Cholesterol retention in Alzheimer's brain is responsible for high β- and γ-secretase activities and Aβ production

Alzheimer's disease (AD) is characterized by overproduction of Aβ derived from APP cleavage via β- and γ-secretase pathway. Recent evidence has linked altered cholesterol metabolism to AD pathogenesis. In this study, we show that AD brain had significant cholesterol retention and high β- and γ-secretase activities as compared to age-matched non-demented controls (ND). Over one-half of AD patients had an apoE4 allele but none of the ND. β- and γ-secretase activities were significantly stimulated in vitro by 40 and 80 μM cholesterol in AD and ND brains, respectively. Both secretase activities in AD brain were more sensitive to cholesterol (40 μM) than those of ND (80 μM). Filipin-stained cholesterol overlapped with BACE and Aβ in AD brain sections. Cholesterol (10–80 μM) added to N2a cultures significantly increased cellular cholesterol, β- and γ-secretase activities and Aβ secretion. Similarly, addition of cholesterol (20–80 μM) to cell lysates stimulated both in vitro secretase activities. Ergosterol slightly decreased β-secretase activity at 20–80 μM, but strongly inhibited γ-secretase activity at 40 μM. Cholesterol depletion reduced cellular cholesterol, β-secretase activity and Aβ secretion. Transcription factor profiling shows that several key nuclear receptors involving cholesterol metabolism were significantly altered in AD brain, including decreased LXR-β, PPAR and TR, and increased RXR. Treatment of N2a cells with LXR, RXR or PPAR agonists strongly stimulated cellular cholesterol efflux to HDL and reduced cellular cholesterol and β-/γ-secretase activities. This study provides direct evidence that cholesterol homeostasis is impaired in AD brain and suggests that altered levels or activities of nuclear receptors may contribute to cholesterol retention which likely enhances β- and γ-secretase activities and Aβ production in human brain.