Characterization of nascent HDL particles and microparticles formed by ABCA1-mediated efflux of cellular lipids to apoA-I

Abstract

The nascent HDL created by ABCA1-mediated efflux of cellular phospholipid (PL) and free (unesterified) cholesterol (FC) to apolipoprotein A-I (apoA-I) has not been defined. To address this issue, we characterized the lipid particles released when J774 mouse macrophages and human skin fibroblasts in which ABCA1 is activated are incubated with human apoA-I. In both cases, three types of nascent HDL containing two, three, or four molecules of apoA-I per particle are formed. With J774 cells, the predominant species have hydrodynamic diameters of z9 and 12 nm. These discoidal HDL particles have different FC contents and PL compositions, and the presence of acidic PL causes them to exhibit a-electrophoretic mobility. These results are consistent with ABCA1 located in more than one membrane microenvironment being responsible for the production of the heterogeneous HDL. Activation of ABCA1 also leads to the release of apoA-I-free plasma membrane vesicles (microparticles). These larger, spherical particles released from J774 cells have the same PL composition as the 12 nm HDL and contain CD14 and ganglioside, consistent with their origin being plasma membrane raft domains. The various HDL particles and microparticles are created concurrently, and there is no precursor-product relationship between them. Importantly, a large fraction of the cellular FC effluxed from these cells by ABCA1 is located in microparticles. Collectively, these results show that the products of the apoA-I/ ABCA1 interaction include discoidal HDL particles containing different numbers of apoA-I molecules. The cellular PLs and cholesterol incorporated into these nascent HDL particles originate from different cell membrane domains.

Cholesterol homeostasis and the escape tendency (activity) of plasma membrane cholesterol

“…e) ABCA1 might transfer bites of plasma membrane to nascent HDL particles [138]. This concept is consistent with the fact that phospholipids from both leaflets are represented in newly-formed HDL particles [131]. It is suggested that ABCA1 uses the energy of ATP hydrolysis to pump cytoplasmic phospholipids to the outer surface, thereby bending the bilayer outward.…”

“…Given that the ATP binding domain and putative substrate entry site of ABCA1 are oriented toward the endofacial leaflet, one might expect that cytoplasmic phospholipids would preferentially be transported to apoA-I. This hypothesis is undercut by the observation that, although ABCA1 might promote the transfer of cytoplasmic phospholipids to the cell surface, nascent HDL particles have a preponderance of exofacial lipid types [131][132][133]. c) It has been suggested that, because HDL particles are enriched in cholesterol, export draws selectively from sterol rich domains (rafts or caveolae).…”

ApoA1 and ApoA1-specific self-antibodies in cardiovascular disease

“…67 Nascent HDL particles (7-17 nm in diameter) are originally discoidal and include 2-4 ApoA1 molecules. 68 In a case of high phospholipid content, generation of larger HDL particles is enhanced. 69,70 As ApoA1 is directly involved in lipid metabolism, this property is important for considering this apolipoprotein as one of key molecular players in the pathogenesis of cardiometabolic diseases, such as atherosclerosis.…”

Lipid Internal Dynamics Probed in Nanodiscs

“…Recent investigations into membrane proteins4, 19, 22, 25 have revealed that nanodiscs are a suitable tool to reconstitute membrane proteins in a native environment to solve their three‐dimensional structure with atomic resolution. They are derived from apolipoprotein A1 (ApoA‐1), which solubilises hydrophobic molecules, such as lipids and cholesterol, to deliver them to the liver through the blood and interstitial fluid as high‐density lipoprotein (HDL) particles 16, 27. A recent study by NMR spectroscopy shows how the structure of ApoA‐1 wraps around the hydrophobic part of the lipids to form a lipid‐bilayer patch in solution 5.…”