Oxidative Stress-Induced Unscheduled CDK1-Cyclin B1 Activity Impairs ER-Mitochondria-Mediated Bioenergetic Metabolism.

Targeting the activities of endoplasmic reticulum (ER)-mitochondrial-dependent metabolic reprogramming is considered one of the most promising strategies for cancer treatment. Here, we present biochemical subcellular fractionation, coimmunoprecipitation, gene manipulation, and pharmacologic evidence that induction of mitochondria-localized phospho (p)-cyclin dependent kinase 1 (CDK1) (Thr 161)-cyclin B1 complexes by apigenin in nasopharyngeal carcinoma (NPC) cells impairs the ER-mitochondrial bioenergetics and redox regulation of calcium (Ca++) homeostasis through suppressing the B cell lymphoma 2 (BCL-2)/BCL-2/B-cell lymphoma-extra large (BCL-xL)-modulated anti-apoptotic and metabolic functions. Using a specific inducer, inhibitor, or short hairpin RNA for acid sphingomyelinase (ASM) demonstrated that enhanced lipid raft-associated ASM activity confers alteration of the lipid composition of lipid raft membranes, which leads to perturbation of protein trafficking, and induces formation of p110α free p85α-unphosphorylated phosphatase and tensin homolog deleted from chromosome 10 complexes in the lipid raft membranes, causing disruption of phosphatidylinositol 3-kinase (PI3K)-protein kinase B (Akt)-GTP-ras-related C3 botulinum toxin substrate 1 (Rac1)-mediated signaling, thus triggering the p-CDK1 (Thr 161))-cyclin B1-mediated BCL-2 (Thr 69/Ser 87)/BCL-xL (Ser 62) phosphorylation and accompanying impairment of ER-mitochondria-regulated bioenergetic, redox, and Ca++ homeostasis. Inhibition of apigenin-induced reactive oxygen species (ROS) generation by a ROS scavenger N-acetyl-L-cysteine blocked the lipid raft membrane localization and activation of ASM and formation of ceramide-enriched lipid raft membranes, returned PI3K-Akt-GTP-Rac1-modulated CDK1-cyclin B1 activity, and subsequently restored the BCL-2/BCL-xL-regulated ER-mitochondrial bioenergetic activity. Thus, this study reveals a novel molecular mechanism of the pro-apoptotic activity of ASM controlled by oxidative stress to modulate the ER-mitochondrial bioenergetic metabolism, as well as suggests the disruption of CDK1-cyclin B1-mediated BCL-2/BCL-xL oncogenic activity by triggering oxidative stress-ASM-induced PI3K-Akt-GTP-Rac1 inactivation as a therapeutic approach for NPC.

CYCLIN-B1/2 and -D1 act in opposition to coordinate cortical progenitor self-renewal and lineage commitment.

The sequential generation of layer-specific cortical neurons requires radial glia cells (RGCs) to precisely balance self-renewal and lineage commitment. While specific cell-cycle phases have been associated with these decisions, the mechanisms linking the cell-cycle machinery to cell-fate commitment remain obscure. Using single-cell RNA-sequencing, we find that the strongest transcriptional signature defining multipotent RGCs is that of G2/M-phase, and particularly CYCLIN-B1/2, while lineage-committed progenitors are enriched in G1/S-phase genes, including CYCLIN-D1. These data also reveal cell-surface markers that allow us to isolate RGCs and lineage-committed progenitors, and functionally confirm the relationship between cell-cycle phase enrichment and cell fate competence. Finally, we use cortical electroporation to demonstrate that CYCLIN-B1/2 cooperate with CDK1 to maintain uncommitted RGCs by activating the NOTCH pathway, and that CYCLIN-D1 promotes differentiation. Thus, this work establishes that cell-cycle phase-specific regulators act in opposition to coordinate the self-renewal and lineage commitment of RGCs via core stem cell regulatory pathways.

Requirement for CCNB1 in mouse spermatogenesis.

Spermatogenesis, which involves mitosis and meiosis of male germ cells, is a highly complicated and coordinately ordered process. Cyclin B1 (CCNB1), an important regulator in cell cycle machinery, is proved essential for mouse embryonic development. However, the role of CCNB1 in mammalian spermatogenesis remains unclear. Here we tested the requirement for CCNB1 using conditional knockout mice lacking CCNB1 in male germ cells. We found that ablation of CCNB1 in gonocytes and spermatogonia led to mouse sterile caused by the male germ cells' depletion. Gonocyte and spermatogonia without CCNB1 is unable to proliferate normally and apoptosis increased. Moreover, CCNB1 ablation in spermatogonia may promote their differentiation by downregulating Lin28a and upregulating let-7 miRNA. However, ablation of CCNB1 in premeiotic male germ cells did not have an effect on meiosis of spermatocytes and male fertility, suggesting that CCNB1 may be dispensable for meiosis of spermatocytes. Collectively, these results indicate that CCNB1 is critically required for the proliferation of gonocytes and spermatogonia but may be redundant in meiosis of spermatocytes in mouse spermatogenesis.

Prognostic role of cyclin B1 in solid tumors: a meta-analysis.

Cyclin B1 is a key mitotic cyclin in the G2-M phase transition of the cell cycle and is overexpressed in various malignant tumors. Numerous studies have reported contradictory evidences of the correlation between cyclin B1 expression and prognosis in human solid tumors. To address this discrepancy, we conducted a meta-analysis with 17 published studies searched from PubMed and Medline. Cyclin B1 overexpression was significantly associated with poor 3-year overall survival (OS) (OR = 2.05, 95% CI = 1.20 to 3.50, P = 0.009) and 5-year OS (OR = 2.11, 95% CI = 1.33 to 3.36, P = 0.002) of solid tumors. Subgroup analysis revealed that elevated cyclin B1 expression was associated with worse prognosis of lung cancer and esophageal cancer but better prognosis of colorectal cancer. In summary, overexpression of cyclin B1 is correlated with poor survival in most solid tumors, which suggests that the expression status of cyclin B1 is a significant prognostic parameter in solid tumors.

DRG2 Regulates G2/M Progression via the Cyclin B1-Cdk1 Complex.

Developmentally regulated GTP-binding protein 2 (DRG2) plays an important role in cell growth. Here we explored the linkage between DRG2 and G2/M phase checkpoint function in cell cycle progression. We observed that knockdown of DRG2 in HeLa cells affected growth in a wound-healing assay, and tumorigenicity in nude mice xenografts. Flow cytometry assays and [(3)H] incorporation assays indicated that G2/M phase arrest was responsible for the decreased proliferation of these cells. Knockdown of DRG2 elicited down-regulation of the major mitotic promoting factor, the cyclin B1/Cdk1 complex, but up-regulation of the cell cycle arresting proteins, Wee1, Myt1, and p21. These findings identify a novel role of DRG2 in G2/M progression.

Tributyltin induces G2/M cell cycle arrest via NAD(+)-dependent isocitrate dehydrogenase in human embryonic carcinoma cells.

Organotin compounds, such as tributyltin (TBT), are well-known endocrine-disrupting chemicals (EDCs). We have recently reported that TBT induces growth arrest in the human embryonic carcinoma cell line NT2/D1 at nanomolar levels by inhibiting NAD(+)-dependent isocitrate dehydrogenase (NAD-IDH), which catalyzes the irreversible conversion of isocitrate to α-ketoglutarate. However, the molecular mechanisms by which NAD-IDH mediates TBT toxicity remain unclear. In the present study, we examined whether TBT at nanomolar levels affects cell cycle progression in NT2/D1 cells. Propidium iodide staining revealed that TBT reduced the ratio of cells in the G1 phase and increased the ratio of cells in the G2/M phase. TBT also reduced cell division cycle 25C (cdc25C) and cyclin B1, which are key regulators of G2/M progression. Furthermore, apigenin, an inhibitor of NAD-IDH, mimicked the effects of TBT. The G2/M arrest induced by TBT was abolished by NAD-IDHα knockdown. Treatment with a cell-permeable α-ketoglutarate analogue recovered the effect of TBT, suggesting the involvement of NAD-IDH. Taken together, our data suggest that TBT at nanomolar levels induced G2/M cell cycle arrest via NAD-IDH in NT2/D1 cells. Thus, cell cycle analysis in embryonic cells could be used to assess cytotoxicity associated with nanomolar level exposure of EDCs.

Enrichment of nuclear S100A4 during G2/M in colorectal cancer cells: possible association with cyclin B1 and centrosomes.

S100A4 promotes metastasis in several types of cancer, but the involved molecular mechanisms are still incompletely described. The protein is associated with a wide variety of biological functions and it locates to different subcellular compartments, including nuclei, cytoplasm and extracellular space. Nuclear expression of S100A4 has been associated with more advanced disease stage as well as poor outcome in colorectal cancer (CRC). The present study was initiated to investigate the nuclear function of S100A4 and thereby unravel potential biological mechanisms linking nuclear expression to a more aggressive phenotype. CRC cell lines show heterogeneity in nuclear S100A4 expression and preliminary experiments revealed cells in G2/M to have increased nuclear accumulation compared to G1 and S cells, respectively. Synchronization experiments validated nuclear S100A4 expression to be most prominent in the G2/M phase, but manipulating nuclear levels of S100A4 using lentiviral modified cells failed to induce changes in cell cycle distribution and proliferation. Proximity ligation assay did, however, demonstrate proximity between S100A4 and cyclin B1 in vitro, while confocal microscopy showed S100A4 to localize to areas corresponding to centrosomes in mitotic cells prior to chromosome segregation. This might indicate a novel and uncharacterized function of the metastasis-associated protein in CRC cells.

Positive and negative regulation of vertebrate separase by Cdk1-cyclin B1 may explain why securin is dispensable.

Sister chromatid cohesion is established during replication by entrapment of both dsDNAs within the cohesin ring complex. It is dissolved in anaphase when separase, a giant cysteine endopeptidase, cleaves the Scc1/Rad21 subunit of cohesin, thereby triggering chromosome segregation. Separase is held inactive by association with securin until this anaphase inhibitor is destroyed at the metaphase-to-anaphase transition by ubiquitin-dependent degradation. The relevant ubiquitin ligase, the anaphase-promoting complex/cyclosome, also targets cyclin B1, thereby causing inactivation of Cdk1 and mitotic exit. Although separase is essential, securin knock-out mice are surprisingly viable and fertile. Capitalizing on our previous finding that Cdk1-cyclin B1 can also bind and inhibit separase, we investigated whether this kinase might be suitable to maintain faithful timing and execution of anaphase in the absence of securin. We found that, similar to securin, Cdk1-cyclin B1 regulates separase in both a positive and negative manner. Although securin associates with nascent separase to co-translationally assist proper folding, Cdk1-cyclin B1 acts on native state separase. Upon entry into mitosis, Cdk1-cyclin B1-dependent phosphorylation of Ser-1126 renders separase prone to inactivation by aggregation/precipitation. Stable association of Cdk1-cyclin B1 with phosphorylated separase counteracts this tendency and stabilizes separase in an inhibited yet activatable state. These opposing effects are suited to prevent premature cleavage of cohesin in early mitosis while ensuring timely activation of separase by anaphase-promoting complex/cyclosome-dependent degradation of cyclin B1. Coupling sister chromatid separation with subsequent exit from mitosis by this simplified mode might have been the common scheme of mitotic control prior to the evolution of securin.

SOX2 regulates self-renewal and tumorigenicity of stem-like cells of head and neck squamous cell carcinoma.

BACKGROUND: Head and neck squamous cell carcinomas (HNSCCs) display cellular heterogeneity and contain cancer stem cells (CSCs). Sex-determining region Y [SRY]-box (SOX)2 is an important regulator of embryonic stem cell fate and is aberrantly expressed in several types of human tumours. Nonetheless, the role of SOX2 in HNSCC remains unclear. METHODS: We created cells ectopically expressing SOX2 from previously established HNSCC cells and examined the cell proliferation, self-renewal capacity, and chemoresistance of these cells compared with control cells. In addition, we knocked down SOX2 in primary spheres obtained from HNSCC tumour tissue and assessed the attenuation of stemness-associated traits in these cells in vitro and in vivo. Furthermore, we examined the clinical relevance of SOX2 expression in HNSCC patients. RESULTS: SOX2 is aberrantly expressed in primary tissue of HNSCC patients but not in healthy tissue. SOX2 expression correlated with tumour recurrence and poor prognosis of HNSCC patients. Ectopic expression of SOX2 induced cell proliferation via cyclin B1 expression and stemness-associated features, such as self-renewal and chemoresistance. In addition, a knockdown of SOX2 in HNSCC CSCs attenuated their self-renewal capacity, chemoresistance (through ABCG2 suppression), invasion capacity (via snail downregulation), and in vivo tumorigenicity. CONCLUSIONS: These results suggest that SOX2 may have important roles in the 'stemness' and progression of HNSCC. Targeting SOX2-positive tumour cells (CSCs) could be a new therapeutic strategy in HNSCCs.

Cdk1 inactivation terminates mitotic checkpoint surveillance and stabilizes kinetochore attachments in anaphase.

Two mechanisms safeguard the bipolar attachment of chromosomes in mitosis. A correction mechanism destabilizes erroneous attachments that do not generate tension across sister kinetochores [1]. In response to unattached kinetochores, the mitotic checkpoint delays anaphase onset by inhibiting the anaphase-promoting complex/cyclosome (APC/C(Cdc20)) [2]. Upon satisfaction of both pathways, the APC/C(Cdc20) elicits the degradation of securin and cyclin B [3]. This liberates separase triggering sister chromatid disjunction and inactivates cyclin-dependent kinase 1 (Cdk1) causing mitotic exit. How eukaryotic cells avoid the engagement of attachment monitoring mechanisms when sister chromatids split and tension is lost at anaphase is poorly understood [4]. Here we show that Cdk1 inactivation disables mitotic checkpoint surveillance at anaphase onset in human cells. Preventing cyclin B1 proteolysis at the time of sister chromatid disjunction destabilizes kinetochore-microtubule attachments and triggers the engagement of the mitotic checkpoint. As a consequence, mitotic checkpoint proteins accumulate at anaphase kinetochores, the APC/C(Cdc20) is inhibited, and securin reaccumulates. Conversely, acute pharmacological inhibition of Cdk1 abrogates the engagement and maintenance of the mitotic checkpoint upon microtubule depolymerization. We propose that the simultaneous destruction of securin and cyclin B elicited by the APC/C(Cdc20) couples chromosome segregation to the dissolution of attachment monitoring mechanisms during mitotic exit.

Restoring p53 function in human melanoma cells by inhibiting MDM2 and cyclin B1/CDK1-phosphorylated nuclear iASPP.

Nearly 90% of human melanomas contain inactivated wild-type p53, the underlying mechanisms for which are not fully understood. Here, we identify that cyclin B1/CDK1-phosphorylates iASPP, which leads to the inhibition of iASPP dimerization, promotion of iASPP monomer nuclear entry, and exposure of its p53 binding sites, leading to increased p53 inhibition. Nuclear iASPP is enriched in melanoma metastasis and associates with poor patient survival. Most wild-type p53-expressing melanoma cell lines coexpress high levels of phosphorylated nuclear iASPP, MDM2, and cyclin B1. Inhibition of MDM2 and iASPP phosphorylation with small molecules induced p53-dependent apoptosis and growth suppression. Concurrent p53 reactivation and BRAFV600E inhibition achieved additive suppression in vivo, presenting an alternative for melanoma therapy.

Pituitary tumor-transforming gene 1 enhances proliferation and suppresses early differentiation of keratinocytes.

The epidermis is a self-renewing tissue, the homeostasis of which is dependent upon the tight balance between proliferation and differentiation based on appropriate regulation of the cell cycle. The cell cycle regulation is dependent on the interactions among a number of cell cycle regulatory molecules, including the pituitary tumor-transforming gene 1 (PTTG1), also known as securin, a regulator of sister chromatid separation and transition from metaphase to anaphase. This study was conducted to clarify the less-known functions of PTTG1 in the epidermis by the use of keratinocytes cultured under two-dimensional (2D) or three-dimensional (3D) conditions. Forced overexpression of PTTG1 caused upregulation of cyclin B1, cyclin-dependent kinase 1 (CDK1), and c-Myc, resulting in enhanced proliferation and suppression of early differentiation without apparent alterations in terminal differentiation, and the exogenous PTTG1 was downregulated in association with cell cycle exit. In contrast, depletion of PTTG1 caused their downregulation and constrained proliferation with retention of differentiation capacity. These findings suggested that PTTG1 could alter the proliferation status by modulating the expression levels of the other cell cycle regulatory proteins, and excess PTTG1 primarily affects early differentiation of keratinocytes under the stability regulation associated with cell cycle exit.

Metastasis-associated protein 3 (MTA3) regulates G2/M progression in proliferating mouse granulosa cells.

Metastasis-associated protein 3 (MTA3) is a constituent of the Mi-2/nucleosome remodeling and deacetylase (NuRD) protein complex that regulates gene expression by altering chromatin structure and can facilitate cohesin loading onto DNA. The biological function of MTA3 within the NuRD complex is unknown. Herein, we show that MTA3 was expressed highly in granulosa cell nuclei of all ovarian follicle stages and at lower levels in corpora lutea. We tested the hypothesis that MTA3-NuRD complex function is required for granulosa cell proliferation. In the ovary, MTA3 interacted with NuRD proteins CHD4 and HDAC1 and the core cohesin complex protein RAD21. In cultured mouse primary granulosa cells, depletion of endogenous MTA3 using RNA interference slowed cell proliferation; this effect was rescued by coexpression of exogenous MTA3. Slowing of cell proliferation correlated with a significant decrease in cyclin B1 and cyclin B2 expression. Granulosa cell populations lacking MTA3 contained a significantly higher percentage of cells in G2/M phase and a lower percentage in S phase compared with control cells. Furthermore, MTA3 depletion slowed entry into M phase as indicated by reduced phosphorylation of histone H3 at serine 10. These findings provide the first evidence to date that MTA3 interacts with NuRD and cohesin complex proteins in the ovary in vivo and regulates G2/M progression in proliferating granulosa cells.

Cyclin B1 interacts with the BH3-only protein Bim and mediates its phosphorylation by Cdk1 during mitosis.

Protracted mitotic arrest leads to cell death; however, the molecular signals that link these distinct processes remain poorly understood. Here we report that the pro-apoptotic BH3-only family member Bim undergoes phosphorylation in K562 cells following treatment with the microtubule targeting agents Taxol and Nocodazole. The phosphorylation of two Bim isoforms, BimEL and BimL, at the mitochondria correlates with mitotic arrest and precedes cell death induced by Taxol. It was also found that Bim undergoes transient phosphorylation during normal mitosis in K562 cells. In addition, siRNA silencing of Bim reduces sensitivity to Taxol-induced cell death. The transition of K562 cells from mitosis to G1 results in the loss of BimEL and BimL phosphorylation and correlates with the degradation of cyclin B1. The Cdk1 inhibitors, RO-3306 and Purvalanol A, block Bim phosphorylation in mitotically arrested cells. Importantly, it was found that cyclin B1 co-immunoprecipitates with endogenous Bim in mitotic extracts. Furthermore, active recombinant Cdk1/cyclin B1 phosphorylates BimEL and BimL in vitro and Serine 44 on BimL has been identified as a Cdk1 phosphorylation site. Collectively, these results suggest that Cdk1/cyclin B1-dependent hyper-phosphorylation of Bim during prolonged mitotic arrest is an important cell death signal.

Quantitative proteomics reveals the basis for the biochemical specificity of the cell-cycle machinery.

Cyclin-dependent kinases comprise the conserved machinery that drives progress through the cell cycle, but how they do this in mammalian cells is still unclear. To identify the mechanisms by which cyclin-cdks control the cell cycle, we performed a time-resolved analysis of the in vivo interactors of cyclins E1, A2, and B1 by quantitative mass spectrometry. This global analysis of context-dependent protein interactions reveals the temporal dynamics of cyclin function in which networks of cyclin-cdk interactions vary according to the type of cyclin and cell-cycle stage. Our results explain the temporal specificity of the cell-cycle machinery, thereby providing a biochemical mechanism for the genetic requirement for multiple cyclins in vivo and reveal how the actions of specific cyclins are coordinated to control the cell cycle. Furthermore, we identify key substrates (Wee1 and c15orf42/Sld3) that reveal how cyclin A is able to promote both DNA replication and mitosis.

Phosphatases: providing safe passage through mitotic exit.

The mitosis-to-interphase transition involves dramatic cellular reorganization from a state that supports chromosome segregation to a state that complies with all functions of an interphase cell. This process, termed mitotic exit, depends on the removal of mitotic phosphorylations from a broad range of substrates. Mitotic exit regulation involves inactivation of mitotic kinases and activation of counteracting protein phosphatases. The key mitotic exit phosphatase in budding yeast, Cdc14, is now well understood. By contrast, in animal cells, it is now emerging that mitotic exit relies on distinct regulatory networks, including the protein phosphatases PP1 and PP2A.

Involvement of MAPK and PI3K signaling pathway in sterigmatocystin-induced G2 phase arrest in human gastric epithelium cells.

SCOPE: Sterigmatocystin (ST), a mycotoxin commonly found in foodstuff and feedstuff, has been shown to be a carcinogenic mycotoxin in animal models. Many studies showed that the high level of ST contamination in grains might be related to the high incidence of gastric carcinoma in rural areas of China. However, up to now, the potential effects of ST on human gastric epithelium cells remain largely unknown. In this study, we explored the effects of ST on cell-cycle distribution and the regulatory mechanism in immortalized human gastric epithelium cells (GES-1). METHODS AND RESULTS: The effects of ST on the cell cycle distribution of GES-1 cells were determined with flow cytometric (FCM) analysis, Giemsa staining and immunofluorescence staining, while that on the expression of related gene-Cdc25C, Cdc2, CyclinB1 and the complex of CyclinB1-Cdc2 were studied with Western blot, reverse transcription polymerase chain reaction (RT-PCR) and immunoprecipitation assay respectively. We found that ST induced GES-1 cells arrested at G2 phase by regulating the expression of Cdc25C, Cdc2, CyclinB1 and the formation of CyclinB1-Cdc2 complex. Further study suggested JNK, ERK and PI3K/AKT/mTOR pathways to be involved in the process of G2 arrest induced by ST. The specific inhibitors of JNK and ERK reversed the role of ST, whereas that of PI3K/AKT/mTOR reinforced the effect of ST on cell-cycle distribution. CONCLUSION: This study demonstrates that JNK, ERK and PI3K/AKT/mTOR pathways participated in the G2 arrest induced by ST through the deregulation of CyclinB1, Cdc2 and Cdc25C. It may play some roles in the gastric carcinogenesis in ST exposure populations.

Phosphorylation of dynamin II at serine-764 is associated with cytokinesis.

Calcineurin is a phosphatase that is activated at the last known stage of mitosis, abscission. Among its many substrates, it dephosphorylates dynamin II during cytokinesis at the midbody of dividing cells. However, dynamin II has several cellular roles including clathrin-mediated endocytosis, centrosome cohesion and cytokinesis. It is not known whether dynamin II phosphorylation plays a role in any of these functions nor have the phosphosites involved in cytokinesis been directly identified. We now report that dynamin II from rat lung is phosphorylated to a low stoichiometry on a single major site, Ser-764, in the proline-rich domain. Phosphorylation on Ser-764 also occurred in asynchronously growing HeLa cells and was greatly increased upon mitotic entry. Tryptic phospho-peptides isolated by TiO(2) chromatography revealed only a single phosphosite in mitotic cells. Mitotic phosphorylation was abolished by roscovitine, suggesting the mitotic kinase is cyclin-dependent kinase 1. Cyclin-dependent kinase 1 phosphorylated full length dynamin II and Glutathione-S-Transferase-tagged-dynamin II-proline-rich domain in vitro, and mutation of Ser-764 to alanine reduced proline-rich domain phosphorylation by 80%, supporting that there is only a single major phosphosite. Ser-764 phosphorylation did not affect clathrin-mediated endocytosis or bulk endocytosis using penetratin-based phospho-deficient or phospho-mimetic peptides or following siRNA depletion/rescue experiments. Phospho-dynamin II was enriched at the mitotic centrosome, but this targeting was unaffected by the phospho-deficient or phospho-mimetic peptides. In contrast, the phospho-mimetic peptide displaced endogenous dynamin II, but not calcineurin, from the midbody and induced cytokinesis failure. Therefore, phosphorylation of dynamin II primarily occurs on a single site that regulates cytokinesis downstream of calcineurin, rather than regulating endocytosis or centrosome function.

The Aurora kinase inhibitor ZM447439 accelerates first meiosis in mouse oocytes by overriding the spindle assembly checkpoint.

Previous studies have established that when maturing mouse oocytes are continuously incubated with the Aurora inhibitor ZM447439, meiotic maturation is blocked. In this study, we observe that by altering the time of addition of the inhibitor, oocyte maturation can actually be accelerated by 1 h as measured by the timing of polar body extrusion. ZM447439 also had the ability to overcome a spindle assembly checkpoint (SAC) arrest caused by nocodazole and so rescue polar body extrusion. Consistent with the ability of the SAC to inhibit cyclin B1 degradation by blocking activation of the anaphase-promoting complex, we could also observe a rescue in cyclin B1 degradation when ZM447439 was added to nocodazole-treated oocytes. The acceleration of the first meiotic division by ZM447439, which has not been achieved previously, and its effects on the SAC are all consistent with the proposed mitotic role of Aurora B in activating the SAC. We hypothesize that Aurora kinase activity controls the SAC in meiosis I, despite differences to the mitotic cell cycle division in spindle architecture brought about by the meiotic mono-orientation of sister kinetochores.