Neuroprotective Mechanisms of Ginkgo Biloba extract (EGb761) in Alzheimer's Disease.

摘要

EGb761, a Ginkgo biloba extract, is a medicinal product for the treatment and prevention of cardiovascular and neuronal diseases, including Alzheimer's disease (AD). While considerable researches have documented its neuroprotective effects, its clinical effect is inconclusive and the precise neuroprotective mechanisms are not clearly known.;EGb761 consists of two major groups of substances, flavonoids and terpenoids. Using human neuroblastoma SH-SY5Y cells, the present study demonstrated that, EGb761 could block Abeta-42 (a 42-amino acid cytoxic form of beta amyloid protein)-induced cell apoptosis, reactive oxygen species (ROS) accumulation, mitochondrial dysfunction and activation of c-jun N-terminal kinase (JNK), extracellular signal-regulated kinase 1/2 (ERK1/2) and Akt signaling pathways, possibly via its antioxidant and platelet activating factor (PAF) antagonizing activities. Two active constituents of EGb761, quercetin (a flavonoid) and ginkgolide B (a terpenoid) might contribute to the protective effects of EGb761. Quercetin but not ginkgolide B might be responsible for the antioxidant action of EGb761. Both compounds might be involved in the PAF antagonist activity of EGb761.;The effective dosage of EGb761 in the brain remains undetermined. Using SH-SY5Y cells, this study demonstrated that low doses of EGb761 (50--100 mug/ml) inhibited hydrogen peroxide (H2O2)-induced cell apoptosis via inactivation of Alet, JNK and caspase 3 while high doses of EGb761 (250--500 flg/ml) enhanced H2O2 toxicities via inactivation of Akt and enhancement of activation of JNK and caspase 3. Additional experiments suggested that the dosage effect of EGb761 on apoptotic signaling proteins might be correlated with regulation of the cell redox state.;The ability of EGb761 to cross the blood brain barrier (BBB) is unclear. In this study, the ability of EGb761 to cross the BBB was speculated through comparison of the effects of EGb761 on mitochondrial function between platelets and central nervous system in two animal models, the senescence accelerated prone 8 (SAMP8) mouse strain and ovariectomized rats. Mitochondrial function was evaluated as cytochrome c oxidase (COX) activity, mitochondrial ATP content and mitochondrial glutathione (GSH) content. SAMP8 mice have been widely used as a model of age-related cognitive decline with relevance to biochemical and genetic alterations in AD. Using two age groups (3-week-old and 40-week-old) of SAMP8 mice, this study found that, EGb761 protected against mitochondrial dysfunction in both platelets and hippocampi of old mice, but only showed protective effects on platelet mitochondria of young mice. Estrogen withdrawal was suggested to play a primary role in the onset of post-menopausal AD. Using ovariectomized middle-aged rats to mimic the post-menopausal pathophysiological changes, this study also demonstrated that, EGb761 protected against mitochondrial dysfunction in both platelets and hippocampi of ovariectomized rats. In contrast, in sham-operated rats, EGb761 increased mitochondrial GSH content in platelets but failed to show similar effect on hippocampi. These results suggested that the effects of EGb761 on the brain might be interfered by the BBB permeability.;In conclusion, EGb761 may have beneficial effects in treatment and prevention of neurodegenerative diseases like AD. Its neuroprotective effects may be associated with constituent multiplicity, the dosage and BBB permeability.