Polo-like kinase 1 mediates BRCA1 phosphorylation and recruitment at DNA double-strand breaks

The pivotal role of irradiation-induced apoptosis in the pathogenesis and therapy of medulloblastoma

BACKGROUND

Medulloblastoma (MB) is a rare primitive neuroectodermal tumors originating from the cerebellum. MB is the most common malignant primary brain tumor of childhood. MB originates from neural precursor cells in distinctive regions of the rhombic lip, and their maturation occurs in the cerebellum or the brain stem during embryonal development. Also, apoptosis is a programmed cell death associated with numerous physiological as well as pathological regulations.

RECENT FINDINGS

Irradiation (IR)-induce apoptosis triggers cell death, with or without intervening mitosis within a few hours of IR and these share different morphologic alteration such as, loss of normal nuclear structure as well as degradation of DNA. Moreover, MB is strikingly sensitive to DNA-damaging therapies and the role of apoptosis a key treatment modality. Furthermore, in MB, the apoptotic pathways are made up of several triggers, modulators, as well as effectors. Notably, IR-induced apoptotic mechanisms in MB therapy are very complex and they either induce radiosensitivity or inhibit radioresistance leading to potential effective treatment strategies for MB.

CONCLUSION

This review explicitly explores the pivotal roles of IR-induced apoptosis in the pathogenesis and therapy of MB.

A new wave of innovations within the DNA damage response

Genome instability has been identified as one of the enabling hallmarks in cancer. DNA damage response (DDR) network is responsible for maintenance of genome integrity in cells. As cancer cells frequently carry DDR gene deficiencies or suffer from replicative stress, targeting DDR processes could induce excessive DNA damages (or unrepaired DNA) that eventually lead to cell death. Poly (ADP-ribose) polymerase (PARP) inhibitors have brought impressive benefit to patients with breast cancer gene (BRCA) mutation or homologous recombination deficiency (HRD), which proves the concept of synthetic lethality in cancer treatment. Moreover, the other two scenarios of DDR inhibitor application, replication stress and combination with chemo- or radio- therapy, are under active clinical exploration. In this review, we revisited the progress of DDR targeting therapy beyond the launched first-generation PARP inhibitors. Next generation PARP1 selective inhibitors, which could maintain the efficacy while mitigating side effects, may diversify the application scenarios of PARP inhibitor in clinic. Albeit with unavoidable on-mechanism toxicities, several small molecules targeting DNA damage checkpoints (gatekeepers) have shown great promise in preliminary clinical results, which may warrant further evaluations. In addition, inhibitors for other DNA repair pathways (caretakers) are also under active preclinical or clinical development. With these progresses and efforts, we envision that a new wave of innovations within DDR has come of age.

Maintaining Genome Integrity: Protein Kinases and Phosphatases Orchestrate the Balancing Act of DNA Double-Strand Breaks Repair in Cancer

DNA double-strand breaks (DSBs) are the most lethal DNA damages which lead to severe genome instability. Phosphorylation is one of the most important protein post-translation modifications involved in DSBs repair regulation. Kinases and phosphatases play coordinating roles in DSB repair by phosphorylating and dephosphorylating various proteins. Recent research has shed light on the importance of maintaining a balance between kinase and phosphatase activities in DSB repair. The interplay between kinases and phosphatases plays an important role in regulating DNA-repair processes, and alterations in their activity can lead to genomic instability and disease. Therefore, study on the function of kinases and phosphatases in DSBs repair is essential for understanding their roles in cancer development and therapeutics. In this review, we summarize the current knowledge of kinases and phosphatases in DSBs repair regulation and highlight the advancements in the development of cancer therapies targeting kinases or phosphatases in DSBs repair pathways. In conclusion, understanding the balance of kinase and phosphatase activities in DSBs repair provides opportunities for the development of novel cancer therapeutics.

PARylation of BRCA1 limits DNA break resection through BRCA2 and EXO1

The nucleolytic processing (resection) of a DNA double-strand break (DSB) is a critical step to repair the lesion by homologous recombination (HR). PARylation, which is the attachment of poly(ADP-ribose) (PAR) units to specific targets by PAR polymerases (PARPs), regulates many steps of HR, including resection. Here, we show that preventing PARylation of the oncosuppressor BRCA1 induces hyper-resection of DSBs through BRCA2 and the EXO1 nuclease. Upon expression of the unPARylatable variant of BRCA1, we observe a reduced 53BP1-RIF1 barrier for resection accompanied by an increase in the recruitment of the RAD51 recombinase. Similar results are observed when cells are treated with the clinically approved PARP inhibitor olaparib. We propose that PARylation of BRCA1 is important to limit the formation of excessively extended DNA filaments, thereby reducing illegitimate chromosome rearrangements. Our results shed light on molecular aspects of HR and on the mechanisms of PARP inhibitor treatment.

Present and Future Perspective on PLK1 Inhibition in Cancer Treatment

Polo-like kinase 1 (PLK1) is the principle member of the well conserved serine/threonine kinase family. PLK1 has a key role in the progression of mitosis and recent evidence suggest its important involvement in regulating the G2/M checkpoint, in DNA damage and replication stress response, and in cell death pathways. PLK1 expression is tightly spatially and temporally regulated to ensure its nuclear activation at the late S-phase, until the peak of expression at the G2/M-phase. Recently, new roles of PLK1 have been reported in literature on its implication in the regulation of inflammation and immunological responses. All these biological processes are altered in tumors and, considering that PLK1 is often found overexpressed in several tumor types, its targeting has emerged as a promising anti-cancer therapeutic strategy. In this review, we will summarize the evidence suggesting the role of PLK1 in response to DNA damage, including DNA repair, cell cycle progression, epithelial to mesenchymal transition, cell death pathways and cancer-related immunity. An update of PLK1 inhibitors currently investigated in preclinical and clinical studies, in monotherapy and in combination with existing chemotherapeutic drugs and targeted therapies will be discussed.

A New Risk Model Based on 7 Quercetin-Related Target Genes for Predicting the Prognosis of Patients With Lung Adenocarcinoma

Background: Studies have reported that quercetin inhibits the growth and migration of lung adenocarcinoma (LUAD). This study aimed to explore the roles and mechanisms of quercetin target genes in the progression of LUAD.

Methods: The quercetin structure and potential target genes of quercetin were explored in the Traditional Chinese Medicine Systems Pharmacology and SwissTargetPrediction databases. The differentially expressed quercetin target genes were identified in The Cancer Genome Atlas (TCGA) database, and the clinical values of quercetin target genes were explored. Subsequently, a risk model was constructed via the Cox regression and survival analysis to evaluate the potential effects and possible mechanisms of quercetin target genes.

Results: The quercetin differential target genes involved in biological processes such as the oxidation-reduction process, cell proliferation, G2/M transition of the mitotic cell cycle, and were related to the lung cancer. NEK2, TOP2A, PLK1, CA4, CDK5R1, AURKB, and F2 were related to the prognosis, and were independent factors influencing the prognosis of LUAD patients. The risk model was related to the gender, clinical stage, T stage, lymph node metastasis, and survival status of LUAD patients, and was independent risk factor associated with poor prognosis. In the high-risk group, the risk model involved signaling pathways such as cell cycle, DNA replication, spliceosome, and homologous recombination.

Conclusion: The quercetin potential target genes NEK2, TOP2A, PLK1, CA4, CDK5R1, AURKB, and F2 were related to the diagnosis and prognosis of LUAD patients. A risk model based on 7 quercetin target genes could be used to assess the prognosis of patients with LUAD.

PLK1 depletion alters homologous recombination and synaptonemal complex disassembly events during mammalian spermatogenesis

Homologous recombination (HR) is an essential meiotic process that contributes to the genetic variation of offspring and ensures accurate chromosome segregation. Recombination is facilitated by the formation and repair of programmed DNA double-strand breaks. These DNA breaks are repaired via recombination between maternal and paternal homologous chromosomes and a subset result in the formation of crossovers. HR and crossover formation is facilitated by synapsis of homologous chromosomes by a proteinaceous scaffold structure known as the synaptonemal complex (SC). Recent studies in yeast and worms have indicated that polo-like kinases (PLKs) regulate several events during meiosis, including DNA recombination and SC dynamics. Mammals express four active PLKs (PLK1-4), and our previous work assessing localization and kinase function in mouse spermatocytes suggested that PLK1 coordinates nuclear events during meiotic prophase. Therefore, we conditionally mutated Plk1 in early prophase spermatocytes and assessed stages of HR, crossover formation, and SC processes. Plk1 mutation resulted in increased RPA foci and reduced RAD51/DMC1 foci during zygonema, and an increase of both class I and class II crossover events. Furthermore, the disassembly of SC lateral elements was aberrant. Our results highlight the importance of PLK1 in regulating HR and SC disassembly during spermatogenesis.

A novel PLK1 inhibitor onvansertib effectively sensitizes MYC-driven medulloblastoma to radiotherapy

BACKGROUND

Group 3 medulloblastoma (MB) is often accompanied by MYC amplification. PLK1 is an oncogenic kinase that controls cell cycle and proliferation and has been preclinically validated as a cancer therapeutic target. Onvansertib (PCM-075) is a novel, orally available PLK1 inhibitor, which shows tumor growth inhibition in various types of cancer. We aim to explore the effect of onvansertib on MYC-driven medulloblastoma as a monotherapy or in combination with radiation.

METHODS

Crisper-Cas9 screen was used to discover essential genes for MB tumor growth. Microarray and immunohistochemistry on pediatric patient samples were performed to examine the expression of PLK1. The effect of onvansertib in vitro was measure by cell viability, colony-forming assays, extreme limiting dilution assay, and RNA-Seq. ALDH activity, cell-cycle distribution, and apoptosis were analyzed by flow cytometry. DNA damage was assessed by immunofluorescence staining. Medulloblastoma xenografts were generated to explore the monotherapy or radio-sensitizing effect.

RESULTS

PLK1 is overexpressed in Group 3 MB. The IC50 concentrations of onvansertib in Group 3 MB cell lines were in a low nanomolar range. Onvansertib reduced colony formation, cell proliferation, stem cell renewal and induced G2/M arrest in vitro. Moreover, onvansertib in combination with radiation increased DNA damage and apoptosis compared with radiation treatment alone. The combination radiotherapy resulted in marked tumor regression in xenografts.

CONCLUSIONS

These findings demonstrate the efficacy of a novel PLK1 inhibitor onvansertib in vitro and in xenografts of Group 3 MB, which suggests onvansertib is an effective strategy as monotherapy or in combination with radiotherapy in MB.

Polo-like kinase 1 (PLK1) signaling in cancer and beyond

PLK1 is an evolutionary conserved Ser/Thr kinase that is best known for its role in cell cycle regulation and is expressed predominantly during the G2/S and M phase of the cell cycle. PLK1-mediated phosphorylation of specific substrates controls cell entry into mitosis, centrosome maturation, spindle assembly, sister chromatid cohesion and cytokinesis. In addition, a growing body of evidence describes additional roles of PLK1 beyond the cell cycle, more specifically in the DNA damage response, autophagy, apoptosis and cytokine signaling. PLK1 has an indisputable role in cancer as it controls several key transcription factors and promotes cell proliferation, transformation and epithelial-to-mesenchymal transition. Furthermore, deregulation of PLK1 results in chromosome instability and aneuploidy. PLK1 is overexpressed in many cancers, which is associated with poor prognosis, making PLK1 an attractive target for cancer treatment. Additionally, PLK1 is involved in immune and neurological disorders including Graft versus Host Disease, Huntington's disease and Alzheimer's disease. Unfortunately, newly developed small compound PLK1 inhibitors have only had limited success so far, due to low therapeutic response rates and toxicity. In this review we will highlight the current knowledge about the established roles of PLK1 in mitosis regulation and beyond. In addition, we will discuss its tumor promoting but also tumor suppressing capacities, as well as the available PLK1 inhibitors, elaborating on their efficacy and limitations.

ATR/ATM-mediated phosphorylation of BRCA1 T1394 promotes homologous recombinational repair and G2/M checkpoint maintenance

BRCA1 maintains genome integrity and suppresses tumorigenesis by promoting homologous recombination (HR)-mediated repair of DNA double strand breaks (DSB) and DNA damage-induced cell cycle checkpoints. Phosphorylation of BRCA1 by ATM, ATR, CHK2, CDK, and PLK1 kinases has been reported to regulate its functions. Here we show that ATR and ATM-mediated phosphorylation of BRCA1 on T1394, a highly conserved but functionally uncharacterized site, is a key modification for its function in the DNA damage response. Following DNA damage, T1394 phosphorylation ensured faithful repair of DSBs by promoting HR and preventing single strand annealing, a deletion-generating repair process. BRCA1 T1394 phosphorylation further safeguarded chromosomal integrity by maintaining the G2/M checkpoint. Moreover, multiple patient-derived BRCA1 variants of unknown significance were shown to affect T1394 phosphorylation. These results establish an important regulatory mechanism of BRCA1 function in the DNA damage response and may have implications in the development or prognosis of BRCA1-associated cancers.

From Laboratory Studies to Clinical Trials: Temozolomide Use in IDH-Mutant Gliomas

In this review, we discuss the use of the alkylating agent temozolomide (TMZ) in the treatment of IDH-mutant gliomas. We describe the challenges associated with TMZ in clinical (drug resistance and tumor recurrence) and preclinical settings (variabilities associated with in vitro models) in treating IDH-mutant glioma. Lastly, we summarize the emerging therapeutic targets that can potentially be used in combination with TMZ.

Breast Cancer and the Other Non-Coding RNAs

Breast cancer is very heterogenous and the most common gynaecological cancer, with various factors affecting its development. While its impact on human lives and national health budgets is still rising in almost all global areas, many molecular mechanisms affecting its onset and development remain unclear. Conventional treatments still prove inadequate in some aspects, and appropriate molecular therapeutic targets are required for improved outcomes. Recent scientific interest has therefore focused on the non-coding RNAs roles in tumour development and their potential as therapeutic targets. These RNAs comprise the majority of the human transcript and their broad action mechanisms range from gene silencing to chromatin remodelling. Many non-coding RNAs also have altered expression in breast cancer cell lines and tissues, and this is often connected with increased proliferation, a degraded extracellular environment, and higher endothelial to mesenchymal transition. Herein, we summarise the known abnormalities in the function and expression of long non-coding RNAs, Piwi interacting RNAs, small nucleolar RNAs and small nuclear RNAs in breast cancer, and how these abnormalities affect the development of this deadly disease. Finally, the use of RNA interference to suppress breast cancer growth is summarised.

Small-molecule inhibitors of breast cancer-related targets: Potential therapeutic agents for breast cancer

Despite dramatic advances in cancer research and therapy, breast cancer remains a tricky health problem and represents a top biomedical research priority. Nowadays, breast cancer is still the leading cause of malignancy-related deaths in women, and incidence and mortality rates of it are expected to increase significantly the next years. Currently more and more researchers are interested in the study of breast cancer by its arising in young women. The common treatment options of breast cancer are chemotherapy, immunotherapy, hormone therapy, surgery, and radiotherapy. Most of them require chemical agents, such as PARP inhibitors, CDK4/6 inhibitors, and HER2 inhibitors. Recent studies suggest that some targets or pathways, including BRD4, PLK1, PD-L1, HDAC, and PI3K/AKT/mTOR, are tightly related to the occurrence and development of breast cancer. This article reviews the interplay between these targets and breast cancer and summarizes the progress of current research on small molecule inhibitors of these anti-breast cancer targets. The review aims to provide structural and theoretical basis for designing novel anti-breast cancer agents.

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.

High expression of polo-like kinase 1 is associated with TP53 inactivation, DNA repair deficiency, and worse prognosis in ER positive Her2 negative breast cancer

Polo-like kinase 1 (PLK1), the most investigated member of the PLK family, plays a pivotal role both in the p53-mediated regulation of DNA damage repair and in mitosis, especially in the G2/M phase. However, the evidence on the clinical and prognostic relevance of PLK1 is limited to triple negative subtype among breast cancer (BC). We hypothesized that high expression of PLK1 is associated with TP53 inactivation, DNA repair deficiency, and worse prognosis in ER positive in BC in a large-scale cohort should clarify its clinical relevance for each BC subtype. Total of 3173 BC cases; 1025 from TCGA cohort, 1904 from METABRIC, and 244 from neoadjuvant chemotherapy (NAC) cohort from Gene Expression Omnibus dataset, GSE32603, were analyzed. PLK1 expressions were significantly higher in high Nottingham Grade and triple negative BC. High expression of PLK1 was significantly associated with TP53 mutation, high expression of TP53 mRNA as well as protein, and it significantly correlated with the homologous recombination deficiency score. High PLK1 expression significantly enriched cell cycle related gene sets (G2/M check point, E2F targets), MTORC1 signaling, and MYC target gene sets in the Gene Set Enrichment Analysis. High expression of PLK1 was significantly associated with tumor infiltrating lymphocytes and tumor associated macrophages (high levels of CD8+ T cells, M0 and M1 macrophage, and low levels of M2 macrophage), and high immune cytolytic activity. While high expression of PLK1 did not associate with pathological complete response after NAC, it was associated with poor prognosis in the whole cohort and in the ER-positive/HER2-negative subtype of TCGA. High expression of PLK1 is significantly associated with TP53 mutations, DNA repair deficiency and worse prognosis in BC particularly in HR+HER2- subtype. Using bioinformatics methods with large cohorts.

MicroRNA-1181 supports the growth of hepatocellular carcinoma by repressing AXIN1

Micro-RNAs regulate multiple biological behaviors of cancers, making them potential targets of new cancer therapies. MiR-1181 has been demonstrated to perform oncogenic or tumor-suppressing function in a tissue-dependent way, but its role in hepatocellular carcinoma (HCC) was unclear. Here, we showed that miR-1181 was significantly overexpressed in HCC tissues when compared with tumor-adjacent normal ones or normal liver tissues from donated organ, and that inhibition of miR-1181 could repress the growth of HCC cells. Through bioinformatics analysis and luciferase reporter assays, we found that axis inhibition protein 1 (AXIN1) was a direct target of miR-1181, and the expression of AXIN1 showed a negative correlation with that of miR-1181 in HCC. Therefore, these data indicated an oncogenic function of miRNA-1181 in the development of HCC and a potential target for the clinical treatment of HCC.

Therapeutic potential of PLK1 inhibition in triple-negative breast cancer

Triple negative breast cancer (TNBC) is responsible for significant number of breast cancer-associated deaths because of lacking of successful molecular-targeted therapy. To explore a therapeutic target for TNBC, we performed a siRNA-mediated knockdown screening and identified Polo-like kinase 1 (PLK1) as a potential therapeutic target for TNBC. Knockdown of PLK1 as well as a small compound inhibitor for PLK1, BI-2536, induced G2/M arrest and created polyploid cell population, shown by increased DNA content and nuclear size. Inhibition of PLK1 eventually triggered apoptosis in multiple TNBC cell lines. In addition, we confirmed that PLK1 was significantly overexpressed in the tissues from TNBC patients compared with the tissues of normal mammary glands and benign breast tumors. Our data indicated that PLK1 plays a pivotal role in the regulation of mitosis of TNBC cells. Although future in vivo studies are warranted, targeting PLK1 by a selective inhibitor such as BI-2536 can be an attractive molecular-targeted therapy for TNBC.

Polo-like Kinase 1 Inhibition as a Therapeutic Approach to Selectively Target BRCA1-Deficient Cancer Cells by Synthetic Lethality Induction

PURPOSE

BRCA1 and BRCA2 deficiencies are widespread drivers of human cancers that await the development of targeted therapies. We aimed to identify novel synthetic lethal relationships with therapeutic potential using BRCA-deficient isogenic backgrounds.

EXPERIMENTAL DESIGN

We developed a phenotypic screening technology to simultaneously search for synthetic lethal (SL) interactions in BRCA1- and BRCA2-deficient contexts. For validation, we developed chimeric spheroids and a dual-tumor xenograft model that allowed the confirmation of SL induction with the concomitant evaluation of undesired cytotoxicity on BRCA-proficient cells. To extend our results using clinical data, we performed retrospective analysis on The Cancer Genome Atlas (TCGA) breast cancer database.

RESULTS

The screening of a kinase inhibitors library revealed that Polo-like kinase 1 (PLK1) inhibition triggers strong SL induction in BRCA1-deficient cells. Mechanistically, we found no connection between the SL induced by PLK1 inhibition and PARP inhibitors. Instead, we uncovered that BRCA1 downregulation and PLK1 inhibition lead to aberrant mitotic phenotypes with altered centrosomal duplication and cytokinesis, which severely reduced the clonogenic potential of these cells. The penetrance of PLK1/BRCA1 SL interaction was validated using several isogenic and nonisogenic cellular models, chimeric spheroids, and mice xenografts. Moreover, bioinformatic analysis revealed high-PLK1 expression in BRCA1-deficient tumors, a phenotype that was consistently recapitulated by inducing BRCA1 deficiency in multiple cell lines as well as in BRCA1-mutant cells.

CONCLUSIONS

We uncovered an unforeseen addiction of BRCA1-deficient cancer cells to PLK1 expression, which provides a new means to exploit the therapeutic potential of PLK1 inhibitors in clinical trials, by generating stratification schemes that consider this molecular trait in patient cohorts.

Adaptation in replicative senescence: a risky business

Cell proliferation is tightly regulated to avoid propagating DNA damage and mutations, which can lead to pathologies such as cancer. To ensure genome integrity, cells activate the DNA damage checkpoint in response to genotoxic lesions to block cell cycle progression. This surveillance mechanism provides time to repair the damage before resuming cell cycle with an intact genome. When the damage is not repaired, cells can, in some conditions, override the cell cycle arrest and proceed with proliferation, a phenomenon known as adaptation to DNA damage. A subpopulation of adapted cells might eventually survive, but only at the cost of extensive genome instability. How and in which context adaptation operates the trade-off between survival and genome stability is a fascinating question. After a brief review of the current knowledge on adaptation to DNA damage in budding yeast, we will discuss a new role of adaptation in the context of telomerase-negative cells and replicative senescence. We highlight the idea that, in all settings studied so far, survival through adaptation is a double-edged sword as it comes with increased genomic instability.

PLK1 targets CtIP to promote microhomology-mediated end joining

Proper DNA double-strand break (DSB) repair is essential for maintaining genome integrity. Microhomology-mediated end joining (MMEJ) is an error-prone repair mechanism, which introduces mutations at break sites and contributes to chromosomal translocations and telomere fusions, thus driving carcinogenesis. Mitotic kinases PLK1, CDK1 and Aurora A are important for supporting MMEJ and are often overexpressed in various tumors. However, the functional interplay between these kinases and MMEJ has not been explored. Here, we found that MMEJ is preferentially employed to fix DSBs in cells arrested in mitosis following nocodazole treatment. We further showed that the DSB repair factor CtIP is jointly phosphorylated by CDK1/Aurora A and PLK1. CDK1/Aurora A-mediated CtIP phosphorylation at serine 327 triggers CtIP binding to the PLK1 polo-box domain, which in turn facilitates PLK1 to phosphorylate CtIP mainly at serine 723. A PLK1 phosphor-mimic CtIP mutant fails to initiate extended end resection and is thus unable to mediate homologous recombination and the G2/M checkpoint but can mediate MMEJ. These data imply that PLK1 may target CtIP to promote error-prone MMEJ and inactivate the G2/M checkpoint. These findings have helped elucidate the oncogenic roles of these factors.

Aurora-PLK1 cascades as key signaling modules in the regulation of mitosis

Mitosis is controlled by reversible protein phosphorylation involving specific kinases and phosphatases. A handful of major mitotic protein kinases, such as the cyclin B-CDK1 complex, the Aurora kinases, and Polo-like kinase 1 (PLK1), cooperatively regulate distinct mitotic processes. Research has identified proteins and mechanisms that integrate these kinases into signaling cascades that guide essential mitotic events. These findings have important implications for our understanding of the mechanisms of mitotic regulation and may advance the development of novel antimitotic drugs. We review collected evidence that in vertebrates, the Aurora kinases serve as catalytic subunits of distinct complexes formed with the four scaffold proteins Bora, CEP192, INCENP, and TPX2, which we deem "core" Aurora cofactors. These complexes and the Aurora-PLK1 cascades organized by Bora, CEP192, and INCENP control crucial aspects of mitosis and all pathways of spindle assembly. We compare the mechanisms of Aurora activation in relation to the different spindle assembly pathways and draw a functional analogy between the CEP192 complex and the chromosomal passenger complex that may reflect the coevolution of centrosomes, kinetochores, and the actomyosin cleavage apparatus. We also analyze the roles and mechanisms of Aurora-PLK1 signaling in the cell and centrosome cycles and in the DNA damage response.

Akt1 Stimulates Homologous Recombination Repair of DNA Double-Strand Breaks in a Rad51-Dependent Manner

Akt1 is known to promote non-homologous end-joining (NHEJ)-mediated DNA double-strand break (DSB) repair by stimulation of DNA-PKcs. In the present study, we investigated the effect of Akt1 on homologous recombination (HR)-dependent repair of radiation-induced DSBs in non-small cell lung cancer (NSCLC) cells A549 and H460. Akt1-knockdown (Akt1-KD) significantly reduced Rad51 protein level, Rad51 foci formation and its colocalization with γH2AX foci after irradiation. Moreover, Akt1-KD decreased clonogenicity after treatment with Mitomycin C and HR repair, as tested by an HR-reporter assay. Double knockdown of Akt1 and Rad51 did not lead to a further decrease in HR compared to the single knockdown of Rad51. Consequently, Akt1-KD significantly increased the number of residual DSBs after irradiation partially independent of the kinase activity of DNA-PKcs. Likewise, the number of residual BRCA1 foci, indicating unsuccessful HR events, also significantly increased in the irradiated cells after Akt1-KD. Together, the results of the study indicate that Akt1 seems to be a regulatory component in the HR repair of DSBs in a Rad51-dependent manner. Thus, based on this novel role of Akt1 in HR and the previously described role of Akt1 in NHEJ, we propose that targeting Akt1 could be an effective approach to selectively improve the killing of tumor cells by DSB-inducing cytotoxic agents, such as ionizing radiation.

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

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

Dasatinib synergises with irinotecan to suppress hepatocellular carcinoma via inhibiting the protein synthesis of PLK1

BACKGROUND

Hepatocellular carcinoma (HCC) is one of the most common types of malignant tumour and has poor prognosis. Currently, systematic chemotherapy is the only approach to prolong survival. Thus the development of new treatment regimens is urgently needed to improve the therapeutic efficacy. Our study intended to assess the combination of dasatinib and irinotecan against HCC and made an effort to develop a potential medical choice for advanced HCC patients.

METHODS

We used SRB colorimetric assay and clonogenic assay to assess antitumour effect in vitro and HCC xenograft model to assess antitumour effect in vivo. We applied flow cytometry and western blotting to explore the mechanism of the combined therapy. Knockdown and overexpression of PLK1 are also applied for validation.

RESULTS

We confirmed that dasatinib has synergistic effect with irinotecan (or SN38) on HCC both in vitro and in vivo. The effect is due to arisen apoptosis rate of HCC cells that is accompanied by mitochondria dysfunction. The enhanced antitumour efficacy of SN38 could be explained by additional inhibition of PLK1, which is triggered by dasatinib. Unlike existed PLK1 inhibitors, dasatinib does not inhibit PLK1 activity in a direct way. Instead, we found that dasatinib reduces PLK1 level by interfering with its protein synthesis progress. We validated that this kind of downregulation of PLK1 level has a key role in the synergistic effect of the two agents.

CONCLUSIONS

Dasatinib is able to reinforce the anti-HCC efficacy of irinotecan/SN38 by downregulation of PLK1 synthesis. The combination of the two agents might be a potential medical choice for HCC therapy.

PLK1 inhibition enhances temozolomide efficacy in IDH1 mutant gliomas

Despite multimodal therapy with radiation and the DNA alkylating agent temozolomide (TMZ), malignant gliomas remain incurable. Up to 90% of grades II-III gliomas contain a single mutant isocitrate dehydrogenase 1 (IDH1) allele. IDH1 mutant-mediated transformation is associated with TMZ resistance; however, there is no clinically available means of sensitizing IDH1 mutant tumors to TMZ. In this study we sought to identify a targetable mechanism of TMZ resistance in IDH1 mutant tumors to enhance TMZ efficacy. IDH1 mutant astrocytes rapidly bypassed the G2 checkpoint with unrepaired DNA damage following TMZ treatment. Checkpoint adaptation was accompanied by PLK1 activation and IDH1 mutant astrocytes were more sensitive to treatment with BI2536 and TMZ in combination (<20% clonogenic survival) than either TMZ (~60%) or BI2536 (~75%) as single agents. In vivo, TMZ or BI2536 alone had little effect on tumor size. Combination treatment caused marked tumor shrinkage in all mice and complete tumor regression in 5 of 8 mice. Mutant IDH1 promotes checkpoint adaptation which can be exploited therapeutically with the combination of TMZ and a PLK1 inhibitor, indicating PLK1 inhibitors may be clinically valuable in the treatment of IDH1 mutant gliomas.

PLK1, A Potential Target for Cancer Therapy

Polo-like kinase 1 (PLK1) plays an important role in the initiation, maintenance, and completion of mitosis. Dysfunction of PLK1 may promote cancerous transformation and drive its progression. PLK1 overexpression has been found in a variety of human cancers and was associated with poor prognoses in cancers. Many studies have showed that inhibition of PLK1 could lead to death of cancer cells by interfering with multiple stages of mitosis. Thus, PLK1 is expected to be a potential target for cancer therapy. In this article, we examined PLK1’s structural characteristics, its regulatory roles in cell mitosis, PLK1 expression, and its association with survival prognoses of cancer patients in a wide variety of cancer types, PLK1 interaction networks, and PLK1 inhibitors under investigation. Finally, we discussed the key issues in the development of PLK1-targeted cancer therapy.

Cellular response to DNA interstrand crosslinks: the Fanconi anemia pathway

Interstrand crosslinks (ICLs) are a highly toxic form of DNA damage. ICLs can interfere with vital biological processes requiring separation of the two DNA strands, such as replication and transcription. If ICLs are left unrepaired, it can lead to mutations, chromosome breakage and mitotic catastrophe. The Fanconi anemia (FA) pathway can repair this type of DNA lesion, ensuring genomic stability. In this review, we will provide an overview of the cellular response to ICLs. First, we will discuss the origin of ICLs, comparing various endogenous and exogenous sources. Second, we will describe FA proteins as well as FA-related proteins involved in ICL repair, and the post-translational modifications that regulate these proteins. Finally, we will review the process of how ICLs are repaired by both replication-dependent and replication-independent mechanisms.