Table of Contents

Volume 4, Issue 9

  September 2005

  Pages 1131 - 1304

View covers archive

 Issue Catalogue (MARC XML)

EXTRA VIEWS

Open Access Article

New and Old Functions of STAT3: A Pivitol Target for Individualized Treatment of Cancer

G. Inghirami, R. Chiarle, W.J. Simmons, R. Piva, K. Schlessinger and D.E. Levy
Pages 1131 - 1133
http://dx.doi.org/10.4161/cc.4.9.1985
Abstract | PDF

Signal transducers and activators of transcription (STAT) regulate a plethora of cytokine responses. Recently, aberrant signaling by STAT proteins has been demonstrated to play important roles in the pathogenesis of many neoplasms, by promoting cell cycle progression and survival, stimulating angiogenesis, and impairing immunological responses and tumor surveillance. We have developed genetic tools to evaluate STAT-dependent malignancy and showed that survival and growth of lymphoid malignancies requires expression of STAT3. In contrast, loss of STAT3 in normal cells does not impair their growth or survival; but in spite of this apparent dispensability of STAT3, STAT3-null fibroblasts are resistant to transformation by a variety of oncogenes. The precise molecular mechanisms responsible for the tumorigenic activity of STAT3 have been only partially elucidated. While the tyrosine phosphorylation of STAT3, which is indicative of its signal-dependent activation, is a common occurrence in tumors, and appears to play a crucial role in some malignancies, a variety of new data suggest that it can be dispensable under some circumstances and STAT3 can participate in transformation through novel and non-canonical mechanisms. The discovery and dissection of non-canonical modes of STAT3 action will open new avenues for the design of effective therapeutics capable of neutralizing the tumorigenic properties of this molecule.

Open Access Article

The Role of CREB as a Proto-oncogene in Hematopoiesis

Kentaro Kinjo, Salemiz Sandoval, Kathleen M. Sakamoto and Deepa B. Shankar
Pages 1134 - 1135
http://dx.doi.org/10.4161/cc.4.9.1991
Abstract | PDF

Cyclic-AMP response element binding protein (CREB) is a transcription factor that functions in glucose homeostasis, growth-factor- dependent cell survival, proliferation and memory. Signaling by hematopoietic growth factors, such as GM-CSF, results in activation of CREB and upregulation of CREB target genes. Data from our laboratory shows that a majority of patients with acute lymphoid and myeloid leukemia overexpress CREB in the bone marrow. CREB overexpression is associated with poor initial outcome of clinical disease in AML patients. To study its role in hematopoiesis, we overexpressed CREB in leukemia cell lines and in mice. CREB overexpression resulted in increased survival and proliferation of myeloid cells and blast-transformation of bone marrow progenitor cells from transgenic mice expressing CREB in the myeloid lineage. CREB transgenic mice also develop myeloproliferative disease after one year. Thus, CREB acts as a proto-oncogene to regulate hematopoiesis and contributes to the leukemia phenotype. Our results suggest that CREB-dependent pathways may serve as targets for directed therapies in leukemia in the future.

Open Access Article

Search for Testicular Cancer Gene Hits Dead-End

Angabin Matin and Joseph H. Nadeau
Pages 1136 - 1138
http://dx.doi.org/10.4161/cc.4.9.1992
Abstract | PDF

Testicular germ cell tumors (TGCTs) are the first tumors where the cell of origin, and the time of transformation were precisely defined. TGCTs in mice originate from primordial germ cells (PGCs) and develop within the testis during fetal development. TGCTs occur at an appreciable frequency (5%) only in the 129 family of inbred strains of laboratory mice, suggesting strong genetic control. These developmental and genetic properties make the 129 strains an exceptional model system to dissect TGCT pathogenesis. Ter is one of the most potent cancer modifiers genes known; it is a single gene mutation that causes progressive loss of PGCs on all inbred strain backgrounds and dramatically increased susceptibility to spontaneous TGCTs only on the 129 background. We recently showed that inactivation of the Dead end gene causes the Ter phenotype. Sequence analysis of Dead end encoded protein indicates it is homologous to factors involved in gene editing. The identity of Ter as Dead end and its function in PGCs will help clarify the role of editing in PGC biology and elucidate the causes of TGCTs in mice and humans.

Open Access Article

Hedgehogs as Negative Regulators of the Cell Cycle

Carl Neumann
Pages 1139 - 1140
http://dx.doi.org/10.4161/cc.4.9.1999
Abstract | PDF

During the development of multicellular animals, cell proliferation must be precisely controlled, as deregulated proliferation can lead to overgrowth and cancer. In addition, proliferation must be tightly integrated with pattern formation and differentiation to generate the required number of cells in the right organs, and at the right time. All major signaling pathways employed during embryogenesis have been implicated in cell cycle regulation, indicating that no single pathway has been dedicated to this task. Also, the precise role of a particular signaling pathway in regulating proliferation is highly dependent on the cellular context, and may have opposite effects on cell-cycle progression in different cells and tissues. The Hedgehog (Hh) family of signaling proteins is known to control both differentiation and proliferation during development. So far, studies addressing the effect of Hh signaling on proliferation have shown it to have a stimulatory effect on cell-cycle progression. Here we review several recent studies indicating that Hh signaling can also have the opposite effect, directing cell-cycle exit in a number of cell types in vertebrate and in invertebrate embryos.

Open Access Article

Apoptosis Through Targeted Activation of Caspase8 (“ATTAC-mice”): Novel Mouse Models of Inducible and Reversible Tissue Ablation

Maria E. Trujillo, Utpal B. Pajvani and Philipp E. Scherer
Pages 1141 - 1145
http://dx.doi.org/10.4161/cc.4.9.2030
Abstract | PDF

Recent studies identifying obesity as a significant and increasingly more common cause of morbidity and mortality have intensified research efforts aimed at increasing our understanding of adipose tissue biology. These efforts have culminated in the discovery of several adipokines, or adipose tissue-derived hormones, that have been implicated in the regulation of multiple physiological functions, as well as the realization that adipose tissue dysfunction plays an important role in the pathogenesis of diseases such as obesity and diabetes. To better understand the role of adipose tissue in these physiological/pathological events, several studies have employed transgenic strategies to eliminate adipose tissue. However, these mouse models of congenital lipoatrophy/lipodystrophy exhibit severe metabolic and somatic cell dysfunction. To circumvent this limitation, we have characterized the first inducible fatless mouse. The FAT-ATTAC mouse is a transgenic model whereby expression of a myristoylated caspase 8-FKBP fusion protein enables selective ablation of adipocytes via induction of apoptosis that occurs upon treatment with a chemical dimerizer. The FAT-ATTAC mouse model not only has the advantage that adipocyte ablation be induced at any time during development, but it is also fully reversible, as adipose tissue regenerates after cessation of dimerizer treatment. The inducibility of this fatless mouse model holds potential for revealing novel physiological roles for adipose tissue as well as its contribution to the etiology and pathogenesis of various disease states. Here we describe several ongoing areas of research employing the FAT-ATTAC mouse; in addition we describe potential uses of the targeted transgenic apoptotic approach to study other cell types of interest.

Open Access Article

The Making of the Liver: Competence in the Foregut Endoderm and Induction of Liver-Specific Genes

Klaus H. Kaestner
Pages 1146 - 1148
http://dx.doi.org/10.4161/cc.4.9.2033
Abstract | PDF

The making of the vertebrate liver occurs in a two-step process, beginning with the establishment of competence in the foregut endoderm to respond to signals from cardiac mesoderm, followed by the induction of liver-specific gene expression. Two winged helix transcription factors, Foxa1 and Foxa2, act in concert in hepatic specification. In a mouse model engineered to lack both of these genes in the foregut endoderm, no liver bud is formed and expression of even the earliest known hepatoblast markers does not occur. Furthermore, foregut endoderm derived from double mutant embryos is not responsive to inductive signals in vitro. The Foxa1/Foxa2 model is the first of a completely “liver-less mouse” and provides strong evidence for the competence model of hepatic induction.

Open Access Article

Microtubule Associated Protein (MAP)-Tau: A Novel Mediator of Paclitaxel Sensitivity In Vitro and In Vivo

P. Wagner, B. Wang, E. Clark, H. Lee, R. Rouzier and L. Pusztai
Pages 1149 - 1152
http://dx.doi.org/10.4161/cc.4.9.2038
Abstract | PDF

Microtubule binding protein Tau was recently identified through gene expression analysis of human breast cancer tissues as a novel marker of response to paclitaxel.1 This article reviews these recent findings and provides additional information to support the role of Tau as an emerging marker and mediator of paclitaxel sensitivity. Low expression of Tau is associated with increased sensitivity to paclitaxel in human breast cancer as well as in a broad range of cell lines. Down regulation of Tau in cell lines by siRNA increases their sensitivity to paclitaxel but not to anthracycline chemotherapy. We propose that this is due to increased paclitaxel binding to microtubules when microtubules are assembled in the presence of low concentrations (or absence) of Tau compared to microtubules that are formed in the presence of physiological (or higher) levels of Tau.

Open Access Article

Regulation of HIF by Prolyl hydroxylases: Recruitment of the Candidate Tumor Suppressor Protein ING4

Abdullah Ozer and Richard K. Bruick
Pages 1153 - 1156
http://dx.doi.org/10.4161/cc.4.9.2040
Abstract | PDF

Many cellular responses to changes in O2 availability are mediated through the hypoxia inducible transcription factor, HIF. HIF regulation is largely dependent on the activity of O2-dependent prolyl hydroxylases. Under normoxic conditions, these enzymes modify HIF to promote its proteasomal degradation. Recently we proposed a second function for the HIF prolyl hydroxylases – recruitment of the candidate tumor suppressor protein ING4 to HIF under hypoxic conditions. Rather than affecting hydroxylase activity or HIF stability, ING4 mediates the ability of HIF to activate transcription of its downstream target genes. This additional mechanism of HIF regulation may serve to fine-tune the magnitude of the cellular hypoxic response under physiological conditions. Furthermore, the identification of a link between ING4 and HIF may shed light on a mechanism by which misregulation of ING4 in tumors promotes angiogenesis, loss of contact inhibition, and changes in cellular proliferation and survival.

Open Access Article

Assembly of GMPCPP-Bound Tubulin into Helical Ribbons and Tubes and Effect of Colchicine

Hong-Wei Wang, Sydney Long, K. Rose Finley and Eva Nogales
Pages 1157 - 1160
http://dx.doi.org/10.4161/cc.4.9.2042
Abstract | PDF

Microtubule assembly and disassembly is a complex structural process that does not proceed by simple addition and subtraction of individual subunits to and from a helical polymer, as would be the case for actin and other helical assemblies. The dynamic process of microtubule growth and shrinking involves short-lasting polymer forms that differ substantially from the microtubule itself and constitute crucial assembly and disassembly intermediates. Structural characterization thus depends on the stabilization of these brief intermediates and their preservation as polymeric assemblies. This paper gives experimental details on the polymerization of GMPCPPtubulin into low-temperature stable polymers that we propose to correspond to the early stages in microtubule assembly and includes new data on the effect of colchicine on GMPCPP-tubulin polymerization. Finally, we include our thoughts on the possible biological meaning of tubulin polymerization versatility.




PERSPECTIVES

Open Access Article

Ran-GTP regulates Kinetochore Attachment in Somatic Cells

Alexei Arnaoutov and Mary Dasso
Pages 1161 - 1165
http://dx.doi.org/10.4161/cc.4.9.1979
Abstract | PDF

The Ran GTPase controls multiple mitotic processes in Xenopus egg extracts, including mitotic checkpoints, spindle assembly and post-mitotic nuclear envelope reassembly. We have analyzed Ran’s role in somatic cells. We uncovered a novel mitotic role of Ran-GTP, involving the Crm1 nuclear export receptor. This pathway is an important mode of Ran-GTP function during mitosis in mammalian somatic cells, which mediates the recruitment of the RanGAP1/RanBP2 complex to kinetochores and maintains the microtubule-based fibers connecting kinetochores to spindle poles (kfibers). Here we discuss potential implications of these findings for normal k-fiber assembly.

Open Access Article

ATM-Mediated Phosphorylations Inhibit Mdmx/Mdm2 Stabilization by HAUSP in Favor of p53 Activation

Erik Meulmeester, Yaron Pereg, Yosef Shiloh and Aart G. Jochemsen
Pages 1166 - 1170
http://dx.doi.org/10.4161/cc.4.9.1981
Abstract | PDF

The p53 tumor suppressor protein has a major role in protecting genome integrity. Under normal circumstances Mdmx and Mdm2 control the activity of p53. Both proteins inhibit the transcriptional regulation by p53, while Mdm2 also functions as an E3 ubiquitin ligase to target both p53 and Mdmx for proteasomal degradation. HAUSP counteracts the destabilizing effect of Mdm2 by direct deubiquitination of p53. Subsequently, HAUSP was shown to deubiquitinate Mdm2 and Mdmx, thereby stabilizing these proteins. The ATM protein kinase is a key regulator of the p53 pathway in response to double strand breaks (DSBs) in the DNA. ATM fine-tunes p53's response to DNA damage by directly phosphorylating it, by regulating additional post-translational modifications of this protein, and by affecting two p53 regulators: Mdm2 and Mdmx. ATM directly and indirectly induces Mdm2 and Mdmx phosphorylation, resulting in decreased activity and stability of these proteins. We recently provided a mechanism for the reduced stability of Mdm2 and Mdmx by showing that ATM-dependent phosphorylation lowers their affinity for the deubiquitinating enzyme HAUSP. Altogether, the emerging picture portrays an elaborate, but fine-tuned, ATM-mediated control of p53 activation and stabilization following DNA damage. Further insight into the mechanism by which ATM switches the interactions between HAUSP, Mdmx, Mdm2 and p53, to favor p53 activation may offer new tools for therapeutic intervention in the p53 pathway for cancer treatment.

Open Access Article

Death-From-Cancer Signatures and Stem Cell Contribution to Metastatic Cancer

Gennadi V. Glinsky
Pages 1171 - 1175
http://dx.doi.org/10.4161/cc.4.9.2001
Abstract | PDF

Genome-wide expression profiling studies reveal a transcriptionally distinct sub-type of human solid tumors with a marked propensity toward metastatic dissemination, highly malignant clinical behavior, and poor therapy outcome in cancer patients diagnosed with the early stage carcinomas of various origins. This sub-set of tumors acquires full metastatic potential, including an emergence and seeding of potent metastasis precursor cells, early in tumor progression. Collectively, these data suggest an early involvement in development of this transcriptionally defined sub-type of human carcinomas of a highly malignant combination of mutant alleles conferring the proclivity to metastasize and/or an engagement in transformation and tumor progression of stem cells and/or early progenitor cells. Enrichment of primary tumors with normal stem cells increases the likelihood of horizontal genomic transfer (large-scale transfer of DNA and chromatin) between stem cells and cancer cells via cell fusion and/or uptake of apoptotic bodies and generation of reprogrammed somatic cell hybrids with self-renewing highly malignant phenotype.

Open Access Article

Poly(ADP-ribose) Polymerase (PARP-1) in Homologous Recombination and as a Target for Cancer Therapy

Thomas Helleday, Helen E. Bryant and Niklas Schultz
Pages 1176 - 1178
http://dx.doi.org/10.4161/cc.4.9.2031
Abstract | PDF

Poly(ADP-ribose) polymerase (PARP-1) binds to DNA breaks to facilitate DNA repair. However, the role of PARP-1 in DNA repair appears to not be critical since PARP-1 knockout mice are viable, fertile and do not develop early onset tumours. Cells isolated from these mice show an increased level of homologous recombination. There is an intricate link between homologous recombination and PARP-1 and a possible role for PARP-1 in DNA double-strand break repair. Although PARP-1 appears not to be required for homologous recombination itself, it regulates the process through its involvement in the repair of DNA single-strand breaks (SSBs). SSBs persisting into the S phase of the cell cycle collapse replication forks, triggering homologous recombination for replication restart. We discuss the recent discoveries on the use of PARP-1 inhibitors as a targeted cancer therapy for recombination deficient cancers, such as BRCA2 tumours.

Open Access Article

MicroRNAs: Critical Regulators of Development, Cellular Physiology and Malignancy

Joshua T. Mendell
Pages 1179 - 1184
http://dx.doi.org/10.4161/cc.4.9.2032
Abstract | PDF

MicroRNAs (miRNAs) are 18-24 nucleotide RNA molecules that regulate the stability or translational efficiency of target mRNAs. These regulatory RNAs function by acting as sequence-specific guides which recruit a large protein complex known as the RNA-induced silencing complex, or RISC, to target mRNAs which are subsequently silenced. Diverse functions have been attributed to miRNAs including the regulation of cellular differentiation, proliferation, and apoptosis. Moreover, significant evidence has accumulated implicating a fundamental role for miRNAs in the development of cancer. We recently demonstrated that the oncogenic transcription factor c-Myc regulates a group of miRNAs known as the mir-17 cluster. This represents the first documented example of a mammalian transcription factor that regulates miRNA expression. Moreover, it was independently demonstrated that the mir-17 cluster accelerates c-Myc-induced lymphomagenesis in an in vivo mouse model. Together, these studies support an important role for this group of miRNAs in c-Myc-mediated tumorigenesis. We have also demonstrated that two miRNAs in this cluster regulate the pro-proliferative, pro-apoptotic transcription factor E2F1. Herein, we propose a model in which the mir-17 cluster prevents excessive E2F1 activity, and thereby apoptosis, in response to activation of c-Myc.

Open Access Article

Crosstalk Between Pten and Ras Signaling Pathways in Tumor Development

Minh D. To, Jesús Pérez-Losada, Jian-Hua Mao and Allan Balmain
Pages 1185 - 1188
http://dx.doi.org/10.4161/cc.4.9.2039
Abstract | PDF

The Pten and Ras pathways are disrupted or activated, respectively, in a substantial proportion of cancers. Skin tumors induced by the classical two stage carcinogenesis protocols show consistent activating mutations of the H-ras gene, but in tumors from Pten heterozygous mice, the frequency of these mutations is markedly decreased, suggesting some redundancy between these pathways. Pten heterozygous mice develop more papillomas and have earlier onset of carcinomas than their control counterparts, but molecular analysis of these tumors indicated that complete loss of Pten and activation of H-ras are mutually exclusive. Pten loss is however not functionally equivalent to H-ras activation, as Pten(-/-) tumors occur earlier and are generally more aggressive. Tumors with Pten loss or H-ras activation have different biochemical properties, suggestive of alternative routes to malignancy. These findings in this mouse model have important implications for the rational design of new targeted therapies for human tumors.

Open Access Article

Activating p38 MAPK: New Tricks for an Old Kinase

Paul R. Mittelstadt, Jesús M. Salvador, Albert J. Fornace, Jr. and Jonathan D. Ashwell
Pages 1189 - 1192
http://dx.doi.org/10.4161/cc.4.9.2043
Abstract | PDF

Mitogen-activated protein kinases (MAPKs) participate in signaling initiated by a wide variety of extracellular stimuli. MAPKs are most commonly activated by a series of phosphorylation events in which one kinase phosphorylates another, the “MAPK cascade”. The cascade concludes with the dual phosphorylation of MAPKs on a conserved Thr-X-Tyr motif. In the case of the p38 MAPK, an exception to this paradigm has been found when signaling via the T cell antigen receptor (TCR). Rather than trigger the MAPK cascade, TCR-mediated stimulation activates proximal tyrosine kinases, which results in the phosphorylation of p38 on a noncanonical activating residue, Tyr-323. This phosphorylation activates p38 to phosphorylate third party substrates as well as its own Thr-X-Tyr motif. Here we discuss the structural and functional implications of this alternative p38 activation pathway, which may provide a new target for tissue-specific pharmacologic inhibition.




REVIEWS

Open Access Article

Repair of Double-Strand DNA Breaks by the Human Nonhomologous DNA End Joining Pathway: The Iterative Processing Model

Yunmei Ma, Haihui Lu, Klaus Schwarz and Michael R. Lieber
Pages 1193 - 1200
http://dx.doi.org/10.4161/cc.4.9.1977
Abstract | PDF

Naturally-occurring ionizing radiation and reactive oxygen species (ROS) from oxidative metabolism are factors that have challenged all life forms during the course of evolution. Ionizing radiation (IR) and reactive oxygen species cause a diverse set of double-strand DNA end configurations. Nonhomologous DNA end joining (NHEJ) is an optimal DNA repair pathway for dealing with such a diverse set of DNA lesions. NHEJ can carry out nucleolytic, polymerase, and ligation operations on each strand independently. This iterative processing nature of NHEJ is ideal for repair of pathologic and physiologic double-strand breaks because it permits sequential action of the NHEJ enzymes on each DNA end and on each strand. The versatility of the Artemis:DNA-PKcs endonuclease in cleaving 5’ and 3’ overhangs, hairpins, gaps, flaps, and various loop conformations makes it well-suited for DNA end modifications on oxidized overhangs. In addition, the ability to cleave stem-loop and hairpin structures permits it to open terminal fold-back configurations that may arise at DNA ends after IR damage. The ability of the XRCC4:DNA ligase IV complex to ligate one strand without ligation of the other permits additional end joining flexibility in NHEJ and raises the possibility of optional involvement of repair proteins from other pathways.

Open Access Article

Molecular Targets and Anticancer Potential of Indole-3-Carbinol and Its Derivatives

Bharat B. Aggarwal and Haruyo Ichikawa
Pages 1201 - 1215
http://dx.doi.org/10.4161/cc.4.9.1993
Abstract | PDF

Indole-3-carbinol (I3C) is produced by members of the family Cruciferae, and particularly members of the genus Brassica (e.g., cabbage, radishes, cauliflower, broccoli, Brussels sprouts, and daikon). Under acidic conditions, 13C is converted to a series of oligomeric products (among which 3,3’-diindolylmethane is a major component) thought to be responsible for its biological effects in vivo. In vitro, 13C has been shown to suppress the proliferation of various tumor cells including breast cancer, prostate cancer, endometrial cancer, colon cancer, and leukemic cells; induce G1/S arrest of the cell cycle, and induce apoptosis. The cell cycle arrest involves downregulation of cyclin D1, cyclin E, cyclin-dependent kinase (CDK)2, CDK4, and CDK6 and upregulation of p15, p21, and p27. Apoptosis by I3C involves downregulation antiapoptotic gene products, including bcl-2, bcl-xl, survivin, inhibitor-of-apoptosis protein (IAP), X chromosome-linked IAP (XIAP), and Fas-associated death domain protein-like interleukin-1-b-converting enzyme inhibitory protein (FLIP); upregulation of proapoptotic protein bax; release of micochondrial cytochrome C; and activation of caspase-9 and caspase-3. This agent inhibits the activation of various transcription factors including nuclear factor-kappaB, SP1, estrogen receptor, androgen receptor and nuclear factor-E2-related factor 2 (Nrf2). This indole potentiates the effects of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) through induction of death receptors and synergises with chemotherapeutic agents through downregulation of P-glycoprotein (P-gp). In vivo, I3C was found to be a potent chemopreventive agent for hormonal-dependent cancers such as breast and cervical cancer. These effects are mediated through its ability to induce apoptosis, inhibit DNA-carcinogen adduct formation, and suppress free-radical production, stimulate 2-hydroxylation of estradiol, inhibit invasion and angiogenesis. Numerous studies have indicated that I3C also has a strong hepatoprotective activity against various carcinogens. Initial clinical trials in women have shown that I3C is a promising agent against breast and cervical cancers.

Open Access Article

Good Timing in the Cell Cycle for Precise DNA Repair by BRCA1

Stephen T. Durant and Jac A Nickoloff
Pages 1216 - 1222
http://dx.doi.org/10.4161/cc.4.9.2027
Abstract | PDF

It is now clear that large DNA-binding proteins have evolved in mammals to orchestrate the relatively ancient processes of DNA recombinational repair. These proteins are recruited to accurately repair DNA double-strand breaks (DSBs) - the frequent, potentially lethal and mutagenic lesions in the genomes of all organisms. An essential mammalian regulator of DSB repair is BRCA1. Heterozygous BRCA1 mutations predispose individuals to breast, ovarian and other secondary cancers. BRCA1-defective cells exhibit reduced DSB repair, sensitivity to a wide range of DNA damaging agents, genomic instability and defects in the S-phase checkpoint, transcription and chromatin remodelling. DSBs can be repaired by Rad51/RPA-dependent homologous recombination (HR) or DNA-PK-dependent non-homologous end-joining (NHEJ). Both of these pathways can be imprecise and mutagenic. BRCA1 plays a central role in promoting accurate repair by both HR and NHEJ. Consistent with recent evidence, we have assembled a novel cell-cycle-dependent model in which DNA-PK inhibits RPA in S-phase of the cell cycle, while BRCA1 inhibits the exonuclease processivity of the MRE11/RAD50/NBS1 (MRN) complex and facilitates the removal of RPA in S and G2 phase. This model provides an explanation for how BRCA1 promotes accurate DSB repair during various phases of the cell cycle and also accounts for the dual effects that BRCA1 and MRN activity have upon DNA repair and S-phase arrest.




BRIEF REPORTS

Open Access Article

DNA Replication Initiates at Different Sites in Early and Late S Phase within Human Ribosomal RNA Genes

Frederick D. Coffman, Mai He, Mai-Ling Diaz and Stanley Cohen
Pages 1223 - 1226
http://dx.doi.org/10.4161/cc.4.9.1961
Abstract | PDF

Metazoan replication origins often contain multiple potential initiation sites, and the selection of which of the potential sites are used appears to be dependent upon multiple factors, including the state of differentiation, cell metabolism, and local transcriptional activity. Numerous studies have shown that a replication origin exists within the nontranscribed spacer region of the human ribosomal RNA gene. We here analyze nascent leading strand DNA from S phase human lymphoid cells, and find that while the majority of rDNA replicates in mid- and late S phase and preferentially initiates replication 6 kbp from the transcription start site, in very early S phase the preferred initiation site is much closer to the transcription start site and may involve rDNA promoter sequences. This early site is coincident with a minimum GC skew value, diagnostic for replication origins in bacteria and yeast. These results suggest that replication timing can influence initiation site selection. The timing and nucleolar localization of rDNA further suggest that this site likely participates in the small number of perinucleolar initiation foci observed in very early S phase cells that represent the beginning of cellular DNA replication.

Open Access Article

The Abnormal Spindle-like, Microcephaly-associated (ASPM) Gene Encodes a Centrosomal Protein

Xueyan Zhong, Limin Liu, Ailian Zhao, Gerd P. Pfeifer and Xingzhi Xu
Pages 1227 - 1229
http://dx.doi.org/10.4161/cc.4.9.2029
Abstract | PDF

Homozygous mutations in the abnormal spindle-like, microcephaly-associated ASPM gene are the leading cause of autosomal recessive primary microcephaly. ASPM is the putative human ortholog of the Drosophila melanogaster abnormal spindles gene (asp), which is essential for mitotic spindle function. Here, we report that downregulation of endogenous ASPM by siRNA decreases protein levels of endogenous BRCA1. ASPM localizes to the centrosome in interphase and to the spindle poles from prophase through telophase. These findings indicate that ASPM may be involved in mitotic spindle function, possibly, through regulation of BRCA1.

Open Access Article

Involvement of the SCF Complex in the Control of Cdh1 Degradation in S-phase

Ramla Benmaamar and Michele Pagano
Pages 1230 - 1232
http://dx.doi.org/10.4161/cc.4.9.2048
Abstract | PDF

The anaphase promoting complex/cyclosome (APC/C) is a multisubunit ubiquitin ligase that acts as a key regulator in the progression through mitosis (when mostly in complex with Cdc20) and as a stabilizer of the G1 phase (when in complex with Cdh1). Cdh1 is an activator of APC/C, and it has previously been reported that it is capable of mediating its own degradation during Go and G1. Herein, we show that the SCF complex (Skp1/Cul1/F-box protein/Roc1) intervenes in the surveillance of Cdh1 cellular abundance in S-phase.




REPORTS

Open Access Article

CDC25B Phosphorylation by Aurora A Occurs at the G2/M Transition and is Inhibited by DNA Damage

Martine Cazales, Estelle Schmitt, Emilie Montembault, Christine Dozier, Claude Prigent and Bernard Ducommun
Pages 1233 - 1238
http://dx.doi.org/10.4161/cc.4.9.1964
Abstract | PDF

CDC25B is one of the three human dual-specificity phosphatases involved in the activation of cyclin-dependent kinases at key stages of the cell division cycle. CDC25B that is responsible for the activation of CDK1-cyclin B1 is regulated by phosphorylation. The STK15/Aurora-A kinase locally phosphorylates CDC25B on serine 353 at the centrosome during the G2/M transition. Here we have investigated this phosphorylation event during the cell cycle, and in response to activation of the G2 DNA damage checkpoint. We show that accumulation of the S353-phosphorylated form of CDC25B at the centrosome correlates with the relocalisation of cyclin B1 to the nucleus and the activation of CDK1 at entry into mitosis. Upon activation of the G2/M checkpoint by DNA damage, we demonstrate that Aurora-A is not activated and consequently CDC25B is not phosphorylated. We show that ectopic expression of Aurora-A results in a bypass of the checkpoint that partially overcome by a S353A mutant of CDC25B. Finally, we show that bypass of the G2/M checkpoint by the CHK1 kinase inhibitor UCN-01 results in the activation of Aurora-A and phosphorylation of CDC25B on S353. These results strongly suggest that Aurora-A-mediated phosphorylation of CDC25B at the centrosome is an important step contributing to the earliest events inducing mitosis, upstream of CDK1-cyclin B1 activation.

Open Access Article

The Intracellular Tyrosine Kinase Brk Sensitizes Nontransformed Cells to Inducers of Apoptosis

Andrea Haegebarth, Rafael Nunez and Angela L. Tyner
Pages 1239 - 1246
http://dx.doi.org/10.4161/cc.4.9.1965
Abstract | PDF

The intracellular tyrosine kinase Brk is expressed in regenerating epithelial tissues with highest levels in the gastrointestinal tract and skin. In these tissues, Brk is restricted to epithelial cells that are exiting the cell cycle and undergoing terminal differentiation. While not expressed in mammary gland, Brk expression is often induced in primary breast tumors and breast tumor cell lines. To identify potential oncogenic functions of Brk, we utilized the immortalized Rat1a rat fibroblast cell line that is highly sensitive to transformation. We generated Rat1a cell lines that stably overexpress wild-type and activated Brk, and we analyzed their growth properties and ability to undergo apoptosis in response to stress and DNA damage. Overexpression of Brk did not induce anchorage-independent growth, and no changes in cell morphology or cell cycle progression were observed. However, when constitutively expressed, Brk sensitized Rat1a cells to a variety of apoptotic stimuli including serum deprivation and a combination of UV irradiation and serum starvation. These findings indicate that Brk does not promote proliferation of nontransformed cells, but plays a positive role in the regulation of the apoptotic response to DNA-damage and stress.

Open Access Article

Blockage of NF-κB Induces Serine 15 Phosphorylation of Mutant p53 by JNK Kinase in Prostate Cancer Cells

Luiz F. Zerbini, Yihong Wang, Ricardo G. Correa, Je-Yoel Cho and Towia A. Libermann
Pages 1247 - 1253
http://dx.doi.org/10.4161/cc.4.9.1966
Abstract | PDF

The p53 tumor suppressor gene plays an important role during induction of apoptosis in cancer. In contrast, NF-κB prevents apoptosis in response to chemotherapeutic agents and is a critical regulator of cell survival. Despite the riches of information on the regulation of wild-type p53 function by phosphorylation, nothing is known about the modulation of mutant p53 activity by phosphorylation. Here we report that inhibition of NF-κB in DU145 prostate cancer cells results in p53 mutant phosphorylation at serine 15 (Ser15), leading to an increase of p53 stability, DNA binding and gain of function. Serine 15-phosphorylation is due to GADD45a-dependent induction of JNK kinase, which can be blocked by SP600125, a JNK kinase inhibitor. Furthermore, inhibition of GADD45a by small interfering RNA blocks JNK activation and abrogates Ser15 phosphorylation. Together, these results highlight the importance of Ser15 phosphorylation in regulating the oncogenic function of mutant p53 and apoptosis induction in the context of the NF-κB/IκB signaling pathway.

Open Access Article

Human CDK2 Inhibition Modifies the Dynamics of Chromatin-Bound Minichromosome Maintenance Complex and Replication Protein A

Yonghong Zhu, Yukio Ishimi, Marcel Tanudji and Emma Lees
Pages 1254 - 1263
http://dx.doi.org/10.4161/cc.4.9.1975
Abstract | PDF

Minichromosome maintenance (MCM) proteins form a complex and possess helicase activity to unwind the DNA duplex and establish a replication fork. To assure that origins only fire once per cell cycle, the MCM complex is removed from chromatin and inactivated as cells exit S phase. In this report, we demonstrate that CDK2 depletion in human cells leads to an overall phosphorylation defect at mitosis with increased rereplication, correlated with the accumulation of chromatin-bound MCM proteins. We show that CDK2 suppression results in decreased MCM4 phosphorylation at multiple serine and threonine sites. In addition, CDK2 inhibition induces an increase in chromatin-bound replication protein A (RPA) which should bind to single-stranded DNA regions, possibly establishing a replication intermediate that activates the ATR cascade. Finally, we observe that loss of CDK2 function in G1 delays replication initiation while it promotes rereplication in G2/M. Thus, by modulating the phospho-status of MCM4 and regulating origin firing, S phase CDK2 appears to be an integrated component of cellular machinery required for temporally controlling replication activity and maintaining genomic stability.

Open Access Article

p53 Codon 72 Alleles Influence the Response to Anticancer Drugs in Cells from Aged People by Regulating the Cell Cycle Inhibitor p21^WAF1

Stefano Salvioli, Massimiliano Bonafé, Cristiana Barbi, Gianluca Storci, Chiara Trapassi, Francesca Tocco, Silvia Gravina, Matteo Rossi, Luca Tiberi, Chiara Mondello, Daniela Monti and Claudio Franceschi
Pages 1264 - 1271
http://dx.doi.org/10.4161/cc.4.9.1978
Abstract | PDF

A common polymorphism at codon 72 in p53 gene leads to an arginine to proline aminoacidic substitution which affects in an age-dependent manner the susceptibility of cells to undergo apoptosis after oxidative stress. Here we report that dermal fibroblasts from Proline allele carriers (Pro+) display a higher expression of p21WAF1 gene, in both basal conditions and after treatment with doxorubicin and camptothecin. This phenomenon is accompanied by a lower susceptibility of Pro+ cells to undergo apoptosis, a lower capability to over cross G1-S transition and an increased propensity to express markers of cell senescence, with respect to fibroblasts from Arginine homozygotes (Pro-). All these phenomena are particularly evident in cells from centenarians. We conclude that the functional difference between the two p53 codon 72 alleles exerts a broad impact on the capability of cell from aged people to respond to stressors such as cytotoxic drugs.

Open Access Article

Retinoblastoma Family 2 is Required In Vivo for the Tissue-Specific Repression of dE2F2 target Genes

Olivier Stevaux, Dessislava K. Dimova, Jun-Yuan Ji, Nam Sung Moon, Maxim V. Frolov and Nicholas J. Dyson
Pages 1272 - 1280
http://dx.doi.org/10.4161/cc.4.9.1982
Abstract | PDF

In higher eukaryotes, the Retinoblastoma and E2F families of proteins control the transcription of a large number of target genes. Here, we have mutated the second Drosophila Retinoblastoma family gene (Rbf2), and contrasted the in vivo molecular functions of RBF2 with dE2F2, the only E2F partner of RBF2. Previous studies failed to uncover a unique role for RBF2 in E2F regulation. Here we find that RBF2 functions in concert with dE2F2 in vivo to repress the expression of differentiation markers in ovaries and embryos where RBF2 is highly expressed. We have compared the profiles of transcripts that are mis-expressed in ovaries, embryos and S2 cells where RBF2 function has been ablated and find that RBF2 and dE2F2 control strikingly different transcriptional programs in each situation. In vivo promoter occupancy studies point to the redistribution of dE2F/RBF complexes to different promoters in different cell types as one mechanism governing the tissue-specific regulation of dE2F/RBF target genes. These results demonstrate that RBF2 has a unique function in repressing E2F-regulated differentiation markers and that dE2F2 and RBF2 are required to regulate different sets of target genes in different tissues.

Open Access Article

APOBEC4, a New Member of the AID/APOBEC Family of Polynucleotide (Deoxy)Cytidine Deaminases Predicted by Computational Analysis

Igor B. Rogozin, Malay K. Basu, I. King Jordan, Youri I. Pavlov and Eugene V. Koonin
Pages 1281 - 1285
http://dx.doi.org/10.4161/cc.4.9.1994
Abstract | PDF

Using iterative database searches, we identified a new subfamily of the AID/APOBEC family of RNA/DNA editing cytidine deaminases. The new subfamily, which is represented by readily identifiable orthologs in mammals, chicken, and frog, but not fishes, was designated APOBEC4. The zinc-coordinating motifs involved in catalysis and the secondary structure of the APOBEC4 deaminase domain are evolutionarily conserved, suggesting that APOBEC4 proteins are active polynucleotide (deoxy)cytidine deaminases. In reconstructed maximum likelihood phylogenetic trees, APOBEC4 forms distinct clade with a high statistical support. APOBEC4 and APOBEC1 are joined in a moderately supported cluster clearly separated from AID, APOBEC2 and APOBEC3 subfamilies. In mammals, APOBEC4 is expressed primarily in testis which suggests the possibility that it is an editing enzyme for mRNAs involved in spermatogenesis.

Open Access Article

Role of the Cell Cycle in the Pathobiology of Central Nervous System Trauma

Ibolja Cernak, Bogdan A. Stoica, Kimberly R. Byrnes, Simone Di Giovanni and Alan I. Faden
Pages 1286 - 1293
http://dx.doi.org/10.4161/cc.4.9.1996
Abstract | PDF

Up-regulation of cell cycle proteins occurs in both mitotic and post-mitotic neural cells after central nervous system (CNS) injury in adult animals. In mitotic cells, such as astroglia and microglia, they induce proliferation, whereas in post-mitotic cells such as neurons they initiate caspase-related apoptosis. We recently reported that early central administration of the cell cycle inhibitor flavopiridol after experimental traumatic brain injury (TBI) significantly reduced lesion volume, scar formation and neuronal cell death, while promoting near complete behavioral recovery. Here we show that in primary neuronal or astrocyte cultures structurally different cell cycle inhibitors (flavopiridol, roscovitine, and olomoucine) significantly reduce up-regulation of cell cycle proteins, attenuate neuronal cell death induced by etoposide, and decrease astrocyte proliferation. Flavopiridol, in a concentration dependent manner, also attenuates proliferation/activation of microglia. In addition, we demonstrate that central administration of flavopiridol improves functional outcome in dose-dependent manner after fluid percussion induced brain injury in rats. Moreover, delayed systemic administration of flavopiridol significantly reduces brain lesion volume and edema development after TBI. These data provide further support for the therapeutic potential of cell cycle inhibitors for the treatment of clinical CNS injury and that protective mechanisms likely include reduction of neuronal cell death, inhibition of glial proliferation and attenuation of microglial activation.

Open Access Article

Topoisomerase II Suppresses the Temperature Sensitivity of Saccharomyces cerevisiae pds5 Mutants, but not the Defect in Sister Chromatid Cohesion

Cristina Aguilar, Christian Davidson, Melissa Dix, Kristen Stead, Ke Zheng, Theresa Hartman and Vincent Guacci
Pages 1294 - 1304
http://dx.doi.org/10.4161/cc.4.9.1997
Abstract | PDF

Sister chromatid cohesion enables chromosomes to achieve bipolar attachment to the mitotic spindle and its dissolution is required for chromosome segregation. The cohesin complex serves as the primary molecular glue responsible for cohesion. Pds5p binds to the same chromosomal loci as the cohesin complex but plays a distinct role as a regulator of cohesion maintenance. Catenation between sister chromatids must also be removed by Topoisomerase II (Top2p) enzymatic activity to enable chromosome segregation. We identified TOP2 as a high-copy suppressor of the temperature sensitivity of pds5 mutants. TOP2 suppression is specific for pds5 mutants as it does not suppress mutants in the cohesin complex. TOP2 suppresses mini-chromosome loss in pds5 mutants indicating that it rescues a chromosome segregation defect. Surprisingly, TOP2 over-expression fails to suppress the cohesion defect of pds5 mutants, suggesting that it suppresses an additional and as yet uncharacterized defect in pds5 mutants that is essential for viability. A catalytically dead TOP2 allele suppresses pds5 temperature sensitivity, suggesting that suppression is unrelated to Top2p enzymatic function. Consistent with this idea, when the pds5 mutant is combined with the top2-4 mutant, which accumulates DNA catenanes due to defective enzymatic activity, the double mutants exhibit synthetic sickness indicating that increased catenation is toxic to pds5 cells. Our results suggest that Pds5p and Top2p cooperate to promote proper chromosome segregation by a mechanism unrelated to either cohesion or catenation/decatenation.