Down-regulation of Polo-like kinase 1 elevates drug sensitivity of breast cancer cells in vitro and in vivo

Antitumor Effects of Poplar Propolis on DLBCL SU-DHL-2 Cells

Propolis is resinous natural product produced by Western honeybees using beeswax and plant and bud exudates, which has a wide range of biological activities, including antioxidation, antibacterial, anti-inflammation, immune regulation, antitumor, and so on. Diffuse large B-cell lymphoma (DLBCL) is an aggressive cancer, and accounts for about 30% of all lymphomas. The effect of poplar propolis on DLBCL has not been reported. The IC₅₀ of propolis on the proliferation of DLBCL SU-DHL-2 cell line and its proteins and gene expressions were detected by CCK-8 kit, label-free proteomic, and RT-PCR. The results showed that the IC₅₀ of propolis at the 5 × l05/mL cell for 24 h was 5.729 μg/mL. Label-free-based proteomics analysis showed that there were 115 differentially expressed proteins (61 up-regulated and 54 down-regulated proteins) between IC₅₀ dose-treated and solvent control groups. There were 32.47% differential proteins located in the nucleus, 20.78% in the cytoplasm, and 14.29% in mitochondria. The most significant different pathway (p = 0.0016) of protein enrichment was ferroptosis (including glutamate-cysteine ligase regulatory subunit, ferritin, and heme oxygenase). The relative expression trend of 17 of the total 22 genes selected according to proteomics results was in line with their encoded protein. The highest protein-protein interaction was serine/threonine-protein kinase PLK, which interacted with 16 differential proteins. In conclusion, poplar propolis inhibited SU-DHL-2 cells via ferroptosis pathway, accelerating cell death and down-regulated serine/threonine-protein kinase PLK1, affecting apoptosis of cell. This result provides a theoretical basis for the treatment of DLBCL using propolis.

Bioinformatics Analysis Highlight Differentially Expressed CCNB1 and PLK1 Genes as Potential Anti-Breast Cancer Drug Targets and Prognostic Markers

Breast cancer is one of the most common malignant tumors in women worldwide. Early diagnosis, treatment, and prognosis of breast cancer are global challenges. Identification of valid predictive diagnosis and prognosis biomarkers and drug targets are crucial for breast cancer prevention. This study characterizes differentially expressed genes (DEGs) based on the TCGA database by using DESeq2, edgeR, and limma. A total of 2032 DEGs, including 1026 up-regulated genes and 1006 down-regulated genes were screened. Followed with WGCNA, PPI analysis, GEPIA 2, and HPA database verification, thirteen hub genes including CDK1, BUB1, BUB1B, CDC20, CCNB2, CCNB1, KIF2C, NDC80, CDCA8, CENPF, BIRC5, AURKB, PLK1, MAD2L1, and CENPE were obtained, and they may serve as potential therapeutic targets of breast cancer. Especially, overexpression of CCNB1 and PLK1 are strongly associated with the low survival rate of breast cancer patients, demonstrating their potentiality as prognostic markers. Moreover, CCNB1 and PLK1 are highly expressed in all breast cancer stages, suggesting that they could be further studied as potential drug targets. Taken together, our study highlights CCNB1 and PLK1 as potential anti-breast cancer drug targets and prognostic markers.

Internalization of Foldamer-Based DNA Mimics through a Site-Specific Antibody Conjugate to Target HER2-Positive Cancer Cells

Inhibition of protein-DNA interactions represents an attractive strategy to modulate essential cellular functions. We reported the synthesis of unique oligoamide-based foldamers that adopt single helical conformations and mimic the negatively charged phosphate moieties of B-DNA. These mimics alter the activity of DNA interacting enzymes used as targets for cancer treatment, such as DNA topoisomerase I, and they are cytotoxic only in the presence of a transfection agent. The aim of our study was to improve internalization and selective delivery of these highly charged molecules to cancer cells. For this purpose, we synthesized an antibody-drug conjugate (ADC) using a DNA mimic as a payload to specifically target cancer cells overexpressing HER2. We report the bioconjugation of a 16-mer DNA mimic with trastuzumab and its functional validation in breast and ovarian cancer cells expressing various levels of HER2. Binding of the ADC to HER2 increased with the expression of the receptor. The ADC was internalized into cells and was more efficient than trastuzumab at inhibiting their growth in vitro. These results provide proof of concept that it is possible to site-specifically graft high molecular weight payloads such as DNA mimics onto monoclonal antibodies to improve their selective internalization and delivery in cancer cells.

Exploration of Prognostic Biomarkers among Replication Factor C Family in the Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is one of the common cancers with a high incidence and mortality. The human replication factor C (RFC) family contains 5 subunits that play an important role in DNA replication and DNA damage repair. RFCs are abnormally expressed in a variety of cancers; some of them are differentially expressed in HCC tissues and related to tumor growth. However, the expression, prognostic value, and effect targets of the whole RFC family in HCC are still unclear. To address these issues, we performed a multidimensional analysis of RFCs in HCC patients by Oncomine, UALCAN, GEPIA, Human protein atlas, Kaplan-Meier plotter, cBioPortal, GeneMANIA, String, and LinkedOmics. mRNA expression of RFCs was significantly increased in HCC tissues. There was a significant correlation between the expression of RFC2/3/4/5 and tumor stage of HCC patients. Besides, high mRNA expression of RFC2/4 was associated with worse overall survival (OS). Moreover, genetic alterations of RFCs were associated with worse OS in HCC patients. We found that genes co-expressed with RFC2/4 were mainly involved in biological processes, such as chromosome segregation, mitotic cell cycle phase transition, and telomere organization and they activated the cell cycle and spliceosome pathways. The gene set is mainly enriched in cancer-related kinases AURKA, ATR, CDK1, PLK1, and CHEK1. E2F family members were the key transcription factors for RFCs. Our results suggest that differentially expressed RFC2 and RFC4 are potential prognostic biomarkers in HCC and may act on E2F transcription factors and some kinase targets to dysregulate the cell cycle pathway. These efforts may provide new research directions for prognostic biomarkers and therapeutic targets in HCC.

Non-mitotic functions of polo-like kinases in cancer cells

Inhibitors of mitotic protein kinases are currently being developed as non-neurotoxic alternatives of microtubule-targeting agents (taxanes, vinca alkaloids) which provide a substantial survival benefit for patients afflicted with different types of solid tumors. Among the mitotic kinases, the cyclin-dependent kinases, the Aurora kinases, the kinesin spindle protein and Polo-like kinases (PLKs) have emerged as attractive targets of cancer therapeutics. The functions of mammalian PLK1-5 are traditionally linked to the regulation of the cell cycle and to the stress response. Especially the key role of PLK1 and PLK4 in cellular growth and proliferation, their overexpression in multiple types of human cancer and their druggability, make them appealing targets for cancer therapy. Inhibitors for PLK1 and PLK4 are currently being tested in multiple cancer trials. The clinical success of microtubule-targeting agents is attributed not solely to the induction of a mitotic arrest in cancer cells, but also to non-mitotic effects like targeting intracellular trafficking on microtubules. This raises the question whether new cancer targets like PLK1 and PLK4 regulate critical non-mitotic functions in tumor cells. In this article we summarize the important roles of PLK1-5 for the regulation of non-mitotic signaling. Due to these functions it is conceivable that inhibitors for PLK1 or PLK4 can target interphase cells, which underscores their attractive potential as cancer drug targets. Moreover, we also describe the contribution of the tumor-suppressors PLK2, PLK3 and PLK5 to cancer cell signaling outside of mitosis. These observations highlight the urgent need to develop highly specific ATP-competitive inhibitors for PLK4 and for PLK1 like the 3rd generation PLK-inhibitor Onvansertib to prevent the inhibition of tumor-suppressor PLKs in- and outside of mitosis. The remarkable feature of PLKs to encompass a unique druggable domain, the polo-box-domain (PBD) that can be found only in PLKs offers the opportunity for the development of inhibitors that target PLKs exclusively. Beyond the development of mono-specific ATP-competitive PLK inhibitors, the PBD as drug target will support the design of new drugs that eradicate cancer cells based on the mitotic and non-mitotic function of PLK1 and PLK4.

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.

CDK1-PLK1/SGOL2/ANLN pathway mediating abnormal cell division in cell cycle may be a critical process in hepatocellular carcinoma

This study aims to investigate the potential mechanisms and identify core biomarkers of Hepatocellular carcinoma (HCC). The profile GSE113850 was downloaded to analyze the differentially expressed genes. Gene ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), and protein-protein interaction network analysis were used to reveal the main signal pathways of the differentially expressed genes (DEGs) and hub genes. The correlation between core gene expression and pathological stages, the disease-free survival analysis, the overall survival analysis were analyzed by Gene Expression Profiling Interactive Analysis. Furthermore, we reidentified the expression level of core genes of carcinoma tissues and para-carcinoma tissues from 14 HCC patients with real-time reverse transcription-polymerase chain reaction analysis (RT-PCR) and western blotting. After SK-Hep1 cell was treated with cyclin-dependent kinase 1 (CDK1) siRNA for 72 h, we detected the expression of the core genes and fluorescence-activated cell sorting analysis. A total of 378 DEGs were found. GO and KEGG analysis revealed that the DEGs were mainly enriched in the cell cycle. There were positive correlations among CDK1, polo-like kinase 1, shugoshin2 and anillin actin-binding protein. Moreover, the expression levels of four core genes were related to the HCC occurrence, pathological stages, and survivorship curve. The clinical HCC specimens verified the higher expression level of core genes by real-time RT-PCR. The transfection of siCDK1 in SK-Hep1 resulted in a disordered cell cycle. Furthermore, CDK1 knockdown suppressed the expression of PLK1, ANLN, and SGOL2. The CDK1-PLK1/SGOL2/ANLN pathway mediating abnormal cell division in the cell cycle might be a critical process in HCC.

PLK1 Knockdown Inhibits Cell Proliferation and Cell Apoptosis, and PLK1 Is Negatively Regulated by miR-4779 in Osteosarcoma Cells

Polo-like kinase 1 (PLK1) is a ubiquitous serine/threonine protein kinase. It is reported to be involved in the occurrence and progression of various human cancers. In the present study, we explored the role and molecular mechanism of PLK1 in the proliferation of osteosarcoma (OS) cells. We found that PLK1 expression was higher in MG63/Dox cells than in MG63 cells, while inhibiting or interfering with the level of PLK1 suppressed cell proliferation of MG63/Dox cells. TargetScan analysis predicted that miR-4779 would interact with the 3'-UTR of PLK1 mRNAs and also inhibit cell autophagy of MG63/Dox cells. The data demonstrated that miR-4779 negatively regulates the expression of PLK1, and both miR-4779 and PLK1 regulate cell proliferation and cell apoptosis of MG63/Dox cells, processes that are involved in the drug resistance of OS cells.

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.

Mitosis inhibitors in anticancer therapy: When blocking the exit becomes a solution

Current microtubule-targeting agents (MTAs) remain amongst the most important antimitotic drugs used against a broad range of malignancies. By perturbing spindle assembly, MTAs activate the spindle assembly checkpoint (SAC), which induces mitotic arrest and subsequent apoptosis. However, besides toxic side effects and resistance, mitotic slippage and failure in triggering apoptosis in various cancer cells are limiting factors of MTAs efficacy. Alternative strategies to target mitosis without affecting microtubules have, thus, led to the identification of small molecules, such as those that target spindle Kinesins, Aurora and Polo-like kinases. Unfortunately, these so-called second-generation of antimitotics, encompassing mitotic blockers and mitotic drivers, have failed in clinical trials. Our recent understanding regarding the mechanisms of cell death during a mitotic arrest pointed out apoptosis as the main variable, providing an opportunity to control the cell fates and influence the effectiveness of antimitotics. Here, we provide an overview on the second-generation of antimitotics, and discuss possible strategies that exploit SAC activity, mitotic slippage/exit and apoptosis induction, in order to improve the efficacy of anticancer strategies that target mitosis.

Targeting forkhead box M1 transcription factor in breast cancer

Breast cancer continues to be the most commonly diagnosed malignancy and second most common cause of cancer-related deaths among women in the United States. Improved understanding of the molecular heterogeneity of breast tumors and the approval of multiple targeted therapies have revolutionized the treatment landscape and long-term survival rates for patients with breast cancer. Despite the development of highly effective targeted agents, drug resistance and disease progression remain major clinical concerns. Improved understanding of the molecular mechanisms mediating drug resistance will allow new treatments to be developed. The forkhead box M1 (FoxM1) transcription factor is overexpressed in breast cancer and strongly associated with resistance to targeted therapies and chemotherapy. FoxM1 regulates all hallmarks of cancer, including proliferation, mitosis, EMT, invasion, and metastasis. Inhibition of FoxM1 transcription factor function is a potential strategy for overcoming breast cancer progression. In this research update, we review the role of FoxM1 in breast cancer and pharmacological approaches for blocking FoxM1 transcription factor function. Future preclinical studies should evaluate combination drug strategies to inhibit FoxM1 function and upstream kinase signaling pathways as potential strategies to treat resistant and metastatic breast cancers.

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.

The Emerging Role of Polo-Like Kinase 1 in Epithelial-Mesenchymal Transition and Tumor Metastasis

Polo-like kinase 1 (PLK1) is a serine/threonine kinase that plays a key role in the regulation of the cell cycle. PLK1 is overexpressed in a variety of human tumors, and its expression level often correlates with increased cellular proliferation and poor prognosis in cancer patients. It has been suggested that PLK1 controls cancer development through multiple mechanisms that include canonical regulation of mitosis and cytokinesis, modulation of DNA replication, and cell survival. However, emerging evidence suggests novel and previously unanticipated roles for PLK1 during tumor development. In this review, we will summarize the recent advancements in our understanding of the oncogenic functions of PLK1, with a focus on its role in epithelial-mesenchymal transition and tumor invasion. We will further discuss the therapeutic potential of these functions.

The generation of compartmentalized nanoparticles containing siRNA and cisplatin using a multi-needle electrohydrodynamic strategy

This study outlines a novel manufacturing technique for the generation of compartmentalized trilayered nanoparticles loaded with an anti-cancer agent and siRNA as a platform for the combination treatment of cancers. More specifically, we describe the use of a multi-needle electrohydrodynamic approach to produce nanoparticles with high size specificity and scalable output, while allowing suitable environments for each therapeutic agent. The inner polylactic-glycolic-acid (PLGA) layer was loaded with cisplatin while the middle chitosan layer was loaded with siRNA. The corresponding polymeric solutions were characterized for their viscosity, surface tension and conductivity, while particle size was determined using dynamic light scattering. The internal structure was studied using transmission electron microscopy (TEM) and Structured Illumination Microscopy (SIM). The inclusion of cisplatin was studied using electron dispersive spectroscopy (EDS). We were able to generate nanoparticles of approximate size 130 nm with three distinct layers containing an outer protective PLGA layer, a middle layer of siRNA and an inner layer of cisplatin. These particles have the potential not only for uptake into tumors via the enhanced permeability and retention (EPR) effect but also the sequential release of the siRNA and chemotherapeutic agent, thereby providing a means of overcoming challenges of targeting and tumor drug resistance.

lncRNA HOXD-AS1 Regulates Proliferation and Chemo-Resistance of Castration-Resistant Prostate Cancer via Recruiting WDR5

Castration-resistant prostate cancer (CRPC) that occurs after the failure of androgen deprivation therapy is the leading cause of deaths in prostate cancer patients. Thus, there is an obvious and urgent need to fully understand the mechanism of CRPC and discover novel therapeutic targets. Long noncoding RNAs (lncRNAs) are crucial regulators in many human cancers, yet their potential roles and molecular mechanisms in CRPC are poorly understood. In this study, we discovered that an lncRNA HOXD-AS1 is highly expressed in CRPC cells and correlated closely with Gleason score, T stage, lymph nodes metastasis, and progression-free survival. Knockdown of HOXD-AS1 inhibited the proliferation and chemo-resistance of CRPC cells in vitro and in vivo. Furthermore, we identified several cell cycle, chemo-resistance, and castration-resistance-related genes, including PLK1, AURKA, CDC25C, FOXM1, and UBE2C, that were activated transcriptionally by HOXD-AS1. Further investigation revealed that HOXD-AS1 recruited WDR5 to directly regulate the expression of target genes by mediating histone H3 lysine 4 tri-methylation (H3K4me3). In conclusion, our findings indicate that HOXD-AS1 promotes proliferation, castration resistance, and chemo-resistance in prostate cancer by recruiting WDR5. This sheds a new insight into the regulation of CRPC by lncRNA and provides a potential approach for the treatment of CRPC.

Therapeutic relevance of the protein phosphatase 2A in cancer

Chromosomal Instability (CIN) is regarded as a unifying feature of heterogeneous tumor populations, driving intratumoral heterogeneity. Polo-Like Kinase 1 (PLK1), a serine-threonine kinase that is often overexpressed across multiple tumor types, is one of the key regulators of CIN and is considered as a potential therapeutic target. However, targeting PLK1 has remained a challenge due to the off-target effects caused by the inhibition of other members of the polo-like family. Here we use synthetic dosage lethality (SDL), where the overexpression of PLK1 is lethal only when another, normally non-lethal, mutation or deletion is present. Rather than directly inhibiting PLK1, we found that inhibition of PP2A causes selective lethality to PLK1-overexpressing breast, pancreatic, ovarian, glioblastoma, and prostate cancer cells. As PP2A is widely regarded as a tumor suppressor, we resorted to gene expression datasets from cancer patients to functionally dissect its therapeutic relevance. We identified two major classes of PP2A subunits that negatively correlated with each other. Interestingly, most mitotic regulators, including PLK1, exhibited SDL interactions with only one class of PP2A subunits (PPP2R1A, PPP2R2D, PPP2R3B, PPP2R5B and PPP2R5D). Validation studies and other functional cell-based assays showed that inhibition of PPP2R5D affects both levels of phospho-Rb as well as sister chromatid cohesion in PLK1-overexpressing cells. Finally, analysis of clinical data revealed that patients with high expression of mitotic regulators and low expression of Class I subunits of PP2A improved survival. Overall, these observations point to a context-dependent role of PP2A that warrants further exploration for therapeutic benefits.

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.

Cytotoxic mechanism of PLK1 inhibitor GSK461364 against osteosarcoma: Mitotic arrest, apoptosis, cellular senescence, and synergistic effect with paclitaxel

Polo-like kinase 1 (PLK1), a serine/threonine kinase and an oncogene, is crucial in regulating cell cycle progression. PLK1 also has been demonstrated as a potential target of osteosarcoma (OS) by using short hairpin RNA libraries in lentiviral vectors for screening of protein kinase. In preclinical studies, GSK461364, a potent and selective ATP-competitive PLK1 inhibitor, showed antiproliferative activity against multiple tumor cell lines. In the present study, we evaluated the expression level of PLK1 in OS and explored the cytotoxic mechanism of GSK461364 against OS. PLK1 was significantly overexpressed in OS compared with normal osteoblasts and other types of sarcoma. GSK461364 inhibited PLK1 and caused mitotic arrest by inducing G2/M arrest in OS cells. Moreover, GSK461364 exerted a cytotoxic effect by inducing apoptosis in OS, and induced cellular senescence in OS cell lines, as indicated by an increased senescence-associated β-galactosidase activity and enhanced DcR2 and interleukin-1α expression. In addition, we demonstrated a synergistic cytotoxic effect of GSK461364 and paclitaxel, possibly resulting from combined mitotic arrest. In conclusion, the present study revealed that PLK1 was overexpressed in OS and that GSK461364 exerted its cytotoxic effect on OS by inducing mitotic arrest and subsequent apoptosis and induced cellular senescence; therefore, senescence-associated markers can be used as treatment biomarkers, and a combination of GSK461364 and paclitaxel can potentially treat OS.

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.

Phase I trial of volasertib, a Polo-like kinase inhibitor, plus platinum agents in solid tumors: safety, pharmacokinetics and activity

BACKGROUND

This trial evaluated the maximum tolerated dose (MTD), safety, pharmacokinetics, and activity of volasertib, a selective Polo-like kinase 1 inhibitor that induces mitotic arrest and apoptosis, combined with cisplatin or carboplatin in patients with advanced/metastatic solid tumors (NCT00969761; 1230.6).

METHODS

Sequential patient cohorts (3 + 3 dose-escalation design) received a single infusion of volasertib (100-350 mg) with cisplatin (60-100 mg/m(2)) or carboplatin (area under the concentration versus time curve [AUC]4-AUC6) on day 1 every 3 weeks for up to six cycles. Sixty-one patients received volasertib/cisplatin (n = 30) or volasertib/carboplatin (n = 31) for a median of 3.5 (range, 1-6) and 2.0 (range, 1-6) treatment cycles, respectively.

RESULTS

The most common cycle 1 dose-limiting toxicities (DLTs) were thrombocytopenia, neutropenia and fatigue. MTDs (based on cycle 1 DLTs) were determined to be volasertib 300 mg plus cisplatin 100 mg/m(2) and volasertib 300 mg plus carboplatin AUC6. Co-administration did not affect the pharmacokinetics of each drug. Partial responses were observed in two patients in each arm. Stable disease was achieved in 11 and six patients treated with volasertib/cisplatin and volasertib/carboplatin, respectively.

CONCLUSIONS

Volasertib plus cisplatin or carboplatin at full single-agent doses was generally manageable and demonstrated activity in heavily pretreated patients with advanced solid tumors.

Mechanical activation of mammalian target of rapamycin pathway is required for cartilage development

Mechanical stress regulates development by modulating cell signaling and gene expression. However, the cytoplasmic components mediating mechanotransduction remain unclear. In this study, elimination of muscle contraction during chicken embryonic development resulted in a reduction in the activity of mammalian target of rapamycin (mTOR) in the cartilaginous growth plate. Inhibition of mTOR activity led to significant inhibition of chondrocyte proliferation, cartilage tissue growth, and expression of chondrogenic genes, including Indian hedgehog (Ihh), a critical mediator of mechanotransduction. Conversely, cyclic loading (1 Hz, 5% matrix deformation) of embryonic chicken growth plate chondrocytes in 3-dimensional (3D) collagen scaffolding induced sustained activation of mTOR. Mechanical activation of mTOR occurred in serum-free medium, indicating that it is independent of growth factor or nutrients. Treatment of chondrocytes with Rapa abolished mechanical activation of cell proliferation and Ihh gene expression. Cyclic loading of chondroprogenitor cells deficient in SH2-containing protein tyrosine phosphatase 2 (Shp2) further enhanced mechanical activation of mTOR, cell proliferation, and chondrogenic gene expression. This result suggests that Shp2 is an antagonist of mechanotransduction through inhibition of mTOR activity. Our data demonstrate that mechanical activation of mTOR is necessary for cell proliferation, chondrogenesis, and cartilage growth during bone development, and that mTOR is an essential mechanotransduction component modulated by Shp2 in the cytoplasm.

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.

An RNAi-based screen reveals PLK1, CDK1 and NDC80 as potential therapeutic targets in malignant pleural mesothelioma

BACKGROUND

Malignant pleural mesothelioma (MPM) is an aggressive tumour originating in the thoracic mesothelium. Prognosis remains poor with 9- to 12-month median survival, and new targets for treatments are desperately needed.

METHODS

Utilising an RNA interference (RNAi)-based screen of 40 genes overexpressed in tumours, including genes involved in the control of cell cycle, DNA replication and repair, we investigated potential therapeutic targets for MPM. Following in vitro characterisation of the effects of target silencing on MPM cells, candidates were assessed in tumour samples from 154 patients.

RESULTS

Gene knockdown in MPM cell lines identified growth inhibition following knockdown of NDC80, CDK1 and PLK1. Target knockdown induced cell-cycle arrest and increased apoptosis. Using small-molecule inhibitors specific for these three proteins also led to growth inhibition of MPM cell lines, and Roscovitine (inhibitor of CDK1) sensitised cells to cisplatin. Protein expression was also measured in tumour samples, with markedly variable levels of CDK1 and PLK1 noted. PLK1 expression in over 10% of cells correlated significantly with a poor prognosis.

CONCLUSION

These results suggest that RNAi-based screening has utility in identifying new targets for MPM, and that inhibition of NDC80, CDK1 and PLK1 may hold promise for treatment of this disease.

Impact of anti-PLK1 siRNA-containing F3-targeted liposomes on the viability of both cancer and endothelial cells

We have previously described the development of novel sterically stabilized F3-targeted pH-sensitive liposomes, which exhibited the ability to target both cancer and endothelial cells. Herein, the therapeutic potential of those liposomes was assessed upon encapsulation of a siRNA against a well-validated molecular target, PLK1. Treatment of prostate cancer (PC3) and angiogenic endothelial (HMEC-1) cells with F3-targeted liposomes containing anti-PLK1 siRNA resulted in a significant decrease in cell viability, which was mediated by a marked PLK1 silencing, both at the mRNA and protein levels. Furthermore, pre-treatment of PC3 cells with F3-targeted liposomes containing anti-PLK1 siRNA enabled a 3-fold reduction of paclitaxel IC50 and a 2.5-fold augment of the percentage of cancer cells in G2/mitosis arrest, which ultimately culminated in cell death. Overall, the F3-targeted nanocarrier containing an anti-PLK1 siRNA might constitute a valuable system for prostate cancer treatment, either applied in a single schedule or combined with conventional chemotherapy.

Mitosis as an anti-cancer drug target

Suppression of cell proliferation by targeting mitosis is one potential cancer intervention. A number of existing chemotherapy drugs disrupt mitosis by targeting microtubule dynamics. While efficacious, these drugs have limitations, i.e. neuropathy, unpredictability and development of resistance. In order to overcome these issues, a great deal of effort has been spent exploring novel mitotic targets including Polo-like kinase 1, Aurora kinases, Mps1, Cenp-E and KSP/Eg5. Here we summarize the latest developments in the discovery and clinical evaluation of new mitotic drug targets.

Plk1-dependent microtubule dynamics promotes androgen receptor signaling in prostate cancer

BACKGROUND

The androgen receptor (AR) signaling continues to be essential in castrate-resistant prostate cancer (CRPC). Taxel-based chemotherapy is the current standard treatment for CRPC patients. Unfortunately, almost all patients eventually develop resistance toward this chemotherapy. Significantly, it was recently found that the anti-tumor effect of paclitaxel in CRPC is due to its inhibition of AR activity via its inhibition of microtubule dynamics. Polo-like kinase 1 (Plk1), a critical regulator in many cell cycle events, is elevated in prostate cancer (PCa) and linked to tumor grades. Of note, we have previously shown that Plk1 phosphorylates CLIP-170 and p150(Glued) , two important regulators of microtubule dynamics.

METHODS

We compared paclitaxel-mediated phenotypes (inhibition of the AR signaling, decrease of microtubule dynamics and cell death) of PCa cells expressing different forms of CLIP-170 and p150(Glued) with different Plk1 phosphorylation states.

RESULTS

We show that Plk1 phosphorylation of CLIP-170 and p150(Glued) affects cellular responses to paclitaxel. Expression of Plk1-unphosphorylatable mutants of CLIP-170 and p150(Glued) results in increased paclitaxel-induced apoptosis, increased protein degradation of the AR, and decreased nuclear accumulation of the AR in response to androgen in prostate cancer cells. Finally, we show that cells expressing unphosphorylatable mutants of CLIP-170 have defective microtubule dynamics, thus providing a new mechanism to understand how Plk1-associated kinase activity promotes constitutive activation of AR signaling in CRPC.

CONCLUSIONS

Our data suggest that a combination of inhibition of Plk1 and paclitaxel might be a novel avenue for treatment of CRPC.

Expression of aryl hydrocarbon receptor nuclear translocator enhances cisplatin resistance by upregulating MDR1 expression in cancer cells

The identification of molecular pathways in cancer cells is important for understanding the cells' underlying biology and for designing effective cancer therapies. We demonstrate that the expression of aryl hydrocarbon receptor nuclear translocator (ARNT) is critical during the development of cisplatin resistance. The reduced expression of ARNT was correlated with cisplatin-induced cell death in drug-sensitive cells. In addition, suppression of ARNT reversed the characteristics of cisplatin-resistant cells, making these cells cisplatin-sensitive, and significantly enhanced caspase-3 activation, DNA fragmentation, and apoptosis. The inhibition of colony formation, regulated by cisplatin, was more significant in ARNT-knockdown cells than in parental cells. In a xenograft analysis of severe combined immunodeficiency mice, cisplatin also efficiently inhibited ARNT-deficient c4 tumors but not ARNT-containing vT2 tumor formation. Furthermore, the downregulation of multidrug resistance 1 (MDR1) expression and retention of drugs in cells caused by suppression of ARNT, resulting in the resensitization of drug-resistant cells to cisplatin, was observed. When overexpressed, ARNT interacted with Sp1 to enhance the expression of MDR1 through Sp1-binding sites on the MDR1 promoter, resulting in a reversal of the effect of cisplatin on cell death. In addition, ARNT-induced MDR1 expression was inhibited in Sp1-knockdown cells. These results reveal previously unrecognized, multifaceted functions of ARNT in establishing the drug-resistant properties of cancer cells by the upregulation of MDR1, highlighting ARNT's potential as a therapeutic target in an important subset of cancers.

Battle of the eternal rivals: restoring functional p53 and inhibiting Polo-like kinase 1 as cancer therapy

Polo-like kinase 1, a pivotal regulator of mitosis and cytokinesis, is highly expressed in a broad spectrum of tumors and its expression correlates often with poor prognosis, suggesting its potential as a therapeutic target. p53, the guardian of the genome, is the most important tumor suppressor. In this review, we address the intertwined relationship of these two key molecules by fighting each other as eternal rivals in many signaling pathways. p53 represses the promoter of Polo-like kinase 1, whereas Polo-like kinase 1 inhibits p53 and its family members p63 and p73 in cancer cells lacking functional p53. Plk1 inhibitors target all rapidly dividing cells irrespective of tumor cells or non-transformed normal but proliferating cells. Upon treatment with Plk1 inhibitors, p53 in tumor cells is activated and induces strong apoptosis, whereas tumor cells with inactive p53 arrest in mitosis with DNA damage. Thus, inactive p53 is not associated with a susceptible cytotoxicity of Polo-like kinase 1 inhibition and could rather foster the induction of polyploidy/aneuploidy in surviving cells. In addition, compared to the mono-treatment, combination of Polo-like kinase 1 inhibition with anti-mitotic or DNA damaging agents boosts more severe mitotic defects, effectually triggers apoptosis and strongly inhibits proliferation of cancer cells with functional p53. In this regard, restoration of p53 in tumor cells with loss or mutation of p53 will reinforce the cytotoxicity of combined Polo-like kinase 1 therapy and provide a proficient strategy for combating relapse and metastasis of cancer.

Targeted co-delivery of docetaxel and siPlk1 by herceptin-conjugated vitamin E TPGS based immunomicelles

We developed a drug delivery system of herceptin-conjugated micelles, which consist of vitamin E TPGS and TPGS-siRNA conjugates, for targeted co-delivery of docetaxel and polo-like kinase 1 siRNA to achieve synergistic effects between the anticancer drug and the small interfering RNA responsible for multidrug resistance. The TPGS-siRNA conjugate is made through disulfide bond that could enable a pH-sensitive intracellular release. The load ratio between siPlk1 and docetaxel could be controlled by adjusting the siPlk1-TPGS to TPGS ratio as well as the drug to polymer ratio. NIH3T3, MCF7, and SK-BR-3 cell lines, which are of low, moderate and high HER2 overexpression, were employed to obtain proof-of-concept experimental results for the advantages of such a design. It has been shown that the IC(50), which is the drug concentration needed to kill 50% of the cancer cells in a designated time period, was 1.72, 0.042, 0.0032 and 0.000671 μg/mL for SK-BR-3 cells after 24 h treatment by Taxotere(®), and docetaxel formulated in the TPGS micelles, the TPGS-siPlk1/TPGS micelles and the herceptin-conjugated TPGS-siPlk1/TPGS micelles, respectively.

Functionalized graphene oxide mediated adriamycin delivery and miR-21 gene silencing to overcome tumor multidrug resistance in vitro

Multidrug resistance (MDR) is a major impediment to successful cancer chemotherapy. Co-delivery of novel MDR-reversing agents and anticancer drugs to cancer cells holds great promise for cancer treatment. MicroRNA-21 (miR-21) overexpression is associated with the development and progression of MDR in breast cancer, and it is emerging as a novel and promising MDR-reversing target. In this study, a multifunctional nanocomplex, composed of polyethylenimine (PEI)/poly(sodium 4-styrenesulfonates) (PSS)/graphene oxide (GO) and termed PPG, was prepared using the layer-by-layer assembly method to evaluate the reversal effects of PPG as a carrier for adriamycin (ADR) along with miR-21 targeted siRNA (anti-miR-21) in cancer drug resistance. ADR was firstly loaded onto the PPG surface (PPG_ADR) by physical mixing and anti-miR-21 was sequentially loaded onto PPG_ADR through electric absorption to form ^anti-miR-21PPG_ADR. Cell experiments showed that PPG significantly enhanced the accumulation of ADR in MCF-7/ADR cells (an ADR resistant breast cancer cell line) and exhibited much higher cytotoxicity than free ADR, suggesting that PPG could effectively reverse ADR resistance of MCF-7/ADR. Furthermore, the enhanced therapeutic efficacy of PPG could be correlated with effective silencing of miR-21 and with increased accumulation of ADR in drug-resistant tumor cells. The endocytosis study confirmed that PPG could effectively carry drug molecules into cells via the caveolae and clathrin-mediated endocytosis pathways. These results suggest that this PPG could be a potential and efficient non-viral vector for reversing MDR, and the strategy of combining anticancer drugs with miRNA therapy to overcome MDR could be an attractive approach in cancer treatment.

Enhanced gemcitabine-mediated cell killing of human lung adenocarcinoma by vector-based RNA interference against PLK1

Specific PLK1 silencing may be an effective gene therapy modality of treating PLK1-overexpressed cancers. In this study, we first explored the anticancer efficacy of three different short hairpin-expressing plasmids targeting PLK1 in animal model, and then determined the combination therapy effect of gemcitabine with PLK1-shRNA as an adjuvant. Transfection of the PLK1-shRNAs to A549 lung cancer cells induced significant PLK1 depletion, growth inhibition and apoptosis. In vivo administration of PLK1-shRNA constructs to tumor-bearing mice resulted in xenograft regression. Moreover, the combination of PLK1-shRNA plus low-dose gemcitabine (GEM) produced an additive antitumor activity on the lung tumors owing to an inhibition of cancer cell survival and augmented apoptosis. These results indicated a feasible bio-chemotherapeutic strategy for cancer.

OTOTOXIC EFFECTS OF CARBOPLATIN IN ORGANOTYPIC CULTURES IN CHINCHILLAS AND RATS

Carboplatin, a second-generation platinum chemotherapeutic drug, is considerably less ototoxic than cisplatin. While common laboratory species such as mice, guinea pigs and rats are highly resistant to carboplatin ototoxicity, the chinchilla stands out as highly susceptible. Moreover, carboplatin causes an unusual gradient of cell death in chinchillas. Moderate doses selectively damage type I spiral ganglion neurons (SGN) and inner hair cells (IHC) and the lesion tends to be relatively uniform along the length of the cochlea. Higher doses eventually damage outer hair cells (OHC), but the lesion follows the traditional gradient in which damage is more severe in the base than the apex. While carboplatin ototoxicity has been well documented in adult animals in vivo, little is known about its in vitro toxicity. To elucidate the ototoxic effects of carboplatin in vitro, we prepared cochlear and vestibular organotypic cultures from postnatal day 3 rats and adult chinchillas. Chinchilla cochlear and vestibular cultures were treated with carboplatin concentrations ranging from 50 µM to 10 mM for 48 h. Consistent with in vivo data, carboplatin selectively damaged IHC at low concentrations (50-100 µM). Surprisingly, IHC loss decreased at higher doses and IHC were intact at doses exceeding 500 µM. The mechanisms underlying this nonlinear response are unclear but could be related to a decrease in carboplatin uptake via active transport mechanisms (e.g., copper). Unlike the cochlea, the carboplatin dose-response function increased with dose with the highest dose destroying all chinchilla vestibular hair cells. Cochlear hair cells and auditory nerve fibers in rat cochlear organotypic cultures were unaffected by carboplatin concentrations <10 µM; however, the damage in OHC were more severe than IHC once the dose reached 100 µM. A dose at 500 µM destroyed all the cochlear hair cells, but hair cell loss decreased at high concentrations and nearly all the cochlear hair cells were present at the highest dose, 5 mM. Unlike the nonlinear dose-response seen with cochlear hair cells, rat auditory nerve fiber and spiral ganglion losses increased with doses above 50 µM with the highest dose destroying virtually all SGN. The remarkable species differences seen in vitro suggest that chinchilla IHC and type I SGN posse some unique biological mechanism that makes them especially vulnerable to carboplatin toxicity.

4E-BP1 participates in maintaining spindle integrity and genomic stability via interacting with PLK1

The essential function of eIF4E-binding protein 1 (4E-BP1) in translation initiation has been well established; however, the role of 4E-BP1 in normal cell cycle progression is coming to attention. Here, we revealed the role of 4E-BP1 on mitotic regulation and chromosomal DNA dynamics during mitosis. First, we have observed the co-localization of the phosphorylated 4E-BP1 at T37/46 with Polo-like kinase 1 (PLK1) at the centrosomes during. Depression of 4E-BP1 by small interfering RNA in HepG2 or HeLa cells resulted in an increased outcome of polyploidy and aberrant mitosis, including chromosomal DNA misaligned and multi-polar spindles or multiple centrosomes. We observed that 4E-BP1 interacted with PLK1 directly in vitro and in vivo in mitotic cells, and the C-terminal aa 77-118 of 4E-BP1 mediates its interaction with PLK1. PLK1 can phosphorylate 4E-BP1 in vitro. Furthermore, the depletion of 4E-BP1 sensitized HepG2 and HeLa cells to the microtubule disruption agent paclitaxel. These results demonstrate that 4E-BP1, beyond its role in translation regulation, can function as a regulator of mitosis via interacting with PLK1, and possibly plays a role in genomic stability maintaining.

FoxM1 inhibition sensitizes resistant glioblastoma cells to temozolomide by downregulating the expression of DNA-repair gene Rad51

PURPOSE

Recurrent glioblastoma multiforme (GBM) is characterized by resistance to radiotherapy and chemotherapy and a poor clinical prognosis. In this study, we investigated the role of the oncogenic transcription factor FoxM1 in GBM cells' resistance to alkylator temozolomide (TMZ) and its potential molecular mechanism.

EXPERIMENTAL DESIGN

FoxM1 expression levels were measured by immunohistochemical analysis in 38 pairs of primary and recurrent GBM tumor samples. Expression levels were also measured in primary recurrent GBM cell lines, and their responses to TMZ were characterized. In a mechanistic study, an siRNA array was used to identify downstream genes, and a chromatin immunoprecipitation assay was used to confirm transcriptional regulation.

RESULTS

Recurrent tumors that were TMZ resistant expressed higher levels of FoxM1 than did primary tumors. Recurrent GBM cell lines expressed higher levels of FoxM1 and the DNA damage repair gene Rad51 and were resistant to TMZ. TMZ treatment led to increased FoxM1 and Rad51 expression. FoxM1 knockdown inhibited Rad51 expression and sensitized recurrent GBM cells to TMZ cytotoxicity. FoxM1 directly regulated Rad51 expression through 2 FoxM1-specific binding sites in its promoter. Rad51 reexpression partially rescued TMZ resistance in FoxM1-knockdown recurrent GBM cells. A direct correlation between FoxM1 expression and Rad51 expression was evident in recurrent GBM tumor samples.

CONCLUSION

Targeting the FoxM1-Rad51 axis may be an effective method to reverse TMZ resistance in recurrent GBM.

Clinicopathological significance of Polo-like kinase 1 (PLK1) expression in human malignant glioma

Polo-like kinase 1 (PLK1), a variety of serine/threonine-protein kinase, has been reported to play important roles in malignant transformation. The purpose of this study was to investigate the clinicopathological significance of PLK1 expression in malignant glioma. A semi-quantitative RT-PCR assay was performed to detect the expression of PLK1 mRNA in 68 cases of glioma tissues and corresponding non-cancerous brain tissues. Additionally, the correlation of PLK1 mRNA expression with clinicopathological factors or prognosis of glioma patients was statistically analyzed. Multivariate analysis of prognostic factors was performed using the Cox proportional hazard model. Small interfering RNA was used to knockdown PLK1 expression in a glioma cell line and analyze the effects of PLK1 inhibition on growth, cell cycle, apoptosis and chemo- or radiosensitivity of glioma cells. Results showed that the expression of PLK1 mRNA was significantly higher in glioma tissues than in corresponding normal brain tissues. The expression of PLK1 mRNA was closely correlated with WHO grade, KPS and tumor recurrence of glioma patients (P=0.022, 0.030 and 0.041, respectively). Meanwhile, the disease-free and overall survival rates of patients with high PLK1 mRNA expression were obviously lower than those of patients with low PLK1 mRNA expression. Multivariate analysis showed that high PLK1 mRNA expression was a poor prognostic factor for glioma patients (P=0.028). The expression of PLK1 mRNA and protein was significantly down-regulated in stably transfected U251-S cells. PLK1 down-regulation could inhibit growth, induce cell arrest in G2/M phase of cell cycle and apoptosis enhancement in glioma cells. Further, PLK1 down-regulation could enhance the sensitivity of glioma cells to cisplatin or irradiation. Thus, the status of PLK1 mRNA expression might be an independent prognostic factor for glioma patients and targeting PLK1 could be a novel strategy for chemo- or radiosensitization of human malignant gliomas.

Polo-like kinases in AML

New therapies targeting critical elements of the cell cycle open novel and attractive avenues for the treatment of cancer patients. At present, the number of clinical trials that are registered with the European Organization for Research and Treatment of Cancer (EORTC) and with the US National Cancer Institute, which investigate the efficacy of Polo-like kinase 1 (Plk1) inhibitors against solid tumors and different types of leukemia is growing. Plks are important regulators of mitotic progression. Plk1, the best characterized mammalian Plk, has become an attractive target for cancer drug development, because most types of cancer appear to be addicted to the non-oncogene Plk1. Here, the authors discuss the role of Plk1 and the potential tumor suppressor gene Plk2 in acute myeloid leukemia (AML).

Polo-like kinase 1 is overexpressed in colorectal cancer and participates in the migration and invasion of colorectal cancer cells

BACKGROUND

Polo-like kinase 1 (PLK1) is an important molecule in proliferation of many human cancers. The aim of study is to clarify the expression patterns and potential function of PLK1 in colorectal cancers.

MATERIAL/METHODS

Fifty-six colorectal cancers samples were collected and arranged onto a tissue array and the expression of PLK1 were detected by immunohistochemistry and correlated with clinico-pathological characteristics and expression of PCNA. Expression of PLK1 in 9 colorectal cancer cells lines was investigated by RT-PCR and Western blot, then SW1116 cells lines were treated with PLK1 siRNA and the efficiency was examined by Western blot. Transwell test was applied to detect the migration and invasion capability of cancer cells by counting the number of cells passing through the membranes. Cell proliferation and apoptosis were examined by Cell Counting Kit-8 (CCK-8) and Annexin-V Kit.

RESULTS

PLK1 was positively expressed in 73.2% (41/56) of colorectal cancers tissues, but in only 3.6% (2/56) of normal tissues, and was associated with Duke's stage (P<0.01), tumor size (P<0.01), invasion extent (P<0.05) and lymphatic metastasis (P<0.01). The expression of PLK1 was correlated with expression of PCNA (R=0.553, P<0.01). PLK1 was inhibited in SW1116 cells by treating with PLK1 siRNA oligos, which resulted in a decreased number of cells passing through the membrane as compared with control groups (P<0.01) at 24 hours after transfection. Cell proliferation was inhibited from 48 hours after transfection, while cells apoptosis was induced from 72 hours after transfection.

CONCLUSIONS

PLK1 could be a progression marker for colorectal cancer patients and PLK1 depletion can inhibit migration and invasion capability of colorectal cancer cells SW1116, suggesting that PLK1 might be involved in metastasis and invasion of colorectal cancer. Therapeutic strategies targeting PLK1 may be a new approach to colorectal cancer.

Overexpression of centrosomal protein Nlp confers breast carcinoma resistance to paclitaxel

Nlp (ninein-like protein), an important molecule involved in centrosome maturation and spindle formation, plays an important role in tumorigenesis and its abnormal expression was recently observed in human breast and lung cancers. In this study, the correlation between overexpression of Nlp and paclitaxel chemosensitivity was investigated to explore the mechanisms of resistance to paclitaxel and to understand the effect of Nlp upon apoptosis induced by chemotherapeutic agents. Nlp expression vector was stably transfected into breast cancer MCF-7 cells. With Nlp overexpression, the survival rates, cell cycle distributions and apoptosis were analyzed in transfected MCF-7 cells by MTT test and FCM approach. The immunofluorescent assay was employed to detect the changes of microtubule after paclitaxel treatment. Immunoblotting analysis was used to examine expression of centrosomal proteins and apoptosis associated proteins. Subsequently, Nlp expression was retrospectively examined with 55 breast cancer samples derived from paclitaxel treated patients. Interestingly, the survival rates of MCF-7 cells with Nlp overexpressing were higher than that of control after paclitaxel treatment. Nlp overexpression promoted G2-M arrest and attenuated apoptosis induced by paclitaxel, which was coupled with elevated Bcl-2 protein. Nlp expression significantly lessened the microtubule polymerization and bundling elicited by paclitaxel attributing to alteration on the structure or dynamics of β-tubulin but not on its expression. The breast cancer patients with high expression of Nlp were likely resistant to the treatment of paclitaxel, as the response rate in Nlp negative patients was 62.5%, whereas was 58.3 and 15.8% in Nlp (+) and Nlp (++) patients respectively (p = 0.015). Nlp expression was positive correlated with those of Plk1 and PCNA. These findings provide insights into more rational chemotherapeutic regimens in clinical practice, and more effective approaches might be developed through targeting Nlp to increase chemotherapeutic sensitivity.

Efficient inhibition of human colorectal carcinoma growth by RNA interference targeting polo-like kinase 1 in vitro and in vivo

Polo-like kinase 1 (PLK1) showing a high expression in various kinds of tumors is considered a candidate target for cancer therapy. The aim of our study was to explore the effects of silencing PLK1 gene on human colorectal carcinoma cell line HCT-116 in vitro and in vivo. In vitro, the plasmids generating short hairpin RNA (shRNA)-targeting PLK1 were transfected into HCT-116 by using FugeneHD reagent, and the silencing potency was measured by RT-PCR, western blot, flow cytometry, and Caspase-Glo 3/7 assay, respectively. In vivo, the growth inhibition capacity of PLK1-shRNA on HCT-116 xenograft was measured in nude mice. Then, the silencing effect of PLK1 was analyzed by RT-PCR, western blot, and immunohistochemistry, respectively. Apoptosis, angiogenesis, and proliferation in tumor tissues were measured by TUNEL, CD31, and PCNA stain, respectively. The RNA interference targeting PLK1 significantly decreased the expression of PLK1 in vitro. More importantly, anti-PLK1 treatment in HCT-116 xenograft decreased tumor weight by 81.58% compared with the control group (p<0.001), accompanied with decreased PLK1 mRNA and protein expression, increased cell apoptosis, and reduced angiogenesis and proliferation (p<0.001). Our study showed that knockdown of PLK1 by shRNA might be the potential therapeutic approach against human colorectal carcinoma.

High-content, high-throughput analysis of cell cycle perturbations induced by the HSP90 inhibitor XL888

BACKGROUND

Many proteins that are dysregulated or mutated in cancer cells rely on the molecular chaperone HSP90 for their proper folding and activity, which has led to considerable interest in HSP90 as a cancer drug target. The diverse array of HSP90 client proteins encompasses oncogenic drivers, cell cycle components, and a variety of regulatory factors, so inhibition of HSP90 perturbs multiple cellular processes, including mitogenic signaling and cell cycle control. Although many reports have investigated HSP90 inhibition in the context of the cell cycle, no large-scale studies have examined potential correlations between cell genotype and the cell cycle phenotypes of HSP90 inhibition.

METHODOLOGY/PRINCIPAL FINDINGS

To address this question, we developed a novel high-content, high-throughput cell cycle assay and profiled the effects of two distinct small molecule HSP90 inhibitors (XL888 and 17-AAG [17-allylamino-17-demethoxygeldanamycin]) in a large, genetically diverse panel of cancer cell lines. The cell cycle phenotypes of both inhibitors were strikingly similar and fell into three classes: accumulation in M-phase, G2-phase, or G1-phase. Accumulation in M-phase was the most prominent phenotype and notably, was also correlated with TP53 mutant status. We additionally observed unexpected complexity in the response of the cell cycle-associated client PLK1 to HSP90 inhibition, and we suggest that inhibitor-induced PLK1 depletion may contribute to the striking metaphase arrest phenotype seen in many of the M-arrested cell lines.

CONCLUSIONS/SIGNIFICANCE

Our analysis of the cell cycle phenotypes induced by HSP90 inhibition in 25 cancer cell lines revealed that the phenotypic response was highly dependent on cellular genotype as well as on the concentration of HSP90 inhibitor and the time of treatment. M-phase arrest correlated with the presence of TP53 mutations, while G2 or G1 arrest was more commonly seen in cells bearing wt TP53. We draw upon previous literature to suggest an integrated model that accounts for these varying observations.

Simultaneous delivery of siRNA and paclitaxel via a "two-in-one" micelleplex promotes synergistic tumor suppression

Combination of two or more therapeutic strategies with different mechanisms can cooperatively prohibit cancer development. Combination of chemotherapy and small interfering RNA (siRNA)-based therapy represents an example of this approach. Hypothesizing that the chemotherapeutic drug and the siRNA should be simultaneously delivered to the same tumoral cell to exert their synergistic effect, the development of delivery systems that can efficiently encapsulate two drugs and successfully deliver payloads to targeted sites via systemic administration has proven to be challenging. Here, we demonstrate an innovative "two-in-one" micelleplex approach based on micellar nanoparticles of a biodegradable triblock copolymer poly(ethylene glycol)-b-poly(ε-caprolactone)-b-poly(2-aminoethyl ethylene phosphate) to systemically deliver the siRNA and chemotherapeutic drug. We show clear evidence that the micelleplex is capable of delivering siRNA and paclitaxel simultaneously to the same tumoral cells both in vitro and in vivo. We further demonstrate that systemic administration of the micelleplex carrying polo-like kinase 1 (Plk1) specific siRNA and paclitaxel can induce a synergistic tumor suppression effect in the MDA-MB-435s xenograft murine model, requiring a thousand-fold less paclitaxel than needed for paclitaxel monotherapy delivered by the micelleplex and without activation of the innate immune response or generation of carrier-associated toxicity.

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

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

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

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

FoxM1 mediates resistance to herceptin and paclitaxel

Inherent and acquired therapeutic resistance in breast cancer remains a major clinical challenge. In human breast cancer samples, overexpression of the oncogenic transcription factor FoxM1 has been suggested to be a marker of poor prognosis. In this study, we report that FoxM1 overexpression confers resistance to the human epidermal growth factor receptor 2 monoclonal antibody Herceptin and microtubule-stabilizing drug paclitaxel, both as single agents and in combination. FoxM1 altered microtubule dynamics to protect tumor cells from paclitaxel-induced apoptosis. Mechanistic investigations revealed that the tubulin-destabilizing protein Stathmin, whose expression also confers resistance to paclitaxel, is a direct transcriptional target of FoxM1. Significantly, attenuating FoxM1 expression by small interfering RNA or an alternate reading frame (ARF)-derived peptide inhibitor increased therapeutic sensitivity. Our findings indicate that targeting FoxM1 could relieve therapeutic resistance in breast cancer.

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.

Overexpression of polo-like kinase1 predicts a poor prognosis in hepatocellular carcinoma patients

AIM

To elucidate the role of overexpressed polo-like kinase1 (PLK1) in hepatocellular carcinoma (HCC).

METHODS

We prospectively collected clinicopathological, immunohistochemical and semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) data from 135 HCC patients undergoing successful hepatectomy. The correlations between PLK1 mRNA expression and clinicopathologic variables were analyzed by Mann-Whitney U test. Prognostic factors were identified by univariate and multivariate analyses.

RESULTS

Immunohistochemical results showed overexpression of PLK1 was mainly found in tumor tissues compared with tumor-free tissue. A similar mRNA result was obtained by semi-quantitative RT-PCR. A total of 111 samples were positive for PLK1 mRNA expression. The positive expression was correlated with venous invasion, tumor nodules and Edmondson grade. Furthermore, 1, 3, 5-year survival rates in the positive expression group were significantly lower than the negative control group. Multivariate analysis showed that positive PLK1 expression was an independent risk factor for HCC.

CONCLUSION

PLK1 could be a potential biomarker for diagnosis and therapy for HCC.

Uptake of plasmid-loaded nanoparticles in breast cancer cells and effect on Plk1 expression

The development of nucleic acid-based drugs for cancer therapeutic application has shown promising results in the past. However the delivery of these drugs to target cells is one problem which remains to be resolved. Nanoparticles have been described as promising strategies to deliver drugs into target cells. Human serum albumin (HSA) nanoparticles conjugated to trastuzumab for a cell type-specific targeting of human epidermal growth factor receptor 2 (HER2)-overexpressing cells were developed with incorporated expression plasmids for small hairpin RNAs (shRNAs) targeting polo-like kinase 1 (Plk1). Plk1 is a promising target for such an approach because it is overexpressed in all known cancer types and is a negative prognostic factor. Receptor-mediated uptake of the trastuzumab-modified nanoparticles into HER2-positive cells could be observed leading to reduced Plk1 expression. Taken together, HSA nanoparticles represent promising tools to deliver expression plasmids for shRNAs into target cells and should be further evaluated with regard to a therapeutic application of RNA interference in cancer therapy.

Association of genetic variation in mitotic kinases with breast cancer risk

An RNAi-based functional screening of mitotic kinases in Drosophila recently identified a number of members of the kinome that are required for normal cell division. Depletion of these kinases resulted in a number of different mitotic abnormalities including spindle malformation, chromosome mis-segregation, centrosome amplification and failure of cytokinesis (Bettencourt-Dias et al. in Nature 432:980-987, 2004). Since mitotic defects are commonly observed in cancer cells, these kinases may contribute to tumor development and/or progression. To investigate whether common genetic variation in the mitotic kinases are associated with breast cancer risk, we genotyped 386 single nucleotide polymorphisms (SNPs) from 44 mitotic kinase genes, in 798 breast cancer cases and 843 unaffected controls from a clinic-based study. A total of 22 SNPs from 13 kinase genes displayed significant associations with breast cancer risk (P(trend) < or = 0.05), including two SNPs from FYN (rs6914091 and rs1465061) that remained of interest after accounting for multiple testing (q = 0.06). These associations were stronger when evaluating cases with estrogen and progesterone receptor positive tumors. In addition, haplotype-based tests identified significant associations with risk for common haplotypes of the MAST2 (P = 0.04) and MAP2K4 (P = 0.006) genes. Although requiring replication, these findings suggest that genetic polymorphisms in mitotic kinases that have been implicated in chromosome instability and aneuploidy may contribute to the development of breast cancer.