Novel sulfonamide-indolinone hybrids targeting mitochondrial respiration of breast cancer cells

Breast cancer (BC) still poses a threat worldwide which demands continuous efforts to present safer and efficacious treatment options via targeted therapy. Beside kinases' aberrations as Aurora B kinase which controls cell division, BC adopts distinct metabolic profiles to meet its high energy demands. Accordingly, targeting both aurora B kinase and/or metabolic vulnerability presents a promising approach to tackle BC. Based on a previously reported indolinone-based Aurora B kinase inhibitor (III), and guided by structural modification and SAR investigation, we initially synthesized 11 sulfonamide-indolinone hybrids (5a-k), which showed differential antiproliferative activities against the NCI-60 cell line panel with BC cells displaying preferential sensitivity. Nonetheless, modest activity against Aurora B kinase (18-49% inhibition) was noted at 100 nM. Screening of a representative derivative (5d) against 17 kinases, which are overexpressed in BC, failed to show significant activity at 1 μM concentration, suggesting that kinase inhibitory activity only played a partial role in targeting BC. Bioinformatic analyses of genome-wide transcriptomics (RNA-sequencing), metabolomics, and CRISPR loss-of-function screens datasets suggested that indolinone-completely responsive BC cell lines (MCF7, MDA-MB-468, and T-47D) were more dependent on mitochondrial oxidative phosphorylation (OXPHOS) compared to partially responsive BC cell lines (MDA-MB-231, BT-549, and HS 578 T). An optimized derivative, TC11, obtained by molecular hybridization of 5d with sunitinib polar tail, manifested superior antiproliferative activity and was used for further investigations. Indeed, TC11 significantly reduced/impaired the mitochondrial respiration, as well as mitochondria-dependent ROS production of MCF7 cells. Furthermore, TC11 induced G0/G1 cell cycle arrest and apoptosis of MCF7 BC cells. Notably, anticancer doses of TC11 did not elicit cytotoxic effects on normal cardiomyoblasts and hepatocytes. Altogether, these findings emphasize the therapeutic potential of targeting the metabolic vulnerability of OXPHOS-dependent BC cells using TC11 and its related sulfonamide-indolinone hybrids. Further investigation is warranted to identify their precise/exact molecular target.

Design, synthesis, and evaluation of dual EGFR/AURKB inhibitors as anticancer agents for non-small cell lung cancer

The epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs) are first-line agents for mutant EGFR-positive (mEGFR+) NSCLC. However, secondary resistant mutations develop following therapy that prevent EGFR-TKI binding. The EGFR-TKIs are rendered ineffective in NSCLC expressing EGFR resistant mutations (rmEGFR+). Mutations in Kirsten rat sarcoma virus protein (mKRAS) support persistent signaling downstream of EGFR regardless of EGFR-TKI earlier in the signaling cascade. The EGFR-TKIs are ineffective in mKRAS+ NSCLC. Thus, newer anticancer agents are needed for rmEGFR+ and mKRAS+ NSCLC. Aurora kinase B (AURKB) is a mitosis related kinase that is overexpressed in NSCLC and supports cancer cell proliferation and survival. Literature reports have suggested that AURKB inhibitors if given concurrently with an EGFR-TKI could overcome EGFR-TKI resistance in mKRAS+ NSCLC and rmEGFR + NSCLC, and showed improved anticancer effects compared to current single-targeted EGFR-TKIs. Molecular modeling was used to identify similarities between the kinase pockets of EGFR and AURKB. An overlap was observed for the inactive conformation of EGFR and the active conformation of AURKB. Compounds 3-7 were synthesized as dual EGFR/AURKB inhibitors for mKRAS+ and rmEGFR+ NSCLC. Compounds 5, 6 and 7 were identified as dual EGFR/AURKB inhibitors. Compound 5 demonstrated modest micromolar inhibition of rmEGFR+ NSCLC. All investigated compounds showed moderate inhibition of mKRAS+ NSCLC cells. Compound 7 demonstrated single-digit micromolar inhibition of mKRAS+ NSCLC.

microRNA-486-5p Regulates DNA Damage Inhibition and Cisplatin Resistance in Lung Adenocarcinoma by Targeting AURKB

Lung adenocarcinoma (LUAD) severely affects human health, and cisplatin (DDP) resistance is the main obstacle in LUAD treatment, the mechanism of which is unknown. Bioinformatics methods were utilized to predict expression and related pathways of AURKB in LUAD tissues, as well as the upstream regulated microRNAs. qRT-PCR assayed expression of AURKB and microRNA-486-5p. RIP and dual-luciferase experiments verified the binding and interaction between the two genes. CCK-8 was used to detect cell proliferation ability and IC50 values. Flow cytometry was utilized to assess the cell cycle. Comet assay and western blot tested DNA damage and γ-H2AX protein expression, respectively. In LUAD, AURKB was upregulated, but microRNA-486-5p was downregulated. The targeted relationship between the two was confirmed by RIP and dual-luciferase experiments. Cell experiments showed that AURKB knock-down inhibited cell proliferation, reduced IC50 values, induced cell cycle arrest, and caused DNA damage. The rescue experiment presented that high expression of microRNA-486-5p could weaken the impact of AURKB overexpression on LUAD cell behavior and DDP resistance. microRNA-486-5p regulated DNA damage to inhibit DDP resistance in LUAD by targeting AURKB, implying that microRNA-486-5p/AURKB axis may be a possible therapeutic target for DDP resistance in LUAD patients.

Proteomic and Phosphoproteomic Reprogramming in Epithelial Ovarian Cancer Metastasis

Epithelial ovarian cancer (EOC) is a high-risk cancer presenting with heterogeneous tumors. The high incidence of EOC metastasis from primary tumors to nearby tissues and organs is a major driver of EOC lethality. We used cellular models of spheroid formation and readherence to investigate cellular signaling dynamics in each step toward EOC metastasis. In our system, adherent cells model primary tumors, spheroid formation represents the initiation of metastatic spread, and readherent spheroid cells represent secondary tumors. Proteomic and phosphoproteomic analyses show that spheroid cells are hypoxic and show markers for cell cycle arrest. Aurora kinase B abundance and downstream substrate phosphorylation are significantly reduced in spheroids and readherent cells, explaining their cell cycle arrest phenotype. The proteome of readherent cells is most similar to spheroids, yet greater changes in the phosphoproteome show that spheroid cells stimulate Rho-associated kinase 1 (ROCK1)-mediated signaling, which controls cytoskeletal organization. In spheroids, we found significant phosphorylation of ROCK1 substrates that were reduced in both adherent and readherent cells. Application of the ROCK1-specific inhibitor Y-27632 to spheroids increased the rate of readherence and altered spheroid density. The data suggest ROCK1 inhibition increases EOC metastatic potential. We identified novel pathways controlled by Aurora kinase B and ROCK1 as major drivers of metastatic behavior in EOC cells. Our data show that phosphoproteomic reprogramming precedes proteomic changes that characterize spheroid readherence in EOC metastasis.

N6-methyladenosine methyltransferase METTL3 modulates the cell cycle of granulosa cells via CCND1 and AURKB in Haimen goats

N6-methyladenosine (m6A) plays a crucial role in many bioprocesses across species, but its function in granulosa cells during oocyte maturation is not well understood in animals, especially domestic animals. We observed an increase in m6A methyltransferase-like 3 (METTL3) in granulosa cells during oocyte maturation in Haimen goats. Our results showed that knockdown of METTL3 disrupted the cell cycle in goat granulosa cells, leading to aggravated cell apoptosis and inhibition of cell proliferation and hormone secretion. Mechanistically, METTL3 may regulate the cell cycle in goat granulosa cells by mediating Aurora kinase B (AURKB) mRNA degradation in an m6A-YTH N6-methyladenosine RNA binding protein 2 (YTHDF2) manner and participating in AURKB transcription via the Cyclin D1 (CCND1)-Retinoblastoma protein (RB)-E2F transcription factor 1 (E2F1) pathway. Overall, our study highlights the essential role of METTL3 in granulosa cells during oocyte maturation in Haimen goats. These findings provide a theoretical basis and technical means for understanding how RNA methylation participates in oocyte maturation through granulosa cells.

Barasertib impedes chondrocyte senescence and alleviates osteoarthritis by mitigating the destabilization of heterochromatin induced by AURKB

Osteoarthritis (OA) is a common joint disease characterized by progressive cartilage loss that causes disability worldwide. The accumulation of senescent chondrocytes in aging human cartilage contributes to the high incidence of OA. Heterochromatin instability, the hallmark and driving factor of senescence, regulates the expression of the senescence-associated secretory phenotype that induces inflammation and cartilage destruction. However, the role of heterochromatin instability in OA progression remains unclear. In this work, we identified AURKB as a key senescence-associated chromatin regulator using bioinformatics methods. We found that AURKB was upregulated in OA cartilage and chondrocytes exposed to abnormal mechanical strain. Overexpression of AURKB could cause senescence and heterochromatin instability. Furthermore, the AURKB inhibitor Barasertib reversed senescence and heterochromatin instability in chondrocytes and alleviated OA in a rat model. Mechanistically, abnormal mechanical strain increased AURKB levels through the Piezo1/Ca2+ signaling axis. Blocking Piezo1/Ca2+ signaling by short interfering RNA against Piezo1 and Ca2+ chelator BAPTA could reduce the expression of AURKB and alleviate senescence in chondrocytes exposed to abnormal mechanical strain. In conclusion, our data confirmed that abnormal mechanical strain increases the expression of AURKB by activating the Piezo1/Ca2+ signaling axis, leading to destabilized heterochromatin and senescence in chondrocytes, whereas Barasertib consolidates heterochromatin, counteracts senescence and alleviates OA.

Perceived motor competence in early childhood predicts perceived and actual motor competence in middle childhood

The study aims were to (1) identify perceived motor competence (PMC) and actual motor competence (AMC) profiles in children at two time points (early and middle childhood) 3 years apart, (2) explore transitions between the profiles from T1 to T2, and (3) investigate how PMC-AMC profiles at T1 differ in their mean values for AMC and PMC variables at T2. PMC was assessed by the pictorial scale of Perceived Movement Skill Competence for young children (PMSC). At T1, AMC was measured with Test of Gross Motor Development-third version (TGMD-3), and at T2, a shortened TGMD-3 was used. To identify the PMC-AMC profiles using latent profile analysis, the Mplus statistical package (version 8.7) was used. For aim 3, the Bolck-Croon-Hagenaars (BCH) method was used. There were 480 children (mean age 6.26 years, 51.9% boys) at T1, 647 children (mean age 8.76 years, 48.8% boys) at T2 (some children were too young to have the PMC assessment at T1), and 292 at both time points. For aim 1, three profiles were identified at each time point for each gender. Boys had two realistic profiles with medium and low levels of PMC-AMC, and an overestimation profile. Girls had a medium realistic profile, an overestimation, and an underestimation profile. The PMC-AMC profile in early childhood predicted the PMC-AMC profile (aim 2) and AMC and PMC variables (aim 3) in middle childhood, especially if a child had low PMC in early childhood. Children with low PMC in early childhood are at risk of low PMC and less AMC development in middle childhood.

Drosophila Screening Identifies Dual Inhibition of MEK and AURKB as an Effective Therapy for Pancreatic Ductal Adenocarcinoma

Significant progress has been made in understanding the pathogenesis of pancreatic ductal adenocarcinoma (PDAC) by generating and using murine models. To accelerate drug discovery by identifying novel therapeutic targets on a systemic level, here we generated a Drosophila model mimicking the genetic signature in PDAC (KRAS, TP53, CDKN2A, and SMAD4 alterations), which is associated with the worst prognosis in patients. The '4-hit' flies displayed epithelial transformation and decreased survival. Comprehensive genetic screening of their entire kinome revealed kinases including MEK and AURKB as therapeutic targets. Consistently, a combination of the MEK inhibitor trametinib and the AURKB inhibitor BI-831266 suppressed the growth of human PDAC xenografts in mice. In patients with PDAC, the activity of AURKB was associated with poor prognosis. This fly-based platform provides an efficient whole-body approach that complements current methods for identifying therapeutic targets in PDAC.

SIGNIFICANCE

Development of a Drosophila model mimicking genetic alterations in human pancreatic ductal adenocarcinoma provides a tool for genetic screening that identifies MEK and AURKB inhibition as a potential treatment strategy.

Lymphocyte antigen 6K signaling to aurora kinase promotes advancement of the cell cycle and the growth of cancer cells, which is inhibited by LY6K-NSC243928 interaction

Lymphocyte antigen 6K (LY6K) is a small GPI-linked protein that is normally expressed in testes. Increased expression of LY6K is significantly associated with poor survival outcomes in many solid cancers, including cancers of the breast, ovary, gastrointestinal tract, head and neck, brain, bladder, and lung. LY6K is required for ERK-AKT and TGF-β pathways in cancer cells and is required for in vivo tumor growth. In this report, we describe a novel role for LY6K in mitosis and cytokinesis through aurora B kinase and its substrate histone H3 signaling axis. Further, we describe the structural basis of the molecular interaction of small molecule NSC243928 with LY6K protein and the disruption of LY6K-aurora B signaling in cell cycle progression due to LY6K-NSC243928 interaction. Overall, disruption of LY6K function via NSC243928 led to failed cytokinesis, multinucleated cells, DNA damage, senescence, and apoptosis of cancer cells. LY6K is not required for vital organ function, thus inhibition of LY6K signaling is an ideal therapeutic approach for hard-to-treat cancers that lack targeted therapy such as triple-negative breast cancer.

Pharmacophore Synergism in Diverse Scaffold Clinches in Aurora Kinase B

Aurora kinase B (AKB) is a crucial signaling kinase with an important role in cell division. Therefore, inhibition of AKB is an attractive approach to the treatment of cancer. In the present work, extensive quantitative structure-activity relationships (QSAR) analysis has been performed using a set of 561 structurally diverse aurora kinase B inhibitors. The Organization for Economic Cooperation and Development (OECD) guidelines were used to develop a QSAR model that has high statistical performance (R²_tr = 0.815, Q²_LMO = 0.808, R²_ex = 0.814, CCC_ex = 0.899). The seven-variable-based newly developed QSAR model has an excellent balance of external predictive ability (Predictive QSAR) and mechanistic interpretation (Mechanistic QSAR). The QSAR analysis successfully identifies not only the visible pharmacophoric features but also the hidden features. The analysis indicates that the lipophilic and polar groups-especially the H-bond capable groups-must be present at a specific distance from each other. Moreover, the ring nitrogen and ring carbon atoms play important roles in determining the inhibitory activity for AKB. The analysis effectively captures reported as well as unreported pharmacophoric features. The results of the present analysis are also supported by the reported crystal structures of inhibitors bound to AKB.

Airborne particulate matter induces mitotic slippage and chromosomal missegregation through disruption of the spindle assembly checkpoint (SAC)

Particulate matter (PM) is a risk factor for lung cancer development and chromosomal missegregation and cell cycle disruptions are key cellular events that trigger tumorigenesis. We aimed to study the effect of PM₁₀ (PM with an aerodynamic diameter ≤10 μm) on mitotic arrest and chromosomal segregation, evaluating the spindle assembly checkpoint (SAC) protein dynamics in the human lung A549 adenocarcinoma cell line. For this purpose, synchronized cells were exposed to PM₁₀ for 24 h to obtain the frequency of micronucleated (MN) and trinucleated (TN) cells. Then, the efficiency of the mitotic arrest after PM₁₀ exposure was analyzed. To elucidate the effect of PM₁₀ in chromosomal segregation, the levels and subcellular localization of essential SAC proteins were evaluated. Results indicated that A549 cells exposed to PM₁₀ exhibited an increase in MN and TN cells and a decrease in mitotic indexes and G2/M phase. A549 cells treated with PM₁₀ showed reduced protein levels of MDC1 and NEK2 (38% and 35% respectively), which is required for MAD2 recruitment to kinetochores, MAD2 and BUBR1, effectors of the SAC (25% and 18% respectively), and CYCLIN B1, required during G2/M phase (35%). Besides, PM₁₀ exposure increase the levels of AURORA B and SURVIVIN, required for SAC activation through chromosome-microtubule attachment errors (85% and 74% respectively). We suggest that PM₁₀ causes mitotic slippage due to alterations in MAD2 localization. Thus, PM₁₀ causes inadequate chromosomal segregation and deficient mitotic arrest by altering SAC protein levels, predisposing A549 cells to chromosomal instability, a common feature observed in cancer.

Detection and Correction of Merotelic Kinetochore Orientation by Aurora B and its Partners

Merotelic kinetochore orientation is a kinetochore-microtubule mis-attachment in which a single kinetochore binds microtubules to both spindle poles, rather than just one. Merotelic attachments occur frequently in early mitosis and can induce anaphase lagging chromosomes and aneuploidy if not corrected before anaphase onset. Merotelic kinetochore orientation does not interfere with chromosome alignment at the metaphase plate and does not activate the mitotic spindle checkpoint. However, a correction mechanism for merotelic attachment reduces the number of merotelic kinetochores entering anaphase, thus preventing chromosome mis-segregation. Results from many different studies support the idea that Aurora B kinase plays a critical role in this merotelic correction mechanism by phosphorylating key substrates at the kinetochore and promoting turnover of kinetochore microtubules. In addition, recent studies are starting to identify the possible 'sensors' of the system that would be able to detect the mis-attachment and communicate this to Aurora B. Here, I review these studies and discuss a model for how merotelic kinetochore orientation could be detected and corrected before anaphase onset.

Human Kid is Degraded by the APC/CCdh1 but Not by the APC/CCdc20

The APC/C(Cdh1) (anaphase promoting complex/cyclosome) targets numerous cell cycle proteins for ubiquitin mediated degradation in late mitosis and G1. The KEN box is one of two major recognition motifs of APC/C(Cdh1) substrates. This motif is however very common and shared by a tenth of the human proteome, the vast majority of which are obviously not APC/C substrates. We have observed that most known functional KEN boxes are followed by a proline residue and show that this proline plays a role in APC/C(Cdh1) specific degradation. This insight can be instrumental for identifying novel APC/C(Cdh1) substrates. We used this KENxP motif to identify human Aurora B and Kid as APC/C(Cdh1) substrates. The degradation of Xenopus XKid at metaphase by APC/C(Cdc20) is essential for chromatid segregation. Human Kid in contrast is degraded later and its APC/C(Cdh1) specific degradation is not required for mitotic progress. It is thus likely that Kid inactivation in G1 takes place both by nuclear sequestration and degradation by the APC/C(Cdh1).

Aurora B suppresses microtubule dynamics and limits central spindle size by locally activating KIF4A

Anaphase central spindle formation is controlled by the microtubule-stabilizing factor PRC1 and the kinesin KIF4A. We show that an MKlp2-dependent pool of Aurora B at the central spindle, rather than global Aurora B activity, regulates KIF4A accumulation at the central spindle. KIF4A phosphorylation by Aurora B stimulates the maximal microtubule-dependent ATPase activity of KIF4A and promotes its interaction with PRC1. In the presence of phosphorylated KIF4A, microtubules grew more slowly and showed long pauses in growth, resulting in the generation of shorter PRC1-stabilized microtubule overlaps in vitro. Cells expressing only mutant forms of KIF4A lacking the Aurora B phosphorylation site overextended the anaphase central spindle, demonstrating that this regulation is crucial for microtubule length control in vivo. Aurora B therefore ensures that suppression of microtubule dynamic instability by KIF4A is restricted to a specific subset of microtubules and thereby contributes to central spindle size control in anaphase.

Aurora B and Cdk1 mediate Wapl activation and release of acetylated cohesin from chromosomes by phosphorylating Sororin

Sister chromatid cohesion depends on Sororin, a protein that stabilizes acetylated cohesin complexes on DNA by antagonizing the cohesin release factor Wings-apart like protein (Wapl). Cohesion is essential for chromosome biorientation but has to be dissolved to enable sister chromatid separation. To achieve this, the majority of cohesin is removed from chromosome arms in prophase and prometaphase in a manner that depends on Wapl and phosphorylation of cohesin's subunit stromal antigen 2 (SA2), whereas centromeric cohesin is cleaved in metaphase by the protease separase. Here we show that the mitotic kinases Aurora B and Cyclin-dependent kinase 1 (Cdk1) destabilize interactions between Sororin and the cohesin subunit precocious dissociation of sisters protein 5 (Pds5) by phosphorylating Sororin, leading to release of acetylated cohesin from chromosome arms and loss of cohesion. At centromeres, the cohesin protector shugoshin (Sgo1)-protein phosphatase 2A (PP2A) antagonizes Aurora B and Cdk1 partly by dephosphorylating Sororin and thus maintains cohesion until metaphase. We propose that the stepwise loss of cohesion between chromosome arms and centromeres is caused by local regulation of Wapl activity, which is controlled by the phosphorylation state of Sororin.

Stage I of a phase 2 study assessing the efficacy, safety, and tolerability of barasertib (AZD1152) versus low-dose cytosine arabinoside in elderly patients with acute myeloid leukemia

BACKGROUND

In this phase 2 study, the authors evaluated the efficacy, safety, and tolerability of the Aurora B kinase inhibitor barasertib compared with low-dose cytosine arabinoside (LDAC) in patients aged ≥ 60 years with acute myeloid leukemia (AML).

METHODS

Patients were randomized 2:1 to receive either open-label barasertib 1200 mg (as a 7-day intravenous infusion) or LDAC 20 mg (subcutaneously twice daily for 10 days) in 28-day cycles. The primary endpoint was the objective complete response rate (OCRR) (complete responses [CR] plus confirmed CRs with incomplete recovery of neutrophils or platelets [CRi] according to Cheson criteria [also requiring reconfirmation of CRi ≥21 days after the first appearance and associated with partial recovery of platelets and neutrophils]). Secondary endpoints included overall survival (OS) and safety.

RESULTS

In total, 74 patients (barasertib, n = 48; LDAC, n = 26) completed ≥1 cycle of treatment. A significant improvement in the OCRR was observed with barasertib (35.4% vs 11.5%; difference, 23.9%; 95% confidence interval, 2.7%-39.9%; P < .05). Although the study was not formally sized to compare OS data, the median OS with barasertib was 8.2 months versus 4.5 months with LDAC (hazard ratio, 0.88; 95% confidence interval, 0.49-1.58; P = .663). Stomatitis and febrile neutropenia were the most common adverse events with barasertib versus LDAC (71% vs 15% and 67% vs 19%, respectively).

CONCLUSIONS

Barasertib produced a significant improvement in the OCRR versus LDAC and had a more toxic but manageable safety profile, consistent with previous studies.

Cyclin E drives human keratinocyte growth into differentiation

Human epidermis is continuously exposed to environmental mutagenic hazard and is the most frequent target of human cancer. How the epidermis coordinates proliferation with differentiation to maintain homeostasis, even in hyperproliferative conditions, is unclear. For instance, overactivation of the proto-oncogene MYC in keratinocytes stimulates differentiation. Here we explore the cell cycle regulation as proliferating human keratinocytes commit to terminal differentiation upon loss of anchorage or overactivation of MYC. The S-phase of the cell cycle is deregulated as mitotic regulators are inhibited in the onset of differentiation. Experimental inhibition of mitotic kinase cdk1 or kinases of the mitosis spindle checkpoint Aurora B or Polo-like Kinase, triggered keratinocyte terminal differentiation. Furthermore, hyperactivation of the cell cycle by overexpressing the DNA replication regulator Cyclin E induced mitosis failure and differentiation. Inhibition of Cyclin E by shRNAs attenuated the induction of differentiation by MYC. In addition, we present evidence that Cyclin E induces DNA damage and the p53 pathway. The results provide novel clues for the mechanisms committing proliferative keratinocytes to differentiate, with implications for tissue homeostasis maintenance, HPV amplification and tumorigenesis.

The transcription factor YY1 is a novel substrate for Aurora B kinase at G2/M transition of the cell cycle

Yin Yang 1 (YY1) is a ubiquitously expressed and highly conserved multifunctional transcription factor that is involved in a variety of cellular processes. Many YY1-regulated genes have crucial roles in cell proliferation, differentiation, apoptosis, and cell cycle regulation. Numerous mechanisms have been shown to regulate the function of YY1, such as DNA binding affinity, subcellular localization, and posttranslational modification including phosphorylation. Polo-like kinase 1(Plk1) and Casein kinase 2α (CK2 α) were the first two kinases identified to phosphorylate YY1. In this study, we identify a third kinase. We report that YY1 is a novel substrate of the Aurora B kinase both in vitro and in vivo. Serine 184 phosphorylation of YY1 by Aurora B is cell cycle regulated and peaks at G2/M and is rapidly dephosphorylated, likely by protein phosphatase 1 (PP1) as the cells enter G1. Aurora A and Aurora C can also phosphorylate YY1 in vitro, but at serine/threonine residues other than serine 184. We present evidence that phosphorylation of YY1 in the central glycine/alanine (G/A)-rich region is important for DNA binding activity, with a potential phosphorylation/acetylation interplay regulating YY1 function. Given their importance in mitosis and overexpression in human cancers, Aurora kinases have been identified as promising therapeutic targets. Increasing our understanding of Aurora substrates will add to the understanding of their signaling pathways.

Multimodal microtubule binding by the Ndc80 kinetochore complex

The Ndc80 complex is a key site of kinetochore-microtubule attachment during cell division. The human complex engages microtubules with a globular 'head' formed by tandem calponin-homology domains and an 80-amino-acid unstructured 'tail' that contains sites of phosphoregulation by the Aurora B kinase. Using biochemical, cell biological and electron microscopy analyses, we dissected the roles of the tail in binding of microtubules and mediation of cooperative interactions between Ndc80 complexes. Two segments of the tail that contain Aurora B phosphorylation sites become ordered at interfaces; one with tubulin and the second with an adjacent Ndc80 head on the microtubule surface, forming interactions that are disrupted by phosphorylation. We propose a model in which Ndc80's interaction with either growing or shrinking microtubule ends can be tuned by the phosphorylation state of its tail.

Aurora B-dependent regulation of class IIa histone deacetylases by mitotic nuclear localization signal phosphorylation

Class IIa histone deacetylases (HDACs 4/5/7/9) are transcriptional regulators with critical roles in cardiac disease and cancer. HDAC inhibitors are promising anticancer agents, and although they are known to disrupt mitotic progression, the underlying mechanisms of mitotic regulation by HDACs are not fully understood. Here we provide the first identification of histone deacetylases as substrates of Aurora B kinase (AurB). Our study identifies class IIa HDACs as a novel family of AurB targets and provides the first evidence that HDACs are temporally and spatially regulated by phosphorylation during the cell cycle. We define the precise site of AurB-mediated phosphorylation as a conserved serine within the nuclear localization signals of HDAC4, HDAC5, and HDAC9 at Ser265, Ser278, and Ser242, respectively. We establish that AurB interacts with these HDACs in vivo, and that this association increases upon disruption of 14-3-3 binding. We observe colocalization of endogenous, phosphorylated HDACs with AurB at the mitotic midzone in late anaphase and the midbody during cytokinesis, complemented by a reduction in HDAC interactions with components of the nuclear corepressor complex. We propose that AurB-dependent phosphorylation of HDACs induces sequestration within a phosphorylation gradient at the midzone, maintaining separation from re-forming nuclei and contributing to transcriptional control.

Integration of kinase and phosphatase activities by BUBR1 ensures formation of stable kinetochore-microtubule attachments

Maintenance of chromosomal stability depends on error-free chromosome segregation. The pseudokinase BUBR1 is essential for this, because it is a core component of the mitotic checkpoint and is required for formation of stable kinetochore-microtubule attachments. We have identified a conserved and highly phosphorylated domain (KARD) in BUBR1 that is crucial for formation of kinetochore-microtubule attachments. Deletion of this domain or prevention of its phosphorylation abolishes formation of kinetochore microtubules, which can be reverted by inhibiting Aurora B activity. Phosphorylation of KARD by PLK1 promotes direct interaction of BUBR1 with the PP2A-B56α phosphatase that counters excessive Aurora B activity at kinetochores. As a result, removal of BUBR1 from mitotic cells or inhibition of PLK1 reduces PP2A-B56α kinetochore binding and elevates phosphorylation of Aurora B substrates on the outer kinetochore. We propose that PLK1 and BUBR1 cooperate to stabilize kinetochore-microtubule interactions by regulating PP2A-B56α-mediated dephosphorylation of Aurora B substrates at the kinetochore-microtubule interface.

Aurora B is regulated by acetylation/deacetylation during mitosis in prostate cancer cells

Protein acetylation has been implicated in playing an important role during mitotic progression. Aurora B kinase is known to play a critical role in mitosis. However, whether Aurora B is regulated by acetylation is not known. Using IP with an anti-acetyl lysine antibody, we identified Aurora B as an acetylated protein in PC3 prostate cancer cells. Knockdown of HDAC3 or inhibiting HDAC3 deacetylase activity led to a significant increase (P<0.01 and P<0.05, respectively) in Aurora B acetylation as compared to siLuc or vehicle-treated controls. Increased Aurora B acetylation is correlated with a 30% reduction in Aurora B kinase activity in vitro and resulted in significant defects in Aurora B-dependent mitotic processes, including kinetochore-microtubule attachment and chromosome congression. Furthermore, Aurora B transiently interacts with HDAC3 at the kinetochore-microtubule interface of congressing chromosomes during prometaphase. This window of interaction corresponded with a transient but significant reduction (P=0.02) in Aurora B acetylation during early mitosis. Together, these results indicate that Aurora B is more active in its deacetylated state and further suggest a new mechanism by which dynamic acetylation/deacetylation acts as a rheostat to fine-tune Aurora B activity during mitotic progression.

In vivo FRET imaging revealed a regulatory role of RanGTP in kinetochore-microtubule attachments via Aurora B kinase

Under the fluctuating circumstances provided by the innate dynamics of microtubules and opposing tensions resulted from microtubule-associated motors, it is vital to ensure stable kinetochore-microtubule attachments for accurate segregation. However, a comprehensive understanding of how this regulation is mechanistically achieved remains elusive. Using our newly designed live cell FRET time-lapse imaging, we found that post-metaphase RanGTP is crucial in the maintenance of stable kinetochore-microtubule attachments by regulating Aurora B kinase via the NES-bearing Mst1. More importantly, our study demonstrates that by ensuring stable alignment of metaphase chromosomes prior to segregation, RanGTP is indispensible in governing the genomic integrity and the fidelity of cell cycle progression. Our findings suggest an additional role of RanGTP beyond its known function in mitotic spindle assembly during the prometaphase-metaphase transition.

LAB-1 targets PP1 and restricts Aurora B kinase upon entrance into meiosis to promote sister chromatid cohesion

At the onset of the first meiotic division, the protein LAB-1 recruits the PP1 phosphatase to cohesion complexes, preventing Aurora B kinase from targeting cohesins for degradation prematurely and thereby ensuring proper progression of meiotic events in C. elegans.

Successful execution of the meiotic program depends on the timely establishment and removal of sister chromatid cohesion. LAB-1 has been proposed to act in the latter by preventing the premature removal of the meiosis-specific cohesin REC-8 at metaphase I in C. elegans, yet the mechanism and scope of LAB-1 function remained unknown. Here we identify an unexpected earlier role for LAB-1 in promoting the establishment of sister chromatid cohesion in prophase I. LAB-1 and REC-8 are both required for the chromosomal association of the cohesin complex subunit SMC-3. Depletion of lab-1 results in partial loss of sister chromatid cohesion in rec-8 and coh-4 coh-3 mutants and further enhanced chromatid dissociation in worms where all three kleisins are mutated. Moreover, lab-1 depletion results in increased Aurora B kinase (AIR-2) signals in early prophase I nuclei, coupled with a parallel decrease in signals for the PP1 homolog, GSP-2. Finally, LAB-1 directly interacts with GSP-1 and GSP-2. We propose that LAB-1 targets the PP1 homologs to the chromatin at the onset of meiosis I, thereby antagonizing AIR-2 and cooperating with the cohesin complex to promote sister chromatid association and normal progression of the meiotic program.

A critical step for achieving successful cell division is the regulation of how the cohesin complexes that bind sister chromatids are initially deposited, then maintained, and finally removed to allow the chromatids to separate into daughter cells. This is particularly challenging during meiosis, when the sister chromatids must remain partially connected to each other through the first division. In organisms that have a single focal centromere on each chromosome, such as mammals and flies, cohesin is protected through the first meiotic division by the protein Shugoshin, which binds the PP2A phosphatase. PP2A counteracts phosphorylation by the Aurora B kinase; if certain cohesins are phosphorylated by Aurora B they become targeted for removal, which allows the chromatids to separate. In the nematode C. elegans, the chromosomes lack a localized centromere and the predicted Shugoshin homolog is not required for protection of cohesins; instead, this function is executed in metaphase of the first meiotic division by the protein LAB-1. But it is not completely understood what leads to the deposition of cohesin prior to entry into meiosis and to its maintenance throughout early meiosis I. In this study, we show that LAB-1 is also required for the loading and maintenance of the cohesin complex. LAB-1 ensures that the chromatids are not separated prematurely, and thus enables the proper progression of events through prophase I of meiosis. We propose that LAB-1 may act at the onset of meiosis in a manner akin to Shugoshin, by recruiting the PP1 phosphatase to counteract Aurora B kinase, thereby ensuring sister chromatid cohesion.

Polo-like kinase-activating kinases: Aurora A, Aurora B and what else?

The events of cell division are regulated by a complex interplay between kinases and phosphatases. Cyclin-dependent kinases (Cdks), polo-like kinases (Plks) and Aurora kinases play central roles in this process. Polo kinase (Plk1 in humans) regulates a wide range of events in mitosis and cytokinesis. To ensure the accuracy of these processes, polo activity itself is subject to complex regulation. Phosphorylation of polo in its T loop (or activation loop) increases its kinase activity several-fold. It has been shown that Aurora A kinase, with its co-factor Bora, activates Plk1 in G(2), and that this is essential for recovery from cell cycle arrest induced by DNA damage. In a recent article published in PLoS Biology, we report that Drosophila polo is activated by Aurora B kinase at centromeres, and that this is crucial for polo function in regulating chromosome dynamics in prometaphase. Our results suggest that this regulatory pathway is conserved in humans. Here, we propose a model for the collaboration between Aurora B and polo in the regulation of kinetochore attachment to microtubules in early mitosis. Moreover, we suggest that Aurora B could also function to activate Polo/Plk1 in cytokinesis. Finally, we discuss recent findings and open questions regarding the activation of polo and polo-like kinases by different kinases in mitosis, cytokinesis and other processes.

Expression of constitutively active CDK1 stabilizes APC-Cdh1 substrates and potentiates premature spindle assembly and checkpoint function in G1 cells

Mitotic progression in eukaryotic cells depends upon the activation of cyclin-dependent kinase 1 (CDK1), followed by its inactivation through the anaphase-promoting complex (APC)/cyclosome-mediated degradation of M-phase cyclins. Previous work revealed that expression of a constitutively active CDK1 (CDK1AF) in HeLa cells permitted their division, but yielded G1 daughter cells that underwent premature S-phase and early mitotic events. While CDK1AF was found to impede the sustained activity of APC-Cdh1, it was unknown if this defect improperly stabilized mitotic substrates and contributed to the occurrence of these premature M phases. Here, we show that CDK1AF expression in HeLa cells improperly stabilized APC-Cdh1 substrates in G1-phase daughter cells, including mitotic kinases and the APC adaptor, Cdc20. Division of CDK1AF-expressing cells produced G1 daughters with an accelerated S-phase onset, interrupted by the formation of premature bipolar spindles capable of spindle assembly checkpoint function. Further characterization of these phenotypes induced by CDK1AF expression revealed that this early spindle formation depended upon premature CDK1 and Aurora B activities, and their inhibition induced rapid spindle disassembly. Following its normal M-phase degradation, we found that the absence of Wee1 in these prematurely cycling daughter cells permitted the endogenous CDK1 to contribute to these premature mitotic events, since expression of a non-degradable Wee1 reduced the number of cells that exhibited premature cyclin B1oscillations. Lastly, we discovered that Cdh1-ablated cells could not be forced into a premature M phase, despite cyclin B1 overexpression and proteasome inhibition. Together, these results demonstrate that expression of constitutively active CDK1AF hampers the destruction of critical APC-Cdh1 targets, and that this type of condition could prevent newly divided cells from properly maintaining a prolonged interphase state. We propose that this more subtle type of defect in activity of the APC-driven negative-feedback loop may have implications for triggering genome instability and tumorigenesis.

Evolution of resistance to Aurora kinase B inhibitors in leukaemia cells

Aurora kinase inhibitors are new mitosis-targeting drugs currently in clinical trials for the treatment of haematological and solid malignancies. However, knowledge of the molecular factors that influence sensitivity and resistance remains limited. Herein, we developed and characterised an in vitro leukaemia model of resistance to the Aurora B inhibitor ZM447439. Human T-cell acute lymphoblastic leukaemia cells, CCRF-CEM, were selected for resistance in 4 µM ZM447439. CEM/AKB4 cells showed no cross-resistance to tubulin-targeted and DNA-damaging agents, but were hypersensitive to an Aurora kinase A inhibitor. Sequencing revealed a mutation in the Aurora B kinase domain corresponding to a G160E amino acid substitution. Molecular modelling of drug binding in Aurora B containing this mutation suggested that resistance is mediated by the glutamate substitution preventing formation of an active drug-binding motif. Progression of resistance in the more highly selected CEM/AKB8 and CEM/AKB16 cells, derived sequentially from CEM/AKB4 in 8 and 16 µM ZM447439 respectively, was mediated by additional defects. These defects were independent of Aurora B and multi-drug resistance pathways and are associated with reduced apoptosis mostly likely due to reduced inhibition of the catalytic activity of aurora kinase B in the presence of drug. Our findings are important in the context of the use of these new targeted agents in treatment regimes against leukaemia and suggest resistance to therapy may arise through multiple independent mechanisms.

Acute sensitization of colon cancer cells to inflammatory cytokines by prophase arrest

Understanding how colon cancer cells survive within the inflammatory milieu of a tumor, and developing approaches that increase their sensitivity to inflammatory cytokines, may ultimately lead to novel approaches for colon cancer therapy and prevention. Analysis of a number of chemopreventive and therapeutic agents reveal that HDAC inhibitors are particularly adept at sensitizing colon cancer cells TNF or TRAIL mediated apoptosis. In vivo data are consistent with an interaction between SAHA and TNF in inducing apoptosis, as AOM-induced colon tumors express elevated levels of TNF and are more sensitive to SAHA administration. Cell cycle analysis and time-lapse imaging indicated a close correspondence between SAHA-induced prophase arrest and TNF or TRAIL-induced apoptosis. Prophase arrest induced by the Aurora kinase inhibitor VX680 likewise sensitized cells to TNF and TRAIL, with siRNA analysis pointing to Aurora kinase A (and not Aurora kinase B) as being the relevant target for this sensitization. We propose that agents that promote prophase arrest may help sensitize cancer cells to TNF and other inflammatory cytokines. We also discuss how circumvention of an early mitotic checkpoint may facilitate cancer cell survival in the inflammatory micro-environment of the tumor.

Aurora-B mediated ATM serine 1403 phosphorylation is required for mitotic ATM activation and the spindle checkpoint

The ATM kinase plays a critical role in the maintenance of genetic stability. ATM is activated in response to DNA damage and is essential for cell-cycle checkpoints. Here, we report that ATM is activated in mitosis in the absence of DNA damage. We demonstrate that mitotic ATM activation is dependent on the Aurora-B kinase and that Aurora-B phosphorylates ATM on serine 1403. This phosphorylation event is required for mitotic ATM activation. Further, we show that loss of ATM function results in shortened mitotic timing and a defective spindle checkpoint, and that abrogation of ATM Ser1403 phosphorylation leads to this spindle checkpoint defect. We also demonstrate that mitotically activated ATM phosphorylates Bub1, a critical kinetochore protein, on Ser314. ATM-mediated Bub1 Ser314 phosphorylation is required for Bub1 activity and is essential for the activation of the spindle checkpoint. Collectively, our data highlight mechanisms of a critical function of ATM in mitosis.

Enhanced HSP70 lysine methylation promotes proliferation of cancer cells through activation of Aurora kinase B

Although heat-shock protein 70 (HSP70), an evolutionarily highly conserved molecular chaperone, is known to be post-translationally modified in various ways such as phosphorylation, ubiquitination and glycosylation, physiological significance of lysine methylation has never been elucidated. Here we identify dimethylation of HSP70 at Lys-561 by SETD1A. Enhanced HSP70 methylation was detected in various types of human cancer by immunohistochemical analysis, although the methylation was barely detectable in corresponding non-neoplastic tissues. Interestingly, methylated HSP70 predominantly localizes to the nucleus of cancer cells, whereas most of the HSP70 protein locates to the cytoplasm. Nuclear HSP70 directly interacts with Aurora kinase B (AURKB) in a methylation-dependent manner and promotes AURKB activity in vitro and in vivo. We also find that methylated HSP70 has a growth-promoting effect in cancer cells. Our findings demonstrate a crucial role of HSP70 methylation in human carcinogenesis.

Phase 1/2 study to assess the safety, efficacy, and pharmacokinetics of barasertib (AZD1152) in patients with advanced acute myeloid leukemia

The primary objective of this 2-part phase 1/2 study was to determine the maximum-tolerated dose (MTD) of the potent and selective Aurora B kinase inhibitor barasertib (AZD1152) in patients with newly diagnosed or relapsed acute myeloid leukemia (AML). Part A determined the MTD of barasertib administered as a continuous 7-day infusion every 21 days. In part B, the efficacy of barasertib was evaluated at the MTD. In part A, 32 patients were treated with barasertib 50 mg (n = 3), 100 mg (n = 3), 200 mg (n = 3), 400 mg (n = 4), 800 mg (n = 7), 1200 mg (n = 6), and 1600 mg (n = 6). Dose-limiting toxicities (stomatitis/mucosal inflammation events) were reported in the 800 mg (n = 1), 1200 mg (n = 1), and 1600 mg (n = 2) groups. The MTD was defined as 1200 mg. In part B, 32 patients received barasertib 1200 mg. In each part of the study, 8 of 32 patients had a hematologic response according to Cheson AML criteria. The most commonly reported grade ≥ 3 events were febrile neutropenia (n = 24) and stomatitis/mucosal inflammation (n = 16). We concluded that the MTD of barasertib is 1200 mg in patients with relapsed or newly diagnosed AML. Toxicity was manageable and barasertib treatment resulted in an overall hematologic response rate of 25%. This study is registered at www.ClinicalTrials.gov as NCT00497991.

Overexpression of active Aurora-C kinase results in cell transformation and tumour formation

Aurora kinases belong to a conserved family of serine/threonine kinases key regulators of cell cycle progression. Aurora-A and Aurora-B are expressed in somatic cells and involved mainly in mitosis while Aurora-C is expressed during spermatogenesis and oogenesis and is involved in meiosis. Aurora-C is hardly detectable in normal somatic cells. However all three kinases are overexpressed in many cancer lines. Aurora-A possesses an oncogenic activity while Aurora-B does not. Here we investigated whether Aurora-C possesses such an oncogenic activity. We report that overexpression of Aurora-C induces abnormal cell division resulting in centrosome amplification and multinucleation in both transiently transfected cells and in stable cell lines. Only stable NIH3T3 cell clones overexpressing active Aurora-C formed foci of colonies when grown on soft agar, indicating that a gain of Aurora-C activity is sufficient to transform cells. Furthermore, we reported that NIH-3T3 stable cell lines overexpressing Aurora-C induced tumour formation when injected into nude mice, demonstrating the oncogenic activity of enzymatically active Aurora kinase C. Interestingly enough tumor aggressiveness was positively correlated with the quantity of active kinase, making Aurora-C a potential anti-cancer therapeutic target.

Temporal control of nuclear envelope assembly by phosphorylation of lamin B receptor

The nuclear envelope of metazoans disassembles during mitosis and reforms in late anaphase after sister chromatids have well separated. The coordination of these mitotic events is important for genome stability, yet the temporal control of nuclear envelope reassembly is unknown. Although the steps of nuclear formation have been extensively studied in vitro using the reconstitution system from egg extracts, the temporal control can only be studied in vivo. Here, we use time-lapse microscopy to investigate this process in living HeLa cells. We demonstrate that Cdk1 activity prevents premature nuclear envelope assembly and that phosphorylation of the inner nuclear membrane protein lamin B receptor (LBR) by Cdk1 contributes to the temporal control. We further identify a region in the nucleoplasmic domain of LBR that inhibits premature chromatin binding of the protein. We propose that this inhibitory effect is partly mediated by Cdk1 phosphorylation. Furthermore, we show that the reduced chromatin-binding ability of LBR together with Aurora B activity contributes to nuclear envelope breakdown. Our studies reveal for the first time a mechanism that controls the timing of nuclear envelope reassembly through modification of an integral nuclear membrane protein.

Formation of stable attachments between kinetochores and microtubules depends on the B56-PP2A phosphatase

Error-free chromosome segregation depends on the precise regulation of phosphorylation to stabilize kinetochore-microtubule attachments (K-fibres) on sister chromatids that have attached to opposite spindle poles (bi-oriented). In many instances, phosphorylation correlates with K-fibre destabilization. Consistent with this, multiple kinases, including Aurora B and Plk1, are enriched at kinetochores of mal-oriented chromosomes when compared with bi-oriented chromosomes, which have stable attachments. Paradoxically, however, these kinases also target to prometaphase chromosomes that have not yet established spindle attachments and it is therefore unclear how kinetochore-microtubule interactions can be stabilized when kinase levels are high. Here we show that the generation of stable K-fibres depends on the B56-PP2A phosphatase, which is enriched at centromeres/kinetochores of unattached chromosomes. When B56-PP2A is depleted, K-fibres are destabilized and chromosomes fail to align at the spindle equator. Strikingly, B56-PP2A depletion increases the level of phosphorylation of Aurora B and Plk1 kinetochore substrates as well as Plk1 recruitment to kinetochores. Consistent with increased substrate phosphorylation, we find that chemical inhibition of Aurora or Plk1 restores K-fibres in B56-PP2A-depleted cells. Our findings reveal that PP2A, an essential tumour suppressor, tunes the balance of phosphorylation to promote chromosome-spindle interactions during cell division.

BPR1K653, a novel Aurora kinase inhibitor, exhibits potent anti-proliferative activity in MDR1 (P-gp170)-mediated multidrug-resistant cancer cells

Background

Over-expression of Aurora kinases promotes the tumorigenesis of cells. The aim of this study was to determine the preclinical profile of a novel pan-Aurora kinase inhibitor, BPR1K653, as a candidate for anti-cancer therapy. Since expression of the drug efflux pump, MDR1, reduces the effectiveness of various chemotherapeutic compounds in human cancers, this study also aimed to determine whether the potency of BPR1K653 could be affected by the expression of MDR1 in cancer cells.

Principal Findings

BPR1K653 specifically inhibited the activity of Aurora-A and Aurora-B kinase at low nano-molar concentrations in vitro. Anti-proliferative activity of BPR1K653 was evaluated in various human cancer cell lines. Results of the clonogenic assay showed that BPR1K653 was potent in targeting a variety of cancer cell lines regardless of the tissue origin, p53 status, or expression of MDR1. At the cellular level, BPR1K653 induced endo-replication and subsequent apoptosis in both MDR1-negative and MDR1-positive cancer cells. Importantly, it showed potent activity against the growth of xenograft tumors of the human cervical carcinoma KB and KB-derived MDR1-positive KB-VIN10 cells in nude mice. Finally, BPR1K653 also exhibited favorable pharmacokinetic properties in rats.

Conclusions and Significance

BPR1K653 is a novel potent anti-cancer compound, and its potency is not affected by the expression of the multiple drug resistant protein, MDR1, in cancer cells. Therefore, BPR1K653 is a promising anti-cancer compound that has potential for the management of various malignancies, particularly for patients with MDR1-related drug resistance after prolonged chemotherapeutic treatments.

Aurora B kinase activation requires survivin priming phosphorylation by PLK1

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

Feedback control in sensing chromosome biorientation by the Aurora B kinase

Maintenance of genome stability during cell division depends on establishing correct attachments between chromosomes and spindle microtubules. Correct, bioriented attachments are stabilized, whereas incorrect attachments are selectively destabilized. This process relies largely on increased phosphorylation of kinetochore substrates of Aurora B kinase at misaligned versus aligned kinetochores. Current models explain this differential phosphorylation by spatial changes in the position of substrates relative to a constant pool of kinase at the inner centromere. However, these models are based on studies in aneuploid cells. We show that normal diploid cells have a more robust error-correction machinery. Aurora B is enriched at misaligned centromeres in these cells, and the dynamic range of Aurora B substrate phosphorylation at misaligned versus aligned kinetochores is increased. These findings indicate that in addition to Aurora B regulating kinetochore-microtubule binding, the kinetochore also controls Aurora B recruitment to the inner centromere. We show that this recruitment depends on both activity of Plk1, a kinetochore-localized kinase, and activity of Aurora B itself. Our results suggest a feedback mechanism in which Aurora B both regulates and is regulated by chromosome attachment to the spindle, which amplifies the differential phosphorylation of kinetochore substrates and increases the efficiency of error correction.

Quantitative phosphoproteomics identifies substrates and functional modules of Aurora and Polo-like kinase activities in mitotic cells

Mitosis is a process involving a complex series of events that require careful coordination. Protein phosphorylation by a small number of kinases, in particular Aurora A, Aurora B, the cyclin-dependent kinase-cyclin complex Cdk1/cyclinB, and Polo-like kinase 1 (Plk1), orchestrates almost every step of cell division, from entry into mitosis to cytokinesis. To discover more about the functions of Aurora A, Aurora B, and kinases of the Plk family, we mapped mitotic phosphorylation sites to these kinases through the combined use of quantitative phosphoproteomics and selective targeting of kinase activities by small-molecule inhibitors. Using this integrated approach, we connected 778 phosphorylation sites on 562 proteins with these enzymes in cells arrested in mitosis. By connecting the kinases to protein complexes, we associated these kinases with functional modules. In addition to predicting previously unknown functions, this work establishes additional substrate-recognition motifs for these kinases and provides an analytical template for further use in dissecting kinase signaling events in other areas of cellular signaling and systems biology.

The Ndc80 complex uses a tripartite attachment point to couple microtubule depolymerization to chromosome movement

In kinetochores, the Ndc80 complex couples the energy in a depolymerizing microtubule to perform the work of moving chromosomes. The complex directly binds microtubules using an unstructured, positively charged N-terminal tail located on Hec1/Ndc80. Hec1/Ndc80 also contains a calponin homology domain (CHD) that increases its affinity for microtubules in vitro, yet whether it is required in cells and how the tail and CHD work together are critical unanswered questions. Human kinetochores containing Hec1/Ndc80 with point mutations in the CHD fail to align chromosomes or form productive microtubule attachments. Kinetochore architecture and spindle checkpoint protein recruitment are unaffected in these mutants, and the loss of CHD function cannot be rescued by removing Aurora B sites from the tail. The interaction between the Hec1/Ndc80 CHD and a microtubule is facilitated by positively charged amino acids on two separate regions of the CHD, and both are required for kinetochores to make stable attachments to microtubules. Chromosome congression in cells also requires positive charge on the Hec1 tail to facilitate microtubule contact. In vitro binding data suggest that charge on the tail regulates attachment by directly increasing microtubule affinity as well as driving cooperative binding of the CHD. These data argue that in vertebrates there is a tripartite attachment point facilitating the interaction between Hec1/Ndc80 and microtubules. We discuss how such a complex microtubule-binding interface may facilitate the coupling of depolymerization to chromosome movement.

Temporal and spatial regulation of targeting aurora B to the inner centromere

Successful partitioning of chromosomes in mitosis relies on the bipolar attachment of sister chromatids at metaphase. For this biorientation, the chromosomal passenger complex (CPC), composed of catalytic kinase Aurora B and regulatory components (INCENP, Survivin, and Borealin), must be localized at the center of paired kinetochores, the site called the inner centromere. It is largely unknown what defines the inner centromere and how the CPC is targeted to this site. Recent studies point out that the shugoshin protein (SGO), originally identified as a cohesin protector, also acts as a conserved centromeric adapter of the CPC. Phosphorylation of the CPC by Cdk1 promotes direct binding with shugoshin, thus explaining how the CPC is targeted to the centromere in a timely manner at prometaphase during the cell cycle. Moreover, the phosphorylation of histone H3 threonine 3 (H3-pT3) mediated by Haspin cooperates with Bub1-mediated H2A-S121 phosphorylation in targeting the CPC to the inner centromere. H3-pT3 promotes nucleosome binding of Survivin, whereas H2A-pS121 facilitates the binding of shugoshin. Haspin colocalizes with cohesin by associating with Pds5, a cohesin-binding protein, and Bub1 localizes at kinetochores. Thus, the inner centromere is defined by the spatial intersection of two histone marks mediated by cohesin- and kinetochore-associated kinases.

Effects of AZD1152, a selective Aurora B kinase inhibitor, on Burkitt's and Hodgkin's lymphomas

We studied the effects of AZD1152, an Aurora B kinase inhibitor, on Burkitt's lymphoma (BL) and Hodgkin's lymphoma (HL) in human tissues and cell cultures and in a murine xenograft model of lymphoma. Aurora kinase A and B levels were assessed by RT-PCR and immunohistochemistry. They were aberrantly expressed in BL and HL cell lines, and in lymph nodes from patients with BL and HL. Next, activation of the Aurora B promoter was detected by reporter gene assays. The promoter activity of Aurora B kinase was high in BL cell lines and the Aurora B promoter contained a positive regulatory region between -74 and -104 from the transcription initiation site. AZD1152-hQPA had antiproliferative effects in the BL and HL cell lines studied; inhibited the phosphorylation of histone H3 and retinoblastoma proteins, and resulted in cells with > 4N DNA content. AZD1152-hQPA induced caspase-dependent apoptosis of some cell lines, demonstrated by loss of mitochondrial membrane potential, activation of caspase-9, followed by activation of caspase-3. This effect was accompanied by the inhibition of survivin expression. In vivo efficacy was determined in NOD/SCID/γc(null) mice implanted with the Ramos human BL cell line. AZD1152 had anti-tumour effects in this murine xenograft model. There preclinical data suggest that the inhibition of Aurora B kinase is a potentially useful therapeutic strategy in BL and HL.

Aurora kinase-A regulates microtubule organizing center (MTOC) localization, chromosome dynamics, and histone-H3 phosphorylation in mouse oocytes

Aurora kinases (AURKs) are conserved serine/threonine kinases, crucial in regulating cell cycle events. Mammalian oocytes express all three Aurk isoforms throughout meiosis, with AurkA being the predominant isoform. Inhibition of all AURK isoforms by pharmacological means disrupts oocyte meiosis. Therefore, AurkA short interfering RNA (siRNA) was performed to silence AurkA gene expression in mouse oocytes and to further assess the function of AurkA during meiosis by analyzing subsequent loss-of-function oocyte phenotypes. Results indicated that AurkA siRNA applied in our experiments specifically knocked down both AurkA gene and protein expression without influencing transcript levels of AurkB/AurkC and other endogenous protein expression, such as GAPDH and ERK-2. AURKA was not essential for resumption of meiosis, but it potentiated oocyte meiotic progression. Knockdown of AurkA led to a significant reduction in the number of oocytes proceeding to metaphase II (MII). AurkA siRNA resulted in abnormal spindle assembly, improper localization of microtubule organizing centers (MTOCs) and misalignment of chromosomes in metaphase I (MI) oocytes. Co-immunoprecipitations demonstrated that AURKA was physically associated with phospho-Histone H3 ser10 in meiotic oocytes. AurkA siRNA dramatically reduced Histone H3 ser10 phosphorylation, but not ser28, and resulted in a significant increase of abnormal chromosome segregation in MII oocytes. In conclusion, as a predominant isoform among Aurks in oocytes, AurkA plays critical roles in mouse oocyte meiosis by regulating spindle and chromosome dynamics.

Aurora B interaction of centrosomal Nlp regulates cytokinesis

Cytokinesis is a fundamental cellular process, which ensures equal abscission and fosters diploid progenies. Aberrant cytokinesis may result in genomic instability and cell transformation. However, the underlying regulatory machinery of cytokinesis is largely undefined. Here, we demonstrate that Nlp (Ninein-like protein), a recently identified BRCA1-associated centrosomal protein that is required for centrosomes maturation at interphase and spindle formation in mitosis, also contributes to the accomplishment of cytokinesis. Through immunofluorescent analysis, Nlp is found to localize at midbody during cytokinesis. Depletion of endogenous Nlp triggers aborted division and subsequently leads to multinucleated phenotypes. Nlp can be recruited by Aurora B to the midbody apparatus via their physical association at the late stage of mitosis. Disruption of their interaction induces aborted cytokinesis. Importantly, Nlp is characterized as a novel substrate of Aurora B and can be phosphorylated by Aurora B. The specific phosphorylation sites are mapped at Ser-185, Ser-448, and Ser-585. The phosphorylation at Ser-448 and Ser-585 is likely required for Nlp association with Aurora B and localization at midbody. Meanwhile, the phosphorylation at Ser-185 is vital to Nlp protein stability. Disruptions of these phosphorylation sites abolish cytokinesis and lead to chromosomal instability. Collectively, these observations demonstrate that regulation of Nlp by Aurora B is critical for the completion of cytokinesis, providing novel insights into understanding the machinery of cell cycle progression.

The chromosome passenger complex is required for fidelity of chromosome transmission and cytokinesis in meiosis of mouse oocytes

The existence of two forms of the chromosome passenger complex (CPC) in the mammalian oocyte has meant that its role in female meiosis has remained unclear. Here we use loss- and gain-of function approaches to assess the meiotic functions of one of the shared components of these complexes, INCENP, and of the variable kinase subunits, Aurora B or Aurora C. We show that either the depletion of INCENP or the combined inhibition of Aurora kinases B and C activates the anaphase-promoting complex or cyclosome (APC/C) before chromosomes have properly congressed in meiosis I and also prevents cytokinesis and hence extrusion of the first polar body. Overexpression of Aurora C also advances APC/C activation and results in cytokinesis failure in a high proportion of oocytes, indicative of a dominant effect on CPC function. Together, this points to roles for the meiotic CPC in functions similar to the mitotic roles of the complex: correcting chromosome attachment to microtubules, facilitating the spindle-assembly checkpoint (SAC) function and enabling cytokinesis. Surprisingly, overexpression of Aurora B leads to a failure of APC/C activation, stabilization of securin and consequently a failure of chiasmate chromosomes to resolve - a dominant phenotype that is completely suppressed by depletion of INCENP. Taken together with the differential distribution of Aurora proteins B and C on chiasmate chromosomes, this points to differential functions of the two forms of CPC in regulating the separation of homologous chromosomes in meiosis I.

Shared and separate functions of polo-like kinases and aurora kinases in cancer

Large numbers of inhibitors for polo-like kinases and aurora kinases are currently being evaluated as anticancer drugs. Interest in these drugs is fuelled by the idea that these kinases have unique functions in mitosis. Within the polo-like kinase family, the emphasis for targeted therapies has been on polo-like kinase 1 (PLK1), and in the aurora kinase family drugs have been developed to specifically target aurora kinase A (AURKA; also known as STK6) and/or aurora kinase B (AURKB; also known as STK12). Information on the selectivity of these compounds in vivo is limited, but it is likely that off-target effects within the same kinase families will affect efficacy and toxicity profiles. In addition, it is becoming clear that interplay between polo-like kinases and aurora kinases is much more extensive than initially anticipated, and that both kinase families are important factors in the response to classical chemotherapeutics that damage the genome or the mitotic spindle. In this Review we discuss the implications of these novel insights on the clinical applicability of polo-like kinase and aurora kinase inhibitors.

Context-dependent cell cycle checkpoint abrogation by a novel kinase inhibitor

Background

Checkpoint kinase 1 and 2 (Chk1/Chk2), and the Aurora kinases play a critical role in the activation of the DNA damage response and mitotic spindle checkpoints. We have identified a novel inhibitor of these kinases and utilized this molecule to probe the functional interplay between these two checkpoints.

Principal Findings

Fragment screening, structure guided design, and kinase cross screening resulted in the identification of a novel, potent small molecule kinase inhibitor (VER-150548) of Chk1 and Chk2 kinases with IC₅₀s of 35 and 34 nM as well as the Aurora A and Aurora B kinases with IC₅₀s of 101 and 38 nM. The structural rationale for this kinase specificity could be clearly elucidated through the X-ray crystal structure. In human carcinoma cells, VER-150548 induced reduplication and the accumulation of cells with >4N DNA content, inhibited histone H3 phosphorylation and ultimately gave way to cell death after 120 hour exposure; a phenotype consistent with cellular Aurora inhibition. In the presence of DNA damage induced by cytotoxic chemotherapeutic drugs, VER-150548 abrogated DNA damage induced cell cycle checkpoints. Abrogation of these checkpoints correlated with increased DNA damage and rapid cell death in p53 defective HT29 cells. In the presence of DNA damage, reduplication could not be observed. These observations are consistent with the Chk1 and Chk2 inhibitory activity of this molecule.

Conclusions

In the presence of DNA damage, we suggest that VER-150548 abrogates the DNA damage induced checkpoints forcing cells to undergo a lethal mitosis. The timing of this premature cell death induced by Chk1 inhibition negates Aurora inhibition thereby preventing re-entry into the cell cycle and subsequent DNA reduplication. This novel kinase inhibitor therefore serves as a useful chemical probe to further understand the temporal relationship between cell cycle checkpoint pathways, chemotherapeutic agent induced DNA damage and cell death.

Role of a novel coiled-coil domain-containing protein CCDC69 in regulating central spindle assembly

The formation of the central spindle (or the spindle midzone) is essential for cytokinesis in animal cells. In this study, we report that coiled-coil domain-containing protein 69 (CCDC69) is implicated in controlling the assembly of central spindles and the recruitment of midzone components. Exogenous expression of CCDC69 in HeLa cells interfered with microtubule polymerization and disrupted the formation of bipolar mitotic spindles. Endogenous CCDC69 proteins were localized to the central spindle during anaphase. RNA interference (RNAi)-mediated knockdown of CCDC69 led to the formation of aberrant central spindles and disrupted the localization of midzone components such as aurora B kinase, protein regulator of cytokinesis 1 (PRC1), MgcRacGAP/HsCYK-4, and polo-like kinase 1 (Plk1) at the central spindle. Aurora B kinase was found to bind to CCDC69 and this binding depended on the coiled-coil domains at the C-terminus of CCDC69. Further, disruption of aurora B function in HeLa cells by treatment with a small chemical inhibitor led to the mislocalization of CCDC69 at the central spindle. Our results indicate that CCDC69 acts as a scaffold to regulate the recruitment of midzone components and the assembly of central spindles.

Two histone marks establish the inner centromere and chromosome bi-orientation

For proper partitioning of chromosomes in mitosis, the chromosomal passenger complex (CPC) including Aurora B and survivin must be localized at the center of paired kinetochores, at the site called the inner centromere. It is largely unknown what defines the inner centromere and how the CPC is targeted to this site. Here, we show that the phosphorylation of histone H3-threonine 3 (H3-pT3) mediated by Haspin cooperates with Bub1-mediated histone 2A-serine 121 (H2A-S121) phosphorylation in targeting the CPC to the inner centromere in fission yeast and human cells. H3-pT3 promotes nucleosome binding of survivin, whereas phosphorylated H2A-S121 facilitates the binding of shugoshin, the centromeric CPC adaptor. Haspin colocalizes with cohesin by associating with Pds5, whereas Bub1 localizes at kinetochores. Thus, the inner centromere is defined by intersection of two histone kinases.