Cell cycle: how, when and why cells get rid of cyclin A

A review on regulation of cell cycle by extracellular matrix

The extracellular matrix (ECM) is a network of structural proteins, glycoproteins and proteoglycans that assists independent cells in aggregating and forming highly organized functional structures. ECM serves numerous purposes and is an essential component of tissue structure and functions. Initially, the role of ECM was considered to be confined to passive functions like providing mechanical strength and structural identity to tissues, serving as barriers and platforms for cells. The doors to understanding ECM's proper role in tissue functioning opened with the discovery of cellular receptors, integrins to which ECM components binds and influences cellular activities. Understanding and utilizing ECM's potential to control cellular function has become a topic of much interest in recent decades, providing different outlooks to study processes involved in developmental programs, wound healing and tumour progression. On another front, the regulatory mechanisms operating to prevent errors in the cell cycle have been topics of a titanic amount of studies. This is expected as many diseases, most infamously cancer, are associated with defects in their functioning. This review focuses on how ECM, through different methods, influences the progression of the somatic cell cycle and provides deeper insights into molecular mechanisms of functional communication between adhesion complex, signalling pathways and cell cycle machinery.

Aneuploidy in Oocytes Is Prevented by Sustained CDK1 Activity through Degron Masking in Cyclin B1

Successful mitosis requires that cyclin B1:CDK1 kinase activity remains high until chromosomes are correctly aligned on the mitotic spindle. It has therefore been unclear why, in mammalian oocyte meiosis, cyclin B1 destruction begins before chromosome alignment is complete. Here, we resolve this paradox and show that mouse oocytes exploit an imbalance in the ratio of cyclin B1 to CDK1 to control CDK1 activity; early cyclin B1 destruction reflects the loss of an excess of non-CDK1-bound cyclin B1 in late prometaphase, while CDK1-bound cyclin B1 is destroyed only during metaphase. The ordered destruction of the two forms of cyclin B1 is brought about by a previously unidentified motif that is accessible in free cyclin B1 but masked when cyclin B1 is in complex with CDK1. This protects the CDK1-bound fraction from destruction in prometaphase, ensuring a period of prolonged CDK1 activity sufficient to achieve optimal chromosome alignment and prevent aneuploidy.

•

In mouse oocytes, an excess of cyclin B1 preserves CDK1 activity

•

A motif in non-CDK1-bound cyclin B1 confers preferential APC/C targeting

•

Non-CDK1-bound cyclin B1 is gradually destroyed before CDK1-bound cyclin B1

•

Prolonged CDK1 activity assists the spindle checkpoint and prevents aneuploidy

Pan-class I PI3-kinase inhibitor BKM120 induces MEK1/2-dependent mitotic catastrophe in non-Hodgkin lymphoma leading to apoptosis or polyploidy determined by Bax/Bak and p53

Constitutive signaling of PI3K/Akt/mTOR plays a prominent role in malignant transformation and progression of B-cell non-Hodgkin lymphomas (B-NHL) underscoring the need for PI3K targeted therapies. The pan-class I PI3-kinase inhibitor BKM120 has shown preclinical activity in distinct malignancies and is currently tested in clinical trials. Intratumor heterogeneity is an intrinsic property of cancers that contributes to drug resistance and tumor recurrence. Here, we demonstrate that inhibition of PI3-kinases by BKM120 attenuates growth and survival of B-NHL cell lines by inducing mitotic arrest with subsequent induction of intrinsic apoptosis. BKM120-mediated downregulation of Cyclin A and activation of the CDK1/Cyclin B1 complex facilitates mitotic entry. In addition, concomitant BKM120-mediated upregulation of Cyclin B1 expression attenuates completion of mitosis, which results in mitotic catastrophe and apoptotic cell death. In Bax and Bak deficient B-NHL, which are resistant to BKM120-induced apoptosis, BKM120-induced mitotic catastrophe results in polyploidy. Upon re-expression of wt p53 in these p53 mutated cells, BKM120-induced polyploidy is strongly reduced demonstrating that the genetic status of the cells determines the outcome of a BKM120-mediated pathway inhibition. Mitotic catastrophe and unfavorable induction of polyploidy can be prevented in this setting by additional inhibition of MEK1/2 signaling. Combining MEK1/2 inhibitors with BKM120 enhances the anti-tumor effects of BKM120, prevents prognostic unfavorable polyploidy and might be a potential strategy for the treatment of B-NHL.

G2/M Cell Cycle Arrest and Tumor Selective Apoptosis of Acute Leukemia Cells by a Promising Benzophenone Thiosemicarbazone Compound

Anti-mitotic therapies have been considered a hallmark in strategies against abnormally proliferating cells. Focusing on the extensively studied family of thiosemicarbazone (TSC) compounds, we have previously identified 4,4'-dimethoxybenzophenone thiosemicarbazone (T44Bf) as a promising pharmacological compound in a panel of human leukemia cell lines (HL60, U937, KG1a and Jurkat). Present findings indicate that T44Bf-mediated antiproliferative effects are associated with a reversible chronic mitotic arrest caused by defects in chromosome alignment, followed by induced programmed cell death. Furthermore, T44Bf selectively induces apoptosis in leukemia cell lines when compared to normal peripheral blood mononuclear cells. The underlying mechanism of action involves the activation of the mitochondria signaling pathway, with loss of mitochondrial membrane potential and sustained phosphorylation of anti-apoptotic protein Bcl-xL as well as increased Bcl-2 (enhanced phosphorylated fraction) and pro-apoptotic protein Bad levels. In addition, ERK signaling pathway activation was found to be a requisite for T44Bf apoptotic activity. Our findings further describe a novel activity for a benzophenone thiosemicarbazone and propose T44Bf as a promising anti-mitotic prototype to develop chemotherapeutic agents to treat acute leukemia malignancies.

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.

Regulation of cyclin A localization downstream of Par-1 function is critical for the centrosome orientation checkpoint in Drosophila male germline stem cells

Male germline stem cells (GSCs) in Drosophila melanogaster divide asymmetrically by orienting the mitotic spindle with respect to the niche, a microenvironment that specifies stem cell identity. The spindle orientation is prepared during interphase through stereotypical positioning of the centrosomes. We recently demonstrated that GSCs possess a checkpoint ("the centrosome orientation checkpoint") that monitors correct centrosome orientation prior to mitosis to ensure an oriented spindle and thus asymmetric outcome of the division. Here, we show that Par-1, a serine/threonine kinase that regulates polarity in many systems, is involved in this checkpoint. Par-1 shows a cell cycle-dependent localization to the spectrosome, a germline-specific, endoplasmic reticulum-like organelle. Furthermore, the localization of cyclin A, which is normally localized to the spectrosome, is perturbed in par-1 mutant GSCs. Interestingly, overexpression of mutant cyclin A that does not localize to the spectrosome and mutation in hts, a core component of the spectrosome, both lead to defects in the centrosome orientation checkpoint. We propose that the regulation of cyclin A localization via Par-1 function plays a critical role in the centrosome orientation checkpoint.

The ATAC acetyl transferase complex controls mitotic progression by targeting non-histone substrates

All DNA-related processes rely on the degree of chromatin compaction. The highest level of chromatin condensation accompanies transition to mitosis, central for cell cycle progression. Covalent modifications of histones, mainly deacetylation, have been implicated in this transition, which also involves transcriptional repression. Here, we show that the Gcn5-containing histone acetyl transferase complex, Ada Two A containing (ATAC), controls mitotic progression through the regulation of the activity of non-histone targets. RNAi for the ATAC subunits Ada2a/Ada3 results in delayed M/G1 transition and pronounced cell division defects such as centrosome multiplication, defective spindle and midbody formation, generation of binucleated cells and hyperacetylation of histone H4K16 and alpha-tubulin. We show that ATAC localizes to the mitotic spindle and controls cell cycle progression through direct acetylation of Cyclin A/Cdk2. Our data describes a new pathway in which the ATAC complex controls Cyclin A/Cdk2 mitotic function: ATAC/Gcn5-mediated acetylation targets Cyclin A for degradation, which in turn regulates the SIRT2 deacetylase activity. Thus, we have uncovered an essential function for ATAC in regulating Cyclin A activity and consequent mitotic progression.

Role for Brm in Cell Growth Control

Recently, we have shown implication of Brm, the catalytic subunit of the SWI/SNF chromatin remodeling complex, in repression of cyclin A expression in quiescent cells. Here, we have examined the fate of cells lacking Brm throughout the cycle. We find that despite elevated levels of cyclins A and E, these cells can respond to serum starvation, however, without reaching a canonical G(0) phase as they continue to express high levels of c-Myc and have an abnormally large average size. The response to serum starvation can be correlated with increased levels of Rb proteins p130 and p107 as well as increased association of p27 with the cyclin-dependent kinases, possibly compensating for the higher levels of G(1) cyclins by reducing their associated kinase activity. After serum stimulation, reentry into the cycle occurs normally, but the S phase is delayed and shorter. In addition, the M phase has an increased duration, and we observed frequent faulty chromosome segregation events in anaphase. Altogether, our data suggest that cells can partially overcome the absence of Brm by activating several compensatory mechanisms to control the cell cycle. However, they remain profoundly affected, unable to enter a canonical quiescent state, presenting a shorter S phase, and finally unable to perform correct chromosome segregation.

Hypoxia induces major effects on cell cycle kinetics and protein expression inDrosophila melanogasterembryos

Hypoxia induces a stereotypic response in Drosophila melanogaster embryos: depending on the time of hypoxia, embryos arrest cell cycle activity either at metaphase or just before S phase. To understand the mechanisms underlying hypoxia-induced arrest, two kinds of experiments were conducted. First, embryos carrying a kinesin-green fluorescent protein construct, which permits in vivo confocal microscopic visualization of the cell cycle, showed a dose-response relation between O2level and cell cycle length. For example, mild hypoxia (Po2∼55 Torr) had no apparent effect on cell cycle length, whereas severe hypoxia (Po2∼25–35 Torr) or anoxia (Po2= 0 Torr) arrested the cell cycle. Second, we utilized Drosophila embryos carrying a heat shock promoter driving the string ( cdc25) gene (HS-STG3), which permits synchronization of embryos before the start of mitosis. Under conditions of anoxia, we induced a stabilization or an increase in the expression of several G1/S (e.g., dE2F1, RBF2) and G2/M (e.g., cyclin A, cyclin B, dWee1) proteins. This study suggests that, in fruit fly embryos, 1) there is a dose-dependent relationship between cell cycle length and O2levels in fruit fly embryos, and 2) stabilized cyclin A and E2F1 are likely to be the mediators of hypoxia-induced arrest at metaphase and pre-S phase.

Cyclin A repression in quiescent cells is associated with chromatin remodeling of its promoter and requires Brahma/SNF2alpha

Cell cycle-dependent expression of cyclin A is controlled by transcriptional repression in early phase of the cell cycle. In this study, we directly examine the chromatin structure of the mouse cyclin A promoter through in vivo micrococcal nuclease footprinting. We describe here that cyclin A repression is associated with two positioned nucleosomes and that histones progressively lose DNA contact synchronously with gene activation. This particular nucleosomal organization is disrupted by mutations of the cyclin A bipartite repressor sequence. Moreover, the same sequence recruits the chromatin remodeling factor Brahma/SNF2alpha (Brm) onto the cyclin A promoter. Accordingly, cyclin A proximal promoter is not wrapped around nucleosomes and not repressed in quiescent cells lacking Brm. These results provide molecular explanations for the transcriptional repression state of cyclin A, as well as insights into the action of Brm chromatin remodeling factor as cell cycle regulator.

Epstein-Barr virus nuclear antigen 3C regulates cyclin A/p27 complexes and enhances cyclin A-dependent kinase activity

Epstein-Barr virus (EBV) nuclear antigen 3C (EBNA3C) is essential for primary B-cell transformation. In this report we show that cyclin A, an activator of S phase progression, bound tightly to EBNA3C. EBNA3C interacted with cyclin A in vitro and associated with cyclin A complexes in EBV-transformed lymphoblastoid cell lines. Importantly, EBNA3C stimulated cyclin A-dependent kinase activity and rescued p27-mediated inhibition of cyclin A/Cdk2 kinase activity by decreasing the molecular association between cyclin A and p27 in cells. Additionally, phosphorylation of the retinoblastoma protein, a major regulator of cell cycle progression, was enhanced both in vitro and in vivo in the presence of EBNA3C. Cyclin A interacted with a region of the carboxy terminus of EBNA3C, shown to be important both for stimulation of cyclin A-dependent kinase activity and for cell cycle progression. This provides the first evidence of an essential EBV latent antigen's directly targeting a cell cycle regulatory protein and suggests a novel mechanism by which EBV deregulates the mammalian cell cycle, which is of critical importance in B-cell transformation.

The developmental expression of cell cycle regulators in Xenopus laevis

Several cyclins and cdks have been cloned in Xenopus, but their developmental expression has not been thoroughly examined. We have analyzed the temporal and spatial expression of cdk1, cdk2, cdk4 and cyclins D1, D2, E, A1, A2 and B1 by in situ hybridization. The transcripts of these cyclins and cdks exhibit striking tissue-restricted expression patterns very early in development that cannot be strictly correlated with proliferation. While the cdks and their activating cyclins are expressed in somewhat overlapping patterns, they are not precisely coincident. Additionally, maternal and zygotic cyclin forms demonstrate markedly different expression patterns.

DNA damage during mitosis in human cells delays the metaphase/anaphase transition via the spindle-assembly checkpoint

BACKGROUND

DNA damage during mitosis triggers an ATM kinase-mediated cell cycle checkpoint pathway in yeast and fly embryos that delays progression through division. Recent data suggest that this is also true for mammals. Here we used laser microsurgery and inhibitors of topoisomerase IIalpha to break DNA in various mammalian cells after they became committed to mitosis. We then followed the fate of these cells and emphasized the timing of mitotic progression, spindle structure, and chromosome behavior.

RESULTS

We find that DNA breaks generated during late prophase do not impede entry into prometaphase. If the damage is minor, cells complete mitosis on time. However, more significant damage substantially delays exit from mitosis in many cell types. In human (HeLa, CFPAC-1, and hTERT-RPE) cells, this delay occurs during metaphase, after the formation of a bipolar spindle and the destruction of cyclin A, and it is not dependent on a functional p53 pathway. Pretreating cells with ATM kinase inhibitors does not abrogate the metaphase delay due to chromosome damage. Immunofluorescence studies reveal that cells blocked in metaphase by chromosome damage contain one or more Mad2-positive kinetochores, and the block is rapidly overridden when the cells are microinjected with a dominant-negative construct of Mad2 (Mad2deltaC).

CONCLUSIONS

We conclude that the delay in mitosis induced by DNA damage is not due to an ATM-mediated DNA damage checkpoint pathway. Rather, the damage leads to defects in kinetochore attachment and function that, in turn, maintain the intrinsic Mad-2-based spindle assembly checkpoint.

The anaphase-promoting complex: proteolysis in mitosis and beyond

Key events in mitosis such as sister chromatid separation and subsequent inactivation of cyclin-dependent kinase 1 are regulated by ubiquitin-dependent proteolysis. These events are mediated by the anaphase-promoting complex (APC), a cell cycle-regulated ubiquitin ligase that assembles multiubiquitin chains on regulatory proteins such as securin and cyclins and thereby targets them for destruction by the 26S proteasome.

mei-41 and bub1 block mitosis at two distinct steps in response to incomplete DNA replication in Drosophila embryos

Drosophila double park encodes a homolog of Cdt1 that functions in initiation of DNA replication in fission yeast and Xenopus. dup mutants complete the first 15 embryonic cell cycles, presumably via maternal dup products, and show defects in the 16(th) S phase (S16). Cells carrying dup(a1) allele forgo S16 altogether but enter mitosis 16 (M16). We find that the timing of entry into M16 is similar in dup(a1) and heterozygous or wild-type (wt) controls. In contrast, we find that mutant cells carrying another allele, dup(a3), undergo a partial S16 and delay the entry into M16. Thus, initiation of S16 appears necessary for delaying M16. This delay is absent in double mutants of dup(a3) and mei-41 (Drosophila ATR), indicating that a mei-41-dependent checkpoint acts to delay the entry into mitosis in response to incomplete DNA replication. dup(a3) and dup(a1) mutant cells that enter M16 become arrested in M16. We find that mitotic cyclins are stabilized and that a spindle checkpoint protein, Bub1, localizes onto chromosomes during mitotic arrest in dup mutants. These features suggest an arrest prior to metaphase-anaphase transition. dup(a3) bub1 double mutant cells exit M16, indicating that a bub1-mediated checkpoint acts to block mitotic exit in dup mutants. To our knowledge, this is the first report of (1) incomplete DNA replication affecting both the entry into and the exit from mitosis in a single cell cycle via different mechanisms and (2) the role of bub1 in regulating mitotic exit in response to incomplete DNA replication.