Molecular mechanisms of cellular cholesterol efflux

Abstract

Most types of cells in the body do not express the capability of catabolizing cholesterol, so cholesterol efflux is essential for homeostasis. For instance, macrophages possess four pathways for exporting free (unesterified) cholesterol to extracellular high density lipoprotein (HDL). The passive processes include simple diffusion via the aqueous phase and facilitated diffusion mediated by scavenger receptor class B, type 1 (SR-BI). Active pathways are mediated by the ATP-binding cassette (ABC) transporters ABCA1 and ABCG1, which are membrane lipid translocases. The efflux of cellular phospholipid and free cholesterol to apolipoprotein A-I promoted by ABCA1 is essential for HDL biogenesis. Current understanding of the molecular mechanisms involved in these four efflux pathways is presented in this minireview.

Keywords: ABC Transporter; Cholesterol; Efflux; High Density Lipoprotein (HDL); Macrophage; Phospholipid; Plasma Membrane; Scavenger Receptor.

FIGURE 1.

Summary of steps involved in the exchange of cholesterol molecules between PL-containing donor and acceptor particles by the aqueous diffusion mechanism. The rate of transfer of the highly hydrophobic cholesterol molecule from donor to acceptor by this simple diffusion process is limited by the rate of desorption into the aqueous phase. As shown at the left of the diagram, the transition (activated) state involves an almost completely desorbed cholesterol molecule; the free energy of such a molecule that is attached to the donor particle surface by its nonpolar end but has most of its hydrophobic surface exposed to water is high (see the free energy profile). This state is achieved by oscillatory motions of the cholesterol molecule in the plane perpendicular to the surface of the particle. Most of the time, the free energy of a cholesterol molecule in this transition state is reduced by relaxation of the molecule back into the donor particle where the cholesterol molecule is fully solvated by PL acyl chains. Occasionally, a cholesterol molecule desorbs completely into the aqueous phase (net free energy change, ΔG^transfer) where, because of its small size, it diffuses relatively quickly until a collision with an acceptor particle leads to rapid absorption and capture. The flux of cholesterol mass out of the donor particle is given by the product (rate constant for desorption, k_off) × (mass of cholesterol in the donor particle). Cholesterol molecules can diffuse in both directions between donor and acceptor particles with the direction of net mass transfer being determined by the concentration (activity) gradient (which approximates to the difference in the cholesterol/PL ratios of the two particles). The physical state of the PL in the particle surface influences the activity (fugacity) of the cholesterol molecules so that k_off is dependent on parameters such as degree of PL acyl chain unsaturation and the content of sphingomyelin. See under “Aqueous Diffusion Efflux Pathway” for further details.

FIGURE 2.

Mechanisms of cellular cholesterol efflux by facilitated diffusion. A, this schematic shows that the presence of the integral membrane proteins, SR-BI and ABCG1, leads to formation of an activated pool of cholesterol in the plasma membrane. The higher activity of this cholesterol leads to enhanced desorption (elevated k_off) and increased efflux by the aqueous diffusion mechanism (cf. Fig. 1). Under this condition, efflux is not influenced by changes in binding of HDL acceptor particles to either SR-BI or ABCG1. Active transport of cholesterol from the cell interior to the plasma membrane mediated by ABCG1 contributes to the formation of the activated cholesterol pool in the plasma membrane by this transporter. B, HDL binds to SR-BI with high affinity, and at low extracellular concentrations of HDL, this interaction promotes cholesterol efflux to the docked HDL particles. The facilitated movement of cholesterol molecules between the PL bilayer of the plasma membrane and the HDL particle bound in the appropriate conformation occurs by diffusion through a nonpolar channel (tunnel) formed by the extracellular domain of the SR-BI molecule. Because the concentration of free (unesterified) cholesterol is higher in the plasma membrane than in the HDL particle, efflux of free cholesterol is favored. It should be noted that because the concentration gradient of cholesterol ester is in the opposite direction, the net influx of cholesterol ester from bound HDL particle to the plasma membrane is favored (the so-called selective uptake process). See under “SR-BI Efflux Pathway” and “ABCG1 Efflux Pathway” for further details.

FIGURE 3.

Summary of the molecular mechanism by which ABCA1 activity in the plasma membrane of cells promotes efflux of PL and cholesterol to extracellular apoA-I and formation of nascent HDL particles. As shown at the top of the diagram, direct apoA-I/ABCA1 interaction and apoA-I/membrane lipid interactions occur with the former leading to transporter stabilization and the latter leading to HDL particle assembly. The activated lipid domain to which apoA-I binds is created as a consequence of the PL translocation induced by ABCA1. As shown in the lower part of the figure, the activated lipid domain is formed by membrane bending and comprises an exovesiculated segment of the plasma membrane. Amphipathic α-helices in the apoA-I molecule confer detergent-like properties on the protein, allowing it to solubilize PL by binding to lattice defects in highly curved PL bilayer surfaces, thereby inducing bilayer fragmentation and formation of discoidal nascent HDL particles. These particles comprise small segments of PL/cholesterol bilayer (containing on the order of 100 PL molecules) that are most frequently stabilized by either two or three apoA-I molecules. The solubilization step mediated by apoA-I is rate-limiting for the overall efflux of PL and cholesterol from the cell. The catalytic efficiency (V_max/K_m) of apoA-I is highest for the lipid-free protein so that its efficiency is reduced by prior phospholipidation. See under “Mechanism of PL/FC Efflux and Nascent HDL Particle Formation” for further details.