S-nitrosylation from GSNOR deficiency impairs DNA repair and promotes hepatocarcinogenesis

S-nitrosylation drives cell senescence and aging in mammals by controlling mitochondrial dynamics and mitophagy

S-nitrosylation, a prototypic redox-based posttranslational modification, is frequently dysregulated in disease. S-nitrosoglutathione reductase (GSNOR) regulates protein S-nitrosylation by functioning as a protein denitrosylase. Deficiency of GSNOR results in tumorigenesis and disrupts cellular homeostasis broadly, including metabolic, cardiovascular, and immune function. Here, we demonstrate that GSNOR expression decreases in primary cells undergoing senescence, as well as in mice and humans during their life span. In stark contrast, exceptionally long-lived individuals maintain GSNOR levels. We also show that GSNOR deficiency promotes mitochondrial nitrosative stress, including excessive S-nitrosylation of Drp1 and Parkin, thereby impairing mitochondrial dynamics and mitophagy. Our findings implicate GSNOR in mammalian longevity, suggest a molecular link between protein S-nitrosylation and mitochondria quality control in aging, and provide a redox-based perspective on aging with direct therapeutic implications.

Chronicles of a reductase: Biochemistry, genetics and physio-pathological role of GSNOR

S-nitrosylation is a major redox posttranslational modification involved in cell signaling. The steady state concentration of S-nitrosylated proteins depends on the balance between the relative ability to generate nitric oxide (NO) via NO synthase and to reduce nitrosothiols by denitrosylases. Numerous works have been published in last decades regarding the role of NO and S-nitrosylation in the regulation of protein structure and function, and in driving cellular activities in vertebrates. Notwithstanding an increasing number of observations indicates that impairment of denitrosylation equally affects cellular homeostasis, there is still no report providing comprehensive knowledge on the impact that denitrosylation has on maintaining correct physiological processes and organ activities. Among denitrosylases, S-nitrosoglutathione reductase (GSNOR) represents the prototype enzyme to disclose how denitrosylation plays a crucial role in tuning NO-bioactivity and how much it deeply impacts on cell homeostasis and human patho-physiology. In this review we attempt to illustrate the history of GSNOR discovery and provide the evidence so far reported in support of GSNOR implications in development and human disease.

Nitric oxide-regulated proteolysis of human CYP2B6 via the ubiquitin-proteasome system

We showed previously that rat cytochrome P450 CYP2B1 undergoes NO-dependent proteasomal degradation in response to inflammatory stimuli, and that the related human enzyme CYP2B6 is also down-regulated by NO in primary human hepatocytes. To investigate the mechanism of CYP2B6 down-regulation, we made several cell lines (HeLa and HuH7 cells) in which native CYP2B6 or CYP2B6 with a C-terminal V5 tag (CYP2B6V5) are expressed from a lentiviral vector with a cytomegalovirus promoter. Native CYP2B6 protein was rapidly down-regulated in HeLa cells within 3h of treatment with the NO donor (Z)-1-[2-(2-Aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate, while its mRNA level was not down-regulated. Treatment of the cells with the NO donor (Z)-1-[N-(3-aminopropyl)-N-(3-ammoniopropyl)amino]diazen-1-ium-1,2-diolate also resulted in rapid down-regulation of CYP2B6 activity, measured as the formation of 7-hydroxy-4-trifluoromethylcoumarin, as well as 2B6 protein in the CYP2B6 HeLa cell line. CYP2B6V5 was also down-regulated by NO donors in HuH7 cells. Down-regulation was observed in the presence of cycloheximide, demonstrating that this occurs via a post-translational mechanism. We generated a HeLa cell line expressing both CYP2B6V5 and human nitric oxide synthase 2 (NOS2), the latter under positive control by tetracycline. The cellular NO produced by doxycycline treatment also effectively down-regulated CYP2B6 protein, which was blocked by the co-treatment with the NOS2 competitive inhibitor L-N^G-nitroarginine methyl ester (L-NAME). We next investigated the proteolytic enzymes responsible for NO-dependent CYP2B6 degradation. Neither calpain inhibitors (N-Acetyl-L-leucyl-L-leucyl-L-norleucinal, carbobenzoxy-valinyl-phenylalaninal), nor lysosomal protease inhibitors (3-methyladenine and chloroquine) inhibited the NO dependent CYP2B6V5 down-regulation. The proteasome inhibitors MG132 and bortezomib attenuated, but did not completely block the NO-induced down-regulation in the HuH7 cell line. However, when cells were co-treated with NO donor and proteasome inhibitors, high molecular mass species could be detected on native CYP2B6 as well as CYP2B6V5 Western blots. Further investigation demonstrated that CYP2B6 protein was polyubiquitinated and this was dramatically enhanced by co-treatment with NO donor and bortezomib. Taken together, our data demonstrate that CYP2B6 is down-regulated in an NO-dependent manner via ubiquitination and proteasomal degradation.

The role of S-nitrosoglutathione reductase (GSNOR) in human disease and therapy

S-nitrosoglutathione reductase (GSNOR), or ADH5, is an enzyme in the alcohol dehydrogenase (ADH) family. It is unique when compared to other ADH enzymes in that primary short-chain alcohols are not its principle substrate. GSNOR metabolizes S-nitrosoglutathione (GSNO), S-hydroxymethylglutathione (the spontaneous adduct of formaldehyde and glutathione), and some alcohols. GSNOR modulates reactive nitric oxide (•NO) availability in the cell by catalyzing the breakdown of GSNO, and indirectly regulates S-nitrosothiols (RSNOs) through GSNO-mediated protein S-nitrosation. The dysregulation of GSNOR can significantly alter cellular homeostasis, leading to disease. GSNOR plays an important regulatory role in smooth muscle relaxation, immune function, inflammation, neuronal development and cancer progression, among many other processes. In recent years, the therapeutic inhibition of GSNOR has been investigated to treat asthma, cystic fibrosis and interstitial lung disease (ILD). The direct action of •NO on cellular pathways, as well as the important regulatory role of protein S-nitrosation, is closely tied to GSNOR regulation and defines this enzyme as an important therapeutic target.

Nitrosothiol-Trapping-Based Proteomic Analysis of S-Nitrosylation in Human Lung Carcinoma Cells

Nitrosylation of cysteines residues (S-nitrosylation) mediates many of the cellular effects of nitric oxide in normal and diseased cells. Recent research indicates that S-nitrosylation of certain proteins could play a role in tumor progression and responsiveness to therapy. However, the protein targets of S-nitrosylation in cancer cells remain largely unidentified. In this study, we used our recently developed nitrosothiol trapping approach to explore the nitrosoproteome of human A549 lung carcinoma cells treated with S-nitrosocysteine or pro-inflammatory cytokines. Using this approach, we identified about 300 putative nitrosylation targets in S-nitrosocysteine-treated A549 cells and approximately 400 targets in cytokine-stimulated cells. Among the more than 500 proteins identified in the two screens, the majority represent novel targets of S-nitrosylation, as revealed by comparison with publicly available nitrosoproteomic data. By coupling the trapping procedure with differential thiol labeling, we identified nearly 300 potential nitrosylation sites in about 150 proteins. The proteomic results were validated for several proteins by an independent approach. Bioinformatic analysis highlighted important cellular pathways that are targeted by S-nitrosylation, notably, cell cycle and inflammatory signaling. Taken together, our results identify new molecular targets of nitric oxide in lung cancer cells and suggest that S-nitrosylation may regulate signaling pathways that are critically involved in lung cancer progression.

Comparative protein profiling of B16 mouse melanoma cells susceptible and non-susceptible to alphavirus infection: Effect of the tumor microenvironment

Alphavirus vectors are promising tools for cancer treatment. However, relevant entry mechanisms and interactions with host cells are still not clearly understood. The first step toward a more effective therapy is the identification of novel intracellular alterations that could be associated with cancer aggressiveness and could affect the therapeutic potential of these vectors. In this study, we observed that alphaviruses efficiently infected B16 mouse melanoma tumors/tumor cells in vivo, whereas their transduction efficiency in B16 cells under in vitro conditions was blocked. Therefore, we further aimed to understand the mechanisms pertaining to the differential transduction efficacy of alphaviruses in B16 tumor cells under varying growth conditions. We hypothesized that the tumor microenvironment might alter gene expression in B16 cells, leading to an up-regulation of the expression of virus-binding receptors or factors associated with virus entry and replication. To test our hypothesis, we performed a proteomics analysis of B16 cells cultured in vitro and of B16 cells isolated from tumors, and we identified 277 differentially regulated proteins. A further in-depth analysis to identify the biological and molecular functions of the detected proteins revealed a set of candidate genes that could affect virus infectivity. Importantly, we observed a decrease in the expression of interferon α (IFN-α) in tumor-isolated cells that resulted in the suppression of several IFN-regulated genes, thereby abrogating host cell antiviral defense. Additionally, differences in the expression of genes that regulate cytoskeletal organization caused significant alterations in cell membrane elasticity. Taken together, our findings demonstrated favorable intracellular conditions for alphavirus transduction/replication that occurred during tumor transformation. These results pave the way for optimizing the development of strategies for the application of alphaviral vectors as a potent cancer therapy.

Nitrosative Stress, Hypernitrosylation, and Autoimmune Responses to Nitrosylated Proteins: New Pathways in Neuroprogressive Disorders Including Depression and Chronic Fatigue Syndrome

Nitric oxide plays an indispensable role in modulating cellular signaling and redox pathways. This role is mainly effected by the readily reversible nitrosylation of selective protein cysteine thiols. The reversibility and sophistication of this signaling system is enabled and regulated by a number of enzymes which form part of the thioredoxin, glutathione, and pyridoxine antioxidant systems. Increases in nitric oxide levels initially lead to a defensive increase in the number of nitrosylated proteins in an effort to preserve their function. However, in an environment of chronic oxidative and nitrosative stress (O&NS), nitrosylation of crucial cysteine groups within key enzymes of the thioredoxin, glutathione, and pyridoxine systems leads to their inactivation thereby disabling denitrosylation and transnitrosylation and subsequently a state described as "hypernitrosylation." This state leads to the development of pathology in multiple domains such as the inhibition of enzymes of the electron transport chain, decreased mitochondrial function, and altered conformation of proteins and amino acids leading to loss of immune tolerance and development of autoimmunity. Hypernitrosylation also leads to altered function or inactivation of proteins involved in the regulation of apoptosis, autophagy, proteomic degradation, transcription factor activity, immune-inflammatory pathways, energy production, and neural function and survival. Hypernitrosylation, as a consequence of chronically elevated O&NS and activated immune-inflammatory pathways, can explain many characteristic abnormalities observed in neuroprogressive disease including major depression and chronic fatigue syndrome/myalgic encephalomyelitis. In those disorders, increased bacterial translocation may drive hypernitrosylation and autoimmune responses against nitrosylated proteins.

S-nitrosylation of the Mitochondrial Chaperone TRAP1 Sensitizes Hepatocellular Carcinoma Cells to Inhibitors of Succinate Dehydrogenase

S-nitrosoglutathione reductase (GSNOR) represents the best-documented denitrosylase implicated in regulating the levels of proteins posttranslationally modified by nitric oxide on cysteine residues by S-nitrosylation. GSNOR controls a diverse array of physiologic functions, including cellular growth and differentiation, inflammation, and metabolism. Chromosomal deletion of GSNOR results in pathologic protein S-nitrosylation that is implicated in human hepatocellular carcinoma (HCC). Here we identify a metabolic hallmark of aberrant S-nitrosylation in HCC and exploit it for therapeutic gain. We find that hepatocyte GSNOR deficiency is characterized by mitochondrial alteration and by marked increases in succinate dehydrogenase (SDH) levels and activity. We find that this depends on the selective S-nitrosylation of Cys(501) in the mitochondrial chaperone TRAP1, which mediates its degradation. As a result, GSNOR-deficient cells and tumors are highly sensitive to SDH inhibition, namely to α-tocopheryl succinate, an SDH-targeting molecule that induced RIP1/PARP1-mediated necroptosis and inhibited tumor growth. Our work provides a specific molecular signature of aberrant S-nitrosylation in HCC, a novel molecular target in SDH, and a first-in-class therapy to treat the disease. Cancer Res; 76(14); 4170-82. ©2016 AACR.

To eat, or NOt to eat: S-nitrosylation signaling in autophagy

Autophagy is the main catabolic cellular process through which cells adapt their needs (e.g., growth and proliferation) to environmental availability of nutrients (e.g., amino acid and glucose) and growth factors. The rapid activation of the autophagy response essentially depends on protein post-translational modifications (PTMs), which act as molecular switches triggering signaling cascades. Deregulation of autophagy contributes to pathological conditions, such as cancer and neurodegeneration. Therefore, understanding how PTMs affect the occurrence of autophagy is of the highest importance for clinical applications. Besides phosphorylation and ubiquitylation, which represent the best known examples of PTMs, redox-based modifications are also emerging as contributing to the regulation of intracellular signaling. Of note, S-nitrosylation of cysteine residues is a redox PTM and is the principal mechanism of nitric oxide-based signaling. Results emerging in recent years suggest that NO has a role in modulating autophagy. However, the function of S-nitrosylation in autophagy regulation remains still unveiled. By this review, we describe the upstream events regulating autophagy activation focusing on recently published evidence implying a S-nitrosylation-dependent regulation.

Nitrogen chemistry and lung physiology

The versatile chemistry of nitrogen is important to pulmonary physiology. Indeed, almost all redox forms of nitrogen are relevant to pulmonary physiology and to pathophysiology. Here we review the relevance to pulmonary biology of (a) elemental nitrogen; (b) reduced forms of nitrogen such as amines, ammonia, and hydroxylamine; and (c) oxidized forms of nitrogen such as the nitroxyl anion, the nitric oxide free radical, and S-nitrosothiols. Our focus is on oxidized nitrogen in the form of S-nitrosothiol bond-containing species, which are now appreciated to be important to every type of cell-signaling process in the lung. We also review potential clinical applications of nitrogen oxide biochemistry. These principles are being translated into clinical practice as diagnostic techniques and therapies for a range of pulmonary diseases including asthma, cystic fibrosis, adult respiratory distress syndrome, primary ciliary dyskinesia, and pulmonary hypertension.

Endogenous Formaldehyde Is a Hematopoietic Stem Cell Genotoxin and Metabolic Carcinogen

Endogenous formaldehyde is produced by numerous biochemical pathways fundamental to life, and it can crosslink both DNA and proteins. However, the consequences of its accumulation are unclear. Here we show that endogenous formaldehyde is removed by the enzyme alcohol dehydrogenase 5 (ADH5/GSNOR), and Adh5(-/-) mice therefore accumulate formaldehyde adducts in DNA. The repair of this damage is mediated by FANCD2, a DNA crosslink repair protein. Adh5(-/-)Fancd2(-/-) mice reveal an essential requirement for these protection mechanisms in hematopoietic stem cells (HSCs), leading to their depletion and precipitating bone marrow failure. More widespread formaldehyde-induced DNA damage also causes karyomegaly and dysfunction of hepatocytes and nephrons. Bone marrow transplantation not only rescued hematopoiesis but, surprisingly, also preserved nephron function. Nevertheless, all of these animals eventually developed fatal malignancies. Formaldehyde is therefore an important source of endogenous DNA damage that is counteracted in mammals by a conserved protection mechanism.

S-Nitrosoglutathione Reductase Plays Opposite Roles in SH-SY5Y Models of Parkinson's Disease and Amyotrophic Lateral Sclerosis

Oxidative and nitrosative stresses have been reported as detrimental phenomena concurring to the onset of several neurodegenerative diseases. Here we reported that the ectopic modulation of the denitrosylating enzyme S-nitrosoglutathione reductase (GSNOR) differently impinges on the phenotype of two SH-SY5Y-based in vitro models of neurodegeneration, namely, Parkinson's disease (PD) and familial amyotrophic lateral sclerosis (fALS). In particular, we provide evidence that GSNOR-knocking down protects SH-SY5Y against PD toxins, while, by contrast, its upregulation is required for G93A-SOD1 expressing cells resistance to NO-releasing drugs. Although completely opposite, both conditions are characterized by Nrf2 localization in the nuclear compartment: in the first case induced by GSNOR silencing, while in the second one underlying the antinitrosative response. Overall, our results demonstrate that GSNOR expression has different effect on neuronal viability in dependence on the stimulus applied and suggest that GSNOR could be a responsive gene downstream of Nrf2 activation.

Nitric oxide in liver diseases

Nitric oxide (NO) and its derivatives play important roles in the physiology and pathophysiology of the liver. Despite its diverse and complicated roles, certain patterns of the effect of NO on the pathogenesis and progression of liver diseases are observed. In general, NO derived from endothelial NO synthase (eNOS) in liver sinusoidal endothelial cells (LSECs) is protective against disease development, while inducible NOS (iNOS)-derived NO contributes to pathological processes. This review addresses the roles of NO in the development of various liver diseases with a focus on recently published articles. We present here two recent advances in understanding NO-mediated signaling - nitrated fatty acids (NO2-FAs) and S-guanylation - and conclude with suggestions for future directions in NO-related studies on the liver.

S-Nitrosoglutathione Reductase Deficiency Enhances the Proliferative Expansion of Adult Heart Progenitors and Myocytes Post Myocardial Infarction

Background

Mammalian heart regenerative activity is lost before adulthood but increases after cardiac injury. Cardiac repair mechanisms, which involve both endogenous cardiac stem cells (CSCs) and cardiomyocyte cell-cycle reentry, are inadequate to achieve full recovery after myocardial infarction (MI). Mice deficient in S-nitrosoglutathione reductase (GSNOR^−⁄−), an enzyme regulating S-nitrosothiol turnover, have preserved cardiac function after MI. Here, we tested the hypothesis that GSNOR activity modulates cardiac cell proliferation in the post-MI adult heart.

Methods and Results

GSNOR^−⁄− and C57Bl6/J (wild-type [WT]) mice were subjected to sham operation (n=3 GSNOR^−⁄−; n=3 WT) or MI (n=41 GSNOR^−⁄−; n=65 WT). Compared with WT,GSNOR^−⁄− mice exhibited improved survival, cardiac performance, and architecture after MI, as demonstrated by higher ejection fraction (P<0.05), lower endocardial volumes (P<0.001), and diminished scar size (P<0.05). In addition, cardiomyocytes from post-MI GSNOR^−⁄− hearts exhibited faster calcium decay and sarcomeric relaxation times (P<0.001). Immunophenotypic analysis illustrated that post-MI GSNOR^−⁄− hearts demonstrated enhanced neovascularization (P<0.001), c-kit⁺ CSC abundance (P=0.013), and a ≈3-fold increase in proliferation of adult cardiomyocytes and c-kit⁺/CD45⁻ CSCs (P<0.0001 and P=0.023, respectively) as measured by using 5-bromodeoxyuridine.

Conclusions

Loss of GSNOR confers enhanced post-MI cardiac regenerative activity, characterized by enhanced turnover of cardiomyocytes and CSCs. Endogenous denitrosylases exert an inhibitory effect over cardiac repair mechanisms and therefore represents a potential novel therapeutic target.

Flavone inhibits nitric oxide synthase (NOS) activity, nitric oxide production and protein S-nitrosylation in breast cancer cells

As the core structure of flavonoids, flavone has been proved to possess anticancer effects. Flavone's growth inhibitory functions are related to NO. NO is synthesized by nitric oxide synthase (NOS), and generally increased in a variety of cancer cells. NO regulates multiple cellular responses by S-nitrosylation. In this study, we explored flavone-induced regulations on nitric oxide (NO)-related cellular processes in breast cancer cells. Our results showed that, flavone suppresses breast cancer cell proliferation and induces apoptosis. Flavone restrains NO synthesis by does-dependent inhibiting NOS enzymatic activity. The decrease of NO generation was detected by fluorescence microscopy and flow cytometry. Flavone-induced inhibitory effect on NOS activity is dependent on intact cell structure. For the NO-induced protein modification, flavone treatment significantly down-regulated protein S-nitrosylation, which was detected by "Biotin-switch" method. The present study provides a novel, NO-related mechanism for the anticancer function of flavone.

Phenotype of asthmatics with increased airway S-nitrosoglutathione reductase activity

S-Nitrosoglutathione is an endogenous airway smooth muscle relaxant. Increased airway S-nitrosoglutathione breakdown occurs in some asthma patients. We asked whether patients with increased airway catabolism of this molecule had clinical features that distinguished them from other asthma patients. We measured S-nitrosoglutathione reductase expression and activity in bronchoscopy samples taken from 66 subjects in the Severe Asthma Research Program. We also analysed phenotype and genotype data taken from the program as a whole. Airway S-nitrosoglutathione reductase activity was increased in asthma patients (p=0.032). However, only a subpopulation was affected and this subpopulation was not defined by a "severe asthma" diagnosis. Subjects with increased activity were younger, had higher IgE and an earlier onset of symptoms. Consistent with a link between S-nitrosoglutathione biochemistry and atopy: 1) interleukin 13 increased S-nitrosoglutathione reductase expression and 2) subjects with an S-nitrosoglutathione reductase single nucleotide polymorphism previously associated with asthma had higher IgE than those without this single nucleotide polymorphism. Expression was higher in airway epithelium than in smooth muscle and was increased in regions of the asthmatic lung with decreased airflow. An early-onset, allergic phenotype characterises the asthma population with increased S-nitrosoglutathione reductase activity.

Arsenic modulates posttranslational S-nitrosylation and translational proteome in keratinocytes

Arsenic is a class I human carcinogen (such as inducing skin cancer) by its prominent chemical interaction with protein thio (-SH) group. Therefore, arsenic may compromise protein S-nitrosylation by competing the -SH binding activity. In the present study, we aimed to understand the influence of arsenic on protein S-nitrosylation and the following proteomic changes. By using primary human skin keratinocyte, we found that arsenic treatment decreased the level of protein S-nitrosylation. This was coincident to the decent expressions of endothelial nitric oxide synthase (eNOS) and inducible nitric oxide synthase (iNOS). By using LC-MS/MS, around twenty S-nitrosoproteins were detected in the biotin-switched eluent. With the interest that arsenic not only regulates posttranslational S-nitrosylation but also separately affects protein's translation expression, we performed two-dimensional gel electrophoresis and found that 8 proteins were significantly decreased during arsenic treatment. Whether these decreased proteins are the consequence of protein S-nitrosylation will be further investigated. Taken together, these results provide a finding that arsenic can deplete the binding activity of NO and therefore reduce protein S-nitrosylation.

S-nitrosoglutathione reductase deficiency-induced S-nitrosylation results in neuromuscular dysfunction

AIMS

Nitric oxide (NO) production is implicated in muscle contraction, growth and atrophy, and in the onset of neuropathy. However, many aspects of the mechanism of action of NO are not yet clarified, mainly regarding its role in muscle wasting. Notably, whether NO production-associated neuromuscular atrophy depends on tyrosine nitration or S-nitrosothiols (SNOs) formation is still a matter of debate. Here, we aim at assessing this issue by characterizing the neuromuscular phenotype of S-nitrosoglutathione reductase-null (GSNOR-KO) mice that maintain the capability to produce NO, but are unable to reduce SNOs.

RESULTS

We demonstrate that, without any sign of protein nitration, young GSNOR-KO mice show neuromuscular atrophy due to loss of muscle mass, reduced fiber size, and neuropathic behavior. In particular, GSNOR-KO mice show a significant decrease in nerve axon number, with the myelin sheath appearing disorganized and reduced, leading to a dramatic development of a neuropathic phenotype. Mitochondria appear fragmented and depolarized in GSNOR-KO myofibers and myotubes, conditions that are reverted by N-acetylcysteine treatment. Nevertheless, although atrogene transcription is induced, and bulk autophagy activated, no removal of damaged mitochondria is observed. These events, alongside basal increase of apoptotic markers, contribute to persistence of a neuropathic and myopathic state.

INNOVATION

Our study provides the first evidence that GSNOR deficiency, which affects exclusively SNOs reduction without altering nitrotyrosine levels, results in a clinically relevant neuromuscular phenotype.

CONCLUSION

These findings provide novel insights into the involvement of GSNOR and S-nitrosylation in neuromuscular atrophy and neuropathic pain that are associated with pathological states; for example, diabetes and cancer.

Involvement of DNA damage response pathways in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) has been known as one of the most lethal human malignancies, due to the difficulty of early detection, chemoresistance, and radioresistance, and is characterized by active angiogenesis and metastasis, which account for rapid recurrence and poor survival. Its development has been closely associated with multiple risk factors, including hepatitis B and C virus infection, alcohol consumption, obesity, and diet contamination. Genetic alterations and genomic instability, probably resulted from unrepaired DNA lesions, are increasingly recognized as a common feature of human HCC. Dysregulation of DNA damage repair and signaling to cell cycle checkpoints, known as the DNA damage response (DDR), is associated with a predisposition to cancer and affects responses to DNA-damaging anticancer therapy. It has been demonstrated that various HCC-associated risk factors are able to promote DNA damages, formation of DNA adducts, and chromosomal aberrations. Hence, alterations in the DDR pathways may accumulate these lesions to trigger hepatocarcinogenesis and also to facilitate advanced HCC progression. This review collects some of the most known information about the link between HCC-associated risk factors and DDR pathways in HCC. Hopefully, the review will remind the researchers and clinicians of further characterizing and validating the roles of these DDR pathways in HCC.

S-nitrosation and ubiquitin-proteasome system interplay in neuromuscular disorders

Protein S-nitrosation is deemed as a prototype of posttranslational modifications governing cell signaling. It takes place on specific cysteine residues that covalently incorporate a nitric oxide (NO) moiety to form S-nitrosothiol derivatives and depends on the ratio between NO produced by NO synthases and nitrosothiol removal catalyzed by denitrosating enzymes. A large number of cysteine-containing proteins are found to undergo S-nitrosation and, among them, the enzymes catalyzing ubiquitination, mainly the class of ubiquitin E3 ligases and the 20S component of the proteasome, have been reported to be redox modulated in their activity. In this review we will outline the processes regulating S-nitrosation and try to debate whether and how it affects protein ubiquitination and degradation via the proteasome. In particular, since muscle and neuronal health largely depends on the balance between protein synthesis and breakdown, here we will discuss the impact of S-nitrosation in the efficiency of protein quality control system, providing lines of evidence and speculating about its involvement in the onset and maintenance of neuromuscular dysfunctions.

Proteomic approaches to evaluate protein S-nitrosylation in disease

Many of nitric oxide (NO) actions are mediated through the coupling of a nitroso moiety to a reactive cysteine leading to the formation of a S-nitrosothiol (SNO), a process known as S-nitrosylation or S-nitrosation. In many cases this reversible post-translational modification is accompanied by altered protein function and aberrant S-nitrosylation of proteins, caused by altered production of NO and/or impaired SNO homeostasis, has been repeatedly reported in a variety of pathophysiological settings. A growing number of studies are directed to the identification and characterization of those proteins that undergo S-nitrosylation and the analysis of S-nitrosoproteomes under pathological conditions is beginning to be reported. The study of these S-nitrosoproteomes has been fueled by advances in proteomic technologies that are providing researchers with improved tools for exploring this post-translational modification. Here we review novel refinements and improvements to these methods, and some recent studies of the S-nitrosoproteome in disease.

Nitric oxide and interleukin-1β stimulate the proteasome-independent degradation of the retinoic acid hydroxylase CYP2C22 in primary rat hepatocytes

CYP2C22 was recently described as a retinoic acid-metabolizing cytochrome P450 enzyme whose transcription is induced by all-trans-retinoic acid (atRA) in hepatoma cells (Qian L, Zolfaghari R, and Ross AC (2010) J Lipid Res 51:1781-1792). We identified CYP2C22 as a putative nitric oxide (NO)-regulated protein in a proteomic screen and raised specific polyclonal antibodies to CYP2C22 to study its protein expression. We found that CYP2C22 is a liver-specific protein that was not significantly induced by activators of the pregnane X receptor, constitutive androstane receptor, or peroxisome proliferator-activated receptor-α, but was downregulated to <25% of control by the aryl hydrocarbon receptor agonist β-naphthoflavone in cultured rat hepatocytes. CYP2C22 protein and its mRNA both were induced by atRA in hepatocytes, with EC50 of 100-300 nM, whereas the maximal extent of mRNA induction was twice that of the protein. CYP2C22 protein, but not its mRNA, was rapidly downregulated in hepatocytes by interleukin-1 (IL-1) or NO-donating compounds, and the downregulation by IL-1 was blocked by inhibition of NO synthases. The NO donor (Z)-1-[N-(3-aminopropyl)-N-(3-ammoniopropyl)amino]diazen-1-ium-1,2-diolate reduced the half-life of CYP2C22 from 8.7 to 3.4 hours in the presence of cycloheximide, demonstrating that NO-dependent downregulation is due to stimulated proteolysis. No intermediate degradation products were detected. However, this degradation was insensitive to inhibitors of calpains or the canonical proteasomal or lysosomal pathways, indicating that NO-dependent degradation of CYP2C22 proceeds via a novel pathway.

Post-translational regulation of endothelial nitric oxide synthase in vascular endothelium

Nitric oxide (NO) is a short-lived gaseous signaling molecule. In blood vessels, it is synthesized in a dynamic fashion by endothelial nitric oxide synthase (eNOS) and influences vascular function via two distinct mechanisms, the activation of soluble guanylyl cyclase (sGC)/cyclic guanosine monophosphate (cGMP)-dependent signaling and the S-nitrosylation of proteins with reactive thiols (S-nitrosylation). The regulation of eNOS activity and NO bioavailability is critical to maintain blood vessel function. The activity of eNOS and ability to generate NO is regulated at the transcriptional, posttranscriptional, and posttranslational levels. Post-translational modifications acutely impact eNOS activity and dysregulation of these mechanisms compromise eNOS activity and foster the development of cardiovascular diseases (CVDs). This review will intergrate past and current literature on the post-translational modifications of eNOS in both health and disease.

Increased susceptibility to Klebsiella pneumonia and mortality in GSNOR-deficient mice

S-nitrosoglutathione reductase (GSNOR) is a key denitrosylase and critically important for protecting immune and other cells from nitrosative stress. Pharmacological inhibition of GSNOR is being actively pursued as a therapeutic approach to increase S-nitrosoglutathione levels for the treatment of asthma and cystic fibrosis. In the present study, we employed GSNOR-deficient (GSNOR(-/-)) mice to investigate whether inactivation of GSNOR may increase susceptibility to pulmonary infection by Klebsiella pneumoniae, a common cause of nosocomial pneumonia. We found that compared to wild-type mice, bacterial colony forming units 48 h after intranasal infection with K. pneumoniae were increased over 4-folds in lung and spleen and strikingly, over a 1000-folds in blood of GSNOR(-/-) mice. Lung injury was comparable between infected wild-type and GSNOR(-/-) mice, but inflammation and injury was significantly elevated in spleen of GSNOR(-/-) mice. Whereas all wild-type mice survived 48 h after infection, 10 of 23 GSNOR(-/-) mice died. Thus, GSNOR appears to play a crucial role in controlling pulmonary and systemic infection by K. pneumoniae. Our results suggest that patients treated in clinical trials with inhibitors of GSNOR should be carefully monitored for signs of infection.

Specificity in S-nitrosylation: a short-range mechanism for NO signaling?

SIGNIFICANCE

Nitric oxide (NO) classical and less classical signaling mechanisms (through interaction with soluble guanylate cyclase and cytochrome c oxidase, respectively) operate through direct binding of NO to protein metal centers, and rely on diffusibility of the NO molecule. S-Nitrosylation, a covalent post-translational modification of protein cysteines, has emerged as a paradigm of nonclassical NO signaling.

RECENT ADVANCES

Several nonenzymatic mechanisms for S-nitrosylation formation and destruction have been described. Enzymatic mechanisms for transnitrosylation and denitrosylation have been also studied as regulators of the modification of specific subsets of proteins. The advancement of modification-specific proteomic methodologies has allowed progress in the study of diverse S-nitrosoproteomes, raising clues and questions about the parameters for determining the protein specificity of the modification.

CRITICAL ISSUES

We propose that S-nitrosylation is mainly a short-range mechanism of NO signaling, exerted in a relatively limited range of action around the NO sources, and tightly related to the very controlled regulation of subcellular localization of nitric oxide synthases. We review the nonenzymatic and enzymatic mechanisms that support this concept, as well as physiological examples of mammalian systems that illustrate well the precise compartmentalization of S-nitrosylation.

FUTURE DIRECTIONS

Individual and proteomic studies of protein S-nitrosylation-based signaling should take into account the subcellular localization in order to gain further insight into the functional role of this modification in (patho)physiological settings.

Specificity in S-nitrosylation: a short-range mechanism for NO signaling?

SIGNIFICANCE

Nitric oxide (NO) classical and less classical signaling mechanisms (through interaction with soluble guanylate cyclase and cytochrome c oxidase, respectively) operate through direct binding of NO to protein metal centers, and rely on diffusibility of the NO molecule. S-Nitrosylation, a covalent post-translational modification of protein cysteines, has emerged as a paradigm of nonclassical NO signaling.

RECENT ADVANCES

Several nonenzymatic mechanisms for S-nitrosylation formation and destruction have been described. Enzymatic mechanisms for transnitrosylation and denitrosylation have been also studied as regulators of the modification of specific subsets of proteins. The advancement of modification-specific proteomic methodologies has allowed progress in the study of diverse S-nitrosoproteomes, raising clues and questions about the parameters for determining the protein specificity of the modification.

CRITICAL ISSUES

We propose that S-nitrosylation is mainly a short-range mechanism of NO signaling, exerted in a relatively limited range of action around the NO sources, and tightly related to the very controlled regulation of subcellular localization of nitric oxide synthases. We review the nonenzymatic and enzymatic mechanisms that support this concept, as well as physiological examples of mammalian systems that illustrate well the precise compartmentalization of S-nitrosylation.

FUTURE DIRECTIONS

Individual and proteomic studies of protein S-nitrosylation-based signaling should take into account the subcellular localization in order to gain further insight into the functional role of this modification in (patho)physiological settings.

New insights into the link between DNA damage and apoptosis

SIGNIFICANCE

When lesions are unrepaired or there are defects in the DNA repair system, DNA damage is often correlated to apoptosis. However, different kinds of lesions and different degrees of lesion severity can trigger numerous signaling responses.

RECENT ADVANCES

DNA repair proteins involved in specific DNA repair pathways can modulate the function or activity of some apoptotic factors, further emphasizing the crosstalk between DNA damage and cell death.

CRITICAL ISSUES

Here, we discuss the signaling networks that link DNA damage to apoptosis, and we focus on post-translational modifications, leading to crucial changes in protein behavior, following various kinds of DNA damage. Moreover, we analyze the existence of apoptosis-related functions of typical repair proteins, leading to diverse, often-overlapping, DNA damage responses.

FUTURE DIRECTIONS

The better understanding of the regulation and the functionality of key DNA repair proteins, also involved in apoptosis regulation, has the potential of modulating the cell outcomes on DNA damage, particularly in the context of cancer treatment.

S-sulfhydration/desulfhydration and S-nitrosylation/denitrosylation: a common paradigm for gasotransmitter signaling by H2S and NO

Sulfhydryl groups on protein Cys residues undergo an array of oxidative reactions and modifications, giving rise to a virtual redox zip code with physiological and pathophysiological relevance for modulation of protein structure and functions. While over two decades of studies have established NO-dependent S-nitrosylation as ubiquitous and fundamental for the regulation of diverse protein activities, proteomic methods for studying H2S-dependent S-sulfhydration have only recently been described and now suggest that this is also an abundant modification with potential for global physiological importance. Notably, protein S-sulfhydration and S-nitrosylation bear striking similarities in terms of their chemical and biological determinants, as well as reversal of these modifications via group-transfer to glutathione, followed by the removal from glutathione by enzymes that have apparently evolved to selectively catalyze denitrosylation and desulfhydration. Here we review determinants of protein and low-molecular-weight thiol S-sulfhydration/desulfhydration, similarities with S-nitrosylation/denitrosylation, and methods that are being employed to investigate and quantify these gasotransmitter-mediated cell signaling systems.

Protein expression profiling of nuclear membrane protein reveals potential biomarker of human hepatocellular carcinoma

Background

Complex molecular events lead to development and progression of liver cirrhosis to HCC. Differentially expressed nuclear membrane associated proteins are responsible for the functional and structural alteration during the progression from cirrhosis to carcinoma. Although alterations/ post translational modifications in protein expression have been extensively quantified, complementary analysis of nuclear membrane proteome changes have been limited. Deciphering the molecular mechanism that differentiate between normal and disease state may lead to identification of biomarkers for carcinoma.

Results

Many proteins displayed differential expression when nuclear membrane proteome of hepatocellular carcinoma (HCC), fibrotic liver, and HepG2 cell line were assessed using 2-DE and ESI-Q-TOF MS/MS. From the down regulated set in HCC, we have identified for the first time a 15 KDa cytochrome b5A (CYB5A), ATP synthase subunit delta (ATPD) and Hemoglobin subunit beta (HBB) with 11, 5 and 22 peptide matches respectively. Furthermore, nitrosylation studies with S-nitrosocysteine followed by immunoblotting with anti SNO-cysteine demonstrated a novel and biologically relevant post translational modification of thiols of CYB5A in HCC specimens only. Immunofluorescence images demonstrated increased protein S-nitrosylation signals in the tumor cells and fibrotic region of HCC tissues. The two other nuclear membrane proteins which were only found to be nitrosylated in case of HCC were up regulated ATP synthase subunit beta (ATPB) and down regulated HBB. The decrease in expression of CYB5A in HCC suggests their possible role in disease progression. Further insight of the functional association of the identified proteins was obtained through KEGG/ REACTOME pathway analysis databases. String 8.3 interaction network shows strong interactions with proteins at high confidence score, which is helpful in characterization of functional abnormalities that may be a causative factor of liver pathology.

Conclusion

These findings may have broader implications for understanding the mechanism of development of carcinoma. However, large scale studies will be required for further verification of their critical role in development and progression of HCC.

Hepatocarcinogenesis driven by GSNOR deficiency is prevented by iNOS inhibition

Hepatocellular carcinoma (HCC) is one of the most common and deadly human cancers and it remains poorly managed. Human HCC development is often associated both with elevated expression of inducible nitric oxide synthase (iNOS) and with genetic deletion of the major denitrosylase S-nitrosoglutathione reductase (GSNOR/ADH5). However, their causal involvement in human HCC is not established. In mice, GSNOR deficiency causes S-nitrosylation and depletion of the DNA repair protein O6-alkylguanine-DNA-alkyltransferase (AGT) and increases rates of both spontaneous and DEN carcinogen-induced HCC. Here, we report that administration of 1400W, a potent and highly selective inhibitor of iNOS, blocked AGT depletion and rescued the repair of mutagenic O6-ethyldeoxyguanosines following DEN challenge in livers of GSNOR-deficient (GSNOR(-/-)) mice. Notably, short-term iNOS inhibition following DEN treatment had little effect on carcinogenesis in wild-type mice, but was sufficient to reduce HCC multiplicity, maximal size, and burden in GSNOR(-/-) mice to levels comparable with wild-type controls. Furthermore, increased HCC susceptibility in GSNOR(-/-) mice was not associated with an increase in interleukin 6, tumor necrosis factor-α, oxidative stress, or hepatocellular proliferation. These results suggested that GSNOR deficiency linked to defective DNA damage repair likely acts at the tumor initiation stage to promote HCC carcinogenesis. Together, our findings provide the first proof of principle that HCC development in the context of uncontrolled nitrosative stress can be blocked by pharmacologic inhibition of iNOS, possibly providing an effective therapy for patients with HCC.

S-nitrosoglutathione reductase deficiency increases mutagenesis from alkylation in mouse liver

In human hepatocellular carcinoma (HCC) and many other cancers, somatic point mutations are highly prevalent, yet the mechanisms critical in their generation remain poorly understood. S-nitrosoglutathione reductase (GSNOR), a key regulator of protein S-nitrosylation, is frequently deficient in human HCC. Targeted deletion of the GSNOR gene in mice can reduce the activity of the DNA repair protein O (6)-alkylguanine-DNA alkyltransferase (AGT) and promote both carcinogen-induced and spontaneous HCC. In this study, we report that following exposure to the environmental carcinogen diethylnitrosamine, the mutation frequency of a transgenic reporter in the liver of GSNOR-deficient mice (GSNOR(-/-)) is significantly higher than that in wild-type control. In wild-type mice, diethylnitrosamine treatment does not significantly increase the frequency of the transition from G:C to A:T, a mutation deriving from diethylnitrosamine-induced O (6)-ethylguanines that are normally repaired by AGT. In contrast, the frequency of this transition from diethylnitrosamine is increased ~20 times in GSNOR(-/-) mice. GSNOR deficiency also significantly increases the frequency of the transversion from A:T to T:A, a mutation not affected by AGT. GSNOR deficiency in our experiments does not significantly affect either the frequencies of the other diethylnitrosamine-induced point mutations or hepatocyte proliferation. Thus, GSNOR deficiency, through both AGT-dependent and AGT-independent pathways, significantly raises the rates of specific types of DNA mutations. Our results demonstrate a critical role for GSNOR in maintaining genomic integrity in mice and support the hypothesis that GSNOR deficiency is an important cause of the widespread mutations in human HCC.

Pharmacological inhibition of S-nitrosoglutathione reductase improves endothelial vasodilatory function in rats in vivo

Nitric oxide (NO) exerts a wide range of cellular effects in the cardiovascular system. NO is short lived, but S-nitrosoglutathione (GSNO) functions as a stable intracellular bioavailable NO pool. Accordingly, increased levels can facilitate NO-mediated processes, and conversely, catabolism of GSNO by the regulatory enzyme GSNO reductase (GSNOR) can impair these processes. Because dysregulated GSNOR can interfere with processes relevant to cardiovascular health, it follows that inhibition of GSNOR may be beneficial. However, the effect of GSNOR inhibition on vascular activity is unknown. To study the effects of GSNOR inhibition on endothelial function, we treated rats with a small-molecule inhibitor of GSNOR (N6338) that has vasodilatory effects on isolated aortic rings and assessed effects on arterial flow-mediated dilation (FMD), an NO-dependent process. GSNOR inhibition with a single intravenous dose of N6338 preserved FMD (15.3 ± 5.4 vs. 14.2 ± 6.3%, P = nonsignificant) under partial NO synthase inhibition that normally reduces FMD by roughly 50% (14.1 ± 2.9 vs. 7.6 ± 4.4%, P < 0.05). In hypertensive rats, daily oral administration of N6338 for 14 days reduced blood pressure (170.0 ± 5.3/122.7 ± 6.4 vs. 203.8 ± 1.9/143.7 ± 7.5 mmHg for vehicle, P < 0.001) and vascular resistance index (1.5 ± 0.4 vs. 3.2 ± 1.0 mmHg · min · l(-1) for vehicle, P < 0.001), and restored FMD from an initially impaired state (7.4 ± 1.7%, day 0) to a level (13.0 ± 3.1%, day 14, P < 0.001) similar to that observed in normotensive rats. N6338 also reversed the pathological kidney changes exhibited by the hypertensive rats. GSNOR inhibition preserves FMD under conditions of impaired NO production and protects against both microvascular and conduit artery dysfunction in a model of hypertension.

S-nitrosoglutathione reductases are low-copy number, cysteine-rich proteins in plants that control multiple developmental and defense responses in Arabidopsis

S-nitrosoglutathione reductase (GSNOR) is believed to modulate effects of reactive oxygen and nitrogen species through catabolism of S-nitrosoglutathione (GSNO). We combined bioinformatics of plant GSNOR genes, localization of GSNOR in Arabidopsis thaliana, and microarray analysis of a GSNOR null mutant to gain insights into the function and regulation of this critical enzyme in nitric oxide (NO) homeostasis. GSNOR-encoding genes are known to have high homology across diverse eukaryotic taxa, but contributions of specific conserved residues have not been assessed. With bioinformatics and structural modeling, we show that plant GSNORs likely localize to the cytosol, contain conserved, solvent-accessible cysteines, and tend to be encoded by a single gene. Arabidopsis thaliana homozygous for GSNOR loss-of-function alleles exhibited defects in stem and trichome branching, and complementation with Green fluorescent protein (GFP) -tagged GSNOR under control of the native promoter quantitatively rescued these phenotypes. GSNOR-GFP showed fluorescence throughout Arabidopsis seedlings, consistent with ubiquitous expression of the protein, but with especially high fluorescence in the root tip, apical meristem, and flowers. At the cellular level we observed cytosolic and nuclear fluorescence, with exclusion from the nucleolus. Microarray analysis identified 99 up- and 170 down-regulated genes (≥2-fold; p ≤ 0.01) in a GSNOR null mutant compared to wild type. Six members of the plant specific, ROXY glutaredoxins and three BHLH transcription factors involved in iron homeostasis were strongly upregulated, supporting a role for GSNOR in redox and iron metabolism. One third of downregulated genes are linked to pathogen resistance, providing further basis for the reported pathogen sensitivity of GSNOR null mutants. Together, these findings indicate GSNOR regulates multiple developmental and metabolic programs in plants and offer insight into putative routes of post-translational GSNOR regulation.

Maintenance of S-nitrosothiol homeostasis plays an important role in growth suppression of estrogen receptor-positive breast tumors

Introduction

Protein denitrosylation by thioredoxin reductase (TrxR) is key for maintaining S-nitrosothiol (SNO) homeostasis, although its role in tumor progression is unknown. Therefore, the present study aimed to assess the role of altered SNO homeostasis in breast cancer cells.

Methods

The impairment of SNO homeostasis in breast cancer cells was achieved with the highly specific TrxR inhibitor auranofin and/or exposure to S-nitroso-L-cysteine. S-nitrosylated proteins were detected using the biotin switch assay. Estrogen receptor (ER) alpha knockdown was achieved using RNA silencing technologies and subcellular localization of ERα was analyzed by confocal microscopy. The Oncomine database was explored for TrxR1 (TXNRD1) expression in breast tumors and TrxR1, ER and p53 expression was analyzed by immunohistochemistry in a panel of breast tumors.

Results

The impairment of SNO homeostasis enhanced cell proliferation and survival of ER+ MCF-7 cells, but not of MDA-MB-231 (ER-, mut p53) or BT-474 (ER+, mut p53) cells. This enhanced cell growth and survival was associated with Akt, Erk1/2 phosphorylation, and augmented cyclin D₁ expression and was abolished by the ER antagonist fulvestrant or the p53 specific inhibitor pifithrin-α. The specific silencing of ERα expression in MCF-7 cells also abrogated the growth effect of TrxR inhibition. Estrogenic deprivation in MCF-7 cells potentiated the pro-proliferative effect of impaired SNO homeostasis. Moreover, the subcellular distribution of ERα was altered, with a predominant nuclear localization associated with phosphorylation at Thr311 in those cells with impaired SNO homeostasis. The impairment of SNO homeostasis also expanded a cancer stem cell-like subpopulation in MCF-7 cells, as indicated by the increase of percentage of CD44⁺ cells and the augmented capability to form mammospheres in vitro. Notably, ER+ status in breast tumors was significantly associated with lower TXNDR1 mRNA expression and immunohistochemical studies confirmed this association, particularly when p53 abnormalities were absent.

Conclusion

The ER status in breast cancer may dictate tumor response to different nitrosative environments. Impairment of SNO homeostasis confers survival advantages to ER+ breast tumors, and these molecular mechanisms may also participate in the development of resistance against hormonal therapies that arise in this type of mammary tumors.

Proteomic analysis of the role of S-nitrosoglutathione reductase in lipopolysaccharide-challenged mice

S-Nitrosoglutathione reductase (GSNOR) is a key regulator of protein S-nitrosylation, the covalent modification of cysteine residues by nitric oxide that can affect activities of many proteins. We recently discovered that excessive S-nitrosylation from GSNOR deficiency in mice under inflammation inactivates the key DNA repair protein O(6) -alkylguanine-DNA alkyltransferase and promotes both spontaneous and carcinogen-induced hepatocellular carcinoma. To explore further the mechanism of tumorigenesis due to GSNOR deficiency, we compared the protein expression profiles in the livers of wild-type and GSNOR-deficient (GSNOR(-/-) ) mice that were challenged with lipopolysaccharide to induce inflammation and expression of inducible nitric oxide synthase (iNOS). Two-dimensional difference gel electrophoresis analysis identified 38 protein spots of significantly increased intensity and 31 protein spots of significantly decreased intensity in the GSNOR(-/-) mice compared to those in the wild-type mice. We subsequently identified 19 upregulated and 19 downregulated proteins in GSNOR(-/-) mice using mass spectrometry. Immunoblot analysis confirmed in GSNOR(-/-) mice a large increase in the expression of the pro-inflammatory mediator S100A9, a protein previously implicated in human liver carcinogenesis. We also found a decrease in the expression of multiple members of the protein disulfide-isomerase (PDI) family and an alteration in the expression pattern of the endoplasmic reticulum (ER) chaperones in GSNOR(-/-) mice. Furthermore, altered expression of these proteins from GSNOR deficiency was prevented in mice lacking both GSNOR and iNOS. In addition, we detected S-nitrosylation of two members of the PDI protein family. These results suggest that S-nitrosylation resulting from GSNOR deficiency may promote carcinogenesis under inflammatory conditions in part through the disruption of inflammatory and ER stress responses.

Nitric oxide-dependent CYP2B degradation is potentiated by a cytokine-regulated pathway and utilizes the immunoproteasome subunit LMP2

CYP2B proteins in rat hepatocytes undergo NO-dependent proteolytic degradation, but the mechanisms and the reasons for the specificity towards only certain P450 (cytochrome P450) enzymes are yet unknown. In the present study we found that down-regulation of CYP2B proteins by the NO donor NOC-18 is accelerated by pretreatment of the hepatocytes with IL-1 (interleukin-1β) in the presence of an NO synthase inhibitor, suggesting that an NO-independent action of IL-1 contributes to the lability of CYP2B proteins. The immunoproteasome subunit LMP2 (large multifunctional peptidase 2) was significantly expressed in hepatocytes under basal conditions, and IL-1 induced LMP2 within 6-12 h of treatment. CYP2B protein degradation in response to IL-1 was attenuated by the selective LMP2 inhibitor UK-101, but not by the LMP7 inhibitor IPSI. The results show that LMP2 contributes to the NO-dependent degradation of CYP2B proteins, and suggest that induction of LMP2 may be involved in the potentiation of this degradation by IL-1.

Ets-1 is a transcriptional mediator of oncogenic nitric oxide signaling in estrogen receptor-negative breast cancer

Introduction

The Ets-1 transcription factor is a candidate breast cancer oncogene that regulates the expression of genes involved in tumor progression and metastasis. Ets-1 signaling has also been linked to the development of a basal-like breast cancer phenotype. We recently described a nitric oxide (NO)-induced gene signature that is associated with poor disease outcome in estrogen receptor-negative (ER-) breast cancer and contains both stem cell-like and basal-like components. Thus, we examined the role of Ets-1 in NO signaling and NO-induced phenotypes in ER- human breast cancer cells.

Methods

Promoter region analyses were performed on genes upregulated in inducible nitric oxide synthase (NOS2) high expressing tumors for Ets-binding sites. In vitro mechanisms were examined in human basal-like breast cancer cells lines. NO signaling effects were studied using either forced NOS2 expression or the use of a chemical NO-donor, diethlylenetriamine NONOate (DETANO).

Results

Promoter region analysis of genes that are up-regulated in human ER-negative breast tumors with high NOS2 expression revealed that the Ets-binding sequence is the only common promoter element present in all of these genes, indicating that Ets-1 is the key transcriptional factor down-stream of oncogenic NOS2-signaling. Accordingly, both forced NOS2 over-expression and exposure to NO-donors resulted in significant Ets-1 transcriptional activation in ER- breast cancer cells. Functional studies showed that NO activated Ets-1 transcriptional activity via a Ras/MEK/ERK signaling pathway by a mechanism that involved Ras S-nitrosylation. RNA knock-down of Ets-1 suppressed NO-induced expression of selected basal-like breast cancer markers such as P-cadherin, S100A8, IL-8 and αβ-crystallin. Additionally, Ets-1 knock-down reduced NO-mediated cellular proliferation, matrix metalloproteinase and cathepsin B activities, as well as matrigel invasion.

Conclusions

These data show that Ets-1 is a key transcriptional mediator of oncogenic NO signaling that promotes the development of an aggressive disease phenotype in ER- breast cancer in an Ets-1 and Ras-dependent manner, providing novel clues of how NOS2 expression in human breast tumors is functionally linked to poor patient survival.

Can nitric oxide-based therapy prevent bronchopulmonary dysplasia?

A growing understanding of endogenous nitric oxide (NO) biology is helping to explain how and when exogenous NO may confer benefit or harm; this knowledge is also helping to identify new better-targeted NO-based therapies. In this review, results of the bronchopulmonary dysplasia clinical trials that used inhaled NO in the preterm population are placed in context, the biologic basis for novel NO therapeutics is considered, and possible future directions for NO-focused clinical and basic research in developmental lung disease are identified.

Established Principles and Emerging Concepts on the Interplay between Mitochondrial Physiology and S-(De)nitrosylation: Implications in Cancer and Neurodegeneration

S-nitrosylation is a posttranslational modification of cysteine residues that has been frequently indicated as potential molecular mechanism governing cell response upon redox unbalance downstream of nitric oxide (over)production. In the last years, increased levels of S-nitrosothiols (SNOs) have been tightly associated with the onset of nitroxidative stress-based pathologies (e.g., cancer and neurodegeneration), conditions in which alterations of mitochondrial homeostasis and activation of cellular processes dependent on it have been reported as well. In this paper we aim at summarizing the current knowledge of mitochondria-related proteins undergoing S-nitrosylation and how this redox modification might impact on mitochondrial functions, whose impairment has been correlated to tumorigenesis and neuronal cell death. In particular, emphasis will be given to the possible, but still neglected implication of denitrosylation reactions in the modulation of mitochondrial SNOs and how they can affect mitochondrion-related cellular process, such as oxidative phosphorylation, mitochondrial dynamics, and mitophagy.

Increase of novel biomarkers for oxidative stress in patients with plasma cell disorders and in multiple myeloma patients with bone lesions

OBJECTIVES

Protein oxidation plays a key role in the pathogenesis of oncological diseases. In this study, we analyzed the oxidative stress in untreated multiple myeloma (MM) patients and in patients affected by monoclonal gammopathy of uncertain significance (MGUS).

METHODS

We evaluated serum levels of advanced oxidation protein products (AOPPs), advanced glycation end products (AGEs), and protein nitrosylation in patients with monoclonal gammopathy and in control subjects.

RESULTS

Serum levels of AOPPs and S-nitrosylated proteins were significantly increased in MM patients in comparison to controls and to MGUS subjects. Moreover, in MM patients the levels of AOPPs, AGEs and S-nitrosylated proteins were significantly higher in patients with bone lesions compared with those without lytic bone lesions.

CONCLUSIONS

MM is closely associated with oxidative stress and further investigation might provide an insight to understand a putative causal link between oxidative stress and MM disease onset and progression or MM complications.

S-nitrosoglutathione reductase in human lung cancer

S-Nitrosoglutathione (GSNO) reductase regulates cell signaling pathways relevant to asthma and protects cells from nitrosative stress. Recent evidence suggests that this enzyme may prevent human hepatocellular carcinoma arising in the setting of chronic hepatitis. We hypothesized that GSNO reductase may also protect the lung against potentially carcinogenic reactions associated with nitrosative stress. We report that wild-type Ras is S-nitrosylated and activated by nitrosative stress and that it is denitrosylated by GSNO reductase. In human lung cancer, the activity and expression of GSNO reductase are decreased. Further, the distribution of the enzyme (including its colocalization with wild-type Ras) is abnormal. We conclude that decreased activity of GSNO reductase could leave the human lung vulnerable to the oncogenic effects of nitrosative stress, as is the case in the liver. This potential should be considered when developing therapies that inhibit pulmonary GSNO reductase to treat asthma and other conditions.

Proteomic characterization of the cellular response to nitrosative stress mediated by s-nitrosoglutathione reductase inhibition

The S-nitrosoglutathione-metabolizing enzyme, GSNO reductase (GSNOR), has emerged as an important regulator of protein S-nitrosylation. GSNOR ablation is protective in models of asthma and heart failure, raising the idea that GSNOR inhibitors might hold therapeutic value. Here, we investigated the effects of a small molecule inhibitor of GSNOR (GSNORi) in mouse RAW 264.7 macrophages. We found that GSNORi increased protein S-nitrosylation in cytokine-stimulated cells, and we utilized stable isotope labeling of amino acids in cell culture (SILAC) to quantify the cellular response to this "nitrosative stress". The expression of several cytokine-inducible immunomodulators, including osteopontin, cyclooxygenase-2, and nitric oxide synthase isoform 2 (NOS2), were decreased by GSNORi. In addition, selective targets of the redox-regulated transcription factor, nuclear factor (erythroid-derived 2)-like 2 (Nrf2)-including heme oxygenase 1 (HO-1) and glutamate cysteine ligase modulatory subunit-were induced by GSNORi in a NOS2- and Nrf2-dependent manner. In cytokine-stimulated cells, Nrf2 protected from GSNORi-induced glutathione depletion and cytotoxicity and HO-1 activity was required for down-regulation of NOS2. Interestingly, GSNORi also affected a marked increase in NOS2 protein stability. Collectively, these data provide the most complete description of the global effects of GSNOR inhibition and demonstrate several important mechanisms for inducible response to GSNORi-mediated nitrosative stress.

Enzymatic mechanisms regulating protein S-nitrosylation: implications in health and disease

Nitric oxide participates in cellular signal transduction largely through S-nitrosylation of allosteric and active-site cysteine thiols within proteins, forming S-nitroso-proteins (SNO-proteins). S-nitrosylation of proteins has been demonstrated to affect a broad range of functional parameters including enzymatic activity, subcellular localization, protein-protein interactions, and protein stability. Analogous to other ubiquitous posttranslational modifications that are regulated enzymatically, including phosphorylation and ubiquitinylation, accumulating evidence suggests the existence of enzymatic mechanisms for regulating protein S-nitrosylation. In particular, studies have led to the identification of multiple enzymes (nitrosylases and denitrosylases) that participate in targeted S-nitrosylation or denitrosylation of proteins in physiological settings. Nitrosylases are best characterized in the context of transnitrosylation in which a SNO-protein transfers an NO group to an acceptor protein (Cys-to-Cys transfer), but examples of transnitrosylation catalyzed by metalloproteins (Metal-to-Cys transfer) also exist. By contrast, denitrosylases remove the NO group from SNO-proteins, ultimately using reducing equivalents derived from NADH or NADPH. Here, we focus on the recent discoveries of nitrosylases and denitrosylases and the notion that their aberrant activities may play roles in health and disease.

Established Principles and Emerging Concepts on the Interplay between Mitochondrial Physiology and S-(De)nitrosylation: Implications in Cancer and Neurodegeneration

S-nitrosylation is a posttranslational modification of cysteine residues that has been frequently indicated as potential molecular mechanism governing cell response upon redox unbalance downstream of nitric oxide (over)production. In the last years, increased levels of S-nitrosothiols (SNOs) have been tightly associated with the onset of nitroxidative stress-based pathologies (e.g., cancer and neurodegeneration), conditions in which alterations of mitochondrial homeostasis and activation of cellular processes dependent on it have been reported as well. In this paper we aim at summarizing the current knowledge of mitochondria-related proteins undergoing S-nitrosylation and how this redox modification might impact on mitochondrial functions, whose impairment has been correlated to tumorigenesis and neuronal cell death. In particular, emphasis will be given to the possible, but still neglected implication of denitrosylation reactions in the modulation of mitochondrial SNOs and how they can affect mitochondrion-related cellular process, such as oxidative phosphorylation, mitochondrial dynamics, and mitophagy.

Predict and analyze S-nitrosylation modification sites with the mRMR and IFS approaches

S-nitrosylation (SNO) is one of the most important and universal post-translational modifications (PTMs) which regulates various cellular functions and signaling events. Identification of the exact S-nitrosylation sites in proteins may facilitate the understanding of the molecular mechanisms and biological function of S-nitrosylation. Unfortunately, traditional experimental approaches used for detecting S-nitrosylation sites are often laborious and time-consuming. However, computational methods could overcome this demerit. In this work, we developed a novel predictor based on nearest neighbor algorithm (NNA) with the maximum relevance minimum redundancy (mRMR) method followed by incremental feature selection (IFS). The features of physicochemical/biochemical properties, sequence conservation, residual disorder, amino acid occurrence frequency, second structure and the solvent accessibility were utilized to represent the peptides concerned. Feature analysis showed that the features except residual disorder affected identification of the S-nitrosylation sites. It was also shown via the site-specific feature analysis that the features of sites away from the central cysteine might contribute to the S-nitrosylation site determination through a subtle manner. It is anticipated that our prediction method may become a useful tool for identifying the protein S-nitrosylation sites and that the features analysis described in this paper may provide useful insights for in-depth investigation into the mechanism of S-nitrosylation.

FK506 neuroprotection after cavernous nerve injury is mediated by thioredoxin and glutathione redox systems

INTRODUCTION

Immunophilin ligands such as FK506 (FK) preserve erectile function (EF) following cavernous nerve injury (CNI), although the precise mechanisms are unclear. We examined whether the thioredoxin (Trx) and glutathione (GSH) redox systems mediate this effect after CNI.

AIM

To investigate the roles of Trx reductase 2 (TrxR2) and S-Nitrosoglutathione reductase (GSNOR) as antioxidative/nitrosative and antiapoptotic mediators of the neuroprotective effect of FK in the penis after CNI.

METHODS

Adult male rats, wild-type (WT) mice, and GSNOR deficient (GSNOR -/-) mice were divided into four groups: sham surgery (CN [cavernous nerves] exposure only) + vehicle; sham surgery + FK (5 mg/kg/day/rat or 2 mg/kg/day/mouse, for 2 days, subcutaneous); CNI + vehicle; and CNI + FK. At day 4 after injury, electrically stimulated changes in intracavernosal pressure (ICP) were measured. Penises were collected for Western blot analysis of TrxR2, GSNOR, and Bcl-2, and for immunolocalization of TrxR2 and GSNOR.

MAIN OUTCOME MEASURES

EF assessment represented by maximal ICP and total ICP in response to electrical stimulation. Evaluation of protein expression levels and distribution patterns of antioxidative/nitrosative and antiapoptotic factors in penile tissue.

RESULTS

EF decreased after CNI compared with sham surgery values in both rats (P < 0.01) and WT and GSNOR -/- mice (P < 0.05). FK treatment preserved EF after CNI compared with vehicle treatment in rats (P < 0.01) and WT mice (P < 0.05) but not in GSNOR -/- mice. In rats, GSNOR (P < 0.01) and Bcl-2 (P < 0.05) expressions were significantly decreased after CNI. FK treatment in CN-injured rats restored expression of GSNOR and upregulated TrxR2 (P < 0.001) and Bcl-2 (P < 0.001) expressions compared with vehicle treatment. Localizations of proteins in the penis were observed for TrxR2 (endothelium, smooth muscle) and for GSNOR (nerves, endothelium, smooth muscle).

CONCLUSIONS

The neuroprotective effect of FK in preserving EF after CNI involves antioxidative/nitrosative and antiapoptotic mechanisms mediated, to some extent, by Trx and GSH systems.