Table of Contents

Volume 6, Issue 24

  December 15, 2007

  Pages 2997 - 3147

View covers archive

CONCEPTUAL REVIEW

Open Access Article

Paradoxes of Aging

Mikhail V. Blagosklonny
Pages 2997 - 3003
http://dx.doi.org/10.4161/cc.6.24.5124
Abstract | PDF

Insightful articles by Kirkwood and other outstanding scientists reveal paradoxes of aging. The source of paradoxes is an assumption that aging is caused by random accumulation of molecular damage. Here I demonstrate that a concept of TOR-driven program-like aging almost automatically resolves eleven paradoxes of aging. This article discusses why the accumulation of molecular damage does not limit life span, why calorie restriction and inhibition of protein synthesis extend life span, why non-existing ‘program’ for aging is nevertheless robust, why a key gene for aging cannot be found by knocking it out, why low insulin is associated with good health but low insulin response with bad health, why aging is not a disease but can be treated as a disease, why ‘healthy’ aging is slow aging, and how do we know that calorie restriction actually slows aging in humans.




EXTRA VIEWS

Open Access Article

A Cul3-Based E3 Ligase Regulates Mitosis and is Required to Maintain the Spindle Assembly Checkpoint in Human Cells

Izabela Sumara and Matthias Peter
Pages 3004 - 3010
http://dx.doi.org/10.4161/cc.6.24.5068
Abstract | PDF

The spindle assembly checkpoint (SAC) is a mechanism that prevents premature chromosome segregation in anaphase before all chromosomes are correctly attached to the mitotic spindle. Errors in chromosome segregation lead to aneuploidy, which may be causally involved in tumorgenesis. Kinetochore complexes are the structural components of the SAC, which are tightly regulated by various mechanisms including phosphorylation and ubiquitin-dependent proteolysis. Recent studies shed new light on the regulatory pathways of the ubiquitin proteasome system involved in SAC signaling. Here we present evidence that a Cul3-based E3 ubiquitin-ligase is required to maintain SAC signaling in human cells. Inactivation of the Cul3/KLHL9/KLHL13 ligase leads to premature degradation of Cyclin B and exit from the mitotic state in the presence of microtubule poisons. We discuss possible mechanisms how Cul3 may be required to maintain SAC activity by ubiquitination of the chromosomal passenger protein Aurora B.

Open Access Article

All Roads Lead to mTOR: Integrating Inflammation and Tumor Angiogenesis

Dung-Fang Lee and Mien-Chie Hung
Pages 3011 - 3014
http://dx.doi.org/10.4161/cc.6.24.5085
Abstract | PDF

Mammalian target of rapamycin (mTOR) is a crucial molecule in the control of cell size and proliferation; dysregulation of the mTOR pathway is commonly found in human cancers. Many cancer-promoting kinases have been identified as regulators of mTOR activity through phosphorylation and inactivation of the TSC1–TSC2 complex. Tumor-associated macrophages (TAMs) are tumor-promoting factors in inflammation-mediated tumor development, and the signaling molecules involved in TAMs-mediated tumor angiogenesis are not well understood. Therefore, it is urgent to elucidate the cross-talk between inflammatory cells and cancers and to explore the precise pathways involved in TAMs-induced tumor angiogenesis. Recently IKKβ was found to activate the mTOR pathway and to promote tumor angiogenesis through inactivation of the TSC1–TSC2 complex by phosphorylating TSC1. This finding provides critical insights into and suggests one mechanism behind inflammation-mediated tumor angiogenesis. In this extra-view, we briefly discuss the possible influence of TAMs-released proangiogenic factors on mTOR activation and propose a model of the cross-talk between tumors and TAMs in tumor angiogenesis.

Open Access Article

Multiple Mechanisms for p27^Kip1 Translocation and Degradation

Etsuo Susaki and Keiichi I. Nakayama
Pages 3015 - 3020
http://dx.doi.org/10.4161/cc.6.24.5087
Abstract | PDF

The nuclear export and rapid degradation of p27^Kip1 at the G₀–G₁ transition are critical events for effective progression of the cell cycle. Several pathways have been proposed at the molecular level for the export of this cyclin-dependent kinase inhibitor from the nucleus. However, the addition of each new pathway renders the situation more complicated. We recently showed that cyclin D2 links growth signals to the cytoplasmic translocation and degradation of p27 at the G₀–G₁ transition. Here we describe our findings and discuss how the multiple potential mechanisms for p27 translocation that precedes its degradation might be integrated in the context of growth stimulation and G1 progression.

Open Access Article

Chemosensitizing Anti-Cancer Activity of LFM-A13, a Leflunomide Metabolite Analog Targeting Polo-like Kinases

Fatih M. Uckun
Pages 3021 - 3026
http://dx.doi.org/10.4161/cc.6.24.5096
Abstract | PDF

LFM-A13 (α-cyano-β-hydroxy-β-methyl-N-(2,5-dibromophenyl)propenamide) has recently been identified as an inhibitor of Polo-like kinases (Plk). LFM-A13 does not inhibit other serine/threonine kinases including CDK, CHK, RAF, DAPK, IKK, IRAK, JNK, MAPK, PKC, and SAPK. LFM-A13-treated human cancer cells develop abnormal mitotic spindles and G₂/M-arrest during cell cycle progression. LFM-A13 was not toxic to rodents or dogs at daily dose levels as high as 100 mg/kg. Notably, at a low dose level of 10 mg/kg, which does not result in delayed tumor progression in the MMTV/neu transgenic mouse model of HER2 positive breast cancer, LFM-A13 markedly enhanced the anti-cancer activity of the mitotic spindle poison paclitaxel. These results indicate that LFM-A13 may be useful in the treatment of cancer patients.

Open Access Article

The Dissection of Human Autosomal Recessive Osteopetrosis Identifies an Osteoclast-Poor Form Due to RANKL Deficiency

Annalisa Frattini, Paolo Vezzoni, Anna Villa and Cristina Sobacchi
Pages 3027 - 3033
http://dx.doi.org/10.4161/cc.6.24.5105
Abstract | PDF

Genetic dissection of human recessive osteopetroses (ARO) has identified specific subsets due to a defect in molecules linked to the effector function of mature osteoclasts. While an impairment in osteoclast differentiation in mouse leads to osteopetrosis, the four genes identified so far in classical human ARO (TCIRG1, CLCN7, OSTM1 and PLEKHM1) are all involved in the resorption and/or intracellular traffic of the minerals solubilised from bone matrix. The recent finding that the RANKL gene is mutated in a subset of ARO patients whose biopsies did not show any osteoclast shows that a differentiation defect can be responsible for human ARO and paves the way to a potential rational therapy of this rare disease by soluble RANKL administration.

Open Access Article

Revealing a Role of MicroRNAs in the Regulation of the Biological Clock

Hai-Ying Mary Cheng and Karl Obrietan
Pages 3034 - 3038
http://dx.doi.org/10.4161/cc.6.24.5106
Abstract | PDF

In the last decade numerous studies have unveiled the pervasive role of microRNAs (miRNAs), a class of small, non-coding transcripts, in post-transcriptional gene regulation in biological processes ranging from development to cancer. Until recently, the circadian clock has been modeled as simple, interlocking transcriptional feedback loops that drive rhythmic gene expression of a few core ‘clock’ determinants. The biological implications of miRNAs are extended further by our recent discovery that miRNAs are expressed in the suprachiasmatic nuclei (SCN), the master circadian clock in mammals, in a rhythmic and inducible fashion, and modulate the intrinsic pacemaker activity and resetting capacity of the SCN. In this review, we will discuss the specific roles of miRNA- (miR-)132 and miR-219 in the SCN, as well as a more general outlook on this newly elucidated layer of circadian clock regulation: inducible translation control via miRNAs.




PERSPECTIVES

Open Access Article

CDC25A Levels Determine the Balance of Proliferation and Checkpoint Response

Dipankar Ray and Hiroaki Kiyokawa
Pages 3039 - 3042
http://dx.doi.org/10.4161/cc.6.24.5104
Abstract | PDF

Current evidence suggests that CDC25A is not only a major regulator of both G1/S and G2/M transition during unperturbed cell cycle progression, but also a critical checkpoint mediator. While CDC25A is overexpressed in a variety of human cancers, a key question remained unanswered whether such overexpression of this CDK-activating phosphatase was a mechanism or consequence of accelerated proliferation and other malignant phenotypes. Recent studies on the tumor suppressive roles of checkpoint proteins suggest that overriding checkpoint response leads normal or pre-cancerous cells to genomic instability and cumulative malignant changes. Here we provide our views on the role of CDC25A in cancer development and genomic stability, discussing insights from our recent studies on Cdc25A knockout mice and MMTV-CDC25A transgenic mice.

Open Access Article

Regulation of Mitochondrial Morphological Dynamics During Apoptosis by Bcl-2 family proteins: A Key in Bak?

Craig Brooks and Zheng Dong
Pages 3043 - 3047
http://dx.doi.org/10.4161/cc.6.24.5115
Abstract | PDF

Early during apoptosis, the mitochondrial network collapses into short punctate fragments. The seemingly morphological change, called mitochondrial fragmentation, contributes to mitochondrial injury. Mitochondrial morphology is dictated by two opposing processes, fission and fusion. It is unclear how the fission-fusion balance is tilted during apoptosis, resulting in mitochondrial fragmentation. Emerging evidence has now suggested a regulation of mitochondrial morphological dynamics by Bcl-2 family proteins. In this regulation, Bak appears to be a key. Through interaction with mitofusins, Bak may block mitochondrial fusion to induce fragmentation. By this function, Bak may collaborate with Bax to permeabilize mitochondrial outer membrane, leading to the release of apoptogenic factors.

Open Access Article

Does Caspase Inhibition Promote Clonogenic Tumor Growth?

Ute Fischer, Katja Janssen and Klaus Schulze-Osthoff
Pages 3048 - 3053
http://dx.doi.org/10.4161/cc.6.24.5118
Abstract | PDF

Defects in the control of cell death are a major cause of resistance to tumor therapy. Until recently, components of the intrinsic apoptotic pathway that act downstream of mitochondria, such as the caspases, have been apportioned only a minor share in this business. Thus, defects in mitochondrial caspase activation were suggested to cause apoptosis inhibition but not to confer clonogenic survival. This assumption was based on the finding that chemotherapeutic agents provoke mitochondrial damage even the absence of caspases, resulting in the release various toxic mediators and a subsequent caspase-independent cell death. In contrast to these earlier observations, we recently showed that in the absence of active caspases tumor cells do not necessarily undergo caspase-independent cell death but may even survive a chemotherapeutic insult. Our findings suggest that caspase inhibition can indeed promote clonogenic tumor growth which might be not only relevant for tumor therapy but should be also considered when evaluating the safety of therapeutic caspases inhibitors.

Open Access Article

Prostaglandin E2: Making More of Your Marrow

Allegra M. Lord, Trista E. North and Leonard I. Zon
Pages 3054 - 3057
http://dx.doi.org/10.4161/cc.6.24.5129
Abstract | PDF

We have recently demonstrated through a chemical screen in the zebrafish embryo that prostaglandin E2 (PGE2) is an evolutionarily conserved regulator of hematopoietic stem cell (HSC) number. These results have further been confirmed by in vitro and in vivo studies in the murine model. Bioactive PGE2 derivatives have potential clinical application to accelerate recovery of the hematopoietic system following chemotherapy or irradiation. Ex vivo expansion of HSCs prior to stem cell transplantation may improve reconstitution of hematopoiesis and immune function. This article aims to summarize current knowledge of PGE2-mediated regulation of blood cell homeostasis as well as to discuss the proposed use of PGE2 to expand hematopoietic stem cells for transplantation in the clinical setting.




REVIEW

Open Access Article

Phosphatases, DNA Damage Checkpoints and Checkpoint Deactivation

Johanna Heideker, Ewa T. Lis and Floyd E. Romesberg
Pages 3058 - 3064
http://dx.doi.org/10.4161/cc.6.24.5100
Abstract | PDF

Cells have evolved intricate and specialized responses to DNA damage, central to which are the DNA damage checkpoints that arrest cell cycle progression and facilitate the repair process. Activation of these damage checkpoints relies heavily on the activity of Ser/Thr kinases, such as Chk1 and Chk2 (Saccharomyces cerevisiae Rad53), which are themselves activated by phosphorylation. Only more recently have we begun to understand how cells disengage the checkpoints to reenter the cell cycle. Here, we review progress toward understanding the functions of phosphatases in checkpoint deactivation in S. cerevisiae, focusing on the non-redundant roles of the type 2A phosphatase Pph3 and the PP2C phosphatases Ptc2 and Ptc3 in the deactivation of Rad53. We discuss how these phosphatases may specifically recognize different phosphorylated forms of Rad53 and how each may independently regulate different facets of the checkpoint response. In conjunction with the independent dephosphorylation of other checkpoint proteins, such regulation may allow a more tailored response to DNA damage that is coordinated with the repair process, ultimately resulting in the resumption of growth.




BRIEF REPORT

Open Access Article

CDK2 is Dispensable for Adult Hippocampal Neurogenesis

Renaud Vandenbosch, Laurence Borgs, Pierre Beukelaers, Agnès Foidart, Laurent Nguyen, Gustave Moonen, Cyril Berthet, Philipp Kaldis, Vittorio Gallo, Shibeshih Belachew and Brigitte Malgrange
Pages 3065 - 3069
http://dx.doi.org/10.4161/cc.6.24.5048
Abstract | PDF

Granule neurons of the dentate gyrus (DG) of the hippocampus undergo continuous renewal throughout life. Among cell-cycle regulators, cyclin-dependent kinase 2 (Cdk2) is considered as a major regulator of S-phase entry. We used Cdk2-deficient mice to decipher the requirement of Cdk2 for the generation of new neurons in the adult hippocampus. The quantification of cell cycle markers first revealed that the lack of Cdk2 activity does not influence spontaneous or seizure-induced proliferation of neural progenitor cells (NPC) in the adult DG. Using bromodeoxyuridine incorporation assays, we showed that the number of mature newborn granule neurons generated de novo was similar in both wild-type (WT) and Cdk2-deficient adult mice. Moreover, the apparent lack of cell output reduction in Cdk2-/- mice DG did not result from a reduction in apoptosis of newborn granule cells as analyzed by TUNEL assays. Our results therefore suggest that Cdk2 is dispensable for NPC proliferation, differentiation and survival of adult-born DG granule neurons in vivo. These data emphasize that functional redundancies between Cdks also occur in the adult brain at the level of neural progenitor cell cycle regulation during hippocampal neurogenesis.




REPORTS

Open Access Article

The E3 Ubiquitin Ligase EDD Regulates S-Phase and G₂/M DNA Damage Checkpoints

Marcia A. Munoz, Darren N. Saunders, Michelle J. Henderson, Jennifer L. Clancy, Amanda J. Russell, Gillian Lehrbach, Elizabeth A. Musgrove, Colin K.W. Watts and Robert L. Sutherland
Pages 3070 - 3077
http://dx.doi.org/10.4161/cc.6.24.5021
Abstract | PDF

The cellular response to DNA damage is critical for maintenance of genomic integrity and inhibition of tumorigenesis. Mutations or aberrant expression of the E3 ubiquitin ligase EDD have been observed in a number of carcinomas and we recently reported that EDD modulates activity of the DNA damage checkpoint kinase, CHK2. Here, we demonstrate that EDD is necessary for G1/S and intra S phase DNA damage checkpoint activation and for the maintenance of G2/M arrest after double strand DNA breaks. Defective checkpoint activation in EDD-depleted cells led to radio-resistant DNA synthesis, premature entry into mitosis, accumulation of polyploid cells, and cell death via mitotic catastrophe. In addition to decreased CHK2 activation in EDD-depleted cells, the expression of several key cell cycle mediators including Cdc25A/C and E2F1 was altered, suggesting that these checkpoint defects may be both CHK2-dependent and -independent. These data support a role for EDD in the maintenance of genomic stability, emphasising the potential importance of dysregulated EDD expression and/or function in the evolution of cancer.

Open Access Article

Bilirubin Inhibits Tumor Cell Growth via Activation of ERK

Robert Ollinger, Pamela Kogler, Jakob Troppmair, Martin Hermann, Martin Wurm, Astrid Drasche, Ingmar Konigsrainer, Albert Amberger, Helmut Weiss, Dietmar Ofner, Fritz H. Bach and Raimund Margreiter
Pages 3078 - 3085
http://dx.doi.org/10.4161/cc.6.24.5022
Abstract | PDF

Bilirubin for decades was considered a potentially toxic waste product of heme degradation until the discovery that it is a potent antioxidant. Accumulating data from observations in humans and experimental studies indicate that the bile pigment may be protective against certain diseases. Based on our own observations that bilirubin induces cell cycle arrest in abnormally proliferating vascular smooth muscle cells and clinical observations describing a lesser incidence of cancer in healthy individuals with high normal or slightly elevated serum bilirubin levels, we hypothesized that bilirubin might suppress tumor cell proliferation in vitro and in vivo. As possible effectors we analyzed key proteins that are involved in cell cycle progression and apoptosis. In vivo tumor growth was assessed in BALB/c nude mice bearing HRT-18 colon cancer xenografts that were treated with bilirubin. In vitro, we investigated the effect of bilirubin on various cell lines and the signaling pathways involved in bilirubin action on tumor cell proliferation in HRT-18 cells using western blots. Bilirubin potently inhibited tumor cell proliferation in vivo and acted cytostatic and pro-apoptotic in vitro. The signaling cascades responsible for this action involved induction of p53, p27, hypophosphorylation of the retinoblastoma tumor suppressor protein as well as caspase activation. These effects were dependent on ERK 1/2. Our study demonstrates that bilirubin may play a role in the defense against cancer by interfering with pro-cancerogenic signaling pathways.

Open Access Article

Staurosporine Sensitizes T Lymphoma Cells to Glucocorticoid-Induced Apoptosis: Role of Nur77 and Bcl-2

Shlomit Kfir, Ronit Vogt Sionov, Elazar Zafrirv, Yael Zilberman and Eitan Yefenof
Pages 3086 - 3096
http://dx.doi.org/10.4161/cc.6.24.5023
Abstract | PDF

Glucocorticoids (GCs) are used for treatment of various hematopoietic malignancies owing to their ability to induce apoptosis. A major obstacle in leukemia therapy is the emergence of GC-resistant cells. Hence, combinatory treatment protocols should be developed that convert GC-resistant leukemia cells into sensitive ones. Here we demonstrate that the broad-acting kinase inhibitor staurosporine (STS) confers GC-sensitivity on GC-resistant T lymphoma cells expressing elevated levels of either Bcl-2 or Bcl-XL, but not on GC-resistant myelogenic leukemia cells expressing Mcl-1 in addition to Bcl-2 and/or Bcl-XL. In T lymphoma cells, STS induces the expression of the pro-apoptotic orphan receptor Nur77 that overcomes the anti-apoptotic effect of Bcl-2, thus enabling GC-induced apoptosis. However, in the myelogenic leukemia cells, STS does not up-regulate Nur77. In these cells, the glucocorticoid receptor (GR) is rapidly downregulated by GC and the anti-apoptotic Mcl-1 protein is upregulated by STS, thereby leading to an even more resistant phenotype. Altogether, our data provide a molecular basis for the differential apoptotic response of T lymphoma versus myelogenic leukemia cells to STS and GC. The former being sensitized to GC-induced apoptosis by STS, whereas in the latter, STS intensifies GC resistance. The cell type specific responses should be taken into consideration when combinatory therapy is used for treating hematopoietic malignancies.

Open Access Article

Cytostatic and Cytotoxic Properties of Amphinase: A Novel Cytotoxic Ribonuclease from Rana pipiens Oocytes

Barbara Ardelt, Wojciech Ardelt, Piotr Pozarowski, Jan Kunicki, Kuslima Shogen and Zbigniew Darzynkiewicz
Pages 3097 - 3102
http://dx.doi.org/10.4161/cc.6.24.5045
Abstract | PDF

Onconase (Onc), is a novel amphibian cytotoxic ribonuclease with antitumor activity, and is currently in a confirmatory phase III clinical trial for the treatment of malignant mesothelioma. It was recently reported that Rana pipiens oocytes contain still another ribonuclease, named Amphinase (Amph). Amph shows 38 – 40 % amino acid sequence identity with Onc; presents as four variants varying between themselves from 87 to 99 % in amino acid sequence identity and has a molecular mass ~ 13,000. In the present study we describe the effects of Amph on growth of several tumor cell lines. All four variants demonstrated cytostatic and cytotoxic activity against human promyelocytic HL-60-, Jurkat T-cell- and U-937 monocytic leukemia cells. The pattern of Amph activity to certain extent resembled that of Onc. Thus, cell proliferation was suppressed at 0.5 – 10.0 µg/ml (40 – 80 nM) Amph concentration with distinct accumulation of cells in G1 phase of the cell cycle. In addition, the cells were undergoing apoptosis, which manifested by DNA fragmentation (presence of “sub-G1” cells, TUNEL-positivity), caspases and serine proteases activation as well as activation of transglutaminase. The cytotostatic and cytotoxic effects of Amph required its ribonuclease activity: the enzymatically inactive Amph-2 having histidine at the active site alkylated was ineffective. The effectiveness and cell cycle specificity was generally similar for all four Amph variants and at the equimolar concentrations was somewhat more pronounced than that of Onc. The observed cytostatic and cytotoxic activity of Amph against tumor cell lines suggests that similar to Onc this cytotoxic ribonuclease may have antitumor activity and find an application in clinical oncology.

Open Access Article

Apaf-1 Deficiency Causes Chromosomal Instability

Shahul Mouhamad, Lorenzo Galluzzi, Yael Zermati, Maria Castedo and Guido Kroemer
Pages 3103 - 3107
http://dx.doi.org/10.4161/cc.6.24.5046
Abstract | PDF

Apaf-1 is an essential component of the apoptosome, the molecular complex assembled in response to mitochondrial cytochrome c release that promotes caspase activation. Apaf-1 expression is suppressed in some malignant tumors, in particular melanoma as well as cervical and colorectal carcinoma, in which the loss of Apaf-1 expression marks tumor progression and poor prognosis. Recent results from our laboratory demonstrate that Apaf-1 has an apoptosis-unrelated function that may well account for its role as a tumor suppressor. The knockout of apaf-1 (in mice), the knockdown of Apaf-1 (in human cells) and loss of function mutations of ced-4 (the Caenorhabditis elegans ortholog of Apaf-1) compromise the arrest of DNA synthesis in response to DNA damage, in a context in which apoptosis does not occur. Here, we show that the depletion of Apaf-1 also sensitizes cells to chromosomal instability induced by different types of DNA damage such as cisplatin, UVC light and γ-irradiation. These results unravel a hitherto unsuspected role for Apaf-1 in the maintenance of genomic stability, independently from its function in the cell death machinery.

Open Access Article

High NaCl Promotes Cellular Senescence

Natalia I. Dmitrieva and Maurice B. Burg
Pages 3108 - 3113
http://dx.doi.org/10.4161/cc.6.24.5084
Abstract | PDF

High extracellular NaCl was previously shown to increase the number of DNA breaks in mammalian cells in tissue culture, renal medullary cells in vivo, C. elegans, and marine invertebrates. It was also shown to increase reactive oxygen species in renal cells, resulting in oxidation of proteins and DNA. Cellular senescence is a common response to such damage. Therefore, in the present studies we looked for signs of senescence in cells exposed to high NaCl. We find that (1) The rate of proliferation of HeLa cells exposed to high NaCl decreases gradually to the point of arrest, and the cells display signs of senescence, including hypertrophy and increased auto fluorescence. (2) High NaCl accelerates the appearance of senescence in primary mouse embryonic fibroblasts, as measured by β-galactosidase activity (SA-β-gal). (3) High NaCl retards growth and markedly decreases the life span of C. elegans, accompanied by features of accelerated aging, such as decreased locomotion and increased number of SA-β-gal positive cells. (4) Mouse renal medullary cells, which are normally continuously exposed to high NaCl, express increased p16^INK4 (another indicator of senescence) much earlier than do cells in the renal cortex, which has the same level of NaCl as peripheral blood. We conclude that high NaCl accelerates cellular senescence and aging, most likely secondary to the DNA breaks and oxidative damage that it causes.

Open Access Article

The Cyclin-Dependent Kinase Inhibitor Seliciclib (R-roscovitine; CYC202) Decreases the Expression of Mitotic Control Genes and Prevents Entry into Mitosis

Steven R. Whittaker, Robert H. te Poele, Florence Chan, Spiros Linardopoulos, Michael I. Walton, Michelle D. Garrett and Paul Workman
Pages 3114 - 3131
http://dx.doi.org/10.4161/cc.6.24.5142
Abstract | PDF

The cyclin-dependent kinase (CDK) inhibitor seliciclib (R-roscovitine, CYC202) shows promising antitumor activity in preclinical models and is currently undergoing phase II clinical trials. Inhibition of the CDKs by seliciclib could contribute to cell cycle arrest and apoptosis seen with the drug. However, it is common for drugs to exert multiple effects on gene expression and biochemical pathways. To further our understanding of the molecular pharmacology of seliciclib, we employed cDNA microarrays to determine changes in gene expression profiles induced by the drug in HT29 human colon cancer cells. Concentrations of seliciclib were used that inhibited RB phosphorylation and cell proliferation. An increase in the mRNA expression for CJUN and EGR1 was confirmed by Western blotting, consistent with activation of the ERK1/2 MAPK pathway by seliciclib. Transcripts of key genes required for the progression through mitosis showed markedly reduced expression, including Aurora-A/B (AURK-A/B), Polo-like kinase (PLK), cyclin B2 (CCNB2), WEE1 and CDC25C. Reduced expression of these mitotic genes was also seen at the protein level. siRNA-mediated depletion of Aurora-A protein led to an arrest of cells in the G2/M phase, consistent with the effects of seliciclib treatment. Inhibition of mitotic entry following seliciclib treatment was indicated by a reduction of histone H3 phosphorylation, which is catalyzed by Aurora-B, and by decreased expression of mitotic markers, including phospho-protein phosphatase 1α. The results indicate a potential mechanism through which seliciclib prevents entry into mitosis. Gene expression profiling has generated hypotheses that led to an increase in our knowledge of the cellular effects of seliciclib and could provide potential pharmacodynamic or response biomarkers for use in animal models and clinical trials.

Open Access Article

Nitric Oxide Coordinates Cell Proliferation and Cell Movements During Early Development of Xenopus

Natalia Peunova, Vladimir Scheinker, Kandasamy Ravi and Grigori Enikolopov
Pages 3132 - 3144
http://dx.doi.org/10.4161/cc.6.24.5146
Abstract | PDF

The establishment of a vertebrate body plan during embryogenesis is achieved through precise coordination of cell proliferation and morphogenetic cell movements. Here we show that nitric oxide (NO) suppresses cell division and facilitates cell movements during early development of Xenopus, such that inhibition of NO synthase (NOS) increases proliferation in the neuroectoderm and suppresses convergent extension in the axial mesoderm and neuroectoderm. NO controls cell division and cell movement through two separate signaling pathways. Both rely on RhoA-ROCK signaling but can be distinguished by the involvement of either guanylate cyclase or the planar cell polarity regulator Dishevelled. Through the cGMP-dependent pathway, NO suppresses cell division by negatively regulating RhoA and controlling the nuclear distribution of ROCK and p21WAF1. Through the cGMP-independent pathway, NO facilitates cell movement by regulating the intracellular distribution and level of Dishevelled and the activity of RhoA, thereby controlling the activity of ROCK and regulating actin cytoskeleton remodeling and cell polarization. Concurrent control by NO helps ensure that the crucial processes of cell proliferation and morphogenetic movements are coordinated during early development.




LETTER TO THE EDITOR

Open Access Article

Multiple Kinases and System Robustness: A Link Between Cdc37 and Genome Integrity

Avrom J. Caplan, Avi Ma'ayan and Ian M. Willis
Pages 3145 - 3147
http://dx.doi.org/10.4161/cc.6.24.5147
Abstract | PDF

We identified 38 genes as having a genetic interaction with a mutant form of the kinase specific chaperone, Cdc37, using a genome-wide synthetic screening approach. The identified genes included a sub-network of highly interacting genes enriched for functions in genome integrity and comprising multiple components of several discrete molecular machines. A network analysis approach related these machines to a small group of cell cycle checkpoint kinases.