Thiamine pyrophosphate (TPP) and the activities of thiamine-dependent enzymes are reduced in Alzheimer's disease (AD) patients. In this study, we analyzed the relationship between thiamine deficiency (TD) and amyloid precursor protein (APP) processing in both cellular and animal models of TD. In SH-SY5Y neuroblastoma cells overexpressing APP, TD promoted maturation of β-site APP cleaving enzyme 1 (BACE1) and increased β-secretase activity which resulted in elevated levels of β-amyloid (Aβ) as well as β-secretase cleaved C-terminal fragment (β-CTF). An inhibitor of β-secretase efficiently reduced TD-induced up-regulation of Aβ and β-CTF. Importantly, thiamine supplementation reversed the TD-induced alterations. Furthermore, TD treatment caused a significant accumulation of reactive oxygen species (ROS); antioxidants suppressed ROS production and maturation of BACE1, as well as TD-induced Aβ accumulation. On the other hand, exogenous Aβ(1-40) enhanced TD-induced production of ROS. A study on mice indicated that TD also caused Aβ accumulation in the brain, which was reversed by thiamine supplementation. Taken together, our study suggests that TD could enhance Aβ generation by promoting β-secretase activity, and the accumulation of Aβ subsequently exacerbated TD-induced oxidative stress.
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