Role for Plk1 phosphorylation of Hbo1 in regulation of replication licensing

The Eyes Absent family: At the intersection of DNA repair, mitosis, and replication

The Eyes Absent family (EYA1-4) are a group of dual function proteins that act as both tyrosine phosphatases and transcriptional co-activators. EYA proteins play a vital role in development, but are also aberrantly overexpressed in cancers, where they often confer an oncogenic effect. Precisely how the EYAs impact cell biology is of growing interest, fuelled by the therapeutic potential of an expanding repertoire of EYA inhibitors. Recent functional studies suggest that the EYAs are important players in the regulation of genome maintenance pathways including DNA repair, mitosis, and DNA replication. While the characterized molecular mechanisms have predominantly been ascribed to EYA phosphatase activities, EYA co-transcriptional activity has also been found to impact the expression of genes that support these pathways. This indicates functional convergence of EYA phosphatase and co-transcriptional activities, highlighting the emerging importance of the EYA protein family at the intersection of genome maintenance mechanisms. In this review, we discuss recent progress in defining EYA protein substrates and transcriptional effects, specifically in the context of genome maintenance. We then outline future directions relevant to the field and discuss the clinical utility of EYA inhibitors.

HBO1/KAT7/MYST2 HAT complex regulates human adenovirus replicative cycle

Human adenoviruses (HAdV) belong to a small DNA tumor virus family that continues as valuable models in understanding the viral strategies of usurping cell growth regulation. A number of HAdV type 2/5 early viral gene products interact with a variety of cellular proteins to build a conducive environment that promotes viral replication. Here we show that HBO1 (Histone Acetyltransferase Binding to ORC1), a member of the MYST histone acetyltransferase (HAT) complex (also known as KAT7 and MYST2) that acetylates most of the histone H3 lysine 14, is essential for HAdV5 growth. HBO1/MYST2/KAT7 HAT complexes are critical for a variety of cellular processes including control of cell proliferation. In HBO1 downregulated human cells, HAdV5 infection results in reduced expression of E1A and other viral early genes, virus growth is also reduced significantly. Importantly, HBO1 downregulation reduced H3 lysine 14 acetylation at viral promoters during productive infection, likely driving reduced viral gene expression. HBO1 was also associated with viral promoters during infection and co-localized with viral replication centers in the nuclei of infected cells. In transiently transfected cells, overexpression of E1A along with HBO1 stimulated histone acetyltransferase activity of HBO1. E1A also co-immunoprecipitated with HBO1 in transiently transfected cells. In summary, our results demonstrate that HAdV recruits the HBO1 HAT complex to aid in viral replication.

Histone acetylation by HBO1 (KAT7) activates Wnt/β-catenin signaling to promote leukemogenesis in B-cell acute lymphoblastic leukemia

B-cell acute lymphoblastic leukemia (B-ALL) is an aggressive hematological disorder with a dismal prognosis. The dysregulation of histone acetylation is of great significance in the pathogenesis and progression of B-ALL. Regarded as a fundamental acetyltransferase gene, the role of HBO1 (lysine acetyltransferase 7/KAT7) in B-ALL has not been investigated. Herein, we found that HBO1 expression was elevated in human B-ALL cells and associated with poor disease-free survival. Strikingly, HBO1 knockdown inhibited viability, proliferation, and G1-S cycle progression in B-ALL cells, while provoking apoptosis. In contrast, ectopic overexpression of HBO1 enhanced cell viability and proliferation but inhibited apoptotic activation. The results of in vivo experiments also certificated the inhibitory effect of HBO1 knockdown on tumor growth. Mechanistically, HBO1 acetylated histone H3K14, H4K8, and H4K12, followed by upregulating CTNNB1 expression, resulting in activation of the Wnt/β-catenin signaling pathway. Moreover, a novel small molecule inhibitor of HBO1, WM-3835, potently inhibited the progression of B-ALL. Our data identified HBO1 as an efficacious regulator of CTNNB1 with therapeutic potential in B-ALL.

Post-translational modifications of histones: Mechanisms, biological functions, and therapeutic targets

Histones are DNA-binding basic proteins found in chromosomes. After the histone translation, its amino tail undergoes various modifications, such as methylation, acetylation, phosphorylation, ubiquitination, malonylation, propionylation, butyrylation, crotonylation, and lactylation, which together constitute the "histone code." The relationship between their combination and biological function can be used as an important epigenetic marker. Methylation and demethylation of the same histone residue, acetylation and deacetylation, phosphorylation and dephosphorylation, and even methylation and acetylation between different histone residues cooperate or antagonize with each other, forming a complex network. Histone-modifying enzymes, which cause numerous histone codes, have become a hot topic in the research on cancer therapeutic targets. Therefore, a thorough understanding of the role of histone post-translational modifications (PTMs) in cell life activities is very important for preventing and treating human diseases. In this review, several most thoroughly studied and newly discovered histone PTMs are introduced. Furthermore, we focus on the histone-modifying enzymes with carcinogenic potential, their abnormal modification sites in various tumors, and multiple essential molecular regulation mechanism. Finally, we summarize the missing areas of the current research and point out the direction of future research. We hope to provide a comprehensive understanding and promote further research in this field.

The phosphorylation to acetylation/methylation cascade in transcriptional regulation: how kinases regulate transcriptional activities of DNA/histone-modifying enzymes

Transcription factors directly regulate gene expression by recognizing and binding to specific DNA sequences, involving the dynamic alterations of chromatin structure and the formation of a complex with different kinds of cofactors, like DNA/histone modifying-enzymes, chromatin remodeling factors, and cell cycle factors. Despite the significance of transcription factors, it remains unclear to determine how these cofactors are regulated to cooperate with transcription factors, especially DNA/histone modifying-enzymes. It has been known that DNA/histone modifying-enzymes are regulated by post-translational modifications. And the most common and important modification is phosphorylation. Even though various DNA/histone modifying-enzymes have been classified and partly explained how phosphorylated sites of these enzymes function characteristically in recent studies. It still needs to find out the relationship between phosphorylation of these enzymes and the diseases-associated transcriptional regulation. Here this review describes how phosphorylation affects the transcription activity of these enzymes and other functions, including protein stability, subcellular localization, binding to chromatin, and interaction with other proteins.

BRPF1-KAT6A/KAT6B Complex: Molecular Structure, Biological Function and Human Disease

The bromodomain and PHD finger–containing protein1 (BRPF1) is a member of family IV of the bromodomain-containing proteins that participate in the post-translational modification of histones. It functions in the form of a tetrameric complex with a monocytic leukemia zinc finger protein (MOZ or KAT6A), MOZ-related factor (MORF or KAT6B) or HAT bound to ORC1 (HBO1 or KAT7) and two small non-catalytic proteins, the inhibitor of growth 5 (ING5) or the paralog ING4 and MYST/Esa1-associated factor 6 (MEAF6). Mounting studies have demonstrated that all the four core subunits play crucial roles in different biological processes across diverse species, such as embryonic development, forebrain development, skeletal patterning and hematopoiesis. BRPF1, KAT6A and KAT6B mutations were identified as the cause of neurodevelopmental disorders, leukemia, medulloblastoma and other types of cancer, with germline mutations associated with neurodevelopmental disorders displaying intellectual disability, and somatic variants associated with leukemia, medulloblastoma and other cancers. In this paper, we depict the molecular structures and biological functions of the BRPF1-KAT6A/KAT6B complex, summarize the variants of the complex related to neurodevelopmental disorders and cancers and discuss future research directions and therapeutic potentials.

Damsel in distress calling on her knights: Illuminating the pioneering role of E3 ubiquitin ligases in guarding the genome integrity

The maintenance of genomic integrity is of utmost importance for the organisms to survive and to accurately inherit traits to their progenies. Any kind of DNA damage either due to defect in DNA duplication and/ or uncontrolled cell division or intracellular insults or environment radiation can result in gene mutation, chromosomal aberration and ultimately genomic instability, which may cause several diseases including cancers. Therefore, cells have evolved machineries for the surveillance of genomic integrity. Enormous exciting studies in the past indicate that ubiquitination (a posttranslational modification of proteins) plays a crucial role in maintaining the genomic integrity by diverse ways. In fact, various E3 ubiquitin ligases catalyse ubiquitination of key proteins to control their central role during cell cycle, DNA damage response (DDR) and DNA repair. Some E3 ligases promote genomic instability while others prevent it, deregulation of both of which leads to several malignancies. In this review, we consolidate the recent findings wherein the role of ubiquitination in conferring genome integrity is highlighted. We also discuss the latest discoveries on the mechanisms utilized by various E3 ligases to preserve genomic stability, with a focus on their actions during cell cycle progression and different types of DNA damage response as well as repair pathways.

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.

The histone acetyltransferase HBO1 promotes efficient tip cell sprouting during angiogenesis

Blood vessel growth and remodelling are essential during embryonic development and disease pathogenesis. The diversity of endothelial cells (ECs) is transcriptionally evident and ECs undergo dynamic changes in gene expression during vessel growth and remodelling. Here, we investigated the role of the histone acetyltransferase HBO1 (KAT7), which is important for activating genes during development and for histone H3 lysine 14 acetylation (H3K14ac). Loss of HBO1 and H3K14ac impaired developmental sprouting angiogenesis and reduced pathological EC overgrowth in the retinal endothelium. Single-cell RNA sequencing of retinal ECs revealed an increased abundance of tip cells in Hbo1-deficient retinas, which led to EC overcrowding in the retinal sprouting front and prevented efficient tip cell migration. We found that H3K14ac was highly abundant in the endothelial genome in both intra- and intergenic regions, suggesting that HBO1 acts as a genome organiser that promotes efficient tip cell behaviour necessary for sprouting angiogenesis. This article has an associated 'The people behind the papers' interview.

Polo-like kinase 1 (Plk1) regulates DNA replication origin firing and interacts with Rif1 in Xenopus

The activation of eukaryotic DNA replication origins needs to be strictly controlled at multiple steps in order to faithfully duplicate the genome and to maintain its stability. How the checkpoint recovery and adaptation protein Polo-like kinase 1 (Plk1) regulates the firing of replication origins during non-challenged S phase remained an open question. Using DNA fiber analysis, we show that immunodepletion of Plk1 in the Xenopus in vitro system decreases replication fork density and initiation frequency. Numerical analyses suggest that Plk1 reduces the overall probability and synchrony of origin firing. We used quantitative chromatin proteomics and co-immunoprecipitations to demonstrate that Plk1 interacts with firing factors MTBP/Treslin/TopBP1 as well as with Rif1, a known regulator of replication timing. Phosphopeptide analysis by LC/MS/MS shows that the C-terminal domain of Rif1, which is necessary for its repressive action on origins through protein phosphatase 1 (PP1), can be phosphorylated in vitro by Plk1 on S2058 in its PP1 binding site. The phosphomimetic S2058D mutant interrupts the Rif1-PP1 interaction and modulates DNA replication. Collectively, our study provides molecular insights into how Plk1 regulates the spatio-temporal replication program and suggests that Plk1 controls origin activation at the level of large chromatin domains in vertebrates.

Co-Targeting Plk1 and DNMT3a in Advanced Prostate Cancer

Because there is no effective treatment for late-stage prostate cancer (PCa) at this moment, identifying novel targets for therapy of advanced PCa is urgently needed. A new network-based systems biology approach, XDeath, is developed to detect crosstalk of signaling pathways associated with PCa progression. This unique integrated network merges gene causal regulation networks and protein-protein interactions to identify novel co-targets for PCa treatment. The results show that polo-like kinase 1 (Plk1) and DNA methyltransferase 3A (DNMT3a)-related signaling pathways are robustly enhanced during PCa progression and together they regulate autophagy as a common death mode. Mechanistically, it is shown that Plk1 phosphorylation of DNMT3a leads to its degradation in mitosis and that DNMT3a represses Plk1 transcription to inhibit autophagy in interphase, suggesting a negative feedback loop between these two proteins. Finally, a combination of the DNMT inhibitor 5-Aza-2'-deoxycytidine (5-Aza) with inhibition of Plk1 suppresses PCa synergistically.

HBO1 overexpression is important for hepatocellular carcinoma cell growth

Hepatocellular carcinoma (HCC) is a common primary liver malignancy lacking effective molecularly-targeted therapies. HBO1 (lysine acetyltransferase 7/KAT7) is a member of MYST histone acetyltransferase family. Its expression and potential function in HCC are studied. We show that HBO1 mRNA and protein expression is elevated in human HCC tissues and HCC cells. HBO1 expression is however low in cancer-surrounding normal liver tissues and hepatocytes. In HepG2 and primary human HCC cells, shRNA-induced HBO1 silencing or CRISPR/Cas9-induced HBO1 knockout potently inhibited cell viability, proliferation, migration, and invasion, while provoking mitochondrial depolarization and apoptosis induction. Conversely, ectopic overexpression of HBO1 by a lentiviral construct augmented HCC cell proliferation, migration and invasion. In vivo, xenografts-bearing HBO1-KO HCC cells grew significantly slower than xenografts with control HCC cells in severe combined immunodeficient mice. These results suggest HBO1 overexpression is important for HCC cell progression.

Identification and culture of proliferative cells in abnormal Taenia solium larvae: Role in the development of racemose neurocysticercosis

Racemose neurocysticercosis is an aggressive disease caused by the aberrant expansion of the cyst form of Taenia solium within the subarachnoid spaces of the human brain and spinal cord resulting in a mass effect and chronic inflammation. Although expansion is likely caused by the proliferation and growth of the parasite bladder wall, there is little direct evidence of the mechanisms that underlie these processes. Since the development and growth of cysts in related cestodes involves totipotential germinative cells, we hypothesized that the expansive growth of the racemose larvae is organized and maintained by germinative cells. Here, we identified proliferative cells expressing the serine/threonine-protein kinase plk1 by in situ hybridization. Proliferative cells were present within the bladder wall of racemose form and absent from the homologous tissue surrounding the vesicular form. Cyst proliferation in the related model species Taenia crassiceps (ORF strain) occurs normally by budding from the cyst bladder wall and proliferative cells were concentrated within the growth buds. Cells isolated from bladder wall of racemose larvae were established in primary cell culture and insulin stimulated their proliferation in a dose-dependent manner. These findings indicate that the growth of racemose larvae is likely due to abnormal cell proliferation. The different distribution of proliferative cells in the racemose larvae and their sensitivity to insulin may reflect significant changes at the cellular and molecular levels involved in their tumor-like growth. Parasite cell cultures offer a powerful tool to characterize the nature and formation of the racemose form, understand the developmental biology of T. solium, and to identify new effective drugs for treatment.

Polo-like Kinase 1 as an emerging drug target: structure, function and therapeutic implications

Polo-like kinase 1 (PLK1) is a conserved mitotic serine-threonine protein kinase, functions as a regulatory protein, and is involved in the progression of the mitotic cycle. It plays important roles in the regulation of cell division, maintenance of genome stability, in spindle assembly, mitosis, and DNA-damage response. PLK1 is consist of a N-terminal serine-threonine kinase domain, and a C-terminal Polo-box domain (regulatory site). The expression of PLK1 is controlled by transcription repressor in the G1 stage and transcription activators in the G2 stage of the cell cycle. Overexpression of PLK1 results in undermining of checkpoints causes excessive cellular division resulting in abnormal cell growth, leading to the development of cancer. Blocking the expression of PLK1 by an antibody, RNA interference, or kinase inhibitors, causes a subsequent reduction in the proliferation of tumour cells and induction of apoptosis in tumour cells without affecting the healthy cells, suggesting an attractive target for drug development. In this review, we discuss detailed information on expression, gene and protein structures, role in different diseases, and progress in the design and development of PLK1 inhibitors. We have performed an in-depth analysis of the PLK1 inhibitors and their therapeutic implications with special focus to the cancer therapeutics.

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.

Protein kinase D1 phosphorylation of KAT7 enhances its protein stability and promotes replication licensing and cell proliferation

The histone acetyltransferase (HAT) KAT7/HBO1/MYST2 plays a crucial role in the pre-replication complex (pre-RC) formation, DNA replication and cell proliferation via acetylation of histone H4 and H3. In a search for protein kinase D1 (PKD1)-interacting proteins, we have identified KAT7 as a potential PKD1 substrate. We show that PKD1 directly interacts and phosphorylates KAT7 at Thr97 and Thr331 in vitro and in vivo. PKD1-mediated phosphorylation of KAT7 enhances its expression levels and stability by reducing its ubiquitination-mediated degradation. Significantly, the phospho-defective mutant KAT7-Thr97/331A attenuates histone H4 acetylation levels, MCM2/6 loading on the chromatin, DNA replication and cell proliferation. Similarly, PKD1 knockdown decreases, whereas the constitutive active mutant PKD1-CA increases histone H4 acetylation levels and MCM2/6 loading on the chromatin. Overall, these results suggest that PKD1-mediated phosphorylation of KAT7 may be required for pre-RC formation and DNA replication.

Polo-like kinase 1 (Plk1) is a positive regulator of DNA replication in the Xenopus in vitro system

Polo-like kinase 1 (Plk1) is a cell cycle kinase essential for mitosis progression, but also important for checkpoint recovery and adaptation in response to DNA damage and replication stress. However, although Plk1 is expressed in S phase, little is known about its function during unperturbed DNA replication. Using Xenopus laevis egg extracts, mimicking early embryonic replication, we demonstrate that Plk1 is simultaneously recruited to chromatin with pre-replication proteins where it accumulates throughout S phase. Further, we found that chromatin-bound Plk1 is phosphorylated on its activating site T201, which appears to be sensitive to dephosphorylation by protein phosphatase 2A. Extracts immunodepleted of Plk1 showed a decrease in DNA replication, rescued by wild type recombinant Plk1. Inversely, modest Plk1 overexpression accelerated DNA replication. Plk1 depletion led to an increase in Chk1 phosphorylation and to a decrease in Cdk2 activity, which strongly suggests that Plk1 could inhibit the ATR/Chk1-dependent intra-S phase checkpoint during normal S phase. In addition, we observed that phosphorylated Plk1 levels are high during the rapid, early cell cycles of Xenopus development but decrease after the mid-blastula transition when the cell cycle and the replication program slow down along with more active checkpoints. These data shed new light on the role of Plk1 as a positive regulating factor for DNA replication in early, rapidly dividing embryos.

Deciphering structure, function and mechanism of lysine acetyltransferase HBO1 in protein acetylation, transcription regulation, DNA replication and its oncogenic properties in cancer

HBO1 complexes are major acetyltransferase responsible for histone H4 acetylation in vivo, which belongs to the MYST family. As the core catalytic subunit, HBO1 consists of an N-terminal domain and a C-terminal MYST domain that are in charge of acetyl-CoA binding and acetylation reaction. HBO1 complexes are multimeric and normally consist of two native subunits MEAF6, ING4 or ING5 and two kinds of cofactors as chromatin reader: Jade-1/2/3 and BRPF1/2/3. The choices of subunits to form the HBO1 complexes provide a regulatory switch to potentiate its activity between histone H4 and H3 tails. Thus, HBO1 complexes present multiple functions in histone acetylation, gene transcription, DNA replication, protein ubiquitination, and immune regulation, etc. HBO1 is a co-activator for CDT1 to facilitate chromatin loading of MCM complexes and promotes DNA replication licensing. This process is regulated by mitotic kinases such as CDK1 and PLK1 by phosphorylating HBO1 and modulating its acetyltransferase activity, therefore, connecting histone acetylation to regulations of cell cycle and DNA replication. In addition, both gene amplification and protein overexpression of HBO1 confirmed its oncogenic role in cancers. In this paper, we review the recent advances and discuss our understanding of the multiple functions, activity regulation, and disease relationship of HBO1.

HBO1 (KAT7) Does Not Have an Essential Role in Cell Proliferation, DNA Replication, or Histone 4 Acetylation in Human Cells

HBO1 (MYST2/KAT7) is essential for histone 3 lysine 14 acetylation (H3K14ac) but is dispensable for H4 acetylation and DNA replication in mouse tissues. In contrast, previous studies using small interfering RNA (siRNA) knockdown in human cell lines have suggested that HBO1 is essential for DNA replication. To determine if HBO1 has distinctly different roles in immortalized human cell lines and normal mouse cells, we performed siRNA knockdown of HBO1. In addition, we used CRISPR/Cas9 to generate 293T, MCF7, and HeLa cell lines lacking HBO1. Using both techniques, we show that HBO1 is essential for all H3K14ac in human cells and is unlikely to have a direct effect on H4 acetylation and only has minor effects on cell proliferation. Surprisingly, the loss of HBO1 and H3K14ac in HeLa cells led to the secondary loss of almost all H4 acetylation after 4 weeks. Thus, HBO1 is dispensable for DNA replication and cell proliferation in immortalized human cells. However, while cell proliferation proceeded without HBO1 and H3K14ac, HBO1 gene deletion led to profound changes in cell adhesion, particularly in 293T cells. Consistent with this phenotype, the loss of HBO1 in both 293T and HeLa principally affected genes mediating cell adhesion, with comparatively minor effects on other cellular processes.

The Plk1 kinase negatively regulates the Hedgehog signaling pathway by phosphorylating Gli1

Hedgehog (Hh) signaling is a highly conserved cell signaling pathway important for cell life, development and tumorigenesis. Increasing evidence suggests that the Hh signaling pathway functions in certain phases of the cell cycle. However, the coordination between Hh signaling and cell cycle control remains poorly understood. Here, we show that polo-like kinase-1 (Plk1), a critical protein kinase regulating many processes during the cell cycle, also regulates Hh signaling by phosphorylating and inhibiting Gli1, a downstream transcription factor of the Hh signaling pathway. Gli1 expression increases along with Hh signaling activation, leading to upregulation of Hh target genes, including cyclin E, during the G1 and S phases. Gli1 is phosphorylated at S481 by Plk1, and this phosphorylation facilitates the nuclear export and binding of Gli1 with its negative regulator Sufu, leading to a reduction in Hh signaling activity. Inhibition of Plk1 kinase activity led to Gli1 maintaining is role in promoting downstream gene expression. Collectively, our data reveal a novel mechanism regarding the crosstalk between Hh signaling and cell cycle control.

PLK1 has tumor-suppressive potential in APC-truncated colon cancer cells

The spindle assembly checkpoint (SAC) acts as a molecular safeguard in ensuring faithful chromosome transmission during mitosis, which is regulated by a complex interplay between phosphatases and kinases including PLK1. Adenomatous polyposis coli (APC) germline mutations cause aneuploidy and are responsible for familial adenomatous polyposis (FAP). Here we study the role of PLK1 in colon cancer cells with chromosomal instability promoted by APC truncation (APC-ΔC). The expression of APC-ΔC in colon cells reduces the accumulation of mitotic cells upon PLK1 inhibition, accelerates mitotic exit and increases the survival of cells with enhanced chromosomal abnormalities. The inhibition of PLK1 in mitotic, APC-∆C-expressing cells reduces the kinetochore levels of Aurora B and hampers the recruitment of SAC component suggesting a compromised mitotic checkpoint. Furthermore, Plk1 inhibition (RNAi, pharmacological compounds) promotes the development of adenomatous polyps in two independent Apc Min/+ mouse models. High PLK1 expression increases the survival of colon cancer patients expressing a truncated APC significantly.

Polo-like kinase 1 (PLK1)-dependent phosphorylation of methylenetetrahydrofolate reductase (MTHFR) regulates replication via histone methylation

Methylenetetrahydrofolate reductase (MTHFR) is a key enzyme regulating the folate cycle and its genetic variations have been associated with various human diseases. Previously we identified that MTHFR is phosphorylated by cyclin-dependent kinase 1 (CDK1) at T34 and MTHFR underlies heterochromatin maintenance marked by H3K9me3 levels. Herein we demonstrate that pT34 creates a binding motif that docks MTHFR to the polo-binding domain (PBD) of polo-like kinase 1 (PLK1), a fundamental kinase that orchestrates many cell cycle events. We show that PLK1 phosphorylates MTHFR at T549 in vitro and in vivo. Further, we uncovered a role of MTHFR in replication. First, MTHFR depletion increased the fraction of cells in S phase. This defect could not be rescued by siRNA resistant plasmids harboring T549A, but could be restored by overproduction of Suv4-20H2, the H4K20 methyltransferase. Moreover, siMTHFR attenuated H4K20me3 levels, which could be rescued by Suv4-20H2 overproduction. More importantly, we also investigated MTHFR-E429A, the protein product of an MTHFR single nucleotide variant. MTHFR-E429A overexpression also increased S phase cells and decreased H4K20me3 levels, and it is linked to a poor glioma prognosis in the Chinese population. Collectively, we have unveiled a vital role of PLK1-dependent phosphorylation of MTHFR in replication via histone methylation, and implicate folate metabolism with glioma.

Lmo2 (LIM-Domain-Only 2) Modulates Sphk1 (Sphingosine Kinase) and Promotes Endothelial Cell Migration

OBJECTIVE

Lmo (LIM-domain-only)2 transcription factor is involved in hematopoiesis and vascular remodeling. Sphk (sphingosine kinase)1 phosphorylates sphingosine to S1P (sphingosine-1-phosphate). We hypothesized that Lmo2 regulates Sphk1 to promote endothelial cell (EC) migration and vascular development. APPROACH AND RESULTS: Lmo2 and Sphk1 knockdown (KD) were performed in Tg(fli1:EGFP) ^y1 zebrafish and in human umbilical vein EC. Rescue of phenotypes or overexpression of these factors were achieved using mRNA encoding Lmo2 or Sphk1. EC proliferation in vivo was assessed by BrdU (bromodeoxyuridine) immunostaining and fluorescence-activated cell sorter analysis of dissociated Tg(fli1:EGFP) ^y1 embryos. Cell migration was assessed by scratch assay in human umbilical vein EC and mouse aortic rings. Lmo2 interactions with Sphk1 promoter were assessed by ChIP-PCR (chromatin immunoprecipitation-polymerase chain reaction). Lmo2 or Sphk1 KD reduced number and length of intersegmental vessels. There was no reduction in the numbers of GFP⁺ (green fluorescent protein) ECs after Lmo2 KD. However, reduced numbers of BrdU⁺GFP⁺ nuclei were observed along the dysmorphic intersegmental vessels, accumulating instead at the sprouting origin of the intersegmental vessels. This anomaly was likely because of impaired EC migration, which was confirmed in migration assays using Lmo2 KD human umbilical vein ECs and mouse aortic rings. Both in vivo and in vitro, Lmo2 KD reduced Sphk1 gene expression, associated with less Lmo2 binding to the Sphk1 promoter as assessed by ChIP-PCR. Sphk1 mRNA rescued the Lmo2 KD phenotype.

CONCLUSIONS

Our data showed that Lmo2 is necessary for Sphk1 gene expression in ECs. Lmo2 KD reduced Lmo2-Sphk1 gene interaction, impaired intersegmental vessels formation, and reduced cell migration. We identified for the first time Sphk1 as downstream effector of Lmo2.

Plk1 Phosphorylation of Mre11 Antagonizes the DNA Damage Response

The mitotic kinase Plk1 contributes to the DNA damage response (DDR) by targeting multiple factors downstream of the core responder kinase ATM/ATR. In this study, we show that Polo-like kinase 1 (Plk1) also phosphorylates key factors upstream of ATM/ATR and regulates their DDR-related functions. Plk1 phosphorylated Mre11, a component of the Mre11/Rad50/Nbs1 (MRN) complex, at serine 649 (S649) during DDR. Phosphorylation of Mre11-S649 by Plk1 primed subsequent CK2-mediated phosphorylation at Mre11-serine 688 (S688). Phosphorylation of Mre11 at S649/S688 inhibited loading of the MRN complex to damaged DNA, leading to both premature DNA damage checkpoint termination and inhibition of DNA repair. Tumors expressing phosphomimetic Mre11 were more sensitive to the PARP inhibitor olaparib, compared with those expressing unphosphorylatable Mre11, suggesting that patients with elevated Plk1 expression might benefit from olaparib treatment. Cancer Res; 77(12); 3169-80. ©2017 AACR.

Regulatory functional territory of PLK-1 and their substrates beyond mitosis

Polo-like kinase 1 (PLK-1) is a well-known (Ser/Thr) mitotic protein kinase and is considered as a proto-oncogene. As hyper-activation of PLK-1 is broadly associated with poor prognosis and cancer progression, it is one of the most extensively studied mitotic kinases. During mitosis, PLK-1 regulates various cell cycle events, such as spindle pole maturation, chromosome segregation and cytokinesis. However, studies have demonstrated that the role of PLK-1 is not only restricted to mitosis, but PLK-1 can also regulate other vital events beyond mitosis, including transcription, translation, ciliogenesis, checkpoint adaptation and recovery, apoptosis, chromosomes dynamics etc. Recent reviews have tried to define the regulatory role of PLK-1 during mitosis progression and tumorigenesis, but its' functional role beyond mitosis is still largely unexplored. PLK-1 can regulate the activity of many proteins that work outside of its conventional territory. The dysregulation of these proteins can cause diseases such as Alzheimer's disease, tumorigenesis etc. and may also lead to drug resistance. Thus, in this review, we discussed the versatile role of PLK-1 and tried to collect data to validate its' functional role in cell cycle regulation apart from mitosis.

The GSK461364 PLK1 inhibitor exhibits strong antitumoral activity in preclinical neuroblastoma models

Polo-like kinase 1 (PLK1) is a serine/threonine kinase that promotes G2/M-phase transition, is expressed in elevated levels in high-risk neuroblastomas and correlates with unfavorable patient outcome. Recently, we and others have presented PLK1 as a potential drug target for neuroblastoma, and reported that the BI2536 PLK1 inhibitor showed antitumoral actvity in preclinical neuroblastoma models. Here we analyzed the effects of GSK461364, a competitive inhibitor for ATP binding to PLK1, on typical tumorigenic properties of preclinical in vitro and in vivo neuroblastoma models. GSK461364 treatment of neuroblastoma cell lines reduced cell viability and proliferative capacity, caused cell cycle arrest and massively induced apoptosis. These phenotypic consequences were induced by treatment in the low-dose nanomolar range, and were independent of MYCN copy number status. GSK461364 treatment strongly delayed established xenograft tumor growth in nude mice, and significantly increased survival time in the treatment group. These preclinical findings indicate PLK1 inhibitors may be effective for patients with high-risk or relapsed neuroblastomas with upregulated PLK1 and might be considered for entry into early phase clinical trials in pediatric patients.

Plk1 Inhibitors in Cancer Therapy: From Laboratory to Clinics

Polo-like kinase 1 (Plk1) overexpression has been shown to occur in a wide range of tumors, prompting research and development of Plk1 inhibitors as a means of cancer treatment. This review discusses recent advances in the development of Plk1 inhibitors for cancer management. Plk1 inhibition has been shown to cause mitotic block and apoptosis of cells with higher mitotic index and therefore higher Plk1 expression. The potential of Plk1 inhibitors as cancer therapeutics has been widely investigated. However, a complete understanding of Plk1 biology/mechanism is yet to be fully achieved. Resistance to certain chemotherapeutic drugs has been linked to Plk1 overexpression, and Plk1-mediated mitotic events such as microtubule rearrangement have been found to reduce the efficacy of chemotherapeutic agents. The Plk1 inhibitor volasertib has shown considerable promise in clinical studies, having reached phase III trials. However, preclinical success with Plk1 inhibitors has not translated well into clinical success. In our view, combined therapies targeting other relevant pathways together with Plk1 may be vital to combat issues observed with monotherapy, especially resistance. In addition, research should also be directed toward understanding the mechanisms of Plk1 and designing additional next generations of specific, potent Plk1 inhibitors to target cancer. Mol Cancer Ther; 15(7); 1427-35. ©2016 AACR.

PLK-1: Angel or devil for cell cycle progression

PLK-1 is a key player in the eukaryotic cell cycle. Cell cycle progression is precisely controlled by cell cycle regulatory kinases. PLK-1 is a mitotic kinase that actively regulates the G2/M transition, mitosis, mitotic exit, and cytokinesis. During cell cycle progression, PLK-1 controls various events related to the cell cycle maturation, directly and/or indirectly. On the contrary, aberrant expression of PLK-1 is strongly associated with tumorigenesis and its poor prognosis. The misexpression of PLK-1 causes the abnormalities including aneuploidy, mitotic defects, leading to tumorigenesis through inhibiting the p53 and pRB genes. Therefore, we reviewed the role of PLK-1 in the cell cycle progression and in the tumorigenesis either as a cell cycle regulator or on an attractive anti-cancer drug target.

FOXM1 participates in PLK1-regulated cell cycle progression in renal cell cancer cells

The regulation of entry into and progression through mitosis is important for cell proliferation. Polo-like kinase 1 (PLK1) is involved in multiple stages of mitosis. Forkhead box protein M1 (FOXM1) has multiple functions in tumorigenesis and, in elevated levels, is frequently associated with cancer progression. The present study reports that FOXM1, a substrate of PLK1, controls the transcription mechanism that mediates the PLK1-dependent regulation of the cell cycle. The present study investigated the expression of PLK1 and FOXM1 in the clear renal cell carcinoma 769-P and ACHN cell lines, and indicated that the expression of PLK1 and FOXM1 are correlated in human renal cell cancer cell lines and that the suppression of PLK1 may decrease the expression of FOXM1. The knockdown of FOXM1 or PLK1 in renal cell cancer cell lines caused cell cycle progression to be blocked. As a result, the present study indicated the involvement of FOXM1 in PLK1-regulated cell cycle progression.

Mumps Virus Nucleoprotein Enhances Phosphorylation of the Phosphoprotein by Polo-Like Kinase 1

The viral RNA-dependent RNA polymerases (vRdRps) of nonsegmented, negative-sense viruses (NNSVs) consist of the enzymatic large protein (L) and the phosphoprotein (P). P is heavily phosphorylated, and its phosphorylation plays a critical role in viral RNA synthesis. Since NNSVs do not encode kinases, P is phosphorylated by host kinases. In this study, we investigate the roles that viral proteins play in the phosphorylation of mumps virus (MuV) P. We found that nucleoprotein (NP) enhances the phosphorylation of P. We have identified the serine/threonine kinase Polo-like kinase 1 (PLK1) as a host kinase that phosphorylates P and have found that phosphorylation of P by PLK1 is enhanced by NP. The PLK1 binding site in MuV P was mapped to residues 146 to 148 within the S(pS/T)P motif, and the phosphorylation site was identified as residues S292 and S294.

IMPORTANCE

It has previously been shown that P acts as a chaperone for NP, which encapsidates viral genomic RNA to form the NP-RNA complex, the functional template for viral RNA synthesis. Thus, it is assumed that phosphorylation of P may regulate NP's ability to form the NP-RNA complex, thereby regulating viral RNA synthesis. Our work demonstrates that MuV NP affects phosphorylation of P, suggesting that NP can regulate viral RNA synthesis by regulating phosphorylation of P.

The LMO2 oncogene regulates DNA replication in hematopoietic cells

Oncogenic transcription factors are commonly activated in acute leukemias and subvert normal gene expression networks to reprogram hematopoietic progenitors into preleukemic stem cells, as exemplified by LIM-only 2 (LMO2) in T-cell acute lymphoblastic leukemia (T-ALL). Whether or not these oncoproteins interfere with other DNA-dependent processes is largely unexplored. Here, we show that LMO2 is recruited to DNA replication origins by interaction with three essential replication enzymes: DNA polymerase delta (POLD1), DNA primase (PRIM1), and minichromosome 6 (MCM6). Furthermore, tethering LMO2 to synthetic DNA sequences is sufficient to transform these sequences into origins of replication. We next addressed the importance of LMO2 in erythroid and thymocyte development, two lineages in which cell cycle and differentiation are tightly coordinated. Lowering LMO2 levels in erythroid progenitors delays G1-S progression and arrests erythropoietin-dependent cell growth while favoring terminal differentiation. Conversely, ectopic expression in thymocytes induces DNA replication and drives these cells into cell cycle, causing differentiation blockade. Our results define a novel role for LMO2 in directly promoting DNA synthesis and G1-S progression.

Jade-1S phosphorylation induced by CK1α contributes to cell cycle progression

The PHD zinc finger protein Jade-1S is a component of the HBO1 histone acetyltransferase complex and binds chromatin in a cell cycle-dependent manner. Jade-1S also acts as an E3 ubiquitin ligase for the canonical Wnt effector protein β-catenin and is influenced by CK1α-mediated phosphorylation. To further elucidate the functional impact of this phosphorylation, we used a stable, low-level expression system to express either wild-type or mutant Jade-1S lacking the N-terminal CK1α phosphorylation motif. Interactome analyses revealed that the Jade-1S mutant unable to be phosphorylated by CK1α has an increased binding affinity to proteins involved in chromatin remodelling, histone deacetylation, transcriptional repression, and ribosome biogenesis. Interestingly, cells expressing the mutant displayed an elongated cell shape and a delay in cell cycle progression. Finally, phosphoproteomic analyses allowed identification of a Jade-1S site phosphorylated in the presence of CK1α but closely resembling a PLK1 phosphorylation motif. Our data suggest that Jade-1S phosphorylation at an N-terminal CK1α motif creates a PLK1 phospho-binding domain. We propose CK1α phosphorylation of Jade 1S to serve as a molecular switch, turning off chromatin remodelling functions of Jade-1S and allowing timely cell cycle progression. As Jade-1S protein expression in the kidney is altered upon renal injury, this could contribute to understanding mechanisms underlying epithelial injury repair.

Plk1 inhibition enhances the efficacy of gemcitabine in human pancreatic cancer

Gemcitabine is the standard-of-care for chemotherapy in patients with pancreatic adenocarcinoma and it can directly incorporate into DNA or inhibit ribonucleotide reductase to prevent DNA replication and, thus, tumor cell growth. Most pancreatic tumors, however, develop resistance to gemcitabine. Polo-like kinase 1 (Plk1), a critical regulator in many cell cycle events, is significantly elevated in human pancreatic cancer. In this study, we show that Plk1 is required for the G1/S transition and that inhibition of Plk1 significantly reduces the DNA synthesis rate in human pancreatic cancer cells. Furthermore, the combined effect of a specific Plk1 inhibitor GSK461364A with gemcitabine was examined. We show that inhibition of Plk1 significantly potentiates the anti-neoplastic activity of gemcitabine in both cultured pancreatic cancer cells and Panc1-derived orthotopic pancreatic cancer xenograft tumors. Overall, our study demonstrates that co-targeting Plk1 can significantly enhance the efficacy of gemcitabine, offering a promising new therapeutic option for the treatment of gemcitabine-resistant human pancreatic cancer.

Understanding the Polo Kinase machine

The Polo Kinase is a central regulator of cell division required for several events of mitosis and cytokinesis. In addition to a kinase domain (KD), Polo-like kinases (Plks) comprise a Polo-Box domain (PBD), which mediates protein interactions with targets and regulators of Plks. In all organisms that contain Plks, one Plk family member fulfills several essential functions in the regulation of cell division, and here we refer to this conserved protein as Polo Kinase (Plk1 in humans). The PBD and the KD are capable of both cooperation and mutual inhibition in their functions. Crystal structures of the PBD, the KD and, recently, a PBD-KD complex have helped understanding the inner workings of the Polo Kinase. In parallel, an impressive array of molecular mechanisms has been found to mediate the regulation of the protein. Moreover, the targeting of Polo Kinase in the development of anti-cancer drugs has yielded several molecules with which to chemically modulate Polo Kinase to study its biological functions. Here we review our current understanding of the protein function and regulation of Polo Kinase as a fascinating molecular device in control of cell division.

Preclinical Childhood Sarcoma Models: Drug Efficacy Biomarker Identification and Validation

Over the past 35 years, cure rates for pediatric cancers have increased dramatically. However, it is clear that further dose intensification using cytotoxic agents or radiation therapy is not possible without enhancing morbidity and long-term effects. Consequently, novel, less genotoxic, agents are being sought to complement existing treatments. Here, we discuss preclinical human tumor xenograft models of pediatric cancers that may be used practically to identify novel agents for soft tissue and bone sarcomas, and "omics" approaches to identifying biomarkers that may identify sensitive and resistant tumors to these agents.

DNA sequence templates adjacent nucleosome and ORC sites at gene amplification origins in Drosophila

Eukaryotic origins of DNA replication are bound by the origin recognition complex (ORC), which scaffolds assembly of a pre-replicative complex (pre-RC) that is then activated to initiate replication. Both pre-RC assembly and activation are strongly influenced by developmental changes to the epigenome, but molecular mechanisms remain incompletely defined. We have been examining the activation of origins responsible for developmental gene amplification in Drosophila. At a specific time in oogenesis, somatic follicle cells transition from genomic replication to a locus-specific replication from six amplicon origins. Previous evidence indicated that these amplicon origins are activated by nucleosome acetylation, but how this affects origin chromatin is unknown. Here, we examine nucleosome position in follicle cells using micrococcal nuclease digestion with Ilumina sequencing. The results indicate that ORC binding sites and other essential origin sequences are nucleosome-depleted regions (NDRs). Nucleosome position at the amplicons was highly similar among developmental stages during which ORC is or is not bound, indicating that being an NDR is not sufficient to specify ORC binding. Importantly, the data suggest that nucleosomes and ORC have opposite preferences for DNA sequence and structure. We propose that nucleosome hyperacetylation promotes pre-RC assembly onto adjacent DNA sequences that are disfavored by nucleosomes but favored by ORC.

Polo-like Kinase 1 (Plk1) Up-regulates Telomerase Activity by Affecting Human Telomerase Reverse Transcriptase (hTERT) Stability

Maintenance of telomere is regulated by active telomerase complex, including telomerase holoenzyme and its associated proteins. The activity of telomerase is precisely controlled in cells, and its dysregulation is one of the hallmarks of cancer. The telomerase catalytic subunit human telomerase reverse transcriptase (hTERT) plays a central role for telomerase activity. In this study, we indentified that Polo-like kinase 1 (Plk1) is a novel telomerase-associated protein. Plk1 can interact with hTERT independently of its kinase activity. More importantly, we found that Plk1 is associated with active telomerase complex. In addition, we demonstrated that knockdown of Plk1 caused the reduction of telomerase activity, whereas overexpression of Plk1 increased telomerase activity. Further analysis showed that overexpression of Plk1 led to a significant increase of hTERT protein by prolonging its half-life but did not affect the level of hTERT mRNA. Furthermore, we found that Plk1 enhanced the chromatin loading of hTERT and inhibited its ubiquitination. This implied that Plk1 affected hTERT stability by inhibiting its ubiquitin-mediated degradation. Collectively, these observations suggested that Plk1 is a positive modulator of telomerase by enhancing the stability of hTERT.

Low-dose arsenic-mediated metabolic shift is associated with activation of Polo-like kinase 1 (Plk1)

Arsenic is a well-established human carcinogen associated with cancers of the skin, liver, lung, kidney, and bladder. Although numerous carcinogenic pathways have been proposed, the molecular mechanisms underlying arsenic-associated cancer etiology are still elusive. The cellular responses to arsenic exposure are dose dependent. It was recently shown that low-dose arsenic leads to a metabolic shift from mitochondrial respiration to aerobic glycolysis via inactivation of tumor suppressor p53 and activation of NF-κB. However, how inactivation of p53, activation of NF-κB, and metabolic change are coordinated in response to low-dose arsenic exposure is still not completely understood. Polo-like kinase 1 (Plk1) is a well- documented regulator in many cell cycle-related events. Herein, we showed that low-dose arsenic leads to elevation of Plk1 in an NF-κB-dependent manner and that elevation of Plk1 contributes to the metabolic change from oxidative phosphorylation to glycolysis via activation of the PI3K/AKT/mTOR pathway. Furthermore, we showed that inhibition/depletion of Plk1 reverses low-dose arsenic-associated phenotypes, including enhanced cell proliferation, activation of the PI3K/AKT/mTOR pathway, and increased glycolysis. Finally, inhibition of the PI3K/AKT/mTOR pathway also antagonizes the enhanced glycolytic influx due to low-dose arsenic exposure. Our studies support the notion that Plk1 likely plays a critical role in cellular responses to low-dose arsenic.

Plk1 inhibition enhances the efficacy of androgen signaling blockade in castration-resistant prostate cancer

Prostate cancer is thought to be driven by oxidative stress, lipid metabolism, androgen receptor (AR) signaling, and activation of the PI3K-AKT-mTOR pathway, but it is uncertain how they may become coordinated during progression to castration-resistant disease that remains incurable. The mitotic kinase polo-like kinase 1 (Plk1) is elevated in prostate cancer, where its expression is linked to tumor grade. Notably, Plk1 signaling and lipid metabolism were identified recently as two of the top five most upregulated pathways in a mouse xenograft model of human prostate cancer. Herein, we show that oxidative stress activates both the PI3K-AKT-mTOR pathway and AR signaling in a Plk1-dependent manner in prostate cells. Inhibition of the PI3K-AKT-mTOR pathway prevented oxidative stress-induced activation of AR signaling. Plk1 modulation also affected cholesteryl ester accumulation in prostate cancer via the SREBP pathway. Finally, Plk1 inhibition enhanced cellular responses to androgen signaling inhibitors (ASI) and overcame ASI resistance in both cultured prostate cancer cells and patient-derived tumor xenografts. Given that activation of AR signaling and the PI3K-AKT-mTOR pathway is sufficient to elevate SREBP-dependent expression of key lipid biosynthesis enzymes in castration-resistant prostate cancer (CRPC), our findings argued that Plk1 activation was responsible for coordinating and driving these processes to promote and sustain the development of this advanced stage of disease. Overall, our results offer a strong mechanistic rationale to evaluate Plk1 inhibitors in combination drug trials to enhance the efficacy of ASIs in CRPC.

Plk1 phosphorylation of PTEN causes a tumor-promoting metabolic state

One outcome of activation of the phosphatidylinositol 3-kinase (PI3K) pathway is increased aerobic glycolysis, but the upstream signaling events that regulate the PI3K pathway, and thus the Warburg effect, are elusive. Increasing evidence suggests that Plk1, a cell cycle regulator, is also involved in cellular events in addition to mitosis. To test whether Plk1 contributes to activation of the PI3K pathway, and thus aerobic glycolysis, we examined potential targets of Plk1 and identified PTEN as a Plk1 substrate. We hypothesize that Plk1 phosphorylation of PTEN leads to its inactivation, activation of the PI3K pathway, and the Warburg effect. Our data show that overexpression of Plk1 leads to activation of the PI3K pathway and enhanced aerobic glycolysis. In contrast, inhibition of Plk1 causes markedly reduced glucose metabolism in mice. Mechanistically, we show that Plk1 phosphorylation of PTEN and Nedd4-1, an E3 ubiquitin ligase of PTEN, results in PTEN inactivation. Finally, we show that Plk1 phosphorylation of PTEN promotes tumorigenesis in both its phosphatase-dependent and -independent pathways, revealing potentially new drug targets to arrest tumor cell growth.

Polo-like kinases: structural variations lead to multiple functions

Members of the polo-like kinase (PLK) family are crucial regulators of cell cycle progression, centriole duplication, mitosis, cytokinesis and the DNA damage response. PLKs undergo major changes in abundance, activity, localization and structure at different stages of the cell cycle. They interact with other proteins in a tightly controlled spatiotemporal manner as part of a network that coordinates key cell cycle events. Their essential roles are highlighted by the fact that alterations in PLK function are associated with cancers and other diseases. Recent knowledge gained from PLK crystal structures, evolution and interacting molecules offers important insights into the mechanisms that underlie their regulation and activity, and suggests novel functions unrelated to cell cycle control for this family of kinases.

SCF(FBXW7α) modulates the intra-S-phase DNA-damage checkpoint by regulating Polo like kinase-1 stability

The intra-S-checkpoint is essential to control cell progression through S phase under normal conditions and in response to replication stress. When DNA lesions are detected, replication fork progression is blocked allowing time for repair to avoid genomic instability and the risk of cancer. DNA replication initiates at many origins of replication in eukaryotic cells, where a series of proteins form pre-replicative complexes (pre-RCs) that are activated to become pre-initiation complexes and ensure a single round of replication in each cell cycle. PLK1 plays an important role in the regulation of DNA replication, contributing to the regulation of pre-RCs formation by phosphorylating several proteins, under both normal and stress conditions. Here we report that PLK1 is ubiquitinated and degraded by SCFFBXW7α/proteasome. Moreover, we identified a new Cdc4 phosphodegron in PLK1, conserved from yeast to humans, whose mutation prevents PLK1 destruction. We established that endogenous SCFFBXW7α degrades PLK1 in the G1 and S phases of an unperturbed cell cycle and in S phase following UV irradiation. Furthermore, we showed that FBXW7α overexpression or UV irradiation prevented the loading of proteins onto chromatin to form pre-RCs and, accordingly, reduced cell proliferation. We conclude that PLK1 degradation mediated by SCFFBXW7α modulates the intra-S-phase checkpoint.

Phosphoproteome dynamics in onset and maintenance of oncogene-induced senescence

Expression of the BRAF(V600E) oncoprotein is known to cause benign lesions, such as melanocytic nevi (moles). Despite the oncogenic function of mutant BRAF, these lesions are arrested by a cell-autonomous mechanism called oncogene-induced senescence. Infrequently, nevi can progress to malignant melanoma, through mechanisms that are incompletely understood. To gain more insight into this vital tumor-suppression mechanism, we performed a mass-spectrometry-based screening of the proteome and phosphoproteome in cycling and senescent cells and in cells with abrogated senescence. Proteome analysis of senescent cells revealed the up-regulation of established senescence biomarkers, including specific cytokines, but also several proteins not previously associated with senescence, including extracellular matrix-interacting. Using both general and targeted phosphopeptide enrichment by Ti(4+)-IMAC and phosphotyrosine antibody enrichment, we identified over 15,000 phosphorylation sites. Among the regulated phosphorylation sites we encountered components of the interleukin, BRAF/MAPK, and CDK-retinoblastoma pathways and several other factors. The extensive proteome and phosphoproteome dataset of BRAF(V600E)-expressing senescent cells provides molecular clues as to how oncogene-induced senescence is initiated, maintained, or evaded, serving as a comprehensive proteomic basis for functional validation.

Sequential Cdk1 and Plk1 phosphorylation of caspase-8 triggers apoptotic cell death during mitosis

Caspase-8 is crucial for cell death induction, especially via the death receptor pathway. The dysregulated expression or function of caspase-8 can promote tumor formation, progression and treatment resistance in different human cancers. Here, we show procaspase-8 is regulated during the cell cycle through the concerted inhibitory action of Cdk1/cyclin B1 and polo-like kinase 1 (Plk1). By phosphorylating S387 in procaspase-8 Cdk1/cyclin B1 generates a phospho-epitope for the binding of the PBD of Plk1. Subsequently, S305 in procaspase-8 is phosphorylated by Plk1 during mitosis. Using an RNAi-based strategy we could demonstrate that the extrinsic cell death is increased upon Fas-stimulation when endogenous caspase-8 is replaced by a mutant (S305A) mimicking the non-phosphorylated form. Together, our data show that sequential phosphorylation by Cdk1/cyclin B1 and Plk1 decreases the sensitivity of cells toward stimuli of the extrinsic pathway during mitosis. Thus, the clinical Plk1 inhibitor BI 2536 decreases the threshold of different cancer cell types toward Fas-induced cell death.

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

Polo-like kinase 1 (Plk1) is a well-established mitotic regulator with a diverse range of biologic functions continually being identified throughout the cell cycle. Preclinical evidence suggests that the molecular targeting of Plk1 could be an effective therapeutic strategy in a wide range of cancers; however, that success has yet to be translated to the clinical level. The lack of clinical success has raised the question of whether there is a true oncogenic addiction to Plk1 or if its overexpression in tumors is solely an artifact of increased cellular proliferation. In this review, we address the role of Plk1 in carcinogenesis by discussing the cell cycle and DNA damage response with respect to their associations with classic oncogenic and tumor suppressor pathways that contribute to the transcriptional regulation of Plk1. A thorough examination of the available literature suggests that Plk1 activity can be dysregulated through key transformative pathways, including both p53 and pRb. On the basis of the available literature, it may be somewhat premature to draw a definitive conclusion on the role of Plk1 in carcinogenesis. However, evidence supports the notion that oncogene dependence on Plk1 is not a late occurrence in carcinogenesis and it is likely that Plk1 plays an active role in carcinogenic transformation.

Centrosome dysfunction contributes to chromosome instability, chromoanagenesis, and genome reprograming in cancer

The unique ability of centrosomes to nucleate and organize microtubules makes them unrivaled conductors of important interphase processes, such as intracellular payload traffic, cell polarity, cell locomotion, and organization of the immunologic synapse. But it is in mitosis that centrosomes loom large, for they orchestrate, with clockmaker's precision, the assembly and functioning of the mitotic spindle, ensuring the equal partitioning of the replicated genome into daughter cells. Centrosome dysfunction is inextricably linked to aneuploidy and chromosome instability, both hallmarks of cancer cells. Several aspects of centrosome function in normal and cancer cells have been molecularly characterized during the last two decades, greatly enhancing our mechanistic understanding of this tiny organelle. Whether centrosome defects alone can cause cancer, remains unanswered. Until recently, the aggregate of the evidence had suggested that centrosome dysfunction, by deregulating the fidelity of chromosome segregation, promotes and accelerates the characteristic Darwinian evolution of the cancer genome enabled by increased mutational load and/or decreased DNA repair. Very recent experimental work has shown that missegregated chromosomes resulting from centrosome dysfunction may experience extensive DNA damage, suggesting additional dimensions to the role of centrosomes in cancer. Centrosome dysfunction is particularly prevalent in tumors in which the genome has undergone extensive structural rearrangements and chromosome domain reshuffling. Ongoing gene reshuffling reprograms the genome for continuous growth, survival, and evasion of the immune system. Manipulation of molecular networks controlling centrosome function may soon become a viable target for specific therapeutic intervention in cancer, particularly since normal cells, which lack centrosome alterations, may be spared the toxicity of such therapies.

Hbo1 is a cyclin E/CDK2 substrate that enriches breast cancer stem-like cells

Expression of cyclin E proteolytic cleavage products, low-molecular weight cyclin E (LMW-E), is associated with poor clinical outcome in patients with breast cancer and it enhances tumorigenecity in mouse models. Here we report that LMW-E expression in human mammary epithelial cells induces an epithelial-to-mesenchymal transition phenotype, increases the CD44(hi)/CD24(lo) population, enhances mammosphere formation, and upregulates aldehyde dehydrogenase expression and activity. We also report that breast tumors expressing LMW-E have a higher proportion of CD44(hi)/CD24(lo) tumor cells as compared with tumors expressing only full-length cyclin E. In order to explore how LMW-E enriches cancer stem cells in breast tumors, we conducted a protein microarray analysis that identified the histone acetyltransferase (HAT) Hbo1 as a novel cyclin E/CDK2 substrate. The LMW-E/CDK2 complex phosphorylated Hbo1 at T88 without affecting its HAT activity. When coexpressed with LMW-E/CDK2, wild-type Hbo1 promoted enrichment of cancer stem-like cells (CSC), whereas the T88 Hbo1 mutant reversed the CSC phenotype. Finally, doxorubicin and salinomycin (a CSC-selective cytotoxic agent) synergized to kill cells expressing LMW-E, but not full-length cyclin E. Collectively, our results suggest that the heightened oncogenecity of LMW-E relates to its ability to promote CSC properties, supporting the design of therapeutic strategies to target this unique function.

Show me your license, please: deregulation of centriole duplication mechanisms that promote amplification

Centrosomes are organelles involved in generating and organizing the interphase microtubule cytoskeleton, mitotic spindles and cilia. At the centrosome core are a pair of centrioles, structures that act as the duplicating elements of this organelle. Centrioles function to recruit and organize pericentriolar material which nucleates microtubules. While centrioles are relatively simple in construction, the mechanics of centriole biogenesis remain an important yet poorly understood process. More mysterious still are the regulatory mechanisms that oversee centriole assembly. The fidelity of centriole duplication is critical as defects in either the assembly or number of centrioles promote aneuploidy, primary microcephaly, birth defects, ciliopathies and tumorigenesis. In addition, some pathogens employ mechanisms to promote centriole overduplication to the detriment of the host cell. This review summarizes our current understanding of this important topic, highlighting the need for further study if new therapeutics are to be developed to treat diseases arising from defects of centrosome duplication.

SCF(Fbxw15) mediates histone acetyltransferase binding to origin recognition complex (HBO1) ubiquitin-proteasomal degradation to regulate cell proliferation

Histone acetyltransferase binding to origin recognition complex (HBO1) plays a crucial role in DNA replication licensing and cell proliferation, yet its molecular regulation in cells is relatively unknown. Here an uncharacterized protein, Fbxw15, directly interacts with HBO1, a labile protein (t½ = ∼3 h), to mediate its ubiquitination (Lys(338)) and degradation in the cytoplasm. Fbxw15-mediated HBO1 depletion required mitogen-activated protein kinase 1 (Mek1), which was sufficient to trigger HBO1 phosphorylation and degradation in cells. Mek1 ability to produce HBO1 degradation was blocked by Fbxw15 silencing. Lipopolysaccharide induced HBO1 degradation, an effect abrogated by Fbxw15 or Mek1 cellular depletion. Modulation of Fbxw15 levels was able to differentially regulate histone H3K14 acetylation and cellular proliferation by altering HBO1 levels. These studies authenticate Fbxw15 as a ubiquitin E3 ligase subunit that mediates endotoxin-induced HBO1 depletion in cells, thereby controlling cell replicative capacity.