S-Glutathiolation in redox-sensitive signaling

S-Glutathionylation and Redox Protein Signaling in Drug Addiction

Drug addiction is a chronic relapsing disorder that comes at a high cost to individuals and society. Therefore understanding the mechanisms by which drugs exert their effects is of prime importance. Drugs of abuse increase the production of reactive oxygen and nitrogen species resulting in oxidative stress. This change in redox homeostasis increases the conjugation of glutathione to protein cysteine residues; a process called S-glutathionylation. Although traditionally regarded as a protective mechanism against irreversible protein oxidation, accumulated evidence suggests a more nuanced role for S-glutathionylation, namely as a mediator in redox-sensitive protein signaling. The reversible modification of protein thiols leading to alteration in function under different physiologic/pathologic conditions provides a mechanism whereby change in redox status can be translated into a functional response. As such, S-glutathionylation represents an understudied means of post-translational protein modification that may be important in the mechanisms underlying drug addiction. This review will discuss the evidence for S-glutathionylation as a redox-sensing mechanism and how this may be involved in the response to drug-induced oxidative stress. The function of S-glutathionylated proteins involved in neurotransmission, dendritic spine structure, and drug-induced behavioral outputs will be reviewed with specific reference to alcohol, cocaine, and heroin.

Redox regulation of SERCA2 is required for vascular endothelial growth factor-induced signaling and endothelial cell migration

AIMS

Vascular endothelial growth factor (VEGF) increases angiogenesis by stimulating endothelial cell (EC) migration. VEGF-induced nitric oxide ((•)NO) release from (•)NO synthase plays a critical role, but the proteins and signaling pathways that may be redox-regulated are poorly understood. The aim of this work was to define the role of (•)NO-mediated redox regulation of the sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA) in VEGF-induced signaling and EC migration.

RESULTS

VEGF-induced EC migration was prevented by the (•)NO synthase inhibitor, N (G)-nitro-L-arginine methyl ester (LNAME). Either VEGF or (•)NO stimulated endoplasmic reticulum (ER) (45)Ca(2+) uptake, a measure of SERCA activity, and knockdown of SERCA2 prevented VEGF-induced EC migration and (45)Ca(2+) uptake. S-glutathione adducts on SERCA2b, identified immunochemically, were increased by VEGF, and were prevented by LNAME or overexpression of glutaredoxin-1 (Glrx-1). Furthermore, VEGF failed to stimulate migration of ECs overexpressing Glrx-1. VEGF or (•)NO increased SERCA S-glutathiolation and stimulated migration of ECs in which wild-type (WT) SERCA2b was overexpressed with an adenovirus, but did neither in those overexpressing a C674S SERCA2b mutant, in which the reactive cysteine-674 was mutated to a serine. Increased EC Ca(2+) influx caused by VEGF or (•)NO was abrogated by overexpression of Glrx-1 or the C674S SERCA2b mutant. ER store-emptying through the ryanodine receptor (RyR) and Ca(2+) entry through Orai1 were also required for VEGF- and (•)NO-induced EC Ca(2+) influx.

INNOVATION AND CONCLUSIONS

These results demonstrate that (•)NO-mediated activation of SERCA2b via S-glutathiolation of cysteine-674 is required for VEGF-induced EC Ca(2+) influx and migration, and establish redox regulation of SERCA2b as a key component in angiogenic signaling.

Superoxide induces endothelial nitric-oxide synthase protein thiyl radical formation, a novel mechanism regulating eNOS function and coupling

An increase in production of reactive oxygen species resulting in a decrease in nitric oxide bioavailability in the endothelium contributes to many cardiovascular diseases, and these reactive oxygen species can oxidize cellular macromolecules. Protein thiols are critical reducing equivalents that maintain cellular redox state and are primary targets for oxidative modification. We demonstrate endothelial NOS (eNOS) oxidant-induced protein thiyl radical formation from tetrahydrobiopterin-free enzyme or following exposure to exogenous superoxide using immunoblotting, immunostaining, and mass spectrometry. Spin trapping with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) followed by immunoblotting using an anti-DMPO antibody demonstrated the formation of eNOS protein radicals, which were abolished by superoxide dismutase and L-NAME, indicating that protein radical formation was due to superoxide generation from the eNOS heme. With tetrahydrobiopterin-reconstituted eNOS, eNOS protein radical formation was completely inhibited. Using mass spectrometric and mutagenesis analysis, we identified Cys-908 as the residue involved in protein radical formation. Mutagenesis of this key cysteine to alanine abolished eNOS thiyl radical formation and uncoupled eNOS, leading to increased superoxide generation. Protein thiyl radical formation leads to oxidation or modification of cysteine with either disulfide bond formation or S-glutathionylation, which induces eNOS uncoupling. Furthermore, in endothelial cells treated with menadione to trigger cellular superoxide generation, eNOS protein radical formation, as visualized with confocal microscopy, was increased, and these results were confirmed by immunoprecipitation with anti-eNOS antibody, followed by immunoblotting with an anti-DMPO antibody. Thus, eNOS protein radical formation provides the basis for a mechanism of superoxide-directed regulation of eNOS, involving thiol oxidation, defining a unique pathway for the redox regulation of cardiovascular function.

Differential effects of peroxynitrite on contractile protein properties in fast- and slow-twitch skeletal muscle fibers of rat

Oxidative modification of contractile proteins is thought to be a key factor in muscle weakness observed in many pathophysiological conditions. In particular, peroxynitrite (ONOO−), a potent short-lived oxidant, is a likely candidate responsible for this contractile dysfunction. In this study ONOO− or 3-morpholinosydnonimine (Sin-1, a ONOO− donor) was applied to rat skinned muscle fibers to characterize the effects on contractile properties. Both ONOO− and Sin-1 exposure markedly reduced maximum force in slow-twitch fibers but had much less effect in fast-twitch fibers. The rate of isometric force development was also reduced without change in the number of active cross bridges. Sin-1 exposure caused a disproportionately large decrease in Ca2+ sensitivity, evidently due to coproduction of superoxide, as it was prevented by Tempol, a superoxide dismutase mimetic. The decline in maximum force with Sin-1 and ONOO− treatments could be partially reversed by DTT, provided it was applied before the fiber was activated. Reversal by DTT indicates that the decrease in maximum force was due at least in part to oxidation of cysteine residues. Ascorbate caused similar reversal, further suggesting that the cysteine residues had undergone S-nitrosylation. The reduction in Ca2+ sensitivity, however, was not reversed by either DTT or ascorbate. Western blot analysis showed cross-linking of myosin heavy chain (MHC) I, appearing as larger protein complexes after ONOO− exposure. The findings suggest that ONOO− initially decreases maximum force primarily by oxidation of cysteine residues on the myosin heads, and that the accompanying decrease in Ca2+ sensitivity is likely due to other, less reversible actions of hydroxyl or related radicals.

Vascular oxidative stress: the common link in hypertensive and diabetic vascular disease

Vascular disease in hypertension and diabetes is associated with increased oxidants. The oxidants arise from NADPH oxidase, xanthine oxidase, and mitochondria. Superoxide anion and hydrogen peroxide are produced by both leukocytes and vascular cells. Nitric oxide is produced in excess by inducible nitric oxide synthase, and the potent oxidant, peroxynitrite, is formed from superoxide and nitric oxide. The damage to proteins caused by oxidants is selective, affecting specific oxidant-sensitive amino acid residues. With some important vascular proteins, for example, endothelial nitric oxide synthase, prostacycline synthase, and superoxide dismutase, oxidation of a single susceptible amino acid inactivates the enzyme. The beneficial effects of antioxidants, at least in animal models of hypertension and diabetes, can in part be ascribed to protection of these and other proteins. Mutant proteins lacking their reactive constituent can recapitulate some disease phenotypes suggesting a pathogenic role of the oxidation. Thus, many of the shared functional abnormalities of hypertensive and diabetic blood vessels may be caused by oxidants. Although studies using antioxidants have failed in patients, the successful treatment of vascular disease with HMG-CoA reductase inhibitors, thromboxane A2 antagonists, and polyphenols may depend on their anti-inflammatory effects and ability to decrease production of damaging oxidants.

Modulation of vascular sarco/endoplasmic reticulum calcium ATPase in cardiovascular pathophysiology

Endothelial dysfunction associated with decreased nitric oxide (NO) bioactivity is a major feature of vascular diseases such as atherosclerosis or diabetes. Sodium nitroprusside (SNP)-induced relaxation is entirely dependent on cyclic guanosine monophosphate (cGMP) and preserved in atherosclerosis, suggesting that smooth muscle response to NO donor is intact. However, NO gas activates both cGMP-dependent and -independent signal pathways in vascular smooth muscle cells, and oxidative stress associated with vascular diseases selectively impairs cGMP-independent relaxation to NO. Sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA), which regulates intracellular Ca(2+) levels by pumping Ca(2+) into store, is a major cGMP-independent target for NO. Physiological levels of reactive nitrogen species (RNS) S-glutathiolate SERCA at Cys674 to increase its activity, and the augmentation of RNS in vascular diseases irreversibly oxidizes Cys674 or nitrates tyrosine residues at Tyr296-Tyr297, which are associated with loss of function. S-glutathiolation of various proteins by NO can explain redox-sensitive cGMP-independent actions, and oxidative inactivation of target proteins for NO can be associated with the pathogenesis of cardiovascular diseases. Oxidative inactivation of SERCA is also implicated with dysregulation of smooth muscle migration, promotion of platelet aggregation, and impairment of cardiac function, which can be implicated with restenosis, pathological angiogenesis, thrombosis, as well as heart failure. Analysis of posttranslational oxidative modifications of SERCA and the preservation of SERCA function can be novel strategies against cardiovascular diseases associated with oxidative stress.

Glutathione redox status in mitochondria and cytoplasm differentially and sequentially activates apoptosis cascade in dopamine-melanin-treated SH-SY5Y cells

Neuromelanin (NM)-containing dopaminergic neurons in the substantia nigra are selectively vulnerable in Parkinson's disease (PD), suggesting the involvement of NM in the pathogenesis. NM is composed of protein, lipid, trace metals and melanin component, a mixture of eumelanin produced from dopamine (DA)-quinone and pheomelanin containing 5-S-cyteinyl-DA-quinone. We reported that NM induces mitochondria-mediated apoptosis in human dopaminergic SH-SY5Y cells, which was suppressed completely by Protease K-treatment, suggesting the essential requirement for the protein component. In this paper, the role of the melanin component in NM-dependent apoptosis was studied using SH-SY5Y cells and synthesized DA-melanin (DAM) and L-cysteinyl-DAM (Cys-DAM). DAM oxidatively decreased glutathione (GSH) and sulfhydryl (SH) content in mitochondria, whereas NM increased GSH by de-S-glutathionylation of complex I. DAM induced mitochondrial permeability transition (mPT), leading to membrane potential collapse and cytochrome c release, whereas Cys-DAM did not. However, the cytotoxicity of DAM itself was rather mild and thiol-targeting reducing reagents, including GSH, dithiothreitol and N-acetyl-cysteine, increased apoptosis significantly. The reducing SH reagents activated caspase 3 and induced apoptosis, but did not affect mPT. On the other hand, NM itself activated mitochondria-initiated apoptotic cascade, which GSH suppressed completely. The results indicate that DAM induces apoptosis through the sequential activation by oxidation of SH status in mitochondria and reduction in cytoplasm, in contrast to the case with NM. The regulation of apoptotic processing by SH redox state is discussed in relation to degeneration of nigra-striatal DA neurons in aging and PD, where oxidative stress is increased with impaired antioxidant capacity.

Role of intracellular calcium and S-glutathionylation in cell death induced by a mixture of isothiazolinones in HL60 cells

Previously we reported that brief exposure of HL60 cells to a mixture of 5-chloro-2-methyl-4-isothiazolin-3-one (CMI) and 2-methyl-4-isothiazolin-3-one (MI) shifts the cells into a state of oxidative stress that induces apoptosis and necrosis. In this study, flow cytometric analysis showed that CMI/MI induces early perturbation of calcium homeostasis, increasing cytosolic and mitochondrial calcium and depleting the intracellular endoplasmic reticulum (ER) stores. The calcium chelator BAPTA-AM reduced necrosis and secondary necrosis, the loss of DeltaPsim and S-glutathionylation induced by necrotic doses of CMI/MI, but did not protect against CMI/MI-induced apoptosis, mitochondrial calcium uptake and mitochondrial hyperpolarization. This indicates that increased cytoplasmic calcium does not have a causal role in the induction of apoptosis, while cross-talk between the ER and mitochondria could be responsible for the induction of apoptosis. GSH-OEt pretreatment, which enhances cellular GSH content, reduced S-glutathionylation and cytosolic and mitochondrial calcium levels, thus protecting against both apoptosis and necrosis shifting to apoptosis. Therefore, the degree of GSH depletion, paralleled by the levels of protein S-glutathionylation, may have a causal role in increasing calcium levels. The mitochondrial calcium increase could be responsible for apoptosis, while necrosis is associated with cytoplasmic calcium overload. These findings suggest that S-glutathionylation of specific proteins acts as a molecular linker between calcium and redox signalling.

Redox features of the cell: a gender perspective

Reactive oxygen and nitrogen species have been implicated in diverse subcellular activities, including cell proliferation,differentiation and, in some instances, cell injury and death. The implications of reactive species inhuman pathology have also been studied in detail. However, although the role of free radicals in the pathogenesis of human diseases has been extensively analyzed in different systems (i.e., in vitro, ex vivo, and in vivo),it is still far from elucidated. In particular, the possible role of gender 4 differences in human pathophysiology associated with reactive species is a promising new field of investigation. Although the complex scenario this presents is still incomplete, important gender-associated "redox features" of cells have already been described in the literature. Here we summarize the different aspects of redox-associated molecules and enzymes in regard to gender differences in terms of the intracellular production and biochemical activity of reactive species. These are often associated with the pathogenetic mechanisms underlying several human morbidities(e.g., degenerative diseases) and can represent a specific target for new pharmacologic strategies. Gender differences may thus pose an important challenge for future studies aimed at the clinical management of diseases characterized by a redox imbalance.

Thiol oxidation in signaling and response to stress: detection and quantification of physiological and pathophysiological thiol modifications

Cysteine thiol modifications are increasingly recognized to occur under both physiological and pathophysiological conditions, making their accurate detection, identification, and quantification of growing importance. Among free cysteines, the bulk of modifications occurs on a subset of cysteines that are more reactive. These exist as thiolate anions at physiological pH because of their surrounding electrostatic environment. Reagents with iodoacetamide-active groups can be used to selectively label these reactive thiols with a high degree of selectivity. Thiol adducts can be detected by the failure to label with iodoacetamide or other reagents; restoration of labeling by specific reducing agents (e.g., ascorbate or glutaredoxin) can be used to detect reversible S-nitroso and S-glutathione adducts. These adducts also may be detected with radiolabels and antibodies. S-Glutathiolation in response to physiological stimuli may be detected in cells and tissues with glutathione ester labeled with biotin. Mass spectrometry can identify thiol modifications with precision, and with isotope-coded affinity tags, used to quantify modification of specific thiols. Combinations of these methods increase sensitivity and specificity, and enable quantification and precise identification of thiol modifications that occur under physiological and pathological conditions.