Abstract 29: Alpha-1-antitrypsin Protects High Density Lipoprotein From Functional Inactivation by Elastase

Abstract

High density lipoproteins (HDL) are complexes of lipid and protein with several known atheroprotective functions. These functions are driven by specific lipids and proteins contained on the HDL particle and include reverse cholesterol transport, suppression of inflammation, and modulation of endothelial function. These activities are most important within atherosclerotic plaque, a harsh environment where HDL interact with macrophage foam cells, activated neutrophils, and dysfunctional endothelial cells. Neutrophils and macrophages secrete proteases, such as elastase, which damage structural components and soluble proteins and propagate inflammatory signaling. It has been suggested that, in plaque, HDL become damaged and dysfunctional. We recently characterized a subspecies of HDL that carries the protein alpha-1-antitrypsin (A1AT), an abundant plasma serine protease inhibitor. In the current study, we tested the hypothesis that A1AT enriched HDL are protected from proteolytic damage and functional inactivation by elastase, the main protease inhibited by A1AT. Human HDL was isolated by ultracentrifugation and was enriched with A1AT by co-incubation and unbound A1AT was removed. Treatment of native HDL with elastase resulted in significant proteolytic degradation of both apoA-I and apoA-II, visualized by coomassie stained SDS-PAGE. Analysis of lipoprotein size by one dimensional native gel electrophoresis revealed that pre-beta HDL were completely degraded by elastase. Compared to native HDL, A1AT enriched HDL samples were protected from protein and pre-beta particle degradation by elastase. We next tested the effect of elastase treatment on HDL function. In native HDL, elastase had damaging effects on ABCA1 mediated cholesterol efflux (-32%; p<0.0001) and the ability to esterify free cholesterol (-14%; p<0.02). A1AT enriched HDL displayed no loss of functionality upon treatment with elastase. Both of these activities are required for HDL to perform what is thought to be its most important function, reverse cholesterol transport. In conclusion, the data presented indicate that HDL particles which contain A1AT may represent a functionally important species of HDL, which have an advantage in the protease-rich plaque environment.