A comparison of the roles of the low density lipoprotein (LDL) receptor and the LDL receptor-related protein/alpha 2-macroglobulin receptor in chylomicron remnant removal in the mouse in vivo

Abstract

Two cell surface molecules, the low density lipoprotein (LDL) receptor and the LDL receptor-related protein (LRP)/alpha 2-macroglobulin receptor, have been found to have a role in the removal of apoE-rich lipoproteins. To further study this process and to assess the relative contributions of these proteins to the removal of chylomicron remnants, we used an anti-LDL receptor antibody, activated alpha 2-macroglobulin and the 39-kDa receptor-associated protein (RAP) as inhibitors of chylomicron remnant removal in intact mice. The advantage of this study is that we were able to use the same litters of animals and batches of lipoprotein for the experiment. In cultured Chinese hamster ovary cells, the anti-LDL receptor antibody inhibited remnant uptake and degradation about 80% as well as did unlabeled remnants. It did not affect activated alpha 2-macroglobulin uptake or degradation. Activated alpha 2-macroglobulin did not inhibit remnant uptake in normal cells but is reported to block uptake in cells that lack LDL receptors. Chylomicron remnants blocked activated alpha 2-macroglobulin uptake as effectively as unlabeled activated alpha 2-macroglobulin. This confirmed that the two ligands are cross-competitors but suggests that the LDL receptor is the primary mediator of remnant uptake in cultured cells that express the LDL receptor. In vivo, pretreatment with the anti-LDL receptor antibody decreased remnant uptake 5 min after injection by one-third. The antibody did not affect the removal of activated alpha 2-macroglobulin. Activated alpha 2-macroglobulin had a small, but reproducible effect on remnant removal, decreasing it by about 7% at 5 min. Injection of chylomicron remnants affected activated alpha 2-macroglobulin removal slightly. A similar pattern, but somewhat greater effect was seen on the hepatic uptake of the ligands. Anti-LDL receptor antibody reduced chylomicron remnant uptake by about half, and activated alpha 2-macroglobulin reduced it by about 15%. Studies with RAP provided results generally similar to those with activated alpha 2-macroglobulin, although RAP appears to bind to a site not recognized by either remnants or activated alpha 2-macroglobulin in addition to sharing a site with these ligands. Together, these results add support for the hypothesis that, although both receptors can play a role in chylomicron remnant removal, in the normal mouse in vivo the LDL receptor plays a substantially greater role, making the role of the LRP difficult to appreciate. The same is true in cells that express LDL receptors.(ABSTRACT TRUNCATED AT 400 WORDS)