Serum amyloid A (SAA) is an inflammatory marker that predicts cardiovascular disease events. SAA levels are elevated in the metabolic syndrome, which is characterized by a markedly increased risk for premature cardiovascular disease. We recently found that dietary cholesterol increases circulating SAA levels in mice and that circulating SAA levels predict atherosclerosis better than cholesterol levels. SAA circulates predominantly on high density lipoprotein (HDL). It is produced mainly by the liver, but also by extra-hepatic cells such as macrophages and vascular smooth muscle cells. SAA has proteoglycan-binding domains that can serve as bridges between lipoproteins and vascular proteoglycans. Apo A-I, the major apolipoprotein of HDL, is abundant in atherosclerotic lesions in both mice and humans, where it co-localizes with both SAA and proteoglycans. These observations suggest that retention of HDL by SAA in the vascular intima might inhibit the atheroprotective effects of HDL and play a role in atherogenesis. We propose that SAA is a mediator rather than a marker of atherosclerosis and that factors that increase circulating SAA levels, such as dietary cholesterol and the metabolic syndrome, might promote atherosclerosis by facilitating the binding of lipoproteins, including HDL, to vascular proteoglycans. Therefore, we plan to (1) further investigate the dietary and metabolic factors that regulate SAA levels in blood and to determine the impact of elevated SAA levels on atherosclerosis, (2) determine which isoforms of SAA are produced by vascular smooth muscle cells and macrophages and whether SAA produced by vascular cells contributes to circulating SAA, and (3) using both in vitro and techniques and mice models in which SAA is overexpressed, to establish whether SAA is a mediator rather than merely a marker of atherosclerosis. The proposed studies will provide important information about factors that regulate SAA levels in blood and predispose humans to cardiovascular disease. They will also establish whether SAA promotes HDL retention in the artery wall.
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