Cell Mitotic Arrest Research Articles

Mitotic Arrest in Teratoma Susceptible Fetal Male Germ Cells

Formation of germ cell derived teratomas occurs in mice of the 129/SvJ strain, but not in C57Bl/6 inbred or CD1 outbred mice. Despite this, there have been few comparative studies aimed at determining the similarities and differences between teratoma susceptible and non-susceptible mouse strains. This study examines the entry of fetal germ cells into the male pathway and mitotic arrest in 129T2/SvJ mice. We find that although the entry of fetal germ cells into mitotic arrest is similar between 129T2/SvJ, C57Bl/6 and CD1 mice, there were significant differences in the size and germ cell content of the testis cords in these strains. In 129T2/SvJ mice germ cell mitotic arrest involves upregulation of p27KIP1, p15INK4B, activation of RB, the expression of male germ cell differentiation markers NANOS2, DNMT3L and MILI and repression of the pluripotency network. The germ-line markers DPPA2 and DPPA4 show reciprocal repression and upregulation, respectively, while FGFR3 is substantially enriched in the nucleus of differentiating male germ cells. Further understanding of fetal male germ cell differentiation promises to provide insight into disorders of the testis and germ cell lineage, such as testis tumour formation and infertility.

USP44 Is Highly Over-Expresed In T-Cell Acute Lymphoblastic Leukemia and Leads to the Development of Chromosome Missegregation and Aneupoloidy

AbstractAbstract 29Acute lymphoblastic leukemia (ALL) cells are frequently characterized by abnormal numbers of chromosomes, a condition known as aneuploidy. Aneuploidy in ALL is not just a curiosity, but impacts on the prognosis and risk stratification in children. Additionally, aneuploidy itself contributes to the development of cancer by dysregulating gene expression, and driving the loss of heterozygosity of mutated tumor suppressor genes. The molecular underpinnings of aneuploidy in ALL are largely unknown. The mitotic checkpoint is a complex pathway that acts in mitosis to prevent the premature onset of anaphase that leads to improper segregation of chromosomes and aneuploidy. Through the concerted action of many intermediates, the mitotic checkpoint functions to inhibit the activity of a large ubiquitin E3 ligase complex known as the anaphase promoting complex/cyclosome (APC/C) that drives the separation of paired sister chromatids by effecting the ubiquitin-dependent degradation of cyclin B1 and securin. The de-ubiquitinating enzyme USP44 was recently identified as an important regulator of mitotic progression that assists in maintaining mitotic checkpoint signaling until all chromosomes are properly attached to the mitotic spindle. Microarray studies of childhood acute leukemias have observed high levels of USP44 in T-cell ALL, but the significance of this observation is unclear. Here we show that USP44 is indeed over-expressed in T-ALL in children and that its presence may contribute to aneuploidy that is frequently observed in this disease. We compared the expression of USP44 in a series of 24 samples from patients with T-ALL to that found in ten samples of peripheral T-cells isolated from healthy donors, using quantitative real-time PCR. There was a dramatic increase in the expression of USP44 in T-ALL with 18 out of 24 cases having at least 3-fold increase, with an average of 16-fold increase, in USP44 mRNA. We next studied the consequences of USP44 over-expression in primary cells using live-cell microscopy. We find that excess USP44 leads to dramatic and significant increase in chromosome missegregation with most defects involving lagging chromosomes due to improper chromosome attachment to the mitotic spindle. We observe that these mitotic errors lead to the development of aneuploidy, through a mechanism that requires USP44 catalytic activity. These effects are accompanied by reinforced mitotic checkpoint signaling that leads to delayed progression through mitosis, and a prolonged duration of mitotic arrest in cells exposed to spindle poisons such as nocodazole or paclitaxel. At the molecular level, we observe that excess USP44 leads to enhanced binding of the mitotic checkpoint effector Mad2 to the APC/C and increased levels of cyclin B1 in early mitosis. Taken together, we conclude that USP44 is a chromosome instability gene that is frequently expressed at high levels in T-ALL cells, and that this may contribute to the pathogenesis of T-ALL by leading to chromosome shuffling and aneuploidy. These data have important implications in our understanding of the pathogenesis of this disease and may contribute to understanding the therapeutic effectiveness of novel aurora inhibitors in cancer. Disclosures:No relevant conflicts of interest to declare.

Up-regulation of stathmin induces growth arrest of esophageal squamous cell carcinoma EC9706 cell

To construct a eukaryotic expression vector of stathmin gene and investigate its effect on esophageal squamous cell carcinoma (ESCC) EC9706 cell line. Stathmin cDNA coding sequence was amplified by RT-PCR and cloned into a eukaryotic expression vector pcDNA3.1(+). EC9706 cells were transfected with this recombinant plasmid pcDNA3.1-stathmin by lipofectamine. And the stable transfectants were selected with G418 medium. Stathmin protein expression was detected with Western blot in transfected EC9706 cell lines. Morphologic change of stable transfectants was observed under microscope. The proliferation of transfected cells was measured by cell counting, MTT and in vitro formation assay of flat. Flow cytometry was used to detect the cell cycle. Nude mice were adopted to investigate the in vivo tumorigenic characteristics of the transfected cells. A 450 bp coding sequence of stathmin cDNA was amplified by RT-PCR and then cloned into pcDNA3.1(+) plasmid to harvest the recombinant plasmid pcDNA3.1-stathmin. The recombinant plasmid pcDNA3.1-stathmin and blank vector were transfected respectively into EC9706 cells. The up-regulated expression of stathmin protein was validated by Western blot (P < 0.01). Compared with control, EC9706 cells transfected with pcDNA3.1-stathmin appeared swollen and multi-nuclear with a cell mitotic arrest; doubling generation time of pcDNA3.1-stathmin transfectants was prolonged (25 - 28 h); The in vitro cell proliferation ability and clone formation rate (34.5% ± 6.9%) decreased, cell cleavage was blocked at G(2)/M phase (21.7% ± 3.4%) and the oncogenicity of inoculated cells in nude mice decreased (all P < 0.01). The up-regulated expression of stathmin protein triggered by the recombinant plasmid pcDNA3.1-stathmin can inhibit the proliferation and oncogenicity of ESCC EC9706 cells. This molecule may be a promising therapeutic target in ESCC patients.

Discovery of tetrahydro-β-carbolines as inhibitors of the mitotic kinesin KSP

Inhibitors of kinesin spindle protein (KSP) are a promising class of anticancer agents that cause mitotic arrest in cells from a failure to form functional bipolar mitotic spindles. Here, we report the synthesis and biological evaluation of a novel series of tetrahydro-β-carboline analogs based on the structure of the known KSP inhibitor HR22C16. Preferred compounds 11b, 12a and 19b were identified as potent inhibitors in a KSP ATPase assay with good anti-proliferative activity in A549 cells.

Natural Product Celastrol Destabilizes Tubulin Heterodimer and Facilitates Mitotic Cell Death Triggered by Microtubule-Targeting Anti-Cancer Drugs

BackgroundMicrotubule drugs are effective anti-cancer agents, primarily due to their ability to induce mitotic arrest and subsequent cell death. However, some cancer cells are intrinsically resistant or acquire a resistance. Lack of apoptosis following mitotic arrest is thought to contribute to drug resistance that limits the efficacy of the microtubule-targeting anti-cancer drugs. Genetic or pharmacological agents that selectively facilitate the apoptosis of mitotic arrested cells present opportunities to strengthen the therapeutic efficacy.Methodology and Principal FindingsWe report a natural product Celastrol targets tubulin and facilitates mitotic cell death caused by microtubule drugs. First, in a small molecule screening effort, we identify Celastrol as an inhibitor of neutrophil chemotaxis. Subsequent time-lapse imaging analyses reveal that inhibition of microtubule-mediated cellular processes, including cell migration and mitotic chromosome alignment, is the earliest events affected by Celastrol. Disorganization, not depolymerization, of mitotic spindles appears responsible for mitotic defects. Celastrol directly affects the biochemical properties of tubulin heterodimer in vitro and reduces its protein level in vivo. At the cellular level, Celastrol induces a synergistic apoptosis when combined with conventional microtubule-targeting drugs and manifests an efficacy toward Taxol-resistant cancer cells. Finally, by time-lapse imaging and tracking of microtubule drug-treated cells, we show that Celastrol preferentially induces apoptosis of mitotic arrested cells in a caspase-dependent manner. This selective effect is not due to inhibition of general cell survival pathways or mitotic kinases that have been shown to enhance microtubule drug-induced cell death.Conclusions and SignificanceWe provide evidence for new cellular pathways that, when perturbed, selectively induce the apoptosis of mitotic arrested cancer cells, identifying a potential new strategy to enhance the therapeutic efficacy of conventional microtubule-targeting anti-cancer drugs.

The Transcriptional Repressor Kaiso Localizes at the Mitotic Spindle and Is a Constituent of the Pericentriolar Material

Kaiso is a BTB/POZ zinc finger protein known as a transcriptional repressor. It was originally identified through its in vitro association with the Armadillo protein p120ctn. Subcellular localization of Kaiso in cell lines and in normal and cancerous human tissues revealed that its expression is not restricted to the nucleus. In the present study we monitored Kaiso's subcellular localization during the cell cycle and found the following: (1) during interphase, Kaiso is located not only in the nucleus, but also on microtubular structures, including the centrosome; (2) at metaphase, it is present at the centrosomes and on the spindle microtubules; (3) during telophase, it accumulates at the midbody. We found that Kaiso is a genuine PCM component that belongs to a pericentrin molecular complex. We analyzed the functions of different domains of Kaiso by visualizing the subcellular distribution of GFP-tagged Kaiso fragments throughout the cell cycle. Our results indicate that two domains are responsible for targeting Kaiso to the centrosomes and microtubules. The first domain, designated SA1 for spindle-associated domain 1, is located in the center of the Kaiso protein and localizes at the spindle microtubules and centrosomes; the second domain, SA2, is an evolutionarily conserved domain situated just before the zinc finger domain and might be responsible for localizing Kaiso towards the centrosomal region. Constructs containing both SA domains and Kaiso's aminoterminal BTB/POZ domain triggered the formation of abnormal centrosomes. We also observed that overexpression of longer or full-length Kaiso constructs led to mitotic cell arrest and frequent cell death. Knockdown of Kaiso accelerated cell proliferation. Our data reveal a new target for Kaiso at the centrosomes and spindle microtubules during mitosis. They also strongly imply that Kaiso's function as a transcriptional regulator might be linked to the control of the cell cycle and to cell proliferation in cancer.

Sexually dimorphic gene expression of the Zimp7 and Zimp10 genes in embryonic gonads

Members of the PIAS (protein inhibitor of activated STAT) family perform essential functions in modulating the activity of transcriptional regulators. Zimp7 and Zimp10 are two proteins that together form a subfamily of the PIAS. Like the other members of this family, they contain the zinc-binding SP-RING/Miz domain, which confers SUMO-conjugating activity. Both proteins have been shown to stimulate androgen receptor-mediated transcription. Previously, we reported that both Zimp7 and Zimp10 genes are extensively expressed and dynamically regulated in the developing mouse embryo. In this work, we investigated the expression of these genes during gonadal development. We found that their expression is sex-specific. Both genes initiate their transcription at early stages in the embryonic male gonad, reaching their peak at 13.5 days post coitum, which coincides with the process of sex-specific germ cell mitotic arrest. Zimp7 is expressed in germ cells of the embryonic gonad and the adult testis. Immunofluorescence of spermatogenic cells revealed that Zimp7 protein localizes to nuclear territories in meiotic spermatocytes, including the XY bodies. On the other hand, Zimp10 is found in somatic cells, outside the testis cords and ceases to be expressed in the adult testis.

A Versatile Viral System for Expression and Depletion of Proteins in Mammalian Cells

The ability to express or deplete proteins in living cells is crucial for the study of biological processes. Viral vectors are often useful to deliver DNA constructs to cells that are difficult to transfect by other methods. Lentiviruses have the additional advantage of being able to integrate into the genomes of non-dividing mammalian cells. However, existing viral expression systems generally require different vector backbones for expression of cDNA, small hairpin RNA (shRNA) or microRNA (miRNA) and provide limited drug selection markers. Furthermore, viral backbones are often recombinogenic in bacteria, complicating the generation and maintenance of desired clones. Here, we describe a collection of 59 vectors that comprise an integrated system for constitutive or inducible expression of cDNAs, shRNAs or miRNAs, and use a wide variety of drug selection markers. These vectors are based on the Gateway technology (Invitrogen) whereby the cDNA, shRNA or miRNA of interest is cloned into an Entry vector and then recombined into a Destination vector that carries the chosen viral backbone and drug selection marker. This recombination reaction generates the desired product with >95% efficiency and greatly reduces the frequency of unwanted recombination in bacteria. We generated Destination vectors for the production of both retroviruses and lentiviruses. Further, we characterized each vector for its viral titer production as well as its efficiency in expressing or depleting proteins of interest. We also generated multiple types of vectors for the production of fusion proteins and confirmed expression of each. We demonstrated the utility of these vectors in a variety of functional studies. First, we show that the FKBP12 Destabilization Domain system can be used to either express or deplete the protein of interest in mitotically-arrested cells. Also, we generate primary fibroblasts that can be induced to senesce in the presence or absence of DNA damage. Finally, we determined that both isoforms of the AT-Rich Interacting Domain 4B (ARID4B) protein could induce G1 arrest when overexpressed. As new technologies emerge, the vectors in this collection can be easily modified and adapted without the need for extensive recloning.

Diallyl trisulfide‐induced apoptosis in human cancer cells is linked to checkpoint kinase 1‐mediated mitotic arrest

Growth suppressive effect of diallyl trisulfide (DATS), a promising cancer chemopreventive constituent of garlic, against cultured human cancer cells correlates with checkpoint kinase 1 (Chk1)-mediated mitotic arrest, but the fate of the cells arrested in mitosis remains elusive. Using LNCaP and HCT-116 human cancer cells as a model, we now demonstrate that the Chk1-mediated mitotic arrest resulting from DATS exposure leads to apoptosis. The DATS exposure resulted in G(2) phase and mitotic arrest in both LNCaP and HCT-116 cell lines. The G(2) arrest was accompanied by downregulation of cyclin-dependent kinase 1 (Cdk1), cell division cycle (Cdc) 25B, and Cdc25C leading to Tyr15 phosphorylation of Cdk1 (inactivation). The DATS-mediated mitotic arrest correlated with inactivation of anaphase-promoting complex/cyclosome as evidenced by accumulation of its substrates cyclinB1 and securin. The DATS treatment increased activating phosphorylation of Chk1 (Ser317) and transient transfection with Chk1-targeted siRNA conferred significant protection against DATS-induced mitotic arrest in both cell lines. The Chk1 protein knockdown also afforded partial yet statistically significant protection against apoptotic DNA fragmentation and caspase-3 activation resulting from DATS exposure in both LNCaP and HCT-116 cells. Even though DATS treatment resulted in stabilization and Ser15 phosphorylation of p53, the knockdown of p53 protein failed to rescue DATS-induced mitotic arrest. In conclusion, the results of the present study indicate that Chk1 dependence of DATS-induced mitotic arrest in human cancer cells is not influenced by the p53 status and cells arrested in mitosis upon DATS exposure are driven to apoptotic DNA fragmentation.

K858, a Novel Inhibitor of Mitotic Kinesin Eg5 and Antitumor Agent, Induces Cell Death in Cancer Cells

The aim of this study was to investigate the mechanism of inhibition of Eg5 (kinesin spindle protein), a mitotic kinesin that plays an essential role in establishing mitotic spindle bipolarity, by the novel small molecule inhibitor K858. K858 was selected in a phenotype-based forward chemical genetics screen as an antimitotic agent, and subsequently characterized as an inhibitor of Eg5. K858 blocked centrosome separation, activated the spindle checkpoint, and induced mitotic arrest in cells accompanied by the formation of monopolar spindles. Long-term continuous treatment of cancer cells with K858 resulted in antiproliferative effects through the induction of mitotic cell death, and polyploidization followed by senescence. In contrast, treatment of nontransformed cells with K858 resulted in mitotic slippage without cell death, and cell cycle arrest in G(1) phase in a tetraploid state. In contrast to paclitaxel, K858 did not induce the formation of micronuclei in either cancer or nontransformed cells, suggesting that K858 has minimal effects on abnormalities in the number and structure of chromosomes. K858 exhibited potent antitumor activity in xenograft models of cancer, and induced the accumulation of mitotic cells with monopolar spindles in tumor tissues. Importantly, K858, unlike antimicrotubule agents, had no effect on microtubule polymerization in cell-free and cell-based assays, and was not neurotoxic in a motor coordination test in mice. Taken together, the Eg5 inhibitor K858 represents an important compound for further investigation as a novel anticancer therapeutic.

Arsenic trioxide suppresses paclitaxel‐induced mitotic arrest

To understand if there exists a functional interaction between arsenic trioxide and paclitaxel in vitro. HeLa and HCT116 (rho53(+/+) and rho53(-/-)) cells were treated with As2O3 and/or paclitaxel for various times. Treated cells were collected for analyses using a combination of flow cytometry, fluorescence microscopy and Western blotting. Because As(2)O(3) is capable of inhibiting tubulin polymerization and inducing mitotic arrest, we examined whether there existed any functional interaction between As(2)O(3) and paclitaxel, a well-known microtubule poison. Flow cytometry and fluorescence microscopy revealed that although As(2)O(3) alone caused a moderate level of mitotic arrest, it greatly attenuated paclitaxel-induced mitotic arrest in cells with p53 deficiency. Western blot analysis showed that As(2)O(3) significantly blocked phosphorylation of BubR1, Cdc20, and Cdc27 in cells treated with paclitaxel, suggesting that arsenic compromised the activation of the spindle checkpoint. Our further studies revealed that the attenuation of paclitaxel-induced mitotic arrest by As(2)O(3) resulted primarily from sluggish cell cycle progression at S phase but not enhanced mitotic exit. The observations that As(2)O(3) has a negative impact on the cell cycle checkpoint activation by taxol should have significant clinical implications because the efficacy of taxol in the clinics is associated with its ability to induce mitotic arrest and subsequent mitotic catastrophe.

SIRT2 downregulation confers resistance to microtubule inhibitors by prolonging chronic mitotic arrest

We previously identified SIRT2, a deacetylase for tubulin and histone H4, as a protein down-regulated in gliomas, and reported that exogenously-expressed SIRT2 arrests the cell cycle prior to entry into mitosis to prevent chromosomal instability in response to microtubule inhibitors (MTIs) such as nocodazole, characteristics previously reported for the CHFR protein. We herein investigated the effects of SIRT2 downregulation on sensitivity to MTIs using HCT116 cells, a mitotic checkpoint-proficient near-diploid cancer cell line used for studying checkpoints. We found that SIRT2 downregulation confers resistance to MTIs as well as that of BubR1, a well-characterized mitotic checkpoint protein, though by a different mechanism. While BubR1 suppression abolished spindle checkpoint functions, which is a requirement for cell death after release from the spindle checkpoint, SIRT2 downregulation prolonged chronic mitotic arrest from sustained activation of the mitotic checkpoint and consequently prevented a shift to secondary outcomes, including cell death, after release from chronic mitotic arrest. Consistent with this notion, BubR1 downregulation was dominant over SIRT2 knockdown in regard to mitotic regulation in the presence of nocodazole. These results suggest that SIRT2 functions to release chronic mitotic arrest in cells treated with MTIs, leading to other outcomes. We also found that SIRT2 downregulation caused centrosome fragmentation in response to nocodazole prior to the alteration in spindle checkpoint function, implying not only a novel function of SIRT2 for centrosome maintenance upon exposure to mitotic stress caused by MTIs, but also the existence of a centrosome-mediated signaling pathway to sustain the spindle checkpoint. Therefore, this study highlights a novel pathway leading to resistance to MTIs, in which SIRT2 downregulation participates.

9‐bromonoscapine‐induced mitotic arrest of cigarette smoke condensate‐transformed breast epithelial cells

In the present investigation, we determined the chemotherapeutic efficacy of 9-bromonoscapine (Br-Nos), a more potent noscapine analog, on MCF10A, spontaneously immortalized human normal breast epithelial cells and MCF10A-CSC3, cigarette smoke condensate (CSC)-transformed cells. The results from cytogenetic analysis showed that Br-Nos induced polyploidy and telomeric association in MCF10A-CSC3 cells, while MCF10A cells remained unaffected. Our immunofluorescence data further demonstrated that MCF10A-CSC3 cells were susceptible to mitotic catastrophe on exposure to Br-Nos and failed to recover after drug withdrawal. MCF10A-CSC3 cells exhibited Br-Nos-induced aberrant multipolar spindle formation, which irreversibly impaired the alignment of replicated chromosome to the equatorial plane and finally culminated in cell death. Although MCF10A cells upon Br-Nos treatment showed bipolar spindles with some uncongressed chromosomes, these cells recovered fairly well after drug withdrawal. Our flow-cytometry analysis data reconfirmed that MCF10A-CSC3 cells were more susceptible to cell death compared to MCF10A cells. Furthermore, our results suggest that decreased levels of cdc2/cyclin B1 and cdc2 kinase activity are responsible for Br-Nos-induced mitotic cell arrest leading to cell death in MCF10A-CSC3 cells. This study thus explores the underlying mechanism of Br-Nos-induced mitotic catastrophe in CSC-transformed MCF10A-CSC3 cells and its potential usefulness as a chemotherapeutic agent for prevention of cigarette smoke-induced breast cancer growth.

AIP regulates stability of Aurora-A at early mitotic phase coordinately with GSK-3β

Glycogen synthase kinase-3 (GSK-3beta) regulates microtubule dynamics and cellular polarity through phosphorylating various microtubule associating proteins and plus-end tracking proteins. Although it was also reported that GSK-3beta is inactivated by protein kinase B at the spindle poles, functions and targets of GSK-3beta in the mitotic phase are unknown. Here, we identified Aurora-A-interacting protein (AIP), a negative regulator of Aurora-A, as a binding partner of GSK-3beta. AIP was colocalized with Aurora-A and GSK-3beta to the spindle poles in metaphase, and its depletion in cells stabilized and activated Aurora-A in early mitotic phase and caused mitotic cell arrest. Treatment of the cells with a GSK-3beta inhibitor reduced the protein level of Aurora-A and this reduction was suppressed by AIP knockdown. AIP was phosphorylated by GSK-3beta, and an AIP mutant in which the GSK-3beta phosphorylation site was mutated could bind and downregulate Aurora-A more efficiently. These results suggest that GSK-3beta modulates the early mitotic Aurora-A level through binding and phosphorylating AIP.

Retinoic acid prevents germ cell mitotic arrest in mouse fetal testes

During mouse fetal development, meiosis is initiated in female germ cells only, with male germ cells undergoing mitotic arrest. Retinoic acid (RA) is degraded by Cyp26b1 in the embryonic testis but not in the ovary where it initiates the mitosis/meiosis transition. However the role of RA status in fetal germ cell proliferation has not been elucidated. As expected, using organ cultures, we observed that addition of RA in 11.5 days post-conception (dpc) testes induced Stra8 expression and meiosis. Surprisingly, in 13.5 dpc testes although RA induced Stra8 expression it did not promote meiosis. On 11.5 and 13.5 dpc, RA prevented male germ cell mitotic arrest through PI3K signaling. Therefore 13.5 dpc testes appeared as an interesting model to investigate RA effects on germ cell proliferation/differentiation independently of RA effect on the meiosis induction. At this stage, RA delayed SSEA-1 extinction, p63γ expression and DNA hypermethylation which normally occur in male mitotic arrested germ cells. In vivo, in the fetal male gonad, germ cells cease their proliferation and loose SSEA-1 earlier than in female gonad and RA administration maintained male germ cell proliferation. Lastly, inhibition of endogenous Cyp26 activity in 13.5 dpc cultured testes also prevented male germ cell mitotic arrest. Our data demonstrate that the reduction of RA levels, which occurs specifically in the male fetal gonad and was known to block meiosis initiation, is also necessary to allow the establishment of the germ cell mitotic arrest and the correct further differentiation of the fetal germ cells along the male pathway.

Defective in Mitotic Arrest 1/Ring Finger 8 Is a Checkpoint Protein That Antagonizes the Human Mitotic Exit Network

A molecular pathway homologous to the S. cerevisiae mitotic exit network (MEN) and S. pombe septation initiation network has recently been described in higher eukaryotes and involves the tumor suppressor kinase LATS1 and its subunit MOB1A. The yeast MEN/septation initiation network pathways are regulated by the ubiquitin ligase defective in mitotic arrest 1 (Dma1p), a checkpoint protein that helps maintain prometaphase arrest when cells are exposed to microtubule poisons. We identified here the RING domain protein ring finger 8 (RNF8) as the human orthologue of the yeast protein Dma1p. Like its yeast counterparts, human DMA1/RNF8 localized at the midbody and its depletion by siRNA compromised mitotic arrest of nocodazole-treated cells in a manner dependent on the MEN. Depletion of MAD2, a spindle checkpoint protein, also compromised mitotic arrest, but in a MEN-independent manner. Thus, two distinct checkpoint pathways maintain mitotic arrest in cells exposed to microtubule poisons.

The Plk-1 Inhibitor BI 2536 Counteracts the Growth of Neoplastic Mast Cells and Synergizes with the KIT D816V-Targeting Drug Midostaurin (PKC412) in Producing Growth-Inhibition.

Systemic mastocytosis (SM) is a myeloid neoplasm characterized by abnormal growth and accumulation of mast cells (MC) in various internal organs. In most patients, the D816V-mutated variant of c-KIT, which mediates resistance against several tyrosine kinase (TK) inhibitors like imatinib, is found. In advanced SM, the response of neoplastic MC to conventional drugs is poor and the prognosis is grave. Therefore current research is attempting to identify novel targets in neoplastic MC. Polo-like kinase 1 (Plk-1) is a serine/threonine kinase that plays an essential role in mitosis and has recently been introduced as a new target in myeloid leukemias. In the present study, we analyzed expression and function of Plk-1 in neoplastic human MC, and asked whether Plk-1 can serve as a target of therapy in SM. As determined by immunohistochemistry, primary neoplastic MC were found to display activated/phosphorylated Plk-1 in all patients examined (n=5). The human MC leukemia cell line HMC-1 was also found to exhibit activated Plk-1. In addition, we found that primary neoplastic MC as well as HMC-1 cells express Plk-1 mRNA in RT-PCR experiments. As assessed by 3H-thymidine-uptake experiments, the Plk-1-targeting drug BI 2536 (Boehringer Ingelheim GmbH, Germany) was found to inhibit the proliferation of HMC-1 cells in a dose-dependent manner (IC50 5–15 nM). The effect of BI 2536 was seen in both subclones of HMC-1, i.e. in HMC-1.1 cells displaying KIT G560V (but not KIT D816V), and HMC-1.2 cells exhibiting both KIT G560V and KIT D816V, with comparable IC50 values. Moreover, BI 2536 was found to inhibit the proliferation of primary neoplastic cells, with IC50 values ranging between 5 and 50 nM. The growth-inhibitory effects of BI 2536 on HMC-1 cells were found to be associated with mitotic arrest and G2-M cell cycle arrest as well as consecutive apoptosis. In normal bone marrow or peripheral blood mononuclear cells, neither mitotic cell arrest nor apoptosis were observed after treatment with BI 2536. In a consecutive phase of the study, we asked whether combined targeting of KIT D816V and Plk-1 would lead to synergistic drug-interactions. For this purpose, HMC-1 cells and primary neoplastic MC were coincubated with BI 2536 and midostaurin (PKC412), a multitargeted kinase inhibitor that blocks KIT D816V TK activity. In these experiments, BI 2536 was found to synergize with midostaurin in counteracting the proliferation of HMC-1 cells and primary neoplastic MC. In conclusion, our data show that activated Plk-1 is detectable in MC neoplasms and plays a role in cell cycle progression and viability of neoplastic MC. Targeting of Plk-1 with BI 2536 leads to growth inhibition and apoptosis in neoplastic MC. Furthermore, BI 2536 synergizes with midostaurin in counteracting growth of neoplastic MC. Targeting of Plk-1 may be an attractive new pharmacologic concept in advanced SM.

The Plk-1 Inhibitor BI 2536 Counteracts Proliferation and Viability of CML Cells and Synergizes with Imatinib and Nilotinib (AMN107) in Producing Growth Inhibition.

Chronic myeloid leukemia (CML) is a hematopoietic stem cell disease in which BCR/ABL enhances growth and survival of leukemic cells. In most patients, the disease can be kept under control by the BCR/ABL tyrosine kinase inhibitor imatinib (STI571; Novartis Basel, Switzerland). However, resistance or intolerance against imatinib may occur during therapy. Therefore, current research is focusing on novel targets and targeted drugs in CML. Polo-like kinase 1 (Plk-1) is a serine/threonine kinase that plays an essential role in mitosis and is expressed in activated/phosphorylated form in various malignancies including acute myeloid leukemia (AML). BI 2536 (Boehringer Ingelheim GmbH, Germany) is a novel selective inhibitor of Plk-1, that is currently tested in AML-trials. In this study, we have evaluated expression and the potential role of Plk-1 as a novel target in CML cells. As assessed by PCR, Plk-1 mRNA was found to be expressed abundantly in primary CML cells and in the CML cell line K562, whereas normal peripheral blood cells did not express detectable levels of Plk-1 mRNA. The Plk-1 protein was detected in primary CML cells and K562 cells by immunocytochemistry. In consecutive experiments, we were able to show that CML cells display phosphorylated Plk-1. As assessed by 3H-thymidine-uptake experiments, BI 2536 was found to inhibit the proliferation of K562 cells in a dose-dependent manner (IC50 5–15 nM). Moreover, BI 2536 was found to inhibit the proliferation of both imatinib-naive (n=6) and imatinib-resistant (n=3) primary CML cells (IC50: 1–15 nM). The growth-inhibitory effect of BI 2536 on CML cells was found to be associated with mitotic arrest, a G2-M cell cycle arrest, and consecutive apoptosis. In normal bone marrow or peripheral blood mononuclear cells, neither mitotic cell arrest nor apoptosis were observed after exposure to BI 2536. In further experiments, primary CML cells were coincubated with BI 2536 plus imatinib or with BI 2536 plus nilotinib (AMN107; Novartis) at fixed ratio of drug concentrations. In these experiments, BI 2536 was found to synergize with both tyrosine kinase inhibitors in counteracting the proliferation of CML cells. In conclusion, our data show that Plk-1 is expressed in activated form in CML cells and plays a role in cell cycle progression and cell viability. Targeting Plk-1 with BI 2536 leads to mitotic arrest, growth inhibition, and apoptosis in imatinib-naive and imatinib-resistant leukemic cells. Moreover, BI 2536 synergizes with imatinib and nilotinib in counteracting the growth of neoplastic cells in CML. Targeting of Plk-1 may be a novel interesting pharmacologic approach to counteract growth of CML cells.

Polo-like kinase 1 regulates mitotic arrest after UV irradiation through dephosphorylation of p53 and inducing p53 degradation

Ultraviolet (UV) irradiation can result in cell cycle arrest. The reactivation of Polo-like kinase 1 (Plk1) is necessary for cell cycle reentry. But the mechanism of how Plk1 regulates p53 in UV-induced mitotic arrest cells remained elusive. Here we find that UV treatment leads HEK293 cells to inverse changes of Plk1 and p53. Over-expression of Plk1 rescue UV-induced mitotic arrest cells by inhibiting p53 activation. Plk1 could also inhibit p53 phosphorylation at Ser15, thus facilitates its nuclear export and degradation. Further examination shows that Plk1, p53 and Cdc25C can form a large complex. Plk1 could bind to the sequence-specific DNA-binding domain of p53 and active Cdc25C by hyperphosphorylation. These results hypothesize that Plk1 and Cdc25C participate in recovery the mitotic arrest through binding to the different domain of p53. Cdc25C may first be actived by Plk1, and then its phosphatase activity makes p53 dephosphorylated at Ser15.

Transcriptional silencing of the mouse mammary tumor virus promoter through chromatin remodeling is concomitant with histone H1 phosphorylation and histone H3 hyperphosphorylation at M phase

We examined histone phosphorylation and their effects on glucocorticoid receptor (GR)-mediated activation of the mouse mammary tumor virus promoter (MMTV) in synchronized cells. In vivo protein expression studies suggest that both histones H1 and H3 are highly phosphorylated in mitotic-arrested cells in which GR is unable to remodel chromatin and recruit transcription factor NF1 to the promoter. Postmitotic cells show an open chromatin structure and efficient binding of NF1 to the promoter accompanied by reversing histone H1 and H3 phosphorylation level. In contrast, the acetylation status of histone H3 and H4 did not change in either condition. These results suggest that hyperphosphorylation of histone H1 and H3 leads to inhibition of GR-mediated chromatin remodeling and inactivation of MMTV by preventing the association of transcription factors to the promoter in vivo.