Endocytosis of the recombinant tissue plasminogen activator alteplase by hepatic endothelial cells

The glycoprotein tissue-type plasminogen activator (t-PA, alteplase, CAS 105857-23-6) is a serine protease consisting of 527 amino acids and can activate plasminogen to plasmin, which subsequently dissolves the fibrin network of a thrombus. This activation occurs selectively on the thrombus, making recombinant t-PA a very effective agent in the treatment of thromboembolic disorders. t-PA has a short in vivo half-life and is rapidly removed from the circulation by the liver. The catabolism of t-PA involves receptor-mediated endocytosis and intracellular degradation in several cell types of the liver namely hepatic endothelial, parenchymal and Kupffer cells. Liver endothelial cells have been reported to possess a specific uptake system for t-PA based on the recognition of the high mannose carbohydrate structures on Asn117. To further elucidate the involvement of the mannose receptor on sinusoidal endothelial cells in the hepatic catabolism of t-PA and to identify the mechanisms involved, biochemical as well as electron microscopic studies were performed. The biochemical studies revealed that the removal of the mannose side chain in t-PA significantly reduced its clearance and degradation in isolated perfused livers. The binding of t-PA to preparations of primary hepatocytes and liver cell membranes could not be competed for by various sugars and glycoproteins, and was not dependent on the presence of carbohydrates on the molecule. This ruled out a major relevance of the sugar moieties of t-PA in its recognition by liver cells that were not of endothelial origin.(ABSTRACT TRUNCATED AT 250 WORDS)