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Chen J, Su Y, Pi S, Hu B, Mao L. The Dual Role of Low-Density Lipoprotein Receptor-Related Protein 1 in Atherosclerosis. Front Cardiovasc Med 2021; 8:682389. [PMID: 34124208 PMCID: PMC8192809 DOI: 10.3389/fcvm.2021.682389] [Citation(s) in RCA: 13] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/18/2021] [Accepted: 05/05/2021] [Indexed: 12/26/2022] Open
Abstract
Low-density lipoprotein receptor–related protein-1 (LRP1) is a large endocytic and signaling receptor belonging to the LDL receptor (LDLR) gene family and that is widely expressed in several tissues. LRP1 comprises a large extracellular domain (ECD; 515 kDa, α chain) and a small intracellular domain (ICD; 85 kDa, β chain). The deletion of LRP1 leads to embryonic lethality in mice, revealing a crucial but yet undefined role in embryogenesis and development. LRP1 has been postulated to participate in numerous diverse physiological and pathological processes ranging from plasma lipoprotein homeostasis, atherosclerosis, tumor evolution, and fibrinolysis to neuronal regeneration and survival. Many studies using cultured cells and in vivo animal models have revealed the important roles of LRP1 in vascular remodeling, foam cell biology, inflammation and atherosclerosis. However, its role in atherosclerosis remains controversial. LRP1 not only participates in the removal of atherogenic lipoproteins and proatherogenic ligands in the liver but also mediates the uptake of aggregated LDL to promote the formation of macrophage- and vascular smooth muscle cell (VSMC)-derived foam cells, which causes a prothrombotic transformation of the vascular wall. The dual and opposing roles of LRP1 may also represent an interesting target for atherosclerosis therapeutics. This review highlights the influence of LRP1 during atherosclerosis development, focusing on its dual role in vascular cells and immune cells.
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Affiliation(s)
- Jiefang Chen
- Department of Neurology, Tongji Medical College, Union Hospital, Huazhong University of Science and Technology, Wuhan, China
| | - Ying Su
- Department of Neurology, Tongji Medical College, Union Hospital, Huazhong University of Science and Technology, Wuhan, China
| | - Shulan Pi
- Department of Neurology, Tongji Medical College, Union Hospital, Huazhong University of Science and Technology, Wuhan, China
| | - Bo Hu
- Department of Neurology, Tongji Medical College, Union Hospital, Huazhong University of Science and Technology, Wuhan, China
| | - Ling Mao
- Department of Neurology, Tongji Medical College, Union Hospital, Huazhong University of Science and Technology, Wuhan, China
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Schnieder J, Mamazhakypov A, Birnhuber A, Wilhelm J, Kwapiszewska G, Ruppert C, Markart P, Wujak L, Rubio K, Barreto G, Schaefer L, Wygrecka M. Loss of LRP1 promotes acquisition of contractile-myofibroblast phenotype and release of active TGF-β1 from ECM stores. Matrix Biol 2019; 88:69-88. [PMID: 31841706 DOI: 10.1016/j.matbio.2019.12.001] [Citation(s) in RCA: 32] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/07/2019] [Revised: 12/06/2019] [Accepted: 12/07/2019] [Indexed: 12/16/2022]
Abstract
In healing tissue, fibroblasts differentiate to α-smooth muscle actin (SMA)-expressing contractile-myofibroblasts, which pull the wound edges together ensuring proper tissue repair. Uncontrolled expansion of the myofibroblast population may, however, lead to excessive tissue scarring and finally to organ dysfunction. Here, we demonstrate that the loss of low-density lipoprotein receptor-related protein (LRP) 1 overactivates the JNK1/2-c-Jun-Fra-2 signaling pathway leading to the induction of α-SMA and periostin expression in human lung fibroblasts (hLF). These changes are accompanied by increased contractility of the cells and the integrin- and protease-dependent release of active transforming growth factor (TGF)-β1 from the extracellular matrix (ECM) stores. Liberation of active TGF-β1 from the ECM further enhances α-SMA and periostin expression thus accelerating the phenotypic switch of hLF. Global gene expression profiling of LRP1-depleted hLF revealed that the loss of LRP1 affects cytoskeleton reorganization, cell-ECM contacts, and ECM production. In line with these findings, fibrotic changes in the skin and lung of Fra-2 transgenic mice were associated with LRP1 depletion and c-Jun overexpression. Altogether, our results suggest that dysregulation of LRP1 expression in fibroblasts in healing tissue may lead to the unrestrained expansion of contractile myofibroblasts and thereby to fibrosis development. Further studies identifying molecules, which regulate LRP1 expression, may provide new therapeutic options for largely untreatable human fibrotic diseases.
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Affiliation(s)
- Jennifer Schnieder
- Departments of Biochemistry and Internal Medicine, Universities of Giessen and Marburg Lung Center, Giessen, Germany
| | - Argen Mamazhakypov
- Departments of Biochemistry and Internal Medicine, Universities of Giessen and Marburg Lung Center, Giessen, Germany
| | - Anna Birnhuber
- Ludwig Boltzmann Institute for Lung Vascular Research, Graz, Austria
| | - Jochen Wilhelm
- Departments of Internal Medicine, Universities of Giessen and Marburg Lung Center, Giessen, Germany
| | | | - Clemens Ruppert
- Departments of Internal Medicine, Universities of Giessen and Marburg Lung Center, Giessen, Germany
| | - Philipp Markart
- Department of Pulmonary Medicine, Fulda Hospital, University Medicine Marburg, Campus Fulda, Fulda, Germany
| | - Lukasz Wujak
- Departments of Biochemistry and Internal Medicine, Universities of Giessen and Marburg Lung Center, Giessen, Germany
| | - Karla Rubio
- Lung Cancer Epigenetic, Max-Planck-Institute for Heart and Lung Research, Bad Nauheim, Germany
| | - Guillermo Barreto
- Lung Cancer Epigenetic, Max-Planck-Institute for Heart and Lung Research, Bad Nauheim, Germany; Brain and Lung Epigenetics, Laboratoire Croissance, Réparation et Régénération Tissulaires (CRRET), Université Paris Est Créteil (UPEC), Créteil, France
| | - Liliana Schaefer
- Institute of Pharmacology and Toxicology, Goethe University, Frankfurt Am Main, Germany
| | - Malgorzata Wygrecka
- Departments of Biochemistry and Internal Medicine, Universities of Giessen and Marburg Lung Center, Giessen, Germany.
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Bres EE, Faissner A. Low Density Receptor-Related Protein 1 Interactions With the Extracellular Matrix: More Than Meets the Eye. Front Cell Dev Biol 2019; 7:31. [PMID: 30931303 PMCID: PMC6428713 DOI: 10.3389/fcell.2019.00031] [Citation(s) in RCA: 37] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/13/2018] [Accepted: 02/25/2019] [Indexed: 12/12/2022] Open
Abstract
The extracellular matrix (ECM) is a biological substrate composed of collagens, proteoglycans and glycoproteins that ensures proper cell migration and adhesion and keeps the cell architecture intact. The regulation of the ECM composition is a vital process strictly controlled by, among others, proteases, growth factors and adhesion receptors. As it appears, ECM remodeling is also essential for proper neuronal and glial development and the establishment of adequate synaptic signaling. Hence, disturbances in ECM functioning are often present in neurodegenerative diseases like Alzheimer’s disease. Moreover, mutations in ECM molecules are found in some forms of epilepsy and malfunctioning of ECM-related genes and pathways can be seen in, for example, cancer or ischemic injury. Low density lipoprotein receptor-related protein 1 (Lrp1) is a member of the low density lipoprotein receptor family. Lrp1 is involved not only in ligand uptake, receptor mediated endocytosis and lipoprotein transport—functions shared by low density lipoprotein receptor family members—but also regulates cell surface protease activity, controls cellular entry and binding of toxins and viruses, protects against atherosclerosis and acts on many cell signaling pathways. Given the plethora of functions, it is not surprising that Lrp1 also impacts the ECM and is involved in its remodeling. This review focuses on the role of Lrp1 and some of its major ligands on ECM function. Specifically, interactions with two Lrp1 ligands, integrins and tissue plasminogen activator are described in more detail.
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Affiliation(s)
- Ewa E Bres
- Department of Cell Morphology and Molecular Neurobiology, Ruhr University Bochum, Bochum, Germany
| | - Andreas Faissner
- Department of Cell Morphology and Molecular Neurobiology, Ruhr University Bochum, Bochum, Germany
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4
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Wujak L, Schnieder J, Schaefer L, Wygrecka M. LRP1: A chameleon receptor of lung inflammation and repair. Matrix Biol 2017; 68-69:366-381. [PMID: 29262309 DOI: 10.1016/j.matbio.2017.12.007] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/15/2017] [Revised: 12/12/2017] [Accepted: 12/12/2017] [Indexed: 12/17/2022]
Abstract
The lung displays a remarkable capability to regenerate following injury. Considerable effort has been made thus far to understand the cardinal processes underpinning inflammation and reconstruction of lung tissue. However, the factors determining the resolution or persistence of inflammation and efficient wound healing or aberrant remodeling remain largely unknown. Low density lipoprotein receptor-related protein 1 (LRP1) is an endocytic/signaling cell surface receptor which controls cellular and molecular mechanisms driving the physiological and pathological inflammatory reactions and tissue remodeling in several organs. In this review, we will discuss the impact of LRP1 on the consecutive steps of the inflammatory response and its role in the balanced tissue repair and aberrant remodeling in the lung.
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Affiliation(s)
- Lukasz Wujak
- Department of Biochemistry, Justus Liebig University, Friedrichstrasse 24, 35392 Giessen, Germany
| | - Jennifer Schnieder
- Department of Biochemistry, Justus Liebig University, Friedrichstrasse 24, 35392 Giessen, Germany
| | - Liliana Schaefer
- Goethe University School of Medicine, University Hospital, Theodor-Stern Kai 7, 60590 Frankfurt am Main, Germany
| | - Malgorzata Wygrecka
- Department of Biochemistry, Justus Liebig University, Friedrichstrasse 24, 35392 Giessen, Germany; Member of the German Center for Lung Research (DZL), Germany.
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Safina D, Schlitt F, Romeo R, Pflanzner T, Pietrzik CU, Narayanaswami V, Edenhofer F, Faissner A. Low-density lipoprotein receptor-related protein 1 is a novel modulator of radial glia stem cell proliferation, survival, and differentiation. Glia 2016; 64:1363-80. [PMID: 27258849 DOI: 10.1002/glia.23009] [Citation(s) in RCA: 37] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/10/2015] [Revised: 04/19/2016] [Accepted: 05/04/2016] [Indexed: 12/22/2022]
Abstract
The LDL family of receptors and its member low-density lipoprotein receptor-related protein 1 (LRP1) have classically been associated with a modulation of lipoprotein metabolism. Current studies, however, indicate diverse functions for this receptor in various aspects of cellular activities, including cell proliferation, migration, differentiation, and survival. LRP1 is essential for normal neuronal function in the adult CNS, whereas the role of LRP1 in development remained unclear. Previously, we have observed an upregulation of LewisX (LeX) glycosylated LRP1 in the stem cells of the developing cortex and demonstrated its importance for oligodendrocyte differentiation. In the current study, we show that LeX-glycosylated LRP1 is also expressed in the stem cell compartment of the developing spinal cord and has broader functions in the developing CNS. We have investigated the basic properties of LRP1 conditional knockout on the neural stem/progenitor cells (NSPCs) from the cortex and the spinal cord, created by means of Cre-loxp-mediated recombination in vitro. The functional status of LRP1-deficient cells has been studied using proliferation, differentiation, and apoptosis assays. LRP1 deficient NSPCs from both CNS regions demonstrated altered differentiation profiles. Their differentiation capacity toward oligodendrocyte progenitor cells (OPCs), mature oligodendrocytes and neurons was reduced. In contrast, astrocyte differentiation was promoted. Moreover, LRP1 deletion had a negative effect on NSPCs proliferation and survival. Our observations suggest that LRP1 facilitates NSPCs differentiation via interaction with apolipoprotein E (ApoE). Upon ApoE4 stimulation wild type NSPCs generated more oligodendrocytes, but LRP1 knockout cells showed no response. The effect of ApoE seems to be independent of cholesterol uptake, but is rather mediated by downstream MAPK and Akt activation. GLIA 2016 GLIA 2016;64:1363-1380.
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Affiliation(s)
- Dina Safina
- Department of Cell Morphology and Molecular Neurobiology, Faculty of Biology and Biotechnology, Ruhr-University, Bochum, D-44780, Germany.,International Graduate School of Neuroscience, Ruhr-University Bochum, Bochum, D-44780, Germany
| | - Frederik Schlitt
- Department of Cell Morphology and Molecular Neurobiology, Faculty of Biology and Biotechnology, Ruhr-University, Bochum, D-44780, Germany
| | - Ramona Romeo
- Department of Cell Morphology and Molecular Neurobiology, Faculty of Biology and Biotechnology, Ruhr-University, Bochum, D-44780, Germany
| | - Thorsten Pflanzner
- Institute for Pathobiochemistry, University Medical Center of the Johannes Gutenberg-University Mainz, Mainz, D-55099, Germany
| | - Claus U Pietrzik
- Institute for Pathobiochemistry, University Medical Center of the Johannes Gutenberg-University Mainz, Mainz, D-55099, Germany
| | - Vasanthy Narayanaswami
- Department of Chemistry and Biochemistry, California State University Long Beach, Long Beach, California, 90840
| | - Frank Edenhofer
- Institute of Anatomy and Cell Biology, University Wuerzburg, Koellikerstraße 6, Wuerzburg, D-97070, Germany
| | - Andreas Faissner
- Department of Cell Morphology and Molecular Neurobiology, Faculty of Biology and Biotechnology, Ruhr-University, Bochum, D-44780, Germany
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Lähdesmäki K, Öörni K, Alanne-Kinnunen M, Jauhiainen M, Hurt-Camejo E, Kovanen PT. Acidity and lipolysis by group V secreted phospholipase A2 strongly increase the binding of apoB-100-containing lipoproteins to human aortic proteoglycans. Biochim Biophys Acta Mol Cell Biol Lipids 2012; 1821:257-67. [DOI: 10.1016/j.bbalip.2011.10.014] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/14/2010] [Revised: 10/11/2011] [Accepted: 10/17/2011] [Indexed: 11/16/2022]
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7
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Plihtari R, Kovanen PT, Öörni K. Acidity increases the uptake of native LDL by human monocyte-derived macrophages. Atherosclerosis 2011; 217:401-6. [DOI: 10.1016/j.atherosclerosis.2011.04.013] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/17/2010] [Revised: 04/11/2011] [Accepted: 04/12/2011] [Indexed: 12/11/2022]
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Chen CL, Hou WH, Liu IH, Hsiao G, Huang SS, Huang JS. Inhibitors of clathrin-dependent endocytosis enhance TGFbeta signaling and responses. J Cell Sci 2009; 122:1863-71. [PMID: 19461075 DOI: 10.1242/jcs.038729] [Citation(s) in RCA: 103] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Clathrin-dependent endocytosis is believed to be involved in TGFbeta-stimulated cellular responses, but the subcellular locus at which TGFbeta induces signaling remains unclear. Here, we demonstrate that inhibitors of clathrin-dependent endocytosis, which are known to arrest the progression of endocytosis at coated-pit stages, inhibit internalization of cell-surface-bound TGFbeta and promote colocalization and accumulation of TbetaR-I and SARA at the plasma membrane. These inhibitors enhance TGFbeta-induced signaling and cellular responses (Smad2 phosphorylation/nuclear localization and expression of PAI-1). Dynasore, a newly identified inhibitor of dynamin GTPase activity, is one of the most potent inhibitors among those tested and, furthermore, is a potent enhancer of TGFbeta. Dynasore ameliorates atherosclerosis in the aortic endothelium of hypercholesterolemic ApoE-null mice by counteracting the suppressed TGFbeta responsiveness caused by the hypercholesterolemia, presumably acting through its effect on TGFbeta endocytosis and signaling in vascular cells.
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Affiliation(s)
- Chun-Lin Chen
- Department of Biochemistry, Saint Louis University School of Medicine, Doisy Research Center, St Louis, MO 63104, USA
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9
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Wei X, Wu L, Ling J, Liu L, Liu S, Liu W, Li M, Xiao Y. Differentially expressed protein profile of human dental pulp cells in the early process of odontoblast-like differentiation in vitro. J Endod 2009; 34:1077-84. [PMID: 18718369 DOI: 10.1016/j.joen.2008.06.014] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/19/2008] [Revised: 06/17/2008] [Accepted: 06/24/2008] [Indexed: 12/14/2022]
Abstract
Dental pulp cells (DPCs) are capable of differentiating into odontoblasts that secrete reparative dentin after pulp injury. The molecular mechanisms governing reparative dentinogenesis are yet to be fully understood. Here we investigated the differential protein profile of human DPCs undergoing odontogenic induction for 7 days. Using two-dimensional differential gel electrophoresis coupled with matrix-assisted laser adsorption ionization time of flight mass spectrometry, 23 protein spots related to the early odontogenic differentiation were identified. These proteins included cytoskeleton proteins, nuclear proteins, cell membrane-bound molecules, proteins involved in matrix synthesis, and metabolic enzymes. The expression of four identified proteins, which were heteronuclear ribonuclear proteins C, annexin VI, collagen type VI, and matrilin-2, was confirmed by Western blot and real-time real-time polymerase chain reaction analyses. This study generated a proteome reference map during odontoblast-like differentiation of human DPCs, which will be valuable to better understand the underlying molecular mechanisms in odontoblast-like differentiation.
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Affiliation(s)
- Xi Wei
- Department of Operative Dentistry and Endodontics, Guanghua School of Stomatology, Sun Yat-sen University, Guangzhou, Guangdong, China
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John H, Schulz S, Forssmann WG. Comparative in vitro degradation of the human hemorphin LVV-H7 in mammalian plasma analysed by capillary zone electrophoresis and mass spectrometry. Biopharm Drug Dispos 2007; 28:73-85. [PMID: 17173283 DOI: 10.1002/bdd.533] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
The human hemorphin LVV-H7 (L32VVYPWTQRF41) is a hemoglobin-beta, -gamma, -delta or -epsilon chain derived cationic decapeptide of the micro-opioid receptor binding family. It exhibits potential pharmacological value relevant, for example, for blood pressure regulation, learning performance and Alzheimer's disease. The regulatory potency is strictly dependent on the length of the amino acid sequence which is sensitive towards proteinases from tissues and plasma. To analyse LVV-H7 in vitro degradation in mammalian plasma, a novel multi-component quantitative capillary zone electrophoretic (CZE) procedure was applied, combined with qualitative metabolite profiling by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). In all types of plasma, LVV-H7 was N-terminally truncated generating four metabolites (M1-M4) with an intact C-terminus: M1 (V33VYPWTQRF41), M2 (V34YPWTQRF41), M3 (Y35PWTQRF41) and M4 (W37TQRF41). In EDTA plasma these degradation products were detected exclusively, whereas in citrate and heparin plasma four further metabolites appeared resulting from additional C-terminal cleavage of the dipeptide R40F41: M5 (L32VVYPWTQ39), M6 (V33VYPWTQ39), M7 (V34YPWTQ39) and M8 (Y35PWTQ39). In the presence of selective proteinase inhibitors aminopeptidase M and angiotensin-converting enzyme (for N- and C-terminal truncation, respectively) were identified as plasma enzymes responsible for hemorphin degradation. Furthermore, striking inter-mammalian species distinctions were detected revealing strongly differing degradation velocities but similar metabolite patterns.
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Affiliation(s)
- Harald John
- IPF PharmaCeuticals GmbH, Feodor-Lynen-Str. 31, Hannover, Germany.
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Campana WM, Li X, Dragojlovic N, Janes J, Gaultier A, Gonias SL. The low-density lipoprotein receptor-related protein is a pro-survival receptor in Schwann cells: possible implications in peripheral nerve injury. J Neurosci 2006; 26:11197-207. [PMID: 17065459 PMCID: PMC6674644 DOI: 10.1523/jneurosci.2709-06.2006] [Citation(s) in RCA: 79] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
Schwann cells undergo phenotypic modulation in peripheral nerve injury. In the adult rodent, Schwann cells are resistant to death-promoting challenges. The responsible receptors and signaling pathways are incompletely understood. In this study, we demonstrate that low-density lipoprotein receptor-related protein-1 (LRP-1) is expressed in adult sciatic nerve. After crush injury, LRP-1 is lost from the axoplasm and substantially upregulated in Schwann cells. Increased LRP-1 mRNA expression was observed locally at the injury site in multiple forms of sciatic nerve injury, including crush injury, chronic constriction injury, and axotomy. Endogenously produced tumor necrosis factor-alpha (TNF-alpha) was mostly responsible for the increase in LRP-1 expression; this activity was reproduced by direct injection of TNF-alpha into injured nerves in the TNF-alpha gene knock-out mouse. TNF receptor II was primarily involved. TNF-alpha also increased LRP-1 mRNA in Schwann cells in primary culture. Silencing of Schwann cell LRP-1 with siRNA decreased phosphorylated Akt and increased activated caspase-3. Equivalent changes in cell signaling were observed in LRP-1-deficient murine embryonic fibroblasts. Schwann cell death was induced in vitro by serum withdrawal or TNF-alpha, to a greater extent when LRP-1 was silenced. Schwann cell death was induced in vivo by injecting the LRP-1 antagonist, receptor-associated protein, into axotomy sites in adult rats. These results support a model in which LRP-1 functions as a pro-survival receptor in Schwann cells.
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Affiliation(s)
- W Marie Campana
- Department of Anesthesiology, University of California, San Diego School of Medicine, La Jolla, California 92093-0629, USA.
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Forsten-Williams K, Cassino TR, Delo LJ, Bellis AD, Robinson AS, Ryan TE. Enhanced insulin-like growth factor-I (IGF-I) cell association at reduced pH is dependent on IGF binding protein-3 (IGFBP-3) interaction. J Cell Physiol 2006; 210:298-308. [PMID: 17044083 DOI: 10.1002/jcp.20849] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
The cellular microenvironment impacts how signals are transduced by cells and plays a key role in tissue homeostasis. Although pH is generally well regulated, there are a number of situations where acidosis occurs and our work addresses how low pH impacts cell association of insulin-like growth factor-I (IGF-I) in the presence of IGF binding protein-3 (IGFBP-3). We have previously shown that IGF-I cell binding was enhanced in the presence of IGFBP-3 at low pH and now show that this binding is IGFBP-mediated as it is inhibited by Y60L-IGF-I, a mutant with reduced affinity for the IGF receptor (IGF-IR), and unaffected by insulin, which binds but not IGFBPs. Using surface plasmon resonance (SPR), we show that direct binding between IGF-I and IGFBP-3 is pH sensitive. Despite this, the key step in the process appears to be IGFBP-3 cell surface association as Long-R(3)-IGF-I, a mutant with reduced affinity for IGFBPs, shows a similar increase in cell association at pH 5.8 in the presence of IGFBP-3 but does not exhibit pH-dependent binding by SPR. Further, analysis indicates a large increase in low-affinity binding sites for IGF-I in the presence of IGFBP-3 and an elimination of IGF-I enhanced binding when a non-cell associating mutant of IGFBP-3 is added in place of IGFBP-3. That the IGFBP-3-mediated binding localizes IGF-I away from IGF-IR is suggested by triton-solubility testing and indicates additional complexities to IGF-I regulation by IGFBP-3. Identifying the pH-dependent binding partner(s) for IGFBP-3 is a necessary next step in deciphering this process.
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Affiliation(s)
- Kimberly Forsten-Williams
- Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA.
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