Activation of Cdc2/cyclin B and inhibition of centrosome amplification in cells depleted of Plk1 by siRNA

Mining the Giardia genome and proteome for conserved and unique basal body proteins

Giardia lamblia is a flagellated protozoan parasite and a major cause of diarrhoea in humans. Its microtubular cytoskeleton mediates trophozoite motility, attachment and cytokinesis, and is characterised by an attachment disk and eight flagella that are each nucleated in a basal body. To date, only 10 giardial basal body proteins have been identified, including universal signalling proteins that are important for regulating mitosis or differentiation. In this study, we have exploited bioinformatics and proteomic approaches to identify new Giardia basal body proteins and confocal microscopy to confirm their localisation in interphase trophozoites. This approach identified 75 homologs of conserved basal body proteins in the genome including 65 not previously known to be associated with Giardia basal bodies. Thirteen proteins were confirmed to co-localise with centrin to the Giardia basal bodies. We also demonstrate that most basal body proteins localise to additional cytoskeletal structures in interphase trophozoites. This might help to explain the roles of the four pairs of flagella and Giardia-specific organelles in motility and differentiation. A deeper understanding of the composition of the Giardia basal bodies will contribute insights into the complex signalling pathways that regulate its unique cytoskeleton and the biological divergence of these conserved organelles.

Control of centrin stability by Aurora A

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

Rapid functional screening of effective siRNAs against Plk1 and its growth inhibitory effects in laryngeal carcinoma cells

Plk 1 is overexpressed in many human malignancies including laryngeal carcinoma. However, its therapeutic potential has been never examined in laryngeal carcinoma. In the present study, a simple cellular morphology-based strategy was firstly proposed for rapidly screening the effective siRNAs against Plk1. Furthermore, we investigated the effects of Plk1 depletion via a novel identified effective siRNA against Plk1, Plk1 siRNA-607, on human laryngeal carcinoma Hep-2 cells. The results indicated that Plk1 siRNA-607 transfection resulted in a significant inhibition in Plk1 expression in cells, and subsequently caused a dramatic mitotic cell cycle arrest followed by massive apoptotic cell death, and eventually resulted in a significant decrease in growth and viability of the laryngeal carcinoma cells. Taken together, our present study not only suggests a simple strategy for rapidly screening effective siRNAs against Plk1 but also implicates that Plk1 may serve as a potential therapeutic target in human laryngeal carcinoma.

Plk1 negatively regulates Cep55 recruitment to the midbody to ensure orderly abscission

Cytokinesis requires a membrane-remodeling and fission event termed abscission that occurs after chromosome segregation, cleavage furrow formation, and contraction have completed. In this study, we show how abscission factor recruitment is controlled by the Polo-like kinase 1 (Plk1). At the metaphase-anaphase transition, Plk1 initiates cleavage furrow formation and is then progressively degraded during mitotic exit. During this period, Plk1 phosphorylates the abscission factor Cep55 in trans and prevents its untimely recruitment to the anaphase spindle. A Plk1 phosphorylation site mutant of Cep55 is prematurely recruited to the anaphase spindle and fails to support abscission. Endogenous Cep55 behaves similarly after Plk1 inhibition by the drugs BI2536 or GW842862. Only once Plk1 is degraded can Cep55 target to the midbody and promote abscission. Blocking Plk1 degradation leads to elevated levels of Plk1 at the midbody and the failure of Cep55 recruitment. Thus, Plk1 activity negatively regulates Cep55 to ensure orderly abscission factor recruitment and ensures that this occurs only once cell contraction has completed.

p53-dependent repression of polo-like kinase-1 (PLK1)

PLK1 is a critical mediator of G₂/M cell cycle transition that is inactivated and depleted as part of the DNA damage-induced G₂/M checkpoint. Here we show that downregulation of PLK1 expression occurs through a transcriptional repression mechanism and that p53 is both necessary and sufficient to mediate this effect. Repression of PLK1 by p53 occurs independently of p21 and of arrest at G₁/S where PLK1 levels are normally repressed in a cell cycle-dependent manner through a CDE/CHR element. Chromatin immunoprecipitation analysis indicates that p53 is present on the PLK1 promoter at two distinct sites termed p53RE1 and p53RE2. Recruitment of p53 to p53RE2, but not to p53RE1, is stimulated in response to DNA damage and/or p53 activation and is coincident with repression-associated changes in the chromatin. Downregulation of PLK1 expression by p53 is relieved by the histone deacetylase inhibitor, trichostatin A, and involves recruitment of histone deacetylase to the vicinity of p53RE2, further supporting a transcriptional repression mechanism. Additionally, wild type, but not mutant, p53 represses expression of the PLK1 promoter when fused upstream of a reporter gene. Silencing of PLK1 expression by RNAi interferes with cell cycle progression consistent with a role in the p53-mediated checkpoint. These data establish PLK1 as a direct transcriptional target of p53, independently of p21, that is required for efficient G₂/M arrest.

Polo-like kinase 1 activated by the hepatitis B virus X protein attenuates both the DNA damage checkpoint and DNA repair resulting in partial polyploidy

Hepatitis B virus X protein (pX), implicated in hepatocarcinogenesis, induces DNA damage because of re-replication and allows propagation of damaged DNA, resulting in partial polyploidy and oncogenic transformation. The mechanism by which pX allows cells with DNA damage to continue proliferating is unknown. Herein, we show pX activates Polo-like kinase 1 (Plk1) in the G(2) phase, thereby attenuating the DNA damage checkpoint. Specifically, in the G(2) phase of pX-expressing cells, the checkpoint kinase Chk1 was inactive despite DNA damage, and protein levels of claspin, an adaptor of ataxia telangiectasia-mutated and Rad3-related protein-mediated Chk1 phosphorylation, were reduced. Pharmacologic inhibition or knockdown of Plk1 restored claspin protein levels, Chk1 activation, and p53 stabilization. Also, protein levels of DNA repair protein Mre11 were decreased in the G(2) phase of pX-expressing cells but not with Plk1 knockdown. Interestingly, in pX-expressing cells, Mre11 co-immunoprecipitated with transfected Plk1 Polo-box domain, and inhibition of Plk1 increased Mre11 stability in cycloheximide-treated cells. These results suggest that pX-activated Plk1 by down-regulating Mre11 attenuates DNA repair. Importantly, concurrent inhibition of Plk1, p53, and Mre11 increased the number of pX-expressing cells with DNA damage entering mitosis, relative to Plk1 inhibition alone. By contrast, inhibition or knockdown of Plk1 reduced pX-induced polyploidy while increasing apoptosis. We conclude Plk1, activated by pX, allows propagation of DNA damage by concurrently attenuating the DNA damage checkpoint and DNA repair, resulting in polyploidy. We propose this novel Plk1 mechanism initiates pX-mediated hepatocyte transformation.

Multifaceted polo-like kinases: drug targets and antitargets for cancer therapy

The polo-like kinase 1 (PLK1) acts in concert with cyclin-dependent kinase 1-cyclin B1 and Aurora kinases to orchestrate a wide range of critical cell cycle events. Because PLK1 has been preclinically validated as a cancer target, small-molecule inhibitors of PLK1 have become attractive candidates for anticancer drug development. Although the roles of the closely related PLK2, PLK3 and PLK4 in cancer are less well understood, there is evidence showing that PLK2 and PLK3 act as tumour suppressors through their functions in the p53 signalling network, which guards the cell against various stress signals. In this article, recent insights into the biology of PLKs will be reviewed, with an emphasis on their role in malignant transformation, and progress in the development of small-molecule PLK1 inhibitors will be examined.

Centriole reduplication during prolonged interphase requires procentriole maturation governed by Plk1

Supernumerary centrioles lead to abnormal mitosis, which in turn promotes tumorigenesis. Thus, centriole duplication must be coordinated with the cell cycle to ensure that the number of centrioles in the cell doubles precisely during each cell cycle. However, in some transformed cells, centrioles undergo multiple rounds of duplication (reduplication) during prolonged interphase. Mechanisms responsible for centriole reduplication are poorly understood. Here, we report that centrioles reduplicate consistently in cancerous and nontransformed human cells during G2 arrests and that this reduplication requires the activity of Polo-like kinase 1 (Plk1). We also find that a cell's ability to reduplicate centrioles during S arrests depends on the presence of activated (Thr210-phosphorylated) Plk1 at the centrosome. In the absence of activated Plk1, nascent procentrioles remain associated with mother centrioles, which prevents centriole reduplication. In contrast, if Plk1(pT210) appears at the centrosome, procentrioles mature, disengage from mother centrioles, and ultimately duplicate. Plk1 activity is not required for the assembly of procentrioles, however. Thus, the role of Plk1 is to coordinate the centriole duplication cycle with the cell cycle. Activation of Plk1 during late S/G2 induces procentriole maturation, and after this point, the centriole cycle can be completed autonomously, even in the absence of cell-cycle progression.

Distinct pools of cdc25C are phosphorylated on specific TP sites and differentially localized in human mitotic cells

Background

The dual specificity phosphatase cdc25C was the first human cdc25 family member found to be essential in the activation of cdk1/cyclin B1 that takes place at the entry into mitosis. Human cdc25C is phosphorylated on Proline-dependent SP and TP sites when it becomes active at mitosis and the prevalent model is that this phosphorylation/activation of cdc25C would be part of an amplification loop with cdk1/cyclin B1.

Methodology/Principal Findings

Using highly specific antibodies directed against cdc25C phospho-epitopes, pT67 and pT130, we show here that these two phospho-forms of cdc25C represent distinct pools with differential localization during human mitosis. Phosphorylation on T67 occurs from prophase and the cdc25C-pT67 phospho-isoform closely localizes with condensed chromosomes throughout mitosis. The phospho-T130 form of cdc25C arises in late G2 and associates predominantly with centrosomes from prophase to anaphase B where it colocalizes with Plk1. As shown by immunoprecipitation of each isoform, these two phospho-forms are not simultaneously phosphorylated on the other mitotic TP sites or associated with one another. Phospho-T67 cdc25C co-precipitates with MPM2-reactive proteins while pT130-cdc25C is associated with Plk1. Interaction and colocalization of phosphoT130-cdc25C with Plk1 demonstrate in living cells, that the sequence around pT130 acts as a true Polo Box Domain (PBD) binding site as previously identified from in vitro peptide screening studies. Overexpression of non-phosphorylatable alanine mutant forms for each isoform, but not wild type cdc25C, strongly impairs mitotic progression showing the functional requirement for each site-specific phosphorylation of cdc25C at mitosis.

Conclusions/Significance

These results show for the first time that in human mitosis, distinct phospho-isoforms of cdc25C exist with different localizations and interacting partners, thus implying that the long-standing model of a cdc25C/cdk1 multi-site auto amplification loop is implausible.

From zero to many: control of centriole number in development and disease

Centrioles are essential for the formation of microtubule-derived structures, including cilia, flagella and centrosomes. These structures are involved in a variety of functions, from cell motility to division. In most dividing animal cells, centriole formation is coupled to the chromosome cycle. However, this is not the case in certain specialized divisions, such as meiosis, and in some differentiating cells. For example, oocytes loose their centrioles upon differentiation, whereas multiciliated epithelial cells make several of those structures after they exit the cell cycle. Aberrations of centriole number are seen in many cancer cells. Recent studies began to shed light on the molecular control of centriole number, its variations in development, and how centriole number changes in human disease. Here we review the recent developments in this field.

Cdk1 activity is required for mitotic activation of aurora A during G2/M transition of human cells

In mammalian cells entry into and progression through mitosis are regulated by multiple mitotic kinases. How mitotic kinases interact with each other and coordinately regulate mitosis remains to be fully understood. Here we employed a chemical biology approach using selective small molecule kinase inhibitors to dissect the relationship between Cdk1 and Aurora A kinases during G(2)/M transition. We find that activation of Aurora A first occurs at centrosomes at late G(2) and is required for centrosome separation independently of Cdk1 activity. Upon entry into mitosis, Aurora A then becomes fully activated downstream of Cdk1 activation. Inactivation of Aurora A or Plk1 individually during a synchronized cell cycle shows no significant effect on Cdk1 activation and entry into mitosis. However, simultaneous inactivation of both Aurora A and Plk1 markedly delays Cdk1 activation and entry into mitosis, suggesting that Aurora A and Plk1 have redundant functions in the feedback activation of Cdk1. Together, our data suggest that Cdk1, Aurora A, and Plk1 mitotic kinases participate in a feedback activation loop and that activation of Cdk1 initiates the feedback loop activity, leading to rapid and timely entry into mitosis in human cells. In addition, live cell imaging reveals that the nuclear cycle of cells becomes uncoupled from cytokinesis upon inactivation of both Aurora A and Aurora B kinases and continues to oscillate in a Cdk1-dependent manner in the absence of cytokinesis, resulting in multinucleated, polyploidy cells.

Plk1 phosphorylation of Topors is involved in its degradation

Topors is a DNA topoisomerase I- and p53-binding protein, and mainly functions as a p53 regulator. Accumulating evidence also supports the notion that Topors plays the role as a negative regulator of cell growth, and possibly as a tumor suppressor. Here, we demonstrated that Topors is also involved in normal mitotic progression, since Topors depletion delays mitotic entry and affects mitotic progression. Furthermore, Topors is degradated in response to the activation of the spindle checkpoint. Significantly, Polo-like kinase 1 (Plk1)-associated phosphorylation of Topors at S718 is essential for nocodazole-induced degradation of Topors.

Significance of Plk1 regulation by miR-100 in human nasopharyngeal cancer

Polo-like kinase 1 (Plk1) is a critical regulator of many stages of mitosis; increasing evidence indicates that Plk1 overexpression correlates with poor clinical outcome, yet its mechanism of regulation remains unknown. Hence, a detailed evaluation was undertaken of Plk1 expression in human nasopharyngeal cancer (NPC), the cellular effects of targeting Plk1 using siRNA in combination with ionizing radiation (RT) and potential upstream microRNAs (miRs) that might regulate Plk1 expression. Using immunohistochemistry, Plk1 was observed to be overexpressed in 28 of 40 (70%) primary NPC biopsies, which in turn was associated with a higher likelihood of recurrence (p = 0.018). SiPlk1 significantly inhibited Plk1 mRNA and protein expression, and decreased Cdc25c levels in NPC cell lines. This depletion resulted in cytotoxicity of C666-1 cells, enhanced by the addition of RT, mediated by G2/M arrest, increased DNA double-strand breaks, apoptosis, and caspase activation. Immunofluorescence demonstrated that the G2/M arrest was associated with aberrant spindle formation, leading to mitotic arrest. In vivo, transfection of C666-1 cells and systemic delivery of siPlk1 decreased tumour growth. MicroRNA-100 (miR-100) was predicted to target Plk1 mRNA, which was indeed underexpressed in C666-1 cells, inversely correlating with Plk1 expression. Using luciferase constructs containing the 3'-UTR of Plk1 sequence, we document that miR-100 can directly target Plk1. Hence, our data demonstrate for the first time that underexpressed miR-100 leads to Plk1 overexpression, which in turn contributes to NPC progression. Targeting Plk1 will cause mitotic catastrophe, with significant cytotoxicity both in vitro and in vivo, underscoring the important therapeutic opportunity of Plk1 in NPC.

Stepwise evolution of the centriole-assembly pathway

The centriole and basal body (CBB) structure nucleates cilia and flagella, and is an essential component of the centrosome, underlying eukaryotic microtubule-based motility, cell division and polarity. In recent years, components of the CBB-assembly machinery have been identified, but little is known about their regulation and evolution. Given the diversity of cellular contexts encountered in eukaryotes, but the remarkable conservation of CBB morphology, we asked whether general mechanistic principles could explain CBB assembly. We analysed the distribution of each component of the human CBB-assembly machinery across eukaryotes as a strategy to generate testable hypotheses. We found an evolutionarily cohesive and ancestral module, which we term UNIMOD and is defined by three components (SAS6, SAS4/CPAP and BLD10/CEP135), that correlates with the occurrence of CBBs. Unexpectedly, other players (SAK/PLK4, SPD2/CEP192 and CP110) emerged in a taxon-specific manner. We report that gene duplication plays an important role in the evolution of CBB components and show that, in the case of BLD10/CEP135, this is a source of tissue specificity in CBB and flagella biogenesis. Moreover, we observe extreme protein divergence amongst CBB components and show experimentally that there is loss of cross-species complementation among SAK/PLK4 family members, suggesting species-specific adaptations in CBB assembly. We propose that the UNIMOD theory explains the conservation of CBB architecture and that taxon- and tissue-specific molecular innovations, gained through emergence, duplication and divergence, play important roles in coordinating CBB biogenesis and function in different cellular contexts.

Overexpression of Plk1 promotes malignant progress in human esophageal squamous cell carcinoma

PURPOSE

Plk1, belonging to a family of serine/threonine kinases, is involved in spindle formation and chromosome segregation during mitosis and therefore, in the regulation of cell cycle. Plk1 was found to be overexpressed in various human tumors. In the present work, we investigated the expression of human esophageal squamous cell carcinoma (ESCC) to determine whether Plk1 has a role in malignant progress.

METHODS

Immunohistochemistry and Western blotting were performed to define the expressions of Plk1 in ESCC tissues and normal adjacent tissues. Transfection of cells with small interference RNA, growth suppression assay, and Transwell assay were used to determine the potential role of Plk1 in ESCC malignant progress.

RESULTS

Plk1 was overexpressed in 69.6% of the ESCC tissues. In addition, the extent of Plk1 expression was closely correlated with differentiation grades and invasiveness in ESCC. We also found that the downregulation of endogenous Plk1 in human ESSC cell lines Eca-109 and EC9706 significantly decreased cells proliferation and migrating ability.

CONCLUSIONS

Our results show that Plk1 expression is elevated in ESCC tissues and is associated with differentiation grades and invasiveness in ESCC, indicating that overexpression of Plk1 may play an important role in carcinogenesis and malignant progress of ESCC.

Targeted depletion of Polo-like kinase (Plk) 1 through lentiviral shRNA or a small-molecule inhibitor causes mitotic catastrophe and induction of apoptosis in human melanoma cells

Melanoma, one of the most lethal forms of skin cancer, remains resistant to currently available treatments. Therefore, additional target-based approaches are needed for the management of this neoplasm. Polo-like kinase 1 (Plk1) has been shown to be a crucial regulator of mitotic entry, progression, and exit. Elevated Plk1 level has been associated with aggressiveness of several cancer types and with poor disease prognosis. However, the role of Plk1 in melanoma is not well established. Here, we show that Plk1 is overexpressed in both clinical tissue specimens and cultured human melanoma cells (WM115, A375, and HS294T) when compared with normal skin tissues and cultured normal melanocytes, respectively. Furthermore, Plk1 gene knockdown through Plk1-specific shRNA or its activity inhibition by a small-molecule inhibitor resulted in a significant decrease in the viability and growth of melanoma cells without affecting normal human melanocytes. In addition, Plk1 inhibition resulted in a significant (i) decrease in clonogenic survival, (ii) multiple mitotic errors, (iii) G(2)/M cell-cycle arrest, and (iv) apoptosis of melanoma cells. This study suggests that Plk1 may have a functional relevance toward melanoma development and/or progression. We suggest that the targeting of Plk1 may be a viable approach for the treatment of melanoma.

Effects of small interference RNAs targeting Polo-like kinase 1 in human liver cancer cell line HepG2

AIM: To explore the role of PLK1 in carcinogenesis and development of liver cancer and its clinical significance for cancer treatment with the technique of RNA interference (RNAi).

METHODS: HepG2 cells were transfected with small interfering RNAs (siRNAs) targeting against the human PLK1 by chemosynthesis. The PLK1 mRNA levels of the cells transfected with siRNAs were monitored with real time PCR. Cell proliferation was evaluated by direct cell counting with trypan blue staining. Cell cycle and apoptosis were examined by flow cytometry.

RESULTS: The PLK1 mRNA levels of transfected liver cancer cells were greatly lowered as compared with that of the control groups in 48 h after the siRNAs transfection (49.7% ± 3.6%), (P < 0.01). Cell proliferation was reduced from 24 h after transfection. Cell cycle distribution was changed and showed a strong G₂/M arrest in 48 h and 72 h after transfection, and cell apoptosis was increased. There was significant difference as compared with the control group (both P < 0.05).

CONCLUSION: PLK1 plays an important role in the carcinogenesis and development of liver cancer. siRNAs targeting against human PLK1 may specially suppress the expression of PLK1 mRNA in liver cancer cells, inhibit cell proliferation and enhance cell apoptosis. The RNAi targeting PLK1 is expected to be used in gene therapy for human liver cancers.

BubR1 localizes to centrosomes and suppresses centrosome amplification via regulating Plk1 activity in interphase cells

BubR1 is a critical component of the mitotic checkpoint that delays the onset of anaphase until all chromosomes have established bipolar attachment to the microtubules. We previously reported that mutations of the BUB1B gene (encoding BubR1) caused premature chromatid separation (PCS) syndrome, a condition characterized by constitutional aneuploidy and a high risk of childhood cancer. We here report that the cells from PCS syndrome patients have loss of regulation of the centrosome duplication machinery, resulting in centrosome amplification and multipolar mitosis. PCS syndrome cells show increased activity of Polo-like kinase 1 (Plk1), whose knockdown suppresses centrosome amplification. BubR1 localizes to centrosomes, physically interacts with Plk1 and inhibits Plk1 phosphorylation and its kinase activity during interphase. These results unravel a crucial role of BubR1 in preventing centrosome reduplication through negative regulation of Plk1 in interphase cells.

Identification of a kinase profile that predicts chromosome damage induced by small molecule kinase inhibitors

Kinases are heavily pursued pharmaceutical targets because of their mechanistic role in many diseases. Small molecule kinase inhibitors (SMKIs) are a compound class that includes marketed drugs and compounds in various stages of drug development. While effective, many SMKIs have been associated with toxicity including chromosomal damage. Screening for kinase-mediated toxicity as early as possible is crucial, as is a better understanding of how off-target kinase inhibition may give rise to chromosomal damage. To that end, we employed a competitive binding assay and an analytical method to predict the toxicity of SMKIs. Specifically, we developed a model based on the binding affinity of SMKIs to a panel of kinases to predict whether a compound tests positive for chromosome damage. As training data, we used the binding affinity of 113 SMKIs against a representative subset of all kinases (290 kinases), yielding a 113x290 data matrix. Additionally, these 113 SMKIs were tested for genotoxicity in an in vitro micronucleus test (MNT). Among a variety of models from our analytical toolbox, we selected using cross-validation a combination of feature selection and pattern recognition techniques: Kolmogorov-Smirnov/T-test hybrid as a univariate filter, followed by Random Forests for feature selection and Support Vector Machines (SVM) for pattern recognition. Feature selection identified 21 kinases predictive of MNT. Using the corresponding binding affinities, the SVM could accurately predict MNT results with 85% accuracy (68% sensitivity, 91% specificity). This indicates that kinase inhibition profiles are predictive of SMKI genotoxicity. While in vitro testing is required for regulatory review, our analysis identified a fast and cost-efficient method for screening out compounds earlier in drug development. Equally important, by identifying a panel of kinases predictive of genotoxicity, we provide medicinal chemists a set of kinases to avoid when designing compounds, thereby providing a basis for rational drug design away from genotoxicity.

Absence of tumor-specific over-expression of Polo-like kinase 1 (Plk1) in major non-Hodgkin lymphoma and relatively low expression of Plk1 in nasal NK/T cell lymphoma

Based on the presence of the tumor-specific over-expression of Plk1 (polo-like kinases) in various malignancies, we examined Plk1 expression in nine cases of reactive follicular hyperplasia (RFH), 42 of diffuse large B cell lymphoma (DLBCL), 16 of follicular lymphoma (FL), and 10 of nasal NK/T lymphoma. There was no significant difference in the Plk1-positive cell percentage between RFH and DLBCL. The Plk1-positive cell percentage ranged from 6 to 20% with a median of 12.9% in DLBCL. In FL, Plk1-positivity was at most 7%. Plk1-positivity in nasal NK/T cell lymphoma (4.7-14.1% with a median of 9.2%) was significantly higher than that of FL and tended to be lower than DLBCL (p < 0.001, p = 0.05, respectively). Although a strong correlation between positive cell percentages for Plk1 and Ki-67 in these three lymphomas specified Plk1 as a proliferation marker (r = 0.83-0.91), the Plk1-positive cell percentage relative to the other proliferation markers tended to be particularly low in nasal NK/T cell lymphoma. In 41 cases of DLBCL, the positive cell percentages of Plk1 and Ki-67 were both correlated with overall survival. The 4-year overall survival rates by Kaplan-Meier analysis for Plk1-negative and positive patients were 80 and 38%, respectively (p = 0.02).

5'-nitro-indirubinoxime induces G2/M cell cycle arrest and apoptosis in human KB oral carcinoma cells

Our previous study demonstrated that the novel indirubin derivative, 5'-nitro-indirubinoxime (5'-NIO), effectively arrested the tumor growth through the inhibition of cell proliferation and the induction of apoptosis. However, the precise molecular mechanisms underlying 5'-NIO-induced antitumor activity remain unclear. Here, we report that 5'-NIO inhibits the proliferation of human KB oral carcinoma cells via the cell cycle arrest in G2/M phase. 5'-NIO reduced the activity of Cdc2/cyclin B complex through the inhibition of the PLK1 expression. Partially, 5'-NIO also arrested cell cycle in G1/S phase via the reduction of CDK4 and cyclin D1/D3 levels by p16 and induction of the level of p21waf1. Using flow cytometry analysis, we showed that 5'-NIO-induced cell cycle arrest is followed by apoptosis. We determined further that 5'-NIO-induced apoptosis is accomplished by the mitochondria-dependent activation of the caspase cascade. Overall, these observations suggest the potential value of 5'-NIO as a candidate for a therapeutic modality for the treatment of oral cancer.

RNA interference-mediated silencing of the polo-like kinase 1 gene enhances chemosensitivity to gemcitabine in pancreatic adenocarcinoma cells

Gemcitabine is the first-line chemotherapeutic agent for advanced adenocarcinoma of the pancreas; however, chemoresistance to gemcitabine remains a major cause of failure for the clinical treatment of this disease. Polo-like kinase 1 (Plk-1) is highly expressed in pancreatic cancer cell lines and pancreatic tumour tissues, and is involved in a wide variety of cell cycle processes. Nevertheless, its biological role and implication for gemcitabine resistance are not clearly defined. In this study, we used RNA-interference (RNAi)-mediated depletion of Plk-1 to determine its potential for sensitizing pancreatic tumour cells to gemcitabine. We showed that the level of Plk-1 protein was correlated significantly with gemcitabine resistance in human pancreatic adenocarcinoma cells and that overexpression of Plk-1 reduced sensitivity to gemcitabine in these cells. In addition, small interfering RNA (siRNA)-mediated knockdown of Plk-1 caused cell cycle arrest at G2/M and the reduction of cellular proliferation. More importantly, the treatment of pancreatic cancer cells with Plk-1 siRNA followed by exposure to gemcitabine dramatically decreased cell viability and increased cellular apoptosis, as compared with treatment with either agent alone. These observations indicate that down-regulation of Plk-1 expression by RNAi enhances gemcitabine sensitivity and increases gemcitabine cytotoxicity in pancreatic tumour cells. This is the first demonstration that the combination of Plk-1 gene therapy and gemcitabine chemotherapy has synergistic anti-tumour activity against pancreatic carcinoma in vitro. This combination treatment warrants further investigation as an effective therapeutic regimen for patients with resistant pancreatic cancer and other tumours.

Phosphorylation of Plk1 at Ser326 regulates its functions during mitotic progression

Polo-like kinase 1 (Plk1), the best characterized member of the mammalian polo-like kinase family, is well regulated throughout the cell cycle at the protein expression level. Moreover, it is known that Plk1 kinase activity is also regulated at the post-translational level through phosphorylation. However, the upstream kinases of Plk1 have not been identified. Although the involvement of the p38 MAP kinase pathway in cellular responses to stress has been well documented, the role of this pathway in normal cell cycle progression is unclear. Here, we show that phosphorylated p38 and MAP kinase-activated protein kinase 2 (MK2) are colocalized with Plk1 to the spindle poles during prophase and metaphase. Specific depletion of various members of the p38 MAP kinase pathway by the use of RNA interference revealed that the pathway is required for mitotic progression under normal growth conditions. Furthermore, MK2 directly phosphorylates Ser326 of Plk1. Ectopic expression of Plk1-S326A completely blocked cells at mitosis, likely due to the defect of bipolar spindle formation and subsequent activation of the spindle checkpoint. Only Plk1-S326E, but not the Plk1-S326A, efficiently rescued the p38 or MK2-depletion-induced mitotic defects, further solidifying the requirement of S326 phosphorylation during mitotic progression.

Erucylphosphocholine shows a strong anti-growth activity in human endometrial and ovarian cancer cells

OBJECTIVES

A membrane-targeted, lipophilic ether lipid of synthetic phospholipid analog, erucylphosphocholine (ErPC) induces apoptosis in some lines of human tumor cells. We investigated the effect of ErPC on three endometrial cancer cell lines, two ovarian cancer cell lines, and normal human endometrial epithelial cells.

METHODS

Endometrial and ovarian cancer cells were treated with various concentrations of ErPC, and its effect on cell growth, cell cycle, apoptosis, and related measurements was investigated.

RESULTS

The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay showed that all endometrial and ovarian cancer cell lines were sensitive to the growth-inhibitory effect of ErPC, although normal endometrial epithelial cells were viable after treatment with the same doses of ErPC that induced growth inhibition in endometrial and ovarian cancer cells. Cell cycle analysis indicated that their exposure to ErPC decreased the proportion of cells in the S-phase and increased the proportion in the G2/M phases of the cell cycle. Induction of apoptosis was confirmed by annexin V staining of externalized phosphatidylserine and loss of the transmembrane potential of mitochondria. This induction occurred in concert with altered expression of genes related to cell growth, malignant phenotype, and apoptosis.

CONCLUSIONS

These results suggest that the anticancer activity of ErPC may occur with higher sensitivity of cancer cells compared with normal healthy cells, when using low concentration, rising hopes that ErPC may become a useful adjuvant therapy for endometrial and ovarian cancers.

Phosphorylation of MyoGEF on Thr-574 by Plk1 promotes MyoGEF localization to the central spindle

We reported previously that a guanine nucleotide exchange factor, MyoGEF, localizes to the central spindle, activates RhoA, and is required for cytokinesis. In this study, we have found that Plk1 (polo-like kinase 1) can phosphorylate MyoGEF, thereby recruiting MyoGEF to the central spindle as well as enhancing MyoGEF activity toward RhoA. The in vitro kinase assay shows that Plk1 can phosphorylate MyoGEF on threonine 574. Immunoprecipitation/immunoblot analysis demonstrates that mutation of threonine 574 to alanine dramatically decreases threonine phosphorylation of MyoGEF in transfected HeLa cells, suggesting that threonine 574 is phosphorylated in vivo. Consistent with these observations, immunofluorescence shows that Plk1 and MyoGEF colocalize at the spindle pole and central spindle during mitosis and cytokinesis. Importantly, RNA interference-mediated depletion of Plk1 interferes with the localization of MyoGEF at the spindle pole and central spindle. Moreover, mutation of threonine 574 to alanine in MyoGEF or depletion of Plk1 by RNA interference leads to a decrease in MyoGEF activity toward RhoA in HeLa cells. Therefore, our results suggest that Plk1 can regulate MyoGEF activity and localization, contributing to the regulation of cytokinesis.

Mechanisms of procentriole formation

The centrosome comprises a pair of centrioles and associated pericentriolar material, and it is the principal microtubule-organizing centre of most animal cells. Like the genetic material, the centrosome is duplicated once and only once during the cell cycle. Despite the fact that both doubling events are crucial for genome integrity, the understanding of the mechanisms governing centrosome duplication has lagged behind the fuller knowledge of DNA replication. Here, we review recent findings that provide important mechanistic insights into how a single procentriole forms next to each centriole once per cell cycle, thus ensuring that one centrosome becomes two.

Myosin phosphatase-targeting subunit 1 regulates mitosis by antagonizing polo-like kinase 1

Myosin phosphatase-targeting subunit 1 (MYPT1) binds to the catalytic subunit of protein phosphatase 1 (PP1C). This binding is believed to target PP1C to specific substrates including myosin II, thus controlling cellular contractility. Surprisingly, we found that during mitosis, mammalian MYPT1 binds to polo-like kinase 1 (PLK1). MYPT1 is phosphorylated during mitosis by proline-directed kinases including cdc2, which generates the binding motif for the polo box domain of PLK1. Depletion of PLK1 by small interfering RNAs is known to result in loss of gamma-tubulin recruitment to the centrosomes, blocking centrosome maturation and leading to mitotic arrest. We found that codepletion of MYPT1 and PLK1 reinstates gamma-tubulin at the centrosomes, rescuing the mitotic arrest. MYPT1 depletion increases phosphorylation of PLK1 at its activating site (Thr210) in vivo, explaining, at least in part, the rescue phenotype by codepletion. Taken together, our results identify a previously unrecognized role for MYPT1 in regulating mitosis by antagonizing PLK1.

CRM1-mediated nuclear export is required for 26 S proteasome-dependent degradation of the TRIP-Br2 proto-oncoprotein

Overexpression of the proto-oncogene TRIP-Br2 (SERTAD2) has been shown to induce E2F activity and promote tumorigenesis, whereas ablation of TRIP-Br2 arrests cell proliferation. Timely degradation of many cell cycle regulators is fundamental to the maintenance of proper cell cycle progression. Here we report novel mechanism(s) that govern the tight regulation of TRIP-Br2 levels during cell cycle progression. TRIP-Br2 was observed to be a short-lived protein in which the expression level peaks at the G(1)/S boundary. TRIP-Br2 accumulated in cells treated with 26 S proteasome inhibitors. Co-immunoprecipitation studies revealed that TRIP-Br2 forms ubiquitin conjugates. In silico analysis identified a putative leucine-rich nuclear export signal (NES) motif that overlaps with the PHD-Bromo interaction domain in the acidic C-terminal transactivation domain (TAD) of TRIP-Br2. This NES motif is highly conserved in widely divergent species and in all TRIP-Br family members. TRIP-Br2 was shown to be stabilized in G(2)/M phase cells through nuclear entrapment, either by deletion of the acidic C-terminal TAD, which includes the NES motif, or by leptomycin B-mediated inhibition of the CRM1-dependent nuclear export machinery. Mutation of leucine residue 238 of this NES motif abolished the interaction between CRM1 and TRIP-Br2, as well as the nuclear export of TRIP-Br2 and its subsequent 26 S proteasome-dependent degradation. These data suggest that CRM1-mediated nuclear export may be required for the proper execution of ubiquitin-proteasome-dependent degradation of TRIP-Br2.

Plk3 phosphorylates topoisomerase IIalpha at Thr(1342), a site that is not recognized by Plk1

The Plk (polo-like kinase) family is involved in cell-cycle machinery. Despite the possible overlapping involvement of Plk1 and Plk3 in cell-cycle distribution, the precise role of each Plk might be different. To investigate mechanisms that may differentiate their physiological roles, we compared the substrate specificities of Plk1 and Plk3 using synthetic peptides. Among these substrate peptides, topoisomerase IIalpha EKT(1342)DDE-containing synthetic peptide was strongly phosphorylated by Plk3 but not by Plk1. By modulating the topoisomerase IIalpha peptide, we identified residues at positions +1, +2 and +4 as determinants of differential substrate recognition between Plk1 and Plk3. Acidic residues at positions +2 and +4 appear to be a positive determinant for Plk3 but not Plk1. Variation at position +1 appears to be tolerated by Plk3, while a hydrophobic residue at +1 is critical for Plk1 activity. The direct phosphorylation of Thr(1342) of topoisomerase IIalpha by Plk3 was demonstrated with an in vitro kinase assay, and overexpression of Plk3 induced the phosphorylation of Thr(1342) in cellular topoisomerase IIalpha. Furthermore, the physical interaction between Plk3 and topoisomerase IIalpha was also demonstrated in cells in addition to phosphorylation. These data suggest that topoisomerase IIalpha is a novel physiological substrate for Plk3 and that Plk1 and Plk3 play different roles in cell-cycle regulation.

Suppression of adenine nucleotide translocase-2 by vector-based siRNA in human breast cancer cells induces apoptosis and inhibits tumor growth in vitro and in vivo

INTRODUCTION

Adenine nucleotide translocator (ANT) 2 is highly expressed in proliferative cells, and ANT2 induction in cancer cells is known to be directly associated with glycolytic metabolisms and carcinogenesis. In addition, ANT2 repression results in the growth arrest of human cells, implying that ANT2 is a candidate for cancer therapy based on molecular targeting.

METHODS

We utilized an ANT2-specific RNA interference approach to inhibit ANT2 expression for evaluating its antitumor effect in vitro and in vivo. Specifically, to investigate the therapeutic potential of ANT2 repression, we used a DNA vector-based RNA interference approach by expressing shRNA to knockdown ANT2 in breast cancer cell lines overexpressing ANT2.

RESULTS

ANT2 shRNA treatment in breast cancer cell line MDA-MB-231 repressed cell growth as well as proliferation. In addition, cell cycle arrest, ATP depletion and apoptotic cell death characterized by the potential disruption of mitochondrial membrane were observed from the ANT2 shRNA-treated breast cancer cells. Apoptotic breast cancer cells transfected with ANT2 shRNA also induced a cytotoxic bystander effect that generates necrotic cell death to the neighboring cells. The intracellular levels of TNFalpha and TNF-receptor I were increased in ANT2 shRNA transfected cells and the bystander effect was partly blocked by anti-TNFalpha antibody. Ultimately, ANT2 shRNA effectively inhibited tumor growth in vivo.

CONCLUSION

These results suggest that vector-based ANT2 RNA interference could be an efficient molecular therapeutic method for breast cancer with high expression of ANT2.

Effect of antisense RNA targeting Polo-like kinase 1 on cell growth in A549 lung cancer cells

In order to investigate the effect of Polo-like kinase-1 (Plk1) depletion on cell cycle progression and cell growth in lung cancer cells, a recombinant plasmid containing antisense RNA targeting Plk1 (pcDNA3-Plk1) was transfected into A549 cells by lipofectine. RT-PCR and Western-blot were used to detect the Plk1 gene expression. Cell proliferation was evaluated by direct cell counting and bromodeoxyuridine (BrdU) labeling. Cell cycle distribution and apoptosis were examined by flow cytometry, and the inhibition rate (IR) by vinorebline (NVB) was determined by MTT assay. The results showed that after transfection of pcDNA3-Plk1 into A549 cells, the expression levels of Plk1 mRNA and protein were greatly decreased. In pcDNA3-Plk1 transfected groups, abnormal morphological changes of cells and growth inhibition were observed, and the BrdU labeling index was significantly lower than in the control groups (P<0.05). Cells in pcDNA3-Plk1 transfected groups were arresed in G2/M phase and apoptosis was detectable 72 h post transfection. IR induced by vinorebline in pcDNA3-Plk1 transfected groups was significantly higher than in other groups. These data suggested that antisense RNA targeting Plk1 could suppress the Plk1 expression, and therefore, significantly inhibit cell proliferation and induce cell cycle arrest and apoptosis. Moreover, it sensitized lung cancer cells to chemotherapy.

Polo-like kinase 1 is involved in invasion through extracellular matrix

Polo-like kinase 1 (PLK1) has important functions in maintaining genome stability via its role in mitosis. Because PLK1 is up-regulated in many invasive carcinomas, we asked whether it may also play a role in acquisition of invasiveness, a crucial step in transition to malignancy. In a model of metaplastic basal-like breast carcinoma progression, we found that PLK1 expression is necessary but not sufficient to induce invasiveness through laminin-rich extracellular matrix. PLK1 mediates invasion via vimentin and beta1 integrin, both of which are necessary. We observed that PLK1 phosphorylates vimentin on Ser82, which in turn regulates cell surface levels of beta1 integrin. We found PLK1 to be also highly expressed in preinvasive in situ carcinomas of the breast. These results support a role for the involvement of PLK1 in the invasion process and point to this pathway as a potential therapeutic target for preinvasive and invasive breast carcinoma treatment.

Polo-like kinase 1 may regulate G2/M transition of mouse fertilized eggs by means of inhibiting the phosphorylation of Tyr 15 of Cdc2

Polo-like kinase 1(Plk1) has been reported to be a multifunctional kinase that plays pivotal regulatory roles in microtubule assembly during mammalian early embryonic mitosis. In the present study, we examined the expression of Plk1 at protein and mRNA level in mouse fertilized eggs by Western blot and RT-PCR. We also examined the kinase activity of Plk1. At various developmental phases of mouse one-cell stage embryos, both the protein and the mRNA of Plk1 were uniformly distributed; but the kinase activity of Plk1 increased at G2/M phase and decreased at the end of M phase. At the meantime, the phosphorylation of Tyr 15 of Cdc2 was inhibited at M phase. To investigate its function in mammalian fertilized eggs further, we used specific short hairpin RNAs (shRNA) and scytonemin, the putative inhibitor of Plk1 to suppress the activity of Plk1 in mouse fertilized eggs. Upon blockage of the activation of with Plk1 shRNA and scytonemin in mouse one-cell stage embryos, the cleavage rate decreased and the phosphorylation level of Tyr 15 of Cdc2 increased. These results imply that the Plk1 may regulate cell cycle progression of mouse fertilized eggs by means of inhibiting the phosphorylation of Tyr 15 of Cdc2.

Changes in mRNA gene expression during growth in the femoral head of the young rat

The rate of physeal growth slows as an animal matures with changes in mRNA gene expression due to the altered cellular activity. To measure the change in gene expression during the juvenile growth period, the femoral head, enclosing the proximal femoral physis, primary spongiosa, and articular cartilage, was collected from both femora of 16 female Sprague-Dawley rats between 4 and 10 weeks of age. One femur of each rat had had a mid-diaphyseal femoral fracture at 4 weeks of age. RNA was extracted and hybridized to 16 Affymetrix Rat Genomic 230 2.0 GeneChip microarrays with probe sets for 31,000 genes of which 18,200 were expressed. Of these, 8002 genes had a significant change in gene expression during growth, about half increasing and half decreasing. These changes included up-regulation with time of genes related to cartilage, blood vessels, osteoprotegerin, osteomodulin, and most ribosomal proteins. There was down-regulation with maturity of genes related to bone, growth-promoting cytokines, G proteins, GTPase-mediated signal transduction factors, cytokine receptors, mitosis, integrin-linked kinase, and the cytoskeleton. In summary, the slowing of growth with maturity was associated with changes in mRNA gene expression in the femoral head for a large number of genes. These changes in gene expression between young and mature rats suggest factors which are important for the support of the rapid linear growth during early life.

Cdc25 and Wee1: analogous opposites?

Movement through the cell cycle is controlled by the temporally and spatially ordered activation of cyclin-dependent kinases paired with their respective cyclin binding partners. Cell cycle events occur in a stepwise fashion and are monitored by molecular surveillance systems to ensure that each cell cycle process is appropriately completed before subsequent events are initiated. Cells prevent entry into mitosis while DNA replication is ongoing, or if DNA is damaged, via checkpoint mechanisms that inhibit the activators and activate the inhibitors of mitosis, Cdc25 and Wee1, respectively. Once DNA replication has been faithfully completed, Cdc2/Cyclin B is swiftly activated for a timely transition from interphase into mitosis. This sharp transition is propagated through both positive and negative feedback loops that impinge upon Cdc25 and Wee1 to ensure that Cdc2/Cyclin B is fully activated. Recent reports from a number of laboratories have revealed a remarkably complex network of kinases and phosphatases that coordinately control Cdc25 and Wee1, thereby precisely regulating the transition into mitosis. Although not all factors that inhibit Cdc25 have been shown to activate Wee1 and vice versa, a number of regulatory modules are clearly shared in common. Thus, studies on either the Cdc25 or Wee1-regulatory arm of the mitotic control pathway should continue to shed light on how both arms are coordinated to smoothly regulate mitotic entry.

Lats2 is an essential mitotic regulator required for the coordination of cell division

Tumor suppressor Lats2 is a member of the conserved Dbf2 kinase family. It localizes to the centrosome and has been implicated in regulation of the cell cycle and apoptosis. However, the in vivo function of this kinase remains unclear. Here, we show that complete disruption of the gene encoding Lats2 in mice causes developmental defects in the nervous system and embryonic lethality. Furthermore, mutant cells derived from total LATS2-knock-out embryos exhibit mitotic defects including centrosome fragmentation and cytokinesis defects, followed by nuclear enlargement and multinucleation. We show that the Mob1 family, a regulator of mitotic exit, associates with Lats2 to induce its activation. We also show that the complete LATS2-knock-out cells exhibit an acceleration of exit from mitosis and marked down-regulation of critical mitotic regulators. These results suggest that Lats2 plays an essential mitotic role in coordinating accurate cytokinesis completion, governing the stabilization of other mitotic regulators.

Chemical genetics reveals the requirement for Polo-like kinase 1 activity in positioning RhoA and triggering cytokinesis in human cells

Polo-like kinases (Plks) play crucial roles in mitosis and cell division. Whereas lower eukaryotes typically contain a single Plk, mammalian cells express several closely related but functionally distinct Plks. We describe here a chemical genetic system in which a single Plk family member, Plk1, can be inactivated with high selectivity and temporal resolution by using an allele-specific, small-molecule inhibitor, as well as the application of this system to dissect Plk1's role in cytokinesis. To do this, we disrupted both copies of the PLK1 locus in human cells through homologous recombination and then reconstituted Plk1 activity by using either the wild-type kinase (Plk1(wt)) or a mutant version whose catalytic pocket has been enlarged to accommodate bulky purine analogs (Plk1(as)). When cultured in the presence of these analogs, Plk1(as) cells accumulate in prometaphase with defects that parallel those found in PLK1(Delta/Delta) cells. In addition, acute treatment of Plk1(as) cells during anaphase prevents recruitment of both Plk1 itself and the Rho guanine nucleotide exchange factor (RhoGEF) Ect2 to the central spindle, abolishes RhoA GTPase localization to the equatorial cortex, and suppresses cleavage furrow formation and cell division. Our studies define and illuminate a late mitotic function of Plk1 that, although difficult or impossible to detect in Plk1-depleted cells, is readily revealed with chemical genetics.

RGC32, a novel p53-inducible gene, is located on centrosomes during mitosis and results in G2/M arrest

To identify target genes for the hemizygous deletions of chromosome 13 that are recurrently observed in malignant gliomas, we performed genome-wide DNA copy-number analysis using array-based comparative genomic hybridization and gene expression analysis using an oligonucleotide-array. The response gene to complement 32 (RGC32) at 13q14.11 was identified as a deletion target, and its expression was frequently silenced in glioma cell lines compared with normal brain. Levels of RGC32 mRNA tended to decrease toward higher grades of primary astrocytomas, especially in tumors with mutations of p53. Expression of RGC32 mRNA was dramatically increased by exogenous p53 in a p53-mutant glioma cell line, and also by endogenous p53 in response to DNA damage in p53+/+ colon-cancer cells, but not in isogenic p53-/- cells. Chromatin immunoprecipitation and reporter assays demonstrated binding of endogenous p53 protein to the promoter region of the RGC32 gene, implying p53-dependent transcriptional activity. Transiently and stably overexpressed RGC32 suppressed the growth of glioma cells, probably owing to induction of G2/M arrest. Immunocytochemical analysis revealed a concentration of RGC32 protein at the centrosome during mitosis. RGC32 formed a protein complex with polo-like kinase 1 and was phosphorylated in vitro. These observations implied a novel mechanism by which p53 might negatively regulate cell-cycle progression by way of this newly identified transcriptional target. Our results provide the first evidence that RGC32 might be a possible tumor-suppressor for glioma, that it is directly induced by p53, and that it mediates the arrest of mitotic progression.

Nitric oxide down-regulates polo-like kinase 1 through a proximal promoter cell cycle gene homology region

Polo-like kinase 1 (PLK1) is an evolutionarily conserved serine/threonine kinase essential for cell mitosis. As a master cell cycle regulator, p21/Waf1 plays a critical role in cell cycle progression. Nitric oxide (NO.) has been shown to down-regulate PLK1 and up-regulate p21/Waf1 independent of cGMP. Here, the respective roles of p38 MAPK and p21/Waf1 in NO.-mediated PLK1 repression were investigated using differentiated U937 cells that lack soluble guanylate cyclase. NO. was shown to down-regulate both PLK1 mRNA and protein. Nuclear run-on assays and mRNA stability studies demonstrated that the effect of NO. on PLK1 expression was associated with decreased transcription without changes in transcript stability. SB202190, a p38 MAPK inhibitor, prevented transcriptional repression of PLK1 by NO.. Transfection with dominant-negative p38 MAPK mutant eliminated the NO. effect on both p21/Waf1 and PLK1 gene expression. Knockdown of p21/Waf1 with siRNA also substantially reduced the regulatory effect of NO. on PLK1. Reporter gene experiments showed that NO. decreased activity of the PLK1 proximal promoter, an effect that was blocked by p38 MAPK inhibitor. Deletion or mutation of the CDE/CHR promoter site, an element regulated by p21/Waf1, increased base-line promoter activity and abolished NO. repression of the PLK1 promoter. Likewise, electrophoretic mobility shift assays with CDE/CHR probe revealed a NO.-mediated change in protein-probe complex formation. Competition with various unlabeled CDE/CHR mutant sequences showed that NO. increased nuclear protein binding to intact CHR. These results demonstrate that a NO.-p38 MAPK-p21/Waf1 signal transduction pathway represses PLK1 through a canonical CDE/CHR promoter element.

Small molecules keep mitotic kinases in check

The remarkable progress in the discovery of small molecules that target protein kinases continues, and two recent reports have described alternative approaches to this task: homology modeling and phenotype-based screening. Compounds targeting the nucleotide binding pockets of polo-like kinases in particular provide significant new insights into the molecular mechanisms controlling cell-division phenotypes.

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

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

Tumor inhibition by genomically integrated inducible RNAi-cassettes

RNA interference (RNAi) has emerged as a powerful tool to induce loss-of-function phenotypes by post-transcriptional silencing of gene expression. In this study we wondered whether inducible RNAi-cassettes integrated into cellular DNA possess the power to trigger neoplastic growth. For this purpose inducible RNAi vectors containing tetracycline (Tet)-responsive derivatives of the H1 promoter for the conditional expression of short hairpin RNA (shRNA) were used to target human polo-like kinase 1 (Plk1), which is overexpressed in a broad spectrum of human tumors. In the absence of doxycycline (Dox) HeLa clones expressing TetR, that carry the RNAi-cassette stably integrated, exhibited no significant alteration in Plk1 expression levels. In contrast, exposure to Dox led to marked downregulation of Plk1 mRNA to 3% and Plk1 protein to 14% in cell culture compared to mismatch shRNA/Plk1-expressing cells. As a result of Plk1 depletion cell proliferation decreased to 17%. Furthermore, for harnessing RNAi for silencing disease-related genes in vivo we transplanted inducible RNAi-HeLa cells onto nude mice. After administration of Dox knockdown of Plk1 expression was observed correlating to a significant inhibition of tumor growth. Taken together, our data revealed that genomically integrated RNAi-elements are suitable to hamper tumor growth by conditional expression of shRNA.

Cdc28-dependent regulation of the Cdc5/Polo kinase

Polo kinase is activated as cells enter mitosis and plays a central role in coordinating diverse mitotic events, yet the mechanisms leading to activation of Polo kinase are poorly understood . Work in Xenopus meiotic cell cycles has suggested that Polo kinase functions in a pathway that helps trigger activation of Cdk1 . However, studies in other organisms have suggested that activation of Polo kinase is dependent upon Cdk1 and therefore occurs downstream of Cdk1 activation . In this study, we have investigated the role of Cdk1 in the activation of budding yeast Polo kinase. The budding yeast homologs of Cdk1 and Polo kinase are referred to as Cdc28 and Cdc5. We show that signaling from Cdc28 is required to maintain Cdc5 activity in vivo. Furthermore, purified Cdc28 associated with the mitotic cyclin Clb2 is sufficient to activate purified Cdc5 in vitro. A single Cdc28 consensus phosphorylation site found at threonine 242 in the activation loop segment of Cdc5 is required for Cdc5 function in vivo and for kinase activity in vitro, whereas four other Cdc28 consensus sites are dispensable. Analysis of Cdc5 phosphorylation by mass spectrometry indicates that threonine 242 is phosphorylated in vivo. These results suggest that Cdc28 activates Cdc5 via phosphorylation of threonine 242.

Stable gene silencing of cyclin B1 in tumor cells increases susceptibility to taxol and leads to growth arrest in vivo

Cyclin B1 is the regulatory subunit of cyclin-dependent kinase 1 (Cdk1) and is critical for the initiation of mitosis. Accumulating data indicate that the deregulation of cyclin B1 is tightly linked to neoplastic transformation. To study the phenotype and the potential preclinical relevance, we generated HeLa cell lines stably transfected with the plasmids encompassing short hairpin RNA (shRNA) targeting cyclin B1. We demonstrate that the reduction of cyclin B1 caused inhibition of proliferation by arresting cells in G2 phase and by inducing apoptosis. Cells, entering mitosis, were impaired in chromosome condensation and alignment. Importantly, HeLa cells with reduced cyclin B1 were more susceptible to the treatment of small interfering RNA targeting Polo-like kinase 1 (Plk1) and to the administration of the chemotherapeutic agent taxol. Finally, HeLa cells with reduced cyclin B1 showed inhibited tumor growth in nude mice compared to that of control cells. In summary, our data indicate that cyclin B1 is an essential molecule for tumor cell survival and aggressive proliferation, suggesting that the downregulation of cyclin B1, especially in combination with other molecular targets, might become an interesting strategy for antitumor intervention.

Expression of polo-like kinase 1 (PLK1) protein predicts the survival of patients with gastric carcinoma

OBJECTIVE

To assess the prognostic value of polo-like kinase 1 (PLK1), an important regulator of cell cycle progression, in patients with gastric carcinoma.

METHODS

PLK1 expression was determined in 160 gastric carcinoma patients by immunohistochemistry and compared with p53 expression and the proliferating cell nuclear antigen-labeling index (PCNA-LI) to evaluate the effect of PLK1 on tumor progression. Furthermore, PLK mRNA expression was determined in 26 advanced gastric cancer patients by reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS

PLK mRNA expression was detected in 25 (96.2%) patients by RT-PCR. Immunohistochemical staining revealed PLK1 expression in 84 (52.5%) patients. There were no significant relationships between PLK1 expression and various clinicopathological factors. PLK1 expression was significantly correlated with the PCNA-LI, but not p53 expression. The prognosis of patients with PLK1-positive tumors was significantly worse than that of patients with PLK1-negative tumors (p < 0.05). Moreover, multivariate analysis revealed that PLK1 expression was an independent prognostic factor. Patients with PLK1-positive and high PCNA-LI tumors showed a significantly poorer prognosis than patients with PLK1-negative and/or low PCNA-LI tumors. Furthermore, the prognosis of patients with PLK1- and p53-positive tumors was significantly worse than that of patients with PLK1- and p53-negative or PLK1-negative and p53-positive tumors.

CONCLUSION

PLK1 expression may be a critical indicator of a poor prognosis in patients with gastric carcinoma.

Polo box domain of Plk3 functions as a centrosome localization signal, overexpression of which causes mitotic arrest, cytokinesis defects, and apoptosis

Polo-like kinase 3 (Plk3), an immediate early response gene product, plays an important role in the regulation of mitosis, DNA damage checkpoint activation, and Golgi dynamics. Similar to other members of the Plk family, Plk3 has a conserved kinase domain at the N terminus and a Polo box domain consisting of two Polo boxes at the C terminus. In this study, we demonstrate that the Polo box domain of Plk3 is sufficient for subcellular localization of this kinase to the centrosomes, the spindle poles, and the midbody when ectopically expressed in HeLa and U2OS cells. Both Polo boxes are required for the subcellular localization. Overexpression of the Polo box domain, not the kinase domain, of Plk3 causes significant cell cycle arrest and cytokinesis defects, eventually leading to mitotic catastrophe/apoptosis. Interestingly, the Polo box domain of Plk3 is more potent in inhibiting cell proliferation and inducing apoptosis than that of Plk1, suggesting that this domain can provide an additional structural basis for discovery of new anticancer drugs given the current emphasis on Plk1 as a therapeutic target.

Targeting polo-like kinase 1 for cancer therapy

Human polo-like kinase 1 (PLK1) is essential during mitosis and in the maintenance of genomic stability. PLK1 is overexpressed in human tumours and has prognostic potential in cancer, indicating its involvement in carcinogenesis and its potential as a therapeutic target. The use of different PLK1 inhibitors has increased our knowledge of mitotic regulation and allowed us to assess their ability to suppress tumour growth in vivo. We address the structural features of the kinase domain and the unique polo-box domain of PLK1 that are most suited for drug development and discuss our current understanding of the therapeutic potential of PLK1.

Normal cells, but not cancer cells, survive severe Plk1 depletion

We previously reported the phenotype of depletion of polo-like kinase 1 (Plk1) using RNA interference (RNAi) and showed that p53 is stabilized in Plk1-depleted cancer cells. In this study, we further analyzed the Plk1 depletion-induced phenotype in both cancer cells and primary cells. The vector-based RNAi approach was used to evaluate the role of the p53 pathway in Plk1 depletion-induced apoptosis in cancer cells with different p53 backgrounds. Although DNA damage and cell death can occur independently of p53, p53-deficient cancer cells were much more sensitive to Plk1 depletion than cancer cells with functional p53. Next, the lentivirus-based RNAi approach was used to generate a series of Plk1 hypomorphs. In HeLa cells, two weak hypomorphs showed only slight G2/M arrest, a medium hypomorph arrested with 4N DNA content, followed later by apoptosis, and a strong Plk1 hypomorph underwent serious mitotic catastrophe. In well-synchronized HeLa cells, a medium level of Plk1 depletion caused a 2-h delay of mitotic progression, and a high degree of Plk1 depletion significantly delayed mitotic entry and completely blocked cells at mitosis. In striking contrast, normal hTERT-RPE1 and MCF10A cells were much less sensitive to Plk1 depletion than HeLa cells; no apparent cell proliferation defect or cell cycle arrest was observed after Plk1 depletion in these cells. Therefore, these data further support suggestions that Plk1 may be a feasible cancer therapy target.

Complex formation of Plk1 and INCENP required for metaphase-anaphase transition

Mitotic chromosomal dynamics is regulated by the coordinated activities of many mitotic kinases, such as cyclin-dependent kinase 1 (Cdk1), Aurora-B or Polo-like kinase 1 (Plk1), but the mechanisms of their coordination remain unknown. Here, we report that Cdk1 phosphorylates Thr 59 and Thr 388 on inner centromere protein (INCENP), which regulates the localization and kinase activity of Aurora-B from prophase to metaphase. INCENP depletion disrupts Plk1 localization specifically at the kinetochore. This phenotype is rescued by the exogenous expression of INCENP wild type and INCENP mutated at Thr 59 to Ala (T59A), but not at Thr 388 to Ala (T388A). The replacement of endogenous INCENP with T388A resulted in the delay of progression from metaphase to anaphase. We propose that INCENP phosphorylation by Cdk1 is necessary for the recruitment of Plk1 to the kinetochore, and that the complex formation of Plk1 and Aurora-B on INCENP may play crucial roles in the regulation of chromosomal dynamics.