Differential regulation of polo-like kinase 1, 2, 3, and 4 gene expression in mammalian cells and tissues

Potential Tumor Suppressor Role of Polo-like Kinase 5 in Cancer

The polo-like kinase (PLK) family of serine/threonine kinases contains five members (PLK1-5). Most PLKs are involved in cell cycle regulation and DNA damage response. However, PLK5 is different as it lacks a functional kinase domain and is not involved in cell cycle control. PLK5 remains the least-studied family member, and its role in oncogenesis remains enigmatic. Here, we identified tissues with high PLK5 expression by leveraging the Protein Atlas and GTEx databases with relevant literature and selected ovarian, lung, testis, endometrium, cervix, and fallopian tube tissues as candidates for further investigation. Subsequently, we performed immunohistochemical staining for PLK5 on multiple tissue microarrays followed by Vectra scanning and quantitative inForm analysis. This revealed consistently downregulated PLK5 expression in these cancers compared to normal tissues. To validate and extend our findings, we performed pan-cancer analysis of PLK5 expression using public RNAseq databases (TCGA and GTEx). We found PLK5 is downregulated in 18 cancer types, including our selected candidates. Interestingly, we also observed PLK5 expression remains consistently low in later stages of cancer, suggesting PLK5 may have a greater role in tumor initiation than cancer progression. Overall, our study demonstrates PLK5 downregulation in multiple cancers, highlighting its role as a tumor suppressor.

Design, synthesis, and biological evaluation of a potent PLK4 inhibitor WY29 with 1H-pyrazolo[3,4-d]pyrimidine scaffold

Centriole duplication occurs once per cell cycle and is regulated by Polo-like kinase 4 (PLK4). Overexpression of PLK4 in somatic cells can lead to the excessive formation of centrioles, directly causing chromosome segregation errors and tumorigenesis. In this study, we described our efforts to develop a series of PLK4 inhibitors with 1H-pyrazolo[3,4-d]pyrimidine core, and further structure- and receptor-based design and optimization resulted in a potent inhibitor WY29 (IC₅₀  = 0.027 μM), which exhibited good selectivity to other PLK family members (PLK1-3). At the cellular level, compound WY29 showed excellent antiproliferative activity against three breast cancer cell lines (MCF-7, BT474, and MDA-MB-231) while weak inhibitory activity was found on normal cell line HUVECs. In addition, the in vitro preliminary drug-like properties evaluation of compound WY29 showed outstanding stability in human plasma and liver microsomes, and weak inhibitory activity against the major subtypes of human cytochrome P450. Also, the drug-like properties prediction of compound WY29 displayed remarkable drug-like properties (drug-likeness mode score: 1.06). In conclusion, these results support the further development of compound WY29 as a lead compound for PLK4-targeted anticancer drug discovery.

Discovery of CZS-241: A Potent, Selective, and Orally Available Polo-Like Kinase 4 Inhibitor for the Treatment of Chronic Myeloid Leukemia

Recent studies demonstrate that PLK4 has emerged as a therapeutic target for the treatment of multiple cancers owing to its indispensable role in cell division. Herein, starting from previously identified effective compound CZS-034, based on rational drug design strategies, tyrosine kinase receptor A (TRKA) selectivity- and metabolic stability-guided structure-activity relationship (SAR) exploration were carried out to discover a highly potent (IC₅₀ = 2.6 nM) and selective (SF = 1054.4 over TRKA) PLK4 inhibitor B43 (CZS-241) with acceptable human liver microsome stability (t_1/2 = 31.5 min). Moreover, compound B43 effectively inhibited leukemia cells in 29 tested cell lines, especially chronic myeloid leukemia (CML) cell lines K562 and KU-812. Pharmacokinetic characteristics revealed that compound B43 possessed over 4 h of half-life and 70.8% bioavailability in mice. In the K562 cells xenograft mouse model, a 20 mg/kg/day dosage treatment obviously suppressed tumor progression. As a potential and novel PLK4-targeted candidate drug for CML, compound B43 is undergoing extensive preclinical safety evaluation.

Drugging Hijacked Kinase Pathways in Pediatric Oncology: Opportunities and Current Scenario

Childhood cancer is considered rare, corresponding to ~3% of all malignant neoplasms in the human population. The World Health Organization (WHO) reports a universal occurrence of more than 15 cases per 100,000 inhabitants around the globe, and despite improvements in diagnosis, treatment and supportive care, one child dies of cancer every 3 min. Consequently, more efficient, selective and affordable therapeutics are still needed in order to improve outcomes and avoid long-term sequelae. Alterations in kinases' functionality is a trademark of cancer and the concept of exploiting them as drug targets has burgeoned in academia and in the pharmaceutical industry of the 21st century. Consequently, an increasing plethora of inhibitors has emerged. In the present study, the expression patterns of a selected group of kinases (including tyrosine receptors, members of the PI3K/AKT/mTOR and MAPK pathways, coordinators of cell cycle progression, and chromosome segregation) and their correlation with clinical outcomes in pediatric solid tumors were accessed through the R2: Genomics Analysis and Visualization Platform and by a thorough search of published literature. To further illustrate the importance of kinase dysregulation in the pathophysiology of pediatric cancer, we analyzed the vulnerability of different cancer cell lines against their inhibition through the Cancer Dependency Map portal, and performed a search for kinase-targeted compounds with approval and clinical applicability through the CanSAR knowledgebase. Finally, we provide a detailed literature review of a considerable set of small molecules that mitigate kinase activity under experimental testing and clinical trials for the treatment of pediatric tumors, while discuss critical challenges that must be overcome before translation into clinical options, including the absence of compounds designed specifically for childhood tumors which often show differential mutational burdens, intrinsic and acquired resistance, lack of selectivity and adverse effects on a growing organism.

Development of hyaluronic acid-anchored polycaprolactone nanoparticles for efficient delivery of PLK1 siRNA to breast cancer

PlK1 has a significant role in the development of breast cancer. Thus, silencing of PlK1 gene may arrest the growth of breast cancer. However, the in vivo stability of PlK1 siRNA after injection remains a challenge to target the specific site. The delivery of siPlK1 RNA via viral vector and amine group-terminated dendrimer is associated with immune reaction and cellular cytotoxicity. Thus, in the present study, hyaluronic acid-functionalized and -thiolated polycaprolactone nanoparticles (SH-HPP NPs) were developed for enhancing the targeting capabilities of siRNA towards human breast cancer cells. NPs displayed size in the range of 180-217 nm, and with sustain and pH-dependent release of siRNA up to 120 h. The in vitro treatments with siRNA-containing NPs showed the high number of necrotic cells and the cell cycle arrest at the G2/M phase. The gene expression analysis depicts the decrease of endogenous PLK1 siRNA expression on MCF-7 cells upon PLK1 NPs treatment. In vitro cytotoxicity experiments demonstrated effective anticancer properties against MCF-7. Finally, in vivo results showed that substantial tumor inhibition was achieved with PLK1 siRNA-containing SH-HPP NPs in comparison of the control group. Hence, HPP NPs have enormous potential for the selective delivery of siRNA, i.e., breast cancer cells.

Exploring the molecular and functional cellular response to hydrazine via transcriptomics and DNA repair mutant cell lines

Hydrazine is a rodent carcinogen and is classified as a probable human carcinogen by IARC. Though hydrazine is positive in both in vitro and in vivo DNA strand break (comet) assays, hydrazine was reported to be negative in an in vitro mutation Muta Mouse lung epithelial cell (FE1) test, as well as in a regulatory-compliant, in vivo Big Blue mouse mutation test. In this article, mechanistic studies explored the cellular response to hydrazine. When tested in a regulatory-compliant mouse lymphoma assay, hydrazine yielded unusual, weakly positive results. This prompted an investigation into the transcriptional response to hydrazine in FE1 cells via RNA sequencing. Amongst the changes identified was a dose-dependent increase in G2/M DNA damage checkpoint activation associated genes. Flow cytometric experiments in FE1 cells revealed that hydrazine exposure led to S-phase cell cycle arrest. Clonogenic assays in a variety of cell lines harboring key DNA repair protein deficiencies indicated that hydrazine could sensitize cells lacking homology dependent repair proteins (Brca2 and Fancg). Lastly, hprt assays with hydrazine were conducted to determine whether a lack of DNA repair could lead to mutagenicity. However, no robust, dose-dependent induction of mutations was noted. The transcriptional and cell cycle response to hydrazine, coupled with functional investigations of DNA repair-deficient cell lines support the inconsistencies noted in the genetic toxicology regulatory battery. In summary, while hydrazine may be genotoxic, transcriptional and functional processes involved in cell cycle regulation and DNA repair appear to play a nuanced role in mediating the mutagenic potential.

Recent Progress on the Localization of PLK1 to the Kinetochore and Its Role in Mitosis

The accurate distribution of the replicated genome during cell division is essential for cell survival and healthy organismal development. Errors in this process have catastrophic consequences, such as birth defects and aneuploidy, a hallmark of cancer cells. PLK1 is one of the master kinases in mitosis and has multiple functions, including mitotic entry, chromosome segregation, spindle assembly checkpoint, and cytokinesis. To dissect the role of PLK1 in mitosis, it is important to understand how PLK1 localizes in the specific region in cells. PLK1 localizes at the kinetochore and is essential in spindle assembly checkpoint and chromosome segregation. However, how PLK1 localizes at the kinetochore remains elusive. Here, we review the recent literature on the kinetochore recruitment mechanisms of PLK1 and its roles in spindle assembly checkpoint and attachment between kinetochores and spindle microtubules. Together, this review provides an overview of how the local distribution of PLK1 could regulate major pathways in mitosis.

Recent advances in lipid nanoparticles for delivery of nucleic acid, mRNA, and gene editing-based therapeutics

Lipid nanoparticles (LNPs) are becoming popular as a means of delivering therapeutics, including those based on nucleic acids and mRNA. The mRNA-based coronavirus disease 2019 vaccines are perfect examples to highlight the role played by drug delivery systems in advancing human health. The fundamentals of LNPs for the delivery of nucleic acid- and mRNA-based therapeutics, are well established. Thus, future research on LNPs will focus on addressing the following: expanding the scope of drug delivery to different constituents of the human body, expanding the number of diseases that can be targeted, and studying the change in the pharmacokinetics of LNPs under physiological and pathological conditions. This review article provides an overview of recent advances aimed at expanding the application of LNPs, focusing on the pharmacokinetics and advantages of LNPs. In addition, analytical techniques, library construction and screening, rational design, active targeting, and applicability to gene editing therapy have also been discussed.

Heterogeneity of tumor microenvironment is associated with clinical prognosis of non-clear cell renal cell carcinoma: a single-cell genomics study

Non-clear renal cell carcinomas (nccRCCs) are less frequent in kidney cancer with histopathological heterogeneity. A better understanding of the tumor biology of nccRCC can provide more effective treatment paradigms for different subtypes. To reveal the heterogeneity of tumor microenvironment (TME) in nccRCC, we performed 10x sing-cell genomics on tumor and normal tissues from patients with papillary renal cell carcinoma (pRCC), chromophobe RCC (chrRCC), collecting duct carcinoma (CDRCC) and sarcomatoid RCC (sarRCC). 15 tissue samples were finally included. 34561 cells were identified as 16 major cell clusters with 34 cell subtypes. Our study presented the sing-cell landscape for four types of nccRCC, and demonstrated that CD8+ T cells exhaustion, tumor-associated macrophages (TAMs) and sarcomatoid process were the pivotal factors in immunosuppression of nccRCC tissues and were closely correlated with poor prognosis. Abnormal metabolic patterns were present in both cancer cells and tumor-infiltrating stromal cells, such as fibroblasts and endothelial cells. Combined with CIBERSORTx tool, the expression data of bulk RNA-seq from TCGA were labeled with cell types of our sing-cell data. Calculation of the relative abundance of cell types revealed that greater proportion of exhausted CD8+ T cells, TAMs and sarRCC derived cells were correlated with poor prognosis in the cohort of 274 nccRCC patients. To the best of our knowledge, this is the first study that provides a more comprehensive sight about the heterogeneity and tumor biology of nccRCC, which may potentially facilitate the development of more effective therapies for nccRCC.

Centriole is the pivot coordinating dynamic signaling for cell proliferation and organization during early development in the vertebrates

Vertebrates have an elaborate and functionally segmented body. It evolves from a single cell by systematic cell proliferation but attains a complex body structure with exquisite precision. This development requires two cellular events: cell cycle and ciliogenesis. For these events, the dynamic molecular signaling is converged at the centriole. The cell cycle helps in cell proliferation and growth of the body and is a highly regulated and integrated process. Its errors cause malignancies and developmental disorders. The cells newly proliferated are organized during organogenesis. For a cellular organization, dedicated signaling hubs are developed in the cells, and most often cilia are utilized. The cilium is generated from one of the centrioles involved in cell proliferation. The developmental signaling pathways hosted in cilia are essential for the elaboration of the body plan. The cilium's compartmental seclusion is ideal for noise-free molecular signaling and is essential for the precision of the body layout. The dysfunctional centrioles and primary cilia distort the development of body layout that manifest as serious developmental disorders. Thus, centriole has a dual role in the growth and cellular organization. It organizes dynamically expressed molecules of cell cycle and ciliogenesis and plays a balancing act to generate new cells and organize them during development. A putative master molecule may regulate and coordinate the dynamic gene expression at the centrioles. The convergence of many critical signaling components at the centriole reiterates the idea that centriole is a major molecular workstation involved in elaborating the structural design and complexity in vertebrates. This article is protected by copyright. All rights reserved.

Effects of photodynamic therapy mediated by emodin in cervical carcinoma cells

Cervical cancer is a worldwide public health problem, and improved selective therapies and anticancer drugs are urgently needed. In recent years, emodin has attracted considerable attention due to its anti-inflammatory, antineoplastic, and proapoptotic effects. Furthermore, emodin may be used as a photosensitizing agent in photodynamic therapy. Interest in photodynamic therapy for cancer treatment has increased due to its efficiency in causing tumor cell death. This study aimed to analyze the effect of emodin combined with photodynamic therapy in cervical carcinoma cell lines. At first, emodin presented cytotoxicity in concentration and time-dependent manners in all the specific cell lines analyzed. SiHa, CaSki, and HaCaT cancer cells presented more than 80% cell viability in concentrations below 30 µmol/L. Fluorescence microscopy images showed efficient cellular uptake of emodin in all analyzed cell lines. A significant decrease in cell viability was observed in SiHa, CaSki, and HaCaT cell lines after treatment of emodin combined with photodynamic therapy. These decreases were accompanied by increased ROS production, caspase-3 activity, and fluorescence intensity of autophagic vacuoles. This suggests increased ROS production led to cell death by apoptosis and autophagy. Additionally, after the combination of emodin and photodynamic therapy in SiHa cells, we observed the overexpression of 22 target genes and downregulation of two target genes of anti-cancer drugs. These results show the promising potential for applications that combine emodin with photodynamic therapy for cervical cancer treatment.

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.

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

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

PLK1 promotes proliferation and suppresses apoptosis of renal cell carcinoma cells by phosphorylating MCM3

Minichromosome maintenance 3 (MCM3) protein has been widely studied due to its essential role in DNA replication. In addition, it is overexpressed in several human tumor types. However, the role of this protein in renal cell carcinoma (RCC) is not widely known. In this study, we demonstrated that polo-like kinase 1 (PLK1)-mediated MCM3 phosphorylation regulates proliferation and apoptosis in RCC. Our results confirm that PLK1 and phospho-MCM3 (p-MCM3) are highly expressed in renal cell carcinoma. The expression of PLK1 is closely related to the clinical characteristics of renal cell carcinoma. They play important roles in the proliferation and apoptosis of RCC. In vitro, after overexpression of PLK1 or MCM3, the proliferation of RCC cells was significantly enhanced and cell apoptosis was inhibited, while after knockout, the proliferation of RCC cells was weakened and cell apoptosis was promoted. In addition, Mn2+-Phos-tag SDS-PAGE, western blotting, and immunofluorescence were utilized to determine that MCM3 is a physiological substrate of PLK1, which is phosphorylated on serine 112 (Ser112) in a PLK1-dependent manner. PLK1-mediated MCM3 phosphorylation promotes RCC cell cycle proliferation and suppresses apoptosis in vitro. Moreover, we found that PLK1-mediated MCM3 phosphorylation induced cellular proliferation and decreased apoptosis, as well as tumor growth in mice. Overall, we conclude that PLK1-mediated MCM3 phosphorylation is a novel mechanism to regulate RCC proliferation and apoptosis.

A homozygous G insertion in MPLKIP leads to TTDN1 with the hypergonadotropic hypogonadism symptom

Background

Trichothiodystrophy nonphotosensitive 1 (TTDN1) is a disease with mental retardation, brittle hair. Some cases of the diseases are caused by mutations of the MPLKIP gene.

Methods

We carefully identified the clinic characteristics, the sulfur level and pattern of the hair shafts of a female patient of with the symptom of hypergonadotropic hypogonadism, and of her parents and brother whose are healthy. We also collected the blood sample of the patient and performed the exon sequencing. One G insertion in MPLKIP was identified after analyzing the obtained exon sequencing profile. The G insertion sites in the patient, her parents and brother, were verified using Sanger sequencing. The G insertion in MPLKIP were compared to the dbSNP.

Results

The female patient of TTDN1 carries a homozygous G insertion (rs747470385) in the MPLKIP gene. The parents and brother of the patient are heterozygous carriers of the same mutation, but are healthy. The hair shafts of the patient had a tiger-tail pattern with relatively low sulfur levels. To the best of our knowledge, this is the first report that autosomal recessive inheritance of the G insertion in the MPLKIP gene results in TTDN1.

Conclusion

Our results indicate that the homozygotic G insertion in MPLKIP results in the TTDN1 with hypergonadotropic hypogonadism, while heterozygous carriers of the same mutation have no symptoms and healthy. These results provide novel insights into the association of mutations in MPLKIP and TTDN1 with hypergonadotropic hypogonadism.

GSK461364A, a Polo-Like Kinase-1 Inhibitor Encapsulated in Polymeric Nanoparticles for the Treatment of Glioblastoma Multiforme (GBM)

Glioblastoma Multiforme (GBM) is a common primary brain cancer with a poor prognosis and a median survival of less than 14 months. Current modes of treatment are associated with deleterious side effects that reduce the life span of the patients. Nanomedicine enables site-specific delivery of active pharmaceutical ingredients and facilitates entrapment inside the tumor. Polo-like kinase 1 (PLK-1) inhibitors have shown promising results in tumor cells. GSK461364A (GSK) is one such targeted inhibitor with reported toxicity issues in phase 1 clinical trials. We have demonstrated in our study that the action of GSK is time dependent across all concentrations. There is a distinct 15−20% decrease in cell viability via apoptosis in U87-MG cells dosed with GSK at low concentrations (within the nanomolar and lower micromolar range) compared to higher concentrations of the drug. Additionally, we have confirmed that PLGA-PEG nanoparticles (NPs) containing GSK have shown significant reduction in cell viability of tumor cells compared to their free equivalents. Thus, this polymeric nanoconstruct encapsulating GSK can be effective even at low concentrations and could improve the effectiveness of the drug while reducing side effects at the lower effective dose. This is the first study to report a PLK-1 inhibitor (GSK) encapsulated in a nanocarrier for cancer applications.

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.

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.

Plk2 Loss Commonly Occurs in Colorectal Carcinomas but not Adenomas: Relationship to mTOR Signaling

Plk2 is a target of p53. Our previous studies demonstrated that with wild-type p53, Plk2 impacts mTOR signaling in the same manner as TSC1, and Plk2-deficient tumors grew larger than control. Other investigators have demonstrated that Plk2 phosphorylates mutant p53 in a positive feedback loop. We investigated Plk2’s tumor suppressor functions in relationship to mTOR signaling. Archival specimens from 12 colorectal adenocarcinomas were stained for markers including Plk2, phosphorylated mTOR (serine 2448) and ribosomal S6 (Serine 235/236). We show that Plk2 is expressed in normal colon, with a punctate staining pattern in supranuclear cytoplasm. In colorectal adenocarcinoma, Plk2 demonstrates complete or partial loss of expression. Strong expression of phosphorylated mTOR is observed in the invasive front. Phosphorylated S6 expression partially correlates with phosphorylated mTOR expression but appears more diffuse in some cases. p53 and Ki67 expression is diffuse, in the subset of cases examined. In order to determine whether Plk2 is lost prior to the development of invasive cancer, 8 colon polyps from 6 patients were evaluated for Plk2 expression. All polyps are positive for Plk2. A Cancer Genome Atlas search identified Plk2 mutations to be infrequent in colorectal adenocarcinomas. Neither Plk2 methylation (in the gene body) nor copy number variations correlated with changes in mRNA expression levels. Loss of Plk2 expression along with accentuated expression of phosphorylated mTOR and phosphorylated S6 at the invasive front in some colorectal carcinomas is consistent with previous findings that an interaction between Plk2 and TSC1 / mTOR signaling molecules plays a role in tumor suppression. Plk2 protein expression is lost at the same stage in colorectal carcinogenesis as p53. The p53 dependence of Plk2 loss and tumor suppressor function in relationship to mTOR signaling may have therapeutic implications.

Polo-like kinase 3 and phosphoT273 caspase-8 are associated with improved local tumor control and survival in patients with anal carcinoma treated with concomitant chemoradiotherapy

We have recently shown that caspase-8 is a new substrate of Polo-like kinase 3 (Plk3) that phosphorylates the protein on residue T273 thereby promoting its pro-apoptotic function. In the present study we aimed to investigate the clinical relevance of Plk3 expression and phosphorylation of caspase-8 at T273 in patients with anal squamous cell carcinoma (SSC) treated with 5-fluorouracil and mitomycin C-based chemoradiotherapy (CRT). Immunohistochemical detection of the markers was performed in pretreatment biopsy specimens of 95 patients and was correlated with clinical/histopathologic characteristics including HPV-16 virus load/p16^INK4a expression and cumulative incidence of local and distant failure, cancer specific survival (CSS), and overall survival (OS). We observed significant positive correlations between Plk3 expression, pT273 caspase-8 signal, and levels of HPV-16 virus DNA load/p16^INK4a detection. Patients with high scores of Plk3 and pT273 caspase-8 showed increased local control (p = 0.011; p = 0.001), increased CSS (p = 0.011; p = 0.013) and OS (p = 0.024; p = 0.001), while the levels of pT273 caspase-8 were significantly associated (p = 0.033) with distant metastases. In multivariate analyses Plk3 expression remained significant for local failure (p = 0.018), CSS (p = 0.016) and OS (p = 0.023). Moreover, a combined HPV16 DNA load and Plk3 or pT273 caspase-8 variable revealed a significant correlation to decreased local failure (p = 0.001; p = 0.009), increased CSS (p = 0.016; p = 0.023) and OS (p = 0.003; p = 0.003). In conclusion these data indicate that elevated levels of Plk3 and pT273 caspase-8 are correlated with favorable clinical outcome in patients with anal SCC treated with concomitant CRT.

The CDK-PLK1 axis targets the DNA damage checkpoint sensor protein RAD9 to promote cell proliferation and tolerance to genotoxic stress

Genotoxic stress causes proliferating cells to activate the DNA damage checkpoint, to assist DNA damage recovery by slowing cell cycle progression. Thus, to drive proliferation, cells must tolerate DNA damage and suppress the checkpoint response. However, the mechanism underlying this negative regulation of checkpoint activation is still elusive. We show that human Cyclin-Dependent-Kinases (CDKs) target the RAD9 subunit of the 9-1-1 checkpoint clamp on Thr292, to modulate DNA damage checkpoint activation. Thr292 phosphorylation on RAD9 creates a binding site for Polo-Like-Kinase1 (PLK1), which phosphorylates RAD9 on Thr313. These CDK-PLK1-dependent phosphorylations of RAD9 suppress checkpoint activation, therefore maintaining high DNA synthesis rates during DNA replication stress. Our results suggest that CDK locally initiates a PLK1-dependent signaling response that antagonizes the ability of the DNA damage checkpoint to detect DNA damage. These findings provide a mechanism for the suppression of DNA damage checkpoint signaling, to promote cell proliferation under genotoxic stress conditions.

PLK4: a link between centriole biogenesis and cancer

INTRODUCTION

Polo like kinase (PLK) is known to play a pivotal role in various cell cycle processes to perpetuate proper division and growth of the cells. Polo like kinase-4 (PLK4) is one such kinase that appears in low abundance and plays a well-characterized role in centriole duplication. PLK4 deregulation (i.e. both overexpression and depletion of PLK4), leads to altered mitotic fidelity and thereby triggers tumorigenesis. Hence, over the last few years PLK4 has emerged as a potential therapeutic target for the treatment of various advanced cancers. Areas covered: In this review, we discuss the basic structure, expression, localization and functions of PLK4 along with its regulation by various proteins. We also discuss the role of altered PLK4 activity in the onset of cancer and the current pre-clinical and clinical inhibitors to regulate PLK4. Expert opinion: PLK4 mediated centriole duplication has a crucial role in maintaining mitotic correctness in normal cells, while its deregulation has a greater impact on genesis of cancer. Henceforth, a deep knowledge of the PLK4 levels, its role and interactions with various proteins in cancer is required to design effective inhibitors for clinical use.

Polo-Like Kinase 2 is Dynamically Regulated to Coordinate Proliferation and Early Lineage Specification Downstream of Yes-Associated Protein 1 in Cardiac Progenitor Cells

BACKGROUND

Recent studies suggest that adult cardiac progenitor cells (CPCs) can produce new cardiac cells. Such cell formation requires an intricate coordination of progenitor cell proliferation and commitment, but the molecular cues responsible for this regulation in CPCs are ill defined.

METHODS AND RESULTS

Extracellular matrix components are important instructors of cell fate. Using laminin and fibronectin, we induced two slightly distinct CPC phenotypes differing in proliferation rate and commitment status and analyzed the early transcriptomic response to CPC adhesion (<2 hours). Ninety-four genes were differentially regulated on laminin versus fibronectin, consisting of mostly downregulated genes that were enriched for Yes-associated protein (YAP) conserved signature and TEA domain family member 1 (TEAD1)-related genes. This early gene regulation was preceded by the rapid cytosolic sequestration and degradation of YAP on laminin. Among the most strongly regulated genes was polo-like kinase 2 (Plk2). Plk2 expression depended on YAP stability and was enhanced in CPCs transfected with a nuclear-targeted mutant YAP. Phenotypically, the early downregulation of Plk2 on laminin was succeeded by lower cell proliferation, enhanced lineage gene expression (24 hours), and facilitated differentiation (3 weeks) compared with fibronectin. Finally, overexpression of Plk2 enhanced CPC proliferation and knockdown of Plk2 induced the expression of lineage genes.

CONCLUSIONS

Plk2 acts as coordinator of cell proliferation and early lineage commitment in CPCs. The rapid downregulation of Plk2 on YAP inactivation marks a switch towards enhanced commitment and facilitated differentiation. These findings link early gene regulation to cell fate and provide novel insights into how CPC proliferation and differentiation are orchestrated.

Polo-like kinase-1 immunoreactivity is associated with metastases in cutaneous melanoma

BACKGROUND

Polo-like kinase-1 (PLK-1) is one of the key regulators of cell cycle progression. Increased expression of PLK-1 was observed in several tumor types.

METHODS

We immunohistochemically assessed PLK-1 expression in neoplastic and stromal compartments of 96 cutaneous melanomas, and analyzed associations between PLK-1 expression and clinicopathological characteristics.

RESULTS

PLK-1 expression in cancer cells was not associated with basic clinical (eg, age, gender and tumor location) or histopathological (eg, Breslow thickness, mitotic rate and ulceration) parameters. However, increased PLK-1 was more frequent in tumors with concurrent regional nodal metastases and positive sentinel lymph node biopsy status. All primary tumors associated with co-existing distant metastases exhibited high PLK-1 expression in melanoma cells. Conversely, PLK-1 expression in stromal cells was more frequent in tumors without nodal metastases. PLK-1 expression in both compartments was not associated with survival.

CONCLUSION

PLK-1 expression is associated with metastatic potential in cutaneous melanoma.

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

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

Plk1 regulates contraction of postmitotic smooth muscle cells and is required for vascular homeostasis

Polo-like kinase 1 (PLK1), an essential regulator of cell division, is currently undergoing clinical evaluation as a target for cancer therapy. We report an unexpected function of Plk1 in sustaining cardiovascular homeostasis. Plk1 haploinsufficiency in mice did not induce obvious cell proliferation defects but did result in arterial structural alterations, which frequently led to aortic rupture and death. Specific ablation of Plk1 in vascular smooth muscle cells (VSMCs) led to reduced arterial elasticity, hypotension, and an impaired arterial response to angiotensin II in vivo. Mechanistically, we found that Plk1 regulated angiotensin II-dependent activation of RhoA and actomyosin dynamics in VSMCs in a mitosis-independent manner. This regulation depended on Plk1 kinase activity, and the administration of small-molecule Plk1 inhibitors to angiotensin II-treated mice led to reduced arterial fitness and an elevated risk of aneurysm and aortic rupture. We thus conclude that a partial reduction of Plk1 activity that does not block cell division can nevertheless impair aortic homeostasis. Our findings have potentially important implications for current approaches aimed at PLK1 inhibition for cancer therapy.

G1/S phase progression is regulated by PLK1 degradation through the CDK1/βTrCP axis

Polo-like kinase 1 (PLK1) is a serine/threonine kinase involved in several stages of the cell cycle, including the entry and exit from mitosis, and cytokinesis. Furthermore, it has an essential role in the regulation of DNA replication. Together with cyclin A, PLK1 also promotes CDH1 phosphorylation to trigger its ubiquitination and degradation, allowing cell cycle progression. The PLK1 levels in different type of tumors are very high compared to normal tissues, which is consistent with its role in promoting proliferation. Therefore, several PLK1 inhibitors have been developed and tested for the treatment of cancer. Here, we further analyzed PLK1 degradation and found that cytoplasmic PLK1 is ubiquitinated and subsequently degraded by the SCFβTrCP/proteasome. This procedure is triggered when heat shock protein (HSP) 90 is inhibited with geldanamycin, which results in misfolding of PLK1. We also identified CDK1 as the major kinase involved in this degradation. Our work shows for the first time that HSP90 inhibition arrests cell cycle progression at the G1/S transition. This novel mechanism inhibits CDH1 degradation through CDK1-dependent PLK1 destruction by the SCFβTrCP/proteasome. In these conditions, CDH1 substrates do not accumulate and cell cycle arrests, providing a novel pathway for regulation of the cell cycle at the G1-to-S boundary.-Giráldez, S., Galindo-Moreno, M., Limón-Mortés, M. C., Rivas, A. C., Herrero-Ruiz, J., Mora-Santos, M., Sáez, C., Japón, M. Á., Tortolero, M., Romero, F. G1/S phase progression is regulated by PLK1 degradation through the CDK1/βTrCP axis.

Mutual regulation between Polo-like kinase 3 and SIAH2 E3 ubiquitin ligase defines a regulatory network that fine-tunes the cellular response to hypoxia and nickel

Elevated cellular response to hypoxia, which contributes to cell transformation and tumor progression, is a prominent feature of malignant cells in solid tumors. Polo-like kinase 3 (Plk3) is a serine/threonine protein kinase known to inhibit the cellular response to hypoxia and tumorigenesis. Nickel compounds are well-established human carcinogens that induce tumorigenesis partly through their hypoxia-mimicking effects. Despite previous research efforts, the role of Plk3 in the hypoxic response induced by hypoxia or nickel is not completely understood. Here, we show that NiCl₂ (Ni(II)) or hypoxia reduces the protein level and shortens the half-life of cytoplasmic Plk3 in a ubiquitin-proteasome-dependent manner. We identify SIAH2, a RING finger E3 ubiquitin ligase associated with the cellular hypoxic response, to be the ubiquitin E3 ligase that mediates the degradation of Plk3. We show that SIAH2 binds to Plk3 and mediates its ubiquitination primarily through its polo-box domain. We report that USP28, a deubiquitinase known to be inhibitable by Ni(II) or hypoxia, may also contribute to the suppression of the Plk3 protein by Ni(II). We also show that Plk3 in turn suppresses the SIAH2 protein level in a kinase activity-dependent manner. Our study revealed an interesting mutual regulation between Plk3 and SIAH2 and uncovered a regulatory network that functions to fine-tune the cellular hypoxic response. We propose that suppression of Plk3 expression contributes to carcinogenesis and tumor progression induced by nickel compounds.

PLK1-associated microRNAs are correlated with pediatric medulloblastoma prognosis

PURPOSE

Medulloblastoma (MB) is the most common malignant tumor of the central nervous system (CNS) in children. Despite its relative good survival rates, treatment can cause long time sequels and may impair patients' lifespan and quality, making the search for new treatment options still necessary. Polo like kinases (PLKs) constitute a five-member serine/threonine kinases family (PLK 1-5) that regulates different stages during cell cycle. Abnormal PLKs expression has been observed in several cancer types, including MB. As gene regulators, miRNAs have also been described with variable expression in cancer.

METHODS

We evaluated gene expression profiles of all PLK family members and related miRNAs (miR-100, miR-126, miR-219, and miR-593*) in MB cell lines and tumor samples.

RESULTS

RT-qPCR analysis revealed increased levels of PLK1-4 in all cell lines and in most MB samples, while PLK5 was found underexpressed. In parallel, miR-100 was also found upregulated while miR-129, miR-216, and miR-593* were decreased in MB cell lines. Variable miRNAs expression patterns were observed in MB samples. However, a correlation between miR-100 and PLK4 expression was observed, and associations between miR-100, miR-126, and miR-219 expression and overall and event free survival were also evinced in our cohort. Moreover, despite the lack of association with clinico-pathological features, when comparing primary tumors to those relapsed, we found a consistent decrease on PLK2, miR-219, and miR-598* and an increase on miR-100 and miR-126.

CONCLUSION

Specific dysregulation on PLKs and associated miRNAs may be important in MB and can be used to predict prognosis. Although miRNAs sequences are fundamental to predict its target, the cell type may also be consider once that mRNA repertoire can define different roles for specific miRNA in a given cell.

Targeted Treatment of Metastatic Breast Cancer by PLK1 siRNA Delivered by an Antioxidant Nanoparticle Platform

Metastatic breast cancer is developed in about 20% to 30% of newly diagnosed patients with early-stage breast cancer despite treatments. Herein, we report a novel nanoparticle platform with intrinsic antimetastatic properties for the targeted delivery of Polo-like kinase 1 siRNA (siPLK1). We first evaluated it in a triple-negative breast cancer (TNBC) model, which shows high metastatic potential. PLK1 was identified as the top therapeutic target for TNBC cells and tumor-initiating cells in a kinome-wide screen. The platform consists of a 50-nm mesoporous silica nanoparticle (MSNP) core coated layer-by-layer with bioreducible cross-linked PEI and PEG polymers, conjugated with an antibody for selective uptake into cancer cells. siRNA is loaded last and fully protected under the PEG layer from blood enzymatic degradation. The material has net neutral charge and low nonspecific cytotoxicity. We have also shown for the first time that the MSNP itself inhibited cancer migration and invasion in TNBC cells owing to its ROS- and NOX4-modulating properties. In vivo, siPLK1 nanoconstructs (six doses of 0.5 mg/kg) knocked down about 80% of human PLK1 mRNA expression in metastatic breast cancer cells residing in mouse lungs and reduced tumor incidence and burden in lungs and other organs of an experimental metastasis mouse model. Long-term treatment significantly delayed the onset of death in mice and improved the overall survival. The platform capable of simultaneously inhibiting the proliferative and metastatic hallmarks of cancer progression is unique and has great therapeutic potential to also target other metastatic cancers beyond TNBC. Mol Cancer Ther; 16(4); 763-72. ©2017 AACR.

Anticancer activity of a novel small molecule tubulin inhibitor STK899704

We have identified the small molecule STK899704 as a structurally novel tubulin inhibitor. STK899704 suppressed the proliferation of cancer cell lines from various origins with IC50 values ranging from 0.2 to 1.0 μM. STK899704 prevented the polymerization of purified tubulin in vitro and also depolymerized microtubule in cultured cells leading to mitotic arrest, associated with increased Cdc25C phosphorylation and the accumulation of both cyclin B1 and polo-like kinase 1 (Plk1), and apoptosis. Unlike many anticancer drugs such as Taxol and doxorubicin, STK899704 effectively displayed antiproliferative activity against multidrug-resistant cancer cell lines. The proposed binding mode of STK899704 is at the interface between αβ-tubulin heterodimer overlapping with the colchicine-binding site. Our in vivo carcinogenesis model further showed that STK 899704 is potent in both the prevention and regression of tumors, remarkably reducing the number and volume of skin tumor by STK899704 treatment. Moreover, it was significant to note that the efficacy of STK899704 was surprisingly comparable to 5-fluorouracil, a widely used anticancer therapeutic. Thus, our results demonstrate the potential of STK899704 to be developed as an anticancer chemotherapeutic and an alternative candidate for existing therapies.

Dissecting the Molecular Pathway Involved in PLK2 Kinase-mediated α-Synuclein-selective Autophagic Degradation

Increasing lines of evidence support the causal link between α-synuclein (α-syn) accumulation in the brain and Parkinson's disease (PD) pathogenesis. Therefore, lowering α-syn protein levels may represent a viable therapeutic strategy for the treatment of PD and related disorders. We recently described a novel selective α-syn degradation pathway, catalyzed by the activity of the Polo-like kinase 2 (PLK2), capable of reducing α-syn protein expression and suppressing its toxicity in vivo However, the exact molecular mechanisms underlying this degradation route remain elusive. In the present study we report that among PLK family members, PLK3 is also able to catalyze α-syn phosphorylation and degradation in living cells. Using pharmacological and genetic approaches, we confirmed the implication of the macroautophagy on PLK2-mediated α-syn turnover, and our observations suggest a concomitant co-degradation of these two proteins. Moreover, we showed that the N-terminal region of α-syn is important for PLK2-mediated α-syn phosphorylation and degradation and is implicated in the physical interaction between the two proteins. We also demonstrated that PLK2 polyubiquitination is important for PLK2·α-syn protein complex degradation, and we hypothesize that this post-translational modification may act as a signal for the selective recognition by the macroautophagy machinery. Finally, we observed that the PD-linked mutation E46K enhances PLK2-mediated α-syn degradation, suggesting that this mutated form is a bona fide substrate of this degradation pathway. In conclusion, our study provides a detailed description of the new degradation route of α-syn and offers new opportunities for the development of therapeutic strategies aiming to reduce α-syn protein accumulation and toxicity.

Clinical Development of Anti-mitotic Drugs in Cancer

Mitosis is one of the most fundamental processes of life by which a mammalian cell divides into two daughter cells. Mitosis has been an attractive target for anticancer therapies since fast proliferation was identified as one of the hallmarks of cancer cells. Despite efforts into developing specific inhibitors for mitotic kinases and kinesins, very few drugs have shown the efficiency of microtubule targeting-agents in cancer cells with paclitaxel being the most successful. A deeper translational research accompanying clinical trials of anti-mitotic drugs will help in identifying potent biomarkers predictive for response. Here, we review the current knowledge of mitosis targeting agents that have been tested so far in the clinics.

Identification of Differential Gene Expression Patterns after Acute Exposure to High and Low Doses of Low-LET Ionizing Radiation in a Reconstituted Human Skin Tissue

In this study we utilized a systems biology approach to identify dose- (0.1, 2.0 and 10 Gy) and time- (3 and 8 h) dependent responses to acute ionizing radiation exposure in a complex tissue, reconstituted human skin. The low dose used here (0.1 Gy) falls within the range of certain medical diagnostic procedures. Of the two higher doses used, 2.0 Gy is typically administered for radiotherapy, while 10 Gy is lethal. Because exposure to any of these doses is possible after an intentional or accidental radiation events, biomarkers are needed to rapidly and accurately triage potentially exposed individuals. Here, tissue samples were acutely exposed to X-ray-generated low-linear-energy transfer (LET) ionizing radiation, and direct RNA sequencing (RNA-seq) was used to quantify altered transcripts. The time points used for this study aid in assessing early responses to exposure, when key signaling pathways and biomarkers can be identified, which precede and regulate later phenotypic alterations that occur at high doses, including cell death. We determined that a total of 1,701 genes expressed were significantly affected by high-dose radiation, with the majority of genes affected at 10 Gy. Expression levels of a group of 29 genes, including GDF15, BBC3, PPM1D, FDXR, GADD45A, MDM2, CDKN1A, TP53INP1, CYCSP27, SESN1, SESN2, PCNA and AEN, were similarly altered at both 2 and 10 Gy, but not 0.1 Gy, at both time points. A much larger group of upregulated genes, including those involved in inflammatory responses, was significantly altered only after 10 Gy irradiation. At high doses, downregulated genes were associated with cell cycle regulation and exhibited an apparent linear response between 2 and 10 Gy. While only a few genes were significantly affected by 0.1 Gy irradiation, using stringent statistical filters, groups of related genes regulating cell cycle progression and inflammatory responses consistently exhibited opposite trends in their regulation compared to high-dose irradiated groups. Differential regulation of PLK1 signaling at low- and high-dose irradiation was confirmed using qRT-PCR. These results indicate that some alterations in gene expression are qualitatively different at low and high doses of ionizing radiation in this model system. They also highlight potential biomarkers for radiation exposure that may precede the development of overt physiological symptoms in exposed individuals.

Spotlight on Volasertib: Preclinical and Clinical Evaluation of a Promising Plk1 Inhibitor

Considering the important side effects of conventional microtubule targeting agents, more and more research focuses on regulatory proteins for the development of mitosis-specific agents. Polo-like kinase 1 (Plk1), a master regulator of several cell cycle events, has arisen as an intriguing target in this research field. The observed overexpression of Plk1 in a broad range of human malignancies has given rise to the development of several potent and specific small molecule inhibitors targeting the kinase. In this review, we focus on volasertib (BI6727), the lead agent in category of Plk1 inhibitors at the moment. Numerous preclinical experiments have demonstrated that BI6727 is highly active across a variety of carcinoma cell lines, and the inhibitor has been reported to induce tumor regression in several xenograft models. Moreover, volasertib has shown clinical efficacy in multiple tumor types. As a result, Food and Drug Administration (FDA) has recently awarded volasertib the Breakthrough Therapy status after significant benefit was observed in acute myeloid leukemia (AML) patients treated with the Plk1 inhibitor. Here, we discuss both preclinical and clinical data available for volasertib administered as monotherapy or in combination with other anticancer therapies in a broad range of tumor types.

Genetic depletion of Polo-like kinase 1 leads to embryonic lethality due to mitotic aberrancies

Polo-like kinase 1 (PLK1) is a serine/threonine kinase that plays multiple and essential roles during the cell division cycle. Its inhibition in cultured cells leads to severe mitotic aberrancies and cell death. Whereas previous reports suggested that Plk1 depletion in mice leads to a non-mitotic arrest in early embryos, we show here that the bi-allelic Plk1 depletion in mice certainly results in embryonic lethality due to extensive mitotic aberrations at the morula stage, including multi- and mono-polar spindles, impaired chromosome segregation and cytokinesis failure. In addition, the conditional depletion of Plk1 during mid-gestation leads also to severe mitotic aberrancies. Our data also confirms that Plk1 is completely dispensable for mitotic entry in vivo. On the other hand, Plk1 haploinsufficient mice are viable, and Plk1-heterozygous fibroblasts do not harbor any cell cycle alterations. Plk1 is overexpressed in many human tumors, suggesting a therapeutic benefit of inhibiting Plk1, and specific small-molecule inhibitors for this kinase are now being evaluated in clinical trials. Therefore, the different Plk1 mouse models here presented are a valuable tool to reexamine the relevance of the mitotic kinase Plk1 during mammalian development and animal physiology.

Mitral regurgitation as a phenotypic manifestation of nonphotosensitive trichothiodystrophy due to a splice variant in MPLKIP

Background

Nonphotosensitive trichothiodystrophy (TTDN) is a rare autosomal recessive disorder of neuroectodermal origin. The condition is marked by hair abnormalities, intellectual impairment, nail dystrophies and susceptibility to infections but with no UV sensitivity.

Methods

We identified three consanguineous Pakistani families with varied TTDN features and used homozygosity mapping, linkage analysis, and Sanger and exome sequencing in order to identify pathogenic variants. Haplotype analysis was performed and haplotype age estimated. A splicing assay was used to validate the effect of the MPLKIP splice variant on expression.

Results

Affected individuals from all families exhibit several TTDN features along with a heart-specific feature, i.e. mitral regurgitation. Exome sequencing in the probands from families ED168 and ED241 identified a homozygous splice mutation c.339 + 1G > A within MPLKIP. The same splice variant co-segregates with TTDN in a third family ED210. The MPLKIP splice variant was not found in public databases, e.g. the Exome Aggregation Consortium, and in unrelated Pakistani controls. Functional analysis of the splice variant confirmed intron retention, which leads to protein truncation and loss of a phosphorylation site. Haplotype analysis identified a 585.1-kb haplotype which includes the MPLKIP variant, supporting the existence of a founder haplotype that is estimated to be 25,900 years old.

Conclusion

This study extends the allelic and phenotypic spectra of MPLKIP-related TTDN, to include a splice variant that causes cardiomyopathy as part of the TTDN phenotype.

The role of Plk3 in oncogenesis

The polo-like kinases (Plks) encompass a family of five serine/threonine protein kinases that play essential roles in many cellular processes involved in the control of the cell cycle, including entry into mitosis, DNA replication and the response to different types of stress. Plk1, which has been validated as a cancer target, came into the focus of many pharmaceutical companies for the development of small-molecule inhibitors as anticancer agents. Recently, FDA (Food and Drug Administration) has granted a breakthrough therapy designation to the Plk inhibitor BI 6727 (volasertib), which provided a survival benefit for patients suffering from acute myeloid leukemia. However, the various ATP-competitive inhibitors of Plk1 that are currently in clinical development also inhibit the activities of Plk2 and Plk3, which are considered as tumor suppressors. Plk3 contributes to the control and progression of the cell cycle while acting as a mediator of apoptosis and various types of cellular stress. The aberrant expression of Plk3 was found in different types of tumors. Recent progress has improved our understanding of Plk3 in regulating stress signaling and tumorigenesis. When using ATP-competitive Plk1 inhibitors, the biological roles of Plk1-related family members like Plk3 in cancer cells need to be considered carefully to improve treatment strategies against cancer.

Response of the hepatic transcriptome to aflatoxin B1 in ducklings

This study was conducted to determine the effects of aflatoxin B1 (AFB1) on the hepatic transcriptome in ducklings through RNA-sequencing (RNA-Seq). Twenty four, 1-day-old ducklings were divided into 4 treatment groups. Each group received an oral dose of AFB1 at 0, 10, 20, 40 μg/kg BW per day for 2 weeks. Administration of 20 and 40 μg/kg BW of AFB1 significantly decreased body weight, feed intake, serum total protein and albumin, while increasing serum aspartate aminotransferase and alanine aminotransferase activities, and hepatic histopathological lesions. Furthermore, RNA was extracted from the liver of ducklings administrated 0 and 40 μg/kg BW of AFB1. Two RNA-Seq libraries were created from pooled samples and produced over 149 M reads, totaling 14.9 Gb of sequence. Approximately 96,953 predicted transcripts were assembled, 749 of which had significant differential expressions (≥ 2-fold) between the control and AFB1 treatment. GO and KEGG pathway analysis results showed that many genes involved in phase I metabolism, phase II detoxification, oxidation-reduction process, carcinogenesis, apoptosis and cell cycle, and fatty acid metabolism were affected by AFB1 exposure. Conclusion, this study determined the hepatic transcriptome responded to AFB1 exposure, and provide candidate genes can be targeted to prevent and/or reduce aflatoxicosis in ducklings.

Polo-like kinase 3 is associated with improved overall survival in cholangiocarcinoma

BACKGROUND & AIMS

Cholangiocarcinomas (CCA) paradoxically express the death ligand TRAIL and thus, are dependent on effective survival signals to circumvent apoptosis. Hedgehog signalling exerts major survival signals in CCA by regulating serine/threonine kinase polo-like kinase (PLK)2. We here aimed to examine the role of PLK1/2/3 expression for CCA tumour biology.

METHODS

We employed CCA samples from 73 patients and human HUCCT-1/Mz-CHA1/KMCH-1 CCA cells. Immunohistochemistry for PLK1/2/3 was performed using tissue microarrays from representative tumour areas.

RESULTS

PLK1/2/3-immunoreactive cancer cells were present in most of the CCA samples. However, only PLK1 and especially PLK3 were expressed in higher amounts within CCA cells as compared to normal liver. Given that fibroblast growth factor (FGF) can induce PLK3 expression and also is present in CCA, we examined the effect of FGF on PLK3 in vitro. Indeed, rhFGF rapidly increased PLK3 mRNA expression all three CCA cell lines giving an explanation for the abundant PLK3 presence in the tissue samples. Clinicopathologically, PLK3 expression was associated with decreased tumour cell migration and lymph/blood vessel infiltration whereas higher levels of PLK1 were correlated with larger tumour sizes. Moreover, strong PLK3 expression was associated with prolonged overall survival.

CONCLUSIONS

The results suggest that PLK3 predominantly is expressed in CCA cells and that high PLK3 expression correlates with prolonged overall survival. These observations might have implications for prognosis prediction of human CCA as well as the potential therapeutic use of polo-like kinase inhibitors (i.e., PLK1/2 specifity).

Putting a bit into the polo-box domain of polo-like kinase 1

Polo-like kinase 1 (Plk1) plays key roles in regulating various mitotic processes that are critical for cellular proliferation. A growing body of evidence suggests that Plk1 overexpression is tightly associated with the development of human cancers. Interestingly, various types of cancer cells are shown to be addicted to a high level of Plk1, and the reversal of Plk1 addiction appears to be an effective strategy for selectively killing cancer cells, but not normal cells. Therefore, Plk1 is considered an attractive anticancer drug target. Over the years, a large number of inhibitors that target the catalytic activity of Plk1 have been developed. However, these inhibitors exhibit significant levels of cross-reactivity with related kinases, including Plk2 and Plk3. Consequently, as an alternative approach for developing anti-Plk1 therapeutics, substantial effort is under way to develop inhibitors that target the C-terminal protein–protein interaction domain of Plk1, called the polo-box domain (PBD). In this communication, I will discuss the pros and cons of targeting the PBD in comparison to those of targeting the ATP-binding site within the kinase domain.

Volasertib for AML: clinical use and patient consideration

Acute myeloid leukemia (AML) is a disease diagnosed mostly in patients >65 years of age. Despite its heterogeneous nature, the different types of AMLs are still managed by standard induction chemotherapy for those who can tolerate it in the beginning. For the elderly and infirm patients, however, this approach leads to unacceptably high induction mortality rate. This article reviews past and current efforts searching for low-intensiveness treatments for the elderly and infirm patients who cannot tolerate the standard induction regimen. Volasertib, currently in Phase III clinical trials in combination with cytarabine, is reviewed as a promising agent for this patient population with AML, from the viewpoints of potential compliance and efficacy.

Targeting polo-like kinase 1 in acute myeloid leukemia

Polo-like kinase 1 (Plk1) plays a number of important roles in the passage of cells through mitosis. It is expressed at high levels in a variety of malignancies, including acute myeloid leukemia (AML). Inhibition of Plk1 results in cell cycle arrest and apoptosis, and has anti-tumor effects in pre-clinical models. A number of Plk1 inhibitors have been developed, some of which have entered clinical trials. Of these, volasertib (BI6727) has been most extensively studied clinically in AML. Volasertib has demonstrated antileukemic activity in AML, both as a single agent and when combined with low-dose cytarabine. It is well tolerated, with the major toxicity being reversible myelosuppression. A recently completed phase III clinical trial in older AML patients will address the question of whether adding this agent to low-dose cytarabine is associated with a survival advantage.

Mammalian Polo-like kinase 1 (Plk1) promotes proper chromosome segregation by phosphorylating and delocalizing the PBIP1·CENP-Q complex from kinetochores

Mammalian Plk1 is critically required for proper M phase progression. Plk1 is self-recruited to prekinetochores/kinetochores by phosphorylating and binding to the Thr-78 motif of a kinetochore scaffold protein, PBIP1 (also called CENP-U/50), which forms a stable complex with another kinetochore component, CENP-Q. However, the mechanism regulating Plk1 localization to this site remains largely unknown. Here, we demonstrate that the PBIP1·CENP-Q complex became hyperphosphorylated and rapidly delocalized from kinetochores as cells entered mitosis. Plk1 phosphorylated the CENP-Q subunit of the PBIP1·CENP-Q complex at multiple sites, and mutation of nine Plk1-dependent phosphorylation sites to Ala (9A) enhanced CENP-Q association with chromatin and prolonged CENP-Q localization to kinetochores. Conversely, mutation of the nine sites to phospho-mimicking Asp/Glu (9D/E) residues dissociated CENP-Q from chromatin and kept the CENP-Q(9D/E) mutant from localizing to interphase prekinetochores. Strikingly, both the 9A and 9D/E mutants induced a defect in proper chromosome segregation, suggesting that both timely localization of the PBIP1·CENP-Q complex to prekinetochores and delocalization from kinetochores are critical for normal M phase progression. Notably, although Plk1 did not alter the level of PBIP1 and CENP-Q ubiquitination, Plk1-dependent phosphorylation and delocalization of these proteins from kinetochores appeared to indirectly lead to their degradation in the cytosol. Thus, we propose that Plk1 regulates the timing of the delocalization and ultimate destruction of the PBIP1·CENP-Q complex and that these processes are important not only for promoting Plk1-dependent mitotic progression, but also for resetting the timing of Plk1 recruitment to prekinetochores in the next cell cycle.