Altered centrosome structure is associated with abnormal mitoses in human breast tumors

Loss of cyclin-dependent kinase 2 (CDK2) inhibitory phosphorylation in a CDK2AF knock-in mouse causes misregulation of DNA replication and centrosome duplication

Cyclin-dependent kinase 1 (CDK1) inhibitory phosphorylation controls the onset of mitosis and is essential for the checkpoint pathways that prevent the G(2)- to M-phase transition in cells with unreplicated or damaged DNA. To address whether CDK2 inhibitory phosphorylation plays a similar role in cell cycle regulation and checkpoint responses at the start of the S phase, we constructed a mouse strain in which the two CDK2 inhibitory phosphorylation sites, threonine 14 and tyrosine 15, were changed to alanine and phenylalanine, respectively (CDK2AF). This approach showed that inhibitory phosphorylation of CDK2 had a major role in controlling cyclin E-associated kinase activity and thus both determined the timing of DNA replication in a normal cell cycle and regulated centrosome duplication. Further, DNA damage in G(1) CDK2AF cells did not downregulate cyclin E-CDK2 activity when the CDK inhibitor p21 was also knocked down. We were surprised to find that this was insufficient to cause cells to bypass the checkpoint and enter the S phase. This led to the discovery of two previously unrecognized pathways that control the activity of cyclin A at the G(1) DNA damage checkpoint and may thereby prevent S-phase entry even when cyclin E-CDK2 activity is deregulated.

How mitotic errors contribute to karyotypic diversity in cancer

Aneuploidy is a common feature of cancer cells, and is believed to play a critical role in tumorigenesis and cancer progression. Most cancer cells also exhibit high rates of mitotic chromosome mis-segregation, a phenomenon known as chromosomal instability, which leads to high variability of the karyotype. Here, we describe the nature, nuances, and implications of cancer karyotypic diversity. Moreover, we summarize recent studies aimed at identifying the mitotic defects that may be responsible for inducing chromosome mis-segregation in cancer cells. These include kinetochore attachment errors, spindle assembly checkpoint dysfunction, mitotic spindle defects, and other cell division inaccuracies. Finally, we discuss how such mitotic errors generate karyotypic diversity in cancer cells.

Interplay between BRCA1 and RHAMM regulates epithelial apicobasal polarization and may influence risk of breast cancer

Differentiated mammary epithelium shows apicobasal polarity, and loss of tissue organization is an early hallmark of breast carcinogenesis. In BRCA1 mutation carriers, accumulation of stem and progenitor cells in normal breast tissue and increased risk of developing tumors of basal-like type suggest that BRCA1 regulates stem/progenitor cell proliferation and differentiation. However, the function of BRCA1 in this process and its link to carcinogenesis remain unknown. Here we depict a molecular mechanism involving BRCA1 and RHAMM that regulates apicobasal polarity and, when perturbed, may increase risk of breast cancer. Starting from complementary genetic analyses across families and populations, we identified common genetic variation at the low-penetrance susceptibility HMMR locus (encoding for RHAMM) that modifies breast cancer risk among BRCA1, but probably not BRCA2, mutation carriers: n = 7,584, weighted hazard ratio ((w)HR) = 1.09 (95% CI 1.02-1.16), p(trend) = 0.017; and n = 3,965, (w)HR = 1.04 (95% CI 0.94-1.16), p(trend) = 0.43; respectively. Subsequently, studies of MCF10A apicobasal polarization revealed a central role for BRCA1 and RHAMM, together with AURKA and TPX2, in essential reorganization of microtubules. Mechanistically, reorganization is facilitated by BRCA1 and impaired by AURKA, which is regulated by negative feedback involving RHAMM and TPX2. Taken together, our data provide fundamental insight into apicobasal polarization through BRCA1 function, which may explain the expanded cell subsets and characteristic tumor type accompanying BRCA1 mutation, while also linking this process to sporadic breast cancer through perturbation of HMMR/RHAMM.

A clinical overview of centrosome amplification in human cancers

The turn of the 21st century had witnessed a surge of interest in the centrosome and its causal relation to human cancer development - a postulate that has existed for almost a century. Centrosome amplification (CA) is frequently detected in a growing list of human cancers, both solid and haematological, and is a candidate "hallmark" of cancer cells. Several lines of evidence support the progressive involvement of CA in the transition from early to advanced stages of carcinogenesis, being also found in pre-neoplastic lesions and even in histopathologically-normal tissue. CA constitutes the major mechanism leading to chromosomal instability and aneuploidy, via the formation of multipolar spindles and chromosomal missegregation. Clinically, CA may translate to a greater risk for initiation of malignant transformation, tumour progression, chemoresistance and ultimately, poor patient prognosis. As mechanisms underlying CA are progressively being unravelled, the centrosome has emerged as a novel candidate target for cancer treatment. This Review summarizes mainly the clinical studies performed to date focusing on the mechanisms underlying CA in human neoplasia, and highlights the potential utility of centrosomes in the diagnosis, prognosis and treatment of human cancers.

Control of centrin stability by Aurora A

Aurora A is an oncogenic serine/threonine kinase which can cause cell transformation and centrosome amplification when over-expressed. Human breast tumors show excess Aurora A and phospho-centrin in amplified centrosomes. Here, we show that Aurora A mediates the phosphorylation of and localizes with centrin at the centrosome, with both proteins reaching maximum abundance from prophase through metaphase, followed by their precipitous loss in late stages of mitosis. Over-expression of Aurora A results in excess phospho-centrin and centrosome amplification. In contrast, centrosome amplification is not seen in cells over-expressing Aurora A in the presence of a recombinant centrin mutant lacking the serine phosphorylation site at residue 170. Expression of a kinase dead Aurora A results in a decrease in mitotic index and abrogation of centrin phosphorylation. Finally, a recombinant centrin mutation that mimics centrin phosphorylation increases centrin's stability against APC/C-mediated proteasomal degradation. Taken together, these results suggest that the stability of centrin is regulated in part by Aurora A, and that excess phosphorylated centrin may promote centrosome amplification in cancer.

Mitotic catastrophe in malignant epithelial tumors: the pathologist's viewpoint

Mitotic catastrophe is a common phenomenon occurring in tumor cells with impaired p53 function exposed to various cytotoxic and genotoxic agents. The defective p53 checkpoint causes improper segregation of chromosomes, resulting in aberrant mitosis, multiple micronuclei, multinucleate giant cells, and eventual necrosis-like death and centrosome aberration. Although various descriptions explaining mitotic catastrophe exist, there is still no generally accepted definition of this phenomenon. However, the syndrome of mitotic catastrophe may be a unifying morphological concept of particular interest to cancer research, as it integrally links cell death to checkpoints of the cell cycle. Morphological findings compatible with mitotic catastrophe may be found in pleomorphic, giant cell carcinomas--neoplasms characterized by a poor prognosis. The inclusion of mitotic catastrophe as part of the microscopic evaluation of tumors will add further insight to the pathobiology of tumor progression and in novel therapeutic designs. Finally, the possibility of assimilating mitotic catastrophe into a prognostic score is discussed.

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.

Human embryonic stem cells suffer from centrosomal amplification

Propagation of human embryonic stem cells (hESCs) in culture tends to alter karyotype, potentially limiting the prospective use of these cells in patients. The chromosomal instability of some malignancies is considered to be driven, at least in part, by centrosomal overamplification, perturbing balanced chromosome segregation. Here, we report, for the first time, that very high percentage of cultured hESCs has supernumerary centrosomes during mitosis. Supernumerary centrosomes were strictly associated with an undifferentiated hESC state and progressively disappeared on prolonged propagation in culture. Improved attachment to culture substratum and inhibition of CDK2 and Aurora A (key regulators of centrosomal metabolism) diminished the frequency of multicentrosomal mitoses. Thus, both attenuated cell attachment and deregulation of machinery controlling centrosome number contribute to centrosomal overamplification in hESCs. Linking the excessive number of centrosomes in mitoses to the ploidy indicated that both overduplication within a single cell cycle and mitotic failure contributed to generation of numerical centrosomal abnormalities in hESCs. Collectively, our data indicate that supernumerary centrosomes are a significant risk factor for chromosome instability in cultured hESCs and should be evaluated when new culture conditions are being implemented.

Influence of centriole number on mitotic spindle length and symmetry

The functional role of centrioles or basal bodies in mitotic spindle assembly and function is currently unclear. Although supernumerary centrioles have been associated with multipolar spindles in cancer cells, suggesting centriole number might dictate spindle polarity, bipolar spindles are able to assemble in the complete absence of centrioles, suggesting a level of centriole-independence in the spindle assembly pathway. In this report we perturb centriole number using mutations in Chlamydomonas reinhardtii, and measure the response of the mitotic spindle to these perturbations in centriole number. Although altered centriole number increased the frequency of monopolar and multipolar spindles, the majority of spindles remained bipolar regardless of the centriole number. But even when spindles were bipolar, abnormal centriole numbers led to asymmetries in tubulin distribution, half-spindle length and spindle pole focus. Half spindle length correlated directly with number of centrioles at a pole, such that an imbalance in centriole number between the two poles of a bipolar spindle correlated with increased asymmetry between half spindle lengths. These results are consistent with centrioles playing an active role in regulating mitotic spindle length. Mutants with centriole number alteration also show increased cytokinesis defects, but these do not correlate with centriole number in the dividing cell and may therefore reflect downstream consequences of defects in preceding cell divisions.

Antizyme restrains centrosome amplification by regulating the accumulation of Mps1 at centrosomes

The failure to degrade Mps1 at centrosomes causes centrosome overproduction, but the factors that target Mps1 for degradation are unknown. This study shows that antizyme, a mediator of ubiquitin-independent degradation, binds to Mps1 and modulates centrosomal Mps1 via the proteasome, revealing a role for Mps1 in procentriole assembly.

Extra centrosomes are found in many tumors, and their appearance is an early event that can generate aberrant mitotic spindles and aneuploidy. Because the failure to appropriately degrade the Mps1 protein kinase correlates with centrosome overproduction in tumor-derived cells, defects in the factors that promote Mps1 degradation may contribute to extra centrosomes in tumors. However, while we have recently characterized an Mps1 degradation signal, the factors that regulate Mps1 centrosomal Mps1 are unknown. Antizyme (OAZ), a mediator of ubiquitin-independent degradation and a suspected tumor suppressor, was recently shown to localize to centrosomes and modulate centrosome overproduction, but the known OAZ substrates were not responsible for its effect on centrosomes. We have found that OAZ exerts its effect on centrosomes via Mps1. OAZ promotes the removal of Mps1 from centrosomes, and centrosome overproduction caused by reducing OAZ activity requires Mps1. OAZ binds to Mps1 via the Mps1 degradation signal and modulates the function of Mps1 in centrosome overproduction. Moreover, OAZ regulates the canonical centrosome duplication cycle, and reveals a function for Mps1 in procentriole assembly. Together, our data suggest that OAZ restrains the assembly of centrioles by controlling the levels of centrosomal Mps1 through the Cdk2-regulated Mps1 degradation signal.

A critical role of integrin-linked kinase, ch-TOG and TACC3 in centrosome clustering in cancer cells

Many cancer cells contain more than two centrosomes, which imposes a potential for multipolar mitoses, leading to cell death. To circumvent this, cancer cells develop mechanisms to cluster supernumerary centrosomes to form bipolar spindles, enabling successful mitosis. Disruption of centrosome clustering thus provides a selective means of killing supernumerary centrosome-harboring cancer cells. Although the mechanisms of centrosome clustering are poorly understood, recent genetic analyses have identified requirements for both actin and tubulin regulating proteins. In this study, we demonstrate that the integrin-linked kinase (ILK), a protein critically involved in actin and mitotic microtubule organization, is required for centrosome clustering. Inhibition of ILK expression or activity inhibits centrosome clustering in several breast and prostate cancer cell lines that have centrosome amplification. Furthermore, cancer cells with supernumerary centrosomes are significantly more sensitive to ILK inhibition than cells with two centrosomes, demonstrating that inhibiting ILK offers a selective means of targeting cancer cells. Live cell analysis shows ILK perturbation leads cancer cells to undergo multipolar anaphases, mitotic arrest and cell death in mitosis. We also show that ILK performs its centrosome clustering activity in a focal adhesion-independent, but centrosome-dependent, manner through the microtubule regulating proteins TACC3 and ch-TOG. In addition, we identify a specific TACC3 phosphorylation site that is required for centrosome clustering and demonstrate that ILK regulates this phosphorylation in an Aurora-A-dependent manner.

A novel role of the aryl hydrocarbon receptor (AhR) in centrosome amplification - implications for chemoprevention

BACKGROUND

Centrosome aberrations can cause genomic instability and correlate with malignant progression in common human malignancies such as breast and prostate cancer. Deregulation of cyclin/cyclin-dependent kinase 2 (CDK2) activity has previously been shown to be critically involved in centrosome overduplication. We therefore test here whether small molecule CDK inhibitors derived from the bis-indole indirubin can be used to suppress centrosome aberrations as a novel approach to chemoprevention of malignant progression.

RESULTS

As expected, we found that the CDK inhibitor indirubin-3'-oxime (IO) suppresses centrosome amplification in breast cancer cells. However, we made the unexpected discovery that indirubin-derived compounds that have been chemically modified to be inactive as kinase inhibitors such as 1-methyl-indirubin-3'-oxime (MeIO) still significantly reduced centrosome amplification. All indirubins used in the present study are potent agonists of the aryl hydrocarbon receptor (AhR), which is known for its important role in the cellular metabolism of xenobiotics. To corroborate our results, we first show that the coincidence of nuclear AhR overexpression, reflecting a constitutive activation, and numerical centrosome aberrations correlates significantly with malignancy in mammary tissue specimens. Remarkably, a considerable proportion (72.7%) of benign mammary tissue samples scored also positive for nuclear AhR overexpression. We furthermore provide evidence that continued expression of endogenous AhR is critical to promote centriole overduplication induced by cyclin E and that AhR and cyclin E may function in the same pathway. Overexpression of the AhR in the absence of exogenous ligands was found to rapidly disrupt centriole duplication control. Nonetheless, the AhR agonists IO and MeIO were still found to significantly reduce centriole overduplication stimulated by ectopic AhR expression.

CONCLUSIONS

Our results indicate that continued expression of endogenous AhR promotes centrosome amplification in breast cancer cells in a pathway that involves cyclin E. AhR agonists such as indirubins inhibit centrosome amplification even when stimulated by ectopic expression of the AhR suggesting that these compounds are potentially useful for the chemoprevention of centrosome-mediated cell division errors and malignant progression in neoplasms in which the AhR is overexpressed. Future studies are warranted to determine whether individuals in which nuclear AhR overexpression is detected in benign mammary tissue are at a higher risk for developing pre-cancerous or cancerous breast lesions.

Low molecular weight cyclin E overexpression shortens mitosis, leading to chromosome missegregation and centrosome amplification

Overexpression of the low molecular weight isoforms (LMW-E) of cyclin E induces chromosome instability; however, the degree to which these tumor-specific forms cause genomic instability differs from that of full-length cyclin E (EL), and the underlying mechanism(s) has yet to be elucidated. Here, we show that EL and LMW-E overexpression impairs the G(2)-M transition differently and leads to different degrees of chromosome instability in a breast cancer model system. First, the most significant difference is that EL overexpression prolongs cell cycle arrest in prometaphase, whereas LMW-E overexpression reduces the length of mitosis and accelerates mitotic exit. Second, LMW-E-overexpressing cells are binucleated or multinucleated with amplified centrosomes, whereas EL-overexpressing cells have the normal complement of centrosomes. Third, LMW-E overexpression causes mitotic defects, chromosome missegregation during metaphase, and anaphase bridges during anaphase, most of which are not detected on EL induction. LMW-E induces additional mitotic defects in cooperation with p53 loss in both normal and tumor cells. Fourth, LMW-E-overexpressing cells fail to arrest in the presence of nocodazole. Collectively, the mitotic defects mediated by LMW-E induction led to failed cytokinesis and polyploidy, suggesting that LMW-E expression primes cells to accrue chromosomal instability by shortening the length of mitosis. Lastly, LMW-E expression in human breast cancer tissues correlates with centrosome amplification and higher nuclear grade. These results suggest that LMW-E overexpression leads to higher centrosome numbers in breast cancer, which is a prerequisite for genomic instability.

Primary cilia are decreased in breast cancer: analysis of a collection of human breast cancer cell lines and tissues

Primary cilia (PC) are solitary, sensory organelles that are critical for several signaling pathways. PC were detected by immunofluorescence of cultured cells and breast tissues. After growth for 7 days in vitro, PC were detected in ∼70% of breast fibroblasts and in 7-19% of epithelial cells derived from benign breast (184A1 and MCF10A). In 11 breast cancer cell lines, PC were present at a low frequency in four (from 0.3% to 4% of cells), but were absent in the remainder. The cancer cell lines with PC were all of the basal B subtype, which is analogous to the clinical triple-negative breast cancer subtype. Furthermore, the frequency of PC decreased with increasing degree of transformation/progression in the MCF10 and MDA-MB-435/LCC6 isogenic models of cancer progression. In histologically normal breast tissues, PC were frequent in fibroblasts and myoepithelial cells and less common in luminal epithelial cells. Of 26 breast cancers examined, rare PC were identified in cancer epithelial cells of only one cancer, which was of the triple-negative subtype. These data indicate a decrease or loss of PC in breast cancer and an association of PC with the basal B subtype. This manuscript contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.

Aurora kinase inhibitors--rising stars in cancer therapeutics?

Standard therapeutic approaches of cytotoxics and radiation in cancer are not only highly toxic, but also of limited efficacy in treatment of a significant number of cancer patients. The molecular analysis of the cancer genomes have shown a remarkable complexity and pointed to key genomic and epigenomic alterations in cancer. These discoveries are paving the way for targeted therapy approaches. However, although there are a large number of potential targets, only a few can regulate key cellular functions and intersect multiple signaling networks. The Aurora kinase family members (A, B, and C) are a collection of highly related and conserved serine-threonine kinases that fulfill these criteria, being key regulators of mitosis and multiple signaling pathways. Alterations in Aurora kinase signaling are associated with mitotic errors and have been closely linked to chromosomal aneuploidy in cancer cells. Several studies have shown amplification and/or overexpression of Aurora kinase A and B in hematologic malignancies and solid tumors. Over the past several years, Aurora kinases have become attractive targets. Several ongoing clinical trials and bench-based research are assessing the unique therapeutic potential of Aurora-based targeted therapy.

Polyploidization of liver cells

Eukaryotic organisms usually contain a diploid complement of chromosomes. However, there are a number of exceptions. Organisms containing an increase in DNA content by whole number multiples of the entire set of chromosomes are defined as polyploid. Cells that contain more than two sets of chromosomes were first observed in plants about a century ago and it is now recognized that polyploidy cells form in many eukaryotes under a wide variety of circumstance. Although it is less common in mammals, some tissues, including the liver, show a high percentage of polyploid cells. Thus, during postnatal growth, the liver parenchyma undergoes dramatic changes characterized by gradual polyploidization during which hepatocytes of several ploidy classes emerge as a result of modified cell-division cycles. This process generates the successive appearance of tetraploid and octoploid cell classes with one or two nuclei (mononucleated or binucleated). Liver cells polyploidy is generally considered to indicate terminal differentiation and senescence and to lead both to the progressive loss of cell pluripotency and a markedly decreased replication capacity. In adults, liver polyploidization is differentially regulated upon loss of liver mass and liver damage. Interestingly, partial hepatectomy induces marked cell proliferation followed by an increase in liver ploidy. In contrast, during hepatocarcinoma (HCC), growth shifts to a nonpolyploidizing pattern and expansion of the diploid hepatocytes population is observed in neoplastic nodules. Here we review the current state of understanding about how polyploidization is regulated during normal and pathological liver growth and detail by which mechanisms hepatocytes become polyploid.

Centrosomes, polyploidy and cancer

Cancer cells are frequently characterized by ploidy changes including tetra-, poly- or aneuploidy. At the same time, malignant cells often contain supernumerary centrosomes. Aneuploidy and centrosome alterations are both hallmarks of tumor aggressiveness and increase with malignant progression. It has been proposed that aneuploidy results from a sequence of events in which failed mitoses produce tetra-/polyploid cells that enter a subsequent cell division with an increased number of centrosomes and hence with an increased risk for multipolar spindle formation and chromosome missegregation. Although this model attempts to integrate several common findings in cancer cells, it has been difficult to prove. Findings that centrosome aberrations can arise in diploid cells and the uncertain proliferative potential of polyploid cells suggest that alternative routes to chromosomal instability may exist. We discuss here recent results on centrosome biogenesis and the possible link between ploidy changes, centrosome aberrations and cancer.

Primary ciliogenesis defects are associated with human astrocytoma/glioblastoma cells

Background

Primary cilia are non-motile sensory cytoplasmic organelles that have been implicated in signal transduction, cell to cell communication, left and right pattern embryonic development, sensation of fluid flow, regulation of calcium levels, mechanosensation, growth factor signaling and cell cycle progression. Defects in the formation and/or function of these structures underlie a variety of human diseases such as Alström, Bardet-Biedl, Joubert, Meckel-Gruber and oral-facial-digital type 1 syndromes. The expression and function of primary cilia in cancer cells has now become a focus of attention but has not been studied in astrocytomas/glioblastomas. To begin to address this issue, we compared the structure and expression of primary cilia in a normal human astrocyte cell line with five human astrocytoma/glioblastoma cell lines.

Methods

Cultured normal human astrocytes and five human astrocytoma/glioblastoma cell lines were examined for primary cilia expression and structure using indirect immunofluorescence and electron microscopy. Monospecific antibodies were used to detect primary cilia and map the relationship between the primary cilia region and sites of endocytosis.

Results

We show that expression of primary cilia in normal astrocytes is cell cycle related and the primary cilium extends through the cell within a unique structure which we show to be a site of endocytosis. Importantly, we document that in each of the five astrocytoma/glioblastoma cell lines fully formed primary cilia are either expressed at a very low level, are completely absent or have aberrant forms, due to incomplete ciliogenesis.

Conclusions

The recent discovery of the importance of primary cilia in a variety of cell functions raises the possibility that this structure may have a role in a variety of cancers. Our finding that the formation of the primary cilium is disrupted in cells derived from astrocytoma/glioblastoma tumors provides the first evidence that altered primary cilium expression and function may be part of some malignant phenotypes. Further, we provide the first evidence that ciliogenesis is not an all or none process; rather defects can arrest this process at various points, particularly at the stage subsequent to basal body association with the plasma membrane.

Centrosomes and myeloma; aneuploidy and proliferation

Multiple myeloma is the second most common hematological malignancy in the United States. The disease is characterized by an accumulation of clonal plasma cells. Clinically, patients present with anemia, lytic bone lesions, hypercalcaemia, or renal impairment. The genome of the malignant plasma cells is extremely unstable and is typically aneuploid and characterized by a complex combination of structure and numerical abnormalities. The basis of the genomic instability underlying myeloma is unclear. In this regard, centrosome amplification is present in about a third of myeloma and may represent a mechanism leading to genomic instability in myeloma. Centrosome amplification is associated with high-risk features and poor prognosis. Understanding the underlying etiology of centrosome amplification in myeloma may lead to new therapeutic avenues.

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.

Aurora A, centrosome structure, and the centrosome cycle

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

Centrosomes and cancer: how cancer cells divide with too many centrosomes

Precise control of centrosome number is crucial for bipolar spindle assembly and accurate transmission of genetic material to daughter cells. Failure to properly control centrosome number results in supernumerary centrosomes, which are frequently found in cancer cells. This presents a paradox: during mitosis, cells with more than two centrosomes are prone to multipolar mitoses and cell death, however, cancer cells possessing extra centrosomes usually divide successfully. One mechanism frequently utilized by cancer cells to escape death caused by multipolar mitoses is the clustering of supernumerary centrosomes into bipolar arrays. An understanding of the molecular mechanisms by which cancer cells can suppress multipolar mitoses is beginning to emerge. Here, we review what's currently known about centrosome clustering mechanisms and discuss potential strategies to target these mechanisms for the selective killing of cancer cells.

Gamma-tubulin-containing abnormal centrioles are induced by insufficient Plk4 in human HCT116 colorectal cancer cells

Cancer cells frequently induce aberrant centrosomes, which have been implicated in cancer initiation and progression. Human colorectal cancer cells, HCT116, contain aberrant centrioles composed of disorganized cylindrical microtubules and displaced appendages. These cells also express unique centrosome-related structures associated with a subset of centrosomal components, including gamma-tubulin, centrin and PCM1. During hydroxyurea treatment, these abnormal structures become more abundant and undergo a change in shape from small dots to elongated fibers. Although gamma-tubulin seems to exist as a ring complex, the abnormal structures do not support microtubule nucleation. Several lines of evidence suggest that the fibers correspond to a disorganized form of centriolar microtubules. Plk4, a mammalian homolog of ZYG-1 essential for initiation of centriole biogenesis, is not associated with the gamma-tubulin-specific abnormal centrosomes. The amount of Plk4 at each centrosome was less in cells with abnormal centrosomes than cells without gamma-tubulin-specific abnormal centrosomes. In addition, the formation of abnormal structures was abolished by expression of exogenous Plk4, but not SAS6 and Cep135/Bld10p, which are downstream regulators required for the organization of nine-triplet microtubules. These results suggest that HCT116 cells fail to organize the ninefold symmetry of centrioles due to insufficient Plk4.

Mouse embryonic fibroblasts null for the Krüppel-like factor 4 gene are genetically unstable

Krüppel-like factor 4 (KLF4) is a zinc-finger transcription factor with tumor suppressive activity in colorectal cancer. Here, we investigated whether KLF4 is involved in maintaining genetic stability in mouse embryonic fibroblasts (MEFs) isolated from mice wild type (+/+), heterozygous (+/-), or homozygous (-/-) for the Klf4 alleles. Compared to Klf4(+/+) and Klf4(+/-) MEFs, Klf4(-/-) MEFs had both a higher level of apoptosis and rate of proliferation. Quantification of chromosome numbers showed that Klf4(-/-) MEFs were aneuploid. A higher number of Klf4(-/-) MEFs exhibited gamma-H2AX foci and had higher amounts of gamma-H2AX compared to controls. Cytogenetic analysis demonstrated the presence of numerous chromosome aberrations including dicentric chromosomes, chromatid breaks, and double minute chromosomes in Klf4(-/-) cells but in few, if any, Klf4(+/+) or Klf4(+/-) MEFs. Approximately 25% of Klf4(-/-) MEFs exhibited centrosome amplification in contrast to the less than 5% of Klf4(+/+) or Klf4(+/-) MEFs. Finally, only Klf4(-/-) MEFs were capable of anchorage-independent growth. Taken together, these findings demonstrate that MEFs null for the Klf4 alleles are genetically unstable, as evidenced by the presence of aneuploidy, chromosome aberration and centrosome amplification. The results support a crucial role for KLF4 in maintaining genetic stability and as a tumor suppressor.

Role of CAP350 in centriolar tubule stability and centriole assembly

Background

Centrioles are microtubule-based cylindrical structures composed of nine triplet tubules and are required for the formation of the centrosome, flagella and cilia. Despite theirs importance, centriole biogenesis is poorly understood. Centrosome duplication is initiated at the G1/S transition by the sequential recruitment of a set of conserved proteins under the control of the kinase Plk4. Subsequently, the procentriole is assembled by the polymerization of centriolar tubules via an unknown mechanism involving several tubulin paralogs.

Methodology/Principal Findings

Here, we developed a cellular assay to study centrosome duplication and procentriole stability based on its sensitivity to the microtubule-depolymerizing drug nocodazole. By using RNA interference experiments, we show that the stability of growing procentrioles is regulated by the microtubule-stabilizing protein CAP350, independently of hSAS-6 and CPAP which initiate procentriole growth. Furthermore, our analysis reveals the critical role of centriolar tubule stability for an efficient procentriole growth.

Conclusions/Significance

CAP350 belongs to a new class of proteins which associate and stabilize centriolar tubules to control centriole duplication.

Mechanisms to suppress multipolar divisions in cancer cells with extra centrosomes

Multiple centrosomes in tumor cells create the potential for multipolar divisions that can lead to aneuploidy and cell death. Nevertheless, many cancer cells successfully divide because of mechanisms that suppress multipolar mitoses. A genome-wide RNAi screen in Drosophila S2 cells and a secondary analysis in cancer cells defined mechanisms that suppress multipolar mitoses. In addition to proteins that organize microtubules at the spindle poles, we identified novel roles for the spindle assembly checkpoint, cortical actin cytoskeleton, and cell adhesion. Using live cell imaging and fibronectin micropatterns, we found that interphase cell shape and adhesion pattern can determine the success of the subsequent mitosis in cells with extra centrosomes. These findings may identify cancer-selective therapeutic targets: HSET, a normally nonessential kinesin motor, was essential for the viability of certain extra centrosome-containing cancer cells. Thus, morphological features of cancer cells can be linked to unique genetic requirements for survival.

Increased expression of Nlp, a potential oncogene in ovarian cancer, and its implication in carcinogenesis

OBJECTIVE

Nlp (Ninein-like protein), a novel centrosome protein involved in microtubule nucleation, has been studied extensively in our laboratory, and its overexpression has been found in some human tumors. To understand the role of Nlp in human ovarian cancer development, we studied the correlation of Nlp expression with clinicopathological parameters and survival in epithelial ovarian cancer, and the impact of Nlp overexpression on ovarian cancer cells.

METHODS

Nlp expression in normal, borderline, benign and malignant epithelial ovarian tissues was examined by immunohistochemistry. The correlation between Nlp expression and tumor grade, FIGO stage and histological type was also evaluated. Survival was calculated using Kaplan-Meier estimates. Cell proliferation and apoptosis were assayed after stable transfection of pEGFP-C3-Nlp or empty vector in human ovarian cancer cell line SKOV3.

RESULTS

Nlp was positive in 1 of 10 (10%) normal ovarian tissues, 5 of 34 (14.7%) benign tumors, 9 of 26 (34.6%) borderline tumors and 73 of 131 (56.0%) ovarian tumors. Nlp immunoreactivity intensity significantly correlated with tumor grade, but not with FIGO stage or histological type. Kaplan-Meier curves showed that Nlp overexpression was marginally associated with decreased overall survival. Overexpression of Nlp enhanced proliferation and inhibited apoptosis induced by paclitaxel in the SKOV3 cell line.

CONCLUSIONS

Overexpression of Nlp in ovarian tumors raises the possibility that Nlp may play a role in ovarian carcinogenesis.

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.

Analysis of the cellular centrosome in fine-needle aspirations of the breast

Background

The purpose of the present investigation is to determine whether centrosome amplifications are present in breast tumor cells, whether there are differences of centrosome amplification between benign breast lesions and breast carcinomas, and whether centrosomal analysis can be of value in the diagnosis and prognosis of breast carcinoma.

Methods

Using immunofluorescence analysis with an antibody against γ-tubulin, we analyzed centrosome abnormalities in fine-needle aspirations of 100 breast lesions (25 cases with benign lesions and 75 cases with carcinomas).

Results

We found that centrosome amplifications, including numerical centrosome amplification and structural centrosome amplification, were present in most breast tumors. Cells with numerical centrosome amplification were found in 23 of 25 benign lesions, and in all 75 cases of breast carcinomas. Cells with structural centrosome amplification were found in three of 25 benign lesions, and in 69 of 75 breast carcinomas. The breast carcinomas showed a mean percentage of cells with numerical centrosome amplification of 4.86% and a mean percentage of cells with structural centrosome amplification of 3.98%. These percentages were significantly higher than those in benign lesions, with a numerical centrosome amplification of 2.77% and a structural centrosome amplification of 0.10%. Furthermore, the mean percentage of cells with structural centrosome amplification was significantly associated with HER2/neu overexpression (P < 0.05) and with negative estrogen receptor status (P < 0.05), and had a borderline association with negative progesterone receptor status (P = 0.056) in breast carcinomas.

Conclusion

Structural centrosome amplification may bear a close relationship with breast carcinoma and may be a potential biomarker for diagnosis and prognosis of breast carcinoma.

Ovarian hyperstimulation induces centrosome amplification and aneuploid mammary tumors independently of alterations in p53 in a transgenic mouse model of breast cancer

Aneuploidy and genomic instability are common features of human cancers, including breast cancer; however, mechanisms by which such abnormalities develop are not understood. The exquisite dependence of the mammary gland on hormones for growth and development as well as hormonal contributions to breast cancer risk and progression suggest that tumorigenic mechanisms in the breast should be considered in the context of hormonal stimulation. We used transgenic mice that overexpress luteinizing hormone with subsequent ovarian hyperstimulation as a model to identify mechanisms involved in hormone-induced mammary cancer. Tumor pathology in these mice is highly variable, suggesting individual tumors undergo distinct initiating or promoting events. Supporting this notion, hormone-induced tumors display considerable chromosomal instability and aneuploidy, despite the presence of functional p53. The presence of extensive centrosome amplification in tumors and hyperplastic glands prior to tumor formation suggests that alterations in the ovarian hormonal milieu dysregulate the centrosome cycle in mammary epithelial cells, leading to aneuploidy and cancer.

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.

Centrin1 is required for organelle segregation and cytokinesis in Trypanosoma brucei

Centrin is a calcium-binding centrosome/basal body-associated protein involved in duplication and segregation of these organelles in eukaryotes. We had shown that disruption of one of the centrin genes (centrin1) in Leishmania amastigotes resulted in failure of both basal body duplication and cytokinesis. Here, we undertook to define the role of centrin1 (TbCen1) in the duplication and segregation of basal body and its associated organelles kinetoplast and Golgi, as well as its role in cytokinesis of the procyclic form of Trypanosoma brucei by depleting its protein using RNA inhibition methodology. TbCen1-depleted cells showed significant reduction in growth compared with control cells. Morphological analysis of these cells showed they were large and pleomorphic with multiple detached flagella. Both immunofluorescence assays using organelle-specific antibodies and electron microscopic analysis showed that TbCen1-deficient cells contained multiple basal bodies, kinetoplasts, Golgi, and nuclei. These multiple organelles were, however, closely clustered together, indicating duplication without segregation in the absence of centrin. This failure in organelle segregation may be the likely cause of inhibition of cytokinesis, suggesting for the first time a new and unique role for centrin in the segregation of organelles without affecting their multiplication in the procyclic form of T. brucei.

Centrosome-Associated NDR Kinase Regulates Centrosome Duplication

Human NDR kinases are upregulated in some cancer types, yet their functions still remain undefined. Here, we report the first known function of a mammalian NDR kinase by demonstrating that human NDR directly contributes to centrosome duplication. A subpopulation of endogenous NDR localizes to centrosomes in a cell-cycle-dependent manner. Overexpression of NDR resulted in centrosome overduplication in a kinase-activity-dependent manner, while expression of kinase-dead NDR or depletion of NDR by small interfering RNA (siRNA) negatively affected centrosome duplication. By targeting NDR to the centrosome, we show that the centrosomal pool of NDR is sufficient to generate supernumerary centrosomes. Furthermore, our data indicate that NDR-driven centrosome duplication requires Cdk2 activity and that Cdk2-induced centrosome amplification is affected upon reduction of NDR activity. Overall, considering that centrosome overduplication is linked to cellular transformation, our observations may also provide a molecular link between mammalian NDR kinases and cancer.

High rate of centrosome aberrations and correlation with proliferative activity in patients with untreated B-cell chronic lymphocytic leukemia

B-cell chronic lymphocytic leukemia (CLL) is characterized by a high rate of clonal genomic alterations and a low proliferative activity with cell cycle arrest in G(0)/G(1) phase. Recently, centrosome aberrations have been described as a possible cause of chromosomal instability and aneuploidy in many human malignancies. To investigate whether centrosome aberrations do occur in CLL and whether they correlate with common prognostic factors and disease activity, we examined peripheral blood mononuclear cells (PBMC) from 70 patients with previously untreated CLL using an antibody to gamma-tubulin. All 70 CLL samples displayed significantly more cells with centrosome aberrations (median: 26.0%, range 11.0-41.5%) as compared to peripheral blood B lymphocytes from 20 age-matched, healthy individuals (median: 2.0%, range 0-6%; p < 0.001). The extent of centrosome aberrations correlated with the proliferative activity of the CLL cases as measured by lymphocyte doubling time (p = 0.02) as well as with time to first treatment (p = 0.05). Accordingly, more centrosome aberrations were found in PHA-stimulated T lymphocytes from healthy individuals as well as in B cells from surgically removed tonsil tissue of patients with acute tonsillitis as compared to the peripheral blood B lymphocytes from the control group. In contrast, no correlation was observed between centrosome aberrations and immunoglobulin VH gene mutation status or cytogenetically defined risk groups. These findings suggest that, despite the common observation of most CLL cells remaining in G(0)/G(1) phase, their centrosome replication process is deregulated and correlates to the proliferative activity of CLL cells.

Genetic and epigenetic changes in mammary epithelial cells identify a subpopulation of cells involved in early carcinogenesis

Morphologically normal foci of epithelial cells exhibiting p16 inactivation have been found in several tissues and may be precursors to cancer. Our previous work demonstrates that cells lacking p16(INK4A) activity exhibit phenotypes associated with malignancy (Romanov et al. 2001). The acquisition of genomic instability occurs through the activation of telomeric and centrosomal dysfunction. Additionally, the activation of stress pathways such as COX-2 provides these cells with the mutagenic potential to survive adverse environments as well as the ability to migrate, evade apoptosis and immune surveillance, and summon sustaining vasculature. Examination of archived tissue from women with DCIS (ductal carcinoma in situ) reveals epithelial cells that overexpress markers of premalignant stress activation pathways and mirror the distinctive expression patterns of these markers observed in vitro. These epithelial cells are found within the premalignant lesion as well as in the field of morphologically normal tissue that surrounds the lesion. Here, we show that p16(INK4A)-silenced vHMEC cells exhibit a gene expression profile which is distinct, reproducible, and extends beyond the changes mediated by p16(INK4A) inactivation. The present work suggests that cells lacking p16(INK4A) activity exhibit critical activities which allow cells to evade differentiation processes that would be expected to terminate proliferation. All of these properties are critical to malignancy. These events may be useful biomarkers to detect the earliest events in breast cancer.

Mitotic kinases: the key to duplication, segregation, and cytokinesis errors, chromosomal instability, and oncogenesis

Chromosomal instability (CIN) and aneuploidy are commonly observed in the vast majority of human solid tumors and in many hematological malignancies. These features are considered defining characteristics of human breast, bladder and kidney cancers since they markedly exceed a 50% aneuploidy frequency. The detection of persistent mitotic kinase over-expression, particularly the Aurora family, and centrosome amplification in precursor/pre-malignant stages, strongly implicate these molecular changes in precipitating the aneuploidy seen in many human neoplasms. Mitotic spindle checkpoint defects may also lead to aneuploid tumors. However, the sustained over-expression and activity of various members of the mitotic kinase families, including Aurora (Aur) (A, B, C), Polo-like (Plk1-4), and Nek (NIMA1-11) in diverse human tumors strongly indicate that these entities are intimately involved in the development of errors in centrosome duplication, chromosome segregation, and cytokinesis. Mitotic kinases have also been implicated in regulating the centrosome cycle, spindle checkpoint and microtubule-kinetochore attachment, spindle assembly, and chromosome condensation. These mitotic kinases are modulated by de-novo synthesis, stability factors, phosphorylation, and ubiquitin-dependent proteolysis. They, in turn, phosphorylate a myriad of centrosomal/mitotic protein substrates, and have the ability to behave as oncogenes (i.e. Aur-A, Plk-1), providing a compelling link between errors in mitosis and oncogenic processes. The recent development of selective small molecule inhibitors of Aurora kinases, in particular, will provide useful tools to ascertain more precisely their role in cancer development. Potent inhibitors of mitotic kinases, when fully developed, have the promise to be effective agents against tumor growth, and possibly, tumor prevention as well.

Targeting carcinogenesis: a role for the prolyl isomerase Pin1?

Phosphorylation of proteins on serine or threonine residues that immediately precede proline (pSer/Thr-Pro) is a central signaling mechanism in cell proliferation and transformation. Recent studies indicate that certain pSer/Thr-Pro motifs in native proteins exist in two completely distinct conformations, cis and trans, whose conversion is markedly slowed down upon phosphorylation, but specifically catalyzed by the peptidyl-prolyl cis/trans isomerase Pin1. Importantly, such Pin1-catalyzed conformational changes can have profound effects on the function of many phosphorylation signaling pathways, thereby playing an important role in various cellular processes. Moreover, increasing evidence indicates that aberrant Pin1 function plays an important role in the pathogenesis of some human diseases. Notably, Pin1 is not only overexpressed in a large number of human cancers, but also is an excellent prognostic marker in some cancers. Furthermore, Pin1 overexpression can function as a critical catalyst that amplifies multiple oncogenic signaling pathways during oncogenesis. Moreover, Pin1 overexpression causes cell transformation, centrosome amplification, genomic instability, and tumor development. In contrast, Pin1 knockout in mice prevents certain oncogenes from inducing tumors and Pin1 inhibition in cancer cells suppresses their cell proliferation, transformed phenotype and tumorigenicity in nude mice as well as increases the response to other anticancer agents. These results suggest that Pin1-mediated postphosphorylation regulation may provide a unique opportunity for disrupting oncogenic pathways, and thereby represent an appealing target for novel anticancer therapies.

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.

Characterization of Su48, a centrosome protein essential for cell division

The centrosome functions as the major microtubule-organizing center and plays a vital role in guiding chromosome segregation during mitosis. Centrosome abnormalities are frequently seen in a variety of cancers, suggesting that dysfunction of this organelle may contribute to malignant transformation. In our efforts to identify the protein components of the centrosome and to understand the structure features involved in the assembly and functions of this organelle, we cloned and characterized a centrosome-associated protein called Su48. We found that a coiled coil-containing subdomain of Su48 was both sufficient and required for its centrosome localization. In addition, this structure also modulates Su48 dimerization. Moreover, ectopic expression of Su48 causes abnormal mitosis, and a mutant form of Su48 disrupts the localization of gamma-tubulin to the centrosome. Finally, by microinjection of an anti-Su48 antibody, we found that disruption of normal Su48 functions leads to mitotic failure, possibly due to centrosome defects or incomplete cytokinesis. Thus, Su48 represents a previously unrecognized centrosome protein that is essential for cell division. We speculate that Su48 abnormalities may cause aberrant chromosome segregation and may contribute to aneuploidy and malignant transformation.

Pin1 regulates centrosome duplication, and its overexpression induces centrosome amplification, chromosome instability, and oncogenesis

Phosphorylation on Ser/Thr-Pro motifs is a major mechanism regulating many events involved in cell proliferation and transformation, including centrosome duplication, whose defects have been implicated in oncogenesis. Certain phosphorylated Ser/Thr-Pro motifs can exist in two distinct conformations whose conversion in certain proteins is catalyzed specifically by the prolyl isomerase Pin1. Pin1 is prevalently overexpressed in human cancers and is important for the activation of multiple oncogenic pathways, and its deletion suppresses the ability of certain oncogenes to induce cancer in mice. However, little is known about the role of Pin1 in centrosome duplication and the significance of Pin1 overexpression in cancer development in vivo. Here we show that Pin1 overexpression correlates with centrosome amplification in human breast cancer tissues. Furthermore, Pin1 localizes to and copurifies with centrosomes in interphase but not mitotic cells. Moreover, Pin1 ablation in mouse embryonic fibroblasts drastically delays centrosome duplication without affecting DNA synthesis and Pin1 inhibition also suppresses centrosome amplification in S-arrested CHO cells. In contrast, overexpression of Pin1 drives centrosome duplication and accumulation, resulting in chromosome missegregation, aneuploidy, and transformation in nontransformed NIH 3T3 cells. More importantly, transgenic overexpression of Pin1 in mouse mammary glands also potently induces centrosome amplification, eventually leading to mammary hyperplasia and malignant mammary tumors with overamplified centrosomes. These results demonstrate for the first time that the phosphorylation-specific isomerase Pin1 regulates centrosome duplication and its deregulation can induce centrosome amplification, chromosome instability, and oncogenesis.

Induction of centrosome amplification in p53 siRNA-treated human fibroblast cells by radiation exposure

Centrosome amplification can be detected in the tissues of p53(-/-) mice. In contrast, loss of p53 does not induce centrosome amplification in cultured human cells. However, examination of human cancer tissues and cultured cells has revealed a significant correlation between loss or mutational inactivation of p53 and occurrence of centrosome amplification, supporting the notion that p53 mutation alone is insufficient to induce centrosome amplification in human cells, and that additional regulatory mechanisms are involved. It has recently been shown that gamma irradiation of tumor cells induces centrosome amplification. However, the precise mechanism of radiation-induced centrosome amplification is not fully understood. In the present study, CCD32SK diploid normal human fibroblasts were transfected transiently with short interfering RNA (siRNA) specific for human p53 (CCD/p53i). There was a small increase in the frequency of centrosome amplification in CCD/p53i cells (4.0%) without irradiation. In contrast, CCD/p53i cells after 5-Gy irradiation showed a marked increase in abnormal nuclear shapes and pronounced amplification of centrosomes (46.0%). At 12 h after irradiation, irradiated CCD/p53i cells were arrested in G(2) phase. By laser scanning cytometry, abnormal mitosis with amplified centrosomes was observed frequently in the accumulating G(2)/M population at 48 h after irradiation. In the present study, we found that siRNA-mediated silencing of p53 in normal human fibroblasts, together with DNA damage by irradiation, efficiently induced centrosome amplification and nuclear fragmentation, but these phenomena were not observed with either siRNA-mediated silencing of p53 or irradiation alone.

p16(INK4a) prevents centrosome dysfunction and genomic instability in primary cells

Aneuploidy, frequently observed in premalignant lesions, disrupts gene dosage and contributes to neoplastic progression. Theodor Boveri hypothesized nearly 100 years ago that aneuploidy was due to an increase in centrosome number (multipolar mitoses) and the resultant abnormal segregation of chromosomes. We performed immunocytochemistry, quantitative immunofluorescence, karyotypic analysis, and time-lapse microscopy on primary human diploid epithelial cells and fibroblasts to better understand the mechanism involved in the production of supernumerary centrosomes (more than two microtubule nucleating bodies) to directly demonstrate that the presence of supernumerary centrosomes in genomically intact cells generates aneuploid daughter cells. We show that loss of p16^INK4a generates supernumerary centrosomes through centriole pair splitting. Generation of supernumerary centrosomes in human diploid epithelial cells was shown to nucleate multipolar spindles and directly drive production of aneuploid daughter cells as a result of unequal segregation of the genomic material during mitosis. Finally, we demonstrate that p16^INK4a cooperates with p21 through regulation of cyclin-dependent kinase activity to prevent centriole pair splitting. Cells with loss of p16^INK4a activity have been found in vivo in histologically normal mammary tissue from a substantial fraction of healthy, disease-free women. Demonstration of centrosome dysfunction in cells due to loss of p16^INK4a suggests that, under the appropriate conditions, these cells can become aneuploid. Gain or loss of genomic material (aneuploidy) may provide the necessary proproliferation and antiapoptotic mechanisms needed for the earliest stages of tumorigenesis.

Here the authors show that aneuploidy (where a cell has an abnormal number of chromosomes) can be caused by cells having an abnormal number of centrosomes, which leads to asymmetric cell division.

Deregulation of the centrosome cycle and the origin of chromosomal instability in cancer

Although we have begun to tap into the mechanisms behind Boveri's initial observation that supernumerary centrosomes cause chromosome missegregation in sea urchin eggs, there is still much left to discover with regard to chromosomal instability in cancer. Many of the molecular players involved in regulation of the centrosome and cell cycles, and the coupling of the two cycles to produce a bipolar mitotic spindle have been identified. One theme that has become apparent is that cross talk and interrelatedness of the pathways serve to provide redundant mechanisms to maintain genomic integrity. In spite of this, cells occasionally fall prey to insults that initiate and maintain the chromosomal instability that results in viable malignant tumours. Deregulation of centrosome structure is an integral aspect of the origin of chromosomal instability in many cancers. There are numerous routes to centrosome amplification including: environmental insults such as ionising radiation and exposure to estrogen (Li et al., 2005); failure of cytokinesis; and activating mutations in key regulators of centrosome structure and function. There are two models for initiation of centrosome amplification (Figure 2). In the first, centrosome duplication and chromosome replication remain coupled and cells enter G2 with 4N chromosomes and duplicated centrosomes. However, these cells may fail to complete mitosis, and thus reenter G1 as tetraploid cells with amplified centrosomes. In the second, the centrosome cycle is uncoupled from chromosome replication and cells go through one or more rounds of centriole/centrosome duplication in the absence of chromosome replication. If these cells then go through chromosome replication accompanied by another round of centrosome duplication, cells complete G2 with 4N chromosomes and more than 2 centrosomes, and therefore are predisposed to generate multipolar mitotic spindles. Fragmentation of centrosomes due to ionising radiation is a variation of the second model. Once centrosome amplification is present, even in a diploid cell, that cell has the potential to yield viable aneuploid progeny. The telophase cell in Figure 3C illustrates this scenario. In a normal telophase configuration, the total number of chromosomes is 92 (resulting from the segregation of 46 pairs of chromatids), with each daughter nucleus containing 46 individual chromosomes. Based on the number of kinetochore signals present, the lower nucleus in Figure 3C has approximately 28 chromosomes, and the elongate upper nucleus has approximately 60, for a total of 88. Due to superimposition of kinetochores in this maximum projection image, 88 is an underestimate of the actual number of kinetochores and is not significantly different from the expected total of 92. A cell resulting from the lower nucleus with only around 28 chromosomes would probably not be viable, much as Boveri's experiments indicated. However, the upper nucleus with at least 60 chromosomes could be viable. This cell would enter G1 as hypotriploid (69 chromosomes = triploid) with 2 centrosomes. During S and G2, the centrosomes and chromosomes would double, and the following mitosis could be tetrapolar with a 6N chromosome content. When centrosome amplification is accompanied by permissive lapses in cell cycle checkpoints, the potential for malignant growth is present. These lapses could result from specific genetic mutations and amplifications, epigenetic gene silencing, or from massive chromosomal instability caused by the centrosome amplification. Centrosome amplification, therefore, can serve to exacerbate and/or generate genetic instabilities associated with cancers.

Clinical implication of centrosome amplification in plasma cell neoplasm

The mechanisms underlying aneuploidy in multiple myeloma (MM) are unclear. Centrosome amplification has been implicated as the cause of chromosomal instability in a variety of tumors and is a potential mechanism causing aneuploidy in MM. Using immunofluorescent (IF) staining, centrosome amplification was detected in 67% of monoclonal gammopathies, including monoclonal gammopathy of undetermined significance (MGUS). We also investigated the gene expression of centrosome proteins. Overall, gene expression data correlated well with IF-detected centrosome amplification, allowing us to derive a gene expression-based centrosome index (CI) as a surrogate for centrosome amplification. Clinically, MM patients with high CI (> 4) are associated with poor prognostic genetic and clinical subtypes (chromosome 13 deletion, t(4; 14), t(14;16), and PCLI > 1%, P < .05) and are shown here to have short survival (11.1 months versus 39.1 months, P < .001). On multivariate regression, a high CI is an independent prognostic factor. Given that centrosome amplification is already observed in MGUS and probably integral to early chromosomal instability and myeloma genesis, and patients with more extensive centrosome amplification have shorter survival, the mechanisms leading to centrosome amplification should be investigated because these may offer new avenues for therapeutic intervention.

Evaluation of genotoxic effects of Apitol® (cymiazole hydrochloride) in vitro by measurement of sister chromatid exchange

Apitol, with cymiazole hydrochloride as the active ingredient, is used in bee-keeping against the ectoparasitic mite Varroa destructor. The preparation was evaluated for genotoxicity in cultured human peripheral blood lymphocytes. Sister chromatid exchange, the mitotic index and the cell proliferation index were determined for three experimental concentrations of Apitol (0.001, 0.01 and 0.1 mg/ml). All concentrations significantly (p < 0.001) increased the mitotic index (MI = 7.35+/-0.18%, 8.31+/-0.20% and 12.33+/-0.25%, respectively), the proliferative index (PI = 1.83+/-0.01, 1.84+/-0.01 and 1.88+/-0.02, respectively) and the frequency of sister chromatid exchange (SCE = 8.19+/-1.81, 8.78+/-1.80 and 13.46+/-1.88, respectively), suggesting that cymiazole hydrochloride has genotoxic potential.

Centrosome amplification, chromosome instability and cancer development

During mitosis, two centrosomes form spindle poles and direct the formation of bipolar mitotic spindles, which is an essential event for accurate chromosome segregation into daughter cells. The presence of more than two centrosomes (centrosome amplification), severely disturbs mitotic process and cytokinesis via formation of more than two spindle poles, resulting in an increased frequency of chromosome segregation errors (chromosome instability). Destabilization of chromosomes by centrosome amplification aids acquisition of further malignant phenotypes, hence promoting tumor progression. Centrosome amplification occurs frequently in almost all types of cancer, and is considered as the major contributing factor for chromosome instability in cancer cells. Upon cytokinesis, each daughter cell receives one centrosome, and thus centrosome must duplicate once, and only once, before the next mitosis. If centrosomes duplicate more than once within a single cell cycle, centrosome amplification occurs, which is frequently seen in cells harboring mutations in some tumor suppressor proteins such as p53 and BRCA1. The recent studies have provided critical information for understanding how loss of these proteins allows multiple rounds of centrosome duplication. In this review, how centrosome amplification destabilizes chromosomes, how loss of certain tumor suppressor proteins leads to centrosome amplification, and the role of centrosome amplification in cancer development will be discussed.

Centrosome aberrations in chronic myeloid leukemia correlate with stage of disease and chromosomal instability

Centrosome abnormalities are hallmarks of various cancers and have been implicated in chromosome missegregation, chromosomal instability, and aneuploidy. Since genetic instability is a common feature in chronic myeloid leukemia (CML), we sought to investigate whether centrosome aberrations occur and correlate with disease stage and cytogenetic findings in CML. We examined 34 CML samples including CD 34+Ph+cells of 18 newly diagnosed patients (chronic phase (CP)) and 16 blast crisis (BC) specimens by using a centrosome-specific antibody to pericentrin. All CP and BC samples displayed centrosome alterations as compared with corresponding CD 34+control cells. Centrosome abnormalities were detected in 29.1+/-5.9% of CP blasts and in 54.3+/-4.8% of BC blasts, but in only 2.4+/-1.1% of controls (P<0.0001). Additional karyotypic alterations to the t(9;22) translocation were found in only 1/18 CML-CP patients. In contrast, 11/16 (73%) CML-BC patients displayed additional karyotype alterations in 48.7% of analyzed cells, correlating with an abnormal centrosome status (P=0.0005). Our results indicate that centrosome defects are a common and early detectable feature in CML that may contribute to acquisition of chromosomal aberrations and aneuploidy. They may be considered as the driving force of disease progression and could serve as future prognostic markers.

Krüppel-like factor 4 prevents centrosome amplification following gamma-irradiation-induced DNA damage

Centrosome duplication is a carefully controlled process in the cell cycle. Previous studies indicate that the tumor suppressor, p53, regulates centrosome duplication. Here, we present evidence for the involvement of the mammalian Krüppel-like transcription factor, KLF4, in preventing centrosome amplification following DNA damage caused by gamma-irradiation. The colon cancer cell line HCT116, which contains wild-type p53 alleles (HCT116 p53+/+), displayed stable centrosome numbers following gamma-irradiation. In contrast, HCT116 cells null for the p53 alleles (HCT116 p53-/-) exhibited centrosome amplification after irradiation. In the latter cell line, KLF4 was not activated following gamma-irradiation due to the absence of p53. However, centrosome amplification could be suppressed in irradiated HCT116 p53-/- cells by conditional induction of exogenous KLF4. Conversely, in a HCT116 p53+/+ cell line stably transfected with small hairpin RNA (shRNA) designed to specifically inhibit KLF4, gamma-irradiation induced centrosome amplification. In these cells, the inability of KLF4 to become activated in response to DNA damage was directly associated with an increase in cyclin E level and Cdk2 activity, both essential for regulating centrosome duplication. Cotransfection experiments showed that KLF4 overexpression suppressed the promoter activity of the cyclin E gene. The results of this study demonstrated that KLF4 is both necessary and sufficient in preventing centrosome amplification following gamma-radiation-induced DNA damage and does so by transcriptionally suppressing cyclin E expression.