S-nitrosylation of fatty acid synthase regulates its activity through dimerization

Activation of hepatic acetyl-CoA carboxylase by S-nitrosylation in response to diet

Nitric oxide (NO), produced primarily by nitric oxide synthase enzymes, is known to influence energy metabolism by stimulating fat uptake and oxidation. The effects of NO on de novo lipogenesis (DNL), however, are less clear. Here we demonstrate that hepatic expression of endothelial nitric oxide synthase is reduced following prolonged administration of a hypercaloric high-fat diet. This results in marked reduction in the amount of S-nitrosylation of liver proteins including notably acetyl-CoA carboxylase (ACC), the rate-limiting enzyme in DNL. We further show that ACC S-nitrosylation markedly increases enzymatic activity. Diminished endothelial nitric oxide synthase expression and ACC S-nitrosylation may thus represent a physiological adaptation to caloric excess by constraining lipogenesis. Our findings demonstrate that S-nitrosylation of liver proteins is subject to dietary control and suggest that DNL is coupled to dietary and metabolic conditions through ACC S-nitrosylation.

S-Nitrosylation at the intersection of metabolism and autophagy: Implications for cancer

Metabolic plasticity, which determines tumour growth and metastasis, is now understood to be a flexible and context-specific process in cancer metabolism. One of the major pathways contributing to metabolic adaptations in eucaryotic cells is autophagy, a cellular degradation and recycling process that is activated during periods of starvation or stress to maintain metabolite and biosynthetic intermediate levels. Consequently, there is a close association between the metabolic adaptive capacity of tumour cells and autophagy-related pathways in cancer. Additionally, nitric oxide regulates protein function and signalling through S-nitrosylation, a post-translational modification that can also impact metabolism and autophagy. The primary objective of this review is to provide an up-to-date overview of the role of S-nitrosylation at the intersection of metabolism and autophagy in cancer. First, we will outline the involvement of S-nitrosylation in the metabolic adaptations that occur in tumours. Then, we will discuss the multifaceted role of autophagy in cancer, the interplay between metabolism and autophagy during tumour progression, and the contribution of S-nitrosylation to autophagic dysregulation in cancer. Finally, we will present insights into relevant therapeutic aspects and discuss prospects for the future.

Insights on the functional dualism of nitric oxide in the hallmarks of cancer

Nitric oxide (NO), a gaseous radical, governs a variety of physiological and pathological processes, including cancer, pro-inflammatory signalling, and vasodilation. The family of nitric oxide synthases (NOS), which comprises the constitutive forms, nNOS and eNOS, and the inducible form, iNOS, produces NO enzymatically. Additionally, NO can be generated non-enzymatically from the nitrate-nitrite-NO pathway. The anti- and pro-oxidant properties of NO and its functional dualism in cancer is due to its highly reactive nature. Numerous malignancies have NOS expression, which interferes with the tumour microenvironment to modulate the tumour's growth in both favourable and unfavourable ways. NO regulates a number of mechanisms in the tumour microenvironment, including metabolism, cell cycle, DNA repair, angiogenesis, and apoptosis/necrosis, depending on its concentration and spatiotemporal profile. This review focuses on the bi-modal impact of nitric oxide on the alteration of a few cancer hallmarks.

Atomic model for core modifying region of human fatty acid synthase in complex with Denifanstat

Fatty acid synthase (FASN) catalyzes the de novo synthesis of palmitate, a 16-carbon chain fatty acid that is the primary precursor of lipid metabolism and an important intracellular signaling molecule. FASN is an attractive drug target in diabetes, cancer, fatty liver diseases, and viral infections. Here, we develop an engineered full-length human FASN (hFASN) that enables isolation of the condensing and modifying regions of the protein post-translation. The engineered protein enables electron cryo-microscopy (cryoEM) structure determination of the core modifying region of hFASN to 2.7 Å resolution. Examination of the dehydratase dimer within this region reveals that unlike its close homolog, porcine FASN, the catalytic cavity is close-ended and is accessible only through one opening in the vicinity of the active site. The core modifying region exhibits two major global conformational variabilities that describe long-range bending and twisting motions of the complex in solution. Finally, we solved the structure of this region bound to an anti-cancer drug, Denifanstat (i.e., TVB-2640), demonstrating the utility of our approach as a platform for structure guided design of future hFASN small molecule inhibitors.

S-nitroso-glutathione (GSNO) inhibits hydrogen peroxide production by alpha-ketoglutarate dehydrogenase: An investigation into sex and diet effects

Pyruvate dehydrogenase (PDH) and α-ketoglutarate dehydrogenase (KGDH) are vital sources of hydrogen peroxide (H₂O₂) and key sites for redox regulation. Here, we report KGDH is more sensitive to inhibition by S-nitroso-glutathione (GSNO) when compared to PDH and deactivation of both enzymes by nitro modification is affected by sex and diet. Liver mitochondria from male C57BL/6N mice displayed a robust inhibition of H₂O₂ production after exposure to 500-2000 μM GSNO. H₂O₂ genesis by PDH was not significantly affected by GSNO. Purified KGDH of porcine heart origin displayed a ∼82% decrease in H₂O₂ generating activity at 500 μM GSNO, which was mirrored by a decrease in NADH production. By contrast, H₂O₂- and NADH-producing activity of purified PDH was only minimally affected by an incubation in 500 μM GSNO. Incubations in GSNO had no significant effect on the H₂O₂-generating activity of KGDH and PDH in female liver mitochondria when compared to samples collected from males, which was attributed to higher GSNO reductase (GSNOR) activity. High fat feeding augmented the GSNO-mediated inhibition of KGDH in liver mitochondria from male mice. Exposure of male mice to a HFD also resulted in a significant decrease in the GSNO-mediated inhibition of H₂O₂ genesis by PDH, an effect not observed in mice fed a control-matched diet (CD). Female mice displayed higher resistance to the GSNO-induced inhibition of H₂O₂ production, regardless of being fed a CD or HFD. However, exposure to a HFD did result in a small but significant decrease in H₂O₂ production by KGDH and PDH when female liver mitochondria were treated with GSNO. Although, the effect was less when compared to their male counterparts. Collectively, we show for the first time GSNO deactivates H₂O₂ production by α-keto acid dehydrogenases and we demonstrate that sex and diet are determinants for the nitro-inhibition of both KGDH and PDH.

Hydroxylated polymethoxyflavones reduce the activity of pancreatic lipase, inhibit adipogenesis and enhance lipolysis in 3T3-L1 mouse embryonic fibroblast cells

Hydroxylated polymethoxyflavones (HPMFs) have been shown to possess various anti-disease effects, including against obesity. This study investigates the anti-obesity effects of HPMFs in further detail, aiming to gain understanding of their mechanism of action in this context. The current study demonstrates that two HPMFs; 3'-hydroxy-5,7,4',5'-tetramethoxyflavone (3'OH-TetMF) and 4'-hydroxy-5,7,3',5'-tetramethoxyflavone (4'OH-TetMF) possess anti-obesity effects. They both significantly reduced pancreatic lipase activity in a competitive manner as demonstrated by molecular docking and kinetic studies. In cell studies, it was revealed that both of the HPMFs suppress differentiation of 3T3-L1 mouse embryonic fibroblast cells during the early stages of adipogenesis. They also reduced expression of key adipogenic and lipogenic marker genes, namely peroxisome proliferator-activated receptor-gamma (PPAR-γ), CCAAT/enhancer-binding protein α and β (C/EBP α and β), adipocyte binding protein 2 (aP2), fatty acid synthase (FASN), and sterol regulatory element-binding protein 1 (SREBF 1). They also enhanced the expression of cell cycle genes, i.e., cyclin D1 (CCND1) and C-Myc, and reduced cyclin A2 expression. When further investigated, it was also observed that these HPMFs accelerate lipid breakdown (lipolysis) and enhance lipolytic gene expression. Moreover, they also reduced the secretion of proteins (adipokines), including pro-inflammatory cytokines, from mature adipocytes. Taken together, this study concludes that these HPMFs have anti-obesity effects, which are worthy of further investigation.

Regulation of tumor metabolism by post translational modifications on metabolic enzymes

Metabolic reprogramming is a hallmark of cancer development, progression, and metastasis. Several metabolic pathways such as glycolysis, tricarboxylic acid (TCA) cycle, lipid metabolism, and glutamine catabolism are frequently altered to support cancer growth. Importantly, the activity of the rate-limiting metabolic enzymes in these pathways are specifically modulated in cancer cells. This is achieved by transcriptional, translational, and post translational regulations that enhance the expression, activity, stability, and substrate sensitivity of the rate-limiting enzymes. These mechanisms allow the enzymes to retain increased activity supporting the metabolic needs of rapidly growing tumors, sustain their survival in the hostile tumor microenvironments and in the metastatic lesions. In this review, we primarily focused on the post translational modifications of the rate-limiting enzymes in the glucose and glutamine metabolism, TCA cycle, and fatty acid metabolism promoting tumor progression and metastasis.

Proteomic and phosphoproteomic analysis reveal threonine deficiency increases hepatic lipid deposition in Pekin ducks via reducing STAT phosphorylation

Dietary threonine (Thr) deficiency enhances triglyceride (TG) deposition in the liver of Pekin ducks, which injures hepatic function and impairs growth performance. However, the underlying molecular mechanisms remain unclear. In the present study, we investigated the effects of dietary Thr deficiency on the expressions of proteins and phosphoproteins in liver of Pekin ducks, to identify the underlying molecular changes. A total of 300 one-day-old ducklings were divided into 3 groups with 10 replicates of 10 birds. All ducks were fed corn-wheat-peanut meal diets containing 0.46%, 0.71%, and 0.96% Thr, respectively, from 1 to 21 days of age. Growth performance, serum parameters, hepatic TG content, and expression of genes involved in lipid metabolism of Pekin ducks were determined. A Thr deficiency group (Thr-D, 0.46% Thr) and a Thr sufficiency group (Thr-S, 0.71% Thr) were selected for subsequent proteomic and phosphoproteomic analysis. The results showed that Thr-D reduced the growth performance (P < 0.001), and increased the plasma concentrations of cholesterol, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, and hepatic TG (P < 0.05). Thr-D increased gene expression related to fatty acid and TG synthesis (P < 0.05). A total of 176 proteins and 259 phosphosites (containing 198 phosphoproteins) were observed to be differentially expressed as a result of Thr-D. The upregulated proteins were enriched in the pathway related to amino acid metabolism, peroxisome. The downregulated proteins were enriched in linolenic and arachidonic acid metabolism, and the Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling pathway. The upregulated phosphoproteins were enriched in the pathways related to fatty acid biosynthesis, fructose and mannose metabolism, and glycolysis/gluconeogenesis. Thr-D reduced the phosphorylation of STAT1 at S729 and STAT3 at S728, and expression of STAT5B. In contrast, Thr-D increased non-receptor tyrosine-protein kinase (TYK2) expression and STAT1 phosphorylation at S649. Taken together, dietary Thr-D increased hepatic TG accumulation by upregulating the expression of genes and proteins, and phosphoproteins related to fatty acid and triglyceride synthesis. Furthermore, these processes might be regulated by the JAK-STAT signaling pathway, especially the phosphorylation of STAT1 and STAT3.

[Fatty acid synthase, a "multi-FASet" enzyme]

In mammals, fatty acids are supplied by diet or synthesized de novo by fatty acid synthase (FASN). Beyond its key role in energy storage, FASN is involved in many biological processes. It actively participates in the synthesis of membrane components necessary for cell division, protein modification, cell signaling and cell proliferation. In this review, we discuss the various physiological functions of FASN as well as its involvement in cancer, the expression of the lipogenic enzyme being particularly high in this disease.

Hepatocyte-like cells differentiated from methylmalonic aciduria cblB type induced pluripotent stem cells: A platform for the evaluation of pharmacochaperoning

Methylmalonic aciduria cblB type (MMA cblB type, MMAB OMIM #251110), caused by a deficiency in the enzyme ATP:cob(I)alamin adenosyltransferase (ATR, E.C_2. 5.1.17), is a severe metabolic disorder with a poor prognosis despite treatment. We recently described the potential therapeutic use of pharmacological chaperones (PCs) after increasing the residual activity of ATR in patient-derived fibroblasts. The present work reports the successful generation of hepatocyte-like cells (HLCs) differentiated from two healthy and two MMAB induced pluripotent stem cell (iPSC) lines, and the use of this platform for testing the effects of PCs. The MMAB cells produced little ATR, showed reduced residual ATR activity, and had higher concentrations of methylmalonic acid compared to healthy HLCs. Differential proteome analysis revealed the two MMAB HCLs to show reproducible differentiation, but this was not so for the healthy HLCs. Interestingly, PC treatment in combination with vitamin B₁₂ increased the amount of ATR available, and subsequently ATR activity, in both MMAB HLCs. More importantly, the treatment significantly reduced the methylmalonic acid content of both. In summary, the HLC model would appear to be an excellent candidate for the pharmacological testing of the described PCs, for analyzing the effects of new drugs, and investigating the repurposing of older drugs, before testing in animal models.

Protein S-Nitrosation: Biochemistry, Identification, Molecular Mechanisms, and Therapeutic Applications

Protein S-nitrosation (SNO), a posttranslational modification (PTM) of cysteine (Cys) residues elicited by nitric oxide (NO), regulates a wide range of protein functions. As a crucial form of redox-based signaling by NO, SNO contributes significantly to the modulation of physiological functions, and SNO imbalance is closely linked to pathophysiological processes. Site-specific identification of the SNO protein is critical for understanding the underlying molecular mechanisms of protein function regulation. Although careful verification is needed, SNO modification data containing numerous functional proteins are a potential research direction for druggable target identification and drug discovery. Undoubtedly, SNO-related research is meaningful not only for the development of NO donor drugs but also for classic target-based drug design. Herein, we provide a comprehensive summary of SNO, including its origin and transport, identification, function, and potential contribution to drug discovery. Importantly, we propose new views to develop novel therapies based on potential protein SNO-sourced targets.

ADH5-mediated NO bioactivity maintains metabolic homeostasis in brown adipose tissue

Brown adipose tissue (BAT) thermogenic activity is tightly regulated by cellular redox status, but the underlying molecular mechanisms are incompletely understood. Protein S-nitrosylation, the nitric-oxide-mediated cysteine thiol protein modification, plays important roles in cellular redox regulation. Here we show that diet-induced obesity (DIO) and acute cold exposure elevate BAT protein S-nitrosylation, including UCP1. This thermogenic-induced nitric oxide bioactivity is regulated by S-nitrosoglutathione reductase (GSNOR; alcohol dehydrogenase 5 [ADH5]), a denitrosylase that balances the intracellular nitroso-redox status. Loss of ADH5 in BAT impairs cold-induced UCP1-dependent thermogenesis and worsens obesity-associated metabolic dysfunction. Mechanistically, we demonstrate that Adh5 expression is induced by the transcription factor heat shock factor 1 (HSF1), and administration of an HSF1 activator to BAT of DIO mice increases Adh5 expression and significantly improves UCP1-mediated respiration. Together, these data indicate that ADH5 controls BAT nitroso-redox homeostasis to regulate adipose thermogenesis, which may be therapeutically targeted to improve metabolic health.

Nitrosative Stress and Human Disease: Therapeutic Potential of Denitrosylation

Proteins dynamically contribute towards maintaining cellular homeostasis. Posttranslational modification regulates the function of target proteins through their immediate activation, sudden inhibition, or permanent degradation. Among numerous protein modifications, protein nitrosation and its functional relevance have emerged. Nitrosation generally initiates nitric oxide (NO) production in association with NO synthase. NO is conjugated to free thiol in the cysteine side chain (S-nitrosylation) and is propagated via the transnitrosylation mechanism. S-nitrosylation is a signaling pathway frequently involved in physiologic regulation. NO forms peroxynitrite in excessive oxidation conditions and induces tyrosine nitration, which is quite stable and is considered irreversible. Two main reducing systems are attributed to denitrosylation: glutathione and thioredoxin (TRX). Glutathione captures NO from S-nitrosylated protein and forms S-nitrosoglutathione (GSNO). The intracellular reducing system catalyzes GSNO into GSH again. TRX can remove NO-like glutathione and break down the disulfide bridge. Although NO is usually beneficial in the basal context, cumulative stress from chronic inflammation or oxidative insult produces a large amount of NO, which induces atypical protein nitrosation. Herein, we (1) provide a brief introduction to the nitrosation and denitrosylation processes, (2) discuss nitrosation-associated human diseases, and (3) discuss a possible denitrosylation strategy and its therapeutic applications.

Understanding the role of S-nitrosylation/nitrosative stress in inflammation and the role of cellular denitrosylases in inflammation modulation: Implications in health and diseases

S-nitrosylation is a very fundamental post-translational modification of protein and non-protein thiols due the involvement of it in a variety of cellular processes including activation/inhibition of several ion channels such as ryanodine receptor in the cardiovascular system; blood vessel dilation; cGMP signaling and neurotransmission. S-nitrosothiol homeostasis in the cell is tightly regulated and perturbations in homeostasis result in an altered redox state leading to a plethora of disease conditions. However, the exact role of S-nitrosylated proteins and nitrosative stress metabolites in inflammation and in inflammation modulation is not well-reviewed. The cell utilizes its intricate defense mechanisms i.e. cellular denitrosylases such as Thioredoxin (Trx) and S-nitrosoglutathione reductase (GSNOR) systems to combat nitric oxide (NO) pathology which has also gained current attraction as novel anti-inflammatory molecules. This review attempts to provide state-of-the-art knowledge from past and present research on the mechanistic role of nitrosative stress intermediates (RNS, OONO^-, PSNO) in pulmonary and autoimmune diseases and how cellular denitrosylases particularly GSNOR and Trx via imparting opposing effects can modulate and reduce inflammation in several health and disease conditions. This review would also bring into notice the existing gaps in current research where denitrosylases can be utilized for ameliorating inflammation that would leave avenues for future therapeutic interventions.

Exploiting S-nitrosylation for cancer therapy: facts and perspectives

S-nitrosylation, the post-translational modification of cysteines by nitric oxide, has been implicated in several cellular processes and tissue homeostasis. As a result, alterations in the mechanisms controlling the levels of S-nitrosylated proteins have been found in pathological states. In the last few years, a role in cancer has been proposed, supported by the evidence that various oncoproteins undergo gain- or loss-of-function modifications upon S-nitrosylation. Here, we aim at providing insight into the current knowledge about the role of S-nitrosylation in different aspects of cancer biology and report the main anticancer strategies based on: (i) reducing S-nitrosylation-mediated oncogenic effects, (ii) boosting S-nitrosylation to stimulate cell death, (iii) exploiting S-nitrosylation through synthetic lethality.

Nitrosative Stress and Its Association with Cardiometabolic Disorders

Reactive nitrogen species (RNS) are formed when there is an abnormal increase in the level of nitric oxide (NO) produced by the inducible nitric oxide synthase (iNOS) and/or by the uncoupled endothelial nitric oxide synthase (eNOS). The presence of high concentrations of superoxide anions (O₂⁻) is also necessary for their formation. RNS react three times faster than O₂⁻ with other molecules and have a longer mean half life. They cause irreversible damage to cell membranes, proteins, mitochondria, the endoplasmic reticulum, nucleic acids and enzymes, altering their activity and leading to necrosis and to cell death. Although nitrogen species are important in the redox imbalance, this review focuses on the alterations caused by the RNS in the cellular redox system that are associated with cardiometabolic diseases. Currently, nitrosative stress (NSS) is implied in the pathogenesis of many diseases. The mechanisms that produce damage remain poorly understood. In this paper, we summarize the current knowledge on the participation of NSS in the pathology of cardiometabolic diseases and their possible mechanisms of action. This information might be useful for the future proposal of anti-NSS therapies for cardiometabolic diseases.

Evaluation of FASN inhibitors by a versatile toolkit reveals differences in pharmacology between human and rodent FASN preparations and in antiproliferative efficacy in vitro vs. in situ in human cancer cells

De novo synthesis of fatty acids is essential to maintain intensive proliferation of cancer cells. Unlike normal cells that utilize food-derived circulating lipids for their fuel, cancer cells rely on heightened lipogenesis irrespective of exogenous lipid availability. Overexpression and activity of the multidomain enzyme fatty acid synthase (FASN) is crucial in supplying palmitate for protumorigenic activity. Therefore, FASN has been proposed as an attractive target for drug development. As an effort to set up an effective toolkit to study FASN inhibitors in human and rodent tissues, we validated activity-based protein profiling (ABPP) as a viable approach to unveil inhibitors targeting FASN thioesterase domain (FASN-TE). ABPP was combined with multi-well plate-assays designed for classical substrate-based FASN activity analysis together with powerful monitoring of cancer cell proliferation using IncuCyte® Live Cell Analyzing System. FASN-TE inhibitors were identified by competitive ABPP using HEK293 cell lysates in a screen of in-house compounds (200+) designed to target serine hydrolase (SH) family. The identified compounds were tested for their inhibitor potencies in vitro using a substrate-based activity assay monitoring FASN-dependent NADPH consumption in LNCaP prostate cancer cell preparation, in parallel with selected reference inhibitors, including orlistat (THL), GSK2194069, GSK837149A, platensimycin and BI-99179. LNCaP lysate supernatant was validated as a reliable native preparation to monitor FASN-dependent NADPH consumption as opposed to human glioma GAMG cells, whereas FASN enrichment was a prerequisite for accurate assays. While inhibitor pharmacology was identical between human prostate and glioma cancer cell FASN preparations, notable differences were revealed between human and rodent FASN preparations, especially for inhibitors targeting FASN-TE. ABPP combined with substrate-based assays facilitated identification of pan thiol-reactive inhibitor scaffolds, exemplified by the 1,2,4-thiadiazole moiety. Finally, selected compounds were evaluated for their antiproliferative efficacy in situ using GAMG cells. These studies revealed that while the tested compounds acted as potent FASN inhibitors in vitro, only a few showed antiproliferative efficacy in situ. To conclude, we describe a versatile toolkit to study FASN inhibitors in vitro and in situ using human cancer cells and reveal dramatic pharmacological differences between human and rodent FASN preparations.

The metabolic regulator small heterodimer partner contributes to the glucose and lipid homeostasis abnormalities induced by hepatitis C virus infection

BACKGROUND

Chronic hepatitis C virus (HCV) infection can predispose the host to metabolic abnormalities. The orphan nuclear receptor small heterodimer partner (SHP; NR0B2) has been identified as a key transcriptional regulatory factor of genes involved in diverse metabolic pathways. The protective effects of SHP against HCV-induced hepatic fibrosis have been reported. However, the exact mechanisms of its role on metabolism are largely unknown. We investigated the role of hepatic SHP in regulating glucose and lipid homeostasis, particularly in the metabolic stress response caused by HCV infection.

MATERIALS AND METHODS

Gluconeogenesis and lipogenesis levels and SHP expression were measured in HCV-infected cells, as well as in liver samples from HCV-infected patients and persistently HCV-infected mice.

RESULTS

We demonstrated that SHP is involved in gluconeogenesis via the acetylation of the Forkhead box O (FoxO) family transcription factor FoxO1, which is mediated by histone deacetylase 9 (HDAC9). Meanwhile, SHP regulates lipogenesis in the liver via suppressing the induction of sterol regulatory element-binding protein-1c (SREBP-1c) expression by the SUMOylation of Liver X receptor α (LXRα) at the SREBP-1c promoter. In particular, SHP can be strongly reduced upon stimulation, such as by HCV infection. The SHP expression levels were decreased in the livers from the CHC patients and persistently HCV-infected mice, and a negative correlation was observed between the SHP expression levels and gluconeogenic or lipogenic activities, emphasizing the clinical relevance of these results.

CONCLUSIONS

Our results suggest that SHP is involved in HCV-induced abnormal glucose and lipid homeostasis and that SHP could be a major target for therapeutic interventions targeting HCV-associated metabolic diseases.

Peroxisomal regulation of redox homeostasis and adipocyte metabolism

Peroxisomes are ubiquitous cellular organelles required for specific pathways of fatty acid oxidation and lipid synthesis, and until recently their functions in adipocytes have not been well appreciated. Importantly, peroxisomes host many oxygen-consumption reactions and play a major role in generation and detoxification of reactive oxygen species (ROS) and reactive nitrogen species (RNS), influencing whole cell redox status. Here, we review recent progress in peroxisomal functions in lipid metabolism as related to ROS/RNS metabolism and discuss the roles of peroxisomal redox homeostasis in adipogenesis and adipocyte metabolism. We provide a framework for understanding redox regulation of peroxisomal functions in adipocytes together with testable hypotheses for developing therapies for obesity and the related metabolic diseases.

An unexpected link between fatty acid synthase and cholesterol synthesis in proinflammatory macrophage activation

Different immune activation states require distinct metabolic features and activities in immune cells. For instance, inhibition of fatty acid synthase (FASN), which catalyzes the synthesis of long-chain fatty acids, prevents the proinflammatory response in macrophages; however, the precise role of this enzyme in this response remains poorly defined. Consistent with previous studies, we found here that FASN is essential for lipopolysaccharide-induced, Toll-like receptor (TLR)-mediated macrophage activation. Interestingly, only agents that block FASN upstream of acetoacetyl-CoA synthesis, including the well-characterized FASN inhibitor C75, inhibited TLR4 signaling, while those acting downstream had no effect. We found that acetoacetyl-CoA could overcome C75's inhibitory effect, whereas other FASN metabolites, including palmitate, did not prevent C75-mediated inhibition. This suggested an unexpected role for acetoacetyl-CoA in inflammation that is independent of its role in palmitate synthesis. Our evidence further suggested that acetoacetyl-CoA arising from FASN activity promotes cholesterol production, indicating a surprising link between fatty acid synthesis and cholesterol synthesis. We further demonstrate that this process is required for TLR4 to enter lipid rafts and facilitate TLR4 signaling. In conclusion, we have uncovered an unexpected link between FASN and cholesterol synthesis that appears to be required for TLR signal transduction and proinflammatory macrophage activation.

S-Nitrosoglutathione Reductase Dysfunction Contributes to Obesity-Associated Hepatic Insulin Resistance via Regulating Autophagy

Obesity is associated with elevated intracellular nitric oxide (NO) production, which promotes nitrosative stress in metabolic tissues such as liver and skeletal muscle, contributing to insulin resistance. The onset of obesity-associated insulin resistance is due, in part, to the compromise of hepatic autophagy, a process that leads to lysosomal degradation of cellular components. However, it is not known how NO bioactivity might impact autophagy in obesity. Here, we establish that S-nitrosoglutathione reductase (GSNOR), a major protein denitrosylase, provides a key regulatory link between inflammation and autophagy, which is disrupted in obesity and diabetes. We demonstrate that obesity promotes S-nitrosylation of lysosomal proteins in the liver, thereby impairing lysosomal enzyme activities. Moreover, in mice and humans, obesity and diabetes are accompanied by decreases in GSNOR activity, engendering nitrosative stress. In mice with a GSNOR deletion, diet-induced obesity increases lysosomal nitrosative stress and impairs autophagy in the liver, leading to hepatic insulin resistance. Conversely, liver-specific overexpression of GSNOR in obese mice markedly enhances lysosomal function and autophagy and, remarkably, improves insulin action and glucose homeostasis. Furthermore, overexpression of S-nitrosylation-resistant variants of lysosomal enzymes enhances autophagy, and pharmacologically and genetically enhancing autophagy improves hepatic insulin sensitivity in GSNOR-deficient hepatocytes. Taken together, our data indicate that obesity-induced protein S-nitrosylation is a key mechanism compromising the hepatic autophagy, contributing to hepatic insulin resistance.

Nitric Oxide: The Forgotten Child of Tumor Metabolism

Nitric oxide (NO) is a signaling molecule with pleiotropic physiological roles in normal cells and pathophysiological roles in cancer. NO synthetase expression and NO synthesis are linked to altered metabolism, neoplasticity, invasiveness, chemoresistance, immune evasion, and ultimately to poor prognosis of cancer patients. Exogenous NO in the microenvironment facilitates paracrine signaling, mediates immune responses, and triggers angiogenesis. NO regulates posttranslational protein modifications, S-nitrosation, and genome-wide epigenetic modifications that can have both tumor-promoting and tumor-suppressing effects. We review mechanisms that link NO to cancer hallmarks, with a perspective of co-targeting NO metabolism with first-line therapies for improved outcome. We highlight the need for quantitative flux analysis to study NO in tumors.

Nitric oxide inhibits the mitochondrial carnitine/acylcarnitine carrier through reversible S-nitrosylation of cysteine 136

S-nitrosylation of the mitochondrial carnitine/acylcarnitine transporter (CACT) has been investigated on the native and the recombinant proteins reconstituted in proteoliposomes, and on intact mitochondria. The widely-used NO-releasing compound, GSNO, strongly inhibited the antiport measured in proteoliposomes reconstituted with the native CACT from rat liver mitochondria or the recombinant rat CACT over-expressed in E. coli. Inhibition was reversed by the reducing agent dithioerythritol, indicating a reaction mechanism based on nitrosylation of Cys residues of the CACT. The half inhibition constant (IC50) was very similar for the native and recombinant proteins, i.e., 74 and 71μM, respectively. The inhibition resulted to be competitive with respect the substrate, carnitine. NO competed also with NEM, correlating well with previous data showing interference of NEM with the substrate transport path. Using a site-directed mutagenesis approach on Cys residues of the recombinant CACT, the target of NO was identified. C136 plays a major role in the reaction mechanism. The occurrence of S-nitrosylation was demonstrated in intact mitochondria after treatment with GSNO, immunoprecipitation and immunostaining of CACT with a specific anti NO-Cys antibody. In parallel samples, transport activity of CACT measured in intact mitochondria, was strongly inhibited after GSNO treatment. The possible physiological and pathological implications of the post-translational modification of CACT are discussed.