Calcium-dependent release of NO from intracellular S-nitrosothiols

Driver Mutations of Pancreatic Cancer Affect Ca2+ Signaling and ATP Production

Glandular pancreatic epithelia of the acinar or ductal phenotype may seem terminally differentiated, but they are characterized by remarkable cell plasticity. Stress-induced trans-differentiation of these cells has been implicated in the mechanisms of carcinogenesis. Current consensus links pancreatic ductal adenocarcinoma with onco-transformation of ductal epithelia, but under the presence of driver mutations in Kras and Trp53, also with trans-differentiation of pancreatic acini. However, we do not know when, in the course of cancer progression, physiological functions are lost by mutant acinar cells, nor can we assess their capacity for the production of pancreatic juice components. Here, we investigated whether two mutations-KrasG12D and Trp53R172H-present simultaneously in acinar cells of KPC mice (model of oncogenesis) influence cytosolic Ca2+ signals. Since Ca2+ signals control the cellular handling of digestive hydrolases, any changes that affect intracellular signaling events and cell bioenergetics might have an impact on the physiology of the pancreas. Our results showed that physiological doses of acetylcholine evoked less regular Ca2+ oscillations in KPC acinar cells compared to the control, whereas responses to supramaximal concentrations were markedly reduced. Menadione elicited Ca2+ signals of different frequencies in KPC cells compared to control cells. Finally, Ca2+ extrusion rates were significantly inhibited in KPC cells, likely due to the lower basal respiration and ATP production. Cumulatively, these findings suggest that driver mutations affect the signaling capacity of pancreatic acinar cells even before the changes in the epithelial cell morphology become apparent.

Biological Mechanisms of S-Nitrosothiol Formation and Degradation: How Is Specificity of S-Nitrosylation Achieved?

The modification of protein cysteine residues underlies some of the diverse biological functions of nitric oxide (NO) in physiology and disease. The formation of stable nitrosothiols occurs under biologically relevant conditions and time scales. However, the factors that determine the selective nature of this modification remain poorly understood, making it difficult to predict thiol targets and thus construct informatics networks. In this review, the biological chemistry of NO will be considered within the context of nitrosothiol formation and degradation whilst considering how specificity is achieved in this important post-translational modification. Since nitrosothiol formation requires a formal one-electron oxidation, a classification of reaction mechanisms is proposed regarding which species undergoes electron abstraction: NO, thiol or S-NO radical intermediate. Relevant kinetic, thermodynamic and mechanistic considerations will be examined and the impact of sources of NO and the chemical nature of potential reaction targets is also discussed.

NADPH diaphorase detects S-nitrosylated proteins in aldehyde-treated biological tissues

NADPH diaphorase is used as a histochemical marker of nitric oxide synthase (NOS) in aldehyde-treated tissues. It is thought that the catalytic activity of NOS promotes NADPH-dependent reduction of nitro-blue tetrazolium (NBT) to diformazan. However, it has been argued that a proteinaceous factor other than NOS is responsible for producing diformazan in aldehyde-treated tissues. We propose this is a NO-containing factor such as an S-nitrosothiol and/or a dinitrosyl-iron (II) cysteine complex or nitrosated proteins including NOS. We now report that (1) S-nitrosothiols covalently modify both NBT and TNBT, but only change the reduction potential of NBT after modification, (2) addition of S-nitrosothiols or β- or α-NADPH to solutions of NBT did not elicit diformazan, (3) addition of S-nitrosothiols to solutions of NBT plus β- or α-NADPH elicited rapid formation of diformazan in the absence or presence of paraformaldehyde, (4) addition of S-nitrosothiols to solutions of NBT plus β- or α-NADP did not produce diformazan, (5) S-nitrosothiols did not promote NADPH-dependent reduction of tetra-nitro-blue tetrazolium (TNBT) in which all four phenolic rings are nitrated, (6) cytoplasmic vesicles in vascular endothelial cells known to stain for NADPH diaphorase were rich in S-nitrosothiols, and (7) procedures that accelerate decomposition of S-nitrosothiols, markedly reduced NADPH diaphorase staining in tissue sections subsequently subjected to paraformaldehyde fixation. Our results suggest that NADPH diaphorase in aldehyde-fixed tissues is not enzymatic but is due to the presence of NO-containing factors (free SNOs or nitrosated proteins such as NOS), which promote NADPH-dependent reduction of NBT to diformazan.

Nitric oxide signals are interlinked with calcium signals in normal pancreatic stellate cells upon oxidative stress and inflammation

The mammalian diffuse stellate cell system comprises retinoid-storing cells capable of remarkable transformations from a quiescent to an activated myofibroblast-like phenotype. Activated pancreatic stellate cells (PSCs) attract attention owing to the pivotal role they play in development of tissue fibrosis in chronic pancreatitis and pancreatic cancer. However, little is known about the actual role of PSCs in the normal pancreas. These enigmatic cells have recently been shown to respond to physiological stimuli in a manner that is markedly different from their neighbouring pancreatic acinar cells (PACs). Here, we demonstrate the capacity of PSCs to generate nitric oxide (NO), a free radical messenger mediating, for example, inflammation and vasodilatation. We show that production of cytosolic NO in PSCs is unambiguously related to cytosolic Ca²⁺ signals. Only stimuli that evoke Ca²⁺ signals in the PSCs elicit consequent NO generation. We provide fresh evidence for the striking difference between signalling pathways in PSCs and adjacent PACs, because PSCs, in contrast to PACs, generate substantial Ca²⁺-mediated and NOS-dependent NO signals. We also show that inhibition of NO generation protects both PSCs and PACs from necrosis. Our results highlight the interplay between Ca²⁺ and NO signalling pathways in cell–cell communication, and also identify a potential therapeutic target for anti-inflammatory therapies.

Functional research and molecular mechanism of Kainic acid-induced denitrosylation of thioredoxin-1 in rat hippocampus

Thioredoxin-1 (Trx1) has long been recognized as a redox regulator, and is implicated in the inhibition of cell apoptosis. Trx1 is essential for the maintenance of the S-nitrosylation of molecules in cells. The S-nitrosylation of Trx1 is essential for the physiological function such as preservation of the redox regulatory activity. The mechanisms underlying Trx1 denitrosylation induced by kainate acid (KA) injection still remain uncharacterized. Our results showed that the S-nitrosylation levels of Trx1 were decreased subsequent to KA injection and that the glutamate receptor 6 (GluR6) antagonist NS102 could inhibit the denitrosylation of Trx1. Moreover, the denitrosylation of Trx1 following KA treatment could be suppressed by the Fas ligand (FasL) antisense oligodeoxynucleotides (AS-ODNs), the Trx reductase (TrxR) inhibitor dinitrochlorobenzene (DNCB), or the Nitric oxide (NO) donors sodium nitroprusside (SNP) and S-nitrosoglutathione (GSNO). Subsequently, these mechanisms were morphologically validated by cresyl violet staining, in situ TUNEL staining to detect the survival of CA1 and CA3/DG pyramidal neurons. NS102, FasL AS-ODNs, GSNO and SNP could provide neuroprotection of the pyramidal neurons of CA1 and CA3/dentate gyrus (DG) regions by attenuating Trx1 denitrosylation. Our results also showed that the denitrosylation of Trx1 induced by KA injection can active the caspase-3 which results in apoptosis.

RhoA S-nitrosylation as a regulatory mechanism influencing endothelial barrier function in response to G+-bacterial toxins

Disruption of the endothelial barrier in response to Gram positive (G⁺) bacterial toxins is a major complication of acute lung injury (ALI) and can be further aggravated by antibiotics which stimulate toxin release. The integrity of the pulmonary endothelial barrier is mediated by the balance of disruptive forces such as the small GTPase RhoA, and protective forces including endothelium-derived nitric oxide (NO). How NO protects against the barrier dysfunction is incompletely understood and our goal was to determine whether NO and S-nitrosylation can modulate RhoA activity and whether this mechanism is important for G⁺ toxin-induced microvascular permeability. We found that the G⁺ toxin listeriolysin-O (LLO) increased RhoA activity and that NO and S-NO donors inhibit RhoA activity. RhoA was robustly S-nitrosylated as determined by biotin-switch and mercury column analysis. MS revealed that three primary cysteine residues are S-nitrosylated including cys16, cys20 and cys159. Mutation of these residues to serine diminished S-nitrosylation to endogenous NO and mutant RhoA was less sensitive to inhibition by S-NO. G⁺-toxins stimulated the denitrosylation of RhoA which was not mediated by S-nitrosoglutathione reductase (GSNOR), thioredoxin (TRX) or thiol-dependent enzyme activity but was instead stimulated directly by elevated calcium levels. Calcium-promoted the direct denitrosylation of WT but not mutant RhoA and mutant RhoA adenovirus was more effective than WT in disrupting the barrier integrity of human lung microvascular endothelial cells. In conclusion, we reveal a novel mechanism by which NO and S-nitrosylation reduces RhoA activity which may be of significance in the management of pulmonary endothelial permeability induced by G⁺-toxins.

Gaseous modulators in the control of the hypothalamic neurohypophyseal system

Nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S) are gaseous molecules produced by the brain. Within the hypothalamus, gaseous molecules have been highlighted as autocrine and paracrine factors regulating endocrine function. Therefore, in the present review, we briefly discuss the main findings linking NO, CO, and H2S to the control of body fluid homeostasis at the hypothalamic level, with particular emphasis on the regulation of neurohypophyseal system output.

Nitric oxide is required for the auxin-induced activation of NADPH-dependent thioredoxin reductase and protein denitrosylation during root growth responses in arabidopsis

BACKGROUND AND AIMS

Auxin is the main phytohormone controlling root development in plants. This study uses pharmacological and genetic approaches to examine the role of auxin and nitric oxide (NO) in the activation of NADPH-dependent thioredoxin reductase (NTR), and the effect that this activity has on root growth responses in Arabidopsis thaliana.

METHODS

Arabidopsis seedlings were treated with auxin with or without the NTR inhibitors auranofin (ANF) and 1-chloro-2, 4-dinitrobenzene (DNCB). NTR activity, lateral root (LR) formation and S-nitrosothiol content were measured in roots. Protein S-nitrosylation was analysed by the biotin switch method in wild-type arabidopsis and in the double mutant ntra ntrb.

KEY RESULTS

The auxin-mediated induction of NTR activity is inhibited by the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (CPTIO), suggesting that NO is downstream of auxin in this regulatory pathway. The NTR inhibitors ANF and DNCB prevent auxin-mediated activation of NTR and LR formation. Moreover, ANF and DNCB also inhibit auxin-induced DR5 : : GUS and BA3 : : GUS gene expression, suggesting that the auxin signalling pathway is compromised without full NTR activity. Treatment of roots with ANF and DNCB increases total nitrosothiols (SNO) content and protein S-nitrosylation, suggesting a role of the NTR-thioredoxin (Trx)-redox system in protein denitrosylation. In agreement with these results, the level of S-nitrosylated proteins is increased in the arabidopsis double mutant ntra ntrb as compared with the wild-type.

CONCLUSIONS

The results support for the idea that NTR is involved in protein denitrosylation during auxin-mediated root development. The fact that a high NO concentration induces NTR activity suggests that a feedback mechanism to control massive and unregulated protein S-nitrosylation could be operating in plant cells.

Novel lipophilic probe for detecting near-membrane reactive oxygen species responses and its application for studies of pancreatic acinar cells: effects of pyocyanin and L-ornithine

AIMS

The aim of this study was to develop a fluorescent reactive oxygen species (ROS) probe, which is preferentially localized in cellular membranes and displays a strong change in fluorescence upon oxidation. We also aimed to test the performance of this probe for detecting pathophysiologically relevant ROS responses in isolated cells.

RESULTS

We introduced a novel lipophilic ROS probe dihydrorhodamine B octadecyl ester (H2RB-C18). We then applied the new probe to characterize the ROS changes triggered by inducers of acute pancreatitis in pancreatic acinar cells. We resolved ROS changes produced by L-ornithine, L-arginine, cholecystokinin-8, acetylcholine, taurolithocholic acid 3-sulfate, palmitoleic acid ethyl ester, and the bacterial toxin pyocyanin. Particularly prominent ROS responses were induced by pyocyanin and L-ornithine. These ROS responses were accompanied by changes in cytosolic Ca(2+)concentration ([Ca(2+)]i), mitochondrial membrane potential (ΔΨ), and NAD(P)H concentration.

INNOVATION

The study describes a novel sensitive lipophilic ROS probe. The probe is particularly suitable for detecting ROS in near-membrane regions and therefore for reporting the ROS environment of plasma membrane channels and pumps.

CONCLUSIONS

In our experimental conditions, the novel probe was more sensitive than 5-(and-6)-chloromethyl-2',7'-dichlorodihydrofluorescein (CM-H2DCF) and dihydrorhodamine123 (H2R123) and allowed us to resolve ROS responses to secretagogues, pyocyanin, and L-ornithine. Changes in the fluorescence of the new probe were particularly prominent in the peripheral plasma membrane-associated regions. Our findings suggest that the new probe will be a useful tool in studies of the contribution of ROS to the pathophysiology of exocrine pancreas and other organs/tissues.

Expression of human cationic trypsinogen (PRSS1) in murine acinar cells promotes pancreatitis and apoptotic cell death

Hereditary pancreatitis (HP) is an autosomal dominant disease that displays the features of both acute and chronic pancreatitis. Mutations in human cationic trypsinogen (PRSS1) are associated with HP and have provided some insight into the pathogenesis of pancreatitis, but mechanisms responsible for the initiation of pancreatitis have not been elucidated and the role of apoptosis and necrosis has been much debated. However, it has been generally accepted that trypsinogen, prematurely activated within the pancreatic acinar cell, has a major role in the initiation process. Functional studies of HP have been limited by the absence of an experimental system that authentically mimics disease development. We therefore developed a novel transgenic murine model system using wild-type (WT) human PRSS1 or two HP-associated mutants (R122H and N29I) to determine whether expression of human cationic trypsinogen in murine acinar cells promotes pancreatitis. The rat elastase promoter was used to target transgene expression to pancreatic acinar cells in three transgenic strains that were generated: Tg(Ela-PRSS1)NV, Tg(Ela-PRSS1*R122H)NV and Tg(Ela-PRSS1*N29I)NV. Mice were analysed histologically, immunohistochemically and biochemically. We found that transgene expression is restricted to pancreatic acinar cells and transgenic PRSS1 proteins are targeted to the pancreatic secretory pathway. Animals from all transgenic strains developed pancreatitis characterised by acinar cell vacuolisation, inflammatory infiltrates and fibrosis. Transgenic animals also developed more severe pancreatitis upon treatment with low-dose cerulein than controls, displaying significantly higher scores for oedema, inflammation and overall histopathology. Expression of PRSS1, WT or mutant, in acinar cells increased apoptosis in pancreatic tissues and isolated acinar cells. Moreover, studies of isolated acinar cells demonstrated that transgene expression promotes apoptosis rather than necrosis. We therefore conclude that expression of WT or mutant human PRSS1 in murine acinar cells induces apoptosis and is sufficient to promote spontaneous pancreatitis, which is enhanced in response to cellular insult.

Nitric oxide metabolites in the lumbosacral spinal cord interstice and cerebrospinal fluid in female rats with acute cyclophosphamide-induced cystitis. An in vivo microdialysis study

Objective:

To determine the concentration of nitrate/nitrite in the cerebrospinal fluid and in the dorsal horn interstice of the L6-S1 spinal cord boundary in rats with or without cystitis induced by cyclophosphamide.

Methods:

All experiments were conducted using Wistar female rats. A microdialysis probe was implanted in the subarachnoid space or in the spinal cord tissue at the L6-S1 segments (confirmed histologically). Two days later, the microdialysis probe was perfused with artificial cerebrospinal fluid, containing or not NGmonomethyl-L-arginine. Samples were collected every 15 minutes and kept at −20°C. Nitrite/nitrate concentrations were determined by chemiluminescence.

Results:

In normal animals, the mean values of nitrite/nitrate concentrations in the first microdialysate sample of the cerebrospinal fluid and of the spinal cord interstice were similar (482.5±90.2pmol/75μL, n=20, and 505.7±11.5pmol/75μL, n=6, respectively), whereas, in the samples from rats with cystitis, these values were significantly greater (955.5±66.3pmol/75μL, n=8, and 926.5±131.7pmol/75μL, n=11, respectively). In both groups, NGmonomethyl-L- arginine caused a significant reduction in the nitrite/nitrate concentration. Interestingly, the maximal reduction of nitrite/nitrate concentration caused by NG-monomethyl-L- arginine was no greater than 30% of the initial values.

Conclusions:

These results constitute the first demonstration that nitrite/nitrate concentrations in the cerebrospinal fluid and spinal cord interstice are elevated between 20- and 22 hours after cyclophosphamide-induced cystitis, and indicate that cystitis is associated with changes in the production of nitric oxide in the spinal cord segments, where most primary bladder afferents end.

N-methyl-D-aspartate receptor-dependent denitrosylation of neuronal nitric oxide synthase increase the enzyme activity

Our laboratory once reported that neuronal nitric oxide synthase (nNOS) S-nitrosylation was decreased in rat hippocampus during cerebral ischemia-reperfusion, but the underlying mechanism was unclear. In this study, we show that nNOS activity is dynamically regulated by S-nitrosylation. We found that overexpressed nNOS in HEK293 (human embryonic kidney) cells could be S-nitrosylated by exogenous NO donor GSNO and which is associated with the enzyme activity decrease. Cys³³¹, one of the zinc-tetrathiolate cysteines, was identified as the key site of nNOS S-nitrosylation. In addition, we also found that nNOS is highly S-nitrosylated in resting rat hippocampal neurons and the enzyme undergos denitrosylation during the process of rat brain ischemia/reperfusion. Intrestingly, the process of nNOS denitrosylation is coupling with the decrease of nNOS phosphorylation at Ser⁸⁴⁷, a site associated with nNOS activation. Further more, we document that nNOS denitrosylation could be suppressed by pretreatment of neurons with MK801, an antagonist of NMDAR, GSNO, EGTA, BAPTA, W-7, an inhibitor of calmodulin as well as TrxR1 antisense oligonucleotide (AS-ODN) respectively. Taken together, our data demonstrate that the denitrosylation of nNOS induced by calcium ion influx is a NMDAR-dependent process during the early stage of ischemia/reperfusion, which is majorly mediated by thioredoxin-1 (Trx1) system. nNOS dephosphorylation may be induced by the enzyme denitrosylation, which suggest that S-nitrosylation/denitrosylation of nNOS may be an important mechanism in regulating the enzyme activity.

Region-specific changes in activities of cell death-related proteases and nitric oxide metabolism in rat brain in a chronic unpredictable stress model

Effects of a chronic combined unpredictable stress on activities of two cell death-related proteases, calpain and cathepsin B, were studied along with indices of nitrergic system in rat brain structures. Male Wistar rats were subjected to a 2-week-long combined stress (combination of unpaired flash light and moderate footshock associated with a white noise session). Stress resulted in a significant loss in the body and thymus weight and increased defecation in the open field test, though neither motor and exploratory activity, nor plasma corticosterone differed from the respective control levels. Decreased calpain activity and increased cathepsin B activity were demonstrated in the hippocampus of stressed rats (previously we have shown that caspase-3 activity was significantly suppressed in the brain of rats subjected to same type of stress). A significant reduction in the number of NOS-containing neurons was accompanied by a chronic stressinduced decline in NOS activity in the neocortex. Similar changes were observed in the hippocampus. However, levels of NO metabolites were elevated in both structures. Thus, stress-induced structural modifications in the brain may be mediated by disturbances in the nitrergic system and increased lysosomal proteolysis.

Kinetic analysis of DAF-FM activation by NO: toward calibration of a NO-sensitive fluorescent dye

Nitric oxide (NO) research in biomedicine has been hampered by the absence of a method that will allow quantitative measurement of NO in biological tissues with high sensitivity and selectivity, and with adequate spatial and temporal resolution. 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM) is a NO sensitive fluorescence probe that has been used widely for qualitative assessment of cellular NO production. However, calibration of the fluorescent signal and quantification of NO concentration in cells and tissues using fluorescent probes, have provided significant challenge. In this study we utilize a combination of mathematical modeling and experimentation to elucidate the kinetics of NO/DAF-FM reaction in solution. Modeling and experiments suggest that the slope of fluorescent intensity (FI) can be related to NO concentration according to the equation: ddtFI=2αk(1)NO(2)O(2)DAF-FMkNO+DAF-FM where α is a proportionality coefficient that relates FI to unit concentration of activated DAF-FM, k(1) is the NO oxidation rate constant, and k was estimated to be 4.3±0.6. The FI slope exhibits saturation kinetics with DAF-FM concentration. Interestingly, the effective half-maximum constant (EC(50)) increases proportionally to NO concentration. This result is not in agreement with the proposition that N(2)O(3) is the NO oxidation byproduct that activates DAF-FM. Kinetic analysis suggests that the reactive intermediate should exhibit NO-dependent consumption and thus NO(2)() is a more likely candidate. The derived rate law can be used for the calibration of DAF-FM fluorescence and the quantification of NO concentration in biological tissues.

Mitochondrial function and malfunction in the pathophysiology of pancreatitis

As a primary energy producer, mitochondria play a fundamental role in pancreatic exocrine physiology and pathology. The most frequent aetiology of acute pancreatitis is either gallstones or heavy alcohol consumption. Repeated episodes of acute pancreatitis can result in the development of chronic pancreatitis and increase the lifetime risk of pancreatic cancer 100-fold. Pancreatic cancer is one of the most common causes of cancer mortality with only about 3-4 % of patients surviving beyond 5 years. It has been shown that acute pancreatitis involves Ca²⁺ overload and overproduction of reactive oxygen species in pancreatic acinar cells. Both factors significantly affect mitochondria and lead to cell death. The pathogenesis of inflammation in acute and chronic pancreatitis is tightly linked to the induction of necrosis and apoptosis. There is currently no specific therapy for pancreatitis, but recent findings of an endogenous protective mechanism against Ca²⁺ overload--and particularly the potential to boost this protection--bring hope of new therapeutic approaches.

The role of nitric oxide in the physiology and pathophysiology of the exocrine pancreas

SIGNIFICANCE

Nitric oxide (NO), a ubiquitous gaseous signaling molecule, contributes to both pancreatic physiology and pathophysiology.

RECENT ADVANCES

The present review provides a general overview of NO synthesis, signaling, and function. Further, it specifically discusses NO metabolism and its effects in the exocrine pancreas and focuses on the role of NO in the pathogenesis of acute pancreatitis and pancreatic ischemia/reperfusion injury.

CRITICAL ISSUES

Unfortunately, the role of NO in pancreatic physiology and pathophysiology remains controversial in numerous areas. Many questions regarding the messenger molecule still remain unanswered.

FUTURE DIRECTIONS

Probably the least is known about the downstream targets of NO, which need to be identified, especially at the molecular level.

Hippocampal neuronal nitric oxide synthase mediates the stress-related depressive behaviors of glucocorticoids by downregulating glucocorticoid receptor

The molecular mechanisms underlying the behavioral effects of glucocorticoids are poorly understood. We report here that hippocampal neuronal nitric oxide synthase (nNOS) is a crucial mediator. Chronic mild stress and glucocorticoids exposures caused hippocampal nNOS overexpression via activating mineralocorticoid receptor. In turn, hippocampal nNOS-derived nitric oxide (NO) significantly downregulated local glucocorticoid receptor expression through both soluble guanylate cyclase (sGC)/cGMP and peroxynitrite (ONOO(-))/extracellular signal-regulated kinase signal pathways, and therefore elevated hypothalamic corticotrophin-releasing factor, a peptide that governs the hypothalamic-pituitary-adrenal axis. More importantly, nNOS deletion or intrahippocampal nNOS inhibition and NO-cGMP signaling blockade (using NO scavenger or sGC inhibitor) prevented the corticosterone-induced behavioral modifications, suggesting that hippocampal nNOS is necessary for the role of glucocorticoids in mediating depressive behaviors. In addition, directly delivering ONOO(-) donor into hippocampus caused depressive-like behaviors. Our findings reveal a role of hippocampal nNOS in regulating the behavioral effects of glucocorticoids.

Selective killing of cancer cells by a small molecule targeting the stress response to ROS

Malignant transformation, driven by gain-of-function mutations in oncogenes and loss-of-function mutations in tumour suppressor genes, results in cell deregulation that is frequently associated with enhanced cellular stress (for example, oxidative, replicative, metabolic and proteotoxic stress, and DNA damage). Adaptation to this stress phenotype is required for cancer cells to survive, and consequently cancer cells may become dependent upon non-oncogenes that do not ordinarily perform such a vital function in normal cells. Thus, targeting these non-oncogene dependencies in the context of a transformed genotype may result in a synthetic lethal interaction and the selective death of cancer cells. Here we used a cell-based small-molecule screening and quantitative proteomics approach that resulted in the unbiased identification of a small molecule that selectively kills cancer cells but not normal cells. Piperlongumine increases the level of reactive oxygen species (ROS) and apoptotic cell death in both cancer cells and normal cells engineered to have a cancer genotype, irrespective of p53 status, but it has little effect on either rapidly or slowly dividing primary normal cells. Significant antitumour effects are observed in piperlongumine-treated mouse xenograft tumour models, with no apparent toxicity in normal mice. Moreover, piperlongumine potently inhibits the growth of spontaneously formed malignant breast tumours and their associated metastases in mice. Our results demonstrate the ability of a small molecule to induce apoptosis selectively in cells that have a cancer genotype, by targeting a non-oncogene co-dependency acquired through the expression of the cancer genotype in response to transformation-induced oxidative stress.

TRPV3 regulates nitric oxide synthase-independent nitric oxide synthesis in the skin

Nitric oxide (NO) is an unstable signalling molecule synthesized de novo mainly from L-arginine by NO synthase (NOS) enzymes. Nitrite reduction can also produce NO, predominantly within body fluids (for example, saliva, sweat and blood plasma) and under extreme hypoxic and acidic conditions. It remains unknown if intracellular canonical signalling pathways regulate nitrite-dependent NO production. Here we examine NO production in the skin, a hypoxic tissue enriched in nitrites wherein NO has important roles in wound healing and other biological processes. We show that activation of TRPV3, a heat-activated transient receptor potential ion channel expressed in keratinocytes, induces NO production via a nitrite-dependent pathway. TRPV3 and nitrite are involved in keratinocyte migration in vitro and in wound healing and thermosensory behaviours in vivo. Our study demonstrates that activation of an ion channel can induce NOS-independent NO production in keratinocytes.

S-nitrosothiol signals in the enteric nervous system: lessons learnt from big brother

Nitric oxide (NO) is a functionally important neurotransmitter signaling molecule generated by mammalian and bacterial nitric oxide synthases (NOS), and by chemical conversion of dietary nitrite in the gastrointestinal (GI) tract. Neuronal NOS (nNOS) is the most abundant isoenzyme in the enteric nervous system, and targeted deletion in transgenic mice has clearly demonstrated its importance in normal gut function. Enteric neuropathy is also often associated with abnormal NO production, for example in achalasia and diabetic gastroparesis. Not surprisingly therefore, aberrant nNOS activity is widely implicated in enteric disease, and represents a potential molecular target for therapeutic intervention. One physiological signaling mechanism of NO bioactivity is through chemical reaction with the heme center of guanylyl cyclase, resulting in the conversion of cGMP from GTP. This second messenger nucleotide signal activates cGMP-dependent protein kinases, phosphodiesterases, and ion channels, and is implicated in the neuronal control of GI function. However, few studies in the GI tract have fully related NO bioactivity with specific molecular targets of NO-derived signals. In the central nervous system (CNS), it is now increasingly appreciated that NO bioactivity is often actively transduced via S-nitrosothiol (SNO) signals rather than via activation of guanylyl cyclase. Moreover, aberrant S-nitrosylation of specific molecular targets is implicated in CNS pathology. S-nitrosylation refers to the post-translational modification of a protein cysteine thiol by NO, forming an endogenous SNO. Because cysteine residues are often key regulators of protein function, S-nitrosylation represents a physiologically important signaling mechanism analogous to other post-translational modifications, such as O-phosphorylation. This article provides an overview of how neurotransmitter NO is produced by nNOS as this represents the most prominent and well defined source of SNO production in the enteric nervous system. Further, it provides a perspective of how S-nitrosylation signals derived from multiple diverse sources may potentially transduce NO bioactivity in the GI tract. Possible lessons that might be learnt from the CNS, such as SNO mediated auto-inhibition of nNOS activity and modulation of neuronal cell death, are also explored as these may have pathophysiological relevance in enteric neuropathy. Thus, S-nitrosylation may mediate previously underappreciated NO-derived signals in the enteric nervous system that regulate homeostatic gut functions and disease susceptibility.

Activation of GluR6-containing kainate receptors induces ubiquitin-dependent Bcl-2 degradation via denitrosylation in the rat hippocampus after kainate treatment

We previously showed that Bcl-2 (B-cell lymphoma 2) is down-regulated in a kainate (KA)-induced rat epileptic seizure model. The underlying mechanism had remained largely unknown, but we here report for the first time that denitrosylation and ubiquitination are involved. Our results show that the S-nitrosylation levels of Bcl-2 are down-regulated after KA injection and that the GluR6 (glutamate receptor 6) antagonist NS102 can inhibit the denitrosylation of Bcl-2. Moreover, the ubiquitin-dependent degradation of Bcl-2 was found to be promoted after KA treatment, which could be suppressed by the proteasome inhibitor MG132 and the NO donors, sodium nitroprusside and S-nitrosoglutathione. In addition, experiments based on siRNA transfections were performed in the human SH-SY5Y neuroblastoma cell line to verify that the stability of Bcl-2 is causal to neuronal survival. At the same time, it was found that the exogenous NO donor GSNO could protect neurons when Bcl-2 is targeted. Subsequently, these mechanisms were morphologically validated by immunohistochemistry, cresyl violet staining, and in situ TUNEL staining to analyze the expression of Bcl-2 as well as the survival of CA1 and CA3/DG pyramidal neurons. NS102, GSNO, sodium nitroprusside, and MG132 contribute to the survival of CA1 and CA3/DG pyramidal neurons by attenuating Bcl-2 denitrosylation. Taken together, our data reveal that Bcl-2 ubiquitin-dependent degradation is induced by Bcl-2 denitrosylation during neuronal apoptosis after KA treatment.

A mechanism of release of calreticulin from cells during apoptosis

Calreticulin (CRT) is an endoplasmic reticulum (ER) chaperone responsible for glycoprotein folding and Ca(2+) homeostasis. CRT also has extracellular functions, e.g. tumor and apoptotic cell recognition and wound healing, but the mechanism of CRT extracellular release is unknown. Cytosolic localization of CRT is determined by signal peptide and subsequent retrotranslocation of CRT into the cytoplasm. Here, we show that under apoptotic stress conditions, the cytosolic concentration of CRT increases and associates with phosphatidylserine (PS) in a Ca(2)(+)-dependent manner. PS distribution is regulated by aminophospholipid translocase (APLT), which maintains PS on the cytosolic side of the cell membrane. APLT is sensitive to redox modifications of its SH groups by reactive nitrogen species. During apoptosis, both CRT expression and the concentration of nitric oxide (NO) increase. By using S-nitroso-l-cysteine-ethyl-ester, an intracellular NO donor and inhibitor of APLT, we showed that PS and CRT externalization occurred together in an S-nitrosothiol-dependent and caspase-independent manner. Furthermore, the CRT and PS are relocated as punctate clusters on the cell surface. Thus, CRT induced nitrosylation and its externalization with PS could explain how CRT acts as a bridging molecule during apoptotic cell clearance.

S-nitrosylation in cardiovascular signaling

Well over 2 decades have passed since the endothelium-derived relaxation factor was reported to be the gaseous molecule nitric oxide (NO). Although soluble guanylyl cyclase (which generates cyclic guanosine monophosphate, cGMP) was the first identified receptor for NO, it has become increasingly clear that NO exerts a ubiquitous influence in a cGMP-independent manner. In particular, many, if not most, effects of NO are mediated by S-nitrosylation, the covalent modification of a protein cysteine thiol by an NO group to generate an S-nitrosothiol (SNO). Moreover, within the current framework of NO biology, endothelium-derived relaxation factor activity (ie, G protein-coupled receptor-mediated, or shear-induced endothelium-derived NO bioactivity) is understood to involve a central role for SNOs, acting both as second messengers and signal effectors. Furthermore, essential roles for S-nitrosylation have been implicated in virtually all major functions of NO in the cardiovascular system. Here, we review the basic biochemistry of S-nitrosylation (and denitrosylation), discuss the role of S-nitrosylation in the vascular and cardiac functions of NO, and identify current and potential clinical applications.

Release and elementary mechanisms of nitric oxide in hair cells

The enzyme nitric oxide (NO) synthase, that produces the signaling molecule NO, has been identified in several cell types in the inner ear. However, it is unclear whether a measurable quantity of NO is released in the inner ear to confer specific functions. Indeed, the functional significance of NO and the elementary cellular mechanism thereof are most uncertain. Here, we demonstrate that the sensory epithelia of the frog saccule release NO and explore its release mechanisms by using self-referencing NO-selective electrodes. Additionally, we investigated the functional effects of NO on electrical properties of hair cells and determined their underlying cellular mechanism. We show detectable amounts of NO are released by hair cells (>50 nM). Furthermore, a hair-cell efferent modulator acetylcholine produces at least a threefold increase in NO release. NO not only attenuated the baseline membrane oscillations but it also increased the magnitude of current required to generate the characteristic membrane potential oscillations. This resulted in a rightward shift in the frequency-current relationship and altered the excitability of hair cells. Our data suggest that these effects ensue because NO reduces whole cell Ca(2+) current and drastically decreases the open probability of single-channel events of the L-type and non L-type Ca(2+) channels in hair cells, an effect that is mediated through direct nitrosylation of the channel and activation of protein kinase G. Finally, NO increases the magnitude of Ca(2+)-activated K(+) currents via direct NO nitrosylation. We conclude that NO-mediated inhibition serves as a component of efferent nerve modulation of hair cells.

Alteration of the nuclear pore complex in Ca(2+)-mediated cell death

Cell death requires coordinated intracellular signalling before disassembly of cell architecture by degradative enzymes. Although the death signalling cascades that involve the mitochondria, the ER and the plasma membrane have been extensively characterized, only a handful of studies have examined the functional and structural alterations of the nuclear pore complex (NPC) during neuronal death. Here, we show that during excitotoxic neuronal degeneration calpains redistributed across the nuclear envelope and mediated the degradation of NPC components causing altered permeability of the nuclear membrane. In primary dissociated neurons, simultaneous recording of cytosolic [Ca(2+)] and localization of fluorescent proteins showed that the onset of Ca(2+) overload signalled a progressive increase in the diffusion of small reporter molecules across the nuclear envelope. Later, calpain-mediated changes in nuclear pore permeability allowed accumulation of large proteins in the nucleus. Further, in a model of excitotoxic neuronal degeneration in Caenorhabditis elegans, we found similar nuclear changes and redistribution of fluorescent probes across the nuclear membrane in dying neurons. Our findings strongly suggest that increased leakiness of the nuclear barrier affects nucleocytoplasmic transport, alters the localization of proteins across the nuclear envelope and it is likely to be involved in Ca(2+)-dependent cell death, including ischemic neuronal demise.

Protein denitrosylation: enzymatic mechanisms and cellular functions

S-Nitrosylation, the redox-based modification of Cys thiol side chains by nitric oxide, is a common mechanism in signal transduction. Dysregulated S-nitrosylation contributes to a range of human pathologies. New roles for protein denitrosylation in regulating S-nitrosylation are being revealed. Recently, several denitrosylases - the enzymes that mediate Cys denitrosylation - have been discovered, of which two enzyme systems in particular, the S-nitrosoglutathione reductase and thioredoxin systems, have been shown to be physiologically relevant. These highly conserved enzymes regulate signalling through multiple classes of receptors and influence diverse cellular responses. In addition, they protect from nitrosative stress in microorganisms, mammals and plants, thereby exerting profound effects on host-microbe interactions and innate immunity.

GOSPEL: a neuroprotective protein that binds to GAPDH upon S-nitrosylation

We recently reported a cell death cascade whereby cellular stressors activate nitric oxide formation leading to S-nitrosylation of GAPDH that binds to Siah and translocates to the nucleus. The nuclear GAPDH/Siah complex augments p300/CBP-associated acetylation of nuclear proteins, including p53, which mediate cell death. We report a 52 kDa cytosolic protein, GOSPEL, which physiologically binds GAPDH, in competition with Siah, retaining GAPDH in the cytosol and preventing its nuclear translocation. GOSPEL is neuroprotective, as its overexpression prevents NMDA-glutamate excitotoxicity while its depletion enhances death in primary neuron cultures. S-nitrosylation of GOSPEL at cysteine 47 enhances GAPDH-GOSPEL binding and the neuroprotective actions of GOSPEL. In intact mice, virally delivered GOSPEL selectively diminishes NMDA neurotoxicity. Thus, GOSPEL may physiologically regulate the viability of neurons and other cells.

A genetic analysis of nitrosative stress

Nitrosative stress is induced by pathophysiological levels of nitric oxide (NO) and S-nitrosothiols (e.g., S-nitrosoglutathione, GSNO) and arises, at least in significant part, from the nitrosylation of critical protein Cys thiols (S-nitrosylation) and metallocofactors. However, the mechanisms by which NO and GSNO mediate nitrosative stress are not well understood. Using yeast Saccharomyces cerevisiae strains lacking NO- and/or GSNO-consuming enzymes (flavohemoglobin and GSNO reductase, respectively), we measured the individual and combined effects of NO and GSNO on both cell growth and the formation of protein-bound NO species. Our results suggest an intracellular equilibrium between NO and GSNO, dependent in part on cell-catalyzed release of NO from GSNO (i.e., "SNO-lyase" activity). However, whereas NO induces multiple types of protein-based modifications, levels of which correlate with inhibition of cell growth, GSNO mainly affects protein S-nitrosylation, and the relationship between S-nitrosylation and nitrosative stress is more complex. These data support the idea of multiple classes of protein-SNO, likely reflected in divergent routes of synthesis and degradation. Indeed, a significant fraction of protein S-nitrosylation by NO occurs in the absence of O(2), which is commonly assumed to drive this reaction but instead is apparently dependent in substantial part upon protein-bound transition metals. Additionally, our findings suggest that nitrosative stress is mediated principally via the S-nitrosylation of a subset of protein targets, which include protein SNOs that are stable to cellular glutathione (and thus are not metabolized by GSNO reductase). Collectively, these results provide new evidence for the mechanisms through which NO and GSNO mediate nitrosative stress as well as the cellular pathways of protein S-nitrosylation and denitrosylation involving metalloproteins, SNO lyase(s) and GSNO reductase.

Nitrosothiol reactivity profiling identifies S-nitrosylated proteins with unexpected stability

Nitric oxide (NO) regulates protein function by S-nitrosylation of cysteine to form nitrosothiols. Nitrosothiols are highly susceptible to nonenzymatic degradation by cytosolic reducing agents. Here we show that although most protein nitrosothiols are rapidly degraded by cytosolic reductants, a small subset form unusually stable S-nitrosylated proteins. Our findings suggest that stable S-nitrosylation reflects a protein conformation change that shields the nitrosothiol. To identify stable protein nitrosothiols, we developed a proteomic method for profiling S-nitrosylation. We examined the stability of over 100 S-nitrosylated proteins, and identified 10 stable nitrosothiols. These proteins remained S-nitrosylated in cells after NO synthesis was inhibited, unlike most S-nitrosylated proteins. Taken together, our data identify a class of NO targets that form stable nitrosothiols in the cell and are likely to mediate the persistent cellular effects of NO.

A key role for redox signaling in rapid P2X7 receptor-induced IL-1 beta processing in human monocytes

P2X(7) receptors (P2X(7)Rs) are ATP-gated ion channels that trigger caspase-1 activation in the presence of TLR ligands. Inflammatory caspase-1 is responsible for the proteolytic activation of IL-1beta. However, the signaling events that couple P2X(7)Rs to caspase-1 activation remain undefined. In this study we demonstrate that ATP-induced cellular oxidation is critical for caspase-1 activation and subsequent IL-1beta processing. Purinergic receptor stimulation, including P2X(7)Rs, of endotoxin-primed human monocytes augments NADPH oxidase activity whereas concurrent purinergic receptor stimulation triggers protein denitroyslation, leading to the formation of peroxynitrite. IL-1beta cleavage is blocked under conditions where superoxide anion formation is blocked or monocytes are treated with antioxidants or a peroxynitrite scavenger. Nigericin, a K(+)/H(+) antiporter, also increases NADPH oxidase activity, leading to IL-1beta and caspase-1 processing that is blocked by a peroxynitrite scavenger or inhibition of NADPH oxidase. These data demonstrate that signaling via NADPH oxidase activity is fundamental for the processing of mature IL-1beta induced by P2X(7)R stimulation.

Pancreatitis and calcium signalling: report of an international workshop

Pancreatitis and Calcium Signalling was an international research workshop organized by the authors and held at the Liverpool Medical Institution, Liverpool, United Kingdom, from Sunday 12th to Tuesday 14th November 2006. The overall goal of the workshop was to review progress and explore new opportunities for understanding the mechanisms of acute pancreatitis with an emphasis on the role of pathological calcium signaling. The participants included those with significant interest and expertise in pancreatitis research and others who are in fields outside gastroenterology but with significant expertise in areas of cell biology relevant to pancreatitis. The workshop was designed to enhance interchange of ideas and collaborations, to engage and encourage younger researchers in the field, and promote biomedical research through the participating and supporting organizations and societies. The workshop was divided into 8 topic-oriented sessions. The sessions were: (1) Physiology and pathophysiology of calcium signaling; (2) Interacting signaling mechanisms; (3) Premature digestive enzyme activation; (4) Physiology Society Lecture: Aberrant Ca2+ signaling, bicarbonate secretion, and pancreatitis; (5) NFkappaB, cytokines, and immune mechanisms; (6) Mitochondrial injury; (7) Cell death pathways; and (8) Overview of areas for future research. In each session, speakers presented work appropriate to the topic followed by discussion of the material presented by the group. The publication of these proceedings is intended to provide a platform for enhancing research and therapeutic development for acute pancreatitis.

Endothelial cilia are fluid shear sensors that regulate calcium signaling and nitric oxide production through polycystin-1

BACKGROUND

When challenged with extracellular fluid shear stress, vascular endothelial cells are known to release nitric oxide, an important vasodilator. Here, we show that the ability of cultured endothelial cells to sense a low range of fluid shear depends on apical membrane organelles, called cilia, and that cilia are compartments required for proper localization and function of the mechanosensitive polycystin-1 molecule.

METHODS AND RESULTS

Cells with the Pkd1(null/null) or Tg737(orpk/orpk) mutation encoded for polycystin-1 or polaris, respectively, are unable to transmit extracellular shear stress into intracellular calcium signaling and biochemical nitric oxide synthesis. Cytosolic calcium and nitric oxide recordings further show that fluid shear sensing is a cilia-specific mechanism because other mechanical or pharmacological stimulation does not abolish calcium and nitric oxide signaling in polycystin-1 and polaris mutant endothelial cells. Polycystin-1 localized in the basal body of Tg737(orpk/orpk) endothelial cells is insufficient for a fluid shear stress response. Furthermore, the optimal shear stress to which the cells respond best does not alter the apical cilia structure but modifies the responsiveness of cells to higher shear stresses through proteolytic modification of polycystin-1.

CONCLUSIONS

We demonstrate for the first time that polycystin-1 (required for cilia function) and polaris (required for cilia structure) are crucial mechanosensitive molecules in endothelial cells. We propose that a distinctive communication with the extracellular microenvironment depends on the proper localization and function of polycystin-1 in cilia.

Visualizing formation and dynamics of vacuoles in living cells using contrasting dextran-bound indicator: endocytic and nonendocytic vacuoles

Here we describe a technique that allows us to visualize in real time the formation and dynamics (fusion, changes of shape, and translocation) of vacuoles in living cells. The technique involves infusion of a dextran-bound fluorescent probe into the cytosol of the cell via a patch pipette, using the whole-cell patch-clamp configuration. Experiments were conducted on pancreatic acinar cells stimulated with supramaximal concentrations of cholecystokinin (CCK). The vacuoles, forming in the cytoplasm of the cell, were revealed as dark imprints on a bright fluorescence background, produced by the probe and visualized by confocal microscopy. A combination of two dextran-bound probes, one infused into the cytosol and the second added to the extracellular solution, was used to identify endocytic and nonendocytic vacuoles. The cytosolic dextran-bound probe was also used together with a Golgi indicator to illustrate the possibility of combining the probes and identifying the localization of vacuoles with respect to other cellular organelles in pancreatic acinar cells. Combinations of cytosolic dextran-bound probes with endoplasmic reticulum (ER) or mitochondrial probes were also used to simultaneously visualize vacuoles and corresponding organelles. We expect that the new technique will also be applicable and useful for studies of vacuole dynamics in other cell types.

Flavin adenine dinucleotide may release preformed stores of nitrosyl factors from the vascular endothelium of conscious rats

This study determined whether flavin adenine dinucleotide (FAD) may elicit vasodilation in conscious rats via release of preformed endothelium-derived nitrosyl factors. Injections 1-6 (inj(1-6)) of FAD (2.5 micromol/kg, IV) elicited pronounced and equivalent vasodilator responses in saline-treated rats. Inj(1) of FAD elicited pronounced vasodilation in L-NAME-treated rats pretreated with the nitric oxide (NO) synthesis inhibitor, NG-nitro-L-arginine (L-NAME; 50 micromol/kg, IV), whereas Inj(2-6) elicited progressively smaller responses such that inj(6) elicited minor responses. The vasodilator responses elicited by the endothelium-dependent agonist, acetylcholine, were markedly attenuated in L-NAME-treated rats that had received inj(1-6) of FAD but not in saline-treated rats that had received inj(1-6) of FAD. The vasodilator actions of L-S-nitrosocysteine and the NO donor, sodium nitroprusside, were not diminished after the injections of FAD in saline- or in L-NAME-treated rats. Binding studies demonstrated that the densities of muscarinic M3 receptors were increased in thoracic aorta endothelium of rats treated with L-NAME + inj(1-6) of saline or L-NAME + inj(1-6) of FAD as compared to rats treated with saline + inj(1-6) of saline or saline + inj(1-6) of FAD. The progressive loss of response to injections of FAD in L-NAME-treated rats coupled with the loss of response to acetylcholine suggests that FAD elicits the use-dependent depletion of vesicular pools of nitrosyl factors in endothelial cells that cannot be replenished in the absence of NO synthesis.

Caspase-8-mediated apoptosis induced by oxidative stress is independent of the intrinsic pathway and dependent on cathepsins

Cell-death programs executed in the pancreas under pathological conditions remain largely undetermined, although the severity of experimental pancreatitis has been found to depend on the ratio of apoptosis to necrosis. We have defined mechanisms by which apoptosis is induced in pancreatic acinar cells by the oxidant stressor menadione. Real-time monitoring of initiator caspase activity showed that caspase-9 (66% of cells) and caspase-8 (15% of cells) were activated within 30 min of menadione administration, but no activation of caspase-2, -10, or -12 was detected. Interestingly, when caspase-9 activation was inhibited, activation of caspase-8 was increased. Half-maximum activation (t(0.5)) of caspase-9 occurred within approximately 2 min and was identified at or in close proximity to mitochondria, whereas t(0.5) for caspase-8 occurred within approximately 26 min of menadione application and was distributed homogeneously throughout cells. Caspase-9 but not caspase-8 activation was blocked completely by the calcium chelator BAPTA or bongkrekic acid, an inhibitor of the mitochondrial permeability transition pore. In contrast, caspase-8 but not caspase-9 activation was blocked by the destruction of lysosomes (preincubation with Gly-Phe beta-naphthylamide, a cathepsin C substrate), loss of lysosomal acidity (bafilomycin A1), or inhibition of cathepsin L or D. Using pepstatin A-BODIPY FL conjugate, we confirmed translocation of cathepsin D out of lysosomes in response to menadione. We conclude that the oxidative stressor menadione induces two independent apoptotic pathways within pancreatic acinar cells: the classical mitochondrial calcium-dependent pathway that is initiated rapidly in the majority of cells, and a slower, caspase-8-mediated pathway that depends on the lysosomal activities of cathepsins and is used when the caspase-9 pathway is disabled.

Contribution of glutamatergic signaling to nitrosative stress-induced protein misfolding in normal brain aging and neurodegenerative diseases

Glutamatergic hyperactivity, associated with Ca2+ influx and consequent production of nitric oxide (NO), is potentially involved in both normal brain aging and age-related neurodegenerative disorders. Many neurodegenerative diseases are characterized by conformational changes in proteins that result in their misfolding and aggregation. Normal protein degradation by the ubiquitin-proteasome system can prevent accumulation of aberrantly folded proteins. Our recent studies have linked nitrosative stress to protein misfolding and neuronal cell death. In particular, molecular chaperones - such as protein disulfide isomerase, glucose regulated protein 78, and heat shock proteins - can provide neuroprotection from misfolded proteins by facilitating proper folding and thus preventing aggregation. Here, we present evidence for the hypothesis that NO contributes to normal brain aging and degenerative conditions by S-nitrosylating specific chaperones that would otherwise prevent accumulation of misfolded proteins.

Assessment and application of the biotin switch technique for examining protein S-nitrosylation under conditions of pharmacologically induced oxidative stress

Protein S-nitrosylation has emerged as a principal mechanism by which nitric oxide exerts biological effects. Among methods for studying protein S-nitrosylation, the biotin switch technique (BST) has rapidly gained popularity because of the ease with which it can detect individual S-nitrosylated (SNO) proteins in biological samples. The identification of SNO sites by the BST relies on the ability of ascorbate to generate a thiol from an S-nitrosothiol, but not from alternatively S-oxidized thiols (e.g. disulfides, sulfenic acids). However, the specificity of this reaction has recently been challenged, prompting several claims that the BST may produce false-positive results and raising concerns about the application of the BST under oxidizing conditions. Here we perform a comparative analysis of the BST using differentially S-oxidized and S-nitrosylated forms of protein tyrosine phosphatase 1B, as well as intact and lysed human embryonic kidney 293 cells treated with S-oxidizing and S-nitrosylating agents, and verify that the assay is highly specific for SNO. Strikingly, exposure of samples to indirect sunlight from a laboratory window resulted in artifactual ascorbate-dependent signals that are likely promoted by the semidehydroascorbate radical; protection from sunlight eliminated the artifact. In contrast, exposure of SNO proteins to a strong ultraviolet light source (SNO photolysis) prior to the BST provided independent verification of assay specificity. By combining BST with photolysis, we have shown that anti-cancer drug-induced oxidative stress facilitates the S-nitrosylation of the major apoptotic effector glyceraldehyde-3-phosphate dehydrogenase. Collectively, these experiments demonstrate that SNO-dependent signaling pathways can be modulated by oxidative conditions and suggest a potential role for S-nitrosylation in antineoplastic drug action.

Assessment of nitric oxide signals by triiodide chemiluminescence

Nitric oxide (NO) bioactivity is mainly conveyed through reactions with iron and thiols, furnishing iron nitrosyls and S-nitrosothiols with wide-ranging stabilities and reactivities. Triiodide chemiluminescence methodology has been popularized as uniquely capable of quantifying these species together with NO byproducts, such as nitrite and nitrosamines. Studies with triiodide, however, have challenged basic ideas of NO biochemistry. The assay, which involves addition of multiple reagents whose chemistry is not fully understood, thus requires extensive validation: Few protein standards have in fact been characterized; NO mass balance in biological mixtures has not been verified; and recovery of species that span the range of NO-group reactivities has not been assessed. Here we report on the performance of the triiodide assay vs. photolysis chemiluminescence in side-by-side assays of multiple nitrosylated standards of varied reactivities and in assays of endogenous Fe- and S-nitrosylated hemoglobin. Although the photolysis method consistently gives quantitative recoveries, the yields by triiodide are variable and generally low (approaching zero with some standards and endogenous samples). Moreover, in triiodide, added chemical reagents, changes in sample pH, and altered ionic composition result in decreased recoveries and misidentification of NO species. We further show that triiodide, rather than directly and exclusively producing NO, also produces the highly potent nitrosating agent, nitrosyliodide. Overall, we find that the triiodide assay is strongly influenced by sample composition and reactivity and does not reliably identify, quantify, or differentiate NO species in complex biological mixtures.

Nitrosothiol stores in vascular tissue: modulation by ultraviolet light, acetylcholine and ionomycin

Our previous studies demonstrated that light-induced vascular relaxation (photorelaxation) was mediated by a tissue source of nitric oxide that was independent of endothelial nitric oxide synthase (eNOS), but sensitive to inhibitors of soluble guanylate cyclase, extracellular nitric oxide scavengers and possessed the properties of a nitrosothiol. In the present study we describe High Performance Liquid Chromatography and spectrofluorometric techniques that allowed us to measure tissue levels of the nitrosothiol, S-nitrosoglutathione and its modulation in mouse aortic tissues, smooth muscle cells and human umbilical vein endothelial cells (HUVECs) following exposure to exogenous S-nitrosoglutathione, light and chemical stimuli. Basal levels of S-nitrosoglutathione were similar in control mouse aortae and HUVECs and the store size could be enhanced by exposure of tissues/cells to nitric oxide solution. No basal S-nitrosoglutathione was detected in tissue from diabetic db/db mice; however, ultraviolet light was still able to elicit relaxation of aortic tissues. Ultraviolet light induced the release of nitric oxide from the S-nitrosoglutathione store with an associated increase in the concentration of nitrite. The release of nitric oxide from the store in HUVECs was modulated by extracellular oxidative stress induced by xanthine/xanthine oxidase and also, in an atropine-sensitive process, by acetylcholine, as well as by the calcium ionophore, ionomycin. These interventions resulted in a reduced S-nitrosoglutathione store and elevated levels of nitrite. These data suggest that endothelial and vascular smooth muscle cells possess stores of nitric oxide that, in part, exist in the form of S-nitrosoglutathione. Furthermore, these stores, albeit small, may provide an additional mechanism for the regulation of vascular tone, especially under conditions, such as diabetes, in which nitric oxide generation or bioavailability is compromised; however, additional studies are required to determine not only whether there are additional chemical storage forms of nitric oxide, but also the location of such stores.

Inflammatory mediators leading to protein misfolding and uncompetitive/fast off-rate drug therapy for neurodegenerative disorders

Inflammatory mediators, including free radicals such as nitric oxide (NO) and reactive oxygen species (ROS), can contribute to neurodegenerative diseases in part by triggering protein misfolding. In this chapter, we will discuss a newly discovered pathway for this phenomenon and possible novel treatments. Excitotoxicity, defined as overstimulation of glutamate receptors, has been implicated in a final common pathway contributing to neuronal injury and death in a wide range of acute and chronic neurological disorders, ranging from Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), multiple sclerosis, and Alzheimer's disease (AD) to stroke and trauma. Excitotoxic cell death is due, at least in part, to excessive activation of N-methyl-d-aspartate (NMDA)-type glutamate receptors, leading to excessive Ca(2+) influx through the receptor's associated ion channel and subsequent free radical production, including NO and ROS. These free radicals can trigger a variety of injurious pathways, but newly discovered evidence suggests that some proteins are S-nitrosylated (transfer of NO to a critical thiol group), and this reaction can mimic the effect of rare genetic mutations. This posttranslational modification can contribute to protein misfolding, triggering neurodegenerative diseases. One such molecule affected is protein disulfide isomerase (PDI), an enzyme responsible for normal protein folding in the endoplasmic reticulum (ER). We found that when PDI is S-nitrosylation (forming SNO-PDI), the function of the enzyme is compromised, leading to misfolded proteins and contributing to neuronal cell injury and loss. Moreover, SNO-PDI occurs at pathological levels in several human diseases, including AD and PD. This discovery thus links protein misfolding to excitotoxicity and free radical formation in a number of neurodegenerative disorders. Another molecule whose S-nitrosylation can lead to abnormal protein accumulation is the E3 ubiquitin ligase, parkin, which contributes to the pathogenesis of PD. One way to ameliorate excessive NO production and hence abnormal S-nitrosylations would be to inhibit NMDA receptors. In fact, blockade of excessive NMDA receptor activity can in large measure protect neurons from this type of injury and death. However, inhibition of the NMDA receptor by high-affinity antagonists also blocks the receptor's normal function in synaptic transmission and leads to unacceptable side effects. For this reason, many NMDA receptor antagonists have disappointingly failed in advanced clinical trials. Our group was the first to demonstrate that gentle blockade of NMDA receptors by memantine, via a mechanism of uncompetitive open-channel block with a rapid "off-rate," can prevent this type of damage in a clinically efficacious manner without substantial side effects. For these Uncompetitive/Fast Off-rate therapeutics, we use the term "UFO drugs" because like Unidentified Flying Objects, they leave very quickly as soon as their job is finished. As a result, memantine blocks excessive NMDA receptor activity without disrupting normal activity. Memantine does this by preferentially entering the receptor-associated ion channel when it is excessively open, and, most importantly, when its off-rate from the channel is relatively fast so that it does not accumulate to interfere with normal synaptic transmission. Hence, memantine is clinically well tolerated, has been used in Europe for PD for many years, and recently passed multiple phase III trials for dementia, leading to its approval by the FDA and European Union for moderate-to-severe AD. Clinical studies of memantine for additional neurological disorders, including other dementias, neuropathic pain, and glaucoma, are underway. We have also developed a series of second-generation drugs that display greater neuroprotective properties than memantine. These second-generation drugs take advantage of the fact that the NMDA receptor has other modulatory sites, including critical thiol groups that are S-nitrosylated. In this case, in contrast to PDI or parkin, S-nitrosylation proves to be neuroprotective by decreasing excessive NMDA receptor activity. Targeted S-nitrosylation of the NMDA receptor can be achieved by coupling NO to memantine, yielding second-generation "UFO drugs" known as NitroMemantines.

Failure of calcium microdomain generation and pathological consequences

Normal physiological regulation depends on Ca(2+) microdomains, because there is a need to spatially separate Ca(2+) regulation of different cellular processes. It is only possible to generate local Ca(2+) signals transiently; so, there is an important functional link between Ca(2+) spiking and microdomains. The pancreatic acinar cell provides a useful cell biological model, because of its clear structural and functional polarization. Although local Ca(2+) spiking in the apical (granular) microdomain regulates fluid and enzyme secretion, prolonged global elevations of the cytosolic Ca(2+) concentration are associated with the human disease acute pancreatitis, in which proteases in the granular region become inappropriately activated and digest the pancreas and its surroundings. A major cause of pancreatitis is alcohol abuse and it has now been established that fatty acid ethyl esters and fatty acids, non-oxidative alcohol metabolites, are principally responsible for causing the acinar cell damage. The fatty acid ethyl esters release Ca(2+) from the endoplasmic reticulum and the fatty acids inhibit markedly mitochondrial ATP generation, which prevents the acinar cell from disposing of the excess Ca(2+) in the cytosol. Because of the abolition of ATP-dependent Ca(2+) pump activity, all intracellular Ca(2+) concentration gradients disappear and the most important part of the normal regulatory machinery is thereby destroyed. The end stage is necrosis.

What is the significance of vascular hydrogen sulphide (H2S)?

The important role of nitric oxide (NO) in the regulation of vascular tone has been well studied. By contrast, the vascular significance of another gaseous mediator, hydrogen sulphide (H2S), is still poorly understood. A study published in this issue of the British Journal of Pharmacology now provides evidence that in addition to the vasorelaxant effects of H2S reported in vitro, low concentrations of H2S also cause arterial vasoconstriction, reverse NO-mediated vasorelaxation and cause an NO-dependent pressor effect in vivo. This commentary discusses the implications and questions raised by these results.