ISOLATION AND CHARACTERIZATION OF A PLATELET-DERIVED MACROPHAGE-BINDING PROTEOGLYCAN

A macromolecule in human platelet secretory products was demonstrated previously to inhibit the binding and uptake of acetoacetylated (AcAc) low density lipoproteins (LDL) by scavenger receptors on mouse perito neal macrophages. In the current study, this macromolecule was purifie d to apparent homogeneity by DEAE-Sephacel chromatography, Sephacryl S -300 chromatography, and sucrose gradient centrifugation. SDS-polyacry lamide gel electrophoresis revealed a single band with an apparent mol ecular mass of approximately 120 kDa. Chemical analysis indicated that the macromolecule (designated platelet-derived macrophage-binding pro teoglycan (PDMBP)) was a chondroitin 4-sulfate proteoglycan with an ap proximately 32-kDa core protein. A polyclonal antibody produced agains t this proteoglycan identified only PDMBP on Western blots of platelet secretory products and removed all ability of these products to inhib it the binding of AcAc LDL to macrophages. Treatment of purified PDMBP with protease or chondroitinase AC or A.BC abolished the ability of t he proteoglycan to inhibit the binding of AcAc LDL to macrophages. Bin ding studies using radiolabeled PDMBP demonstrated that the proteoglyc an bound directly to the macrophage cell surface and was competitively inhibited by AcAc LDL, acetyl-LDL, fucoidin, and unlabeled PDMBP. PDM BP inhibited binding of I-125-labeled AcAc LDL to macrophages but had no effect on binding to endothelial cells. The finding that PDMBP bind s to the scavenger receptor on macrophages suggests a mechanism for th e inhibition of foam cell formation and suggests that the receptor cou ld be involved in the plasma clearance of chondroitin sulfate proteogl ycans.