Nuclear export of cyclin B1 and its possible role in the DNA damage-induced G2 checkpoint

CDC6 as a Key Inhibitory Regulator of CDK1 Activation Dynamics and the Timing of Mitotic Entry and Progression

Timely mitosis is critically important for early embryo development. It is regulated by the activity of the conserved protein kinase CDK1. The dynamics of CDK1 activation must be precisely controlled to assure physiologic and timely entry into mitosis. Recently, a known S-phase regulator CDC6 emerged as a key player in mitotic CDK1 activation cascade in early embryonic divisions, operating together with Xic1 as a CDK1 inhibitor upstream of the Aurora A and PLK1, both CDK1 activators. Herein, we review the molecular mechanisms that underlie the control of mitotic timing, with special emphasis on how CDC6/Xic1 function impacts CDK1 regulatory network in the Xenopus system. We focus on the presence of two independent mechanisms inhibiting the dynamics of CDK1 activation, namely Wee1/Myt1- and CDC6/Xic1-dependent, and how they cooperate with CDK1-activating mechanisms. As a result, we propose a comprehensive model integrating CDC6/Xic1-dependent inhibition into the CDK1-activation cascade. The physiological dynamics of CDK1 activation appear to be controlled by the system of multiple inhibitors and activators, and their integrated modulation ensures concomitantly both the robustness and certain flexibility of the control of this process. Identification of multiple activators and inhibitors of CDK1 upon M-phase entry allows for a better understanding of why cells divide at a specific time and how the pathways involved in the timely regulation of cell division are all integrated to precisely tune the control of mitotic events.

Pan-cancer analysis reveals the prognostic and immunotherapeutic value of cytoskeleton-associated protein 2-like

Cytoskeleton-associated protein 2-like (CKAP2L), a cell cycle-related protein, is correlated to tumor progression in some tumors. But there were no pan-cancer studies on CKAP2L, and its role in cancer immunotherapy is also unclear. The expression levels, expression activity, genomic alterations, DNA methylation and functions of CKAP2L in various tumors, as well as the associations between CKAP2L expression and patient prognosis, chemotherapy sensitivity, and tumor immune microenvironment, were all analyzed in a comprehensive pan-cancer analysis of CKAP2L by various databases, analysis websites, and R software. The experiments were also conducted to verify the analysis results. In the majority of cancers, CKAP2L expression and activity were markedly elevated. Elevated CKAP2L expression led to poor prognostic outcomes in patients, and is an independent risk factor for most tumors. Elevated CKAP2L causes decreased sensitivity to chemotherapeutic agents. Knockdown of CKAP2L significantly inhibited the proliferation and metastasis capacity of the KIRC cell lines and resulted in cell cycle G2/M arrest. In addition, CKAP2L was closely related to immune subtypes, immune cell infiltration, immunomodulators and immunotherapy markers (TMB, MSI), patients with high CKAP2L expression were more sensitive to immunotherapy in the IMvigor210 cohort. The results indicate that CKAP2L is a pro-cancer gene that serves as a potential biomarker for predicting patient outcomes. By inducing cells to transition from the G2 phase to the M phase, CKAP2L may promote cell proliferation and metastasis. Furthermore, CKAP2L is closely related to the tumor immune microenvironment and can be used as a biomarker to predict tumor immunotherapy.

The clinical significance of cyclin B1 (CCNB1) in invasive breast cancer with emphasis on its contribution to lymphovascular invasion development

BACKGROUND

Lymphovascular invasion (LVI) is regulated through complex molecular mechanisms. Cyclin B1 (CCNB1) was previously determined as being associated with LVI using large cohorts of breast cancer (BC) and artificial neural network (ANN) technique. In this study, we aimed to assess the association between CCNB1 and LVI, other clinicopathological and other LVI-related biomarkers at the molecular (RNA transcriptomic) and proteomic levels in BC.

METHODS

Two transcriptomic BC cohorts (n = 2834) were used to assess the association between the expression of CCNB1 at the mRNA level and clinicopathological characteristics and patient outcome. Tissue microarrays (TMAs) from a well-characterised BC cohort (n = 2480) with long-term outcome were also used to assess the clinical significance of CCNB1 protein expression using immunohistochemistry.

RESULTS

High CCNB1 mRNA expression was associated with aggressive tumour behaviour, including LVI, larger size, higher tumour grade, high lymph nodal stage, hormonal receptor negativity, HER2 positivity and poor clinical outcome (all p < 0.0001). Similarly, high CCNB1 protein expression was associated with higher tumour grade, hormonal receptor negativity and HER2 positivity (all p < 0.0001). Additionally, there was a significant association between CCNB1- and LVI-related biomarkers including N-cadherin, P-cadherin and TWIST2 at the transcriptomic and proteomic level. Multivariate analysis revealed that CCNB1 was an independent predictor of shorter BC-specific survival (HR = 1.3; 95% CI 1.2-1.5; p = 0.010).

CONCLUSION

CCNB1 is a key gene associated with LVI in BC and has prognostic value. More functional studies are warranted to unravel the mechanistic role of CCNB1 in the development of LVI.

Cyclin A and Cks1 promote kinase consensus switching to non-proline-directed CDK1 phosphorylation

Ordered protein phosphorylation by CDKs is a key mechanism for regulating the cell cycle. How temporal order is enforced in mammalian cells remains unclear. Using a fixed cell kinase assay and phosphoproteomics, we show how CDK1 activity and non-catalytic CDK1 subunits contribute to the choice of substrate and site of phosphorylation. Increases in CDK1 activity alter substrate choice, with intermediate- and low-sensitivity CDK1 substrates enriched in DNA replication and mitotic functions, respectively. This activity dependence is shared between Cyclin A- and Cyclin B-CDK1. Cks1 has a proteome-wide role as an enhancer of multisite CDK1 phosphorylation. Contrary to the model of CDK1 as an exclusively proline-directed kinase, we show that Cyclin A and Cks1 enhance non-proline-directed phosphorylation, preferably on sites with a +3 lysine residue. Indeed, 70% of cell-cycle-regulated phosphorylations, where the kinase carrying out this modification has not been identified, are non-proline-directed CDK1 sites.

Nuclear tension controls mitotic entry by regulating cyclin B1 nuclear translocation

As cells prepare to divide, they must ensure that enough space is available to assemble the mitotic machinery without perturbing tissue homeostasis. To do so, cells undergo a series of biochemical reactions regulated by cyclin B1-CDK1 that trigger cytoskeletal reorganization and ensure the coordination of cytoplasmic and nuclear events. Along with the biochemical events that control mitotic entry, mechanical forces have recently emerged as important players in cell-cycle regulation. However, the exact link between mechanical forces and the biochemical pathways that control mitotic progression remains unknown. Here, we identify a tension-dependent signal on the nucleus that sets the time for nuclear envelope permeabilization (NEP) and mitotic entry. This signal relies on actomyosin contractility, which unfolds the nucleus during the G2-M transition, activating the stretch-sensitive cPLA2 on the nuclear envelope and regulating the nuclear translocation of cyclin B1. Our data demonstrate how nuclear tension during the G2-M transition contributes to timely and efficient mitotic spindle assembly and prevents chromosomal instability.

Nucleosome Assembly Protein 1, Nap1, Is Required for the Growth, Development, and Pathogenicity of Magnaporthe oryzae

Magnaporthe oryzae is the causal agent of rice blast, leading to significant reductions in rice and wheat productivity. Nap1 is a conserved protein in eukaryotes involved in diverse physiological processes, such as nucleosome assembly, histone shuttling between the nucleus and cytoplasm, transcriptional regulation, and the cell cycle. Here, we identified Nap1 and characterized its roles in fungal development and virulence in M. oryzae. MoNap1 is involved in aerial hyphal and conidiophore differentiation, sporulation, appressorium formation, plant penetration, and virulence. ΔMonap1 generated a small, elongated, and malformed appressorium with an abnormally organized septin ring on hydrophobic surfaces. ΔMonap1 was more sensitive to cell wall integrity stresses but more resistant to microtubule stresses. MoNap1 interacted with histones H₂A and H₂B and the B-type cyclin (Cyc1). Moreover, a nuclear export signal (NES) domain is necessary for Nap1’s roles in the regulation of the growth and pathogenicity of M. oryzae. In summary, NAP1 is essential for the growth, appressorium formation, and pathogenicity of M. oryzae.

B1-type cyclins control microtubule organization during cell division in Arabidopsis

Flowering plants contain a large number of cyclin families, each containing multiple members, most of which have not been characterized to date. Here, we analyzed the role of the B1 subclass of mitotic cyclins in cell cycle control during Arabidopsis development. While we reveal CYCB1;5 to be a pseudogene, the remaining four members were found to be expressed in dividing cells. Mutant analyses showed a complex pattern of overlapping, development-specific requirements of B1-type cyclins with CYCB1;2 playing a central role. The double mutant cycb1;1 cycb1;2 is severely compromised in growth, yet viable beyond the seedling stage, hence representing a unique opportunity to study the function of B1-type cyclin activity at the organismic level. Immunolocalization of microtubules in cycb1;1 cycb1;2 and treating mutants with the microtubule drug oryzalin revealed a key role of B1-type cyclins in orchestrating mitotic microtubule networks. Subsequently, we identified the GAMMA-TUBULIN COMPLEX PROTEIN 3-INTERACTING PROTEIN 1 (GIP1/MOZART) as an in vitro substrate of B1-type cyclin complexes and further genetic analyses support a potential role in the regulation of GIP1 by CYCB1s.

CDC20 regulates the cell proliferation and radiosensitivity of P53 mutant HCC cells through the Bcl-2/Bax pathway

Purpose: The incidence of hepatocellular carcinoma (HCC) is extremely high, and China accounts for approximately 50% of global liver cancer cases. Previous studies reported that CDC20 is involved in the occurrence and progression of a variety of malignant tumors. So, whether CDC20 will affect the development of HCC, we have conducted in-depth research on this. Methods: We selected Hep3B and HepG2 for cell culture, and performed siRNA transfection, lentiviral infection, western blot, MTS determination, cell cycle determination, apoptosis test, immunodeficiency test, clone survival test and subcutaneous parthenogenesis in nude mice. Results: Knockdown of CDC20 greatly enhanced the radiation efficacy on the growth retardation in HepG2, and protein level of CDC20 was decreased for the activation of P53 by radiation. Downregulation of CDC20 combined with radiation can inhibit proliferation, aggravate DNA damage, increase G2/M arrest, and promote apoptosis of HCC cells to a greater extent, and the relative survival fraction of HCC cells was gradually reduced with radiation dose increased in P53 mutated Hep3B cells. After knocking down CDC20 in HCC, Bcl-2 was down-regulated and Bax expression increased. Down-regulation of CDC20 can inhibit further invasion by promoting the radiosensitivity of HCC. Conclusion: In this study, we found that that CDC20 was highly expressed in HCC and participated in radio resistance of HCC cells with P53 mutation Bcl-2/Bax via signaling pathway. This study is the first to present evidence that CDC20 may play a role in improving the efficacy of radiotherapy in HCC.

Using a Simple Cellular Assay to Map NES Motifs in Cancer-Related Proteins, Gain Insight into CRM1-Mediated NES Export, and Search for NES-Harboring Micropeptides

The nuclear export receptor CRM1 (XPO1) recognizes and binds specific sequence motifs termed nuclear export signals (NESs) in cargo proteins. About 200 NES motifs have been identified, but over a thousand human proteins are potential CRM1 cargos, and most of their NESs remain to be identified. On the other hand, the interaction of NES peptides with the "NES-binding groove" of CRM1 was studied in detail using structural and biochemical analyses, but a better understanding of CRM1 function requires further investigation of how the results from these in vitro studies translate into actual NES export in a cellular context. Here we show that a simple cellular assay, based on a recently described reporter (SRVB/A), can be applied to identify novel potential NESs motifs, and to obtain relevant information on different aspects of CRM1-mediated NES export. Using cellular assays, we first map 19 new sequence motifs with nuclear export activity in 14 cancer-related proteins that are potential CRM1 cargos. Next, we investigate the effect of mutations in individual NES-binding groove residues, providing further insight into CRM1-mediated NES export. Finally, we extend the search for CRM1-dependent NESs to a recently uncovered, but potentially vast, set of small proteins called micropeptides. By doing so, we report the first NES-harboring human micropeptides.

Mitotic kinase anchoring proteins: the navigators of cell division

The coordinated activities of many protein kinases, acting on multiple protein substrates, ensures the error-free progression through mitosis of eukaryotic cells. Enormous research effort has thus been devoted to studying the roles and regulation of these mitotic kinases, and to the identification of their physiological substrates. Central for the timely deployment of specific protein kinases to their appropriate substrates during the cell division cycle are the many anchoring proteins, which serve critical regulatory roles. Through direct association, anchoring proteins are capable of modulating the catalytic activity and/or sub-cellular distribution of the mitotic kinases they associate with. The key roles of some anchoring proteins in cell division are well-established, whilst others are still being unearthed. Here, we review the current knowledge on anchoring proteins for some mitotic kinases, and highlight how targeting anchoring proteins for inhibition, instead of the mitotic kinases themselves, could be advantageous for disrupting the cell division cycle.

Nuclear Export of Cyclin B Mediated by the Nup62 Complex Is Required for Meiotic Initiation in Drosophila Males

BACKGROUND

The central channel of the nuclear pore complex plays an important role in the selective transport of proteins between the nucleus and cytoplasm. Previous studies have demonstrated that the depletion of the Nup62 complex, constructing the nuclear pore channel in premeiotic Drosophila cells, resulted in the absence of meiotic cells. We attempted to understand the mechanism underlying the cell cycle arrest before meiosis.

METHODS

We induced dsRNAs against the nucleoporin mRNAs using the Gal4/UAS system in Drosophila.

RESULTS

The cell cycle of the Nup62-depleted cells was arrested before meiosis without CDK1 activation. The ectopic over-expression of CycB, but not constitutively active CDK1, resulted in partial rescue from the arrest. CycB continued to exist in the nuclei of Nup62-depleted cells and cells depleted of exportin encoded by emb. Protein complexes containing CycB, Emb, and Nup62 were observed in premeiotic spermatocytes. CycB, which had temporally entered the nucleus, was associated with Emb, and the complex was transported back to the cytoplasm through the central channel, interacting with the Nup62 complex. Conclusion: We proposed that CycB is exported with Emb through the channel interacting with the Nup62 complex before the onset of meiosis. The nuclear export ensures the modification and formation of sufficient CycB-CDK1 in the cytoplasm.

Human papillomavirus type 16 E6 and E7 oncoproteins interact with the nuclear p53-binding protein 1 in an in vitro reconstructed 3D epithelium: new insights for the virus-induced DNA damage response

Background

Despite vaccination and screening measures, anogenital cancer, mainly promoted by HPV16 oncoproteins, still represents the fourth tumor and the second cause of death among women.

Cell replication fidelity is the result of the host DNA damage response (DDR). Unlike many DNA viruses that promote their life cycle through the DDR inactivation, HR-HPVs encourage cells proliferation despite the DDR turned on. Why and how it occurs has been only partially elucidated.

During HPV16 infection, E6 links and degrades p53 via the binding to the E6AP LXXLL sequence; unfortunately, E6 direct role in the DDR response has not clearly identified yet.

Similarly, E7 increases DDR by competing with E2F1-pRb interaction, thus leading to the inactivation of pRb, and promotion, E2F1 mediated, of DDR genes translation, by binding to the pRb-like proteins CBP/p300 and p107, that also harbour LXXLL sequence, and via the interaction and activation of several DDR proteins.

Methods

To gain information regarding E6 and E7 contribution in DDR activation, we produced an in vitro 3D HPV16-E6E7 infected epithelium, already consolidated study model for HPVs, and validated it by assessing H&E staining and BrdU, HPV16 DNA, E6E7 proteins and γH2A.X/53BP1 double-strand break (DSBs) sensors expression; then we made an immuno-colocalization of E6 and E7 with cyclin E2 and B1.

Since 53BP1, like E6 and E7, also binds p53 and pRb, we supposed their possible direct binding. To explore this hypothesis, we performed a double immunofluorescence of E6 and E7 with 53BP1, a sequence analysis of 53BP1 within its BRCT2 domain and then an in situ PLA within CaSki, E6E7HPV16 NHEKs and the 3D model.

Results

The in vitro epithelium resembled the histology and the events typical of in vivo infected tissues. E6E7HPV16 were both expressed in basal and differentiated strata and induced H2A.X phosphorylation and 53BP1 increment into nuclear foci.

After highlighting E6 and E7 co-expression with 53BP1 and a LKVLL sequence within the 53BP1 BRCT2 domain, we demonstrated the bindings via the PLA technique.

Conclusions

Our results reinforce E6 and E7 role in cellular function control providing potentially new insights into the activity of this tumor virus.

Cyclin B1 is essential for mitosis in mouse embryos, and its nuclear export sets the time for mitosis

There is remarkable redundancy between the Cyclin-Cdk complexes that comprise the cell cycle machinery. None of the mammalian A-, D-, or E-type cyclins are required in development until implantation, and only Cdk1 is essential for early cell divisions. Cyclin B1 is essential for development, but whether it is required for cell division is contentious. Here, we used a novel imaging approach to analyze Cyclin B1-null embryos from fertilization onward. We show that Cyclin B1-/- embryos arrest in G2 phase after just two divisions. This is the earliest arrest of any Cyclin known and places Cyclin B1 with cdk1 as the essential regulators of the cell cycle. We reintroduced mutant proteins into this genetically null background to determine why Cyclin B1 is constantly exported from the nucleus. We found that Cyclin B1 must be exported from the nucleus for the cell to prevent premature entry to mitosis, and retaining Cyclin B1-Cdk1 at the plasma membrane precludes entry to mitosis.

Antitumor activity and expression profiles of genes induced by sulforaphane in human melanoma cells

Purpose

Human melanoma is a highly aggressive incurable cancer due to intrinsic cellular resistance to apoptosis, reprogramming, proliferation and survival during tumour progression. Sulforaphane (SFN), an isothiocyanate found in cruciferous vegetables, plays a role in carcinogenesis in many cancer types. However, the cytotoxic molecular mechanisms and gene expression profiles promoted by SFN in human melanoma remain unknown.

Methods

Three different cell lines were used: two human melanoma A375 and 501MEL and human epidermal melanocytes (HEMa). Cell viability and proliferation, cell cycle analysis, cell migration and invasion and protein expression and phosphorylation status of Akt and p53 upon SFN treatment were determined. RNA-seq of A375 was performed at different time points after SFN treatment.

Results

We demonstrated that SFN strongly decreased cell viability and proliferation, induced G₂/M cell cycle arrest, promoted apoptosis through the activation of caspases 3, 8, 9 and hampered migration and invasion abilities in the melanoma cell lines. Remarkably, HEMa cells were not affected by SFN treatment. Transcriptomic analysis revealed regulation of genes involved in response to stress, apoptosis/cell death and metabolic processes. SFN upregulated the expression of pro-apoptotic genes, such as p53, BAX, PUMA, FAS and MDM2; promoted cell cycle inhibition and growth arrest by upregulating EGR1, GADD45B, ATF3 and CDKN1A; and simultaneously acted as a potent inhibitor of genotoxicity by launching the stress-inducible protein network (HMOX1, HSPA1A, HSPA6, SOD1).

Conclusion

Overall, the data show that SFN cytotoxicity in melanoma derives from complex and concurrent mechanisms during carcinogenesis, which makes it a promising cancer prevention agent.

Electronic supplementary material

The online version of this article (doi:10.1007/s00394-017-1527-7) contains supplementary material, which is available to authorized users.

Unique Regulatory Mechanisms for the Human Embryonic Stem Cell Cycle

The cell cycle in pluripotent human embryonic stem cells is governed by unique mechanisms that support unrestricted proliferation and competency for endodermal, mesodermal, and ectodermal differentiation. The abbreviated G1 period with retention of uncompromised fidelity for genetic and epigenetic mechanisms operative in control of proliferation support competency for expansion of the pluripotent cell population that is fundamental for initial stages of development. Regulatory events during the G1 period of the pluripotent cell cycle are decisive for the transition from pluripotency to lineage commitment. Recent findings indicate that a G2 cell cycle pause is present in both endodermal and mesodermal lineage cells, and is obligatory for differentiation to endoderm. J. Cell. Physiol. 232: 1254-1257, 2017. © 2016 Wiley Periodicals, Inc.

Immunohistochemical Expression of Cyclin B1 in Epithelial Hyperplasia, Dysplasia and Oral Squamous Cell Carcinomas - A Comparative Study

INTRODUCTION

Cyclin B1 is important in the cell cycle progression from G2 to M phase. Cyclin B1 binds to CDC2, which then becomes dephosphorylated and gets relocated to the nucleus, ensuring the transition toward mitosis.

AIM

Over expression of Cyclin B1, has been reported more recently in breast, colon, prostate, oral and esophageal carcinomas. Thus, the aim of the present study was to examine the expression of Cyclin B1 in hyperplasia, dysplasia and Oral Squamous Cell Carcinomas (OSCC).

MATERIALS AND METHODS

A total of 64 histopathologically diagnosed cases of epithelial hyperplasias, dysplastic oral epithelium and OSCC were included in the study. Immunohistochemical procedure was carried out using the monoclonal mouse Cyclin B1 antibody (Clone V-152). The Cyclin B1 positive tumor cells counted were expressed as percentage of positive tumor cells. Nuclear and cytoplasmic labeling index (n&cLI) were calculated. The results were tabulated and statistically analyzed by Kruskal Wallis test- One Way ANOVA and Mann Whitney U- test.

RESULTS

Combined n&cLI was considered only in 28.57% of epithelial hyperplasias, 40.7% of oral epithelial dysplasias and 72% of OSCC showed over expression of Cyclin B1 with p value being 0.029. Cyclin B1 expression was not significantly different between the grades of dysplasia, between the grades of OSCC and between the marginal groups.

CONCLUSION

The present study demonstrates more than 50% of the study group showing less than 20% of nuclear staining. The importance of such variations within a type of lesion requires further investigation, since Cyclin B1 has proved useful in many studies from esophageal and laryngeal squamous cell carcinoma as a prognostic indicator, an indicator of recurrence and as an indicator for tumor sensitivity to radiotherapy. Further studies are to be extended towards evaluating the role of Cyclin B1 as a prognostic indicator.

RCC1-dependent activation of Ran accelerates cell cycle and DNA repair, inhibiting DNA damage-induced cell senescence

The coordination of cell cycle progression with the repair of DNA damage supports the genomic integrity of dividing cells. The function of many factors involved in DNA damage response (DDR) and the cell cycle depends on their Ran GTPase-regulated nuclear-cytoplasmic transport (NCT). The loading of Ran with GTP, which is mediated by RCC1, the guanine nucleotide exchange factor for Ran, is critical for NCT activity. However, the role of RCC1 or Ran⋅GTP in promoting cell proliferation or DDR is not clear. We show that RCC1 overexpression in normal cells increased cellular Ran⋅GTP levels and accelerated the cell cycle and DNA damage repair. As a result, normal cells overexpressing RCC1 evaded DNA damage-induced cell cycle arrest and senescence, mimicking colorectal carcinoma cells with high endogenous RCC1 levels. The RCC1-induced inhibition of senescence required Ran and exportin 1 and involved the activation of importin β-dependent nuclear import of 53BP1, a large NCT cargo. Our results indicate that changes in the activity of the Ran⋅GTP-regulated NCT modulate the rate of the cell cycle and the efficiency of DNA repair. Through the essential role of RCC1 in regulation of cellular Ran⋅GTP levels and NCT, RCC1 expression enables the proliferation of cells that sustain DNA damage.

Discovery of a potent microtubule-targeting agent: Synthesis and biological evaluation of water-soluble amino acid prodrug of combretastatin A-4 derivatives

Amino acid prodrugs are known to be very useful for improving the aqueous solubility of sparingly water soluble drugs (Drug Discovery Today 2013, 18, 93). Therefore, we synthesized eleven novel combretastatin A-4 amino acid derivatives and evaluated their anti-tumor activities in vitro and in vivo. Among them, compound 15 (valine attached to compound 3, which was shown to be a potent tubulin polymerization inhibitor in our previous study) exhibited high efficacy in tumor-bearing mice, and pharmacokinetic analysis in rats indicated that compound 15 was an effective prodrug as well. Besides, compound 15 significantly inhibited tubulin polymerization in vitro and in vivo by binding to the colchicine binding site. In addition, compound 15 induced cell cycle arrest in the G2/M phase and triggered apoptosis in a caspase-dependent manner. In conclusion, our study showed that compound 15 could have significant anti-tumor activity as a novel microtubule polymerization disrupting agent with improved aqueous solubility.

Induction of DNA damage and G2 cell cycle arrest by diepoxybutane through the activation of the Chk1-dependent pathway in mouse germ cells

1,2:3,4-Diepoxybutane (DEB) is a major carcinogenic metabolite of 1,3-butadiene (BD), which has been shown to cause DNA strand breaks in cells through its potential genotoxicity. The adverse effect of DEB on male reproductive cells in response to DNA damage has not been thoroughly studied, and the related mechanism is yet to be elucidated. Using mouse spermatocyte-derived GC-2 cells, we demonstrated in the present study that DEB caused the proliferation inhibition and marked cell cycle arrest at the G2 phase but not apoptosis. DEB also induced DNA damage as evidenced by γ-H2AX expression, the comet assay, and the cytokinesis-block micronucleus assay. Meanwhile, DEB triggered the Chk1/Cdc25c/Cdc2 signal pathway, which could be abated in the presence of UCN-01 or Chk1 siRNA. GC-2 cells exposed to DEB experienced ROS generation and pretreatment of N-acetyl-l-cysteine, partly attenuated DEB-induced DNA damage, and G2 arrest. Furthermore, measurement of testicular cells showed an increased proportion of tetraploid cells in mice administrated with DEB, alongside the enhanced expression of p-Chk1. Also, the defective reproductive phenotypes, including reduced sperm motility, increased sperm malformation, and histological abnormality of testes, were observed. In conclusion, these results suggest DEB induces DNA damage and G2 cell cycle arrest by activating the Chk1-dependent pathway, while oxidative stress may be associated with eliciting toxicity in male reproductive cells.

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.

Homeostatic control of polo-like kinase-1 engenders non-genetic heterogeneity in G2 checkpoint fidelity and timing

The G2 checkpoint monitors DNA damage, preventing mitotic entry until the damage can be resolved. The mechanisms controlling checkpoint recovery are unclear. Here, we identify non-genetic heterogeneity in the fidelity and timing of damage-induced G2 checkpoint enforcement in individual cells from the same population. Single-cell fluorescence imaging reveals that individual damaged cells experience varying durations of G2 arrest, and recover with varying levels of remaining checkpoint signal or DNA damage. A gating mechanism dependent on polo-like kinase-1 (PLK1) activity underlies this heterogeneity. PLK1 activity continually accumulates from initial levels in G2-arrested cells, at a rate inversely correlated to checkpoint activation, until it reaches a threshold allowing mitotic entry regardless of remaining checkpoint signal or DNA damage. Thus, homeostatic control of PLK1 by the dynamic opposition between checkpoint signalling and pro-mitotic activities heterogeneously enforces the G2 checkpoint in each individual cell, with implications for cancer pathogenesis and therapy.

The kinetics of G2 and M transitions regulated by B cyclins

B cyclins regulate G2-M transition. Because human somatic cells continue to cycle after reduction of cyclin B1 (cycB1) or cyclin B2 (cycB2) by RNA interference (RNAi), and because cycB2 knockout mice are viable, the existence of two genes should be an optimization. To explore this idea, we generated HeLa BD™ Tet-Off cell lines with inducible cyclin B1- or B2-EGFP that were RNAi resistant. Cultures were treated with RNAi and/or doxycycline (Dox) and bromodeoxyuridine. We measured G2 and M transit times and 4C cell accumulation. In the absence of ectopic B cyclin expression, knockdown (kd) of either cyclin increased G2 transit. M transit was increased by cycB1 kd but decreased by cycB2 depletion. This novel difference was further supported by time-lapse microscopy. This suggests that cycB2 tunes mitotic timing, and we speculate that this is through regulation of a Golgi checkpoint. In the presence of endogenous cyclins, expression of active B cyclin-EGFPs did not affect G2 or M phase times. As previously shown, B cyclin co-depletion induced G2 arrest. Expression of either B cyclin-EGFP completely rescued knockdown of the respective endogenous cyclin in single kd experiments, and either cyclin-EGFP completely rescued endogenous cyclin co-depletion. Most of the rescue occurred at relatively low levels of exogenous cyclin expression. Therefore, cycB1 and cycB2 are interchangeable for ability to promote G2 and M transition in this experimental setting. Cyclin B1 is thought to be required for the mammalian somatic cell cycle, while cyclin B2 is thought to be dispensable. However, residual levels of cyclin B1 or cyclin B2 in double knockdown experiments are not sufficient to promote successful mitosis, yet residual levels are sufficient to promote mitosis in the presence of the dispensible cyclin B2. We discuss a simple model that would explain most data if cyclin B1 is necessary.

Identification of CRM1-dependent Nuclear Export Cargos Using Quantitative Mass Spectrometry

Chromosome region maintenance 1/exportin1/Exp1/Xpo1 (CRM1) is the major transport receptor for the export of proteins from the nucleus. It binds to nuclear export signals (NESs) that are rich in leucines and other hydrophobic amino acids. The prediction of NESs is difficult because of the extreme recognition flexibility of CRM1. Furthermore, proteins can be exported upon binding to an NES-containing adaptor protein. Here we present an approach for identifying targets of the CRM1-export pathway via quantitative mass spectrometry using stable isotope labeling with amino acids in cell culture. With this approach, we identified >100 proteins from HeLa cells that were depleted from cytosolic fractions and/or enriched in nuclear fractions in the presence of the selective CRM1-inhibitor leptomycin B. Novel and validated substrates are the polyubiquitin-binding protein sequestosome 1, the cancerous inhibitor of protein phosphatase 2A (PP2A), the guanine nucleotide-binding protein-like 3-like protein, the programmed cell death protein 2-like protein, and the cytosolic carboxypeptidase 1 (CCP1). We identified a functional NES in CCP1 that mediates direct binding to the export receptor CRM1. The method will be applicable to other nucleocytoplasmic transport pathways, as well as to the analysis of nucleocytoplasmic shuttling proteins under different growth conditions.

The control of meiotic maturation in mammalian oocytes

Mammalian oocytes spend the majority of their lives in a dormant state, residing in primordial follicles. This arrest, most analogous to the G2 stage of the mitotic cell cycle division, is only broken in the hours preceding ovulation, when a hormonal rise induces meiotic resumption and entry into the first meiotic division. At a molecular level, this event is triggered by CDK1 activity, and here, we examine how CDK1 is suppressed during meiotic arrest and raised for oocyte maturation. We focus on signaling: intercellular signaling between the oocyte and the somatic cells of the follicle, and spatial signaling involving the anaphase-promoting complex (APC) within the oocyte. Meiotic arrest is achieved through APC(FZR1)-mediated cyclin B1 degradation. Once meiotic resumption resumes, CDK1 levels rise, but its activity eventually needs to be suppressed for completion of the first meiotic division. This is achieved by APC(CDC20), whose activity is critically regulated by the spindle assembly checkpoint, and which induces both a loss in CDK1 activity as well as the cohesive ties holding chromosomes together.

BRCA1 targets G2/M cell cycle proteins for ubiquitination and proteasomal degradation

The BRCA1 tumor suppressor protein heterodimerizes with its partner protein, BARD1, via the RING domain present in both proteins. The heterodimer contains an E3 ubiquitin ligase activity and participates in multiple cellular functions such as cell cycle control, DNA repair and regulation of gene transcription, collectively aimed at maintaining genomic stability and tumor suppression. Yet, the precise role of BRCA1 E3 ligase in these cellular functions is poorly understood. We present data showing that BRCA1 ubiquitinates G2/M cell cycle proteins, cyclin B and Cdc25C, leading to their accelerated degradation via a mechanism that is independent of APC/C. BRCA1-dependent degradation of cyclin B and Cdc25C is reversed by proteasome inhibitors and is enhanced following DNA damage, which may represent a possible mechanism to prevent cyclin B and Cdc25C accumulation, a requirement for mitotic entry. Our data provide mechanistic insight into how BRCA1 E3 ligase activity regulates the G2/M cell cycle checkpoint and, thus, contributes to maintenance of genomic stability.

Pathways for genome integrity in G2 phase of the cell cycle

The maintenance of genome integrity is important for normal cellular functions, organism development and the prevention of diseases, such as cancer. Cellular pathways respond immediately to DNA breaks leading to the initiation of a multi-facetted DNA damage response, which leads to DNA repair and cell cycle arrest. Cell cycle checkpoints provide the cell time to complete replication and repair the DNA damage before it can continue to the next cell cycle phase. The G2/M checkpoint plays an especially important role in ensuring the propagation of error-free copies of the genome to each daughter cell. Here, we review recent progress in our understanding of DNA repair and checkpoint pathways in late S and G2 phases. This review will first describe the current understanding of normal cell cycle progression through G2 phase to mitosis. It will also discuss the DNA damage response including cell cycle checkpoint control and DNA double-strand break repair. Finally, we discuss the emerging concept that DNA repair pathways play a major role in the G2/M checkpoint pathway thereby blocking cell division as long as DNA lesions are present.

Convergent starvation signals and hormone crosstalk in regulating nutrient mobilization upon germination in cereals

Germination is a unique developmental transition from metabolically quiescent seed to actively growing seedling that requires an ensemble of hydrolases for coordinated nutrient mobilization to support heterotrophic growth until autotrophic photosynthesis is established. This study reveals two crucial transcription factors, MYBS1 and MYBGA, present in rice (Oryza sativa) and barley (Hordeum vulgare), that function to integrate diverse nutrient starvation and gibberellin (GA) signaling pathways during germination of cereal grains. Sugar represses but sugar starvation induces MYBS1 synthesis and its nuclear translocation. GA antagonizes sugar repression by enhancing conuclear transport of the GA-inducible MYBGA with MYBS1 and the formation of a stable bipartite MYB-DNA complex to activate the α-amylase gene. We further discovered that not only sugar but also nitrogen and phosphate starvation signals converge and interconnect with GA to promote the conuclear import of MYBS1 and MYBGA, resulting in the expression of a large set of GA-inducible but functionally distinct hydrolases, transporters, and regulators associated with mobilization of the full complement of nutrients to support active seedling growth in cereals.

DAT-230, a novel microtubule inhibitor, exhibits potent anti-tumor activity by inducing G2/M phase arrest, apoptosis in vitro and perfusion decrease in vivo to HT-1080

PURPOSE

The anti-mitotic agent, combretastatin A-4 (CA-4), is the lead compound of a new class of anti-cancer drugs that target tumor vasculature. 2-Methoxy-5-(2-(3, 4, 5-trimethoxyphenyl) thiophen-3-yl) aniline (DAT-230) is a structurally novel CA-4 analog with more stability. We investigated its anti-tumor activity and mechanisms in vitro and in vivo for the first time.

METHODS

Cytotoxicity was measured by MTT method. Apoptosis, mitochondria membrane potential (ΔΨm) and NO generation were measured by flow cytometry. Intracellular microtubule network was detected by immunofluorescence experiments. Protein expression was analyzed by Western blotting. In vivo, the anti-tumor activity was assessed using fibrosarcoma xenografts subcutaneously established in BALB/c nude mice. Vasculature perfusion was identified using fluorescent DNA-binding compound Hoechst 33342.

RESULTS

DAT-230 exhibited potent anti-proliferative activity against various cancer cells. DAT-230-treatment in HT-1080 cells resulted in microtubule de-polymerization and G2/M phase arrest preceding apoptosis. Phosphor-cdc2 (thr14/tyr15) reduction, cyclin B1 accumulation and aberrant spindles denoted the cyclin B1-cdc2 complex active and M phase arrest in HT-1080 cells treated with DAT-230. Apoptosis induced by DAT-230 was related with the activation of caspase-9, caspase-3 and PARP cleavage, which were at the downstream of mitochondria. The decrease ratio of Bcl-2/Bax, elevation of NO and disruption of ΔΨm confirmed the causal relationship between DAT-230 and mitochondrial pathway. In vivo, DAT-230 delayed tumor growth, induced tumor perfusion decrease and extensive hemorrhagic-necrosis.

CONCLUSIONS

DAT-230 is a promising microtubule inhibitor that has great potential for the treatment of fibrosarcoma in vitro and in vivo. Its potential to be a candidate of anti-cancer agent is worth being further investigated.

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.

Accumulation of DNA damage and cell death after fractionated irradiation

The purpose of this work was to determine how fractionated radiation used in the treatment of tumors affects the ability of cancer as well as normal cells to repair induced DNA double-strand breaks (DSBs) and how cells that have lost this ability die. Lymphocytic leukemia cells (MOLT4) were used as an experimental model, and the results were compared to those for normal cell types. The results show that cancer and normal cells were mostly unable to repair all DSBs before the next radiation dose induced new DNA damage. Accumulation of DSBs was observed in normal human fibroblasts and healthy lymphocytes irradiated in vitro after the second radiation dose. The lymphocytic leukemia cells irradiated with 4 × 1 Gy and a single dose of 4 Gy had very similar survival; however, there was a big difference between human fibroblasts irradiated with 4 × 1.5 Gy and a single dose of 6 Gy. These results suggest that exponentially growing lymphocytic leukemia cells, similar to rapidly proliferating tumors, are not very sensitive to fraction size, in contrast to the more slowly growing fibroblasts and most late-responding (radiation therapy dose-limiting) normal tissues, which have a low proliferation index.

Human papillomavirus (HPV) E7 induces prolonged G2 following S phase reentry in differentiated human keratinocytes

The productive program of human papillomaviruses occurs in differentiated squamous keratinocytes. We have previously shown that HPV-18 DNA amplification initiates in spinous cells in organotypic cultures of primary human keratinocytes during prolonged G(2) phase, as signified by abundant cytoplasmic cyclin B1 (Wang, H. K., Duffy, A. A., Broker, T. R., and Chow, L. T. (2009) Genes Dev. 23, 181-194). In this study, we demonstrated that the E7 protein, which induces S phase reentry in suprabasal cells by destabilizing the p130 pocket protein (Genovese, N. J., Banerjee, N. S., Broker, T. R., and Chow, L. T. (2008) J. Virol. 82, 4862-4873), also elicited extensive G(2) responses. Western blots and indirect immunofluorescence assays were used to probe for host proteins known to control G(2)/M progression. E7 expression induced cytoplasmic accumulation of cyclin B1 and cdc2 in the suprabasal cells. The elevated cdc2 had inactivating phosphorylation on Thr(14) or Tyr(15), and possibly both, due to an increase in the responsible Wee1 and Myt1 kinases. In cells that harbored cytoplasmic cyclin B1 or cdc2, there was also an accumulation of the phosphatase-inactive cdc25C phosphorylated on Ser(216), unable to activate cdc2. Moreover, E7 expression induced elevated expression of phosphorylated ATM (Ser(1981)) and the downstream phosphorylated Chk1, Chk2, and JNKs, kinases known to inactivate cdc25C. Similar results were observed in primary human keratinocyte raft cultures in which the productive program of HPV-18 took place. Collectively, this study has revealed the mechanisms by which E7 induces prolonged G(2) phase in the differentiated cells following S phase induction.

Spatiotemporal regulations of Wee1 at the G2/M transition

Wee1 is highly dynamic at the SPB during the G2/M transition. Wee1 accumulates at the nuclear face of the SPB when cyclin B–Cdc2 peaks at the SPB and disappears from the SPB during spindle assembly. This dynamic behavior of Wee1 at the SPB is important for regulation of cyclin B–Cdc2 activity and proper mitotic entry and progression.

Wee1 is a protein kinase that negatively regulates mitotic entry in G2 phase by suppressing cyclin B–Cdc2 activity, but its spatiotemporal regulations remain to be elucidated. We observe the dynamic behavior of Wee1 in Schizosaccharomyces pombe cells and manipulate its localization and kinase activity to study its function. At late G2, nuclear Wee1 efficiently suppresses cyclin B–Cdc2 around the spindle pole body (SPB). During the G2/M transition when cyclin B–Cdc2 is highly enriched at the SPB, Wee1 temporally accumulates at the nuclear face of the SPB in a cyclin B–Cdc2-dependent manner and locally suppresses both cyclin B–Cdc2 activity and spindle assembly to counteract a Polo kinase–dependent positive feedback loop. Then Wee1 disappears from the SPB during spindle assembly. We propose that regulation of Wee1 localization around the SPB during the G2/M transition is important for proper mitotic entry and progression.

Molecular characterization of apoptosis induced by CARF silencing in human cancer cells

Collaborator of ARF (CARF) was cloned as an ARF-interacting protein and shown to regulate the p53-p21(WAF1)-HDM2 pathway, which is central to tumor suppression via senescence and apoptosis. We had previously reported that CARF inhibition in cancer cells led to polyploidy and caspase-dependent apoptosis, however, the mechanisms governing this phenomenon remained unknown. Thus, we examined various cell death and survival pathways including the mitochondrial stress, ataxia telangiectasia mutated (ATM)-ATR, Ras-MAP kinase and retinoblastoma cascades. We found that CARF is a pleiotropic regulator with widespread effects; its suppression affected all investigated pathways. Most remarkably, it protected the cells against genotoxicity; CARF knockdown elicited DNA damage response as evidenced by increased levels of phosphorylated ATM and γH2AX, leading to induction of mitotic arrest and eventual apoptosis. We also show that the CARF-silencing-induced apoptosis in vitro translates to in vivo. In a human tumor xenograft mouse model, treatment of developing tumors with short hairpin RNA (shRNA) against CARF via an adenovirus carrier induced complete suppression of tumor growth, suggesting that CARF shRNA is a strong candidate for an anticancer reagent. We demonstrate that CARF has a vital role in genome preservation and tumor suppression and CARF siRNA is an effective novel cancer therapeutic agent.

Activation of cyclin B1-Cdk1 synchronizes events in the nucleus and the cytoplasm at mitosis

The cyclin B-Cdk1 kinase triggers mitosis in most eukaryotes. In animal cells, cyclin B shuttles between the nucleus and cytoplasm in interphase before rapidly accumulating in the nucleus at prophase, which promotes disassembly of the nuclear lamina and nuclear envelope breakdown (NEBD). What triggers the nuclear accumulation of cyclin B1 is presently unclear, although the prevailing view is that the Plk1 kinase inhibits its nuclear export. In this study, we use a biosensor specific for cyclin B1-Cdk1 activity to show that activating cyclin B1-Cdk1 immediately triggers its rapid accumulation in the nucleus through a 40-fold increase in nuclear import that remains dependent on Cdk1 activity until NEBD. Nevertheless, a substantial proportion of cyclin B1-Cdk1 remains in the cytoplasm. The increase in nuclear import is driven by changes in the nuclear import machinery that require neither Plk1 nor inhibition of nuclear export. Thus, the intrinsic link between cyclin B1-Cdk1 activation and its rapid nuclear import inherently coordinates the reorganization of the nucleus and the cytoplasm at mitotic entry.

Prophase I arrest and progression to metaphase I in mouse oocytes: comparison of resumption of meiosis and recovery from G2-arrest in somatic cells

Mammalian oocytes are arrested at prophase I until puberty when luteinizing hormone (LH) induces resumption of meiosis of follicle-enclosed oocytes. Resumption of meiosis is tightly coupled with regulating cyclin-dependent kinase 1 (CDK1) activity. Prophase I arrest depends on inhibitory phosphorylation of CDK1 and anaphase-promoting complex-(APC-CDH1)-mediated regulation of cyclin B levels. Prophase I arrest is maintained by endogenously produced cyclic adenosine monophosphate (cAMP), which activates protein kinase A (PKA) that in turn phosphorylates (and activates) the nuclear kinase WEE2. In addition, PKA-mediated phosphorylation of the phosphatase CDC25B results in its cytoplasmic retention. The combined effect maintains low levels of CDK1 activity that are not sufficient to initiate resumption of meiosis. LH triggers synthesis of epidermal growth factor-like factors in mural granulosa cells and leads to reduced cGMP transfer from cumulus cells to oocytes via gap junctions that couple the two cell types. cGMP inhibits oocyte phosphodiesterase 3A (PDE3A) and a decline in oocyte cGMP results in increased PDE3A activity. The ensuing decrease in oocyte cAMP triggers maturation by alleviating the aforementioned phosphorylations of WEE2 and CDC25B. As a direct consequence CDC25B translocates into the nucleus. The resulting activation of CDK1 also promotes extrusion of WEE2 from the nucleus thereby providing a positive amplification mechanism for CDK1 activation. Other kinases, e.g. protein kinase B, Aurora kinase A and polo-like kinase 1, also participate in resumption of meiosis. Mechanisms governing meiotic prophase I arrest and resumption of meiosis share common features with DNA damage-induced mitotic G2-checkpoint arrest and checkpoint recovery, respectively. These common features include CDC14B-dependent activation of APC-CDH1 in prophase I arrested oocytes or G2-arrested somatic cells, and CDC25B-dependent cell cycle resumption in both oocytes and somatic cells.

Mutation of the DEAD-box helicase belle downregulates the cyclin-dependent kinase inhibitor Dacapo

The retinoblastoma protein (pRB) negatively regulates cell proliferation by limiting the activity of the family of E2F transcription factors. In Drosophila, mutation of the DEAD-box helicase belle (bel) relieves an E2F/pRB induced G(1) cell cycle arrest; however, the mechanism of this rescue is unknown. Here, we show that the level of the cyclin-dependent kinase inhibitor Dacapo (Dap), homolog of mammalian p21/p27, is strongly reduced both in bel mutant cells in vivo and in tissue culture cells depleted of Bel by RNA interference. Interestingly, the loss of bel also partially alleviates an ectopically induced G(1) cell cycle arrest. Additionally, we show that Bel undergoes nucleocytoplasmic shuttling. Thus, inactivation of bel renders cells less sensitive to several anti-proliferative signals inducing G(1) arrest.

BCL2L12A localizes to the cell nucleus and induces growth inhibition through G2/M arrest in CHO cells

BCL2L12, a newly identified member of Bcl-2 family, and its transcript variant BCL2L12A have been found to be associated with favorable prognosis in breast cancer patients while correlated with tumorigenesis of glioblastoma and colon cancer. However, the biological functions of BCL2L12 and especially those of BCL2L12A are largely unknown. Here, we report that, unlike other Bcl-2 family proteins, BCL2L12 and its transcript variant BCL2L12A are nuclear proteins. Interestingly, BCL2L12 forms speckle patterns in the nuclei and potently induces apoptosis in CHO cells. BCL2L12A had a diffuse distribution in the nuclei and inhibits cell growth by inducing cell cycle arrested at G2/M transition in CHO cells. More importantly, BCL2L12A-induced G2/M arrest was associated with a slight up-regulation of cyclin B1 and significant down-regulation of an active form of cyclin B1 phosphorylated at Ser147. Taken together, our study suggests that both BCL2L12 and BCL2L12A have negative effects on CHO cell growths, and that BCL2L12A is a potential cell cycle regulator that interferes with G2-M transition.

Safeguarding entry into mitosis: the antephase checkpoint

Maintenance of genomic stability is needed for cells to survive many rounds of division throughout their lifetime. Key to the proper inheritance of intact genome is the tight temporal and spatial coordination of cell cycle events. Moreover, checkpoints are present that function to monitor the proper execution of cell cycle processes. For instance, the DNA damage and spindle assembly checkpoints ensure genomic integrity by delaying cell cycle progression in the presence of DNA or spindle damage, respectively. A checkpoint that has recently been gaining attention is the antephase checkpoint that acts to prevent cells from entering mitosis in response to a range of stress agents. We review here what is known about the pathway that monitors the status of the cells at the brink of entry into mitosis when cells are exposed to insults that threaten the proper inheritance of chromosomes. We highlight issues which are unresolved in terms of our understanding of the antephase checkpoint and provide some perspectives on what lies ahead in the understanding of how the checkpoint functions.

Mammalian TIMELESS is required for ATM-dependent CHK2 activation and G2/M checkpoint control

Timeless (Tim), a core circadian clock gene in Drosophila, is retained in mammals but has no apparent mammalian circadian clock function. Mammalian TIM is essential for ATR-dependent Chk1 activation and S-phase arrest. We report that TIM is likewise essential for ATM-dependent Chk2-mediated signaling of doxorubicin-induced DNA double strand breaks. TIM depletion attenuates doxorubicin-induced G(2)/M cell cycle arrest and sensitizes cancer cells to doxorubicin-induced cytotoxicity. TIM is, thereby, a potential novel anticancer drug target whose inhibition may enhance the therapeutic cytotoxicity of agents that activate DNA damage pathways as part of their mechanism.

Large T antigen promotes JC virus replication in G2-arrested cells by inducing ATM- and ATR-mediated G2 checkpoint signaling

Large T antigen (TAg) of the human polyomavirus JC virus (JCV) possesses DNA binding and helicase activities, which, together with various cellular proteins, are required for replication of the viral genome. We now show that JCV-infected cells expressing TAg accumulate in the G(2) phase of the cell cycle as a result of the activation of ATM- and ATR-mediated G(2) checkpoint pathways. Transient transfection of cells with a TAg expression vector also induced G(2) checkpoint signaling and G(2) arrest. Analysis of TAg mutants with different subnuclear localizations suggested that the association of TAg with cellular DNA contributes to the induction of G(2) arrest. Abrogation of G(2) arrest by inhibition of ATM and ATR, Chk1, and Wee1 suppressed JCV genome replication. In addition, abrogation of the G(2)-M transition by Cdc2 depletion disabled Wee1 depletion-induced suppression of JCV genome replication, suggesting that JCV replication is facilitated by G(2) arrest resulting from G(2) checkpoint signaling. Moreover, inhibition of ATM and ATR by caffeine suppressed JCV production. The observation that oligodendrocytes productively infected with JCV in vivo also undergo G(2) arrest suggests that G(2) checkpoint inhibitors such as caffeine are potential therapeutic agents for JCV infection.

Mammalian cell-cycle regulation: several Cdks, numerous cyclins and diverse compensatory mechanisms

After a decade of extensive work on gene knockout mouse models of cell-cycle regulators, the classical model of cell-cycle regulation was seriously challenged. Several unexpected compensatory mechanisms were uncovered among cyclins and Cdks in these studies. The most astonishing observation is that Cdk2 is dispensable for the regulation of the mitotic cell cycle with both Cdk4 and Cdk1 covering for Cdk2's functions. Similar to yeast, it was recently discovered that Cdk1 alone can drive the mammalian cell cycle, indicating that the regulation of the mammalian cell cycle is highly conserved. Nevertheless, cell-cycle-independent functions of Cdks and cyclins such as in DNA damage repair are still under investigation. Here we review the compensatory mechanisms among major cyclins and Cdks in mammalian cell-cycle regulation.

Hedgehog signalling: emerging evidence for non-canonical pathways

Hedgehog (HH) signalling is involved in the development of numerous embryonic tissues. In humans,germline mutations in hedgehog pathway components cause congenital malformations and somatic mutations are associated with cancers. The basic framework of the HH pathway was elucidated in the fruitfly, Drosophila melanogaster, and this pathway is largely conserved in vertebrates, although some important differences have been noted. The current paradigm of the "canonical" pathway views HH signalling as a series of repressive interactions which culminates in GLI-mediated transcriptional regulation of a variety of cellular processes. Definitions of "non-canonical" signalling stem from examples where the response to HH morphogen deviates from this paradigm and, according to current reports, three general scenarios of noncanonical HH signalling can be defined: (1) Signalling that involves HH pathway components but which is independent of GLI-mediated transcription; (2) Direct interaction of HH signalling components with components of other molecular pathways; and (3) "Non-contiguous" or "atypical" interaction of core HH pathway components with one another. Currently, the evidence supporting non-canonical HH signalling is not conclusive. However, Sonic hedgehog (SHH) has been shown to regulate cell migration and axon guidance in several contexts, and some of these processes are independent of downstream components of the HH pathway, and presumably the transcriptional response to morphogen. Furthermore, biochemical studies have shown that the HH receptor, PTCH1, can directly interact both with Cyclin B1 and caspases, to inhibit cell proliferation and to promote apoptosis, respectively, and that these functions are inhibited in the presence of morphogen. Genetic analysis of orthologues of the HH pathway in nematode worms further supports the notion that PTCH1-related molecules can function independently of other components of the canonical HH pathway, and the phenotypes of mice with point mutations in the Ptch1 gene offer clues as to the processes that non-canonical HH signalling might regulate. While none of these evidences are conclusive,collectively they point to the existence of added complexity in the HH pathway in the form of non-canonical pathways. A major difficulty in studying this problem is that canonical and non-canonical pathways are likely to act in parallel, and so in many situations it will not be possible to implicate non-canonical responses in certain cellular processes simply by excluding a role for the canonical pathway-directed analyses of non-canonical HH signalling are therefore necessary. The aim of this review is to present the cumulative evidence supporting non-canonical HH signalling, with the hope of promoting further enquiry into this area.

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

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

Crocetin inhibits pancreatic cancer cell proliferation and tumor progression in a xenograft mouse model

Crocetin, a carotenoid compound derived from saffron, has long been used as a traditional ancient medicine against different human diseases including cancer. The aim of the series of experiments was to systematically determine whether crocetin significantly affects pancreatic cancer growth both in vitro and/or in vivo. For the in vitro studies, first, MIA-PaCa-2 cells were treated with crocetin and in these sets of experiments, a proliferation assay using H(3)-thymidine incorporation and flow cytometric analysis suggested that crocetin inhibited proliferation. Next, cell cycle proteins were investigated. Cdc-2, Cdc-25C, Cyclin-B1, and epidermal growth factor receptor were altered significantly by crocetin. To further confirm the findings of inhibition of proliferation, H(3)-thymidine incorporation in BxPC-3, Capan-1, and ASPC-1 pancreatic cancer cells was also significantly inhibited by crocetin treatment. For the in vivo studies, MIA-PaCa-2 as highly aggressive cells than other pancreatic cancer cells used in this study were injected into the right hind leg of the athymic nude mice and crocetin was given orally after the development of a palpable tumor. The in vivo results showed significant regression in tumor growth with inhibition of proliferation as determined by proliferating cell nuclear antigen and epidermal growth factor receptor expression in the crocetin-treated animals compared with the controls. Both the in vitro pancreatic cancer cells and in vivo athymic nude mice tumor, apoptosis was significantly stimulated as indicated by Bax/Bcl-2 ratio. This study indicates that crocetin has a significant antitumorigenic effect in both in vitro and in vivo on pancreatic cancer.