Antibody microinjection reveals an essential role for human polo-like kinase 1 (Plk1) in the functional maturation of mitotic centrosomes

Breaking the ties that bind: new advances in centrosome biology

The centrosome, which consists of two centrioles and the surrounding pericentriolar material, is the primary microtubule-organizing center (MTOC) in animal cells. Like chromosomes, centrosomes duplicate once per cell cycle and defects that lead to abnormalities in the number of centrosomes result in genomic instability, a hallmark of most cancer cells. Increasing evidence suggests that the separation of the two centrioles (disengagement) is required for centrosome duplication. After centriole disengagement, a proteinaceous linker is established that still connects the two centrioles. In G2, this linker is resolved (centrosome separation), thereby allowing the centrosomes to separate and form the poles of the bipolar spindle. Recent work has identified new players that regulate these two processes and revealed unexpected mechanisms controlling the centrosome cycle.

Polo-like kinase 1 inhibition kills glioblastoma multiforme brain tumor cells in part through loss of SOX2 and delays tumor progression in mice

Glioblastoma multiforme (GBM) ranks among the deadliest types of cancer and given these new therapies are urgently needed. To identify molecular targets, we queried a microarray profiling 467 human GBMs and discovered that polo-like kinase 1 (PLK1) was highly expressed in these tumors and that it clustered with the proliferative subtype. Patients with PLK1-high tumors were more likely to die from their disease suggesting that current therapies are inactive against such tumors. This prompted us to examine its expression in brain tumor initiating cells (BTICs) given their association with treatment failure. BTICs isolated from patients expressed 110-470 times more PLK1 than normal human astrocytes. Moreover, BTICs rely on PLK1 for survival because the PLK1 inhibitor BI2536 inhibited their growth in tumorsphere cultures. PLK1 inhibition suppressed growth, caused G(2) /M arrest, induced apoptosis, and reduced the expression of SOX2, a marker of neural stem cells, in SF188 cells. Consistent with SOX2 inhibition, the loss of PLK1 activity caused the cells to differentiate based on elevated levels of glial fibrillary acidic protein and changes in cellular morphology. We then knocked glial fibrillary acidic protein (GFAP) down SOX2 with siRNA and showed that it too inhibited cell growth and induced cell death. Likewise, in U251 cells, PLK1 inhibition suppressed cell growth, downregulated SOX2, and induced cell death. Furthermore, BI2536 delayed tumor growth of U251 cells in an orthotopic brain tumor model, demonstrating that the drug is active against GBM. In conclusion, PLK1 level is elevated in GBM and its inhibition restricts the growth of brain cancer cells.

Polo-like kinase-activating kinases: Aurora A, Aurora B and what else?

The events of cell division are regulated by a complex interplay between kinases and phosphatases. Cyclin-dependent kinases (Cdks), polo-like kinases (Plks) and Aurora kinases play central roles in this process. Polo kinase (Plk1 in humans) regulates a wide range of events in mitosis and cytokinesis. To ensure the accuracy of these processes, polo activity itself is subject to complex regulation. Phosphorylation of polo in its T loop (or activation loop) increases its kinase activity several-fold. It has been shown that Aurora A kinase, with its co-factor Bora, activates Plk1 in G(2), and that this is essential for recovery from cell cycle arrest induced by DNA damage. In a recent article published in PLoS Biology, we report that Drosophila polo is activated by Aurora B kinase at centromeres, and that this is crucial for polo function in regulating chromosome dynamics in prometaphase. Our results suggest that this regulatory pathway is conserved in humans. Here, we propose a model for the collaboration between Aurora B and polo in the regulation of kinetochore attachment to microtubules in early mitosis. Moreover, we suggest that Aurora B could also function to activate Polo/Plk1 in cytokinesis. Finally, we discuss recent findings and open questions regarding the activation of polo and polo-like kinases by different kinases in mitosis, cytokinesis and other processes.

Polo-like kinase is necessary for flagellum inheritance in Trypanosoma brucei

Polo-like kinases play an important role in a variety of mitotic events in mammalian cells, ranging from centriole separation and chromosome congression to abscission. To fulfill these roles, Polo-like kinase homologs move to different cellular locations as the cell cycle progresses, starting at the centrosome, progressing to the spindle poles and then the midbody. In the protist parasite Trypanosoma brucei, the single polo-like kinase homolog T. brucei PLK (TbPLK) is essential for cytokinesis and is necessary for the correct duplication of a centrin-containing cytoskeletal structure known as the bilobe. We show that TbPLK has a dynamic localization pattern during the cell cycle. The kinase localizes to the basal body, which nucleates the flagellum, and then successively localizes to a series of cytoskeletal structures that regulate the position and attachment of the flagellum to the cell body. The kinase localizes to each of these structures as they are duplicating. TbPLK associates with a specialized set of microtubules, known as the microtubule quartet, which might transport the kinase during its migration. Depletion of TbPLK causes defects in basal body segregation and blocks the duplication of the regulators that position the flagellum, suggesting that its presence on these structures might be necessary for their proper biogenesis. TbPLK migrates throughout the cell in T. brucei, but the specific locations to which it targets and its functions are geared towards the inheritance of a properly positioned and attached flagellum.

TAK-960, a novel, orally available, selective inhibitor of polo-like kinase 1, shows broad-spectrum preclinical antitumor activity in multiple dosing regimens

Polo-like kinase 1 (PLK1) is a serine/threonine protein kinase involved in key processes during mitosis. Human PLK1 has been shown to be overexpressed in various human cancers, and elevated levels of PLK1 have been associated with poor prognosis, making it an attractive target for anticancer therapy. TAK-960 [4-[(9-cyclopentyl-7,7-difluoro-5-methyl-6-oxo-6,7,8,9-tetrahydro-5H-pyrimido[4,5-b][1,4]diazepin-2-yl)amino]-2-fluoro-5-methoxy-N-(1-methylpiperidin-4-yl) benzamide] is a novel, investigational, orally bioavailable, potent, and selective PLK1 inhibitor that has shown activity in several tumor cell lines, including those that express multidrug-resistant protein 1 (MDR1). Consistent with PLK1 inhibition, TAK-960 treatment caused accumulation of G(2)-M cells, aberrant polo mitosis morphology, and increased phosphorylation of histone H3 (pHH3) in vitro and in vivo. TAK-960 inhibited proliferation of multiple cancer cell lines, with mean EC(50) values ranging from 8.4 to 46.9 nmol/L, but not in nondividing normal cells (EC(50) >1,000 nmol/L). The mutation status of TP53 or KRAS and MDR1 expression did not correlate with the potency of TAK-960 in the cell lines tested. In animal models, oral administration of TAK-960 increased pHH3 in a dose-dependent manner and significantly inhibited the growth of HT-29 colorectal cancer xenografts. Treatment with once daily TAK-960 exhibited significant efficacy against multiple tumor xenografts, including an adriamycin/paclitaxel-resistant xenograft model and a disseminated leukemia model. TAK-960 has entered clinical evaluation in patients with advanced cancers.

Polo-like kinase 1, on the rise from cell cycle regulation to prostate cancer development

Polo-like kinase 1 (Plk1), a well-characterized member of serine/threonine kinases Plk family, has been shown to play pivotal roles in mitosis and cytokinesis in eukaryotic cells. Recent studies suggest that Plk1 not only controls the process of mitosis and cytokinesis, but also, going beyond those previously described functions, plays critical roles in DNA replication and Pten null prostate cancer initiation. In this review, we briefly summarize the functions of Plk1 in mitosis and cytokinesis, and then mainly focus on newly discovered functions of Plk1 in DNA replication and in Pten-null prostate cancer initiation. Furthermore, we briefly introduce the architectures of human and mouse prostate glands and the possible roles of Plk1 in human prostate cancer development. And finally, the newly chemotherapeutic development of small-molecule Plk1 inhibitors to target Plk1 in cancer treatment and their translational studies are also briefly reviewed.

Plk2 regulates centriole duplication through phosphorylation-mediated degradation of Fbxw7 (human Cdc4)

Polo-like kinases (Plks) perform crucial functions during mitosis, cytokinesis and centriole duplication. Plk2 is activated in early G1 phase and is involved in the reproduction of centrosomes. However, the mechanisms underlying Plk2-induced centriole duplication are incompletely understood. Here, we show that Plk2 directly targets the F-box protein F-box/WD repeat-containing protein 7 (Fbxw7), which is a regulator of the ubiquitin-mediated degradation of cyclin E. Plk2 phosphorylates Fbxw7 on serine 176 and the two proteins form a complex in vitro and in vivo. Phosphorylation of Fbxw7 by Plk2 induces destabilization of the F-box protein resulting in accumulation of cyclin E and increased potential for centriole reproduction. In addition, loss of Fbxw7 in human cells leads to uncontrolled centriole duplication, highlighting the importance of Fbxw7 regulation by Plk2. These findings define a previously unknown Plk2-dependent pathway involved at the onset of S phase and in centrosome duplication.

Plk1-mediated phosphorylation of UAP56 regulates the stability of UAP56

Polo-like kinase 1 (Plk1) is a conserved serine/threonine protein kinase that plays pivotal roles during the cell cycle and cell proliferation. Although a number of important targets have been identified, the mechanism of Plk1-regulated pathways and the bulk of the Plk1 interactome are largely unknown. Here, we demonstrate that Plk1 interacts with the DExH/D RNA helicase, UAP56. The protein levels of UAP56 and Plk1 are inversely correlated during the cell cycle. We also show that Plk1 phosphorylates UAP56 in vitro and in vivo and that Plk1-dependent phosphorylation of UAP56 triggers ubiquitination and degradation of UAP56 through proteasomes. This result suggests that Plk1-mediated phosphorylation of UAP56 regulates the stability of UAP56. Our results will be helpful in further understanding mRNA metabolism, cell cycle progression, and the link between mRNA metabolism and cellular function.

The APC/C Ubiquitin Ligase: From Cell Biology to Tumorigenesis

The ubiquitin proteasome system (UPS) is required for normal cell proliferation, vertebrate development, and cancer cell transformation. The UPS consists of multiple proteins that work in concert to target a protein for degradation via the 26S proteasome. Chains of an 8.5-kDa protein called ubiquitin are attached to substrates, thus allowing recognition by the 26S proteasome. Enzymes called ubiquitin ligases or E3s mediate specific attachment to substrates. Although there are over 600 different ubiquitin ligases, the Skp1-Cullin-F-box (SCF) complexes and the anaphase promoting complex/cyclosome (APC/C) are the most studied. SCF involvement in cancer has been known for some time while APC/C's cancer role has recently emerged. In this review we will discuss the importance of APC/C to normal cell proliferation and development, underscoring its possible contribution to transformation. We will also examine the hypothesis that modulating a specific interaction of the APC/C may be therapeutically attractive in specific cancer subtypes. Finally, given that the APC/C pathway is relatively new as a cancer target, therapeutic interventions affecting APC/C activity may be beneficial in cancers that are resistant to classical chemotherapy.

G Protein-coupled receptor kinase 5 is localized to centrosomes and regulates cell cycle progression

G protein-coupled receptor kinases (GRKs) are important regulators of G protein-coupled receptor function and mediate receptor desensitization, internalization, and signaling. While GRKs also interact with and/or phosphorylate many other proteins and modify their function, relatively little is known about the cellular localization of endogenous GRKs. Here we report that GRK5 co-localizes with γ-tubulin, centrin, and pericentrin in centrosomes. The centrosomal localization of GRK5 is observed predominantly at interphase and although its localization is not dependent on microtubules, it can mediate microtubule nucleation of centrosomes. Knockdown of GRK5 expression leads to G2/M arrest, characterized by a prolonged G2 phase, which can be rescued by expression of wild type but not catalytically inactive GRK5. This G2/M arrest appears to be due to increased expression of p53, reduced activity of aurora A kinase and a subsequent delay in the activation of polo-like kinase 1. Overall, these studies demonstrate that GRK5 is localized in the centrosome and regulates microtubule nucleation and normal cell cycle progression.

Overexpression of polo-like kinase 1 and its clinical significance in human non-small cell lung cancer

Polo-like kinase 1 is a serine/threonine kinase which plays an essential role in mitosis and malignant transformation. The aim of this study was to investigate the prognostic significance of polo-like kinase 1 expression and determine its possibility as a therapeutic target in non-small cell lung cancer. Semi-quantitative RT-PCR assay was performed to detect polo-like kinase 1 mRNA expression in non-small cell lung cancer cells or tissues. Immunohistochemistry was performed to detect polo-like kinase 1 protein expression in 100 non-small cell lung cancer tissue samples, and the associations of polo-like kinase 1 expression with clinicopathological factors or prognosis of non-small cell lung cancer patients were evaluated. RNA interference was employed to inhibit endogenous polo-like kinase 1 expression and analyzed the effects of polo-like kinase 1 inhibition on the malignant phenotypes of non-small cell lung cancer cells including growth, apoptosis, radio- or chemoresistance. Also, the possible molecular mechanisms were also investigated. The levels of polo-like kinase 1 mRNA expression in non-small cell lung cancer cell lines or tissues were significantly higher than those in normal human bronchial epithelial cell line or corresponding non-tumor tissues. High polo-like kinase 1 expression was significantly correlated with advanced clinical stage, higher tumor classification and lymph node metastasis of non-small cell lung cancer patients (P=0.001, 0.004 and 0.001, respectively). Meanwhile, high polo-like kinase 1 protein expression was also an independent prognostic molecular marker for non-small cell lung cancer patients (hazard ratio: 2.113; 95% confidence interval: 1.326-3.557; P=0.017). Polo-like kinase 1 inhibition could significantly inhibit in vitro and in vivo proliferation, induce cell arrest of G(2)/M phase and apoptosis enhancement in non-small cell lung cancer cells, which might be activation of the p53 pathway and the Cdc25C/cdc2/cyclin B1 feedback loop. Further, inhibition of polo-like kinase 1 could enhance the sensitivity of non-small cell lung cancer cells to taxanes or irradiation. Thus, polo-like kinase 1 might be a prognostic marker and a chemo- or radiotherapeutic target for non-small cell lung cancer.

PLK1 phosphorylation of pericentrin initiates centrosome maturation at the onset of mitosis

The microtubule-organizing activity of the centrosome oscillates during the cell cycle, reaching its highest level at mitosis. At the onset of mitosis, the centrosome undergoes maturation, which is characterized by a drastic expansion of the pericentriolar matrix (PCM) and a robust increase in microtubule-organizing activity. It is known that PLK1 is critical for the initiation of centrosome maturation. In this paper, we report that pericentrin (PCNT), a PCM protein, was specifically phosphorylated by PLK1 during mitosis. Phosphoresistant point mutants of PCNT did not recruit centrosomal proteins, such as CEP192, GCP-WD (γ-complex protein with WD repeats), γ-tubulin, Aurora A, and PLK1, into the centrosome during mitosis. However, centrosomal recruitment of CEP215 depended on PCNT irrespective of its phosphorylation status. Furthermore, ectopic expression of PLK1-PCNT fusion proteins induced the centrosomal accumulation of CEP192, GCP-WD, and γ-tubulin even in interphase cells, mimicking centrosome maturation. Based on these results, we propose that PLK1-mediated phosphorylation of PCNT initiates centrosome maturation by organizing the spindle pole-specific PCM lattice.

The Polo-like kinase PLKA in Aspergillus nidulans is not essential but plays important roles during vegetative growth and development

The Polo-like kinases (Plks) are conserved, multifunctional cell cycle regulators that are induced in many forms of cancer and play additional roles in metazoan development. We previously identified plkA in Aspergillus nidulans, the only Plk investigated in filamentous fungi to date, and partially characterized its function through overexpression. Here, we report the plkA null phenotype. Surprisingly, plkA was not essential, unlike Plks in other organisms that contain a single homologue. A subset of cells lacking PLKA contained defects in spindle formation and chromosome organization, supporting some conservation in cell cycle function. However, septa were present, suggesting that PLKA, unlike other Plks, is not a central regulator of septation. Colonies lacking PLKA were compact with multibranched hyphae, implying a role for this factor in aspects of hyphal morphogenesis. These defects were suppressed by high temperature or low concentrations of benomyl, suggesting that PLKA may function during vegetative growth by influencing microtubule dynamics. However, the colonies also showed reduced conidiation and precocious formation of sexual Hülle cells in a benomyl- and temperature-insensitive manner. This result suggests that PLKA may influence reproduction through distinct mechanisms and represents the first example of a link between Plk function and development in fungi. Finally, filamentous fungal Plks have distinct features, and phylogenetic analyses reveal that they may group more closely with metazoan PLK4. In contrast, yeast Plks are more similar to metazoan proteins PLK1 to PLK3. Thus, A. nidulans PLKA shows some conservation in cell cycle function but may also play novel roles during hyphal morphogenesis and development.

Protein phosphatases and the regulation of mitosis

Dynamic control of protein phosphorylation is necessary for the regulation of many cellular processes, including mitosis and cytokinesis. Indeed, although the central role of protein kinases is widely appreciated and intensely studied, the importance of protein phosphatases is often overlooked. Recent studies, however, have highlighted the considerable role of protein phosphatases in both the spatial and temporal control of protein kinase activity, and the modulation of substrate phosphorylation. Here, we will focus on recent advances in our understanding of phosphatase structure, and the importance of phosphatase function in the control of mitotic spindle formation, chromosome architecture and cohesion, and cell division.

Quantitative phospho-proteomics to investigate the polo-like kinase 1-dependent phospho-proteome

Polo-like kinase 1 (PLK1) is a key regulator of mitotic progression and cell division, and small molecule inhibitors of PLK1 are undergoing clinical trials to evaluate their utility in cancer therapy. Despite this importance, current knowledge about the identity of PLK1 substrates is limited. Here we present the results of a proteome-wide analysis of PLK1-regulated phosphorylation sites in mitotic human cells. We compared phosphorylation sites in HeLa cells that were or were not treated with the PLK1-inhibitor BI 4834, by labeling peptides via methyl esterification, fractionation of peptides by strong cation exchange chromatography, and phosphopeptide enrichment via immobilized metal affinity chromatography. Analysis by quantitative mass spectrometry identified 4070 unique mitotic phosphorylation sites on 2069 proteins. Of these, 401 proteins contained one or multiple phosphorylation sites whose abundance was decreased by PLK1 inhibition. These include proteins implicated in PLK1-regulated processes such as DNA damage, mitotic spindle formation, spindle assembly checkpoint signaling, and chromosome segregation, but also numerous proteins that were not suspected to be regulated by PLK1. Analysis of amino acid sequence motifs among phosphorylation sites down-regulated under PLK1 inhibition in this data set identified two potential novel variants of the PLK1 consensus motif.

Proteomic identification of Hsp70 as a new Plk1 substrate in arsenic trioxide-induced mitotically arrested cells

We previously demonstrated that when arsenic trioxide (ATO)-induced mitotically arrested HeLa S3 cells (AIMACs) were treated with staurosporine (SSP) the cells rapidly exited mitosis. To better define the cellular targets and the underlying mechanisms of AIMACs, we applied 2-D DIGE followed by LC-MS/MS analysis and showed that SSP induced a significant change in the phosphoproteome of AIMACs. Among the proteins whose phosphorylation was modulated by SSP, we identified Hsp70, Rad 23B, and eukaryotic translation initiation factor 4B as potentially new substrates of polo-like kinase 1 (Plk1), an essential serine/threonine kinase with versatile mitotic functions. Since Hsp70 is a stress protein responsible for ATO treatment, we further identified Thr(13) , Ser(362) , Ser(631) , and Ser(633) on Hsp70 intracellularly phosphorylated in AIMACs by combining TiO(2) phospho-peptides enrichment and MS/MS analysis. Using antibody specifically against phosph-Ser(631) Hsp70 and further aided by expression of kinase-dead Plk1 and pharmacological inhibition of Plk1, we concluded that Ser(631) on Hsp70 is phosphorylated by Plk1 in AIMACs. By immnuofluorescent staining, we found the colocalization of Hsp70 and Plk1 in AIMACs but not in interphase cells. In addition, Plk1-mediated phosphorylation of Hsp70 prevented AIMACs from mitotic death. Our results reveal that Hsp70 is a novel substrate of Plk1 and that its phosphorylation contributes to attenuation of ATO-induced mitotic abnormalities.

Polo-like kinase 1 regulates cell proliferation and is targeted by miR-593* in esophageal cancer

Polo-like kinase 1 (PLK1) is overexpressed in various human cancers. However, the biological functions and the post-transcriptional regulations of PLK1 in esophageal cancer (EC) are still unknown. The purposes of our study are to determine whether PLK1 can be a molecular target of EC therapy and to identify a microRNA (miRNA) targeting PLK1. We performed loss-of-function and gain-of-function experiments regarding cell proliferation, cell cycle, apoptosis, in vivo tumor formation and luciferase reporter assays, using siRNAs against PLK1 and miRNA. PLK1 protein was expressed in all 11 EC cell lines, but not in normal esophageal epithelial cells (HEEpiC). Knockdown of PLK1 in EC cells induced G2/M arrest (p < 0.001) in cell cycle assay and reduced cell proliferation (p = 0.019) and tumor formation ability in vivo (p < 0.0001). MiR-593*, identified as a miRNA targeting PLK1 by a database search, was less expressed especially in six EC cell lines than HEEpiC cells. Moreover, miR-593* expression level was inversely correlated with PLK1 mRNA level in 48 clinical tissue specimens of EC (p = 0.006). Introduction of synthetic miR-593* suppressed PLK1 expression by 69-73%, reduced cell proliferation (p = 0.008) and increased cell proportion of G2/M phase (p = 0.01) in HSA/c (an EC cells), whereas a miR-593* inhibitor upregulated PLK1 expression by 11-55%. Additionally, luciferase assay demonstrated that miR-593* interacted two binding sites in the PLK1 3'-UTR and reduced 56.8-71.5% of luciferase activity by degrading luciferase mRNA in HSA/c cells. In conclusion, PLK1 is post-transcriptionally regulated by miR-593* and could be a promising molecular target for EC treatment.

Polo-box domain inhibitor poloxin activates the spindle assembly checkpoint and inhibits tumor growth in vivo

Polo-like kinase 1 (Plk1) is widely established as one of the most promising targets in oncology. Although the protein kinase domain of Plk1 is highly conserved, the polo-box domain (PBD) of Plk1 provides a much more compelling site to specifically inhibit the localization and target binding of Plk1. We recently identified, via fluorescence polarization assay, the natural product derivative, Poloxin, as the first small-molecule inhibitor specifically targeting the function of the Plk1 PBD. In this study, we characterized its mitotic phenotype and its function in vitro and in vivo. Poloxin induces centrosome fragmentation and abnormal spindle and chromosome misalignment, which activate the spindle assembly checkpoint and prolong mitosis. Notably, centrosomal fragmentation induced by Poloxin is partially attributable to dysfunctional Kizuna, a key substrate of Plk1 at centrosomes. Moreover, Poloxin strongly inhibits proliferation of a panel of cancer cells by inducing mitotic arrest, followed by a surge of apoptosis. More important, we report, for the first time to our knowledge, that the PBD inhibitor, Poloxin, significantly suppresses tumor growth of cancer cell lines in xenograft mouse models by lowering the proliferation rate and triggering apoptosis in treated tumor tissues. The data highlight that targeting the PBD by Poloxin is a powerful approach for selectively inhibiting Plk1 function in vitro and in vivo.

Small interfering RNA-mediated knockdown of polo-like kinase 1 promotes apoptosis in human hepatocellular carcinoma cell line BCL-7402

AIM: To investigate the effect of small interfering RNA (siRNA)-mediated Polo-like kinase 1 (Plk1) gene silencing on p53 expression and cell apoptosis in human hepatocellular carcinoma cell line BCL-7402, and to explore the feasibility of targeting the human Plk1 gene as a therapeutic strategy for hepatocellular carcinoma.

METHODS: Two siRNA sequences (siRNA1 and siRNA2) targeting the human Plk1 gene were designed and synthesized. BCL-7402 cells were transfected with blank control, negative control, siRNA1 or siRNA2 via lipofection. After transfection, reverse transcription-polymerase chain reaction (RT- PCR) was used to examine the expression of Plk1 mRNA , and Western blot was used to examine the expression of Plk1 and P53 proteins in transfected BCL-7402 cells. Cell cycle distribution and apoptosis of transfected cells were monitored by flow cytometry (FCM). The ultrastructural changes of transfected BCL-7402 cells were observed by transmission electron microscopy (TEM).

RESULTS: BCL-7402 cells transfected with low doses of siRNAs targeting the Plk1 gene showed greatly decreased levels of Plk1 mRNA and protein. In the siRNA1 group, Plk1 mRNA expression was reduced by 51% and 62% and Plk1 protein expression by 65% and 81% 24 and 48 h after transfection (all P < 0.01). In the siRNA2 group, Plk1 mRNA expression was reduced by 42% and 56% and Plk1 protein expression by 51% and 65% 24 and 48 h after transfection (all P < 0.01). P53 protein levels increased obviously with the decrease in Plk1 protein levels (P < 0.01). The percentage of cells at G₂/M phase increased obviously 24 h after transfection (P < 0.01). Apoptosis rate increased remarkably and apoptotic phenotypes could be seen by TEM. in cells 48 h after transfection.

CONCLUSION: SiRNAs targeting the human Plk1 gene remarkably inhibited Plk1 expression, increased p53 gene expression, and promoted apoptosis, suggesting that the Plk1 gene plays important roles in cell cycle control and apoptosis of BCL-7402 cells.

Phosphorylation of Ran-binding protein-1 by Polo-like kinase-1 is required for interaction with Ran and early mitotic progression

Polo-like kinase-1 (Plk1) is essential for progression of mitosis and localizes to centrosomes, central spindles, midbody, and kinetochore. Ran, a small GTPase of the Ras superfamily, plays a role in microtubule dynamics and chromosome segregation during mitosis. Although Ran-binding protein-1 (RanBP1) has been reported as a regulator of RanGTPase for its mitotic functions, the action mechanism between Ran and RanBP1 during mitosis is still unknown. Here, we demonstrated in vitro and in vivo phosphorylation of RanBP1 by Plk1 as well as the importance of phosphorylation of RanBP1 in the interaction between Plk1 and Ran during early mitosis. Both phosphorylation-defective and N-terminal deletion mutant constructs of RanBP1 disrupted the interaction with Ran, and depletion of Plk1 also disrupted the formation of a complex between Ran and RanBP1. In addition, the results from both ectopic expression of phosphorylation-defective mutant construct and a functional complementation on RanBP1 deficiency with this mutant indicated that phosphorylation of RanBP1 by Plk1 might be crucial to microtubule nucleation and spindle assembly during mitosis.

Toxicity modelling of Plk1-targeted therapies in genetically engineered mice and cultured primary mammalian cells

High attrition rates of novel anti-cancer drugs highlight the need for improved models to predict toxicity. Although polo-like kinase 1 (Plk1) inhibitors are attractive candidates for drug development, the role of Plk1 in primary cells remains widely unexplored. Therefore, we evaluated the utility of an RNA interference-based model to assess responses to an inducible knockdown (iKD) of Plk1 in adult mice. Here we show that Plk1 silencing can be achieved in several organs, although adverse events are rare. We compared responses in Plk1-iKD mice with those in primary cells kept under controlled culture conditions. In contrast to the addiction of many cancer cell lines to the non-oncogene Plk1, the primary cells' proliferation, spindle assembly and apoptosis exhibit only a low dependency on Plk1. Responses to Plk1-depletion, both in cultured primary cells and in our iKD-mouse model, correspond well and thus provide the basis for using validated iKD mice in predicting responses to therapeutic interventions.

From Plk1 to Plk5: functional evolution of polo-like kinases

Mammalian polo-like kinases (Plks) are characterized by the presence of an N-terminal protein kinase domain and a C-terminal polo-box domain (PBD) involved in substrate binding and regulation of kinase activity. Plk1-4 have traditionally been linked to cell cycle progression, genotoxic stress and, more recently, neuron biology. Recently, a fifth mammalian Plk family member, Plk5, has been characterized in murine and human cells. Plk5 is expressed mainly in differentiated tissues such as the cerebellum. Despite apparent loss of catalytic activity and a stop codon in the middle of the human gene, Plk5 proteins retain important functions in neuron biology. Notably, its expression is silenced by epigenetic alterations in brain tumors, such as glioblastomas, and its re-expression prevents cell proliferation of these tumor cells. In this review, we will focus on the non-cell cycle roles of Plks, the biology of the new member of the family and the possible kinase- and PBD-independent functions of polo-like kinases.

Plk1 controls the Nek2A-PP1γ antagonism in centrosome disjunction

In human cells, separation of the centrosomes and formation of a bipolar spindle are essential for correct chromosome segregation [1]. During interphase, centrosomes are joined together by the linker proteins C-Nap1 and rootletin [2-4]. At the onset of mitosis, these linker proteins are phosphorylated and displaced from centrosomes by the Nek2A kinase, which is regulated by two Hippo pathway components, Mst2 kinase and the scaffold protein hSav1. The kinesin-5 motor protein Eg5 promotes centrosome separation in a parallel pathway to Nek2A [5]. Here, we report that Polo-like kinase 1 (Plk1) functions upstream of the Mst2-Nek2A kinase module in centrosome disjunction as well as being important for Eg5 localization at centrosomes. Plk1 regulates Mst2-Nek2A-induced centrosome disjunction by phosphorylating Mst2. The absence of Plk1 phosphorylation of Mst2 promotes assembly of Nek2A-PP1γ-Mst2 complexes, in which PP1γ counteracts Nek2A kinase activity. In contrast, Plk1 phosphorylation of Mst2 prevents PP1γ binding to Mst2-Nek2A, allowing Nek2A activity to promote centrosome disjunction. We propose that centrosome disjunction is regulated by Plk1, providing a well-balanced control between the counteracting Nek2A and PP1γ activities on the centrosome linker.

Current progress of siRNA/shRNA therapeutics in clinical trials

Through a mechanism known as RNA interference (RNAi), small interfering RNA (siRNA) molecules can target complementary mRNA strands for degradation, thus specifically inhibiting gene expression. The ability of siRNAs to inhibit gene expression offers a mechanism that can be exploited for novel therapeutics. Indeed, over the past decade, at least 21 siRNA therapeutics have been developed for more than a dozen diseases, including various cancers, viruses, and genetic disorders. Like other biological drugs, RNAi-based therapeutics often require a delivery vehicle to transport them to the targeted cells. Thus, the clinical advancement of numerous siRNA drugs has relied on the development of siRNA carriers, including biodegradable nanoparticles, lipids, bacteria, and attenuated viruses. Most therapies permit systemic delivery of the siRNA drug, while others use ex vivo delivery by autologous cell therapy. Advancements in bioengineering and nanotechnology have led to improved control of delivery and release of some siRNA therapeutics. Likewise, progress in molecular biology has allowed for improved design of the siRNA molecules. Here, we provide an overview of siRNA therapeutics in clinical trials, including their clinical progress, the challenges they have encountered, and the future they hold in the treatment of human diseases.

Spatial exclusivity combined with positive and negative selection of phosphorylation motifs is the basis for context-dependent mitotic signaling

The timing and localization of events during mitosis are controlled by the regulated phosphorylation of proteins by the mitotic kinases, which include Aurora A, Aurora B, Nek2 (never in mitosis kinase 2), Plk1 (Polo-like kinase 1), and the cyclin-dependent kinase complex Cdk1/cyclin B. Although mitotic kinases can have overlapping subcellular localizations, each kinase appears to phosphorylate its substrates on distinct sites. To gain insight into the relative importance of local sequence context in kinase selectivity, identify previously unknown substrates of these five mitotic kinases, and explore potential mechanisms for substrate discrimination, we determined the optimal substrate motifs of these major mitotic kinases by positional scanning oriented peptide library screening (PS-OPLS). We verified individual motifs with in vitro peptide kinetic studies and used structural modeling to rationalize the kinase-specific selection of key motif-determining residues at the molecular level. Cross comparisons among the phosphorylation site selectivity motifs of these kinases revealed an evolutionarily conserved mutual exclusion mechanism in which the positively and negatively selected portions of the phosphorylation motifs of mitotic kinases, together with their subcellular localizations, result in proper substrate targeting in a coordinated manner during mitosis.

Study of the docking-dependent PLK1 phosphorylation of the CDC25B phosphatase

CDC25 (A, B and C) phosphatases control cell cycle progression through the timely dephosphorylation and activation of cyclin-dependent kinases (CDK). At mitosis the CDC25B phosphatase activity is dependent on its phosphorylation by multiple kinases impinging on its localisation, stability and catalytic activity. Here we report that prior phosphorylation of CDC25B by CDK1 enhances its substrate properties for PLK1 in vitro, and we also show that phosphorylated S50 serves as a docking site for PLK1. Using a sophisticated strategy based on the sequential phosphorylation of CDC25B with (16)O and (18)O ATP prior to nanoLC-MS/MS analysis we identified 13 sites phosphorylated by PLK1. This study illustrates the complexity of the phosphorylation pattern and of the subsequent regulation of CDC25B activity.

Nek9 is a Plk1-activated kinase that controls early centrosome separation through Nek6/7 and Eg5

The NIMA-family kinases Nek9/Nercc1, Nek6 and Nek7 form a signalling module required for mitotic spindle assembly. Nek9, the upstream kinase, is activated during prophase at centrosomes although the details of this have remained elusive. We now identify Plk1 as Nek9 direct activator and propose a two-step activation mechanism that involves Nek9 sequential phosphorylation by CDK1 and Plk1. Furthermore, we show that Plk1 controls prophase centrosome separation through the activation of Nek9 and ultimately the phosphorylation of the mitotic kinesin Eg5 at Ser1033, a Nek6/7 site that together with the CDK1 site Thr926 we establish contributes to the accumulation of Eg5 at centrosomes and is necessary for subsequent centrosome separation and timely mitosis. Our results provide a basis to understand signalling downstream of Plk1 and shed light on the role of Eg5, Plk1 and the NIMA-family kinases in the control of centrosome separation and normal mitotic progression.

Mammalian polo-like kinase 1-dependent regulation of the PBIP1-CENP-Q complex at kinetochores

Mammalian polo-like kinase 1 (Plk1) plays a pivotal role during M-phase progression. Plk1 localizes to specific subcellular structures through the targeting activity of the C-terminal polo-box domain (PBD). Disruption of the PBD function results in improper bipolar spindle formation, chromosome missegregation, and cytokinesis defect that ultimately lead to the generation of aneuploidy. It has been shown that Plk1 recruits itself to centromeres by phosphorylating and binding to a centromere scaffold, PBIP1 (also called MLF1IP and CENP-U[50]) through its PBD. However, how PBIP1 itself is targeted to centromeres and what roles it plays in the regulation of Plk1-dependent mitotic events remain unknown. Here, we demonstrated that PBIP1 directly interacts with CENP-Q, and this interaction was mutually required not only for their stability but also for their centromere localization. Plk1 did not appear to interact with CENP-Q directly. However, Plk1 formed a ternary complex with PBIP1 and CENP-Q through a self-generated p-T78 motif on PBIP1. This complex formation was central for Plk1-dependent phosphorylation of PBIP1-bound CENP-Q and delocalization of the PBIP1-CENP-Q complex from mitotic centromeres. This study reveals a unique mechanism of how PBIP1 mediates Plk1-dependent phosphorylation event onto a third protein, and provides new insights into the mechanism of how Plk1 and its recruitment scaffold, PBIP1-CENP-Q complex, are localized to and delocalized from centromeres.

Differential control of Eg5-dependent centrosome separation by Plk1 and Cdk1

Cyclin-dependent kinase 1 (Cdk1) is thought to trigger centrosome separation in late G2 phase by phosphorylating the motor protein Eg5 at Thr927. However, the precise control mechanism of centrosome separation remains to be understood. Here, we report that in G2 phase polo-like kinase 1 (Plk1) can trigger centrosome separation independently of Cdk1. We find that Plk1 is required for both C-Nap1 displacement and for Eg5 localization on the centrosome. Moreover, Cdk2 compensates for Cdk1, and phosphorylates Eg5 at Thr927. Nevertheless, Plk1-driven centrosome separation is slow and staggering, while Cdk1 triggers fast movement of the centrosomes. We find that actin-dependent Eg5-opposing forces slow down separation in G2 phase. Strikingly, actin depolymerization, as well as destabilization of interphase microtubules (MTs), is sufficient to remove this obstruction and to speed up Plk1-dependent separation. Conversely, MT stabilization in mitosis slows down Cdk1-dependent centrosome movement. Our findings implicate the modulation of MT stability in G2 and M phase as a regulatory element in the control of centrosome separation.

Continuous polo-like kinase 1 activity regulates diffusion to maintain centrosome self-organization during mitosis

Whether mitotic structures like the centrosome can self-organize from the regulated mobility of their dynamic protein components remains unclear. Here, we combine fluorescence spectroscopy and chemical genetics to study in living cells the diffusion of polo-like kinase 1 (PLK1), an enzyme critical for centrosome maturation at the onset of mitosis. The cytoplasmic diffusion of a functional EGFP-PLK1 fusion correlates inversely with known changes in its enzymatic activity during the cell cycle. Specific EGFP-PLK1 inhibition using chemical genetics enhances mobility, as do point mutations inactivating the polo-box or kinase domains responsible for substrate recognition and catalysis. Spatial mapping of EGFP-PLK1 diffusion across living cells, using raster image correlation spectroscopy and line scanning, detects regions of low mobility in centrosomes. These regions exhibit characteristics of increased transient recursive EGFP-PLK1 binding, distinct from the diffusion of stable EGFP-PLK1-containing complexes in the cytoplasm. Chemical genetic suppression of mitotic EGFP-PLK1 activity, even after centrosome maturation, causes defects in centrosome structure, which recover when activity is restored. Our findings imply that continuous PLK1 activity during mitosis maintains centrosome self-organization by a mechanism dependent on its reaction and diffusion, suggesting a model for the formation of stable mitotic structures using dynamic protein kinases.

Polo-like kinases and DNA damage checkpoint: beyond the traditional mitotic functions

Polo-like kinases (Plks) are a family of serine-threonine kinases that play a pivotal role in cell cycle progression and in cellular response to DNA damage. The Plks are highly conserved from yeast to mammals. There are five Plk family members (Plk1-5) in humans, of which Plk1, is the best characterized. The Plk1 isoform is being aggressively pursued as a target for cancer therapy, following observations that this protein is overexpressed in human tumors and is actively involved in malignant transformation. The roles of Plks in mitotic entry, spindle pole functions and cytokinesis are well established and have been the subject of several recent reviews. In this review, we discuss functions of Plks other than their classical roles in mitotic progression. When cells incur DNA damage, they activate checkpoint mechanisms that result in cell cycle arrest and allow time for repair. If the damage is extensive and cannot be repaired, cells will undergo cell death by apoptosis. If the damage is repaired, cells can resume cycling, as part of the process known as checkpoint recovery. If the damage is not repaired or incompletely repaired, cells can override the checkpoint and resume cycling with damaged DNA, a process called checkpoint adaptation. The Plks play a role in all three outcomes and their involvement in these processes will be the subject of this review.

The role of centrosomal Nlp in the control of mitotic progression and tumourigenesis

The human centrosomal ninein-like protein (Nlp) is a new member of the γ-tubulin complexes binding proteins (GTBPs) that is essential for proper execution of various mitotic events. The primary function of Nlp is to promote microtubule nucleation that contributes to centrosome maturation, spindle formation and chromosome segregation. Its subcellular localisation and protein stability are regulated by several crucial mitotic kinases, such as Plk1, Nek2, Cdc2 and Aurora B. Several lines of evidence have linked Nlp to human cancer. Deregulation of Nlp in cell models results in aberrant spindle, chromosomal missegregation and multinulei, and induces chromosomal instability and renders cells tumourigenic. Overexpression of Nlp induces anchorage-independent growth and immortalised primary cell transformation. In addition, we first demonstrate that the expression of Nlp is elevated primarily due to NLP gene amplification in human breast cancer and lung carcinoma. Consistently, transgenic mice overexpressing Nlp display spontaneous tumours in breast, ovary and testicle, and show rapid onset of radiation-induced lymphoma, indicating that Nlp is involved in tumourigenesis. This review summarises our current knowledge of physiological roles of Nlp, with an emphasis on its potentials in tumourigenesis.

Polo-like kinase 1 (Plk1) in cutaneous T-cell lymphoma

BACKGROUND

Polo-like kinase 1 (Plk1) has multiple functions throughout mitosis. Plk1 levels are high in a number of cancers and haematological malignancies while being low in most differentiated tissues.

OBJECTIVES

To assess the immunoreactivity of Plk1 in cutaneous T-cell lymphoma (CTCL) as a potential therapeutic target, to differentiate Plk1 levels among lesion types and to compare the detection level of Plk1 in fresh frozen (f) vs. paraffin-embedded (p) tissue.

METHODS

Immunohistochemical staining of CTCL skin lesions with anti-Plk1 antibody was performed in a total of 65 biopsies from 49 patients with CTCL. Both f and p tissue was available for comparison in 46 biopsies.

RESULTS

Tumour-stage CTCL lesions displayed significantly more Plk1 (mean f 7·7%, p 8·8%) than patch (mean f 0·7%, p 2·0%) and plaque-stage lesions (mean f 1·1%, p 2·0%) (P < 0·05). Plk1 ranged from 0% to 18% in f and 0% to 24% in p samples. p tissue revealed a higher mean Plk1 detection rate of 4·4% compared with 2·9% in f tissue with no statistical significance.

CONCLUSIONS

Our results indicate that in CTCL, Plk1 is increased mainly in advanced lesions. Several Plk1 inhibitors have already shown promising results in preclinical and clinical phase I and II trials for different types of cancers with low adverse effects. Immunohistochemical detection of high Plk1 levels in patients with CTCL could help select individuals who might benefit from treatment with small molecule Plk1 inhibitors.

Absence of polo-like kinase 3 in mice stabilizes Cdc25A after DNA damage but is not sufficient to produce tumors

The polo-like kinases (Plks1-5) are emerging as an important class of proteins involved in many facets of cell cycle regulation and response to DNA damage and stress. Here we show that Plk3 phosphorylates the key cell cycle protein phosphatase Cdc25A on two serine residues in its cyclinB/cdk1 docking domain and regulates its stability in response to DNA damage. We generated a Plk3 knock-out mouse and show that Cdc25A protein from Plk3-deficient cells is less susceptible to DNA damage-mediated degradation than cells with functional Plk3. We also show that absence of Plk3 correlates with loss of the G1/S cell cycle checkpoint. However, neither this compromised DNA damage checkpoint nor reduced susceptibility to proteasome-mediated degradation after DNA damage translated into a significant increase in tumor incidence in the Plk3-deficient mice.

The Stil protein regulates centrosome integrity and mitosis through suppression of Chfr

Stil (Sil, SCL/TAL1 interrupting locus) is a cytosolic and centrosomal protein expressed in proliferating cells that is required for mouse and zebrafish neural development and is mutated in familial microcephaly. Recently the Drosophila melanogaster ortholog of Stil was found to be important for centriole duplication. Consistent with this finding, we report here that mouse embryonic fibroblasts lacking Stil are characterized by slow growth, low mitotic index and absence of clear centrosomes. We hypothesized that Stil regulates mitosis through the tumor suppressor Chfr, an E3 ligase that blocks mitotic entry in response to mitotic stress. Mouse fibroblasts lacking Stil by genomic or RNA interference approaches, as well as E9.5 Stil(-/-) embryos, express high levels of the Chfr protein and reduced levels of the Chfr substrate Plk1. Exogenous expression of Stil, knockdown of Chfr or overexpression of Plk1 reverse the abnormal mitotic phenotypes of fibroblasts lacking Stil. We further demonstrate that Stil increases Chfr auto-ubiquitination and reduces its protein stability. Thus, Stil is required for centrosome organization, entry into mitosis and cell proliferation, and these functions are at least partially mediated by Chfr and its targets. This is the first identification of a negative regulator of the Chfr mitotic checkpoint.

WNK1 is required for mitosis and abscission

WNK [with no lysine (K)] protein kinases are found in all sequenced multicellular and many unicellular organisms. WNKs influence ion balance. Two WNK family members are associated with a single gene form of hypertension. RNA interference screens have implicated WNKs in survival and growth, and WNK1 is essential for viability of mice. We found that the majority of WNK1 is localized on cytoplasmic puncta in resting cells. During cell division, WNK1 localizes to mitotic spindles. Therefore, we analyzed mitotic phenotypes in WNK1 knockdown cells. A large percentage of WNK1 knockdown cells fail to complete cell division, displaying defects in mitotic spindles and also in abscission and cell survival. One of the best-characterized WNK1 targets is the protein kinase OSR1 (oxidative stress responsive 1). OSR1 regulates ion cotransporters, is activated in response to osmotic stress by WNK family members, and is largely associated with WNK1. In resting cells, the majority of OSR1, like WNK1, is on cytoplasmic puncta. OSR1 is also in nuclei. In contrast to WNK1, however, OSR1 does not concentrate around spindles during mitosis and does not show a WNK1-like localization pattern in mitotic cells. Knockdown of OSR1 has only a modest effect on cell survival and does not lead to spindle defects. We conclude that decreased cell survival associated with loss of WNK1 is attributable to defects in chromosome segregation and abscission and is independent of the effector kinase OSR1.

The Plk1-dependent phosphoproteome of the early mitotic spindle

Polo-like kinases regulate many aspects of mitotic and meiotic progression from yeast to man. In early mitosis, mammalian Polo-like kinase 1 (Plk1) controls centrosome maturation, spindle assembly, and microtubule attachment to kinetochores. However, despite the essential and diverse functions of Plk1, the full range of Plk1 substrates remains to be explored. To investigate the Plk1-dependent phosphoproteome of the human mitotic spindle, we combined stable isotope labeling by amino acids in cell culture with Plk1 inactivation or depletion followed by spindle isolation and mass spectrometry. Our study identified 358 unique Plk1-dependent phosphorylation sites on spindle proteins, including novel substrates, illustrating the complexity of the Plk1-dependent signaling network. Over 100 sites were validated by in vitro phosphorylation of peptide arrays, resulting in a broadening of the Plk1 consensus motif. Collectively, our data provide a rich source of information on Plk1-dependent phosphorylation, Plk1 docking to substrates, the influence of phosphorylation on protein localization, and the functional interaction between Plk1 and Aurora A on the early mitotic spindle.

Protein phosphatase 6 regulates mitotic spindle formation by controlling the T-loop phosphorylation state of Aurora A bound to its activator TPX2

Many protein kinases are activated by a conserved regulatory step involving T-loop phosphorylation. Although there is considerable focus on kinase activator proteins, the importance of specific T-loop phosphatases reversing kinase activation has been underappreciated. We find that the protein phosphatase 6 (PP6) holoenzyme is the major T-loop phosphatase for Aurora A, an essential mitotic kinase. Loss of PP6 function by depletion of catalytic or regulatory subunits interferes with spindle formation and chromosome alignment because of increased Aurora A activity. Aurora A T-loop phosphorylation and the stability of the Aurora A-TPX2 complex are increased in cells depleted of PP6 but not other phosphatases. Furthermore, purified PP6 acts as a T-loop phosphatase for Aurora A-TPX2 complexes in vitro, whereas catalytically inactive mutants cannot dephosphorylate Aurora A or rescue the PPP6C depletion phenotype. These results demonstrate a hitherto unappreciated role for PP6 as the T-loop phosphatase regulating Aurora A activity during spindle formation and suggest the general importance of this form of regulation.

Female infertility in PDE3A(-/-) mice: polo-like kinase 1 (Plk1) may be a target of protein kinase A (PKA) and involved in meiotic arrest of oocytes from PDE3A(-/-) mice

Mechanisms of cAMP/PKA-induced meiotic arrest in oocytes are not completely identified. In cultured, G2/M-arrested PDE3A(-/-) murine oocytes, elevated PKA activity was associated with inactivation of Cdc2 and Plk1, and inhibition of phosphorylation of histone H3 (S10) and of dephosphorylation of Cdc25B (S323) and Cdc2 (Thr14/Tyr15). In cultured WT oocytes, PKA activity was transiently reduced and then increased to that observed in PDE3A(-/-) oocytes; Cdc2 and Plk1 were activated, phosphorylation of histone H3 (S10) and dephosphorylation of Cdc25B (S323) and Cdc2 (Thr14/Tyr15) were observed. In WT oocytes, PKAc were rapidly translocated into nucleus, and then to the spindle apparatus, but in PDE3A(-/-) oocytes, PKAc remained in the cytosol. Plk1 was reactivated by incubation of PDE3A(-/-) oocytes with PKA inhibitor, Rp-cAMPS. PDE3A was co-localized with Plk1 in WT oocytes, and co-immunoprecipitated with Plk1 in WT ovary and Hela cells. PKAc phosphorylated rPlk1 and Hela cell Plk1 and inhibited Plk1 activity in vitro. Our results suggest that PKA-induced inhibition of Plk1 may be critical in oocyte meiotic arrest and female infertility in PDE3A(-/-) mice.

Shared and separate functions of polo-like kinases and aurora kinases in cancer

Large numbers of inhibitors for polo-like kinases and aurora kinases are currently being evaluated as anticancer drugs. Interest in these drugs is fuelled by the idea that these kinases have unique functions in mitosis. Within the polo-like kinase family, the emphasis for targeted therapies has been on polo-like kinase 1 (PLK1), and in the aurora kinase family drugs have been developed to specifically target aurora kinase A (AURKA; also known as STK6) and/or aurora kinase B (AURKB; also known as STK12). Information on the selectivity of these compounds in vivo is limited, but it is likely that off-target effects within the same kinase families will affect efficacy and toxicity profiles. In addition, it is becoming clear that interplay between polo-like kinases and aurora kinases is much more extensive than initially anticipated, and that both kinase families are important factors in the response to classical chemotherapeutics that damage the genome or the mitotic spindle. In this Review we discuss the implications of these novel insights on the clinical applicability of polo-like kinase and aurora kinase inhibitors.

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.

Dishevelled, a Wnt signalling component, is involved in mitotic progression in cooperation with Plk1

Wnt signalling is known to promote G1/S progression through the stimulation of gene expression, but whether this signalling regulates mitotic progression is not clear. Here, the function of dishevelled 2 (Dvl2), which transmits the Wnt signal, in mitosis was examined. Dvl2 localized to the spindles and spindle poles during mitosis. When cells were treated with nocodazole, Dvl2 was observed at the kinetochores (KTs). Dvl2 bound to and was phosphorylated at Thr206 by a mitotic kinase, Polo-like kinase 1 (Plk1), and this phosphorylation was required for spindle orientation and stable microtubule (MT)-KT attachment. Dvl2 was also found to be involved in the activation of a spindle assembly checkpoint (SAC) kinase, Mps1, and the recruitment of other SAC components, Bub1 and BubR1, to the KTs. However, the phosphorylation of Dvl2 by Plk1 was dispensable for SAC. Furthermore, Wnt receptors were involved in spindle orientation, but not in MT-KT attachment or SAC. These results suggested that Dvl2 is involved in mitotic progression by regulating the dynamics of MT plus-ends and the SAC in Plk1-dependent and -independent manners.

The Plk1 inhibitor BI 2536 temporarily arrests primary cardiac fibroblasts in mitosis and generates aneuploidy in vitro

BI 2536 is a new anti-mitotic drug that targets polo-like kinase 1 (Plk1) and is currently under clinical development for cancer therapy. The effect of this drug on cancer cells has been extensively investigated, but information about the effects on primary dividing cells and differentiated non-dividing cells is scarce. We have investigated the effects of this drug on primary neonatal rat cardiac fibroblasts and on differentiated cardiomyocytes and explored the possibility to use this drug to enrich differentiated cell populations in vitro. BI 2536 had a profound effect on cardiac fibroblast proliferation in vitro and arrested these cells in mitosis with an IC50 of about 43 nM. Similar results were observed with primary human cells (HUVEC, IC50  =  30 nM), whereas the cancer cell line HeLa was more sensitive (IC50 of 9 nM). Further analysis revealed that prolonged mitotic arrest resulted in cell death for about 40% of cardiac fibroblasts. The remaining cells showed an interphase morphology with mostly multi- and micro-nucleated nuclei. This indicates that a significant number of primary fibroblasts are able to escape BI 2536 induced mitotic arrest and apparently become aneuploid. No effects were observed on cardiomyocytes and hypertrophic response (growth) upon endothelin-1 and phenylephrine stimulation was normal in the presence of BI 2536. This indicates that BI 2536 has no adverse effects on terminally differentiated cells and still allows proliferation independent growth induction in these cells. In conclusion, cardiomyocytes could be enriched using BI 2536, but the formation of aneuploidy in proliferating cells most likely limits this in vitro application and does not allow its use in putative cell based therapies.

Inhibition of Plk1 induces mitotic infidelity and embryonic growth defects in developing zebrafish embryos

Polo-like kinase 1 (Plk1) is central to cell division. Here, we report that Plk1 is critical for mitosis in the embryonic development of zebrafish. Using a combination of several cell biology tools, including single-cell live imaging applied to whole embryos, we show that Plk1 is essential for progression into mitosis during embryonic development. Plk1 morphant cells displayed mitotic infidelity, such as abnormal centrosomes, irregular spindle assembly, hypercondensed chromosomes, and a failure of chromosome arm separation. Consequently, depletion of Plk1 resulted in mitotic arrest and finally death by 6days post-fertilization. In comparison, Plk2 or Plk3 morphant embryos did not display any significant abnormalities. Treatment of embryos with the Plk1 inhibitor, BI 2536, caused a block in mitosis, which was more severe when used to treat plk1 morphants. Finally, using an assay to rescue the Plk1 morphant phenotype, we found that the kinase domain and PBD domains are both necessary for Plk1 function in zebrafish development. Our studies demonstrate that Plk1 is required for embryonic proliferation because its activity is crucial for mitotic integrity. Furthermore, our study suggests that zebrafish will be an efficient and economical in vivo system for the validation of anti-mitotic drugs.

Centrobin/NIP2 is a microtubule stabilizer whose activity is enhanced by PLK1 phosphorylation during mitosis

Centrobin/NIP2 is a centrosomal protein that is required for centrosome duplication. It is also critical for microtubule organization in both interphase and mitotic cells. In the present study, we observed that centrobin is phosphorylated in a cell cycle stage-specific manner, reaching its maximum at M phase. PLK1 is a kinase that is responsible for M phase-specific phosphorylation of centrobin. The microtubule forming activity of centrobin was enhanced by PLK1 phosphorylation. Furthermore, mitotic spindles were not assembled properly with the phospho-resistant mutant of centrobin. Based on these results, we propose that centrobin functions as a microtubule stabilizing factor and PLK1 enhances centrobin activity for proper spindle formation during mitosis.

Polo-like kinase 1 as predictive marker and therapeutic target for radiotherapy in rectal cancer

The ability to predict tumor sensitivity toward radiotherapy may significantly impact the selection of patients for preoperative combined-modality therapy. The aim of the present study was to test the predictive value of Polo-like kinase 1 (PLK1) in rectal cancer patients and to investigate whether PLK1 plays a direct role in mediating radiation sensitivity. PLK1 expression was evaluated by immunohistochemistry (n = 76) or Affymetrix HG133 microarray (n = 20) on pretreatment biopsies of patients with advanced rectal cancer. Expression was correlated with both tumor regression in the resected specimen and long-term clinical outcome. Furthermore, we used small interfering RNAs (siRNAs) to down-regulate PLK1 expression in colorectal cancer cells and analyzed the effects of PLK1-specific siRNAs by Western blot and quantitative real-time PCR analysis, FACScan analysis, caspase 3/7 assays, and colony-forming assays. We observed that increased PLK1 protein expression was significantly related to a poorer tumor regression and a higher risk of local recurrence in uni- and multivariate analysis. A significant decrease of PLK1 expression by siRNAs in combination with ionizing radiation induced an increased percentage of apoptotic cells and increased caspase 3/7 activity. Furthermore, enhanced G(2)-M levels, decreased cellular viability, and reduced clonogenic survival were demonstrated, indicating a radiosensitizing effect of PLK1 depletion. Therefore, PLK1 may be a novel predictive marker for radiation response as well as a promising therapeutic target in rectal cancer patients.

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.

Lentiviral shRNA screen of human kinases identifies PLK1 as a potential therapeutic target for osteosarcoma

We describe an optimized systematic screen of known kinases using osteosarcoma cell lines (KHOS and U-2OS) and a lentiviral-based short hairpin RNA (shRNA) human kinase library. CellTiter 96(R)AQueous One Solution Cell Proliferation Assay was used to measure cell growth and survival. We identified several kinases, including human polo-like kinase (PLK1), which inhibit cell growth and induce apoptosis in osteosarcoma cells when knocked down. cDNA rescue and synthetic siRNA assays confirm that the observed phenotypic changes result from the loss of PLK1 gene expression. Furthermore, a small molecule inhibitor to PLK1 inhibited osteosarcoma cell growth and induced apoptosis. Western blot analysis confirmed that PLK1 is highly expressed and activated in several osteosarcoma cell lines as well as in resected tumor samples. Immunohistochemistry analysis showed that patients with high PLK1 tumor expression levels correlated with significantly shorter survival than patients with lower levels of tumor PLK1 expression. These results demonstrate the capability and feasibility of a high-throughput screen with a large collection of lentiviral kinases and its effectiveness in identifying potential drug targets. The development of more potent inhibitors that target PLK1 may open doors to a new range of anti-cancer strategies in osteosarcoma.