2-PKM2 interaction through the Chk2 SCD (SQ/TQ cluster domain) and the PKM2 C domain. Furthermore, Chk2 promotes the nuclear export of PKM2 by phosphorylating PKM2 at Ser100. P-PKM2 S100 reduces VM formation by decreasing glucose flux, and the PKM2 S100A mutation abolishes the inhibition of glucose flux and VM formation induced by Chk2 activation. Overall, this study proposes a novel strategy of VM suppression through Chk2 induction, which prevents PKM2-mediated glucose flux in p53-mutated TNBC.

The intra-S phase checkpoint directly regulates replication elongation to preserve the integrity of stalled replisomes

DNA replication is dramatically slowed down under replication stress. The regulation of replication speed is a conserved response in eukaryotes and, in fission yeast, requires the checkpoint kinases Rad3ATR and Cds1Chk2 However, the underlying mechanism of this checkpoint regulation remains unresolved. Here, we report that the Rad3ATR-Cds1Chk2 checkpoint directly targets the Cdc45-MCM-GINS (CMG) replicative helicase under replication stress. When replication forks stall, the Cds1Chk2 kinase directly phosphorylates Cdc45 on the S275, S322, and S397 residues, which significantly reduces CMG helicase activity. Furthermore, in cds1Chk2 -mutated cells, the CMG helicase and DNA polymerases are physically separated, potentially disrupting replisomes and collapsing replication forks. This study demonstrates that the intra-S phase checkpoint directly regulates replication elongation, reduces CMG helicase processivity, prevents CMG helicase delinking from DNA polymerases, and therefore helps preserve the integrity of stalled replisomes and replication forks.

PDS5A and PDS5B in Cohesin Function and Human Disease

Precocious dissociation of sisters 5 (PDS5) is an associate protein of cohesin that is conserved from yeast to humans. It acts as a regulator of the cohesin complex and plays important roles in various cellular processes, such as sister chromatid cohesion, DNA damage repair, gene transcription, and DNA replication. Vertebrates have two paralogs of PDS5, PDS5A and PDS5B, which have redundant and unique roles in regulating cohesin functions. Herein, we discuss the molecular characteristics and functions of PDS5, as well as the effects of its mutations in the development of diseases and their relevance for novel therapeutic strategies.

Use of the Polo-like kinase 4 (PLK4) inhibitor centrinone to investigate intracellular signalling networks using SILAC-based phosphoproteomics

Polo-like kinase 4 (PLK4) is the master regulator of centriole duplication in metazoan organisms. Catalytic activity and protein turnover of PLK4 are tightly coupled in human cells, since changes in PLK4 concentration and catalysis have profound effects on centriole duplication and supernumerary centrosomes, which are associated with aneuploidy and cancer. Recently, PLK4 has been targeted with a variety of small molecule kinase inhibitors exemplified by centrinone, which rapidly induces inhibitory effects on PLK4 and leads to on-target centrosome depletion. Despite this, relatively few PLK4 substrates have been identified unequivocally in human cells, and PLK4 signalling outside centriolar networks remains poorly characterised. We report an unbiased mass spectrometry (MS)-based quantitative analysis of cellular protein phosphorylation in stable PLK4-expressing U2OS human cells exposed to centrinone. PLK4 phosphorylation was itself sensitive to brief exposure to the compound, resulting in PLK4 stabilisation. Analysing asynchronous cell populations, we report hundreds of centrinone-regulated cellular phosphoproteins, including centrosomal and cell cycle proteins and a variety of likely 'non-canonical' substrates. Surprisingly, sequence interrogation of ∼300 significantly down-regulated phosphoproteins reveals an extensive network of centrinone-sensitive [Ser/Thr]Pro phosphorylation sequence motifs, which based on our analysis might be either direct or indirect targets of PLK4. In addition, we confirm that NMYC and PTPN12 are PLK4 substrates, both in vitro and in human cells. Our findings suggest that PLK4 catalytic output directly controls the phosphorylation of a diverse set of cellular proteins, including Pro-directed targets that are likely to be important in PLK4-mediated cell signalling.

p50 mono-ubiquitination and interaction with BARD1 regulates cell cycle progression and maintains genome stability

p50, the mature product of NFKB1, is constitutively produced from its precursor, p105. Here, we identify BARD1 as a p50-interacting factor. p50 directly associates with the BARD1 BRCT domains via a C-terminal phospho-serine motif. This interaction is induced by ATR and results in mono-ubiquitination of p50 by the BARD1/BRCA1 complex. During the cell cycle, p50 is mono-ubiquitinated in S phase and loss of this post-translational modification increases S phase progression and chromosomal breakage. Genome-wide studies reveal a substantial decrease in p50 chromatin enrichment in S phase and Cycln E is identified as a factor regulated by p50 during the G1 to S transition. Functionally, interaction with BARD1 promotes p50 protein stability and consistent with this, in human cancer specimens, low nuclear BARD1 protein strongly correlates with low nuclear p50. These data indicate that p50 mono-ubiquitination by BARD1/BRCA1 during the cell cycle regulates S phase progression to maintain genome integrity.

p50 is a constitutively produced NF-κB subunit that modulates the response to DNA damage. Here, the authors show that activation of ATR during S phase induces p50 interaction with BARD1 resulting in p50 mono-ubiquitination, facilitating cell cycle progression and promoting chromosome integrity.

Regulation of Mitotic Exit by Cell Cycle Checkpoints: Lessons From Saccharomyces cerevisiae

In order to preserve genome integrity and their ploidy, cells must ensure that the duplicated genome has been faithfully replicated and evenly distributed before they complete their division by mitosis. To this end, cells have developed highly elaborated checkpoints that halt mitotic progression when problems in DNA integrity or chromosome segregation arise, providing them with time to fix these issues before advancing further into the cell cycle. Remarkably, exit from mitosis constitutes a key cell cycle transition that is targeted by the main mitotic checkpoints, despite these surveillance mechanisms being activated by specific intracellular signals and acting at different stages of cell division. Focusing primarily on research carried out using Saccharomyces cerevisiae as a model organism, the aim of this review is to provide a general overview of the molecular mechanisms by which the major cell cycle checkpoints control mitotic exit and to highlight the importance of the proper regulation of this process for the maintenance of genome stability during the distribution of the duplicated chromosomes between the dividing cells.

A novel alkylating deacetylase inhibitor molecule EDO-S101 in combination with cytarabine synergistically enhances apoptosis of acute myeloid leukemia cells

Acute myeloid leukemia (AML) is a devastating disease. Hybrid agents with dual activity, which have been shown to possess anti-cancer effect, are expected to potentially improve the prognosis of AML patients. EDO-S101 is a novel alkylating deacetylase inhibitor molecule synthesized by the addition of the hydroxamic acid of histone deacetylases inhibitor vorinostat into bendamustine, a DNA-damaging agent. However, the effect of EDO-S101 in combination with traditional chemotherapy drugs has not been studied in AML. In this study, we investigated the effect of EDO-S101 in combination with cytarabine in treating AML cells. The synergic activity against AML was identified by remarkable reduction of cell viability, significant apoptosis enhancement and the upregulation of the cleaved PARP, Casepase-3 and -7 proteins compared with monotherapy. To explain the drivers, we detected the DNA damage pathway including DNA double-strand breaks marker γ-H2AX and DNA damage checkpoint proteins, which was supposed to be responsible for the enhanced apoptosis activity. In summary, our data demonstrated that EDO-S101 in combination with cytarabine could synergistically induce the apoptosis of AML cells and it might be a potential regimen for treating leukemia.

The kinase domain of CK1δ can be phosphorylated by Chk1

Members of the casein kinase 1 (CK1) family are key regulators in numerous cellular signal transduction pathways and in order to prevent the development of certain diseases, CK1 kinase activity needs to be tightly regulated. Modulation of kinase activity by site-specific phosphorylation within the C-terminal regulatory domain of CK1δ has already been shown for several cellular kinases. By using biochemical methods, we now identified residues T161, T174, T176, and S181 within the kinase domain of CK1δ as target sites for checkpoint kinase 1 (Chk1). At least residues T176 and S181 show full conservation among CK1δ orthologues from different eukaryotic species. Enzyme kinetic analysis furthermore led to the hypothesis that site-specific phosphorylation within the kinase domain finally contributes to fine-tuning of CK1δ kinase activity. These data provide a basis for the extension of our knowledge about the role of site-specific phosphorylation for regulation of CK1δ and associated signal transduction pathways.

Quantitative phosphoproteomics reveals mitotic function of the ATR activator ETAA1

Bass and Cortez use comparative quantitative mass spectrometry analyses of cells lacking either ATR activator, ETAA1 or TOPBP1. They identify a role for ETAA1 and ATR activation in the regulation of chromosome alignment and segregation in mitosis through Aurora B activity.

The ATR kinase controls cell cycle transitions and the DNA damage response. ATR activity is regulated through two ATR-activating proteins, ETAA1 and TOPBP1. To examine how each activator contributes to ATR signaling, we used quantitative mass spectrometry to identify changes in protein phosphorylation in ETAA1- or TOPBP1-deficient cells. We identified 724, 285, and 118 phosphosites to be regulated by TOPBP1, ETAA1, or both ATR activators, respectively. Gene ontology analysis of TOPBP1- and ETAA1-dependent phosphoproteins revealed TOPBP1 to be a primary ATR activator for replication stress, while ETAA1 regulates mitotic ATR signaling. Inactivation of ATR or ETAA1, but not TOPBP1, results in decreased Aurora B kinase activity during mitosis. Additionally, ATR activation by ETAA1 is required for proper chromosome alignment during metaphase and for a fully functional spindle assembly checkpoint response. Thus, we conclude that ETAA1 and TOPBP1 regulate distinct aspects of ATR signaling with ETAA1 having a dominant function in mitotic cells.

Combination of Proteasome and Histone Deacetylase Inhibitors Overcomes the Impact of Gain-of-Function p53 Mutations

Mutations in the “guardian of the genome” TP53 predominate in solid tumors. In addition to loss of tumor suppressor activity, a specific subset of missense mutations confers additional oncogenic properties. These “gain-of-function” (GOF) mutations portend poor prognosis across cancer types regardless of treatment. Our objective in this study was to identify novel therapeutic opportunities to overcome the deleterious effects of GOF TP53 mutants. Using gynecologic cancer cell lines with known TP53 mutational status, we established that treatment with a proteasome inhibitor induced cell death in cells with two recurrent GOF TP53 mutations (R175H and R248Q), and addition of a histone deacetylase inhibitor (HDACi) enhanced this effect. By contrast, p53-null cancer cells were relatively resistant to the combination. Proteasome inhibition promoted apoptosis of cells with TP53 GOF mutations, potentially through induction of the unfolded protein response. In line with the reported hyperstabilization of GOF p53 protein, cells treated with HDACi exhibited reduced levels of p53 protein. Together, these data form the basis for future clinical studies examining therapeutic efficacy in a preselected patient population with GOF TP53 mutations.

IRTKS is correlated with progression and survival time of patients with gastric cancer

BACKGROUND AND OBJECTIVES

IRTKS functions as a novel regulator of tumour suppressor p53; however, the role of IRTKS in pathogenesis of gastric cancer is unclear.

DESIGN

We used immunohistochemistry to detect IRTKS levels in 527 human gastric cancer specimens. We generated both IRTKS-deficient and p53-deficient mice to observe survival time of these mice and to isolate mouse embryonic fibroblasts (MEFs) for evaluating in vivo tumorigenicity. Co-immunoprecipitation was used to study the interaction among p53, MDM2 and IRTKS, as well as the ubiquitination of p53.

RESULTS

IRTKS was significantly overexpressed in human gastric cancer, which was conversely associated with wild-type p53 expression. Among patients with wild-type p53 (n=206), those with high IRTKS expression (n=141) had a shorter survival time than those with low IRTKS (n=65) (p=0.0153). Heterozygous p53^+/- mice with IRTKS deficiency exhibited significantly delayed tumorigenesis and an extended tumour-free survival time. p53^+/- MEFs without IRTKS exhibited attenuated in vivo tumorigenicity. IRTKS depletion upregulated p53 and its target genes, such as BAX and p21. Intriguingly, IRTKS overexpression promoted p53 ubiquitination and degradation in MEFs and gastric cancer cells. Under DNA damage conditions, IRTKS was phosphorylated at Ser331 by the activated Chk2 kinase and then dissociated from p53, along with the p53-specific E3 ubiquitin ligase MDM2, resulting in attenuated p53 ubiquitination and degradation.

CONCLUSION

IRTKS overexpression is negatively correlated with progression and overall survival time of patients with gastric cancer with wild-type p53 through promotion of p53 degradation via the ubiquitin/proteasome pathway.

Anti-proliferative effect of isorhamnetin on HeLa cells through inducing G2/M cell cycle arrest

As a major cancer type in females, cervical cancer has been explored in depth by researchers. HeLa is a cervical cancer cell line. Isorhamnetin is an O-methylated flavonol that is primarily extracted from sea buckthorn. In the present study, the anti-proliferative effect of isorhamnetin on HeLa cells was evaluated using a Trypan blue dye exclusion assay. Isorhamnetin inhibited the cell proliferation in a time- and dose-dependent manner. Flow cytometric analysis of the cell cycle distribution revealed that isorhamnetin inhibited the cell cycle progression of HeLa by causing G2/M phase arrest and decreasing the proportion of cells in G1 phase. In addition, western blot analysis was performed to evaluate the presence of certain cell cycle-associated proteins. It was demonstrated that isorhamnetin inhibited the protein expression of cyclin B1, cell division cycle 25C (Cdc25C) and Cdc2, but enhanced checkpoint kinase 2 (Chk2), Cdc25C and Cdc2 phosphorylation. In addition, tubulin depolymerization participated in the isorhamnetin-induced cell cycle arrest in G2/M phase. In conclusion, the present results indicated that the anti-proliferative action of isorhamnetin is associated with arrest of the cell cycle in G2/M phase, which is a consequence of activation of the ataxia telangiectasia mutated Chk2 pathway and disruption of microtubule function.

Identification of three signaling molecules required for calcineurin-dependent monopolar growth induced by the DNA replication checkpoint in fission yeast

Cell polarity is coordinately regulated with the cell cycle. Growth polarity of the fission yeast Schizosaccharomyces pombe transits from monopolar to bipolar during G2 phase, termed NETO (new end take off). Upon perturbation of DNA replication, the checkpoint kinase Cds1/CHK2 induces NETO delay through activation of Ca²⁺/calmodulin-dependent protein phosphatase calcineurin (CN). CN in turn regulates its downstream targets including the microtubule (MT) plus-end tracking CLIP170 homologue Tip1 and the Casein kinase 1γ Cki3. However, whether and which Ca²⁺ signaling molecules are involved in the NETO delay remains elusive. Here we show that 3 genes (trp1322, vcx1 and SPAC6c3.06c encoding TRP channel, antiporter and P-type ATPase, respectively) play vital roles in the NETO delay. Upon perturbation of DNA replication, these 3 genes are required for not only the NETO delay but also for the maintenance of cell viability. Trp1322 and Vcx1 act downstream of Cds1 and upstream of CN for the NETO delay, whereas SPAC6c3.06c acts downstream of CN. Consistently, Trp1322 and Vcx1, but not SPAC6c3.06c, are essential for activation of CN. Interestingly, we have found that elevated extracellular Ca²⁺ per se induces a NETO delay, which depends on CN and its downstream target genes. These findings imply that Ca²⁺-CN signaling plays a central role in cell polarity control by checkpoint activation.

Checkpoint-dependent phosphorylation of Med1/TRAP220 in response to DNA damage

Mediator complex subunit 1 (Med1)/Thyroid hormone receptor-associated protein 220 (TRAP220), an essential component of thyroid hormone receptor-associated proteins (TRAP)/mediator, plays important roles in hormone responses and tumorigenesis. However, the role of Med1 in the DNA damage response has not been studied. In this study, we found that DNA damage, resulted from γ-irradiation, ultraviolet (UV)-irradiation, or hydroxyurea, induced phosphorylation of Med1 in vivo. Phosphorylation of Med1 was abrogated by either caffeine or wortmannin treatment, suggesting that Med1 is phosphorylated through the DNA damage checkpoint pathway. A checkpoint kinase 1 (Chk1)/checkpoint kinase 2 (Chk2) consensus phosphorylation motif was identified at Serine 671 of Med1 and Ser671 motif was primarily phosphorylated by Chk2 in vitro. Moreover, the in vivo phosphorylation of Med1 was abrogated by a Chk2 inhibitor, and physical interaction between Chk2 and Med1 was observed, confirming that Chk2 is responsible for Med1 phosphorylation upon DNA damage. These results suggest that Med1 is a novel target for the DNA damage checkpoint pathway and may participate in the DNA damage response. Consistent with this notion, knockdown of Med1 expression caused a significant increase in cellular sensitivity to UV irradiation. Moreover, microarray analysis revealed that the UV-induced activation of the transcription of important regulators of cell cycle control and DNA repair, including p21, Gadd45, Rad50, DnaJ, and RecQL, was impaired upon Med1 knockdown. Taken together, our data suggest that Med1 is a novel target for Chk2-mediated phosphorylation and may play a role in cellular DNA damage responses by mediating proper induction of gene transcription upon DNA damage.

Activation of the Chicken Anemia Virus Apoptin Protein by Chk1/2 Phosphorylation Is Required for Apoptotic Activity and Efficient Viral Replication

Chicken anemia virus (CAV) is a single-stranded circular DNA virus that carries 3 genes, the most studied of which is the gene encoding VP3, also known as apoptin. This protein has been demonstrated to specifically kill transformed cells while leaving normal cells unharmed in a manner that is independent of p53 status. Although the mechanistic basis for this differential activity is unclear, it is evident that the subcellular localization of the protein is important for the difference. In normal cells, apoptin exists in filamentous networks in the cytoplasm, whereas in transformed cells, apoptin is present in the nucleus and appears as distinct foci. We have previously demonstrated that DNA damage signaling through the ataxia telangiectasia mutated (ATM) pathway induces the translocation of apoptin from the cytoplasm to the nucleus, where it induces apoptosis. We found that apoptin contains four checkpoint kinase consensus sites and that mutation of either threonine 56 or 61 to alanine restricts apoptin to the cytoplasm. Furthermore, treatment of tumor cells expressing apoptin with inhibitors of checkpoint kinase 1 (Chk1) and Chk2 causes apoptin to localize to the cytoplasm. Importantly, silencing of Chk2 rescues cancer cells from the cytotoxic effects of apoptin. Finally, treatment of virus-producing cells with Chk inhibitor protects them from virus-mediated toxicity and reduces the titer of progeny virus. Taken together, our results indicate that apoptin is a sensor of DNA damage signaling through the ATM-Chk2 pathway, which induces it to migrate to the nucleus during viral replication.

IMPORTANCE

The chicken anemia virus (CAV) protein apoptin is known to induce tumor cell-specific death when expressed. Therefore, understanding its regulation and mechanism of action could provide new insights into tumor cell biology. We have determined that checkpoint kinase 1 and 2 signaling is important for apoptin regulation and is a likely feature of both tumor cells and host cells producing virus progeny. Inhibition of checkpoint signaling prevents apoptin toxicity in tumor cells and attenuates CAV replication, suggesting it may be a future target for antiviral therapy.

Identification of S-phase DNA damage-response targets in fission yeast reveals conservation of damage-response networks

The cellular response to DNA damage during S-phase regulates a complicated network of processes, including cell-cycle progression, gene expression, DNA replication kinetics, and DNA repair. In fission yeast, this S-phase DNA damage response (DDR) is coordinated by two protein kinases: Rad3, the ortholog of mammalian ATR, and Cds1, the ortholog of mammalian Chk2. Although several critical downstream targets of Rad3 and Cds1 have been identified, most of their presumed targets are unknown, including the targets responsible for regulating replication kinetics and coordinating replication and repair. To characterize targets of the S-phase DDR, we identified proteins phosphorylated in response to methyl methanesulfonate (MMS)-induced S-phase DNA damage in wild-type, rad3∆, and cds1∆ cells by proteome-wide mass spectrometry. We found a broad range of S-phase-specific DDR targets involved in gene expression, stress response, regulation of mitosis and cytokinesis, and DNA replication and repair. These targets are highly enriched for proteins required for viability in response to MMS, indicating their biological significance. Furthermore, the regulation of these proteins is similar in fission and budding yeast, across 300 My of evolution, demonstrating a deep conservation of S-phase DDR targets and suggesting that these targets may be critical for maintaining genome stability in response to S-phase DNA damage across eukaryotes.

The Chromosome Axis Mediates Feedback Control of CHK-2 to Ensure Crossover Formation in C. elegans

CHK-2 kinase is a master regulator of meiosis in C. elegans. Its activity is required for homolog pairing and synapsis and for double-strand break formation, but how it drives and coordinates these pathways to ensure crossover formation remains unknown. Here we show that CHK-2 promotes pairing and synapsis by phosphorylating a family of zinc finger proteins that bind to specialized regions on each chromosome known as pairing centers, priming their recruitment of the Polo-like kinase PLK-2. This knowledge enabled the development of a phospho-specific antibody as a tool to monitor CHK-2 activity. When either synapsis or crossover formation is impaired, CHK-2 activity is prolonged, and meiotic progression is delayed. We show that this common feedback circuit is mediated by interactions among a network of HORMA domain proteins within the chromosome axis and generates a graded signal. These findings reveal conserved regulatory mechanisms that ensure faithful meiotic chromosome segregation in diverse species.

Bimodal regulation of p21(waf1) protein as function of DNA damage levels

Human p21^Waf1 protein is well known for being transcriptionally induced by p53 and activating the cell cycle checkpoint arrest in response to DNA breaks. Here we report that p21^Waf1 protein undergoes a bimodal regulation, being upregulated in response to low doses of DNA damage but rapidly and transiently degraded in response to high doses of DNA lesions. Responsible for this degradation is the checkpoint kinase Chk1, which phosphorylates p21^Waf1 on T145 and S146 residues and induces its proteasome-dependent proteolysis. The initial p21^Waf1 degradation is then counteracted by the ATM-Chk2 pathway, which promotes the p53-dependent accumulation of p21^Waf1 at any dose of damage. We also found that p21^Waf1 ablation favors the activation of an apoptotic program to eliminate otherwise irreparable cells. These findings support a model in which in human cells a balance between ATM-Chk2-p53 and the ATR-Chk1 pathways modulates p21^Waf1 protein levels in relation to cytostatic and cytotoxic doses of DNA damage.

Reversal of female infertility by Chk2 ablation reveals the oocyte DNA damage checkpoint pathway

Genetic errors in meiosis can lead to birth defects and spontaneous abortions. Checkpoint mechanisms of hitherto unknown nature eliminate oocytes with unrepaired DNA damage, causing recombination-defective mutant mice to be sterile. Here, we report that checkpoint kinase 2 (Chk2 or Chek2), is essential for culling mouse oocytes bearing unrepaired meiotic or induced DNA double-strand breaks (DSBs). Female infertility caused by a meiotic recombination mutation or irradiation was reversed by mutation of Chk2. Both meiotically programmed and induced DSBs trigger CHK2-dependent activation of TRP53 (p53) and TRP63 (p63), effecting oocyte elimination. These data establish CHK2 as essential for DNA damage surveillance in female meiosis and indicate that the oocyte DSB damage response primarily involves a pathway hierarchy in which ataxia telangiectasia and Rad3-related (ATR) signals to CHK2, which then activates p53 and p63.

Strategies for Molecularly Enhanced Chemotherapy to Achieve Synthetic Lethality in Endometrial Tumors with Mutant p53

Serous uterine endometrial carcinomas are aggressive type II cancers with poor outcomes for which new treatment strategies are urgently needed, in particular, strategies that augment sensitivity to established chemotherapy regimens. The tumor suppressor gene TP53 is dysregulated in more than 90% of serous tumors, altering master regulators of the G2/M cell cycle checkpoint in unique and predictable ways and desensitizing cells to chemotherapy. We hypothesized that synthetic lethality can be achieved in endometrial cancer cells with mutant p53 by combining paclitaxel with agents to overcome G2/M arrest and induce mitotic catastrophe. The combination of BIBF1120, an investigational VEGFR, PDGFR, and FGFR multityrosine kinase inhibitor with established anti-angiogenic activity, with paclitaxel abrogated the G2/M checkpoint in p53-null endometrial cancer cells via modulation of G2/M checkpoint regulators followed by induction of mitotic cell death. In endometrial cancer cells harboring an oncogenic gain-of-function p53 mutation, synthetic lethality was created by combining paclitaxel with BIBF1120 and a histone deacetylase inhibitor, which serves to destabilize mutant p53. These cells were also sensitive to an inhibitor of the G2/M kinase Wee1 in combination with paclitaxel. These findings reveal that, in addition to antiangiogenic activity, the angiokinase inhibitor BIBF1120 can be used to restore sensitivity to paclitaxel and induce mitotic cell death in endometrial cancer cells with non-functional p53. These preclinical data serve as a critical platform for the creative design of future clinical trials utilizing molecularly enhanced chemotherapy to achieve synthetic lethality based on the mutational landscape.

The DNA damage and the DNA replication checkpoints converge at the MBF transcription factor

DNA damage and DNA replication checkpoints regulate differently the G1-to-S phase transcriptional program, resulting in the repression or induction, respectively, of the same set of genes. When this signaling is disrupted, cells are unable to cope with DNA-damaging agents, leading to increased cell lethality.

In fission yeast cells, Cds1 is the effector kinase of the DNA replication checkpoint. We previously showed that when the DNA replication checkpoint is activated, the repressor Yox1 is phosphorylated and inactivated by Cds1, resulting in activation of MluI-binding factor (MBF)–dependent transcription. This is essential to reinitiate DNA synthesis and for correct G1-to-S transition. Here we show that Cdc10, which is an essential part of the MBF core, is the target of the DNA damage checkpoint. When fission yeast cells are treated with DNA-damaging agents, Chk1 is activated and phosphorylates Cdc10 at its carboxy-terminal domain. This modification is responsible for the repression of MBF-dependent transcription through induced release of MBF from chromatin. This inactivation of MBF is important for survival of cells challenged with DNA-damaging agents. Thus Yox1 and Cdc10 couple normal cell cycle regulation in unperturbed conditions and the DNA replication and DNA damage checkpoints into a single transcriptional complex.

CK1δ kinase activity is modulated by Chk1-mediated phosphorylation

CK1δ, a member of the casein kinase 1 family, is involved in the regulation of various cellular processes and has been associated with the pathophysiology of neurodegenerative diseases and cancer. Therefore recently, interest in generating highly specific inhibitors for personalized therapy has increased enormously. However, the efficacy of newly developed inhibitors is affected by the phosphorylation state of CK1δ. Cellular kinases phosphorylating CK1δ within its C-terminal domain have been identified but still more information regarding the role of site-specific phosphorylation in modulating the activity of CK1δ is required. Here we show that Chk1 phosphorylates rat CK1δ at serine residues 328, 331, 370, and threonine residue 397 as well as the human CK1δ transcription variants 1 and 2. CK1δ mutant proteins bearing one, two or three mutations at these identified phosphorylation sites exhibited significant differences in their kinetic properties compared to wild-type CK1δ. Additionally, CK1δ co-precipitates with Chk1 from HT1080 cell extracts and activation of cellular Chk1 resulted in a significant decrease in cellular CK1δ kinase activity. Taken together, these data point towards a possible regulatory relationship between Chk1 and CK1δ.

Roles of Chk1 in cell biology and cancer therapy

The evolutionally conserved DNA damage response (DDR) and cell cycle checkpoints preserve genome integrity. Central to these genome surveillance pathways is a protein kinase, Chk1. DNA damage induces activation of Chk1, which then transduces the checkpoint signal and facilitates cell cycle arrest and DNA damage repair. Significant progress has been made recently toward our understanding of Chk1 regulation and its implications in cancer etiology and therapy. Specifically, a model that involves both spatiotemporal and conformational changes of proteins has been proposed for Chk1 activation. Further, emerging evidence suggests that Chk1 does not appear to be a tumor suppressor; instead, it promotes tumor growth and may contribute to anticancer therapy resistance. Recent data from our laboratory suggest that activating, but not inhibiting, Chk1 in the absence of chemotherapy might represent an innovative approach to suppress tumor growth. These findings suggest unique regulation of Chk1 in cell biology and cancer etiology, pointing to novel strategies for targeting Chk1 in cancer therapy.

Global analysis of phosphorylation of tau by the checkpoint kinases Chk1 and Chk2 in vitro

Hyperphosphorylation of microtubule-associated protein tau is thought to contribute to Alzheimer's disease (AD) pathogenesis. We previously showed that DNA damage-activated cell cycle checkpoint kinases Chk1 and Chk2 phosphorylate tau at an AD-related site and enhance tau toxicity, suggesting potential roles of these kinases in AD. The purpose of this study is to systematically identify which sites in tau are directly phosphorylated by Chk1 and Chk2. Using recombinant human tau phosphorylated by Chk1 and Chk2 in vitro, we first analyzed tau phosphorylation at the AD-related sites by Western blot with phospho-tau-specific antibodies. Second, to globally identify phosphorylated sites in tau, liquid chromatography-tandem mass spectrometry (LC-MS(3)) was employed. These systematic analyses identified a total of 27 Ser/Thr residues as Chk1- or Chk2- target sites. None of them were proline-directed kinase targets. Many of these sites are located within the microtubule-binding domain and C-terminal domain, whose phosphorylation has been shown to reduce tau binding to microtubules and/or has been implicated in tau toxicity. Among these 27 sites, 13 sites have been identified to be phosphorylated in AD brains. Since DNA damage is accumulated in diseased brains, Chk1 and Chk2 may be involved in tau phosphorylation and toxicity in AD pathogenesis.

Chk1-mediated phosphorylation of receptor-associated late transducer at serine 250 increases its stability by stimulating its interaction with 14-3-3

Receptor-associated late transducer (RALT) acts as a negative feedback inhibitor of ErbB receptor signaling via physical interaction with ErbB. Although RALT contains a 14-3-3 binding motif (247-RSHSGP-252), little is known about the molecular basis and significance of binding to 14-3-3. Here, we report that 14-3-3 interacts with RALT in H9c2 and COS-7 cells in a Ser-250 phosphorylation-dependent manner. An in vitro kinase assay showed that RALT is a substrate for checkpoint kinase 1 (Chk1). Interaction between ectopically expressed RALT and endogenous 14-3-3 was partially suppressed by pretreatment with the Chk1 inhibitor, UCN-01. In addition, expression of constitutively active Chk1 (Chk11-365 ) resulted in increased phosphorylation of the RALT 14-3-3 binding motif and enhanced the interaction between RALT and 14-3-3θ. Furthermore, fluorescence microscopy revealed that rapid trafficking of RALT to endosome-like vesicle structures was decelerated by coexpression of Chk11-365 , whereas this coexpression had no significant impact on trafficking of the RALT S250A mutant. Finally, a cycloheximide chase assay indicated that coexpression of Chk11-365 decelerated the degradation of ectopically expressed RALT, but not that of the S250A mutant. Collectively, these results suggest that Chk1 plays a role in regulating RALT protein stability by facilitating the interaction between 14-3-3 and RALT.

Lats2 phosphorylates p21 after UV irradiation and regulates apoptosis

Lats2 (Large tumor suppressor 2), a member of the conserved AGC Ser/Thr (S/T) kinase family, is a human tumor suppressor gene. Here we show that in response to ultraviolet radiation, Lats2 is phosphorylated by Chk1 at Ser835 (S835), which is located in the kinase domain of Lats2. This phosphorylation enhances Lats2 kinase activity. Subsequently, Lats2 phosphorylates p21 at S146. p21 is a cyclin-dependent kinase (CDK) inhibitor, which not only regulates cell cycle by CDK inhibition but also inhibits apoptosis by binding to procaspase-3 in the cytoplasm. Phosphorylation by Lats2 induces p21 degradation and promotes apoptosis. Accordingly, Lats2 overexpression induces p21 degradation, caspase-3/9 activation and apoptosis. These findings describe a novel Lats2-dependent mechanism for induction of cell death in response to severe DNA damage.

CHK2 kinase promotes pre-mRNA splicing via phosphorylating CDK11(p110)

Checkpoint kinase 2 (CHK2) kinase is a key mediator in many cellular responses to genotoxic stresses, including ionizing radiation (IR) and topoisomerase inhibitors. Upon IR, CHK2 is activated by ataxia telangiectasia mutated kinase and regulates the S-phase and G1-S checkpoints, apoptosis and DNA repair by phosphorylating downstream target proteins, such as p53 and Brca1. In addition, CHK2 is thought to be a multi-organ cancer susceptibility gene. In this study, we used a tandem affinity purification strategy to identify proteins that interact with CHK2 kinase. Cyclin-dependent kinase 11 (CDK11)(p110) kinase, implicated in pre-mRNA splicing and transcription, was identified as a CHK2-interacting protein. CHK2 kinase phosphorylated CDK11(p110) on serine 737 in vitro. Unexpectedly, CHK2 kinase constitutively phosphorylated CDK11(p110) in a DNA damage-independent manner. At a molecular level, CDK11(p110) phosphorylation was required for homodimerization without affecting its kinase activity. Overexpression of CHK2 promoted pre-mRNA splicing. Conversely, CHK2 depletion decreased endogenous splicing activity. Mutation of the phosphorylation site in CDK11(p110) to alanine abrogated its splicing-activating activity. These results provide the first evidence that CHK2 kinase promotes pre-mRNA splicing via phosphorylating CDK11(p110).

Putting the brakes on p53-driven apoptosis

Following genotoxic stress, cells activate a complex, kinase-based signaling network to arrest the cell cycle and initiate DNA repair or apoptosis. The tumor suppressor p53 lies at the heart of this DNA damage response. p53 mediates the transactivation of both cell cycle-regulating and pro-apoptotic clusters of target genes. However, it remains incompletely understood which signaling molecules dictate the choice between these two opposing p53-dependent cellular outcomes. Over recent years, numerous regulatory mechanisms impacting on the cellular outcome of p53 signaling have been described. However, no single dominant mechanism has thus far been identified to regulate the cellular choice between p53-driven apoptosis or senescence. The transcriptional regulator AATF has recently emerged as a novel factor impacting on the cellular outcome of the p53 response. Upon genotoxic stress, cytoplasmic pools of MRLC-bound AATF are phosphorylated through the p38MAPK/MK2 checkpoint kinase complex. This AATF phosphorylation results in the disruption of cytoplasmic MRLC3:AATF complexes followed by rapid nuclear localization of AATF. Once in the nucleus, AATF binds to the PUMA, BAX and BAK promoters to repress the DNA damage-induced expression of these pro-apoptotic p53 target genes. Depletion of AATF in tumor cells results in a dramatically enhanced response to DNA-damaging chemotherapeutics, both in vitro and in vivo. Furthermore, focal copy number gains at the AATF locus in neuroblastoma correlate with adverse prognosis and reduced overall survival in this typically p53-proficient malignancy. These data identify the p38/MK2/AATF signaling pathway as a critical repressor of p53-driven apoptosis in tumor cells and implicate this signaling cascade as a novel target for chemotherapy-sensitizing therapeutic efforts.

The intra-S phase checkpoint targets Dna2 to prevent stalled replication forks from reversing

When replication forks stall at damaged bases or upon nucleotide depletion, the intra-S phase checkpoint ensures they are stabilized and can restart. In intra-S checkpoint-deficient budding yeast, stalling forks collapse, and ∼10% form pathogenic chicken foot structures, contributing to incomplete replication and cell death (Lopes et al., 2001; Sogo et al., 2002; Tercero and Diffley, 2001). Using fission yeast, we report that the Cds1(Chk2) effector kinase targets Dna2 on S220 to regulate, both in vivo and in vitro, Dna2 association with stalled replication forks in chromatin. We demonstrate that Dna2-S220 phosphorylation and the nuclease activity of Dna2 are required to prevent fork reversal. Consistent with this, Dna2 can efficiently cleave obligate precursors of fork regression-regressed leading or lagging strands-on model replication forks. We propose that Dna2 cleavage of regressed nascent strands prevents fork reversal and thus stabilizes stalled forks to maintain genome stability during replication stress.

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.

Kruppel-associated Box (KRAB)-associated co-repressor (KAP-1) Ser-473 phosphorylation regulates heterochromatin protein 1β (HP1-β) mobilization and DNA repair in heterochromatin

The DNA damage response encompasses a complex series of signaling pathways that function to regulate and facilitate the repair of damaged DNA. Recent studies have shown that the repair of transcriptionally inactive chromatin, named heterochromatin, is dependent upon the phosphorylation of the co-repressor, Krüppel-associated box (KRAB) domain-associated protein (KAP-1), by the ataxia telangiectasia-mutated (ATM) kinase. Co-repressors, such as KAP-1, function to regulate the rigid structure of heterochromatin by recruiting histone-modifying enzymes, such HDAC1/2, SETDB1, and nucleosome-remodeling complexes such as CHD3. Here, we have characterized a phosphorylation site in the HP1-binding domain of KAP-1, Ser-473, which is phosphorylated by the cell cycle checkpoint kinase Chk2. Expression of a nonphosphorylatable S473A mutant conferred cellular sensitivity to DNA-damaging agents and led to defective repair of DNA double-strand breaks in heterochromatin. In addition, cells expressing S473A also displayed defective mobilization of the HP1-β chromodomain protein. The DNA repair defect observed in cells expressing S473A was alleviated by depletion of HP1-β, suggesting that phosphorylation of KAP-1 on Ser-473 promotes the mobilization of HP1-β from heterochromatin and subsequent DNA repair. These results suggest a novel mechanism of KAP-1-mediated chromatin restructuring via Chk2-regulated HP1-β exchange from heterochromatin, promoting DNA repair.

CHK1 targets spleen tyrosine kinase (L) for proteolysis in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most prevalent malignancies resistant to current chemotherapies or radiotherapies, which makes it urgent to identify new therapeutic targets for HCC. In this study, we found that checkpoint kinase 1 (CHK1) was frequently overexpressed and correlated with poor clinical outcome in patients with HCC. We further showed that the CHK1 inhibitor GÖ6976 was capable of sensitizing HCC cells to cisplatin, indicating that CHK1 may have oncogenic function in HCC. We found that CHK1 phosphorylated the tumor suppressor spleen tyrosine kinase (L) (SYK[L]) and identified the phosphorylation site at Ser295. Furthermore, CHK1 phosphorylation of SYK(L) promoted its subsequent proteasomal degradation. Expression of a nonphosphorylated mutant of SYK(L) was more efficient at suppressing proliferation, colony formation, mobility, and tumor growth in HCC lines. Importantly, a strong inverse correlation between the expression levels of CHK1 and SYK(L) was observed in patients with HCC. Collectively, our data demonstrate that SYK(L) is a substrate of CHK1 in tumor cells and suggest that targeting the CHK1/SYK(L) pathway may be a promising strategy for treating HCC.

Chk1 phosphorylates the tumour suppressor Mig-6, regulating the activation of EGF signalling

The tumour suppressor gene product Mig-6 acts as an inhibitor of epidermal growth factor (EGF) signalling. However, its posttranslational modifications and regulatory mechanisms have not been elucidated. Here, we investigated the phosphorylation of human Mig-6 and found that Chk1 phosphorylated Mig-6 in vivo as well as in vitro. Moreover, EGF stimulation promoted phosphorylation of Mig-6 without DNA damage and the phosphorylation was inhibited by depletion of Chk1. EGF also increased Ser280-phosphorylated Chk1, a cytoplasmic-tethering form, via PI3K pathway. Mass spectrometric analyses suggested that Ser 251 of Mig-6 was a major phosphorylation site by Chk1 in vitro and in vivo. Substitution of Ser 251 to alanine increased inhibitory activity of Mig-6 against EGF receptor (EGFR) activation. Moreover, EGF-dependent activation of EGFR and cell growth were inhibited by Chk1 depletion, and were rescued by co-depletion of Mig-6. Our results suggest that Chk1 phosphorylates Mig-6 on Ser 251, resulting in the inhibition of Mig-6, and that Chk1 acts as a positive regulator of EGF signalling. This is a novel function of Chk1.

c-Jun N-terminal kinase-mediated Rad18 phosphorylation facilitates Polη recruitment to stalled replication forks

The E3 ubiquitin ligase Rad18 chaperones DNA polymerase η (Polη) to sites of UV-induced DNA damage and monoubiquitinates proliferating cell nuclear antigen (PCNA), facilitating engagement of Polη with stalled replication forks and promoting translesion synthesis (TLS). It is unclear how Rad18 activities are coordinated with other elements of the DNA damage response. We show here that Ser-409 residing in the Polη-binding motif of Rad18 is phosphorylated in a checkpoint kinase 1-dependent manner in genotoxin-treated cells. Recombinant Rad18 was phosphorylated specifically at S409 by c-Jun N-terminal kinase (JNK) in vitro. In UV-treated cells, Rad18 S409 phosphorylation was inhibited by a pharmacological JNK inhibitor. Conversely, ectopic expression of JNK and its upstream kinase mitogen-activated protein kinase kinase 4 led to DNA damage-independent Rad18 S409 phosphorylation. These results identify Rad18 as a novel JNK substrate. A Rad18 mutant harboring a Ser → Ala substitution at S409 was compromised for Polη association and did not redistribute Polη to nuclear foci or promote Polη-PCNA interaction efficiently relative to wild-type Rad18. Rad18 S409A also failed to fully complement the UV sensitivity of Rad18-depleted cells. Taken together, these results show that Rad18 phosphorylation by JNK represents a novel mechanism for promoting TLS and DNA damage tolerance.

Chk1 phosphorylation of Metnase enhances DNA repair but inhibits replication fork restart

Chk1 both arrests replication forks and enhances repair of DNA damage by phosphorylating downstream effectors. Although there has been a concerted effort to identify effectors of Chk1 activity, underlying mechanisms of effector action are still being identified. Metnase (also called SETMAR) is a SET and transposase domain protein that promotes both DNA double-strand break (DSB) repair and restart of stalled replication forks. In this study, we show that Metnase is phosphorylated only on Ser495 (S495) in vivo in response to DNA damage by ionizing radiation. Chk1 is the major mediator of this phosphorylation event. We had previously shown that wild-type (wt) Metnase associates with chromatin near DSBs and methylates histone H3 Lys36. Here we show that a Ser495Ala (S495A) Metnase mutant, which is not phosphorylated by Chk1, is defective in DSB-induced chromatin association. The S495A mutant also fails to enhance repair of an induced DSB when compared with wt Metnase. Interestingly, the S495A mutant demonstrated increased restart of stalled replication forks compared with wt Metnase. Thus, phosphorylation of Metnase S495 differentiates between these two functions, enhancing DSB repair and repressing replication fork restart. In summary, these data lend insight into the mechanism by which Chk1 enhances repair of DNA damage while at the same time repressing stalled replication fork restart.

Chk1 and Hsp90 cooperatively regulate phosphorylation of endothelial nitric oxide synthase at serine 1179

The effects of DNA damage on NO production have not been completely elucidated. Using ultraviolet (UV) irradiation as a DNA-damaging agent, we studied its effect on NO production in bovine aortic endothelial cells (BAEC). UV irradiation acutely increased NO production, the phosphorylation of endothelial NO synthase (eNOS) at serine 1179, and eNOS activity. No alterations in eNOS expression nor phosphorylation at eNOS Thr(497) or eNOS Ser(116) were found. SB218078, a checkpoint kinase 1 (Chk1) inhibitor, inhibited UV-irradiation-stimulated eNOS-Ser(1179) phosphorylation and NO production. Similarly, ectopic expression of small interference RNA for Chk1 or a dominant-negative Chk1 repressed the UV-irradiation stimulatory effect, whereas wild-type Chk1 increased basal eNOS-Ser(1179) phosphorylation. Purified Chk1 directly phosphorylated eNOS Ser(1179) in vitro. Confocal microscopy and coimmunoprecipitation studies revealed a colocalization of eNOS and Chk1. In basal BAEC, heat shock protein 90 (Hsp90) predominantly interacted with Chk1. This interaction, which decreased significantly in response to UV irradiation, was accompanied by increased interaction of Hsp90 with eNOS. The Hsp90 inhibitor geldanamycin attenuated UV-irradiation-stimulated eNOS-Ser(1179) phosphorylation by dissociating Hsp90 from eNOS. UV irradiation and geldanamycin did not alter the interaction between eNOS and Chk1. Overall, this is the first study demonstrating that Chk1 directly phosphorylates eNOS Ser(1179) in response to UV irradiation, which is dependent on Hsp90 interaction.

Pairing centers recruit a Polo-like kinase to orchestrate meiotic chromosome dynamics in C. elegans

Faithful segregation of homologous chromosomes during meiosis requires pairing, synapsis, and crossing-over. In C. elegans, homolog pairing and synapsis depend on pairing centers (PCs), special regions near one end of each chromosome that interact with the nuclear envelope (NE) and cytoplasmic microtubules. Here, we report that PCs are required for nuclear reorganization at the onset of meiosis. We demonstrate that PCs recruit the Polo-like kinase PLK-2 to induce NE remodeling, chromosome pairing, and synapsis. Recruitment of PLK-2 is also required to mediate a cell cycle delay and selective apoptosis of nuclei containing unsynapsed chromosomes, establishing a molecular link between these two quality control mechanisms. This work reveals unexpected functions for the conserved family of Polo-like kinases, and advances our understanding of how meiotic processes are properly coordinated to ensure transmission of genetic information from parents to progeny.

Gossypin induces G2/M arrest in human malignant glioma U251 cells by the activation of Chk1/Cdc25C pathway

Gossypin is a flavone that was originally isolated from Hibiscus vitifolius and has traditionally been used for the treatment of diabetes, jaundice, and inflammation. Recently, gossypin was found to have potent anticancer properties; however, its effect on human gliomas still remain unknown. To investigate the potential anticancer effects of gossypin on malignant gliomas and analyze the associated molecular mechanisms, we treated human glioma U251 cells with gossypin. Our study showed that the treatment of U251 cells with gossypin inhibited cell proliferation in a dose- and time-dependent manner and was observed to be minimally toxic to normal human astrocytes. Gossypin's effect on cell cycle distribution was observed, and we found that it induced G2/M-phase arrest in U251 cells. An analysis of cell-cycle regulatory proteins indicated that the arresting effect of gossypin on the cell cycle at G2/M phase was involved in the phosphorylation of cell division cycle 25C (Cdc25C) tyrosine phosphatase via the activation of checkpoint kinase 1 (Chk1). These findings indicate that gossypin is a potential treatment of gliomas because of gossypin's potential to regulate the proliferation of U251 cells via the cell-cycle regulatory proteins Chk1 and Cdc25C.

A phospho-proteomic screen identifies substrates of the checkpoint kinase Chk1

Background

The cell-cycle checkpoint kinase Chk1 is essential in mammalian cells due to its roles in controlling processes such as DNA replication, mitosis and DNA-damage responses. Despite its paramount importance, how Chk1 controls these functions remains unclear, mainly because very few Chk1 substrates have hitherto been identified.

Results

Here, we combine a chemical genetics approach with high-resolution mass spectrometry to identify novel Chk1 substrates and their phosphorylation sites. The list of targets produced reveals the potential impact of Chk1 function not only on processes where Chk1 was already known to be involved, but also on other key cellular events such as transcription, RNA splicing and cell fate determination. In addition, we validate and explore the phosphorylation of transcriptional co-repressor KAP1 Ser473 as a novel DNA-damage-induced Chk1 site.

Conclusions

By providing a substantial set of potential Chk1 substrates, we present opportunities for studying unanticipated functions for Chk1 in controlling a wide range of cellular processes. We also refine the Chk1 consensus sequence, facilitating the future prediction of Chk1 target sites. In addition, our identification of KAP1 Ser473 phosphorylation as a robust readout for Chk1 activity could be used to explore the in vivo effects of Chk1 inhibitors that are being developed for clinical evaluation.