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

PLK1 inhibition down-regulates polycomb group protein BMI1 via modulation of the miR-200c/141 cluster

The polycomb group protein BMI1 is an important regulator of cancer stem cell (CSC) phenotype and is often overexpressed in cancer cells. Its overexpression leads to increase in CSC fraction and therapy resistance in tumors. BMI1 functions via polycomb repressive complex 1 (PRC1)-mediated gene silencing and also via PRC1-independent transcriptional activities. At present, very little is known about the therapy reagents that can efficiently inhibit BMI1 expression, and the CSC phenotype. Here, we report that the polo-like kinase 1 (PLK1) regulates BMI1 expression, and that its inhibition can efficiently down-regulate BMI1 expression and PRC1 activity, and induce premature senescence in breast cancer cells. We also show that the exogenous BMI1 overexpression mitigates anti-oncogenic effects of PLK1 inhibition and overcomes senescence induction by PLK1 inhibitors. We further show that PLK1 inhibition down-regulates BMI1 by upregulating the miRNA-200c/141 cluster, which encodes miR-200c and miR-141, both of which are known to post-transcriptionally downregulate BMI1 expression. Thus, our data suggest that PLK1 inhibitors can be successfully used to inhibit growth of tumors in which PcG protein BMI1 is overexpressed or the PRC1 activity is deregulated.

The same, only different - DNA damage checkpoints and their reversal throughout the cell cycle

Cell cycle checkpoints activated by DNA double-strand breaks (DSBs) are essential for the maintenance of the genomic integrity of proliferating cells. Following DNA damage, cells must detect the break and either transiently block cell cycle progression, to allow time for repair, or exit the cell cycle. Reversal of a DNA-damage-induced checkpoint not only requires the repair of these lesions, but a cell must also prevent permanent exit from the cell cycle and actively terminate checkpoint signalling to allow cell cycle progression to resume. It is becoming increasingly clear that despite the shared mechanisms of DNA damage detection throughout the cell cycle, the checkpoint and its reversal are precisely tuned to each cell cycle phase. Furthermore, recent findings challenge the dogmatic view that complete repair is a precondition for cell cycle resumption. In this Commentary, we highlight cell-cycle-dependent differences in checkpoint signalling and recovery after a DNA DSB, and summarise the molecular mechanisms that underlie the reversal of DNA damage checkpoints, before discussing when and how cell fate decisions after a DSB are made.

PLK1 phosphorylates PAX3-FOXO1, the inhibition of which triggers regression of alveolar Rhabdomyosarcoma

Pediatric tumors harbor very low numbers of somatic mutations and therefore offer few targets to improve therapeutic management with targeted drugs. In particular, outcomes remain dismal for patients with metastatic alveolar rhabdomyosarcoma (aRMS), where the chimeric transcription factor PAX3/7-FOXO1 has been implicated but problematic to target. In this report, we addressed this challenge by developing a two-armed screen for druggable upstream regulatory kinases in the PAX3/7-FOXO1 pathway. Screening libraries of kinome siRNA and small molecules, we defined PLK1 as an upstream-acting regulator. Mechanistically, PLK1 interacted with and phosphorylated PAX3-FOXO1 at the novel site S503, leading to protein stabilization. Notably, PLK1 inhibition led to elevated ubiquitination and rapid proteasomal degradation of the PAX3-FOXO1 chimeric oncoprotein. On this basis, we embarked on a preclinical validation of PLK1 as a target in a xenograft mouse model of aRMS, where the PLK1 inhibitor BI 2536 reduced PAX3-FOXO1-mediated gene expression and elicited tumor regression. Clinically, analysis of human aRMS tumor biopsies documented high PLK1 expression to offer prognostic significance for both event-free survival and overall survival. Taken together, these preclinical studies validate the PLK1-PAX3-FOXO1 axis as a rational target to treat aRMS.

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.

Identification of the PLK2-dependent phosphopeptidome by quantitative proteomics [corrected]

Polo-like kinase 2 (PLK2) has been recently recognized as the major enzyme responsible for phosphorylation of α-synuclein at S129 in vitro and in vivo, suggesting that this kinase may play a key role in the pathogenesis of Parkinson's disease and other synucleinopathies. Moreover PLK2 seems to be implicated in cell division, oncogenesis, and synaptic regulation of the brain. However little is known about the phosphoproteome generated by PLK2 and, consequently the overall impact of PLK2 on cellular signaling. To fill this gap we exploited an approach based on in vitro kinase assay and quantitative phosphoproteomics. A proteome-derived peptide library obtained by digestion of undifferentiated human neuroblastoma cell line was exhaustively dephosphorylated by lambda phosphatase followed by incubation with or without PLK2 recombinant kinase. Stable isotope labeling based quantitative phosphoproteomics was applied to identify the phosphosites generated by PLK2. A total of 98 unique PLK2-dependent phosphosites from 89 proteins were identified by LC-MS/MS. Analysis of the primary structure of the identified phosphosites allowed the detailed definition of the kinase specificity and the compilation of a list of potential PLK2 targets among those retrieved in PhosphositePlus, a curated database of in cell/vivo phosphorylation sites.

CK2 phosphorylates and inhibits TAp73 tumor suppressor function to promote expression of cancer stem cell genes and phenotype in head and neck cancer

Cancer stem cells (CSC) and genes have been linked to cancer development and therapeutic resistance, but the signaling mechanisms regulating CSC genes and phenotype are incompletely understood. CK2 has emerged as a key signal serine/threonine kinase that modulates diverse signal cascades regulating cell fate and growth. We previously showed that CK2 is often aberrantly expressed and activated in head and neck squamous cell carcinomas (HNSCC), concomitantly with mutant (mt) tumor suppressor TP53, and inactivation of its family member, TAp73. Unexpectedly, we observed that classical stem cell genes Nanog, Sox2, and Oct4, are overexpressed in HNSCC with inactivated TAp73 and mtTP53. However, the potential relationship between CK2, TAp73 inactivation, and CSC phenotype is unknown. We reveal that inhibition of CK2 by pharmacologic inhibitors or siRNA inhibits the expression of CSC genes and side population (SP), while enhancing TAp73 mRNA and protein expression. Conversely, CK2 inhibitor attenuation of CSC protein expression and the SP by was abrogated by TAp73 siRNA. Bioinformatic analysis uncovered a single predicted CK2 threonine phosphorylation site (T27) within the N-terminal transactivation domain of TAp73. Nuclear CK2 and TAp73 interaction, confirmed by co-immunoprecipitation, was attenuated by CK2 inhibitor, or a T27A point-mutation of this predicted CK2 threonine phospho-acceptor site of TAp73. Further, T27A mutation attenuated phosphorylation, while enhancing TAp73 function in repressing CSC gene expression and SP cells. A new CK2 inhibitor, CX-4945, inhibited CSC related SP cells, clonogenic survival, and spheroid formation. Our study unveils a novel regulatory mechanism whereby aberrant CK2 signaling inhibits TAp73 to promote the expression of CSC genes and phenotype.

Gene expression changes during repopulation in a head and neck cancer xenograft

BACKGROUND/PURPOSE

To investigate temporal changes in global gene expression and pathways involved in the response to irradiation during phases of growth inhibition, recovery and repopulation in a human head and neck squamous cell cancer (HNSCC) xenograft.

METHODS AND MATERIALS

Low passage head and neck squamous cancer cells (UT-14-SCC) were injected into the flanks of female nu/nu mice to generate xenografts. After tumors reached a size of 500 mm3, they were treated with either sham RT or 15Gy in one fraction. At different time points, days 0, 3, and 10 for controls and days 4, 7, 12, and 21 after irradiation, the tumors were harvested for global gene expression analysis and pathway analysis.

RESULTS

The tumors showed growth inhibition through days 4-7 and began the transition to regrowth around the day 12 time point. When comparing the pooled controls to each day of treatment, there were 22, 119, 125, and 25 differentially expressed genes on days 4, 7, 12, and 21 respectively using a p⩽0.01 and a 2-fold cut-off. Gene Ontology (GO), gene set enrichment analysis (GSEA) and sub-network enrichment analysis (SNEA) identified different biological processes, cell process pathways and expression targets to be active on each time point after irradiation. An important observation was that the molecular events on day 12 which represented the transition from growth inhibition to regrowth identified interferon and cytokine related genes and signaling pathways as the most prominent.

CONCLUSION

The findings in this study compliment research which has identified components of interferon-related signaling pathways to be involved in radioresistance. Further work will be required to understand the significance of these genes in both radioresistance and treatment response leading to new therapeutic strategies and prognostic tools.

Bufalin inhibits proliferation and invasion of human liver cancer cells

AIM: To assess the effect of bufalin on cell proliferation, migration and invasion in human hepatocellular carcinoma cell line BEL-7402.

METHODS: BEL-7402 cells were cultured in vitro. The effect of bufalin on cell proliferation and cell cycle was measured by CCK-8 colorimetric method and flow cytometry. Cell migration and invasion were assessed by Transwell assay.

RESULTS: Bufalin significantly inhibited the growth and proliferation of BEL-7402 cells in a dose- and time-dependent manner. Bufalin at a dose of 0.085 μg/mL blocked the cell cycle of BEL-7402 cells in G₂/S phase [48 h (G₂: t = -6.618, P < 0.01; S: t = -5.339, P < 0.01), 72 h (G₂: t = -14.273, P < 0.01; S: t = -4.812, P < 0.01)]. Treatment with bufalin at a dose of 0.085 μg/mL for 72 h significantly inhibited the migration (t = 11.717, P < 0.01) and invasion (t = 5.437, P < 0.01) of BEL-7402 cells.

CONCLUSION: Bufalin inhibits the migration and invasion of human liver cancer cells. Besides, bufalin can inhibit the proliferation of human liver cancer cells by blocking the cell cycle in G₂/S phase. The anti-proliferative effect of bufalin is dose- and time-dependent.

PLK1 is a binding partner and a negative regulator of FOXO3 tumor suppressor

FOXO family members (FOXOs: FOXO1, FOXO3, FOXO4 and FOXO6) are important transcription factors and tumor suppressors controlling cell homeostasis and cell fate. They are characterized by an extraordinary functional diversity, being involved in regulation of cell cycle, proliferation, apoptosis, DNA damage response, oxidative detoxification, cell differentiation and stem cell maintenance, cell metabolism, angiogenesis, cardiac and other organ's development, aging, and other critical cellular processes. FOXOs are tightly regulated by reversible phosphorylation, ubiquitination, acetylation and methylation. Interestingly, the known kinases phosphorylate only a small percentage of the known or predicted FOXOs phosphorylation sites, suggesting that additional kinases that phosphorylate and control FOXOs activity exist. In order to identify novel regulators of FOXO3, we have employed a proteomics screening strategy. Using HeLa cancer cell line and a Tandem Affinity Purification followed by Mass Spectrometry analysis, we identified several proteins as binding partners of FOXO3. Noteworthy, Polo Like Kinase 1 (PLK1) proto-oncogene was one of the identified FOXO3 binding partners. PLK1 plays a critical role during cell cycle (G2-M transition and all phases of mitosis) and in maintenance of genomic stability. Our experimental results presented in this manuscript demonstrate that FOXO3 and PLK1 exist in a molecular complex through most of the phases of the cell cycle, with a higher occurrence in the G2-M cell cycle phases. PLK1 induces translocation of FOXO3 from the nucleus to the cytoplasm and suppresses FOXO3 activity, measured by the decrease in the pro-apoptotic Bim protein levels and in the cell cycle inhibitor protein p27. Furthermore, PLK1 can directly phosphorylate FOXO3 in an in vitro kinase assay. These results present the discovery of PLK1 proto-oncogene as a binding partner and a negative regulator of FOXO3 tumor suppressor.