Metabolism of activated complement component C3 is mediated by the low density lipoprotein receptor-related protein/alpha(2)-macroglobulin receptor.
Complement component 3 (C3) and alpha(2)-macroglobulin evolved from a common, evolutionarily old, ancestor gene. Low density lipoprotein-receptor-related protein/alpha(2)-macroglobulin receptor (LRP/alpha(2)MR), a member of the low density lipoprotein receptor family, is responsible for the clearance of alpha(2)-macroglobulin-protease complexes. In this study, we examined whether C3 has conserved affinity for LRP/alpha(2)MR. Ligand blot experiments with human (125)I-C3 on endosomal proteins show binding to a 600-kDa protein, indistinguishable from LRP/alpha(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/alpha(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/alpha(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/alpha(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 nM. 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/alpha(2)MR to bind and endocytose C3 and provide evidence for an LRP/alpha(2)MR-mediated system participating in C3 metabolism.
Meilinger
M
M
Department of Medical Chemistry, University of Vienna, Währingerstrasse 10, A-1090 Vienna, Austria.
Gschwentner
C
C
Burger
I
I
Haumer
M
M
Wahrmann
M
M
Szollar
L
L
Nimpf
J
J
Huettinger
M
M
D000818:Animals; D003167:Complement Activation; D003176:Complement C3 / Q000008:administration & dosage / Q000378:metabolism*; D004705:Endocytosis; D006801:Humans; D007700:Kinetics; D008099:Liver / Q000378:metabolism; D026503:Low Density Lipoprotein Receptor-Related Protein-1; D051379:Mice; D011485:Protein Binding; D051381:Rats; D011971:Receptors, Immunologic / Q000378:metabolism / Q000502:physiology*; D011973:Receptors, LDL / Q000378:metabolism*; D000511:alpha-Macroglobulins / Q000378:metabolism*
D003176:Complement C3; D026503:Low Density Lipoprotein Receptor-Related Protein-1; D011971:Receptors, Immunologic; D011973:Receptors, LDL; D000511:alpha-Macroglobulins