Internalization of beta-amyloid causes downregulation of apolipoprotein E mRNA expression in neuroblastoma cells

Apolipoprotein (apo) E, like beta-amyloid (Abeta), is a key component of the senile plaques that characterize Alzheimer's disease (AD). Understanding how apoE participates in the formation of senile plaques is necessary to clarify the pathogenesis of AD; however, the mechanism remains unknown. In this study, we investigated the changes of cellular apoE and its mRNA level induced by addition of extracellular Abeta to neuroblastoma cells. The presence of > or = 1.0 micromol/L of Abeta induced a decrease of apoE mRNA expression and an increase in the immunofluorescence reactivity for intracellular apoE. Both Abeta and apoE were observed by electron-microscopy to be localized within lysosomes. The levels of intracellular apoE and its mRNA returned to the steady state time-dependently. These changes were attenuated by treatments with heparinase I or receptor-associated protein. These findings suggest that the internalized Abeta, along with cellular apoE, induces downregulation of apoE mRNA via a pathway possibly mediated by apoE receptors and heparin sulfate proteoglycans. A disorder of this physiological response could be linked to the development of AD.