Polo-like kinases and oncogenesis

Structural regulation of PLK1 activity: implications for cell cycle function and drug discovery

Polo Like Kinase 1 (PLK1), a key regulator of mitosis whose overexpression is often associated with poor survival rates in cancer, continues to be widely investigated as an oncology drug target with clinical trials evaluating second and third generation inhibitors. In addition to the conserved N-terminal kinase domain (KD), a unique characteristic of the Polo-Like kinase family is the C-terminal polo-box domain (PBD). The PBD contains a phosphopeptide binding site that recognizes substrates primed by other kinases and furthermore is responsible for subcellular localization of PLK1 to specific sites in the nucleus including centrosomes and kinetochores. Another role of the PBD is its regulatory ability through domain-domain interactions with the KD to maintain an autoinhibited state of PLK1. Insights into post translational modifications and the PBD - KD domain-domain association have been obtained and show that key events in PLK1 regulation include phosphosubstrate binding, T210 phosphorylation and engagement with the Bora protein. These can induce an open and active conformation where the domain-domain inhibitory interactions no longer dominate. Further regulatory events recently described include the interchange between monomeric and dimeric forms, which can also serve to inhibit or activate PLK1 during the cell cycle. Different oligomeric forms of PLK1, existing as homodimers and heterodimers with PLK2, have been identified and likely play context dependent roles. This review provides an overview of recent information describing structural and mechanistic insights into inhibition of PLK1 and the temporal and spatial requirements of its activation and regulation. It also covers recent insights into the conformational regulation of other members of the Polo-Like kinase family. The implications of the conformational regulation of PLK1 with respect to cell cycle function and drug discovery are significant and are therefore discussed in detail.

Pharmacological inhibition of PLK1/PRC1 triggers mitotic catastrophe and sensitizes lung cancers to chemotherapy

Polo-like kinase 1 (PLK1) signaling drives tumor malignancy and chemotherapy resistance, which is an unmet clinical need. Recruiting PLK1 to the central spindle during anaphase is necessary for its function in promoting cancer cell proliferation, which is achieved by binding to microtubule-associated protein regulating of cytokinesis (PRC1) located in the spindle. However, the role of PLK1/PRC1 signaling in chemotherapy resistance is unknown. In this study, we identified a small molecule B4 which inhibited PLK1/PRC1 signaling through disrupting the formation of PLK1/PRC1 protein complexes. In the presence of blocking PLK1/PRC1 signaling, enhanced sensitivity of drug-resistant tumors to traditional chemotherapy was found. Suppression of PLK1 activity by B4 inhibited disease progression in allograft models, and combination with cisplatin elicited dramatic regression of drug-resistant tumors. Our findings provide a promising strategy to target the PLK1 signaling cascade and demonstrate a potential modality to enhance sensitivity to chemotherapy in non-small cell lung cancer (NSCLC).

Cyclin C promotes development and progression of B-cell acute lymphoblastic leukemia by counteracting p53-mediated stress responses

Despite major therapeutic advances in the treatment of acute lymphoblastic leukemia (ALL), resistances and long-term toxicities still pose significant challenges. Cyclins and their associated cyclin-dependent kinases are one focus of cancer research when looking for targeted therapies. We discovered cyclin C to be a key factor for B-cell ALL (B-ALL) development and maintenance. While cyclin C is not essential for normal hematopoiesis, CcncΔ/Δ BCR::ABL1+ B-ALL cells fail to elicit leukemia in mice. RNA sequencing experiments revealed a p53 pathway deregulation in CcncΔ/Δ BCR::ABL1+ cells resulting in the inability of the leukemic cells to adequately respond to stress. A genome-wide CRISPR/Cas9 loss-of-function screen supplemented with additional knock-outs unveiled a dependency of human B-lymphoid cell lines on CCNC. High cyclin C levels in B-cell precursor (BCP) ALL patients were associated with poor event-free survival and increased risk of early disease recurrence after remission. Our findings highlight cyclin C as a potential therapeutic target for B-ALL, particularly to enhance cancer cell sensitivity to stress and chemotherapy.

The multi-kinase inhibitor CG-806 exerts anti-cancer activity against acute myeloid leukemia by co-targeting FLT3, BTK, and aurora kinases

Despite the development of several Fms-like tyrosine kinase 3 (FLT3) inhibitors that have improved outcomes in patients with FLT3-mutant acute myeloid leukemia (AML), drug resistance is frequently observed, which may be associated with the activation of additional pro-survival pathways, such as those regulated by BTK, aurora kinases (AuroK), and potentially others, in addition to acquired tyrosine kinase domain (TKD) mutations of FLT3 gene. FLT3 may not always be a driver mutation. We evaluated the anti-leukemia efficacy of the novel multi-kinase inhibitor CG-806, which targets FLT3 and other kinases, to circumvent drug resistance and target FLT3 wild-type (WT) cells. The anti-leukemia activity of CG-806 was investigated by measuring apoptosis induction and analyzing the cell cycle using flow cytometry in vitro. CG-806 demonstrated superior anti-leukemia efficacy compared to commercially available FLT3 inhibitors, both in vitro and in vivo, regardless of FLT3 mutational status. The mechanism of action of CG-806 may involve its broad inhibitory profile against FLT3, BTK, and AuroK. In FLT3 mutant cells, CG-806 induced G1 phase blockage, whereas in FLT3 WT cells, it resulted in G2/M phase arrest. Targeting FLT3 and Bcl-2 and/or Mcl-1 simultaneously results in a synergistic pro-apoptotic effect in FLT3 mutant leukemia cells. The results of this study suggest that CG-806 is a promising multi-kinase inhibitor with anti-leukemic efficacy regardless of FLT3 mutational status. A phase 1 clinical trial of CG-806 for the treatment of AML has been initiated (NCT04477291).

PLK1 and PARP positively correlate in Middle Eastern breast cancer and their combined inhibition overcomes PARP inhibitor resistance in triple negative breast cancer

Background

Despite advancements in treatment approaches, patients diagnosed with aggressive breast cancer (BC) subtypes typically face an unfavorable prognosis. Globally, these cancers continue to pose a significant threat to women's health, leading to substantial morbidity and mortality. Consequently, there has been a significant struggle to identify viable molecular targets for therapeutic intervention in these patients. Polo-like Kinase-1 (PLK1) represents one of these molecular targets currently undergoing rigorous scrutiny for the treatment of such tumors. Yet, its role in the pathogenesis of BC in Middle Eastern ethnicity remains unexplored.

Methods

We investigated the expression of PLK1 protein in a cohort of more than 1500 Middle Eastern ethnicity BC cases by immunohistochemistry. Association with clinicopathological parameters and prognosis were performed. In vitro studies were conducted using the PLK1 inhibitor volasertib and the PARP inhibitor olaparib, either alone or in combination, in PTC cell lines.

Results

Overexpression of PLK1 was detected in 27.4% of all BC cases, and this was notably correlated with aggressive clinicopathological markers. PLK1 was enriched in the triple-negative breast cancer (TNBC) subtype and exhibited poor overall survival (p = 0.0347). Notably, there was a positive correlation between PLK1 and PARP overexpression, with co-expression of PLK1 and PARP observed in 15.7% of cases and was associated with significantly poorer overall survival (OS) compared to the overexpression of either protein alone (p = 0.0050). In vitro, we studied the effect of PLK1 and PARP inhibitors either single or combined treatments in two BRCA mutated, and one BRCA proficient TNBC cell lines. We showed that combined inhibition significantly reduced cell survival and persuaded apoptosis in TNBC cell lines. Moreover, our findings indicate that inhibition of PLK1 can reinstate sensitivity in PARP inhibitor (PARPi) resistant TNBC cell lines.

Conclusion

Our results shed light on the role of PLK1 in the pathogenesis and prognosis of Middle Eastern BC and support the potential clinical development of combined inhibition of PLK1 and PARP, a strategy that could potentially broaden the use of PLK1 and PARP inhibitors beyond BC cases lacking BRCA.

Design, synthesis, and biological evaluation of novel pyrimidin-2-amine derivatives as potent PLK4 inhibitors

Serine/threonine protein kinase PLK4 is a master regulator of centriole duplication, which is significant for maintaining genome integrity. Accordingly, due to the detection of PLK4 overexpression in a variety of cancers, PLK4 has been identified as a candidate anticancer target. Thus, it is a very meaningful to find effective and safe PLK4 inhibitors for the treatment of cancer. However, the reported PLK4 inhibitors are scarce and have potential safety issues. In this study, a series of novel and potent PLK4 inhibitors with an aminopyrimidine core was obtained utilizing the scaffold hopping strategy. The in vitro enzyme activity results showed that compound 8h (PLK4 IC50 = 0.0067 μM) displayed high PLK4 inhibitory activity. In addition, compound 8h exhibited a good plasma stability (t1/2 > 289.1 min), liver microsomal stability (t1/2 > 145 min), and low risk of DDIs. At the cellular level, it presented excellent antiproliferative activity against breast cancer cells. Taken together, these results suggest that compound 8h has potential value in the further research of PLK4-targeted anticancer drugs.

Nanocarriers for photodynamic-gene therapy

The use of nanotechnology in medicine has important potential applications, including in anticancer strategies. Nanomedicine has made it possible to overcome the limitations of conventional monotherapies, in addition to improving therapeutic results by means of synergistic or cumulative effects. A highlight is the combination of gene therapy (GT) and photodynamic therapy (PDT), which are alternative anticancer approaches that have attracted attention in the last decade. In this review, strategies involving the combination of PDT and GT will be discussed, together with the role of nanocarriers (nonviral vectors) in this synergistic therapeutic approach, including aspects related to the design of nanomaterials, responsiveness, the interaction of the nanomaterial with the biological environment, and anticancer performance in studies in vitro and in vivo.

GSK461364A suppresses proliferation of gastric cancer cells and induces apoptosis

Polo-like kinase-1 (PLK1) is crucial in regulating cell division and has been shown to have an oncogenic function in several cancers. Since PLK1 overexpression is closely related to tumorigenesis and has been correlated with poor clinical outcomes, specific inhibition of PLK1 in cancer cells is a promising approach for developing new anticancer drugs. In this context, the aim of the present study was to evaluated the potential cytotoxic effects of GSK461364A, a competitive inhibitor for PLK1, in gastric cancer cell line SNU-1 cells and explored its cytotoxic mechanism. The cells were exposed to GSK461364A at different concentrations ranging from 1 to 40 μM for 24 h, and it showed considerable cytotoxicity with an IC50 value of 4.34 μM. The treatment of SNU-1 cells with GSK461364A results in cell cycle arrest at the G2/M phase, decreased mitochondrial membrane potential, and increased apoptosis as indicated by Annexin V binding assay. In addition, GSK461364A treatment significantly increased the total oxidant (TOS) level, a signal of oxidative stress, and increased cleaved PARP and 8-oxo-dG levels as an indicator of DNA damage. ELISA experiments evaluating Bax, BCL-2, and cleaved caspase-3 also confirmed the apoptotic effect of GSK461364A. Current findings suggest that GSK461364A may be a chemotherapeutic agent in patients with gastric cancer. Nevertheless, more research is needed to evaluate GSK461364A as a cancer treatment drug.

The toxicity of dysregulated Plk1 activity revealed by its suppressor mutations

Polo-like kinase 1 (Plk1) is a mitotic kinase that has multiple functions throughout the cell cycle. Catalytic activation of Plk1 is known to be regulated by phosphorylation of the kinase domain, including Thr210, and by releasing the kinase domain from its inhibitory polo-box domain. However, how Plk1 is activated to fulfill its proper roles, in time and space, is not well understood. In this study, we unintentionally found that the expression of a constitutively active form of human Plk1 is toxic to bacterial cells, such that cells contained point mutations that alleviate the kinase activity. Structural prediction revealed that these mutations are adjacent to the amino acids supporting the kinase activity. When human cells express these mutants, we found decreased levels of Plk1's substrate phosphorylation, resulting in mitotic defects. Moreover, unlike in bacterial cells, the expression of activated Plk1 mutants did not affect cell proliferation in human cells unless localized at the right place in mitosis. Our observations identified new suppressor mutations and underscored the importance of spatiotemporal regulation in Plk1, providing a basis for how we might intervene in this kinase for therapeutic purpose in human cells.

The multi-kinase inhibitor CG-806 exerts anti-cancer activity against acute myeloid leukemia by co-targeting FLT3, BTK, and Aurora kinases

Background

Despite the development of several FLT3 inhibitors that have improved outcomes in patients with FLT3-mutant acute myeloid leukemias (AML), drug resistance is frequently observed, which may be associated with the activation of additional pro-survival pathways such as those regulated by BTK, aurora kinases, and potentially others in addition to acquired tyrosine kinase domains (TKD) mutations of FLT3 gene. FLT3may not always be a driver mutation.

Objective

To evaluate the anti-leukemia efficacy of the novel multi-kinase inhibitor CG-806, which targets FLT3 and other kinases, in order to circumvent drug resistance and target FLT3 wild-type (WT) cells.

Methods

The anti-leukemia activity of CG-806 was investigated by measuring apoptosis induction and analyzing cell cycle with flow cytometry in vitro, and its anti-leukemia.

Results

CG-806 demonstrated superior anti-leukemia efficacy compared to commercially available FLT3 inhibitors, both in vitro and in vivo, regardless of FLT3 mutational status. The mechanism of action of CG-806 may involve its broad inhibitory profile of FLT3, BTK, and aurora kinases. InFLT3 mutant cells, CG-806 induced G1 phase blockage, while in FLT3WT cells, it resulted in G2/M arrest. Targeting FLT3 and Bcl-2 and/or Mcl-1 simultaneously resulted in a synergistic pro-apoptotic effect in FLT3mutant leukemia cells.

Conclusion

The results of this study suggest that CG-806 is a promising multi-kinase inhibitor with anti-leukemia efficacy, regardless of FLT3 mutational status. A phase 1 clinical trial of CG-806 for the treatment of AML has been initiated (NCT04477291).

HO-1089 and HO-1197, Novel Herbal Formulas, Have Antitumor Effects via Suppression of PLK1 (Polo-like Kinase 1) Expression in Hepatocellular Carcinoma

The treatment for hepatocellular carcinoma (HCC), a severe cancer with a very high mortality rate, begins with the surgical resection of the primary tumor. For metastasis or for tumors that cannot be resected, sorafenib, a multi-tyrosine protein kinase inhibitor, is usually the drug of choice. However, typically, neither resection nor sorafenib provides a cure. The drug discovery strategy for HCC therapy is shifting from monotherapies to combination regimens that combine an immuno-oncology agent with an angiogenesis inhibitor. Herbal formulas can be included in the combinations used for this personalized medicine approach. In this study, we evaluated the HCC anticancer efficacy of the new herbal formula, HO-1089. Treatment with HO-1089 inhibited HCC tumor growth by inducing DNA damage-mediated apoptosis and by arresting HCC cell replication during the G2/M phase. HO-1089 also attenuated the migratory capacity of HCC cells via the inhibition of the expression of EMT-related proteins. Biological pathways involved in metabolism and the mitotic cell cycle were suppressed in HO-1089-treated HCC cells. HO-1089 attenuated expression of the G2/M phase regulatory protein, PLK1 (polo-like kinase 1), in HCC cells. HCC xenograft mouse models revealed that the daily oral administration of HO-1089 retarded tumor growth without systemic toxicity in vivo. The use of HO-1197, a novel herbal formula derived from HO-1089, resulted in statistically significant improved anticancer efficacy relative to HO-1089 in HCC. These results suggest that HO-1089 is a safe and potent integrated natural medicine for HCC therapy.

Escape from G1 arrest during acute MEK inhibition drives the acquisition of drug resistance

Mutations and gene amplifications that confer drug resistance emerge frequently during chemotherapy, but their mechanism and timing are poorly understood. Here, we investigate BRAFV600E amplification events that underlie resistance to the MEK inhibitor selumetinib (AZD6244/ARRY-142886) in COLO205 cells, a well-characterized model for reproducible emergence of drug resistance, and show that BRAF amplifications acquired de novo are the primary cause of resistance. Selumetinib causes long-term G1 arrest accompanied by reduced expression of DNA replication and repair genes, but cells stochastically re-enter the cell cycle during treatment despite continued repression of pERK1/2. Most DNA replication and repair genes are re-expressed as cells enter S and G2; however, mRNAs encoding a subset of factors important for error-free replication and chromosome segregation, including TIPIN, PLK2 and PLK3, remain at low abundance. This suggests that DNA replication following escape from G1 arrest in drug is more error prone and provides a potential explanation for the DNA damage observed under long-term RAF-MEK-ERK1/2 pathway inhibition. To test the hypothesis that escape from G1 arrest in drug promotes de novo BRAF amplification, we exploited the combination of palbociclib and selumetinib. Combined treatment with selumetinib and a dose of palbociclib sufficient to reinforce G1 arrest in selumetinib-sensitive cells, but not to impair proliferation of resistant cells, delays the emergence of resistant colonies, meaning that escape from G1 arrest is critical in the formation of resistant clones. Our findings demonstrate that acquisition of MEK inhibitor resistance often occurs through de novo gene amplification and can be suppressed by impeding cell cycle entry in drug.

In vivo delivery of CRISPR-Cas9 genome editing components for therapeutic applications

Since its mechanism discovery in 2012 and the first application for mammalian genome editing in 2013, CRISPR-Cas9 has revolutionized the genome engineering field and created countless opportunities in both basic science and translational medicine. The first clinical trial of CRISPR therapeutics was initiated in 2016, which employed ex vivo CRISPR-Cas9 edited PD-1 knockout T cells for the treatment of non-small cell lung cancer. So far there have been dozens of clinical trials registered on ClinicalTrials.gov in regard to using the CRISPR-Cas9 genome editing as the main intervention for therapeutic applications; however, most of these studies use ex vivo genome editing approach, and only a few apply the in vivo editing strategy. Compared to ex vivo editing, in vivo genome editing bypasses tedious procedures related to cell isolation, maintenance, selection, and transplantation. It is also applicable to a wide range of diseases and disorders. The main obstacles to the successful translation of in vivo therapeutic genome editing include the lack of safe and efficient delivery system and safety concerns resulting from the off-target effects. In this review, we highlight the therapeutic applications of in vivo genome editing mediated by the CRISPR-Cas9 system. Following a brief introduction of the history, biology, and functionality of CRISPR-Cas9, we showcase a series of exemplary studies in regard to the design and implementation of in vivo genome editing systems that target the brain, inner ear, eye, heart, liver, lung, muscle, skin, immune system, and tumor. Current challenges and opportunities in the field of CRISPR-enabled therapeutic in vivo genome editing are also discussed.

Sequential Targeting of PLK1 and PARP1 Reverses the Resistance to PARP Inhibitors and Enhances Platin-Based Chemotherapy in BRCA-Deficient High-Grade Serous Ovarian Cancer with KRAS Amplification

Ovarian cancer (OC) accounts for approximately 4% of cancer deaths in women worldwide and is the deadliest gynecologic malignancy. High-grade serous ovarian cancer (HGSOC) is the most predominant ovarian cancer, in which BRCA1/2 gene mutation ranges from 3 to 27%. PARP inhibitors (PARPi) have shown promising results as a synthetically lethal therapeutic approach for BRCA mutant and recurrent OC in clinical use. However, emerging data indicate that BRCA-deficient cancers may be resistant to PARPi, and the mechanisms of this resistance remain elusive. We found that amplification of KRAS likely underlies PARPi resistance in BRCA2-deficient HGSOC. Our data suggest that PLK1 inhibition restores sensitivity to PARPi in HGSOC with KRAS amplification. The sequential combination of PLK1 inhibitor (PLK1i) and PARPi drastically reduces HGSOC cell survival and increases apoptosis. Furthermore, we were able to show that a sequential combination of PLK1i and PARPi enhanced the cellular apoptotic response to carboplatin-based chemotherapy in KRAS-amplified resistant HGSOC cells and 3D spheroids derived from recurrent ovarian cancer patients. Our results shed new light on the critical role of PLK1 in reversing PARPi resistance in KRAS-amplified HGSOC, and offer a new therapeutic strategy for this class of ovarian cancer patients where only limited options currently exist.

Shedding light on the binding mechanism of kinase inhibitors BI-2536, Volasetib and Ro-3280 with their pharmacological target PLK1

In the present work, the interactions of the novel kinase inhibitors BI-2536, Volasetib (BI-6727) and Ro-3280 with the pharmacological target PLK1 have been studied by fluorescence spectroscopy and molecular dynamics calculations. High Stern-Volmer constants were found in fluorescence experiments suggesting the formation of stable protein-ligand complexes. In addition, it was observed that the binding constant between BI-2536 and PLK1 increases about 100-fold in presence of the phosphopeptide Cdc25C-p that docks to the polo box domain of the protein and releases the kinase domain. All the determined binding constants are higher for the kinase inhibitors than for their competitor for the active center (ATP) being BI-2536 and Volasertib the inhibitors that showed more affinity for PLK1. Calculated binding free energies confirmed the higher affinity of PLK1 for BI-2536 and Volasertib than for ATP. The higher affinity of the inhibitors to PLK1 compared to ATP was mainly attributed to stronger van der Waals interactions. Results may help with the challenge of designing and developing new kinase inhibitors more effective in clinical cancer therapy.

PLK2 targets GSK3β to protect against cisplatin-induced acute kidney injury

Cisplatin-induced acute kidney injury (AKI), which is accompanied by a rapid decline in renal function and a high risk of death, is a complex critical illness with no effective or specific treatment. Polo-like kinase 2 (PLK2), a serine/threonine kinase, is involved in the progression of multiple diseases, including cancers, cardiac fibrosis, diabetic nephropathy, etc. Here, by integrating two Gene Expression Omnibus (GEO) datasets of cisplatin-induced AKI animal models, we identified PLK2 as a significantly up-regulated gene in AKI renal tissues, which was then verified in different AKI animal models and cell models. Suppressing PLK2 using siRNAs or inhibitors could enhance cisplatin-induced AKI by inducing severe apoptosis and oxidative stress damage, while enforced PLK2 expression could prevent renal dysfunction induced by cisplatin. We further discovered that PLK2 might phosphorylate glycogen synthase kinase 3β (GSK3β) in the pathogenesis of AKI. In conclusion, our results show that PLK2 play a protective role in cisplatin-induced AKI and may be a new protective target of cisplatin nephrotoxicity.

Elevated ASCL1 activity creates de novo regulatory elements associated with neuronal differentiation

Background

The pro-neural transcription factor ASCL1 is a master regulator of neurogenesis and a key factor necessary for the reprogramming of permissive cell types to neurons. Endogenously, ASCL1 expression is often associated with neuroblast stem-ness. Moreover, ASCL1-mediated reprogramming of fibroblasts to differentiated neurons is commonly achieved using artificially high levels of ASCL1 protein, where ASCL1 acts as an “on-target” pioneer factor. However, the genome-wide effects of enhancing ASCL1 activity in a permissive neurogenic environment has not been thoroughly investigated. Here, we overexpressed ASCL1 in the neuronally-permissive context of neuroblastoma (NB) cells where modest endogenous ASCL1 supports the neuroblast programme.

Results

Increasing ASCL1 in neuroblastoma cells both enhances binding at existing ASCL1 sites and also leads to creation of numerous additional, lower affinity binding sites. These extensive genome-wide changes in ASCL1 binding result in significant reprogramming of the NB transcriptome, redirecting it from a proliferative neuroblastic state towards one favouring neuronal differentiation. Mechanistically, ASCL1-mediated cell cycle exit and differentiation can be increased further by preventing its multi-site phosphorylation, which is associated with additional changes in genome-wide binding and gene activation profiles.

Conclusions

Our findings show that enhancing ASCL1 activity in a neurogenic environment both increases binding at endogenous ASCL1 sites and also results in additional binding to new low affinity sites that favours neuronal differentiation over the proliferating neuroblast programme supported by the endogenous protein. These findings have important implications for controlling processes of neurogenesis in cancer and cellular reprogramming.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08495-8.

Keep Calm and Carry on with Extra Centrosomes

Simple Summary

Precise chromosome segregation during mitosis is a vital event orchestrated by formation of bipolar spindle poles. Supernumerary centrosomes, caused by centrosome amplification, deteriorates mitotic processes, resulting in segregation defects leading to chromosomal instability (CIN). Centrosome amplification is frequently observed in various types of cancer and considered as a significant contributor to destabilization of chromosomes. This review provides a comprehensive overview of causes and consequences of centrosome amplification thoroughly describing molecular mechanisms.

Abstract

Aberrations in the centrosome number and structure can readily be detected at all stages of tumor progression and are considered hallmarks of cancer. Centrosome anomalies are closely linked to chromosome instability and, therefore, are proposed to be one of the driving events of tumor formation and progression. This concept, first posited by Boveri over 100 years ago, has been an area of interest to cancer researchers. We have now begun to understand the processes by which these numerical and structural anomalies may lead to cancer, and vice-versa: how key events that occur during carcinogenesis could lead to amplification of centrosomes. Despite the proliferative advantages that having extra centrosomes may confer, their presence can also lead to loss of essential genetic material as a result of segregational errors and cancer cells must deal with these deadly consequences. Here, we review recent advances in the current literature describing the mechanisms by which cancer cells amplify their centrosomes and the methods they employ to tolerate the presence of these anomalies, focusing particularly on centrosomal clustering.

Identification of polo-like kinase 1 as a therapeutic target in murine lupus

Introduction

The signalling cascades that contribute to lupus pathogenesis are incompletely understood. We address this by using an unbiased activity‐based kinome screen of murine lupus.

Methods

An unbiased activity‐based kinome screen (ABKS) of 196 kinases was applied to two genetically different murine lupus strains. Systemic and renal lupus were evaluated following in vivo PLK1blockade. The upstream regulators and downstream targets of PLK1 were also interrogated.

Results

Multiple signalling cascades were noted to be more active in murine lupus spleens, including PLK1. In vivo administration of a PLK1‐specific inhibitor ameliorated splenomegaly, anti‐dsDNA antibody production, proteinuria, BUN and renal pathology in MRL.lpr mice (P < 0.05). Serum IL‐6, IL‐17 and kidney injury molecule 1 (KIM‐1) were significantly decreased after PLK1 inhibition. PLK1 inhibition reduced germinal centre and marginal zone B cells in the spleen, but changes in T cells were not significant. In vitro, splenocytes were treated with anti‐mouse CD40 Ab or F(ab’)2 fragment anti‐mouse IgM. After 24‐h stimulation, IL‐6 secretion was significantly reduced upon PLK1 blockade, whereas IL‐10 production was significantly increased. The phosphorylation of mTOR was assessed in splenocyte subsets, which revealed a significant change in myeloid cells. PLK1 blockade reduced phosphorylation associated with mTOR signalling, while Aurora‐A emerged as a potential upstream regulator of PLK1.

Conclusion

The Aurora‐A → PLK1 → mTOR signalling axis may be central in lupus pathogenesis, and emerges as a potential therapeutic target.

Tumor suppressor PLK2 may serve as a biomarker in triple-negative breast cancer for improved response to PLK1 therapeutics

Polo-like kinase (PLK) family members play important roles in cell cycle regulation. The founding member PLK1 is oncogenic and preclinically validated as a cancer therapeutic target. Paradoxically, frequent loss of chromosome 5q11-35 which includes PLK2 is observed in basal-like breast cancer. In this study, we found that PLK2 was tumor suppressive in breast cancer, preferentially in basal-like and triple-negative breast cancer (TNBC) subtypes. Knockdown of PLK1 rescued phenotypes induced by PLK2-loss both in vitro and in vivo. We also demonstrated that PLK2 directly interacted with PLK1 at prometaphase through the kinase but not the polo-box domains of PLK2, suggesting PLK2 functioned at least partially through the interaction with PLK1. Furthermore, an improved treatment response was seen in both Plk2-deleted/low mouse preclinical and PDX TNBC models using the PLK1 inhibitor volasertib alone or in combination with carboplatin. Re-expression of PLK2 in an inducible PLK2-null mouse model reduced the therapeutic efficacy of volasertib. In summary, this study delineates the effects of chromosome 5q loss in TNBC that includes PLK2, the relationship between PLK2 and PLK1, and how this may render PLK2-deleted/low tumors more sensitive to PLK1 inhibition in combination with chemotherapy.

At a Crossroads to Cancer: How p53-Induced Cell Fate Decisions Secure Genome Integrity

P53 is known as the most critical tumor suppressor and is often referred to as the guardian of our genome. More than 40 years after its discovery, we are still struggling to understand all molecular details on how this transcription factor prevents oncogenesis or how to leverage current knowledge about its function to improve cancer treatment. Multiple cues, including DNA-damage or mitotic errors, can lead to the stabilization and nuclear translocation of p53, initiating the expression of multiple target genes. These transcriptional programs may be cell-type- and stimulus-specific, as is their outcome that ultimately imposes a barrier to cellular transformation. Cell cycle arrest and cell death are two well-studied consequences of p53 activation, but, while being considered critical, they do not fully explain the consequences of p53 loss-of-function phenotypes in cancer. Here, we discuss how mitotic errors alert the p53 network and give an overview of multiple ways that p53 can trigger cell death. We argue that a comparative analysis of different types of p53 responses, elicited by different triggers in a time-resolved manner in well-defined model systems, is critical to understand the cell-type-specific cell fate induced by p53 upon its activation in order to resolve the remaining mystery of its tumor-suppressive function.

PLK1 Inhibition Induces Immunogenic Cell Death and Enhances Immunity against NSCLC

PLK1 inhibitors were shown, in vitro and in vivo, to possess inhibitory activities against non-small cell lung cancer (NSCLC), and such inhibition has been proven by clinical trials. However, it remains unclear whether and how the immune microenvironment is associated with the action. In this study, we found that inhibiting PLK1 could alter the tumor immune microenvironment by increasing DC maturation, and enriching T cells infiltration. PLK1 inhibitors, serving as immunogenic cell death (ICD) inducers, indirectly activated DCs, instead of directly acting on DC cells, through the surface expression of costimulatory molecules on and enhanced phagocytosis by DCs. Furthermore, upon targeting PLK1, tumor cells that had undergone ICD were converted into an endogenous vaccine, which triggered the immune memory responses and protected the mice from tumor challenge. Collectively, these results suggested that the PLK1 inhibitor might function as an immune modulator in antitumor treatment.

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.

Regulation of the SIAH2-HIF-1 Axis by Protein Kinases and Its Implication in Cancer Therapy

The cellular response to hypoxia is a key biological process that facilitates adaptation of cells to oxygen deprivation (hypoxia). This process is critical for cancer cells to adapt to the hypoxic tumor microenvironment resulting from rapid tumor growth. Hypoxia-inducible factor 1 (HIF-1) is a transcription factor and a master regulator of the cellular response to hypoxia. The activity of HIF-1 is dictated primarily by its alpha subunit (HIF-1α), whose level and/or activity are largely regulated by an oxygen-dependent and ubiquitin/proteasome-mediated process. Prolyl hydroxylases (PHDs) and the E3 ubiquitin ligase Von Hippel-Lindau factor (VHL) catalyze hydroxylation and subsequent ubiquitin-dependent degradation of HIF-1α by the proteasome. Seven in Absentia Homolog 2 (SIAH2), a RING finger-containing E3 ubiquitin ligase, stabilizes HIF-1α by targeting PHDs for ubiquitin-mediated degradation by the proteasome. This SIAH2-HIF-1 signaling axis is important for maintaining the level of HIF-1α under both normoxic and hypoxic conditions. A number of protein kinases have been shown to phosphorylate SIAH2, thereby regulating its stability, activity, or substrate binding. In this review, we will discuss the regulation of the SIAH2-HIF-1 axis via phosphorylation of SIAH2 by these kinases and the potential implication of this regulation in cancer biology and cancer therapy.

Connectivity Mapping Identifies BI-2536 as a Potential Drug to Treat Diabetic Kidney Disease

Diabetic kidney disease (DKD) remains the most common cause of kidney failure, and the treatment options are insufficient. Here, we used a connectivity mapping approach to first collect 15 gene expression signatures from 11 DKD-related published independent studies. Then, by querying the Library of Integrated Network-based Cellular Signatures (LINCS) L1000 data set, we identified drugs and other bioactive small molecules that are predicted to reverse these gene signatures in the diabetic kidney. Among the top consensus candidates, we selected a PLK1 inhibitor (BI-2536) for further experimental validation. We found that PLK1 expression was increased in the glomeruli of both human and mouse diabetic kidneys and localized largely in mesangial cells. We also found that BI-2536 inhibited mesangial cell proliferation and extracellular matrix in vitro and ameliorated proteinuria and kidney injury in DKD mice. Further pathway analysis of the genes predicted to be reversed by the PLK1 inhibitor was of members of the TNF-α/NF-κB, JAK/STAT, and TGF-β/Smad3 pathways. In vitro, either BI-2536 treatment or knockdown of PLK1 dampened the NF-κB and Smad3 signal transduction and transcriptional activation. Together, these results suggest that the PLK1 inhibitor BI-2536 should be further investigated as a novel therapy for DKD.

Non-mitotic functions of polo-like kinases in cancer cells

Inhibitors of mitotic protein kinases are currently being developed as non-neurotoxic alternatives of microtubule-targeting agents (taxanes, vinca alkaloids) which provide a substantial survival benefit for patients afflicted with different types of solid tumors. Among the mitotic kinases, the cyclin-dependent kinases, the Aurora kinases, the kinesin spindle protein and Polo-like kinases (PLKs) have emerged as attractive targets of cancer therapeutics. The functions of mammalian PLK1-5 are traditionally linked to the regulation of the cell cycle and to the stress response. Especially the key role of PLK1 and PLK4 in cellular growth and proliferation, their overexpression in multiple types of human cancer and their druggability, make them appealing targets for cancer therapy. Inhibitors for PLK1 and PLK4 are currently being tested in multiple cancer trials. The clinical success of microtubule-targeting agents is attributed not solely to the induction of a mitotic arrest in cancer cells, but also to non-mitotic effects like targeting intracellular trafficking on microtubules. This raises the question whether new cancer targets like PLK1 and PLK4 regulate critical non-mitotic functions in tumor cells. In this article we summarize the important roles of PLK1-5 for the regulation of non-mitotic signaling. Due to these functions it is conceivable that inhibitors for PLK1 or PLK4 can target interphase cells, which underscores their attractive potential as cancer drug targets. Moreover, we also describe the contribution of the tumor-suppressors PLK2, PLK3 and PLK5 to cancer cell signaling outside of mitosis. These observations highlight the urgent need to develop highly specific ATP-competitive inhibitors for PLK4 and for PLK1 like the 3rd generation PLK-inhibitor Onvansertib to prevent the inhibition of tumor-suppressor PLKs in- and outside of mitosis. The remarkable feature of PLKs to encompass a unique druggable domain, the polo-box-domain (PBD) that can be found only in PLKs offers the opportunity for the development of inhibitors that target PLKs exclusively. Beyond the development of mono-specific ATP-competitive PLK inhibitors, the PBD as drug target will support the design of new drugs that eradicate cancer cells based on the mitotic and non-mitotic function of PLK1 and PLK4.

Elucidation of PLK1 Linked Biomarkers in Oesophageal Cancer Cell Lines: A Step Towards Novel Signaling Pathways by p53 and PLK1-Linked Functions Crosstalk

BACKGROUND

Oesophgeal adenocarcinoma (OAC) is the most frequent cause of cancer death. POLO-like kinase 1 (PLK1) is overexpressed in broad spectrum of tumors and has prognostic value in many cancers including esophageal cancer, suggesting its potential as a therapeutic target. p53, the guardian of genome is the most important tumor suppressors that represses the promoter of PLK1, whereas tumor cells with inactive p53 are arrested in mitosis due to DNA damage. PLK1 expression has been linked to the elevated p53 expression and has been shown to act as a biomarker that predicts poor prognosis in OAC.

OBJECTIVES

The aim of the present study was identification of PLK1 associated phosphorylation targets in p53 mutant and p53 normal cells to explore the downstream signaling evets.

METHODS

Here we develop a proof-of-concept phospho-proteomics approach to identify possible biomarkers that can be used to identify mutant p53 or wild-type p53 pathways. We treated PLK1 asynchronously followed by mass spectrometry data analysis. Protein networking and motif analysis tools were used to identify the significant clusters and potential biomarkers.

RESULTS

We investigated approximately 1300 potential PLK1-dependent phosphopeptides by LCMS/ MS. In total, 2216 and 1155 high confidence phosphosites were identified in CP-A (p53+) and OE33 (p53-) cell lines owing to PLK1 inhibition. Further clustering and motif assessment uncovered many significant biomarkers with known and novel link to PLK1.

CONCLUSION

Taken together, our study suggests that PLK1 may serve as a potential therapeutic target in human OAC. The data highlight the efficacy and specificity of small molecule PLK1 kinase inhibitors to identify novel signaling pathways in vivo.

Delivery of CRISPR/Cas systems for cancer gene therapy and immunotherapy

The clustered, regularly interspaced, short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas) systems are efficient and versatile gene editing tools, which offer enormous potential to treat cancer by editing genome, transcriptome or epigenome of tumor cells and/or immune cells. A large body of works have been done with CRISPR/Cas systems for genetic modification, and 16 clinical trials were conducted to treat cancer by ex vivo or in vivo gene editing approaches. Now, promising preclinical works have begun using CRISPR/Cas systems in vivo. However, efficient and safe delivery of CRISPR/Cas systems in vivo is still a critical challenge for their clinical applications. This article summarizes delivery of CRISPR/Cas systems by physical methods, viral vectors and non-viral vectors for cancer gene therapy and immunotherapy. The prospects for the development of physical methods, viral vectors and non-viral vectors for delivery of CRISPR/Cas systems are reviewed, and promising advances in cancer treatment using CRISPR/Cas systems are discussed.

Integrative public data-mining pipeline for the validation of novel independent prognostic biomarkers for lung adenocarcinoma

Aim: We aimed to develop a candidate-based integrative public data mining strategy for validation of novel prognostic markers in lung adenocarcinoma. Materials & methods: An in silico approach integrating meta-analyses of publicly available clinical information linked RNA expression, gene copy number and mutation datasets combined with independent immunohistochemistry and survival datasets. Results: After validation of pipeline integrity utilizing data from the well-characterized prognostic factor Ki-67, prognostic impact of the calcium- and integrin-binding protein, CIB1, was analyzed. CIB1 was overexpressed in lung adenocarcinoma which correlated with pathological tumor and pathological lymph node status and impaired overall/progression-free survival. In multivariate analyses, CIB1 emerged as UICC stage-independent risk factor for impaired survival. Conclusion: Our pipeline holds promise to facilitate further identification and validation of novel lung cancer-associated prognostic markers.

Dual PLK1 and STAT3 inhibition promotes glioblastoma cells apoptosis through MYC

Glioblastoma (GBM) is the deadliest primary brain tumor that is highly resistant to current treatments. Polo-like kinase 1 (PLK1) and signal transducer and activator of transcription 3 (STAT3) are highly expressed in gliomas, especially GBM. Previous studies have shown reciprocal activation between PLK1 and STAT3 and that they regulate the same pools of MYC downstream. We have demonstrated that PLK1 and STAT3 levels are elevated in gliomas compared with those in normal brain tissues, and high expression of both PLK1 and STAT3 is associated with poor prognosis in TCGA. Moreover, there was direct or indirect reciprocal regulation between PLK1 and STAT3. Furthermore, we found that PLK1 and STAT3 can regulate the same pools of MYC downstream. Compared to monotherapy, combined treatment of glioma cells with PLK1 and STAT3 inhibitors, BI2536 and Stattic, respectively, showed lower expression of MYC, synergistic induction of cell invasion and apoptosis in vitro, and tumor inhibition in xenografts. PLK1 and STAT3 were able to directly regulate the expression of MYC and induce apoptosis of glioma cells through the regulation of MYC. These findings may help develop a therapeutic strategy for dual inhibition of PLK1 and STAT3 against the tumorigenesis of glioma.

Transcriptome Remodeling in Gradual Development of Inverse Resistance between Paclitaxel and Cisplatin in Ovarian Cancer Cells

Resistance to anti-cancer drugs is the main challenge in oncology. In pre-clinical studies, established cancer cell lines are primary tools in deciphering molecular mechanisms of this phenomenon. In this study, we proposed a new, transcriptome-focused approach, utilizing a model of isogenic cancer cell lines with gradually changing resistance. We analyzed trends in gene expression in the aim to find out a scaffold of resistance development process. The ovarian cancer cell line A2780 was treated with stepwise increased concentrations of paclitaxel (PTX) to generate a series of drug resistant sublines. To monitor transcriptome changes we submitted them to mRNA-sequencing, followed by the identification of differentially expressed genes (DEGs), principal component analysis (PCA), and hierarchical clustering. Functional interactions of proteins, encoded by DEGs, were analyzed by building protein-protein interaction (PPI) networks. We obtained human ovarian cancer cell lines with gradually developed resistance to PTX and collateral sensitivity to cisplatin (CDDP) (inverse resistance). In their transcriptomes, we identified two groups of DEGs: (1) With fluctuations in expression in the course of resistance acquiring; and (2) with a consistently changed expression at each stage of resistance development, constituting a scaffold of the process. In the scaffold PPI network, the cell cycle regulator-polo-like kinase 2 (PLK2); proteins belonging to the tumor necrosis factor (TNF) ligand and receptor family, as well as to the ephrin receptor family were found, and moreover, proteins linked to osteo- and chondrogenesis and the nervous system development. Our cellular model of drug resistance allowed for keeping track of trends in gene expression and studying this phenomenon as a process of evolution, reflected by global transcriptome remodeling. This approach enabled us to explore novel candidate genes and surmise that abrogation of the osteomimic phenotype in ovarian cancer cells might occur during the development of inverse resistance between PTX and CDDP.

Target Analysis and Mechanism of Podophyllotoxin in the Treatment of Triple-Negative Breast Cancer

Background

As the original compound of many podophyllotoxin derivatives, podophyllotoxin has a beneficial antitumor effect. The mechanism of podophyllotoxin activity in triple-negative breast cancer still needs to be explored.

Methods

We used cell proliferation assay, scratch and transwell experiments, and cell cycle and apoptosis analyses to observe the intervention effect of podophyllotoxin on breast cancer. Furthermore, we analyzed the differences between GSE31448, GSE65194, and GSE45827 in the Gene Expression Omnibus database (GEO) and explored the differential genes using a STRING database. Centiscape2.2, MCODE cluster analysis and KEGG pathway analysis were used to identify the most significant gene differences. Next, we utilized BATMAN-TCM and TCMSP databases for further screening to identify key genes. Finally, quantitative RT-PCR (qRT-PCR) and Western blotting were performed to detect the expression of key targets.

Results

Our research confirmed that podophyllotoxin could not only inhibit the migration and invasion of triple-negative breast cancer but also affect the cell cycle and induce apoptosis. In total, 566 differential genes were obtained by using the GEO database. After topological network analysis, cluster analysis, and molecular docking screening, we finally identified PLK1, CCDC20, and CDK1 as key target genes. The results of the qRT-PCR assay showed that the mRNA levels of PLK1, CDC20, and CDK1 decreased, while the expression of upstream P53 increased, after drug induction. The Gene Set Enrichment Analysis (GSEA) and conetwork analysis showed that PLK1 is a more critical regulatory factor. Further Western blotting analysis revealed that there was a negative regulatory relationship between the key gene PLK1 and P53 on the protein level. The results were presented as the mean ± standard deviation of triplicate experiments and P<0.05 was considered to indicate a statistically significant difference.

Conclusion

Podophyllotoxin has an intervention effect on the development of triple-negative breast cancer. The expression of PLK1, CDC20, and CDK1 in the cell cycle pathway is inhibited by regulating P53. Our research shows that natural drugs inhibit tumor activity by regulating the expression of cyclins, and the combination of natural drugs and modern extensive database analysis has a wide range of potential applications in the development of antitumor therapies.

Identification of critical genes in gastric cancer to predict prognosis using bioinformatics analysis methods

Background

Ranking fourth in the world in tumor incidence and second in cancer-related death worldwide, gastric cancer (GC) is one of the major malignant tumors, and has a very complicated pathogenesis. In the present study, we aimed to identify new biomarkers to predict the survival rate of GC patients.

Methods

The differentially expressed genes (DEGs) between GC tissues and normal stomach tissues were obtained by using GEO2R, and overlapped DEGs were acquired with Venn diagrams. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis were conducted with R software. Then, the protein-protein interaction (PPI) of these DEGs was visualized by Cytoscape. Gene Expression Profiling Interactive Analysis (GEPIA) was used to verify the expression differences of hub genes in gastric adenocarcinoma tissues and normal tissues. Overall survival (OS) of hub genes was calculated by Kaplan-Meier plotter.

Results

There were a total of 128 consistently expressed genes in the two datasets: 85 upregulated genes were enriched in extra-cellular matrix (ECM)-receptor interaction, protein digestion and absorption, focal adhesion, gastric acid secretion, mineral absorption, systemic lupus erythematosus, amoebiasis, and PI3K-Akt signaling pathway, and 43 downregulated genes were enriched in palate development, blood coagulation, positive regulation of transcription from RNA polymerase II promoter, axonogenesis, receptor internalization, negative regulation of transcription from RNA polymerase II promoter, and in no significant signaling pathways. From the PPI network analyzed by Molecular Complex Detection (MCODE) plug-in, all 27 upregulated genes were selected. Furthermore, to analyze the OS among these genes, Kaplan-Meier analysis was conducted, and 25 genes were associated with remarkably worse survival. For validation in GEPIA, 11 of 25 genes were discovered to be highly expressed in GC tissues compared to normal OS tissues. Furthermore, in the re-analysis of the Database for Annotation, Visualization and Integrated Discovery (DAVID), three genes [G2/miotic-specific cyclin B1 (CCNB1), polo-like kinases 1 (PLK1), and pituitary tumor-transforming gene-1 (PTTG1)] were markedly enriched in the cell cycle pathway, particulary the G1-G1/S phase.

Conclusions

Three remarkably upregulated DEGs with poor prognosis in GC were identified and may serve as new prognostic biomarkers and targets in GC therapy.

Resveratrol activates DNA damage response through inhibition of polo-like kinase 1 (PLK1) in natural killer/T cell lymphoma

Background

Extranodal natural killer/T cell lymphoma (NKTCL) is a highly aggressive non-Hodgkin lymphoma with a poor prognosis. Resveratrol (REV), a natural nontoxic pleiotropic agent, has antitumor effects, yet not being studied in NKTCL.

Methods

We performed immunohistochemical (IHC) staining with NKTCL tumor tissues. Apoptosis and cell cycle of NKTCL cell line NK-92 were detected by using flow cytometry. Then we detected the cellular expression level of polo-like kinase 1 (PLK1) and key molecules in DNA damage response (DDR) pathway by using RNA sequencing (RNA-seq) technology, real-time PCR, and Western blot.

Results

In this study, we found distinguishingly expressed phosphorylated ataxia telangiectasia mutated (ATM) in human NKTCL tumor tissues compared to normal lymph nodes samples. But low levels of phosphorylated checkpoint kinase 2 (Chk2) and phosphorylated p53 were shown, suggesting that DDR pathway is blocked midway in NKTCL. REV inhibited the proliferation of NK-92 cells in a time- and dose-dependent manner, arrested cell cycle at G1 phase, and induced mitochondrial apoptosis. PLK1 was inhibited in both mRNA and protein levels by REV in NK-92 cells. At the same time, phosphorylation levels of Chk2 and p53 were upregulated.

Conclusions

DDR pathway plays an important role in the pathogenesis of NKTCL. REV shows anti-NKTCL activity. The inhibition of PLK1 and the activation of DDR are vital for REV induced tumor cell apoptosis.

Pharmacogenomic Analysis Reveals CCNA2 as a Predictive Biomarker of Sensitivity to Polo-Like Kinase I Inhibitor in Gastric Cancer

Despite recent innovations and advances in early diagnosis, the prognosis of advanced gastric cancer remains poor due to a limited number of available therapeutics. Here, we employed pharmacogenomic analysis of 37 gastric cancer cell lines and 1345 small-molecule pharmacological compounds to investigate biomarkers predictive of cytotoxicity among gastric cancer cells to the tested drugs. We discovered that expression of CCNA2, encoding cyclin A2, was commonly associated with responses to polo-like kinase 1 (PLK1) inhibitors (BI-2536 and volasertib). We also found that elevated CCNA2 expression is required to confer sensitivity to PLK1 inhibitors through increased mitotic catastrophe and apoptosis. Further, we demonstrated that CCNA2 expression is elevated in KRAS mutant gastric cancer cell lines and primary tumors, resulting in an increased sensitivity to PLK1 inhibitors. Our study suggests that CCNA2 is a novel biomarker predictive of sensitivity to PLK1 inhibitors for the treatment of advanced gastric cancer, particularly cases carrying KRAS mutation.

Identification of PLK1 as a New Therapeutic Target in Mucinous Ovarian Carcinoma

Mucinous epithelial ovarian cancer (mEOC) is a rare subset of epithelial ovarian cancer. When diagnosed at a late stage, its prognosis is very poor, as it is quite chemo-resistant. To find new therapeutic options for mEOC, we performed high-throughput screening using a siRNA library directed against human protein kinases in a mEOC cell line, and polo-like kinase1 (PLK1) was identified as the kinase whose downregulation interfered with cell proliferation. Both PLK1 siRNA and two specific PLK1 inhibitors (onvansertib and volasertib) were able to inhibit cell growth, induce apoptosis and block cells in the G2/M phase of the cell cycle. We evaluated, in vitro, the combinations of PLK1 inhibitors and different chemotherapeutic drugs currently used in the treatment of mEOC, and we observed a synergistic effect of PLK1 inhibitors and antimitotic drugs. When translated into an in vivo xenograft model, the combination of onvansertib and paclitaxel resulted in stronger tumor regressions and in a longer mice survival than the single treatments. These effects were associated with a higher induction of mitotic block and induction of apoptosis, similarly to what was observed in vitro. These data suggest that the combination onvansertib/paclitaxel could represent a new active therapeutic option in mEOC.

The dual role of BI 2536, a small-molecule inhibitor that targets PLK1, in induction of apoptosis and attenuation of autophagy in neuroblastoma cells

Neuroblastoma (NB) is the most common extra-cranial solid tumor in childhood with the overall 5 years' survival less than 40%. Polo-like kinase 1 (PLK1) is a serine/threonine-protein kinase expressed during mitosis and over expressed in multiple cancers, including neuroblastoma. We found that higher PLK1 expression related to poor outcome of NB patients. BI2536, a small molecule inhibitor against PLK1, significantly reduced cell viability in a panel of NB cell lines, with IC50 less than 100 nM. PLK1 inhibition by BI 2536 treatment induced cell cycle arrest at G₂/M phase and cell apoptosis in NB cells. Realtime PCR array revealed the PLK1 inhibition related genes, such as BIRC7, TNFSF10, LGALS1 and DAD1 et al. Moreover, autophagy activity was investigated in the NB cells treated with BI 2536. BI 2536 treatment in NB cells increased LC3-II puncta formation and LC3-II expression. Formation of autophagosome induced by BI 2536 was observed by transmission electron microscopy. However, BI2536 abrogated the autophagic flux in NB cells by reducing SQSTM1/p62 expression and AMPKα^T172 phosphorylation. These results provide new clues for the molecular mechanism of cell death induced by BI 2536 and suggest that BI 2536 may act as new candidate drug for neuroblastoma.

Folate-Mediated Targeted Delivery of siPLK1 by Leucine-Bearing Polyethylenimine

BACKGROUND

siRNA-mediated polo-like kinase 1 (PLK1) silencing has been proposed as a promising therapeutic method for multiple cancers. However, the clinic application of this method is still hindered by the low specific delivery of siPLK1 to desired tumor lesions. Herein, folate (FA)-modified and leucine-bearing polyethylenimine was successfully synthesized and showed excellent targeted silencing to folate receptor overexpressed cells.

MATERIALS AND METHODS

The condensation of siPLK1 by FA-N-Ac-L-Leu-PEI (NPF) was detected by the gel retardation assay. The targeted and silencing efficiency was evaluated by flow cytometry and confocal laser scanning microscope. The PLK1 expressions at gene or protein levels were detected by quantitative real-time PCR and Western blotting assay. Further impacts of the PLK1 silencing on cell viability, cell cycle, migration, and invasion were studied by MTT, colony formation, wound healing and transwell assays.

RESULTS

The NPF and siPLK1 could efficiently assemble to stable nanoparticles at a weight ratio of 3.0 and showed excellent condensation and protection effect. Owing to the FA-mediated targeted delivery, the uptake and silencing efficiency of NPF/siPLK1 to SGC-7901 cells was higher than that without FA modification. Moreover, NPF-mediated PLK1 silencing showed significant antitumor activity in vitro. The anti-proliferation effect of PLK1 silencing was induced via the mitochondrial-dependent apoptosis pathway with the cell cycle arrest of 45% at G2 phase and the apoptotic ratio of 28.3%.

CONCLUSION

FA-N-Ac-L-Leu-PEI (NPF) could generate targeted delivery siPLK1 to FA receptor overexpressed cells and dramatically downregulate the expression of PLK1 expression.

Truncated tetrahedral RNA nanostructures exhibit enhanced features for delivery of RNAi substrates

Using RNA as a material for nanoparticle construction provides control over particle size and shape at the nano-scale. RNA nano-architectures have shown promise as delivery vehicles for RNA interference (RNAi) substrates, allowing multiple functional entities to be combined on a single particle in a programmable fashion. Rather than employing a completely bottom-up approach to scaffold design, here multiple copies of an existing synthetic supramolecular RNA nano-architecture serve as building blocks along with additional motifs for the design of a novel truncated tetrahedral RNA scaffold, demonstrating that rationally designed RNA assemblies can themselves serve as modular pieces in the construction of larger rationally designed structures. The resulting tetrahedral scaffold displays enhanced characteristics for RNAi-substrate delivery in comparison to similar RNA-based scaffolds, as evidenced by its increased functional capacity, increased cellular uptake and ultimately an increased RNAi efficacy of its adorned Dicer substrate siRNAs. The unique truncated tetrahedral shape of the nanoparticle core appears to contribute to this particle's enhanced function, indicating the physical characteristics of RNA scaffolds merit significant consideration when designing platforms for delivery of functional RNAs via RNA nanoparticles.

Development of a Novel Cell-Permeable Protein-Protein Interaction Inhibitor for the Polo-box Domain of Polo-like Kinase 1

Polo-like kinase 1 (PLK1) is a key regulator of mitosis and a recognized drug target for cancer therapy. Inhibiting the polo-box domain of PLK1 offers potential advantages of increased selectivity and subsequently reduced toxicity compared with targeting the kinase domain. However, many if not all existing polo-box domain inhibitors have been shown to be unsuitable for further development. In this paper, we describe a novel compound series, which inhibits the protein-protein interactions of PLK1 via the polo-box domain. We combine high throughput screening with molecular modeling and computer-aided design, synthetic chemistry, and cell biology to address some of the common problems with protein-protein interaction inhibitors, such as solubility and potency. We use molecular modeling to improve the solubility of a hit series with initially poor physicochemical properties, enabling biophysical and biochemical characterization. We isolate and characterize enantiomers to improve potency and demonstrate on-target activity in both cell-free and cell-based assays, entirely consistent with the proposed binding model. The resulting compound series represents a promising starting point for further progression along the drug discovery pipeline and a new tool compound to study kinase-independent PLK functions.

Structure-Based Virtual Screening and Biological Evaluation of Peptide Inhibitors for Polo-Box Domain

The polo-box domain of polo-like kinase 1 (PLK1-PBD) is proved to have crucial roles in cell proliferation. Designing PLK1-PBD inhibitors is challenging due to their poor cellular penetration. In this study, we applied a virtual screening workflow based on a combination of structure-based pharmacophore modeling with molecular docking screening techniques, so as to discover potent PLK1-PBD peptide inhibitors. The resulting 9 virtual screening peptides showed affinities for PLK1-PBD in a competitive binding assay. In particular, peptide 5 exhibited an approximately 100-fold increase in inhibitory activity (IC50 = 70 nM), as compared with the control poloboxtide. Moreover, cell cycle experiments indicated that peptide 5 effectively inhibited the expression of p-Cdc25C and cell cycle regulatory proteins by affecting the function of PLK1-PBD, thereby inducing mitotic arrest at the G2/M phase. Overall, peptide 5 can serve as a potent lead for further investigation as PLK1-PBD inhibitors.

Peptide carriers to the rescue: overcoming the barriers to siRNA delivery for cancer treatment

Cancer is a significant health concern worldwide and its clinical treatment presents many challenges. Consequently, much research effort has focused on the development of new anticancer drugs to combat this disease. One area of exploration, in particular, has been in the therapeutic application of RNA interference (RNAi). Although RNAi appears to be an attractive therapeutic tool for the treatment of cancer, one of the primary obstacles towards its pervasive use in the clinic has been cell/tissue type-specific cytosolic delivery of therapeutic small interfering RNA (siRNA) molecules. Consequently, varied drug delivery platforms have been developed and widely explored for siRNA delivery. Among these candidate drug delivery systems, peptides have shown great promise as siRNA carriers due to their varied physiochemical properties and functions, simple formulations, and flexibility in design. In this review, we will focus on distinguishing between the different classes of peptide carriers based on their functions, as well as summarize and discuss the various design strategies and advancements that have been made in circumventing the barriers to siRNA delivery for cancer treatment. Resolution of these challenges by peptide carriers will accelerate the translation of RNAi-based therapies to the clinic.

Phospho‑regulation of Cdc14A by polo‑like kinase 1 is involved in β‑cell function and cell cycle regulation

The objective of the present study was to investigate the effects of polo‑like kinase 1 (PLK1) and the phosphorylation of human cell division cycle protein 14A (Cdc14A) by PLK1 on β‑cell function and cell cycle regulation. Mouse β‑TC3 cells were incubated with small interfering RNA (siRNA) to knock down the expression of PLK1. Cell cycle analysis was performed using flow cytometry, and cell proliferation and apoptosis was determined. Insulin secretion was evaluated by a radioimmunoassay under both low and high glucose conditions. Mouse β‑TC3 cells were transfected with a wild type or a non‑phosphorylatable Cdc14A mutant (Cdc14AS351A/363A; Cdc14AAA) to investigate whether the phosphorylation of Cdc14A is involved in cellular regulation of PLK1 under high glucose conditions. It was found that PLK1 siRNA significantly promoted cellular apoptosis, inhibited cell proliferation, decreased insulin secretion and reduced Cdc14A expression under both low and high glucose conditions. Cdc14A overexpression promoted β‑TC3 cell proliferation and insulin secretion, while Cdc14AAA overexpression inhibited cell proliferation and insulin secretion under high glucose conditions. PLK1 siRNA partially reversed the proliferation‑promoting effects of Cdc14A and further intensified the inhibition of proliferation by Cdc14AAA under high glucose conditions. Similarly, Cdc14A overexpression partially reversed the insulin‑inhibiting effects of PLK1 siRNA, while Cdc14AAA overexpression showed a synergistic inhibitory effect on insulin secretion with PLK1 siRNA under high glucose conditions. In conclusion, PLK1 promoted cell proliferation and insulin secretion while inhibiting cellular apoptosis in β‑TC3 cell lines under both low and high glucose conditions. In addition, the phospho‑regulation of Cdc14A by PLK1 may be involved in β‑TC3 cell cycle regulation and insulin secretion under high glucose conditions.

Enhanced polo-like kinase 1 expression in myelodysplastic syndromes

Background

Cancer is characterized by uncontrolled cellular proliferation, and Polo-like kinase 1 (PLK1), a key regulator of the cell cycle, is overexpressed in many cancers, including acute leukemia and lymphoma. However, the dynamics of PLK1 transcription in myelodysplastic syndromes (MDS) are unknown. This study aimed to investigate the transcript dynamics of PLK1 and determine its role in the pathophysiology of MDS.

Methods

PLK1 mRNA obtained from the bone marrow samples of 67 patients with MDS, 16 patients with secondary acute myeloid leukemia (sAML), and 10 healthy controls were analyzed using quantitative real-time PCR and compared according to various clinical parameters.

Results

The median PLK1 expression levels differed slightly, but not significantly, between MDS and sAML patients [661.21 (range, 29.38-8,987.31) vs. 1,462.05 (32.22-5,734.09), respectively], but were significantly higher (P<0.001) than the levels in the healthy controls [19.0 (1.60-49.90)]. Further analyses of PLK1 levels according to the WHO classification of MDS, prognostic risk groups, karyotype risk groups, marrow blast percentage, and depth of cytopenia did not reveal any significant associations. In patients progressing to sAML, PLK1 expression levels differed significantly according to the presence or absence of resistance to hypomethylation treatment (2,470.58 vs. 415.98, P=0.03).

Conclusion

PLK1 is upregulated in MDS patients; however, its role in the pathophysiology of MDS is unclear. Gene upregulation in cases with pharmacotherapeutic resistance warrants further investigation.

Effect of PLK1 inhibition on cisplatin-resistant gastric cancer cells

OBJECTIVE

This study aims to investigate the effect of polo-like kinase 1 (PLK1) inhibition on cisplatin (DDP)-resistant gastric cancer (GC) cells.

METHODS

The transcriptional level of PLK1 was measured by quantitative reverse-transcription polymerase chain reaction. Expressions of PLK1 and its downstream mediators as well as autophagy-related protein LC3 I/LC3 II were detected by western blot. An 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and 5-ethynyl-2'-deoxyuridine immunofluorescent staining were conducted to evaluate the cell viability and replication activity separately. Flow cytometry was carried out to determine the cell cycle status. The GFP-LC3 vector contributed toward tracking the formation and aggregation of autophagosomes.

RESULTS

Drug-resistant SGC-7901/DDP cells showed insignificant changes in all phases after DDP treatment, including DNA replication, cell proliferation, cell cycle, and apoptosis, whereas DDP could significantly improve the autophagy level of SGC-7901/DDP as well as PLK1expression. By downregulating the expression of PLK1, both BI2536 andsi-PLK1 enhanced SGC-7901/DDP sensitivity to DDP, suppressing the proliferation and autophagy as well as improving the apoptosis rate. PLK1 inhibition also resulted in the repression of cell division regulators CDC25C and cyclin B1.

CONCLUSION

Together, our experimental results illustrated that the DDP resistance of GC cells might be associated with the aberrant overexpression of PLK1. PLK1 inhibition, including si-PLK1 and BI2536 treatment, could restore the chemosensitivity of drug-resistant SGC-7901/DDP cells and enhance the efficacy of DDP, revealing the potential value of PLK1 inhibition in GC chemotherapy.

The Mitotic Cancer Target Polo-Like Kinase 1: Oncogene or Tumor Suppressor?

The master mitotic regulator, Polo-like kinase 1 (Plk1), is an essential gene for the correct execution of cell division. Plk1 has strong clinical relevance, as it is considered a bona fide cancer target, it is found overexpressed in a large collection of different cancer types and this tumoral overexpression often correlates with poor patient prognosis. All these data led the scientific community to historically consider Plk1 as an oncogene. Although there is a collection of scientific reports showing how Plk1 can contribute to tumor progression, recent data from different laboratories using mouse models, show that Plk1 can surprisingly play as a tumor suppressor. Therefore, the fact that Plk1 is an oncogene is now under debate. This review summarizes the proposed mechanisms by which Plk1 can play as an oncogene or as a tumor suppressor, and extrapolates this information to clinical features.

Plk1 regulates spindle orientation by phosphorylating NuMA in human cells

Proper orientation of the mitotic spindle defines the correct division plane and is essential for accurate cell division and development. In metazoans, an evolutionarily conserved complex comprising of NuMA/LGN/Gαi regulates proper orientation of the mitotic spindle by orchestrating cortical dynein levels during metaphase. However, the molecular mechanisms that modulate the spatiotemporal dynamics of this complex during mitosis remain elusive. Here, we report that acute inactivation of Polo-like kinase 1 (Plk1) during metaphase enriches cortical levels of dynein/NuMA/LGN and thus influences spindle orientation. We establish that this impact of Plk1 on cortical levels of dynein/NuMA/LGN is through NuMA, but not via dynein/LGN. Moreover, we reveal that Plk1 inhibition alters the dynamic behavior of NuMA at the cell cortex. We further show that Plk1 directly interacts and phosphorylates NuMA. Notably, NuMA-phosphorylation by Plk1 impacts its cortical localization, and this is needed for precise spindle orientation during metaphase. Overall, our finding connects spindle-pole pool of Plk1 with cortical NuMA and answers a long-standing puzzle about how spindle-pole Plk1 gradient dictates proper spindle orientation for error-free mitosis.

Enhancing Gene-Knockdown Efficiency of Poly(N-isopropylacrylamide) Nanogels

Polo-like-kinase 1 (PLK1), which is a serine-threonine protein kinase overexpressed in cancer cells, is known to regulate tumor growth and have recently gathered attention as a target gene for RNA interference because of the poor bioavailability and nonspecificity of the available inhibitors. However, the lower transfection efficiency of siRNA and its poor stability in biological mileu necessitate the need of efficient siRNA delivery systems. Here, we report efficacious polymeric nanoparticles for the delivery of PLK1 siRNA in mammalian cancer cells. N-Isopropylacrylamide (NIPAm) and N-isopropylmethacrylamide-co-NIPAm nanogels were synthesized and modified using poly-ε-lysine. Furthermore, their ability to induce gene silencing was investigated by flow cytometry and real-time polymerase chain reaction, and the silencing efficiency observed was related to the polymer composition and its effect on the gene loading and protection ability and the endosomal escape capability. This study attempts to leverage the understanding of the cell-material interaction, thus, addressing the bottlenecks of siRNA delivery for enhancing the efficacy of the poly(N-isopropylacrylamide)-based delivery vehicle.