Activation of Cdc2/cyclin B and inhibition of centrosome amplification in cells depleted of Plk1 by siRNA

Hamartin, the tuberous sclerosis complex 1 gene product, interacts with polo-like kinase 1 in a phosphorylation-dependent manner

Tuberous sclerosis complex (TSC) is a tumor suppressor gene syndrome caused by mutations in TSC1 and TSC2. Hamartin and tuberin, the products of TSC1 and TSC2, respectively, form heterodimers and inhibit the mammalian target of rapamycin. Previously, we have shown that hamartin is phosphorylated by CDC2/cyclin B1 during the G(2)/M phase of the cell cycle. Here, we report that hamartin is localized to the centrosome and that phosphorylated hamartin and phosphorylated tuberin co-immunoprecipitate with the mitotic kinase Plk1. Plk1 interacts with the N-terminus of hamartin (amino acids 1-880), which contains two potential Plk1-binding sites (T310 and S332). Phosphorylated hamartin interacts with Plk1 independent of tuberin with all three proteins present in a complex. A non-phosphorylatable hamartin mutant with an alanine substitution at residue T310 does not interact with Plk1, whereas a non-phosphorylatable hamartin mutant at residue S332 in conjunction with alanine mutations at the other CDC2/cyclin B1 sites (T417, S584 and T1047) does not impact hamartin binding to Plk1. Hamartin negatively regulates the protein levels of Plk1. Finally, Tsc1(-/-) mouse embryonic fibroblasts (MEFs) have increased number of centrosomes and increased DNA content, compared to Tsc1(+/+) cells. Both phenotypes are rescued after pre-treatment with the mTOR inhibitor rapamycin. RNAi inhibition of Plk1 in Tsc1(-/-) MEFs failed to rescue the increased centrosome number phenotype. These data reveal a novel subcellular localization for hamartin and a novel interaction partner for the hamartin/tuberin complex and implicate hamartin and mTOR in the regulation of centrosome duplication.

Inhibitory effect of Polo-like kinase 1 depletion on mitosis and apoptosis of gastric cancer cells

AIM

Polo-like kinase 1 (PLK1) serine/threonine kinase plays a vital role in multiple phases of mitosis in gastric cancer cells. To investigate the effect of PLK1 depletion on mitosis and apoptosis of gastric cancer cells.

METHODS

PLK1 expression was blocked by small RNA interference(siRNA). The expression levels of PLK1, cdc2, cyclin B and caspase 3 were detected by Western blotting. Then, PLK1 depletion, cdc2 activity, cell proliferation, cell cycle phase distribution, mitotic spindle structure, and the rate of apoptosis of the PLK1 knockdown cells were observed.

RESULTS

PLK1 gene knockdown was associated with increased cyclin B expression, increased cdc2 activity (but not with the expression levels), accumulation of gastric cancer cells at G2/M, improper mitotic spindle formation, delayed chromosome separation and delayed or arrested cytokinesis. Moreover, PLK1 depletion in gastric cancer cells was associated with decreased proliferation, attenuated pro-caspase 3 levels and increased apoptosis.

CONCLUSION

Blockage of PLK1 expression may lead to decreased mitosis or even apoptosis in gastric cancer cells, indicating that PLK1 may be a valuable therapeutic target for gastric cancer.

The centrosome cycle

Centrosomes are dynamic organelles involved in many aspects of cell function and growth. Centrosomes act as microtubule organizing centers, and provide a site for concerted regulation of cell cycle progression. While there is diversity in microtubule organizing center structure among eukaryotes, many centrosome components, such as centrin, are conserved. Experimental analysis has provided an outline to describe centrosome duplication, and numerous centrosome components have been identified. Even so, more work is needed to provide a detailed understanding of the interactions between centrosome components and their roles in centrosome function and duplication. Precise duplication of centrosomes once during each cell cycle ensures proper mitotic spindle formation and chromosome segregation. Defects in centrosome duplication or function are linked to human diseases including cancer. Here we provide a multifaceted look at centrosomes with a detailed summary of the centrosome cycle.

Different Plk1 functions show distinct dependencies on Polo-Box domain-mediated targeting

Polo-like kinase 1 (Plk1) has multiple important functions during M-phase progression. In addition to a catalytic domain, Plk1 possesses a phosphopeptide-binding motif, the polo-box domain (PBD), which is required for proper localization. Here, we have explored the importance of correct Plk1 subcellular targeting for its mitotic functions. We either displaced endogenous Plk1 through overexpression of the PBD or introduced the catalytic domain of Plk1, lacking the PBD, into Plk1-depleted cells. Both treatments resulted in remarkably similar phenotypes, which were distinct from the Plk1 depletion phenotype. Cells depleted of Plk1 mostly arrested with monoastral spindles, because of inhibition of centrosome maturation and separation. In contrast, these functions were not impaired in cells with mislocalized Plk1. Instead, these latter cells showed a checkpoint-dependent mitotic arrest characterized by impaired chromosome congression. Thus, whereas chromosome congression requires localized Plk1 activity, other investigated Plk1 functions are less dependent on correct PBD-mediated targeting. This opens the possibility that PBD-directed drugs might be developed to selectively interfere with a subset of Plk1 functions.

Phosphorylation by Cdk1 induces Plk1-mediated vimentin phosphorylation during mitosis

Several kinases phosphorylate vimentin, the most common intermediate filament protein, in mitosis. Aurora-B and Rho-kinase regulate vimentin filament separation through the cleavage furrow-specific vimentin phosphorylation. Cdk1 also phosphorylates vimentin from prometaphase to metaphase, but its significance has remained unknown. Here we demonstrated a direct interaction between Plk1 and vimentin-Ser55 phosphorylated by Cdk1, an event that led to Plk1 activation and further vimentin phosphorylation. Plk1 phosphorylated vimentin at approximately 1 mol phosphate/mol substrate, which partly inhibited its filament forming ability, in vitro. Plk1 induced the phosphorylation of vimentin-Ser82, which was elevated from metaphase and maintained until the end of mitosis. This elevation followed the Cdk1-induced vimentin-Ser55 phosphorylation, and was impaired by Plk1 depletion. Mutational analyses revealed that Plk1-induced vimentin-Ser82 phosphorylation plays an important role in vimentin filaments segregation, coordinately with Rho-kinase and Aurora-B. Taken together, these results indicated a novel mechanism that Cdk1 regulated mitotic vimentin phosphorylation via not only a direct enzyme reaction but also Plk1 recruitment to vimentin.

Polo-like kinase 1-mediated phosphorylation stabilizes Pin1 by inhibiting its ubiquitination in human cells

The Polo-like kinase 1 (Plk1) is a key regulator of mitosis. It is reported that the human peptidyl-prolyl cis/trans-isomerase Pin1 binds to Plk1 from mitotic cell extracts in vitro. Here we demonstrate that Ser-65 in Pin1 is the major site for Plk1-specific phosphorylation, and the polo-box domain of Plk1 is required for this phosphorylation. Interestingly, the phosphorylation of Pin1 by Plk1 does not affect its isomerase activity but rather is linked to its protein stability. Pin1 is ubiquitinated in HeLa S3 cells, and substitution of Glu for Ser-65 reduces the ubiquitination of Pin1. Furthermore, inhibition of Plk1 activity by expression of a dominant negative form of Plk1 or by transfection of small interfering RNA targeted to Plk1 enhances the ubiquitination of Pin1 and subsequently reduces the amount of Pin1 in human cancer cells. Since previous reports suggested that Plk1 is a substrate of Pin1, our work adds a new dimension to this interaction of two important mitotic regulators.

The Ste20-like kinase SLK is required for cell cycle progression through G2

We have previously shown that the Ste20-like kinase SLK is a microtubule-associated protein that can regulate actin reorganization during cell adhesion and spreading (Wagner, S., Flood, T. A., O'Reilly, P., Hume, K., and Sabourin, L. A. (2002) J. Biol. Chem. 277, 37685-37692). Because of its association with the microtubule network, we investigated whether SLK plays a role in cell cycle progression, a process that requires microtubule dynamics during mitosis. Consistent with microtubule association in exponentially growing cells, our results showed that SLK co-localizes with the mitotic spindle in cells undergoing mitosis. Expression of a kinase-inactive mutant or SLK small interfering RNAs inhibited cell proliferation and resulted in an accumulation of quiescent cells stimulated to re-enter the cell cycle in the G2 phase. Cultures expressing the mutant SLK displayed a normal pattern of cyclin D, E, and B expression but failed to down-regulate cyclin A levels, suggesting that they cannot proceed through M phase. In addition, these cultures displayed low levels of both phospho-H3 and active p34/cdc2 kinase. Overexpression of active SLK resulted in ectopic spindle assembly and the induction of cell cycle re-entry of Xenopus oocytes, suggesting that SLK is required for progression through G2 upstream of H1 kinase activation.

Polo-like kinase (Plk) 1 as a target for prostate cancer management

Prostate cancer (PCa) is the most commonly occurring cancer in American men, next to skin cancer. Existing treatment options and surgical intervention are unable to effectively manage this cancer. Therefore, continuing efforts are ongoing to establish novel mechanism-based targets and strategies for its management. The serine/threonine kinases Polo-like kinase (Plk) 1 plays a key role in mitotic entry of proliferating cells and regulates many aspects of mitosis which are necessary for successful cytokinesis. Plk1 is over-expressed in many tumor types with aberrant elevation frequently constituting a prognostic indicator of poor disease outcome. This review discusses the studies which indicate that Plk1 could be an excellent target for the treatment as well as chemoprevention of prostate cancer.

Centrosome Duplication and Nematodes: Recent Insights from an Old Relationship

Centrosome duplication is required for proper cell division, and centriole formation is a key step in this process. This review discusses recent studies in C. elegans that have identified five core proteins required for centriole formation, thus shedding light into the mechanisms underlying centrosome duplication in nematodes and beyond.

Centrosome-targeting region of CG-NAP causes centrosome amplification by recruiting cyclin E-cdk2 complex

Centrosome duplication occurs once per cell cycle and is thought to be triggered by cyclin E-cdk2. However, it is largely unknown how the duplication is regulated. Here, we found that the expression of the centrosome-targeting region of CG-NAP (centrosome and Golgi-localized PKN-associated protein), which we designate as CG-NAP/D, increased the number of centrosomes in Chinese hamster ovary (CHO)-K1 cells. The amplified centrosomes co-localized with centrosome markers gamma-tubulin, centrin-2 and kendrin as well as endogenous CG-NAP. When CG-NAP/D was dislocated from centrosomes by deleting the centrosome-targeting domain or by fusing with a membrane-targeting sequence, centrosome amplification was suppressed. CG-NAP/D interacted with exogenously expressed cyclin E, which co-localized at centrosomes. The immunoprecipitates of CG-NAP/D exhibited histone H1 kinase activity, suggesting the co-immunoprecipitation of active cyclin-cdk complexes. Furthermore, centrosome fractions prepared from cells expressing CG-NAP/D contained increased amount of cdk2 compared with those from control cells. Centrosome amplification by CG-NAP/D was suppressed by co-expression of a mutant cyclin E unable to interact with cdk2. These results suggest that CG-NAP/D causes centrosome amplification by anchoring excess amount of cyclin E-cdk2 to centrosomes and, possibly, CG-NAP participates in centrosome duplication by recruiting cyclin E-cdk2 to centrosomes in normal cell cycle.

Divergent roles of c-Src in controlling platelet-derived growth factor-dependent signaling in fibroblasts

The vast complexity of platelet-derived growth factor (PDGF)-induced downstream signaling pathways is well known, but the precise roles of critical players still elude us due to our lack of specific and temporal control over their activities. Accordingly, although Src family members are some of the better characterized effectors of PDGFbeta signaling, considerable controversy still surrounds their precise functions. To address these questions and limitations, we applied a chemical-genetic approach to study the role of c-Src at the cellular level, in defined signaling cascades; we also uncovered novel phosphorylation targets and defined its influence on transcriptional events. The spectacular control of c-Src on actin reorganization and chemotaxis was delineated by global substrate labeling and transcriptional analysis, revealing multiple cytoskeletal proteins and chemotaxis promoting genes to be under c-Src control. Additionally, this tool revealed the contrasting roles of c-Src in controlling DNA synthesis, where it transmits conflicting inputs via the phosphatidylinositol 3 kinase and Ras pathways. Finally, this study reveals a mechanism by which Src family kinases may control PDGF-mediated responses both at transcriptional and translational levels.

Inhibition of cyclin-dependent kinase 1 induces cytokinesis without chromosome segregation in an ECT2 and MgcRacGAP-dependent manner

Cleavage furrow formation marks the onset of cell division during early anaphase. The small GTPase RhoA and its regulators ECT2 and MgcRacGAP have been implicated in furrow ingression in mammalian cells, but the signaling upstream of these molecules remains unclear. We now show that the inhibition of cyclin-dependent kinase (Cdk)1 is sufficient to initiate cytokinesis. When mitotically synchronized cells were treated with the Cdk-specific inhibitor BMI-1026, the initiation of cytokinesis was induced precociously before chromosomal separation. Cytokinesis was also induced by the Cdk1-specific inhibitor purvalanol A but not by Cdk2/Cdk5- or Cdk4-specific inhibitors. Consistent with initiation of precocious cytokinesis by Cdk1 inhibition, introduction of anti-Cdk1 monoclonal antibody resulted in cells with aberrant nuclei. Depolymerization of mitotic spindles by nocodazole inhibited BMI-1026-induced precocious cytokinesis. However, in the presence of a low concentration of nocodazole, BMI-1026 induced excessive membrane blebbing, which appeared to be caused by formation of ectopic cleavage furrows. Depletion of ECT2 or MgcRacGAP by RNA interference abolished both of the phenotypes (precocious furrowing after nocodazole release and excessive blebbing in the presence of nocodazole). RNA interference of RhoA or expression of dominant-negative RhoA efficiently reduced both phenotypes. RhoA was localized at the cleavage furrow or at the necks of blebs. We propose that Cdk1 inactivation is sufficient to activate a signaling pathway leading to cytokinesis, which emanates from mitotic spindles and is regulated by ECT2, MgcRacGAP, and RhoA. Chemical induction of cytokinesis will be a valuable tool to study the initiation mechanism of cytokinesis.

Polo-like kinase 1 creates the tension-sensing 3F3/2 phosphoepitope and modulates the association of spindle-checkpoint proteins at kinetochores

BACKGROUND

In mitosis, a mechanochemical system recognizes tension that is generated by bipolar microtubule attachment to sister kinetochores. This is translated into multiple outputs including the stabilization of microtubule attachments, changes in kinetochore protein dynamics, and the silencing of the spindle checkpoint. How kinetochores sense tension and translate this into various signals represent critical unanswered questions. The kinetochores of chromosomes not under tension are specifically phosphorylated at an epitope recognized by the 3F3/2 monoclonal antibody. Determining the kinase that generates the 3F3/2 phosphoepitope at kinetochores should reveal an important component of this system that regulates mitotic progression.

RESULTS

We demonstrate that Polo-like kinase 1 (Plk1) creates the 3F3/2 phosphoepitope on mitotic kinetochores. In a permeabilized in vitro cell system, the depletion of Xenopus Plk1 from M phase extract leads to the loss of 3F3/2 kinase activity. Purified recombinant Plk1 is sufficient to generate the 3F3/2 phosphoepitope in this system. Using siRNA, we show that the reduction of Plk1 protein levels significantly diminishes 3F3/2 phosphoepitope expression at kinetochores. The consensus phosphorylation sites of Plk1 show strong similarity to the 3F3/2 phosphoepitope sequence determined by phosphopeptide mapping. The inhibition of Plk1 by siRNA alters the normal kinetochore association of Mad2, Cenp-E, Hec1/Ndc80, Spc24, and Cdc20 and induces a spindle-checkpoint-mediated mitotic arrest.

CONCLUSIONS

Plk1 generates the 3F3/2 phosphoepitope at kinetochores that are not under tension and contributes to the normal kinetochore association of several key proteins important in checkpoint signaling. Mechanical tension regulates Plk1 accumulation at kinetochores and possibly its kinase activity.

CCT chaperonin complex is required for the biogenesis of functional Plk1

Experiments from several different organisms have demonstrated that polo-like kinases are involved in many aspects of mitosis and cytokinesis. Here, we provide evidence to show that Plk1 associates with chaperonin-containing TCP1 complex (CCT) both in vitro and in vivo. Silencing of CCT by use of RNA interference (RNAi) in mammalian cells inhibits cell proliferation, decreases cell viability, causes cell cycle arrest with 4N DNA content, and leads to apoptosis. Depletion of CCT in well-synchronized HeLa cells causes cell cycle arrest at G(2), as demonstrated by a low mitotic index and Cdc2 activity. Complete depletion of Plk1 in well-synchronized cells also leads to G(2) block, suggesting that misfolded Plk1 might be responsible for the failure of CCT-depleted cells to enter mitosis. Moreover, partial depletion of CCT or Plk1 leads to mitotic arrest. Finally, the CCT-depleted cells reenter the cell cycle upon reintroduction of the purified constitutively active form of Plk1, indicating that Plk1 might be a CCT substrate.

Conditional inhibition of cancer cell proliferation by tetracycline-responsive, H1 promoter-driven silencing of PLK1

RNA interference (RNAi) is a powerful tool for studying gene function. We developed an inducible genetic element for short interfering RNA-mediated gene silencing. This system uses a tetracycline (Tet)-responsive derivative of the H1 promoter and the Tet repressor (TetR) for conditional expression of short hairpin RNA (shRNA) in HeLa cells. Promoter constructs were generated, which contain the Tet operator (TetO) derived from a prokaryotic Tet resistance transposon upstream and/or downstream of the TATA box. To quantify the response of controllable transcription units for shRNA expression, we examined the functional activity of polo-like kinase 1 (PLK1), a key component of mitotic progression, that is overexpressed in many human tumors. Cotransfection of plasmids for the expression of TetR and shRNA/PLK1 under the control of an H1 promoter-variant carrying TetO upstream of the TATA box did not alter PLK1 expression and proliferation properties of HeLa cells in the absence of doxycycline. Addition of the antibiotic led to marked downregulation of endogenous PLK1 accompanied by strong inhibition of cellular proliferation. Our data indicate that an inducible transcription system for shRNAs based on the human H1 promoter could be a versatile tool for controlled gene silencing in vitro.

Getting in and out of mitosis with Polo-like kinase-1

Research in different species has shown that Polo-like kinases are essential for successful cell division. In human cells, Polo-like kinase-1 (Plk1) has been implicated in the regulation of different processes, including mitotic entry, spindle formation and cytokinesis. Recently, a range of new downstream targets of Plk1 has been identified, as well as a molecular mechanism that explains recruitment of Plk1 to potential substrate proteins through its polo-box domain. On the basis of these reports, we discuss possible mechanisms by which Polo-like kinases can exert their multiple functions during mitosis. Polo-like kinases also function in DNA damage checkpoints. Plk1 has been shown to be a target of the G2 DNA damage checkpoint, while Cdc5, the Polo-like kinase in Saccharomyces cerevisiae, has long been known to be required for adaptation to persistent DNA damage. Just recently, a similar requirement for Polo-like kinases during checkpoint adaptation was demonstrated in multicellular organisms. Moreover, Plk1 was also shown to be required for checkpoint recovery following checkpoint inactivation, that is, in cells where the damage is completely repaired. Thus, Plk1 appears to play a role at multiple points during a restart of the cell cycle following DNA damage. Based on these novel observations, we discuss possible consequences of using Plk1 as a target in anticancer strategies.

Silencing of polo-like kinase (Plk) 1 via siRNA causes induction of apoptosis and impairment of mitosis machinery in human prostate cancer cells: implications for the treatment of prostate cancer

Prostate cancer (PCa) is one of the most common cancers in men. Each year approximately 543,000 new cases are reported worldwide, and the disease kills 200,000 (mostly older men) in developed countries. The existing treatment approaches and surgical intervention have not been able to effectively manage this dreaded cancer and, therefore, continuing efforts are ongoing to explore novel targets and strategies for the management of PCa. The activity of polo-like kinase 1 (Plk1) is elevated in tissues and cells with a high mitotic index, including cancer cells. An increasing body of evidence suggests that the level of Plk1 expression has prognostic value for predicting outcomes in patients with some cancers. A close correlation between Plk1 expression and carcinogenesis has been documented. However, the role of Plk1 in PCa is not known. We propagated a hypothesis that Plk1 inhibition will result in elimination of human PCa cells via a mitotic arrest followed by apoptosis (1). To define the role of Plk1 in PCa, we used the technique of RNA silencing via small interfering RNA (siRNA). First, using a series of human prostate carcinoma cells and normal human prostate epithelial (PrEC) cells, we assessed Plk1 levels in PCa. Immunoblot analyses clearly showed a significant expression of Plk1 in LNCaP, DU145, and PC3 human PCa cells. Interestingly, Plk1 was not detectable in normal PrEC cells. Next, we transfected the PCa cells with Plk 1 siRNA, which resulted in a significant inhibition in Plk1 protein in all PCa cells. Plk1 depletion resulted in a decrease in cell viability and induction of apoptosis in PCa cells but had no appreciable effect in normal PrEC cells. Our data also demonstrated that Plk1 siRNA transfection of PCa cells resulted in 1) a mitotic cell cycle arrest, 2) failure of cytokinesis, and 3) defects in centrosome integrity and maturation. Thus, our study suggested that 1) Plk1 plays a critical role in the process of PCa development and 2) gene therapeutic approaches aimed at Plk1 or the pharmacological inhibitors of Plk1 may be developed for the management of PCa.

Small interfering RNA-mediated Polo-like kinase 1 depletion preferentially reduces the survival of p53-defective, oncogenic transformed cells and inhibits tumor growth in animals

Polo-like kinase 1 (Plk1) is required for multiple stages of mitosis and is up-regulated in many human malignancies. We depleted Plk1 expression using small interfering RNA (siRNA) and showed defects in bipolar spindle formation and cytokinesis, growth inhibition, and apoptosis induction in human cancer cell lines. To our surprise, depletion of Plk1 in normal human cells did not result in obvious cell cycle defects, and did not induce significant inhibition of cell growth for at least two cell cycles. In addition, Plk1 siRNA inhibited colony formation in soft agar and tumorigenesis in a HT1080 xenograft model in a dose-dependent manner. Analysis with isogenic pairs of cell lines, differing in p53 status, revealed that Plk1 depletion preferentially induced mitotic arrest, aneuploidy, and reduced cell survival in the p53-defective cell lines. No obvious defects were observed in most p53 wild-type cells during the first few cell cycles. In addition, long-term survival studies revealed that p53 facilitates survival upon Plk1 depletion. Therefore, short-term inhibition of Plk1 can kill tumor cells while allowing normal cells to survive. These data validate the episodic inhibition of Plk1 as a very useful approach for cancer treatment.

Polo-like kinases and oncogenesis

Polo-like kinases (Plks) play pivotal roles in the regulation of cell cycle progression. Plk1, the best characterized family member among mammalian Plks, strongly promotes the progression of cells through mitosis. Furthermore, Plk1 is found to be overexpressed in a variety of human tumors and its expression correlates with cellular proliferation and prognosis of tumor patients. Although all Plks share two conserved elements, the N-terminal Ser/Thr kinase domain and a highly homologues C-terminal region termed the polo-box motif, their functions diverge considerably. While Plk1 is inhibited by different checkpoint pathways, Plk2 and Plk3 are activated by the spindle checkpoint or the DNA damage checkpoint. Thus, Plk2 and Plk3 seem to inhibit oncogenic transformation. Deregulation of Plk1 activity contributes to genetic instability, which in turn leads to oncogenic transformation. In contrast, Plk2 and Plk3 are involved in checkpoint-mediated cell cycle arrest to ensure genetic stability, thereby inhibiting the accumulation of genetic defects. In this review, we shall discuss the roles of Plks in oncogenesis and Plk1 as a target for therapeutic intervention against cancer.

Xenopus Polo-like kinase Plx1: a multifunctional mitotic kinase

The Xenopus Polo-like kinase Plx1 plays multiple roles in mitosis. Accumulating evidence shows that Plx1 is the trigger kinase for the G2/M transition that phosphorylates and activates the phosphatase Cdc25C, which subsequently dephosphorylates Cdc2/cyclin B and initiates a positive feedback loop between Cdc25C and Cdc2/cyclin B. Recent findings indicate that Plx1 itself is also in a positive feedback loop. It phosphorylates and activates the protein kinase xPlkk1, which itself then phosphorylates and further activates Plx1. Plx1 functions on the centrosome to promote bipolar spindle formation. Plx1 associates with the anaphase-promoting complex/cyclosome (APC/C) and is required to activate the APC/C for degradation of mitotic regulators required for sister chromatid separation and exit from mitosis. Plx1 is also required for cytokinesis and is localized on the midbody of the contractile ring. All known functions of Plx1 require not only its kinase activity but also an intact polo box domain in the C-terminus.

Regulation of cell cycle checkpoints by polo-like kinases

Protein kinases play a pivotal role in execution of cell division. Polo and Polo-like kinases have emerged as major regulators for various cell cycle checkpoints. Early genetic studies have demonstrated that CDC5, a budding yeast counterpart of vertebrate Plks, is essential for successful mitotic progression. Mammalian Plks localize primarily to the centrosome during interphase and the mitotic apparatus during mitosis. Many key cell cycle regulators such as p53, Cdc25C, cyclin B, components of the anaphase-promoting complex, and mitotic motor proteins are directly targeted by Plks. Although the exact mechanism of action of these protein kinases in vivo remains to be elucidated, Plks are important mediators for various cell cycle checkpoints that monitor centrosome duplication, DNA replication, formation of bipolar mitotic spindle, segregation of chromosomes, and mitotic exit, thus protecting cells against genetic instability during cell division.

Polo-like kinases (Plks) and cancer

Deregulated centrosome duplication or maturation often results in increased centrosome size and/or centrosome number, both of which show a positive and significant correlation with aneuploidy and chromosomal instability, thus contributing to cancer formation. Given the role of Polo-like kinases (Plks) in the centrosome cycle, it is not unexpected that deregulated expression of Plks is detected in many types of cancer and is associated with oncogenesis. Extensive studies have shown that Plk1 expression is elevated in non-small-cell lung cancer, head and neck cancer, esophageal cancer, gastric cancer, melanomas, breast cancer, ovarian cancer, endometrial cancer, colorectal cancer, gliomas, and thyroid cancer. Plk1 gene and protein expression has been proposed as a new prognostic marker for many types of malignancies, and Plk1 is a potential target for cancer therapy. In contrast to Plk1, several studies have observed that Plk3 expression is negatively correlated with the development of certain cancers.

Deficiency in SNM1 abolishes an early mitotic checkpoint induced by spindle stress

Spindle poisons represent an important class of anticancer drugs that act by interfering with microtubule polymerization and dynamics and thereby induce mitotic checkpoints and apoptosis. Here we show that mammalian SNM1 functions in an early mitotic stress checkpoint that is distinct from the well-characterized spindle checkpoint that regulates the metaphase-to-anaphase transition. Specifically, we found that compared to wild-type cells, Snm1-deficient mouse embryonic fibroblasts exposed to spindle poisons exhibited elevated levels of micronucleus formation, decreased mitotic delay, a failure to arrest in mitosis prior to chromosome condensation, supernumerary centrosomes, and decreased viability. In addition, we show that both Snm1 and 53BP1, previously shown to interact, coimmunoprecipitate with components of the anaphase-promoting complex (APC)/cyclosome. These findings suggest that Snm1 is a component of a mitotic stress checkpoint that negatively targets the APC prior to chromosome condensation.

Human immunodeficiency virus type 1 Vpr-mediated G2 arrest requires Rad17 and Hus1 and induces nuclear BRCA1 and gamma-H2AX focus formation

Eukaryotic cells have evolved a complex mechanism for sensing DNA damage during genome replication. Activation of this pathway prevents entry into mitosis to allow for either DNA repair or, in the event of irreparable damage, commitment to apoptosis. Under conditions of replication stress, the damage signal is initiated by the ataxia-telangiectasia-mutated and Rad3-related kinase ATR. We recently demonstrated that the human immunodeficiency virus type 1 (HIV-1) gene product viral protein R (Vpr) arrests infected cells in the G(2) phase via the activation of ATR. In the present study, we show that the activation of ATR by Vpr is analogous to activation by certain genotoxic agents, both mechanistically and in its downstream consequences. Specifically, we show a requirement for Rad17 and Hus1 to induce G(2) arrest as well as Vpr-induced phosphorylation of histone 2A variant X (H2AX) and formation of nuclear foci containing H2AX and breast cancer susceptibility protein 1. These results demonstrate that G(2) arrest mediated by the HIV-1 gene product Vpr utilizes the cellular signaling pathway whose physiological function is to recognize replication stress. These findings should contribute to a greater understanding of how HIV-1 manipulates the CD4(+)-lymphocyte cell cycle and apoptosis induction in the progressive CD4(+)-lymphocyte depletion characteristic of HIV-1 pathogenesis.

Suppression of Nek2A in mouse early embryos confirms its requirement for chromosome segregation

Nek2, a mammalian structural homologue of Aspergillus protein kinase NIMA, is predominantly known as a centrosomal kinase that controls centriole-centriole linkage during the cell cycle. However, its dynamic subcellular localization during mitosis suggested that Nek2 might be involved in diverse cell cycle events in addition to the centrosomal cycle. In order to determine the importance of Nek2 during mammalian development, we investigated the expression and function of Nek2 in mouse early embryos. Our results show that both Nek2A and Nek2B were expressed throughout early embryogenesis. Unlike cultured human cells, however, embryonic Nek2A appeared not to be destroyed upon entry into mitosis, suggesting that the Nek2A protein level is controlled in a unique manner during mouse early embryogenesis. Suppression of Nek2 expression by RNAi resulted in developmental defects at the second mitosis. Many of the blastomeres in Nek2-suppressed embryos showed abnormality in nuclear morphology, including dumbbell-like nuclei, nuclear bridges and micronuclei. These results indicate the importance of Nek2 for proper chromosome segregation in embryonic mitoses.

Polo-like kinase-2 is required for centriole duplication in mammalian cells

Centriole duplication initiates at the G1-to-S transition in mammalian cells and is completed during the S and G2 phases. The localization of a number of protein kinases to the centrosome has revealed the importance of protein phosphorylation in controlling the centriole duplication cycle. Here we show that the human Polo-like kinase 2 (Plk2) is activated near the G1-to-S transition of the cell cycle. Endogenous and overexpressed HA-Plk2 localize with centrosomes, and this interaction is independent of Plk2 kinase activity. In contrast, the kinase activity of Plk2 is required for centriole duplication. Overexpression of a kinase-deficient mutant under S-phase arrest blocks centriole duplication. Downregulation of endogenous Plk2 with small hairpin RNAs interferes with the ability to reduplicate centrioles. Furthermore, centrioles failed to duplicate during the cell cycle of human fibroblasts and U2OS cells after overexpression of a Plk2 dominant-negative mutant. These results show that Plk2 is a physiological centrosomal protein and that its kinase activity is likely to be required for centriole duplication near the G1-to-S phase transition.

Filtering of Ineffective siRNAs and Improved siRNA Design Tool

MOTIVATION

Short interfering RNAs (siRNAs) can be used to suppress gene expression and possess many potential applications in therapy, but how to design an effective siRNA is still not clear. Based on the MPI (Max-Planck-Institute) basic principles, a number of siRNA design tools have been developed recently. The set of candidates reported by these tools is usually large and often contains ineffective siRNAs. In view of this, we initiate the study of filtering ineffective siRNAs.

RESULTS

The contribution of this paper is 2-fold. First, we propose a fair scheme to compare existing design tools based on real data in the literature. Second, we attempt to improve the MPI principles and existing tools by an algorithm that can filter ineffective siRNAs. The algorithm is based on some new observations on the secondary structure, which we have verified by AI techniques (decision trees and support vector machines). We have tested our algorithm together with the MPI principles and the existing tools. The results show that our filtering algorithm is effective.

AVAILABILITY

The siRNA design software tool can be found in the website http://www.cs.hku.hk/~sirna/

CONTACT

smyiu@cs.hku.hk

Polo-like kinase-1 is required for bipolar spindle formation but is dispensable for anaphase promoting complex/Cdc20 activation and initiation of cytokinesis

Polo-like kinase-1 (Plk1) performs multiple essential functions during the cell cycle. Here we show that human Plk1-deficient cells are unable to separate their centrosomes, fail to form a bipolar spindle, and undergo a Mad2/BubR1-dependent prometaphase arrest. However, electron microscopy demonstrates that kinetochore-microtubule interactions can be established in cells lacking Plk1. In addition, co-depletion of Plk1 and survivin allows mitotic exit. This indicates that Plk1 depletion does not prevent microtubule attachment, but specifically interferes with the generation of tension, as a consequence of a failure to form a bipolar spindle. Moreover, we find that after silencing of the spindle assembly checkpoint, degradation of cyclin B1 is unaffected in cells lacking Plk1. These data indicate that activation of the anaphase promoting complex or cyclosome (APC/C)-Cdc20 complex that is under control of the spindle assembly checkpoint does not require Plk1 activity. Finally, we find that translocation of chromosome passengers and initiation of cleavage furrow ingression is unaffected in cells depleted of Plk1. Thus, our data confirm an important role of Plk1 in bipolar spindle formation, and also demonstrate that Plk1 is dispensable for APC/C-Cdc20 activation and the initiation of cytokinesis.

B23/nucleophosmin serine 4 phosphorylation mediates mitotic functions of polo-like kinase 1

Phosphoprotein profiling by Kinetworks trade mark analysis of M-phase-arrested HeLa cells by nocodazole treatment revealed that a novel mitosis-specific phosphorylation event occurred in the nucleolar protein B23/nucleophosmin at a conserved Ser-4 residue. Consistent with the resemblance of the Ser-4 phosphorylation site to the Polo-like kinase 1 (Plk1) consensus recognition sequence, inhibition of Plk1 by a kinase-defective mutation (K82M) abrogated B23 Ser-4 phosphorylation, whereas activation of Plk1 by a constitutively active mutation (T210D) enhanced its phosphorylation following in vivo transfection and in vitro phosphorylation assays. Depletion of endogenous Plk1 by RNA interference abolished B23 Ser-4 phosphorylation. The physical interaction of Plk1 and B23 was further demonstrated by their co-immunoprecipitation and glutathione S-transferase fusion protein pull-down assays. Interference of Ser-4 phosphorylation of B23 induced multiple mitotic defects in HeLa cells, including aberrant numbers of centrosomes, elongation and fragmentation of nuclei, and incomplete cytokinesis. The phenotypes of B23 mutants are reminiscent of a subset of those described previously in Plk1 mutants. Our findings provide insights into the biochemical mechanism underlying the role of Plk1 in mitosis regulation through the identification of Ser-4 in B23 as a major physiological substrate of Plk1.

Cyclin A/Cdk2 complexes regulate activation of Cdk1 and Cdc25 phosphatases in human cells

Mitotic entry, a critical decision point for maintaining genetic stability, is governed by the cyclin B/Cyclin dependent kinase 1 (Cdc2) complex. In Xenopus oocytes and early embryos, accumulation of cyclin B activates Cdk1, which then phosphorylates and activates the positive regulator Cdc25 in an autocatalytic feedback loop. However, cyclin B levels do not increase as some human cells approach mitosis, and the key factors regulating Cdk1 activation in human cells are unknown. We report here that reducing cyclin A expression by RNA interference (RNAi) in primary human fibroblasts inhibited activation of Cdc25B and Cdc25C and dephosphorylation of Cdk1 on tyrosine (tyr) 15. These results were reproduced in U2-OS cells by inducing the expression of a dominant-negative (dn) mutant of Cdk2, the principal cyclin A binding partner. Cdk2-dn induction could inhibit Cdc25B activity and foster Cdk1 tyr phosphorylation within the S phase, temporally dissociating these events from Cdk1 activation at mitosis. In contrast, reducing Cdk1 expression delayed mitotic entry without markedly impairing Cdc25B or Cdc25C activity. These results suggest that cyclin A/Cdk2 complexes are key regulators of Cdc25 and Cdk1 activation in human cells. This pathway appears to be commonly deregulated in cancer.

Stat3 activity is required for centrosome duplication in chinese hamster ovary cells

The centrosome is the major microtubule organizing center in mammalian cells. During interphase, the single centrosome is duplicated and the progeny centrosomes then serve as the spindle poles during mitosis. Little is known about the signals that drive centrosome doubling. In these studies, various inhibitors and molecular approaches were used to demonstrate a role for the Stat pathway in regulating the events of centrosome doubling. Both piceatannol and a dominant negative behaving Stat3 adenovirus were able to disrupt centrosome duplication in hydroxyurea-arrested Chinese hamster ovary cells, demonstrating that Stat3 is a key signaling molecule in the events of centrosome duplication. Investigation into the role of Stat3 signaling during centrosome production demonstrated that Stat3 does not directly regulate the transcription of the centrosome genes encoding gamma-tubulin and PCM-1. Instead, Stat3 apparently regulated gamma-tubulin levels through post-transcriptional mechanisms whereas PCM-1 levels actually increased when Stat3 was inhibited, suggesting more complex mechanisms for regulating PCM-1 production. These studies demonstrate that Stat3 plays a vital role in centrosome duplication events, although the downstream targets of Stat3 activation leading to centrosome production remain to be established. The proposed signaling pathway utilizes Stat3 as a fundamental signaling molecule that directs the production of the various centrosome proteins indirectly.

Many commonly used siRNAs risk off-target activity

Using small interfering RNA (siRNA) to induce sequence specific gene silencing is fast becoming a standard tool in functional genomics. As siRNAs in some cases tolerate mismatches with the mRNA target, knockdown of genes other than the intended target could make results difficult to interpret. In an investigation of 359 published siRNA sequences, we have found that about 75% of them have a risk of eliciting non-specific effects. A possible cause for this is the popular BLAST search engine, which is inappropriate for such short oligos as siRNAs. Furthermore, we used new special purpose hardware to do a transcriptome-wide screening of all possible siRNAs, and show that many unique siRNAs exist per target even if several mismatches are allowed. Hence, we argue that the risk of off-target effects is unnecessary and should be avoided in future siRNA design.

Molecular interactions of Polo-like-kinase 1 with the mitotic kinesin-like protein CHO1/MKLP-1

Polo-like kinases and kinesin-like motor proteins are among the many proteins implicated in the execution of cytokinesis. Polo-like-kinase 1 (Plk1) interacts with the mitotic kinesin-like motor protein CHO1/MKLP-1 during anaphase and telophase, and CHO1/MKLP-1 is a Plk1 substrate in vitro. Here, we explore the molecular interactions of these two key contributors to mitosis and cytokinesis. Using the transient transfection approach, we show that the C-terminus of Plk1 binds CHO1/MKLP-1 in a Polo-box-dependent manner and that the stalk domain of CHO1/MKLP-1 is responsible for its binding to Plk1. The stalk domain was found to localize with Plk1 to the mid-body, and Plk1 appears to be mislocalized in CHO1/MKLP-1-depleted cells during late mitosis. We showed that Ser904 and Ser905 are two major Plk1 phosphorylation sites. Using the vector-based RNA interference approach, we showed that depletion of CHO1/MKLP-1 causes the formation of multinucleate cells with more centrosomes, probably because of a defect in the early phase of cytokinesis. Overexpression of a non-Plk1-phosphorylatable CHO1 mutant caused cytokinesis defects, presumably because of dominant negative effect of the construct. Finally, CHO1-depletion-induced multinucleation could be partially rescued by co-transfection of a non-degradable hamster wild-type CHO1 construct, but not an unphosphorylatable mutant. These data provide more detailed information about the interaction between Plk1 and CHO1/MKLP-1, and the significance of this is discussed.

OptiRNAi, an RNAi design tool

RNA interference (RNAi), a recently developed reverse genetics tool, has many advantages compared to traditional gene knockout methods. Appropriate selection of double stranded RNAs identical to a specific region(s) of the target gene is critical for the successful implementation of this technology. Recently, Elbashir et al. [Methods 26 (2002) 199] has established empirical criteria for siRNA sequence selection that significantly improved the success rate for RNAi attempts. We have developed OptiRNAi, a computational tool, which uses the Elbashir et al. criteria to predict appropriate target sequences for siRNA production. Specificity of these siRNAs for the target of interest can then be assessed by the investigator using the embedded Blast search engine optimized for RNAi design. Thus, OptiRNAi is an efficient and user friendly tool for RNAi design based on criteria that are more stringent than other available tools.

Role of Polo-like kinase in the degradation of early mitotic inhibitor 1, a regulator of the anaphase promoting complex/cyclosome

Early mitotic inhibitor 1 (Emi1) inhibits the activity of the anaphase promoting complex/cyclosome (APC/C), which is a multisubunit ubiquitin ligase that targets mitotic regulators for degradation in exit from mitosis. Levels of Emi1 oscillate in the cell cycle: it accumulates in the S phase and is rapidly degraded in prometaphase. The degradation of Emi1 in early mitosis is necessary for the activation of APC/C in late mitosis. Previous studies have shown that Emi1 is targeted for degradation in mitosis by a Skp1-Cullin1 F-box protein (SCF) ubiquitin ligase complex that contains the F-box protein beta-TrCP. As with other substrates of SCF(beta-TrCP), the phosphorylation of Emi1 on a DSGxxS sequence is required for this process. However, the protein kinase(s) involved has not been identified. We find that Polo-like kinase 1 (Plk1), a protein kinase that accumulates in mitosis, markedly stimulates the ligation of Emi1 to ubiquitin by purified SCF(beta-TrCP). Cdk1-cyclin B, another major mitotic protein kinase, has no influence on this process by itself but stimulates the action of Plk1 at low, physiological concentrations. Plk1 phosphorylates serine residues in the DSGxxS sequence of Emi1, as suggested by the reduced phosphorylation of a derivative in which the two serines were mutated to nonphosphorylatable amino acids. Transfection with an small interfering RNA duplex directed against Plk1 caused the accumulation of Emi1 in mitotically arrested HeLa cells. It is suggested that phosphorylation of Emi1 by Plk1 is involved in its degradation in mitosis.

The polo box is required for multiple functions of Plx1 in mitosis

Polo-like kinases comprise a family of evolutionarily conserved serine/threonine protein kinases that play multiple roles in cell cycle regulation. In addition to the N-terminal catalytic domain, polo-like kinases have one or two highly conserved C-terminal non-catalytic regions, termed polo boxes. These motifs are required for targeting these kinases to subcellular mitotic structures. Here we report that kinase-dead Xenopus polo-like kinase (Plx1NA) functions as a competitor of endogenous Plx1 for polo box binding site(s) and inhibits the activation of Cdc25C and the G(2)-M transition in vivo. However, kinase-dead Plx1 with a point mutation in the polo box region (Plx1NAWF) did not have inhibitory effects. The ability of Plx1NA to block activation of the anaphase-promoting complex/cyclosome also requires an intact polo box. Microinjection of Plx1NA but not Plx1NAWF mRNA into Xenopus embryos caused cleavage arrest and formation of monopolar spindles, an effect previously seen in embryos injected with anti-Plx1 antibody. Spindle assembly experiments in vitro also showed that only monopolar spindles formed in Xenopus egg extracts supplemented with recombinant Plx1NA and that the spindle assembly process was delayed. Taken together, these results indicate that the polo box is required for Plx1 function in both the G(2)-M and the metaphase/anaphase transitions during the cell cycle.

Polo-like kinase 3 is Golgi localized and involved in regulating Golgi fragmentation during the cell cycle

The Golgi apparatus undergoes extensive fragmentation during mitosis in animal cells. Protein kinases play a critical role during fragmentation of the Golgi apparatus. We reported here that Polo-like kinase 3 (Plk3) may be an important mediator during Golgi breakdown. Specifically, Plk3 was concentrated at the Golgi apparatus in HeLa and A549 cells during interphase. At the onset of mitosis, Plk3 signals disintegrated and redistributed in a manner similar to those of Golgi stacks. Nocodazole activated Plk3 kinase activity, correlating with redistribution of Plk3 signals and Golgi fragmentation. In addition, treatment with brefeldin A (BFA), a Golgi-specific poison, also resulted in disappearance of concentrated Plk3 signals. Plk3 interacted with giantin, a Golgi-specific protein. Expression of Plk3, but not the kinase-defective Plk3 (Plk3(K52R)), resulted in significant Golgi breakdown. Given its role in cell cycle progression, Plk3 may be a protein kinase involved in regulation of Golgi fragmentation during the cell cycle.

Dysregulation of the polo-like kinase pathway in CD4+ T cells is characteristic of pathogenic simian immunodeficiency virus infection

CD4(+) T-cell dysfunction highlighted by defects within the intracellular signaling cascade and cell cycle has long been characterized as a direct and/or indirect consequence of human immunodeficiency virus (HIV) infection in humans and simian immunodeficiency virus (SIV) infection in rhesus macaques (RM). Dysregulation of the M phase of the cell cycle is a well-documented effect of HIV or SIV infection both in vivo and in vitro. In this study the effect of SIV infection on the modulation of two important regulators of the M phase-polo-like kinases Plk3 and Plk1-was investigated. We have previously shown that Plk3 is markedly downregulated in CD4(+) T cells from SIV-infected disease-susceptible RM but not SIV-infected disease-resistant sooty mangabeys (SM), denoting an association of downregulation with disease progression. Here we show that, in addition to the downregulation, Plk3 exhibits aberrant activation patterns in the CD4(+) T cells from SIV-infected RM following T-cell receptor stimulation. Interestingly, in vitro SIV infection of CD4(+) T cells leads to the upregulation, rather than downregulation, of Plk3, suggesting that different mechanisms operate in vitro and in vivo. In addition, CD4(+) T cells from RM with high viral loads exhibited consistent and significant upregulation of Plk1, concurrent with an aberrant activation-induced Plk1 response, suggesting complex mechanisms of SIV-induced M-phase abnormalities in vivo. Altogether this study presents a novel mechanism underlying M-phase defects observed in CD4(+) T cells from HIV or SIV-infected disease-susceptible humans and RM which may contribute to aberrant T-cell responses and disease pathogenesis.

The human small glutamine-rich TPR-containing protein is required for progress through cell division

Eukaryotic organisms from yeast to human harbor genes encoding the small glutamine-rich tetratricopeptide repeat-containing (SGT) protein. Work presented here demonstrated the presence of human SGT (hSGT) protein in a panel of human cell lines and throughout the cell cycle. To identify cellular processes in which hSGT is involved, knock down populations were analyzed which were generated through transfection of hsgt-specific small interfering RNA. Most strikingly, depletion of hSGT led to reduced proliferation of the affected cell populations while the mitotic index was increased. Time-lapse video microscopy revealed that cells from hSGT-depleted populations were unable to complete cell division due to mitotic arrest which was frequently followed by cell death. Further evidence for a role in cell division was given by the accumulation of hSGT in the midzone and the midbody, and by a mitosis-specific migration pattern of hSGT as detected by Western blotting after SDS-PAGE or two-dimensional gel electrophoresis. In conclusion, results obtained in this study demonstrate that hSGT protein is a constitutive component of all human cell lines tested and that this protein is essential for successful completion of cell division.

The MAP kinase pathway is required for entry into mitosis and cell survival

In this communication, we examined the role of the MAP kinase pathway in the G2/M phase of the cell cycle. Activation of the Plk1 and MAP kinase pathways was initially evaluated in FT210 cells, which arrest at G2 phase at the restrictive temperature (39 degrees C), due to a mutation in the cdc2 gene. Previous studies had shown that these cells enter mitosis at the nonpermissive temperature upon incubation with okadaic acid, a protein phosphatase 1 and 2A inhibitor. We show that treatment of FT210 cells at 39 degrees C with okadaic acid activated Plk1, as shown by hyperphosphorylation and elevated protein kinase activity, and also induced activation of the MAP kinase pathway. The specific Mek inhibitor PD98059 antagonized the okadaic acid-induced activation of both Plk1 and MAP kinases. This suggests that activation of the MAP kinase pathway may contribute to the okadaic acid-induced activation of Plk1 in FT210 cells at 39 degrees C. We also found that PD98059 strongly attenuated progression of HeLa cells through mitosis, and active Mek colocalizes with Plk1 at mitotic structures. To study the potential function of the MAP kinase pathway during mitosis, RNAi was used to specifically deplete five members of this pathway (Raf1, Mek1/2, Erk1/2). Each of these five protein kinases is required for cell proliferation and survival, and depletion of any of these proteins eventually leads to apoptosis. Treatment with Mek inhibitors also inhibited cell proliferation and caused apoptosis. A dramatic increase of Plk1 activities and a moderate increase of Cdc2 activities in Raf1-depleted cells indicate that Raf1-depleted cells arrest in the late G2 or M phase. Mek1 and Erk1 depletion also caused cell cycle arrest at G2, suggesting that these enzymes are required for the G2/M transition, whereas the loss of Mek2 or Erk2 caused arrest at G1.

Polo-like kinase (Plk) 1: a novel target for the treatment of prostate cancer

Second only to skin cancer, cancer of the prostate gland (CaP) is the most commonly occurring cancer in American men. Existing treatment approaches and surgical intervention have been unable to effectively manage this dreaded cancer; therefore, efforts are ongoing to explore novel targets and strategies for the management of CaP. A complete understanding of the genetic control of the processes of cellular proliferation and programmed cell death, or "apoptosis," may provide the basis for the rational design of novel therapeutic strategies against CaP. Key regulators for the mitotic progression in mammalian cells are the polo-like kinases (Plks). The activity of Plk1 is elevated in tissues and cells with a high mitotic index, including cancer cells. An increasing body of evidence suggests that the level of Plk1 expression has prognostic value for predicting outcomes in patients with some cancers such as lung cancer, squamous cell carcinomas of the head and neck, melanomas, and ovarian and endometrial carcinomas. However, the role of Plk1 in CaP is not known. Here, a hypothesis is put forward that Plk 1 plays a critical role in the development of prostate cancer; and the silencing of Plk1 will result in elimination of human CaP cells via an inactivation of cyclin-dependent kinase 1 (Cdc2)/cyclin B 1-mediated mitotic arrest followed by apoptosis. A corollary to this hypothesis is that Plk1 could serve as a target for the intervention of CaP in humans. Therefore, if the hypothesis is tested to be true, it is conceivable that gene therapeutic approaches aimed at Plk1 or the pharmacological inhibitors of Plk1 may be developed for the treatment/management of CaP.

Small interfering RNA (siRNA) inhibits the expression of the Her2/neu gene, upregulates HLA class I and induces apoptosis of Her2/neu positive tumor cell lines

Silencing of a specific mRNA using double stranded RNA oligonucleotides represents one of the newest technologies for suppressing a specific gene product. Small interfering RNA (siRNA) are 21 nucleotides long, double stranded RNA fragments that are identical in sequence to the target mRNA. We designed 3 such siRNA against the Her2/neu (HER2) gene. The HER2 gene is known to play an important role in the oncogenesis of several types of cancers, such as breast, ovarian, colon and gastric cancers. Introduction of the siRNA into HER2 positive tumor lines in vitro greatly reduced the cell surface expression of the HER2 protein. Concurrently, a range of effects on cell physiology, such as growth inhibition or apoptosis, was observed. The expression of HLA class I was observed to be upregulated when HER2 was silenced with siRNA. Treatment of SKBr3 and MCF7/HER2 tumor cell lines with the HER2 siRNA resulted in growth arrest of cells in the late G(1)/S-phase. Our results suggest that siRNA may be an effective method of abrogating the effect of HER2 in tumorigenesis.

RNA interference in biology and medicine

First discovered in plants the nematode Caenorhabditis elegans, the production of small interfering RNAs (siRNAs) that bind to and induce the degradation of specific endogenous mRNAs is now recognized as a mechanism that is widely employed by eukaryotic cells to inhibit protein production at a post-transcriptional level. The endogenous siRNAs are typically 19- to 23-base double-stranded RNA oligonucleotides, produced from much larger RNAs that upon binding to target mRNAs recruit RNases to a protein complex that degrades the targeted mRNA. Methods for expressing siRNAs in cells in culture and in vivo using viral vectors, and for transfecting cells with synthetic siRNAs, have been developed and are being used to establish the functions of specific proteins in various cell types and organisms. RNA interference methods provide several major advantages over prior methods (antisense DNA or antibody-based techniques) for suppressing gene expression. Recent preclinical studies suggest that RNA interference technology holds promise for the treatment of various diseases. Pharmacologists have long dreamed of the ability to selectively antagonize or eliminate the function of individual proteins--RNAi technology may eventually make that dream a reality.

Essential role of junctional adhesion molecule-1 in basic fibroblast growth factor-induced endothelial cell migration

OBJECTIVE

Recently, we have shown that blocking of junctional adhesion molecule-1/A (JAM-1/A) inhibits basic fibroblast growth factor (bFGF)-induced angiogenesis. Because the process of endothelial cell proliferation is a key initial step of neovascularization, we studied the effect of functional knockdown of JAM-1 on human umbilical vein endothelial cell (HUVEC) adhesion and migration induced by bFGF.

METHODS AND RESULTS

We introduced small interfering RNAs specific to JAM-1 in HUVECs, stimulated them with bFGF, and studied the resultant adhesion and migration of these cells on vitronectin and fibronectin. We show that depletion of JAM-1 inhibits bFGF-induced HUVEC migration specifically on vitronectin. This inhibition is not attributable to the failure of junctional organization, because expression and distribution of other junctional proteins remained unaffected. This inhibition was in fact attributed to an inability of JAM-1-depleted HUVECs to adhere and spread on vitronectin. Furthermore, we find that JAM-1-depleted HUVECs failed to activate extracellular signal-related kinase (ERK) in response to bFGF treatment.

CONCLUSIONS

Our results show that JAM-1 is required for the bFGF-induced ERK activation that leads to endothelial cell migration on vitronectin. These data thus implicate JAM-1 as an integral part of both bFGF and ERK signaling pathways in endothelial cells.

Identification of common or distinct genes related to antitumor activities of a medicinal herb and its major component by oligonucleotide microarray

Although the physiological actions of many herbs are gradually being elucidated at the molecular level, it remains unclear how individual components of herbs contribute to their biological activities. In the present study, the antiproliferative activity of Coptidis rhizoma, a medicinal herb, and the major component berberine was investigated in 8 human pancreatic cancer cell lines. Gene expression patterns associated with sensitivities to each agent were analyzed with oligonucleotide arrays that comprised approximately 11,000 genes. We used a tetrazolium dye (MTT) assay to determine ID(50) values after the 8 cell lines were exposed to the 2 agents for 72 hr. The ID(50) value for berberine was correlated positively with that for C. rhizoma (r=0.725, p=0.0401). C. rhizoma killed tumor cells more effectively than purified berberine when normalized to the level of berberine present in the herb. From the oligonucleotide array data, we selected 20 and 13 genes with strong correlations (r(2)>0.81) to ID(50) values for berberine and C. rhizoma, respectively. Among these 33 genes, the levels of expression of 12 were correlated with the ID(50) values of both agents, suggesting that these genes are associated with tumor-killing activity of berberine in C. rhizoma. Expression of the remaining 21 genes was correlated with the ID(50) value of either purified berberine or C. rhizoma. Thus, we identified common and distinct genes responsible for anti-proliferative activities of purified berberine and C. rhizoma. This strategy may improve our understanding of the actions of herbs with antitumor activities.

Role of Plk2 (Snk) in mouse development and cell proliferation

Plk2 (Snk) is a polo-like kinase expressed at G(1) in cultured cells and mainly in the hippocampal neurons in the brains of adult rodents, but its function is poorly understood. We have generated mice deficient in Plk2 by gene targeting. Although Plk2 is not required for postnatal growth, Plk2(-/-) embryos show retarded growth and skeletal development late in gestation. The labyrinthine zone of the placenta is diminished in Plk2(-/-) embryos due to decreased cell proliferation. Cultured Plk2(-/-) embryonic fibroblasts grow more slowly than normal cells and show delayed entry into S phase. These data suggest a role for Plk2 in the cell cycle.

Silencing of the novel p53 target gene Snk/Plk2 leads to mitotic catastrophe in paclitaxel (taxol)-exposed cells

Loss of p53 sensitizes to antimicrotubule agents in human tumor cells, but little is known about its role during mitosis. We have identified the Polo-like kinase family member serum inducible kinase (Snk/Plk2) as a novel p53 target gene. Snk/Plk2 mutagenesis demonstrated that its kinase activity is negatively regulated by its C terminus. Small interfering RNA (siRNA)-mediated Snk/Plk2 silencing in the presence of the mitotic poisons paclitaxel (Taxol) or nocodazole significantly increased apoptosis, similar to p53 mutations, which confer paclitaxel sensitivity. Furthermore, we have demonstrated that the apoptosis due to silencing of Snk/Plk2 in the face of spindle damage occurs in mitotic cells and not in cells that have progressed to a G(1)-like state without dividing. Since siRNA directed against Snk/Plk2 promoted death of paclitaxel-treated cells in mitosis, we envision a mitotic checkpoint wherein p53-dependent activation of Snk/Plk2 prevents mitotic catastrophe following spindle damage. Finally, these studies suggest that disruption of Snk/Plk2 may be of therapeutic value in sensitizing paclitaxel-resistant tumors.

Identification of a consensus motif for Plk (Polo-like kinase) phosphorylation reveals Myt1 as a Plk1 substrate

Plk1 (Polo-like kinase 1), an evolutionarily conserved serine/threonine kinase, is crucially involved in multiple events during the M phase. Here we have identified a consensus phosphorylation sequence for Plk1, by testing the ability of systematically mutated peptides derived from human Cdc25C to serve as a substrate for Plk1. The obtained results show that a hydrophobic amino acid at position +1 carboxyl-terminal of phosphorylated Ser/Thr and an acidic amino acid at position -2 are important for optimal phosphorylation by Plk1. We have then found that Myt1, an inhibitory kinase for MPF, has a number of putative phosphorylation sites for Plk1 in its COOH-terminal portion. While wild-type Myt1 (Myt1-WT) served as a good substrate for Plk1 in vitro, a mutant Myt1 (Myt1-4A), in which the four putative phosphorylation sites are replaced by alanines, did not. In nocodazole-treated cells, Myt1-WT, but not Myt1-4A, displayed its mobility shift in gel electrophoresis, due to phosphorylation. These results suggest that Plk1 phosphorylates Myt1 during M phase. Thus, this study identifies a novel substrate for Plk1 by determining a consensus phosphorylation sequence by Plk1.