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Bartish M, Abraham MJ, Gonçalves C, Larsson O, Rolny C, Del Rincón SV. The role of eIF4F-driven mRNA translation in regulating the tumour microenvironment. Nat Rev Cancer 2023; 23:408-425. [PMID: 37142795 DOI: 10.1038/s41568-023-00567-5] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Accepted: 03/27/2023] [Indexed: 05/06/2023]
Abstract
Cells can rapidly adjust their proteomes in dynamic environments by regulating mRNA translation. There is mounting evidence that dysregulation of mRNA translation supports the survival and adaptation of cancer cells, which has stimulated clinical interest in targeting elements of the translation machinery and, in particular, components of the eukaryotic initiation factor 4F (eIF4F) complex such as eIF4E. However, the effect of targeting mRNA translation on infiltrating immune cells and stromal cells in the tumour microenvironment (TME) has, until recently, remained unexplored. In this Perspective article, we discuss how eIF4F-sensitive mRNA translation controls the phenotypes of key non-transformed cells in the TME, with an emphasis on the underlying therapeutic implications of targeting eIF4F in cancer. As eIF4F-targeting agents are in clinical trials, we propose that a broader understanding of their effect on gene expression in the TME will reveal unappreciated therapeutic vulnerabilities that could be used to improve the efficacy of existing cancer therapies.
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Affiliation(s)
- Margarita Bartish
- Department of Oncology, Faculty of Medicine, McGill University, Montreal, QC, Canada
- Segal Cancer Center, Lady Davis Institute and Jewish General Hospital, Montreal, QC, Canada
- Science for Life Laboratory, Stockholm, Sweden
- Department of Oncology-Pathology, Karolinska Institutet, Stockholm, Sweden
| | - Madelyn J Abraham
- Department of Oncology, Faculty of Medicine, McGill University, Montreal, QC, Canada
- Segal Cancer Center, Lady Davis Institute and Jewish General Hospital, Montreal, QC, Canada
| | - Christophe Gonçalves
- Department of Oncology, Faculty of Medicine, McGill University, Montreal, QC, Canada
- Segal Cancer Center, Lady Davis Institute and Jewish General Hospital, Montreal, QC, Canada
| | - Ola Larsson
- Science for Life Laboratory, Stockholm, Sweden
- Department of Oncology-Pathology, Karolinska Institutet, Stockholm, Sweden
| | - Charlotte Rolny
- Department of Oncology-Pathology, Karolinska Institutet, Stockholm, Sweden.
| | - Sonia V Del Rincón
- Department of Oncology, Faculty of Medicine, McGill University, Montreal, QC, Canada.
- Segal Cancer Center, Lady Davis Institute and Jewish General Hospital, Montreal, QC, Canada.
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2
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Liu Y, Cui J, Hoffman AR, Hu JF. Eukaryotic translation initiation factor eIF4G2 opens novel paths for protein synthesis in development, apoptosis and cell differentiation. Cell Prolif 2023; 56:e13367. [PMID: 36547008 PMCID: PMC9977666 DOI: 10.1111/cpr.13367] [Citation(s) in RCA: 5] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/05/2022] [Revised: 10/21/2022] [Accepted: 11/04/2022] [Indexed: 12/24/2022] Open
Abstract
Protein translation is a critical regulatory event involved in nearly all physiological and pathological processes. Eukaryotic translation initiation factors are dedicated to translation initiation, the most highly regulated stage of protein synthesis. Eukaryotic translation initiation factor 4G2 (eIF4G2, also called p97, NAT1 and DAP5), an eIF4G family member that lacks the binding sites for 5' cap binding protein eIF4E, is widely considered to be a key factor for internal ribosome entry sites (IRESs)-mediated cap-independent translation. However, recent findings demonstrate that eIF4G2 also supports many other translation initiation pathways. In this review, we summarize the role of eIF4G2 in a variety of cap-independent and -dependent translation initiation events. Additionally, we also update recent findings regarding the role of eIF4G2 in apoptosis, cell survival, cell differentiation and embryonic development. These studies reveal an emerging new picture of how eIF4G2 utilizes diverse translational mechanisms to regulate gene expression.
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Affiliation(s)
- Yudi Liu
- Key Laboratory of Organ Regeneration and Transplantation of Ministry of Education, Cancer Center, First Hospital, Jilin University, Changchun, Jilin, P.R. China
| | - Jiuwei Cui
- Key Laboratory of Organ Regeneration and Transplantation of Ministry of Education, Cancer Center, First Hospital, Jilin University, Changchun, Jilin, P.R. China
| | - Andrew R Hoffman
- Stanford University Medical School, VA Palo Alto Health Care System, Palo Alto, California, USA
| | - Ji-Fan Hu
- Key Laboratory of Organ Regeneration and Transplantation of Ministry of Education, Cancer Center, First Hospital, Jilin University, Changchun, Jilin, P.R. China.,Stanford University Medical School, VA Palo Alto Health Care System, Palo Alto, California, USA
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3
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Smith MR, Costa G. RNA-binding proteins and translation control in angiogenesis. FEBS J 2022; 289:7788-7809. [PMID: 34796614 DOI: 10.1111/febs.16286] [Citation(s) in RCA: 10] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/27/2021] [Revised: 10/17/2021] [Accepted: 11/17/2021] [Indexed: 01/14/2023]
Abstract
Tissue vascularization through the process of angiogenesis ensures adequate oxygen and nutrient supply during development and regeneration. The complex morphogenetic events involved in new blood vessel formation are orchestrated by a tightly regulated crosstalk between extra and intracellular factors. In this context, RNA-binding protein (RBP) activity and protein translation play fundamental roles during the cellular responses triggered by particular environmental cues. A solid body of work has demonstrated that key RBPs (such as HuR, TIS11 proteins, hnRNPs, NF90, QKIs and YB1) are implicated in both physiological and pathological angiogenesis. These RBPs are critical for the metabolism of messenger (m)RNAs encoding angiogenic modulators and, importantly, strong evidence suggests that RBP-mRNA interactions can be altered in disease. Lesser known, but not less important, the mechanistic aspects of protein synthesis can also regulate the generation of new vessels. In this review, we outline the key findings demonstrating the implications of RBP-mediated RNA regulation and translation control in angiogenesis. Furthermore, we highlight how these mechanisms of post-transcriptional control of gene expression have led to promising therapeutic strategies aimed at targeting undesired blood vessel formation.
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Affiliation(s)
- Madeleine R Smith
- Wellcome-Wolfson Institute for Experimental Medicine, Queen's University, Belfast, UK
| | - Guilherme Costa
- Wellcome-Wolfson Institute for Experimental Medicine, Queen's University, Belfast, UK
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4
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Steinberger KJ, Forget MA, Bobko AA, Mihalik NE, Gencheva M, Roda JM, Cole SL, Mo X, Hoblitzell EH, Evans R, Gross AC, Moldovan L, Marsh CB, Khramstov VV, Eubank TD. Hypoxia-Inducible Factor α Subunits Regulate Tie2-Expressing Macrophages That Influence Tumor Oxygen and Perfusion in Murine Breast Cancer. JOURNAL OF IMMUNOLOGY (BALTIMORE, MD. : 1950) 2020; 205:2301-2311. [PMID: 32938724 PMCID: PMC7596922 DOI: 10.4049/jimmunol.2000185] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/18/2020] [Accepted: 08/17/2020] [Indexed: 12/27/2022]
Abstract
Tie2-expressing monocytes/macrophages (TEMs) are a distinct subset of proangiogenic monocytes selectively recruited to tumors in breast cancer. Because of the hypoxic nature of solid tumors, we investigated if oxygen, via hypoxia-inducible transcription factors HIF-1α and HIF-2α, regulates TEM function in the hypoxic tumor microenvironment. We orthotopically implanted PyMT breast tumor cells into the mammary fat pads of syngeneic LysMcre, HIF-1α fl/fl /LysMcre, or HIF-2α fl/fl /LysMcre mice and evaluated the tumor TEM population. There was no difference in the percentage of tumor macrophages among the mouse groups. In contrast, HIF-1α fl/fl /LysMcre mice had a significantly smaller percentage of tumor TEMs compared with control and HIF-2α fl/fl /LysMcre mice. Proangiogenic TEMs in macrophage HIF-2α-deficient tumors presented significantly more CD31+ microvessel density but exacerbated hypoxia and tissue necrosis. Reduced numbers of proangiogenic TEMs in macrophage HIF-1α-deficient tumors presented significantly less microvessel density but tumor vessels that were more functional as lectin injection revealed more perfusion, and functional electron paramagnetic resonance analysis revealed more oxygen in those tumors. Macrophage HIF-1α-deficient tumors also responded significantly to chemotherapy. These data introduce a previously undescribed and counterintuitive prohypoxia role for proangiogenic TEMs in breast cancer which is, in part, suppressed by HIF-2α.
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Affiliation(s)
- Kayla J Steinberger
- In Vivo Multifunctional Magnetic Resonance Center, West Virginia University, Morgantown, WV 26506
- Robert C. Byrd Health Sciences Center, West Virginia University, Morgantown, WV 26506
- Department of Microbiology, Immunology, and Cell Biology, West Virginia University, Morgantown, WV 26506
| | - Mary A Forget
- Division of Pulmonary Medicine, Dorothy M. Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210
- Molecular, Cellular and Developmental Biology Program, The Ohio State University, Columbus, OH 43210
| | - Andrey A Bobko
- Robert C. Byrd Health Sciences Center, West Virginia University, Morgantown, WV 26506
- West Virginia University Cancer Institute, Morgantown, WV 26506
| | - Nicole E Mihalik
- Robert C. Byrd Health Sciences Center, West Virginia University, Morgantown, WV 26506
- West Virginia University Cancer Institute, Morgantown, WV 26506
| | - Marieta Gencheva
- In Vivo Multifunctional Magnetic Resonance Center, West Virginia University, Morgantown, WV 26506
- Robert C. Byrd Health Sciences Center, West Virginia University, Morgantown, WV 26506
| | - Julie M Roda
- Division of Pulmonary Medicine, Dorothy M. Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210
| | - Sara L Cole
- Division of Pulmonary Medicine, Dorothy M. Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210
- Campus Microscopy and Imaging Facility, The Ohio State University, Columbus, OH 43210
| | - Xiaokui Mo
- Division of Pulmonary Medicine, Dorothy M. Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210
- Center for Biostatistics, The Ohio State University, Columbus, OH 43210; and
| | - E Hannah Hoblitzell
- In Vivo Multifunctional Magnetic Resonance Center, West Virginia University, Morgantown, WV 26506
- Robert C. Byrd Health Sciences Center, West Virginia University, Morgantown, WV 26506
| | - Randall Evans
- Division of Pulmonary Medicine, Dorothy M. Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210
| | - Amy C Gross
- Division of Pulmonary Medicine, Dorothy M. Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210
| | - Leni Moldovan
- Division of Pulmonary Medicine, Dorothy M. Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210
| | - Clay B Marsh
- Robert C. Byrd Health Sciences Center, West Virginia University, Morgantown, WV 26506
| | - Valery V Khramstov
- In Vivo Multifunctional Magnetic Resonance Center, West Virginia University, Morgantown, WV 26506
- Robert C. Byrd Health Sciences Center, West Virginia University, Morgantown, WV 26506
- West Virginia University Cancer Institute, Morgantown, WV 26506
- Department of Biochemistry, West Virginia University, Morgantown, WV 26506
| | - Timothy D Eubank
- In Vivo Multifunctional Magnetic Resonance Center, West Virginia University, Morgantown, WV 26506;
- Robert C. Byrd Health Sciences Center, West Virginia University, Morgantown, WV 26506
- Department of Microbiology, Immunology, and Cell Biology, West Virginia University, Morgantown, WV 26506
- West Virginia University Cancer Institute, Morgantown, WV 26506
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5
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eIF3i activity is critical for endothelial cells in tumor induced angiogenesis through regulating VEGFR and ERK translation. Oncotarget 2017; 8:19968-19979. [PMID: 28193911 PMCID: PMC5386737 DOI: 10.18632/oncotarget.15274] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/11/2016] [Accepted: 11/30/2016] [Indexed: 02/05/2023] Open
Abstract
Translational control is a critical step in the regulation of gene expression. Accumulating evidence shows that translational control of a subgroup of mRNAs tends to be selective. However, our understanding of the function of selective translational control in endothelial cells is still incomplete. We found that a key translational regulator, eIF3i, is highly expressed in endothelial cells during embryonic and tumor angiogenesis. Knockdown of eIF3i restrained cell proliferation and migration in endothelial cells. In zebrafish angiogenesis model, eIF3i mutant endothelial cells could not respond to induction signals from tumor mass. Mechanistically, we showed that eIF3i knockdown reduced VEGFR/ERK signaling by down-regulating VEGFR2 and ERK protein expression. Gene therapy model suggested that the growth and metastasis of cancer cells were suppressed by eIF3i shRNA. Therefore, our work established a selective translational regulatory mechanism during tumor induced angiogenesis and suggested that targeting eIF3i may be applicable for anticancer therapy.
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6
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Gabrusiewicz K, Liu D, Cortes-Santiago N, Hossain MB, Conrad CA, Aldape KD, Fuller GN, Marini FC, Alonso MM, Idoate MA, Gilbert MR, Fueyo J, Gomez-Manzano C. Anti-vascular endothelial growth factor therapy-induced glioma invasion is associated with accumulation of Tie2-expressing monocytes. Oncotarget 2015; 5:2208-20. [PMID: 24809734 PMCID: PMC4039157 DOI: 10.18632/oncotarget.1893] [Citation(s) in RCA: 67] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
The addition of anti-angiogenic therapy to the few treatments available to patients with malignant gliomas was based on the fact that these tumors are highly vascularized and on encouraging results from preclinical and clinical studies. However, tumors that initially respond to this therapy invariably recur with the acquisition of a highly aggressive and invasive phenotype. Although several myeloid populations have been associated to this pattern of recurrence, a specific targetable population has not been yet identified. Here, we present evidence for the accumulation of Tie2-expressing monocytes/macrophages (TEMs) at the tumor/normal brain interface of mice treated with anti-VEGF therapies in regions with heightened tumoral invasion. Furthermore, we describe the presence of TEMs in malignant glioma surgical specimens that recurred after bevacizumab treatment. Our studies showed that TEMs enhanced the invasive properties of glioma cells and secreted high levels of gelatinase enzymatic proteins. Accordingly, Tie2+MMP9+ monocytic cells were consistently detected in the invasive tumor edge upon anti-VEGF therapies. Our results suggest the presence of a specific myeloid/monocytic subpopulation that plays a pivotal role in the mechanism of escape of malignant gliomas from anti-VEGF therapies and therefore constitutes a new cellular target for combination therapies in patients selected for anti-angiogenesis treatment.
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Affiliation(s)
- Konrad Gabrusiewicz
- Department of Neuro-Oncology, The University of Texas MD Anderson Cancer Center, Houston, Texas, USA
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7
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Abstract
Upstream open reading frames (uORFs) are major gene expression regulatory elements. In many eukaryotic mRNAs, one or more uORFs precede the initiation codon of the main coding region. Indeed, several studies have revealed that almost half of human transcripts present uORFs. Very interesting examples have shown that these uORFs can impact gene expression of the downstream main ORF by triggering mRNA decay or by regulating translation. Also, evidence from recent genetic and bioinformatic studies implicates disturbed uORF-mediated translational control in the etiology of many human diseases, including malignancies, metabolic or neurologic disorders, and inherited syndromes. In this review, we will briefly present the mechanisms through which uORFs regulate gene expression and how they can impact on the organism's response to different cell stress conditions. Then, we will emphasize the importance of these structures by illustrating, with specific examples, how disturbed uORF-mediated translational control can be involved in the etiology of human diseases, giving special importance to genotype-phenotype correlations. Identifying and studying more cases of uORF-altering mutations will help us to understand and establish genotype-phenotype associations, leading to advancements in diagnosis, prognosis, and treatment of many human disorders.
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Affiliation(s)
- Cristina Barbosa
- Departamento de Genética Humana, Instituto Nacional de Saúde Dr. Ricardo Jorge, Lisboa, Portugal
- Center for Biodiversity, Functional and Integrative Genomics, Faculdade de Ciências, Universidade de Lisboa, Lisboa, Portugal
| | - Isabel Peixeiro
- Departamento de Genética Humana, Instituto Nacional de Saúde Dr. Ricardo Jorge, Lisboa, Portugal
- Center for Biodiversity, Functional and Integrative Genomics, Faculdade de Ciências, Universidade de Lisboa, Lisboa, Portugal
| | - Luísa Romão
- Departamento de Genética Humana, Instituto Nacional de Saúde Dr. Ricardo Jorge, Lisboa, Portugal
- Center for Biodiversity, Functional and Integrative Genomics, Faculdade de Ciências, Universidade de Lisboa, Lisboa, Portugal
- * E-mail:
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8
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Nucleotide composition of cellular internal ribosome entry sites defines dependence on NF45 and predicts a posttranscriptional mitotic regulon. Mol Cell Biol 2012; 33:307-18. [PMID: 23129811 DOI: 10.1128/mcb.00546-12] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023] Open
Abstract
The vast majority of cellular mRNAs initiate their translations through a well-defined mechanism of ribosome recruitment that occurs at the 5'-terminal 7-methylguanosine cap with the help of several canonical protein factors. A subset of cellular and viral mRNAs contain regulatory motifs in their 5' untranslated regions (UTRs), termed internal ribosome entry sites (IRES), that sidestep this canonical mode of initiation. On cellular mRNAs, this mechanism requires IRES trans-acting protein factors (ITAFs) that facilitate ribosome recruitment downstream of the cap. While several ITAFs and their target mRNAs have been empirically identified, the in silico prediction of targets has proved difficult. Here, we report that a high AU content (>60%) of the IRES-containing 5' UTRs serves as an excellent predictor of dependence on NF45, a recently identified ITAF. Moreover, we provide evidence that cells deficient in NF45 ITAF activity exhibit reduced IRES-mediated translation of X-linked inhibitor of apoptosis protein (XIAP) and cellular inhibitor of apoptosis protein 1 (cIAP1) mRNAs that, in turn, leads to dysregulated expression of their respective targets, survivin and cyclin E. This specific defect in IRES translation explains in part the cytokinesis impairment and senescence-like phenotype observed in HeLa cells expressing NF45 RNA interference (RNAi). This study uncovers a novel role for NF45 in regulating ploidy and highlights the importance of IRES-mediated translation in cellular homeostasis.
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9
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Li S, Hu GF. Emerging role of angiogenin in stress response and cell survival under adverse conditions. J Cell Physiol 2012; 227:2822-6. [PMID: 22021078 PMCID: PMC3271170 DOI: 10.1002/jcp.23051] [Citation(s) in RCA: 126] [Impact Index Per Article: 10.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Angiogenin (ANG), also known as ribonuclease (RNASE) 5, is a member of the vertebrate-specific, secreted RNASE superfamily. ANG was originally identified as a tumor angiogenic factor, but its biological activity has been extended from inducing angiogenesis to stimulating cell proliferation and more recently, to promoting cell survival. Under growth conditions, ANG is translocated to nucleus where it accumulates in nucleolus and stimulates ribosomal RNA (rRNA) transcription, thus facilitating cell growth and proliferation. Under stress conditions, ANG is accumulated in cytoplasmic compartments and modulates the production of tiRNA, a novel class of small RNA that is derived from tRNA and is induced by stress. tiRNA suppress global protein translation by inhibiting both cap-dependent and -independent translation including that mediated by weak IRESes. However, strong IRES-mediated translation, a mechanism often used by genes involved in pro-survival and anti-apoptosis, is not affected. Thus, ANG-mediated tiRNA reprogram protein translation, save anabolic energy, and promote cell survival. This recently uncovered function of ANG presents a novel mechanism of action in regulating cell growth and survival.
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Affiliation(s)
- Shuping Li
- Molecular Oncology Research Institute, Tufts Medical Center, Boston, MA, USA
- State Key Laboratory for Diagnosis and Treatment of Infectious Disease, The First Affiliated Hospital, Zhejiang University School of Medicine, Hangzhou, China
| | - Guo-Fu Hu
- Molecular Oncology Research Institute, Tufts Medical Center, Boston, MA, USA
- Graduate Program in Biochemistry, Sackler School of Graduate Biomedical Sciences, Tufts University, Boston, MA, USA
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10
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Komar AA, Hatzoglou M. Cellular IRES-mediated translation: the war of ITAFs in pathophysiological states. Cell Cycle 2011; 10:229-40. [PMID: 21220943 DOI: 10.4161/cc.10.2.14472] [Citation(s) in RCA: 297] [Impact Index Per Article: 22.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
Translation of cellular mRNAs via initiation at Internal Ribosome Entry Sites (IRESs) has received increased attention during recent years due to its emerging significance for many physiological and pathological stress conditions in eukaryotic cells. Expression of genes bearing IRES elements in their mRNAs is controlled by multiple molecular mechanisms, with IRES-mediated translation favored under conditions when cap-dependent translation is compromised. In this review, we discuss recent advances in the field and future directions that may bring us closer to understanding the complex mechanisms that guide cellular IRES-mediated expression. We present examples in which the competitive action of IRES-transacting factors (ITAFs) plays a pivotal role in IRES-mediated translation and thereby controls cell-fate decisions leading to either pro-survival stress adaptation or cell death.
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Affiliation(s)
- Anton A Komar
- Center for Gene Regulation in Health and Disease, Department of Biological, Geological and Environmental Sciences, Cleveland State University, Cleveland, OH, USA.
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11
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Makinde TO, Agrawal DK. Increased expression of angiopoietins and Tie2 in the lungs of chronic asthmatic mice. Am J Respir Cell Mol Biol 2010; 44:384-93. [PMID: 20463289 DOI: 10.1165/rcmb.2009-0330oc] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
Angiopoietin (Ang)1 and Ang2 are ligands for Tie2 tyrosine kinase receptor (Tie2). Elevated levels of Ang1 and Ang2 in induced sputum of patients with asthma have been reported, with a positive correlation of Ang2 levels with the severity of airway occlusion. Although studies have shown Tie2-mediated regulation of nonvascular cells in some pathological conditions, current knowledge on Tie2 signaling in asthma is limited to the vasculature. We examined the expression pattern of Ang1, Ang2, vascular endothelial growth factor (VEGF), and Tie2 and their correlation with the degree of airway remodeling in the lung of ovalbumin (OVA)-sensitized and OVA-challenged mice with airway hyperresponsiveness. Lung tissues were isolated from Balb/c mice after OVA sensitization and challenge. Hematoxylin and eosin, periodic acid-Schiff, and trichrome staining were used to show the lung pathology. The expression of Ang1, Ang2, VEGF, and Tie2 was examined using immunofluorescence, Western blot, ELISA, and real-time PCR. In the lung of normal mice, Tie2 expression was detected only in the blood vessels. However, in the lung of OVA-sensitized and OVA-challenged mice, Tie2 was abundantly expressed in airway epithelial cells and in a subset of macrophages in addition to constitutive expression in pulmonary vessels. The increase in Tie2 expression correlated with the severity of airway remodeling. Macrophages and airway epithelial cells express Ang2 and VEGF only in allergic models. Ang1 was constitutively expressed, with a decrease in mRNA level in allergic models. In conclusion, increased expression of Tie2 and Ang2 in allergic airway epithelium and alveolar macrophages correlates with the severity of airway remodeling.
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Affiliation(s)
- Toluwalope O Makinde
- Center for Clinical and Translational Science, Creighton University School of Medicine, Omaha, NE 68178, USA
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12
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He JZ, Ho JJD, Gingerich S, Courtman DW, Marsden PA, Ward ME. Enhanced translation of heme oxygenase-2 preserves human endothelial cell viability during hypoxia. J Biol Chem 2010; 285:9452-9461. [PMID: 20118244 DOI: 10.1074/jbc.m109.077230] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
Heme oxygenases (HOs) -1 and -2 catalyze the breakdown of heme to release carbon monoxide, biliverdin, and ferrous iron, which may preserve cell function during oxidative stress. HO-1 levels decrease in endothelial cells exposed to hypoxia, whereas the effect of hypoxia on HO-2 expression is unknown. The current study was carried out to determine if hypoxia alters HO-2 protein levels in human endothelial cells and whether this enzyme plays a role in preserving their viability during hypoxic stress. Human umbilical vein endothelial cells (HUVECs), human aortic endothelial cells (HAECs), and human blood outgrowth endothelial cells were exposed to 21% or 1% O(2) for 48 or 16 h in the presence or absence of tumor necrosis factor-alpha (10 ng/ml) or H(2)O(2) (100 microm). In all three endothelial cell types HO-1 mRNA and protein levels were decreased following hypoxic incubation, whereas HO-2 protein levels were unaltered. In HUVECs HO-2 levels were maintained during hypoxia despite a 57% reduction in steady-state HO-2 mRNA level and a 43% reduction in total protein synthesis. Polysome profiling revealed increased HO-2 transcript association with polysomes during hypoxia consistent with enhanced translation of these transcripts. Importantly, inhibition of HO-2 expression by small interference RNA increased oxidative stress, exacerbated mitochondrial membrane depolarization, and enhanced caspase activation and apoptotic cell death in cells incubated under hypoxic but not normoxic conditions. These data indicate that HO-2 is important in maintaining endothelial viability and may preserve local regulation of vascular tone, thrombosis, and inflammatory responses during reductions in systemic oxygen delivery.
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Affiliation(s)
- Jeff Z He
- Terrence Donnelly Laboratories, Toronto, Ontario M5B 1W8; Li Ka Shing Knowledge Institute of St. Michael's Hospital, Toronto, Ontario M5B 1W8; Departments of Laboratory Medicine and Pathobiology, Toronto, Ontario M5S 1A8, Canada
| | - J J David Ho
- Medical Biophysics, Toronto, Ontario M5S 1A8, Canada
| | - Sheena Gingerich
- Nephrology, Department of Medicine, University of Toronto, Toronto, Ontario M5S 1A8, Canada
| | - David W Courtman
- Terrence Donnelly Laboratories, Toronto, Ontario M5B 1W8; Li Ka Shing Knowledge Institute of St. Michael's Hospital, Toronto, Ontario M5B 1W8
| | - Philip A Marsden
- Li Ka Shing Knowledge Institute of St. Michael's Hospital, Toronto, Ontario M5B 1W8; Nephrology, Department of Medicine, University of Toronto, Toronto, Ontario M5S 1A8, Canada.
| | - Michael E Ward
- Terrence Donnelly Laboratories, Toronto, Ontario M5B 1W8; Li Ka Shing Knowledge Institute of St. Michael's Hospital, Toronto, Ontario M5B 1W8; Departments of Laboratory Medicine and Pathobiology, Toronto, Ontario M5S 1A8, Canada; Divisions of Respirology, Toronto, Ontario M5S 1A8, Canada
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13
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van Meurs M, Kümpers P, Ligtenberg JJM, Meertens JHJM, Molema G, Zijlstra JG. Bench-to-bedside review: Angiopoietin signalling in critical illness - a future target? CRITICAL CARE : THE OFFICIAL JOURNAL OF THE CRITICAL CARE FORUM 2009; 13:207. [PMID: 19435476 PMCID: PMC2689450 DOI: 10.1186/cc7153] [Citation(s) in RCA: 86] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
Multiple organ dysfunction syndrome (MODS) occurs in response to major insults such as sepsis, severe haemorrhage, trauma, major surgery and pancreatitis. The mortality rate is high despite intensive supportive care. The pathophysiological mechanism underlying MODS are not entirely clear, although several have been proposed. Overwhelming inflammation, immunoparesis, occult oxygen debt and other mechanisms have been investigated, and – despite many unanswered questions – therapies targeting these mechanisms have been developed. Unfortunately, only a few interventions, usually those targeting multiple mechanisms at the same time, have appeared to be beneficial. We clearly need to understand better the mechanisms that underlie MODS. The endothelium certainly plays an active role in MODS. It functions at the intersection of several systems, including inflammation, coagulation, haemodynamics, fluid and electrolyte balance, and cell migration. An important regulator of these systems is the angiopoietin/Tie2 signalling system. In this review we describe this signalling system, giving special attention to what is known about it in critically ill patients and its potential as a target for therapy.
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Affiliation(s)
- Matijs van Meurs
- Department of Critical Care, University Medical Center Groningen, University of Groningen, 9700RB Groningen, The Netherlands
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14
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Cathepsin L expression is up-regulated by hypoxia in human melanoma cells: role of its 5'-untranslated region. Biochem J 2008; 413:125-34. [PMID: 18366346 DOI: 10.1042/bj20071255] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Overexpression of cathepsin L, a cysteine protease, and consequently procathepsin L secretion switch the phenotype of human melanoma cells to highly tumorigenic and strongly metastatic. This led us to identify the DNA regulatory sequences involved in the regulation of cathepsin L expression in highly metastatic human melanoma cells. The results of the present study demonstrated the presence of regulatory sequences in the 3' region downstream of the cathepsin L gene and in the 3'- and 5'-flanking regions of GC/CCAAT sites of its promoter. In addition, we established that the 5'-UTR (untranslated region) was the most important region for cathepsin L expression. This 5'-UTR integrated an alternative promoter and sequences involved in post-transcriptional regulation. Transfection experiments of bicistronic reporter vectors and RNAs demonstrated that the cathepsin L 5'-UTR contained a functional IRES (internal ribosome entry site). This complete IRES was present only in one of the three splice variants, which differed in their 5'-UTR. Then, we analysed cathepsin L expression in this human melanoma cell line grown under hypoxia. We demonstrated that under moderate hypoxic conditions (1% O2) intracellular expression of cathepsin L was up-regulated. Hypoxia significantly increased only the expression of the transcript which contains the complete IRES, but inhibited promoter activity. These results suggest that the presence of an IRES allowed cathepsin L mRNA translation to be efficient under hypoxic conditions. Altogether, our results indicated that in vivo a tumour hypoxic environment up-regulates cathepsin L expression which promotes tumour progression.
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15
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Abstract
Low oxygen (O2) levels are a naturally occurring feature of embryonic development, adult physiology, and diseases such as those of the cardiovascular system. Although many responses to O2 deprivation are mediated by hypoxia-inducible factors (HIFs), researchers are finding a growing number of HIF-independent pathways that promote O2 conformance and hypoxia tolerance. Here, we describe HIF-independent responses and how they impact cardiovascular tissue homeostasis.
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Affiliation(s)
- M Celeste Simon
- Abramson Family Cancer Research Institute, University of Pennsylvania, Philadelphia, PA 19104, USA.
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16
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Young RM, Wang SJ, Gordan JD, Ji X, Liebhaber SA, Simon MC. Hypoxia-mediated selective mRNA translation by an internal ribosome entry site-independent mechanism. J Biol Chem 2008; 283:16309-19. [PMID: 18430730 DOI: 10.1074/jbc.m710079200] [Citation(s) in RCA: 97] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023] Open
Abstract
Although it is advantageous for hypoxic cells to inhibit protein synthesis and conserve energy, it is also important to translate mRNAs critical for adaptive responses to hypoxic stress. Because internal ribosome entry sites (IRES) have been postulated to mediate this preferential synthesis, we analyzed the 5 '-untranslated regions from a panel of stress-regulated mRNAs for m(7)GTP cap-independent translation and identified putative IRES elements in encephalomyocarditis virus, vascular endothelial growth factor, hypoxia-inducible factors (HIFs) 1alpha and 2alpha, glucose transporter-like protein 1, p57(Kip2), La, BiP, and triose phosphate isomerase transcripts. However, when capped and polyadenylated dicistronic RNAs were synthesized in vitro and transfected into cells, cellular IRES-mediated translation accounted for less than 1% that of the level of cap-dependent translation. Moreover, hypoxic stress failed to activate cap-independent synthesis, indicating that it is unlikely that this is the primary mechanism for the maintenance of the translation of these mRNAs under low O(2). Furthermore, although HIF-1alpha is frequently cited as an example of an mRNA that is preferentially translated, we demonstrate that under different levels and durations of hypoxic stress, changes in newly synthesized HIF-1alpha and beta-actin protein levels mirror alterations in corresponding mRNA abundance. In addition, our data suggest that cyclin-dependent kinase inhibitor p57(Kip2) and vascular endothelial growth factor mRNAs are selectively translated by an IRES-independent mechanism under hypoxic stress.
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Affiliation(s)
- Regina M Young
- Abramson Family Cancer Research Institute, Departments of Genetics and Medicine, University of Pennsylvania School of Medicine, and Howard Hughes Medical Institute, University of Pennsylvania, Philadelphia Pennsylvania 19104, USA
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17
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Thomas JD, Dias LM, Johannes GJ. Translational repression during chronic hypoxia is dependent on glucose levels. RNA (NEW YORK, N.Y.) 2008; 14:771-781. [PMID: 18268023 PMCID: PMC2271361 DOI: 10.1261/rna.857308] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/02/2007] [Accepted: 12/20/2007] [Indexed: 05/25/2023]
Abstract
Translation is often repressed in cell lines that are exposed to hypoxic conditions (0.5% - 1.5% O2) but this repression requires prolonged exposure (> 16 h). We report here that prolonged exposure to hypoxia results in the depletion of glucose from the media and that the loss of glucose correlates with the shut down in translation. Furthermore, we show that the addition of glucose or reoxygenation restores translation in hypoxic PC3 cells. This indicates that both glucose depletion and hypoxia are required for translational repression. We also show that eIF2alpha phosphorylation is reversed by glucose addition. Moreover, we present data that strongly indicate that eIF2alpha phosphorylation as well as the translational inhibition that occurs when cells are grown under conditions of glucose depletion and hypoxia is pancreatic eIF2alpha kinase (PERK) independent. We believe this is the first report to show that glucose depletion is required for translational repression under hypoxic conditions and that this explains why prolonged exposure to hypoxia is required for this inhibition. Since the physiological conditions that lead to tumor hypoxia would also likely lead to reduced glucose levels, understanding the interplay of glucose and hypoxia in regulating tumor metabolism will provide important information on the growth and development of solid tumors.
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Affiliation(s)
- Jeff D Thomas
- Department of Pathology and Laboratory Medicine, Drexel University College of Medicine, Philadelphia, Pennsylvania 19102, USA
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18
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Makinde T, Agrawal DK. Intra and extravascular transmembrane signalling of angiopoietin-1-Tie2 receptor in health and disease. J Cell Mol Med 2008; 12:810-28. [PMID: 18266978 PMCID: PMC4401129 DOI: 10.1111/j.1582-4934.2008.00254.x] [Citation(s) in RCA: 56] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023] Open
Abstract
Angiopoietin-1 (Ang-1) is the primary agonist for Tie2 tyrosine kinase receptor (Tie2), and the effect of Ang-1-Tie2 signalling is context-dependent. Deficiency in either Ang-1 or Tie2 protein leads to severe microvascular defects and subsequent embryonic lethality in murine model. Tie2 receptors are expressed in several cell types, including endothelial cells, smooth muscle cells, fibroblasts, epithelial cells, monocytes, neutrophils, eosinophils and glial cells. Ang-1-Tie2 signalling induces a chemotactic effect in smooth muscle cells, neutrophils and eosinophils, and induces differentiation of mesenchymal cells to smooth muscle cells. Additionally, this signalling pathway induces the secretion of serotonin, matrix metalloproteinases (MMPs) and plasmin. Ang-1 inhibits the secretion of tissue inhibitor of matrix metalloproteinase (TIMPs). Aberrant expression and activity of Tie2 in vascular and non-vascular cells may result in the development of rheumatoid arthritis, cancer, hypertension and psoriasis. Ang-1 has an anti-inflammatory effect, when co-localized with vascular endothelial growth factor (VEGF) in the vasculature. Thus, Ang-1 could be potentially important in the therapy of various pathological conditions such as pulmonary hypertension, arteriosclerosis and diabetic retinopathy. In this article, we have summarized and critically reviewed the pathophysiological role of Ang-1-Tie2 signalling pathway.
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Affiliation(s)
- T Makinde
- Department of Biomedical Sciences, Creighton University School of Medicine, Omaha, NE 68178, USA
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19
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Blais JD, Addison CL, Edge R, Falls T, Zhao H, Wary K, Koumenis C, Harding HP, Ron D, Holcik M, Bell JC. Perk-dependent translational regulation promotes tumor cell adaptation and angiogenesis in response to hypoxic stress. Mol Cell Biol 2006; 26:9517-32. [PMID: 17030613 PMCID: PMC1698539 DOI: 10.1128/mcb.01145-06] [Citation(s) in RCA: 229] [Impact Index Per Article: 12.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
It has been well established that the tumor microenvironment can promote tumor cell adaptation and survival. However, the mechanisms that influence malignant progression have not been clearly elucidated. We have previously demonstrated that cells cultured under hypoxic/anoxic conditions and transformed cells in hypoxic areas of tumors activate a translational control program known as the integrated stress response (ISR). Here, we show that tumors derived from K-Ras-transformed Perk(-/-) mouse embryonic fibroblasts (MEFs) are smaller and exhibit less angiogenesis than tumors with an intact ISR. Furthermore, Perk promotes a tumor microenvironment that favors the formation of functional microvessels. These observations were corroborated by a microarray analysis of polysome-bound RNA in aerobic and hypoxic Perk(+/+) and Perk(-/-) MEFs. This analysis revealed that a subset of proangiogenic transcripts is preferentially translated in a Perk-dependent manner; these transcripts include VCIP, an adhesion molecule that promotes cellular adhesion, integrin binding, and capillary morphogenesis. Taken with the concomitant Perk-dependent translational induction of additional proangiogenic genes identified by our microarray analysis, this study suggests that Perk plays a role in tumor cell adaptation to hypoxic stress by regulating the translation of angiogenic factors necessary for the development of functional microvessels and further supports the contention that the Perk pathway could be an attractive target for novel antitumor modalities.
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MESH Headings
- Animals
- Cell Line, Transformed
- Fibroblasts/enzymology
- Gene Expression Profiling
- HT29 Cells
- Humans
- Hypoxia/enzymology
- Hypoxia/genetics
- Hypoxia/physiopathology
- Mice
- Mice, Nude
- Neoplasms, Experimental/enzymology
- Neoplasms, Experimental/genetics
- Neoplasms, Experimental/physiopathology
- Neovascularization, Pathologic/enzymology
- Neovascularization, Pathologic/genetics
- Neovascularization, Pathologic/physiopathology
- Oligonucleotide Array Sequence Analysis
- Protein Biosynthesis
- Stress, Physiological/enzymology
- Stress, Physiological/genetics
- Stress, Physiological/physiopathology
- eIF-2 Kinase/deficiency
- eIF-2 Kinase/genetics
- eIF-2 Kinase/physiology
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Affiliation(s)
- Jaime D Blais
- Ottawa Health Research Institute, Ottawa, ON K1H 1C4, Canada
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20
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Spevak CC, Park EH, Geballe AP, Pelletier J, Sachs MS. her-2 upstream open reading frame effects on the use of downstream initiation codons. Biochem Biophys Res Commun 2006; 350:834-41. [PMID: 17045969 PMCID: PMC1668710 DOI: 10.1016/j.bbrc.2006.09.128] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/22/2006] [Accepted: 09/11/2006] [Indexed: 11/30/2022]
Abstract
The her-2 (neu, erbB-2) oncogene encodes a 185-kDa transmembrane receptor tyrosine kinase. HER2 overexpression occurs in numerous primary human tumors and contributes to 25-30% of breast and ovarian carcinomas. Synthesis of HER2 is controlled in part by an upstream open reading frame (uORF) present in the transcript. We used synthetic capped and polyadenylated mRNAs containing sequences derived from the 5' region of the her-2 transcript fused to a firefly luciferase (LUC) reporter to examine this uORF's effect on translation in cell-free systems derived from reticulocytes, wheat germ and Neurospora crassa, and in RNA-transfected HeLa cells. The uORF reduced translation of the downstream cistron in all systems. [(35)S]Met labeling of in vitro translation products obtained indicated that the uORF also affected downstream start-site selection. Primer extension inhibition (toeprint) assays of ribosomes loaded at initiation codons in reticulocyte lysates indicated that the uORF affected the interaction of ribosomes with the primary her-2 AUG codon.
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Affiliation(s)
- Christina C. Spevak
- Department of Environmental & Biomolecular Systems,
Oregon Health and Science University, Beaverton, OR 97006
| | - Eun-Hee Park
- Department of Biochemistry and McGill Cancer Center, McGill
University, Montreal, Quebec H3G 1Y6
| | - Adam P. Geballe
- Divisions of Human Biology and Clinical Research, C2-023, Fred
Hutchinson Cancer Research Center, Seattle, Washington 98109; Departments of
Medicine and Microbiology University of Washington, Seattle, WA 98115
| | - Jerry Pelletier
- Department of Biochemistry and McGill Cancer Center, McGill
University, Montreal, Quebec H3G 1Y6
- McGill Cancer Center, McGill University, Montreal, Quebec H3G
1Y6
| | - Matthew S. Sachs
- Department of Environmental & Biomolecular Systems,
Oregon Health and Science University, Beaverton, OR 97006
- Department of Molecular Microbiology and Immunology, Oregon
Health & Science University, Portland, Oregon 97201
- Address correspondence to: Matthew S. Sachs, Department of
Environmental and Biomolecular Systems, Oregon Health & Science
University, 20000 NW Walker Road, Beaverton OR, 97006-8921, Tel. 503-748-1487;
Fax 214 648-6899; E-mail
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21
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Abstract
The cell has many ways to regulate the production of proteins. One mechanism is through the changes to the machinery of translation initiation. These alterations favor the translation of one subset of mRNAs over another. It was first shown that internal ribosome entry sites (IRESes) within viral RNA genomes allowed the production of viral proteins more efficiently than most of the host proteins. The RNA secondary structure of viral IRESes has sometimes been conserved between viral species even though the primary sequences differ. These structures are important for IRES function, but no similar structure conservation has yet to be shown in cellular IRES. With the advances in mathematical modeling and computational approaches to complex biological problems, is there a way to predict an IRES in a data set of unknown sequences? This review examines what is known about cellular IRES structures, as well as the data sets and tools available to examine this question. We find that the lengths, number of upstream AUGs, and %GC content of 5'-UTRs of the human transcriptome have a similar distribution to those of published IRES-containing UTRs. Although the UTRs containing IRESes are on the average longer, almost half of all 5'-UTRs are long enough to contain an IRES. Examination of the available RNA structure prediction software and RNA motif searching programs indicates that while these programs are useful tools to fine tune the empirically determined RNA secondary structure, the accuracy of de novo secondary structure prediction of large RNA molecules and subsequent identification of new IRES elements by computational approaches, is still not possible.
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Affiliation(s)
- Stephen D Baird
- Department of Biochemistry, Microbiology and Immunology, University of Ottawa, Ontario K1H 8M5, Canada
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22
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Park EH, Lee JM, Pelletier J. The Tie2 5' untranslated region is inhibitory to 5' end-mediated translation initiation. FEBS Lett 2006; 580:1309-19. [PMID: 16457819 DOI: 10.1016/j.febslet.2006.01.049] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/13/2005] [Accepted: 01/12/2006] [Indexed: 11/20/2022]
Abstract
Tie2 is an endothelium-specific receptor tyrosine kinase required for normal blood vessel maturation, remodeling, and stability. Tie2 expression is also upregulated in various cancers implicating a role in tumor angiogenesis. Its mRNA transcript contains an unusually long (372 nucleotides) 5' untranslated region (UTR) with five upstream open reading frames (uORFs) and an internal ribosome entry site (IRES) that allows this mRNA to be translated under hypoxic conditions. This sets up an alternative initiation pathway with the potential to clash with 5' end-mediated initiation from the same template. Herein, we define experimental conditions under which the Tie2 IRES is not active, allowing us to assess the contribution of the 5' UTR to cap-dependent translation on the Tie2 transcript. We find that the Tie2 5' UTR is inhibitory to translation initiation with ribosome flow decreasing following encounters with each uORF. No single uORF was found to harbor significant cis-acting inhibitory activity. Our results suggest that the uORFs within the Tie2 5' UTR serve to decrease the percent of ribosomes competent for reinitiation as these traverse the mRNA 5' UTR, thus minimizing interference with the IRES.
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Affiliation(s)
- Eun-Hee Park
- Department of Biochemistry, McIntyre Medical Sciences Building, Room 810, 3655 Promenade Sir William Osler, McGill University, Montreal, Que., Canada H3G 1Y6
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23
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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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