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.

Inhibition instead of enhancement of lipid peroxidation by pretreatment with the carcinogenic peroxisome proliferator nafenopin in rat liver exposed to a high single dose of corn oil.

Oxidative stress is discussed as a possible hepatocarcinogenic mechanism of peroxisome proliferators (PP) in rodents and is suggested to result from the induction of peroxisomal beta-oxidation (PBOX) by PP. The induced PBOX is assumed to produce excessive H2O2 from the degradation of fatty acids, ultimately leading to oxidative stress and lipid peroxidation. In the present short term-study, we attempted to stimulate lipid peroxidation in male Wistar rats by (1) inducing PBOX enzymes with the peroxisome proliferator nafenopin at 90 mg/kg body weight per day in the diet for 10-11 days, and (2) by supplying the induced PBOX with an abundant amount of fatty acid as substrate, using a corn oil gavage at 20 ml/kg body weight. The corn-oil gavage alone, i.e. without preceding nafenopin treatment, enhanced liver triacylglycerol nine- to tenfold and hepatic lipid peroxidation, measured as thiobarbituric acid reactive substances (TBARS), was increased 50% compared with controls. Both observations were made after 18 h when the peak elevations occurred. Upon pretreatment with nafenopin, associated with a sevenfold induction of PBOX, the corn oil gavage however caused only a threefold maximal increase in hepatic triacylglycerol, also at the 18 h time-point; TBARS remained almost at control levels, as monitored at seven time points over 24-25 h. These results suggest that nafenopin reduces rather than enhances lipid peroxidation, despite the provision, in a short term study, of high doses of substrate to the induced enzyme system that is hypothetically causing oxidative stress in the liver.