Inhibition of centrosome separation after DNA damage: a role for Nek2

Nek10 mediates G2/M cell cycle arrest and MEK autoactivation in response to UV irradiation

Appropriate cell cycle checkpoint control is essential for the maintenance of cell and organismal homeostasis. Members of the Nek (NIMA-related kinase) family of serine/threonine protein kinases have been implicated in the regulation of various aspects of the cell cycle. We explored the cellular functions of Nek10, a novel member of the Nek family, and demonstrate a role for Nek10 in the cellular UV response. Nek10 was required for the activation of extracellular signal-regulated kinase 1/2 (ERK1/2) signaling upon UV irradiation but not in response to mitogens, such as epidermal growth factor stimulation. Nek10 physically associated with Raf-1 and MEK1 in a Raf-1-dependent manner, and the formation of this complex was necessary for Nek10-mediated MEK1 activation. Nek10 did not affect the kinase activity of Raf-1 but instead promoted the autophosphorylation-dependent activation of MEK1. The appropriate maintenance of the G(2)/M checkpoint following UV irradiation required Nek10 expression and ERK1/2 activation. Taken together, our results uncover a role for Nek10 in the cellular response to UV irradiation.

Identification by high-throughput screening of viridin analogs as biochemical and cell-based inhibitors of the cell cycle-regulated nek2 kinase

Nek2 is a serine/threonine protein kinase that localizes to the centrosome and is implicated in mitotic regulation. Overexpression of Nek2 induces premature centrosome separation and nuclear defects indicative of mitotic errors, whereas depletion of Nek2 interferes with cell growth. As Nek2 expression is upregulated in a range of cancer cell lines and primary human tumors, inhibitors of Nek2 may have therapeutic value in cancer treatment. The authors used a radiometric proximity assay in a high-throughput screen to identify small-molecule inhibitors of Nek2 kinase activity. The assay was based on the measurement of the radiolabeled phosphorylated product of the kinase reaction brought into contact with the surface of wells of solid scintillant-coated microplates. Seventy nonaggregating hits were identified from approximately 73,000 compounds screened and included a number of toxoflavins and a series of viridin/wortmannin-like compounds. The viridin-like compounds were >70-fold selective for Nek2 over Nek6 and Nek7 and inhibited the growth of human tumor cell lines at concentrations consistent with their biochemical potencies. An automated mechanism-based microscopy assay in which centrosomes were visualized using pericentrin antibodies confirmed that 2 of the viridin inhibitors reduced centrosome separation in a human tumor cell line. The data presented show that pharmacological inhibition of Nek2 kinase results in the expected phenotype of disruption to centrosome function associated with growth inhibition and further supports Nek2 as a target for cancer drug discovery.

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.

Centrosome-associated regulators of the G(2)/M checkpoint as targets for cancer therapy

In eukaryotic cells, control mechanisms have developed that restrain cell-cycle transitions in response to stress. These regulatory pathways are termed cell-cycle checkpoints. The G(2)/M checkpoint prevents cells from entering mitosis when DNA is damaged in order to afford these cells an opportunity to repair the damaged DNA before propagating genetic defects to the daughter cells. If the damage is irreparable, checkpoint signaling might activate pathways that lead to apoptosis. Since alteration of cell-cycle control is a hallmark of tumorigenesis, cell-cycle regulators represent potential targets for therapy. The centrosome has recently come into focus as a critical cellular organelle that integrates G(2)/M checkpoint control and repairs signals in response to DNA damage. A growing number of G(2)/M checkpoint regulators have been found in the centrosome, suggesting that centrosome has an important role in G(2)/M checkpoint function. In this review, we discuss centrosome-associated regulators of the G(2)/M checkpoint, the dysregulation of this checkpoint in cancer, and potential candidate targets for cancer therapy.

Nek2 as a novel molecular target for the treatment of breast carcinoma

We investigated the role of Nek2, a member of the serine-threonine kinase family, in the tumorigenic growth of breast carcinoma. Increased expression of Nek2 was observed in all breast carcinoma cell lines examined (BT20, BT474, Hs578T, MCF7, MDA-MB-231, T47D, and ZR-75-1) by immunoblotting. By treatment with Nek2 short interfering RNA (siRNA), expression of Nek2 was clearly decreased in both estrogen receptor (ER)-positive (MCF7) and ER-negative (MDA-MB-231) breast carcinoma cell lines. Cell growth, colony formation in soft agar, and in vitro invasiveness of these cell lines were substantially suppressed by Nek2 siRNA treatment. In a xenograft nude mouse model with subcutaneous implantation of MCF7 or MDA-MB-231, subcutaneous injection of Nek2 siRNA around the tumor nodules resulted in a reduction of tumor size compared with those of control siRNA injection. Taken together, Nek2 appears to play a pivotal role in tumorigenic growth of breast carcinoma cells, and could be a useful therapeutic molecular target for the treatment of breast carcinoma both in ER-positive and ER-negative cases.

Nek2 as an effective target for inhibition of tumorigenic growth and peritoneal dissemination of cholangiocarcinoma

We investigated the role of Nek2, a member of the serine/threonine kinase family, Nek, in the tumorigenic growth of cholangiocarcinoma cells. Expression of Nek2 is elevated in cholangiocarcinoma in a tumor-specific manner as compared with that of normal fibroblast cells. Expression of exogenous Nek2 did not perturb the growth of cholangiocarcinoma cells, whereas suppression of the Nek2 expression with siRNA resulted in the inhibition of cell proliferation and induced cell death. In xenograft-nude mouse model, s.c. injection of Nek2 siRNA around the tumor nodules resulted in reduction of tumor size as compared with those of control siRNA injection. In peritoneal dissemination model, Nek2 siRNA-treated mice showed statistically longer survival periods in comparison with those of the control siRNA-treated mice. Taken together, our data indicate a pivotal role of Nek2 in tumorigenic growth of cholangiocarcinoma.

Identification of genetic variants contributing to cisplatin-induced cytotoxicity by use of a genomewide approach

Cisplatin, a platinating agent commonly used to treat several cancers, is associated with nephrotoxicity, neurotoxicity, and ototoxicity, which has hindered its utility. To gain a better understanding of the genetic variants associated with cisplatin-induced toxicity, we present a stepwise approach integrating genotypes, gene expression, and sensitivity of HapMap cell lines to cisplatin. Cell lines derived from 30 trios of European descent (CEU) and 30 trios of African descent (YRI) were used to develop a preclinical model to identify genetic variants and gene expression that contribute to cisplatin-induced cytotoxicity in two different populations. Cytotoxicity was determined as cell-growth inhibition at increasing concentrations of cisplatin for 48 h. Gene expression in 176 HapMap cell lines (87 CEU and 89 YRI) was determined using the Affymetrix GeneChip Human Exon 1.0 ST Array. We identified six, two, and nine representative SNPs that contribute to cisplatin-induced cytotoxicity through their effects on 8, 2, and 16 gene expressions in the combined, Centre d'Etude du Polymorphisme Humain (CEPH), and Yoruban populations, respectively. These genetic variants contribute to 27%, 29%, and 45% of the overall variation in cell sensitivity to cisplatin in the combined, CEPH, and Yoruban populations, respectively. Our whole-genome approach can be used to elucidate the expression of quantitative trait loci contributing to a wide range of cellular phenotypes.

Members of the plant NIMA-related kinases are involved in organ development and vascularization in poplar, Arabidopsis and rice

NIMA-related kinases (Neks) are a family of serine/threonine kinases that have been linked to cell-cycle regulation in fungi and mammals. Information regarding the function of Neks in plants is very limited. We screened the three plant species that have had their genomes sequenced in an attempt to improve our understanding of their role in plants. We retrieved seven members in Arabidopsis thaliana, nine in Populus trichocarpa and six in Oryza sativa. Phylogenetic analysis showed that plant Neks are closely related to each other and contain paralogous genes. Moreover, their chromosome distribution and their exon-intron structure revealed that the actual plant Nek family was derived from a single representative followed by large segmental duplication events. Functional expression analyses in the three species relied on RTqPCR in poplar and publicly available microarray data for Arabidopsis and rice. Although plant Neks are present in every organ analyzed, their expression profiles suggest their involvement in plant development processes. Furthermore, we showed that PNek1, a member of the poplar family, is expressed at sites of free auxin synthesis and is specifically involved during the vascularization process.

Protein phosphatase-1alpha regulates centrosome splitting through Nek2

ATM is a central mediator of the cellular response to the DNA damage produced by ionizing radiation. We recently showed that protein phosphatase 1 (PP1) is activated by ATM. Because Nek2 is activated by autophosphorylation, and because its dephosphorylation is catalyzed by PP1, we asked if the radiation damage signal to Nek2 was mediated by PP1. Overexpression of Nek2 induces premature centrosome splitting probably by phosphorylating centrosome cohesion proteins C-Nap1 and Rootletin. In this study, we show isoform specificity of PP1 binding and regulation of Nek2. Although both PP1alpha and PP1gamma coimmunoprecipitated with Nek2, only PP1alpha regulated Nek2 function. Ionizing radiation inhibited Nek2 activity, and this response was dependent on ATM and on PP1 binding to Nek2 and coincident with Thr(320) dephosphorylation of PP1. Radiation-induced inhibition of centrosome splitting was abrogated in cells expressing Nek2 mutated in the PP1-binding motif outside the kinase domain. Conversely, cells depleted of PP1alpha by small interfering RNA showed enhanced centrosome splitting and loss of radiation-induced inhibition of centrosome splitting. The identification of a PP1-specific isoform mediating a checkpoint response opens up the possibility of selectively targeting phosphatases as novel radiation sensitizers.

Structure and regulation of the human Nek2 centrosomal kinase

The dimeric Ser/Thr kinase Nek2 regulates centrosome cohesion and separation through phosphorylation of structural components of the centrosome, and aberrant regulation of Nek2 activity can lead to aneuploid defects characteristic of cancer cells. Mutational analysis of autophosphorylation sites within the kinase domain identified by mass spectrometry shows a complex pattern of positive and negative regulatory effects on kinase activity that are correlated with effects on centrosomal splitting efficiency in vivo. The 2.2-A resolution x-ray structure of the Nek2 kinase domain in complex with a pyrrole-indolinone inhibitor reveals an inhibitory helical motif within the activation loop. This helix presents a steric barrier to formation of the active enzyme and generates a surface that may be exploitable in the design of specific inhibitors that selectively target the inactive state. Comparison of this "auto-inhibitory" conformation with similar arrangements in cyclin-dependent kinase 2 and epidermal growth factor receptor kinase suggests a role for dimerization-dependent allosteric regulation that combines with autophosphorylation and protein phosphatase 1c phosphatase activity to generate the precise spatial and temporal control required for Nek2 function in centrosomal maturation.

Structure meets function—Centrosomes, genome maintenance and the DNA damage response

Centrosomes are cytoplasmic organelles playing a fundamental role in organizing both the interphase cytoskeleton and the bipolar mitotic spindle. In addition, the centrosome has recently come into focus as part of the network that integrates cell cycle arrest and repair signals in response to genotoxic stress--the DNA damage response. One important mediator of this response, the checkpoint kinase Chk1, has been shown to negatively regulate the G(2)/M transition via its centrosomal localization. Moreover, there is growing evidence that a centrosome inactivation checkpoint exists, which utilizes DNA damage-induced centrosome fragmentation or amplification to provoke a "mitotic catastrophe" and eliminate damaged cells. Candidate regulators of this centrosomal checkpoint include the checkpoint kinase Chk2 and its upstream regulators ATM and ATR. In addition, a growing number of other proteins have been implicated in centrosomal regulation of the DNA damage response, e.g. the tumor suppressor p53, the breast cancer susceptibility gene product BRCA1 and mitotic regulators such as Aurora A, Nek2 and the Polo-like kinases Plk1 and Plk3. However, many missing links and discrepancies between different model systems remain.

The centrosome and the DNA damage induced checkpoint

The centrosome, the microtubule-organizing center of the cell, acts as a localization point, where signaling molecules are able to interact. Many kinases and phosphatases critical for regulation of DNA damage signaling pathways localize to the centrosome. This review will discuss the possible involvement of the centrosome in mediating DNA damage checkpoint control, in particular the effect of DNA damage signaling pathways involved in initiation or maintenance of cell cycle arrest on the centrosome. The mechanisms that lead to centrosome abnormalities such as centrosome hyperamplification and multipolarity in response to DNA damage will also be addressed.

Caught Nek-ing: cilia and centrioles

The Nek family of cell-cycle kinases is widely represented in eukaryotes and includes numerous proteins that were described only recently and remain poorly characterized. Comparing Neks in the context of clades allows us to examine the question of whether microbial eukaryotic Neks, although not strictly orthologs of their vertebrate counterparts, can provide clues to ancestral functions that might be retained in the vertebrate Neks. Relatives of the Nek2/NIMA proteins play important roles at the G2-M transition in nuclear envelope breakdown and centromere separation. Nek6, Nek7 and Nek9 also seem to regulate mitosis. By contrast, Nek1 and Nek8 have been linked with polycystic kidney disease. Results of statistical analysis indicate that the family coevolved with centrioles that function as both microtubule-organizing centers and the basal bodies of cilia. This evolutionary perspective, taken together with functional studies of microbial Neks, provides new insights into the cellular roles of the proteins and disease with which some of them have been linked.

The role of Polo-like kinase 1 in the inhibition of centrosome separation after ionizing radiation

Activation of the G2/M cell cycle checkpoint by DNA damage prevents cells from entering mitosis. Centrosome separation is initiated in G2 phase and completed in M phase. This critical process for cell division is targeted by G2/M checkpoint. Here we show that Plk1 signaling plays an important role in regulation of centrosome separation after DNA damage. Constitutively active Plk1 overrides the inhibition of centrosome separation induced by DNA damage. This inhibition is dependent on ATM, but not on Chk2 or Chk1. Nek2 is a key regulator of centrosome separation and is a target of Plk1 in blocking centrosome separation. We found that Plk1 can phosphorylate Nek2 in vitro and interacts with Nek2 in vivo. Down-regulation of Plk1 with RNA interference prevents Nek2-induced centrosome splitting. DNA damage is known to inhibit Plk1 activity. We propose that the DNA damage-induced inhibition of Plk1 leads to inhibition of Nek2 activity and thus prevents centrosome separation.

Laboratory variability does not preclude identification of biological functions impacted by hydroxyurea

The multi-lab International Life Sciences Institute (ILSI) project on the application of genomics to risk assessment offered the unique opportunity to investigate the influence of variability within and between laboratories on identifying biologically relevant gene expression changes. We assessed the gene expression profiles of mouse lymphoma L5178Y cells treated with hydroxyurea (HU) in three independent studies from two different laboratories, Sanofi Aventis and Procter and Gamble. Cells were dosed for 4 hr and harvested immediately at the end of the treatment or after a 20-hr recovery period. Cytotoxicity and genotoxicity were evaluated by standard assays. Statistical analysis of these data revealed that, while gene expression responses to HU treatment were markedly different at 4 hr vs. 24 hr, there was otherwise a consistent pattern of dose-response across the three studies. Therefore, the studies were merged and each time point was evaluated separately. At 4 hr, we identified 173 (P < 0.0001) dose-responsive genes with a common trend in all three studies. These were mainly associated with the cell cycle, DNA repair and DNA metabolism, and in particular, the intra-S and G2/M phase checkpoints. At 24 hr, we identified 434 dose-responsive genes common across studies. These genes were involved in lymphocyte-specific activities and the activation of apoptosis via the caspase cascade. Our results show that despite inter-laboratory variability, combining the three studies in a single statistical analysis identifies more significantly-modulated genes than in any of the individual studies, due to improved statistical sensitivity. The genes identified in our study provide information that is relevant to HU biology.

Nek2 kinase in chromosome instability and cancer

Aneuploidy and chromosome instability are two of the most common abnormalities in cancer cells. They arise through defects in cell division and, specifically, in the unequal segregation of chromosomes between daughter cells during mitosis. A number of cell cycle dependent protein kinases have been identified that control mitotic progression and chromosome segregation. Some of these localize to the centrosome and regulate mitotic spindle formation. One such protein is Nek2, a member of the NIMA-related serine/threonine kinase family. Data are emerging that Nek2 is abnormally expressed in a wide variety of human cancers. In this review, we summarize current knowledge on the expression, regulation and function of Nek2, consider how Nek2 may contribute to chromosome instability, and ask whether it might make an attractive target for chemotherapeutic intervention in human cancer.

Live cell imaging reveals distinct roles in cell cycle regulation for Nek2A and Nek2B

Two splice variants of Nek2 kinase, a member of the NIMA-related family, have been identified as Nek2A and Nek2B. Nek2A regulates centrosome disjunction, spindle formation checkpoint signaling, and faithful chromosome segregation. A specific role for Nek2B has not yet been identified. Here, we have examined the distinct roles of Nek2A and Nek2B using timelapse video microscopy to follow the fate of cells progressing through the cell cycle in the absence of either Nek2A or Nek2B. We show that the down-regulation of Nek2B leads to a mitotic delay in the majority of cells. Upon exiting mitosis, cells exhibit mitotic defects such as the formation of multinucleated cells. Such phenotypes are not observed in cells that exit mitosis in the absence of Nek2A. These observations suggest that Nek2B may be required for the execution of mitotic exit.