Serglycin-binding proteins in activated macrophages and platelets

The major proteoglycan in macrophages and platelets is the chondroitin sulfate proteoglycan serglycin. To study the biological role of serglycin, its binding to secreted and cell-associated proteins from macrophages and blood platelets was examined. Affinity chromatography with serglycin-Sepharose and chondroitin sulfate-Sepharose was used to isolate proteoglycan-binding proteins from macrophages and platelets. Antibodies against human macrophage inflammatory protein-1 alpha (MIP-1 alpha) precipitated a 14-kDa S-35, methionine-labeled protein among the chondroitin sulfate binding proteins secreted from the macrophage-like U937 cells after stimulation. Two proteins from murine macrophage J774 cells with molecular masses of similar to 10 and 14 kDa were precipitated by an antiserum against the murine MIP-1 alpha. Protein sequencing of fragments obtained by trypsin digestion of a 14-kDa chondroitin sulfate-binding protein from cell extracts of stimulated U937 cells revealed 100% homology with lysozyme, a bacteriolytic enzyme. Fragment of one other protein with approximate molecular mass of 8 kDa showed high homology with bone morphogenetic protein. Inhibition studies showed that chondroitin 6-sulfate inhibited the bacteriolytic activity of lysozyme in a competitive manner more efficiently than heparin and chondroitin 4-sulfate. Amino-terminal sequencing of two proteins from platelet extracts that bound to serglycin-Sepharose revealed that they corresponded to multimeric forms of human platelet factor 4 (PF4). Chondroitin sulfate-Sepharose was shown to be equally efficient in retaining PF4 from platelet extracts as serglycin-Sepharose, indicating that the glycosaminoglycan chains mediate the binding to PF4 in the intact proteoglycan molecule. Competition experiments showed that serglycin was as efficient as heparan sulfate in blocking the binding of [H-3]chondroitin sulfate to PF4, whereas heparin was one order of magnitude more efficient. Affinity measurements using fluoresceinamine-labeled glycosaminoglycans showed that the affinity of heparin for PF4 is on the order of 30 nM, whereas chondroitin sulfate has an affinity of 260 nM. Both PF4, MIP-1 alpha, and lysozyme play important roles in different types of inflammatory reactions. The interaction with serglycin may indicate that this proteoglycan is involved in the regulation of the inflammatory response.