NAD+ therapy in age-related degenerative disorders: A benefit/risk analysis Author links open overlay panel

Abstract

Nicotinamide adenine dinucleotide (NAD+) is an essential pyridine nucleotide that is present in all living cells. NAD+ acts as an important cofactor and substrate for a multitude of biological processes including energy production, DNA repair, gene expression, calcium-dependent secondary messenger signalling and immunoregulatory roles. The de novo synthesis of NAD+ is primarily dependent on the kynurenine pathway (KP), although NAD+ can also be recycled from nicotinic acid (NA), nicotinamide (NAM) and nicotinamide riboside (NR). NAD+ levels have been reported to decline during ageing and age-related diseases. Recent studies have shown that raising intracellular NAD+ levels represents a promising therapeutic strategy for age-associated degenerative diseases in general and to extend lifespan in small animal models. A systematic review of the literature available on Medline, Embase and Pubmed was undertaken to evaluate the potential health and/or longevity benefits due to increasing NAD+ levels. A total of 1545 articles were identified and 147 articles (113 preclinical and 34 clinical) met criteria for inclusion. Most studies indicated that the NAD+ precursors NAM, NR, nicotinamide mononucleotide (NMN), and to a lesser extent NAD+ and NADH had a favourable outcome on several age-related disorders associated with the accumulation of chronic oxidative stress, inflammation and impaired mitochondrial function. While these compounds presented with a limited acute toxicity profile, evidence is still quite limited and long-term human clinical trials are still nascent in the current literature. Potential risks in raising NAD+ levels in various clinical disorders using NAD+ precursors include the accumulation of putative toxic metabolites, tumorigenesis and promotion of cellular senescence. Therefore, NAD+ metabolism represents a promising target and further studies are needed to recapitulate the preclinical benefits in human clinical trials.

Introduction

The kynurenine (KYN) pathway is the principle route of catabolism of the amino acid tryptophan (TRYP). The KP also represents the de novo synthesis pathway of the essential coenzyme and pyridine nucleotide, nicotinamide adenine dinucleotide (NAD+) (Fig. 1) (reviewed in Braidy et al., 2019). We and others have demonstrated that activation of the KP may represent a compensatory mechanism to replenish NAD+ depletion in activated pro-inflammatory cells such as macrophages, astrocytes and microglia (reviewed in Braidy and Grant, 2017). NAD+ is an essential cofactor in several important biological processes, including oxidative phosphorylation and production of adenosine triphosphate (ATP). NAD+ is also an important substrate for DNA repair, secondary messenger signalling and epigenetic regulation of gene expression (reviewed in Braidy et al., 2019). Intracellular NAD+ and ATP levels are crucial for linking cellular energy status to a wide variety of molecular processes that regulate cell survival (Eguchi et al., 1997; Leist et al., 1997). Although NAD+ can be recycled from the acid, amide or riboside form of vitamin B3, the de novo synthesis of NAD+ occurs by the KP (reviewed in Braidy et al., 2019).

Cellular NAD+ levels are critical factors for cell survival in several models and NAD+ may serve as a longevity assurance factor (Braidy et al., 2011; Braidy et al., 2014; Braidy et al., 2008). Our group has previously demonstrated a significant decline in NAD+ levels in catabolic tissue in physiologically aged rats (Braidy et al., 2011; Braidy et al., 2014), aged human pelvic skin biopsies (Massudi et al., 2012) and human plasma (Clement et al., 2018). NAD+ decline has also been reported in other degenerative diseases associated with the accumulation of oxidative stress and inflammation including neurodegenerative diseases such as multiple sclerosis (Braidy et al., 2013). Energy restriction and impaired metabolic function following depletion of NAD+ stores can lead to cell death due to impaired ATP synthesis. The de novo synthesis of NAD+ is dependent of the KP, and reduced NAD+ levels due to KP inhibition is likely to explain the observed increases in apoptotic cell death in neuroinflammatory conditions (Grant et al., 1999; Grant et al., 2000). In contrast, KP activation has been shown to enhance NAD+ levels in activated macrophages and microglial cells.

Apart from the deleterious effects on NAD+ levels, inhibition of the KP may have detrimental effects on cell survival. For example, inhibition of the KP was reported to induce accumulation of kynurenic acid (KYNA) at the low micromolar concentrations in rats infected with pneuomococcal meningitis (Bellac et al., 2010). KYN is an antagonist of the N-methyl-D-aspartic acid (NMDA) receptor and the nicotinic acetylcholine receptor. These receptors contribute to brain excitation and KYNA can antagonise the cytotoxic effects of glutamate, quinolinic acid (QUIN), d-serine and other NMDA receptor agonists that are elevated in inflammatory conditions (Lopes et al., 2007; Vesce et al., 2007) (Fig. 1). While some studies have reported attenuation of cellular damage following exposure to KYNA in a variety of models (Obrenovitch and Urenjak, 2000; Urenjak and Obrenovitch, 2000), some studies have reported increased apoptotic neural cell death because of impaired glutamate mediated stimulation of excitatory receptors (Biegon et al., 2004; Potter et al., 2010). Additionally, increased apoptotic neuronal death has been reported following treatment with dextromethorphan, a non-competitive NMDA receptor antagonist in the infant rat model of pneumococcal meningitis (Sellner et al., 2008). This suggests that the resulting apoptosis following KP inhibition in pneumococcal meningitis may be independent of modulation of CSF inflammation.

‘Inflammaging’ is a new term used to explain the link between ageing and inflammation. Inflammaging involves the regulation of several genes/proteins and small molecules (Franceschi et al., 2017). Activated glial cells lead to increased production of nuclear factor-κB (NF-κB), cyclooxygenase-2 (COX2) and inducible nitric oxide synthase (iNOS) levels leading to further production and release pro-inflammatory cytokines, such as interleukin-6 (IL-6), interleukin-1β (IL-1β) and reactive oxidative species (ROS) and tumor necrosis factor-α (TNF-α), which contribute to cell death manifested in age-related degenerative diseases (Agostinho et al., 2010; Dantzer et al., 2008). Increased NF-κB activation via Toll Like Receptors 4 (TLR4) and the Innate Immune Signal Transduction Adaptor (MYD88), enhances the release of pro-inflammatory cytokines which promotes inflammatory processes. It has been hypothesised that molecules that upregulate of the vitagene system can inhibit this pathogenic process and slow down the progression of age-related disorders (Scuto et al., 2019). Basal levels of oxidants are essential for the maintenance of adaptive cellular responses such as vitagenes associated with cell survival (Calabrese et al., 2010). However, at higher levels, these oxidants have deleterious effects on cells, promoting ageing and progression of various age-related diseases (Calabrese et al., 2015). The “vitagene” system includes heat shock proteins (HSP70) and heme oxygenase-1 (HO-1), thioredoxin/thioredoxin reductase (Trx/TrxR), γ-glutamyl cysteine synthetase (γ-GCS), and NAD-dependent histone deacetylases (sirtuins particularly SIRT1) (Scuto et al., 2019). Increased lymphocyte levels of HO-1, HSP70, Trx and TrxR-1, and reduced levels of SIRT1 and SIRT2 protein have been reported in diabetic patients compared to healthy controls. This suggests that patients affected by type 2 diabetes are exposed to conditions of systemic oxidative stress, although the exact significance of reduced sirtuin protein remains to be fully elucidated (Calabrese et al., 2012).

Aside from its role as a cofactor in over 400 oxidoreductase enzymes including lactate and alcohol dehydrogenases, NAD+ is substrate for at least 4 main enzymes known as NAD+ consumers: that is, poly(ADP-ribose) polymerases (PARPs), mono(ADP-ribosyl) transferases, bifunctional ADP-ribosyl cyclases/cyclic ADP-ribose hydrolases (CD38), and NAD+ − dependent histone deacetylases (sirtuins) (reviewed in Braidy et al., 2019). Although little is known regarding the contribution of these enzymes to the pathobiology of age-related degenerative disorders, NAD+ consumption competes for the availability of NAD+ for other processes. Declining NAD+ levels with age reduces SIRT1 function (Braidy et al., 2011), which can be restored by increasing NAD+ levels (Cantó et al., 2012). While the activity of NAD+ consumers is affected by various conditions, CD38 has been hypothesised as the main regulator of age-related NAD+ decline in ageing and metabolic disorders (Braidy et al., 2014). As well, PARP activity strongly correlated with mammalian lifespan, suggesting that the NAD+/PARP1/SIRT1 axis may provide a converging link for decreased NAD+ levels and increased DNA damage with epigenomic DNA methylation clocks (reviewed in Braidy et al., 2019). Pharmacological inhibition of PARP-1, which is activated in response to increased oxidative DNA damage has been shown to rescue damaged injured cells, maintain NAD+ levels and the activity of NAD-dependent processes, and provide symptomatic relief in several studies (Clark et al., 2007; Koedel et al., 2002). However, while PARP-1 inhibition may rescue cells by preventing cellular NAD+ depletion, PARP inhibition induces genomic instability (Beneke et al., 2004). Therefore, the clinical benefits of PARP inhibitors and their potential negative effects on genomic instability have important implications for therapies to be used for the management and/or prevention of non-oncologic indications.

Consistent with the strategic approach that maintenance of intracellular NAD+ homeostasis is beneficial for normal cellular survival, supplementation of NAD+ with either exogenous NAD+ and its reduced form NADH, and NAD+ precursors such as NAM, NMN or NR have shown varying degrees of health benefits in different paradigms of degenerative diseases (Fig. 2). However, many of these benefits have been reported in preclinical animal models, and human studies have reported increases in blood NAD+ levels following supplementation with NAM and NR (Conze et al 2019; Trammell et al., 2016; Elhassan et al., 2019). Moreover, the risks of raising NAD+ remain unclear. This systematic review examines all relevant preclinical and clinical studies currently available in the literature, in order to elucidate the potential risk/benefits of NAD+ supplementation as a holistic approach to promote healthspan.

Section snippets

Search strategy

We identified studies through searches of Medline, Embase, Pubmed and the Cochrane Library (1990 till now) databases. The search was combined with terms of: NAD+ or nicotinamide adenine dinucleotide, including supplementation OR benefits OR adverse effects OR humans OR clinical trials; NMN OR nicotinamide mononucleotide OR NR OR nicotinamide riboside OR NAM OR nicotinamide AND clinical trials. We identified relevant trials by reviewing titles and abstracts of identified articles and supplements

Search results

A total of 114 preclinical and 36 clinical studies that met the inclusion criteria were included following review of title and abstracts. The main characteristics of the preclinical (condition, intervention, reported benefits, sample size, dose, animal model) and clinical studies (condition, intervention, reported benefits, sample size, age, study design) are included in Table 1, Table 2 respectively. (See Table 3.)

Age-related skin pathologies

We previously demonstrated that NAD+ levels are reduced in pelvic skin samples

Discussion

NAD+ precursors, including NR and NMN represent likely candidates for supplementation due to their beneficial effects on energy production, DNA repair, cell signalling and delayed degenerative effects. Since raising NAD+ has been shown to modulate biological processes such as DNA repair and necessary for stress responses and energy metabolism, NAD+ precursors should be examined further as potential investigative agents for further drug development for various age-related disorders. Imperative

Conclusion

NAD+ metabolism represents a promising therapeutic target for the treatment of metabolic and age-related disorders, such as obesity, diabetes, cardiovascular and neurodegenerative diseases. Modulation of NAD+ biosynthesis has shown that NAD+ depletion may play a contributory role in the aetiology of several metabolic disorders, at least in murine models. Surmounting evidence has suggested that raising NAD+ levels using NAD+ precursors could slow down and reduce symptoms of metabolic stress and

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Source: https://www.sciencedirect.com/science/article/abs/pii/S0531556519307582