Centrosome function in normal and tumor cells

Adverse Toxic Effects of Tyrosine Kinase Inhibitors on Non-Target Zebrafish Liver (ZFL) Cells

Over the past 20 years, numerous tyrosine kinase inhibitors (TKIs) have been introduced for targeted therapy of various types of malignancies. Due to frequent and increasing use, leading to eventual excretion with body fluids, their residues have been found in hospital and household wastewaters as well as surface water. However, the effects of TKI residues in the environment on aquatic organisms are poorly described. In the present study, we investigated the cytotoxic and genotoxic effects of five selected TKIs, namely erlotinib (ERL), dasatinib (DAS), nilotinib (NIL), regorafenib (REG), and sorafenib (SOR), using the in vitro zebrafish liver cell (ZFL) model. Cytotoxicity was determined using the MTS assay and propidium iodide (PI) live/dead staining by flow cytometry. DAS, SOR, and REG decreased ZFL cell viability dose- and time-dependently, with DAS being the most cytotoxic TKI studied. ERL and NIL did not affect viability at concentrations up to their maximum solubility; however, NIL was the only TKI that significantly decreased the proportion of PI negative cells as determined by the flow cytometry. Cell cycle progression analyses showed that DAS, ERL, REG, and SOR caused the cell cycle arrest of ZFL cells in the G0/G1 phase, with a concomitant decrease of cells in the S-phase fraction. No data could be obtained for NIL due to severe DNA fragmentation. The genotoxic activity of the investigated TKIs was evaluated using comet and cytokinesis block micronucleus (CBMN) assays. The dose-dependent induction of DNA single strand breaks was induced by NIL (≥2 μM), DAS (≥0.006 μM), and REG (≥0.8 μM), with DAS being the most potent. None of the TKIs studied induced micronuclei formation. These results suggest that normal non-target fish liver cells are sensitive to the TKIs studied in a concentration range similar to those previously reported for human cancer cell lines. Although the TKI concentrations that induced adverse effects in exposed ZFL cells are several orders of magnitude higher than those currently expected in the aquatic environment, the observed DNA damage and cell cycle effects suggest that residues of TKIs in the environment may pose a hazard to non-intentionally exposed organisms living in environments contaminated with TKIs.

A look into the link between centrosome amplification and breast cancer

Centrosome amplification (CA) is a common feature of human tumors, but it is not clear whether this is a cause or a consequence of cancer. The centrosome amplification observed in tumor cells may be explained by a series of events, such as failure of cell division, dysregulation of centrosome cycle checkpoints, and de novo centriole biogenesis disorder. The formation and progression of breast cancer are characterized by genomic abnormality. The centrosomes in breast cancer cells show characteristic structural aberrations, caused by centrosome amplification, which include: an increase in the number and volume of centrosomes, excessive increase of pericentriolar material (PCM), inappropriate phosphorylation of centrosomal molecular, and centrosome clustering formation induced by the dysregulation of important genes. The mechanism of intracellular centrosome amplification, the impact of which on breast cancer and the latest breast cancer target treatment options for centrosome amplification are exhaustively elaborated in this review.

During mitosis ZEB1 "switches" from being a chromatin-bound epithelial gene repressor, to become a microtubule-associated protein

Microtubules are polymers of α/β-tubulin, with microtubule organization being regulated by microtubule-associated proteins (MAPs). Herein, we describe a novel role for the epithelial gene repressor, zinc finger E-box-binding homeobox 1 (ZEB1), that "switches" from a chromatin-associated protein during interphase, to a MAP that associates with α-, β- and γ-tubulin during mitosis. Additionally, ZEB1 was also demonstrated to associate with γ-tubulin at the microtubule organizing center (MTOC). Using confocal microscopy, ZEB1 localization was predominantly nuclear during interphase, with α/β-tubulin being primarily cytoplasmic and the association between these proteins being minimal. However, during the stages of mitosis, ZEB1 co-localization with α-, β-, and γ-tubulin was significantly increased, with the association commonly peaking during metaphase in multiple tumor cell-types. ZEB1 was also observed to accumulate in the cleavage furrow during cytokinesis. The increased interaction between ZEB1 and α-tubulin during mitosis was also confirmed using the proximity ligation assay. In contrast to ZEB1, its paralog ZEB2, was mainly perinuclear and cytoplasmic during interphase, showing some co-localization with α-tubulin during mitosis. Considering the association between ZEB1 with α/β/γ-tubulin during mitosis, studies investigated ZEB1's role in the cell cycle. Silencing ZEB1 resulted in a G₂-M arrest, which could be mediated by the up-regulation of p21^Waf1/Cip1 and p27^Kip1 that are known downstream targets repressed by ZEB1. However, it cannot be excluded the G₂/M arrest observed after ZEB1 silencing is not due to its roles as a MAP. Collectively, ZEB1 plays a role as a MAP during mitosis and could be functionally involved in this process.

AURKA mRNA expression is an independent predictor of poor prognosis in patients with non-small cell lung cancer

Deregulation of mitotic spindle genes has been reported to contribute to the development and progression of malignant tumours. The aim of the present study was to explore the association between the expression profiles of Aurora kinases (AURKA, AURKB and AURKC), cytoskeleton-associated protein 5 (CKAP5), discs large-associated protein 5 (DLGAP5), kinesin-like protein 11 (KIF11), microtubule nucleation factor (TPX2), monopolar spindle 1 kinase (TTK), and β-tubulins (TUBB) and (TUBB3) genes and clinicopathological characteristics in human non-small cell lung carcinoma (NSCLC). Reverse transcription-quantitative polymerase chain reaction-based RNA gene expression profiles of 132 NSCLC and 44 adjacent wild-type tissues were generated, and Cox's proportional hazard regression was used to examine associations. With the exception of AURKC, all genes exhibited increased expression in NSCLC tissues. Of the 10 genes examined, only AURKA was significantly associated with prognosis in NSCLC. Multivariate Cox's regression analysis demonstrated that AURKA mRNA expression [hazard ratio (HR), 1.81; 95% confidence interval (CI), 1.16-2.84; P=0.009], age (HR, 1.03; 95% CI, 1.00-1.06; P=0.020), pathological tumour stage 2 (HR, 2.43; 95% CI, 1.16-5.10; P=0.019) and involvement of distal nodes (pathological node stage 2) (HR, 3.14; 95% CI, 1.24-7.99; P=0.016) were independent predictors of poor prognosis in patients with NSCLC. Poor prognosis of patients with increased AURKA expression suggests that those patients may benefit from surrogate therapy with AURKA inhibitors.

Revisiting the tubulin folding pathway: new roles in centrosomes and cilia

Centrosomes and cilia are critical eukaryotic organelles which have been in the spotlight in recent years given their implication in a myriad of cellular and developmental processes. Despite their recognized importance and intense study, there are still many open questions about their biogenesis and function. In the present article, we review the existing data concerning members of the tubulin folding pathway and related proteins, which have been identified at centrosomes and cilia and were shown to have unexpected roles in these structures.

Aberrant expression of NEK2 and its clinical significance in non-small cell lung cancer

The purpose of the present study was to identify a potential biomarker that is more effective than those already available for the prognosis of non-small cell lung cancer (NSCLC) patients. The expression of never in mitosis gene A (NIMA)-related kinase 2 (NEK2), minichromosome maintenance complex component 7 (Mcm7) and Ki67 was evaluated in 270 NSCLC tissues using immunohistochemical and immunofluorescence techniques. Associations between protein expression and clinicopathological characters were assessed, and the impact on overall survival was analyzed. High levels of NEK2, Mcm7 and Ki67 expression were detected in 25.9, 35.2 and 24.4% of the NSCLC tissues. Overexpression of NEK2 was detected more frequently in cases with high T and N stages (P<0.0001 and P=0.011, respectively). Correlations were present between the expression of NEK2, Mcm7 and Ki67. Kaplan-Meier curves indicated that the patients with overexpressed NEK2, Mcm7 and Ki67 had a poorer overall survival time compared to those with low expression for all stages (P<0.0001). In particular, the patients with NEK2 overexpression had a poorer prognosis. Multivariate Cox regression analysis showed that NEK2, Mcm7 and Ki67 are independent prognostic indicators for NSCLC. In conclusion, the data indicate that compared with Mcm7 and Ki67, NEK2 may be a more effective tumor proliferation marker of poor prognosis for NSCLC patients, and that NEK2 may represent a novel potential target for NSCLC therapeutic intervention.

CEP55 contributes to human gastric carcinoma by regulating cell proliferation

Centrosomal protein 55 (CEP55) is the latest found member in the centrosomal relative protein family, which participates in cell-cycle regulation. CEP55 exists in many kinds of normal tissues and tumour cells such as hepatocellular carcinoma, and is important in carcinogenesis. However, the role of CEP55 in the pathogenesis of gastric cancer (GC) remains unclear. The mRNA levels of CEP55 in GC tissues and GC cell lines were examined by quantitative real-time PCR, and the protein expression of CEP55 in GC tissues was detected by Western blot and immunohistochemistry. The role of CEP55 in regulating the proliferation of GC cell lines was investigated both in vitro and in vivo. CEP55 was strongly upregulated in human GC, indicating that CEP55 contributed to carcinogenesis and progression of GC. Ectopic overexpression of CEP55 enhanced the cell proliferation, colony formation, and tumourigenicity of GC cells, whereas CEP55 knockdown inhibited these effects. We discovered that cell transformation induced by CEP55 was mediated by the AKT signalling pathway. Overexpression of CEP55 enhanced the phosphorylation of AKT and inhibited the activity of p21 WAF1/Cip1. In addition, cellular proliferation was suppressed as a result of cell cycle arrest at the G2/M phase in CEP55-knockdown cells. CEP55 expression was elevated in GC compared with normal control tissues. Credible evidence showed that CEP55 can be a potential therapeutic target in GC.

Overexpression of WDR62 is associated with centrosome amplification in human ovarian cancer

PURPOSE

To assess the clinical significance of WD40 repeat containing 62 (WDR62), a novel centrosome abnormalities-associated gene, in ovarian cancer.

MATERIALS AND METHODS

In this study, WDR62 expression was assessed by western blot (6 ovarian cancer cell lines) and immunohistochemistry (primary epithelial ovarian cancer clinical specimens), and clinical variables were collected by retrospective chart review. Centrosome amplification was assessed by immunofluorescence staining in ovarian cancer cell lines, and by immunohistochemistry staining in ovarian cancer samples.

RESULTS

Six ovarian cancer cell lines exhibited significant WDR62 protein overexpression, and amplification of centrosome. High-grade ovarian cancer specimens exhibited significantly stronger nuclear staining of WDR62 than low-grade ovarian carcinoma specimens (80.4% vs 41.3%; P<0.012). High WDR62 expression was strongly associated with supernumerary centrosome count in tumor cells (P < 0.001).

CONCLUSION

Our findings suggest that WDR62 overexpression is related to centrosome amplification in ovarian cancer. It may be a novel useful differentiation biomarker and a potential therapy target for OC. Further assessment of WDR62 expression is highly warranted in large, prospective studies.

Fanconi anemia complementation group A (FANCA) localizes to centrosomes and functions in the maintenance of centrosome integrity

Fanconi anemia (FA) proteins are known to play roles in the cellular response to DNA interstrand cross-linking lesions; however, several reports have suggested that FA proteins play additional roles. To elucidate novel functions of FA proteins, we used yeast two-hybrid screening to identify binding partners of the Fanconi anemia complementation group A (FANCA) protein. The candidate proteins included never-in-mitosis-gene A (NIMA)-related kinase 2 (Nek2), which functions in the maintenance of centrosome integrity. The interaction of FANCA and Nek2 was confirmed in human embryonic kidney (HEK) 293T cells. Furthermore, FANCA interacted with γ-tubulin and localized to centrosomes, most notably during the mitotic phase, confirming that FANCA is a centrosomal protein. Knockdown of FANCA increased the frequency of centrosomal abnormalities and enhanced the sensitivity of U2OS osteosarcoma cells to nocodazole, a microtubule-interfering agent. In vitro kinase assays indicated that Nek2 can phosphorylate FANCA at threonine-351 (T351), and analysis with a phospho-specific antibody confirmed that this phosphorylation occurred in response to nocodazole treatment. Furthermore, U2OS cells overexpressing the phosphorylation-defective T351A FANCA mutant showed numerical centrosomal abnormalities, aberrant mitotic arrest, and enhanced nocodazole sensitivity, implying that the Nek2-mediated T351 phosphorylation of FANCA is important for the maintenance of centrosomal integrity. Taken together, this study revealed that FANCA localizes to centrosomes and is required for the maintenance of centrosome integrity, possibly through its phosphorylation at T351 by Nek2.

Abnormal expression of Nek2 and β-catenin in breast carcinoma: clinicopathological correlations

AIMS

NIMA-related kinase 2 (Nek2) and β-catenin are important centrosome regulatory factors. The aim of this study was to detect the possible disparity in their expression among normal breast tissue, invasive ductal carcinoma (IDC), concomitant ductal carcinoma in situ (DCIS), and pure DCIS, and to explore its correlation with clinicopathological factors.

METHODS AND RESULTS

We used immunohistochemistry to detect protein expression of Nek2 and β-catenin in breast cancer tissues from 60 cases of pure DCIS, 348 cases of IDC and 137 cases of concomitant DCIS with that in normal breast tissues from the same 137 concomitant DCIS patients as controls. As compared with normal tissue, expression of Nek2 and β-catenin in the cytoplasm was significantly increased in IDC and DCIS (P < 0.05), and variation in expression was also observed in different grades of IDC (P < 0.01). Also, cytoplasmic expression of Nek2 and and of β-catenin were correlated with each other in IDC and DCIS (P < 0.01). In addition, they were both related to Ki67 immunoreactivity (P < 0.05). Furthermore, our study also revealed a correlation between their expression and some clinicopathological factors. We found that Nek2 cytoplasmic expression was associated with grade and tumour size (P < 0.01) in IDC, whereas β-catenin cytomembrane expression showed significant variation with grades, TNM stages, lymphoid node status, oestrogen receptor status, and molecular subtype (P < 0.05); a difference in expression was also observed between IDC and DCIS (P < 0.05). Also, β-catenin cytoplasmic expression was associated with TNM stage (P < 0.05). Expression of Nek2 at the mRNA level was detected in 50 pairs of breast cancer specimens and matched normal tissues by reverse transcriptase polymerase chain reaction, and the result showed increased expression in IDC.

CONCLUSIONS

This study suggests that abnormal expression of Nek2 and β-catenin might be one of the mechanisms of tumorigenesis, especially of abnormal tumour proliferation. They may represent new potential targets for therapeutic intervention.

Chromosome aberrations associated with centrosome defects: a study of comparative genomic hybridization in breast cancer

Centrosome abnormalities occur frequently in various tumors and can cause chromosomal instability and eventually promote cancer development. We investigated the chromosome aberrations associated with centrosome abnormalities in 30 cases of breast cancer, combining immunohistochemical staining and comparative genomic hybridization. Except for some common chromosome alterations (including gains of 1q, 8q, 17q, 20q, and Xq and losses of 8p, 11q, 13q, 14q, 16q, 17p, 22q, and Xp) that have also been seen more frequently in other studies, we discovered some new changes that have rarely been reported, including gains at 2p, 5p, 10p, 15q, 16p, 18q, 21q, and 22q and losses at 6p, 8p23, 11p13-pter, 13q34, and 14q32-qter. We also identified some changes (such as gains of 17q, 20q, and Xq and losses of 17p, 13q, and 14q) harboring candidate genes. We also explored the expression of centrosome protein in different molecular subtypes of breast cancer. Our findings provide a new way to explore the molecular mechanisms of breast tumorigenesis and accordingly potential new targets for therapy for this disease.

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.

Mitogen-activated protein/extracellular signal-regulated kinase kinase 1act/tubulin interaction is an important determinant of mitotic stability in cultured HT1080 human fibrosarcoma cells

Activation of the mitogen-activated protein kinase (MAPK) pathway plays a major role in neoplastic cell transformation. Using a proteomics approach, we identified alpha tubulin and beta tubulin as proteins that interact with activated MAP/extracellular signal-regulated kinase kinase 1 (MEK1), a central MAPK regulatory kinase. Confocal analysis revealed spatiotemporal control of MEK1-tubulin colocalization that was most prominent in the mitotic spindle apparatus in variant HT1080 human fibrosarcoma cells. Peptide arrays identified the critical role of positively charged amino acids R108, R113, R160, and K157 on the surface of MEK1 for tubulin interaction. Overexpression of activated MEK1 caused defects in spindle arrangement, chromosome segregation, and ploidy. In contrast, chromosome polyploidy was reduced in the presence of an activated MEK1 mutant (R108A, R113A) that disrupted interactions with tubulin. Our findings indicate the importance of signaling by activated MEK1-tubulin in spindle organization and chromosomal instability.

Arpc1b, a centrosomal protein, is both an activator and substrate of Aurora A

In addition to its function as an Arp2/3 complex subunit, Arp1cb interacts with and stimulates Aurora A at centrosomes, functioning in cell cycle progression.

Here we provide evidence in support of an inherent role for Arpc1b, a component of the Arp2/3 complex, in regulation of mitosis and demonstrate that its depletion inhibits Aurora A activation at the centrosome and impairs the ability of mammalian cells to enter mitosis. We discovered that Arpc1b colocalizes with γ-tubulin at centrosomes and stimulates Aurora A activity. Aurora A phosphorylates Arpc1b on threonine 21, and expression of Arpc1b but not a nonphosphorylatable Arpc1b mutant in mammalian cells leads to Aurora A kinase activation and abnormal centrosome amplification in a Pak1-independent manner. Together, these findings reveal a new function for Arpc1b in centrosomal homeostasis. Arpc1b is both a physiological activator and substrate of Aurora A kinase and these interactions help to maintain mitotic integrity in mammalian cells.

Delocalization of gamma-tubulin due to increased solubility in human breast cancer cell lines

The centrosome is the major organelle responsible for the nucleation and organization of microtubules into arrays. Recent studies demonstrate that microtubules can nucleate outside the centrosome. The molecular mechanisms controlling acentrosomal microtubule nucleation are currently poorly defined, and the function of this type of microtubule regulation in tumor cell biology is particularly unclear. Since microtubule nucleation is initiated by the gamma-tubulin protein, we examined the regulation of gamma-tubulin in a panel of human breast tumor cell lines, ranging from non-tumorigenic to highly aggressive. We have identified a more dispersive subcellular localization of gamma-tubulin in aggressive breast cancer cell lines, while gamma-tubulin localization remains largely centrosomal in non-aggressive cell lines. Delocalization of gamma-tubulin occurs independently from changes in protein expression and is therefore regulated at the post-translational level. Subcellular fractionation revealed that tumor cell lines show an aberrantly increased release of gamma-tubulin into a soluble cytoplasmic fraction, with the most dramatic changes observed in tumor cell lines of greater aggressiveness. Extraction of soluble gamma-tubulin revealed acentrosomal incorporation of gamma-tubulin in cytoplasmic microtubules and along cell junctions. Moreover, acentrosomal delocalization of gamma-tubulin yielded resistance to colchicine-mediated microtubule collapse. These findings support a model where the solubility of gamma-tubulin can be altered through post-translational modification and provides a new mechanism for microtubule dysregulation in breast cancer. Gamma-tubulin that is delocalized from the centrosome can still clearly be incorporated into filaments, and defines a novel mechanism for tumor cells to develop resistance to microtubule-targeted chemotherapies.

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.

Understanding the role of aneuploidy in tumorigenesis

The role of aneuploidy in tumorigenesis remains poorly understood, although the two have been known to be linked for more than 100 years. Recent studies indicate that aneuploidy can promote tumour cell growth and cell death and that the cellular outcome is dependent on the extent of aneuploidy induced. The mitotic checkpoint plays a pivotal role in the maintenance of genome stability and has been the focus of work investigating the distinct outcomes of aneuploidy. In the present article, we review the molecular mechanisms involved and discuss the potential of the mitotic checkpoint as a therapeutic target in cancer therapy.

Emerging connection between centrosome and DNA repair machinery

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

Inactivation of the Nijmegen breakage syndrome gene leads to excess centrosome duplication via the ATR/BRCA1 pathway

Nijmegen breakage syndrome is characterized by genomic instability and a predisposition for lymphoma and solid tumors. Nijmegen breakage syndrome 1 (NBS1), the protein which is mutated in these patients, functions in association with BRCA1 and ATR as part of the cellular response to DNA double-strand breaks. We show here that NBS1 forms foci at the centrosomes via an interaction with gamma-tubulin. Down-regulation of NBS1 by small interfering RNA induces supernumerary centrosomes, and this was confirmed with experiments using Nbs1 knockout mouse cells; the introduction of wild-type NBS1 (wt-NBS1) cDNA into these knockout mouse cells reduced the number of supernumerary centrosomes to normal levels. This phenotype in NBS1-deficient cells is caused by both centrosome duplication and impaired separation of centrioles, which have been observed in BRCA1-inhibited cells. In fact, supernumerary centrosomes were observed in Brca1 knockout mouse cells, and the frequency was not affected by NBS1 down-regulation, suggesting that NBS1 maintains centrosomes via a common pathway with BRCA1. This is consistent with findings that NBS1 physically interacts with BRCA1 at the centrosomes and is required for BRCA1-mediated ubiquitination of gamma-tubulin. Moreover, the ubiquitination of gamma-tubulin is compromised by either ATR depletion or an NBS1 mutation in the ATR interacting (FHA) domain, which is essential for ATR activation. These results suggest that, although centrosomes lack DNA, the NBS1/ATR/BRCA1 repair machinery affects centrosome behavior, and this might be a crucial role in the prevention of malignances.

Centrosome-associated regulators of the G(2)/M checkpoint as targets for cancer therapy

In eukaryotic cells, control mechanisms have developed that restrain cell-cycle transitions in response to stress. These regulatory pathways are termed cell-cycle checkpoints. The G(2)/M checkpoint prevents cells from entering mitosis when DNA is damaged in order to afford these cells an opportunity to repair the damaged DNA before propagating genetic defects to the daughter cells. If the damage is irreparable, checkpoint signaling might activate pathways that lead to apoptosis. Since alteration of cell-cycle control is a hallmark of tumorigenesis, cell-cycle regulators represent potential targets for therapy. The centrosome has recently come into focus as a critical cellular organelle that integrates G(2)/M checkpoint control and repairs signals in response to DNA damage. A growing number of G(2)/M checkpoint regulators have been found in the centrosome, suggesting that centrosome has an important role in G(2)/M checkpoint function. In this review, we discuss centrosome-associated regulators of the G(2)/M checkpoint, the dysregulation of this checkpoint in cancer, and potential candidate targets for cancer therapy.

Increased gamma-tubulin expression and P16INK4A promoter methylation occur together in preinvasive lesions and carcinomas of the breast

BACKGROUND

Loss of p16(INK4A) due to promoter hypermethylation is correlated with the ability to acquire centrosomal abnormalities in variant human mammary epithelial cells. gamma-Tubulin is a highly conserved component of centrosome in most animal cells and gamma-tubulin protein overexpression could lead to centrosome aberration.

MATERIALS AND METHODS

A large series of breast premalignant lesions and carcinoma was analyzed. Real-time quantitative PCR and immunohistochemistry were carried out to measure gamma-tubulin copy numbers and protein expression. MethyLight and immunohistochemistry were carried out to determine p16(INK4A) methylation and protein expression.

RESULTS

gamma-Tubulin protein expression was concordant with gene amplification; both of them were found to increase with atypical ductal hyperplasia-carcinoma sequence. The median value and positive rate of p16(INK4a) methylation increased while protein expression displayed a decreasing trend. P16(INK4a) methylation showed a firm association with gamma-tubulin gene amplification.

CONCLUSION

gamma-Tubulin gene amplification and the concomitant protein overexpression present not only in invasive carcinoma but also in a significant fraction of atypical hyperplasia and in situ carcinomas. P16(INK4a) methylation and gamma-tubulin gene amplification had a synergistic effect on tumor progression. The synergism might arise as a result of the combined influence that p16(INK4a) and gamma-tubulin have on the G1-S cell cycle checkpoints and centrosome.

Regulation of centrosomes by the BRCA1-dependent ubiquitin ligase

Centrosomes are the organelles that organize microtubule networks and establish the bipolar mitotic spindle, which is essential for the segregation of chromosomes during cell division. Proper duplication of centrosomes is necessary to prevent genetic instability, thus control of this organelle is important in the suppression of tumorigenesis. The BRCA1 dependent ubiquitination activity regulates centrosome number in breast derived cell lines and this activity is likely critical for the tumor suppression activity of BRCA1. This review will focus on the importance of controlling centrosome number and on the effect of BRCA1 on the centrosome duplication cycle in mammary cells.

Association of loss of BRCA1 expression with centrosome aberration in human breast cancer

PURPOSE

Centrosome aberration in number and/or size is reportedly often observed in human breast cancer. The aim of this study was to investigate the relationship between centrosome aberration and chromosomal instability as well as the expression of centrosome regulators such as BRCA1, Aurora-A, and p53.

METHODS

Centrosome aberration in number and size was determined immunohistochemically using the anti-gamma-tubulin antibody, and chromosomal instability was evaluated by fluorescence in situ hybridization analysis of chromosomes 1, 11, and 17 in paraffin sections from 50 human breast cancers. Immunohistochemical examination of BRCA1, Aurora-A, and p53 was also performed to examine the relationship of their expression with centrosome aberration.

RESULTS

Percentage of tumor cells with centrosome aberration in size varied from 0.9 to 30.4% (median 9.5%) and in number it varied from 0.5 to 86.5% (median 34.5%) in each tumor. No significant association in number or size, however, was observed between chromosomal instability and centrosome aberration. Numerical centrosome aberration was significantly associated with negative BRCA1 expression (P = 0.001). Breast tumors (n = 3) from patients with a proven BRCA1 germline mutation also showed a significant relationship with numerical centrosome aberration (P = 0.011). On the other hand, expression of Aurora-A or p53 was not significantly associated with centrosome aberration in either number or size.

CONCLUSIONS

Centrosome aberration is not associated with chromosomal instability, indicating the importance of other mechanisms in the induction of chromosomal instability in human breast cancer. BRCA1, but not Aurora-A and p53, is significantly involved in the pathogenesis of centrosome aberration.

Aurora-A kinase regulates breast cancer associated gene 1 inhibition of centrosome-dependent microtubule nucleation

Breast cancer-associated gene 1 (BRCA1) regulates the duplication and the function of centrosomes in breast cells. We have previously shown that BRCA1 ubiquitin ligase activity directly inhibits centrosome-dependent microtubule nucleation. However, there is a paradox because centrosome microtubule nucleation potential is highest during mitosis, a phase when BRCA1 is most abundant at the centrosome. In this study, we resolve this conundrum by testing whether centrosomes from cells in M phase are regulated differently by BRCA1 when compared with other phases of the cell cycle. We observed that BRCA1-dependent inhibition of centrosome microtubule nucleation was high in S phase but was significantly lower during M phase. The cell cycle-specific effects of BRCA1 on centrosome-dependent microtubule nucleation were detected in living cells and in cell-free experiments using centrosomes purified from cells at specific stages of the cell cycle. We show that Aurora-A kinase modulates the BRCA1 inhibition of centrosome function by decreasing the E3 ubiquitin ligase activity of BRCA1. In addition, dephosphorylation of BRCA1 by protein phosphatase 1 alpha enhances the E3 ubiquitin ligase activity of BRCA1. These observations reveal that the inhibition of centrosome microtubule nucleation potential by the BRCA1 E3 ubiquitin ligase is controlled by Aurora-A kinase and protein phosphatase 1 alpha-mediated phosphoregulation through the different phases of the cell cycle.

BRCA1 regulates gamma-tubulin binding to centrosomes

Centrosomes are the cellular organelles that nucleate microtubules (MTs) via the activity of gamma-tubulin ring complex(s) (gammaTuRC) bound to the pericentriolar material of the centrosomes. BRCA1, the breast and ovarian cancer specific tumor suppressor, inhibits centrosomal MT nucleation via its ubiquitin ligase activity, and one of the known BRCA1 substrates is the key gammaTuRC component, gamma-tubulin. We analyzed the mechanism by which BRCA1 regulates centrosome function using an in vitro reconstitution assay, which includes separately staged steps. Our results are most consistent with a model by which the BRCA1 ubiquitin ligase modifies both gamma-tubulin plus a second centrosomal protein that controls localization of gammaTuRC to the centrosome. We suggest that this second protein is an adapter protein or protein complex that docks gamma-TuRC to the centrosome. By controlling gamma-TuRC localization, BRCA1 appropriately inhibits centrosome function, and loss of BRCA1 would result in centrosome hyperactivity, supernumerary centrosomes and, possibly, aneuploidy.

Human papillomavirus type 16 E7 oncoprotein associates with the centrosomal component gamma-tubulin

Expression of a high-risk human papillomavirus (HPV) E7 oncoprotein is sufficient to induce aberrant centrosome duplication in primary human cells. The resulting centrosome-associated mitotic abnormalities have been linked to the development of aneuploidy. HPV type 16 (HPV16) E7 induces supernumerary centrosomes through a mechanism that is at least in part independent of the inactivation of the retinoblastoma tumor suppressor pRb and is dependent on cyclin-dependent kinase 2 activity. Here, we show that HPV16 E7 can concentrate around mitotic spindle poles and that a small pool of HPV16 E7 is associated with centrosome fractions isolated by sucrose density gradient centrifugation. The targeting of HPV16 E7 to the centrosome, however, was not sufficient for centrosome overduplication. Nonetheless, we found that HPV16 E7 can associate with the centrosomal regulator gamma-tubulin and that the recruitment of gamma-tubulin to the centrosome is altered in HPV16 E7-expressing cells. Since the association of HPV16 E7 with gamma-tubulin is independent of pRb, p107, and p130, our results suggest that the association with gamma-tubulin contributes to the pRb/p107/p130-independent ability of HPV16 E7 to subvert centrosome homeostasis.

Centrosomal localization of DNA damage checkpoint proteins

During mitosis, the phosphatidylinositol-3 (PI-3) family-related DNA damage checkpoint kinases ATM and ATR were found on the centrosomes of human cells. ATRIP, an interaction partner of ATR, as well as Chk1 and Chk2, the downstream targets of ATR or ATM, were also localized to the centrosomes. Surprisingly, the DNA-PK inhibitor vanillin enhanced the level of ATM on centrosomes. Accordingly, DNA-PKcs, the catalytic subunit of DNA-PK, was also found on the centrosomes. Vanillin altered the phosphorylation of Chk2 in the centrosomes and in whole cell extracts. Nucleoplasmic ATM co-immunoprecipitated with Ku70/86, the DNA binding subunits of DNA-PK, while vanillin diminished this association. Vanillin did not affect microtubule polymerization at the centrosomes but, surprisingly, caused a transient enhancement of alpha-tubulin foci in the nucleus. Interestingly, gamma-tubulin was also present in the nucleus and co-immunoprecipitated with ATR or BRCA1. DNA damage led to a reduction of the mentioned checkpoint proteins on the centrosomes but increased the level of gamma-tubulin at this organelle. Taken together, these results indicate that DNA damage checkpoint proteins may control the formation of gamma-tubulin and/or the kinetics of microtubule formation at the centrosomes, and thereby couple them to the DNA damage response.

Expression of the p210BCR-ABL oncoprotein drives centrosomal hypertrophy and clonal evolution in human U937 cells

Centrosomes play fundamental roles in mitotic spindle organization, chromosome segregation and maintenance of genetic stability. Recently, we have shown that centrosome aberrations occur early in chronic myeloid leukemia (CML) and are induced by imatinib in normal fibroblasts in vitro. To investigate the influence of BCR-ABL on centrosomes, we performed long-term in vitro experiments employing the conditionally p210BCR-ABL-expressing (tetracycline-inducible promoter) human monocytic cell line U937p210BCR-ABL/c6 as a model of CML chronic phase. Centrosome hypertrophy was detectable after 4 weeks of transgene expression onset, increasing up to a rate of 25.7% aberrant cells within 13 weeks of propagation. This concurred with clonal expansion of aneuploid cells displaying a hyperdiploid phenotype with 57 chromosomes. Partial reversibility of centrosome aberrations (26-8%) was achieved under prolonged propagation (14 weeks) after abortion of induction and bcr-abl silencing using small interfering RNA. Therapeutic doses of imatinib did not revert the aberrant phenotype, but counteracted the observed reverting effect of bcr-abl gene expression switch off. Suggesting a mechanistic model that features distinct abl-related tyrosine kinase activity levels as essential determinants of centrosomal integrity, this is the first report mechanistically linking p210BCR-ABL oncoprotein activity to centrosomal hypertrophy.

RINT-1 serves as a tumor suppressor and maintains Golgi dynamics and centrosome integrity for cell survival

Faithful mitotic partitioning of the Golgi apparatus and the centrosome is critical for proper cell division. Although these two cytoplasmic organelles are probably coordinated during cell division, supporting evidence of this coordination is still largely lacking. Here, we show that the RAD50-interacting protein, RINT-1, is localized at the Golgi apparatus and the centrosome in addition to the endoplasmic reticulum. To examine the biological roles of RINT-1, we found that the homozygous deletion of Rint-1 caused early embryonic lethality at embryonic day 5 (E5) to E6 and the failure of blastocyst outgrowth ex vivo. About 81% of the Rint-1 heterozygotes succumbed to multiple tumor formation with haploinsufficiency during their average life span of 24 months. To pinpoint the cellular function of RINT-1, we found that RINT-1 depletion by RNA interference led to the loss of the pericentriolar positioning and dispersal of the Golgi apparatus and concurrent centrosome amplification during the interphase. Upon mitotic entry, RINT-1-deficient cells exhibited multiple abnormalities, including aberrant Golgi dynamics during early mitosis and defective reassembly at telophase, increased formation of multiple spindle poles, and frequent chromosome missegregation. Mitotic cells often underwent cell death in part due to the overwhelming cellular defects. Taken together, these findings suggest that RINT-1 serves as a novel tumor suppressor essential for maintaining the dynamic integrity of the Golgi apparatus and the centrosome, a prerequisite to their proper coordination during cell division.

Suppressors of zyg-1 define regulators of centrosome duplication and nuclear association in Caenorhabditis elegans

In Caenorhabditis elegans, the kinase ZYG-1 is required for centrosome duplication. To identify factors that interact with ZYG-1, we used a classical genetic approach and identified 21 szy (suppressor of zyg-1) genes that when mutated restore partial viability to a zyg-1 mutant. None of the suppressors render animals completely independent of zyg-1 activity and analysis of a subset of the suppressors indicates that all restore the normal process of centrosome duplication to zyg-1 mutants. Thirteen of these suppressor mutations confer phenotypes of their own and cytological examination reveals that these genes function in a variety of cellular processes including cell cycle timing, microtubule organization, cytokinesis, chromosome segregation, and centrosome morphology. Interestingly, several of the szy genes play a role in attaching the centrosome to the nuclear envelope. We have found that one such szy gene is sun-1, a gene encoding a nuclear envelope component. We further show that the role of SUN-1 in centrosome duplication is distinct from its role in attachment. Our approach has thus identified numerous candidate regulators of centrosome duplication and uncovered an unanticipated regulatory mechanism involving factors that tether the centrosome to the nucleus.

The Influence of Nuclear Content on Developmental Competence of Gaur × Cattle Hybrid In Vitro Fertilized and Somatic Cell Nuclear Transfer Embryos1

In nondomestic and endangered species, the use of domestic animal oocytes as recipients for exotic donor nuclei causes the normal pattern of cytoplasmic inheritance to be disrupted, resulting in the production of nuclear-cytoplasmic hybrids. Evidence suggests that conflict between nuclear and cytoplasmic control elements leads to a disruption of normal cellular processes, including metabolic function and cell division. This study investigated the effects of nuclear-cytoplasmic interactions on the developmental potential of interspecies embryos produced by in vitro fertilization and somatic cell nuclear transfer: cattle x cattle, gaur x cattle, hybrid x cattle. Cattle control and hybrid embryos were examined for development to the blastocyst stage and blastocyst quality, as determined by cell number and allocation, apoptosis incidence, and expression patterns of mitochondria-related genes. These analyses demonstrated that a 100% gaur nucleus within a domestic cattle cytoplasmic environment was not properly capable of directing embryo development in the later preimplantation stages. Poor blastocyst development accompanied by developmental delay, decreased cell numbers, and aberrant apoptotic and related gene expression profiles, all signs of disrupted cellular processes associated with mitochondrial function, were observed. Developmental potential was improved when at least a portion of the nuclear genome corresponded to the inherited cytoplasm, indicating that recognition of cytoplasmic components by the nucleus is crucial for proper cellular function and embryo development. A better understanding of the influence of the cytoplasmic environment on embryonic processes is necessary before interspecies somatic cell nuclear transfer can be considered a viable alternative for endangered species conservation.

Cloning in companion animal, non-domestic and endangered species: can the technology become a practical reality?

Somatic cell nuclear transfer (SCNT) can provide a unique alternative for the preservation of valuable individuals, breeds and species. However, with the exception of a handful of domestic animal species, successful production of healthy cloned offspring has been challenging. Progress in species that have little commercial or research interest, including many companion animal, non-domestic and endangered species (CANDES), has lagged behind. In this review, we discuss the current and future status of SCNT in CANDES and the problems that must be overcome to improve pre- and post-implantation embryo survival in order for this technology to be considered a viable tool for assisted reproduction in these species.