Current Pharmaceutical Design - Volume 15, Issue 1, 2009

Nicotinamide adenine dinucleotide (NAD) research has progressed from the initial vitamin discovery phase in 1937 with the cure for the NAD deficiency disease pellagra [1], to discoveries in the 1950s of activities observed after application at pharmacological doses. These latter applications include benefits to mental health [2, 3] and lipodystrophy [4], where nicotinic acid remains the most effective means of elevating HDL levels to this current day. Basic research reveals that NAD functions as a co-factor in over 200 redox reactions and as a substrate for three categorical classes of enzymes: NAD-dependent deacetylases (Sirtuins), ADP-ribosyl transferases (most prominently, PARP-1), and ADP cyclases (e.g. CD38). The effects of NAD deficiency are made evident as the symptoms of the dreaded disease pellagra the symptoms of which are often generalized and easily remembered by the four D's: dermatitis, diarrhea, dementia, and death. These symptoms represent the wide range of functions of NAD spanning from auto-immunity to mental function. Modern molecular biology has revealed undeniably profound trophic neuronal benefits afforded by maintaining NAD levels working through SIRT-1 in studies of Slow Wallerian Degeneration mouse [5]. These benefits extend to a wide range of neurodegenerative disease models [6]. Two NAD-utilizing enzymes have emerged as tremendous focus areas of pharmaceutical drug target investment: PARP-1 (e.g. Inotek Pharmaceuticals bought by Genentech last year) and SIRT-1 (e.g. Sirtris Pharmaceuticals, bought by GSK). However, there are now known to be up to 18 potential ADP-ribosyl transferase related genes and seven Sirtuin related genes, some with specific nuclear or mitochondrial restricted subcellular locations. While SIRT- 1 and PARP-1 have mostly been studied for their roles in cancer biology to date, the role of NAD in auto-immune, neurodegenerative, and infectious disease is rapidly increasing. Looking to the future, this issue of Current Pharmaceutical Design features leaders in the field relating NAD biology to specific disease states. Focus is given to cancer, cardiovascular disease, diabetes, multiple sclerosis, and tuberculosis-schizophrenia. James B. Kirkland presents detailed analysis of endogenous and pharmacological concentrations for respective NAD precursors with emphasis on relevant pathway signaling, while Weihai Ying et al. present a mechanisms of action regulating NADPH oxidase and apoptosis. Shin-ichiro Imai covers the rate-limiting enzyme controlling NAD recycling, NAMPT [7]. As the only known extracellular enzyme in the NAD salvage pathway nicotinamide mononucleotide working through NAMPT has unique potential for regulating the immune system and beta cell function. Cancer research has led the way in NAD experimental investigations for many years. In this issue, Claudia Benavente with Elaine and Myron Jacobson highlight increasingly appreciated NAD-centric pathways including niacin-deficient activation of NADPH oxidase, utilization of glutamine as an energy source during loss of glycolysis, and the potential importance of Sirtuins in UV damage responses. Meanwhile, Jo Milner brings to light the importance of increased SIRT1 concentrations in cancer with new insight on regulatory mechanisms involving micro-RNAs.

Dietary niacin deficiency, and pharmacological excesses of nicotinic acid or nicotinamide, have dramatic effects on cellular NAD pools, ADP-ribose metabolism, tissue function and health. ADP-ribose metabolism is providing new targets for pharmacological intervention, and it is important to consider how the supply of vitamin B3 may directly influence ADP-ribosylation reactions, or create interactions with other drugs designed to influence these pathways. In addition to its redox roles, NAD+ is used as a substrate for mono-, poly- and cyclic ADP-ribose formation. During niacin deficiency, not all of these processes can be maintained, and dramatic changes in tissue function and clinical condition take place. Conversely, these reactions may be differentially enhanced by pharmacological intakes of vitamin B3, and potentially by changing expression of specific NAD generating enzymes. A wide range of metabolic changes can take place following pharmacological supplementation of nicotinic acid or nicotinamide. As niacin status decreases towards a deficient state, the function of other types of pharmaceutical agents may be modified, including those that target ADP-ribosylation reactions, apoptosis and inflammation. This article will explore what is known and yet to be learned about the response of tissues, cells and subcellular compartments to excessive and limiting supplies of niacin, and will discuss the etiology of the resulting pathologies.

A rapidly growing body of information has suggested that NAD (including NAD+ and NADH) and NADP (including NADP+ and NADPH) could be new fundamental factors in cell death: Many studies have indicated key roles of poly (ADP-ribose) polymerases and sirtuins --- two families of NAD-dependent enzymes --- in cell death; and NAD may also affect cell survival by influencing mitochondrial permeability transition, apoptosis-inducing factor and GAPDH. NAD may further influence cell survival by its effects on calcium homeostasis, gene expression and immunological functions. Due to the crucial roles of oxidative stress in cell death, NADPH may mediate cell death by its major effects on oxidative stress: NADPH is a key factor in cellular antioxidation systems; and NADPH oxidase is also a major generator of oxidative stress. With growing information about the novel biological properties of NAD and NADP, it is likely that new roles of NAD and NADP in cell death and various diseases will be elucidated. The elucidation may not only improve our understanding about the fundamental mechanisms of cell death, but also suggest new therapeutic targets for a variety of diseases.

New interest in NAD biology has recently been revived, and enzymes involved in NAD biosynthetic pathways have been identified and characterized in mammals. Among them, nicotinamide phosphoribosyltransferase (Nampt) has drawn much attention in several different fields, including NAD biology, metabolism, and immunomodulatory response. The research history of this protein is peculiar and controversial, and its physiological function has been a matter of debate. Nampt has both intra- and extracellular forms in mammals. Intracellular Nampt (iNampt) is an essential enzyme in the NAD biosynthetic pathway starting from nicotinamide. On the other hand, an extracellular form of this protein has been reported to act as a cytokine named PBEF, an insulin-mimetic hormone named visfatin, or an extracellular NAD biosynthetic enzyme named eNampt. This review article summarizes the research history and reported functions of this unique protein and discusses the pathophysiological significance of Nampt as an NAD biosynthetic enzyme vs. a potential inflammatory cytokine in diverse biological contexts.

The maintenance and regulation of cellular NAD(P)(H) content and its influence on cell function involves many metabolic pathways, some of which remain poorly understood. Niacin deficiency in humans, which leads to low NAD status, causes sun sensitivity in skin, indicative of deficiencies in responding to UV damage. Animal models of niacin deficiency demonstrate genomic instability and increased cancer development in sensitive tissues including skin. Cell culture models of niacin deficiency have allowed the identification of NAD-dependent signaling events critical in early skin carcinogenesis. Niacin restriction in immortalized keratinocytes leads to an increased expression and activity of NADPH oxidase resulting in an accumulation of ROS, providing a potential survival mechanism as has been shown to occur in cancer cells. Niacin deficient keratinocytes are more sensitive to photodamage, as both poly(ADP-ribose) polymerases and Sirtuins are inhibited by the unavailability of their substrate, NAD+, leading to unrepaired DNA damage upon photodamage and a subsequent increase in cell death. Furthermore, the identification of the nicotinic acid receptor in human skin keratinocytes provides a further link to niacin's role as a potential skin cancer prevention agent and suggests the nicotinic acid receptor as a potential target for skin cancer prevention agents. The new roles for niacin as a modulator of differentiation and photo-immune suppression and niacin status as a critical resistance factor for UV damaged skin cells are reviewed here.

The intersection between regulatory pathways responsive to metabolic fluctuation on one hand, and to cellular stress on the other, is a fascinating area within which NAD/NADH responsive proteins play a major role [1, 2]. A key player amongst these is SIRT1, a member of the mammalian sirtuin family (SIRT1-7). SIRT1 is an NAD-dependent deacetylase with critical functions in the maintenance of homeostasis and cell survival. In this review I shall focus upon (i) the cellular regulation of SIRT1 expression and (ii) the cellular regulation of SIRT1 activity. In addition the distinction between basal and stress-induced functions will be addressed: do they simply reflect a sliding scale of response, or are they mechanistically distinct? Elevated levels of SIRT1 are evident in cancer and SIRT1 can function as a cancer-specific survival factor in human cell lines. However, in a mouse model SIRT1 is reported to function as a tumour suppressor. Possible explanations for this apparent discrepancy will be considered. Given the high profile of SIRT1 as a potential therapeutic target it is clearly important to clarify its basal functioning in relation to differentiation, cell type, intercellular communication, and to age-related disease states including neurodegeneration and cancer.

The sirtuins are protein modifying enzymes widely distributed in all forms of life. The sirtuins are principally NAD+-dependent protein acetyl-lysine deacetylases that reverse acetyl-modifications of proteins. The sirtuins are implicated in a variety of adaptations to reduced nutritional intake, and increase lifespan in several model organisms, such as yeast, flies and worms. The human sirtuins (SIRT1-7) have been identified to regulate a variety of biological processes, such as glucose homeostasis, gluconeogenesis, mitochondrial biogenesis, insulin secretion, adipogenesis and adipolysis, apoptosis, senescence and metabolism. The potency of sirtuins in regulating mammalian biological processes invites consideration of them as potential drug targets. This review considers small molecule approaches to activate sirtuins in a biochemical and biological context. These approaches include allosteric activation, which has been demonstrated for the SIRT1 enzyme. Another approach discussed is enhancement of NAD+ levels in cells, since sirtuins appear to be responsive to increased cellular NAD+. Finally, a sirtuin-specific approach is considered that is called nicotinamide derepression. This approach is designed to antagonize physiologic nicotinamide inhibition of sirtuins as a means to upregulate sirtuin function. Biological data that provides evidence of effectiveness of these approaches in in vitro and in vivo contexts is presented along with a discussion of the theoretical considerations that underpin these strategies. Efficacies and shortcomings of the various approaches are also discussed.

CD38 is a multifunctional enzyme that uses nicotinamide adenine dinucleotide (NAD) as a substrate to generate second messengers. Recently, CD38 was also identified as one of the main cellular NADases in mammalian tissues and appears to regulate cellular levels of NAD in multiple tissues and cells. Due to the emerging role of NAD as a key molecule in multiple signaling pathways, and metabolic conditions it is imperative to determine the cellular mechanisms that regulate the synthesis and degradation of this nucleotide. In fact, recently it has been shown that NAD participates in multiple physiological processes such as insulin secretion, control of energy metabolism, neuronal and cardiac cell survival, airway constriction, asthma, aging and longevity. The discovery of CD38 as the main cellular NADase in mammalian tissues, and the characterization of its role on the control of cellular NAD levels indicate that CD38 may serve as a pharmacological target for multiple conditions.

The etiology of multiple sclerosis (MS) is unknown but it manifests as a chronic inflammatory demyelinating disease in the central nervous system (CNS). During chronic CNS inflammation, nicotinamide adenine dinucleotide (NAD) concentrations are altered by (T helper) Th1-derived cytokines through the coordinated induction of both indoleamine 2,3-dioxygenase (IDO) and the ADP cyclase CD38 in pathogenic microglia and lymphocytes. While IDO activation may keep auto-reactive T cells in check, hyper-activation of IDO can leave neuronal CNS cells starving for extracellular sources of NAD. Existing data indicate that glia may serve critical functions as an essential supplier of NAD to neurons during times of stress. Administration of pharmacological doses of non-tryptophan NAD precursors ameliorates pathogenesis in animal models of MS. Animal models of MS involve artificially stimulated autoimmune attack of myelin by experimental autoimmune encephalomyelitis (EAE) or by viral-mediated demyelination using Thieler's murine encephalomyelitis virus (TMEV). The WldS mouse dramatically resists razor axotomy mediated axonal degeneration. This resistance is due to increased efficiency of NAD biosynthesis that delays stress-induced depletion of axonal NAD and ATP. Although the WldS genotype protects against EAE pathogenesis, TMEV-mediated pathogenesis is exacerbated. In this review, we contrast the role of NAD in EAE versus TMEV demyelinating pathogenesis to increase our understanding of the pharmacotherapeutic potential of NAD signal transduction pathways. We speculate on the importance of increased SIRT1 activity in both PARP-1 inhibition and the potentially integral role of neuronal CD200 interactions through glial CD200R with induction of IDO in MS pathogenesis. A comprehensive review of immunomodulatory control of NAD biosynthesis and degradation in MS pathogenesis is presented. Distinctive pharmacological approaches designed for NAD-complementation or targeting NAD-centric proteins (SIRT1, SIRT2, PARP-1, GPR109a, and CD38) are outlined towards determining which approach may work best in the context of clinical application.

Schizophrenia is a common, debilitating mental illness that has persisted over the generations. For a disease with a strong genetic component, such prevalence has been difficult to understand in evolutionary terms. A model for its prevalence as a phenotype is presented in this manuscript, based on reports of specific differences in gene expression, metabolite levels and historical epidemiology. The selective force that underlies the proposed model is tuberculosis, a scourge of huge proportions that itself evolved to interact with the human host in a manner ensuring both its long term persistence in the host and its transfer to other carriers prior to the host's unfortunate death. The focal point of the interaction between humans and M. tuberculosis is hypothesized to be the de novo synthesis of NAD via activation of the kynurenine pathway. The strategy that M. tuberculosis employed to circumvent this aspect of the host's response to mycobacterial infection, and how that strategy interacted with a poor diet to force human evolution towards increased risk for schizophrenia, forms the basic premise of this paper. The model has implications for treatment of both diseases and generates hypotheses to be tested.

Cardiovascular disease (CVD) is the most prevalent disease worldwide and there is intense interest in pharmaceutical approaches to reduce the burden of this chronic, aging-related condition. The sirtuin (SIRT) family of NAD+- dependent protein deacetylases and ADP-ribosyltransferases have emerged as exciting targets for CVD management that can impact the cardiovascular system both directly and indirectly, the latter by modulating whole body metabolism. SIRT1-4 regulate the activities of a variety of transcription factors, coregulators, and enzymes that improve metabolic control in adipose tissue, liver, skeletal muscle, and pancreas, particularly during obesity and aging. SIRT1 and 7 can control myocardial development and resist stress- and aging-associated myocardial dysfunction through the deacetylation of p53 and forkhead box O1 (FoxO1). By modulating the activity of endothelial nitric oxide synthase (eNOS), FoxO1, and p53, and the expression of angiotensin II type 1 receptor (AT1R), SIRT1 also promotes vasodilatory and regenerative functions in endothelial and smooth muscle cells of the vascular wall. Given the array of potentially beneficial effects of SIRT activation on cardiovascular health, interest in developing specific SIRT agonists is well-substantiated. Because SIRT activity depends on cellular NAD+ availability, enzymes involved in NAD+ biosynthesis, including nicotinamide phosphoribosyltransferase (Nampt), may also be valuable pharmaceutical targets for managing CVD. Herein we review the actions of the SIRT proteins on the cardiovascular system and consider the potential of modulating SIRT activity and NAD+ availability to control CVD.