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

The fate of extra centrosomes in newly formed tetraploid cells: should I stay, or should I go?

An increase in centrosome number is commonly observed in cancer cells, but the role centrosome amplification plays along with how and when it occurs during cancer development is unclear. One mechanism for generating cancer cells with extra centrosomes is whole genome doubling (WGD), an event that occurs in over 30% of human cancers and is associated with poor survival. Newly formed tetraploid cells can acquire extra centrosomes during WGD, and a generally accepted model proposes that centrosome amplification in tetraploid cells promotes cancer progression by generating aneuploidy and chromosomal instability. Recent findings, however, indicate that newly formed tetraploid cells in vitro lose their extra centrosomes to prevent multipolar cell divisions. Rather than persistent centrosome amplification, this evidence raises the possibility that it may be advantageous for tetraploid cells to initially restore centrosome number homeostasis and for a fraction of the population to reacquire additional centrosomes in the later stages of cancer evolution. In this review, we explore the different evolutionary paths available to newly formed tetraploid cells, their effects on centrosome and chromosome number distribution in daughter cells, and their probabilities of long-term survival. We then discuss the mechanisms that may alter centrosome and chromosome numbers in tetraploid cells and their relevance to cancer progression following WGD.

Modulation of DNA Damage Response by Sphingolipid Signaling: An Interplay that Shapes Cell Fate

Although once considered as structural components of eukaryotic biological membranes, research in the past few decades hints at a major role of bioactive sphingolipids in mediating an array of physiological processes including cell survival, proliferation, inflammation, senescence, and death. A large body of evidence points to a fundamental role for the sphingolipid metabolic pathway in modulating the DNA damage response (DDR). The interplay between these two elements of cell signaling determines cell fate when cells are exposed to metabolic stress or ionizing radiation among other genotoxic agents. In this review, we aim to dissect the mediators of the DDR and how these interact with the different sphingolipid metabolites to mount various cellular responses.

Centrosomes in the DNA damage response--the hub outside the centre

Here, we review how DNA damage affects the centrosome and how centrosomes communicate with the DNA damage response (DDR) apparatus. We discuss how several proteins of the DDR are found at centrosomes, including the ATM, ATR, CHK1 and CHK2 kinases, the BRCA1 ubiquitin ligase complex and several members of the poly(ADP-ribose) polymerase family. Stereotypical centrosome organisation, in which two centriole barrels are orthogonally arranged in a roughly toroidal pericentriolar material (PCM), is strongly affected by exposure to DNA-damaging agents. We describe the genetic dependencies and mechanisms for how the centrioles lose their close association, and the PCM both expands and distorts after DNA damage. Another consequence of genotoxic stress is that centrosomes undergo duplication outside the normal cell cycle stage, meaning that centrosome amplification is commonly seen after DNA damage. We discuss several potential mechanisms for how centrosome numbers become dysregulated after DNA damage and explore the links between the DDR and the PLK1- and separase-dependent mechanisms that drive centriole separation and reduplication. We also describe how centrosome components, such as centrin2, are directly involved in responding to DNA damage. This review outlines current questions on the involvement of centrosomes in the DDR.

Cellular Pathways in Response to Ionizing Radiation and Their Targetability for Tumor Radiosensitization

During the last few decades, improvements in the planning and application of radiotherapy in combination with surgery and chemotherapy resulted in increased survival rates of tumor patients. However, the success of radiotherapy is impaired by two reasons: firstly, the radioresistance of tumor cells and, secondly, the radiation-induced damage of normal tissue cells located in the field of ionizing radiation. These limitations demand the development of drugs for either radiosensitization of tumor cells or radioprotection of normal tissue cells. In order to identify potential targets, a detailed understanding of the cellular pathways involved in radiation response is an absolute requirement. This review describes the most important pathways of radioresponse and several key target proteins for radiosensitization.

Use of Ciliogenesis to Detect Aneugens: The Role of Primary Cilia

Primary cilia arise from the centrosomes of quiescent or post-mitotic cells, and serve as sensory organelles that communicate mechanical and chemical stimuli from the environment to the interior of the cell. Cilium formation may, therefore, become a useful end point signaling exposure to genotoxins or aneugens. Here we have used the aneugen, zidovudine (AZT), an antiretroviral drug that induces DNA replication arrest and centrosomal amplification (>2 centrosomes per quiescent cell), to evaluate cilia formation in retinal epithelial (pigmented) cells. Since cilia are derived from centrosomes, and aneugens can induce centrosomal amplification, the production of multiple cilia arising from multiple centrosomes may reveal the aneugenic nature of the agents. Cells were exposed to AZT to induce centrosomal amplification, cultured without serum to allow the centrioles to develop cilia, and immunostained to visualize cilia and centrosomes. Nuclear DNA was stained with DAPI. Preliminary observations suggest that cells with multiple centrosomes are able to generate extra cilia.

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

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

Centrosome aberrations associated with cellular senescence and p53 localization at supernumerary centrosomes

Centrosome overduplication or amplification has been observed in many human cancers and in premalignant tissue, but the mechanisms leading to such centrosome aberrations are not fully understood. We previously showed that abnormal mitotic cells with supernumerary centrosomes increase with replicative senescence in human fibroblasts, especially in a polyploid subpopulation. This study examines localization of p53 protein at centrosomes in mitotic cells, which is often observed in association with DNA damage response, to investigate a possible association between p53 localization and numerical centrosome aberrations induced by cellular senescence. Cultures at later passages or the 4th day after exposure to H(2)O(2) showed increased frequencies of mitotic cells with supernumerary centrosomes, especially in a polyploid subpopulation. Immunohistochemical analysis frequently showed p53-positive foci in mitotic cells, and some were localized at centrosomes. The number of p53-positive foci in mitotic cells and its localization to centrosomes increased with replicative and premature senescence. Supernumerary centrosomes showed higher frequencies of p53 localization compared to normally duplicated centrosomes. Centrosome-associated p53 protein was phosphorylated at Ser15. These data suggest a possible association between localization of p53 protein and numerical centrosome aberrations in replicatively or prematurely senescent cells.

Differential regulation of cutaneous oncoprotein HPVE6 by wtp53, mutant p53R248W and ΔNp63α is HPV type dependent

UV exposure and p53 mutations are major factors in non-melanoma skin cancer, whereas a role for HPV infections has not been defined. Previous data demonstrated the wtp53-mediated degradation of cutaneous HPV20E6 by caspase-3. ΔNp63α and hot-spot mutant p53R248W conveyed a protective effect on HPV20E6 under these conditions. We demonstrate a differential regulation by wtp53 of the E6 genes of cutaneous types HPV4, HPV5, HPV7, HPV27, HPV38, HPV48, HPV60 and HPV77. Caspase- or proteasome-mediated down-regulation was HPV type dependent. Mutant p53R248W up-regulated expression of all these E6 proteins as did ΔNp63α except for HPV38E6 which was down-regulated by the latter. None of these cellular proteins affected HPV41E6 expression. Ectopic expression of both mutp53R248W and ΔNp63α in the normal NIKS keratinocyte cell line harbouring endogenous p53 and p63however led to a down-regulation of HPV20E6. We demonstrate that HPV20E6 expression in these cells is modulated by additional, yet unidentified, cellular protein(s), which are not necessarily involved in apoptosis or autophagy. We further demonstrate proliferation of HPV20E6-expressing keratinocytes. Levels of proteins involved in cell cycle control, cyclin-D1, cdk6 and p16^INK4a, phosphorylated pRB, as well as c-Jun and p-c-Jun, were all increased in these cells. HPV20E6 did not compete for the interaction between p16^INK4a with cyclin-D1 or cdk6. Phosphorylation of pRB in the HPV20E6 expressing cells seems to be sufficient to override the cytokenetic block induced by the p16^INK4a/pRB pathway. The present study demonstrates the diverse influence of p53 family members on individual cutaneous HPVE6 proteins. HPV20E6 expression also resulted in varying protein levels of factors involved in proliferation and differentiation.

The distribution of phosphorylated SR proteins and alternative splicing are regulated by RANBP2

SR splicing factors are distributed in the speckled pattern in the nucleus. Alternative pre-mRNA splicing is regulated through nuclear distribution of phosphorylated SR splicing factors, which is specifically regulated by the RANBP2 system in mammalian cell lines, as well as in mouse tissues.

The mammalian cell nucleus is functionally compartmentalized into various substructures. Nuclear speckles, also known as interchromatin granule clusters, are enriched with SR splicing factors and are implicated in gene expression. Here we report that nuclear speckle formation is developmentally regulated; in certain cases phosphorylated SR proteins are absent from the nucleus and are instead localized at granular structures in the cytoplasm. To investigate how the nuclear architecture is formed, we performed a phenotypic screen of HeLa cells treated with a series of small interfering RNAs. Depletion of Ran-binding protein 2 induced cytoplasmic intermediates of nuclear speckles in G1 phase. Detailed analyses of these structures suggested that a late step in the sequential nuclear entry of mitotic interchromatin granule components was disrupted and that phosphorylated SR proteins were sequestered in an SR protein kinase–dependent manner. As a result, the cells had an imbalanced subcellular distribution of phosphorylated and hypophosphorylated SR proteins, which affected alternative splicing patterns. This study demonstrates that the speckled distribution of phosphorylated pre-mRNA processing factors is regulated by the nucleocytoplasmic transport system in mammalian cells and that it is important for alternative splicing.

Laser scanning cytometry of mitosis: state and stage analysis

Here we use a concept of cell state, which can be defined as the conjunction of expression levels of an arbitrary number of biomolecules or modifications thereof that oscillate, to classify mitotic cells. We describe detection of cell states with quantitative immunofluorescence measurements performed by laser scanning cytometry. This platform allows both measurement of the cell states, capture of cell images within those states, and subsequent analysis of each image to classify by traditional mitotic stages based on nuclear morphology.

Radiation-induced cell death mechanisms

The main goal when treating malignancies with radiation therapy is to deprive tumor cells of their reproductive potential. One approach to achieve this is by inducing tumor cell apoptosis. Accumulating evidences suggest that induction of apoptosis alone is insufficient to account for the therapeutic effect of radiotherapy. It has become obvious in the last few years that inhibition of the proliferative capacity of malignant cells following irradiation, especially with solid tumors, can occur via alternative cell death modalities or permanent cell cycle arrests, i.e., senescence. In this review, apoptosis and mitotic catastrophe, the two major cell deaths induced by radiation, are described and dissected in terms of activating mechanisms. Furthermore, treatment-induced senescence and its relevance for the outcome of radiotherapy of cancer will be discussed. The importance of p53 for the induction and execution of these different types of cell deaths is highlighted. The efficiency of radiotherapy and radioimmunotherapy has much to gain by understanding the cell death mechanisms that are induced in tumor cells following irradiation. Strategies to use specific inhibitors that will manipulate key molecules in these pathways in combination with radiation might potentiate therapy and enhance tumor cell kill.

Iodine-131 induces mitotic catastrophes and activates apoptotic pathways in HeLa Hep2 cells

Iodine-131 (131I) has been used both in unconjugated form and conjugated to antibody derivates (i.e., radioimmunotherapy; RIT) to treat malignant diseases. The mechanisms by which 131I-irradiation causes growth retardation are, however, inadequately understood. The aim of this study was to elucidate the sequential molecular and cellular events that initiate cell death in HeLa Hep2 cells exposed to 131I. In this paper, HeLa Hep2 cells were found to display a transient G2-M arrest following irradiation, but then reentered the cell cycle still containing unrepaired cellular damage. An increase of multipolar mitotic spindles, as well as a significant increase in centrosome numbers from 8.8% +/- 1.9% in controls to 54.7% +/- 2.2% in irradiated cells, was observed (p < 0.0001). A subsequent failure of cytokinesis caused the cells to progress into mitotic catastrophe. This was accompanied by the formation of giant cells with multiple nuclei, multilobulated nuclei, and an increased frequency of polyploidy cells. A fraction of the cells also displayed apoptotic features, including the activation of initiator caspases-2, -8, -9, and effector caspase-3, as well as cleavage of poly(ADP-ribose) polymerase, a cell-death substrate for active caspase-3. These findings demonstrate that mitotic catastrophes and the activation of a delayed type of apoptosis might be important mechanisms involved in cell death following the RIT of solid tumors with -emitting radionuclides, such as 131I.

Differential regulation of centrosome integrity by DNA damage response proteins

MDC1 and BRIT1 have been shown to function as key regulators in response to DNA damage. However, their roles in centrosomal regulation haven't been elucidated. In this study, we demonstrated the novel functions of these two molecules in regulating centrosome duplication and mitosis. We found that MDC1 and BRIT1 were integral components of the centrosome that colocalize with gamma-tubulin. Depletion of either protein led to centrosome amplification. However, the mechanisms that allow them to maintain centrosome integrity are different. MDC1-depleted cells exhibited centrosome overduplication, leading to multipolar mitosis, chromosome missegregation, and aneuploidy, whereas BRIT1 depletion led to misaligned spindles and/or lagging chromosomes with defective spindle checkpoint activation that resulted in defective cytokinesis and polyploidy. We further illustrated that both MDC1 and BRIT1 were negative regulators of Aurora A and Plk1, two centrosomal kinases involved in centrosome maturation and spindle assembly. Moreover, the levels of MDC1 and BRIT1 inversely correlated with centrosome amplification, defective mitosis and cancer metastasis in human breast cancer. Together, MDC1 and BRIT1 may function as tumor-suppressor genes, at least in part by orchestrating proper centrosome duplication and mitotic spindle assembly.

Roles of cyclins A and E in induction of centrosome amplification in p53-compromised cells

Abnormal amplification of centrosomes, which occurs frequently in cancers, leads to high frequencies of mitotic defect and chromosome segregation error, profoundly affecting the rate of tumor progression. Centrosome amplification results primarily from overduplication of centrosomes, and p53 is involved in the regulation of centrosome duplication partly through controlling the activity of cyclin-dependent kinase (CDK) 2-cyclin E, a kinase complex critical for the initiation of centrosome duplication. Thus, loss or mutational inactivation of p53 leads to an increased frequency of centrosome amplification. Moreover, the status of cyclin E greatly influences the frequency of centrosome amplification in cells lacking functional p53. Here, we dissected the roles of CDK2-associating cyclins, namely cyclins E and A, in centrosome amplification in the p53-negative cells. We found that loss of cyclin E was readily compensated by cyclin A for triggering the initiation of centrosome duplication, and thus the centrosome duplication kinetics was not significantly altered in cyclin E-deficient cells. It has been shown that cells lacking functional p53, when arrested in either early S-phase or late G(2) phase, continue to reduplicate centrosomes, resulting in centrosome amplification. In cells arrested in early S phase, cyclin E, but not cyclin A, is important in centrosome amplification, whereas in the absence of cyclin E, cyclin A is important for centrosome amplification. In late G(2)-arrested cells, cyclin A is important in centrosome amplification irrespective of the cyclin E status. These findings advance our understandings of the mechanisms underlying the numeral abnormality of centrosomes and consequential genomic instability associated with loss of p53 function and aberrant expression of cyclins E and A in cancer cells.

The presence of high-risk chromosome aberrations in chronic lymphocytic leukaemia does not correlate with centrosome aberrations

Chromosome aberrations are frequently found in B-cell chronic lymphocytic leukaemia (B-CLL), and specific chromosome aberrations identify poor prognostic subgroups. Almost all the aberrations identified in B-CLL involve loci where genes with a role in the regulation of centrosome duplication have been mapped. Centrosome aberrations have been described as a possible cause of numerical chromosome abnormalities in both solid and haematological tumours. However, little is known about the possible role of centrosome aberrations in B-CLL. To investigate whether centrosome aberrations do occur in B-CLL and correlate with cytogenetically defined prognostic subgroups, we examined a set of 64 B-CLL samples by immunofluorescent staining. B-CLL cases differed significantly from controls in the mean frequency of cells with centrosome aberrations, while no difference was found between subgroups with or without specific chromosome aberrations. Our results indicated that although centrosome aberrations were a common feature in B-CLL, they did not represent a reliable prognostic marker.

P53, cyclin-dependent kinase and abnormal amplification of centrosomes

Centrosomes play a critical role in formation of bipolar mitotic spindles, an essential event for accurate chromosome segregation into daughter cells. Numeral abnormalities of centrosomes (centrosome amplification) occur frequently in cancers, and are considered to be the major cause of chromosome instability, which accelerates acquisition of malignant phenotypes during tumor progression. Loss or mutational inactivation of p53 tumor suppressor protein, one of the most common mutations found in cancers, results in a high frequency of centrosome amplification in part via allowing the activation of the cyclin-dependent kinase (CDK) 2-cyclin E (as well as CDK2-cyclin A) which is a key factor for the initiation of centrosome duplication. In this review, the role of centrosome amplification in tumor progression, and mechanistic view of how centrosomes are amplified in cells through focusing on loss of p53 and aberrant activities of CDK2-cyclins will be discussed.

Oncogenes and tumour suppressors take on centrosomes

Chromosome instability, which is equated to mitotic defects and consequential chromosome segregation errors, provides a formidable basis for the acquisition of further malignant phenotypes during tumour progression. Centrosomes have a crucial role in the formation of bipolar mitotic spindles, which are essential for accurate chromosome segregation. Mutations of certain oncogenic and tumour-suppressor proteins directly induce chromosome instability by disrupting the normal function and numeral integrity of centrosomes. How these proteins control centrosome duplication and function, and how their mutational activation and/or inactivation results in numeral and functional centrosome abnormalities, is discussed in this Review.

The centrosome and cell proliferation

Centrosomes are frequently amplified in cancer cells. Increased numbers of centrosomes can give rise to multipolar spindles in mitosis, and thereby lead to the formation of aneuploid daughter cells. However, whether centrosome amplification is a cause or a consequence of cancer is unclear. In contrast, loss of a functional centrosome has been shown to lead to cell cycle arrest. In this review, the potential mechanisms underlying centrosome amplification and centrosome-dependent cell cycle regulation are discussed.