p21CIP1 and Cdc25A: competition between an inhibitor and an activator of cyclin-dependent kinases

Positively charged specificity site in cyclin B1 is essential for mitotic fidelity

Phosphorylation of substrates by cyclin-dependent kinases (CDKs) is the driving force of cell cycle progression. Several CDK-activating cyclins are involved, yet how they contribute to substrate specificity is still poorly understood. Here, we discover that a positively charged pocket in cyclin B1, which is exclusively conserved within B-type cyclins and binds phosphorylated serine- or threonine-residues, is essential for correct execution of mitosis. HeLa cells expressing pocket mutant cyclin B1 are strongly delayed in anaphase onset due to multiple defects in mitotic spindle function and timely activation of the E3 ligase APC/C. Pocket integrity is essential for APC/C phosphorylation particularly at non-consensus CDK1 sites and full in vitro ubiquitylation activity. Our results support a model in which cyclin B1's pocket facilitates sequential substrate phosphorylations involving initial priming events that assist subsequent pocket-dependent phosphorylations even at non-consensus CDK1 motifs.

Cryo-EM structure of the CDK2-cyclin A-CDC25A complex

The cell division cycle 25 phosphatases CDC25A, B and C regulate cell cycle transitions by dephosphorylating residues in the conserved glycine-rich loop of CDKs to activate their activity. Here, we present the cryo-EM structure of CDK2-cyclin A in complex with CDC25A at 2.7 Å resolution, providing a detailed structural analysis of the overall complex architecture and key protein-protein interactions that underpin this 86 kDa complex. We further identify a CDC25A C-terminal helix that is critical for complex formation. Sequence conservation analysis suggests CDK1/2-cyclin A, CDK1-cyclin B and CDK2/3-cyclin E are suitable binding partners for CDC25A, whilst CDK4/6-cyclin D complexes appear unlikely substrates. A comparative structural analysis of CDK-containing complexes also confirms the functional importance of the conserved CDK1/2 GDSEID motif. This structure improves our understanding of the roles of CDC25 phosphatases in CDK regulation and may inform the development of CDC25-targeting anticancer strategies.

CIP/KIP and INK4 families as hostages of oncogenic signaling

CIP/KIP and INK4 families of Cyclin-dependent kinase inhibitors (CKIs) are well-established cell cycle regulatory proteins whose canonical function is binding to Cyclin-CDK complexes and altering their function. Initial experiments showed that these proteins negatively regulate cell cycle progression and thus are tumor suppressors in the context of molecular oncology. However, expanded research into the functions of these proteins showed that most of them have non-canonical functions, both cell cycle-dependent and independent, and can even act as tumor enhancers depending on their posttranslational modifications, subcellular localization, and cell state context. This review aims to provide an overview of canonical as well as non-canonical functions of CIP/KIP and INK4 families of CKIs, discuss the potential avenues to promote their tumor suppressor functions instead of tumor enhancing ones, and how they could be utilized to design improved treatment regimens for cancer patients.

Quadra-Stable Dynamics of p53 and PTEN in the DNA Damage Response

Cell fate determination is a complex process that is frequently described as cells traveling on rugged pathways, beginning with DNA damage response (DDR). Tumor protein p53 (p53) and phosphatase and tensin homolog (PTEN) are two critical players in this process. Although both of these proteins are known to be key cell fate regulators, the exact mechanism by which they collaborate in the DDR remains unknown. Thus, we propose a dynamic Boolean network. Our model incorporates experimental data obtained from NSCLC cells and is the first of its kind. Our network's wild-type system shows that DDR activates the G2/M checkpoint, and this triggers a cascade of events, involving p53 and PTEN, that ultimately lead to the four potential phenotypes: cell cycle arrest, senescence, autophagy, and apoptosis (quadra-stable dynamics). The network predictions correspond with the gain-and-loss of function investigations in the additional two cell lines (HeLa and MCF-7). Our findings imply that p53 and PTEN act as molecular switches that activate or deactivate specific pathways to govern cell fate decisions. Thus, our network facilitates the direct investigation of quadruplicate cell fate decisions in DDR. Therefore, we concluded that concurrently controlling PTEN and p53 dynamics may be a viable strategy for enhancing clinical outcomes.

Transcriptome Analysis Identifies Novel Mechanisms Associated with the Antitumor Effect of Chitosan-Stabilized Selenium Nanoparticles

Selenium nanoparticles (SeNPs) have been receiving special attention in recent years due to their antioxidant capacity and antitumor properties. However, the mechanisms associated with these properties remain to be elucidated. For this reason, a global transcriptome analysis has been designed in this work and it was carried out using human hepatocarcinoma cells and chitosan-stabilized SeNPs (Ch-SeNPs) to identify new targets and pathways related to the antitumor mechanisms associated with Ch-SeNPs. The results obtained confirm the alteration of the cell cycle and the effect of Ch-SeNPs on different tumor suppressors and other molecules involved in key mechanisms related to cancer progression. Furthermore, we demonstrated the antioxidant properties of these nanoparticles and their capacity to induce senescence, which was further confirmed through the measurement of β-galactosidase activity.

Multifaceted p21 in carcinogenesis, stemness of tumor and tumor therapy

Cancer cells possess metabolic properties that are different from those of benign cells. p21, encoded by CDKN1A gene, also named p21^Cip1/WAF1, was first identified as a cyclin-dependent kinase regulator that suppresses cell cycle G1/S phase and retinoblastoma protein phosphorylation. CDKN1A (p21) acts as the downstream target gene of TP53 (p53), and its expression is induced by wild-type p53 and it is not associated with mutant p53. p21 has been characterized as a vital regulator that involves multiple cell functions, including G1/S cell cycle progression, cell growth, DNA damage, and cell stemness. In 1994, p21 was found as a tumor suppressor in brain, lung and colon cancer by targeting p53 and was associated with tumorigenesis and metastasis. Notably, p21 plays a significant role in tumor development through p53-dependent and p53-independent pathways. In addition, expression of p21 is closely related to the resting state or terminal differentiation of cells. p21 is also associated with cancer stem cells and acts as a biomarker for such cells. In cancer therapy, given the importance of p21 in regulating the G1/S and G2 check points, it is not surprising that p21 is implicated in response to many cancer treatments and p21 promotes the effect of oncolytic virotherapy.

p21cip1/waf1 Coordinate Autophagy, Proliferation and Apoptosis in Response to Metabolic Stress

Cancer cells possess metabolic properties that are different from benign cells. These unique characteristics have become attractive targets that are being actively investigated for cancer therapy. p21cip1/waf1, also known as Cyclin-Dependent Kinase inhibitor 1A, is encoded by the CDKN1A gene. It is a major p53 target gene involved in cell cycle progression that has been extensively evaluated. To date, p21 has been reported to regulate various cell functions, both dependent and independent of p53. Besides regulating the cell cycle, p21 also modulates apoptosis, induces senescence, and maintains cellular quiescence in response to various stimuli. p21 transcription is induced in response to stresses, including those from oxidative and chemotherapeutic treatment. A recent study has shown that in response to metabolic stresses such as nutrient and energy depletion, p21 expression is induced to regulate various cell functions. Despite the biological significance, the mechanism of p21 regulation in cancer adaptation to metabolic stress is underexplored and thus represents an exciting field. This review focuses on the recent development of p21 regulation in response to metabolic stress and its impact in inducing cell cycle arrest and death in cancer cells.

Protein phosphatases in the regulation of mitosis

The accurate segregation of genetic material to daughter cells during mitosis depends on the precise coordination and regulation of hundreds of proteins by dynamic phosphorylation. Mitotic kinases are major regulators of protein function, but equally important are protein phosphatases that balance their actions, their coordinated activity being essential for accurate chromosome segregation. Phosphoprotein phosphatases (PPPs) that dephosphorylate phosphoserine and phosphothreonine residues are increasingly understood as essential regulators of mitosis. In contrast to kinases, the lack of a pronounced peptide-binding cleft on the catalytic subunit of PPPs suggests that these enzymes are unlikely to be specific. However, recent exciting insights into how mitotic PPPs recognize specific substrates have revealed that they are as specific as kinases. Furthermore, the activities of PPPs are tightly controlled at many levels to ensure that they are active only at the proper time and place. Here, I will discuss substrate selection and regulation of mitotic PPPs focusing mainly on animal cells and explore how these actions control mitosis, as well as important unanswered questions.

Ciclopirox inhibits cancer cell proliferation by suppression of Cdc25A

Ciclopirox olamine (CPX), an off-patent fungicide, has recently been identified as a novel anticancer agent. However, the molecular mechanism underlying its anticancer action remains to be elucidated. Here we show that CPX inhibits cell proliferation in part by downregulating the protein level of Cdc25A in tumor cells. Our studies revealed that CPX did not significantly reduce Cdc25A mRNA level or Cdc25A protein synthesis, but remarkably promoted Cdc25A protein degradation. This resulted in inhibition of G₁-cyclin dependent kinases (CDKs), as evidenced by increased inhibitory phosphorylation of G₁-CDKs. Since Cdc25A degradation is tightly related to its phosphorylation status, we further examined whether CPX alters Cdc25A phosphorylation. The results showed that CPX treatment increased the phosphorylation of Cdc25A (S76 and S82), but only Cdc25A-S82A mutant was resistant to CPX-induced degradation. Furthermore, ectopic expression of Cdc25A-S82A partially conferred resistance to CPX inhibition of cell proliferation. Therefore, our findings indicate that CPX inhibits cell proliferation at least in part by promoting Cdc25A degradation.

Sapheno-Femoral Junction Pathology: Molecular Mechanism of Saphenous Vein Incompetence

A molecular mechanism responsible for varicose vein occurrence was investigated. The role of potential cell cycle regulator p21 and programmed cell death in the pathology leading to the proximal long saphenous vein (LSV) incompetence was investigated. Proximal LSV specimens were obtained from 40 patients with primary varicose veins who had undergone crossectomy. The expression of the p21, p53, and fas encoding genes was investigated by the means of real-time RT-QPCR. Immunostaining for gene product presence, proliferating cell nuclear antigen (PCNA), and apoptotic cells (TUNEL assay) was carried out. The results were compared to the control healthy vein specimens and correlated with pathologic examination findings (of the valve and vein structure). A significant increase in p21, p53, and fas mRNA expression were reported in the proximal incompetent veins. The expression of p21 correlated with expression of p53 (r = 0.658; p<0.05) and negative correlation between media apoptotic index and p21 mRNA expression was found (r = -0.493; p<0.05). Decrease in the muscular component within the media and disturbances of the local structure in the incompetent LSVs were reported. Fas overexpression did not correlate with p53 expression level and did not correlate with apoptotic cell number in the respective vein layers. PCNA-positive cells were present more frequently in the media of the control veins, especially in young subjects. Apoptosis downregulation, cell cycle inhibition and smooth muscle cell hypertrophy are important factors influencing vein wall disturbances related to sapheno-femoral junction incompetence.

miRNA-497 Negatively Regulates the Growth and Motility of Chondrosarcoma Cells by Targeting Cdc25A

Chondrosarcoma (CHS) is the second most common malignant bone sarcoma with increased risk of invasion and metastasis. However, the regulatory mechanisms of CHS tumorigenesis remain unknown. Here we investigated the novel role of miR-497 in regulating chondrosarcoma cell growth and cell cycle arrest. RT-PCR analysis showed that the expression of miR-497 is aberrantly downregulated in human chondrosarcoma samples and cells. After transfection with miR-497 mimic or antagomir, the proliferation and apoptosis of JJ012 and OUMS-27 chondrosarcoma cells were determined by CCK-8 assay and flow cytometric analysis, respectively. Results showed that the proliferation capacity of JJ012 and OUMS-27 cells was significantly decreased by miR-497 overexpression but increased by miR-497 repression. Apoptosis in both cell types was remarkably enhanced by miR-497 mimic but inhibited by miR-497 antagomir. By bioinformatics and luciferase reporter analysis, Cdc25A was proven to be a direct target of miR-497 in chondrosarcoma cells. Further studies indicated that miR-497 modulates the growth of chondrosarcoma cells by targeting Cdc25A, in which the cell cycle inhibitor p21 is involved through a p53-independent pathway. In conclusion, we demonstrated that miR-497 represents a potential tumor suppressor in human chondrosarcoma that regulates the growth of chondrosarcoma cells by targeting Cdc25A. This may provide a novel therapeutic target for chondrosarcoma.

Motif co-regulation and co-operativity are common mechanisms in transcriptional, post-transcriptional and post-translational regulation

A substantial portion of the regulatory interactions in the higher eukaryotic cell are mediated by simple sequence motifs in the regulatory segments of genes and (pre-)mRNAs, and in the intrinsically disordered regions of proteins. Although these regulatory modules are physicochemically distinct, they share an evolutionary plasticity that has facilitated a rapid growth of their use and resulted in their ubiquity in complex organisms. The ease of motif acquisition simplifies access to basal housekeeping functions, facilitates the co-regulation of multiple biomolecules allowing them to respond in a coordinated manner to changes in the cell state, and supports the integration of multiple signals for combinatorial decision-making. Consequently, motifs are indispensable for temporal, spatial, conditional and basal regulation at the transcriptional, post-transcriptional and post-translational level. In this review, we highlight that many of the key regulatory pathways of the cell are recruited by motifs and that the ease of motif acquisition has resulted in large networks of co-regulated biomolecules. We discuss how co-operativity allows simple static motifs to perform the conditional regulation that underlies decision-making in higher eukaryotic biological systems. We observe that each gene and its products have a unique set of DNA, RNA or protein motifs that encode a regulatory program to define the logical circuitry that guides the life cycle of these biomolecules, from transcription to degradation. Finally, we contrast the regulatory properties of protein motifs and the regulatory elements of DNA and (pre-)mRNAs, advocating that co-regulation, co-operativity, and motif-driven regulatory programs are common mechanisms that emerge from the use of simple, evolutionarily plastic regulatory modules.

Cyclin-dependent kinase inhibitor p21(Waf1): contemporary view on its role in senescence and oncogenesis

p21(Waf1) was identified as a protein suppressing cyclin E/A-CDK2 activity and was originally considered as a negative regulator of the cell cycle and a tumor suppressor. It is now considered that p21(Waf1) has alternative functions, and the view of its role in cellular processes has begun to change. At present, p21(Waf1) is known to be involved in regulation of fundamental cellular programs: cell proliferation, differentiation, migration, senescence, and apoptosis. In fact, it not only exhibits antioncogenic, but also oncogenic properties. This review provides a contemporary understanding of the functions of p21(Waf1) depending on its intracellular localization. On one hand, when in the nucleus, it serves as a negative cell cycle regulator and tumor suppressor, in particular by participating in the launch of a senescence program. On the other hand, when p21(Waf1) is localized in the cytoplasm, it acts as an oncogene by regulating migration, apoptosis, and proliferation.

Tumor suppressors and cell-cycle proteins in lung cancer

The cell cycle is the cascade of events that allows a growing cell to duplicate all its components and split into two daughter cells. Cell cycle progression is mediated by the activation of a highly conserved family of protein kinases, the cyclin-dependent kinases (CDKs). CDKs are also regulated by related proteins called cdk inhibitors grouped into two families: the INK4 inhibitors (p16, p15, p19, and p18) and the Cip/Kip inhibitors (p21, p27, and p53). Several studies report the importance of cell-cycle proteins in the pathogenesis and the prognosis of lung cancer. This paper will review the most recent data from the literature about the regulation of cell cycle. Finally, based essentially on the data generated in our laboratory, the expression, the diagnostic, and prognostic significance of cell-cycle molecules in lung cancer will be examined.

Identification of substrates of an S-phase cell cycle kinase from Leishmania donovani

Despite the importance of cyclin-Cdk related kinases (CRK) in regulation of cell and life cycle of kinetoplastida parasites, only limited knowledge about their substrates are presently available. Here, the potential substrates were searched for an S-phase LdCyc1-CRK3 complex from Leishmania donovani based on the presence of Cdk target phosphorylation site together with the cyclin interacting Cy-motif in genome-derived putative protein sequences. Three substrates could be identified with one of them being a unique protein with no known homologues. Another identified substrate is similar to MYST family of histone acetyl transferase and the third one contains Ku-70 related conserved domains. All the substrates interact directly with LdCyc1 and are phosphorylated in a Cy-motif dependent manner suggesting the importance of Cy-motif for their functions.

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

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

Pro-apoptotic role of Cdc25A: activation of cyclin B1/Cdc2 by the Cdc25A C-terminal domain

Cdc25A is a dual specificity protein phosphatase that activates cyclin/cyclin-dependent protein kinase (Cdk) complexes by removing inhibitory phosphates from conserved threonine and tyrosine in Cdks. To address how Cdc25A promotes apoptosis, Jurkat cells were treated with staurosporine, an apoptosis inducer. Upon staurosporine treatment, a Cdc25A C-terminal 37-kDa fragment, designated C37, was generated by caspase cleavage at Asp-223. Thr-507 in C37 became dephosphorylated, which prevented 14-3-3 binding, as shown previously. C37 exhibited higher phosphatase activity than full-length Cdc25A. C37 with alanine substitution for Thr-507 (C37/T507A) that imitated the cleavage product during staurosporine treatment interacted with Cdc2, Cdk2, cyclin A, and cyclin B1 and markedly activated cyclin B1/Cdc2. The dephosphorylation of Thr-507 might expose the Cdc2/Cdk2-docking site in C37. C37/T507A also induced apoptosis in Jurkat and K562 cells, resulting from activating cyclin B1/Cdc2 but not Cdk2. Thus, this study reveals that Cdc25A is a pro-apoptotic protein that amplifies staurosporine-induced apoptosis through the activation of cyclin B1/Cdc2 by its C-terminal domain.

p21 in cancer: intricate networks and multiple activities

One of the main engines that drives cellular transformation is the loss of proper control of the mammalian cell cycle. The cyclin-dependent kinase inhibitor p21 (also known as p21WAF1/Cip1) promotes cell cycle arrest in response to many stimuli. It is well positioned to function as both a sensor and an effector of multiple anti-proliferative signals. This Review focuses on recent advances in our understanding of the regulation of p21 and its biological functions with emphasis on its p53-independent tumour suppressor activities and paradoxical tumour-promoting activities, and their implications in cancer.

Cdc25A serine 123 phosphorylation couples centrosome duplication with DNA replication and regulates tumorigenesis

The cell division cycle 25A (Cdc25A) phosphatase is a critical regulator of cell cycle progression under normal conditions and after stress. Stress-induced degradation of Cdc25A has been proposed as a major way of delaying cell cycle progression. In vitro studies pointed toward serine 123 as a key site in regulation of Cdc25A stability after exposure to ionizing radiation (IR). To address the role of this phosphorylation site in vivo, we generated a knock-in mouse in which alanine was substituted for serine 123. The Cdc25 S123A knock-in mice appeared normal, and, unexpectedly, cells derived from them exhibited unperturbed cell cycle and DNA damage responses. In turn, we found that Cdc25A was present in centrosomes and that Cdc25A levels were not reduced after IR in knock-in cells. This resulted in centrosome amplification due to lack of induction of Cdk2 inhibitory phosphorylation after IR specifically in centrosomes. Further, Cdc25A knock-in animals appeared sensitive to IR-induced carcinogenesis. Our findings indicate that Cdc25A S123 phosphorylation is crucial for coupling centrosome duplication to DNA replication cycles after DNA damage and therefore is likely to play a role in the regulation of tumorigenesis.

ATP-dependent activation of p21WAF1/CIP1-associated Cdk2 by Cdc6

When cells progressing in mid-S phase are damaged with a base-modifying chemical, they arrest in S phase long after the CHK1 checkpoint signal fades out, partly because of p53-mediated long-lasting induction of the cyclin-dependent kinase inhibitor p21(WAF1/CIP1). We have recently found that enforced expression of Cdc6, the assembler of prereplicative complexes, markedly advances recovery from the prolonged S-phase arrest and reactivation of Cdk2 despite the presence of a high level of induced p21. Here, we report that Cdc6 protein can activate p21-associated Cdk2 in an ATP-dependent manner in vitro. Consistently, Cdc6 mutated for ATPase or a putative cyclin binding motif is no longer able to activate the Cdk2 in vitro or promote reinitiation of S-phase progression and reactivation of Cdk2 in vivo. These results reveal the never anticipated function of Cdc6 and redefine its role in the control of S-phase progression in mammalian cells.

NanoLC-MS/MS analysis provides new insights into the phosphorylation pattern of Cdc25B in vivo: full overlap with sites of phosphorylation by Chk1 and Cdk1/cycB kinases in vitro

NanoLC-MS/MS analysis was used to characterize the phosphorylation pattern in vivo of CDC25B3 (phosphatase splice variant 1) expressed in a human cell line and to compare it to the phosphorylation of CDC25B3 by Cdk1/cyclin B and Chk1 in vitro. Cellular CDC25B3 was purified from U2OS cells conditionally overexpressing the phosphatase. Eighteen sites were detectably phosphorylated in vivo. Nearly all existing (S/T)P sites were phosphorylated in vivo and in vitro. Eight non(S/T)P sites were phosphorylated in vivo. All these sites could be phosphorylated by kinase Chk1, which phosphorylated a total of 11 sites in vitro, with consensus sequence (R/K) X(2-3) (S/P)-non P. Nearly half of the sites identified in this study were not previously described and were not homologous to sites reported to be phosphorylated in other CDC25 species. We also show that in vivo a significant part of CDC25B molecules can be hyperphosphorylated, with up to 13 phosphates per phosphatase molecule.

Use of hyperosmolar stress to measure stress-activated protein kinase activation and function in human HTR cells and mouse trophoblast stem cells

Embryo growth is inversely correlated with hyperosmolar stress-induced stress-activated protein kinase/jun kinase (SAPK/JNK) induction. To examine whether stress has similar effects in stem cells derived from the embryo, the authors test trophoblast stem cells. The stress response of human placental and mouse trophoblast stem cell lines are tested here. Peak phosphorylated SAPK/JNK was induced by 400 mM sorbitol at 0.5 hours. At this dose, there is an SAPK/JNK-dependent decrease in mitogenic, phosphorylated cMyc at 0.5 hours preceding an SAPK/JNK-dependent decrease in cell cycle entrance at 24 hours. At 0.5 hours, SAPK/JNK decreases terminal deoxynucleotidyltransferase dUTP nick end labeling/apoptosis at sorbitol doses from 50 mM to 400 mM and induces phosphorylated cJun prior to an SAPK/JNK-dependent, approximate 8-fold increase in apoptosis by 24 hours at 400 mM. SAPK/JNK phosphorylation peaked at 0.5 to 4 hours and largely subsided by 12 hours. Thus, total SAPK/JNK exists before stress and mediates rapid, homeostatic molecular responses that become biologic consequences after phosphorylated SAPK/JNK ends. This suggests continuity in the homeostatic mechanisms and functions of SAPK/JNK in placental lineage cells during implantation, in which SAPK/JNK is completely responsible for cell cycle arrest and largely responsible for apoptosis.

Leishmania donovani cyclin 1 (LdCyc1) forms a complex with cell cycle kinase subunit CRK3 (LdCRK3) and is possibly involved in S-phase-related activities

Expression of Leishmania donovani cyclin 1 (LdCyc1) mRNA during the cell cycle of promastigotes is S-phase specific. Here, we show that the LdCyc1 protein is periodically expressed and the activity of its associated kinase varies during the cell cycle in line with its expression pattern. In addition, we have shown that LdCRK3, homologous to CRK3 from L. mexicana, is the cognate Cdk partner of LdCyc1 and that the activity of the complex is inhibited specifically by heat stable factor(s) from the parasite.

Galectin-4 is involved in p27-mediated activation of the myelin basic protein promoter

Our previous studies have found that expression of p27 in oligodendrocytes enhances myelin basic protein (MBP) gene expression through a mechanism that involves the transcription factor Sp1. In this study we show that this activation only requires the N-terminal 45 amino acids of p27 containing a functional cyclin-binding motif. In an effort to identify other cofactors that are involved in the p27-mediated activation of MBP gene expression, a yeast two-hybrid assay was performed using an N-terminal truncated p27 and a mouse embryo cDNA library. Galectin-4 was found to interact with p27 in the yeast two-hybrid assay. This novel interaction was also confirmed using a glutathione-S-transferase interaction assay and immunoprecipitation assays. Expression of galectin-4 in primary oligodendrocytes was confirmed by western blot. Additionally, the MBP promoter could be activated by expression of galectin-4 in CG4 oligodendrocytes, similar to the effects of increased p27 levels. We also show that Sp1 and galectin-4 interact in cells, while a complex of all three proteins could not be found. We conclude that galectin-4 is involved in the p27-mediated activation of the MBP gene, possibly through modulation of the glycosylation status of the transcription factor Sp1.

Cdc25 phosphatases: structure, specificity, and mechanism

Cdc25 phosphatases, as activators of the Cdk/cyclins, play critical roles in the regulation of the eukaryotic cell cycle. Because of their overexpression and correlation with poor prognosis in many diverse cancers, Cdc25 phosphatases are attractive targets for anticancer drug development. Over the past few years, much knowledge of the basic enzymology of the Cdc25 phosphatases that may aid in the development of specific inhibitors has been gained. We review herein the structure, specificity, and mechanism of the Cdc25 phosphatases with a special focus on the activity of Cdc25 phosphatases with native protein substrates.

Biochemical characterization of Cdk2-Speedy/Ringo A2

Background

Normal cell cycle progression requires the precise activation and inactivation of cyclin-dependent protein kinases (CDKs), which consist of a CDK and a cyclin subunit. A novel cell cycle regulator called Speedy/Ringo shows no sequence similarity to cyclins, yet can directly bind to and activate CDKs. Speedy/Ringo proteins, which bind to and activate Cdc2 and Cdk2 in vitro, are required for the G2 to M transition during Xenopus oocyte maturation and for normal S-phase entry in cultured human cells.

Results

We have characterized the substrate specificity and enzymatic activity of human Cdk2-Speedy/Ringo A2 in order to gain insights into the possible functions of this complex. In contrast to Cdk2-cyclin A, which has a well-defined consensus target site ((S/T)PX(K/R)) that strongly favors substrates containing a lysine at the +3 position of substrates, Cdk2-Speedy/Ringo A2 displayed a broad substrate specificity at this position. Consequently, Cdk2-Ringo/Speedy A2 phosphorylated optimal Cdk2 substrates such as histone H1 and a KSPRK peptide poorly, only ~0.08% as well as Cdk2-cyclin A, but non-canonical Cdk2 substrates such as a KSPRY peptide relatively well, with an efficiency of ~80% compared to Cdk2-cyclin A. Cdk2-Speedy/Ringo A2 also phosphorylated authentic Cdk2 substrates, such as Cdc25 proteins, which contain non-canonical CDK phosphorylation sites, nearly as well as Cdk2-cyclin A. Phosphopeptide mapping indicated that Cdk2-Speedy/Ringo A2 and Cdk2-cyclin A phosphorylate distinct subsets of sites on Cdc25 proteins. Thus, the low activity that Cdk2-Speedy/Ringo A2 displays when assayed on conventional Cdk2 substrates may significantly underestimate the potential physiological importance of Cdk2-Speedy/Ringo A2 in phosphorylating key subsets of Cdk2 substrates. Unlike Cdk2-cyclin A, whose activity depends strongly on activating phosphorylation of Cdk2 on Thr-160, neither the overall catalytic activity nor the substrate recognition by Cdk2-Speedy/Ringo A2 was significantly affected by this phosphorylation. Furthermore, Cdk2-Speedy/Ringo A2 was not a suitable substrate for metazoan CAK (which phosphorylates Cdk2 at Thr-160), supporting the notion that Speedy/Ringo A2 activates Cdk2 in a CAK-independent manner.

Conclusion

There are major differences in substrate preferences between CDK-Speedy/Ringo A2 and Cdk2-cyclin complexes. These differences may accommodate the CAK-independent activation of Cdk2 by Speedy/Ringo A2 and they raise the possibility that CDK-Speedy/Ringo A2 complexes could phosphorylate and regulate a subset of non-canonical CDK substrates, such as Cdc25 protein phosphatases, to control cell cycle progression.

Chk1, but not Chk2, inhibits Cdc25 phosphatases by a novel common mechanism

Cdc25 phosphatases activate cyclin-dependent kinases (Cdks) and thereby promote cell cycle progression. In vertebrates, Chk1 and Chk2 phosphorylate Cdc25A at multiple N-terminal sites and target it for rapid degradation in response to genotoxic stress. Here we show that Chk1, but not Chk2, phosphorylates Xenopus Cdc25A at a novel C-terminal site (Thr504) and inhibits it from C-terminally interacting with various Cdk-cyclin complexes, including Cdk1-cyclin A, Cdk1-cyclin B, and Cdk2-cyclin E. Strikingly, this inhibition, rather than degradation itself, of Cdc25A is essential for the Chk1-induced cell cycle arrest and the DNA replication checkpoint in early embryos. 14-3-3 proteins bind to Chk1-phosphorylated Thr504, but this binding is not required for the inhibitory effect of Thr504 phosphorylation. A C-terminal site presumably equivalent to Thr504 exists in all known Cdc25 family members from yeast to humans, and its phosphorylation by Chk1 (but not Chk2) can also inhibit all examined Cdc25 family members from C-terminally interacting with their Cdk-cyclin substrates. Thus, Chk1 but not Chk2 seems to inhibit virtually all Cdc25 phosphatases by a novel common mechanism.

p21-Mediated nuclear retention of cyclin B1-Cdk1 in response to genotoxic stress

G2 arrest of cells suffering DNA damage in S phase is crucial to avoid their entry into mitosis, with the concomitant risks of oncogenic transformation. According to the current model, signals elicited by DNA damage prevent mitosis by inhibiting both activation and nuclear import of cyclin B1-Cdk1, a master mitotic regulator. We now show that normal human fibroblasts use additional mechanisms to block activation of cyclin B1-Cdk1. In these cells, exposure to nonrepairable DNA damage leads to nuclear accumulation of inactive cyclin B1-Cdk1 complexes. This nuclear retention, which strictly depends on association with endogenous p21, prevents activation of cyclin B1-Cdk1 by Cdc25 and Cdk-activating kinase as well as its recruitment to the centrosome. In p21-deficient normal human fibroblasts and immortal cell lines, cyclin B1 fails to accumulate in the nucleus and could be readily detected at the centrosome in response to DNA damage. Therefore, in normal cells, p21 exerts a dual role in mediating DNA damage-induced cell cycle arrest and exit before mitosis. In addition to blocking pRb phosphorylation, p21 directly prevents mitosis by inactivating and maintaining the inactive state of mitotic cyclin-Cdk complexes. This, with subsequent degradation of mitotic cyclins, further contributes to the establishment of a permanent G2 arrest.

Subcellular localisation of Cdc25A determines cell fate

Cell division cycle 25A (Cdc25A) was shown to colocalise both with nuclear and cytoplasmic proteins. Recently, we have demonstrated that overexpressed Cdc25A promoted the survival of rat 423 cells through indirect activation of PKB-protein kinase B. Using a Cdc25A:ER fusion protein, which can be shuttled from the cytoplasm into the nucleus, the present investigation evidences that the antiapoptotic effect of Cdc25A was restricted to its cytoplasmic localisation in rat 423 cells. In contrast, nuclear Cdc25A overexpression caused dephosphorylation and nuclear retention of the proapoptotic transcription factor Forkhead in rhabdomyosarcoma-like 1 (FKHRL1) in human N.1 ovarian carcinoma cells. This resulted in the increased constitutive expression of the FKHRL1 targets Fas ligand and Bim, and promoted apoptosis. Thus, the Cdc25A oncogene, which was found to be frequently overexpressed in certain human cancers, can increase or decrease the susceptibility to apoptosis depending on the cell-type-specific subcellular distribution.

Isolation, characterization and expression of a cyclin from Leishmania donovani

We have cloned and sequenced a DNA fragment (approximately 1 kb) containing a complete open reading frame from a cDNA library of Leishmania donovani promastigotes. The alignment of the derived polypeptide sequence and the modeling studies revealed that the protein is highly homologous to the mammalian cyclins having conserved cyclin box and substrate-docking motif. Northern blot analysis of the RNA isolated from synchronized L. donovani promastigotes showed periodic expression of the message with maximum abundance at S-phase suggesting its involvement in the events related to the regulation of DNA replication. The results confirm that we have isolated a cyclin molecule from L. donovani (LdCyc1) which may play an important role in the regulation of the parasite cell cycle.

TGF-beta-induced RhoA and p160ROCK activation is involved in the inhibition of Cdc25A with resultant cell-cycle arrest

The ability of the transforming growth factor beta (TGF-beta) signaling pathways to inhibit proliferation of most cells while stimulating proliferation of others remains a conundrum. In this article, we report that the absence of RhoA and p160ROCK activity in fibroblastic NIH 3T3 cells and its presence in epithelial NMuMG cells can at least partially explain the difference in the TGF-beta growth response. Further, evidence is presented for TGF-beta-stimulated p160ROCK translocation to the nucleus and inhibitory phosphorylation of the cyclin-dependent kinase-activating phosphatase, Cdc25A. The resultant suppression of Cdk2 activity contributes to G1/S inhibition in NMuMG cells. These data provide evidence that signaling through RhoA and p160ROCK is important in TGF-beta inhibition of cell proliferation and links signaling components for epithelial transdifferentiation with regulation of cell-cycle progression.

Chk1 kinase negatively regulates mitotic function of Cdc25A phosphatase through 14-3-3 binding

The order and fidelity of cell cycle events in mammals is intimately linked to the integrity of the Chk1 kinase-Cdc25A phosphatase pathway. Chk1 phosphorylation targets Cdc25A for destruction and, as shown here, inhibits interactions between Cdc25A and its mitotic substrate cyclin B1-Cdk1. Phosphorylation of Cdc25A on serine 178 and threonine 507 facilitates 14-3-3 binding, and Chk1 phosphorylates both residues in vitro. Mutation of T507 to alanine (T507A) enhanced the biological activity of Cdc25A. Cdc25A(T507A) was more efficient in binding to cyclin B1, activating cyclin B1-Cdk1, and promoting premature entry into mitosis. We propose that the Chk1/Cdc25A/14-3-3 pathway functions to prevent cells from entering into mitosis prior to replicating their genomes to ensure the fidelity of the cell division process.

Phosphorylations of cyclin-dependent kinase 2 revisited using two-dimensional gel electrophoresis

To control the G1/S transition and the progression through the S phase, the activation of the cyclin-dependent kinase (CDK) 2 involves the binding of cyclin E then cyclin A, the activating Thr-160 phosphorylation within the T-loop by CDK-activating kinase (CAK), inhibitory phosphorylations within the ATP binding region at Tyr-15 and Thr-14, dephosphorylation of these sites by cdc25A, and release from Cip/Kip family (p27kip1 and p21cip1) CDK inhibitors. To re-assess the precise relationship between the different phosphorylations of CDK2, and the influence of cyclins and CDK inhibitors upon them, we introduce here the use of the high resolution power of two-dimensional gel electrophoresis, combined to Tyr-15- or Thr-160-phosphospecific antibodies. The relative proportions of the potentially active forms of CDK2 (phosphorylated at Thr-160 but not Tyr-15) and inactive forms (non-phosphorylated, phosphorylated only at Tyr-15, or at both Tyr-15 and Thr-160), and their respective association with cyclin E, cyclin A, p21, and p27, were demonstrated during the mitogenic stimulation of normal human fibroblasts. Novel observations modify the current model of the sequential CDK2 activation process: (i) Tyr-15 phosphorylation induced by serum was not restricted to cyclin-bound CDK2; (ii) Thr-160 phosphorylation engaged the entirety of Tyr-15-phosphorylated CDK2 associated not only with a cyclin but also with p27 and p21, suggesting that Cip/Kip proteins do not prevent CDK2 activity by impairing its phosphorylation by CAK; (iii) the potentially active CDK2 phosphorylated at Thr-160 but not Tyr-15 represented a tiny fraction of total CDK2 and a minor fraction of cyclin A-bound CDK2, underscoring the rate-limiting role of Tyr-15 dephosphorylation by cdc25A.

The association of cyclin A and cyclin kinase inhibitor p21 in response to γ-irradiation requires the CDK2 binding region, but not the Cy motif

The cyclin kinase inhibitor p21 associates with and inhibits cyclin-CDKs to retard the progress of the cell cycle in response to DNA damage. The recognition sites for cyclin binding on the various cell cycle-related molecules have been identified as RXL motifs. In the case of p21, the dependence of the Cy1 (18CRRL) or Cy2 (154KRRL) motifs on cyclin E, but not on cyclin A has been demonstrated by in vitro experiments. In this study, to clarify the mechanism of p21 association with cyclin A, we constructed a p21 expression system in mammalian cells. After transfection with an expression vector containing cDNA of various p21-mutants, cells were irradiated with 10 Gy of gamma-rays to introduce DNA damage, followed by quantification of the p21-cyclin A association. The p21-mutant constructs were single or multiple deletions in Cy1, Cy2, and the CDK2 binding region, and a nonphosphorylatable alanine mutant of the C-terminal phosphorylation site. We demonstrated that the association of p21 and cyclin A in response to gamma-irradiation requires the CDK binding region, 49-71 aa, but not the Cy motifs. We believe the mechanism by which p21 inhibits cyclin-CDKs is distinct in each phase of the cell cycle. Furthermore, the increase in the association of p21 and cyclin A was not correlated with the levels of p21. This suggests that DNA damage triggers a signal to the p21 region between 21 and 96 aa to allow cyclin A association.

The RRASK motif in Xenopus cyclin B2 is required for the substrate recognition of Cdc25C by the cyclin B-Cdc2 complex

The FLRRXSK sequence is conserved in the second cyclin box fold of B-type cyclins. We show that this conserved sequence in Xenopus cyclin B2, termed the RRASK motif, is required for the substrate recognition by the cyclin B-Cdc2 complex of Cdc25C. Mutations to charged residues of the RRASK motif of cyclin B2 abolished its ability to activate Cdc2 kinase without affecting its capacity to bind to Cdc2. Cdc2 bound to the cyclin B2 RRASK mutant was not dephosphorylated by Cdc25C, and as a result, the complex was inactive. The cyclin B2 RRASK mutants can form a complex with the constitutively active Cdc2, but a resulting active complex did not phosphorylate a preferred substrate Cdc25C in vitro, although it can phosphorylate the non-specific substrate histone H1. The RRASK mutations prevented the interaction of Cdc25C with the cyclin B2-Cdc2 complex. Consistently, the RRASK mutants neither induced germinal vesicle breakdown in Xenopus oocyte maturation nor activated in vivo Cdc2 kinase during the cell cycle in mitotic extracts. These results suggest that the RRASK motif in Xenopus cyclin B2 plays an important role in defining the substrate specificity of the cyclin B-Cdc2 complex.

Differential gene expression underlying the functional distinctions of primary human CD34+ hematopoietic stem and progenitor cells from peripheral blood and bone marrow

The restorative capacity of human CD34(+) hematopoietic cells is clinically used in the autologous and allogeneic transplant setting to support cytotoxic therapy. We examined gene expression patterns of highly enriched bone marrow CD34(+) (BM-CD34(+)) or G-CSF-mobilized peripheral blood CD34(+) (PB-CD34(+)) cells by cDNA array technology, quantitative real-time RT-PCR, and flow cytometry, to identify molecular causes underlying the functional differences between circulating and sedentary hematopoietic stem and progenitor cells. The greater cell cycle and DNA synthesis activity of BM-CD34(+) compared to PB-CD34(+) cells was reflected by the 2- to 5-fold higher expression of 9 genes involved in cell cycle, 11 genes regulating DNA synthesis, and the cell cycle-initiating transcription factor E2F-1. The 2- to 3-fold greater expression of 5 pro-apoptotic genes in PB-CD34(+) cells indicated a higher apoptotic activity, which could functionally be corroborated by apoptosis assays. Thrombin receptor (PAR1), known to play a role in trafficking of malignant cells, was 3.6-fold higher expressed in circulating CD34(+) cells than in BM-CD34(+) cells. Guidance via thrombin receptor might molecularly mediate stem cell migration. In summary, our study provides gene expression profiles of primary human CD34(+) hematopoietic cells of blood and marrow. Our data molecularly confirm and explain the finding that CD34(+) cells residing in the bone marrow are cycling more rapidly, whereas circulating CD34(+) cells consist of a higher number of quiescent stem and progenitor cells. Moreover, our data give novel molecular insights into stem cell migration and differentiation.

Cycling at the interface between neurodevelopment and neurodegeneration

The discovery of cell cycle regulators has directed cell research into uncharted territory. In dividing cells, cell cycle-associated protein kinases, which are referred to as cyclin-dependent-kinases (Cdks), regulate proliferation, differentiation, senescence and apoptosis. In contrast, all Cdks in post-mitotic neurons, with the notable exception of Cdk5, are silenced. Surprisingly, misregulation of Cdks occurs in neurons in a wide diversity of neurological disorders, including Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis. Ectopic expression of these proteins in neurons potently induces cell death with hallmarks of apoptosis. Deregulation of the unique, cell cycle-unrelated Cdk5 by its truncated co-activator, p25 and p29, contributes to neurodegeneration by altering the phosphorylation state of non-membrane-associated proteins and possibly through the induction of cell cycle proteins. On the other hand, cycling Cdks such as Cdk2, Cdk4 and Cdk6, initiate death pathways by derepressing E2F-1/Rb-dependent transcription at the neuronal G1/S checkpoint. Thus, Cdk5 and cycling Cdks may have little in common in the healthy CNS, but they likely conspire in leading neurons to their demise.

K vitamins, PTP antagonism, and cell growth arrest

The main function of K vitamins is to act as co-factors for gamma-glutamyl carboxylase. However, they have also recently been shown to inhibit cell growth. We have chemically synthesized a series of K vitamin analogs with various side chains at the 2 or 3 position of the core naphthoquinone structure. The analogs with short thio-ethanol side chains are found to be more potent growth inhibitors in vitro of various tumor cell lines. Cpd 5 or [2-(2-mercaptoethanol)-3-methyl-1,4-naphthoquinone] is one of the most potent. The anti-proliferation activity of these compounds is antagonized by exogenous thiols but not by non-thiol antioxidants. This suggests that the growth inhibition is mediated by sulfhydryl arylation of cellular glutathione and cysteine-containing proteins and not by oxidative stress. The protein tyrosine phosphatases (PTP) are an important group of proteins that contain cysteine at their catalytic site. PTPs regulate mitogenic signal transduction and cell cycle progression. PTP inhibition by Cpd 5 results in prolonged tyrosine phosphorylation and activation of several kinases and transcription factors including EGFR, ERK1/2, and Elk1. Cpd 5 could activate ERK1/2 either by signaling from an activated EGFR, which is upstream in the signaling cascade, or by direct inhibition of ERK1/2 phosphatase(s). Prolonged ERK1/2 phosphorylation strongly correlates with Cpd 5-mediated growth inhibition. Cpd 5 can also bind to and inhibit the Cdc25 family of dual specific phosphatases. As a result, several Cdc25 substrates (Cdk1, Cdk2, Cdk4) involved in cell cycle progression are tyrosine phosphorylated and thereby inhibited by its action. Cpd 5 could also inhibit both normal liver regeneration and hepatoma growth in vivo. DNA synthesis during rat liver regeneration following partial hepatectomy, transplantable rat hepatoma cell growth, and glutathione-S-transferase-pi expressing hepatocytes after administration of the chemical carcinogen diethylnitrosamine, are all inhibited by Cpd 5 administration. The growth inhibitory effect during liver regeneration and transplantable tumor growth is also correlated with ERK1/2 phosphorylation induced by Cpd 5. Thus, Cpd 5-mediated inhibition of PTPs, such as Cdc25 leads to cell growth arrest due to altered activity of key cellular kinases involved in signal transduction and cell cycle progression. This prototype K vitamin analog represents a novel class of growth inhibitor based upon its action as a selective PTP antagonist. It is clearly associated with prolonged ERK1/2 phosphorylation, which is in contrast with the transient ERK1/2 phosphorylation induced by growth stimulatory mitogens.

Complementary functions of the antiapoptotic protein A1 and serine/threonine kinase pim-1 in the BCR/ABL-mediated leukemogenesis

BCR/ABL oncogenic tyrosine kinase activates STAT5, which plays an important role in leukemogenesis. The downstream effectors of the BCR/ABL-->STAT5 pathway remain poorly defined. We show here that expression of the antiapoptotic protein A1, a member of the Bcl-2 family, and the serine/threonine kinase pim-1 are enhanced by BCR/ABL. This up-regulation requires activation of STAT5 by the signaling from SH3+SH2 domains of BCR/ABL. Enhanced expression of A1 and pim-1 played a key role in the BCR/ABL-mediated cell protection from apoptosis. In addition, pim-1 promoted proliferation of the BCR/ABL-transformed cells. Both A1 and pim-1 were required to induce interleukin 3-independent cell growth, inhibit activation of caspase 3, and stimulate cell cycle progression. Moreover, simultaneous up-regulation of both A1 and pim-1 was essential for in vitro transformation and in vivo leukemogenesis mediated by BCR/ABL. These data indicate that induction of A1 and pim-1 expression may play a critical role in the BCR/ABL-dependent transformation.

The p21(Cip1) protein, a cyclin inhibitor, regulates the levels and the intracellular localization of CDC25A in mice regenerating livers

Liver cells from p21(Cip1-/-) mice subjected to partial hepatectomy (PH) progress into DNA synthesis faster than those from wild-type mice. These cells also show a premature induction of cyclin E/cyclin-dependent kinase (CDK) 2 activity. We studied the mechanisms whereby cells lacking p21(Cip1) showed a premature induction of this activity. Whereas the levels of CDK2, cyclin E, and p27(Kip1) were similar in both wild-type and p21(Cip1-/-) mice, those of the activator CDC25A were much higher in p21(Cip1-/-) quiescent and regenerating livers than in wild-type animals. Moreover, p21(Cip1-/-) cells also showed a premature translocation of CDC25A from cytoplasm into the nucleus. The ectopic expression of p21(Cip1) into mice embryo fibroblasts from p21(Cip1-/-) mice decreased the levels of CDC25A and delayed its nuclear translocation. The levels of CDC25A messenger RNA in p21(Cip1-/-) cells were higher than in wild-type cells, suggesting that this increase might be responsible, at least in part, for the high levels of CDC25A protein in these cells. Thus, the results reported here indicate that p21(Cip1) regulates the levels and the intracellular localization of CDC25A. We also found a good correlation between CDC25A nuclear translocation and cyclin E/CDK2 activation. In conclusion, premature translocation of CDC25A to the nucleus might be involved in the advanced induction of cyclin E/CDK2 activity and DNA replication in cells from animals lacking p21(Cip1).

Reversal of growth suppression by p107 via direct phosphorylation by cyclin D1/cyclin-dependent kinase 4

p107 functions to control cell division and development through interaction with members of the E2F family of transcription factors. p107 is phosphorylated in a cell cycle-regulated manner, and its phosphorylation leads to its release from E2F. Although it is known that p107 physically associates with E- and A-type cyclin/cyclin-dependent kinase 2 (Cdk2) complexes through a cyclin-binding RXL motif located in the spacer domain, the mechanisms underlying p107 inactivation via phosphorylation remain poorly defined. Recent genetic evidence indicates a requirement for cyclin D1/Cdk4 complexes in p107 inactivation. In this work, we provide direct biochemical evidence for the involvement of cyclin D1/Cdk4 in the inactivation of p107's growth-suppressive function. While coexpression of cyclin D1/Cdk4 can reverse the cell cycle arrest properties of p107 in Saos-2 cells, we find that p107 in which the Lys-Arg-Arg-Leu sequence of the RXL motif is replaced by four alanine residues is largely refractory to inactivation by cyclin D/Cdk4, indicating a role for this motif in p107 inactivation without a requirement for its tight interaction with cyclin D1/Cdk4. We identified four phosphorylation sites in p107 (Thr-369, Ser-640, Ser-964, and Ser-975) that are efficiently phosphorylated by Cdk4 but not by Cdk2 in vitro and are also phosphorylated in tissue culture cells. Growth suppression by p107 containing nonphosphorylatable residues in these four sites is not reversed by coexpression of cyclin D1/Cdk4. In model p107 spacer region peptides, phosphorylation of S640 by cyclin D1/Cdk4 is strictly dependent upon an intact RXL motif, but phosphorylation of this site in the absence of an RXL motif can be partially restored by replacement of S643 by arginine. This suggests that one role for the RXL motif is to facilitate phosphorylation of nonconsensus Cdk substrates. Taken together, these data indicate that p107 is inactivated by cyclin D1/Cdk4 via direct phosphorylation and that the RXL motif of p107 plays a role in its inactivation by Cdk4 in the absence of stable binding.

Gene expression profiling identifies significant differences between the molecular phenotypes of bone marrow-derived and circulating human CD34+ hematopoietic stem cells

CD34+ hematopoietic stem cells are used clinically to support cytotoxic therapy, and recent studies raised hope that they could even serve as a cellular source for nonhematopoietic tissue engineering. Here, we examined in 18 volunteers the gene expressions of 1185 genes in highly enriched bone marrow CD34+ (BM-CD34+) or granulocyte-colony-stimulating factor-mobilized peripheral blood CD34+ (PB-CD34+) cells by means of cDNA array technology to identify molecular causes underlying the functional differences between circulating and sedentary hematopoietic stem and progenitor cells. In total, 65 genes were significantly differentially expressed. Greater cell cycle and DNA synthesis activity of BM-CD34+ than PB-CD34+ cells were reflected by the 2- to 5-fold higher expression of 9 genes involved in cell cycle progression, 11 genes regulating DNA synthesis, and cell cycle-initiating transcription factor E2F-1. Conversely, 9 other transcription factors, including the differentiation blocking GATA2 and N-myc, were expressed 2 to 3 times higher in PB-CD34+ cells than in BM-CD34+ cells. Expression of 5 apoptosis driving genes was also 2 to 3 times greater in PB-CD34+ cells, reflecting a higher apoptotic activity. In summary, our study provides a gene expression profile of primary human CD34+ hematopoietic cells of the blood and marrow. Our data molecularly confirm and explain the finding that CD34+ cells residing in the bone marrow cycle more rapidly, whereas circulating CD34+ cells consist of a higher number of quiescent stem and progenitor cells. Moreover, our data provide novel molecular insight into stem cell physiology.

Phosphorylation of the CCAAT displacement protein (CDP)/Cux transcription factor by cyclin A-Cdk1 modulates its DNA binding activity in G(2)

Stable DNA binding by the mammalian CCAAT displacement protein (CDP)/Cux transcription factor was previously found to be up-regulated at the G(1)/S transition as the result of two events, dephosphorylation by the Cdc25A phosphatase and proteolytic processing, to generate an amino-truncated isoform of 110 kDa. In S phase, CDP/Cux was shown to interact with and repress the core promoter of the p21(WAF1) gene. Here we demonstrate that DNA binding by p110 CDP/Cux is down-modulated as cells progress into G(2). Accordingly, cyclin A-Cdk1 was found to bind to CDP/Cux and modulate its DNA binding activity in vitro and in vivo. Interaction with CDP/Cux required the presence of both cyclin A and a cyclin-dependent kinase (Cdk)-activating kinase-activated Cdk1 and involved the Cut homeodomain and a downstream Cy motif. Phosphorylation of serines 1237 and 1270 caused inhibition of DNA binding in vitro. In cotransfection studies, cyclin A-Cdk1 inhibited CDP/Cux stable DNA binding and prevented repression of the p21(WAF1) reporter. In contrast, mutant CDP/Cux proteins in which serines 1237 and 1270 were replaced with alanines were not affected by cyclin A-Cdk1. In summary, our results suggest that the phosphorylation of CDP/Cux by cyclin A-Cdk1 contributes to down-modulate CDP/Cux activity as cells progress into the G(2) phase of the cell cycle.

A low abundance pool of nascent p21WAF1/Cip1 is targeted by estrogen to activate cyclin E*Cdk2

Estrogens regulate cell proliferation in target tissues, including breast cancer by stimulating G(1)-S phase transition. Activation of cyclin E.Cdk2 through abrogation of the ability of p21(WAF1/Cip1) to bind to and inhibit cyclin-CDKs is a pivotal event in this process in MCF-7 breast cancer cells. A proposed mechanism is p21 sequestration into cyclin D1.Cdk4/6 complexes driven by estrogen-induced transcriptional activation of cyclin D1 gene expression. However, we now show that some E(2)-induced cyclin E.Cdk2 activation occurs in the absence of increased cyclin D1 levels and requires decreased p21 protein synthesis. Both mechanisms operate in the absence of major changes in total p21 protein levels and instead target a low abundance subset of newly synthesized p21. E(2)-induced activation of cyclin E.Cdk2 is mimicked by targeted inhibition of nascent p21 expression by antisense p21 oligonucleotides. Cyclin E.Cdk2 activation is completely inhibited by a combination of antisense cyclin D1 oligonucleotide transfection and elimination of the decrease in nascent p21 by infection with adenoviral-p21. These findings strongly support a central role for p21 in the early phase of E(2)-induced mitogenesis and highlight a major functional role for newly synthesized CDK inhibitory proteins.

Involvement of the interaction between p21 and proliferating cell nuclear antigen for the maintenance of G2/M arrest after DNA damage

Although a major effect of p21, a cyclin-dependent kinase inhibitor, is considered to be exerted during G(1) phase of the cell cycle, p21 gene knock-out studies suggested its involvement in G(2)/M checkpoint as well. Here we demonstrate evidence that p21 is required for the cell cycle arrest at G(2) upon DNA damage. We found that expression of wild-type p21 (p21(WT)), not mutant p21 (p21(PCNA-)) lacking the interaction with proliferating cell nuclear antigen (PCNA), caused G(2) cell cycle arrest in p53-deficient DLD1 colon cancer cell line after the DNA damage by treatment with cis-diamminedichloroplatinum (II). We also found that p21(WT) was associated with Cdc2/cyclin B1 together with PCNA. Furthermore, coimmunoprecipitation experiments revealed that PCNA interacted with Cdc25C at the G(2)/M transition, and this interaction was abolished when p21(WT) was expressed presumably due to the competition between p21(WT) and Cdc25C in the binding to PCNA. These findings suggest that p21 plays a regulatory role in the maintenance of cell cycle arrest at G(2) by blocking the interaction of Cdc25C with PCNA.

Antitumor action of the PKC activator gnidimacrin through cdk2 inhibition

Daphnane-type diterpene gnidimacrin (NSC252940), isolated from a Chinese plant, exhibited antitumor activity against murine leukemias and solid tumors. At concentrations of 10(-9) to 10(-10) M, this agent strongly inhibited the growth of human tumor cell lines. In sensitive human leukemia K562 cells, gnidimacrin is a PKC activator that arrests the cell cycle in the G(1) phase by inhibiting cdk2 activity. A 4 hr exposure of K562 cells to gnidimacrin induced the CDK inhibitor p21(WAF1/Cip1), but this effect was transient and did not correlate temporally with the onset of G(1) arrest. Expression of cdc25A, a phosphatase that activates cdk2, was reduced during 24-hr exposure to gnidimacrin. Moreover, the suppression corresponded in a concentration- and time-dependent manner to both the inhibition of cdk2 activity and the mobility shift observed when cdk2 was electrophoresed on SDS-PAGE, indicating that the phosphorylation state of cdk2 must change. Cyclin E, the other regulator of cdk2 activity, was not influenced by gnidimacrin. These results suggest that gnidimacrin exerts antitumor activity through suppression of cdc25A and inhibition of cdk2 activity.

Estrogens and cell-cycle regulation in breast cancer

Clinical and experimental data have established that the leading cause of sporadic female breast cancer is exposure to estrogens, predominantly 17beta-estradiol. Recent advances in the understanding of cell-cycle control mechanisms have been applied to outline the molecular mechanisms through which estrogens regulate the cell cycle in cultured breast cancer cells, in particular, in MCF-7 cells. Here, we discuss how estrogens exert control over several key G1 phase cell-cycle regulators, namely cyclin D1, Myc, Cdk2, Cdk4, Cdk inhibitors and Cdc25A. Although the molecular mechanisms underlying estrogenic regulation of G1 phase regulators are far from clear, current evidence indicates that estrogens might regulate several key molecules required for S phase entry, this regulation being independent of cell-cycle transit per se.