Human cyclin A is required for mitosis until mid prophase

Novel inhibitors of cyclin-dependent kinases combat hepatocellular carcinoma without inducing chemoresistance

Treatment options for hepatocellular carcinoma using chemotherapeutics at intermediate and advanced stages of disease are limited as patients most rapidly escape from therapy and succumb to disease progression. Mechanisms of the hepatic xenobiotic metabolism are mostly involved in providing chemoresistance to therapeutic compounds. Given the fact that the aberrant activation of cyclin-dependent kinases (CDK) is frequently observed in hepatocellular carcinomas, we focused on the efficacy of the novel compounds BA-12 and BP-14 that antagonize CDK1/2/5/7 and CDK9. Inhibition of those CDKs in human hepatocellular carcinoma cell lines reduced the clonogenicity by arresting cells in S-G2 and G2-M phase of the cell cycle and inducing apoptosis. In contrast, primary human hepatocytes failed to show cytotoxicity and apoptosis. No loss of chemosensitivity was observed in hepatocellular carcinoma cells after long-term exposure to inhibitors. In vivo, treatment of xenografted human hepatocellular carcinomas with BA-12 or BP-14 effectively repressed tumor formation. Moreover, BA-12 or BP-14 significantly diminished diethylnitrosamine (DEN)-induced hepatoma development in mice. These data show that BA-12 or BP-14 exhibit strong antitumorigenic effects in the absence of chemoresistance, resulting in a superior efficacy compared with currently used chemotherapeutics in hepatocellular carcinomas.

Histone deacetylase 3 regulates cyclin A stability

PCAF and GCN5 acetylate cyclin A at specific lysine residues targeting it for degradation at mitosis. We report here that histone deacetylase 3 (HDAC3) directly interacts with and deacetylates cyclin A. HDAC3 interacts with a domain included in the first 171 aa of cyclin A, a region involved in the regulation of its stability. In cells, overexpression of HDAC3 reduced cyclin A acetylation whereas the knocking down of HDAC3 increased its acetylation. Moreover, reduction of HDAC3 levels induced a decrease of cyclin A that can be reversed by proteasome inhibitors. These results indicate that HDAC3 is able to regulate cyclin A degradation during mitosis via proteasome. Interestingly, HDAC3 is abruptly degraded at mitosis also via proteasome thus facilitating cyclin A acetylation by PCAF/GCN5, which will target cyclin A for degradation. Because cyclin A is crucial for S phase progression and mitosis entry, the knock down of HDAC3 affects cell cycle progression specifically at both, S phase and G2/M transition. In summary we propose here that HDAC3 regulates cyclin A stability by counteracting the action of the acetylases PCAF/GCN5.

Mammalian interphase cdks: dispensable master regulators of the cell cycle

Cyclin-dependent kinases (Cdks) drive cell cycle progression in all eukaryotes. Yeasts have a single major Cdk that mediates distinct cell cycle transitions via association with different cyclins. The closest homolog in mammals, Cdk1, drives mitosis. Mammals have additional Cdks-Cdk2, Cdk4, and Cdk6-that represent the major Cdks activated during interphase (iCdks). A large body of evidence has accrued that suggests that activation of iCdks dictates progression though interphase. In apparent contradiction, deficiency in each individual iCdk, respectively, in knockout mice proved to be compatible with live birth and in some instances fertility. Moreover, murine embryos could be derived with Cdk1 as the only functional Cdk. Thus, none of the iCdks is strictly essential for mammalian cell cycle progression, raising the possibility that Cdk1 is the dominant regulator in interphase. However, an absence of iCdks has been accompanied by major shifts in cyclin association to Cdk1, suggesting gain in function. After considerable tweaking, a chemical genetic approach has recently been able to examine the impact of acute inhibition of Cdk2 activity without marked distortion of cyclin/Cdk complex formation. The results suggest that, when expressed at its normal levels, Cdk2 performs essential roles in driving human cells into S phase and maintaining genomic stability. These new findings appear to have restored order to the cell cycle field, bringing it full circle to the view that iCdks indeed play important roles. They also underscore the caveat in knockdown and knockout approaches that protein underexpression can significantly perturb a protein interaction network. We discuss the implications of the new synthesis for future cell cycle studies and anti-Cdk-based therapy of cancer and other diseases.

Cyclins and cell cycle control in cancer and disease

Cyclin D1 overexpression is found in more than 50% of human breast cancers and causes mammary cancer in transgenic mice. Dysregulation of cyclin D1 gene expression or function contributes to the loss of normal cell cycle control during tumorigenesis. Recent studies have demonstrated that cyclin D1 conducts additional specific functions to regulate gene expression in the context of local chromatin, promote cellular migration, and promote chromosomal instability. It is anticipated that these additional functions contribute to the pathology associated with dysregulated cyclin D1 abundance. This article discusses evidence that examines the functional roles that cyclin D1 may play in cancer with an emphasis on other cyclin family members that also may contribute to cancer and disease in a similar fashion.

Cell cycle control across the eukaryotic kingdom

Almost two billion years of evolution have generated a vast and amazing variety of eukaryotic life with approximately 8.7 million extant species. Growth and reproduction of all of these organisms depend on faithful duplication and distribution of their chromosomes to the newly forming daughter cells in a process called the cell cycle. However, most of what is known today about cell cycle control comes from a few model species that belong to the unikonts; that is, to only one of five 'supergroups' that comprise the eukaryotic kingdom. Recently, analyzing species from distantly related clades is providing insights into general principles of cell cycle regulation and shedding light on its evolution. Here, referring to animal and fungal as opposed to non-unikont systems, especially flowering plants from the archaeplastid supergroup, we compare the conservation of central cell cycle regulator functions, the structure of network topologies, and the evolutionary dynamics of substrates of core cell cycle kinases.

Silencing of mutant p53 by siRNA induces cell cycle arrest and apoptosis in human bladder cancer cells

BACKGROUND

p53 is the most frequently mutated tumor-suppressor gene in human cancers. It has been reported that mutations in p53 result not only in the loss of its ability as a tumor suppressor, but also in the gain of novel cancer-related functions that contribute to oncogenesis. The present study evaluated the potential of silencing of mutant p53 by small interfering RNA in the treatment of bladder cancer cells in vitro.

METHODS

We used the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay to assess cell viability and flow cytometry to detect cell cycle alterations and apoptosis. The related molecular mechanisms were assessed by western blotting. We also used the MTT assay and flow cytometry to investigate if silencing of mutant p53 by knockdown with small interfering (si)RNA would change the sensitivity to cisplatin treatment.

RESULTS

Using 5637 and T24 human bladder cancer cell lines characterized by mutations in p53, we found that silencing of the mutant p53 by RNA interference induced evident inhibition of cell proliferation and viability, which was related to the induction of G2 phase cell cycle arrest and apoptosis. Moreover, our study also showed that the p53-targeting siRNA cooperated with cisplatin in the inhibition of bladder cancer cells.

CONCLUSIONS

These findings suggest that RNA interference targeting mutant p53 may be a promising therapeutic strategy for the treatment of bladder cancer.

Eroded human telomeres are more prone to remain uncapped and to trigger a G2 checkpoint response

Telomeres cap the ends of chromosomes and regulate the replicative life span of human somatic cells. Telomere function is lost upon critical shortening and a p53-dependent checkpoint that detects altered telomere states at the G1/S transition was proposed to act as a regulator of the telomere damage response. We show that telomerase-negative human fibroblasts spend more time in G2 phase as they approach senescence and this delay is associated with manifestations of telomere dysfunction and the triggering of an ATM/ATR-dependent DNA damage signal. This correlates with a partial release of telomeric proteins TRF1 and TRF2. Analysis of the consequences of TRF1 and TRF2 depletion or over-expression of mutated versions revealed that telomere uncapping or telomere replication stress also led to DNA damage signalling in G2. Progression through mitosis of these cells was associated with signs of incomplete telomere terminal processing. We also observed an increase in sister chromatid-type telomere aberrations in senescing fibroblasts indicating that defects of telomere post-replicative events increased as cells age. Our results link a post-replicative damage response at eroded telomeres to G2 arrest signalling and challenge the current paradigm that the checkpoint response to short telomeres occurs primarily at the G1/S transition in human cells.

An intron-retaining splice variant of human cyclin A2, expressed in adult differentiated tissues, induces a G1/S cell cycle arrest in vitro

Background

Human cyclin A2 is a key regulator of S phase progression and entry into mitosis. Alternative splice variants of the G1 and mitotic cyclins have been shown to interfere with full-length cyclin functions to modulate cell cycle progression and are therefore likely to play a role in differentiation or oncogenesis. The alternative splicing of human cyclin A2 has not yet been studied.

Methodology/Principal Findings

Sequence-specific primers were designed to amplify various exon–intron regions of cyclin A2 mRNA in cell lines and human tissues. Intron retaining PCR products were cloned and sequenced and then overexpressed in HeLa cells. The subcellular localization of the splice variants was studied using confocal and time-lapse microscopy, and their impact on the cell cycle by flow cytometry, immunoblotting and histone H1 kinase activity. We found a splice variant of cyclin A2 mRNA called A2V6 that partly retains Intron 6. The gene expression pattern of A2V6 mRNA in human tissues was noticeably different from that of wild-type cyclin A2 (A2WT) mRNA. It was lower in proliferating fetal tissues and stronger in some differentiated adult tissues, especially, heart. In transfected HeLa cells, A2V6 localized exclusively in the cytoplasm whereas A2WT accumulated in the nucleus. We show that A2V6 induced a clear G1/S cell cycle arrest associated with a p21 and p27 upregulation and an inhibition of retinoblastoma protein phosphorylation. Like A2WT, A2V6 bound CDK2, but the A2V6/CDK2 complex did not phosphorylate histone H1.

Conclusion/Significance

This study has revealed that some highly differentiated human tissues express an intron-retaining cyclin A2 mRNA that induced a G1/S block in vitro. Contrary to full-length cyclin A2, which regulates cell proliferation, the A2V6 splice variant might play a role in regulating nondividing cell states such as terminal differentiation or senescence.

Identification of novel mitosis regulators through data mining with human centromere/kinetochore proteins as group queries

Background

Proteins functioning in the same biological pathway tend to be transcriptionally co-regulated or form protein-protein interactions (PPI). Multiple spatially and temporally regulated events are coordinated during mitosis to achieve faithful chromosome segregation. The molecular players participating in mitosis regulation are still being unravelled experimentally or using in silico methods.

Results

An extensive literature review has led to a compilation of 196 human centromere/kinetochore proteins, all with experimental evidence supporting the subcellular localization. Sixty-four were designated as “core” centromere/kinetochore components based on peak expression and/or well-characterized functions during mitosis. By interrogating and integrating online resources, we have mined for genes/proteins that display transcriptional co-expression or PPI with the core centromere/kinetochore components. Top-ranked hubs in either co-expression or PPI network are not only enriched with known mitosis regulators, but also contain candidates whose mitotic functions are not yet established. Experimental validation found that KIAA1377 is a novel centrosomal protein that also associates with microtubules and midbody; while TRIP13 is a novel kinetochore protein and directly interacts with mitotic checkpoint silencing protein p31^comet.

Conclusions

Transcriptional co-expression and PPI network analyses with known human centromere/kinetochore proteins as a query group help identify novel potential mitosis regulators.

Triptolide inhibits colon-rectal cancer cells proliferation by induction of G1 phase arrest through upregulation of p21

Triptolide, a diterpene triepoxide compound extracted from the traditional Chinese medicine herb Tripterygium wilfordii Hook F., is a potential cancer chemotherapeutic for tumors. However, the mechanism of anti-proliferative mechanism of triptolide in colon cancer cells is not entirely clear. Triptolide markedly inhibited HT29 and SW480 cells proliferation in a dose- and time-dependent manner. Triptolide decreased ERK and AKT phosphorylation, and GABPα expression in colon cancer cells. Beta-catenin expression and phosphorylation were not altered by incubation of triptolide. However, we found that triptolide repressed expression of LEF/TCF. Although it did not significantly affect cells apoptosis, triptolide induced G1 phase arrest dose-dependently. Further detection for the expression of cell cycle-related proteins suggesting that triptolide stimulate expression of p21 and repress cyclin A1. Increased p21 binded to CDK4/CDK6, therefore blocked function of CDK4/CDK6, and subsequently contribute to the G1 arrest. These data suggested that triptolide is a potential agent for treatment of colon cancer, and its anti-proliferation effect mainly occur through G1 phase arrest.

Distinct activities of the anaphase-promoting complex/cyclosome (APC/C) in mouse embryonic cells

The first differentiation event in mammalian development gives rise to the blastocyst, consisting of two cell lineages that have also segregated in how the cell cycle is structured. Pluripotent cells of the inner cell mass divide mitotically to retain a diploid DNA content, but the outer trophoblast cells can amplify their genomes more than 500-fold by undergoing multiple rounds of DNA replication, completely bypassing mitosis. Central to this striking divergence in cell cycle control is the E3 ubiquitin-ligase activity of the anaphase-promoting complex or cyclosome (APC/C). Extended suppression of APC/C activity during interphase of mouse pluripotent cells promotes rapid cell cycle progression by allowing stabilization of cyclins, whereas unopposed APC/C activity during S phase of mouse trophoblast cells triggers proteasomal-mediated degradation of geminin and giant cell formation. While differential APC/C activity might govern the atypical cell cycles observed in pre-implantation mouse embryos, geminin is a critical APC/C substrate that: (1) escapes degradation in pluripotent cells to maintain expression of Oct4, Sox2 and Nanog; and (2) mediates specification and endoreduplication when targeted for ectopic destruction in trophoblast. Thus, in contrast to trophoblast giant cells that lack geminin, geminin is preserved in both mouse pluripotent cells and non-endoreduplicating human cytotrophoblast cells.

Quantitative phosphoproteomics of transforming growth factor-β signaling in colon cancer cells

The transforming growth factor-β (TGF-β) signaling pathway progresses through a series of protein phosphorylation regulated steps. Smad4 is a key mediator of the classical TGF-β signaling pathway; however, reports suggest that TGF-β can activate other cellular pathways independent of Smad4. By investigating the TGF-β-regulated phosphoproteome, we aimed to uncover new functions controlled by TGF-β. We applied titanium dioxide to enrich phosphopeptides from stable isotope labeling with amino acids in cell culture (SILAC)-labeled SW480 cells stably expressing Smad4 and profiled them by mass spectrometry. TGF-β stimulation for 30 min resulted in the induction of 17 phosphopeptides and the repression of 8 from a total of 149 unique phosphopeptides. Proteins previously not known to be phosphorylated by TGF-β including programmed cell death protein 4, nuclear ubiquitous casein and cyclin-dependent kinases substrate, hepatoma-derived growth factor and cell division kinases amongst others were induced following TGF-β stimulation, while the phosphorylation of TRAF2 and NCK-interacting protein kinase are examples of proteins whose phosphorylation status was repressed. This phosphoproteomic screen has identified new TGF-β-modulated phosphorylation responses in colon carcinoma cells.

Coupling of T161 and T14 phosphorylations protects cyclin B-CDK1 from premature activation

Mitosis is triggered by the abrupt dephosphorylation of inhibitory Y15 and T14 residues of cyclin B1-bound cyclin-dependent kinase (CDK)1 that is also phosphorylated at T161 in its activation loop. The sequence of events leading to the accumulation of fully phosphorylated cyclin B1-CDK1 complexes remains unclear. Two-dimensional gel electrophoresis allowed us to determine whether T14, Y15, and T161 phosphorylations occur on same CDK1 molecules and to characterize the physiological occurrence of their seven phosphorylation combinations. Intriguingly, in cyclin B1-CDK1, the activating T161 phosphorylation never occurred without the T14 phosphorylation. This strict association could not be uncoupled by a substantial reduction of T14 phosphorylation in response to Myt1 knockdown, suggesting some causal relationship. However, T14 phosphorylation was not directly required for T161 phosphorylation, because Myt1 knockdown did uncouple these phosphorylations when leptomycin B prevented cyclin B1-CDK1 complexes from accumulating in cytoplasm. The coupling mechanism therefore depended on unperturbed cyclin B1-CDK1 traffic. The unexpected observation that the activating phosphorylation of cyclin B1-CDK1 was tightly coupled to its T14 phosphorylation, but not Y15 phosphorylation, suggests a mechanism that prevents premature activation by constitutively active CDK-activating kinase. This explained the opposite effects of reduced expression of Myt1 and Wee1, with only the latter inducing catastrophic mitoses.

The impact of multi-targeted cyclin-dependent kinase inhibition in breast cancer cells: clinical implications

INTRODUCTION

The progression of the mammalian cell cycle is driven by the transient activation of complexes consisting of cyclins and cyclin-dependent kinases (CDKs). Loss of control over the cell cycle results in accelerated cell division and malignant transformation and can be caused by the upregulation of cyclins, the aberrant activation of CDKs or the inactivation of cellular CDK inhibitors. For these reasons, cell cycle regulators are regarded as very promising therapeutic targets for the treatment of human malignancies.

AREAS COVERED

This review covers the structures and anti-breast cancer activity of selected pharmacological pan-specific CDK inhibitors. Multi-targeted CDK inhibitors affect CDKs involved in the regulation of both cell cycle progression and transcriptional control. The inhibition of CDK7/CDK9 has a serious impact on the activity of RNA polymerase II; when its carboxy-terminal domain is unphosphorylated, it is unable to recruit the cofactors required for transcriptional elongation, resulting in a global transcriptional block. Multi-targeted inhibition of CDKs represses anti-apoptotic proteins and thus promotes the induction of apoptosis. Moreover, the inhibition of CDK7 in estrogen receptor (ER)-positive breast cancer cells prevents activating phosphorylation of ER-α.

EXPERT OPINION

These diverse modes of action make multi-targeted CDK inhibitors promising drugs for the treatment of breast cancers.

Efficient Wnt mediated intestinal hyperproliferation requires the cyclin D2-CDK4/6 complex

Inactivation of the gene encoding the adenomatous polyposis coli (APC) tumour suppressor protein is recognized as the key early event in the development of colorectal cancers (CRC). Apc loss leads to nuclear localization of beta-catenin and constitutive activity of the beta-catenin-Tcf4 transcription complex. This complex drives the expression of genes involved in cell cycle progression such as c-Myc and cyclin D2. Acute loss of Apc in the small intestine leads to hyperproliferation within the intestinal crypt, increased levels of apoptosis, and perturbed differentiation and migration. It has been demonstrated that c-Myc is a critical mediator of the phenotypic abnormalities that follow Apc loss in the intestine. As it may be difficult to pharmacologically inhibit transcription factors such as c-Myc, investigating more druggable targets of the Wnt-c-Myc pathway within the intestine may reveal potential therapeutic targets for CRC. Recent work in our laboratory has shown that the cyclin D2-cyclin-dependent kinase 4/6 (CDK4/6) complex promotes hyperproliferation in Apc deficient intestinal tissue and ApcMin/+ adenomas. We showed that the hyperproliferative phenotype associated with Apc loss in vivo was partially dependent on the expression of cyclin D2. Most importantly, tumour growth and development in ApcMin/+ mice was strongly perturbed in mice lacking cyclin D2. Furthermore, pharmacological inhibition of CDK4/6 suppressed the proliferation of adenomatous cells. This commentary discusses the significance of this work in providing evidence for the importance of the cyclin D2-CDK4/6 complex in colorectal adenoma formation. It also argues that inhibition of this complex may be an effective chemopreventative strategy in CRC.

Cell cycle regulation during proliferation and differentiation of mammalian muscle precursor cells

Proliferation and differentiation of muscle precursor cells are intensively studied not only in the developing mouse embryo but also using models of skeletal muscle regeneration or analyzing in vitro cultured cells. These analyses allowed to show the universality of the cell cycle regulation and also uncovered tissue-specific interplay between major cell cycle regulators and factors crucial for the myogenic differentiation. Examination of the events accompanying proliferation and differentiation leading to the formation of functional skeletal muscle fibers allows understanding the molecular basis not only of myogenesis but also of skeletal muscle regeneration. This chapter presents the basis of the cell cycle regulation in proliferating and differentiating muscle precursor cells during development and after muscle injury. It focuses at major cell cycle regulators, myogenic factors, and extracellular environment impacting on the skeletal muscle.

Function of the A-type cyclins during gametogenesis and early embryogenesis

The cyclins and their cyclin-dependent kinase partners, the Cdks, are the basic components of the machinery that regulates the passage of cells through the cell cycle. Among the cyclins, those known as the A-type cyclins are unique in that in somatic cells, they appear to function at two stages of the cell cycle, at the G1-S transition and again as the cells prepare to enter M-phase. Higher vertebrate organisms have two A-type cyclins, cyclin A1 and cyclin A2, both of which are expressed in the germ line and/or early embryo, following highly specialized patterns that suggest functions in both mitosis and meiosis. Insight into their in vivo functions has been obtained from gene targeting experiments in the mouse model. Loss of cyclin A1 results in disruption of spermatogenesis and male sterility due to cell arrest in the late diplotene stage of the meiotic cell cycle. In contrast, cyclin A2-deficiency is marked by early embryonic lethality; thus, understanding the function of cyclin A2 in the adult germ line awaits conditional mutagenesis or other approaches to knock down its expression.

Genomic evolution and complexity of the Anaphase-promoting Complex (APC) in land plants

BACKGROUND

The orderly progression through mitosis is regulated by the Anaphase-Promoting Complex (APC), a large multiprotein E3 ubiquitin ligase that targets key cell-cycle regulators for destruction by the 26 S proteasome. The APC is composed of at least 11 subunits and associates with additional regulatory activators during mitosis and interphase cycles. Despite extensive research on APC and activator functions in the cell cycle, only a few components have been functionally characterized in plants.

RESULTS

Here, we describe an in-depth search for APC subunits and activator genes in the Arabidopsis, rice and poplar genomes. Also, searches in other genomes that are not completely sequenced were performed. Phylogenetic analyses indicate that some APC subunits and activator genes have experienced gene duplication events in plants, in contrast to animals. Expression patterns of paralog subunits and activators in rice could indicate that this duplication, rather than complete redundancy, could reflect initial specialization steps. The absence of subunit APC7 from the genome of some green algae species and as well as from early metazoan lineages, could mean that APC7 is not required for APC function in unicellular organisms and it may be a result of duplication of another tetratricopeptide (TPR) subunit. Analyses of TPR evolution suggest that duplications of subunits started from the central domains.

CONCLUSIONS

The increased complexity of the APC gene structure, tied to the diversification of expression paths, suggests that land plants developed sophisticated mechanisms of APC regulation to cope with the sedentary life style and its associated environmental exposures.

Inducing chromosome pairing through premature condensation: analysis of wheat interspecific hybrids

At the onset of meiosis, chromosomes first decondense and then condense as the process of recognition and intimate pairing occurs between homologous chromosomes. We show here that okadaic acid, a drug known to induce chromosome condensation, can be introduced into wheat interspecific hybrids prior to meiosis to induce chromosome pairing. This pairing occurs in the presence of the Ph1 locus, which usually suppresses pairing of related chromosomes and which we show here delays condensation. Thus the timing of chromosome condensation during the onset of meiosis is an important factor in controlling chromosome pairing.

Suppression of cell-cycle progression by Jun dimerization protein-2 (JDP2) involves downregulation of cyclin-A2

We report here a novel role for Jun dimerization protein-2 (JDP2) as a regulator of the progression of normal cells through the cell cycle. To determine the role of JDP2 in vivo, we generated Jdp2-knockout (Jdp2KO) mice by targeting exon-1 to disrupt the site of initiation of transcription. The epidermal thickening of skin from the Jdp2KO mice after treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA) proceeded more rapidly than that of control mice, and more proliferating cells were found at the epidermis. Fibroblasts derived from embryos of Jdp2KO mice proliferated faster and formed more colonies than fibroblasts from wild-type mice. JDP2 was recruited to the promoter of the gene for cyclin-A2 (ccna2) at the AP-1 site. Cells lacking Jdp2 had elevated levels of cyclin-A2 mRNA. Furthermore, reintroduction of JDP2 resulted in the repression of transcription of ccna2 and of cell-cycle progression. Thus, transcription of the gene for cyclin-A2 appears to be a direct target of JDP2 in the suppression of cell proliferation.

The roles of cyclin A2, B1, and B2 in early and late mitotic events

This paper presents evidence that chromatin condensation, like nuclear envelope breakdown, is brought about through the combined effects of cyclins A2 and B1, and that cyclins B1 and B2 are largely responsible for maintenance of a spindle assembly checkpoint arrest.

Here we have used siRNAs and time-lapse epifluorescence microscopy to examine the roles of various candidate mitotic cyclins in chromatin condensation in HeLa cells. Knocking down cyclin A2 resulted in a substantial (∼7 h) delay in chromatin condensation and histone H3 phosphorylation, and expressing an siRNA-resistant form of cyclin A2 partially rescued chromatin condensation. There was no detectable delay in DNA replication in the cyclin A2 knockdowns, arguing that the delay in chromatin condensation is not secondary to a delay in S-phase completion. Cyclin A2 is required for the activation and nuclear accumulation of cyclin B1-Cdk1, raising the possibility that cyclin B1-Cdk1 mediates the effects of cyclin A2. Consistent with this possibility, we found that chromatin condensation was tightly associated temporally with the redistribution of cyclin B1 to the nucleus. Moreover, a constitutively nuclear cyclin B1 rescued chromatin condensation in cyclin A2 knockdown cells. On the other hand, knocking down cyclin B1 delayed chromatin condensation by only about one hour. Our working hypothesis is that active, nuclear cyclin B1-Cdk1 normally cooperates with cyclin A2 to bring about early mitotic events. Because cyclin A2 is present only during the early stages of mitosis, we asked whether cyclin B knockdown might have more dramatic defects on late mitotic events. Consistent with this possibility, we found that cyclin B1- and cyclin B1/B2-knockdown cells had difficulty in maintaining a mitotic arrest in the presence of nocodazole. Taken together, these data suggest that cyclin A2 helps initiate mitosis, in part through its effects on cyclin B1, and that cyclins B1 and B2 are particularly critical for the maintenance of the mitotic state.

Anti-pancreatic cancer effects of a polar extract from the edible sea cucumber, Cucumaria frondosa

OBJECTIVES

To investigate the effects and mechanism of Frondanol-A5P, a polar extract from Cucumaria frondosa, on growth inhibition and apoptosis in S2013 and AsPC-1 human pancreatic cancer cells.

METHODS

The effects of Frondanol-A5P on proliferation, cell cycle, expression of cell cycle proteins and p21, phosphorylation of MAP kinases, annexin V binding, and caspase-3 activation were examined.

RESULTS

Frondanol-A5P inhibited proliferation and induced G2/M phase cell cycle arrest in both cell lines with decreased expression of cyclin A, cyclin B, and cdc25c. Frondanol-A5P induced phosphorylation of stress-activated protein kinase and Janus kinase (SAPK/JAK) and p38 mitogen-activated protein kinase (MAP) within 5 minutes. Frondanol-A5P markedly increased expression of p21 messenger RNA and protein at 3 hours in both cell lines. This effect was reduced by the p38 kinase inhibitor, SB203580. Frondanol-A5P markedly increased annexin V binding and activated caspase-3.

CONCLUSIONS

Frondanol-A5 causes cell cycle arrest and apoptosis in human pancreatic cancer cells. These changes are associated with decreased expression of cyclin A, cyclin B, and cdc25c and increased expression of p21 that, at least in part, is mediated by a p38 kinase-dependent mechanism. Because Frondanol-A5P is derived from an edible, nontoxic, sea cucumber, it may be valuable for nutritional therapy or prevention of pancreatic cancer.

Gauchos and ochos: a Wee1-Cdk tango regulating mitotic entry

The kinase Wee1 has been recognized for a quarter century as a key inhibitor of Cyclin dependent kinase 1 (Cdk1) and mitotic entry in eukaryotes. Nonetheless, Wee1 regulation is not well understood and its large amino-terminal regulatory domain (NRD) has remained largely uncharted. Evidence has accumulated that cyclin B/Cdk1 complexes reciprocally inhibit Wee1 activity through NRD phosphorylation. Recent studies have identified the first functional NRD elements and suggested that vertebrate cyclin A/Cdk2 complexes also phosphorylate the NRD. A short NRD peptide, termed the Wee box, augments the activity of the Wee1 kinase domain. Cdk1/2-mediated phosphorylation of the Wee box (on T239) antagonizes kinase activity. A nearby region harbors a conserved RxL motif (RxL1) that promotes cyclin A/Cdk2 binding and T239 phosphorylation. Mutation of either T239 or RxL1 bolsters the ability of Wee1 to block mitotic entry, consistent with negative regulation of Wee1 through these sites. The region in human somatic Wee1 that encompasses RxL1 also binds Crm1, directing Wee1 export from the nucleus. These studies have illuminated important aspects of Wee1 regulation and defined a specific molecular pathway through which cyclin A/Cdk2 complexes foster mitotic entry. The complexity, speed, and importance of regulation of mitotic entry suggest that there is more to be learned.

Functional characterisation of cell cycle-related kinase (CCRK) in colorectal cancer carcinogenesis

Cell cycle-related kinase (CCRK) is a newly identified protein kinase homologous to Cdk7. We have previously shown that CCRK is a candidate oncogene in human glioblastoma. However, whether CCRK is a bona fide oncogene remains to be tested. The aim of this study was to investigate the role of CCRK in human colorectal cancer carcinogenesis. By Western blotting, we analysed the expression profile of CCRK protein in 10 colorectal cancer tissue samples and their adjacent normal colon tissues and in seven colorectal cancer cell lines. CCRK protein expression was also investigated by immunohistochemistry in a colorectal tissue microarray, which contained 120 cases of primary colorectal cancer and adjacent normal colorectal mucosa. The effects of CCRK knock-down on cell cycle profile and proliferation of colorectal cancer cells were examined by transfecting LoVo and DLD1 human colorectal cancer cell lines by either short-hairpin RNA (shCCRK) or small interfering RNA targeting CCRK (siCCRK). We found that CCRK protein levels were elevated by more than 1.5-fold in 70% of colorectal cancer patient samples examined and CCRK was detectable in all seven colorectal cancer cell lines tested. Colorectal tissue microarray indicated that overexpression of CCRK was detected in 62/109 (56.9%) of informative colorectal cancer cases and was significantly associated with the tumour pT and pN status (p<0.05). Suppression of CCRK by siCCRK led to G1 phase cell cycle arrest and reduced cell growth. Consistently, stable clones of LoVo and DLD1 cells expressing shCCRK exhibited decreased cell proliferation rates. Furthermore, we showed that CCRK is required for the phosphorylation of Cdk2 (on Thr-160) and Rb (on Ser-795) and the expression of cyclin E. These results suggest for the first time that CCRK is involved in colorectal cancer carcinogenesis and G1/S cell cycle transition by regulating Cdk2, cyclin E and Rb.

Quantitative reconstitution of mitotic CDK1 activation in somatic cell extracts

The regulation of mitotic entry in somatic cells differs from embryonic cells, yet it is only for embryonic cells that we have a quantitative understanding of this process. To gain a similar insight into somatic cells, we developed a human cell extract system that recapitulates CDK1 activation and nuclear envelope breakdown in response to mitotic cyclins. As cyclin B concentrations increase, CDK1 activates in a three-stage nonlinear response, creating an ordering of substrate phosphorylations. This response is established by dual regulatory feedback loops involving WEE1/MYT1, which impose a cyclin B threshold, and CDC25, which allows CDK1 to escape the WEE1/MYT1 inhibition. This system also exhibits a complex response to cyclin A. Cyclin A promotes WEE1 phosphorylation to weaken the negative feedback loop and primes mitotic entry through cyclin B. This observation explains the requirement of both cyclin A and cyclin B to initiate mitosis in somatic cells.

Cdc25 phosphatases are required for timely assembly of CDK1-cyclin B at the G2/M transition

Progression through mitosis requires the coordinated regulation of Cdk1 kinase activity. Activation of Cdk1 is a multistep process comprising binding of Cdk1 to cyclin B, relocation of cyclin-kinase complexes to the nucleus, activating phosphorylation of Cdk1 on Thr(161) by the Cdk-activating kinase (CAK; Cdk7 in metazoans), and removal of inhibitory Thr(14) and Tyr(15) phosphorylations. This dephosphorylation is catalyzed by the dual specific Cdc25 phosphatases, which occur in three isoforms in mammalian cells, Cdc25A, -B, and -C. We find that expression of Cdc25A leads to an accelerated G(2)/M phase transition. In Cdc25A-overexpressing cells, Cdk1 exhibits high kinase activity despite being phosphorylated on Tyr(15). In addition, Tyr(15)-phosphorylated Cdk1 binds more cyclin B in Cdc25A-overexpressing cells compared with control cells. Consistent with this observation, we demonstrate that in human transformed cells, Cdc25A and Cdc25B, but not Cdc25C phosphatases have an effect on timing and efficiency of cyclin-kinase complex formation. Overexpression of Cdc25A or Cdc25B promotes earlier assembly and activation of Cdk1-cyclin B complexes, whereas repression of these phosphatases by short hairpin RNA has a reverse effect, leading to a substantial decrease in amounts of cyclin B-bound Cdk1 in G(2) and mitosis. Importantly, we find that Cdc25A overexpression leads to an activation of Cdk7 and increase in Thr(161) phosphorylation of Cdk1. In conclusion, our data suggest that complex assembly and dephosphorylation of Cdk1 at G(2)/M is tightly coupled and regulated by Cdc25 phosphatases.

Acetylation of cyclin A: a new cell cycle regulatory mechanism

Cyclin A must be degraded at prometaphase in order to allow mitosis progression. Nevertheless, the signals that trigger cyclin A degradation at mitosis have been largely elusive. In the present paper, we review the status of cyclin A degradation in the light of recent evidence indicating that acetylation plays a role in cyclin A stability. The emerging model proposes that the acetyltransferase PCAF [p300/CREB (cAMP-response-element-binding protein)-binding protein-associated factor] [perhaps also its homologue GCN5 (general control non-derepressible 5)] acetylates cyclin A at Lys(54), Lys(68), Lys(95) and Lys(112) during mitosis, leading to its ubiquitylation by the anaphase-promoting factor/cyclosome and its subsequent degradation via proteasome. Interestingly, these four lysine residues in cyclin A also participate in the regulation of cyclin A-Cdk (cyclin-dependent kinase) activity by modulating its interaction with Cdks.

Combination of nutlin-3 and VX-680 selectively targets p53 mutant cells with reversible effects on cells expressing wild-type p53

Chemotherapeutics (e.g., aurora kinase inhibitors) designed to target proliferative cells are often nonspecific for tumor cells as normal cycling cells are also susceptible. Indeed, one of the major dose-limiting toxicities of aurora kinase inhibitors is a dangerous depletion of neutrophils in patients. In this study we proposed a strategy to selectively target p53 mutant cells while sparing normal ones. The strategy is based on the understanding that normal cells have an intact p53 pathway but not tumor cells carrying p53 mutations. Nongenotoxic activation of p53 using nutlin led to a reversible activation of G1 and G2 arrest in normal cells, which prevents them from entering mitosis, thus protecting them from the side effects of aurora kinase inhibition (VX-680), namely endoreduplication and apoptosis. Cells carrying mutant p53 are selectively killed by the nutlin/VX-680 combination, whereas p53 wild-type cells retain their proliferative capacity. The major implications drawn from these results are: (1) reversible nongenotoxic activation of p53 may be used as a strategy for the chemoprotection of normal tissues, and (2) aurora kinase inhibitors may have alleviated side effects when used in combination with nutlin-like inhibitors. We highlight the distinct roles of p53 and p73 in mediating the cellular responses to VX-680 and suggest that dual protection by p53 and p73 are needed to guard against endoreduplication and polyploidy.

Cdk2 is required for p53-independent G2/M checkpoint control

The activation of phase-specific cyclin-dependent kinases (Cdks) is associated with ordered cell cycle transitions. Among the mammalian Cdks, only Cdk1 is essential for somatic cell proliferation. Cdk1 can apparently substitute for Cdk2, Cdk4, and Cdk6, which are individually dispensable in mice. It is unclear if all functions of non-essential Cdks are fully redundant with Cdk1. Using a genetic approach, we show that Cdk2, the S-phase Cdk, uniquely controls the G₂/M checkpoint that prevents cells with damaged DNA from initiating mitosis. CDK2-nullizygous human cells exposed to ionizing radiation failed to exclude Cdk1 from the nucleus and exhibited a marked defect in G₂/M arrest that was unmasked by the disruption of P53. The DNA replication licensing protein Cdc6, which is normally stabilized by Cdk2, was physically associated with the checkpoint regulator ATR and was required for efficient ATR-Chk1-Cdc25A signaling. These findings demonstrate that Cdk2 maintains a balance of S-phase regulatory proteins and thereby coordinates subsequent p53-independent G₂/M checkpoint activation.

Metazoan cells contain multiple Cdks that regulate cell cycle progression. Recent studies have shown that mouse cells can grow normally with just Cdk1. The roles of the non-essential Cdks remain a fundamental question. In this study, we describe the generation and detailed characterization of CDK2-knockout human somatic cells. Our study demonstrates that Cdk2 is required for robust DNA damage checkpoint signaling. Loss of Cdk2 caused a marked deficiency in the G₂/M arrest—a basic response to DNA damage—in cells that were also nullizygous for P53. We propose that the multiple Cdks present in metazoan cells provide a mechanism by which the cell cycle can be efficiently halted after DNA damage. Significantly, this study reveals a heretofore unrecognized dependence for Cdk2 in p53-deficient cancer cells.

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.

A bifunctional regulatory element in human somatic Wee1 mediates cyclin A/Cdk2 binding and Crm1-dependent nuclear export

Sophisticated models for the regulation of mitotic entry are lacking for human cells. Inactivating human cyclin A/Cdk2 complexes through diverse approaches delays mitotic entry and promotes inhibitory phosphorylation of Cdk1 on tyrosine 15, a modification performed by Wee1. We show here that cyclin A/Cdk2 complexes physically associate with Wee1 in U2OS cells. Mutation of four conserved RXL cyclin A/Cdk binding motifs (RXL1 to RXL4) in Wee1 diminished stable binding. RXL1 resides within a large regulatory region of Wee1 that is predicted to be intrinsically disordered (residues 1 to 292). Near RXL1 is T239, a site of inhibitory Cdk phosphorylation in Xenopus Wee1 proteins. We found that T239 is phosphorylated in human Wee1 and that this phosphorylation was reduced in an RXL1 mutant. RXL1 and T239 mutants each mediated greater Cdk phosphorylation and G(2)/M inhibition than the wild type, suggesting that cyclin A/Cdk complexes inhibit human Wee1 through these sites. The RXL1 mutant uniquely also displayed increased nuclear localization. RXL1 is embedded within sequences homologous to Crm1-dependent nuclear export signals (NESs). Coimmunoprecipitation showed that Crm1 associated with Wee1. Moreover, treatment with the Crm1 inhibitor leptomycin B or independent mutation of the potential NES (NESm) abolished Wee1 nuclear export. Export was also reduced by Cdk inhibition or cyclin A RNA interference, suggesting that cyclin A/Cdk complexes contribute to Wee1 export. Somewhat surprisingly, NESm did not display increased G(2)/M inhibition. Thus, nuclear export of Wee1 is not essential for mitotic entry though an important functional role remains likely. These studies identify a novel bifunctional regulatory element in Wee1 that mediates cyclin A/Cdk2 association and nuclear export.

A peptide fragment of azurin induces a p53-mediated cell cycle arrest in human breast cancer cells

We report that amino acids 50 to 77 of azurin (p28) preferentially enter the human breast cancer cell lines MCF-7, ZR-75-1, and T47D through a caveolin-mediated pathway. Although p28 enters p53 wild-type MCF-7 and the isogenic p53 dominant-negative MDD2 breast cancer cell lines, p28 only induces a G(2)-M-phase cell cycle arrest and apoptosis in MCF-7 cells. p28 exerts its antiproliferative activity by reducing proteasomal degradation of p53 through formation of a p28:p53 complex within a hydrophobic DNA-binding domain (amino acids 80-276), increasing p53 levels and DNA-binding activity. Subsequent elevation of the cyclin-dependent kinase inhibitors p21 and p27 reduces cyclin-dependent kinase 2 and cyclin A levels in a time-dependent manner in MCF-7 cells but not in MDD2 cells. These results suggest that p28 and similar peptides that significantly reduce proteasomal degradation of p53 by a MDM2-independent pathway(s) may provide a unique series of cytostatic and cytotoxic (apoptotic) chemotherapeutic agents.

Dietary feeding of dibenzoylmethane inhibits prostate cancer in transgenic adenocarcinoma of the mouse prostate model

Dibenzoylmethane (DBM), a minor beta-diketone constituent of licorice, has been shown to exhibit antineoplastic effects in prostate cancer cell lines by induction of cell cycle arrest and regulation of androgen receptor expression. In the present study, we investigated the in vitro and in vivo efficacy of DBM using TRAMP-C1 cell lines and TRAMP mice. DBM was found to arrest TRAMP-C1 cells at G(2)-M phase of cell cycle and suppressed phosphorylated retinoblastoma, cyclin D1, and cyclin A. Importantly, DBM was found to be equally effective in suppression of prostate tumor progression in TRAMP mice. At 8 or 12 weeks of age, mice were fed control or 1% DBM-supplemented diets until 24 weeks of age. Our results show that DBM-fed groups had a lower incidence of palpable tumor and high-grade prostatic intraepithelial neoplasia. Subsequent mechanistic studies show that the expression of phosphorylated retinoblastoma, c-myc, cyclin D1, cyclin A, phosphorylated Akt, phosphorylated PDK-1, and phosphorylated S6 was significantly reduced by DBM. Our findings suggest that DBM blocks the growth and progression of prostate cancer in TRAMP mice via modulation of tumor cell cycle regulation and therefore merits its consideration for future clinical intervention of human prostate cancer.

Cyclin A/cdk2 regulates adenomatous polyposis coli-dependent mitotic spindle anchoring

Mutations in adenomatous polyposis coli (APC) protein is a major contributor to tumor initiation and progression in several tumor types. These mutations affect APC function in the Wnt-beta-catenin signaling and influence mitotic spindle anchoring to the cell cortex and orientation. Here we report that the mitotic anchoring and orientation function of APC is regulated by cyclin A/cdk2. Knockdown of cyclin A and inhibition of cdk2 resulted in cells arrested in mitosis with activation of the spindle assembly checkpoint. The mitotic spindle was unable to form stable attachments to the cell cortex, and this resulted in the spindles failing to locate to the central position in the cells and undergo dramatic rotation. We have demonstrated that cyclin A/cdk2 specifically associates with APC in late G2 phase and phosphorylates it at Ser-1360, located in the mutation cluster region of APC. Mutation of APC Ser-1360 to Ala results in identical off-centered mitotic spindles. Thus, this cyclin A/cdk2-dependent phosphorylation of APC affects astral microtubule attachment to the cortical surface in mitosis.

Cyclin A is redundant in fibroblasts but essential in hematopoietic and embryonic stem cells

Cyclins are regulatory subunits of cyclin-dependent kinases. Cyclin A, the first cyclin ever cloned, is thought to be an essential component of the cell-cycle engine. Mammalian cells encode two A-type cyclins, testis-specific cyclin A1 and ubiquitously expressed cyclin A2. Here, we tested the requirement for cyclin A function using conditional knockout mice lacking both A-type cyclins. We found that acute ablation of cyclin A in fibroblasts did not affect cell proliferation, but led to prolonged expression of another cyclin, cyclin E, across the cell cycle. However, combined ablation of all A- and E-type cyclins extinguished cell division. In contrast, cyclin A function was essential for cell-cycle progression of hematopoietic and embryonic stem cells. Expression of cyclin A is particularly high in these compartments, which might render stem cells dependent on cyclin A, whereas in fibroblasts cyclins A and E play redundant roles in cell proliferation.

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.

Degradation of cyclin A is regulated by acetylation

Cyclin A accumulates at the onset of S phase, remains high during G(2) and early mitosis and is degraded at prometaphase. Here, we report that the acetyltransferase P/CAF directly interacts with cyclin A that as a consequence becomes acetylated at lysines 54, 68, 95 and 112. Maximal acetylation occurs simultaneously to ubiquitylation at mitosis, indicating importance of acetylation on cyclin A stability. This was further confirmed by the observation that the pseudoacetylated cyclin A mutant can be ubiquitylated whereas the nonacetylatable mutant cannot. The nonacetylatable mutant is more stable than cyclin A WT (cycA WT) and arrests cell cycle at mitosis. Moreover, in cells treated with histone deacetylase inhibitors cyclin A acetylation increases and its stability decreases, thus supporting the function of acetylation on cyclin A degradation. Although the nonacetylatable mutant cannot be ubiquitylated, it interacts with the proteins needed for its degradation (cdks, Cks, Cdc20, Cdh1 and APC/C). In fact, its association with cdks is increased and its complexes with these kinases display higher activity than control cycA WT-cdk complexes. All these results indicate that cyclin A acetylation at specific lysines is crucial for cyclin A stability and also has a function in the regulation of cycA-cdk activity.

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.

Inappropriate activation of cyclin-dependent kinases by the phosphatase Cdc25b results in premature mitotic entry and triggers a p53-dependent checkpoint

The dual specificity phosphatase Cdc25B is capable of inhibiting cellular proliferation, and this occurs in a manner dependent upon its catalytic activity. Here it is shown that this is accompanied by inappropriate cyclin-dependent kinase activation and premature mitotic entry, leading to both p53-dependent and independent checkpoints. Forced expression of Cdc25B inappropriately up-regulated the activity of Cdk1 and Cdk2, by reducing levels of inhibitory phosphorylation. In cells lacking p14ARF, p53 is induced, and components of the ATM and ATR pathways are activated. Cdc25B triggers cell cycle arrest in the G(1) and G(2) phases that is p53- and p21-dependent and is inhibited by caffeine. Cdc25B also causes cells with an S phase DNA content to enter mitosis prematurely in a p53-independent manner. Synchronization of cells with aphidicolin results in these cells undergoing apoptosis. Thus, inappropriate cell cycle progression and premature mitotic entry via dysregulation of cyclin-dependent kinases results in activation of both p53-dependent and independent responses. Because Cdc25B is known to have oncogenic activity, this provides insight into the multistep nature of cancer development and why there is p53 loss during tumorigenesis.

Cyclin E deregulation is an early event in the development of breast cancer

Cyclin E has been shown to be overexpressed in some human breast cancers, however, data to support deregulation of cyclin E as an early event in human mammary tumor development is lacking. We analyzed surgical specimens from 183 patients with breast carcinomas and evaluated cyclin E expression in areas of invasive carcinoma, adjacent carcinoma in situ (CIS), and non-neoplastic breast parenchyma. Overexpression of cyclin E was seen in one-third of invasive carcinoma samples, one-third of the CIS component and nearly half of the non-neoplastic breast epithelial cells adjacent to carcinoma (44% vs. 33%, P < or = 0.05). Nuclear labeling for cyclin E was highly concordant between areas of in invasive carcinoma, CIS and non-neoplastic breast epithelial cells from the same patient (P < 0.0001). Localization of cyclin E to the cytoplasm was seen in a small proportion of tumor samples. Our findings suggest that cyclin E deregulation is an early event in the progression from histologically benign mammary epithelial cells to invasive carcinoma and occurs through both overexpression and altered cellular localization.

Transient nutlin-3a treatment promotes endoreduplication and the generation of therapy-resistant tetraploid cells

p53 Activity is controlled in large part by MDM2, an E3 ubiquitin ligase that binds p53 and promotes its degradation. The MDM2 antagonist Nutlin-3a stabilizes p53 by blocking its interaction with MDM2. Several studies have supported the potential use of Nutlin-3a in cancer therapy. Two different p53 wild-type cancer cell lines (U2OS and HCT116) treated with Nutlin-3a for 24 hours accumulated 2N and 4N DNA content, suggestive of G(1) and G(2) phase cell cycle arrest. This coincided with increased p53 and p21 expression, hypophosphorylation of pRb, and depletion of Cyclin B1, Cyclin A, and CDC2. Upon removal of Nutlin-3a, 4N cells entered S phase and re-replicated their DNA without an intervening mitotic division, a process known as endoreduplication. p53-p21 pathway activation was required for the depletion of Cyclin B1, Cyclin A, and CDC2 in Nutlin-3a-treated cells and for endoreduplication after Nutlin-3a removal. Stable tetraploid clones could be isolated from Nutlin-3a treated cells, and these tetraploid clones were more resistant to ionizing radiation and cisplatin-induced apoptosis than diploid counterparts. These data indicate that transient Nutlin-3a treatment of p53 wild-type cancer cells can promote endoreduplication and the generation of therapy-resistant tetraploid cells. These findings have important implications regarding the use of Nutlin-3a in cancer therapy

FHA-RING ubiquitin ligases in cell division cycle control

Despite the common occurrence of forkhead associated (FHA) phosphopeptide-binding domains and really interesting new gene (RING) E3 ubiquitin ligase domains, gene products containing both an N-terminal FHA domain and C-terminal RING domain constitute a highly distinctive intersection. Characterized FHA-RING ligases include the two vertebrate proteins, Checkpoint with FHA and RING (Chfr) and RING finger 8 (Rnf8), as well as three fungal proteins, Defective in mitosis (Dma1), Chf1 and Chf2. These FHA-RING ligases play roles in negative regulation of the cell division cycle, apparently by coupling protein phosphorylation events to specific ubiquitylation of target proteins. Here, the available data on upstream and downstream regulation of and by FHA-RING ligases are reviewed.

Ustilago maydis, a new fungal model system for cell biology

The use of fungal model systems, such as Saccharomyces cerevisisae and Schizosaccharomyces pombe, has contributed enormously to our understanding of essential cellular processes in animals. Here, we introduce the corn smut fungus Ustilago maydis as a new model organism for studying cell biological processes. Genome-wide analysis demonstrates that U. maydis is more closely related to humans than to budding yeast, and numerous proteins are shared only by U. maydis and Homo sapiens. Growing evidence suggests that basic principles of long-distance transport, mitosis and motor-based microtubule organization are conserved between U. maydis and humans. The fungus U. maydis, therefore, offers a unique system for the study of certain mammalian processes.

cAMP signaling regulates histone H3 phosphorylation and mitotic entry through a disruption of G2 progression

cAMP signaling is known to have significant effects on cell growth, either inhibitory or stimulatory depending on the cell type. Study of cAMP-induced growth inhibition in mammalian somatic cells has focused mainly on the combined role of protein kinase A (PKA) and mitogen-activated protein (MAP) kinases in regulation of progression through the G1 phase of the cell cycle. Here we show that cAMP signaling regulates histone H3 phosphorylation in a cell cycle-dependent fashion, increasing it in quiescent cells but dramatically reducing it in cycling cells. The latter is due to a rapid and dramatic loss of mitotic histone H3 phosphorylation caused by a disruption in G2 progression, as evidenced by the inhibition of mitotic entry and decreased activity of the CyclinB/Cdk1 kinase. The inhibition of G2 progression induced through cAMP signaling is dependent on expression of the catalytic subunit of PKA and is highly sensitive to intracellular cAMP concentration. The mechanism by which G2 progression is inhibited is independent of both DNA damage and MAP kinase signaling. Our results suggest that cAMP signaling activates a G2 checkpoint by a unique mechanism and provide new insight into normal cellular regulation of G2 progression.

JunB breakdown in mid-/late G2 is required for down-regulation of cyclin A2 levels and proper mitosis

JunB, a member of the AP-1 family of dimeric transcription factors, is best known as a cell proliferation inhibitor, a senescence inducer, and a tumor suppressor, although it also has been attributed a cell division-promoting activity. Its effects on the cell cycle have been studied mostly in G1 and S phases, whereas its role in G2 and M phases still is elusive. Using cell synchronization experiments, we show that JunB levels, which are high in S phase, drop during mid- to late G2 phase due to accelerated phosphorylation-dependent degradation by the proteasome. The forced expression of an ectopic JunB protein in late G2 phase indicates that JunB decay is necessary for the subsequent reduction of cyclin A2 levels in prometaphase, the latter event being essential for proper mitosis. Consistently, abnormal JunB expression in late G2 phase entails a variety of mitotic defects. As these aberrations may cause genetic instability, our findings contrast with the acknowledged tumor suppressor activity of JunB and reveal a mechanism by which the deregulation of JunB might contribute to tumorigenesis.

Cyclin A/cdk2 coordinates centrosomal and nuclear mitotic events

Cyclin A/cdk2 has a role in progression through S phase, and a large pool is also activated in G2 phase. Here we report that this G2 phase pool regulates the timing of progression into mitosis. Knock down of cyclin A by siRNA or addition of a specific cdk2 small molecule inhibitor delayed entry into mitosis by delaying cells in G2 phase. The G2 phase-delayed cells contained elevated levels of inactive cyclin B/cdk1. However, increased microtubule nucleation at the centrosomes was observed, and the centrosomes stained for markers of cyclin B/cdk1 activity. Both microtubule nucleation at the centrosomes and the phosphoprotein markers were lost with short-term treatment of the cdk1/2 inhibitor roscovitine but not the Mek1/2 inhibitor U0126. Cyclin A/cdk2 localized at the centrosomes in late G2 phase after separation of the centrosomes but before the start of prophase. Thus G2 phase cyclin A/cdk2 controls the timing of entry into mitosis by controlling the subsequent activation of cyclin B/cdk1, but also has an unexpected role in coordinating the activation of cyclin B/cdk1 at the centrosome and in the nucleus.

Finishing mitosis, one step at a time

The final stages of mitosis begin in anaphase, when the mitotic spindle segregates the duplicated chromosomes. Mitotic exit is then completed by disassembly of the spindle and packaging of chromosomes into daughter nuclei. The successful completion of mitosis requires that these events occur in a strict order. Two main mechanisms govern progression through late mitosis: dephosphorylation of cyclin-dependent kinase (Cdk) substrates and destruction of the substrates of the anaphase-promoting complex (APC). Here, we discuss the hypothesis that the order of late mitotic events depends, at least in part, on the order in which different Cdk and APC substrates are dephosphorylated or destroyed, respectively.