Amyloid-β Secretion, Generation, and Lysosomal Sequestration in Response to Proteasome Inhibition: Involvement of Autophagy

The proteasome is important for degradation of worn out and misfolded proteins. Decreased proteasome activity has been implicated in Alzheimer's disease (AD). Proteasome inhibition induces autophagy, but it is still unknown whether autophagy is beneficial or deleterious to AD neurons, as the autophagosome has been suggested as a site of amyloid-beta (A beta) generation. In this study, we investigated the effect of proteasome inhibition on A beta accumulation and secretion, as well as the processing of amyloid-beta protein precursor (A beta PP) in A beta PPSwe transfected SH-SY5Y neuroblastoma cells. We show that proteasome inhibition resulted in autophagy-dependent accumulation of A beta in lysosomes, and increased levels of intracellular and secreted A beta. The enhanced levels of A beta could not be explained by increased amounts of A beta PP. Instead, reduced degradation of the C-terminal fragment of A beta PP (C99) by the proteasome makes C99 available for gamma-secretase cleavage, leading to A beta generation. Inhibition of autophagy after proteasome inhibition led to reduced levels of intracellular, but not secreted A beta, and tended to further increase the C99 to A beta PP ratio, supporting involvement of the autophagosome in A beta generation. Furthermore, proteasome inhibition caused a reduction in cellular viability, which was reverted by inhibition of autophagy. Dysfunction of the proteasome could cause lysosomal accumulation of A beta, as well as increased generation and secretion of A beta, which is partly facilitated by autophagy. As a decrease in cellular viability was also detected, it is possible that upregulation of autophagy is an unsuccessful rescue mechanism, which instead of being protective, contributes to AD pathogenesis.