Figure 5.

SIRT1 protects from genotoxic stress-induced cell death through APE1. Apoptotic death with oxidative stress (H₂O₂ for 24 h) and abasic DNA damage (MMS for 24 h) in HeLa cells in which SIRT1 expression (A) or APE1 expression (B) is knocked down with SIRT1 or APE1 siRNA (black bars). Scrambled (scr) siRNA (white bars) was used as control. Apoptosis is expressed as fold change compared with untreated cells. *P < 0.05 and **P < 0.01 compared with control siRNA. Knockdown of SIRT1 and APE1 is shown at bottom. (C) APE1 overexpression rescues cells with SIRT1 down-regulation from MMS-induced apoptosis. Apoptotic cell death induced by MMS (200 µM, 24 h) in HeLa cells treated with SIRT1 siRNA (white bars) or scrambled (scr) siRNA (black bars) were infected with a control virus (AdLacZ) that expresses the inert E. coli LacZ gene, or and adenovirus that expresses APE1 (AdAPE1). Apoptosis is expressed as fold change compared with control siRNA. *P < 0.05 compared with cells infected with AdLacZ. Knockdown of SIRT1 and adenoviral overexpression of APE1 is shown at right. (D) SIRT1 plays a role in abasic DNA repair. SIRT1 or APE1 was knocked down in HeLa cells with siRNA. Apurinic/apyrimidinic DNA sites were quantified in untreated cells and cells treated with MMS for 3 h. *P < 0.05 compared with control siRNA.