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Raven JF, Baltzis D, Wang S, Mounir Z, Papadakis AI, Gao HQ, Koromilas AE. PKR and PKR-like Endoplasmic Reticulum Kinase Induce the Proteasome-dependent Degradation of Cyclin D1 via a Mechanism Requiring Eukaryotic Initiation Factor 2α Phosphorylation. J Biol Chem 2008; 283:3097-3108. [PMID: 18063576 DOI: 10.1074/jbc.m709677200] [Citation(s) in RCA: 75] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Affiliation(s)
- Jennifer F Raven
- Department of Oncology, Faculty of Medicine, McGill University, Montreal, Quebec H2W 1S6, Canada; Lady Davis Institute for Medical Research, Sir Mortimer B. Davis-Jewish General Hospital, Montreal, Quebec H3T 1E2, Canada
| | - Dionissios Baltzis
- Department of Oncology, Faculty of Medicine, McGill University, Montreal, Quebec H2W 1S6, Canada; Lady Davis Institute for Medical Research, Sir Mortimer B. Davis-Jewish General Hospital, Montreal, Quebec H3T 1E2, Canada
| | - Shuo Wang
- Lady Davis Institute for Medical Research, Sir Mortimer B. Davis-Jewish General Hospital, Montreal, Quebec H3T 1E2, Canada
| | - Zineb Mounir
- Department of Oncology, Faculty of Medicine, McGill University, Montreal, Quebec H2W 1S6, Canada; Lady Davis Institute for Medical Research, Sir Mortimer B. Davis-Jewish General Hospital, Montreal, Quebec H3T 1E2, Canada
| | - Andreas I Papadakis
- Department of Oncology, Faculty of Medicine, McGill University, Montreal, Quebec H2W 1S6, Canada; Lady Davis Institute for Medical Research, Sir Mortimer B. Davis-Jewish General Hospital, Montreal, Quebec H3T 1E2, Canada
| | - Hong Qing Gao
- Lady Davis Institute for Medical Research, Sir Mortimer B. Davis-Jewish General Hospital, Montreal, Quebec H3T 1E2, Canada
| | - Antonis E Koromilas
- Department of Oncology, Faculty of Medicine, McGill University, Montreal, Quebec H2W 1S6, Canada; Lady Davis Institute for Medical Research, Sir Mortimer B. Davis-Jewish General Hospital, Montreal, Quebec H3T 1E2, Canada.
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152
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Abstract
The repression of translation in environmentally stressed eukaryotic cells causes the sequestration of translation initiation factors and the 40S ribosomal subunit into discrete cytoplasmic foci called stress granules (SGs). Most components of the preinitiation complex, such as eIF3, eIF4A, eIF4E, eIF4G, and poly(A)-binding protein, congregate into SGs under stress conditions. However, the molecular basis of translation factor sequestration into SGs has not been clearly elucidated. Here, we report that proline-rich transcript in brain (PRTB) protein interacts with eIF4G and participates in SG formation. PRTB was recruited to SG under sodium arsenite and heat stress conditions. When overexpressed, PRTB inhibited global translation and formed SGs containing TIA-1, eIF4G, and eIF3. Knockdown of PRTB reduced the SG formation induced by sodium arsenite. These results suggest that PRTB not only is a component of SG formed by cellular stresses but also plays an important role in SG formation via an interaction with the scaffold protein eIF4G, which is associated with many translation factors and mRNAs.
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153
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154
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Graff JR, Konicek BW, Vincent TM, Lynch RL, Monteith D, Weir SN, Schwier P, Capen A, Goode RL, Dowless MS, Chen Y, Zhang H, Sissons S, Cox K, McNulty AM, Parsons SH, Wang T, Sams L, Geeganage S, Douglass LE, Neubauer BL, Dean NM, Blanchard K, Shou J, Stancato LF, Carter JH, Marcusson EG. Therapeutic suppression of translation initiation factor eIF4E expression reduces tumor growth without toxicity. J Clin Invest 2007; 117:2638-48. [PMID: 17786246 PMCID: PMC1957541 DOI: 10.1172/jci32044] [Citation(s) in RCA: 294] [Impact Index Per Article: 17.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/07/2007] [Accepted: 06/27/2007] [Indexed: 12/14/2022] Open
Abstract
Expression of eukaryotic translation initiation factor 4E (eIF4E) is commonly elevated in human and experimental cancers, promoting angiogenesis and tumor growth. Elevated eIF4E levels selectively increase translation of growth factors important in malignancy (e.g., VEGF, cyclin D1) and is thereby an attractive anticancer therapeutic target. Yet to date, no eIF4E-specific therapy has been developed. Herein we report development of eIF4E-specific antisense oligonucleotides (ASOs) designed to have the necessary tissue stability and nuclease resistance required for systemic anticancer therapy. In mammalian cultured cells, these ASOs specifically targeted the eIF4E mRNA for destruction, repressing expression of eIF4E-regulated proteins (e.g., VEGF, cyclin D1, survivin, c-myc, Bcl-2), inducing apoptosis, and preventing endothelial cells from forming vessel-like structures. Most importantly, intravenous ASO administration selectively and significantly reduced eIF4E expression in human tumor xenografts, significantly suppressing tumor growth. Because these ASOs also target murine eIF4E, we assessed the impact of eIF4E reduction in normal tissues. Despite reducing eIF4E levels by 80% in mouse liver, eIF4E-specific ASO administration did not affect body weight, organ weight, or liver transaminase levels, thereby providing the first in vivo evidence that cancers may be more susceptible to eIF4E inhibition than normal tissues. These data have prompted eIF4E-specific ASO clinical trials for the treatment of human cancers.
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Affiliation(s)
- Jeremy R Graff
- Lilly Research Labs, Eli Lilly and Company, Indianapolis, Indiana 46285, USA.
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155
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Thumma SC, Kratzke RA. Translational control: a target for cancer therapy. Cancer Lett 2007; 258:1-8. [PMID: 17923280 DOI: 10.1016/j.canlet.2007.08.022] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/25/2007] [Revised: 08/21/2007] [Accepted: 08/31/2007] [Indexed: 12/22/2022]
Abstract
Cap-mediated translation is the default mechanism for the synthesis of proteins in eukaryotic cells. Increasingly, malignant cells have been found to have deregulation of this process. Return of normal translational control is associated with loss of tumorigenic potential in pre-clinical models. Currently, a variety of novel therapeutics are in development targeting this mechanism as a treatment for cancer.
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Affiliation(s)
- Saritha C Thumma
- The Division of Hematology-Oncology-Transplant, Department of Medicine, The University of Minnesota Medical School, Minneapolis, MN 55455, USA
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156
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Phillips A, Blaydes JP. MNK1 and EIF4E are downstream effectors of MEKs in the regulation of the nuclear export of HDM2 mRNA. Oncogene 2007; 27:1645-9. [PMID: 17828301 DOI: 10.1038/sj.onc.1210785] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Regulation of the synthesis, function and degradation of HDM2 (Mdm2 in mouse) plays a key role in controlling the abundance and activity of the transcription factor p53, with consequent implications for the proliferation and survival of normal and cancer cells. We have previously identified the regulation of export of HDM2 mRNA from the nucleus as a novel point of control of HDM2 synthesis. This process is dependent on the activity of the growth factor-regulated MAP-kinase kinases (MEKs). Here, we provide evidence that the eIF4E kinase MNK1 is a key downstream effector of MEKs in this regulatory pathway. We show that HDM2 mRNA export in breast cancer cells is promoted by overexpressed eIF4E in a MEK- and MNK1-dependent manner, and inhibition of MNK1 suppresses endogenous HDM2 mRNA export pathways. This MNK1- and eIF4E-dependent HDM2 regulation occurs through sequences in the 3' untranslated region of HDM2 mRNA, and consequently HDM2 mRNA transcripts from both the constitutive P1 and inducible P2 promoters are regulated by this pathway. eIF4E is a known oncogene that is overexpressed in human tumours, including the majority of breast cancers. This pathway, therefore, may play an important role in the dysregulation of HDM2 oncoprotein expression that occurs in many human tumours.
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Affiliation(s)
- A Phillips
- Cancer Sciences Division, University of Southampton, Southampton General Hospital, Southampton, UK
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157
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O'Loghlen A, González VM, Jurado T, Salinas M, Martín ME. Characterization of the activity of human MAP kinase-interacting kinase Mnk1b. BIOCHIMICA ET BIOPHYSICA ACTA 2007; 1773:1416-27. [PMID: 17590453 DOI: 10.1016/j.bbamcr.2007.05.009] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/09/2007] [Revised: 05/18/2007] [Accepted: 05/21/2007] [Indexed: 02/08/2023]
Abstract
Human mitogen-activated protein (MAP) kinase interacting kinase 1b (Mnk1b) is a splice variant of human Mnk1a, which has been identified in our laboratory [A. O'Loghlen, V.M. Gonzalez, D. Pineiro, M.I. Perez-Morgado, M. Salinas, M.E. Martin, Identification and molecular characterization of Mnk1b, a splice variant of human MAP kinase-interacting kinase Mnk1, Exp. Cell Res. 299 (2004) 343-355]. Mnk1b has much higher basal eIF4E kinase activity than Mnk1a. Because Mnk1b presents different features in its C-terminus with respect to Mnk1a, we have studied in this paper the potential role of these structural differences in determining the higher basal eIF4E kinase activity as well as the subcellular localization of Mnk1b. In this paper, we demonstrate that phosphorylation of the Thr209 and Thr214 in the activation loop of Mnk1b is necessary for its activation. However, the different kinase activity between Mnk1a and Mnk1b is independent of the phosphorylation status of the activation loop residues. By deletion of the C-terminal tail in Mnk1a, we confirmed that the absence of this sequence is not responsible for the higher eIF4E kinase activity present in Mnk1b. Moreover, our findings support a crucial role of the 12 amino acids, particularly the Ala344, in the C-terminal specific region of Mnk1b (Mnk1bSR), on the kinase activity of the protein.
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Affiliation(s)
- Ana O'Loghlen
- Servicio de Bioquímica-Investigación, Hospital Ramón y Cajal, Ctra. Colmenar km. 9,100. E-28034 Madrid, Spain
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158
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Averous J, Fonseca BD, Proud CG. Regulation of cyclin D1 expression by mTORC1 signaling requires eukaryotic initiation factor 4E-binding protein 1. Oncogene 2007; 27:1106-13. [PMID: 17724476 DOI: 10.1038/sj.onc.1210715] [Citation(s) in RCA: 152] [Impact Index Per Article: 8.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
There is currently substantial interest in the regulation of cell function by mammalian target of rapamycin (mTOR), especially effects linked to the rapamycin-sensitive mTOR complex 1 (mTORC1). Rapamycin induces G(1) arrest and blocks proliferation of many tumor cells, suggesting that the inhibition of mTORC1 signaling may be useful in cancer therapy. In MCF7 breast adenocarcinoma cells, rapamycin decreases levels of cyclin D1, without affecting cytoplasmic levels of its mRNA. In some cell-types, rapamycin does not affect cyclin D1 levels, whereas the starvation for leucine (which impairs mTORC1 signaling more profoundly than rapamycin) does. This pattern correlates with the behavior of eukaryotic initiation factor 4E-binding protein 1 (4E-BP1, an mTORC1 target that regulates translation initiation). siRNA-mediated knock-down of 4E-BP1 abrogates the effect of rapamycin on cyclin D1 expression and increases the polysomal association of the cyclin D1 mRNA. Our data identify 4E-BP1 as a key regulator of cyclin D1 expression, indicate that this effect is not mediated through the changes in cytoplasmic levels of cyclin D1 mRNA and suggest that, in some cell types, interfering with the amino acid input to mTORC1, rather than using rapamycin, may inhibit proliferation.
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Affiliation(s)
- J Averous
- Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, BC, Canada
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159
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Larsson O, Li S, Issaenko OA, Avdulov S, Peterson M, Smith K, Bitterman PB, Polunovsky VA. Eukaryotic translation initiation factor 4E induced progression of primary human mammary epithelial cells along the cancer pathway is associated with targeted translational deregulation of oncogenic drivers and inhibitors. Cancer Res 2007; 67:6814-24. [PMID: 17638893 DOI: 10.1158/0008-5472.can-07-0752] [Citation(s) in RCA: 125] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Pathologic redirection of translational control by constitutive activation of eukaryotic translation initiation factor 4F (eIF4F), the cap-dependent translation initiation apparatus, is an obligatory step in oncogenesis; however, its mechanism remains undefined. Here, we simulate this pro-oncogenic state by overexpressing eIF4E, the rate-limiting component of eIF4F, in primary human mammary epithelial cells (HMECs) and examine the resultant changes in cell biology and gene expression profiles of total and polyribosome-bound mRNA genome wide. Overexpressed eIF4E rescues primary HMECs from telomere-independent growth arrest and disables checkpoints governing S-phase entry as well as apoptosis in HMECs immortalized by telomerase, imparting cells with proliferative and survival autonomy. Although the transcriptional response to increased eIF4E was modest, the translational response was large, selective, and bidirectional. In addition to translational activation of known and novel eIF4E-responsive oncogenic drivers regulating cell growth and survival, our data unveil previously unrecognized cellular defenses including translational activation of tumor suppressors, translational repression of transcripts enriched with miRNA target sites, and translational modulation of genes governing translation itself. These findings provide insight into the proneoplastic and compensatory mechanisms embedded in the oncogenic translational program. They support a model whereby deregulated eIF4E moves human epithelial cells along the cancer pathway by profoundly altering ribosomal recruitment to cancer-related transcripts, and eIF4E-modified cells counter these potentially oncogenic alterations with a compensatory translational mechanism that mitigates acquisition of malignancy.
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Affiliation(s)
- Ola Larsson
- Department of Medicine, University of Minnesota, 420 Delaware Street, Minneapolis, MN 55455, USA
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160
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Elia A, Constantinou C, Clemens MJ. Effects of protein phosphorylation on ubiquitination and stability of the translational inhibitor protein 4E-BP1. Oncogene 2007; 27:811-22. [PMID: 17653084 DOI: 10.1038/sj.onc.1210678] [Citation(s) in RCA: 53] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
Abstract
The availability of the eukaryotic polypeptide chain initiation factor 4E (eIF4E) for protein synthesis is regulated by the 4E-binding proteins (4E-BPs), which act as inhibitors of cap-dependent mRNA translation. The ability of the 4E-BPs to sequester eIF4E is regulated by reversible phosphorylation at multiple sites. We show here that, in addition, 4E-BP1 is a substrate for polyubiquitination and that some forms of 4E-BP1 are simultaneously polyubiquitinated and phosphorylated. In Jurkat cells inhibition of proteasomal activity by MG132 enhances the level of hypophosphorylated, unmodified 4E-BP1 but only modestly increases the accumulation of high-molecular-weight, phosphorylated forms of 4E-BP1. In contrast, inhibition of protein phosphatase activity with calyculin A reduces the level of unmodified 4E-BP1 but strongly enhances the amount of phosphorylated, high-molecular-weight 4E-BP1. Turnover measurements in the presence of cycloheximide show that, whereas 4E-BP1 is normally a very stable protein, calyculin A decreases the apparent half-life of the normal-sized protein. Affinity chromatography on m(7)GTP-Sepharose indicates that the larger forms of 4E-BP1 bind very poorly to eIF4E. We suggest that the phosphorylation of 4E-BP1 may play a dual role in the regulation of protein synthesis, both reducing the affinity of 4E-BP1 for eIF4E and promoting the conversion of 4E-BP1 to alternative, polyubiquitinated forms.
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Affiliation(s)
- A Elia
- Translational Control Group, Division of Basic Medical Sciences, Centre for Molecular and Metabolic Signalling, St George's, University of London, London, UK
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161
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Kolb S, Fritsch R, Saur D, Reichert M, Schmid RM, Schneider G. HMGA1 controls transcription of insulin receptor to regulate cyclin D1 translation in pancreatic cancer cells. Cancer Res 2007; 67:4679-86. [PMID: 17510394 DOI: 10.1158/0008-5472.can-06-3308] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
The HMGA1 proteins act as architectural transcription factors and are involved in the regulation of genes important in the process of carcinogenesis. Although HMGA1 proteins are overexpressed in most types of cancer, signaling circuits regulated by HMGA1 are not clarified in detail. In this study, we show that HMGA1 proteins promote proliferation of pancreatic cancer cells by accelerating G(1) phase progression. Transfection of HMGA1-specific small interfering RNA (siRNA) activates the RB-dependent G(1)-phase checkpoint due to the impaired expression of cyclin D1. Down-regulation of cyclin D1 after the HMGA1 knockdown is due to translational control and involves the repressor of the eukaryotic translation initiation factor 4E (eIF4E) 4E-BP1. We show that 4E-BP1 and cyclin D1 act downstream of the insulin receptor (IR) in pancreatic cancer cells. At the molecular level transcription of the IR is controlled by a CAAT/enhancer binding protein beta (C/EBPbeta)/HMGA1 complex. Together, this work defines a novel pathway regulated by HMGA1, which contributes to the proliferation of pancreatic cancer cells.
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Affiliation(s)
- Sebastian Kolb
- Department of Internal Medicine II, Technical University of Munich, Ismaninger, Munich, Germany
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162
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Meloche S, Pouysségur J. The ERK1/2 mitogen-activated protein kinase pathway as a master regulator of the G1- to S-phase transition. Oncogene 2007; 26:3227-39. [PMID: 17496918 DOI: 10.1038/sj.onc.1210414] [Citation(s) in RCA: 820] [Impact Index Per Article: 48.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
The Ras-dependent extracellular signal-regulated kinase (ERK)1/2 mitogen-activated protein (MAP) kinase pathway plays a central role in cell proliferation control. In normal cells, sustained activation of ERK1/ERK2 is necessary for G1- to S-phase progression and is associated with induction of positive regulators of the cell cycle and inactivation of antiproliferative genes. In cells expressing activated Ras or Raf mutants, hyperactivation of the ERK1/2 pathway elicits cell cycle arrest by inducing the accumulation of cyclin-dependent kinase inhibitors. In this review, we discuss the mechanisms by which activated ERK1/ERK2 regulate growth and cell cycle progression of mammalian somatic cells. We also highlight the findings obtained from gene disruption studies.
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Affiliation(s)
- S Meloche
- Departments of Pharmacology and Molecular Biology, Institut de Recherche en Immunologie et Cancérologie, Université de Montréal, Montreal, Quebec, Canada.
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163
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Miao XP, Li JS, Li HY, Zeng SP, Zhao Y, Zeng JZ. Expression of ornithine decarboxylase in precancerous and cancerous gastric lesions. World J Gastroenterol 2007; 13:2867-71. [PMID: 17569126 PMCID: PMC4395642 DOI: 10.3748/wjg.v13.i20.2867] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/29/2007] [Revised: 02/05/2007] [Accepted: 02/14/2007] [Indexed: 02/06/2023] Open
Abstract
AIM To investigate the expression of ornithine decarboxylase (ODC) in precancerous and cancerous gastric lesions. METHODS We studied the expression of ODC in gastric mucosa from patients with chronic superficial gastritis (CSG, n=32), chronic atrophic gastritis [CAG, n=43; 15 with and 28 without intestinal metaplasia (IM)], gastric dysplasia (DYS, n=11) and gastric cancer (GC, n=48) tissues using immunohistochemical staining. All 134 biopsy specimens of gastric mucosa were collected by gastroscopy. METHODS The positive rate of ODC expression was 34.4%, 42.9%, 73.3%, 81.8% and 91.7% in cases with CSG, CAG without IM, CAG with IM, DYS and GC, respectively (P<0.01), The positive rate of ODC expression increased in the order of CSG < CAG (without IM) < CAG (with IM) < DYS and finally, GC. In addition, ODC positive immunostaining rate was lower in well-differentiated GC than in poorly-differentiated GC (P<0.05). CONCLUSION The expression of ODC is positively correlated with the degree of malignity of gastric mucosa and development of gastric lesions. This finding indicates that ODC may be used as a good biomarker in the screening and diagnosis of precancerous lesions.
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Affiliation(s)
- Xin-Pu Miao
- Department of Gastroenterology, West China Hospital of Sichuan University, 17 Renmin Avenue, Chengdu 610041, Sichuan Province, China.
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164
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Origanti S, Shantz LM. Ras Transformation of RIE-1 Cells Activates Cap-Independent Translation of Ornithine Decarboxylase: Regulation by the Raf/MEK/ERK and Phosphatidylinositol 3-Kinase Pathways. Cancer Res 2007; 67:4834-42. [PMID: 17510413 DOI: 10.1158/0008-5472.can-06-4627] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Ornithine decarboxylase (ODC) is the first and generally rate-limiting enzyme in polyamine biosynthesis. Deregulation of ODC is critical for oncogenic growth, and ODC is a target of Ras. These experiments examine translational regulation of ODC in RIE-1 cells, comparing untransformed cells with those transformed by an activated Ras12V mutant. Analysis of the ODC 5' untranslated region (5'UTR) revealed four splice variants with the presence or absence of two intronic sequences. All four 5'UTR species were found in both cell lines; however, variants containing intronic sequences were more abundant in Ras-transformed cells. All splice variants support internal ribosome entry site (IRES)-mediated translation, and IRES activity is markedly elevated in cells transformed by Ras. Inhibition of Ras effector targets indicated that the ODC IRES element is regulated by the phosphorylation status of the translation factor eIF4E. Dephosphorylation of eIF4E by inhibition of mitogen-activated protein/extracellular signal-regulated kinase (ERK) kinase (MEK) or the eIF4E kinase Mnk1/2 increases ODC IRES activity in both cell lines. When both the Raf/MEK/ERK and phosphatidylinositol 3-kinase/mammalian target of rapamycin pathways are inhibited in normal cells, ODC IRES activity is very low and cells arrest in G(1). When these pathways are inhibited in Ras-transformed cells, cell cycle arrest does not occur and ODC IRES activity increases, helping to maintain high ODC activity.
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Affiliation(s)
- Sofia Origanti
- Department of Cellular and Molecular Physiology, Penn State College of Medicine, The Milton S. Hershey Medical Center, Hershey, Pennsylvania 17033, USA
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165
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Shenberger JS, Zhang L, Hughlock MK, Ueda T, Watanabe-Fukunaga R, Fukunaga R. Roles of mitogen-activated protein kinase signal-integrating kinases 1 and 2 in oxidant-mediated eIF4E phosphorylation. Int J Biochem Cell Biol 2007; 39:1828-42. [PMID: 17689282 PMCID: PMC2001257 DOI: 10.1016/j.biocel.2007.05.001] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2006] [Revised: 04/16/2007] [Accepted: 05/01/2007] [Indexed: 01/21/2023]
Abstract
Oxidative stress alters cellular metabolic processes including protein synthesis. The eukaryotic initiation factor, eIF4E, acts in the rate-limiting steps of initiation and promotes nuclear export. Phosphorylation of eIF4E by mitogen activated protein kinase signal-integrating kinases 1 and 2 (Mnk) influences the affinity of eIF4E for the 5'-mRNA cap and fosters nuclear export activity. Although phosphorylation of eIF4E on Ser209 is observed following oxidant exposure, the contribution of Mnk isoforms and the significance of phosphorylation remain elusive. Using a Mnk inhibitor and fibroblasts derived from Mnk knockout mice, we demonstrate that that H2O2 enhances eIF4E phosphorylation in cells containing Mnk1. In contrast, cells containing only Mnk2 show little change or a decrease in eIF4E phosphorylation in response to H2O2. H2O2 also shifted eIF4GI protein from the nucleus to the cytoplasm suggesting that the increases in eIF4E phosphorylation may reflect enhanced substrate availability to cytoplasmic Mnk1. In Mnk1(+/+) cells, H2O2 also enhanced eIF4E phosphorylation in the nucleus to a greater degree than in the cytoplasm, an effect not observed in cells containing Mnk2. In response to H2O2, all MEFs showed increased eIF4E:4E-BP1 and 4E-BP2:eIF4E binding and reduced eIF4E:eIF4GI binding. We also observed a dramatic increase in the amount of Mnk1 associated with eIF4E following affinity chromatography. These changes coincided with a smaller reduction in global protein synthesis in response to H2O2 in the DKO cells. These findings suggest that changes in eIF4GI distribution may enhance eIF4E phosphorylation and that the presence of either Mnk1 or 2 or any degree of eIF4E phosphorylation negatively regulates global protein synthesis in response to oxidant stress.
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Affiliation(s)
- Jeffrey S Shenberger
- Department of Pediatrics, The Pennsylvania State University College of Medicine, USA.
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166
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Shantz LM, Levin VA. Regulation of ornithine decarboxylase during oncogenic transformation: mechanisms and therapeutic potential. Amino Acids 2007; 33:213-23. [PMID: 17443268 DOI: 10.1007/s00726-007-0531-2] [Citation(s) in RCA: 79] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/15/2006] [Accepted: 02/01/2007] [Indexed: 01/10/2023]
Abstract
The activity of ornithine decarboxylase (ODC(1)), the first enzyme in polyamine biosynthesis, is induced during carcinogenesis by a variety of oncogenic stimuli. Intracellular levels of ODC and the polyamines are tightly controlled during normal cell growth, and regulation occurs at the levels of transcription, translation and protein degradation. Several known proto-oncogenic pathways appear to control ODC transcription and translation, and dysregulation of pathways downstream of ras and myc result in the constitutive elevation of ODC activity that occurs with oncogenesis. Inhibition of ODC activity reverts the transformation of cells in vitro and reduces tumor growth in several animal models, suggesting high levels of ODC are necessary for the maintenance of the transformed phenotype. The ODC irreversible inactivator DFMO has proven to be not only a valuable tool in the study of ODC in cancer, but also shows promise as a chemopreventive and chemotherapeutic agent in certain types of malignancies.
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Affiliation(s)
- L M Shantz
- Department of Cellular and Molecular Physiology, The Pennsylvania State University College of Medicine, Hershey, PA 17033, USA.
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167
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Bilanges B, Stokoe D. Mechanisms of translational deregulation in human tumors and therapeutic intervention strategies. Oncogene 2007; 26:5973-90. [PMID: 17404576 DOI: 10.1038/sj.onc.1210431] [Citation(s) in RCA: 69] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Abstract
Analysis of the recurrent genetic aberrations present in human tumors provides insight into how normal cells escape appropriate proliferation and survival cues. Commonly mutated genes encode proteins that monitor DNA damage (e.g., p53), proteins that regulate the cell cycle (such as Rb), and proteins that regulate signal transduction pathways (such as APC, PTEN and Ras). Analysis of the relevant targets and downstream events of these genes in normal and tumor cells will clearly highlight important pathways for tumorigenesis. However, more infrequent mutations are also informative in defining events critical for the process of tumorigenesis, and often delineate important pathways lying downstream of commonly mutated oncogenes and tumor suppressors. Together, these studies have led to the conclusion that deregulated protein synthesis plays an important role in human cancer. This review will discuss the evidence implicating mRNA translation as an important downstream consequence of signal transduction pathways initiated by mutated oncogenes and tumor suppressors, as well as additional genetic findings implicating the importance of global and specific translational control in human cancer. It will also discuss therapeutic strategies that take advantage of differences in translational regulation between normal and tumor cells.
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Affiliation(s)
- B Bilanges
- UCSF Cancer Research Institute, San Francisco, CA 94115, USA.
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168
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Proud CG. Signalling to translation: how signal transduction pathways control the protein synthetic machinery. Biochem J 2007; 403:217-34. [PMID: 17376031 DOI: 10.1042/bj20070024] [Citation(s) in RCA: 380] [Impact Index Per Article: 22.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
Recent advances in our understanding of both the regulation of components of the translational machinery and the upstream signalling pathways that modulate them have provided important new insights into the mechanisms by which hormones, growth factors, nutrients and cellular energy status control protein synthesis in mammalian cells. The importance of proper control of mRNA translation is strikingly illustrated by the fact that defects in this process or its control are implicated in a number of disease states, such as cancer, tissue hypertrophy and neurodegeneration. Signalling pathways such as those involving mTOR (mammalian target of rapamycin) and mitogen-activated protein kinases modulate the phosphorylation of translation factors, the activities of the protein kinases that act upon them and the association of RNA-binding proteins with specific mRNAs. These effects contribute both to the overall control of protein synthesis (which is linked to cell growth) and to the modulation of the translation or stability of specific mRNAs. However, important questions remain about both the contributions of individual regulatory events to the control of general protein synthesis and the mechanisms by which the translation of specific mRNAs is controlled.
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Affiliation(s)
- Christopher G Proud
- Department of Biochemistry and Molecular Biology, University of British Columbia, 2350 Health Sciences Mall, Vancouver, BC, Canada V6T 1Z3.
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169
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Guan L, Song K, Pysz MA, Curry KJ, Hizli AA, Danielpour D, Black AR, Black JD. Protein kinase C-mediated down-regulation of cyclin D1 involves activation of the translational repressor 4E-BP1 via a phosphoinositide 3-kinase/Akt-independent, protein phosphatase 2A-dependent mechanism in intestinal epithelial cells. J Biol Chem 2007; 282:14213-25. [PMID: 17360714 DOI: 10.1074/jbc.m610513200] [Citation(s) in RCA: 70] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023] Open
Abstract
We reported previously that protein kinase Calpha (PKCalpha), a negative regulator of cell growth in the intestinal epithelium, inhibits cyclin D1 translation by inducing hypophosphorylation/activation of the translational repressor 4E-BP1. The current study explores the molecular mechanisms underlying PKC/PKCalpha-induced activation of 4E-BP1 in IEC-18 nontransformed rat ileal crypt cells. PKC signaling is shown to promote dephosphorylation of Thr(45) and Ser(64) on 4E-BP1, residues directly involved in its association with eIF4E. Consistent with the known role of the phosphoinositide 3-kinase (PI3K)/Akt/mTOR pathway in regulation of 4E-BP1, PKC signaling transiently inhibited PI3K activity and Akt phosphorylation in IEC-18 cells. However, PKC/PKCalpha-induced activation of 4E-BP1 was not prevented by constitutively active mutants of PI3K or Akt, indicating that blockade of PI3K/Akt signaling is not the primary effector of 4E-BP1 activation. This idea is supported by the fact that PKC activation did not alter S6 kinase activity in these cells. Further analysis indicated that PKC-mediated 4E-BP1 hypophosphorylation is dependent on the activity of protein phosphatase 2A (PP2A). PKC signaling induced an approximately 2-fold increase in PP2A activity, and phosphatase inhibition blocked the effects of PKC agonists on 4E-BP1 phosphorylation and cyclin D1 expression. H(2)O(2) and ceramide, two naturally occurring PKCalpha agonists that promote growth arrest in intestinal cells, activate 4E-BP1 in PKC/PKCalpha-dependent manner, supporting the physiological significance of the findings. Together, our studies indicate that activation of PP2A is an important mechanism underlying PKC/PKCalpha-induced inhibition of cap-dependent translation and growth suppression in intestinal epithelial cells.
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Affiliation(s)
- Lingjie Guan
- Department of Pharmacology and Therapeutics, Roswell Park Cancer Institute, Buffalo, NY 14263, USA
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170
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Hu YL, Passegué E, Fong S, Largman C, Lawrence HJ. Evidence that the Pim1 kinase gene is a direct target of HOXA9. Blood 2007; 109:4732-8. [PMID: 17327400 PMCID: PMC1885524 DOI: 10.1182/blood-2006-08-043356] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022] Open
Abstract
The HOXA9 homeoprotein exerts dramatic effects in hematopoiesis. Enforced expression of HOXA9 enhances proliferation of primitive blood cells, expands hematopoietic stem cells (HSCs), and leads to myeloid leukemia. Conversely, loss of HOXA9 inhibits proliferation and impairs HSC function. The pathways by which HOXA9 acts are largely unknown, and although HOXA9 is a transcription factor, few direct target genes have been identified. Our previous study suggested that HOXA9 positively regulates Pim1, an oncogenic kinase. The hematologic phenotypes of Hoxa9- and Pim1-deficient animals are strikingly similar. Here we show that HOXA9 protein binds to the Pim1 promoter and induces Pim1 mRNA and protein in hematopoietic cells. Pim1 protein is diminished in Hoxa9(-/-) cells, and Hoxa9 and Pim1 mRNA levels track together in early hematopoietic compartments. Induction of Pim1 protein by HOXA9 increases the phosphorylation and inactivation of the proapoptotic BAD protein, a target of Pim1. Hoxa9(-/-) cells show increased apoptosis and decreased proliferation, defects that are ameliorated by reintroduction of Pim1. Thus Pim1 appears to be a direct transcriptional target of HOXA9 and a mediator of its antiapoptotic and proproliferative effects in early cells. Since HOXA9 is frequently up-regulated in acute myeloid leukemia, Pim1 may be a therapeutic target in human disease.
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Affiliation(s)
- Yu-Long Hu
- Hematology Research, Medical Service, UCSF Veterans Affairs Medical Center, University of California-San Francisco, 4150 Clement Street, San Francisco, CA 94121, USA
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171
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Beauchemin C, Boutet N, Laliberté JF. Visualization of the interaction between the precursors of VPg, the viral protein linked to the genome of turnip mosaic virus, and the translation eukaryotic initiation factor iso 4E in Planta. J Virol 2007; 81:775-82. [PMID: 17079311 PMCID: PMC1797466 DOI: 10.1128/jvi.01277-06] [Citation(s) in RCA: 121] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/17/2006] [Accepted: 10/20/2006] [Indexed: 01/20/2023] Open
Abstract
The RNA genome of Turnip mosaic virus is covalently linked at its 5' end to a viral protein known as VPg. This protein binds to the translation eukaryotic initiation factor iso 4E [eIF(iso)4E]. This interaction has been shown to be important for virus infection, although its exact biological function(s) has not been elucidated. In this study, we investigated the subcellular site of the VPg-eIF(iso)4E interaction using bimolecular fluorescence complementation (BiFC). As a first step, eIF(iso)4E, 6K-VPg-Pro, and VPg-Pro were expressed as full-length green fluorescent protein (GFP) fusions in Nicotiana benthamiana, and their subcellular localizations were visualized by confocal microscopy. eIF(iso)4E was predominantly associated with the endoplasmic reticulum (ER), and VPg-Pro was observed in the nucleus and possibly the nucleolus, while 6K-VPg-Pro-GFP induced the formation of cytoplasmic vesicles budding from the ER. In BiFC experiments, reconstituted green fluorescence was observed throughout the nucleus, with a preferential accumulation in subnuclear structures when the GFP split fragments were fused to VPg-Pro and eIF(iso)4E. On the other hand, the interaction of 6K-VPg-Pro with eIF(iso)4E was observed in cytoplasmic vesicles embedded in the ER. These data suggest that the association of VPg with the translation factor might be needed for two different functions, depending of the VPg precursor involved in the interaction. VPg-Pro interaction with eIF(iso)4E may be involved in perturbing normal cellular functions, while 6K-VPg-Pro interaction with the translation factor may be needed for viral RNA translation and/or replication.
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Affiliation(s)
- Chantal Beauchemin
- Institut Armand-Frappier, Institut National de la Recherche Scientifique, 531 Boulevard des Prairies, Laval, Québec, Canada H7V 1B7
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172
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Wang Y, Zhu W, Levy DE. Nuclear and cytoplasmic mRNA quantification by SYBR green based real-time RT-PCR. Methods 2006; 39:356-62. [PMID: 16893657 DOI: 10.1016/j.ymeth.2006.06.010] [Citation(s) in RCA: 90] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/01/2006] [Accepted: 06/15/2006] [Indexed: 11/18/2022] Open
Abstract
Measurement of the steady-state abundance of nuclear and cytoplasmic RNA requires efficient subcellular fractionation and RNA recovery coupled with accurate quantification of individual RNA species. Detergent lysis of tissue culture cells provides a simple fractionation procedure that can be optimized to individual cell lines. The large dynamic range, extreme sensitivity, high sequence-specificity, and fast turn-around time has allowed real-time reverse transcription polymerase chain reaction (real-time RT-PCR) to become a standard tool for mRNA quantification. Among the different chemistries used for PCR product detection during amplification, DNA binding dyes such as SYBR Green I are simple, versatile, and yet highly reliable and least expensive. With attention to primer design and cycling conditions, virtually any mRNA species can be accurately quantified from even minute quantities of starting RNA. This method provides an accurate and efficient procedure for estimating the relative ratios of nuclear and cytoplasmic RNA concentrations.
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Affiliation(s)
- Yaming Wang
- Department of Pathology, NYU Cancer Institute, New York University School of Medicine, 550 1st Avenue, New York, NY 10016, USA
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173
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Abstract
There is currently a high level of interest in signalling through the mammalian target of rapamycin (mTOR). This reflects both its key role in many cell functions and its involvement in disease states such as cancers. The best understood targets for mTOR signalling are proteins involved in controlling the translational machinery, including the ribosomal protein S6 kinases and proteins that regulate the initiation and elongation phases of translation. Indeed, there is compelling evidence that at least one of these targets of mTOR (eukaryotic initiation factor eIF4E) plays a key role in tumorigenesis. It is regulated through the mTOR-dependent phosphorylation of inhibitory proteins such as eIF4E-binding protein 1. Thus, targeting mTOR signalling may be an effective anticancer strategy, in at least a significant subset of tumours. Not all effects of mTOR are sensitive to the classical anti-mTOR drug rapamycin, and this compound also interferes with other processes besides eIF4E function. Developing new approaches to targeting mTOR for cancer therapy requires more detailed knowledge of signalling downstream of mTOR. Such advances are likely to come from further work to understand the regulation of mTOR targets such as components of the translational apparatus.
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Affiliation(s)
- J Averous
- Unité de Nutrition Humaine, INRA de Theix, Saint Genès Champanelle, France
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174
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Culjkovic B, Topisirovic I, Skrabanek L, Ruiz-Gutierrez M, Borden KLB. eIF4E is a central node of an RNA regulon that governs cellular proliferation. J Cell Biol 2006; 175:415-26. [PMID: 17074885 PMCID: PMC2064519 DOI: 10.1083/jcb.200607020] [Citation(s) in RCA: 214] [Impact Index Per Article: 11.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/06/2006] [Accepted: 10/06/2006] [Indexed: 11/25/2022] Open
Abstract
This study demonstrates that the eukaryotic translation initiation factor eIF4E is a critical node in an RNA regulon that impacts nearly every stage of cell cycle progression. Specifically, eIF4E coordinately promotes the messenger RNA (mRNA) export of several genes involved in the cell cycle. A common feature of these mRNAs is a structurally conserved, approximately 50-nucleotide element in the 3' untranslated region denoted as an eIF4E sensitivity element. This element is sufficient for localization of capped mRNAs to eIF4E nuclear bodies, formation of eIF4E-specific ribonucleoproteins in the nucleus, and eIF4E-dependent mRNA export. The roles of eIF4E in translation and mRNA export are distinct, as they rely on different mRNA elements. Furthermore, eIF4E-dependent mRNA export is independent of ongoing RNA or protein synthesis. Unlike the NXF1-mediated export of bulk mRNAs, eIF4E-dependent mRNA export is CRM1 dependent. Finally, the growth-suppressive promyelocytic leukemia protein (PML) inhibits this RNA regulon. These data provide novel perspectives into the proliferative and oncogenic properties of eIF4E.
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Affiliation(s)
- Biljana Culjkovic
- Institute for Research in Immunology and Cancer, Université de Montréal, Montréal, Québec H4M 1J6, Canada
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175
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Cho S, Park SM, Kim TD, Kim JH, Kim KT, Jang SK. BiP internal ribosomal entry site activity is controlled by heat-induced interaction of NSAP1. Mol Cell Biol 2006; 27:368-83. [PMID: 17074807 PMCID: PMC1800651 DOI: 10.1128/mcb.00814-06] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023] Open
Abstract
TheBiP protein, a stress response protein, plays an important role in the proper folding and assembly of nascent protein and in the scavenging of misfolded proteins in the endoplasmic reticulum lumen. Translation of BiP is directed by an internal ribosomal entry site (IRES) in the 5' nontranslated region of the BiP mRNA. BiP IRES activity increases when cells are heat stressed. Here we report that NSAP1 specifically enhances the IRES activity of BiP mRNA by interacting with the IRES element. Overexpression of NSAP1 in 293T cells increased the IRES activity of BiP mRNA, whereas knockdown of NSAP1 by small interfering RNA (siRNA) reduced the IRES activity of BiP mRNA. The amount of NSAP1 bound to the BiP IRES increased under heat stress conditions, and the IRES activity of BiP mRNA was increased. Moreover, the increase in BiP IRES activity with heat treatment was not observed in cells lacking NSAP1 after siRNA treatment. BiP mRNAs were redistributed from the heavy polysome to the light polysome in NSAP1 knockdown cells. Together, these data indicate that NSAP1 modulates IRES-dependent translation of BiP mRNA through an RNA-protein interaction under heat stress conditions.
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Affiliation(s)
- Sungchan Cho
- Department of Life Science, Pohang University of Science and Technology, San 31, Hyoja Dong, Pohang, Kyungbuk 790-784, Republic of Korea
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176
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Volpon L, Osborne MJ, Topisirovic I, Siddiqui N, Borden KLB. Cap-free structure of eIF4E suggests a basis for conformational regulation by its ligands. EMBO J 2006; 25:5138-49. [PMID: 17036047 PMCID: PMC1630411 DOI: 10.1038/sj.emboj.7601380] [Citation(s) in RCA: 80] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/12/2006] [Accepted: 09/13/2006] [Indexed: 12/31/2022] Open
Abstract
The activity of the eukaryotic translation initiation factor eIF4E is modulated through conformational response to its ligands. For example, eIF4G and eIF4E-binding proteins (4E-BPs) modulate cap affinity, and thus physiological activity of eIF4E, by binding a site distal to the 7-methylguanosine cap-binding site. Further, cap binding substantially modulates eIF4E's affinity for eIF4G and the 4E-BPs. To date, only cap-bound eIF4E structures were reported. In the absence of structural information on the apo form, the molecular underpinnings of this conformational response mechanism cannot be established. We report here the first cap-free eIF4E structure. Apo-eIF4E exhibits structural differences in the cap-binding site and dorsal surface relative to cap-eIF4E. Analysis of structure and dynamics of apo-eIF4E, and changes observed upon ligand binding, reveal a molecular basis for eIF4E's conformational response to these ligands. In particular, alterations in the S4-H4 loop, distal to either the cap or eIF4G binding sites, appear key to modulating these effects. Mutation in this loop mimics these effects. Overall, our studies have important implications for the regulation of eIF4E.
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Affiliation(s)
- Laurent Volpon
- Department of Pathology and Cell Biology, Institute for Research in Immunology and Cancer (IRIC), Université de Montréal, Pavillion Marcelle-Coutu, Chemin Polytechnique, Montréal, QC, Canada
| | - Michael J Osborne
- Department of Pathology and Cell Biology, Institute for Research in Immunology and Cancer (IRIC), Université de Montréal, Pavillion Marcelle-Coutu, Chemin Polytechnique, Montréal, QC, Canada
| | - Ivan Topisirovic
- Department of Pathology and Cell Biology, Institute for Research in Immunology and Cancer (IRIC), Université de Montréal, Pavillion Marcelle-Coutu, Chemin Polytechnique, Montréal, QC, Canada
| | - Nadeem Siddiqui
- Department of Pathology and Cell Biology, Institute for Research in Immunology and Cancer (IRIC), Université de Montréal, Pavillion Marcelle-Coutu, Chemin Polytechnique, Montréal, QC, Canada
| | - Katherine LB Borden
- Department of Pathology and Cell Biology, Institute for Research in Immunology and Cancer (IRIC), Université de Montréal, Pavillion Marcelle-Coutu, Chemin Polytechnique, Montréal, QC, Canada
- Department of Pathology and Cell Biology, Institute for Research in Immunology and Cancer (IRIC), Université de Montréal, Pavillion Marcelle-Coutu, 2950, Chemin Polytechnique, Montréal, Québec, Canada H3T 1J4. Tel.: +1 514 343 6291; Fax: +1 514 343 5839; E-mail:
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177
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Carroll M, Dyer J, Sossin WS. Serotonin increases phosphorylation of synaptic 4EBP through TOR, but eukaryotic initiation factor 4E levels do not limit somatic cap-dependent translation in aplysia neurons. Mol Cell Biol 2006; 26:8586-98. [PMID: 16982686 PMCID: PMC1636790 DOI: 10.1128/mcb.00955-06] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023] Open
Abstract
The target of rapamycin (TOR) plays an important role in memory formation in Aplysia californica. Here, we characterize one of the downstream targets of TOR, the eukaryotic initiation factor 4E (eIF4E) binding protein (4EBP) from Aplysia. Aplysia 4EBP contains the four critical phosphorylation sites regulated by TOR as well as an N-terminal RAIP motif and a C-terminal TOS site. Aplysia 4EBP was hypophosphorylated in synaptosomes, and serotonin addition caused a rapamycin-sensitive increase in 4EBP phosphorylation both in synaptosomes and in isolated neurites. Aplysia 4EBP was regulated in a fashion similar to that of mammalian 4EBPs, binding to eIF4E when dephosphorylated and releasing eIF4E after phosphorylation. Overexpression of 4EBP in the soma of Aplysia neurons caused a specific decrease in cap-dependent translation that was rescued by concomitant overexpression of eIF4E. However, eIF4E overexpression by itself did not increase cap-dependent translation, suggesting that increasing levels of free eIF4E by phosphorylating 4EBP is not important in regulating cap-dependent translation in the cell soma. Total levels of eIF4E were also regulated by 4EBP, suggesting that 4EBP can also act as an eIF4E chaperone. These studies demonstrate the conserved nature of 4EBP regulation and its role in cap-dependent translation and suggest differential roles of 4EBP phosphorylation in the soma and synapse.
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Affiliation(s)
- Matthew Carroll
- Department of Neurology and Neurosurgery, McGill University, Montreal Neurological Institute, BT 110, 3801 University Street, Montreal, Quebec H3A 2B4, Canada
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178
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Leung KM, van Horck FPG, Lin AC, Allison R, Standart N, Holt CE. Asymmetrical beta-actin mRNA translation in growth cones mediates attractive turning to netrin-1. Nat Neurosci 2006; 9:1247-56. [PMID: 16980963 PMCID: PMC1997306 DOI: 10.1038/nn1775] [Citation(s) in RCA: 368] [Impact Index Per Article: 20.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/17/2006] [Accepted: 08/28/2006] [Indexed: 11/09/2022]
Abstract
Local protein synthesis regulates the turning of growth cones to guidance cues, yet little is known about which proteins are synthesized or how they contribute to directional steering. Here we show that beta-actin mRNA resides in Xenopus laevis retinal growth cones where it binds to the RNA-binding protein Vg1RBP. Netrin-1 induces the movement of Vg1RBP granules into filopodia, suggesting that it may direct the localization and translation of mRNAs in growth cones. Indeed, a gradient of netrin-1 activates a translation initiation regulator, eIF-4E-binding protein 1 (4EBP), asymmetrically and triggers a polarized increase in beta-actin translation on the near side of the growth cone before growth cone turning. Inhibition of beta-actin translation abolishes both the asymmetric rise in beta-actin and attractive, but not repulsive, turning. Our data suggest that newly synthesized beta-actin, concentrated near sites of signal reception, provides the directional bias for polymerizing actin in the direction of an attractive stimulus.
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Affiliation(s)
- Kin-Mei Leung
- Department of Physiology, Development and Neuroscience, University of Cambridge, Downing Street, Cambridge CB2 3DY, UK
| | - Francisca PG van Horck
- Department of Physiology, Development and Neuroscience, University of Cambridge, Downing Street, Cambridge CB2 3DY, UK
| | - Andrew C Lin
- Department of Physiology, Development and Neuroscience, University of Cambridge, Downing Street, Cambridge CB2 3DY, UK
| | - Rachel Allison
- Department of Biochemistry, University of Cambridge, Downing Street, Cambridge CB2 3DY, UK
| | - Nancy Standart
- Department of Biochemistry, University of Cambridge, Downing Street, Cambridge CB2 3DY, UK
| | - Christine E Holt
- Department of Physiology, Development and Neuroscience, University of Cambridge, Downing Street, Cambridge CB2 3DY, UK
- Corresponding author: Department of Physiology, Development and Neuroscience, University of Cambridge, Downing Street, Cambridge, CB2 3DY, United Kingdom, Tel: (+44) 1223 766229, Fax: (+44) 1223 333786, E-mail:
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179
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Faivre S, Kroemer G, Raymond E. Current development of mTOR inhibitors as anticancer agents. Nat Rev Drug Discov 2006; 5:671-88. [PMID: 16883305 DOI: 10.1038/nrd2062] [Citation(s) in RCA: 734] [Impact Index Per Article: 40.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
Mammalian target of rapamycin (mTOR) is a kinase that functions as a master switch between catabolic and anabolic metabolism and as such is a target for the design of anticancer agents. The most established mTOR inhibitors--rapamycin and its derivatives--showed long-lasting objective tumour responses in clinical trials, with CCI-779 being a first-in-class mTOR inhibitor that improved the survival of patients with advanced renal cell carcinoma. This heralded the beginning of extensive clinical programmes to further evaluate mTOR inhibitors in several tumour types. Here we review the clinical development of this drug class and look at future prospects for incorporating these agents into multitarget or multimodality strategies against cancer.
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Affiliation(s)
- Sandrine Faivre
- Service Inter Hospitalier de Cancrologie, Beaujon University Hospital, 100 Boulevard du General Leclerc, 92118 Clichy Cedex, France
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180
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Larsson O, Perlman DM, Fan D, Reilly CS, Peterson M, Dahlgren C, Liang Z, Li S, Polunovsky VA, Wahlestedt C, Bitterman PB. Apoptosis resistance downstream of eIF4E: posttranscriptional activation of an anti-apoptotic transcript carrying a consensus hairpin structure. Nucleic Acids Res 2006; 34:4375-86. [PMID: 16936314 PMCID: PMC1636353 DOI: 10.1093/nar/gkl558] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
Aberrant activation of the translation initiation machinery is a common property of malignant cells, and is essential for breast carcinoma cells to manifest a malignant phenotype. How does sustained activation of the rate limiting step in protein synthesis so fundamentally alter a cell? In this report, we test the post transcriptional operon theory as a possible mechanism, employing a model system in which apoptosis resistance is conferred on NIH 3T3 cells by ectopic expression of eIF4E. We show (i) there is a set of 255 transcripts that manifest an increase in translational efficiency during eIF4E-mediated escape from apoptosis; (ii) there is a novel prototype 55 nt RNA consensus hairpin structure that is overrepresented in the 5′-untranslated region of translationally activated transcripts; (iii) the identified consensus hairpin structure is sufficient to target a reporter mRNA for translational activation under pro-apoptotic stress, but only when eIF4E is deregulated; and (iv) that osteopontin, one of the translationally activated transcripts harboring the identified consensus hairpin structure functions as one mediator of the apoptosis resistance seen in our model. Our findings offer genome-wide insights into the mechanism of eIF4E-mediated apoptosis resistance and provide a paradigm for the systematic study of posttranscriptional control in normal biology and disease.
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Affiliation(s)
- Ola Larsson
- To whom correspondence should be addressed. Tel: +1 612 626 6848; Fax: +1 612 625 2174;
| | | | | | - Cavan S. Reilly
- School of Public Health Division of Biostatistics, University of MinnesotaMinneapolis, MN 55455, USA
| | | | - Cecilia Dahlgren
- Department of Cell and Molecular Biology (CMB), Programme for Genomics and Bioinformatics, Karolinska InstitutetBerzelius väg 35, 171 77 Stockholm, Sweden
| | - Zicai Liang
- Department of Cell and Molecular Biology (CMB), Programme for Genomics and Bioinformatics, Karolinska InstitutetBerzelius väg 35, 171 77 Stockholm, Sweden
| | | | | | | | - Peter B. Bitterman
- To whom correspondence should be addressed. Tel: +1 612 626 6848; Fax: +1 612 625 2174;
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181
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Chen JM, Férec C, Cooper DN. A systematic analysis of disease-associated variants in the 3' regulatory regions of human protein-coding genes II: the importance of mRNA secondary structure in assessing the functionality of 3' UTR variants. Hum Genet 2006; 120:301-33. [PMID: 16807757 DOI: 10.1007/s00439-006-0218-x] [Citation(s) in RCA: 99] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/03/2006] [Accepted: 05/29/2006] [Indexed: 12/13/2022]
Abstract
In an attempt both to catalogue 3' regulatory region (3' RR)-mediated disease and to improve our understanding of the structure and function of the 3' RR, we have performed a systematic analysis of disease-associated variants in the 3' RRs of human protein-coding genes. We have previously analysed the variants that have occurred in two specific domains/motifs of the 3' untranslated region (3' UTR) as well as in the 3' flanking region. Here we have focused upon 83 known variants within the upstream sequence (USS; between the translational termination codon and the upstream core polyadenylation signal sequence) of the 3' UTR. To place these variants in their proper context, we first performed a comprehensive survey of known cis-regulatory elements within the USS and the mechanisms by which they effect post-transcriptional gene regulation. Although this survey supports the view that RNA regulatory elements function within the context of specific secondary structures, there are no general rules governing how secondary structure might exert its influence. We have therefore addressed this question by systematically evaluating both functional and non-functional (based upon in vitro reporter gene and/or electrophoretic mobility shift assay data) USS variant-containing sequences against known cis-regulatory motifs within the context of predicted RNA secondary structures. This has allowed us not only to establish a reliable and objective means to perform secondary structure prediction but also to identify consistent patterns of secondary structural change that could potentiate the discrimination of functional USS variants from their non-functional counterparts. The resulting rules were then used to infer potential functionality in the case of some of the remaining functionally uncharacterized USS variants, from their predicted secondary structures. This not only led us to identify further patterns of secondary structural change but also several potential novel cis-regulatory motifs within the 3' UTRs studied.
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182
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Xi Y, Nakajima G, Schmitz JC, Chu E, Ju J. Multi-level gene expression profiles affected by thymidylate synthase and 5-fluorouracil in colon cancer. BMC Genomics 2006; 7:68. [PMID: 16584549 PMCID: PMC1448211 DOI: 10.1186/1471-2164-7-68] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2005] [Accepted: 04/03/2006] [Indexed: 11/24/2022] Open
Abstract
Background Thymidylate synthase (TS) is a critical target for cancer chemotherapy and is one of the most extensively studied biomarkers for fluoropyrimidine-based chemotherapy. In addition to its critical role in enzyme catalysis, TS functions as an RNA binding protein to regulate the expression of its own mRNA translation and other cellular mRNAs, such as p53, at the translational level. In this study, a comprehensive gene expression analysis at the levels of both transcriptional and post-transcriptional regulation was conducted to identify response markers using human genome array with TS-depleted human colon cancer HCT-C18 (TS-) cells and HCT-C18 (TS+) cells stably transfected with the human TS cDNA expression plasmid. Results A total of 38 genes were found to be significantly affected by TS based on the expression profiles of steady state mRNA transcripts. However, based on the expression profiles of polysome associated mRNA transcripts, over 149 genes were affected by TS overexpression. This indicates that additional post-transcriptionally controlled genes can be captured with profiling polysome associated mRNA population. This unique approach provides a comprehensive overview of genes affected by TS. Additional novel post-transcriptionally regulated genes affected by 5-fluorouracil (5-FU) treatment were also discovered via similar approach. Conclusion To our knowledge, this is the first time that a comprehensive gene expression profile regulated by TS and 5-FU was analyzed at the multiple steps of gene regulation. This study will provide candidate markers that can be potentially used for predicting therapeutic outcomes for fluoropyrimidine-based cancer chemotherapy.
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Affiliation(s)
- Yaguang Xi
- Mitchell Cancer Institute, University of South Alabama, Mobile, AL, 36688, USA
| | - Go Nakajima
- Mitchell Cancer Institute, University of South Alabama, Mobile, AL, 36688, USA
| | - John C Schmitz
- Yale Cancer Center, Yale University School of Medicine, New Haven, CT 06501, USA
| | - Edward Chu
- Yale Cancer Center, Yale University School of Medicine, New Haven, CT 06501, USA
| | - Jingfang Ju
- Mitchell Cancer Institute, University of South Alabama, Mobile, AL, 36688, USA
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183
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Hizli AA, Black AR, Pysz MA, Black JD. Protein kinase C alpha signaling inhibits cyclin D1 translation in intestinal epithelial cells. J Biol Chem 2006; 281:14596-603. [PMID: 16556598 DOI: 10.1074/jbc.m601959200] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023] Open
Abstract
Cyclin D1 is a key regulator of cell proliferation, acting as a mitogen sensor and linking extracellular signaling to the cell cycle machinery. Strict control of cyclin D1 levels is critical for maintenance of tissue homeostasis. We have reported previously that protein kinase C alpha (PKCalpha), a negative regulator of cell growth in the intestinal epithelium, promotes rapid down-regulation of cyclin D1 (Frey, M. R., Clark, J. A., Leontieva, O., Uronis, J. M., Black, A. R., and Black, J. D. (2000) J. Cell Biol. 151, 763-778). The current study explores the mechanisms underlying PKCalpha-induced loss of cyclin D1 protein in non-transformed intestinal epithelial cells. Our findings exclude several mechanisms previously implicated in down-regulation of cyclin D1 during cell cycle exit/differentiation, including alterations in cyclin D1 mRNA expression and protein turnover. Instead, we identify PKCalpha as a novel repressor of cyclin D1 translation, acting at the level of cap-dependent initiation. Inhibition of cyclin D1 translation initiation is mediated by PKCalpha-induced hypophosphorylation/activation of the translational suppressor 4E-BP1, association of 4E-BP1 with the mRNA cap-binding protein eIF4E, and sequestration of cyclin D1 mRNA in 4E-BP1-associated complexes. Together, these post-transcriptional effects ensure rapid disappearance of the potent mitogenic molecule cyclin D1 during PKCalpha-induced cell cycle withdrawal in the intestinal epithelium.
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Affiliation(s)
- A Asli Hizli
- Department of Pharmacology and Therapeutics, Roswell Park Cancer Institute, Elm and Carlton Streets, Buffalo, NY 14263, USA
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184
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Abstract
Cyclins are the regulatory subunits of kinases that control progress through the cell cycle. This review focuses on cyclins that are targets for extracellular signaling and frequently deregulated during oncogenesis, particularly cyclin D1. Receptor tyrosine kinases and adhesion molecules act through various effector pathways to modulate cyclin D1 abundance at multiple levels including transcription, translation and protein stability. In contrast, cyclin E-Cdk2 activity appears to be more commonly regulated by means other than regulation of cyclin E abundance. The importance of these pathways during oncogenesis is illustrated by the dependence of oncogenes such as Ras and Neu/ErbB2 on cyclin D1. Thus, understanding the roles of cyclins in growth factor and adhesion signaling is important for understanding the biology of both normal and neoplastic cells.
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Affiliation(s)
- Elizabeth A Musgrove
- Cancer Research Program, Garvan Institute of Medical Research, Darlinghurst, NSW, Australia.
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185
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Kentsis A, Volpon L, Topisirovic I, Soll CE, Culjkovic B, Shao L, Borden KLB. Further evidence that ribavirin interacts with eIF4E. RNA (NEW YORK, N.Y.) 2005; 11:1762-6. [PMID: 16251386 PMCID: PMC1370864 DOI: 10.1261/rna.2238705] [Citation(s) in RCA: 73] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/05/2023]
Abstract
This commentary discusses the recent reports in RNA by Yan and colleagues and Westman and colleagues of the apparent failure of ribavirin to bind to recombinant eIF4E and inhibit 7-methyl guanosine cap-dependent exogenous mRNA translation of cell extracts in vitro. Measuring binding by using affinity chromatography of matrix-immobilized proteins and by using protein emission fluorescence spectroscopy in the presence of nucleotide ligands, as well as limitations of using cell extracts for the assessment of mechanisms of mRNA translation are discussed. Possible reasons for the discordant findings of Yan and colleagues and Westman and colleagues are suggested, and direct observation of the specific binding of ribavirin to eIF4E by using mass spectrometry is presented.
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Affiliation(s)
- Alex Kentsis
- Mount Sinai School of Medicine, New York University, New York, NY 10029, USA
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186
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Kozak M. A second look at cellular mRNA sequences said to function as internal ribosome entry sites. Nucleic Acids Res 2005; 33:6593-602. [PMID: 16314320 PMCID: PMC1298923 DOI: 10.1093/nar/gki958] [Citation(s) in RCA: 129] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/05/2005] [Accepted: 10/26/2005] [Indexed: 01/27/2023] Open
Abstract
This review takes a second look at a set of mRNAs that purportedly employ an alternative mechanism of initiation when cap-dependent translation is reduced during mitosis or stress conditions. A closer look is necessary because evidence cited in support of the internal initiation hypothesis is often flawed. When putative internal ribosome entry sequences (IRESs) are examined more carefully, they often turn out to harbor cryptic promoters or splice sites. This undermines the dicistronic assay, wherein IRES activity is measured by the ability to support translation of the 3' cistron. Most putative IRESs still have not been checked carefully to determine whether the dicistronic vector produces only the intended dicistronic mRNA. The widespread use of the pRF vector is a major problem because this vector, which has Renilla luciferase as the 5' cistron and firefly luciferase as the 3' cistron, has been found to generate spliced transcripts. RNA transfection assays could theoretically circumvent these problems, but most candidate IRESs score very weakly in that test. The practice of calling even very weak results 'positive' is one of the problems discussed herein. The extremely low efficiency of putative IRESs is inconsistent with their postulated biological roles.'
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Affiliation(s)
- Marilyn Kozak
- Department of Biochemistry, Robert Wood Johnson Medical School, 675 Hoes Lane, Piscataway, NJ 08854, USA.
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187
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Ferraiuolo MA, Basak S, Dostie J, Murray EL, Schoenberg DR, Sonenberg N. A role for the eIF4E-binding protein 4E-T in P-body formation and mRNA decay. ACTA ACUST UNITED AC 2005; 170:913-24. [PMID: 16157702 PMCID: PMC2171455 DOI: 10.1083/jcb.200504039] [Citation(s) in RCA: 187] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
4E-transporter (4E-T) is one of several proteins that bind the mRNA 5′cap-binding protein, eukaryotic initiation factor 4E (eIF4E), through a conserved binding motif. We previously showed that 4E-T is a nucleocytoplasmic shuttling protein, which mediates the import of eIF4E into the nucleus. At steady state, 4E-T is predominantly cytoplasmic and is concentrated in bodies that conspicuously resemble the recently described processing bodies (P-bodies), which are believed to be sites of mRNA decay. In this paper, we demonstrate that 4E-T colocalizes with mRNA decapping factors in bona fide P-bodies. Moreover, 4E-T controls mRNA half-life, because its depletion from cells using short interfering RNA increases mRNA stability. The 4E-T binding partner, eIF4E, also is localized in P-bodies. 4E-T interaction with eIF4E represses translation, which is believed to be a prerequisite for targeting of mRNAs to P-bodies. Collectively, these data suggest that 4E-T interaction with eIF4E is a priming event in inducing messenger ribonucleoprotein rearrangement and transition from translation to decay.
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Affiliation(s)
- Maria A Ferraiuolo
- Department of Biochemistry, McGill University, Montréal, Québec H3G 1Y6, Canada
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188
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Smagulova F, Maurel S, Morichaud Z, Devaux C, Mougel M, Houzet L. The highly structured encapsidation signal of MuLV RNA is involved in the nuclear export of its unspliced RNA. J Mol Biol 2005; 354:1118-28. [PMID: 16289115 DOI: 10.1016/j.jmb.2005.10.021] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/06/2005] [Revised: 10/03/2005] [Accepted: 10/05/2005] [Indexed: 11/22/2022]
Abstract
The encapsidation signal (Psi) of retroviruses is located in the 5' UTR of the viral genomic unspliced transcript and is highly structured. In the Psi of murine leukaemia virus (MuLV), four stem-loops, called A, B, C and D, promote dimerization and encapsidation of the MuLV unspliced RNA into virions. Through analysis of Psi-deleted transcripts, we found that the AB and CD motifs independently enhanced the cytoplasmic accumulation of RNAs. Furthermore, we showed that over-expression of the Psi sequence in the infected cells led to a competition with the nuclear export of unspliced MuLV transcripts, revealing a new function for these stem-loops in the transport of viral intron-containing RNAs from the nucleus to the cytoplasm.
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MESH Headings
- 5' Untranslated Regions
- Active Transport, Cell Nucleus
- Animals
- Capsid/metabolism
- Cell Fractionation
- Cell Nucleus/metabolism
- Cytoplasm/metabolism
- Dimerization
- Fluorescent Dyes
- In Situ Hybridization, Fluorescence
- Indoles
- Introns
- Mice
- Microscopy, Fluorescence
- Moloney murine leukemia virus/chemistry
- Moloney murine leukemia virus/genetics
- Mutation
- NIH 3T3 Cells
- Nucleic Acid Conformation
- RNA Splicing
- RNA, Ribosomal/analysis
- RNA, Ribosomal/chemistry
- RNA, Small Nucleolar/analysis
- RNA, Small Nucleolar/metabolism
- RNA, Transfer/analysis
- RNA, Transfer/chemistry
- RNA, Viral/analysis
- RNA, Viral/chemistry
- RNA, Viral/isolation & purification
- RNA, Viral/metabolism
- Transfection
- Virion/chemistry
- Virion/genetics
- Virus Replication/genetics
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Affiliation(s)
- Fatima Smagulova
- CNRS UMR5121-UM1, IFR122, Institut de Biologie, 34960 Montpellier, France
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189
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Othumpangat S, Kashon M, Joseph P. Sodium arsenite-induced inhibition of eukaryotic translation initiation factor 4E (eIF4E) results in cytotoxicity and cell death. Mol Cell Biochem 2005; 279:123-31. [PMID: 16283521 DOI: 10.1007/s11010-005-8284-2] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Exposure to arsenic (As) is a risk factor for the development of diabetes, vascular diseases and cancer. Several theories have been proposed to account for the mechanisms potentially responsible for As toxicity and carcinogenesis. Currently, we have investigated whether the eukaryotic translation initiation factor 4E (eIF4E), the mRNA cap binding and rate limiting factor required for translation, is a target for As-induced cytotoxicity and cell death. We have also investigated the potential cellular mechanisms underlying the As-induced de-regulation of expression of eIF4E that are most likely responsible for the cytotoxicity and cell death induced by As. Exposure of four different human cell lines - HCT15 (colorectal adenocarcinoma), PLC/PR/5 (hepatocellular carcinoma), HeLa (cervical adenocarcinoma) and Chang (likely derived from HeLa cells) to sodium arsenite (NaAsO2) for time intervals up to 24 h resulted in a concentration-dependent cytotoxicity and cell death. All the NaAsO2-treated cells exhibited significant inhibition of eIF4E gene (protein). The potential involvement of eIF4E gene expression in the NaAsO2-induced cytotoxicity and cell death was investigated by silencing the cellular expression of the eIF4E gene by employing a small interfering RNA (SiRNA) specifically targeting the eIF4E gene's expression. The SiRNA-mediated silencing of eIF4E gene expression also resulted in significant cytotoxicity and cell death suggesting that the toxicity noticed among the NaAsO2-treated cells was probably due to the chemically induced inhibition of eIF4E gene expression. The potential involvement of inhibition of eIF4E gene expression in the NaAsO2-induced cytotoxicity and cell death was further investigated by employing transgenic cell lines overexpressing the eIF4E gene. Overexpression of the eIF4E gene in the Chinese hamster ovary cell line was protective against the NaAsO2-induced cytotoxicity and cell death. Additional studies conducted to understand the potential mechanisms responsible for NaAsO2-induced inhibition of eIF4E gene expression demonstrated that exposure to NaAsO2 resulted in transcriptional down-regulation of the eIF4E gene only in HCT-15 and HeLa cells, while in the NaAsO2-treated and PLC/PR/5 and Chang cells, the eIF4E mRNA expression level was comparable to those of the corresponding control cells. Cellular levels of ubiquitin and the process of ubiquitination were significantly higher in the NaAsO2-treated cells compared with the control cells. Immunoprecipitation of lysates obtained from the NaAsO2-treated cells and the subsequent western blot analysis of the immunoprecipitated protein(s) using the eIF4E antibody detected the presence of eIF4E protein in the immunoprecipitate suggesting possible ubiquitination of eIF4E protein in the NaAsO2-treated cells. Pre-exposure of the NaAsO2-treated cells to proteasome inhibitors blocked the inhibition of eIF4E gene expression as well as the resulting cytotoxicity and cell death. Furthermore, exposure of cells to NaAsO2 resulted in a significant inhibition of expression of the cell cycle and growth regulating gene, cyclin D1. Whether or not the inhibition of cyclin D1 in the NaAsO2-treated cells is mediated through the inhibition of eIF4E was tested by silencing the expression of eIF4E gene in the cells. Transfection of cells with SiRNA specifically targeting eIF4E gene expression resulted in a significant inhibition of cyclin D1 gene suggesting that the observed inhibition of cyclin D1 gene in the NaAsO2-treated cells is most likely mediated through inhibition of eIF4E gene. Taken together, our results indicate that the exposure of cells to NaAsO2 resulted in cytotoxicity and cell death, at least in part, due to the inhibition of eIF4E gene expression leading to diminished cellular levels of critical genes such as cyclin D1.
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Affiliation(s)
- Sreekumar Othumpangat
- Molecular Carcinogenesis Laboratory, Toxicology and Molecular Biology Branch, National Institute for Occupational Safety and Health, Morgantown, West Virginia 26505, USA
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190
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Kozak M. Regulation of translation via mRNA structure in prokaryotes and eukaryotes. Gene 2005; 361:13-37. [PMID: 16213112 DOI: 10.1016/j.gene.2005.06.037] [Citation(s) in RCA: 540] [Impact Index Per Article: 28.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/25/2005] [Revised: 05/31/2005] [Accepted: 06/27/2005] [Indexed: 01/19/2023]
Abstract
The mechanism of initiation of translation differs between prokaryotes and eukaryotes, and the strategies used for regulation differ accordingly. Translation in prokaryotes is usually regulated by blocking access to the initiation site. This is accomplished via base-paired structures (within the mRNA itself, or between the mRNA and a small trans-acting RNA) or via mRNA-binding proteins. Classic examples of each mechanism are described. The polycistronic structure of mRNAs is an important aspect of translational control in prokaryotes, but polycistronic mRNAs are not usable (and usually not produced) in eukaryotes. Four structural elements in eukaryotic mRNAs are important for regulating translation: (i) the m7G cap; (ii) sequences flanking the AUG start codon; (iii) the position of the AUG codon relative to the 5' end of the mRNA; and (iv) secondary structure within the mRNA leader sequence. The scanning model provides a framework for understanding these effects. The scanning mechanism also explains how small open reading frames near the 5' end of the mRNA can down-regulate translation. This constraint is sometimes abrogated by changing the structure of the mRNA, sometimes with clinical consequences. Examples are described. Some mistaken ideas about regulation of translation that have found their way into textbooks are pointed out and corrected.
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Affiliation(s)
- Marilyn Kozak
- Department of Biochemistry, Robert Wood Johnson Medical School, Piscataway, NJ 08854, USA.
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191
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Gu X, Jones L, Lowery-Norberg M, Fowler M. Expression of eukaryotic initiation factor 4E in astrocytic tumors. Appl Immunohistochem Mol Morphol 2005; 13:178-83. [PMID: 15894932 DOI: 10.1097/01.pai.0000135615.07325.a2] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
Upregulation of expression of the eukaryotic initiation factor 4E (eIF4E) has been identified in breast carcinomas, squamous cell carcinomas in the head and neck regions, and transitional cell carcinomas of urinary bladder. In this immunohistochemical study, eIF4E protein expression was investigated in human brain tissue from patients without central nervous system diseases and brain biopsy tissues from patients with anaplastic astrocytoma and glioblastoma multiforme. Expression of eIF4E protein was observed in normal pyramidal neurons but not in neuroglial components. In anaplastic astrocytoma and glioblastoma multiforme, there was diffuse uniform expression of eIF4E immunoreactivity in malignant astrocytes. A similar pattern of immunoreactivity was also present in proliferative endothelial cells and vascular lining endothelial cells in glioblastoma multiforme. This study provides evidence that eIF4E is upregulated in high-grade astrocytic tumors. As in other malignancies, a high level of eIF4E may play an important role in the neoplastic transformation, angiogenesis, and tumor growth in astrocytic tumors. Because eIF4E is crucial in regulation of tumor growth, eIF4E could be a potential target for inhibitors as an adjuvant therapy for brain tumors.
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Affiliation(s)
- Xin Gu
- Department of Pathology, Louisiana University Health Sciences Center-Shreveport, Shreveport, LA 71130, USA.
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192
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Rahmani M, Davis EM, Bauer C, Dent P, Grant S. Apoptosis induced by the kinase inhibitor BAY 43-9006 in human leukemia cells involves down-regulation of Mcl-1 through inhibition of translation. J Biol Chem 2005; 280:35217-27. [PMID: 16109713 DOI: 10.1074/jbc.m506551200] [Citation(s) in RCA: 246] [Impact Index Per Article: 12.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
BAY 43-9006 is a kinase inhibitor that induces apoptosis in a variety of tumor cells. Here we report that treatment with BAY 43-9006 results in marked cytochrome c and AIF release into the cytosol, caspase-9, -8, -7, and -3 activation, and apoptosis in human leukemia cells (U937, Jurkat, and K562). Pronounced apoptosis was also observed in blasts from patients with acute myeloid leukemia. These events were accompanied by ERK1/2 inactivation and caspase-independent down-regulation of Mcl-1. Inducible expression of a constitutively active MEK1 construct did not prevent Mcl-1 down-regulation, suggesting that this event is not related to MEK/ERK pathway inactivation. Furthermore, BAY 43-9006 did not induce major changes in Mcl-1 mRNA levels monitored by real-time PCR or Mcl-1 promoter activity demonstrated by luciferase reporter assays, but it did enhance Mcl-1 down-regulation in actinomycin D-treated cells. Inhibition of protein synthesis by cycloheximide or proteasome function with MG132 and pulse-chase studies with [35S]methionine demonstrated that BAY 43-9006 did not diminish Mcl-1 protein stability, nor did it enhance Mcl-1 ubiquitination, but instead markedly attenuated Mcl-1 translation in association with the rapid and potent dephosphorylation of the eIF4E translation initiation factor. Finally, ectopic expression of Mcl-1 in leukemic cells markedly inhibited BAY 43-9006-mediated cytochrome c cytosolic release, caspase-9, -7, and -3 activation, as well as cell death, indicating that Mcl-1 operates upstream of cytochrome c release and caspase activation. Together, these findings demonstrate that BAY 43-9006 mediates cell death in human leukemia cells, at least in part, through down-regulation of Mcl-1 via inhibition of translation.
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Affiliation(s)
- Mohamed Rahmani
- Departments of Medicine, Biochemistry, and Pharmacology, Virginia Commonwealth University, School of Medicine, Richmond, Virginia 23298
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193
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Othumpangat S, Kashon M, Joseph P. Eukaryotic Translation Initiation Factor 4E Is a Cellular Target for Toxicity and Death Due to Exposure to Cadmium Chloride. J Biol Chem 2005; 280:25162-9. [PMID: 15878868 DOI: 10.1074/jbc.m414303200] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023] Open
Abstract
Whether translation initiation factor 4E (eIF4E), the mRNA cap binding and rate-limiting factor required for translation, is a target for cytotoxicity and cell death induced by cadmium, a human carcinogen, was investigated. Exposure of human cell lines, HCT15, PLC/PR/5, HeLa, and Chang, to cadmium chloride resulted in cytotoxicity and cell death, and this was associated with a significant decrease in eIF4E protein levels. Similarly, specific silencing of the expression of the eIF4E gene, caused by a small interfering RNA, resulted in significant cytotoxicity and cell death. On the other hand, overexpression of the eIF4E gene was protective against the cadmium-induced cytotoxicity and cell death. Further studies revealed the absence of alterations in the eIF4E mRNA level in the cadmium-treated cells despite their decreased eIF4E protein level. In addition, exposure of cells to cadmium resulted in enhanced ubiquitination of eIF4E protein while inhibitors of proteasome activity reversed the cadmium-induced decrease of eIF4E protein. Exposure of cells to cadmium, as well as the specific silencing of eIF4E gene, also resulted in decreased cellular levels of cyclin D1, a critical cell cycle and growth regulating gene, suggesting that the observed inhibition of cyclin D1 gene expression in the cadmium-treated cells is most likely due to decreased cellular level of eIF4E. Taken together, our results demonstrate that the exposure of cells to cadmium chloride resulted in cytotoxicity and cell death due to enhanced ubiquitination and consequent proteolysis of eIF4E protein, which in turn diminished cellular levels of critical genes such as cyclin D1.
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Affiliation(s)
- Sreekumar Othumpangat
- Molecular Carcinogenesis Laboratory, Toxicology and Molecular Biology Branch, Biostatistics and Epidemiology Branch, Health Effects Laboratory Division, National Institute for Occupational Safety and Health, Morgantown, West Virginia 26505, USA
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194
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Moody CA, Scott RS, Amirghahari N, Nathan CA, Young LS, Dawson CW, Sixbey JW. Modulation of the cell growth regulator mTOR by Epstein-Barr virus-encoded LMP2A. J Virol 2005; 79:5499-506. [PMID: 15827164 PMCID: PMC1082717 DOI: 10.1128/jvi.79.9.5499-5506.2005] [Citation(s) in RCA: 99] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Control of translation initiation is one means by which cells regulate growth and proliferation, with components of the protein-synthesizing machinery having oncogenic potential. Expression of latency protein LMP2A by the human tumor virus Epstein-Barr virus (EBV) activates phosphatidylinositol 3-kinase/Akt located upstream of an essential mediator of growth signals, mTOR (mammalian target of rapamycin). We show that mTOR is activated by expression of LMP2A in carcinoma cells, leading to wortmannin- and rapamycin-sensitive inhibition of the negative regulator of translation, eukaryotic initiation factor 4E-binding protein 1, and increased c-Myc protein translation. Intervention by this DNA tumor virus in cellular translational controls is likely to be an integral component of EBV tumorigenesis.
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Affiliation(s)
- Cary A Moody
- Microbiology and Immunology, Louisiana State University Health Sciences Center, 1501 Kings Highway, Shreveport, LA 71130, USA
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195
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Hu HY, Liu XX, Jiang CY, Lu Y, Liu SL, Bian JF, Wang XM, Geng Z, Zhang Y, Zhang B. Ornithine decarboxylase gene is overexpressed in colorectal carcinoma. World J Gastroenterol 2005; 11:2244-8. [PMID: 15818733 PMCID: PMC4305806 DOI: 10.3748/wjg.v11.i15.2244] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 02/06/2023] Open
Abstract
AIM: To investigate the ornithine decarboxylase (ODC) gene expression in colorectal carcinoma, ODC mRNA was assayed by RT-PCR and ODC protein was detected by a monoclonal antibody against fusion of human colon ODC prepared by hybridoma technology.
METHODS: Total RNA was extracted from human colorectal cancer tissues and their normal counterpart tissues. ODC mRNA levels were examined by RT-PCR. ODC genes amplified from RT-PCR were cloned into a prokaryotic vector pQE-30. The expressed proteins were purified by chromatography. Anti-ODC mAb was prepared with classical hybridoma techniques and used to determine the ODC expression in colon cancer tissues by immunohistochemical and Western blotting assay.
RESULTS: A cell line, which could steadily secrete anti-ODC mAb, was selected through subcloning four times. Western blotting reconfirmed the mAb and ELISA showed that its subtype was IgG2a. RT-PCR showed that the ODC mRNA level increased greatly in colon cancer tissues (P<0.01). Immunohistochemical staining showed that colorectal carcinoma cells expressed a significantly higher level of ODC than normal colorectal mucosa (98.6±1.03% vs 5.26±5%, P<0.01).
CONCLUSION: ODC gene overexpression is significantly related to human colorectal carcinoma. ODC gene expression may be a marker for the gene diagnosis and therapy of colorectal carcinoma.
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Affiliation(s)
- Hai-Yan Hu
- Experimental Center of Medical Molecular Biology, School of Medicine, Shandong University, Jinan 250012, Shandong Province, China
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196
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Dong J, Peng J, Zhang H, Mondesire WH, Jian W, Mills GB, Hung MC, Meric-Bernstam F. Role of glycogen synthase kinase 3beta in rapamycin-mediated cell cycle regulation and chemosensitivity. Cancer Res 2005; 65:1961-72. [PMID: 15753396 DOI: 10.1158/0008-5472.can-04-2501] [Citation(s) in RCA: 82] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
The mammalian target of rapamycin is a serine-threonine kinase that regulates cell cycle progression. Rapamycin and its analogues inhibit the mammalian target of rapamycin and are being actively investigated in clinical trials as novel targeted anticancer agents. Although cyclin D1 is down-regulated by rapamycin, the role of this down-regulation in rapamycin-mediated growth inhibition and the mechanism of cyclin D1 down-regulation are not well understood. Here, we show that overexpression of cyclin D1 partially overcomes rapamycin-induced cell cycle arrest and inhibition of anchorage-dependent growth in breast cancer cells. Rapamycin not only decreases endogenous cyclin D1 levels but also decreases the expression of transfected cyclin D1, suggesting that this is at least in part caused by accelerated proteolysis. Indeed, rapamycin decreases the half-life of cyclin D1 protein, and the rapamycin-induced decrease in cyclin D1 levels is partially abrogated by proteasome inhibitor N-acetyl-leucyl-leucyl-norleucinal. Rapamycin treatment leads to an increase in the kinase activity of glycogen synthase kinase 3beta (GSK3beta), a known regulator of cyclin D1 proteolysis. Rapamycin-induced down-regulation of cyclin D1 is inhibited by the GSK3beta inhibitors lithium chloride, SB216763, and SB415286. Rapamycin-induced G1 arrest is abrogated by nonspecific GSK3beta inhibitor lithium chloride but not by selective inhibitor SB216763, suggesting that GSK3beta is not essential for rapamycin-mediated G1 arrest. However, rapamycin inhibits cell growth significantly more in GSK3beta wild-type cells than in GSK3beta-null cells, suggesting that GSK3beta enhances rapamycin-mediated growth inhibition. In addition, rapamycin enhances paclitaxel-induced apoptosis through the mitochondrial death pathway; this is inhibited by selective GSK3beta inhibitors SB216763 and SB415286. Furthermore, rapamycin significantly enhances paclitaxel-induced cytotoxicity in GSK3beta wild-type but not in GSK3beta-null cells, suggesting a critical role for GSK3beta in rapamycin-mediated paclitaxel-sensitization. Taken together, these results show that GSK3beta plays an important role in rapamycin-mediated cell cycle regulation and chemosensitivity and thus significantly potentiates the antitumor effects of rapamycin.
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Affiliation(s)
- Jinjiang Dong
- Department of Surgical Oncology, University of Texas M.D. Anderson Cancer Center, Houston, Texas 77030, USA
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197
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Culjkovic B, Topisirovic I, Skrabanek L, Ruiz-Gutierrez M, Borden KLB. eIF4E promotes nuclear export of cyclin D1 mRNAs via an element in the 3'UTR. ACTA ACUST UNITED AC 2005; 169:245-56. [PMID: 15837800 PMCID: PMC2171863 DOI: 10.1083/jcb.200501019] [Citation(s) in RCA: 145] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
The eukaryotic translation initiation factor eIF4E is a critical modulator of cellular growth with functions in the nucleus and cytoplasm. In the cytoplasm, recognition of the 5' m(7)G cap moiety on all mRNAs is sufficient for their functional interaction with eIF4E. In contrast, we have shown that in the nucleus eIF4E associates and promotes the nuclear export of cyclin D1, but not GAPDH or actin mRNAs. We determined that the basis of this discriminatory interaction is an approximately 100-nt sequence in the 3' untranslated region (UTR) of cyclin D1 mRNA, we refer to as an eIF4E sensitivity element (4E-SE). We found that cyclin D1 mRNA is enriched at eIF4E nuclear bodies, suggesting these are functional sites for organization of specific ribonucleoproteins. The 4E-SE is required for eIF4E to efficiently transform cells, thereby linking recognition of this element to eIF4E mediated oncogenic transformation. Our studies demonstrate previously uncharacterized fundamental differences in eIF4E-mRNA recognition between the nuclear and cytoplasmic compartments and further a novel level of regulation of cellular proliferation.
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Affiliation(s)
- Biljana Culjkovic
- Institute for Research in Immunology and Cancer, University of Montreal, Quebec, Canada
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198
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Park EH, Lee JM, Blais JD, Bell JC, Pelletier J. Internal translation initiation mediated by the angiogenic factor Tie2. J Biol Chem 2005; 280:20945-53. [PMID: 15802272 DOI: 10.1074/jbc.m412744200] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Tie2 is an endothelium-specific receptor tyrosine kinase required for normal blood vessel maturation. We report that Tie2 mRNA translation is maintained under hypoxic conditions. To identify the mechanism responsible for this, we undertook structure/function analysis of the Tie2 5'-untranslated region (UTR). Transcription start site mapping indicates the existence of a several mRNA isoforms containing unusually long 5'-UTRs (>350 nucleotides) with five upstream open reading frames. We find internal ribosome binding activity that allows the Tie2 mRNA to initiate in a cap-independent fashion. Our data provide a framework for understanding how Tie2 mRNA is translated despite a cumbersome structured 5'-UTR and how its production is secured under unfavorable environmental conditions.
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Affiliation(s)
- Eun-Hee Park
- Department of Biochemistry, McGill University, Montreal, Quebec H3G 1Y6, Canada
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199
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Topisirovic I, Kentsis A, Perez JM, Guzman ML, Jordan CT, Borden KLB. Eukaryotic translation initiation factor 4E activity is modulated by HOXA9 at multiple levels. Mol Cell Biol 2005; 25:1100-12. [PMID: 15657436 PMCID: PMC544005 DOI: 10.1128/mcb.25.3.1100-1112.2005] [Citation(s) in RCA: 72] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The eukaryotic translation initiation factor 4E (eIF4E) alters gene expression on multiple levels. In the cytoplasm, eIF4E acts in the rate-limiting step of translation initiation. In the nucleus, eIF4E facilitates nuclear export of a subset of mRNAs. Both of these functions contribute to eIF4E's ability to oncogenically transform cells. We report here that the homeodomain protein, HOXA9, is a positive regulator of eIF4E. HOXA9 stimulates eIF4E-dependent export of cyclin D1 and ornithine decarboxylase (ODC) mRNAs in the nucleus, as well as increases the translation efficiency of ODC mRNA in the cytoplasm. These activities depend on direct interactions of HOXA9 with eIF4E and are independent of the role of HOXA9 in transcription. At the biochemical level, HOXA9 mediates these effects by competing with factors that repress eIF4E function, in particular the proline-rich homeodomain PRH/Hex. This competitive mechanism of eIF4E regulation is disrupted in a subset of leukemias, where HOXA9 displaces PRH from eIF4E, thereby contributing to eIF4E's dysregulation. In regard to these results and our previous finding that approximately 200 homeodomain proteins contain eIF4E binding sites, we propose that homeodomain modulation of eIF4E activity is a novel means through which this family of proteins implements their effects on growth and development.
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MESH Headings
- Active Transport, Cell Nucleus/genetics
- Acute Disease
- Amino Acid Sequence
- Bone Marrow Cells/metabolism
- Cell Nucleus/genetics
- Cell Nucleus/metabolism
- Cyclin D1/genetics
- Cyclin D1/metabolism
- Cytoplasm/genetics
- Cytoplasm/metabolism
- Eukaryotic Initiation Factor-4E/genetics
- Eukaryotic Initiation Factor-4E/metabolism
- Homeodomain Proteins/genetics
- Homeodomain Proteins/metabolism
- Humans
- Leukemia, Myelogenous, Chronic, BCR-ABL Positive/genetics
- Leukemia, Myelogenous, Chronic, BCR-ABL Positive/metabolism
- Leukemia, Myeloid/genetics
- Leukemia, Myeloid/metabolism
- Molecular Sequence Data
- Ornithine Decarboxylase/genetics
- Ornithine Decarboxylase/metabolism
- Sequence Homology, Amino Acid
- Transcription Factors
- Transcription, Genetic
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Affiliation(s)
- Ivan Topisirovic
- Institute for Research in Immunovirology and Cancer, University of Montreal, Montreal, Quebec H3T 1J4, Canada
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200
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Worch J, Tickenbrock L, Schwäble J, Steffen B, Cauvet T, Mlody B, Buerger H, Koeffler HP, Berdel WE, Serve H, Müller-Tidow C. The serine-threonine kinase MNK1 is post-translationally stabilized by PML-RARalpha and regulates differentiation of hematopoietic cells. Oncogene 2005; 23:9162-72. [PMID: 15516979 DOI: 10.1038/sj.onc.1208164] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Microarray analyses were performed to identify target genes that are shared by the acute myeloid leukemia (AML) translocation products PML-RARalpha, PLZF-RARalpha and AML1-ETO in inducibly transfected U937 cell lines. The cytoplasmic serine and threonine kinase MNK1 was identified as one of the target genes. At the protein level, MNK1 was significantly induced by each of the three fusion proteins. Protein half-life analyses showed that PML-RARalpha enhanced MNK1 protein stability in U937 cells and ATRA exposure decreased MNK1 half-life in NB4 cells. EIF4E, the main MNK1 substrate, plays a role in the pathogenesis of a variety of cancers. Upon MNK1 overexpression, eIF4E phosphorylation increased as a sign of functional activation. Interestingly, MNK1 protein expression decreased during myeloid differentiation. Inhibition of MNK1 activity by a specific inhibitor (CGP57380) enhanced differentiation of HL60 and 32D cells, further suggesting a role for MNK1 in the myeloid differentiation. In addition, kinase dead mutants of MNK1 significantly impaired proliferation of 32D cells. Immunohistochemistry of primary AML bone marrow biopsies showed strong cytoplasmic MNK1 expression in 25 of 99 AML specimens (25%). MNK1 expression was associated with high levels of c-myc expression. Taken together, we identified MNK1 as a target gene of several leukemogenic fusion proteins in AML. MNK1 plays a role in myeloid differentiation. These data suggest a role for MNK1 in the AML fusion protein-associated differentiation block.
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Affiliation(s)
- Jennifer Worch
- Department of Medicine, Hematology and Oncology, University of Münster, Domagkstr. 3, 48129 Münster, Germany
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