Table of Contents

Volume 6, Issue 19

  October 1, 2007

  Pages 2319 - 2424

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 Issue Catalogue (MARC XML)

EXTRA VIEWS

Open Access Article

p18^Hamlet Mediates Different p53-Dependent Responses to DNA Damage Inducing Agents

Vanesa Lafarga, Ana Cuadrado and Angel R. Nebreda
Pages 2319 - 2322
http://dx.doi.org/10.4161/cc.6.19.4741
Abstract | PDF

Cells organize appropriate responses to environmental cues by activating specific signaling networks. Two proteins that play key roles in coordinating stress responses are the kinase p38α (MAPK14) and the transcription factor p53 (TP53). Depending on the nature and the extent of the stress-induced damage, cells may respond by arresting the cell cycle or by undergoing cell death, and these responses are usually associated with the phosphorylation of particular substrates by p38α as well as the activation of specific target genes by p53. We recently characterized a new p38α substrate, named p18^Hamlet (ZNHIT1), which mediates p53-dependent responses to different genotoxic stresses. Thus, cisplatin or UV light induce stabilization of the p18Hamlet protein, which then enhances the ability of p53 to bind to and activate the promoters of pro-apoptotic genes such as NOXA and PUMA leading to apoptosis induction. In a similar way, we report here that ^Hamlet can also mediate the cell cycle arrest induced in response to γ-irradiation, by participating in the p53-dependent up-regulation of the cell cycle inhibitor p21Cip1 (CDKN1A).

Open Access Article

Regulation of RNA Polymerase III Transcription During Mammalian Cell Growth

Sarah J. Goodfellow and Robert J. White
Pages 2323 - 2326
http://dx.doi.org/10.4161/cc.6.19.4767
Abstract | PDF

RNA polymerase (pol) III manufactures transfer RNAs, 5S ribosomal RNA and several other untranslated RNA molecules that are essential components of the biosynthetic process. Accordingly, transcription by pol III is closely coupled to cellular growth rate. In mammals, stringent regulation of pol III output is achieved through the concerted action of various mechanisms that target the essential pol III-specific transcription factor TFIIIB. Positive regulators of growth, including ERK and c-Myc, directly bind and activate TFIIIB, thus increasing pol III output when growth demands are high. In contrast, TFIIIB is inactive when bound by RB. Growth stimulation leads to RB hyperphosphorylation, which alleviates this repression. These TFIIIB-directed mechanisms regulate pol III transcription in proliferating fibroblasts, and this is likely to contribute to the tight coordination of cell growth with division. Recent evidence indicates that these same pol III-regulatory mechanisms operate in terminally differentiated cells, where growth occurs in the absence of division, leading to hypertrophic enlargement. This cell division-independent regulation of pol III transcription, and hence biosynthetic capacity, is consistent with a direct involvement of these proteins in controlling cell growth. ERK-mediated induction of expression of the TFIIIB subunit Brf1 was identified as an additional mechanism for raising pol III output in terminally differentiated cardiomyocytes. Brf1 levels are limiting for pol III transcription in resting cardiomyocytes and so hypertrophic stimuli induce Brf1 expression as part of the pol III response in this context. The complex network of strategies that couple pol III transcription with cell growth suggest that stringent control of this system is of fundamental importance.

Open Access Article

Fbw7 and Usp28 Regulate Myc Protein Stability in Response to DNA Damage

Nikita Popov, Steffi Herold, Maria Llamazares, Christina Schülein and Martin Eilers
Pages 2327 - 2331
http://dx.doi.org/10.4161/cc.6.19.4804
Abstract | PDF

The cellular levels of the Myc oncoprotein are critical determinants of cell proliferation, cell growth and apoptosis and are tightly regulated by external growth factors. Levels of Myc oncoprotein also decline in response to intracellular stress signals such as DNA damage. We show here that this decline is in part due to proteasomal degradation and that it is mediated by the Fbw7 ubiquitin ligase. We have shown previously that the ubiquitin-specific protease Usp28, binds to the nucleoplasmic isoform of Fbw7, Fbw7α, and counteracts its function in mammalian cells. Usp28 dissociates from Fbw7α in response to UV irradiation, providing a mechanism how Fbw7-mediated degradation of Myc is enhanced upon DNA damage. Our data extend previous observations that link Myc function to the cellular response to DNA damage.




PERSPECTIVES

Open Access Article

Breast Tumor Heterogeneity: Cancer Stem Cells or Clonal Evolution?

Lauren L. Campbell and Kornelia Polyak
Pages 2332 - 2338
http://dx.doi.org/10.4161/cc.6.19.4914
Abstract | PDF

Breast tumors are composed of a variety of cell types with distinct morphologies and behaviors. It is not clear how this tumor heterogeneity comes about. Two popular concepts that attempt to explain this are the cancer stem cell hypothesis and the clonal evolution model. Each of these ideas has been investigated for some time, leading to the accumulation of numerous findings that are used to support one or the other. Although the two views share some similarities, they are fundamentally different notions with very different clinical implications. Analysis of the research backing each concept, along with a review of the results of our recent study investigating putative breast cancer stem cells, suggests how the cancer stem cell hypothesis and the clonal evolution model may be involved in generating breast tumor heterogeneity. An understanding of this process will allow the development of more effective ways to treat and prevent breast cancer.




SPOTLIGHT

Open Access Article

Causes and Consequences of the DNA Damage Response

André Nussenzweig
Pages 2339 - 2340
http://dx.doi.org/10.4161/cc.6.19.4995
Abstract | PDF

A prerequisite for maintaining genome stability in all cell types is the accurate repair and efficient signaling of DNA double strand breaks (DSBs). It is believed that DSBs are initially detected by damage sensors that trigger the activation of transducing kinases. These transducers amplify the damage signal, which is then relayed to effector proteins, which regulate the progression of the cell cycle, DNA repair and apoptosis. Errors in the execution of the repair and/or signaling of DSBs can give rise to multi-systemic disorders characterized by tissue degeneration, infertility, immune system dysfunction, age-related pathologies and cancer. This special Spotlight issue of Cell Cycle highlights recent advances in our understanding of the biology and significance of the DNA damage response. A range of issues are addressed including mechanistic ones: what is the aberrant DNA structure that triggers the activation of the checkpoint - how does chromatin structure influence the recruitment of repair and checkpoint proteins- how does chromosomal instability contribute to the evolution of cancer. In addition, questions related to the physiology of the DNA damage response in normal and abnormal cells is explored: what is the in vivo consequence of altering specific amino acids in a DNA damage sensor- does DNA damage accumulation in stem cells cause aging- how is neurodegeneration linked to deficiencies in specific DNA repair pathways, and finally, what is the biological basis for selection of aberrant DNA damage responses in cancer cells?

Open Access Article

Chromosomal Instability in Cancer: Causality and Interdependence

Ashok R. Venkitaraman
Pages 2341 - 2343
http://dx.doi.org/10.4161/cc.6.19.4753
Abstract | PDF

Aberrations in the number and structure of chromosomes are a common feature of cells from solid tumours. In this brief review, I summarize the evidence that there is a causal relationship between chromosomal instability and the initiation or progression of cancer, and argue that the co-existence of both forms of instability in many solid tumours reflects mechanistic and functional interdependencies that remain poorly investigated.

Open Access Article

DNA Damage Response as an Anti-Cancer Barrier: Damage Threshold and the Concept of 'Conditional Haploinsufficiency'

Jiri Bartek, Jiri Lukas and Jirina Bartkova
Pages 2344 - 2347
http://dx.doi.org/10.4161/cc.6.19.4754
Abstract | PDF

DNA damage response (DDR) emerges as a biological tumorigenesis barrier in early stages of cancer development, and a selective pressure that favors outgrowth of malignant clones with defects in the genome maintenance machinery, such as mutations of p53 and other DDR components. Recent studies indicate that the DDR barrier is not alarmed universally among early noninvasive lesions, but rather responds to high-risk tumorigenic threats that occur in high-grade, pre-malignant lesions that are generally more likely to develop into bona fide malignancies. In addition, while the DDR barrier appears to operate in major types of cancer, such as carcinomas of the lung, breast and colon, DDR activation is rare at any stage of progression among testicular germ-cell tumors. Together with observations that several, but not all oncogenic insults are capable of activating the DDR machinery, these new results point to existence of a critical threshold of such oncogene-induced DNA damage. It seems that only cells and lesions that experience DNA replication stress and DNA damage above such threshold activate the cellular senescence or cell death pathways within the DDR machinery. The higher load of DNA damage may also contribute to cancer predisposition in families with inherited heterozygous defects in the DDR barrier, such as in ATM, BRCA1, BRCA2, p53 and other genes. We propose that carriers of such DDR defects may be more prone to malignancy due to ‘conditional haploinsufficiency’: such partial defects may be asymptomatic in normal tissues, yet they may become manifest under conditions of supra-threshold endogenous DNA damage in oncogene-driven pre-malignant lesions.

Open Access Article

Probing ATR Activation with Model DNA Templates

Karlene A. Cimprich
Pages 2348 - 2354
http://dx.doi.org/10.4161/cc.6.19.4755
Abstract | PDF

The ATR kinase is a critical upstream component of a checkpoint pathway that responds to many forms of damaged and incompletely replicated DNA. Cellular processes such as DNA replication and repair are thought to convert these DNA lesions into a common DNA intermediate that activates this signaling pathway. Indeed, numerous studies have shown that two DNA structures formed during these processes – single-stranded DNA (ssDNA) and junctions between double-stranded DNA (dsDNA) and ssDNA – are important components of the ATR-activating structure. However, an unanswered question is whether primed ssDNA is sufficient for activation of the ATR response. We recently demonstrated that primed ssDNA is sufficient to induce a bona fide checkpoint response in Xenopus egg extracts. This is the first well-defined DNA structure capable of eliciting ATR activation. Using this structure, we examined the contribution of ds/ssDNA junctions and ssDNA to checkpoint activation. Our results indicate the context in which the checkpoint-activating structure is generated may contribute significantly to its signaling properties. Here we discuss the implications of our findings, in the context of other recent work in the field, on our understanding of checkpoint signaling.

Open Access Article

DNA Damage Leaves its Mark on Chromatin

Sophie E. Polo and Geneviève Almouzni
Pages 2355 - 2359
http://dx.doi.org/10.4161/cc.6.19.4756
Abstract | PDF

DNA organization into chromatin has a major influence on the cellular response to DNA damage. Recent studies in various systems ranging from yeast to human cells stress the importance of chromatin not simply as a barrier to DNA repair processes but also as an active contributor to the DNA damage response. Indeed, modulations of chromatin organization involving various degrees of rearrangements, such as histone modifications and even nucleosome displacement, can promote efficient repair and also participate in checkpoint signaling. Here, we survey recent progress in delineating how chromatin rearrangements provide crosstalk with the DNA damage response. In particular, we highlight new data on histone dynamics at damage sites and discuss their functional importance for the stable propagation of specific chromatin states.

Open Access Article

DNA Repair Deficiency and Neurodegeneration

Sachin Katyal and Peter J. McKinnon
Pages 2360 - 2365
http://dx.doi.org/10.4161/cc.6.19.4757
Abstract | PDF

Defects in the response to DNA single-strand or double-strand breaks underpin many human diseases associated with disorders of the nervous system. During nervous system development endogenous DNA damage often results in apoptosis, although cell replacement can occur from germinal zones within this rapidly proliferating tissue. However, if this damage surveillance is faulty, cells with genomic damage may inappropriately become incorporated into the nervous system, and the subsequent demise of these cells may result in neurodegeneration. Ataxia telangiectasia results from defective DNA double strand break signaling, and during development a failure to eliminate damaged neural precursor cells may cause the neurodegeneration present in this disease. In contrast, postmitotic neurons in the mature brain are faced with a less facile option, and in this situation DNA breaks may interfere with transcription required for cellular survival. This scenario may reflect neurodegeneration that occurs in spinocerebellar ataxia with axonal neuropathy, in which single strand break repair is defective. Therefore, the response to DNA damage in the nervous system occurs in a distinct spatiotemporal manner utilizing different DNA repair pathways to ensure genomic stability and to prevent disease.

Open Access Article

The NBS1-ATM Connection Revisited

Simone Difilippantonio and André Nussenzweig
Pages 2366 - 2370
http://dx.doi.org/10.4161/cc.6.19.4758
Abstract | PDF

Nijmegen Breakage syndrome (NBS) is a rare autosomal recessive disorder characterized by microcephaly, immunodeficiency, and increased predisposition to the development of malignancy.^1,2 Due to the overlap of clinical and cellular features of patients with ataxia telangiectasia (AT), NBS was described as an AT variant syndrome until the underlying gene product mutation was identified.^3-5 Cells from both AT and NBS patients show increased sensitivity to ionizing radiation (IR), genomic instability and cell cycle checkpoint defects following DNA damage,^6,7 suggesting that both gene products participate in the same DNA damage response pathway. Here we highlight recent developments and refinements in our understanding of the interplay between NBS1 and ATM in vivo.

Open Access Article

Hematopoietic Stem Cell Quiescence Attenuates DNA Damage Response and Permits DNA Damage Accumulation During Aging

Derrick J. Rossi, Jun Seita, Agnieszka Czechowicz, Deepta Bhattacharya, David Bryder and Irving L. Weissman
Pages 2371 - 2376
http://dx.doi.org/10.4161/cc.6.19.4759
Abstract | PDF

The aging of tissue-specific stem and progenitor cells is believed to be central to the pathophysiological conditions arising in aged individuals. While the mechanisms driving stem cell aging are poorly understood, mounting evidence points to age-dependent DNA damage accrual as an important contributing factor. While it has been postulated that DNA damage may deplete stem cell numbers with age, recent studies indicate that murine hematopoietic stem cell (HSC) reserves are in fact maintained despite the accrual of genomic damage with age. Evidence suggests this to be a result of the quiescent (G0) cell cycle status of HSC, which results in an attenuation of checkpoint control and DNA damage responses for repair or apoptosis. When aged stem cells that have acquired damage are called into cycle under conditions of stress or tissue regeneration however, their functional capacity was shown to be severely impaired. These data suggest that age-dependent DNA damage accumulation may underlie the diminished capacity of aged stem cells to mediate a return to homeostasis after acute stress or injury. Moreover, the cytoprotection afforded by stem cell quiescence in stress-free, steady-state conditions suggests a mechanism through which potentially dangerous lesions can accumulate in the stem cell pool with age.




REPORTS

Open Access Article

Mitochondrial Protein Phosphatase 2A Regulates Cell Death Induced by Simulated Ischemia in Kidney NRK-52E Cells

Chun Chui Tsao, Alina Felicia Nica, Svitlana M. Kurinna, Tilahun Jiffar, Marc Mumby and Peter P. Ruvolo
Pages 2377 - 2385
http://dx.doi.org/10.4161/cc.6.19.4737
Abstract | PDF

Acute renal failure can occur after an ischemic injury and results in significant mortality. The stress-signaling pathways that are activated during renal ischemia are unknown. PP2A has emerged as an important regulator of cell death. To study the role of PP2A in ischemia-induced cell death, we used an in vitro model of simulated ischemia. In the present study, simulated ischemia in rat renal tubule epithelial NRK-52E cells (a) results in cell death that involves both necrosis and apoptosis, (b) activates PP2A, and (c) up-regulates the PP2A B56 α regulatory subunit. Previous data have shown that PKC α negatively regulates B56 α protein expression. Consistent with this finding, simulated ischemia suppressed PKC α and up-regulated B56 α. Treatment of NRK-52E cells with ceramide suppressed PKC α and activated PP2A in a manner that mimicked simulated ischemia. A role for PP2A in simulated ischemia-induced cell death is likely since inhibition of PP2A protected NRK-52E cells. In addition, overexpression of exogenous B56 α but not B55 in NRK-52E cells enhanced simulated ischemia-induced cell death. These findings suggest that activation of a PP2A isoform that contains the B56 α regulatory subunit is required for ischemia-induced cell death in kidney epithelial proximal tubule cells.

Open Access Article

Molecular Basis for the Inhibition of p53 by Mdmx

Grzegorz M. Popowicz, Anna Czarna, Ulli Rothweiler, Aleksandra Szwagierczak, Marcin Krajewski, Lutz Weber and Tad A. Holak
Pages 2386 - 2392
http://dx.doi.org/10.4161/cc.6.19.4740
Abstract | PDF

The oncoprotein Mdm2, and the recently intensely studied, homologues protein Mdmx, are principal negative regulators of the p53 tumor uppressor. The mechanisms by which they regulate the stability and activity of p53 are not fully established. We have determined the crystal structure of the N-terminal domain of Mdmx bound to a 15-residue p53 peptide. The structure reveals that although the principle features of the Mdm2-p53 interaction are preserved in the Mdmx-p53 complex, the Mdmx hydrophobic cleft on which the p53 peptide binds is significantly altered: a part of the cleft is blocked by sidechains of Met and Tyr of the p53-binding pocket of Mdmx. Thus specific inhibitors of Mdm2-p53 would not be optimal for binding to Mdmx. Our binding assays show indeed that nutlins, the newly discovered, potent antagonists of the Mdm2-p53 interaction, are not capable to efficiently disrupt the Mdmx-p53 interaction. To achieve full activation of p53 in tumor cells, compounds that are specific for Mdmx are necessary to complement the Mdm2 specific binders.

Open Access Article

Replication Timing Profile Reflects the Distinct Functional and Genomic Features of the MHC Class II Region

Petros Takousis, Peter Johonnett, Jill Williamson, Peter Sasieni, Gary Warnes, Tim Forshew, Veronique Azuara, Amanda Fisher, Pei-Jun Wu, Tania Jones, Radost Vatcheva, Stephan Beck and Denise Sheer
Pages 2393 - 2398
http://dx.doi.org/10.4161/cc.6.19.4762
Abstract | PDF

The timing of DNA replication generally correlates with transcription, gene density and sequence composition. How is the timing affected if a genomic region has a combination of features that individually correlate with either early or late replication? The Major Histocompatibility Complex (MHC) class II region is an AT-rich isochore that would be expected to replicate late, but it also contains coordinately regulated genes that are highly expressed in antigen-presenting cells and are strongly inducible in other cell types. Using cytological and biochemical assays, we find that the entire MHC replicates within the first half of S-phase, and that the class II region replicates slightly later than the adjacent regions irrespective of gene expression. These data suggest that despite AT-richness, an early-to-middle replication time in the class II region is defined by an open chromatin conformation that allows rapid transcriptional activation as a defence against pathogens.

Open Access Article

Cell Cycle- and Proteasome-Dependent Formation of Etoposide-Induced Replication Protein A (RPA) or Mre11/Rad50/Nbs1 (MRN) Complex Repair Foci

Jacob G. Robison, Kathleen Dixon and John J. Bissler
Pages 2399 - 2407
http://dx.doi.org/10.4161/cc.6.19.4772
Abstract | PDF

In response to DNA damage, cells activate a complex protein network designed to sustain genomic integrity. Many of the proteins involved in the network form discrete repair foci, the composition of which is determined by the specific type of damage. Replication protein A (RPA) and the Mre11/Rad50/Nbs1 (MRN) complex both participate in foci and co-localize at certain types of lesions. Following etoposide (ETOP) treatment, cells form foci containing either RPA or the MRN complex, but not both. To investigate this preferential foci formation, we used cell cycle synchronization experimentation. We demonstrate that cells in S phase contain RPA foci but lack phospho-Nbs1 foci. This is consistent with RPA’s role in homologous recombination repair of DNA double-strand breaks (DSBs), the predominant form of repair during S phase. Cells synchronized at G0/G1 phase contain phospho-Nbs1 foci, consistent with the MRN complex involvement in non-homologous end joining, the predominant form of repair in G1 phase. Treatment of cells with the proteasome inhibitor MG132 dramatically reduced the percentage of cells forming phospho-Nbs1 foci but did not alter the percentage of cells containing RPA or phospho-RPA foci. ETOP induced similar amounts of damage in all phases of the cell cycle as measured by the comet assay. These data suggest that in response to DNA DSBs, cell cycle-preferred repair pathways differentially engage RPA and the MRN complex in repair foci.

Open Access Article

Replication Protein A is Required for Etoposide-Induced Assembly of MRE11/RAD50/NBS1 Complex Repair Foci

Jacob G. Robison, John J. Bissler and Kathleen Dixon
Pages 2408 - 2416
http://dx.doi.org/10.4161/cc.6.19.4773
Abstract | PDF

The presence of DNA damage activates a specific response cascade culminating in DNA repair activity and cell cycle checkpoints. Although the type of lesion dictates what proteins are involved in the response, replication protein A (RPA) and the Mre11/Rad50/Nbs1 complex (MRN) respond to most types of lesions. To examine the relationship of RPA and the MRN complex in DNA damage responses, we used siRNA-mediated protein depletion of RPA-p70 and Mre11. Depletion of RPA-p70 decreased the ability of cells to form phospho-Nbs1 foci and increased levels of DNA double-strand breaks (DSBs) following treatment with etoposide (ETOP). In contrast, depletion of Mre11 led to increased levels of RPA-p34 foci formation, but abrogated phospho-RPA-p34 foci formation. These data support a role for RPA as an initial signal/sensor for DNA damage that facilitates recruitment of MRN and ATM/ATR to sites of damage, where they then work together to fully activate the DNA damage response.

Open Access Article

Mx1 Gene Protects Mice Against the Highly Lethal Human H5N1 Influenza Virus

Rachelle Salomon, Peter Staeheli, Georg Kochs, Hui-Ling Yen, John Franks, Jerold E. Rehg, Robert G. Webster and Erich Hoffmann
Pages 2417 - 2421
http://dx.doi.org/10.4161/cc.6.19.4779
Abstract | PDF

We investigated the importance of the host Mx1 gene in protection against highly pathogenic H5N1 avian influenza virus. Mice expressing the Mx1 gene survived infection with the lethal human H5N1 isolate A/Vietnam/1203/04 and with reassortants combining its genes with those of the non-lethal virus A/chicken/Vietnam/C58/04, while all Mx1–/– mice succumbed. Mx1-expressing mice showed lower organ virus titers, fewer lesions, and less pulmonary inflammation. Our data support the hypothesis that Mx1 expression protects mice against the high pathogenicity of H5N1 virus through inhibition of viral polymerase activity ultimately resulting in reduced viral growth and spread. Drugs that mimic this mechanism may be protective in humans.




LETTER TO THE EDITOR

Open Access Article

An Unexpected Link Between Energy Metabolism, Calcium, Chromatin Condensation and Cell Cycle

Robert M. Martin, Sabine M. Görisch, Heinrich Leonhardt and M. Cristina Cardoso
Pages 2422 - 2424
http://dx.doi.org/10.4161/cc.6.19.4738
Abstract | PDF

Not yet available.