Regulation of cholesterol efflux by amyloid beta secretion

Amyloid beta (Abeta) is a key molecule in the pathogenesis of Alzheimer's disease, but its physiological function remains unclear. Abeta is produced from amyloid precursor protein (APP) by beta- and gamma-secretases, which is enhanced by high levels of cellular cholesterol, so cholesterol is a risk factor for Alzheimer's disease. This linkage led us to hypothesize that Abeta is produced to regulate cellular cholesterol levels in response to high-cholesterol stimulation. Here we show that Abeta production caused a reduction of cellular cholesterol levels in transfected HEK293 cells and neuronal IMR-32 and Neuro2a cells, which was accompanied by an increase in efflux of cholesterol from cells. Fractionation of the culture media by ultracentrifugation and subsequent immunoelectron microscopic observation revealed that Abeta assembled high-density lipoprotein-like particles with cellular cholesterol during its secretion. This assembly was mediated by the ATP-binding cassette transporter A1. APP transgenic and knockout mice exhibited lower and higher levels of cellular cholesterol in their brains, suggesting that Abeta-mediated regulation of cellular cholesterol is physiological. Furthermore, we found that, when injected into mouse cerebral ventricle, reconstituted lipoproteins with Abeta were excreted into the peripheral tissues more efficiently than those without Abeta. This result suggests that Abeta mediates cholesterol transport from the brain to the circulation. We propose, based on these findings, a novel, apolipoprotein-like function for Abeta that is involved in maintenance of cellular and cerebral cholesterol homeostasis.