Mitotic Kinase Aurora Research Articles

Kava, a Tonic for Relieving the Irrational Development of Natural Preventive Agents

The last two decades have witnessed explosive growth in the study of natural and other cancer chemopreventive agents ([1][1], [2][2]). Extensive preclinical data have been generated for natural agents, and many (such as green tea, curcumin, phenyl isothiocyanate, indole-3-carbinol, silibinin,

A Mitotic GlcNAcylation/Phosphorylation Signaling Complex Alters the Posttranslational State of the Cytoskeletal Protein Vimentin

O-linked beta-N-acetylglucosamine (O-GlcNAc) is a highly dynamic intracellular protein modification responsive to stress, hormones, nutrients, and cell cycle stage. Alterations in O-GlcNAc addition or removal (cycling) impair cell cycle progression and cytokinesis, but the mechanisms are not well understood. Here, we demonstrate that the enzymes responsible for O-GlcNAc cycling, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) are in a transient complex at M phase with the mitotic kinase Aurora B and protein phosphatase 1. OGT colocalized to the midbody during telophase with Aurora B. Furthermore, these proteins coprecipitated with each other in a late mitotic extract. The complex was stable under Aurora inhibition; however, the total cellular levels of O-GlcNAc were increased and the localization of OGT was decreased at the midbody after Aurora inhibition. Vimentin, an intermediate filament protein, is an M phase substrate for both Aurora B and OGT. Overexpression of OGT or OGA led to defects in mitotic phosphorylation on multiple sites, whereas OGT overexpression increased mitotic GlcNAcylation of vimentin. OGA inhibition caused a decrease in vimentin late mitotic phosphorylation but increased GlcNAcylation. Together, these data demonstrate that the O-GlcNAc cycling enzymes associate with kinases and phosphatases at M phase to regulate the posttranslational status of vimentin.

Loss of CHFR in Human Mammary Epithelial Cells Causes Genomic Instability by Disrupting the Mitotic Spindle Assembly Checkpoint

CHFR is an E3 ubiquitin ligase and an early mitotic checkpoint protein implicated in many cancers and in the maintenance of genomic stability. To analyze the role of CHFR in genomic stability, by siRNA, we decreased its expression in genomically stable MCF10A cells. Lowered CHFR expression quickly led to increased aneuploidy due to many mitotic defects. First, we confirmed that CHFR interacts with the mitotic kinase Aurora A to regulate its expression. Furthermore, we found that decreased CHFR led to disorganized multipolar mitotic spindles. This was supported by the finding that CHFR interacts with α-tubulin and can regulate its ubiquitination in response to nocodazole and the amount of acetylated α-tubulin, a component of the mitotic spindle. Finally, we found a novel CHFR interacting protein, the spindle checkpoint protein MAD2. Decreased CHFR expression resulted in the mislocalization of both MAD2 and BUBR1 during mitosis and impaired MAD2/CDC20 complex formation. Further evidence of a compromised spindle checkpoint was the presence of misaligned metaphase chromosomes, lagging anaphase chromosomes, and defective cytokinesis in CHFR knockdown cells. Importantly, our results suggest a novel role for CHFR regulating chromosome segregation where decreased expression, as seen in cancer cells, contributes to genomic instability by impairing the spindle assembly checkpoint.

ZM 447439 inhibition of aurora kinase induces Hep2 cancer cell apoptosis in three-dimensional culture

Mitotic Aurora kinases are essential for accurate chromosome segregation during cell division. Forced over-expression of Aurora kinase results in centrosome amplification and multipolar spindles, causing aneuploidy, a hallmark of cancer. ZM447439 (ZM), an Aurora selective ATP-competitive inhibitor, interferes with the spindle integrity checkpoint and chromosome segregation. Here, we showed that inhibition of Aurora kinase by ZM reduced histone H3 phosphorylation at Ser10 in Hep2 carcinoma cells. Multipolar spindles were induced in these ZM-treated G2/M-arrested cells with accumulation of 4N/8N DNA, similar to cells with genetically suppressed Aur-B. Cells subsequently underwent apoptosis, as assessed by cleavage of critical apoptotic associated protein PARP. Hep2 cells formed a tumor-like cell mass in 3-dimensional matrix culture; inhibition of Aurora kinase by ZM either destructed the preformed cell mass or prevented its formation, by inducing apoptotic cell death as stained for cleaved caspase-3. Lastly, ZM inhibition of Aurora kinase was potently in association with decrease of Akt phosphorylation at Ser473 and its substrates GSK3α/beta; phosphorylation at Ser21 and Ser9. Together, we demonstrated that Aurora kinase served as a potential molecular target of ZM for more selective therapeutic cancer treatment.

Target Validation and Biomarker Identification in Oncology

The strong link between gene expression of mitotic Aurora kinases and cancer has stimulated a very high interest in developing Aurora kinase inhibitors for cancer therapy. Validation of Aurora kinases as targets, and development of pharmacodynamic biomarkers for inhibitors of Aurora kinases, provides an example of how target validation can help the drug discovery process, and also of how to interpret results depending on the technology used. In this review, we outline the principal tools, concepts, and strategies of target and biomarker validation for Aurora kinases, with emphasis on validation results derived from RNA-interference experiments. These data were essential for the decision to enter the next steps in drug development and for the selection of the appropriate biomarkers for clinical trials.

VX-680 Inhibits Aurora A and Aurora B Kinase Activity in Human Cells

VX-680, also known as MK-0457, is a member of a diverse group of small molecules that inhibit the Aurora kinases, and has shown significant potential as an anti-cancer agent. In keeping with many protein kinase inhibitors, this compound is not a monospecific agent, and its cellular specificity remains largely unknown. In cells, VX-680 blocks mitotic Histone H3 phosphorylation and induces polyploidy and apoptosis, consistent with inhibition of the mitotic protein kinase Aurora B. In this study, we have investigated the effects of VX-680 in proliferating human cancer cells, and demonstrate that it blocks the phosphorylation and activation of both Aurora A and B. Additionally, VX-680 suppresses the phosphorylation of specific substrates of each enzyme, including the Aurora A target TACC3 on Ser558. Exposure to VX-680 induces a monopolar spindle phenotype, delays mitotic progression and rapidly overrides the spindle assembly checkpoint in the presence of spindle poisons. VX-680 also exhibits potent cytotoxicity when compared to the well documented Aurora B inhibitor ZM447439. Taken together, these data identify Aurora A and Aurora B as dual intracellular targets of VX-680.

Aurora kinase small molecule inhibitor destroys mitotic spindle, suppresses cell growth, and induces apoptosis in oral squamous cancer cells

Mitotic Aurora kinases are required for accurate chromosome segregation during cell division. Ectopic expression of Aurora-A (Aur-A) kinase results in centrosome amplification, aberrant spindles, and consequent aneuploidy. In the present study, we showed that Aurora kinase inhibitory small molecule VX-680 inhibited histone H3 phosphorylation at Ser10, a known in vivo substrate residue of Aurora kinase, in oral squamous cell carcinoma (OSCC) KB cells. In addition, monopolar spindle structures, typical abnormalities induced by inhibition of Aur-A, were generated in VX-680-treated cells. Inhibition of Aurora kinase led to reduced KB cell growth, as assessed by MTT assay. Western blot analysis revealed that VX-680 caused cleavage of two critical apoptotic associated proteins, PARP and caspase-3. In contrast, expression of cell survival factor Bcl-2 was reduced by VX-680 treatment in a dose-dependent manner. Subsequently, nuclear characteristic of DNA fragmentation, indicative of apoptotic cell death, was clearly observed in these OSCC cells with Aurora kinase inhibitory VX-680. Taken together, we showed that Aurora kinase inhibitory VX-680 led to apoptotic cell death in OSCC cells, suggesting a novel therapeutic target in oral cancer.

The Transforming Acidic Coiled Coil 3 Protein Is Essential for Spindle-dependent Chromosome Alignment and Mitotic Survival

Cancer-associated centrosomal transforming acidic coiled coil (TACC) proteins are involved in mitotic spindle function. By employing gene targeting, we have recently described a nonredundant and essential role of TACC3 in regulating cell proliferation. In this study, we used an inducible RNA interference approach to characterize the molecular function of TACC3 and its role in mitotic progression and cell survival. Our data demonstrate that a TACC3 knockdown arrests G(1) checkpoint-compromised HeLa cells prior to anaphase with aberrant spindle morphology and severely misaligned chromosomes. Interestingly, TACC3-depleted cells fail to accumulate the mitotic kinase Aurora B and the checkpoint protein BubR1 to normal levels at kinetochores. Moreover, localization of the structural protein Ndc80 at outer kinetochores is reduced, indicating a defective kinetochore-microtubule attachment in TACC3-deficient cells. As a consequence of prolonged TACC3 depletion, cells undergo caspase-dependent cell death that relies on a spindle checkpoint-dependent mitotic arrest. TACC3 knockdown cells that escape from this arrest by mitotic slippage become highly polyploid and accumulate supernumerary centrosomes. Similarly, deficiency of the post-mitotic cell cycle inhibitor p21(WAF) exacerbates the effects of TACC3 depletion. Our findings therefore point to an essential role of TACC3 in spindle assembly and cellular survival and identify TACC3 as a potential therapeutic target in cancer cells.

GEF-H1 Modulates Localized RhoA Activation during Cytokinesis under the Control of Mitotic Kinases

Formation of the mitotic cleavage furrow is dependent upon both microtubules and activity of the small GTPase RhoA. GEF-H1 is a microtubule-regulated exchange factor that couples microtubule dynamics to RhoA activation. GEF-H1 localized to the mitotic apparatus in HeLa cells, particularly at the tips of cortical microtubules and the midbody, and perturbation of GEF-H1 function induced mitotic aberrations, including asymmetric furrowing, membrane blebbing, and impaired cytokinesis. The mitotic kinases Aurora A/B and Cdk1/Cyclin B phosphorylate GEF-H1, thereby inhibiting GEF-H1 catalytic activity. Dephosphorylation of GEF-H1 occurs just prior to cytokinesis, accompanied by GEF-H1-dependent GTP loading on RhoA. Using a live cell biosensor, we demonstrate distinct roles for GEF-H1 and Ect2 in regulating Rho activity in the cleavage furrow, with GEF-H1 catalyzing Rho activation in response to Ect2-dependent localization and initiation of cell cleavage. Our results identify a GEF-H1-dependent mechanism to modulate localized RhoA activation during cytokinesis under the control of mitotic kinases.

Aurora B controls the association of condensin I but not condensin II with mitotic chromosomes

The assembly of mitotic chromosomes is controlled by condensin complexes. In vertebrates, condensin I binds to chromatin in prometaphase, confers rigidity to chromosomes and enables the release of cohesin complexes from chromosome arms, whereas condensin II associates with chromosomes in prophase and promotes their condensation. Both complexes are essential for chromosome segregation in anaphase. Although the association of condensins with chromatin is important for the assembly and segregation of mitotic chromosomes, it is poorly understood how this process is controlled. Here we show that the mitotic kinase Aurora B regulates the association of condensin I, but not the interaction of condensin II with chromatin. Quantitative time-lapse imaging of cells expressing GFP-tagged condensin subunits revealed that Aurora B is required for efficient loading of condensin I onto chromosomes in prometaphase and for maintenance of the complex on chromosomes in later stages of mitosis. The three non-SMC subunits of condensin I are Aurora B substrates in vitro and their mitosis-specific phosphorylation depends on Aurora B in vivo. Our data indicate that Aurora B contributes to chromosome rigidity and segregation by promoting the binding of condensin I to chromatin. We have also addressed how Aurora B might mediate the dissociation of cohesin from chromosome arms.

Preclinical pharmacodynamic studies of Aurora A inhibition by MLN8054

13059 Background: The mitotic kinase Aurora A is implicated in the development of multiple tumor types. MLN8054 is an oral, potent and selective small-molecule inhibitor of Aurora A with broad efficacy in preclinical models of cancer. Inhibition of Aurora A by MLN8054 induces accumulation of mitotic cells, followed by apoptosis. This study explores relationships between Aurora A inhibition, mitotic index, and tumor growth inhibition for xenograft models with different sensitivity to MLN8054. The marker response in mouse skin was also studied. Methods: Mice bearing subcutaneous xenografts were dosed orally qd or bid with MLN8054 for 21 days. Pharmacodynamic markers were studied after 1–2 doses. Formalin-fixed xenograft tissues were stained with the mitotic markers pHisH3 and MPM2, or with an antibody to the T288 autophosphorylation site on Aurora A. Tumor growth inhibition (TGI) was calculated using the formula 100 - [ΔT/ΔC * 100], where ΔT is the volume change for treated tumors, and ΔC is the volume change for control tumors. Results: HCT116 human colon xenografts were sensitive to MLN8054 on a qd or bid schedule (84% and 96% TGI respectively for 30mg/kg dose). The T288 autophosphorylation site was used to directly demonstrate inhibition of Aurora A, which resulted in dose-dependent duration of the elevation in mitotic index. Efficacy was similar for qd vs bid dosing of 30mg/kg MLN8054, and accordingly we found that a single dose was sufficient to elevate the mitotic index for about 20–24h in this model. SW480 human colon xenografts have MLN8054 sensitivity similar to that of HCT116, but more modest effects on mitotic index were observed. The mitotic index profile of SW480 is similar to that of MDA-MB-231 xenografts, the most insensitive model studied. Elevated mitotic index was also observed in mouse skin. Conclusions: We found that mitotic index measurements coupled with the T288 autophosphorylation site as a direct marker of Aurora A activity are useful for monitoring inhibition of Aurora A by MLN8054 in tumor and/or skin biopsies. In a sensitive model, greater duration of mitotic index elevation results in greater efficacy. Our continuing work aims to better understand the differences in marker and efficacy responses between xenograft lines, incorporating the pT288 antibody as a direct marker of Aurora A inhibition. [Table: see text]

Biased spindle formation

A microtubule-stabilizing protein called HURP forms a gradient around chromosomes that stabilizes spindles, say Wong and Fang (page 879). Figure 1 HURP (green) is localized close to chromosomes (blue). HURP was previously associated with hepatocellular carcinoma, but Wong and Fang picked it out as being induced during mitosis and showing covariation with known mitotic regulators. Almost half of all HURP-depleted cells had one or more unaligned chromosome during metaphase. The kinetochores of these chromosomes were not attached to microtubules, and even attached kinetochores were under less tension than in a normal spindle. HURP has also shown up recently as part of a complex from frog egg extracts that is required for the conversion of aster-like to spindle-like structures (Koffa, M.D., et al. 2006. Curr. Biol. 16:743-754). The complex consists of two characterized microtubule-associated proteins (TPX2 and XMAP215), a plus end-directed microtubule motor (Eg5), a mitotic kinase (Aurora A), and HURP. This complex is dependent on the activity of chromatin-localized Ran-GTP, thus helping to focus the spindle around chromosomes. This is consistent with Wong and Fang's finding that HURP is found on microtubules and in a gradient that peaks around chromosomes. They also found that HURP binds to and stabilizes microtubules. HURP's activity may be needed as, based on mathematical modeling, unbiased microtubule growth is too inefficient for kinetochore capture given the time constraints of mitosis. The HeLa cells depleted of HURP eventually escaped from their cell cycle arrest induced by unattached chromosomes. The same was true if the cells were treated with low levels of microtubule depolymerizing drugs that induced some chromosome detachment. This suggests that the mitotic checkpoint is weak in certain tumor-derived cells, which would promote genomic instability.

HURP Is Part of a Ran-Dependent Complex Involved in Spindle Formation

GTP-loaded Ran induces the assembly of microtubules into aster-like and spindle-like structures in Xenopus egg extract. The microtubule-associated protein (MAP), TPX2, can mediate Ran's role in aster formation, but factors responsible for the transition from aster-like to spindle-like structures have not been described. Here we identify a complex that is required for the conversion of aster-like to spindle-like structures. The complex consists of two characterized MAPs (TPX2, XMAP215), a plus end-directed motor (Eg5), a mitotic kinase (Aurora A), and HURP, a protein associated with hepatocellular carcinoma. Formation and function of the complex is dependent on Aurora A activity. HURP protein was further characterized and shown to bind microtubules and affect their organization both in vitro and in vivo. In egg extract, anti-HURP antibodies disrupt the formation of both Ran-dependent and chromatin and centrosome-induced spindles. HURP is also required for the proper formation and function of mitotic spindles in HeLa cells. HURP is a new and essential component of the mitotic apparatus. HURP acts as part of a multicomponent complex that affects the growth or stability of spindle MTs and is required for spindle MT organization.

Regulation of HP1–chromatin binding by histone H3 methylation and phosphorylation

Tri-methylation of histone H3 lysine 9 is important for recruiting heterochromatin protein 1 (HP1) to discrete regions of the genome, thereby regulating gene expression, chromatin packaging and heterochromatin formation. Here we show that HP1alpha, -beta, and -gamma are released from chromatin during the M phase of the cell cycle, even though tri-methylation levels of histone H3 lysine 9 remain unchanged. However, the additional, transient modification of histone H3 by phosphorylation of serine 10 next to the more stable methyl-lysine 9 mark is sufficient to eject HP1 proteins from their binding sites. Inhibition or depletion of the mitotic kinase Aurora B, which phosphorylates serine 10 on histone H3, causes retention of HP1 proteins on mitotic chromosomes, suggesting that H3 serine 10 phosphorylation is necessary for the dissociation of HP1 from chromatin in M phase. These findings establish a regulatory mechanism of protein-protein interactions, through a combinatorial readout of two adjacent post-translational modifications: a stable methylation and a dynamic phosphorylation mark.

An Essential Function of the C. elegans Ortholog of TPX2 Is to Localize Activated Aurora A Kinase to Mitotic Spindles

In vertebrates, the microtubule binding protein TPX2 is required for meiotic and mitotic spindle assembly. TPX2 is also known to bind to and activate Aurora A kinase and target it to the spindle. However, the relationship between the TPX2-Aurora A interaction and the role of TPX2 in spindle assembly is unclear. Here, we identify TPXL-1, a C. elegans protein that is the first characterized invertebrate ortholog of TPX2. We demonstrate that an essential role of TPXL-1 during mitosis is to activate and target Aurora A to microtubules. Our data suggest that this targeting stabilizes microtubules connecting kinetochores to the spindle poles. Thus, activation and targeting of Aurora A appears to be an ancient and conserved function of TPX2 that plays a central role in mitotic spindle assembly.

Chfr is required for tumor suppression and Aurora A regulation

Tumorigenesis is a consequence of loss of tumor suppressors and activation of oncogenes. Expression of the mitotic checkpoint protein Chfr is lost in 20-50% of primary tumors and tumor cell lines. To explore whether downregulation of Chfr contributes directly to tumorigenesis, we generated Chfr knockout mice. Chfr-deficient mice are cancer-prone, develop spontaneous tumors and have increased skin tumor incidence after treatment with dimethylbenz(a)anthracene. Chfr deficiency leads to chromosomal instability in embryonic fibroblasts and regulates the mitotic kinase Aurora A, which is frequently upregulated in a variety of tumors. Chfr physically interacts with Aurora A and ubiquitinates Aurora A both in vitro and in vivo. Collectively, our data suggest that Chfr is a tumor suppressor and ensures chromosomal stability by controlling the expression levels of key mitotic proteins such as Aurora A.

Polo kinase and progression through M phase in Drosophila: a perspective from the spindle poles

Genes for the mitotic kinases Polo and Aurora A were first identified in Drosophila through screens of maternal effect lethal mutations for defects in spindle pole behaviour. These enzymes have been shown to be highly conserved and required for multiple functions in mitosis. Polo is stabilized at the centrosome by association with Hsp90. It is required for centrosome maturation on M-phase entry in order to recruit the gamma-tubulin ring complex and activate the abnormal spindle protein, Asp. These events facilitate the nucleation of minus ends of microtubules at the centrosome. The localization of Polo at the kinetochore and the mid-zone of the central spindle together with the phenotypes of polo mutants point to functions at the metaphase to anaphase transition and in cytokinesis. The latter are mediated, at least in part, through the Pavarotti kinesin-like motor protein and its conserved counterparts in other metazoans.

Requirements for the destruction of human Aurora-A

The mitotic kinase Aurora A (Aur-A) is overexpressed in a high proportion of human tumors, often in the absence of gene amplification. In somatic cells, Aur-A protein levels fall following mitosis or upon overexpression of Cdh1, an activator of the ubiquitin ligase APC/C. Thus, mutations that reduce or block the rate of Aur-A destruction might also be expected to contribute to its oncogenic potential. Previous work had defined two short sequences of Xenopus Aur-A that are required for its Cdh1-inducible destruction in extracts of Xenopus eggs, an N-terminal A box and a C-terminal D box, and a serine residue within the A box whose phosphorylation might inhibit destruction. Here, we show that these same sequences are required for the destruction of human Aur-A during mitotic exit and G1 in the somatic cell cycle. Expression of a dominant negative Cdh1 protein leads to accumulation of Aur-A, further indicating that the Cdh1-activated form of the APC/C is responsible for destruction of Aur-A during the somatic cell cycle in vivo. During the course of this work, we found some previously unsuspected problems in commonly used in vitro destruction assays, which can result in misleading results. Potentially confounding factors include: (i) the presence of D-box- and A-box-dependent destruction-promoting activities in the reticulocyte in vitro translation mix that is used to produce radiolabeled substrates for destruction assays; and (ii) the ability of green-fluorescent-protein tags to reduce the destruction rate of Aur-A substantially. These findings have direct relevance for studies of Aur-A destruction itself, and for broader approaches that use in vitro translation products in screens for additional APC/C targets.

REGULATION OF THE CHROMATIN PROTEIN HP1γ BY THE MITOTIC AURORA ONCOPROTEINS.

Numerous kinase and phosphatase activities coordinate the dynamic events of mitosis, such as chromosome condensation, nuclear envelope breakdown, centrosome separation, formation of bipolar spindles, chromosome segregation and cytokinesis. The Aurora kinases are key regulators during these mitotic processes. Deregulation of Aurora kinases is implicated in oncogenesis as a consequence of overriding the spindle checkpoint to cause chromosome missegregation, and these proteins are overexpressed in a variety of human tumors, including pancreatic cancer. In the current study, we characterize the functional link between the mitotic Aurora kinases and the chromatin proteins, HP1γ, β, and γ, along with their participation in chromosomal stability. We find that Aurora-A, and to a lesser extent Aurora-B, specifically phosphorylate the HP1γ isoform at residue serine 83, which is located within the linker region between the chromo- and chromoshadow-domains. Aurora activation is dependent upon the activation of the mitotic cyclin-dependent kinases. Correspondingly, we demonstrate that treatment of pancreatic cancer cells with roscovitine, a cdk inhibitor, abolishes HP1γ phosphorylation at S83. Immunofluorescence of this phosphorylated form of HP1γ shows a punctate, euchromatic localization during interphase. Interestingly, at the earliest stages of chromatin condensation during prophase, phosphorylated HP1γ increases and associates with entire chromosomes, which continues through metaphase. In conjunction, cell cycle analyses reveal that the levels of Aurora-modified HP1γ are highest during mitosis, while the total amount of HP1γ remains constant throughout the cell cycle. Together, these results suggest a novel, dynamic model for an integral chromatin protein, HP1γ, and Aurora kinase-mediated regulation of chromosomal stability in pancreatic cancer cells. RU is supported by the Mayo Cancer Center and NIH Grants DK52913 and DK5662.

Aurora B -TACC1 protein complex in cytokinesis.

Taxins are a family of centrosomal proteins important for the regulation of mitosis and microtubule dynamics. Cytokinesis, the last step of M phase, is essential for chromosomal integrity and cell division. It is highly regulated and involves a reorganization of microtubules and actin filaments. We show here that TACC1 localizes diffusely to the midzone spindle in anaphase and strongly to the midbody during cytokinesis, indicating a possible involvement of this protein in the exit of M phase. TACC1 also relocalizes to the nucleolus in interphase. We demonstrate that TACC1 and the mitotic kinase Aurora B belong to the same complex during cytokinesis. We further show that Aurora B knocked down by RNA-mediated interference prevents the formation of the midbody - and consequently affects TACC1 localization at this site - and leads to abnormal cell division and multinucleated cells.