Table of Contents

Volume 7, Issue 2

  January 15, 2008

  Pages 127 - 271

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EXTRA VIEWS

Open Access Article

Ras nanoclusters: Combining digital and analog signaling

Angus Harding and John F. Hancock
Pages 127 - 134
http://dx.doi.org/10.4161/cc.7.2.5237
Abstract | PDF

Cellular signaling pathways respond to external inputs to drive pivotal cellular decisions. Far from being mere data relay systems, signaling cascades form complex interacting networks with multiple layers of feedback and feed-forward control loops regulated in both space and time. While it may be intuitively obvious that this complexity allows cells to assess and respond appropriately to a myriad of external cues, untangling the wires to understand precisely how complex networks function as control and computational systems presents a daunting challenge to theoretical and experimental biologists alike. In this review we have focused on activation of the canonical MAP kinase cascade by receptor tyrosine kinases (RTKs) in order to examine some of the fundamental design principles used to build biological circuits and control systems. In particular, we explore how cells can reconfigure signaling cascades to generate distinct biological outputs by utilizing the unique spatial constraints available in biological membranes.

Open Access Article

Niche players: Spermatogonial progenitors marked by GPR125

Marco Seandel, Ilaria Falciatori, Sergey V. Shmelkov, Jiyeon Kim, Daylon James and Shahin Rafii
Pages 135 - 140
http://dx.doi.org/10.4161/cc.7.2.5248
Abstract | PDF

The undifferentiated spermatogonia of adult mouse testes are composed of both true stem cells and committed progenitors. It is unclear what normally prevents these adult germ cells from manifesting multipotency. The critical elements of the spermatogonial stem cell niche, while poorly understood, are thought to be composed of Sertoli cells with several other somatic cell types in close proximity. We recently discovered a novel orphan G-protein coupled receptor (GPR125) that is restricted to undifferentiated spermatogonia within the testis. GPR125 expression was maintained when the progenitor cells were extracted from the in vivo niche and propagated under growth conditions that recapitulate key elements of the niche. Such conditions preserved the ability of the cells to generate multipotent derivatives, known as multipotent adult spermatogonial derived progenitor cells (MASCs). Upon differentiation, the latter produced a variety tissues including functional endothelium, illustrating the potential applications of such cells. Thus, GPR125 represents a novel target for purifying adult stem and progenitors from tissues, with the goal of developing autologous multipotent cell lines.

Open Access Article

Mechanisms of mammalian polo-like kinase 1 (Plk1) localization: Self-versus non-self-priming

Kyung S. Lee, Jung-Eun Park, Young Hwi Kang, Wendy Zimmerman, Nak-Kyun Soung, Yeon-Sun Seong, Sahng-June Kwak and Raymond L. Erikson
Pages 141 - 145
http://dx.doi.org/10.4161/cc.7.2.5272
Abstract | PDF

Mammalian polo-like kinase 1 (Plk1) has been studied intensively as a key element in regulating diverse mitotic events during M-phase progression. Plk1 is spatially regulated through the targeting activity of the conserved polo-box domain (PBD) present in the C-terminal non-catalytic region. Over the years, studies have demonstrated that the PBD forms a phospho-epitope binding module and the PBD-dependent interaction is critical for proper subcellular localization of Plk1. The current prevailing model is that the PBD binds to a phospho-epitope generated by Cdc2 or other Pro-directed kinases. Here we discuss a recent finding that Plk1 also self-promotes its localization by generating its own PBD-docking site.

Open Access Article

Control of the Cdc6 replication licensing factor in metazoa: The role of nuclear export and the CUL4 ubiquitin ligase

Jihyun Kim and Edward T. Kipreos
Pages 146 - 150
http://dx.doi.org/10.4161/cc.7.2.5282
Abstract | PDF

A central requirement to maintain genome stability is that DNA replication must be tightly controlled so that genomic DNA is replicated only once in a single cell cycle. The prevention of DNA re-replication is achieved by restricting the assembly of pre-replicative complexes (pre RCs) to the period prior to S phase, and ensuring that pre-RCs cannot reform during S phase. The regulation of the replication licensing factors Cdt1 and Cdc6 during S phase is critical to prevent the reformation of pre-RCs. In yeast, Cdc6 is degraded during S phase to block DNA re-replication. In mammals, Cdc6 is exported from the nucleus; however, a variable percentage of endogenous Cdc6 remains nuclear throughout S phase. The perdurance of nuclear Cdc6 has led a number of groups to question whether the nuclear export of Cdc6 is relevant in restricting its activity. A recent study in C. elegans shows that the nuclear export of Cdc6 is in fact critical to prevent DNA re-replication. This work also identifies the CUL-4 ubiquitin ligase as a master regulator that controls DNA replication by regulating both Cdt1 and Cdc6 replication licensing factors.

Open Access Article

What makes centromeric cohesion resistant to separase cleavage during meiosis I but not during meiosis II?

Juraj Gregan, Cornelia Rumpf, Zhang Li and Lubos Cipak
Pages 151 - 153
http://dx.doi.org/10.4161/cc.7.2.5325
Abstract | PDF

Segregation of chromosomes during meiosis I is triggered by separase cleavage of the cohesin’s Rec8 subunit along chromosome arms. Centromeric cohesin is protected from separase cleavage during meiosis I by Sgo1/MEI-S332 proteins in complex with protein phosphatase 2A (PP2A). This retention of centromeric sister chromatid cohesion is essential for faithful segregation of chromatids during the second meiotic division. While Sgo1/PP2A complex is required for protecting centromeric sister chromatid cohesion during meiosis I, it is not known what renders the centromeric cohesion sensitive to separase cleavage during meiosis II. Our data suggest that the absence of Sgo1 and PP2A from meiosis II centromeres is not sufficient to render centromeric cohesion sensitive to cleavage by separase and additional factors are required to ensure the removal of centromeric cohesion during meiosis II.




PERSPECTIVES

Open Access Article

p53 promoter selection: Choosing between life and death

Sanjeev Das, Sarah A. Boswell, Stuart A. Aaronson and Sam W. Lee
Pages 154 - 157
http://dx.doi.org/10.4161/cc.7.2.5236
Abstract | PDF

A crucial unresolved issue about the genotoxic stress response is how the activation of the p53 tumor suppressor can lead either to cell cycle arrest and DNA repair or to apoptosis. p53 is one of the most important tumor suppressor proteins in the cell to prevent to heritable transfer of damaged DNA. In response to different stress conditions p53 rapidly accumulates and functions as a sequence specific DNA-binding transcription factor to regulate a large number of target genes. Activation of p53 has two major outcomes: cell cycle arrest or apoptosis. In this review we attempt to enumerate the different modifications and co-factors that influence p53 promoter selection and demonstrate how p53 chooses life or death for the cell.

Open Access Article

The common germline Arg72Pro polymorphism of p53 and increased longevity in humans

Stig E. Bojesen and Børge G. Nordestgaard
Pages 158 - 163
http://dx.doi.org/10.4161/cc.7.2.5249
Abstract | PDF

More than ten million single nucleotide polymorphisms (SNPs) have been identified in humans; however, the importance of most SNPs for health and disease is not understood. Most SNPs are indeed unimportant and because of often inadequately powered studies, many observations on SNP effects can not be repeated by other researchers. SNPs are at best shown to influence protein function or level, rarely to influence risk of disease, and almost never to influence total mortality, the ultimate endpoint. A well-known functional SNP in the tumor suppressor TP53 gene leads to increased longevity: in the Danish general population (n=9219) homozygotes for the minor allele versus homozygotes for the major allele had an increase in median survival of 3 years. This is partly explained by increased survival after development of cancer or other diseases, in accordance with the observation that this Arg72Pro substitution in the p53 protein has important influence on cell death via increased apoptosis. Thus, the increased longevity may be due to a generally increased robustness after a diagnosis of any life-threatening disease. In contrast to widespread skepticism on the importance of SNPs in humans, this gain-of-function p53 SNP of importance for cell repair mechanisms has a profound influence on longevity.

Open Access Article

The Wip1 phosphatase and Mdm2: Cracking the "Wip" on p53 stability

Xiongbin Lu, Thuy-Ai Nguyen, Xinna Zhang and Lawrence A. Donehower
Pages 164 - 168
http://dx.doi.org/10.4161/cc.7.2.5299
Abstract

The p53 tumor suppressor is essential in maintaining genomic integrity in response to cellular stresses. In response to DNA damage, p53 is activated and stabilized largely through post-translational modifications, including phosphorylation by DNA damage responsive kinases such as ATM and ATR. Activated p53 transactivates a battery of genes that can mediate either cell cycle arrest or apoptosis. In those instances where p53 facilitates cell cycle arrest, a means to return the cell to a pre-stress state with low p53 levels is important. The E3 ubiquitin ligase Mdm2 is one p53 transcriptional target that accumulates after damage and promotes p53 ubiquitination and degradation. Thus, p53 and Mdm2 form a critical negative feedback regulatory loop that helps to maintain appropriate p53 levels in the presence or absence of stress. We propose here that Wip1 (Wildtype p53-Induced Phosphatase 1), also known as PPM1D, plays an important role in the p53-Mdm2 autoregulatory loop. We have recently shown that Wip1, also a p53 target gene, dephosphorylates Mdm2 at Ser395 (an ATM target site), resulting in stabilization of Mdm2, enhanced Mdm2-p53 binding, and enhanced ubiquitination of p53 by Mdm2. Thus, Wip1 facilitates Mdm2-mediated degradation of p53. The p53 inhibitory role of Wip1 implicates it as a potential oncogene and indeed Wip1 is amplified and overexpressed in a number of human cancers. Wip1 may inhibit p53 signaling by multiple mechanisms, but our data suggests that its largest effects are due to dephosphorylation of Mdm2.

Open Access Article

MAP kinase p38 and its relation to T cell anergy and suppressor function of regulatory T cells

Henric S. Adler and Kerstin Steinbrink
Pages 169 - 175
http://dx.doi.org/10.4161/cc.7.2.5312
Abstract | PDF

Diverse regulatory T cell populations (Treg) are important for the control of self tolerance and immune homeostasis. These include naturally occurring CD4+CD25+ Treg (nTreg) and induced Treg (iTreg). Tolerogenic dendritic cells, modulated by IL-10, are able to convert peripheral T cells into iTreg. These are anergic and characterized by a G1 cell cycle arrest, dependent on elevated levels of the cdk inhibitor p27Kip1. Novel data revealed a distinct pattern of MAP kinase activation in iTreg different from clonal T cell anergy, with enhanced activation of the p38-MAPKAP-K2/3 pathway. p38 is involved in cell cycle control and its activity is a prerequisite for the induction and maintenance of the anergic state in iTreg. Inhibition of p38 leads to down regulation of p27Kip1, cell cycle progress and loss of regulatory T cell function. Here, we discuss these data in light of the role of p38 and p27Kip1 in T cell activation, anergy induction and cell cycle control.

Open Access Article

Environmental restrictions within tumor ecosystems select for a convergent, hypoxia-resistant phenotype of cancer stem cells

Massimo Olivotto and Persio Dello Sbarba
Pages 176 - 187
http://dx.doi.org/10.4161/cc.7.2.5315
Abstract | PDF

Tumors are ecosystems which develop from stem cells endowed with unlimited cell renewal and genetic instability, under the effects of mutagenesis and natural selection imposed by environmental changes. While changes and variations made possible by genetic instability are practically unlimited, the microenvironment progressively reduces those possibilities in the struggle for life imposed by hypoxia and nutrient shortage typical of tumor environments. This entails the tendency to evolve a convergent phenotype resistant to microenvironmental restrictions (first of all hypoxia), which progressively dominates the clonal selection. It is shown that adaptation to hypoxia, rather than being a peculiarity of cancer stem cells is also a characteristic of normal hematopoietic stem cells, and may thus be described as a general feature of the stem cell phenotype. The metabolic orientation of this phenotype closely resembles the orientation of highly anaplastic ascites hepatomas, showing that, in restricted environments, stem cell recruitment to growth is limited by mitochondrial reoxidation of reducing equivalents produced in folate redox steps connected with purine synthesis. Finally, a review of earlier research into glucose metabolism in cancer leads to the reinterpretation of Warburg’s aerobic glycolysis as a defence mechanism which disposes of glycolytic products able to negatively interfere with the crucial role of mitochondrial respiration in cell recruitment for growth.

Open Access Article

Metastatic cancer stem cells: A new target for anti-cancer therapy?

Patrick C. Hermann, Stephan L. Huber and Christopher Heeschen
Pages 188 - 193
http://dx.doi.org/10.4161/cc.7.2.5326
Abstract | PDF

Over the past few years, supporting evidence for the cancer stem cell hypothesis has been provided for an increasing number of tumor entities. According to this hypothesis, only a small population of undifferentiated cells with stem cell characteristics has the ability to form tumors through asymmetric division and subsequent differentiation of the progeny into the heterogeneous cell types that comprise a tumor. Recently, we were able to show that cancer stem cells are not only responsible for tumorigenesis, but that they contain a subpopulation characterized by CXCR4 expression which is exclusively capable of disseminating and subsequently providing the substrate for tumor metastasis. Of note, these recent advances in our understanding of cancer stem cell biology raise more questions than they answer. Some of these arising questions regarding the targeted elimination of these cancer stem cells will be addressed in this perspective.

Open Access Article

TNF signaling gets FLIPped off: TNF-induced regulation of FLIP

Stuart A. Rushworth, Alison Taylor, Susana Langa and David J. MacEwan
Pages 194 - 199
http://dx.doi.org/10.4161/cc.7.2.5159
Abstract | PDF

One of the major guardians of death receptor mediated programmed cell death is FLIP (Fas-associated protein with death domain-like IL-1β-converting enzyme (FLICE) inhibitory protein). As an important modulator of apoptosis, FLIP regulates life and death in various types of normal cells and tissues, such as lymphoid cells, and renders resistance to death receptor-mediated apoptosis in many types of tumor cells. Therefore, understanding the signals that control FLIP is vital to understanding and controlling diseases such as cancer. Here, we discuss the mechanisms that regulate FLIP activation by tumor necrosis factor (TNF), and their functional significance in regulating apoptotic processes.




REVIEW

Open Access Article

Genetic similarities between organogenesis and tumorigenesis of the lung

Scott Powers and David Mu
Pages 200 - 204
http://dx.doi.org/10.4161/cc.7.2.5284
Abstract | PDF

In hematological malignancies, there are numerous examples of lymphocyte developmental genes playing a role in cancer formation. In this article, we discuss how processes of fetal organogenesis are also operant in lung tumorigenesis. We first review four pathways important in lung cancer (MYC, Hedgehog, Rb, and Wnt) and describe the experimental evidence linking them to lung development. Then, we review genome-wide analysis approaches of both RNA (gene expression profiling) and DNA (copy number alterations) and how they have uncovered links between lung cancer and fetal lung development. Finally, the recent discovery of three closely linked developmental transcription factors, which are co-amplified as cooperating lung oncogenes, is discussed. We suggest that inhibition of the fetal developmental pathways selectively reactivated in cancer cells is a research area of interest for novel anti-cancer therapies in light of the presumed low toxicity of such therapies to the nearby normal adult cells.




PRIORITY REPORT

Open Access Article

Specific isoforms of p73 control the induction of cell death induced by the viral proteins, E1A or Apoptin

Poramaporn Klanrit, Marcella B. Flinterman, Edward W. Odell, Gerry Melino, Richard Killick, James S. Norris and Mahvash Tavassoli
Pages 205 - 215
http://dx.doi.org/10.4161/cc.7.2.5361
Abstract | PDF

A member of the p53 family, p73, has several isoforms and differentially regulates transcription of genes involved in the control of the cell cycle and apoptosis. We have previously shown efficient and p53-independent, tumour-specific cell death induced by the viral proteins E1A and Apoptin. Here, we demonstrate that the induction of apoptosis by these viral proteins involves activation of TAp73. Both E1A and Apoptin induced expression of endogenous TAp73 and the p53/p73 BH3-only pro-apoptotic target, PUMA, independently of the p53 function. Furthermore, exogenous expression of TAp73 isoforms, particularly TAp73β, sensitized cells to killing by both E1A and Apoptin, while expression of ΔNp73α blocked this activity. Besides, knockout of the p73 regulator, c-Abl, attenuated E1A-induced apoptosis. In accordance with the role of p73 in apoptosis induced by these viral proteins, overexpression of TAp73β strongly induced apoptosis in p53-deficient cancer cells in vitro and in HNSCC xenografts. Using a doxycycline-inducible system, we provide evidence for target selectivity and significant differences in protein stability for specific p73 isoforms, suggesting a diverse and pivotal role for p73 in response to various genotoxic agents. Collectively, our data show that in the absence of the p53 function, viral proteins E1A and Apoptin utilize the p73 pathway to induce efficient tumour cell death.




REPORTS

Open Access Article

Basal c-Jun N-terminal kinases promote mitotic progression through histone H3 phosphorylation

Kyunghee Lee and Kiwon Song
Pages 216 - 221
http://dx.doi.org/10.4161/cc.7.2.5155
Abstract | PDF

Phosphorylation of histone H3 at serine 10 (S10) is essential for the onset of mitosis. Here, we show that basal c-Jun N-terminal kinases (JNKs) are required for mitotic histone H3 S10 phosphorylation in human primary fibroblast IMR90 cells. Inhibition of JNKs by specific pharmacologic inhibitors, expression of dominant-negative JNK1 and 2 mutants, or RNAi of JNK1 and 2 prevented phosphorylation of histone H3 at S10 in vivo. The JNK-specific inhibitor SP600125 blocked mitotic entry, as shown by its ability to prevent CDK1 dephosphorylation and cyclin A degradation. Basal JNK phosphorylation increased at G2/M-phase, although total JNK protein levels remained unchanged. In addition, basal JNKs were localized in nuclei and centrosomes during this time, suggesting that the nuclear localization of JNKs during G2/M is tightly coupled with histone H3 phosphorylation. Basal JNKs were able to phosphorylate histone H3 in vitro and co-precipitation of histone H3 and JNKs was only detected at G2/M. Taken together, these data strongly suggest that basal JNKs play a key role in controlling histone H3 phosphorylation for mitotic entry at G2/M-phase.

Open Access Article

Specific inhibition of Mdm2-mediated neddylation by Tip60

Christoph Dohmesen, Max Koeppel and Matthias Dobbelstein
Pages 222 - 231
http://dx.doi.org/10.4161/cc.7.2.5185
Abstract | PDF

Tip60 is a histone acetyl transferase (HAT) and a cofactor of transcription, but also an interaction partner of the Mdm2 oncoprotein. The functional consequences of this interaction are only partially understood and were further explored in this study. We found that Tip60 is capable of selectively inhibiting the Mdm2-mediated conjugation of Nedd8 to p53, whereas it did not affect p53 ubiquitination. In contrast, the known Mdm2 antagonist p14arf preferentially blocked Ubiquitin conjugation by Mdm2. To identify underlying mechanisms, we studied the intracellular localization of Tip60 and Mdm2. Both proteins relocalized each other to the PML nuclear bodies, but a similar localization pattern was observed even in the absence of PML. Analysis of Tip60 deletion mutants revealed that some mutants, while still interacting with Mdm2, failed to relocalize it and to inhibit Mdm2-mediated neddylation, suggesting that these two phenomena require biochemical activities in addition to the mere interaction between the two proteins. For both activities, the HAT domain of Tip60 was not required. We propose that Tip60 can act as a selective antagonist to Mdm2-mediated neddylation but not ubiquitination. Hence, the two different E3 ligase activities of Mdm2 can be regulated individually.

Open Access Article

Synergystic induction of HIF-1α transcriptional activity by hypoxia and lipopolysaccharide in macrophages

Zenghui Mi, Annamaria Rapisarda, Lynn Taylor, Alan Brooks, Mark Creighton-Gutteridge, Giovanni Melillo and Luigi Varesio
Pages 232 - 241
http://dx.doi.org/10.4161/cc.7.2.5193
Abstract | PDF

Hypoxia Inducible Factor-1 (HIF-1) is activated by a variety of stimuli, including inflammatory mediators. In this report we investigated the role that bacterial lipopolysaccharide (LPS) and hypoxia play in the regulation of HIF-1-dependent gene expression in macrophages. We report that murine macrophages stimulated with low concentrations of LPS (1-10 ng/ml) expressed significantly higher levels of inducible nitric oxide synthase (iNOS) mRNA when cultured under hypoxic compared to normoxic conditions. Functional studies of the iNOS promoter demonstrated that the synergistic interaction between LPS and hypoxia was mediated, at least in part, by the NFκB and the HIF-1 binding sites. In addition, transient transfection experiments using a Hypoxia Response Element (HRE)-containing plasmid showed that LPS and hypoxia synergistically induced HIF-1-dependent transcriptional activity. Interestingly, LPS did not significantly affect HIF-1α protein levels or HIF-1 DNA binding activity relative to hypoxic induction. HIF-1α, but not HIF-2α, was critical for the synergistic induction of HRE-dependent transcriptional activity in macrophages, as indicated by experiments using siRNA targeting HIF-1α or HIF-2α. Addition of ROS-scavengers completely abrogated the synergistic induction of HIF-1 transcriptional activity by LPS and hypoxia, but neither inhibited HIF-1 transcriptional activity induced by hypoxia alone nor affected HIF-1α protein levels or HIF-1 DNA binding induced by hypoxia alone or hypoxia plus LPS. Taken together, our results demonstrate that LPS and hypoxia act synergistically to induce HIF-1α-transcriptional activity and they emphasize the existence of a cross talk between hypoxic and non-hypoxic signaling pathways in the regulation of macrophages gene expression.

Open Access Article

Functional analyses of the cancer stem cell-like properties of human endometrial tumor initiating cells

Anne M. Friel, Petra A. Sergent, Christine Patnaude, Paul P. Szotek, Esther Oliva, David T. Scadden, Michael V. Seiden, Rosemary Foster and Bo R. Rueda
Pages 242 - 249
http://dx.doi.org/10.4161/cc.7.2.5207
Abstract | PDF

Recent data suggest that rare stem cell populations with the capacity to self renew and drive tumor formation are a feature of solid tumors. Several investigators have identified putative stem cells from solid tumors and cancer cell lines following isolation of a side population (SP) defined by dye exclusion. We investigated this parameter in our efforts to identify an endometrial cancer (EnCa) stem cell population. Multiple EnCa cell lines were assessed and verapamil sensitive SP and non-SP cells were isolated from two human EnCa cell lines. The functional significance of the SP and non-SP derived from AN3CA was evaluated in vitro and in vivo. SP cells proliferated at a significantly slower rate than the non-SP fraction, and a larger proportion of the SP cells were in G1 phase of the cell cycle as compared to the non-SP fraction. The SP fraction was more resistant to the chemotherapeutic agent paclitaxel. The SP comprised ~0.02% of the initial AN3CA cell population and this proportion of SP cells was maintained within the larger heterogeneous population following repeated passages of purified SP cells. These findings suggest that SP cells derived from the AN3CA cell line have the stem cell properties of low proliferative activity, chemoresistance, and self-renewal. We also tested relative tumor formation activity of the SP and non-SP fractions. Only the SP fraction was tumorigenic. Additionally, we identified SP fractions in primary EnCa. Together these results are consistent with the hypothesis that EnCa contain a subpopulation of tumor initiating cells with stem like properties.

Open Access Article

A transcriptional profiling meta-analysis reveals a core EWS-FLI gene expression signature

Jeffrey D. Hancock and Stephen L. Lessnick
Pages 250 - 256
http://dx.doi.org/10.4161/cc.7.2.5229
Abstract | PDF

Ewing's sarcomas are characterized by recurrent chromosomal translocations expressing EWS-ETS fusion proteins, the most common of which is EWS-FLI1-5. EWS-FLI is an oncogenic transcription factor that regulates genes involved in tumorigenesis.6,7 Because the Ewing's sarcoma cell of origin remains unknown, a variety of model systems have been developed to study EWS-FLI fusions,8-14 and multiple microarray experiments describing potential EWS-FLI target genes have been reported.8,10,11,13,15-21 Each model has potential benefits and drawbacks, but a large-scale comparison of these has not been reported. Herein we report a meta-analysis of the genes that are dysregulated by EWS-FLI in Ewing’s sarcoma model systems. In general, EWS-FLI gain- and loss-of-function models in human cell types were well correlated to patient-derived tumor samples, while murine models were not. Using frequency analysis of dysregulated genes across multiple model systems, we identified a conserved “core” EWS-FLI transcriptional signature. This signature contained many of the genes known to be involved in the tumorigenic phenotype of Ewing’s sarcoma, and also contained genes that have not been previously reported. Comparisons between the core EWS-FLI signature and published mesenchymal stem cell data support the recent assertion that mesenchymal stem cells are likely the Ewing’s sarcoma precursor cell15. These results demonstrate the utility of using comparative analysis to validate model systems and emphasize the unique potential of this approach to identify both oncogenic and background cell signatures.

Open Access Article

PI3K/Akt/FOXO3a pathway contributes to thrombopoietin-induced proliferation of primary megakaryocytes in vitro and in vivo via modulation of p27Kip1

Takafumi Nakao, Amy E. Geddis, Norma E. Fox and Kenneth Kaushansky
Pages 257 - 266
http://dx.doi.org/10.4161/cc.7.2.5148
Abstract | PDF

Thrombopoietin (TPO), the primary regulator of megakaryocyte (MK) and platelet formation, modulates the activity of multiple signal transduction molecules, including those in the Jak/STAT, p42/p44 MAPK, and phosphatidylinositol 3-kinase (PI3K)/Akt pathways. We previously demonstrated that PI3K and Akt are necessary for TPO-induced cell cycle progression of primary MK progenitors. However, the molecular events secondary to the activation of PI3K/Akt responsible for MK proliferation remain unclear. In this study we show that FOXO3a and its downstream target p27Kip1 play an important role in TPO-induced proliferation of MK progenitors. We found that TPO down-modulates p27Kip1 expression at both the mRNA and protein levels in primary MKs in a PI3K dependent fashion. UT-7/TPO, a megakaryocytic cell line, stably expressing constitutively active Akt or a dominant-negative form of FOXO3a failed to reduce p27Kip1 expression after TPO stimulation, and fail to induce p27Kip1 expression following TPO withdrawal. Induced expression of an active form of FOXO3a resulted in increased p27Kip1 expression in this cell line. In addition, the number of MKs is significantly increased in bone marrow from Foxo3a-deficient mice. Taken together with the previous observation that p27Kip1-deficient mice also display increased numbers of MK progenitors, our findings indicate that the PI3K/Akt/FOXO3a/p27Kip1 pathway contributes to normal TPO-induced MK proliferation.




LETTERS TO THE EDITOR

Open Access Article

A new mitotic-cell specific monoclonal antibody

Martine Cazales, Muriel Quaranta, Valérie Lobjois, Denis Jullien, Talal Al Saati, Georges Delsol and Bernard Ducommun
Pages 267 - 268
http://dx.doi.org/10.4161/cc.7.2.5306
Abstract | PDF

No abstract.

Open Access Article

HSV-1 UL45 encodes a carbohydrate binding C-type lectin protein

Lucjan S. Wyrwicz, Krzysztof Ginalski and Leszek Rychlewski
Pages 269 - 271
http://dx.doi.org/10.4161/cc.7.2.5324
Abstract | PDF

No abstract.