Vitamin E Neuroprotection: Novel Molecular Mechanisms

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DESCRIPTION (provided by applicant): Neuroprotective properties of vitamin E have been established from the study of alpha-tocopherol, one of 8 members of the vitamin E family broadly categorized as tocopherols and tocotrienols. This proposal is based on a striking observation that alpha-tocotrienol, but not aipha-tocopherol, confers potent neuroprotection at nanomolar concentrations through regulation of signal transduction pathways independent of its antioxidant property. Thus, it was found that at 50-100 nM (one-tenth of plasma-concentration in supplemented humans) alpha-tocotrienol prevented glutamate-induced death of cultured HT4 cells as well as that of both immature and mature primary cortical neurons. Murine HT hippocampal neuronal cells, lacking intrinsic excitotoxicity-pathway, have been validated as a model to characterize oxidative glutamate toxicity. Neuroprotective effects of alpha-tocotrienol were also observed when homocysteic acid or buthionine sulfoximine (inhibitor of GSH synthesis) was used to challenge HT4 or primary rat fetal cortical neurons. Tocotrienols occur naturally and has been safely consumed by humans, particularly in Southeast Asia, for years. We observed that dietary alpha-tocotrienol contained in palm oil fed to pregnant rats reaches the brain of mother rats. Availability of this form of vitamin E is multi-fold higher in fetal brain compared to that of the mother. The objective of this proposal is to characterize the mechanisms by which alpha-tocotrienol exerts its neuroprotective action using pharmacological, biochemical and genetic approaches. In addition, the dogma that the neuroprotective function of vitamin E is solely mediated by its antioxidant property will be revisited. A commonly used criterion to establish the involvement of reactive oxygen species (ROS) in a cell-response system is sensitivity of that system to antioxidants. It is often ignored that antioxidant molecules may have potent antioxidant-independent properties. Our working hypothesis is that elevated levels of extracellular glutamate trigger the following events: i) activation of pp6O c-Src, extracellular signal regulated kinase (ERK) and 12-lipoxygenase (LOX), and ii) depletes [GSH]i resulting in high [ROS]i, high [Ca2+]i, mitochondrial dysfunction eventually causing neuronal death. Tocotrienol controls these responses at two levels: early and late. In a time frame where >90 percent of the cells are dead after 12h of glutamate challenge, inhibition of inducible c-Src, ERK and 12-LOX activation within the first 1 h of glutamate treatment by nM (50-250) alpha-tocotrienol constitutes one of the early control mechanisms that is antioxidant-independent. Depletion of [GSH]i, elevation of [ROSJ]i and subsequent mitochondrial dysfunction are all tightly linked and are subject to late-control by higher (= or> 1 micromolar) concentrations of alpha-tocotrienol by virtue of its antioxidant property. This study will lay the foundation for a safe and efficient approach to prevent death and secure normal functioning of neurons under challenging conditions.