The effects of ABCA1 on cholesterol efflux and Ab levels in vitro and in vivo Mark P. Burns,* Lilit Vardanian,* Ahdeah Pajoohesh-Ganji,* Lili Wang, Matthew Cooper,* Donnie C. Harris,* Karen Duff and G. William Rebeck* *Department of Neuroscience, Georgetown University Medical Center, Washington, USA  Center for Dementia Research, Nathan S. Kline Institute, Orangeburg, New York, USA Abstract ABCA1 promotes cholesterol efflux from cells and is required for maintaining plasma cholesterol levels. Cholesterol home- ostasis is important in the production of b-amyloid (Ab), a peptide that is overproduced in Alzheimer’s disease (AD). Overexpression of ABCA1 can be achieved by stimulating Liver X Receptors (LXR), and changes in Ab have been reported after LXR stimulation in vitro. To determine whether ABCA1 could alter endogenous Ab levels, we used two dif- ferent in vivo systems. We first examined the effects of an LXR agonist (TO-901317) on wild-type mice and found an increase in brain ABCA1 and apoE levels, which caused an increase in plasma cholesterol. This was accompanied by a decrease in brain Ab levels. We then examined endogenous Ab levels in ABCA1 knockout mice and found that, despite having no ABCA1, lowered brain apoE levels, and lowered plasma cholesterol, there was no change in Ab levels. To assess these in vivo models in an in vitro system, we designed a model in which cholesterol transport via ABCA1 (or related transporters) was prevented. Switching off cholesterol efflux, even in the presence of TO-901317, caused no change in Ab levels. However, when efflux capability was restored, TO-901317 reduced Ab levels. These data show that pro- moting cholesterol efflux is a viable target for Ab reducing strategies; however, knockout of cholesterol transporters is not sufficient to alter Ab in vitro or in vivo. Keywords: Alzheimer, amyloid, apoE, oxysterol. J. Neurochem. (2006) 98, 792–800. ABCA1 is an ATP-binding cassette protein that promotes efflux of cholesterol and phospholipids from intracellular compartments to extracellular cholesterol acceptors. ABCA1 is regulated by the liver X receptor (LXR), a nuclear receptor activated by oxysterols to regulate cholesterol homeostasis (Oram and Heinecke 2005). While these pathways have been extensively examined in the periphery, there is less know- ledge of this process in the central nervous system. Regulation of cholesterol homeostasis in the brain may affect the etiology of Alzheimer’s disease (AD). Excessive accumulation of the neurotoxic peptide amyloid-b (Ab) is a key pathological event in AD. Ab is the product of sequential proteolytic cleavage of the amyloid precursor protein (APP), and both enzymes involved in this cleavage are known to be affected by cholesterol levels, such that an increase in cholesterol causes an increase in Ab production and decreasing cholesterol reduces Ab production (Simons et al. 1998; Wahrle et al. 2002; Urano et al. 2005). Changes in cholesterol storage and distribution of cholesterol within the cell also affect Ab levels. Inhibiting the conversion of cholesterol to cholesterol esters within cells reduces Ab levels both in vivo and in vitro (Hutter-Paier et al. 2004; Puglielli et al. 2001). Prevention of cholesterol transport from late endosomes/lysosomes to the endoplasmic reticulum in vitro increases Ab levels (Yamazaki et al. 2001; Runz et al. 2002), and mutations of the NPC gene, which controls this transport in mice and humans leads to increased APP C-terminal fragments and increased Ab levels in vivo (Burns et al. 2003; Jin et al. 2004). Finally, retrospective epidemi- ological studies suggest that use of inhibitors of cholesterol synthesis to lower plasma cholesterol in humans can reduce the risk of developing AD (Jick et al. 2000; Wolozin et al. 2000; Rockwood et al. 2002). Received January 6, 2006; revised manuscript received February 27, 2006; accepted March 20, 2006. Address correspondence and reprint requests to G. William Rebeck, Ph.D., Georgetown University Medical Center, Department of Neuro- science, New Research Building-WP10, 3970 Reservoir Road NW, Washington, DC 20007, USA. E-mail: gwr2@georgetown.edu Abbreviations used: AD, Alzheimer’s disease; APP, amyloid precursor protein; CHO, Chinese hamster ovary; DEA, diethylamine; LXR, Liver X Receptors. Journal of Neurochemistry , 2006, 98, 792–800 doi:10.1111/j.1471-4159.2006.03925.x 792 Journal Compilation Ó 2006 International Society for Neurochemistry, J. Neurochem. (2006) 98, 792–800 Ó 2006 The Authors