Cholesterol independent effect of LXR agonist TO-901317 on gamma-secretase

Abstract

The balance of intracellular cholesterol has proven to be critical to the production of β-amyloid (Aβ). Reducing cholesterol in vitro leads to decreased production of Aβ, whereas an increase in cellular cholesterol induces Aβ production. Liver X Receptor (LXR) agonists are known to increase cholesterol efflux from cells, but there are conflicting reports as to the effects of these agonists on Aβ production. We therefore examined the effects of efflux-inducing agents on Aβ production in vitro. We used methyl-β-cyclodextrin and an LXR agonist (TO-901317) to induce cholesterol efflux and studied the resulting Aβ production in a stable amyloid precursor protein (APP) -transfected cell line. When cholesterol efflux was induced with methyl-β-cyclodextrin there was a >60% decrease in Aβ40 and Aβ42 production. However, while activation of LXR using TO-901317-induced cholesterol efflux in the presence of a cholesterol acceptor, no changes in Aβ levels were recorded. When cells were incubated with TO-901317 above the concentration required for maximal cholesterol efflux, there was a 150% increase in Aβ42 levels. The absence of a cholesterol acceptor from the culture media (preventing cholesterol efflux) did not blunt this increase in Aβ42, suggesting that the effects of TO-901317 on Aβ42 are efflux independent. These results were confirmed in APP stably transfected human H4 cells, which revealed in addition to a 200% increase in Aβ42 levels, a concomitant 80% reduction in Aβ38. A cell-free γ-secretase assay confirmed that TO-901317 can directly alter γ-secretase activity. These data demonstrate that TO-901317 can directly modulate the site of cleavage of APP by γ-secretase in vitro. © 2007 The Authors.