Apolipoprotein A-I Induces Translocation of Cholesterol, Phospholipid, and Caveolin-1 to Cytosol in Rat Astrocytes

Abstract

Intercellular cholesterol transport in the brain is carried by high density lipoprotein (HDL) generated in situ by cellular interaction with the apolipoprotein apoE, which is mainly synthesized by astrocytes, and with apoA-I secreted by cells such as endothelial cells. Rat astrocytes in fact generate HDL with extracellular apoA-I in addition to releasing HDL with endogenously synthesized apoE, seemingly by the same mechanism as the HDL assembly for systemic circulation. Relating to this reaction, apoA-I induced translocation of newly synthesized cholesterol and phospholipid to the cytosol prior to extracellular assembly of HDL, accompanied by an increase of caveolin-1 in the cytosol, activation of sterol regulatory element-binding protein, and enhancement of cholesterol synthesis. The lipid translocated into the cytosol was recovered in the fraction with a density of 1.09 -1.16 g/ml as well as caveolin-1 and cyclophilin A. Cyclosporin A inhibited these apoA-I-mediated reactions and suppressed apoA-I-mediated cholesterol release. The findings suggest that such translocation of cholesterol and phospholipid into the cytosol is related to the apo A-I-mediated HDL assembly in astrocytes through functional association with caveolin-1 and a cyclosporin A-sensitive cyclophilin protein(s). The central nervous system (CNS)1 is sheltered from interaction with the lipoproteins of the systemic circulation by the blood-brain barrier. Therefore, extracellular cholesterol transport in the CNS is mediated by its own lipoprotein system, consisting mainly of the particles equivalent to plasma HDL (1). The main apolipoproteins are apoE produced by astrocytes (2), microglias (3), and apoA-I from an unknown source but reportedly secreted by brain endothelial cells (4). The astrocytes were shown to generate HDL, not only with endogenously synthesized apoE but also with exogenous apoE and apoA-I (5).The apolipoprotein-cell interaction that generates HDL is common for many somatic cells from various origins and is distinct from the diffusion-mediated cholesterol efflux from the cell surface (6). The reaction is a main source of plasma lipoprotein (7) and is also one of the major pathways for cholesterol release from the cells (8). Generation of HDL with cellular phospholipid seems to require a cellular interaction site with apolipoprotein. An intracellular cholesterol trafficking system linked to such interaction is responsible for incorporation of cholesterol into the HDL (9, 10). Mutations in the ABCA1 transporter protein were identified in patients with plasma HDL deficiency who lack the ability to generate HDL by this reaction (11-13). Thus, the reaction depends on the cellular system to export materials. A specific intracellular cholesterol transport system is important to make the HDL cholesterolrich. In macrophages and smooth muscle cells, protein kinase C was shown to be involved in this trafficking (9, 10). It is not known whether the mechanism for generation of HDL in somatic cells is different from that for the HDL assembly i...

A novel model of cholesterol efflux from lipid-loaded cells

“…Apo-A1 is involved not only in the transmembrane transportation of cholesterol but also in the transport of intracellular and extracellular cholesterol. After binding to the HDL receptor, the complex consisting of Apo-A1 and phospholipids triggers the transfer of free cholesterol from the intracellular cholesterol pool to the cell membrane [97] . The regulation network controlling the four transport systems is very complex.…”

“…Activated calcineurin suppresses the expression and activity of PPAR-γ, a regulator of the expression of SR-B1, ABC-A1 and CD36. Cyclosporin A, an inhibitor of cyclophilin A, suppresses apo-A1/HDL-induced RCT [97] . Therefore, cyclophilin A participates not only in the formation of the caveolin-1-cholesterol transport complex but also in the regulation of SR-B1-and ABC-A1-mediated cholesterol transmembrane transportation.…”

Cytoplasmic Lipid Droplets Are Sites of Convergence of Proteasomal and Autophagic Degradation of Apolipoprotein B

“…Finally, as a third possibility, the particles could be VLDL and/or pre-VLDL particles themselves. It is difficult to explain how they could extravasate from the luminal side, but lipidic particles bearing apolipoproteins can exist in the cytoplasm (Ito et al, 2002).…”

Endogenous adipocyte apolipoprotein E is colocalized with caveolin at the adipocyte plasma membrane

“…by guest, on May 10, 2018 www.jlr.org Downloaded from colocalization for apoE and cav-1 ( 8,(42)(43)(44). The C-terminal domain of apoE is highly a -helical and is thought to be primarily involved in mediating the association of apoE with phospholipid ( 42 ).…”

“…The C-terminal domain of apoE is highly a -helical and is thought to be primarily involved in mediating the association of apoE with phospholipid ( 42 ). Cav-1 has high affi nity for sterol and is thought to exist not only in a membrane-associated form but also in a cytoplasmic and secreted form ( 8,43,44 ). In each of the latter two forms, cav-1 appears to exist as part of a high-density lipoprotein particle that can contain apoA1, which like apoE, is also a soluble amphipathic a -helical apolipoprotein.…”