Serum Amyloid A Induces Monocyte Tissue Factor

Abstract

C-reactive protein (CRP) and serum amyloid A (SAA) increase in the blood of patients with inflammatory conditions and CRP-induced monocyte tissue factor (TF) may contribute to inflammation-associated thrombosis. This study demonstrates that SAA is a potent and rapid inducer of human monocyte TF. SAA induced TF mRNA in PBMC within 30 min and optimal procoagulant activity within 4 h, whereas CRP (25 μg/ml)-induced activity was minimal at this time. Unlike CRP, SAA did not synergize with LPS. Procoagulant activity was inhibited by anti-TF and was dependent on factors VII and X, and TF Ag levels were elevated on CD14+ monocytes. Responses were optimal with lymphocytes, although these were not obligatory. Inhibitor studies indicate activation of NF-κB through the ERK1/2 and p38 MAPK pathways; the cyclo-oxygenase pathway was not involved. SAA-induced TF was partially inhibited by high-density lipoprotein, but not by low-density lipoprotein or by apolipoprotein A-I. SAA is a ligand for the receptor for advanced glycation end products (RAGE), and TF generation was suppressed by ∼50% by a RAGE competitor, soluble RAGE, and by ∼85% by anti-RAGE IgG. However, another RAGE ligand, high mobility group box-1 protein, capable of inducing monocyte chemotactic protein-1 mRNA in 2 h, did not induce TF within 24 h. Cross-linking studies confirmed SAA binding to soluble RAGE. Elevated SAA is a marker of disease activity in patients with rheumatoid arthritis, and PBMC from patients with rheumatoid arthritis were more sensitive to SAA than normals, suggesting a new link between inflammation and thrombosis.

Serum Amyloid A Stimulates PKR Expression and HMGB1 Release Possibly through TLR4/RAGE Receptors

“…The infrequently expressed SAA, unlike the most abundant SAA1, significantly induced HMGB1 release in TLR4/RAGE-and PKR-dependent mechanisms. These novel findings were consistent with several studies that echoed their dramatic differences in stimulating other cytokines/chemokines (22)(23)(24)(25). Although SAA contained minute amount of endotoxins, this trivial contamination was not likely the underlying cause for SAAmediated HMGB1 release, because (a) Ultrapure LPS (free from bacterial proteins and nucleic acids; catalog # tlrl-pelps, InvivoGen) fails to trigger HMGB1 release even when given up to 10 μg/mL (29,30); (b) the similarly prepared SAA1 that contained comparable amounts of endotoxin still failed to induce HMGB1 release; (c) endotoxin-neutralizing agent (for example, polymyxin B) effectively abrogated LPS-induced (0.5 μg/mL), but not SAAinduced (0.5 μg/mL), HMGB1 release; and (d) the disruption of TLR4 receptor impaired crude LPS-, but not SAAinduced, HMGB1 release.…”

“…SAAs can signal via multiple receptors including the TLR2, TLR4 (18)(19)(20)43) and RAGE (17,25) to activate macrophages and monocytes. To identify the receptor(s) responsible for SAA-mediated HMGB1 release, we compared the levels of SAAinduced HMGB1 release between primary peritoneal macro phages from wild-type and mutant mice, respectively, deficient in TLR2, TLR4, RAGE, TLR2/RAGE or TLR4/RAGE.…”

“…Clinically, SAAs have been implicated as biomarkers in cardiovascular disorders (14), ulcerative colitis (15) and sepsis (16). Extracellular SAA signals via a family of receptors including the receptor for advanced glycation end products (RAGE) (17), TLR2 (18,19) and TLR4 (20) to activate NLRP3 inflammasome (21) and to induce various cytokines and chemokines (22)(23)(24)(25).…”

The Receptor for Advanced Glycation End Products Impairs Host Defense in Pneumococcal Pneumonia

“…Administration of soluble RAGE in vivo for 6 wk in a model of chronic vascular inflammation in diabetic apolipoprotein E-deficient mice suppressed levels of tissue factor, the main initiator of coagulation in sepsis in general (52) and in this model of pneumococcal pneumonia in particular (53), in the aorta (51). Furthermore, soluble RAGE, anti-RAGE, or antisense RAGE have been reported to inhibit tissue factor expression by monocytes or endothelial cells stimulated with advanced glycation end products or serum amyloid A in vitro (49,50).…”

Serum- and HDL3-serum amyloid A and HDL3-LCAT activity are influenced by increased CVD-burden

“…It also enhances cholesterol ester uptake into macrophages and reduces reverse cholesterol transport (RCT) [9,10], as demonstrated by the ex vivo and in vitro experiments of Heinecke's group [11]. Dysfunctional HDL induces the release of pro-inflammatory cytokines [12], enhances the recruitment of monocytes into vascular plaques [13] and causes macrophage accumulation [14], thus helping propagate the inflammatory process. Furthermore, SAA impairs the antioxidant function of HDL [15], while the activity of HDL remodelling enzymes, such as lecithin cholesterol acyltransferase (LCAT), an enzyme that is responsible for the esterification of free cholesterol within HDL3 and thus its maturation to HDL2, is reduced by the presence of SAA [16].…”