Role of Polo-Like Kinase 4 (PLK4) in Epithelial Cancers and Recent Progress in its Small Molecule Targeting for Cancer Management

Centrosomes and associated proteins in pathogenesis and treatment of breast cancer

Breast cancer is the most prevalent malignancy among women worldwide. Despite significant advances in treatment, it remains one of the leading causes of female mortality. The inability to effectively treat advanced and/or treatment-resistant breast cancer demonstrates the need to develop novel treatment strategies and targeted therapies. Centrosomes and their associated proteins have been shown to play key roles in the pathogenesis of breast cancer and thus represent promising targets for drug and biomarker development. Centrosomes are fundamental cellular structures in the mammalian cell that are responsible for error-free execution of cell division. Centrosome amplification and aberrant expression of its associated proteins such as Polo-like kinases (PLKs), Aurora kinases (AURKs) and Cyclin-dependent kinases (CDKs) have been observed in various cancers, including breast cancer. These aberrations in breast cancer are thought to cause improper chromosomal segregation during mitosis, leading to chromosomal instability and uncontrolled cell division, allowing cancer cells to acquire new genetic changes that result in evasion of cell death and the promotion of tumor formation. Various chemical compounds developed against PLKs and AURKs have shown meaningful antitumorigenic effects in breast cancer cells in vitro and in vivo. The mechanism of action of these inhibitors is likely related to exacerbation of numerical genomic instability, such as aneuploidy or polyploidy. Furthermore, growing evidence demonstrates enhanced antitumorigenic effects when inhibitors specific to centrosome-associated proteins are used in combination with either radiation or chemotherapy drugs in breast cancer. This review focuses on the current knowledge regarding the roles of centrosome and centrosome-associated proteins in breast cancer pathogenesis and their utility as novel targets for breast cancer treatment.

PLK4 reflects extrathyroidal invasion, high tumor stage and poor prognosis in papillary thyroid carcinoma patients

Objective: This study aimed to assess the value of PLK4 as a biomarker in papillary thyroid carcinoma (PTC). Methods: This study reviewed 230 PTC patients receiving surgical resections. PLK4 was detected in tumor tissues and samples of normal thyroid gland tissues by immunohistochemistry. Results: PLK4 was elevated in tumor tissues versus normal thyroid gland tissues (p < 0.001). Tumor PLK4 was linked with extrathyroidal invasion (p = 0.036), higher pathological tumor stage (p = 0.030), node stage (p = 0.045) and tumor/node/metastasis stage (p = 0.022) in PTC patients. Tumor PLK4 immunohistochemistry score >3 was linked with shortened disease-free survival (p = 0.026) and overall survival (p = 0.028) and independently predicted poorer disease-free survival (hazard ratio: 2.797; p = 0.040). Conclusion: Tumor PLK4 reflects extrathyroidal invasion, higher tumor stage and shortened survival in PTC.

Prolonged overexpression of PLK4 leads to formation of centriole rosette clusters that are connected via canonical centrosome linker proteins

Centrosome amplification is a hallmark of cancer and PLK4 is one of the responsible factors for cancer associated centrosome amplification. Increased PLK4 levels was also shown to contribute to generation of cells with centriole amplification in mammalian tissues as olfactory neuron progenitor cells. PLK4 overexpression generates centriole rosette (CR) structures which harbor more than two centrioles each. Long term PLK4 overexpression results with centrosome amplification, but the maturation of amplified centrioles in CRs and linking of PLK4 induced amplified centrosomes has not yet been investigated in detail. Here, we show evidence for generation of large clustered centrosomes which have more than 2 centriole rosettes and define these structures as centriole rosette clusters (CRCs) in cells that have high PLK4 levels for 2 consecutive cell cycles. In addition, we show that PLK4 induced CRs follow normal centrosomal maturation processes and generate CRC structures that are inter-connected with canonical centrosomal linker proteins as C-Nap1, Rootletin and Cep68 in the second cell cycle after PLK4 induction. Increased PLK4 levels in cells with C-Nap1 and Rootletin knock-out resulted with distanced CRs and CRCs in interphase, while Nek2 knock-out inhibited separation of CRCs in prometaphase, providing functional evidence for the binding of CRC structures with centrosomal linker proteins. Taken together, these results suggest a cell cycle dependent model for PLK4 induced centrosome amplification which occurs in 2 consecutive cell cycles: (i) CR state in the first cell cycle, and (ii) CRC state in the second cell cycle.

PLK4 as a potential target to enhance radiosensitivity in triple-negative breast cancer

Radioresistance is one of the barriers to developing more effective therapies against the most aggressive, triple-negative, breast cancer (TNBC) subtype. In our previous studies, we showed that inhibition of Polo-like Kinase 4 (PLK4) by a novel drug, CFI-400945 significantly enhances the anticancer effects of radiotherapy (RT) compared to single treatment alone. Here we further investigate the role of PLK4 in enhancing radiation effects in TNBC and explore mechanisms of PLK4 inhibition and radiation combinatorial antiproliferative effects. To assess cellular proliferation in response to treatments, we used colony formation assays in TNBC cell lines and patient-derived organoids (PDOs). Downregulation of PLK4 expression was achieved using siRNA silencing in TNBC cell lines. Immunofluorescence against centrin was used to assess the alteration of centriole amplification in response to treatments. We observed that inhibition of PLK4 by CFI-400945 or Centrinone B or its downregulation by siRNA, when combined with RT, resulted in a significant increase in antiproliferative effect in TNBC cells lines and PDOs compared to untreated or single-treated cells. Anticancer synergy was observed using a response matrix in PDOs treated with CFI-400945 and RT. We show that the overamplification of centrioles might be involved in the combined antiproliferative action of RT and PLK4 inhibition. Our data suggest that PLK4 is a promising target for enhancing the anticancer effects of RT in TNBC that, at least in part, is modulated by the overamplification of centrioles. These results support further mechanistic and translational studies of anti-PLK4 agents and RT as an anticancer combination treatment strategy.

FOXM1 Participates in Trophoblast Migration and Early Trophoblast Invasion: Potential Role in Blastocyst Implantation

Successful implantation requires coordinated migration and invasion of trophoblast cells into a receptive endometrium. Reduced forkhead box M1 (FOXM1) expression limits trophoblast migration and angiogenesis in choriocarcinoma cell lines, and in a rat model, placental FOXM1 protein expression was significantly upregulated in the early stages of pregnancy compared to term pregnancy. However, the precise role of FOXM1 in implantation events remains unknown. By analyzing mice blastocysts at embryonic day (E3.5), we have demonstrated that FOXM1 is expressed as early as the blastocyst stage, and it is expressed in the trophectoderm of the blastocyst. Since controlled oxygen tension is determinant for achieving normal implantation and placentation and a chronic hypoxic environment leads to shallow trophoblast invasion, we evaluated if FOXM1 expression changes in response to different oxygen tensions in the HTR-8/SVneo first trimester human trophoblast cell line and observed that FOXM1 expression was significantly higher when trophoblast cells were cultured at 3% O2, which coincides with oxygen concentrations in the uteroplacental interface at the time of implantation. Conversely, FOXM1 expression diminished in response to 1% O2 that resembles a hypoxic environment in utero. Migration and angiogenesis were assessed following FOXM1 knockdown and overexpression at 3% O2 and 1% O2, respectively, in HTR-8/SVneo cells. FOXM1 overexpression increased transmigration ability and tubule formation. Using a 3D trophoblast invasion model with trophospheres from HTR-8/SVneo cells cultured on a layer of MATRIGEL and of mesenchymal stem cells isolated from menstrual fluid, we observed that trophospheres obtained from 3D trophoblast invasion displayed higher FOXM1 expression compared with pre-invasion trophospheres. Moreover, we have also observed that FOXM1-overexpressing trophospheres increased trophoblast invasion compared with controls. HTR-8/SVneo-FOXM1-depleted cells led to a downregulation of PLK4, VEGF, and MMP2 mRNA expression. Our current findings suggest that FOXM1 participates in embryo implantation by contributing to trophoblast migration and early trophoblast invasion, by inducing transcription activation of genes involved in these processes. Maternal-fetal communication is crucial for trophoblast invasion, and maternal stromal cells may induce higher levels of FOXM1 in trophoblast cells.

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.

Downregulation of Polo-like kinase 4 induces cell apoptosis and G2/M arrest in acute myeloid leukemia

BACKGROUND

Polo-like kinase 4 (PLK4) is a crucial regulator for centriole replication and is reported to be aberrantly expressed in various cancers, where it participates to tumorigenesis. However, PLK4 effect in acute myeloid leukemia (AML), is still uncertain. This study investigates the function of PLK4 in AML.

METHODS

Quantitative real-time PCR was used to measure the level of PLK4. Centrinone, a selective PLK4 small molecule inhibitor, was used for PLK4 inhibition and explore its effect in AML cells. The cell growth was detected by the CCK8, while the cell cycle and apoptosis were assessed by flow cytometry. The level of proteins associated with apoptosis, cell cycle and endoplasmic reticulum (ER) stress were analyzed by western blotting.

RESULTS

PLK4 was overexpressed in AML cells. PLK4 knockdown or its specific inhibition by centrinone induced G2/M phase arrest via suppressing the expression of cyclin B1 and Cdc2 and promoting the level of proapoptotic proteins. Moreover, PLK4 targeting enhanced the level of proteins related to ER stress, such as GRP78, ATF4, ATF6, and CHOP.

CONCLUSION

These findings demonstrated that targeting PLK4 can induce apoptosis, G2/M and ER stress in AML cells.

PLK4 initiates crosstalk between cell cycle, cell proliferation and macrophages infiltration in gliomas

Tumor immune microenvironment plays an important role in tumorigenesis and metastasis. Polo-like kinases 4 (PLK4) is a crucial regulatory factor in the process of cell cycle, and its abnormal regulation often leads to a variety of diseases including tumorigenesis. We have previously explored the function of PLK4 in sensitizing chemotherapy in glioma, but there are few studies on the correlation between PLK4 and tumor immune microenvironment. PLK4 was found to be highly expressed in various types of cancers, including glioma and closely related to histological and genetic features in public databases. Kaplan-Meier survival analysis and Cox regression analysis revealed that higher PLK4 expression is associated with poorer prognosis. GO and KEGG functional enrichment analysis showed that PLK4 expression level was significantly correlated with regulation of immune microenvironment, cell cycle and genomic instability. Immune infiltration analysis showed that high expression of PLK4 resulted in reduced infiltration of macrophages. M1 macrophage infiltration assays showed that PLK4 knockdown GBM cell lines promoted the recruitment of M1-type macrophages via altering expression of chemokines. And in intracranial tumor mouse models, PLK4 inhibition increased tumor-infiltrating M1 macrophages. In summary, our results demonstrated the correlation between high PLK4 expression level and malignant progression of gliomas, and the possible involvement of PLK4 in regulation of cell cycle, cell proliferation and macrophages infiltration in gliomas.

Downregulation of PLK4 expression induces apoptosis and G0/G1-phase cell cycle arrest in keloid fibroblasts

Objectives

Keloids are benign fibroproliferative tumors that display many cancer‐like characteristics, such as progressive uncontrolled growth, lack of spontaneous regression, and extremely high rates of recurrence. Polo‐like kinase 4 (PLK4) was recently identified as a master regulator of centriole replication, and its aberrant expression is closely associated with tumorigenesis. This study aimed to investigate the expression and biological role of PLK4 in the pathogenesis of keloids.

Materials and Methods

We evaluated the expression of PLK4 in keloids and adjacent normal skin tissue samples. Then, we established PLK4 knockdown and overexpression cell lines in keloid fibroblasts (KFs) and normal skin fibroblasts (NFs), respectively, to investigate the roles of PLK4 in the regulation of proliferation, migration, invasion, apoptosis, and cell cycle in KFs. Centrinone B (Cen‐B), a highly selective PLK4 inhibitor, was used to inhibit PLK4 activity in KFs to evaluate the therapeutic effect on KFs.

Results

We discovered that PLK4 was overexpressed in keloid dermal samples and KFs compared with adjacent normal skin samples and NFs derived from the same patients. High PLK4 expression was positively associated with the proliferation, migration, and invasion of KFs. Furthermore, knockdown of PLK4 expression or inhibition of PLK4 activity by Cen‐B suppressed KF growth, induced KF apoptosis via the caspase‐9/3 pathway, and induced cell cycle arrest at the G0/G1 phase in vitro.

Conclusions

These findings demonstrate that PLK4 is a critical regulator of KF proliferation, migration, and invasion, and thus, Cen‐B is a promising candidate drug for keloid treatment.

Genetic Basis for Resistance Against Viral Nervous Necrosis: GWAS and Potential of Genomic Prediction Explored in Farmed European Sea Bass (Dicentrarchus labrax)

Viral nervous necrosis (VNN) is an infectious disease caused by the red-spotted grouper nervous necrosis virus (RGNNV) in European sea bass and is considered a serious concern for the aquaculture industry with fry and juveniles being highly susceptible. To understand the genetic basis for resistance against VNN, a survival phenotype through the challenge test against the RGNNV was recorded in populations from multiple year classes (YC2016 and YC2017). A total of 4,851 individuals from 181 families were tested, and a subset (n∼1,535) belonging to 122 families was genotyped using a ∼57K Affymetrix Axiom array. The survival against the RGNNV showed low to moderate heritability with observed scale estimates of 0.18 and 0.25 obtained using pedigree vs. genomic information, respectively. The genome-wide association analysis showed a strong signal of quantitative trait loci (QTL) at LG12 which explained ∼33% of the genetic variance. The QTL region contained multiple genes (ITPK1, PLK4, HSPA4L, REEP1, CHMP2, MRPL35, and SCUBE) with HSPA4L and/or REEP1 genes being highly relevant with a likely effect on host response in managing disease-associated symptoms. The results on the accuracy of predicting breeding values presented 20–43% advantage in accuracy using genomic over pedigree-based information which varied across model types and applied validation schemes.

Fraxetin down-regulates polo-like kinase 4 (PLK4) to inhibit proliferation, migration and invasion of prostate cancer cells through the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) pathway

Fraxetin, a natural product isolated from herb Cortex Fraxini, has been demonstrated to exhibit anti-cancer effects on various cancers. The aim of this work is to investigate the anti-tumor effect of Fraxetin in prostate cancer and the potential mechanisms. In this study, the prostatic epithelial cell RWPE-1 and prostate cancer cell DU145 were exposed to Fraxetin (10, 20, 40, and 80 μM) to detect the changes in cell viability using cell counting kit-8 (CCK-8) assay. Fraxetin (10, 20, and 40 μM) was utilized to treat DU145 cell, then the changes in cell proliferation, apoptosis, migration, and invasion were assessed. Western blot assay was employed to detect the expression of proteins that participate in the above cellular processes as well as Polo-like kinase 4 (PLK4), phosphatidylinositol 3-kinase (PI3K). In addition to 40 μM Fraxetin treatment, DU145 cells were overexpressed with PLK4, and then the above experiments were repeated. Results revealed that Fraxetin markedly decreased DU145 cell viability, but didn’t affect the cell viability of RWPE-1. Fraxetin suppressed cell proliferation, migration, invasion, and induced apoptosis of DU145 cells in a concentration-dependent manner. Furthermore, the expression of PLK4 and phosphorylated PI3K and protein kinase B (Akt) were reduced upon Fraxetin treatment. Finally, PLK4 overexpression significantly reversed all the effects of Fraxetin on DU145 cells. Collectively, Fraxetin acted as a cancer suppressor in prostate cancer through inhibiting PLK4 expression thereby inactivating PI3K/Akt signaling.

Role of Hub Genes in the Occurrence and Development of Testicular Cancer Based on Bioinformatics

Background

Testicular cancer severely affects male health, so finding effective diagnosis and prognostic indicators and exploring its pathogenesis are very important.

Purpose

This study aims to explore the hub genes that play important roles in the occurrence and development of testicular germ cell tumor (TGCT).

Methods

Data were obtained from Gene Expression Omnibus datasets (GSE3218 and GSE1818) and verified in The Cancer Genome Atlas database and the Genotype-Tissue Expression database and the Human Protein Atlas database. A protein–protein interaction network was constructed to obtain hub genes. GEO2R, R software and packages were used to analyze differentially expressed genes (DEGs), receiver operating characteristic curve assessment, Cox regression analysis, Kaplan–Meier survival curve assessment, Gene Ontology analysis, Kyoto Encyclopedia of Genes and Genomes analysis, the relationship with clinicopathological information, gene set enrichment analysis, the correlation with immune cells’ infiltration, and the expression in pan-cancers of the hub genes.

Results

PLK4, TRIP13, TPR, KIF18A, CDKN3, HMMR, PBK, PTTG1, CKS2, SYCP1, HSPA2, and MKI67 were selected as the hub genes. mRNA of PLK4, TRIP13, CDKN3, SYCP1, HSPA2, and MKI67 had high diagnostic values, and higher expression of CDKN3 and HSPA2 mRNA were poor prognostic factors for progression-free interval of TGCT. The hub genes involved organelle division and cell cycle, chromosome and centromeric region, heat shock protein binding, and more. Downregulated TPR and PLK4 were selected as research targets for continued study, and they may participate in multiple signaling pathways. The expression of TPR and PLK4 correlated with the infiltration of a variety of immune cells and differed in pan-cancers.

Conclusion

The mRNA levels of multiple hub genes have high diagnostic and prognostic values for TGCT. TPR and PLK4 may play a role in the occurrence and development of TGCT through cancer-related signaling pathways.

Integration of Genomic Profiling and Organoid Development in Precision Oncology

Precision oncology involves an innovative personalized treatment strategy for each cancer patient that provides strategies and options for cancer treatment. Currently, personalized cancer medicine is primarily based on molecular matching. Next-generation sequencing and related technologies, such as single-cell whole-transcriptome sequencing, enable the accurate elucidation of the genetic landscape in individual cancer patients and consequently provide clinical benefits. Furthermore, advances in cancer organoid models that represent genetic variations and mutations in individual cancer patients have direct and important clinical implications in precision oncology. This review aimed to discuss recent advances, clinical potential, and limitations of genomic profiling and the use of organoids in breast and ovarian cancer. We also discuss the integration of genomic profiling and organoid models for applications in cancer precision medicine.

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.