Low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor is an hepatic receptor for tissue-type plasminogen activator.
Tissue-type plasminogen activator (t-PA), a serine protease that catalyzes the initial and rate-limiting step in the fibrinolytic cascade, is cleared rapidly in vivo by the liver. Using chemical crosslinking, we have recently identified a plasminogen-activator inhibitor type 1 (PAI-1)-independent t-PA clearance receptor on rat hepatoma MH1C1 cells with a relative molecular mass of approximately 500 kDa. Another recently identified membrane receptor, low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor (LRP/alpha 2MR), was also detected on MH1C1 hepatoma cells by using immunoprecipitation with anti-LRP/alpha 2MR antibody. When analyzed by SDS/PAGE, we found the t-PA receptor identified on MH1C1 cells comigrated with the large subunit of LRP/alpha 2MR. The t-PA receptor was immunoprecipitated by an anti-LRP/alpha 2MR antibody after chemical crosslinking of specifically bound 125I-labeled t-PA to its receptor. Through chemical crosslinking studies, we found that t-PA and methylamine-activated alpha 2-macroglobulin could bind to LRP/alpha 2MR simultaneously without competing with one another for binding, suggesting that the two ligands bound to two independent sites on the LRP/alpha 2MR molecule. Furthermore, a 39-kDa protein, which modulates ligand binding to LRP/alpha 2MR, was also found to inhibit t-PA binding to its receptor. These data thus show that the t-PA clearance receptor identified on MH1C1 hepatoma cells is LRP/alpha 2MR.
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Edward Mallinckrodt Department of Pediatrics, Washington University School of Medicine, St. Louis, MO 63110.
Williams
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Strickland
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Schwartz
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D000818:Animals; D000906:Antibodies; D003429:Cross Reactions; D003432:Cross-Linking Reagents; D007700:Kinetics; D008024:Ligands; D008114:Liver Neoplasms, Experimental / Q000378:metabolism*; D026503:Low Density Lipoprotein Receptor-Related Protein-1; D051381:Rats; D011971:Receptors, Immunologic / Q000302:isolation & purification / Q000378:metabolism*; D013388:Succinimides; D010959:Tissue Plasminogen Activator / Q000302:isolation & purification / Q000378:metabolism*; D014407:Tumor Cells, Cultured
D000906:Antibodies; D003432:Cross-Linking Reagents; D008024:Ligands; D026503:Low Density Lipoprotein Receptor-Related Protein-1; D011971:Receptors, Immunologic; D013388:Succinimides; D010959:Tissue Plasminogen Activator; C011240:dithiobis(succinimidylpropionate)