细胞周期调控的研究进展

细胞周期是一种非常复杂和精细的调节过程,有大量调节蛋白参与其中。此过程的核心是细胞周期依赖性蛋白激酶（CDKs）。CDKs的激活又依赖于另一类呈细胞周期特异性或时相性表达的细胞周期蛋白（cyclins）,而CDKs调节的关键步骤是细胞周期检查点。PLKs是多种细胞周期检查点的主要调节因子,Aurora蛋白激酶主要在细胞有丝分裂期起作用。本文就上述因素在细胞周期进程中的作用作一综述。     

Cdk inhibitors: on the threshold of checkpoints and development

The regulation of cyclin-dependent kinases is at the heart of cell cycle control and, by inference, the control of cell proliferation. Recent advances in regulation of these kinases have uncovered a group of small proteins that bind to and inhibit them, thus preventing cell cycle progression. Linking these proteins to tumor suppressor functions has provided a much sought after connection between cancer and cell cycle control.     

Revisiting the “Cdk-Centric” View of the Mammalian Cell Cycle

Since the early genetic studies in yeast, regulation of the cell cycle has been associated to the sequential activation of several proline-directed serine-threonine protein kinases by cyclins. From yeast to humans, the activiy of these cyclin-dependent kinases (Cdks) have been thought to be essential for cell cycle regulation. Recent gene-targeted mouse models for different cyclins and Cdks have shown that members of these families show a certain level of redundancy and that specific complexes are not required for the mitotic cell cycle. However, the complexity of the Cdk-cyclin network and the promiscuity of their members makes it difficult to understand the relative contribution of these proteins to the mammalian cell division cycle. Compensatory roles by non-Cdk activities and Cdk-independent functions of cyclins are increasing the complexity of the current simplistic models. We still do not know whether at least one cyclin-dependent kinase activity is required for cell cycle progression in mammalian cells. Indeed, a relevant question for cancer therapy.     

Degradation of the cyclin-dependent kinase inhibitor KRP1 is regulated by two different ubiquitin E3 ligases

In animals and fungi, a group of proteins called the cyclin-dependent kinase inhibitors play a key role in cell cycle regulation. However, comparatively little is known about the role of these proteins in plant cell cycle regulation. To gain insight into the mechanisms by which the plant cell cycle is regulated, we studied the cyclin-dependent kinase inhibitor KRP1 in Arabidopsis. KRP1 interacts with the CDKA;1/CYCD2;1 complex in planta and functions in the G1–S transition of the cell cycle. Furthermore, we show that KRP1 is a likely target of the ubiquitin/proteasome pathway. Two different ubiquitin protein ligases, SCF^SKP2 and the RING protein RKP, contribute to its degradation. These results suggest that SCF^SKP2b and RPK play an important role in the cell cycle through regulating KRP1 protein turnover.     

Investigating the hows and whys of DNA endoreduplication

Endoreduplication is a form of nuclear polyploidization thatresults in multiple, uniform copies of chromosomes. This processis common in plants and animals, especially in tissues withhigh metabolic activity, and it generally occurs in cells thatare terminally differentiated. In plants, endoreduplicationis well documented in the endosperm and cotyledons of developingseeds, but it also occurs in many tissues throughout the plant.It is thought that endoreduplication provides a mechanism toincrease the level of gene expression, but the function of thisprocess has not been thoroughly investigated. Numerous observationshave been made of endoreduplication, or at least extra cyclesof S-phase, as a consequence of mutations in genes controllingseveral aspects of cell cycle regulation. However, until recentlythere were few studies directed at the molecular mechanismsresponsible for this specialized cell cycle. It is suggestedthat endoreduplication requires nothing more elaborate thana loss of M-phase cyclin-dependent kinase activity and oscillationsin the activity of S-phase cyclin-dependent kinase. Key words: Endoreduplication, gene expression, cell cycle regulation, cyclin-dependent kinase.     

A Novel Member of the Cyclin-Dependent Kinase Family in Paramecium tetraurelia

Passage through the cell cycle in eukaryotes requires the successive activation of different cyclin-dependent protein kinases. Here, we describe the identification and characterization of a novel class of cyclin-dependent protein kinase, termed Cdk2, in the ciliate Paramecium tetraurelia. It is 301 amino acids long, 7 amino acids shorter than Cdk1, the CDK that is associated with macronuclear DNA synthesis. All the catalytic domains typical of protein kinases can be located within the sequence and putative regulatory phosphorylation sites equivalent to Thr14, Tyr15, and Thr161 in human CDK1 are also conserved. The 'PSTAIRE' region characteristic of most CDKs is perfectly conserved. Cdk2 shares only 48% homology to Cdk1 at the amino acid level, suggesting that the evolutionary separation of Cdk1 and Cdk2 is ancient, and implying that they have different roles in cell cycle regulation. Like Cdk1, Cdk2 does not bind to yeast p13suc1, even though it has better conservation of p13suc1 binding sites than Cdk1 does. The Cdk2 protein level is relatively constant throughout the vegetative cell cycle. Cdk2 exhibits kinase activity towards bovine histone H1 in vitro with the maximal level late in the cell cycle, suggesting it may be involved in the regulation of cytokinesis. Our results further support the view that an analogue of the cyclin-dependent kinase cell cycle regulatory system like that of yeast and higher eukaryotic cells operates in Paramecium and that a family of cyclin-dependent kinases may control different aspects of the Paramecium cell cycle.     

Recent advances on cyclins, CDKs and CDK inhibitors

In eukaryotes, cell division is controlled by cyclin-dependent kinases (CDKs). Here we summarize a few new developments on the regulation of the cell cycle by CDK-cyclin complexes. We have focused on three aspects in which there has been recent progress: the structural analysis of these complexes, the phenotypes of mice carrying knockouts of CDK inhibitors and the role of proteolysis in the regulation of the cell cycle.     

Human glioma PKC-iota and PKC-betaII phosphorylate cyclin-dependent kinase activating kinase during the cell cycle

Cell cycle phase transition is regulated in part by the trimeric enzyme, cyclin-dependent kinase activating kinase (CAK) which phosphorylates and activates cyclin-dependent kinases (cdks). Protein kinase C (PKC) inhibitors prevent cell cycle phase transition, suggesting a fundamental role for PKCs in cell cycle regulation. We report that in glioma cells, CAK (cdk7) is constitutively associated with PKC-iota. In vitro phosphorylation, co-immunoprecipitation, and analysis of phosphorylated proteins by autoradiography indicate that CAK (cdk7) is a substrate for PKC-iota and PKC-betaII hyperphosphorylation. These results establish a role for PKC-iota and PKC-betaII in the activation of CAK during the glioma cell cycle.     

Cell cycle control in normal and neoplastic cells.

Recent studies of cell cycle control suggest that cyclin-dependent protein kinases play a central role in the cell's commitment to a new division cycle in late G1. The regulation of these kinases in normal and neoplastic growth is becoming clear.     

The cyclin-dependent kinase inhibitors p15INK4B and p21CIP1 are critical regulators of fibrillar collagen-induced tumor cell cycle arrest

The extracellular matrix is a crucial component in determining cell fate. Fibrillar collagen in its native form inhibits cell proliferation, whereas in its monomeric form it stimulates proliferation. The observation of elevated levels of p27(KIP1) in cells plated in the presence of fibrillar collagen has led to the assumption that this kinase inhibitor was responsible for cell cycle arrest on fibrillar collagen. Here we provide evidence that p15(INK4b), rather than p27(KIP1), is the cyclin-dependent kinase inhibitor responsible for G0/G1 arrest of human melanoma cells grown on fibrillar collagen. Additionally, we demonstrate that fibrillar collagen can also arrest cells at the G2 phase, which is mediated in part by p21(CIP1). Our data, in addition to identifying cyclin-dependent kinase inhibitors important in cell cycle arrest mediated by fibrillar collagen, demonstrate the complexity of cell cycle regulation and indicate that modulating a single cyclin-dependent kinase inhibitor does not disrupt cell proliferation in the presence of fibrillar collagen.     

Analysis of site-specific phosphorylation of the retinoblastoma protein during cell cycle progression

Differential phosphorylation of the retinoblastoma protein plays a pivotal role in cell cycle regulation. The retinoblastoma protein is specifically phosphorylated during the cell cycle by cyclin-dependent kinase complexes which intersect with many cellular signaling networks. Since the loss of the retinoblastoma signaling pathways occurs in a wide variety of human tumors, understanding the significance of site-specific phosphorylation can clarify the role of selected cyclin-dependent kinase complexes during cell cycle progression. Here we describe the phosphospecificity and cellular characterization of a panel of polyclonal antibodies that recognize unique phosphorylation sites within the retinoblastoma protein. These reagents were used to validate authentic cellular retinoblastoma phosphorylation sites at amino acids 780, 795, and 807/811 correlating with the G1-S transition.     

Cyclins,cyclin-dependent kinases and differentiation

Cyclin-dependent kinases and their regulatory subunits, the cyclins, are known to regulate progression through the cell cycle. Yet these same proteins are often expressed in non-cycling, differentiated cells. This review surveys the available information about cyclins and cyclin-dependent kinases in differentiated cells and explores the possibility that these proteins may have important functions that are independent of cell cycle regulation.     

CDK1-cyclin B1 mediates the inhibition of proliferation induced by omega-3 fatty acids in MDA-MB-231 breast cancer cells

Long-chain omega-3 polyunsaturated fatty acids are thought to inhibit the development of breast cancer. We investigated the effects of docosahexaenoic and eicosapentaenoic acids on the proliferation of MDA-MB-231 human mammary epithelial cells. Both docosahexaenoic and eicosapentaenoic acids decreased cell growth with a higher efficiency for docosahexaenoic acid (87% at 100 microM versus 74% for eicosapentaenoic acid). The effect on specific cell cycle phases was studied. G2/M duration was markedly increased by docosahexaenoic and by eicosapentaenoic acids (respectively by more than seven- and six-fold at 50 microM) when cells were synchronized at the G1/S boundary and released in the cell cycle. In contrast, there was no alteration of G1 or S phases. The expression of cyclin A, cyclin B1 and cyclin-dependent kinase 1, the regulators required for the progression from G2 to mitosis, were all decreased by these fatty acids (western blot). Since omega-3 fatty acids had no effect on the S phase, thus ruling out an involvement of cyclin A in their anti-proliferative effect, we examined whether the regulation of the cyclin-dependent kinase 1-cyclin B1 complex was altered. Upon omega-3 fatty acids treatment, cyclin B1 phosphorylation was inhibited and the expression of the cell division cycle 25C phosphatase, which dephosphorylates cyclin-dependent kinase 1, was decreased. We conclude that the anti-proliferative effect of omega-3 fatty acids occurs via the regulation of the cyclin-dependent kinase 1-cyclin B1 complex.     

The retinoblastoma pathway in plant cell cycle and development

The activity of cyclin-dependent kinases (CDKs) on specific targets mediates the temporal regulation of plant cell cycle transitions. The sequential activity of CDKs and the spatial regulation of cell proliferation during plant development, however, are still poorly understood. Understanding these aspects depends on the identification of the downstream targets and upstream modulators of CDKs and their regulation in response to mitogenic and/or differentiation signals. Current efforts to elucidate the answers to these questions are very promising; in particular, recent works reveal the essential role that the retinoblastoma pathway plays in controlling cell cycle progression and, presumably, some developmental events.     

植物细胞周期调控因子及其功能的研究进展

细胞周期调控因子能通过影响细胞周期对植物细胞的生长、分裂和分化产生作用,进而调节植物的生长发育。本文综述了近几年来植物细胞周期调控因子中细胞周期蛋白（cyclin,CYC）、周期蛋白依赖激酶（cyclin-dependent kinase,CDK）等的作用机理及研究进展,阐述了各调控因子在植物生长发育过程中的作用。     

The cell cycle: a review of regulation,deregulation and therapeutic targets in cancer

The cell cycle is controlled by numerous mechanisms ensuring correct cell division. This review will focus on these mechanisms, i.e. regulation of cyclin-dependent kinases (CDK) by cyclins, CDK inhibitors and phosphorylating events. The quality checkpoints activated after DNA damage are also discussed. The complexity of the regulation of the cell cycle is also reflected in the different alterations leading to aberrant cell proliferation and development of cancer. Consequently, targeting the cell cycle in general and CDK in particular presents unique opportunities for drug discovery. This review provides an overview of deregulation of the cell cycle in cancer. Different families of known CDK inhibitors acting by ATP competition are also discussed. Currently, at least three compounds with CDK inhibitory activity (flavopiridol, UCN-01, roscovitine) have entered clinical trials.     

Genetic,molecular, and humoral endocycle-regulating mechanisms

The review presents data on the molecular genetic mechanisms controlling endoreduplication. The issues concerning the activity of the main cycle cell regulators, such as cyclins, cyclin-dependent kinases, and their inhibitors, under conditions of a modified cell cycle of polytene cells are considered. Specific features of regulation at the replication origin points and the role of hormones and phytohormones in the ontogenetic control of endoreduplication are analyzed.     

Protein kinases and cell cycle control

Protein kinases play a central role in the regulation of the eukaryotic cell cycle. Recent research has concentrated on a particular family of protein kinases, the cyclin-dependent kinases (CDKs), and their involvement in regulating particular cell cycle transitions, such as the initiation of DNA synthesis (S phase) or of cell division (mitosis). One can think of these enzymes as the basic machinery of the cell; their activity is then modulated by proteins which transduce signals from the external environment, and by proteins that monitor the progress of events such as DNA replication or the formation of the mitotic spindle. This review will be structured so as to introduce the cyclin-CDK motif, outline which cyclin-CDKs are involved at different cell cycle stages, their direct regulation by other protein kinases and phosphatases, and lastly the importance of other protein kinases in the cell cycle.