Recent Advances and New Strategies in Targeting Plk1 for Anticancer Therapy

Discovery of a novel PLK1 inhibitor with high inhibitory potency using a combined virtual screening strategy

PLK1 is essential for cell cycle regulation and proliferation, and its elevated expression in prostate cancer is associated with high tumour grade. Therefore, PLK1 inhibition is considered a promising strategy for the treatment of prostate cancer. Here, we identified five compounds (Hits 1-5) targeting the kinase domain (KD) of PLK1 using a combined virtual screening approach. Hits 1-5 all had picomolar (pM) inhibitory potency against PLK1. Notably, Hit-4 showed the strongest inhibitory activity against PLK1 (IC50 = 22.61 ± 1.12 pM) and displayed high selectivity for PLK1. Meanwhile, molecular dynamics (MD) simulations revealed that the complex formed by Hit-4 and PLK1 remained stable. Importantly, Hit-4 exhibited potent inhibitory effects on the proliferation of DU-145 prostate cancer cells (IC50 = 0.09 ± 0.01 nM). In conclusion, Hit-4 is a potent and highly selective antitumor candidate with therapeutic potential for prostate cancer.

Uncovering the underlying mechanism of yuanhuacine against colorectal cancer by transcriptomics and experimental investigations

BACKGROUND

Colorectal cancer (CRC) holds the third position in terms of incidence and ranks behind lung cancer in terms of mortality worldwide. Yuanhuacine, one of the main active ingredients of genkwa flos, has demonstrated promising application prospects in the field of cancer treatment. However, its underlying mechanism against CRC has not been fully clarified.

PURPOSE

This study aimed to investigate anti-tumor activity of yuanhuacine and clarify its underlying mechanism in CRC.

METHODS

CRC HCT116, HT-29, Caco-2, SW480, and LS174T cells were used to assess the in vitro anti-tumor activity of yuanhuacine by cell viability, proliferation, apoptosis, cycle distribution, migration, and colony formation assays. Meanwhile, an HT-29 xenograft mouse model was successfully constructed to investigate the anti-tumor effect of yuanhuacine in vivo. Transcriptomic assay and network pharmacology were applied to explore the underlying mechanism of yuanhuacine in combating CRC, which was further verified by quantitative reverse transcription polymerase chain reaction, western blot. The interaction of yuanhuacine with protein was performed by molecular docking, molecular dynamics simulation, and cell thermal shift assays.

RESULTS

Yuanhuacine significantly induced apoptosis and reduced viability of CRC cells with IC50 values ranging from 28.09 to 56.16 μM. Moreover, it suppressed the colony formation ability of CRC cells and inhibited the expression of proliferation marker Ki67 in CRC cells and tissues. Meanwhile, the impairment of cell migration by yuanhuacine has been identified by wound healing assay and transwell migration assay. Furthermore, cell cycle assay showed that yuanhuacine resulted in significant G2/M phase arrest. Yuanhuacine significantly inhibited the tumor growth of HT-29 xenograft mice without obvious pathological changes in major organs. Mechanistically, the differentially expressed genes were enriched in cell cycle by both Kyoto Encyclopedia of Genes and Genomes and Gene Ontology enrichment analyses. The mRNA and protein expressions of PLK1, CCNA2, and TTK were inhibited by yuanhuacine. Cell thermal shift assay further validated the direct interactions between yuanhuacine and each of PLK1, CCNA2, and TTK. The anti-proliferation activity and cell cycle arrest induced by yuanhuacine were reversed by overexpression of PLK1.

CONCLUSIONS

Yuanhuacine is a promising candidate compound in combating CRC by inhibiting proliferation of CRC cells. The major underlying mechanism involves regulating PLK1, which results in G2/M phase arrest.

Fine-tuning probes for fluorescence polarization binding assays of bivalent ligands against polo-like kinase 1 using full-length protein

Polo-like kinase 1 (Plk1) is an important cell cycle regulator that is a recognized target for development of anti-cancer therapeutics. Plk1 is composed of a catalytic kinase domain (KD), a flexible interdomain linker and a polo-box domain (PBD). Intramolecular protein-protein interactions (PPIs) between the PBD and KD result in "auto-inhibition" that is an essential component of proper Plk1 function. Recently, we developed high-affinity PBD-binding inhibitors using a bivalent approach. These ligands contain the low-nanomolar affinity Plk1 KD-binding inhibitors BI2536 or Wortmannin tethered to the PBD-binding peptide, PLH*SpT (H* represents a -(CH2)8Ph group on the histidine side chain π-nitrogen). Due to the extremely high affinity of these bivalent inhibitors, to avoid bottoming out in competitive binding assays, it was necessary to use PLH*SpT in the affinity probe. As reported herein, we have developed fluorescence polarization assays using a new fluorescent probe based on the Plk1 PBD-binding peptide, FDPPLHSpTA. We applied the assay to evaluate the affinities of bivalent inhibitors that possess a variety of PBD-binding peptides having much lower PBD-affinities than PLH*SpT. Tethering BI2536 in these bivalent inhibitors resulted in significant affinity enhancements as compared to the parent monovalent peptides.

Structure-Based Discovery of a Highly Selective, Oral Polo-Like Kinase 1 Inhibitor with Potent Antileukemic Activity

Polo-like kinase 1 (PLK1) plays pivotal roles in cell division and cancer pathogenesis, making it a highly coveted therapeutic target for anticancer strategies. This article reports a series of PLK1 inhibitors developed using a structure-based strategy, culminating in the discovery of compound B31, a novel isoform-specific PLK1 inhibitor with excellent kinome selectivity. In vitro, this compound exhibited superior anticancer potency across a broad spectrum of cell lines, particularly against K562, achieving a remarkable IC50 value of 0.08 nM. In a mouse model harboring subcutaneous K562 tumors, oral administration of B31 at dosages of 10 or 20 mg/kg twice weekly exhibited remarkable antileukemic activity. B31 had minimal impact on HEK293T cells and very weak inhibitory activity against the hERG channel. Furthermore, in the acute toxicity test, this compound demonstrated an extraordinary safety profile even at a dosage of 500 mg/kg, highlighting its potential as a novel antileukemic agent.

Synergistic two-step inhibition approach using a combination of trametinib and onvansertib in KRAS and TP53-mutated colorectal adenocarcinoma

Colorectal malignancies associated with KRAS and TP53 mutations led us to investigate the effects of combination therapy targeting KRAS, MEK1, or PLK1 in colorectal cancer. MEK1 is downstream of RAS in the MAPK pathway, whereas PLK1 is a mitotic kinase of the cell cycle activated by MAPK and regulated by p53. Bioinformatics analysis revealed that patients with colorectal cancer had a high expression of MAP2K1 and PLK1. Furthermore, PLK1 and MEK1 activity in human colorectal adenocarcinoma (COAD) tissues was found to be highly upregulated compared to healthy tissues. To determine the sensitivity of KRAS or/and TP53-mutated cancer to KRAS, MEK1, or PLK1-targeted therapy, the inhibitors salirasib, trametinib, volasertib, and onvansertib were used in COAD cells with different KRAS and TP53 status. The results showed that combinations with trametinib and PLK1 inhibitors were more potent than combinations with salirasib. A combination of MEK1 and PLK1 inhibitors exhibited significant therapeutic effects on KRAS or/and TP53-mutated COAD cells. Notably, the combination of trametinib and onvansertib effectively suppressed tumor growth in a xenograft mouse model of KRAS and TP53-mutated COAD. This treatment induced G1 and G2/M arrest, respectively, and showed the strongest synergistic effect in KRAS and TP53-mutated SW48 cells expressing mutant KRASG13D and transduced with TP53 shRNA, ultimately leading to apoptotic cell death. These effects are attributed to two-step inhibition mechanism that blocks the MAPK signaling pathway and disrupts mitosis in KRAS and TP53-mutated COAD cells.

Progress with polo-like kinase (PLK) inhibitors: a patent review (2018-present)

INTRODUCTION

Polo-like kinases (PLKs) have five isoforms, all of which play crucial roles in cell cycle and cell proliferation, offering opportunities for drug design and treatment of cancers and other related diseases. Notably, PLK1 and PLK4 have been extensively investigated as cancer drug targets. One distinctive feature of PLKs is the presence of a unique polo-box domain (PBD), which regulates kinase activity and subcellular localization. This provides possibilities for specifically targeting PLKs.

AREA COVERED

This article provides an overview of the roles of PLKs in various cancers and related diseases, as well as the drug development involving PLKs, with a particular focus on PLK1 and PLK4. It summarizes the PLK1 and PLK4 inhibitors that have been disclosed in patents or literature (from 2018 - present), which were sourced from SciFinder and WIPO database.

EXPERT OPINION

After two decades of drug development on PLKs, several drugs progressed into clinical trials for the treatment of many cancers; however, none of them has been approved yet. Further elucidating the mechanisms of PLKs and identifying and developing highly selective ATP-competitive inhibitors, highly potent drug-like PBD inhibitors, degraders, etc. may provide new opportunities for cancer therapy and the treatment for several nononcologic diseases. PLKs inhibition-based combination therapies can be another helpful strategy.

Affinity enhancement of polo-like kinase 1 polo box domain-binding ligands by a bivalent approach using a covalent kinase-binding component

The polo-like kinase 1 (Plk1) is an important cell cycle regulator that is recognized as a target molecule for development of anti-cancer agents. Plk1 consists of a catalytic kinase domain (KD) and a polo-box domain (PBD), which engages in protein-protein interactions (PPIs) essential to proper Plk1 function. Recently, we developed extremely high-affinity PBD-binding inhibitors based on a bivalent approach using the Plk1 KD-binding inhibitor, BI2536, and a PBD-binding peptide. Certain of the resulting bivalent constructs exhibited more than 100-fold Plk1 affinity enhancement relative to the best monovalent PBD-binding ligands. Herein, we report an extensive investigation of bivalent ligands that utilize the non-selective kinase inhibitor Wortmannin as a Plk1 KD-binding component. We found that bivalent ligands incorporating Wortmannin demonstrated affinity enhancements that could be similar to what we had obtained with BI2536 and that they could tightly bind to the protein. This suggests that these tight binding ligands might be useful for structural analysis of full-length Plk1.

Pan-cancer analysis of polo-like kinase family genes reveals polo-like kinase 1 as a novel oncogene in kidney renal papillary cell carcinoma

BACKGROUND

Polo-like kinases (PLKs) are a kinase class of serine/threonine with five members that play crucial roles in cell cycle regulation. However, their biological functions, regulation, and expression remain unclear. This study revealed the molecular properties, oncogenic role, and clinical significance of PLK genes in pan-cancers, particularly in kidney renal papillary cell carcinoma (KIRP).

METHODS

We evaluated the mutation landscape, expression level, and prognostic values of PLK genes using bioinformatics analyses and explored the association between the expression level of PLK genes and tumor microenvironment (TME), immune subtype, cancer immunotherapy, tumor stemness, and drug sensitivity. Finally, we verified the prognostic value in patients with KIRP through univariate and multivariate analyses and nomogram construction.

RESULTS

PLK genes are extensively altered in pan-cancer, which may contribute to tumorigenesis. These genes are aberrantly expressed in some types of cancer, with PLK1 being overexpressed in 31 cancers. PLK expression is closely associated with the prognosis of various cancers. The expression level of PLK genes is related with sensitivity to diverse drugs and cancer immunity as well as cancer immunotherapy. Importantly, we verified that PLK1 was overexpressed in KIRP tissues and could be an unfavorable prognostic biomarker in patients with KIRP. Hence, PLK1 may serve as an oncogenic gene in KIRP and should be explored in future studies.

CONCLUSIONS

Our study comprehensively reports the molecular characteristics and biological functions of PLK family gens across human cancers and recommends further investigation of these genes as potential biomarkers and therapeutic targets, especially in KIRP.

Degradation of Polo-like Kinase 1 by the Novel Poly-Arginine N-Degron Pathway PROTAC Regulates Tumor Growth in Nonsmall Cell Lung Cancer

Polo-like kinase 1 (PLK1), which is crucial in cell cycle regulation, is considered a promising anticancer drug target. Herein, we present the N-degron pathway-based proteolysis targeting chimera (PROTAC) for PLK1 degradation, targeting the Polo-box domain (PBD). We identified DD-2 as the most potent PROTAC that selectively induces PLK1 degradation in cancer cells, including HeLa and nonsmall cell lung cancer (NSCLC), through the N-degron pathway. DD-2 exhibited significant in vitro anticancer effects, inducing G2/M arrest and apoptosis in HeLa and NSCLC cell lines. DD-2 showed significant tumor growth inhibition in a xenograft mouse model using HeLa and NSCLC cell lines, highlighting its potential in cancer treatment. Furthermore, the combination of DD-2 with tyrosine kinase inhibitor (TKI), osimertinib, effectively suppressed tumor growth in double-mutated H1975 cell lines, emphasizing DD-2's potential in combination cancer therapies. Collectively, this study demonstrates the potential of the N-degron pathway, especially using DD-2, for targeted cancer therapies.

A review of nuclear Dbf2-related kinase 1 (NDR1) protein interaction as promising new target for cancer therapy

Nuclear Dbf2-related kinase 1 (NDR1) is a nuclear Dbf2-related (NDR) protein kinase family member, which regulates cell functions and participates in cell proliferation and differentiation through kinase activity. NDR1 regulates physiological functions by interacting with different proteins. Protein-protein interactions (PPIs) are crucial for regulating biological processes and controlling cell fate, and as a result, it is beneficial to study the actions of PPIs to elucidate the pathological mechanism of diseases. The previous studies also show that the expression of NDR1 is deregulated in numerous human cancer samples and it needs the context-specific targeting strategies for NDR1. Thus, a comprehensive understanding of the direct interaction between NDR1 and varieties of proteins may provide new insights into cancer therapies. In this review, we summarize recent studies of NDR1 in solid tumors, such as prostate cancer and breast cancer, and explore the mechanism of action of PPIs of NDR1 in tumors.

Application of a Fluorescence Recovery-Based Polo-Like Kinase 1 Binding Assay to Polo-Like Kinase 2 and Polo-Like Kinase 3

Assay systems for evaluating compound protein-binding affinities are essential for developing agonists and/or antagonists. Targeting individual members of a protein family can be extremely important and for this reason it is critical to have methods for evaluating selectivity. We have previously reported a fluorescence recovery assay that employs a fluorescein-labelled probe to determine IC50 values of ATP-competitive type 1 inhibitors of polo-like kinase 1 (Plk1). This probe is based on the potent Plk1 inhibitor BI2536 [fluorescein isothiocyanate (FITC)-polyethylene glycol (PEG)-lysine (Lys) (BI2536) 1]. Herein, we extend this approach to the highly homologous Plk2 and Plk3 members of this kinase family. Our results suggest that this assay system is suitable for evaluating binding affinities against Plk2 and Plk3 as well as Plk1. The new methodology represents the first example of evaluating N-terminal catalytic kinase domain (KD) affinities of Plk2 and Plk3. It represents a simple and cost-effective alternative to traditional kinase assays to explore the KD-binding compounds against Plk2 and Plk3 as well as Plk1.

HILPS, a long noncoding RNA essential for global oxygen sensing in humans

Adaptation to low levels of oxygen (hypoxia) is a universal biological feature across metazoans. However, the unique mechanisms how different species sense oxygen deprivation remain unresolved. Here, we functionally characterize a novel long noncoding RNA (lncRNA), LOC105369301, which we termed hypoxia-induced lncRNA for polo-like kinase 1 (PLK1) stabilization (HILPS). HILPS exhibits appreciable basal expression exclusively in a wide variety of human normal and cancer cells and is robustly induced by hypoxia-inducible factor 1α (HIF1α). HILPS binds to PLK1 and sequesters it from proteasomal degradation. Stabilized PLK1 directly phosphorylates HIF1α and enhances its stability, constituting a positive feed-forward circuit that reinforces oxygen sensing by HIF1α. HILPS depletion triggers catastrophic adaptation defect during hypoxia in both normal and cancer cells. These findings introduce a mechanism that underlies the HIF1α identity deeply interconnected with PLK1 integrity and identify the HILPS-PLK1-HIF1α pathway as a unique oxygen-sensing axis in the regulation of human physiological and pathogenic processes.

Combination of QSAR Modeling and Hybrid-Based Consensus Scoring to Identify Dual-Targeting Inhibitors of PLK1 and p38γ

Polo-like kinase 1 (PLK1) and p38γ mitogen-activated protein kinase (p38γ) play important roles in cancer pathogenesis by controlling cell cycle progression and are therefore attractive cancer targets. The design of multitarget inhibitors may offer synergistic inhibition of distinct targets and reduce the risk of drug-drug interactions to improve the balance between therapeutic efficacy and safety. We combined deep-learning-based quantitative structure-activity relationship (QSAR) modeling and hybrid-based consensus scoring to screen for inhibitors with potential activity against the targeted proteins. Using this combination strategy, we identified a potent PLK1 inhibitor (compound 4) that inhibited PLK1 activity and liver cancer cell growth in the nanomolar range. Next, we deployed both our QSAR models for PLK1 and p38γ on the Enamine compound library to identify dual-targeting inhibitors against PLK1 and p38γ. Likewise, the identified hits were subsequently subjected to hybrid-based consensus scoring. Using this method, we identified a promising compound (compound 14) that could inhibit both PLK1 and p38γ activities. At nanomolar concentrations, compound 14 inhibited the growth of human hepatocellular carcinoma and hepatoblastoma cells in vitro. This study demonstrates the combined screening strategy to identify novel potential inhibitors for existing targets.

Leveraging the Fragment Molecular Orbital Method to Explore the PLK1 Kinase Binding Site and Polo-Box Domain for Potent Small-Molecule Drug Design

Polo-like kinase 1 (PLK1) plays a pivotal role in cell division regulation and emerges as a promising therapeutic target for cancer treatment. Consequently, the development of small-molecule inhibitors targeting PLK1 has become a focal point in contemporary research. The adenosine triphosphate (ATP)-binding site and the polo-box domain in PLK1 present crucial interaction sites for these inhibitors, aiming to disrupt the protein's function. However, designing potent and selective small-molecule inhibitors can be challenging, requiring a deep understanding of protein-ligand interaction mechanisms at these binding sites. In this context, our study leverages the fragment molecular orbital (FMO) method to explore these site-specific interactions in depth. Using the FMO approach, we used the FMO method to elucidate the molecular mechanisms of small-molecule drugs binding to these sites to design PLK1 inhibitors that are both potent and selective. Our investigation further entailed a comparative analysis of various PLK1 inhibitors, each characterized by distinct structural attributes, helping us gain a better understanding of the relationship between molecular structure and biological activity. The FMO method was particularly effective in identifying key binding features and predicting binding modes for small-molecule ligands. Our research also highlighted specific "hot spot" residues that played a critical role in the selective and robust binding of PLK1. These findings provide valuable insights that can be used to design new and effective PLK1 inhibitors, which can have significant implications for developing anticancer therapeutics.

The effect of PLK1 inhibitor in osimertinib resistant non-small cell lung carcinoma cells

OBJECTIVES

To investigate the effects of PLK1 inhibitors on osimertinib-resistant non-small cell lung carcinoma (NSCLC) cells and the anti-tumor effect combined with osimertinib.

METHODS

An osimertinib resistant NCI-H1975 cell line was induced by exposure to gradually increasing drug concentrations. Osimertinib-resistant cells were co-treated with compounds from classical tumor pathway inhibitor library and osimertinib to screen for compounds with synergistic effects with osimertinib. The Gene Set Enrichment Analysis (GSEA) was used to investigate the activated signaling pathways in osimertinib-resistant cells; sulforhodamine B (SRB) staining was used to investigate the effect of PLK1 inhibitors on osimertinib-resistant cells and the synergistic effect of PLK1 inhibitors combined with osimertinib.

RESULTS

Osimertinib-resistance in NCI-H1975 cell (resistance index=43.45) was successfully established. The PLK1 inhibitors GSK 461364 and BI 2536 had synergistic effect with osimertinib. Compared with osimertinib-sensitive cells, PLK1 regulatory pathway and cell cycle pathway were significantly activated in osimertinib-resistant cells. In NSCLC patients with epidermal growth factor receptor mutations treated with osimertinib, PLK1 mRNA levels were negatively correlated with progression free survival of patients (R=－0.62, P<0.05), indicating that excessive activation of PLK1 in NSCLC cells may cause cell resistant to osimertinib. Further in vitro experiments showed that IC50 of PLK1 inhibitors BI 6727 and GSK 461364 in osimertinib-resistant cells were lower than those in sensitive ones. Compared with the mono treatment of osimertinib, PLK1 inhibitors combined with osimertinib behaved significantly stronger effect on the proliferation of osimertinib-resistant cells.

CONCLUSIONS

PLK1 inhibitors have a synergistic effect with osimertinib on osimertinib-resistant NSCLC cells which indicates that they may have potential clinical value in the treatment of NSCLC patients with osimertinib resistance.

Single-cell analysis characterizes PLK1 as a catalyst of an immunosuppressive tumor microenvironment in LUAD

PLK1 (Polo-like kinase 1) plays a critical role in the progression of lung adenocarcinoma (LUAD). Recent studies have unveiled that targeting PLK1 improves the efficacy of immunotherapy, highlighting its important role in the regulation of tumor immunity. Nevertheless, our understanding of the intricate interplay between PLK1 and the tumor microenvironment (TME) remains incomplete. Here, using genetically engineered mouse model and single-cell RNA-seq analysis, we report that PLK1 promotes an immunosuppressive TME in LUAD, characterized with enhanced M2 polarization of tumor associated macrophages (TAM) and dampened antigen presentation process. Mechanistically, elevated PLK1 coincides with increased secretion of CXCL2 cytokine, which promotes M2 polarization of TAM and diminishes expression of class II major histocompatibility complex (MHC-II) in professional antigen-presenting cells. Furthermore, PLK1 negatively regulates MHC-II expression in cancer cells, which has been shown to be associated with compromised tumor immunity and unfavorable patient outcomes. Taken together, our results reveal PLK1 as a novel modulator of TME in LUAD and provide possible therapeutic interventions.

Specific inhibition of an anticancer target, polo-like kinase 1, by allosterically dismantling its mechanism of substrate recognition

Polo-like kinase 1 (Plk1) is considered an attractive target for anticancer therapy. Over the years, studies on the noncatalytic polo-box domain (PBD) of Plk1 have raised the expectation of generating highly specific protein-protein interaction inhibitors. However, the molecular nature of the canonical PBD-dependent interaction, which requires extensive water network-mediated interactions with its phospholigands, has hampered efforts to identify small molecules suitable for Plk1 PBD drug discovery. Here, we report the identification of the first allosteric inhibitor of Plk1 PBD, called Allopole, a prodrug that can disrupt intracellular interactions between PBD and its cognate phospholigands, delocalize Plk1 from centrosomes and kinetochores, and induce mitotic block and cancer cell killing. At the structural level, its unmasked active form, Allopole-A, bound to a deep Trp-Phe-lined pocket occluded by a latch-like loop, whose adjoining region was required for securely retaining a ligand anchored to the phospho-binding cleft. Allopole-A binding completely dislodged the L2 loop, an event that appeared sufficient to trigger the dissociation of a phospholigand and inhibit PBD-dependent Plk1 function during mitosis. Given Allopole's high specificity and antiproliferative potency, this study is expected to open an unexplored avenue for developing Plk1 PBD-specific anticancer therapeutic agents.

Inhibition of Polo-like kinase 1 (PLK1) triggers cell apoptosis via ROS-caused mitochondrial dysfunction in colorectal carcinoma

BACKGROUND

Colorectal cancer (CRC) is one of the most frequently diagnosed cancers. Polo-like kinase 1 (PLK1), a member of the serine/threonine kinase PLK family, is the most investigated and essential in the regulation of cell cycle progression, including chromosome segregation, centrosome maturation and cytokinesis. However, the nonmitotic role of PLK1 in CRC is poorly understood. In this study, we explored the tumorigenic effects of PLK1 and its potential as a therapeutic target in CRC.

METHODS

GEPIA database and immunohistochemistry analysis were performed to evaluate the abnormal expression of PLK1 in CRC patients. MTT assay, colony formation and transwell assay were performed to assess cell viability, colony formation ability and migration ability after inhibiting PLK1 by RNAi or the small molecule inhibitor BI6727. Cell apoptosis, mitochondrial membrane potential (MMP) and ROS levels were evaluated by flow cytometry. Bioluminescence imaging was performed to evaluate the impact of PLK1 on CRC cell survival in a preclinical model. Finally, xenograft tumor model was established to study the effect of PLK1 inhibition on tumor growth.

RESULTS

First, immunohistochemistry analysis revealed the significant accumulation of PLK1 in patient-derived CRC tissues compared with adjacent healthy tissues. Furthermore, PLK1 inhibition genetically or pharmacologically significantly reduced cell viability, migration and colony formation, and triggered apoptosis of CRC cells. Additionally, we found that PLK1 inhibition elevated cellular reactive oxygen species (ROS) accumulation and decreased the Bcl2/Bax ratio, which led to mitochondrial dysfunction and the release of Cytochrome c, a key process in initiating cell apoptosis.

CONCLUSION

These data provide new insights into the pathogenesis of CRC and support the potential value of PLK1 as an appealing target for CRC treatment. Overall, the underlying mechanism of inhibiting PLK1-induced apoptosis indicates that the PLK1 inhibitor BI6727 may be a novel potential therapeutic strategy in the treatment of CRC.

The emerging roles and mechanism of N6-methyladenosine (m6A) modifications in urologic tumours progression

Prostate cancer (PCa), bladder cancer (BC), and renal cell cancer (RCC) are the most common urologic tumours in males. N6-methyladenosine (m⁶A), adenosine N6 methylation, is the most prevalent RNA modification in mammals. Increasing evidence suggests that m⁶A plays a crucial role in cancer development. In this review, we comprehensively analyzed the influence of m⁶A methylation on Prostate cancer, bladder cancer, and renal cell cancer and the relationship between the expression of relevant regulatory factors and their development and occurrence, which provides new insights and approaches for the early clinical diagnosis and targeted therapy of urologic malignancies.

Structural Optimization and Anticancer Activity of Polo-like Kinase 1 (Plk1) Polo-Box Domain (PBD) Inhibitors and Their Prodrugs

Polo-like kinase 1 (Plk1), a mitotic kinase whose activity is widely upregulated in various human cancers, is considered an attractive target for anticancer drug discovery. Aside from the kinase domain, the C-terminal noncatalytic polo-box domain (PBD), which mediates the interaction with the enzyme's binding targets or substrates, has emerged as an alternative target for developing a new class of inhibitors. Various reported small molecule PBD inhibitors exhibit poor cellular efficacy and/or selectivity. Here, we report structure-activity relationship (SAR) studies on triazoloquinazolinone-derived inhibitors, such as 43 (a 1-thioxo-2,4-dihydrothieno[2,3-e][1,2,4]triazolo[4,3-a]pyrimidin-5(1H)-one) that effectively block Plk1, but not Plk2 and Plk3 PBDs, with improved affinity and drug-like properties. The range of prodrug moieties needed for thiol group masking of the active drugs has been expanded to increase cell permeability and mechanism-based cancer cell (L363 and HeLa) death. For example, a 5-thio-1-methyl-4-nitroimidazolyl prodrug 80, derived from 43, showed an improved cellular potency (GI50 4.1 μM). As expected, 80 effectively blocked Plk1 from localizing to centrosomes and kinetochores and consequently induced potent mitotic block and apoptotic cell death. Another prodrug 78 containing 9-fluorophenyl in place of the thiophene-containing heterocycle in 80 also induced a comparable degree of anti-Plk1 PBD effect. However, orally administered 78 was rapidly converted in the bloodstream to parent drug 15, which was shown be relatively stable toward in vivo oxidation due to its 9-fluorophenyl group in comparison to unsubstituted phenyl. Further derivatization of these inhibitors, particularly to improve the systemic prodrug stability, could lead to a new class of therapeutics against Plk1-addicted cancers.

Unusually High Affinity of the PLK Inhibitors RO3280 and GSK461364 to HSA and Its Possible Pharmacokinetic Implications

The binding processes of two Polo-like kinase inhibitors, RO3280 and GSK461364, to the human serum albumin (HSA) protein as well as the protonation equilibria of both compounds have been studied combining absorbance and fluorescence spectroscopy experiments together with density functional theory calculations. We found that the charge states of RO3280 and GSK461364 are +2 and +1, respectively, at the physiological pH. Nevertheless, RO3280 binds to HSA in the charge state +1 prior to a deprotonation pre-equilibrium. Binding constants to site I of HSA of 2.23 × 10⁶ and 8.80 × 10⁴ M^-1 were determined for RO3280 and GSK461364, respectively, at 310 K. The binding processes of RO3280 and GSK461364 to HSA are entropy- and enthalpy-driven, respectively. The positive enthalpy found for the RO3280-HSA complex formation could be related to a proton pre-equilibrium of RO3280.

Polo-like kinase 1 (PLK1) O-GlcNAcylation is essential for dividing mammalian cells and inhibits uterine carcinoma

The O-linked β-N-acetylglucosamine (O-GlcNAc) transferase (OGT) mediates intracellular O-GlcNAcylation modification. O-GlcNAcylation occurs on Ser/Thr residues and is important for numerous physiological processes. OGT is essential for dividing mammalian cells and is involved in many human diseases; however, many of its fundamental substrates during cell division remain unknown. Here, we focus on the effect of OGT on polo-like kinase 1 (PLK1), a mitotic master kinase that governs DNA replication, mitotic entry, chromosome segregation, and mitotic exit. We show that PLK1 interacts with OGT and is O-GlcNAcylated. By utilizing stepped collisional energy/higher-energy collisional dissociation mass spectrometry, we found a peptide fragment of PLK1 that is modified by O-GlcNAc. Further mutation analysis of PLK1 shows that the T291A mutant decreases O-GlcNAcylation. Interestingly, T291N is a uterine carcinoma mutant in The Cancer Genome Atlas. Our biochemical assays demonstrate that T291A and T291N both increase PLK1 stability. Using stable H2B-GFP cells, we found that PLK1-T291A and PLK1-T291N mutants display chromosome segregation defects and result in misaligned and lagging chromosomes. In mouse xenograft models, we demonstrate that the O-GlcNAc-deficient PLK1-T291A and PLK1-T291N mutants enhance uterine carcinoma in animals. Hence, we propose that OGT partially exerts its mitotic function through O-GlcNAcylation of PLK1, which might be one mechanism by which elevated levels of O-GlcNAc promote tumorigenesis.

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.

A novel thinking: DDR axis refines the classification of ccRCC with distinctive prognosis, multi omics landscape and management strategy

Background

DNA damage response and repair (DDR) related signatures play an important role in maintaining genome stability and other biological processes. It also affects the occurrence, development, and treatment of cancer. However, in renal cell carcinoma (RCC), especially clear cell renal carcinoma (ccRCC), the potential association between DDR-related signatures and tumor heterogeneity and tumor microenvironment (TME) remains unclear.

Methods

Utilizing unsupervised clustering algorithm, we divided RCC into two subgroups, DCS1 and DCS2, according to the differences in DDR gene expression, and compared the characteristics of the two subgroups through multiple dimensions.

Results

Compared with DCS1, DCS2 patients have higher clinical stage/grade and worse prognosis, which may be related to active metabolic status and immunosuppression status. At the same time, the high mutation rate in DCS2 may also be an important reason for the prognosis. We also analyzed the sensitivity of the two subgroups to different therapeutic agents and established a subtypes' biomarkers-based prognostic system with good validation results to provide ideas for clinical diagnosis and treatment. Finally, we identified a pivotal role for DDX1 in the DDR gene set, which may serve as a future therapeutic target.

Conclusion

This study showed that DDR has an important impact on the development and treatment of RCC. DCS2 subtypes have a poor prognosis, and more personalized treatment and follow-up programs may be needed. The assessment of DDR gene mutations in patients may be helpful for clinical decision-making. DDX1 may be one of the effective targets for RCC treatment in the future.

A Novel Risk Score Model of Lactate Metabolism for Predicting over Survival and Immune Signature in Lung Adenocarcinoma

Simple Summary

Since the discovery of the WarBurg effect, the veil of the tumorigenic role of lactic acid has been gradually revealed. Recently, it was proposed that lactic acid that is produced by tumor cells was secreted into the extracellular space to create immunosuppressive tumor microenvironment (TME) in a variety of ways. However, the intersection genes and the association with immunotherapy are unclear. At present, we identified six lactate-metabolism-associated genes, which were thought to enable tumor progression, that were related to LUAD immunotherapy and we constructed an LAR-score risk model.

Abstract

Background: The role of lactate acid in tumor progression was well proved. Recently, it was found that lactate acid accumulation induced an immunosuppressive microenvironment. However, these results were based on a single gene and it was unclear that lactate acid genes were associated with immunotherapy and able to predict overall survival. Methods: Genes and survival data were acquired from TCGA, GEO and GENECARDS. PCA and TSNE were used to distinguish sample types according to lactate metabolism-associated gene expression. A Wilcox-test examined the expression differences between normal and tumor samples. The distribution in chromatin and mutant levels were displayed by Circo and MAfTools. The lactate metabolism-associated gene were divided into categories by consistent clustering and visualized by Cytoscape. Immune cell infiltration was evaluated by CIBERSORT and LM22 matrix. Enrichment analysis was performed by GSVA. We used the ConsensusClusterPlus package for consistent cluster analysis. A prognostic model was constructed by Univariate Cox regression and Lasso regression analysis. Clinical specimens were detected their expression of genes in model by IHC. Results: Most lactate metabolism-associated gene were significantly differently expressed between normal and tumor samples. There was a strong correlation between the expression of lactate metabolism-associated gene and the abundance of immune cells. We divided them into two clusters (lactate.cluster A,B) with significantly different survival. The two clusters showed a difference in signal, immune cells, immune signatures, chemokines, and clinical features. We identified 162 differential genes from the two clusters, by which the samples were divided into three categories (gene.cluster A,B,C). They also showed a difference in OS and immune infiltration. Finally, a risk score model that was composed of six genes was constructed. There was significant difference in the survival between the high and low risk groups. ROC curves of 1, 3, 5, and 10 years verified the model had good predictive efficiency. Gene expression were correlated with ORR and PFS in patients who received anti-PD-1/L1. Conclusion: The lactate metabolism-associated genes in LUAD were significantly associated with OS and immune signatures. The risk scoring model that was constructed by us was able to well identify and predict OS and were related with anti-PD-1/L1 therapy outcome.

Polo-like Kinase 1 Inhibitors in Human Cancer Therapy: Development and Therapeutic Potential

Polo-like kinase 1 (PLK1) plays an important role in a variety of cellular functions, including the regulation of mitosis, DNA replication, autophagy, and the epithelial-mesenchymal transition (EMT). PLK1 overexpression is often associated with cell proliferation and poor prognosis in cancer patients, making it a promising antitumor target. To date, at least 10 PLK1 inhibitors (PLK1i) have been entered into clinical trials, among which the typical kinase domain (KD) inhibitor BI 6727 (volasertib) was granted "breakthrough therapy designation" by the FDA in 2013. Unfortunately, many other KD inhibitors showed poor specificity, resulting in dose-limiting toxicity, which has greatly impeded their development. Researchers recently discovered many PLK1i with higher selectivity, stronger potency, and better absorption, distribution, metabolism, and elimination (ADME) characteristics. In this review, we emphasize the structure-activity relationships (SARs) of PLK1i, providing insights into new drugs targeting PLK1 for antitumor clinical practice.

Shedding light on the binding mechanism of kinase inhibitors BI-2536, Volasetib and Ro-3280 with their pharmacological target PLK1

In the present work, the interactions of the novel kinase inhibitors BI-2536, Volasetib (BI-6727) and Ro-3280 with the pharmacological target PLK1 have been studied by fluorescence spectroscopy and molecular dynamics calculations. High Stern-Volmer constants were found in fluorescence experiments suggesting the formation of stable protein-ligand complexes. In addition, it was observed that the binding constant between BI-2536 and PLK1 increases about 100-fold in presence of the phosphopeptide Cdc25C-p that docks to the polo box domain of the protein and releases the kinase domain. All the determined binding constants are higher for the kinase inhibitors than for their competitor for the active center (ATP) being BI-2536 and Volasertib the inhibitors that showed more affinity for PLK1. Calculated binding free energies confirmed the higher affinity of PLK1 for BI-2536 and Volasertib than for ATP. The higher affinity of the inhibitors to PLK1 compared to ATP was mainly attributed to stronger van der Waals interactions. Results may help with the challenge of designing and developing new kinase inhibitors more effective in clinical cancer therapy.

Present and Future Perspective on PLK1 Inhibition in Cancer Treatment

Polo-like kinase 1 (PLK1) is the principle member of the well conserved serine/threonine kinase family. PLK1 has a key role in the progression of mitosis and recent evidence suggest its important involvement in regulating the G2/M checkpoint, in DNA damage and replication stress response, and in cell death pathways. PLK1 expression is tightly spatially and temporally regulated to ensure its nuclear activation at the late S-phase, until the peak of expression at the G2/M-phase. Recently, new roles of PLK1 have been reported in literature on its implication in the regulation of inflammation and immunological responses. All these biological processes are altered in tumors and, considering that PLK1 is often found overexpressed in several tumor types, its targeting has emerged as a promising anti-cancer therapeutic strategy. In this review, we will summarize the evidence suggesting the role of PLK1 in response to DNA damage, including DNA repair, cell cycle progression, epithelial to mesenchymal transition, cell death pathways and cancer-related immunity. An update of PLK1 inhibitors currently investigated in preclinical and clinical studies, in monotherapy and in combination with existing chemotherapeutic drugs and targeted therapies will be discussed.

Nano-hydroxyapatite-evoked immune response synchronized with controllable immune adjuvant release for strengthening melanoma-specific growth inhibition

Concerns about the potential systematic toxicity limit the extensive application of traditional therapeutic drugs for melanoma therapy, nano-hydroxyapatite (nHA) with good biocompatibility and anti-tumor ability could be an alternative choice. In this study, nHA was employed as an anti-tumor biomaterial due to its tumor-specific toxicity. Meanwhile, granulocyte-macrophage colony-stimulating factor (GM-CSF) served as the immune adjuvant to activate the immune response. The delivery platform was fabricated by co-encapsulation of both nHA and GM-CSF into a biocompatible thermosensitive PLGA-PEG-PLGA hydrogel. The results showed that the bio-activities of nHA and GM-CSF could be well-maintained within the hydrogel. Interestingly, the addition of nHA could attenuate the burst release of GM-CSF due to possible protein absorption capacity of nHA, which is beneficial for GM-CSF sustainable release at the tumor site, achieving boosted and prolonged anti-tumor immunity. The in vitro and in vivo data demonstrated that nHA/GM-CSF hydrogel exhibited greater potency to inhibit tumor growth via enhanced CD8⁺ T-cell response compared with hydrogel and nHA hydrogel groups, contributed by the synergistic effects of nHA and GM-CSF. Overall, the strategy combining nHA and immune adjuvant shows great promise, which largely broadens the choice of combinational therapies for melanoma. STATEMENT OF SIGNIFICANCE: Nano-hydroxyapatite (nHA) has been confirmed to specifically inhibit melanoma tumor growth and induce immune response. However, its antitumor efficiency and immunity-evoking capacity are limited. In this study, granulocyte-macrophage colony-stimulating factor (GM-CSF) was introduced to serve as the immune adjuvant. Both of them were encapsulated into a biocompatible thermosensitive PLGA-PEG-PLGA hydrogel. The addition of nHA could attenuate the burst release of GM-CSF due to the interaction with nHA, which is beneficial for GM-CSF sustainable release at tumor site, achieving boosted and prolonged anti-tumor immunity. Anti-tumor immune response could be activated due to the release of tumor-associated antigen and tumor debris induced by the specifically tumor inhibition effect of nHA and GM-CSF. The combination of nHA and GM-CSF could play synergistic inhibiting effect on tumor growth via boosting and prolonging anti-tumor immunity.

Inhibition of Polo-like Kinase 1 by HMN-214 Blocks Cell Cycle Progression and Inhibits Neuroblastoma Growth

Polo-like kinase 1 (PLK1) is an essential cell cycle mitotic kinase component that plays an important role in cell cycle progression and has been reported to be involved in various cancers, including neuroblastoma (NB). PLK1 also regulates G2/M transition, chromosomal segregation, spindle assembly maturation, and mitotic exit. NB is an early embryonic-stage heterogeneous solid tumor and accounts for 15% of all pediatric cancer-related deaths. Therefore, we aimed to develop a targeting strategy for PLK1 by repurposing HMN-214 in NB. HMN-214 is a prodrug of HMN-176 and is known to selectively interfere with PLK1 function. In the present study, we performed the transcriptomic analysis of a large cohort of primary NB patient samples and revealed that PLK1 expression is inversely correlated with the overall survival of NB patients. Additionally, we found that PLK1 strongly correlates with NB disease and stage progression. HMN-214 significantly inhibited NB proliferation and colony formation in both MYCN-amplified and -nonamplified cell lines in a dose-dependent manner. Furthermore, HMN-214 induces apoptosis and significantly obstructs the cell cycle at the G2/M phase in NB cells by inhibiting multiple cell-cycle-related genes, such as PLK1, WEE1, CDK1, CDK2, Cyclin B1, CHK1, and CHK2. HMN-214 significantly inhibits cell cycle regulator CDK1 and the phosphorylation and activation of PLK1 in NB. In the NB 3D spheroid tumor model, HMN-214 significantly and in a dose-dependent manner inhibits spheroid tumor mass and growth. Overall, our study highlights that targeting PLK1 using HMN-214 is a novel therapeutic approach for NB.

PLK1 Mitigates Intervertebral Disc Degeneration by Delaying Senescence of Nucleus Pulposus Cells

Intervertebral disc degeneration (IVDD) is the primary cause of low back pain; however, the molecular mechanisms involved in the pathogenesis of IVDD are not fully understood. Polo-like kinase 1 (PLK1) plays numerous roles in the cell cycle, including in cell proliferation and senescence. To investigate the involvement of PLK1 in IVDD, we used patient tissues and an animal model of IVDD. Samples were analyzed via immunoblotting, quantitative real-time polymerase chain reaction (qPCR), immunofluorescence, and immunohistochemistry. Our results demonstrated that PLK1 expression was decreased in nucleus pulposus cells (NPCs) of degenerative IVDs. The inhibition of PLK1 kinase activity in normal NPCs increased the expression of p53 protein, inhibited cell proliferation, and induced senescence. Our results suggest that PLK1 regulates the degeneration of the IVD through p53, revealing the function and mechanism of PLK1 in IVDD and providing a theoretical basis and experimental evidence for the potential treatment of low back pain.

Research Progress on Gene Editing Based on Nano-Drug Delivery Vectors for Tumor Therapy

Malignant tumors pose a serious threat to human health and have high fatality rates. Conventional clinical anti-tumor treatment is mainly based on traditional surgery, chemotherapy, radiotherapy, and interventional therapy, and even though these treatment methods are constantly updated, a satisfactory efficacy is yet to be obtained. Therefore, research on novel cancer treatments is being actively pursued. We review the classification of gene therapies of malignant tumors and their advantages, as well as the development of gene editing techniques. We further reveal the nano-drug delivery carrier effect in improving the efficiency of gene editing. Finally, we summarize the progress in recent years of gene editing techniques based on nano-drug delivery carriers in the treatment of various malignant tumors, and analyze the prospects of the technique and its restricting factors.

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.

Development of ultra-high affinity bivalent ligands targeting the polo-like kinase 1

The polo-like kinase 1 (Plk1) is an important mediator of cell cycle regulation and a recognized anti-cancer molecular target. In addition to its catalytic kinase domain (KD), Plk1 contains a polo-box domain (PBD), which engages in protein–protein interactions (PPIs) essential to proper Plk1 function. We have developed a number of extremely high-affinity PBD-binding peptide inhibitors. However, we have reached an apparent limit to increasing the affinities of these monovalent ligands. Accordingly, we undertook an extensive investigation of bivalent ligands, designed to engage both KD and PBD regions of Plk1. This has resulted in bivalent constructs exhibiting more than 100-fold Plk1 affinity enhancement relative to the best monovalent PBD-binding ligands. Startlingly, and in contradiction to widely accepted notions of KD–PBD interactions, we have found that full affinities can be retained even with minimal linkers between KD and PBD-binding components. In addition to significantly advancing the development of PBD-binding ligands, our findings may cause a rethinking of the structure – function of Plk1.

The polo-like kinase 1 (Plk1) is an important mediator of cell cycle regulation and a recognized anti-cancer molecular target.

SiRNA-templated 3D framework nucleic acids for chemotactic recognition, and programmable and visualized precise delivery for synergistic cancer therapy

Developments in framework nucleic acids (FNAs) are limited by complicated synthesis, by-product interference, and low framework utilization. Herein, simple core-shell spherical 3D FNAs (ST-SFNAs) preparation is presented based on siRNA-templated linear polymerization followed by hybridization chain reaction branched polymerization. Without by-products, all components exhibited their special functions to obtain high space utilization of ST-SFNAs. ST-SFNAs were covered by catalase and folic acid-functionalized liposome membranes. The catalase endowed ST-SFNAs with chemotactic activities in the H₂O₂ reaction catalyzed by catalase. Furthermore, combined with functionalized folic acids' targeting folate receptors, the synergistic chemotactic recognition of cancer cells was obtained. This dramatically promoted targeted cellular uptakes compared with traditional active or passive targeting pathways. Subsequently, the cascaded-logical programmable release of drugs was precisely controlled by targeting glutathione and ATP (via S-S bond and ATP aptamer on the inner g-DNA cover). This was visualized by "turn on" fluorescent signals generated by special hybridization of released hairpin DNAs with survivin mRNA biomarkers. Simultaneously, biocompatible synergistic therapy was achieved by simultaneously releasing doxorubicin and siRNA. With its high utilization for synergistic chemotactic recognition, programmable and visualized delivery, as well as synergistic therapy, an efficient platform for maximizing the therapeutic efficacy has been developed. This would initiate further FNA-based material development for a variety of biological applications.

Design and synthesis of a new orthogonally protected glutamic acid analog and its use in the preparation of high affinity polo-like kinase 1 polo-box domain - binding peptide macrocycles

Targeting protein - protein interactions (PPIs) has emerged as an important area of discovery for anticancer therapeutic development. In the case of phospho-dependent PPIs, such as the polo-like kinase 1 (Plk1) polo-box domain (PBD), a phosphorylated protein residue can provide high-affinity recognition and binding to target protein hot spots. Developing antagonists of the Plk1 PBD can be particularly challenging if one relies solely on interactions within and proximal to the phospho-binding pocket. Fortunately, the affinity of phospho-dependent PPI antagonists can be significantly enhanced by taking advantage of interactions in both the phospho-binding site and hidden "cryptic" pockets that may be revealed on ligand binding. In our current paper, we describe the design and synthesis of macrocyclic peptide mimetics directed against the Plk1 PBD, which are characterized by a new glutamic acid analog that simultaneously serves as a ring-closing junction that provides accesses to a cryptic binding pocket, while at the same time achieving proper orientation of a phosphothreonine (pT) residue for optimal interaction in the signature phospho-binding pocket. Macrocycles prepared with this new amino acid analog introduce additional hydrogen-bonding interactions not found in the open-chain linear parent peptide. It is noteworthy that this new glutamic acid-based amino acid analog represents the first example of extremely high affinity ligands where access to the cryptic pocket from the pT-2 position is made possible with a residue that is not based on histidine. The concepts employed in the design and synthesis of these new macrocyclic peptide mimetics should be useful for further studies directed against the Plk1 PBD and potentially for ligands directed against other PPI targets.

Therapeutic Targeting PLK1 by ON-01910.Na Is Effective in Local Treatment of Retinoblastoma

Cell cycle deregulation is involved in the pathogenesis of many cancers and is often associated with protein kinase aberrations, including the polo-like kinase 1 (PLK1). We used retinoblastoma, an intraocular malignancy that lacks targeted therapy, as a disease model and set out to reveal targetability of PLK1 with a small molecular inhibitor ON-01910.Na. First, transcriptomic analysis on patient retinoblastoma tissues suggested that cell cycle progression was deregulated and confirmed that PLK1 pathway was upregulated. Next, antitumor activity of ON-01910.Na was investigated in both cellular and animal levels. Cytotoxicity induced by ON-01910.Na was tumor specific and dose dependent in retinoblastoma cells, while nontumor cells were minimally affected. In three-dimensional culture, ON-01910.Na demonstrated efficient drug penetrability with multilayer cell death. Posttreatment transcriptomic findings revealed that cell cycle arrest and MAPK cascade activation were induced following PLK1 inhibition and eventually resulted in apoptotic cell death. In Balb/c nude mice, a safe threshold of 0.8 nmol intravitreal dosage of ON-01910.Na was established for intraocular safety, which was demonstrated by structural integrity and functional preservation. Furthermore, intraocular and subcutaneous xenograft were significantly reduced with ON-01910.Na treatments. For the first time, we demonstrated targetability of PLK1 in retinoblastoma by efficiently causing cell cycle arrest and apoptosis. Our study is supportive that local treatment of ON-01910.Na may be a novel, effective modality benefiting patients with PLK1-aberrant tumors.

Inhibition of polo-like kinase 1 (PLK1) facilitates reactivation of gamma-herpesviruses and their elimination

Both Kaposi's sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV) establish the persistent, life-long infection primarily at the latent status, and associate with certain types of tumors, such as B cell lymphomas, especially in immuno-compromised individuals including people living with HIV (PLWH). Lytic reactivation of these viruses can be employed to kill tumor cells harboring latently infected viral episomes through the viral cytopathic effects and the subsequent antiviral immune responses. In this study, we identified that polo-like kinase 1 (PLK1) is induced by KSHV de novo infection as well as lytic switch from KSHV latency. We further demonstrated that PLK1 depletion or inhibition facilitates KSHV reactivation and promotes cell death of KSHV-infected lymphoma cells. Mechanistically, PLK1 regulates Myc that is critical to both maintenance of KSHV latency and support of cell survival, and preferentially affects the level of H3K27me3 inactive mark both globally and at certain loci of KSHV viral episomes. Furthremore, we recognized that PLK1 inhibition synergizes with STAT3 inhibition to efficiently induce KSHV reactivation. We also confirmed that PLK1 depletion or inhibition yields the similar effect on EBV lytic reactivation and cell death of EBV-infected lymphoma cells. Lastly, we noticed that PLK1 in B cells is elevated in the context of HIV infection and caused by HIV Nef protein to favor KSHV/EBV latency.

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.

Modelling the Functions of Polo-Like Kinases in Mice and Their Applications as Cancer Targets with a Special Focus on Ovarian Cancer

Polo-like kinases (PLKs) belong to a five-membered family of highly conserved serine/threonine kinases (PLK1-5) that play differentiated and essential roles as key mitotic kinases and cell cycle regulators and with this in proliferation and cellular growth. Besides, evidence is accumulating for complex and vital non-mitotic functions of PLKs. Dysregulation of PLKs is widely associated with tumorigenesis and by this, PLKs have gained increasing significance as attractive targets in cancer with diagnostic, prognostic and therapeutic potential. PLK1 has proved to have strong clinical relevance as it was found to be over-expressed in different cancer types and linked to poor patient prognosis. Targeting the diverse functions of PLKs (tumor suppressor, oncogenic) are currently at the center of numerous investigations in particular with the inhibition of PLK1 and PLK4, respectively in multiple cancer trials. Functions of PLKs and the effects of their inhibition have been extensively studied in cancer cell culture models but information is rare on how these drugs affect benign tissues and organs. As a step further towards clinical application as cancer targets, mouse models therefore play a central role. Modelling PLK function in animal models, e.g., by gene disruption or by treatment with small molecule PLK inhibitors offers promising possibilities to unveil the biological significance of PLKs in cancer maintenance and progression and give important information on PLKs' applicability as cancer targets. In this review we aim at summarizing the approaches of modelling PLK function in mice so far with a special glimpse on the significance of PLKs in ovarian cancer and of orthotopic cancer models used in this fatal malignancy.

RNAi prodrugs decrease elevated mRNA levels of Polo-like kinase 1 in ex vivo cultured primary cells from pediatric acute myeloid leukemia patients

Polo-like kinase 1 (Plk1) is an important regulator of the cell cycle and it is frequently overexpressed in cancer cells. Several small molecule inhibitors have been developed to target Plk1 and some of them have reached clinical trials in adults with acute myeloid leukemia (AML). Pediatric AML patients have a poor prognosis and survivors suffer from long-term side effects. As adult AML cells have an elevated expression of Plk1, AML is a disease candidate for Plk1 inhibition. However, the relative success of clinical trials have been hampered by adverse reactions. Herein, PLK1-targeting RNA interference (RNAi) prodrugs that enter cells without a transfection reagent are used to target PLK1 selectively in primary cells from pediatric AML patients. We show that PLK1 and PLK4 mRNA expression are significantly higher in pediatric AML patients when compared to healthy donors and that PLK1 is downregulated by on average 50% using RNAi prodrugs without a significant effect on other PLK family members. In addition, the RNAi prodrug-induced decrease in PLK1 can be used to potentiate the effect of cytarabine. In summary, PLK1-targeting RNAi prodrugs can decrease the elevated levels of PLK1 in primary cells from pediatric AML patients and sensitize pediatric AML cells to chemotherapeutics.

Polo-like Kinase 1 as an emerging drug target: structure, function and therapeutic implications

Polo-like kinase 1 (PLK1) is a conserved mitotic serine-threonine protein kinase, functions as a regulatory protein, and is involved in the progression of the mitotic cycle. It plays important roles in the regulation of cell division, maintenance of genome stability, in spindle assembly, mitosis, and DNA-damage response. PLK1 is consist of a N-terminal serine-threonine kinase domain, and a C-terminal Polo-box domain (regulatory site). The expression of PLK1 is controlled by transcription repressor in the G1 stage and transcription activators in the G2 stage of the cell cycle. Overexpression of PLK1 results in undermining of checkpoints causes excessive cellular division resulting in abnormal cell growth, leading to the development of cancer. Blocking the expression of PLK1 by an antibody, RNA interference, or kinase inhibitors, causes a subsequent reduction in the proliferation of tumour cells and induction of apoptosis in tumour cells without affecting the healthy cells, suggesting an attractive target for drug development. In this review, we discuss detailed information on expression, gene and protein structures, role in different diseases, and progress in the design and development of PLK1 inhibitors. We have performed an in-depth analysis of the PLK1 inhibitors and their therapeutic implications with special focus to the cancer therapeutics.

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

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

Identification of a New Heterocyclic Scaffold for Inhibitors of the Polo-Box Domain of Polo-like Kinase 1

As a mitotic-specific target widely deregulated in various human cancers, polo-like kinase 1 (Plk1) has been extensively explored for anticancer activity and drug discovery. Although multiple catalytic domain inhibitors were tested in preclinical and clinical studies, their efficacies are limited by dose-limiting cytotoxicity, mainly from off-target cross reactivity. The C-terminal noncatalytic polo-box domain (PBD) of Plk1 has emerged as an attractive target for generating new protein-protein interaction inhibitors. Here, we identified a 1-thioxo-2,4-dihydro-[1,2,4]triazolo[4,3-a]quinazolin-5(1H)-one scaffold that efficiently inhibits Plk1 PBD but not its related Plk2 and Plk3 PBDs. Structure-activity relationship studies led to multiple inhibitors having ≥10-fold higher inhibitory activity than the previously characterized Plk1 PBD-specific phosphopeptide, PLHSpT (Kd ∼ 450 nM). In addition, S-methyl prodrugs effectively inhibited mitotic progression and cell proliferation and their metabolic stability was determined. These data describe a novel class of small-molecule inhibitors that offer a promising avenue for future drug discovery against Plk1-addicted cancers.

Uncoupling the Folding-Function Paradigm of Lytic Peptides to Deliver Impermeable Inhibitors of Intracellular Protein-Protein Interactions

Here, we describe the use of peptide backbone N-methylation as a new strategy to transform membrane-lytic peptides (MLPs) into cytocompatible intracellular delivery vehicles. The ability of lytic peptides to engage with cell membranes has been exploited for drug delivery to carry impermeable cargo into cells, but their inherent toxicity results in narrow therapeutic windows that limit their clinical translation. For most linear MLPs, a prerequisite for membrane activity is their folding at cell surfaces. Modification of their backbone with N-methyl amides inhibits folding, which directly correlates to a reduction in lytic potential but only minimally affects cell entry. We synthesized a library of N-methylated peptides derived from MLPs and conducted structure-activity studies that demonstrated the broad utility of this approach across different secondary structures, including both β-sheet and helix-forming peptides. Our strategy is highlighted by the delivery of a notoriously difficult class of protein-protein interaction inhibitors that displayed on-target activity within cells.

Activation of AMPK suppresses S1P-induced airway smooth muscle cells proliferation and its potential mechanisms

The aims of the present study were to investigate the signaling mechanisms for sphingosine-1-phosphate (S1P)-induced airway smooth muscle cells (ASMCs) proliferation and to explore the effect of activation of adenosine monophosphate-activated protein kinase (AMPK) on S1P-induced ASMCs proliferation and its underlying mechanisms. S1P phosphorylated signal transducer and activator of transcription 3 (STAT3) through binding to S1PR_2/3, and this further sequentially up-regulated polo-like kinase 1 (PLK1) and inhibitor of differentiation 2 (ID2) protein expression. Pretreatment of cells with S1PR₂ antagonist JTE-013, S1PR₃ antagonist CAY-10444, knockdown of STAT3, PLK1 and ID2 attenuated S1P-triggered ASMCs proliferation. In addition, activation of AMPK by metformin inhibited S1P-induced ASMCs proliferation by suppressing STAT3 phosphorylation and therefore suppression of PLK1 and ID2 protein expression. Our study suggests that S1P promotes ASMCs proliferation by stimulating S1PR_2/3/STAT3/PLK1/ID2 axis, and activation of AMPK suppresses ASMCs proliferation by targeting on STAT3 signaling pathway. Activation of AMPK might benefit asthma by inhibiting airway remodeling.

Gravin-associated kinase signaling networks coordinate γ-tubulin organization at mitotic spindle poles

Mitogenic signals that regulate cell division often proceed through multienzyme assemblies within defined intracellular compartments. The anchoring protein Gravin restricts the action of mitotic kinases and cell-cycle effectors to defined mitotic structures. In this report we discover that genetic deletion of Gravin disrupts proper accumulation and asymmetric distribution of γ-tubulin during mitosis. We utilize a new precision pharmacology tool, Local Kinase Inhibition, to inhibit the Gravin binding partner polo-like kinase 1 at spindle poles. Using a combination of gene-editing approaches, quantitative imaging, and biochemical assays, we provide evidence that disruption of local polo-like kinase 1 signaling underlies the γ-tubulin distribution defects observed with Gravin loss. Our study uncovers a new role for Gravin in coordinating γ-tubulin recruitment during mitosis and illuminates the mechanism by which signaling enzymes regulate this process at a distinct subcellular location.

A new genre of fluorescence recovery assay to evaluate polo-like kinase 1 ATP-competitive inhibitors

Using a probe consisting of a fluorescein-labeled variant of the potent polo-like kinase 1 (Plk1) inhibitor BI2536 [FITC-PEG-Lys(BI2536) 4], we were able to determine half maximal inhibitory concentration (IC50) of ATP-competitive Type 1 inhibitors of Plk1 by means of a fluorescence recovery assay. This methodology represents a cost-effective and simple alternative to traditional kinase assays for initial screening of potential Plk1 inhibitors.