Astrocyte׳s endogenous apoE generates HDL-like lipoproteins using previously synthesized cholesterol through interaction with ABCA1.

Apolipoprotein E (apoE) in the brain is predominantly synthesized in and secreted from astrocytes to generate apoE-containing high-density lipoprotein-like particles (apoE/HDL). However, the mechanism underlying generation of apoE/HDL has not been completely understood. The newly synthesized cholesterol, which is synthesized in rat astrocytes within 24 h using [(3)H]-acetate as a cholesterol precursor, was assembled as lipoproteins with densities of 1.12-1.17 g/mL (higher density HDL), although apoE was secreted as lipoproteins with lower densities of 1.08-1.12 g/mL from the cells. This finding suggests that the newly synthesized cholesterol is released without the association with apoE, which is like that from apoE-deficient mouse (apoE-KO) astrocytes. The cholesterol released from rat astrocytes at 3 days after the onset of its synthesis (previously synthesized cholesterol) was assembled as apoE/HDL with the densities of 1.08-1.12 g/mL (lower density HDL). These findings indicate that the endogenous apoE participates in the release of previously synthesized cholesterol but not newly synthesized one. Whereas, exogenously added human apoE induced release of both newly synthesized and previously synthesized cholesterols to generate apoE/HDL with lower density, suggesting that the cellular pool of cholesterol released by endogenous and exogenous apoE is different. The endogenous apoE was distributed in the caveolin-1-rich domain along with ATP-binding cassette transporter A1 (ABCA1) in the membrane fraction and immuno-precipitated using an anti-ABCA1 antibody. However, this is not the case for ABCA1-KO astrocytes. These findings suggest that endogenous apoE generates lower density HDL to produce more lipidated HDL using previously synthesized cholesterol through the interaction with ABCA1 in caveolin-1-rich domain of astrocytes.