FIGURE 1.

NR, caffeine, or NR+Caff increase NAD co‐factor availability in human skin fibroblasts. (a) Schematic of NAD⁺ de novo synthesis or recycling. Dietary products, including nicotinic acid (NA) and nicotinamide riboside (NMR or NR), are converted to nicotinic acid mononucleotide (NAMN) or nicotinamide mononucleotide (NMN) by nicotinic acid phosphoribosyltransferase (NAPRT) or nicotinamide riboside kinase 1 (NMRK1 or NRK1), respectively. NAMN and NMN are substrates for nicotinamide mononucleotide transferases subtypes 1–3 (NMNAT1‐3) to produce the NAD precursor nicotinic acid adenine dinucleotide (NAAD) which is converted to NAD by NAD synthase (NADS). Oxidized NAD (NAD⁺) or NR are co‐substrates for SIRTs, PARPs, ARTs, CD38, BST1, and SARM1, and consumed to nicotinamide (NAM) which is recycled to NMN by nicotinamide phosphotransferase (NAMPT). Caffeine has been proposed to induce the transcription of NMNAT2 (Ali et al., 2017). (b) Changes of tNAD, NAD⁺, and NADH in skin fibroblasts from a YC (21 years), OC (65 years), and LOAD (76 years) sample after treatment with NR, caffeine, or NR+Caff for 24 hrs plotted as percent change from untreated controls. (c) tNAD, NAD⁺, and NADH in YC (n = 5, average age 25), OC (n = 8, average age 67), and LOAD (n = 8, average age 70) fibroblasts in untreated or NR+Caff treated cells for 24 h plotted as relative values luminescence (left panel) or as percent change NR+Caff over untreated condition (right panel). (d) Calculated RR. Data are means ± SEM from two repeat experiments. *p < 0.1; **p < 0.05; ***p < 0.01 using one‐way ANOVA, depicting significant changes between treated and untreated cells or groups