Protein phosphatase 2A regulates bim expression via the Akt/FKHRL1 signaling pathway in amyloid-β peptide-induced cerebrovascular endothelial cell death

Amyloid-beta peptide (A beta)-induced death in cerebral endothelial cells (CECs) is preceded by mitochondrial dysfunction and signaling events characteristic of apoptosis. Mitochondria-dependent apoptosis engages Bcl-2 family proteins, especially the BH3-only homologues, which play a key role in initiating the apoptotic cascade. Here, we report that the expression of bim, but not other BH3-only members, was selectively increased in cerebral microvessels isolated from 18-month-old APPsw (Tg2576) mice, a model of cerebral amyloid angiopathy (CAA), suggesting a pivotal role for Bim in A beta-induced cerebrovascular degeneration in vivo. A similar expression profile was observed in A beta-treated CECs. Furthermore, A beta induction of bim expression involved a pro-apoptotic transcription factor, FKHRL1. FKHRL1 bound to a consensus sequence in the bim promoter region and was activated by A beta before bim expression. FKHRL1 activity was negatively regulated by phosphorylation catalyzed by Akt, an anti-apoptotic kinase. Akt upregulation by adenoviral gene transfer inhibited A beta-induced FKHRL1 activation and bim induction. In addition, A beta increased the activity of protein phosphatase 2A (PP2A), a ceramide-activated protein phosphatase. Suppression of PP2A activity by RNA interference or a specific inhibitor, okadaic acid, effectively suppressed A beta-induced Akt inactivation and FKHRL1 activation, leading to an attenuation of bim expression and cell death in CECs. Coimmunoprecipitation experiments revealed that A beta enhanced the binding of the PP2A regulatory subunit PP2AC alpha beta to Akt. These results implicate PP2A as an early regulator of A beta-induced bim expression and CEC apoptosis via the Akt/FKHRL1 signaling pathway. We raise the possibility that this pathway may play a role in cerebrovascular degeneration in CAA.