Serum amyloid A impairs the antiinflammatory properties of HDL

ApoCIII enrichment in HDL impairs HDL-mediated cholesterol efflux capacity

Apolipoprotein CIII (apoCIII) has been reported to be tightly associated with triglyceride metabolism and the susceptibility to coronary artery disease (CAD). Besides, apoCIII has also been found to affect the anti-apoptotic effects of HDL. However, the effect of apoCIII on HDL-mediated cholesterol efflux, the crucial function of HDL, has not been reported. A hospital-based case-control study was conducted to compare the apoCIII distribution in lipoproteins between CAD patients and nonCAD controls and to explore the relationship between HDL-associated apoCIII (apoCIII_HDL) and HDL-mediated cholesterol efflux. One hundred forty CAD patients and nighty nine nonCAD controls were included. Plasma apoCIII, apoCIII_HDL and cholesterol efflux capacity was measured. The apoCIII_HDL ratio (apoCIII_HDL over plasma apoCIII) was significantly higher in CAD patients than that in control group (0.52 ± 0.24 vs. 0.43 ± 0.22, P = 0.004). Both apoCIII_HDL and apoCIII_HDL ratio were inversely correlated with cholesterol efflux capacity (r = −0.241, P = 0.0002; r = −0.318, P < 0.0001, respectively). Stepwise multiple regression analysis revealed that the apoCIII_HDL ratio was an independent contributor to HDL-mediated cholesterol efflux capacity (standardized β = −0.325, P < 0.001). This study indicates that the presence of apoCIII in HDL may affect HDL-mediated cholesterol efflux capacity, implying the alternative role of apoCIII in the atherogenesis.

High density lipoproteins are modulators of protease activity: Implications in inflammation, complement activation, and atherothrombosis

High density lipoproteins (HDL) represent a compositionally diverse population of particles in the circulation, containing a wide variety of lipids and proteins. Gene ontology functional analysis of the 96 commonly identified HDL binding proteins reveals that almost half of these proteins are either proteases or have known roles in protease regulation. Here, we discuss the activities of some of these proteins in regard to their roles in regulating proteases involved in inflammation, coagulation, and complement activation, particularly in the context of atherosclerosis. The overall goal of this review is to discuss potential functional roles of HDL in protease regulatory pathways based on current literature and known functions of HDL binding proteins and to promote the consideration of HDL as a global modulator of proteolytic equilibrium.

Inflammation, remodeling, and other factors affecting HDL cholesterol efflux

PURPOSE OF REVIEW

The ability of HDL to promote cholesterol efflux from macrophages is a predictor of cardiovascular risk independent of HDL cholesterol levels. However, the molecular determinants of HDL cholesterol efflux capacity (CEC) are largely unknown.

RECENT FINDINGS

The term HDL defines a heterogeneous population of particles with distinct size, shape, protein, and lipid composition. Cholesterol efflux is mediated by multiple pathways that may be differentially modulated by HDL composition. Furthermore, different subpopulations of HDL particles mediate CEC via specific pathways, but the molecular determinants of CEC, either proteins or lipids, are unclear. Inflammation promotes a profound remodeling of HDL and impairs overall HDL CEC while improving ATP-binding cassette transporter G1-mediated efflux. This review discusses recent findings that connect HDL composition and CEC.

SUMMARY

Data from recent animal and human studies clearly show that multiple factors associate with CEC including individual proteins, lipid composition, as well as specific particle subpopulations. Although acute inflammation remodels HDL and impairs CEC, chronic inflammation has more subtle effects. Standardization of assays measuring HDL composition and CEC is a necessary prerequisite for understanding the factors controlling HDL CEC. Unraveling these factors may help the development of new therapeutic interventions improving HDL function.

Paradoxical effects of SAA on lipoprotein oxidation suggest a new antioxidant function for SAA

Oxidative stress and inflammation, which involve a dramatic increase in serum amyloid A (SAA) levels, are critical in the development of atherosclerosis. Most SAA circulates on plasma HDL particles, altering their cardioprotective properties. SAA-enriched HDL has diminished anti-oxidant effects on LDL, which may contribute to atherogenesis. We determined combined effects of SAA enrichment and oxidation on biochemical changes in HDL. Normal human HDLs were incubated with SAA, oxidized by various factors (Cu²⁺, myeloperoxidase, H₂O₂, OCl^-), and analyzed for lipid and protein modifications and biophysical remodeling. Three novel findings are reported: addition of SAA reduces oxidation of HDL and LDL lipids; oxidation of SAA-containing HDL in the presence of OCl^- generates a covalent heterodimer of SAA and apoA-I that resists the release from HDL; and mild oxidation promotes spontaneous release of proteins (SAA and apoA-I) from SAA-enriched HDL. We show that the anti-oxidant effects of SAA extend to various oxidants and are mediated mainly by the unbound protein. We propose that free SAA sequesters lipid hydroperoxides and delays lipoprotein oxidation, though much less efficiently than other anti-oxidant proteins, such as apoA-I, that SAA displaces from HDL. These findings prompt us to reconsider the role of SAA in lipid oxidation in vivo.

Serum amyloid A and atherosclerosis

PURPOSE OF REVIEW

Atherosclerosis is a chronic inflammation associated with increased expression of the acute phase isoforms of serum amyloid A (SAA) and in humans is a plasma biomarker for future cardiovascular events. However, whether SAA is only a biomarker or participates in the development of cardiovascular disease is not well characterized. The purpose of this review is to summarize putative functions of SAA relevant to atherogenesis and in-vivo murine studies that directly examine the effect of SAA on atherosclerosis.

RECENT FINDINGS

Modulation of the expression of SAA1 and/or SAA2 in murine models of atherosclerosis suggests that SAA promotes early atherogenesis. SAA secreted from bone-marrow-derived cells contributes to this antiatherogenic phenotype. SAA also promotes angiotensin-induced abdominal aneurysm in atherogenic mouse models. The reduction in atherosclerosis may be due, at least in part, to remodeling of the acute phase HDL to reduce its capacity to promote cholesterol efflux and reduce its anti-inflammatory ability.

SUMMARY

SAA is more than a marker of cardiovascular disease and is a participant in the early atherogenic process.

Heparin interactions with apoA1 and SAA in inflammation-associated HDL

Apolipoprotein A1 (apoA1) is the main protein component responsible for transportation of cholesterol on high-density lipoprotein (HDL). Serum amyloid A (SAA) is an acute phase protein associated with HDL. Apart from their physiological functions, both apoA1 and SAA have been identified as 'amyloidogenic peptides'. We report herein that the polysaccharide heparin interacts with both apoA1 and SAA in HDL isolated from plasma of inflamed mice. The reaction is rapid, forming complex aggregates composed of heparin, apoA1 and SAA as revealed by gel electrophoresis. This interaction is dependent on the size and concentration of added heparin. Mass spectrometry analysis of peptides derived from chemically crosslinked HDL-SAA particles detected multiple crosslinks between apoA1 and SAA, indicating close proximity (within 25 Å) of these two proteins on the HDL surface, providing a molecular and structural mechanism for the simultaneous binding of heparin to apoA1 and SAA.

Structure of serum amyloid A suggests a mechanism for selective lipoprotein binding and functions: SAA as a hub in macromolecular interaction networks

Serum amyloid A is a major acute-phase plasma protein that modulates innate immunity and cholesterol homeostasis. We combine sequence analysis with x-ray crystal structures to postulate that SAA acts as an intrinsically disordered hub mediating interactions among proteins, lipids and proteoglycans. A structural model of lipoprotein-bound SAA monomer is proposed wherein two α-helices from the N-domain form a concave hydrophobic surface that binds lipoproteins. A C-domain, connected to the N-domain via a flexible linker, binds polar/charged ligands including cell receptors, bridging them with lipoproteins and rerouting cholesterol transport. Our model is supported by the SAA cleavage in the interdomain linker to generate the 1-76 fragment deposited in reactive amyloidosis. This model sheds new light on functions of this enigmatic protein.