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 initial recognition and clearance of chylomicron-remnants and beta-migrating very-low-density lipoprotein (beta-VLDL) is under active dispute. We have investigated the effect of the 39-kDa receptor-associated protein (RAP) on the recognition site for activated alpha(2)-macroglobulin and beta-VLDL on rat liver parenchymal cells in vivo and in vitro in order to analyze whether both substrates are recognized and internalized by the same receptor system. Radiolabelled trypsin-activated alpha(2)-macroglobulin (alpha(2)M-T) was cleared rapidly by the liver (maximal 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 decay was strongly delayed after injection of 50 mg GST-RAP/kg rat but this did not affect the serum decay and liver uptake of I-125-beta-VLDL. Binding studies with isolated liver parenchymal cells in vitro demonstrated 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-apoE) 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 association and degradation of alpha(2)M-T lasted for 1-2 h of incubation at 37 degrees C. The binding of the radioiodinated RAP to isolated liver parenchymal cells was highly efficiently coupled to lysosomal degradation. Upon in vivo injection into rats, I-125-labeled RAP is rapidly cleared from the serum and taken up by the liver, which is also coupled to efficient degradation. Since RAP blocks binding of all known ligands to the alpha(2)-macroglobulin receptor/low-density lipoprotein receptor-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 mediated by the alpha(2)Mr/LRP. The properties of binding of beta-VLDL to rat liver parenchymal cells points to an apoE-specific recognition site for lipoprotein remnants which differs from the alpha(2)Mr/LRP, proteoglycans and the LDL receptor and is tentatively called the lipoprotein remnant receptor.