Targeted depletion of Polo-like kinase (Plk) 1 through lentiviral shRNA or a small-molecule inhibitor causes mitotic catastrophe and induction of apoptosis in human melanoma cells

PLK1 inhibition leads to mitotic arrest and triggers apoptosis in cholangiocarcinoma cells

Cholangiocarcinoma (CCA) is a lethal cancer originating from the epithelial cells within the bile duct and ranks as the second most prevalent form of liver cancer in Thailand. Polo-like kinase 1 (PLK1), a protein serine/threonine kinase, regulates a number of steps in cell mitosis and is upregulated in several types of cancer, including CCA. Our previous study identified PLK1 as a biomarker of the C1 subtype, correlating with poor prognosis in intrahepatic CCA. The present study aimed to examine the effect of PLK1 inhibition on CCA cells. Different CCA cell lines developed from Thai patients, HuCCA1, KKU055, KKU100 and KKU213A, were treated with two PLK1 inhibitors, BI2536 and BI6727, and were transfected with small interfering RNA, followed by analysis of cell proliferation, cell cycle distribution and cell apoptosis. It was discovered that BI2536 and BI6727 inhibited cell proliferation and caused G2/M-phase arrest in CCA cells. Furthermore, the number of total apoptotic cells was increased in PLK1 inhibitor-treated CCA cells. The expression levels of mitotic proteins, aurora kinase A, phosphorylated PLK1 (T210) and cyclin B1, were augmented in PLK1-inhibited CCA cells. Additionally, inhibition of PLK1 led to increased DNA damage, as determined by the upregulated levels of γH2AX and increased cleavage of poly (ADP-ribose) polymerase, an apoptotic marker. These results suggested that inhibiting PLK1 prolonged mitotic arrest and subsequently triggered cell apoptosis. Validation of the antiproliferative effects of PLK1 inhibition was accomplished through silencing of the PLK1 gene. In conclusion, targeting PLK1 provided promising results for further study as a potential candidate for targeted therapy in CCA.

Unlocking Mitochondrial Dysfunction-Associated Senescence (MiDAS) with NAD + - a Boolean Model of Mitochondrial Dynamics and Cell Cycle Control

The steady accumulation of senescent cells with aging creates tissue environments that aid cancer evolution. Aging cell states are highly heterogeneous. 'Deep senescent' cells rely on healthy mitochondria to fuel a strong proinflammatory secretome, including cytokines, growth and transforming signals. Yet, the physiological triggers of senescence such as the reactive oxygen species (ROS) can also trigger mitochondrial dysfunction, and sufficient energy deficit to alter their secretome and cause chronic oxidative stress - a state termed Mitochondrial Dysfunction-Associated Senescence (MiDAS). Here, we offer a mechanistic hypothesis for the molecular processes leading to MiDAS, along with testable predictions. To do this we have built a Boolean regulatory network model that qualitatively captures key aspects of mitochondrial dynamics during cell cycle progression (hyper-fusion at the G1/S boundary, fission in mitosis), apoptosis (fission and dysfunction) and glucose starvation (reversible hyper-fusion), as well as MiDAS in response to SIRT3 knockdown or oxidative stress. Our model reaffirms the protective role of NAD + and external pyruvate. We offer testable predictions about the growth factor- and glucose-dependence of MiDAS and its reversibility at different stages of reactive oxygen species (ROS)-induced senescence. Our model provides mechanistic insights into the distinct stages of DNA-damage induced senescence, the relationship between senescence and epithelial-to-mesenchymal transition in cancer and offers a foundation for building multiscale models of tissue aging.

Highlights

Boolean regulatory network model reproduces mitochondrial dynamics during cell cycle progression, apoptosis, and glucose starvation. Model offers a mechanistic explanation for the positive feedback loop that locks in Mitochondrial Dysfunction-Associated Senescence (MiDAS), involving autophagy-resistant, hyperfused, dysfunctional mitochondria. Model reproduces ROS-mediated mitochondrial dysfunction and suggests that MiDAS is part of the early phase of damage-induced senescence. Model predicts that cancer-driving mutations that bypass the G1/S checkpoint generally increase the incidence of MiDAS, except for p53 loss.

PLK1 and its substrate MISP facilitate intrahepatic cholangiocarcinoma progression by promoting lymphatic invasion and impairing E-cadherin adherens junctions

Intrahepatic cholangiocarcinoma (iCCA) is a subtype of CCA and has a high mortality rate and a relatively poor prognosis. However, studies focusing on increased cell motility and loss of epithelial integrity during iCCA progression remain relatively scarce. We collected seven fresh tumor samples from four patients to perform RNA sequencing (RNA-seq) and assay for transposase-accessible chromatin using sequencing (ATAC-seq) to determine the transcriptome profile and chromatin accessibility of iCCA. The increased expression of cell cycle regulators, including PLK1 and its substrate MISP, was identified. Ninety-one iCCA patients were used to validate the clinical significance of PLK1 and MISP. The upregulation of PLK1 and MISP was determined in iCCA tissues. Increased expression of PLK1 and MISP was significantly correlated with tumor number, N stage, and lymphatic invasion in an iCCA cohort. Knockdown of PLK1 or MISP reduced trans-lymphatic endothelial migration and wound healing and affected focal adhesions in vitro. In cell‒cell junctions, MISP localized to adherens junctions and suppressed E-cadherin dimerization. PLK1 disrupted adherens junctions in a myosin-dependent manner. Furthermore, PLK1 and MISP promoted cell proliferation in vitro and tumorigenesis in vivo. In iCCA, PLK1 and MISP promote aggressiveness by increasing lymphatic invasion, tumor growth, and motility through the repression of E-cadherin adherens junctions.

Role of PLK1/NUMB/NOTCH in epithelial-mesenchymal transition in human melanoma

Polo-like kinase 1 (PLK1), a serine/threonine kinase, is overexpressed in melanoma and its expression has been associated with poor disease prognosis. PLK1 has been shown to interact with NUMB, a NOTCH antagonist. However, the exact role of PLK1, NUMB, and NOTCH signaling in epithelial-mesenchymal transition (EMT) in melanoma progression is unclear. In this study, Affymetrix microarray analysis was performed to determine differentially expressed genes following shRNA-mediated knockdown of PLK1 in human melanoma cells that showed significant modulations in EMT and metastasis-related genes. Using multiple PLK1-modulated melanoma cell lines, we found that PLK1 is involved in the regulation of cell migration, invasion, and EMT via its kinase activity and NOTCH activation. In vitro kinase assay and mass spectrometry analysis demonstrated a previously unknown PLK1 phosphorylation site (Ser413) on NUMB. Overexpression of non-phosphorylatable (S413A) and phosphomimetic (S413D) mutants of NUMB in melanoma cells implicated the involvement of NUMB-S413 phosphorylation in cell migration and invasion, which was independent of NOTCH activation. To determine the clinical relevance of these findings, immunohistochemistry was performed using melanoma tissue microarray, which indicated a strong positive correlation between PLK1 and N-cadherin, a protein required for successful EMT. These findings were supported by TCGA analysis, where expression of high PLK1 with low NUMB or high NOTCH or N-cadherin showed a significant decrease in survival of melanoma patients. Overall, these results suggest a potential role of PLK1 in EMT, migration, and invasion of melanoma cells. Our findings support the therapeutic targeting of PLK1, NUMB, and NOTCH for melanoma management.

Resistance to BRAF Inhibitors: EZH2 and Its Downstream Targets as Potential Therapeutic Options in Melanoma

Activating BRAF mutations occurs in 50-60% of malignant melanomas. Although initially treatable, the development of resistance to BRAF-targeted therapies (BRAFi) is a major challenge and limits their efficacy. We have previously shown that the BRAF_V600E signaling pathway mediates the expression of EZH2, an epigenetic regulator related to melanoma progression and worse overall survival. Therefore, we wondered whether inhibition of EZH2 would be a way to overcome resistance to vemurafenib. We found that the addition of an EZH2 inhibitor to vemurafenib improved the response of melanoma cells resistant to BRAFi with regard to decreased viability, cell-cycle arrest and increased apoptosis. By next-generation sequencing, we revealed that the combined inhibition of BRAF and EZH2 dramatically suppresses pathways of mitosis and cell cycle. This effect was linked to the downregulation of Polo-kinase 1 (PLK1), a key regulator of cell cycle and proliferation. Subsequently, when we inhibited PLK1, we found decreased cell viability of melanoma cells resistant to BRAFi. When we inhibited both BRAF and PLK1, we achieved an improved response of BRAFi-resistant melanoma cells, which was comparable to the combined inhibition of BRAF and EZH2. These results thus reveal that targeting EZH2 or its downstream targets, such as PLK1, in combination with BRAF inhibitors are potential novel therapeutic options in melanomas with BRAF mutations.

Knockdown of GSG2 inhibits the development and progression of non-small cell lung cancer in vitro and in vivo

Lung cancer has been recognized as the most common malignant neoplasm of the respiratory system with extremely high morbidity, among which non-small cell lung cancer (NSCLC) accounts for the majority. Many published literatures have revealed the roles of GSG2 in the progression of ovarian cancer, bladder cancer and breast cancer. However, there were no reports on the relationship between GSG2 and NSCLC. Herein, GSG2 was identified as a potential tumor promoter in NSCLC development, whose abundant expression was observed in NSCLC tissues compared with adjacent nonmalignant tissues and statistically correlated with more advanced tumor stage, more malignant grade and higher risk of lymphatic metastasis. Subsequent in vitro loss-of-function experiments indicated that GSG2 depletion could arrest cell cycle and suppress cell proliferation and migration while enhancing cell apoptosis. At the same time, the suppressive effects of GSG2 depletion on NSCLC development were verified by in vivo experiments. In conclusion, the current study identified GSG2 as a tumor promoter in development and progression of NSCLC, which could work as a novel therapeutic target for NSCLC.

Necroptosis-associated classification combined with tumor microenvironment characteristic analysis of cutaneous melanoma

Necroptosis is a mode of programmed cell death that overcomes apoptotic resistance. The accurate prognosis of cutaneous melanoma is complicated to predict due to tumor heterogeneity. Necroptosis contributes to the regulation of oncogenesis and cancer immunity. We comprehensively investigated different necroptosis patterns by the non-negative matrix factorization (NMF) clustering analysis and explored the relationships among necroptosis patterns, infiltered immune cells, and tumor microenvironment (TME) scores. Two different necroptosis patterns were identified, and the two clusters could predict prognosis and immune landscape. A four-gene signature was successfully constructed and validated its predictive capability of overall survival (OS) in cutaneous melanoma patients. The prognostic value of the signature was further enhanced by incorporating other independent prognostic factors such as age and clinicopathological stages in a nomogram-based prediction model. Patients with lower risk scores tended to have better OS, higher TME score, immune checkpoints, immunophenoscore (IPS), and lower Tumor Immune Dysfunction and Exclusion (TIDE), which indicated better responses to immunotherapy. In addition, the pigmentation score of the high-risk group was visibly higher than those of the low-risk group. In conclusion, the necroptosis-related signature indicated favorable predictive performance in cutaneous melanoma patients, which provides guidance for immunotherapy and provide novel insights into precision medicine.

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

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

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.

PLK1 and NOTCH Positively Correlate in Melanoma and Their Combined Inhibition Results in Synergistic Modulations of Key Melanoma Pathways

Melanoma is one of the most serious forms of skin cancer, and its increasing incidence coupled with nonlasting therapeutic options for metastatic disease highlights the need for additional novel approaches for its management. In this study, we determined the potential interactions between polo-like kinase 1 (PLK1, a serine/threonine kinase involved in mitotic regulation) and NOTCH1 (a type I transmembrane protein deciding cell fate during development) in melanoma. Employing an in-house human melanoma tissue microarray (TMA) containing multiple cases of melanomas and benign nevi, coupled with high-throughput, multispectral quantitative fluorescence imaging analysis, we found a positive correlation between PLK1 and NOTCH1 in melanoma. Furthermore, The Cancer Genome Atlas database analysis of patients with melanoma showed an association of higher mRNA levels of PLK1 and NOTCH1 with poor overall, as well as disease-free, survival. Next, utilizing small-molecule inhibitors of PLK1 and NOTCH (BI 6727 and MK-0752, respectively), we found a synergistic antiproliferative response of combined treatment in multiple human melanoma cells. To determine the molecular targets of the overall and synergistic responses of combined PLK1 and NOTCH inhibition, we conducted RNA-sequencing analysis employing a unique regression model with interaction terms. We identified the modulations of several key genes relevant to melanoma progression/metastasis, including MAPK, PI3K, and RAS, as well as some new genes such as Apobec3G, BTK, and FCER1G, which have not been well studied in melanoma. In conclusion, our study demonstrated a synergistic antiproliferative response of concomitant targeting of PLK1 and NOTCH in melanoma, unraveling a potential novel therapeutic approach for detailed preclinical/clinical evaluation.

Gene Expression Signature of BRAF Inhibitor Resistant Melanoma Spheroids

In vitro cell cultures are frequently used to define the molecular background of drug resistance. The majority of currently available data have been obtained from 2D in vitro cultures, however, 3D cell culture systems (spheroids) are more likely to behave similarly to in vivo conditions. Our major aim was to compare the gene expression signature of 2D and 3D cultured BRAFV600E mutant melanoma cell lines. We successfully developed BRAF-drug resistant cell lines from paired primary/metastatic melanoma cell lines in both 2D and 3D in vitro cultures. Using Affymetrix Human Gene 1.0 ST arrays, we determined the gene expression pattern of all cell lines. Our analysis revealed 1049 genes (562 upregulated and 487 downregulated) that were differentially expressed between drug-sensitive cells grown under different cell cultures. Pathway analysis showed that the differently expressed genes were mainly associated with the cell cycle, p53, and other cancer-related pathways. The number of upregulated genes (72 genes) was remarkably fewer when comparing the resistant adherent cells to cells that grow in 3D, and were associated with cell adhesion molecules and IGF1R signalling. Only 1% of the upregulated and 5.6% of the downregulated genes were commonly altered between the sensitive and the resistant spheroids. Interestingly, we found several genes (BNIP3, RING1 and ABHD4) with inverse expression signature between sensitive and resistant spheroids, which are involved in anoikis resistance and cell cycle regulation. In summary, our study highlights gene expression alterations that might help to understand the development of acquired resistance in melanoma cells in tumour tissue.

The role of polo-like kinase 3 in the response of BRAF-mutant cells to targeted anticancer therapies

The activation of oncogenic mitogen-activated protein kinase cascade via mutations in BRAF is often observed in human melanomas. Targeted inhibitors of BRAF (BRAFi), alone or as a part of a combination therapy, offer a significant benefit to such patients. Unfortunately, some cases are initially nonresponsive to these drugs, while others become refractory in the course of treatment, underscoring the need to understand and mitigate the underlying resistance mechanisms. We report that interference with polo-like kinase 3 (PLK3) reduces the tolerance of BRAF-mutant melanoma cells to BRAFi, while increased PLK3 expression has the opposite effect. Accordingly, PLK3 expression correlates with tolerance to BRAFi in a panel of BRAF-mutant cell lines and is elevated in a subset of recurring BRAFi-resistant melanomas. In PLK3-expressing cells, R406, a kinase inhibitor whose targets include PLK3, recapitulates the sensitizing effects of genetic PLK3 inhibitors. The findings support a role for PLK3 as a predictor of BRAFi efficacy and suggest suppression of PLK3 as a way to improve the efficacy of targeted therapy.

In Vivo ERK1/2 Reporter Predictively Models Response and Resistance to Combined BRAF and MEK Inhibitors in Melanoma

Combined BRAF and MEK inhibition is a standard of care in patients with advanced BRAF-mutant melanoma, but acquired resistance remains a challenge that limits response durability. Here, we quantitated in vivo ERK1/2 activity and tumor response associated with resistance to combined BRAF and MEK inhibition in mutant BRAF xenografts. We found that ERK1/2 pathway reactivation preceded the growth of resistant tumors. Moreover, we detected a subset of cells that not only persisted throughout long-term treatment but restored ERK1/2 signaling and grew upon drug removal. Cell lines derived from combination-resistant tumors (CRT) exhibited elevated ERK1/2 phosphorylation, which were sensitive to ERK1/2 inhibition. In some CRTs, we detected a tandem duplication of the BRAF kinase domain. Monitoring ERK1/2 activity in vivo was efficacious in predicting tumor response during intermittent treatment. We observed maintained expression of the mitotic regulator, polo-like kinase 1 (Plk1), in melanoma resistant to BRAF and MEK inhibitors. Plk1 inhibition induced apoptosis in CRTs, leading to slowed growth of BRAF and MEK inhibitor-resistant tumors in vivo These data demonstrate the utility of in vivo ERK1/2 pathway reporting as a tool to optimize clinical dosing schemes and establish suppression of Plk1 as potential salvage therapy for BRAF inhibitor and MEK inhibitor-resistant melanoma.

Boolean model of growth signaling, cell cycle and apoptosis predicts the molecular mechanism of aberrant cell cycle progression driven by hyperactive PI3K

The PI3K/AKT signaling pathway plays a role in most cellular functions linked to cancer progression, including cell growth, proliferation, cell survival, tissue invasion and angiogenesis. It is generally recognized that hyperactive PI3K/AKT1 are oncogenic due to their boost to cell survival, cell cycle entry and growth-promoting metabolism. That said, the dynamics of PI3K and AKT1 during cell cycle progression are highly nonlinear. In addition to negative feedback that curtails their activity, protein expression of PI3K subunits has been shown to oscillate in dividing cells. The low-PI3K/low-AKT1 phase of these oscillations is required for cytokinesis, indicating that oncogenic PI3K may directly contribute to genome duplication. To explore this, we construct a Boolean model of growth factor signaling that can reproduce PI3K oscillations and link them to cell cycle progression and apoptosis. The resulting modular model reproduces hyperactive PI3K-driven cytokinesis failure and genome duplication and predicts the molecular drivers responsible for these failures by linking hyperactive PI3K to mis-regulation of Polo-like kinase 1 (Plk1) expression late in G2. To do this, our model captures the role of Plk1 in cell cycle progression and accurately reproduces multiple effects of its loss: G2 arrest, mitotic catastrophe, chromosome mis-segregation / aneuploidy due to premature anaphase, and cytokinesis failure leading to genome duplication, depending on the timing of Plk1 inhibition along the cell cycle. Finally, we offer testable predictions on the molecular drivers of PI3K oscillations, the timing of these oscillations with respect to division, and the role of altered Plk1 and FoxO activity in genome-level defects caused by hyperactive PI3K. Our model is an important starting point for the predictive modeling of cell fate decisions that include AKT1-driven senescence, as well as the non-intuitive effects of drugs that interfere with mitosis.

ATP-competitive Plk1 inhibitors induce caspase 3-mediated Plk1 cleavage and activation in hematopoietic cell lines

Polo-like kinases (Plks) define a highly conserved family of Ser/Thr kinases with crucial roles in the regulation of cell division. Here we show that Plk1 is cleaved by caspase 3, but not by other caspases in different hematopoietic cell lines treated with competitive inhibitors of the ATP-binding pocket of Plk1. Intriguingly, Plk1 was not cleaved in cells treated with Rigosertib, a non-competitive inhibitor of Plk1, suggesting that binding of the inhibitor to the ATP binding pocket of Plk1 triggers a conformational change and unmasks a cryptic caspase 3 cleavage site on the protein. Cleavage occurs after Asp-404 in a DYSD/K sequence and separates the kinase domain from the two PBDs of Plk1. All Plk1 inhibitors triggered G2/M arrest, activation of caspases 2 and 3, polyploidy, multiple nuclei and mitotic catastrophe, albeit at higher concentrations in the case of Rigosertib. Upon BI-2536 treatment, Plk1 cleavage occurred only in the cytosolic fraction and cleaved Plk1 accumulated in this subcellular compartment. Importantly, the cleaved N-Terminal fragment of Plk1 exhibited a higher enzymatic activity than its non-cleaved counterpart and accumulated into the cytoplasm conversely to the full length and the C-Terminal Plk1 fragments that were found essentially into the nucleus. Finally, the DYSD/K cleavage site was highly conserved during evolution from c. elegans to human. In conclusion, we described herein for the first time a specific cleavage of Plk1 by caspase 3 following treatment of cancer cells with ATP-competitive inhibitors of Plk1.

SUMOylation regulates the localization and activity of Polo-like kinase 1 during cell cycle in the silkworm, Bombyx mori

Polo-like kinase 1 (Plk1) is a crucial cell cycle regulator by its specific localization and activity during cell cycle. It has been shown that the phosphorylation and ubiquitylation of Plk1 are required for its own activation and localization. Here, we report that SUMOylation regulates the activity of Plk1 in the lepidopteran insect of Bombyx mori. In the absence of SUMOylation, it causes the lost localization of Plk1 on centrosomes and kinetochores, as well as an uneven distribution in midzone. We further identify that the putative SUMOylation site of Bombyx Plk1 at lysine 466 is required for its localization on centrosomes, and K466 mutation in Plk1 could influence its interaction with Smt3/Ubc9 complex. These findings are also confirmed by Drosophila Polo and human Plk1, which together reveals a conserved role of Plk1 SUMOylation in mammals. Moreover, conjugation of Smt3 to Plk1 SUMOylation mutant promotes its localization on centrosomes and kinetochores, and rescues functional defects of chromosome alignment in cells depleted of endogenous Plk1. Altogether, the present data indicate that the SUMOylation of Plk1 could participate in proper chromosome alignment and segregation during mitosis, and provides a novel layer for the regulation of Plk1 localization and activity throughout cell cycle.

RNA interference-mediated knockdown of SIRT1 and/or SIRT2 in melanoma: Identification of downstream targets by large-scale proteomics analysis

Melanoma is the most notorious and fatal of all skin cancers and the existing treatment options have not been proven to effectively manage this neoplasm, especially the metastatic disease. Sirtuin (SIRT) proteins have been shown to be differentially expressed in melanoma. We have shown that SIRTs 1 and 2 were overexpressed in melanoma and inhibition of SIRT1 imparts anti-proliferative responses in human melanoma cells. To elucidate the impact of SIRT 1 and/or 2 in melanoma, we created stable knockdowns of SIRTs 1, 2, and their combination using shRNA mediated RNA interference in A375 human melanoma cells. We found that SIRT1 and SIRT1&2 combination knockdown caused a decreased cellular proliferation in melanoma cells. Further, the knockdown of SIRT 1 and/or 2 resulted in a decreased colony formation in melanoma cells. To explore the downstream targets of SIRTs 1 and/or 2, we employed a label-free quantitative nano-LC-MS/MS proteomics analysis using the stable lines. We found aberrant levels of proteins involved in many vital cellular processes, including cytoskeletal organization, ribosomal activity, oxidative stress response, and angiogenesis. These findings provide clear evidence of cellular systems undergoing alterations in response to sirtuin inhibition, and have unveiled several excellent candidates for future study.

SIGNIFICANCE

Melanoma is the deadliest form of skin cancer, due to its aggressive nature, metastatic potential, and a lack of sufficient treatment options for advanced disease. Therefore, detailed investigations into the molecular mechanisms of melanoma growth and progression are needed. In the search for candidate genes to serve as therapeutic targets, the sirtuins show promise as they have been found to be upregulated in melanoma and they regulate a large number of proteins involved in cellular processes known to affect tumor growth, such as DNA damage repair, cell cycle arrest, and apoptosis. In this study, we used a large-scale label-free comparative proteomics system to identify novel protein targets that are affected following knockdown of SIRT1 and/or 2 in A375 metastatic melanoma cell line. Our study offers important insight into the potential downstream targets of SIRTs 1 and/or 2. This may unravel new potential areas of exploration in melanoma research.

Cell Cycle Regulation and Melanoma

Dysregulation of cell cycle control is a hallmark of melanomagenesis. Agents targeting the G1-S and G2-M checkpoints, as well as direct anti-mitotic agents, have all shown promising preclinical activity in melanoma. However, in vivo, standalone single agents targeting cell cycle regulation have only demonstrated modest efficacy in unselected patients. The advent of specific CDK 4/6 inhibitors targeting the G1-S transition, with an improved therapeutic index, is a significant step forward. Potential synergy exists with the combination of CDK4/6 inhibitors with existing therapies targeting the MAPK pathway, particularly in subsets of metastatic melanomas such as NRAS and BRAF mutants. This reviews summaries of the latest developments in both preclinical and clinical data with cell cycle-targeted therapies in melanoma.

Targeted knockdown of polo-like kinase 1 alters metabolic regulation in melanoma

A limited number of studies have indicated an association of the mitotic kinase polo-like kinase 1 (PLK1) and cellular metabolism. Here, employing an inducible RNA interference approach in A375 melanoma cells coupled with a PCR array and multiple validation approaches, we demonstrated that PLK1 alters a number of genes associated with cellular metabolism. PLK1 knockdown resulted in a significant downregulation of IDH1, PDP2 and PCK1 and upregulation of FBP1. Ingenuity Pathway Analysis (IPA) identified that 1) glycolysis and the pentose phosphate pathway are major canonical pathways associated with PLK1, and 2) PLK1 inhibition-modulated genes were largely associated with cellular proliferation, with FBP1 being the key modulator. Further, BI 6727-mediated inhibition of PLK1 caused a decrease in PCK1 and increase in FBP1 in A375 melanoma cell implanted xenografts in vivo. Furthermore, an inverse correlation between PLK1 and FBP1 was found in melanoma cells, with FBP1 expression significantly downregulated in a panel of melanoma cells. In addition, BI 6727 treatment resulted in an upregulation in FBP1 in A375, Hs294T and G361 melanoma cells. Overall, our study suggests that PLK1 may be an important regulator of metabolism maintenance in melanoma cells.

Synergistic interactions between PLK1 and HDAC inhibitors in non-Hodgkin's lymphoma cells occur in vitro and in vivo and proceed through multiple mechanisms

Interactions between the polo-like kinase 1 (PLK1) inhibitor volasertib and the histone deacetylase inhibitor (HDACI) belinostat were examined in diffuse large B-cell lymphoma (DLBCL) and mantle cell lymphoma (MCL) cells in vitro and in vivo. Exposure of DLBCL cells to very low concentrations of volasertib in combination with belinostat synergistically increased cell death (apoptosis). Similar interactions occurred in GC-, ABC-, double-hit DLBCL cells, MCL cells, bortezomib-resistant cells and primary lymphoma cells. Co-exposure to volasertib/belinostat induced a marked increase in M-phase arrest, phospho-histone H3, mitotic errors, cell death in M-phase, and DNA damage. Belinostat diminished c-Myc mRNA and protein expression, an effect significantly enhanced by volasertib co-exposure. c-Myc knock-down increased DNA damage and cell death in response to volasertib, arguing that c-Myc down-regulation plays a functional role in the lethality of this regimen. Notably, PLK1 knock-down in DLBCL cells significantly increased belinostat-induced M-phase accumulation, phospho-histone H3, γH2AX, and cell death. Co-administration of volasertib and belinostat dramatically reduced tumor growth in an ABC-DLBCL flank model (U2932) and a systemic double-hit lymphoma model (OCI-Ly18), accompanied by a pronounced increase in survival without significant weight loss or other toxicities. Together, these findings indicate that PLK1/HDAC inhibition warrants attention as a therapeutic strategy in NHL.

Small molecule inhibition of polo-like kinase 1 by volasertib (BI 6727) causes significant melanoma growth delay and regression in vivo

The objective of this study was to determine the therapeutic potential of polo-like kinase 1 (Plk1) inhibition in melanoma, in vivo. Employing Vectra technology, we assessed the Plk1 expression profile in benign nevi, malignant (stages I-IV) and metastatic melanomas. We found a significant elevation of Plk1 immunostaining in melanoma tissues. Further, a second generation small molecule Plk1 inhibitor, BI 6727, resulted in reductions in growth, viability and clonogenic survival, as well as an increase in apoptosis of A375 and Hs 294T melanoma cells. BI 6727 treatment also resulted in a G2/M-as well as S-phase cell cycle arrest in melanoma cells. Importantly, BI 6727 (intravenous injection; 10 and 25 mg/kg body weight) treatment resulted in significant tumor growth delay and regression in vivo in A375-and Hs 294T-implanted xenografts in athymic nude mice. These anti-melanoma effects were accompanied with a decreased cellular proliferation (Ki-67 staining) and induction of apoptosis (caspase 3 activation). In addition, BI 6727 treatment caused a marked induction of p53 and p21 in vitro as well as in vivo. Overall, we suggest that Plk1 inhibition may be a useful approach as a monotherapy as well as in combination with other existing therapeutics, for melanoma management.

Identification of Polo-like kinase 1 interaction inhibitors using a novel cell-based assay

Polo-like kinase 1 (Plk1) plays several roles in cell division and it is a recognized cancer drug target. Plk1 levels are elevated in cancer and several types of cancer cells are hypersensitive to Plk1 inhibition. Small molecule inhibitors of the kinase domain (KD) of Plk1 have been developed. Their selectivity is limited, which likely contributes to their toxicity. Polo-like kinases are characterized by a Polo-Box Domain (PBD), which mediates interactions with phosphorylation substrates or regulators. Inhibition of the PBD could allow better selectivity or result in different effects than inhibition of the KD. In vitro screens have been used to identify PBD inhibitors with mixed results. We developed the first cell-based assay to screen for PBD inhibitors, using Bioluminescence Resonance Energy Transfer (BRET). We screened through 112 983 compounds and characterized hits in secondary biochemical and biological assays. Subsequent Structure-Activity Relationship (SAR) analysis on our most promising hit revealed that it requires an alkylating function for its activity. In addition, we show that the previously reported PBD inhibitors thymoquinone and Poloxin are also alkylating agents. Our cell-based assay is a promising tool for the identification of new PBD inhibitors with more drug-like profiles using larger and more diverse chemical libraries.

Tumour-specific delivery of siRNA-coupled superparamagnetic iron oxide nanoparticles, targeted against PLK1, stops progression of pancreatic cancer

OBJECTIVE

Pancreatic ductal adenocarcinoma (PDAC) is one of the most aggressive malignancies and is projected to be the second leading cause of cancer-related death by 2030. Despite extensive knowledge and insights into biological properties and genetic aberrations of PDAC, therapeutic options remain temporary and ineffective. One plausible explanation for the futile response to therapy is an insufficient and non-specific delivery of anticancer drugs to the tumour site.

DESIGN

Superparamagnetic iron oxide nanoparticles (SPIONs) coupled with siRNA directed against the cell cycle-specific serine-threonine-kinase, Polo-like kinase-1 (siPLK1-StAv-SPIONs), could serve a dual purpose for delivery of siPLK1 to the tumour and for non-invasive assessment of efficiency of delivery in vivo by imaging the tumour response. siPLK1-StAv-SPIONs were designed and synthesised as theranostics to function via a membrane translocation peptide with added advantage of driving endosomal escape for mediating transportation to the cytoplasm (myristoylated polyarginine peptides) as well as a tumour-selective peptide (EPPT1) to increase intracellular delivery and tumour specificity, respectively.

RESULTS

A syngeneic orthotopic as well as an endogenous cancer model was treated biweekly with siPLK1-StAv-SPIONs and tumour growth was monitored by small animal MRI. In vitro and in vivo experiments using a syngeneic orthotopic PDAC model as well as the endogenous LSL-KrasG12D, LSL-Trp53R172H, Pdx-1-Cre model revealed significant accumulation of siPLK1-StAv-SPIONs in PDAC, resulting in efficient PLK1 silencing. Tumour-specific silencing of PLK1 halted tumour growth, marked by a decrease in tumour cell proliferation and an increase in apoptosis.

CONCLUSIONS

Our data suggest siPLK1-StAv-SPIONs with dual specificity residues for tumour targeting and membrane translocation to represent an exciting opportunity for targeted therapy in patients with PDAC.

Cross Talk between Wnt/β-Catenin and CIP2A/Plk1 Signaling in Prostate Cancer: Promising Therapeutic Implications

Aberrant activation of the Wnt/β-catenin pathway and polo-like kinase 1 (Plk1) overexpression represent two common events in prostate cancer with relevant functional implications. This minireview analyzes their potential therapeutic significance in prostate cancer based on their role as androgen receptor (AR) signaling regulators and the pivotal role of the tumor suppressor protein phosphatase 2A (PP2A) modulating these pathways.

A novel ATM-dependent checkpoint defect distinct from loss of function mutation promotes genomic instability in melanoma

Melanomas have high levels of genomic instability that can contribute to poor disease prognosis. Here, we report a novel defect of the ATM-dependent cell cycle checkpoint in melanoma cell lines that promotes genomic instability. In defective cells, ATM signalling to CHK2 is intact, but the cells are unable to maintain the cell cycle arrest due to elevated PLK1 driving recovery from the arrest. Reducing PLK1 activity recovered the ATM-dependent checkpoint arrest, and over-expressing PLK1 was sufficient to overcome the checkpoint arrest and increase genomic instability. Loss of the ATM-dependent checkpoint did not affect sensitivity to ionizing radiation demonstrating that this defect is distinct from ATM loss of function mutations. The checkpoint defective melanoma cell lines over-express PLK1, and a significant proportion of melanomas have high levels of PLK1 over-expression suggesting this defect is a common feature of melanomas. The inability of ATM to impose a cell cycle arrest in response to DNA damage increases genomic instability. This work also suggests that the ATM-dependent checkpoint arrest is likely to be defective in a higher proportion of cancers than previously expected.

Radiosensitization in esophageal squamous cell carcinoma: Effect of polo-like kinase 1 inhibition

PURPOSE

This study examined the efficacy of polo-like kinase 1 (PLK1) inhibition on radiosensitivity in vitro and in vivo by a pharmacologic approach using the highly potent PLK1 inhibitor volasertib.

METHODS AND MATERIALS

Human esophageal squamous cell carcinoma (ESCC) cell lines KYSE 70 and KYSE 150 were used to evaluate the synergistic effect of volasertib and irradiation in vitro using cell viability assay, colony formation assay, cell cycle phase analysis, and western blot, and in vivo using ectopic tumor models.

RESULTS

Volasertib decreased ESCC cell proliferation in a dose- and time-dependent manner. Combination of volasertib and radiation caused G2/M cell cycle arrest, increased cyclin B levels, and induced apoptosis. Volasertib significantly enhanced radiation-induced death in ESCC cells by a mechanism involving the enhancement of histone H3 phosphorylation and significant cell cycle interruption. The combination of volasertib plus irradiation delayed the growth of ESCC tumor xenografts markedly compared with either treatment modality alone.

CONCLUSIONS

The in vitro results suggested that targeting PLK1 might be a viable approach to improve the effects of radiation in ESCC. In vivo studies showed that PLK1 inhibition with volasertib during irradiation significantly improved local tumor control when compared to irradiation or drug treatment alone.

Playing Polo-Like Kinase in NRAS-Mutant Melanoma

NRAS-mutant melanomas are extremely aggressive and highly resistant to currently available therapeutic modalities. Hence, new targets and therapeutic strategies for NRAS-driven melanomas are needed. As blocking NRAS directly has not been possible thus far, targeting downstream NRAS effectors, such as MEK, is being evaluated as an alternative therapeutic approach. However, blocking this pathway alone has limited efficacy. In this issue, Posch et al. report on a combination approach co-targeting PLK1 and MEK in NRAS-mutant melanomas. This combination triggers a dual blockade of the cell cycle machinery, leading to apoptosis, and it may provide a new strategy to treat NRAS-mutant melanoma.

Combined Inhibition of MEK and Plk1 Has Synergistic Antitumor Activity in NRAS Mutant Melanoma

About one-third of cancers harbor activating mutations in rat sarcoma viral oncogene homolog (RAS) oncogenes. In melanoma, aberrant neuroblastoma-RAS (NRAS) signaling fuels tumor progression in about 20% of patients. Current therapeutics for NRAS-driven malignancies barely affect overall survival. To date, pathway interference downstream of mutant NRAS seems to be the most promising approach. In this study, data revealed that mutant NRAS induced Polo-like kinase 1 (Plk1) expression, and pharmacologic inhibition of Plk1 stabilized the size of NRAS mutant melanoma xenografts. The combination of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK) and Plk1 inhibitors resulted in a significant growth reduction of NRAS mutant melanoma cells in vitro, and regression of xenografted NRAS mutant melanoma in vivo. Independent cell cycle arrest and increased induction of apoptosis underlies the synergistic effect of this combination. Data further suggest that the p53 signaling pathway is of key importance to the observed therapeutic efficacy. This study provides in vitro, in vivo, and first mechanistic data that an MEK/Plk1 inhibitor combination might be a promising treatment approach for patients with NRAS-driven melanoma. As mutant NRAS signaling is similar across different malignancies, this inhibitor combination could also offer a previously unreported treatment modality for NRAS mutant tumors of other cell origins.

Chk1 and Wee1 control genotoxic-stress induced G2-M arrest in melanoma cells

In the present report, the role of ATR-Chk1-Wee1 and ATM-Chk2-p53-p21 pathways in stress-induced cell cycle control is analysed in melanoma cells. Treatment of p53 wild-type melanoma cells with the genotoxic agent doxorubicin induces G2-M arrest, inhibitory phosphorylation of cell cycle kinase Cdc2 (CDK1) and enhanced expression of p53/p21. Wee1 inhibition under doxorubicin pulse-treatment reduces G2-M arrest and induces apoptosis. Inhibition of upstream kinase Chk1 under doxorubicin treatment almost completely abolishes stress-induced G2-M arrest and induces enhanced apoptosis. Interestingly, Chk1 inhibition alone even further increases apoptosis. While Chk1 inhibition alone almost completely abolishes G0-G1 arrest, combined treatment with doxorubicin re-establishes G0-G1 arrest. Moreover, Chk1 inhibition alone induces only a slight p53/p21 induction, while a strong induction of both proteins is observed by the combination with doxorubicin. These findings are suggestive for a particular role of p53/p21 in G0-G1, and Chk1 in G0-G1 and G2-M arrest. In line with this, the p53-mutant SK-Mel-28 melanoma cells do not mount a significant G0-G1 arrest under combined doxorubicin and Chk1 inhibitor treatment but rather show extensive apoptosis. Moreover, knockdown of p21 dramatically reduces stress-induced G0-G1 arrest under doxorubicin and Chk1 inhibitor treatment accompanied by massive DNA damage and apoptosis induction. Treatment of melanoma cells with an inhibitor of Chk2 upstream kinase ATM and doxorubicin almost completely abolishes G0-G1 arrest. Taken together, both Chk1 and Wee1 are mediators of G2-M arrest, while p53, p21 and Chk1 are mediators of G0-G1 arrest in melanoma cells. Combined treatment with chemotherapeutic agents such as doxorubicin and Chk1 inhibitors may help to overcome apoptosis resistance of p53-proficient melanoma cells. But treatment with Chk1 inhibitor alone may even be more efficient.

Large-scale label-free comparative proteomics analysis of polo-like kinase 1 inhibition via the small-molecule inhibitor BI 6727 (Volasertib) in BRAF(V600E) mutant melanoma cells

Polo-like kinase 1 (Plk1) is a serine/threonine kinase that plays a key role during the cell cycle by regulating mitotic entry, progression, and exit. Plk1 is overexpressed in a variety of human cancers and is essential to sustained oncogenic proliferation, thus making Plk1 an attractive therapeutic target. However, the clinical efficacy of Plk1 inhibition has not emulated the preclinical success, stressing an urgent need for a better understanding of Plk1 signaling. This study addresses that need by utilizing a quantitative proteomics strategy to compare the proteome of BRAF(V600E) mutant melanoma cells following treatment with the Plk1-specific inhibitor BI 6727. Employing label-free nano-LC-MS/MS technology on a Q-exactive followed by SIEVE processing, we identified more than 20 proteins of interest, many of which have not been previously associated with Plk1 signaling. Here we report the down-regulation of multiple metabolic proteins with an associated decrease in cellular metabolism, as assessed by lactate and NAD levels. Furthermore, we have also identified the down-regulation of multiple proteasomal subunits, resulting in a significant decrease in 20S proteasome activity. Additionally, we have identified a novel association between Plk1 and p53 through heterogeneous ribonucleoprotein C1/C2 (hnRNPC), thus providing valuable insight into Plk1's role in cancer cell survival.

Plk1 inhibition causes post-mitotic DNA damage and senescence in a range of human tumor cell lines

Plk1 is a checkpoint protein whose role spans all of mitosis and includes DNA repair, and is highly conserved in eukaryotes from yeast to man. Consistent with this wide array of functions for Plk1, the cellular consequences of Plk1 disruption are diverse, spanning delays in mitotic entry, mitotic spindle abnormalities, and transient mitotic arrest leading to mitotic slippage and failures in cytokinesis. In this work, we present the in vitro and in vivo consequences of Plk1 inhibition in cancer cells using potent, selective small-molecule Plk1 inhibitors and Plk1 genetic knock-down approaches. We demonstrate for the first time that cellular senescence is the predominant outcome of Plk1 inhibition in some cancer cell lines, whereas in other cancer cell lines the dominant outcome appears to be apoptosis, as has been reported in the literature. We also demonstrate strong induction of DNA double-strand breaks in all six lines examined (as assayed by γH2AX), which occurs either during mitotic arrest or mitotic-exit, and may be linked to the downstream induction of senescence. Taken together, our findings expand the view of Plk1 inhibition, demonstrating the occurrence of a non-apoptotic outcome in some settings. Our findings are also consistent with the possibility that mitotic arrest observed as a result of Plk1 inhibition is at least partially due to the presence of unrepaired double-strand breaks in mitosis. These novel findings may lead to alternative strategies for the development of novel therapeutic agents targeting Plk1, in the selection of biomarkers, patient populations, combination partners and dosing regimens.

PLK1 expression and BI 2536 effects in childhood acute lymphoblastic leukemia

BACKGROUND

Polo-like kinase 1 (PLK1) is a conserved kinase that mediates various mitotic events. Compelling data have repeatedly demonstrated its upregulation in different neoplasia, being frequently associated with poor prognosis. However, in childhood acute lymphoblastic leukemia (ALL), no studies have yet been conducted.

PROCEDURE

PLK1 expression and association with biological features were evaluated in 65 consecutively diagnosed childhood ALL samples by quantitative real-time PCR. Moreover, the effects of a specific PLK1 inhibitor, BI 2536, was tested against a panel of nine ALL cell lines at nanomolar concentrations (10, 50, 100 nM).

RESULTS

The mRNA expression of PLK1 showed great variability in pediatric ALL, but no difference was evidenced compared to normal bone marrow. Additionally, no association was found between PLK1 mRNA expression with any clinical or biological features. Alternatively, high mRNA expression of PLK1 was present in ALL cell lines. In vitro treatment with BI 2536 strongly diminished growth, while presenting significant reduction in colony formation capacity and increased apoptosis rates. Moreover, strong G2/M arrest was detected suggesting important impaired proliferation after treatment.

CONCLUSIONS

PLK1 mRNA expression level is not associated with prognosis in childhood ALL; however, considering the great variability observed in the sample and the in vitro experiments presented herein, BI 2536 treatment might serve as a promising therapeutic to enhance the efficacy of conventional treatment modalities in some childhood ALL cases.

Current assessment of polo-like kinases as anti-tumor drug targets

INTRODUCTION

Polo-like kinase (PLK)1 is the most studied of the PLK family and is a serine/threonine kinase that plays pivotal roles in many aspects of mitosis and hence its deregulation is prevalent in various malignant tumor types.

AREAS COVERED

In this review, the authors discuss the relevancy of PLK1 and other PLK members as oncology targets in light of known roles of these kinases and the observed phenotypic consequence of downregulating their activity, depending on how they are targeted. Furthermore, they also discuss the pathways mutated in cancer that have been shown to enhance sensitivity toward PLK1 inhibitors in the context of tumor types that possess these molecular defects. They also summarize preclinical and clinical investigations that have been undertaken for both ATP and non-ATP competitive inhibitors.

EXPERT OPINION

PLKs 2, 3 and 5 are primarily linked with tumor suppressor functions and as PLK1 is the most validated anticancer drug target, selective inhibitors for its activities are most likely to result in effective therapeutics with reduced side effects. In this regard, the polo box domain can be targeted to generate selective inhibitors of PLK1 while preventing inhibition of kinases outside of this family. Recent studies confirming the synthetic lethality of other molecular defects with PLK1 can be exploited to obtain tumor selective apoptosis in p53, KRAS and PTEN mutant cancers.

Aromatic diacylhydrazine derivatives as a new class of polo-like kinase 1 (PLK1) inhibitors

A novel class of aromatic diacylhydrazine derivatives was designed as PLK1 inhibitors. All the 19 new synthesized compounds were assayed for antitumor activity against the respective cervical cancer cells. In which, nine compounds with better antitumor activities were further tested for their PLK1 inhibitory activity. Last, we have successfully found that compound 7k showed both the promising antitumor activity with IC50 of 0.17 μM against the cervical cancer cells, and also processed the most potent PLK1 inhibitory activity with IC50 of 0.03 μM. In addition, docking simulation also carried out in this study to give a potent prediction binding mode between the small molecule and PKL1 (PDB code: 1umw) protein.

The role of polo-like kinase 1 in carcinogenesis: cause or consequence?

Polo-like kinase 1 (Plk1) is a well-established mitotic regulator with a diverse range of biologic functions continually being identified throughout the cell cycle. Preclinical evidence suggests that the molecular targeting of Plk1 could be an effective therapeutic strategy in a wide range of cancers; however, that success has yet to be translated to the clinical level. The lack of clinical success has raised the question of whether there is a true oncogenic addiction to Plk1 or if its overexpression in tumors is solely an artifact of increased cellular proliferation. In this review, we address the role of Plk1 in carcinogenesis by discussing the cell cycle and DNA damage response with respect to their associations with classic oncogenic and tumor suppressor pathways that contribute to the transcriptional regulation of Plk1. A thorough examination of the available literature suggests that Plk1 activity can be dysregulated through key transformative pathways, including both p53 and pRb. On the basis of the available literature, it may be somewhat premature to draw a definitive conclusion on the role of Plk1 in carcinogenesis. However, evidence supports the notion that oncogene dependence on Plk1 is not a late occurrence in carcinogenesis and it is likely that Plk1 plays an active role in carcinogenic transformation.

High expression of polo-like kinase 1 is associated with the metastasis and recurrence in urothelial carcinoma of bladder

OBJECTIVES

Polo-like kinase 1 (Plk1) has been widely pursued as an oncology target because it is overexpressed in several human tumor types. To investigate whether Plk1 plays a general role in bladder urothelial carcinoma, we examined the expression of Plk1 protein in bladder urothelial carcinoma and cell lines, and analyzed the relationship among Plk1 protein expression, metastasis, and recurrence of urinary bladder urothelial carcinoma.

METHODS

Immunohistochemistry was used to detect the expression of Plk1 in 120 bladder urothelial carcinoma. Moreover, the expression of Plk1 was analyzed by Western blot in 60 bladder urothelial carcinoma and 21 normal epithelial tissues. MTT assay and flow cytometry and transwell assay were used to examine the proliferative and invasive ability of bladder cancer cells with the treatment of scytonemin (the inhibitor of Plk1). Statistical analysis was used to discuss the association between Plk1 expression and clinicopathologic parameters, tumor metastasis and recurrence, and the proliferative and invasive ability and cell cycle process of the bladder cancer cells.

RESULTS

There was a significantly higher Plk1expressions in bladder urothelial carcinoma and highly invasive bladder T24 cells than those in bladder normal tissues and the superficial bladder BIU-87 cells. Plk1 expression was positively correlated with histologic grade, pT stage, recurrence, and metastasis. With the increasing concentration of scytonemin, we found that not only the cell proliferation and invasion activity decreased significantly, but also the cell cycle was blocked at G2/M stage.

CONCLUSION

Plk1 expression status was closely correlated with important histopathologic characteristics (grades and stages) and the recurrence and metastasis of bladder urothelial carcinomas. Furthermore, Plk1 played an important function on the bladder cancer cells' proliferation by regulating the cancer cell cycle from G1/S to G2/M and probably promoted the invasion and metastasis of bladder cancer.

Downregulation of Polo-like kinase 1 induces cellular senescence in human primary cells through a p53-dependent pathway

Polo-like kinase 1 (PLK1) plays a key role in various stages of mitosis from entry into M phase to exit from mitosis. However, its role in cellular senescence remains to be determined. Therefore, the effects of PLK1 on cellular senescence in human primary cells were investigated. We found that expression of PLK1 decreased in human dermal fibroblasts and human umbilical vein endothelial cells under replicative senescence and premature senescence induced by adriamycin. PLK1 knockdown with PLK1 small interfering RNAs in young cells induced premature senescence. In contrast, upregulation of PLK1 in old cells partially reversed senescence phenotypes. Cellular senescence by PLK1 inhibition was observed in p16 knockdown cells but not in p53 knockdown cells. Our data suggest that PLK1 repression might result in cellular senescence in human primary cells via a p53-dependent pathway.

Inhibition of polo-like kinase 1 induces cell cycle arrest and sensitizes glioblastoma cells to ionizing radiation

Despite efforts to improve surgical, radiologic, and chemotherapeutic strategies, the outcome of patients with glioblastoma (GBM) is still poor. Polo-like kinase 1 (PLK1) is a serine/threonine kinase that plays key roles in cell cycle control and has been associated with tumor growth and prognosis. Here, we aimed at testing the radiosensitizing effects of the PLK1 inhibitor BI 2536 on eight GBM cell lines. For cell cycle analysis, T98G, U251, U343 MG-a, LN319, SF188, U138 MG, and U87 MG cell lines were treated with 10, 50, or 100 nM of BI 2536 for 24 hours. In addition, cell cultures exposed to BI 2536 50 nM for 24 hours were irradiated with γ-rays from (60)Cobalt source at final doses of 2, 4, and 6 Gy. Combinatorial effects were evaluated through proliferation and clonogenic capacity assays. Treatment with BI 2536 caused mitotic arrest after 24 hours, and increased apoptosis in GBM cells. Moreover, our results demonstrate that pretreatment with this drug sensitized six out of seven GBM cell lines to different doses of γ-irradiation as shown by decreased growth and abrogation of colony-formation capacity. Our data suggest that PLK1 blockage has a radiosensitizing effect on GBM, which could improve treatment strategies for this devastating tumor.

In vitro targeting of Polo-like kinase 1 in bladder carcinoma: comparative effects of four potent inhibitors

Despite the improvements in neoadjuvant chemotherapy, the outcome of patients with advanced bladder cancer has changed very little over the past 30 years. In the present study we tested and compared the in vitro antitumor activities of four different inhibitors of Polo-like kinase 1 (PLK1) (BI 2536, BI 6727, GW843682X, and GSK461364), against 3 bladder carcinoma cell lines RT4, 5637 and T24. The impact on radiosensitivity and drug interactions in simultaneous treatments with cisplatin, methotrexate, and doxorubicin were also investigated. Our results showed that PLK1 inhibition prevented cell proliferation and clonogenicity, causing significant inhibition of invasion of tumor cells, though modest differences were observed between drugs. Moreover, all PLK1 inhibitors induced G 2/M arrest, with the subsequent induction of death in all 3 cell lines. Drug interactions studies showed auspicious results for all PLK1 inhibitors when combined with the commonly used cisplatin and methotrexate, though combinations with doxorubicin showed mostly antagonistic effects. Comparably, the four PLK1 inhibitors efficiently sensitized cells to ionizing radiation. Our findings demonstrate that irrespective of the inhibitor used, the pharmacological inhibition of PLK1 constrains bladder cancer growth and dissemination, providing new opportunities for future therapeutic intervention. However, further laboratorial and pre-clinical tests are still needed to corroborate the usefulness of using them in combination with other commonly used chemotherapeutic drugs.

Plk1 is upregulated in androgen-insensitive prostate cancer cells and its inhibition leads to necroptosis

Castration-resistant prostate cancer (PCa) is refractory to hormone therapy and new strategies for treatment are urgently needed. We found that androgen-insensitive (AI) PCa cells, LNCaP-AI, are reprogrammed to upregulate the mitotic kinase Plk1 (Polo-like kinase 1) and other M-phase cell-cycle proteins, which may underlie AI PCa growth. In androgen-depleted media, LNCaP-AI cells showed exquisite sensitivity to growth inhibition by subnanomolar concentrations of a small molecule inhibitor of Plk1, BI2536, suggesting that these cells are dependent on Plk1 for growth. In contrast, the androgen-responsive parental LNCaP cells showed negligible responses to BI2536 treatment under the same condition. BI2536 treatment of LNCaP-AI cells resulted in an increase in cell death marker PARP-1 (polymerase-1) but did not activate caspase-3, an apoptosis marker, suggesting that the observed cell death was caspase-independent. BI2536-treated LNCaP-AI cells formed multinucleated giant cells that contain clusters of nuclear vesicles indicative of mitotic catastrophe. Live-cell time-lapse imaging revealed that BI2536-treated giant LNCaP-AI cells underwent necroptosis, as evidenced by 'explosive' cell death and partial reversal of cell death by a necroptosis inhibitor. Our studies suggest that LNCaP-AI cells underwent reprogramming in both their cell growth and cell death pathways, rendering them highly sensitive to Plk1 inhibition that induces necroptosis. Harnessing necroptosis through Plk1 inhibition may be explored for therapeutic intervention of castration-resistant PCa.

Depletion of Aurora-A in zebrafish causes growth retardation due to mitotic delay and p53-dependent cell death

Aurora-A is a serine/threonine mitotic kinase that is required for centrosome maturation. Many cancer cells over-express Aurora-A, and several reports have suggested that Aurora-A has prognostic value in the clinical treatment of cancer. Therefore, inhibitors for Aurora-A kinase have been developed. However, studies on Aurora-A are largely performed in cancer cell lines and are sometimes controversial. For effective evaluation of Aurora-A inhibitors in cancer treatment, it is essential to understand its function at the organism level. Here, we report the crucial functions of Aurora-A in homeostasis of spindle organization in mitosis using zebrafish embryogenesis as a model system. Using morpholino technology, we show that depletion of Aurora-A in zebrafish embryogenesis results in short bent trunks, accompanied by growth retardation and eventual cell death. Live-imaging and immunofluorescence analyses of the embryos revealed that the developmental defects are due to problems in mitosis, manifested through monopolar and disorganized spindle formation. Aurora-A-depleted cells exhibited mitotic arrest with congression failure, leading to activation of the spindle assembly checkpoint. Cell death in the absence of Aurora-A was partially rescued by co-injection of the p53 morpholino, suggesting that apoptosis after Aurora-A depletion is p53-dependent. The clinical implications of these results relate to the indication that Aurora-A inhibitors may be effective towards cancers with intact p53.

Antiproliferative in vitro effects of BI 2536-mediated PLK1 inhibition on cervical adenocarcinoma cells

Cervical adenocarcinoma is one of the most common gynecological malignancies. Despite the improvements in multimodality treatment, advanced disease is still associated with a significantly poor prognosis making the search for more effective therapeutic agents imperative. BI 2536, an unambiguous inhibitor of Polo-like kinase 1 (PLK1), has shown anticancer activity in a variety of tumor cell types. Herein, we present more evidence of the antiproliferative effects of this drug on HeLa cells. Nanomolar concentrations (10-100 nmol/l) of the drug significantly decreased cell proliferation and clonogenic capacity. Our results also demonstrate that inhibition of PLK1 promoted G2/M arrest and resulted in a dramatic increase in the mitotic index after 24 h of treatment. Apoptosis onset was evinced by the accumulation of a sub-G1 population as well as by a significant increase in caspase-3 activity at longer periods of exposure. Taken together, our results reinforce the prospect of directing against PLK1 as a potential therapeutic target to be evaluated in different preclinical models for cervical carcinoma.

Sequential Cdk1 and Plk1 phosphorylation of protein tyrosine phosphatase 1B promotes mitotic cell death

Mitotic cell death following prolonged arrest is an important death mechanism that is not completely understood. This study shows that Protein Tyrosine Phosphatase 1B (PTP1B) undergoes phosphorylation during mitotic arrest induced by microtubule-targeting agents (MTAs) in chronic myeloid leukaemia cells. Inhibition of cyclin-dependent kinase 1 (Cdk1) or polo-like kinase 1 (Plk1) during mitosis prevents PTP1B phosphorylation, implicating these kinases in PTP1B phosphorylation. In support of this, Cdk1 and Plk1 co-immunoprecipitate with endogenous PTP1B from mitotic cells. In addition, active recombinant Cdk1-cyclin B1 directly phosphorylates PTP1B at serine 386 in a kinase assay. Recombinant Plk1 phosphorylates PTP1B on serine 286 and 393 in vitro, however, it requires a priming phosphorylation by Cdk1 at serine 386 highlighting a novel co-operation between Cdk1 and Plk1 in the regulation of PTP1B. Furthermore, overexpression of wild-type PTP1B induced mitotic cell death, which is potentiated by MTAs. Moreover, mutation of serine 286 abrogates the cell death induced by PTP1B, whereas mutation of serine 393 does not, highlighting the importance of serine 286 phosphorylation in the execution of mitotic cell death. Finally, phosphorylation on serine 286 enhanced PTP1B phosphatase activity. Collectively, these data reveal that PTP1B activity promotes mitotic cell death and is regulated by the co-operative action of Cdk1 and Plk1 during mitotic arrest.

Phosphorylation of Tara by Plk1 is essential for faithful chromosome segregation in mitosis

Trio-associated repeat on actin (Tara) is an F-actin binding protein and regulates actin cytoskeletal organization. In our previous study, we have found that Tara associates with telomeric repeat binding factor 1 (TRF1) and mediates the function of TRF1 in mitotic regulation. We also found that overexpression HECTD3, a member of HECT E3 ubiquitin ligases, enhances the ubiquitination of Tara in vivo and promotes the degradation of Tara, and such degradation of Tara facilitates cell cycle progression. However, less is known about the post-translational modification of Tara in mitosis. Here we show that Tara is a novel Polo-like kinase 1 (Plk1) target protein. Plk1 interacts with and phosphorylates Tara in vivo and in vitro. Actually, the Thr-457 in Tara was a bona fide in vivo phosphorylation site for Plk1. Interestingly, we found that the centrosomal localization of Tara depended on the Thr-457 phosphorylation and the kinase activity of Plk1. Furthermore, overexpression of non-phosphorylatable mutant of Tara caused aberrant mitosis delay in HeLa cells. Our study demonstrated that Plk1-mediated phospho-dependent centrosomal localization of Tara is important for faithful chromosome segregation, and provided novel insights into understanding on the role of Plk1 in cooperation with Tara in mitotic progression.

Validation of Polo-like kinase 1 as a therapeutic target in pancreatic cancer cells

Polo-like kinase 1 (PLK1) is a serine/threonine protein kinase and plays a critical role in mitosis. PLK1 has also been regarded as a valuable target for cancer treatment, and several PLK1 inhibitors are currently undergoing clinical investigations. In this study, our data show that the expression level of PLK1 is upregulated in human pancreatic cancer cells. Molecular modeling studies indicate that DMTC inhibits PLK1 activity through competitive displacement of ATP from its binding pocket. Our data further show that DMTC suppresses the proliferation of pancreatic cancer cells and induces the formation of multinucleated cells, ultimately resulting in apoptosis. In addition, combination index analysis demonstrates that DMTC acts synergistically with the chemotherapeutic drug gemcitabine in inhibiting the proliferation of pancreatic cancer cells. These results thus suggest a potential of using PLK1 inhibitors for the treatment of pancreatic cancer.

Numb regulates stability and localization of the mitotic kinase PLK1 and is required for transit through mitosis

Numb functions in progenitor cell fate determination and early development, but it is also expressed in postdevelopmental tissues and cancers where its role is unclear. In this study, we report that a targeted knockdown of Numb expression causes a G(2)-M arrest and reduced cell growth in human melanoma cells. Co-immunoprecipitation and colocalization studies showed that Numb interacts with the serine/threonine polo-like kinase Plk1 and Numb cycles in a cell-cycle-dependent fashion along with this mitotic regulator. Interestingly, Numb expression was required for Plk1 protein stability and localization to the spindle poles during mitosis. Reduction in Numb expression resulted in mislocalization of Plk1 at both metaphase and anaphase, leading to disorganized γ-tubulin recruitment in centrosomes. Together, our findings present a novel function for Numb during symmetric cell division. We suggest that dysregulation of Numb expression results in mislocalized Plk1 and poor centrosomal γ-tubulin recruitment, potentially contributing to mitotic errors, aneuploidy, and cancer development.

NMS-P937, an orally available, specific small-molecule polo-like kinase 1 inhibitor with antitumor activity in solid and hematologic malignancies

Polo-like kinase 1 (PLK1) is a serine/threonine protein kinase considered to be the master player of cell-cycle regulation during mitosis. It is indeed involved in centrosome maturation, bipolar spindle formation, chromosome separation, and cytokinesis. PLK1 is overexpressed in a variety of human tumors and its overexpression often correlates with poor prognosis. Although five different PLKs are described in humans, depletion or inhibition of kinase activity of PLK1 is sufficient to induce cell-cycle arrest and apoptosis in cancer cell lines and in xenograft tumor models. NMS-P937 is a novel, orally available PLK1-specific inhibitor. The compound shows high potency in proliferation assays having low nanomolar activity on a large number of cell lines, both from solid and hematologic tumors. NMS-P937 potently causes a mitotic cell-cycle arrest followed by apoptosis in cancer cell lines and inhibits xenograft tumor growth with clear PLK1-related mechanism of action at well-tolerated doses in mice after oral administration. In addition, NMS-P937 shows potential for combination in clinical settings with approved cytotoxic drugs, causing tumor regression in HT29 human colon adenocarcinoma xenografts upon combination with irinotecan and prolonged survival of animals in a disseminated model of acute myelogenous leukemia in combination with cytarabine. NMS-P937, with its favorable pharmacologic parameters, good oral bioavailability in rodent and nonrodent species, and proven antitumor activity in different preclinical models using a variety of dosing regimens, potentially provides a high degree of flexibility in dosing schedules and warrants investigation in clinical settings.

Management of cutaneous T cell lymphoma: new and emerging targets and treatment options

Cutaneous T cell lymphomas (CTCL) clinically and biologically represent a heterogeneous group of non-Hodgkin lymphomas, with mycosis fungoides and Sézary syndrome being the most common subtypes. Over the last decade, new immunological and molecular pathways have been identified that not only influence CTCL phenotype and growth, but also provide targets for therapies and prognostication. This review will focus on recent advances in the development of therapeutic agents, including bortezomib, the histone deacetylase inhibitors (vorinostat and romidepsin), and pralatrexate in CTCL.

5-(2-amino-pyrimidin-4-yl)-1H-pyrrole and 2-(2-amino-pyrimidin-4-yl)-1,5,6,7-tetrahydro-pyrrolo[3,2-c]pyridin-4-one derivatives as new classes of selective and orally available Polo-like kinase 1 inhibitors

The discovery and characterization of two new chemical classes of potent and selective Polo-like kinase 1 (PLK1) inhibitors is reported. For the most interesting compounds, we discuss the biological activities, crystal structures and preliminary pharmacokinetic parameters. The more advanced compounds inhibit PLK1 in the enzymatic assay at the nM level and exhibit good activity in cell proliferation on A2780 cells. Furthermore, these compounds showed high levels of selectivity on a panel of unrelated kinases, as well as against PLK2 and PLK3 isoforms. Additionally, the compounds show acceptable oral bioavailability in mice making these inhibitors suitable candidates for further in vivo activity studies.