Oxidation sensitivity of the catalytic cysteine of the protein-tyrosine phosphatases SHP-1 and SHP-2

Nutraceutical Aid for Allergies - Strategies for Down-Regulating Mast Cell Degranulation

Interactions of antigens with the mast cell FcεRI-IgE receptor complex induce degranulation and boost synthesis of pro-inflammatory lipid mediators and cytokines. Activation of spleen tyrosine kinase (Syk) functions as a central hub in this signaling. The tyrosine phosphatase SHP-1 opposes Syk activity; stimulation of NADPH oxidase by FcεRI activation results in the production of oxidants that reversibly inhibit SHP-1, up-regulating the signal from Syk. Activated AMPK can suppress Syk activation by the FcεRI receptor, possibly reflecting its ability to phosphorylate the FcεRI beta subunit. Cyclic GMP, via protein kinase G II, enhances the activity of SHP-1 by phosphorylating its C-terminal region; this may explain its inhibitory impact on mast cell activation. Hydrogen sulfide (H₂S) likewise opposes mast cell activation; H₂S can boost AMPK activity, up-regulate cGMP production, and trigger Nrf2-mediated induction of Phase 2 enzymes - including heme oxygenase-1, whose generation of bilirubin suppresses NADPH oxidase activity. Phycocyanobilin (PCB), a chemical relative of bilirubin, shares its inhibitory impact on NADPH oxidase, rationalizing reported anti-allergic effects of PCB-rich spirulina ingestion. Phase 2 inducer nutraceuticals can likewise oppose the up-regulatory impact of NADPH oxidase on FcεRI signaling. AMPK can be activated with the nutraceutical berberine. High-dose biotin can boost cGMP levels in mast cells via direct stimulation of soluble guanylate cyclase. Endogenous generation of H₂S in mast cells can be promoted by administering N-acetylcysteine and likely by taurine, which increases the expression of H₂S-producing enzymes in the vascular system. Mast cell stabilization by benifuuki green tea catechins may reflect the decreased surface expression of FcεRI.

Diabetes Impaired Ischemia-Induced PDGF (Platelet-Derived Growth Factor) Signaling Actions and Vessel Formation Through the Activation of Scr Homology 2-Containing Phosphatase-1

Objective

Critical limb ischemia is a major complication of diabetes characterized by insufficient collateral vessel development and proper growth factor signaling unresponsiveness. Although mainly deactivated by hypoxia, phosphatases are important players in the deregulation of proangiogenetic pathways. Previously, SHP-1 (Scr homology 2-containing phosphatase-1) was found to be associated with the downregulation of growth factor actions in the diabetic muscle. Thus, we aimed to gain further understanding of the impact of SHP-1 on smooth muscle cell (SMC) function under hypoxic and diabetic conditions.

Approach and Results

Despite being inactivated under hypoxic conditions, high glucose level exposure sustained SHP-1 phosphatase activity in SMC and increased its interaction with PDGFR (platelet-derived growth factor receptor)-β, thus reducing PDGF proangiogenic actions. Overexpression of an inactive form of SHP-1 fully restored PDGF-induced proliferation, migration, and signaling pathways in SMC exposed to high glucose and hypoxia. Nondiabetic and diabetic mice with deletion of SHP-1 specifically in SMC were generated. Ligation of the femoral artery was performed, and blood flow was measured for 4 weeks. Blood flow reperfusion, vascular density and maturation, and limb survival were all improved while vascular apoptosis was attenuated in diabetic SMC-specific SHP-1 null mice as compared to diabetic mice.

Conclusions

Diabetes and high glucose level exposure maintained SHP-1 activity preventing hypoxia-induced PDGF actions in SMC. Specific deletion of SHP-1 in SMC partially restored blood flow reperfusion in the diabetic ischemic limb. Therefore, local modulation of SHP-1 activity in SMC could represent a potential therapeutic avenue to improve the proangiogenic properties of SMC under ischemia and diabetes.

NRF2-dependent stress defense in tumor antioxidant control and immune evasion

The transcription factor NRF2 is known as the master regulator of the oxidative stress response. Tumor entities presenting oncogenic activation of NRF2, such as lung adenocarcinoma, are associated with drug resistance, and accumulating evidence demonstrates its involvement in immune evasion. In other cancer types, the KEAP1/NRF2 pathway is not commonly mutated, but NRF2 is activated by other means such as radiation, oncogenic activity, cytokines, or other pro-oxidant triggers characteristic of the tumor niche. The obvious effect of stress-activated NRF2 is the protection from oxidative or electrophilic damage and the adaptation of the tumor metabolism to changing conditions. However, data from melanoma also reveal a role of NRF2 in modulating differentiation and suppressing anti-tumor immunity. This review summarizes the function of NRF2 in this tumor entity and discusses the implications for current tumor therapies.

Hydrogen peroxide as a signal for skeletal muscle adaptations to exercise: What do concentrations tell us about potential mechanisms?

Hydrogen peroxide appears to be the key reactive oxygen species involved in redox signalling, but comparisons of the low concentrations of hydrogen peroxide that are calculated to exist within cells with those previously shown to activate common signalling events in vitro indicate that direct oxidation of key thiol groups on "redox-sensitive" signalling proteins is unlikely to occur. A number of potential mechanisms have been proposed to explain how cells overcome this block to hydrogen peroxide-stimulated redox signalling and these will be discussed in the context of the redox-stimulation of specific adaptations of skeletal muscle to contractile activity and exercise. It is argued that current data implicate a role for currently unidentified effector molecules (likely to be highly reactive peroxidases) in propagation of the redox signal from sites of hydrogen peroxide generation to common adaptive signalling pathways.

Dioscin Promotes Prostate Cancer Cell Apoptosis and Inhibits Cell Invasion by Increasing SHP1 Phosphorylation and Suppressing the Subsequent MAPK Signaling Pathway

Dioscin possesses antioxidant effects and has anticancer ability in many solid tumors including prostate cancer (PCa). Nevertheless, its effect and mechanism of anti-PCa action remain unclear. The tyrosine protein phosphatase SHP1, which contains an oxidation-sensitive domain, has been confirmed as a target for multicancer treatment. Further studies are needed to determine whether dioscin inhibits PCa through SHP1. We performed in vitro studies using androgen-sensitive (LNCaP) and androgen-independent (LNCaP -C81) cells to investigate the anticancer effects and possible mechanisms of dioscin after administering interleukin-6 (IL-6) and dihydrotestosterone (DHT). Our results show that dioscin inhibited cell growth and invasion by increasing SHP1 phosphorylation [p-SHP1 (Y536)] and inhibiting the subsequent P38 mitogen-activated protein kinase signaling pathway. Further in vivo studies confirmed that dioscin promoted caspase-3 and Bad-related cell apoptosis in these two cell lines. Our research suggests that the anticancer effects of dioscin on PCa may occur through SHP1. Dioscin may be useful to treat androgen-sensitive and independent PCa in the future.

Redox regulation of tyrosine kinase signalling: more than meets the eye

Protein kinases are essential mediators of cellular signal transduction and are often dysregulated in disease. Among these, protein tyrosine kinases (PTKs) have received specific interest due to their common roles in various diseases including cancer, and emerging observations indicating that PTK signalling pathways are susceptible to regulation by reactive oxygen species (ROS), which are also frequently implicated in disease pathology. While it is well recognized that ROS can impact on tyrosine kinase signalling by inhibiting tyrosine phosphatases, more recent studies highlight additional modes of redox-based regulation of tyrosine kinase signalling by direct redox modification of non-catalytic cysteines within tyrosine kinases or other protein components of this signalling pathway. In this review, we will present recent advancements with respect to redox-based mechanisms in regulating PTK signalling, with a specific focus on recent studies demonstrating direct redox regulation of Src-family kinases and epidermal growth factor receptor kinases. Importantly, redox-based modulation of tyrosine kinases may be relevant for many other kinases and has implications for current approaches to develop pharmacological inhibitors for these proteins.

Regulation of the Proteolytic Activity of Cysteine Cathepsins by Oxidants

Besides their primary involvement in the recycling and degradation of proteins in endo-lysosomal compartments and also in specialized biological functions, cysteine cathepsins are pivotal proteolytic contributors of various deleterious diseases. While the molecular mechanisms of regulation via their natural inhibitors have been exhaustively studied, less is currently known about how their enzymatic activity is modulated during the redox imbalance associated with oxidative stress and their exposure resistance to oxidants. More specifically, there is only patchy information on the regulation of lung cysteine cathepsins, while the respiratory system is directly exposed to countless exogenous oxidants contained in dust, tobacco, combustion fumes, and industrial or domestic particles. Papain-like enzymes (clan CA, family C1, subfamily C1A) encompass a conserved catalytic thiolate-imidazolium pair (Cys25-His159) in their active site. Although the sulfhydryl group (with a low acidic pKa) is a potent nucleophile highly susceptible to chemical modifications, some cysteine cathepsins reveal an unanticipated resistance to oxidative stress. Besides an introductory chapter and peculiar attention to lung cysteine cathepsins, the purpose of this review is to afford a concise update of the current knowledge on molecular mechanisms associated with the regulation of cysteine cathepsins by redox balance and by oxidants (e.g., Michael acceptors, reactive oxygen, and nitrogen species).

The Importance of Tyrosine Phosphorylation Control of Cellular Signaling Pathways in Respiratory Disease: pY and pY Not

Reversible phosphorylation of proteins on tyrosine residues is an essential signaling mechanism by which diverse cellular processes are closely regulated. The tight temporal and spatial control of the tyrosine phosphorylation status of proteins by protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs) is critical to cellular homeostasis as well as to adaptations to the external environment. Via regulation of cellular signaling cascades involving other protein kinases and phosphatases, receptors, adaptor proteins, and transcription factors, PTKs and PTPs closely control diverse cellular processes such as proliferation, differentiation, migration, inflammation, and maintenance of cellular barrier function. Given these key regulatory roles, it is not surprising that dysfunction of PTKs and PTPs is important in the pathogenesis of human disease, including many pulmonary diseases. The roles of various PTKs and PTPs in acute lung injury and repair, pulmonary fibrosis, pulmonary vascular disease, and inflammatory airway disease are discussed in this review. It is important to note that although there is overlap among many of these proteins in various disease states, the mechanisms by which they influence the pathogenesis of these conditions differ, suggesting wide-ranging roles for these enzymes and their potential as therapeutic targets.

Hyperglycaemia-induced methylglyoxal accumulation potentiates VEGF resistance of diabetic monocytes through the aberrant activation of tyrosine phosphatase SHP-2/SRC kinase signalling axis

Diabetes mellitus (DM) is a major cardiovascular risk factor contributing to cardiovascular complications by inducing vascular cell dysfunction. Monocyte dysfunction could contribute to impaired arteriogenesis response in DM patients. DM monocytes show blunted chemotactic responses to arteriogenic stimuli, a condition termed as vascular endothelial growth factor (VEGF) resistance. We hypothesize that methylglyoxal (MG), a glucose metabolite, induces monocyte dysfunction and aimed to elucidate the underlying molecular mechanisms. Human monocytes exposed to MG or monocytes from DM patients or mice (db/db) showed VEGF-resistance secondary to a pro-migratory phenotype. Mechanistically, DM conditions or MG exposure resulted in the upregulation of the expression of SHP-2 phosphatase. This led to the enhanced activity of SHP-2 and aided an interaction with SRC kinase. SHP-2 dephosphorylated the inhibitory phosphorylation site of SRC leading to its abnormal activation and phosphorylation of cytoskeletal protein, paxillin. We demonstrated that MG-induced molecular changes could be reversed by pharmacological inhibitors of SHP-2 and SRC and by genetic depletion of SHP-2. Finally, a SHP-2 inhibitor completely reversed the dysfunction of monocytes isolated from DM patients and db/db mice. In conclusion, we identified SHP-2 as a hitherto unknown target for improving monocyte function in diabetes. This opens novel perspectives for treating diabetic complications associated with impaired monocyte function.

Detection of Intracellular Reduced (Catalytically Active) SHP-1 and Analyses of Catalytically Inactive SHP-1 after Oxidation by Pervanadate or H2O2

Oxidative inactivation of cysteine-dependent Protein Tyrosine Phosphatases (PTPs) by cellular reactive oxygen species (ROS) plays a critical role in regulating signal transduction in multiple cell types. The phosphatase activity of most PTPs depends upon a 'signature' cysteine residue within the catalytic domain that is maintained in the de-protonated state at physiological pH rendering it susceptible to ROS-mediated oxidation. Direct and indirect techniques for detection of PTP oxidation have been developed (Karisch and Neel, 2013). To detect catalytically active PTPs, cell lysates are treated with iodoacetyl-polyethylene glycol-biotin (IAP-biotin), which irreversibly binds to reduced (S^-) cysteine thiols. Irreversible oxidation of SHP-1 after treatment of cells with pervanadate or H₂O₂ is detected with antibodies specific for the sulfonic acid (SO₃H) form of the conserved active site cysteine of PTPs. In this protocol, we describe a method for the detection of the reduced (S^-; active) or irreversibly oxidized (SO₃H; inactive) form of the hematopoietic PTP SHP-1 in thymocytes, although this method is applicable to any cysteine-dependent PTP in any cell type.

Redox Regulation of a Gain-of-Function Mutation (N308D) in SHP2 Noonan Syndrome

SHP2 (Src homology 2 domain-containing protein tyrosine phosphatase 2; PTPN11) is a ubiquitous multidomain, nonreceptor protein tyrosine phosphatase (PTP) that plays an important role in diseases such as cancer, diabetes, and Noonan syndrome (NS). NS is one of the most common genetic disorders associated with congenital heart disease, and approximately half of the patients with Noonan syndrome have gain-of-function mutations in SHP2. One of the most common NS mutations is N308D. The activity of SHP2, like that of most PTPs, is reversibly inactivated by reactive oxygen species (ROS). However, the molecular basis of this inactivation and the consequences of NS-related mutations in PTPN11 on ROS-mediated inhibition are poorly understood. Here, we investigated the mechanistic and structural details of the reversible oxidation of the NS variant SHP2_N308D. We show that SHP2_N308D is more sensitive to oxidation when compared with wild-type SHP2. We also show that although the SHP2_N308D catalytic domain can be reactivated by dithiothreitol as effectively as the wild-type, full-length SHP2_N308D is only poorly reactivated by comparison. To understand the mechanism of oxidation at a molecular level, we determined the crystal structure of oxidized SHP2_N308D. The structure shows that the catalytic Cys459 residue forms a disulfide bond with Cys367, which confirms that Cys367 functions as the "backdoor" cysteine in SHP2. Together, our data suggest that the reversible oxidation of SHP2 contributes negligibly, if at all, to the symptoms associated with NS.

Redox regulation in tumor cell epithelial-mesenchymal transition: molecular basis and therapeutic strategy

Epithelial–mesenchymal transition (EMT) is recognized as a driving force of cancer cell metastasis and drug resistance, two leading causes of cancer recurrence and cancer-related death. It is, therefore, logical in cancer therapy to target the EMT switch to prevent such cancer metastasis and recurrence. Previous reports have indicated that growth factors (such as epidermal growth factor and fibroblast growth factor) and cytokines (such as the transforming growth factor beta (TGF-β) family) are major stimulators of EMT. However, the mechanisms underlying EMT initiation and progression remain unclear. Recently, emerging evidence has suggested that reactive oxygen species (ROS), important cellular secondary messengers involved in diverse biological events in cancer cells, play essential roles in the EMT process in cancer cells by regulating extracellular matrix (ECM) remodeling, cytoskeleton remodeling, cell–cell junctions, and cell mobility. Thus, targeting EMT by manipulating the intracellular redox status may hold promise for cancer therapy. Herein, we will address recent advances in redox biology involved in the EMT process in cancer cells, which will contribute to the development of novel therapeutic strategies by targeting redox-regulated EMT for cancer treatment.

Canonical Transient Receptor Potential 6 Channel: A New Target of Reactive Oxygen Species in Renal Physiology and Pathology

SIGNIFICANCE

Regulation of Ca2+ signaling cascade by reactive oxygen species (ROS) is becoming increasingly evident and this regulation represents a key mechanism for control of many fundamental cellular functions. Canonical transient receptor potential (TRPC) 6, a member of Ca2+-conductive channel in the TRPC family, is widely expressed in kidney cells, including glomerular mesangial cells, podocytes, tubular epithelial cells, and vascular myocytes in renal microvasculature. Both overproduction of ROS and dysfunction of TRPC6 channel are involved in renal injury in animal models and human subjects. Although regulation of TRPC channel function by ROS has been well described in other tissues and cell types, such as vascular smooth muscle, this important cell regulatory mechanism has not been fully reviewed in kidney cells. Recent Advances: Accumulating evidence has shown that TRPC6 is a redox-sensitive channel, and modulation of TRPC6 Ca2+ signaling by altering TRPC6 protein expression or TRPC6 channel activity in kidney cells is a downstream mechanism by which ROS induce renal damage.

CRITICAL ISSUES

This review highlights how recent studies analyzing function and expression of TRPC6 channels in the kidney and their response to ROS improve our mechanistic understanding of oxidative stress-related kidney diseases.

FUTURE DIRECTIONS

Although it is evident that ROS regulate TRPC6-mediated Ca2+ signaling in several types of kidney cells, further study is needed to identify the underlying molecular mechanism. We hope that the newly identified ROS/TRPC6 pathway will pave the way to new, promising therapeutic strategies to target kidney diseases such as diabetic nephropathy. Antioxid. Redox Signal. 25, 732-748.

The SOD1 transgene expressed in erythroid cells alleviates fatal phenotype in congenic NZB/NZW-F1 mice

Oxidative stress due to a superoxide dismutase 1 (SOD1) deficiency causes anemia and autoimmune responses, which are phenotypically similar to autoimmune hemolytic anemia (AIHA) and systemic lupus erythematosus (SLE) in C57BL/6 mice and aggravates AIHA pathogenesis in New Zealand black (NZB) mice. We report herein on an evaluation of the role of reactive oxygen species (ROS) in a model mouse with inherited SLE, that is, F1 mice of the NZB × New Zealand white (NZW) strain. The ROS levels within red blood cells (RBCs) of the F1 mice were similar to the NZW mice but lower compared to the NZB mice throughout adult period. Regarding SLE pathogenesis, we examined the effects of an SOD1 deficiency or the overexpression of human SOD1 in erythroid cells by establishing corresponding congenic F1 mice. A SOD1 deficiency caused an elevation in ROS production, methemoglobin content, and hyperoxidation of peroxiredoxin in RBC of the F1 mice, which were all consistent with elevated oxidative stress. However, while the overexpression of human SOD1 in erythroid cells extended the life span of the congenic F1 mice, the SOD1 deficiency had no effect on life span compared to wild-type F1 mice. It is generally recognized that NZW mice possess a larval defect in the immune system and that NZB mice trigger an autoimmune reaction in the F1 mice. Our results suggest that the oxidative insult originated from the NZB mouse background has a functional role in triggering an aberrant immune reaction, leading to fatal responses in F1 mice.

Resveratrol inhibits collagen-induced platelet stimulation through suppressing NADPH oxidase and oxidative inactivation of SH2 domain-containing protein tyrosine phosphatase-2

Reactive oxygen species (ROS) produced upon collagen stimulation are implicated in propagating various platelet-activating pathways. Among ROS-producing enzymes, NADPH oxidase (NOX) is largely responsible for collagen receptor-dependent ROS production. Therefore, NOX has been proposed as a novel target for the development of antiplatelet agent. We here investigate whether resveratrol inhibits collagen-induced NOX activation and further examine the effects of resveratrol on ROS-dependent signaling pathways in collagen-stimulated platelets. Collagen-induced superoxide anion production in platelets was inhibited by resveratrol. Resveratrol suppressed collagen-induced phosphorylation of p47(phox), a major regulatory subunit of NOX. Correlated with the inhibitory effects on NOX, resveratrol protected SH2 domain-containing protein tyrosine phosphatase-2 (SHP-2) from ROS-mediated inactivation and subsequently attenuated the specific tyrosine phosphorylation of key components (spleen tyrosine kinase, Vav1, Bruton's tyrosine kinase, and phospholipase Cγ2) for collagen receptor signaling cascades. Resveratrol also inhibited downstream responses such as cytosolic calcium elevation, P-selectin surface exposure, and integrin-αIIbβ3 activation. Furthermore, resveratrol inhibited platelet aggregation and adhesion in response to collagen. The antiplatelet effects of resveratrol through the inhibition of NOX-derived ROS production and subsequent oxidative inactivation of SHP-2 suggest that resveratrol is a potential compound for prevention and treatment of thrombovascular diseases.

Dynamic Redox Regulation of IL-4 Signaling

Quantifying the magnitude and dynamics of protein oxidation during cell signaling is technically challenging. Computational modeling provides tractable, quantitative methods to test hypotheses of redox mechanisms that may be simultaneously operative during signal transduction. The interleukin-4 (IL-4) pathway, which has previously been reported to induce reactive oxygen species and oxidation of PTP1B, may be controlled by several other putative mechanisms of redox regulation; widespread proteomic thiol oxidation observed via 2D redox differential gel electrophoresis upon IL-4 treatment suggests more than one redox-sensitive protein implicated in this pathway. Through computational modeling and a model selection strategy that relied on characteristic STAT6 phosphorylation dynamics of IL-4 signaling, we identified reversible protein tyrosine phosphatase (PTP) oxidation as the primary redox regulatory mechanism in the pathway. A systems-level model of IL-4 signaling was developed that integrates synchronous pan-PTP oxidation with ROS-independent mechanisms. The model quantitatively predicts the dynamics of IL-4 signaling over a broad range of new redox conditions, offers novel hypotheses about regulation of JAK/STAT signaling, and provides a framework for interrogating putative mechanisms involving receptor-initiated oxidation.

Incomplete reduction of oxygen during respiration results in the formation of highly reactive molecules known as reactive oxygen species (ROS) that react indiscriminately with cellular components and adversely affect cellular function. For a long time ROS were thought solely to be undesirable byproducts of respiration. Indeed, high levels of ROS are associated with a number of diseases. Despite these facts, antioxidants, agents that neutralize ROS, have not shown any clinical benefits when used as oral supplements. This paradox is partially explained by discoveries over the last two decades demonstrating that ROS are not always detrimental and may be essential for controlling physiological processes like cell signaling. However, the mechanisms by which ROS react with biomolecules are not well understood. In this work we have combined biological experiments with novel computational methods to identify the most important mechanisms of ROS-mediated regulation in the IL-4 signaling pathway of the immune system. We have also developed a detailed computer model of the IL-4 pathway and its regulation by ROS dependent and independent methods. Our work enhances the understanding of principles underlying regulation of cell signaling by ROS and has potential implications in advancing therapeutic methods targeting ROS and their adverse effects.

Kaempferol suppresses collagen-induced platelet activation by inhibiting NADPH oxidase and protecting SHP-2 from oxidative inactivation

Reactive oxygen species (ROS) generated upon collagen stimulation act as second messengers to propagate various platelet-activating events. Among the ROS-generating enzymes, NADPH oxidase (NOX) plays a prominent role in platelet activation. Thus, NOX has been suggested as a novel target for anti-platelet drug development. Although kaempferol has been identified as a NOX inhibitor, the influence of kaempferol on the activation of platelets and the underlying mechanism have never been investigated. Here, we studied the effects of kaempferol on NOX activation, ROS-dependent signaling pathways, and functional responses in collagen-stimulated platelets. Superoxide anion generation stimulated by collagen was significantly inhibited by kaempferol in a concentration-dependent manner. More importantly, kaempferol directly bound p47(phox), a major regulatory subunit of NOX, and significantly inhibited collagen-induced phosphorylation of p47(phox) and NOX activation. In accordance with the inhibition of NOX, ROS-dependent inactivation of SH2 domain-containing protein tyrosine phosphatase-2 (SHP-2) was potently protected by kaempferol. Subsequently, the specific tyrosine phosphorylation of key components (Syk, Vav1, Btk, and PLCγ2) of collagen receptor signaling pathways was suppressed by kaempferol. Kaempferol also attenuated downstream responses, including cytosolic calcium elevation, P-selectin surface exposure, and integrin-αIIbβ3 activation. Ultimately, kaempferol inhibited platelet aggregation and adhesion in response to collagen in vitro and prolonged in vivo thrombotic response in carotid arteries of mice. This study shows that kaempferol impairs collagen-induced platelet activation through inhibition of NOX-derived ROS production and subsequent oxidative inactivation of SHP-2. This effect suggests that kaempferol has therapeutic potential for the prevention and treatment of thrombovascular diseases.

Oxidative stress as a potential causal factor for autoimmune hemolytic anemia and systemic lupus erythematosus

The kidneys and the blood system mutually exert influence in maintaining homeostasis in the body. Because the kidneys control erythropoiesis by producing erythropoietin and by supporting hematopoiesis, anemia is associated with kidney diseases. Anemia is the most prevalent genetic disorder, and it is caused by a deficiency of glucose 6-phosphate dehydrogenase (G6PD), for which sulfhydryl oxidation due to an insufficient supply of NADPH is a likely direct cause. Elevated reactive oxygen species (ROS) result in the sulfhydryl oxidation and hence are another potential cause for anemia. ROS are elevated in red blood cells (RBCs) under superoxide dismutase (SOD1) deficiency in C57BL/6 mice. SOD1 deficient mice exhibit characteristics similar to autoimmune hemolytic anemia (AIHA) and systemic lupus erythematosus (SLE) at the gerontic stage. An examination of AIHA-prone New Zealand Black (NZB) mice, which have normal SOD1 and G6PD genes, indicated that ROS levels in RBCs are originally high and further elevated during aging. Transgenic overexpression of human SOD1 in erythroid cells effectively suppresses ROS elevation and ameliorates AIHA symptoms such as elevated anti-RBC antibodies and premature death in NZB mice. These results support the hypothesis that names oxidative stress as a risk factor for AIHA and other autoimmune diseases such as SLE. Herein we discuss the association between oxidative stress and SLE pathogenesis based mainly on the genetic and phenotypic characteristics of NZB and New Zealand white mice and provide insight into the mechanism of SLE pathogenesis.

Thiol-based redox proteomics in cancer research

Cancer cells maintain their intracellular ROS concentrations at required levels for their survival. Changes in ROS concentrations can regulate biochemical signaling mechanisms that control cell function. It has been demonstrated that ROS regulate the cellular events through redox regulation of redox-sensitive proteins (redox sensors). Upon oxidative stress, redox sensors undergo redox modifications that cause the allosteric changes of these proteins and endow them with different functions. Understanding the altered functions of redox sensors and the underlying mechanisms is critical for the development of novel cancer therapeutics. Recently, a series of high-throughput proteomics approaches have been developed for screening redox processes. In this manuscript, we review these methodologies and discuss the important redox sensors recently identified that are related to cancer.

Inhibitor of the tyrosine phosphatase STEP reverses cognitive deficits in a mouse model of Alzheimer's disease

STEP (STriatal-Enriched protein tyrosine Phosphatase) is a neuron-specific phosphatase that regulates N-methyl-D-aspartate receptor (NMDAR) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) trafficking, as well as ERK1/2, p38, Fyn, and Pyk2 activity. STEP is overactive in several neuropsychiatric and neurodegenerative disorders, including Alzheimer's disease (AD). The increase in STEP activity likely disrupts synaptic function and contributes to the cognitive deficits in AD. AD mice lacking STEP have restored levels of glutamate receptors on synaptosomal membranes and improved cognitive function, results that suggest STEP as a novel therapeutic target for AD. Here we describe the first large-scale effort to identify and characterize small-molecule STEP inhibitors. We identified the benzopentathiepin 8-(trifluoromethyl)-1,2,3,4,5-benzopentathiepin-6-amine hydrochloride (known as TC-2153) as an inhibitor of STEP with an IC50 of 24.6 nM. TC-2153 represents a novel class of PTP inhibitors based upon a cyclic polysulfide pharmacophore that forms a reversible covalent bond with the catalytic cysteine in STEP. In cell-based secondary assays, TC-2153 increased tyrosine phosphorylation of STEP substrates ERK1/2, Pyk2, and GluN2B, and exhibited no toxicity in cortical cultures. Validation and specificity experiments performed in wild-type (WT) and STEP knockout (KO) cortical cells and in vivo in WT and STEP KO mice suggest specificity of inhibitors towards STEP compared to highly homologous tyrosine phosphatases. Furthermore, TC-2153 improved cognitive function in several cognitive tasks in 6- and 12-mo-old triple transgenic AD (3xTg-AD) mice, with no change in beta amyloid and phospho-tau levels.

Reactive oxygen species play a critical role in collagen-induced platelet activation via SHP-2 oxidation

AIMS

The collagen-stimulated generation of reactive oxygen species (ROS) regulates signal transduction in platelets, although the mechanism is unclear. The major targets of ROS include protein tyrosine phosphatases (PTPs). ROS-mediated oxidation of the active cysteine site in PTPs abrogates the PTP catalytic activity. The aim of this study was to elucidate whether collagen-induced ROS generation leads to PTP oxidation, which promotes platelet stimulation.

RESULTS

SH2 domain-containing PTP-2 (SHP-2) is oxidized in platelets by ROS produced upon collagen stimulation. The oxidative inactivation of SHP-2 leads to the enhanced tyrosine phosphorylation of spleen tyrosine kinase (Syk), Vav1, and Bruton's tyrosine kinase (Btk) in the linker for the activation of T cells signaling complex, which promotes the tyrosine phosphorylation-mediated activation of phospholipase Cγ2 (PLCγ2). Moreover, we found that, relative to wild-type platelets, platelets derived from glutathione peroxidase 1 (GPx1)/catalase double-deficient mice showed enhanced cellular ROS levels, oxidative inactivation of SHP-2, and tyrosine phosphorylation of Syk, Vav1, Btk, and PLCγ2 in response to collagen, which subsequently led to increased intracellular calcium levels, degranulation, and integrin αIIbβ3 activation. Consistent with these findings, GPx1/catalase double-deficiency accelerated the thrombotic response in FeCl3-injured carotid arteries.

INNOVATION

The present study is the first to demonstrate that SHP-2 is targeted by ROS produced in collagen-stimulated platelets and suggests that a novel mechanism for the regulation of platelet activation by ROS is due to oxidative inactivation of SHP-2.

CONCLUSION

We conclude that collagen-induced ROS production leads to SHP-2 oxidation, which promotes platelet activation by upregulating tyrosine phosphorylation-based signal transduction.

Regulation of protein tyrosine phosphatase oxidation in cell adhesion and migration

SIGNIFICANCE

Redox-regulated control of protein tyrosine phosphatases (PTPs) through inhibitory reversible oxidation of their active site is emerging as a novel and general mechanism for control of cell surface receptor-activated signaling. This mechanism allows for a previously unrecognized crosstalk between redox regulators and signaling pathways, governed by, for example, receptor tyrosine kinases and integrins, which control cell proliferation and migration.

RECENT ADVANCES

A large number of different molecules, in addition to hydrogen peroxide, have been found to induce PTP inactivation, including lipid peroxides, reactive nitrogen species, and hydrogen sulfide. Characterization of oxidized PTPs has identified different types of oxidative modifications that are likely to display differential sensitivity to various reducing systems. Accumulating evidence demonstrates that PTP oxidation occurs in a temporally and spatially restricted manner. Studies in cell and animal models indicate altered PTP oxidation in models of common diseases, such as cancer and metabolic/cardiovascular disease. Novel methods have appeared that allow characterization of global PTP oxidation.

CRITICAL ISSUES

As the understanding of the molecular and cellular biology of PTP oxidation is developing, it will be important to establish experimental procedures that allow analyses of PTP oxidation, and its regulation, in physiological and pathophysiological settings. Future studies should also aim to establish specific connections between various oxidants, specific PTPs, and defined signaling contexts.

FUTURE DIRECTIONS

Modulation of PTP activity still appears as a valid strategy for correction or inhibition of dys-regulated cell signaling. Continued studies on PTP oxidation might present yet unrecognized means to exploit this regulatory mechanism for pharmacological purposes.

Redox-regulated growth factor survival signaling

SIGNIFICANCE

Once the thought of as unwanted byproducts of cellular respiration in eukaryotes, reactive oxygen species (ROS) have been shown to facilitate essential physiological roles. It is now understood that ROS are critical mediators of intracellular signaling. Control of signal transduction downstream of growth factor receptors by ROS is a complex process whose details are only recently coming to light.

RECENT ADVANCES

Indeed, recent evidence points to control of signal propagation by ROS at multiple levels in the typical cascade. Growth factor stimulation activates nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (Noxs) at the membrane, producing superoxide in the extracellular matrix, which is catalyzed to the membrane-permeable hydrogen peroxide (H2O2) that mediates intracellular signaling events.

CRITICAL ISSUES

The potential for H2O2, however, to disrupt cellular functions by damaging proteins and nucleic acids demands that its levels are kept in check by receptor-associated peroxiredoxins. This interplay of Nox and peroxiredoxin activity moderates levels of H2O2 sufficiently to modify signaling partners locally. Among the best studied of these partners are redox-controlled phosphatases that are inactivated by H2O2. Phosphatases regulate signal propagation downstream of receptors, and thus their inactivation allows a further level of control. Transmission of information further downstream to targets such as transcription factors, themselves regulated by ROS, completes this pathway.

FUTURE DIRECTIONS

Thus, signal propagation or attenuation can be dictated by ROS at multiple points. Given the complex nature of these processes, we envisage the emerging trends in the field of redox signaling in the context of growth factor stimulation.

Thiol-dependent recovery of catalytic activity from oxidized protein tyrosine phosphatases

Protein tyrosine phosphatases (PTPs) play an important role in the regulation of mammalian signal transduction. During some cell signaling processes, the generation of endogenous hydrogen peroxide inactivates selected PTPs via oxidation of the enzyme's catalytic cysteine thiolate group. Importantly, low-molecular weight and protein thiols in the cell have the potential to regenerate the catalytically active PTPs. Here we examined the recovery of catalytic activity from two oxidatively inactivated PTPs (PTP1B and SHP-2) by various low-molecular weight thiols and the enzyme thioredoxin. All monothiols examined regenerated the catalytic activity of oxidized PTP1B, with apparent rate constants that varied by a factor of approximately 8. In general, molecules bearing low-pKa thiol groups were particularly effective. The biological thiol glutathione repaired oxidized PTP1B with an apparent second-order rate constant of 0.023 ± 0.004 M(-1) s(-1), while the dithiol dithiothreitol (DTT) displayed an apparent second-order rate constant of 0.325 ± 0.007 M(-1) s(-1). The enzyme thioredoxin regenerated the catalytic activity of oxidized PTP1B at a substantially faster rate than DTT. Thioredoxin (2 μM) converted oxidized PTP1B to the active form with an observed rate constant of 1.4 × 10(-3) s(-1). The rates at which these agents regenerated oxidized PTP1B followed the order Trx > DTT > GSHand comparable values observed at 2 μM Trx, 4 mM DTT, and 60 mM GSH. Various disulfides that are byproducts of the reactivation process did not inactivate native PTP1B at concentrations of 1-20 mM. The common biochemical reducing agent tris(2-carboxyethyl)phosphine regenerates enzymatic activity from oxidized PTP1B somewhat faster than the thiol-based reagents, with a rate constant of 1.5 ± 0.5 M(-1) s(-1). We observed profound kinetic differences between the thiol-dependent regeneration of activity from oxidized PTP1B and SHP-2, highlighting the potential for structural differences in various oxidized PTPs to play a significant role in the rates at which low-molecular weight thiols and thiol-containing enzymes such as thioredoxin and glutaredoxin return catalytic activity to these enzymes during cell signaling events.

Redox regulation of Janus kinase: The elephant in the room

The redox regulation of Janus kinases (JAKs) is a complex subject. Due to other redox-sensitive kinases in the kinome, redox-sensitive phosphatases, and cellular antioxidant systems and reactive oxygen species (ROS) production systems, the net biological outcomes of oxidative stress on JAK-dependent signal transduction vary according to the specific biological system examined. This review begins with a discussion of the biochemical evidence for a cysteine-based redox switch in the catalytic domain of JAKs, proceeds to consider direct and indirect regulatory mechanisms involved in biological experiments, and ends with a discussion of the role(s) of redox regulation of JAKs in various diseases.

Redox regulation of protein kinases

Reactive oxygen species (ROS) have been long regarded as by-products of a cascade of reactions stemming from cellular oxygen metabolism, which, if they accumulate to toxic levels, can have detrimental effects on cellular biomolecules. However, more recently, the recognition of ROS as mediators of cellular communications has led to their classification as signalling mediators in their own right. The prototypic redox-regulated targets downstream of ROS are the protein tyrosine phosphatases, and the wealth of research that has focused on this area has come to shape our understanding of how redox-signalling contributes to and facilitates protein tyrosine phosphorylation signalling cascades. However, it is becoming increasingly apparent that there is more to this system than simply the negative regulation of protein tyrosine phosphatases. Identification of redox-sensitive kinases such as Src led to the slow emergence of a role for redox regulation of tyrosine kinases. A flow of evidence, which has increased exponentially in recent times as a result of the development of new methods for the detection of oxidative modifications, demonstrates that, by concurrent oxidative activation of tyrosine kinases, ROS fine tune the duration and amplification of the phosphorylation signal. A more thorough understanding of the complex regulatory mechanism of redox-modification will allow targeting of both the production of ROS and their downstream effectors for therapeutic purposes. The present review assesses the most relevant recent literature that demonstrates a role for kinase regulation by oxidation, highlights the most significant findings and proposes future directions for this crucial area of redox biology.

Advanced glycation end product Nε-carboxymethyllysine induces endothelial cell injury: the involvement of SHP-1-regulated VEGFR-2 dephosphorylation

N(ε)-carboxymethyllysine (CML), a major advanced glycation end product, plays a crucial role in diabetes-induced vascular injury. The roles of protein tyrosine phosphatases and vascular endothelial growth factor (VEGF) receptors in CML-related endothelial cell injury are still unclear. Human umbilical vein endothelial cells (HUVECs) are a commonly used human EC type. Here, we tested the hypothesis that NADPH oxidase/reactive oxygen species (ROS)-mediated SH2 domain-containing tyrosine phosphatase-1 (SHP-1) activation by CML inhibits the VEGF receptor-2 (VEGFR-2, KDR/Flk-1) activation, resulting in HUVEC injury. CML significantly inhibited cell proliferation and induced apoptosis and reduced VEGFR-2 activation in parallel with the increased SHP-1 protein expression and activity in HUVECs. Adding recombinant VEGF increased forward biological effects, which were attenuated by CML. The effects of CML on HUVECs were abolished by SHP-1 siRNA transfection. Exposure of HUVECs to CML also remarkably escalated the integration of SHP-1 with VEGFR-2. Consistently, SHP-1 siRNA transfection and pharmacological inhibitors could block this interaction and elevating [(3)H]thymidine incorporation. CML also markedly activated the NADPH oxidase and ROS production. The CML-increased SHP-1 activity in HUVECs was effectively attenuated by antioxidants. Moreover, the immunohistochemical staining of SHP-1 and CML was increased, but phospho-VEGFR-2 staining was decreased in the aortic endothelium of streptozotocin-induced and high-fat diet-induced diabetic mice. We conclude that a pathway of tyrosine phosphatase SHP-1-regulated VEGFR-2 dephosphorylation through NADPH oxidase-derived ROS is involved in the CML-triggered endothelial cell dysfunction/injury. These findings suggest new insights into the development of therapeutic approaches to reduce diabetic vascular complications.

Protein tyrosine phosphatase structure-function relationships in regulation and pathogenesis

Protein phosphorylation on tyrosine residues is tightly controlled by protein tyrosine phosphatases (PTPs) at multiple levels: spatio-temporal expression, subcellular localization and post-translational modification. Structural and functional analysis of the PTP domains has provided insight into catalysis and regulatory mechanisms that control the enzymatic activity. Understanding the molecular basis of PTP regulation is of fundamental importance to dissect the pleiotropic effect of these enzymes in both health and disease. Here, we review recent insights into the regulation of receptor-like PTPs by extracellular ligands and into regulation by reversible oxidation that impairs catalysis directly. The physiological roles of PTPs are essential in homeostasis in eukaryotic cells and pertubation of their functional attributes causes different disease states. As an example, we discuss recent findings indicating how inappropriate oxidation of PTPs in cancer cells may contribute to cell transformation. On the other hand, PTPs from many pathogens are key virulence factors and manipulate signalling pathways in the host cells to promote invasion and survival of the microorganisms. This research area has received relatively little attention but has advanced remarkably. We review the structural features of pathogenic PTPs, their similarities and differences with eukaryotic PTPs, and the possible exploitation of this knowledge for therapeutic intervention.

Methods to monitor classical protein-tyrosine phosphatase oxidation

Reactive oxygen species (ROS), particularly hydrogen peroxide (H(2)O(2)), act as intracellular second messengers in many signaling pathways. Protein-tyrosine phosphatases (PTPs) are now believed to be important targets of ROS. PTPs contain a conserved catalytic cysteine with an unusually low pK(a). This property allows PTPs to execute nucleophilic attack on substrate phosphotyrosyl residues, but also renders them highly susceptible to oxidation. Reversible oxidation, which inactivates PTPs, is emerging as an important cellular regulatory mechanism and might contribute to human diseases, including cancer. Given their potential toxicity, it seems likely that ROS generation is highly controlled within cells to restrict oxidation to those PTPs that must be inactivated for signaling to proceed. Thus, identifying ROS-inactivated PTPs could be tantamount to finding the PTP(s) that critically regulate a specific signaling pathway. This article provides an overview of the methods currently available to identify and quantify PTP oxidation and outlines future challenges in redox signaling.

Reactive oxygen species (ROS) homeostasis and redox regulation in cellular signaling

Reactive oxygen species (ROS) are generated during mitochondrial oxidative metabolism as well as in cellular response to xenobiotics, cytokines, and bacterial invasion. Oxidative stress refers to the imbalance due to excess ROS or oxidants over the capability of the cell to mount an effective antioxidant response. Oxidative stress results in macromolecular damage and is implicated in various disease states such as atherosclerosis, diabetes, cancer, neurodegeneration, and aging. Paradoxically, accumulating evidence indicates that ROS also serve as critical signaling molecules in cell proliferation and survival. While there is a large body of research demonstrating the general effect of oxidative stress on signaling pathways, less is known about the initial and direct regulation of signaling molecules by ROS, or what we term the "oxidative interface." Cellular ROS sensing and metabolism are tightly regulated by a variety of proteins involved in the redox (reduction/oxidation) mechanism. This review focuses on the molecular mechanisms through which ROS directly interact with critical signaling molecules to initiate signaling in a broad variety of cellular processes, such as proliferation and survival (MAP kinases, PI3 kinase, PTEN, and protein tyrosine phosphatases), ROS homeostasis and antioxidant gene regulation (thioredoxin, peroxiredoxin, Ref-1, and Nrf-2), mitochondrial oxidative stress, apoptosis, and aging (p66Shc), iron homeostasis through iron-sulfur cluster proteins (IRE-IRP), and ATM-regulated DNA damage response.

Global proteomic assessment of the classical protein-tyrosine phosphatome and "Redoxome"

Protein-tyrosine phosphatases (PTPs), along with protein-tyrosine kinases, play key roles in cellular signaling. All Class I PTPs contain an essential active site cysteinyl residue, which executes a nucleophilic attack on substrate phosphotyrosyl residues. The high reactivity of the catalytic cysteine also predisposes PTPs to oxidation by reactive oxygen species, such as H(2)O(2). Reversible PTP oxidation is emerging as an important cellular regulatory mechanism and might contribute to diseases such as cancer. We exploited these unique features of PTP enzymology to develop proteomic methods, broadly applicable to cell and tissue samples, that enable the comprehensive identification and quantification of expressed classical PTPs (PTPome) and the oxidized subset of the PTPome (oxPTPome). We find that mouse and human cells and tissues, including cancer cells, display distinctive PTPomes and oxPTPomes, revealing additional levels of complexity in the regulation of protein-tyrosine phosphorylation in normal and malignant cells.

Regulation of protein tyrosine phosphatases by reversible oxidation

Oxidation of the catalytic cysteine of protein-tyrosine phosphatases (PTP), which leads to their reversible inactivation, has emerged as an important regulatory mechanism linking cellular tyrosine phosphorylation and signalling by reactive-oxygen or -nitrogen species (ROS, RNS). This review focuses on recent findings about the involved pathways, enzymes and biochemical mechanisms. Both the general cellular redox state and extracellular ligand-stimulated ROS production can cause PTP oxidation. Members of the PTP family differ in their intrinsic susceptibility to oxidation, and different types of oxidative modification of the PTP catalytic cysteine can occur. The role of PTP oxidation for physiological signalling processes as well as in different pathologies is described on the basis of well-investigated examples. Criteria to establish the causal involvement of PTP oxidation in a given process are proposed. A better understanding of mechanisms leading to selective PTP oxidation in a cellular context, and finding ways to pharmacologically modulate these pathways are important topics for future research.

Redox regulation of protein tyrosine phosphatases: structural and chemical aspects

Protein tyrosine phosphatases (PTPs) are important targets of the H(2)O(2) that is produced during mammalian signal transduction. H(2)O(2)-mediated inactivation of PTPs also may be important in various pathophysiological conditions involving oxidative stress. Here we review the chemical and structural biology of redox-regulated PTPs. Reactions of H(2)O(2) with PTPs convert the catalytic cysteine thiol to a sulfenic acid. In PTPs, the initially generated sulfenic acid residues have the potential to undergo secondary reactions with a neighboring amide nitrogen or cysteine thiol residue to yield a sulfenyl amide or disulfide, respectively. The chemical mechanisms by which formation of sulfenyl amide and disulfide linkages can protect the catalytic cysteine residue against irreversible overoxidation to sulfinic and sulfonic oxidation states are described. Due to the propensity for back-door and distal cysteine residues to engage with the active-site cysteine after oxidative inactivation, differences in the structures of the oxidatively inactivated PTPs may stem, to a large degree, from differences in the number and location of cysteine residues surrounding the active site of the enzymes. PTPs with key cysteine residues in structurally similar locations may be expected to share similar mechanisms of oxidative inactivation.

The myeloperoxidase-derived oxidant HOSCN inhibits protein tyrosine phosphatases and modulates cell signalling via the mitogen-activated protein kinase (MAPK) pathway in macrophages

MPO (myeloperoxidase) catalyses the oxidation of chloride, bromide and thiocyanate by hydrogen peroxide to HOCl (hypochlorous acid), HOBr (hypobromous acid) and HOSCN (hypothiocyanous acid) respectively. Specificity constants indicate that SCN⁻ is a major substrate for MPO. HOSCN is also a major oxidant generated by other peroxidases including salivary, gastric and eosinophil peroxidases. While HOCl and HOBr are powerful oxidizing agents, HOSCN is a less reactive, but more specific, oxidant which targets thiols and especially low pK_a species. In the present study we show that HOSCN targets cysteine residues present in PTPs (protein tyrosine phosphatases) with this resulting in a loss of PTP activity for the isolated enzyme, in cell lysates and intact J774A.1 macrophage-like cells. Inhibition also occurs with MPO-generated HOCl and HOBr, but is more marked with MPO-generated HOSCN, particularly at longer incubation times. This inhibition is reversed by dithiothreitol, particularly at early time points, consistent with the reversible oxidation of the active site cysteine residue to give either a cysteine–SCN adduct or a sulfenic acid. Inhibition of PTP activity is associated with increased phosphorylation of p38a and ERK2 (extracellular-signal-regulated kinase 2) as detected by Western blot analysis and phosphoprotein arrays, and results in altered MAPK (mitogen-activated protein kinase) signalling. These data indicate that the highly selective targeting of some protein thiols by HOSCN can result in perturbation of cellular phosphorylation and altered cell signalling. These changes occur with (patho)physiological concentrations of SCN⁻ ions, and implicate HOSCN as an important mediator of inflammation-induced oxidative damage, particularly in smokers who have elevated plasma levels of SCN⁻.

A stress survival response in retinal cells mediated through inhibition of the serine/threonine phosphatase PP2A

Cell survival signalling involving the PI3K/Akt survival pathway can be negatively regulated by several phosphatases including PP2A. When retinal-derived 661W cells were subjected to trophic factor deprivation this initiated a survival response through inhibition of the activity of PP2A and subsequent upregulation of the Erk and Akt survival pathways. We show this survival response via inhibition of PP2A activity was due in part to increased reactive oxygen species production when retinal cells were deprived of trophic factors. Inhibition of PP2A activity was mediated by a rapid and transient increase in phosphorylation at Tyr307, accompanied by an increase in demethylation and a decrease in the methylated form. Pre-treatment with N-acetyl-L-cysteine, which is involved in scavenging reactive oxygen species, prevented PP2A inhibition and subsequent upregulation of survival pathways. Pre-treatment with the Src family kinase inhibitor PP2 resulted in approximately 50% reduction in cellular levels of phospho-PP2A in trophic factor-deprived 661W cells, suggesting an Src tyrosine kinase had a role to play in this redox regulation of cell survival. We observed similar events in the rd10 mouse retina where there was an increased survival response prior to retinal cell death mediated through an increase in both phospho-PP2A and phospho-Gsk. Together, these results demonstrate that when retinal cells are stressed there is an initial struggle to survive, mediated through inhibition of PP2A and subsequent upregulation of survival pathways, and that these events occur simultaneously with production of reactive oxygen species, thus suggesting an important cell-signalling role for reactive oxygen species.

Dynamic active-site protection by the M. tuberculosis protein tyrosine phosphatase PtpB lid domain

The Mycobacterium tuberculosis protein tyrosine phosphatase PtpB shows resistance to the oxidative conditions that prevail within an infected host macrophage, but the mechanism of this molecular adaptation is unknown. Crystal structures of PtpB revealed previously that a closed, two-helix lid covers the active site. By measuring single-molecule Forster-type resonance energy transfer to probe the dynamics of two helices that constitute the lid, we obtained direct evidence for large, spontaneous opening transitions of PtpB with the closed form of both helices favored approximately 3:1. Despite similar populations of conformers, the two helices move asynchronously as demonstrated by different opening and closing rates under our experimental conditions. Assuming that lid closure excludes oxidant, the rates of opening and closing quantitatively accounted for the slow observed rate of oxidative inactivation. Increasing solvent viscosity using glycerol but not PEG8000 resulted in higher rates of oxidative inactivation due to an increase in the population of open conformers. These results establish that the rapid conformational gating of the PtpB lid constitutes a reversible physical blockade that transiently masks the active site and retards oxidative inactivation.

Canonical transient receptor potential 6 (TRPC6), a redox-regulated cation channel

This study examined the effect of H₂O₂ on the TRPC6 channel and its underlying mechanisms using a TRPC6 heterologous expression system. In TRPC6-expressing HEK293T cells, H₂O₂ significantly stimulated Ca²⁺ entry in a dose-dependent manner. Electrophysiological experiments showed that H₂O₂ significantly increased TRPC6 channel open probability and whole-cell currents. H₂O₂ also evoked a robust inward current in A7r5 vascular smooth muscle cells, which was nearly abolished by knockdown of TRPC6 using a small interfering RNA. Catalase substantially attenuated arginine vasopressin (AVP)-induced Ca²⁺ entry in cells co-transfected with TRPC6 and AVP V1 receptor. N-Ethylmaleimide and thimerosal were able to simulate the H₂O₂ response. Dithiothreitol or glutathione-reduced ethyl ester significantly antagonized the response. Furthermore, both N-ethylmaleimide- and H₂O₂-induced TRPC6 activations were only observed in the cell-attached patches but not in the inside-out patches. Moreover, 1-oleoyl-2-acetyl-sn-glycerol effect on TRPC6 was significantly greater in the presence of H₂O₂. Biotinylation assays revealed a significant increase in cell surface TRPC6 in response to H₂O₂. Similarly, in cells transfected with TRPC6-EGFP, confocal microscopy showed a significant increase in fluorescence intensity in the region of the cell membrane and adjacent to the membrane. AVP also increased the fluorescence intensity on the surface of the cells co-transfected with TRPC6-EGFP and V1 receptor, and this response was inhibited by catalase. These data indicate that H₂O₂ activates TRPC6 channels via modification of thiol groups of intracellular proteins. This cysteine oxidation-dependent pathway not only stimulates the TRPC6 channel by itself but also sensitizes the channels to diacylglycerol and promotes TRPC6 trafficking to the cell surface.

Down-regulation of protein-tyrosine phosphatases activates an immune receptor in the absence of its translocation into lipid rafts

The earliest known biochemical step that occurs after ligand binding to the multichain immune recognition receptor is tyrosine phosphorylation of the receptor subunits. In mast cells and basophils activated by multivalent antigen-IgE complexes, this step is mediated by Src family kinase Lyn, which phosphorylates the high affinity IgE receptor (Fc epsilonRI). However, the exact molecular mechanism of this phosphorylation step is incompletely understood. In this study, we tested the hypothesis that changes in activity and/or topography of protein-tyrosine phosphatases (PTPs) could play a major role in the Fc epsilonRI triggering. We found that exposure of rat basophilic leukemia cells or mouse bone marrow-derived mast cells to PTP inhibitors, H(2)O(2) or pervanadate, induced phosphorylation of the Fc epsilonRI subunits, similarly as Fc epsilonRI triggering. Interestingly, and in sharp contrast to antigen-induced activation, neither H(2)O(2) nor pervanadate induced any changes in the association of Fc epsilonRI with detergent-resistant membranes and in the topography of Fc epsilonRI detectable by electron microscopy on isolated plasma membrane sheets. In cells stimulated with pervanadate, H(2)O(2) or antigen, enhanced oxidation of active site cysteine of several PTPs was detected. Unexpectedly, most of oxidized phosphatases bound to the plasma membrane were associated with the actin cytoskeleton. Several PTPs (SHP-1, SHP-2, hematopoietic PTP, and PTP-MEG2) showed changes in their enzymatic activity and/or oxidation state during activation. Based on these and other data, we propose that down-regulation of enzymatic activity of PTPs and/or changes in their accessibility to the substrates play a key role in initial tyrosine phosphorylation of the Fc epsilonRI and other multichain immune receptors.

Expanding the role of Src and protein-tyrosine phosphatases balance in modulating osteoblast metabolism: lessons from mice

The widespread nature of protein phosphorylation/dephosphorylation underscores its key role in cell signaling metabolism, growth and differentiation. Tyrosine phosphorylation of cytoplasmic proteins is a critical event in the regulation of intracellular signaling pathways activated by external stimuli. An adequate balance in protein phosphorylation is a major factor in the regulation of osteoclast and osteoblast activities involved in bone metabolism. However, although phosphorylation is widely recognized as an important regulatory pathway in skeletal development and maintenance, the mechanisms involved are not fully understood. Among the putative protein-tyrosine kinases (ptk) and protein-tyrosine phosphatases (ptp) involved in this phenomenon there is increasing evidence that Src and low molecular weight-ptps play a central role in a range of osteoblast activities, from adhesion to differentiation. A role for Src in bone metabolism was first demonstrated in Src-deficient mice and has since been confirmed using low molecular weight Src inhibitors in animal models of osteoporosis. Several studies have shown that Src is important for cellular proliferation, adhesion and motility. In contrast, few studies have assessed the importance of the ptk/ptp balance in driving osteoblast metabolism. In this review, we summarize our current knowledge of the functional importance of the ptk/ptp balance in osteoblast metabolism, and highlight directions for future research that should improve our understanding of these critical signaling molecules.

Calpain-mediated degradation of reversibly oxidized protein-tyrosine phosphatase 1B

Protein-tyrosine phosphatases (PTPs) are regulated by reversible inactivating oxidation of the catalytic-site cysteine. We have previously shown that reversible oxidation upon UVA irradiation is followed by calpain-mediated PTP degradation. Here, we address the mechanism of regulated cleavage and the physiological function of PTP degradation. Reversible oxidation of PTP1B in vitro strongly facilitated the association with calpain and led to greatly increased calpain-dependent inactivating cleavage. Both oxidation-induced association and cleavage depended exclusively on the presence of the catalytic (reversibly oxidized) cysteine residue 215. A major cleavage site was identified preceding amino acid position Ala77. In calpain-deficient cells, insulin signaling was apparently diminished, consistent with a possible role for calpain in removing a negative regulator of insulin signaling. Reversibly oxidized PTP1B may be a target of calpain in this context.

NADPH oxidase Nox2 is required for hypoxia-induced mobilization of endothelial progenitor cells

RATIONALE

Endothelial progenitor cells (EPCs, defined as sca-1(+)flk-1(+)lin(-) mononuclear blood cells) contribute to vascular repair. The role of hypoxia and reactive oxygen species (ROS) in mobilization and function of these cells is incompletely understood.

OBJECTIVE

We studied the contribution of the NADPH oxidase Nox2, an important vascular source of ROS in this context.

METHODS AND RESULTS

Hypoxia (10% oxygen) induced the mobilization of EPCs in wild-type (WT) and Nox1 but not in Nox2 knockout (Nox2(y/-)) mice. As erythropoietin (EPO) is known to induce EPC mobilization, we focused on this hormone. EPO induced the mobilization of EPCs in WT and Nox1(y/-) but not Nox2(y/-) animals. Transplantation of bone marrow from Nox2(y/-) mice into WT-mice blocked mobilization in response to hypoxia and EPO, whereas transplantation of WT bone marrow into Nox2(y/-) mice restored mobilization. Reendothelialization of the injured mouse carotid artery was enhanced by hypoxia as well as by EPO, and this effect was not observed in Nox2(y/-) mice or after transplantation of Nox2(y/-) bone marrow. In cultured EPCs from WT but not Nox2(y/-) mice, EPO induced ROS production, migration, and proliferation. EPO signaling involves the STAT5 transcription factor. EPO-induced STAT5-dependent reporter gene expression was absent in Nox2-deficient cells. siRNA against the redox-sensitive phosphatase SHP-2 restored EPO-mediated STAT5 induction and inhibition of SHP-2 restored EPO-induced migration in Nox2-deficient cells

CONCLUSIONS

We conclude that Nox2-derived ROS inactivate SHP-2 and thereby facilitate EPO signaling in EPCs to promote hypoxia-induced mobilization and vascular repair by these cells.

Redox regulation of the human dual specificity phosphatase YVH1 through disulfide bond formation

YVH1 was one of the first eukaryotic dual specificity phosphatases cloned, and orthologues poses a unique C-terminal zinc-coordinating domain in addition to a cysteine-based phosphatase domain. Our recent results revealed that human YVH1 (hYVH1) protects cells from oxidative stress. This function requires phosphatase activity and the zinc binding domain. This current study provides evidence that the thiol-rich zinc-coordinating domain may act as a redox sensor to impede the active site cysteine from inactivating oxidation. Furthermore, using differential thiol labeling and mass spectrometry, it was determined that hYVH1 forms intramolecular disulfide bonds at the catalytic cleft as well as within the zinc binding domain to avoid irreversible inactivation during severe oxidative stress. Importantly, zinc ejection is readily reversible and required for hYVH1 activity upon returning to favorable conditions. This inimitable mechanism provides a means for hYVH1 to remain functionally responsive for protecting cells during oxidative stimuli.

CD8+ T cell responses to a viral escape mutant epitope: active suppression via altered SHP-1 activity

One mechanism viruses use to subvert immune surveillance is through mutation of MHC contact residues of antigenic epitopes that weaken T cell recognition to the point that the immune system is ignorant of the infection. However, in contrast to ignorance, results presented herein demonstrate that intracellular signaling does occur upon stimulation with a lymphocytic choriomeningitis virus-derived escape mutant as demonstrated by the sustained activation of Src homology 2 domain-containing protein tyrosine phosphatase (SHP-1). In addition to the increased SHP-1 activity, we found that the mutated epitope failed to induce oxidation of SHP-1, further enhancing enzymatic activity. Sustained activation of SHP-1 in a reduced form correlated with ERK and early growth response gene 1 activation and failure of T cells to commit to the effector lineage. Thus, instead of immune ignorance, these studies demonstrate the activation of a negative signaling pathway that actively suppresses T cell responses and limits recognition of viral escape mutants.

Cell cycle regulation by oncogenic tyrosine kinases in myeloid neoplasias: from molecular redox mechanisms to health implications

Neoplastic expansion of myeloid cells is associated with specific genetic changes that lead to chronic activation of signaling pathways, as well as altered metabolism. It has become increasingly evident that transformation relies on the interdependency of both events. Among the various genetic changes, the oncogenic BCR-ABL tyrosine kinase in patients with Philadelphia chromosome positive chronic myeloid leukemia (CML) has been a focus of extensive research. Transformation by this oncogene is associated with elevated levels of intracellular reactive oxygen species (ROS). ROS have been implicated in processes that promote viability, cell growth, and regulation of other biological functions such as migration of cells or gene expression. Currently, the BCR-ABL inhibitor imatinib mesylate (Gleevec) is being used as a first-line therapy for the treatment of CML. However, BCR-ABL transformation is associated with genomic instability, and disease progression or resistance to imatinib can occur. Imatinib resistance is not known to cause or significantly alter signaling requirements in transformed cells. Elevated ROS are crucial for transformation, making them an ideal additional target for therapeutic intervention. The underlying mechanisms leading to elevated oxidative stress are reviewed, and signaling mechanisms that may serve as novel targeted approaches to overcome ROS-dependent cell growth are discussed.

Redox-sensitive signaling by angiotensin II involves oxidative inactivation and blunted phosphorylation of protein tyrosine phosphatase SHP-2 in vascular smooth muscle cells from SHR

Angiotensin II (Ang II) signaling in vascular smooth muscle cells (VSMCs) involves reactive oxygen species (ROS) through unknown mechanisms. We propose that Ang II induces phosphorylation of growth signaling kinases by redox-sensitive regulation of protein tyrosine phosphatases (PTP) in VSMCs and that augmented Ang II signaling in spontaneously hypertensive rats (SHRs) involves oxidation/inactivation and blunted phosphorylation of the PTP, SHP-2. PTP oxidation was assessed by the in-gel PTP method. SHP-2 expression and activity were evaluated by immunoblotting and by a PTP activity assay, respectively. SHP-2 and Nox1 were downregulated by siRNA. Ang II induced oxidation of multiple PTPs, including SHP-2. Basal SHP-2 content was lower in SHRs versus WKY. Ang II increased SHP-2 phosphorylation and activity with blunted responses in SHRs. Ang II-induced SHP-2 effects were inhibited by valsartan (AT(1)R blocker), apocynin (NAD(P)H oxidase inhibitor), and Nox1 siRNA. Ang II stimulation increased activation of ERK1/2, p38MAPK, and AKT, with enhanced effects in SHR. SHP-2 knockdown resulted in increased AKT phosphorylation, without effect on ERK1/2 or p38MAPK. Nox1 downregulation attenuated Ang II-mediated AKT activation in SHRs. Hence, Ang II regulates PTP/SHP-2 in VSMCs through AT(1)R and Nox1-based NAD(P)H oxidase via two mechanisms, oxidation and phosphorylation. In SHR Ang II-stimulated PTP oxidation/inactivation is enhanced, basal SHP-2 expression is reduced, and Ang II-induced PTP/SHP-2 phosphorylation is blunted. These SHP-2 actions are associated with augmented AKT signaling. We identify a novel redox-sensitive SHP-2-dependent pathway for Ang II in VSMCs. SHP-2 dysregulation by increased Nox1-derived ROS in SHR is associated with altered Ang II-AKT signaling.

Protein tyrosine phosphorylation and protein tyrosine nitration in redox signaling

Reversible phosphorylation of protein tyrosine residues by polypeptide growth factor-receptor protein tyrosine kinases is implicated in the control of fundamental cellular processes including the cell cycle, cell adhesion, and cell survival, as well as cell proliferation and differentiation. During the last decade, it has become apparent that receptor protein tyrosine kinases and the signaling pathways they activate belong to a large signaling network. Such a network can be regulated by various extracellular cues, which include cell adhesion, agonists of G protein-coupled receptors, and oxidants. It is well documented that signaling initiated by receptor protein tyrosine kinases is directly dependent on the intracellular production of oxidants, including reactive oxygen and nitrogen species. Accumulated evidence indicates that the intracellular redox environment plays a major role in the mechanisms underlying the actions of growth factors. Oxidation of cysteine thiols and nitration of tyrosine residues on signaling proteins are described as posttranslational modifications that regulate, positively or negatively, protein tyrosine phosphorylation (PTP). Early observations described the inhibition of PTP activities by oxidants, resulting in increased levels of proteins phosphorylated on tyrosine. Therefore, a redox circuitry involving the increasing production of intracellular oxidants associated with growth-factor stimulation/cell adhesion, oxidative reversible inhibition of protein tyrosine phosphatases, and the activation of protein tyrosine kinases can be delineated.

Protein tyrosine phosphatases: regulatory mechanisms

Protein-tyrosine phosphatases are tightly controlled by various mechanisms, ranging from differential expression in specific cell types to restricted subcellular localization, limited proteolysis, post-translational modifications affecting intrinsic catalytic activity, ligand binding and dimerization. Here, we review the regulatory mechanisms found to control the classical protein-tyrosine phosphatases.