Endocrine Abstracts

A decline in skeletal muscle nicotinamide adenine dinucleotide (NAD+) can decrease mitochondrial function and energy metabolism in age-related metabolic disease. Restoration of NAD+ using the precursor nicotinamide riboside (NR) may serve to support age and disease driven impairment of mitochondrial energy metabolism. Manipulating NAD+, and consequently cellular pyridine nucleotide NAD(P)(H) pools, may impact the flux of glucose through intermediary energy metabolism pathways. To examine this we have used NMR spectroscopy and metabolic tracer analysis in NAD+ depleted (10 μM FK866 (inhibitor of the NAD⁺ salvage enzyme nicotinamide phosphoribosyltransferase NAMPT) for 48 h), and NAD+ replete (0.5 mM NR) C2C12 myotubes grown in 10 mM ¹³C₂-[1,2]-D-Glucose. FK866 treatment impaired cellular energetic status, reducing NAD+(>90%), NADP+(>50%), ATP (>30%), and basal mitochondrial respiration (50% using seahorse technology), but without inducing apoptosis or affecting cell viability. Compensatory adaptations in redox-sensitive metabolic pathways were observed, including a reduction in use of the pentose phosphate pathway (PPP) and a block in glycolysis at the NAD+ dependant glyceraldehyde-3-phosphate dehydrogenase step. Supplementing NR to FK866 treated cells for only 4 h rescued NAD⁺ levels and normalised these metabolic pathways. NAD⁺ repletion in ‘healthy’ cells supplemented with NR for 4h in the absence of FK866 resulted in a 53% increase in NAD⁺ and 20% increase in NADP+ without affecting ATP or basal mitochondrial respiration. The excess NAD⁺ reduced PPP flux while increasing contributions to glycolytic flux and TCA cycle activity from non-glucose carbon sources. Our results show NAMPT as a critical enzyme for NAD⁺ homeostasis, with low NAD⁺ impairing glucose flux and TCA cycle activity, providing mechanistic information as to why age-related decline in NAD+ affects overall health. While NR supplementation may be effective to ‘normalise’ glucose flux in the low NAD+ state, excess NAD+ may have unintended consequences for glucose turnover and intermediary energy metabolism in muscle requiring further evaluation.