The role of the centrosome in the development of malignant tumors

The Impact of Centrosome Pathologies on Ovarian Cancer Development and Progression with a Focus on Centrosomes as Therapeutic Target

The impact of centrosome abnormalities on cancer cell proliferation has been recognized as early as 1914 (Boveri, Zur Frage der Entstehung maligner Tumoren. Jena: G. Fisher, 1914), but vigorous research on molecular levels has only recently started when it became fully apparent that centrosomes can be targeted for new cancer therapies. While best known for their microtubule-organizing capabilities as MTOC (microtubule organizing center) in interphase and mitosis, centrosomes are now further well known for a variety of different functions, some of which are related to microtubule organization and consequential activities such as cell division, migration, maintenance of cell shape, and vesicle transport powered by motor proteins, while other functions include essential roles in cell cycle regulation, metabolic activities, signal transduction, proteolytic activity, and several others that are now heavily being investigated for their role in diseases and disorders (reviewed in Schatten and Sun, Histochem Cell Biol 150:303-325, 2018; Schatten, Adv Anat Embryol Cell Biol 235:43-50, 2022a; Schatten, Adv Anat Embryol Cell Biol 235:17-35, 2022b).Cancer cell centrosomes differ from centrosomes in noncancer cells in displaying specific abnormalities that include phosphorylation abnormalities, overexpression of specific centrosomal proteins, abnormalities in centriole and centrosome duplication, formation of multipolar spindles that play a role in aneuploidy and genomic instability, and several others that are highlighted in the present review on ovarian cancer. Ovarian cancer cell centrosomes, like those in other cancers, display complex abnormalities that in part are based on the heterogeneity of cells in the cancer tissues resulting from different etiologies of individual cancer cells that will be discussed in more detail in this chapter.Because of the critical role of centrosomes in cancer cell proliferation, several lines of research are being pursued to target centrosomes for therapeutic intervention to inhibit abnormal cancer cell proliferation and control tumor progression. Specific centrosome abnormalities observed in ovarian cancer will be addressed in this chapter with a focus on targeting such aberrations for ovarian cancer-specific therapies.

Functions and dysfunctions of the mammalian centrosome in health, disorders, disease, and aging

Since its discovery well over 100 years ago (Flemming, in Sitzungsber Akad Wissensch Wien 71:81-147, 1875; Van Beneden, in Bull Acad R Belg 42:35-97, 1876) the centrosome is increasingly being recognized as a most impactful organelle for its role not only as primary microtubule organizing center (MTOC) but also as a major communication center for signal transduction pathways and as a center for proteolytic activities. Its significance for cell cycle regulation has been well studied and we now also know that centrosome dysfunctions are implicated in numerous diseases and disorders including cancer, Alstrom syndrome, Bardet-Biedl syndrome, Huntington's disease, reproductive disorders, and several other diseases and disorders. The present review is meant to build on information presented in the previous review (Schatten, in Histochem Cell Biol 129:667-686, 2008) and to highlight functions of the mammalian centrosome in health, and dysfunctions in disorders, disease, and aging with six sections focused on (1) centrosome structure and functions, and new insights into the role of centrosomes in cell cycle progression; (2) the role of centrosomes in tumor initiation and progression; (3) primary cilia, centrosome-primary cilia interactions, and consequences for cell cycle functions in health and disease; (4) transitions from centrosome to non-centrosome functions during cellular polarization; (5) other centrosome dysfunctions associated with the pathogenesis of human disease; and (6) centrosome functions in oocyte germ cells and dysfunctions in reproductive disorders and reproductive aging.

Increased cathepsin D protein expression is a biomarker for osteosarcomas, pulmonary metastases and other bone malignancies

Cancer proteomics provide a powerful approach to identify biomarkers for personalized medicine. Particularly, biomarkers for early detection, prognosis and therapeutic intervention of bone cancers, especially osteosarcomas, are missing. Initially, we compared two-dimensional gel electrophoresis (2-DE)-based protein expression pattern between cell lines of fetal osteoblasts, osteosarcoma and pulmonary metastasis derived from osteosarcoma. Two independent statistical analyses by means of PDQuest® and SameSpot® software revealed a common set of 34 differentially expressed protein spots (p < 0.05). 17 Proteins were identified by mass spectrometry and subjected to Ingenuity Pathway Analysis resulting in one high-ranked network associated with Gene Expression, Cell Death and Cell-To-Cell Signaling and Interaction. Ran/TC4-binding protein (RANBP1) and Cathepsin D (CTSD) were further validated by Western Blot in cell lines while the latter one showed higher expression differences also in cytospins and in clinical samples using tissue microarrays comprising osteosarcomas, metastases, other bone malignancies, and control tissues. The results show that protein expression patterns distinguish fetal osteoblasts from osteosarcomas, pulmonary metastases, and other bone diseases with relevant sensitivities between 55.56% and 100% at ≥87.50% specificity. Particularly, CTSD was validated in clinical material and could thus serve as a new biomarker for bone malignancies and potentially guide individualized treatment regimes.

Epidermal development, growth control, and homeostasis in the face of centrosome amplification

As nucleators of the mitotic spindle and primary cilium, centrosomes play crucial roles in equal segregation of DNA content to daughter cells, coordination of growth and differentiation, and transduction of homeostatic cues. Whereas the majority of mammalian cells carry no more than two centrosomes per cell, exceptions to this rule apply in certain specialized tissues and in select disease states, including cancer. Centrosome amplification, or the condition of having more than two centrosomes per cell, has been suggested to contribute to instability of chromosomes, imbalance in asymmetric divisions, and reorganization of tissue architecture; however, the degree to which these conditions are a direct cause of or simply a consequence of human disease is poorly understood. Here we addressed this issue by generating a mouse model inducing centrosome amplification in a naturally proliferative epithelial tissue by elevating Polo-like kinase 4 (Plk4) expression in the skin epidermis. By altering centrosome numbers, we observed multiciliated cells, spindle orientation errors, and chromosome segregation defects within developing epidermis. None of these defects was sufficient to impart a proliferative advantage within the tissue, however. Rather, impaired mitoses led to p53-mediated cell death and contributed to defective growth and stratification. Despite these abnormalities, mice remained viable and healthy, although epidermal cells with centrosome amplification were still appreciable. Moreover, these abnormalities were insufficient to disrupt homeostasis and initiate or enhance tumorigenesis, underscoring the powerful surveillance mechanisms in the skin.

Link between DNA damage and centriole disengagement/reduplication in untransformed human cells

The radiation and radiomimetic drugs used to treat human tumors damage DNA in both cancer cells and normal proliferating cells. Centrosome amplification after DNA damage is well established for transformed cell types but is sparsely reported and not fully understood in untransformed cells. We characterize centriole behavior after DNA damage in synchronized untransformed human cells. One hour treatment of S phase cells with the radiomimetic drug, Doxorubicin, prolongs G2 by at least 72 h, though 14% of the cells eventually go through mitosis in that time. By 72 h after DNA damage we observe a 52% incidence of centriole disengagement plus a 10% incidence of extra centrioles. We find that either APC/C or Plk activities can disengage centrioles after DNA damage, though they normally work in concert. All disengaged centrioles are associated with γ-tubulin and maturation markers and thus, should in principle be capable of reduplicating and organizing spindle poles. The low incidence of reduplication of disengaged centrioles during G2 is due to the p53-dependent expression of p21 and the consequent loss of Cdk2 activity. We find that 26% of the cells going through mitosis after DNA damage contain disengaged or extra centrioles. This could produce genomic instability through transient or persistent spindle multipolarity. Thus, for cancer patients the use of DNA damaging therapies raises the chances of genomic instability and evolution of transformed characteristics in proliferating normal cell populations.

On Centrioles, Microtubules, and Cellular Electromagnetism

This paper describes the inner workings of centrioles (a pair of small organelles adjacent to the nucleus) as they create cell electropolarity, engage in cell division (mitosis), but in going awry, also promote the development of cancers. The electropolarity arises from vibrations of microtubules composing the centrioles. Mitosis begins as each centrioles duplicates itself by growing a daughter centriole on its side. If during duplication more than one daughter is grown, cancer can occur and the cells divide uncontrollably. Cancer cells with supernumerary centrioles have high electropolarity which can serve as an attractor for charged therapeutic nanoparticles.

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.

Overexpression of centrosomal protein Nlp confers breast carcinoma resistance to paclitaxel

Nlp (ninein-like protein), an important molecule involved in centrosome maturation and spindle formation, plays an important role in tumorigenesis and its abnormal expression was recently observed in human breast and lung cancers. In this study, the correlation between overexpression of Nlp and paclitaxel chemosensitivity was investigated to explore the mechanisms of resistance to paclitaxel and to understand the effect of Nlp upon apoptosis induced by chemotherapeutic agents. Nlp expression vector was stably transfected into breast cancer MCF-7 cells. With Nlp overexpression, the survival rates, cell cycle distributions and apoptosis were analyzed in transfected MCF-7 cells by MTT test and FCM approach. The immunofluorescent assay was employed to detect the changes of microtubule after paclitaxel treatment. Immunoblotting analysis was used to examine expression of centrosomal proteins and apoptosis associated proteins. Subsequently, Nlp expression was retrospectively examined with 55 breast cancer samples derived from paclitaxel treated patients. Interestingly, the survival rates of MCF-7 cells with Nlp overexpressing were higher than that of control after paclitaxel treatment. Nlp overexpression promoted G2-M arrest and attenuated apoptosis induced by paclitaxel, which was coupled with elevated Bcl-2 protein. Nlp expression significantly lessened the microtubule polymerization and bundling elicited by paclitaxel attributing to alteration on the structure or dynamics of β-tubulin but not on its expression. The breast cancer patients with high expression of Nlp were likely resistant to the treatment of paclitaxel, as the response rate in Nlp negative patients was 62.5%, whereas was 58.3 and 15.8% in Nlp (+) and Nlp (++) patients respectively (p = 0.015). Nlp expression was positive correlated with those of Plk1 and PCNA. These findings provide insights into more rational chemotherapeutic regimens in clinical practice, and more effective approaches might be developed through targeting Nlp to increase chemotherapeutic sensitivity.

Repeated cleavage failure does not establish centrosome amplification in untransformed human cells

Transient cleavage failure in dividing cells is not sufficient to establish stable populations of cells with extra centrosomes.

We tested whether cleavage failure as a transient event establishes an incidence of centrosome amplification in cell populations. Five rounds of ∼30% cytochalasin-induced cleavage failure in untransformed human cell cultures did not establish centrosome amplification in the short or long terms. The progeny of binucleate cells progressively dropped out of the cell cycle and expressed p53/p21, and none divided a fourth time. We also tested whether cleavage failure established centrosome amplification in transformed cell populations. Tetraploid HCT116 p53^−/− cells eventually all failed cleavage repeatedly and ceased proliferating. HeLa cells all died or arrested within four cell cycles. Chinese hamster ovary cells proliferated after cleavage failure, but five rounds of induced cleavage failure produced a modest increase in the incidence of centrosome amplification in the short term, which did not rise with more cycles of cleavage failure. This incidence dropped to close to control values in the long term despite a 2–6% rate of spontaneous cleavage failure in the progeny of tetraploid cells.

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.

Mps1 phosphorylation sites regulate the function of centrin 2 in centriole assembly

We show that while Centrin2 is dispensable for centriole assembly, it is an Mps1 substrate that stimulates canonical and aberrant centriole assembly by two different Mps1-dependent mechanisms, HsSas-6–dependent and –independent. Centrin2 phosphorylation is also required for the ability of Mps1 to drive production of mature centrioles.

The nondegradable Mps1^Δ12/13 protein drives centriole overproduction, suggesting that Mps1 phosphorylates a subset of centrosomal proteins to drive the assembly of new centrioles. Here we identify three Mps1 phosphorylation sites within the centriolar protein Centrin 2 (Cetn2). Although centrioles can be assembled in the absence of Cetn2, centriole assembly is attenuated in the absence of Cetn2. While wild-type Cetn2 can compensate for this attenuation, a nonphosphorylatable version cannot. In addition, overexpressing Cetn2 causes Mps1-dependent centriole overproduction that requires each of the three Mps1 phosphorylation sites within Cetn2 and is greatly exacerbated by mimicking phosphorylation at any of these sites. Wild-type Cetn2 generates excess foci that are competent as mitotic spindle poles in HsSas-6–depleted cells, suggesting that Cetn2 can organize a subset of centriolar proteins independently of cartwheels. However, centriole overproduction caused by a phosphomimetic Cetn2 mutant requires HsSas-6, suggesting that Cetn2 phosphorylation stimulates the canonical centriole assembly pathway. Moreover, in the absence of Cetn2, Mps1^Δ12/13 cannot drive the production of mature centrioles capable of recruiting γ-Tubulin, and a nonphosphorylatable Cetn2 mutant cannot compensate for this defect and exacerbates Cetn2 depletion. Together, our data suggest that Mps1-dependent phosphorylation of Cetn2 stimulates the canonical centriole assembly pathway.

High temperature requirement A3 (HtrA3) promotes etoposide- and cisplatin-induced cytotoxicity in lung cancer cell lines

Lung cancer is the leading cause of cancer-related deaths worldwide. Here we show for the first time that HtrA3 is a mitochondrial stress-response factor that promotes cytotoxicity to etoposide and cisplatin in lung cancer cell lines. Exogenous expression of wild type HtrA3 domain variants significantly attenuated cell survival with etoposide and cisplatin treatment in lung cancer cell lines H157 and A549 compared with expression of protease inactive mutants (S305A) or vector control. Conversely, HtrA3 suppression promoted cell survival with etoposide and cisplatin treatment in lung cancer cell lines Hop62 and HCC827. Survival was attenuated by re-expression of wild type HtrA3 variants during treatment but not by protease inactive mutants or vector control. HtrA3 also co-fractionated and co-localized with mitochondrial markers with both endogenous and exogenous expression in normal lung and lung cancer cell lines but was translocated from mitochondria following etoposide treatment. Moreover, HtrA3 translocation from mitochondria correlated with an increase in cell death that was attenuated by either HtrA3 suppression or Bcl-2 overexpression. Taken together, these results suggest that HtrA3 may be a previously uncharacterized mitochondrial cell death effector whose serine protease function may be crucial to modulating etoposide- and cisplatin-induced cytotoxicity in lung cancer cell lines.

Acute myeloid leukemia with mutated nucleophosmin (NPM1): molecular, pathological, and clinical features

The NPM1 gene encodes for nucleophosmin, a nucleolus-located shuttling protein that is involved in multiple cell functions, including regulation of ribosome biogenesis, control of centrosome duplication and preservation of ARF tumor suppressor integrity. The NPM1 gene is specifically mutated in about 30% acute myeloid leukemia (AML) but not in other human neoplasms. Mutations cause crucial changes at the C-terminus of the NPM1 protein that are responsible for the aberrant nuclear export and accumulation of NPM1 mutants in the cytoplasm of leukemic cells. Diagnosis of AML with mutated NPM1 can be done using molecular techniques, immunohistochemistry (looking at cytoplasmic dislocation of nucleophosmin that is predictive of NPM1 mutations) and Western blotting with antibodies specifically directed against NPM1 mutants. Because of its distinctive molecular, pathological, immunophenotypic and prognostic features, AML with mutated NPM1 (synonym: NPMc+ AML) has been included, as a new provisional entity, in the 2008 World Health Organization (WHO) classification of myeloid neoplasms.

Mps1 as a link between centrosomes and genomic instability

Centrosomes are microtubule-organizing centers that must be precisely duplicated before mitosis. Centrosomes regulate mitotic spindle assembly, and the presence of excess centrosomes leads to the production of aberrant mitotic spindles which generate chromosome segregation errors. Many human tumors possess excess centrosomes that lead to the production of abnormal spindles in situ. In some tumors, these extra centrosomes appear before aneuploidy, suggesting that defects in centrosome duplication might promote genomic instability and tumorigenesis. The Mps1 protein kinase is required for centrosome duplication, and preventing the proteasome-dependent degradation of Mps1 at centrosomes increases its local concentration and causes the production of excess centrosomes during a prolonged S-phase. Here, we show that Mps1 degradation is misregulated in two tumor-derived cell lines, and that the failure to appropriately degrade Mps1 correlates with the ability of these cells to produce extra centrosomes during a prolonged S-phase. In the 21NT breast-tumor derived cell line, a mutant Mps1 protein that is normally constitutively degraded can accumulate at centrosomes and perturb centrosome duplication, suggesting that these cells have a defect in the mechanisms that target Mps1 to the proteasome. In contrast, the U2OS osteosarcoma cell line expresses a nondegradable form of Mps1, which we show causes the dose-dependent over duplication of centrioles even at very low levels of expression. Our data demonstrate that defects in Mps1 degradation can occur through multiple mechanisms, and suggest that Mps1 may provide a link between the control of centrosome duplication and genomic instability.

Localization of TEIF in the centrosome and its functional association with centrosome amplification in DNA damage, telomere dysfunction and human cancers

Centrosome amplification and telomere shortening, which are commonly detected in human cancers, have been implicated in the induction of chromosome instability in tumorigenesis. The functions of these two structures are closely related to DNA damage repair machinery, and some factors that operate in the maintenance of telomeres also take part in the regulation of centrosome status, suggesting they are functionally linked. We report that TEIF (telomerase transcriptional elements-interacting factor), a transactivator of the hTERT (human telomerase reverse transcriptase subunit) gene, is distributed in the centrosome throughout the cell cycle, but its transport into the centrosome is increased under some conditions, and its distribution is dependent on its C-terminal domain. Experimental modulation of TEIF expression through overexpression, polypeptide expression or depletion affected centrosome status and increased abnormalities of cell mitosis. Localization of TEIF to the centrosome was also stimulated by treatment with genotoxic agents and experimental telomere dysfunction, accompanying centrosome amplification. Moreover, we demonstrated that the expression level of TEIF is not only closely correlated with centrosome amplification in soft tissue sarcomas but it is also significantly related to tumor histologic grade. Our data confirmed TEIF functions as a centrosome regulator. Its participation in DNA damage response, including telomere dysfunction and tumorigenesis, indicates TEIF is likely to be a factor involved in linking centrosome amplification and telomere dysfunction in cancer development.

Cyclin E in centrosome duplication and reduplication in sea urchin zygotes

When protein synthesis is completely blocked from before fertilization, the sea urchin zygote arrests in first S phase and the paternal centrosome reduplicates multiple times. However, when protein synthesis is blocked starting in prophase of first mitosis, the zygote divides and the blastomeres arrest in a G1-like state. The centrosome inherited from this mitosis duplicates only once in each blastomere for reasons that are not understood. The late G1 rise in cyclin E/cdk2 kinase activity initiates centrosome duplication in mammalian cells and its activity is needed for centrosome duplication in Xenopus egg extracts. Since the half-time for cyclin E turnover is normally approximately 1 h in sea urchin zygotes, the different behaviors of centrosomes during G1 and S phase arrests could be due to differential losses of cyclin E and its associated kinase activities at these two arrest points. To better understand the mechanisms that limit centrosome duplication, we characterize the levels of cyclin E and its associated kinase activity at the S phase and G1 arrest points. We first demonstrate that cyclin E/cdk2 kinase activity is required for centrosome duplication and reduplication in sea urchin zygotes. Next we find that cyclin E levels and cyclin E/cdk2 kinase activities are both constitutively and equivalently elevated during both the S phase and G1 arrests. This indicates that centrosome duplication during the G1 arrest is limited by a block to reduplication under conditions permissive for duplication. The cytoplasmic conditions of S phase, however, abrogate this block to reduplication.

Centrosome amplification can initiate tumorigenesis in flies

Centrosome amplification is a common feature of many cancer cells, and it has been previously proposed that centrosome amplification can drive genetic instability and so tumorigenesis. To test this hypothesis, we generated Drosophila lines that have extra centrosomes in approximately 60% of their somatic cells. Many cells with extra centrosomes initially form multipolar spindles, but these spindles ultimately become bipolar. This requires a delay in mitosis that is mediated by the spindle assembly checkpoint (SAC). As a result of this delay, there is no dramatic increase in genetic instability in flies with extra centrosomes, and these flies maintain a stable diploid genome over many generations. The asymmetric division of the larval neural stem cells, however, is compromised in the presence of extra centrosomes, and larval brain cells with extra centrosomes can generate metastatic tumors when transplanted into the abdomens of wild-type hosts. Thus, centrosome amplification can initiate tumorigenesis in flies.

The mammalian centrosome and its functional significance

Primarily known for its role as major microtubule organizing center, the centrosome is increasingly being recognized for its functional significance in key cell cycle regulating events. We are now at the beginning of understanding the centrosome’s functional complexities and its major impact on directing complex interactions and signal transduction cascades important for cell cycle regulation. The centrosome orchestrates entry into mitosis, anaphase onset, cytokinesis, G1/S transition, and monitors DNA damage. Recently, the centrosome has also been recognized as major docking station where regulatory complexes accumulate including kinases and phosphatases as well as numerous other cell cycle regulators that utilize the centrosome as platform to coordinate multiple cell cycle-specific functions. Vesicles that are translocated along microtubules to and away from centrosomes may also carry enzymes or substrates that use centrosomes as main docking station. The centrosome’s role in various diseases has been recognized and a wealth of data has been accumulated linking dysfunctional centrosomes to cancer, Alstrom syndrome, various neurological disorders, and others. Centrosome abnormalities and dysfunctions have been associated with several types of infertility. The present review highlights the centrosome’s significant roles in cell cycle events in somatic and reproductive cells and discusses centrosome abnormalities and implications in disease.

Tastin is required for bipolar spindle assembly and centrosome integrity during mitosis

Tastin was previously characterized as an accessory protein for cell adhesion that participates in early embryo implantation. Here, we report that tastin is also required for spindle assembly during mitosis. Tastin protein levels peaked in the G(2)/M phase and abruptly declined after cell division. Microscopy showed that tastin is primarily localized on the microtubules, centrosomes, and the mitotic spindle during the cell cycle. Tastin interacted with the dynein intermediate chain, p150(Glued), and gamma-tubulin in addition to Tctex-1 (the light chain of dynein). Overexpression of tastin led to monopolar spindle formation, whereas loss of tastin expression caused profound mitotic block and preferentially induced multipolar spindles. These multipolar spindles were generated through a loss of cohesion in mitotic centrosomes; specifically, tastin depletion caused the fragmentation of pericentrosomal material and the splitting of the centrioles at the spindle poles. Tastin depletion induced centrosome abnormalities exclusively during mitosis and required both microtubule integrity and Eg5 activity. However, tastin depletion did not disrupt the organization of spindle poles, as revealed by localization of nuclear mitotic apparatus protein (NuMA) and the p150(Glued) component of dynactin. These data indicate that the major function of tastin during mitosis is to maintain the structural and dynamic features of centrosomes, thereby contributing to spindle bipolarity.

Defective cohesin is associated with age-dependent misaligned chromosomes in oocytes

Aneuploidy often results from chromosome misalignment at metaphases. Oocytes from senescence-accelerated mice (SAM) exhibit increased chromosome misalignment with age, which originates from nuclear factors. This work sought to further characterize the underlying defects of chromosome misalignments. Using immunofluorescence microscopy with specific antibodies, several specific components associated with spindles or chromosomes, including centrosomes, centromeres and cohesin complex were examined. No obvious differences were found in the distribution of centrosome focus at the spindle pole of oocytes from young and aged SAM, regardless of chromosome alignments, although cytoplasmic centrosome foci were significantly reduced in aged SAM (P < 0.0001). Oocytes from both young and aged SAM exhibited centromere-associated protein-E (CENP-E) at centromeres of all chromosomes, including misaligned chromosomes from aged SAM, demonstrating that CENP-E did not contribute to chromosome misalignments. Notably, both meiotic cohesin proteins located between sister chromatids, REC8 (recombinant 8), STAG3 (stromal antigen 3) and SMC1beta, were remarkably reduced in oocytes from aged SAM. Further, degradation of the cohesin was even more obvious in SAM than in hybrid F1 mice with age, which may explain why SAM are vulnerable to aneuploidy. This natural ageing mouse model shows that defective cohesin coincides with increased incidence of chromosome misalignment and precocious separations of sister chromatids.

Preventing the degradation of mps1 at centrosomes is sufficient to cause centrosome reduplication in human cells

Supernumerary centrosomes promote the assembly of abnormal mitotic spindles in many human tumors. In human cells, overexpression of the cyclin-dependent kinase (Cdk)2 partner cyclin A during a prolonged S phase produces extra centrosomes, called centrosome reduplication. Cdk2 activity protects the Mps1 protein kinase from proteasome-mediated degradation, and we demonstrate here that Mps1 mediates cyclin A-dependent centrosome reduplication. Overexpression of cyclin A or a brief proteasome inhibition increases the centrosomal levels of Mps1, whereas depletion of Cdk2 leads to the proteasome-dependent loss of Mps1 from centrosomes only. When a Cdk2 phosphorylation site within Mps1 (T468) is mutated to alanine, Mps1 cannot accumulate at centrosomes or participate in centrosome duplication. In contrast, phosphomimetic mutations at T468 or deletion of the region surrounding T468 prevent the proteasome-dependent removal of Mps1 from centrosomes in the absence of Cdk2 activity. Moreover, cyclin A-dependent centrosome reduplication requires Mps1, and these stabilizing Mps1 mutations cause centrosome reduplication, bypassing cyclin A. Together, our data demonstrate that the region surrounding T468 contains a motif that regulates the accumulation of Mps1 at centrosomes. We suggest that phosphorylation of T468 attenuates the degradation of Mps1 at centrosomes and that preventing this degradation is necessary and sufficient to cause centrosome reduplication in human cells.

Spindle orientation, asymmetric division and tumour suppression in Drosophila stem cells

Recent genetic studies in flies have added further support to an increasing body of evidence that suggests that stem cells might be the cell-of-origin of certain tumours. Malfunction of the mechanisms that control the division of stem cells and the developmental fate of the two resulting daughters could be one of the initial events that steers cells into malignant transformation. These studies suggest a role for controlled spindle orientation in suppressing stem-cell overgrowth. In parallel, the machinery that drives asymmetry in stem cells has been further characterized, identifying new components and uncovering the unique, highly sophisticated behaviour of centrosomes in these cells.

MYCN-Directed Centrosome Amplification Requires MDM2-Mediated Suppression of p53 Activity in Neuroblastoma Cells

The MYC family oncogenes cause transformation and tumor progression by corrupting multiple cellular pathways, altering cell cycle progression, apoptosis, and genomic instability. Several recent studies show that MYCC (c-Myc) expression alters DNA repair mechanisms, cell cycle checkpoints, and karyotypic stability, and this is likely partially due to alterations in centrosome replication control. In neuroblastoma cell lines, MYCN (N-Myc) expression induces centrosome amplification in response to ionizing radiation. Centrosomes are cytoplasmic domains that critically regulate cytokinesis, and aberrations in their number or structure are linked to mitotic defects and karyotypic instability. Whereas centrosome replication is linked to p53 and Rb/E2F-mediated cell cycle progression, the mechanisms downstream of MYCN that generate centrosome amplification are incompletely characterized. We hypothesized that MDM2, a direct transcriptional target of MYCN with central inhibitory effects on p53, plays a role in MYC-mediated genomic instability by altering p53 responses to DNA damage, facilitating centrosome amplification. Herein we show that MYCN mediates centrosome amplification in a p53-dependent manner. Accordingly, inhibition of the p53-MDM2 interaction with Nutlin 3A (which activates p53) completely ablates the MYCN-dependent contribution to centrosome amplification after ionizing radiation. We further show that modulating MDM2 expression levels by overexpression or RNA interference-mediated posttranscriptional inhibition dramatically affects centrosome amplification in MYCN-induced cells, indicating that MDM2 is a necessary and sufficient mediator of MYCN-mediated centrosome amplification. Finally, we show a significant correlation between centrosome amplification and MYCN amplification in primary neuroblastoma tumors. These data support the hypothesis that elevated MDM2 levels contribute to MYCN-induced genomic instability through altered regulation of centrosome replication in neuroblastoma.

Nucleophosmin and human cancer

Nucleophosmin (NPM) is a nucleolar phosphoprotein that shuttles between the nucleus and cytoplasm during the cell cycle. NPM has several interacting partners and diverse cellular functions, including the processing of ribosomal RNA, centrosome duplication and the control of cellular processes to ensure genomic stability. Subcellular localization of NPM appears to be strongly correlated with NPM functions and cell proliferation. NPM is phosphorylated mainly at its central acidic domain by several upstream kinases, and its phosphorylation appears to be involved in regulating its functions in ribosome biogenesis and centrosome duplication. Recent studies suggest that NPM may act as a licensing factor to maintain proper centrosome duplication and that the Ran/CRM1 nucleocytoplasmic complex regulates local trafficking of NPM to centrosomes by interacting through its nuclear export sequence motif. Here, we provide a brief overview of NPM functions and its roles in human carcinogenesis, and discuss our recent findings related to the potential mechanisms underlying its regulation of centrosome duplication.

Cyclin G2 is a centrosome-associated nucleocytoplasmic shuttling protein that influences microtubule stability and induces a p53-dependent cell cycle arrest

Cyclin G2 is an atypical cyclin that associates with active protein phosphatase 2A. Cyclin G2 gene expression correlates with cell cycle inhibition; it is significantly upregulated in response to DNA damage and diverse growth inhibitory stimuli, but repressed by mitogenic signals. Ectopic expression of cyclin G2 promotes cell cycle arrest, cyclin dependent kinase 2 inhibition and the formation of aberrant nuclei [Bennin, D. A., Don, A. S., Brake, T., McKenzie, J. L., Rosenbaum, H., Ortiz, L., DePaoli-Roach, A. A., and Horne, M. C. (2002). Cyclin G2 associates with protein phosphatase 2A catalytic and regulatory B' subunits in active complexes and induces nuclear aberrations and a G(1)/S-phase cell cycle arrest. J Biol Chem 277, 27449-67]. Here we report that endogenous cyclin G2 copurifies with centrosomes and microtubules (MT) and that ectopic G2 expression alters microtubule stability. We find exogenous and endogenous cyclin G2 present at microtubule organizing centers (MTOCs) where it colocalizes with centrosomal markers in a variety of cell lines. We previously reported that cyclin G2 forms complexes with active protein phosphatase 2A (PP2A) and colocalizes with PP2A in a detergent-resistant compartment. We now show that cyclin G2 and PP2A colocalize at MTOCs in transfected cells and that the endogenous proteins copurify with isolated centrosomes. Displacement of the endogenous centrosomal scaffolding protein AKAP450 that anchors PP2A at the centrosome resulted in the depletion of centrosomal cyclin G2. We find that ectopic expression of cyclin G2 induces microtubule bundling and resistance to depolymerization, inhibition of polymer regrowth from MTOCs and a p53-dependent cell cycle arrest. Furthermore, we determined that a 100 amino acid carboxy-terminal region of cyclin G2 is sufficient to both direct GFP localization to centrosomes and induce cell cycle inhibition. Colocalization of endogenous cyclin G2 with only one of two GFP-centrin-tagged centrioles, the mature centriole present at microtubule foci, indicates that cyclin G2 resides primarily on the mother centriole. Copurification of cyclin G2 and PP2A subunits with microtubules and centrosomes, together with the effects of ectopic cyclin G2 on cell cycle progression, nuclear morphology and microtubule growth and stability, suggests that cyclin G2 may modulate the cell cycle and cellular division processes through modulation of PP2A and centrosomal associated activities.

Methotrexate resistance in vitro is achieved by a dynamic selectionprocess of tumor cell variants emerging during treatment

Genetic instability leads to tumor heterogeneity, which in turn provides a source of cell variants responsible for drug resistance. However, the source of resistant cells during the process of acquired resistance is poorly understood. Our aim has been to characterize the mechanism by which acquired resistance to methotrexate emerges during the course of cancer cell treatment in vitro. We recently demonstrated that, in vitro, HT-29 colon cancer cells become transiently sensitive to methotrexate by depleting the extracellular milieu of survival factors; on the other hand, the cell population under treatment can reversibly adapt to grow below a critical cell density in the presence of the drug. Here, we show that this adapted cell population gives rise to permanent resistant populations through repeated cycles of cell death and growth. This increased cell turnover, but not merely cell proliferation, is required for the appearance of increasing degrees of stable resistance that are progressively selected by drug pressure. Such a process, taking place in multiple steps, is here designated "dynamic selection." The analysis of sensitive and resistant HT-29 cell populations revealed that methotrexate induces genomic instability--characterized by centrosome amplification and aberrant chromosome recombination--leading to a low-level amplification of the 5q chromosome arm as one of the earliest genetic events selected during treatment. Therefore, this model provides a mechanism by which a tumor cell population lacking resistant subpopulations before treatment is able to acquire the genetic changes required for stable drug resistance.

The centrosome and the DNA damage induced checkpoint

The centrosome, the microtubule-organizing center of the cell, acts as a localization point, where signaling molecules are able to interact. Many kinases and phosphatases critical for regulation of DNA damage signaling pathways localize to the centrosome. This review will discuss the possible involvement of the centrosome in mediating DNA damage checkpoint control, in particular the effect of DNA damage signaling pathways involved in initiation or maintenance of cell cycle arrest on the centrosome. The mechanisms that lead to centrosome abnormalities such as centrosome hyperamplification and multipolarity in response to DNA damage will also be addressed.

Centrosomal proteins Nde1 and Su48 form a complex regulated by phosphorylation

The centrosome modulates spindle formation and plays a critical role in guiding proper segregation of chromosomes during cell division. Centrosome aberrations, frequently seen in human tumors, may cause abnormal chromosome segregation and contribute to malignant transformation. To explore the components of the centrosomes, we previously identified a novel centrosomal protein called Su48. To further characterize the Su48-containing protein ensemble in the centrosome, we performed yeast two-hybrid screens and isolated a number of Su48-interacting molecules, including the centrosomal protein Nde1. Here, we demonstrate that Su48 can associate with Nde1. Moreover, we found that Nde1 is subjected to phosphorylation in vivo. In particular, we identified six putative Cdc2 phosphorylation sites in Nde1 and found that alteration of these sites diminishes phosphorylation by Cdc2 in vitro and affects the stability of Su48-Nde1 interactions and the centrosomal localization of Nde1. Ablation of Nde1 by gene specific small interfering RNA causes mitotic delay and cell death, coupled with a modest decrease in the incidence of the cells that harbor excessive centrosomes. Collectively, our findings indicate that Nde1 can form a protein complex with Su48 in the centrosome and plays an important role for successful mitosis.

The structure of the human centrin 2-xeroderma pigmentosum group C protein complex

Human centrin-2 plays a key role in centrosome function and stimulates nucleotide excision repair by binding to the xeroderma pigmentosum group C protein. To determine the structure of human centrin-2 and to develop an understanding of molecular interactions between centrin and xeroderma pigmentosum group C protein, we characterized the crystal structure of calcium-loaded full-length centrin-2 complexed with a xeroderma pigmentosum group C peptide. Our structure shows that the carboxyl-terminal domain of centrin-2 binds this peptide and two calcium atoms, whereas the amino-terminal lobe is in a closed conformation positioned distantly by an ordered alpha-helical linker. A stretch of the amino-terminal domain unique to centrins appears disordered. Two xeroderma pigmentosum group C peptides both bound to centrin-2 also interact to form an alpha-helical coiled-coil. The interface between centrin-2 and each peptide is predominantly nonpolar, and key hydrophobic residues of XPC have been identified that lead us to propose a novel binding motif for centrin.

Temporal and spatial control of nucleophosmin by the Ran-Crm1 complex in centrosome duplication

Centrosome duplication is tightly controlled during faithful cell division, and unnecessary reduplication can lead to supernumerary centrosomes and multipolar spindles that are associated with most human cancer cells. In addition to nucleocytoplasmic transport, the Ran-Crm1 network is involved in regulating centrosome duplication to ensure the formation of a bipolar spindle. Here, we discover that nucleophosmin (NPM) may be a Ran-Crm1 substrate that controls centrosome duplication. NPM contains a functional nuclear export signal (NES) that is responsible for both its nucleocytoplasmic shuttling and its association with centrosomes, which are Ran-Crm1-dependent as they are sensitive to Crm1-specific nuclear export inhibition, either by leptomycin B (LMB) or by the expression of a Ran-binding protein, RanBP1. Notably, LMB treatment induces premature centrosome duplication in quiescent cells, which coincides with NPM dissociation from centrosomes. Moreover, deficiency of NPM by RNA interference results in supernumerary centrosomes, which can be reversed by reintroducing wild-type but not NES-mutated NPM. Mutation of a potential proline-dependent kinase phosphorylation site at residue 95, from threonine to aspartic acid (T95D) within the NES motif, abolishes NPM association and inhibition of centrosome duplication. Our results are consistent with the hypothesis that the Ran-Crm1 complex may promote a local enrichment of NPM on centrosomes, thereby preventing centrosome reduplication.

Allelotypes and Fluorescence In situ Hybridization Profiles of Poorly Differentiated Endocrine Carcinomas of Different Sites

PURPOSE

The aim of this work was to investigate the genotypic profiles of 36 poorly differentiated endocrine carcinoma (PDEC) of different sites to verify if their very similar phenotype may reflect similar pattern of genetic anomalies and if useful diagnostic or prognostic markers may be pointed out.

EXPERIMENTAL DESIGN

All tumors were microallelotyped at 57 microsatellite on 11 autosomes and the allelotypes of a selected panel of tumors were validated by interphasic fluorescence in situ hybridization with centromeric probes for chromosomes 1, 3, 6, 11, 17, and 18 and a probe specific for p53.

RESULTS

Regardless of the primary sites, PDECs exhibit very complex allelotypes (86%) and TP53 allelic imbalance (89%). Among these cases, fluorescence in situ hybridization analysis confirmed the presence of multiple aneusomies and a chromosome instability phenotype. Very low percentage of allelic imbalance (AI) and few aneuploidies were detected in only five PDECs for which an overall longer survival was observed. We found recurrent AI on 3p, 5, and 11q13 in lung PDECs, on 5q21, 8p, and 18q21 in colorectal PDECs and on 7 and 11q22 in gastric PDECs. Significantly better outcome was observed in patients with PDEC exhibiting 8q AIs and absence of AI at chromosome regions 6q25 and 6p.

CONCLUSIONS

The concurrence of p53 inactivation and aneuploidies or chromosome instability are the main features of PDECs. However, the specific allelotypes observed in relation to primary site support the hypothesis that PDECs and exocrine carcinomas of all sites may share early pathogenetic mechanisms. Molecular markers of potential diagnostic and prognostic values for PDECs of different sites have been identified.

Lats2/Kpm is required for embryonic development, proliferation control and genomic integrity

The Drosophila melanogaster warts/lats tumour suppressor has two mammalian counterparts LATS1/Warts-1 and LATS2/Kpm. Here, we show that mammalian Lats orthologues exhibit distinct expression profiles according to germ cell layer origin. Lats2(-/-) embryos show overgrowth in restricted tissues of mesodermal lineage; however, lethality ultimately ensues on or before embryonic day 12.5 preceded by defective proliferation. Lats2(-/-) mouse embryonic fibroblasts (MEFs) acquire growth advantages and display a profound defect in contact inhibition of growth, yet exhibit defective cytokinesis. Lats2(-/-) embryos and MEFs display centrosome amplification and genomic instability. Lats2 localizes to centrosomes and overexpression of Lats2 suppresses centrosome overduplication induced in wild-type MEFs and reverses centrosome amplification inherent in Lats2(-/-) MEFs. These findings indicate an essential role of Lats2 in the integrity of processes that govern centrosome duplication, maintenance of mitotic fidelity and genomic stability.

Functional STK15 Phe31Ile polymorphism is associated with the occurrence and advanced disease status of esophageal squamous cell carcinoma

STK15/BTAK/Aurora-A involved in regulating centrosomes and chromosome segregation is amplified and overexpressed in human cancers. A T91A polymorphism in STK15 causes Phe31Ile substitution, and the 31Ile variant has been shown to be preferentially amplified and associated with degree of aneuploidy in human tumors. We genotyped 656 patients with esophageal squamous cell carcinoma (ESCC) and 656 controls for the polymorphism to examine the hypothesis that the STK15 variation may affect individual susceptibility to the occurrence and aggression of ESCC. It was found that the Ile/Ile genotype was significantly associated with increased risk of ESCC occurrence [odds ratio (OR) = 1.97, 95% confidence interval (CI) = 1.36-2.85] compared with the Phe/Phe genotype. The 31Ile allele frequency significantly increased as ESCC stage increased (trend test, P = 0.006). Patients with the Ile/Ile genotype had an increased risk for invasive disease (stage II-IV; OR = 2.13, 95% CI = 1.04-4.39) or metastatic disease (stage III and IV; OR = 2.31, 95% CI = 1.06-5.05) compared with those with the Phe/Phe genotype. A positive correlation between the Ile/Ile genotype and high ESCC grade was also observed. Our results demonstrate for the first time that the STK15 polymorphism is a genetic susceptibility factor for the occurrence and aggression of ESCC.

The good, the bad and the ugly: the practical consequences of centrosome amplification

Centrosome amplification (the presence of more than two centrosomes at mitosis) is characteristic of many human cancers. Extra centrosomes can cause the assembly of multipolar spindles, which unequally distribute chromosomes to daughter cells; the resulting genetic imbalances may contribute to cellular transformation. However, this raises the question of how a population of cells with centrosome amplification can survive such chaotic mitoses without soon becoming non-viable as a result of chromosome loss. Recent observations indicate that a variety of mechanisms partially mute the practical consequences of centrosome amplification. Consequently, populations of cells propagate with good efficiency, despite centrosome amplification, yet have an elevated mitotic error rate that can fuel the evolution of the transformed state.

Recapitulation of the effects of the human papillomavirus type 16 E7 oncogene on mouse epithelium by somatic Rb deletion and detection of pRb-independent effects of E7 in vivo

Although the human papillomavirus (HPV) E7 oncogene is known to contribute to the development of human cervical cancer, the mechanisms of its carcinogenesis are poorly understood. The first identified and most recognized function of E7 is its binding to and inactivation of the retinoblastoma tumor suppressor (pRb), but at least 18 other biological activities have also been reported for E7. Thus, it remains unclear which of these many activities contribute to the oncogenic potential of E7. We used a Cre-lox system to abolish pRb expression in the epidermis of transgenic mice and compared the outcome with the effects of E7 expression in the same tissue at early ages. Mice lacking pRb in epidermis showed epithelial hyperplasia, aberrant DNA synthesis, and improper differentiation. In addition, Rb-deleted epidermis (i.e., epidermis composed of cells with Rb deleted) exhibited centrosomal abnormalities and failed to arrest the cell cycle in response to ionizing radiation. Transgenic mice expressing E7 in skin display the same range of phenotypes. In sum, few differences were detected between Rb-deleted epidermis and E7-expressing epidermis in young mice. However, when both E7 was expressed and Rb was deleted in the same tissue, increased hyperplasia and dysplasia were observed. These findings indicate that inactivation of the Rb pathway can largely account for E7's phenotypes at an early age, but that pRb-independent activities of E7 are detectable in vivo.

The Centrosome in Higher Organisms: Structure, Composition, and Duplication

The centrosome found in higher organisms is an organelle with a complex and dynamic architecture and composition. This organelle not only functions as a microtubule-organizing center, but also is integrated with or impacts a number of cellular processes. Defects associated with this organelle have been linked to a variety of human diseases including several forms of cancer. Here we review the emerging picture of how the structure, composition, duplication, and function of the centrosome found in higher organisms are interrelated.

Molecular and functional analysis of the dictyostelium centrosome

The centrosome is a nonmembranous, nucleus-associated organelle that functions not only as the main microtubule-organizing center but also as a cell cycle control unit. How the approximately 100 different proteins that make up a centrosome contribute to centrosome function is still largely unknown. Considerable progress in the understanding of centrosomal functions can be expected from comparative cell biology of morphologically different centrosomal structures fulfilling conserved functions. Dictyostelium is an alternative model organism for centrosome research in addition to yeast and animal cells. With the elucidation of morphological changes and dynamics of centrosome duplication, the establishment of a centrosome isolation protocol, and the identification of many centrosomal components, there is a solid basis for understanding the biogenesis and function of this fascinating organelle. Here we give an overview of the prospective protein inventory of the Dictyostelium centrosome based on database searches. Moreover, we focus on the comparative cell biology of known components of the Dictyostelium centrosome including the gamma-tubulin complex and the homologues of centrin, Nek2, XMAP215, and EB1.

Emerging roles of DNA tumor viruses in cell proliferation: new insights into genomic instability

The small DNA virus proteins E1A and E1B from human Adenovirus, E6 and E7 from human papillomavirus, and large T and small T antigens from SV40, are multifaceted molecular tools that can carry out an impressive number of tasks in the host cell. These viral factors, collectively termed 'oncoproteins' for their ability to induce cancer, can be viewed as paradigmatic oncogenic factors which can disrupt checkpoint controls at multiple levels--they interfere with both 'gatekeeper' cellular functions, including major control pathways of cell cycle and apoptosis, and with 'caretaker' functions, thereby inducing mitotic abnormalities and increasing genomic instability. Both E1A and E7 have been recently found to interact physically with the Ran GTPase. This interaction is key in uncoupling the centrosome cycle from the cell cycle, highlighting a direct link between viral infection and the induction of genomic instability. Further expanding our current knowledge in this field will be crucial to elucidate viral strategies leading to cellular transformation and cancer progression, as well as design novel preventive or therapeutic approaches to human cancer.

Involvement of Crm1 in hepatitis B virus X protein-induced aberrant centriole replication and abnormal mitotic spindles

Hepatitis B virus (HBV) includes an X gene (HBx gene) that plays a critical role in liver carcinogenesis. Because centrosome abnormalities are associated with genomic instability in most human cancer cells, we examined the effect of HBx on centrosomes. We found that HBx induced supernumerary centrosomes and multipolar spindles. This effect was independent of mutations in the p21 gene. Furthermore, the ability of HBV to induce supernumerary centrosomes was dependent on the presence of physiological HBx expression. We recently showed that HBx induces cytoplasmic sequestration of Crm1, a nuclear export receptor that binds to Ran GTPase, thereby inducing nuclear localization of NF-kappaB. Consistently, supernumerary centrosomes were observed in cells treated with a Crm1-specific inhibitor but not with an HBx mutant that lacked the ability to sequester Crm1 in the cytoplasm. Moreover, a fraction of Crm1 was found to be localized at the centrosomes. Immunocytochemical and ultrastructural examination of these supernumerary centrosomes revealed that inactivation of Crm1 was associated with abnormal centrioles. The presence of more than two centrosomes led to an increased frequency of defective mitoses and chromosome transmission errors. Based on this evidence, we suggest that Crm1 is actively involved in maintaining centrosome integrity and that HBx disrupts this process by inactivating Crm1 and thus contributes to HBV-mediated carcinogenesis.

Centrosome number is controlled by a centrosome-intrinsic block to reduplication

The centrosome duplicates once in S phase. To determine whether there is a block in centrosome reduplication, we used a cell fusion assay to compare the duplication potential of unduplicated G1 centrosomes and recently duplicated G2 centrosomes. By fusing cells in different cell cycle stages, we found that G2 centrosomes were unable to reduplicate in a cellular environment that supports centrosome duplication. Furthermore, G2 cytoplasm did not inhibit centrosome duplication in fused cells, indicating that the block to reduplication is intrinsic to the centrosomes rather than the cytoplasm. To test the underlying mechanism, we created mononucleate G1 cells with two centrosomes by fusing cells with enucleated cytoplasts. Both centrosomes duplicated, indicating that the block is not controlled by centrosome:nucleus ratio. We also found that human primary cells have tight control over centrosome number during prolonged S-phase arrest and that this control is partially abrogated in transformed cells. This suggests a link between the control of centrosome duplication and maintenance of genomic stability.

Higher order structure of the PCM adjacent to the centriole

The centrosome is the major microtubule organizing center in most animal cells. This cytoplasmic organelle consists of two components : a mature centriole (or a pair of centrioles) and a mass of pericentriolar material (PCM). The PCM has been described as either a cloud of material that encases the entire centriole or as a cluster of proteins divided into two subsets, one that adheres to the lateral surface of the centriole and another that extends outward from this region as a cloud of material. In contrast to these protein distribution patterns, we demonstrated in a previous study that a subset of proteins present within the PCM is integrated together to form a tube (PCM tube) with an open and closed end that is duplicated in concert with centrosome duplication. The present study was undertaken to determine if this tubular conformation represents proteins that are confined to the surface of the centriole or if it represents a subset of proteins within the cloud of material that extends outward from the centriole. We document that : (1) the PCM tube represents a portion of the PCM directly associated with the centriole; (2) the PCM tube has a specific and reproducible relationship to the polar structure of the centriole; (3) the tube is a site of cytoplasmic microtubule organization, and has a structure that influences the initial pattern of microtubule assembly within the juxta-centriolar region; and (4) the PCM tube has a structural relationship with respect to the centriole, which allows the simultaneous expression of centriole- and PCM-based functions (e.g., ciliogenesis and cytoplasmic microtubule organization). Based on these findings, we propose a new model of the PCM at the centriole. This model highlights the role played by the proximal end of the centriole in the nucleation and organization of centriole-associated PCM, and indicates that the centrosome has an overall polarity in the region of the centriole.

Centrosomes and tumour suppressors

Centrosomes are microtubule organising centres that act as spindle poles during mitosis. Recent work implicates centrosomes in many other processes, and shows that centrosome defects can cause genetic instability. Many regulators of mammalian centrosome function were predicted from studies of model systems. Surprisingly, some well-known tumour suppressors have recently been found at centrosomes, where they influence centrosome duplication and function, suggesting that control of centrosome function is central to genetic stability.

The centrosome cycle

The centrosome is the major microtubule-organizing center of animal cells. It influences cell shape and polarity and directs the formation of the bipolar mitotic spindle. Numerical and structural centrosome aberrations have been implicated in disease, notably cancer. In dividing cells, centrosomes need to be duplicated and segregated in synchrony with chromosomes. This centrosome cycle requires a series of structural and functional transitions that are regulated by both phosphorylation and proteolysis. Here we summarize recent information on the regulation of the centrosome cycle and its coordination with the chromosomal cell cycle.