A detailed analysis of cyclin A accumulation at the G(1)/S border in normal and transformed cells

Analysis of the multiparametric cell cycle data

This chapter was originally written in 2011. The idea was to give some history of cell cycle analysis before and after flow cytometry became widely accessible; provide references to educational material for single parameter DNA content analysis, introduce and discuss multiparameter cell cycle analysis in a methodological style, and in a casual style, discuss aspects of the work over the last 40years that we have given thought, performing some experiments, but didn't publish. It feels like there is a linear progression that moves from counting cells for growth curves, to counting labeled mitotic cells by autoradiography, to DNA content analysis, to cell cycle states defined by immunofluorescence plus DNA content analysis, to extraction of cell cycle expression profiles, and finally to probability state modeling, which should be the "right" way to analyze cytometric cell cycle data. This is the sense of this chapter. In 2023, we have updated it, but the exciting, expansive aspects brought about by spectral and mass cytometry are still young and developing, and thus have not been vetted, reviewed, and presented in mature form.

Meiotic Cell Cycle Progression in Mouse Oocytes: Role of Cyclins

All eukaryotic cells, including oocytes, utilize an engine called cyclin-dependent kinase (Cdk) to drive the cell cycle. Cdks are activated by a co-factor called cyclin, which regulates their activity. The key Cdk-cyclin complex that regulates the oocyte cell cycle is known as Cdk1-cyclin B1. Recent studies have elucidated the roles of other cyclins, such as B2, B3, A2, and O, in oocyte cell cycle regulation. This review aims to discuss the recently discovered roles of various cyclins in mouse oocyte cell cycle regulation in accordance with the sequential progression of the cell cycle. In addition, this review addresses the translation and degradation of cyclins to modulate the activity of Cdks. Overall, the literature indicates that each cyclin performs unique and redundant functions at various stages of the cell cycle, while their expression and degradation are tightly regulated. Taken together, this review provides new insights into the regulatory role and function of cyclins in oocyte cell cycle progression.

A Truncated Form of the p27 Cyclin-Dependent Kinase Inhibitor Translated from Pre-mRNA Causes G2-Phase Arrest

Pre-mRNA splicing is an indispensable mechanism for eukaryotic gene expression. Splicing inhibition causes cell cycle arrest at the G₁ and G₂/M phases, and this is thought to be one of the reasons for the potent antitumor activity of splicing inhibitors. However, the molecular mechanisms underlying the cell cycle arrest have many unknown aspects. In particular, the mechanism of G₂/M-phase arrest caused by splicing inhibition is completely unknown. Here, we found that lower and higher concentrations of pladienolide B caused M-phase and G₂-phase arrest, respectively. We analyzed protein levels of cell cycle regulators and found that a truncated form of the p27 cyclin-dependent kinase inhibitor, named p27*, accumulated in G₂-arrested cells. Overexpression of p27* caused partial G₂-phase arrest. Conversely, knockdown of p27* accelerated exit from G₂/M phase after washout of splicing inhibitor. These results suggest that p27* contributes to G₂/M-phase arrest caused by splicing inhibition. We also found that p27* bound to and inhibited M-phase cyclins, although it is well known that p27 regulates the G₁/S transition. Intriguingly, p27*, but not full-length p27, was resistant to proteasomal degradation and remained in G₂/M phase. These results suggest that p27*, which is a very stable truncated protein in G₂/M phase, contributes to G₂-phase arrest caused by splicing inhibition.

CREB1 promotes proliferation and differentiation by mediating the transcription of CCNA2 and MYOG in bovine myoblasts

The cAMP response element binding protein 1 (CREB1) is an important nuclear transcription factor in eukaryotes. To explore the potential role of CREB1 on Qinchuan bovine skeletal myoblasts, we investigated the function of CREB1 on proliferation and differentiation. In this study, we found that CREB1 promoted cell proliferation by promoting DNA synthesis in S phase and cell division in G2 phase and promoted myogenic differentiation process in bovine myoblasts. Through dual luciferase experiments, we found that CREB1 can bind to the proximal promoter regions of CCNA2 and MyoG, indicating that CREB1 can play a positive regulatory role in the proliferation and differentiation of myoblasts by mediating the transcription of CCNA2 and MyoG. In addition, through downstream target gene analysis and transcriptome sequencing, we found that CREB1 plays a role in cell proliferation, myogenic differentiation, skeletal muscle repair and other related pathways.

Acetylshikonin induces autophagy-dependent apoptosis through the key LKB1-AMPK and PI3K/Akt-regulated mTOR signalling pathways in HL-60 cells

Acetylshikonin (ASK) is a natural naphthoquinone derivative of traditional Chinese medicine Lithospermum erythrorhyzon. It has been reported that ASK has bactericidal, anti‐inflammatory and antitumour effects. However, whether ASK induces apoptosis and autophagy in acute myeloid leukaemia (AML) cells and the underlying mechanism are still unclear. Here, we explored the roles of apoptosis and autophagy in ASK‐induced cell death and the potential molecular mechanisms in human AML HL‐60 cells. The results demonstrated that ASK remarkably inhibited the cell proliferation, viability and induced apoptosis in HL‐60 cells through the mitochondrial pathway, and ASK promoted cell cycle arrest in the S‐phase. In addition, the increased formation of autophagosomes, the turnover from light chain 3B (LC3B) I to LC3B II and decrease of P62 suggested the induction of autophagy by ASK. Furthermore, ASK significantly decreased PI3K, phospho‐Akt and p‐p70S6K expression, while enhanced phospho‐AMP‐activated protein kinase (AMPK) and phospho‐liver kinase B1(LKB1) expression. The suppression of ASK‐induced the conversion from LC3B I to LC3B II caused by the application of inhibitors of AMPK (compound C) demonstrated that ASK‐induced autophagy depends on the LKB1/AMPK pathway. These data suggested that the autophagy induced by ASK were dependent on the activation of LKB1/AMPK signalling and suppression of PI3K/Akt/mTOR pathways. The cleavage of the apoptosis‐related markers caspase‐3 and caspase‐9 and the activity of caspase‐3 induced by ASK were markedly reduced by inhibitor of AMPK (compound C), an autophagy inhibitor 3‐methyladenine (3‐MA) and another autophagy inhibitor chloroquine (CQ). Taken together, our data reveal that ASK‐induced HL‐60 cell apoptosis is dependent on the activation of autophagy via the LKB1/AMPK and PI3K/Akt‐regulated mTOR signalling pathways.

Induction of Mitosis Delay and Apoptosis by CDDO-TFEA in Glioblastoma Multiforme

Background: Glioblastoma multiforme (GBM) is the vicious malignant brain tumor in adults. Despite advances multi-disciplinary treatment, GBM constinues to have a poor overall survival. CDDO-trifluoroethyl-amide (CDDO-TEFA), a trifluoroethylamidederivative of CDDO, is an Nrf2/ARE pathway activator. CDDO-TEFEA is used to inhibit proliferation and induce apoptosis in glioma cells. However, it not clear what effect it may have on tumorigenesis in GBM. Methods: This in vitro study evaluated the effects of CDDO-TFEA on GBM cells. To do this, we treated GBM8401 cell lines with CDDO-TFEA and assessed apoptosis, cell cycle. DNA content and induction of apoptosis were analyzed by flow cytometry and protein expression by Western blot analysis. Results: CDDO-TFEA significantly inhibited the cell viability and induced cell apoptosis on GBM 8401 cell line. The annexin-FITC/PI assay revealed significant changes in the percentage of apoptotic cells. Treatment with CDDO-TFEA led to a significant reduction in the GBM8401 cells' mitochondrial membrane potential. A significant rise in the percentage of caspase-3 activity was detected in the treated cells. In addition, treatment with CDDO-TFEA led to an accumulation of G2/M-phase cells. In addition, these results suggest that regarding increased protein synthesis during mitosis in the MPM-2 staining, indicative of a delay in the G2 checkpoint. An analysis of Cyclin B1, CDK1, Cyclin B1/CDK1 complex and CHK1 and CHK2 expression suggested that cell cycle progression seems also to be regulated by CDDO-TFEA. Therefore, CDDO-TFEA may not only induce cell cycle G2/M arrest, it may also exert apoptosis in established GBM cells. Conclusion: CDDO-TFEA can inhibit proliferation, cell cycle progression and induce apoptosis in GBM cells in vitro, possibly though its inhibition of Cyclin B1, CDK1 expression, and Cyclin B1/CDK1 association and the promotion of CHK1 and CHK2 expression.

CDCA7 Facilitates Tumor Progression by Directly Regulating CCNA2 Expression in Esophageal Squamous Cell Carcinoma

Background

CDCA7 is a copy number amplified gene identified not only in esophageal squamous cell carcinoma (ESCC) but also in various cancer types. Its clinical relevance and underlying mechanisms in ESCC have remained unknown.

Methods

Tissue microarray data was used to analyze its expression in 179 ESCC samples. The effects of CDCA7 on proliferation, colony formation, and cell cycle were tested in ESCC cells. Real-time PCR and Western blot were used to detect the expression of its target genes. Correlation of CDCA7 with its target genes in ESCC and various SCC types was analyzed using GSE53625 and TCGA data. The mechanism of CDCA7 was studied by chromatin immunoprecipitation (ChIP), luciferase reporter assays, and rescue assay.

Results

The overexpression of CDCA7 promoted proliferation, colony formation, and cell cycle in ESCC cells. CDCA7 affected the expression of cyclins in different cell phases. GSE53625 and TCGA data showed CCNA2 expression was positively correlated with CDCA7. The knockdown of CCNA2 reversed the malignant phenotype induced by CDCA7 overexpression. Furthermore, CDCA7 was found to directly bind to CCNA2, thus promoting its expression.

Conclusions

Our results reveal a novel mechanism of CDCA7 that it may act as an oncogene by directly upregulating CCNA2 to facilitate tumor progression in ESCC.

Functions of cyclins and CDKs in mammalian gametogenesis†

Cyclins and cyclin-dependent kinases (CDKs) are key regulators of the cell cycle. Most of our understanding of their functions has been obtained from studies in single-cell organisms and mitotically proliferating cultured cells. In mammals, there are more than 20 cyclins and 20 CDKs. Although genetic ablation studies in mice have shown that most of these factors are dispensable for viability and fertility, uncovering their functional redundancy, CCNA2, CCNB1, and CDK1 are essential for embryonic development. Cyclin/CDK complexes are known to regulate both mitotic and meiotic cell cycles. While some mechanisms are common to both types of cell divisions, meiosis has unique characteristics and requirements. During meiosis, DNA replication is followed by two successive rounds of cell division. In addition, mammalian germ cells experience a prolonged prophase I in males or a long period of arrest in prophase I in females. Therefore, cyclins and CDKs may have functions in meiosis distinct from their mitotic functions and indeed, meiosis-specific cyclins, CCNA1 and CCNB3, have been identified. Here, we describe recent advances in the field of cyclins and CDKs with a focus on meiosis and early embryogenesis.

Connections between the cell cycle, cell adhesion and the cytoskeleton

Cell division, the purpose of which is to enable cell replication, and in particular to distribute complete, accurate copies of genetic material to daughter cells, is essential for the propagation of life. At a morphological level, division not only necessitates duplication of cellular structures, but it also relies on polar segregation of this material followed by physical scission of the parent cell. For these fundamental changes in cell shape and positioning to be achieved, mechanisms are required to link the cell cycle to the modulation of cytoarchitecture. Outside of mitosis, the three main cytoskeletal networks not only endow cells with a physical cytoplasmic skeleton, but they also provide a mechanism for spatio-temporal sensing via integrin-associated adhesion complexes and site-directed delivery of cargoes. During mitosis, some interphase functions are retained, but the architecture of the cytoskeleton changes dramatically, and there is a need to generate a mitotic spindle for chromosome segregation. An economical solution is to re-use existing cytoskeletal molecules: transcellular actin stress fibres remodel to create a rigid cortex and a cytokinetic furrow, while unipolar radial microtubules become the primary components of the bipolar spindle. This remodelling implies the existence of specific mechanisms that link the cell-cycle machinery to the control of adhesion and the cytoskeleton. In this article, we review the intimate three-way connection between microenvironmental sensing, adhesion signalling and cell proliferation, particularly in the contexts of normal growth control and aberrant tumour progression. As the morphological changes that occur during mitosis are ancient, the mechanisms linking the cell cycle to the cytoskeleton/adhesion signalling network are likely to be primordial in nature and we discuss recent advances that have elucidated elements of this link. A particular focus is the connection between CDK1 and cell adhesion. This article is part of a discussion meeting issue 'Forces in cancer: interdisciplinary approaches in tumour mechanobiology'.

The positive circadian regulators CLOCK and BMAL1 control G2/M cell cycle transition through Cyclin B1

We previously identified a tight bidirectional phase coupling between the circadian clock and the cell cycle. To understand the role of the CLOCK/BMAL1 complex, representing the main positive regulator of the circadian oscillator, we knocked down Bmal1 or Clock in NIH3T33C mouse fibroblasts (carrying fluorescent reporters for clock and cell cycle phase) and analyzed timing of cell division in individual cells and cell populations. Inactivation of Bmal1 resulted in a loss of circadian rhythmicity and a lengthening of the cell cycle, originating from delayed G2/M transition. Subsequent molecular analysis revealed reduced levels of Cyclin B1, an important G2/M regulator, upon suppression of Bmal1 gene expression. In complete agreement with these experimental observations, simulation of Bmal1 knockdown in a computational model for coupled mammalian circadian clock and cell cycle oscillators (now incorporating Cyclin B1 induction by BMAL1) revealed a lengthening of the cell cycle. Similar data were obtained upon knockdown of Clock gene expression. In conclusion, the CLOCK/BMAL1 complex controls cell cycle progression at the level of G2/M transition through regulation of Cyclin B1 expression.

The Temporal Regulation of S Phase Proteins During G1

Successful DNA replication requires intimate coordination with cell-cycle progression. Prior to DNA replication initiation in S phase, a series of essential preparatory events in G₁ phase ensures timely, complete, and precise genome duplication. Among the essential molecular processes are regulated transcriptional upregulation of genes that encode replication proteins, appropriate post-transcriptional control of replication factor abundance and activity, and assembly of DNA-loaded protein complexes to license replication origins. In this chapter we describe these critical G₁ events necessary for DNA replication and their regulation in the context of both cell-cycle entry and cell-cycle progression.

Preterm labor in the absence of acute histologic chorioamnionitis is characterized by cellular senescence of the chorioamniotic membranes

BACKGROUND

Decidual senescence has been considered a mechanism of disease for spontaneous preterm labor in the absence of severe acute inflammation. Yet, signs of cellular senescence have also been observed in the chorioamniotic membranes from women who underwent the physiological process of labor at term.

OBJECTIVE

We aimed to investigate whether, in the absence of acute histologic chorioamnionitis, the chorioamniotic membranes from women who underwent spontaneous preterm labor or labor at term exhibit signs of cellular senescence.

STUDY DESIGN

Chorioamniotic membrane samples were collected from women who underwent spontaneous preterm labor or labor at term. Gestational age-matched nonlabor controls were also included. Senescence-associated genes/proteins were determined using reverse transcription quantitative polymerase chain reaction analysis (n = 7-9 each for array; n = 26-28 each for validation), enzyme-linked immunosorbent assays (n = 7-9 each), immunoblotting (n = 6-7 each), and immunohistochemistry (n = 7-8 each). Senescence-associated β-galactosidase activity (n = 7-11 each) and telomere length (n = 15-22 each) were also evaluated.

RESULTS

In the chorioamniotic membranes without acute histologic chorioamnionitis: (1) the expression profile of senescence-associated genes was different between the labor groups (term in labor and preterm in labor) and the nonlabor groups (term no labor and preterm no labor), yet there were differences between the term in labor and preterm in labor groups; (2) most of the differentially expressed genes among the groups were closely related to the tumor suppressor protein (TP53) pathway; (3) the expression of TP53 was down-regulated in the term in labor and preterm in labor groups compared to their nonlabor counterparts; (4) the expression of CDKN1A (gene coding for p21) was up-regulated in the term in labor and preterm in labor groups compared to their nonlabor counterparts; (5) the expression of the cyclin kinase CDK2 and cyclins CCNA2, CCNB1, and CCNE1 was down-regulated in the preterm in labor group compared to the preterm no labor group; (6) the concentration of TP53 was lower in the preterm in labor group than in the preterm no labor and term in labor groups; (7) the senescence-associated β-galactosidase activity was greater in the preterm in labor group than in the preterm no labor and term in labor groups; (8) the concentration of phospho-S6 ribosomal protein was reduced in the term in labor group compared to its nonlabor counterpart, but no differences were observed between the preterm in labor and preterm no labor groups; and (9) no significant differences were observed in relative telomere length among the study groups (term no labor, term in labor, preterm no labor, and preterm in labor).

CONCLUSION

In the absence of acute histologic chorioamnionitis, signs of cellular senescence are present in the chorioamniotic membranes from women who underwent spontaneous preterm labor compared to those who delivered preterm in the absence of labor. However, the chorioamniotic membranes from women who underwent spontaneous labor at term did not show consistent signs of cellular senescence in the absence of histologic chorioamnionitis. These results suggest that different pathways are implicated in the pathological and physiological processes of labor.

Cyclin A2 modulates kinetochore-microtubule attachment in meiosis II

Cyclin A2 is a crucial mitotic Cdk regulatory partner that coordinates entry into mitosis and is then destroyed in prometaphase within minutes of nuclear envelope breakdown. The role of cyclin A2 in female meiosis and its dynamics during the transition from meiosis I (MI) to meiosis II (MII) remain unclear. We found that cyclin A2 decreases in prometaphase I but recovers after the first meiotic division and persists, uniquely for metaphase, in MII-arrested oocytes. Conditional deletion of cyclin A2 from mouse oocytes has no discernible effect on MI but leads to disrupted MII spindles and increased merotelic attachments. On stimulation of exit from MII, there is a dramatic increase in lagging chromosomes and an inhibition of cytokinesis. These defects are associated with an increase in microtubule stability in MII spindles, suggesting that cyclin A2 mediates the fidelity of MII by maintaining microtubule dynamics during the rapid formation of the MII spindle.

New perspectives of cobalt tris(bipyridine) system: anti-cancer effect and its collateral sensitivity towards multidrug-resistant (MDR) cancers

Platinating compounds including cisplatin, carboplatin, and oxaliplatin are common chemotherapeutic agents, however, patients developed resistance to these clinical agents after initial therapeutic treatments. Therefore, different approaches have been applied to identify novel therapeutic agents, molecular mechanisms, and targets for overcoming drug resistance. In this study, we have identified a panel of cobalt complexes that were able to specifically induce collateral sensitivity in taxol-resistant and p53-deficient cancer cells. Consistently, our reported anti-cancer functions of cobalt complexes 1-6 towards multidrug-resistant cancers have suggested the protective and non-toxic properties of cobalt metal-ions based compounds in anti-cancer therapies. As demonstrated in xenograft mouse model, our results also confirmed the identified cobalt complex 2 was able to suppress tumor growth in vivo. The anti-cancer effect of the cobalt complex 2 was further demonstrated to be exerted via the induction of autophagy, cell cycle arrest, and inhibition of cell invasion and P-glycoprotein (P-gp) activity. These data have provided alternative metal ion compounds for targeting drug resistance cancers in chemotherapies.

APC/C and SCF(cyclin F) Constitute a Reciprocal Feedback Circuit Controlling S-Phase Entry

The anaphase promoting complex/cyclosome (APC/C) is an ubiquitin ligase and core component of the cell-cycle oscillator. During G1 phase, APC/C binds to its substrate receptor Cdh1 and APC/C(Cdh1) plays an important role in restricting S-phase entry and maintaining genome integrity. We describe a reciprocal feedback circuit between APC/C and a second ubiquitin ligase, the SCF (Skp1-Cul1-F box). We show that cyclin F, a cell-cycle-regulated substrate receptor (F-box protein) for the SCF, is targeted for degradation by APC/C. Furthermore, we establish that Cdh1 is itself a substrate of SCF(cyclin F). Cyclin F loss impairs Cdh1 degradation and delays S-phase entry, and this delay is reversed by simultaneous removal of Cdh1. These data indicate that the coordinated, temporal ordering of cyclin F and Cdh1 degradation, organized in a double-negative feedback loop, represents a fundamental aspect of cell-cycle control. This mutual antagonism could be a feature of other oscillating systems.

Blockage of Autophagy Rescues the Dual PI3K/mTOR Inhibitor BEZ235-induced Growth Inhibition of Colorectal Cancer Cells

Molecular targeting for the altered signaling pathways has been proven to be effective for the treatment ofmany types of human cancer, including colorectal cancer (CRC). The dual phosphatidylinositol-3-kinase (PI3K) and mammalian target of rapamycin (mTOR) inhibitor BEZ235 has shown to exhibit potent antitumor activity against solid tumors. Autophagy is a cellular lysosomal catabolic process to maintain metabolic homeostasis, which has been known to be induced in response to many therapeutic agents in cancer cells. This process is negatively regulated by mTOR and often acts as prosurvival or prodeath mechanism following cancer therapeutics. The current study was designed to investigate the antiproliferation activity of BEZ235 and to evaluate the role of autophagy induced by BEZ235 using HCT15 CRC cells bearing ras oncogene mutation. We found that BEZ235 decreases cell viability, which was mostly dependent on G1 arrest of cell cycle via suppression of cyclin A expression. BEZ235 affects PI3K/Akt/mTOR signaling pathway by increasing the phosphorylation of AKT at Ser(473) and RAS/RAF/MEK/ERK pathway by decreasing the phosphorylation of ERK at Tyr(204). BEZ235 also stimulated autophagy induction as evidenced by the increased expression of LC3-II and abundant acidic vesicular organelles (AVOs) in the cytoplasm. In addition, the combination of BEZ235 with autophagy inhibitor chloroquine, a known antagonist of autophagy, counteracted the antiproliferation effect of BEZ235. Thus, our study indicates that autophagy induced in response to BEZ235 treatment appears to act as cell death mechanism in HCT15 CRC cells.

Tissue-Specific Cell Cycle Indicator Reveals Unexpected Findings for Cardiac Myocyte Proliferation

RATIONALE

Discerning cardiac myocyte cell cycle behavior is challenging owing to commingled cell types with higher proliferative activity.

OBJECTIVE

To investigate cardiac myocyte cell cycle activity in development and the early postnatal period.

METHODS AND RESULTS

To facilitate studies of cell type-specific proliferation, we have generated tissue-specific cell cycle indicator BAC transgenic mouse lines. Experiments using embryonic fibroblasts from CyclinA2-LacZ-floxed-EGFP, or CyclinA2-EGFP mice, demonstrated that CyclinA2-βgal and CyclinA2-EGFP were expressed from mid-G1 to mid-M phase. Using Troponin T-Cre;CyclinA2-LacZ-EGFP mice, we examined cardiac myocyte cell cycle activity during embryogenesis and in the early postnatal period. Our data demonstrated that right ventricular cardiac myocytes exhibited reduced cell cycle activity relative to left ventricular cardiac myocytes in the immediate perinatal period. Additionally, in contrast to a recent report, we could find no evidence to support a burst of cardiac myocyte cell cycle activity at postnatal day 15.

CONCLUSIONS

Our data highlight advantages of a cardiac myocyte-specific cell cycle reporter for studies of cardiac myocyte cell cycle regulation.

A standardized method for quantifying proliferation by Ki-67 and cyclin A immunohistochemistry in breast cancer

Immunohistochemical analysis of proliferation markers such as Ki-67 and cyclin A is widely used in clinical evaluation as a prognostic factor in breast cancer. The proliferation status of tumors is guiding the decision of whether or not a patient should be treated with chemotherapy because low-proliferative tumors are less sensitive by such treatment. However, the lack of optimal cutoff points and selection of tumor areas hamper its use in clinical practice. This study was performed to compare the Ki-67 and cyclin A expression counted in hot-spot vs average counting based on 5 to 14 random tumor areas in 613 breast carcinomas. We correlated the findings with 10-year follow-up in order to standardize the evaluation of proliferation markers in clinical practice. A significant correlation was found between the percentage of positive cells estimated by Ki-67 and cyclin A both by hot-spot and by average counting. Both methods showed that high expression of Ki-67 and cyclin A is associated with more adverse tumor stage. The cutoff value for Ki-67 for distant metastases was set to 22% and to 15%, using hot-spot and average counting, respectively. For cyclin A, the values were set to 14% and 8% using the respective methods. Survival curves revealed that patients with a high hot-spot proliferation index had a significantly greater risk of shorter tumor-free survival. Our findings suggest that the determination of proliferation markers in breast cancer should be standardized to hot-spot counting and that specific cutoff values for proliferation could be useful as prognostic markers in clinical practice. Moreover, we suggest that expression levels of cyclin A could be used as a complementary marker to estimate the proliferation status in tumors, especially those with "borderline" expression levels of Ki-67, in order to more accurately estimate the proliferations status of the tumors.

Loss of Cdk2 and cyclin A2 impairs cell proliferation and tumorigenesis

Cell-cycle inhibition has yet to offer a generally effective approach to cancer treatment, but a full evaluation of different combinations of cell-cycle inhibitors has not been evaluated. Cyclin A2, a core component of the cell cycle, is often aberrantly expressed in cancer where it may impact cell proliferation. In this study, we investigated the role of cyclin A2 in tumorigenesis using a conditional genetic knockout mouse model. Cyclin A2 deletion in oncogene-transformed mouse embryonic fibroblasts (MEF) suppressed tumor formation in immunocompromised mice. These findings were confirmed in mice with cyclin A2-deficient hepatocytes, where a delay in liver tumor formation was observed. Because cyclin A2 acts in complex with Cdk2 in the cell cycle, we explored a hypothesized role for Cdk2 dysregulation in this effect through conditional deletions of both genes. In oncogene-transformed MEFs lacking both genes, tumor formation was strongly suppressed in a manner associated with decreased proliferation, premature senescence, and error-prone recovery from serum deprivation after immortalization. Whereas loss of cyclin A2 led to a compensatory increase in Cdk1 activity, this did not occur with loss of both Cdk2 and cyclin A2. Our work offers a rationale to explore combinations of Cdk1 and Cdk2 inhibitors as a general approach in cancer therapy.

Reciprocal activation of transcription factors underlies the dichotomy between proliferation and invasion of glioma cells

Histology of malignant glioma depicts dense proliferative areas rich in angiogenesis as well as dissemination of neoplastic cells into adjacent brain tissue. Although the mechanisms that trigger transition from proliferative to invasive phenotypes are complex, the dichotomy of cell proliferation and migration, the “Go or Grow” hypothesis, argues for specific and coordinated regulation of these phenotypes. We investigated transcriptional elements that accompany the phenotypes of migration and proliferation, and consider the therapeutic significance of the “Go or Grow” hypothesis. Interrogation of matched core and rim regions from human glioblastoma biopsy specimens in situ (n = 44) revealed higher proliferation (Ki67 labeling index) in cells residing at the core compared to the rim. Profiling activated transcription factors in a panel of migration-activated versus migration-restricted GBM cells portrayed strong NF-κB activity in the migratory cell population. In contrast, increased c-Myc activity was found in migration-restricted proliferative cells. Validation of transcriptional activity by NF-κB- or c-Myc-driven GFP or RFP, respectively, showed an increased NF-κB activity in the active migrating cells, whereas the proliferative, migration restricted cells displayed increased c-Myc activity. Immunohistochemistry on clinical specimens validated a robust phosphorylated c-Myc staining in tumor cells at the core, whereas increased phosphorylated NF-κB staining was detected in the invasive tumor cells at the rim. Functional genomics revealed that depletion of c-Myc expression by siRNA oligonucleotides reduced cell proliferation in vitro, but surprisingly, cell migration was enhanced significantly. Conversely, inhibition of NF-κB by pharmacological inhibitors, SN50 or BAY-11, decreased both cell migration in vitro and invasion ex vivo. Notably, inhibition of NF-κB was found to have no effect on the proliferation rate of glioma cells. These findings suggest that the reciprocal and coordinated suppression/activation of transcription factors, such as c-Myc and NF-κB may underlie the shift of glioma cells from a “growing-to-going” phenotype.

Dynamic expression profiles from static cytometry data: component fitting and conversion to relative, "same scale" values

Background

Cytometry of asynchronous proliferating cell populations produces data with an extractable time-based feature embedded in the frequency of clustered, correlated events. Here, we present a specific case of general methodology for calculating dynamic expression profiles of epitopes that oscillate during the cell cycle and conversion of these values to the same scale.

Methods

Samples of K562 cells from one population were labeled by direct and indirect antibody methods for cyclins A2 and B1 and phospho-S10-histone H3. The same indirect antibody was used for both cyclins. Directly stained samples were counter-stained with 4′6-diamidino-2-phenylindole and indirectly stained samples with propidium to label DNA. The S phase cyclin expressions from indirect assays were used to scale the expression of the cyclins of the multi-variate direct assay. Boolean gating and two dimensional, sequential regions set on bivariate displays of the directly conjugated sample data were used to untangle and isolate unique, unambiguous expression values of the cyclins along the four-dimensional data path through the cell cycle. The median values of cyclins A2 and B1 from each region were correlated with the frequency of events within each region.

Results

The sequential runs of data were plotted as continuous multi-line linear equations of the form y  =  [(y_i+1−y_i)/(x_i+1−x_i)]x + y_i−[(y_i+1−y_i)/(x_i+1−x_i)]x_i (line between points (x_i,y_i) and (x_i+1, y_i+1)) to capture the dynamic expression profile of the two cyclins.

Conclusions

This specific approach demonstrates the general methodology and provides a rule set from which the cell cycle expression of any other epitopes could be measured and calculated. These expression profiles are the “state variable” outputs, useful for calibrating mathematical cell cycle models.

Oncogenic RAS regulates BRIP1 expression to induce dissociation of BRCA1 from chromatin, inhibit DNA repair, and promote senescence

Here, we report a cell-intrinsic mechanism by which oncogenic RAS promotes senescence while predisposing cells to senescence bypass by allowing for secondary hits. We show that oncogenic RAS inactivates the BRCA1 DNA repair complex by dissociating BRCA1 from chromatin. This event precedes senescence-associated cell cycle exit and coincides with the accumulation of DNA damage. Downregulation of BRIP1, a physiological partner of BRCA1 in the DNA repair pathway, triggers BRCA1 chromatin dissociation. Conversely, ectopic BRIP1 rescues BRCA1 chromatin dissociation and suppresses RAS-induced senescence and the DNA damage response. Significantly, cells undergoing senescence do not exhibit a BRCA1-dependent DNA repair response when exposed to DNA damage. Overall, our study provides a molecular basis by which oncogenic RAS promotes senescence. Because DNA damage has the potential to produce additional "hits" that promote senescence bypass, our findings may also suggest one way a small minority of cells might bypass senescence and contribute to cancer development.

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.

Clusters, factories and domains: The complex structure of S-phase comes into focus

During S-phase of the cell cycle, chromosomal DNA is replicated according to a complex replication timing program, with megabase-sized domains replicating at different times. DNA fibre analysis reveals that clusters of adjacent replication origins fire near-synchronously. Analysis of replicating cells by light microscopy shows that DNA synthesis occurs in discrete foci or factories. The relationship between timing domains, origin clusters and replication foci is currently unclear. Recent work, using a hybrid Xenopus/hamster replication system, has shown that when CDK levels are manipulated during S-phase the activation of replication factories can be uncoupled from progression through the replication timing program. Here, we use data from this hybrid system to investigate potential relationships between timing domains, origin clusters and replication foci. We suggest that each timing domain typically comprises several replicon clusters, which are usually processed sequentially by replication factories. We discuss how replication might be regulated at different levels to create this complex organisation and the potential involvement of CDKs in this process.

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.

Aberrant expression of cyclin E in low-risk node negative breast cancer

BACKGROUND

Cyclin E is a cell cycle regulatory protein which occurs in G1, peaks in late G1 and is degraded in early S-phase. Cyclin E overexpression appears to be an independent prognostic factor for overall survival in breast cancer. Nuclear cyclin A is a reliable marker for S-and G2-phases. Consequently, aberrant expression of cyclin E can be detected by simultaneous immunostainings for cyclin A and cyclin E. Studies have shown that aberrant cyclin E might provide additional prognostic information compared to that of cyclin E alone. This study aimed to investigate cyclin E and aberrant cyclin E expression in low-risk node negative breast cancer.

MATERIAL AND METHODS

We compared women that died from their breast cancer (n=17) with women free from relapse > 8 years after initial diagnosis (n=24). All women had stage I, low risk breast cancer. The groups were matched regarding tumour size, receptor status, adjuvant chemotherapy and tumour differentiation. Tumour samples were analysed regarding expression of cyclin A, cyclin E and double-stained tumour cells using immunoflourescence staining and digital microscopy.

RESULTS

No differences were seen regarding expression of cyclin E or aberrant cyclin E in cases compared to controls.

DISCUSSION

We conclude that neither cyclin E nor aberrant cyclin E is a prognostic factor in low-risk node negative breast cancer patients.

Genomic instability and proliferative activity as risk factors for distant metastases in breast cancer

The role of genomic instability and proliferative activity for development of distant metastases in breast cancer was analysed, and the relative contribution of these two risk factors was quantified. A detailed quantitative comparison was performed between Ki67 and cyclin A as proliferative markers. The frequency of Ki67 and cyclin A-positive cells was scored in the same microscopic areas in 428 breast tumours. The frequency of Ki67-positive cells was found to be highly correlated with the frequency of cyclin A-positive cells, and both proliferation markers were equally good to predict risk of distant metastases. The relative contribution of degree of aneuploidy and proliferative activity as risk markers for developing distant metastases was studied independently. Although increased proliferative activity in general was associated with an increased risk of developing distant metastases, ploidy level was found to be an independent and even stronger marker when considering the group of small (T1) node negative tumours. By combining proliferative activity and ploidy level, a large group of low risk breast tumours (39%) could be identified in which only a few percentage of the tumours (5%) developed distant metastases during the 9-year follow-up time period.

DIURNAL CELL DIVISION REGULATED BY GATING THE G1 /S TRANSITION IN ENTEROMORPHA COMPRESSA (CHLOROPHYTA)(1)

The cell-cycle progression of Enteromorpha compressa (L.) Nees (=Ulva compressa L.) was diurnally regulated by gating the G1 /S transition. When the gate was open, the cells were able to divide if they had attained a sufficient size. However, the cells were not able to divide while the gate was closed, even if the cells had attained sufficient size. The diurnal rhythm of cell division immediately disappeared when the thalli were transferred to continuous light or darkness. When the thalli were transferred to a shifted photoperiod, the rhythm of cell division immediately and accurately synchronized with the shifted photoperiod. These data support a gating-system model regulated by light:dark (L:D) cycles rather than an endogenous circadian clock. A dark phase of 6 h or longer was essential for gate closing, and a light phase of 14 h was required to renew cell division after a dark phase of >6 h.

Regulation of multiple cell cycle events by Cdc14 homologues in vertebrates

Whereas early cytokinesis events have been relatively well studied, little is known about its final stage, abscission. The Cdc14 phosphatase is involved in the regulation of multiple cell cycle events, and in all systems studied Cdc14 misexpression leads to cytokinesis defects. In this work, we have cloned two CDC14 cDNA from Xenopus, including a previously unreported CDC14B homologue. We use Xenopus and human cell lines and demonstrate that localization of Cdc14 proteins is independent of both cell-type and species specificity. Ectopically expressed XCdc14A is centrosomal in interphase and localizes to the midbody in cytokinesis. By using XCdc14A misregulation, we confirm its control over different cell cycle events and unravel new functions during abscission. XCdc14A regulates the G1/S and G2/M transitions. We show that Cdc25 is an in vitro substrate for XCdc14A and might be its target at the G2/M transition. Upregulated wild-type or phosphatase-dead XCdc14A arrest cells in a late stage of cytokinesis, connected by thin cytoplasmic bridges. It does not interfere with central spindle formation, nor with the relocalization of passenger protein and centralspindlin complexes to the midbody. We demonstrate that XCdc14A upregulation prevents targeting of exocyst and SNARE complexes to the midbody, both essential for abscission to occur.

Epstein-Barr virus nuclear protein EBNA3C is required for cell cycle progression and growth maintenance of lymphoblastoid cells

Epstein-Barr virus (EBV) infection converts primary human B cells into continuously proliferating lymphoblastoid cell lines (LCLs). To examine the role of EBV nuclear antigen (EBNA) 3C in the proliferation of LCLs, we established LCLs infected with an EBV recombinant that expresses EBNA3C with a C-terminal fusion to a 4-hydroxytamoxifen (4HT)-dependent mutant estrogen receptor, E3C-HT. In the presence of 4HT, LCLs expressed the E3C-HT protein and grew like WT LCLs. When E3C-HT EBV-infected LCLs were transferred to medium without 4HT, E3C-HT protein slowly disappeared, and the LCLs gradually ceased growing. WT EBNA3C expression from an oriP plasmid transfected into E3C-HT LCLs protected the LCLs from growth arrest in medium without 4HT, whereas expression of EBNA3A or EBNA3B did not. The expression of other EBNA proteins and of LMP1, CD21, CD23, and c-myc was unaffected by EBNA3C inactivation. However, EBNA3C inactivation resulted in the accumulation of p16INK4A, a decrease in the hyperphosphorylated form of the retinoblastoma protein, and a decrease in the proportion of cells in S or G2/M phase. These results indicate that EBNA3C has an essential role in cell cycle progression and the growth maintenance of LCLs.

Parallel cyclin E and cyclin A expression in neoplastic lesions of the uterine cervix

Cyclin E levels are high during late G1 and early S-phase in normal cells. The cyclin E expression over the cell cycle in tumours is not fully known. The impact on patient outcome by high cyclin E levels during other parts of the cell cycle than late G1- and early S-phase is unknown. We set out to study the expression of cyclin E over the cell cycle in cervical carcinomas. Using immunofluorescence staining of cyclin A, digital microscopy, and digital image analysis, we determined which cells in a tissue section that were in S- or G2-phase. M-phase cells were detected by morphology. By simultaneously staining for cyclin E, we investigated the variation in cyclin E levels over the cell cycle in cervical carcinoma lesions. In a case–control study, in which each deceased patient was matched with a patient still alive and well after >5 years of follow-up, we found that the deceased patients had a considerably higher fraction of cyclin A-positive cells staining for cyclin E than the survivors (n=36). We conclude that parallel cyclin E and cyclin A expression is an indicator for poor outcome in cervical carcinomas. In addition, we investigated the expression pattern of cyclin E and cyclin A in consecutive biopsy samples from cervical carcinomas at different stages, as well as in human papillomavirus positive or negative adenocarcinomas in order to further study the cyclin E and cyclin A expression pattern in neoplastic lesions of the uterine cervix.

Low doses of cisplatin or gemcitabine plus Photofrin/photodynamic therapy: Disjointed cell cycle phase-related activity accounts for synergistic outcome in metastatic non–small cell lung cancer cells (H1299)

We compared the effects of monotherapy (photodynamic therapy or chemotherapy) versus combination therapy (photodynamic therapy plus a specific drug) on the non-small cell lung cancer cell line H1299. Our aim was to evaluate whether the additive/synergistic effects of combination treatment were such that the cytostatic dose could be reduced without affecting treatment efficacy. Photodynamic therapy was done by irradiating Photofrin-preloaded H1299 p53/p16-null cells with a halogen lamp equipped with a bandpass filter. The cytotoxic drugs used were cis-diammine-dichloroplatinum [II] (CDDP or cisplatin) and 2',2'-difluoro-2'-deoxycytidine (gemcitabine). Various treatment combinations yielded therapeutic effects (trypan blue dye exclusion test) ranging from additive to clearly synergistic, the most effective being a combination of photodynamic therapy and CDDP. To gain insight into the cellular response mechanisms underlying favorable outcomes, we analyzed the H1299 cell cycle profiles and the expression patterns of several key proteins after monotherapy. In our conditions, we found that photodynamic therapy with Photofrin targeted G0-G1 cells, thereby causing cells to accumulate in S phase. In contrast, low-dose CDDP killed cells in S phase, thereby causing an accumulation of G0-G1 cells (and increased p21 expression). Like photodynamic therapy, low-dose gemcitabine targeted G0-G1 cells, which caused a massive accumulation of cells in S phase (and increased cyclin A expression). Although we observed therapeutic reinforcement with both drugs and photodynamic therapy, reinforcement was more pronounced when the drug (CDDP) and photodynamic therapy exert disjointed phase-related cytotoxic activity. Thus, if photodynamic therapy is appropriately tuned, the dose of the cytostatic drug can be reduced without compromising the therapeutic response.

Distinct mechanisms control the stability of the related S-phase cyclins Clb5 and Clb6

The yeast S-phase cyclins Clb5 and Clb6 are closely related proteins that are synthesized late in G1. Although often grouped together with respect to function, Clb5 and Clb6 exhibit differences in their ability to promote S-phase progression. DNA replication is significantly slowed in clb5Delta mutants but not in clb6Delta mutants. We have examined the basis for the differential functions of Clb5 and Clb6 and determined that unlike Clb5, which is stable until mitosis, Clb6 is degraded rapidly at the G1/S border. N-terminal deletions of CLB6 were hyperstabilized, suggesting that the sequences responsible for directing the destruction of Clb6 reside in the N terminus. Clb6 lacks the destruction box motif responsible for the anaphase promoting complex-mediated destruction of Clb5 but contains putative Cdc4 degron motifs in the N terminus. Clb6 was hyperstabilized in cdc34-3 and cdc4-3 mutants at restrictive temperatures and when S/T-P phosphorylation sites in the N terminus were mutated to nonphosphorylatable residues. Efficient degradation of Clb6 requires the activities of both Cdc28 and Pho85. Finally, hyperstabilized Clb6 expressed from the CLB6 promoter rescued the slow S-phase defect exhibited by clb5Delta cells. Taken together, these findings suggest that the SCF(Cdc4) ubiquitin ligase complex regulates Clb6 turnover and that the functional differences exhibited by Clb5 and Clb6 arise from the distinct mechanisms controlling their stability.

The number of PML nuclear bodies increases in early S phase by a fission mechanism

Promyelocytic leukemia (PML) nuclear bodies have been implicated in a variety of cellular processes including apoptosis, tumour suppression, anti-viral response, DNA repair and transcriptional regulation. PML nuclear bodies are both positionally and structurally stable over extended periods of interphase. As demonstrated in this study, the structural stability is lost as cells enter S phase, evidenced both by distortions in shape and by fission and fusion events. At the end of this period of structural instability, the number of PML nuclear bodies has increased by a factor of twofold. Association of the fission products with chromatin implies that the PML nuclear bodies respond to changes in chromatin organisation or topology, and thus could play a role in monitoring genome integrity during DNA synthesis or in the continued maintenance of functional chromosomal domains prior to mitosis.

Single cell analysis of G1 check points-the relationship between the restriction point and phosphorylation of pRb

Single cell analysis allows high resolution investigation of temporal relationships between transition events in G1. It has been suggested that phosphorylation of the retinoblastoma tumor suppressor protein (pRb) is the molecular mechanism behind passage through the restriction point (R). We performed a detailed single cell study of the temporal relationship between R and pRb phosphorylation in human fibroblasts using time lapse video-microscopy combined with immunocytochemistry. Four principally different criteria for pRb phosphorylation were used, namely (i) phosphorylation of residues Ser795 and Ser780, (ii) degree of pRb-association with the nuclear structure, a property that is closely related with pRb phosphorylation status, (iii) release of the transcription factor E2F-1 from pRb, and (iv) accumulation of cyclin E, which is dependent on phosphorylation of pRb. The analyses of individual cells revealed that passage through R preceded phosphorylation of pRb, which occurs in a gradually increasing proportion of cells in late G1. Our data clearly suggest that pRb phosphorylation is not the molecular mechanism behind the passage through R. The restriction point and phosphorylation of pRb thus seem to represent two separate check point in G1.

S-phase modulation by irinotecan: pilot studies in advanced solid tumors

Two studies of irinotecan (CPT-11) followed 24 h later by an antimetabolite were conducted. The objectives of the studies were: (1) to determine whether the increase in S-phase in tumor cells seen 24 h after CPT-11 administration in animal studies is seen in advanced solid tumors in patients, (2) to determine the dose of CPT-11 required to produce this effect, (3) to compare two methods (immunohistochemistry, IHC, for cyclin A, and DNA flow cytometry, FC) for evaluating S-phase in tumor biopsies from patients, and (4) to establish the maximum tolerated dose (MTD) and dose-limiting toxicity (DLT) of CPT-11, given 24 h before gemcitabine (GEM, 1000 mg/m(2)). In one study CPT-11 was followed 24 h later by 5-fluorouracil (5-FU), 400 mg/m(2) per week for 4 weeks every 6 weeks. Tumor biopsies were obtained before and 24 h after CPT-11 administration before administration of 5-FU and assayed for S-phase by IHC for cyclin A and by FC. The starting dose of CPT-11 was 80 mg/m(2) per week with subsequent exploration of 40 and 60 mg/m(2) per week to establish the dose-effect relationship of the increase in tumor cells in S-phase. In the second study, CPT-11 was given 24 h before GEM 1000 mg/m(2) per week for 2 weeks every 3 weeks. Doses of 20-80 mg/m(2) were explored to establish the MTD and DLT and to study tumor cell S-phase in selected patients. CPT-11 80 mg/m(2) produced a mean increase in S-phase by IHC for cyclin A of 137%. Lesser increases were seen with 40 and 60 mg/m(2). CPT-11 followed 24 h later by 5-FU 400 mg/m(2) per week for 4 weeks was well tolerated. In the study of CPT-11 followed by GEM 1000 mg/m(2), 60 mg/m(2) of CPT-11 was the MTD.

Characterisation of Cdc25B localisation and nuclear export during the cell cycle and in response to stress

Cdc25 phosphatases are essential regulators of the cell cycle. In mammalian cells, the Cdc25B isoform activates cyclin A- and cyclin B1-containing complexes and is necessary for entry into mitosis. In this report, we characterise the subcellular localisation of Cdc25B by immunofluorescence in combination with RNA interference to identify specific antibody staining. We find that endogenous Cdc25B is mainly nuclear, but a fraction resides in the cytoplasm during the G2 phase of the cell cycle. Cdc25B starts to appear in S-phase cells and accumulates until prophase, after which the protein disappears. We characterise a nuclear export sequence in the N-terminus of Cdc25B (amino acids 54-67) that, when mutated, greatly reduces the ability of Cdc25B to shuttle in a fluorescence loss in photobleaching assay. Mutation of the nuclear export sequence makes Cdc25B less efficient in inducing mitosis, suggesting that an important mitotic function of Cdc25B occurs in the cytoplasm. Furthermore, we find that when cells are exposed to cycloheximide or ultraviolet irradiation, Cdc25B partially translocates to the cytoplasm. The dependence of this translocation event on a functional nuclear export sequence, an intact serine 323 residue (a 14-3-3 binding site) and p38 mitogen-activated protein kinase activity indicates that the p38 pathway regulates Cdc25B localisation in different situations of cellular stress.

Autonomous regulation of the anaphase-promoting complex couples mitosis to S-phase entry

Oscillations in cyclin-dependent kinase (CDK) activity drive the somatic cell cycle. After entry into mitosis, CDKs activate the anaphase-promoting complex (APC), which then promotes cyclin degradation and mitotic exit. The re-accumulation of cyclin A causes the inactivation of APC and entry into S phase, but how cyclin A can accumulate in the presence of active APC has remained unclear. Here we show that, during G1, APC autonomously switches to a state permissive for cyclin A accumulation. Crucial to this transition is the APC(Cdh1)-dependent autoubiquitination and proteasomal degradation of the ubiquitin-conjugating enzyme (E2) UbcH10. Because APC substrates inhibit the autoubiquitination of UbcH10, but not its E2 function, APC activity is maintained as long as G1 substrates are present. Thus, through UbcH10 degradation and cyclin A stabilization, APC autonomously downregulates its activity. This indicates that the core of the metazoan cell cycle could be described as a self-perpetuating but highly regulated oscillator composed of alternating CDK and APC activities.

Role for Cdk1 (Cdc2)/cyclin A in preventing the mammalian origin recognition complex's largest subunit (Orc1) from binding to chromatin during mitosis

The eukaryotic origin recognition complex (ORC) selects the genomic sites where prereplication complexes are assembled and DNA replication begins. In proliferating mammalian cells, ORC activity appears to be regulated by reducing the affinity of the Orc1 subunit for chromatin during S phase and then preventing reformation of a stable ORC-chromatin complex until mitosis is completed and a nuclear membrane is assembled. Here we show that part of the mechanism by which this is accomplished is the selective association of Orc1 with Cdk1 (Cdc2)/cyclin A during the G(2)/M phase of cell division. This association accounted for the appearance in M-phase cells of hyperphosphorylated Orc1 that was subsequently dephosphorylated during the M-to-G(1) transition. Moreover, inhibition of Cdk activity in metaphase cells resulted in rapid binding of Orc1 to chromatin. However, chromatin binding was not mediated through increased affinity of Orc1 for Orc2, suggesting that additional events are involved in the assembly of functional ORC-chromatin sites. These results reveal that the same cyclin-dependent protein kinase that initiates mitosis in mammalian cells also concomitantly inhibits assembly of functional ORC-chromatin sites.

Cell cycle-dependent regulation of a human DNA helicase that localizes in DNA damage foci

Mutational studies of human DNA helicase B (HDHB) have suggested that its activity is critical for the G1/S transition of the cell cycle, but the nature of its role remains unknown. In this study, we show that during G1, ectopically expressed HDHB localizes in nuclear foci induced by DNA damaging agents and that this focal pattern requires active HDHB. During S and G2/M, HDHB localizes primarily in the cytoplasm. A carboxy-terminal domain from HDHB confers cell cycle-dependent localization, but not the focal pattern, to a reporter protein. A cluster of potential cyclin-dependent kinase phosphorylation sites in this domain was modified at the G1/S transition and maintained through G2/M of the cell cycle in vivo, coincident with nuclear export of HDHB. Serine 967 of HDHB was the major site phosphorylated in vivo and in vitro by cyclin-dependent kinases. Mutational analysis demonstrated that phosphorylation of serine 967 is crucial in regulating the subcellular localization of ectopically expressed HDHB. We propose that the helicase of HDHB operates primarily during G1 to process endogenous DNA damage before the G1/S transition, and it is largely sequestered in the cytoplasm during S/G2.

Mutation of hCDC4 Leads to Cell Cycle Deregulation of Cyclin E in Cancer

hCDC4, the gene that encodes the F-box protein responsible for targeting cyclin E for ubiquitin-mediated proteolysis, has been found to be mutated in a number of primary cancers and cancer-derived cell lines. We have observed that functional inactivation of hCDC4 does not necessarily correlate with elevated levels of cyclin E in tumors. Here we show, however, that hCDC4 mutation in primary tumors correlates strongly with loss of cell cycle regulation of cyclin E. Similarly, a breast carcinoma-derived cell line mutated for hCDC4 exhibits cell cycle deregulation of cyclin E, but periodic expression is restored by reintroducing hCDC4 via retroviral transduction. Conversely, small interfering RNA-mediated silencing of hCdc4 deregulates cyclin E with respect to the cell cycle. These results indicate that hCdc4 function is an absolute prerequisite for cell cycle regulation of cyclin E levels, and loss of hCdc4 function is sufficient to deregulate cyclin E.

Differential prognostic impact of the cyclins E and B in premenopausal and postmenopausal women with lymph node-negative breast cancer

Searching for new prognostic factors, we investigated the influence of cyclin expression on breast cancer prognosis. A total of 273 archival tumor specimens from patients with pT1/pT2 N0 breast cancers treated by surgery and local irradiation were immunostained for cyclins E, A and B. Outcome was evaluated as metastasis-free (MFS) and disease-specific survival (DSS) over a median observation period of 99 months. In postmenopausal women, DSS was significantly predicted by cyclin E, and in premenopausal patients by cyclin B. No statistical significance was found for cyclin A. When the prognostic impact of cyclins was compared to that of standard prognostic indicators in a multivariate analysis, both cyclin E and cyclin B were selected as independent predictors of survival in postmenopausal and premenopausal patients, respectively. After inclusion of Ki-67 in the model, cyclin E lost its significance, whereas cyclin B remained the only independent prognostic factor with a hazard ratio of 4.5 (p = 0.026) for tumor-related death. Assessment of cyclin expression may, therefore, refine current prognostic models if considered in relation to menopausal status. The prognostic relevance of cyclins is likely attributable to an influence on proliferation, cell survival and genetic instability. Awareness of the molecular mechanisms leading to deregulated cyclin expression may guide decisions for risk-adapted therapy regimens.

Cell Cycle Dysregulation Induced by Cytoplasm of Lactococcus lactis ssp. lactis in SNUC2A, a Colon Cancer Cell Line

The anticancer effect of cytoplasmic fraction from Lactococcus lactis ssp. lactis, which had showed strong antiproliferative activity against SNUC2A human colon cancer cell line in the previous study, was investigated. The proliferation of SNUC2A was inhibited by the treatment with cytoplasmic fraction of Lactococcus lactis ssp. lactis in a dose-dependent and partially reversible manner. After exposure to the cytoplasmic fraction of Lc. lactis for 72 h, strong antiproliferative activity was efficiently induced through S-phase accumulation in SNUC2A cells. Analysis of cell cycle regulatory proteins demonstrated that the cytoplasmic fraction enhanced the levels of p21CIP1 and cyclin A, decreased cyclin E protein, and slightly reduced the activity of cyclin-dependent kinase 2 (CDK2).

Abnormal expression pattern of cyclin E in tumour cells

The expression pattern of cyclin E during the cell cycle was studied in normal and tumour cells in culture and in tumour biopsies. This pattern was found to be abnormal in tumour cells. A triple immunostaining protocol, digital microscopy and image analysis were used to find the position of the individual cells in the cell cycle and to measure the nuclear cyclin E levels. In normal cells, the number of cyclin E-positive cells decreased rapidly when the cells entered the S-phase. In the tumour cell lines, cyclin E was not downregulated in early S-phase, as in normal cells. Instead the number of cyclin E-positive cells remained high throughout S-phase, and the cyclin E staining intensity per cell often increased during S-phase. In about half of the analysed tumour cell lines, many cells stained positive for cyclin E even in the G(2)-phase. This abnormal expression over the cell cycle of cyclin E was also found in tumour biopsies from cervical, breast and prostatic carcinomas, even though it varied greatly between individual tumours. In some tumours, the expression pattern of cyclin E was similar to that of normal cells in culture, whereas in others high cyclin E levels could be seen in S-phase cells, as in the transformed cell lines. A high percentage of cells expressing cyclin E during S- or G(2)-phase was found to be related to poor outcome (p < 0.025) in a small group of cervical carcinoma patients (n = 12).

Distinct roles for cyclins E and A during DNA replication complex assembly and activation

Initiation of DNA replication is regulated by cyclin-dependent protein kinase 2 (Cdk2) in association with two different regulatory subunits, cyclin A and cyclin E (reviewed in ref. 1). But why two different cyclins are required and why their order of activation is tightly regulated are unknown. Using a cell-free system for initiation of DNA replication that is based on G1 nuclei, G1 cytosol and recombinant proteins, we find that cyclins E and A have specialized roles during the transition from G0 to S phase. Cyclin E stimulates replication complex assembly by cooperating with Cdc6, to make G1 nuclei competent to replicate in vitro. Cyclin A has two separable functions: it activates DNA synthesis by replication complexes that are already assembled, and it inhibits the assembly of new complexes. Thus, cyclin E opens a 'window of opportunity' for replication complex assembly that is closed by cyclin A. The dual functions of cyclin A ensure that the assembly phase (G1) ends before DNA synthesis (S) begins, thereby preventing re-initiation until the next cell cycle.

S-phase kinase-associated protein 2 expression in non-Hodgkin's lymphoma inversely correlates with p27 expression and defines cells in S phase

The protein expression of the cyclin-dependent kinase inhibitor p27 is often deregulated in human tumors. In lymphomas the inactivation of p27 is achieved through either increased degradation(1) or sequestration via D cyclins,(2) and p27 protein levels have been shown to have a prognostic significance.(1,3) Recently, S-phase kinase-associated protein 2 (Skp2) has been proved to mediate p27 degradation in normal cells(4-7) and to have oncogenetic properties.(8,9) In this study, B-, T-, and myeloid hematopoietic cell lines and a well-characterized panel of human lymphomas (n = 244) were studied for the expression of Skp2. In human lymphomas, the expression of Skp2 strongly related to the grade of malignancy, being low in indolent tumors and very high in aggressive lymphomas. Moreover, the percentages of Skp2- and S-phase-positive cells, as measured by DNA content or BrdU labeling, strictly matched and closely parallel that of Ki-67 and cyclin A. An inverse correlation between Skp2 and p27 was found in the majority of lymphoma subtypes. Nonetheless, most mantle cell lymphomas and a subset of diffuse large cell lymphomas failed to show this correlation, suggesting that alternative pathway(s) for the regulation of p27 might exist. The detection of Skp2 protein either by flow cytometry or by immunohistochemistry represents a simple method to precisely assess the S phase of lymphomas. The potential diagnostic and prognostic value of Skp2 is discussed.