The NAD metabolome--a key determinant of cancer cell biology

High glucose promotes gastric cancer chemoresistance in vivo and in vitro

The aim of the present study was to determine whether gastric cancer chemoresistance was increased under high glucose conditions by means of a clinical case study and experimental cytology. The expression of nicotinamide phosphoribosyltransferase (Nampt), silent information regulator 1 (sirt1), p53, p-glycoprotein (P-gp) and topoisomerase (topo)-IIα was evaluated in gastric cancer tissues and gastric cancer with diabetes tissues by immunohistochemistry. Subsequently, the survival time of the patients was assessed. For further investigation, the human gastric cancer cell line SGC7901 was subjected to different glucose concentrations and the aforementioned proteins were detected using reverse transcription-quantitative polymerase chain reaction and western blot analysis. Finally, cell sensitivity to chemotherapy treatment was examined in order to elucidate the role of high glucose in MDR. Positive expression of Nampt, Sirt1, p53, P-gp and Topo-IIα was observed to be higher in gastric cancer with diabetes patients compared with gastric cancer patients (P=0.01, 0.003, 0.0025, 0.016 and 0.336, respectively) with reduced survival time. Similar results were observed in SGC7901 cells. Additionally, cell proliferation rates of SGC7901 cells increased at glucose concentrations of 4,500 and 9,000 mg/l. Notably, the inhibition rates of 5-fluorouracil on cells decreased over 48 h when treated with 4,500 and 9,000 mg/l glucose compared with 1,000 mg/l. In conclusion, patients suffering from gastric cancer and diabetes exhibited greater negative effects, such as a poorer response to chemotherapy and had a lower survival time. High glucose conditions promoted gastric cancer cell proliferation and reduced susceptibility to chemotherapy drugs. These data provided a potential diagnostic and therapeutic strategy for gastric cancer chemoresistance.

Pharmacological NAD-Boosting Strategies Improve Mitochondrial Homeostasis in Human Complex I-Mutant Fibroblasts

Mitochondrial disorders are devastating genetic diseases for which efficacious therapies are still an unmet need. Recent studies report that increased availability of intracellular NAD obtained by inhibition of the NAD-consuming enzyme poly(ADP-ribose) polymerase (PARP)-1 or supplementation with the NAD-precursor nicotinamide riboside (NR) ameliorates energetic derangement and symptoms in mouse models of mitochondrial disorders. Whether these pharmacological approaches also improve bioenergetics of human cells harboring mitochondrial defects is unknown. It is also unclear whether the same signaling cascade is prompted by PARP-1 inhibitors and NR supplementation to improve mitochondrial homeostasis. Here, we show that human fibroblasts mutant for the NADH dehydrogenase (ubiquinone) Fe-S protein 1 (NDUFS1) subunit of respiratory complex I have similar ATP, NAD, and mitochondrial content compared with control cells, but show reduced mitochondrial membrane potential. Interestingly, mutant cells also show increased transcript levels of mitochondrial DNA but not nuclear DNA respiratory complex subunits, suggesting activation of a compensatory response. At variance with prior work in mice, however, NR supplementation, but not PARP-1 inhibition, increased intracellular NAD content in NDUFS1 mutant human fibroblasts. Conversely, PARP-1 inhibitors, but not NR supplementation, increased transcription of mitochondrial transcription factor A and mitochondrial DNA-encoded respiratory complexes constitutively induced in mutant cells. Still, both NR and PARP-1 inhibitors restored mitochondrial membrane potential and increased organelle content as well as oxidative activity of NDUFS1-deficient fibroblasts. Overall, data provide the first evidence that in human cells harboring a mitochondrial respiratory defect exposure to NR or PARP-1, inhibitors activate different signaling pathways that are not invariantly prompted by NAD increases, but equally able to improve energetic derangement.

Inhibition of the mevalonate pathway affects epigenetic regulation in cancer cells

The mevalonate pathway provides metabolites for post-translational modifications such as farnesylation, which are critical for the activity of RAS downstream signaling. Subsequently occurring regulatory processes can induce an aberrant stimulation of DNA methyltransferase (DNMT1) as well as changes in histone deacetylases (HDACs) and microRNAs in many cancer cell lines. Inhibitors of the mevalonate pathway are increasingly recognized as anticancer drugs. Extensive evidence indicates an intense cross-talk between signaling pathways, which affect growth, differentiation, and apoptosis either directly or indirectly via epigenetic mechanisms. Herein, we show data obtained by novel transcriptomic and corresponding methylomic or proteomic analyses from cell lines treated with pharmacologic doses of respective inhibitors (i.e., simvastatin, ibandronate). Metabolic pathways and their epigenetic consequences appear to be affected by a changed concentration of NADPH. Moreover, since the mevalonate metabolism is part of a signaling network, including vitamin D metabolism or fatty acid synthesis, the epigenetic activity of associated pathways is also presented. This emphasizes the far-reaching epigenetic impact of metabolic therapies on cancer cells and provides some explanation for clinical observations, which indicate the anticancer activity of statins and bisphosphonates.

Targeting respiratory complex I to prevent the Warburg effect

In the last 10 years, studies of energetic metabolism in different tumors clearly indicate that the definition of Warburg effect, i.e. the glycolytic shift cells undergo upon transformation, ought to be revisited considering the metabolic plasticity of cancer cells. In fact, recent findings show that the shift from glycolysis to re-established oxidative metabolism is required for certain steps of tumor progression, suggesting that mitochondrial function and, in particular, respiratory complex I are crucial for metabolic and hypoxic adaptation. Based on these evidences, complex I can be considered a lethality target for potential anticancer strategies. In conclusion, in this mini review we summarize and discuss why it is not paradoxical to develop pharmacological and genome editing approaches to target complex I as novel adjuvant therapies for cancer treatment. This article is part of a Directed Issue entitled: Energy Metabolism Disorders and Therapies.

Enhancing metabolomics research through data mining

Metabolomics research, like other disciplines utilizing high-throughput technologies, generates a large amount of data for every sample. Although handling this data is a challenge and one of the biggest bottlenecks of the metabolomics workflow, it is also the clue to accomplish valuable results. This work has been designed to supply methodological data mining guidelines, describing systematically the steps to be followed in metabolomics data exploration. Instrumental raw data refinement in the pre-processing step and assessment of the statistical assumptions in pre-treatment directly affect the results of subsequent univariate and multivariate analyses. A study of aging in a healthy population was selected to represent this data mining process. Multivariate analysis of variance and linear regression methods were used to analyze the metabolic changes underlying aging. Selection of both multivariate methods aims to illustrate the treatment of age from two rather different perspectives, as a categorical variable and a continuous variable.

BIOLOGICAL SIGNIFICANCE

Metabolomics is a discipline involving the analysis of a large amount of data to gather relevant information. Researchers in this field have to overcome the challenges of complex data processing and statistical analysis issues. A wide range of tasks has to be executed, from the minimization of batch-to-batch/systematic variations in pre-processing, to the application of common data analysis techniques relying on statistical assumptions. In this work, a real-data metabolic profiling research on aging was used to illustrate the proposed workflow and suggest a set of guidelines for analyzing metabolomics data. This article is part of a Special Issue entitled: HUPO 2014.

FK866-induced NAMPT inhibition activates AMPK and downregulates mTOR signaling in hepatocarcinoma cells

BACKGROUND

Nicotinamide phosphoribosyltransferase (NAMPT) is the key enzyme of the NAD salvage pathway starting from nicotinamide. Cancer cells have an increased demand for NAD due to their high proliferation and DNA repair rate. Consequently, NAMPT is considered as a putative target for anti-cancer therapies. There is evidence that AMP-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) become dysregulated during the development of hepatocellular carcinoma (HCC). Here, we investigated the effects of NAMPT inhibition by its specific inhibitor FK866 on the viability of hepatocarcinoma cells and analyzed the effects of FK866 on the nutrient sensor AMPK and mTOR complex1 (mTORC1) signaling.

RESULTS

FK866 markedly decreased NAMPT activity and NAD content in hepatocarcinoma cells (Huh7 cells, Hep3B cells) and led to delayed ATP reduction which was associated with increased cell death. These effects could be abrogated by administration of nicotinamide mononucleotide (NMN), the enzyme product of NAMPT. Our results demonstrated a dysregulation of the AMPK/mTOR pathway in hepatocarcinoma cells compared to non-cancerous hepatocytes with a higher expression of mTOR and a lower AMPKα activation in hepatocarcinoma cells. We found that NAMPT inhibition by FK866 significantly activated AMPKα and inhibited the activation of mTOR and its downstream targets p70S6 kinase and 4E-BP1 in hepatocarcinoma cells. Non-cancerous hepatocytes were less sensitive to FK866 and did not show changes in AMPK/mTOR signaling after FK866 treatment.

CONCLUSION

Taken together, these findings reveal an important role of the NAMPT-mediated NAD salvage pathway in the energy homeostasis of hepatocarcinoma cells and suggest NAMPT inhibition as a potential treatment option for HCC.

Global profiling of metabolic adaptation to hypoxic stress in human glioblastoma cells

Oncogenetic events and unique phenomena of the tumor microenvironment together induce adaptive metabolic responses that may offer new diagnostic tools and therapeutic targets of cancer. Hypoxia, or low oxygen tension, represents a well-established and universal feature of the tumor microenvironment and has been linked to increased tumor aggressiveness as well as resistance to conventional oncological treatments. Previous studies have provided important insights into hypoxia induced changes of the transcriptome and proteome; however, how this translates into changes at the metabolite level remains to be defined. Here, we have investigated dynamic, time-dependent effects of hypoxia on the cancer cell metabolome across all families of macromolecules, i.e., carbohydrate, protein, lipid and nucleic acid, in human glioblastoma cells. Using GC/MS and LC/MS/MS, 345 and 126 metabolites were identified and quantified in cells and corresponding media, respectively, at short (6 h), intermediate (24 h), and prolonged (48 h) incubation at normoxic or hypoxic (1% O₂) conditions. In conjunction, we performed gene array studies with hypoxic and normoxic cells following short and prolonged incubation. We found that levels of several key metabolites varied with the duration of hypoxic stress. In some cases, metabolic changes corresponded with hypoxic regulation of key pathways at the transcriptional level. Our results provide new insights into the metabolic response of glioblastoma cells to hypoxia, which should stimulate further work aimed at targeting cancer cell adaptive mechanisms to microenvironmental stress.

Mitochondrial dependency in progression of acute myeloid leukemia

Acute myeloid leukemia (AML) is a clonal hematopoietic malignant disorder which arises due to dysregulated differentiation, uncontrolled growth and inhibition of apoptosis leading to the accumulation of immature myeloid progenitor in the bone marrow. The heterogeneity of the disease at the molecular and cytogenetic level has led to the identification of several alteration of biological and clinical significance. One of the alterations which have gained attention in recent times is the altered energy and metabolic dependency of cancer originally proposed by Warburg. Mitochondria are important cell organelles regulating cellular energetic level, metabolism and apoptosis which in turn can affect cell proliferation and differentiation, the major manifestations of diseases like AML. In recent times the importance of mitochondrial generated ATP and mitochondrial localized metabolic pathways has been shown to play important role in the progression of AML. These studies have also demonstrated the clinical significance of mitochondrial targets for its effectiveness in combating relapsed or refractory AML. Here we review the importance of the mitochondrial dependency for the progression of AML and the emergence of the mitochondrial molecular targets which holds therapeutic importance.

NAMPT inhibition sensitizes pancreatic adenocarcinoma cells to tumor-selective, PAR-independent metabolic catastrophe and cell death induced by β-lapachone

Nicotinamide phosphoribosyltransferase (NAMPT) inhibitors (e.g., FK866) target the most active pathway of NAD(+) synthesis in tumor cells, but lack tumor-selectivity for use as a single agent. Reducing NAD(+) pools by inhibiting NAMPT primed pancreatic ductal adenocarcinoma (PDA) cells for poly(ADP ribose) polymerase (PARP1)-dependent cell death induced by the targeted cancer therapeutic, β-lapachone (β-lap, ARQ761), independent of poly(ADP ribose) (PAR) accumulation. β-Lap is bioactivated by NADPH:quinone oxidoreductase 1 (NQO1) in a futile redox cycle that consumes oxygen and generates high levels of reactive oxygen species (ROS) that cause extensive DNA damage and rapid PARP1-mediated NAD(+) consumption. Synergy with FK866+β-lap was tumor-selective, only occurring in NQO1-overexpressing cancer cells, which is noted in a majority (∼85%) of PDA cases. This treatment strategy simultaneously decreases NAD(+) synthesis while increasing NAD(+) consumption, reducing required doses and treatment times for both drugs and increasing potency. These complementary mechanisms caused profound NAD(P)(+) depletion and inhibited glycolysis, driving down adenosine triphosphate levels and preventing recovery normally observed with either agent alone. Cancer cells died through an ROS-induced, μ-calpain-mediated programmed cell death process that kills independent of caspase activation and is not driven by PAR accumulation, which we call NAD(+)-Keresis. Non-overlapping specificities of FK866 for PDA tumors that rely heavily on NAMPT-catalyzed NAD(+) synthesis and β-lap for cancer cells with elevated NQO1 levels affords high tumor-selectivity. The concept of reducing NAD(+) pools in cancer cells to sensitize them to ROS-mediated cell death by β-lap is a novel strategy with potential application for pancreatic and other types of NQO1+ solid tumors.

A microfluidic coculture and multiphoton FAD analysis assay provides insight into the influence of the bone microenvironment on prostate cancer cells

In prostate cancer, bone is a frequent site of metastasis; however, the molecular mechanisms of this tumor tropism remain unclear. Here, we integrate a microfluidic coculture platform with multi-photon imaging based techniques to assess both phenotypic cell behavior and FAD fluorescence intensity and fluorescence lifetime in the same cell. This platform combines two independent assays normally performed with two different cell populations into a single device, allowing us to simultaneously assess both phenotypic cell behavior and enzyme activity. We observed that the osteotropic prostate cancer cell line (C4-2B), when in a coculture with bone marrow stromal cells (MC3T3-E1), has increased protrusive phenotype and increased total and protein-bound FAD compared to its parent cell line (LNCaP). We hypothesized that an increase in ROS-generating APAO activity may be responsible for these effects, and found that the effects were decreased in the presence of the antioxidant N-Acetyl Cysteine (NAC). This suggests that an ROS-related signaling mechanism at the bone metastatic site may be correlated with and play a role in increased invasion of metastasizing prostate cancer cells. The studies performed using this combined platform will lead to new insights into the mechanisms that drive prostate cancer metastasis.

Identification of a novel NAMPT inhibitor by combinatorial click chemistry and chemical refinement

The identification of compounds able to inhibit the NAD salvage pathway is experiencing a growing popularity as it has been proposed to be a novel target for antitumoral and anti-inflammatory drugs.

Prediction of intracellular metabolic states from extracellular metabolomic data

Metabolic models can provide a mechanistic framework to analyze information-rich omics data sets, and are increasingly being used to investigate metabolic alternations in human diseases. An expression of the altered metabolic pathway utilization is the selection of metabolites consumed and released by cells. However, methods for the inference of intracellular metabolic states from extracellular measurements in the context of metabolic models remain underdeveloped compared to methods for other omics data. Herein, we describe a workflow for such an integrative analysis emphasizing on extracellular metabolomics data. We demonstrate, using the lymphoblastic leukemia cell lines Molt-4 and CCRF-CEM, how our methods can reveal differences in cell metabolism. Our models explain metabolite uptake and secretion by predicting a more glycolytic phenotype for the CCRF-CEM model and a more oxidative phenotype for the Molt-4 model, which was supported by our experimental data. Gene expression analysis revealed altered expression of gene products at key regulatory steps in those central metabolic pathways, and literature query emphasized the role of these genes in cancer metabolism. Moreover, in silico gene knock-outs identified unique control points for each cell line model, e.g., phosphoglycerate dehydrogenase for the Molt-4 model. Thus, our workflow is well-suited to the characterization of cellular metabolic traits based on extracellular metabolomic data, and it allows the integration of multiple omics data sets into a cohesive picture based on a defined model context.

Extracellular nicotinamide phosphoribosyltransferase (NAMPT) promotes M2 macrophage polarization in chronic lymphocytic leukemia

CLL lymphocytes show high intracellular and extracellular NAMPT levels, further increased upon activation. eNAMPT prompts differentiation of CLL monocytes into M2 macrophages that sustain CLL survival and reduce T-cell proliferation.

Identification of nicotinamide phosphoribosyltransferase (NAMPT) inhibitors with no evidence of CYP3A4 time-dependent inhibition and improved aqueous solubility

Herein we report the optimization efforts to ameliorate the potent CYP3A4 time-dependent inhibition (TDI) and low aqueous solubility exhibited by a previously identified lead compound from our NAMPT inhibitor program (1, GNE-617). Metabolite identification studies pinpointed the imidazopyridine moiety present in 1 as the likely source of the TDI signal, and replacement with other bicyclic systems was found to reduce or eliminate the TDI finding. A strategy of reducing the number of aromatic rings and/or lowering cLogD7.4 was then employed to significantly improve aqueous solubility. These efforts culminated in the discovery of 42, a compound with no evidence of TDI, improved aqueous solubility, and robust efficacy in tumor xenograft studies.

NAD(+) supplementation as a novel approach to cURIng HCC?

In this issue of Cancer Cell, Tummala and colleagues demonstrate that unconventional prefoldin RPB5 interactor (URI) expression in hepatocytes leads to hepatocellular carcinoma (HCC) development by interacting with L-tryptophan/kynurenine/nicotinamide adenine dinucleotide (NAD(+)) metabolism. The results suggest that supplementation of NAD(+) may be a prophylactic or therapeutic approach in HCC.

Metabolic profiling of alternative NAD biosynthetic routes in mouse tissues

NAD plays essential redox and non-redox roles in cell biology. In mammals, its de novo and recycling biosynthetic pathways encompass two independent branches, the "amidated" and "deamidated" routes. Here we focused on the indispensable enzymes gating these two routes, i.e. nicotinamide mononucleotide adenylyltransferase (NMNAT), which in mammals comprises three distinct isozymes, and NAD synthetase (NADS). First, we measured the in vitro activity of the enzymes, and the levels of all their substrates and products in a number of tissues from the C57BL/6 mouse. Second, from these data, we derived in vivo estimates of enzymes'rates and quantitative contributions to NAD homeostasis. The NMNAT activity, mainly represented by nuclear NMNAT1, appears to be high and nonrate-limiting in all examined tissues, except in blood. The NADS activity, however, appears rate-limiting in lung and skeletal muscle, where its undetectable levels parallel a relative accumulation of the enzyme's substrate NaAD (nicotinic acid adenine dinucleotide). In all tissues, the amidated NAD route was predominant, displaying highest rates in liver and kidney, and lowest in blood. In contrast, the minor deamidated route showed higher relative proportions in blood and small intestine, and higher absolute values in liver and small intestine. Such results provide the first comprehensive picture of the balance of the two alternative NAD biosynthetic routes in different mammalian tissues under physiological conditions. This fills a gap in the current knowledge of NAD biosynthesis, and provides a crucial information for the study of NAD metabolism and its role in disease.

The macrophage paradox

Macrophages are a diverse population of phagocytic cells that reside in tissues throughout the body. At sites of infection, macrophages encounter and engulf invading microbes. Accordingly, macrophages possess specialized effector functions to kill or coordinate the elimination of their prey. Nevertheless, many intracellular bacterial pathogens preferentially replicate inside macrophages. Here we consider explanations for what we call "the macrophage paradox:" why do so many pathogenic bacteria replicate in the very cells equipped to destroy them? We ask whether replication in macrophages is an unavoidable fate that essentially defines a key requirement to be a pathogen. Conversely, we consider whether fundamental aspects of macrophage biology provide unique cellular or metabolic environments that pathogens can exploit. We conclude that resolution of the macrophage paradox requires acknowledgment of the richness and complexity of macrophages as a replicative niche.

Sequence divergence and diversity suggests ongoing functional diversification of vertebrate NAD metabolism

NAD is not only an important cofactor in redox reactions but has also received attention in recent years because of its physiological importance in metabolic regulation, DNA repair and signaling. In contrast to the redox reactions, these regulatory processes involve degradation of NAD and therefore necessitate a constant replenishment of its cellular pool. NAD biosynthetic enzymes are common to almost all species in all clades, but the number of NAD degrading enzymes varies substantially across taxa. In particular, vertebrates, including humans, have a manifold of NAD degrading enzymes which require a high turnover of NAD. As there is currently a lack of a systematic study of how natural selection has shaped enzymes involved in NAD metabolism we conducted a comprehensive evolutionary analysis based on intraspecific variation and interspecific divergence. We compare NAD biosynthetic and degrading enzymes in four eukaryotic model species and subsequently focus on human NAD metabolic enzymes and their orthologs in other vertebrates. We find that the majority of enzymes involved in NAD metabolism are subject to varying levels of purifying selection. While NAD biosynthetic enzymes appear to experience a rather high level of evolutionary constraint, there is evidence for positive selection among enzymes mediating NAD-dependent signaling. This is particularly evident for members of the PARP family, a diverse protein family involved in DNA damage repair and programmed cell death. Based on haplotype information and substitution rate analysis we pinpoint sites that are potential targets of positive selection. We also link our findings to a three dimensional structure, which suggests that positive selection occurs in domains responsible for DNA binding and polymerization rather than the NAD catalytic domain. Taken together, our results indicate that vertebrate NAD metabolism is still undergoing functional diversification.

A novel crosstalk between BRCA1 and sirtuin 1 in ovarian cancer

BRCA mutations are the main known hereditary factors for ovarian cancer. Notably, emerging evidence has led to considerable interest in the role of sirtuin 1 (SIRT1) in ovarian cancer development. However, dynamic crosstalk between BRCA1 and SIRT1 is poorly understood. Here, we showed that: (i) BRCA1 inactivation events (mutation, promoter methylation, or knockdown) were accompanied by decreased SIRT1 levels and increased nicotinamide adenine dinucleotide (NAD) levels and a subsequent increase in SIRT1 activity; (ii) overexpression of BRCA1 resulted in increased SIRT1 levels, an impairment in NAD synthesis, and a subsequent inhibition of SIRT1 activity; and (iii) intracellular NAD levels were largely responsible for regulating SIRT1 activity, and BRCA1 expression patterns correlated with SIRT1 levels and NAD levels correlated with SIRT1 activity in human ovarian cancer specimens. Interestingly, although BRCA1 inactivation events inhibited SIRT1 expression, they led to a substantial increase in NAD levels that enhanced NAD-related SIRT1 activity. This is a special BRCA1-mediated compensatory mechanism for the maintenance of SIRT1 function. Therefore, these results highlight a novel interaction between BRCA1 and SIRT1, which may be beneficial for the dynamic balance between BRCA1-related biologic processes and SIRT1-related energy metabolism and stress response.

Age-dependent metabolic dysregulation in cancer and Alzheimer's disease

Age is the main risk factor for cancer and neurodegeneration; two radically divergent diseases. Yet selective pressure to meet cellular metabolic needs may provide a common mechanism linking these two disorders. The exclusive use of glycolysis, despite the presence of oxygen, is commonly referred to as aerobic glycolysis and is the primary metabolic pathway of cancer cells. Recent evidence suggests that aerobic glycolysis is also a key regulator of synaptic plasticity in the brain that may positively influence cognition. Elevated aerobic glycolysis is a contributing factor to the development of cancer as increased glycolytic flux plays an important role in the biosynthesis of macromolecules and promotes proliferation. In contrast, decreased aerobic glycolysis in the brain occurs with age and could lead to a loss of cell survival mechanisms that counter pathogenic processes underlying neurodegeneration. In this review we discuss the recent findings from epidemiological studies demonstrating an inverse comorbidity of cancer and Alzheimer's disease. We summarize evidence linking the two diseases through changes in metabolism over the course of normal aging. We discuss the key steps and regulatory mechanisms of aerobic glycolysis and mitochondrial oxidative phosphorylation which could be exploited for the development of novel therapies. In addition, we outline the regulation of aerobic glycolysis at the transcriptional level by HIF-1α and Pin1 and their roles in cancer and neurodegeneration. Finally, we provide a possible explanation for metabolic dysregulation that occurs with age, and how it may be a contributing factor to age-related diseases. Determining how metabolism becomes dysregulated over time could lead to the development of effective interventions for ensuring metabolic homeostasis and healthy aging.

Novel assay for simultaneous measurement of pyridine mononucleotides synthesizing activities allows dissection of the NAD(+) biosynthetic machinery in mammalian cells

The redox coenzyme NAD(+) is also a rate-limiting co-substrate for several enzymes that consume the molecule, thus rendering its continuous re-synthesis indispensable. NAD(+) biosynthesis has emerged as a therapeutic target due to the relevance of NAD(+) -consuming reactions in complex intracellular signaling networks whose alteration leads to many neurologic and metabolic disorders. Distinct metabolic routes, starting from various precursors, are known to support NAD(+) biosynthesis with tissue/cell-specific efficiencies, probably reflecting differential expression of the corresponding rate-limiting enzymes, i.e. nicotinamide phosphoribosyltransferase, quinolinate phosphoribosyltransferase, nicotinate phosphoribosyltransferase and nicotinamide riboside kinase. Understanding the contribution of these enzymes to NAD(+) levels depending on the tissue/cell type and metabolic status is necessary for the rational design of therapeutic strategies aimed at modulating NAD(+) availability. Here we report a simple, fast and sensitive coupled fluorometric assay that enables simultaneous determination of the four activities in whole-cell extracts and biological fluids. Its application to extracts from various mouse tissues, human cell lines and plasma yielded for the first time an overall picture of the tissue/cell-specific distribution of the activities of the various enzymes. The screening enabled us to gather novel findings, including (a) the presence of quinolinate phosphoribosyltransferase and nicotinamide riboside kinase in all examined tissues/cell lines, indicating that quinolinate and nicotinamide riboside are relevant NAD(+) precursors, and (b) the unexpected occurrence of nicotinate phosphoribosyltransferase in human plasma.

ARTD1 (PARP1) activation and NAD(+) in DNA repair and cell death

Nicotinamide adenine dinucleotide, NAD(+), is a small metabolite coenzyme that is essential for the progress of crucial cellular pathways including glycolysis, the tricarboxylic acid cycle (TCA) and mitochondrial respiration. These processes consume and produce both oxidative and reduced forms of NAD (NAD(+) and NADH). NAD(+) is also important for ADP(ribosyl)ation reactions mediated by the ADP-ribosyltransferase enzymes (ARTDs) or deacetylation reactions catalyzed by the sirtuins (SIRTs) which use NAD(+) as a substrate. In this review, we highlight the significance of NAD(+) catabolism in DNA repair and cell death through its utilization by ARTDs and SIRTs. We summarize the current findings on the involvement of ARTD1 activity in DNA repair and most specifically its involvement in the trigger of cell death mediated by ARTD1 activation and energy depletion. By sharing the same substrate, the activities of ARTDs and SIRTs are tightly linked, are dependent on each other and are thereby involved in the same cellular processes that play an important role in cancer biology, inflammatory diseases and ischaemia/reperfusion.

Browning of white adipose cells by intermediate metabolites: an adaptive mechanism to alleviate redox pressure

The presence of brown adipose tissue (BAT) in human adults opens attractive perspectives to treat metabolic disorders. Indeed, BAT dissipates energy as heat via uncoupling protein (UCP)1. Brown adipocytes are located in specific deposits or can emerge among white fat through the so-called browning process. Although numerous inducers have been shown to drive this process, no study has investigated whether it could be controlled by specific metabolites. Here, we show that lactate, an important metabolic intermediate, induces browning of murine white adipose cells with expression of functional UCP1. Lactate-induced browning also occurs in human cells and in vivo. Lactate controls Ucp1 expression independently of hypoxia-inducible factor-1α and PPARα pathways but requires active PPARγ signaling. We demonstrate that the lactate effect on Ucp1 is mediated by intracellular redox modifications as a result of lactate transport through monocarboxylate transporters. Further, the ketone body β-hydroxybutyrate, another metabolite that impacts redox state, is also a strong browning inducer. Because this redox-dependent increase in Ucp1 expression promotes an oxidative phenotype with mitochondria, browning appears as an adaptive mechanism to alleviate redox pressure. Our findings open new perspectives for the control of adipose tissue browning and its physiological relevance.

Targeting metabolic changes in cancer: novel therapeutic approaches

Therapeutic strategies designed to target cancer metabolism are an area of intense research. Antimetabolites, first used to treat patients in the early twentieth century, served as an early proof of concept for such therapies. We highlight strategies that attempt to improve on the anti-metabolite approach as well as new metabolic drug targets. Some of these targets have the advantage of a strong genetic anchor to drive patient selection (isocitrate dehydrogenase 1/2, Enolase 2). Additional approaches described here derive from hypothesis-driven and systems biology efforts designed to exploit tumor cell metabolic dependencies (fatty acid oxidation, nicotinamide adenine dinucleotide synthesis, glutamine biology).

Nicotinamide phosphoribosyltransferase can affect metastatic activity and cell adhesive functions by regulating integrins in breast cancer

NAD(+) metabolism is an essential regulator of cellular redox reactions, energy pathways, and a substrate provider for NAD(+) consuming enzymes. We recently demonstrated that enhancement of NAD(+)/NADH levels in breast cancer cells with impaired mitochondrial NADH dehydrogenase activity, through augmentation of complex I or by supplementing tumor cell nutrients with NAD(+) precursors, inhibits tumorigenicity and metastasis. To more fully understand how aberrantly low NAD(+) levels promote tumor cell dissemination, we here asked whether inhibition of NAD(+) salvage pathway activity by reduction in nicotinamide phosphoribosyltransferase (NAMPT) expression can impact metastasis and tumor cell adhesive functions. We show that knockdown of NAMPT, the enzyme catalyzing the rate-limiting step of the NAD(+) salvage pathway, enhances metastatic aggressiveness in human breast cancer cells and involves modulation of integrin expression and function. Reduction in NAMPT expression is associated with upregulation of select adhesion receptors, particularly αvβ3 and β1 integrins, and results in increased breast cancer cell attachment to extracellular matrix proteins, a key function in tumor cell dissemination. Interestingly, NAMPT downregulation prompts expression of integrin αvβ3 in a high affinity conformation, known to promote tumor cell adhesive interactions during hematogenous metastasis. NAMPT has been selected as a therapeutic target for cancer therapy based on the essential functions of this enzyme in NAD(+) metabolism, cellular redox, DNA repair and energy pathways. Notably, our results indicate that incomplete inhibition of NAMPT, which impedes NAD(+) metabolism but does not kill a tumor cell can alter its phenotype to be more aggressive and metastatic. This phenomenon could promote cancer recurrence, even if NAMPT inhibition initially reduces tumor growth.

Metabolic effect of TAp63α: enhanced glycolysis and pentose phosphate pathway, resulting in increased antioxidant defense

TAp63α is a member of the p53 family, which plays a central role in epithelial cancers. Recently, a role has emerged for p53 family members in cancer metabolic modulation. In order to assess whether TAp63α plays a role in cancer metabolism, we exploited p53-null osteosarcoma Tet-On Saos-2 cells, in which the expression of TAp63α was dependent on doxycycline supplementation to the medium. Metabolomics labeling experiments were performed by incubating the cells in 13C-glucose or 13C15N-glutamine-labeled culture media, as to monitor metabolic fluxes upon induced expression of TAp63α. Induced expression of TAp63α resulted in cell cycle arrest at the G1 phase. From a metabolic standpoint, expression of Tap63α promoted glycolysis and the pentose phosphate pathway, which was uncoupled from nucleotide biosynthesis, albeit prevented oxidative stress in the form of oxidized glutathione. Double 13C-glucose and 13C15N-glutamine metabolic labeling confirmed that induced expression of TAp63α corresponded to a decreased flux of pyruvate to the Krebs cycle and decreased utilization of glutamine for catabolic purposes in the TCA cycle. Results were not conclusive in relation to anabolic utilization of labeled glutamine, since it is unclear to what extent the observed minor TAp63α-dependent increases of glutamine-derived labeling in palmitate could be tied to increased rates of reductive carboxylation and de novo synthesis of fatty acids. Finally, bioinformatics elaborations highlighted a link between patient survival rates and the co-expression of p63 and rate limiting enzymes of the pentose phosphate pathway, G6PD and PGD.

NAMPT and NAPRT1: novel polymorphisms and distribution of variants between normal tissues and tumor samples

Nicotinamide phosphoribosyltransferase (NAMPT) and nicotinate phosphoribosyltransferase domain containing 1 (NAPRT1) are the main human NAD salvage enzymes. NAD regulates energy metabolism and cell signaling, and the enzymes that control NAD availability are linked to pathologies such as cancer and neurodegeneration. Here, we have screened normal and tumor samples from different tissues and populations of origin for mutations in human NAMPT and NAPRT1, and evaluated their potential pathogenicity. We have identified several novel polymorphisms and showed that NAPRT1 has a greater genetic diversity than NAMPT, where any alteration can have a greater functional impact. Some variants presented different frequencies between normal and tumor samples that were most likely related to their population of origin. The novel mutations described that affect protein structure or expression levels can be functionally relevant and should be considered in a disease context. Particularly, mutations that decrease NAPRT1 expression can predict the usefulness of Nicotinic Acid in tumor treatments with NAMPT inhibitors.

PARP inhibition delays progression of mitochondrial encephalopathy in mice

Mitochondrial disorders are deadly childhood diseases for which therapeutic remedies are an unmet need. Given that genetic suppression of the nuclear enzyme poly (adenine diphosphate-ribose) polymerase(PARP)-1 improves mitochondrial functioning, we investigated whether pharmacological inhibition of the enzyme affords protection in a mouse model of a mitochondrial disorder. We used mice lacking the Ndufs4 subunit of the respiratory complex I (Ndufs4 knockout [ KO] mice); these mice undergo progressive encephalopathy and die around postnatal day 50. Mice were treated daily with the potent PARP inhibitor N-(6-oxo-5,6-dihydrophenanthridin-2-yl)-(N,N-dimethylamino)acetamide hydrochloride (PJ34); neurological parameters, PARP activity, and mitochondrial homeostasis were evaluated. We found that mice receiving N-(6-oxo-5,6-dihydrophenanthridin-2-yl)-(N,N-dimethylamino)acetamide hydrochloride from postnatal day 30 to postnatal day 50 show reduced neurological impairment, and increased exploratory activity and motor skills compared with vehicle-treated animals. However, drug treatment did not delay or reduce death. We found no evidence of increased PARP activity within the brain of KO mice compared with heterozygous, healthy controls. Conversely, a 10-day treatment with the PARP inhibitor significantly reduced basal poly(ADP-ribosyl)ation in different organs of the KO mice, including brain, skeletal muscle, liver, pancreas, and spleen. In keeping with the epigenetic role of PARP-1, its inhibition correlated with increased expression of mitochondrial respiratory complex subunits and organelle number. Remarkably, pharmacological targeting of PARP reduced astrogliosis in olfactory bulb and motor cortex, but did not affect neuronal loss of KO mice. In light of the advanced clinical development of PARP inhibitors, these data emphasize their relevance to treatment of mitochondrial respiratory defects.

Detection of cerebral NAD(+) by in vivo (1)H NMR spectroscopy

Nicotinamide adenine dinucleotide (NAD(+)) plays a central role in cellular metabolism both as a coenzyme for electron-transfer enzymes as well as a substrate for a wide range of metabolic pathways. In the current study NAD(+) was detected on rat brain in vivo at 11.7T by 3D localized (1)H MRS of the NAD(+) nicotinamide protons in the 8.7-9.5 ppm spectral region. Avoiding water perturbation was critical to the detection of NAD(+) as strong, possibly indirect cross-relaxation between NAD(+) and water would lead to a several-fold reduction of the NAD(+) intensity in the presence of water suppression. Water perturbation was minimized through the use of localization by adiabatic spin-echo refocusing (LASER) in combination with frequency-selective excitation. The NAD(+) concentration in the rat cerebral cortex was determined at 296 ± 28 μm, which is in good agreement with recently published (31) P NMR-based results as well as results from brain extracts in vitro (355 ± 34 μm). The T1 relaxation time constants of the NAD(+) nicotinamide protons as measured by inversion recovery were 280 ± 65 and 1136 ± 122 ms in the absence and presence of water inversion, respectively. This confirms the strong interaction between NAD(+) nicotinamide and water protons as observed during water suppression. The T2 relaxation time constants of the NAD(+) nicotinamide protons were determined at 60 ± 13 ms after confounding effects of scalar coupling evolution were taken into account. The simplicity of the MR sequence together with the robustness of NAD(+) signal detection and quantification makes the presented method a convenient choice for studies on NAD(+) metabolism and function. As the method does not critically rely on magnetic field homogeneity and spectral resolution it should find immediate applications in rodents and humans even at lower magnetic fields.

The complex landscape of pancreatic cancer metabolism

Pancreatic ductal adenocarcinomas (PDA) are extremely aggressive cancers and currently available therapies are only minimally effective in treating this disease. Tackling this devastating cancer has been a major challenge to the scientific and medical communities, in part due to its intense therapeutic resistance. One of the aspects of this tumor that contributes to its aggressive behavior is its altered cellular metabolism. Indeed, PDA cells seem to possess the ability to adapt their metabolism to the particular environment to which they are exposed, including utilizing diverse fuel sources depending on their availability. Moreover, PDA tumors are efficient at recycling various metabolic substrates through activation of different salvage pathways such as autophagy and macropinocytosis. Together, these diverse metabolic adaptations allow PDA cells to survive and thrive in harsh environments that may lack nutrients and oxygen. Not surprisingly, given its central role in the pathogenesis of this tumor, oncogenic Kras plays a critical role in much of the metabolic reprogramming seen in PDA. In this review, we discuss the metabolic landscape of PDA tumors, including the molecular underpinnings of the key regulatory nodes, and describe how such pathways can be exploited for future diagnostic and therapeutic approaches.

Possible role of hemichannels in cancer

In humans, connexins (Cxs) and pannexins (Panxs) are the building blocks of hemichannels. These proteins are frequently altered in neoplastic cells and have traditionally been considered as tumor suppressors. Alteration of Cxs and Panxs in cancer cells can be due to genetic, epigenetic and post-transcriptional/post-translational events. Activated hemichannels mediate the diffusional membrane transport of ions and small signaling molecules. In the last decade hemichannels have been shown to participate in diverse cell processes including the modulation of cell proliferation and survival. However, their possible role in tumor growth and expansion remains largely unexplored. Herein, we hypothesize about the possible role of hemichannels in carcinogenesis and tumor progression. To support this theory, we summarize the evidence regarding the involvement of hemichannels in cell proliferation and migration, as well as their possible role in the anti-tumor immune responses. In addition, we discuss the evidence linking hemichannels with cancer in diverse models and comment on the current technical limitations for their study.

Poly(ADP-ribose): a signaling molecule in different paradigms of cell death

Poly(ADP-ribosylation) results from the conversion of NAD(+) into ADP-ribose and the following addition of ADP-ribose units to form polymers, further bound to acceptor proteins; once post-translationally ADP-ribosylated, proteins could change their function in basic processes. Poly(ADP-ribosylation) is activated under critical situations represented by DNA damage and cellular stress, and modulated in different paradigms of cell death. The hallmarks of the main death processes, i.e. apoptosis, parthanatos, necroptosis and autophagy, will be described, focusing on the role of poly(ADP-ribose) as a signaling molecule.

Nicotinamide phosphoribosyltransferase in malignancy: a review

Nicotinamide phosphoribosyltransferase (Nampt) catalyzes the rate-limiting step of nicotinamide adenine dinucleotide (NAD) synthesis. Both intracellular and extracellular Nampt (iNampt and eNampt) levels are increased in several human malignancies and some studies demonstrate increased iNampt in more aggressive/invasive tumors and in tumor metastases. Several different molecular targets have been identified that promote carcinogenesis following iNampt overexpression, including SirT1, CtBP, and PARP-1. Additionally, eNampt is elevated in several human cancers and is often associated with a higher tumor stage and worse prognoses. Here we review the roles of Nampt in malignancy, some of the known mechanisms by which it promotes carcinogenesis, and discuss the possibility of employing Nampt inhibitors in cancer treatment.

Deficiency of nicotinamide mononucleotide adenylyltransferase 3 (nmnat3) causes hemolytic anemia by altering the glycolytic flow in mature erythrocytes

NAD biosynthesis is of substantial interest because of its important roles in regulating various biological processes. Nicotinamide mononucleotide adenylyltransferase 3 (Nmnat3) is considered a mitochondria-localized NAD synthesis enzyme involved in de novo and salvage pathways. Although the biochemical properties of Nmnat3 are well documented, its physiological function in vivo remains unclear. In this study, we demonstrated that Nmnat3 was localized in the cytoplasm of mature erythrocytes and critically regulated their NAD pool. Deficiency of Nmnat3 in mice caused splenomegaly and hemolytic anemia, which was associated with the findings that Nmnat3-deficient erythrocytes had markedly lower ATP levels and shortened lifespans. However, the NAD level in other tissues were not apparently affected by the deficiency of Nmnat3. LC-MS/MS-based metabolomics revealed that the glycolysis pathway in Nmnat3-deficient erythrocytes was blocked at a glyceraldehyde 3-phosphate dehydrogenase (GAPDH) step because of the shortage of the coenzyme NAD. Stable isotope tracer analysis further demonstrated that deficiency of Nmnat3 resulted in glycolysis stall and a shift to the pentose phosphate pathway. Our findings indicate the critical roles of Nmnat3 in maintenance of the NAD pool in mature erythrocytes and the physiological impacts at its absence in mice.

Resveratrol differentially regulates NAMPT and SIRT1 in Hepatocarcinoma cells and primary human hepatocytes

Resveratrol is reported to possess chemotherapeutic properties in several cancers. In this study, we wanted to investigate the molecular mechanisms of resveratrol-induced cell cycle arrest and apoptosis as well as the impact of resveratrol on NAMPT and SIRT1 protein function and asked whether there are differences in hepatocarcinoma cells (HepG2, Hep3B cells) and non-cancerous primary human hepatocytes. We found a lower basal NAMPT mRNA and protein expression in hepatocarcinoma cells compared to primary hepatocytes. In contrast, SIRT1 was significantly higher expressed in hepatocarcinoma cells than in primary hepatocytes. Resveratrol induced cell cycle arrest in the S- and G2/M- phase and apoptosis was mediated by activation of p53 and caspase-3 in HepG2 cells. In contrast to primary hepatocytes, resveratrol treated HepG2 cells showed a reduction of NAMPT enzymatic activity and increased p53 acetylation (K382). Resveratrol induced NAMPT release from HepG2 cells which was associated with increased NAMPT mRNA expression. This effect was absent in primary hepatocytes where resveratrol was shown to function as NAMPT and SIRT1 activator. SIRT1 inhibition by EX527 resembled resveratrol effects on HepG2 cells. Furthermore, a SIRT1 overexpression significantly decreased both p53 hyperacetylation and resveratrol-induced NAMPT release as well as S-phase arrest in HepG2 cells. We could show that NAMPT and SIRT1 are differentially regulated by resveratrol in hepatocarcinoma cells and primary hepatocytes and that resveratrol did not act as a SIRT1 activator in hepatocarcinoma cells.

RIP1 maintains DNA integrity and cell proliferation by regulating PGC-1α-mediated mitochondrial oxidative phosphorylation and glycolysis

Aerobic glycolysis or the Warburg effect contributes to cancer cell proliferation; however, how this glucose metabolism pathway is precisely regulated remains elusive. Here we show that receptor-interacting protein 1 (RIP1), a cell death and survival signaling factor, regulates mitochondrial oxidative phosphorylation and aerobic glycolysis. Loss of RIP1 in lung cancer cells suppressed peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) expression, impairing mitochondrial oxidative phosphorylation and accelerating glycolysis, resulting in spontaneous DNA damage and p53-mediated cell proliferation inhibition. Thus, although aerobic glycolysis within a certain range favors cancer cell proliferation, excessive glycolysis causes cytostasis. Our data suggest that maintenance of glycolysis by RIP1 is pivotal to cancer cell energy homeostasis and DNA integrity and may be exploited for use in anticancer therapy.

Daratumumab granted breakthrough drug status

Multiple myeloma (MM) remains incurable despite important recent advances in treatment due to its inherent resistance, characterized by highly complex and heterogeneous molecular abnormalities, as well as the support from myeloma bone marrow (BM) microenvironment. A novel therapeutic strategy that effectively targets specific molecules on myeloma cells and also potentially overcomes tumor microenvironment-mediated drug resistance and the downstream effects of genetic instability is thus urgently needed. Over the last 2 years, an anti-CD38 monoclonal antibody daratumumab (DARA) has emerged as a breakthrough targeted therapy for patients with MM. Early-stage clinical trials have found DARA to be safe and to have encouraging clinical activity as a single agent and in combination with lenalidomide in heavily pretreated, relapsed patients in whom other novel agents (such as bortezomib, thalidomide and lenalidomide) as well as stem cell transplant has already failed. DARA may, therefore, be the first mAb with significant anti-MM activity both as a monotherapy and in combination. It is currently being further evaluated both alone and in combination with conventional and novel anti-MM agents as part of prospective clinical trials. This review discusses the preclinical and clinical development of DARA, its pathophysiological basis, and its prospects for future use in MM.

Alteration of mitochondrial proteome due to activation of Notch1 signaling pathway

The Notch signaling pathway, a known regulator of cell fate decisions, proliferation, and apoptosis, has recently been implicated in the regulation of glycolysis, which affects tumor progression. However, the impact of Notch on other metabolic pathways remains to be elucidated. To gain more insights into the Notch signaling and its role in regulation of metabolism, we studied the mitochondrial proteome in Notch1-activated K562 cells using a comparative proteomics approach. The proteomic study led to the identification of 10 unique proteins that were altered due to Notch1 activation. Eight of these proteins belonged to mitochondria-localized metabolic pathways like oxidative phosphorylation, glutamine metabolism, Krebs cycle, and fatty acid oxidation. Validation of some of these findings showed that constitutive activation of Notch1 deregulated glutamine metabolism and Complex 1 of the respiratory chain. Furthermore, the deregulation of glutamine metabolism involved the canonical Notch signaling and its downstream effectors. The study also reports the effect of Notch signaling on mitochondrial function and status of high energy intermediates ATP, NADH, and NADPH. Thus our study shows the effect of Notch signaling on mitochondrial proteome, which in turn affects the functioning of key metabolic pathways, thereby connecting an important signaling pathway to the regulation of cellular metabolism.

Metabolic implication of tumor:stroma crosstalk in breast cancer

The metabolic properties of cancer cells significantly differ from those of normal cells. In particular, cancer cells are largely dependent on aerobic glycolysis, a phenomenon that has been exploited clinically by using labelled glucose for positron emission tomography imaging. Importantly, cancer-associated alterations in metabolism are not merely due to the resulting response to cell proliferation and survival. Indeed, direct metabolic regulation could be driven by tumor oncogenes and/or suppressors, as demonstrated in several solid tumors, including breast cancer. Despite the fact that most breast cancer studies have focused on the intrinsic characteristics of breast tumor cells, it is now widely accepted that tumor microenvironment plays an important role in defining and reprogramming cancer cell metabolism. Tumor:stroma crosstalk, as well as inflammatory cues, concurs to outlining the cancer metabolism, impact on cancer aggressiveness and ultimately on patient survival and therapeutic responses. The aim of this review is to (i) gather the most recent data regarding the metabolic alterations in breast cancer, (ii) describe the role of tumor microenvironment in breast cancer cell metabolic reprogramming, and (iii) contemplate how targeting metabolic pathways aberrantly activated in breast cancer could help current therapeutic regimens.

Fragment-based identification of amides derived from trans-2-(pyridin-3-yl)cyclopropanecarboxylic acid as potent inhibitors of human nicotinamide phosphoribosyltransferase (NAMPT)

Potent, trans-2-(pyridin-3-yl)cyclopropanecarboxamide-containing inhibitors of the human nicotinamide phosphoribosyltransferase (NAMPT) enzyme were identified using fragment-based screening and structure-based design techniques. Multiple crystal structures were obtained of initial fragment leads, and this structural information was utilized to improve the biochemical and cell-based potency of the associated molecules. Many of the optimized compounds exhibited nanomolar antiproliferative activities against human tumor lines in in vitro cell culture experiments. In a key example, a fragment lead (13, KD = 51 μM) was elaborated into a potent NAMPT inhibitor (39, NAMPT IC50 = 0.0051 μM, A2780 cell culture IC50 = 0.000 49 μM) which demonstrated encouraging in vivo efficacy in an HT-1080 mouse xenograft tumor model.

Fragment-based design of 3-aminopyridine-derived amides as potent inhibitors of human nicotinamide phosphoribosyltransferase (NAMPT)

The fragment-based identification of two novel and potent biochemical inhibitors of the nicotinamide phosphoribosyltransferase (NAMPT) enzyme is described. These compounds (51 and 63) incorporate an amide moiety derived from 3-aminopyridine, and are thus structurally distinct from other known anti-NAMPT agents. Each exhibits potent inhibition of NAMPT biochemical activity (IC50=19 and 15 nM, respectively) as well as robust antiproliferative properties in A2780 cell culture experiments (IC50=121 and 99 nM, respectively). However, additional biological studies indicate that only inhibitor 51 exerts its A2780 cell culture effects via a NAMPT-mediated mechanism. The crystal structures of both 51 and 63 in complex with NAMPT are also independently described.

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.

bHLH-PAS proteins in cancer

Mammalian basic HLH (helix-loop-helix)-PER-ARNT-SIM (bHLH-PAS) proteins are heterodimeric transcription factors that sense and respond to environmental signals (such as pollutants) or to physiological signals (for example, hypoxia and circadian rhythms) through their two PAS domains. PAS domains form a generic three-dimensional fold, which commonly contains an internal cavity capable of small-molecule binding and outer surfaces adept at protein-protein interactions. These proteins are important in several pro-tumour and antitumour pathways and their activities can be modulated by both natural metabolites and oncometabolites. Recently determined structures and successful small-molecule screening programmes are now providing new opportunities to discover selective agonists and antagonists directed against this multitasking family of transcription factors.

Nicotinamide phosphoribosyltransferase (NAMPT) activity is essential for survival of resting lymphocytes

NAD biosynthesis is emerging as a key regulator of immune cell functions. Accordingly, inhibitors of the NAD-synthesizing enzyme nicotinamide phosphoribosyltransferase (NAMPT) have anti-inflammatory effects, counteract hematological malignancies and are being tested in clinical trials. Still, their effect on different cell types still waits to be fully investigated. Here we show that the NAMPT inhibitor FK866 induces NAD depletion in various mouse organs but selectively causes dramatic atrophy of the spleen red pulp. Accordingly, in cultured mouse lymphocytes exposed to FK866, NAD contents drop to 50% of basal values within 2 days, a condition sufficient to prompt complete cell death. Cultures of human lymphocytes are more resistant to FK866 and sustain a 50% NAD reduction for 5 days before dying. Death of both cell types can be prevented by different NAD precursors, indicating critical NAD homeostasis in lymphocytes. Indeed, inhibition of the NAD-consuming enzyme poly(ADP-ribose) polimerase-1 suffices to prevent FK866-induced NAD depletion and death of both lymphocyte types. Poly(ADP-ribose) polymerase-1-null lymphocytes also undergo lower NAD depletion and reduced cell death when exposed to the drug. At variance with other cell types, neither apoptosis nor autophagy are exclusively responsible for lymphocyte death by FK866, consistent with a general impairment of lymphocyte homeostasis following NAD depletion. Data demonstrate a unique sensitivity of resting lymphocytes to NAD-depleting agents, providing new hints of relevance to lymphocyte biology and therapeutic interventions with NAMPT inhibitors.

Carbohydrate metabolism during vertebrate appendage regeneration: what is its role? How is it regulated?: A postulation that regenerating vertebrate appendages facilitate glycolytic and pentose phosphate pathways to fuel macromolecule biosynthesis

We recently examined gene expression during Xenopus tadpole tail appendage regeneration and found that carbohydrate regulatory genes were dramatically altered during the regeneration process. In this essay, we speculate that these changes in gene expression play an essential role during regeneration by stimulating the anabolic pathways required for the reconstruction of a new appendage. We hypothesize that during regeneration, cells use leptin, slc2a3, proinsulin, g6pd, hif1α expression, receptor tyrosine kinase (RTK) signaling, and the production of reactive oxygen species (ROS) to promote glucose entry into glycolysis and the pentose phosphate pathway (PPP), thus stimulating macromolecular biosynthesis. We suggest that this metabolic shift is integral to the appendage regeneration program and that the Xenopus model is a powerful experimental system to further explore this phenomenon. Also watch the Video Abstract.

The NADH oxidase ENOX1, a critical mediator of endothelial cell radiosensitization, is crucial for vascular development

ENOX1 is a highly conserved NADH oxidase that helps to regulate intracellular nicotinamide adenine dinucleotide levels in many cell types, including endothelial cells. Pharmacologic and RNA interference (RNAi)-mediated suppression of ENOX1 impairs surrogate markers of tumor angiogenesis/vasculogenesis, providing support for the concept that ENOX1 represents an antiangiogenic druggable target. However, direct genetic evidence that demonstrates a role for ENOX1 in vascular development is lacking. In this study, we exploited a zebrafish embryonic model of development to address this question. Whole-mount in situ hybridization coupled with immunofluorescence performed on zebrafish embryos demonstrate that enox1 message and translated protein are expressed in most tissues, and its expression is enriched in blood vessels and heart. Morpholino-mediated suppression of Enox1 in Tg(fli1-eGFP) and Tg(flk1-eGFP) zebrafish embryos significantly impairs the development of vasculature and blood circulation. Using in vivo multiphoton microscopy, we show that morpholino-mediated knockdown of enox1 increases NADH levels, consistent with loss of enzyme. VJ115 is a small-molecule inhibitor of Enox1's oxidase activity shown to increase intracellular NADH in endothelial cells; we used VJ115 to determine if the oxidase activity was crucial for vascular development. We found that VJ115 suppressed vasculogenesis in Tg(fli1-eGFP) embryos and impaired circulation. Previously, it was shown that suppression of ENOX1 radiosensitizes proliferating tumor vasculature, a consequence of enhanced endothelial cell apoptosis. Thus, our current findings, coupled with previous research, support the hypothesis that ENOX1 represents a potential cancer therapy target, one that combines molecular targeting with cytotoxic sensitization.

Insight into molecular and functional properties of NMNAT3 reveals new hints of NAD homeostasis within human mitochondria

Among the enzymes involved in NAD homeostasis, nicotinamide mononucleotide adenylyltransferases (NMNAT1-3) are central to intracellular NAD formation. Although NMNAT3 is postulated to be a mitochondrial enzyme contributing to NAD-dependent organelle functioning, information on endogenous proteins is lacking. We report that in human cells a single gene nmnat3 localized on chromosome 3 codes for two mRNA splice variants NMNATv1 and FKSG76, whereas the previously reported NMNAT3v2 transcript is not present. However, NMNAT3v1 and FKSG76 proteins are not detectable, consistent with the finding that an upstream ORF in their mRNAs negatively regulates translation. NMNAT3v1 transfection demonstrates that the protein is cytosolic and inactive, whereas FKSG76 is mitochondrial but operates NAD cleavage rather than synthesis. In keeping with the lack of NMNAT3, we show that extracellular NAD, but not its metabolic precursors, sustains mitochondrial NAD pool in an ATP-independent manner. Data of the present study modify the scenario of the origin of mitochondrial NAD by showing that, in human cells, NMNAT3 is absent in mitochondria, and, akin to plants and yeast, cytosolic NAD maintains the mitochondrial NAD pool.