Abstract
In addition to functioning as an activator of fibrinolysis, tissue-type plasminogen activator (tPA) interacts with neurons and regulates multiple aspects of neuronal cell physiology. In this study, we examined the mechanism by which tPA initiates cell signaling in PC12 and N2a neuron-like cells. We demonstrate that enzymatically active and inactive tPA (EI-tPA) activate ERK1/2 in a biphasic manner. Rapid ERK1/2 activation is dependent on LDL receptor-related protein-1 (LRP1). In the second phase, ERK1/2 is activated by tPA independently of LRP1. The length of the LRP1-dependent phase varied inversely with the tPA concentration. Rapid ERK1/2 activation in response to EI-tPA and activated α2-macroglobulin (α2M*) required the NMDA receptor and Trk receptors, which assemble with LRP1 into a single pathway. Assembly of this signaling system may have been facilitated by the bifunctional adapter protein, PSD-95, which associated with LRP1 selectively in cells treated with EI-tPA or α2M*. Myelin-associated glycoprotein binds to LRP1 with high affinity but failed to induce phosphorylation of TrkA or ERK1/2. Instead, myelin-associated glycoprotein recruited p75 neurotrophin receptor (p75NTR) into a complex with LRP1 and activated RhoA. p75NTR was not recruited by other LRP1 ligands, including EI-tPA and α2M*. Lactoferrin functioned as an LRP1 signaling antagonist, inhibiting Trk receptor phosphorylation and ERK1/2 activation in response to EI-tPA. These results demonstrate that LRP1-initiated cell signaling is ligand-dependent. Proteins that activate cell signaling by binding to LRP1 assemble different co-receptor systems. Ligand-specific co-receptor recruitment provides a mechanism by which one receptor, LRP1, may trigger different signaling responses.
Keywords:
Cell Signaling; ERK; Lipoprotein-like Receptor (LRP); Protease Inhibitor; Tissue-type Plasminogen Activator (tPA).
Publication types
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Research Support, N.I.H., Extramural
MeSH terms
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Animals
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Disks Large Homolog 4 Protein
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Guanylate Kinases / genetics
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Guanylate Kinases / metabolism
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Humans
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Intracellular Signaling Peptides and Proteins / genetics
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Intracellular Signaling Peptides and Proteins / metabolism
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Lactoferrin / genetics
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Lactoferrin / metabolism
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Low Density Lipoprotein Receptor-Related Protein-1 / genetics
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Low Density Lipoprotein Receptor-Related Protein-1 / metabolism*
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MAP Kinase Signaling System / physiology*
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Membrane Proteins / genetics
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Membrane Proteins / metabolism
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Mice
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Mitogen-Activated Protein Kinase 1 / genetics
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Mitogen-Activated Protein Kinase 1 / metabolism
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Mitogen-Activated Protein Kinase 3 / genetics
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Mitogen-Activated Protein Kinase 3 / metabolism
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Myelin-Associated Glycoprotein / genetics
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Myelin-Associated Glycoprotein / metabolism*
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Nerve Tissue Proteins / genetics
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Nerve Tissue Proteins / metabolism
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PC12 Cells
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Rats
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Receptors, Growth Factor
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Receptors, LDL / genetics
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Receptors, LDL / metabolism*
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Receptors, Nerve Growth Factor / genetics
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Receptors, Nerve Growth Factor / metabolism
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Tissue Plasminogen Activator / genetics
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Tissue Plasminogen Activator / metabolism*
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Tumor Suppressor Proteins / genetics
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Tumor Suppressor Proteins / metabolism*
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alpha-Macroglobulins / genetics
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alpha-Macroglobulins / metabolism
Substances
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DLG4 protein, human
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Disks Large Homolog 4 Protein
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Dlg4 protein, mouse
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Dlg4 protein, rat
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Intracellular Signaling Peptides and Proteins
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LRP1 protein, human
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Low Density Lipoprotein Receptor-Related Protein-1
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Lrp1 protein, mouse
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Ltf protein, mouse
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MAG protein, human
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Mag protein, mouse
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Membrane Proteins
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Myelin-Associated Glycoprotein
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NGFR protein, human
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Nerve Tissue Proteins
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Receptors, Growth Factor
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Receptors, LDL
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Receptors, Nerve Growth Factor
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Ngfr protein, mouse
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Tumor Suppressor Proteins
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alpha-Macroglobulins
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Ngfr protein, rat
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MAPK1 protein, human
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Mitogen-Activated Protein Kinase 1
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Mitogen-Activated Protein Kinase 3
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Guanylate Kinases
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Lactoferrin
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Tissue Plasminogen Activator