Whether exogenously provided NAD or overexpression of NAD biosynthetic enzymes can reduce paraquat-induced cytotoxicity is still unclear. NAD performs a variety of roles in the cell. By transferring electrons, NAD plays an important part in energy production by involving in the tricarboxylic acid cycle and the electron transport chain in mitochondria . NAD also acts as a substrate for various enzymes including cADP-ribose synthases, poly polymerase-1 and the sirtuin family to be involved in the processes of cell signal transduction, DNA repair, gene transcription, and cell death . NAD can be taken up from the extracellular surroundings, or can be synthesized either through de novo pathway, or through salvage pathway by recycling NAD derivatives such as nicotinic acid, nicotinamide, and nicotinamide riboside back to NAD . Nicotinamide phosphoribosyltransferase and Nmnat sequentially form a major salvage pathway to synthesize NAD from nicotinamide via the intermediate nicotinamide mononucleotide in mammalian cells . Knockdown or inhibition of Nampt can directly induce apoptosis, which can be reversed by exogenously provided NAD or its derivatives . Furthermore, providing sufficient extracellular NAD or overexpressing Nmnat1 or Nampt could replenish the NSC 136476 Hedgehog inhibitor decrease of intracellular NAD, and therefore conferred protection against cell death under various conditions . These findings indicate that maintenance of intracellular NAD levels should be beneficial for cell survival. SIRT1 is an NAD-dependent protein deacetylase of sirtuin family, and its activity is sensitively influenced by alteration of intracellular NAD concentration . SIRT1 has been reported to be a key regulator of cell defense and survival in response to various kinds of stress such as DNA damage, oxidative stress, heat shock and ionizing radiation . Consistently, previous report has shown that SIRT1 is a key link between NAD depletion and PARP-mediated cardiac myocyte cell death . The synergic role of NAD and SIRT1 in cytotoxicity under FDA-approved Compound Library different conditions is yet to be further elucidated. In this study, we investigated whether WldS could confer protection to cytotoxicity induced by various toxic chemicals. We demonstrate that WldS reduces paraquat-induced cytotoxicity via an NAD dependent deacetylase SIRT1 by attenuating the depletion of NAD. To investigate whether WldS has cellular protective function, we used various toxic chemicals to treat mouse embryonic fibroblasts isolated from wild-type and WldS embryos.