Inhibition of HSP90-Dependent Telomerase Activity in Amyloid β-Induced Apoptosis of Cerebral Endothelial Cells

Although apoptosis induced by amyloid beta (A beta) has been identified, the effect of A beta on telomerase activity in relation to apoptosis induction remains unclear. In the present study, A beta(1-40) and A beta(25-35), but not A beta(1-16) and A beta(35-25), reduce the viability of primary cerebral endothelial cells (CECs) in accordance with apoptosis induction. Increases in caspase 3 and PARP protein cleavage with reductions of the Bcl-2/Bax protein ratio accompanied by a loss in the mitochondria membrane potential were identified in A beta(1-40) and A beta(25-35)-treated CECs. A significant decrease in intracellular telomerase activity by A beta(1-40) and A beta(25-35) was detected; meanwhile, reduced telomerase activity by telomerase reverse transcriptase (TERT) siRNA enhanced the cytotoxic effect of A beta. The addition of serum might block the A beta(25-35)-induced cytotoxic effect via elevated telomerase activity in according with stimulating phospho-AKT protein expression, which was blocked by adding AKT inhibitor LY294002. Decreases in heat shock protein 90 (HSP90) and its client proteins including TERT, AKT, p53, CDK4 were observed in A beta(1-40) and A beta(25-35), but not A beta(1-16) and A beta(35-25), -treated CECs. The knockdown of HSP90 gene expression by HSP90 siRNA significantly inhibits telomerase activity with decreasing TERT protein expression. The application of HSP90 activity inhibitor geldanamycin (GA) and radicicol (RD) potentiates the telomerase inhibition and apoptosis induction of A beta in CECs. An increase in protein ubiquitination by A beta(25-35), but not A beta(35-25), treatment was examined, and A beta-inhibited HSP90 and TERT protein expression and telomerase activity was reversed by adding proteasome inhibitor, MG132. Additionally, increased TERT protein ubiquitination by A beta(25-35) was detected in CECs via immunoprecipitation/Western blotting analysis. The data of the present study firstly demonstrates that telomerase inhibition contributes to the apoptosis induction of A beta in CECs. J. Cell. Physiol. 226: 2041-2051, 2011. (C) 2010 Wiley-Liss, Inc.