Cyclin B1-Cdk1 activation continues after centrosome separation to control mitotic progression

LOX is a novel mitotic spindle-associated protein essential for mitosis

LOX regulates cancer progression in a variety of human malignancies. It is overexpressed in aggressive cancers and higher expression of LOX is associated with higher cancer mortality. Here, we report a new function of LOX in mitosis. We show that LOX co-localizes to mitotic spindles from metaphase to telophase, and p-H3(Ser10)-positive cells harbor strong LOX staining. Further, purification of mitotic spindles from synchronized cells show that LOX fails to bind to microtubules in the presence of nocodazole, whereas paclitaxel treated samples showed enrichment in LOX expression, suggesting that LOX binds to stabilized microtubules. LOX knockdown leads to G2/M phase arrest; reduced p-H3(Ser10), cyclin B1, CDK1, and Aurora B. Moreover, LOX knockdown significantly increased sensitivity of cancer cells to chemotherapeutic agents that target microtubules. Our findings suggest that LOX has a role in cancer cell mitosis and may be targeted to enhance the activity of microtubule inhibitors for cancer therapy.

Tumor suppressor BLU exerts growth inhibition by blocking ERK signaling and disrupting cell cycle progression through RAS pathway interference

We have previously reported that the 3p21 tumor suppressor BLU regulates cell cycle by blocking JNK/MAPK signaling. Another member of the MAPK family, extracellular signal response kinase (ERK), is induced by the RAS-RAF-MEK-ERK pathway and is targeted in anticancer therapy. The effects of BLU on tumor growth were evaluated by measuring the size of nasopharyngeal carcinoma (NPC) xenografted tumors intra-tumorally injected with BLU adenovirus 5 (BLU Ad5) and the viability of NPC cells transferred with BLU. Tumor size was correlated with downregulation of the ERK pathway by BLU. Phosphorylation of ERK and Elk reporter activities were assayed. The regulated cyclins D1 and B1 were measured by CCND1 and CCNB1 gene promoter activity by co-transfection of BLU, RAS V12G, together with BLU+RAS V12G, pCD316+RAS V12G. The cell cycle phase distribution was determined by FACS-based DNA content assay. The data showed that growth of the xenografted tumor was inhibited and viability of HONE-1 cells was reduced by recombinant BLU. BLU down-regulated ERK signaling by reducing protein substrate phosphorylation, inhibiting Elk reporter activity, and blocking promoter activities of the CCND1 gene and reduced cyclins D1 expression to arrest the cell cycle at the G1 phase. The population of G2/M cells was also remarkably decreased. HRAS V12G activated ERK and cyclin D1 and B1 promoters, and the effects were antagonized by BLU. Taken together, our results suggested that BLU inhibited ERK signaling, downregulated cyclins D1 and B1, and prevented cell cycle progression through interfering with HRAS V12G signaling to exert tumor suppression.

Knockdown of NF-κB1 by shRNA Inhibits the Growth of Renal Cell Carcinoma In Vitro and In Vivo

Renal cell carcinoma (RCC) accounts for approximately 2%–3% of human malignancies and is the most aggressive among urologic tumors. Biological heterogeneity, drug resistance, and chemotherapy side effects are the biggest obstacles to the effective treatment of RCC. The NF-κB transcription factor is one of several molecules identified to be responsible for the aggressive phenotype of this tumor. In the past decade, several studies have demonstrated the activation of NF-κB in RCC, and many have implicated NF-κB1 (p50) as an important molecule in tumor progression and metastasis. In the present study, a lentivirus was used to deliver shRNA targeting NF-κB1 into mouse RCC (Renca) cells. It was determined that the knockdown of the NF-κB1 gene led to a reduction in cell proliferation and late apoptosis/necrosis in vitro. Flow cytometry analysis demonstrated G₂/M arrest in the cells. In addition, immunoblotting analysis revealed a significant increase in cyclin B1 and Bax. In vivo experiments showed that Renca-shRNA-NF-κB1 cells have significantly diminished tumorigenicity. Moreover, immunohistochemical analysis revealed an increase in necrotic areas of Renca-shRNA-NF-κB1 tumors. Thus, this study indicates that downregulation of NF-κB1 can suppress RCC tumorigenesis by inducing late apoptosis/necrosis. Therefore, NF-κB1 may be a potential therapeutic target for RCC.

Regulation of G2/M Transition by Inhibition of WEE1 and PKMYT1 Kinases

In the cell cycle, there are two checkpoint arrests that allow cells to repair damaged DNA in order to maintain genomic integrity. Many cancer cells have defective G1 checkpoint mechanisms, thus depending on the G2 checkpoint far more than normal cells. G2 checkpoint abrogation is therefore a promising concept to preferably damage cancerous cells over normal cells. The main factor influencing the decision to enter mitosis is a complex composed of Cdk1 and cyclin B. Cdk1/CycB is regulated by various feedback mechanisms, in particular inhibitory phosphorylations at Thr14 and Tyr15 of Cdk1. In fact, Cdk1/CycB activity is restricted by the balance between WEE family kinases and Cdc25 phosphatases. The WEE kinase family consists of three proteins: WEE1, PKMYT1, and the less important WEE1B. WEE1 exclusively mediates phosphorylation at Tyr15, whereas PKMYT1 is dual-specific for Tyr15 as well as Thr14. Inhibition by a small molecule inhibitor is therefore proposed to be a promising option since WEE kinases bind Cdk1, altering equilibria and thus affecting G2/M transition.

Taming the Beast: Control of APC/CCdc20-Dependent Destruction

The anaphase-promoting complex/cyclosome (APC/C) is a large multisubunit ubiquitin ligase that triggers the metaphase-to-anaphase transition in the cell cycle by targeting the substrates cyclin B and securin for destruction. APC/C activity toward these two key substrates requires the coactivator Cdc20. To ensure that cells enter mitosis and partition their duplicated genome with high accuracy, APC/C^Cdc20 activity must be tightly controlled. Here, we discuss the mechanisms that regulate APC/C^Cdc20 activity both before and during mitosis. We focus our discussion primarily on the chromosomal pathways that both accelerate and delay APC/C activation by targeting Cdc20 to opposing fates. The findings discussed provide an overview of how cells control the activation of this major cell cycle regulator to ensure both accurate and timely cell division.

MCPH1, mutated in primary microcephaly, is required for efficient chromosome alignment during mitosis

MCPH1 gene, mutated in primary microcephaly, regulates cell progression into mitosis. While this role has been extensively investigated in the context of DNA damage, its function during unperturbed cell cycles has been given less attention. Here we have analyzed the dynamics of chromosome condensation and cell cycle progression in MCPH1 deficient cells under undamaging conditions. Our study demonstrates that chromosome condensation is uncoupled from cell cycle progression when MCPH1 function is lacking, resulting in cells that prematurely condense their chromosomes during mid G2-phase and delay decondensation at the completion of mitosis. However, mitosis onset occurs on schedule in MCPH1 deficient cells. We also revealed active Cdk1 to be mandatory for the premature onset of chromosome condensation during G2 and the maintenance of the condensed state thereafter. Interestingly, a novel cellular phenotype was observed while monitoring cell cycle progression in cells lacking MCPH1 function. Specifically, completion of chromosome alignment at the metaphase plate was significantly delayed. This deficiency reveals that MCPH1 is required for efficient chromosome biorientation during mitosis.

The hnRNP RALY regulates transcription and cell proliferation by modulating the expression of specific factors including the proliferation marker E2F1

The heterogeneous nuclear ribonucleoproteins (hnRNP) form a large family of RNA-binding proteins that exert numerous functions in RNA metabolism. RALY is a member of the hnRNP family that binds poly-U-rich elements within several RNAs and regulates the expression of specific transcripts. RALY is up-regulated in different types of cancer, and its down-regulation impairs cell cycle progression. However, the RALY's role in regulating RNA levels remains elusive. Here, we show that numerous genes coding for factors involved in transcription and cell cycle regulation exhibit an altered expression in RALY-down-regulated HeLa cells, consequently causing impairments in transcription, cell proliferation, and cell cycle progression. Interestingly, by comparing the list of RALY targets with the list of genes affected by RALY down-regulation, we found an enrichment of RALY mRNA targets in the down-regulated genes upon RALY silencing. The affected genes include the E2F transcription factor family. Given its role as proliferation-promoting transcription factor, we focused on E2F1. We demonstrate that E2F1 mRNA stability and E2F1 protein levels are reduced in cells lacking RALY expression. Finally, we also show that RALY interacts with transcriptionally active chromatin in both an RNA-dependent and -independent manner and that this association is abolished in the absence of active transcription. Taken together, our results highlight the importance of RALY as an indirect regulator of transcription and cell cycle progression through the regulation of specific mRNA targets, thus strengthening the possibility of a direct gene expression regulation exerted by RALY.

Quantitative Systems Biology to decipher design principles of a dynamic cell cycle network: the "Maximum Allowable mammalian Trade-Off-Weight" (MAmTOW)

Network complexity is required to lend cellular processes flexibility to respond timely to a variety of dynamic signals, while simultaneously warranting robustness to protect cellular integrity against perturbations. The cell cycle serves as a paradigm for such processes; it maintains its frequency and temporal structure (although these may differ among cell types) under the former, but accelerates under the latter. Cell cycle molecules act together in time and in different cellular compartments to execute cell type-specific programs. Strikingly, the timing at which molecular switches occur is controlled by abundance and stoichiometry of multiple proteins within complexes. However, traditional methods that investigate one effector at a time are insufficient to understand how modulation of protein complex dynamics at cell cycle transitions shapes responsiveness, yet preserving robustness. To overcome this shortcoming, we propose a multidisciplinary approach to gain a systems-level understanding of quantitative cell cycle dynamics in mammalian cells from a new perspective. By suggesting advanced experimental technologies and dedicated modeling approaches, we present innovative strategies (i) to measure absolute protein concentration in vivo, and (ii) to determine how protein dosage, e.g., altered protein abundance, and spatial (de)regulation may affect timing and robustness of phase transitions. We describe a method that we name “Maximum Allowable mammalian Trade–Off–Weight” (MAmTOW), which may be realized to determine the upper limit of gene copy numbers in mammalian cells. These aspects, not covered by current systems biology approaches, are essential requirements to generate precise computational models and identify (sub)network-centered nodes underlying a plethora of pathological conditions.

Drosophila p115 is required for Cdk1 activation and G2/M cell cycle transition

Golgi complex inheritance and its relationship with the cell cycle are central in cell biology. Golgi matrix proteins, known as golgins, are one of the components that underlie the shape and functionality of this organelle. In mammalian cells, golgins are phosphorylated during mitosis to allow fragmentation of the Golgi ribbon and they also participate in spindle dynamics; both processes are required for cell cycle progression. Little is known about the function of golgins during mitosis in metazoans in vivo. This is particularly significant in Drosophila, in which the Golgi architecture is distributed in numerous units scattered throughout the cytoplasm, in contrast with mammalian cells. We examined the function of the ER/cis-Golgi golgin p115 during the proliferative phase of the Drosophila wing imaginal disc. Knockdown of p115 decreased tissue size. This phenotype was not caused by programmed cell death or cell size reductions, but by a reduction in the final cell number due to an accumulation of cells at the G2/M transition. This phenomenon frequently allows mitotic bypass and re-replication of DNA. These outcomes are similar to those observed following the partial loss of function of positive regulators of Cdk1 in Drosophila. In agreement with this, Cdk1 activation was reduced upon p115 knockdown. Interestingly, these phenotypes were fully rescued by Cdk1 overexpression and partially rescued by Myt1 depletion, but not by String (also known as Cdc25) overexpression. Additionally, we confirmed the physical interaction between p115 and Cdk1, suggesting that the formation of a complex where both proteins are present is essential for the full activation of Cdk1 and thus the correct progression of mitosis in proliferating tissues.

A Mathematical Framework for Kinetochore-Driven Activation Feedback in the Mitotic Checkpoint

Proliferating cells properly divide into their daughter cells through a process that is mediated by kinetochores, protein-complexes that assemble at the centromere of each sister chromatid. Each kinetochore has to establish a tight bipolar attachment to the spindle apparatus before sister chromatid separation is initiated. The spindle assembly checkpoint (SAC) links the biophysical attachment status of the kinetochores to mitotic progression and ensures that even a single misaligned kinetochore keeps the checkpoint active. The mechanism by which this is achieved is still elusive. Current computational models of the human SAC disregard important biochemical properties by omitting any kind of feedback loop, proper kinetochore signals, and other spatial properties such as the stability of the system and diffusion effects. To allow for more realistic in silico study of the dynamics of the SAC model, a minimal mathematical framework for SAC activation and silencing is introduced. A nonlinear ordinary differential equation model successfully reproduces bifurcation signaling switches with attachment of all 92 kinetochores and activation of APC/C by kinetochore-driven feedback. A partial differential equation model and mathematical linear stability analyses indicate the influence of diffusion and system stability. The conclusion is that quantitative models of the human SAC should account for the positive feedback on APC/C activation driven by the kinetochores which is essential for SAC silencing. Experimental diffusion coefficients for MCC subcomplexes are found to be insufficient for rapid APC/C inhibition. The presented analysis allows for systems-level understanding of mitotic control, and the minimal new model can function as a basis for developing further quantitative-integrative models of the cell division cycle.

A cyclin-dependent kinase inhibitor, dinaciclib in preclinical treatment models of thyroid cancer

Background

We explored the therapeutic effects of dinaciclib, a cyclin-dependent kinase (CDK) inhibitor, in the treatment of thyroid cancer.

Materials and methods

Seven cell lines originating from three pathologic types of thyroid cancer (papillary, follicular and anaplastic) were studied. The cytotoxicity of dinaciclib was measured using a lactate dehydrogenase assay. The expression of proteins associated with cell cycle and apoptosis was assessed using Western blot analysis and immunofluorescence microscopy. Cell cycle distribution was measured by flow cytometry and immunofluorescence microscopy. Apoptosis and caspase-3 activity were measured by flow cytometry and fluorometric assay. Mice bearing flank anaplastic thyroid cancer (ATC) were treated with intraperitoneal injections of dinaciclib.

Results

Dinaciclib inhibited thyroid cancer cell proliferation in a dose-dependent manner. Dinaciclib had a low median-effect dose (≤ 16.0 nM) to inhibit cell proliferation in seven thyroid cancer cell lines. Dinaciclib decreased CDK1, cyclin B1, and Aurora A expression, induced cell cycle arrest in the G2/M phase, and induced accumulation of prophase mitotic cells. Dinaciclib decreased Mcl-1, Bcl-x_L and survivin expression, activated caspase-3 and induced apoptosis. In vivo, the growth of ATC xenograft tumors was retarded in a dose-dependent fashion with daily dinaciclib treatment. Higher-dose dinaciclib (50 mg/kg) caused slight, but significant weight loss, which was absent with lower-dose dinaciclib (40 mg/kg) treatment.

Conclusions

Dinaciclib inhibited thyroid cancer proliferation both in vitro and in vivo. These findings support dinaciclib as a potential drug for further studies in clinical trials for the treatment of patients with refractory thyroid cancer.

Anti-proliferative and apoptotic effects of S1, a tetrandrine derivative, in human gastric cancer BGC-823 cells

The aim of the study was to investigate the anti-proliferation and apoptosis-inducing effects of S1, a novel tetrandrine derivative, in human gastric cancer BGC-823 cells and explore the possible mechanism of action. The anti-proliferative activity was determined by MTT assay; the induction of cell cycle arrest and apoptosis were detected by flow cytometry. Quantitative real time RT-PCR and Western blotting were used to evaluate the mRNA and protein expression levels in mitochondrial pathway. S1 significantly reduced cell viability and induced a G2/M phase arrest and apoptosis in dose- and time-dependent manner. Further studies showed that S1 increased mRNA and protein expression of Bax and the Bax/Bcl-2 ratio. Moreover, S1 decreased the protein expression of procaspase-9 and procaspase-3, suggesting that the induction of apoptosis may be related to the alteration of the ratio of Bax/Bcl-2 and the activation of caspases. These findings suggested that S1 merits further investigation as a novel therapeutic agent for the treatment of human gastric cancer.

Sirt1 regulates glial progenitor proliferation and regeneration in white matter after neonatal brain injury

Regenerative processes in brain pathologies require the production of distinct neural cell populations from endogenous progenitor cells. We have previously demonstrated that oligodendrocyte progenitor cell (OPC) proliferation is crucial for oligodendrocyte (OL) regeneration in a mouse model of neonatal hypoxia (HX) that reproduces diffuse white matter injury (DWMI) of premature infants. Here we identify the histone deacetylase Sirt1 as a Cdk2 regulator in OPC proliferation and response to HX. HX enhances Sirt1 and Sirt1/Cdk2 complex formation through HIF1α activation. Sirt1 deacetylates retinoblastoma (Rb) in the Rb/E2F1 complex, leading to dissociation of E2F1 and enhanced OPC proliferation. Sirt1 knockdown in culture and its targeted ablation in vivo suppresses basal and HX-induced OPC proliferation. Inhibition of Sirt1 also promotes OPC differentiation after HX. Our results indicate that Sirt1 is an essential regulator of OPC proliferation and OL regeneration after neonatal brain injury. Therefore, enhancing Sirt1 activity may promote OL recovery after DWMI.

Oligodendrocyte progenitor cell (OPC) proliferation is crucial for regeneration after hypoxic lesions in mice, a model of diffuse white matter injury of premature infants. Here, the authors show that the histone deacetylase Sirt1 is a Cdk2-dependent mediator of OPC proliferation and OPC response to hypoxia.

Positive Feedback Keeps Duration of Mitosis Temporally Insulated from Upstream Cell-Cycle Events

Cell division is characterized by a sequence of events by which a cell gives rise to two daughter cells. Quantitative measurements of cell-cycle dynamics in single cells showed that despite variability in G1-, S-, and G2 phases, duration of mitosis is short and remarkably constant. Surprisingly, there is no correlation between cell-cycle length and mitotic duration, suggesting that mitosis is temporally insulated from variability in earlier cell-cycle phases. By combining live cell imaging and computational modeling, we showed that positive feedback is the molecular mechanism underlying the temporal insulation of mitosis. Perturbing positive feedback gave rise to a sluggish, variable entry and progression through mitosis and uncoupled duration of mitosis from variability in cell cycle length. We show that positive feedback is important to keep mitosis short, constant, and temporally insulated and anticipate it might be a commonly used regulatory strategy to create modularity in other biological systems.

Mitochondrial Ribosomal Protein L10 Associates with Cyclin B1/Cdk1 Activity and Mitochondrial Function

Mitochondrial ribosomal proteins are important for mitochondrial-encoded protein synthesis and mitochondrial function. In addition to their roles in mitoribosome assembly, several mitochondrial ribosome proteins are also implicated in cellular processes like cell cycle regulation, apoptosis, and mitochondrial homeostasis regulation. Here, we demonstrate that MRPL10 regulates cyclin B1/Cdk1 (cyclin-dependent kinase 1) activity and mitochondrial protein synthesis in mammalian cells. In Drosophila, inactivation of mRpL10 (the Drosophila ortholog of mammalian MRPL10) in eyes results in abnormal eye development. Furthermore, expression of human cyclin B1 suppresses eye phenotypes and mitochondrial abnormality of mRpL10 knockdown Drosophila. This study identified that the physiological regulatory pathway of MRPL10 and providing new insights into the role of MRPL10 in growth control and mitochondrial function.

Recombinant adenovirus of human p66Shc inhibits MCF-7 cell proliferation

The aim of this work was to construct a human recombinant p66Shc adenovirus and to investigate the inhibition of recombinant p66Shc adenovirus on MCF-7 cells. The recombinant adenovirus expression vector was constructed using the Adeno-X Adenoviral System 3. Inhibition of MCF-7 cell proliferation was determined by MTT. Intracellular ROS was measured by DCFH-DA fluorescent probes, and 8-OHdG was detected by ELISA. Cell apoptosis and the cell cycle were assayed by flow cytometry. Western blot were used to observe protein expression. p66Shc expression was upregulated in 4 cell lines after infection. The inhibitory effect of p66Shc recombinant adenovirus on MCF-7 cells was accompanied by enhanced ROS and 8-OHdG. However, no significant differences were observed in the cell apoptosis rate. The ratio of the cell cycle G2/M phase showed a significant increase. Follow-up experiments demonstrated that the expressions of p53, p-p53, cyclin B1 and CDK1 were upregulated with the overexpression of p66Shc. The Adeno-X Adenoviral System 3 can be used to efficiently construct recombinant adenovirus containing p66Shc gene, and the Adeno-X can inhibit the proliferation of MCF-7 cells by inducing cell cycle arrest at the G2/M phase. These results suggested that p66Shc may be a key target for clinical cancer therapy.

Bistability of mitotic entry and exit switches during open mitosis in mammalian cells

Mitotic entry and exit are switch-like transitions that are driven by the activation and inactivation of Cdk1 and mitotic cyclins. This simple on/off reaction turns out to be a complex interplay of various reversible reactions, feedback loops, and thresholds that involve both the direct regulators of Cdk1 and its counteracting phosphatases. In this review, we summarize the interplay of the major components of the system and discuss how they work together to generate robustness, bistability, and irreversibility. We propose that it may be beneficial to regard the entry and exit reactions as two separate reversible switches that are distinguished by differences in the state of phosphatase activity, mitotic proteolysis, and a dramatic rearrangement of cellular components after nuclear envelope breakdown, and discuss how the major Cdk1 activity thresholds could be determined for these transitions.

Application of flow cytometry for evaluating clinical prognosis and histopathological grade of human glioma

OBJECTIVES

Flow cytometry was applied to predict the biological parameters of tumor behavior based on the DNA content distribution of tumors. We used flow cytometry to determine the number of cell cycles for the characterization of intracranial gliomas and its possible prognostic role.

METHODS

Flow cytometric analysis of the DNA content was performed for 37 fresh operative glioma specimens. The expression of Ki-67 in glioma specimens was detected using immunohistochemistry staining. The check points of G2/M-phase fractions, cyclin B, and pCdk1 (Y15) were analyzed using Western immunoblotting.

RESULTS

Compared to low-grade (grade I/II) gliomas, significant differences in the Ki-67, cyclin B, G2/M-phase, and S+G2/M-phase expressions were found in high-grade (grade III/IV) gliomas. Furthermore, receiver operating characteristic (ROC) analysis indicated optimal cutoff points for the G2/M-phase and S+G2/M-phase fractions of 13.47 and 17.26%, respectively, which can be used to differentiate cases with low- and high-grade gliomas. Additionally, both G2/M-phase and S+G2/M-phase fractions had significant association with the expression of Ki-67 in the gliomas. The gliomas were classified by the DNA content. We found that patients with high-grade glioma had worse survival rate than patients with low-grade glioma. Meanwhile, ROC curve analysis gave cutoffs for G2/M-phase of 9.4% and for S+G2/M-phase fractions of 15.04% as best predicting survival. The patients with glioma had poor survival when the levels of G2/M-phase and S+G2/M-phase were more than 9.4 and 15.04%, respectively. In contrast, no significant association between the DNA content of glioma patients and their age, tumor recurrence, and tumor size was found.

DISCUSSION

Our results indicate that flow cytometry analysis for G2/M-phase and S+G2/M-phase fractions can be used for tumor grading for rapidly differentiating low- from high-grade gliomas.

14-3-3γ Prevents Centrosome Amplification and Neoplastic Progression

More than 80% of malignant tumors show centrosome amplification and clustering. Centrosome amplification results from aberrations in the centrosome duplication cycle, which is strictly coordinated with DNA-replication-cycle. However, the relationship between cell-cycle regulators and centrosome duplicating factors is not well understood. This report demonstrates that 14-3-3γ localizes to the centrosome and 14-3-3γ loss leads to centrosome amplification. Loss of 14-3-3γ results in the phosphorylation of NPM1 at Thr-199, causing early centriole disjunction and centrosome hyper-duplication. The centrosome amplification led to aneuploidy and increased tumor formation in mice. Importantly, an increase in passage of the 14-3-3γ-knockdown cells led to an increase in the number of cells containing clustered centrosomes leading to the generation of pseudo-bipolar spindles. The increase in pseudo-bipolar spindles was reversed and an increase in the number of multi-polar spindles was observed upon expression of a constitutively active 14-3-3-binding-defective-mutant of cdc25C (S216A) in the 14-3-3γ knockdown cells. The increase in multi-polar spindle formation was associated with decreased cell viability and a decrease in tumor growth. Our findings uncover the molecular basis of regulation of centrosome duplication by 14-3-3γ and inhibition of tumor growth by premature activation of the mitotic program and the disruption of centrosome clustering.

Targeting of tubulin polymerization and induction of mitotic blockage by Methyl 2-(5-fluoro-2-hydroxyphenyl)-1H-benzo[d]imidazole-5-carboxylate (MBIC) in human cervical cancer HeLa cell

Background

Microtubule Targeting Agents (MTAs) including paclitaxel, colchicine and vinca alkaloids are widely used in the treatment of various cancers. As with most chemotherapeutic agents, adverse effects and drug resistance are commonly associated with the clinical use of these agents. Methyl 2-(5-fluoro-2-hydroxyphenyl)-1H- benzo[d]imidazole-5-carboxylate (MBIC), a benzimidazole derivative displays greater toxicity against various cancer compared to normal human cell lines. The present study, focused on the cytotoxic effects of MBIC against HeLa cervical cancer cells and possible actions on the microtubule assembly.

Methods

Apoptosis detection and cell-cycle assays were performed to determine the type of cell death and the phase of cell cycle arrest in HeLa cells. Tubulin polymerization assay and live-cell imaging were performed to visualize effects on the microtubule assembly in the presence of MBIC. Mitotic kinases and mitochondrial-dependent apoptotic proteins were evaluated by Western blot analysis. In addition, the synergistic effect of MBIC with low doses of selected chemotherapeutic actions were examined against the cancer cells.

Results

Results from the present study showed that following treatment with MBIC, the HeLa cells went into mitotic arrest comprising of multi-nucleation and unsegregated chromosomes with a prolonged G₂-M phase. In addition, the HeLa cells showed signs of mitochondrial-dependant apoptotic features such as the release of cytochrome c and activation of caspases. MBIC markedly interferes with tubulin polymerization. Western blotting results indicated that MBIC affects mitotic regulatory machinery by up-regulating BubR1, Cyclin B1, CDK1 and down-regulation of Aurora B. In addition, MBIC displayed synergistic effect when given in combination with colchicine, nocodazole, paclitaxel and doxorubicin.

Conclusion

Taken together, our study demonstrated the distinctive microtubule destabilizing effects of MBIC against cervical cancer cells in vitro. Besides that, MBIC exhibited synergistic effects with low doses of selected anticancer drugs and thus, may potentially reduce the toxicity and drug resistance to these agents.

Electronic supplementary material

The online version of this article (doi:10.1186/s13046-016-0332-0) contains supplementary material, which is available to authorized users.

Transcriptome analysis of phycocyanin inhibitory effects on SKOV-3 cell proliferation

Phycocyanin (PC) from Spirulina platensis has inhibitory effects on tumor cell growth. In this research, the transcriptome study was designed to investigate the underlying molecular mechanisms of PC inhibition on human ovarian cancer cell SKOV-3 proliferation. The PC IC50 was 216.6μM and 163.8μM for 24h and 48h exposure, respectively, as determined by CCK-8 assay. The morphological changes of SKOV-3 cells after PC exposure were recorded using HE staining. Cells arrested in G2/M stages as determined by flow cytometry. The transcriptome analysis showed that 2031 genes (with > three-fold differences) were differentially expressed between the untreated and the PC-treated cells, including 1065 up-regulated and 966 down-regulated genes. Gene ontology and KEGG pathway analysis identified 18 classical pathways that were remarkably enriched, such as neurotrophin signaling pathway, VEGF signaling pathway and P53 signaling pathway. qPCR results further showed that PTPN12, S100A2, RPL26, and LAMA3 increased while HNRNPA1P10 decreased in PC-treated cells. Molecules and genes in those pathways may be potential targets to develop treatments for ovarian cancer.

A novel microtubule inhibitor, MT3-037, causes cancer cell apoptosis by inducing mitotic arrest and interfering with microtubule dynamics

We investigated the anticancer potential of a new synthetic compound, 7-(3-fluorophenyl)-4-methylpyrido-[2,3-d]pyrimidin-5(8H)-one (MT3-037). We found that MT3-037 effectively decreased the cancer cell viability by inducing apoptosis. MT3-037 treatments led to cell cycle arrest at M phase, with a marked increase in both expression of cyclin B1 and cyclin-dependent kinase 1 (CDK1) as well as in CDK1 kinase activity. Key proteins that regulate mitotic spindle dynamics, including survivin, Aurora A/B kinases, and polo-like kinase 1 (PLK1), were activated in MT3-037-treated cells. MT3-037-induced apoptosis was accompanied by activation of a pro-apoptotic factor, FADD, and the inactivation of apoptosis inhibitors, Bcl-2 and Bcl-xL, resulting in the cleavage/activation of caspases. The activation of c-Jun N-terminal kinase (JNK) was associated with MT3-037-induced CDK1 and Aurora A/B activation and apoptosis. Immunofluorescence staining of tubulin indicated that MT3-037 altered tubulin networks in cancer cells. Moreover, an in vitro tubulin polymerization assay revealed that MT3-037 inhibited the tubulin polymerization by direct binding to tubulin. Molecular docking studies and binding site completion assays revealed that MT3-037 binds to the colchicine-binding site. Furthermore, MT3-037 significantly inhibited the tumor growth in both MDAMB-468 and Erlotinib-resistant MDA-MB-468 xenograft mouse models. In addition, MT3-037 inhibited the angiogenesis and disrupted the tube formation by human endothelial cells. Our study demonstrates that MT3-037 is a potential tubulin-disrupting agent for antitumor therapy.

Herp depletion arrests the S phase of the cell cycle and increases estradiol synthesis in mouse granulosa cells

The endoplasmic reticulum (ER) stress response has been implicated in the development, atresia and luteinization of ovarian follicles. However, there have been few reports concerning the role of Herp, an ER stress-induced protein, in follicular development. The present study aims to detect the distribution and cyclic variations of Herp during the estrous cycle and to reveal the roles of Herp in regulating the cell cycle, apoptosis and steroid hormone biosynthesis in mouse granulosa cells. In this study, immunohistochemistry staining showed that Herp expression was primarily in the granulosa cells and oocytes. Furthermore, we constructed recombinant lentiviral vectors for Herp short hairpin interfering RNA (shRNA) expression; immunofluorescence staining, real-time quantitative PCR (RT-qPCR) and western blot analysis revealed that Herp was successfully knocked down. Flow cytometry showed that knockdown of Herp arrested granulosa cells at the S phase of the cell cycle. More importantly, ELISA analysis revealed that Herp knockdown significantly upregulated the concentration of estradiol (E2) in the culture supernatants. RT-qPCR was performed to determine the regulatory mechanism of Herp knockdown in the cell cycle, and in steroid synthesis, RT-qPCR analysis revealed that Herp knockdown upregulated the mRNA expression of steroidogenic enzymes (Cyp19a1) and downregulated metabolic enzymes (Cyp1b1) and cell cycle factors (cyclin A1, cyclin B1 and cyclin D2). These results suggest that Herp may regulate the cell cycle and hormone secretions in mouse granulosa cells. The present study helps to elucidate the physiological functions of Herp as they relate to reproduction.

Cross-Talk between AURKA and Plk1 in Mitotic Entry and Spindle Assembly

The Aurora kinase A (AURKA) is involved in different aspects of mitotic control, from mitotic entry to cytokinesis. Consistent with its pleiotropic roles, several AURKA interactors are able to modulate its activity, the best characterized being the microtubule-binding protein TPX2, the centrosomal protein Cep192, and Bora. Bora has been described as an essential cofactor of AURKA for phosphorylation-mediated activation of the mitotic kinase polo-like kinase 1 (Plk1) at the G2/M transition. A complex AURKA/Plk1 signaling axis is emerging, with multiple involved actors; recent data suggest that this control network is not restricted to mitotic entry only, but operates throughout mitosis. Here, we integrate available data from the literature to depict the complex interplay between AURKA and Plk1 in G2 and mitosis and how it contributes to their mitotic functions. We will particularly focus on how the activity of specifically localized AURKA/Plk1 pools is modulated in time and space by their reciprocal regulation to ensure the timely and coordinated unfolding of downstream mitotic events.

The lethal response to Cdk1 inhibition depends on sister chromatid alignment errors generated by KIF4 and isoform 1 of PRC1

Cyclin-dependent kinase 1 (Cdk1) is absolutely essential for cell division. Complete ablation of Cdk1 precludes the entry of G2 phase cells into mitosis, and is early embryonic lethal in mice. Dampening Cdk1 activation, by reducing gene expression or upon treatment with cell-permeable Cdk1 inhibitors, is also detrimental for proliferating cells, but has been associated with defects in mitotic progression, and the formation of aneuploid daughter cells. Here, we used a large-scale RNAi screen to identify the human genes that critically determine the cellular toxicity of Cdk1 inhibition. We show that Cdk1 inhibition leads to fatal sister chromatid alignment errors and mitotic arrest in the spindle checkpoint. These problems start early in mitosis and are alleviated by depletion of isoform 1 of PRC1 (PRC1-1), by gene ablation of its binding partner KIF4, or by abrogation of KIF4 motor activity. Our results show that, normally, Cdk1 activity must rise above the level required for mitotic entry. This prevents KIF4-dependent PRC1-1 translocation to astral microtubule tips and safeguards proper chromosome congression. We conclude that cell death in response to Cdk1 inhibitors directly relates to chromosome alignment defects generated by insufficient repression of PRC1-1 and KIF4 during prometaphase.

A novel ER-microtubule-binding protein, ERLIN2, stabilizes Cyclin B1 and regulates cell cycle progression

The gene encoding endoplasmic reticulum (ER) lipid raft-associated protein 2 (ERLIN2) is amplified in human breast cancers. ERLIN2 gene mutations were also found to be associated with human childhood progressive motor neuron diseases. Yet, an understanding of the physiological function and mechanism for ERLIN2 remains elusive. In this study, we reveal that ERLIN2 is a spatially and temporally regulated ER–microtubule-binding protein that has an important role in cell cycle progression by interacting with and stabilizing the mitosis-promoting factors. Whereas ERLIN2 is highly expressed in aggressive human breast cancers, during normal development ERLIN2 is expressed at the postnatal stage and becomes undetectable in adulthood. ERLIN2 interacts with the microtubule component α-tubulin, and this interaction is maximal during the cell cycle G2/M phase where ERLIN2 simultaneously interacts with the mitosis-promoting complex Cyclin B1/Cdk1. ERLIN2 facilitates K63-linked ubiquitination and stabilization of Cyclin B1 protein in G2/M phase. Downregulation of ERLIN2 results in cell cycle arrest, represses breast cancer proliferation and malignancy and increases sensitivity of breast cancer cells to anticancer drugs. In summary, our study revealed a novel ER–microtubule-binding protein, ERLIN2, which interacts with and stabilizes mitosis-promoting factors to regulate cell cycle progression associated with human breast cancer malignancy.

Alkaloids from beach spider lily (Hymenocallis littoralis) induce apoptosis of HepG-2 cells by the fas-signaling pathway

Alkaloids are the most extensively featured compounds of natural anti-tumor herbs, which have attracted much attention in pharmaceutical research. In our previous studies, a mixture of major three alkaloid components (5, 6-dihydrobicolorine, 7-deoxy-trans-dihydronarciclasine, littoraline) from Hymenocallis littoralis were extracted, analyzed and designated as AHL. In this paper, AHL extracts were added to human liver hepatocellular cells HepG-2, human gastric cancer cell SGC-7901, human breast adenocarcinoma cell MCF-7 and human umbilical vein endothelial cell EVC-304, to screen one or more AHL-sensitive tumor cell. Among these cells, HepG-2 was the most sensitive to AHL treatment, a very low dose (0.8μg/ml) significantly inhibiting proliferation . The non- tumor cell EVC-304, however, was not apparently affected. Effect of AHL on HepG-2 cells was then explored. We found that the AHL could cause HepG-2 cycle arrest at G2/M checkpoint, induce apoptosis, and interrupt polymerization of microtubules. In addition, expression of two cell cycle-regulated proteins, CyclinB1 and CDK1, was up-regulated upon AHL treatment. Up-regulation of the Fas, Fas ligand, Caspase-8 and Caspase-3 was observed as well, which might imply roles for the Fas/FsaL signaling pathway in the AHL-induced apoptosis of HepG-2 cells.

Fenofibrate increases radiosensitivity in head and neck squamous cell carcinoma via inducing G2/M arrest and apoptosis

Radiation therapy is an important treatment for head and neck squamous cell carcinoma (HNSCC). However, how to promote radiation sensitivity in HNSCC remains a challenge. This study aimed to investigate the radiosensitizing effects of fenofibrate on HNSCC and explore the underlying mechanisms. HNSCC cell lines CNE-2 and KB were subjected to ionizing radiation (IR), in the presence or absence of fenofibrate treatment. Cell growth and survival, apoptosis and cell cycle were evaluated. In addition, CNE-2 cells were xenografted into nude mice and subjected to IR and/ or fenofibrate treatment. The expression of cyclinB and CDK1 was detected by Western blotting. Our results showed that fenofibrate efficiently radiosensitized HNSCC cells and xenografts in mice, and induced apoptosis and G2/M arrest via reducing the activity of the CDK1/cyclinB1 kinase complex. These data suggest that fenofibrate could be a promising radiosensitizer for HNSCC radiotherapy.

Wip1 knockout inhibits the proliferation and enhances the migration of bone marrow mesenchymal stem cells

Mesenchymal stem cells (MSCs), a unique population of multipotent adult progenitor cells originally found in bone marrow (BM), are extremely useful for multifunctional therapeutic approaches. However, the growth arrest and premature senescence of MSCs in vitro prevent the in-depth characterization of these cells. In addition, the regulatory factors involved in MSCs migration remain largely unknown. Given that protein phosphorylation is associated with the processes of MSCs proliferation and migration, we focused on wild-type p53-inducible phosphatase-1 (Wip1), a well-studied modulator of phosphorylation, in this study. Our results showed that Wip1 knockout significantly inhibited MSCs proliferation and induced G2-phase cell-cycle arrest by reducing cyclinB1 expression. Compared with WT-MSCs, Wip1(-/-) MSCs displayed premature growth arrest after six passages in culture. Transwell and scratch assays revealed that Wip1(-/-) MSCs migrate more effectively than WT-MSCs. Moreover, the enhanced migratory response of Wip1(-/-) MSCs may be attributed to increases in the induction of Rac1-GTP activity, the pAKT/AKT ratio, the rearrangement of filamentous-actin (f-actin), and filopodia formation. Based on these results, we then examined the effect of treatment with a PI3K/AKT and Rac1 inhibitor, both of which impaired the migratory activity of MSCs. Therefore, we propose that the PI3K/AKT/Rac1 signaling axis mediates the Wip1 knockout-induced migration of MSCs. Our findings indicate that the principal function of Wip1 in MSCs transformation is the maintenance of proliferative capacity. Nevertheless, knocking out Wip1 increases the migratory capacity of MSCs. This dual effect of Wip1 provides the potential for purposeful routing of MSCs.

Module organization and variance in protein-protein interaction networks

A module is a group of closely related proteins that act in concert to perform specific biological functions through protein–protein interactions (PPIs) that occur in time and space. However, the underlying module organization and variance remain unclear. In this study, we collected module templates to infer respective module families, including 58,041 homologous modules in 1,678 species, and PPI families using searches of complete genomic database. We then derived PPI evolution scores and interface evolution scores to describe the module elements, including core and ring components. Functions of core components were highly correlated with those of essential genes. In comparison with ring components, core proteins/PPIs were conserved across multiple species. Subsequently, protein/module variance of PPI networks confirmed that core components form dynamic network hubs and play key roles in various biological functions. Based on the analyses of gene essentiality, module variance, and gene co-expression, we summarize the observations of module organization and variance as follows: 1) a module consists of core and ring components; 2) core components perform major biological functions and collaborate with ring components to execute certain functions in some cases; 3) core components are more conserved and essential during organizational changes in different biological states or conditions.

The biochemistry of mitosis

In this article, we will discuss the biochemistry of mitosis in eukaryotic cells. We will focus on conserved principles that, importantly, are adapted to the biology of the organism. It is vital to bear in mind that the structural requirements for division in a rapidly dividing syncytial Drosophila embryo, for example, are markedly different from those in a unicellular yeast cell. Nevertheless, division in both systems is driven by conserved modules of antagonistic protein kinases and phosphatases, underpinned by ubiquitin-mediated proteolysis, which create molecular switches to drive each stage of division forward. These conserved control modules combine with the self-organizing properties of the subcellular architecture to meet the specific needs of the cell. Our discussion will draw on discoveries in several model systems that have been important in the long history of research on mitosis, and we will try to point out those principles that appear to apply to all cells, compared with those in which the biochemistry has been specifically adapted in a particular organism.

AMPKα1 deficiency promotes cellular proliferation and DNA damage via p21 reduction in mouse embryonic fibroblasts

Emerging evidence suggests that activation of adenosine monophosphate-activated protein kinase (AMPK), an energy gauge and redox sensor, controls the cell cycle and protects against DNA damage. However, the molecular mechanisms by which AMPKα isoform regulates DNA damage remain largely unknown. The aim of this study was to determine if AMPKα deletion contributes to cellular hyperproliferation by reducing p21(WAF1/Cip1) (p21) expression thereby leading to accumulated DNA damage. The markers for DNA damage, cell cycle proteins, and apoptosis were monitored in cultured mouse embryonic fibroblasts (MEFs) isolated from wild type (WT, C57BL/6J), AMPKα1, or AMPKα2 homozygous deficient (AMPKα1(-/-), AMPKα2(-/-)) mice by Western blot, flow cytometry, and cellular immunofluorescence staining. Deletion of AMPKα1, the predominant AMPKα isoform, but not AMPKα2 in immortalized MEFs led to spontaneous DNA double-strand breaks (DSB) which corresponded to repair protein p53-binding protein 1 (53BP1) foci formation and subsequent apoptosis. Furthermore, AMPKα1 localizes to chromatin and AMPKα1 deletion down-regulates cyclin-dependent kinase inhibitor, p21, an important protein that plays a role in decreasing the incidence of spontaneous DSB via inhibition of cell proliferation. In addition, AMPKα1 null cells exhibited enhanced cell proliferation. Finally, p21 overexpression partially blocked the cellular hyperproliferation of AMPKα1-deleted MEFs via the inhibition of cyclin-dependent kinase 2 (CDK2). Taken together, our results suggest that AMPKα1 plays a fundamental role in controlling the cell cycle thereby affecting DNA damage and cellular apoptosis.

Spatial-temporal model for silencing of the mitotic spindle assembly checkpoint

The spindle assembly checkpoint arrests mitotic progression until each kinetochore secures a stable attachment to the spindle. Despite fluctuating noise, this checkpoint remains robust and remarkably sensitive to even a single unattached kinetochore among many attached kinetochores; moreover, the checkpoint is silenced only after the final kinetochore-spindle attachment. Experimental observations have shown that checkpoint components stream from attached kinetochores along microtubules toward spindle poles. Here, we incorporate this streaming behavior into a theoretical model that accounts for the robustness of checkpoint silencing. Poleward streams are integrated at spindle poles, but are diverted by any unattached kinetochore; consequently, accumulation of checkpoint components at spindle poles increases markedly only when every kinetochore is properly attached. This step-change robustly triggers checkpoint silencing after, and only after, the final kinetochore-spindle attachment. Our model offers a conceptual framework that highlights the role of spatiotemporal regulation in mitotic spindle checkpoint signaling and fidelity of chromosome segregation.

PCTAIRE1 phosphorylates p27 and regulates mitosis in cancer cells

PCTAIRE1 is distant relative of the cyclin-dependent kinase family that has been implicated in spermatogenesis and neuronal development, but it has not been studied in cancer. Here, we report that PCTAIRE1 is expressed in prostate, breast, and cervical cancer cells, where its RNAi-mediated silencing causes growth inhibition with aberrant mitosis due to defects in centrosome dynamics. PCTAIRE1 was not similarly involved in proliferation of nontransformed cells, including diploid human IMR-90 fibroblasts. Through yeast two-hybrid screening, we identified tumor suppressor p27 as a PCTAIRE1 interactor. In vitro kinase assays showed PCTAIRE1 phosphorylates p27 at Ser10. PCTAIRE1 silencing modulated Ser10 phosphorylation on p27 and led to its accumulation in cancer cells but not in nontransformed cells. In a mouse xenograft model of PPC1 prostate cancer, conditional silencing of PCTAIRE1 restored p27 protein expression and suppressed tumor growth. Mechanistic studies in HeLa cells showed that PCTAIRE1 phosphorylates p27 during the S and M phases of the cell cycle. Notably, p27 silencing was sufficient to rescue cells from mitotic arrest caused by PCTAIRE1 silencing. Clinically, PCTAIRE1 was highly expressed in primary breast and prostate tumors compared with adjacent normal epithelial tissues. Together our findings reveal an unexpected role for PCTAIRE1 in regulating p27 stability, mitosis, and tumor growth, suggesting PCTAIRE1 as a candidate cancer therapeutic target.

Knockdown of CEBPβ by RNAi in porcine granulosa cells resulted in S phase cell cycle arrest and decreased progesterone and estradiol synthesis

Cultured ovarian granulosa cells (GCs) are essential models to study molecular mechanisms of gene regulation during folliculogenesis. CCAAT enhancer binding proteins β (CEBPβ) has been identified in the ovary and is critical for follicular growth, ovulation and luteinization in mice. In the present study, hormonal treatment indicated that luteinizing hormone (LH) and exogenous human chorionic gonadotropins (hCG) significantly increased the expression of CEBPβ in porcine GCs. By RNAi-Ready pSIREN-RetroQ-ZsGreen Vector mediated recombinant pshRNA vectors, CEBPβ gene was successfully knocked down in porcine GCs, confirmed by mRNA and protein level analyzed by real time PCR and western blot, respectively. We further found that knockdown of CEBPβ significantly increased the expression of p-ERK1/2. Furthermore, CEBPβ knockdown arrested the GCs at S phase of cell cycle, but had no effects on cell apoptosis. More importantly, it markedly down regulated the concentration of estradiol (E2) and progesterone (P4) in the culture medium. To uncover the regulatory mechanism of CEBPβ knockdown on cell cycle and steroids synthesis, we found that the mRNA expression of bcl-2 (anti-apoptosis), StAR and Runx2 (steroid hormone synthesis) was up-regulated, while genes related to apoptosis (Caspase-3 and p53), hormonal synthesis (CYP11A1) and cell cycle (cyclinA1, cyclinB1, cyclinD1) were down-regulated, suggesting that knockdown of CEBPβ may inhibit apoptosis, regulate cell cycle and hormone secretions at the transcriptional level in porcine GCs. Furthermore, knockdown of CEBPβ significantly increased the expression of PTGS2 and decreased the expression of IGFBP4, Has2 and PTGFR which are important for folliculogenesis in porcine GCs. In conclusion, this study reveals that CEBPβ is a key regulator of porcine GCs through modulation of cell cycle, apoptosis, steroid synthesis, and other regulators of folliculogenesis.

Engineering light-inducible nuclear localization signals for precise spatiotemporal control of protein dynamics in living cells

The function of many eukaryotic proteins is regulated by highly dynamic changes in their nucleocytoplasmic distribution. The ability to precisely and reversibly control nuclear translocation would, therefore, allow dissecting and engineering cellular networks. Here we develop a genetically encoded, light-inducible nuclear localization signal (LINuS) based on the LOV2 domain of Avena sativa phototropin 1. LINuS is a small, versatile tag, customizable for different proteins and cell types. LINuS-mediated nuclear import is fast and reversible, and can be tuned at different levels, for instance, by introducing mutations that alter AsLOV2 domain photo-caging properties or by selecting nuclear localization signals (NLSs) of various strengths. We demonstrate the utility of LINuS in mammalian cells by controlling gene expression and entry into mitosis with blue light.

Designing inducible and reversible nuclear localization signals would enable researchers to dissect and engineer cellular networks. Here Niopek et al. create a light-inducible nuclear localization signal to regulate gene expression and mitosis in mammalian cells, using blue light.

CDC2 mediates progestin initiated endometrial stromal cell proliferation: a PR signaling to gene expression independently of its binding to chromatin

Although non-genomic steroid receptor pathways have been studied over the past decade, little is known about the direct gene expression changes that take place as a consequence of their activation. Progesterone controls proliferation of rat endometrial stromal cells during the peri-implantation phase of pregnancy. We showed that picomolar concentration of progestin R5020 mimics this control in UIII endometrial stromal cells via ERK1-2 and AKT activation mediated by interaction of Progesterone Receptor (PR) with Estrogen Receptor beta (ERb) and without transcriptional activity of endogenous PR and ER. Here we identify early downstream targets of cytoplasmic PR signaling and their possible role in endometrial stromal cell proliferation. Microarray analysis of global gene expression changes in UIII cells treated for 45 min with progestin identified 97 up- and 341 down-regulated genes. The most over-represented molecular functions were transcription factors and regulatory factors associated with cell proliferation and cell cycle, a large fraction of which were repressors down-regulated by hormone. Further analysis verified that progestins regulate Ccnd1, JunD, Usf1, Gfi1, Cyr61, and Cdkn1b through PR-mediated activation of ligand-free ER, ERK1-2 or AKT, in the absence of genomic PR binding. ChIP experiments show that progestin promoted the interaction of USF1 with the proximal promoter of the Cdc2 gene. Usf1 knockdown abolished Cdc2 progestin-dependent transcriptional regulation and cell proliferation, which also blocked Cdc2 knockdown. We conclude that progestin-induced proliferation of endometrial stromal cells is mediated by ERK1-2 and AKT dependent early regulation of USF1, which directly induces Cdc2. To our knowledge, this is the first description of early target genes of progestin-activated classical PR via crosstalk with protein kinases and independently of hormone receptor binding to the genomic targets.

Polo-like kinase-1 triggers histone phosphorylation by Haspin in mitosis

Histone modifications coordinate the chromatin localization of key regulatory factors in mitosis. For example, mitotic phosphorylation of Histone H3 threonine-3 (H3T3ph) by Haspin creates a binding site for the chromosomal passenger complex (CPC). However, how these histone modifications are spatiotemporally controlled during the cell cycle is unclear. Here we show that Plk1 binds to Haspin in a Cdk1-phosphorylation-dependent manner. Reducing Plk1 activity decreases the phosphorylation of Haspin and inhibits H3T3ph, particularly in prophase, suggesting that Plk1 is required for initial activation of Haspin in early mitosis. These studies demonstrate that Plk1 can positively regulate CPC recruitment in mitosis.

Assessing kinetics from fixed cells reveals activation of the mitotic entry network at the S/G2 transition

During the cell cycle, DNA duplication in S phase must occur before a cell divides in mitosis. In the intervening G2 phase, mitotic inducers accumulate, which eventually leads to a switch-like rise in mitotic kinase activity that triggers mitotic entry. However, when and how activation of the signaling network that promotes the transition to mitosis occurs remains unclear. We have developed a system to reduce cell-cell variation and increase accuracy of fluorescence quantification in single cells. This allows us to use immunofluorescence of endogenous marker proteins to assess kinetics from fixed cells. We find that mitotic phosphorylations initially occur at the completion of S phase, showing that activation of the mitotic entry network does not depend on protein accumulation through G2. Our data show insights into how mitotic entry is linked to the completion of S phase and forms a quantitative resource for mathematical models of the human cell cycle.

Phenethyl isothiocyanate and paclitaxel synergistically enhanced apoptosis and alpha-tubulin hyperacetylation in breast cancer cells

Combination of phenethyl isothiocyanate (PEITC) and paclitaxel (taxol) has been shown to work synergistically to increase apoptosis and cell cycle arrest in breast cancer cells. In this report, we further explored the mechanisms for the synergistic activity of PEITC and taxol in MCF7 and MDA-MB-231 (MB) breast cancer cell lines. By Western blotting analysis, treatment of MCF7 cells with both PEITC and taxol led to a 10.4-fold and 5.96-fold increase in specific acetylation of alpha-tubulin over single agent PEITC and taxol, respectively. This synergistic effect on acetylation of alpha-tubulin was also seen in MB cells. The combination of PEITC and taxol also reduced expressions of cell cycle regulator Cdk1, and anti-apoptotic protein bcl-2, enhanced expression of Bax and cleavage of PARP proteins. In conclusion, this study provided biochemical evidence for the mechanism of synergistic effect between the epigenetic agent PEITC and the chemotherapeutic agent taxol.

Interplay between the cell cycle and double-strand break response in mammalian cells

The cell cycle is intimately associated with the ability of cells to sense and respond to and repair DNA damage. Understanding how cell cycle progression, particularly DNA replication and cell division, are regulated and how DNA damage can affect these processes has been the subject of intense research. Recent evidence suggests that the repair of DNA damage is regulated by the cell cycle, and that cell cycle factors are closely associated with repair factors and participate in cellular decisions regarding how to respond to and repair damage. Precise regulation of cell cycle progression in the presence of DNA damage is essential to maintain genomic stability and avoid the accumulation of chromosomal aberrations that can promote tumor formation. In this review, we discuss the current understanding of how mammalian cells induce cell cycle checkpoints in response to DNA double-strand breaks. In addition, we discuss how cell cycle factors modulate DNA repair pathways to facilitate proper repair of DNA lesions.

Cyclin B1 destruction box-mediated protein instability: the enhanced sensitivity of fluorescent-protein-based reporter gene system

The periodic expression and destruction of several cyclins are the most important steps for the exact regulation of cell cycle. Cyclins are degraded by the ubiquitin-proteasome system during cell cycle. Besides, a short sequence near the N-terminal of cyclin B called the destruction box (D-box; CDB) is also required. Fluorescent-protein-based reporter gene system is insensitive to analysis because of the overly stable fluorescent proteins. Therefore, in this study, we use human CDB fused with both enhanced green fluorescent protein (EGFP) at C-terminus and red fluorescent protein (RFP, DsRed) at N-terminus in the transfected human melanoma cells to examine the effects of CDB on different fluorescent proteins. Our results indicated that CDB-fused fluorescent protein can be used to examine the slight gene regulations in the reporter gene system and have the potential to be the system for screening of functional compounds in the future.

p21Waf1/Cip1 deficiency causes multiple mitotic defects in tumor cells

As a multifaceted molecule, p21 plays multiple critical roles in cell cycle regulation, differentiation, apoptosis, DNA repair, senescence, aging and stem cell reprogramming. The important roles of p21 in the interphase of the cell cycle have been intensively investigated. The function of p21 in mitosis has been proposed but not systematically studied. We show here that p21 is abundant in mitosis and binds to and inhibits the activity of Cdk1/cyclin B1. Deficiency of p21 prolongs the duration of mitosis by extending metaphase, anaphase and cytokinesis. The activity of Aurora B is reduced and the localization of Aurora B on the central spindle is disturbed in anaphase cells without p21. Moreover, HCT116 p21-/-, HeLa and Saos-2 cells depleted of p21 encounter problems in chromosome segregation and cytokinesis. Gently inhibiting the mitotic Cdk1 or add-back of p21 rescues segregation defect in HCT116 p21-/- cells. Our data demonstrate that p21 is important for a fine-tuned control of the Cdk1 activity in mitosis, and its proper function facilitates a smooth mitotic progression. Given that p21 is downregulated in the majority of tumors, either by the loss of tumor suppressors like p53 or by hyperactive oncogenes such as c-myc, this finding also sheds new light on the molecular mechanisms by which p21 functions as a tumor suppressor.

Inhibition of LDH-A by oxamate induces G2/M arrest, apoptosis and increases radiosensitivity in nasopharyngeal carcinoma cells

An elevated rate of glucose consumption and the dependency on aerobic glycolysis for ATP generation have long been observed in cancer cells, a phenomenon known as the Warburg effect. the altered energy metabolism in cancer cells provides an attractive opportunity for developing novel cancer therapeutic strategies. Lactate dehydrogenase (LDH), which catalyzes the transformation of pyruvate to lactate, plays a vital role in the process of glycolysis. It has been reported that the level of LDH-A expression is increased both in head and neck cancer cells and in the blood serum of nasopharyngeal carcinoma (NPC) patients, and is associated with poor prognosis. However, the effect of LDH-A inhibition on NPC cells remains unknown. Here, in the present study, we found that oxamate, a classical inhibitor of LDH-A, suppressed cell proliferation in a dose- and time-dependent manner both in CNE-1 and CNE-2 cells, two NPC cancer cell lines. LDH inhibition by oxamate induced G2/M cell cycle arrest via downregulation of the CDK1/cyclin B1 pathway and promoted apoptosis through enhancement of mitochondrial ROS generation. N-acetylcysteine, a specific scavenger of ROS, significantly blocked the growth inhibition effect induced by oxamate. We also identified that oxamate increased sensitivity to ionizing radiation in the two NPC cancer cell lines. Furthermore, we verified similar results in tumor xenograft models. collectively, these results suggest that LDH-A may serve as a promising therapeutic target for NPC treatment.

A critical function of Mad2l2 in primordial germ cell development of mice

The development of primordial germ cells (PGCs) involves several waves of epigenetic reprogramming. A major step is following specification and involves the transition from the stably suppressive histone modification H3K9me2 to the more flexible, still repressive H3K27me3, while PGCs are arrested in G2 phase of their cycle. The significance and underlying molecular mechanism of this transition were so far unknown. Here, we generated mutant mice for the Mad2l2 (Mad2B, Rev7) gene product, and found that they are infertile in both males and females. We demonstrated that Mad2l2 is essential for PGC, but not somatic development. PGCs were specified normally in Mad2l2^−/− embryos, but became eliminated by apoptosis during the subsequent phase of epigenetic reprogramming. A majority of knockout PGCs failed to arrest in the G2 phase, and did not switch from a H3K9me2 to a H3K27me3 configuration. By the analysis of transfected fibroblasts we found that the interaction of Mad2l2 with the histone methyltransferases G9a and GLP lead to a downregulation of H3K9me2. The inhibitory binding of Mad2l2 to Cyclin dependent kinase 1 (Cdk1) could arrest the cell cycle in the G2 phase, and also allowed another histone methyltransferase, Ezh2, to upregulate H3K27me3. Together, these results demonstrate the potential of Mad2l2 in the regulation of both cell cycle and the epigenetic status. The function of Mad2l2 is essential in PGCs, and thus of high relevance for fertility.

Primordial germ cells (PGCs) are the origin of sperm and oocytes, and are responsible for transferring genetic information to the next generation faithfully. PGCs are first specified from pluripotent epiblast cells early in embryonic development. Second, they reprogram their epigenetic signature by changing histone modifications. This developmental event is specific to germ cells but not somatic cells. Although many players in the specification of PGCs are identified, only little is known about the genes essential for the regulation of the second phase. Here, we report that the Mad2l2 gene product plays an important role in the epigenetic reprogramming of PGCs. In wild type PGCs the cell cycle is arrested, and the methylation of histone 3 on residue K9 is replaced by methylation on K27. Our findings indicate that Mad2l2 is involved in this coordination of cell cycle and epigenetic reprogramming. The elucidation of this mechanism would help to identify the genetic basis of infertility.

A Wee1 checkpoint inhibits anaphase onset

The budding yeast Wee1 kinase Swe1 restrains the metaphase-to-anaphase transition by preventing the Cdk1-dependent phosphorylation and activation of APC^Cdc20.

Cdk1 drives both mitotic entry and the metaphase-to-anaphase transition. Past work has shown that Wee1 inhibition of Cdk1 blocks mitotic entry. Here we show that the budding yeast Wee1 kinase, Swe1, also restrains the metaphase-to-anaphase transition by preventing Cdk1 phosphorylation and activation of the mitotic form of the anaphase-promoting complex/cyclosome (APC^Cdc20). Deletion of SWE1 or its opposing phosphatase MIH1 (the budding yeast cdc25⁺) altered the timing of anaphase onset, and activation of the Swe1-dependent morphogenesis checkpoint or overexpression of Swe1 blocked cells in metaphase with reduced APC activity in vivo and in vitro. The morphogenesis checkpoint also depended on Cdc55, a regulatory subunit of protein phosphatase 2A (PP2A). cdc55Δ checkpoint defects were rescued by mutating 12 Cdk1 phosphorylation sites on the APC, demonstrating that the APC is a target of this checkpoint. These data suggest a model in which stepwise activation of Cdk1 and inhibition of PP2A^Cdc55 triggers anaphase onset.

Evidence for reciliation of RPE1 cells in late G1 phase, and ciliary localisation of cyclin B1

The primary cilium, an organelle that transduces extracellular signals important for development and tissue homeostasis, is typically assembled upon cell cycle exit and disassembled upon cell cycle re-entry. Cilium assembly is thought to be suppressed in cycling cells, however the extent of suppression is not clear. For example, primary cilia are present in certain proliferating cells during development, and a period of reciliation has been reported to occur in late G1 in murine 3T3 cells released from serum starvation-induced quiescence. Human retinal pigmented epithelial (hTERT-RPE1; herein, RPE1) cells are commonly used to investigate pathways regulating cilium disassembly, however the ciliary disassembly profile of these cells remains uncertain. A period of reciliation has not been observed. Here, we analyse the ciliary disassembly profile of RPE1 cells by immunofluorescence microscopy. The results suggest a profile similar to 3T3 cells, including a period of reciliation in late G1 and a second wave of deciliation in S phase. We present evidence that arresting cells in early S phase with hydroxyurea or excess thymidine prevents the second wave of deciliation, and that deciliation is initiated shortly after release from a thymidine block, consistent with coupling to DNA replication. These findings support the often overlooked notion that cilium formation can occur in late G1, and suggest that RPE1 cells could serve as a model system for studying the molecular pathways that direct this process, in addition to those that stimulate cilium disassembly. We also present immunofluorescence data indicating that cyclin B1 localises to primary cilia.

Cdk1 modulation ensures the coordination of cell-cycle events during the switch from meiotic prophase to mitosis

BACKGROUND

Budding yeast cells that enter the developmental path of meiosis do not commit to finishing meiosis until after prophase I and the realization of such meiosis-specific events as pairing of homologous chromosomes and initiation of recombination. If the meiosis-inducing signal is withdrawn prior to commitment, cells exit meiosis and return to mitosis. The timing of this transition poses a singular problem for maintaining genome integrity. Cells in meiotic prophase have already replicated their DNA, but they have not undergone the morphological changes intrinsic to mitosis, including budding. Successful re-entry into mitosis requires that these cells bud but not rereplicate their DNA, reversing the normal order of mitosis. This study focuses on the cellular mechanisms that permit this dramatically altered order of cell-cycle events.

RESULTS

By developing a microfluidics assay to monitor individual cells, we show that the successful transition from meiotic prophase to mitosis requires the modulation of Cdk1 activity to coordinate cell-cycle events. The S. cerevisiae Wee1 homolog Swe1 prevents the formation of multinucleate cells by restraining M phase CDK activity to allow bud formation prior to nuclear division. The remaining S phase CDK activity promotes bud formation and prevents origin licensing so that DNA cannot rereplicate between bud formation and nuclear division. Once a bud has formed, M phase CDK drives cells through a normal mitotic division.

CONCLUSIONS

Our study uncovers the essential requirement of Swe1 to modulate CDK activity to coordinate cell-cycle events and maintain genome integrity during the transition from meiotic prophase to mitosis.