Adhesion of human platelets to serum amyloid A

Abstract

Serum amyloid A (SAA) is an acute phase reactant, and its level in the blood is elevated to 1000-fold in response of the body to trauma, infection, inflammation, and neoplasia. SAA was reported to inhibit platelet aggregation and to induce adhesion of leukocytes. This study looked at adhesion of human platelets to SAA. Immobilized SAA supported the adhesion of human washed platelets; level of adhesion to SAA was comparable to fibronectin and lower than to fibrinogen. Adhesion to SAA was further enhanced by Mn 2؉ and the physiological agonist, thrombin. Platelet adhesion to SAA was completely abolished by anti-SAA antibody. SAA-induced adhesion was inhibited by antibodies against the integrin receptor ␣IIb␤3, by the peptide GRGDSP and by SAA-derived peptide containing YIGSR-like and RGD-like adhesion motifs (amino acids 29 to 42). Adhesion was not inhibited by control immunoglobulin G, by antibody against the integrin receptor ␣V␤3, by the peptide GRGESP, and by SAA-derived peptide that includes incomplete RGD motif. SAA-derived peptide 29 to 42 also inhibited platelet adhesion to fibronectin. Transfected human melanoma cells expressing ␣IIb␤3 adhered to SAA, whereas transfected cells expressing ␣V␤3 did not. By using flow cytometry, the ␣IIb␤3 cells displayed significantly higher levels of binding of soluble SAA than the ␣V␤3 cells. These data indicate that human platelets specifically adhere to SAA in an RGD-and ␣IIb␤3- IntroductionSerum amyloid A (SAA) is composed of a family of proteins, and its level in the blood is elevated to 1000-fold as part of the response of the body to various injuries, including trauma, infection, inflammation, and neoplasia. 1,2 The SAA proteins are expressed primarily in the liver, but extrahepatic expression was described, including in cells of human atherosclerotic lesions, ie, smooth muscle cells, endothelial cells, and monocytes/macrophages, 3,4 and in many histologically normal human tissues, predominantly by the epithelium. 5 SAA contains binding sites for the extracellular matrix (ECM) components laminin 6 and heparin/heparan sulfate 7 as well as YIGSR-like and RGD-like adhesion motifs (YIGSDKYF-HARGNY, residues 29 to 42). The latter is present at the turn of an assumed ␤-sheet, in a region that is highly conserved through evolution. 8 SAA was reported to inhibit platelet aggregation, 9 to induce adhesion of mononuclear and polymorphonuclear leukocytes, 10,11 to bind to ECM proteins, 12,13 to induce matrix metalloproteinases, and to serve as their substrate. 14,15 SAA-derived peptides were reported to inhibit T-lymphocyte attachment to ECM proteins. 16 Nevertheless, the physiologic role of SAA as an ECMassociated and/or adhesion protein is not well established.The major integrin receptor expressed on platelet membrane, ␣IIb␤3 (platelet receptor GPIIb/IIIa), plays a critical role in hemostasis and thrombosis by mediating platelet-platelet and platelet-matrix protein interactions. 17 The RGD (arginine-glycineaspartic acid) adhesion motif is present in fibronectin and other ma...

Serum Amyloid A Induces Monocyte Tissue Factor

“…SAA may also modulate platelet adhesion at sites of vascular injury. It can reduce thrombin-activated platelet aggregation (23), but supports platelet adhesion to fibronectin, thus suggesting a regulatory role in thrombosis and hemostasis (24). Together with its effects on platelets, we propose that SAA plays a key role in thrombus formation and fibrin deposition in inflammatory lesions.…”

Murine apolipoprotein serum amyloid A in solution forms a hexamer containing a central channel

“…The observation that peptides derived from the C terminus of SAA can also bind to heparan sulfate in vitro (35) suggests that hexameric SAA is not required for this interaction. SAA also contains a highly conserved RGD-like adhesion motif (RGN, residues 39-41 in SAA2.2) that adheres to various cells including platelets (36). Peptides derived from SAA and comprising amino acids 29-42 also exhibited platelet-adhesion function, suggesting that hexameric SAA is not required for this function.…”

Pathogenic Serum Amyloid A 1.1 Shows a Long Oligomer-rich Fibrillation Lag Phase Contrary to the Highly Amyloidogenic Non-pathogenic SAA2.2

“…Thus, the structural malleability of SAA1.1 and SAA2.2 we observed in vitro may reflect the in vivo properties of SAA proteins to allow the regulation of their many putative functions related to cholesterol metabolism and the inflammatory responses (34). Furthermore, it would help explain how a small (103-104 residues) and marginally stable protein like SAA is able to interact in vivo with many binding partners, including HDL, cholesterol (37), outer membrane protein (38), heparin (39), extracellular matrix glycoproteins (40), and platelets (41).…”