Poly(ethylene glycol)-Induced Fusion and Destabilization of Human Plasma High-Density Lipoproteins

Abstract

High-density lipoproteins (HDL) are macromolecular complexes of specific proteins and lipids that mediate the removal of cholesterol from peripheral tissues. Chemical unfolding revealed that HDL fusion and rupture are the two main kinetic steps in HDL denaturation. Here we test the hypothesis that lipid fusogens such as poly(ethylene glycol) (PEG) may promote lipoprotein fusion and rupture and thereby destabilize HDL. We analyze thermal disruption of spherical HDL in 0-15% PEG-8000 by calorimetric, spectroscopic, electron microscopic, and light scattering techniques. We demonstrate that the two irreversible high-temperature endothermic HDL transitions involve particle enlargement and show a heating rate dependence characteristic of kinetically controlled reactions with high activation energy. The first calorimetric transition reflects HDL fusion and dissociation of lipid-poor apolipoprotein A-1 (apoA-1), and the second transition reflects HDL rupture and release of the apolar lipid core. Neither transition involves substantial protein unfolding; thus, the transition heat originates from lipid and/or protein dissociation and repacking. At room temperature, PEG-8000 induces HDL fusion that is distinct from the heat-, denaturant-, or enzyme-induced fusion since it leads to formation of larger particles and does not involve apoA-1 dissociation. Increasing the PEG concentration in solution from 0 to 15% leads to low-temperature shifts by approximately -18 degrees C in the two calorimetric HDL transitions without altering their nature. Thus, consistent with our hypothesis, PEG-8000 induces fusion and reduces the thermal stability of HDL. Our results suggest that PEG is useful for the analysis of the molecular events involved in metabolic HDL remodeling and fusion.

Spontaneous remodeling of HDL particles at acidic pH enhances their capacity to induce cholesterol efflux from human macrophage foam cells

“…Given the dominance of apoA-I in mature HDL particles, the changes in the CD spectra at acidic pH are likely to be attributable mainly to conformational changes of apoA-I rather than of other apolipoproteins present in HDL particles ( 44 ). The indispensability of the presence of apoA-I for HDL particle stability well agrees with the concept that HDL fusion can be triggered by an imbalance between the hydrophobic core and the particle surface, particularly when the latter becomes protein-defi cient ( 39 ). Actually, heat-induced and chemically induced HDL fusion ( 38,42,43 ) have been ascribed to dissociation of apoA-I.…”

“…1 and 2 ). Considering the changes in the size of the acidic pH-modifi ed HDL particles, we could calculate that the formation of larger particles (11-13 nm) had resulted from the fusion of two to three native HDL particles (7-9 nm), consistent with fusion of HDL particles induced by chemical and thermal denaturation ( 38,39 ). Further analysis indicated that about 50% of the particles analyzed individually by negative staining electron microscopy had an increased diameter; i.e., they were fused particles containing the core lipid of two or more native HDL particles.…”

Thermal Transitions in Human Very-Low-Density Lipoprotein: Fusion, Rupture, and Dissociation of HDL-like Particles

“…4B, C) and thereby provides a mechanism for removing excess surface material from the fused particles. Thus, different modes of VLDL perturbation (such as lipase-mediated TG hydrolysis or heatinduced VLDL fusion) may lead to transient formation of similar apoE-containing HDL-like particles.Earlier studies of HDL and LDL have shown that thermal and solute-induced perturbations may mimic aspects of enzymatic lipoprotein remodeling such as particle fusion, apolipoprotein dissociation and phospholipid transfer (10,11,36,37). The results reported here extend this notion to VLDL and suggest that thermal perturbation of VLDL mimics aspects of protein and lipid transfer among TG-rich lipoproteins and HDL pools in plasma.…”

Apolipoprotein E•dipalmitoylphosphatidylcholine particles are ellipsoidal in solution

“…A second possibility is that high concentrations of polyethylene glycol 1000 (PEG 1000) or low pH in the crystallization buffer may have caused some minor remodeling of the particles. Moreextensive remodeling of apoA-I&DMPC particles has been observed with higher-molecular-weight PEGs (35). However, particles from dissolved apoE&DPPC crystals have an electrophoretic mobility identical to that of native apoE& DPPC crystals under nondenaturing conditions, ruling out any large changes in particle morphology (22).…”