Tournefolic acid B attenuates amyloid β protein-mediated toxicity by abrogating the calcium overload in mitochondria and retarding the caspase 8-truncated Bid-cytochrome c pathway in rat cortical neurons

The effect of tournefolic acid B (TAB) on amyloid β protein-mediated neurotoxicity and the underlying mechanisms were investigated. Amyloid β protein 25–35 elicited neuronal death as determined by calcein/ethidium homodimer-1 staining. 10 µM amyloid β protein 25–35 caused cell death at a level of 41.5 ± 3.8% by MTT reduction. 50 µM TAB attenuated the amyloid β protein 25–35-induced cell death by 49.7 ± 11.1%. TAB also abrogated amyloid β protein-induced activation of caspases 8 and 9 by about 50–60%. Furthermore, TAB significantly diminished the amyloid β protein 25–35-induced elevation of calcium level in mitochondria, whereas it did not affect the calcium level in cytosol or endoplasmic reticulum. TAB markedly retarded the amyloid β protein-mediated release of cytochrome c from mitochondria. Amyloid β protein 25–35 elevated mitochondrial truncated BH3 interacting domain death agonist (tBid) and decreased the level of B-cell leukemia/lymphoma-2α (Bcl-2α) in mitochondria. Moreover, amyloid β protein induced a slight up-regulation of Bcl-2 agonist killer 1 (Bak) in cytosol. 50 µM TAB decreased the amyloid β protein-induced elevation of mitochondrial tBid and the level of Bak, whereas it did not affect the amyloid β protein-mediated decrease in mitochondrial Bcl-2α. Caspase 8 inhibitor significantly inhibited the amyloid β protein-mediated increase in mitochondrial tBid and the release of cytochrome c. Therefore, TAB blocked the overload of calcium in mitochondria and impaired the amyloid β protein-mediated activation of the caspase 8–tBid–cytochrome c pathway, thereby conferring its neuroprotective effects on amyloid β protein-mediated neurotoxicity.