The neuroprotective effects of phytoestrogens on amyloid β protein-induced toxicity are mediated by abrogating the activation of caspase cascade in rat cortical neurons

Amyloid beta protein (A beta) elicits a toxic effect on neurons in vitro and in vivo. In present study we attempt to elucidate the mechanism by which A beta confers its neurotoxicity. The neuroprotective effects of phytoestrogens on A beta -mediated toxicity were also investigated. Cortical neurons treated with 5 muM A beta-(25-35) for 40 h decreased the cell viability by 45.5 +/- 4.6% concomitant with the appearance of apoptotic morphology. 50 muM kaempferol and apigenin decreased the A beta -induced cell death by 81.5 +/- 9.4% and 49.2 +/- 9.9%, respectively. A beta increased the activity of caspase 3 by 10.6-fold and to a lesser extent for caspase 2, 8, and 9. The A beta -induced activation of caspase 3 and release of cytochrome c showed a biphasic pattern. Apigenin abrogated A beta -induced cytochrome c release, and the activation of caspase cascade. Kaempferol showed a similar effect but to a less extent. Kaempferol was also capable of eliminating A beta -induced accumulation of reactive oxygen species. These two events accounted for the remarkable effect of kaempferol on neuroprotection. Quercetin and probucol did not affect the A beta -mediated neurotoxicity, However, they potentiated the protective effect of apigenin. Therefore, these results demonstrate that A beta elicited activation of caspase cascades and reactive oxygen species accumulation, thereby causing neuronal death. The blockade of caspase activation conferred the major neuroprotective effect of phytoestrogens. The antioxidative activity of phytoestrogens also modulated their neuroprotective effects on A beta -mediated toxicity.