PLK1-mediated phosphorylation of ß-catenin enhances its stability and transcriptional activity for extracellular matrix remodeling in metastatic NSCLC.

Rationale: ß-catenin is a component for cell adhesion and a transcriptional coactivator in epithelial-mesenchymal transition (EMT). Previously we found that catalytically active PLK1 drives EMT in non-small cell lung cancer (NSCLC), upregulating extracellular matrix factors including TSG6, laminin γ2, and CD44. To understand the underlying mechanism and clinical significance of PLK1 and ß-catenin in NSCLC, their relationship and function in metastatic regulation were investigated. Methods: The clinical relevance between the survival rate of NSCLC patients and the expression of PLK1 and ß-catenin was analyzed by a KM plot. Immunoprecipitation, kinase assay, LC-MS/MS spectrometry, and site-directed mutagenesis were performed to reveal their interaction and phosphorylation. A lentiviral doxycycline-inducible system, Transwell-based 3D culture, tail-vein injection model, confocal microscopy, and chromatin immunoprecipitation assays were used to elucidate the function of phosphorylated ß-catenin in the EMT of NSCLC. Results: Clinical analysis revealed that the high expression of CTNNB1/PLK1 was inversely correlated with the survival rates of 1,292 NSCLC patients, especially in metastatic NSCLC. In TGF-ß-induced or active PLK1-driven EMT, ß-catenin, PLK1, TSG6, laminin γ2, and CD44 were concurrently upregulated. ß-catenin is a binding partner of PLK1 in TGF-ß-induced EMT and is phosphorylated at S311. Phosphomimetic ß-catenin promotes cell motility, invasiveness of NSCLC cells, and metastasis in a tail-vein injection mouse model. Its upregulated stability by phosphorylation enhances transcriptional activity through nuclear translocation for the expression of laminin γ2, CD44, and c-Jun, therefore enhancing PLK1 expression by AP-1. Conclusions: Our findings provide evidence for the critical role of the PLK1/ß-catenin/AP-1 axis in metastatic NSCLC, implying that ß-catenin and PLK1 may serve as a molecular target and prognostic indicator of the therapeutic response in metastatic NSCLC patients.

Dual Targeting of EGFR with PLK1 Exerts Therapeutic Synergism in Taxane-Resistant Lung Adenocarcinoma by Suppressing ABC Transporters.

To overcome the limitations of chemoresistance, combination therapies using druggable targets have been investigated. Our previous studies led us to hypothesize that the downregulation of PLK1 expression or activity can be one strategy to overcome the hurdles of taxane resistance by the downregulation of ABC transporters. To explore this, various versions of PLK1 including a constitutively active version, kinase-dead form, and polo-box domain mutant were expressed in paclitaxel-resistant lung adenocarcinoma (LUADTXR). Targeting PLK1 using shRNA or non-functional mutants downregulated ABCB1, ABCC9, and ABCG2 in LUADTXR cells, which was similar to the downregulation effects from treatment with PLK1 inhibitors. The high expression of EGFR in LUAD led us to administer gefitinib, showing a markedly reduced EGFR level in LUADTXR cells. When gefitinib and PLK1 inhibitors were combined, LUADTXR cells tended to undergo apoptosis more effectively than parental cells, showing a synergistic effect on the downregulation of ABC transporters through c-Myc and AP-1. Clinical data provide evidence for the relevance between survival rates and expressions of PLK1 and EGFR in LUAD patients. Based on these results, we suggest that a combination of gefitinib and PLK1 inhibitors exerts strong synergism in LUADTXR, which helps to overcome the limitations associated with taxanes.

PLK1/vimentin signaling facilitates immune escape by recruiting Smad2/3 to PD-L1 promoter in metastatic lung adenocarcinoma.

The prerequisite function of vimentin for the epithelial-mesenchymal transition (EMT) is not clearly elucidated yet. Here, we show that vimentin phosphorylated by PLK1, triggers TGF-ß-signaling, which consequently leads to metastasis and PD-L1 expression for immune suppression in lung adenocarcinoma. The clinical correlation between expression of both vimentin and PLK1, and overall survival rates of patients was significant in lung adenocarcinoma but not in squamous cell carcinoma. The phosphorylation of vimentin was accompanied by the activation of PLK1 during TGF-ß-induced EMT in lung adenocarcinoma. Among the several phosphorylation sites determined by phospho-proteomic analysis and the site-specific mutagenesis, the phosphorylation at S339 displayed the most effective metastasis and tumourigenesis with the highest expression of PD-L1, compared with that of wild-type and other versions in both 3D cell culture and tail-vein injection metastasis models. Phosphomimetic vimentin at S339 interacted with p-Smad2 for its nuclear localization, leading to the expression of PD-L1. Clinical relevance revealed the inverse correlation between the survival rates of patients and the expressions of VIM, PLK1, and CD274 in primary and metastatic lung adenocarcinoma. Thus, PLK1-mediated phosphorylation of vimentin activates TGF-ß signaling pathway, leading to the metastasis and immune escape through the expression of PD-L1, functioning as a shuttling protein in lung adenocarcinoma.

Combination of Inhibitors of USP7 and PLK1 has a Strong Synergism against Paclitaxel Resistance.

USP7 is a promising target for the development of cancer treatments because of its high expression and the critical functions of its substrates in carcinogenesis of several different carcinomas. Here, we demonstrated the effectiveness of targeting USP7 in advanced malignant cells showing high levels of USP7, especially in taxane-resistant cancer. USP7 knockdown effectively induced cell death in several cancer cells of lung, prostate, and cervix. Depletion of USP7 induced multiple spindle pole formation in mitosis, and, consequently, resulted in mitotic catastrophe. When USP7 was blocked in the paclitaxel-resistant lung cancer NCI-H460TXR cells, which has resistance to mitotic catastrophe, NCI-H460TXR cells underwent apoptosis effectively. Furthermore, combination treatment with the mitotic kinase PLK1 inhibitor volasertib and the USP7 inhibitor P22077 showed a strong synergism through down-regulation of MDR1/ABCB1 in paclitaxel-resistant lung cancer. Therefore, we suggest USP7 is a promising target for cancer therapy, and combination therapy with inhibitors of PLK1 and USP7 may be valuable for treating paclitaxel-resistant cancers, because of their strong synergism.

Active PLK1-driven metastasis is amplified by TGF-ß signaling that forms a positive feedback loop in non-small cell lung cancer.

Early findings that PLK1 is highly expressed in cancer have driven an exploration of its functions in metastasis. However, whether PLK1 induces metastasis in vivo and its underlying mechanisms in NSCLC have not yet been determined. Here, we show that the expression of active PLK1 phosphorylated at T210, abundant in TGF-ß-treated lung cells, potently induced metastasis in a tail-vein injection model. Active PLK1 with intact polo-box and ATP-binding domains accelerated cell motility and invasiveness by triggering EMT reprogramming, whereas a phosphomimetic version of p-S137-PLK1 did not, indicating that the phosphorylation status of PLK1 may determine the cell traits. Active PLK1-driven invasiveness upregulated TGF-ß signaling and TSG6 encoded by TNFAIP6. Loss of TNFAIP6 disturbed the metastatic activity induced by active PLK1 or TGF-ß. Clinical relevance shows that PLK1 and TNFAIP6 are strong predictors of poor survival rates in metastatic NSCLC patients. Therefore, we suggest that active PLK1 promotes metastasis by upregulating TGF-ß signaling, which amplifies its metastatic properties by forming a positive feedback loop and that the PLK1/TGF-ß-driven metastasis is effectively blocked by targeting PLK1 and TSG6, providing PLK1 and TSG6 as negative markers for prognostics and therapeutic targets in metastatic NSCLC.

Correction: Active PLK1-driven metastasis is amplified by TGF-ß signaling that forms a positive feedback loop in non-small cell lung cancer.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Cotargeting Plk1 and androgen receptor enhances the therapeutic sensitivity of paclitaxel-resistant prostate cancer.

BACKGROUNDS: Despite the clinical success of taxanes, they still have limitations, such as chemoresistance. To overcome the limitations of paclitaxel, genetic alterations and targeting effects of altered genes were observed in paclitaxel-resistant cancer. Because paclitaxel-resistant cancer shows high levels of Plk1, a promising target in chemotherapy, the effectiveness of Plk1 inhibitors in paclitaxel-resistant cancer cells has been investigated. METHODS: Paclitaxel-resistant cancer cells were developed by exposure of stepwise escalating levels of paclitaxel. Genetic alterations were detected by quantitative reverse transcription polymerase chain reaction (qRT-PCR) and immunoblotting. Using a cell viability assay, combined targeting effects for Plk1 and androgen receptor (AR) were determined. Clinical data were analyzed to understand the relationship between Plk1 and AR in prostate cancer patients. RESULTS: Treatment with Plk1 inhibitors markedly reduced the expression of MDR1, MRP1, and Plk1 in the paclitaxel-resistant cancer. Among Plk1 inhibitors, genistein, recently found as a direct Plk1 inhibitor, tended to be more effective in the paclitaxel-resistant prostate cancer than the parental cancer cells, which was related to the suppression of the AR, as well as inhibition of Plk1 activity. A combination of Plk1 inhibitors and AR antagonist bicalutamide exhibited a synergistic effect in LNCaPTXR, as well as LNCaP cells, by inhibiting Plk1 and AR. Analysis of clinical data provides evidence for the relevance between Plk1 and AR in prostate cancer patients, showing that Plk1 and AR are strong predictors of poor survival rates. CONCLUSIONS: We suggest that cotargeting Plk1 and AR would be effective in advanced chemoresistant prostate cancer cells to overcome the limitations associated with paclitaxel.

Nrf2-lncRNA controls cell fate by modulating p53-dependent Nrf2 activation as an miRNA sponge for Plk2 and p21cip1.

Long noncoding RNA (lncRNA) capable of controlling antioxidative capacity remains to be investigated. Nuclear factor erythroid-2-related factor 2 (Nrf2) is a central molecule for cellular defense that increases antioxidative capacity. We identified a novel lncRNA named Nrf2-activating lncRNA (Nrf2-lncRNA) transcribed from an upstream region of the microRNA 122 gene (MIR122). Nrf2-lncRNA existed in the cytoplasm, suggestive of its function as a competing endogenous RNA [ceRNA, microRNA (miRNA) sponge]. Nrf2-lncRNA served as a ceRNA for polo-like kinase (Plk) 2 and cyclin-dependent kinase inhibitor 1 (p21cip1) through binding of miRNA 128 and miRNA 224, inducing Plk2/Nrf2/p21cip1 complexation for Nrf2 activation in the cells under p53-activating conditions (i.e., DNA damage and serum deprivation). Nrf2-lncRNA expression was suppressed with the initiation of apoptosis, being a rheostat for cell fate determination. Nrf2-lncRNA levels correlated with the recurrence-free postsurgery survival rate of patients with hepatocellular carcinoma. Collectively, Nrf2-lncRNA promotes Plk2 and p21cip1 translation by competing for specific miRNAs and activating Nrf2 under surviving conditions from oxidative stress, implying that Nrf2-lncRNA serves as a fine-tuning rheostat for cell fate decision.-Joo, M. S., Shin, S.-B., Kim, E. J., Koo, J. H., Yim, H., Kim, S. G. Nrf2-lncRNA controls cell fate by modulating p53-dependent Nrf2 activation as an miRNA sponge for Plk2 and p21cip1.

Oligopeptide Competition Assay for Phosphorylation Site Determination.

Protein phosphorylation at specific sites determines its conformation and interaction with other molecules. Thus, protein phosphorylation affects biological functions and characteristics of the cell. Currently, the most common method for discovering phosphorylation sites is by liquid chromatography/mass spectrometry (LC/MS) analysis, a rapid and sensitive method. However, relatively labile phosphate moieties are often released from phosphopeptides during the fragmentation step, which often yields false-negative signals. In such cases, a traditional in vitro kinase assay using site-directed mutants would be more accurate, but this method is laborious and time-consuming. Therefore, an alternative method using peptide competition may be advantageous. The consensus recognition motif of 5' adenosine monophosphate-activated protein kinase (AMPK) has been established1 and was validated using a positional scanning peptide library assay2. Thus, AMPK phosphorylation sites for a novel substrate could be predicted and confirmed by the peptide competition assays. In this report, we describe the detailed steps and procedures for the in vitro oligopeptide-competing kinase assay by illustrating AMPK-mediated nuclear factor erythroid 2-related factor 2 (Nrf2) phosphorylation. To authenticate the phosphorylation site, we carried out a sequential in vitro kinase assay using a site-specific mutant. Overall, the peptide competition assay provides a method to screen multiple potential phosphorylation sites and to identify sites for validation by the phosphorylation site mutants.

Comparative Analysis of a FRET-based PLK1 Kinase Assay to Identify PLK1 inhibitors for Chemotherapy.

Advanced techniques for detecting kinase inhibitors are in demand due to limitations of traditional approaches. Here, we used a fluorescence resonance energy transfer (FRET)-based kinase assay, a sensitive fluorescence turn-on biosensing platform, to identify a Polo-like kinase 1 (PLK1) inhibitor. The assay was developed with the Z'-Lyte™ FRET-peptide and PLK1 kinase purified from a baculovirus expression system. Using PLK1 inhibitors, sensitivity and efficiency of this FRET-based PLK1 kinase assay were compared to those of radioisotope-based and immunoblot-based assays. Although the inhibitory activity of BI 2536 against PLK1 kinase in each assay was almost the same, the FRET-based PLK1 kinase assay was much easier, faster, safer, and more convenient than a radioisotope-based assay or an immunoblot-based traditional kinase assay. From our findings, we suggest that a FRET-based PLK1 kinase assay is an advanced tool which overcomes the limitations of previous traditional kinase assays to detect kinase inhibitors for the development of anticancer drugs.

Sensitivity of TP53-Mutated Cancer Cells to the Phytoestrogen Genistein Is Associated With Direct Inhibition of Plk1 Activity.

Polo-like kinase 1 (Plk1), a conserved Ser/Thr mitotic kinase, has been identified as a promising target for anticancer drug development because its overexpression is correlated with malignancy. Here, we found that genistein, an isoflavone, inhibits Plk1 kinase activity directly. Previously the mitotic disturbance phenomenon induced by treatment with genistein was not fully explained by its inhibitory effect on EGFR. In kinase profiling assays, it showed selectivity relative to a panel of kinases, including EGFR. Treatment with genistein induced cell death in a concentration-dependent manner in cancer cells from diverse tissue origins, but not in non-transformed cells such as hTERT-RPE or MCF10A cells. We also observed that genistein tended to be more selective against cancer cells with mutations in the TP53 gene. TP53-depeleted LNCaP and NCI-H460 cells using shRNA targeting human TP53 were more sensitive to cell death by treatment of genistein. Furthermore, genistein induced mitotic arrest by inhibiting Plk1 activity and, consequently, led to mitotic catastrophe and apoptosis. These data suggest that genistein may be a promising anticancer drug candidate due to its inhibitory activity against Plk1 as well as EGFR and effectiveness toward cancer cells, especially those with p53-mutation. J. Cell. Physiol. 232: 2818-2828, 2017. © 2016 Wiley Periodicals, Inc.

Differential Cellular Effects of Plk1 Inhibitors Targeting the ATP-binding Domain or Polo-box Domain.

The expression of polo-like kinase 1 (Plk1) correlates with malignancy and is thus recognized as a target for cancer therapy. In addition to the development of ATP-competitive Plk1 inhibitors, the polo-box domain (PBD), a unique functional domain of PLKs, is being targeted to develop Plk1-specific inhibitors. However, the action mechanisms of these two classes of Plk1 inhibitors have not been thoroughly evaluated. Here, we evaluate the differences in cellular effects of ATP-binding domain inhibitors (BI 2536, GSK 461364) and PBD inhibitors (poloxin, thymoquinone) to determine their mechanisms of Plk1 inhibition. Our data show that BI 2536 and GSK461364 increased the population of cells in the G2/M phase compared with controls, while treatment with poloxin and thymoquinone increased cell population in the S phase as well as in G2/M, in a p53-independent manner. The population of cells staining positively for p-Histone H3 and MPM2, mitotic index, was increased by treatment with BI 2536 or GSK461364, but not by treatment with poloxin or thymoquinone. Furthermore, treatment with BI 2536 or GSK461364 resulted in activation of the BubR1 spindle checkpoint kinase, suggesting that treatment with ATP-binding domain inhibitors induces metaphase arrest. However, the administration of poloxin and thymoquinone resulted in an increase in p21(WAF1) and S arrest, indicating that PBD inhibitors also affected interphase before mitotic entry. Taken together, these data suggest that the PDB of Plk1 plays a role in S phase progression through interaction with other proteins, while its ATP-binding domain is important for regulating mitotic progression mediated by its catalytic activity involving consumption of ATP.