Polo-like kinase 1 and Chk2 interact and co-localize to centrosomes and the midbody

PLK1 inhibition leads to mitotic arrest and triggers apoptosis in cholangiocarcinoma cells

Cholangiocarcinoma (CCA) is a lethal cancer originating from the epithelial cells within the bile duct and ranks as the second most prevalent form of liver cancer in Thailand. Polo-like kinase 1 (PLK1), a protein serine/threonine kinase, regulates a number of steps in cell mitosis and is upregulated in several types of cancer, including CCA. Our previous study identified PLK1 as a biomarker of the C1 subtype, correlating with poor prognosis in intrahepatic CCA. The present study aimed to examine the effect of PLK1 inhibition on CCA cells. Different CCA cell lines developed from Thai patients, HuCCA1, KKU055, KKU100 and KKU213A, were treated with two PLK1 inhibitors, BI2536 and BI6727, and were transfected with small interfering RNA, followed by analysis of cell proliferation, cell cycle distribution and cell apoptosis. It was discovered that BI2536 and BI6727 inhibited cell proliferation and caused G2/M-phase arrest in CCA cells. Furthermore, the number of total apoptotic cells was increased in PLK1 inhibitor-treated CCA cells. The expression levels of mitotic proteins, aurora kinase A, phosphorylated PLK1 (T210) and cyclin B1, were augmented in PLK1-inhibited CCA cells. Additionally, inhibition of PLK1 led to increased DNA damage, as determined by the upregulated levels of γH2AX and increased cleavage of poly (ADP-ribose) polymerase, an apoptotic marker. These results suggested that inhibiting PLK1 prolonged mitotic arrest and subsequently triggered cell apoptosis. Validation of the antiproliferative effects of PLK1 inhibition was accomplished through silencing of the PLK1 gene. In conclusion, targeting PLK1 provided promising results for further study as a potential candidate for targeted therapy in CCA.

Dynamic Phosphorylation of G9a Regulates its Repressive Activity on Chromatin Accessibility and Mitotic Progression

Phosphorylation of Ser10 of histone H3 (H3S10p), together with the adjacent methylation of Lys9 (H3K9me), has been proposed to function as a 'phospho-methyl switch' to regulate mitotic chromatin architecture. Despite of immense understanding of the roles of H3S10 phosphorylation, how H3K9me2 are dynamically regulated during mitosis is poorly understood. Here, it is identified that Plk1 kinase phosphorylates the H3K9me1/2 methyltransferase G9a/EHMT2 at Thr1045 (pT1045) during early mitosis, which attenuates its catalytic activity toward H3K9me2. Cells bearing Thr1045 phosphomimic mutant of G9a (T1045E) show decreased H3K9me2 levels, increased chromatin accessibility, and delayed mitotic progression. By contrast, dephosphorylation of pT1045 during late mitosis by the protein phosphatase PPP2CB reactivates G9a activity and upregulates H3K9me2 levels, correlated with decreased levels of H3S10p. Therefore, the results provide a mechanistic explanation of the essential of a 'phospho-methyl switch' and highlight the importance of Plk1 and PPP2CB-mediated dynamic regulation of G9a activity in chromatin organization and mitotic progression.

Maintaining Genome Integrity: Protein Kinases and Phosphatases Orchestrate the Balancing Act of DNA Double-Strand Breaks Repair in Cancer

DNA double-strand breaks (DSBs) are the most lethal DNA damages which lead to severe genome instability. Phosphorylation is one of the most important protein post-translation modifications involved in DSBs repair regulation. Kinases and phosphatases play coordinating roles in DSB repair by phosphorylating and dephosphorylating various proteins. Recent research has shed light on the importance of maintaining a balance between kinase and phosphatase activities in DSB repair. The interplay between kinases and phosphatases plays an important role in regulating DNA-repair processes, and alterations in their activity can lead to genomic instability and disease. Therefore, study on the function of kinases and phosphatases in DSBs repair is essential for understanding their roles in cancer development and therapeutics. In this review, we summarize the current knowledge of kinases and phosphatases in DSBs repair regulation and highlight the advancements in the development of cancer therapies targeting kinases or phosphatases in DSBs repair pathways. In conclusion, understanding the balance of kinase and phosphatase activities in DSBs repair provides opportunities for the development of novel cancer therapeutics.

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.

An updated view on the centrosome as a cell cycle regulator

The centrosome is a multifunctional organelle that is known primarily for its microtubule organising function. Centrosomal defects caused by changes in centrosomal structure or number have been associated with human diseases ranging from congenital defects to cancer. We are only beginning to appreciate how the non-microtubule organising roles of the centrosome are related to these clinical conditions. In this review, we will discuss the historical evidence that led to the proposal that the centrosome participates in cell cycle regulation. We then summarize the body of work that describes the involvement of the mammalian centrosome in triggering cell cycle progression and checkpoint signalling. Then we will highlight work from the fission yeast model organism, revealing the molecular details that explain how the spindle pole body (SPB, the yeast functional equivalent of the centrosome), participates in these cell cycle transitions. Importantly, we will discuss some of the emerging questions from recent discoveries related to the role of the centrosome as a cell cycle regulator.

An ATM-Chk2-INCENP pathway activates the abscission checkpoint

During cell division, in response to chromatin bridges, the chromosomal passenger complex (CPC) delays abscission to prevent chromosome breakage or tetraploidization. Here, we show that inhibition of ATM or Chk2 kinases impairs CPC localization to the midbody center, accelerates midbody resolution in normally segregating cells, and correlates with premature abscission and chromatin breakage in cytokinesis with trapped chromatin. In cultured human cells, ATM activates Chk2 at late midbodies. In turn, Chk2 phosphorylates human INCENP-Ser91 to promote INCENP binding to Mklp2 kinesin and CPC localization to the midbody center through Mklp2 association with Cep55. Expression of truncated Mklp2 that does not bind to Cep55 or nonphosphorylatable INCENP-Ser91A impairs CPC midbody localization and accelerates abscission. In contrast, expression of phosphomimetic INCENP-Ser91D or a chimeric INCENP protein that is targeted to the midbody center rescues the abscission delay in Chk2-deficient or ATM-deficient cells. Furthermore, the Mre11–Rad50–Nbs1 complex is required for ATM activation at the midbody in cytokinesis with chromatin bridges. These results identify an ATM–Chk2–INCENP pathway that imposes the abscission checkpoint by regulating CPC midbody localization.

Aurora kinases and DNA damage response

It is well established that Aurora kinases perform critical functions during mitosis. It has become increasingly clear that the Aurora kinases also perform a myriad of non-mitotic functions including DNA damage response. The available evidence indicates that inhibition Aurora kinase A (AURKA) may contribute to the G₂ DNA damage checkpoint through AURKA's functions in PLK1 and CDC25B activation. Both AURKA and Aurora kinase B (AURKB) are also essential in mitotic DNA damage response that guard against DNA damage-induced chromosome segregation errors, including the control of abscission checkpoint and prevention of micronuclei formation. Dysregulation of Aurora kinases can trigger DNA damage in mitosis that is sensed in the subsequent G₁ by a p53-dependent postmitotic checkpoint. Aurora kinases are themselves linked to the G₁ DNA damage checkpoint through p53 and p73 pathways. Finally, several lines of evidence provide a connection between Aurora kinases and DNA repair and apoptotic pathways. Although more studies are required to provide a comprehensive picture of how cells respond to DNA damage, these findings indicate that both AURKA and AURKB are inextricably linked to pathways guarding against DNA damage. They also provide a rationale to support more detailed studies on the synergism between small-molecule inhibitors against Aurora kinases and DNA-damaging agents in cancer therapies.

AR phosphorylation and CHK2 kinase activity regulates IR-stabilized AR-CHK2 interaction and prostate cancer survival

We have previously demonstrated that checkpoint kinase 2 (CHK2) is a critical negative regulator of androgen receptor (AR) transcriptional activity, prostate cancer (PCa) cell growth, and androgen sensitivity. We have now uncovered that the AR directly interacts with CHK2 and ionizing radiation (IR) increases this interaction. This IR-induced increase in AR-CHK2 interactions requires AR phosphorylation and CHK2 kinase activity. PCa associated CHK2 mutants with impaired kinase activity reduced IR-induced AR-CHK2 interactions. The destabilization of AR - CHK2 interactions induced by CHK2 variants impairs CHK2 negative regulation of cell growth. CHK2 depletion increases transcription of DNAPK and RAD54, increases clonogenic survival, and increases resolution of DNA double strand breaks. The data support a model where CHK2 sequesters the AR through direct binding decreasing AR transcription and suppressing PCa cell growth. CHK2 mutation or loss of expression thereby leads to increased AR transcriptional activity and survival in response to DNA damage.

DNA Replication Stress and Chromosomal Instability: Dangerous Liaisons

Chromosomal instability (CIN) is associated with many human diseases, including neurodevelopmental or neurodegenerative conditions, age-related disorders and cancer, and is a key driver for disease initiation and progression. A major source of structural chromosome instability (s-CIN) leading to structural chromosome aberrations is "replication stress", a condition in which stalled or slowly progressing replication forks interfere with timely and error-free completion of the S phase. On the other hand, mitotic errors that result in chromosome mis-segregation are the cause of numerical chromosome instability (n-CIN) and aneuploidy. In this review, we will discuss recent evidence showing that these two forms of chromosomal instability can be mechanistically interlinked. We first summarize how replication stress causes structural and numerical CIN, focusing on mechanisms such as mitotic rescue of replication stress (MRRS) and centriole disengagement, which prevent or contribute to specific types of structural chromosome aberrations and segregation errors. We describe the main outcomes of segregation errors and how micronucleation and aneuploidy can be the key stimuli promoting inflammation, senescence, or chromothripsis. At the end, we discuss how CIN can reduce cellular fitness and may behave as an anticancer barrier in noncancerous cells or precancerous lesions, whereas it fuels genomic instability in the context of cancer, and how our current knowledge may be exploited for developing cancer therapies.

Working on Genomic Stability: From the S-Phase to Mitosis

Fidelity in chromosome duplication and segregation is indispensable for maintaining genomic stability and the perpetuation of life. Challenges to genome integrity jeopardize cell survival and are at the root of different types of pathologies, such as cancer. The following three main sources of genomic instability exist: DNA damage, replicative stress, and chromosome segregation defects. In response to these challenges, eukaryotic cells have evolved control mechanisms, also known as checkpoint systems, which sense under-replicated or damaged DNA and activate specialized DNA repair machineries. Cells make use of these checkpoints throughout interphase to shield genome integrity before mitosis. Later on, when the cells enter into mitosis, the spindle assembly checkpoint (SAC) is activated and remains active until the chromosomes are properly attached to the spindle apparatus to ensure an equal segregation among daughter cells. All of these processes are tightly interconnected and under strict regulation in the context of the cell division cycle. The chromosomal instability underlying cancer pathogenesis has recently emerged as a major source for understanding the mitotic processes that helps to safeguard genome integrity. Here, we review the special interconnection between the S-phase and mitosis in the presence of under-replicated DNA regions. Furthermore, we discuss what is known about the DNA damage response activated in mitosis that preserves chromosomal integrity.

DNA damage response proteins regulating mitotic cell division: double agents preserving genome stability

The DNA damage response recognizes DNA lesions and coordinates a cell cycle arrest with the repair of the damaged DNA, or removal of the affected cells to prevent the passage of genetic alterations to the next generation. The mitotic cell division, on the other hand, is a series of processes that aims to accurately segregate the genomic material from the maternal to the two daughter cells. Despite their great importance in safeguarding genomic integrity, the DNA damage response and the mitotic cell division were long viewed as unrelated processes, mainly because animal cells that are irradiated during mitosis continue cell division without repairing the broken chromosomes. However, recent studies have demonstrated that DNA damage proteins play an important role in mitotic cell division. This is performed through regulation of the onset of mitosis, mitotic spindle formation, correction of misattached kinetochore-microtubules, spindle checkpoint signaling, or completion of cytokinesis (abscission), in the absence of DNA damage. In this review, we summarize the roles of DNA damage proteins in unperturbed mitosis, analyze the molecular mechanisms involved, and discuss the potential implications of these findings in cancer therapy.

Building bridges between chromosomes: novel insights into the abscission checkpoint

In the presence of chromatin bridges, mammalian cells delay completion of cytokinesis (abscission) to prevent chromatin breakage or tetraploidization by regression of the cleavage furrow. This abscission delay is called "the abscission checkpoint" and is dependent on Aurora B kinase. Furthermore, cells stabilize the narrow cytoplasmic canal between the two daughter cells until the DNA bridges are resolved. Impaired abscission checkpoint signaling or unstable intercellular canals can lead to accumulation of DNA damage, aneuploidy, or generation of polyploid cells which are associated with tumourigenesis. However, the molecular mechanisms involved have only recently started to emerge. In this review, we focus on the molecular pathways of the abscission checkpoint and describe newly identified triggers, Aurora B-regulators and effector proteins in abscission checkpoint signaling. We also describe mechanisms that control intercellular bridge stabilization, DNA bridge resolution, or abscission checkpoint silencing upon satisfaction, and discuss how abscission checkpoint proteins can be targeted to potentially improve cancer therapy.

Chk2-dependent phosphorylation of myosin phosphatase targeting subunit 1 (MYPT1) regulates centrosome maturation

Checkpoint kinase 2 (Chk2) is a pivotal effector kinase in the DNA damage response, with an emerging role in mitotic chromosome segregation. In this study, we show that Chk2 interacts with myosin phosphatase targeting subunit 1 (MYPT1), the targeting subunit of protein phosphatase 1cβ (PP1cβ). Previous studies have shown that MYPT1 is phosphorylated by CDK1 at S473 during mitosis, and subsequently docks to the polo-binding domain of PLK1 and dephosphorylates PLK1. Herein we present data that Chk2 phosphorylates MYPT1 at S507 in vitro and in vivo, which antagonizes pS473. Chk2 inhibition results in failure of γ-tubulin recruitment to the centrosomes, phenocopying Plk1 inhibition defects. These aberrancies were also observed in the MYPT1-S507A stable transfectants, suggesting that Chk2 exerts its effect on centrosomes via MYPT1. Collectively, we have identified a Chk2-MYPT1-PLK1 axis in regulating centrosome maturation. Abbreviations: Chk2: checkpoint kinase 2; MYPT1: myosin phosphatase targeting subunit 1; PP1cβ: protein phosphatase 1c β; Noc: nocodazole; IP: immunoprecipitation; IB: immunoblotting; LC-MS/MS: liquid chromatography-tandem mass spectrometry; Chk2: checkpoint kinase 2; KD: kinase domain; WT: wild type; Ub: ubiquitin; DAPI: 4',6-diamidino-2-phenylindole; IF: Immunofluorescence; IR: ionizing radiation; siCHK2: siRNA targeting CHK2.

Discovery of Small-Molecule Inhibitors of the HSP90-Calcineurin-NFAT Pathway against Glioblastoma

Glioblastoma (GBM) is among the most common and malignant types of primary brain tumors in adults, with a dismal prognosis. Although alkylating agents such as temozolomide are widely applied as the first-line treatment for GBM, they often cause chemoresistance and remain ineffective with recurrent GBM. Alternative therapeutics against GBM are urgently needed in the clinic. We report herein the discovery of a class of inhibitors (YZ129 and its derivatives) of the calcineurin-NFAT pathway that exhibited potent anti-tumor activity against GBM. YZ129-induced GBM cell-cycle arrest at the G2/M phase promoted apoptosis and inhibited tumor cell proliferation and migration. At the molecular level, YZ129 directly engaged HSP90 to antagonize its chaperoning effect on calcineurin to abrogate NFAT nuclear translocation, and also suppressed other proto-oncogenic pathways including hypoxia, glycolysis, and the PI3K/AKT/mTOR signaling axis. Our data highlight the potential for targeting the cancer-promoting HSP90 chaperone network to treat GBM.

Constitutional mutations of the CHEK2 gene are a risk factor for MDS, but not for de novo AML

CHEK2 plays a key role in cellular response to DNA damage, and also in regulation of mitosis and maintenance of chromosomal stability. In patients newly diagnosed with myelodysplastic syndrome (MDS, n = 107) or acute myeloid leukemia (AML, n = 117) congenital CHEK2 mutations (c.444 + 1G > A, c.1100delC, del5395, p.I157 T) were tested by PCR and sequencing analysis. The karyotype of bone marrow cells of each patient was assessed at disease diagnosis using classical cytogenetic methods and fluorescence in situ hybridization. The CHEK2 mutations were strongly associated with the risk of MDS (p < 0.0001) but not with the risk of de novo AML (p = 0.798). In CHEK2-positive MDS patients, two times higher frequency of aberrant karyotypes than in CHEK2-negative patients was found (71% vs. 37%, p = 0.015). In CHEK2-positive patients with cytogenetic abnormalities, subtypes of MDS: refractory anemia with excess blasts-1 or 2, associated with unfavorable disease prognosis, were diagnosed two times more often than in CHEK2-negative cases with aberrations (78% vs. 44%). In conclusion, the congenital CHEK2 inactivation is strongly associated with the risk of MDS and with a poorer prognosis of the disease. However, the chromosomal instability in AML is not correlated with the hereditary dysfunction of CHEK2.

The Emerging Role of Polo-Like Kinase 1 in Epithelial-Mesenchymal Transition and Tumor Metastasis

Polo-like kinase 1 (PLK1) is a serine/threonine kinase that plays a key role in the regulation of the cell cycle. PLK1 is overexpressed in a variety of human tumors, and its expression level often correlates with increased cellular proliferation and poor prognosis in cancer patients. It has been suggested that PLK1 controls cancer development through multiple mechanisms that include canonical regulation of mitosis and cytokinesis, modulation of DNA replication, and cell survival. However, emerging evidence suggests novel and previously unanticipated roles for PLK1 during tumor development. In this review, we will summarize the recent advancements in our understanding of the oncogenic functions of PLK1, with a focus on its role in epithelial-mesenchymal transition and tumor invasion. We will further discuss the therapeutic potential of these functions.

Dialogue between centrosomal entrance and exit scaffold pathways regulates mitotic commitment

The fission yeast scaffold molecule Sid4 anchors the septum initiation network to the spindle pole body (SPB, centrosome equivalent) to control mitotic exit events. A second SPB-associated scaffold, Cut12, promotes SPB-associated Cdk1-cyclin B to drive mitotic commitment. Signals emanating from each scaffold have been assumed to operate independently to promote two distinct outcomes. We now find that signals from Sid4 contribute to the Cut12 mitotic commitment switch. Specifically, phosphorylation of Sid4 by NIMAFin1 reduces Sid4 affinity for its SPB anchor, Ppc89, while also enhancing Sid4's affinity for casein kinase 1δ (CK1δ). The resulting phosphorylation of Sid4 by the newly docked CK1δ recruits Chk2Cds1 to Sid4. Chk2Cds1 then expels the Cdk1-cyclin B antagonistic phosphatase Flp1/Clp1 from the SPB. Flp1/Clp1 departure can then support mitotic commitment when Cdk1-cyclin B activation at the SPB is compromised by reduction of Cut12 function. Such integration of signals emanating from neighboring scaffolds shows how centrosomes/SPBs can integrate inputs from multiple pathways to control cell fate.

Mitotic DNA Damage Response: At the Crossroads of Structural and Numerical Cancer Chromosome Instabilities

DNA double-strand breaks (DSBs) prevent cells from entering mitosis allowing cells to repair their genomic damage. Little is known about the response to DSBs once cells have already committed to mitosis. Here, we review the genome-protective role of the mitotic DNA damage response (DDR) and evidence suggesting that its untimely activation induces chromosome segregation errors and paradoxically undermines genomic integrity. In contrast to normal cells, cancer cells coopt this pathway to propagate structural and numerical chromosomal instabilities. Cells derived from genomically unstable tumors exhibit evidence for a partially activated DDR during mitosis, which leads to ongoing chromosome segregation errors. Thus, a thorough understanding of the consequences of mitotic DNA damage is key to our ability to devise novel anticancer therapeutic strategies.

Stabilization of 4E-BP1 by PI3K kinase and its involvement in CHK2 phosphorylation in the cellular response to radiation

Objectives: 4E-BP1 is a family member of eIF4E binding proteins (4E-BPs) which act as the suppressors of cap-dependent translation of RNA via competitively associating with cap-bound eIF4E. RNA translation regulation is an important manner to control the cellular responses to a series of stress conditions such as ionizing radiation (IR)-induced DNA damage response and cell cycle controlling. This study aimed to determine the mechanism of 4E-BP1 stabilization and its potential downstream target(s) in the response to IR. Methods: PI3Ks kinase inhibitors were used to determine the signaling control of 4E-BP1 phosphorylation and protein stability. shRNA strategy was employed to silence the expression of 4E-BP1 in HeLa and HepG2 cells, and determine its effect on the irradiation-induced CHK2 phosphorylation. The protein degradation/stability was investigated by western blotting on the condition of blocking novel protein synthesis by cycloheximide (CHX). Results: The phosphorylation of 4E-BP1 at Thr37/46 was significantly increased in both HepG2 and HeLa cells by ionizing radiation. Depression of 4E-BP1 by shRNA strategy resulted in an incomplete G2 arrest at the early stage of 2 hours post-irradiation, as well as a higher accumulation of mitotic cells at 10 and 12 hours post-irradiation as compared to the control cells. Consistently, the CHK2 phosphorylation at Thr68 induced by IR was also attenuated by silencing 4E-BP1 expression. Both PI3K and DNA-PKcs kinase inhibitors significantly decreased the protein level of 4E-BP1, which was associated with the accelerated degradation mediated by ubiquitination-proteasome pathway. Conclusion: PI3K kinase activity is necessary for maintaining 4E-BP1 stability. Our results also suggest 4E-BP1 a novel biological role of regulating cell cycle G2 checkpoint in responding to IR stress in association with controlling CHK2 phosphorylation.

PLK1, A Potential Target for Cancer Therapy

Polo-like kinase 1 (PLK1) plays an important role in the initiation, maintenance, and completion of mitosis. Dysfunction of PLK1 may promote cancerous transformation and drive its progression. PLK1 overexpression has been found in a variety of human cancers and was associated with poor prognoses in cancers. Many studies have showed that inhibition of PLK1 could lead to death of cancer cells by interfering with multiple stages of mitosis. Thus, PLK1 is expected to be a potential target for cancer therapy. In this article, we examined PLK1’s structural characteristics, its regulatory roles in cell mitosis, PLK1 expression, and its association with survival prognoses of cancer patients in a wide variety of cancer types, PLK1 interaction networks, and PLK1 inhibitors under investigation. Finally, we discussed the key issues in the development of PLK1-targeted cancer therapy.

Centrosomes in the DNA damage response--the hub outside the centre

Here, we review how DNA damage affects the centrosome and how centrosomes communicate with the DNA damage response (DDR) apparatus. We discuss how several proteins of the DDR are found at centrosomes, including the ATM, ATR, CHK1 and CHK2 kinases, the BRCA1 ubiquitin ligase complex and several members of the poly(ADP-ribose) polymerase family. Stereotypical centrosome organisation, in which two centriole barrels are orthogonally arranged in a roughly toroidal pericentriolar material (PCM), is strongly affected by exposure to DNA-damaging agents. We describe the genetic dependencies and mechanisms for how the centrioles lose their close association, and the PCM both expands and distorts after DNA damage. Another consequence of genotoxic stress is that centrosomes undergo duplication outside the normal cell cycle stage, meaning that centrosome amplification is commonly seen after DNA damage. We discuss several potential mechanisms for how centrosome numbers become dysregulated after DNA damage and explore the links between the DDR and the PLK1- and separase-dependent mechanisms that drive centriole separation and reduplication. We also describe how centrosome components, such as centrin2, are directly involved in responding to DNA damage. This review outlines current questions on the involvement of centrosomes in the DDR.

CHK2-BRCA1 tumor-suppressor axis restrains oncogenic Aurora-A kinase to ensure proper mitotic microtubule assembly

BRCA1 (breast cancer type 1 susceptibility protein) is a multifunctional tumor suppressor involved in DNA damage response, DNA repair, chromatin regulation, and mitotic chromosome segregation. Although the nuclear functions of BRCA1 have been investigated in detail, its role during mitosis is little understood. It is clear, however, that loss of BRCA1 in human cancer cells leads to chromosomal instability (CIN), which is defined as a perpetual gain or loss of whole chromosomes during mitosis. Moreover, our recent work has revealed that the mitotic function of BRCA1 depends on its phosphorylation by the tumor-suppressor kinase Chk2 (checkpoint kinase 2) and that this regulation is required to ensure normal microtubule plus end assembly rates within mitotic spindles. Intriguingly, loss of the positive regulation of BRCA1 leads to increased oncogenic Aurora-A activity, which acts as a mediator for abnormal mitotic microtubule assembly resulting in chromosome missegregation and CIN. However, how the CHK2-BRCA1 tumor suppressor axis restrains oncogenic Aurora-A during mitosis to ensure karyotype stability remained an open question. Here we uncover a dual molecular mechanism by which the CHK2-BRCA1 axis restrains oncogenic Aurora-A activity during mitosis and identify BRCA1 itself as a target for Aurora-A relevant for CIN. In fact, Chk2-mediated phosphorylation of BRCA1 is required to recruit the PP6C-SAPS3 phosphatase, which acts as a T-loop phosphatase inhibiting Aurora-A bound to BRCA1. Consequently, loss of CHK2 or PP6C-SAPS3 promotes Aurora-A activity associated with BRCA1 in mitosis. Aurora-A, in turn, then phosphorylates BRCA1 itself, thereby inhibiting the mitotic function of BRCA1 and promoting mitotic microtubule assembly, chromosome missegregation, and CIN.

Combined Inhibition of MEK and Plk1 Has Synergistic Antitumor Activity in NRAS Mutant Melanoma

About one-third of cancers harbor activating mutations in rat sarcoma viral oncogene homolog (RAS) oncogenes. In melanoma, aberrant neuroblastoma-RAS (NRAS) signaling fuels tumor progression in about 20% of patients. Current therapeutics for NRAS-driven malignancies barely affect overall survival. To date, pathway interference downstream of mutant NRAS seems to be the most promising approach. In this study, data revealed that mutant NRAS induced Polo-like kinase 1 (Plk1) expression, and pharmacologic inhibition of Plk1 stabilized the size of NRAS mutant melanoma xenografts. The combination of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK) and Plk1 inhibitors resulted in a significant growth reduction of NRAS mutant melanoma cells in vitro, and regression of xenografted NRAS mutant melanoma in vivo. Independent cell cycle arrest and increased induction of apoptosis underlies the synergistic effect of this combination. Data further suggest that the p53 signaling pathway is of key importance to the observed therapeutic efficacy. This study provides in vitro, in vivo, and first mechanistic data that an MEK/Plk1 inhibitor combination might be a promising treatment approach for patients with NRAS-driven melanoma. As mutant NRAS signaling is similar across different malignancies, this inhibitor combination could also offer a previously unreported treatment modality for NRAS mutant tumors of other cell origins.

EXO1 is critical for embryogenesis and the DNA damage response in mice with a hypomorphic Nbs1 allele

The maintenance of genome stability is critical for the suppression of diverse human pathologies that include developmental disorders, premature aging, infertility and predisposition to cancer. The DNA damage response (DDR) orchestrates the appropriate cellular responses following the detection of lesions to prevent genomic instability. The MRE11 complex is a sensor of DNA double strand breaks (DSBs) and plays key roles in multiple aspects of the DDR, including DNA end resection that is critical for signaling and DNA repair. The MRE11 complex has been shown to function both upstream and in concert with the 5′-3′ exonuclease EXO1 in DNA resection, but it remains unclear to what extent EXO1 influences DSB responses independently of the MRE11 complex. Here we examine the genetic relationship of the MRE11 complex and EXO1 during mammalian development and in response to DNA damage. Deletion of Exo1 in mice expressing a hypomorphic allele of Nbs1 leads to severe developmental impairment, embryonic death and chromosomal instability. While EXO1 plays a minimal role in normal cells, its loss strongly influences DNA replication, DNA repair, checkpoint signaling and damage sensitivity in NBS1 hypomorphic cells. Collectively, our results establish a key role for EXO1 in modulating the severity of hypomorphic MRE11 complex mutations.

ATR and a Chk1-Aurora B pathway coordinate postmitotic genome surveillance with cytokinetic abscission

Aurora B is a key regulator of cytokinetic abscission, yet the full range of cellular events and signaling factors that regulate Aurora B at this stage remains to be elucidated. Here it is shown that postmitotic genome surveillance is linked to the abscission checkpoint. This connection is dependent on ATR and a central Chk1-Aurora B signaling node.

Aurora B regulates cytokinesis timing and plays a central role in the abscission checkpoint. Cellular events monitored by this checkpoint are beginning to be elucidated, yet signaling pathways upstream of Aurora B in this context remain poorly understood. Here we reveal a new connection between postmitotic genome surveillance and cytokinetic abscission. Underreplicated DNA lesions are known to be transmitted through mitosis and protected in newly formed nuclei by recruitment of 53BP1 and other proteins until repair takes place. We find that this genome surveillance initiates before completion of cytokinesis. Elevating replication stress increases this postmitotic process and delays cytokinetic abscission by keeping the abscission checkpoint active. We further find that ATR activity in midbody-stage cells links postmitotic genome surveillance to abscission timing and that Chk1 integrates this and other signals upstream of Aurora B to regulate when the final step in the physical separation of daughter cells occurs.

CHK2 kinase in the DNA damage response and beyond

The serine/threonine kinase CHK2 is a key component of the DNA damage response. In human cells, following genotoxic stress, CHK2 is activated and phosphorylates >20 proteins to induce the appropriate cellular response, which, depending on the extent of damage, the cell type, and other factors, could be cell cycle checkpoint activation, induction of apoptosis or senescence, DNA repair, or tolerance of the damage. Recently, CHK2 has also been found to have cellular functions independent of the presence of nuclear DNA lesions. In particular, CHK2 participates in several molecular processes involved in DNA structure modification and cell cycle progression. In this review, we discuss the activity of CHK2 in response to DNA damage and in the maintenance of the biological functions in unstressed cells. These activities are also considered in relation to a possible role of CHK2 in tumorigenesis and, as a consequence, as a target of cancer therapy.

Polo-like kinase 1 (PLK1) and protein phosphatase 6 (PP6) regulate DNA-dependent protein kinase catalytic subunit (DNA-PKcs) phosphorylation in mitosis

The protein kinase activity of the DNA-PKcs (DNA-dependent protein kinase catalytic subunit) and its autophosphorylation are critical for DBS (DNA double-strand break) repair via NHEJ (non-homologous end-joining). Recent studies have shown that depletion or inactivation of DNA-PKcs kinase activity also results in mitotic defects. DNA-PKcs is autophosphorylated on Ser2056, Thr2647 and Thr2609 in mitosis and phosphorylated DNA-PKcs localize to centrosomes, mitotic spindles and the midbody. DNA-PKcs also interacts with PP6 (protein phosphatase 6), and PP6 has been shown to dephosphorylate Aurora A kinase in mitosis. Here we report that DNA-PKcs is phosphorylated on Ser3205 and Thr3950 in mitosis. Phosphorylation of Thr3950 is DNA-PK-dependent, whereas phosphorylation of Ser3205 requires PLK1 (polo-like kinase 1). Moreover, PLK1 phosphorylates DNA-PKcs on Ser3205 in vitro and interacts with DNA-PKcs in mitosis. In addition, PP6 dephosphorylates DNA-PKcs at Ser3205 in mitosis and after IR (ionizing radiation). DNA-PKcs also phosphorylates Chk2 on Thr68 in mitosis and both phosphorylation of Chk2 and autophosphorylation of DNA-PKcs in mitosis occur in the apparent absence of Ku and DNA damage. Our findings provide mechanistic insight into the roles of DNA-PKcs and PP6 in mitosis and suggest that DNA-PKcs' role in mitosis may be mechanistically distinct from its well-established role in NHEJ.

Chk2 prevents mitotic exit when the majority of kinetochores are unattached

As an essential step of the spindle checkpoint, Chk2 stabilizes Mps1 and phosphorylates Aurora B to prevent mitotic exit when most kinetochores remain unattached.

The spindle checkpoint delays exit from mitosis in cells with spindle defects. In this paper, we show that Chk2 is required to delay anaphase onset when microtubules are completely depolymerized but not in the presence of relatively few unattached kinetochores. Mitotic exit in Chk2-deficient cells correlates with reduced levels of Mps1 protein and increased Cdk1–tyrosine 15 inhibitory phosphorylation. Chk2 localizes to kinetochores and is also required for Aurora B–serine 331 phosphorylation in nocodazole or unperturbed early prometaphase. Serine 331 phosphorylation contributed to prometaphase accumulation in nocodazole after partial Mps1 inhibition and was required for spindle checkpoint establishment at the beginning of mitosis. In addition, expression of a phosphomimetic S331E mutant Aurora B rescued chromosome alignment or segregation in Chk2-deficient cells. We propose that Chk2 stabilizes Mps1 and phosphorylates Aurora B–serine 331 to prevent mitotic exit when most kinetochores are unattached. These results highlight mechanisms of an essential function of Chk2 in mitosis.

DNA-PK/Chk2 induces centrosome amplification during prolonged replication stress

The antineoplastic drug hydroxyurea (HU), when used at subtoxic doses, induces prolonged replication stress and centrosome amplification. This causes genomic instability and increases the malignancy of the recurring tumor. The mechanism of centrosome amplification induced by prolonged replication stress, however, is still unclear. Here, we examined the involvement of ataxia telangiectasia, mutated (ATM), ataxia telangiectasia, mutated and Rad3-related (ATR) and DNA-dependent protein kinase (DNA-PK) and found that HU-induced centrosome amplification was inhibited by the depletion of DNA-PKcs, but not ATM and ATR. Inactivation of ATM/ATR in U2OS cells instead caused aneuploidy and cell death. We found DNA-PKcs depletion also abrogated ATM phosphorylation, indicating that ATM activation during prolonged replication stress depends on DNA-PK. Depletion of DNA-PK abrogated checkpoint kinase (Chk)2 activation and partially reduced Chk1 activation. Chk2 depletion blocked HU-induced centrosome amplification, indicating a function of Chk2 in centrosome amplification. We further found that Chk2 was phosphorylated at Thr68 on the mother centriole at late G2 and mitosis when unstressed and on all amplified centrioles induced by HU. In summary, we have elucidated that DNA-PK/Chk2 signaling induces centrosome amplification upon long-term HU treatment, therefore increasing our insight into tumor recurrence after initial chemotherapy.

DNA-PKcs activates the Chk2-Brca1 pathway during mitosis to ensure chromosomal stability

The catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) is known to have a critical role in DNA double-strand break repair. We have previously reported that DNA-PKcs is activated when cells enter mitosis and functions in mitotic spindle assembly and chromosome segregation. Here we report that DNA-PKcs is the upstream regulator of the Chk2-Brca1 pathway, which impacts microtubule dynamics, kinetochore attachment and chromosomal segregation in mitosis. Downstream from Chk2, Brca1 promotes monoubiquitination of γ-tubulin to inhibit microtubule nucleation and growth. We found that DNA-PKcs is essential for mitotic Chk2 phosphorylation at Thr68. As in Chk2- and Brca1-deficient cells, loss of DNA-PKcs resulted in chromosome misalignment and lagging during anaphase owing to elevation in microtubule dynamics. Importantly, these mitotic aberrations in DNA-PKcs-defective cells were alleviated by the overexpression of phosphomimetic Chk2 or Brca1 mutant proteins but not their wild-type counterparts. Taken together, these results demonstrate that DNA-PKcs regulates mitotic spindle organization and chromosomal instability via the Chk2-Brca1 signaling pathway.

Chk2 regulates cell cycle progression during mouse oocyte maturation and early embryo development

As a tumor suppressor homologue during mitosis, Chk2 is involved in replication checkpoints, DNA repair, and cell cycle arrest, although its functions during mouse oocyte meiosis and early embryo development remain uncertain. We investigated the functions of Chk2 during mouse oocyte maturation and early embryo development. Chk2 exhibited a dynamic localization pattern; Chk2 expression was restricted to germinal vesicles at the germinal vesicle (GV) stage, was associated with centromeres at pro-metaphase I (Pro-MI), and localized to spindle poles at metaphase I (MI). Disrupting Chk2 activity resulted in cell cycle progression defects. First, inhibitor-treated oocytes were arrested at the GV stage and failed to undergo germinal vesicle breakdown (GVBD); this could be rescued after Chk2 inhibition release. Second, Chk2 inhibition after oocyte GVBD caused MI arrest. Third, the first cleavage of early embryo development was disrupted by Chk2 inhibition. Additionally, in inhibitor-treated oocytes, checkpoint protein Bub3 expression was consistently localized at centromeres at the MI stage, which indicated that the spindle assembly checkpoint (SAC) was activated. Moreover, disrupting Chk2 activity in oocytes caused severe chromosome misalignments and spindle disruption. In inhibitor-treated oocytes, centrosome protein γ-tubulin and Polo-like kinase 1 (Plk1) were dissociated from spindle poles. These results indicated that Chk2 regulated cell cycle progression and spindle assembly during mouse oocyte maturation and early embryo development.

Phospho-Ser/Thr-binding domains: navigating the cell cycle and DNA damage response

Coordinated progression through the cell cycle is a complex challenge for eukaryotic cells. Following genotoxic stress, diverse molecular signals must be integrated to establish checkpoints specific for each cell cycle stage, allowing time for various types of DNA repair. Phospho-Ser/Thr-binding domains have emerged as crucial regulators of cell cycle progression and DNA damage signalling. Such domains include 14-3-3 proteins, WW domains, Polo-box domains (in PLK1), WD40 repeats (including those in the E3 ligase SCF(βTrCP)), BRCT domains (including those in BRCA1) and FHA domains (such as in CHK2 and MDC1). Progress has been made in our understanding of the motif (or motifs) that these phospho-Ser/Thr-binding domains connect with on their targets and how these interactions influence the cell cycle and DNA damage response.

Inhibition of the mitotic exit network in response to damaged telomeres

When chromosomal DNA is damaged, progression through the cell cycle is halted to provide the cells with time to repair the genetic material before it is distributed between the mother and daughter cells. In Saccharomyces cerevisiae, this cell cycle arrest occurs at the G2/M transition. However, it is also necessary to restrain exit from mitosis by maintaining Bfa1-Bub2, the inhibitor of the Mitotic Exit Network (MEN), in an active state. While the role of Bfa1 and Bub2 in the inhibition of mitotic exit when the spindle is not properly aligned and the spindle position checkpoint is activated has been extensively studied, the mechanism by which these proteins prevent MEN function after DNA damage is still unclear. Here, we propose that the inhibition of the MEN is specifically required when telomeres are damaged but it is not necessary to face all types of chromosomal DNA damage, which is in agreement with previous data in mammals suggesting the existence of a putative telomere-specific DNA damage response that inhibits mitotic exit. Furthermore, we demonstrate that the mechanism of MEN inhibition when telomeres are damaged relies on the Rad53-dependent inhibition of Bfa1 phosphorylation by the Polo-like kinase Cdc5, establishing a new key role of this kinase in regulating cell cycle progression.

γH2AX foci formation in the absence of DNA damage: mitotic H2AX phosphorylation is mediated by the DNA-PKcs/CHK2 pathway

Phosphorylated H2AX is considered to be a biomarker for DNA double-strand breaks (DSB), but recent evidence suggests that γH2AX does not always indicate the presence of DSB. Here we demonstrate the bimodal dynamic of H2AX phosphorylation induced by ionizing radiation, with the second peak appearing when G2/M arrest is induced. An increased level of γH2AX occurred in mitotic cells, and this increase was attenuated by DNA-PKcs inactivation or Chk2 depletion, but not by ATM inhibition. The phosphorylation-mimic CHK2-T68D abrogated the attenuation of mitotic γH2AX induced by DNA-PKcs inactivation. Thus, the DNA-PKcs/CHK2 pathway mediates the mitotic phosphorylation of H2AX in the absence of DNA damage.

Cell cycle-dependent localization of CHK2 at centrosomes during mitosis

Background

Centrosomes function primarily as microtubule-organizing centres and play a crucial role during mitosis by organizing the bipolar spindle. In addition to this function, centrosomes act as reaction centers where numerous key regulators meet to control cell cycle progression. One of these factors involved in genome stability, the checkpoint kinase CHK2, was shown to localize at centrosomes throughout the cell cycle.

Results

Here, we show that CHK2 only localizes to centrosomes during mitosis. Using wild-type and CHK2^−/− HCT116 human colon cancer cells and human osteosarcoma U2OS cells depleted for CHK2 with small hairpin RNAs we show that several CHK2 antibodies are non-specific and cross-react with an unknown centrosomal protein(s) by immunofluorescence. To characterize the localization of CHK2, we generated cells expressing inducible GFP-CHK2 and Flag-CHK2 fusion proteins. We show that CHK2 localizes to the nucleus in interphase cells but that a fraction of CHK2 associates with the centrosomes in a Polo-like kinase 1-dependent manner during mitosis, from early mitotic stages until cytokinesis.

Conclusion

Our findings demonstrate that a subpopulation of CHK2 localizes at the centrosomes in mitotic cells but not in interphase. These results are consistent with previous reports supporting a role for CHK2 in the bipolar spindle formation and the timely progression of mitosis.

Mitotic catastrophe and cell death induced by depletion of centrosomal proteins

Mitotic catastrophe, which refers to cell death or its prologue triggered by aberrant mitosis, can be induced by a heterogeneous group of stimuli, including chromosome damage or perturbation of the mitotic apparatus. We investigated the mechanism of mitotic catastrophe and cell death induced by depletion of centrosomal proteins that perturbs microtubule organization. We transfected cells harboring wild-type or mutated p53 with siRNAs targeting Aurora A, ninein, TOG, TACC3, γ-tubulin, or pericentriolar material-1, and monitored the effects on cell death. Knockdown of Aurora A, ninein, TOG, and TACC3 led to cell death, regardless of p53 status. Knockdown of Aurora A, ninein, and TOG, led to aberrant spindle formation and subsequent cell death, which was accompanied by several features of apoptosis, including nuclear condensation and Annexin V binding in HeLa cells. During this process, cleavage of poly(ADP-ribose) polymerase-1, caspase-3, and caspase-9 was detected, but cleavage of caspase-8 was not. Cell death, monitored by time-lapse imaging, occurred during both interphase and M phase. In cells depleted of a centrosomal protein (Aurora A, ninein, or TOG), the rate of cell death was higher if the cells were cotransfected with siRNA against BubR1 or Mad2 than if they were transfected with siRNA against Bub1 or a control siRNA. These results suggest that metaphase arrest is necessary for the mitotic catastrophe and cell death caused by depletion of centrosomal proteins. Knockdown of centrosomal proteins led to increased phosphorylation of Chk2. Enhanced p-Chk2 localization was also observed at the centrosome in cells arrested in M phase, as well as in the nuclei of dying cells. Cotransfection of siRNAs against Chk2, in combination with depletion of a centrosomal protein, decreased the amount of cell death. Thus, Chk2 activity is indispensable for apoptosis after mitotic catastrophe induced by depletion of centrosomal proteins that perturbs microtubule organization.

Mutations in LRRC50 predispose zebrafish and humans to seminomas

Seminoma is a subclass of human testicular germ cell tumors (TGCT), the most frequently observed cancer in young men with a rising incidence. Here we describe the identification of a novel gene predisposing specifically to seminoma formation in a vertebrate model organism. Zebrafish carrying a heterozygous nonsense mutation in Leucine-Rich Repeat Containing protein 50 (lrrc50 also called dnaaf1), associated previously with ciliary function, are found to be highly susceptible to the formation of seminomas. Genotyping of these zebrafish tumors shows loss of heterozygosity (LOH) of the wild-type lrrc50 allele in 44.4% of tumor samples, correlating with tumor progression. In humans we identified heterozygous germline LRRC50 mutations in two different pedigrees with a family history of seminomas, resulting in a nonsense Arg488* change and a missense Thr590Met change, which show reduced expression of the wild-type allele in seminomas. Zebrafish in vivo complementation studies indicate the Thr590Met to be a loss-of-function mutation. Moreover, we show that a pathogenic Gln307Glu change is significantly enriched in individuals with seminoma tumors (13% of our cohort). Together, our study introduces an animal model for seminoma and suggests LRRC50 to be a novel tumor suppressor implicated in human seminoma pathogenesis.

Modulators of sensitivity and resistance to inhibition of PI3K identified in a pharmacogenomic screen of the NCI-60 human tumor cell line collection

The phosphoinositide 3-kinase (PI3K) signaling pathway is significantly altered in a wide variety of human cancers, driving cancer cell growth and survival. Consequently, a large number of PI3K inhibitors are now in clinical development. To begin to improve the selection of patients for treatment with PI3K inhibitors and to identify de novo determinants of patient response, we sought to identify and characterize candidate genomic and phosphoproteomic biomarkers predictive of response to the selective PI3K inhibitor, GDC-0941, using the NCI-60 human tumor cell line collection. In this study, sixty diverse tumor cell lines were exposed to GDC-0941 and classified by GI(50) value as sensitive or resistant. The most sensitive and resistant cell lines were analyzed for their baseline levels of gene expression and phosphorylation of key signaling nodes. Phosphorylation or activation status of both the PI3K-Akt signaling axis and PARP were correlated with in vitro response to GDC-0941. A gene expression signature associated with in vitro sensitivity to GDC-0941 was also identified. Furthermore, in vitro siRNA-mediated silencing of two genes in this signature, OGT and DDN, validated their role in modulating sensitivity to GDC-0941 in numerous cell lines and begins to provide biological insights into their role as chemosensitizers. These candidate biomarkers will offer useful tools to begin a more thorough understanding of determinants of patient response to PI3K inhibitors and merit exploration in human cancer patients treated with PI3K inhibitors.

Elevated cyclin G2 expression intersects with DNA damage checkpoint signaling and is required for a potent G2/M checkpoint arrest response to doxorubicin

To maintain genomic integrity DNA damage response (DDR), signaling pathways have evolved that restrict cellular replication and allow time for DNA repair. CCNG2 encodes an unconventional cyclin homolog, cyclin G2 (CycG2), linked to growth inhibition. Its expression is repressed by mitogens but up-regulated during cell cycle arrest responses to anti-proliferative signals. Here we investigate the potential link between elevated CycG2 expression and DDR signaling pathways. Expanding our previous finding that CycG2 overexpression induces a p53-dependent G(1)/S phase cell cycle arrest in HCT116 cells, we now demonstrate that this arrest response also requires the DDR checkpoint protein kinase Chk2. In accord with this finding we establish that ectopic CycG2 expression increases phosphorylation of Chk2 on threonine 68. We show that DNA double strand break-inducing chemotherapeutics stimulate CycG2 expression and correlate its up-regulation with checkpoint-induced cell cycle arrest and phospho-modification of proteins in the ataxia telangiectasia mutated (ATM) and ATM and Rad3-related (ATR) signaling pathways. Using pharmacological inhibitors and ATM-deficient cell lines, we delineate the DDR kinase pathway promoting CycG2 up-regulation in response to doxorubicin. Importantly, RNAi-mediated blunting of CycG2 attenuates doxorubicin-induced cell cycle checkpoint responses in multiple cell lines. Employing stable clones, we test the effect that CycG2 depletion has on DDR proteins and signals that enforce cell cycle checkpoint arrest. Our results suggest that CycG2 contributes to DNA damage-induced G(2)/M checkpoint by enforcing checkpoint inhibition of CycB1-Cdc2 complexes.

CHEK2 genomic and proteomic analyses reveal genetic inactivation or endogenous activation across the 60 cell lines of the US National Cancer Institute

CHEK2 encodes a serine/threonine kinase (Chk2) activated by ATM in response to DNA double-strand breaks. On the one hand, CHEK2 has been described as a tumor suppressor with proapoptotic, cell-cycle checkpoint and mitotic functions. On the other hand, Chk2 is also commonly activated (phosphorylated at T68) in cancers and precancerous lesions. Here, we report an extensive characterization of CHEK2 across the panel of 60 established cancer cell lines from the NCI Anticancer Screen (the NCI-60) using genomic and proteomic analyses, including exon-specific mRNA expression, DNA copy-number variation (CNV) by aCGH, exome sequencing, as well as western blot analyses for total and activated (pT68-Chk2) Chk2. We show that the high heterogeneity of Chk2 levels in cancer cells is primarily due to its inactivation (owing to low gene expression, alternative splicing, point mutations, copy-number alterations and premature truncation) or reduction of protein levels. Moreover, we observe that a significant percentage of cancer cells (12% of the NCI-60 and HeLa cells) show high endogenous Chk2 activation, which is always associated with p53 inactivation, and which is accompanied by downregulation of the Fanconi anemia and homologous recombination pathways. We also report the presence of activated Chk2 (pT68-Chk2) along with histone γ-H2AX in centrosomes.

Centrosomal Chk2 in DNA damage responses and cell cycle progression

Two major control systems regulate early stages of mitosis: activation of Cdk1 and anaphase control through assembly and disassembly of the mitotic spindle. In parallel to cell cycle progression, centrosomal duplication is regulated through proteins including Nek2. Recent studies suggest that centrosome-localized Chk1 forestalls premature activation of centrosomal Cdc25b and Cdk1 for mitotic entry, whereas Chk2 binds centrosomes and arrests mitosis only after activation by ATM and ATR in response to DNA damage. Here, we show that Chk2 centrosomal binding does not require DNA damage, but varies according to cell cycle progression. These and other data suggest a model in which binding of Chk2 to the centrosome at multiple cell cycle junctures controls co-localization of Chk2 with other cell cycle and centrosomal regulators.

Give me a break, but not in mitosis: the mitotic DNA damage response marks DNA double-strand breaks with early signaling events

: DNA double-strand breaks (DSBs) are extremely cytotoxic with a single unrepaired DSB being sufficient to induce cell death. A complex signalling cascade, termed the DNA damage response (DDR), is in place to deal with such DNA lesions and maintain genome stability. Recent work by us and others has found that the signalling cascade activated by DSBs in mitosis is truncated, displaying apical, but not downstream, components of the DDR. The E3 Ubiquitin ligases RNF8, RNF168 and BRCA1, along with the DDR mediator 53BP1, are not recruited to DSB sites in mitosis, and activation of downstream checkpoint kinases is also impaired. Here, we show that RNF8 and RNF168 are recruited to DNA damage foci in late mitosis, presumably to prime sites for 53BP1 recruitment in early G1. Interestingly, we show that, although RNF8, RNF168 and 53BP1 are excluded from DSB sites during most of mitosis, they associate with mitotic structures such as the kinetochore, suggesting roles for these DDR factors during mitotic cell division. We discuss these and other recent findings and suggest how these novel data collectively contribute to our understanding of mitosis and how cells deal with DNA damage during this crucial cell cycle stage.

Loss of the tumour-suppressor genes CHK2 and BRCA1 results in chromosomal instability

CHK2 (checkpoint kinase 2) and BRCA1 (breast cancer early-onset 1) are tumour-suppressor genes that have been implicated previously in the DNA damage response. Recently, we have identified CHK2 and BRCA1 as genes required for the maintenance of chromosomal stability and have shown that a Chk2-mediated phosphorylation of Brca1 is required for the proper and timely assembly of mitotic spindles. Loss of CHK2, BRCA1 or inhibition of its Chk2-mediated phosphorylation inevitably results in the transient formation of abnormal spindles that facilitate the establishment of faulty microtubule-kinetochore attachments associated with the generation of lagging chromosomes. Importantly, both CHK2 and BRCA1 are lost at very high frequency in aneuploid lung adenocarcinomas that are typically induced in knockout mice exhibiting chromosomal instability. Thus these results suggest novel roles for Chk2 and Brca1 in mitosis that might contribute to their tumour-suppressor functions.

TOPORS, implicated in retinal degeneration, is a cilia-centrosomal protein

We recently reported that mutations in the widely expressed nuclear protein TOPORS (topoisomerase I-binding arginine/serine rich) are associated with autosomal dominant retinal degeneration. However, the precise localization and a functional role of TOPORS in the retina remain unknown. Here, we demonstrate that TOPORS is a novel component of the photoreceptor sensory cilium, which is a modified primary cilium involved with polarized trafficking of proteins. In photoreceptors, TOPORS localizes primarily to the basal bodies of connecting cilium and in the centrosomes of cultured cells. Morpholino-mediated silencing of topors in zebrafish embryos demonstrates in another species a comparable retinal problem as seen in humans, resulting in defective retinal development and failure to form outer segments. These defects can be rescued by mRNA encoding human TOPORS. Taken together, our data suggest that TOPORS may play a key role in regulating primary cilia-dependent photoreceptor development and function. Additionally, it is well known that mutations in other ciliary proteins cause retinal degeneration, which may explain why mutations in TOPORS result in the same phenotype.

Female infertility in PDE3A(-/-) mice: polo-like kinase 1 (Plk1) may be a target of protein kinase A (PKA) and involved in meiotic arrest of oocytes from PDE3A(-/-) mice

Mechanisms of cAMP/PKA-induced meiotic arrest in oocytes are not completely identified. In cultured, G2/M-arrested PDE3A(-/-) murine oocytes, elevated PKA activity was associated with inactivation of Cdc2 and Plk1, and inhibition of phosphorylation of histone H3 (S10) and of dephosphorylation of Cdc25B (S323) and Cdc2 (Thr14/Tyr15). In cultured WT oocytes, PKA activity was transiently reduced and then increased to that observed in PDE3A(-/-) oocytes; Cdc2 and Plk1 were activated, phosphorylation of histone H3 (S10) and dephosphorylation of Cdc25B (S323) and Cdc2 (Thr14/Tyr15) were observed. In WT oocytes, PKAc were rapidly translocated into nucleus, and then to the spindle apparatus, but in PDE3A(-/-) oocytes, PKAc remained in the cytosol. Plk1 was reactivated by incubation of PDE3A(-/-) oocytes with PKA inhibitor, Rp-cAMPS. PDE3A was co-localized with Plk1 in WT oocytes, and co-immunoprecipitated with Plk1 in WT ovary and Hela cells. PKAc phosphorylated rPlk1 and Hela cell Plk1 and inhibited Plk1 activity in vitro. Our results suggest that PKA-induced inhibition of Plk1 may be critical in oocyte meiotic arrest and female infertility in PDE3A(-/-) mice.

Cell-cycle regulation in embryonic stem cells: centrosomal decisions on self-renewal

Embryonic stem cells seem to have the intriguing capacity to divide indefinitely while retaining their pluripotency. This self-renewal is accomplished by specialized mechanisms of cell-cycle control. In the last few years, several studies have provided evidence for a direct link between cell-cycle regulation and cell-fate decisions in stem cells. In this review, we discuss the peculiarities of embryonic stem cell-cycle control mechanisms, implicate their involvement in cell-fate decisions, and distinguish centrosomes as important players in the self-renewal versus differentiation roulette.

Multifaceted polo-like kinases: drug targets and antitargets for cancer therapy

The polo-like kinase 1 (PLK1) acts in concert with cyclin-dependent kinase 1-cyclin B1 and Aurora kinases to orchestrate a wide range of critical cell cycle events. Because PLK1 has been preclinically validated as a cancer target, small-molecule inhibitors of PLK1 have become attractive candidates for anticancer drug development. Although the roles of the closely related PLK2, PLK3 and PLK4 in cancer are less well understood, there is evidence showing that PLK2 and PLK3 act as tumour suppressors through their functions in the p53 signalling network, which guards the cell against various stress signals. In this article, recent insights into the biology of PLKs will be reviewed, with an emphasis on their role in malignant transformation, and progress in the development of small-molecule PLK1 inhibitors will be examined.

Inactivation of DNA-dependent protein kinase leads to spindle disruption and mitotic catastrophe with attenuated checkpoint protein 2 Phosphorylation in response to DNA damage

DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is well known as a critical component involving the nonhomologous end joining pathway of DNA double-strand breaks repair. Here, we showed another important role of DNA-PKcs in stabilizing spindle formation and preventing mitotic catastrophe in response to DNA damage. Inactivation of DNA-PKcs by small interfering RNA or specific inhibitor NU7026 resulted in an increased outcome of polyploidy after 2-Gy or 4-Gy irradiation. Simultaneously, a high incidence of multinucleated cells and multipolar spindles was detected in DNA-PKcs-deficient cells. Time-lapse video microscopy revealed that depression of DNA-PKcs results in mitotic catastrophe associated with mitotic progression failure in response to DNA damage. Moreover, DNA-PKcs inhibition led to a prolonged G(2)-M arrest and increased the outcome of aberrant spindles and mitotic catastrophe in Ataxia-telangiectasia mutated kinase (ATM)-deficient AT5BIVA cells. We have also revealed the localizations of phosphorylated DNA-PKcs/T2609 at the centrosomes, kinetochores, and midbody during mitosis. We have found that the association of DNA-PKcs and checkpoint kinase 2 (Chk2) is driven by Ku70/80 heterodimer. Inactivation of DNA-PKcs strikingly attenuated the ionizing radiation-induced phosphorylation of Chk2/T68 in both ATM-efficient and ATM-deficient cells. Chk2/p-T68 was also shown to localize at the centrosomes and midbody. These results reveal an important role of DNA-PKcs on stabilizing spindle formation and preventing mitotic catastrophe in response to DNA damage and provide another prospect for understanding the mechanism coupling DNA repair and the regulation of mitotic progression.

Rad9A is required for G2 decatenation checkpoint and to prevent endoreduplication in response to topoisomerase II inhibition

The Rad9A checkpoint protein interacts with and is required for proper localization of topoisomerase II-binding protein 1 (TopBP1) in response to DNA damage. Topoisomerase II (Topo II), another binding partner of TopBP1, decatenates sister chromatids that become intertwined during replication. Inhibition of Topo II by ICRF-193 (meso-4,4'-(3,2-butanediyl)-bis-(2,6-piperazinedione)), a catalytic inhibitor that does not induce DNA double-strand breaks, causes a mitotic delay known as the G(2) decatenation checkpoint. Here, we demonstrate that this checkpoint, dependent on ATR and BRCA1, also requires Rad9A. Analysis of different Rad9A phosphorylation mutants suggests that these modifications are required to prevent endoreduplication and to maintain decatenation checkpoint arrest. Furthermore, Rad9A Ser(272) is phosphorylated in response to Topo II inhibition. ICRF-193 treatment also causes phosphorylation of an effector kinase downstream of Rad9A in the DNA damage checkpoint pathway, Chk2, at Thr(68). Both of these sites are major targets of phosphorylation by the ATM kinase, although it has previously been shown that ATM is not required for the decatenation checkpoint. Examination of ataxia telangectasia (A-T) cells demonstrates that ATR does not compensate for ATM loss, suggesting that phosphorylation of Rad9A and Chk2 by ATM plays an additional role in response to Topo II inhibition than checkpoint function alone. Finally, we have shown that murine embryonic stem cells deficient for Rad9A have higher levels of catenated mitotic spreads than wild-type counterparts. Together, these results emphasize the importance of Rad9A in preserving genomic integrity in the presence of catenated chromosomes and all types of DNA aberrations.

The transcription factor SP1 regulates centriole function and chromosomal stability through a functional interaction with the mammalian target of rapamycin/raptor complex

Specificity protein 1 (SP1) is an essential transcription factor implicated in the regulation of genes that control multiple cellular processes, including cell cycle, apoptosis, and DNA damage. Very few nontranscriptional roles for SP1 have been reported thus far. Using confocal microscopy and centrosome fractionation, we identified SP1 as a centrosomal protein. Sp1-deficient mouse embryonic fibroblasts and cells depleted of SP1 by RNAi have increased centrosome number associated with centriole splitting, decreased microtubule nucleation, chromosome misalignment, formation of multipolar mitotic spindles and micronuclei, and increased incidence of aneuploidy. Using mass spectrometry, we identified P70S6K, an effector of the mTOR/raptor (mTORC1) kinase complex, as a novel interacting protein of SP1. We found that SP1-deficient cells have increased phosphorylation of the P70S6K effector ribosomal protein S6, suggesting that SP1 participates in the regulation of the mTORC1/P70S6K/S6 signaling pathway. We previously reported that aberrant mTORC1 activation leads to supernumerary centrosomes, a phenotype rescued by the mTORC1 inhibitor rapamycin. Similarly, treatment with rapamycin rescued the multiple centrosome phenotype of SP1-deficient cells. Taken together, these data strongly support the hypothesis that SP1 is involved in the control of centrosome number via regulation of the mTORC1 pathway, and predict that loss of SP1 function can lead to aberrant centriole splitting, deregulated mTORC1 signaling, and aneuploidy, thereby contributing to malignant transformation.