The LDL receptor and LRP are receptors for beta VLDL on pigeon monocyte-derived macrophages

Abstract

Receptors for the lipoprotein, beta very low density lipoprotein (beta VLDL), have been identified through the binding of beta VLDL-gold conjugates on two ligand-induced regions of pigeon monocyte-derived macrophages. These regions were microvilli/retraction fibers and membrane ruffles. The present study investigated the location and identity of beta VLDL receptors using an antiserum directed against the epidermal growth factor (EGF) precursor region of the human low density lipoprotein (LDL) receptor. The anti-receptor serum recognized two membrane proteins from pigeon monocyte-derived macrophages, a 116 kDa (LDL receptor) protein and a 600 kDa (low density lipoprotein receptor-related protein; LRP) protein. Ligand blot analysis demonstrated that pigeon beta VLDL bound to both the LDL receptor and LRP. Immuno-gold electron microscopy using the anti-receptor serum resulted in immunoglobulin localization on the same two ligand-induced regions, microvilli/retraction fibers and membrane ruffles, to which the ligand had bound. Furthermore, simultaneous immunogold localization of the lipoprotein receptor antigens and beta VLDL-gold (ligand) binding substantiated co-localization of the receptor antigens and beta VLDL on the ligand-induced regions. Cross-competition studies with the anti-receptor serum and beta VLDL-gold conjugate documented that increasing concentration of the anti-receptor serum resulted in 70% inhibition of beta VLDL-gold conjugate binding. These data suggest that pigeon monocyte-derived macrophages utilize both the LDL receptor and LRP as receptors for pigeon beta VLDL.