Metabolism of Activated Complement Component C3 Is Mediated by the Low Density Lipoprotein Receptor-related Protein/α2-Macroglobulin Receptor

Complement component 3 (C3) and α 2 -macroglobulin evolved from a common, evolutionarily old, ancestor gene. Low density lipoprotein-receptor-related protein/α 2 -macroglobulin receptor (LRP/α 2 MR), a member of the low density lipoprotein receptor family, is responsible for the clearance of α 2 -macroglobulin-protease complexes. In this study, we examined whether C3 has conserved affinity for LRP/α 2 MR. Ligand blot experiments with human 125 I-C3 on endosomal proteins show binding to a 600-kDa protein, indistinguishable from LRP/α 2 MR by the following criteria: it is competed by receptor-associated protein (the 39-kDa receptor-associated protein that impairs binding of all ligands to LRP/α 2 MR) and by lactoferrin and Pseudomonas exotoxin, other well known ligands of the multifunctional receptor. Binding of C3 is sensitive to reduction of the receptor and is Ca 2+ -dependent. All these features are typical for cysteine-rich binding repeats of the low density lipoprotein receptor family. In LRP/α 2 MR, they are found in four cassettes (2, 8, 10, and 11 repeats). Ligand blotting to chicken LR8 demonstrates that a single 8-fold repeat is sufficient for binding. Confocal microscopy visualizes initial surface labeling of human fibroblasts incubated with fluorescent labeled C3, which changes after 5 min to an intracellular vesicular staining pattern that is abolished in the presence of receptor-associated protein. Cell uptake is abolished in mouse fibroblasts deficient in LRP/α 2 MR. Native plasma C3 is not internalized. We demonstrate that the capacity to internalize C3 is saturable and exhibits a K D value of 17 n m . After intravenous injection, rat hepatocytes accumulate C3 in sedimentable vesicles with a density typical for endosomes. In conclusion, our ligand blot and uptake studies demonstrate the competence of the LRP/α 2 MR to bind and endocytose C3 and provide evidence for an LRP/α 2 MR-mediated system participating in C3 metabolism.