Amyloid Beta-Peptide Increases BACE1 Translation through the Phosphorylation of the Eukaryotic Initiation Factor-2α

Alzheimer's disease (AD) is tightly linked to oxidative stress since amyloid beta-peptide (A beta) aggregates generate free radicals. Moreover, the aggregation of A beta is increased by oxidative stress, and the neurotoxicity induced by the oligomers and fibrils is in part mediated by free radicals. Interestingly, it has been reported that oxidative stress can also induce BACE1 transcription and expression. BACE1 is the key enzyme in the cleavage of the amyloid precursor protein to produce A beta, and the expression of this enzyme has been previously shown to be enhanced in the brains of Alzheimer's patients. Here, we have found that BACE1 expression is increased in the hippocampi from AD patients at both the early (Braak stage II) and late (Braak stage VI) stages of the disease as studied by immunohistochemistry and western blot. To address the role of A beta and oxidative stress in the regulation of BACE1 expression, we have analyzed the effect of subtoxic concentrations of A oligomers (0.25 mu M) and H2O2(10 mM) on a human neuroblastoma cell line. Firstly, our results show that A beta oligomers and H2O2 induce an increase ofBACE1mRNA as we studied by qPCR. Regarding BACE1 translation, it is dependent on the phosphorylation of the eukaryotic initiation factor 2 alpha(eIF2 alpha), sinceBACE1mRNA bears a 5 ' UTR that avoids its translation under basal conditions. BACE1 5 ' UTR contains four upstream initiating codons (uAUGs), and its translation is activated when eIF2 alpha is phosphorylated. Consistently, we have obtained that A beta oligomers and H2O2 increase the levels of BACE1 and p-eIF2 alpha assayed by western blot and confocal microscopy. Our results suggest that A beta oligomers increase BACE1 translation by phosphorylating eIF2 alpha in a process that involves oxidative stress and conforms a pathophysiological loop, where the A beta once aggregated favors its own production continuously by the increase in BACE1 expression as observed in AD patients.