Processing of Endogenous A beta PP in Blood-Brain Barrier Endothelial Cells is Modulated by Liver-X Receptor Agonists and Altered Cellular Cholesterol Homeostasis

Impaired clearance of cerebral amyloid-beta (A beta) across the blood-brain barrier (BBB) may facilitate the onset and progression of Alzheimer's disease (AD). Additionally, experimental evidence suggests a central role for cellular cholesterol in amyloid-beta protein precursor (A beta PP) processing. The present study investigated whether brain capillary endothelial cells (BCEC; the anatomical basis of the BBB) are capable of endogenous A beta PP synthesis and whether and to what extent A beta PP synthesis and processing is under control of cellular cholesterol homeostasis. Intracellular cholesterol metabolism was pharmacologically manipulated by using natural and synthetic liver-X receptor (LXR) agonists. Using an in vitro model of the BBB consisting of primary porcine BCEC (pBCEC), we demonstrate that endogenous full-length A beta PP synthesis by pBCEC is significantly increased while the amount of cell-associated, amyloidogenic A beta oligomers is decreased in response to 24(S)-hydroxycholesterol (24OH-C) or 27OH-C, TO901317, cholesterol, or simvastatin treatment. Oxysterols, as well as simvastatin, enhanced the secretion of non-amyloidogenic sA beta PP alpha up to 2.5-fold. In parallel, LXR agonists reduced cholesterol biosynthesis by 30-80% while stimulating esterification (up to 2.5-fold) and efflux (up to 2.5-fold) of cellular cholesterol by modifying hydroxymethylglutaryl-CoA reductase (HMGCR), sterol regulatory element-binding protein (SREBP-2), acyl-CoA: cholesterol acyltransferase 2 (ACAT-2), and ATP binding cassette transporter A1 (ABCA1) expression levels. In a polarized in vitro model mimicking the BBB, pBCEC secreted sA beta PP alpha preferentially to the basolateral compartment. In summary, endothelial cells of the BBB actively synthesize A beta PP, A beta oligomers, and secrete A beta PP alpha in a polarized manner. A beta PP processing by pBCEC is regulated by LXR agonists, which have been proven beneficial in experimental AD models.