Evaluating the role of niacin in human carcinogenesis

The Central Role of the NAD+ Molecule in the Development of Aging and the Prevention of Chronic Age-Related Diseases: Strategies for NAD+ Modulation

The molecule NAD+ is a coenzyme for enzymes catalyzing cellular redox reactions in several metabolic pathways, encompassing glycolysis, TCA cycle, and oxidative phosphorylation, and is a substrate for NAD+-dependent enzymes. In addition to a hydride and electron transfer in redox reactions, NAD+ is a substrate for sirtuins and poly(adenosine diphosphate-ribose) polymerases and even moderate decreases in its cellular concentrations modify signaling of NAD+-consuming enzymes. Age-related reduction in cellular NAD+ concentrations results in metabolic and aging-associated disorders, while the consequences of increased NAD+ production or decreased degradation seem beneficial. This article reviews the NAD+ molecule in the development of aging and the prevention of chronic age-related diseases and discusses the strategies of NAD+ modulation for healthy aging and longevity.

NAD precursors cycle between host tissues and the gut microbiome

Nicotinamide adenine dinucleotide (NAD) is an essential redox cofactor in mammals and microbes. Here we use isotope tracing to investigate the precursors supporting NAD synthesis in the gut microbiome of mice. We find that dietary NAD precursors are absorbed in the proximal part of the gastrointestinal tract and not available to microbes in the distal gut. Instead, circulating host nicotinamide enters the gut lumen and supports microbial NAD synthesis. The microbiome converts host-derived nicotinamide into nicotinic acid, which is used for NAD synthesis in host tissues and maintains circulating nicotinic acid levels even in the absence of dietary consumption. Moreover, the main route from oral nicotinamide riboside, a widely used nutraceutical, to host NAD is via conversion into nicotinic acid by the gut microbiome. Thus, we establish the capacity for circulating host micronutrients to feed the gut microbiome, and in turn be transformed in a manner that enhances host metabolic flexibility.

Balancing NAD+ deficits with nicotinamide riboside: therapeutic possibilities and limitations

Alterations in cellular nicotinamide adenine dinucleotide (NAD+) levels have been observed in multiple lifestyle and age-related medical conditions. This has led to the hypothesis that dietary supplementation with NAD+ precursors, or vitamin B3s, could exert health benefits. Among the different molecules that can act as NAD+ precursors, Nicotinamide Riboside (NR) has gained most attention due to its success in alleviating and treating disease conditions at the pre-clinical level. However, the clinical outcomes for NR supplementation strategies have not yet met the expectations generated in mouse models. In this review we aim to provide a comprehensive view on NAD+ biology, what causes NAD+ deficits and the journey of NR from its discovery to its clinical development. We also discuss what are the current limitations in NR-based therapies and potential ways to overcome them. Overall, this review will not only provide tools to understand NAD+ biology and assess its changes in disease situations, but also to decide which NAD+ precursor could have the best therapeutic potential.

NAD+ Precursors: A Questionable Redundancy

The last decade has seen a strong proliferation of therapeutic strategies for the treatment of metabolic and age-related diseases based on increasing cellular NAD⁺ bioavailability. Among them, the dietary supplementation with NAD⁺ precursors—classically known as vitamin B3—has received most of the attention. Multiple molecules can act as NAD⁺ precursors through independent biosynthetic routes. Interestingly, eukaryote organisms have conserved a remarkable ability to utilize all of these different molecules, even if some of them are scarcely found in nature. Here, we discuss the possibility that the conservation of all of these biosynthetic pathways through evolution occurred because the different NAD⁺ precursors might serve specialized purposes.

Enzymology of extracellular NAD metabolism

Extracellular NAD represents a key signaling molecule in different physiological and pathological conditions. It exerts such function both directly, through the activation of specific purinergic receptors, or indirectly, serving as substrate of ectoenzymes, such as CD73, nucleotide pyrophosphatase/phosphodiesterase 1, CD38 and its paralog CD157, and ecto ADP ribosyltransferases. By hydrolyzing NAD, these enzymes dictate extracellular NAD availability, thus regulating its direct signaling role. In addition, they can generate from NAD smaller signaling molecules, like the immunomodulator adenosine, or they can use NAD to ADP-ribosylate various extracellular proteins and membrane receptors, with significant impact on the control of immunity, inflammatory response, tumorigenesis, and other diseases. Besides, they release from NAD several pyridine metabolites that can be taken up by the cell for the intracellular regeneration of NAD itself. The extracellular environment also hosts nicotinamide phosphoribosyltransferase and nicotinic acid phosphoribosyltransferase, which inside the cell catalyze key reactions in NAD salvaging pathways. The extracellular forms of these enzymes behave as cytokines, with pro-inflammatory functions. This review summarizes the current knowledge on the extracellular NAD metabolome and describes the major biochemical properties of the enzymes involved in extracellular NAD metabolism, focusing on the contribution of their catalytic activities to the biological function. By uncovering the controversies and gaps in their characterization, further research directions are suggested, also to better exploit the great potential of these enzymes as therapeutic targets in various human diseases.

NAD+ homeostasis in health and disease

The conceptual evolution of nicotinamide adenine dinucleotide (NAD⁺) from being seen as a simple metabolic cofactor to a pivotal cosubstrate for proteins regulating metabolism and longevity, including the sirtuin family of protein deacylases, has led to a new wave of scientific interest in NAD⁺. NAD⁺ levels decline during ageing, and alterations in NAD⁺ homeostasis can be found in virtually all age-related diseases, including neurodegeneration, diabetes and cancer. In preclinical settings, various strategies to increase NAD⁺ levels have shown beneficial effects, thus starting a competitive race to discover marketable NAD⁺ boosters to improve healthspan and lifespan. Here, we review the basics of NAD⁺ biochemistry and metabolism, and its roles in health and disease, and we discuss current challenges and the future translational potential of NAD⁺ research.

The NAD+-Dependent Family of Sirtuins in Cerebral Ischemia and Preconditioning

SIGNIFICANCE

Sirtuins are an evolutionarily conserved family of NAD+-dependent lysine deacylases and ADP ribosylases. Their requirement for NAD+ as a cosubstrate allows them to act as metabolic sensors that couple changes in the energy status of the cell to changes in cellular physiological processes. NAD+ levels are affected by several NAD+-producing and NAD+-consuming pathways as well as by cellular respiration. Thus their intracellular levels are highly dynamic and are misregulated in a spectrum of metabolic disorders including cerebral ischemia. This, in turn, compromises several NAD+-dependent processes that may ultimately lead to cell death. Recent Advances: A number of efforts have been made to replenish NAD+ in cerebral ischemic injuries as well as to understand the functions of one its important mediators, the sirtuin family of proteins through the use of pharmacological modulators or genetic manipulation approaches either before or after the insult. Critical Issues and Future Directions: The results of these studies have regarded the sirtuins as promising therapeutic targets for cerebral ischemia. Yet, additional efforts are needed to understand the role of some of the less characterized members and to address the sex-specific effects observed with some members. Sirtuins also exhibit cell-type-specific expression in the brain as well as distinct subcellular and regional localizations. As such, they are involved in diverse and sometimes opposing cellular processes that can either promote neuroprotection or further contribute to the injury; which also stresses the need for the development and use of sirtuin-specific pharmacological modulators. Antioxid. Redox Signal. 28, 691-710.

Modulating NAD+ metabolism, from bench to bedside

Discovered in the beginning of the 20^th century, nicotinamide adenine dinucleotide (NAD⁺) has evolved from a simple oxidoreductase cofactor to being an essential cosubstrate for a wide range of regulatory proteins that include the sirtuin family of NAD⁺-dependent protein deacylases, widely recognized regulators of metabolic function and longevity. Altered NAD⁺ metabolism is associated with aging and many pathological conditions, such as metabolic diseases and disorders of the muscular and neuronal systems. Conversely, increased NAD⁺ levels have shown to be beneficial in a broad spectrum of diseases. Here, we review the fundamental aspects of NAD⁺ biochemistry and metabolism and discuss how boosting NAD⁺ content can help ameliorate mitochondrial homeostasis and as such improve healthspan and lifespan.

It takes two to tango: NAD+ and sirtuins in aging/longevity control

The coupling of nicotinamide adenine dinucleotide (NAD⁺) breakdown and protein deacylation is a unique feature of the family of proteins called ‘sirtuins.’ This intimate connection between NAD⁺ and sirtuins has an ancient origin and provides a mechanistic foundation that translates the regulation of energy metabolism into aging and longevity control in diverse organisms. Although the field of sirtuin research went through intensive controversies, an increasing number of recent studies have put those controversies to rest and fully established the significance of sirtuins as an evolutionarily conserved aging/longevity regulator. The tight connection between NAD⁺ and sirtuins is regulated at several different levels, adding further complexity to their coordination in metabolic and aging/longevity control. Interestingly, it has been demonstrated that NAD⁺ availability decreases over age, reducing sirtuin activities and affecting the communication between the nucleus and mitochondria at a cellular level and also between the hypothalamus and adipose tissue at a systemic level. These dynamic cellular and systemic processes likely contribute to the development of age-associated functional decline and the pathogenesis of diseases of aging. To mitigate these age-associated problems, supplementation of key NAD⁺ intermediates is currently drawing significant attention. In this review article, we will summarize these important aspects of the intimate connection between NAD⁺ and sirtuins in aging/longevity control.

Randomized, double-blinded, vehicle-controlled, split-face study to evaluate the effects of topical application of a Gold Silk Sericin/Niacinamide/Signaline complex on biophysical parameters related to skin ageing

OBJECTIVE

To investigate the effects of topical application of a Gold Silk Sericin (GSS) complex on biophysical parameters related to skin ageing.

METHODS

A range of non-invasive bioengineering methods were deployed in an 8-week randomized, double-blinded, vehicle-controlled, split-face study among 40 female subjects aged 40-70. Endpoints measured included expert grades of skin condition, stratum corneum (SC) hydration, SC barrier function, elasticity and surface topography.

RESULTS

The GSS complex produced significant single-variable (P < 0.05) improvements in SC hydration, barrier function, elasticity and surface topography compared with the Vehicle control.

CONCLUSION

The GSS complex examined in this study represents an interesting new cosmetic topical technology with which to address multiple aspects of aged/photoaged female facial skin.

NAD(+) Metabolism and the Control of Energy Homeostasis: A Balancing Act between Mitochondria and the Nucleus

NAD(+) has emerged as a vital cofactor that can rewire metabolism, activate sirtuins, and maintain mitochondrial fitness through mechanisms such as the mitochondrial unfolded protein response. This improved understanding of NAD(+) metabolism revived interest in NAD(+)-boosting strategies to manage a wide spectrum of diseases, ranging from diabetes to cancer. In this review, we summarize how NAD(+) metabolism links energy status with adaptive cellular and organismal responses and how this knowledge can be therapeutically exploited.

Nicotinamide overcomes pluripotency deficits and reprogramming barriers

Crosstalk between intracellular signaling pathways has been extensively studied to understand the pluripotency of human pluripotent stem cells (hPSCs), including human embryonic stem cells and human induced pluripotent stem cells (hiPSCs); however, the contribution of NAD(+) -dependent pathways remains largely unknown. Here, we show that NAD(+) depletion by FK866 (a potent inhibitor of NAD(+) biosynthesis) was fatal in hPSCs, particularly when deriving pluripotent cells from somatic cells and maintaining pluripotency. NAD and its precursors (nicotinamide [NAM] and nicotinic acid) fully replenished the NAD(+) depletion by FK866 in hPSCs. However, only NAM effectively enhanced the reprogramming efficiency and kinetics of hiPSC generation and was also significantly advantageous for the maintenance of undifferentiated hPSCs. Our molecular and functional studies reveal that NAM lowers the barriers to reprogramming by accelerating cell proliferation and protecting cells from apoptosis and senescence by alleviating oxidative stress, reactive oxygen species accumulation, and subsequent mitochondrial membrane potential collapse. We provide evidence that the positive effects of NAM (occurring at concentrations well above the physiological range) on pluripotency control are molecularly associated with the repression of p53, p21, and p16. Our findings establish that adequate intracellular NAD(+) content is crucial for pluripotency; the distinct effects of NAM on pluripotency may be dependent not only on its metabolic advantage as a NAD(+) precursor but also on the ability of NAM to enhance resistance to cellular stress.

Nicotinamide and neurocognitive function

Nicotinamide, or vitamin B3, is a precursor of nicotinamide adenine dinucleotide (NAD(+)) and is involved in a multitude of intra- and inter-cellular processes, which regulate some of the cell's metabolic, stress, and immune responses to physiological or pathological signals. As a precursor of NAD(+), which is a key coenzyme in the production of adenosine triphosphate or cellular energy, nicotinamide has been investigated for potential neuroprotective effects in cellular, animal, and human studies. Objectives We aimed to summarize the current evidence on the effect of dietary and supplemental nicotinamide on cognitive function. Methods A literature review was conducted on the effects of nicotinamide and its derivatives as a preventive and therapeutic agent for disorders of neurocognitive function. Specific conditions examined include age-related cognitive decline, Alzheimer's disease, Parkinson's disease, and ischaemic and traumatic brain injury. Results Data from animal and human interventional studies and epidemiological research suggests that nicotinamide may be beneficial in preserving and enhancing neurocognitive function. Discussion Nicotinamide is non-toxic, inexpensive and widely available, and interventional studies in humans, using supplemental doses of nicotinamide, are now warranted.

Mitochondrial dysfunction and NAD(+) metabolism alterations in the pathophysiology of acute brain injury

Mitochondrial dysfunction is commonly believed to be one of the major players in mechanisms of brain injury. For several decades, pathologic mitochondrial calcium overload and associated opening of the mitochondrial permeability transition (MPT) pore were considered a detrimental factor causing mitochondrial damage and bioenergetics failure. Mitochondrial and cellular bioenergetic metabolism depends on the enzymatic reactions that require NAD(+) or its reduced form NADH as cofactors. Recently, it was shown that NAD(+) also has an important function as a substrate for several NAD(+) glycohydrolases whose overactivation can contribute to cell death mechanisms. Furthermore, downstream metabolites of NAD(+) catabolism can also adversely affect cell viability. In contrast to the negative effects of NAD(+)-catabolizing enzymes, enzymes that constitute the NAD(+) biosynthesis pathway possess neuroprotective properties. In the first part of this review, we discuss the role of MPT in acute brain injury and its role in mitochondrial NAD(+) metabolism. Next, we focus on individual NAD(+) glycohydrolases, both cytosolic and mitochondrial, and their role in NAD(+) catabolism and brain damage. Finally, we discuss the potential effects of downstream products of NAD(+) degradation and associated enzymes as well as the role of NAD(+) resynthesis enzymes as potential therapeutic targets.

The ways and means that fine tune Sirt1 activity

Sirt1 is the most evolutionarily conserved mammalian sirtuin. It plays a vital role in the regulation of metabolism, stress responses, genome stability, and ultimately aging. Although much attention has focused on the identification of the cellular targets and functional networks controlled by Sirt1, the mechanisms that regulate Sirt1 activity by biological stimuli have only recently begun to emerge. As an enzyme, the activity of Sirt1 can be controlled by the availability of its substrates, post-translational modifications, interactions with other proteins, or changes in its expression levels. In this review, we briefly discuss the ways and means by which the activity of Sirt1 is fine-tuned under different conditions.

Nicotinamide reduces photodynamic therapy-induced immunosuppression in humans

BACKGROUND

The immune suppressive effects of topical photodynamic therapy (PDT) are potential contributors to treatment failure after PDT for nonmelanoma skin cancer. Nicotinamide (vitamin B(3) ) prevents immune suppression by ultraviolet radiation, but its effects on PDT-induced immunosuppression are unknown.

OBJECTIVES

To determine the effects of topical and oral nicotinamide on PDT-induced immunosuppression in humans.

METHODS

Twenty healthy Mantoux-positive volunteers received 5% nicotinamide lotion or vehicle to either side of the back daily for 3 days. Another group of 30 volunteers received 500 mg oral nicotinamide or placebo twice daily for 1 week in a randomized, double-blinded, crossover design. In each study, methylaminolaevulinate cream was applied to discrete areas on the back, followed by narrowband red light irradiation (37 J cm(-2) ) delivered at high (75 mW cm(-2) ) or low (15 mW cm(-2) ) irradiance rates. Adjacent, nonirradiated sites served as controls. Delayed-type hypersensitivity (Mantoux) reactions were assessed at treatment and control sites to determine immunosuppression.

RESULTS

High irradiance rate PDT with vehicle or with placebo caused significant immunosuppression (equivalent to 48% and 50% immunosuppression, respectively; both P < 0·0001); topical and oral nicotinamide reduced this immunosuppression by 59% and 66%, respectively (both P < 0·0001). Low irradiance rate PDT was not significantly immunosuppressive in the topical nicotinamide study (15% immunosuppression, not significant), but caused 22% immunosuppression in the oral study (placebo arm; P = 0·006); nicotinamide reduced this immunosuppression by 69% (P = 0·045).

CONCLUSIONS

While the clinical relevance of these findings is currently unknown, nicotinamide may provide an inexpensive means of preventing PDT-induced immune suppression and enhancing PDT cure rates.

Dietary B vitamin and methionine intakes and lung cancer risk among female never smokers in China

PURPOSE

B vitamins and methionine have been postulated to have potential effects on carcinogenesis; however, findings from previous epidemiologic studies on B vitamins, methionine, and lung cancer risk are inconsistent. We investigated associations of dietary intakes of B vitamins (i.e., riboflavin, niacin, vitamin B6, folate, and vitamin B12) and methionine with lung cancer risk among female never smokers.

METHODS

The Shanghai Women's Health Study, a population-based, prospective cohort study, included 74,941 women. During a median follow-up of 11.2 years, 428 incident lung cancer cases accrued among 71,267 women with no history of smoking or cancer at baseline. Baseline dietary intakes were derived from a validated, interviewer-administered food frequency questionnaire. Cancer incidence and vital status were ascertained through annual linkage to the Shanghai Cancer Registry and Shanghai Vital Statistics Registry databases and through biennial in-person follow-ups with participants. Adjusted hazard ratios (HR) and 95 % confidence intervals (CI) were calculated using Cox regression.

RESULTS

Dietary riboflavin intake was inversely associated with lung cancer risk (HR = 0.62; 95 % CI = 0.43-0.89; p trend = 0.03 for the highest quartile compared with the lowest). A higher than median intake of methionine was associated with lower risk of lung cancer (HR = 0.78; 95 % CI = 0.60-0.99); however, there was no dose-response relation. Intakes of other B vitamins were not associated with lung cancer risk.

CONCLUSIONS

Our study suggests that dietary riboflavin intake may be inversely associated with lung cancer risk among female never smokers, which warrants further investigation.

Identification of gene expression signature modulated by nicotinamide in a mouse bladder cancer model

BACKGROUND

Urinary bladder cancer is often a result of exposure to chemical carcinogens such as cigarette smoking. Because of histological similarity, chemically-induced rodent cancer model was largely used for human bladder cancer studies. Previous investigations have suggested that nicotinamide, water-soluble vitamin B3, may play a key role in cancer prevention through its activities in cellular repair. However, to date, evidence towards identifying the genetic alterations of nicotinamide in cancer prevention has not been provided. Here, we search for the molecular signatures of cancer prevention by nicotinamide using a N-butyl-N-(4-hydroxybutyl)-nitrosamine (BBN)-induced urinary bladder cancer model in mice.

METHODOLOGY/PRINCIPAL FINDINGS

Via microarray gene expression profiling of 20 mice and 233 human bladder samples, we performed various statistical analyses and immunohistochemical staining for validation. The expression patterns of 893 genes associated with nicotinamide activity in cancer prevention were identified by microarray data analysis. Gene network analyses of these 893 genes revealed that the Myc and its associated genes may be the most important regulator of bladder cancer prevention, and the gene expression signature correlated well with protein expression data. Comparison of gene expression between human and mouse revealed that BBN-induced mouse bladder cancers exhibited gene expression profiles that were more similar to those of invasive human bladder cancers than to those of non-invasive human bladder cancers.

CONCLUSIONS/SIGNIFICANCE

This study demonstrates that nicotinamide plays an important role as a chemo-preventive and therapeutic agent in bladder cancer through the regulation of the Myc oncogenic signature. Nicotinamide may represent a promising therapeutic modality in patients with muscle-invasive bladder cancer.

Role of nicotinamide in DNA damage, mutagenesis, and DNA repair

Nicotinamide is a water-soluble amide form of niacin (nicotinic acid or vitamin B3). Both niacin and nicotinamide are widely available in plant and animal foods, and niacin can also be endogenously synthesized in the liver from dietary tryptophan. Nicotinamide is also commercially available in vitamin supplements and in a range of cosmetic, hair, and skin preparations. Nicotinamide is the primary precursor of nicotinamide adenine dinucleotide (NAD⁺), an essential coenzyme in ATP production and the sole substrate of the nuclear enzyme poly-ADP-ribose polymerase-1 (PARP-1). Numerous in vitro and in vivo studies have clearly shown that PARP-1 and NAD⁺ status influence cellular responses to genotoxicity which can lead to mutagenesis and cancer formation. This paper will examine the role of nicotinamide in the protection from carcinogenesis, DNA repair, and maintenance of genomic stability.

Ex vivo supplementation with nicotinic acid enhances cellular poly(ADP-ribosyl)ation and improves cell viability in human peripheral blood mononuclear cells

Poly(ADP-ribosyl)ation is a posttranslational modification of proteins, which is mainly catalyzed by poly(ADP-ribose) polymerase-1 (PARP-1) by using NAD(+) as substrate and is directly triggered by DNA strand breaks. Under mild genotoxic stress poly(ADP-ribose) (PAR) formation plays an important role in DNA repair whereas severe genotoxic stress and the ensuing overactivation of PARP-1 induce cellular NAD(+) depletion, energy failure and ultimately cell death. We are interested in studying the consequences of moderately enhanced enzymatic activity under conditions of DNA damage. Here we chose supplementation of cells with the NAD(+) precursor nicotinic acid (NA) as a strategy. In order to reliably assess PAR accumulation in living cells we first developed a novel, sensitive flow-cytometric method for the rapid analysis of poly(ADP-ribose) accumulation (RAPARA). Our data showed that ex vivo supplementation of human peripheral blood mononuclear cells (PBMC) with low concentrations of NA significantly raised cellular NAD(+) levels by 2.1-fold. Upon X-irradiation or exposure to hydrogen peroxide or N-methyl-N'-nitro-N-nitrosoguanidine, PAR accumulation was significantly increased and sustained in NA-supplemented cells. Furthermore, NA-supplemented PBMC displayed significantly higher cell viability due to a lower rate of necrotic cell death. In summary, ex vivo supplementation of human PBMC with NA increases cellular NAD(+) levels, boosts the cellular poly(ADP-ribosyl)ation response to genotoxic treatment, and protects from DNA-damage-induced cell death.

New hypotheses for the health-protective mechanisms of whole-grain cereals: what is beyond fibre?

Epidemiological studies have clearly shown that whole-grain cereals can protect against obesity, diabetes, CVD and cancers. The specific effects of food structure (increased satiety, reduced transit time and glycaemic response), fibre (improved faecal bulking and satiety, viscosity and SCFA production, and/or reduced glycaemic response) and Mg (better glycaemic homeostasis through increased insulin secretion), together with the antioxidant and anti-carcinogenic properties of numerous bioactive compounds, especially those in the bran and germ (minerals, trace elements, vitamins, carotenoids, polyphenols and alkylresorcinols), are today well-recognised mechanisms in this protection. Recent findings, the exhaustive listing of bioactive compounds found in whole-grain wheat, their content in whole-grain, bran and germ fractions and their estimated bioavailability, have led to new hypotheses. The involvement of polyphenols in cell signalling and gene regulation, and of sulfur compounds, lignin and phytic acid should be considered in antioxidant protection. Whole-grain wheat is also a rich source of methyl donors and lipotropes (methionine, betaine, choline, inositol and folates) that may be involved in cardiovascular and/or hepatic protection, lipid metabolism and DNA methylation. Potential protective effects of bound phenolic acids within the colon, of the B-complex vitamins on the nervous system and mental health, of oligosaccharides as prebiotics, of compounds associated with skeleton health, and of other compounds such as alpha-linolenic acid, policosanol, melatonin, phytosterols and para-aminobenzoic acid also deserve to be studied in more depth. Finally, benefits of nutrigenomics to study complex physiological effects of the 'whole-grain package', and the most promising ways for improving the nutritional quality of cereal products are discussed.

Photoprotective effects of nicotinamide

Sun protective measures can reduce numbers of both precancerous actinic keratoses and cutaneous squamous cell carcinomas within relatively short periods of time even in high-risk populations. Sunscreens, which tend to provide greater protection against shortwave UVB than against longer wavelength UVA radiation, can however provide only partial protection from the mutagenic and immune suppressive effects of sunlight. In large part, this reflects poor compliance with proper sunscreen application and reapplication. Skin cancer is by far the most common malignancy in Caucasian populations, and additional strategies to reduce the morbidity and economic burden of this disease are now urgently needed. Nicotinamide, the amide form of vitamin B3, is an inexpensive agent which is used for a variety of dermatological applications with little or no toxicity even at high doses. Nicotinamide has photoprotective effects against carcinogenesis and immune suppression in mice, and is photoimmunoprotective in humans when used as a lotion or orally. UV irradiation depletes keratinocytes of cellular energy and nicotinamide, which is a precursor of nicotinamide adenine dinucleotide, may act at least in part by providing energy repletion to irradiated cells.

The secret life of NAD+: an old metabolite controlling new metabolic signaling pathways

A century after the identification of a coenzymatic activity for NAD(+), NAD(+) metabolism has come into the spotlight again due to the potential therapeutic relevance of a set of enzymes whose activity is tightly regulated by the balance between the oxidized and reduced forms of this metabolite. In fact, the actions of NAD(+) have been extended from being an oxidoreductase cofactor for single enzymatic activities to acting as substrate for a wide range of proteins. These include NAD(+)-dependent protein deacetylases, poly(ADP-ribose) polymerases, and transcription factors that affect a large array of cellular functions. Through these effects, NAD(+) provides a direct link between the cellular redox status and the control of signaling and transcriptional events. Of particular interest within the metabolic/endocrine arena are the recent results, which indicate that the regulation of these NAD(+)-dependent pathways may have a major contribution to oxidative metabolism and life span extension. In this review, we will provide an integrated view on: 1) the pathways that control NAD(+) production and cycling, as well as its cellular compartmentalization; 2) the signaling and transcriptional pathways controlled by NAD(+); and 3) novel data that show how modulation of NAD(+)-producing and -consuming pathways have a major physiological impact and hold promise for the prevention and treatment of metabolic disease.

Niacin and fibrates in atherogenic dyslipidemia: pharmacotherapy to reduce cardiovascular risk

Although statin therapy represents a cornerstone of cardiovascular disease (CVD) prevention, a major residual CVD risk (60-70% of total relative risk) remains, attributable to both modifiable and non-modifiable risk factors. Among the former, low levels of HDL-C together with elevated triglyceride (TG)-rich lipoproteins and their remnants represent major therapeutic targets. The current pandemic of obesity, metabolic syndrome, and type 2 diabetes is intimately associated with an atherogenic dyslipidemic phenotype featuring low HDL-C combined with elevated TG-rich lipoproteins and small dense LDL. In this context, there is renewed interest in pharmacotherapeutic strategies involving niacin and fibrates in monotherapy and in association with statins. This comprehensive, critical review of available data in dyslipidemic subjects indicates that niacin is more efficacious in raising HDL-C than fibrates, whereas niacin and fibrates reduce TG-rich lipoproteins and LDL comparably. Niacin is distinguished by its unique capacity to effectively lower Lp(a) levels. Several studies have demonstrated anti-atherosclerotic action for both niacin and fibrates. In contrast with statin therapy, the clinical benefit of fibrates appears limited to reduction of nonfatal myocardial infarction, whereas niacin (frequently associated with statins and/or other agents) exerts benefit across a wider range of cardiovascular endpoints in studies involving limited patient numbers. Clearly the future treatment of atherogenic dyslipidemias involving the lipid triad, as exemplified by the occurrence of the mixed dyslipidemic phenotype in metabolic syndrome, type 2 diabetes, renal, and auto-immune diseases, requires integrated pharmacotherapy targeted not only to proatherogenic particles, notably VLDL, IDL, LDL, and Lp(a), but also to atheroprotective HDL.

The role of dietary niacin intake and the adenosine-5'-diphosphate-ribosyl cyclase enzyme CD38 in spatial learning ability: is cyclic adenosine diphosphate ribose the link between diet and behaviour?

The pyridine nucleotide NAD+ is derived from dietary niacin and serves as the substrate for the synthesis of cyclic ADP-ribose (cADPR), an intracellular Ca signalling molecule that plays an important role in synaptic plasticity in the hippocampus, a region of the brain involved in spatial learning. cADPR is formed in part via the activity of the ADP-ribosyl cyclase enzyme CD38, which is widespread throughout the brain. In the present review, current evidence of the relationship between dietary niacin and behaviour is presented following investigations of the effect of niacin deficiency, pharmacological nicotinamide supplementation and CD38 gene deletion on brain nucleotides and spatial learning ability in mice and rats. In young male rats, both niacin deficiency and nicotinamide supplementation significantly altered brain NAD+ and cADPR, both of which were inversely correlated with spatial learning ability. These results were consistent across three different models of niacin deficiency (pair feeding, partially restricted feeding and niacin recovery). Similar changes in spatial learning ability were observed in Cd38- / - mice, which also showed decreases in brain cADPR. These findings suggest an inverse relationship between spatial learning ability, dietary niacin intake and cADPR, although a direct link between cADPR and spatial learning ability is still missing. Dietary niacin may therefore play a role in the molecular events regulating learning performance, and further investigations of niacin intake, CD38 and cADPR may help identify potential molecular targets for clinical intervention to enhance learning and prevent or reverse cognitive decline.

NAD and axon degeneration: from the Wlds gene to neurochemistry

Neurodegenerative diseases have become a global issue due to the aging population. These disorders affect a vast patient population and represent a huge area of unmet therapeutic need. Axon degeneration is a common pathological character of those neurodegenerative diseases. It results in the loss of communication between neurons. Two decades ago, the Wallerian degeneration slow (Wlds) mouse strain was identified, in which the degeneration of transected axons is delayed. The phenotype is attributed to the overexpression of a chimeric protein Wlds which contains a short fragment of the ubiquitin assembly protein UFD2 and the full-length nicotinamide adenine dinucleotide (NAD) synthetic enzyme Nicotinamide mononucleotide adenylyl-transferase-1 (Nmnat-1). However, the underlying molecular mechanism remains largely unknown. Recently, it's reported by independent researchers that the full length coding sequence of mouse Nmnat-1 could mimic the axonal protective effect of the Wlds gene when overexpressed in primary neural cultures. Together with a significant number of subsequential reports, this finding highlighted the substantial role of nicotinamide adenine dinucleotide (NAD) in the process of axon degeneration. Here we reviewed the history of axon degeneration research from a neurochemical standpoint and discuss the potential involvement of NAD synthesis, NAD consumption and NAD-dependent proteins and small molecules in axon degeneration.

Niacin for dairy cattle: a review

Due to the incorporation of niacin into the coenzymes NAD and NADP, niacin is of great importance for the metabolism of man and animals. Apart from niacin in feed and endogenous formation, microbial niacin synthesis in the rumen is an important source for dairy cows. But the amount synthesised seems to differ greatly, which might be influenced by the ration fed. Many studies revealed a positive impact of a niacin supplementation on rumen protozoa, but microbial protein synthesis or volatile fatty acid production in the rumen showed inconsistent reactions to supplemental niacin. The amount of niacin reaching the duodenum is usually higher when niacin is fed. However, not the whole quantity supplemented reaches the duodenum, indicating degradation or absorption before the duodenal cannula. Furthermore, supplementation of niacin did not always lead to a higher niacin concentration in blood. Effects on other blood parameters have been inconsistent, but might be more obvious when cows are in a tense metabolic situation, for example, ketosis or if high amounts are infused post-ruminally, since ruminal degradation appears to be substantial. The same is valid for milk parameters. In the few studies where blood niacin and milk parameters have been investigated, enhanced niacin concentrations in blood did not necessarily affect milk production or composition. These results are discussed in the present review, gaps of knowledge of niacin's mode of action on the metabolism of dairy cows are identified and directions for future research are suggested.

Dietary intake of selected B vitamins in relation to risk of major cancers in women

Although folic acid has been investigated for its potential to inhibit carcinogenesis, few epidemiologic studies have assessed the effects of intake of thiamin, riboflavin, and niacin, which may reduce cancer risk by acting as cofactors in folate metabolism or by other mechanisms. Using data from a large cohort of Canadian women, we examined the association of dietary intake of these nutrients, as well as intake of folate, methionine, and alcohol, with cancers of the breast, endometrium, ovary, colorectum, and lung ascertained during an average of 16.4 years of follow-up. After exclusions, the following numbers of incident cases were available for analysis: breast, n=2491; endometrium, n=426; ovary, n=264; colorectum, n=617; and lung, n=358. Cox proportional hazard models were used to estimate risk of each cancer with individual nutrients and to explore possible effect modification by combinations of nutrients on cancer risk. Few significant associations of intake of individual B vitamins with the five cancers were observed. Alcohol consumption showed a modest positive association with breast cancer risk but not with risk of the other cancers. There was no evidence of effect modification among the nutrients. This large study provides little support for an association of dietary intake thiamin, riboflavin, niacin, folate, or methionine with five major cancers in women.

Niacin deficiency delays DNA excision repair and increases spontaneous and nitrosourea-induced chromosomal instability in rat bone marrow

We have shown that niacin deficiency impairs poly(ADP-ribose) formation and enhances sister chromatid exchanges and micronuclei formation in rat bone marrow. We designed the current study to investigate the effects of niacin deficiency on the kinetics of DNA repair following ethylation, and the accumulation of double strand breaks, micronuclei (MN) and chromosomal aberrations (CA). Weanling male Long-Evans rats were fed niacin deficient (ND), or pair fed (PF) control diets for 3 weeks. We examined repair kinetics by comet assay in the 36h following a single dose of ethylnitrosourea (ENU) (30mg/kg bw). There was no effect of ND on mean tail moment (MTM) before ENU treatment, or on the development of strand breaks between 0 and 8h after ENU. Repair kinetics between 12 and 30h were significantly delayed by ND, with a doubling of area under the MTM curve during this period. O(6)-ethylation of guanine peaked by 1.5h, was largely repaired by 15h, and was also delayed in bone marrow cells from ND rats. ND significantly enhanced double strand break accumulation at 24h after ENU. ND alone increased chromosome and chromatid breaks (four- and two-fold). ND alone caused a large increase in MN, and this was amplified by ENU treatment. While repair kinetics suggest that ND may be acting by creating catalytically inactive PARP molecules with a dominant-negative effect on repair processes, the effect of ND alone on O(6)-ethylation, MN and CA, in the absence of altered comet results, suggests additional mechanisms are also leading to chromosomal instability. These data support the idea that the bone marrow cells of niacin deficient cancer patients may be more sensitive to the side effects of genotoxic chemotherapy, resulting in acute bone marrow suppression and chronic development of secondary leukemias.

Synthesis, physicochemical properties and in vitro cytotoxicity of nicotinic acid ester prodrugs intended for pulmonary delivery using perfluorooctyl bromide as vehicle

This study explores perfluorooctyl bromide (PFOB) as a potential vehicle for the pulmonary delivery of a series of prodrugs of nicotinic acid using cell culture studies. The prodrugs investigated have PFOB-water (logK(p)=0.78 to >2.2), perfluoromethylcyclohexane-toluene (logK(p)=-2.62 to 0.13) and octanol-water (logK(p)=0.90-10.2) partition coefficients spanning several orders of magnitude. In confluent NCI-H358 human lung cancer cells, the toxicity of prodrugs administered in culture medium or PFOB depends on the medium of administration, with EC20's above 8 mM and 2.5 mM for culture medium and PFOB, respectively. Short-chain nicotinates administered both in PFOB and medium increase cellular NAD/NADP levels at 1mM nicotinate concentrations. Long-chain nicotinates, which could not be administered in medium due to their poor aqueous solubility, increased NAD/NADP levels if administered in PFOB at concentrations > or =10 mM. These findings suggest that even highly lipophilic prodrugs can partition out of the PFOB phase into cells, where nicotinic acid is released and converted to NAD. Thus, PFOB may be a novel and biocompatible vehicle for the delivery of lipophilic prodrugs of nicotinic acid and other drugs directly to the lung of laboratory animals and humans.

Water maze performance in young male Long-Evans rats is inversely affected by dietary intakes of niacin and may be linked to levels of the NAD+ metabolite cADPR

Niacin is converted in tissues to NAD(+), which is required for synthesis of the intracellular calcium signaling molecule cyclic ADP-ribose (cADPR). cADPR is involved in many aspects of cognitive function, including long-term depression, in the hippocampus, a brain region that regulates spatial learning ability. The objective of this study was to determine whether niacin deficiency and pharmacological nicotinamide supplementation have an effect on spatial learning ability in young male Long-Evans rats as assessed by the Morris Water Maze, and whether brain NAD(+) and cADPR are modified by dietary niacin intake. We investigated 3 models of niacin deficiency: niacin deficient (ND) vs. pair fed (PF), ND vs. partially feed restricted (PFR), and ND vs. niacin recovered (REC). ND rats showed an improvement in spatial learning ability relative to PF, PFR, and REC rats. ND rats also showed a decrease in both NAD(+) and cADPR relative to PF and REC rats. We also investigated 1 model of pharmacological supplementation, niacin-supplemented vs. control. The niacin-supplemented group showed a small but significant spatial learning impairment relative to controls, and an increase in brain cADPR and NAD(+). Changes in neural function related to the NAD(+) associated calcium signaling molecule, cADPR, may be the link between diet and behavior.

The regulation of nicotinamide adenine dinucleotide biosynthesis by Nampt/PBEF/visfatin in mammals

PURPOSE OF REVIEW

Nicotinamide adenine dinucleotide (NAD) is a classic coenzyme in cellular redox reactions. Recently, NAD biochemistry has also been implicated in a broader range of biological functions in mammals, but the regulation of NAD biosynthesis has been poorly investigated. Recent progress in the field of NAD biochemistry has fueled new interest in the NAD biosynthetic pathways from its precursors and their physiological roles in metabolism. This review summarizes the latest knowledge on the NAD biosynthetic pathways and focuses on one of the key NAD biosynthetic enzymes, namely, nicotinamide phosphoribosyltransferase.

RECENT FINDINGS

Mammals predominantly use nicotinamide rather than nicotinic acid as a precursor for NAD biosynthesis. Nicotinamide phosphoribosyltransferase (Nampt) is the rate-limiting enzyme that converts nicotinamide to nicotinamide mononucleotide in the NAD biosynthetic pathway from nicotinamide in mammals. The same protein has also been identified as a cytokine (pre-B-cell colony-enhancing factor or PBEF) or an insulin-mimetic hormone (visfatin).

SUMMARY

We propose that the presumed multiple effects of Nampt/PBEF/visfatin may be entirely explained by its role as an intra and extracellular NAD biosynthetic enzyme. We also propose a new model of Namp/PBEF/visfatin-mediated systemic NAD biosynthesis and its possible physiological significance. Our model provides an important insight into developing preventive/therapeutic interventions for metabolic complications, such as obesity and diabetes.

Simultaneous determination of myristyl nicotinate, nicotinic acid, and nicotinamide in rabbit plasma by liquid chromatography-tandem mass spectrometry using methyl ethyl ketone as a deproteinization solvent

Myristyl nicotinate (Nia-114) is an ester prodrug being developed for delivery of nicotinic acid (NIC) into the skin for prevention of actinic keratosis and its progression to skin cancer. To facilitate dermal studies of Nia-114, a novel liquid chromatography-tandem mass spectrometry (LC-MS/MS) method using methyl ethyl ketone (MEK) as a deproteinization solvent was developed and validated for the simultaneous determination of Nia-114, NIC, and nicotinamide (NAM) in rabbit plasma. NAM is the principal metabolite of NIC, which is also expected to have chemopreventive properties. The analytes were chromatographically separated using a Spherisorb Cyano column under isocratic conditions, and detected by multiple reaction monitoring (MRM) in positive-ion electrospray ionization mode with a run time of 9 min. The method utilized a plasma sample volume of 0.2 ml and isotope-labeled D4 forms of each analyte as internal standards. The method was linear over the concentration range of 2-1000, 8-1000, and 75-1000 ng/ml, for Nia-114, NIC, and NAM, respectively. The intra- and inter-day assay accuracy and precision were within +/-15% for all analytes at low, medium, and high quality control standard levels. The relatively high value for the lower limit of quantitation (LLOQ) of NAM was demonstrated to be due to the high level of endogenous NAM in the rabbit plasma (about 350 ng/ml). Endogenous levels of NIC and NAM in human, dog, rat, and mouse plasma were also determined, and mean values ranged from <2 ng/ml NIC and 38.3 ng/ml NAM in human, to 233 ng/ml NIC and 622 ng/ml NAM in mouse. Nia-114 was generally unstable in rabbit plasma, as evidenced by loss of 44-50% at room temperature by 2 h, and loss of 64-70% upon storage at -20 degrees C for 1 week, whereas it was stable (<7% loss) upon storage at -80 degrees C for 1 month.

The NAD biosynthesis pathway mediated by nicotinamide phosphoribosyltransferase regulates Sir2 activity in mammalian cells

Recent studies have revealed new roles for NAD and its derivatives in transcriptional regulation. The evolutionarily conserved Sir2 protein family requires NAD for its deacetylase activity and regulates a variety of biological processes, such as stress response, differentiation, metabolism, and aging. Despite its absolute requirement for NAD, the regulation of Sir2 function by NAD biosynthesis pathways is poorly understood in mammals. In this study, we determined the kinetics of the NAD biosynthesis mediated by nicotinamide phosphoribosyltransferase (Nampt) and nicotinamide/nicotinic acid mononucleotide adenylyltransferase (Nmnat), and we examined its effects on the transcriptional regulatory function of the mouse Sir2 ortholog, Sir2alpha, in mouse fibroblasts. We found that Nampt was the rate-limiting component in this mammalian NAD biosynthesis pathway. Increased dosage of Nampt, but not Nmnat, increased the total cellular NAD level and enhanced the transcriptional regulatory activity of the catalytic domain of Sir2alpha recruited onto a reporter gene in mouse fibroblasts. Gene expression profiling with oligonucleotide microarrays also demonstrated a significant correlation between the expression profiles of Nampt- and Sir2alpha-overexpressing cells. These findings suggest that NAD biosynthesis mediated by Nampt regulates the function of Sir2alpha and thereby plays an important role in controlling various biological events in mammals.

Niacin and carcinogenesis

The dietary status of niacin (vitamin B3) has the potential to influence DNA repair, genomic stability, and the immune system, eventually having an impact on cancer risk, as well as the side effects of chemotherapy in the cancer patient. In addition to its well-known redox functions in energy metabolism, niacin, in the form of NAD, participates in a wide variety of ADP-ribosylation reactions. Poly(ADP-ribose) is a negatively charged polymer synthesized, predominantly on nuclear proteins, by at least seven different enzymes. Poly(ADP-ribose) polymerase-1 (PARP-1) is responsible for the majority of polymer synthesis and plays important roles in DNA damage responses, including repair, maintenance of genomic stability, and signaling events for stress responses such as apoptosis. NAD is also used in the synthesis of mono(ADP-ribose), often on G proteins, with poorly understood roles in signal transduction. Last, NAD and NADP are required for the synthesis of cyclic ADP-ribose and nicotinic acid adenine dinucleotide (NAADP), two mediators of intracellular calcium signaling pathways. Disruption of any of these processes has the potential to impair genomic stability and deregulate cell division, leading to enhanced cancer risk. There are various sources of evidence that niacin status does have an impact on cancer risk, including animal models of leukemogenesis and skin cancer, as well as epidemiological data from human populations.

Inhibition of human P450 enzymes by nicotinic acid and nicotinamide

Nicotinic acid has been used as a cholesterol-lowering agent for a few decades already, whereas the cytoprotective and antiviral properties of nicotinamide are slowly gaining attention. In both cases however, very high doses are needed to achieve a therapeutic effect, resulting in blood concentrations sometimes as high as 15 mM. Based on their common pyridine functionality, we hypothesized that these two molecules could inhibit human P450 enzymes. In vitro inhibition studies demonstrate that, at their therapeutic concentrations, both nicotinic acid and nicotinamide inhibit CYP2D6 (Ki = 3.8 +/- 0.3 and 19 +/- 4 mM, respectively). Nicotinamide also inhibits CYP3A4 (Ki = 13 +/- 3 mM) and CYP2E1 (Ki = 13 +/- 8 mM). As expected for nitrogen-containing heteroaromatic molecules, spectrophotometric analysis indicates that the inhibition occurs via coordination of the pyridine nitrogen atom to the heme iron.

Reconstructing eukaryotic NAD metabolism

In addition to its well-known role as a coenzyme in oxidation-reduction reactions, the distinct role of NAD as a precursor for molecules involved in cell regulation has been clearly established. The involvement of NAD in these regulatory processes is based on its ability to function as a donor of ADP-ribose; NAD synthesis is therefore required to avoid depletion of the intracellular pool. The rising interest in the biosynthetic routes leading to NAD formation and the highly conserved nature of the enzymes involved prompted us to reconstruct the NAD biosynthetic routes operating in distinct eukaryotic organisms. The evidence obtained from biochemical and computational analysis provides a good example of how complex metabolic pathways may evolve. In particular, it is proposed that the development of several NAD biosynthetic routes during evolution has led to partial functional redundancy, allowing a given pathway to freely acquire novel functions unrelated to NAD biosynthesis.

Nicotinamide: an oral antimicrobial agent with activity against both Mycobacterium tuberculosis and human immunodeficiency virus

Coinfection with Mycobacterium tuberculosis and human immunodeficiency virus (HIV) is responsible for one-third of all deaths due to acquired immunodeficiency syndrome. More than 99% of cases of HIV-M. tuberculosis coinfection occur in the developing world, where limited resources add urgency to the search for effective and affordable therapies. Although antimicrobial agents against each of these infections are available, single agents that have activity against both M. tuberculosis and HIV are uncommon. The activity of nicotinamide has been evaluated in 2 different eras: in anti-M. tuberculosis studies performed during 1945-1961 and in anti-HIV studies performed from 1991 to the present. This review brings together these 2 bodies of inquiry and raises the possibility that, with more study, this small molecule could emerge at the beginning of the 21st century either as a therapeutic agent in itself or as the lead compound for a new class of agents with activity against both M. tuberculosis and HIV.

Effects of fatty liver induced by niacin-free diet with orotic acid on the metabolism of tryptophan to niacin in rats

The effects of dietary orotic acid on the metabolism of tryptophan to niacin in weaning rats was investigated. The rats were fed with a niacin-free, 20% casein diet containing 0% (control diet) or 1% orotic acid diet (test diet) for 29 d. Retardation of growth, development of fatty liver, and enlargement of liver were observed in the test group in comparison with the control group. The concentrations of NAD and NADP in liver significantly decreased, while these in blood did not decrease compared to the control group. The formation of the upper metabolites of tryptophan to niacin such as anthranilic acid, kynurenic acid, and 3-hydroxyanthranilic acid were not affected, but the quinolinic acid and beyond, such as nicotinamide, N1-methylnicotinamide, N1-methyl-2-pyridone-5-carboxamide, and N1-methyl-4-pyridone-3-carboxamide, were significantly reduced by the administration of orotic acid. Therefore, the conversion ratio of tryptophan to niacin significantly decreased in the test group in comparison with the control group.

Adenocarcinomas of the esophagus and gastric cardia: the role of diet

The incidence of adenocarcinomas of the esophagus and gastric cardia (ACEGC) has been increasing for the past 10-15 years in the United States. The reason for this increase is unknown. This hospital-based case-control study was conducted to assess the effects of dietary and nutritional factors on the risk of ACECG. A total of 95 incident cases with pathological diagnosis and 132 cancer-free controls were included in the study. Patients were recruited at Memorial Sloan-Kettering Cancer Center from 1 November 1992 to 1 November 1994. Epidemiologic data were collected by a modified National Cancer Institute Health Habits History Questionnaire. Nutritional and dietary factors were analyzed using a logistic regression model. Increased risk of ACEGC was significantly related to higher intake of dietary calories and fat after controlling for several potential confounding factors. Decreased risk of ACEGC was significantly associated with high ingestion of dietary fiber, lutein, niacin, vitamin B6, iron, and zinc. Higher intakes of vitamin A, beta-carotene, vitamin E, folate, phosphorus, and potassium were associated with a decreased risk of the disease, but these were not statistically significant. The study suggests that ACEGC can be preventable through dietary interventions.

Induced mono-(ADP)-ribosylation of rat liver cytosolic proteins by lipid peroxidant agents

We have studied the effect of free radical generating agents on the mono-(ADP)-ribosylation of rat liver cytosolic proteins. Our results show that this post-translational modification, whose physiological significance is still unclear, is activated by lipid peroxidant agents via activation of cytoplasmatic mono-(ADP)-ribosyltransferases. The implication of free radicals in this process is demonstrated by the fact that mono-(ADP)-ribosylation can be prevented by melatonin, N-tert-butyl-alpha-phenylnitrone and dithiothreitol. On the basis of our results, we discuss the modification of proteins caused by free radicals as a possible mechanism by which they damage the cell.