Abstract
NAD is a multifunctional molecule involved in both metabolic processes and signaling pathways. Such signalling pathways consume NAD which is replenished via one of several biosynthesis pathways. We show that influx of NAD across the plasma membrane may be able to contribute to the homeostasis of intracellular NAD levels. Indeed, extracellular application of NAD was able to replete NAD levels that had been lowered pharmacologically using the novel drug FK866 and was also able to rescue cells from FK866-induced cell death. A marked lag between the drop in NAD levels and cell death prompted us to investigate the mechanism of cell death. We were unable to find evidence of apoptosis as assessed by immunoblotting for the Caspase 3 activation fragment and immunostaining for cytochrome C and AIF translocation. We, therefore, investigated whether autophagy was initiated by FK866. Indeed, we were able to observe the formation of LC3-positive vesicles that had fused with lysosomes in FK866-treated but not control cells. Furthermore, this autophagic phenotype could be reverted by the addition of NAD to the extracellular medium.