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 almost-equal-to 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 I-125-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.