Upregulation of Polo-like kinase 2 gene expression by GATA-1 acetylation in human osteosarcoma MG-63 cells

Polo-Like Kinase 2: From Principle to Practice

Polo-like kinase (PLK) 2 is an evolutionarily conserved serine/threonine kinase that shares the n-terminal kinase catalytic domain and the C-terminal Polo Box Domain (PBD) with other members of the PLKs family. In the last two decades, mounting studies have focused on this and tried to clarify its role in many aspects. PLK2 is essential for mitotic centriole replication and meiotic chromatin pairing, synapsis, and crossing-over in the cell cycle; Loss of PLK2 function results in cell cycle disorders and developmental retardation. PLK2 is also involved in regulating cell differentiation and maintaining neural homeostasis. In the process of various stimuli-induced stress, including oxidative and endoplasmic reticulum, PLK2 may promote survival or apoptosis depending on the intensity of stimulation and the degree of cell damage. However, the role of PLK2 in immunity to viral infection has been studied far less than that of other family members. Because PLK2 is extensively and deeply involved in normal physiological functions and pathophysiological mechanisms of cells, its role in diseases is increasingly being paid attention to. The effect of PLK2 in inhibiting hematological tumors and fibrotic diseases, as well as participating in neurodegenerative diseases, has been gradually recognized. However, the research results in solid organ tumors show contradictory results. In addition, preliminary studies using PLK2 as a disease predictor and therapeutic target have yielded some exciting and promising results. More research will help people better understand PLK2 from principle to practice.

Selective Targeting of Class I Histone Deacetylases in a Model of Human Osteosarcoma

Dysregulation of histone deacetylases (HDACs) is associated with the pathogenesis of human osteosarcoma, which may present an epigenetic vulnerability as well as a therapeutic target. Domatinostat (4SC-202) is a next-generation class I HDAC inhibitor that is currently being used in clinical research for certain cancers, but its impact on human osteosarcoma has yet to be explored. In this study, we report that 4SC-202 inhibits osteosarcoma cell growth in vitro and in vivo. By analyzing cell function in vitro, we show that the anti-tumor effect of 4SC-202 involves the combined induction of cell-cycle arrest at the G2/M phase and apoptotic program, as well as a reduction in cell invasion and migration capabilities. We also found that 4SC-202 has little capacity to promote osteogenic differentiation. Remarkably, 4SC-202 revised the global transcriptome and induced distinct signatures of gene expression in vitro. Moreover, 4SC-202 decreased tumor growth of established human tumor xenografts in immunodeficient mice in vivo. We further reveal key targets regulated by 4SC-202 that contribute to tumor cell growth and survival, and canonical signaling pathways associated with progression and metastasis of osteosarcoma. Our study suggests that 4SC-202 may be exploited as a valuable drug to promote more effective treatment of patients with osteosarcoma and provide molecular insights into the mechanism of action of class I HDAC inhibitors.

Circular RNA 0102049 suppresses the progression of osteosarcoma through modulating miR-520g-3p/PLK2 axis

Circular RNAs (circRNAs) are a type of non-coding RNAs generated from back splicing to enhance or inhibit the progression of multiple human cancers including osteosarcoma (OS). Although circ_0102049 has been found to be highly expressed in OS cell lines, the role and specific mechanism of circ_0102049 in OS remains unclear. Here, we found that silence of circ_0102049 could significantly exacerbate the tumorigenesis of OS in vivo through sponging microRNA-520g-3p. Polo-like kinase 2 (PLK2) was predicted to be a target of miR-520g-3p, and luciferase reporter assay revealed that overexpression of miR-520g-3p dramatically suppressed the expression of PLK2, whereas miR-520g-3p inhibitor promoted the PLK2 expression. Moreover, the silence of circ_0102049 could markedly promote the proliferation, invasion, migration and cell-cycle promotion while inhibiting the apoptosis of OS cell line MG63 cells in vitro through regulating miR-520g-3p/PLK2 axis. Taken together, the present study indicated that circ_0102049 suppressed the progression of osteosarcoma via modulating miR-520g-3p/PLK2/TAp73 axis, providing a potential therapeutic target for OS.

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.

Low expression or hypermethylation of PLK2 might predict favorable prognosis for patients with glioblastoma multiforme

Background

As the most aggressive brain tumor, patients with glioblastoma multiforme (GBM) have a poor prognosis. Our purpose was to explore prognostic value of Polo-like kinase 2 (PLK2) in GBM, a member of the PLKs family.

Methods

The expression profile of PLK2 in GBM was obtained from The Cancer Genome Atlas database. The PLK2 expression in GBM was tested. Kaplan–Meier curves were generated to assess the association between PLK2 expression and overall survival (OS) in patients with GBM. Furthermore, to assess its prognostic significance in patients with primary GBM, we constructed univariate and multivariate Cox regression models. The association between PLK2 expression and its methylation was then performed. Differentially expressed genes correlated with PLK2 were identified by Pearson test and functional enrichment analysis was performed.

Results

Overall survival results showed that low PLK2 expression had a favorable prognosis of patients with GBM (P-value = 0.0022). Furthermore, PLK2 (HR = 0.449, 95% CI [0.243–0.830], P-value = 0.011) was positively associated with OS by multivariate Cox regression analysis. In cluster 5, DNA methylated PLK2 had the lowest expression, which implied that PLK2 expression might be affected by its DNA methylation status in GBM. PLK2 in CpG island methylation phenotype (G-CIMP) had lower expression than non G-CIMP group (P = 0.0077). Regression analysis showed that PLK2 expression was negatively correlated with its DNA methylation (P = 0.0062, Pearson r = −0.3855). Among all differentially expressed genes of GBM, CYGB (r = 0.5551; P < 0.0001), ISLR2 (r = 0.5126; P < 0.0001), RPP25 (r = 0.5333; P < 0.0001) and SOX2 (r = −0.4838; P < 0.0001) were strongly correlated with PLK2. Functional enrichment analysis results showed that these genes were enriched several biological processes or pathways that were associated with GBM.

Conclusion

Polo-like kinase 2 expression is regulated by DNA methylation in GBM, and its low expression or hypermethylation could be considered to predict a favorable prognosis for patients with GBM.

Therapeutic applications of histone deacetylase inhibitors in sarcoma

Sarcomas are a rare group of malignant tumors originating from mesenchymal stem cells. Surgery, radiation and chemotherapy are currently the only standard treatments for sarcoma. However, their response rates to chemotherapy are quite low. Toxic side effects and multi-drug chemoresistance make treatment even more challenging. Therefore, better drugs to treat sarcomas are needed. Histone deacetylase inhibitors (HDAC inhibitors, HDACi, HDIs) are epigenetic modifying agents that can inhibit sarcoma growth in vitro and in vivo through a variety of pathways, including inducing tumor cell apoptosis, causing cell cycle arrest, impairing tumor invasion and preventing metastasis. Importantly, preclinical studies have revealed that HDIs can not only sensitize sarcomas to chemotherapy and radiotherapy, but also increase treatment responses when combined with other chemotherapeutic drugs. Several phase I and II clinical trials have been conducted to assess the efficacy of HDIs either as monotherapy or in combination with standard chemotherapeutic agents or targeted therapeutic drugs for sarcomas. Combination regimen for sarcomas appear to be more promising than monotherapy when using HDIs. This review summarizes our current understanding and therapeutic applications of HDIs in sarcomas.

Up-regulation of IRF-3 expression through GATA-1 acetylation by histone deacetylase inhibitor in lung adenocarcinoma A549 cells

Interferon regulatory factor 3 (IRF-3) is an important transcription factor for interferon genes. Although its functional activation by viral infection has been widely explicated, the regulatory mechanism of IRF-3 gene expression in cancer cells is poorly understood. In this study, we demonstrated treatment of lung adenocarcinoma A549 cells with trichostatin A (TSA) and valproic acid (VPA), two different classes of histone deacetylase inhibitors, strongly stimulated IRF-3 gene expression. Truncated and mutated IRF-3 promoter indicated that a specific GATA-1 element was responsible for TSA-induced activation of IRF-3 promoter. Chromatin immunoprecipitation and electrophoretic mobility shift assay showed that TSA treatment increased the binding affinity of GATA-1 to IRF-3 promoter. Using immunoprecipitation assay and immunoblotting, we demonstrated that TSA increased the level of acetylated GATA-1 in A549 cells. In summary, our study implied that TSA enhanced IRF-3 gene expression through increased GATA-1 recruitment to IRF-3 promoter and the acetylation level of GATA-1 in lung adenocarcinoma A549 cells.

PLK2 phosphorylates and inhibits enriched TAp73 in human osteosarcoma cells

TAp73, a member of the p53 tumor suppressor family, can substitute for p53 function, especially in p53‐null and p53‐mutant cells. However, TAp73 enrichment and phosphorylation change its transcriptional activity. Previously, we found that the antitumor function of TAp73 was reactivated by dephosphorylation. Polo‐like kinase 2 (PLK2) plays an important role in bone development. Using a biological information database and phosphorylation prediction software, we hypothesized that PLK2 phosphorylates TAp73 and inhibits TAp73 function in osteosarcomas. Actually,we determined that PLK2 physically binds to and phosphorylates TAp73 when TAp73 protein abundance is up‐regulated by cisplatin. PLK2‐phosphorylated TAp73 at residue Ser48 within the TA domain; phosphorylation of TAp73 was abolished by mutating this residue. Moreover, PLK2 inhibition combined with cisplatin treatment in osteosarcoma Saos2 cells up‐regulated p21 and puma mRNA expression to a greater extent than cisplatin treatment alone. Inhibiting PLK2 in TAp73‐enriched Saos2 cells resulted in inhibited cell proliferation, increased apoptosis, G1 phase arrest, and decreased cell invasion. However, these changes did not occur in TAp73 knockdown Saos2 cells. In conclusion, these findings reveal a novel PLK2 function in the phosphorylation of TAp73, which prevents TAp73 activity in osteosarcoma cells. Thereby, this research provides an insight into the clinical treatment of malignant tumors overexpressing TAp73.

Polo-like kinase 2 acting as a promoter in human tumor cells with an abundance of TAp73

Background

TAp73, a member of the p53 tumor suppressor family, is frequently overexpressed in malignant tumors in humans. TAp73 abundance and phosphorylation modification result in variations in transcriptional activity. In a previous study, we found that the antitumor function of TAp73 was reactivated by dephosphorylation in head and neck squamous cell carcinomas. Polo-like kinase 2 (PLK2) displayed a close relationship with the p53 family in affecting the fate of cells. Herein, we investigate the hypothesis that PLK2 phosphorylates TAp73 and inhibits TAp73 function.

Materials and methods

Head and neck squamous cell carcinoma cell lines and osteosarcoma cell lines were used as natural models of the different expression levels of TAp73. Phosphorylation predictor software Scansite 3.0 and the predictor GPS-polo 1.0 were used to analyze the phosphorylation sites. Coimmunoprecipitation, phosphor-tag Western blot, metabolic labeling, and indirect immunofluorescence assays were used to determine the interactions between PLK2 and TAp73. TAp73 activity was assessed by Western blot and reverse transcription polymerase chain reaction, which we used to detect P21 and PUMA, both downstream genes of TAp73. The physiological effects of PLK2 cross talk with TAp73 on cell cycle progress and apoptosis were observed by flow cytometry and terminal deoxynucleotidyl transferase dUTP nick end labeling assays.

Results

PLK2 binds to and phosphorylates TAp73. PLK2 phosphorylates TAp73 at residue Ser48 and prohibits TAp73 translocation to the nucleus. Additionally, PLK2 inhibition combined with a DNA-damaging drug upregulated p21 and PUMA mRNA expression to a greater extent than DNA-damaging drug treatment alone. Inhibiting PLK2 in TAp73-enriched cells strengthened the effects of the DNA-damaging drug on both G1 phase arrest and apoptosis. Pretreatment with TAp73-siRNA weakened these effects.

Conclusion

These findings reveal a novel PLK2 function (catalyzed phosphorylation of TAp73) which suppresses TAp73 functions. PLK2 promotes the survival of human tumor cells, a novel insight into the workings of malignant tumors characterized by TAp73 overexpression, and one that could speed the development of therapies.

Establishment and analysis of osteosarcoma cell sublines with different metastatic characteristics

The aim of this study is to establish human osteosarcoma MG-63 cell sublines with different metastatic characteristics and provide good experimental models for mechanism study of osteosarcoma metastasis. Six cell sublines were screened and established by using the in vitro cloning technology. In vitro invasion experiments, cellular electrophoretic mobility determinations, cellular proliferation rate determinations, and soft agar clone formation assays were used to compare, analyze, and identify the metastatic characteristics of various cell sublines. The results indicated that the invasion capacity, cellular electrophoretic mobility, cell proliferation, and soft agar clone formation capacity of A2, A3, and A16 sublines were higher than those of A1, A6, and A20 sublines. There was no significant difference in various values determined among A2, A3, and A16, and among A1, A6, and A20 (P > 0.05), but there were significant differences in various values determined between the former three and the latter three (P < 0.05). In conclusion, MG-63 cell sublines with different metastatic characteristics can be established by combining several technologies and can contribute to further research on the mechanism of osteosarcoma metastasis.

Polo-like kinase 1 inhibition causes decreased proliferation by cell cycle arrest, leading to cell death in glioblastoma

Glioblastoma (GBM) is one of the most aggressive central nervous system tumors with a patient's median survival of <1 year. Polo-like kinases (PLKs) are a family of serine/threonine kinases that have key roles in cell cycle control and DNA-damage response. We evaluated PLK1, 2, 3 and 4 gene expression in 8 GBM cell lines and 17 tumor samples, and analyzed the effect of the PLK1 inhibition on SF188 and T98G GBM cell lines and 13 primary cultures. Our data showed PLK1 overexpression and a variable altered expression of PLK2, 3 and 4 genes in GBM tumor samples and cell lines. Treatments with nanomolar concentrations of BI 2536, BI 6727, GW843682X or GSK461364 caused a significant decrease in GBM cells proliferation. Colony formation was also found to be inhibited (P<0.05), whereas apoptosis rate and mitotic index were significantly increased (P<0.05) after PLK1 inhibition in both GBM cell lines. Cell cycle analysis showed an arrest at G2 (P<0.05) and cell invasion was also decreased after PLK1 inhibition. Furthermore, simultaneous combinations of BI 2536 and temozolomide produced synergistic effects for both the cell lines after 48 h of treatment. Our findings suggest that PLK1 might be a promising target for the treatment of GBMs.