Table of Contents

Volume 7, Issue 24

  December 15, 2008

  Pages 3789 - 3951

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EDITORIAL

Open Access Article

You never know: Cdk inhibitors as anti-cancer drugs

Tim Hunt
Pages 3789 - 3790
http://dx.doi.org/10.4161/cc.7.24.7515
Abstract | PDF

Ever since it emerged that cyclin-dependent protein kinases catalysed cell cycle transitions, and with cancer seen as “A disease of the cell cycle”, people have pursued the aim of testing kinase inhibitors as anti-cancer drugs 1-4. Quite early on, Laurent Meijer and his colleagues discovered roscovitine as a potent inhibitor of Cdk1 5, and the compound went into clinical trials (as CYC202 or Seliciclib) whose outcomes are awaited 6-9. It was never clear to me that cancer was really a disease of the cell cycle (strictly speaking—considering that cancer cells have no trouble dividing), or how inhibiting cell cycle progression could reveal a window of therapeutic advantage between normal and neoplastic cells. Everyone knows what happens if you permanently block cell division in humans: they die. Yet, at the same time as harbouring doubts about the rationale for using anti-Cdk drugs for cancer therapy, I would also be the first to admit that our understanding of cell cycle control is so far from complete, that, given the relative ease of developing specific protein kinase inhibitors, it is not a bad idea to try and see if they have selective effects on tumours. You never know.




NEWS

Open Access Article

c-Abl, an additional tyrosine kinase required for T cell development and function

Wenfang Wu and Leslie J. Berg
Page 3791
http://dx.doi.org/10.4161/cc.7.24.7514
PDF




EXTRA VIEWS

Open Access Article

Chromatin reassembly signals the end of DNA repair

Chin-chuan Chen and Jessica Tyler
Pages 3792 - 3797
http://dx.doi.org/10.4161/cc.7.24.7188
Abstract | PDF

The packaging of the eukaryotic genome into chromatin provides a formidable obstacle to the machinery that mediates genomic processes such as transcription, repair and replication. The cell solves this accessibility problem during double-strand DNA repair by removing the histone proteins flanking the DNA lesion. Presumably then, the repaired DNA is subsequently reassembled into chromatin in order to allow the epigenetic information to be restored and to repackage and protect the genome. Our recent work has shown that chromatin is indeed reassembled following double-strand break repair in budding yeast. Furthermore, the assembly of the repaired DNA into chromatin is driven by acetylation within the globular domain of histone H3, on lysine 56 (H3 K56Ac). Unexpectedly, we also discovered that H3 K56Ac and chromatin assembly onto the repaired DNA is essential to turn off the DNA damage cell cycle checkpoint. This work demonstrates that reformation of the chromatin structure, not DNA repair per se, is the elusive signal that tells the cell when DNA repair is complete.

Open Access Article

Stem cell aging: Survival of the laziest?

Christa Muller-Sieburg and Hans B. Sieburg
Pages 3798 - 3804
http://dx.doi.org/10.4161/cc.7.24.7214
Abstract | PDF

The question whether stem cells age remains an enigma. Traditionally, aging was thought to change the properties of hematopoietic stem cell (HSC). We discuss here a new model of stem cell aging that challenges this view. It is now well-established that the HSC compartment is heterogeneous, consisting of epigenetically fixed subpopulations of HSC that differ in self-renewal and differentiation capacity. New data show that the representation of these HSC subsets changes during ageing. HSC that generate lymphocyte-rich progeny are depleted, while myeloid-biased HSC are enriched in the aged HSC compartment. Myeloid-biased HSC, even when isolated from young donors, have most of the characteristics that had been attributed to aged HSC. Thus, the distinct behavior of the HSC isolated from aged hosts is due to the accumulation of myeloid-biased HSC. By extension this means that the properties of individual HSC are not substantially changed during the lifespan of the organism and that aged hosts do not contain many aged HSC. Myeloid-biased HSC give rise to mature cells slowly but contribute for a long time to peripheral hematopoiesis. We propose that such slow, “lazy” HSC are less likely to be transformed and therefore may safely sustain hematopoiesis for a long time.

Open Access Article

Deconstructing feedback-signaling networks to improve anticancer therapy with mTORC1 inhibitors

Arkaitz Carracedo, Jose Baselga and Pier Paolo Pandolfi
Pages 3805 - 3809
http://dx.doi.org/10.4161/cc.7.24.7244
Abstract | PDF

Targeting mTOR complex 1 (mTORC1), which regulates general protein translation, represents one of the most attractive approaches to treating cancer, since up-regulation of this pathway is a common hallmark in many tumors. Nevertheless, the use of rapamycin and its analogs in the clinic has revealed that mTORC1 pathway is embedded in a network of signaling cross-talks and feedbacks which might reduce its effectiveness in cancer. We have recently described a novel signaling feedback stemming from mTORC1 inhibition, which leads to the activation of ERK-MAPK (MAPK) pathway. The observation that MAPK is activated by rapamycin and its analogs in vitro, in mouse models, and cancer patient biopsies sets the rationale for the combined use of MAPK and mTORC1 inhibitors in cancer therapy. In this extra-view, we integrate our findings into the mTORC1 signaling network and discuss its relevance for the design of combinatorial therapies with mTORC1 inhibitors.

Open Access Article

Chronic lymphocytic leukemia and treatment resistance in cancer: The role of the p53 pathway

Thorsten Zenz, Axel Benner, Hartmut Döhner and Stephan Stilgenbauer
Pages 3810 - 3814
http://dx.doi.org/10.4161/cc.7.24.7245
Abstract | PDF

The importance of studying p53 pathway defects in CLL has been fostered by the demonstration of the fundamentally different clinical course of patients with 17p deletion, where the clinical course is, contrary to most patients with CLL, very poor. The demonstration of the resistance to chemotherapy and mutation of the remaining TP53 allele explains the clinical presentation of CLL with 17p deletion. Here we review recent evidence that TP53 mutation in the absence of the deletion of 17p shows a similar clinical and biological course in CLL. In addition, there is growing evidence that alterations of other components of the DNA damage response pathway, such as ATM and miR34a are associated with treatment resistance.




PERSPECTIVES

Open Access Article

Wnt/β-Catenin signaling acts at multiple developmental stages to promote mammalian cardiogenesis

Chulan Kwon, Kimberly R. Cordes and Deepak Srivastava
Pages 3815 - 3818
http://dx.doi.org/10.4161/cc.7.24.7189
Abstract | PDF

Despite decades of progress in cardiovascular biology, heart disease remains the leading cause of death in the developed world. Recently, cell-based therapy has emerged as a promising avenue for future therapeutics. However, the molecular signals that regulate cardiac progenitor cells are not well-understood. Wnt/β-catenin signaling is essential for expansion and differentiation of cardiac progenitors in mouse embryos and in the embryonic stem cell system. Studies from our laboratory and others highlight the pivotal roles of Wnt/β-catenin signaling in the multiple steps of cardiogenesis and provide insights into understanding the complex regulation of cardiac progenitors. Here we discuss the required roles of Wnt/β-catenin signaling at the different stages of heart development.

Open Access Article

CDK1-FOXO1: A mitotic signal takes center stage in post-mitotic neurons

Albert H. Kim and Azad Bonni
Pages 3819 - 3822
http://dx.doi.org/10.4161/cc.7.24.7215
Abstract | PDF

An emerging theme in molecular neurobiology is the discovery of post-mitotic functions for proteins classically associated with mitotic transition in cycling cells. Although neurons have departed the cell cycle, they surprisingly express molecules in the cell cycle apparatus throughout development. The major mitotic cyclin-dependent kinase Cdk1 plays a critical role during the period of naturally occurring neuronal death in the nervous system and has been suggested to contribute to the pathogenesis of neurodegenerative diseases. However, the mechanisms by which Cdk1 promotes neuronal apoptosis are incompletely understood. A recent report by Yuan et al. (2008) has identified a direct relationship between this mitotic kinase and forkhead transcription factor FOXO1, a protein previously implicated in cell death, DNA damage repair, and tumor suppression. Here we will discuss the key findings of this report and consider the implications of this mechanism to the regulation of other signal transduction pathways in brain development and diseases.

Open Access Article

Using kinomics to delineate signaling pathways: Control of crtc2/torc2 by the ampk family

Accalia Fu and Robert A. Screaton
Pages 3823 - 3828
http://dx.doi.org/10.4161/cc.7.24.7241
Abstract | PDF

The classical role of AMP-activated protein kinase (AMPK) as an energy status sensor is expanding to include other members of the AMPK family. Recent genetic and cell biological evidence points to a role for MAP/microtubule affinity-regulating kinase 2 (MARK2/EMK/Par1b) in the regulation of metabolic events as well as in the control of CREB-dependent transcription activated by glucose in pancreatic islet beta cells. We have recently developed an in vitro kinase screening platform to identify novel kinase: Substrate pairs, the building blocks of signal transduction pathways. Application of this technology led us to identify MARK2 as the kinase that targets a novel glucose-regulated phosphorylation site on Transducer of Regulated CREB Activity 2 (TORC2, referred to as CREB-Regulated Transcriptional Coactivator 2, or CRTC2), a transcriptional coactivator essential for CREB activity in beta cells. We discuss these recent developments and suggest a model whereby members of the AMPK family integrate numerous signals to coordinate energy metabolism and cellular polarity with gene expression to regulate cell function/proliferation.




REVIEWS

Open Access Article

Clever cancer strategies with FoxO transcription factors

Kenneth Maiese, Zhao Zhong Chong, Yan Chen Shang and Jinling Hou
Pages 3829 - 3839
http://dx.doi.org/10.4161/cc.7.24.7231
Abstract | PDF

Given that cancer and related disorders affect a wide spectrum of the world's population, and in most cases are progressive in nature, it is essential that future care must overcome the present limitations of existing therapies in the absence of toxic side effects. Mammalian forkhead transcription factors of the O class (FoxOs) may fill this niche since these proteins are increasingly considered to represent unique cellular targets directed against human cancer in light of their pro-apoptotic effects and ability to lead to cell cycle arrest. Yet, FoxOs also can significantly affect normal cell survival and longevity, requiring new treatments for neoplastic growth to modulate novel pathways that integrate cell proliferation, metabolism, inflammation, and survival. In this respect, members of the FoxO family are extremely compelling to consider since these transcription factors have emerged as versatile proteins that can control angiogenesis, stem cell proliferation, cell adhesion, and autoimmune disease. Further elucidation of FoxO protein function during neoplastic growth should continue to lay the foundation for the successful translation of these transcription factors into novel and robust clinical therapies for cancer.

Open Access Article

The p21 and PCNA partnership: A new twist for an old plot

Carol Prives and Vanesa Gottifredi
Pages 3840 - 3846
http://dx.doi.org/10.4161/cc.7.24.7243
Abstract | PDF

The contribution of error-prone DNA polymerases to the DNA damage response has been a subject of great interest in the last decade. Error-prone polymerases are required for translesion DNA synthesis (TLS), a process that involves synthesis past a DNA lesion. Under certain circumstances, TLS polymerases can achieve bypass with good efficiency and fidelity. However, they can also in some cases be mutagenic, and so negative regulators of TLS polymerases would have the important function of inhibiting their recruitment to undamaged DNA templates. Recently work from Livneh’s and our groups have provided evidence regarding the role of the cyclin kinase inhibitor p21 as a negative regulator of TLS. Interestingly, both the cyclin dependent kinase (CDK) and proliferating cell nuclear antigen (PCNA) binding domains of p21 are involved in different aspects of the modulation of TLS, affecting both the interaction between PCNA and the TLS-specific pol η as well as PCNA ubiquitination status. In line with this, p21 was shown to reduce the efficiency but increase the accuracy of TLS. Hence, in absence of DNA damage p21 may work to impede accidental loading of pol η to undamaged DNA and avoid consequential mutagenesis. After UV irradiation, when TLS plays a decisive role, p21 is progressively degraded. This might allow gradual release of replication fork blockage by TLS polymerases. For these reasons, in higher eukaryotes p21 might represent a key regulator of the equilibrium between mutagenesis and cell survival.




REPORTS

Open Access Article

T cell survival and function requires the c-Abl tyrosine kinase

Isabelle Silberman, Ronit Vogt Sionov, Valentina Zuckerman, Sue Haupt, Zehavit Goldberg, Andreas Strasser, Zami S. Ben-Sasson, Michal Baniyash, Anthony J. Koleske and Ygal Haupt
Pages 3847 - 3857
http://dx.doi.org/10.4161/cc.7.24.7267
Abstract | PDF

C-Abl (Abl) regulates multiple cellular processes, including proliferation, survival, shape determination and motility, and participates in cellular responses to genotoxic and oxidative stress stimuli. Mice lacking Abl exhibit retarded growth, osteoporosis and defects in the immune system resulting in lymphopoenia and susceptibility to infections, leading to early death. To define the role of Abl in the regulation of adult T cells we ablated Abl exclusively in T cells by generating mice with floxed abl alleles and expressing an Lck-Cre transgene (Abl-T-/-). These mice exhibited thymic atrophy and abnormally reduced T cell numbers in the periphery. The thymic atrophy was caused by increased susceptibility of thymocytes to cell death. Importantly, Abl deficient T cells displayed abnormally reduced response to mitogenic stimulation in vitro. Consequently, Abl-T-/- mice exhibited impaired ability to reject syngeneic tumor, to induce T-mediated tumor cell killing, and to generate anti-tumor antibodies. These results demonstrate a cell-autonomous role for Abl in T cell function and survival.

Open Access Article

Tyrosine phosphatase SHP-2 is a regulator of p27^Kip1 tyrosine phosphorylation

Irini Tossidou, Marc Dangers, Alexandra Koch, Dominique T. Brandt, Mario Schiffer and Christian Kardinal
Pages 3858 - 3868
http://dx.doi.org/10.4161/cc.7.24.7260
Abstract | PDF

Tyrosine phosphorylation of the cell cycle regulator p27^Kip1 plays a crucial role in its binding to cyclin dependent kinases and its subcellular localization. While Src and Bcr-Abl were shown to be responsible for tyrosine phosphorylation, no data are available on the dephosphorylation of p27^Kip1 and the phosphatase involved. Considering the associated dephosphorylation as a pivotal event in the regulation of cell cycle proteins, we focused on the tyrosine phosphatase SHP-2, which is regulated in promyelocytic leukemia cells on G-CSF stimulation. SHP-2 was thus found in association with p27^Kip1 and the G-CSF receptor, and we observed a nuclear translocation of SHP-2 on G-CSF stimulation. Using a catalytically inactive form of SHP-2 and siRNA directed against SHP-2, we could demonstrate the involvement of SHP-2 in tyrosine dephosphorylation of p27^Kip1. Moreover, SHP-2 was strongly activated on G-CSF stimulation and specifically dephosphorylated p27^Kip1 in vitro. Most importantly, we could illustrate that SHP-2 modulates p27^Kip1 stability and contributes to p27^Kip1-mediated cell cycle progression. Taken together, our results demonstrate that SHP-2 is a key regulator of p27^Kip1 tyrosine phosphorylation.

Open Access Article

Insulin-like growth factor-1 inhibits STS-induced cell death and increases functional recovery of in vitro differentiated neurons

Bruna Pucci, Francesca Bertani, Manuela Indelicato, Patrizio Sale, Emanuela Lococo, Francesca Grassi, Francesca Pagani, Valeria Colafrancesco, Emanuela Morgante, Marco Tafani, Massimo Fini and Matteo A. Russo
Pages 3869 - 3877
http://dx.doi.org/10.4161/cc.7.24.7261
Abstract | PDF

NG108-15 cells differentiate into neurons by 1 mM sodium butyrate (NaB) treatment. Differentiated cells resulted more resistant to staurosporine (STS) than proliferating cells. In particular, STS treatment decreased Bcl-2 and Bcl-xL content in mitochondria of proliferating cells, but not in mitochondria of differentiated cells. Bad was phosphorylated and down-regulated only in differentiated cells. Bax accumulated in the mitochondria of proliferating but not differentiated cells. Mitochondrial release of cytochrome c was observed in proliferating cells, whereas mitochondria of differentiated cells retained cytochrome c. Proliferating cells treated with STS accumulated Endo G and AIF in the nucleus. By contrast, differentiated cells did not show such nuclear accumulation. Treatment of differentiated cells with Insulin-like Growth Factor-1 (IGF-1) and STS resulted in a 17,1% increase of cell viability. The survival role of IGF-1 was demonstrated by treating differentiated cells with an anti-IGF-1 neutralizing antibody. Such treatment significantly increased STS-induced cell death. Electrophysiology studies showed that in STS-treated cells membrane potential oscillations were reduced in amplitude without giving rise to spontaneous action potentials (APs). However, the percentage of cells yielding overshooting APs returned to 100% after STS removal. It is concluded that neuronal differentiation of NG108-15 cells induces resistance to apoptotic cell death and that IGF-1 plays a central role in sustaining this mechanism.

Open Access Article

ATRA and KL promote differentiation toward the meiotic program of male germ cells.

Manuela Pellegrini, Doria Filipponi, Manuele Gori, Florencia Barrios, Francesca Lolicato, Paola Grimaldi, Pellegrino Rossi, Emmanuele A. Jannini, Raffaele Geremia and Susanna Dolci
Pages 3878 - 3888
http://dx.doi.org/10.4161/cc.7.24.7262
Abstract | PDF

While it is known that Retinoic Acid (RA) induces meiosis in mouse female fetal gonads, the mechanisms which regulate this process during spermatogenesis are poorly understood. We show that the All trans RA derivative (ATRA) and Kit Ligand (KL) increase meiotic entry of postnatal mouse spermatogonia in vitro without synergism. Competence to enter meiosis is reached by spermatogonia only at the stage in which they undergo Kit-dependent divisions. Besides increasing Kit expression in spermatogonia, ATRA also upregulates KL expression in Sertoli cells. Both ATRA and KL increase the expression of Stimulated by Retinoic Acid Gene 8 and Dmc1, an early meiotic marker. A specific Kit tyrosine kinase inhibitor prevents the increase in the number of meiotic cells induced by both the two factors, suggesting that they converge on common Kit-dependent signalling pathways. Meiotic entry induced by ATRA and KL is independent from their ability to affect germ cell viability, and is mediated by the activation of PI3K and MAPK pathways through Kit autophosphorylation. ATRA-induced phosphorylation of the two downstream kinases is mediated by a non-genomic mechanism.
These data suggest that RA may control the timing of meiosis by influencing both the somatic and the germ cell compartment of the postnatal testis through the activation of the KL/Kit system.

Open Access Article

Stem cell marker expression, proliferation and apoptosis of vascular smooth muscle cells

Augusto Orlandi, Antonio Di Lascio, Arianna Francesconi, Maria Giovanna Scioli, Gaetano Arcuri, Amedeo Ferlosio and Luigi Giusto Spagnoli
Pages 3889 - 3897
http://dx.doi.org/10.4161/cc.7.24.7323
Abstract | PDF

Vascular endothelial Flt-1 and other stem cell markers are variably expressed in vascular smooth muscle cells (SMCs) during normal and pathological conditions, but their biological role remains uncertain. In normal rat aorta, rare flt-1+ and c-kit+ SMCs were detected. Fifteen days after injury, 61.8+3.8, 45.7+3% of the intimal cells resulted flt-1+ and c-kit+ and expressed low level of alpha-smooth muscle actin; CD133+ cells were 5.6+0.7%. BrDU+/flt-1+ largely predominated in the neointima, whereas  BrDU+/CD133+ cells were rare. Forty-five and sixty days after injury, intimal proliferation such as BrDU+ cells was greatly reduced. After sixty days, intimal stem marker expression had almost disappeared whereas alpha-smooth muscle actin was restored. Flk-1 and Oct-4 SMC immunodection was consistently negative. In vitro, intimal cells obtained fifteen days after injury exhibited an epithelioid phenotype and increased flt-1 and c-kit protein and mRNA and low smooth muscle markers compared to spindle-shaped medial and intimal SMCs obtained after sixty days. Epithelioid clones, independently from layer of origin, were similar in stem cell marker expression. The anti-flt-1 blocking antibody added to epithelioid SMC cultures reduced serum-deprived apoptosis and migration but not PDGF-BB-induced proliferation, and increased cell-populated collagen lattice contraction. In conclusion, stem marker expression in vascular SMCs was variable, chronologically regulated and prevailed in epithelioid populations and clones; among stem markers, flt-1 expression critically regulates intimal SMC response to microenviromental changes.

Open Access Article

Transient treatment with CDK inhibitors eliminates proliferative potential even when their abilities to evoke apoptosis and DNA damage are blocked

Simon F. Scrace, Peter Kierstan, Jenifer Borgognoni, Lan-Zhen Wang, Sarah Denny, Joanne Wayne, Carol Bentley, Andrew D. Cansfield, Philip S. Jackson, Andrea M. Lockie, Nicola J. Curtin, David R. Newell, Douglas S. Williamson and Jonathan D. Moore
Pages 3898 - 3907
http://dx.doi.org/10.4161/cc.7.24.7345
Abstract | PDF

Transient treatment with small molecule CDK inhibitors is toxic to cancer cells and leads to depletion of anti-apoptotic proteins and Chk1, coupled with DNA damage and induction of apoptosis. Here we have examined, which of these phenomena are necessary for CDK inhibitors to have an anti-proliferative effect. We find that 24 hours treatment with either a primarily CDK2-specific, or a primarily CDK7/9-specific, antagonist eliminates proliferative potential even if apoptosis is blocked and the tendency of CDK inhibition to result in DNA damage is overcome by expression of recombinant Chk1. Loss of proliferative potential is correlated with irreversible suppression of biomarkers of cell cycle progression. CDK inhibitors dramatically reduced levels of the anti-apoptotic proteins, Mcl-1 and XIAP, but siRNA-mediated suppression of Mcl-1 and XIAP did not induce cell death in the osteosarcoma cells used in this study. Finally, we found that many literature CDK inhibitors do not effectively suppress the CDK/cyclin complexes responsible for cell-cycle progression at the minimum doses required to block proliferation: some are only effective after a substantial delay and may act via inhibition of CDK7.

Open Access Article

HAT cofactor TRRAP mediates β-Catenin ubiquitination on the chromatin and the regulation of the canonical Wnt pathway

Martin G. Finkbeiner, Carla Sawan, Maria Ouzounova, Rabih Murr and Zdenko Herceg
Pages 3908 - 3914
http://dx.doi.org/10.4161/cc.7.24.7354
Abstract | PDF

The Wnt pathway is a key regulator of embryonic development and stem cell self-renewal, and hyperactivation of the Wnt signalling is associated with many human cancers. The central player in the Wnt pathway is β-Catenin, a cytoplasmic protein whose function is under tight control by ubiquitination and degradation, however the precise regulation of β-Catenin stability/degradation remains elusive. Here, we report a new mechanism of β-Catenin ubiquitination acting in the context of chromatin. This mechanism is mediated by the histone acetyltransferase (HAT) complex component TRRAP and Skp1, an invariable component of the Skp-Cullin-F-box (SCF) ubiquitin ligase complex. TRRAP interacts with Skp1/SCF and mediates its recruitment to β-Catenin target promoter in chromatin. TRRAP deletion leads to a reduced level of β-Catenin ubiquitination, lower degradation rate and accumulation of β-Catenin protein. Furthermore, recruitment of Skp1 to chromatin and ubiquitination of chromatin-bound β-Catenin is abolished upon TRRAP knock-down, leading to an abnormal retention of β-Catenin at chromatin and concomitant hyperactivation of the canonical Wnt pathway. These results demonstrate that there is a distinct regulatory mechanism for β-Catenin ubiquitination/destruction acting in the nucleus which functionally complements cytoplasmic destruction of β-Catenin and prevents oncogenic stabilization of β-Catenin and chronic activation of the canonical Wnt pathway.

Open Access Article

E2F2 represses cell cycle regulators to maintain quiescence

Arantza Infante, Usua Laresgoiti, Jon Fernández-Rueda, Asier Fullaondo, Javier Galán, Ramón Díaz-Uriarte, Marcos Malumbres, Seth J. Field and Ana M. Zubiaga
Pages 3915 - 3927
http://dx.doi.org/10.4161/cc.7.24.7379
Abstract | PDF

E2F transcription factors control diverse biological processes through regulation of target gene expression.  However, the mechanism by which this regulation is established, and the relative contribution of each E2F member are still poorly defined.  We have investigated the role of E2F2 in regulating cellular proliferation.  We show that E2F2 is required for the normal G0/G1 phase because targeted disruption of the E2F2 gene causes T cells to enter S phase early and to undergo accelerated cell division. A large set of E2F target genes involved in DNA replication and cell cycle progression (such as Mcm’s, cyclins and Cdc2a) that are silent in G0 and typically transcribed late in G1 phase are already actively expressed in quiescent T cells and MEFs lacking E2F2.  The classic E2F activators, E2F1 and E2F3, are largely dispensable for this process because compound loss of E2F1-/- and E2F2-/- produces a comparably shortened G0/G1 phase, with early S phase entry. Likewise, shRNA knockdown of E2F3 does not alter significantly the E2F2-/- phenotype.  Chromatin immunoprecipitation analysis indicates that in wild-type cells the promoters of the aberrantly early-transcribed genes are occupied by E2F2 in G0, suggesting a direct role for E2F2 in transcriptional repression.  We conclude that E2F2 functions to transcriptionally repress cell cycle genes to establish the G0 state.

Open Access Article

Positional analyses of BRCA1-dependent expression in Saccharomyces cerevisiae

Robert V. Skibbens, Danielle N. Ringhoff, Jutta Marzillier, Lynne Cassimeris and Laura Eastman
Pages 3928 - 3934
http://dx.doi.org/10.4161/cc.7.24.7380
Abstract | PDF

Mutations in BRCA1 account for a significant proportion of hereditary breast and ovarian cancers, but analysis of BRCA1 function is complicated by pleiotropic effects and binding partners (Pol II holoenzyme and transcription factors, chromatin remodelers, recombination complexes and E3 ligases). In vertebrate cells, efforts to elucidate BRCA1 transcriptional effects have focused on specific genes or restricted portions of the genome – limiting analyses of BRCA1 effects on adjoining DNA sequences and along chromosome lengths. Here, we use microarray analyses on the genetically tractable yeast
cell system to elucidate BRCA1-dependent genome-wide positional effects on both gene induction and repression. Yeast responses may be of clinical relevance based on findings that BRCA1 severely diminishes yeast growth kinetics but that BRCA1 mutated at sites identified from breast tumors is no longer able to retard yeast. Our analysis reveals that BRCA1 acts through both transcription factors to up-regulate specific loci and chromatin remodeling complexes to effect global changes in gene expression. BRCA1 also exhibits gene repression activities. Cluster-functional analysis reveals that these repressed factors are required for mitotic stability and provide a novel molecular explanation for the conditional lethality observed between BRCA1 and chromosome segregation genes.

Open Access Article

The let-7 microRNA target gene, Mlin41/Trim71 is required for mouse embryonic survival and neural tube closure

Betsy R. Maller Schulman, Xianping Liang, Carlos Stahlhut, Casey DelConte, Giovanni Stefani and Frank J. Slack
Pages 3935 - 3942
http://dx.doi.org/10.4161/cc.7.24.7397
Abstract | PDF

In the nematode Caenorhabditis elegans, the let-7 microRNA (miRNA) controls the timing of key developmental events and terminal differentiation in part by directly regulating lin-41. C. elegans lin-41 mutants display precocious cell cycle exit and terminal differentiation of epidermal skin cells. lin-41 orthologues are found in more complex organisms including both mice and humans, but their roles are not known. We generated Mlin41 mouse mutants to ascertain a functional role for Mlin41. Strong loss of function Mlin41 gene-trap mutants demonstrated a striking neural tube closure defect during development, and embryonic lethality. Like C. elegans lin-41, Mlin41 also appears to be regulated by the let-7 and mir-125 miRNAs. Since Mlin41 is required for neural tube closure and survival it points to human lin-41 (HLIN41/TRIM71) as a potential human development and disease gene.

Open Access Article

Unexpected link between anaphase promoting complex and the toxicity of expanded polyglutamines expressed in yeast

Natalia A. Bocharova, Svyatoslav S. Sokolov, Dmitry A. Knorre, Vladimir P. Skulachev and Fedor F. Severin
Pages 3943 - 3946
http://dx.doi.org/10.4161/cc.7.24.7398
Abstract | PDF

Protein aggregation is intimately linked to a number of neurodegenerative diseases. Expansion of the huntingtin polyglutamine-rich domain causes protein aggregation and neuronal degeneration. Recently we found that, similar to neurons, yeast expressing the expanded domain show markers of programmed cell death. Here we showed that deletion of yeast metacaspase gene YCA1 partly rescues the toxic effect of the domain overexpression. We also performed genetic screen for other genes deletions alleviating the toxic effect and found ASE1. Ase1 is a substrate of the Cdh1 form of anaphase promoting complex, APC/Cdh1. We tested Cdh1 overexpression and the deletion of CLB2 (mitotic cyclin, substrate of APC/Cdh1) and found that both mutations had a rescuing effect on the expanded polyglutamine toxicity. Our data suggest that the toxic effect of aggregated proteins is partly indirect. We speculate that cellular attempt to degrade the aggregates overloads the proteasome, and this leads to pathological accumulation of APC substrates.




LETTERS TO THE EDITOR

Open Access Article

Concordance of exon array and real-time PCR assessment of gene expression following cancer cell cytotoxic drug exposure

Alvin J.X. Lee, Phil East, Stuart Pepper, Barbara Nicke, Zoltan Szallasi, Aron C. Eklund, Julian Downward and Charles Swanton
Pages 3947 - 3948
http://dx.doi.org/10.4161/cc.7.24.7217
PDF

Open Access Article

New origins of DNA replication identified near the human IFNA2 gene

Gennady Kholodii, Olga Dantsevich and Vyacheslav Tarantul
Pages 3949 - 3951
http://dx.doi.org/10.4161/cc.7.24.7242
PDF