Cooperative phosphorylation including the activity of polo-like kinase 1 regulates the subcellular localization of cyclin B1

Cyclers' kinases in cell division: from molecules to cancer therapy

Faithful eucaryotic cell division requires spatio-temporal orchestration of multiple sequential events. To ensure the dynamic nature of these molecular and morphological transitions, a swift modulation of key regulatory pathways is necessary. The molecular process that most certainly fits this description is phosphorylation, the post-translational modification provided by kinases, that is crucial to allowing the progression of the cell cycle and that culminates with the separation of two identical daughter cells. In detail, from the early stages of the interphase to the cytokinesis, each critical step of this process is tightly regulated by multiple families of kinases including the Cyclin-dependent kinases (CDKs), kinases of the Aurora, Polo, Wee1 families, and many others. While cell-cycle-related CDKs control the timing of the different phases, preventing replication machinery errors, the latter modulate the centrosome cycle and the spindle function, avoiding karyotypic abnormalities typical of chromosome instability. Such chromosomal abnormalities may result from replication stress (RS) and chromosome mis-segregation and are considered a hallmark of poor prognosis, therapeutic resistance, and metastasis in cancer patients. Here, we discuss recent advances in the understanding of how different families of kinases concur to govern cell cycle, preventing RS and mitotic infidelity. Additionally, considering the growing number of clinical trials targeting these molecules, we review to what extent and in which tumor context cell-cycle-related kinases inhibitors are worth exploiting as an effective therapeutic strategy.

CDC6 as a Key Inhibitory Regulator of CDK1 Activation Dynamics and the Timing of Mitotic Entry and Progression

Timely mitosis is critically important for early embryo development. It is regulated by the activity of the conserved protein kinase CDK1. The dynamics of CDK1 activation must be precisely controlled to assure physiologic and timely entry into mitosis. Recently, a known S-phase regulator CDC6 emerged as a key player in mitotic CDK1 activation cascade in early embryonic divisions, operating together with Xic1 as a CDK1 inhibitor upstream of the Aurora A and PLK1, both CDK1 activators. Herein, we review the molecular mechanisms that underlie the control of mitotic timing, with special emphasis on how CDC6/Xic1 function impacts CDK1 regulatory network in the Xenopus system. We focus on the presence of two independent mechanisms inhibiting the dynamics of CDK1 activation, namely Wee1/Myt1- and CDC6/Xic1-dependent, and how they cooperate with CDK1-activating mechanisms. As a result, we propose a comprehensive model integrating CDC6/Xic1-dependent inhibition into the CDK1-activation cascade. The physiological dynamics of CDK1 activation appear to be controlled by the system of multiple inhibitors and activators, and their integrated modulation ensures concomitantly both the robustness and certain flexibility of the control of this process. Identification of multiple activators and inhibitors of CDK1 upon M-phase entry allows for a better understanding of why cells divide at a specific time and how the pathways involved in the timely regulation of cell division are all integrated to precisely tune the control of mitotic events.

A Review of the Regulatory Mechanisms of N-Myc on Cell Cycle

Neuroblastoma has obvious heterogeneity. It is one of the few undifferentiated malignant tumors that can spontaneously degenerate into completely benign tumors. However, for its high-risk type, even with various intensive treatment options, the prognosis is still unsatisfactory. At the same time, a large number of research data show that the abnormal amplification and high-level expression of the MYCN gene are positively correlated with the malignant progression, poor prognosis, and mortality of neuroblastoma. In this context, this article explores the role of the N-Myc, MYCN gene expression product on its target genes related to the cell cycle and reveals its regulatory network in promoting tumor proliferation and malignant progression. We hope it can provide ideas and direction for the research and development of drugs targeting N-Myc and its downstream target genes.

Virtual screening of natural compounds as selective inhibitors of polo-like kinase-1 at C-terminal polo box and N-terminal catalytic domain

The over-expression of Polo-like kinase-1 (PLK1) is associated with cancer prognosis due to its pivotal role in cell proliferation. The N-terminal catalytic domain (NCD) and C-terminal polo box domain (PBD) of PLK1 are critical for the activity of the protein. Drugs that inhibit PLK1 by targeting these domains are on clinical trials, but so far, none has been approved by FDA. Thus, this study targets the two domains of PLK1 to identify compounds with inhibitory potential. Four validated e-pharmacophore models from NCD (PDB ID: 2OU7 and 4J52) and PBD (PDB ID: 5NEI and 5NN2) were used to screen over 26,000 natural compounds from NPASS database. Hits were identified after the well-fitted compounds were subjected to molecular docking study and ADME prediction. The pIC50 and electronic behaviour of the identified hits selectively targeting NCD and PBD of PLK1 were predicted via an externally validated QSAR model and quantum mechanics. The results showed that CAA180504, CAA197326, CAA74619, CAA328856 modulating PLK1 at NCD, and CBB130581, CBB230713, CBB206123, CBB12656 and CBB267117 modulating PLK1 at PBD had better molecular docking scores, pharmacokinetics and drug-like properties than NCD (volasertib) and PBD (purpurogallin) reference inhibitors. The compounds all had satisfactory inhibitory (pIC50) values which range from 6.187 to 7.157. The electronic behaviours of understudied compounds using HOMO/LUMO and global descriptive parameters revealed the atomic portion of the compounds prone to donating and accepting electrons. In conclusion, the hit compounds identified from the library of natural compounds are worthy of further experimental validation.Communicated by Ramaswamy H. Sarma.

Piperine Increases Pentagamavunon-1 Anti-cancer Activity on 4T1 Breast Cancer Through Mitotic Catastrophe Mechanism and Senescence with Sharing Targeting on Mitotic Regulatory Proteins

Pentagamavunon-1 performs more potent anti-cancer effects than curcumin against various cancer cells, but it remains to be optimized. Piperine shows the activity as an enhancer of a therapeutic agent. This study expects to achieve higher effectiveness of PGV-1 on 4T1 breast cancer cells through co-treatment with piperine with exploring the effect of cytotoxicity, mitotic catastrophe, cellular senescence, and target proteins of PGV-1 and piperine on the regulation of mitosis in TNBC cells (4T1). The assays emphasize MTT assay, May Grünwald-Giemsa staining, SA-β-galactosidase assay, and bioinformatics analysis, respectively, to elicit the respected activities. The results revealed that PGV-1 performed a cytotoxic effect with an IC50 value of 9 µM while piperine showed a lower cytotoxic effect with an IC50 value of 800 µM on 4T1 cells 24 h treatment. However, the combination treatment of both showed a synergistic cytotoxic enhancement effect with an average CI value < 1. Furthermore, the combination of PGV-1 and piperine induced mitotic catastrophe and senescence better than the single treatment. Treatment of 1 µM of PGV-1 and 400 µM of piperine increased the percentage of senescent cells by 33%. Bioinformatics analysis revealed that PGV-1 and piperine target proteins play a role in mitotic regulation, namely CDK1, KIF11, AURKA, AURKB, and PLK1, to contribute to mitotic catastrophe. Therefore, piperine increases the effectiveness of PGV-1 to suppress 4T1 cells growth synergistically that may occur through mitotic catastrophe and senescence targeting on mitotic regulatory proteins.

PLK1-dependent phosphorylation restrains EBNA2 activity and lymphomagenesis in EBV-infected mice

While Epstein-Barr virus (EBV) establishes a life-long latent infection in apparently healthy human immunocompetent hosts, immunodeficient individuals are at particular risk to develop lymphoproliferative B-cell malignancies caused by EBV. A key EBV protein is the transcription factor EBV nuclear antigen 2 (EBNA2), which initiates B-cell proliferation. Here, we combine biochemical, cellular, and in vivo experiments demonstrating that the mitotic polo-like kinase 1 (PLK1) binds to EBNA2, phosphorylates its transactivation domain, and thereby inhibits its biological activity. EBNA2 mutants that impair PLK1 binding or prevent EBNA2 phosphorylation are gain-of-function mutants. They exhibit enhanced transactivation capacities, accelerate the proliferation of infected B cells, and promote the development of monoclonal B-cell lymphomas in infected mice. Thus, PLK1 coordinates the activity of EBNA2 to attenuate the risk of tumor incidences in favor of the establishment of latency in the infected but healthy host.

Transmembrane topology and oligomeric nature of an astrocytic membrane protein, MLC1

MLC1 is a membrane protein mainly expressed in astrocytes, and genetic mutations lead to the development of a leukodystrophy, megalencephalic leukoencephalopathy with subcortical cysts disease. Currently, the biochemical properties of the MLC1 protein are largely unknown. In this study, we aimed to characterize the transmembrane (TM) topology and oligomeric nature of the MLC1 protein. Systematic immunofluorescence staining data revealed that the MLC1 protein has eight TM domains and that both the N- and C-terminus face the cytoplasm. We found that MLC1 can be purified as an oligomer and could form a trimeric complex in both detergent micelles and reconstituted proteoliposomes. Additionally, a single-molecule photobleaching experiment showed that MLC1 protein complexes could consist of three MLC1 monomers in the reconstituted proteoliposomes. These results can provide a basis for both the high-resolution structural determination and functional characterization of the MLC1 protein.

Polo-like kinase 1 (PLK1) signaling in cancer and beyond

PLK1 is an evolutionary conserved Ser/Thr kinase that is best known for its role in cell cycle regulation and is expressed predominantly during the G2/S and M phase of the cell cycle. PLK1-mediated phosphorylation of specific substrates controls cell entry into mitosis, centrosome maturation, spindle assembly, sister chromatid cohesion and cytokinesis. In addition, a growing body of evidence describes additional roles of PLK1 beyond the cell cycle, more specifically in the DNA damage response, autophagy, apoptosis and cytokine signaling. PLK1 has an indisputable role in cancer as it controls several key transcription factors and promotes cell proliferation, transformation and epithelial-to-mesenchymal transition. Furthermore, deregulation of PLK1 results in chromosome instability and aneuploidy. PLK1 is overexpressed in many cancers, which is associated with poor prognosis, making PLK1 an attractive target for cancer treatment. Additionally, PLK1 is involved in immune and neurological disorders including Graft versus Host Disease, Huntington's disease and Alzheimer's disease. Unfortunately, newly developed small compound PLK1 inhibitors have only had limited success so far, due to low therapeutic response rates and toxicity. In this review we will highlight the current knowledge about the established roles of PLK1 in mitosis regulation and beyond. In addition, we will discuss its tumor promoting but also tumor suppressing capacities, as well as the available PLK1 inhibitors, elaborating on their efficacy and limitations.

Potent Synergistic Effect on C-Myc-Driven Colorectal Cancers Using a Novel Indole-Substituted Quinoline with a Plk1 Inhibitor

Developing effective treatments for colorectal cancers through combinations of small-molecule approaches and immunotherapies present intriguing possibilities for managing these otherwise intractable cancers. During a broad-based, screening effort against multiple colorectal cancer cell lines, we identified indole-substituted quinolines (ISQ), such as N7,N7 -dimethyl-3-(1-methyl-1H-indol-3-yl)quinoline-2,7-diamine (ISQ-1), as potent in vitro inhibitors of several cancer cell lines. We found that ISQ-1 inhibited Wnt signaling, a main driver in the pathway governing colorectal cancer development, and ISQ-1 also activated adenosine monophosphate kinase (AMPK), a cellular energy-homeostasis master regulator. We explored the effect of ISQs on cell metabolism. Seahorse assays measuring oxygen consumption rate (OCR) indicated that ISQ-1 inhibited complex I (i.e., NADH ubiquinone oxidoreductase) in the mitochondrial, electron transport chain (ETC). In addition, ISQ-1 treatment showed remarkable synergistic depletion of oncogenic c-Myc protein level in vitro and induced strong tumor remission in vivo when administered together with BI2536, a polo-like kinase-1 (Plk1) inhibitor. These studies point toward the potential value of dual drug therapies targeting the ETC and Plk-1 for the treatment of c-Myc-driven cancers.

The Cytomegalovirus Protein Kinase pUL97:Host Interactions, Regulatory Mechanisms and Antiviral Drug Targeting

Human cytomegalovirus (HCMV) expresses a variety of viral regulatory proteins that undergo close interaction with host factors including viral-cellular multiprotein complexes. The HCMV protein kinase pUL97 represents a viral cyclin-dependent kinase ortholog (vCDK) that determines the efficiency of HCMV replication via phosphorylation of viral and cellular substrates. A hierarchy of functional importance of individual pUL97-mediated phosphorylation events has been discussed; however, the most pronounced pUL97-dependent phenotype could be assigned to viral nuclear egress, as illustrated by deletion of the UL97 gene or pharmacological pUL97 inhibition. Despite earlier data pointing to a cyclin-independent functionality, experimental evidence increasingly emphasized the role of pUL97-cyclin complexes. Consequently, the knowledge about pUL97 involvement in host interaction, viral nuclear egress and additional replicative steps led to the postulation of pUL97 as an antiviral target. Indeed, validation experiments in vitro and in vivo confirmed the sustainability of this approach. Consequently, current investigations of pUL97 in antiviral treatment go beyond the known pUL97-mediated ganciclovir prodrug activation and henceforward include pUL97-specific kinase inhibitors. Among a number of interesting small molecules analyzed in experimental and preclinical stages, maribavir is presently investigated in clinical studies and, in the near future, might represent a first kinase inhibitor applied in the field of antiviral therapy.

Clinicopathological and prognostic implications of polo-like kinase 1 expression in colorectal cancer: A systematic review and meta-analysis

BACKGROUND

Polo-like kinase 1 (PLK1) is a potential prognostic marker in colorectal cancer (CRC). Nevertheless, the clinicopathological and prognostic roles of PLK1 in CRC are still undefined. Therefore, we performed a meta-analysis to investigate the clinicopathological and prognostic relevance of PLK1 expression in CRC patients.

METHODS

Studies published between 2003 and 2016 were selected for the meta-analysis based on an electronic literature search (PubMed, EMBASE and Chinese databases). Studies that investigated the clinicopathological and prognostic impacts of PLK1 expression in CRC patients were included for this analysis.

RESULTS

Eleven studies that enrolled 1147 CRC patients were included in our meta-analysis. The effect of PLK1 level on overall survival (OS) was reported in five studies, which included 702 patients. Ten studies investigated the clinicopathological role of PLK1 expression in CRC patients. Consequently, PLK1 overexpression was associated with poorer OS in CRC patients. Furthermore, the results revealed that higher PLK1 levels were also observed in CRC tissues compared with that of normal colorectal tissues. In addition, this meta-analysis also revealed positive correlations between PLK1 upregulation and lymph node metastasis or invasion. PLK1 overexpression was significantly correlated with advanced TNM stages and higher Dukes stages.

CONCLUSION

This meta-analysis strongly supports the hypothesis that PLK1 might serve as an important factor in evaluating the biological behavior and prognosis of CRC.

Mechanisms for the temporal regulation of substrate ubiquitination by the anaphase-promoting complex/cyclosome

The anaphase-promoting complex/cyclosome (APC/C) is a multi-subunit, multifunctional ubiquitin ligase that controls the temporal degradation of numerous cell cycle regulatory proteins to direct the unidirectional cell cycle phases. Several different mechanisms contribute to ensure the correct order of substrate modification by the APC/C complex. Recent advances in biochemical, biophysical and structural studies of APC/C have provided a deep mechanistic insight into the working of this complex ubiquitin ligase. This complex displays remarkable conformational flexibility in response to various binding partners and post-translational modifications, which together regulate substrate selection and catalysis of APC/C. Apart from this, various features and modifications of the substrates also influence their recognition and affinity to APC/C complex. Ultimately, temporal degradation of substrates depends on the kind of ubiquitin modification received, the processivity of APC/C, and other extrinsic mechanisms. This review discusses our current understanding of various intrinsic and extrinsic mechanisms responsible for ‘substrate ordering’ by the APC/C complex.

Mitotic Exit Dysfunction through the Deregulation of APC/C Characterizes Cisplatin-Resistant State in Epithelial Ovarian Cancer

Purpose: Acquired resistance to cisplatin is a major barrier to success in treatment of various cancers, and understanding mitotic mechanisms unique to cisplatin-resistant cancer cells can provide the basis for developing novel mitotic targeted therapies aimed at eradicating these cells.Experimental Design: Using cisplatin-resistant models derived from primary patient epithelial ovarian cancer (EOC) cells, we have explored the status of mitotic exit mechanisms in cisplatin-resistant cells.Results: We have uncovered an unexpected role of long-term cisplatin treatment in inducing mitotic exit vulnerability characterized by increased spindle checkpoint activity and functional dependency on Polo-like kinase 1 (PLK1) for mitotic exit in the presence of anaphase promoting complex/cyclosome (APC/C) dysfunction in a cisplatin-resistant state. Accordingly, PLK1 inhibition decreased the survival of cisplatin-resistant cells in vitro and in vivo and exacerbated spindle checkpoint response in these cells. APC/CCDC20 inhibition increased sensitivity to pharmacologic PLK1 inhibition, further confirming the existence of APC/C dysfunction in cisplatin-resistant cells. In addition, we uncovered that resistance to volasertib, PLK1 inhibitor, is due to maintenance of cells with low PLK1 expression. Accordingly, stable PLK1 downregulation in cisplatin-resistant cells induced tolerance to volasertib.Conclusions: We provide the first evidence of APC/C dysfunction in cisplatin-resistant state, suggesting that understanding APC/C functions in cisplatin-resistant state could provide a basis for developing novel mitotic exit-based therapies to eradicate cisplatin-resistant cancer cells. Our results also show that PLK1 downregulation could underlie emergence of resistance to PLK1-targeted therapies in cancers. Clin Cancer Res; 24(18); 4588-601. ©2018 AACR.

Reverse the Resistance to PARP Inhibitors

One of the DNA repair machineries is activated by Poly (ADP-ribose) Polymerase (PARP) enzyme. Particularly, this enzyme is involved in repair of damages to single-strand DNA, thus decreasing the chances of generating double-strand breaks in the genome. Therefore, the concept to block PARP enzymes by PARP inhibitor (PARPi) was appreciated in cancer treatment. PARPi has been designed and tested for many years and became a potential supplement for the conventional chemotherapy. However, increasing evidence indicates the appearance of the resistance to this treatment. Specifically, cancer cells may acquire new mutations or events that overcome the positive effect of these drugs. This paper describes several molecular mechanisms of PARPi resistance which were reported most recently, and summarizes some strategies to reverse this type of drug resistance.

Targeting polo-like kinase 1 suppresses essential functions of alloreactive T cells

Acute graft-versus-host disease (aGvHD) is still a major cause of transplant-related mortality after allogeneic stem cell transplantation (ASCT). It requires immunosuppressive treatments that broadly abrogate T cell responses including beneficial ones directed against tumor cells or infective pathogens. Polo-like kinase 1 (PLK1) is overexpressed in many cancer types including leukemia, and clinical studies demonstrated that targeting PLK1 using selective PLK1 inhibitors resulted in inhibition of proliferation and induction of apoptosis predominantly in tumor cells, supporting the feasibility of PLK1 as target for anticancer therapy. Here, we show that activation of alloreactive T cells (Tallo) up-regulate expression of PLK1, suggesting that PLK1 is a potential new candidate for dual therapy of aGvHD and leukemia after ASCT. Inhibition of PLK1, using PLK1-specific inhibitor GSK461364A selectively depletes Tallo by preventing activation and by inducing apoptosis in already activated Tallo, while memory T cells are preserved. Activated Tallo cells which survive exposure to PLK1 undergo inhibition of proliferation by induction of G2/M cell cycle arrest, which is accompanied by accumulation of cell cycle regulator proteins p21(WAF/CIP1), p27(Kip1), p53 and cyclin B1, whereas abundance of CDK4 decreased. We also show that suppressive effects of PLK1 inhibition on Tallo were synergistically enhanced by concomitant inhibition of molecular chaperone Hsp90. Taken together, our data suggest that PLK1 inhibition represents a reasonable dual strategy to suppress residual tumor growth and efficiently deplete Tallo, and thus provide a rationale to selectively prevent and treat aGvHD.

PLK-1: Angel or devil for cell cycle progression

PLK-1 is a key player in the eukaryotic cell cycle. Cell cycle progression is precisely controlled by cell cycle regulatory kinases. PLK-1 is a mitotic kinase that actively regulates the G2/M transition, mitosis, mitotic exit, and cytokinesis. During cell cycle progression, PLK-1 controls various events related to the cell cycle maturation, directly and/or indirectly. On the contrary, aberrant expression of PLK-1 is strongly associated with tumorigenesis and its poor prognosis. The misexpression of PLK-1 causes the abnormalities including aneuploidy, mitotic defects, leading to tumorigenesis through inhibiting the p53 and pRB genes. Therefore, we reviewed the role of PLK-1 in the cell cycle progression and in the tumorigenesis either as a cell cycle regulator or on an attractive anti-cancer drug target.

Alisertib Induces Cell Cycle Arrest, Apoptosis, Autophagy and Suppresses EMT in HT29 and Caco-2 Cells

Colorectal cancer (CRC) is one of the most common malignancies worldwide with substantial mortality and morbidity. Alisertib (ALS) is a selective Aurora kinase A (AURKA) inhibitor with unclear effect and molecular interactome on CRC. This study aimed to evaluate the molecular interactome and anticancer effect of ALS and explore the underlying mechanisms in HT29 and Caco-2 cells. ALS markedly arrested cells in G₂/M phase in both cell lines, accompanied by remarkable alterations in the expression level of key cell cycle regulators. ALS induced apoptosis in HT29 and Caco-2 cells through mitochondrial and death receptor pathways. ALS also induced autophagy in HT29 and Caco-2 cells, with the suppression of phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt)/mammalian target of rapamycin (mTOR), but activation of 5′ AMP-activated protein kinase (AMPK) signaling pathways. There was a differential modulating effect of ALS on p38 MAPK signaling pathway in both cell lines. Moreover, induction or inhibition of autophagy modulated basal and ALS-induced apoptosis in both cell lines. ALS potently suppressed epithelial to mesenchymal transition (EMT) in HT29 and Caco-2 cells. Collectively, it suggests that induction of cell cycle arrest, promotion of apoptosis and autophagy, and suppression of EMT involving mitochondrial, death receptor, PI3K/Akt/mTOR, p38 MAPK, and AMPK signaling pathways contribute to the cancer cell killing effect of ALS on CRC cells.

Indirect p53-dependent transcriptional repression of Survivin, CDC25C, and PLK1 genes requires the cyclin-dependent kinase inhibitor p21/CDKN1A and CDE/CHR promoter sites binding the DREAM complex

The transcription factor p53 is central to cell cycle control by downregulation of cell cycle-promoting genes upon cell stress such as DNA damage. Survivin (BIRC5), CDC25C, and PLK1 encode important cell cycle regulators that are repressed following p53 activation. Here, we provide evidence that p53-dependent repression of these genes requires activation of p21 (CDKN1A, WAF1, CIP1). Chromatin immunoprecipitation (ChIP) data indicate that promoter binding of B-MYB switches to binding of E2F4 and p130 resulting in a replacement of the MMB (Myb-MuvB) by the DREAM complex. We demonstrate that this replacement depends on p21. Furthermore, transcriptional repression by p53 requires intact DREAM binding sites in the target promoters. The CDE and CHR cell cycle promoter elements are the sites for DREAM binding. These elements as well as the p53 response of Survivin, CDC25C, and PLK1 are evolutionarily conserved. No binding of p53 to these genes is detected by ChIP and mutation of proposed p53 binding sites does not alter the p53 response. Thus, a mechanism for direct p53-dependent transcriptional repression is not supported by the data. In contrast, repression by DREAM is consistent with most previous findings and unifies models based on p21-, E2F4-, p130-, and CDE/CHR-dependent repression by p53. In conclusion, the presented data suggest that the p53-p21-DREAM-CDE/CHR pathway regulates p53-dependent repression of Survivin, CDC25C, and PLK1.

Survival of primary, but not of cancer cells after combined Plk1-HDAC inhibition

In the current study we examined the combination of SAHA and SBE13 in cancer and non-cancer cells. HeLa cells displayed a synergistically reduced cell proliferation, which was much weaker in hTERT-RPE1 or NIH-3T3 cells. Cell cycle distribution differed in HeLa, hTERT-RPE1 and NIH-3T3 cells. SAHA-treated HeLa cells showed slightly increasing cell numbers in G2/M phase, but after combination with SBE13 strongly elevated cell numbers in G2/M and S phase, accompanied by decreasing G0/G1 percentages. hTERT-RPE1 and NIH-3T3 cells showed strongly enriched cell numbers in G0/G1 phase. Western blot and quantitative real time analyses revealed reduced Plk1 mRNA and protein in all cells. p21 protein was strongly induced in cancer, but not in non-cancer cells, corresponding to a different localization in immunofluorescence studies. Additionally, these revealed an abundantly present pRb protein in HeLa cells after any treatment but almost completely vanished pRb staining in treated hTERT-RPE1 cells. These differences could be approved in Western blots against Parp and Caspase 3, which were activated in HeLa, but not in hTERT-RPE1 cells. Thus, we observed for the first time a differential effect of cancer versus non-cancer cells after treatment with SAHA and SBE13, which might be due to the dual role of p21.

Regulation of Greatwall kinase by protein stabilization and nuclear localization

Greatwall (Gwl) functions as an essential mitotic kinase by antagonizing protein phosphatase 2A. In this study we identified Hsp90, Cdc37 and members of the importin α and β families as the major binding partners of Gwl. Both Hsp90/Cdc37 chaperone and importin complexes associated with the N-terminal kinase domain of Gwl, whereas an intact glycine-rich loop at the N-terminus of Gwl was essential for binding of Hsp90/Cdc37 but not importins. We found that Hsp90 inhibition led to destabilization of Gwl, a mechanism that may partially contribute to the emerging role of Hsp90 in cell cycle progression and the anti-proliferative potential of Hsp90 inhibition. Moreover, in agreement with its importin association, Gwl exhibited nuclear localization in interphase Xenopus S3 cells, and dynamic nucleocytoplasmic distribution during mitosis. We identified KR456/457 as the locus of importin binding and the functional NLS of Gwl. Mutation of this site resulted in exclusion of Gwl from the nucleus. Finally, we showed that the Gwl nuclear localization is indispensable for the biochemical function of Gwl in promoting mitotic entry.

The activity regulation of the mitotic centromere-associated kinesin by Polo-like kinase 1

The mitotic centromere-associated kinesin (MCAK), a potent microtubule depolymerase, is involved in regulating microtubule dynamics. The activity and subcellular localization of MCAK are tightly regulated by key mitotic kinases, such as Polo-like kinase 1 (Plk1) by phosphorylating multiple residues in MCAK. Since Plk1 phosphorylates very often different residues of substrates at different stages, we have dissected individual phosphorylation of MCAK by Plk1 and characterized its function in more depth. We have recently shown that S621 in MCAK is the major phosphorylation site of Plk1, which is responsible for regulating MCAK's degradation by promoting the association of MCAK with APC/CCdc20. In the present study, we have addressed another two residues phosphorylated by Plk1, namely S632/S633 in the C-terminus of MCAK. Our data suggest that Plk1 phosphorylates S632/S633 and regulates its catalytic activity in mitosis. This phosphorylation is required for proper spindle assembly during early phases of mitosis. The subsequent dephosphorylation of S632/S633 might be necessary to timely align the chromosomes onto the metaphase plate. Therefore, our studies suggest new mechanisms by which Plk1 regulates MCAK: the degradation of MCAK is controlled by Plk1 phosphorylation on S621, whereas its activity is modulated by Plk1 phosphorylation on S632/S633 in mitosis.

Mitotic arrest and slippage induced by pharmacological inhibition of Polo-like kinase 1

Exposure to drugs that interfere with microtubule dynamics block cell cycle progression at mitosis by prolonged activation of the spindle assembly checkpoint (SAC). Cells can evade mitotic arrest and proceed to interphase without chromosome segregation by a process termed mitotic slippage that involves Cyclin B1 degradation without checkpoint inactivation. Here, we explored the cellular response to small-molecule inhibitors of Polo-like kinase 1 (Plk1), an important regulator of cell division. We found that the clinical Plk1 inhibitors BI 2536 and BI 6727, both unexpectedly, induced a dose-dependent cellular drug response: While mitotic arrest was induced in cancer cell lines and primary non-transformed cells across the entire range of concentrations tested, only high concentrations seemed to promote mitotic slippage. Since this observation contrasts with the effects expected from studies reporting RNAi-mediated Plk1 depletion in cancer cells, we wondered whether both ATP-competitive inhibitors target unknown kinases that are involved in signaling from the spindle assembly checkpoint (SAC) and might contribute to the mitotic slippage. A chemical proteomics approach used to profile the selectivity of both inhibitors revealed that SAC kinases are not targeted directly. Still, the activities of Cdk1/Cyclin B1 and Aurora B, which plays important roles in the error correction of false microtubule-kinetochore attachments and in checkpoint signaling, were shown to be downregulated at high inhibitor concentrations. Our data suggest that the inhibition of Plk1 activity below a certain threshold influences Aurora B activity via reduced phosphorylation of Fox M1 and Survivin leading to diminished levels of Aurora B protein and alteration of its subcellular localization. Within the spectrum of SAC proteins that are degraded during mitotic slippage, the degradation of Cyclin B1 and the downregulation of Aurora B activity by Plk1 inhibition seem to be critical promoters of mitotic slippage. The results indicate that careful dose-finding studies in cancer trials are necessary to limit or even prevent mitotic slippage, which could be associated with improved cancer cell survival.

Turnover of protein phosphorylation evolving under stabilizing selection

Most proteins are regulated by posttranslational modifications and changes in these modifications contribute to evolutionary changes as well as to human diseases. Phosphorylation of serines, threonines, and tyrosines are the most common modifications identified to date in eukaryotic proteomes. While the mode of action and the function of most phosphorylation sites remain unknown, functional studies have shown that phosphorylation affects protein stability, localization and ability to interact. Two broad modes of action have been described for protein phosphorylation. The first mode corresponds to the canonical and qualitative view whereby single phosphorylation sites act as molecular switches that either turn on or off specific protein functions through direct or allosteric effects. The second mode is more akin to a rheostat than a switch. In this case, a group of phosphorylation sites in a given protein region contributes collectively to the modification of the protein, irrespective of the precise position of individual sites, through an aggregate property. Here we discuss these two types of regulation and examine how they affect the rate and patterns of protein phosphorylation evolution. We describe how the evolution of clusters of phosphorylation sites can be studied under the framework of complex traits evolution and stabilizing selection.

Transient activation of p53 in G2 phase is sufficient to induce senescence

DNA damage can result in a transient cell-cycle arrest or lead to permanent cell-cycle withdrawal. Here we show that the decision to irreversibly withdraw from the cell cycle is made within a few hours following damage in G2 cells. This permanent arrest is dependent on induction of p53 and p21, resulting in the nuclear retention of Cyclin B1. This rapid response is followed by the activation of the APC/C(Cdh1) (the anaphase-promoting complex/cyclosome and its coactivator Cdh1) several hours later. Inhibition of APC/C(Cdh1) activity fails to prevent cell-cycle withdrawal, whereas preventing nuclear retention of Cyclin B1 does allow cells to remain in cycle. Importantly, transient induction of p53 in G2 cells is sufficient to induce senescence. Taken together, these results indicate that a rapid and transient pulse of p53 in G2 can drive nuclear retention of Cyclin B1 as the first irreversible step in the onset of senescence.

Identification of a high affinity selective inhibitor of Polo-like kinase 1 for cancer chemotherapy by computational approach

Polo-like kinase (Plk)1 is a key regulator of the cell cycle during mitotic phase and is an attractive anti-mitotic drug target for cancer. Plk1 is a member of Ser/Thr kinase family which also includes Plk2-4 in human. Plk1 promotes the cell division whereas Plk2 and Plk3 are reported to act as tumour suppressors. The available inhibitors of Plk1 also suppress Plk2 and Plk3 activity significantly resulting in the cell death of normal cells in addition to the cancer cells. Hence, it is imperative to explore Plk1 specific inhibitors as anti-cancer drugs. In this work, a selective potential inhibitor of Plk1 has been identified by molecular docking based high throughput virtual screening. The identified compound exploits the subtle differences between the binding sites of Plk1 and other Ser/Thr kinases including Plk2-4. The predicted binding affinity of identified inhibitor is higher than available inhibitors with a 100-fold selectivity towards Plk1 over Plk2-4 and several cell cycle kinases. It also satisfies the Lipinski's criteria of drug-like molecules and passes the other ADMET filters. This triazole compound with aryl substituent belongs to a novel class of potential inhibitor for Plk1. The suggested potential lead molecule can thus be tested and developed further as a potent and selective anti-cancer drug.

Combinatorial inhibition of Plk1 and PKCβ in cancer cells with different p53 status

PKCβ and Plk1 are fascinating targets in cancer therapy. Therefore, we combined Enzastaurin targeting PKCβ and SBE13 targeting Plk1 to test synergistic effects in cells with different p53 status. We analyzed cell proliferation and apoptosis induction, and did Western blot and FACScan analyses to examine the combined PKCβ and Plk1 inhibition. p53-wild-type cells are more resistant to the combinatorial treatment than p53-deficient cells, which displayed a synergistic reduction of cell proliferation after the combination. HeLa, MCF-7 and HCT116(p53wt) and HCT116(p53-/-) cells differed in their cell cycle distribution after combinatorial treatment in dependence on a functional p53-dependent G1/S checkpoint (p53-deficient cells showed an enrichment in S and G2/M, p53-wild-type cells in G0/G1 phase). hTERT-RPE1 cells did not show the synergistic effects of cancer cells. Thus, we demonstrate for the first time that Plk1 inhibition using SBE13 enhances the effects of Enzastaurin in cancer cells. HCT116(p53wt) and HCT116(p53-/-) cells confirmed the p53-dependence of different effects after Plk1 and PKCβ inhibition observed in HeLa and MCF-7 cells. Obviously, p53 protects cells from the cytotoxicity of Enzastaurin in combination with SBE13. For that reason this combination can be useful to treat p53-deficient cancers, without displaying toxicity to normal cells, which all have functional p53.

p21Waf1/Cip1 deficiency causes multiple mitotic defects in tumor cells

As a multifaceted molecule, p21 plays multiple critical roles in cell cycle regulation, differentiation, apoptosis, DNA repair, senescence, aging and stem cell reprogramming. The important roles of p21 in the interphase of the cell cycle have been intensively investigated. The function of p21 in mitosis has been proposed but not systematically studied. We show here that p21 is abundant in mitosis and binds to and inhibits the activity of Cdk1/cyclin B1. Deficiency of p21 prolongs the duration of mitosis by extending metaphase, anaphase and cytokinesis. The activity of Aurora B is reduced and the localization of Aurora B on the central spindle is disturbed in anaphase cells without p21. Moreover, HCT116 p21-/-, HeLa and Saos-2 cells depleted of p21 encounter problems in chromosome segregation and cytokinesis. Gently inhibiting the mitotic Cdk1 or add-back of p21 rescues segregation defect in HCT116 p21-/- cells. Our data demonstrate that p21 is important for a fine-tuned control of the Cdk1 activity in mitosis, and its proper function facilitates a smooth mitotic progression. Given that p21 is downregulated in the majority of tumors, either by the loss of tumor suppressors like p53 or by hyperactive oncogenes such as c-myc, this finding also sheds new light on the molecular mechanisms by which p21 functions as a tumor suppressor.

Transcriptome of Atlantic cod (Gadus morhua L.) early embryos from farmed and wild broodstocks

Significant efforts have been made to elucidate factors affecting egg quality in fish. Recently, we have shown that eggs originating from wild broodstock (WB) of Atlantic cod (Gadus morhua L.) are of superior quality to those derived from farmed broodstock (FB), and this is associated with differences in the chemical composition of egg yolk. However, maternal transcripts, accumulated during oogenesis, have not been studied extensively in fish. The aim of the present study was to characterize putative maternal mRNA transcriptome in fertilized eggs of Atlantic cod and to compare transcript pools between WB and FB in order to investigate the relation between egg developmental potential and putative maternal mRNA deposits. We performed high-throughput 454 pyrosequencing. For each WB and FB group, five cDNA libraries were individually tagged and sequenced, resulting in 98,687 (WB) and 119,333 (FB) average reads per library. Sequencing reads were de novo assembled, annotated, and mapped. Out of 13,726 identified isotigs, 238 were differentially expressed between WB and FB, with 155 isotigs significantly upregulated in WB. The sequence reads were mapped to 11,340 different Atlantic cod transcripts and 158 sequences were differentially expressed between the 2 groups. Important transcripts involved in fructose metabolism, fatty acid metabolism, glycerophospholipid metabolism, and oxidative phosphorylation were differentially represented between the two broodstock groups, showing potential as biomarkers of egg quality in teleosts. Our findings contribute to the hypothesis that maternal mRNAs affect egg quality and, consequently, the early development of fish.

Targeting of polo-like kinases and their cross talk with Aurora kinases--possible therapeutic strategies in human acute myeloid leukemia?

INTRODUCTION

Five human polo-like kinases (PLKs) have been identified, and PLK1 - 4 seem to interact with Aurora kinases and act as cell cycle regulators in both normal and malignant human cells.

AREAS COVERED

The present review describes i) experimental evidence for a role for PLKs and Aurora kinases in human leukemogenesis and ii) the results from clinical studies of PLK and Aurora kinase inhibitors in the treatment of human acute myeloid leukemia (AML). The review was based on searches in the PubMed and the ClinicalTrials.gov databases. These inhibitors have antiproliferative and proapoptotic effects in AML cells. Hematological and gastrointestinal toxicities are frequently dose limiting, and this may limit the use of these agents in combination with conventional AML therapy. Aurora kinase inhibitors seem to be most effective for patients with high expression of the target kinases, and the same may be true for PLK inhibitors.

EXPERT OPINION

PLK inhibition is a promising strategy for the treatment of AML. Future clinical studies have to clarify i) whether this strategy is most effective for certain subsets of patients; ii) whether multikinase inhibitors targeting several cell cycle regulators should be preferred; and iii) how this therapeutic strategy eventually should be combined with conventional antileukemic chemotherapy.

HIV-1 Tat impairs cell cycle control by targeting the Tip60, Plk1 and cyclin B1 ternary complex

HIV-1 Tat triggers intrinsic and extrinsic apoptosis pathways in both infected and uninfected cells and plays an important role in the pathogenesis of AIDS. Knocking down Tip60, an interactive protein of Tat, leads to the impairment of cell cycle progression, indicating a key role of Tip60 in cell cycle control. We found that Tip60 interacts with Plk1 through its ZnFMYST domain, and that this interaction is enhanced in the G 2/M phase. In addition, cyclin B1 was confirmed to interact with the ZnF domain of Tip60. Immunofluorescence imaging showed that Tip60 co-localizes with both Plk1 and cyclin B1 at the centrosome during the mitotic phase and to the mid-body during cytokinesis. Further experiments revealed that Tip60 forms a ternary complex with Plk1 and cyclin B1 and acetylates Plk1 but not cyclin B1. HIV-1 Tat likely forms a quaternary complex with Tip60, cyclin B1 and Plk1. Fluorescent microscopy showed that Tat causes an unscheduled nuclear translocation of both cyclin B1 and Plk1, causing their co-localization with Tip60 in the nucleus. Tat, Tip60, cyclin B1 and Plk1 interactions provide new a mechanistic explanation for Tat-mediated cell cycle dysregulation and apoptosis.

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.

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.

How protein kinases co-ordinate mitosis in animal cells

Mitosis is associated with profound changes in cell physiology and a spectacular surge in protein phosphorylation. To accomplish these, a remarkably large portion of the kinome is involved in the process. In the present review, we will focus on classic mitotic kinases, such as cyclin-dependent kinases, Polo-like kinases and Aurora kinases, as well as more recently characterized players such as NIMA (never in mitosis in Aspergillus nidulans)-related kinases, Greatwall and Haspin. Together, these kinases co-ordinate the proper timing and fidelity of processes including centrosomal functions, spindle assembly and microtubule-kinetochore attachment, as well as sister chromatid separation and cytokinesis. A recurrent theme of the mitotic kinase network is the prevalence of elaborated feedback loops that ensure bistable conditions. Sequential phosphorylation and priming phosphorylation on substrates are also frequently employed. Another important concept is the role of scaffolds, such as centrosomes for protein kinases during mitosis. Elucidating the entire repertoire of mitotic kinases, their functions, regulation and interactions is critical for our understanding of normal cell growth and in diseases such as cancers.

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.

Activation of cyclin B1-Cdk1 synchronizes events in the nucleus and the cytoplasm at mitosis

The cyclin B-Cdk1 kinase triggers mitosis in most eukaryotes. In animal cells, cyclin B shuttles between the nucleus and cytoplasm in interphase before rapidly accumulating in the nucleus at prophase, which promotes disassembly of the nuclear lamina and nuclear envelope breakdown (NEBD). What triggers the nuclear accumulation of cyclin B1 is presently unclear, although the prevailing view is that the Plk1 kinase inhibits its nuclear export. In this study, we use a biosensor specific for cyclin B1-Cdk1 activity to show that activating cyclin B1-Cdk1 immediately triggers its rapid accumulation in the nucleus through a 40-fold increase in nuclear import that remains dependent on Cdk1 activity until NEBD. Nevertheless, a substantial proportion of cyclin B1-Cdk1 remains in the cytoplasm. The increase in nuclear import is driven by changes in the nuclear import machinery that require neither Plk1 nor inhibition of nuclear export. Thus, the intrinsic link between cyclin B1-Cdk1 activation and its rapid nuclear import inherently coordinates the reorganization of the nucleus and the cytoplasm at mitotic entry.

Cyclin B-Cdk1 activates its own pump to get into the nucleus

The transition to mitosis requires extensive nuclear and cytoplasmic rearrangements that must be spatially and temporally coordinated. In this issue, Gavet and Pines (2010a. J. Cell Biol. doi:10.1083/jcb.200909144) report on a simple yet elegant mechanism as to how this is achieved. By monitoring the activity of cyclin B-Cdk1 in real time, the authors show that concomitant with its activation in the cytoplasm, the kinase complex is rapidly imported into the nucleus by modifying the activity of the nucleocytoplasmic transport machinery. Thus, cyclin B-Cdk1 activates its own pump to get into the nucleus.

Functional and spatial regulation of mitotic centromere-associated kinesin by cyclin-dependent kinase 1

Mitotic centromere-associated kinesin (MCAK) plays an essential role in spindle formation and in correction of improper microtubule-kinetochore attachments. The localization and activity of MCAK at the centromere/kinetochore are controlled by Aurora B kinase. However, MCAK is also abundant in the cytosol and at centrosomes during mitosis, and its regulatory mechanism at these sites is unknown. We show here that cyclin-dependent kinase 1 (Cdk1) phosphorylates T537 in the core domain of MCAK and attenuates its microtubule-destabilizing activity in vitro and in vivo. Phosphorylation of MCAK by Cdk1 promotes the release of MCAK from centrosomes and is required for proper spindle formation. Interfering with the regulation of MCAK by Cdk1 causes dramatic defects in spindle formation and in chromosome positioning. This is the first study demonstrating that Cdk1 regulates the localization and activity of MCAK in mitosis by directly phosphorylating the catalytic core domain of MCAK.

Multiplexed random peptide library and phospho-specific antibodies facilitate human polo-like kinase 1 inhibitor screen

One of the challenges to develop time-resolved fluorescence resonance energy transfer (TR-FRET) assay for serine/threonine (Ser/Thr) protein kinase is to select an optimal peptide substrate and a specific phosphor Ser/Thr antibody. This report describes a multiplexed random screen-based development of TR-FRET assay for ultra-high-throughput screening (uHTS) of small molecule inhibitors for a potent cancer drug target polo-like kinase 1 (Plk1). A screen of a diverse peptide library in a 384-well plate format identified several highly potent substrates that share the consensus motif for phosphorylation by Plk1. Their potencies were comparable to FKD peptide, a designed peptide substrate derived from well-described Plk1 substrate Cdc25C. A specific anti-phosphor Ser/Thr antibody p(S/T)F antibody that detects the phosphorylation of FKD peptide was screened out of 87 antibodies with time-resolved fluorometry technology in a 96-well plate format. Using FKD peptide and p(S/T)F antibody, we successfully developed a robust TR-FRET assay in 384-well plate format, and further miniaturized this assay to 1,536-well plate format to perform uHTS. We screened about 1.2 million compounds for Plk1 inhibitors using a Plk1 deletion mutant that only has the kinase domain and subsequently screened the same compound library using a full-length active-mutant Plk1. These uHTSs identified a number of hit compounds, and some of them had selectivity to either the deletion mutant or the full-length protein. Our results prove that a combination of random screen for substrate peptide and phospho-specific antibodies is very powerful strategy to develop TR-FRET assays for protein kinases.

ERK1/2-mediated Schwann cell proliferation in the regenerating sciatic nerve by treadmill training

Proliferation of Schwann cells in the injured peripheral nerve supports axonal regeneration, and physical training in experimental animals has been shown to promote nerve regeneration. Extracellular signal-regulated kinase 1/2 (ERK1/2) activity can mediate neuronal responses to lesion signals, but its role in non-neuronal cells in the injured area is largely unknown. Here we report that treadmill training (TMT) facilitates axonal regeneration via the upregulation of phospho-ERK1/2 protein levels in Schwann cells in the injured sciatic nerve. Low-intensity, but not high-intensity, TMT increased neurite outgrowth of dorsal root ganglion (DRG) sensory neurons and potentiated Schwann cell proliferation. TMT elevated levels of GAP-43 mRNA and protein, and phospho-ERK1/2 protein in the injured sciatic nerves. TMT also enhanced phospho-c-Jun protein levels in the injured nerve. In-vivo administration of the ERK1/2 inhibitor PD98059 eliminated phospho-c-Jun, suggesting ERK1/2 phosphorylation of the c-Jun protein. PD98059 treatment decreased levels of BrdU-labeled proliferating Schwann cells in the distal portion of the injured nerve, and delayed the axonal regrowth that was promoted by TMT. The present data suggest that increased ERK1/2 activity in Schwann cells may play an important role in TMT-mediated enhancement of axonal regeneration in the injured peripheral nerve.

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.

Polo-like kinase inhibitors: an emerging opportunity for cancer therapeutics

IMPORTANCE OF THE FIELD

The Polo-like kinase (Plk) family has emerged as an important regulator in cell cycle progression. Plks belong to a family of serine/threonine kinases and exist in four isoforms Plk1- 4. However, only one of these isoforms, Plk1, is shown to be involved in the activation of Cdc2, chromosome segregation, centrosome maturation, bipolar spindle formation and execution of cytokinesis. The activity of Plk1 is elevated in tissues and cells with a high mitotic index. In patients, Plk1 is overexpressed in tumors including those derived from lung, breast, colon, pancreas, prostate and ovary. Plk1 depletion is associated with the decrease in cell viability and induction of apoptosis in various cancerous cells. Several Plk1 inhibitors are in different phases of clinical development for anticancer therapy.

AREAS COVERED IN THIS REVIEW

The focus of present review is to highlight Plk1 as a promising therapeutic approach for the treatment of cancer. The review discusses the role of Plk1 in cancer and the current status of Plk1 inhibitors, as well as highlighting the possible beneficial effect of inhibition of Plk1 as compared to other mitotic targets.

WHAT THE READER WILL GAIN

Readers will get a comprehensive overview of Plk1 as a novel anticancer drug target. This review will also update readers about the progress made in the field of Plk1 inhibitors.

TAKE HOME MESSAGE

The current literature about Plk1 inhibitors and knockout studies favor Plk1 inhibition as a potential antitumor therapy.

Cell cycle genes and ovarian cancer susceptibility: a tagSNP analysis

Background:

Dysregulation of the cell cycle is a hallmark of many cancers including ovarian cancer, a leading cause of gynaecologic cancer mortality worldwide.

Methods:

We examined single nucleotide polymorphisms (SNPs) (n=288) from 39 cell cycle regulation genes, including cyclins, cyclin-dependent kinases (CDKs) and CDK inhibitors, in a two-stage study. White, non-Hispanic cases (n=829) and ovarian cancer-free controls (n=941) were genotyped using an Illumina assay.

Results:

Eleven variants in nine genes (ABL1, CCNB2, CDKN1A, CCND3, E2F2, CDK2, E2F3, CDC2, and CDK7) were associated with risk of ovarian cancer in at least one genetic model. Seven SNPs were then assessed in four additional studies with 1689 cases and 3398 controls. Association between risk of ovarian cancer and ABL1 rs2855192 found in the original population [odds ratio, OR_{BB vs AA} 2.81 (1.29–6.09), P=0.01] was also observed in a replication population, and the association remained suggestive in the combined analysis [OR_{BB vs AA} 1.59 (1.08–2.34), P=0.02]. No other SNP associations remained suggestive in the replication populations.

Conclusion:

ABL1 has been implicated in multiple processes including cell division, cell adhesion and cellular stress response. These results suggest that characterization of the function of genetic variation in this gene in other ovarian cancer populations is warranted.

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.

Functional dynamics of Polo-like kinase 1 at the centrosome

Polo-like kinase 1 (Plk1) functions as a key regulator of mitotic events by phosphorylating substrate proteins on centrosomes, kinetochores, the mitotic spindle, and the midbody. Through mechanisms that are incompletely understood, Plk1 is released from and relocalizes to different mitotic structures as cells proceed through mitosis. We used fluorescence recovery after photobleaching to examine the kinetics of this process in more detail. We observed that Plk1 displayed a range of different recovery rates that differ at each mitotic substructure and depend on both the Polo-box domain and a functional kinase domain. Upon mitotic entry, centrosomal Plk1 becomes more dynamic, a process that is directly enhanced by Plk1 kinase activity. In contrast, Plk1 displays little dynamic exchange at the midbody, a process that again is modulated by the kinase activity of Plk1. Our findings suggest that the intrinsic kinase activity of Plk1 triggers its release from early mitotic structures and its relocalization to late mitotic structures. To assess the importance of Plk1 dynamic relocalization, Plk1 was persistently tethered to the centrosome. This resulted in a G(2) delay, followed by a prominent prometaphase arrest, as a consequence of defective spindle formation and activation of the spindle checkpoint. The dynamic release of Plk1 from early mitotic structures is thus crucial for mid- to late-stage mitotic events and demonstrates the importance of a fully dynamic Plk1 at the centrosome for proper cell cycle progression. This dependence on dynamic Plk1 was further observed during the mitotic reentry of cells after a DNA damage G(2) checkpoint, as this process was significantly delayed upon centrosomal tethering of Plk1. These results indicate that mitotic progression and control of mitotic reentry after DNA damage resides, at least in part, on the dynamic behavior of Plk1.

The decision to enter mitosis: feedback and redundancy in the mitotic entry network

The decision to enter mitosis is mediated by a network of proteins that regulate activation of the cyclin B–Cdk1 complex. Within this network, several positive feedback loops can amplify cyclin B–Cdk1 activation to ensure complete commitment to a mitotic state once the decision to enter mitosis has been made. However, evidence is accumulating that several components of the feedback loops are redundant for cyclin B–Cdk1 activation during normal cell division. Nonetheless, defined feedback loops become essential to promote mitotic entry when normal cell cycle progression is perturbed. Recent data has demonstrated that at least three Plk1-dependent feedback loops exist that enhance cyclin B–Cdk1 activation at different levels. In this review, we discuss the role of various feedback loops that regulate cyclin B–Cdk1 activation under different conditions, the timing of their activation, and the possible identity of the elusive trigger that controls mitotic entry in human cells.

High expression levels of cyclin B1 and Polo-like kinase 1 in ectopic endometrial cells associated with abnormal cell cycle regulation of endometriosis

OBJECTIVE

To investigate the possible roles of cyclin B1/cyclin-dependent kinase (cdc2) and Polo-like kinase 1 (Plk1) in the pathogenesis of endometriosis.

DESIGN

A case-control study.

SETTING

University hospital.

PATIENT(S)

Patients with or without endometriosis were diagnosed by pathological examination or laparoscopy. The patients with the following criteria within the past 6 months were excluded: endocrine or inflammatory diseases, pregnancy or lactation, hormonal therapy, and neoplasm in the uterine cavity.

INTERVENTION(S)

Eutopic and ectopic endometria were obtained at the time of surgery. Blood was collected on the same day as surgery.

MAIN OUTCOME MEASURE(S)

The mRNA/protein expression and localization of cyclin B1, cdc2, and Plk1 in endometrium, and serum levels of E(2) and P.

RESULT(S)

The expression levels of cyclin B1 and Plk1, but not cdc2, in ectopic endometria were significantly higher than in eutopic endometria. The immunohistochemical staining of cyclin B1 and Plk1 was detected in the nuclei of ectopic and eutopic endometrial cells. Furthermore, ectopic endometrial expression levels of cyclin B1 or Plk1 were positively correlated with serum E(2) levels.

CONCLUSION(S)

Cyclin B1 and Plk1 may play important roles in the pathogenesis of endometriosis by mediating ectopic endometrial cell proliferation under regulation of ovarian hormones.

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

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