Role Of Cyclin-dependent Kinase Research Articles

Endoreduplication of human smooth muscle cells induced by 2-methoxyestradiol: a role for cyclin-dependent kinase 2

Endoreduplication has been suggested to contribute to the development of hypertrophy of smooth muscle cells (SMCs) in hypertension. However, endoreduplication in vascular SMCs and the underlying molecular mechanisms are not clear. Treatment of human SMCs with 10 microM 2-methoxyestradiol (2-ME) for 24 h induces accumulation of cells with > or =4N DNA content, and some polyploid/aneuploid cells actively synthesize their DNA, suggesting the occurrence of endoreduplication. In addition, 2-ME treatment upregulates the expression of cyclin-dependent kinase 2 (Cdk2). The present study was designed to characterize endoreduplication of human SMCs and explore the potential roles of Cdk2 in endoreduplication induced by 2-ME. Treatment with 2-ME (10 microM) for 2-4 days not only caused increases in >4N cells and their reentry into S phase but also induced overduplication of chromosomes. Furthermore, 2-ME increased the kinase activity of Cdk2 and its interaction with cyclin E. Inducible overexpression of dominant-negative Cdk2 in human SMCs inhibited both DNA synthesis of >4N cells and the accumulation of >4N cells induced by 2-ME. We conclude that 2-ME induces endoreduplication of human SMCs and Cdk2 plays an important role in endoreduplication in response to 2-ME.

Metaphase Arrest by Cyclin E-Cdk2 Requires the Spindle-Checkpoint Kinase Mps1

Cytostatic factor (CSF) arrests vertebrate eggs in metaphase of meiosis II through several pathways that inhibit activation of the anaphase-promoting complex/cyclosome (APC/C). In Xenopus, the Mos-MEK1-MAPK-p90(Rsk) cascade utilizes spindle-assembly-checkpoint components to effect metaphase arrest. Another pathway involves cyclin E-Cdk2, and sustained cyclin E-Cdk2 activity in egg extracts causes metaphase arrest in the absence of Mos; this latter finding suggests that an independent pathway contributes to CSF arrest. Here, we demonstrate that metaphase arrest with cyclin E-Cdk2, but not with Mos, requires the spindle-checkpoint kinase monopolar spindles 1 (Mps1), a cyclin E-Cdk2 target that is also implicated in centrosome duplication. xMps1 is synthesized and activated during oocyte maturation and inactivated upon CSF release. In egg extracts, CSF release by calcium was inhibited by constitutively active cyclin E-Cdk2 and delayed by wild-type xMps1. Ablation of cyclin E by antisense oligonucleotides blocked accumulation of xMps1, suggesting that cyclin E-Cdk2 controls Mps1 levels. During meiosis II, activated cyclin E-Cdk2 significantly inhibited the APC/C even in the absence of the Mos-MAPK pathway, but this inhibition was not sufficient to suppress S phase between meiosis I and II. These results uniquely place xMps1 downstream of cyclin E-Cdk2 in mediating a pathway of APC/C inhibition and metaphase arrest.

Reconstructing the Regulatory Kinase Pathways of Myogenesis from Phosphopeptide Data

Multiple kinase activities are required for skeletal muscle differentiation. However, the mechanisms by which these kinase pathways converge to coordinate the myogenic process are unknown. Using multiple phosphoprotein and phosphopeptide enrichment techniques we obtained phosphopeptides from growing and differentiating C2C12 muscle cells and determined specific peptide sequences using LC-MS/MS. To place these phosphopeptides into a rational context, a bioinformatics approach was used. Phosphorylation sites were matched to known site-specific and to site non-specific kinase-substrate interactions, and then other substrates and upstream regulators of the implicated kinases were incorporated into a model network of protein-protein interactions. The model network implicated several kinases of known relevance to myogenesis including AKT, GSK3, CDK5, p38, DYRK, and MAPKAPK2 kinases. This combination of proteomics and bioinformatics technologies should offer great utility as the volume of protein-protein and kinase-substrate information continues to increase.

Cyclin A1/Cdk2 is Sufficient but not Required for the Induction of Apoptosis in Early Xenopus laevis Embryos

The role of cyclin-dependent kinases in cell proliferation is well characterized, whereas their somewhat paradoxical role in catalyzing apoptosis is less understood. One Cdk complex implicated in both cell proliferation and cell death is cyclin A/Cdk2. During early embryonic development of Xenopus laevis, distinct isoforms of cyclin A are expressed at different times. From fertilization through gastrulation, cyclin A1 is the predominant isoform. Cyclin A1 dimerizes with Cdk2 but not Cdk1. In contrast, cyclin A2 is expressed at a low level until gastrulation, when it becomes the major A-type cyclin and associates with both Cdk1 and Cdk2. When Xenopus embryos are treated with ionizing radiation (IR) prior to the midblastula transition (MBT), cyclin A1 protein persists beyond the MBT and forms an active complex with Cdk2. During this window of cyclin A1/Cdk2 activity, the embryo undergoes apoptosis. To test the hypothesis that cyclin A1-associated activity is a mediator of apoptosis, cyclin A1 protein level and associated kinase activity were measured in embryos treated with aphidicolin to induce apoptosis. Both cyclin A1 content and associated kinase activity were sustained after the MBT as embryos underwent apoptosis. To determine whether cyclin A1/Cdk2 was sufficient to induce apoptosis, recombinant cyclin A1/Cdk2 complex was injected into single-celled embryos, which induced apoptosis after the MBT. However, morpholinos targeting translation of cyclins A1 and A2 did not block apoptosis in embryos treated with X-rays or aphidicolin. These data indicate that cyclin A1/Cdk2 is sufficient, but not required for apoptosis during early development.

The Roles of Cyclin-dependent Kinase 5 and Glycogen Synthase Kinase 3 in Tau Hyperphosphorylation

Hyperphosphorylation of the microtubule-associated protein tau is a characteristic feature of neurodegenerative tauopathies including Alzheimer disease. Over-activation of proline-directed kinases, such as cyclin-dependent kinase 5 (Cdk5) and glycogen synthase kinase 3 (GSK3), has been implicated in the aberrant phosphorylation of tau at proline-directed sites. In this study we tested the roles of Cdk5 and GSK3 in tau hyperphosphorylation in vivo using transgenic mice with p25-induced Cdk5 over-activation. We found that over-activation of Cdk5 in young transgenic animals does not induce tau hyperphosphorylation at sites recognized by the antibodies AT8, AT100, PHF-1, and TG3. In fact, we observed that Cdk5 over-activation leads to inhibition of GSK3. However, in old transgenic animals the inhibition of GSK3 is lost and results in increased GSK3 activity, which coincides with tau hyperphosphorylation at the AT8 and PHF-1 sites. Pharmacological inhibition of GSK3 in old transgenic mice by chronic treatment with lithium leads to a reduction of the age-dependent increase in tau hyperphosphorylation. Furthermore, we found that Cdk5, GSK3, and PP2A co-immunoprecipitate, suggesting a functional association of these molecules. Together, these results reveal the role of GSK3 as a key mediator of tau hyperphosphorylation, whereas Cdk5 acts as a modulator of tau hyperphosphorylation via the inhibitory regulation of GSK3. Furthermore, these findings suggest that disruption of regulation of GSK3 activity underlies tau hyperphosphorylation in neurodegenerative tauopathies. Hence, GSK3 may be a prime target for therapeutic intervention in tauopathies including Alzheimer disease.

Regulating Dendritic Spine Morphology

Noting the emerging role for cyclin-dependent kinase 5 (Cdk5) in the developing and mature brain, Kim et al. searched rat striatal extracts for proteins that interacted with Cdk5 together with its neuron-specific regulatory subunit, P35 (P35/Cdk5). They identified two proteins known to exist in a complex with WAVE1 [Wiskott-Aldrich syndrome protein (WASP)-family verprolin homologous protein 1], a protein implicated in the regulation of actin dynamics. WAVE1 itself was also present in the complex of P35/Cdk5-binding proteins and interacted directly with P35. Cdk5 phosphorylated WAVE1 in vitro, and treatment of mouse striatal slices with a Cdk5 inhibitor led to reduced WAVE1 phosphorylation; moreover, primary cultures of cortical neurons from mice lacking Cdk5 showed reduced WAVE1 phosphorylation compared with wild-type neurons. Preincubation with p35/Cdk5 and MgATP attenuated WAVE1-mediated actin polymerization, whereas preincubation with a protein phosphatase enhanced it. Striatal neurons from mice lacking WAVE1 showed a decrease in mature dendritic spines, as did cultured hippocampal neurons in which WAVE1 expression was inhibited with RNAi. Expression of an RNAi-resistant WAVE1 mutant reversed the effects of RNAi on spine morphology; a dephosphorylation-mimicking mutant had a greater effect than the wild-type mutant, whereas a phosphorylation-mimicking mutant had little effect. Treatment with a dopamine D1 receptor agonist or with forskolin (which stimulates adenylyl cyclase) decreased WAVE1 phosphorylation in striatal slices. Forskolin also decreased WAVE1 phosphorylation in cultured neurons and increased the number of mature dendritic spines (an effect that depended on WAVE1). Thus, the authors conclude that WAVE1 phosphorylation (by Cdk5) and dephosphorylation (in response to cAMP signaling) is likely involved in regulating the morphology of dendritic spines. Y. Kim, J. Y. Sung, I. Ceglia, K.-W. Lee, J.-H. Ahn, J. M. Halford, A. M. Kim, S. P. Kwak, J. B. Park, S. H. Ryu, A. Schenck, B. Bardoni, J. D. Scott, A. C. Nairn, P. Greengard, Phosphorylation of WAVE1 regulates actin polymerization and dendritic spine morphology. Nature 442 , 814-817 (2006). [PubMed]

The Role of Cyclin-Dependent Kinases in T-Cell Development, Proliferation, and Function

The transit of T lymphocytes through the cell cycle in response to extracellular signals is controlled in large part by the ordered expression and degradation of cyclins and cyclin-dependent kinases and their negative regulators, the cyclin-dependent kinase inhibitors. This review outlines findings that have provided insights into how T lymphocytes integrate signals from their antigen, costimulatory, and cytokine receptors to drive cell cycle progression and discusses how the coordinated activities of these families of proteins influence multiple aspects of T-cell function, from thymic development and peripheral homeostasis to antigen-driven responses and the induction of T-cell memory and tolerance.

Cyclin-dependent kinases in C. elegans

Cell division is an inherent part of organismal development, and defects in this process can lead to developmental abnormalities as well as cancerous growth. In past decades, much of the basic cell-cycle machinery has been identified, and a major challenge in coming years will be to understand the complex interplay between cell division and multicellular development. Inevitably, this requires the use of more complex multicellular model systems. The small nematode Caenorhabditis elegans is an excellent model system to study the regulation of cell division in a multicellular organism, and is poised to make important contributions to this field. The past decade has already seen a surge in cell-cycle research in C. elegans, yielding information on the function of many basic cell-cycle regulators, and making inroads into the developmental control of cell division. This review focuses on the in vivo roles of cyclin-dependent kinases in C. elegans, and highlights novel findings implicating CDKs in coupling development to cell-cycle progression.

Novel Role for Cyclin-dependent Kinase 2 in Neuregulin-induced Acetylcholine Receptor ϵ Subunit Expression in Differentiated Myotubes

Cyclin-dependent kinases (CDKs) are a family of evolutionarily conserved serine/threonine kinases. CDK2 acts as a checkpoint for the G(1)/S transition in the cell cycle. Despite a down-regulation of CDK2 activity in postmitotic cells, many cell types, including muscle cells, maintain abundant levels of CDK2 protein. This led us to hypothesize that CDK2 may have a function in postmitotic cells. We show here for the first time that CDK2 can be activated by neuregulin (NRG) in differentiated C2C12 myotubes. In addition, this activity is required for expression of the acetylcholine receptor (AChR) epsilon subunit. The switch from the fetal AChRgamma subunit to the adult-type AChRepsilon is required for synapse maturation and the neuromuscular junction. Inhibition of CDK2 activity with either the specific CDK2 inhibitory peptide Tat-LFG or by RNA interference abolished neuregulin-induced AChRepsilon expression. Neuregulin-induced activation of CDK2 also depended on the ErbB receptor, MAPK, and PI3K, all of which have previously been shown to be required for AChRepsilon expression. Neuregulin regulated CDK2 activity through coordinating phosphorylation of CDK2 on Thr-160, accumulation of CDK2 in the nucleus, and down-regulation of the CDK2 inhibitory protein p27 in the nucleus. In addition, we also observed a novel mechanism of regulation of CDK2 activity by a low molecular weight variant of cyclin E in response to NRG. These findings establish CDK2 as an intermediate molecule that integrates NRG-activated signals from both the MAPK and PI3K pathways to AChRepsilon expression and reveal an undiscovered physiological role for CDK2 in postmitotic cells.

Control of cyclin‐dependent kinase 5 (Cdk5) activity by glutamatergic regulation of p35 stability

Although the roles of cyclin-dependent kinase 5 (Cdk5) in neurodevelopment and neurodegeneration have been studied extensively, regulation of Cdk5 activity has remained largely unexplored. We report here that glutamate, acting via NMDA or kainate receptors, can induce a transient Ca(2+)/calmodulin-dependent activation of Cdk5 that results in enhanced autophosphorylation and proteasome-dependent degradation of a Cdk5 activator p35, and thus ultimately down-regulation of Cdk5 activity. The relevance of this regulation to synaptic plasticity was examined in hippocampal slices using theta burst stimulation. p35(-/-) mice exhibited a lower threshold for induction of long-term potentiation. Thus excitatory glutamatergic neurotransmission regulates Cdk5 activity through p35 degradation, and this pathway may contribute to plasticity.

Role of cyclin-dependent kinase 5 and its activator P35 in local axon and growth cone stabilization

Axons elongate and perform steering reactions with their growth cones constantly undergoing local collapse and stabilization. Our previous studies have shown that a type-1 phosphorylated form of microtubule-associated protein 1B, recognized by monoclonal antibody 1E11 (mab1E11), is present in stable regions and absent from unstable regions of turning growth cones of retinal ganglion cells. In contrast, the total population of microtubule-associated protein 1B is present in the entire growth cone. Here we demonstrate that inhibition of cyclin-dependent kinase 5 (Cdk5) results in loss of mab1E11 binding whereas inhibition of glycogen synthase kinase 3 has no such effect, revealing that mab1E11 recognizes a Cdk5 phosphorylation site on type-1 phosphorylated form of microtubule-associated protein 1B. We moreover show that kinase Cdk5 as well as its activator P35 is present in retinal ganglion cells in the early developing chick embryo retina and enriched in their extending axons. Cdk5 and P35 are concentrated in the youngest, distal axon region and the growth cone as also seen for Cdk5-phosphorylated type-1 phosphorylated form of microtubule-associated protein 1B. Inhibition of Cdk5 by antibodies or inhibitor Roscovitine results in growth cone collapse and axon retraction and prevents substantial axon outgrowth. In contrast, glycogen synthase kinase 3 inhibition causes only a transient axon retraction which is soon recovered and allows for axon formation. In growth cones induced to turn at substrate borders, where stable and instable parts of the growth cone are clearly defined, Cdk5 is present in the entire growth cone. P35, in contrast, is restricted to the stable parts of the growth cone, which do not collapse but instead transform into new distal axon. The local presence of Cdk5-phosphorylated type-1 phosphorylated form of microtubule-associated protein 1B in stabilized growth cone areas can be therefore attributed to the local activation of Cdk5 by P35 in these regions. Together our data demonstrate a crucial role of Cdk5 and its activator P35 in elongation and maintenance of axons as well as for stability and steering of their growth cones.

Roles of cyclin-dependent kinase 4 and p53 in neuronal cell death induced by doxorubicin on cerebellar granule neurons in mouse

Cell cycle regulators such as cyclin-dependent kinases (Cdks) and their inhibitors (Ckis) have been reported to be involved in neuronal cell death (NCD) induced by a variety of insults such as ischemia, UV-irradiation, nerve growth factor (NGF)-withdrawal, and anticancer therapeutics. But their precise interactive regulation has still to be unveiled. In the present study, we focused on cell cycle regulators such as Cdk4, p21 WAF1 and p53 to clarify their regulatory mechanisms, using NCD induced by doxorubicin (D-NCD) in mouse cerebellar granule neurons as a model. Doxorubicin induced NCD in a dose-dependent manner, a typical feature of apoptosis as determined by TUNEL assay. Doxorubicin increased the protein expression of p53 in time- and dose-dependent manners. The protein expression of p21 WAF1, a Cki of Cdk4, was stimulated by doxorubicin at low concentrations, but it disappeared at high concentrations. Doxorubicin activated the kinase activity of Cdk4 without the enhancement of Cdk4 protein. 3-Amino-9-thio(10 H)-acridone (3-ATA), the specific inhibitor of Cdk4, prevented D-NCD in a dose-dependent manner. Wortmannin, an inhibitor of ATM (ataxia telangiectasia, mutated) that has high homology with the phosphatidyl-inositol-3-kinase (PI3K) family and has protein kinase activity for the induction of p53 with specificity for serine and threonine residues, inhibited the activation of Cdk4 without the induction of p53 in D-NCD. These data suggest that (1) Cdk4 is one of the essential components for inducing NCD, that (2) p53 may prevent D-NCD through the induction of p21 WAF1 at low concentrations of doxorubicin, and that (3) Cdk4 might be activated by the same signal-molecules, like ATM, that are necessary for the activation of p53 in D-NCD.

Regulation of contextual fear conditioning by baseline and inducible septo-hippocampal cyclin-dependent kinase 5

In this work, we confirm the novel role of cyclin-dependent kinase (Cdk) 5 in associative learning by demonstrating that injection of the Cdk5 inhibitor butyrolactone I into the lateral septum or hippocampus profoundly impaired context-dependent fear conditioning of C57BL/6J mice. However, unlike the inducible up-regulation of Cdk5 and its regulator p35 observed in Balb/c mice, high baseline levels, which were not affected by fear conditioning, were found in C57BL/6J mice. Surprisingly, microinjections of butyrolactone I into the lateral septum or hippocampus significantly decreased baseline Cdk5 activity within the entire septo-hippocampal circuitry, suggesting a functional link between septal and hippocampal Cdk5 activity. Significantly higher levels of the transcription factor Sp4 in the septo-hippocampal system of C57BL/6J mice may account for the high baseline Cdk5/p35 production. On the other hand, the stronger cFos production observed in the lateral septum of fear conditioned Balb/c mice may be responsible, at least in part, for the inducible up-regulation of Cdk5 in this strain. These results suggest that the role of Cdk5 in memory consolidation is strain independent and functionally related to the septo-hippocampal circuitry. However, the molecular regulation of baseline and inducible Cdk5 protein might be different among individual mouse strains and possibly other species.

Role for cyclin-dependent kinase 2 in mitosis exit

Mitosis requires cyclin-dependent kinase (cdk) 1-cyclin B activity [1]. Exit from mitosis depends on the inactivation of the complex by the degradation of cyclin B [2]. Cdk2 is also active during mitosis [3, 4]. In Xenopus egg extracts, cdk2 is primarily in complex with cyclin E, which is stable [5]. At the end of mitosis, downregulation of cdk2-cyclin E activity is accompanied by inhibitory phosphorylation of cdk2 [6]. Here, we show that cdk2-cyclin E activity maintains cdk1-cyclin B during mitosis. At mitosis exit, cdk2 is inactivated prior to cdk1. The loss of cdk2 activity follows and depends upon an increase in protein kinase A (PKA) activity. Prematurely inactivating cdk2 advances the time of cyclin B degradation and cdk1 inactivation. Blocking PKA, instead, stabilizes cdk2 activity and inhibits cyclin B degradation and cdk1 inactivation. The stabilization of cdk1-cyclin B is also induced by a mutant cdk2-cyclin E complex that is resistant to inhibitory phosphorylation. P21-Cip1, which inhibits both wild-type and mutant cdk2-cyclin E, reverses mitotic arrest under either condition. Our findings indicate that the proteolysis-independent downregulation of cdk2 activity at the end of mitosis depends on PKA and is required to activate the proteolysis cascade that leads to mitosis exit.

Induction of anergy in Th1 cells associated with increased levels of cyclin-dependent kinase inhibitors p21Cip1 and p27Kip1.

Th1 cells exposed to Ag and the G(1) blocker n-butyrate in primary cultures lose their ability to proliferate in Ag-stimulated secondary cultures. The ability of n-butyrate to induce anergy in Ag-stimulated, but not resting, Th1 cells was shown here to be blocked by cycloheximide. Subsequent experiments to delineate the nature of the protein apparently required for n-butyrate-induced Th1 cell anergy focused on the role of cyclin-dependent kinase (cdk) inhibitors p21(Cip1) and p27(Kip1). Normally, entry into S phase by Th1 cells occurs around 24 h after Ag stimulation and corresponds with relatively low levels of both p21(Cip1) and p27(Kip1). However, unlike control Th1 cells, anergic Th1 cells contained high levels of both p21(Cip1) and p27(Kip1) when examined 24 h after Ag stimulation. The increase in p21(Cip1) observed in Ag-stimulated anergic Th1 cells appeared to be initiated in primary cultures. In contrast, the increase in p27(Kip1) observed in these anergic Th1 cells appears to represent a re-expression of the protein much earlier than control cells following Ag stimulation in secondary cultures. The anergic Th1 cells contained functionally active cdk inhibitors capable of inhibiting the activity of both endogenous and exogenous cdks. Consequently, it appears that n-butyrate-induced anergy in Th1 cells correlated with the up-regulation of p21(Cip1) and perhaps the downstream failure to maintain low levels of p27(Kip1). Increased levels of both p21(Cip1) and p27(Kip1) at the end of G(1) could prevent cdk-mediated entry into S phase, and thus help maintain the proliferative unresponsiveness found in the anergic Th1 cells.

Cyclin-dependent kinase 5 colocalizes with phosphorylated tau in human inclusion-body myositis paired-helical filaments and may play a role in tau phosphorylation

To investigate the possible role of cyclin-dependent kinase 5 (cdk5) in the formation of paired helical filaments (PHFs) in muscle of patients with inclusion-body myositis (IBM), we immunolocalized cdk5, by light- and electron- microscopy, in muscle biopsies of six IBM patients. Approximately 80–90% of IBM vacuolated muscle fibers, and 10–15% of non-vacuolated fibers, contained well defined cdk5-immunoreactive inclusions that colocalized with phosphorylated tau in 70–80% of those fibers. Immunoelectronmicroscopy revealed the association of cdk5 with tau-immunoreactive PHFs. In all biopsies that contained them, regenerating muscle fibers had diffuse, moderate to strong cdk5 immunoreactivity. At all neuromuscular junctions, there was strong cdk5 immunoreactivity postsynaptically. Our study suggests that cdk5: (1) plays a role in IBM pathogenesis, possibly mediating phosphorylation of PHF-related tau; (2) is involved in muscle regeneration; and (3) has a novel function at normal neuromuscular junctions.

Molecular switches that govern the balance between proliferation and apoptosis.

Tissue modelling during embryogenesis and tissue homeostasis during adult life is governed by a dynamic equilibrium between growth and programmed cell death (apoptosis). Growth control and apoptosis are intimately associated, and a disturbance of the balance between these two processes often leads to pathological situations, such as for example cell accumulations in cancer. To date many of the molecular mechanisms controlling growth control on the one hand, and apoptosis on the other hand are known, whereas the switch that controls the decision between both pathways remains elusive. A cell is continuously exposed to multiple opposing "death" and "survival" triggers. A challenging question is how a cell senses these signals and decides to live or die. A decision in favour of survival should automatically result in a shut down of the death pathways. Alternatively, a decision for death should result in inhibition of futile attempts to survive. The molecular events controlling this balance of signals will be discussed with special emphasis on the role of cyclin-dependent kinases and the ubiquitin-dependent and proteasome-mediated protein degradation pathway.

Distinct Roles for PP1 and PP2A in Phosphorylation of the Retinoblastoma Protein: PP2A REGULATES THE ACTIVITIES OF G1 CYCLIN-DEPENDENT KINASES

The function of the retinoblastoma protein (pRB) in controlling the G(1) to S transition is regulated by phosphorylation and dephosphorylation on serine and threonine residues. While the roles of cyclin-dependent kinases in phosphorylating and inactivating pRB have been characterized in detail, the roles of protein phosphatases in regulating the G(1)/S transition are not as well understood. We used cell-permeable inhibitors of protein phosphatases 1 and 2A to assess the contributions of these phosphatases in regulating cyclin-dependent kinase activity and pRB phosphorylation. Treating asynchronously growing Balb/c 3T3 cells with PP2A-selective concentrations of either okadaic acid or calyculin A caused a time- and dose-dependent decrease in pRB phosphorylation. Okadaic acid and calyculin A had no effect on pRB phosphatase activity even though PP2A was completely inhibited. The decrease in pRB phosphorylation correlated with inhibitor-induced suppression of G(1) cyclin-dependent kinases including CDK2, CDK4, and CDK6. The inhibitors also caused decreases in the levels of cyclin D2 and cyclin E, and induction of the cyclin-dependent kinase inhibitors p21(Cip1) and p27(Kip1). The decrease in cyclin-dependent kinase activities were not dependent on induction of cyclin-dependent kinase inhibitors since CDK inhibition still occurred in the presence of actinomycin D or cycloheximide. In contrast, selective inhibition of protein phosphatase 1 with tautomycin inhibited pRB phosphatase activity and maintained pRB in a highly phosphorylated state. The results show that protein phosphatase 1 and protein phosphatase 2A, or 2A-like phosphatases, play distinct roles in regulating pRB function. Protein phosphatase 1 is associated with the direct dephosphorylation of pRB while protein phosphatase 2A is involved in pathways regulating G(1) cyclin-dependent kinase activity.

Yoshio Masui and cell cycle control, past, present and future

Yoshio Masui is a gentle, modest traditional biologist whose contributions have had a major influence on the recent advances that have taken place in understanding how the cell cycle is controlled. His work forms part of a long tradition of experimental zoologists beginning at the end of the last century, who have studied the process of cell division and its control. In this essay I want to place Yoshio Masui's contributions in perspective, as the successor to these early experimentalists, and as the forerunner of the molecular biologists now working on cell cycle control. For a fuller treatment of much of the same historical material, I recommend Yoshio Masui's own scholarly review (Masui (1992) Biochem Cell Biol 70, 920–945). After this I will briefly summarise the central role of cyclin dependent kinases (CDK's) in controlling the cell cycle, following Yoshio Masui's own example by emphasising their biological significance. Finally I will sketch out some of the unsolved problems of the future in this area of research as I see them.

Yoshio Masui and cell cycle control, past, present and future

Yoshio Masui is a gentle, modest traditional biologist whose contributions have had a major influence on the recent advances that have taken place in understanding how the cell cycle is controlled. His work forms part of a long tradition of experimental zoologists beginning at the end of the last century, who have studied the process of cell division and its control. In this essay I want to place Yoshio Masui's contributions in perspective, as the successor to these early experimentalists, and as the forerunner of the molecular biologists now working on cell cycle control. For a fuller treatment of much of the same historical material, I recommend Yoshio Masui's own scholarly review (Masui (1992) Biochem Cell Biol 70, 920–945). After this I will briefly summarise the central role of cyclin dependent kinases (CDK's) in controlling the cell cycle, following Yoshio Masui's own example by emphasising their biological significance. Finally I will sketch out some of the unsolved problems of the future in this area of research as I see them.