The cause of neuronal degeneration in Alzheimer's disease (AD) has not been completely clari®ed, but has been variously attributed to increases in cytosolic calcium and increased generation of reactive oxygen species (ROS). The b-amyloid fragment (Ab) of the amyloid precursor protein induces calcium in¯ux, ROS and apoptosis. Homocysteine (HC), a neurotoxic amino acid that accumulates in neurological disorders including AD, also induces calcium in¯ux and oxidative stress, which has been shown to enhance neuronal excitotoxicity, leading to apoptosis. We examined the possibility that HC may augment Ab neurotoxicity. HC potentiated the Ab-induced increase in cytosolic calcium and apoptosis in differentiated SH-SY-5Y human neuroblastoma cells. The antioxidant vitamin E and the glutathione precursor N-acetyl-Lcysteine blocked apoptosis following cotreatment with HC and Ab, indicating that apoptosis is associated with oxidative stress. These ®ndings underscore that moderate accumulation of excitotoxins at concentrations that alone do not appear to initiate adverse events may enhance the effects of other factors known to cause neurodegeneration such as Ab.

No abstract

Exposure of neurons to amyloid-beta (Abeta) is accompanied by a cascade of oxidative damage that initiates with lipid peroxidation followed by subsequent generation of cytosolic free radicals and reactive oxygen species (ROS). The antioxidant vitamin E has been utilized to counteract Abetainduced oxidative stress. We considered herein whether or not the lipid-solubility of vitamin E limits its neuroprotection to membrane-related oxidative damage, and renders it relatively ineffective where prior lipid peroxidation has already generated cytosolic free radicals and ROS. To test this possibility, we treated differentiated SH-SY-5Y human neuroblastoma with vitamin E or a cell-permeant antioxidant, Nacetyl cysteine (NAC), simultaneously with or 15 min after the application of Abeta. Both vitamin E and NAC prevented Abeta-induced ROS generation when applied simultaneously with Abeta, but only NAC prevented Abeta-induced ROS generation when added to cultures that had previously been exposed to Abeta. These results support the hypothesis that vitamin E can quench Abeta-induced lipid peroxidation, but cannot effectively quench ROS generated by prior lipid peroxidation. These findings in cell culture may provide limited insight into why vitamin E is not fully effective against neurodegeneration in AD in clinical settings, since some neuronal populations are likely to already have been compromised by prior Abeta exposure before vitamin E treatment was initiated.