Abstract 426: The Remodeling of HDL-SAA by Macrophage Surface Heparan Sulfate is Necessary for Efficient Cholesterol Efflux

Abstract

Serum amyloid A (SAA) is an acute-phase protein that circulates associated with high density lipoprotein (HDL) and can replace apoA-I as the predominant apoprotein. SAA can increase HDL’s binding affinity for macrophages and we provide evidence that this targeting process is dependent on cell surface heparan sulfate (HS) and facilitated by at least one of two HS binding sites that we have been previously identified in SAA. Additional studies utilizing various assays including native-PAGE, turbidity, thioflavin T fluorescence and surface plasmon resonance have demonstrated that HDL-SAA binding to HS (or heparin) causes SAA to dissociate from the HDL particle, leaving behind novel HDL species with enhanced cholesterol carrying capacities. In the present study we explore this remodeling phenomenon further and assess its potential importance for cholesterol efflux from macrophages. The chemical disruption of HDL-SAA with chaotropic agents, temperatures below 37°C and lipid oxidation with copper all interfere with HDL-SAA/heparin remodeling. The oxidation of HDL-SAA also significantly reduces its cholesterol efflux potential and SAA’s ability to be converted into AA-amyloid fibrils in cell culture, a process that requires the dissociation of SAA from HDL. Finally, we demonstrate that the removal of cell surface HS chains, by either heparinase treatment or by preventing HS biosynthesis with sodium chlorate, impairs cholesterol efflux rates from macrophages to HDL-SAA. Our interpretation of these findings is that the reduction in HDL-lipid fluidity (low temperatures or conjugated dienes) significantly reduces the ability of HS:SAA interactions to dissociate SAA from HDL. We postulate that HS normally plays an important role in separating SAA from HDL but during chronic inflammatory diseases with elevated lipoprotein oxidation, HDL-SAA species are generated in which SAA is more tightly associated with HDL, thus preventing its dissociation from HDL and impairing atheroprotective functions.