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

Elevating PLK1 overcomes BETi resistance in prostate cancer via triggering BRD4 phosphorylation-dependent degradation in mitosis

Bromodomain-containing protein 4 (BRD4) has emerged as a promising therapeutic target in prostate cancer (PCa). Understanding the mechanisms of BRD4 stability could enhance the clinical response to BRD4-targeted therapy. In this study, we report that BRD4 protein levels are significantly decreased during mitosis in a PLK1-dependent manner. Mechanistically, we show that BRD4 is primarily phosphorylated at T1186 by the CDK1/cyclin B complex, recruiting PLK1 to phosphorylate BRD4 at S24/S1100, which are recognized by the APC/CCdh1 complex for proteasome pathway degradation. We find that PLK1 overexpression lowers SPOP mutation-stabilized BRD4, consequently rendering PCa cells re-sensitized to BRD4 inhibitors. Intriguingly, we report that sequential treatment of docetaxel and JQ1 resulted in significant inhibition of PCa. Collectively, the results support that PLK1-phosphorylated BRD4 triggers its degradation at M phase. Sequential treatment of docetaxel and JQ1 overcomes BRD4 accumulation-associated bromodomain and extra-terminal inhibitor (BETi) resistance, which may shed light on the development of strategies to treat PCa.

Multifunctional Fluoropolymer-Engineered Magnetic Nanoparticles to Facilitate Blood-Brain Barrier Penetration and Effective Gene Silencing in Medulloblastoma

Patients with brain cancers including medulloblastoma lack treatments that are effective long-term and without side effects. In this study, a multifunctional fluoropolymer-engineered iron oxide nanoparticle gene-therapeutic platform is presented to overcome these challenges. The fluoropolymers are designed and synthesized to incorporate various properties including robust anchoring moieties for efficient surface coating, cationic components to facilitate short interference RNA (siRNA) binding, and a fluorinated tail to ensure stability in serum. The blood-brain barrier (BBB) tailored system demonstrates enhanced BBB penetration, facilitates delivery of functionally active siRNA to medulloblastoma cells, and delivers a significant, almost complete block in protein expression within an in vitro extracellular acidic environment (pH 6.7) - as favored by most cancer cells. In vivo, it effectively crosses an intact BBB, provides contrast for magnetic resonance imaging (MRI), and delivers siRNA capable of slowing tumor growth without causing signs of toxicity - meaning it possesses a safe theranostic function. The pioneering methodology applied shows significant promise in the advancement of brain and tumor microenvironment-focused MRI-siRNA theranostics for the better treatment and diagnosis of medulloblastoma.

PLK1 inhibition leads to mitotic arrest and triggers apoptosis in cholangiocarcinoma cells

Cholangiocarcinoma (CCA) is a lethal cancer originating from the epithelial cells within the bile duct and ranks as the second most prevalent form of liver cancer in Thailand. Polo-like kinase 1 (PLK1), a protein serine/threonine kinase, regulates a number of steps in cell mitosis and is upregulated in several types of cancer, including CCA. Our previous study identified PLK1 as a biomarker of the C1 subtype, correlating with poor prognosis in intrahepatic CCA. The present study aimed to examine the effect of PLK1 inhibition on CCA cells. Different CCA cell lines developed from Thai patients, HuCCA1, KKU055, KKU100 and KKU213A, were treated with two PLK1 inhibitors, BI2536 and BI6727, and were transfected with small interfering RNA, followed by analysis of cell proliferation, cell cycle distribution and cell apoptosis. It was discovered that BI2536 and BI6727 inhibited cell proliferation and caused G2/M-phase arrest in CCA cells. Furthermore, the number of total apoptotic cells was increased in PLK1 inhibitor-treated CCA cells. The expression levels of mitotic proteins, aurora kinase A, phosphorylated PLK1 (T210) and cyclin B1, were augmented in PLK1-inhibited CCA cells. Additionally, inhibition of PLK1 led to increased DNA damage, as determined by the upregulated levels of γH2AX and increased cleavage of poly (ADP-ribose) polymerase, an apoptotic marker. These results suggested that inhibiting PLK1 prolonged mitotic arrest and subsequently triggered cell apoptosis. Validation of the antiproliferative effects of PLK1 inhibition was accomplished through silencing of the PLK1 gene. In conclusion, targeting PLK1 provided promising results for further study as a potential candidate for targeted therapy in CCA.

Genetic enhancers of partial PLK1 inhibition reveal hypersensitivity to kinetochore perturbations

Polo-like kinase 1 (PLK1) is a serine/threonine kinase required for mitosis and cytokinesis. As cancer cells are often hypersensitive to partial PLK1 inactivation, chemical inhibitors of PLK1 have been developed and tested in clinical trials. However, these small molecule inhibitors alone are not completely effective. PLK1 promotes numerous molecular and cellular events in the cell division cycle and it is unclear which of these events most crucially depend on PLK1 activity. We used a CRISPR-based genome-wide screening strategy to identify genes whose inactivation enhances cell proliferation defects upon partial chemical inhibition of PLK1. Genes identified encode proteins that are functionally linked to PLK1 in multiple ways, most notably factors that promote centromere and kinetochore function. Loss of the kinesin KIF18A or the outer kinetochore protein SKA1 in PLK1-compromised cells resulted in mitotic defects, activation of the spindle assembly checkpoint and nuclear reassembly defects. We also show that PLK1-dependent CENP-A loading at centromeres is extremely sensitive to partial PLK1 inhibition. Our results suggest that partial inhibition of PLK1 compromises the integrity and function of the centromere/kinetochore complex, rendering cells hypersensitive to different kinetochore perturbations. We propose that KIF18A is a promising target for combinatorial therapies with PLK1 inhibitors.

Inhibitors of the PLK1 polo-box domain: drug design strategies and therapeutic opportunities in cancer

INTRODUCTION

Polo Like Kinase 1 (PLK1) is a key regulator of mitosis and its overexpression is frequently observed in a wide variety of human cancers, while often being associated with poor survival rates. Therefore, it is considered a potential and attractive target for cancer therapeutic development. The Polo like kinase family is characterized by the presence of a unique C terminal polobox domain (PBD) involved in regulating kinase activity and subcellular localization. Among the two functionally essential, druggable sites with distinct properties that PLK1 offers, targeting the PBD presents an alternative approach for therapeutic development.

AREAS COVERED

Significant progress has been made in progressing from the peptidic PBD inhibitors first identified, to peptidomimetic and recently drug-like small molecules. In this review, the rationale for targeting the PBD over the ATP binding site is discussed, along with recent progress, challenges, and outlook.

EXPERT OPINION

The PBD has emerged as a viable alternative target for the inhibition of PLK1, and progress has been made in using compounds to elucidate mechanistic aspects of activity regulation and in determining roles of the PBD. Studies have resulted in proof of concept of in vivo efficacy suggesting promise for PBD binders in clinical development.

Development of a nanoparticle-based immunotherapy targeting PD-L1 and PLK1 for lung cancer treatment

Immune checkpoint inhibitors (ICIs) targeting PD-L1 and PD-1 have improved survival in a subset of patients with advanced non-small cell lung cancer (NSCLC). However, only a minority of NSCLC patients respond to ICIs, highlighting the need for superior immunotherapy. Herein, we report on a nanoparticle-based immunotherapy termed ARAC (Antigen Release Agent and Checkpoint Inhibitor) designed to enhance the efficacy of PD-L1 inhibitor. ARAC is a nanoparticle co-delivering PLK1 inhibitor (volasertib) and PD-L1 antibody. PLK1 is a key mitotic kinase that is overexpressed in various cancers including NSCLC and drives cancer growth. Inhibition of PLK1 selectively kills cancer cells and upregulates PD-L1 expression in surviving cancer cells thereby providing opportunity for ARAC targeted delivery in a feedforward manner. ARAC reduces effective doses of volasertib and PD-L1 antibody by 5-fold in a metastatic lung tumor model (LLC-JSP) and the effect is mainly mediated by CD8+ T cells. ARAC also shows efficacy in another lung tumor model (KLN-205), which does not respond to CTLA-4 and PD-1 inhibitor combination. This study highlights a rational combination strategy to augment existing therapies by utilizing our nanoparticle platform that can load multiple cargo types at once.

Only a minority of patients with non-small cell lung cancer (NSCLC) respond to immune checkpoint inhibitors. Here the authors design a nanosystem for the co-delivery of a PLK1 inhibitor and PD-L1 antibody, showing anti-tumor immune responses in preclinical lung cancer models.

The kinase PLK1 promotes the development of <i>Kras</i>/<i>Tp53</i>-mutant lung adenocarcinoma through transcriptional activation of the receptor RET

Increased abundance of polo-like kinase 1 (PLK1) is observed in various tumor types, particularly in lung adenocarcinoma (LUAD). Here, we found that PLK1 accelerated the progression of LUAD through a mechanism that was independent of its role in mediating mitotic cell division. Analysis of human tumor databases revealed that increased PLK1 abundance in LUAD correlated with mutations in KRAS and p53, with tumor stage, and with reduced survival in patients. In a mouse model of KRAS^G12D-driven, p53-deficient LUAD, PLK1 overexpression increased tumor burden, decreased tumor cell differentiation, and reduced animal survival. PLK1 overexpression in cultured cells and mice indirectly increased the expression of the gene encoding the receptor tyrosine kinase RET by phosphorylating the transcription factor TTF-1. Signaling by RET and mutant KRAS in these tumors converged to activate the mitogen-activated protein kinase (MAPK) pathway. Pharmacological inhibition of the MAPK pathway kinase MEK combined with inhibition of either RET or PLK1 markedly suppressed tumor growth. Our findings show that PLK1 can amplify MAPK signaling and reveal a potential target for stemming progression in lung cancers with high PLK1 abundance.

Icaritin inhibits PLK1 to activate DNA damage response in NK/T cell lymphoma and increases sensitivity to GELOX regime

Natural killer/T cell lymphoma (NKTCL) is a highly aggressive subtype of non-Hodgkin lymphoma. Gemcitabine, oxaliplatin, and L-asparaginase (GELOX) is one of the first-line chemotherapy regimens of NKTCL. Yet, the prognosis of NKTCL is poor. Icaritin is an herb-derived monomer from icariin with antitumor effects. We found that icaritin induced proliferation inhibition and apoptosis of NKTCL both in vitro and in vivo. Moreover, icaritin inhibited the dissemination of NKTCL in vivo. RNA sequencing revealed the Polo-like kinase 1 (PLK1) gene and DNA damage response (DDR) as the targets of icaritin. Mechanistically, icaritin inhibited PLK1 to promote checkpoint kinase 2 (Chk2) homodimerization and its T387 phosphorylation, which further activated p53, leading to the activation of the DDR pathway. Moreover, inhibiting PLK1 increased Forkhead box O3a nuclear localization, the latter of which activated ataxia telangiectasia mutated (ATM), an early sensor of DNA damage. Then ATM phosphorylated Chk2 T68 and initiated Chk2 activation. Remarkably, the combined treatment of icaritin and GELOX achieved better antitumor efficacy than single treatment in vivo. In summary, our results proved the efficacy of icaritin treating NKTCL, provided insights into its antitumor molecular mechanism, and revealed the application value of icaritin in facilitating clinical NKTCL treatment.

Synergistic apoptotic effect of miR-183-5p and Polo-Like kinase 1 inhibitor NMS-P937 in breast cancer cells

MicroRNAs (miRNAs) are small noncoding RNAs that act as endogenous regulatory molecules targeting specific mRNAs for translational repression. Studies of breast cancer genomics indicate that breast cancer subtypes are distinguished and regulated by specific sets of miRNAs which affect activities such as tumor initiation, progression, and even drug response. Polo-like Kinase 1 (PLK1) is widely considered to be a proto-oncogene due to its increased expression in multiple tumor types, as well as its crucial role in regulating mitosis. Pharmacological inhibition of PLK1 can reduce tumor volume and induce tumor cell death in solid and hematologic malignancies. This prompted us to investigate how PLK1 inhibition with the target-specific inhibitor NMS-P937 would impact breast cancer cells, and how miRNAs may influence the overall response of these cells to this inhibition. We found that miR-183-5p targets PLK1 gene, effectively reducing its protein expression. Such miRNA-driven regulation of PLK1 expression sensitizes breast cancer cells to NMS-P937, resulting in synergistically increased apoptosis. We also show that the miRNA-regulated reduction of PLK1 influences the expression of apoptosis-related key proteins and possibly inducing further indirect PLK1 downmodulation through a DNMT1-p53 axis. These results suggest a potential biologically significant link between the expression of miR-183-5p and the efficacy of PLK1-specific inhibitors in breast cancer cells. Our work further elucidates how miR-183-5p regulates PLK1 gene while also enhancing NMS-P937 effect in breast cancer. Future studies assessing the role of miR-183-5p as a novel biomarker for anti-PLK1 chemotherapy agents are warranted.

Structure-activity and mechanistic studies of non-peptidic inhibitors of the PLK1 polo box domain identified through REPLACE

Polo-like kinase 1 (PLK1) is a serine/threonine-protein kinase involved in cell cycle regulation and mitotic progression. Studies have shown that PLK1 is upregulated in many tumors and high levels are adversely related to a poor prognosis. Knocking down or inhibiting PLK1 results in synthetic lethality in PTEN deficient prostate tumors and Kras mutant colorectal tumors, further validating PLK1 as an oncotarget. Substrate recognition by PLK1 occurs through the Polo-Box Domain (PBD), which is a phospho-peptide binding site also responsible for subcellular localization. Much effort has been directed to target this kinase therapeutically through the ATP-binding site, and a few such inhibitors have advanced to clinical trials however with limited clinical efficacy. Moreover, it has been shown that a point mutation in PLK1 (C67V) confers dramatic cellular resistance to catalytic site inhibitors. An alternative approach to target PLK1 potently and selectively is through the PBD to block its protein-protein interactions. Through the REPLACE strategy, for converting peptide inhibitors into more drug-like non peptidic compounds, a PBD targeting compound series ("ABBAs"), has been identified and the key determinants of potency and selectivity elucidated through structure-activity relationship studies. In cellular experiments, the ABBAs were shown to lead to profound effects on the cell cycle, to inhibit tumor proliferation and overcome resistance of cells expressing the PLK1 C67V mutant to ATP-based inhibitors. These non-ATP competitive inhibitors of PLK1 were also used chemical biology probes to investigate the gene regulatory effects of PLK1, known to act on transcription factors such as p53.

Crosstalk between Plk1, p53, cell cycle, and G2/M DNA damage checkpoint regulation in cancer: computational modeling and analysis

Different cancer cell lines can have varying responses to the same perturbations or stressful conditions. Cancer cells that have DNA damage checkpoint-related mutations are often more sensitive to gene perturbations including altered Plk1 and p53 activities than cancer cells without these mutations. The perturbations often induce a cell cycle arrest in the former cancer, whereas they only delay the cell cycle progression in the latter cancer. To study crosstalk between Plk1, p53, and G2/M DNA damage checkpoint leading to differential cell cycle regulations, we developed a computational model by extending our recently developed model of mitotic cell cycle and including these key interactions. We have used the model to analyze the cancer cell cycle progression under various gene perturbations including Plk1-depletion conditions. We also analyzed mutations and perturbations in approximately 1800 different cell lines available in the Cancer Dependency Map and grouped lines by genes that are represented in our model. Our model successfully explained phenotypes of various cancer cell lines under different gene perturbations. Several sensitivity analysis approaches were used to identify the range of key parameter values that lead to the cell cycle arrest in cancer cells. Our resulting model can be used to predict the effect of potential treatments targeting key mitotic and DNA damage checkpoint regulators on cell cycle progression of different types of cancer cells.

Dual Targeting of EGFR with PLK1 Exerts Therapeutic Synergism in Taxane-Resistant Lung Adenocarcinoma by Suppressing ABC Transporters

Simple Summary

Our previous studies led us to hypothesize that downregulation of PLK1 expression or its activity can overcome the hurdles of taxane resistance by downregulating ABC transporters. Targeting PLK1 with shRNA or non-functional mutants downregulated ABCB1, ABCC9, and ABCG2 in paclitaxel-resistant lung adenocarcinoma (LUAD^TXR), similar to the downregulation effects from treatment with PLK1 inhibitors. Since EGFR is highly expressed in LUAD^TXR cells, gefitinib was combined with PLK1 inhibitors. Under these conditions, LUAD^TXR cells tend to undergo apoptosis more effectively than parental cells, showing a synergistic effect on downregulation of ABC transporters through c-Myc or AP-1. Clinical data provide evidence for the relationship between survival rates and expressions of PLK1 and EGFR in LUAD patients. Taken together, our data suggest that a combination of gefitinib and PLK1 inhibitors exerts strong synergism in LUAD^TXR, providing a benefit to overcome the limitations associated with taxanes.

Abstract

To overcome the limitations of chemoresistance, combination therapies using druggable targets have been investigated. Our previous studies led us to hypothesize that the downregulation of PLK1 expression or activity can be one strategy to overcome the hurdles of taxane resistance by the downregulation of ABC transporters. To explore this, various versions of PLK1 including a constitutively active version, kinase-dead form, and polo-box domain mutant were expressed in paclitaxel-resistant lung adenocarcinoma (LUAD^TXR). Targeting PLK1 using shRNA or non-functional mutants downregulated ABCB1, ABCC9, and ABCG2 in LUAD^TXR cells, which was similar to the downregulation effects from treatment with PLK1 inhibitors. The high expression of EGFR in LUAD led us to administer gefitinib, showing a markedly reduced EGFR level in LUAD^TXR cells. When gefitinib and PLK1 inhibitors were combined, LUAD^TXR cells tended to undergo apoptosis more effectively than parental cells, showing a synergistic effect on the downregulation of ABC transporters through c-Myc and AP-1. Clinical data provide evidence for the relevance between survival rates and expressions of PLK1 and EGFR in LUAD patients. Based on these results, we suggest that a combination of gefitinib and PLK1 inhibitors exerts strong synergism in LUAD^TXR, which helps to overcome the limitations associated with taxanes.

A comprehensive overview of β-carbolines and its derivatives as anticancer agents

β-Carboline alkaloids are a family of natural and synthetic products with structural diversity and outstanding antitumor activities. This review summarizes research developments of β-carboline and its derivatives as anticancer agents, which focused on both natural and synthetic monomers as well as dimers. In addition, the structure-activity relationship (SAR) analysis of β-carboline monomers and dimers are summarized and mechanism of action of β-carboline and its derivatives are also presented. A few possible research directions, suggestions and clues for future work on the development of novel β-carboline-based anticancer agents with improved expected activities and lesser toxicity are also provided.

Targeting cytoskeletal phosphorylation in cancer

Phosphorylation of cytoskeletal proteins regulates the dynamics of polymerization, stability, and disassembly of the different types of cytoskeletal polymers. These control the ability of cells to migrate and divide. Mutations and alterations of the expression levels of multiple protein kinases are hallmarks of most forms of cancer. Thus, altered phosphorylation of cytoskeletal proteins is observed in most cancer cells. These alterations potentially control the ability of cancer cells to divide, invade and form distal metastasis. This review highlights the emergent role of phosphorylation in the control of the function of the different cytoskeletal polymers in cancer cells. It also addresses the potential effect of targeted inhibitors in the normalization of cytoskeletal function.

Dual-target Inhibitors Based on BRD4: Novel Therapeutic Approaches for Cancer

BACKGROUND

Currently, cancer continues being a dramatically increasing and serious threat to public health. Although many anti-tumor agents have been developed in recent years, the survival rate of patients is not satisfactory. The poor prognosis of cancer patients is closely related to the occurrence of drug resistance. Therefore, it is urgent to develop new strategies for cancer treatment. Multi-target therapies aim to have additive or synergistic effects and reduce the potential for the development of resistance by integrating different pharmacophores into a single drug molecule. Given the fact that majority of diseases are multifactorial in nature, multi-target therapies are being exploited with increasing intensity, which has brought improved outcomes in disease models and obtained several compounds that have entered clinical trials. Thus, it is potential to utilize this strategy for the treatment of BRD4 related cancers. This review focuses on the recent research advances of dual-target inhibitors based on BRD4 in the aspect of anti-tumor.

METHODS

We have searched the recent literatures about BRD4 inhibitors from the online resources and databases, such as pubmed, elsevier and google scholar.

RESULTS

In the recent years, many efforts have been taken to develop dual-target inhibitors based on BRD4 as anti-cancer agents, such as HDAC/BRD4 dual inhibitors, PLK1/BRD4 dual inhibitors and PI3K/BRD4 dual inhibitors and so on. Most compounds display good anti-tumor activities.

CONCLUSION

Developing new anti-cancer agents with new scaffolds and high efficiency is a big challenge for researchers. Dual-target inhibitors based on BRD4 are a class of important bioactive compounds. Making structural modifications on the active dual-target inhibitors according to the corresponding structure-activity relationships is of benefit to obtain more potent anti-cancer leads or clinical drugs. This review will be useful for further development of new dual-target inhibitors based on BRD4 as anti-cancer agents.

Oxidized DNA Precursors Cleanup by NUDT1 Contributes to Vascular Remodeling in Pulmonary Arterial Hypertension

Rationale: Pulmonary arterial hypertension (PAH) is a life-threatening condition characterized by abnormally elevated pulmonary pressures and right ventricular failure. Excessive proliferation and resistance to apoptosis of pulmonary artery smooth muscle cells (PASMCs) is one of the most important drivers of vascular remodeling in PAH, for which available treatments have limited effectiveness.Objectives: To gain insights into the mechanisms leading to the development of the disease and identify new actionable targets.Methods: Protein expression profiling was conducted by two-dimensional liquid chromatography coupled to tandem mass spectrometry in isolated PASMCs from controls and patients with PAH. Multiple molecular, biochemical, and pharmacologic approaches were used to decipher the role of NUDT1 (nudrix hyrolase 1) in PAH.Measurements and Main Results: Increased expression of the detoxifying DNA enzyme NUDT1 was detected in cells and tissues from patients with PAH and animal models. In vitro, molecular or pharmacological inhibition of NUDT1 in PAH-PASMCs induced accumulation of oxidized nucleotides in the DNA, irresolvable DNA damage (comet assay), disruption of cellular bioenergetics (Seahorse), and cell death (terminal deoxynucleotidyl transferase dUTP nick end labeling assay). In two animal models with established PAH (i.e., monocrotaline and Sugen/hypoxia-treated rats), pharmacological inhibition of NUDT1 using (S)-Crizotinib significantly decreased pulmonary vascular remodeling and improved hemodynamics and cardiac function.Conclusions: Our results indicate that, by overexpressing NUDT1, PAH-PASMCs hijack persistent oxidative stress in preventing incorporation of oxidized nucleotides into DNA, thus allowing the cell to escape apoptosis and proliferate. Given that NUDT1 inhibitors are under clinical investigation for cancer, they may represent a new therapeutic option for PAH.

Phase 1 dose escalation trial of volasertib in combination with decitabine in patients with acute myeloid leukemia

Polo-like kinase 1 (PLK1) regulates mitotic checkpoints and cell division. PLK1 overexpression is reported in numerous cancers, including acute myeloid leukemia (AML), and is associated with poor prognosis. Volasertib is a selective, potent cell-cycle kinase inhibitor that targets PLK to induce mitotic arrest and apoptosis. This phase 1 trial investigated the maximum tolerated dose (MTD), safety, pharmacokinetics, and anti-leukemic activity of volasertib in combination with decitabine in AML patients aged ≥ 65 years. Thirteen patients were treated with escalating volasertib doses (3 + 3 design; 300 mg, 350 mg, and 400 mg) plus standard-dose decitabine. Dose-limiting toxicity was reported in one patient in cycle 1; the MTD of volasertib in combination with decitabine was determined as 400 mg. The most common treatment-emergent adverse events were febrile neutropenia, pneumonia, and decreased appetite. Objective response rate was 23%. The combination was well tolerated, and the adverse event profile was in line with previous findings.

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.

Targeting cell cycle by β-carboline alkaloids in vitro: Novel therapeutic prospects for the treatment of cancer

Cell cycle dysregulation is the mainstay of aberrant cell proliferation, which leads to tumor progression. Mutations in tumor cells initiate various dysregulated pathways and spontaneous over-proliferation with genomic/chromosomal instability. Despite advances in cancer therapy, it has remained a medicinal challenge to treat. Besides, the complexity of pathophysiological pathways behind cancer raises the need for novel multi-target agents, possessing fewer side effects. Alkaloid-based therapies have been explored so far to target cell division in cancer, including vinca alkaloids. As a class of hopeful β-carboline derivatives, growing evidence has indicated their auspicious roles in combating cancer by inhibiting topoisomerase (TOPO), kinesin Eg5, telomerase, cyclin-dependent kinase (CDK), IκB kinase (IKK), and polo-like kinase-1 (PLK1) in the transition phases of cell cycle. In this review, in vitro potential of β-carboline has been revealed through targeting cell division cycle at different phases. In conclusion, β-carboline alkaloids could be introduced as novel candidates in cancer therapy.

Inhibition of Polo-like kinase 1 (PLK1) facilitates the elimination of HIV-1 viral reservoirs in CD4+ T cells ex vivo

Although combination antiretroviral therapy is effective in controlling HIV-1 infection, latent HIV-1 proviruses cannot be eliminated. HIV-1 reactivation induced by the mere use of latency-reversing agents is insufficient to render death of reservoir cells, indicating that certain intrinsic survival mechanisms exist. We report that Polo-like kinase 1 (PLK1) plays a critical role in survival of CD4+ T cells that undergo HIV-1 reactivation from latency or de novo infection. PLK1 is elevated in both scenarios, which requires HIV-1 Nef. HIV-1 enhances PLK1 SUMOylation, causing its nuclear translocation and protein stabilization. Inhibition or knockdown of PLK1 markedly facilitates death of HIV-1-infected CD4+ T cells. Furthermore, PLK1 inhibitors strikingly reduce the size of HIV-1 latent reservoirs in primary CD4+ T cells. Our findings demonstrate that HIV-1 infection hijacks PLK1 to prevent cell death induced by viral cytopathic effects, and that PLK1 is a promising target for chemical "killing" of HIV-1 reservoir cells.

Therapeutic opportunities for PLK1 inhibitors: Spotlight on BRCA1-deficiency and triple negative breast cancers

Polo-Like Kinases (PLKs) are central players of mitotic progression in Eukaryotes. Given the intimate relationship between cell cycle progression and cancer development, PLKs in general and PLK1 in particular have been thoroughly studied as biomarkers and potential therapeutic targets in oncology. The oncogenic properties of PLK1 overexpression across different types of human cancers are attributed to its roles in promoting mitotic entry, centrosome maturation, spindle assembly and cytokinesis. While several academic labs and pharmaceutical companies were able to develop potent and selective inhibitors of PLK1 (PLK1i) for preclinical research, such compounds have reached only limited success in clinical trials despite their great pharmacokinetics. Even though this could be attributed to multiple causes, the housekeeping roles of PLK1 in both normal and cancer cells are most likely the main reason for clinical trials failure and withdraw due to toxicities issues. Therefore, great efforts are being invested to position PLK1i in the treatment of specific types of cancers with revised dosages schemes. In this mini review we focus on two potential niches for PLK1i that are supported by recent evidence: triple negative breast cancers (TNBCs) and BRCA1-deficient cancers. On the one hand, we recollect several lines of strong evidence indicating that TNBCs are among the cancers with highest PLK1 expression and sensitivity to PLK1i. These findings are encouraging because of the limited therapeutics options available for TNBC patients, which rely mainly on classic chemotherapy. On the other hand, we discuss recent evidence that unveils synthetic lethality induction by PLK1 inhibition in BRCA1-deficient cancers cells. This previously unforeseen therapeutic link between PLK1 and BRCA1 is promising because it defines novel therapeutic opportunities for PLK1i not only for breast cancer (i.e. TNBCs with BRCA1 deficiencies), but also for other types of cancers with BRCA1-deficiencies, such as pancreatic and prostate cancers.

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.

Identification of inhibitors of the polo-box domain of polo-like kinase 1 from natural and semisynthetic compounds

PLK1 has an important role in the regulation of cell cycle and represents an important target for cancer treatment. This enzyme belongs to the Polo-like kinases family, which is characterized by a regulatory domain named Polo-box domain (PBD). Rather than regular kinase inhibitors, this domain provides high selectivity to PLK1. Here, we report on four novel PLK1 PBD inhibitors identified by cytotoxicity screening and fluorescence polarization assay of a chemical library of natural and semisynthetic compounds. These compounds revealed two- to three-fold higher selectivity to the PDB of PLK1 than to those of the related family members, PLK2 and PLK3. These four substances inhibited tumor cell growth of sensitive CCRF-CEM and multidrug-resistant CEM/ADR5000 leukemia cells. The tested compounds increased the apoptotic cell fraction, which indicates apoptosis as a major mechanism of cell death. Cell cycle analysis showed compound (5) arrested the cell cycle of CCRF-CEM cells in the G2/M phase, while the other three molecules ((compound (3), compound (4), and compound (6)) exerted pronounced cytotoxicity with an increase of cells in the sub-G1 population. Molecular docking was performed for the understanding of ligand-protein interaction, the tested candidates showed strong binding affinity to PLK1 PBD. In conclusion, we identified four new chemical scaffolds that may serve as lead compounds for the development of selective PLK1 inhibitors in the future.

Development of a Novel Cell-Permeable Protein-Protein Interaction Inhibitor for the Polo-box Domain of Polo-like Kinase 1

Polo-like kinase 1 (PLK1) is a key regulator of mitosis and a recognized drug target for cancer therapy. Inhibiting the polo-box domain of PLK1 offers potential advantages of increased selectivity and subsequently reduced toxicity compared with targeting the kinase domain. However, many if not all existing polo-box domain inhibitors have been shown to be unsuitable for further development. In this paper, we describe a novel compound series, which inhibits the protein-protein interactions of PLK1 via the polo-box domain. We combine high throughput screening with molecular modeling and computer-aided design, synthetic chemistry, and cell biology to address some of the common problems with protein-protein interaction inhibitors, such as solubility and potency. We use molecular modeling to improve the solubility of a hit series with initially poor physicochemical properties, enabling biophysical and biochemical characterization. We isolate and characterize enantiomers to improve potency and demonstrate on-target activity in both cell-free and cell-based assays, entirely consistent with the proposed binding model. The resulting compound series represents a promising starting point for further progression along the drug discovery pipeline and a new tool compound to study kinase-independent PLK functions.

Active PLK1-driven metastasis is amplified by TGF-β signaling that forms a positive feedback loop in non-small cell lung cancer

Early findings that PLK1 is highly expressed in cancer have driven an exploration of its functions in metastasis. However, whether PLK1 induces metastasis in vivo and its underlying mechanisms in NSCLC have not yet been determined. Here, we show that the expression of active PLK1 phosphorylated at T210, abundant in TGF-β-treated lung cells, potently induced metastasis in a tail-vein injection model. Active PLK1 with intact polo-box and ATP-binding domains accelerated cell motility and invasiveness by triggering EMT reprogramming, whereas a phosphomimetic version of p-S137-PLK1 did not, indicating that the phosphorylation status of PLK1 may determine the cell traits. Active PLK1-driven invasiveness upregulated TGF-β signaling and TSG6 encoded by TNFAIP6. Loss of TNFAIP6 disturbed the metastatic activity induced by active PLK1 or TGF-β. Clinical relevance shows that PLK1 and TNFAIP6 are strong predictors of poor survival rates in metastatic NSCLC patients. Therefore, we suggest that active PLK1 promotes metastasis by upregulating TGF-β signaling, which amplifies its metastatic properties by forming a positive feedback loop and that the PLK1/TGF-β-driven metastasis is effectively blocked by targeting PLK1 and TSG6, providing PLK1 and TSG6 as negative markers for prognostics and therapeutic targets in metastatic NSCLC.

Phase I dose-escalation study of volasertib in pediatric patients with acute leukemia or advanced solid tumors

BACKGROUND

Volasertib induces mitotic arrest and apoptosis by targeting Polo-like kinases. In this phase I dose-escalation study, the maximum tolerated dose (MTD), pharmacokinetics (PK), and preliminary efficacy of volasertib were determined in pediatric patients.

METHODS

Patients aged 2 to <18 years with relapsed/refractory acute leukemia/advanced solid tumors (ST) without available effective treatments were enrolled-cohort C1 (aged 2 to <12 years); cohort C2 (aged 12 to <18 years). The patients received volasertib intravenously (starting dose: 200 mg/m² body surface area on day 1, every 14 days). The primary endpoint was the pediatric MTD for further development.

RESULTS

Twenty-two patients received treatment (C1: leukemia, n = 4; ST, n = 8; C2: leukemia, n = 3; ST, n = 7). No dose-limiting toxicities (DLTs) occurred up to 300 mg/m² volasertib in C1; two patients in C2, at 250 mg/m² volasertib, had DLTs in cycle 1, one of which led to death; therefore, the MTD of volasertib in C2 was 200 mg/m² . The most common grade 3/4 adverse events (all patients) were febrile neutropenia, thrombocytopenia, and neutropenia (41% each). Stable disease (SD) was the best objective response (leukemia, n = 5; ST, n = 2); the duration of SD was short in all patients, except in one with an ST. PK profiles were generally comparable across dose groups and were consistent with those in adults.

CONCLUSION

The pediatric MTD/dose for further development was identified. There were no unexpected safety or PK findings; limited antitumor/antileukemic activity was demonstrated.

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.

Polo-like kinase 1 inhibition in NSCLC: mechanism of action and emerging predictive biomarkers

Non-small cell lung cancer (NSCLC) is the leading cause of cancer death worldwide. Due to often unspecific disease symptoms, locally advanced or metastatic disease is diagnosed in the majority of all cases. Palliative treatment options comprise of conventional cytotoxic agents, immunotherapy with checkpoint inhibitors and the use of specific small-molecule tyrosine kinase inhibitors (TKI). However, these TKIs are mainly restricted to a small proportion of patients with lung cancer that harbor activating driver mutations. Still, the effectiveness and favorable safety profile of these compounds have prompted a systematic search for specific driver mechanisms of tumorigenesis and moreover the development of corresponding kinase inhibitors. In recent years, the Polo-like kinase (PLK) family has emerged as a key regulator in mitotic regulation. Its role in cell proliferation and the frequently observed overexpression in various tumor entities have raised much interest in basic and clinical oncology aiming to attenuate tumor growth by targeting the PLK. In this review, we give a comprehensive summary on the (pre-) clinical development of the different types of PLK inhibitors in lung cancer and summarize their mechanisms of action, safety and efficacy data and give an overview on translational research aiming to identify predictive biomarkers for a rational use of PLK inhibitors.

Molecular and enzoinformatics perspectives of targeting Polo-like kinase 1 in cancer therapy

Cancer is a disease that has been the focus of scientific research and discovery and continues to remain so. Polo-like kinases (PLKs) are basically serine/threonine kinase enzymes that control cell cycle from yeast to humans. PLK-1 stands for 'Polo-like kinase-1'. It is the most investigated protein among PLKs. It is crucial for intracellular processes, hence a 'hot' anticancer drug-target. Accelerating innovations in Enzoinformatics and associated molecular visualization tools have made it possible to literally perform a 'molecular level walk' traversing through and observing the minutest contours of the active site of relevant enzymes. PLK-1 as a protein consists of a kinase domain at the protein N-terminal and a Polo Box Domain (PBD) at the C-terminal connected by a short inter-domain linking region. PBD has two Polo-Boxes. PBD of PLK-1 gives the impression of "a small clamp sandwiched between two clips", where the two Polo Boxes are the 'clips' and the 'phosphopeptide' is the small 'clamp'. Broadly, two major sites of PLK-1 can be potential targets: one is the adenosine-5'-triphosphate (ATP)-binding site in the kinase domain and the other is PBD (more preferred due to specificity). Targeting PLK-1 RNA and the interaction of PLK-1 with a key binding partner can also be approached. However, the list of potent small molecule inhibitors targeting the PBD site of PLK-1 is still not long enough and needs due input from the scientific community. Recently, eminent scientists have proposed targeting the 'Y'-shaped pocket of PLK-1-PBD and encouraged design of ligands that should be able to concurrently bind to two or more modules of the 'Y' pocket. Hence, it is suggested that during molecular interaction analyses, particular focus should be kept on the moiety in each ligand/drug candidate which directly interacts with the amino acid residue(s) that belong(s) to one of the three binding modules which together create this Y-shaped cavity. This obviously includes (but it is not limited to) the 'shallow cleft'-forming residues i.e. Trp414, H538 and K540, as significance of these binding residues has been consistently highlighted by many studies. The present article attempts to give a concise yet critically updated overview of targeting PLK-1 for cancer therapy.

Molecular Targets of Genistein and Its Related Flavonoids to Exert Anticancer Effects

Increased health awareness among the public has highlighted the health benefits of dietary supplements including flavonoids. As flavonoids target several critical factors to exert a variety of biological effects, studies to identify their target-specific effects have been conducted. Herein, we discuss the basic structures of flavonoids and their anticancer activities in relation to the specific biological targets acted upon by these flavonoids. Flavonoids target several signaling pathways involved in apoptosis, cell cycle arrest, mitogen-activated protein kinase (MAPK), phosphoinositide 3-kinase (PI3K)/AKT kinase, and metastasis. Polo-like kinase 1 (PLK1) has been recognized as a valuable target in cancer treatment due to the prognostic implication of PLK1 in cancer patients and its clinical relevance between the overexpression of PLK1 and the reduced survival rates of several carcinoma patients. Recent studies suggest that several flavonoids, including genistein directly inhibit PLK1 inhibitory activity. Later, we focus on the anticancer effects of genistein through inhibition of PLK1.

In vitro study of the Polo-like kinase 1 inhibitor volasertib in non-small-cell lung cancer reveals a role for the tumor suppressor p53

Polo-like kinase 1 (Plk1), a master regulator of mitosis and the DNA damage response, is considered to be an intriguing target in the research field of mitotic intervention. The observation that Plk1 is overexpressed in multiple human malignancies, including non-small-cell lung cancer (NSCLC), gave rise to the development of several small-molecule inhibitors. Volasertib, presently the most extensively studied Plk1 inhibitor, has been validated to efficiently reduce tumor growth in preclinical settings. Unfortunately, only modest antitumor activity against solid tumors was reported in clinical trials. This discrepancy prompted research into the identification of predictive biomarkers. In this study, we investigated the therapeutic effect of volasertib monotherapy (i.e., cytotoxicity, cell cycle distribution, apoptotic cell death, cellular senescence, and migration) in a panel of NSCLC cell lines differing in p53 status under both normal and reduced oxygen tension (<0.1% O2 ). A strong growth inhibitory effect was observed in p53 wild-type cells (A549 and A549-NTC), with IC50 values significantly lower than those in p53 knockdown/mutant cells (A549-920 and NCI-H1975) (P < 0.001). While mitotic arrest was significantly greater in cells with nonfunctional p53 (P < 0.005), apoptotic cell death (P < 0.026) and cellular senescence (P < 0.021) were predominantly induced in p53 wild-type cells. Overall, the therapeutic effect of volasertib was reduced under hypoxia (P < 0.050). Remarkably, volasertib inhibited cell migration in all cell lines tested (P < 0.040), with the exception of for the NCI-H1975 p53 mutant cell line. In conclusion, our results show an important difference in the therapeutic effect of Plk1 inhibition in NSCLC cells with versus without functional p53. Overall, the p53 wild-type cell lines were more sensitive to volasertib treatment, suggesting that p53 might be a predictive biomarker for Plk1 inhibition in NSCLC. Moreover, our results pave the way for new combination strategies with Plk1 inhibitors to enhance antitumor activity.

An oncoinformatics study to predict the inhibitory potential of recent FDA-approved anti-cancer drugs against human Polo-like kinase 1 enzyme: a step towards dual-target cancer medication

Cancer prevalence has increased at an alarming rate worldwide. Complexity, resistance mechanism and multiple compensatory survival pathways of cancer cells have abated the response of currently available cancer medications. Therefore, multi-target agents rather than single target might provide a better solution to these cancer therapy issues. In the present study, anti-PLK1 (Polo-like kinase 1) potential of the eight FDA-approved (2017) anti-cancer drugs have been explored using molecular docking approach. Out of all the tested drugs, brigatinib, niraparib and ribociclib showed better binding affinity towards the 'kinase domain' of PLK1. The Gibbs free binding energy (ΔG) and inhibition constant (K _i) values for brigatinib, niraparib and ribociclib interaction with the kinase domain of PLK1 were '- 8.05 kcal/mol and 1.26 µM', '- 8.35 kcal/mol and 0.729 µM' and '- 7.29 kcal/mol and 4.52 µM', respectively. Interestingly, the docking results of these three drugs were better than the known PLK1 inhibitors (BI-2536 and rigosertib). The ΔG and K _i values for BI-2536 and rigosertib interaction with the kinase domain of PLK1 were '- 6.8 kcal/mol and 10.38 µM' and '- 6.6 kcal/mol and 14.51 µM', respectively. Brigatinib, niraparib and ribociclib have been approved by FDA for the treatment of non-small cell lung cancer, ovarian/fallopian tube cancer and breast cancer, respectively. PLK1 is regarded as a potential cancer target, and it is specifically over-expressed in different types of cancer cells, including aforementioned cancers. Actually, the target enzymes for anti-cancer action of brigatinib, niraparib and ribociclib are tyrosine kinase, poly(ADP-ribose) polymerase and cyclin-dependent kinase 4/6, respectively. However, based on our outcomes, we could safely state that PLK1 might plausibly emerge as an add-on target for each of these three anti-cancer drugs. We strongly believe that this study would assist in the development of better dual-targeting cancer therapeutic agent in the near future.

Targeting the CINful genome: Strategies to overcome tumor heterogeneity

Genomic instability, and more specifically chromosomal instability (CIN), arises from a number of processes that are defective in cancer, such as aberrant mitotic cell division, replication stress, defective DNA damage repair, and ineffective telomere maintenance. CIN is an emerging hallmark of cancer that contributes to tumor heterogeneity through increased rates of genetic alterations. As genetic heterogeneity within a single tumor and between tumors is a key challenge leading to treatment failures, this brings to question, whether therapeutic approaches should aim at the genetic diversity or a specific mutation present within these tumors. Answering this question will determine the future of personalized targeted therapies. Here we discuss, how the genetic diversity associated with CIN in tumor cells can be used as a therapeutic advantage and targeted by exploiting the genetic concepts of synthetic lethality and synthetic dosage lethality. Given that a number of CIN-related pathways work together to fix the DNA damage within our genome and ensure proper segregation of chromosomes, we specifically focus on the genetic interactions amongst these pathways and their potential therapeutic applicability in cancer. We also discuss, how tumor genetic heterogeneity can be targeted in emerging immunotherapeutic approaches.

Polo-like kinase 1 inhibitor BI 6727 induces DNA damage and exerts strong antitumor activity in small cell lung cancer

The prognosis of small cell lung cancer (SCLC) is poor despite its good initial response to chemotherapy. Polo-like kinase 1 (PLK1) is a crucial mitotic regulator that is overexpressed in many tumors, and its overexpression is associated with tumor aggressiveness and a poor prognosis. However, its role in SCLC is still poorly characterized. Based on immunohistochemistry findings, the PLK1 protein is expressed at higher levels in SCLC tumor samples than in normal lung tissue samples. The selective PLK1 inhibitor BI 6727 significantly induced the inhibition of proliferation and apoptosis in a dose-dependent manner in SCLC cell lines. FACS analysis showed an increase in the population of cells in the G2/M phase, followed by DNA damage and the consequent activation of the ataxia telangiectasia and Rad3-related (ATR)/ataxia telangiectasia mutated (ATM)-Chk1/Chk2 checkpoint pathway. In addition, BI 6727 treatment resulted in clearly attenuated growth and apoptosis in NCI-H446 xenografts. The level of histone H2AX phosphorylation at serine-139 (γH2AX) was markedly increased both in vitro and in vivo. Our findings indicate that BI 6727 has therapeutic potential for SCLC patients.

Hsa-let-7b inhibits cell proliferation by targeting PLK1 in HCC

Previous studies have shown that high levels of PLK1 are expressed in HCC, and PLK1 inhibitors are being tested in clinical trials. However, the mechanisms, which regulate PLK1 expression in HCC, have not been clarified. Here, we show that induction of let-7b over-expression inhibits the PLK1-regulated luciferase activity in HEK-293T cells, and decreases the levels of PLK1 expression in HCC cells. Furthermore, the levels of let-7b expression were negatively correlated with PLK1 expression in HCC tissues. Let-7b over-expression inhibited the proliferation of HCC cells and promoted their apoptosis, which were partially rescued by increased PLK1 expression. Let-7b over-expression decreased the levels of PLK1, CDC25C and Survivin phosphorylation and CDC2, β-catenin, TCF-4 expression, which were mitigated by increased PLK1 expression in MHCC-97H cells. Let-7b over-expression inhibited the development and growth of implanted HCC tumors in mice by decreasing PLK1 and Survivin expression in the tumors. Together, our data indicated that let-7b targeted PLK1 to inhibit HCC growth and induce their apoptosis by attenuating the PLK1-mediated Survivin phosphorylation. Our findings may provide new insights into the pathogenesis of HCC.

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 resveratrol-curcumin hybrid derivative inhibits mitosis progression in estrogen positive MCF-7 breast cancer cells

Curcumin (1) and resveratrol (2) are bioactive natural compounds that display wide pharmacological properties, including antitumor activity. However, their clinical application has been limited due to their low solubility and bioavailability. Nevertheless, independent studies have considered these compounds as interesting prototypes for developing new chemical structures useful for anticancer therapy. Here in, we report the synthesis of novel curcumin-like hydrazide analogues (3a and 3b), and a series of curcumin-resveratrol hybrid compounds (4a-f), and the evaluation of their cytotoxic potential on three tumor cell lines MCF-7 (breast), A549 (lung), and HepG2 (liver). Cell viability was significantly reduced in all tested cell lines when compounds 4c-4e were used. The IC50 values for these compounds on MCF-7 cells were lower than those for curcumin, resveratrol, or curcumin combined with resveratrol. We evidenced that 4c promoted a drastic increase of G2/M population. The accumulation of cells in mitosis onset in treated cultures was due to, at least in part, the ability of 4c to modulate nuclear kinase proteins, which orchestrate important events in mitosis progression. We have also observed significant reduction of the relative RNAm abundance of CCNB1, PLK1, AURKA, AURKB in samples treated with 4c, with concomitant increase of CDKN1A (p21). Thus, compound 4c is a promising multi-target antitumor agent that should be considered for further in vivo studies.

MASTL inhibition promotes mitotic catastrophe through PP2A activation to inhibit cancer growth and radioresistance in breast cancer cells

BACKGROUND

Although MASTL (microtubule-associated serine/threonine kinase-like) is a key mitotic kinase that regulates mitotic progression through the inactivation of tumor suppressor protein phosphatase 2A (PP2A), the antitumor mechanism of MASTL targeting in cancer cells is still unclear.

METHODS

MASTL expression was evaluated by using breast cancer tissue microarrays and public cancer databases. The effects of MASTL depletion with siRNAs were evaluated in various breast cancer cells or normal cells. Various methods, including cell viability, cell cycle, soft agar, immunoblotting, immunofluorescence, PP2A activity, live image, and sphere forming assay, were used in this study.

RESULTS

This study showed the oncosuppressive mechanism of MASTL targeting that promotes mitotic catastrophe through PP2A activation selectively in breast cancer cells. MASTL expression was closely associated with tumor progression and poor prognosis in breast cancer. The depletion of MASTL reduced the oncogenic properties of breast cancer cells with high MASTL expression, but did not affect the viability of non-transformed normal cells with low MASTL expression. With regard to the underlying mechanism, we found that MASTL inhibition caused mitotic catastrophe through PP2A activation in breast cancer cells. Furthermore, MASTL depletion enhanced the radiosensitivity of breast cancer cells with increased PP2A activity. Notably, MASTL depletion dramatically reduced the formation of radioresistant breast cancer stem cells in response to irradiation.

CONCLUSION

Our data suggested that MASTL inhibition promoted mitotic catastrophe through PP2A activation, which led to the inhibition of cancer cell growth and a reversal of radioresistance in breast cancer cells.

Targeting Polo-like kinase 1 and TRAIL enhances apoptosis in non-small cell lung cancer

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) can selectively induce apoptosis in cancer cells without causing damage to normal cells. However, some tumors are resistant to TRAIL monotherapy and clinical studies assessing targeted agents towards the TRAIL receptor have failed to show robust therapeutic activity. Evidence has shown that standard anti-mitotic drugs can induce synergistic apoptosis upon combination with TRAIL via cell cycle arrest. Polo like kinase-1 (PLK1) plays a critical role in different stages of cell cycle progression and mitosis. A number of investigations have demonstrated that PLK1 inhibition causes cell cycle arrest and mitotic catastrophe in non-small cell lung cancer (NSCLC), and we thus postulated that PLK1 inhibition could enhance TRAIL-induced apoptosis. We demonstrate that the combination of a TRAIL receptor agonist and a PLK1 inhibitor synergistically reduces cell viability, and strongly increases apoptosis in NSCLC cellular models. Consistent with our in vitro observations, this drug combination also significantly reduces tumor growth in vivo. Our data additionally reveal that G2/M cell cycle arrest and downregulation of Mcl-1 and signal transducer and activator of transcription 3 (STAT3) activity following PLK1 inhibition may contribute to the sensitization of TRAIL-induced apoptosis in NSCLC. Together, these data support the further exploration of combined TRAIL and PLK1 inhibition in the treatment of NSCLC.

Targeting Polo-like Kinase 1 by a Novel Pyrrole-Imidazole Polyamide-Hoechst Conjugate Suppresses Tumor Growth In Vivo

The serine/threonine kinase Polo-like kinase 1 (Plk1) plays a pivotal role in cell proliferation and has been validated as a promising anticancer drug target. However, very limited success has been achieved in clinical applications using existing Plk1 inhibitors, due to lack of sufficient specificity toward Plk1. To develop a novel Plk1 inhibitor with high selectivity and efficacy, we designed and synthesized a pyrrole-imidazole polyamide-Hoechst conjugate, PIP3, targeted to specific DNA sequence in the PLK1 promoter. PIP3 could specifically inhibit the cell cycle-regulated Plk1 expression and consequently retard tumor cell growth. Cancer cells treated with PIP3 exhibited severe mitotic defects and increased apoptosis, whereas normal cells were not affected by PIP3 treatment. Furthermore, subcutaneous injection of PIP3 into mice bearing human cancer xenografts induced significant tumor growth suppression with low host toxicity. Therefore, PIP3 exhibits the potential as an effective agent for targeted cancer therapy. Mol Cancer Ther; 17(5); 988-1002. ©2018 AACR.

Stable aneuploid tumors cells are more sensitive to TTK inhibition than chromosomally unstable cell lines

Inhibition of the spindle assembly checkpoint kinase TTK causes chromosome mis-segregation and tumor cell death. However, high levels of TTK correlate with chromosomal instability (CIN), which can lead to aneuploidy. We show that treatment of tumor cells with the selective small molecule TTK inhibitor NTRC 0066-0 overrides the mitotic checkpoint, irrespective of cell line sensitivity. In stable aneuploid cells NTRC 0066-0 induced acute CIN, whereas in cells with high levels of pre-existing CIN there was only a small additional fraction of cells mis-segregating their chromosomes. In proliferation assays stable aneuploid cells were more sensitive than cell lines with pre-existing CIN. Tetraploids are thought to be an intermediate between diploid and unstable aneuploid cells. TTK inhibitors had the same potency on post-tetraploid and parental diploid cells, which is remarkable because the post-tetraploids are more resistant to mitotic drugs. Finally, we confirm that the reference compound reversine is a TTK inhibitor and like NTRC 0066-0, inhibits the proliferation of patient-derived colorectal cancer organoids. In contrast, treatment with TTK inhibitor did not reduce the viability of non-proliferating T cell acute lymphoblastic leukemia cells samples. Consequently, TTK inhibitor therapy is expected to spare non-dividing cells, and may be used to target stable aneuploid tumors.

Targeting PLK1 overcomes T-DM1 resistance via CDK1-dependent phosphorylation and inactivation of Bcl-2/xL in HER2-positive breast cancer

Trastuzumab-refractory, HER2 (human epidermal growth factor receptor 2)-positive breast cancer is commonly treated with trastuzumab emtansine (T-DM1), an antibody-drug conjugate of trastuzumab and the microtubule-targeting agent, DM1. However, drug response reduces greatly over time due to acquisition of resistance whose molecular mechanisms are mostly unknown. Here, we uncovered a novel mechanism of resistance against T-DM1 by combining whole transcriptome sequencing (RNA-Seq), proteomics and a targeted small interfering RNA (siRNA) sensitization screen for molecular level analysis of acquired and de novo T-DM1-resistant models of HER2-overexpressing breast cancer. We identified Polo-like kinase 1 (PLK1), a mitotic kinase, as a resistance mediator whose genomic as well as pharmacological inhibition restored drug sensitivity. Both acquired and de novo resistant models exhibited synergistic growth inhibition upon combination of T-DM1 with a selective PLK1 inhibitor, volasertib, at a wide concentration range of the two drugs. Mechanistically, T-DM1 sensitization upon PLK1 inhibition with volasertib was initiated by a spindle assembly checkpoint (SAC)-dependent mitotic arrest, leading to caspase activation, followed by DNA damage through CDK1-dependent phosphorylation and inactivation of Bcl-2/xL. Furthermore, we showed that Ser70 phosphorylation of Bcl-2 directly regulates apoptosis by disrupting the binding to and sequestration of the pro-apoptotic protein Bim. Importantly, T-DM1 resistance signature or PLK1 expression correlated with cell cycle progression and DNA repair, and predicted a lower sensitivity to taxane/trastuzumab combination in HER2-positive breast cancer patients. Finally, volasertib in combination with T-DM1 greatly synergized in models of T-DM1 resistance in terms of growth inhibition both in three dimensional (3D) cell culture and in vivo. Altogether, our results provide promising pre-clinical evidence for potential testing of T-DM1/volasertib combination in T-DM1 refractory HER2-positive breast cancer patients for whom there is currently no treatment available.

PLK1 has tumor-suppressive potential in APC-truncated colon cancer cells

The spindle assembly checkpoint (SAC) acts as a molecular safeguard in ensuring faithful chromosome transmission during mitosis, which is regulated by a complex interplay between phosphatases and kinases including PLK1. Adenomatous polyposis coli (APC) germline mutations cause aneuploidy and are responsible for familial adenomatous polyposis (FAP). Here we study the role of PLK1 in colon cancer cells with chromosomal instability promoted by APC truncation (APC-ΔC). The expression of APC-ΔC in colon cells reduces the accumulation of mitotic cells upon PLK1 inhibition, accelerates mitotic exit and increases the survival of cells with enhanced chromosomal abnormalities. The inhibition of PLK1 in mitotic, APC-∆C-expressing cells reduces the kinetochore levels of Aurora B and hampers the recruitment of SAC component suggesting a compromised mitotic checkpoint. Furthermore, Plk1 inhibition (RNAi, pharmacological compounds) promotes the development of adenomatous polyps in two independent Apc Min/+ mouse models. High PLK1 expression increases the survival of colon cancer patients expressing a truncated APC significantly.

A kinome-wide RNAi screen identifies ALK as a target to sensitize neuroblastoma cells for HDAC8-inhibitor treatment

The prognosis of advanced stage neuroblastoma patients remains poor and, despite intensive therapy, the 5-year survival rate remains less than 50%. We previously identified histone deacetylase (HDAC) 8 as an indicator of poor clinical outcome and a selective drug target for differentiation therapy in vitro and in vivo. Here, we performed kinome-wide RNAi screening to identify genes that are synthetically lethal with HDAC8 inhibitors. These experiments identified the neuroblastoma predisposition gene ALK as a candidate gene. Accordingly, the combination of the ALK/MET inhibitor crizotinib and selective HDAC8 inhibitors (3–6 µM PCI-34051 or 10 µM 20a) efficiently killed neuroblastoma cell lines carrying wildtype ALK (SK-N-BE(2)-C, IMR5/75), amplified ALK (NB-1), and those carrying the activating ALK F1174L mutation (Kelly), and, in cells carrying the activating R1275Q mutation (LAN-5), combination treatment decreased viable cell count. The effective dose of crizotinib in neuroblastoma cell lines ranged from 0.05 µM (ALK-amplified) to 0.8 µM (wildtype ALK). The combinatorial inhibition of ALK and HDAC8 also decreased tumor growth in an in vivo zebrafish xenograft model. Bioinformatic analyses revealed that the mRNA expression level of HDAC8 was significantly correlated with that of ALK in two independent patient cohorts, the Academic Medical Center cohort (n = 88) and the German Neuroblastoma Trial cohort (n = 649), and co-expression of both target genes identified patients with very poor outcome. Mechanistically, HDAC8 and ALK converge at the level of receptor tyrosine kinase (RTK) signaling and their downstream survival pathways, such as ERK signaling. Combination treatment of HDAC8 inhibitor with crizotinib efficiently blocked the activation of growth receptor survival signaling and shifted the cell cycle arrest and differentiation phenotype toward effective cell death of neuroblastoma cell lines, including sensitization of resistant models, but not of normal cells. These findings reveal combined targeting of ALK and HDAC8 as a novel strategy for the treatment of neuroblastoma.

PRKAR2B plays an oncogenic role in the castration-resistant prostate cancer

Castration-resistant prostate cancer (CRPC) is an advanced form of prostate cancer. Despite some progresses have been made, the mechanism of CRPC development is still largely unknown, including the genes involved in its development have not been well defined. Here, we identifiedPRKAR2B to be a gene over-expressingin castration-resistant prostate cancer by analyzing the different online databases. Followed functional validation experiments showed that PRKAR2B promoted CRPC cell proliferation and invasion, and inhibited CRPC cell apoptosis. Whole genome transcriptome and GO enrichment analyses of the knock-down of PRKAR2B in CRPC cells showed that PRKAR2B mainly accelerated cell cycle biological process and modulated multiple cell cycle genes, such as CCNB1, MCM2, PLK1 and AURKB. Our study firstly identified PRKAR2B as a novel oncogenic gene involved in CRPC development and suggested it is a promising target for the future investigation and the treatment of CRPC.

Plk1 phosphorylation of Numb leads to impaired DNA damage response

Although Numb is well-recognized as a cell-fate determinant in stem/progenitor cells, accumulating evidence supports that Numb also has a critical role in adult tissues and cancers, in particular, in the context of regulation of tumor suppressor p53. Herein, we identified Numb as a novel substrate of Polo-like kinase 1 (Plk1). Of significance, we showed that Plk1-mediated phosphorylation of Numb leads to its enhanced proteasomal degradation and impaired Numb/p53 pathway, thus providing another mechanism how Plk1 antagonizes p53 during DNA damage response. In addition, the novel phosphorylation event identified by us further supports the notion that post-translational modifications of Numb uncouple Numb from p53 and lead to p53 destabilization. Finally, our data generated from both human cancer cell lines and mouse xenograft model showed that cancer cells carrying the unphosphorylated form of Numb by Plk1 are more sensitive to doxorubicin, a classical chemotherapeutic drug. Therefore, our work may provide future strategies for improving the efficacy of chemotherapy by targeting Numb phosphorylation by Plk1.

Antitumor activity of the polo-like kinase inhibitor, TAK-960, against preclinical models of colorectal cancer

BACKGROUND

Polo-like kinase 1 (Plk1) is a serine/threonine kinase that is a key regulator of multiple stages of mitotic progression. Plk1 is upregulated in many tumor types including colorectal cancer (CRC) and portends a poor prognosis. TAK-960 is an ATP-competitive Plk1 inhibitor that has demonstrated efficacy across a broad range of cancer cell lines, including CRC. In this study, we investigated the activity of TAK-960 against a large collection of CRC models including 55 cell lines and 18 patient-derived xenografts.

METHODS

Fifty-five CRC cell lines and 18 PDX models were exposed to TAK-960 and evaluated for proliferation (IC50) and Tumor Growth Inhibition Index, respectively. Additionally, 2 KRAS wild type and 2 KRAS mutant PDX models were treated with TAK-960 as single agent or in combination with cetuximab or irinotecan. TAK-960 mechanism of action was elucidated through immunoblotting and cell cycle analysis.

RESULTS

CRC cell lines demonstrated a variable anti-proliferative response to TAK-960 with IC50 values ranging from 0.001 to > 0.75 μmol/L. Anti-proliferative effects were sustained after removal of drug. Following TAK-960 treatment a highly variable accumulation of mitotic (indicating cell cycle arrest) and apoptotic markers was observed. Cell cycle analysis demonstrated that TAK-960 treatment induced G2/M arrest and polyploidy. Six out of the eighteen PDX models responded to single agent TAK-960 therapy (TGII< 20). The addition of TAK-960 to standard of care chemotherapy resulted in largely additive antitumor effects.

CONCLUSION

TAK-960 is an active anti-proliferative agent against CRC cell lines and PDX models. Collectively, these data suggest that TAK-960 may be of therapeutic benefit alone or in combination with other agents, although future work should focus on the development of predictive biomarkers and hypothesis-driven rational combinations.

The responses of cancer cells to PLK1 inhibitors reveal a novel protective role for p53 in maintaining centrosome separation

Polo-like kinase-1 (PLK1) plays a major role in driving mitotic events, including centrosome disjunction and separation, and is frequently over-expressed in human cancers. PLK1 inhibition is a promising therapeutic strategy and works by arresting cells in mitosis due to monopolar spindles. The p53 tumour suppressor protein is a short-lived transcription factor that can inhibit the growth, or stimulate the death, of developing cancer cells. Curiously, although p53 normally acts in an anti-cancer capacity, it can offer significant protection against inhibitors of PLK1, but the events underpinning this effect are not known. Here, we show that functional p53 reduces the sensitivity to PLK1 inhibitors by permitting centrosome separation to occur, allowing cells to traverse mitosis and re-enter cycle with a normal complement of 2N chromosomes. Protection entails the activation of p53 through the DNA damage-response enzymes, ATM and ATR, and requires the phosphorylation of p53 at the key regulatory site, Ser15. These data highlight a previously unrecognised link between p53, PLK1 and centrosome separation that has therapeutic implications for the use of PLK1 inhibitors in the clinic.