The family of polo-like kinases

Identification of tumor antigens and immunogenic cell death-related subtypes for the improvement of immunotherapy of breast cancer

The current immunotherapy strategy for breast cancer is limited. Tumor neoantigens have been proven to be a promising biomarker and potential target of immunotherapy in a variety of tumors. However, their effectiveness for breast cancer remains unclear. Immunogenic cell death (ICD) is a regulated form of cell death that can reshape the tumor immune microenvironment and activate adaptive immune responses. To this end, we screened potential antigens that could be used both for the development of immunotherapy and differentiating the patient-specific immune responses based on ICD-related risk signatures, in order to formulate an accurate scheme for breast cancer immunotherapy. We retrieved the gene expression profiles of the breast invasive cancer cohort and their corresponding clinical control data from The Cancer Genome Atlas. The Gene Expression Profiling Interactive Analysis (GEPIA) database was used to evaluate tumor antigen expression, the cBioPortal program was used to identify genetic variations, and the TIMER website was used to estimate the immune infiltration signatures. The risk score predictive model based on the ICD-related genes was constructed using the least absolute shrinkage and selection operator (LASSO) Cox regression algorithm, and the cohort was divided into low- and high-risk score groups. Two tumor antigens, namely, CCNE1 and PLK1, were associated with poor prognosis and infiltration of antigen-presenting cells. Furthermore, the ICD-related risk signature could significantly predict survival outcomes. The risk groups based on the ICD-related signature predictive model showed diverse immune infiltration and molecular and clinical features. The high-risk group was associated with low immune cell infiltration, immune score, expression of immune checkpoints, and human leukocyte antigen genes but high levels of CCNE1 and PLK1 and poor survival outcome. In conclusion, CCNE1 and PLK1 were identified as potential antigens in breast cancer. The ICD-related prognostic model distinguished immune response heterogeneity and predicted prognosis. Patients with high ICD-related risk scores were suitable to receive combination treatments based on CCNE1 or PLK1 and immune checkpoint inhibitors. In the future, these results will help us develop more accurate treatment schemes for patients with breast cancer.

The DDR-related gene signature with cell cycle checkpoint function predicts prognosis, immune activity, and chemoradiotherapy response in lung adenocarcinoma

BACKGROUND

As a DNA surveillance mechanism, cell cycle checkpoint has recently been discovered to be closely associated with lung adenocarcinoma (LUAD) prognosis. It is also an essential link in the process of DNA damage repair (DDR) that confers resistance to radiotherapy. Whether genes that have both functions play a more crucial role in LUAD prognosis remains unclear.

METHODS

In this study, DDR-related genes with cell cycle checkpoint function (DCGs) were selected to investigate their effects on the prognosis of LUAD. The TCGA-LUAD cohort and two GEO external validation cohorts (GSE31210 and GSE42171) were performed to construct a prognosis model based on the least absolute shrinkage and selection operator (LASSO) regression. Patients were divided into high-risk and low-risk groups based on the model. Subsequently, the multivariate COX regression was used to construct a prognostic nomogram. The ssGSEA, CIBERSORT algorithm, TIMER tool, CMap database, and IC50 of chemotherapeutic agents were used to analyze immune activity and responsiveness to chemoradiotherapy.

RESULTS

4 DCGs were selected as prognostic signatures, and patients in the high-risk group had a lower overall survival (OS). The lower infiltration levels of immune cells and the higher expression levels of immune checkpoints appeared in the high-risk group. The damage repair pathways were upregulated, and chemotherapeutic agent sensitivity was poor in the high-risk group.

CONCLUSIONS

The 4-DCGs signature prognosis model we constructed could predict the survival rate, immune activity, and chemoradiotherapy responsiveness of LUAD patients.

Vaccinia-related kinase 2 drives pancreatic cancer progression by protecting Plk1 from Chfr-mediated degradation

As a key cell cycle regulator, polo-like kinase 1 (Plk1) has been recognized as a crucial factor involved in the progression of pancreatic cancer (PC). However, its regulatory mechanism is poorly understood. Here, we present evidence that Plk1 is a novel substrate of vaccinia-related kinase 2 (VRK2), a serine-threonine kinase that is highly expressed and predicts poor prognosis in PC. VRK2 phosphorylates Plk1 at threonine 210 and protects it from ubiquitin-dependent proteasomal degradation. We showed that mechanistically complement factor H-related protein (CFHR), as a major E3 ligase, promotes Plk1 degradation by ubiquitinating it at lysine 209. Phosphorylation of Plk1 at threonine 210 by VRK2 interferes with the interaction of Chfr with Plk1 and antagonizes Plk1 ubiquitination, thereby stabilizing the Plk1 protein. Taken together, our data reveal a mechanism of Plk1 overexpression in PC and provide evidence for targeting VRK2 as a potential therapeutic strategy.

Principal Postulates of Centrosomal Biology. Version 2020

The centrosome, which consists of two centrioles surrounded by pericentriolar material, is a unique structure that has retained its main features in organisms of various taxonomic groups from unicellular algae to mammals over one billion years of evolution. In addition to the most noticeable function of organizing the microtubule system in mitosis and interphase, the centrosome performs many other cell functions. In particular, centrioles are the basis for the formation of sensitive primary cilia and motile cilia and flagella. Another principal function of centrosomes is the concentration in one place of regulatory proteins responsible for the cell's progression along the cell cycle. Despite the existing exceptions, the functioning of the centrosome is subject to general principles, which are discussed in this review.

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

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

Exploration of the Combination of PLK1 Inhibition with Immunotherapy in Cancer Treatment

Background

PLK1 overexpression is oncogenic and is associated with poor prognosis in various cancers. However, the current PLK1 inhibitors have achieved limited clinical successes. On the other hand, although immunotherapies are demonstrating efficacy in treating many refractory cancers, a substantial number of patients do not respond to such therapies. The potential of combining PLK1 inhibition with immunotherapy for cancer treatment is worthy of exploration.

Methods

We analyzed the associations of PLK1 expression with tumor immunity in 33 different cancer types. Moreover, we analyzed the associations of the drug sensitivities of PLK1 inhibitors with tumor immunity in cancer cell lines. Furthermore, we explored the mechanism underlying the significant associations between PLK1 and tumor immunity. Finally, we experimentally verified some findings from bioinformatics analysis.

Results

The cancers with higher PLK1 expression levels tended to have lower immune activities, such as lower HLA expression and decreased B cells, NK cells and tumor-infiltrating lymphocytes infiltration. On the other side, elevated tumor immunity likely increased the sensitivity of cancer cells to PLK1 inhibitors. The main mechanism underlying the associations between PLK1 and tumor immunity may lie in the aberrant cell cycle and p53 pathways in cancers.

Conclusions

Our findings implicate that the PLK1 inhibition and immunotherapy combination may achieve a synergistic antitumor efficacy.

Targeting oncogenic Myc as a strategy for cancer treatment

The MYC family oncogene is deregulated in >50% of human cancers, and this deregulation is frequently associated with poor prognosis and unfavorable patient survival. Myc has a central role in almost every aspect of the oncogenic process, orchestrating proliferation, apoptosis, differentiation, and metabolism. Although Myc inhibition would be a powerful approach for the treatment of many types of cancers, direct targeting of Myc has been a challenge for decades owing to its "undruggable" protein structure. Hence, alternatives to Myc blockade have been widely explored to achieve desirable anti-tumor effects, including Myc/Max complex disruption, MYC transcription and/or translation inhibition, and Myc destabilization as well as the synthetic lethality associated with Myc overexpression. In this review, we summarize the latest advances in targeting oncogenic Myc, particularly for cancer therapeutic purposes.

PLK1, A Potential Target for Cancer Therapy

Polo-like kinase 1 (PLK1) plays an important role in the initiation, maintenance, and completion of mitosis. Dysfunction of PLK1 may promote cancerous transformation and drive its progression. PLK1 overexpression has been found in a variety of human cancers and was associated with poor prognoses in cancers. Many studies have showed that inhibition of PLK1 could lead to death of cancer cells by interfering with multiple stages of mitosis. Thus, PLK1 is expected to be a potential target for cancer therapy. In this article, we examined PLK1’s structural characteristics, its regulatory roles in cell mitosis, PLK1 expression, and its association with survival prognoses of cancer patients in a wide variety of cancer types, PLK1 interaction networks, and PLK1 inhibitors under investigation. Finally, we discussed the key issues in the development of PLK1-targeted cancer therapy.

Attenuated monocyte apoptosis, a new mechanism for osteoporosis suggested by a transcriptome-wide expression study of monocytes

BACKGROUND

Osteoporosis is caused by excessive bone resorption (by osteoclasts) over bone formation (by osteoblasts). Monocytes are important to osteoporosis by serving as progenitors of osteoclasts and produce cytokines for osteoclastogenesis.

AIM

To identify osteoporosis-related genes, we performed microarray analyses of monocytes using Affymetrix 1.0 ST arrays in 42 (including 16 pre- and 26 postmenopausal) high hip BMD (bone mineral density) vs. 31 (including 15 pre- and 16 postmenopausal) low hip BMD Caucasian female subjects. Here, high vs. low BMD is defined as belonging to top vs. bottom 30% of BMD values in population.

METHOD

Differential gene expression analysis in high vs. low BMD subjects was conducted in the total cohort as well as pre- and post-menopausal subjects. Focusing on the top differentially expressed genes identified in the total, the pre- and the postmenopausal subjects (with a p <5E-03), we performed replication of the findings in 3 independent datasets of microarray analyses of monocytes (total N = 125).

RESULTS

We identified (in the 73 subjects) and successfully replicated in all the 3 independent datasets 2 genes, DAXX and PLK3. Interestingly, both genes are apoptosis induction genes and both down-regulated in the low BMD subjects. Moreover, using the top 200 genes identified in the meta-analysis across all of the 4 microarray datasets, GO term enrichment analysis identified a number of terms related to induction of apoptosis, for which the majority of component genes are also down-regulated in the low BMD subjects. Overall, our result may suggest that there might be a decreased apoptosis activity of monocytes in the low BMD subjects.

CONCLUSION

Our study for the first time suggested a decreased apoptosis rate (hence an increased survival) of monocytes, an important osteoclastogenic cell, as a novel mechanism for osteoporosis.

The scaffold protein TANK/I-TRAF inhibits NF-kappaB activation by recruiting polo-like kinase 1

TANK/I-TRAF is a TRAF-binding protein that negatively regulates NF-kappaB activation. The underlying mechanism of this activity remains unclear. Here we show that TANK directly interacts with PLK1, a conserved cell cycle-regulated kinase. PLK1 inhibits NF-kappaB transcriptional activation induced by TNF-alpha, IL-1beta, or several activators, but not by nuclear transcription factor p65. PLK1 expression reduces the DNA-binding activity of NF-kappaB induced by TNF-alpha. Moreover, endogenous activation of PLK1 reduces the TNF-induced phosphorylation of endogenous IkappaBalpha. PLK1 is bound to NEMO (IKKgamma) through TANK to form a ternary complex in vivo. We describe a new regulatory mechanism for PLK1: PLK1 negatively regulates TNF-induced IKK activation by inhibiting the ubiquitination of NEMO. These findings reveal that the scaffold protein TANK recruits PLK1 to negatively regulate NF-kappaB activation and provide direct evidence that PLK1 is required for the repression function of TANK.

Plx1 is required for chromosomal DNA replication under stressful conditions

Polo-like kinase (Plk)1 is required for mitosis progression. However, although Plk1 is expressed throughout the cell cycle, its function during S-phase is unknown. Using Xenopus laevis egg extracts, we demonstrate that Plx1, the Xenopus orthologue of Plk1, is required for DNA replication in the presence of stalled replication forks induced by aphidicolin, etoposide or reduced levels of DNA-bound Mcm complexes. Plx1 binds to chromatin and suppresses the ATM/ATR-dependent intra-S-phase checkpoint that inhibits origin firing. This allows Cdc45 loading and derepression of DNA replication initiation. Checkpoint activation increases Plx1 binding to the Mcm complex through its Polo box domain. Plx1 recruitment to chromatin is independent of checkpoint mediators Tipin and Claspin. Instead, ATR-dependent phosphorylation of serine 92 of Mcm2 is required for the recruitment of Plx1 to chromatin and for the recovery of DNA replication under stress. Depletion of Plx1 leads to accumulation of chromosomal breakage that is prevented by the addition of recombinant Plx1. These data suggest that Plx1 promotes genome stability by regulating DNA replication under stressful conditions.

Inhibitory effect of Polo-like kinase 1 depletion on mitosis and apoptosis of gastric cancer cells

AIM

Polo-like kinase 1 (PLK1) serine/threonine kinase plays a vital role in multiple phases of mitosis in gastric cancer cells. To investigate the effect of PLK1 depletion on mitosis and apoptosis of gastric cancer cells.

METHODS

PLK1 expression was blocked by small RNA interference(siRNA). The expression levels of PLK1, cdc2, cyclin B and caspase 3 were detected by Western blotting. Then, PLK1 depletion, cdc2 activity, cell proliferation, cell cycle phase distribution, mitotic spindle structure, and the rate of apoptosis of the PLK1 knockdown cells were observed.

RESULTS

PLK1 gene knockdown was associated with increased cyclin B expression, increased cdc2 activity (but not with the expression levels), accumulation of gastric cancer cells at G2/M, improper mitotic spindle formation, delayed chromosome separation and delayed or arrested cytokinesis. Moreover, PLK1 depletion in gastric cancer cells was associated with decreased proliferation, attenuated pro-caspase 3 levels and increased apoptosis.

CONCLUSION

Blockage of PLK1 expression may lead to decreased mitosis or even apoptosis in gastric cancer cells, indicating that PLK1 may be a valuable therapeutic target for gastric cancer.

Silencing of polo-like kinase (Plk) 1 via siRNA causes induction of apoptosis and impairment of mitosis machinery in human prostate cancer cells: implications for the treatment of prostate cancer

Prostate cancer (PCa) is one of the most common cancers in men. Each year approximately 543,000 new cases are reported worldwide, and the disease kills 200,000 (mostly older men) in developed countries. The existing treatment approaches and surgical intervention have not been able to effectively manage this dreaded cancer and, therefore, continuing efforts are ongoing to explore novel targets and strategies for the management of PCa. The activity of polo-like kinase 1 (Plk1) is elevated in tissues and cells with a high mitotic index, including cancer cells. An increasing body of evidence suggests that the level of Plk1 expression has prognostic value for predicting outcomes in patients with some cancers. A close correlation between Plk1 expression and carcinogenesis has been documented. However, the role of Plk1 in PCa is not known. We propagated a hypothesis that Plk1 inhibition will result in elimination of human PCa cells via a mitotic arrest followed by apoptosis (1). To define the role of Plk1 in PCa, we used the technique of RNA silencing via small interfering RNA (siRNA). First, using a series of human prostate carcinoma cells and normal human prostate epithelial (PrEC) cells, we assessed Plk1 levels in PCa. Immunoblot analyses clearly showed a significant expression of Plk1 in LNCaP, DU145, and PC3 human PCa cells. Interestingly, Plk1 was not detectable in normal PrEC cells. Next, we transfected the PCa cells with Plk 1 siRNA, which resulted in a significant inhibition in Plk1 protein in all PCa cells. Plk1 depletion resulted in a decrease in cell viability and induction of apoptosis in PCa cells but had no appreciable effect in normal PrEC cells. Our data also demonstrated that Plk1 siRNA transfection of PCa cells resulted in 1) a mitotic cell cycle arrest, 2) failure of cytokinesis, and 3) defects in centrosome integrity and maturation. Thus, our study suggested that 1) Plk1 plays a critical role in the process of PCa development and 2) gene therapeutic approaches aimed at Plk1 or the pharmacological inhibitors of Plk1 may be developed for the management of PCa.

Role of Plk2 (Snk) in mouse development and cell proliferation

Plk2 (Snk) is a polo-like kinase expressed at G(1) in cultured cells and mainly in the hippocampal neurons in the brains of adult rodents, but its function is poorly understood. We have generated mice deficient in Plk2 by gene targeting. Although Plk2 is not required for postnatal growth, Plk2(-/-) embryos show retarded growth and skeletal development late in gestation. The labyrinthine zone of the placenta is diminished in Plk2(-/-) embryos due to decreased cell proliferation. Cultured Plk2(-/-) embryonic fibroblasts grow more slowly than normal cells and show delayed entry into S phase. These data suggest a role for Plk2 in the cell cycle.

Targeting the cell cycle for cancer therapy

Most if not all neoplasias show a directly or indirectly deregulated cell cycle. Targeting its regulatory molecules, the cyclin-dependent kinases, as a therapeutic mode to develop new anticancer drugs, is being currently explored in both academia and pharmaceutical companies. The development of new compounds is being focused on the many features of the cell cycle with promising preclinical data in most fields. Moreover, a few compounds have entered clinical trials with excellent results maintaining the high hopes. Thus, although too early to provide a cell cycle target based new commercial drug, there is no doubt that it will be an excellent source of new anticancer compounds.

Identification and cloning of Lmairk, a member of the Aurora/Ipl1p protein kinase family, from the human protozoan parasite Leishmania

Lmairk, a gene encoding a member of the Aurora/Ipl1p family of protein kinases (AIRK), was cloned from the protozoan parasite Leishmania major. Aurora kinases are key enzymes involved in the regulation of normal chromosome segregation during mitosis and cytokenesis of eukaryotic cells. This single-copy gene located on L. major chromosome 28 encodes a 301 amino acid polypeptide. All 11 conserved eukaryotic protein kinase catalytic subdomains are present and the proposed AIRK signature sequence was identified in the activation loop between subdomains VII and VIII. Lmairk is expressed, as an approximately 2.4 kb message, in at least three different species of Leishmania. This report represents the first identification of an AIRK from the trypanosomatid family of early divergent eukaryotes.

Increased human polo-like kinase-1 expression in gliomas

PLK-1 (polo-like kinase) belongs to the family of serine/threonine kinases and is involved in spindle formation, centrosome cycles and chromosome segregation. Hence, the kinase is tightly linked to cell proliferation. We could detect immunohistochemically highly expressed PLK protein in astrocytic tumours depending on the grade of anaplasia, in commercially available human glioma cell lines (U87MG, U118MG, U138MG), in one immortalized cell culture derived from a glioblastoma patient and in a primary culture derived from a glioblastoma patient. The highest labelling of PLK-1 was demonstrated in glioblastomas. There was a significant correlation between the PLK expression and the nuclear immunoreactivity of MIB-1. PLK-mRNA, found in all tumour specimens investigated emphasizes the close correlation to proliferation and growth. Furthermore, the relation of the PLK-1 expression to the Mitogen-activated Protein Kinase Cascades was studied by applying various highly specific inhibitors. While all inhibitors minimized the cell density, only the PLCy inhibitor clearly lead to a reduced PLK-1 expression in the three cell lines U87MG, U118MG, U138MG.

The disappearance of cyclin B at the end of mitosis is regulated spatially in Drosophila cells

We have followed the behaviour of a cyclin B-green fluorescent protein (GFP) fusion protein in living Drosophila embryos in order to study how the localization and destruction of cyclin B is regulated in space and time. We show that the fusion protein accumulates at centrosomes in interphase, in the nucleus in prophase, on the mitotic spindle in prometaphase and on the microtubules that overlap in the middle of the spindle in metaphase. In cellularized embryos, toward the end of metaphase, the spindle-associated cyclin B-GFP disappears from the spindle in a wave that starts at the spindle poles and spreads to the spindle equator; when the cyclin B-GFP on the spindle is almost undetectable, the chromosomes enter anaphase, and any remaining cytoplasmic cyclin B-GFP then disappears over the next few minutes. The endogenous cyclin B protein appears to behave in a similar manner. These findings suggest that the inactivation of cyclin B is regulated spatially in Drosophila cells. We show that the anaphase-promoting complex/cyclosome (APC/C) specifically interacts with microtubules in embryo extracts, but it is not confined to the spindle in mitosis, suggesting that the spatially regulated disappearance of cyclin B may reflect the spatially regulated activation of the APC/C.