Objective:
The ability of high density lipoprotein (HDL) to promote cholesterol efflux from ATP binding cassette transporter A1 (ABCA1) is inversely associated with HDL particle size. The exchangeability of HDL’s primary protein constituent, apolipoprotein A-I (apoA-I), is positively correlated with cholesterol efflux capacity. However, it is unknown whether HDL particle size similarly affects the exchangeability of apoA-I. In this study, we investigated the effect of HDL particle size on the exchangeability of apoA-I on reconstituted (rHDL) and human-derived plasma HDL (pHDL) particles.
Method and Results:
Discoidal rHDL particles of well-defined size (17.0, 12.2, 9.6, 8.4 and 7.8 nm) were prepared from POPC and apoA-I by cholate dialysis and purified by size exclusion chromatography. Human HDL was isolated by subclass (HDL2b, 2a, 3a, 3b, 3c) from the plasma of healthy volunteers by sequential density gradient ultracentrifugation. HDL-apoA-I exchange (HAE) was measured by electron paramagnetic resonance (EPR) and non-denaturing gradient gel electrophoresis (NDGGE) following incubation with either spin-labeled or fluorescently-labeled, lipid-free apoA-I. Size-dependent effects were observed with rHDL particles, with the largest rHDL particles exhibiting 3-fold higher HAE compared to the smallest particles. In contrast, the HAE of human-derived pHDL particles was relatively constant across all subclasses at constant apoA-I concentration.
Conclusions:
HDL-apoA-I exchange is size-dependent in rHDL, with HAE positively associated with rHDL particle size, but it is size-independent in human pHDL. Our findings suggest that additional factors such as remodeling enzymes may facilitate apoA-I exchange from human pHDL.
Characterization of apolipoprotein A-I- and A-II-containing lipoproteins in a new case of high density lipoprotein deficiency resembling Tangier disease and their effects on intracellular cholesterol efflux.