Nicotinamide Adenine Dinucleotide in Aging Biology: Potential Applications and Many Unknowns

Abstract

Recent research has unveiled an expansive role of NAD+ in cellular energy generation, redox reactions, and as a substrate or cosubstrate in signaling pathways that regulate health span and aging. This review provides a critical appraisal of the clinical pharmacology and the preclinical and clinical evidence for therapeutic effects of NAD+ precursors for age-related conditions, with a particular focus on cardiometabolic disorders, and discusses gaps in current knowledge. NAD+ levels decrease throughout life; age-related decline in NAD+ bioavailability has been postulated to be a contributor to many age-related diseases. Raising NAD+ levels in model organisms by administration of NAD+ precursors improves glucose and lipid metabolism; attenuates diet-induced weight gain, diabetes, diabetic kidney disease, and hepatic steatosis; reduces endothelial dysfunction; protects heart from ischemic injury; improves left ventricular function in models of heart failure; attenuates cerebrovascular and neurodegenerative disorders; and increases health span. Early human studies show that NAD+ levels can be raised safely in blood and some tissues by oral NAD+ precursors and suggest benefit in preventing nonmelanotic skin cancer, modestly reducing blood pressure and improving lipid profile in older adults with obesity or overweight; preventing kidney injury in at-risk patients; and suppressing inflammation in Parkinson disease and SARS-CoV-2 infection. Clinical pharmacology, metabolism, and therapeutic mechanisms of NAD+ precursors remain incompletely understood. We suggest that these early findings provide the rationale for adequately powered randomized trials to evaluate the efficacy of NAD+ augmentation as a therapeutic strategy to prevent and treat metabolic disorders and age-related conditions.

Keywords: NAD; NAD boosters; NAD metabolism; NAD precursors; aging; clinical applications of NAD boosters; geroscience; mechanisms of aging; pharmacologic approaches for augmenting NAD; pharmacology of NAD boosters.

Graphical Abstract

Figure 1.

The structures and some important uses of NAD⁺, NADH, NADP⁺, and NADPH in the cells. The redox couples—NAD⁺ and NADH, and NADP⁺ and NADPH—play an important role in redox reactions in which NAD and NADP serve as electron carriers during exchange of reducing equivalents in molecular reactions inside the cell. NAD⁺ also serves as a cofactor/cosubstrate for sirtuins, PARPs, CD38 and other proteins in many signaling pathways and is the only nicotinamide derived cofactor that is directly consumed with the concomitant release of nicotinamide and signaling molecules during these reactions. NADH is stable to processes responsible for the cleavage of the glycosidic bond and, therefore, serves as a protected pool of NAD⁺ in the cell. Among other processes, NADH is oxidized to NAD⁺ by complex I of the ETC and LDH. Mitochondrial NAD⁺ can be oxidized by NNT with the concomitant reduction of NADP to NADPH. NADP⁺ is also reduced to NADPH in the cytosol by the PPP enzymes, G6PDH and 6PGDH, and by reductases of folate metabolism. Crucially, NADPH is central to ROS homeostasis and is employed in anabolic processes such as the synthesis of fatty acids, nucleic acids, and steroid hormones. SIRTs, sirtuins; ARTs, ADP ribosyltransferases; SARM1, Sterile alpha and Toll/interleukin-1 receptor motif-containing protein 1 (SARM1), CD38 and CD73, cluster of differentiation factor 38 and cluster of differentiation factor 73. TCA, tricarboxylic acid pathway; LDH, lactate dehydrogenase; ETC, electron transport chain; G6PDH, glucose-6-phosphate dehydrogenase; DHFR, dihydrofolate reductase; MTHFR, methylene tetrahydrofolate reductase; DPYD, dihydropyrimidine dehydrogenase; TRXR, thioredoxin reductase; NOX, nicotinamide adenine dinucleotide phosphate (NADPH) oxidase; CAT, catalase; GPX, glutathione peroxidase; FASN, fatty acid synthase; HMGCR, 3-hydroxy-3-methylglutaryl coenzyme A reductase; POR, Cytochrome P450 oxidoreductase.

Figure 2.

Biochemical pathways for the synthesis of NAD⁺ and related metabolites. Biosynthetic intermediates and the salvage (recycling) pathway for NAD⁺ synthesis are shown in blue. The salvage pathway (recycling pathway) for NAD⁺ generation uses direct metabolites of NAD⁺ degradation. Nicotinamide (NAM) is generated from NAD⁺-consuming reactions, while nicotinamide riboside (NR) is produced either from NRH (the reduced form of NR) oxidation by N-ribosyl dihydronicotinamide : quinone reductase 2 (NQO2) or by nucleoside diphosphate linked to moiety-X (NUDIX) phosphatase-catalyzed NAD⁺ hydrolysis. The biosynthetic intermediate common to both biosynthetic precursors, nicotinamide mononucleotide (NMN), is a substrate for nicotinamide mononucleotide adenylyl transferase (NMNAT). Levels of NMN are usually low compared with NAM or NAD⁺. NAD⁺ is the only precursor to the 3′-phospho-NAD⁺, NADP. De novo biosynthetic pathway for NAD synthesis and its intermediates are shown in green. Dietary tryptophan (Trp), nicotinic acid (NA; generated by bacterial NAM deamination), aspartate (Asp), and nicotinic acid riboside (NAR) are all converted to the same biosynthetic intermediate, nicotinic acid mononucleotide (NAMN). NAMN is a substrate for NMNAT, just like NMN, and is converted to NAAD. Unlike NAD⁺, NAAD is neither a substrate for NAD⁺-consuming enzymes nor a redox cofactor. Instead, it is a direct biosynthetic precursor to NAD⁺ via NAD⁺ synthase. The expression of NAD⁺ synthase regulates this pathway. Redox-coupled biosynthetic intermediates (NR and NRH, NMN and NMNH, NAD⁺ and NADH, NADP and NADPH) are shown in black. Reduction of NAD(P) generates the NAD(P)H pools. NRH (reduced form of NR), a precursor to NMNH (reduced form of NMN) and NADH, uses adenosine kinase (AK) to enter the NAD(P)(H) biosynthetic pool. There is no known feedback inhibition of AK by either of the NAD(P)/NADPH pools; as such, once NRH enters the cell and is phosphorylated, it is retained by the cell and supplies a reliable source of NADH precursor, NMNH. As NADH is not a substrate for NAD⁺-consuming enzymes, redox biology shields the NAD(H) pools from being overly depleted. Transient metabolites of NAD⁺ are shown in purple. Nicotinic acid adenine dinucleotide phosphate (NAADP) and its reduced form NAADPH are transient species that remain under investigation, including the endogenous origin of NAADP. However, some ubiquitous redox enzymes that use NADP as a cofactor, such as G6PDH, can generate NAADPH from NAADP and abolish the signaling properties of NAADP. Several additional important byproducts of NAD⁺ consuming enzymatic reactions such as adenosine diphosphate ribose (ADPR) and cyclic adenosine diphosphate ribose (cADPR), and poly adenosine diphosphate riboside (PAR) have not been shown in this figure as they do not incorporate a nicotinoyl moiety. Nicotinic acid adenine dinucleotide phosphate exists only transiently as a signaling intermediate and is shown in magenta. As described in the text, these by-products of NAD(P)-consuming processes are major carriers of ribose and adenosine units and are shown in Fig. 2.

Figure 3.

Metabolism of NAD⁺ and its related intermediates and precursors. (A) Most readily detected metabolites of NAD⁺ and its intermediates. Dietary nicotinamide (Nam) and Nam generated from supplemented nicotinamide riboside (NR), and maybe NMN, are substrates for bacterial nicotinamidase in the gut and converted to nicotinic acid (NA). Once absorbed and in circulation, NA is rapidly converted to NAD⁺ by the Preiss-Handler Pathway unless it is in excess. Excess NA is conjugated to glycine to form nicotinuric acid (NUA) in phase II metabolism. NUA is excreted in the urine and is the only catabolite of NA. Trigonelline, N-Me-NA, is not considered an endogenous metabolite of NAD⁺ and is thought to be acquired from dietary sources. Nam that is not recycled to NAD⁺ by the salvage pathway can be methylated by nicotinamide N-methyltransferase (NNMT). This methylation process requires the methyl-donor S-adenosyl methionine (SAM) as a cosubstrate. In cells, excessive methylation of Nam promotes SAM depletion and genomic hypomethylation. Nam and methyl-Nam are readily oxidized to N-oxy-Nam and the N-methylated pyridone series, respectively. The relative abundance and product distribution of these catabolites are specific for each animal species and differ in model organisms from that in humans. Nicotinuric acid can only be generated from NA. Trigonelline (N-Me-NA), found in torrefied coffee, is detected in biospecimens like serum and urine, but its origin is unclear, and its relationship with the rest of the B3 vitaminome remains unknown. Overall, the biological properties of these circulating catabolites remain incompletely understood. 1-Methylnicotinamide, N-methyl-2-pyridone-3-carboxamide (Me-2PY), N-methyl-4-pyridone-3-carboxamide (Me-4PY), and N-methyl-6-pyridone-3-carboxamide (Me-6PY) are the major circulating and urinary metabolites of NAD⁺. Once generated, none of these catabolites can be recycled to NAD⁺. (B) Products of hyperoxidation of ribosylated nicotinamide derivatives. NAD⁺ and NADP⁺ are acceptors of electrons, in the form of hydrides, hydroxide radicals, and superoxide. While the former leads to NAD(P)H, the latter leads to the formation of hyperoxidized NAD(P) species. The hydroxide radicals and superoxide can add on 3 different positions of the nicotinamide ring and generate 3 different chemical series. Each can be metabolized into simpler units. The phosphorylated species are detected in blood and tissues, while the nucleosides and the nucleobases are found in serum and urine. While the 2, 4 and 6-isomers of 3-carboxamide pyridone ribosides have been detected, the 4-isomer of the triphosphate and dinucleotide species is the isomer most often reported. Some biochemical conversions remain uncharacterized and are indicated with dashed arrows. 2PY, 2-pyridone-3-carboxamide (2-hydroxynicotinamide); 4PY, 4-pyridone-3-carboxamide; 6PY, 6-pyridone-3-carboxamide (2-pyridone-5-carboxamide; 6-hydroxynicotinamide); 2-PYR, 2-pyridone-3-carboxamide riboside; 4-PYR, 4-pyridone-3-carboxamide riboside; 6-PYR, 6-pyridone-3-carboxamide riboside; 2-ox-NAD, 6-pyridone-3-carboxamide adenine dinucleotide; 4-ox-NAD, 4-pyridone-3-carboxamide adenine dinucleotide; 6-ox-NAD, 6-pyridone-3-carboxamide adenine dinucleotide phosphate.

Figure 4.

Unstable transient degradation products of NADH and NADPH. Isomerization of NAD(P)(H) generates isomers that are potent inhibitors of redox enzymes. Renalase catalyzes the oxidation of all NAD(P)H isomers to NAD(P); however, renalase is mainly found in kidneys and extracellularly, but it is not known whether other dehydrogenases can perform this conversion since renalase is not an obligate enzyme. Hydration of NAD(P)(H) also occurs chemically and is favored by acid conditions. Once generated, it can fully degrade to glycating species unless it is converted back to NAD(P)H by a specific epimerase-dehydratase. The absence of this enzyme is embryonically lethal.