Regulation of intracellular levels of NAD: a novel role for CD38

CD38 mediates the intracellular ATP levels and cell survival of C6 glioma cells

CD38 is a multifunctional enzyme that can act as a NADase, generate cyclic adenosine diphosphate-ribose (cADPR) - a key Ca²⁺-mobilizing second messenger - and transport cADPR into cells. There have only been a small number of studies on the functions of CD38 in the central nervous system. In this study, we applied CD38 small interfering RNA (siRNA) to determine the effects of decreased CD38 on the intracellular ATP levels and survival of C6 glioma cells. Our study showed that both CD38 siRNA and 8-bromo-cADPR - a ryanodine receptor antagonist - can lead to a significant increase in the intracellular ATP levels of C6 glioma cells. We further found that the siRNA-induced CD38 reductions can induce decreases in the number of surviving cells. Our study has also suggested that P2X receptors contribute toward the CD38 siRNA-induced decreases in the cell survival. In summary, our study has provided novel evidence suggesting that CD38 mediates both the intracellular ATP levels and the survival of C6 glioma cells, suggesting that CD38 may become a therapeutic target for gliomas.

Cyclic adenosine 5'-diphosphate ribose analogs without a "southern" ribose inhibit ADP-ribosyl cyclase-hydrolase CD38

Cyclic adenosine 5'-diphosphate ribose (cADPR) analogs based on the cyclic inosine 5'-diphosphate ribose (cIDPR) template were synthesized by recently developed stereo- and regioselective N1-ribosylation. Replacing the base N9-ribose with a butyl chain generates inhibitors of cADPR hydrolysis by the human ADP-ribosyl cyclase CD38 catalytic domain (shCD38), illustrating the nonessential nature of the "southern" ribose for binding. Butyl substitution generally improves potency relative to the parent cIDPRs, and 8-amino-N9-butyl-cIDPR is comparable to the best noncovalent CD38 inhibitors to date (IC50 = 3.3 μM). Crystallographic analysis of the shCD38:8-amino-N9-butyl-cIDPR complex to a 2.05 Å resolution unexpectedly reveals an N1-hydrolyzed ligand in the active site, suggesting that it is the N6-imino form of cADPR that is hydrolyzed by CD38. While HPLC studies confirm ligand cleavage at very high protein concentrations, they indicate that hydrolysis does not occur under physiological concentrations. Taken together, these analogs confirm that the "northern" ribose is critical for CD38 activity and inhibition, provide new insight into the mechanism of cADPR hydrolysis by CD38, and may aid future inhibitor design.

PARP and other prospective targets for poisoning cancer cell metabolism

Increasing evidence indicates that cancer cells rewire their metabolism during tumorigenesis. The high intracellular levels of lactate and reactive oxygen species (ROS) generated during enhanced aerobic glycolysis and mitochondrial oxidative phosphorylation respectively led to oxidative stress. The detoxification of these accumulating metabolites and the equilibrium between reduced and oxidized nicotine adenine dinucleotide (NADH and NAD(+)) are two prominent mechanisms regulating redox status and hence energy homeostasis in tumors. Targeting both processes may thus be selectively cytotoxic for cancer cells. In this context, the impact of poly(ADP-ribose) polymerase (PARP) inhibitors, a class of anticancer agents employed for the treatment of DNA repair deficient tumors, on energy homeostasis and mitochondrial respiration regulation has potential clinical implications. Here we provide an overview of the metabolic reprogramming occurring in cancer cells and discuss the translational perspectives of targeting tumor metabolism and redox balance for antineoplastic therapy.

ATRA-induced cellular differentiation and CD38 expression inhibits acquisition of BCR-ABL mutations for CML acquired resistance

Acquired resistance through genetic mutations is a major obstacle in targeted cancer therapy, but the underlying mechanisms are poorly understood. Here we studied mechanisms of acquired resistance of chronic myeloid leukemia (CML) to tyrosine kinase inhibitors (TKIs) by examining genome-wide gene expression changes in KCL-22 CML cells versus their resistant KCL-22M cells that acquire T315I BCR-ABL mutation following TKI exposure. Although T315I BCR-ABL is sufficient to confer resistance to TKIs in CML cells, surprisingly we found that multiple drug resistance pathways were activated in KCL-22M cells along with reduced expression of a set of myeloid differentiation genes. Forced myeloid differentiation by all-trans-retinoic acid (ATRA) effectively blocked acquisition of BCR-ABL mutations and resistance to the TKIs imatinib, nilotinib or dasatinib in our previously described in vitro models of acquired TKI resistance. ATRA induced robust expression of CD38, a cell surface marker and cellular NADase. High levels of CD38 reduced intracellular nicotinamide adenine dinucleotide (NAD+) levels and blocked acquired resistance by inhibiting the activity of the NAD+-dependent SIRT1 deacetylase that we have previously shown to promote resistance in CML cells by facilitating error-prone DNA damage repair. Consequently, ATRA treatment decreased DNA damage repair and suppressed acquisition of BCR-ABL mutations. This study sheds novel insight into mechanisms underlying acquired resistance in CML, and suggests potential benefit of combining ATRA with TKIs in treating CML, particularly in advanced phases.

NAD+ and sirtuins in aging and disease

Nicotinamide adenine dinucleotide (NAD(+)) is a classical coenzyme mediating many redox reactions. NAD(+) also plays an important role in the regulation of NAD(+)-consuming enzymes, including sirtuins, poly-ADP-ribose polymerases (PARPs), and CD38/157 ectoenzymes. NAD(+) biosynthesis, particularly mediated by nicotinamide phosphoribosyltransferase (NAMPT), and SIRT1 function together to regulate metabolism and circadian rhythm. NAD(+) levels decline during the aging process and may be an Achilles' heel, causing defects in nuclear and mitochondrial functions and resulting in many age-associated pathologies. Restoring NAD(+) by supplementing NAD(+) intermediates can dramatically ameliorate these age-associated functional defects, counteracting many diseases of aging, including neurodegenerative diseases. Thus, the combination of sirtuin activation and NAD(+) intermediate supplementation may be an effective antiaging intervention, providing hope to aging societies worldwide.

Critical role for NAD glycohydrolase in regulation of erythropoiesis by hematopoietic stem cells through control of intracellular NAD content

NAD glycohydrolases (NADases) catalyze the hydrolysis of NAD to ADP-ribose and nicotinamide. Although many members of the NADase family, including ADP-ribosyltransferases, have been cloned and characterized, the structure and function of NADases with pure hydrolytic activity remain to be elucidated. Here, we report the structural and functional characterization of a novel NADase from rabbit reticulocytes. The novel NADase is a glycosylated, glycosylphosphatidylinositol-anchored cell surface protein exclusively expressed in reticulocytes. shRNA-mediated knockdown of the NADase in bone marrow cells resulted in a reduction of erythroid colony formation and an increase in NAD level. Furthermore, treatment of bone marrow cells with NAD, nicotinamide, or nicotinamide riboside, which induce an increase in NAD content, resulted in a significant decrease in erythroid progenitors. These results indicate that the novel NADase may play a critical role in regulating erythropoiesis of hematopoietic stem cells by modulating intracellular NAD.

Mapping NAD(+) metabolism in the brain of ageing Wistar rats: potential targets for influencing brain senescence

Over the last decade, the importance of NAD(+) has expanded beyond its role as an essential cofactor for energy metabolism. NAD(+) has emerged as a major signalling molecule that serves as the sole substrate for several enzymatic reactions including the DNA repair enzyme, poly(ADP-ribose) polymerase (PARP), NAD-dependent protein deacetylases or CD38, and transcriptional factors by a new class of histone deacetylases known as sirtuins. NAD(+) levels are regulated by the metabolic status and cellular stress caused by oxidative stress and DNA damage. Since a detailed study of NAD(+) metabolism in the healthy ageing mammalian brain is nascent, we examined the effect of ageing on intracellular NAD(+) metabolism in different brain regions in female Wistar rats in young (3 months), middle aged (12 months) and older adults (24 months). Our results are the first to show a significant decline in intracellular NAD(+) levels and NAD:NADH ratio with ageing in the CNS, occurring in parallel to an increase in lipid peroxidation and protein oxidation (o- and m-tyrosine) and a decline in total antioxidant capacity. Hyperphosphorylation of H2AX levels was also observed together with increased PARP-1 and PARP-2 expression, and CD38 activity, concomitantly with reduced NAD(+) and ATP levels and SIRT1 function in the cortex, brainstem, hippocampus and cerebellum. Reduced activity of mitochondrial complex I-IV and impaired maximum mitochondrial respiration rate were also observed in the ageing rat brain. Among the multiple physiological pathways associated with NAD(+) catabolism, our discovery of CD38 as the major regulator of cellular NAD(+) levels in rat neurons indicates that CD38 is a promising therapeutic target for the treatment of age-related neurodegenerative diseases.

Overexpression of CD38 decreases cellular NAD levels and alters the expression of proteins involved in energy metabolism and antioxidant defense

Nicotinamide adenine dinucleotide (NAD) is a coenzyme found in all living cells and mediates multiple cellular signaling pathways. In the present study, a 35% decrease of cellular NAD level is achieved by stable expression of the N-terminal truncated CD38, a NAD hydrolase. CD38-expressing (CD38(+)) cells have the lower growth rate and are more susceptive to oxidative stress than the wild type cells and empty vector-transfected (CD38(-)) cells. Quantitative proteomic analysis shows that 178 proteins are down-regulated in CD38(+) cells, which involve in diverse cellular processes including glycolysis, RNA processing and protein synthesis, antioxidant, and DNA repair. Down regulation of six selected proteins is confirmed by Western blotting. However, down-regulation of mRNA expressions of genes associated with glycolysis, antioxidant, and DNA repair is less significant than the corresponding change in protein expression, suggesting the low NAD level impairs the protein translational machinery in CD38(+) cells. Down-regulation of antioxidant protein and DNA-repair protein expression contributes to the susceptibility of CD38(+) cells to oxidative stress. Taken together, these results demonstrate that CD38(+) cells are a useful model to study effects of the cellular NAD levels on cellular processes and establish a new linker between cellular NAD levels and oxidative stress.

Roles of NAD (+) , PARP-1, and Sirtuins in Cell Death, Ischemic Brain Injury, and Synchrotron Radiation X-Ray-Induced Tissue Injury

NAD(+) plays crucial roles in a variety of biological processes including energy metabolism, aging, and calcium homeostasis. Multiple studies have also shown that NAD(+) administration can profoundly decrease oxidative cell death and ischemic brain injury. A number of recent studies have further indicated that NAD(+) administration can decrease ischemic brain damage, traumatic brain damage and synchrotron radiation X-ray-induced tissue injury by such mechanisms as inhibiting inflammation, decreasing autophagy, and reducing DNA damage. Our latest study that applies nano-particles as a NAD(+) carrier has also provided first direct evidence demonstrating a key role of NAD(+) depletion in oxidative stress-induced ATP depletion. Poly(ADP-ribose) polymerase-1 (PARP-1) and sirtuins are key NAD(+)-consuming enzymes that mediate multiple biological processes. Recent studies have provided new information regarding PARP-1 and sirtuins in cell death, ischemic brain damage and synchrotron radiation X-ray-induced tissue damage. These findings have collectively supported the hypothesis that NAD(+) metabolism, PARP-1 and sirtuins play fundamental roles in oxidative stress-induced cell death, ischemic brain injury, and radiation injury. The findings have also supported "the Central Regulatory Network Hypothesis", which proposes that a fundamental network that consists of ATP, NAD(+) and Ca(2+) as its key components is the essential network regulating various biological processes.

Properties of human blood monocytes. II. Monocytes from healthy adults are highly heterogeneous within and among individuals

BACKGROUND

Human blood monocytes are known to include subsets defined by the expression of CD14 and CD16 but otherwise are often assumed to be relatively homogeneous. However, we had observed additional heterogeneity that led us to a more extensive examination of monocytes.

METHODS

Blood samples from 200 healthy adults without known immunological abnormalities were examined by analysis with a hematology analyzer and by flow cytometry (FCM) to determine leukocyte differential counts, to identify subsets and to measure expression of monocyte-associated molecules.

RESULTS

The estimated cell counts of monocytes, neutrophils, total lymphocytes, and T cells all varied to a similar extent, that is, ±30-35%. The fractions of monocyte subsets defined by CD14 and CD16 or by CD163 expression also varied among individuals. FCM examinations showed that all the monocyte-associated molecules that were examined varied in expression in this increasing order-CD244, CD4, CD38, CD91, CD11b, toll-like receptor 2 (TLR2), TIA-1, CD14 (on CD14(Br+) cells), CD86, CD80, HLA-DQ, CD33, and HLA-DR.

CONCLUSIONS

Human blood monocytes are heterogeneous among healthy adults with respect to cell counts, subsets, and the levels of expression of monocyte-associated molecules. An increase in the "non-classical" (CD14(Lo/Neg) /CD16(+) ) monocyte subset or in the expression of CD11b or TLR2 have known diagnostic/prognostic implications. CD244 and CD4 have well-defined functions on lymphocytes but perform unknown activities on monocytes although their expression appears more narrowly controlled. Together, these data suggest that monocytes should be more extensively examined in both clinical and basic contexts.

Regulation of SIRT1 by microRNAs

Sirtuin 1 (SIRT1) is an NAD(+)-dependent deacetylase that connects cellular energy levels to homeostatic responses by deacetylating and modulating the activities of many transcriptional regulators. Discovered as a longevity protein in yeast, the mammalian SIRT1 has been intensively studied because of its great potential as a therapeutic target to benefit human health by preventing and improving many age-related diseases. There has been, therefore, substantial interest in developing agents that upregulate SIRT1 expression and activity. SIRT1 is regulated at multiple levels, including post-transcriptionally by microRNAs (miRs), powerful regulators of diverse biological pathways. Here we discuss how expression and activity of SIRT1 and other sirtuins are inhibited by miRs and further discuss the therapeutic potential of targeting miRs for age-related diseases that involve SIRT1 dysfunction, focusing on obesityrelated diseases.

Targeting of NAD metabolism in pancreatic cancer cells: potential novel therapy for pancreatic tumors

PURPOSE

Here, we describe a novel interplay between NAD synthesis and degradation involved in pancreatic tumor growth.

EXPERIMENTAL DESIGN

We used human pancreatic cancer cells, both in vitro (cell culture experiments) and in vivo (xenograft experiments), to demonstrate the role of NAD synthesis and degradation in tumor cell metabolism and growth.

RESULTS

We demonstrated that pharmacologic and genetic targeting of Nampt, the key enzyme in the NAD salvage synthesis pathway, inhibits cell growth and survival of pancreatic cancer cells. These changes were accompanied by a reduction of NAD levels, glycolytic flux, lactate production, mitochondrial function, and levels of ATP. The massive reduction in overall metabolic activity induced by Nampt inhibition was accompanied by a dramatic decrease in pancreatic tumor growth. The results of the mechanistic experiments showed that neither the NAD-dependent enzymes PARP-1 nor SIRT1 play a significant role on the effect of Nampt inhibition on pancreatic cancer cells. However, we identified a role for the NAD degradation pathway mediated by the NADase CD38 on the sensitivity to Nampt inhibition. The responsiveness to Nampt inhibition is modulated by the expression of CD38; low levels of this enzyme decrease the sensitivity to Nampt inhibition. In contrast, its overexpression decreased cell growth in vitro and in vivo, and further increased the sensitivity to Nampt inhibition.

CONCLUSIONS

Our study demonstrates that NAD metabolism is essential for pancreatic cancer cell survival and proliferation and that targeting NAD synthesis via the Nampt pathway could lead to novel therapeutic treatments for pancreatic cancer.

Protein lysine acetylation guards metabolic homeostasis to fight against cancer

Properly coordinated metabolism and maintained metabolite homeostasis are important because altered metabolite homeostasis has a causal role in many human diseases, including cancer. Metabolite homeostasis is maintained by fine-tuned coordination of metabolite generation and utilization. Metabolite deregulation has recently been shown to alter the signaling pathways and reprogram epigenetic factors associated with tumorigenesis. Protein lysine acetylation is emerging as a metabolism-coordinating mechanism. Mechanistic studies have shown that acetylation may have roles in nutrient adaptation and in maintaining metabolite homeostasis by exerting regulatory effects on metabolic enzymes, metabolic pathways and metabolic networks. Here we review recent progress in the determination of the role of acetylation regulation in metabolism coordination. In particular, we review links between deregulated acetylation in metabolic enzymes and tumorigenesis. We further hypothesize on applications of the mediation of acetylation to restore deregulated metabolism coordination and thus develop novel means of cancer treatment.

Flavonoid apigenin is an inhibitor of the NAD+ ase CD38: implications for cellular NAD+ metabolism, protein acetylation, and treatment of metabolic syndrome

Metabolic syndrome is a growing health problem worldwide. It is therefore imperative to develop new strategies to treat this pathology. In the past years, the manipulation of NAD(+) metabolism has emerged as a plausible strategy to ameliorate metabolic syndrome. In particular, an increase in cellular NAD(+) levels has beneficial effects, likely because of the activation of sirtuins. Previously, we reported that CD38 is the primary NAD(+)ase in mammals. Moreover, CD38 knockout mice have higher NAD(+) levels and are protected against obesity and metabolic syndrome. Here, we show that CD38 regulates global protein acetylation through changes in NAD(+) levels and sirtuin activity. In addition, we characterize two CD38 inhibitors: quercetin and apigenin. We show that pharmacological inhibition of CD38 results in higher intracellular NAD(+) levels and that treatment of cell cultures with apigenin decreases global acetylation as well as the acetylation of p53 and RelA-p65. Finally, apigenin administration to obese mice increases NAD(+) levels, decreases global protein acetylation, and improves several aspects of glucose and lipid homeostasis. Our results show that CD38 is a novel pharmacological target to treat metabolic diseases via NAD(+)-dependent pathways.

Seven sirtuins for seven deadly diseases of aging

Sirtuins are a class of NAD(+)-dependent deacetylases having beneficial health effects. This extensive review describes the numerous intracellular actions of the seven mammalian sirtuins, their protein targets, intracellular localization, the pathways they modulate, and their role in common diseases of aging. Selective pharmacological targeting of sirtuins is of current interest in helping to alleviate global disease burden. Since all sirtuins are activated by NAD(+), strategies that boost NAD(+) in cells are of interest. While most is known about SIRT1, the functions of the six other sirtuins are now emerging. Best known is the involvement of sirtuins in helping cells adapt energy output to match energy requirements. SIRT1 and some of the other sirtuins enhance fat metabolism and modulate mitochondrial respiration to optimize energy harvesting. The AMP kinase/SIRT1-PGC-1α-PPAR axis and mitochondrial sirtuins appear pivotal to maintaining mitochondrial function. Downregulation with aging explains much of the pathophysiology that accumulates with aging. Posttranslational modifications of sirtuins and their substrates affect specificity. Although SIRT1 activation seems not to affect life span, activation of some of the other sirtuins might. Since sirtuins are crucial to pathways that counter the decline in health that accompanies aging, pharmacological agents that boost sirtuin activity have clinical potential in treatment of diabetes, cardiovascular disease, dementia, osteoporosis, arthritis, and other conditions. In cancer, however, SIRT1 inhibitors could have therapeutic value. Nutraceuticals such as resveratrol have a multiplicity of actions besides sirtuin activation. Their net health benefit and relative safety may have originated from the ability of animals to survive environmental changes by utilizing these stress resistance chemicals in the diet during evolution. Each sirtuin forms a key hub to the intracellular pathways affected.

The epigenetic language of circadian clocks

Epigenetic control, which includes DNA methylation and histone modifications, leads to chromatin remodeling and regulated gene expression. Remodeling of chromatin constitutes a critical interface of transducing signals, such as light or nutrient availability, and how these are interpreted by the cell to generate permissive or silenced states for transcription. CLOCK-BMAL1-mediated activation of clock-controlled genes (CCGs) is coupled to circadian changes in histone modification at their promoters. Several chromatin modifiers, such as the deacetylases SIRT1 and HDAC3 or methyltransferase MLL1, have been shown to be recruited to the promoters of the CCGs in a circadian manner. Interestingly, the central element of the core clock machinery, the transcription factor CLOCK, also possesses histone acetyltransferase activity. Rhythmic expression of the CCGs is abolished in the absence of these chromatin modifiers. Here we will discuss the evidence demonstrating that chromatin remodeling is at the crossroads of circadian rhythms and regulation of metabolism and cellular proliferation.

Implication of CD38 gene in podocyte epithelial-to-mesenchymal transition and glomerular sclerosis

CD38 is a multifunctional protein involving in a number of signalling pathways. Given that the lack of CD38 is considered as a dedifferentiation marker of lymphocytes and other cells, we hypothesized that CD38 and its signalling pathway may participate in the epithelial-to-mesenchymal transition (EMT) process of podocytes and thereby regulates the integrity of glomerular structure and function. Western blot analysis and RT-PCR demonstrated that renal tissue CD38 expression was lacking in CD38(-/-) mice or substantially reduced in renal CD38 shRNA-transfected WT (CD38-shRNA) mice compared to CD38(+/+) littermates. Confocal fluorescent microscopy demonstrated the reduced expression of epithelial markers (P-Cadherin, ZO-1 and podocin) and increased expression of mesenchymal markers (FSP-1, α-SMA and desmin) in the glomeruli of CD38(-/-) and CD38-shRNA mice compared to CD38(+/+) mice. Morphological examinations showed profound injury in the glomeruli of CD38(-/-) or CD38-shRNA mice compared to CD38(+/+) mice. This enhanced glomerular injury in CD38(-/-) or CD38-shRNA mice was accompanied by increased albuminuria and proteinuria. DOCA/high salt treatment further decreased the expression of epithelial markers and increased the abundance of mesenchymal markers, which were accompanied by more increased glomerular damage index and mean arterial pressure in CD38(-/-) and CD38-shRNA mice than CD38(+/+) mice. In vitro studies showed that inhibition of CD38 enhances the EMT in podocytes. In conclusion, our observations reveal that the normal expression of CD38 importantly contributes to the differentiation and function of podocytes and the defect of this gene expression may be a critical mechanism inducing EMT and consequently resulting in glomerular injury and sclerosis.

A candidate gene for autoimmune myasthenia gravis

OBJECTIVE

We sought to identify a causative mutation in a previously reported kindred with parental consanguinity and 5 of 10 siblings with adult-onset autoimmune myasthenia gravis.

METHODS

We performed genome-wide homozygosity mapping, and sequenced all known genes in the one region of extended homozygosity. Quantitative and allele-specific reverse transcriptase PCR (RT-PCR) were performed on a candidate gene to determine the RNA expression level in affected siblings and controls and the relative abundance of the wild-type and mutant alleles in a heterozygote.

RESULTS

A region of shared homozygosity at chromosome 13q13.3-13q14.11 was found in 4 affected siblings and 1 unaffected sibling. A homozygous single nucleotide variant was found in the 3'-untranslated region of the ecto-NADH oxidase 1 gene (ENOX1). No other variants likely to be pathogenic were found in genes in this region or elsewhere. The ENOX1 sequence variant was not found in 764 controls. Quantitative RT-PCR showed that expression of ENOX1 decreased to about 20% of normal levels in lymphoblastoid cells from individuals homozygous for the variant and to about 50% in 2 unaffected heterozygotes. Allele-specific RT-PCR showed a 55%-60% reduction in the level of the variant transcript in heterozygous cells due to reduced mRNA stability.

CONCLUSION

These results indicate that this sequence variant in ENOX1 may contribute to the familial autoimmune myasthenia in these patients.

NAD+ metabolism and oxidative stress: the golden nucleotide on a crown of thorns

In the twentieth century, NAD+ research generated multiple discoveries. Identification of the important role of NAD+ as a cofactor in cellular respiration and energy production was followed by discoveries of numerous NAD+ biosynthesis pathways. In recent years, NAD+ has been shown to play a unique role in DNA repair and protein deacetylation. As discussed in this review, there are close interactions between oxidative stress and immune activation, energy metabolism, and cell viability in neurodegenerative disorders and ageing. Profound interactions with regard to oxidative stress and NAD+ have been highlighted in the present work. This review emphasizes the pivotal role of NAD+ in the regulation of DNA repair, stress resistance, and cell death, suggesting that NAD+ synthesis through the kynurenine pathway and/or salvage pathway is an attractive target for therapeutic intervention in age-associated degenerative disorders. NAD+ precursors have been shown to slow down ageing and extend lifespan in yeasts, and protect severed axons from degeneration in animal models neurodegenerative diseases.

Role of deleted in breast cancer 1 (DBC1) protein in SIRT1 deacetylase activation induced by protein kinase A and AMP-activated protein kinase

The NAD(+)-dependent deacetylase SIRT1 is a key regulator of several aspects of metabolism and aging. SIRT1 activation is beneficial for several human diseases, including metabolic syndrome, diabetes, obesity, liver steatosis, and Alzheimer disease. We have recently shown that the protein deleted in breast cancer 1 (DBC1) is a key regulator of SIRT1 activity in vivo. Furthermore, SIRT1 and DBC1 form a dynamic complex that is regulated by the energetic state of the organism. Understanding how the interaction between SIRT1 and DBC1 is regulated is therefore essential to design strategies aimed to activate SIRT1. Here, we investigated which pathways can lead to the dissociation of SIRT1 and DBC1 and consequently to SIRT1 activation. We observed that PKA activation leads to a fast and transient activation of SIRT1 that is DBC1-dependent. In fact, an increase in cAMP/PKA activity resulted in the dissociation of SIRT1 and DBC1 in an AMP-activated protein kinase (AMPK)-dependent manner. Pharmacological AMPK activation led to SIRT1 activation by a DBC1-dependent mechanism. Indeed, we found that AMPK activators promote SIRT1-DBC1 dissociation in cells, resulting in an increase in SIRT1 activity. In addition, we observed that the SIRT1 activation promoted by PKA and AMPK occurs without changes in the intracellular levels of NAD(+). We propose that PKA and AMPK can acutely activate SIRT1 by inducing dissociation of SIRT1 from its endogenous inhibitor DBC1. Our experiments provide new insight on the in vivo mechanism of SIRT1 regulation and a new avenue for the development of pharmacological SIRT1 activators targeted at the dissociation of the SIRT1-DBC1 complex.

Multifunctional roles of NAD⁺ and NADH in astrocytes

The control and maintenance of the intracellular redox state is an essential task for cells and organisms. NAD(+) and NADH constitute a redox pair crucially involved in cellular metabolism as a cofactor for many dehydrogenases. In addition, NAD(+) is used as a substrate independent of its redox-carrier function by enzymes like poly(ADP)ribose polymerases, sirtuins and glycohydrolases like CD38. The activity of these enzymes affects the intracellular pool of NAD(+) and depends in turn on the availability of NAD(+). In addition, both NAD(+) and NADH as well as the NAD(+)/NADH redox ratio can modulate gene expression and Ca(2+) signals. Therefore, the NAD(+)/NADH redox state constitutes an important metabolic node involved in the control of many cellular events ranging from the regulation of metabolic fluxes to cell fate decisions and the control of cell death. This review summarizes the different functions of NAD(+) and NADH with a focus on astrocytes, a pivotal glial cell type contributing to brain metabolism and signaling.

Circadian rhythms and memory formation: regulation by chromatin remodeling

Epigenetic changes, such as DNA methylation or histone modification, can remodel the chromatin and regulate gene expression. Remodeling of chromatin provides an efficient mechanism of transducing signals, such as light or nutrient availability, to regulate gene expression. CLOCK:BMAL1 mediated activation of clock-controlled genes (CCGs) is coupled to circadian changes in histone modification at their promoters. Several chromatin modifiers, such as the deacetylases SIRT1 and HDAC3 or methyltransferase MLL1, have been shown to be recruited to the promoters of the CCGs in a circadian manner. Interestingly, the central element of the core clock machinery, the transcription factor CLOCK, also possesses histone acetyltransferase activity. Rhythmic expression of the CCGs is abolished in the absence of these chromatin modifiers. Recent research has demonstrated that chromatin remodeling is at the cross-roads of circadian rhythms and regulation of metabolism and aging. It would be of interest to identify if similar pathways exist in the epigenetic regulation of memory formation.

Nicotinamide, NAD(P)(H), and Methyl-Group Homeostasis Evolved and Became a Determinant of Ageing Diseases: Hypotheses and Lessons from Pellagra

Compartmentalized redox faults are common to ageing diseases. Dietary constituents are catabolized to NAD(H) donating electrons producing proton-based bioenergy in coevolved, cross-species and cross-organ networks. Nicotinamide and NAD deficiency from poor diet or high expenditure causes pellagra, an ageing and dementing disorder with lost robustness to infection and stress. Nicotinamide and stress induce Nicotinamide-N-methyltransferase (NNMT) improving choline retention but consume methyl groups. High NNMT activity is linked to Parkinson's, cancers, and diseases of affluence. Optimising nicotinamide and choline/methyl group availability is important for brain development and increased during our evolution raising metabolic and methylome ceilings through dietary/metabolic symbiotic means but strict energy constraints remain and life-history tradeoffs are the rule. An optimal energy, NAD and methyl group supply, avoiding hypo and hyper-vitaminoses nicotinamide and choline, is important to healthy ageing and avoids utilising double-edged symbionts or uncontrolled autophagy or reversions to fermentation reactions in inflammatory and cancerous tissue that all redistribute NAD(P)(H), but incur high allostatic costs.

Bioinformatic selection of putative epigenetically regulated loci associated with obesity using gene expression data

There is considerable interest in defining the relationship between epigenetic variation and the risk of common complex diseases. Strategies which assist in the prioritisation of target loci that have the potential to be epigenetically regulated might provide a useful approach in identifying concrete examples of epigenotype-phenotype associations. Focusing on the postulated role of epigenetic factors in the aetiopathogenesis of obesity this report outlines an approach utilising gene expression data and a suite of bioinformatic tools to prioritise a list of target candidate genes for more detailed experimental scrutiny. Gene expression microarrays were performed using peripheral blood RNA from children aged 11-13years selected from the Newcastle Preterm Birth Growth Study which were grouped by body mass index (BMI). Genes showing ≥2.0 fold differential expression between low and high BMI groups were selected for in silico analysis. Several bioinformatic tools were used for each following step; 1) a literature search was carried out to identify whether the differentially expressed genes were associated with adiposity phenotypes. Of those obesity-candidate genes, putative epigenetically regulated promoters were identified by 2) defining the promoter regions, 3) then by selecting promoters with a CpG island (CGI), 4) and then by identifying any transcription factor binding modules covering CpG sites within the CGI. This bioinformatic processing culminated in the identification of a short list of target obesity-candidate genes putatively regulated by DNA methylation which can be taken forward for experimental analysis. The proposed workflow provides a flexible, versatile and low cost methodology for target gene prioritisation that is applicable to multiple species and disease contexts.

Altered behavioral and metabolic circadian rhythms in mice with disrupted NAD+ oscillation

The Intracellular levels of nicotinamide adenine dinucleotide (NAD(+)) are rhythmic and controlled by the circadian clock. However, whether NAD(+) oscillation in turn contributes to circadian physiology is not fully understood. To address this question we analyzed mice mutated for the NAD(+) hydrolase CD38. We found that rhythmicity of NAD(+) was altered in the CD38-deficient mice. The high, chronic levels of NAD(+) results in several anomalies in circadian behavior and metabolism. CD38-null mice display a shortened period length of locomotor activity and alteration in the rest-activity rhythm. Several clock genes and, interestingly, genes involved in amino acid metabolism were deregulated in CD38-null livers. Metabolomic analysis identified alterations in the circadian levels of several amino acids, specifically tryptophan levels were reduced in the CD38-null mice at a circadian time paralleling with elevated NAD(+) levels. Thus, CD38 contributes to behavioral and metabolic circadian rhythms and altered NAD(+) levels influence the circadian clock.

CD38 is a key enzyme for the survival of mouse microglial BV2 cells

CD38 is a multifunctional enzyme that can not only generate cyclic adenosine diphosphate-ribose (cADPR) - a key Ca(2+) -mobilizing second messenger - by consuming NAD(+), but also hydrolyze extracellular NAD(+). There have been only a small number of studies on the functions of CD38 in the CNS. Brain inflammation plays critical roles in ischemic brain injury and multiple other neurological diseases, in which microglia activation is a key event. In this study we determined the roles of CD38 in the basal survival of mouse BV2 microglia cells by applying CD38 siRNA. Our study found that silencing of CD38 led to significantly decreased survival of the cells. We also found that decreased CD38 levels can lead to apoptosis of the microglial cells, as assessed by flow cytometry-based Annexin V/7-AAD assay, caspase-3 immunostaining and Hoechst staining assays. Our study has further indicated that the CD38 silencing-induced apoptosis is mainly caspase 3-dependent. Collectively, our study has provided the first evidence suggesting that CD38 plays a critical role in the basal survival of microglia, and decreased CD38 can lead to caspase 3-dependent apoptosis of the cells. These results suggest that CD38 may become a therapeutic target for modulating microglial survival in neurological diseases.

Regulation of metabolism: the circadian clock dictates the time

Circadian rhythms occur with a periodicity of approximately 24h and regulate a wide array of metabolic and physiologic functions. Accumulating epidemiological and genetic evidence indicates that disruption of circadian rhythms can be directly linked to many pathological conditions, including sleep disorders, depression, metabolic syndrome and cancer. Intriguingly, several molecular gears constituting the clock machinery have been found to establish functional interplays with regulators of cellular metabolism. Although the circadian clock regulates multiple metabolic pathways, metabolite availability and feeding behavior can in turn regulate the circadian clock. An in-depth understanding of this reciprocal regulation of circadian rhythms and cellular metabolism may provide insights into the development of therapeutic intervention against specific metabolic disorders.

Disruption of CD38 gene enhances cardiac functions by elevating serum testosterone in the male null mice

AIMS

Gender-related phenotypes in the cardiovascular system have been observed in various genetically modified mice. Here, we report that cardiac functions are significantly improved only in male CD38-null mice and we explore the potential mechanisms of the sexual dimorphism mediated by CD38 deficiency.

MAIN METHODS

Cardiac functions of mice were measured by pressure-volume conductance catheter technique and echocardiography. Serum sex steroids were determined by radioimmunoassay. Relative mRNA levels of myocardial contractile-associated proteins in cardiomyocytes were analyzed by real-time PCR analysis. To clarify the effects of testosterone on the sexual dimorphism, flutamide, an androgen receptor antagonist, was subcutaneously infused into the male null mice for 6 weeks with an osmotic mini-pump.

KEY FINDINGS

The myocardial contractility, contraction and relaxation velocities were significantly enhanced only in male CD38-null mice, in which the levels of serum testosterone were markedly elevated. The elevated testosterone levels in the null mice were correlated to an obvious decrease in expression of androgen receptor and dramatic increases in expressions of major genes involved in myocardial contraction, including ryanodine receptor type 2 (RyR2), sarcoplasmic reticular Ca(2+) ATPase (SERCA2) and Na(+)/Ca(2+)-exchanger protein 1 (NCX1), and α myosin heavy chain (α-MHC). More importantly, all of the alternations that were observed in the male null mice were almost completely restored by flutamide administration.

SIGNIFICANCE

Elevated serum level of testosterone in the male CD38(-/-) mice enhances cardiac functions through upregulation of major calcium regulatory proteins, which improve our understanding on sex disparities and molecular mechanisms in the incidence and manifestation of heart diseases.

Mitochondrial dysfunction and nicotinamide dinucleotide catabolism as mechanisms of cell death and promising targets for neuroprotection

Both acute and chronic neurodegenerative diseases are frequently associated with mitochondrial dysfunction as an essential component of mechanisms leading to brain damage. Although loss of mitochondrial functions resulting from prolonged activation of the mitochondrial permeability transition (MPT) pore has been shown to play a significant role in perturbation of cellular bioenergetics and in cell death, the detailed mechanisms are still elusive. Enzymatic reactions linked to glycolysis, the tricarboxylic acid cycle, and mitochondrial respiration are dependent on the reduced or oxidized form of nicotinamide dinucleotide [NAD(H)] as a cofactor. Loss of mitochondrial NAD(+) resulting from MPT pore opening, although transient, allows detrimental depletion of mitochondrial and cellular NAD(+) pools by activated NAD(+) glycohydrolases. Poly(ADP-ribose) polymerase (PARP) is considered to be a major NAD(+) degrading enzyme, particularly under conditions of extensive DNA damage. We propose that CD38, a main cellular NAD(+) level regulator, can significantly contribute to NAD(+) catabolism. We discuss NAD(+) catabolic and NAD(+) synthesis pathways and their role in different strategies to prevent cellular NAD(+) degradation in brain, particularly following an ischemic insult. These therapeutic approaches are based on utilizing endogenous intermediates of NAD(+) metabolism that feed into the NAD(+) salvage pathway and also inhibit CD38 activity.

CD38 expression, function, and gene resequencing in a human lymphoblastoid cell line-based model system

CD38 is an ecto-enzyme that hydrolyzes NAD. Its expression is a prognostic marker for chronic lymphocytic leukemia. We have characterized individual variation in CD38 expression in lymphoblastoid cell lines from 288 healthy subjects of three ethnicities. Expression varied widely, with significant differences among ethnic groups, and was correlated significantly with CD38 enzymatic activity and protein levels. The CD38 gene was then resequenced using DNA from the same cell lines, with the identification of 53 single nucleotide polymorphisms (SNPs) and one indel, 39 novel. One SNP, rs1130169, was significantly associated with CD38 mRNA expression and explained a portion of the difference in expression among ethnic groups. EMS assay showed nuclear protein binding at or near this SNP. We also determined that variation in CD38 expression in these cell lines was associated with variation in antineoplastic drug sensitivity. These results represent a step toward understanding mechanisms involved in CD38 expression.

Mouse embryonic fibroblasts from CD38 knockout mice are resistant to oxidative stresses through inhibition of reactive oxygen species production and Ca(2+) overload

CD38 is a multifunctional enzyme that has both ADP-ribosyl cyclase and cADPR hydrolase activities, being capable of cleaving NAD(+) to cyclic ADP ribose (cADPR) and hydrolyzing cADPR to ADPR. It has been reported that there is markedly a reduction of cADPR and elevation of NAD in many tissues from CD38 knockout (CD38(-/-)) mice. Cyclic ADPR is a potent second messenger for intracellular Ca(2+) mobilization, and NAD is a key cellular metabolite for cellular energetic and a crucial regulator for multiple signaling pathways in cells. We hypothesize that CD38 knockout may have a protective effect in oxidative stresses through elevating NAD and decreasing cADPR. In the present study, we observed that the mouse embryonic fibroblasts (MEFs) from CD38(-/-) mice were significantly resistant to oxidative stress such as H(2)O(2) injury and hypoxia/reoxygenation compared with wild type MEFs (WT MEFs). We further found that production of reactive oxygen species (ROS) and concentrations of intracellular Ca(2+) ([Ca(2+)](i)) in CD38(-/-) MEFs were markedly reduced compared with WT MEFs during hypoxia/reoxygenation. Coincidence with these results, a remarkably lower mRNA level of Nox1, one of the enzymes responsible for ROS generation, was observed in CD38(-/-) MEFs. Furthermore, we found that transcription of Nox1 mRNA in WT MEFs could be elevated by calcium ionophore ionomycin in a dose-dependent manner, indicating that the expression of Nox1 mRNA can be regulated by elevation of intracellular [Ca(2+)]. Therefore we concluded that CD38(-/-) MEFs are resistant to oxidative stresses through inhibiting intracellular Ca(2+) overload and ROS production which may be regulated by Ca(2+)-mediated inhibition of Nox1 expression. Our data should provide an insight for elucidating the roles of CD38 in oxidative stresses and a novel perspective of dealing with the ischemia/reperfusion-related diseases.

CD38/cyclic ADP-ribose system: a new player for oxytocin secretion and regulation of social behaviour

Oxytocin is important for regulating a number of physiological processes. Disruption of the secretion, metabolism or action of oxytocin results in an impairment of reproductive function, social and sexual behaviours, and stress responses. This review discusses current views on the regulation and autoregulation of oxytocin release in the hypothalamic-neurohypophysial system, with special focus on the activity of the CD38/cADP-ribose system as a new component in this regulation. Data from our laboratories indicate that an impairment of this system results in alterations of oxytocin secretion and abnormal social behaviour, thus suggesting new clues that help in our understanding of the pathogenesis of neurodevelopmental disorders.

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.

Deleted in breast cancer-1 regulates SIRT1 activity and contributes to high-fat diet-induced liver steatosis in mice

The enzyme sirtuin 1 (SIRT1) is a critical regulator of many cellular functions, including energy metabolism. However, the precise mechanisms that modulate SIRT1 activity remain unknown. As SIRT1 activity in vitro was recently found to be negatively regulated by interaction with the deleted in breast cancer-1 (DBC1) protein, we set out to investigate whether DBC1 regulates SIRT1 activity in vivo. We found that DBC1 and SIRT1 colocalized and interacted, and that DBC1 modulated SIRT1 activity, in multiple cell lines and tissues. In mouse liver, increased SIRT1 activity, concomitant with decreased DBC1-SIRT1 interaction, was detected after 24 hours of starvation, whereas decreased SIRT1 activity and increased interaction with DBC1 was observed with high-fat diet (HFD) feeding. Consistent with the hypothesis that DBC1 is crucial for HFD-induced inhibition of SIRT1 and for the development of experimental liver steatosis, genetic deletion of Dbc1 in mice led to increased SIRT1 activity in several tissues, including liver. Furthermore, DBC1-deficient mice were protected from HFD-induced liver steatosis and inflammation, despite the development of obesity. These observations define what we believe to be a new role for DBC1 as an in vivo regulator of SIRT1 activity and liver steatosis. We therefore propose that the DBC1-SIRT1 interaction may serve as a new target for therapies aimed at nonalcoholic liver steatosis.

CD38 as a regulator of cellular NAD: a novel potential pharmacological target for metabolic conditions

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.

Postischemic oxidative stress promotes mitochondrial metabolic failure in neurons and astrocytes

Oxidative stress and mitochondrial dysfunction have been closely associated in many subcellular, cellular, animal, and human studies of both acute brain injury and neurodegenerative diseases. Our animal models of brain injury caused by cardiac arrest illustrate this relationship and demonstrate that both oxidative molecular modifications and mitochondrial metabolic impairment are exacerbated by reoxygenation of the brain using 100% ventilatory O(2) compared to lower levels that maintain normoxemia. Numerous molecular mechanisms may be responsible for mitochondrial dysfunction caused by oxidative stress, including oxidation and inactivation of mitochondrial proteins, promotion of the mitochondrial membrane permeability transition, and consumption of metabolic cofactors and intermediates, for example, NAD(H). Moreover, the relative contribution of these mechanisms to cell injury and death is likely different among different types of brain cells, for example, neurons and astrocytes. In order to better understand these oxidative stress mechanisms and their relevance to neurologic disorders, we have undertaken studies with primary cultures of astrocytes and neurons exposed to O(2) and glucose deprivation and reoxygenation and compared the results of these studies to those using a rat model of neonatal asphyxic brain injury. These results support the hypothesis that release and or consumption of mitochondrial NAD(H) is at least partially responsible for respiratory inhibition, particularly in neurons.

The importance of NAD in multiple sclerosis

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 Wld(S) 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 Wld(S) 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.

Transformation and action of extracellular NAD+ in perfused rat and mouse livers

AIM

Transformation and possible metabolic effects of extracellular NAD+ were investigated in the livers of mice (Mus musculus; Swiss strain) and rats (Rattus novergicus; Holtzman and Wistar strains).

METHODS

The livers were perfused in an open system using oxygen-saturated Krebs/Henseleit-bicarbonate buffer (pH 7.4) as the perfusion fluid. The transformation of NAD+ was monitored using high-performance liquid chromatography.

RESULTS

In the mouse liver, the single-pass metabolism of 100 micromol/L NAD+ was almost complete; ADP-ribose and nicotinamide were the main products in the outflowing perfusate. In the livers of both Holtzman and Wistar rats, the main transformation products were ADP-ribose, uric acid and nicotinamide; significant amounts of inosine and AMP were also identified. On a weight basis, the transformation of NAD+ was more efficient in the mouse liver. In the rat liver, 100 micromol/L NAD+ transiently inhibited gluconeogenesis and oxygen uptake. Inhibition was followed by a transient stimulation. Inhibition was more pronounced in the Wistar strain and stimulation was more pronounced in the Holtzman strain. In the mouse liver, no clear effects on gluconeogenesis and oxygen uptake were found even at 500 micromol/L NAD+.

CONCLUSION

It can be concluded that the functions of extracellular NAD+ are species-dependent and that observations in one species are strictly valid for that species. Interspecies extrapolations should thus be made very carefully. Actually, even variants of the same species can demonstrate considerably different responses.

Identification of an unusual AT(D)Pase-like activity in multifunctional NAD glycohydrolase from the venom of Agkistrodon acutus

NAD-glycohydrolases (NADases) are ubiquitous enzymes that possess NAD glycohydrolase, ADPR cyclase or cADPR hydrolase activity. All these activities are attributed to the NADase-catalyzed cleavage of C-N glycosyl bond. AA-NADase purified from the venom of Agkistrodon acutus is different from the known NADases, for it consists of two chains linked with disulfide-bond(s) and contains one Cu(2+) ion. Here, we show that AA-NADase is not only able to cleave the C-N glycosyl bond of NAD to produce ADPR and nicotinamide, but also able to cleave the phosphoanhydride linkages of ATP, ADP and AMP-PNP to yield AMP. AA-NADase selectively cleaves the P-O-P bond of ATP, ADP and AMP-PNP without the cleavage of P-O-P bond of NAD. The hydrolysis reactions of NAD, ATP and ADP catalyzed by AA-NADase are mutually competitive. ATP is the excellent substrate for AA-NADase with the highest specificity constant k(cat)/K(m) of 293+/-7mM(-1)s(-1). AA-NADase catalyzes the hydrolysis of ATP to produce AMP with an intermediate ADP. AA-NADase binds with one AMP with high affinity determined by isothermal titration calorimetry (ITC). AMP is an efficient inhibitor against NAD. AA-NADase has so far been identified as the first unique multicatalytic enzyme with both NADase and AT(D)Pase-like activities.

Evolution and function of the ADP ribosyl cyclase/CD38 gene family in physiology and pathology

The membrane proteins CD38 and CD157 belong to an evolutionarily conserved family of enzymes that play crucial roles in human physiology. Expressed in distinct patterns in most tissues, CD38 (and CD157) cleaves NAD(+) and NADP(+), generating cyclic ADP ribose (cADPR), NAADP, and ADPR. These reaction products are essential for the regulation of intracellular Ca(2+), the most ancient and universal cell signaling system. The entire family of enzymes controls complex processes, including egg fertilization, cell activation and proliferation, muscle contraction, hormone secretion, and immune responses. Over the course of evolution, the molecules have developed the ability to interact laterally and frontally with other surface proteins and have acquired receptor-like features. As detailed in this review, the loss of CD38 function is associated with impaired immune responses, metabolic disturbances, and behavioral modifications in mice. CD38 is a powerful disease marker for human leukemias and myelomas, is directly involved in the pathogenesis and outcome of human immunodeficiency virus infection and chronic lymphocytic leukemia, and controls insulin release and the development of diabetes. Here, the data concerning diseases are examined in view of potential clinical applications in diagnosis, prognosis, and therapy. The concluding remarks try to frame all of the currently available information within a unified working model that takes into account both the enzymatic and receptorial functions of the molecules.

Dual role of CD38 in microglial activation and activation-induced cell death

Microglia, the resident immune cells of the CNS, are normally quiescent but become activated after infection or injury. Their properties then change, and they promote both repair and damage processes. The extent of microglial activation is regulated, in part, by activation-induced cell death (AICD). Although many apoptotic aspects of the microglial AICD mechanism have been elucidated, little is known about the connection between the activation step and the death process. Using mouse primary microglial cultures, we show that the ectoenzyme CD38, via its calcium-mobilizing metabolite cyclic-ADP-ribose (cADPR), helps promote microglial activation and AICD induced by LPS plus IFN-gamma (LPS/IFN-gamma), suggesting that CD38 links the two processes. Accordingly, CD38 expression and activity, as well as the intracellular calcium concentration ([Ca2+]i) in the primary microglia were increased by LPS/IFN-gamma treatment. Moreover, CD38 deficiency or treatment with cADPR antagonists conferred partial resistance to LPS/IFN-gamma-induced AICD and also reduced [Ca2+]i. Microglial activation, indicated by induced expression of NO synthase-2 mRNA and production of NO, secretion and mRNA expression of TNF-alpha and IL-12 p40, and expression of IL-6 mRNA, was attenuated by CD38 deficiency or cADPR-antagonist treatment. The observed effects of CD38 on microglial activation are probably mediated via a cADPR-dependent increase in [Ca2+]i and the effect on AICD by regulation of NO production. Our results thus suggest that CD38 significantly affects regulation of the amount and function of activated microglia, with important consequences for injury and repair processes in the brain.

Transformation and actions of extracellular NADP(+) in the rat liver

The possible actions and transformation of extracellular NADP(+) in the rat liver have not yet been studied. Considering the various effects of its analogue NAD(+) in the liver, however, effects of NADP(+) can equally be expected. In the present work, this question was approached in the isolated perfused rat liver to get a preliminary picture of the action of extracellular NADP(+) in this organ. NADP(+) (100 microM) produced transient increases in the portal perfusion pressure. Glucose release (glycogenolysis) and lactate production from endogenous glycogen were transiently increased in antegrade and retrograde perfusion. Oxygen uptake was stimulated after a transient inhibition in antegrade perfusion, which was practically absent in retrograde perfusion. Pyruvate production was transiently inhibited. In the absence of Ca(2+), all of these effects were no longer observed. Bromophenacyl bromide, an inhibitor of eicosanoid synthesis, almost abolished all effects. Suramin, a non-specific purinergic P2(YX) antagonist, also inhibited the action of NADP(+). Single pass transformation of 75 microM NADP(+) was equal to 92%. Besides nicotinamide, at least two additional transformation products were detected: 2'-phospho-ADP-ribose and a non-identified component, the former being more important (67% of the transformed NADP(+)). Nicotinic acid adenine dinucleotide phosphate (NAADP) was not found in the outflowing perfusate. It was concluded that NADP(+), like NAD(+), acts on perfusion pressure and glycogen catabolism in the liver mainly via eicosanoid synthesis mediated by purinergic P2(YX) receptors.

NAD+/NADH and NADP+/NADPH in cellular functions and cell death: regulation and biological consequences

Accumulating evidence has suggested that NAD (including NAD+ and NADH) and NADP (including NADP+ and NADPH) could belong to the fundamental common mediators of various biological processes, including energy metabolism, mitochondrial functions, calcium homeostasis, antioxidation/generation of oxidative stress, gene expression, immunological functions, aging, and cell death: First, it is established that NAD mediates energy metabolism and mitochondrial functions; second, NADPH is a key component in cellular antioxidation systems; and NADH-dependent reactive oxygen species (ROS) generation from mitochondria and NADPH oxidase-dependent ROS generation are two critical mechanisms of ROS generation; third, cyclic ADP-ribose and several other molecules that are generated from NAD and NADP could mediate calcium homeostasis; fourth, NAD and NADP modulate multiple key factors in cell death, such as mitochondrial permeability transition, energy state, poly(ADP-ribose) polymerase-1, and apoptosis-inducing factor; and fifth, NAD and NADP profoundly affect aging-influencing factors such as oxidative stress and mitochondrial activities, and NAD-dependent sirtuins also mediate the aging process. Moreover, many recent studies have suggested novel paradigms of NAD and NADP metabolism. Future investigation into the metabolism and biological functions of NAD and NADP may expose fundamental properties of life, and suggest new strategies for treating diseases and slowing the aging process.

Mechanisms of impaired mitochondrial energy metabolism in acute and chronic neurodegenerative disorders

Altered mitochondrial energy metabolism contributes to the pathophysiology of acute brain injury caused by ischemia, trauma, and neurotoxins and by chronic neurodegenerative disorders such as Parkinson's and Huntington's diseases. Although much evidence supports that the electron transport chain dysfunction in these metabolic abnormalities has both genetic and intracellular environmental causes, alternative mechanisms are being explored. These include direct, reversible inhibition of cytochrome oxidase by nitric oxide, release of mitochondrial cytochrome c, oxidative inhibition of mitochondrial matrix dehydrogenases and adenine nucleotide transport, the availability of NAD for dehydrogenase reactions, respiratory uncoupling by activities such as that of the permeability transition pore, and altered mitochondrial structure and intracellular trafficking. This review focuses on the catabolism of neuronal NAD and the release of neuronal mitochondrial NAD as important contributors to metabolic dysfunction. In addition, the relationship between apoptotic signaling cascades and disruption of mitochondrial energy metabolism is considered in light of the fine balance between apoptotic and necrotic neural cell death.

Sirtuins: critical regulators at the crossroads between cancer and aging

Sirtuins (SIRTs 1-7), or class III histone deacetylases (HDACs), are protein deacetylases/ADP ribosyltransferases that target a wide range of cellular proteins in the nucleus, cytoplasm, and mitochondria for post-translational modification by acetylation (SIRT1, -2, -3 and -5) or ADP ribosylation (SIRT4 and -6). The orthologs of sirtuins in lower organisms play a critical role in regulating lifespan. As cancer is a disease of aging, we discuss the growing implications of the sirtuins in protecting against cancer development. Sirtuins regulate the cellular responses to stress and ensure that damaged DNA is not propagated and that mutations do not accumulate. SIRT1 also promotes replicative senescence under conditions of chronic stress. By participating in the stress response to genomic insults, sirtuins are thought to protect against cancer, but they are also emerging as direct participants in the growth of some cancers. Here, we review the growing implications of sirtuins both in cancer prevention and as specific and novel cancer therapeutic targets.

Transformation products of extracellular NAD(+) in the rat liver: kinetics of formation and metabolic action

The perfused rat liver responds in several ways to NAD(+) infusion (20-100 microM). Increases in portal perfusion pressure and glycogenolysis and transient inhibition of oxygen consumption and gluconeogenesis are some of the effects that were observed. Extracellular NAD(+) is also extensively transformed in the liver. The purpose of the present work was to determine the main products of extracellular NAD(+) transformation under various conditions and to investigate the possible contribution of these products for the metabolic effects of the parent compound. The experiments were done with the isolated perfused rat liver. The NAD(+) transformation was monitored by HPLC. Confirming previous findings, the single-pass transformation of 100 microM NAD(+) ranged between 75% at 1.5 min after starting infusion to 95% at 8 min. The most important products of single-pass NAD(+) transformation appearing in the outflowing perfusate were nicotinamide, ADP-ribose, uric acid, and inosine. The relative proportions of these products presented some variations with the time after initiation of NAD(+) infusion and the perfusion conditions, but ADP-ribose was always more abundant than uric acid and inosine. Cyclic ADP-ribose (cADP-ribose) as well as adenosine were not detected in the outflowing perfusate. The metabolic effects of ADP-ribose were essentially those already described for NAD(+). These effects were sensitive to suramin (P2(XY) purinergic receptor antagonist) and insensitive to 3,7-dimethyl-1-(2-propargyl)-xanthine (A2 purinergic receptor antagonist). Inosine, a known purinergic A3 agonist, was also active on metabolism, but uric acid and nicotinamide were inactive. It was concluded that the metabolic and hemodynamic effects of extracellular NAD(+) are caused mainly by interactions with purinergic receptors with a highly significant participation of its main transformation product ADP-ribose.