![[Feedback]](/icons/banner/feedbackACT.gif){kind=icon}
![[For Subscribers]](/icons/banner/subscriberACT.gif){kind=icon}
![[Archive]](/icons/banner/archiveACT.gif){kind=icon}
![[Search]](/icons/banner/searchACT.gif){kind=icon}
![[Contents]](/icons/banner/tocACT.gif){kind=icon}
|
|
|
|
|
||
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
PERSPECTIVES IN PHARMACOLOGY
Department of Pharmacology, Weill Medical College of Cornell University, New York, New York
Received June 14, 2007; accepted December 27, 2007.
 |
Abstract |
|---|
|
). Furthermore, ADPR transfer modifies protein nucleophilic side chains (Hassa et al., 2006). The ADPR-transfer enzymes fall into three distinct families, the mono-ADP-ribosyltransferases (Hassa et al., 2006), the poly(ADP-ribosyl) polymerases (PARPs) (Virag and Szabo, 2002; Hassa et al., 2006), and the sirtuins (Sauve et al., 2006). The sirtuins transiently ADP-ribosylate acetyllysines of proteins causing protein deacetylation, releasing an acetyl transfer product, acetyl-ADP-ribose (Sauve et al., 2006). However, some sirtuins are not deacetylases and appear to catalyze protein ADPR transfer in the ordinary sense. Collectively, these enzymes regulate numerous signaling pathways and respond to changes in NAD+ metabolism. They exert profound influences on apoptosis, metabolism, proliferation, DNA repair, senescence, endocrine signaling, and lifespan (Guarente, 2006).
In addition to being nutrients, nicotinamide and nicotinic acid are clinically applied pharmacological agents. Nicotinic acid is administered in large doses to lower serum lipids and cholesterol (Schachter, 2005). Nicotinamide has recently been used for prevention of type 1 diabetes (Gale et al., 2004) and is being evaluated for prevention of neurotoxicity and for treatment of ischemia. High-dose nicotinic acid and nicotinamide enter metabolism and increase NAD+ pools but also bind to proteins in cells to elicit their effects. For example, nicotinic acid has a cognate receptor, which is implicated in some of its antilipid effects (Soudijn et al., 2007). Nicotinamide inhibits PARP, leading to decreased NAD+ turnover, to provide beneficial effects in degenerative states where PARP activity is overactivated (Virag and Szabo, 2002).
The multiplicity of functions attributed to nicotinamide, nicotinic acid, and the dinucleotides, as well as the linkage of powerful signaling components to NAD+ metabolism via the ADPR-transferases, has provided a surge of interest in the therapeutic possibilities inherent to targeting NAD+ metabolism for therapy (see Table 1). NAD+ metabolism has been a topic of several recent reviews (Magni et al., 2004a; Yang and Sauve, 2006; Yang et al., 2006; Belenky et al., 2007; Revollo et al., 2007). Herein, we survey knowledge of NAD+ metabolism in humans and microbes. We examine the properties of nicotinamide and nicotinic acid as nutrients and as pharmacologic agents. We consider other precursors of NAD+ distinct from nicotinamide and nicotinic acid, such as nicotinamide riboside. Finally, we explore current and potential applications of therapeutics that target NAD+ metabolism and consider how future therapies could develop.
|
|
NAD+ Metabolism |
|---|
). |
De Novo Pathways in Humans Microbes and Bacteria |
|---|
). The reaction of aspartate and dihydroxyacetone-phosphate leads to efficient synthesis of QA, catalyzed by aspartate oxidase and QA synthase (Fig. 2). Alternatively, yeast, humans, and some bacterial microbes make QA via an aerobic pathway from tryptophan (Kurnasov et al., 2003). Molecular oxygen as a substrate oxidizes tryptophan to downstream metabolites of the kynurenine pathway, 3-hydroxyanthranilate, and finally QA (Fig. 3) (Kurnasov et al., 2003). The respective de novo pathways are important sources of vitamin B3 in most bacteria and in humans. In humans, 1 of 67 mg of tryptophan eventually ends up as nicotinamide if another B3 source is not available in the diet (Fukuwatari et al., 2004).
|
|
|
Recycling in Bacteria |
|---|
). Once formed, nicotinamide is converted to nicotinic acid with nicotinamidase. Hydrolysis of nicotinamide is an obligate step of recycling in most bacteria, and a nicotinic acid phosphoribosyltransferase couples nicotinic acid to PRPP to form NAMN (Fig. 2). The conversions of nicotinic acid to NAD+ are known as the Preiss-Handler pathway. NAMN adenylyltransferase catalyzes formation of nicotinic acid adenine dinucleotide (Fig. 2), and NAD+ synthetase completes synthesis of NAD+ by converting the carboxylate to the amide (Fig. 2). Most plants and eukaryotes (with the exception of mammals) catalyze resynthesis of NAD+ from nicotinamide, similar to bacteria, via obligate breakdown of nicotinamide to nicotinic acid followed by synthesis NAMN (Kurnasov et al., 2003).
A second major pathway of NAD+ decomposition in bacteria involves breakage of the phosphate anhydride bond to form NMN and AMP (Foster et al., 1979). This latter reaction is catalyzed by pyrophosphatases as well as bacterial NAD+-dependent ligases (Fig. 2) (Wilkinson et al., 2001). NMN is resynthesized into NAD+ via NMN adenylate transferases or further converted to nicotinamide.
|
Recycling in Humans |
|---|
). Nicotinamide formed upon NAD+ turnover is not hydrolyzed but is coupled directly to PRPP to form NMN by nicotinamide phosphoribosyltransferase (nampt, also known as PBEF) (Fig. 3). This enzyme activity is found only in mammals and some classes of bacteria. It is distinct from the enzyme that couples nicotinic acid to PRPP (Yang et al., 2006; Revollo et al., 2007). Although an activity that converted nicotinamide to NAD+ independently of nicotinic acid was indicated for sometime, the enzyme was only recently identified (Rongvaux et al., 2002).
The human genome also encodes a Preiss-Handler pathway, which converts nicotinic acid to NAD+ via NAMN and nicotinic acid adenine dinucleotide (Fig. 3). Humans use both nicotinic acid and nicotinamide recycling to synthesize NAD+ but utilize different pathways to achieve this. An enzyme in common between the pathways is the adenylation enzyme nmnat (Fig. 3). This enzyme has three isoforms in humans (nmnat-1, nmnat-2, and nmnat-3). nmnat-1 is localized to nuclei as determined by immunofluorescence and was recently shown to stimulate PARP-1 (Schweiger et al., 2001; Berger et al., 2007). nmnat-2 is in Golgi, and nmnat-3 is in mitochondria (Berger et al., 2005). All isoforms exhibit dual specificity for both NAMN and NMN as a substrate (Raffaelli et al., 2002; Magni et al., 2004b; Berger et al., 2005). nmnat-1 is the most proficient catalyst as determined by catalytic velocity (Vmax) and efficiency (Vmax/Km). The distribution of NAD in cells and the locations of NAD+ synthesis have recently received new consideration. Implied from the fact that nmnat activity is required to complete all salvage and de novo pathways of NAD+ biosynthesis, mammalian cell NAD+ synthesis is compartmentalized. Indeed, there are stable NAD+ pools within distinct subcellular compartments. Evidence to support this idea is available from cell fractionation studies that confirm that mitochondria maintain relatively high NAD+ concentrations and that mitochondrial NAD+ does not readily leak across the inner mitochondrial membrane (Di Lisa and Ziegler, 2001). On the other hand, the majority of cytosolic NAD+ is probably made within the nucleus of cells and then redistributed to the cytosol by passive diffusion through nuclear pores (Berger et al., 2005). It has been asserted that the relative distribution of total NAD+ in cells is largely mitochondrial (Di Lisa and Ziegler, 2001; Di Lisa et al., 2001), although this premise derives mostly from data obtained on myocytes (Di Lisa et al., 2001), which are rich in mitochondria. In contrast, in hepatocytes, 30 to 40% of total cellular NAD+ is mitochondrial, whereas the majority is cytosolic (Tischler et al., 1977). On the extreme, erythrocytes have reasonably high concentrations of NAD+ but have no mitochondria at all. It is apparent that relative NAD+ contents in cellular compartments are probably cell- and tissue-specific. It is important to point out that, although whole-cell NAD+, nicotinamide, and nicotinic acid measurements can monitor NAD+ metabolism at a gross level, knowledge of the metabolite concentrations in subcellular compartments, such as the nucleus, cytoplasm, and mitochondria, is crucial to gauge how NAD+ metabolism affects sirtuin and PARP functions at different cellular loci. Technical and experimental progress in this area is needed before it will be possible to describe just how NAD+ metabolism is coupled to NAD+-dependent signaling processes.
|
Pathways Involving Nicotinamide Riboside |
|---|
). The pathway leads from the metabolite nicotinamide riboside (NR), the dephosphorylated form of NMN. A highly biologically conserved nicotinamide riboside kinase is able to use NR as a substrate and can convert NR to NMN in cells (Bieganowski and Brenner, 2004). This activity allows NR to enter NAD+ metabolism via NMN and then to NAD+. Thus, NR is converted to NAD+ in only two metabolic steps (Figs. 2 and 3) (Bieganowski and Brenner, 2004). Yeast deficient in de novo or B3-recycling pathways but retaining an intact nmnat activity survive with NR as their only source of B3, indicating that yeast can efficiently salvage this nucleoside and synthesize adequate amounts of NAD+ via this pathway (Bieganowski and Brenner, 2004). In humans, two isoforms of the kinase (Nrk1 and Nrk2) have been cloned, although little is known about the biochemical properties of these enzymes. |
The Role of NAD in Energy Metabolism and Oxidation Processes |
|---|
). Each mole of NADH consumed by the mitochondria can furnish energy for the formation of three moles of ATP from 3 mol of ADP (Pollak et al., 2007). NADH formation in cells is tightly regulated and typically represents approximately 10% of total cellular NADH and NAD+ content (Williamson et al., 1967). NADH is highly depleted in the cytosol, where it typically represents less than 1% of the combined total of NADH and NAD+ that is not in complexation with proteins (Williamson et al., 1967). In the mitochondria, NADH represents approximately 15% of the dinucleotide content uncomplexed to proteins (Williamson et al., 1967; Tischler et al., 1977). The uncomplexed dinucleotides represent the cellular pool able to interact with unliganded proteins in cells (Williamson et al., 1967). As such, the relatively low concentrations of NADH available in uncomplexed form, particularly in the cytosol, suggest that direct NADH interactions with enzymes such as sirtuins (such as nuclear SIRT1) may be insufficient to explain changes in activity attributed to NADH/NAD+ ratio (Guarente, 2006).
The phosphorylated dinucleotide NADP+ in the reduced form plays important roles in biosynthesis. In contrast to NADH/NAD+, the uncomplexed and total (complexed and uncomplexed) NADPH/NADP ratios in cells are maintained high in the cytosolic and mitochondrial compartments (Tischler et al., 1977). This appears to stem from the importance of NADPH to biosynthesis and because NADPH provides several cell protective functions. For instance, NADPH is an important cofactor for P450 enzymes that detoxify xenobiotics (Pollak et al., 2007). In oxidative defense, NADPH acts as a terminal reductant for glutathione reductase, which maintains reduced glutathione. Enhanced formation of NADPH via up-regulation of glucose-6-phosphate dehydrogenase appears to increase reduced glutathione concentrations. Conversely, deletion of glucose-6-phosphate dehydrogenase causes increased sensitivity of cells to oxidative stress (Pollak et al., 2007). Finally, NADPH serves as a substrate for NADPH oxidase, which generates peroxides for release in oxidative burst processes of the immune system (Pollak et al., 2007).
|
Clinical Manifestations of Niacin Deficiency |
|---|
). However, poor diets, alcoholism, AIDS, and other diseases can cause niacin deficiency or pellagra. Symptoms of pellagra include dermatitis, dementia, and diarrhea (Revollo et al., 2007). Niacin deficiency is also associated with an increased risk of cancer (Kirkland, 2003) and has been shown to increase toxicity caused by reactive oxygen species (Pollak et al., 2007). |
Prospects for Drugs Targeted to Inhibition of NAD+ Biosynthesis |
|---|
). It is problematic that nicotinamide is relatively abundant in mammalian tissues (Qin et al., 2006; Yang and Sauve, 2006), and it is not certain how an inhibition of upstream steps of NAD+ biosynthesis would affect infectivity or virulence if recycling pathways in bacteria can biochemically salvage nicotinamide and nicotinic acid from the host. The effect of targeting this pathway on bacterial growth in a mammalian host is still undetermined.
Targeting nicotinamide/nicotinic acid recycling for antibiotics may be effective because some human pathogens (e.g., Borrelia burgdorferei, Plasmodium falciparum) do not seem to encode a de novo NAD+-biosynthetic pathway. In these cases, salvage of host nicotinamide and nicotinic acid pools to complete NAD+ biosynthesis is probably required for parasite viability. It is undetermined whether small molecule inhibition of nicotinamide recycling reduces virulence or infectivity in microbial infections, and to date, no potent inhibitors of nicotinamidases have been reported. On the other hand, genetic studies have validated the importance of nicotinamidases for infectivity in pathogens that cause human disease. B. burgdorferei (Purser et al., 2003) and Brucella abortus (Kim et al., 2004) have been shown to be less infective and less pathogenic if their nicotinamidase genes are deleted. In Leishmania infantum, nicotinamide is able to restrict growth in vitro (Sereno et al., 2005). It is interesting that deletion of nicotinamidase causes abnormally high nicotinamide levels in yeast (Anderson et al., 2003; Gallo et al., 2004; Sauve et al., 2005). Thus, disruption of nicotinamidase in L. infantum may have an antileishmanial effect if it causes elevated intracellular nicotinamide concentrations.
Because bacteria must use NAMN adenylation and NAD+ synthetase activity to complete both recycling and de novo pathways to NAD+ (with the exception of recycling NMN), it is likely that each of these two enzymes could be targeted for drug design with the prospect of antibiotic effects. These enzyme activities are essential for growth of most bacteria and have been identified as broad spectrum drug targets (Gerdes et al., 2002). With respect to the adenylating enzyme, humans require their own versions (nmnat-1, nmnat-2, and nmnat-3) in both recycling and de novo pathways. It is surprising that the sequence similarity of the human and bacterial enzymes is quite low, suggesting that small molecule inhibitors could be developed that are specific toward the bacterial forms (Gerdes et al., 2002). Finally, NAD+ synthetase activity is not required to recycle nicotinamide in humans, and its central role in recycling in microbes suggests that it may be an excellent target for antimicrobials. NAD+ synthetase inhibitors have proven antibiotic properties, killing Gram-positive bacteria (Velu et al., 2003).
Anticancer Agents
NAD+ metabolism plays a vital role in maintaining the genome, via PARPs and sirtuins, and proliferating cells appear to have higher demands for NAD+ biosynthesis and greater turnover of NAD+. The role of PARP as a protector of genomic stability has stimulated investigation of its inhibition as a way to make cancer cells more susceptible to genotoxicity (Virag and Szabo, 2002). Alternatively, compounds directed specifically to inhibition of human NAD metabolism have recently been developed. Specifically, an inhibitor of nampt/PBEF (FK-866) has recently been shown to have potent anticancer activity in cell culture and causes acute sensitivity to alkylating agents and increased apoptosis (Pogrebniak et al., 2006). It is currently in early clinical trials as an anticancer therapy.
|
Regulation of NAD in a Model Microbe: Yeast |
|---|
; Gallo et al., 2004). Conversely, pnc
strains exhibit defective gene silencing and decreased replicative lifespan (Anderson et al., 2003; Gallo et al., 2004). PNC1 expression levels are subject to transcriptional regulation in response to stress, and PNC1 regulates nicotinamide concentrations (Anderson et al., 2003; Gallo et al., 2004; Sauve et al., 2005). Nicotinamide is a potent sirtuin inhibitor, implying that sirtuin catalytic activity is regulated by nicotinamide concentrations in yeast, which are in turn controlled by PNC1 expression levels (Anderson et al., 2003). Stimuli that increase PNC1 expression also extend lifespan (Anderson et al., 2003; Gallo et al., 2004), an effect that can be reproduced genetically by overexpression of SIR2. Thus, changes in NAD+ metabolism represent a mechanism for regulating heterochromatin formation and lifespan mediated by sirtuins. |
Regulation of NAD+ Metabolism in Humans |
|---|
). For example, NAD+ concentrations in liver increase 30% with fasting (Guarente, 2006). Thus, NAD+ metabolism is dynamically regulated by organism nutrient intake and genotoxic stress. Changes in NAD+ metabolism are now thought to initiate signaling events coupled to sirtuins or other NAD+-consuming enzymes, such as PARPs, via concentration changes of NAD+ and its metabolites, such as NADH and nicotinamide (Guarente, 2006).
The mechanisms that regulate NAD+ biosynthesis in mammalian cells have recently come under increased investigation. Because the human genome does not encode a nicotinamidase, the regulation of NAD+ metabolism must be different from that of yeast. It is interesting that the nicotinamide-recycling enzyme, nampt/PBEF, is a likely regulator for both nicotinamide and NAD+ levels in cells. This enzyme is transcriptionally regulated in various conditions, and studies show that expression levels of nampt/PBEF are correlated to NAD+ concentrations in cultured cells (Revollo et al., 2007). The generality of nampt/PBEF as a determinant for NAD+ concentrations in tissues of the body and its role in activating signaling via sirtuins and other ADP-ribosyltransferases is still poorly determined to date. nampt/PBEF does up-regulate SIRT1 catalytic function in cultured cells (Revollo et al., 2007). It remains to be determined whether nampt/PBEF regulates NAD+ concentrations in liver where increased NAD+ concentrations associated with fasting stimulate SIRT1 and peroxisome proliferator-activated receptor 
coactivator 1-
-mediated gluconeogenesis (Guarente, 2006). In general, the mechanisms that alter human NAD+ metabolism probably include multiple processes, but the understandings of these mechanisms are currently very unclear and a considerable effort in this area is required before we know how NAD+ metabolism is controlled, how changes in NAD+ metabolism influence physiology, and how NAD+ metabolism might be manipulated for therapeutic benefit.
|
Pharmacology of NAD+ Increasing Agents |
|---|
). Nicotinamide at high doses has been reported to be protective of β-cell functions before the onset of type I diabetes. However, a large clinical study in Europe failed to show decreases in incidence of onset of type I diabetes with long-term nicotinamide dosing (Gale et al., 2004).
High doses of nicotinamide administered orally or through injection are transiently metabolized in liver to increase NAD+. However, nicotinamide at elevated doses can cause hepatotoxicity. Nicotinamide is methylated to form 1-methylnicotinamide and downstream oxidized pyridones as metabolic end products (Knip et al., 2000). Large doses of nicotinamide cause methyl donor depletion (Knip et al., 2000). A large portion of nicotinamide administered to rats at 500 mg/kg was excreted unchanged within 12 h after injection. The remainder of nicotinamide was generally excreted as methylated or oxidized forms of the pyridine (Knip et al., 2000). At nonpharmacologic doses, nicotinamide is lost, mostly by excretion of the catabolic products, rather than as the unmetabolized vitamin.
Nicotinic Acid
Nicotinic acid is widely used in high doses to lower serum cholesterol, and it also lowers serum triglyceride levels (Capuzzi et al., 2000; Kamanna and Kashyap, 2000). This effect is unique to nicotinic acid and is not observed with high-dose nicotinamide. The doses required typically cause uncomfortable flushing in immediate release formulations (Capuzzi et al., 2000; Kamanna and Kashyap, 2000). Slow release formulations of nicotinic acid have been developed, which provide less discomfort from flushing but retain the desired lipid-lowering effects (Capuzzi et al., 2000). Nicotinic acid is rapidly metabolized by the liver and can be catabolized by glycine conjugation to nicotinuric acid (Capuzzi et al., 2000). Nicotinic acid increases NAD+ content in liver but is generally no more effective than nicotinamide in this respect (Jackson et al., 1995), indicating that NAD+ biosynthesis in liver is not a likely explanation for nicotinic acid correction effects in hyperlipidemia.
The principle effects of nicotinic acid in lowering cholesterol have been proposed to stem from four basic causes: 1) inhibition of lipolysis in fat; 2) increased HDL levels; 3) lowering of serum lipoprotein-a; and 4) inhibition of synthesis and secretion of very low density lipoprotein in liver (Capuzzi et al., 2000). Some of nicotinic acids effects could be from a described interaction with the G protein HM74a (Capuzzi et al., 2000; Soudijn et al., 2007). This affinity was recently shown to be quite potent (100–200 nM); nicotinic acid binding antagonizes forskolin-mediated increase of cAMP production and inhibits lipolysis in differentiated 3T3L adipocytes (Capuzzi et al., 2000). The decrease in adipose lipolysis is hypothesized to limit liver uptake of free fatty acids, which reduces synthesis of very low density lipoprotein, intermediate density lipoprotein, and low density lipoproteins (Capuzzi et al., 2000). Nicotinic acid interferes with HDL-ApoA1-mediated uptake by hepatocytes, without interfering with uptake of cholesterol esters (Capuzzi et al., 2000). This inhibition of removal of HDL-ApoA1 has been proposed to increase cholesterol efflux from peripheral tissues (increased reverse cholesterol transport), mediated by an increased amount of HDL particles (Capuzzi et al., 2000). The relative importance of these mechanisms in explaining the beneficial effects of nicotinic acid, as well as the exact molecular mechanisms that explain these effects, are still under investigation. Nevertheless, it is known that nicotinic acid dose-response profiles are different for different serum lipotypes, suggesting different pharmacological mechanisms for the effects seen. Clinically, high-dose nicotinic acid leads to reduced lipidemias, reduced progression of coronary heart disease, and reduced mortality (Capuzzi et al., 2000; Kamanna and Kashyap, 2000).
|
Effects NA and NAM on NAD+ in Tissues |
|---|
). Both blood (packed red blood cells) and liver were responsive to increased dosages of nicotinamide or nicotinic acid, leading to increases of 40 to 60% in NAD+ content for both tissues for either B3. Smaller increases in NAD+ concentrations not exceeding 15% were observed for 1000 mg kg–1 doses of nicotinamide in heart, lung, and kidneys. These findings, on the one hand, appear to confirm that nampt/PBEF activity, which is responsible for recycling nicotinamide to NAD+, is typically not rate-limited by nicotinamide concentrations in some but not all tissues. In cell culture, nampt/PBEF controls NAD+ concentrations independent of exogenous nicotinamide concentrations (Revollo et al., 2004). nampt/PBEF has a very low Km for nicotinamide (<2 µM), suggesting that it is readily saturated by endogenous nicotinamide concentrations (Revollo et al., 2004). On the other hand, the ability of nicotinamide to stimulate NAD+ synthesis in liver and blood suggests that nicotinamide is convertible to alternative forms of B3 that ultimately increase nicotinamide bioavailability and/or that nicotinamide treatment causes cellular adaptations that lead to improved NAD+ biosynthesis. Why nicotinamide is efficiently utilized in some but not all tissues for NAD+ biosynthesis is currently unexplained.
Jackson et al. (1995) also showed that nicotinic acid increases NAD+ concentrations in liver and blood, similar to nicotinamide. In addition, NAD+ biosynthesis was increased in heart (50%) and kidney (100%) as well. These results show that nicotinic acid generally has a broader effect than nicotinamide for NAD+ increases in the body. These results also indicate that the Preiss-Handler pathway is typically operating below saturation in most tissues.
|
Genome Stability |
|---|
). Of particular importance in this respect is the involvement of PARP-1 as a DNA damage sensor, which cooperates in the DNA damage and repair process. The importance of NAD+ and PARP is highlighted by studies that show that vitamin B3 deficiency is associated with reduced tissue NAD+ concentrations and that a reduced ability of tissues to maintain poly-ADPR concentrations at normal levels (Boyonoski et al., 2002a). In the absence of toxins, B3-deficient bone marrow showed a 6.2-fold increase in micronucleus formation and a 2.8-fold increase in sister chromatid exchange (Boyonoski et al., 2002a). With DNA-damaging agents, animals show reduced ability to synthesize poly-ADPR in bone marrow and were more susceptible to the formation of DNA strand breaks, as measured by comet assay (Boyonoski et al., 2002a). B3 deficiency also results in reduced latency to leukemia in animals treated with ethylnitrosourea, which is used as a model for secondary carcinogenesis arising from chemotherapies (Henning et al., 1997). Conversely, pharmacological doses of nicotinamide or nicotinic acid supplementation increase NAD+ in bone marrow and also increase poly-ADPR levels (Boyonoski et al., 2002b). This latter result provides evidence against the idea that PARP-1 is efficiently inhibited by nicotinamide concentrations at high doses, as is widely assumed, because observed poly-ADPR levels in marrow and even liver were increased by nicotinamide and nicotinic acid similarly compared to controls. B3 treatments were able to retard ethylnitrosourea-induced carcinogenesis and led to increased lifespan for animals on a normal diet (Boyonoski et al., 2002b).
In vitro results indicate that PARP-1 inhibition leads to delayed DNA repair, particularly base excision repair (Hassa et al., 2006). Consistent with a role for PARP in DNA repair, PARP–/– animals exhibit hypersensitivity to alkylating agents and ionizing radiation (Hassa et al., 2006). Some data appear to indicate that a normal if not an augmented NAD+ level in tissues aids in DNA repair and may reduce carcinogenesis. Some hints that this may be true are found in epidemiological studies that show that PARP-1 activity levels are lower in families predisposed to cancer (Decker and Muller, 2002) and that some cancers are found to have reduced PARP activities (Decker and Muller, 2002). Another finding of interest is that PARP activity may be generally higher in long-lived people, suggesting that PARP activity levels may have an antiaging effect (Decker and Muller, 2002).
It is interesting that there is growing evidence that the body naturally adapts to genotoxic, hypoxic, and caloric restriction stress by increasing NAD+ biosynthesis. These evidences suggest that physiological responses to stress may be partly cued by increased NAD+ levels. Consistent with this view, sirtuin signaling has been shown to respond to increased physiological NAD+ concentrations (Guarente, 2006). Although increased vitamin B3 intake may seem beneficial, higher dosages of nicotinamide or nicotinic acid have undesirable side effects. In addition to hepatotoxicity, nicotinamide at high doses can adversely affect thymine biosynthesis and cause an increase of DNA damage caused by depleted thymidine levels in the cell (ApSimon et al., 1995).
|
Ischemia and Stroke |
|---|
). In turn, NAD+ must be resynthesized using ATP, PRPP, and other high-energy precursors. It is believed that the demands of resynthesizing NAD+ in large quantities place serious strains on energy resources in the cell, causing the cell to die from energy depletion (Hassa et al., 2006). Evidence that this model of cell death is considerably accurate has come from many sources. The data include observations that PARP–/– mice experience significantly reduced tissue damage in cerebral ischemia, with corresponding protection of NAD+ metabolism (Hassa et al., 2006). PARP inhibitors appear to have similar effects (Virag and Szabo, 2002). Nicotinamide, which is a micromolar inhibitor of PARP, is also protective. There remain questions about the mechanisms of action of nicotinamide in this respect, because some studies show that nicotinamide may not inhibit PARP activities, as determined by ADP-ribosyl polymer measurements (Boyonoski et al., 2002b). Nicotinamide effects may also play a role in enhancing NAD+ synthesis. Nevertheless, the effects of nicotinamide are distinctly more beneficial than those of nicotinic acid in ischemia models, suggestive of its effect as a PARP-1 inhibitor (Virag and Szabo, 2002). |
Nicotinamide in Fetal Ischemia and Fetal Alcohol Syndrome |
|---|
). From a medical perspective, this issue is an important one, because alcohol abuse is considered the leading cause of mental retardation in children (Ieraci and Herrera, 2006). Few interventions are known that mitigate this damage. Recent studies have looked at nicotinamide as a potential intervention as a means to protect the fetal brain cells during this developmental stage. In fetal mice whose mothers were treated with alcohol, a single nicotinamide treatment of the mother provided protection against oxidative stress markers in the fetal brain, such as lipid peroxidation, and also prevented apoptosis (Ieraci and Herrera, 2006). When assessed for behavior, offspring whose mothers were administered nicotinamide performed better in a number of tests for anxiety, a typical side effect of fetal alcohol syndrome in mice, than their nicotinamide-untreated controls (Ieraci and Herrera, 2006). Likewise, in fetal ischemia, nicotinamide treatment has been shown to prevent neural damage versus untreated controls, suggesting that nicotinamide could represent a reasonable intervention for early neuron damage during development (Feng et al., 2006). |
Alzheimer's Disease and Neurodegenerative Disorders |
|---|
). Of recent note, Milbrandt and co-workers showed that the process of axon degeneration, which occurs when an axon is severed, can be significantly slowed when NAD+ or other NAD+ precursors are present (Araki et al., 2004). Subsequent work by other laboratories has verified that NAD+ metabolism can protect severed neurons from degeneration. It is interesting that a mouse with slowed axon degeneration has a triplicate chimeric gene consisting of a partial ubiquitin ligase gene fused to a full nmnat-1 gene (Mack et al., 2001). nmnat-1 is nuclear-localized and couples NMN or NAMN with ATP to form NAD+ or nicotinic acid adenine dinucleotide, respectively (Fig. 3). Some controversy has emerged regarding the significance of the nmnat-1 biochemical activity in slowing axon degeneration, because nmnat-1 overexpression has not been shown to increase intracellular NAD+ concentrations (Mack et al., 2001) and overexpression of nmnat-1 does not reproduce the slow degeneration phenotype in a transgenic mouse that overexpressed this enzyme (Conforti et al., 2007). Nevertheless, NAD+ appears to be protective to neural cells, and it has been reported that NAD+, NR, NMN, nicotinamide, and NA all protect neurons under different conditions in cell culture experiments.
Chronic disease states such as Parkinson's and Alzheimer's, are still somewhat poorly understood. Nevertheless, recent evidence is starting to suggest that chronic neurodegenerative disorders affect NAD+ metabolism adversely and may respond favorably to interventions that target NAD+ metabolism. For example, it has been known that Parkinson's disease results in increased methylnicotinamide excretion, suggesting enhanced NAD+ breakdown. Recently, we participated in a study in which NAD metabolism was examined in transgenic mice that have a gene encoding a human amyloid precursor protein (APP). These animals develop some neuropathology of Alzheimer's disease, such as plaque formation. Upon assay of brain tissue, NAD+ levels were decreased, and nicotinamide levels were increased in animals affected severely by disease who were on normal diets compared with animals on calorie restriction diets where the neuropathology was less severe (Qin et al., 2006). NAD+ itself was implicated in mitigating disease, and exogenous NAD+ redirected how cells process amyloid precursor protein (Qin et al., 2006). It was shown that NAD+-treated cells produced less plaque-associated forms of processed APP (Aβ) through a mechanism involving up-regulation of -secretase, which cleaves APP competitively with β- and -secretases preventing Aβ formation (Qin et al., 2006). Increased sirtuin (SIRT1) catalytic activity was also implicated in mediating the enhanced protection from neuropathology in cell culture and in mouse brains (Qin et al., 2006). SIRT1 is transcriptionally up-regulated in neurons by calorie restriction and is activated directly by NAD+. Although the work in the area of vitamin B3 effects in neurodegenerative disorders is still very preliminary, it invites the question of how NAD+ metabolism affects long-term neurodegenerative processes and whether enhancements/modulations to NAD+ metabolism can provide therapeutically meaningful changes in long-term outcomes in these notoriously difficult to treat diseases.
|
Conclusions |
|---|
|
Footnotes |
|---|
A.S. is a consultant for Sirtris Pharmaceuticals and has financial interests related to some of the topics discussed in this review.
Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
ABBREVIATIONS: ADPR, adenosine diphosphate ribose; APP, amyloid precursor protein; NAMN, nicotinic acid mononucleotide; nampt, nicotinamide phosphoribosyltransferase; NMN, nicotinamide mononucleotide; nmnat, nicotinamide/nicotinate mononucleotide adenylyltransferase; NR, nicotinamide riboside; PARP, poly(ADP-ribose) polymerase; PBEF, pre-B-cell colony-enhancing factor; PRPP, 5-phosphoryl-ribose-1-pyrophosphate; QA, quinolinic acid; Sir2, silencing information regulator 2; HDL, high-density lipoprotein; SIRT1, mammalian sirtuin 1; FK-866, (E)-N-[4-(1-benzoylpiperidine-4-yl)butyl]-3-(pyridin-3-yl)acrylamide.
Address correspondence to: Anthony A. Sauve, Department of Pharmacology, Weill Medical College of Cornell University, 1300 York Avenue, New York, NY 10021. E-mail: aas2004{at}med.cornell.edu
|
References |
|---|
Anderson RM, Bitterman KJ, Wood JG, Medvedik O, and Sinclair DA (2003) Nicotinamide and PNC1 govern lifespan extension by calorie restriction in Saccharomyces cerevisiae. Nature 423: 181–185.[CrossRef][Medline]
ApSimon MM, Rawling JM, and Kirkland JB (1995) Nicotinamide megadosing increases hepatic poly(ADP-ribose) levels in choline-deficient rats. J Nutr 125: 1826–1832.
Araki T, Sasaki Y, and Milbrandt J (2004) Increased nuclear NAD biosynthesis and SIRT1 activation prevent axonal degeneration. Science 305: 1010–1013.
Belenky P, Bogan KL, and Brenner C (2007) NAD+ metabolism in health and disease. Trends Biochem Sci 32: 12–19.[CrossRef][Medline]
Berger F, Lau C, Dahlmann M, and Ziegler M (2005) Subcellular compartmentation and differential catalytic properties of the three human nicotinamide mononucleotide adenylyltransferase isoforms. J Biol Chem 280: 36334–36341.
Berger F, Lau C, and Ziegler M (2007) Regulation of poly(ADP-ribose) polymerase 1 activity by the phosphorylation state of the nuclear NAD biosynthetic enzyme NMN adenylyl transferase 1. Proc Natl Acad Sci U S A 104: 3765–3770.
Bieganowski P and Brenner C (2004) Discoveries of nicotinamide riboside as a nutrient and conserved NRK genes establish a Preiss-Handler independent route to NAD+ in fungi and humans. Cell 117: 495–502.[CrossRef][Medline]
Blander G and Guarente L (2004) The Sir2 family of protein deacetylases. Annu Rev Biochem 73: 417–435.[CrossRef][Medline]
Boyonoski AC, Spronck JC, Gallacher LM, Jacobs RM, Shah GM, Poirier GG, and Kirkland JB (2002a) Niacin deficiency decreases bone marrow poly(ADP-ribose) and the latency of ethylnitrosourea-induced carcinogenesis in rats. J Nutr 132: 108–114.
Boyonoski AC, Spronck JC, Jacobs RM, Shah GM, Poirier GG, and Kirkland JB (2002b) Pharmacological intakes of niacin increase bone marrow poly(ADP-ribose) and the latency of ethylnitrosourea-induced carcinogenesis in rats. J Nutr 132: 115–120.
Capuzzi DM, Morgan JM, Brusco OA Jr, and Intenzo CM (2000) Niacin dosing: relationship to benefits and adverse effects. Curr Atheroscler Rep 2: 64–71.[Medline]
Carlson LA (2004) Niaspan, the prolonged release preparation of nicotinic acid (niacin), the broad-spectrum lipid drug. Int J Clin Pract 58: 706–713.[CrossRef][Medline]
Conforti L, Fang G, Beirowski B, Wang MS, Sorci L, Asress S, Adalbert R, Silva A, Bridge K, Huang XP, et al. (2007) NAD(+) and axon degeneration revisited: nmnat1 cannot substitute for Wld(S) to delay Wallerian degeneration. Cell Death Differ 14: 116–127.[CrossRef][Medline]
Decker P and Muller S (2002) Modulating poly (ADP-ribose) polymerase activity: potential for the prevention and therapy of pathogenic situations involving DNA damage and oxidative stress. Curr Pharm Biotechnol 3: 275–283.[CrossRef][Medline]
Di Lisa F, Menabo R, Canton M, Barile M, and Bernardi P (2001) Opening of the mitochondrial permeability transition pore causes depletion of mitochondrial and cytosolic NAD+ and is a causative event in the death of myocytes in postischemic reperfusion of the heart. J Biol Chem 276: 2571–2575.
Di Lisa F and Ziegler M (2001) Pathophysiological relevance of mitochondria in NAD(+) metabolism. FEBS Lett 492: 4–8.[CrossRef][Medline]
Feng Y, Paul IA, and LeBlanc MH (2006) Nicotinamide reduces hypoxic ischemic brain injury in the newborn rat. Brain Res Bull 69: 117–122.[CrossRef][Medline]
Foster JW, Kinney DM, and Moat AG (1979) Pyridine nucleotide cycle of Salmonella typhimurium: isolation and characterization of pncA, pncB, and pncC mutants and utilization of exogenous nicotinamide adenine dinucleotide. J Bacteriol 137: 1165–1175.
Fukuwatari T, Ohta M, Kimtjra N, Sasaki R, and Shibata K (2004) Conversion ratio of tryptophan to niacin in Japanese women fed a purified diet conforming to the Japanese Dietary Reference Intakes. J Nutr Sci Vitaminol (Tokyo) 50: 385–391.[Medline]
Gale EA, Bingley PJ, Emmett CL, and Collier T (2004) European Nicotinamide Diabetes Intervention Trial (ENDIT): a randomised controlled trial of intervention before the onset of type 1 diabetes. Lancet 363: 925–931.[CrossRef][Medline]
Gallo CM, Smith DL Jr, and Smith JS (2004) Nicotinamide clearance by Pnc1 directly regulates Sir2-mediated silencing and longevity. Mol Cell Biol 24: 1301–1312.
Gerdes SY, Scholle MD, D'Souza M, Bernal A, Baev MV, Farrell M, Kurnasov OV, Daugherty MD, Mseeh F, Polanuyer BM, et al. (2002) From genetic footprinting to antimicrobial drug targets: examples in cofactor biosynthetic pathways. J Bacteriol 184: 4555–4572.
Graham GG (1993) Starvation in the modern world. N Engl J Med 328: 1058–1061.
Guarente L (2006) Sirtuins as potential targets for metabolic syndrome. Nature 444: 868–874.[CrossRef][Medline]
Hassa PO, Haenni SS, Elser M, and Hottiger MO (2006) Nuclear ADP-ribosylation reactions in mammalian cells: where are we today and where are we going? Microbiol Mol Biol Rev 70: 789–829.
Henning SM, Swendseid ME, and Coulson WF (1997) Male rats fed methyl- and folate-deficient diets with or without niacin develop hepatic carcinomas associated with decreased tissue NAD concentrations and altered poly(ADP-ribose) polymerase activity. J Nutr 127: 30–36.
Ieraci A and Herrera DG (2006) Nicotinamide protects against ethanol-induced apoptotic neurodegeneration in the developing mouse brain. PLoS Med 3: e101.[CrossRef][Medline]
Ijichi H, Ichiyama A, and Hayaishi O (1966) Studies on the biosynthesis of nicotinamide adenine dinucleotide. 3. Comparative in vivo studies on nicotinic acid, nicotinamide, and quinolinic acid as precursors of nicotinamide adenine dinucleotide. J Biol Chem 241: 3701–3707.
Jackson TM, Rawling JM, Roebuck BD, and Kirkland JB (1995) Large supplements of nicotinic acid and nicotinamide increase tissue NAD+ and poly(ADP-ribose) levels but do not affect diethylnitrosamine-induced altered hepatic foci in Fischer-344rats. J Nutr 125: 1455–1461.
Kamanna VS and Kashyap ML (2000) Mechanism of action of niacin on lipoprotein metabolism. Curr Atheroscler Rep 2: 36–46.[Medline]
Kim S, Kurokawa D, Watanabe K, Makino S, Shirahata T, and Watarai M (2004) Brucella abortus nicotinamidase (PncA) contributes to its intracellular replication and infectivity in mice. FEMS Microbiol Lett 234: 289–295.[CrossRef][Medline]
Kirkland JB (2003) Niacin and carcinogenesis. Nutr Cancer 46: 110–118.[CrossRef][Medline]
Knip M, Douek IF, Moore WP, Gillmor HA, McLean AE, Bingley PJ, and Gale EA (2000) Safety of high-dose nicotinamide: a review. Diabetologia 43: 1337–1345.[CrossRef][Medline]
Kurnasov O, Goral V, Colabroy K, Gerdes S, Anantha S, Osterman A, and Begley TP (2003) NAD biosynthesis: identification of the tryptophan to quinolinate pathway in bacteria. Chem Biol 10: 1195–1204.[CrossRef][Medline]
Mack TG, Reiner M, Beirowski B, Mi W, Emanuelli M, Wagner D, Thomson D, Gillingwater T, Court F, Conforti L, et al. (2001) Wallerian degeneration of injured axons and synapses is delayed by a Ube4b/Nmnat chimeric gene. Nat Neurosci 4: 1199–1206.[Medline]
Magni G, Amici A, Emanuelli M, Orsomando G, Raffaelli N, and Ruggieri S (2004a) Enzymology of NAD+ homeostasis in man. Cell Mol Life Sci 61: 19–34.[CrossRef][Medline]
Magni G, Amici A, Emanuelli M, Orsomando G, Raffaelli N, and Ruggieri S (2004b) Structure and function of nicotinamide mononucleotide adenylyltransferase. Curr Med Chem 11: 873–885.[CrossRef][Medline]
Pogrebniak A, Schemainda I, Azzam K, Pelka-Fleischer R, Nussler V, and Hasmann M (2006) Chemopotentiating effects of a novel NAD biosynthesis inhibitor, FK866, in combination with antineoplastic agents. Eur J Med Res 11: 313–321.[Medline]
Pollak N, Dolle C, and Ziegler M (2007) The power to reduce: pyridine nucleotides–small molecules with a multitude of functions. Biochem J 402: 205–218.[CrossRef][Medline]
Purser JE, Lawrenz MB, Caimano MJ, Howell JK, Radolf JD, and Norris SJ (2003) A plasmid-encoded nicotinamidase (PncA) is essential for infectivity of Borrelia burgdorferi in a mammalian host. Mol Microbiol 48: 753–764.[CrossRef][Medline]
Qin W, Yang T, Ho L, Zhao Z, Wang J, Chen L, Zhao W, Thiyagarajan M, MacGrogan D, Rodgers JT, et al. (2006) Neuronal SIRT1 activation as a novel mechanism underlying the prevention of Alzheimer disease amyloid neuropathology by calorie restriction. J Biol Chem 281: 21745–21754.
Raffaelli N, Sorci L, Amici A, Emanuelli M, Mazzola F, and Magni G (2002) Identification of a novel human nicotinamide mononucleotide adenylyltransferase. Biochem Biophys Res Commun 297: 835–840.[CrossRef][Medline]
Revollo JR, Grimm AA, and Imai S (2004) The NAD biosynthesis pathway mediated by nicotinamide phosphoribosyltransferase regulates Sir2 activity in mammalian cells. J Biol Chem 279: 50754–50763.
Revollo JR, Grimm AA, and Imai S (2007) The regulation of nicotinamide adenine dinucleotide biosynthesis by Nampt/PBEF/visfatin in mammals. Curr Opin Gastroenterol 23: 164–170.[Medline]
Rongvaux A, Shea RJ, Mulks MH, Gigot D, Urbain J, Leo O, and Andris F (2002) Pre-B-cell colony-enhancing factor, whose expression is up-regulated in activated lymphocytes, is a nicotinamide phosphoribosyltransferase, a cytosolic enzyme involved in NAD biosynthesis. Eur J Immunol 32: 3225–3234.[CrossRef][Medline]
Sauve AA, Moir RD, Schramm VL, and Willis IM (2005) Chemical activation of Sir2-dependent silencing by relief of nicotinamide inhibition. Mol Cell 17: 595–601.[CrossRef][Medline]
Sauve AA, Wolberger C, Schramm VL, and Boeke JD (2006) The biochemistry of sirtuins. Annu Rev Biochem 75: 435–465.[CrossRef][Medline]
Schachter M (2005) Strategies for modifying high-density lipoprotein cholesterol: a role for nicotinic acid. Cardiovasc Drugs Ther 19: 415–422.[CrossRef][Medline]
Schweiger M, Hennig K, Lerner F, Niere M, Hirsch-Kauffmann M, Specht T, Weise C, Oei SL, and Ziegler M (2001) Characterization of recombinant human nicotinamide mononucleotide adenylyl transferase (NMNAT), a nuclear enzyme essential for NAD synthesis. FEBS Lett 492: 95–100.[CrossRef][Medline]
Sereno D, Alegre AM, Silvestre R, Vergnes B, and Ouaissi A (2005) In vitro anti-leishmanial activity of nicotinamide. Antimicrob Agents Chemother 49: 808–812.
Soudijn W, van Wijngaarden I, and Ijzerman AP (2007) Nicotinic acid receptor subtypes and their ligands. Med Res Rev 27: 417–433.[CrossRef][Medline]
Tischler ME, Friedrichs D, Coll K, and Williamson JR (1977) Pyridine nucleotide distributions and enzyme mass action ratios in hepatocytes from fed and starved rats. Arch Biochem Biophys 184: 222–236.[CrossRef][Medline]
Velu SE, Cristofoli WA, Garcia GJ, Brouillette CG, Pierson MC, Luan CH, DeLucas LJ, and Brouillette WJ (2003) Tethered dimers as NAD synthetase inhibitors with antibacterial activity. J Med Chem 46: 3371–3381.[CrossRef][Medline]
Virag L and Szabo C (2002) The therapeutic potential of poly(ADP-ribose) polymerase inhibitors. Pharmacol Rev 54: 375–429.
Wilkinson A, Day J, and Bowater R (2001) Bacterial DNA ligases. Mol Microbiol 40: 1241–1248.[CrossRef][Medline]
Williamson DH, Lund P, and Krebs HA (1967) The redox state of free nicotinamide-adenine dinucleotide in the cytoplasm and mitochondria of rat liver. Biochem J 103: 514–526.[Medline]
Yang H, Lavu S, and Sinclair DA (2006) Nampt/PBEF/Visfatin: a regulator of mammalian health and longevity? Exp Gerontol 41: 718–726.[CrossRef][Medline]
Yang T and Sauve AA (2006) NAD metabolism and sirtuins: metabolic regulation of protein deacetylation in stress and toxicity. AAPS J 8: E632–E643.[CrossRef][Medline]
This article has been cited by other articles:
|
|
 |
|
  L. Formentini, A. Macchiarulo, G. Cipriani, E. Camaioni, E. Rapizzi, R. Pellicciari, F. Moroni, and A. Chiarugi Poly(ADP-ribose) Catabolism Triggers AMP-dependent Mitochondrial Energy Failure J. Biol. Chem., June 26, 2009; 284(26): 17668 - 17676. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S.-P. Lu, M. Kato, and S.-J. Lin Assimilation of Endogenous Nicotinamide Riboside Is Essential for Calorie Restriction-mediated Life Span Extension in Saccharomyces cerevisiae J. Biol. Chem., June 19, 2009; 284(25): 17110 - 17119. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
V. Balan, G. S. Miller, L. Kaplun, K. Balan, Z.-Z. Chong, F. Li, A. Kaplun, M. F. A. VanBerkum, R. Arking, D. C. Freeman, et al. Life Span Extension and Neuronal Cell Protection by Drosophila Nicotinamidase J. Biol. Chem., October 10, 2008; 283(41): 27810 - 27819. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 |
 |
 |
|
 |
 |
 |
Top
Abstract
NAD+ Metabolism
