Serum amyloid A (SAA), the precursor protein in inflammation-associated reactive amyloidosis (AA-type), is an acute phase reactant whose level in the blood increases in response to various insults. It is expressed in the liver, but its physiological role is not well understood. Recently, a broader view of SAA expression and function has been emerging. Expression studies show local production of SAA proteins in histologically normal, atherosclerotic, Alzheimer, inflammatory, and tumor tissues. Binding sites in the SAA protein for high density lipoproteins, calcium, laminin, and heparin/heparan-sulfate were described. Adhesion motifs were identified and new functions, affecting cell adhesion, migration, proliferation and aggregation have been described. These findings emphasize the importance of SAA in various physiological and pathological processes, including inflammation, atherosclerosis, thrombosis, AA-amyloidosis, rheumatoid arthritis, and neoplasia. In addition, recent experiments suggest that SAA may play a "housekeeping" role in normal human tissues.

SUMMARY Serum amyloid A (SAA) is an acute-phase reactant whose level in the blood is elevated to 1000-fold as part of the body's responses to various injuries, including trauma, infection, inflammation, and neoplasia. As an acute-phase reactant, the liver has been considered to be the primary site of expression. However, limited extrahepatic SAA expression was described in mouse tissues and in cells of human atherosclerotic lesions. Here we describe nonradioactive in situ hybridization experiments revealing that the SAA mRNA is widely expressed in many histologically normal human tissues. Expression was localized predominantly to the epithelial components of a variety of tissues, including breast, stomach, small and large intestine, prostate, lung, pancreas, kidney, tonsil, thyroid, pituitary, placenta, skin epidermis, and brain neurons. Expression was also observed in lymphocytes, plasma cells, and endothelial cells. RT-PCR analysis of selected tissues revealed expression of the SAA1, SAA2, and SAA4 genes but not of SAA3, consistent with expression of these genes in the liver. Immunohistochemical staining revealed SAA protein expression that colocalized with SAA mRNA expression. These data indicate local production of the SAA proteins in histologically normal human extrahepatic tissues.

Freshly isolated human neutrophils were investigated for their ability to degrade heparan sulfate proteoglycans in the subendothelial extracellular matrix (ECM) produced by cultured corneal and vascular endothelial cells. The ECM was metabolically labeled with Na2(35S)04 and labeled degradation products were analyzed by gel filtration over Sepharose 6B. More than 90% of the released radioactivity consisted of heparan sulfate fragments 5-6 times smaller than intact heparan sulfate side chains released from the ECM by either papain or alkaline borohydride. These fragments were sensitive to deamination with nitrous acid and were not produced in the presence of either heparin or serine protease inhibitors. In contrast, degradation of soluble high molecular weight heparan sulfate proteoglycan, which was first released from the ECM, was inhibited by heparin but there was no effect of protease inhibitors. These results indicate that interaction of human neutrophils with the subendothelial ECM is associated with degradation of its heparan sulfate by means of a specific, newly identified, heparanase activity and that this degradation is facilitated to a large extent by serine proteases.The neutrophil heparanase was readily and preferentially released (15-25% of the cellular content in 60 min) by simply incubating the cells at 40C in the absence of added stimuli. Under these conditions, <5% of the cellular content of lactate dehydrogenase, lysozyme, and globin degrading proteases was released. Further purification of the neutrophil heparanase was achieved by its binding to heparin-Sepharose and elution at 1 M NaCI. It is suggested that heparanase activity is involved in the early events of extravasation and diapedesis of neutrophils in response to a threshold signal from an extravascular inflamed organ.

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...