Phosphorylation of CDC25B by Aurora-A at the centrosome contributes to the G2-M transition

Effects of stable knockdown of Aurora kinase A on proliferation, migration, chromosomal instability, and expression of focal adhesion kinase and matrix metalloproteinase-2 in HEp-2 cells

Overexpression of Aurora kinase A (AURKA) is frequently observed in various cancers, including laryngeal squamous cell carcinoma (LSCC). We investigated the effects of knockdown of AURKA on laryngeal cancer HEp-2 cells both in vitro and in vivo. A plasmid containing short hairpin (sh)RNA against AURKA was constructed and transfected into HEp-2. Measurements included the CCK-8 assay for viability and proliferation, flow cytometry for apoptosis and effects on the mitotic checkpoint, a trans-well assay for migration, immunofluorescence for assessment of genomic instability, and western blotting for protein expression. AURKA knockdown inhibited proliferation, migration, and colony formation in vitro and tumorigenicity in vivo. The knockdown induced the accumulation of cells in G2-M phase and eventual apoptosis. Knockdown of AURKA caused delayed entry into mitosis after treatment with nocodazole, reduced chromosomal instability, and decreased expression of focal adhesion kinase (FAK), phosphorylated FAK, and matrix metalloproteinase-2 (MMP-2), key regulators in cell adhesion and invasion. Knockdown of AURKA inhibits the growth and invasiveness of this LSCC cell line both in vitro and in vivo. These effects may partially result from the reduced expression of FAK and MMP-2. Knockdown of AURKA expression may represent a promising therapeutic strategy for the treatment of LSCC.

How protein kinases co-ordinate mitosis in animal cells

Mitosis is associated with profound changes in cell physiology and a spectacular surge in protein phosphorylation. To accomplish these, a remarkably large portion of the kinome is involved in the process. In the present review, we will focus on classic mitotic kinases, such as cyclin-dependent kinases, Polo-like kinases and Aurora kinases, as well as more recently characterized players such as NIMA (never in mitosis in Aspergillus nidulans)-related kinases, Greatwall and Haspin. Together, these kinases co-ordinate the proper timing and fidelity of processes including centrosomal functions, spindle assembly and microtubule-kinetochore attachment, as well as sister chromatid separation and cytokinesis. A recurrent theme of the mitotic kinase network is the prevalence of elaborated feedback loops that ensure bistable conditions. Sequential phosphorylation and priming phosphorylation on substrates are also frequently employed. Another important concept is the role of scaffolds, such as centrosomes for protein kinases during mitosis. Elucidating the entire repertoire of mitotic kinases, their functions, regulation and interactions is critical for our understanding of normal cell growth and in diseases such as cancers.

A translational regulator, PUM2, promotes both protein stability and kinase activity of Aurora-A

Aurora-A, a centrosomal serine-threonine kinase, orchestrates several key aspects of cell division. However, the regulatory pathways for the protein stability and kinase activity of Aurora-A are still not completely understood. In this study, PUM2, an RNA-binding protein, is identified as a novel substrate and interacting protein of Aurora-A. Overexpression of the PUM2 mutant which fails to interact with Aurora-A, and depletion of PUM2 result in a decrease in the amount of Aurora-A. PUM2 physically binds to the D-box of Aurora-A, which is recognized by APC/C^Cdh1. Overexpression of PUM2 prevents ubiquitination and enhances the protein stability of Aurora-A, suggesting that PUM2 protects Aurora-A from APC/C^Cdh1-mediated degradation. Moreover, association of PUM2 with Aurora-A not only makes Aurora-A more stable but also enhances the kinase activity of Aurora-A. Our study suggests that PUM2 plays two different but important roles during cell cycle progression. In interphase, PUM2 localizes in cytoplasm and plays as translational repressor through its RNA binding domain. However, in mitosis, PUM2 physically associates with Aurora-A to ensure enough active Aurora-A at centrosomes for mitotic entry. This is the first time to reveal the moonlight role of PUM2 in mitosis.

Overexpression of Aurora-A in primary cells interferes with S-phase entry by diminishing Cyclin D1 dependent activities

Background

Aurora-A is a bona-fide oncogene whose expression is associated with genomic instability and malignant transformation. In several types of cancer, gene amplification and/or increased protein levels of Aurora-A are a common feature.

Results

In this report, we describe that inhibition of cell proliferation is the main effect observed after transient overexpression of Aurora-A in primary human cells. In addition to the known cell cycle block at the G2/M transition, Aurora-A overexpressing cells fail to overcome the restriction point at the G1/S transition due to diminished RB phosphorylation caused by reduced Cyclin D1 expression. Consequently, overexpression of Cyclin D1 protein is able to override the Aurora-A mediated G1 block. The Aurora-A mediated cell cycle arrest in G2 is not influenced by Cyclin D1 and as a consequence cells accumulate in G2. Upon deactivation of p53 part of the cells evade this premitotic arrest to become aneuploid.

Conclusion

Our studies describe that an increase of Aurora-A expression levels on its own has a tumor suppressing function, but in combination with the appropriate altered intracellular setting it might exert its oncogenic potential. The presented data indicate that deactivation of the tumor suppressor RB is one of the requirements for overriding a cell cycle checkpoint triggered by increased Aurora-A levels.

Shared and separate functions of polo-like kinases and aurora kinases in cancer

Large numbers of inhibitors for polo-like kinases and aurora kinases are currently being evaluated as anticancer drugs. Interest in these drugs is fuelled by the idea that these kinases have unique functions in mitosis. Within the polo-like kinase family, the emphasis for targeted therapies has been on polo-like kinase 1 (PLK1), and in the aurora kinase family drugs have been developed to specifically target aurora kinase A (AURKA; also known as STK6) and/or aurora kinase B (AURKB; also known as STK12). Information on the selectivity of these compounds in vivo is limited, but it is likely that off-target effects within the same kinase families will affect efficacy and toxicity profiles. In addition, it is becoming clear that interplay between polo-like kinases and aurora kinases is much more extensive than initially anticipated, and that both kinase families are important factors in the response to classical chemotherapeutics that damage the genome or the mitotic spindle. In this Review we discuss the implications of these novel insights on the clinical applicability of polo-like kinase and aurora kinase inhibitors.

Centrosomal protein of 192 kDa (Cep192) promotes centrosome-driven spindle assembly by engaging in organelle-specific Aurora A activation

Centrosomes are primary microtubule (MT)-organizing centers (MTOCs). During mitosis, they dramatically increase their size and MT-nucleating activity and participate in spindle assembly from spindle poles. These events require the serine/threonine kinase, Aurora A (AurA), and the centrosomal protein of 192 kDa (Cep192)/spindle defective 2 (Spd-2), but the underlying mechanism remains unclear. We have found that Cep192, unlike targeting protein for Xklp2 (TPX2), a known MT-localizing AurA activator, is an AurA cofactor in centrosome-driven spindle assembly. Cep192, through a direct interaction, targets AurA to mitotic centrosomes where the locally accumulating AurA forms homodimers or oligomers. The dimerization of endogenous AurA, in the presence of bound Cep192, triggers potent kinase activation that, in turn, drives MT assembly. Depletion of Cep192 or specific interference with AurA-Cep192 binding did not prevent AurA oligomerization on MTs but abrogated AurA recruitment to centrosomes and its activation by either sperm nuclei or anti-AurA antibody (αAurA)-induced dimerization. In these settings, MT assembly by both centrosomes and αAurA-coated beads was also abolished or severely compromised. Hence, Cep192 activates AurA by a mechanism different from that previously described for TPX2. The Cep192-mediated mechanism maximizes AurA activity at centrosomes and appears essential for the function of these organelles as MTOCs.

Human Cdc14A phosphatase modulates the G2/M transition through Cdc25A and Cdc25B

The Cdc14 family of serine-threonine phosphatases antagonizes CDK activity by reversing CDK-dependent phosphorylation events. It is well established that the yeast members of this family bring about the M/G1 transition. Budding yeast Cdc14 is essential for CDK inactivation at the end of mitosis and fission yeast Cdc14 homologue Flp1/Clp1 down-regulates Cdc25 to ensure the inactivation of mitotic CDK complexes to trigger cell division. However, the functions of human Cdc14 homologues remain poorly understood. Here we have tested the hypothesis that Cdc14A might regulate Cdc25 mitotic inducers in human cells. We found that increasing levels of Cdc14A delay entry into mitosis by inhibiting Cdk1-cyclin B1 activity. By contrast, lowering the levels of Cdc14A accelerates mitotic entry. Biochemical analyses revealed that Cdc14A acts through key Cdk1-cyclin B1 regulators. We observed that Cdc14A directly bound to and dephosphorylated Cdc25B, inhibiting its catalytic activity. Cdc14A also regulated the activity of Cdc25A at the G2/M transition. Our results indicate that Cdc14A phosphatase prevents premature activation of Cdk1 regulating Cdc25A and Cdc25B at the entry into mitosis.

Aurora kinases orchestrate mitosis; who are the players?

The Aurora are a conserved family of serine/threonine kinases with essential functions in cell division. In mitosis, Aurora kinases are required for chromosome segregation, condensation and orientation in the metaphase plate, spindle assembly, and the completion of cytokinesis. This review presents the Aurora kinases, their partners and how their interactions impact on the different mitotic functions.

Prophase I arrest and progression to metaphase I in mouse oocytes: comparison of resumption of meiosis and recovery from G2-arrest in somatic cells

Mammalian oocytes are arrested at prophase I until puberty when luteinizing hormone (LH) induces resumption of meiosis of follicle-enclosed oocytes. Resumption of meiosis is tightly coupled with regulating cyclin-dependent kinase 1 (CDK1) activity. Prophase I arrest depends on inhibitory phosphorylation of CDK1 and anaphase-promoting complex-(APC-CDH1)-mediated regulation of cyclin B levels. Prophase I arrest is maintained by endogenously produced cyclic adenosine monophosphate (cAMP), which activates protein kinase A (PKA) that in turn phosphorylates (and activates) the nuclear kinase WEE2. In addition, PKA-mediated phosphorylation of the phosphatase CDC25B results in its cytoplasmic retention. The combined effect maintains low levels of CDK1 activity that are not sufficient to initiate resumption of meiosis. LH triggers synthesis of epidermal growth factor-like factors in mural granulosa cells and leads to reduced cGMP transfer from cumulus cells to oocytes via gap junctions that couple the two cell types. cGMP inhibits oocyte phosphodiesterase 3A (PDE3A) and a decline in oocyte cGMP results in increased PDE3A activity. The ensuing decrease in oocyte cAMP triggers maturation by alleviating the aforementioned phosphorylations of WEE2 and CDC25B. As a direct consequence CDC25B translocates into the nucleus. The resulting activation of CDK1 also promotes extrusion of WEE2 from the nucleus thereby providing a positive amplification mechanism for CDK1 activation. Other kinases, e.g. protein kinase B, Aurora kinase A and polo-like kinase 1, also participate in resumption of meiosis. Mechanisms governing meiotic prophase I arrest and resumption of meiosis share common features with DNA damage-induced mitotic G2-checkpoint arrest and checkpoint recovery, respectively. These common features include CDC14B-dependent activation of APC-CDH1 in prophase I arrested oocytes or G2-arrested somatic cells, and CDC25B-dependent cell cycle resumption in both oocytes and somatic cells.

Assessment of the in vivo antitumor effects of ENMD-2076, a novel multitargeted kinase inhibitor, against primary and cell line-derived human colorectal cancer xenograft models

PURPOSE

This in vivo study was designed to investigate the efficacy of ENMD-2076, a small-molecule kinase inhibitor with activity against the Aurora kinases A and B, and several other tyrosine kinases linked to cancer, including vascular endothelial growth factor receptor 2, cKit, and fibroblast growth factor receptor 1, against murine xenograft models of human colorectal cancer (CRC).

EXPERIMENTAL DESIGN

HT-29 CRC cell line xenografts were treated with either vehicle or ENMD-2076 (100 or 200 mg/kg) orally daily for 28 days. Tumor growth inhibition, dynamic contrast-enhanced magnetic resonance imaging, and (18)FDG-positron emission tomography were conducted to assess the antiproliferative, antiangiogenic, and antimetabolic responses, respectively. Effects on proliferation were also analyzed by immunohistochemical methods. Additionally, three patient-derived xenografts from primary and metastatic sites were treated with ENMD-2076 (100 mg/kg) and assessed for tumor growth inhibition.

RESULTS

In the HT-29 xenograft model, ENMD-2076 induced initial tumor growth inhibition followed by regression. Treatment was associated with significant tumor blanching, indicating a loss of vascularity and substantial reductions in tumor vascular permeability and perfusion as measured by dynamic contrast-enhanced magnetic resonance imaging. Positron emission tomography scanning showed significant decreases in (18)FDG uptake at days 3 and 21 of treatment, which was associated with a marked reduction in proliferation as assessed by Ki-67. All three of the patient-derived xenografts tested were sensitive to treatment with ENMD 2076 as measured by tumor growth inhibition.

CONCLUSIONS

ENMD-2076 showed robust antitumor activity against cell line and patient-derived xenograft models of CRC that is detectable by functional imaging, supporting clinical investigation of this agent in CRC.

Making the Auroras glow: regulation of Aurora A and B kinase function by interacting proteins

The conserved Aurora family of protein kinases have emerged as crucial regulators of mitosis and cytokinesis. Despite their high degree of homology, Aurora A and B have very distinctive localisations and functions: Aurora A associates with the spindle poles to regulate entry into mitosis, centrosome maturation and spindle assembly; Aurora B is a member of the Chromosomal Passenger Complex (CPC) that transfers from the inner centromere in early mitosis to the spindle midzone, equatorial cortex and midbody in late mitosis and cytokinesis. Aurora B functions include regulation of chromosome–microtubule interactions, cohesion, spindle stability and cytokinesis. This review will focus on how interacting proteins make this functional diversity possible by targeting the kinases to different subcellular locations and regulating their activity.

Biology of Aurora A kinase: implications in cancer manifestation and therapy

The Aurora A kinase belongs to serine/threonine group of kinases, well known for its role in cell cycle, especially in the regulation of mitosis. Numerous substrates of Aurora A kinase have been identified, which are predominantly related to cell cycle progression while some of them are transcription factors. Aurora A-mediated phosphorylation can either directly or indirectly regulate the function of its substrates. There are overwhelming evidences which report overexpression and gene amplification of Aurora A in several human cancers, and suggest that Aurora A could be a bona fide oncogene involved in tumorigenesis. Hence, Aurora A plays wide-ranging roles in both mitosis and its deregulation manifests in cancer progression. These observations have favored the choice of Aurora kinases as a target for cancer therapy. Recently, numerous small molecules have been discovered against Aurora kinases and many have entered clinical trials. Most of these small-molecule modulators designed are specific against either Aurora A or Aurora B, but some are dual inhibitors targeting the ATP-binding site which is highly conserved among the three human homologues of Aurora kinase. In this review, we discuss the physiological functions of Aurora A, interactions between Aurora A kinase and its cellular substrates, tumorigenesis mediated by Aurora A kinase upon overexpression, and small-molecule modulators of Aurora kinase as targets for cancer therapy.

Identification of Myb-binding protein 1A (MYBBP1A) as a novel substrate for aurora B kinase

Aurora kinases are mitotic enzymes involved in centrosome maturation and separation, spindle assembly and stability, and chromosome condensation, segregation, and cytokinesis and represent well known targets for cancer therapy because their deregulation has been linked to tumorigenesis. The availability of suitable markers is of crucial importance to investigate the functions of Auroras and monitor kinase inhibition in in vivo models and in clinical trials. Extending the knowledge on Aurora substrates could help to better understand their biology and could be a source for clinical biomarkers. Using biochemical, mass spectrometric, and cellular approaches, we identified MYBBP1A as a novel Aurora B substrate and serine 1303 as the major phosphorylation site. MYBBP1A is phosphorylated in nocodazole-arrested cells and is dephosphorylated upon Aurora B silencing or by treatment with Danusertib, a small molecule inhibitor of Aurora kinases. Furthermore, we show that MYBBP1A depletion by RNA interference causes mitotic progression delay and spindle assembly defects. MYBBP1A has until now been described as a nucleolar protein, mainly involved in transcriptional regulation. The results presented herein show MYBBP1A as a novel Aurora B kinase substrate and reveal a not yet recognized link of this nucleolar protein to mitosis.

Temporal gene expression changes induced by a low concentration of benzo[a]pyrene diol epoxide in a normal human cell line

(+ or -)-anti-Benzo[a]pyrene-7,8-diol-9,10-epoxide (BPDE), which causes bulky-adduct DNA damage, is well-characterized as the ultimate carcinogen of benzo[a]pyrene (BaP). In this study, we have employed Affymetrix HG-U133 Plus 2.0 microarray and quantitative real-time RT-PCR methods to investigate a temporal transcriptomic response triggered by a low concentration (0.05 microM) of BPDE at 1, 10, and 22 h after exposure in normal human cells. The differential gene expression profiles at the three time points varied greatly, and generally reflected a cellular responsive process from initiation to progression and to recovery after the BPDE-caused damage. The dynamic regulation of the genes related with cell cycle progression and cell fate exhibited a tendency from inhibition to survival, which was accordant with the cell cycle arrest and cytotoxicity data induced by the low-dose BPDE exposure. In silico comparison of the genomic data revealed that BPDE and ultraviolet induced a panel of common transcriptional responses, which might be related with a series of similar molecular processes elicited by these two DNA-damaging agents. In conclusion, this whole-genome time-course study has identified a dynamically regulated transcriptional signature after low-dose BPDE exposure, which may help to understand the complex mechanisms of mutagenesis and carcinogenesis induced by BPDE.

Aurora-A kinase: a novel target both for cellular immunotherapy and molecular target therapy against human leukemia

BACKGROUND

Although cellular immunotherapy still remains in its infancy, it is one of the important treatment options against cancer. The marked improvement of its clinical efficacy requires a 'better' target antigen, which is well recognized by cancer-cell-specific cytotoxic T lymphocytes. We have recently demonstrated the potential of Aurora-A kinase (Aurora-A) as such a 'better' target for cellular immunotherapy against human leukemia. Aurora-A is a member of the serine/threonine kinase family that properly regulates the cell division process, and has recently been implicated in tumorigenesis. On the other hand, small-molecule inhibitors targeting Aurora-A have recently been developed and preliminary but promising observations from Phase I clinical trials have been reported. These facts highlight the attractiveness of Aurora-A as an important target of comprehensive cancer therapies.

OBJECTIVE/METHODS

In this review, we cover Aurora-A in the areas of immunotherapy and small-molecule inhibitor therapy against cancers.

RESULTS/CONCLUSIONS

Aurora-A kinase is an attractive molecule not only as a target for small-molecule inhibitors, but also as a potential target for immunotherapy against cancer.

Function of TopBP1 in genome stability

Human DNA topoisomerase IIbeta-binding protein 1 (TopBP1) and its orthologues in other organisms are proteins consisting of multiple BRCT modules that have acquired several functions during evolution. These proteins execute their tasks by interacting with a great variety of proteins involved in nuclear processes. TopBP1 is an essential protein that has numerous roles in the maintenance of the genomic integrity. In particular, it is required for the activation of ATM and Rad3-related (ATR), a vital regulator of DNA replication and replication stress response. The orthologues from yeast to human are involved in DNA replication and DNA damage response, while only proteins from higher eukaryotes are also involved in complex regulation of transcription, which is related to cell proliferation, damage response and apoptosis. We review here the recent progress in research aimed at elucidating the multiple cellular functions of TopBP1, focusing on metazoan systems.

Coupling the cell cycle to development

The core machinery that drives the eukaryotic cell cycle has been thoroughly investigated over the course of the past three decades. It is only more recently, however, that light has been shed on the mechanisms by which elements of this core machinery are modulated to alter cell cycle progression during development. It has also become increasingly clear that, conversely, core cell cycle regulators can play a crucial role in developmental processes. Here, focusing on findings from Drosophila melanogaster and Caenorhabditis elegans, we review the importance of modulating the cell cycle during development and discuss how core cell cycle regulators participate in determining cell fates.

Aurora A, centrosome structure, and the centrosome cycle

The centrosome, also known as the microtubule organizing center of the cell, is a membrane-less organelle composed of a pair of barrel-shaped centrioles surrounded by electron-dense pericentriolar material. The centrosome progresses through the centrosome cycle in step with the cell cycle such that centrosomes are duplicated in time to serve as the spindle poles during mitosis and that each resultant daughter cell contains a single centrosome. Regulation of the centrosome cycle with relation to the cell cycle is an essential process to maintain the ratio of one centrosome per new daughter cell. Numerous mitosis-specific kinases have been implicated in this regulation, and phosphorlyation plays an important role in coordinating the centrosome and cell cycles. Centrosome amplification can occur when the cycles are uncoupled, and this amplification is associated with cancer and with an increase in the levels of chromosomal instability. The aurora kinases A, B, and C are serine/threonine kinases that are active during mitosis. Aurora A is associated with centrosomes, being localized at the centrosome just prior to the onset of mitosis and for the duration of mitosis. Overexpression of aurora A leads to centrosome amplification and cellular transformation. The activity of aurora A is regulated by phosphorlyation and proteasomal degradation.

Aurora kinase A is a target of Wnt/beta-catenin involved in multiple myeloma disease progression

Multiple myeloma (MM) is a cancer of plasma cells with complex molecular characteristics that evolves from monoclonal gammopathy of undetermined significance, a highly prevalent premalignant condition. MM is the second most frequent hematologic cancer in the United States, and it remains incurable, thereby highlighting the need for new therapeutic approaches, particularly those targeting common molecular pathways involved in disease progression and maintenance, shared across different MM subtypes. Here we report that Wnt/beta-catenin is one such pathway. We document the involvement of beta-catenin in cell-cycle regulation, proliferation, and invasion contributing to enhanced proliferative and metastatic properties of MM. The pleiotropic effects of beta-catenin in MM correlate with its transcriptional function, and we demonstrate regulation of a novel target gene, Aurora kinase A, implicating beta-catenin in G2/M regulation. beta-catenin and Aurora kinase A are present in most MM but not in normal plasma cells and are expressed in a pattern that parallels progression from monoclonal gammopathy of undetermined significance to MM. Our data provide evidence for a novel functional link between beta-catenin and Aurora kinase A, underscoring a critical role of these pathways in MM disease progression.

Emerging connection between centrosome and DNA repair machinery

Centrosomes function in proper cell division in animal cells. The centrosome consists of a pair of centrioles and the surrounding pericentriolar matrix (PCM). After cytokinesis, daughter cells each acquire one centrosome, which subsequently duplicates at the G1/S phase in a manner that is dependent upon CDK2/cyclin-E activity. Defects in the regulation of centrosome duplication lead to tumorigenesis through abnormal cell division and resulting inappropriate chromosome segregation. Therefore, maintenance of accurate centrosome number is important for cell fate. Excess number of centrosomes can be induced by several factors including ionizing radiation (IR). Recent studies have shown that several DNA repair proteins localize to the centrosome and are involved in the regulation of centrosome number possibly through cell cycle checkpoints or direct modification of centrosome proteins. Furthermore, it has been reported that the development of microcephaly is likely caused by defective expression of centrosome proteins, such as ASPM, which are also involved in the response to IR. The present review highlights centrosome duplication in association with genotoxic stresses and the regulatory mechanism mediated by DNA repair proteins.Translated and modified from Radiat. Biol. Res. Comm. Vol.43; 343-356 (2008.12, in Japanese).

Microcephalin and pericentrin regulate mitotic entry via centrosome-associated Chk1

Primary microcephaly, Seckel syndrome, and microcephalic osteodysplastic primordial dwarfism type II (MOPD II) are disorders exhibiting marked microcephaly, with small brain sizes reflecting reduced neuron production during fetal life. Although primary microcephaly can be caused by mutations in microcephalin (MCPH1), cells from patients with Seckel syndrome and MOPD II harbor mutations in ataxia telangiectasia and Rad3 related (ATR) or pericentrin (PCNT), leading to disturbed ATR signaling. In this study, we show that a lack of MCPH1 or PCNT results in a loss of Chk1 from centrosomes with subsequently deregulated activation of centrosomal cyclin B–Cdk1.

Functional dynamics of Polo-like kinase 1 at the centrosome

Polo-like kinase 1 (Plk1) functions as a key regulator of mitotic events by phosphorylating substrate proteins on centrosomes, kinetochores, the mitotic spindle, and the midbody. Through mechanisms that are incompletely understood, Plk1 is released from and relocalizes to different mitotic structures as cells proceed through mitosis. We used fluorescence recovery after photobleaching to examine the kinetics of this process in more detail. We observed that Plk1 displayed a range of different recovery rates that differ at each mitotic substructure and depend on both the Polo-box domain and a functional kinase domain. Upon mitotic entry, centrosomal Plk1 becomes more dynamic, a process that is directly enhanced by Plk1 kinase activity. In contrast, Plk1 displays little dynamic exchange at the midbody, a process that again is modulated by the kinase activity of Plk1. Our findings suggest that the intrinsic kinase activity of Plk1 triggers its release from early mitotic structures and its relocalization to late mitotic structures. To assess the importance of Plk1 dynamic relocalization, Plk1 was persistently tethered to the centrosome. This resulted in a G(2) delay, followed by a prominent prometaphase arrest, as a consequence of defective spindle formation and activation of the spindle checkpoint. The dynamic release of Plk1 from early mitotic structures is thus crucial for mid- to late-stage mitotic events and demonstrates the importance of a fully dynamic Plk1 at the centrosome for proper cell cycle progression. This dependence on dynamic Plk1 was further observed during the mitotic reentry of cells after a DNA damage G(2) checkpoint, as this process was significantly delayed upon centrosomal tethering of Plk1. These results indicate that mitotic progression and control of mitotic reentry after DNA damage resides, at least in part, on the dynamic behavior of Plk1.

Aurora-A and hBora join the game of Polo

Overactivation of both Polo-like kinase-1 (Plk1) and Aurora-A is linked to cancer development, and small-molecule inhibitors that target these kinases are currently tested as anticancer drugs. Here, we discuss recent advances in the understanding of the functional crosstalk between Plk1 and Aurora-A before and during mitosis. Several recent findings have led to a better appreciation of how the activities of these distinct mitotic kinases are intertwined. Such insight is important for the expected utility of small-molecule inhibitors targeting Plk1 or Aurora-A, and it might help us to improve their application.

Alternative functions of core cell cycle regulators in neuronal migration, neuronal maturation, and synaptic plasticity

Recent studies have demonstrated that boundaries separating a cycling cell from a postmitotic neuron are not as concrete as expected. Novel and unique physiological functions in neurons have been ascribed for proteins fundamentally required for cell cycle progression and control. These "core" cell cycle regulators serve diverse postmitotic functions that span various developmental stages of a neuron, including neuronal migration, axonal elongation, axon pruning, dendrite morphogenesis, and synaptic maturation and plasticity. In this review, we detail the nonproliferative postmitotic roles that these cell cycle proteins have recently been reported to play, the significance of their expression in neurons, mechanistic insight when available, and future prospects.

The decision to enter mitosis: feedback and redundancy in the mitotic entry network

The decision to enter mitosis is mediated by a network of proteins that regulate activation of the cyclin B–Cdk1 complex. Within this network, several positive feedback loops can amplify cyclin B–Cdk1 activation to ensure complete commitment to a mitotic state once the decision to enter mitosis has been made. However, evidence is accumulating that several components of the feedback loops are redundant for cyclin B–Cdk1 activation during normal cell division. Nonetheless, defined feedback loops become essential to promote mitotic entry when normal cell cycle progression is perturbed. Recent data has demonstrated that at least three Plk1-dependent feedback loops exist that enhance cyclin B–Cdk1 activation at different levels. In this review, we discuss the role of various feedback loops that regulate cyclin B–Cdk1 activation under different conditions, the timing of their activation, and the possible identity of the elusive trigger that controls mitotic entry in human cells.

Tumor suppressor protein C53 antagonizes checkpoint kinases to promote cyclin-dependent kinase 1 activation

Cyclin-dependent kinase 1 (Cdk1)/cyclin B1 complex is the driving force for mitotic entry, and its activation is tightly regulated by the G2/M checkpoint. We originally reported that a novel protein C53 (also known as Cdk5rap3 and LZAP) potentiates DNA damage-induced cell death by modulating the G2/M checkpoint. More recently, Wang et al. (2007) found that C53/LZAP may function as a tumor suppressor by way of inhibiting NF-kappaB signaling. We report here the identification of C53 protein as a novel regulator of Cdk1 activation. We found that knockdown of C53 protein causes delayed Cdk1 activation and mitotic entry. During DNA damage response, activation of checkpoint kinase 1 and 2 (Chk1 and Chk2) is partially inhibited by C53 overexpression. Intriguingly, we found that C53 interacts with Chk1 and antagonizes its function. Moreover, a portion of C53 protein is localized at the centrosome, and centrosome-targeting C53 potently promotes local Cdk1 activation. Taken together, our results strongly suggest that C53 is a novel negative regulator of checkpoint response. By counteracting Chk1, C53 promotes Cdk1 activation and mitotic entry in both unperturbed cell-cycle progression and DNA damage response.

The polo-like kinase 1 regulates CDC25B-dependent mitosis entry

Activation of cyclin-dependent kinase complexes (CDK) at key cell cycle transitions is dependent on their dephosphorylation by CDC25 dual-specificity phosphatases (CDC25A, B and C in human). The CDC25B phosphatase plays an essential role in controlling the activity of CDK1-cyclin B complexes at the entry into mitosis and together with polo-like kinase 1 (PLK1) in regulating the resumption of cell cycle progression after DNA damage-dependent checkpoint arrest in G2. In this study, we analysed the regulation of CDC25B-dependent mitosis entry by PLK1. We demonstrate that PLK1 activity is essential for the relocation of CDC25B from the cytoplasm to the nucleus. By gain and loss of function analyses, we show that PLK1 stimulates CDC25B-induced mitotic entry in both normal conditions and after DNA-damage induced G2/M arrest. Our results support a model in which the relocalisation of CDC25B to the nucleus at the G2-M transition by PLK1 regulates its mitotic inducing activity.

IRC-083864, a novel bis quinone inhibitor of CDC25 phosphatases active against human cancer cells

CDC25 phosphatases are key actors in cyclin-dependent kinases activation whose role is essential at various stages of the cell cycle. CDC25 expression is upregulated in a number of human cancers. CDC25 phosphatases are therefore thought to represent promising novel targets in cancer therapy. Here, we report the identification and the characterization of IRC-083864, an original bis-quinone moiety that is a potent and selective inhibitor of CDC25 phosphatases in the low nanomolar range. IRC-083864 inhibits cell proliferation of a number of cell lines, regardless of their resistance to other drugs. It irreversibly inhibits cell proliferation and cell cycle progression and prevents entry into mitosis. In addition, it inhibits the growth of HCT-116 tumor spheroids with induction of p21 and apoptosis. Finally, IRC-083864 reduced tumor growth in mice with established human prostatic and pancreatic tumor xenografts. This study describes a novel compound, which merits further study as a potential anticancer agent.

Human Cdc25A phosphatase has a non-redundant function in G2 phase by activating Cyclin A-dependent kinases

Cdc25 phosphatases activate Cdk/Cyclin complexes by dephosphorylation and thus promote cell cycle progression. We observed that the peak activity of Cdc25A precedes the one of Cdc25B in prophase and the maximum of Cyclin/Cdk kinase activity. Furthermore, Cdc25A activates both Cdk1-2/Cyclin A and Cdk1/Cyclin B complexes while Cdc25B seems to be involved only in activation of Cdk1/Cyclin B. Concomitantly, repression of Cdc25A led to a decrease in Cyclin A-associated kinase activity and attenuated Cdk1 activation. Our results indicate that Cdc25A acts before Cdc25B - at least in cancer cells, and has non-redundant functions in late G2/early M-phase as a major regulator of Cyclin A/kinase complexes.

Aurora-A kinase is essential for bipolar spindle formation and early development

Aurora-A is a conserved kinase implicated in mitotic regulation and carcinogenesis. Aurora-A was previously implicated in mitotic entry and spindle assembly, although contradictory results prevented a clear understanding of the roles of Aurora-A in mammals. We developed a conditional null mutation in the mouse Aurora-A gene to investigate Aurora-A functions in primary cells ex vivo and in vivo. We show here that conditional Aurora-A ablation in cultured embryonic fibroblasts causes impaired mitotic entry and mitotic arrest with a profound defect in bipolar spindle formation. Germ line Aurora-A deficiency causes embryonic death at the blastocyst stage with pronounced cell proliferation failure, mitotic arrest, and monopolar spindle formation. Aurora-A deletion in mid-gestation embryos causes an increase in mitotic and apoptotic cells. These results indicate that murine Aurora-A facilitates, but is not absolutely required for, mitotic entry in murine embryonic fibroblasts and is essential for centrosome separation and bipolar spindle formation in vitro and in vivo. Aurora-A deletion increases apoptosis, suggesting that molecular therapies targeting Aurora-A may be effective in inducing tumor cell apoptosis. Aurora-A conditional mutant mice provide a valuable system for further defining Aurora-A functions and for predicting effects of Aurora-A therapeutic intervention.

Inappropriate activation of cyclin-dependent kinases by the phosphatase Cdc25b results in premature mitotic entry and triggers a p53-dependent checkpoint

The dual specificity phosphatase Cdc25B is capable of inhibiting cellular proliferation, and this occurs in a manner dependent upon its catalytic activity. Here it is shown that this is accompanied by inappropriate cyclin-dependent kinase activation and premature mitotic entry, leading to both p53-dependent and independent checkpoints. Forced expression of Cdc25B inappropriately up-regulated the activity of Cdk1 and Cdk2, by reducing levels of inhibitory phosphorylation. In cells lacking p14ARF, p53 is induced, and components of the ATM and ATR pathways are activated. Cdc25B triggers cell cycle arrest in the G(1) and G(2) phases that is p53- and p21-dependent and is inhibited by caffeine. Cdc25B also causes cells with an S phase DNA content to enter mitosis prematurely in a p53-independent manner. Synchronization of cells with aphidicolin results in these cells undergoing apoptosis. Thus, inappropriate cell cycle progression and premature mitotic entry via dysregulation of cyclin-dependent kinases results in activation of both p53-dependent and independent responses. Because Cdc25B is known to have oncogenic activity, this provides insight into the multistep nature of cancer development and why there is p53 loss during tumorigenesis.

Aurora kinases: structure, functions and their association with cancer

BACKGROUND

Aurora kinases are a recently discovered family of kinases (A, B & C) consisting of highly conserved serine\threonine protein kinases found to be involved in multiple mitotic events: regulation of spindle assembly checkpoint pathway, function of centrosomes and cytoskeleton, and cytokinesis. Aberrant expression of Aurora kinases may lead to cancer. For this reason the Aurora kinases are potential targets in the treatment of cancer. In this review we discuss the biology of these kinases: structure, function, regulation and association with cancer.

METHODS AND RESULTS

A literature search.

CONCLUSION

Many of the multiple functions of mitosis are mediated by the Aurora kinases. Their aberrant expression can lead to the deregulation of cell division and cancer. For this reason, the Aurora kinases are currently one of the most interesting targets for cancer therapy. Some Aurora kinase inhibitors in the clinic have proven effectively on a wide range of tumor types. The clinical data are very encouraging and promising for development of novel class of structurally different Aurora kinase inhibitors. Hopefully the Aurora kinases will be potentially useful in drug targeted cancer treatment.

In vivo roles of CDC25 phosphatases: biological insight into the anti-cancer therapeutic targets

CDC25 phosphatases are not only rate-limiting activators of cyclin-dependent kinases (CDKs) but also important targets of the CHK1/CHK2-mediated checkpoint pathway. Each isoform of the mammalian CDC25 family seems to exert unique biological functions. CDC25A is a critical regulator for both G1-S and G2-M transitions and essential for embryonic cell proliferation after the blastocyst stage. CDC25B is dispensable for embryogenesis but required for meiotic progression of oocytes in a manner analogous to Drosophila Twine or C. elegans cdc-25.1. Moreover, CDC25A and CDC25B appear to regulate different events or stages of mitosis. CDC25B may mediate the activation of CDK1/Cyclin B at the centrosome during prophase, while CDC25A may be required for the subsequent full activation of nuclear CDK1/Cyclin B. CDC25C is dispensable for both mitotic and meiotic divisions, although it is highly regulated during the processes. Excessive levels of CDC25A and CDC25B are often observed in various human cancer tissues. Deregulated expression of these phosphatases allows cells to overcome DNA damage-induced checkpoint, leading to genomic instability. Studies using mouse models demonstrated that deregulated expression of CDC25A significantly promotes RAS- or NEU-induced mammary tumor development with chromosomal aberrations, whereas decreased CDC25A expression in heterozygous knockout mice delays tumorigenesis. These biological properties of CDC25 phosphatases provide significant insight into the pathobiology of cancer and scientific foundation for anti-CDC25 therapeutic intervention.

Aurora A is essential for early embryonic development and tumor suppression

Aurora A is a serine/threonine kinase that functions in various stages of mitosis. Accumulating evidence has demonstrated that gene amplification and overexpression of Aurora A are linked to tumorigenesis, suggesting that Aurora A is an oncogene. In addition, Aurora A overexpression has been used as a negative prognostic marker, because it is associated with resistance to anti-mitotic agents commonly used for cancer therapy. To understand the physiological functions of Aurora A, we generated Aurora A knock-out mice. Aurora A null mice die early during embryonic development before the 16-cell stage. These Aurora A null embryos have defects in mitosis, particularly in spindle assembly, supporting critical functions of Aurora A during mitotic transitions. Interestingly, Aurora A heterozygosity results in a significantly increased tumor incidence in mice, suggesting that Aurora A may also act as a haploinsufficient tumor suppressor. Consistently, Aurora A heterozygous mouse embryonic fibroblasts have higher rates of aneuploidy. We further discovered that VX-680, an Aurora kinase inhibitor currently in phase II clinical trials for cancer treatment, could induce aneuploidy in wild type mouse embryonic fibroblasts. We conclude that a balanced Aurora A level is critical for maintaining genomic stability and one needs to be fully aware of the potential side effects of anti-cancer therapy based on the use of Aurora A-specific inhibitors.

Localization of aurora A and aurora B kinases during interphase: role of the N-terminal domain

Aurora kinases possess a conserved catalytic domain (CD) and a N-terminal domain (ND) that varies in size and sequence. We have previously reported that the N-terminal domain of AuroraA (AurA) participates in the localization of the kinase to the centrosome in interphase. AuroraB (AurB) is a chromosome passenger protein and its N-terminal domain is not necessary for its localization or function during mitosis. Using various combinations of GFP-AurA and AurB protein domains we show that AurB N-terminal domain is required for nuclear localization in Xenopus XL2 cells in interphase. In human cells, however, we found both AurA and AurB kinases in the nucleus, AurA being mainly cytoplasmic and AurB mainly nuclear. Both proteins are actively excluded from the nucleus by a CRM1 dependent pathway. Interestingly, at a functional level, in interphase, every combination of Aurora kinase domains (ND-CD) rescues histone H3 Serine10 phosphorylation defect induced by AurB knockdown. This clearly indicates the presence of a functional AurA in the nucleus. However, the chimera ND-AurA/CD-AurB was much more efficient than the ND-AurB/ CD-AurA to rescue multinucleation also induced by AurB knockdown. This indicates that the catalytic domain of AurB is required to fulfill specific functions during mitosis that cannot be fulfilled by the catalytic domain of AurA, probably for localization reasons during mitosis.

Polo-like kinase-1 is activated by aurora A to promote checkpoint recovery

Polo-like kinase-1 (PLK1) is an essential mitotic kinase regulating multiple aspects of the cell division process. Activation of PLK1 requires phosphorylation of a conserved threonine residue (Thr 210) in the T-loop of the PLK1 kinase domain, but the kinase responsible for this has not yet been affirmatively identified. Here we show that in human cells PLK1 activation occurs several hours before entry into mitosis, and requires aurora A (AURKA, also known as STK6)-dependent phosphorylation of Thr 210. We find that aurora A can directly phosphorylate PLK1 on Thr 210, and that activity of aurora A towards PLK1 is greatly enhanced by Bora (also known as C13orf34 and FLJ22624), a known cofactor for aurora A (ref. 7). We show that Bora/aurora-A-dependent phosphorylation is a prerequisite for PLK1 to promote mitotic entry after a checkpoint-dependent arrest. Importantly, expression of a PLK1-T210D phospho-mimicking mutant partially overcomes the requirement for aurora A in checkpoint recovery. Taken together, these data demonstrate that the initial activation of PLK1 is a primary function of aurora A.

Evaluation of Polo-like Kinase 1 inhibition on the G2/M checkpoint in Acute Myelocytic Leukaemia

Polo-Kinase 1 (PLK1) is a key cell cycle regulator that is necessary for checkpoint recovery after DNA damage-induced G2 arrest. We have examined the effects of PLK inhibition in Acute Myelocytic Leukaemia (AML) cells, whose resistance to genotoxic agents is thought to be associated with checkpoint reinforcement. We report that in U937 AML cells, PLK1 participates in checkpoint recovery, and that inhibition of PLK by the GW843682X compound results in mitotic accumulation and apoptosis. We also found that when challenged with VP-16, inhibition of PLK1 prevented U937 cells from checkpoint exit. Finally, we found that treatment with GW843682X slightly reduced genotoxic-induced inhibition of colony formation efficiency of primary leukaemia cells (CFU-L) from AML patients.

P53, cyclin-dependent kinase and abnormal amplification of centrosomes

Centrosomes play a critical role in formation of bipolar mitotic spindles, an essential event for accurate chromosome segregation into daughter cells. Numeral abnormalities of centrosomes (centrosome amplification) occur frequently in cancers, and are considered to be the major cause of chromosome instability, which accelerates acquisition of malignant phenotypes during tumor progression. Loss or mutational inactivation of p53 tumor suppressor protein, one of the most common mutations found in cancers, results in a high frequency of centrosome amplification in part via allowing the activation of the cyclin-dependent kinase (CDK) 2-cyclin E (as well as CDK2-cyclin A) which is a key factor for the initiation of centrosome duplication. In this review, the role of centrosome amplification in tumor progression, and mechanistic view of how centrosomes are amplified in cells through focusing on loss of p53 and aberrant activities of CDK2-cyclins will be discussed.

Aurora kinase A controls meiosis I progression in mouse oocytes

Aurora kinase A (AURKA), which is a centrosome-localized serine/threonine kinase crucial for cell cycle control, is critically involved in centrosome maturation and spindle assembly in somatic cells. Active T288 phosphorylated AURKA localizes to the centrosome in the late G(2) and also spreads to the minus ends of mitotic spindle microtubules. AURKA activates centrosomal CDC25B and recruits cyclin B1 to centrosomes. We report here functions for AURKA in meiotic maturation of mouse oocytes, which is a model system to study the G(2) to M transition. Whereas AURKA is present throughout the entire GV-stage oocyte with a clear accumulation on microtubule organizing centers (MTOC), active AURKA becomes entirely localized to MTOCs shortly before germinal vesicle breakdown. In contrast to somatic cells in which active AURKA is present at the centrosomes and minus ends of microtubules, active AURKA is mainly located on MTOCs at metaphase I (MI) in oocytes. Inhibitor studies using Roscovitine (CDK1 inhibitor), LY-294002 (PI3K inhibitor) and SH-6 (PKB inhibitor) reveal that activation of AURKA localized on MTOCs is independent on PI3K-PKB and CDK1 signaling pathways and MOTC amplification is observed in roscovitine- and SH-6-treated oocytes that fail to undergo nuclear envelope breakdown. Moreover, microinjection of Aurka mRNA into GV-stage oocytes cultured in 3-isobutyl-1-methyl xanthine (IBMX)-containing medium to prevent maturation also results in MOTC amplification in the absence of CDK1 activation. Overexpression of AURKA also leads to formation of an abnormal MI spindle, whereas RNAi-mediated reduction of AURKA interferes with resumption of meiosis and spindle assembly. Results of these experiments indicate that AURKA is a critical MTOC-associated component involved in resumption of meiosis, MTOC multiplication, proper spindle formation and the metaphase I-metaphase II transition.

Gene expression study of Aurora-A reveals implication during bladder carcinogenesis and increasing values in invasive urothelial cancer

OBJECTIVES

Urothelial carcinoma is a frequent and aggressive cancer. We wanted to gain better insight into the early molecular mechanisms of bladder carcinogenesis by evaluating Aurora-A gene expression, which is implicated in genomic stability and essential for mitosis.

MATERIALS

This study, using real-time quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), analyzed the expression levels of three selected genes in dissected tissues from normal bladder, noninvasive cancers, and muscle-invasive bladder carcinomas (n = 49). We compared gene expression levels of three genes (Aurora-A, and as control uroplakin II (UPII) and TBP, respectively) at different stages of bladder cancer. We used multivariate analysis, receiver operating characteristic curves and the nonparametric Mann-Whitney test.

RESULTS

The expression of Aurora-A gene studied was significantly deregulated, with an increasing level in cancer versus normal tissue Aurora-A. This development was linear. Aurora-A was already deregulated in early stages of carcinogenesis (pTa/pT1) (P = 0.0004) and displayed even more deregulation in muscle-invasive stages (pT2 to pT4). Immunohistochemistry performed on the same samples using Aurora-A antibody confirmed results of RT-PCR, with statistically significant values when comparing m-RNA expression and immunohistochemical values (P = 0.0001).

CONCLUSIONS

This study highlights the fact that Aurora-A gene expression is already strongly deregulated in early stages of urothelial carcinoma with abnormal expression, and might be considered a biomarker of tumor aggression. The increase in Aurora-A expression might provide further information regarding the behavior of bladder cancer in daily practice.

CDC25A phosphatase controls meiosis I progression in mouse oocytes

CDK1 is a pivotal regulator of resumption of meiosis and meiotic maturation of oocytes. CDC25A/B/C are dual-specificity phosphatases and activate cyclin-dependent kinases (CDKs). Although CDC25C is not essential for either mitotic or meiotic cell cycle regulation, CDC25B is essential for CDK1 activation during resumption of meiosis. Cdc25a -/- mice are embryonic lethal and therefore a role for CDC25A in meiosis is unknown. We report that activation of CDK1 results in a maturation-associated decrease in the amount of CDC25A protein, but not Cdc25a mRNA, such that little CDC25A is present by metaphase I. In addition, expression of exogenous CDC25A overcomes cAMP-mediated maintenance of meiotic arrest. Microinjection of Gfp-Cdc25a and Gpf-Cdc25b mRNAs constructs reveals that CDC25A is exclusively localized to the nucleus prior to nuclear envelope breakdown (NEBD). In contrast, CDC25B localizes to cytoplasm in GV-intact oocytes and translocates to the nucleus shortly before NEBD. Over-expressing GFP-CDC25A, which compensates for the normal maturation-associated decrease in CDC25A, blocks meiotic maturation at MI. This MI block is characterized by defects in chromosome congression and spindle formation and a transient reduction in both CDK1 and MAPK activities. Lastly, RNAi-mediated reduction of CDC25A results in fewer oocytes resuming meiosis and reaching MII. These data demonstrate that CDC25A behaves differently during female meiosis than during mitosis, and moreover, that CDC25A has a function in resumption of meiosis, MI spindle formation and the MI-MII transition. Thus, both CDC25A and CDC25B are critical for meiotic maturation of oocytes.

Aurora-A: the maker and breaker of spindle poles

The gene encoding the Aurora-A protein kinase is located in the 20q13 breast cancer amplicon and is also overexpressed in colorectal, pancreatic and gastric tumours. Although Aurora-A may not be a bona fide oncoprotein in humans, it is a promising drug target in cancer therapy. Thus, it is surprising that so little is known of its role in normal cells. The primary function of Aurora-A is to promote bipolar spindle assembly, but the molecular details of this process remained obscure until recently. The discovery of several novel Aurora-A-binding proteins and substrates has implicated Aurora-A in centrosome maturation and separation, acentrosomal and centrosomal spindle assembly, kinetochore function, cytokinesis and in cell fate determination. Here we discuss recent advances in determining the early mitotic role of Aurora-A, with a strong emphasis on its function at the mitotic spindle poles.

Differential phosphorylation of Cdc25C phosphatase in mitosis

Cdc25 dual-specificity phosphatases coordinate entry into mitosis through activating dephosphorylation of the Mitosis-Promoting Factor, Cdk1-cyclin B1. Activation of Cdc25C at the G2/M transition, involves its dissociation from 14-3-3, together with its hyperphosphorylation on several sites within its regulatory N-terminal domain, mediated by cyclin-dependent kinases and Plk1. Growing evidence suggests that phosphorylation intermediates are likely to precede complete hyperphosphorylation of Cdc25C. To address whether such variants occur in mitotic cells, we raised antibodies directed against different mitotic phosphorylation sites of human Cdc25C, and characterized the phosphorylated species detectable in HeLa cells. In the present study, we provide first-time evidence for the existence of multiple species of Cdc25C in mitotic cell extracts, including full-length and splice variants with different phosphorylation patterns, thereby revealing an intricate network of Cdc25C phosphatases, likely to have distinct biological functions.

Cyclin A/cdk2 coordinates centrosomal and nuclear mitotic events

Cyclin A/cdk2 has a role in progression through S phase, and a large pool is also activated in G2 phase. Here we report that this G2 phase pool regulates the timing of progression into mitosis. Knock down of cyclin A by siRNA or addition of a specific cdk2 small molecule inhibitor delayed entry into mitosis by delaying cells in G2 phase. The G2 phase-delayed cells contained elevated levels of inactive cyclin B/cdk1. However, increased microtubule nucleation at the centrosomes was observed, and the centrosomes stained for markers of cyclin B/cdk1 activity. Both microtubule nucleation at the centrosomes and the phosphoprotein markers were lost with short-term treatment of the cdk1/2 inhibitor roscovitine but not the Mek1/2 inhibitor U0126. Cyclin A/cdk2 localized at the centrosomes in late G2 phase after separation of the centrosomes but before the start of prophase. Thus G2 phase cyclin A/cdk2 controls the timing of entry into mitosis by controlling the subsequent activation of cyclin B/cdk1, but also has an unexpected role in coordinating the activation of cyclin B/cdk1 at the centrosome and in the nucleus.

Targeted disruption of Aurora A causes abnormal mitotic spindle assembly, chromosome misalignment and embryonic lethality

Aurora A (also known as STK15/BTAK in humans), a putative oncoprotein naturally overexpressed in many human cancers, is a member of the conserved Aurora protein serine/threonine kinase family that is implicated in the regulation of G(2)-M phases of the cell cycle. In vitro studies utilizing antibody microinjection, siRNA silencing and small molecule inhibitors have indicated that Aurora A functions in early as well as late stages of mitosis. However, due to limitations in specificity of the techniques, exact functional roles of the kinase remain to be clearly elucidated. In order to identify the physiological functions in vivo, we have generated Aurora A null mouse embryos, which show severe defects at 3.5 d.p.c. (days post-coitus) morula/blastocyst stage and lethality before 8.5 d.p.c. Null embryos at 3.5 d.p.c. reveal growth retardation with cells in mitotic disarray manifesting disorganized spindle, misaligned and lagging chromosomes as well as micronucleated cells. These findings provide the first unequivocal genetic evidence for an essential physiological role of Aurora A in normal mitotic spindle assembly, chromosome alignment segregation and maintenance of viability in mammalian embryos.

NanoLC-MS/MS analysis provides new insights into the phosphorylation pattern of Cdc25B in vivo: full overlap with sites of phosphorylation by Chk1 and Cdk1/cycB kinases in vitro

NanoLC-MS/MS analysis was used to characterize the phosphorylation pattern in vivo of CDC25B3 (phosphatase splice variant 1) expressed in a human cell line and to compare it to the phosphorylation of CDC25B3 by Cdk1/cyclin B and Chk1 in vitro. Cellular CDC25B3 was purified from U2OS cells conditionally overexpressing the phosphatase. Eighteen sites were detectably phosphorylated in vivo. Nearly all existing (S/T)P sites were phosphorylated in vivo and in vitro. Eight non(S/T)P sites were phosphorylated in vivo. All these sites could be phosphorylated by kinase Chk1, which phosphorylated a total of 11 sites in vitro, with consensus sequence (R/K) X(2-3) (S/P)-non P. Nearly half of the sites identified in this study were not previously described and were not homologous to sites reported to be phosphorylated in other CDC25 species. We also show that in vivo a significant part of CDC25B molecules can be hyperphosphorylated, with up to 13 phosphates per phosphatase molecule.

G2/M checkpoint stringency is a key parameter in the sensitivity of AML cells to genotoxic stress

Acute myeloid leukemia (AML) cells exposed to genotoxic agents arrest their cell cycle at the G2/M checkpoint and are inherently chemoresistant. To understand the mechanism of this chemoresistance, we compared the ability of immature CD34+ versus mature CD34- AML cell lines (KG1a and U937, respectively) to recover from a DNA damage-induced cell cycle checkpoint in G2. Here, we report that KG1a cells have a more stringent G2/M checkpoint response than U937 cells. We show that in both cell types, the CDC25B phosphatase participates in the G2/M checkpoint recovery and that its expression is upregulated. Furthermore, we show that CHK1 inhibition by UCN-01 in immature KG1a cells allows checkpoint exit and induces sensitivity to genotoxic agents. Similarly, UCN-01 treatment potentializes genotoxic-induced inhibition of colony formation efficiency of primary leukemic cells from AML patients. Altogether, our results demonstrate that checkpoint stringency varies during the maturation process and indicate that targeting checkpoint mechanisms might represent an attractive therapeutic opportunity for chemoresistant immature AML cells.