Table of Contents

Volume 6, Issue 11

  June 1, 2007

  Pages 1277 - 1402

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 Issue Catalogue (MARC XML)

CONCEPTUAL REVIEW

Open Access Article

Research by Retrieving Experiments

Mikhail V. Blagosklonny
Pages 1277 - 1283
http://dx.doi.org/10.4161/cc.6.11.4350
Abstract | PDF

Newton did not discover that apples fall: the information was available prior to his gravitational hypothesis. Hypotheses can be tested not only by performing experiments but also by retrieving experiments from the literature (via PubMed, for example). Here I show how disconnected facts from known data, if properly connected, can generate novel predictions testable in turn by other published data. With examples from cell cycle, aging, cancer and other fields of biology and medicine, I discuss how new knowledge was and will be derived from old information. Millions of experiments have been already performed to test unrelated hypotheses and the results of those experiments are available to‘test’ your hypotheses too. But most data (99% by some estimates) remain unpublished, because they were negative, seemed of low priority, or did not fit the story. Yet for other investigators those data may be valuable. The well-known story of Franklin and Watson is a case in point. By making preliminary data widely available, ‘data-owners’ will benefit most, receiving the credit for otherwise unused results. If posted (pre-published) on searchable databases, these data may fuel thousands of projects without the need for repetitive experiments. Enormous ‘pre-published’ databases coupled with Google-like search engines can change the structure of scientific research, and shrinking funding will make this inevitable.




EXTRA VIEWS

Open Access Article

Soothing the Watchman: Telomerase Reduces the p53-dependent Cellular Stress Response

Alain Beliveau and Paul Yaswen
Pages 1284 - 1287
http://dx.doi.org/10.4161/cc.6.11.4298
Abstract | PDF

In addition to conferring an indefinite replicative life span, telomerase renders p16(-) human mammary epithelial cells (HMEC) resistant to growth arrest by TGFβ or by loss of EGF or insulin signaling. In contrast to earlier reports, we recently found that growth factor signaling was not directly affected by telomerase expression. Rather, short dysfunctional or near-dysfunctional telomeres in proliferating telomerase(-) HMEC sensitized the cells to p53-dependent signals for growth arrest. We showed that during serial passage and before any signs of replicative senescence, HMEC lacking telomerase experience enhanced p53 stability and DNA damage signaling, as determined by increased phosphorylation on p53-Ser15 and Chk2-Thr68, and formation of 53BP1/phosphorylated histone H2AX foci at chromosome ends. This heightened activity of the p53 pathway enhanced the efficiency with which cells arrested growth in response to TGFβ or to EGF or insulin withdrawal, and was abolished by ectopic expression of hTERT, the catalytic subunit of telomerase. Telomerase elongated short telomeres, thereby reducing the basal level of activated p53 and raising cellular tolerance for other p53-dependent signals, including those emanating from non-genotoxic sources. These findings explain a number of observed effects of telomerase expression on cell growth and survival without postulating additional functions for telomerase.

Open Access Article

Posttranscriptional Orchestration of an Anti-Apoptotic Program by HuR

Kotb Abdelmohsen, Ashish Lal, Hyeon Ho Kim and Myriam Gorospe
Pages 1288 - 1292
http://dx.doi.org/10.4161/cc.6.11.4299
Abstract | PDF

The RNA-binding protein HuR can stabilize and/or regulate the translation of target mRNAs, thereby affecting the cellular responses to immune, proliferative, and damaging agents. Here, we discuss emerging evidence that HuR elicits a broad anti-apoptotic function through its influence on the expression of multiple target mRNAs. HuR was previously shown to bind to the mRNA encoding the apoptosome inhibitor prothymosin α (ProTα) and enhanced its translation and cytoplasmic abundance. More recently, HuR was shown to increase the stability of a target mRNA encoding the pro-survival deacetylase SIRT1. The discovery that HuR likewise binds to and promotes the expression of mRNAs encoding Bcl-2 and Mcl-1, two major anti-apoptotic effectors, strongly supports HuR’s role as a key upstream coordinator of a constitutive pro-survival program.

Open Access Article

CDK4 Inhibitors and Apoptosis: A Novel Mechanism Requiring Nucleolar Targeting of RelA

Hazel C. Thoms, M.G. Dunlop and L.A. Stark
Pages 1293 - 1297
http://dx.doi.org/10.4161/cc.6.11.4312
Abstract | PDF

Components of the cyclin D-CDK4/6-INK4-Rb pathway are key regulators of the cell cycle and are frequently disrupted in cancer. Defects in this pathway usually manifest as an increase in CDK4 activity, leading to unrestricted proliferation of tumour cells. CDK4 inhibitors have been shown to possess anti-tumour activity in vitro and agents that target the cyclin D1/CDK4 complex are currently the focus of intense scrutiny for clinical application as cancer therapeutics. However, the mechanisms by which these agents mediate their effects remains to be fully elucidated. We recently described a novel mechanism by which a CDK4 inhibitor induces apoptosis in colon cancer cells through activation of the NF-κB signalling pathway. Specific inhibition of CDK4 activity induced translocation of RelA, the principal component of NF-κB, from the cytoplasm to the nucleoplasm and then to the nucleolus. This was accompanied by a repression of NF-κB-driven transcription and apoptosis of the cancer cells. To determine the role of RelA in apoptosis, we utilised a mutant form of the protein, where the critical domain required for nucleolar targeting had been deleted. When cells expressing this mutant protein were treated with the CDK4 inhibitor, RelA translocated from the cytoplasm to the nucleoplasm, but was excluded from the nucleolus. Furthermore, apoptosis induced by CDK4 inhibition was also abrogated in cells expressing mutant RelA protein. Here, we discuss the molecular mechanisms that regulate programmed cell death induced by disruption of the cyclin D1/CDK4 complex and consider the wider implications these findings have for the future development of novel chemotherapeutic agents.

Open Access Article

Genetic Deficiency of p38α Reveals its Critical Role in Myoblast Cell Cycle Exit:The p38α-JNK Connection

Eusebio Perdiguero, Vanessa Ruiz-Bonilla, Antonio L. Serrano and Pura Muñoz-Cánoves
Pages 1298 - 1303
http://dx.doi.org/10.4161/cc.6.11.4315
Abstract | PDF

The regulation of skeletal muscle formation (myogenesis) is essential for normal development as well as in pathological conditions such as muscular dystrophies and inflammatory myopathies. Findings published over the past years have established a key role for the p38 MAP kinase signaling pathway in the control of muscle gene expression and myotube formation. However, the relative contribution of the four p38 MAP kinases (p38α, p38β, p38γ and p38δ) to this process was unknown. We have recently demonstrated that myoblasts lacking p38α, but not those lacking p38β or p38δ, were unable to differentiate and form multinucleated myotubes, while p38γ-deficient myoblasts exhibited an attenuated fusion capacity. Defective myogenesis in the absence of p38α was attributed to delayed cell cycle exit and continuous proliferation in differentiation-promoting conditions, caused by enhanced activation of the JNK/cJun pathway. We discuss these findings in the context of the emerging crosstalk of p38 and JNK signaling pathways in controlling cell growth and differentiation.

Open Access Article

The ARF/Oncogene Pathway Activates p53 Acetylation within the DNA Binding Domain

Hestia Mellert, Stephen M. Sykes, Maureen E. Murphy and Steven B. McMahon
Pages 1304 - 1306
http://dx.doi.org/10.4161/cc.6.11.4343
Abstract | PDF

Stabilization of the p53 tumor suppressor is a critical event in the response to various forms of cellular stress. Two distinct signaling pathways are thought to lead to this stabilization, depending on the type of cellular stress encountered. Genotoxic stress, such as chromosomal breaks or lesions induced by chemotherapeutic agents, result in the activation of the well-characterized DNA damage response pathway. Conversely, cellular stress that results from the aberrant activation of oncogenes triggers p53 stabilization via the induction of the p19ARF pathway. While activation of the DNA damage pathway ultimately causes a complex array of post-translational modifications on p53, activation few if any modifications have been demonstrated to occur following activation of the p19ARF pathway. We and others have recently identified a novel modification on p53, acetylation of lysine 120 within the DNA binding domain. This acetylation event is eliminated by tumor-derived mutations in p53 and its presence is required for the tumor suppressor apoptotic function of p53. We demonstrate here that both the DNA damage response pathway and the p19ARF/oncogene stress pathway induce the acetylation of p53 at lysine 120.

Open Access Article

The Complex Role of AR Signaling After Cytotoxic Insult: Implications for Cell Cycle Based Chemotherapeutics

Clay E.S. Comstock and Karen E. Knudsen
Pages 1307 - 1313
http://dx.doi.org/10.4161/cc.6.11.4353
Abstract | PDF

Prostatic adenocarcinomas are dependent on androgen receptor (AR) signaling for growth and progression, in part through the ability of AR to induce G1-S phase cell cycle transition. Hormonal therapies that inhibit AR activity are the first line of intervention for disseminated disease, and are initially quite effective; however, recurrent, incurable tumors ultimately arise with restored AR function. Given the importance of AR in governing the potentiation of this tumor type, there has been a dedicated interest in dissecting the mechanisms by which AR promotes prostate cancer proliferation and survival. Recent studies have challenged the utility of manipulating AR activity to enhance cell death in combination with genotoxic insult. Herein, the role of AR in controlling cell cycle progression and paradoxical roles of AR in survival signals are considered, as are the potential implications of these findings for chemotherapeutic response. Although there is much to be resolved, the present data suggest that knowledge of AR action in promoting cellular proliferation can be utilized for the design of coordinate strategies that maximize cell death in response to cytotoxic chemotherapeutics.




PERSPECTIVES

Open Access Article

Finding Plk3

Wendy C. Zimmerman and Raymond L. Erikson
Pages 1314 - 1318
http://dx.doi.org/10.4161/cc.6.11.4275
Abstract | PDF

Polo-like kinases (Plks) are a highly conserved family of kinases found in flies, yeast and vertebrates. Plks derive their name from homology to the gene product of polo, a protein kinase first identified in Drosophila. Three polo-like kinases have been identified in vertebrates: Plk1, Plk2 and Plk3. Studies on Plk1 have revealed a great deal of information on its multiple functions, however Plk2 and Plk3 functions have not been fully explored. In this perspective we discuss recent work on Plk3 expression, function and localization in the context of previous reports on Plk3 and in terms of its relationship to Plk1.

Open Access Article

The Roles of Synoviolin in Crosstalk Between Endoplasmic Reticulum Stress-Induced Apoptosis and p53 Pathway

Satoshi Yamasaki, Naoko Yagishita, Kusuki Nishioka and Toshihiro Nakajima
Pages 1319 - 1323
http://dx.doi.org/10.4161/cc.6.11.4277
Abstract | PDF

Endopalsmic reticulum (ER) is specialized organelle to maintain the integrity of secreted and membranous proteins. ER also senses so-called “ER stress”, which is a resulted from a various internal and external stresses, and triggers apoptosis when the diverse attempts to accommodate with the stress are in fail. The impairment these ER functions has been implicated in several human diseases, in which aberrant ER stress induced apoptosis is observed. We discuss about another disease model related with ER mediated apoptosis based on the recent studies about Synoviolin, an E3 ubiquitin ligase inherently utilized for ER associated degradation (ERAD). In addition to its canonical role in ERAD, Synoviolin targets tumor suppressor gene p53 for proteasomal degradation, suggesting the crosstalk between ERAD and p53 mediated apoptotic pathway under ER stress. Together with the anti-apoptotic property of Synoviolin previously elucidated by both in vitro and in vivo analyses, its new function in p53 regulation may provide a new insight into the pathomechanism of proliferative diseases such as cancer or rheumatoid arthritis.

Open Access Article

The Function and Regulation of the JARID1 Family of Histone H3 Lysine 4 Demethylases: the Myc Connection

Julie Secombe and Robert N. Eisenman
Pages 1324 - 1328
http://dx.doi.org/10.4161/cc.6.11.4269
Abstract | PDF

Epigenetic regulation of transcription refers to reversible, heritable changes in gene expression that occur in the absence of changes in DNA sequence. A major epigenetic mechanism involves the covalent modification of nucleosomal histones to create binding sites for transcriptional regulators and chromatin remodeling complexes that mediate activation or repression of transcription. While it has been known for a number of years that many histone modifications are reversible, it has only recently been shown that methyl groups are enzymatically removed from lysine residues. Here we discuss the recent characterization of a new class of demethylase enzyme, the JARID1 family, that catalyzes the removal of methyl groups from lysine 4 of histone H3. We summarize recent findings regarding the function of this family of proteins, focusing on our characterization of Little imaginal discs (Lid), the sole JARID1 family protein in Drosophila, which is rate-limiting for Myc-induced cell growth. Finally, we propose models to explain the role of Lid in Myc-mediated growth and discuss the relevance of these findings to human disease and tumor formation.

Open Access Article

Circadian Clock and Breast Cancer: A Molecular Link

Saurabh Saha and Paolo Sassone-Corsi
Pages 1329 - 1331
http://dx.doi.org/10.4161/cc.6.11.4295
Abstract | PDF

The circadian clock controls a large array of behavioral and physiological systems of fundamental importance to most organisms. Consequently, abnormal functioning of the clock results in severe dysfunctions and pathologies. Although epidemiological studies show a clear correlation between disruption of circadian rhythms and incidence of breast cancer, a molecular interpretation of how clock-related mechanisms may link to tumor development remains elusive. Here we speculate on the molecular pathways that may couple the circadian machinery to breast cancer.

Open Access Article

Pin1 in Neuronal Apoptosis

Esther B.E. Becker and Azad Bonni
Pages 1332 - 1335
http://dx.doi.org/10.4161/cc.6.11.4316
Abstract | PDF

While the role of the prolyl isomerase Pin1 in dividing cells has long been recognized, Pin1’s function in postmitotic neurons is poorly understood. We have identified a novel mechanism by which Pin1 mediates activation of the mitochondrial cell death machinery specifically in neurons. This perspective presents a sophisticated signaling pathway that triggers neuronal apoptosis upon JNK-mediated phosphorylation of the BH3-only protein BIMEL at serine 65. Pin1 is enriched at the mitochondria in neurons together with BIMEL and components of a neuron-specific JNK signaling complex and functions as a molecular switch that couples the phosphorylation of BIMEL by JNK to apoptosis specifically in neurons. We discuss how these findings relate to our understanding of the development of the nervous system and the pathogenesis of neurologic disorders.

Open Access Article

Distinguishing Rational from Irrational Applications of Pharmacogenetic Synergies from the Bench to Clinical Trials

Tomas Hucl, Eike Gallmeier and Scott E. Kern
Pages 1336 - 1341
http://dx.doi.org/10.4161/cc.6.11.4359
Abstract | PDF

Single therapeutic agents very often fail in unselected patients. It is therefore commonplace to combine an agent specifically with a selected patient subgroup or with another agent. To support such efforts, it is useful to clarify the distinctions between the terms and the mathematical models used in analyzing combinations. To incorporate molecular disease classifications, the familiar concept of the therapeutic window is modified to define a pharmacogenetic window, which is an unambiguous numerical measure of the magnitude of interaction produced by a combination, and to define a test of pharmacogenetic synergy. In contrast, certain common comparative methods, such as vertical windows (comparing effects at a given dose) and animal models of mutational targets may be dominated by undesirable features. Although this discussion is oriented towards cancer therapy, an extension of these concepts to other comparative biologic assays is feasible and advisable.




PRIORITY REPORTS

Open Access Article

The Crystal Structure of Human Cyclin B

Edward T. Petri, Alessia Errico, Lourdes Escobedo, Tim Hunt and Ravi Basavappa
Pages 1342 - 1349
http://dx.doi.org/10.4161/cc.6.11.4297
Abstract | PDF

Cyclin B is the key regulatory protein controlling mitosis in all eukaryotes, where it binds cyclin-dependent kinase, cdk1, forming a complex which initiates the mitotic program through phosphorylation of select proteins. Cyclin B regulates the activation, subcellular localization, and substrate specificity of cdk1, and destruction of cyclin B is necessary for mitotic exit. Overexpression of human cyclin B1 has been found in numerous cancers and has been associated with tumor aggressiveness. Here we report the crystal structure of human cyclin B1 to 2.9 Å. Comparison of the structure with cyclin A and cyclin E reveals remarkably similar N-terminal cyclin box motifs but significant differences among the C-terminal cyclin box lobes. Divergence in sequence gives rise to unique interaction surfaces at the proposed cyclin B/ cdk1 interface as well as the ‘RxL’ motif substrate binding site on cyclin B. Examination of the structure provides insight into the molecular basis for differential affinities of protein based cyclin/cdk inhibitors such as p27, substrate recognition, and cdk interaction.

Open Access Article

Cyclin B and Cyclin A Confer Different Substrate Recognition Properties on CDK2

Nick R. Brown, Ed D. Lowe, Ed Petri, Vicky Skamnaki, Robin Antrobus and Louise Johnson
Pages 1350 - 1359
http://dx.doi.org/10.4161/cc.6.11.4278
Abstract | PDF

The transitions of the cell cycle are regulated by the cyclin dependent protein kinases (CDKs). The cyclins activate their respective CDKs and confer substrate recognition properties. We report the structure of phospho-CDK2/cyclin B and show that cyclin B confers M phase-like properties on CDK2, the kinase that is usually associated with S phase. Cyclin B produces an almost identical activated conformation of CDK2 as that produced by cyclin A. There are differences between cyclin A and cyclin B at the recruitment site, which in cyclin A is used to recruit substrates containing an RXL motif. Because of sequence differences this site in cyclin B binds RXL motifs more weakly than in cyclin A. Despite similarity in kinase structures, phospho-CDK2/cyclin B phosphorylates substrates, such as nuclear lamin and a model peptide derived from p107, at sequences SPXX that differ from the canonical CDK2/cyclin A substrate recognition motif, SPXK. CDK2/cyclin B phosphorylation at these non-canonical sites is not dependent on the presence of a RXL recruitment motif. The p107 peptide contained two SP motifs each followed by a noncanonical sequence of which only one site (Ser640) is phosphorylated by pCDK2/cyclin A while two sites are phosphorylated by pCDK2/cyclin B. The second site is too close to the RXL motif to allow the cyclin A recruitment site to be effective, as previous work has shown that there must be at least 16 residues between the catalytic site serine and the RXL motif. Thus the cyclins A and B in addition to their role in promoting the activatory conformational switch in CDK2, also provide differential substrate specificity.




REPORTS

Open Access Article

TGFβ Receptor I Conditional Knockout Mice Develop Spontaneous Squamous Cell Carcinoma

Yasuyuki Honjo, Yansong Bian, Koji Kawakam, Alfredo Molinolo, Glenn Longenecker, Ramanamurthy Boppana, Jonas Larsson, Stefan Karlsson, J. Silvio Gutkind, Raj K. Puri and Ashok B. Kulkarni
Pages 1360 - 1366
http://dx.doi.org/10.4161/cc.6.11.4268
Abstract | PDF

We generated a mouse model with a conditional deletion of TGF-β signaling in the neurons by crossing TGF-β receptor I (TβRI) floxed mice with neurofilament-H (NF-H) Cre mice. 35% of F1 conditional knockout (COKO) mice developed spontaneous squamous cell carcinomas (SCCs) in periorbital and/or perianal regions. Transplantation of these tumors into athymic nude mice resulted in 62% tumorigenicity. To determine whether evasion of the immune response plays any role in this tumorigenesis, we analyzed the expression levels of receptors for interleukin-13 (mIL-13R), a key negative regulator of tumor immunosurveillance, and found that 33% of COKO tumors expressed the IL-13Rα2 chain. Primary cultures of the SCCs expressing IL-13Rα2 were sensitive to the cytotoxic effect of IL-13R-directed cytotoxin treatment. This is the first demonstration that loss of TβRI can lead to spontaneous tumor formation. These mice can serve as a unique mouse model of SCC to evaluate the tumorigenicity and effect of anti-cancer therapeutics.

Open Access Article

BubR1 and CENP-E Have Antagonistic Effects Upon the Stability of Microtubule-Kinetochore Attachments in Drosophila S2 Cell Mitosis

Andre F. Maia, Carla S. Lopes and Claudio E. Sunkel
Pages 1367 - 1378
http://dx.doi.org/10.4161/cc.6.11.4271
Abstract | PDF

The Spindle Assembly Checkpoint ensures the fidelity of chromosome segregation at each cell division cycle. Previous reports have indicated that in higher eukaryotes checkpoint proteins, such as BubR1, are also implicated in chromosome congression, more specifically that BubR1 regulates chromosome-spindle attachments. Also, several studies have shown that BubR1 interacts with the microtubule motor protein CENP-E. Whether this association contributes to the regulation of chromosome-spindle attachments is not yet known. Accordingly, we performed a detailed analysis of microtubule-kinetochore interactions after depletion of BubR1 and the Drosophila CENP-E homologue, CENP-meta by RNAi. We find that depletion of BubR1 affects mitosis very differently from depletion of CENP-meta. While BubR1-depleted cells exit mitosis prematurely due to loss of SAC activity, CENP-meta-depleted cells accumulate in prometaphase and do not exit mitosis after spindle damage. Also, in contrast to cells depleted for CENP-meta, cells depleted for BubR1 very rarely reach full metaphase alignment even if arrested in mitosis with the proteasome inhibitor MG132. More importantly, we show for the first time that BubR1-depleted cells contain a high frequency of either monoriented or fully unattached chromosomes while most CENP-meta dsRNAi-treated cells have chromosomes attached to spindle microtubules. Moreover, simultaneous depletion of both proteins reveals that absence of CENP-meta is able to partially rescue the unattached chromosome phenotype observed after BubR1 depletion. These results strongly suggest that while BubR1 is required to promote stable microtubule kinetochore attachment, CENP-E appears to be required to destabilize kinetochore attachment. Overall our results suggest that activation of the mechanism that corrects inappropriate kinetochore attachment requires the antagonistic effects of BubR1 and CENP-E.

Open Access Article

Silencing of LMP1 Induces Cell Cycle Arrest and Enhances Chemosensitivity Through Inhibition of AKT Signaling Pathway in EBV-Positive Nasopharyngeal Carcinoma Cells

Yu-Ping Mei, Jun-Min Zhou, Yi Wang, He Huang, Rong Deng, Gong-Kan Feng, Yi-Xin Zeng and Xiao-Feng Zhu
Pages 1379 - 1385
http://dx.doi.org/10.4161/cc.6.11.4274
Abstract | PDF

The latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV) is closely associated with nasopharyngeal carcinoma (NPC). In this study, we investigated that the effect of silencing LMP1 on cell cycle distribution and chemosensitivity in EBV-positive nasopharyngeal carcinoma C666-1 cells. Silencing of LMP1 by specific siRNA induced G1 arrest in C666-1 cells. The protein expression of CDK4 and cyclin D1 decreased and P27 was upregulated following LMP1 knockdown. Phosphorylation of AKT and its downstream targets IКB, FKHR was inhibited by LMP1 siRNA. The chemosensitivity of C666-1 cells to bleomycin and cisplatin was enhanced by siRNA targeting LMP1. The cells treated with LMP1 siRNA showed enhanced cleavage of the effector caspase3 and PARP, and Bax had the tendency to exhibit higher expression. Also, co-transfection of constitutive active AKT plasmid with LMP-1 siRNA plasmid abrogates sensitivity of C666-1 to bleomycin and cisplatin. It is reported for the first time that AKT signaling pathway was directly involved in the effects induced by siRNA targeting LMP1. Our findings confirm LMP1 as a rational therapeutic target in NPC.

Open Access Article

Protein Phosphatase PP6 N Terminal Domain Restricts G₁ to S Phase Progression in Human Cancer Cells

Bjarki Stefansson and David L. Brautigan
Pages 1386 - 1392
http://dx.doi.org/10.4161/cc.6.11.4276
Abstract | PDF

Yeast SIT4 is an essential gene encoding a protein Ser/Thr phosphatase conserved throughout eukaryotic evolution, known as PPV in Drosophila and PP6 in vertebrates. Sit4 promotes transcription of G1 cyclins and a sit4ts strain exhibits a G1 arrest at the restrictive temperature. The yeast sit4ts was rescued by expression of PPV or a chimeric phosphatase containing the first 53 residues of PPV fused to Drosophila PP1. The results suggested that the N terminus of the Sit4/PPV protein exerts a specific function in the yeast cell cycle. Here we tested whether the N terminus of human PP6 exerts specific effects on G1-S progression in human cells. The N terminus of PP6 or PP2A was fused to GFP and the proteins transiently expressed in prostate cancer PC-3 cells. Expression of the PP6 fusion protein was restricted to lower levels than either the PP2A fusion protein or GFP. However, the PP6 fusion protein blocked entry into S phase and increased by > 20% the proportion of cells in G1 phase. Expression of the PP6 fusion protein did not significantly change the levels of transcripts for cyclins or ca. 80 other cell cycle genes, but did suppress the levels of cyclin D1 protein in cells in G1 phase and reduce the phosphorylation of RB1 at Ser807/811. Thus, our results provide evidence that PP6 regulates cell cycle progression in human cells at least in part through control of cyclin D1 and the function of PP6 is distinct from its homolog Sit4 in yeast.

Open Access Article

Low Dose Geldanamycin Inhibits Hepatocyte Growth Factor- and Hypoxia-Stimulated Invasion of Cancer Cells

Fumitaka Koga, Shinji Tsutsumi and Leonard M. Neckers
Pages 1393 - 1402
http://dx.doi.org/10.4161/cc.6.11.4296
Abstract | PDF

Hepatocyte growth factor (HGF) receptor Met and hypoxia-inducible factor-1 (HIF-1) signaling pathways are commonly activated in aggressive tumors and promote progression. Since both Met and HIF-1α proteins are heat shock protein (Hsp) 90 clients, Hsp90 inhibitors might be expected to positively impact tumor progression. Here, we systematically evaluated the inhibitory effects of the prototypical Hsp90 inhibitor geldanamycin (GA) on cellular processes involved in invasion and angiogenesis in T24 bladder cancer cells stimulated with HGF and chemical hypoxia. First, we demonstrated the positive feedback loop between Met and HIF-1 pathways, which serves to sustain and amplifies their signaling in T24 cells. GA down-regulated Met by inhibiting new protein maturation, thereby dampening HGF signaling. HGF and chemical hypoxia with CoCl2 cooperatively promoted in vitro invasion and vascular endothelial growth factor (VEGF) secretion, while CoCl2 but not HGF activated urokinase-type plasminogen activator and matrix metalloproteinase 2, both of which promote invasion and angiogenesis. Low dose GA (100 nmol/L) inhibited these processes by suppressing both HGF and HIF-1 pathways. Notably, brief GA pretreatment inhibited in vitro invasion and VEGF secretion induced by HGF as effectively as did continuous treatment. Moreover, we found that GA inhibited activation of focal adhesion kinase, focal adhesion assembly, and actin reorganization induced by HGF and integrin engagement by extracellular matrix. Thus, GA widely suppresses extrinsic stimuli-induced signaling that contribute to tumor invasion and angiogenesis in this bladder carcinoma model, suggesting the utility of Hsp90 inhibitors in preventing tumor progression and metastasis.