Nicotinamide-mediated inhibition of SIRT1 deacetylase is associated with the viability of cancer cells exposed to antitumor agents and apoptosis

Silent mating-type information regulation 2, homolog 1 (SIRT1) represents an NAD+-dependent deacetylase that regulates the processes of stress response and cell survival. However, the functions of SIRT1 in stress- and drug-induced apoptosis remain elusive. The present study was designed to determine the effects of SIRT1 in tumor cells subjected to antitumor agent treatment and to identify the underlying mechanisms during the stress response. Several of the most commonly used antitumor medications [arsenic trioxide (As2O3), Taxol and doxorubicin (doxo)] were selected to treat MCF-7 human breast cancer cells with or without nicotinamide (NAM) inhibition. 3-(4,5-Dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) was used to test cell viability. SIRT1 expression was tested by immunoblot analysis. The typical hallmarks of apoptosis (chromatin condensation, apoptotic bodies, sub G1 change and Annexin V+/PI- stained cells) were detected by Hoechst 33342 staining, flow cytometry and Annexin V+/PI- staining following NAM treatment. The cleavage of poly(ADP-ribose) polymerase (PARP) and caspases 9, 6 and 7 was detected through immunoblot analysis. Augmented SIRT1 expression was observed only at low concentrations (>80% cell viability) and the inhibition of SIRT1 deacetylase by NAM decreased the viability of the cancer cells exposed to low concentrations of antitumor agents. NAM induced typical apoptosis in the MCF-7 tumor cells, accompanied by the activation of the caspase cascade. SIRT1 promotes cellular survival at certain stress levels by its deacetylase function. The SIRT1 deacetylase inhibitor, NAM, triggers the activation of the caspase cascade and induces typical apoptosis in MCF-7 cells.