C1 INHIBITOR-C(1)OVER-BAR-S COMPLEXES ARE INTERNALIZED AND DEGRADED BY THE LOW-DENSITY-LIPOPROTEIN RECEPTOR-RELATED PROTEIN

Like other serpin-enzyme complexes (SECs), proteinase-complexed C1 inh ibitor (C1-INH) is rapidly cleared from the circulation and thought to be a neutrophil chemoattractant, suggesting that complex formation ca uses structural rearrangements exposing a domain which is recognized b y specific cell surface receptors. However, the cellular receptor(s) r esponsible for the catabolism and potential mediation of chemotaxis by C1-INH-protease complexes remained obscure, To determine whether the SEC receptor mediates the binding and potential chemotaxis of C1-INH . C (1) over bar s, we performed binding assays with HepG2 cells, neutr ophils, and monocytes, and the results show that C1-INH . C (1) over b ar s neither bind to these cells nor cause a chemotactic response of n eutrophils and monocytes, Furthermore, C1-INH . C (1) over bar s, the COOH-terminal C1 inhibitor peptide, or the tetrameric C1-INH . C (1) o ver bar s . C (1) over bar r . C1-INH complex were found to be signifi cantly less effective in competing with the SEC receptor ligand I-125- peptide 105Y for the binding to HepG2; cells than unlabeled 105Y, indi cating that the SEC receptor does not sufficiently recognize C1-INH-pr oteaae complexes. The asialoglycoprotein receptor was also ruled out t o be responsible for the removal of the heavily glycosylated C1-INH . C (1) over bar s complex, since asialoorosomucoid did not compete for the clearance of C1-INH .I-125-C (1) over bar s and asialoglycoprotein receptor knockout mice showed no alterations in the C1-INH .I-125-C ( 1) over bar s clearance rate. We found that C1-INH .I-125-C (1) over b ar s 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 bi nding of all known ligands to LRP. Moreover, receptor-associated prote in inhibited the in vivo clearance of C1-INH .I-125-C (1) over bar s . ,and murine fibroblasts genetically deficient for LRP did not degrade C1-INH .I-125-C (1) over bar s. Our results demonstrate that C1-INH . C (1) over bar s complexes do not stimulate neutrophil or monocytic ch emotaxis but are removed by LRP, further underscoring its role as a se rpin-enzyme complex clearance receptor.