Sirtuins: The NAD+-Dependent Multifaceted Modulators of Inflammation

Significance: Sirtuins are NAD+-dependent histone deacetylases regulating important processes in cellular biology such as inflammation, metabolism, oxidative stress, and apoptosis. Recent Advances: Despite initially being discovered to regulate transcription and life span via histone deacetylase activities, emerging data continually uncover new targets and propose additional roles. Due to the outstanding importance of the sirtuins in the control of the inflammatory response, their roles in the pathogenesis of several inflammatory-based diseases have become an area of intense research. Although sirtuins have been traditionally regarded as anti-inflammatory players, several recent findings suggest that their role in inflammation is complex and that in some cases sirtuins can indeed promote inflammation. Critical Issues: In this article, we provide an update on the latest findings concerning the new mechanisms of action and concepts about the role of sirtuins during inflammation. We focus on the impact that inflammatory-based processes exert on the liver, adipose tissue, and nervous system as well as on macrophage function and activation. Also, we discuss available data pointing to the dual role that, in particular contexts, sirtuins may have on inflammation control. Future Directions: Since the knowledge of sirtuin impact on metabolism is continually expanding, new venues of research arise. Besides become being regarded as candidates of therapeutic targets, posttranscriptional control of sirtuin expression by means of microRNAs challenges our traditional concepts of sirtuin regulation; importantly, the emerging role of NAD+ metabolism in aging and longevity has added a new dimension to the interest in sirtuin function. Antioxid. Redox Signal. 39, 1185-1208.

The NADase CD38 may not dictate NAD levels in brain mitochondria of aged mice but regulates hydrogen peroxide generation

Aging is a time-related functional decline that affects many species. One of the hallmarks of aging is mitochondrial dysfunction, which leads to metabolic decline. The NAD decline during aging, in several tissues, correlates with increase in NADase activity of CD38. Knock out or pharmacological inhibition of CD38 activity can rescue mitochondrial function in several tissues, however, the role of CD38 in controlling NAD levels and metabolic function in the aging brain is unknown. In this work, we investigated CD38 NADase activity controlling NAD levels and mitochondrial function in mice brain with aging. We demonstrate that NADase activity of CD38 does not dictate NAD total levels in brain of aging mice and does not control mitochondrial oxygen consumption nor other oxygen parameters markers of mitochondrial dysfunction. However, for the first time we show that CD38 regulates hydrogen peroxide (H2O2) generation, one of the reactive oxygen species (ROS) in aging brain, through regulation of pyruvate dehydrogenase and alfa-ketoglutarate dehydrogenase, as mitochondria H2O2 leakage sites. The effect may be related to mitochondrial calcium handling differences in CD38 absence. Our study highlights a novel role of CD38 in brain energy metabolism and aging.

A nitroalkene derivative of salicylate alleviates diet-induced obesity by activating creatine metabolism and non-shivering thermogenesis

Obesity-related type II diabetes (diabesity) has increased global morbidity and mortality dramatically. Previously, the ancient drug salicylate demonstrated promise for the treatment of type II diabetes, but its clinical use was precluded due to high dose requirements. In this study, we present a nitroalkene derivative of salicylate, 5-(2-nitroethenyl)salicylic acid (SANA), a molecule with unprecedented beneficial effects in diet-induced obesity (DIO). SANA reduces DIO, liver steatosis and insulin resistance at doses up to 40 times lower than salicylate. Mechanistically, SANA stimulated mitochondrial respiration and increased creatine-dependent energy expenditure in adipose tissue. Indeed, depletion of creatine resulted in the loss of SANA action. Moreover, we found that SANA binds to creatine kinases CKMT1/2, and downregulation CKMT1 interferes with the effect of SANA in vivo. Together, these data demonstrate that SANA is a first-in-class activator of creatine-dependent energy expenditure and thermogenesis in adipose tissue and emerges as a candidate for the treatment of diabesity.

Rapid regulation of substrate use for oxidative phosphorylation during a single session of high intensity interval or aerobic exercises in different rat skeletal muscles

Different exercise protocols lead to long-term adaptations that are related to increased mitochondrial content through the activation of mitochondrial biogenesis. However, immediate mitochondrial response to exercise and energetic substrate utilization is still unknown. We evaluate the mitochondrial physiology of two types rat skeletal muscle fibres immediately after a single session of high intensity interval exercise (HIIE) or aerobic exercise (AER). We found AER was able to reduce the ATP synthesis dependent oxygen flux in the tibialis (TA) when stimulated by complex I and II substrates. On the other hand, there was an increase of the maximum velocity (V_max) for glycerol-phosphate oxidation and V_max and affinity (K_M) of palmitoyl-carnitine oxidation (PC). The exercise did not affect oxygen flux coupled to ATP synthesis in red gastrocnemius (RG) but, surprisingly, reduced its affinity for PC, decreasing the apparent catalytic efficiency (V_max/K_M) of oxidation for PC. Neither exercise protocol was able to change the electron transfer system capacity of the mitochondria or markers of mitochondrial content. The AER group had increased H₂O₂ production compared to the SED and HIIE groups, with the mechanism being predominantly the escape of electrons through reverse flux in complex I and other sites in TA, and only through other sites in RG. There were no changes in the activities of antioxidant enzymes. Our results show that mitochondria from different muscles submitted to distinct exercise protocols show alterations in the specific fluxes of substrate utilization and oxygen metabolism, indicating that the dynamics of mitochondria are linked to the metabolic flexibility.

HSulf-1 deficiency dictates a metabolic reprograming of glycolysis and TCA cycle in ovarian cancer

Warburg effect has emerged as a potential hallmark of many cancers. However, the molecular mechanisms that led to this metabolic state of aerobic glycolysis, particularly in ovarian cancer (OVCA) have not been completely elucidated. HSulf-1 predominantly functions by limiting the bioavailability of heparan binding growth factors and hence their downstream signaling. Here we report that HSulf-1, a known putative tumor suppressor, is a negative regulator of glycolysis. Silencing of HSulf-1 expression in OV202 cell line increased glucose uptake and lactate production by upregulating glycolytic genes such as Glut1, HKII, LDHA, as well as metabolites. Conversely, HSulf-1 overexpression in TOV21G cells resulted in the down regulation of glycolytic enzymes and reduced glycolytic phenotype, supporting the role of HSulf-1 loss in enhanced aerobic glycolysis. HSulf-1 deficiency mediated glycolytic enhancement also resulted in increased inhibitory phosphorylation of pyruvate dehydrogenase (PDH) thus blocking the entry of glucose flux into TCA cycle. Consistent with this, metabolomic and isotope tracer analysis showed reduced glucose flux into TCA cycle. Moreover, HSulf-1 loss is associated with lower oxygen consumption rate (OCR) and impaired mitochondrial function. Mechanistically, lack of HSulf-1 promotes c-Myc induction through HB-EGF-mediated p-ERK activation. Pharmacological inhibition of c-Myc reduced HB-EGF induced glycolytic enzymes implicating a major role of c-Myc in loss of HSulf-1 mediated altered glycolytic pathway in OVCA. Similarly, PG545 treatment, an agent that binds to heparan binding growth factors and sequesters growth factors away from their ligand also blocked HB-EGF signaling and reduced glucose uptake in vivo in HSulf-1 deficient cells.

CD38 Dictates Age-Related NAD Decline and Mitochondrial Dysfunction through an SIRT3-Dependent Mechanism

Nicotinamide adenine dinucleotide (NAD) levels decrease during aging and are involved in age-related metabolic decline. To date, the mechanism responsible for the age-related reduction in NAD has not been elucidated. Here we demonstrate that expression and activity of the NADase CD38 increase with aging and that CD38 is required for the age-related NAD decline and mitochondrial dysfunction via a pathway mediated at least in part by regulation of SIRT3 activity. We also identified CD38 as the main enzyme involved in the degradation of the NAD precursor nicotinamide mononucleotide (NMN) in vivo, indicating that CD38 has a key role in the modulation of NAD-replacement therapy for aging and metabolic diseases.

Cardiomyopathy and Worsened Ischemic Heart Failure in SM22-α Cre-Mediated Neuropilin-1 Null Mice: Dysregulation of PGC1α and Mitochondrial Homeostasis

OBJECTIVE

Neuropilin-1 (NRP-1) is a multidomain membrane receptor involved in angiogenesis and development of neuronal circuits, however, the role of NRP-1 in cardiovascular pathophysiology remains elusive.

APPROACH AND RESULTS

In this study, we first observed that deletion of NRP-1 induced peroxisome proliferator-activated receptor γ coactivator 1α in cardiomyocytes and vascular smooth muscle cells, which was accompanied by dysregulated cardiac mitochondrial accumulation and induction of cardiac hypertrophy- and stress-related markers. To investigate the role of NRP-1 in vivo, we generated mice lacking Nrp-1 in cardiomyocytes and vascular smooth muscle cells (SM22-α-Nrp-1 KO), which exhibited decreased survival rates, developed cardiomyopathy, and aggravated ischemia-induced heart failure. Mechanistically, we found that NRP-1 specifically controls peroxisome proliferator-activated receptor γ coactivator 1 α and peroxisome proliferator-activated receptor γ in cardiomyocytes through crosstalk with Notch1 and Smad2 signaling pathways, respectively. Moreover, SM22-α-Nrp-1 KO mice exhibited impaired physical activities and altered metabolite levels in serum, liver, and adipose tissues, as demonstrated by global metabolic profiling analysis.

CONCLUSIONS

Our findings provide new insights into the cardioprotective role of NRP-1 and its influence on global metabolism.

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

PURPOSE

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

EXPERIMENTAL DESIGN

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

RESULTS

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

CONCLUSIONS

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

Insulin prevents mitochondrial generation of H₂O₂ in rat brain

The mitochondrial electron transport system (ETS) is a main source of cellular ROS, including hydrogen peroxide (H₂O₂). The production of H₂O₂ also involves the mitochondrial membrane potential (ΔΨm) and oxygen consumption. Impaired insulin signaling causes oxidative neuronal damage and places the brain at risk of neurodegeneration. We evaluated whether insulin signaling cross-talks with ETS components (complexes I and F₀F₁ATP synthase) and ΔΨm to regulate mitochondrial H₂O₂ production, in tissue preparations from rat brain. Insulin (50 to 100 ng/mL) decreased H₂O₂ production in synaptosomal preparations in high Na(+) buffer (polarized state), stimulated by glucose and pyruvate, without affecting the oxygen consumption. In addition, insulin (10 to 100 ng/mL) decreased H₂O₂ production induced by succinate in synaptosomes in high K(+) (depolarized state), whereas wortmannin and LY290042, inhibitors of the PI3K pathway, reversed this effect; heated insulin had no effect. Insulin decreased H₂O₂ production when complexes I and F₀F₁ATP synthase were inhibited by rotenone and oligomycin respectively suggesting a target effect on complex III. Also, insulin prevented the generation of maximum level of ∆Ψm induced by succinate. The PI3K inhibitors and heated insulin maintained the maximum level of ∆Ψm induced by succinate in synaptosomes in a depolarized state. Similarly, insulin decreased ROS production in neuronal cultures. In mitochondrial preparations, insulin neither modulated H2O2 production or oxygen consumption. In conclusion, the normal downstream insulin receptor signaling is necessary to regulate complex III of ETS avoiding the generation of maximal ∆Ψm and increased mitochondrial H2O2 production.

Cardiac systolic dysfunction in doxorubicin-challenged rats is associated with upregulation of MuRF2 and MuRF3 E3 ligases

Doxorubicin (DOXO) is an efficient and low-cost chemotherapeutic agent. The use of DOXO is limited by its side effects, including cardiotoxicity, that may progress to cardiac failure as a result of multifactorial events that have not yet been fully elucidated. In the present study, the effects of DOXO at two different doses were analyzed to identify early functional and molecular markers of cardiac distress. One group of rats received 7.5 mg/kg of DOXO (low-dose group) and was followed for 20 weeks. A subset of these animals was then subjected to an additional cycle of DOXO treatment, generating a cumulative dose of 20 mg/kg (high-dose group). Physiological and biochemical parameters were assessed in both treatment groups and in a control group that received saline. Systolic dysfunction was observed only in the high-dose group. Mitochondrial function analysis showed a clear reduction in oxidative cellular respiration for animals in both DOXO treatment groups, with evidence of complex I damage being observed. Transcriptional analysis by quantitative polymerase chain reaction revealed an increase in atrial natriuretic peptide transcript in the high-dose group, which is consistent with cardiac failure. Analysis of transcription levels of key components of the cardiac ubiquitin-proteasome system found that the ubiquitin E3 ligase muscle ring finger 1 (MuRF1) was upregulated in both the low- and high-dose DOXO groups. MuRF2 and MuRF3 were also upregulated in the high-dose group but not in the low-dose group. This molecular profile may be useful as an early physiological and energetic cardiac failure indicator for testing therapeutic interventions in animal models.

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

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

Brown adipose tissue mitochondria: modulation by GDP and fatty acids depends on the respiratory substrates

The UCP1 [first UCP (uncoupling protein)] that is found in the mitochondria of brown adipocytes [BAT (brown adipose tissue)] regulates the heat production, a process linked to non-shivering thermogenesis. The activity of UCP1 is modulated by GDP and fatty acids. In this report, we demonstrate that respiration and heat released by BAT mitochondria vary depending on the respiratory substrate utilized and the coupling state of the mitochondria. It has already been established that, in the presence of pyruvate/malate, BAT mitochondria are coupled by faf-BSA (fatty-acid-free BSA) and GDP, leading to an increase in ATP synthesis and mitochondrial membrane potential along with simultaneous decreases in both the rates of respiration and heat production. Oleate restores the uncoupled state, inhibiting ATP synthesis and increasing the rates of both respiration and heat production. We now show that in the presence of succinate: (i) the rates of uncoupled mitochondria respiration and heat production are five times slower than in the presence of pyruvate/malate; (ii) faf-BSA and GDP accelerate heat and respiration as a result and, in coupled mitochondria, these two rates are accelerated compared with pyruvate/malate; (iii) in spite of the differences in respiration and heat production noted with the two substrates, the membrane potential and the ATP synthesized were the same; and (iv) oleate promoted a decrease in heat production and respiration in coupled mitochondria, an effect different from that observed using pyruvate/malate. These effects are not related to the production of ROS (reactive oxygen species). We suggest that succinate could stimulate a new route to heat production in BAT mitochondria.

Reactive oxygen species production by potato tuber mitochondria is modulated by mitochondrially bound hexokinase activity

Potato tuber (Solanum tuberosum) mitochondria (PTM) have a mitochondrially bound hexokinase (HK) activity that exhibits a pronounced sensitivity to ADP inhibition. Here we investigated the role of mitochondrial HK activity in PTM reactive oxygen species generation. Mitochondrial HK has a 10-fold higher affinity for glucose (Glc) than for fructose (KMGlc=140 microM versus KMFrc=1,375 microM). Activation of PTM respiration by succinate led to an increase in hydrogen peroxide (H2O2) release that was abrogated by mitochondrial HK activation. Mitochondrial HK activity caused a decrease in the mitochondrial membrane potential and an increase in oxygen consumption by PTM. Inhibition of Glc phosphorylation by mannoheptulose or GlcNAc induced a rapid increase in H2O2 release. The blockage of H2O2 release sustained by Glc was reverted by oligomycin and atractyloside, indicating that ADP recycles through the adenine nucleotide translocator and F0F1ATP synthase is operative during the mitochondrial HK reaction. Inhibition of mitochondrial HK activity by 60% to 70% caused an increase of 50% in the maximal rate of H2O2 release. Inhibition in H2O2 release by mitochondrial HK activity was comparable to, or even more potent, than that observed for StUCP (S. tuberosum uncoupling protein) activity. The inhibition of H2O2 release in PTM was two orders of magnitude more selective for the ADP produced from the mitochondrial HK reaction than for that derived from soluble yeast (Saccharomyces cerevisiae) HK. Modulation of H2O2 release and oxygen consumption by Glc and mitochondrial HK inhibitors in potato tuber slices shows that hexoses and mitochondrial HK may act as a potent preventive antioxidant mechanism in potato tubers.