Low-Density Lipoprotein Receptor-Related Protein/ α2-Macroglobulin Receptor on Murine Peritoneal Macrophages Mediates the Binding and Catabolism of Low-Density Lipoprotein

Low-density lipoprotein receptor-related protein (LRP)/α2-macroglobulin receptor is a member of the low-density lipoprotein receptor family. It is known to bind a wide variety of unrelated ligands including α2-macroglobulin–proteinase complexes, tissue plasminogen activator, apolipoprotein E-enriched very low density lipoprotein, lipoprotein lipase, andPseudomonasexotoxin A. Receptor-associated protein (RAP), a protein which copurifies with LRP, can inhibit the binding and internalization of all known ligands to LRP. Recent studies have shown that some ligands can bind to more than one receptor in this family. However, the ability of low-density lipoprotein (LDL) to bind to LRP in addition to the LDL receptor has not been demonstrated consistently. In this study we demonstrate that LDL binds with high affinity to macrophage cell surface receptors at 4°C (Kd= 1.8 nM) and competes for the binding of a receptor-recognized form of α2-macroglobulin (α2M*) (Ki= 3 nM). α2M* and RAP can inhibit the binding of LDL to macrophages completely (96 and 100% inhibition, respectively), after cell surface heparin has been removed by treatment with heparinase. Using a solid-phase assay, we show that LDL binds specifically, saturably, and with high affinity to purified LRP (Kd= 5 nM). LDL can also completely inhibit the binding of α2M* to purified LRP. These results indicate that LDL binds directly to LRP. The ability of LDL to cross-compete with α2M* for binding to LRP suggests that LDL binds to a similar or overlapping site as α2M*. In addition, the ability of α2M* to inhibit most of the receptor-mediated binding of LDL to macrophages suggests that LDL receptors on murine peritoneal macrophages are predominantly LRP.