Geriatric, Faculty of Health Science, Department of Clinical and Experimental Medicine, Linköping University, Linköping, Sweden.
J Alzheimers Dis. 2012;31(2):343-58. doi: 10.3233/JAD-2012-120001.
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-β (Aβ) generation. In this study, we investigated the effect of proteasome inhibition on Aβ accumulation and secretion, as well as the processing of amyloid-β protein precursor (AβPP) in AβPP(Swe) transfected SH-SY5Y neuroblastoma cells. We show that proteasome inhibition resulted in autophagy-dependent accumulation of Aβ in lysosomes, and increased levels of intracellular and secreted Aβ. The enhanced levels of Aβ could not be explained by increased amounts of AβPP. Instead, reduced degradation of the C-terminal fragment of AβPP (C99) by the proteasome makes C99 available for γ-secretase cleavage, leading to Aβ generation. Inhibition of autophagy after proteasome inhibition led to reduced levels of intracellular, but not secreted Aβ, and tended to further increase the C99 to AβPP ratio, supporting involvement of the autophagosome in Aβ 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 Aβ, as well as increased generation and secretion of Aβ, 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.
蛋白酶体对于降解磨损和错误折叠的蛋白质很重要。蛋白酶体活性降低与阿尔茨海默病（AD）有关。蛋白酶体抑制诱导自噬，但自噬对 AD 神经元是有益还是有害仍不清楚，因为自噬体被认为是淀粉样β（Aβ）生成的部位。在这项研究中，我们研究了蛋白酶体抑制对 Aβ 积累和分泌以及 Aβ 前体蛋白（AβPP）在 AβPP（Swe）转染的 SH-SY5Y 神经母细胞瘤细胞中的处理的影响。我们表明，蛋白酶体抑制导致 Aβ 在溶酶体中依赖自噬的积累，并增加细胞内和分泌的 Aβ 水平。增强的 Aβ 水平不能用 AβPP 的增加来解释。相反，蛋白酶体对 AβPP 的 C 端片段（C99）的降解减少使 C99 可用于 γ-分泌酶切割，导致 Aβ 的生成。蛋白酶体抑制后自噬的抑制导致细胞内 Aβ 水平降低，但分泌的 Aβ 水平没有降低，并且倾向于进一步增加 C99 与 AβPP 的比值，支持自噬体参与 Aβ 的生成。此外，蛋白酶体抑制导致细胞活力降低，自噬抑制可逆转这种降低。蛋白酶体的功能障碍可能导致 Aβ 在溶酶体中的积累，以及 Aβ 的生成和分泌增加，这部分是通过自噬来促进的。由于还检测到细胞活力下降，因此上调自噬可能是一种不成功的挽救机制，它不仅没有保护作用，反而有助于 AD 的发病机制。
