Expression of polo-like kinase in ovarian cancer is associated with histological grade and clinical stage

The Role of Polo-Like Kinase 1 in Regulating the Forkhead Box Family Transcription Factors

Polo-like kinase 1 (PLK1) is a serine/threonine kinase with more than 600 phosphorylation substrates through which it regulates many biological processes, including mitosis, apoptosis, metabolism, RNA processing, vesicle transport, and G₂ DNA-damage checkpoint recovery, among others. Among the many PLK1 targets are members of the FOX family of transcription factors (FOX TFs), including FOXM1, FOXO1, FOXO3, and FOXK1. FOXM1 and FOXK1 have critical oncogenic roles in cancer through their antagonism of apoptotic signals and their promotion of cell proliferation, metastasis, angiogenesis, and therapeutic resistance. In contrast, FOXO1 and FOXO3 have been identified to have broad functions in maintaining cellular homeostasis. In this review, we discuss PLK1-mediated regulation of FOX TFs, highlighting the effects of PLK1 on the activity and stability of these proteins. In addition, we review the prognostic and clinical significance of these proteins in human cancers and, more importantly, the different approaches that have been used to disrupt PLK1 and FOX TF-mediated signaling networks. Furthermore, we discuss the therapeutic potential of targeting PLK1-regulated FOX TFs in human cancers.

Mitotic protein kinase-driven crosstalk of machineries for mitosis and metastasis

Accumulating evidence indicates that mitotic protein kinases are involved in metastatic migration as well as tumorigenesis. Protein kinases and cytoskeletal proteins play a role in the efficient release of metastatic cells from a tumor mass in the tumor microenvironment, in addition to playing roles in mitosis. Mitotic protein kinases, including Polo-like kinase 1 (PLK1) and Aurora kinases, have been shown to be involved in metastasis in addition to cell proliferation and tumorigenesis, depending on the phosphorylation status and cellular context. Although the genetic programs underlying mitosis and metastasis are different, the same protein kinases and cytoskeletal proteins can participate in both mitosis and cell migration/invasion, resulting in migratory tumors. Cytoskeletal remodeling supports several cellular events, including cell division, movement, and migration. Thus, understanding the contributions of cytoskeletal proteins to the processes of cell division and metastatic motility is crucial for developing efficient therapeutic tools to treat cancer metastases. Here, we identify mitotic kinases that function in cancer metastasis as well as tumorigenesis. Several mitotic kinases, namely, PLK1, Aurora kinases, Rho-associated protein kinase 1, and integrin-linked kinase, are considered in this review, as an understanding of the shared machineries between mitosis and metastasis could be helpful for developing new strategies to treat cancer.

Improving understanding of the mechanisms linking cell division and cancer spread (metastasis) could provide novel strategies for treatment. A group of enzymes involved in cell division (mitosis) are also thought to play critical roles in the spread of cancers. Hyungshin Yim at Hanyang University in Ansan, South Korea, and co-workers in Korea and the USA reviewed the roles of several mitotic enzymes that are connected with metastasis as well as tumorigenesis. They discussed how these enzymes modify cytoskeletal proteins and other substrates during cancer progression. Some regulatory control of cell cytoskeletal structures is required for cancer cells to metastasize. Recent research has uncovered crosstalk between mitotic enzymes and metastatic cytoskeletal molecules in various cancers. Targeting mitotic enzymes and the ways they influence cytoskeletal mechanisms could provide valuable therapeutic strategies for suppressing metastasis.

Folate-mediated targeted PLK1 inhibition therapy for ovarian cancer: A comparative study of molecular inhibitors and siRNA therapeutics

PLK1 is a promising target for clinical treatment of diverse malignancies including ovarian cancer (OC), in which PLK1 over-expression is often correlated with poor prognosis and short survival. PLK1 can be blocked with small molecular inhibitors like volasertib (Vol) or silenced with PLK1-specific siRNA (siPLK1), hence effectively suppressing tumor growth. Surprisingly, despite intensive work on molecular inhibitor and siRNA therapeutics, there is no direct comparison between them reported for targeted tumor therapy. Herein, we employing folate as a ligand and polymersomes as a nanovehicle performed a comparative study on Vol and siPLK1 in inhibiting OC in vitro and in vivo. Folate-targeted polymersomal Vol and siPLK1 (termed as FA-Ps-Vol and FA-Ps-siPLK1, respectively) were both nano-sized and stable, and displayed an optimal FA density of 20% for SKOV-3 cells. Notably, FA-Ps-Vol and FA-Ps-siPLK1 exhibited an IC₅₀ of 193 and 770 nM, respectively, to SKOV-3 cells, indicating a greater potency of Vol than siPLK1. The markedly increased uptake for FA-Ps-Vol and FA-Ps-siPLK1 compared with respective non-targeted controls by SKOV-3 tumor xenografts in mice confirmed that FA mediates strong OC-targeting in vivo. Intriguingly, FA-Ps-Vol while greatly lessening toxic effects of Vol potently repressed tumor growth with a remarkable tumor inhibition rate (TIR) of 97% at 20 mg (i.e. 32.4 µmol) Vol equiv./kg. FA-Ps-siPLK1 achieved effective tumor inhibition (TIR = ca. 87% or 90%) at 2 or 4 mg (i.e. 0.15 or 0.3 µmol) siPLK1 equiv./kg without causing adverse effects. This comparative study highlights that molecular inhibitor has the advantage of easy dose escalation and potent protein inhibition at the expense of certain adverse effects while siRNA therapeutics has low toxicity with moderate protein inhibition in vivo. STATEMENT OF SIGNIFICANCE: PLK1 is a promising target for the development of innovative and specific treatments against diverse malignancies. Interestingly, despite intensive work on molecular inhibitors and siRNA against PLK1, little work has been directed to compare their efficacy in targeted tumor therapy. Here, we employed folate as a ligand and polymersomes as a nanovehicle and have performed a comparative study on volasertib and siPLK1 in inhibiting ovarian cancer in vitro and in vivo. Our data show that the dose of volasertib can be easily escalated to induce prominent antitumor efficacy at the expense of certain adverse effects, while siPLK1 brings about moderate protein inhibition and antitumor therapy without causing toxicity at two-orders-of-magnitude lower dose.

Cytotoxicity and molecular docking analysis of racemolactone I, a new sesquiterpene lactone isolated from Inula racemosa

CONTEXT

Traditionally, Inula racemosa Hook. f. (Asteraceae) has been reported to be effective in cancer treatment which motivated the authors to explore the plant for novel anticancer compounds.

OBJECTIVE

To isolate and characterize new cytotoxic phytoconstituents from I. racemosa roots.

MATERIALS AND METHODS

The column chromatography of I. racemosa ethyl acetate extract furnished a novel sesquiterpene lactone whose structure was established by NMR (1D/2D), ES-MS and its cytotoxic properties were assessed on HeLa, MDAMB-231, and A549 cell lines using MTT and LDH (lactate dehydrogenase) assays. Further, morphological changes were analyzed by flow cytometry, mitochondrial membrane potential, AO-EtBr dual staining, and comet assay. Molecular docking and simulation were performed using Glide and Desmond softwares, respectively, to validate the mechanism of action.

RESULTS

The isolated compound was identified as racemolactone I (compound 1). Amongst the cell lines tested, considerable changes were observed in HeLa cells. Compound 1 (IC50 = 0.9 µg/mL) significantly decreased cell viability (82%) concomitantly with high LDH release (76%) at 15 µg/mL. Diverse morphological alterations along with significant increase (9.23%) in apoptotic cells and decrease in viable cells were observed. AO-EtBr dual staining also confirmed the presence of 20% apoptotic cells. A gradual decrease in mitochondrial membrane potential was observed. HeLa cells showed significantly increased comet tail length (48.4 µm), indicating broken DNA strands. In silico studies exhibited that compound 1 binds to the active site of Polo-like kinase-1 and forms a stable complex.

CONCLUSIONS

Racemolactone I was identified as potential anticancer agent, which can further be confirmed by in vivo investigations.

Elucidation of PLK1 Linked Biomarkers in Oesophageal Cancer Cell Lines: A Step Towards Novel Signaling Pathways by p53 and PLK1-Linked Functions Crosstalk

BACKGROUND

Oesophgeal adenocarcinoma (OAC) is the most frequent cause of cancer death. POLO-like kinase 1 (PLK1) is overexpressed in broad spectrum of tumors and has prognostic value in many cancers including esophageal cancer, suggesting its potential as a therapeutic target. p53, the guardian of genome is the most important tumor suppressors that represses the promoter of PLK1, whereas tumor cells with inactive p53 are arrested in mitosis due to DNA damage. PLK1 expression has been linked to the elevated p53 expression and has been shown to act as a biomarker that predicts poor prognosis in OAC.

OBJECTIVES

The aim of the present study was identification of PLK1 associated phosphorylation targets in p53 mutant and p53 normal cells to explore the downstream signaling evets.

METHODS

Here we develop a proof-of-concept phospho-proteomics approach to identify possible biomarkers that can be used to identify mutant p53 or wild-type p53 pathways. We treated PLK1 asynchronously followed by mass spectrometry data analysis. Protein networking and motif analysis tools were used to identify the significant clusters and potential biomarkers.

RESULTS

We investigated approximately 1300 potential PLK1-dependent phosphopeptides by LCMS/ MS. In total, 2216 and 1155 high confidence phosphosites were identified in CP-A (p53+) and OE33 (p53-) cell lines owing to PLK1 inhibition. Further clustering and motif assessment uncovered many significant biomarkers with known and novel link to PLK1.

CONCLUSION

Taken together, our study suggests that PLK1 may serve as a potential therapeutic target in human OAC. The data highlight the efficacy and specificity of small molecule PLK1 kinase inhibitors to identify novel signaling pathways in vivo.

A New Cytotoxic Dimeric Sesquiterpene Isolated from Inula racemosa Hook. f. (Root): In Vitro and In Silico Analyses

A new dimeric sesquiterpene named disesquicin (compound 1) was isolated from Inula racemosa roots by normal-phase MPLC (Medium Pressure Liquid Chromatography), and its structure was established by using extensive spectral analysis. Compound 1, when tested on different human cancer cell lines, showed marked cytotoxic activity (IC50 (µg/mL): 5.99 (MDA-MB), 9.10 (HeLa), and 12.47 (A549)). Docking study revealed that it binds at the catalytic domain of PLK-1 and interacts with catalytic site residues Leu59, Gly60, Lys61, Gly62, Cys67, Ala80, Lys82, Leu130, Arg136, Ser137, Leu139, Glu140, Lys178, Gly180, Asn181, Phe183, and Asp194. The binding of compound 1 to PLK-1 is spontaneous in nature as evident by a free energy of—8.930 kcal mol−1, corresponding to a binding affinity of 3.54 × 106 M−1. Results showed that compound 1 exhibited cytotoxic potential that was further confirmed by in vivo investigations.

The CINs of Polo-Like Kinase 1 in Cancer

Simple Summary

Many alterations specific to cancer cells have been investigated as targets for targeted therapies. Chromosomal instability is a characteristic of nearly all cancers that can limit response to targeted therapies by ensuring the tumor population is not genetically homogenous. Polo-like Kinase 1 (PLK1) is often up regulated in cancers and it regulates chromosomal instability extensively. PLK1 has been the subject of much pre-clinical and clinical studies, but thus far, PLK1 inhibitors have not shown significant improvement in cancer patients. We discuss the numerous roles and interactions of PLK1 in regulating chromosomal instability, and how these may provide an avenue for identifying targets for targeted therapies. As selective inhibitors of PLK1 showed limited clinical success, we also highlight how genetic interactions of PLK1 may be exploited to tackle these challenges.

Abstract

Polo-like kinase 1 (PLK1) is overexpressed near ubiquitously across all cancer types and dysregulation of this enzyme is closely tied to increased chromosomal instability and tumor heterogeneity. PLK1 is a mitotic kinase with a critical role in maintaining chromosomal integrity through its function in processes ranging from the mitotic checkpoint, centrosome biogenesis, bipolar spindle formation, chromosome segregation, DNA replication licensing, DNA damage repair, and cytokinesis. The relation between dysregulated PLK1 and chromosomal instability (CIN) makes it an attractive target for cancer therapy. However, clinical trials with PLK1 inhibitors as cancer drugs have generally displayed poor responses or adverse side-effects. This is in part because targeting CIN regulators, including PLK1, can elevate CIN to lethal levels in normal cells, affecting normal physiology. Nevertheless, aiming at related genetic interactions, such as synthetic dosage lethal (SDL) interactions of PLK1 instead of PLK1 itself, can help to avoid the detrimental side effects associated with increased levels of CIN. Since PLK1 overexpression contributes to tumor heterogeneity, targeting SDL interactions may also provide an effective strategy to suppressing this malignant phenotype in a personalized fashion.

The Prognostic Relevance of the Proliferation Markers Ki-67 and Plk1 in Early-Stage Ovarian Cancer Patients With Serous, Low-Grade Carcinoma Based on mRNA and Protein Expression

Since type and duration of an appropriate adjuvant chemotherapy in early-stage ovarian cancer (OC) are still being debated, novel markers for a better stratification of these patients are of utmost importance for the design of an improved chemotherapeutical strategy. In contrast to numerous cancer studies on cellular proliferation based on the immunohistochemistry-driven evaluation of protein expression, we compared mRNA and protein expression of two independent markers of cellular proliferation, Ki-67 and Plk1, in a large cohort of 243 early-stage OC and their relationship with clinicopathological features and survival. Based on marker expression we demonstrate that early-stage OC patients (stages I/II, low-grade, serous) with high expression (Ki-67, Plk1) had a significantly shorter progression-free survival (PFS) and overall survival (OS) compared to patients with low expression (Ki-67, Plk1). Remarkably, based on mRNA expression this significant difference got lost in advanced stages (III/IV): At least for PFS, high levels of Ki-67 and Plk1 correlate with moderately better survival compared to patients with low expressing tumors. Our data suggest that in addition to Ki-67, Plk1 is a novel marker for the stratification of early-stage OC patients to maximize therapeutic efforts. Both, Ki-67 and Plk1, seem to be better suited in early-stages (I/II) as therapeutical targets compared to advanced-stages (III/IV) OC.

Constitutive expression of progesterone receptor isoforms promotes the development of hormone-dependent ovarian neoplasms

Differences in the relative abundances of the progesterone receptor (PGR) isoforms PGRA and PGRB are often observed in women with reproductive tract cancers. To assess the importance of the PGR isoform ratio in the maintenance of the reproductive tract, we generated mice that overexpress PGRA or PGRB in all PGR-positive tissues. Whereas few PGRA-overexpressing mice developed reproductive tract tumors, all PGRB-overexpressing mice developed ovarian neoplasms that were derived from ovarian luteal cells. Transcriptomic analyses of the ovarian tumors from PGRB-overexpressing mice revealed enhanced AKT signaling and a gene expression signature similar to those of human ovarian and endometrial cancers. Treating PGRB-overexpressing mice with the PGR antagonist RU486 stalled tumor growth and decreased the expression of cell cycle-associated genes, indicating that tumor growth and cell proliferation were hormone dependent in addition to being isoform dependent. Analysis of the PGRB cistrome identified binding events at genes encoding proteins that are critical regulators of mitotic phase entry. This work suggests a mechanism whereby an increase in the abundance of PGRB relative to that of PGRA drives neoplasia in vivo by stimulating cell cycling.

Analyses of expressions and prognostic values of Polo-like kinases in non-small cell lung cancer

BACKGROUND

Despite great advances in its early diagnosis and treatment, lung cancer is still an intractable disease and the second leading cause of cancer-related deaths and morbidity in the world. The family of Polo-like kinases (PLKs) consists of five serine/threonine kinases, which have been reported to participate in various human diseases. However, the expression and prognostic value of each PLK in human lung cancer have not been fully understood. This study analyzed mRNA expression and prognostic value of different PLKs in human non-small cell lung cancer (NSCLC).

METHODS

First, mRNA expression of PLKs in patients with NSCLC from the Oncomine and the Gene Expression Profiling Interactive Analysis (GEPIA) database was investigated. Then, a Kaplan-Meier plotter was employed for survival analysis. The sequence alteration for PLKs was analyzed using The Cancer Genome Atlas (TCGA) and the cBioPortal database. Additionally, we analyzed the association among different PLKs using the LinkedOmics database. Finally, the enrichment analysis of PLKs was achieved using the DAVID database.

RESULTS

The mRNA expression levels of PLK1 and PLK4 were significantly overexpressed, while mRNA expression level of PLK3 was underexpressed in patients with NSCLC. mRNA expressions of PLK1 and PLK4 were significantly and positively related to the tumor stage of NSCLC. Increased expressions of PLK1, PLK4, and PLK5 and decreased expression of PLK2 were attributed to limited overall survival time in NSCLC. PLK1 was positively correlated with PLK4 via the LinkedOmics database.

CONCLUSIONS

PLKs are relevant targets for NSCLC treatment, especially PLK1 and PLK4.

Uncovering Potential Roles of Differentially Expressed Genes, Upstream Regulators, and Canonical Pathways in Endometriosis Using an In Silico Genomics Approach

Endometriosis is characterized by ectopic endometrial tissue implantation, mostly within the peritoneum, and affects women in their reproductive age. Studies have been done to clarify its etiology, but the precise molecular mechanisms and pathophysiology remain unclear. We downloaded genome-wide mRNA expression and clinicopathological data of endometriosis patients and controls from NCBI’s Gene Expression Omnibus, after a systematic search of multiple independent studies comprising 156 endometriosis patients and 118 controls to identify causative genes, risk factors, and potential diagnostic/therapeutic biomarkers. Comprehensive gene expression meta-analysis, pathway analysis, and gene ontology analysis was done using a bioinformatics-based approach. We identified 1590 unique differentially expressed genes (129 upregulated and 1461 downregulated) mapped by IPA as biologically relevant. The top upregulated genes were FOS, EGR1, ZFP36, JUNB, APOD, CST1, GPX3, and PER1, and the top downregulated ones were DIO2, CPM, OLFM4, PALLD, BAG5, TOP2A, PKP4, CDC20B, and SNTN. The most perturbed canonical pathways were mitotic roles of Polo-like kinase, role of Checkpoint kinase proteins in cell cycle checkpoint control, and ATM signaling. Protein–protein interaction analysis showed a strong network association among FOS, EGR1, ZFP36, and JUNB. These findings provide a thorough understanding of the molecular mechanism of endometriosis, identified biomarkers, and represent a step towards the future development of novel diagnostic and therapeutic options.

YLZ-F5, a novel polo-like kinase 4 inhibitor, inhibits human ovarian cancer cell growth by inducing apoptosis and mitotic defects

PURPOSE

Polo-like kinase 4 (PLK4), a member of the polo-like kinase family, plays several important roles in mitotic regulation, including centrosome duplication, spindle formation, and cytokinesis. PLK4 overexpression is frequently detected in many human cancers, including ovarian cancer, and the inhibition of PLK4 activity results in cancer cell mitotic arrest and apoptosis. Therefore, PLK4 might be a valid therapeutic target for antitumor therapy. In the present study, we aimed to determine if YLZ-F5, a potent small-molecule inhibitor of PLK4, inhibits ovarian cancer cell growth.

METHODS AND RESULTS

MTT assay showed that YLZ-F5 inhibited ovarian cancer cell proliferation in a concentration- and time-dependent manner. The results of colony formation assays were consistent with those of the MTT assay results. In addition, YLZ-F5 induced ovarian cancer cell apoptosis that was associated with activation of caspase-3/caspase-9. Moreover, YLZ-F5 caused aberrant in centriole duplication that was associated with the inhibition of PLK4 phosphorylation. Notably, we showed that YLZ-F5 promoted the accumulation of ovarian cancer cells with mitotic defects (> 4 N DNA content) in a concentration-dependent manner. Furthermore, YLZ-F5 markedly inhibited the migration of A2780 cells.

CONCLUSION

Taken together, these findings suggest that YLZ-F5 is a potential drug candidate for human ovarian cancer.

Identification of PLK1 as a New Therapeutic Target in Mucinous Ovarian Carcinoma

Mucinous epithelial ovarian cancer (mEOC) is a rare subset of epithelial ovarian cancer. When diagnosed at a late stage, its prognosis is very poor, as it is quite chemo-resistant. To find new therapeutic options for mEOC, we performed high-throughput screening using a siRNA library directed against human protein kinases in a mEOC cell line, and polo-like kinase1 (PLK1) was identified as the kinase whose downregulation interfered with cell proliferation. Both PLK1 siRNA and two specific PLK1 inhibitors (onvansertib and volasertib) were able to inhibit cell growth, induce apoptosis and block cells in the G2/M phase of the cell cycle. We evaluated, in vitro, the combinations of PLK1 inhibitors and different chemotherapeutic drugs currently used in the treatment of mEOC, and we observed a synergistic effect of PLK1 inhibitors and antimitotic drugs. When translated into an in vivo xenograft model, the combination of onvansertib and paclitaxel resulted in stronger tumor regressions and in a longer mice survival than the single treatments. These effects were associated with a higher induction of mitotic block and induction of apoptosis, similarly to what was observed in vitro. These data suggest that the combination onvansertib/paclitaxel could represent a new active therapeutic option in mEOC.

CircPLK1 sponges miR-296-5p to facilitate triple-negative breast cancer progression

Aim: To investigate the role of circRNAs in triple-negative breast cancer (TNBC) and the underlying mechanisms. Materials & methods: We performed circRNA microarrays to explore the expression profiles of TNBC cell lines. Experiments in vitro and in vivo were conducted to explore the effects of circPLK1 on tumor proliferation and metastasis as well as the interaction between circPLK1, miR-296-5p and PLK1 in TNBC. Results & conclusion: CircPLK1 was significantly upregulated in TNBC and associated with poor survivals. CircPLK1 knockdown inhibited cell growth and invasion in vitro as well as tumor occurrence and metastasis in vivo. CircPLK1-miR-296-5p-PLK1 axis regulates tumor progression by ceRNA mechanism in TNBC, indicating that circPLK1 may serve as a prognostic factor and novel therapeutic target for TNBC.

Blocking Mitotic Exit of Ovarian Cancer Cells by Pharmaceutical Inhibition of the Anaphase-Promoting Complex Reduces Chromosomal Instability

Paclitaxel is a frontline drug for the treatment of epithelial ovarian cancer (EOC). However, following paclitaxel-platinum based chemotherapy, tumor recurrence occurs in most ovarian cancer patients. Chromosomal instability (CIN) is a hallmark of cancer and represents genetic variation fueling tumor adaptation to cytotoxic effects of anticancer drugs. In this study, our Kaplan-Meier analysis including 263 ovarian cancer patients (stages I/II) revealed that high Polo-like kinase (PLK) 1 expression correlates with bad prognosis. To evaluate the role of PLK1 as potential cancer target within a combinatorial trial, we induced strong mitotic arrest in ovarian cancer cell lines by synergistically co-targeting microtubules (paclitaxel) and PLK1 (BI6727) followed by pharmaceutical inhibition of the Anaphase-Promoting Complex (APC/C) using proTAME. In short- and long-term experiments, this triple treatment strongly activated apoptosis in cell lines and primary ovarian cells derived from cancer patients. Mechanistically, BI6727/paclitaxel/proTAME stabilize Cyclin B1 and trigger mitotic arrest, which initiates mitochondrial apoptosis by inactivation of antiapoptotic BCL-2 family proteins, followed by activation of caspase-dependent effector pathways. This triple treatment prevented endoreduplication and reduced CIN, two mechanisms that are associated with aggressive tumors and the acquisition of drug resistance. This "two-punch strategy" (strong mitotic arrest followed by blocking mitotic exit) has important implications for developing paclitaxel-based combinatorial treatments in ovarian cancer.

An Integrative "Omics" Approach, for Identification of Bona Fides PLK1 Associated Biomarker in Esophageal Adenocarcinoma

BACKGROUND

The rapid expansion of genome-wide profiling techniques offers the opportunity to utilize various types of information collected in the study of human health and disease. Overexpression of Polo like kinase 1 (PLK1) is associated with esophageal adenocarcinoma (OAC), however biological functions and molecular targets of PLK1 in OAC are still unknown.

OBJECTIVES

Here we performed integrative analysis of two "omics" data sources to reveal high-level interactions of PLK1 associated with OAC.

METHODS

Initially, quantitative gene expression (RPKM) was measured from transcriptomics data set of four OAC patients. In parallel, alteration in phosphorylation levels was evaluated in the proteomics data set (mass spectrometry) in OAC cell line (PLK1 inhibited). Next, two "omics" data sets were integrated and through comprehensive analysis possible true PLK1 targets that may serve as OAC biomarkers were assembled.

RESULTS

Through experimental validation, small ubiquitin-related modifier 1 (SUMO1) and heat shock protein beta-1 (HSPB1) were identified as novel phosphorylation targets of PLK1. Consequently in vivo, in situ and in silico experiments clearly demonstrated the interaction of PLK1 with putative novel targets (SUMO1 and HSPB1).

CONCLUSION

Identification of a PLK1 dependent biosignature in OAC with high confidence in two omics levels proven the robustness and efficacy of our integrative approach.

Pharmacoinformatics approach for the identification of Polo-like kinase-1 inhibitors from natural sources as anti-cancer agents

Polo-like kinase-1 (PLK-1) plays a key role in cell cycle progression during mitosis. Overexpression/dysfunction of PLK-1 is directly associated with cancerous transformation and has been reported in different cancer types. Here, we employed high throughput virtual screening and molecular docking to screen Selleck's natural compound library against PLK-1 kinase domain. We have identified eight bioactive compounds (Apigenin, Dihydromyricetin, Diosmetin, Hesperidin, Hesperitin, Naringenin, Phlorizi, and Quercetin) as the potential inhibitors of PLK-1. Further investigation through Molecular Mechanics-Generalized Born Surface Area (MM-GBSA) calculations and 15 ns molecular dynamics simulation revealed that hesperidin formed the most stable complex with PLK-1 kinase domain. Altogether, our results indicate that hesperidin interacted strongly with the key residues of the PLK-1 active site (such as Leu59, Lys61, Lys82, Cys133, Asn181, Asp194, Leu59, Cys67, Ala80, Val114, Leu130, Leu132, Cys133, Leu139, Phe183, and Phe195) through hydrogen bonding and hydrophobic interactions. The Hesperidin-PLK-1 complex was stabilized by Gibb's free energy of -13.235 kcal/mol which corresponded to the binding affinity of 5.095 × 109 M-1. This is the first study wherein hesperidin has been identified as a potential inhibitor of PLK-1. Further design and optimization of the hesperidin scaffold as an inhibitor of PLK-1 kinase domain is highly recommended.

Synthetic lethality in CCNE1-amplified high grade serous ovarian cancer through combined inhibition of Polo-like kinase 1 and microtubule dynamics

The taxanes are effective microtubule-stabilizing chemotherapy drugs that inhibit mitosis, induce apoptosis, and produce regression in a fraction of cancers that arise at many sites including the ovary. Novel therapeutic targets that augment taxane effects are needed to improve clinical chemotherapy response in CCNE1-amplified high grade serous ovarian cancer (HGSOC) cells. In this study, we conducted an siRNA-based kinome screen to identify modulators of mitotic progression in CCNE1-amplified HGSOC cells that may influence clinical paclitaxel response. PLK1 is overexpressed in many types of cancer, which correlates with poor prognosis. Here, we identified a novel synthetic lethal interaction of the clinical PLK1 inhibitor BI6727 and the microtubule-targeting drug paclitaxel in HGSOC cell lines with CCNE1-amplification and elucidated the underlying molecular mechanisms of this synergism. BI6727 synergistically induces apoptosis together with paclitaxel in different cell lines including a patient-derived primary ovarian cancer culture. Moreover, the inhibition of PLK1 reduced the paclitaxel-induced neurotoxicity in a neurite outgrowth assay. Mechanistically, the combinatorial treatment with BI6727/paclitaxel triggers mitotic arrest, which initiates mitochondrial apoptosis by inactivation of anti-apoptotic BCL-2 family proteins, followed by significant loss of the mitochondrial membrane potential and activation of caspase-dependent effector pathways. This conclusion is supported by data showing that BI6727/paclitaxel-co-treatment stabilizes FBW7, a component of SCF-type ubiquitin ligases that bind and regulate key modulators of cell division and growth including MCL-1 and Cyclin E. This identification of a novel synthetic lethality of PLK1 inhibitors and a microtubule-stabilizing drug has important implications for developing PLK1 inhibitor-based combination treatments in CCNE1-amplified HGSOC cells.

Regulation of PLK1 through competition between hnRNPK, miR-149-3p and miR-193b-5p

Polo-like kinase 1 (PLK1) is a critical regulator of cell cycle progression and apoptosis. However, its regulation remains poorly understood. In the present study, we investigated the molecular mechanism underlying the post-transcriptional regulation of PLK1. We observed that heterogeneous nuclear ribonucleoprotein K (hnRNPK) and PLK1 were positively associated in several different cancers and high expression levels of them correlated with poor prognosis in patients with cancer. Knockdown of hnRNPK resulted in reduced expression of PLK1, whereas conversely, PLK1 expression was increased in hnRNPK-overexpressing cells. We found that hnRNPK regulated PLK1 expression through KH1- and KH2-dependent interactions with the 3'UTR of PLK1 mRNA. In addition, microRNA-149-3p (miR-149-3p) and miR-193b-5p suppressed PLK1 expression by targeting the 3'UTR of PLK1 mRNA. MicroRNA-elicited enrichment of PLK1 mRNA in Ago2 immunoprecipitation was altered by the presence or absence of hnRNPK. Furthermore, the deletion of the cytosine (C)-rich sequences of the 3'UTR of PLK1 mRNA abolished the decreased PLK1 expression observed via hnRNPK silencing and administration of miRNAs, a finding that suggests that hnRNPK shares this C-rich motif with miR-149-3p and miR-193b-5p. We also found that downregulation of PLK1 by either silencing hnRNPK or overexpression of miR-149-3p and miR-193b-5p decreased clonogenicity and induced apoptosis. Our findings from this study demonstrate that hnRNPK regulates PLK1 expression by competing with the PLK1-targeting miRNAs, miR-149-3p and miR-193b-5p.

Dacomitinib, a pan-inhibitor of ErbB receptors, suppresses growth and invasive capacity of chemoresistant ovarian carcinoma cells

Epithelial ovarian cancer (EOC) is the most lethal gynaecological malignancy worldwide. Development of chemoresistance and peritoneal dissemination of EOC cells are the major reasons for low survival rate. Targeting signal transduction pathways which promote therapy resistance and metastatic dissemination is the key to successful treatment. Members of the ErbB family of receptors are over-expressed in EOC and play key roles in chemoresistance and invasiveness. Despite this, single-targeted ErbB inhibitors have demonstrated limited activity in chemoresistant EOC. In this report, we show that dacomitinib, a pan-ErbB receptor inhibitor, diminished growth, clonogenic potential, anoikis resistance and induced apoptotic cell death in therapy-resistant EOC cells. Dacominitib inhibited PLK1-FOXM1 signalling pathway and its down-stream targets Aurora kinase B and survivin. Moreover, dacomitinib attenuated migration and invasion of the EOC cells and reduced expression of epithelial-to-mesenchymal transition (EMT) markers ZEB1, ZEB2 and CDH2 (which encodes N-cadherin). Conversely, the anti-tumour activity of single-targeted ErbB agents including cetuximab (a ligand-blocking anti-EGFR mAb), transtuzumab (anti-HER2 mAb), H3.105.5 (anti-HER3 mAb) and erlotinib (EGFR small-molecule tyrosine kinase inhibitor) were marginal. Our results provide a rationale for further investigation on the therapeutic potential of dacomitinib in treatment of the chemoresistant EOC.

Targeted knockdown of polo-like kinase 1 alters metabolic regulation in melanoma

A limited number of studies have indicated an association of the mitotic kinase polo-like kinase 1 (PLK1) and cellular metabolism. Here, employing an inducible RNA interference approach in A375 melanoma cells coupled with a PCR array and multiple validation approaches, we demonstrated that PLK1 alters a number of genes associated with cellular metabolism. PLK1 knockdown resulted in a significant downregulation of IDH1, PDP2 and PCK1 and upregulation of FBP1. Ingenuity Pathway Analysis (IPA) identified that 1) glycolysis and the pentose phosphate pathway are major canonical pathways associated with PLK1, and 2) PLK1 inhibition-modulated genes were largely associated with cellular proliferation, with FBP1 being the key modulator. Further, BI 6727-mediated inhibition of PLK1 caused a decrease in PCK1 and increase in FBP1 in A375 melanoma cell implanted xenografts in vivo. Furthermore, an inverse correlation between PLK1 and FBP1 was found in melanoma cells, with FBP1 expression significantly downregulated in a panel of melanoma cells. In addition, BI 6727 treatment resulted in an upregulation in FBP1 in A375, Hs294T and G361 melanoma cells. Overall, our study suggests that PLK1 may be an important regulator of metabolism maintenance in melanoma cells.

Towards Prognostic Profiling of Non-Small Cell Lung Cancer: New Perspectives on the Relevance of Polo-Like Kinase 1 Expression, the TP53 Mutation Status and Hypoxia

Background: Currently, prognosis of non-small cell lung cancer (NSCLC) patients is based on clinicopathological factors, including TNM stage. However, there are considerable differences in patient outcome within a similar staging group, even when patients received identical treatments. In order to improve prognostic predictions and to guide treatment options, additional parameters influencing outcome are required. Polo-like kinase 1 (Plk1), a master regulator of mitotic cell division and the DNA damage response, is considered as a new potential biomarker in this research area. While several studies reported Plk1 overexpression in a broad range of human malignancies, inconsistent results were published regarding the clinical significance hereof. A prognostic panel, consisting of Plk1 and additional biomarkers that are related to the Plk1 pathway, might further improve prediction of patient prognosis. Methods: In this study, we evaluated for the first time the prognostic value of Plk1 mRNA and protein expression in combination with the TP53 mutation status (next generation sequencing), induction of apoptotic cell death (immunohistochemistry for cleaved caspase 3) and hypoxia (immunohistochemistry for carbonic anhydrase IX (CA IX)) in 98 NSCLC adenocarcinoma patients. Results: Both Plk1 mRNA and protein expression and CA IX protein levels were upregulated in the majority of tumor samples. Plk1 mRNA and protein expression levels were higher in TP53 mutant samples, suggesting that Plk1 overexpression is, at least partially, the result of loss of functional p53 (<0.05). Interestingly, the outcome of patients with both Plk1 mRNA and CA IX protein overexpression, who also harbored a TP53 mutation, was much worse than that of patients with aberrant expression of only one of the three markers (p=0.001). Conclusion: The combined evaluation of Plk1 mRNA expression, CA IX protein expression and TP53 mutations shows promise as a prognostic panel in NSCLC patients. Moreover, these results pave the way for new combination strategies with Plk1 inhibitors.

PLK1, A Potential Target for Cancer Therapy

Polo-like kinase 1 (PLK1) plays an important role in the initiation, maintenance, and completion of mitosis. Dysfunction of PLK1 may promote cancerous transformation and drive its progression. PLK1 overexpression has been found in a variety of human cancers and was associated with poor prognoses in cancers. Many studies have showed that inhibition of PLK1 could lead to death of cancer cells by interfering with multiple stages of mitosis. Thus, PLK1 is expected to be a potential target for cancer therapy. In this article, we examined PLK1’s structural characteristics, its regulatory roles in cell mitosis, PLK1 expression, and its association with survival prognoses of cancer patients in a wide variety of cancer types, PLK1 interaction networks, and PLK1 inhibitors under investigation. Finally, we discussed the key issues in the development of PLK1-targeted cancer therapy.

Study of apoptosis-related interactions in colorectal cancer

Abnormalities in apoptotic functions contribute to the pathogenesis of colorectal cancer. In this study, molecular interactions behind the apoptotic regulation have been explored. For this purpose, enrichment analysis was performed considering microRNAs (miRNAs) that putatively target TP53 and altered during colon cancer. This revealed gene associated with both TP53 and miRNAs. Further analysis showed that a significant molecular interaction between the shortlisted candidates (TP53, miR-143, KRAS, BCL2, and PLK1) exists. Mutation study was conducted to confirm the clinical relevance of candidates. It showed that the mutation extent does not significantly alter survival in patients thus making these candidates suitable as drug targets. Overall, we showed the importance of interactions between TP53, miR-143, KRAS, BCL2, and PLK1 with respect to colorectal cancer using bioinformatics approach.

PTEN regulates PLK1 and controls chromosomal stability during cell division

PTEN functions as a guardian of the genome through multiple mechanisms. We have previously established that PTEN maintains the structural integrity of chromosomes. In this report, we demonstrate a fundamental role of PTEN in controlling chromosome inheritance to prevent gross genomic alterations. Disruption of PTEN or depletion of PTEN protein phosphatase activity causes abnormal chromosome content, manifested by enlarged or polyploid nuclei. We further identify polo-like kinase 1 (PLK1) as a substrate of PTEN phosphatase. PTEN can physically associate with PLK1 and reduce PLK1 phosphorylation in a phosphatase-dependent manner. We show that PTEN deficiency leads to PLK1 phosphorylation and that a phospho-mimicking PLK1 mutant causes polyploidy, imitating functional deficiency of PTEN phosphatase. Inhibition of PLK1 activity or overexpression of a non-phosphorylatable PLK1 mutant reduces the polyploid cell population. These data reveal a new mechanism by which PTEN controls genomic stability during cell division.

Molecular interactions of polo-like kinase 1 in human cancers

Polo-like kinase 1 (PLK1) is an essential protein in communicating cell-cycle progression and DNA damage. Overexpression of PLK1 has been validated as a marker for poor prognosis in many cancers. PLK1 knockdown decreases the survival of cancer cells. PLK1 is therefore an attractive target for anticancer treatments. Several inhibitors have been developed, and some have been clinically tested to show additive effects with conventional therapies. Upstream regulation of PLK1 involves multiple interactions of proteins such as FoxM1, E2F and p21. Other cancer-related proteins such as pRB and p53 also indirectly influence PLK1 expression. With the high mutation rates of these genes seen in cancers, they may be associated with PLK1 deregulation. This raises the question of whether PLK1 overexpression is a cause or a consequence of oncogenesis. In addition, hypomethylation of the CpG island of the PLK1 promoter region contributes to its upregulation. PLK1 expression can be affected by many factors; thus, it is possible that PLK1 deregulation in each individual patient tumours could be due to different underlying mechanisms.

Cross Talk between Wnt/β-Catenin and CIP2A/Plk1 Signaling in Prostate Cancer: Promising Therapeutic Implications

Aberrant activation of the Wnt/β-catenin pathway and polo-like kinase 1 (Plk1) overexpression represent two common events in prostate cancer with relevant functional implications. This minireview analyzes their potential therapeutic significance in prostate cancer based on their role as androgen receptor (AR) signaling regulators and the pivotal role of the tumor suppressor protein phosphatase 2A (PP2A) modulating these pathways.

Satellite Cells in Muscular Dystrophy - Lost in Polarity

Recent findings employing the mdx mouse model for Duchenne muscular dystrophy (DMD) have revealed that muscle satellite stem cells play a direct role in contributing to disease etiology and progression of DMD, the most common and severe form of muscular dystrophy. Lack of dystrophin expression in DMD has critical consequences in satellite cells including an inability to establish cell polarity, abrogation of asymmetric satellite stem-cell divisions, and failure to enter the myogenic program. Thus, muscle wasting in dystrophic mice is not only caused by myofiber fragility but is exacerbated by intrinsic satellite cell dysfunction leading to impaired regeneration. Despite intense research and clinical efforts, there is still no effective cure for DMD. In this review we highlight recent research advances in DMD and discuss the current state of treatment and, importantly, how we can incorporate satellite cell-targeted therapeutic strategies to correct satellite cell dysfunction in DMD.

Increased Expression of Phosphorylated Polo-Like Kinase 1 and Histone in Bypass Vein Graft and Coronary Arteries following Angioplasty

Interventional procedures, including percutaneous transluminal coronary angioplasty (PTCA) and coronary artery bypass surgery (CABG) to re-vascularize occluded coronary arteries, injure the vascular wall and cause endothelial denudation and medial vascular smooth muscle cell (VSMCs) metaplasia. Proliferation of the phenotypically altered SMCs is the key event in the pathogenesis of intimal hyperplasia (IH). Several kinases and phosphatases regulate cell cycle in SMC proliferation. It is our hypothesis that increased expression and activity of polo-like kinase-1 (PLK1) in SMCs, following PTCA and CABG, contributes to greater SMC proliferation in the injured than uninjured blood vessels. Using immunofluorescence (IF), we assessed the expression of PLK1 and phosphorylated-PLK1 (pPLK1) in post-PTCA coronary arteries, and superficial epigastric vein grafts (SEV) and compared it with those in the corresponding uninjured vessels. We also compared the expressions of mitotic marker phospho-histone, synthetic-SMC marker, contractile SMC marker, IFN-γ and phosphorylated STAT-3 in the post-PTCA arteries, SEV-grafts, and the uninjured vessels. Immunostaining demonstrated an increase in the number of cells expressing PLK1 and pPLK1 in the neointima of post PTCA-coronary arteries and SEV-grafts compared to their uninjured counterparts. VSMCs in the neointima showed an increased expression of phospho-histone, synthetic and contractile SMC markers, IFN-γ and phosphorylated STAT-3. However, VSMCs of uninjured coronaries and SEV had no significant expression of the aforementioned proteins. These data suggest that PLK1 might play a critical role in VSMC mitosis in hyperplastic intima of the injured vessels. Thus, novel therapies to inhibit PLK1 could be developed to inhibit the mitogenesis of VSMCs and control neointimal hyperplasia.

Crosstalk between kinases, phosphatases and miRNAs in cancer

Reversible phosphorylation of proteins, performed by kinases and phosphatases, is the major post translational protein modification in eukaryotic cells. This intracellular event represents a critical regulatory mechanism of several signaling pathways and can be related to a vast array of diseases, including cancer. Cancer research has produced increasing evidence that kinase and phosphatase activity can be compromised by mutations and also by miRNA silencing, performed by small non-coding and endogenously produced RNA molecules that lead to translational repression. miRNAs are believed to target about one-third of human mRNAs while a single miRNA may target about 200 transcripts simultaneously. Regulation of the phosphorylation balance by miRNAs has been a topic of intense research over the last years, spanning topics going as far as cancer aggressiveness and chemotherapy resistance. By addressing recent studies that have shown miRNA expression patterns as phenotypic signatures of cancers and how miRNA influence cellular processes such as apoptosis, cell cycle control, angiogenesis, inflammation and DNA repair, we discuss how kinases, phosphatases and miRNAs cooperatively act in cancer biology.

Recently targeted kinases and their inhibitors-the path to clinical trials

Protein kinases have emerged as one of the most important drug target families for the treatment of cancer. To date, 28 inhibitors with reported activity versus one or multiple kinases have been approved for clinical use. However, the majority of new clinical trials are focused on new subindications using already approved kinase inhibitors or target well validated kinase targets with novel inhibitors. In contrast, relatively few clinical trials have been initiated using specific inhibitors that inhibit novel kinase targets, despite significant validation efforts in the public domain. Analysis of the target validation history of first in class kinase inhibitors revealed a long delay between initial disease association and development of inhibitors. As part of this analysis, we have investigated which first in class inhibitor that entered phase I clinical trials over the last five years and also considered which research approaches that were used to validate them.

An enzoinformatics study targeting polo-like kinases-1 enzyme: Comparative assessment of anticancer potential of compounds isolated from leaves of Ageratum houstonianum

Natural products from plant sources, embracing inherently ample structural diversity than synthetic ones are the major sources of anticancer agents and will constantly play as protagonists for discovering new drugs. Polo-like kinases (PLKs) play a leading role in the ordered execution of mitotic events and 4 mammalian PLK family members have been identified. PLK1 is an attractive target for anticancer drugs in mammalian cells, among the four members of PLKs. The present study expresses the molecular interaction of compounds (1,2-Benzenedicarboxylic acid bis (2 ethylhexyl) ester, squalene, 3,5-bis (1,1-dimethylethyl) phenol, Pentamethyl tetrahydro-5H-chromene, (1,4-Cyclohexylphenyl) ethanone and 6-Vinyl-7-methoxy-2,2-dimethylchromene) isolated from methanolic extract of leaves of Ageratum houstonianum with PLK1 enzyme. Docking between PLK1 and each of these compounds (separately) was performed using “Auto dock 4.2.” (1,4-Cyclohexylphenyl) ethanone showed the maximum potential as a promising inhibitor of PLK1 enzyme with reference to ∆G (−6.84 kcal/mol) and Ki (9.77 μM) values. This was sequentially followed by Pentamethyl tetrahydro-5H-chromene (∆G = −6.60 kcal/mol; Ki = 14.58 μM), squalene (∆G = −6.17 kcal/mol; Ki = 30.12 μM), 6-Vinyl-7-methoxy-2,2-dimethylchromene (∆G = −5.91 kcal/mol; Ki = 46.68 μM), 3, 5-bis (1,1-dimethylethyl) phenol (∆G = −5.70 kcal/mol; Ki = 66.68 μM) and 1,2-Benzenedicarboxylic acid bis (2 ethylhexyl) ester (∆G = −5.58 kcal/mol; Ki = 80.80 μM). These results suggest that (1,4-Cyclohexylphenyl) ethanone might be a potent PLK1 inhibitor. Further, in vitro and in vivo rumination are warranted to validate the anticancer potential of (1,4-Cyclohexylphenyl) ethanone.

Plk1-targeted therapies in TP53- or RAS-mutated cancer

Despite advances in treatment, prognosis for many types of carcinoma remains poor. Polo-like kinase 1 (Plk1) has been explored as a target for the development of anticancer drugs. As a mitotic master Ser/Thr kinase, Plk1 is involved in centrosomal maturation, microtubule nucleation, chromosomal segregation, and cytokinesis. Additional functions in interphase and in response to DNA damage have been revealed. The multiple locations of Plk1 correspond to distinct functions, mediated by phosphorylation of multiple substrates. Since it is highly expressed in several carcinomas, and expression of Plk1 is inversely correlated with the survival rate of patients in non-small cell lung, head and neck, and esophageal cancer, Plk1 is recognized as a valid prognostic marker. Connections between Plk1 and p53 or KRAS in carcinoma provide a rationale and several possible routes to the development of therapies. Tumors with both p53-deficiency and high Plk1 expression may be particularly sensitive to Plk1 inhibitors, although some controversial data exist. In KRAS-mutant cancers, on the other hand, Plk1 may be essential for tumor cell survival, but detailed studies as to whether Plk1 inhibitors are more effective in KRAS-mutant cancers must be performed in order to determine whether this is the case. Here, we present evidence for Plk1 as a prognostic marker and potentially effective target for the treatment of patients with carcinoma, to demonstrate the value of Plk1 as a target for the development of cancer treatment, especially for patients with solid tumors. In addition, the effects of Plk1 inhibition in p53- or KRAS-mutated cancer are discussed with respect to clinical implications. Structural specifics of Plk1 are presented, as well as current strategies for discovering new Plk1 inhibitors by targeting the conserved ATP binding site or polo-box domain of Plk1, in order to develop Plk1-specific anticancer drugs.

Lentiviral-mediated RNA interference targeting the PLK1 gene inhibits invasion and metastasis of esophageal squamous cell carcinoma cells

AIM: To investigate the inhibitory effect of lentiviral-mediated RNA interference targeting the PLK1 gene on invasion and metastasis of esophageal squamous cell carcinoma (ESCC) cells.

METHODS: RT-PCR and Western blot were used to detect the expression of PLK1 mRNA and protein in different ESCC cells. Based on the mRNA sequence of human PLK1 gene, interference fragments were designed, and interference efficiency was detected by Western blot. The in vitro effect of PLK1 siRNA on migration and invasion of ESCC cells was assessed by wound-healing assay and Matrigel chemoinvasion assay. The most efficient interference fragment was cloned into the lentiviral vector pGLV/H1/GFP+Puro and sequenced. The resulting recombinant lentiviral vector and packaging plasmids were transfected into 293T cells, and packaged virus particles were used to infect ESCC cells. Interference efficiency was assessed using fluorescence quantitative PCR and Western blot. The in vivo effect of recombinant lentiviral vector on invasion and metastasis of ESCC cells was studied using a nude mouse model of pulmonary metastasis.

RESULTS: The ESCC cell line TE-8 overexpressed PLK1, and the most efficient PLK1 siRNA could obviously inhibit migration and invasion of TE-8 cells in vitro. The lentiviral vector for RNA interference targeting the PLK1 gene was successfully constructed. The prepared recombinant virus particles could infect TE-8 cells and significantly inhibit the metastasis of ESCC cells in vivo.

CONCLUSION: Lentiviral-mediated RNA interference targeting PLK1 could obviously inhibit invasion and metastasis of ESCC cells. PLK1 may promote the malignant development of ESCC.

Asymmetric cell division of stem and progenitor cells during homeostasis and cancer

Stem and progenitor cells are characterized by their ability to self-renew and produce differentiated progeny. A fine balance between these processes is achieved through controlled asymmetric divisions and is necessary to generate cellular diversity during development and to maintain adult tissue homeostasis. Disruption of this balance may result in premature depletion of the stem/progenitor cell pool, or abnormal growth. In many tissues, including the brain, dysregulated asymmetric divisions are associated with cancer. Whether there is a causal relationship between asymmetric cell division defects and cancer initiation is as yet not known. Here, we review the cellular and molecular mechanisms that regulate asymmetric cell divisions in the neural lineage and discuss the potential connections between this regulatory machinery and cancer.

Targeting cell cycle kinases and kinesins in anticancer drug development

The cell cycle is regulated by kinases such as the cyclin-dependent kinases (CDKs) and non-CDKs, which include Aurora and polo-like kinases, as well as checkpoint proteins. Mitotic kinesins are involved in the establishment of the mitotic spindle formation and function, and also play a role in cell cycle control. The disruption of the cell cycle is a hallmark of malignancy. Genetic or epigenetic events result in the upregulation of these kinases and mitotic kinesins in a myriad of tumour types, suggesting that their inhibition could result in preferential targeting of malignant cells. Such findings make the development of these inhibitors a rational and attractive new area for cancer therapeutics. Although challenges of potency and non-specificity have hampered their progress through the clinic, several novel compounds are presently in various phases of clinical trial evaluation.

Polo-like kinase 1 as target for cancer therapy

Polo-like kinase 1 (Plk1) is an interesting molecule both as a biomarker and as a target for highly specific cancer therapy for several reasons. Firstly, it is over-expressed in many cancers and can serve as a biomarker to monitor treatment efficacy of Plk1 inhibitors. Furthermore, the Plk1 enzyme is expressed only in dividing cells and is a major regulator of the cell cycle. It controls entry into mitosis and regulates the spindle checkpoint. The expression of Plk1 in normal cells is not nearly as strong as that in cancer cells, which makes Plk1 a discriminating tartget for the development of cancer-specific small molecule drugs. RNA interference experiments in vitro and in vivo have indicated that downregulation of Plk1 expression represents an attractive concept for cancer therapy. Over the years, a number of Plk1 inhibitors have been discovered. Many of these inhibitors are substances that compete with ATP for the substrate binding site. The ATP-competitive inhibitor BI 6727 is currently being clinically tested in cancer patients. Another drug in development, poloxin, is the first Polo-box domain inhibitor of Plk1. This compound is a derivative of the natural product, thymoquinone, derived from Nigella sativa. A novel and promising strategy is to synthesize bifunctional inhibitors that combine the high binding affinity of ATP inhibitors with the specificity of competitive inhibitors.

A series of beta-carboline derivatives inhibit the kinase activity of PLKs

Polo-like kinases play an essential role in the ordered execution of mitotic events and 4 mammalian PLK family members have been identified. Accumulating evidence indicates that PLK1 is an attractive target for anticancer drugs. In this paper, a series of beta-carboline derivatives were synthesized and three compounds, DH281, DH285 and DH287, were identified as potent new PLK inhibitors. We employed various biochemical and cellular approaches to determine the effects of these compounds on the activity of PLK1 and other mitotic kinases and on cell cycle progression. We found that these three compounds could selectively inhibit the kinase activity of purified PLK1, PLK2 and PLK3 in vitro. They show strong antitumor activity against a number of cancer cell lines with relatively low micromolar IC(50)s, but are relatively less toxic to non-cancer cells (MRC5). Moreover, these compounds could induce obvious accumulation of HeLa cells in G(2)/M and S phases and trigger apoptosis. Although MRC5 cells show clear S-phase arrest after treatment with these compounds, the G2/M arrest and apoptosis are less insignificant, indicating the distinct sensitivity between normal and cancer cells. We also found that HeLa cells treated with these drugs exhibit monopolar spindles and increased Wee1 protein levels, the characteristics of cells treated with PLK1 inhibitors. Together, these results demonstrate that DH281, DH285 and DH287 beta-carboline compounds are new PLK inhibitors with potential for cancer treatment.

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.

Computational design of targeted inhibitors of polo-like kinase 1 (plk1)

Computational design of small molecule putative inhibitors of Polo-like kinase 1 (Plk1) is presented. Plk1, which regulates the cell cycle, is often over expressed in cancers. Down regulation of Plk1 has been shown to inhibit tumor progression. Most kinase inhibitors interact with the ATP binding site on Plk1, which is highly conserved. This makes the development of Plk1-specific inhibitors challenging, since different kinases have similar ATP sites. However, Plk1 also contains a unique region called the polo-box domain (PBD), which is absent from other kinases. In this study, the PBD site was used as a target for designed Plk1 putative inhibitors. Common structural features of several experimentally known Plk1 ligands were first identified. The findings were used to design small molecules that specifically bonded Plk1. Drug likeness and possible toxicities of the molecules were investigated. Molecules with no implied toxicities and optimal drug likeness values were used for docking studies. Several molecules were identified that made stable complexes only with Plk1 and LYN kinases, but not with other kinases. One molecule was found to bind exclusively the PBD site of Plk1. Possible utilization of the designed molecules in drugs against cancers with over expressed Plk1 is discussed.

Common variation in Nemo-like kinase is associated with risk of ovarian cancer

BACKGROUND

Overexpression of mitotic kinases has been associated with prognosis, histologic grade, and clinical stage in ovarian cancer, but the relationship between inherited variation in these genes and ovarian cancer risk has not been well defined.

METHODS

We measured associations between 397 single nucleotide polymorphisms (SNPs) from 67 mitotic kinases and invasive epithelial ovarian cancer risk in two case-control studies (n = 671 cases; n = 939 controls). Thirty-six candidate SNPs (P < 0.05) were assessed in a replication analysis consisting of three additional studies (n = 1,094 cases; n = 829 controls).

RESULTS

In initial analysis, thirty-six SNPs were suggestive of association with risk of serous ovarian cancer, all subtypes of ovarian cancer, or both (P < 0.05). Replication analyses suggested an association between rs2125846 in the Nemo-like kinase (NLK) gene and ovarian cancer (serous OR = 1.36, 95% CI: 1.11-1.67, P = 1.77 × 10(-3); all subtypes OR = 1.30, 95% CI: 1.08-1.56, P = 2.97 × 10(-3)). Furthermore, rs2125846 was associated with risk in the combined discovery and replication sets (serous OR = 1.33, 95% CI: 1.15-1.54; all subtypes OR = 1.27, 95% CI: 1.12-1.45).

CONCLUSIONS

Variation in NLK may be associated with risk of invasive epithelial ovarian cancer. Further studies are needed to confirm and understand the biologic relationship between this mitotic kinase and ovarian cancer risk.

IMPACT

An association between SNPs in NLK and ovarian cancer may provide biologic insight into the development of this disease.

Discovery of a potent and orally bioavailable benzolactam-derived inhibitor of Polo-like kinase 1 (MLN0905)

This article describes the discovery of a series of potent inhibitors of Polo-like kinase 1 (PLK1). Optimization of this benzolactam-derived chemical series produced an orally bioavailable inhibitor of PLK1 (12c, MLN0905). In vivo pharmacokinetic-pharmacodynamic experiments demonstrated prolonged mitotic arrest after oral administration of 12c to tumor bearing nude mice. A subsequent efficacy study in nude mice achieved tumor growth inhibition or regression in a human colon tumor (HT29) xenograft model.

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-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.

Depsides isolated from the Sri Lankan lichen Parmotrema sp. exhibit selective Plk1 inhibitory activity

CONTEXT

Mitotic kinase enzymes regulate critical stages of mitosis and are amenable to pharmacological inhibition. Since natural products have been a rich source of antimitotic inhibitors, we postulated that natural products would also provide effective inhibitors of mitotic kinases.

OBJECTIVE

To explore unique marine and terrestrial natural product sources for new anticancer drug leads, we screened our natural product extract library for polo-like kinase-1 (Plk1) kinase inhibitors.

MATERIALS AND METHODS

Extracts of the lichen Parmotrema sp. (Parmeliaceae) exhibited in vitro inhibitory activity. Bioassay-guided fractionation of the Parmotrema sp. extract led to the isolation of depside inhibitors.

RESULTS

A new depside 1 has been isolated from the Sri Lankan lichen Parmotrema sp. along with the known metabolites 2 (β-collatolic acid) and 3 (β-alectoronic acid). The structure of depside 1 was elucidated by spectroscopic analysis. The three depsides 1-3 exhibited moderate inhibition of purified recombinant Plk1 kinase with IC₅₀ of 2.8, 0.7, and 1.7 µM, respectively, at 1 µM ATP. Inhibitory activity was also observed at high concentrations of ATP, suggesting the potential for activity in a cellular environment. The depsides were also tested against a panel of 23 other recombinant kinases and were found to possess up to 30-fold selectivity toward Plk1.

DISCUSSION AND CONCLUSION

These data suggest that the depsides 1-3 may serve as core structures that can be further explored as potential inhibitors of Plk1 and other kinases.

Polo-like kinase 1 is a therapeutic target in high-risk neuroblastoma

PURPOSE

High-risk neuroblastoma remains a therapeutic challenge for pediatric oncologists. The Polo-like kinase 1 (PLK1) is highly expressed in many human cancers and is a target of the novel small-molecule inhibitor BI 2536, which has shown promising anticancer activity in adult malignancies. Here, we investigated the effect of BI 2536 on neuroblastoma cells in vitro and in vivo to explore PLK1 as a potential target in high-risk neuroblastoma therapy.

EXPERIMENTAL DESIGN

PLK1 transcript levels were analyzed by microarrays in 476 primary neuroblastoma specimens, and correlation with prognostic markers and patient outcome was examined. To explore the effect of PLK1 inhibition on neuroblastoma cells, 7 cell lines were treated with BI 2536 and changes in growth properties were determined. Furthermore, nude mice with IMR-32 and SK-N-AS xenografts were treated with BI 2536.

RESULTS

PLK1 is highly expressed in unfavorable neuroblastoma and in neuroblastoma cell lines. Expression of PLK1 is associated with unfavorable prognostic markers such as stage 4, age >18 months, MYCN amplification, unfavorable gene expression-based classification, and adverse patient outcome (P < 0.001 each). On treatment with nanomolar doses of BI 2536, all neuroblastoma cell lines analyzed showed significantly reduced proliferation, cell cycle arrest, and cell death. Moreover, BI 2536 abrogated growth of neuroblastoma xenografts in nude mice.

CONCLUSIONS

Elevated PLK1 expression is significantly associated with high-risk neuroblastoma and unfavorable patient outcome. Inhibition of PLK1 using BI 2536 exhibits strong antitumor activity on human neuroblastoma cells in vitro and in vivo, opening encouraging new perspectives for the treatment of high-risk neuroblastoma.

Sensitivity of cancer cells to Plk1 inhibitor GSK461364A is associated with loss of p53 function and chromosome instability

Polo-like kinases are a family of serine threonine kinases that are critical regulators of cell cycle progression and DNA damage response. Predictive biomarkers for the Plk1-selective inhibitor GSK461364A were identified by comparing the genomics and genetics of a panel of human cancer cell lines with their response to a drug washout followed by an outgrowth assay. In this assay, cell lines that have lost p53 expression or carry mutations in the TP53 gene tended to be more sensitive to GSK461364A. These more sensitive cell lines also had increased levels of chromosome instability, a characteristic associated with loss of p53 function. Further mechanistic studies showed that p53 wild-type (WT) and not mutant cells can activate a postmitotic tetraploidy checkpoint and arrest at pseudo-G(1) state after GSK461364A treatment. RNA silencing of WT p53 increased the antiproliferative activity of GSK461364A. Furthermore, silencing of p53 or p21/CDKN1A weakened the tetraploidy checkpoint in cells that survived mitotic arrest and mitotic slippage. As many cancer therapies tend to be more effective in p53 WT patients, the higher sensitivity of p53-deficient tumors toward GSK461364A could potentially offer an opportunity to treat tumors that are refractory to other chemotherapies as well as early line therapy for these genotypes.