Table of Contents

Volume 3, Issue 8

  August 2004

  Pages 971 - 1083

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 Issue Catalogue (MARC XML)

EXTRA VIEWS

Open Access Article

A Role for the RSC Chromatin Remodeler in Regulating Cohesion of Sister Chromatid Arms

Jian Huang and Brehon C. Laurent
Pages 971 - 973
http://dx.doi.org/10.4161/cc.3.8.1014
Abstract | PDF

The precise segregation of chromosomes is critical for the proliferation and development of living organisms. Defects in this process can result in tumorigenesis and hereditary diseases. The four-subunit cohesin complex plays an essential role in chromosome segregation and genome integrity. Recently, we reported that the association of cohesin with centromeres and chromosome arms is differentially regulated by the ATP-dependent RSC chromatin-remodeling complex. Here, we propose two models to explain why the cell should have evolved special mechanisms for centromeric and sister arm cohesion and why RSC differentially regulates these processes.

Open Access Article

Nucleophosmin, HDM2 and p53: Players in UV Damage Incited Nucleolar Stress Response

Sari Kurki, Karita Peltonen and Marikki Laiho
Pages 974 - 977
http://dx.doi.org/10.4161/cc.3.8.1015
Abstract | PDF

p53 tumor suppressor protein acts as a critical monitor preventing survival of cells with irreparable genetic damage. Its levels are tightly controlled by its negative regulator HDM2, and are allowed to rise only during cellular stress. In our recent paper (Kurki, et al. Cancer Cell 2004; 5:465-75) we identify a novel mechanism leading to p53 stabilization following UV damage of the cells. This involves UV damage provoked nucleoplasmic relocalization of a nucleolar protein, nucleophosmin (NPM, B23) and its rapid and transient interactions with both p53 and HDM2. We discuss here implications of recent findings that several p53 pathway proteins interact with NPM and find that its participation in cellular damage responses is limited to transcriptional stress but absent in direct ds DNA breaks. These findings suggest divergence in the routes provoking p53 stability and implicate the nucleolus as a central site participating in transcriptional stress responses.

Open Access Article

Metamorphosis from Bone Marrow Derived Primitive Stem Cells to Functional Liver Cells

Yoon-Young Jang and Saul J. Sharkis
Pages 978 - 980
http://dx.doi.org/10.4161/cc.3.8.1018
Abstract | PDF

Both stem cell plasticity and cell fusion have been implicated as physiological responses to tissue injury. It remains the ultimate goal for the future to understand the regulatory control of each during regeneration. In our recent paper by Jang et al. we demonstrate the repair of damaged liver by bone marrow derived stem cells (SCs) in response to microenvironmental cues. Within 48 hrs after transplantation or co-culture, conversion of SCs into liver cells was observed. Fusion was ruled out as a major mechanism of this functional regeneration. Direct differentiation of SCs into liver epithelial cells may be clinically useful. However, if plasticity or fusion results in abnormal genetic changes they could be harmful. Before proceeding with therapeutic applications, the consequences of cellular therapy accompanying both plasticity and fusion must be examined in multiple animal models. Functional repair should also be demonstrated prior to treatment in patients.

Open Access Article

Precise Dosage of an Endogenous Mutagen in the Immune System

Clifford L. Wang and Matthias Wabl
Pages 981 - 983
http://dx.doi.org/10.4161/cc.3.8.1022
Abstract | PDF

Four decades ago, it was hypothesized that lymphocytes mutate the immunoglobulin loci at a rate much greater than the spontaneous rate. Over time this Ig somatic hypermutation became the prime example of in vivo, site-directed mutagenesis. But recent studies have demonstrated that this is not necessarily the case, and that hypermutation occurs not only at the Ig locus but also at various locations throughout the genome. Here, we propose that hypermutation can be genome-wide with minimal injury, if a sufficient mutation rate is coupled with strong clonal selection.

Open Access Article

Complete and Limited Proteolysis in Cell Cycle Progression

Brigitte Goulet and Alain Nepveu
Pages 984 - 987
http://dx.doi.org/10.4161/cc.3.8.1025
Abstract | PDF

An important mechanism of regulation that controls progression through the cell cycle involves the timely degradation of specific regulatory proteins. In parallel to the main degradative pathways, it appears that the function of certain proteins may also be modulated by a process called limited proteolysis. We have recently shown that the CDP/Cux transcription factor is proteolytically processed at the G1/S transition by the cathepsin L protease. Two aspects of these findings are discussed in the context of the cell cycle. Firstly, together with the cohesin subunit Scc1 and the HCF-1 factor, CDP/Cux represents a third example whereby the process of "limited proteolysis" plays a role in the control of cell cycle progression. Secondly, our findings provides compelling evidence that the cathepsin L protease, which was believed to be obligatorily targeted through the endoplasmic reticulum to the lysosomes or the extra-cellular milieu, could also be present in the nucleus and modulate the function of transcription factors.

Open Access Article

A Double Edged Sword: MAD2 Dependent Mitotic Checkpoint Defects in Tumorigenesis and Tumor Cell Death

Loren Michel, Robert Benezra and Elena Diaz-Rodriguez
Pages 988 - 990
http://dx.doi.org/10.4161/cc.3.8.1058
Abstract | PDF

The failure of cell cycle regulatory checkpoints is a common event in human cancer. Defects at the G1-S transition have been widely characterized, but only more recently has aberrant checkpoint signaling during mitotic progression been identified as playing a role in cancer. The metaphase to anaphase transition is regulated by multiple proteins that together comprise the mitotic checkpoint. Previously it has been shown that loss of one copy of MAD2, a mitotic checkpoint gene, results in aneuploidy and tumorigenesis arising from chromosome missegregation. More recently and quite surprisingly, MAD2 has been demonstrated to be an essential gene even in tumor cells such that near complete elimination of this protein from cancer cells results in p53 independent cell death. This is the first identification of a haploinsufficient tumor suppressor gene that is also required for tumor cell survival, and suggests that targeting this checkpoint in cancer might be a viable therapeutic strategy.

Open Access Article

The Production and Localization of GTP-Bound Ran in Mitotic Mammalian Tissue Culture Cells

Hoi-Yeung Li and Yixian Zheng
Pages 991 - 993
http://dx.doi.org/10.4161/cc.3.8.1059
Abstract | PDF

The RanGTPase system has multiple functions in both interphase and mitosis. Extensive studies of Ran-driven nucleocytoplasmic transport have contributed significantly toward our understanding of how RanGTP is produced, hydrolyzed, and localized in interphase. However, there is still a lack of understanding about how this system operates in mitosis. Recent advances have begun to shed light on how RanGTP is produced and localized in mitotic mammalian cells.

Open Access Article

EGFR Autocrine Signaling in a Compliant Interstitial Space: Mechanotransduction from the Outside In

Daniel J. Tschumperlin
Pages 994 - 995
http://dx.doi.org/10.4161/cc.3.8.1061
Abstract | PDF

Cells transduce mechanical forces into biochemical signals; traditionally these processes are thought to occur through direct effects on the cell membrane, the cytoskeleton, or specific transmembrane proteins. In multicellular tissues mechanical forces alter intercellular spacing through redistribution of interstitial fluid. Recent morphological and biochemical observations, supported by analytical modeling, support a new paradigm for mechanotransduction arising from constitutive growth factor shedding into a dynamically regulated interstitial volume.

Open Access Article

Is There a Link Between DNA Polymerase Beta and Cancer?

Daniela Starcevic, Shibani Dalal and Joann B. Sweasy
Pages 996 - 999
http://dx.doi.org/10.4161/cc.3.8.1062
Abstract | PDF

Recent small-scale studies have shown that 30 % of human tumors examined to date express DNA polymerase beta variant proteins. One of the DNA polymerase beta colon cancer-associated mutants, K289M, has been shown to synthesize DNA with a lower fidelity than wild-type Pol beta. Thus, the K289M protein could confer a mutator phenotype to the cell, resulting in genomic instability. Another DNA polymerase beta variant identified in colon carcinoma interferes with base excision repair in cells. This may result in unfilled gaps which can serve as substrates for recombination and result in genomic instability. DNA polymerase beta has also been shown to be overexpressed in a variety of tumors. In some cases, overexpression of polymerase beta in cells confers a transformed phenotype to the cells. In other cases, overexpression results in telomere fusions. Thus, mutant forms or aberrant quantities of polymerase beta confer a mutator phenotype to cells. Combined with the small-scale tumor studies, these mechanistic studies implicate variant forms of DNA polymerase beta in the etiology of human cancer.

Open Access Article

Geminin: A Major DNA Replication Safeguard in Higher Eukaryotes

Marina Melixetian and Kristian Helin
Pages 1000 - 1002
http://dx.doi.org/10.4161/cc.3.8.1063
Abstract | PDF

Eukaryotes have evolved multiple mechanisms to restrict DNA replication to once per cell cycle. These mechanisms prevent relicensing of origins of replication after initiation of DNA replication in S phase until the end of mitosis. Most of our knowledge of mechanisms controlling prereplication complex (pre-RC) formation are based on studies from yeast and Xenopus, while much less is known for mammalian cells. Here we discuss our recent data demonstrating that Geminin is required for preventing rereplication in human normal and cancer cells.

Open Access Article

Elevated Levels of Thymidylate Synthase Linked to Neoplastic Transformation of Mammalian Cells

Donna Voeller, Lambratu Rahman and Maria Zajac-Kaye
Pages 1003 - 1005
http://dx.doi.org/10.4161/cc.3.8.1064
Abstract | PDF

Thymidylate synthase (TS), an enzyme that is essential for DNA synthesis and repair has been identified as an important biomarker for colorectal and other human cancers. The elevated steady-state levels of TS found in many common human malignancies have been thought to represent a secondary event in tumor formation. However, it has recently been demonstrated that the deregulated levels of ectopic TS may also have a causal effect on tumorgenesis since overexpression of human TS transforms immortalized mammalian cells to a malignant phenotype. Since the levels of TS are regulated by E2F-1 and thus are linked to the cell cycle pathway, regulating TS activity may be an important factor for the control of cell cycle progression and for the development of therapeutic strategies and cancer prevention.

Open Access Article

A Novel Function for Human Rad9 Protein as a Transcriptional Activator of Gene Expression

Howard B. Lieberman and Yuxin Yin
Pages 1006 - 1008
http://dx.doi.org/10.4161/cc.3.8.1069
Abstract | PDF

The human Rad9 gene is evolutionarily conserved and plays important roles in several fundamental biological processes, including promotion of resistance to DNA damage, mediating cell cycle checkpoints, acting as a proapoptotic element, and maintaining genomic integrity in the presence or even in the absence of exogenous DNA damaging agents. Rad9 protein was recently found to have the capability of activating transcription of p21, a well-known p53 target gene and universal inhibitor of cyclins and CDKs. Rad9 can bind previously defined p53 consensus sequences in the promoter region of p21. Microarray gene expression screening coupled with northern analyses revealed that Rad9 activates transcription of multiple genes, a subset of which is also controlled by p53. In this review, the newly defined ability of Rad9 to transactivate p21 and other downstream target genes is discussed in terms of the activities already associated with this protein.

Open Access Article

Ubiquitination of PCNA and the Polymerase Switch in Human Cells

Patricia L. Kannouche and Alan R. Lehmann
Pages 1009 - 1011
http://dx.doi.org/10.4161/cc.3.8.1074
Abstract | PDF

Replicative DNA polymerases are blocked by damage in the template DNA. To get past this damage, the cell employs specialised translesion synthesis (TLS) polymerases, which have reduced stringency and are able to bypass different lesions. For example, DNA polymerase ? (pol?) is able to carry out TLS past UV-induced cyclobutane pyrimidine dimers. How does the cell bring about the switch from replicative to TLS polymerase? We have shown that, in human cells, when the replication machinery is blocked at DNA damage, PCNA, the sliding clamp required for DNA replication, is mono-ubiquitinated and that this modified form of PCNA has increased affinity for pol?. This provides a mechanism for the polymerase switch. In this Extra-View, we discuss the possible signals that might trigger ubiquitination of PCNA, whether PCNA becomes de-ubiquitinated after TLS has been accomplished and the role of the hREV1 protein in TLS. We point out some apparent differences between mechanisms in Saccharomyces cerevisiae and human cells.

Open Access Article

Finding and Keeping Your Partner during Meiosis

Florence Couteau, Will Goodyer and Monique Zetka
Pages 1012 - 1014
http://dx.doi.org/10.4161/cc.3.8.1077
Abstract | PDF

HIM-3 is a meiosis-specific protein that localizes to the cores of chromosomes from the earliest stages of prophase I until the metaphase to anaphase I transition in Caenorhabditis elegans. him-3 mutations disrupt homolog alignment, synapsis, and recombination and we propose that the association of HIM-3 with chromosome axes is a critical event in meiotic chromosome morphogenesis that is required for the proper coordination of these processes. The presence of HIM-3-like proteins in other eukaryotes, some of which are known to be required for synapsis and recombination, suggests the existence of a conserved class of axis-associated proteins that function at the junction of essential meiotic processes.




ANALYTICAL REPORT

Open Access Article

Actvation of NF-kappaB by the API2/MALT1 Fusions Inhibits p53 Dependant but Not FAS Induced Apoptosis: A Directional Link between NF-?B and p53

Archontoula Stoffel and Arnold J. Levine
Pages 1015 - 1018
http://dx.doi.org/10.4161/cc.3.8.1060
Abstract | PDF

Interactions between survival pathways and apoptotic cascades play a determinant role in the maintenance of neoplastic clone proliferation and impaired response to apoptosis. Recently, we established a novel interplay between the NF-kappaB survival- and p53 death- pathways in a tumor model system that represents the most common extranodal lymphoid cell neoplasia, MALT (Mucosa Associated Lymphoid Tissue) lymphoma. MALTs are genetically characterized by the t(11;18)(q21;q21) chromosomal translocation that results in API2/MALT1 fusion products. It was shown that distinct API2/MALT1 chimeric proteins function as oncogenes that bilaterally confer a proliferative advantage to the neoplastic clone by activating the NF-kappaB signaling pathway and also inhibiting p53 mediated cell death. Here, we demonstrate that API2/MALT1 mediated inhibition of apoptosis is p53 specific, as distinct API2/MALT1 fusion proteins fail to protect cells from FAS induced cell death. Furthermore, we demonstrate that API2/MALT1 mediated NF-kappaB activation does not alter p53 protein levels or subcellular localization suggesting a post-translational or indirect mechanism of p53 deregulation.




PERSPECTIVES

Open Access Article

The ARTS Connection: Role of ARTS in Apoptosis and Cancer

Sarit Larisch
Pages 1019 - 1021
http://dx.doi.org/10.4161/cc.3.8.1021
Abstract | PDF

ARTS is an unusual septin that is localized to mitochondria in living cells promotes apoptosis by antagonizing IAPs. ARTS functions as a tumor suppressor in Acute Lymphoblatic Leukemia (ALL) and is lost in more than of leukemic patients. The loss of ARTS is specific as levels of H5, a closely related non-apoptotic septin protein derived from the same gene, were unaffected. Thus, ARTS, a new member in the mitochondrial pro-apoptotic arsenal, provides a link between mitochondria, apoptosis and cancer.

Open Access Article

Mammalian DNA Methyltransferase 1: Inspiration for New Directions

Angela H. Ting, Kam-wing Jair, Hiromu Suzuki, Ray-Whay Chiu Yen, Stephen B. Baylin and Kornel E. Schuebel
Pages 1022 - 1024
http://dx.doi.org/10.4161/cc.3.8.1070
Abstract | PDF

The role of DNA methyltransferase 1, DNMT1, in human cancer cells has recently been contested. In this setting, DNMT1’s function as the sole maintenance methyltransferase was based on the assumption that its biological activity is identical to the mouse homologue. However, the application of recent technological advances, including gene targeting and siRNA mediated ablation studies, has cast doubt on this presumed role. Here, we attempt to shed light on these new data within the context of previous experiments.

Open Access Article

Oxygen Sensing by H+: Implications for HIF and Hypoxic Cell Memory

Karim Mekhail, Mireille Khacho, Lakshman Gunaratnam and Stephen Lee
Pages 1025 - 1027
http://dx.doi.org/10.4161/cc.3.8.1075
Abstract | PDF

Hypoxia and acidosis are common features of several physiological and pathological situations, including cancer and stroke. The HIF (hypoxia-inducible factor) transcription factor plays a seminal role in orchestrating cellular responses to alterations in oxygen availability. HIF is degraded in normal oxygen tension by the VHL (von Hippel-Lindau) tumor suppressor protein but stabilized by hypoxia to activate an array of genes implicated in oxygen homeostasis including vascular endothelial growth factor. Cells respond to a comparatively mild decline in oxygen tension by converting to an anaerobic state of respiration and secreting lactic acid. We recently reported that a decrease in environmental pH triggers sequestration of VHL into the nucleolus neutralizing its ability to degrade HIF. This implies that cells have evolved a parallel mechanism of HIF activation that responds to changes in oxygen levels by sensing extracellular [H⁺]. Here we discuss the implications of this new VHL regulatory mechanism on oxygen homeostasis and hypoxic cell memory.

Open Access Article

p53 Mutants: the Achilles’ Heel of Human Cancers?

Rainer K. Brachmann
Pages 1028 - 1032
http://dx.doi.org/10.4161/cc.3.8.1076
Abstract | PDF

Recent scientific discoveries have thrust mutants of the tumor suppressor protein p53 into the forefront of the war on cancer, and hold out eventual hope for a small molecule drug that will be useful in treating human cancers with mutant p53 protein.




REVIEW

Open Access Article

Analysis of FDA Approved Anticancer Drugs Reveals the Future of Cancer Therapy

Mikhail V. Blagosklonny
Pages 1033 - 1040
http://dx.doi.org/10.4161/cc.3.8.1023
Abstract | PDF

This review discusses 26 new anticancer drugs approved by the FDA in the past decade. Based on their targets, these anticancer agents can be divided into three groups. First group contains cancer-selective or semi-selective drugs that are effective in rare kinase-addictive cancers. For other malignancies, semi-selective drugs have to be judiciously combined with non-selective agents. The second group includes analogs of classic cytotoxic agents such as DNA alkylating agents, nucleoside analogs, and anti-microtubule agents. As expected, they have a marginal advantage over the existing cytotoxic drugs, nevertheless are more effective (in common cancers) than semi-selective agents. The third is a diverse group of tissue-selective agents that essentially attack the normal tissues of tumor origin and thus, exploit the tissue-specific similarities between normal and cancer cells. Our analysis predicts that monotherapy with semi-selective agents will be limited to rare cancers. In most cancers however, two anticancer strategies may be most fruitful: (a) combinations of cytotoxic drugs with semi-selective agents aimed at matching targets and (b) tissue-selective therapy aimed at normal and tumor cells of the same tissues.




BRIEF REPORTS

Open Access Article

Induction of p53-Dependent Apoptosis in HCT116 Tumor Cells by RNA Viruses and Possible Implications in Virus-Mediated Oncolysis

Shirley Huang, Li-Ke Qu and Antonis E. Koromilas
Pages 1041 - 1043
http://dx.doi.org/10.4161/cc.3.8.1016
Abstract | PDF

Recent findings showed that type I interferons (IFN-?/?) induce the transcription of tumor suppressor p53 and sensitize primary mouse embryonic fibroblasts (MEFs) to p53-mediated apoptosis by oncolytic viruses. However, the ability of RNA viruses to induce a p53-mediated apoptotic response may differ between primary and tumor cells and may be dependent upon the virus type. We have investigated this hypothesis by analyzing the apoptotic effects of various oncolytic viruses on the human colon carcinoma HCT116 cells and their derivatives lacking either p53 or bax gene. We show that HCT116 cells are resistant to the apoptotic effects of vesicular stomatitis virus, reovirus or poliovirus but activate the p53/Bax apoptotic pathway after infection with Sendai virus. These data substantiate the role of p53 in virus-mediated apoptosis and show that, unlike primary cells, tumor cells may be more selective in the activation of p53 pathway in response to the infection with specific types of viruses.

Open Access Article

The Finger Domain of the Human Deubiquitinating Enzyme HAUSP is a Zinc Ribbon

S. Sri Krishna and Nick V. Grishin
Pages 1044 - 1047
http://dx.doi.org/10.4161/cc.3.8.1017
Abstract | PDF

Human deubiquitinating enzyme HAUSP is a cysteine protease that regulates the levels of the tumor suppressor protein p53. By comparative sequence and structural analysis, we show that the previously uncharacterized finger domain insert to the catalytic core of HAUSP is a zinc ribbon that has lost its zinc-binding ability.




REPORTS

Open Access Article

Paclitaxel Induces Primary and Postmitotic G1 Arrest in Human Arterial Smooth Muscle Cells

Mikhail V. Blagosklonny, Zbigniew Darzynkiewicz, H. Dorota Halicka, Piotr Pozarowski, Zoya N. Demidenko, James J. Barry, Kalpana R. Kamath and Robert A. Herrmann
Pages 1048 - 1054
http://dx.doi.org/10.4161/cc.3.8.986
Abstract | PDF

Paclitaxel (PTX), a microtubule-active drug, causes mitotic arrest leading to apoptosis in certain tumor cell lines. Here we investigated the effects of PTX on human arterial smooth muscle cell (SMC) cells. In SMC, PTX caused both (a) primary arrest in G1 and (b) post-mitotic arrest in G1. Post-mitotic cells were multinucleated (MN) with either 2C (near-diploid) or 4C (tetraploid) DNA content. At PTX concentrations above12 ng/ml, MN cells had 4C DNA content consistent with the lack of cytokinesis during abortive mitosis. Treatment with 6-12 ng/ml PTX yielded MN cells with 2C DNA content. Finally, 1-6 ng/ml of PTX, the lowest concentrations that affected cell proliferation, caused G1 arrest without multinucleation. It is important that PTX did not cause apoptosis in SMC. The absence of apoptosis could be explained by mitotic exit and G1 arrest as well as by low constitutive levels of caspase expression and by p53 and p21 induction. Thus, following transient mitotic arrest, SMC exit mitosis to form MN cells. These post-mitotic cells were subsequently arrested in G1 but maintained normal elongated morphology and were viable for at least 21 days. We conclude that in SMC PTX causes post-mitotic cell cycle arrest rather than cell death.

Open Access Article

Cyclin mRNA Stability Does Not Vary During the Cell Cycle

K. Leigh Eward, Matthew N. Van Ert, Maureen Thornton and Charles E. Helmstetter
Pages 1055 - 1059
http://dx.doi.org/10.4161/cc.3.8.987
Abstract | PDF

The cyclins are tightly regulated elements governing eukaryotic cell cycle progression by means of sequential activation-inactivation of cyclin-dependent kinases. In one manifestation of this regulation, the mRNA levels of several cyclin genes oscillate during the cycle in mammalian cells. Such cycle-dependent fluctuations in transcript levels could result from changes not only in rates of transcription, but also in mRNA stability. Here we used a new, minimally-disturbing method for producing multi-cycle synchronous growth of human MOLT-4 cells, in combination with quantitative real-time RT-PCR, to compare cell cycle-dependent transcript levels and half-lives of cyclin A2, B1, D3, E and PCNA mRNAs. While all mRNA levels except cyclin D3 varied in the cycle, there were no apparent variations in message half-lives. This differs from several previous reports of dramatic fluctuations in the stabilities of cyclin mRNAs, and infers that fluctuations in cyclin mRNA transcript levels during the MOLT-4 cell cycle are not due to variations in half-lives. The discrepancy in mRNA stability determinations could be due to differences in cell types or synchronization methods, but our findings may be representative of mRNA processing in the cycle of cells in unstressed steady-state growth.

Open Access Article

Induction of H2AX Phosphorylation in Pulmonary Cells by Tobacco Smoke: A New Assay for Carcinogens

A.P. Albino, X. Huang, E. Jorgensen, J. Yang, D. Gietl, F. Traganos and Z. Darzynkiewicz
Pages 1060 - 1066
http://dx.doi.org/10.4161/cc.3.8.988
Abstract | PDF

DNA double strand breaks (DSBs) are potentially carcinogenic lesions. The induction of DSBs triggers phosphorylation of histone H2AX. Phosphorylated H2AX, denoted p-H2AX, may be detected immunocytochemically and the intensity of p-H2AX immunofluorescence (IF) reveals the frequency of DSBs. Using this assay we tested whether the exposure of A549 human pulmonary adenocarcinoma cells to tobacco smoke, and normal human bronchial epithelial cells (NHBE) to tobacco smoke condensate, induces DSBs. Cellular p-H2AX IF and DAPI fluorescence of individual cells were measured by laser scanning cytometry (LSC). Exposure of A549 cells to tobacco smoke and NHBE cells to smoke condensate led to H2AX phosphorylation in both a time and dose dependent manner. The maximal rate of H2AX phosphorylation was seen during the initial 4h of cell treatment. At high doses (50 _g/ml of smoke condensate), H2AX phosphorylation continued to increase for up to 24h. No differences in the level of H2AX phosphorylation were apparent between cells in G1 vs S vs G2/M phase of the cell cycle in response to treatment with smoke condensate. The data provide strong evidence that exposure of A549 cells to tobacco smoke or NHBE cells to smoke condensate rapidly induces DSBs in these cells. The present assay to detect and measure DSBs induced by tobacco products complements other mutagenicity assays and may be applied to test potential carcinogens in other products.

Open Access Article

TGF-β Targets the Wnt Pathway Components, APC and β-catenin, as Mv1Lu Cells Undergo Cell Cycle Arrest

Daniel J. Satterwhite and Kristi L. Neufeld
Pages 1067 - 1071
http://dx.doi.org/10.4161/cc.3.8.1072
Abstract | PDF

The highly coordinated interaction of TGF-? and Wnt signaling pathways is critical for normal development. However, the effects of TGF-β on APC and beta-catenin, two key mediators of Wnt signaling in epithelial cells, have been largely unknown. We determined the effect of TGF-? on APC and beta-catenin expression in Mv1Lu, a non-transformed epithelial cell line, in which TGF-β signaling causes a G₁ cell cycle arrest. We found that TGF-β rapidly reduced APC protein levels through a post-transcriptional mechanism. Further, TGF-beta increased beta-catenin mRNA and protein levels, and increased beta-catenin nuclear accumulation. Finally, retrovirus-mediated overexpression of beta-catenin discernibly enhanced the ability of TGF-β to induce a G₁ cell cycle arrest. This is the first report demonstrating that TGF-β mimics the effect of Wnt signaling on beta-catenin in Mv1Lu cells, and that reduction of APC and nuclear accumulation of β-catenin have cooperative effects on mechanisms that mediate TGF-β-induced cell cycle arrest.

Open Access Article

Akt-Dependent T198 Phosphorylation of Cyclin-Dependent Kinase Inhibitor p27^kip1 in Breast Cancer

Maria Letizia Motti, Carmela De Marco, Daniela Califano, Alfredo Fusco and Giuseppe Viglietto
Pages 1072 - 1078
http://dx.doi.org/10.4161/cc.3.8.1073
Abstract | PDF

The localization of the cyclin-dependent kinase inhibitor p27kip1 is dependent on the phosphorylation of one of three key amino acid residues: S10, T157 and T198. However, it was unclear whether endogenous p27kip1 is phosphorylated at T198 in the living cell. In the present work we describe the generation and characterization of a polyclonal antibody able to recognize recombinant, transfected as well as endogenous T198-phosphorylated p27kip1. Using this antibody, we demonstrate that: (i) endogenous p27kip1 is phosphorylated at T198 in 4 breast cancer cells lines (MCF7, MDA-MB231, MDA-MB436 and MDA-MB468); (ii) T198 phosphorylation is increased in breast cancer cells compared with normal mammary epithelial cells (HMEC); (iii) T198-phosphorylated p27kip1 is exclusively cytoplasmic; (iv) T198 phosphorylation is dependent on the activity of the PI3K-PKB/Akt pathway, being it drastically reduced by the pharmacological PI3K inhibitor LY294002 or stimulated by the constitutive activation of PKB/Akt. Finally, in primary human breast carcinomas, cytoplasmic accumulation of T198-phosphorylated p27kip1 parallels Akt activation. We conclude that in breast cancer cells p27kip1 is phosphorylated at T198 in a PI3K/Akt dependent manner and that this phosphorylation may contribute to p27kip1 cytoplasmic mislocalization observed in breast cancer.




MEETING REPORT

Open Access Article

Genomic Instability in Cancer: Biological and Mathematical Approaches

Natalia L. Komarova
Pages 1079 - 1083
http://dx.doi.org/10.4161/cc.3.8.1024
Abstract | PDF

The workshop, organized by Natalia Komarova and Arnold Levine, was sponsored by Institute for Advanced study and DIMACS (Rutgers). Held on the grounds of In-stitute for Advanced Study in Princeton on June 8 and 9, it brought together about 60 distinguished researchers in the areas of Molecular and Cell Biology, Evolution-ary/Theoretical Biology and Mathematics.