Specificity of serine proteinase serpin complex binding to very-low-density lipoprotein receptor and alpha(2)-macroglobulin receptor low-density-lipoprotein-receptor-related protein

Very-low-density lipoprotein receptor (VLDLR) and alpha(2)-macroglobulin receptor/low-density-lipoprotein-receptor-related protein (alpha(2)MR/LRP) are multifunctional endocytosis receptors of the low-density lipoprotein receptor family. Both have been shown to mediate endocytosis and degradation of complex between plasminogen activators and type-1 plasminogen-activator inhibitor (PAI-1) by cultured cells. We have now studied the specificity of binding and endocytosis by VLDLR and alpha(2)MR/LRP among a variety of serine proteinase/serpin complexes, including various combinations of the serine proteinases urokinase-type and tissue-type plasminogen activators, plasmin, thrombin, human leukocyte elastase, cathepsin G, and plasma kallikrein with the serpins PAI-1, horse leukocyte elastase inhibitor, protein C inhibitor, C1-inhibitor, alpha(2)-antiplasmin, alpha(1)-proteinase inhibitor, alpha(1)-antichymotrypsin, protease nexin-1, heparin cofactor II. and antithrombin III. Binding was estimated with radiolabelled ligands in ligand blotting analysis and microtiter well assays. Endocytosis was estimated by measuring receptor-associated protein (RAP)-sensitive degradation of radiolabelled complexes by Chinese hamster ovary cells transfected with VLDLR cDNA and by COS-1 cells, which have a high endogenous expression of alpha(2)MR/LRP. We found that the receptors bind with high affinity to some, but not all, combinations of plasminogen activators and thrombin with PAI-1, protease nexin-1, protein C inhibitor, and antithrombin III, while complexes of many serine proteinases with their primary inhibitor, i.e. plasmin/alpha(2)-antiplasmin complex, do not bind, or bind with a very low affinity. Both the serine proteinase and the serpin moieties contribute to the binding specificity. The binding specificities of VLDLR and alpha(2)MR/LRP are overlapping, but not identical. The results suggest that VLDLR and alpha(2)MR/LRP have different biological functions by having different binding specificities as well as by being expressed by different cell types.