Polo-like Kinase 1 Function Research Articles

Multiple Roles of PLK1 in Mitosis and Meiosis.

Cells are equipped with a diverse network of signaling and regulatory proteins that function as cell cycle regulators and checkpoint proteins to ensure the proper progression of cell division. A key regulator of cell division is polo-like kinase 1 (PLK1), a member of the serine/threonine kinase family that plays an important role in regulating the mitotic and meiotic cell cycle. The phosphorylation of specific substrates mediated by PLK1 controls nuclear envelope breakdown (NEBD), centrosome maturation, proper spindle assembly, chromosome segregation, and cytokinesis. In mammalian oogenesis, PLK1 is essential for resuming meiosis before ovulation and for establishing the meiotic spindle. Among other potential roles, PLK1 regulates the localized translation of spindle-enriched mRNAs by phosphorylating and thereby inhibiting the translational repressor 4E-BP1, a downstream target of the mTOR (mammalian target of rapamycin) pathway. In this review, we summarize the functions of PLK1 in mitosis, meiosis, and cytokinesis and focus on the role of PLK1 in regulating mRNA translation. However, knowledge of the role of PLK1 in the regulation of meiosis remains limited.

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.

PLK1 Mitigates Intervertebral Disc Degeneration by Delaying Senescence of Nucleus Pulposus Cells.

Intervertebral disc degeneration (IVDD) is the primary cause of low back pain; however, the molecular mechanisms involved in the pathogenesis of IVDD are not fully understood. Polo-like kinase 1 (PLK1) plays numerous roles in the cell cycle, including in cell proliferation and senescence. To investigate the involvement of PLK1 in IVDD, we used patient tissues and an animal model of IVDD. Samples were analyzed via immunoblotting, quantitative real-time polymerase chain reaction (qPCR), immunofluorescence, and immunohistochemistry. Our results demonstrated that PLK1 expression was decreased in nucleus pulposus cells (NPCs) of degenerative IVDs. The inhibition of PLK1 kinase activity in normal NPCs increased the expression of p53 protein, inhibited cell proliferation, and induced senescence. Our results suggest that PLK1 regulates the degeneration of the IVD through p53, revealing the function and mechanism of PLK1 in IVDD and providing a theoretical basis and experimental evidence for the potential treatment of low back pain.

LncRNA DANCR improves the dysfunction of the intestinal barrier and alleviates epithelial injury by targeting the miR-1306-5p/PLK1 axis in sepsis.

Intestinal barrier dysfunction often occurs in various acute or chronic pathological conditions and has been identified as an important clinical problem. Herein, we explored the biological role and molecular mechanism of Polo-like kinase 1 (PLK1) and differentiation antagonizing non-protein coding RNA (DANCR) in intestinal barrier dysfunction caused by sepsis. RT-qPCR analysis was used to examine PLK1, miR-1306-5p, and DANCR expression in NCM460 cells after LPS treatment. TUNEL assay and Western blot analysis were performed to explore PLK1 function in cell apoptosis and intestinal barrier in vitro. Hematoxylin and eosin staining, Western blot analysis, and TUNEL assay were used to investigate DANCR function in the intestinal barrier and cell apoptosis in vivo. The interaction between miR-1306-5p and PLK1 (or DANCR) was validated by luciferase reporter assay. As a result, PLK1 overexpression decreased cell apoptosis and promoted intestinal barrier function. Moreover, DANCR was validated as a sponge of miR-1306-5p to target PLK1. In addition, we found that DANCR overexpression decreased intestinal mucosal permeability and colon mucosa epithelial cell apoptosis in vivo. Conclusively, DANCR improved intestinal barrier dysfunction and alleviated epithelial injury by targeting the miR-1306-5p/PLK1 axis in sepsis.

PLK1 inhibition exhibits strong anti-tumoral activity in CCND1-driven breast cancer metastases with acquired palbociclib resistance

A significant proportion of patients with oestrogen receptor (ER) positive breast cancers (BC) develop resistance to endocrine treatments (ET) and relapse with metastatic disease. Here we perform whole exome sequencing and gene expression analysis of matched primary breast tumours and bone metastasis-derived patient-derived xenografts (PDX). Transcriptomic analyses reveal enrichment of the G2/M checkpoint and up-regulation of Polo-like kinase 1 (PLK1) in PDX. PLK1 inhibition results in tumour shrinkage in highly proliferating CCND1-driven PDX, including different RB-positive PDX with acquired palbociclib resistance. Mechanistic studies in endocrine resistant cell lines, suggest an ER-independent function of PLK1 in regulating cell proliferation. Finally, in two independent clinical cohorts of ER positive BC, we find a strong association between high expression of PLK1 and a shorter metastases-free survival and poor response to anastrozole. In conclusion, our findings support clinical development of PLK1 inhibitors in patients with advanced CCND1-driven BC, including patients progressing on palbociclib treatment.

Abstract 925: PDX models of ER+ endocrine-resistant metastatic breast cancer identify Polo-like kinase 1 (PLK1) as a therapeutic target

Abstract Background: Metastatic estrogen receptor positive (ER+) breast cancers (BC) are genetically heterogeneous and a significant proportion of patients display intrinsic or acquired resistance to endocrine treatments, mTOR and CDK4/6 inhibitors. The objective of this study was to identify new therapies in clinically relevant PDX models of metastatic ER+ BC progressing on endocrine treatment and/or palbociclib. Methods: PDX models were generated by engraftment of spinal bone metastases from 120 BC patients. PDXs were molecularly characterized using SNP-arrays, targeted exome sequencing and global gene expression (GEX) analysis. The anti-tumor efficacy of the PLK1 inhibitor volasertib was evaluated in vivo compared to fulvestrant and palbociclib in two PDX models. PLK1 abundance was analyzed by immunohistochemistry in PDXs and compared with the original primary tumor and bone metastasis from each patient. PLK1 expression measured by RT-PCR was assessed in a cohort of 441 BC patients with 15-year follow-up. GEX analysis of PLK1was assessed in patients treated with neoadjuvant anastrazole. Mechanistic studies were performed in a panel of endocrine resistant BC cell lines. Results: Ten PDX have been established, 8 were ER+ and 2 triple-negative (TN). Genomic alterations included mutations in AKT1, PI3KCA, BRCA2, GATA3, NF1, and amplifications of FGFR1, CCND1 and CCNE2. Comparative pathway analysis of bone metastases derived PDX and patients' primary tumors showed enrichment for pathways associated with mitotic nuclear division, chromatid segregation and G2/M transition. PLK1 was the top commonly up-regulated gene within these pathways. Treatment of an endocrine-resistant ER+ PDX, harboring amplification of CCND1, CCNE2, FGFR1 and high expression of CDK1, showed only partial response to palbociclib and resistance to the FGFR inhibitor, AZD4547. However, PLK1 inhibitor volasertib induced rapid tumor shrinkage and complete response within 5 weeks. Importantly, volasertib was also highly effective when tested as second line therapy in palbociclib pre-treated xenografts. In-vitro inhibition of PLK1 by siRNA or volasertib inhibited tumor proliferation without affecting the expression of ER-regulated genes, suggesting an ER-independent function of PLK1 in regulating cell proliferation. Assessment of the clinical validity of PLK1 expression, revealed a strong association with poor metastases free survival (p<0.0001) in ER+ BC, but not in HER2+ or TNBC. Finally, in a cohort of patients treated with neoadjuvant anastrozole, on-treatment gene expression of PLK1 was significantly (p<0.0001) associated with poor response. Conclusion: We show that inhibition of PLK1 is a new potential treatment strategy for metastatic ER+ BC. Additional experiments are ongoing in PDX and cell lines to investigate G2/M phase dependence of ER+ BC and to identify predictive biomarkers. Citation Format: Elodie Montaudon, Joanna Nikitorowicz-Buniak, Laura Sourd, Rania El Botty, Ahmed Dahmani, Léa Huguet, Ludivine Morriset, Pierre Painsec, Fariba Nemati, Sophie Vacher, Sophie Chateau-Joubert, Sunil Pancholi, Camilla Rega, Ricardo Ribas, André Nicolas, Didier Meseure, Anne Salomon, Zakia Tariq, Keltouma Driouch, Florence Coussy, Guillaume Dutertre, Paul Cottu, Ivan Bièche, Lesley-Ann Martin, Elisabetta Marangoni. PDX models of ER+ endocrine-resistant metastatic breast cancer identify Polo-like kinase 1 (PLK1) as a therapeutic target [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 925.

A PIM-CHK1 signaling pathway regulates PLK1 phosphorylation and function during mitosis.

Although the kinase CHK1 is a key player in the DNA damage response (DDR), several studies have recently provided evidence of DDR-independent roles of CHK1, in particular following phosphorylation of its S280 residue. Here, we demonstrate that CHK1 S280 phosphorylation is cell cycle-dependent and peaks during mitosis. We found that this phosphorylation was catalyzed by the kinase PIM2, whose protein expression was also increased during mitosis. Importantly, we identified polo-like kinase 1 (PLK1) as a direct target of CHK1 during mitosis. Genetic or pharmacological inhibition of CHK1 reduced the activating phosphorylation of PLK1 on T210, and recombinant CHK1 was able to phosphorylate T210 of PLK1 in vitro Accordingly, S280-phosphorylated CHK1 and PLK1 exhibited similar specific mitotic localizations, and PLK1 was co-immunoprecipitated with S280-phosphorylated CHK1 from mitotic cell extracts. Moreover, CHK1-mediated phosphorylation of PLK1 was dependent on S280 phosphorylation by PIM2. Inhibition of PIM proteins reduced cell proliferation and mitotic entry, which was rescued by expressing a T210D phosphomimetic mutant of PLK1. Altogether, these data identify a new PIM-CHK1-PLK1 phosphorylation cascade that regulates different mitotic steps independently of the CHK1 DDR function.This article has an associated First Person interview with the first author of the paper.

Abstract 4294: SUMOylation promotes nuclear import and stabilization of polo-like kinase 1 to support its mitotic function

Abstract As a pivotal mitotic regulator, polo-like kinase 1 (PLK1) is under highly coordinated and multi-layered regulation. However, the regulatory pathways that control PLK1's activity and function have just begun to be elucidated. PLK1 has recently been shown to be functionally modulated by post-translational modifications (PTMs), including phosphorylation and ubiquitination. Herein, we report for the first time that a novel PTM, SUMOylation, plays an essential role in regulating PLK1's mitotic function. We found that Ubc9 was recruited to PLK1, upon initial phosphorylation and activation by CDK1/cyclin B. By in vivo and in vitro SUMOylation assays, PLK1 was identified as a physiologically relevant SUMO-targeted protein, preferentially modified by SUMO-1. We further showed that K492 on PLK1 was the lysine residue essential for SUMOylation. SUMOylation not only led to PLK1's nuclear import, but also significantly increased its protein stability by preventing the recruitment of APC/Ccdh1, the ubiquitin E3-ligase of PLK1. Resembling Ubc9 deficiency, cells expressing PLK1-K492R mutant exhibited mitotic aberrations characterized by prolonged mitotic progression, and misaligned and/or mis-segregated chromosomes, which could be rescued by reintroducing SUMOylation modification to PLK1. Collectively, our findings suggest that SUMOylation is another important regulatory mechanism governing PLK1's mitotic function to ensure normal mitotic progression and genomic integrity. Citation Format: Donghua Wen, Jianguo Wu, Lei Wang, Zheng Fu. SUMOylation promotes nuclear import and stabilization of polo-like kinase 1 to support its mitotic function [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4294.

Comprehensive and Integrative Analysis Reveals the Diagnostic, Clinicopathological and Prognostic Significance of Polo-Like Kinase 1 in Hepatocellular Carcinoma

Background/Aims: Liver cancer has the second highest cancer-related death rate globally and has relatively few targeted therapeutics. Polo-like kinase 1 (PLK1) is a fascinating trigger of the cell cycle; however, the still-rudimentary understanding of PLK1 at present is a significant barrier to its clinical applications. Here, we comprehensively clarified the clinicopathological value and potential functions of PLK1 in hepatocellular carcinoma (HCC). Methods: HCC-related microarrays, RNA-sequencing datasets and published studies were deeply mined and integrated from The Cancer Genome Atlas, Gene Expression Omnibus, ArrayExpress, Oncomine, literature databases, and immunohistochemistry experiments. Meanwhile, the associations between PLK1 expression and its clinicopathological implications and prognostic value in HCC patients were assessed. The standardized mean difference, summary receiver operating characteristic curve and the corresponding area under the curve, hazard ratios, odds ratios (ORs), and their 95% confidence intervals (CIs) were examined by STATA 12.0. Additionally, several bioinformatics methods were used to identify the potential function of PLK1 in HCC. Results: Comprehensive analyses revealed that PLK1 was significantly increased in HCC (standardized mean difference = 1.34, 95% CI: 1.03–1.65, P < 0.001). The results of diagnostic tests specified that in the summary receiver operating characteristic curve, the area under the curve was 0.88 (95% CI: 0.85–0.90). Furthermore, an elevated PLK1 level significantly predicted unfavorable overall survival (hazard ratio = 1.78, 95% CI: 1.10–2.88, P = 0.019) and was correlated with female gender (OR = 0.73, 95% CI: 0.56–0.95, P = 0.017), tumor thrombus (OR = 3.97, 95% CI: 1.46–10.78, P < 0.001), metastasis (OR = 3.46, 95% CI: 1.33–9.01, P = 0.011), pathologic stage (OR = 1.56, 95% CI: 1.17–2.07, P = 0.002), Barcelona Clinic Liver Cancer stage (OR = 5.76, 95% CI: 2.17–15.28, P < 0.001) and histologic grade (OR = 2.33, 95% CI: 1.12–487, P = 0.024). Through bioinformatics methods, we determined that enhancing the proliferative effect of PLK1 in HCC was associated with a series of hub genes and the activation of the cell cycle pathway. Conclusions: These findings substantiated that PLK1 may be an independent prognostic biomarker in HCC and may facilitate the development of targeted precision oncology.

SUMOylation Promotes Nuclear Import and Stabilization of Polo-like Kinase 1 to Support Its Mitotic Function

As a pivotal mitotic regulator, polo-like kinase 1 (PLK1) is under highly coordinated and multi-layered regulation. However, the pathways that control PLK1's activity and function have just begun to be elucidated. PLK1 has recently been shown to be functionally modulated by post-translational modifications (PTMs), including phosphorylation and ubiquitination. Herein, we report that SUMOylation plays an essential role in regulating PLK1's mitotic function. We found that Ubc9 was recruited to PLK1 upon initial phosphorylation and activation by CDK1/cyclin B. By invivo and invitro SUMOylation assays, PLK1 was identified as a physiologically relevant small ubiquitin-related modifier (SUMO)-targeted protein, preferentially modified by SUMO-1. We further showed that K492 on PLK1 is essential for SUMOylation. SUMOylation causes PLK1's nuclear import and significantly increases its protein stability, both of which are critical for normal mitotic progression and genomic integrity. Our findings suggest that SUMOylation is an important regulatory mechanism governing PLK1's mitotic function.

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

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

Playing polo during mitosis: PLK1 takes the lead.

Polo-like kinase 1 (PLK1), the prototypical member of the polo-like family of serine/threonine kinases, is a pivotal regulator of mitosis and cytokinesis in eukaryotes. Many layers of regulation have evolved to target PLK1 to different subcellular structures and to its various mitotic substrates in line with its numerous functions during mitosis. Collective work is starting to illuminate an important set of substrates for PLK1: the mitotic kinases that together ensure the fidelity of the cell division process. Amongst these, recent developments argue that PLK1 regulates the activity of the histone kinases Aurora B and Haspin to define centromere identity, of MPS1 to initiate spindle checkpoint signaling, and of BUB1 and its pseudokinase paralog BUBR1 to coordinate spindle checkpoint activation and inactivation. Here, we review the recent work describing the regulation of these kinases by PLK1. We highlight common themes throughout and argue that a major mitotic function of PLK1 is as a master regulator of these key kinases.

PLK1 Inhibition: Prospective Role for the Treatment of Pediatric Tumors.

Over the years, polo-like kinase 1 (PLK1) has garnered great interest as a therapeutic target. The PLK1 is a member of a highly conserved serine/threonine kinase family that plays pivotal roles in mitosis, cytokinesis and DNA damage response in eukaryotic cells. In this review, we summarize the functions of PLK1 during cell cycle progression, its roles in human pediatric cancer and its value as a prognostic factor. Furthermore, we introduce the advances in pharmacological inhibition and the newly chemotherapeutic development of small-molecules to target PLK1 in cancer treatment. Finally, clinical trials with PLK1 inhibitors are briefly reviewed.

Highlights from Recent Cancer Literature

Highlights from Recent Cancer Literature

Polo-like kinase 1 induces epithelial-to-mesenchymal transition and promotes epithelial cell motility by activating CRAF/ERK signaling.

Polo-like kinase 1 (PLK1) is a key cell cycle regulator implicated in the development of various cancers, including prostate cancer. However, the functions of PLK1 beyond cell cycle regulation remain poorly characterized. Here, we report that PLK1 overexpression in prostate epithelial cells triggers oncogenic transformation. It also results in dramatic transcriptional reprogramming of the cells, leading to epithelial-to-mesenchymal transition (EMT) and stimulation of cell migration and invasion. Consistently, PLK1 downregulation in metastatic prostate cancer cells enhances epithelial characteristics and inhibits cell motility. The signaling mechanisms underlying the observed cellular effects of PLK1 involve direct PLK1-dependent phosphorylation of CRAF with subsequent stimulation of the MEK1/2-ERK1/2-Fra1-ZEB1/2 signaling pathway. Our findings highlight novel non-canonical functions of PLK1 as a key regulator of EMT and cell motility in normal prostate epithelium and prostate cancer. This study also uncovers a previously unanticipated role of PLK1 as a potent activator of MAPK signaling.

Downregulation of PLK1 by RNAi attenuates the tumorigenicity of esophageal squamous cell carcinoma cells via promoting apoptosis and inhibiting angiogenesis.

Polo-like kinase 1(PLK1) is essential for the maintenance of genomic stability during mitosis. PLK1 has been reported to be upregulated in several solid tumors, including esophageal squamous cell carcinoma (ESCC). However, the role of PLK1 in tumorigenesis of ESCC remains undetermined. We used siRNA and lentivirus-mediated PLK1 RNA interference to investigate the tumor suppressor function of PLK1 reduction in ESCC cells. Flow cytometry and Terminal deoxynuleotidyl transferase-mediated nick-end labeling assay in vitro, as well as immunohistochemitry analysis of Caspase-3 and CD31 in s.c. tumor tissue section, were performed. Knock down of PLK1 expression significantly suppressed the ability of ESCC cells to form colonies in plastic and soft agar. PLK1 reduction mediated by lentivirus caused growth suppression of ESCC in nude mice. Caspase-3 upregulation further indicated that dysregulated apoptosis might contribute to reduced tumorigenecity. In particular, downregulation of CD31 suggested that PLK1 reduction-induced angiogenesis inhibition may also contribute, at least in part, to attenuated tumorigenecity. These findings indicate that PLK1 might play roles in tumorigenesis of ESCC and that PLK1 might be apotential gene therapy target in ESCC. Apoptosis induction together with decreased angiogenesis might be involved in the mechanism of tumor suppressor function of RNA interference targeting PLK1.

Identification of green tea catechins as potent inhibitors of the polo-box domain of polo-like kinase 1.

Polo-like kinase 1 (PLK1) plays crucial functions in multiple stages of mitosis and is considered to be a potential drug target for cancer therapy. The functions of PLK1 are mediated by its N-terminal kinase domain and C-terminal polo-box domain (PBD). Most inhibitors targeting the kinase domain of PLK1 have a selectivity issue because of a high degree of structural conservation within kinase domains of all protein kinases. Here, we combined virtual and experimental screenings to identify green tea catechins as potent inhibitors of the PLK1 PBD. Initially, (-)-epigallocatechin, one of the main components of green tea polyphenols, was found to significantly block the binding of fluorescein-labeled phosphopeptide to the PBD at a concentration of 10 μm. Next, additional catechins were evaluated for their dose-dependent inhibition of the PBD and preliminary structure-activity relationships were derived. Cellular analysis further showed that catechins interfere with the proper subcellular localization of PLK1, lead to cell-cycle arrest in the S and G2M phases, and induce growth inhibition of several human cancer cell types, such as breast adenocarcinoma (MCF7), lung adenocarcinoma (A549), and cervical adenocarcinoma (HeLa). Our data provides new insight into understanding the anticancer activities of green tea catechins.

A 3'UTR polymorphism modulates mRNA stability of the oncogene and drug target Polo-like Kinase 1.

BackgroundThe Polo-like Kinase 1 (PLK1) protein regulates cell cycle progression and is overexpressed in many malignant tissues. Overexpression is associated with poor prognosis in several cancer entities, whereby expression of PLK1 shows high inter-individual variability. Although PLK1 is extensively studied, not much is known about the genetic variability of the PLK1 gene. The function of PLK1 and the expression of the corresponding gene could be influenced by genomic variations. Hence, we investigated the gene for functional polymorphisms. Such polymorphisms could be useful to investigate whether PLK1 alters the risk for and the course of cancer and they could have an impact on the response to PLK1 inhibitors.MethodsThe coding region, the 5′ and 3′UTRs and the regulatory regions of PLK1 were systematically sequenced. We determined the allele frequencies and genotype distributions of putatively functional SNPs in 120 Caucasians and analyzed the linkage and haplotype structure using Haploview. The functional analysis included electrophoretic mobility shift assay (EMSA) for detected variants of the silencer and promoter regions and reporter assays for a 3′UTR polymorphism.ResultsFour putatively functional polymorphisms were detected and further analyzed, one in the silencer region (rs57973275), one in the core promoter region (rs16972787), one in intron 3 (rs40076) and one polymorphism in the 3′untranslated region (3′UTR) of PLK1 (rs27770). Alleles of rs27770 display different secondary mRNA structures and showed a distinct allele-dependent difference in mRNA stability with a significantly higher reporter activity of the A allele (p < 0.01).ConclusionThe present study provides evidence that at least one genomic variant of PLK1 has functional properties and influences expression of PLK1. This suggests polymorphisms of the PLK1 gene as an interesting target for further studies that might affect cancer risk, tumor progression as well as the response to PLK1 inhibitors.

Abstract 978: Inhibition of Polo-like kinase 1 as a strategy in the treatment of adrenocortical carcinoma

Abstract Adrenocortical carcinoma (ACC) is an aggressive cancer with a 5-year survival rate of 20-30%. Mitotane is the only approved drug for the treatment of patients with ACC. It often carries significant toxicities which result in the discontinuation of treatment. There are no approved second line therapies. The increased incidence of ACC in the cancer predisposition syndrome, Li-Fraumeni, suggests the involvement of the p53 pathway in ACC pathogenesis. Our analysis of the gene expression profiles of 19 ACC samples identified dysregulation of the G2/M transition and the activity of the p53 modulator, MDM2 as important in ACC pathogenesis. Polo-like kinase-1 (PLK1) is involved in the G2/M transition and acts to promote MDM2 activity through its phosphorylation. We observed that PLK1 inhibition by siRNA results in up to a 70% reduction in viability in the ACC cell lines SW-13 and H295R. Therefore, we studied PLK1 inhibition as a potential therapeutic strategy. We used the small molecule inhibitor, BI-2536, to inhibit PLK1 function in SW-13 and H295R. Drug-dose response curves demonstrated that both cell lines are sensitive to pharmacological inhibition of PLK1 (IC50 doses of 0.0094815 μM and 0.062805 μM for SW-13 and H295R, respectively.) Murine xenograft studies demonstrated that BI-2536 resulted in a statistically significant reduction in tumor growth in SW-13 but not H295R. Examination of p53 protein levels in the presence of the drug showed a dose-dependent reduction in p53 levels in SW-13, which carries a homozygous p53 mutation. The same was not true in H295R, which is p53 wild-type. To test the hypothesis that BI-2536 was decreasing mutant p53 levels by promoting its proteasomal degradation, both cell lines were treated with previously determined inhibitory concentrations of BI-2536, either alone or in combination with the proteasome inhibitor, MG132. Western blot analysis showed recovery of p53 protein when cells were concomitantly treated with MG132, supporting a role for BI-2536 as a regulator of proteasomal degradation. Both ACC cell lines are relatively insensitive to MDM2 inhibition by nutlin-3 (NCI60 GI50 values range from ∼4 uM to ∼2 uM versus ACC cell lines @13-15uM), possibly because PLK1 stimulates MDM2 and renders nutlin-3 ineffective. We therefore assessed the ability of BI-2536 to sensitize ACC cell lines to the effects of the MDM2 inhibitor. BI-2636 did show synergy with nutlin-3, shifting the IC50 in SW-13 from 19.78 μM to 6.45 μM and from 12.75 μM to 2.84 μM in H295R. These results demonstrate that inhibition of PLK1 alone or in combination with an MDM2 inhibitor warrants further investigation as a treatment for patients with ACC. Cells with mutant p53 are more sensitive to the growth inhibitory effects of BI-2536. In the context of wild-type p53, the combined inhibition of both PLK1 and MDM2 results in loss of viability. Further clinical development of PLK1 as a target should take p53 mutation status into account. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 978. doi:1538-7445.AM2012-978

Polo-like kinase 1 is overexpressed in colorectal cancer and participates in the migration and invasion of colorectal cancer cells

SummaryBackgroundPolo-like kinase 1 (PLK1) is an important molecule in proliferation of many human cancers. The aim of study is to clarify the expression patterns and potential function of PLK1 in colorectal cancers.Material/MethodsFifty-six colorectal cancers samples were collected and arranged onto a tissue array and the expression of PLK1 were detected by immunohistochemistry and correlated with clinico-pathological characteristics and expression of PCNA. Expression of PLK1 in 9 colorectal cancer cells lines was investigated by RT-PCR and Western blot, then SW1116 cells lines were treated with PLK1 siRNA and the efficiency was examined by Western blot. Transwell test was applied to detect the migration and invasion capability of cancer cells by counting the number of cells passing through the membranes. Cell proliferation and apoptosis were examined by Cell Counting Kit-8 (CCK-8) and Annexin-V Kit.ResultsPLK1 was positively expressed in 73.2% (41/56) of colorectal cancers tissues, but in only 3.6% (2/56) of normal tissues, and was associated with Duke’s stage (P<0.01), tumor size (P<0.01), invasion extent (P<0.05) and lymphatic metastasis (P<0.01). The expression of PLK1 was correlated with expression of PCNA (R=0.553, P<0.01). PLK1 was inhibited in SW1116 cells by treating with PLK1 siRNA oligos, which resulted in a decreased number of cells passing through the membrane as compared with control groups (P<0.01) at 24 hours after transfection. Cell proliferation was inhibited from 48 hours after transfection, while cells apoptosis was induced from 72 hours after transfection.ConclusionsPLK1 could be a progression marker for colorectal cancer patients and PLK1 depletion can inhibit migration and invasion capability of colorectal cancer cells SW1116, suggesting that PLK1 might be involved in metastasis and invasion of colorectal cancer. Therapeutic strategies targeting PLK1 may be a new approach to colorectal cancer.