Phosphorylation of human Sgo1 by NEK2A is essential for chromosome congression in mitosis

Function of NEK2 in clear cell renal cell carcinoma and its effect on the tumor microenvironment

BACKGROUND

Previous studies have revealed the critical functions of NEK2 in controlling the cell cycle which is linked to poor prognosis in multiple tumor types, but less research has been devoted to clear cell renal cell carcinoma (ccRCC).

METHODS

We downloaded clinical data from the gene expression omnibus (GEO) and TCGA databases together with transcriptional and mutational datasets. Strongly coexpressed genes with NEK2 were extracted from TCGA-KIRC cohort, and were submitted to Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) for functional analyses. According to NEK2 levels, the survival status, mutational characteristics, response to immunotherapy and sensitivity to drugs of the patients were studied. The potential correlations between NEK2 levels and immune cell state as well as immune cell infiltration were examined using the GEPIA, TIMER and TISIDB databases. Double immunofluorescence (IF) was performed to identify the NEK2 overexpression and relationship with CD8 in ccRCC.

RESULTS

The NEK2 gene was overexpressed and would enhance the nuclear division and cell cycle activities in ccRCC. ccRCC patients with high NEK2 expression had worse clinical outcomes, higher mutation burden and better therapeutic response. Moreover, NEK2 gene overexpression was positively related to various immune cell marker sets, which was also proved by validation cohort, and more infiltration of various immune cells.

CONCLUSION

ccRCC patients with NEK2 high expression have a poorer prognosis than those with NEK2 low expression, resulting from its function of promoting proliferation, accompanied by increased infiltration of CD8 + T cells and Tregs and T-cell exhaustion and will respond better to proper treatments.

Sgo1 interacts with CENP-A to guide accurate chromosome segregation in mitosis

Shugoshin-1 (Sgo1) is necessary for maintaining sister centromere cohesion and ensuring accurate chromosome segregation during mitosis. It has been reported that the localization of Sgo1 at the centromere is dependent on Bub1-mediated phosphorylation of histone H2A at T120. However, it remains uncertain whether other centromeric proteins play a role in regulating the localization and function of Sgo1 during mitosis. Here, we show that CENP-A interacts with Sgo1 and determines the localization of Sgo1 to the centromere during mitosis. Further biochemical characterization revealed that lysine and arginine residues in the C-terminal domain of Sgo1 are critical for binding CENP-A. Interestingly, the replacement of these basic amino acids with acidic amino acids perturbed the localization of Sgo1 and Aurora B to the centromere, resulting in aberrant chromosome segregation and premature chromatid separation. Taken together, these findings reveal a previously unrecognized but direct link between Sgo1 and CENP-A in centromere plasticity control and illustrate how the Sgo1-CENP-A interaction guides accurate cell division.

Sustained Shugoshin 1 downregulation reduces tumor growth and metastasis in a mouse xenograft tumor model of triple-negative breast cancer

BACKGROUND

Triple-negative breast cancer (TBNC) is an aggressive breast cancer subtype with a poor prognosis. Shugoshin-1 (SGO1) protects chromatids from early separation. Previous studies from our group have demonstrated that transient SGO1 downregulation suppresses early stages of metastasis (the epithelial-to-mesenchymal transition, or EMT, cell invasion, and cell migration) in TNBC cells. Thus, the inhibition of SGO1 activity may represent a potential therapeutic intervention against cancers that progress to metastasis. Therefore, we aimed to investigate the effects of sustained shRNA-mediated SGO1 downregulation on tumor growth and metastasis in TBNC. To that end, female NOD-SCID Gamma (NSG) mice were injected with 2.5 × 10⁶ shRNA Control (n = 10) or shRNA SGO1 (n = 10) MDA-MB-231 cells. After eight weeks, the number of mice with metastasis to the lymph nodes was calculated. Primary and metastatic tumors, as well as lung and liver tissue, were harvested, measured, sectioned, and stained with hematoxylin and eosin (H&E) stain.

RESULTS

Tumor growth and metastasis to the lymph nodes and lungs were significantly reduced in the shRNA SGO1-treated mice group, while metastasis to the liver tends to be lower in cells with downregulated SGO1, but it did not reach statistical significance. Furthermore, sustained SGO1 downregulation significantly reduced cell proliferation, cell migration, and invasion which correlated with lower levels of Snail, Slug, MMP2, MMP3, and MMP9.

CONCLUSION

The supression of SGO1 activity in TNBC harboring dysregulated expression of SGO1 may be a potential target for preventing breast cancer growth and metastasis.

One shoot, three birds: Targeting NEK2 orchestrates chemoradiotherapy, targeted therapy, and immunotherapy in cancer treatment

Combinational therapy has improved the cancer therapeutic landscape but is associated with a concomitant increase in adverse side reactions. Emerging evidence proposes that targeting one core target with multiple critical roles in tumors can achieve combined anti-tumor effects. This review focuses on NEK2, a member of serine/threonine kinases, with broad sequence identity to the mitotic regulator NIMA of the filamentous fungus Aspergillus nidulans. Elevated expression of NEK2 was initially found to promote tumorigeneses through abnormal regulation of the cell cycle. Subsequent studies report that NEK2 is overexpressed in a broad spectrum of tumor types and is associated with tumor progression and therapeutic resistance. Intriguingly, NEK2 has recently been revealed to mediate tumor immune escape by stabilizing the expression of PD-L1. Targeting NEK2 is thus becoming a promising approach for cancer treatment by orchestrating chemoradiotherapy, targeted therapy, and immunotherapy. It represents a novel strategy for inducing combined anti-cancer effects using a mono-agent.

Mitotic kinases as drivers of the epithelial-to-mesenchymal transition and as therapeutic targets against breast cancers

Biological therapies against breast cancer patients with tumors positive for the estrogen and progesterone hormone receptors and Her2 amplification have greatly improved their survival. However, to date, there are no effective biological therapies against breast cancers that lack these three receptors or triple-negative breast cancers (TNBC). TNBC correlates with poor survival, in part because they relapse following chemo- and radio-therapies. TNBC is intrinsically aggressive since they have high mitotic indexes and tend to metastasize to the central nervous system. TNBCs are more likely to display centrosome amplification, an abnormal phenotype that results in defective mitotic spindles and abnormal cytokinesis, which culminate in aneuploidy and chromosome instability (known causes of tumor initiation and chemo-resistance). Besides their known role in cell cycle control, mitotic kinases have been also studied in different types of cancer including breast, especially in the context of epithelial-to-mesenchymal transition (EMT). EMT is a cellular process characterized by the loss of cell polarity, reorganization of the cytoskeleton, and signaling reprogramming (upregulation of mesenchymal genes and downregulation of epithelial genes). Previously, we and others have shown the effects of mitotic kinases like Nek2 and Mps1 (TTK) on EMT. In this review, we focus on Aurora A, Aurora B, Bub1, and highly expressed in cancer (Hec1) as novel targets for therapeutic interventions in breast cancer and their effects on EMT. We highlight the established relationships and interactions of these and other mitotic kinases, clinical trial studies involving mitotic kinases, and the importance that represents to develop drugs against these proteins as potential targets in the primary care therapy for TNBC.

The Greatwall kinase safeguards the genome integrity by affecting the kinome activity in mitosis

Progression through mitosis is balanced by the timely regulation of phosphorylation and dephosphorylation events ensuring the correct segregation of chromosomes before cytokinesis. This balance is regulated by the opposing actions of CDK1 and PP2A, as well as the Greatwall kinase/MASTL. MASTL is commonly overexpressed in cancer, which makes it a potential therapeutic anticancer target. Loss of Mastl induces multiple chromosomal errors that lead to the accumulation of micronuclei and multilobulated cells in mitosis. Our analyses revealed that loss of Mastl leads to chromosome breaks and abnormalities impairing correct segregation. Phospho-proteomic data for Mastl knockout cells revealed alterations in proteins implicated in multiple processes during mitosis including double-strand DNA damage repair. In silico prediction of the kinases with affected activity unveiled NEK2 to be regulated in the absence of Mastl. We uncovered that, RAD51AP1, involved in regulation of homologous recombination, is phosphorylated by NEK2 and CDK1 but also efficiently dephosphorylated by PP2A/B55. Our results suggest that MastlKO disturbs the equilibrium of the mitotic phosphoproteome that leads to the disruption of DNA damage repair and triggers an accumulation of chromosome breaks even in noncancerous cells.

Role of E2Fs and mitotic regulators controlled by E2Fs in the epithelial to mesenchymal transition

The epithelial-to-mesenchymal transition (EMT) is a complex cellular process in which epithelial cells acquire mesenchymal properties. EMT occurs in three biological settings: development, wound healing and fibrosis, and tumor progression. Despite occurring in three independent biological settings, EMT signaling shares some molecular mechanisms that allow epithelial cells to de-differentiate and acquire mesenchymal characteristics that confer cells invasive and migratory capacity to distant sites. Here we summarize the molecular mechanism that delineates EMT and we will focus on the role of E2 promoter binding factors (E2Fs) in EMT during tumor progression. Since the E2Fs are presently undruggable due to their control in numerous pivotal cellular functions and due to the lack of selectivity against individual E2Fs, we will also discuss the role of three mitotic regulators and/or mitotic kinases controlled by the E2Fs (NEK2, Mps1/TTK, and SGO1) in EMT that can be useful as drug targets.

Impact statement

The study of the epithelial to mesenchymal transition (EMT) is an active area of research since it is one of the early intermediates to invasion and metastasis—a state of the cancer cells that ultimately kills many cancer patients. We will present in this review that besides their canonical roles as regulators of proliferation, unregulated expression of the E2F transcription factors may contribute to cancer initiation and progression to metastasis by signaling centrosome amplification, chromosome instability, and EMT. Since our discovery that the E2F activators control centrosome amplification and mitosis in cancer cells, we have identified centrosome and mitotic regulators that may represent actionable targets against EMT and metastasis in cancer cells. This is impactful to all of the cancer patients in which the Cdk/Rb/E2F pathway is deregulated, which has been estimated to be most cancer patients with solid tumors.

Destabilizing NEK2 overcomes resistance to proteasome inhibition in multiple myeloma

Drug resistance remains the key problem in cancer treatment. It is now accepted that each myeloma patient harbors multiple subclones and subclone dominance may change over time. The coexistence of multiple subclones with high or low chromosomal instability (CIN) signature causes heterogeneity and drug resistance with consequent disease relapse. In this study, using a tandem affinity purification-mass spectrometry (TAP-MS) technique, we found that NEK2, a CIN gene, was bound to the deubiquitinase USP7. Binding to USP7 prevented NEK2 ubiquitination resulting in NEK2 stabilization. Increased NEK2 kinase levels activated the canonical NF-κB signaling pathway through the PP1α/AKT axis. Newly diagnosed myeloma patients with activated NF-κB signaling through increased NEK2 activity had poorer event-free and overall survivals based on multiple independent clinical cohorts. We also found that NEK2 activated heparanase, a secreted enzyme, responsible for bone destruction in an NF-κB-dependent manner. Intriguingly, both NEK2 and USP7 inhibitors showed great efficacy in inhibiting myeloma cell growth and overcoming NEK2-induced and -acquired drug resistance in xenograft myeloma mouse models.

Preclinical activity of MBM-5 in gastrointestinal cancer by inhibiting NEK2 kinase activity

NEK2 is a conserved mitotic regulator critical for cell cycle progression. Aberrant expression of NEK2 has been found in a variety of human cancers, making it an attractive molecular target for the design of novel anticancer therapeutics. In the present study, we have identified a novel compound MBM-5, which was found to bind to NEK2 with high affinity by docking simulations study. MBM-5 potently inhibited NEK2 kinase activity in vitro in a concentration-dependent manner. MBM-5 also suppressed cellular NEK2 kinase activity, as evidenced by the decreased phosphorylation of its substrate Hec1 on S165 in a concentration- and time-dependent manner. This inhibition impeded mitotic progression by inducing chromosome segregation defects and cytokinesis failure; therefore leading to accumulation of cells with ≥4N DNA content, which finally underwent apoptosis. More importantly, MBM-5 treatment effectively suppressed the tumor growth of human gastric and colorectal cancer cells xenografts. Taken together, we demonstrated that MBM-5 effectively inhibited the kinase activity of NEK2 and showed a potential application in anti-cancer treatment regimens.

The E2F activators control multiple mitotic regulators and maintain genomic integrity through Sgo1 and BubR1

The E2F1, E2F2, and E2F3a transcriptional activators control proliferation. However, how the E2F activators regulate mitosis to maintain genomic integrity is unclear. Centrosome amplification (CA) and unregulated spindle assembly checkpoint (SAC) are major generators of aneuploidy and chromosome instability (CIN) in cancer. Previously, we showed that overexpression of single E2F activators induced CA and CIN in mammary epithelial cells, and here we show that combined overexpression of E2F activators did not enhance CA. Instead, the E2F activators elevated expression of multiple mitotic regulators, including Sgo1, Nek2, Hec1, BubR1, and Mps1/TTK. cBioPortal analyses of the TCGA database showed that E2F overexpression in lobular invasive breast tumors correlates with overexpression of multiple regulators of chromosome segregation, centrosome homeostasis, and the SAC. Kaplan-Meier plots identified correlations between individual or combined overexpression of E2F1, E2F3a, Mps1/TTK, Nek2, BubR1, or Hec1 and poor overall and relapse-free survival of breast cancer patients. In MCF10A normal mammary epithelial cells co-overexpressing E2Fs, transient Sgo1 knockdown induced CA, high percentages of premature sister chromatid separation, chromosome losses, increased apoptosis, and decreased cell clonogenicity. BubR1 silencing resulted in chromosome losses without CA, demonstrating that Sgo1 and BubR1 maintain genomic integrity through two distinct mechanisms. Our results suggest that deregulated activation of the E2Fs in mammary epithelial cells is counteracted by activation of a Sgo1-dependent mitotic checkpoint.

Targeting NEK2 as a promising therapeutic approach for cancer treatment

Never in Mitosis (NIMA) Related Kinase 2 (NEK2) plays a key role in regulating mitotic processes, including centrosome duplication and separation, microtubule stabilization, kinetochore attachment and spindle assembly checkpoint. NEK2 is aberrantly overexpressed in a wide variety of human cancers and has been implicated in various aspects of malignant transformation, including tumorigenesis, drug resistance and tumor progression. The close relationship between NEK2 and cancer has made it an attractive target for anticancer therapeutic development; however, the mechanisms of how NEK2 coordinates altered signaling to malignant transformation remains unclear. In this paper, we discuss the functional roles of NEK2 in cancer development; highlight some of the significant NEK2 signaling in cancer, and summarize recent advances in the development of NEK2 inhibitors.

Ubiquitin, the centrosome, and chromosome segregation

For over a century, the abnormal movement or number of centrosomes has been linked with errors of chromosomes distribution in mitosis. While not essential for the formation of the mitotic spindle, the presence and location of centrosomes has a major influence on the manner in which microtubules interact with the kinetochores of replicated sister chromatids and the accuracy with which they migrate to resulting daughter cells. A complex network has evolved to ensure that cells contain the proper number of centrosomes and that their location is optimal for effective attachment of emanating spindle fibers with the kinetochores. The components of this network are regulated through a series of post-translational modifications, including ubiquitin and ubiquitin-like modifiers, which coordinate the timing and strength of signaling events key to the centrosome cycle. In this review, we examine the role of the ubiquitin system in the events relating to centriole duplication and centrosome separation, and discuss how the disruption of these functions impacts chromosome segregation.

Nek2 activation of Kif24 ensures cilium disassembly during the cell cycle

Many proteins are known to promote ciliogenesis, but mechanisms that promote primary cilia disassembly before mitosis are largely unknown. Here we identify a mechanism that favours cilium disassembly and maintains the disassembled state. We show that co-localization of the S/G2 phase kinase, Nek2 and Kif24 triggers Kif24 phosphorylation, inhibiting cilia formation. We show that Kif24, a microtubule depolymerizing kinesin, is phosphorylated by Nek2, which stimulates its activity and prevents the outgrowth of cilia in proliferating cells, independent of Aurora A and HDAC6. Our data also suggest that cilium assembly and disassembly are in dynamic equilibrium, but Nek2 and Kif24 can shift the balance toward disassembly. Further, Nek2 and Kif24 are overexpressed in breast cancer cells, and ablation of these proteins restores ciliation in these cells, thereby reducing proliferation. Thus, Kif24 is a physiological substrate of Nek2, which regulates cilia disassembly through a concerted mechanism involving Kif24-mediated microtubule depolymerization.

Most differentiated mammalian cells assemble a primary cilium, which serves as a cellular ‘antenna' for sensing and responding to the extracellular environment. Here the authors show that Nek2-mediated phosphorylation of Kif24 further promotes the loss of primary cilia, triggered by Aurora A and HDAC6 on cell cycle re-entry.

Cancerous inhibitor of protein phosphatase 2A promotes premature chromosome segregation and aneuploidy in prostate cancer cells through association with shugoshin

Yeast two-hybrid (Y2H) studies have shown that cancerous Inhibitor of protein phosphatase 2A (CIP2A) interacted with several proteins, including leucine-rich repeat-containing protein 59 (LRRC59), suggesting that CIP2A may interact with the chromosome maintenance protein, shugoshin (Sgol1). We previously showed that LRRC59 interacted with CIP2A, which was required for CIP2A nuclear localization. Thus, we predicted that CIP2A and Sgol1 may also interact. Sgol1 is a nuclear protein that regulates chromosome segregation during cell division via protection of cohesin ring proteins. Here, we demonstrated that Sgol1 and the C-terminus of CIP2A interact in prostate carcinoma cell lines in a protein phosphatase 2A (PP2A)-dependent manner. Moreover, we demonstrated that depletion of CIP2A in PC-3 cells decreases premature chromosome segregation, whereas overexpression of CIP2A in an immortalized prostate cell line increases premature chromosome segregation. Importantly, we further showed that CIP2A depletion decreases the incidence of aneuploidy and stabilizes cohesin complex proteins, while overexpression of CIP2A destabilizes Sgol1. Thus, our findings strongly suggest that CIP2A promotes cell cycle progression, premature chromosome segregation, and aneuploidy, possibly through a novel interaction with Sgol1.

Role of NEK2A in human cancer and its therapeutic potentials

Chromosome instability (CIN) has been identified as a common feature of most human cancers. A number of centrosomal kinases are thought to cause CIN in cancer cells. Part of those centrosomal kinases exhibit elevated expression in a wide variety of tumours and cancer cell lines. Additionally, critical roles in many aspects of cancer cell growth, proliferation, metastasis, and drug resistance have been assigned to some of these centrosomal kinases, such as polo-like kinase 1 (PLk1) and Aurora-A kinase. Recent studies from our group and others revealed that a centrosomal kinase, Never in Mitosis (NIMA) Related Kinase 2A (NEK2A), is frequently upregulated in multiple types of human cancers. Uncontrolled activity of NEK2A activates several oncogenic pathways and ABC transporters, thereby leading to CIN, cancer cell proliferation, metastasis, and enhanced drug resistance. In this paper, we highlight recent findings on the aberrant expression and functional significance of NEK2A in human cancers and emphasize their significance for therapeutic potentials.

Regulation of the subcellular shuttling of Sgo1 between centromeres and chromosome arms by Aurora B-mediated phosphorylation

A minor fraction of cohesin complexes at chromosome arms is not removed by the prophase pathway, and maintained until metaphase and enriched at centromeres. Sgo1 localizes to chromosome arms from prophase to metaphase, and is indispensable for removing cohesin complexes from chromosome arms. However, it has not been established how the chromosome arm localization of Sgo1 leads to the establishment of cohesion on chromosomes. Here, we report that Aurora B kinase interacts with and phosphorylates Sgo1 in vitro and in vivo. Furthermore, the phosphorylation of Sgol by Aurora B kinase regulated the distribution of Sgo1 between centromeres and chromosome arms, and the expression of Aurora B kinase-dead mutants of Sgo1 caused mislocalization from centromeres to chromosome arms. These results suggest Aurora B kinase directly regulates the subcellular distribution of Sgo1 to facilitate the accurate separation of mitotic chromosomes.

Differential expression of centrosome regulators in Her2+ breast cancer cells versus non-tumorigenic MCF10A cells

Centrosome amplification (CA) amongst particular breast cancer subtypes (Her2+ subtype) is associated with genomic instability and aggressive tumor phenotypes. However, changes in signaling pathways associated with centrosome biology have not been fully explored in subtype specific models. Novel centrosome regulatory genes that are selectively altered in Her2+ breast cancer cells are of interest in discerning why CA is more prevalent in this subtype. To determine centrosome/cell cycle genes that are altered in Her2+ cells that display CA (HCC1954) versus non-tumorigenic cells (MCF10A), we carried out a gene microarray. Expression differences were validated by real-time PCR and Western blotting. After the microarray validation, we pursued a panel of upregulated and downregulated genes based on novelty/relevance to centrosome duplication. Functional experiments measuring CA and BrdU incorporation were completed after genetic manipulation of targets (TTK, SGOL1, MDM2 and SFRP1). Amongst genes that were downregulated in HCC1954 cells, knockdown of MDM2 and SFRP1 in MCF10A cells did not consistently induce CA or impaired BrdU incorporation. Conversely, amongst upregulated genes in HCC1954 cells, knockdown of SGOL1 and TTK decreased CA in breast cancer cells, while BrdU incorporation was only altered by SGOL1 knockdown. We also explored the Kaplan Meier Plot resource and noted that MDM2 and SFRP1 are positively associated with relapse free survival in all breast cancer subtypes, while TTK is negatively correlated with overall survival of Luminal A patients. Based on this functional screen, we conclude that SGOL1 and TTK are important modulators of centrosome function in a breast cancer specific model.

The spatiotemporal dynamics of chromatin protein HP1α is essential for accurate chromosome segregation during cell division

Heterochromatin protein 1α (HP1α) is involved in regulation of chromatin plasticity, DNA damage repair, and centromere dynamics. HP1α detects histone dimethylation and trimethylation of Lys-9 via its chromodomain. HP1α localizes to heterochromatin in interphase cells but is liberated from chromosomal arms at the onset of mitosis. However, the structural determinants required for HP1α localization in interphase and the regulation of HP1α dynamics have remained elusive. Here we show that centromeric localization of HP1α depends on histone H3 Lys-9 trimethyltransferase SUV39H1 activity in interphase but not in mitotic cells. Surprisingly, HP1α liberates from chromosome arms in early mitosis. To test the role of this dissociation, we engineered an HP1α construct that persistently localizes to chromosome arms. Interestingly, persistent localization of HP1α to chromosome arms perturbs accurate kinetochore-microtubule attachment due to an aberrant distribution of chromosome passenger complex and Sgo1 from centromeres to chromosome arms that prevents resolution of sister chromatids. Further analyses showed that Mis14 and perhaps other PXVXL-containing proteins are involved in directing localization of HP1α to the centromere in mitosis. Taken together, our data suggest a model in which spatiotemporal dynamics of HP1α localization to centromere is governed by two distinct structural determinants. These findings reveal a previously unrecognized but essential link between HP1α-interacting molecular dynamics and chromosome plasticity in promoting accurate cell division.

The 68-kDa telomeric repeat binding factor 1 (TRF1)-associated protein (TAP68) interacts with and recruits TRF1 to the spindle pole during mitosis

The telomere capping protein TRF1 is a component of the multiprotein complex "shelterin," which organizes the telomere into a high order structure. Besides telomere maintenance, telomere-associated proteins also have nontelomeric functions. For example, tankyrase 1 and TRF1 are required for the maintenance of faithful mitotic progression. However, the functional relevance of their centrosomal localization has not been established. Here, we report the identification of a TRF1-binding protein, TAP68, that interacts with TRF1 in mitotic cells. TAP68 contains two coiled-coil domains and a structural maintenance of chromosome motifs and co-localizes with TRF1 to telomeres during interphase. Immediately after nuclear envelope breakdown, TAP68 translocates toward the spindle poles followed by TRF1. Dissociation of TAP68 from the telomere is concurrent with the Nek2A-dependent phosphorylation at Thr-221. Biochemical characterization demonstrated that the first coiled-coil domain of TAP68 binds and recruits TRF1 to the centrosome. Inhibition of TAP68 expression by siRNA blocked the localization of TRF1 and tankyrase 1 to the centrosome. Furthermore, siRNA-mediated depletion of TAP68 perturbed faithful chromosome segregation and genomic stability. These findings suggest that TAP68 functions in mediating TRF1-tankyrase 1 localization to the centrosome and in mitotic regulation.

Nek2 and Plk4: prognostic markers, drivers of breast tumorigenesis and drug resistance

The Nek2 and Plk4 kinases serve as crucial regulators of mitotic processes such as the centrosome duplication cycle and spindle assembly. Deregulation of these processes can trigger chromosome instability and aneuploidy, which are hallmarks of many solid tumors, including breast cancer. Emerging data from the literature illustrated various functions of Nek2 in breast cancer models, with compelling evidence of its prognostic value in breast tumors. The two kinases control distinct steps in the centrosome-centriole cycle and their dysregulation lead to centrosome amplification, marked by the presence of more than two centrosomes within the cell. We found single or composite overexpression of these kinases in breast tumor samples, regardless of subtype, which strongly associated with poor prognosis. Interestingly, in a panel of established cell lines, both kinases are highly expressed in Her2-positive breast cancer cells exhibiting centrosome amplification and trastuzumab resistance. In summary, it appears that Nek2 and Plk4 might synergize to promote breast tumorigenesis and may also be involved in tamoxifen and trastuzumab resistance.

The histone acetyltransferase component TRRAP is targeted for destruction during the cell cycle

Chromosomes are dynamic structures that must be reversibly condensed and unfolded to accommodate mitotic division and chromosome segregation. Histone modifications are involved in the striking chromatin reconfiguration taking place during mitosis. However, the mechanisms that regulate activity and function of histone-modifying factors as cells enter and exit mitosis are poorly understood. Here, we show that the anaphase-promoting complex or cyclosome (APC/C) is involved in the mitotic turnover of TRRAP (TRansformation/tRanscription domain-Associated Protein), a common component of histone acetyltransferase (HAT) complexes, and that the pre-mitotic degradation of TRRAP is mediated by the APC/C ubiquitin ligase activators Cdc20 and Cdh1. Ectopic expression of both Cdh1 and Cdc20 reduced the levels of coexpressed TRRAP protein and induced its ubiquitination. TRRAP overexpression or stabilization induces multiple mitotic defects, including lagging chromosomes, chromosome bridges and multipolar spindles. In addition, lack of sister chromatid cohesion and impaired chromosome condensation were found after TRRAP overexpression or stabilization. By using a truncated form of TRRAP, we show that mitotic delay is associated with a global histone H4 hyperacetylation induced by TRRAP overexpression. These results demonstrate that the chromatin modifier TRRAP is targeted for destruction in a cell cycle-dependent fashion. They also suggest that degradation of TRRAP by the APC/C is necessary for a proper condensation of chromatin and proper chromosome segregation. Chromatin compaction mediated by histone modifiers may represent a fundamental arm for APC/C orchestration of the mitotic machinery.

NEK2 induces drug resistance mainly through activation of efflux drug pumps and is associated with poor prognosis in myeloma and other cancers

Using sequential gene expression profiling (GEP) samples, we defined a major functional group related to drug resistance that contains chromosomal instability (CIN) genes. One CIN gene in particular, NEK2, was highly correlated with drug resistance, rapid relapse, and poor outcome in multiple cancers. Overexpressing NEK2 in cancer cells resulted in enhanced CIN, cell proliferation and drug resistance, while targeting NEK2 by NEK2 shRNA overcame cancer cell drug resistance and induced apoptosis in vitro and in a xenograft myeloma mouse model. High expression of NEK2 induced drug resistance mainly through activation of the efflux pumps. Thus, NEK2 represents a strong predictor for drug resistance and poor prognosis in cancer and could be an important target for cancer therapy.

Bod1 regulates protein phosphatase 2A at mitotic kinetochores

Mitotic entry and progression require the activation of several mitotic kinases and the proper regulation and localization of several phosphatases. The activity and localization of each of these enzymes is tightly controlled through a series of specific activators, inhibitors and regulatory subunits. Two proteins, Ensa and Arpp-19, were recently identified as specific inhibitors of PP2A-B55 and are critical for allowing full activity of Cdk1/cyclin B and entry into mitosis. Here we show that Bod1, a protein required for proper chromosome alignment at mitosis, shares sequence similarity with Ensa and Arpp-19 and specifically inhibits the kinetochore-associated PP2A-B56 holoenzyme. PP2A-B56 regulates the stability of kinetochore-microtubule attachments by dephosphorylating several kinetochore proteins. Loss of Bod1 changes the balance of phosphorylation at kinetochores, causing defects in kinetochore function. Bod1, Ensa and Arpp-19 define a family of specific PP2A inhibitors that regulate specific PP2A holoenzymes at distinct locations and points in the cell cycle.

Genetic polymorphisms in centrobin and Nek2 are associated with breast cancer susceptibility in a Chinese Han population

Centrosome aberrations have been suggested to cause chromosomal instability and aneuploidy, and eventually promote cancer development. The Centrobin and Nek2 proteins interact with each other and both are involved in centrosome duplication and chromosome segregation. This study aimed to investigate whether genetic polymorphisms in these two genes may affect breast cancer susceptibility in Chinese Han population using a haplotype-based analysis. Five single nucleotide polymorphisms (SNPs) in centrobin and four SNPs in Nek2 were genotyped in 1,215 cases of infiltrating ductal breast cancer and 1,215 age-matched cancer-free controls from Chinese Han population. The results showed that CATCG haplotype of centrobin was strongly associated with decreased breast cancer risk (adjusted OR = 0.14, 95 % CI = 0.09-0.22), which was mainly driven by the C allele of SNP rs11650083 (A>C, located in exon 12, resulting in Pro578Gln). None of the individual SNPs in Nek2 was associated with breast cancer risk. However, haplotype GTAT of Nek2 was associated with increased risk of breast cancer (adjusted OR = 1.56, 95 % CI = 1.18-2.06) and its risk was significantly elevated among women with both family history of cancer and a longer menarche-first full-term pregnancy (FFTP) interval (>11 years) (adjusted OR = 5.31, 95 % CI = 1.97-14.32). Furthermore, women harboring both at-risk haplotype GTAT of Nek2 and protective haplotype CATCG of centrobin were linked with decreased breast cancer risk, suggesting that the association between genetic variants of Nek2 and increased breast cancer risk was modified by genetic variants of centrobin. Our results indicate that genetic polymorphisms of centrobin and Nek2 are related to breast cancer susceptibility in Chinese Han women.

Nek2A contributes to tumorigenic growth and possibly functions as potential therapeutic target for human breast cancer

Nek2A (NIMA-related kinases 2A) has been known as an important centrosome regulatory factor. The aim of this study was to investigate the expression of Nek2A and the role it played in different stages of breast cancer. We detected the expression of Nek2A in both mRNA and protein levels in MCF10 cell lines including MCF-10A, MCF-10DCIS.com, MCF-10CA1a and in human breast samples which contained normal breast tissue (NBT), breast ductal carcinoma in situ (DCIS), and invasive ductal carcinoma (IDC). Our study revealed that the mRNA and protein expression of Nek2A were significantly up-regulated in MCF-10DCIS.com and MCF-10CA1a cell lines as well as in human primary breast cancer tissue (DCIS and IDC). Our study also presented a correlation between Nek2A mRNA expression and some clinic pathological factors. We found that Nek2A mRNA expression was associated with molecular subtypes, ER, PR and Ki-67 immunoreactivity (P<0.05) in DCIS and associated with histological grade, lymph node metastasis, molecular subtypes, c-erbB-2, and Ki-67 expression (P<0.05) in IDC. In addition, we observed that ectopic expression of Nek2A in "normal" immortalized MCF-10A breast epithelial cell resulted in increased Nek2A which lead to abnormal centrosomes. Furthermore, knockdown of Nek2A in MCF-10DCIS.com could remarkably inhibit cell proliferation and induce cell cycle arrest in MCF-10DCIS.com cell line. These data suggested that Nek2A might bear a close relationship with development and progression of breast carcinoma, and highlighted its role as a novel potential biomarker for diagnosis and a possible therapeutic target for human breast cancer especially for DCIS.

Cohesin: a guardian of genome integrity

Ability to reproduce is one of the hallmark features of all life forms by which new organisms are produced from their progenitors. During this process each cell duplicates its genome and passes a copy of its genome to the daughter cells along with the cellular matrix. Unlike bacteria, in eukaryotes there is a definite time gap between when the genome is duplicated and when it is physically separated. Therefore, for precise halving of the duplicated genome into two, it is required that each pair of duplicated chromosomes, termed sister chromatids, should be paired together in a binary fashion from the moment they are generated. This pairing function between the duplicated genome is primarily provided by a multimeric protein complex, called cohesin. Thus, genome integrity largely depends on cohesin as it ensures faithful chromosome segregation by holding the sister chromatids glued together from S phase to anaphase. In this review, we have discussed the life cycle of cohesin during both mitotic and meiotic cell divisions including the structure and architecture of cohesin complex, relevance of cohesin associated proteins, mechanism of cohesin loading onto the chromatin, cohesion establishment and the mechanism of cohesin disassembly during anaphase to separate the sister chromatids. We have also focused on the role of posttranslational modifications in cohesin biology. For better understanding of the complexity of the cohesin regulatory network to the readers, we have presented an interactome profiling of cohesin core subunits in budding yeast during mitosis and meiosis.

SUV39H1 orchestrates temporal dynamics of centromeric methylation essential for faithful chromosome segregation in mitosis

Histone methylation performs multiple functions such as DNA replication, transcription regulation, heterochromatin formation, and chromatin condensation. How this methylation gradient is orchestrated in the centromere during chromosome segregation is not known. Here we examine the temporal dynamics of protein methylation in the centromere by SUV39H1 methyltransferase, a key mitotic regulator, using fluorescence resonance energy transfer-based sensors in living HeLa cells and immunofluorescence of native SUV39H1 substrates. A quantitative analysis of methylation dynamics, using centromere-targeted sensors, reveals a temporal change during chromosome segregation. These dynamics result in an accurate chromosome congression to and alignment at the equator as an inhibition of methylation dynamics using SUV39H1 inhibitor perturbs chromosome congression in living HeLa cells. Surprisingly, this inhibition of methylation results in a brief increase in Aurora B kinase activity and an enrichment of microtubule depolymerase MCAK in the centromere with a concomitant kinetochore-microtubule destabilization and a reduced tension across the sister kinetochores with ultimate chromosome misalignments. We reason that SUV39H1 generates a gradient of methylation marks at the kinetochore that provides spatiotemporal information essential for accurate chromosome segregation in mitosis.

Shugoshins function as a guardian for chromosomal stability in nuclear division

Accurate chromosome segregation during mitosis and meiosis is regulated and secured by several distinctly different yet intricately connected regulatory mechanisms. As chromosomal instability is a hallmark of a majority of tumors as well as a cause of infertility for germ cells, extensive research in the past has focused on the identification and characterization of molecular components that are crucial for faithful chromosome segregation during cell division. Shugoshins, including Sgo1 and Sgo2, are evolutionarily conserved proteins that function to protect sister chromatid cohesion, thus ensuring chromosomal stability during mitosis and meiosis in eukaryotes. Recent studies reveal that Shugoshins in higher animals play an essential role not only in protecting centromeric cohesion of sister chromatids and assisting bi-orientation attachment at the kinetochores, but also in safeguarding centriole cohesion/engagement during early mitosis. Many molecular components have been identified that play essential roles in modulating/mediating Sgo functions. This review primarily summarizes recent advances on the mechanisms of action of Shugoshins in suppressing chromosomal instability during nuclear division in eukaryotic organisms.

Shugoshins: from protectors of cohesion to versatile adaptors at the centromere

Sister chromatids are held together by a protein complex named cohesin. Shugoshin proteins protect cohesin from cleavage by separase during meiosis I in eukaryotes and from phosphorylation-mediated removal during mitosis in vertebrates. This protection is crucial for chromosome segregation during mitosis and meiosis. Mechanistically, shugoshins shield cohesin by forming a complex with the phosphatase PP2A, which dephosphorylates cohesin, leading to its retention at centromeres during the onset of meiotic anaphase and vertebrate mitotic prophase I. In addition to this canonical function, shugoshins have evolved novel, species-specific cellular functions, the mechanisms of which remain a subject of intense debate, but are likely to involve spatio-temporally coordinated interactions with the chromosome passenger complex, the spindle checkpoint and the anaphase promoting complex. Here, we compare and contrast these remarkable features of shugoshins in model organisms and humans.

Giardia lamblia Nek1 and Nek2 kinases affect mitosis and excystation

The NIMA-related serine/threonine kinases (Neks) function in the cell cycle and regulate ciliary and flagellar length. The Giardia lamblia genome encodes 198 Neks, of which 56 are predicted to be active. Here we believe that we report the first functional analysis of two G. lamblia Neks. The GlNek1 and GlNek2 kinase domains share 57% and 43% identity to the kinase domains of human Nek1 and Nek2, respectively. Both GlNeks are active in vitro, have dynamic relocalisation during the cell cycle, and are expressed throughout the life cycle, with GlNek1 being upregulated in cysts. Over-expression of inactive GlNek1 delays disassembly of the parental attachment disc and cytokinesis, whilst over-expression of either wild type GlNek1 or inactive mutant GlNek2 inhibits excystation.

Aurora B kinase activation requires survivin priming phosphorylation by PLK1

During cell division, chromosome segregation is orchestrated by the interaction of spindle microtubules with the centromere. Accurate attachment of spindle microtubules to kinetochore requires the chromosomal passenger of Aurora B kinase complex with borealin, INCENP and survivin (SUR). The current working model argues that SUR is responsible for docking Aurora B to the centromere whereas its precise role in Aurora B activation has been unclear. Here, we show that Aurora B kinase activation requires SUR priming phosphorylation at Ser20 which is catalyzed by polo-like kinase 1 (PLK1). Inhibition of PLK1 kinase activity or expression of non-phosphorylatable SUR mutant prevents Aurora B activation and correct spindle microtubule attachment. The PLK1-mediated regulation of Aurora B kinase activity was examined in real-time mitosis using fluorescence resonance energy transfer-based reporter and quantitative analysis of native Aurora B substrate phosphorylation. We reason that the PLK1-mediated priming phosphorylation is critical for orchestrating Aurora B activity in centromere which is essential for accurate chromosome segregation and faithful completion of cytokinesis.

Irreversible Nek2 kinase inhibitors with cellular activity

A structure-based approach was used to design irreversible, cysteine-targeted inhibitors of the human centrosomal kinase, Nek2. Potent inhibition of Nek2 kinase activity in biochemical and cell-based assays required a noncatalytic cysteine residue (Cys22), located near the glycine-rich loop in a subset of human kinases. Elaboration of an oxindole scaffold led to our most selective compound, oxindole propynamide 16 (JH295). Propynamide 16 irreversibly inhibited cellular Nek2 without affecting the mitotic kinases, Cdk1, Aurora B, or Plk1. Moreover, 16 did not perturb bipolar spindle assembly or the spindle assembly checkpoint. To our knowledge, 16 is the first small molecule shown to inactivate Nek2 kinase activity in cells.

How protein kinases co-ordinate mitosis in animal cells

Mitosis is associated with profound changes in cell physiology and a spectacular surge in protein phosphorylation. To accomplish these, a remarkably large portion of the kinome is involved in the process. In the present review, we will focus on classic mitotic kinases, such as cyclin-dependent kinases, Polo-like kinases and Aurora kinases, as well as more recently characterized players such as NIMA (never in mitosis in Aspergillus nidulans)-related kinases, Greatwall and Haspin. Together, these kinases co-ordinate the proper timing and fidelity of processes including centrosomal functions, spindle assembly and microtubule-kinetochore attachment, as well as sister chromatid separation and cytokinesis. A recurrent theme of the mitotic kinase network is the prevalence of elaborated feedback loops that ensure bistable conditions. Sequential phosphorylation and priming phosphorylation on substrates are also frequently employed. Another important concept is the role of scaffolds, such as centrosomes for protein kinases during mitosis. Elucidating the entire repertoire of mitotic kinases, their functions, regulation and interactions is critical for our understanding of normal cell growth and in diseases such as cancers.

Involvement of a centrosomal protein kendrin in the maintenance of centrosome cohesion by modulating Nek2A kinase activity

Centrosome cycle is strictly coordinated with chromosome duplication cycle to ensure the faithful segregation of chromosomes. Centrosome duplication occurs from the beginning of S phase, and the duplicated centrosomes are held together by centrosome cohesion to function as a single microtubule organizing center during interphase. At late G2 phase centrosome cohesion is disassembled by Nek2A kinase-mediated phosphorylation and, as a consequence, centrosomes are split and constitute spindle poles in mitosis. It has been reported that depletion of a centrosomal protein kendrin (also named pericentrin) induces premature centrosome splitting in interphase, however, it remains unknown how kendrin contributes to the maintenance of centrosome cohesion. Here we show that kendrin associates with Nek2A kinase, which exhibits considerably low activity. Nek2A kinase activity is inhibited in vitro by addition of the Nek2A-binding region of kendrin in a dose-dependent manner. Furthermore, ectopic expression of the same region decreases the number of the cells with split centrosomes at late G2 phase. Taken together, these results suggest that kendrin anchors Nek2A and suppresses its kinase activity at the centrosomes, and thus, is involved in the mechanism to prevent premature centrosome splitting during interphase.

Novel combination treatment for colorectal cancer using Nek2 siRNA and cisplatin

Nek2 (NIMA-related kinase 2) is involved in cell division and mitotic regulation by centrosome splitting. We previously reported that Nek2 depletion causes growth suppression and cell death in cholangiocarcinoma and breast cancer cells. In this report, we examine the effect of a combination treatment using Nek2 siRNA with the cytotoxic chemotherapeutic agent cisplatin (CDDP) on colorectal cancer. Nek2 was overexpressed in all colorectal cancer cell lines examined (HCT-15, DLD-1, Colo205, and Colo320). Nek2 short-interfering RNA (siRNA) resulted in the inhibition of cell proliferation and the induction of apoptosis in vitro. Nek2 siRNA suppressed tumor growth compared to control siRNA in a xenograft mouse model. To investigate the potential utility of Nek2 siRNA for clinical cancer therapy, we examine the effect of a combination treatment using Nek2 siRNA with CDDP on colorectal cancer. The combined administration of both Nek2 siRNA and CDDP inhibited cell proliferation and induced apoptotic cell death in vitro. Furthermore, the combined administration of both Nek2 siRNA and CDDP suppressed tumor growth compared to either the single administration of Nek2 siRNA or the combined administration of control siRNA and CDDP. Our results suggest that combination treatment using Nek2 siRNA and chemotherapeutic agents may be an effective therapeutic option for colorectal cancer.

Phosphorylation of HsMis13 by Aurora B kinase is essential for assembly of functional kinetochore

Chromosome movements in mitosis are orchestrated by dynamic interactions between spindle microtubules and the kinetochore, a multiprotein complex assembled onto centromeric DNA of the chromosome. Here we show that phosphorylation of human HsMis13 by Aurora B kinase is required for functional kinetochore assembly in HeLa cells. Aurora B interacts with HsMis13 in vitro and in vivo. HsMis13 is a cognate substrate of Aurora B, and the phosphorylation sites were mapped to Ser-100 and Ser-109. Suppression of Aurora B kinase by either small interfering RNA or chemical inhibitors abrogates the localization of HsMis13 but not HsMis12 to the kinetochore. In addition, non-phosphorylatable but not wild type and phospho-mimicking HsMis13 failed to localize to the kinetochore, demonstrating the requirement of phosphorylation by Aurora B for the assembly of HsMis13 to kinetochore. In fact, localization of HsMis13 to the kinetochore is spatiotemporally regulated by Aurora B kinase, which is essential for recruiting outer kinetochore components such as Ndc80 components and CENP-E for functional kinetochore assembly. Importantly, phospho-mimicking mutant HsMis13 restores the assembly of CENP-E to the kinetochore, and tension developed across the sister kinetochores in Aurora B-inhibited cells. Thus, we reason that HsMis13 phosphorylation by Aurora B is required for organizing a stable bi-oriented microtubule kinetochore attachment that is essential for faithful chromosome segregation in mitosis.

The mitotic checkpoint kinase NEK2A regulates kinetochore microtubule attachment stability

Loss or gain of whole chromosome, the form of chromosome instability commonly associated with cancers is thought to arise from aberrant chromosome segregation during cell division. Chromosome segregation in mitosis is orchestrated by the interaction of kinetochores with spindle microtubules. Our studies show that NEK2A is a kinetochore-associated protein kinase essential for faithful chromosome segregation. However, it was unclear how NEK2A ensures accurate chromosome segregation in mitosis. Here we show that NEK2A-mediated Hec1 (highly expressed in cancer) phosphorylation is essential for faithful kinetochore microtubule attachments in mitosis. Using phospho-specific antibody, our studies show that NEK2A phosphorylates Hec1 at Ser165 during mitosis. Although such phosphorylation is not required for assembly of Hec1 to the kinetochore, expression of non-phosphorylatable mutant Hec1(S165) perturbed chromosome congression and resulted in a dramatic increase in microtubule attachment errors, including syntelic and monotelic attachments. Our in vitro reconstitution experiment demonstrated that Hec1 binds to microtubule in low affinity and phosphorylation by NEK2A, which prevents aberrant kinetochore-microtubule connections in vivo, increases the affinity of the Ndc80 complex for microtubules in vitro. Thus, our studies illustrate a novel regulatory mechanism in which NEK2A kinase operates a faithful chromosome attachment to spindle microtubule, which prevents chromosome instability during cell division.