ApoE4 is associated with higher lipid peroxidation but not protein nitration in AD brains

Abstract Background Oxidative damage to proteins is associated with increased lipid peroxidation product hydroxynonenal (HNE) and nitrotyrosine (NT) during Alzheimer’s disease (AD). Because ApoE4 confers higher risk for AD, we assayed HNE and NT in AD and controls by ApoE allele. Oxidative damage was assayed in whole cell lysate and in lipid rafts (LR), where APP is enzymatically processed. Prefrontal cortex (Brodmann 8‐10) was compared with cerebellum, which has minimal AD pathology. Method Postmortem samples were age‐and sex matched for Braak scores of NFT. Homogenates from full thickness prefrontal cortex (n = 52) or cerebellum (n = 58) in isotonic buffer and mild detergent (RIPA; 1% NP40) were centrifuged (10,000g/1h; for the supernate fraction’). The same brain samples were processed for the subcellular LR fraction (Invent Biotechnologies; LR‐039); LR isolations were validated by ultracentrifugal fractionation. Samples were assayed by dot blots with antibodies for HNE or NT (Millipore, Burlington, MA). Result In prefrontal cortex supernates, AD increased HNE (+60%) and NT (+40%) above cognitively normal, while in cerebellum, only HNE increased. For LR, HNE increased in both prefrontal cortex (+165%) and cerebellum (+45%), while NT increased in AD prefrontal cortex (+40%), but not cerebellum. ApoE alleles differed in oxidative damage by brain region: E4 carriers had higher HNE (+35%) only in prefrontal cortex, while LR HNE was higher in E4 carriers of both regions (prefrontal cortex, +80%; cerebellum, +35%). NT did not differ in prefrontal cortex supernatant or LR by ApoE allele. Conclusion These findings extended prior findings on oxidative damage for HNE with AD by showing differences by brain region and by ApoE allele. HNE is formed from iron redox mediated lipid peroxidation, while protein nitration may be stimulated by aggregated amyloid independent of iron. AD was consistently associated with higher HNE and NT. The ApoE4 allele was more robustly associated with higher HNE in prefrontal cortex than cerebellum, while NT had no associations with E4, suggesting lipid peroxidation may be the predominant form of oxidative damage in the AD brain. The LR may be a hotspot for lipid peroxidation, with implications for APP processing which will be examined in future studies.