NAD metabolic dependency in cancer is shaped by gene amplification and enhancer remodelling

The redox cofactor nicotinamide adenine dinucleotide (NAD) plays a central role in metabolism and is a substrate for signaling enzymes including poly-ADP-ribose-polymerases (PARPs) and sirtuins. NAD concentration falls during aging, which has triggered intense interest in strategies to boost NAD levels. A limitation in understanding NAD metabolism has been reliance on concentration measurements. Here, we present isotope-tracer methods for NAD flux quantitation. In cell lines, NAD was made from nicotinamide and consumed largely by PARPs and sirtuins. In vivo , NAD was made from tryptophan selectively in liver, which then excreted nicotinamide. NAD fluxes varied widely across tissues, with high flux in small intestine and spleen and low flux in skeletal muscle. Intravenous administration of nicotinamide riboside or mononucleotide delivered intact molecules to multiple tissues, but the same agents given orally were metabolized to nicotinamide in liver. Thus, flux analysis can reveal tissue-specific NAD metabolism. Tissue concentrations of the redox cofactor NAD change during aging and disease. Liu et al. developed isotope-tracer methods to quantitate NAD fluxes in cell culture and in mice, revealing that the liver makes nicotinamide from tryptophan and from orally delivered nicotinamide riboside, a nutraceutical. In contrast, other tissues rely on circulating nicotinamide for NAD synthesis.

Nicotinamide adenine dinucleotide (NAD+) is a cosubstrate for several enzymes, including the sirtuin family of NAD+-dependent protein deacylases. Beneficial effects of increased NAD+ levels and sirtuin activation on mitochondrial homeostasis, organismal metabolism and lifespan have been established across species. Here we show that α-amino-β-carboxymuconate-ε-semialdehyde decarboxylase (ACMSD), the enzyme that limits the proportion of ACMS able to undergo spontaneous cyclisation in the de novo NAD+ synthesis pathway, controls cellular NAD+ levels via an evolutionary conserved mechanism from C. elegans to the mouse. Genetic and pharmacological inhibition of ACMSD boosts de novo NAD+ synthesis and SIRT1 activity, ultimately enhancing mitochondrial function. We furthermore characterized a series of potent and selective ACMSD inhibitors, which, given the restricted ACMSD expression in kidney and liver, are of high therapeutic interest to protect these tissues from injury. ACMSD hence is a key modulator of cellular NAD+ levels, sirtuin activity, and mitochondrial homeostasis in kidney and liver.

NAD is an obligate co-factor for the catabolism of metabolic fuels in all cell types. However, the availability of NAD in several tissues can become limited during genotoxic stress and the course of natural aging. The point at which NAD restriction imposes functional limitations on tissue physiology remains unknown. We examined this question in murine skeletal muscle by specifically depleting Nampt, an essential enzyme in the NAD salvage pathway. Knockout mice exhibited a dramatic 85% decline in intramuscular NAD content, accompanied by fiber degeneration and progressive loss of both muscle strength and treadmill endurance. Administration of the NAD precursor nicotinamide riboside rapidly ameliorated functional deficits and restored muscle mass despite having only a modest effect on the intramuscular NAD pool. Additionally, lifelong overexpression of Nampt preserved muscle NAD levels and exercise capacity in aged mice, supporting a critical role for tissue-autonomous NAD homeostasis in maintaining muscle mass and function.