LIGAND SELECTIVITY OF 105-KDA AND 130-KDA LIPOPROTEIN-BINDING PROTEINS IN VASCULAR-SMOOTH-MUSCLE-CELL MEMBRANES IS UNIQUE

Using ligand blotting techniques, with low-density lipoprotein (LDL) a s ligand, we have previously described the existence of atypical lipop rotein-binding proteins (105 kDa and 130 kDa) in membranes from human aortic medial tissue. The present study demonstrates that these protei ns are also present in membranes from cultured human (aortic and mesen teric) and rat (aortic) vascular smooth-muscle cells (VSMCs). To asses s the relationship of 105 and 130 kDa lipoprotein-binding proteins to known lipoprotein receptors, ligand binding specificity was studied, W e tested effects of substances known to antagonize ligand binding to e ither the LDL [apolipoprotein B,E (ape B,E)] receptor (dextran sulphat e, heparin, pentosan polysulphate, protamine, spermine, histone), the scavenger receptor (dextran sulphate, fucoidin), the very-low-density- lipoprotein (VLDL) receptor [receptor-associated protein (RAP)], or LD L receptor-related protein (RAP, alpha(2)-macroglobulin, lipoprotein l ipase, exotoxin-A). None of these substances, with the exception of de xtran sulphate, influenced binding of LDL to either 105 or 130 kDa pro teins. Sodium oleate or oleic acid, known stimuli for the lipoprotein binding activity of the lipolysis-stimulated receptor, were also witho ut effect. LDL binding to 105 and 130 kDa proteins was inhibited by an ti-LDL (ape B) antibodies. LDL and VLDL bound to 105 and 130 kDa prote ins with similar affinities (approximate to 50 mu g/ml). The unique li gand selectivity of 105 and 130 kDa proteins supports the existence of a novel lipoprotein-binding protein that is distinct from all other c urrently identified LDL receptor family members. The similar ligand se lectivity of 105 and 130 kDa proteins suggests that they may represent variant forms of an atypical lipoprotein-binding protein.