Table of Contents

Volume 9, Issue 18

  September 15, 2010

  Pages 3633 - 3852

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 Issue Catalogue (MARC XML)

EDITORIALS: CELL CYCLE FEATURES

Open Access Article

Starved cells use mitochondria for autophagosome biogenesis

Angelika S. Rambold and Jennifer Lippincott-Schwartz
Pages 3633 - 3634
http://dx.doi.org/10.4161/cc.9.18.13170
Abstract | PDF

Comment on: Hailey DW, et al. Cell 2010; 141:656-67.

Open Access Article

Neighborly DISCourse: DNA double strand breaks silence transcription

Niraj M. Shanbhag and Roger Greenberg
Pages 3635 - 3636
http://dx.doi.org/10.4161/cc.9.18.13171
Abstract | PDF

Comment on: Shanbhag NM, et al. Cell 2010; 141:970-81.

Open Access Article

A microRNA regulating adult hematopoietic stem cells

Shangqin Guo and David T. Scadden
Pages 3637 - 3638
http://dx.doi.org/10.4161/cc.9.18.13174
Abstract | PDF

Comment on: Guo S, et al. Proc Natl Acad Sci USA 2010; 107:14229-34.

Open Access Article

Prognostic signatures, cancer metastasis and MYC

Anita Wolfer and Sridhar Ramaswamy
Page 3639
http://dx.doi.org/10.4161/cc.9.18.13220
Abstract | PDF

Comment on: Wolfer A, et al. Proc Natl Acad Sci USA 2010; 23:3698-703.

Open Access Article

RNAi knock of HdmX or Hdm2 leads to new insights into p53 signaling

Steven Berberich
Pages 3640 - 3641
http://dx.doi.org/10.4161/cc.9.18.13255
Abstract | PDF

Comment on: Heminger et.al. Aging 2009; 1:89-108.




NEWS & VIEWS

Open Access Article

A new layer of regulation is required to silence the DNA damage checkpoint

Yanchang Wang
Pages 3642 - 3647
http://dx.doi.org/10.4161/cc.9.18.13180
Abstract | PDF

Comment on: Wood MD, et al. Cell Cycle 2010; 9:3354-64.

Open Access Article

A sliding scale: The many faces of Ctf7/Eco1 cohesion establishment factor in DNA Repair

Robert V. Skibbens
Pages 3648 - 3653
http://dx.doi.org/10.4161/cc.9.18.13181
Abstract | PDF

Comment on: Lu S, et al. Cell Cycle 2010; 9:3316-28.

Open Access Article

Additional proof for the importance of Eco1 for DNA double strand break repair

Lena Ström
Pages 3654 - 3659
http://dx.doi.org/10.4161/cc.9.18.13182
Abstract | PDF

Comment on: Lu S, et al. Cell Cycle 2010; 9:3316-28.

Open Access Article

Incapacitating CtBP to kill cancer

Ling-zun Zhao and G. Chinnadurai
Pages 3660 - 3665
http://dx.doi.org/10.4161/cc.9.18.13221
Abstract | PDF

Comment on: Straza MW, et al. Cell Cycle 2010; 9:3740-50.

Open Access Article

The role of p53 in nutrients levels

Yosef Shaul and Nina Reuven
Pages 3666 - 3671
http://dx.doi.org/10.4161/cc.9.18.13222
Abstract | PDF

Comment on: Ashur-Fabian O, et al. Cell Cycle 2010; 9:446-7.




EXTRA VIEWS

Open Access Article

Skeletal aging and the adipocyte program: New insights from an "old" molecule

Beata Lecka-Czernik, Clifford J. Rosen and Masanobu Kawai
Pages 3672 - 3678
http://dx.doi.org/10.4161/cc.9.18.13046
Abstract | PDF

Aging is associated with profound changes in bone mass and body composition. Emerging evidence supports the hypothesis that alterations in mesenchymal stromal cell fate are a critical etiologic factor. In addition, timekeeping at the cellular level is affected as aging progresses, particularly in the adipocyte. In this Extra View we discuss the interactive role of three molecules, PPARγ, nocturnin and IGF-I in regulating stem cell fate in the marrow, and the potential implications of this network for understanding cellular aging.

Open Access Article

Myc, Cdk2 and cellular senescence: Old players, new game

Stefano Campaner, Mirko Doni, Alessandro Verrecchia, Giovanni Fagà, Lucia Bianchi and Bruno Amati
Pages 3679 - 3685
http://dx.doi.org/10.4161/cc.9.18.13049
Abstract | PDF

The aberrant activation of oncogenic pathways promotes tumor progression, but concomitantly elicits compensatory tumor-suppressive responses, such as apoptosis or senescence. For example, Ras induces senescence, while Myc generally triggers apoptosis. Myc is in fact viewed as an anti-senescence oncogene, as it is a potent inducer of cell proliferation and immortalization, bypasses growth-inhibitory signals, and cooperates with Ras in cellular transformation. Recent reports prompt re-evaluation of Myc-induced senescence, and of its role in tumor progression and therapy. We have shown that the cyclin-dependent kinase Cdk2, although redundant for cell cycle progression, has a unique role in suppressing a Myc-induced senescence program: Myc activation elicited expression of p16^INK4a and p21^Cip1, and caused senescence in cell lacking Cdk2, but not in Cdk2-proficient cells. Additional cellular activities have been identified that suppress Myc-induced senescence, including the Wrn helicase, Telomerase and Miz1. These senescence-suppressing activities were critical for tumor progression, as deficiency in Cdk2, telomerase or Miz1 reduced the onset of Myc-induced lymphoma in transgenic mice. Other gene products like p53, SUV39H1 or TGFß promoted senescence, which together with apoptosis contributed to tumor suppression. Paradoxically, Myc directly counteracted the very same senescence program that it potentially elicits, since it positively regulated Wrn, Telomerase and Cdk2 activity, and Cdk2 inhibition re-activated the latent senescence program in Myc expressing cells. Hence, while these molecules are instrumental to the oncogenic action of Myc, they may simultaneously constitute its Achille's heel for therapeutic development.

Open Access Article

GEMC1 is a novel TopBP1-interacting protein involved in chromosomal DNA replication

Gabriele Piergiovanni and Vincenzo Costanzo
Pages 3686 - 3690
http://dx.doi.org/10.4161/cc.9.18.13060
Abstract | PDF

Chromosomal DNA must be precisely replicated in each cell cycle in order to ensure maintenance of genome stability. Most of the factors controlling this process have been identified in lower eukaryotes. Several factors involved in DNA replication are also important for the cell response to stress conditions. However, the regulation of DNA replication in multi-cellular organisms is still poorly understood. Using the Xenopus laevis egg cell-free system, we have recently identified a novel vertebrate protein named GEMC1 required for DNA replication. xGEMC1 is a Cyclin dependent kinase (CDK) target required the Cdc45 loading onto chromatin and it interacts with the checkpoint and replication factor TopBP1, which promotes its binding to chromatin during pre-replication complex formation. Here we discuss our recent findings and we propose possible roles for GEMC1. Interesting, recent studies have identified other proteins with analogous functions, showing a higher level of complexity in metazoan replication control compared to lower eukaryotes.

Open Access Article

Epimorphic regeneration in mice is p53-independent

L. Matthew Arthur, Renee M. Demarest, Lise Clark, Dmitri Gourevitch, Kamila Bedelbaeva, Rhonda Anderson, Andrew R Snyder, Anthony J. Capobianco, Paul M. Lieberman, Lionel Feigenbaum and E. Heber-Katz
Pages 3691 - 3697
http://dx.doi.org/10.4161/cc.9.18.13119
Abstract | PDF

The process of regeneration is most readily studied in species of sponge, hydra, planarian, and salamander (i.e., newt and axolotl).   The closure of MRL mouse ear pinna through-and-through holes provides a model of unusual wound healing/regeneration in which a blastema-like structure closes the ear hole and cartilage and hair follicles are replaced.  Recent studies, based on a broad level of DNA damage and a cell cycle pattern of G2/M "arrest," showed that p21^Cip1/Waf1 was missing from the MRL mouse ear and that a p21-null mouse could close its ear holes.  Given the p53/p21 axis of control of DNA damage, cell cycle arrest, apoptosis, and senescence, we tested the role of p53 in the ear hole regenerative response.  Using backcross mice, we found that loss of p53 in MRL mice did not show reduced healing.  Furthermore, cross sections of MRL.p53^-/- mouse ears at 6 weeks post-injury showed an increased level of adipocytes and chondrocytes in the region of healing whereas MRL or p21^-/- mice show chondrogenesis alone in this same region, though at later time points.  In addition, we also investigated other cell cycle-related mutant mice to determine how p21 was being regulated.  We demonstrate that p16 and Gadd45 null mice show little healing capacity.  Interestingly, a partial healing phenotype in mice with a dual Tgfb/Rag2 knockout mutation was seen.  These data demonstrate an independence of p53 signaling for mouse appendage regeneration and suggest that the role of p21 in this process is possibly through the abrogation of the Tgfb/Smad pathway.

Open Access Article

Damage-specific modification of PCNA

Sapna Das-Bradoo, Hai Dang Nguyen and Anja-Katrin Bielinsky
Pages 3698 - 3703
http://dx.doi.org/10.4161/cc.9.18.13121
Abstract | PDF

Okazaki fragment processing is an integral part of DNA replication.  For a long time, we assumed that the maturation of these small RNA-primed DNA fragments did not necessarily have to occur during S phase, but could be postponed to late in S phase after the bulk of DNA synthesis had been completed.  This view was primarily based on the arrest phenotype of temperature-sensitive DNA ligase I mutants in yeast, which accumulated with an almost fully duplicated set of chromosomes.  However, many temperature-sensitive alleles can be leaky, and the re-evaluation of DNA ligase I-deficient cells has offered new and unexpected insights into how cells keep track of lagging strand synthesis.  It turns out that if Okazaki fragment joining goes awry, cells have their own alarm system in the form of ubiquitin that is conjugated to the replication clamp PCNA.  Although this modification results in mono- and poly-ubiquitination of PCNA, it is genetically distinct from the known post-replicative repair mark at lysine 164.  In this Extra View, we discuss the possibility that eukaryotic cells utilize different enzymatic pathways and ubiquitin attachment sites on PCNA to alert the replication machinery to the accumulation of single-stranded gaps or nicks behind the fork.

Open Access Article

HDAC1 in axonal degeneration: A matter of subcellular localization

Jin Young Kim and Patrizia Casaccia
Pages 3704 - 3708
http://dx.doi.org/10.4161/cc.9.18.12716
Abstract | PDF

Multiple sclerosis (MS) is a disease characterized by inflammatory demyelination and a strong neurodegenerative component.




PERSPECTIVE

Open Access Article

Differential localization of ATM is correlated with activation of distinct downstream signaling pathways

Angela Alexander and Cheryl L. Walker
Pages 3709 - 3710
http://dx.doi.org/10.4161/cc.9.18.13253
Abstract | PDF

ATM, the gene mutated in the genetic disease ataxia telangiectasia (AT), is a well-known protein involved in the DNA double-strand break response, where it plays an important role in sensing damage and signaling to DNA repair machinery and cell cycle checkpoints. However, a number of recent papers, including ours have found that ATM also plays important roles outside of the nucleus, which may explain some of the phenotypic features seen in AT patients. Our research into mechanisms of TSC2 regulation helped uncover a pathway upstream of TSC2 that is regulated by cytoplasmic ATM in response to ROS initiated by ATM activation of LKB1 and AMPK. We found that TSC2 activation results in mTORC1 repression and subsequent induction of autophagy. Elucidation of this stress response pathway provides a molecular mechanism for ATM signaling in the cytoplasm, and lays the groundwork for further studies on how ATM activity is regulated beyond DNA damage in different cellular compartments.




REVIEWS

Open Access Article

Cell growth- and differentiation-dependent regulation of RNA polymerase III transcription

Hélène Dumay-Odelot, Stéphanie Durrieu-Gaillard, Daniel Da Silva, Robert G. Roeder and Martin Teichmann
Pages 3711 - 3723
http://dx.doi.org/10.4161/cc.9.18.13203
Abstract | PDF

RNA polymerase III transcribes small untranslated RNAs that fulfill essential cellular functions in regulating transcription, RNA processing, translation and protein translocation. RNA polymerase III transcription activity is tightly regulated during the cell cycle and coupled to growth control mechanisms. Furthermore, there are reports of changes in RNA polymerase III transcription activity during cellular differentiation, including the discovery of a novel isoform of human RNA polymerase III that has been shown to be specifically expressed in undifferentiated human H1 embryonic stem cells. Here, we review major regulatory mechanisms of RNA polymerase III transcription during the cell cycle, cell growth and cell differentiation.

Open Access Article

The various roles of ubiquitin in Wnt pathway regulation

Daniele V. F. Tauriello and Madelon M. Maurice
Pages 3724 - 3733
http://dx.doi.org/10.4161/cc.9.18.13204
Abstract | PDF

Wnt signaling mediates key developmental and homeostatic processes including stem cell maintenance, growth and cell fate specification, cell polarity and migration. Inappropriate activation of Wnt signaling is linked to a range of human disorders, most notably cancer and neurodegenerative diseases. In the Wnt/β-catenin cascade, signaling events converge on the regulation of ubiquitin-mediated degradation of the crucial transcriptional regulator β-catenin. The emerging mechanisms by which ubiquitin modification of proteins controls cellular pathways comprise both proteolytic and nonproteolytic functions. In nonproteolytic functions, ubiquitin acts as a signaling device in the control of protein activity, subcellular localization and complex formation. Here, we review and discuss recent developments that implicate ubiquitin-mediated mechanisms at multiple steps of Wnt pathway activation.

Open Access Article

Allosteric inhibition of BCR-ABL

A. Quamrul Hassan, Sreenath V. Sharma and Markus Warmuth
Pages 3734 - 3738
http://dx.doi.org/10.4161/cc.9.18.13232
Abstract | PDF

Impaired regulation of kinase activity can lead to a variety of diseases, including cancer. Inhibition of kinase activity has, therefore, been considered an attractive anti-cancer therapeutic strategy. The success of targeted therapy with kinase inhibitors has been well documented with BCR-ABL, where imatinib specifically inhibits kinase activity with impressive pharmacological responses in chronic myelogenous leukemia (CML). However, the success of kinase inhibitors as cancer therapeutics is being challenged clinically by the emergence of acquired resistance. Most kinase inhibitors available today are ATP-competitive. There have been efforts to develop kinase inhibitors with new modes of action. In this review, we highlight the development of 'allosteric kinase inhibitors' that inhibit kinase activity by binding to a site remote from the active site of the kinase. We focus on recent efforts directed towards BCR-ABL, for which, significant progress has been made to develop allosteric inhibitors with promising therapeutic activity, especially in the context of overcoming clinically acquired resistance mutations to the first generation of ATP-competitive kinase inhibitors.




REPORTS

Open Access Article

Activation of HIV-1 LTR by Rad51 in microglial cells

Inna Rom, Armine Darbinyan, Martyn K. White, Jay Rappaport, Bassel E Sawaya, Shohreh Amini and Kamel Khalili
Pages 3739 - 3746
http://dx.doi.org/10.4161/cc.9.18.12930
Abstract | PDF

Infection with HIV-1 induces a variety of biological alterations to the host that are beneficial to the life cycle of the virus but may have adverse effects on the host cell. Here we demonstrate that expression of Rad51, a major component of the homologous recombination-directed DNA repair (HRR) pathway, is induced upon HIV-1 infection of microglial cells. Activation of Rad51 expression positively impacts on HIV-1 LTR transcription through a region of the viral promoter known for binding the inducible transcription factor NF-κB. Rad51 showed the ability to form a complex with the p65 subunit of NF-κB and regulate the level of p65 interaction with LTR DNA encompassing the κB motif. This study provides evidence for reciprocal interaction of HIV-1 and a host DNA repair protein that impacts on expression of the viral genome. These results also point to the ability of HIV-1 to recruit proteins involved in DNA repair that are necessary for retroviral DNA integration, efficient replication and prevention of viral-induced cell death.

Open Access Article

Hypoxia induces PTHrP gene transcription in human cancer cells through the HIF-2α

Michal Manisterski, Maya Golan, Sharon Amir, Yosef Weisman and Nicola Mabjeesh
Pages 3747 - 3753
http://dx.doi.org/10.4161/cc.9.18.12931
Abstract | PDF

Parathyroid hormone-related protein (PTHrP) expressed by human cancer cells enhances tumor cell growth and metastasis in vivo and it is considered as the major factor responsible for humoral hypercalcemia of malignancy. Hypoxia is a widespread feature of most solid tumors. Here, we studied the effects of hypoxia on PTHrP expression. We found that PTHrP is transcriptionally induced by prolonged (48 h) hypoxia in multiple human cancer and endothelial cell lines. Pharmacological up- or down-regulation of hypoxia-inducible factor (HIF) resulted in induction or reduction of PTHrP levels, respectively, implying that PTHrP hypoxic induction is mediated by HIF pathway. Analysis of PTHrP promoter revealed that both HIF-1α and HIF-2α subunits bind to specific hypoxia-responsive elements (HRE) within the P2 promoter of PTHrP. However, only HIF-2α can drive direct transcriptional activation, which can be abolished by mutation in the specific HRE. To the best of our knowledge, these results provide for the first time evidence that PTHrP is regulated by hypoxia in cancer and endothelial cells through the HIF-2 pathway. We suggest that HIF-2 induced by intratumoral hypoxia or by other genetic alterations may contribute to the pathogenesis of hypercalcemia of malignancy and cancer aggressiveness by stimulation of PTHrP expression.

Open Access Article

Recognition of p63 by the E3 ligase ITCH: Effect of an ectodermal dysplasia mutant

Alessia Bellomaria, Gaetano Barbato, Gerry Melino, Maurizio Paci and Sonia Melino
Pages 3754 - 3763
http://dx.doi.org/10.4161/cc.9.18.12933
Abstract | PDF

The E3 ubiquitin ligase Itch mediates the degradation of the p63 protein. Itch contains four WW domains which are pivotal for the substrate recognition process. Indeed, this domain is implicated in several signalling complexes crucially involved in human diseases including Muscular Dystrophy, Alzheimer's Disease, and Huntington Disease. WW domains are highly compact protein-protein binding modules that interact with short proline-rich sequences. The four WW domains present in Itch belong to the Group I type, which binds polypeptides with a PY motif characterized by a PPxY consensus sequence, where x can be any residue. Accordingly, the Itch-p63 interaction results from a direct binding of Itch-WW2 domain with the PY motif of p63. Here, we report a structural analysis of the Itch-p63 interaction by fluorescence, CD and NMR spectroscopy. Indeed, we studied the in vitro interaction between Itch-WW2 domain and p63(534-551), an 18-mer peptide encompassing a fragment of the p63 protein including the PY motif. In addition, we evaluated the conformation and the interaction with Itch-WW2 of a site specific mutant of p63, I549T, that has been reported in both Hay–Wells syndrome and Rapp–Hodgkin syndrome. Based on our results, we propose an extended PPxY motif for the Itch recognition motif (P-P-P-Y-x(4)-[ST]-[ILV]), which includes these C-terminal residues to the PPxY motif.

Open Access Article

Therapeutic targeting of C-terminal binding protein in human cancer

Michael W. Straza, Seema Paliwal, Ramesh C. Kovi, Barur Rajeshkumar, Peter Trenh, Daniel Parker, Giles F. Whalen, Stephen Lyle, Celia A. Schiffer and Steven R. Grossman
Pages 3764 - 3774
http://dx.doi.org/10.4161/cc.9.18.12936
Abstract | PDF

The CtBP transcriptional corepressors promote cancer cell survival and migration/invasion. CtBP senses cellular metabolism via a regulatory dehydrogenase domain, and is antagonized by p14/p19ARF tumor suppressors. The CtBP dehydrogenase substrate 4-methylthio-2-oxobutyric acid (MTOB) can act as a CtBP inhibitor at high concentrations, and is cytotoxic to cancer cells. MTOB induced apoptosis was p53-independent, correlated with the derepression of the pro-apoptotic CtBP repression target Bik, and was rescued by CtBP over-expression or Bik silencing. MTOB did not induce apoptosis in mouse embryonic fibroblasts (MEFs), but was increasingly cytotoxic to immortalized and transformed MEFs, suggesting that CtBP inhibition may provide a suitable therapeutic index for cancer therapy. In human colon cancer cell peritoneal xenografts, MTOB treatment decreased tumor burden and induced tumor cell apoptosis. To verify the potential utility of CtBP as a therapeutic target in human cancer, the expression of CtBP and its negative regulator ARF was studied in a series of resected human colon adenocarcinomas. CtBP and ARF levels were inversely-correlated, with elevated CtBP levels (compared with adjacent normal tissue) observed in greater than 60% of specimens, with ARF absent in nearly all specimens exhibiting elevated CtBP levels. Targeting CtBP may represent a useful therapeutic strategy in human malignancies.

Open Access Article

Spo5 phosphorylation is essential for its own timely degradation and for successful meiosis in Schizosaccharomyces pombe

Daisuke Okuzaki, Takashi Kasama, Aiko Hirata, Ayami Ohtaka, Reiko Kakegawa and Hiroshi Nojima
Pages 3775 - 3784
http://dx.doi.org/10.4161/cc.9.18.12937
Abstract | PDF

Protein phosphorylation is pivotal for meiotic progression, but little is known about its regulatory mechanisms. We show that before meiosis I, the meiosis-specific Schizosaccharomyces pombe protein Spo5 is phosphorylated in vivo on T29, T55, S59 and/or T63. In a mutant strain expressing Spo5 fused to green fluorescent protein with alanine substitutions of these amino acid sites (GFP; Spo5-4A-GFP), the timely degradation of Spo5 at meiosis II was not observed. Additionally, Spo5-4A-GFP signals were retained after metaphase II and were localized to the nucleus. This was accompanied by the nuclear mislocalization of Psy1, a marker of the forespore membrane, (FSM) and the generation of empty cells, in which cytoplasm had leaked from the ruptured membrane, as well as by the appearance of asci harboring deformed spores. Indeed, thin-section electron microscopy (TEM) revealed fragile-looking spo5-4A-GFP ascospores with ruffled spore walls. In contrast, a mutant strain expressing a constitutively-phosphorylated form of Spo5 (Spo5-4D-GFP) was phenotypically indistinguishable from a strain expressing wild-type (WT) protein (Spo5-WT-GFP). Taken together, these results indicate that Spo5 phosphorylation ensures the timely degradation of Spo5 during meiosis and the proper localization of Psy1, leading to the production of viable spores with robust FSMs and strong walls.

Open Access Article

apoB and apobec1, two genes key to lipid metabolism, are transcriptionally regulated by p53

Osnat Ashur-Fabian, Adi Har-Zahav, Aviv Shaish, Hila Wiener Amram, Ofer Margalit, Orly Weizer-Stern, Dan Dominissini, Dror Harats, Ninette Amariglio and Gideon Rechavi
Pages 3785 - 3794
http://dx.doi.org/10.4161/cc.9.18.12993
Abstract | PDF

p53 is an established tumor suppressor gene activating the transcription of multiple target genes. Apolipoprotein B (apo B), a dietary lipid transporter, occurs as apo B-100 and apoB-48, created by a premature stop codon by apo B mRNA-editing enzyme complex 1 (apobec1). We have identified p53 response elements (p53RE) in the genes encoding for apoB and apobec1, cloned these novel p53RE and by performing functionality, chromatin immunoprecipitation (ChIP) and expression assays in cancer cell lines, confirmed that these genes are transcriptionally regulated by p53. In C57bl/6 mice treated with adriamycin, a potent p53 inducer, intestinal/liver mRNA expression of apoB and apobec1 and liver apoB editing levels were elevated. In irradiated wild type C57bl6 mice but not p53 knockout mice, liver and intestine apoB but not apobec1 mRNA expression was elevated. In this work, we have identified that p53 regulates the transcription of two central lipid metabolism players. We further show, for the first time, an involvement of p53 in the RNA editing process, through the transcription of apobec1. Our findings may reveal a previously unknown role for p53 in the direct regulation of atherogenic lipoproteins and a possible role for these genes in classical p53 activities.

Open Access Article

A conserved G₁ regulatory circuit promotes asynchronous behavior of nuclei sharing a common cytoplasm

Dhanalakshmi R. Nair, Cori A. D’Ausilio, Patricia Occhipinti, Mark E. Borsuk and Amy S. Gladfelter
Pages 3795 - 3803
http://dx.doi.org/10.4161/cc.9.18.12999
Abstract | PDF

Synthesis and accumulation of conserved cell cycle regulators such as cyclins are thought to promote G1/S and G2/M transitions in most eukaryotes. ¹ When cells at different stages of the cell cycle are fused to form heterokaryons, the shared complement of regulators in the cytoplasm induces the nuclei to become synchronized.² However, multinucleate fungi often display asynchronous nuclear division cycles, even though the nuclei inhabit a shared cytoplasm. ³ Similarly, checkpoints can induce nuclear asynchrony in multinucleate cells by arresting only the nucleus that receives damage. ^4-6 The cell biological basis for nuclear autonomy in a common cytoplasm is not known.  Here we show that in the filamentous fungus Ashbya gossypii, sister nuclei born from one mitosis immediately lose synchrony in the subsequent G1 interval.  A conserved G1 transcriptional regulatory circuit involving the Rb-analogue Whi5p promotes the asynchronous behavior yetWhi5 protein is uniformly distributed among nuclei throughout the cell cycle.  The homologous Whi5p circuit in S. cerevisiae employs positive feedback to promote robust and coherent entry into the cell cycle. We propose that positive feedback in this same circuit generates timing variability in a multinucleate cell.  These unexpected findings indicate that a regulatory program whose products (mRNA transcripts) are translated in a common cytoplasm can nevertheless promote variability in the individual behavior of sister nuclei.

Open Access Article

Tumor-initiating cells are not enriched in cisplatin-surviving BRCA1;p53-deficient mammary tumor cells in vivo

Marina Pajic, Ariena Kersbergen, Frank van Diepen, Anita Pfauth, Jos Jonkers, Piet Borst and Sven Rottenberg
Pages 3804 - 3815
http://dx.doi.org/10.4161/cc.9.18.13002
Abstract | PDF

Although many breast cancers respond to chemotherapy or hormonal therapy, lack of tumor eradication is a central clinical problem preceding the development of drug resistant tumors. Using the K14cre;Brca1^F5-13/F5-13;p53^F2-10/F2-10 mouse model for hereditary breast cancer, we have previously studied responses of mammary tumors generated in to clinically relevant anti-cancer drugs, including cisplatin. The BRCA1- and p53-deficient tumors generated in this model are hypersensitive to cisplatin and never become resistant to this agent due to the large, irreversible deletion in Brca1. We show here that even dose-dense treatment with a maximum tolerated dose of cisplatin does not result in complete tumor eradication. To explain this result we have addressed the hypothesis that the lack of eradication of drug-sensitive tumors is due to increased in vivo chemotherapy resistance of tumor-initiating cells (TICs). Using the CD24 and CD49f cell surface markers which detect normal mouse mammary stem cells, we have identified tumor-initiating cells in BRCA1- and p53-deficient tumors. In addition to the "OLE_LINK14">Lin^-/CD24⁺/CD49f⁺ subpopulation, we show that a larger population of Lin^-/CD24⁺/CD49f^- cells also has tumor-initiating capability in at least two serial orthotopic transplantations, suggesting that these are not more differentiated transit-amplifying cells. However, we did not find an enrichment of TICs in cisplatin-treated tumor remnants. We conclude that in this model the tolerance of the cisplatin-surviving cells cannot be attributed to special biochemical defense mechanisms of TICs.

 

Open Access Article

Evolutionary dynamics of two related malignant plasma cell lines

David Dingli, Bonnie K. Arendt, Zeljko Bajzer and Diane Jelinek
Pages 3816 - 3821
http://dx.doi.org/10.4161/cc.9.18.13047
Abstract | PDF

Cancer is the consequence of sequential acquisition of mutations within somatic cells. Mutations alter the relative reproductive fitness of cells, enabling the population to evolve in time as a consequence of selection. Cancer therapy itself can select for or against specific subclones. Given the large population of tumor cells, subclones inevitably emerge and their fate will depend on the evolutionary dynamics that define the interactions between such clones. Using a combination of in vitro studies and mathematical modeling, we describe the dynamic behavior of two cell lines isolated from the same patient at different time points of disease progression and show how the two clones relate to one another. We provide evidence that the two clones coexisted at the time of initial presentation. The dominant clone presented with biopsy proven cardiac AL amyloidosis. Initial therapy selected for the second clone that expanded leading to a change in the diagnosis to multiple myeloma. The evolutionary dynamics relating the two cell lines are discussed and a hypothesis is generated in regard to the mechanism of one of the phenotypic characteristics that is shared by these two cell lines.

Open Access Article

Ku80 facilitates chromatin binding of the telomere binding protein, TRF2

Lauren S. Fink, Chad A. Lerner, Paulina F. Torres and Christian Sell
Pages 3822 - 3830
http://dx.doi.org/10.4161/cc.9.18.13129
Abstract | PDF

The Ku70/80 heterodimer is central to non-homologous end joining repair of DNA double-strand breaks and the Ku80 gene appears to be essential for human but not rodent cell survival. The Ku70/80 heterodimer is located at telomeres but its precise function in telomere maintenance is not known.  In order to examine the role of Ku80 fibroblast strain.  Following targeted Ku80 knockdown, telomere defects are observed and the steady state levels of the TRF2 protein are reduced. Inhibitor studies indicate that this loss of TRF2 is mediated by the proteasome, and degradation of TRF2 following Ku depletion appears to involve a decrease in chromatin binding of TRF2, suggesting that the Ku heterodimer enhances TRF2 chromatin association and that non-chromatin bound TRF2 is targeted to the proteasome.

Open Access Article

Metformin regulates breast cancer stem cello ntogeny by transcriptional regulation of the epithelial-mesenchymal transition (EMT) status

Alejandro Vazquez-Martin, Cristina Oliveras-Ferraros, Sílvia Cufí, Sonia Del Barco , Begoña Martin-Castillo and Javier A. Menendez
Pages 3831 - 3838
http://dx.doi.org/10.4161/cc.9.18.13131
Abstract | PDF

The sole overexpression of pivotal regulators of the embryonic Epithelial-Mesenchymal Transition (EMT) genetic program ("EMT status") may be sufficient to efficiently drive the ontogeny of the breast cancer stem cell molecular signature independently of changes in EMT functioning ("EMT phenotype"). Using basal-like breast cancer models naturally enriched in either CD44^posCD24^low/neg or CD44^posCD24^pos tumor-initiating cell populations we herein illustrate that non-cytotoxic concentrations of the anti-diabetic drug metformin efficiently impedes the ontogeny of generating the stem cell phenotype by transcriptionally repressing the stem cell property EMT. Metformin treatment dynamically regulated the CD44^posCD24^neg/low breast cancer stem cell immunophenotype, transcriptionally reprogrammed cells through decreased expression of key drivers of the EMT machinery including the transcription factors ZEB1, TWIST1 and SNAI2 (Slug) and the pleiotrophic cytokines TGFbs, and lastly impeded the propensity of breast cancer stem cells to form multicellular "microtumors" in non-adherent and non-differentiating conditions (i.e. "mammospheres"). These findings, altogether, provide strong motivation for the continued molecular understanding and clinical development of metformin as a non-toxic therapeutic aimed to interdict the breast cancer stem cell phenotype by targeting EMT, a molecular process that is central to the ontogenesis of the breast cancer stem cell molecular signature.

Open Access Article

A screen for deubiquitinating enzymes involved in the G₂/M checkpoint identifies USP50 as a regulator of HSP90-dependent Wee1 stability

Bernadette Aressy, Denis Jullien, Martine Cazales, Marlène Marcellin, Béatrix Bugler, Odile Burlet-Schiltz and Bernard Ducommun
Pages 3839 - 3846
http://dx.doi.org/10.4161/cc.9.18.13133
Abstract | PDF

Tight regulation of cell cycle progression is essential for the maintenance of genomic integrity in response to DNA injury. The aim of this study was to identify new deubiquitinating enzymes (DUBs) involved in the regulation of the G2/M checkpoint. By using an siRNA-based screen to identify DUBs with an inherent ability to enhance a CDC25B-dependent G2/M checkpoint bypass, we have identified 11 candidates whose invalidation compromises checkpoint stringency. We subsequently focused our attention on one of these, the previously uncharacterized USP50. Using a TAP-tag approach associated to mass spectrometry, in addition to a yeast-two-hybrid screen, we identified HSP90 as a major interacting partner for USP50. We also demonstrate USP50 depletion causes a loss in accumulation of the HSP90 client Wee1, which is an essential component of the G2/M cell cycle arrest. Finally, we show that in response to DNA damaging agents, USP50 accumulates in the nucleus. We propose that USP50 may act through a HSP90-dependent mechanism to counteract CDC25B mitotic inducing activity and prevent Wee1 degradation, thereby repressing entry into mitosis following activation of the DNA damage checkpoint.




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