THE LDL RECEPTOR AND LRP ARE RECEPTORS FOR BETA-VLDL ON PIGEON MONOCYTE-DERIVED MACROPHAGES

Receptors for the lipoprotein, beta very low density lipoprotein (beta VLDL), have been identified through the binding of beta VLDL-gold con jugates on two ligand-induced regions of pigeon monocyte-derived macro phages. These regions were microvilli/retraction fibers and membrane r uffles. The present study investigated the location and identity of be ta VLDL receptors using an antiserum directed against the epidermal gr owth factor (EGF) precursor region of the human low density lipoprotei n (LDL) receptor. The anti-receptor serum recognized two membrane prot eins from pigeon monocyte-derived macrophages, a 116 kDa (LDL receptor ) protein and a 600 kDa (low density lipoprotein receptor-related prot ein; LRP) protein. Ligand blot analysis demonstrated that pigeon beta VLDL bound to both the LDL receptor and LRP. Immune-gold electron micr oscopy using the antireceptor serum resulted in immunoglobulin localiz ation on the same two ligand-induced regions, microvilli/retraction fi bers and membrane ruffles, to which the ligand had bound. Furthermore, simultaneous immunogold localization of the lipoprotein receptor anti gens 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 antireceptor serum and beta VLDL-g old conjugates documented that increasing concentration of the anti-re ceptor serum resulted in 70% inhibition of beta VLDL-gold conjugate bi nding. These data suggest that pigeon monocyte-derived macrophages uti lize both the LDL receptor and LRP as receptors for pigeon beta VLDL.