Structural stability and fusion of human low-density lipoproteins
Low-density lipoproteins (LDL) are heterogeneous nanoparticles containing one copy of apolipoprotein B (~550 kDa) and thousands of lipids. LDL are the main plasma carriers of cholesterol and the major risk factor for atherosclerosis, the number one cause of death in the developed world. In atherosclerosis, LDL lipids are deposited in the arterial intima. Fusion of modified LDL in the arterial wall is an important underexplored triggering event in early atherosclerosis. Previous studies from our laboratory showed that thermal denaturation mimics LDL remodeling and fusion, and revealed the kinetic origin of LDL stability. Here, we report the first quantitative kinetic analysis of LDL stability. We show that LDL denaturation monitored by turbidity follows a sigmoidal time course that is unique among lipoproteins, suggesting that slow conformational changes in apoB precede lipoprotein fusion. High activation energy of LDL denaturation, Ea~100 kcal/mol, indicates disruption of extensive protein-protein and protein-lipid interactions involving large apoB domains. Next, we combined size-exclusion chromatography, gel electrophoresis and electron microscopy to show that dimerization is a common early step preceding LDL fusion. Monoclonal antibody binding studies indicated that α-helices in the N-terminal βα1 domain of apoB undergo conformational changes at early stages of LDL aggregation and fusion. Better understanding of these structural changes that prime LDL for fusion is important, as it may help control this pathogenic process before it occurs. We applied the kinetic approach to test how selected factors that are expected to contribute to LDL fusion in vivo affect the rate of LDL fusion and rupture in vitro. The results show that LDL fusion accelerates at pH<7, which may contribute to LDL retention in acidic atherosclerotic lesions. Fusion also accelerates upon increasing LDL concentration in near-physiologic range, which likely contributes to atherogenesis. Further, we showed that thermal stability of LDL decreases with increasing particle size, indicating that the pro-atherogenic properties of small dense LDL do not result from their enhanced fusion. Our work provides the first kinetic approach to measuring LDL stability and suggests that lipid-lowering therapies that reduce LDL concentration but increase the particle size may have opposite effects on LDL fusion.