Low density lipoprotein receptor-related protein/α₂-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 ≈500 kDa. Another recently identified membrane receptor, low density lipoprotein receptor-related protein/α₂-macroglobulin receptor (LRP/α₂MR), was also detected on MH₁C₁ hepatoma cells by using immunoprecipitation with anti-LRP/α₂MR antibody. When analyzed by SDS/PAGE, we found the t-PA receptor identified on MH₁C₁ cells comigrated with the large subunit of LRP/α₂MR. The t-PA receptor was immunoprecipitated by an anti-LRP/α₂MR 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 α₂-macroglobulin could bind to LRP/α₂MR simultaneously without competing with one another for binding, suggesting that the two ligands bound to two independent sites on the LRP/α₂MR molecule. Furthermore, a 39-kDa protein, which modulates ligand binding to LRP/α₂MR, 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/α₂MR.