ApoA-I/HDL Generation and Intracellular Cholesterol Transport through Cytosolic Lipid-Protein Particles in Astrocytes

Abstract

Exogenous apolipoprotein A-I (apoA-I) associates with ATP-binding cassette transporter A1 (ABCA1) on the cell surface of astrocytes like various peripheral cells and enhances the translocation of newly synthesized cholesterol from the endoplasmic reticulum/Golgi apparatus (ER/Golgi) to the cytosol. The cholesterol translocated to the cytosol is incorporated to cytosolic lipid-protein particles (CLPP) together with phospholipids and proteins such as sphingomyelin, phosphatidylcholine, caveolin-1, protein kinase Cα (PK-Cα), and cyclophilin A. The CLPP are high density lipoproteins- (HDL-)like cytosolic lipid-protein complex with densities of 1.09–1.16 g/mL and diameters of 17-18 nm. The association of exogenous apoA-I with cellular ABCA1 induces tyrosine phosphorylation, activation, and translocation to the CLPP of ABCA1-associated phospholipase Cγ (PL-Cγ) in rat astrocytes. Furthermore, PK-Cα is translocated and activated to/in the CLPP through theproduction of diacylglyceride in the CLPP. ApoA-I enhances both the association of CLPP with microtubules and the phosphorylation of α-tubulin as a component of microtubules. The CLPP are dissociated from microtubules after α-tubulin in microtubules is phosphorylated by the CLPP-associated PK-Cα. The association and dissociation between CLPP and microtubules may participate in the intracellular transport of cholesterol to the plasma membrane.

The formation of giant plasma membrane vesicles enable new insights into the regulation of cholesterol efflux

“…4a). Cells were also treated with the cholesterol acceptor ApoA1, which is well-characterized to promote cholesterol efflux 19,48 . Cells were evaluated after 24 and 48 hours of treatment, to monitor efflux progression.…”

“…Multiple cellular cholesterol efflux mechanisms have been identified, including aqueous diffusion, facilitated diffusion by scavenger receptor class B (SR-BI), and movement by the cholesterol transporters ABCA1 and ABCG1 22 . Intracellular cholesterol trafficking is facilitated by vesicular and non-vesicular processes 23,24 , with microtubules playing a role in the movement of cholesterol between intracellular compartments and the cell surface 19,25 . A role for the actin cytoskeleton in cholesterol efflux has been suggested by the observation that binding of the cholesterol acceptor apolipoprotein A-I (ApoA1) stimulates actin remodeling at the cell surface 26 .…”

β-COP as a Component of Transport Vesicles for HDL Apolipoprotein-Mediated Cholesterol Exocytosis

“…Of them, α-actin 2 (α smooth muscle actin) and β-actin are known for vesicle transport and they potentially guide the vesicles moving toward a particular location on the cell membrane [ 36 ]. The α tubulin, β tubulin and clathrin are previously identified as components of coat vesicles [ 37 ], and α tubulin has been shown to participate apoA-1 mediated cholesterol efflux in astrocytes [ 38 ]. Adenylyl cyclase-associated protein 1 is a signaling protein that regulates actin cytoskeleton function [ 39 ].…”

Lipid metabolism and retinal diseases

“…Astrocytes are thought to be the primary site of HDL‐like apolipoprotein synthesis. The mechanism of HDL‐like secretion by astrocytes is still poorly understood and several hypotheses have been put forward (Ito & Michikawa, 2014; Karten et al, 2006). These particles are then remodelled via their interactions with ABCG1/G4 receptors expressed by astrocytes and neurons, for example (Tarr & Edwards, 2008).…”