Proteins required for centrosome clustering in cancer cells

Failure of cell cleavage induces senescence in tetraploid primary cells

Induction of tetraploidy through cleavage failure induces G1 arrest and senescence in primary mammalian cells but not in immortal cells. Induction of senescence occurs without DNA damage, and the capacity to become senescent appears to be a prerequisite of tetraploid arrest.

Tetraploidy can arise from various mitotic or cleavage defects in mammalian cells, and inheritance of multiple centrosomes induces aneuploidy when tetraploid cells continue to cycle. Arrest of the tetraploid cell cycle is therefore potentially a critical cellular control. We report here that primary rat embryo fibroblasts (REF52) and human foreskin fibroblasts become senescent in tetraploid G1 after drug- or small interfering RNA (siRNA)-induced failure of cell cleavage. In contrast, T-antigen–transformed REF52 and p53+/+ HCT116 tumor cells rapidly become aneuploid by continuing to cycle after cleavage failure. Tetraploid primary cells quickly become quiescent, as determined by loss of the Ki-67 proliferation marker and of the fluorescent ubiquitination-based cell cycle indicator/late cell cycle marker geminin. Arrest is not due to DNA damage, as the γ-H2AX DNA damage marker remains at control levels after tetraploidy induction. Arrested tetraploid cells finally become senescent, as determined by SA-β-galactosidase activity. Tetraploid arrest is dependent on p16INK4a expression, as siRNA suppression of p16INK4a bypasses tetraploid arrest, permitting primary cells to become aneuploid. We conclude that tetraploid primary cells can become senescent without DNA damage and that induction of senescence is critical to tetraploidy arrest.

Identification of novel small molecule inhibitors of centrosome clustering in cancer cells

Most normal cells have two centrosomes that form bipolar spindles during mitosis, while cancer cells often contain more than two, or "supernumerary" centrosomes. Such cancer cells achieve bipolar division by clustering their centrosomes into two functional poles, and inhibiting this process then leads to cancer-specific cell death. A major problem with clinically used anti-mitotic drugs, such as paclitaxel, is their toxicity in normal cells. To discover new compounds with greater specificity for cancer cells, we established a high-content screen for agents that block centrosome clustering in BT-549 cells, a breast cancer cell line that harbors supernumerary centrosomes. Using this screen, we identified 14 compounds that inhibit centrosome clustering and induce mitotic arrest. Some of these compounds were structurally similar, suggesting a common structural motif important for preventing centrosome clustering. We next compared the effects of these compounds on the growth of several breast and other cancer cell lines, an immortalized normal human mammary epithelial cell line, and progenitor-enriched primary normal human mammary epithelial cells. From these comparisons, we found some compounds that kill breast cancer cells, but not their normal epithelial counterparts, suggesting their potential for targeted therapy. One of these compounds, N2-(3-pyridylmethyl)-5-nitro-2-furamide (Centrosome Clustering Chemical Inhibitor-01, CCCI-01), that showed the greatest differential response in this screen was confirmed to have selective effects on cancer as compared to normal breast progenitors using more precise apoptosis induction and clonogenic growth endpoints. The concentration of CCCI-01 that killed cancer cells in the clonogenic assay spared normal human bone marrow hematopoietic progenitors in the colony-forming cell assay, indicating a potential therapeutic window for CCCI-01, whose selectivity might be further improved by optimizing the compound. Immunofluorescence analysis showed that treatment with CCCI-01 lead to multipolar spindles in BT-549, while maintaining bipolar spindles in the normal primary human mammary epithelial cells. Since centrosome clustering is a complex process involving multiple pathways, the 14 compounds identified in this study provide a potentially novel means to developing non-cross-resistant anti-cancer drugs that block centrosome clustering.

Mechanisms promoting escape from mitotic stress-induced tumor cell death

Non-small cell lung cancer (NSCLC) is notorious for its paltry responses to first-line therapeutic regimens. In contrast to acquired chemoresistance, little is known about the molecular underpinnings of the intrinsic resistance of chemo-naïve NSCLC. Here we report that intrinsic resistance to paclitaxel in NSCLC occurs at a cell-autonomous level because of the uncoupling of mitotic defects from apoptosis. To identify components that permit escape from mitotic stress-induced death, we used a genome-wide RNAi-based strategy, which combines a high-throughput toxicity screen with a live-cell imaging platform to measure mitotic fate. This strategy revealed that prolonging mitotic arrest with a small molecule inhibitor of the APC/cyclosome could sensitize otherwise paclitaxel-resistant NSCLC. We also defined novel roles for CASC1 and TRIM69 in supporting resistance to spindle poisons. CASC1, which is frequently co-amplified with KRAS in lung tumors, is essential for microtubule polymerization and satisfaction of the spindle assembly checkpoint. TRIM69, which associates with spindle poles and promotes centrosomal clustering, is essential for formation of a bipolar spindle. Notably, RNAi-mediated attenuation of CASC1 or TRIM69 was sufficient to inhibit tumor growth in vivo. On the basis of our results, we hypothesize that tumor evolution selects for a permissive mitotic checkpoint, which may promote survival despite chromosome segregation errors. Attacking this adaptation may restore the apoptotic consequences of mitotic damage to permit the therapeutic eradication of drug-resistant cancer cells.

The translational landscape of the splicing factor SRSF1 and its role in mitosis

The shuttling Serine/Arginine rich (SR) protein SRSF1 (previously known as SF2/ASF) is a splicing regulator that also activates translation in the cytoplasm. In order to dissect the gene network that is translationally regulated by SRSF1, we performed a high-throughput deep sequencing analysis of polysomal fractions in cells overexpressing SRSF1. We identified approximately 1,500 mRNAs that are translational targets of SRSF1. These include mRNAs encoding proteins involved in cell cycle regulation, such as spindle, kinetochore and M phase proteins, which are essential for accurate chromosome segregation. Indeed, we show that translational activity of SRSF1 is required for normal mitotic progression. Furthermore, we found that mRNAs that display alternative splicing changes upon SRSF1 overexpression are also its translational targets; strongly suggesting that SRSF1 couples pre-mRNA splicing and translation. These data provide insights on the complex role of SRSF1 in the control of gene expression at multiple levels and its implications in cancer.

Quantitative multi-parametric evaluation of centrosome declustering drugs: centrosome amplification, mitotic phenotype, cell cycle and death

Unlike normal cells, cancer cells contain amplified centrosomes and rely on centrosome clustering mechanisms to form a pseudobipolar spindle that circumvents potentially fatal spindle multipolarity (MP). Centrosome clustering also promotes low-grade chromosome missegregation, which can drive malignant transformation and tumor progression. Putative ‘centrosome declustering drugs' represent a cancer cell-specific class of chemotherapeutics that produces a common phenotype of centrosome declustering and spindle MP. However, differences between individual agents in terms of efficacy and phenotypic nuances remain unexplored. Herein, we have developed a conceptual framework for the quantitative evaluation of centrosome declustering drugs by investigating their impact on centrosomes, clustering, spindle polarity, cell cycle arrest, and death in various cancer cell lines at multiple drug concentrations over time. Surprisingly, all centrosome declustering drugs evaluated in our study were also centrosome-amplifying drugs to varying extents. Notably, all declustering drugs induced spindle MP, and the peak extent of MP positively correlated with the induction of hypodiploid DNA-containing cells. Our data suggest acentriolar spindle pole amplification as a hitherto undescribed activity of some declustering drugs, resulting in spindle MP in cells that may not have amplified centrosomes. In general, declustering drugs were more toxic to cancer cell lines than non-transformed ones, with some exceptions. Through a comprehensive description and quantitative analysis of numerous phenotypes induced by declustering drugs, we propose a novel framework for the assessment of putative centrosome declustering drugs and describe cellular characteristics that may enhance susceptibility to them.

A genetic screen based on in vivo RNA imaging reveals centrosome-independent mechanisms for localizing gurken transcripts in Drosophila

We have screened chromosome arm 3L for ethyl methanesulfonate−induced mutations that disrupt localization of fluorescently labeled gurken (grk) messenger (m)RNA, whose transport along microtubules establishes both major body axes of the developing Drosophila oocyte. Rapid identification of causative mutations by single-nucleotide polymorphism recombinational mapping and whole-genomic sequencing allowed us to define nine complementation groups affecting grk mRNA localization and other aspects of oogenesis, including alleles of elg1, scaf6, quemao, nudE, Tsc2/gigas, rasp, and Chd5/Wrb, and several null alleles of the armitage Piwi-pathway gene. Analysis of a newly induced kinesin light chain allele shows that kinesin motor activity is required for both efficient grk mRNA localization and oocyte centrosome integrity. We also show that initiation of the dorsoanterior localization of grk mRNA precedes centrosome localization, suggesting that microtubule self-organization contributes to breaking axial symmetry to generate a unique dorsoventral axis.

Genes involved in centrosome-independent mitotic spindle assembly in Drosophila S2 cells

Animal mitotic spindle assembly relies on centrosome-dependent and centrosome-independent mechanisms, but their relative contributions remain unknown. Here, we investigated the molecular basis of the centrosome-independent spindle assembly pathway by performing a whole-genome RNAi screen in Drosophila S2 cells lacking functional centrosomes. This screen identified 197 genes involved in acentrosomal spindle assembly, eight of which had no previously described mitotic phenotypes and produced defective and/or short spindles. All 197 genes also produced RNAi phenotypes when centrosomes were present, indicating that none were entirely selective for the acentrosomal pathway. However, a subset of genes produced a selective defect in pole focusing when centrosomes were absent, suggesting that centrosomes compensate for this shape defect. Another subset of genes was specifically associated with the formation of multipolar spindles only when centrosomes were present. We further show that the chromosomal passenger complex orchestrates multiple centrosome-independent processes required for mitotic spindle assembly/maintenance. On the other hand, despite the formation of a chromosome-enriched RanGTP gradient, S2 cells depleted of RCC1, the guanine-nucleotide exchange factor for Ran on chromosomes, established functional bipolar spindles. Finally, we show that cells without functional centrosomes have a delay in chromosome congression and anaphase onset, which can be explained by the lack of polar ejection forces. Overall, these findings establish the constitutive nature of a centrosome-independent spindle assembly program and how this program is adapted to the presence/absence of centrosomes in animal somatic cells.

Centrosome dysfunction contributes to chromosome instability, chromoanagenesis, and genome reprograming in cancer

The unique ability of centrosomes to nucleate and organize microtubules makes them unrivaled conductors of important interphase processes, such as intracellular payload traffic, cell polarity, cell locomotion, and organization of the immunologic synapse. But it is in mitosis that centrosomes loom large, for they orchestrate, with clockmaker's precision, the assembly and functioning of the mitotic spindle, ensuring the equal partitioning of the replicated genome into daughter cells. Centrosome dysfunction is inextricably linked to aneuploidy and chromosome instability, both hallmarks of cancer cells. Several aspects of centrosome function in normal and cancer cells have been molecularly characterized during the last two decades, greatly enhancing our mechanistic understanding of this tiny organelle. Whether centrosome defects alone can cause cancer, remains unanswered. Until recently, the aggregate of the evidence had suggested that centrosome dysfunction, by deregulating the fidelity of chromosome segregation, promotes and accelerates the characteristic Darwinian evolution of the cancer genome enabled by increased mutational load and/or decreased DNA repair. Very recent experimental work has shown that missegregated chromosomes resulting from centrosome dysfunction may experience extensive DNA damage, suggesting additional dimensions to the role of centrosomes in cancer. Centrosome dysfunction is particularly prevalent in tumors in which the genome has undergone extensive structural rearrangements and chromosome domain reshuffling. Ongoing gene reshuffling reprograms the genome for continuous growth, survival, and evasion of the immune system. Manipulation of molecular networks controlling centrosome function may soon become a viable target for specific therapeutic intervention in cancer, particularly since normal cells, which lack centrosome alterations, may be spared the toxicity of such therapies.

GSI promotes vincristine-induced apoptosis by enhancing multi-polar spindle formation

Gamma secretase inhibitors (GSI), cell-permeable small-molecule inhibitors of gamma secretase activity, had been originally developed for the treatment of Alzheimer disease. In recent years, it has been exploited in cancer research to inhibit Notch signaling that is aberrantly activated in various cancers. We previously found that GSI could synergize with anti-microtubule agent, vincristine (VCR) in a Notch-independent manner. Here, we delineate the underlying cell cycle-related mechanism using HeLa cells, which have strong mitotic checkpoints. GSI enhanced VCR-induced cell death, although GSI alone did not affect cell viability at all. GSI augmented VCR-induced mitotic arrest in a dose-dependent manner, which was preceded by apoptotic cell death, as shown by an increase in Annexin V-positive and caspase-positive cell population. Furthermore, GSI amplified multi-polar spindle formation triggered by VCR. Altogether, we show the evidence that GSI enhances VCR-induced apoptosis in HeLa cells via multi-polar mitotic spindle formation, independent of Notch signaling. These data suggest that one or more GS substrates, yet to be identified, in a post-GS processed form, may play a role in maintaining functional centrosomes/mitotic spindles. More significantly, the synergistic effect of GSI in combination with VCR could be exploited in clinical setting to improve the efficacy of VCR.

Discovery and mechanistic study of a small molecule inhibitor for motor protein KIFC1

Centrosome amplification is observed in many human cancers and has been proposed to be a driver of both genetic instability and tumorigenesis. Cancer cells have evolved mechanisms to bundle multiple centrosomes into two spindle poles to avoid multipolar mitosis that can lead to chromosomal segregation defects and eventually cell death. KIFC1, a kinesin-14 family protein, plays an essential role in centrosomal bundling in cancer cells, but its function is not required for normal diploid cell division, suggesting that KIFC1 is an attractive therapeutic target for human cancers. To this end, we have identified the first reported small molecule inhibitor AZ82 for KIFC1. AZ82 bound specifically to the KIFC1/microtubule (MT) binary complex and inhibited the MT-stimulated KIFC1 enzymatic activity in an ATP-competitive and MT-noncompetitive manner with a Ki of 0.043 μM. AZ82 effectively engaged with the minus end-directed KIFC1 motor inside cells to reverse the monopolar spindle phenotype induced by the inhibition of the plus end-directed kinesin Eg5. Treatment with AZ82 caused centrosome declustering in BT-549 breast cancer cells with amplified centrosomes. Consistent with genetic studies, our data confirmed that KIFC1 inhibition by a small molecule holds promise for targeting cancer cells with amplified centrosomes and provided evidence that functional suppression of KIFC1 by inhibiting its enzymatic activity could be an effective means for developing cancer therapeutics.

CEP proteins: the knights of centrosome dynasty

Centrosome forms the backbone of cell cycle progression mechanism. Recent debates have occurred regarding the essentiality of centrosome in cell cycle regulation. CEP family protein is the active component of centrosome and plays a vital role in centriole biogenesis and cell cycle progression control. A total of 31 proteins have been categorized into CEP family protein category and many more are under candidate evaluation. Furthermore, by the recent advancements in genomics and proteomics researches, several new CEP proteins have also been characterized. Here we have summarized the importance of CEP family proteins and their regulation mechanism involved in proper cell cycle progression. Further, we have reviewed the detailed molecular mechanism behind the associated pathological phenotypes and the possible therapeutic approaches. Proteins such as CEP57, CEP63, CEP152, CEP164, and CEP215 have been extensively studied with a detailed description of their molecular mechanisms, which are among the primary targets for drug discovery. Moreover, CEP27, CEP55, CEP70, CEP110, CEP120, CEP135, CEP192, CEP250, CEP290, and CEP350 also seem promising for future drug discovery approaches. Since the overview implicates that the overall researches on CEP proteins are not yet able to present significant details required for effective therapeutics development, thus, it is timely to discuss the importance of future investigations in this field.

Cell death associated with abnormal mitosis observed by confocal imaging in live cancer cells

Phenanthrene derivatives acting as potent PARP1 inhibitors prevented the bi-focal clustering of supernumerary centrosomes in multi-centrosomal human cancer cells in mitosis. The phenanthridine PJ-34 was the most potent molecule. Declustering of extra-centrosomes causes mitotic failure and cell death in multi-centrosomal cells. Most solid human cancers have high occurrence of extra-centrosomes. The activity of PJ-34 was documented in real-time by confocal imaging of live human breast cancer MDA-MB-231 cells transfected with vectors encoding for fluorescent γ-tubulin, which is highly abundant in the centrosomes and for fluorescent histone H2b present in the chromosomes. Aberrant chromosomes arrangements and de-clustered γ-tubulin foci representing declustered centrosomes were detected in the transfected MDA-MB-231 cells after treatment with PJ-34. Un-clustered extra-centrosomes in the two spindle poles preceded their cell death. These results linked for the first time the recently detected exclusive cytotoxic activity of PJ-34 in human cancer cells with extra-centrosomes de-clustering in mitosis, and mitotic failure leading to cell death. According to previous findings observed by confocal imaging of fixed cells, PJ-34 exclusively eradicated cancer cells with multi-centrosomes without impairing normal cells undergoing mitosis with two centrosomes and bi-focal spindles. This cytotoxic activity of PJ-34 was not shared by other potent PARP1 inhibitors, and was observed in PARP1 deficient MEF harboring extracentrosomes, suggesting its independency of PARP1 inhibition. Live confocal imaging offered a useful tool for identifying new molecules eradicating cells during mitosis.

The novel actin/focal adhesion-associated protein MISP is involved in mitotic spindle positioning in human cells

Accurate mitotic spindle positioning is essential for the regulation of cell fate choices, cell size and cell position within tissues. The most prominent model of spindle positioning involves a cortical pulling mechanism, where the minus end-directed microtubule motor protein dynein is attached to the cell cortex and exerts pulling forces on the plus ends of astral microtubules that reach the cortex. In nonpolarized cultured cells integrin-dependent, retraction fiber-mediated cell adhesion is involved in spindle orientation. Proteins serving as intermediaries between cortical actin or retraction fibers and astral microtubules remain largely unknown. In a recent genome-wide RNAi screen we identified a previously uncharacterized protein, MISP (C19ORF21) as being involved in centrosome clustering, a process leading to the clustering of supernumerary centrosomes in cancer cells into a bipolar mitotic spindle array by microtubule tension. Here, we show that MISP is associated with the actin cytoskeleton and focal adhesions and is expressed only in adherent cell types. During mitosis MISP is phosphorylated by Cdk1 and localizes to retraction fibers. MISP interacts with the +TIP EB1 and p150^glued, a subunit of the dynein/dynactin complex. Depletion of MISP causes mitotic arrest with reduced tension across sister kinetochores, chromosome misalignment and spindle multipolarity in cancer cells with supernumerary centrosomes. Analysis of spindle orientation revealed that MISP depletion causes randomization of mitotic spindle positioning relative to cell axes and cell center. Together, we propose that MISP links microtubules to the actin cytoskeleton and focal adhesions in order to properly position the mitotic spindle.

Never tear us apart--the importance of centrosome clustering

The presence of more than two centrosomes (centrosome amplification) at the onset of mitosis has long been associated with multipolar spindle formation, and with the generation of genetic instability. However, in recent years, several studies have shown that a process termed 'centrosome clustering' actively contributes to bipolar division by promoting the gathering of extra centrosomes in two main poles. In this Commentary, we describe the main proteins that are involved in centriole duplication and discuss how centrosome amplification can be generated both in vitro and in vivo. We then summarize what is currently known about the processes that contribute to bipolar spindle formation when extra centrosomes are present, and which forces contribute to this process. Finally, we discuss how extra centrosomes might contribute to tumorigenesis, giving emphasis to the role of centrosome amplification in promoting genetic instability.

Linking abnormal mitosis to the acquisition of DNA damage

Cellular defects that impair the fidelity of mitosis promote chromosome missegregation and aneuploidy. Increasing evidence reveals that errors in mitosis can also promote the direct and indirect acquisition of DNA damage and chromosome breaks. Consequently, deregulated cell division can devastate the integrity of the normal genome and unleash a variety of oncogenic stimuli that may promote transformation. Recent work has shed light on the mechanisms that link abnormal mitosis with the development of DNA damage, how cells respond to such affronts, and the potential impact on tumorigenesis.

Potential effects of CRM1 inhibition in mantle cell lymphoma

Mantle cell lymphoma (MCL) is an aggressive histotype of B-cell non-Hodgkin lymphoma. The disease has no known cure, which prompts the urgent need for novel therapeutic agents. Chromosomal region maintenance 1 (CRM1) may play a role in human neoplasia and serve as a novel target of cancer treatment. This study summarizes MCL pathogenesis and determines the involvement of CRM1 in the regulation of several vital signaling pathways contributing to MCL pathogenesis, including the pathways of cell cycle progression, DNA damage response, phosphoinositide kinase-3, nuclear factor-κB activation, and chromosomal stability. A preclinical study is also presented to compare the CRM1 status in MCL cell lines and primary MCL cells with normal B cells, as well as the therapeutic efficiency of CRM1 inhibition in MCL in vitro and in vivo, which make these agents potential targets of novel MCL treatments.

Centrosomes, chromosome instability (CIN) and aneuploidy

Each time a cell divides its chromosome content must be equally segregated into the two daughter cells. This critical process is mediated by a complex microtubule based apparatus, the mitotic spindle. In most animal cells the centrosomes contribute to the formation and the proper function of the mitotic spindle by anchoring and nucleating microtubules and by establishing its functional bipolar organization. Aberrant expression of proteins involved in centrosome biogenesis can drive centrosome dysfunction or abnormal centrosome number, leading ultimately to improper mitotic spindle formation and chromosome missegregation. Here we review recent work focusing on the importance of the centrosome for mitotic spindle formation and the relation between the centrosome status and the mechanisms controlling faithful chromosome inheritance.

The centrosome as potential target for cancer therapy and prevention

INTRODUCTION

Cancer initiation and propagation is not possible without cell division. Besides microtubules, which are targeted by taxanes as part of a number of standard chemotherapy regimens, mitosis depends on small cellular organelles known as centrosomes. Centrosome abnormalities are a common finding in tumors including major human malignancies such as prostate or breast cancer. Centrosome aberrations can drive chromosome missegregation and aneuploidy, thereby promoting malignant progression. Nonetheless, these important cellular structures have not yet been directly exploited for targeted interventions.

AREAS COVERED

This review will summarize the current knowledge of normal and aberrant centrosome duplication. We will highlight the principal pathways leading to aberrant centrosome numbers and the evidence for a role of centrosome amplification in malignant progression. Strategies to target centrosome-mediated cell division errors will be discussed. Lastly, we will review the evidence for centrosome clustering as a druggable cellular process.

EXPERT OPINION

Recent advances in the understanding of centrosome biogenesis have revealed a number of potential centrosomal drug targets including Polo-like kinases, Cyclin-dependent kinases, Aurora kinases, and molecular motor proteins. For some of these proteins, targeted inhibitory compounds are available and in vitro experiments have provided the proof-of-concept that blocking centrosome overduplication can result in a reduction of aneuploid cells. In addition, inhibition of centrosomal clustering has antitumor activity in vitro and in vivo. Nonetheless, further in vitro and preclinical studies are required to determine the most effective way to exploit the centrosome for therapeutic or preventive anticancer strategies.

Acentrosomal spindle organization renders cancer cells dependent on the kinesin HSET

Centrosomes represent the major microtubule organizing centers (MTOCs) of animal somatic cells and orchestrate bipolar spindle assembly during mitotic cell division. In meiotic cells, the kinesin HSET compensates for the lack of centrosomes by focusing acentrosomal MTOCs into two spindle poles. By clustering multiple centrosomes into two spindle poles, HSET also mediates bipolar mitosis in cancer cells with supernumerary centrosomes. However, although dispensable in non-transformed human cells, the role of HSET in cancer cells with two centrosomes has remained elusive. In this study, we demonstrate that HSET is required for proper spindle assembly, stable pole-focusing and survival of cancer cells irrespective of normal or supernumerary centrosome number. Strikingly, we detected pronounced acentrosomal MTOC structures in untreated mitotic cancer cells. While in most cancer cells these acentrosomal MTOCs were rapidly incorporated into the assembling bipolar spindle, some cells eventually established bipolar spindles with acentrosomal poles and free centrosomes. These observations demonstrate that acentrosomal MTOCs were functional and that both centrosomal and acentrosomal mechanisms were required for bipolar spindle organization. Our study shows that HSET is critical for clustering acentrosomal and centrosomal MTOCs during spindle formation in human cancer cells with two bona fide centrosomes. Furthermore, we show that in checkpoint-defective cancer cells, acentrosomal spindle formation and HSET-dependence are partially mediated by a constitutive activation of the DNA damage response. In summary, we propose that acentrosomal spindle assembly mechanisms are hyperactive in cancer cells and promote HSET, a key driver of acentrosomal spindle organization, as an attractive target for cancer therapy.

GF-15, a novel inhibitor of centrosomal clustering, suppresses tumor cell growth in vitro and in vivo

In contrast to normal cells, malignant cells are frequently aneuploid and contain multiple centrosomes. To allow for bipolar mitotic division, supernumerary centrosomes are clustered into two functional spindle poles in many cancer cells. Recently, we have shown that griseofulvin forces tumor cells with supernumerary centrosomes to undergo multipolar mitoses resulting in apoptotic cell death. Here, we describe the characterization of the novel small molecule GF-15, a derivative of griseofulvin, as a potent inhibitor of centrosomal clustering in malignant cells. At concentrations where GF-15 had no significant impact on tubulin polymerization, spindle tension was markedly reduced in mitotic cells upon exposure to GF-15. Moreover, isogenic cells with conditional centrosome amplification were more sensitive to GF-15 than parental controls. In a wide array of tumor cell lines, mean inhibitory concentrations (IC(50)) for proliferation and survival were in the range of 1 to 5 μmol/L and were associated with apoptotic cell death. Importantly, treatment of mouse xenograft models of human colon cancer and multiple myeloma resulted in tumor growth inhibition and significantly prolonged survival. These results show the in vitro and in vivo antitumor efficacy of a prototype small molecule inhibitor of centrosomal clustering and strongly support the further evaluation of this new class of molecules.

Transient defects of mitotic spindle geometry and chromosome segregation errors

Assembly of a bipolar mitotic spindle is essential to ensure accurate chromosome segregation and prevent aneuploidy, and severe mitotic spindle defects are typically associated with cell death. Recent studies have shown that mitotic spindles with initial geometric defects can undergo specific rearrangements so the cell can complete mitosis with a bipolar spindle and undergo bipolar chromosome segregation, thus preventing the risk of cell death associated with abnormal spindle structure. Although this may appear as an advantageous strategy, transient defects in spindle geometry may be even more threatening to a cell population or organism than permanent spindle defects. Indeed, transient spindle geometry defects cause high rates of chromosome mis-segregation and aneuploidy. In this review, we summarize our current knowledge on two specific types of transient spindle geometry defects (transient multipolarity and incomplete spindle pole separation) and describe how these mechanisms cause chromosome mis-segregation and aneuploidy. Finally, we discuss how these transient spindle defects may specifically contribute to the chromosomal instability observed in cancer cells.

A phthalimide derivative that inhibits centrosomal clustering is effective on multiple myeloma

Despite the introduction of newly developed drugs such as lenalidomide and bortezomib, patients with multiple myeloma are still difficult to treat and have a poor prognosis. In order to find novel drugs that are effective for multiple myeloma, we tested the antitumor activity of 29 phthalimide derivatives against several multiple myeloma cell lines. Among these derivatives, 2-(2,6-diisopropylphenyl)-5-amino-1H-isoindole-1,3- dione (TC11) was found to be a potent inhibitor of tumor cell proliferation and an inducer of apoptosis via activation of caspase-3, 8 and 9. This compound also showed in vivo activity against multiple myeloma cell line KMS34 tumor xenografts in ICR/SCID mice. By means of mRNA display selection on a microfluidic chip, the target protein of TC11 was identified as nucleophosmin 1 (NPM). Binding of TC11 and NPM monomer was confirmed by surface plasmon resonance. Immunofluorescence and NPM knockdown studies in HeLa cells suggested that TC11 inhibits centrosomal clustering by inhibiting the centrosomal-regulatory function of NPM, thereby inducing multipolar mitotic cells, which undergo apoptosis. NPM may become a novel target for development of antitumor drugs active against multiple myeloma.

Losing balance: the origin and impact of aneuploidy in cancer

Most solid human tumours are aneuploid, that is, they contain an abnormal number of chromosomes. Paradoxically, however, aneuploidy has been reported to induce a stress response that suppresses cellular proliferation in vitro. Here, we review the progress in our understanding of the causes and effects of aneuploidy in cancer and discuss how, in specific contexts, aneuploidy can provide a growth advantage and facilitate cellular transformation. We also explore the emerging possibilities for targeting the cause or consequences of aneuploidy therapeutically.

Let's huddle to prevent a muddle: centrosome declustering as an attractive anticancer strategy

Nearly a century ago, cell biologists postulated that the chromosomal aberrations blighting cancer cells might be caused by a mysterious organelle-the centrosome-that had only just been discovered. For years, however, this enigmatic structure was neglected in oncologic investigations and has only recently reemerged as a key suspect in tumorigenesis. A majority of cancer cells, unlike healthy cells, possess an amplified centrosome complement, which they manage to coalesce neatly at two spindle poles during mitosis. This clustering mechanism permits the cell to form a pseudo-bipolar mitotic spindle for segregation of sister chromatids. On rare occasions this mechanism fails, resulting in declustered centrosomes and the assembly of a multipolar spindle. Spindle multipolarity consigns the cell to an almost certain fate of mitotic arrest or death. The catastrophic nature of multipolarity has attracted efforts to develop drugs that can induce declustering in cancer cells. Such chemotherapeutics would theoretically spare healthy cells, whose normal centrosome complement should preclude multipolar spindle formation. In search of the 'Holy Grail' of nontoxic, cancer cell-selective, and superiorly efficacious chemotherapy, research is underway to elucidate the underpinnings of centrosome clustering mechanisms. Here, we detail the progress made towards that end, highlighting seminal work and suggesting directions for future research, aimed at demystifying this riddling cellular tactic and exploiting it for chemotherapeutic purposes. We also propose a model to highlight the integral role of microtubule dynamicity and the delicate balance of forces on which cancer cells rely for effective centrosome clustering. Finally, we provide insights regarding how perturbation of this balance may pave an inroad for inducing lethal centrosome dispersal and death selectively in cancer cells.

p53 dependent centrosome clustering prevents multipolar mitosis in tetraploid cells

BACKGROUND

p53 abnormality and aneuploidy often coexist in human tumors, and tetraploidy is considered as an intermediate between normal diploidy and aneuploidy. The purpose of this study was to investigate whether and how p53 influences the transformation from tetraploidy to aneuploidy.

PRINCIPAL FINDINGS

Live cell imaging was performed to determine the fates and mitotic behaviors of several human and mouse tetraploid cells with different p53 status, and centrosome and spindle immunostaining was used to investigate centrosome behaviors. We found that p53 dominant-negative mutation, point mutation, or knockout led to a 2∼ 33-fold increase of multipolar mitosis in N/TERT1, 3T3 and mouse embryonic fibroblasts (MEFs), while mitotic entry and cell death were not significantly affected. In p53-/- tetraploid MEFs, the ability of centrosome clustering was compromised, while centrosome inactivation was not affected. Suppression of RhoA/ROCK activity by specific inhibitors in p53-/- tetraploid MEFs enhanced centrosome clustering, decreased multipolar mitosis from 38% to 20% and 16% for RhoA and ROCK, respectively, while expression of constitutively active RhoA in p53+/+ tetraploid 3T3 cells increased the frequency of multipolar mitosis from 15% to 35%.

CONCLUSIONS

p53 could not prevent tetraploid cells entering mitosis or induce tetraploid cell death. However, p53 abnormality impaired centrosome clustering and lead to multipolar mitosis in tetraploid cells by modulating the RhoA/ROCK signaling pathway.

A phenanthrene derived PARP inhibitor is an extra-centrosomes de-clustering agent exclusively eradicating human cancer cells

Background

Cells of most human cancers have supernumerary centrosomes. To enable an accurate chromosome segregation and cell division, these cells developed a yet unresolved molecular mechanism, clustering their extra centrosomes at two poles, thereby mimicking mitosis in normal cells. Failure of this bipolar centrosome clustering causes multipolar spindle structures and aberrant chromosomes segregation that prevent normal cell division and lead to 'mitotic catastrophe cell death'.

Methods

We used cell biology and biochemical methods, including flow cytometry, immunocytochemistry and live confocal imaging.

Results

We identified a phenanthrene derived PARP inhibitor, known for its activity in neuroprotection under stress conditions, which exclusively eradicated multi-centrosomal human cancer cells (mammary, colon, lung, pancreas, ovarian) while acting as extra-centrosomes de-clustering agent in mitosis. Normal human proliferating cells (endothelial, epithelial and mesenchymal cells) were not impaired. Despite acting as PARP inhibitor, the cytotoxic activity of this molecule in cancer cells was not attributed to PARP inhibition alone.

Conclusion

We identified a water soluble phenanthridine that exclusively targets the unique dependence of most human cancer cells on their supernumerary centrosomes bi-polar clustering for their survival. This paves the way for a new selective cancer-targeting therapy, efficient in a wide range of human cancers.

Centrosomes and cilia in human disease

Centrioles are microtubule-derived structures that are essential for the formation of centrosomes, cilia and flagella. The centrosome is the major microtubule organiser in animal cells, participating in a variety of processes, from cell polarisation to cell division, whereas cilia and flagella contribute to several mechanisms in eukaryotic cells, from motility to sensing. Although it was suggested more than a century ago that these microtubule-derived structures are involved in human disease, the molecular bases of this association have only recently been discovered. Surprisingly, there is very little overlap between the genes affected in the different diseases, suggesting that there are tissue-specific requirements for these microtubule-derived structures. Knowledge of these requirements and disease mechanisms has opened new avenues for therapeutical strategies. Here, we give an overview of recent developments in this field, focusing on cancer, diseases of brain development and ciliopathies.

Centrosome clustering and chromosomal (in)stability: a matter of life and death

Centrosome abnormalities occur commonly in cancer, and contribute to chromosomal instability and tumorigenesis. New evidence on a phylogenetically conserved mechanism termed 'centrosomal clustering' provides exciting insights into how cells with supernumerary centrosomes adapt to avoid lethal multipolar divisions. Here, we highlight the emerging molecular basis of centrosome clustering, and its impact on asymmetric divisions of stem cells, chromosomal (in)stability and malignant transformation. Finally, pharmacological inhibition of centrosome clustering promises to selectively target tumor cells.

Tales from an academic RNAi screening facility; FAQs

RNAi technology is now a well-established and widely employed research technique that has been adopted by many researchers for use in large-scale screening campaigns. Here, we offer our experience of genome-wide siRNA screening from the perspective of a facility providing screening as a service to a wide range of researchers with diverse interests and approaches. We have experienced the emotional rollercoaster of screening from the exuberant early promise of a screen, the messy reality of the data through to the recognition of screen data as a potential information goldmine. Here, we use some of the questions we most frequently encounter to highlight the initial concerns of many researchers embarking on a siRNA screen and conclude that an informed view of what can be reasonably expected from a screen is essential to the most effective implementation of the technology. Along the way, we suggest that for this area of research at least, either centralization of the resources or close and open collaboration between interested parties offers distinct advantages.

Molecular docking and pharmacogenomics of vinca alkaloids and their monomeric precursors, vindoline and catharanthine

Vinblastine and vincristine are dimeric indole alkaloids derived from Catharanthus roseus (formerly: Vinca rosea). Their monomeric precursor molecules are vindoline and catharanthine. While vinblastine and vincristine are well-known mitotic spindle poisons, not much is known about vindoline and catharanthine. Vindoline and catharanthine showed weak cytotoxicity, while vinblastine, vincristine, and the semisynthetic vindesine and vinorelbine revealed high cytotoxicity towards cancer cells. This may reflect a general biological principle of poisonous plants. Highly toxic compounds are not only active towards predators, but also towards plant tissues. Hence, plants need mechanisms to protect themselves from their own poisons. One evolutionary strategy to solve this problem is to generate less toxic precursors, which are dimerized to toxic end products when needed. As shown by in silico molecular docking and biochemical approaches, vinblastine, vincristine and vinorelbine bound with high affinity to α/β-tubulin and inhibited tubulin polymerization, whereas the effects of vindoline and catharanthine were weak. Similarly, vinblastine produced high fractions of mono- and multipolar mitotic spindles, while vindoline and catharanthine did only weakly affect bipolar mitotic spindle formation. Here, we show that vinblastine contributes to cell death by interference with spindle polarity. P-glycoprotein-overexpressing multidrug-resistant CEM/VCR1000 cells were highly resistant towards vincristine and cross-resistant to vinblastine, vindesine, and vinorelbine, but not or only weakly cross-resistant to vindoline and catharanthine. In addition to tubulin as primary target, microarray-based mRNA signatures of responsiveness of these compounds have been identified by COMPARE and signaling pathway profiling.

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.

Illicit survival of cancer cells during polyploidization and depolyploidization

Tetraploidy and the depolyploidization of tetraploid cells may contribute to oncogenesis. Several mechanisms have evolved to avoid the generation, survival, proliferation and depolyploidization of tetraploids. Cells that illicitly survive these checkpoints are prone to chromosomal instability and aneuploidization. Along with their replication, tetraploids constantly undergo chromosomal rearrangements that eventually lead to pseudodiploidy by two non-exclusive mechanisms: (i) multipolar divisions and (ii) illicit bipolar divisions in the presence of improper microtubule-kinetochore attachments. Here, we describe the regulation and the molecular mechanisms that underlie such a 'polyploidization-depolyploidization' cascade, while focusing on the role of oncogenes and tumor suppressor genes in tetraploidy-driven tumorigenesis. We speculate that the identification of signaling/metabolic cascades that are required for the survival of tetraploid or aneuploid (but not diploid) cancer cells may pave the way for the development of novel broad-spectrum anticancer agents.

Symplekin specifies mitotic fidelity by supporting microtubule dynamics

Using a pangenomic loss-of-function screening strategy, we have previously identified 76 potent modulators of paclitaxel responsiveness in non-small-cell lung cancer. The top hit isolated from this screen, symplekin, is a well-established component of the mRNA polyadenylation machinery. Here, we performed a high-resolution phenotypic analysis to reveal the mechanistic underpinnings by which symplekin depletion collaborates with paclitaxel. We find that symplekin supports faithful mitosis by contributing to the formation of a bipolar spindle apparatus. Depletion of symplekin attenuates microtubule polymerization activity as well as expression of the critical microtubule polymerization protein CKAP5 (TOGp). Depletion of additional members of the polyadenylation complex induces similar phenotypes, suggesting that polyadenylation machinery is intimately coupled to microtubule function and thus mitotic spindle formation. Importantly, tumor cells depleted of symplekin display reduced fecundity, but the mitotic defects that we observe are not evident in immortalized cells. These results demonstrate a critical connection between the polyadenylation machinery and mitosis and suggest that tumor cells have an enhanced dependency on these components for spindle assembly.