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Disulfide Relays and Phosphorylative Cascades Cell Death and Differentiation (2005) 12, 1555–1563 & 2005 Nature Publishing Group All rights reserved 1350-9047/05 $30.00 www.nature.com/cdd Review Disulfide relays and phosphorylative cascades: partners in redox-mediated signaling pathways G Filomeni*,1, G Rotilio1 and MR Ciriolo1 cyclin dependent kinase; JNK, c-Jun-N-terminal kinase; Bcl-2, B-cell lymphoma-2; MDM2, murine double minute 2; ASK1, 1 Department of Biology, University of Rome ‘Tor Vergata’, Via della Ricerca apoptosis signal-regulating kinase 1; MKK, MAP kinase kinase; Scientifica, 00133 Rome, Italy GST, glutathione-S-transferase; PTPase, protein tyrosine phos- * Corresponding author: G Filomeni, Department of Biology, University of Rome phatases ‘Tor Vergata’, Via della Ricerca Scientifica, 1, 00133 Rome, Italy. Tel: þ 39-06-7259-4360; Fax: þ 39-06-7259-4311; E-mail: fi[email protected] Redox signaling Received 26.5.05; accepted 13.7.05; published online 09.9.05 The discovery in the late 1960s of the catalytic function of Edited by B Zhivotovsky erythrocuprein as the dismutase of superoxide anion1 revolutionized the study of the biological role of O2 giving rise Abstract to the exponentially growing field of research on oxidative stress. In this perspective, oxygen is not only the ultimate Modifications of specific amino-acid residues of proteins electron sink of redox processes downstream consumption of are fundamental in order to modulate different signaling carbon molecules and the source of energy coming from processes among which the cascade of phosphorylation electron transport chain within the mitochondria, but, owing to KÀ K represents the most effective example. Recently, also, the its intermediate redox species (the free radicals O2 ,OH modification of the redox state of cysteine residues of certain plus H2O2 referred as ROS, reactive oxygen species), it proteins, which is a widespread mechanism in the regulation represents a potential source of cellular damage.2 ROS are of protein function, has been proposed to be involved in produced, even under physiological conditions, during the signaling pathways. Growing evidence shows that some reduction of oxygen by the catalysis of enzymes (oxidases transcription factors could be modulated by both oxidation and oxygenases) as well as at the mitochondrial level by the Complex I and III of the electron transport chain.2 Their and phosphorylation. In particular, the pathways regulated unconstrained effects give rise to oxidation and irreversible by the mitogen activated protein (MAP) kinases represent damage of unsaturated fatty acid of membranes, amino-acid well-established examples of the cross talk between redox- residues of proteins, and nucleotide bases.2 mediated signaling and phosphorylative cascades. This The so-called antioxidant enzymes that remove the inter- review will compare the two modes of signal transduction mediates of oxygen reduction, in particular, superoxide and propose an evolutionary model of a partnership of the two dismutase (SOD), catalase and glutathione peroxidase act mechanisms in the eukaryotic cell, with redox-mediated in damage control by preventing initiation and/or propagation signals being more specific and ancestral and phosphory- of oxidizing radical chain reactions together with noncatalytic lative signals being more diffuse but predominant in signal redox-active molecules. The latter ones can be divided into propagation. two different classes: (i) non-thiol compounds, essentially Cell Death and Differentiation (2005) 12, 1555–1563. vitamins (e.g. ascorbate, tocopherols, b-carotene) and poly- phenols, which are able to block the radical chain reaction by doi:10.1038/sj.cdd.4401754; published online 9 September 2005 delocalizing the positive charge formed upon the interception of radicals; (ii) sulfur-containing compounds, such as gluta- Keywords: glutathione; MAP kinases; signal transduction; thione (GSH) and thioredoxin (Trx), which act as donors of phosphorylation; disulfides; redox reducing equivalents either directly or in concert with specific enzymes. Abbreviations: ROS, reactive oxygen species; SOD, super- It should be kept in mind that redox reactions occur not only oxide dismutase; GSH, reduced glutathione; GSSG, glutathione during antioxidant responses to oxidative stimuli, but as the disulfide; GS-R, glutathione mixed disulfide with protein thiols; continuous fine adjustment of a network of redox couples Trx, thioredoxin; NO, nitric oxide; GRX, glutaredoxin; RNS, linked to each other in response of electron movements within reactive nitrogen species; Yap1, yeast activating protein 1; NES, the cell, than the ultimate redox potential of the cell results nuclear export signal; AP-1, activating protein 1; NF-kB, nuclear from the contribution of the redox state of each couple. factor-kB; HSF1, heat shock factor 1; Ref-1, Redox factor 1; Glutathione (GSSG/GSH), nicotinamide adenine dinucleotide STPK, serine/threonine protein kinases; MAP kinase, mitogen- phosphate (NADP þ /NADPH), and thioredoxin [TrxSS/ activated protein kinase; TGF-b, tumor growth factor-b; PTK, Trx(SH)2] couples are among the most important in maintain- protein tyrosine kinase; EGF-R, epidermal growth factor receptor; ing an intracellular reducing environment.3 Actually, since the VEGF-R, vascular endothelial growth factor receptor; IGF-R, GSH concentration is 100- to 10 000-fold higher than the insulin-like growth factor receptor; PKC, protein kinase C; CDK, reduced form of the other couples, the glutathione redox Thiol/disulfide exchange and phosphorylation G Filomeni et al 1556 state usually determines the steady-state value of the type of oxidative alteration is due to physiological mild intracellular redox potential.4 NADPH and Trx redox couples oxidative unbalances, which may be responsible for regula- play a concerted role to kinetically support a constant value of tory processes. In fact, stimuli of different origin, such the GSH/GSSG ratio (Figure 1). The enzyme glutathione as ligand/receptor binding,6 cytokines,7 UV8 and osmotic reductase (GSSG-Red) catalyzes the reduction of GSSG by shock,9 could result in localized, site-specific generation of using the NADPH-supplied reducing equivalents; on the other ROS that escape the antioxidant defense by transferring hand, Trx participates as electron donor in the reduction of their oxidizing equivalent to reactive cysteine residues of mixed disulfide that may have formed between GSH and proteins, which are present as the thiolate form under protein thiols (GS-R).4 physiological pH. Depending on the surrounding environment We can distinguish two different modalities of oxidative sulfenic (R-SOH), or sulfinic (R-SO2H), or sulfonic (R-SO3H) alterations in the cell (Table 1). The first one derives from acid derivatives are formed.10,11 The last two oxidative abrupt ROS overproduction: its prototype is the oxidative modifications irreversibly modify the structure and function burst in response to bacterial infection during which phago- of the protein involved, while sulfenic acid can be either KÀ cytes produce O2 at high rates through the activation reduced back to the thiolate form or transformed in a number of a plasma membrane-associated NADPH oxidase.2 Another of thiol adducts by reactions such as S-nitrosylation (addition condition where ROS could be produced at a high level is in to NO) or S-thiolation (disulfide bridge formation with the case of xenobiotics- or drugs-mediated redox reactions.5 other protein thiols or with GSH). They represent stable These compounds include quinones, bipyridils, antracyclins and reversible derivatives of reactive thiol groups, which can and nitro-imidazol molecules that, due to their chemical be efficiently reduced back through the reaction catalyzed structure, easily undergo one-electron redox cycling with by Trx and peroxiredoxins, such as glutaredoxin (GRX)4 KÀ oxygen, giving rise to O2 production. Usually, this explosive (Figure 2). type of redox changes leads to irreversible cell damage Therefore, different from the pathological effects of heavy and ultimately may give rise to necrotic cell death. The other and severe oxidative stress, mild oxidative unbalance Figure 1 Glutathione redox cycle. In the schematic diagram are represented the reduced (GSH) and oxidized forms (GSSG and GS-R) of glutathione. The reduction back of GSSG and GS-R by the action of GSSG reductase (GSSG-Red) and thioredoxin (Trx) are also shown together with the contribution of NADPH as upstream supplier of reducing equivalents. (GSH-Px ¼ Glutathione peroxidase) Cell Death and Differentiation Thiol/disulfide exchange and phosphorylation G Filomeni et al 1557 Table 1 Different modalities of oxidative stress [ROS] oxidative stress Effect Target Antioxidants implicated Response in cell response GENERAL HEAVY (pathological) Irreversible oxidative Cellular macromolecules Enzymatic defense Nonprogrammed events: damage (Proteins, DNA, lipids) (SOD, GPx, CAT) GSH (cosubstrate and 1. Rescue/survival scavenger) 2. NECROSIS SPECIFIC MILD (physiological) Reversible oxidations Transcription (NF-kB, Programmed events: AP-1, p53) Regulation of gene Regulatory proteins GSH, TRX (mediators of 1. Proliferation transcription (TRX, GST) cell signaling) 2. APOPTOSIS Figure 2 Redox modifications of reactive cysteine. Under physiological pH, a reactive cysteine can exist as thiolate anion, which more easily undergoes reversible oxidation either by the reaction with ROS (S-hydroxylation) or by the reaction of the NO radical (S-nitrosylation), or by the reaction with GSSG
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