Low density lipoprotein receptor-related protein and gp330 bind similar ligands, including plasminogen activator-inhibitor complexes and lactoferrin, an inhibitor of chylomicron remnant clearance

The low density lipoprotein receptor-related protein/ α₂-macroglobulin receptor (LRP) and gp330, two members of the low density lipoprotein receptor gene family, share a multitude of cysteine-rich repeats. LRP has been shown to act as an endocytosis-mediating receptor for several ligands, including protease-antiprotease complexes and plasma lipoproteins. The former include α₂-macroglobulin-protease complexes and plasminogen activator inhibitor-activator complexes. The latter include chylomicron remnant-like particles designated β-very low density lipoproteins (β-VLDL) complexed with apoprotein E or lipoprotein lipase. The binding specificity of gp330 is unknown. In the current studies we show that gp330 from rat kidney membranes binds several of these ligands on nitrocellulose blots. We also show that both LRP and gp330 bind an additional ligand, bovine lactoferrin, which is known to inhibit the hepatic clearance of chylomicron remnants. Lactoferrin blocked the LRP-dependent stimulation of cholesteryl ester synthesis in cultured human fibroblasts elicited by apoprotein E-β-VLDL or lipoprotein lipase-β-VLDL complexes. Cross-competition experiments in fibroblasts showed that the multiple ligands recognize at least three distinct, but partially overlapping sites on the LRP molecule. Binding of all ligands to LRP and gp330 was inhibited by the 39-kDa protein, which co-purifies with the two receptors, suggesting that the 39-kDa protein is a universal regulator of ligand binding to both receptors. The correlation of the inhibitory effects of lactoferrin in vivo and in vitro support the notion that LRP functions as a chylomicron remnant receptor in liver. LRP and gp330 share a multiplicity of binding sites, and both may function as endocytosis-mediating receptors for a large number of ligands in different organs.