CD73 protein as a source of extracellular precursors for sustained NAD+ biosynthesis in FK866-treated tumor cells

NAD+ metabolism, stemness, the immune response, and cancer

NAD⁺ was discovered during yeast fermentation, and since its discovery, its important roles in redox metabolism, aging, and longevity, the immune system and DNA repair have been highlighted. A deregulation of the NAD⁺ levels has been associated with metabolic diseases and aging-related diseases, including neurodegeneration, defective immune responses, and cancer. NAD⁺ acts as a cofactor through its interplay with NADH, playing an essential role in many enzymatic reactions of energy metabolism, such as glycolysis, oxidative phosphorylation, fatty acid oxidation, and the TCA cycle. NAD⁺ also plays a role in deacetylation by sirtuins and ADP ribosylation during DNA damage/repair by PARP proteins. Finally, different NAD hydrolase proteins also consume NAD⁺ while converting it into ADP-ribose or its cyclic counterpart. Some of these proteins, such as CD38, seem to be extensively involved in the immune response. Since NAD cannot be taken directly from food, NAD metabolism is essential, and NAMPT is the key enzyme recovering NAD from nicotinamide and generating most of the NAD cellular pools. Because of the complex network of pathways in which NAD⁺ is essential, the important role of NAD⁺ and its key generating enzyme, NAMPT, in cancer is understandable. In the present work, we review the role of NAD⁺ and NAMPT in the ways that they may influence cancer metabolism, the immune system, stemness, aging, and cancer. Finally, we review some ongoing research on therapeutic approaches.

CD38 ecto-enzyme in immune cells is induced during aging and regulates NAD+ and NMN levels

Decreased NAD+ levels have been shown to contribute to metabolic dysfunction during aging. NAD+ decline can be partially prevented by knockout of the enzyme CD38. However, it is not known how CD38 is regulated during aging, and how its ecto-enzymatic activity impacts NAD+ homeostasis. Here we show that an increase in CD38 in white adipose tissue (WAT) and the liver during aging is mediated by accumulation of CD38+ immune cells. Inflammation increases CD38 and decreases NAD+. In addition, senescent cells and their secreted signals promote accumulation of CD38+ cells in WAT, and ablation of senescent cells or their secretory phenotype decreases CD38, partially reversing NAD+ decline. Finally, blocking the ecto-enzymatic activity of CD38 can increase NAD+ through a nicotinamide mononucleotide (NMN)-dependent process. Our findings demonstrate that senescence-induced inflammation promotes accumulation of CD38 in immune cells that, through its ecto-enzymatic activity, decreases levels of NMN and NAD+.

CD38 downregulation modulates NAD+ and NADP(H) levels in thermogenic adipose tissues

Different strategies to boost NAD⁺ levels are considered promising means to promote healthy aging and ameliorate dysfunctional metabolism. CD38 is a NAD⁺-dependent enzyme involved in the regulation of different cell functions. In the context of systemic energy metabolism, it has been demonstrated that brown adipocytes, the parenchymal cells of brown adipose tissue (BAT) as well as beige adipocytes that emerge in white adipose tissue (WAT) depots in response to catabolic conditions, are important to maintain metabolic homeostasis. In this study we aim to understand the functional relevance of CD38 for NAD⁺ and energy metabolism in BAT and WAT, also using a CD38^-/- mouse model. During cold exposure, an increase in NAD⁺ levels occurred in BAT of wild type mice, together with a marked downregulation of CD38, as detected at the mRNA and protein level. CD38 downregulation was observed also in WAT of cold-exposed mice, where it was accompanied by a strong increase in NADP(H) levels. Accordingly, NAD kinase and glucose-6-phosphate dehydrogenase activities were enhanced in WAT (but not in BAT). Increased NAD⁺ levels were observed in BAT/WAT from CD38^-/- compared with wild type mice, in line with CD38 being a major NAD⁺-consumer in AT. CD38^-/- mice kept at 6 °C had higher levels of Ucp1 and Pgc-1α in BAT and WAT, and increased levels of phosphorylated hormone-sensitive lipase in BAT, compared with wild type mice. These results demonstrate that CD38, by modulating cellular NAD(P)⁺ levels, is involved in the regulation of thermogenic responses in cold-activated BAT and WAT.

NAD+ Metabolism as an Emerging Therapeutic Target for Cardiovascular Diseases Associated With Sudden Cardiac Death

In addition to its central role in mediating oxidation reduction in fuel metabolism and bioenergetics, nicotinamide adenine dinucleotide (NAD⁺) has emerged as a vital co-substrate for a number of proteins involved in diverse cellular processes, including sirtuins, poly(ADP-ribose) polymerases and cyclic ADP-ribose synthetases. The connection with aging and age-associated diseases has led to a new wave of research in the cardiovascular field. Here, we review the basics of NAD⁺ homeostasis, the molecular physiology and new advances in ischemic-reperfusion injury, heart failure, and arrhythmias, all of which are associated with increased risks for sudden cardiac death. Finally, we summarize the progress of NAD⁺-boosting therapy in human cardiovascular diseases and the challenges for future studies.

Nicotinamide Riboside-The Current State of Research and Therapeutic Uses

Nicotinamide riboside (NR) has recently become one of the most studied nicotinamide adenine dinucleotide (NAD+) precursors, due to its numerous potential health benefits mediated via elevated NAD+ content in the body. NAD+ is an essential coenzyme that plays important roles in various metabolic pathways and increasing its overall content has been confirmed as a valuable strategy for treating a wide variety of pathophysiological conditions. Accumulating evidence on NRs' health benefits has validated its efficiency across numerous animal and human studies for the treatment of a number of cardiovascular, neurodegenerative, and metabolic disorders. As the prevalence and morbidity of these conditions increases in modern society, the great necessity has arisen for a rapid translation of NR to therapeutic use and further establishment of its availability as a nutritional supplement. Here, we summarize currently available data on NR effects on metabolism, and several neurodegenerative and cardiovascular disorders, through to its application as a treatment for specific pathophysiological conditions. In addition, we have reviewed newly published research on the application of NR as a potential therapy against infections with several pathogens, including SARS-CoV-2. Additionally, to support rapid NR translation to therapeutics, the challenges related to its bioavailability and safety are addressed, together with the advantages of NR to other NAD+ precursors.

CD38: A Potential Therapeutic Target in Cardiovascular Disease

Substantial research has demonstrated the association between cardiovascular disease and the dysregulation of intracellular calcium, ageing, reduction in nicotinamide adenine dinucleotide NAD+ content, and decrease in sirtuin activity. CD38, which comprises the soluble type, type II, and type III, is the main NADase in mammals. This molecule catalyses the production of cyclic adenosine diphosphate ribose (cADPR), nicotinic acid adenine dinucleotide phosphate (NAADP), and adenosine diphosphate ribose (ADPR), which stimulate the release of Ca²⁺, accompanied by NAD+ consumption and decreased sirtuin activity. Therefore, the relationship between cardiovascular disease and CD38 has been attracting increased attention. In this review, we summarize the structure, regulation, function, targeted drug development, and current research on CD38 in the cardiac context. More importantly, we provide original views about the as yet elusive mechanisms of CD38 action in certain cardiovascular disease models. Based on our review, we predict that CD38 may serve as a novel therapeutic target in cardiovascular disease in the future.

Reversal of endothelial dysfunction by nicotinamide mononucleotide via extracellular conversion to nicotinamide riboside

BACKGROUND

Nicotinamide mononucleotide (NMN) and nicotinamide riboside (NR) are effective substrates for NAD synthesis, which may act as vasoprotective agents. Here, we characterize the effects of NMN and NR on endothelial inflammation and dysfunction and test the involvement of CD73 in these effects.

MATERIALS AND METHODS

The effect of NMN and NR on IL1β- or TNFα-induced endothelial inflammation (ICAM1 and vWF expression), intracellular NAD concentration and NAD-related enzyme expression (NAMPT, CD38, CD73), were studied in HAECs. The effect of NMN and NR on angiotensin II-induced impairment of endothelium-dependent vasodilation was analyzed in murine aortic rings. The involvement of CD73 in NMN and NR effects was tested using CD73 inhibitor-AOPCP, or CD73^-/- mice.

RESULTS

24 h-incubation with NMN and NR induced anti-inflammatory effects in HAEC stimulated by IL1β or TNFα, as evidenced by a reduction in ICAM1 and vWF expression. Effects of exogenous NMN but not NR was abrogated in the presence of AOPCP, that efficiently inhibited extracellular endothelial conversion of NMN to NR, without a significant effect on the metabolism of NMN to NA. Surprisingly, intracellular NAD concentration increased in HAEC stimulated by IL1β or TNFα and this effect was associated with upregulation of NAMPT and CD73, whereas changes in CD38 expression were less pronounced. NMN and NR further increased NAD in IL1β-stimulated HAECs and AOPCP diminished NMN-induced increase in NAD, without an effect on NR-induced response. In ex vivo aortic rings stimulated with angiotensin II for 24 h, NO-dependent vasorelaxation induced by acetylcholine was impaired. NMN and NR, both prevented Ang II-induced endothelial dysfunction in the aorta. In aortic rings taken from CD73^-/- mice NMN effect was lost, whereas NR effect was preserved.

CONCLUSION

NMN and NR modulate intracellular NAD content in endothelium, inhibit endothelial inflammation and improve NO-dependent function by CD73-dependent and independent pathways, respectively. Extracellular conversion of NMN to NR by CD73 localized in the luminal surface of endothelial cells represent important vasoprotective mechanisms to maintain intracellular NAD.

Nicotinamide Mononucleotide: A Promising Molecule for Therapy of Diverse Diseases by Targeting NAD+ Metabolism

NAD+, a co-enzyme involved in a great deal of biochemical reactions, has been found to be a network node of diverse biological processes. In mammalian cells, NAD+ is synthetized, predominantly through NMN, to replenish the consumption by NADase participating in physiologic processes including DNA repair, metabolism, and cell death. Correspondingly, aberrant NAD+ metabolism is observed in many diseases. In this review, we discuss how the homeostasis of NAD+ is maintained in healthy condition and provide several age-related pathological examples related with NAD+ unbalance. The sirtuins family, whose functions are NAD-dependent, is also reviewed. Administration of NMN surprisingly demonstrated amelioration of the pathological conditions in some age-related disease mouse models. Further clinical trials have been launched to investigate the safety and benefits of NMN. The NAD+ production and consumption pathways including NMN are essential for more precise understanding and therapy of age-related pathological processes such as diabetes, ischemia–reperfusion injury, heart failure, Alzheimer’s disease, and retinal degeneration.

Targeting the adenosine pathway for cancer immunotherapy

Suppression of anti-tumor immunity is recognized as a critical step in the development of many types of cancers. Over the past decade, a multitude of immunosuppressive pathways occurring in the tumor microenvironment (TME) have been identified. Amongst them, the hydrolysis of extracellular ATP into adenosine by ecto-nucleotidases has been increasingly documented as new immune checkpoint pathway that can significantly impair anti-tumor immunity of multiple types of cancer. In this review, we summarize past and recent research on the ecto-nucleotidases CD39 and CD73, conducted by our group and others, that recently lead to the development and clinical testing of adenosine targeting agents for cancer immunotherapy.

NAMPT and NAPRT: Two Metabolic Enzymes With Key Roles in Inflammation

Nicotinamide phosphoribosyltransferase (NAMPT) and nicotinate phosphoribosyltransferase (NAPRT) are two intracellular enzymes that catalyze the first step in the biosynthesis of NAD from nicotinamide and nicotinic acid, respectively. By fine tuning intracellular NAD levels, they are involved in the regulation/reprogramming of cellular metabolism and in the control of the activity of NAD-dependent enzymes, including sirtuins, PARPs, and NADases. However, during evolution they both acquired novel functions as extracellular endogenous mediators of inflammation. It is well-known that cellular stress and/or damage induce release in the extracellular milieu of endogenous molecules, called alarmins or damage-associated molecular patterns (DAMPs), which modulate immune functions through binding pattern recognition receptors (PRRs), such as Toll-like receptors (TLRs), and activate inflammatory responses. Increasing evidence suggests that extracellular (e)NAMPT and eNAPRT are novel soluble factors with cytokine/adipokine/DAMP-like actions. Elevated eNAMPT were reported in several metabolic and inflammatory disorders, including obesity, diabetes, and cancer, while eNAPRT is emerging as a biomarker of sepsis and septic shock. This review will discuss available data concerning the dual role of this unique family of enzymes.

NAD- and NADPH-Contributing Enzymes as Therapeutic Targets in Cancer: An Overview

Actively proliferating cancer cells require sufficient amount of NADH and NADPH for biogenesis and to protect cells from the detrimental effect of reactive oxygen species. As both normal and cancer cells share the same NAD biosynthetic and metabolic pathways, selectively lowering levels of NAD(H) and NADPH would be a promising strategy for cancer treatment. Targeting nicotinamide phosphoribosyltransferase (NAMPT), a rate limiting enzyme of the NAD salvage pathway, affects the NAD and NADPH pool. Similarly, lowering NADPH by mutant isocitrate dehydrogenase 1/2 (IDH1/2) which produces D-2-hydroxyglutarate (D-2HG), an oncometabolite that downregulates nicotinate phosphoribosyltransferase (NAPRT) via hypermethylation on the promoter region, results in epigenetic regulation. NADPH is used to generate D-2HG, and is also needed to protect dihydrofolate reductase, the target for methotrexate, from degradation. NAD and NADPH pools in various cancer types are regulated by several metabolic enzymes, including methylenetetrahydrofolate dehydrogenase, serine hydroxymethyltransferase, and aldehyde dehydrogenase. Thus, targeting NAD and NADPH synthesis under special circumstances is a novel approach to treat some cancers. This article provides the rationale for targeting the key enzymes that maintain the NAD/NADPH pool, and reviews preclinical studies of targeting these enzymes in cancers.

CD38 in Neurodegeneration and Neuroinflammation

Neurodegenerative diseases are characterized by neuronal degeneration as well as neuroinflammation. While CD38 is strongly expressed in brain cells including neurons, astrocytes as well as microglial cells, the role played by CD38 in neurodegeneration and neuroinflammation remains elusive. Yet, CD38 expression increases as a consequence of aging which is otherwise the primary risk associated with neurodegenerative diseases, and several experimental data demonstrated that CD38 knockout mice are protected from neurodegenerative and neuroinflammatory insults. Moreover, nicotinamide adenine dinucleotide, whose levels are tightly controlled by CD38, is a recognized and potent neuroprotective agent, and NAD supplementation was found to be beneficial against neurodegenerative diseases. The aims of this review are to summarize the physiological role played by CD38 in the brain, present the arguments indicating the involvement of CD38 in neurodegeneration and neuroinflammation, and to discuss these observations in light of CD38 complex biology.

The NAD+ Salvage Pathway Supports PHGDH-Driven Serine Biosynthesis

NAD⁺ is a key metabolic redox cofactor that is regenerated from nicotinamide through the NAD⁺ salvage pathway. Here, we find that inhibiting the NAD⁺ salvage pathway depletes serine biosynthesis from glucose by impeding the NAD⁺-dependent protein, 3-phosphoglycerate dehydrogenase (PHGDH). Importantly, we find that PHGDH^high breast cancer cell lines are exquisitely sensitive to inhibition of the NAD⁺ salvage pathway. Further, we find that PHGDH protein levels and those of the rate-limiting enzyme of NAD⁺ salvage, NAMPT, correlate in ER-negative, basal-like breast cancers. Although NAD⁺ salvage pathway inhibitors are actively being pursued in cancer treatment, their efficacy has been poor, and our findings suggest that they may be effective for PHGDH-dependent cancers.

Extracellular NAD+ enhances PARP-dependent DNA repair capacity independently of CD73 activity

Changes in nicotinamide adenine dinucleotide (NAD⁺) levels that compromise mitochondrial function trigger release of DNA damaging reactive oxygen species. NAD⁺ levels also affect DNA repair capacity as NAD⁺ is a substrate for PARP-enzymes (mono/poly-ADP-ribosylation) and sirtuins (deacetylation). The ecto-5′-nucleotidase CD73, an ectoenzyme highly expressed in cancer, is suggested to regulate intracellular NAD⁺ levels by processing NAD⁺ and its bio-precursor, nicotinamide mononucleotide (NMN), from tumor microenvironments, thereby enhancing tumor DNA repair capacity and chemotherapy resistance. We therefore investigated whether expression of CD73 impacts intracellular NAD⁺ content and NAD⁺-dependent DNA repair capacity. Reduced intracellular NAD⁺ levels suppressed recruitment of the DNA repair protein XRCC1 to sites of genomic DNA damage and impacted the amount of accumulated DNA damage. Further, decreased NAD⁺ reduced the capacity to repair DNA damage induced by DNA alkylating agents. Overall, reversal of these outcomes through NAD⁺ or NMN supplementation was independent of CD73. In opposition to its proposed role in extracellular NAD⁺ bioprocessing, we found that recombinant human CD73 only poorly processes NMN but not NAD⁺. A positive correlation between CD73 expression and intracellular NAD⁺ content could not be made as CD73 knockout human cells were efficient in generating intracellular NAD⁺ when supplemented with NAD⁺ or NMN.

Degradation of Extracellular NAD+ Intermediates in Cultures of Human HEK293 Cells

Nicotinamide adenine dinucleotide (NAD) is an essential redox carrier, whereas its degradation is a key element of important signaling pathways. Human cells replenish their NAD contents through NAD biosynthesis from extracellular precursors. These precursors encompass bases nicotinamide (Nam) and nicotinic acid and their corresponding nucleosides nicotinamide riboside (NR) and nicotinic acid riboside (NAR), now collectively referred to as vitamin B3. In addition, extracellular NAD+ and nicotinamide mononucleotide (NMN), and potentially their deamidated counterparts, nicotinic acid adenine dinucleotide (NAAD) and nicotinic acid mononucleotide (NAMN), may serve as precursors of intracellular NAD. However, it is still debated whether nucleotides enter cells directly or whether they are converted to nucleosides and bases prior to uptake into cells. Here, we studied the metabolism of extracellular NAD+ and its derivatives in human HEK293 cells using normal and serum-free culture medium. Using medium containing 10% fetal bovine serum (FBS), mono- and dinucleotides were degraded to the corresponding nucleosides. In turn, the nucleosides were cleaved to their corresponding bases. Degradation was also observed in culture medium alone, in the absence of cells, indicating that FBS contains enzymatic activities which degrade NAD+ intermediates. Surprisingly, NR was also rather efficiently hydrolyzed to Nam in the absence of FBS. When cultivated in serum-free medium, HEK293 cells efficiently cleaved NAD+ and NAAD to NMN and NAMN. NMN exhibited rather high stability in cell culture, but was partially metabolized to NR. Using pharmacological inhibitors of plasma membrane transporters, we also showed that extracellular cleavage of NAD+ and NMN to NR is a prerequisite for using these nucleotides to maintain intracellular NAD contents. We also present evidence that, besides spontaneous hydrolysis, NR is intensively metabolized in cell culture by intracellular conversion to Nam. Our results demonstrate that both the cultured cells and the culture medium mediate a rather active conversion of NAD+ intermediates. Consequently, in studies of precursor supplementation and uptake, the culture conditions need to be carefully defined.

Subcellular Characterization of Nicotinamide Adenine Dinucleotide Biosynthesis in Metastatic Melanoma by Using Organelle-Specific Biosensors

Aim: Nicotinamide adenine dinucleotide (NAD⁺) plays central roles in a wide array of normal and pathological conditions. Inhibition of NAD⁺ biosynthesis can be exploited therapeutically in cancer, including melanoma. To obtain quantitation of NAD⁺ levels in live cells and to address the issue of the compartmentalization of NAD⁺ biosynthesis, we exploited a recently described genetically encoded NAD⁺ biosensor (LigA-circularly permutated Venus), which was targeted to the cytosol, mitochondria, and nuclei of BRAF-V600E A375 melanoma cells, a model of metastatic melanoma (MM). Results: FK866, a specific inhibitor of nicotinamide phosphoribosyltransferase (NAMPT), the main NAD⁺-producing enzyme in MM cells, was used to monitor NAD⁺ depletion kinetics at the subcellular level in biosensor-transduced A375 cells. In addition, we treated FK866-blocked A375 cells with NAD⁺ precursors, including nicotinamide, nicotinic acid, nicotinamide riboside, and quinolinic acid, highlighting an organelle-specific capacity of each substrate to rescue from NAMPT block. Expression of NAD⁺ biosynthetic enzymes was then biochemically studied in isolated organelles, revealing the presence of NAMPT in all three cellular compartments, whereas nicotinate phosphoribosyltransferase was predominantly cytosolic and mitochondrial, and nicotinamide riboside kinase mitochondrial and nuclear. In keeping with biosensor data, quinolinate phosphoribosyltransferase was expressed at extremely low levels. Innovation and Conclusions: Throughout this work, we validated the use of genetically encoded NAD⁺ biosensors to characterize subcellular distribution of NAD⁺ production routes in MM. The chance of real-time monitoring of NAD⁺ fluctuations after chemical perturbations, together with a deeper comprehension of the cofactor biosynthesis compartmentalization, strengthens the foundation for a targeted strategy of NAD⁺ pool manipulation in cancer and metabolic diseases.

NAD+ in Brain Aging and Neurodegenerative Disorders

NAD⁺ is a pivotal metabolite involved in cellular bioenergetics, genomic stability, mitochondrial homeostasis, adaptive stress responses, and cell survival. Multiple NAD⁺-dependent enzymes are involved in synaptic plasticity and neuronal stress resistance. Here, we review emerging findings that reveal key roles for NAD⁺ and related metabolites in the adaptation of neurons to a wide range of physiological stressors and in counteracting processes in neurodegenerative diseases, such as those occurring in Alzheimer's, Parkinson's, and Huntington diseases, and amyotrophic lateral sclerosis. Advances in understanding the molecular and cellular mechanisms of NAD⁺-based neuronal resilience will lead to novel approaches for facilitating healthy brain aging and for the treatment of a range of neurological disorders.

Cellular Compartmentation and the Redox/Nonredox Functions of NAD. Antioxid Redox Signal 2019;31:623-642. [PMID: 30784294 PMCID: PMC6657305 DOI: 10.1089/ars.2018.7722]

Significance: Nicotinamide adenine dinucleotide (NAD⁺) spans diverse roles in biology, serving as both an important redox cofactor in metabolism and a substrate for signaling enzymes that regulate protein post-translational modifications (PTMs). Critical Issues: Although the interactions between these different roles of NAD⁺ (and its reduced form NADH) have been considered, little attention has been paid to the role of compartmentation in these processes. Specifically, the role of NAD⁺ in metabolism is compartment specific (e.g., mitochondrial vs. cytosolic), affording a very different redox landscape for PTM-modulating enzymes such as sirtuins and poly(ADP-ribose) polymerases in different cell compartments. In addition, the orders of magnitude differences in expression levels between NAD⁺-dependent enzymes are often not considered when assuming the effects of bulk changes in NAD⁺ levels on their relative activities. Recent Advances: In this review, we discuss the metabolic, nonmetabolic, redox, and enzyme substrate roles of cellular NAD⁺, and the recent discoveries regarding the interplay between these roles in different cell compartments. Future Directions: Therapeutic implications for the compartmentation and manipulation of NAD⁺ biology are discussed. Antioxid. Redox Signal. 31, 623-642.

Nicotinamide riboside, an NAD+ precursor, attenuates the development of liver fibrosis in a diet-induced mouse model of liver fibrosis

OBJECTIVE

Liver fibrosis is part of the non-alcoholic fatty liver disease (NAFLD) spectrum, which currently has no approved pharmacological treatment. In this study, we investigated whether supplementation of nicotinamide riboside (NR), a nicotinamide adenine dinucleotide (NAD+) precursor, can reduce the development of liver fibrosis in a diet-induced mouse model of liver fibrosis.

METHODS

Male C57BL/6 J mice were fed a low-fat control (LF), a high-fat/high-sucrose/high-cholesterol control (HF) or a HF diet supplemented with NR at 400 mg/kg/day (HF-NR) for 20 weeks. Features of liver fibrosis were assessed by histological and biochemical analyses. Whole-body energy metabolism was also assessed using indirect calorimetry. Primary mouse and human hepatic stellate cells were used to determine the anti-fibrogenic effects of NR in vitro.

RESULTS

NR supplementation significantly reduced body weight of mice only 7 weeks after mice were on the supplementation, but did not attenuate serum alanine aminotransferase levels, liver steatosis, or liver inflammation. However, NR markedly reduced collagen accumulation in the liver. RNA-Seq analysis suggested that the expression of genes involved in NAD+ metabolism is altered in activated hepatic stellate cells (HSCs) compared to quiescent HSCs. NR inhibited the activation of HSCs in primary mouse and human HSCs. Indirect calorimetry showed that NR increased energy expenditure, likely by upregulation of β-oxidation in skeletal muscle and brown adipose tissue.

CONCLUSION

NR attenuated HSC activation, leading to reduced liver fibrosis in a diet-induced mouse model of liver fibrosis. The data suggest that NR may be developed as a potential preventative for human liver fibrosis.

Subcellular compartmentalization of NAD+ and its role in cancer: A sereNADe of metabolic melodies

Nicotinamide adenine dinucleotide (NAD+) is an essential biomolecule involved in many critical processes. Its role as both a driver of energy production and a signaling molecule underscores its importance in health and disease. NAD+ signaling impacts multiple processes that are dysregulated in cancer, including DNA repair, cell proliferation, differentiation, redox regulation, and oxidative stress. Distribution of NAD+ is highly compartmentalized, with each subcellular NAD+ pool differentially regulated and preferentially involved in distinct NAD+-dependent signaling or metabolic events. Emerging evidence suggests that targeting NAD+ metabolism is likely to repress many specific mechanisms underlying tumor development and progression, including proliferation, survival, metabolic adaptations, invasive capabilities, heterotypic interactions with the tumor microenvironment, and stress response including notably DNA maintenance and repair. Here we provide a comprehensive overview of how compartmentalized NAD+ metabolism in mitochondria, nucleus, cytosol, and extracellular space impacts cancer formation and progression, along with a discussion of the therapeutic potential of NAD+-targeting drugs in cancer.

Nicotinamide mononucleotide alters mitochondrial dynamics by SIRT3-dependent mechanism in male mice

Nicotinamide adenine dinucleotide (NAD+ ) is a central signaling molecule and enzyme cofactor that is involved in a variety of fundamental biological processes. NAD+ levels decline with age, neurodegenerative conditions, acute brain injury, and in obesity or diabetes. Loss of NAD+ results in impaired mitochondrial and cellular functions. Administration of NAD+ precursor, nicotinamide mononucleotide (NMN), has shown to improve mitochondrial bioenergetics, reverse age-associated physiological decline, and inhibit postischemic NAD+ degradation and cellular death. In this study, we identified a novel link between NAD+ metabolism and mitochondrial dynamics. A single dose (62.5 mg/kg) of NMN, administered to male mice, increases hippocampal mitochondria NAD+ pools for up to 24 hr posttreatment and drives a sirtuin 3 (SIRT3)-mediated global decrease in mitochondrial protein acetylation. This results in a reduction of hippocampal reactive oxygen species levels via SIRT3-driven deacetylation of mitochondrial manganese superoxide dismutase. Consequently, mitochondria in neurons become less fragmented due to lower interaction of phosphorylated fission protein, dynamin-related protein 1 (pDrp1 [S616]), with mitochondria. In conclusion, manipulation of mitochondrial NAD+ levels by NMN results in metabolic changes that protect mitochondria against reactive oxygen species and excessive fragmentation, offering therapeutic approaches for pathophysiologic stress conditions.

CD73: an emerging checkpoint for cancer immunotherapy

CD73 is a novel immune checkpoint associated with adenosine metabolism that promotes tumor progression by suppressing antitumor immune response and promoting angiogenesis. The inhibition of CD73, in combination with immune checkpoint blockade, targeted therapy or conventional therapy, improves antitumor effects in numerous preclinical mouse models of cancer. Emerging evidence suggests that the combination of anti-CD73 and immune checkpoint blockade has promising clinical activity in patients with advanced solid tumors. In this review, we will discuss the specific role of CD73 on both tumor cells and nontumor cells in regulating tumor immunity and tumorigenesis and provide an update on the current view of the antitumor activity of targeting CD73 by mAb or small molecule selective inhibitors in preclinical and clinical settings.

NAD-Biosynthetic and Consuming Enzymes as Central Players of Metabolic Regulation of Innate and Adaptive Immune Responses in Cancer

Cancer cells, particularly in solid tumors, are surrounded by non-neoplastic elements, including endothelial and stromal cells, as well as cells of immune origin, which can support tumor growth by providing the right conditions. On the other hand, local hypoxia, and lack of nutrients induce tumor cells to reprogram their metabolism in order to survive, proliferate, and disseminate: the same conditions are also responsible for building a tumor-suppressive microenvironment. In addition to tumor cells, it is now well-recognized that metabolic rewiring occurs in all cellular components of the tumor microenvironment, affecting epigenetic regulation of gene expression and influencing differentiation/proliferation decisions of these cells. Nicotinamide adenine dinucleotide (NAD) is an essential co-factor for energy transduction in metabolic processes. It is also a key component of signaling pathways, through the regulation of NAD-consuming enzymes, including sirtuins and PARPs, which can affect DNA plasticity and accessibility. In addition, both NAD-biosynthetic and NAD-consuming enzymes can be present in the extracellular environment, adding a new layer of complexity to the system. In this review we will discuss the role of the “NADome” in the metabolic cross-talk between cancer and infiltrating immune cells, contributing to cancer growth and immune evasion, with an eye to therapeutic implications.

Safety and Metabolism of Long-term Administration of NIAGEN (Nicotinamide Riboside Chloride) in a Randomized, Double-Blind, Placebo-controlled Clinical Trial of Healthy Overweight Adults

Nicotinamide riboside (NR) is a newly discovered nicotinamide adenine dinucleotide (NAD+) precursor vitamin. A crystal form of NR chloride termed NIAGEN is generally recognized as safe (GRAS) for use in foods and the subject of two New Dietary Ingredient Notifications for use in dietary supplements. To evaluate the kinetics and dose-dependency of NR oral availability and safety in overweight, but otherwise healthy men and women, an 8-week randomized, double-blind, placebo-controlled clinical trial was conducted. Consumption of 100, 300 and 1000 mg NR dose-dependently and significantly increased whole blood NAD+ (i.e., 22%, 51% and 142%) and other NAD+ metabolites within 2 weeks. The increases were maintained throughout the remainder of the study. There were no reports of flushing and no significant differences in adverse events between the NR and placebo-treated groups or between groups at different NR doses. NR also did not elevate low density lipoprotein cholesterol or dysregulate 1-carbon metabolism. Together these data support the development of a tolerable upper intake limit for NR based on human data.

Differential modulation of SIRT6 deacetylase and deacylase activities by lysine-based small molecules

Sirtuin 6 (SIRT6) is an NAD⁺-dependent deacetylase regulating important functions: modulators of its enzymatic activity have been considered as possible therapeutic agents. Besides the deacetylase activity, SIRT6 also has NAD⁺-dependent deacylase activity, whereby it regulates the secretion of cytokines and proteins. We identified novel SIRT6 modulators with a lysine-based structure: compound 1 enhances SIRT6 deacylase while inhibiting the deacetylase activity. As expected based on the biological effects of SIRT6 deacetylase activity, compound 1 increased histone 3 lysine 9 acetylation and the activity of glycolytic enzymes. Moreover, the fact that compound 1 enhanced SIRT6 deacylase activity was accompanied by an increased TNF-α release. In conclusion, new SIRT6 modulators with a lysine-like structure were identified, with differential effects on specific SIRT6 activities. The novel SIRT6 modulator concomitantly inhibits deacetylase and enhances deacylase activity.

The Multi-faceted Ecto-enzyme CD38: Roles in Immunomodulation, Cancer, Aging, and Metabolic Diseases

CD38 (Cluster of Differentiation 38) is a multifunctional ecto-enzyme that metabolizes NAD+ and mediates nicotinamide dinucleotide (NAD+) and extracellular nucleotide homeostasis as well as intracellular calcium. CD38 is also an emerging therapeutic target under conditions in which metabolism is altered including infection, aging, and tumorigenesis. We describe multiple enzymatic activities of CD38, which may explain the breadth of biological roles observed for this enzyme. Of greatest significance is the role of CD38 as an ecto-enzyme capable of modulating extracellular NAD+ precursor availability: 1 to bacteria unable to perform de novo synthesis of NAD+; and 2 in aged parenchyma impacted by the accumulation of immune cells during the process of ‘inflammaging’. We also discuss the paradoxical role of CD38 as a modulator of intracellular NAD+, particularly in tumor immunity. Finally, we provide a summary of therapeutic approaches to CD38 inhibition and ‘NAD+ boosting’ for treatment of metabolic dysfunction observed during aging and in tumor immunity. The present review summarizes the role of CD38 in nicotinamide nucleotide homeostasis with special emphasis on the role of CD38 as an immunomodulator and druggable target.

Regulation of Cancer and Cancer-Related Genes via NAD. Antioxid Redox Signal 2019;30:906-923. [PMID: 29334761 DOI: 10.1089/ars.2017.7478]

SIGNIFICANCE

NAD⁺ is an essential redox cofactor in cellular metabolism and has emerged as an important regulator of a wide spectrum of disease conditions, most notably, cancers. As such, various strategies targeting NAD⁺ synthesis in cancers are in clinical trials. Recent Advances: Being a substrate required for the activity of various enzyme families, especially sirtuins and poly(adenosine diphosphate [ADP]-ribose) polymerases, NAD⁺-mediated signaling plays an important role in gene expression, calcium release, cell cycle progression, DNA repair, and cell proliferation. Many strategies exploring the potential of interfering with NAD⁺ metabolism to sensitize cancer cells to achieve anticancer benefits are highly promising, and are being pursued.

CRITICAL ISSUES

With the multifaceted roles of NAD⁺ in cancer, it is important to understand how cellular processes are reliant on NAD⁺. This review summarizes how NAD⁺ metabolism regulates various pathophysiological processes in cancer, and how this knowledge can be exploited to devise effective anticancer therapies in clinical settings.

FUTURE DIRECTIONS

In line with the redundant pathways that facilitate NAD⁺ metabolism, further studies should comprehensively understand the roles of the various NAD⁺-synthesizing as well as NAD⁺-utilizing biomolecules to understand its true potential in cancer treatment.

Multi-targeted Effect of Nicotinamide Mononucleotide on Brain Bioenergetic Metabolism

Dysfunctions in NAD⁺ metabolism are associated with neurodegenerative diseases, acute brain injury, diabetes, and aging. Loss of NAD⁺ levels results in impairment of mitochondria function, which leads to failure of essential metabolic processes. Strategies to replenish depleted NAD⁺ pools can offer significant improvements of pathologic states. NAD⁺ levels are maintained by two opposing enzymatic reactions, one is the consumption of NAD⁺ while the other is the re-synthesis of NAD⁺. Inhibition of NAD⁺ degrading enzymes, poly-ADP-ribose polymerase 1 (PARP1) and ectoenzyme CD38, following brain ischemic insult can provide neuroprotection. Preservation of NAD⁺ pools by administration of NAD⁺ precursors, such as nicotinamide (Nam) or nicotinamide mononucleotide (NMN), also offers neuroprotection. However, NMN treatment demonstrates to be a promising candidate as a therapeutic approach due to its multi-targeted effect acting as PARP1 and CD38 inhibitor, sirtuins activator, mitochondrial fission inhibitor, and NAD⁺ supplement. Many neurodegenerative diseases or acute brain injury activate several cellular death pathways requiring a treatment strategy that will target these mechanisms. Since NMN demonstrated the ability to exert its effect on several cellular metabolic pathways involved in brain pathophysiology it seems to be one of the most promising candidates to be used for successful neuroprotection.

NAD Metabolome Analysis in Human Cells Using ¹H NMR Spectroscopy

Nicotinamide adenine dinucleotide (NAD) and its phosphorylated form, NADP, are the major coenzymes of redox reactions in central metabolic pathways. Nicotinamide adenine dinucleotide is also used to generate second messengers, such as cyclic ADP-ribose, and serves as substrate for protein modifications including ADP-ribosylation and protein deacetylation by sirtuins. The regulation of these metabolic and signaling processes depends on NAD availability. Generally, human cells accomplish their NAD supply through biosynthesis using different forms of vitamin B3: Nicotinamide (Nam) and nicotinic acid as well as nicotinamide riboside (NR) and nicotinic acid riboside (NAR). These precursors are converted to the corresponding mononucleotides NMN and NAMN, which are adenylylated to the dinucleotides NAD and NAAD, respectively. Here, we have developed an NMR-based experimental approach to detect and quantify NAD(P) and its biosynthetic intermediates in human cell extracts. Using this method, we have determined NAD, NADP, NMN and Nam pools in HEK293 cells cultivated in standard culture medium containing Nam as the only NAD precursor. When cells were grown in the additional presence of both NAR and NR, intracellular pools of deamidated NAD intermediates (NAR, NAMN and NAAD) were also detectable. We have also tested this method to quantify NAD+ in human platelets and erythrocytes. Our results demonstrate that ¹H NMR spectroscopy provides a powerful method for the assessment of the cellular NAD metabolome.

SIRT6 deacetylase activity regulates NAMPT activity and NAD(P)(H) pools in cancer cells

Nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme in the NAD⁺ salvage pathway from nicotinamide. By controlling the biosynthesis of NAD⁺, NAMPT regulates the activity of NAD⁺-converting enzymes, such as CD38, poly-ADP-ribose polymerases, and sirtuins (SIRTs). SIRT6 is involved in the regulation of a wide number of metabolic processes. In this study, we investigated the ability of SIRT6 to regulate intracellular NAMPT activity and NAD(P)(H) levels. BxPC-3 cells and MCF-7 cells were engineered to overexpress a catalytically active or a catalytically inactive SIRT6 form or were engineered to silence endogenous SIRT6 expression. In SIRT6-overexpressing cells, NAD(H) levels were up-regulated, as a consequence of NAMPT activation. By immunopurification and incubation with recombinant SIRT6, NAMPT was found to be a direct substrate of SIRT6 deacetylation, with a mechanism that up-regulates NAMPT enzymatic activity. Extracellular NAMPT release was enhanced in SIRT6-silenced cells. Also glucose-6-phosphate dehydrogenase activity and NADPH levels were increased in SIRT6-overexpressing cells. Accordingly, increased SIRT6 levels reduced cancer cell susceptibility to H₂O₂-induced oxidative stress and to doxorubicin. Our data demonstrate that SIRT6 affects intracellular NAMPT activity, boosts NAD(P)(H) levels, and protects against oxidative stress. The use of SIRT6 inhibitors, together with agents inducing oxidative stress, may represent a promising treatment strategy in cancer.-Sociali, G., Grozio, A., Caffa, I., Schuster, S., Becherini, P., Damonte, P., Sturla, L., Fresia, C., Passalacqua, M., Mazzola, F., Raffaelli, N., Garten, A., Kiess, W., Cea, M., Nencioni, A., Bruzzone, S. SIRT6 deacetylase activity regulates NAMPT activity and NAD(P)(H) pools in cancer cells.

The Danger Signal Extracellular ATP Is Involved in the Immunomediated Damage of α-Sarcoglycan-Deficient Muscular Dystrophy

In muscular dystrophies, muscle membrane fragility results in a tissue-specific increase of danger-associated molecular pattern molecules (DAMPs) and infiltration of inflammatory cells. The DAMP extracellular ATP (eATP) released by dying myofibers steadily activates muscle and immune purinergic receptors exerting dual negative effects: a direct damage linked to altered intracellular calcium homeostasis in muscle cells and an indirect toxicity through the triggering of the immune response and inhibition of regulatory T cells. Accordingly, pharmacologic and genetic inhibition of eATP signaling improves the phenotype in models of chronic inflammatory diseases. In α-sarcoglycanopathy, eATP effects may be further amplified because α-sarcoglycan extracellular domain binds eATP and displays an ecto-ATPase activity, thus controlling eATP concentration at the cell surface and attenuating the magnitude and/or the duration of eATP-induced signals. Herein, we show that in vivo blockade of the eATP/P2X purinergic pathway by a broad-spectrum P2X receptor-antagonist delayed the progression of the dystrophic phenotype in α-sarcoglycan-null mice. eATP blockade dampened the muscular inflammatory response and enhanced the recruitment of forkhead box protein P3-positive immunosuppressive regulatory CD4⁺ T cells. The improvement of the inflammatory features was associated with increased strength, reduced necrosis, and limited expression of profibrotic factors, suggesting that pharmacologic purinergic antagonism, altering the innate and adaptive immune component in muscle infiltrates, might provide a therapeutic approach to slow disease progression in α-sarcoglycanopathy.

Functional insights into nucleotide-metabolizing ectoenzymes expressed by bone marrow-resident cells in patients with multiple myeloma

Human myeloma cells grow in a hypoxic acidic niche in the bone marrow. Cross talk among cellular components of this closed niche generates extracellular adenosine, which promotes tumor cell survival. This is achieved through the binding of adenosine to purinergic receptors into complexes that function as an autocrine/paracrine signal factor with immune regulatory activities that i) down-regulate the functions of most immune effector cells and ii) enhance the activity of cells that suppress anti-tumor immune responses, thus facilitating the escape of malignant myeloma cells from immune surveillance. Here we review recent findings confirming that the dominant phenotype for survival of tumor cells is that where the malignant cells have been metabolically reprogrammed for the generation of lactic acidosis in the bone marrow niche. Adenosine triphosphate and nicotinamide-adenine dinucleotide extruded from tumor cells, along with cyclic adenosine monophosphate, are the main intracellular energetic/messenger molecules that serve as leading substrates in the extracellular space for membrane-bound ectonucleotidases metabolizing purine nucleotides to signaling adenosine. Within this mechanistic framework, the adenosinergic substrate conversion can vary significantly according to the metabolic environment. Indeed, the neoplastic expansion of plasma cells exploits both enzymatic networks and hypoxic acidic conditions for migrating and homing to a protected niche and for evading the immune response. The expression of multiple specific adenosine receptors in the niche completes the profile of a complex regulatory framework whose signals modify multiple myeloma and host immune responses.

Overexpression of NRK1 ameliorates diet- and age-induced hepatic steatosis and insulin resistance

NAD⁺ is a co-enzyme in redox reactions and a substrate required for activity of various enzyme families, including sirtuins and poly(ADP-ribose) polymerases. Dietary supplementation of NAD⁺ precursors nicotinamide mononucleotide (NMN) or nicotinamide riboside (NR) protects against metabolic disease, neurodegenerative disorders and age-related physiological decline in mammals. Here we sought to identify the roles of nicotinamide riboside kinase 1 (NRK1) plays in regulating hepatic NAD⁺ biosynthesis and lipid metabolism. Using adenovirus mediated gene transduction to overexpress or knockdown NRK1 in mouse liver, we have demonstrated that NRK1 is critical for maintaining hepatic NAD⁺ levels and triglyceride content. We have further shown that the hepatic expression of Nmrk1 mRNA is significantly decreased either in mice treated with high-fat diet or in aged mice. However, adenoviral delivery of NRK1 in these diet- and age-induced mice elevates hepatic NAD⁺ levels, reduces hepatic steatosis, and improves glucose tolerance and insulin sensitivity. Our results provide important insights in targeting NRK1 for treating hepatic steatosis.

Nicotinamide Adenine Dinucleotide Metabolism and Neurodegeneration

SIGNIFICANCE

Nicotinamide adenine dinucleotide (NAD+) participates in redox reactions and NAD+-dependent signaling processes, which involve the cleavage of NAD+ coupled to posttranslational modifications of proteins or the production of second messengers. Either as a primary cause or as a secondary component of the pathogenic process, mitochondrial dysfunction and oxidative stress are prominent features of several neurodegenerative diseases. Activation of NAD+-dependent signaling pathways has a major effect in the capacity of the cell to modulate mitochondrial function and counteract the deleterious effects of increased oxidative stress. Recent Advances: Progress in the understanding of the biological functions and compartmentalization of NAD+-synthesizing and NAD+-consuming enzymes have led to the emergence of NAD+ metabolism as a major therapeutic target for age-related diseases.

CRITICAL ISSUES

Three distinct families of enzymes consume NAD+ as substrate: poly(ADP-ribose) polymerases (PARPs), ADP-ribosyl cyclases (CD38/CD157) and sirtuins. Two main strategies to increase NAD+ availability have arisen. These strategies are based on the utilization of NAD+ intermediates/precursors or the inhibition of the NAD+-consuming enzymes, PARPs and CD38. An increase in endogenous sirtuin activity seems to mediate the protective effect that enhancing NAD+ availability confers in several models of neurodegeneration and age-related diseases.

FUTURE DIRECTIONS

A growing body of evidence suggests the beneficial role of enhancing NAD+ availability in models of neurodegeneration. The challenge ahead is to establish the value and safety of the long-term use of these strategies for the treatment of neurodegenerative diseases. Antioxid. Redox Signal. 28, 1652-1668.

A Potent and Specific CD38 Inhibitor Ameliorates Age-Related Metabolic Dysfunction by Reversing Tissue NAD+ Decline

Aging is characterized by the development of metabolic dysfunction and frailty. Recent studies show that a reduction in nicotinamide adenine dinucleotide (NAD⁺) is a key factor for the development of age-associated metabolic decline. We recently demonstrated that the NADase CD38 has a central role in age-related NAD⁺ decline. Here we show that a highly potent and specific thiazoloquin(az)olin(on)e CD38 inhibitor, 78c, reverses age-related NAD⁺ decline and improves several physiological and metabolic parameters of aging, including glucose tolerance, muscle function, exercise capacity, and cardiac function in mouse models of natural and accelerated aging. The physiological effects of 78c depend on tissue NAD⁺ levels and were reversed by inhibition of NAD⁺ synthesis. 78c increased NAD⁺ levels, resulting in activation of pro-longevity and health span-related factors, including sirtuins, AMPK, and PARPs. Furthermore, in animals treated with 78c we observed inhibition of pathways that negatively affect health span, such as mTOR-S6K and ERK, and attenuation of telomere-associated DNA damage, a marker of cellular aging. Together, our results detail a novel pharmacological strategy for prevention and/or reversal of age-related NAD⁺ decline and subsequent metabolic dysfunction.

Therapeutic Potential of NAD-Boosting Molecules: The In Vivo Evidence

Nicotinamide adenine dinucleotide (NAD), the cell's hydrogen carrier for redox enzymes, is well known for its role in redox reactions. More recently, it has emerged as a signaling molecule. By modulating NAD⁺-sensing enzymes, NAD⁺ controls hundreds of key processes from energy metabolism to cell survival, rising and falling depending on food intake, exercise, and the time of day. NAD⁺ levels steadily decline with age, resulting in altered metabolism and increased disease susceptibility. Restoration of NAD⁺ levels in old or diseased animals can promote health and extend lifespan, prompting a search for safe and efficacious NAD-boosting molecules that hold the promise of increasing the body's resilience, not just to one disease, but to many, thereby extending healthy human lifespan.

NAD+ Intermediates: The Biology and Therapeutic Potential of NMN and NR

Research on the biology of NAD+ has been gaining momentum, providing many critical insights into the pathogenesis of age-associated functional decline and diseases. In particular, two key NAD+ intermediates, nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN), have been extensively studied over the past several years. Supplementing these NAD+ intermediates has shown preventive and therapeutic effects, ameliorating age-associated pathophysiologies and disease conditions. Although the pharmacokinetics and metabolic fates of NMN and NR are still under intensive investigation, these NAD+ intermediates can exhibit distinct behavior, and their fates appear to depend on the tissue distribution and expression levels of NAD+ biosynthetic enzymes, nucleotidases, and presumptive transporters for each. A comprehensive concept that connects NAD+ metabolism to the control of aging and longevity in mammals has been proposed, and the stage is now set to test whether these exciting preclinical results can be translated to improve human health.

The Pharmacology of CD38/NADase: An Emerging Target in Cancer and Diseases of Aging

Recent reports indicate that intracellular NAD levels decline in tissues during chronological aging, and that therapies aimed at increasing cellular NAD levels could have beneficial effects in many age-related diseases. The protein CD38 (cluster of differentiation 38) is a multifunctional enzyme that degrades NAD and modulates cellular NAD homeostasis. At the physiological level, CD38 has been implicated in the regulation of metabolism and in the pathogenesis of multiple conditions including aging, obesity, diabetes, heart disease, asthma, and inflammation. Interestingly, many of these functions are mediated by CD38 enzymatic activity. In addition, CD38 has also been identified as a cell-surface marker in hematologic cancers such as multiple myeloma, and a cytotoxic anti-CD38 antibody has been approved by the FDA for use in this disease. Although this is a remarkable development, killing CD38-positive tumor cells with cytotoxic anti-CD38 antibodies is only one of the potential pharmacological uses of targeting CD38. The present review discusses the biology of the CD38 enzyme and the current state of development of pharmacological tools aimed at CD38, and explores how these agents may represent a novel approach for treating human conditions including cancer, metabolic disease, and diseases of aging.

NAD + biosynthesis, aging, and disease

Nicotinamide adenine dinucleotide (NAD ⁺) biosynthesis and its regulation have recently been attracting markedly increasing interest. Aging is marked by a systemic decrease in NAD ⁺ across multiple tissues. The dysfunction of NAD ⁺ biosynthesis plays a critical role in the pathophysiologies of multiple diseases, including age-associated metabolic disorders, neurodegenerative diseases, and mental disorders. As downstream effectors, NAD ⁺-dependent enzymes, such as sirtuins, are involved in the progression of such disorders. These recent studies implicate NAD ⁺ biosynthesis as a potential target for preventing and treating age-associated diseases. Indeed, new studies have demonstrated the therapeutic potential of supplementing NAD ⁺ intermediates, such as nicotinamide mononucleotide and nicotinamide riboside, providing a proof of concept for the development of an effective anti-aging intervention.

NAD(H) and NADP(H) Redox Couples and Cellular Energy Metabolism

SIGNIFICANCE

The nicotinamide adenine dinucleotide (NAD⁺)/reduced NAD⁺ (NADH) and NADP⁺/reduced NADP⁺ (NADPH) redox couples are essential for maintaining cellular redox homeostasis and for modulating numerous biological events, including cellular metabolism. Deficiency or imbalance of these two redox couples has been associated with many pathological disorders. Recent Advances: Newly identified biosynthetic enzymes and newly developed genetically encoded biosensors enable us to understand better how cells maintain compartmentalized NAD(H) and NADP(H) pools. The concept of redox stress (oxidative and reductive stress) reflected by changes in NAD(H)/NADP(H) has increasingly gained attention. The emerging roles of NAD⁺-consuming proteins in regulating cellular redox and metabolic homeostasis are active research topics.

CRITICAL ISSUES

The biosynthesis and distribution of cellular NAD(H) and NADP(H) are highly compartmentalized. It is critical to understand how cells maintain the steady levels of these redox couple pools to ensure their normal functions and simultaneously avoid inducing redox stress. In addition, it is essential to understand how NAD(H)- and NADP(H)-utilizing enzymes interact with other signaling pathways, such as those regulated by hypoxia-inducible factor, to maintain cellular redox homeostasis and energy metabolism.

FUTURE DIRECTIONS

Additional studies are needed to investigate the inter-relationships among compartmentalized NAD(H)/NADP(H) pools and how these two dinucleotide redox couples collaboratively regulate cellular redox states and cellular metabolism under normal and pathological conditions. Furthermore, recent studies suggest the utility of using pharmacological interventions or nutrient-based bioactive NAD⁺ precursors as therapeutic interventions for metabolic diseases. Thus, a better understanding of the cellular functions of NAD(H) and NADP(H) may facilitate efforts to address a host of pathological disorders effectively. Antioxid. Redox Signal. 28, 251-272.

Emerging potential benefits of modulating NAD+ metabolism in cardiovascular disease

Nicotinamide adenine dinucleotide (NAD⁺) and related metabolites are central mediators of fuel oxidation and bioenergetics within cardiomyocytes. Additionally, NAD⁺ is required for the activity of multifunctional enzymes, including sirtuins and poly(ADP-ribose) polymerases that regulate posttranslational modifications, DNA damage responses, and Ca²⁺ signaling. Recent research has indicated that NAD⁺ participates in a multitude of processes dysregulated in cardiovascular diseases. Therefore, supplementation of NAD⁺ precursors, including nicotinamide riboside that boosts or repletes the NAD⁺ metabolome, may be cardioprotective. This review examines the molecular physiology and preclinical data with respect to NAD⁺ precursors in heart failure-related cardiac remodeling, ischemic-reperfusion injury, and arrhythmias. In addition, alternative NAD⁺-boosting strategies and potential systemic effects of NAD⁺ supplementation with implications on cardiovascular health and disease are surveyed.

Nicotinamide Riboside Preserves Cardiac Function in a Mouse Model of Dilated Cardiomyopathy

BACKGROUND

Myocardial metabolic impairment is a major feature in chronic heart failure. As the major coenzyme in fuel oxidation and oxidative phosphorylation and a substrate for enzymes signaling energy stress and oxidative stress response, nicotinamide adenine dinucleotide (NAD⁺) is emerging as a metabolic target in a number of diseases including heart failure. Little is known on the mechanisms regulating homeostasis of NAD⁺ in the failing heart.

METHODS

To explore possible alterations of NAD⁺ homeostasis in the failing heart, we quantified the expression of NAD⁺ biosynthetic enzymes in the human failing heart and in the heart of a mouse model of dilated cardiomyopathy (DCM) triggered by Serum Response Factor transcription factor depletion in the heart (SRF^HKO) or of cardiac hypertrophy triggered by transverse aorta constriction. We studied the impact of NAD⁺ precursor supplementation on cardiac function in both mouse models.

RESULTS

We observed a 30% loss in levels of NAD⁺ in the murine failing heart of both DCM and transverse aorta constriction mice that was accompanied by a decrease in expression of the nicotinamide phosphoribosyltransferase enzyme that recycles the nicotinamide precursor, whereas the nicotinamide riboside kinase 2 (NMRK2) that phosphorylates the nicotinamide riboside precursor is increased, to a higher level in the DCM (40-fold) than in transverse aorta constriction (4-fold). This shift was also observed in human failing heart biopsies in comparison with nonfailing controls. We show that the Nmrk2 gene is an AMP-activated protein kinase and peroxisome proliferator-activated receptor α responsive gene that is activated by energy stress and NAD⁺ depletion in isolated rat cardiomyocytes. Nicotinamide riboside efficiently rescues NAD⁺ synthesis in response to FK866-mediated inhibition of nicotinamide phosphoribosyltransferase and stimulates glycolysis in cardiomyocytes. Accordingly, we show that nicotinamide riboside supplementation in food attenuates the development of heart failure in mice, more robustly in DCM, and partially after transverse aorta constriction, by stabilizing myocardial NAD⁺ levels in the failing heart. Nicotinamide riboside treatment also robustly increases the myocardial levels of 3 metabolites, nicotinic acid adenine dinucleotide, methylnicotinamide, and N1-methyl-4-pyridone-5-carboxamide, that can be used as validation biomarkers for the treatment.

CONCLUSIONS

The data show that nicotinamide riboside, the most energy-efficient among NAD precursors, could be useful for treatment of heart failure, notably in the context of DCM, a disease with few therapeutic options.

NAD and the aging process: Role in life, death and everything in between

Life as we know it cannot exist without the nucleotide nicotinamide adenine dinucleotide (NAD). From the simplest organism, such as bacteria, to the most complex multicellular organisms, NAD is a key cellular component. NAD is extremely abundant in most living cells and has traditionally been described to be a cofactor in electron transfer during oxidation-reduction reactions. In addition to participating in these reactions, NAD has also been shown to play a key role in cell signaling, regulating several pathways from intracellular calcium transients to the epigenetic status of chromatin. Thus, NAD is a molecule that provides an important link between signaling and metabolism, and serves as a key molecule in cellular metabolic sensoring pathways. Importantly, it has now been clearly demonstrated that cellular NAD levels decline during chronological aging. This decline appears to play a crucial role in the development of metabolic dysfunction and age-related diseases. In this review we will discuss the molecular mechanisms responsible for the decrease in NAD levels during aging. Since other reviews on this subject have been recently published, we will concentrate on presenting a critical appraisal of the current status of the literature and will highlight some controversial topics in the field. In particular, we will discuss the potential role of the NADase CD38 as a driver of age-related NAD decline.

Nicotinamide riboside, a form of vitamin B3, protects against excitotoxicity-induced axonal degeneration

NAD⁺ depletion is a common phenomenon in neurodegenerative pathologies. Excitotoxicity occurs in multiple neurologic disorders and NAD⁺ was shown to prevent neuronal degeneration in this process through mechanisms that remained to be determined. The activity of nicotinamide riboside (NR) in neuroprotective models and the recent description of extracellular conversion of NAD⁺ to NR prompted us to probe the effects of NAD⁺ and NR in protection against excitotoxicity. Here, we show that intracortical administration of NR but not NAD⁺ reduces brain damage induced by NMDA injection. Using cortical neurons, we found that provision of extracellular NR delays NMDA-induced axonal degeneration (AxD) much more strongly than extracellular NAD⁺ Moreover, the stronger effect of NR compared to NAD⁺ depends of axonal stress since in AxD induced by pharmacological inhibition of nicotinamide salvage, both NAD⁺ and NR prevent neuronal death and AxD in a manner that depends on internalization of NR. Taken together, our findings demonstrate that NR is a better neuroprotective agent than NAD⁺ in excitotoxicity-induced AxD and that axonal protection involves defending intracellular NAD⁺ homeostasis.-Vaur, P., Brugg, B., Mericskay, M., Li, Z., Schmidt, M. S., Vivien, D., Orset, C., Jacotot, E., Brenner, C., Duplus, E. Nicotinamide riboside, a form of vitamin B₃, protects against excitotoxicity-induced axonal degeneration.

NAD+ Deficits in Age-Related Diseases and Cancer

The phenomenon of aging has gained widespread attention in recent times. Although significant advances have been made to better understand aging and its related pathologies including cancer, there is not yet a clear mechanism explaining why diseases and cancer are inherent parts of the aging process. Finding a unifying equation that could bridge aging and its related diseases would allow therapeutic development and solve an immense human health problem to live longer and better. In this review, we discuss NAD⁺ reduction as the central mechanism that may connect aging to its related pathologies and cancer. NAD⁺ boosters would ensure and ameliorate health quality during aging.

Nicotinic Acid Phosphoribosyltransferase Regulates Cancer Cell Metabolism, Susceptibility to NAMPT Inhibitors, and DNA Repair

In the last decade, substantial efforts have been made to identify NAD⁺ biosynthesis inhibitors, specifically against nicotinamide phosphoribosyltransferase (NAMPT), as preclinical studies indicate their potential efficacy as cancer drugs. However, the clinical activity of NAMPT inhibitors has proven limited, suggesting that alternative NAD⁺ production routes exploited by tumors confer resistance. Here, we show the gene encoding nicotinic acid phosphoribosyltransferase (NAPRT), a second NAD⁺-producing enzyme, is amplified and overexpressed in a subset of common types of cancer, including ovarian cancer, where NAPRT expression correlates with a BRCAness gene expression signature. Both NAPRT and NAMPT increased intracellular NAD⁺ levels. NAPRT silencing reduced energy status, protein synthesis, and cell size in ovarian and pancreatic cancer cells. NAPRT silencing sensitized cells to NAMPT inhibitors both in vitro and in vivo; similar results were obtained with the NAPRT inhibitor 2-hydroxynicotinic acid. Reducing NAPRT levels in a BRCA2-deficient cancer cell line exacerbated DNA damage in response to chemotherapeutics. In conclusion, NAPRT-dependent NAD⁺ biosynthesis contributes to cell metabolism and to the DNA repair process in a subset of tumors. This knowledge could be used to increase the efficacy of NAMPT inhibitors and chemotherapy. Cancer Res; 77(14); 3857-69. ©2017 AACR.

Role of NAD+ and mitochondrial sirtuins in cardiac and renal diseases

The coenzyme nicotinamide adenine dinucleotide (NAD⁺) has key roles in the regulation of redox status and energy metabolism. NAD⁺ depletion is emerging as a major contributor to the pathogenesis of cardiac and renal diseases and NAD⁺ repletion strategies have shown therapeutic potential as a means to restore healthy metabolism and physiological function. The pleotropic roles of NAD⁺ enable several possible avenues by which repletion of this coenzyme could have therapeutic efficacy. In particular, NAD⁺ functions as a co-substrate in deacylation reactions carried out by the sirtuin family of enzymes. These NAD⁺-dependent deacylases control several aspects of metabolism and a wealth of data suggests that boosting sirtuin activity via NAD⁺ supplementation might be a promising therapy for cardiac and renal pathologies. This Review summarizes the role of NAD⁺ metabolism in the heart and kidney, and highlights the mitochondrial sirtuins as mediators of some of the beneficial effects of NAD⁺-boosting therapies in preclinical animal models. We surmise that modulating the NAD⁺-sirtuin axis is a clinically relevant approach to develop new therapies for cardiac and renal diseases.

Antitumor effect of combined NAMPT and CD73 inhibition in an ovarian cancer model

Nicotinamide phosphoribosyltransferase (NAMPT) is a crucial enzyme in the biosynthesis of intracellular NAD⁺. NAMPT inhibitors have potent anticancer activity in several preclinical models by depleting NAD⁺ and ATP levels. Recently, we demonstrated that CD73 enables the utilization of extracellular NAD⁺/nicotinamide mononucleotide (NMN) by converting them to Nicotinamide riboside (NR), which can cross the plasmamembrane and fuel intracellular NAD⁺ biosynthesis in human cells. These processes are herein confirmed to also occur in a human ovarian carcinoma cell line (OVCAR-3), by means of CD73 or NRK1 specific silencing. Next, we investigated the anti-tumor activity of the simultaneous inhibition of NAMPT (with FK866) and CD73 (with α, β-methylene adenosine 5′-diphosphate, APCP), in an in vivo human ovarian carcinoma model. Interestingly, the combined therapy was found to significantly decrease intratumor NAD⁺, NMN and ATP levels, compared with single treatments. In addition, the concentration of these nucleotides in ascitic exudates was more remarkably reduced in animals treated with both FK866 and APCP compared with single treatments. Importantly, tumors treated with FK866 in combination with APCP contained a statistically significant lower proportion of Ki67 positive proliferating cells and a higher percentage of necrotic area. Finally, a slight but significant increase in animal survival in response to the combined therapy, compared to the single agents, could be demonstrated. Our results indicate that the pharmacological inhibition of CD73 enzymatic activity could be considered as a means to potentiate the anti-cancer effects of NAMPT inhibitors.