Upstream signaling of PKR in beta-amyloid peptide neurotoxicity

Abstract

Beta-amyloid peptide (Aβ) has been proposed to play an important role in the pathogenesis of Alzheimer,s disease (AD). Aβ has been shown to induce intracellular calcium disturbance and activation of caspase-3 and caspase-8 in neurons. PKR, a double-stranded RNA-dependent serine/threonine protein kinase has been demonstrated to be significantly involved in apoptosis. Activated PKR phosphorylates eukaryotic initiation factor 2 alpha (eIF2α), resulting in inhibition of global protein synthesis. We have recently reported that PKR is activated in Aβ-induced neuronal cell death. Degenerating neurons in the brains of AD patients also displayed high immunoreactivity of phosphorylated PKR and eIF2α. However, how the PKR-eIF2α pathway is activated by Aβ is unclear. In the present study, we focus on whether intracellular calcium disturbance and activation of caspase-3 and caspase-8 are involved in PKR activation in Aβ neurotoxicity. Western-blotting analysis showed that PKR activation in cultured cortical neurons exposed to Aβ is mediated by caspase-3 as inhibition of caspase-3 activity by DEVD-CHO significantly reduced PKR activation. Our results also showed that an antagonist of inositol-1,4,5 trisphosphate receptor Xestospongin C which can reduce intracellular calcium increase significantly attenuated PKR activation. Moreover, our data showed that inhibition of early caspase-8 activation decreased Aβ-triggered activation of PKR in neurons. Taken together, both the reduction on intracellular calcium increase and blockade of caspase-8 activity can attenuate caspase-3 activation, suggesting that caspase-3 can play a central role in converging different signals to activate PKR.