Autophagy of amyloid beta-protein in differentiated neuroblastoma cells exposed to oxidative stress

Oxidative stress is considered important for the pathogenesis of Alzheimer disease (AD), which is characterized by the formation of senile plaques rich in amyloid beta-protein (Aβ). Aβ cytotoxicity has been found dependent on lysosomes, which are abundant in AD neurons and are shown to partially co-localize with Aβ. To determine whether oxidative stress has any influence on the relationship between lysosomes and Aβ1–42 (the most toxic form of Aβ), we studied the effect of hyperoxia (40% versus 8% ambient oxygen) on the intracellular localization of Aβ1–42 (assessed by immunocytochemistry) in retinoic acid differentiated SH-SY5Y neuroblastoma cells maintained in serum-free OptiMEM medium. In control cells, Aβ1–42 was mainly localized to small non-lysosomal cytoplasmic granules. Only occasionally Aβ1–42 was found in large (over 1 μm) lysosomal-associated membrane protein 2 positive vacuoles, devoid of the early endosomal marker rab5. These large Aβ1–42-containing lysosomes were not detectable in the presence of serum (known to suppress autophagy), while their number increased dramatically (up to 24-fold) after exposure of cells to hyperoxia during 5 days. Activation of autophagy by hyperoxia was confirmed by transmission electron microscopy. Furthermore, an inhibitor of autophagic sequestration 3-methyladenine prevented the accumulation of Aβ1–42-positive lysosomes due to hyperoxia. In parallel experiments, intralysosomal accumulation of Aβ1–40 following oxidative stress has been found as well. The results suggest that Aβ can be autophagocytosed and its accumulation within neuronal lysosomes is enhanced by oxidative stress.