LOW-DENSITY-LIPOPROTEIN RECEPTOR-RELATED PROTEIN ALPHA(2)-MACROGLOBULIN RECEPTOR ON MURINE PERITONEAL-MACROPHAGES MEDIATES THE BINDING AND CATABOLISM OF LOW-DENSITY-LIPOPROTEIN

Low-density lipoprotein receptor-related protein (LRP)/alpha(2)-macrog lobulin receptor is a member of the low-density lipoprotein receptor f amily. It is known. to bind a wide variety of unrelated ligands includ ing alpha(2)-macroglobulin-proteinase complexes, tissue plasminogen ac tivator, apolipoprotein E-enriched very low density lipoprotein, lipop rotein lipase, and Pseudomonas exotoxin A. Receptor-associated protein (RAP), a protein which copurifies with LRP, can inhibit the binding a nd internalization of all known ligands to LRP. Recent studies have sh own that some ligands can bind to more than one receptor in this famil y. However, the ability of low-density lipoprotein (LDL) to bind to LR P in addition to the LDL receptor has not been demonstrated consistent ly. In this study we demonstrate that LDL binds with high affinity to macrophage cell surface receptors at 4 degrees C (K-d = 1.8 nM) and co mpetes for the binding of a receptor-recognized form of alpha(2)-macro globulin (alpha(2)M) (K-i = 3 nM). (alpha(2)M* and RAP can inhibit th e binding of LDL to macrophages completely (96 and 100% inhibition, re spectively), after cell surface heparin has been removed by treatment with heparinase. Using a solid-phase assay, we show that LDL binds spe cifically, saturably, and with high affinity to purified LRP (K-d = 5 nM). LDL can also completely inhibit the binding of alpha(2)M to puri fied LRP. These results indicate that LDL binds directly to LRP. The a bility of LDL to cross compete with (alpha(2)M for binding to LRP sug gests that LDL binds to a similar or overlapping site as alpha(2)M. I n addition, the ability of alpha(2)M to inhibit most of the receptor- mediated binding of LDL to macrophages suggests that LDL receptors on murine peritoneal macrophages are predominantly LRP. (C) 1996 Academic Press, Inc.