C1 inhibitor-C1s complexes are internalized and degraded by the low density lipoprotein receptor-related protein

Like other serpin-enzyme complexes (SECs), proteinase-complexed C1 inhibitor (C1-INH) is rapidly cleared from the circulation and thought to be a neutrophil chemoattractant, suggesting that complex formation causes structural rearrangements exposing a domain which is recognized by specific cell surface receptors. However, the cellular receptor(s) responsible for the catabolism and potential mediation of chemotaxis by C1-INH-protease complexes remained obscure. To determine whether the SEC receptor mediates the binding end potential chemotaxis of C1-INH-C1s, we performed binding assays with HepG2 cells, neutrophils, and monocytes, end the results show that C1-INH- C1s neither bind to these cells nor cause a chemotactic response of neutrophils and monocytes. Furthermore, C1-INH-C1s, the COOH-terminal C1 inhibitor peptide, or the tetrameric C1-INH-C1s-C1r-C1-INH complex were found to be significantly less effective in competing with the SEC receptor ligand ¹²⁵I-peptide 105Y for the binding to HepG2 cells then unlabeled 105Y, indicating that the SEC receptor does not sufficiently recognize C1-INH- protease complexes. The asialoglycoprotein receptor was also ruled out to be responsible for the removal of the heavily glycosylated C1-INH-C1s complex, since asialoorosomucold did not compete for the clearance of C1-INH-¹²⁵I- C1s and asialoglycoprotein receptor knockout mice showed no alterations in the C1-INH·¹²⁵I-C1s clearance rate. We found that C1-INH·¹²⁵-C1s complexes were efficiently degraded by normal murine fibroblasts expressing the low density lipoprotein receptor-related protein (LRP) and cellular degradation was significantly reduced by chloroquine and the receptor- associated protein, which is a potent inhibitor of the binding of all known ligands to LRP. Moreover, receptor-associated protein inhibited the in vivo clearance of C1-INH·¹²⁵-C1s and murine fibroblasts genetically deficient for LRP did not degrade C1-INH·¹²⁵I-C1s. Our results demonstrate that C1- INH·C1s complexes do not stimulate neutrophil or monocytic chemotaxis but are removed by LRP, further underscoring its role as a serpin-enzyme complex clearance receptor.