DNA damage-inducing agents elicit gamma-secretase activation mediated by oxidative stress.

According to the amyloid cascade hypothesis, Alzheimer’s disease is the consequence of neuronal cell death induced by β-amyloid (Aβ), which accumulates by abnormal clearance or production. On the other hand, recent studies have shown cell death-induced alteration in amyloid precursor protein (APP) processing, suggesting potential mutual interactions between APP processing and cell death. We have shown previously that the cell death caused by DNA damage-inducing agents (DDIAs) facilitated γ-secretase activity and Aβ generation in a Bax/Bcl-2-dependent, but caspase-independent manner. Here, we attempted to elucidate the downstream mechanism that modulates γ-secretase activity in DDIA-treated cells. N-acetyl cysteine, a potent antioxidant, attenuated DDIA-induced enhancement of γ-secretase activity but failed to rescue cell death. Overexpression of heat shock protein 70, which blocks cytochrome c release from mitochondria, also reduced γ-secretase activity. Moreover, glutathione depletion significantly facilitated γ-secretase activity and Aβ generation by enhancing the formation of higher molecular weight γ-secretase complex before signs of cell death developed. Finally, Aβ treatment, a known inducer of oxidative stress, also increased γ-secretase activity. Taken together, these results indicate that DDIA-induced γ-secretase activation is dependent on augmented oxidative stress, and that Aβ and γ-secretase may activate each other. On the basis of these results, we propose a feed-back loop between oxidative stress and Aβ generation mediated by γ-secretase activation.