BLOCKADE OF THE ALPHA(2)-MACROGLOBULIN RECEPTOR LOW-DENSITY-LIPOPROTEIN-RECEPTOR-RELATED PROTEIN ON RAT-LIVER PARENCHYMAL-CELLS BY THE 39-KDA RECEPTOR-ASSOCIATED PROTEIN LEAVES THE INTERACTION OF BETA-MIGRATING VERY-LOW-DENSITY LIPOPROTEIN WITH THE LIPOPROTEIN REMNANT RECEPTOR UNAFFECTED/

The nature of the liver binding site which is responsible for the init ial recognition and clearance of chylomicron-remnants and beta-migrati ng very-low-density lipoprotein (beta-VLDL) is under active dispute. W e have investigated the effect of the 39-kDa receptor-associated prote in (RAP) on the recognition site for activated alpha(2)-macroglobulin and beta-VLDL on rat liver parenchymal cells in vivo and in vitro in o rder to analyze whether both substrates are recognized and internalize d by the same receptor system. Radiolabelled trypsin-activated alpha(2 )-macroglobulin (alpha(2)M-T) was cleared rapidly by the liver (maxima l uptake of 80.8 +/- 1.0 % of the injected dose). Prior injection of 5 , 15, or 50 mg gluthathione-S-transferase-linked RAP (GST-RAP)/kg rat reduced the liver uptake to 62.2 +/- 2.3 %, 59.3 +/- 1.1 %, or 2.9 +/- 0.1 % of the injected dose, respectively. Concurrently the serum deca y was strongly delayed after injection of 50 mg GST-RAP/kg rat but thi s did not affect the serum decay and liver uptake of I-125-beta-VLDL. Binding studies with isolated liver parenchymal cells in vitro demonst rated that the binding of I-125-alpha(2)M-T was 98 % inhibited by GST- RAP with an IC50 of 0.3 mu g/ml (4.2 nM), whereas the binding of I-125 -beta-VLDL and I-125-beta-VLDL + recombinant apolipoprotein E (rec-apo E) was unaffected by GST-RAP up to 50 mu g/ml (700 nM). Also, the cell association and degradation of alpha(2)M-T was blocked by RAP, while the association and degradation of beta-VLDL and beta-VLDL + rec-apoE were not influenced. The inhibitory effect of RAP on the cell associat ion and degradation of alpha(2)M-T lasted for 1-2 h of incubation at 3 7 degrees C. The binding of the radioiodinated RAP to isolated liver p arenchymal cells was highly efficiently coupled to lysosomal degradati on. Upon in vivo injection into rats, I-125-labeled RAP is rapidly cle ared from the serum and taken up by the liver, which is also coupled t o efficient degradation. Since RAP blocks binding of all known ligands to the alpha(2)-macroglobulin receptor/low-density lipoprotein recept or-related protein (the alpha(2)Mr/LRP) and at high concentrations the binding to the LDL receptor, we conclude that the initial binding and internalization of beta-VLDL by rat liver parenchymal cells is not me diated by the alpha(2)Mr/LRP. The properties of binding of beta-VLDL t o rat liver parenchymal cells points to an apoE-specific recognition s ite for lipoprotein remnants which differs from the alpha(2)Mr/LRP, pr oteoglycans and the LDL receptor and is tentatively called the lipopro tein remnant receptor.