High density lipoprotein structure-function and role in reverse cholesterol transport

Abstract

High density lipoprotein (HDL) possesses important anti-atherogenic properties and this review addresses the molecular mechanisms underlying these functions. The structures and cholesterol transport abilities of HDL particles are determined by the properties of their exchangeable apolipoprotein (apo) components. ApoA-I and apoE, which are the best characterized in structural terms, contain a series of amphipathic alpha-helical repeats. The helices located in the amino-terminal two-thirds of the molecule adopt a helix bundle structure while the carboxy-terminal segment forms a separately folded, relatively disorganized, domain. The latter domain initiates lipid binding and this interaction induces changes in conformation; the alpha-helix content increases and the amino-terminal helix bundle can open subsequently. These conformational changes alter the abilities of apoA-I and apoE to function as ligands for their receptors. The apoA-I and apoE molecules possess detergent-like properties and they can solubilize vesicular phospholipid to create discoidal HDL particles with hydrodynamic diameters of ~10 nm. In the case of apoA-I, such a particle is stabilized by two protein molecules arranged in an anti-parallel, double-belt, conformation around the edge of the disc. The abilities of apoA-I and apoE to solubilize phospholipid and stabilize HDL particles enable these proteins to be partners with ABCA1 in mediating efflux of cellular phospholipid and cholesterol, and the biogenesis of HDL particles. ApoA-I-containing nascent HDL particles play a critical role in cholesterol transport in the circulation whereas apoE-containing HDL particles mediate cholesterol transport in the brain. The mechanisms by which HDL particles are remodeled by lipases and lipid transfer proteins, and interact with SR-BI to deliver cholesterol to cells, are reviewed.

Fig. 7.1

A Distribution of amphipathic α-helices in the human exchangeable apolipoproteins, apoA-I and apoE. The letter P below the rectangles indicates positions of all proline residues. B Amphipathic helix classes found in the exchangeable apolipoproteins. Classification is based on the distribution of charged residues (see Section 2.1). These figures were adapted from Segrest et al. (1992)

Fig. 7.2

Crystal structures of human apolipoproteins in the lipid-free state. A The six α-helices in human apoA-I are shown (from Ajees et al., 2006, with permission). The N-terminal anti-parallel four-helix bundle contains helices A (residues 10–39), B (50–84), C (97–137), and D (147–187). The C-terminal domain is formed by the two α-helices E (residues 196–213) and F (219–242). Hydrophobic residues located in the interior of the helix bundles are shown as sticks. B Ribbon model of the structure of the 22-kDa N-terminal domain fragment of human apoE3 (from Weisgraber (1994), with permission). Four of the five helices are arranged in an anti-parallel four-helix bundle. The residues spanned by each helix, together with the region in helix 4 recognized by the LDL receptor, are indicated

Fig. 7.3

Model of the two-step mechanism of apoA-I binding to a spherical lipid particle. In the lipid-free state, apoA-I is organized into two structural domains in which the N-terminal domain forms a helix bundle whereas the C-terminal domain forms a separate, less organized structure. Initial lipid binding occurs through amphipathic α-helices in the C-terminal domain accompanied by an increase in α-helicity probably in the region including residues 187–220. Subsequently, the helix bundle in the N-terminal domain undergoes a conformational opening, converting hydrophobic helix-helix interactions to the helix–lipid interaction; this second step is only slowly reversible. Reproduced with permission from Saito et al. (2003b)

Fig. 7.4

ApoA-I conformation on discoidal and spherical HDL particles. The apoA-I molecules are organized as a double-belt in discoidal particles and as a trefoil in spherical particles. All helix–helix interactions between the two molecules of apoA-I in the disc double-belt arrangement are also present between the three apoA-I molecules in the trefoil organization on the surface of a spherical HDL particle. Reproduced with permission from Silva et al. (2008)

Fig. 7.5

Schematic representation of the spontaneous solubilization of dimyristoyl phosphatidyl-choline (DMPC) multilamellar vesicles (MLV) by apoA-I. When apoA-I is incubated with the turbid MLV suspension at 24°C (the melting temperature of the DMPC acyl chains), the solution becomes optically clear because the DMPC bilayers are solubilized in a few minutes to create discoidal HDL particles that are too small to scatter visible light

Fig. 7.6

Molecular model explaining the two simultaneous kinetic phases of DMPC solubilization by apolipoproteins. The solubilization of DMPC MLV by apolipoprotein molecules involves four states (as indicated). Completion of the first and second of these stages can each occur by two simultaneous alternative pathways, one more rapid than the other, whereas stages three and four comprise a common pathway. Flexible apolipoprotein molecules react more rapidly than inflexible ones. See Segall et al. (2002) for more details. Reproduced with permission from Segall et al. (2002)

Fig. 7.7

Schematic overview of the major pathways involved in HDL-mediated macrophage cholesterol efflux and reverse cholesterol transport to the liver. ApoA-I is produced by the liver and acquires free cholesterol (FC) and phospholipid (PL) from liver and peripheral cells (including macrophages) via the ABCA1 transporter to form nascent (discoidal) HDL particles. Non-lipidated apoA-I is cleared by the kidney. FC efflux from macrophages to HDL particles is also promoted by the ABCG1 transporter and SR-BI. As discussed in Section 7.3.3, the FC in discoidal HDL particles is converted to CE by LCAT activity leading to the formation of mature, spherical HDL particles. PLTP mediates transfer of PL from VLDL into HDL thereby providing PL for the LCAT reaction. Mature HDL particles can be remodeled to smaller particles with the release of apoA-I by the actions of hepatic lipase (HL) and endothelial lipase (EL) which hydrolyze HDL TAG and PL, respectively. In humans, but not rodents, HDL-CE can be transferred to the VLDL/LDL pool by CETP and taken up by endocytosis into hepatocytes via interaction with the LDL receptor (LDLR). HDL-CE and FC are also transferred directly to hepatocytes via SR-BI-mediated selective uptake. Cholesterol taken up by the liver can be recycled back into the ABCA1 pathway, secreted into bile as either FC or bile acids, or assembled into lipoprotein particles that are secreted back into the circulation (not shown)

Fig. 7.8

Mechanism of interaction of apoA-I with ABCA1 and efflux of cellular phospholipids and cholesterol. The reaction in which apoA-I binds to ABCA1 and membrane lipids to create discoidal nascent HDL particles contains three steps. Step 1 involves the high affinity binding of a small amount of apoA-I to ABCA1 located in the plasma membrane PL bilayer; this regulatory pool of apoA-I up-regulates ABCA1 activity, thereby enhancing the active translocation of membrane PL from the cytoplasmic to exofacial leaflet. This translocase activity leads to lateral compression of the PL molecules in the exofacial leaflet and expansion of those in the cytoplasmic leaflet. Step 2 involves the bending of the membrane to relieve the strain induced by the unequal molecular packing density across the membrane and the formation of an exovesiculated domain to which apoA-I can bind with high affinity. This interaction with the highly curved membrane surface involves apoA-I/membrane lipid interactions and creates a relatively large pool of bound apoA-I. Step 3 involves the spontaneous solubilization by the bound apoA-I of membrane PL and cholesterol in the exovesiculated domains to create discoidal HDL particles containing two, three or four apoA-I molecules/particle. In the diagram, the two transmembrane six-helix domains of ABCA1 are represented as rectangles, whereas the two ATPase domains are shown as ovals. The space between the two rectangles represents the chamber in which translocation of PL molecules occurs. Reproduced with permission from Vedhachalam et al. (2007a)

Fig. 7.9

Suggested molecular mechanism for the solubilization of PL bilayers by apoA-I to create discoidal HDL particles. This process is envisaged to underlie the solubilization of DMPC MLV depicted in Figs. 7.5 and 7.6, and Step 3 in the formation of nascent HDL particles in the apoA-I/ABCA1 reaction depicted in Fig. 7.8

Fig. 7.10

Model of SR-BI-mediated selective uptake of cholesteryl ester (CE) from HDL. This model proposes that SR-BI contains a non-aqueous channel, which excludes water, and serves as a conduit for hydrophobic CE molecules diffusing from bound HDL down their concentration gradient to the cell plasma membrane. The scheme depicts a channel formed by a single SR-BI molecule, but it is possible that self-association of SR-BI is required to create the channel. Reproduced with permission from Rodrigueza et al. (1999)