SIRT1 targets lysines 6 and 7 in APE1 for deacetylation. (A) Basal lysine acetylation, and increase in lysine acetylation by treatment with the SIRT1 inhibitor nicotinamide (NAM: 5 mM, 16 h), of epitope-tagged wild-type and non-acetylatable mutants of APE1 expressed in HEK 293 cells. Lysine acetylated/total APE1 was quantified and is expressed relative to wild-type APE1 in untreated cells. *P < 0.05 and #P > 0.05 compared with untreated cells transfected with the same construct. ##P > 0.05, and **P < 0.005 compared with untreated cells transfected with WT APE1. ###P > 0.05, ***P < 0.05, and ΨP < 0.005 compared with NAM-treated cells transfected with WT APE1. N = 4 in all conditions. (B) Targeting of the N-terminal lysines in APE1 for acetylation by the p300 acetyltransferase. Epitope-tagged wild-type (WT) APE1, or the non-acetylatable lysine mutants of APE1, were expressed in HEK 293 cells, with and without overexpression of the p300 acetyltransferase. WCL: whole cell lysate.