Retraction

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.

Visfatin, glucose metabolism and vascular disease: a review of evidence

The adipose tissue is an endocrine organ producing substances called adipocytokines that have different effects on lipid metabolism, metabolic syndrome, and cardiovascular risk. Visfatin was recently described as an adipocytokine with potentially important effects on glucose metabolism and atherosclerosis. Visfatin has been linked to several inflammatory conditions, beta cell function, and cardiovascular disease. The growing number of publications on the subject shall bring further evidence about this adipocytokine. Its findings may contribute in the identification of higher risk individuals for diabetes and cardiovascular disease with a better comprehension about the complex intercorrelation between adiposity, glucose metabolism and vascular disease.

Skeletal muscle NAMPT is induced by exercise in humans

In mammals, nicotinamide phosphoribosyltransferase (NAMPT) is responsible for the first and rate-limiting step in the conversion of nicotinamide to nicotinamide adenine dinucleotide (NAD+). NAD+ is an obligate cosubstrate for mammalian sirtuin-1 (SIRT1), a deacetylase that activates peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha), which in turn can activate mitochondrial biogenesis. Given that mitochondrial biogenesis is activated by exercise, we hypothesized that exercise would increase NAMPT expression, as a potential mechanism leading to increased mitochondrial content in muscle. A cross-sectional analysis of human subjects showed that athletes had about a twofold higher skeletal muscle NAMPT protein expression compared with sedentary obese, nonobese, and type 2 diabetic subjects (P < 0.05). NAMPT protein correlated with mitochondrial content as estimated by complex III protein content (R(2) = 0.28, P < 0.01), MRS-measured maximal ATP synthesis (R(2) = 0.37, P = 0.002), and Vo(2max) (R(2) = 0.63, P < 0.0001). In an exercise intervention study, NAMPT protein increased by 127% in sedentary nonobese subjects after 3 wk of exercise training (P < 0.01). Treatment of primary human myotubes with forskolin, a cAMP signaling pathway activator, resulted in an approximately 2.5-fold increase in NAMPT protein expression, whereas treatment with ionomycin had no effect. Activation of AMPK via AICAR resulted in an approximately 3.4-fold increase in NAMPT mRNA (P < 0.05) as well as modest increases in NAMPT protein (P < 0.05) and mitochondrial content (P < 0.05). These results demonstrate that exercise increases skeletal muscle NAMPT expression and that NAMPT correlates with mitochondrial content. Further studies are necessary to elucidate the pathways regulating NAMPT as well as its downstream effects.

The relationship of visfatin/pre-B-cell colony-enhancing factor/nicotinamide phosphoribosyltransferase in adipose tissue with inflammation, insulin resistance, and plasma lipids

Visfatin/pre-B-cell colony-enhancing factor (PBEF)/nicotinamide phosphoribosyltransferase (Nampt) has been proposed as an insulin-mimicking adipocytokine predominantly secreted from visceral adipose tissue (VAT) and correlated with obesity. However, recent evidence challenged this proposal and instead suggested visfatin/PBEF/Nampt as a proinflammatory cytokine. The study aimed to examine whether visfatin/PBEF/Nampt was predominantly expressed in VAT and was correlated with obesity. The relationship of visfatin/PBEF/Nampt gene expression in adipose tissues with proinflammatory gene expression and metabolic phenotypes was also examined. The relative messenger RNA (mRNA) levels of visfatin/PBEF/Nampt, macrophage-specific marker CD68, and proinflammatory genes were measured in paired abdominal VAT and subcutaneous adipose tissues (SAT) and from 53 nondiabetic adults using quantitative real-time polymerase chain reaction. Fasting glucose, insulin, triglyceride, cholesterol, and uric acid levels were measured; and systemic insulin sensitivity was quantified with modified insulin suppression tests. There was no difference in visfatin/PBEF/Nampt mRNA levels between VAT and SAT, and neither was associated with measures of obesity. Visfatin/PBEF/Nampt mRNA levels were strongly correlated with proinflammatory gene expression including CD68 and tumor necrosis factor-alpha gene in both VAT and SAT. The VAT and SAT visfatin/PBEF/Nampt mRNA expressions were positively correlated with steady-state plasma glucose concentrations measured with modified insulin suppression tests, a direct measurement of systemic insulin resistance (r = 0.42, P = .03 and r = 0.44, P = .03, respectively). The VAT visfatin/PBEF/Nampt mRNA expression was also positively correlated with fasting triglyceride (r = 0.42, P = .002) and total cholesterol levels (r = 0.37, P = .009). Visfatin/PBEF/Nampt is not predominantly secreted from VAT and is not correlated with obesity. Our findings suggest that visfatin/PBEF/Nampt is a proinflammatory marker of adipose tissue associated with systemic insulin resistance and hyperlipidemia.

Extracellular PBEF/NAMPT/visfatin activates pro-inflammatory signalling in human vascular smooth muscle cells through nicotinamide phosphoribosyltransferase activity

AIMS/HYPOTHESIS

Extracellular pre-B cell colony-enhancing factor/nicotinamide phosphoribosyltransferase/visfatin (ePBEF/NAMPT/visfatin) is an adipocytokine, whose circulating levels are enhanced in metabolic disorders, such as diabetes mellitus and obesity. Here, we explored the ability of ePBEF/NAMPT/visfatin to promote vascular inflammation, as a condition closely related to atherothrombotic diseases. We specifically studied the ability of PBEF/NAMPT/visfatin to directly activate pathways leading to inducible nitric oxide synthase (iNOS) induction in cultured human aortic smooth muscle cells, as well as the mechanisms involved.

METHODS

iNOS levels and extracellular signal-regulated kinase (ERK) 1/2 activity were determined by western blotting. Nuclear factor (NF)-kappaB activity was assessed by electrophoretic mobility shift assay.

RESULTS

ePBEF/NAMPT/visfatin (10-250 ng/ml) induced iNOS in a concentration-dependent manner. At a submaximal concentration (100 ng/ml), ePBEF/NAMPT/visfatin time-dependently enhanced iNOS levels up to 18 h after stimulation. Over this time period, ePBEF/NAMPT/visfatin elicited a sustained activation of NF-kappaB and triggered a biphasic ERK 1/2 activation. By using the respective ERK 1/2 and NF-kappaB inhibitors, PD98059 and pyrrolidine dithiocarbamate, we established that iNOS induction by ePBEF/NAMPT/visfatin required the consecutive upstream activation of ERK 1/2 and NF-kappaB. The pro-inflammatory action of ePBEF/NAMPT/visfatin was not prevented by insulin receptor blockade. However, exogenous nicotinamide mononucleotide, the product of NAMPT activity, mimicked NF-kappaB activation and iNOS induction by ePBEF/NAMPT/visfatin, while the NAMPT inhibitor APO866 prevented the effects of ePBEF/NAMPT/visfatin on iNOS and NF-kappaB.

CONCLUSIONS/INTERPRETATION

Through its intrinsic NAMPT activity, ePBEF/NAMPT/visfatin appears to be a direct contributor to vascular inflammation, a key feature of atherothrombotic diseases linked to metabolic disorders.

SIRT1-dependent regulation of chromatin and transcription: linking NAD(+) metabolism and signaling to the control of cellular functions

Sirtuins comprise a family of NAD(+)-dependent protein deacetylases and ADP-ribosyltransferases. Mammalian SIRT1 - a homolog of yeast Sir2, the prototypical member of the sirtuin family - is an important regulator of metabolism, cell differentiation and senescence, stress response, and cancer. As an NAD(+)-dependent enzyme, SIRT1 regulates gene expression programs in response to cellular metabolic status, thereby coordinating metabolic adaptation of the whole organism. Several important mechanisms have emerged for SIRT1-dependent regulation of transcription. First, SIRT1 can modulate chromatin function through direct deacetylation of histones as well as by promoting alterations in the methylation of histones and DNA, leading to the repression of transcription. The latter is accomplished through the recruitment of other nuclear enzymes to chromatin for histone methylation and DNA CpG methylation, suggesting a broader role of SIRT1 in epigenetic regulation. Second, SIRT1 can interact and deacetylate a broad range of transcription factors and coregulators, thereby regulating target gene expression both positively and negatively. Cellular energy state, specifically NAD(+) metabolism, plays a major role in the regulation of SIRT1 activity. Recent studies on the NAD(+) biosynthetic enzymes in the salvage pathway, nicotinamide phosphoribosyltransferase (NAMPT) and nicotinamide mononucleotide adenylyltransferase 1 (NMNAT-1), have revealed important functions for these enzymes in SIRT1-dependent transcription regulation. The collective molecular actions of SIRT1 control specific patterns of gene expression that modulate a wide variety of physiological outcomes.

Adipokines as novel modulators of lipid metabolism

In the mid-1990s, interest in adipose tissue - until then generally regarded as a mere energy reserve - was revived by the discovery of leptin. Since then numerous other cytokine-like hormones have been isolated from white adipose tissue. These adipokines have been investigated in relation to obesity, metabolic syndrome, insulin resistance and other pathological conditions and processes. In addition, it is now established that adipokines play a role in the maintenance of an inflammatory state in adipose tissue and in the development of obesity and comorbidities. The contributions of individual adipokines in the pathophysiological features of obesity have yet to be determined in full, but recent data highlight important roles for adipokines in lipid metabolism.

The small molecule GMX1778 is a potent inhibitor of NAD+ biosynthesis: strategy for enhanced therapy in nicotinic acid phosphoribosyltransferase 1-deficient tumors

GMX1777 is a prodrug of the small molecule GMX1778, currently in phase I clinical trials for the treatment of cancer. We describe findings indicating that GMX1778 is a potent and specific inhibitor of the NAD(+) biosynthesis enzyme nicotinamide phosphoribosyltransferase (NAMPT). Cancer cells have a very high rate of NAD(+) turnover, which makes NAD(+) modulation an attractive target for anticancer therapy. Selective inhibition by GMX1778 of NAMPT blocks the production of NAD(+) and results in tumor cell death. Furthermore, GMX1778 is phosphoribosylated by NAMPT, which increases its cellular retention. The cytotoxicity of GMX1778 can be bypassed with exogenous nicotinic acid (NA), which permits NAD(+) repletion via NA phosphoribosyltransferase 1 (NAPRT1). The cytotoxicity of GMX1778 in cells with NAPRT1 deficiency, however, cannot be rescued by NA. Analyses of NAPRT1 mRNA and protein levels in cell lines and primary tumor tissue indicate that high frequencies of glioblastomas, neuroblastomas, and sarcomas are deficient in NAPRT1 and not susceptible to rescue with NA. As a result, the therapeutic index of GMX1777 can be widended in the treatment animals bearing NAPRT1-deficient tumors by coadministration with NA. This provides the rationale for a novel therapeutic approach for the use of GMX1777 in the treatment of human cancers.

Eicosapentaenoic acid stimulates AMP-activated protein kinase and increases visfatin secretion in cultured murine adipocytes

Visfatin is an adipokine highly expressed in visceral AT (adipose tissue) of humans and rodents, the production of which seems to be dysregulated in excessive fat accumulation and conditions of insulin resistance. EPA (eicosapentaenoic acid), an n-3 PUFA (polyunsaturated fatty acid), has been demonstrated to exert beneficial effects in obesity and insulin resistance conditions, which have been further linked to its reported ability to modulate adipokine production by adipocytes. TNF-alpha (tumour necrosis factor-alpha) is a pro-inflammatory cytokine whose production is increased in obesity and is involved in the development of insulin resistance. Control of adipokine production by some insulin-sensitizing compounds has been associated with the stimulation of AMPK (AMP-activated protein kinase). The aim of the present study was to examine in vitro the effects of EPA on visfatin production and the potential involvement of AMPK both in the absence or presence of TNF-alpha. Treatment with the pro-inflammatory cytokine TNF-alpha (1 ng/ml) did not modify visfatin gene expression and protein secretion in primary cultured rat adipocytes. However, treatment of these primary adipocytes with EPA (200 mumol/l) for 24 h significantly increased visfatin secretion (P<0.001) and mRNA gene expression (P<0.05). Moreover, the stimulatory effect of EPA on visfatin secretion was prevented by treatment with the AMPK inhibitor Compound C, but not with the PI3K (phosphoinositide 3-kinase) inhibitor LY294002. Similar results were observed in 3T3-L1 adipocytes. Moreover, EPA strongly stimulated AMPK phosphorylation alone or in combination with TNF-alpha in 3T3-L1 adipocytes and pre-adipocytes. The results of the present study suggest that the stimulatory action of EPA on visfatin production involves AMPK activation in adipocytes.

Association of plasma visfatin levels with inflammation, atherosclerosis and acute coronary syndromes (ACS) in humans

OBJECTIVES

Visfatin is a new cytokine that act as an insulin analogue on the insulin receptor and may link obesity and insulin resistance. It was recently shown that visfatin plays a role in plaque destabilization. However, the role of visfatin in atherosclerosis remains to be elucidated. We sought to assess whether plasma visfatin level is independently associated with inflammation, atherosclerosis and acute coronary syndromes (ACS).

DESIGN AND PATIENTS

Two hundred and fifty-three patients undergoing coronary angiography were divided into three subgroups: chronic coronary artery disease (CAD) (n = 102), ACS (n = 100) and control patients (n = 51). The plasma samples were thawed and analysed for circulating visfatin, monocyte chemoattractant protein 1 (MCP-1), interleukin-6 (IL-6), high sensitivity C-reactive protein (hs-CRP). The association of visfatin with risk factors, inflammation, atherosclerosis, and ACS was determined.

RESULTS

Plasma visfatin levels were significantly higher in chronic CAD and ACS compared with control patients. Multiple regression analysis demonstrated that plasma visfatin levels correlated with inflammatory factors and were associated with chronic CAD (odds ratio [OR][95% confidence interval], for second, third and fourth quartiles were 1.74 [0.96-2.69], 1.54 [0.85-2.28] and 1.84 [0.98-2.87], respectively) and ACS (ORs for second, third and fourth quartiles were 2.56 [1.57-3.34], 4.61 [1.94-10.96] and 6.52 [2.34-18.12], respectively) following adjustment for established risk factors and other inflammatory factors.

CONCLUSIONS

Plasma visfatin levels are significantly associated with CAD, particularly ACS, independent of well-known CAD risk factors.

Decreased visfatin after exercise training correlates with improved glucose tolerance

Nampt/pre-B-cell colony-enhancing factor/visfatin (visfatin) release from adipocytes has recently been suggested to be nutrient responsive and linked to systemic nicotinamide adenine dinucleotide biosynthesis and regulation of pancreatic beta-cell function.

PURPOSE

We hypothesized that if visfatin does play a role in the insulin response, then the exercise training-induced reduction in insulin response to an oral glucose load would correlate with reduced plasma visfatin.

METHODS

Sixteen obese men and women (age = 65 +/- 1 yr, body mass index = 33.4 +/- 1.5 kg x m(-2)) volunteered to participate in a 12-wk supervised exercise program (5 d x wk(-1), 60 min x d(-1) at 85% of HRmax). Visceral (VAT) and subcutaneous adipose tissue (SAT) were measured by computed tomographic scans. A 2-h 75-g oral glucose tolerance test was performed to determine the effect of exercise training on the insulin response to a glucose load. Fasting plasma visfatin was measured by enzyme-linked immunosorbent assay.

RESULTS

Exercise training resulted in an increase in (.)VO2max (21.1 +/- 0.9 vs 24.2 +/- 1.1 mL x kg(-1) x min(-1), P < 0.001), a decrease in body weight (96.4 +/- 4.1 vs 92.4 +/- 3.7 kg, P < 0.001), VAT (191 +/- 16 vs 144 +/- 16 cm, P < 0.001), and SAT (369 +/- 34 vs 309 +/- 41 cm, P < 0.02). Area under the glucose (450 +/- 31 vs 392 +/- 33 mmol x L(-1) x 2 h(-1), P < 0.01) and insulin (45,767 +/- 6142 vs 35,277 +/- 4997 pmol x L(-1) x 2 h(-1), P < 0.003) response curves were decreased after training. After intervention, plasma visfatin levels were significantly reduced (16.9 +/- 2.2 vs 14.5 +/- 1.8 ng x mL(-1), P < 0.05), and the change in visfatin was associated with the corresponding change in insulin (r = 0.56, P < 0.05) and glucose AUC (r = 0.53, P < 0.05).

CONCLUSION

The exercise-induced reduction of plasma visfatin is most likely the result of weight loss and body composition changes. The potential regulatory role of visfatin in mediating the pancreatic insulin response to oral glucose requires further investigation.

Adipokines in the treatment of diabetes mellitus and obesity

BACKGROUND

The physiology of adipose tissue plays a main role in the pathogenesis of type 2 diabetes mellitus. The secretion of adipocyte-derived hormones, in either an autocrine or a paracrine manner, has been proposed as a relevant mechanism in this process. In this sense, the administration and regulation of hormones derived from adipose tissue arises as an attractive option for treating metabolic disorders.

OBJECTIVE

To review the current understanding of the implication of adipokines in the development of obesity and insulin resistance, as well as their potential use as therapeutic agents.

METHODOLOGY

Review of scientific literature.

CONCLUSIONS

This review describes the role of adipokines in generating insulin resistance and the chronic low-grade inflammatory profile accompanying visceral obesity.

Nampt: linking NAD biology, metabolism and cancer

Nicotinamide phosphoribosyltransferase (Nampt) converts nicotinamide to nicotinamide mononucleotide (NMN), a key nicotinamide adenine dinucleotide (NAD) intermediate. Previously identified as a cytokine pre-B-cell colony-enhancing factor and controversially claimed as an insulin-mimetic hormone visfatin, Nampt has recently drawn much attention in several fields, including NAD biology, metabolism and inflammation. As a NAD biosynthetic enzyme, Nampt regulates the activity of NAD-consuming enzymes such as sirtuins and influences a variety of metabolic and stress responses. Nampt also plays an important part in regulating insulin secretion in pancreatic beta-cells. Nampt seems to have another function as an immunomodulatory cytokine and, therefore, has a role in inflammation. This review summarizes these various functional aspects of Nampt and discusses its potential roles in diseases, including type 2 diabetes and cancer.

The NAD World: a new systemic regulatory network for metabolism and aging--Sirt1, systemic NAD biosynthesis, and their importance

For the past several years, it has been demonstrated that the NAD-dependent protein deacetylase Sirt1 and nicotinamide phosphoribosyltransferase (Nampt)-mediated systemic NAD biosynthesis together play a critical role in the regulation of metabolism and possibly aging in mammals. Based on our recent studies on these two critical components, we have developed a hypothesis of a novel systemic regulatory network, named "NAD World", for mammalian aging. Conceptually, in the NAD World, systemic NAD biosynthesis mediated by intra- and extracellular Nampt functions as a driver that keeps up the pace of metabolism in multiple tissues/organs, and the NAD-dependent deacetylase Sirt1 serves as a universal mediator that executes metabolic effects in a tissue-dependent manner in response to changes in systemic NAD biosynthesis. This new concept of the NAD World provides important insights into a systemic regulatory mechanism that fundamentally connects metabolism and aging and also conveys the ideas of functional hierarchy and frailty for the regulation of metabolic robustness and aging in mammals.

[Adipocytokines: emerging mediators of the immune response and inflammation]

Scientific interest in the biology of white adipose tissue (WAT) has increased since the discovery of leptin in 1994. The description of the effects of leptin, the product of the ob gene, has started to clarify the role of adipose tissue in the physiopathology of obesity related diseases and has helped in the identification of a great number of other molecules (named adipocytokines), some of them with a proinflammatory nature. In rheumatic diseases, adipocytokines derived from the WAT are among the most important factors related to obesity and promote inflammatory and/or autoimmune conditions. In this review we will present the most recent advances in adipocytokine research, with special attention to the role of leptin, adiponectin, resistin and visfatin and inflammatory, autoimmune and rheumatic diseases.

Nicotinamide phosphoribosyltransferase (Nampt): a link between NAD biology, metabolism, and diseases

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

From heterochromatin islands to the NAD World: a hierarchical view of aging through the functions of mammalian Sirt1 and systemic NAD biosynthesis

For the past couple of decades, aging science has been rapidly evolving, and powerful genetic tools have identified a variety of evolutionarily conserved regulators and signaling pathways for the control of aging and longevity in model organisms. Nonetheless, a big challenge still remains to construct a comprehensive concept that could integrate many distinct layers of biological events into a systemic, hierarchical view of aging. The "heterochromatin island" hypothesis was originally proposed 10 years ago to explain deterministic and stochastic aspects of cellular and organismal aging, which drove the author to the study of evolutionarily conserved Sir2 proteins. Since a surprising discovery of their NAD-dependent deacetylase activity, Sir2 proteins, now called "sirtuins," have been emerging as a critical epigenetic regulator for aging. In this review, I will follow the process of conceptual development from the heterochromatin island hypothesis to a novel, comprehensive concept of a systemic regulatory network for mammalian aging, named "NAD World," summarizing recent studies on the mammalian NAD-dependent deacetylase Sirt1 and nicotinamide phosphoribosyltransferase (Nampt)-mediated systemic NAD biosynthesis. This new concept of the NAD World provides critical insights into a systemic regulatory mechanism that fundamentally connects metabolism and aging and also conveys the ideas of functional hierarchy and frailty for the regulation of aging in mammals.

Detection and pharmacological modulation of nicotinamide mononucleotide (NMN) in vitro and in vivo

The emerging key role of NAD-consuming enzymes in cell biology has renewed the interest in NAD resynthesis through the rescue pathways. The first step of the nicotinamide-dependent NAD-rescue pathway is operated by nicotinamide phosphoribosyl transferase (NaPRT) forming nicotinamide mononucleotide (NMN). Because of the difficulties in measuring NMN, numerous open questions exist about the pathophysiological relevance of NaPRT and NMN itself. Here, we describe a new method of fluorimetric NMN detection upon derivatization of its alkylpyridinium group with acetophenone. By adopting this method, we analyzed the kinetics of nicotinamide-dependent NAD recycling in HeLa and U937 cells. Measurement of NMN contents in subcellular fractions revealed that the nucleotide is highly enriched in mitochondria, suggesting intramitochondrial NAD synthesis. NMN increases in cells undergoing hyperactivation of the NAD-consuming enzyme poly(ADP-ribose) polymerase (PARP)-1, or exposed to gallotannin, a putative inhibitor of NMN-adenylyl transferases. Evidence that the inhibitor of NAD resynthesis FK866 selectively inhibits NaPRT having no effect on NMNAT activity is also provided. Importantly, NMN reduces NAD and ATP depletion in cells undergoing PARP-1 hyperactivation, significantly delaying cell death. Finally, we show that a single injection of FK866 in the mouse induces long-lasting (up to 16 h) but mild (approximately 20%) reduction of NMN contents in different organs, suggesting slow rate of basal NAD consumption in vivo. Data provide new information on the biochemistry and pharmacology of NAD biosynthesis, allowing a better understanding of pyridine nucleotide metabolism.

Relationship between cerebrospinal fluid visfatin (PBEF/Nampt) levels and adiposity in humans

OBJECTIVE

Observations of elevated circulating concentrations of visfatin (PBEF/Nampt) in obesity and diabetes suggest that this recently described adipokine is involved in the regulation of body weight and metabolism. We examined in humans whether visfatin is found in cerebrospinal fluid (CSF) and, if so, how CSF visfatin concentrations relate to adiposity and metabolic parameters.

RESEARCH DESIGN AND METHODS

We measured visfatin concentrations in the plasma and CSF of 38 subjects (18 men and 20 women; age 19-80 years) with a wide range of body weight (BMI 16.24-38.10 kg/m2). In addition, anthropometric parameters and endocrine markers were assessed. Bivariate correlation coefficients were determined and stepwise multiple regression analyses were performed to detect associations of CSF and plasma visfatin levels with relevant parameters.

RESULTS

Plasma visfatin levels increased with rising BMI (P < 0.0001) and body fat mass (P < 0.0001). In contrast, CSF visfatin levels decreased with increasing plasma visfatin concentrations (P < 0.03), BMI (P < 0.001), body fat mass (P < 0.0001), and insulin resistance (P < 0.05). Body fat was the only factor independently associated with CSF visfatin, explaining 58% of the variation of CSF visfatin levels (P < 0.0001). Neither plasma (P > 0.13) nor CSF (P > 0.61) visfatin concentrations differed between men and women.

CONCLUSIONS

Our data indicate that visfatin concentrations in human CSF decrease with rising body fat, supporting the assumption that visfatin transport across the blood-brain barrier is impaired in obesity and that central nervous visfatin insufficiency or resistance are linked to pathogenetic mechanisms of obesity.

Visfatin/PBEF/Nampt: structure, regulation and potential function of a novel adipokine

Over the last few years, it has become obvious that obesity and insulin resistance are linked by a variety of proteins secreted by adipocytes. Visfatin/PBEF (pre-B-cell colony-enhancing factor) has recently been identified as a novel adipokine with insulin-mimetic effects. Furthermore, an enzymatic function has been reported that reveals visfatin/PBEF as Nampt (nicotinamide phosphoribosyltransferase; EC 2.4.2.12.). Moreover, reports on the structure and hormonal regulation of visfatin/PBEF/Nampt have given further insights into its potential physiological role. The present review summarizes studies on visfatin/PBEF/Nampt as a novel adipokine.

Therapeutic potential of SIRT1 and NAMPT-mediated NAD biosynthesis in type 2 diabetes

Both genetic and environmental factors contribute to the pathogenesis of type 2 diabetes, and it is critical to understand the interplay between these factors in the regulation of insulin secretion and insulin sensitivity to develop effective therapeutic interventions for type 2 diabetes. For the past several years, studies on the mammalian NAD-dependent protein deacetylase SIRT1 and systemic NAD biosynthesis mediated by nicotinamide phosphoribosyltransferase (NAMPT) have demonstrated that these two regulatory components together play a critical role in the regulation of glucose homeostasis, particularly in the regulation of glucose-stimulated insulin secretion in pancreatic beta cells. These components also contribute to the age-associated decline in beta cell function, which has been suggested to be one of the major contributing factors to the pathogenesis of type 2 diabetes. In this review article, the roles of SIRT1 and NAMPT-mediated systemic NAD biosynthesis in glucose homeostasis and the pathophysiology of type 2 diabetes will be summarized, and their potential as effective targets for the treatment and prevention of type 2 diabetes will be discussed.

Adipocytokines and the metabolic complications of obesity

CONTEXT

Adipose tissue is increasingly recognized as an active endocrine organ with many secretory products and part of the innate immune system. With obesity, macrophages infiltrate adipose tissue, and numerous adipocytokines are released by both macrophages and adipocytes. Adipocytokines play important roles in the pathogenesis of insulin resistance and associated metabolic complications such as dyslipidemia, hypertension, and premature heart disease.

EVIDENCE ACQUISITION

Published literature was analyzed with the intent of addressing the role of the major adipose secretory proteins in human obesity, insulin resistance, and type 2 diabetes.

EVIDENCE SYNTHESIS

This review analyzes the characteristics of different adipocytokines, including leptin, adiponectin, pro-inflammatory cytokines, resistin, retinol binding protein 4, visfatin, and others, and their roles in the pathogenesis of insulin resistance.

CONCLUSIONS

Inflamed fat in obesity secretes an array of proteins implicated in the impairment of insulin signaling. Further studies are needed to understand the triggers that initiate inflammation in adipose tissue and the role of each adipokine in the pathogenesis of insulin resistance.

Visfatin in pregnancy: proposed mechanism of peptide delivery

Visfatin is a newly identified 52 kD adipocytokine that appears to have insulinomimetic properties. We examined visfatin expression in visceral fat from lean and pregnant women. Visfatin gene expression was seven times higher in omental fat of pregnant women than in lean women. Both immunohistochemistry and immunoblot confirmed that visfatin protein was much higher in pregnant women than in nonpregnant women. However, serum visfatin was 20.8 +/- 7.7 ng/ml (n = 7) in lean women as compared to only a slight increase to 40.3 ng/ml in pregnant women (n = 4). We measured visfatin mRNA content of human placenta and found that placenta expresses high levels of visfatin mRNA and protein. At a concentration of 2 nM, visfatin and insulin produced nearly identical increase in glucose transport. The discrepancy between the elevated visfatin expression and tissue visfatin compared to only a small increase in serum visfatin is a matter of controversy. The data on serum visfatin concentrations are replete with contradictory data. Taken together, we suggest that visfatin is not a hormone. Instead, we propose that visfatin acts in either a paracrine or autocrine mode. This hypothesis would explain what various laboratories have found widely discrepant values for serum visfatin. Since visfatin potently and efficaciously induced glucose transport in a cell culture model, any hypothetical role for visfatin in pregnancy should include the possibility that it may play a role in maternal/fetal glucose metabolism or distribution and that it may do so by acting locally.

Extracellular Nampt promotes macrophage survival via a nonenzymatic interleukin-6/STAT3 signaling mechanism

Macrophages play key roles in obesity-associated pathophysiology, including inflammation, atherosclerosis, and cancer, and processes that affect the survival-death balance of macrophages may have an important impact on obesity-related diseases. Adipocytes and other cells secrete a protein called extracellular nicotinamide phosphoribosyltransferase (eNampt; also known as pre-B cell colony enhancing factor or visfatin), and plasma levels of eNampt increase in obesity. Herein we tested the hypothesis that eNampt could promote cell survival in macrophages subjected to endoplasmic reticulum (ER) stress, a process associated with obesity and obesity-associated diseases. We show that eNampt potently blocks macrophage apoptosis induced by a number of ER stressors. The mechanism involves a two-step sequential process: rapid induction of interleukin 6 (IL-6) secretion, followed by IL-6-mediated autocrine/paracrine activation of the prosurvival signal transducer STAT3. The ability of eNampt to trigger this IL-6/STAT3 cell survival pathway did not depend on the presence of the Nampt enzymatic substrate nicotinamide in the medium, could not be mimicked by the Nampt enzymatic product nicotinamide mononucleotide (NMN), was not blocked by the Nampt enzyme inhibitor FK866, and showed no correlation with enzyme activity in a series of site-directed mutant Nampt proteins. Thus, eNampt protects macrophages from ER stress-induced apoptosis by activating an IL-6/STAT3 signaling pathway via a nonenzymatic mechanism. These data suggest a novel action and mechanism of eNampt that could affect the balance of macrophage survival and death in the setting of obesity, which in turn could play important roles in obesity-associated diseases.

Adipokines influencing metabolic and cardiovascular disease are differentially regulated in maintenance hemodialysis

Adipokines including leptin, adiponectin, visfatin, resistin, and interleukin (IL)-6 significantly influence energy metabolism, insulin sensitivity, and cardiovascular health. In the current study, we investigated serum levels of these adipokines in diabetic and nondiabetic patients on maintenance hemodialysis (MD) as compared with controls with a glomerular filtration rate greater than 50 mL/min. Serum leptin, adiponectin, high-molecular-weight (HMW) adiponectin, visfatin, resistin, and IL-6 were determined by enzyme-linked immunosorbent assay in control (n = 60) and MD (n = 60) patients and correlated to clinical and biochemical measures of renal function, glucose, and lipid metabolism, as well as inflammation. Adiponectin, visfatin, resistin, and IL-6 were significantly elevated in MD patients as compared with controls. In multivariate analyses, sex and body mass index were independently correlated with serum leptin levels in both controls and MD patients. Furthermore, insulin resistance was independently and negatively associated with adiponectin and HMW adiponectin in both groups. Moreover, circulating resistin levels were independently correlated with serum visfatin concentrations in control and MD patients. However, various independent associations were only found in either controls or patients on MD. Thus, serum IL-6 levels were strongly and independently associated with C reactive protein and resistin in MD patients but not control subjects. We show that levels of various adipokines are significantly increased in MD patients. Furthermore, regulation of adipokines in vivo strongly depends on renal function. Regulation of HMW adiponectin is similar as compared with total adiponectin in the patients studied.

Effect of antipsychotics on peptides involved in energy balance in drug-naive psychotic patients after 1 year of treatment

Weight gain has become one of the most common and concerning side effects of antipsychotic treatment. The mechanisms whereby antipsychotics induce weight gain are not known. It has been suggested that peptides related to food intake and energy balance could play a role in weight gain secondary to antipsychotic therapy. To better understand the pathophysiology of antipsychotic-induced weight gain, we studied the effects of 3 antipsychotic drugs (haloperidol, olanzapine, and risperidone) on peptides involved in energy balance (insulin, ghrelin, leptin, adiponectin, visfatin, and resistin) in a population of drug-naive patients with first episode of psychosis.A significant increase in weight (10.16 kg [SD, 8.30 kg]; P < 0.001), body mass index (3.56 kg/m [SD, 2.89 kg/m]; P < 0.001), and fasting insulin (3.93 muU/mL [SD, 3.93 muU/mL]; P = 0.028), leptin (6.76 ng/mL [SD, 7.21 ng/mL]; P < 0.001), and ghrelin (15.47 fmol/mL [SD, 47.90 fmol/mL]; P = 0.009) plasma levels were observed. The increments in insulin and leptin concentrations were highly correlated with the increment in weight and body mass index and seem to be a consequence of the higher fat stores. The unexpected increase in ghrelin levels might be related with the causal mechanism of weight gain induced by antipsychotics. Finally, the 3 antipsychotics had similar effects in all parameters evaluated.

Inflammatory mechanisms in the regulation of insulin resistance

Insulin resistance (IR) plays a key role in the pathophysiology of obesity-related diseases such as type 2 diabetes and nonalcoholic fatty liver disease. It has been demonstrated that IR is associated with a state of chronic low-grade inflammation, and several mediators released from various cell types, including immune cells and adipocytes, have been identified as being involved in the development of IR. Among those are several pro-inflammatory cytokines such as tumor necrosis factor-alpha(TNF-alpha), interleukin (IL)-1, IL-6, and various adipocytokines. Furthermore, several transcription factors and kinases such as c-Jun N-terminal kinase (JNK) and inhibitor of kappa B kinase-beta (IKKbeta), a kinase located proximal of nuclear factor-kappaB (NF-kappaB), participate in this process. Hepatocyte-specific overexpression of NF-kappaB is associated with IR and can mimic all features of fatty liver disease. Whereas the evidence for an important role of many pro-inflammatory pathways in IR in in vitro and animal studies is overwhelming, data from interventional studies in humans to prove this concept are still minor. As a complex network of inflammatory cytokines, adipocytokines, transcription factors, receptor molecules, and acute-phase reactants are involved in the development of IR, new therapeutic approaches in IR-related diseases will be based on a better understanding of their complex interactions.

Inflammatory mechanisms in the regulation of insulin resistance

Insulin resistance (IR) plays a key role in the pathophysiology of obesity-related diseases such as type 2 diabetes and nonalcoholic fatty liver disease. It has been demonstrated that IR is associated with a state of chronic low-grade inflammation, and several mediators released from various cell types, including immune cells and adipocytes, have been identified as being involved in the development of IR. Among those are several pro-inflammatory cytokines such as tumor necrosis factor-alpha(TNF-alpha), interleukin (IL)-1, IL-6, and various adipocytokines. Furthermore, several transcription factors and kinases such as c-Jun N-terminal kinase (JNK) and inhibitor of kappa B kinase-beta (IKKbeta), a kinase located proximal of nuclear factor-kappaB (NF-kappaB), participate in this process. Hepatocyte-specific overexpression of NF-kappaB is associated with IR and can mimic all features of fatty liver disease. Whereas the evidence for an important role of many pro-inflammatory pathways in IR in in vitro and animal studies is overwhelming, data from interventional studies in humans to prove this concept are still minor. As a complex network of inflammatory cytokines, adipocytokines, transcription factors, receptor molecules, and acute-phase reactants are involved in the development of IR, new therapeutic approaches in IR-related diseases will be based on a better understanding of their complex interactions.

Pharmacological inhibition of nicotinamide phosphoribosyltransferase/visfatin enzymatic activity identifies a new inflammatory pathway linked to NAD

Nicotinamide phosphoribosyltransferase (NAMPT), also known as visfatin, is the rate-limiting enzyme in the salvage pathway of NAD biosynthesis from nicotinamide. Since its expression is upregulated during inflammation, NAMPT represents a novel clinical biomarker in acute lung injury, rheumatoid arthritis, and Crohn's disease. However, its role in disease progression remains unknown. We report here that NAMPT is a key player in inflammatory arthritis. Increased expression of NAMPT was confirmed in mice with collagen-induced arthritis, both in serum and in the arthritic paw. Importantly, a specific competitive inhibitor of NAMPT effectively reduced arthritis severity with comparable activity to etanercept, and decreased pro-inflammatory cytokine secretion in affected joints. Moreover, NAMPT inhibition reduced intracellular NAD concentration in inflammatory cells and circulating TNFα levels during endotoxemia in mice. In vitro pharmacological inhibition of NAMPT reduced the intracellular concentration of NAD and pro-inflammatory cytokine secretion by inflammatory cells. Thus, NAMPT links NAD metabolism to inflammatory cytokine secretion by leukocytes, and its inhibition might therefore have therapeutic efficacy in immune-mediated inflammatory disorders.

The novel adipocytokine visfatin exerts direct cardioprotective effects

Visfatin is an adipocytokine capable of mimicking the glucose-lowering effects of insulin and activating the pro-survival kinases phosphatidylinositol-3-OH kinase (PI3K)-protein kinase B (Akt) and mitogen-activated protein kinase kinase 1 and 2 (MEK1/2)-extracellular signal-regulated kinase 1 and 2 (Erk 1/2). Experimental studies have demonstrated that the activation of these kinases confers cardioprotection through the inhibition of the mitochondrial permeability transition pore (mPTP). Whether visfatin is capable of exerting direct cardioprotective effects through these mechanisms is unknown and is the subject of the current study. Anaesthetized C57BL/6 male mice were subjected to in situ 30 min. of regional myocardial ischaemia and 120 min. of reperfusion. The administration of an intravenous bolus of visfatin (5 × 10⁻⁶μmol) at the time of myocardial reperfusion reduced the myocardial infarct size from 46.1 ± 4.1% in control hearts to 27.3 ± 4.0% (n≥ 6/group, P < 0.05), an effect that was blocked by the PI3K inhibitor, wortmannin, and the MEK1/2 inhibitor, U0126 (48.8 ± 5.5% and 45.9 ± 8.4%, respectively, versus 27.3 ± 4.0% with visfatin; n≥ 6/group, P < 0.05). In murine ventricular cardiomyocytes subjected to 30 min. of hypoxia followed by 30 min. of reoxygenation, visfatin (100 ng/ml), administered at the time of reoxygenation, reduced the cell death from 65.2 ± 4.6% in control to 49.2 ± 3.7%(n > 200 cells/group, P < 0.05), an effect that was abrogated by wortmannin and U0126 (68.1 ± 5.2% and 59.7 ± 6.2%, respectively; n > 200 cells/group, P > 0.05). Finally, the treatment of murine ventricular cardiomyocytes with visfatin (100 ng/ml) delayed the opening of the mPTP induced by oxidative stress from 81.2 ± 4 sec. in control to 120 ± 7 sec. (n > 20 cells/group, P < 0.05) in a PI3K- and MEK1/2-dependent manner. We report that the adipocytokine, visfatin, is capable of reducing myocardial injury when administered at the time of myocardial reperfusion in both the in situ murine heart and the isolated murine cardiomyocytes. The mechanism appears to involve the PI3K and MEK1/2 pathways and the mPTP.