Functional Interactions Between Lipoproteins and Protease Inhibitors in Atherosclerosis

Project Summary Lipoproteins are dynamic complexes of lipid and protein in the circulation that are correlated with cardiovascular events. Lipoprotein particles can interact with over 100 different proteins in the circulation. However, except for a few core lipid-binding proteins (apolipoproteins), the functional implications of these protein-lipoprotein interactions are not known. We found that about 20% of lipoprotein-associated proteins are protease inhibitors. Proteases are enzymes that cleave other proteins and they play important roles in many physiological pathways. In atherosclerosis, their activity is often associated with inflammation and unstable lesions that may be more prone to rupture. Because lipoproteins accumulate within atherosclerotic lesions and these particles are enriched with protease inhibitors, we propose that lipoprotein-associated protease inhibitor activity may be a novel mechanism by which lipoproteins can suppress vascular inflammation and stabilize atherosclerotic lesions. Alpha-1-antitrypsin (AAT) is an abundant plasma serine protease inhibitor that is detected on both low density and high-density lipoproteins (LDL and HDL, respectively). AAT is an endogenous inhibitor of neutrophil elastase, a protease generated by neutrophils and macrophages within atherosclerotic lesions. Our central hypothesis is that lipoproteins can modulate AAT function and this contributes to lipoprotein-mediated effects on atherosclerosis by influencing vascular inflammation. Our preliminary data suggest a unique structural interaction between AAT and lipoproteins that results in functional preservation of AAT activity. Aim 1 of our proposal will investigate this structure-function interaction in detail and will determine the mechanistic basis for this observation on LDL and HDL. These studies will utilize cross-liking mass spectrometry and detailed in vitro functional assays. The literature and our preliminary data support a potential role of AAT in various cardiovascular diseases, however, the mechanistic contribution of AAT to vascular inflammation in atherosclerosis has not been studied. Aim 2 will determine the role of lipoprotein-associated elastase-inhibitor activity in vascular inflammation and atherosclerosis using in vivo mouse models of atherosclerosis. Furthermore, we have developed a novel lipoprotein-targeting elastase-inhibitor peptide for synthetic enhancement of lipoprotein-associated protease inhibitor activity. Aim 3 will utilize a newly generated AAT-deficient mouse model to examine the impact of AAT on hepatic lipid metabolism and atherosclerosis. Together, these studies will fill a significant knowledge gap in our understanding of how non-apolipoprotein components of lipoproteins can impact atherosclerosis risk by modulating the antiinflammatory potential of lipoproteins. This work will also develop new tools for enhancement of lipoprotein function and data on the potential of this approach for atherosclerosis prevention or stabilization of high-risk lesions.