Identification of phosphorylated residues that affect the activity of the mitotic kinase Aurora-A

Protein-protein interactions: a mechanism regulating the anti-metastatic properties of Nm23-H1

Nm23-H1, also known as NDPK-A, was the first of a class of metastasis suppressor genes to be identified. Overexpression of Nm23-H1 in metastatic cell lines (melanoma, breast carcinoma, prostate, colon, hepatocellular, and oral squamous cell carcinoma) reduced cell motility in in vitro assays and metastatic potential in xenograft models, without a significant effect on primary tumor size. The mechanism of Nm23-H1 suppression of metastasis, however, is incompletely understood. Nm23-H1 has been reported to bind proteins, including those in small G-protein complexes, transcriptional complexes, the Map kinase, the TGF-β signaling pathways and the cytoskeleton. Evidence supporting these associations is presented together with evidence of resultant biochemical and phenotypic consequences of association. Cumulatively, the data suggest that part of the anti-metastatic function of Nm23-H1 lies in pathways that it interrupts via binding and inactivation of proteins.

Control of centrin stability by Aurora A

Aurora A is an oncogenic serine/threonine kinase which can cause cell transformation and centrosome amplification when over-expressed. Human breast tumors show excess Aurora A and phospho-centrin in amplified centrosomes. Here, we show that Aurora A mediates the phosphorylation of and localizes with centrin at the centrosome, with both proteins reaching maximum abundance from prophase through metaphase, followed by their precipitous loss in late stages of mitosis. Over-expression of Aurora A results in excess phospho-centrin and centrosome amplification. In contrast, centrosome amplification is not seen in cells over-expressing Aurora A in the presence of a recombinant centrin mutant lacking the serine phosphorylation site at residue 170. Expression of a kinase dead Aurora A results in a decrease in mitotic index and abrogation of centrin phosphorylation. Finally, a recombinant centrin mutation that mimics centrin phosphorylation increases centrin's stability against APC/C-mediated proteasomal degradation. Taken together, these results suggest that the stability of centrin is regulated in part by Aurora A, and that excess phosphorylated centrin may promote centrosome amplification in cancer.

A kinase-independent role for Aurora A in the assembly of mitotic spindle microtubules in Caenorhabditis elegans embryos

The assembly of a functional mitotic spindle is crucial for achieving successful mitosis. Aurora A kinase is one of the key regulators of mitotic events, including mitotic entry, centrosome maturation and spindle bipolarity. Caenorhabditis elegans Aurora A (AIR-1) is responsible for the assembly of γ-tubulin-independent microtubules in early embryos; however, the mechanism by which AIR-1 contributes to microtubule assembly during mitosis has been unclear. Here we show by live-cell imaging and RNA-mediated interference (RNAi)-based modulation of gene activity that AIR-1 has a crucial role in the assembly of chromatin-stimulated microtubules that is independent of the γ-tubulin complex. Surprisingly, the kinase activity of AIR-1 is dispensable for this process. Although the kinase-inactive form of AIR-1 was detected along the microtubules as well as on centrosomes, the kinase-active form of AIR-1 was restricted to centrosomes. Thus, we propose that AIR-1 has a kinase-dependent role at centrosomes and a kinase-independent role for stabilizing spindle microtubules and that coordination of these two roles is crucial for the assembly of mitotic spindles.

A translational regulator, PUM2, promotes both protein stability and kinase activity of Aurora-A

Aurora-A, a centrosomal serine-threonine kinase, orchestrates several key aspects of cell division. However, the regulatory pathways for the protein stability and kinase activity of Aurora-A are still not completely understood. In this study, PUM2, an RNA-binding protein, is identified as a novel substrate and interacting protein of Aurora-A. Overexpression of the PUM2 mutant which fails to interact with Aurora-A, and depletion of PUM2 result in a decrease in the amount of Aurora-A. PUM2 physically binds to the D-box of Aurora-A, which is recognized by APC/C^Cdh1. Overexpression of PUM2 prevents ubiquitination and enhances the protein stability of Aurora-A, suggesting that PUM2 protects Aurora-A from APC/C^Cdh1-mediated degradation. Moreover, association of PUM2 with Aurora-A not only makes Aurora-A more stable but also enhances the kinase activity of Aurora-A. Our study suggests that PUM2 plays two different but important roles during cell cycle progression. In interphase, PUM2 localizes in cytoplasm and plays as translational repressor through its RNA binding domain. However, in mitosis, PUM2 physically associates with Aurora-A to ensure enough active Aurora-A at centrosomes for mitotic entry. This is the first time to reveal the moonlight role of PUM2 in mitosis.

State of the APC/C: organization, function, and structure

The ubiquitin-proteasome protein degradation system is involved in many essential cellular processes including cell cycle regulation, cell differentiation, and the unfolded protein response. The anaphase-promoting complex/cyclosome (APC/C), an evolutionarily conserved E3 ubiquitin ligase, was discovered 15 years ago because of its pivotal role in cyclin degradation and mitotic progression. Since then, we have learned that the APC/C is a very large, complex E3 ligase composed of 13 subunits, yielding a molecular machine of approximately 1 MDa. The intricate regulation of the APC/C is mediated by the Cdc20 family of activators, pseudosubstrate inhibitors, protein kinases and phosphatases and the spindle assembly checkpoint. The large size, complexity, and dynamic nature of the APC/C represent significant obstacles toward high-resolution structural techniques; however, over the last decade, there have been a number of lower resolution APC/C structures determined using single particle electron microscopy. These structures, when combined with data generated from numerous genetic and biochemical studies, have begun to shed light on how APC/C activity is regulated. Here, we discuss the most recent developments in the APC/C field concerning structure, substrate recognition, and catalysis.

Aurora-A overexpression and aneuploidy predict poor outcome in serous ovarian carcinoma

OBJECTIVE

Aurora-A is a potential oncogene and therapeutic target in ovarian carcinoma. It is involved in mitotic events and overexpression leads to centrosome amplification and chromosomal instability. The objective of this study was to evaluate the clinical significance of Aurora-A and DNA ploidy in serous ovarian carcinoma.

METHODS

Serous ovarian carcinomas were analysed for Aurora-A protein by immunohistochemistry (n=592), Aurora-A copy number by CISH (n=169), Aurora-A mRNA by real-time PCR (n=158) and DNA ploidy by flowcytometry (n=440).

RESULTS

Overexpression of Aurora-A was found in 27% of the tumors, cytoplasmic overexpression in 11% and nuclear in 17%. The cytoplasmic and nuclear overexpression were nearly mutually exclusive. Both cytoplasmic and nuclear overexpression were associated with shorter survival, high grade, high proliferation index and aberrant p53. Interestingly, only cytoplasmic expression was associated with aneuploidy and expression of phosphorylated Aurora-A. DNA ploidy was associated with poor patient outcome as well as aggressive clinicopathological parameters. In multivariate analysis, Aurora-A overexpression appeared as an independent prognostic factor for disease-free survival, together with grade, stage and ploidy.

CONCLUSIONS

Aurora-A protein expression is strongly linked with poor patient outcome and aggressive disease characteristics, which makes Aurora-A a promising biomarker and a potential therapeutic target in ovarian carcinoma. Cytoplasmic and nuclear Aurora-A protein may have different functions. DNA aneuploidy is a strong predictor of poor prognosis in serous ovarian carcinoma.

Aurora-A kinase inhibitor scaffolds and binding modes

Aurora kinases (A-C) belong to the serine/threonine protein kinase family. In recent years, the constitutive or elevated expression of Aurora kinases has been found in cancer cells and oncogene transfected cells. In this review, we summarize the common binding modes of Aurora-A kinase inhibitors, the hot spot residues in the binding sites and the privileged inhibitor structures. Our review of the reported chemical scaffolds of Aurora-A kinase inhibitors and their binding modes could provide a useful framework from which new design strategies for inhibitors might be assessed or developed.

Centrosomal protein of 192 kDa (Cep192) promotes centrosome-driven spindle assembly by engaging in organelle-specific Aurora A activation

Centrosomes are primary microtubule (MT)-organizing centers (MTOCs). During mitosis, they dramatically increase their size and MT-nucleating activity and participate in spindle assembly from spindle poles. These events require the serine/threonine kinase, Aurora A (AurA), and the centrosomal protein of 192 kDa (Cep192)/spindle defective 2 (Spd-2), but the underlying mechanism remains unclear. We have found that Cep192, unlike targeting protein for Xklp2 (TPX2), a known MT-localizing AurA activator, is an AurA cofactor in centrosome-driven spindle assembly. Cep192, through a direct interaction, targets AurA to mitotic centrosomes where the locally accumulating AurA forms homodimers or oligomers. The dimerization of endogenous AurA, in the presence of bound Cep192, triggers potent kinase activation that, in turn, drives MT assembly. Depletion of Cep192 or specific interference with AurA-Cep192 binding did not prevent AurA oligomerization on MTs but abrogated AurA recruitment to centrosomes and its activation by either sperm nuclei or anti-AurA antibody (αAurA)-induced dimerization. In these settings, MT assembly by both centrosomes and αAurA-coated beads was also abolished or severely compromised. Hence, Cep192 activates AurA by a mechanism different from that previously described for TPX2. The Cep192-mediated mechanism maximizes AurA activity at centrosomes and appears essential for the function of these organelles as MTOCs.

ZNF217 confers resistance to the pro-apoptotic signals of paclitaxel and aberrant expression of Aurora-A in breast cancer cells

BACKGROUND

ZNF217 is a candidate oncogene located at 20q13, a chromosomal region frequently amplified in breast cancers. The precise mechanisms involved in ZNF217 pro-survival function are currently unknown, and utmost importance is given to deciphering the role of ZNF217 in cancer therapy response.

RESULTS

We provide evidence that stable overexpression of ZNF217 in MDA-MB-231 breast cancer cells conferred resistance to paclitaxel, stimulated cell proliferation in vitro associated with aberrant expression of several cyclins, and increased tumor growth in mouse xenograft models. Conversely, siRNA-mediated silencing of ZNF217 expression in MCF7 breast cancer cells, which possess high endogenous levels of ZNF217, led to decreased cell proliferation and increased sensitivity to paclitaxel. The paclitaxel resistance developed by ZNF217-overexpressing MDA-MB-231 cells was not mediated by the ABCB1/PgP transporter. However, ZNF217 was able to counteract the apoptotic signals mediated by paclitaxel as a consequence of alterations in the intrinsic apoptotic pathway through constitutive deregulation of the balance of Bcl-2 family proteins. Interestingly, ZNF217 expression levels were correlated with the oncogenic kinase Aurora-A expression levels, as ZNF217 overexpression led to increased expression of the Aurora-A protein, whereas ZNF217 silencing was associated with low Aurora-A expression levels. We showed that a potent Aurora-A kinase inhibitor was able to reverse paclitaxel resistance in the ZNF217-overexpressing cells.

CONCLUSION

Altogether, these data suggest that ZNF217 might play an important role in breast neoplastic progression and chemoresistance, and that Aurora-A might be involved in ZNF217-mediated effects.

Rapid calcium-dependent activation of Aurora-A kinase

Oncogenic hyperactivation of the mitotic kinase Aurora-A (AurA) in cancer is associated with genomic instability. Increasing evidence indicates that AurA also regulates critical processes in normal interphase cells, but the source of such activity has been obscure. We report here that multiple stimuli causing release of Ca²⁺ from intracellular endoplasmic reticulum stores rapidly and transiently activate AurA, without requirement for second messengers. This activation is mediated by direct Ca²⁺-dependent calmodulin (CaM) binding to multiple motifs on AurA. On the basis of structure–function analysis and molecular modelling, we map two primary regions of CaM-AurA interaction to unfolded sequences in the AurA N- and C-termini. This unexpected mechanism for AurA activation provides a new context for evaluating the function of AurA and its inhibitors in normal and cancerous cells.

Aurora-A kinase localizes to centrosomes, is involved in the progression through mitosis and is overexpressed in certain cancers. Here, calcium is shown to induce Aurora-A auto-phosphorylation in a calmodulin-dependent manner, suggesting a novel role for Aurora-A in non-mitotic cells.

Arpc1b, a centrosomal protein, is both an activator and substrate of Aurora A

In addition to its function as an Arp2/3 complex subunit, Arp1cb interacts with and stimulates Aurora A at centrosomes, functioning in cell cycle progression.

Here we provide evidence in support of an inherent role for Arpc1b, a component of the Arp2/3 complex, in regulation of mitosis and demonstrate that its depletion inhibits Aurora A activation at the centrosome and impairs the ability of mammalian cells to enter mitosis. We discovered that Arpc1b colocalizes with γ-tubulin at centrosomes and stimulates Aurora A activity. Aurora A phosphorylates Arpc1b on threonine 21, and expression of Arpc1b but not a nonphosphorylatable Arpc1b mutant in mammalian cells leads to Aurora A kinase activation and abnormal centrosome amplification in a Pak1-independent manner. Together, these findings reveal a new function for Arpc1b in centrosomal homeostasis. Arpc1b is both a physiological activator and substrate of Aurora A kinase and these interactions help to maintain mitotic integrity in mammalian cells.

Preclinical activity of a novel multiple tyrosine kinase and aurora kinase inhibitor, ENMD-2076, against multiple myeloma

ENMD-2076 is a novel, orally-active molecule that has been shown to have significant activity against aurora and multiple receptor tyrosine kinases. We investigated the activity of ENMD-2076 against multiple myeloma (MM) cells in vitro and in vivo. ENMD-2076 showed significant cytotoxicity against MM cell lines and primary cells, with minimal cytotoxicity to haematopoietic progenitors. ENMD-2076 inhibited the phosphoinositide 3-kinase/AKT pathway and downregulated survivin and X-linked inhibitor of apoptosis as early as 6 h after treatment. With longer treatment (24-48 h), ENMD-2076 also inhibited aurora A and B kinases, and induced G(2)/M cell cycle arrest. In non-obese diabetic/severe combined immunodeficient mice implanted with H929 human plasmacytoma xenografts, oral treatment with ENMD-2076 (50, 100, 200 mg/kg per day) resulted in a dose-dependent inhibition of tumour growth. Immunohistochemical staining of excised tumours showed significant reduction in phospho-Histone 3 (pH3), Ki-67, and angiogenesis, and also a significant increase in cleaved caspase-3 at all dose levels compared to tumours from vehicle-treated mice. In addition, a significant reduction in p-FGFR3 was observed on Western blot. ENMD-2076 shows significant activity against MM cells in vitro and in vivo, and acts on several pathways important for myeloma cell growth and survival. These results provide preclinical rationale for clinical investigation of ENMD-2076 in MM.

Aurora A selective inhibitor MLN8237 suppresses the growth and survival of HTLV-1-infected T-cells in vitro

Aurora A kinase plays an essential role in the proper assembly and function of the mitotic spindle. We have shown previously that Aurora A expression is increased aberrantly in human T-cell leukemia virus type 1 (HTLV-1)-infected T-cell lines and primary adult T-cell leukemia cells, and a pan-Aurora kinase inhibitor, which inhibits both Aurora A and Aurora B kinases, reduces viability and induces apoptosis in these cells. However, the specific effects of Aurora A inhibition on HTLV-1-infected T-cells are poorly understood. In this study, we addressed this question by comparing the effects of MLN8237, a selective inhibitor of Aurora A, on cell viability, cell cycle progression, and induction of apoptosis in HTLV-1-infected and -uninfected T-cell lines. MLN8237 reduced the viability of HTLV-1-infected T-cell lines within 24 h, but its effects on that of HTLV-1-uninfected T-cell lines were moderate. MLN8237 induced early apoptosis of HTLV-1-infected T-cell lines without induction of polyploidy. It induced p53 and p21 expression in HTLV-1-infected but not in -uninfected T-cell lines, suggesting that MLN8237-treated HTLV-1-infected T-cell lines exit from mitosis and activate a p53-dependent postmitotic G(1) checkpoint, leading to G(1) arrest followed by the induction of apoptosis. Our results suggest that specific inhibition of Aurora A kinase is a potentially useful therapeutic strategy in the treatment of adult T-cell leukemia and that further in vivo exploration is warranted.

Biology of Aurora A kinase: implications in cancer manifestation and therapy

The Aurora A kinase belongs to serine/threonine group of kinases, well known for its role in cell cycle, especially in the regulation of mitosis. Numerous substrates of Aurora A kinase have been identified, which are predominantly related to cell cycle progression while some of them are transcription factors. Aurora A-mediated phosphorylation can either directly or indirectly regulate the function of its substrates. There are overwhelming evidences which report overexpression and gene amplification of Aurora A in several human cancers, and suggest that Aurora A could be a bona fide oncogene involved in tumorigenesis. Hence, Aurora A plays wide-ranging roles in both mitosis and its deregulation manifests in cancer progression. These observations have favored the choice of Aurora kinases as a target for cancer therapy. Recently, numerous small molecules have been discovered against Aurora kinases and many have entered clinical trials. Most of these small-molecule modulators designed are specific against either Aurora A or Aurora B, but some are dual inhibitors targeting the ATP-binding site which is highly conserved among the three human homologues of Aurora kinase. In this review, we discuss the physiological functions of Aurora A, interactions between Aurora A kinase and its cellular substrates, tumorigenesis mediated by Aurora A kinase upon overexpression, and small-molecule modulators of Aurora kinase as targets for cancer therapy.

Aurora-A kinase: a novel target both for cellular immunotherapy and molecular target therapy against human leukemia

BACKGROUND

Although cellular immunotherapy still remains in its infancy, it is one of the important treatment options against cancer. The marked improvement of its clinical efficacy requires a 'better' target antigen, which is well recognized by cancer-cell-specific cytotoxic T lymphocytes. We have recently demonstrated the potential of Aurora-A kinase (Aurora-A) as such a 'better' target for cellular immunotherapy against human leukemia. Aurora-A is a member of the serine/threonine kinase family that properly regulates the cell division process, and has recently been implicated in tumorigenesis. On the other hand, small-molecule inhibitors targeting Aurora-A have recently been developed and preliminary but promising observations from Phase I clinical trials have been reported. These facts highlight the attractiveness of Aurora-A as an important target of comprehensive cancer therapies.

OBJECTIVE/METHODS

In this review, we cover Aurora-A in the areas of immunotherapy and small-molecule inhibitor therapy against cancers.

RESULTS/CONCLUSIONS

Aurora-A kinase is an attractive molecule not only as a target for small-molecule inhibitors, but also as a potential target for immunotherapy against cancer.

Danusertib (formerly PHA-739358)--a novel combined pan-Aurora kinases and third generation Bcr-Abl tyrosine kinase inhibitor

The Aurora kinases belong to a family of highly conserved serine/threonine protein kinases. They play an essential role as key mitotic regulators, controlling entry into mitosis, centrosome function, chromosome assembly, and segregation. As many other regulators of mitosis, Aurora kinases are frequently found to be aberrantly overexpressed in cancer cells. Therefore, these proteins have become an attractive target for the development of new anticancer therapies. In fact, several small-molecule inhibitors of Aurora kinases have already been developed and some of them have shown promising clinical efficacy in a number of human tumors in Phase I and II clinical trials. Among those, one of the most advanced clinical compound currently is Danusertib (formerly PHA-739358), which exhibits inhibitory activity against all known Aurora kinases as well as other cancer-relevant kinases such as the Bcr-Abl tyrosine kinase, including its multidrug-resistant T315I mutant. This mutation is responsible for up to 25% of all clinically observed resistances in CML patients undergoing Imatinib therapy. However, this particular mutation is predicted to play an even more important clinical role in the future, since in addition to Imatinib, it also confers resistance to second-generation Bcr-Abl inhibitors such as Nilotinib, Dasatinib, and Bosutinib. Therefore, combined Aurora and Bcr-Abl inhibition (the latter including high-grade resistance conferring mutations) with compounds such as Danusertib represents a promising new strategy for treatment of Bcr-Abl positive leukemias, especially those in second and third line of treatment.

Aurora A, centrosome structure, and the centrosome cycle

The centrosome, also known as the microtubule organizing center of the cell, is a membrane-less organelle composed of a pair of barrel-shaped centrioles surrounded by electron-dense pericentriolar material. The centrosome progresses through the centrosome cycle in step with the cell cycle such that centrosomes are duplicated in time to serve as the spindle poles during mitosis and that each resultant daughter cell contains a single centrosome. Regulation of the centrosome cycle with relation to the cell cycle is an essential process to maintain the ratio of one centrosome per new daughter cell. Numerous mitosis-specific kinases have been implicated in this regulation, and phosphorlyation plays an important role in coordinating the centrosome and cell cycles. Centrosome amplification can occur when the cycles are uncoupled, and this amplification is associated with cancer and with an increase in the levels of chromosomal instability. The aurora kinases A, B, and C are serine/threonine kinases that are active during mitosis. Aurora A is associated with centrosomes, being localized at the centrosome just prior to the onset of mitosis and for the duration of mitosis. Overexpression of aurora A leads to centrosome amplification and cellular transformation. The activity of aurora A is regulated by phosphorlyation and proteasomal degradation.

Aurora A is differentially expressed and regulated in chromosomal and microsatellite instable sporadic colorectal cancers

The centrosome-associated kinase aurora A has been shown to be involved in genetic instability and to be (over)expressed in several human carcinomas. This study investigated aurora A gene copy numbers, mRNA and protein expression as well as tumour cell proliferation and aneuploidy in chromosomal and microsatellite instable sporadic colorectal cancers. Case-matched tissues of normal (n=71) and dysplastic (n=49) colorectal epithelium and invasive carcinomas (n=71) were included in this study. PCR-based microsatellite analysis classified 14/71 (20%) of carcinomas as microsatellite instable. A stepwise increase of aurora A mRNA expression (P<0.0001; quantitative RT-PCR) and aurora A protein expressing tumour cells (P=0.0141; immunohistochemistry) occurred in the adenoma-carcinoma sequence. Within invasive carcinomas, aurora A mRNA levels (P=0.0259) and aurora A positive tumour cells (P<0.0001) were closely associated with tumour cell proliferation (Ki-67 specific immunohistochemistry). Compared with chromosomal instable carcinomas, microsatellite instable carcinomas had significantly more aurora A positive tumour cells (P=0.0043) and a higher tumour cell proliferation (P=0.0335). In contrast, only chromosomal instable carcinomas exhibited marked tumour cell aneuploidy (P=0.0004, fluorescence in situ hybridization) and significantly higher aurora A gene copy numbers (P=0.0206) as compared with microsatellite instable carcinomas. This study further supports a role of aurora A in the carcinogenesis of sporadic colorectal cancers. Moreover, it demonstrates that in a minority of predominantly microsatellite instable carcinomas the presence of aurora A positive tumour cells is merely reflecting tumour cell proliferation. In contrast, the large majority of chromosomal instable carcinomas shows additional (de)regulation of aurora A by gene amplification and concomitant tumour cell aneuploidy. Thus, sporadic colorectal cancers exhibit different mechanisms of aurora A regulation and this may impact the efficacy of aurora-targeted therapies.

The decision to enter mitosis: feedback and redundancy in the mitotic entry network

The decision to enter mitosis is mediated by a network of proteins that regulate activation of the cyclin B–Cdk1 complex. Within this network, several positive feedback loops can amplify cyclin B–Cdk1 activation to ensure complete commitment to a mitotic state once the decision to enter mitosis has been made. However, evidence is accumulating that several components of the feedback loops are redundant for cyclin B–Cdk1 activation during normal cell division. Nonetheless, defined feedback loops become essential to promote mitotic entry when normal cell cycle progression is perturbed. Recent data has demonstrated that at least three Plk1-dependent feedback loops exist that enhance cyclin B–Cdk1 activation at different levels. In this review, we discuss the role of various feedback loops that regulate cyclin B–Cdk1 activation under different conditions, the timing of their activation, and the possible identity of the elusive trigger that controls mitotic entry in human cells.

7-Diethylamino-3(2'-benzoxazolyl)-coumarin is a novel microtubule inhibitor with antimitotic activity in multidrug resistant cancer cells

Microtubules are a proven target for anticancer drug development because they are critical for mitotic spindle formation and the separation of chromosomes at mitosis. We here report a novel synthetic microtubule inhibitor 7-diethylamino-3(2'-benzoxazolyl)-coumarin (DBC). DBC causes destabilization of microtubules, leading to a cell cycle arrest at G(2)/M stage. In addition, human cancer cells are more sensitive to DBC (IC(50) 44.8-475.2nM) than human normal fibroblast (IC(50) 7.9microM), and DBC induces apoptotic cell death of cancer cells. Furthermore, our data show that DBC is a poor substrate of drug efflux pumps and effective against multidrug resistant (MDR) cancer cells. Taken together, these results describe a novel pharmacological property of DBC as a microtubule inhibitor, which may make it an attractive new agent for treatment of MDR cancer.

Phosphorylation of p53 is regulated by TPX2-Aurora A in xenopus oocytes

p53 is an important tumor suppressor regulating the cell cycle at multiple stages in higher vertebrates. The p53 gene is frequently deleted or mutated in human cancers, resulting in loss of p53 activity. This leads to centrosome amplification, aneuploidy, and tumorigenesis, three phenotypes also observed after overexpression of the oncogenic kinase Aurora A. Accordingly, recent studies have focused on the relationship between these two proteins. p53 and Aurora A have been reported to interact in mammalian cells, but the function of this interaction remains unclear. We recently reported that Xenopus p53 can inhibit Aurora A activity in vitro but only in the absence of TPX2. Here we investigate the interplay between Xenopus Aurora A, TPX2, and p53 and show that newly synthesized TPX2 is required for nearly all Aurora A activation and for full p53 synthesis and phosphorylation in vivo during oocyte maturation. In vitro, phosphorylation mediated by Aurora A targets serines 129 and 190 within the DNA binding domain of p53. Glutathione S-transferase pull-down studies indicate that the interaction occurs via the p53 transactivation domain and the Aurora A catalytic domain around the T-loop. Our studies suggest that targeting of TPX2 might be an effective strategy for specifically inhibiting the phosphorylation of Aurora A substrates, including p53.

Aurora kinases: structure, functions and their association with cancer

BACKGROUND

Aurora kinases are a recently discovered family of kinases (A, B & C) consisting of highly conserved serine\threonine protein kinases found to be involved in multiple mitotic events: regulation of spindle assembly checkpoint pathway, function of centrosomes and cytoskeleton, and cytokinesis. Aberrant expression of Aurora kinases may lead to cancer. For this reason the Aurora kinases are potential targets in the treatment of cancer. In this review we discuss the biology of these kinases: structure, function, regulation and association with cancer.

METHODS AND RESULTS

A literature search.

CONCLUSION

Many of the multiple functions of mitosis are mediated by the Aurora kinases. Their aberrant expression can lead to the deregulation of cell division and cancer. For this reason, the Aurora kinases are currently one of the most interesting targets for cancer therapy. Some Aurora kinase inhibitors in the clinic have proven effectively on a wide range of tumor types. The clinical data are very encouraging and promising for development of novel class of structurally different Aurora kinase inhibitors. Hopefully the Aurora kinases will be potentially useful in drug targeted cancer treatment.

Aurora A is essential for early embryonic development and tumor suppression

Aurora A is a serine/threonine kinase that functions in various stages of mitosis. Accumulating evidence has demonstrated that gene amplification and overexpression of Aurora A are linked to tumorigenesis, suggesting that Aurora A is an oncogene. In addition, Aurora A overexpression has been used as a negative prognostic marker, because it is associated with resistance to anti-mitotic agents commonly used for cancer therapy. To understand the physiological functions of Aurora A, we generated Aurora A knock-out mice. Aurora A null mice die early during embryonic development before the 16-cell stage. These Aurora A null embryos have defects in mitosis, particularly in spindle assembly, supporting critical functions of Aurora A during mitotic transitions. Interestingly, Aurora A heterozygosity results in a significantly increased tumor incidence in mice, suggesting that Aurora A may also act as a haploinsufficient tumor suppressor. Consistently, Aurora A heterozygous mouse embryonic fibroblasts have higher rates of aneuploidy. We further discovered that VX-680, an Aurora kinase inhibitor currently in phase II clinical trials for cancer treatment, could induce aneuploidy in wild type mouse embryonic fibroblasts. We conclude that a balanced Aurora A level is critical for maintaining genomic stability and one needs to be fully aware of the potential side effects of anti-cancer therapy based on the use of Aurora A-specific inhibitors.

Plk1 regulates mitotic Aurora A function through betaTrCP-dependent degradation of hBora

Polo-like kinase 1 (Plk1) and Aurora A play key roles in centrosome maturation, spindle assembly, and chromosome segregation during cell division. Here we show that the functions of these kinases during early mitosis are coordinated through Bora, a partner of Aurora A first identified in Drosophila. Depletion of human Bora (hBora) results in spindle defects, accompanied by increased spindle recruitment of Aurora A and its partner TPX2. Conversely, hBora overexpression induces mislocalization of Aurora A and monopolar spindle formation, reminiscent of the phenotype seen in Plk1-depleted cells. Indeed, Plk1 regulates hBora. Following Cdk1-dependent recruitment, Plk1 triggers hBora destruction by phosphorylating a recognition site for SCF(Beta-TrCP). Plk1 depletion or inhibition results in a massive accumulation of hBora, concomitant with displacement of Aurora A from spindle poles and impaired centrosome maturation, but remarkably, co-depletion of hBora partially restores Aurora A localization and bipolar spindle formation. This suggests that Plk1 controls Aurora A localization and function by regulating cellular levels of hBora.

Porcine Aurora A accelerates Cyclin B and Mos synthesis and promotes meiotic resumption of porcine oocytes

Full-grown oocytes arrested at germinal vesicle stage contain many dormant maternal mRNAs, and Aurora A has been reported to play a key role for the translation of these maternal mRNAs in Xenopus oocytes. Although the presence of Aurora A has been reported in mammals, the functions of Aurora A on the protein synthesis and the meiotic resumption have never been elucidated in mammalian oocytes. In the present study, the effects of porcine Aurora A on meiotic resumption of porcine oocytes were examined. At first, we cloned porcine Aurora A from total RNA of immature porcine oocytes by RT-PCR and obtained full-length cDNA that was 77%, 86% and 54% homologous with mouse, human and Xenopus Aurora A, respectively. The Aurora A mRNA and large amounts of protein were present throughout maturation period in porcine oocytes. The overexpression of porcine Aurora A by the mRNA injection into immature porcine oocytes had no effects on Cyclin B synthesis and meiotic resumption. Therefore we constructed a mutated Aurora A (AA-Aurora A), which was replaced the expecting inhibitory phosphorylation sites, serines 283 and 284, to non-phosphorylatable alanines. The oocytes expressed AA-Aurora A were accelerated their Cyclin B synthesis and Rsk phosphorylation, an indicator of Mos synthesis, then their meiotic resumption was promoted significantly. These results suggest for the first time in mammalian oocytes that mammalian Aurora A stimulates the protein synthesis and promotes the meiotic resumption. In addition, we identified the inhibitory phosphorylation sites of porcine Aurora A, and indicate the presence of phosphorylation-dependent regulation mechanisms in mammalian Aurora A.

Nuclear epidermal growth factor receptor (EGFR) interacts with signal transducer and activator of transcription 5 (STAT5) in activating Aurora-A gene expression

Loss of the maintenance of genetic material is a critical step leading to tumorigenesis. It was reported that overexpression of Aurora-A and the constitutive activation of the epidermal growth factor (EGF) receptor (EGFR) are implicated in chromosome instability. In this study, we examined that when cells treated with EGF result in centrosome amplification and microtubule disorder, which are critical for genetic instability. Interestingly, the expression of Aurora-A was also increased by EGF stimulus. An immunofluorescence assay indicated that EGF can induce the nuclear translocation of EGFR. Chromatin immunoprecipitation (ChIP) and re-ChIP assays showed significant EGF-induced recruitment of nuclear EGFR and signal transducer and activator of transcription 5 (STAT5) to the Aurora-A promoter. A co-immunoprecipitation assay further demonstrated that EGF induces nuclear interaction between EGFR and STAT5. A small interfering (si)RNA knockdown assay also showed that EGFR and STAT5 are indeed involved in EGF-increased Aurora-A gene expression. Altogether, this study proposes that the nuclear EGFR associates with STAT5 to bind and increase Aurora-A gene expression, which ultimately may lead to chromosome instability and tumorigenesis. The results also provide a novel linkage between the EGFR signaling pathway and overexpression of Aurora-A in tumorigenesis and chromosome instability.

Aurora-A: the maker and breaker of spindle poles

The gene encoding the Aurora-A protein kinase is located in the 20q13 breast cancer amplicon and is also overexpressed in colorectal, pancreatic and gastric tumours. Although Aurora-A may not be a bona fide oncoprotein in humans, it is a promising drug target in cancer therapy. Thus, it is surprising that so little is known of its role in normal cells. The primary function of Aurora-A is to promote bipolar spindle assembly, but the molecular details of this process remained obscure until recently. The discovery of several novel Aurora-A-binding proteins and substrates has implicated Aurora-A in centrosome maturation and separation, acentrosomal and centrosomal spindle assembly, kinetochore function, cytokinesis and in cell fate determination. Here we discuss recent advances in determining the early mitotic role of Aurora-A, with a strong emphasis on its function at the mitotic spindle poles.

Aurora-A kinase Ser349 phosphorylation is required during Xenopus laevis oocyte maturation

Xenopus laevis Aurora-A is phosphorylated in vivo onto three amino acids: Ser53, Thr295 and Ser349. The activation of the kinase depends on its autophosphorylation on Thr295 within the T-loop. The phosphorylation of Ser53 by still unknown kinase(s) prevents its degradation. The present work focused on the regulation of Aurora-A function via Ser349 phosphorylation. Mutagenesis of Ser349 to alanine (S349A) had few impact in vitro on the capability of the kinase to autophosphorylate as well as on its activity. These data in addition to in gel kinase assays and site-specific proteolytic digestion experiments prove that Ser349 is clearly neither a primary autophosphorylation site, nor an autophosphorylation site depending on the priming phosphorylation of Thr295. Using specific antibodies, we also show that the phosphorylation of Aurora-A Ser349 is a physiological event during Xenopus oocyte maturation triggered by progesterone. A peak of phosphorylation paralleled the decrease of Aurora activity observed between meiosis I and II. In response to progesterone, X. laevis stage VI oocytes microinjected with the Aurora-A S349A mutant proceeded normally to germinal vesicle breakdown (GVBD), but degenerated rapidly soon after. Since phosphorylation of Ser349 is responsible for a decrease in kinase activity, our results suggest that a down-regulation of Aurora-A activity involving Ser349 phosphorylation is required in the process of maturation.

Efficient identification of phosphorylation by mass spectrometric phosphopeptide fingerprinting

We describe a rapid and efficient method for the identification of phosphopeptides, which we term mass spectrometric (MS) phosphopeptide fingerprinting. The method involves quantitative comparison of proteolytic peptides from native versus completely dephosphorylated proteins. Dephosphorylation of serine, threonine, and tyrosine residues is achieved by in-gel treatment of the separated proteins with hydrogen fluoride (HF). This chemical dephosphorylation results in enrichment of those unmodified peptides that correspond to previously phosphorylated peptides. Quantitative comparison of the signal-to-noise ratios of peaks in the treated versus untreated samples are used to identify phosphopeptides, which can be confirmed and further studied by tandem mass spectrometry (MS/MS). We have applied this method to identify eight known phosphorylation sites of Xenopus Aurora A kinase, as well as several novel sites in the Xenopus chromosome passenger complex (CPC).

Spatio-Temporal Expression Patterns of Aurora Kinases A, B, and C and Cytoplasmic Polyadenylation-Element-Binding Protein in Bovine Oocytes During Meiotic Maturation1

Maturation of immature bovine oocytes requires cytoplasmic polyadenylation and synthesis of a number of proteins involved in meiotic progression and metaphase-II arrest. Aurora serine-threonine kinases--localized in centrosomes, chromosomes, and midbody--regulate chromosome segregation and cytokinesis in somatic cells. In frog and mouse oocytes, Aurora A regulates polyadenylation-dependent translation of several mRNAs such as MOS and CCNB1, presumably by phosphorylating CPEB, and Aurora B phosphorylates histone H3 during meiosis. We analyzed the expression of three Aurora kinase genes--AURKA, AURKB, and AURKC--in bovine oocytes during meiosis by reverse transcription followed by quantitative real-time PCR and immunodetection. Aurora A was the most abundant form in oocytes, both at mRNA and protein levels. AURKA protein progressively accumulated in the oocyte cytoplasm during antral follicle growth and in vitro maturation. AURKB associated with metaphase chromosomes. AURKB, AURKC, and Thr-phosphorylated AURKA were detected at a contractile ring/midbody during the first polar body extrusion. CPEB, localized in oocyte cytoplasm, was hyperphosphorylated during prophase/metaphase-I transition. Most CPEB degraded in metaphase-II oocytes and remnants remained localized in a contractile ring. Roscovitine, U0126, and metformin inhibited meiotic divisions; they all induced a decrease of CCNB1 and phospho-MAPK3/1 levels and prevented CPEB degradation. However, only metformin depleted AURKA. The Aurora kinase inhibitor VX680 at 100 nmol/L did not inhibit meiosis but led to multinuclear oocytes due to the failure of the polar body extrusion. Thus, in bovine oocyte meiosis, massive destruction of CPEB accompanies metaphase-I/II transition, and Aurora kinases participate in regulating segregation of the chromosomes, maintenance of metaphase-II, and formation of the first polar body.

The role of Aurora-A inhibitors in cancer therapy

Recently, new chemotherapy agents which target the non-structural components of mitosis have been developed. An important protein involved in several mitotic phases is the Aurora-A protein. By means of the phosphorylation of different substrates, Aurora-A regulates the correct development of the various phases of mitosis. The kinase activity of this protein makes Aurora-A an excellent candidate as an oncogene. The first data of Aurora-A involvement in cancer regarded the identification of Aurora-A overexpression in primary breast and colon tumour samples. With regard to the predictive role of Aurora-A, it has been shown that its overexpression disrupts the spindle checkpoint activated by paclitaxel (Taxol) or nocodazole treatment, thus inducing the cells to become resistant to these drugs. The development therefore of small molecules with an Aurora-A inhibition function may make it possible to reduce or block the oncogenic activity of Aurora-A and in addition may improve the survival of oncological patients showing resistance to paclitaxel or nocodazole treatment. Three novel Aurora kinase inhibitors have recently been described--Hesperadin, ZM447439 and VX-680. All these three drugs have been designed to target the ATP-binding site of Aurora kinase, so they inhibit all three Aurora kinase family members showing a similar phenotype when tested in cell-based assays. Among these three different molecules, VX-680 has shown promising results in in vitro and in vivo studies. In conclusion, it is clear that we are entering a new era in anti-mitotic therapy with the identification and now clinical translation of new targets in mitosis beyond tubulin but many questions remain with regard to Aurora function.

Constitutive phosphorylation of aurora-a on ser51 induces its stabilization and consequent overexpression in cancer

BACKGROUND

The serine/threonine kinase Aurora-A (Aur-A) is a proto-oncoprotein overexpressed in a wide range of human cancers. Overexpression of Aur-A is thought to be caused by gene amplification or mRNA overexpression. However, recent evidence revealed that the discrepancies between amplification of Aur-A and overexpression rates of Aur-A mRNA were observed in breast cancer, gastric cancer, hepatocellular carcinoma, and ovarian cancer. We found that aggressive head and neck cancers exhibited overexpression and stabilization of Aur-A protein without gene amplification or mRNA overexpression. Here we tested the hypothesis that aberration of the protein destruction system induces accumulation and consequently overexpression of Aur-A in cancer.

PRINCIPAL FINDINGS

Aur-A protein was ubiquitinylated by APC(Cdh1) and consequently degraded when cells exited mitosis, and phosphorylation of Aur-A on Ser51 was observed during mitosis. Phosphorylation of Aur-A on Ser51 inhibited its APC(Cdh1)-mediated ubiquitylation and consequent degradation. Interestingly, constitutive phosphorylation on Ser51 was observed in head and neck cancer cells with protein overexpression and stabilization. Indeed, phosphorylation on Ser51 was observed in head and neck cancer tissues with Aur-A protein overexpression. Moreover, an Aur-A Ser51 phospho-mimetic mutant displayed stabilization of protein during cell cycle progression and enhanced ability to cell transformation.

CONCLUSIONS/SIGNIFICANCE

Broadly, this study identifies a new mode of Aur-A overexpression in cancer through phosphorylation-dependent inhibition of its proteolysis in addition to gene amplification and mRNA overexpression. We suggest that the inhibition of Aur-A phosphorylation can represent a novel way to decrease Aur-A levels in cancer therapy.

Identification of the auto-inhibitory domains of Aurora-A kinase

Aurora-A is a centrosome-localized serine/threonine kinase that is overexpressed in multiple human cancers. Here, we report an intramolecular inhibitory regulation in Aurora-A between its N-terminal regulatory domain (aa 1-128, Nt) and the C-terminal catalytic domain (aa 129-403, Cd). Removal of Nt results in a significant increase in kinase activity. Nt inhibited the activity of the single C-terminal kinase domain, but had little effect on the activity of the full-length of Aurora-A. PP1 is not involved in this regulation, instead, Nt interacts Cd directly in vitro and in vivo. The non-Aurora box (aa 64-128) in the N-terminal negatively regulated the kinase activity of the C-terminal kinase domain by intramolecular interaction with aa 240-300 within the C-terminal.

MAPK interacts with XGef and is required for CPEB activation during meiosis in Xenopus oocytes

Meiotic progression in Xenopus oocytes, and all other oocytes investigated, is dependent on polyadenylation-induced translation of stockpiled maternal mRNAs. Early during meiotic resumption, phosphorylation of CPE-binding protein (CPEB) is required for polyadenylation-induced translation of mRNAs encoding cell cycle regulators. Xenopus Gef (XGef), a Rho-family guanine-exchange factor, influences the activating phosphorylation of CPEB. An exchange-deficient version of XGef does not, therefore implicating Rho-family GTPase function in early meiosis. We show here that Clostridium difficile Toxin B, a Rho-family GTPase inhibitor, does not impair early CPEB phosphorylation or progression to germinal vesicle breakdown, indicating that XGef does not influence these events through activation of a Toxin-B-sensitive GTPase. Using the inhibitors U0126 for mitogen-activated protein kinase (MAPK), and ZM447439 for Aurora kinase A and Aurora kinase B, we found that MAPK is required for phosphorylation of CPEB, whereas Aurora kinases are not. Furthermore, we do not detect active Aurora kinase A in early meiosis. By contrast, we observe an early, transient activation of MAPK, independent of Mos protein expression. MAPK directly phosphorylates CPEB on four residues (T22, T164, S184, S248), but not on S174, a key residue for activating CPEB function. Notably, XGef immunoprecipitates contain MAPK, and this complex can phosphorylate CPEB. MAPK may prime CPEB for phosphorylation on S174 by an as-yet-unidentified kinase or may activate this kinase.

Mouse Aurora A: expression in Escherichia coli and purification

Aurora kinases have recently become some of the most intensely pursued oncology targets for the design of small-molecule inhibitors. Most of the active Aurora-A protein variants are currently being expressed from baculoviruses in insect cells, while catalytically impaired proteins can also be generated in and purified from Escherichia coli. In this study we present a method of expressing large quantities of active mouse Aurora-A kinase domain as an N-terminal glutathione-S-transferase fusion protein in bacteria and outline a simple purification method that produces greater than 99% pure protein samples suitable for enzymatic assays and X-ray crystallography. The methods described in this report simplify mouse Aurora-A expression and purification, and may aid in the production of other difficult kinases in prokaryotes.

Roles of Aurora kinases in mitosis and tumorigenesis

Aurora kinases, which have been implicated in several vital events in mitosis, represent a protein kinase family highly conserved during evolution. The activity of Aurora kinases is delicately regulated, mainly by phosphorylation and degradation. Deregulation of Aurora kinase activity can result in mitotic abnormality and genetic instability, leading to defects in centrosome function, spindle assembly, chromosome alignment, and cytokinesis. Both the expression level and the kinase activity of Aurora kinases are found to be up-regulated in many human cancers, indicating that these kinases might serve as useful targets for the development of anticancer drugs. This review focuses on recent progress on the roles of Aurora kinases in mitosis and tumorigenesis.

Antitumor activity of MLN8054, an orally active small-molecule inhibitor of Aurora A kinase

Increased Aurora A expression occurs in a variety of human cancers and induces chromosomal abnormalities during mitosis associated with tumor initiation and progression. MLN8054 is a selective small-molecule Aurora A kinase inhibitor that has entered Phase I clinical trials for advanced solid tumors. MLN8054 inhibits recombinant Aurora A kinase activity in vitro and is selective for Aurora A over the family member Aurora B in cultured cells. MLN8054 treatment results in G(2)/M accumulation and spindle defects and inhibits proliferation in multiple cultured human tumor cells lines. Growth of human tumor xenografts in nude mice was dramatically inhibited after oral administration of MLN8054 at well tolerated doses. Moreover, the tumor growth inhibition was sustained after discontinuing MLN8054 treatment. In human tumor xenografts, MLN8054 induced mitotic accumulation and apoptosis, phenotypes consistent with inhibition of Aurora A. MLN8054 is a selective inhibitor of Aurora A kinase that robustly inhibits growth of human tumor xenografts and represents an attractive modality for therapeutic intervention of human cancers.

Aurora-A regulation of nuclear factor-kappaB signaling by phosphorylation of IkappaBalpha

The Aurora-A/STK15 gene encodes a kinase that is frequently amplified in cancer. Overexpression of Aurora-A in mammalian cells leads to centrosome amplification, genetic instability, and transformation. In this study, we show that Aurora-A activates nuclear factor-kappaB (NF-kappaB) via IkappaBalpha phosphorylation. Inhibition of endogenous Aurora-A reduces tumor necrosis factor alpha (TNFalpha)-induced IkappaBalpha degradation. We analyzed primary human breast cancers, and 13.6% of samples showed Aurora-A gene amplification, all of which exhibited nuclear localization of NF-kappaB. We propose that this subgroup of patients with breast cancer might benefit from inhibiting Aurora-A. We also show that down-regulation of NF-kappaB via Aurora-A depletion can enhance cisplatin-dependent apoptosis. These data define a new role for Aurora-A in regulating IkappaBalpha that is critical for the activation of NF-kappaB-directed gene expression and may be partially responsible for the oncogenic effect of Aurora-A when the gene is amplified and overexpressed in human tumors.

Nicotiana tabacum osmotic stress-activated kinase is regulated by phosphorylation on Ser-154 and Ser-158 in the kinase activation loop

NtOSAK (Nicotiana tabacum osmotic stress-activated protein kinase), a member of the SnRK2 subfamily, is activated rapidly in response to hyperosmotic stress. Our previous results as well as data presented by others indicate that phosphorylation is involved in activation of SnRK2 kinases. Here, we have mapped the regulatory phosphorylation sites of NtOSAK by mass spectrometry with collision-induced peptide fragmentation. We show that active NtOSAK, isolated from NaCl-treated tobacco BY-2 cells, is phosphorylated on Ser-154 and Ser-158 in the kinase activation loop. Prediction of the NtOSAK three-dimensional structure indicates that phosphorylation of Ser-154 and Ser-158 triggers changes in enzyme conformation resulting in its activation. The involvement of Ser-154 and Ser-158 phosphorylation in regulation of NtOSAK activity was confirmed by site-directed mutagenesis of NtOSAK expressed in bacteria and in maize protoplasts. Our data reveal that phosphorylation of Ser-158 is essential for NtOSAK activation, whereas phosphorylation of Ser-154 most probably facilitates Ser-158 phosphorylation. The time course of NtOSAK phosphorylation on Ser-154 and Ser-158 in BY-2 cells subjected to osmotic stress correlates with NtOSAK activity, indicating that NtOSAK is regulated by reversible phosphorylation of these residues in vivo. Importantly, Ser-154 and Ser-158 are conserved in all SnRK2 subfamily members, suggesting that phosphorylation at these sites may be a general mechanism for SnRK2 activation.

Mitotic activation of the kinase Aurora-A requires its binding partner Bora

The protein kinase Aurora-A is required for centrosome maturation, spindle assembly, and asymmetric protein localization during mitosis. Here, we describe the identification of Bora, a conserved protein that is required for the activation of Aurora-A at the onset of mitosis. In the Drosophila peripheral nervous system, bora mutants have defects during asymmetric cell division identical to those observed in aurora-A. Furthermore, overexpression of bora can rescue defects caused by mutations in aurora-A. Bora is conserved in vertebrates, and both Drosophila and human Bora can bind to Aurora-A and activate the kinase in vitro. In interphase cells, Bora is a nuclear protein, but upon entry into mitosis, Bora is excluded from the nucleus and translocates into the cytoplasm in a Cdc2-dependent manner. We propose a model in which activation of Cdc2 initiates the release of Bora into the cytoplasm where it can bind and activate Aurora-A.

Phospho-regulation of human protein kinase Aurora-A: analysis using anti-phospho-Thr288 monoclonal antibodies

Mammalian Aurora-A is related to a serine/threonine protein kinase that was originally identified by its close homology with Saccharomyces cerevisiae Ipl1p and Drosophila melanogaster aurora that are key regulators in the orchestration of mitotic events. The protein level of Aurora-A, its peak kinase activity during mitosis, and its activation have been attributed to phosphorylation. Here we show that this enzyme is an arginine-directed kinase and define its substrate specificity. We also found that Thr288 within the activation loop is a critical residue for activating phosphorylation events in vitro and that it is spatiotemporally restricted to a brief window at mitosis on duplicated centrosomes and on spindle microtubules proximal to the poles in vivo. Immunodepletion assays indicated that an upstream kinase(s) of Aurora-A might exist in mammalian cells in addition to autophosphorylation. Furthermore, human activated Aurora-A forms complexes with the negative regulator protein serine/threonine phosphatase type 1 (PP1) that was negatively phosphorylated on Thr320. Interestingly, phospho-specific Aurora-A monoclonal antibodies restrain Aurora-A kinase activity in vitro, providing further therapeutic avenues to explore.

Aurora B is required for mitotic chromatin-induced phosphorylation of Op18/Stathmin

Oncoprotein 18/Stathmin (Op18) is a microtubule-destabilizing protein that is inhibited by phosphorylation in response to many types of signals. During mitosis, phosphorylation of Op18 by cdc2 is necessary but not sufficient for Op18 inhibition. The presence of mitotic chromosomes is additionally required and involves phosphorylation of Ser-16 in Xenopus Op18 (and/or Ser-63 in human). Given that Ser-16 is an excellent Aurora A (Aur-A) kinase consensus phosphorylation site and the Aurora kinase inhibitor ZM447439 (ZM) blocks phosphorylation in the activation loop of Aur-A, we asked whether either Aur-A or Aurora B (Aur-B) might regulate Op18. We find that ZM blocks the ability of mitotic chromatin to induce Op18 hyperphosphorylation in Xenopus egg extracts. Depletion of Aur-B, but not Aur-A, blocks hyperphosphorylation of Op18, and chromatin assembled in the absence of Aur-B fails to induce hyperphosphorylation. These results suggest that Aur-B, which concentrates at centromeres of metaphase chromosomes, contributes to localized regulation of Op18 during the process of spindle assembly.

Inhibition of Aurora A in response to DNA damage

Mitotic kinases are the ultimate target of pathways sensing genotoxic damage and impinging on the cell cycle machinery. Here, we provide evidence that Aurora A (AurA) was inhibited upon generation of double-strand breaks in DNA. We demonstrate that AurA was not downstream of CDK1 and that inhibition of AurA and CDK1 by DNA damage occurred independently. Using a cell line functionally deficient in Chk2, a selective Chk1 inhibitor and siRNA to Chk1, we show that DNA-damage signals were delivered to AurA through a Chk1-dependent pathway. With regard to the molecular mechanism of AurA inhibition, we found that the point mutation Ser(342)>Ala rendered AurA resistant to inhibition by DNA damage. By means of two distinct approaches we examined the impact of reconstitution of AurA activity in DNA-damaged cells: (i) transient expression of wild-type and Ser(342)>Ala mutant, but not kinase-dead, AurA led to bypass of the DNA damage block; (ii) direct transduction of highly active wt-AurA into G2 arrested cells precisely after induction of DNA damage resulted in mitotic entry. We show that the mechanism through which AurA allowed entry into mitosis was reactivation of CDK1, thus indicating that AurA plays a key role upstream of CDK1. A model depicting the possible role of AurA at the onset of mitosis and upon DNA damage is presented.

Dynamic localization and functional implications of Aurora-C kinase during male mouse meiosis

Aurora-C was first identified during screening for kinases expressed in mouse sperm and eggs. Herein, we report for the first time the precise subcellular localization of endogenous Aurora-C during male meiotic division. The localization of Aurora-C was analyzed by immunofluorescence staining on chromosome spreads of mouse spermatocytes or in squashed seminiferous tubules. Aurora-C was first detected at clusters of chromocenters in diplotene spermatocytes and was concentrated at centromeres in metaphase I and II. Interestingly, Aurora-C was also found along the chromosome axes, including both the regions of centromeres and the chromosome arms in diakinesis. During the anaphase I/telophase I and anaphase II/telophase II transitions, Aurora-C was relocalized to the spindle midzone and midbody. A similar distribution pattern was also observed for Aurora-B during male meiotic divisions. Surprisingly, we detected no Aurora-C in mitotic spermatogonia. Furthermore, immunoprecipitation analyses revealed that INCENP associated with Aurora-C in the male testis. We propose that INCENP recruits Aurora-C (or some other factor(s) recruit INCENP and Aurora-C) to meiotic chromosomes, while Aurora-C may either work alone or cooperate with Aurora-B to regulate chromosome segregation during male meiosis.