LAUSR

In addition to metabolic and redox functions, nicotinamide adenine dinucleotide (NAD+) impacts cell signaling via the sirtuin family of lysine deacylases. Several diseases are reported to benefit from pharmacologic stimulation of sirtuin activity by delivery of NAD+ precursors such as nicotinamide mononucleotide (NMN). Stimulation of cytosolic SIRT1 is known to protect the heart against ischemia-reperfusion (IR) injury and, consistent with this, NMN protected perfused mouse hearts against IR (post-IR function recovery: NMN 35±2 %, vs. vehicle 10±4 %). However, NMN protection was absent in hearts perfused without glucose (i.e. with palmitate as respiratory substrate). In isolated cardiomyocytes, NMN induced rapid and large-scale cytosolic acidification (fluorescent pH reporter), with increased glycolytic flux (Seahorse XF) yielding lactate (LC-MS/MS based metabolomics). Glycolysis requires NAD+ and is known to be beneficial during hypoxia. As such, our data suggest that protection afforded by an NMN-induced elevation in NAD+ may simply be due to glycolytic stimulation rather than SIRT1 activation. Consistent with this, western blot for acetyl-lysine (as a surrogate for in-situ sirtuin activity) showed that NMN drove protein deacetylation in mitochondrial and nuclear compartments, but not in cytosol where cardiac SIRT1 is located. We conclude the benefit of NMN in IR and potentially other disease states is mediated via stimulation of glycolysis. Caution is thus urged, in the human therapeutic use of NAD+ boosting dietary supplements.