Centrosomes and cancer: how cancer cells divide with too many centrosomes
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- Godinho, S.A., Kwon, M. & Pellman, D. Cancer Metastasis Rev (2009) 28: 85. doi:10.1007/s10555-008-9163-6
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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.
1 The centrosome
The centrosome, the primary microtubule-organizing center (MTOC) in animal cells, regulates the nucleation of microtubules (MTs) as well as their spatial organization [for review see ]. In addition to its facilitating role in mitosis, the centrosome influences MT-dependent processes in interphase: cell shape, cell polarity and cell motility.
A single centrosome contains two centrioles, joined by fibers connecting their proximal ends. The centrioles are embedded in an electron-dense protein matrix, known as the pericentriolar material (PCM). Centrioles within a pair are structurally different at their distal ends, the mother centriole contains a set of distal appendages important to anchor MTs, while the daughter centriole does not . Centrioles are also important for the recruitment of the PCM, as centriole loss leads to PCM dispersal and loss of centrosome integrity [3–7].
Like DNA, centrioles normally duplicate in a semi-conservative fashion once per cell cycle during S-phase, ensuring that each daughter cell inherits one centrosome at the end of cell division. The centrosome cycle can be subdivided into five distinct steps: centrosome segregation (M-phase), centriole disengagement (late M-phase/G1-phase), centriole duplication (S-phase), centrosome maturation and centrosome separation (G2-phase) [reviewed in [1, 8, 9]. Recently, the PCM has been shown to have an important role in centriole duplication by controlling the formation and stabilization of daughter centrioles [10, 11], suggesting that centrioles and PCM have a symbiotic relationship for their assembly and stabilization.
Coordination of the DNA and centrosome replication cycles is essential to avoid centrosome overduplication . Indeed, defective coordination between the DNA and centrosome cycles is commonly observed in many cancers [12, 13]. The mechanisms that control centrosome duplication are only recently starting to be understood [1, 8, 9, 14]. Similar to the DNA cycle, the control of centrosome duplication involves: (1) centrosome-duplication machinery; (2) licensing molecules essential for centriole duplication; (3) activities that integrate licensing with cell cycle controls, maintaining the coordination of both processes [for review see [1, 8, 14]].
2 Centrosomes and cancer
Extra centrosomes and Cancer
Centrosome amplification (supernumerary centrosomes) is a common characteristic of most solid and hematological cancers examined to date [12, 13, 15–17]. Extra centrosomes in cancers can originate by several mechanisms: cell fusion, centrosome overduplication, de novo synthesis of centrosomes, mitotic slippage and cytokinesis failure [13, 18–20]. One way to distinguish whether centrosome amplification is derived from overduplication of centrosomes or from cytokinesis failure is to specifically label mature centrioles . The presence of only one mature centriole among numerous immature centrioles indicates centriole overduplication rather than cell division failure . Although this type of analysis is an important technical advance, it has yet to be applied to primary tumors on a large-scale basis. Thus, the relative frequency that each of these different paths contributes to centrosome amplification in human tumors remains unknown. Whatever the cause leading to extra centrosomes, the percentage of cancer cells containing extra centrosomes within a given tumor is highly variable. Why some cancer cells maintain their extra centrosomes more than others, how they maintain them, and whether extra centrosomes are beneficial or deleterious to cancer cells remain interesting open questions.
The link between supernumerary centrosomes and tumorigenesis dates back over 100 years. Extra centrosomes may contribute to tumorigenesis by promoting genetic instability through multipolar mitoses or by disrupting cell polarity and asymmetric cell division [13, 15, 19, 22]. Genetic instability in human cancers was first hinted at in 1890 by the German pathologist, David Hansemann, who observed the presence of bipolar divisions but with asymmetric segregation of chromosomes in human epithelial tumors . Following Hansemann’s report, in 1902, Theodor Boveri recognized that multipolar divisions in the presence of extra centrosomes could have similar consequences; each daughter cell inherits an incorrect and unequal set of chromosomes . These findings led Boveri to propose a simple but appealing hypothesis that tumors can originate from premalignant precursors and that aneuploidy resulting from multipolar divisions in cells harboring supernumerary centrosomes has a causal role in tumorigenesis . This idea is further supported by a strong correlative relationship between extra centrosomes, aneuploidy and tumor aggressiveness in tumor-derived cell lines, mouse models and human patients [15, 16, 25–27]. Deregulation of many genes implicated in human cancers leads to centrosome amplification. For example, centrosome overduplication can result from mutation of the p53 tumor suppressor, a genetic alteration associated with more than half of human tumors . Moreover, centrosome overduplication is associated with aneuploidy in nucleophosmin (NPM) haploinsufficient mice. NPM haploinsufficiency induced tumorigenesis and this was accompanied by unrestricted centrosome duplication and genetic instability . Likewise, many cell division errors can lead to cytokinesis failure which is linked, by a series of recent studies, to genetic changes in tumors [20, 30–34]. Fujiwara et al. demonstrated that tetraploid cells containing extra centrosomes could promote genetic instability and tumorigenesis in a mouse mammary model . Several recent studies observed a similar tumor-promoting effect of tetraploidy [18, 36].
Cancer cell division: the extra centrosome paradox
The idea that extra centrosomes lead to multipolar mitoses, aneuploidy, and thus tumor formation is based on the correlation between extra centrosomes, multipolar mitoses and tumorigenesis. However, this does not account for the fact that multipolar mitoses are likely to be detrimental to cancer cells because multipolar mitoses can lead to gross chromosome missegregation and death. A recent study by Weaver et al. highlights the fact that there is no black-and-white answer for the relationship between aneuploidy and tumorigenesis . The degree of aneuploidy and the context in which it occurs can result in different outcomes: moderate levels of genetic instability can induce tumorigenesis, whereas high levels of genetic instability, which might be intolerable to cancer cells, can instead suppress tumorigenesis in vivo . Using mice heterozygous for the mitotic kinesin CENP-E, which is important for chromosome segregation and mitotic checkpoint function, the authors demonstrate that one or few missegregations of whole chromosomes could promote tumorigenesis in some tissues. Interestingly, when this mouse was treated with a chemical mutagen or crossed with a mouse deficient for the tumor suppressor gene ARF which by itself promotes tumor development, suppression of tumorigenesis was observed. Whether or not the level of aneuploidy in those tumors is higher than that of CENP-E heterozygous mice was not examined in this study . Altogether, high levels of aneuploidy expected from a multipolar mitosis might set unfavorable conditions for the survival of cancer cells with supernumerary centrosomes.
How then do cancer cells “cope” with extra centrosomes? For tumors with a small number of extra centrosomes-containing cells, coping mechanisms may not be important. A minor fraction of cells undergoing multipolar mitoses and death might have only minimal effects on the growth of the tumor. However, as the fraction of cells with extra centrosomes within a tumor rises, there might be a stronger requirement to suppress multipolar mitoses.
In addition, supernumerary centrosomes can be inactivated, allowing only two centrosomes to function as MTOCs during mitosis (Fig. 1b). In physiologically polyspermic newt eggs, all sperm nuclei incorporated into the egg develop sperm asters, however eventually only one sperm nucleus and associated centrosomes contribute to bipolar mitosis of the zygote . Several studies demonstrated that a gradient of cyclin B1 and γ-tubulin distribution (high in the animal hemisphere while low in the vegetal hemisphere) is responsible for the preferential accumulation of cyclin B1 and γ-tubulin with the centrosome associated with the principal sperm nucleus . This ensures that only the centrosome associated with the zygote nucleus located in the animal hemisphere contributes to the formation of bipolar spindle whereas centrosomes associated with accessory nuclei degenerate [46, 47]. MTOC activity of the centrosomes is regulated not only by the accumulation of PCM but also by their phosphorylation, dependent upon cdk1/cyclin B . Thus, a gradient of cyclin B1 and γ-tubulin in a large oocyte may confer selectivity to centrosomes to be maintained in this study. In addition, given that PCM also have a important role in centrioles duplication by the formation and stabilization of daughter centrioles [6, 10], loss of PCM could account for the mechanism for centrosome degeneration by limiting their duplication potential in this system. Centrosome inactivation also seems to occur in flies overexpressing SAK/PLK4: centrosomes that are scattered along spindles contain significantly less γ-tubulin than those at the spindle poles . Thus, a decrease in PCM levels may account for, at least partially, the inactivation of extra centrosomes. However, the mechanisms leading to selective removal or inactivation of centrosomes still remain a mystery. How frequently the removal or inactivation of extra centrosomes occurs in mammalian cells and what distinguishes the centrosomes to be removed or inactivated from those that form the MTOCs is largely unknown.
Alternatively, extra centrosomes may be segregated asymmetrically during cell division (Fig. 1c). In this case, the daughter cell that inherited one centrosome would proliferate and take over the population if it harbors sufficient genetic material or any mutation to support their survival . Predominantly in cancer cells, supernumerary centrosomes can be clustered into two groups to achieve a bipolar mitosis (Fig. 1d) [13, 50–52]. Even in Dictyostelium and SAK overexpressing flies, where centrosome removal or inactivation was observed, centrosome clustering appears to be a dominant mechanism to prevent multipolar mitosis [37, 44].
In principle, centrosome clustering could be a unique adaptation mechanism in cancer with extra centrosomes. Alternatively, and perhaps more intuitively, mechanisms to cluster centrosomes may exist in normal cells, but only cells with extra centrosomes depend on this machinery to survive [13, 58, 59]. Consistent with the latter hypothesis, several non-transformed cell types are known to cluster extra centrosomes in various circumstances: after cytokinesis failure [52, 60, 61], during normal development, or in the setting of stress or disease [58, 59]. Nevertheless, the efficiency of centrosome clustering varies among cancer cell lines ; it remains an interesting possibility that under selective pressure for bipolar mitoses, the strength of these pathways is modulated in different tumor types.
3 Mechanisms to facilitate centrosome clustering
Spindle assembly checkpoint (SAC)
The SAC is a signaling network that delays anaphase onset if kinetochores are improperly attached to the mitotic spindle [reviewed in . Recent studies suggest that the SAC is one important factor in suppressing multipolar mitoses. In addition to extra centrosomes, it has been observed by Lambert in 1913 that tumors frequently display a high mitotic index (the ratio between the number of cells in mitosis and the total number of cells) . Multiple subsequent studies suggest that this increased mitotic index might be explained by a prolonged metaphase [64, 65]. Although similar results have now been observed in numerous studies, the precise nature of the delay was unclear. One complicating issue when assessing mitotic index for a given tumor is defining the appropriate “normal” cell for comparison. Nevertheless, using time-lapse microscopy, direct comparison of SV40 transformed human fibroblasts and their counterpart untransformed cells clearly revealed that the delay in transformed cells occurs in metaphase , suggesting that transformation-dependent events might account for the metaphase delay. In SV40 transgene-induced pancreatic cancer model, SV40 transformation generated tetraploid cells with extra centrosomes. Thus, the mitotic delay in tumors could possibly be explained by tetraploidy and/or extra centrosomes that are frequently associated with cancer . Indeed, work from Rieder and colleagues, directly demonstrated that either increased chromosome or centrosome numbers can increase the duration of mitosis in human cells .
Molecular motors and spindle associated proteins
Centrosome clustering in tumor cells relies to a significant degree on MT-based motors and microtubule-associated proteins (MAPs) that organize the spindle poles. Numerous studies have identified proteins required to form spindle poles. Interestingly, the importance of these factors vary from cell type to cell type; loss of the minus-end directed motor dynein in Drosophila S2 cells inhibits the attachment of centrosomes to the spindles, but does not strikingly increase the frequency of multipolar mitoses . In contrast, dynein is absolutely required for the organization of supernumerary centrosomes in many human cell lines . In cancer cells containing extra centrosomes, elevated levels of NuMA (mitotic apparatus protein), a main structural component of the spindle poles, and accompanying loss of dynein from the spindle lead to centrosome de-clustering. Interestingly, this defect was rescued by the titration of NuMA levels, which restore dynein localization to the spindle, supporting the role of the NuMA/dynein complex in clustering supernumerary centrosomes [72, 73].
The actin cytoskeleton
A critical role for the actin cytoskeleton in suppressing multipolar mitoses in cells with extra centrosomes was recently described [52, 78]. In principle, the actin cytoskeleton could affect the distribution of centrosomes by at least four mechanisms: (1) by controlling cell shape; (2) by controlling the contractility and stiffness of the cell cortex; (3) by influencing the cell adhesion pattern and the organization and integrity of retraction fibers (RFs); and (4) via a direct role on the centrosomes and the spindle. Although studies by Kwon and colleagues support a key role for the interphase adhesion pattern and thus the distribution of RFs (see next section), we cannot exclude the possibility that other roles mediated by the actin cytoskeleton could be important for the clustering of supernumerary centrosomes.
Actin, along with myosin II, is important for centrosome separation by providing cortical forces that drive the separation of newly duplicated centrosomes . One model for how actin-based contractility could influence centrosome organization, proposed by Rosenblatt et al., envisions astral MTs attached to a continuously tensioned cortical actin cytoskeleton. Astral MTs are known to inhibit type II myosin-dependent cortical contractility. These authors reasoned that if astral MTs attach to the cortical actin cytoskeleton and stimulate a local relaxation of myosin II, the induced asymmetric contractile forces would drive centrosome separation. If cells have two centrosomes, the expected lowest energy state would be to have two centrosomes at opposite poles. With more than two centrosomes, the situation might be more complex; contractility would drive centrosomes together or apart, depending on their starting positions. Under this sort of model, cortical rigidity would also be expected to be a prerequisite for exerting forces on astral MTs. Indeed, the contribution of cortical rigidity to spindle formation has been recently shown in multiple studies and this appears to be mediated in large part by moesin activation [79, 80]. Moesin is a member of the ezrin/radixin/moesin (ERM) family. The ERM proteins bind to actin and stiffen the cortex by cross linking the actin cortex to the plasma membrane . Depletion of moesin, the sole ERM family member in Drosophila, impairs cortical rigidity and is important for spindle formation and positioning [79, 80]. Strikingly, in mammalian tissue culture cells, the ERM family member ezrin localizes heterogeneously at the cell cortex where RFs form (see next section). RFs are actin-rich structures that form during mitotic round-up at the sites of strong interphase cell adhesion. The regions where RF form are important to promote interaction of the astral MTs with the cortex, which drive spindle orientation . Thus, actin-dependent cortical stiffness during mitosis might be important for centrosome clustering by facilitating astral MTs interaction with the cortex.
An alternative view of how cortical forces act on astral MTs is that the distribution of cortical forces that act on astral MTs is heterogeneous rather than being continuously tensioned. Under this model, local forces rather than global contractility would be the dominant factor in positioning centrosomes during mitosis. This hypothesis is supported by the heterogeneous distribution of ezrin in mammalian tissue culture cells . It is also strongly supported by the influence of the adhesion pattern on the distribution of RFs (see next section). Irrespective of the precise nature of the cortical forces acting on centrosomes, the importance of the actin cytoskeleton and myosin II are clear; actin-based contractility is not merely required, but enhanced cortical contractility can suppress multipolar mitoses in certain circumstances .
The machinery that drives centrosome separation or spindle positioning in cells could also contribute to the organization of multiple centrosomes in extra centrosomal cells. This would suggest a role for the astral MTs, which link the cortex and centrosomes, in the clustering of extra centrosomes. Interactions between astral MTs and the cell cortex are mediated by MT plus-ends proteins, such as MT-associated proteins (MAPs) and plus- and minus-end directed MT motors [for review see ). The MAPs associated with the MT plus-ends, such as CLIP-170, are important for the anchoring of MTs to the cortex; MT motors, such as the minus-end directed motor dynein, are though to be responsible for the pulling forces necessary for spindle positioning [83–85]. More recently, the actin-based motor protein Myosin-10 (Myo10) was demonstrated to be important for spindle positioning in mammalian cells . The unconventional Myo10 contains a motor domain that binds to actin and a C-terminal MyTH4/FERM domain, which in turn binds to MTs. This uncommon characteristic makes Myo10 an ideal candidate to link astral microtubules to cortical actin to promote spindle orientation. However, how and where astral MTs contact the cortex is not well understood. Several studies suggest the spindle is positioned according to adhesion cues that are translated into cortical domains where astral MTs will preferentially bind [82, 87]. Interestingly, depolymerization of astral MTs with low doses of nocodazole affects not only spindle positioning but also centrosome clustering [52, 82]. Furthermore, consistent with a role for the spindle positioning machinery in centrosome clustering, other molecules involved in spindle positioning seem to cause a multipolar phenotype, namely CLIP-170 (Drosophila CLIP-190 homolog), Phosphoinositide-3 kinase (PI3K), p150glued (dynactin subunit), dynein and Myo10 [52, 72, 86–89]. Thus, the spindle positioning machinery that operates in normal cells is involved in the clustering of extra centrosomes.
Similar to actin, adhesion molecules can associate with the centrosomes and directly influence spindle assembly. For example, molecules involved in focal adhesion regulation during interphase, such as HEF1 and PAK1, were shown to associate with centrosomes during mitosis and to regulate Aurora-A activity [96, 97]. Moreover, the integrin-linked kinase (ILK), which functions in integrin-mediated cell adhesion, has also been shown to localize to the centrosomes and to play a role in spindle assembly . Importantly, ILK signaling effectors such as Akt, GSK3 and β-catenin have also been found at the centrosomes and mitotic spindles, suggesting a direct role for ILK in spindle assembly [99–101]. These results suggest an intriguing interplay between adhesion complexes and centrosomes. However, whether there is a genuine functional link or whether the findings represent “moonlighting” jobs for these proteins remains poorly understood. Interestingly, a mutation in the cytoplasmic domain of β1 integrin essential for integrin activation inhibits MT nucleation from the centrosomes and also disrupts cytokinesis, most likely due to spindle defects such as multipolar spindles . Surprisingly, the aforementioned phenotypes can be restored by the addition of an integrin activating antibody that bypasses the requirement of the cytoplasmic domain of β1 integrin for integrin activation . Thus, it seems that, at least in this case, the role of the integrins in cell adhesion and centrosome function might be connected.
4 Interplay between the tumor microenvironment, centrosomes and the stability of the genome
Tumor microenvironments are known to play important roles in the later stages of tumorigenesis, such as invasive behavior and motility of malignant cells. Alterations in geometry/shape, adhesion and stiffness of tumor cells or tissues can lead to changes in the gene expression and signal transduction in cancer cells. One example of how cell shape can control metastatic potential is illustrated in a study by Raz et al. Cells grown in conditions promoting a round morphology (plating cells on a nonadhesive substrate) can lead to an increased metastatic potential when injected to nude mice, by comparison with conditions that promote flattened cell morphology . Moreover, cell shape per se can govern whether cells grow or die. This idea was directly tested by Ingber and colleagues using fibronectin (FN) micropatterns . The authors demonstrated that human capillary endothelial cells can switch from apoptosis to growth when cell spreading increases, suggesting a critical role for cell shape in cell growth control . In addition, tissue stiffness, which can be used as a tumor detection method and is a predictor of tumor risk profiles, can alter signal transduction mechanisms, that promote tumor progression [105–107].
Interestingly, tumor microenvironments can also affect early stages of tumorigenesis. The impact of the tumor microenvironment on genomic stability is receiving growing attention. Several studies highlight the fact that cell adhesion has an impact on genetic instability through multiple routes. Loss of cell adhesion can promote genetic instability by attenuating key genomic surveillance mechanisms. This idea is supported by the findings that the loss of adhesion leads to a decrease in the expression levels of p53 in human epithelial cells , which could attenuate the cell cycle checkpoint controls thus contributing to genomic instability . p53-dependent apoptosis has also been shown to play a role in limiting the accumulation of cells with supernumerary centrosomes and genomic instability . Thus, the loss of cell adhesion and a concomitant decrease in p53 levels could compromise the fidelity of chromosome segregation and bipolar spindle formation. In addition, loss of adhesion could also induce genomic instability by generating genotoxic metabolites such as reactive oxygen species (ROS) , which have previously been implicated in genomic instability and tumorigenesis . Targeted degradation of cell–cell adhesion by overexpression of MMP-3 (matrix metalloproteinase-3) can induce genetic instability and tumors . This MMP-3 effect in promoting genomic instability is mediated by the production of ROS .
Another important mechanism by which cell adhesion can affect genome stability is by influencing centrosome clustering . Using a genome-wide RNAi screen, Kwon et al. unexpectedly found that proteins involved in cell adhesion are important for centrosome clustering. To directly test the role of cell adhesion in centrosome clustering, the authors manipulated cell adhesion using FN micropatterns, instead of genetically modulating proteins involved in cell adhesion, which might also have direct roles in mitosis . Using this method, the authors demonstrated that the interphase cell adhesion pattern can influence the organization of supernumerary centrosomes and thus determine whether mitosis is bipolar or multipolar in cancer cells. Given the reorganization of tissue architecture during tumorigenesis, testing how the shape of cancer cells in three-dimensional space affects centrosome clustering, genomic instability and cell survival is an interesting topic for future research.
5 Targeting cancer cells with extra centrosomes: a novel therapeutic approach?
Many useful chemotherapy drugs target the mitotic spindle. For example, microtubule poisons, such as taxanes and vinca alkaloids, are extensively used and are effective in a wide variety of cancers . Despite their efficacy, targeting MTs as therapeutic agents affects not only cancer cells, but also normal dividing cells, leading to toxic side effects for example myelosuppresion (suppression of both white and red blood cells), or gastrointestinal toxicity. Moreover, MT poisons cause toxicity in non-dividing cells, such as neurons, leading to neuropathy (peripheral nerve damage). While myelosuppression can, to some degree, be managed, neuropathy resulting from inhibiting MT dynamics is irreversible and untreatable [115, 116]. This has provided a rationale for the development of pure mitotic inhibitors that do not affect MTs .
Kinesin is a mechanochemical enzyme that uses ATP to generate force. Many kinesins are essential for accurate cell division and have recently emerged as a promising “druggable” target class . Kinesin-5 (Eg5) inhibitors are some of the first mitotic inhibitors being evaluated in the clinic. Eg5 is an essential kinesin required for bipolar spindle assembly . A cell based-screen developed by Mitchison and colleagues identified a specific Eg5 inhibitor named monastrol . Extensive study of the structure and enzymatic mechanisms of kinesin motors provided the basis for the development of specific chemical inhibitors . Indeed, new Eg5 inhibitors are currently being tested as potential chemotherapeutic agents and while they seem to have little neutoxicity in patients, it is still unknown if they will be efficient in inhibiting tumor progression [122, 123]. Mitotic kinesins might therefore be suitable anti-cancer targets. For similar reasons, mitotic kinases are also attractive chemotherapeutic targets. Recently, inhibitors against the mitotic kinases polo-like kinases (PLKs) and Auroras have been identified and are currently in clinical trials [117, 124, 125]. Whether these new mitotic inhibitors will improve anti-cancer therapies when compared with MT poisons remains unknown. However, Aurora and PLK inhibitors appear to cause no significant neuropathy and the main dose-limiting toxicity observed was neutropaenia (low number of neutrophils), while other haematopoietic lineages seemed to be spared .
Although these new mitotic inhibitors are a promising advance, they still have the drawback of blocking essential functions in normal cells. Therefore, the cancer selectivity or therapeutic window of these agents might be narrow. Ultimately, the ideal therapeutic strategy will rely on the identification of unique requirements for cancer cell survival; effective cancer therapeutics must exploit biological differences between tumor cells and the normal cells from which they arose. One striking difference is that many cancers have extra centrosomes and seem to rely on centrosome clustering mechanisms for their survival . The disruption of these mechanisms could selectively kill tumor cells with extra centrosomes. Indeed, the kinesin HSET, which is non-essential in normal cells , becomes essential for the survival of cancer cells with extra centrosomes . Hence, HSET might be an appealing chemotherapeutic target.
Interestingly, the levels of HSET expression vary in tumors (http://cgap.nci.nih.gov/SAGE/AnatomicViewer). Perhaps in some of those tumors HSET expression is increased as an adaptive response to promote centrosome clustering. Determining if there is a correlation between the levels of HSET expression and the number of extra centrosomes in tumors would be an interesting future direction, and could help define the type of tumors that could be sensitive to HSET inhibition.
The actin cytoskeleton and its interactions with cell–cell and cell–matrix adhesion might similarly be modulated in tumor cells in response to extra centrosomes. Importantly, the strength and organization of interphase adhesion sites strikingly influences whether cells with multiple centrosomes undergo bipolar or multipolar mitoses.
Therefore, other cellular characteristics could influence the extent of the requirement for HSET . For example, N1E-115 neuroblastoma cells are a classic “small round blue-cell” tumor whose cells express low levels of integrin β-1 . The round cell shape and inefficient substrate attachment may therefore contribute to their dramatic requirement for HSET. Likewise, cancer cells with compromised SAC signaling might also be more sensitive to HSET depletion. Current cancer treatments often combine different chemotherapeutic agents in order to improve efficacy and to minimize toxicity. Identifying targets that induce loss of cancer cells viability synergistically with HSET inhibition will be a valuable future direction. For example, PI-3 kinase inhibitor, an important cancer therapeutic target , might be synergistic with HSET inhibition. PI-3 kinase appears to affect centrosome clustering through regulation of the actin cytoskeleton whereas HSET affects spindle MTs bundling. Therefore, inhibition of both pathways could synergistically kill cancer cells.
Despite its potential appeal, HSET inhibitors as chemotherapeutic agents need further validation. What are the long term consequences of HSET inhibition in cells? Even though flies develop normally until adulthood in the absence of Ncd/HSET , whether or not HSET is essential for mouse development and/or survival is still unknown. Furthermore, it is important to stress that such an inhibitor would be efficient only in a subset of tumors, the ones with high percentage of cells containing supernumerary centrosomes. Thus, identifying a strategy that enables cost effective and accurate quantification of the percentage of extra centrosomes in tumor samples will be a major challenge to address in the future.
Many exciting questions lie ahead in the field. It will be interesting to investigate the dynamics of the centrosome number in cancer cells. In a given tumor, are there separate stable populations of cells with either normal centrosome numbers or centrosome amplification? Alternatively, do tumor cells with normal centrosome number generate progeny with centrosome amplification and vice versa? One fascinating open question is why centrosome amplification correlates with the malignancy of the tumor. Are tumors addicted to extra centrosomes or are extra centrosomes a byproduct of defective cell divisions? Do extra centrosomes bring any advantages to the tumor? Addressing these questions would allow us to understand the role of extra centrosomes in tumorigenesis and to potentially define selective therapeutic strategies.
The presence of extra centrosomes is a common feature of many human cancers. Understanding whether extra centrosomes are beneficial or deleterious to cancer cells is an important topic in cancer cell biology. Extra centrosomes can have a causative role in tumorigenesis, at least in flies, however, it is unlikely that this is achieved through multipolar mitoses. Recent studies showed that multipolar mitoses are detrimental for cells and several mechanisms ensure that cells cluster the extra centrosomes to form a bipolar spindle prior to anaphase onset. The fact that extra centrosome-containing cancer cells have unique requirement for some proteins primarily known to function in acentrosomal spindle assembly is intriguing. The fact that cell geometry and the adhesion pattern can influence centrosome clustering demonstrates a new mechanism by which the microenvironment can affect mitosis and the stability of the genome. Centrosome clustering can be a unique requirement for the survival of certain cancer cells. Can we use this knowledge to selectively kill cancer cells? In vitro studies suggest that this may be the case. This could provide an avenue towards the individual tailoring of therapies based on morphological features of cells.
We apologize to the authors whose work was not discussed or cited owing to space constraints. We are grateful to N. Ganem, S. Buttery, K. Crasta, N. Chandhok and M. Thery for critical reading of the manuscript. Our work is supported by a Fundação para a Ciência e Tecnologia grant to S. Godinho, by a Susan Komen postdoctoral fellowship to M. Kwon, and by NIH grant GM083299 to D. Pellman. D.P. is an Investigator of Howard Hughes Medical Institute.