Two vicinal cysteines confer a peculiar redox regulation to low molecular weight protein tyrosine phosphatase in response to platelet-derived growth factor receptor stimulation

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.

Reactive oxygen species and alpha,beta-unsaturated aldehydes as second messengers in signal transduction

Signaling by H(2)O(2), alpha,beta-unsaturated aldehydes, such as 4-hydroxy-2-nonenal (HNE) and related chemical species, is thought to differ from signaling by other second messengers because the oxidants and other electrophiles can readily undergo nonenzymatic reactions and are therefore classified as "reactive." This brief review will describe how and when the chemistry of signaling is similar or differs from classic second messengers, such as cyclic AMP, or posttranslational signaling, such as farnesylation or ubiquitination. The chemistry of cysteine provides a common factor that underlies signaling by H(2)O(2) and HNE. Nonetheless, as H(2)O(2) and HNE are rapidly metabolized in vivo, spatial considerations are extremely important in their actions. Therefore, the locations of sources of H(2)O(2) and alpha,beta-unsaturated aldehydes, the NADPH oxidases, mitochondria, membrane lipids, and redox cycling toxicants, as well as their targets, are key factors. The activation of the JNK pathway by HNE and endogenously generated H(2)O(2) illustrates these principles.

Reversible phosphorylation in haematological malignancies: potential role for protein tyrosine phosphatases in treatment?

Most aspects of leukocyte physiology are under the control of reversible tyrosine phosphorylation. It is clear that excessive phosphorylation of signal transduction elements is a pivotal element of many different pathologies including haematological malignancies and accordingly, strategies that target such phosphorylation have clinically been proven highly successful for treatment of multiple types of leukemias and lymphomas. Cellular phosphorylation status is dependent on the resultant activity of kinases and phosphatases. The cell biology of the former is now well understood; for most cellular phosphoproteins we now know the kinases responsible for their phosphorylation and we understand the principles of their aberrant activity in disease. With respect to phosphatases, however, our knowledge is much patchier. Although the sequences of whole genomes allow us to identify phosphatases using in silico methodology, whereas transcription profiling allows us to understand how phosphatase expression is regulated during disease, most functional questions as to substrate specificity, dynamic regulation of phosphatase activity and potential for therapeutic intervention are still to a large degree open. Nevertheless, recent studies have allowed us to make meaningful statements on the role of tyrosine phosphatase activity in the three major signaling pathways that are commonly affected in leukemias, i.e. the Ras-Raf-ERK1/2, the Jak-STAT and the PI3K-PKB-mTOR pathways. Lessons learned from these pathways may well be applicable elsewhere in leukocyte biology as well.

Francisella tularensis antioxidants harness reactive oxygen species to restrict macrophage signaling and cytokine production

Francisella tularensis is the etiologic agent of the highly infectious animal and human disease tularemia. Its extreme infectivity and virulence are associated with its ability to evade immune detection, which we now link to its robust reactive oxygen species-scavenging capacity. Infection of primary human monocyte-derived macrophages with virulent F. tularensis SchuS4 prevented proinflammatory cytokine production in the presence or absence of IFN-gamma compared with infection with the attenuated live vaccine strain. SchuS4 infection also blocked signals required for macrophage cytokine production, including Akt phosphorylation, IkappaB alpha degradation, and NF-kappaB nuclear localization and activation. Concomitant with SchuS4-mediated suppression of Akt phosphorylation was an increase in the levels of the Akt antagonist PTEN. Moreover, SchuS4 prevented the H(2)O(2)-dependent oxidative inactivation of PTEN compared with a virulent live vaccine strain. Mutation of catalase (katG) sensitized F. tularensis to H(2)O(2) and enhanced PTEN oxidation, Akt phosphorylation, NF-kappaB activation, and inflammatory cytokine production. Together, these findings suggest a novel role for bacterial antioxidants in restricting macrophage activation through their ability to preserve phosphatases that temper kinase signaling and proinflammatory cytokine production.

Posttranslational modification of human glyoxalase 1 indicates redox-dependent regulation

BACKGROUND

Glyoxalase 1 (Glo1) and glyoxalase 2 (Glo2) are ubiquitously expressed cytosolic enzymes that catalyze the conversion of toxic alpha-oxo-aldehydes into the corresponding alpha-hydroxy acids using L-glutathione (GSH) as a cofactor. Human Glo1 exists in various isoforms; however, the nature of its modifications and their distinct functional assignment is mostly unknown.

METHODOLOGY/PRINCIPAL FINDINGS

We characterized native Glo1 purified from human erythrocytes by mass spectrometry. The enzyme was found to undergo four so far unidentified posttranslational modifications: (i) removal of the N-terminal methionine 1, (ii) N-terminal acetylation at alanine 2, (iii) a vicinal disulfide bridge between cysteine residues 19 and 20, and (iv) a mixed disulfide with glutathione on cysteine 139. Glutathionylation of Glo1 was confirmed by immunological methods. Both, N-acetylation and the oxidation state of Cys(19/20), did not impact enzyme activity. In contrast, glutathionylation strongly inhibited Glo1 activity in vitro. The discussed mechanism for enzyme inhibition by glutathionylation was validated by molecular dynamics simulation.

CONCLUSION/SIGNIFICANCE

It is shown for the first time that Glo1 activity directly can be regulated by an oxidative posttranslational modification that was found in the native enzyme, i.e., glutathionylation. Inhibition of Glo1 by chemical reaction with its co-factor and the role of its intramolecular disulfides are expected to be important factors within the context of redox-dependent regulation of glucose metabolism in cells.

Toxicological disruption of signaling homeostasis: tyrosine phosphatases as targets

The protein tyrosine phosphatases (PTPs) consist of a diverse group of enzymes whose activity opposes that of the tyrosine kinases. As such, the PTPs have critical roles in maintaining signaling quiescence in resting cells and in restoring homeostasis by effecting signal termination. Interest in these enzymes has increased in recent years following the discovery that the activity of PTPs is modulated through redox mechanisms during signaling. The molecular features that enable redox regulation of PTPs during physiological signaling also render them highly susceptible to oxidative and electrophilic inactivation by a broad spectrum of structurally disparate xenobiotic compounds. The loss of PTP activity results in a profound disregulation of protein phosphotyrosine metabolism, leading to widespread and persistent activation of signaling cascades in the cell.

Redox control of vascular smooth muscle migration

Vascular smooth muscle cell migration is important during vascular development and contributes to lesion formation in the adult vasculature. The mechanisms regulating migration of this cell type are therefore of great interest. Recent work has shown that reactive oxygen species (ROS) derived from NADPH oxidases are important mediators of promigratory signaling pathways. ROS regulate the intracellular signals responsible for lamellipodia formation, actin cytoskeleton remodeling, focal adhesion turnover, and contraction of the cell body. In addition, they contribute to matrix remodeling, a critical step to initiate and support vascular smooth muscle cell motility. Despite these recent advances in our understanding of the redox mechanisms that contribute to migration, additional work is needed to evaluate fully the potential of ROS-sensitive molecular signals as therapeutic targets to prevent inappropriate smooth muscle cell migration.

Reactive oxygen species-dependent signaling regulates cancer

Historically, it has been assumed that oxidative stress contributes to tumor initiation and progression solely by inducing genomic instability. Recent studies indicate that reactive oxygen species are upregulated in tumors and can lead to aberrant induction of signaling networks that cause tumorigenesis and metastasis. Here we review the role of redox-dependent signaling pathways and transcription factors that regulate tumorigenesis.

Hydrogen peroxide as a cell-survival signaling molecule

Reactive oxygen species (ROS) were seen as destructive molecules, but recently, they have been shown also to act as second messengers in varying intracellular signaling pathways. This review concentrates on hydrogen peroxide (H2O2), as it is a more stable ROS, and delineates its role as a survival molecule. In the first part, the production of H2O2 through the NADPH oxidase (Nox) family is investigated. Through careful examination of Nox proteins and their regulation, it is determined how they respond to stress and how this can be prosurvival rather than prodeath. The pathways on which H2O2 acts to enable its prosurvival function are then examined in greater detail. The main survival pathways are kinase driven, and oxidation of cysteines in the active sites of various phosphatases can thus regulate those survival pathways. Regulation of transcription factors such as p53, NF-kappaB, and AP-1 also are reviewed. Finally, prodeath proteins such as caspases could be directly inhibited through their cysteine residues. A better understanding of the prosurvival role of H2O2 in cells, from the why and how it is generated to the various molecules it can affect, will allow more precise targeting of therapeutics to this pathway.

Sphingosine 1-phosphate increases glucose uptake through trans-activation of insulin receptor

Sphingosine 1-phosphate (S1P) is a bioactive lipid that acts through a family of G-protein-coupled receptors. Herein, we report evidence of a novel redox-based cross-talk between S1P and insulin signaling pathways. In skeletal muscle cells S1P, through engagement of its S1P(2) receptor, is found to produce a transient burst of reactive oxygen species through a calcium-dependent activation of the small GTPase Rac1. S1P-induced redox-signaling is sensed by protein tyrosine phosphatase-1B, the main negative regulator of insulin receptor phosphorylation, which undergoes oxidation and enzymatic inhibition. This redox-based inhibition of the phosphatase provokes a ligand-independent trans-phosphorylation of insulin receptor and a strong increase in glucose uptake. Our results propose a new role of S1P, recognizing the lipid as an insulin-mimetic cue and pointing at reactive oxygen species as critical regulators of the cross-talk between S1P and insulin pathways. Any possible implication of S1P-directed insulin signaling in diabetes and insulin resistance remains to be established.

Weak oligomerization of low-molecular-weight protein tyrosine phosphatase is conserved from mammals to bacteria

The well-characterized self-association of a mammalian low-molecular-weight protein tyrosine phosphatase (lmwPTP) produces inactive oligomers that are in equilibrium with active monomers. A role of the inactive oligomers as supramolecular proenzymes has been suggested. The oligomerization equilibrium of YwlE, a lmwPTP from Bacillus subtilis, was studied by NMR. Chemical shift data and NMR relaxation confirm that dimerization takes place through the enzyme's active site, and is fully equivalent to the dimerization previously characterized in a eukaryotic low-molecular-weight phosphatase, with similarly large dissociation constants. The similarity between the oligomerization of prokaryotic and eukaryotic phosphatases extends beyond the dimer and involves higher order oligomers detected by NMR relaxation analysis at high protein concentrations. The conservation across different kingdoms of life suggests a physiological role for lmwPTP oligomerization in spite of the weak association observed in vitro. Structural data suggest that substrate modulation of the oligomerization equilibrium could be a regulatory mechanism leading to the generation of signaling pulses. The presence of a phenylalanine residue in the dimerization site of YwlE, replacing a tyrosine residue conserved in all eukaryotic lmwPTPs, demonstrates that lmwPTP regulation by oligomerization can be independent from tyrosine phosphorylation.

Reactive oxygen species mediate mitogenic growth factor signaling pathways in human leiomyoma smooth muscle cells

Uterine leiomyomas are benign uterine tumors characterized by extracellular matrix remodeling, increased collagen deposition, and increased smooth muscle cell (SMC) proliferation. The reactive oxygen species (ROS) producing NADPH oxidase complex has been shown to be involved in the signaling pathways of several growth factors, cytokines, and vasoactive agents that stimulate proliferation of a variety of cell types. Our objective was to test the hypothesis that ROS derived from NADPH oxidase is a necessary component of the MAP kinase mitogenic pathway activated by platelet derived growth factor (PDGF) and epidermal growth factor (EGF) in leiomyoma SMCs (LSMCs). Primary cell cultures of LSMCs were used as our experimental model. Our results showed that stimulation of these cells with PDGF or EGF caused a marked increase in intracellular ROS production and that the NADPH oxidase inhibitor, DPI, blocks ROS production. In addition, inhibition of ROS production by NADPH oxidase inhibitors blocked, in a dose-dependent manner, the EGF- and PDGF-induced increase in [(3)H]thymidine incorporation by LSMCs. Furthermore, an exogenous source of ROS, hydrogen peroxide, was sufficient to stimulate [(3)H]thymidine incorporation in LSMCs but did not affect COL1A2 and COL3A1 mRNA levels. Inhibition of the NADPH oxidase complex decreased PDGF-induced MAPK1/MAPK3 activation, whereas exogenous hydrogen peroxide induced MAPK1/MAPK3 activation. This article is the first report suggesting the presence of the NADPH oxidase system and its importance in mitogenic signaling pathways in LSMCs. The necessity of NADPH oxidase-derived ROS for EGF and PDGF signaling pathways leading to cell proliferation points to another potential therapeutic target for treatment and/or prevention of uterine leiomyomas.

Roles of Nox1 and other Nox isoforms in cancer development

The NADPH oxidase (Nox) family of enzymes generates reactive oxygen species (ROS). At low ROS concentration, intracellular signaling is initiated, whereas at high ROS concentration, oxidative stress is induced. The extensive studies over the years have shed light on the mediating roles of the Nox enzymes in a variety of normal physiological processes ranging from bactericidal activity to remodeling of the extracellular matrix. Consequently, imbalance of Nox activities could be the potential cause of acute or chronic diseases. With regard to functional relationships between Nox isoforms and pathogenesis, it is of particular interest to study whether they are involved in carcinogenesis, because overproduction of ROS has long been implicated as a risk factor in cancer development. We see one remarkable example of the causal relationship between Nox1 and cancer in Ras oncogene-induced cell transformation. Other studies also indicate that the Nox family of genes appears to be required for survival and growth of a subset of human cancer cells. Thus, the Nox family will be a focus of attention in cancer biology and etiology over the next couple years.

Crystal structure of the human lymphoid tyrosine phosphatase catalytic domain: insights into redox regulation

The lymphoid tyrosine phosphatase (LYP), encoded by the PTPN22 gene, recently emerged as an important risk factor and drug target for human autoimmunity. Here we solved the structure of the catalytic domain of LYP, which revealed noticeable differences with previously published structures. The active center with a semi-closed conformation binds a phosphate ion, which may represent an intermediate conformation after dephosphorylation of the substrate but before release of the phosphate product. The structure also revealed an unusual disulfide bond formed between the catalytic Cys and one of the two Cys residues nearby, which is not observed in previously determined structures. Our structural and mutagenesis data suggest that the disulfide bond may play a role in protecting the enzyme from irreversible oxidation. Surprisingly, we found that the two noncatalytic Cys around the active center exert an opposite yin-yang regulation on the catalytic Cys activity. These detailed structural and functional characterizations have provided new insights into autoregulatory mechanisms of LYP function.

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.

Redox control of platelet function

There has recently been a dramatic expansion in research in the area of redox biology with systems that utilize thiols to perform redox chemistry being central to redox control. Thiol-based reactions occur in proteins involved in platelet function, including extracellular platelet proteins. The alphaIIbbeta3 fibrinogen receptor contains free thiols that are required for the activation of this receptor to a fibrinogen-binding conformation. This process is under enzymatic control, with protein disulfide isomerase playing a central role in the activation of alphaIIbbeta3. Other integrins, such as the alpha2beta1 collagen receptor on platelets, are also regulated by protein disulfide isomerase and thiol metabolism. Low molecular weight thiols that are found in blood regulate these processes by converting redox sensitive disulfide bonds to thiols and by providing the appropriate redox potential for these reactions. Additional mechanisms of redox control of platelets involve nitric oxide that inhibits platelet responses, and reactive oxygen species that potentiate platelet thrombus formation. Specific nitrosative or oxidative modifications of thiol groups in platelets may modulate platelet function. Since many biologic processes are regulated by redox reactions that involve surface thiols, the extracellular redox state can have an important influence on health and disease status and may be a target for therapeutic intervention.

Redox regulation and trapping sulfenic acid in the peroxide-sensitive human mitochondrial branched chain aminotransferase

The human branched chain aminotransferase enzymes are key regulators of glutamate metabolism in the brain and are among a growing number of redox-sensitive proteins. Studies that use thiol-specific reagents and electrospray ionization mass spectrometry demonstrate that the mitochondrial BCAT enzyme has a redox-active CXXC center, which on oxidation forms a disulfide bond (RSSR), via a cysteine sulfenic acid intermediate. Mechanistic details of this redox regulation were revealed by the use of mass spectrometry and dimedone modification. We discovered that the thiol group at position C315 of the CXXC motif acts a redox sensor, whereas the thiol group at position C318 permits reversible regulation by forming an intrasubunit disulphide bond. Because of their roles in redox regulation and catalysis, there is a growing interest in cysteine sulphenic acids. Therefore, development of chemical tags/methods to trap these transient intermediates is of immense importance.

CD36 modulates migration of mouse and human macrophages in response to oxidized LDL and may contribute to macrophage trapping in the arterial intima

The trapping of lipid-laden macrophages in the arterial intima is a critical but reversible step in atherogenesis. However, the mechanism by which this occurs is not clearly defined. Here, we tested in mice the hypothesis that CD36, a class B scavenger receptor expressed on macrophages, has a role in this process. Using both in vivo and in vitro migration assays, we found that oxidized LDL (oxLDL), but not native LDL, inhibited migration of WT mouse macrophages but not CD36-deficient cells. We further observed a crucial role for CD36 in modulating the in vitro migratory response of human peripheral blood monocyte-derived macrophages to oxLDL. oxLDL also induced rapid spreading and actin polymerization in CD36-sufficient but not CD36-deficient mouse macrophages in vitro. The underlying mechanism was dependent on oxLDL-mediated CD36 signaling, which resulted in sustained activation of focal adhesion kinase (FAK) and inactivation of Src homology 2-containing phosphotyrosine phosphatase (SHP-2). The latter was due to NADPH oxidase-mediated ROS generation, resulting in oxidative inactivation of critical cysteine residues in the SHP-2-active site. Macrophage migration in the presence of oxLDL was restored by both antioxidants and NADPH oxidase inhibitors, which restored the dynamic activation of FAK. We conclude therefore that CD36 signaling in response to oxLDL alters cytoskeletal dynamics to enhance macrophage spreading, inhibiting migration. This may induce trapping of macrophages in the arterial intima and promote atherosclerosis.

Leukocyte antigen-related protein tyrosine phosphatase negatively regulates hydrogen peroxide-induced vascular smooth muscle cell apoptosis

Reactive oxygen species (ROS) have been implicated in vascular smooth muscle cell (VSMC) apoptosis, a hallmark of advanced atherosclerotic lesions. Transient oxidation and inactivation of protein-tyrosine phosphatases play a critical role in cellular response to ROS production. However, the function of leukocyte antigen-related (LAR) protein-tyrosine phosphatase in ROS signaling is not known. To determine the expression of LAR in ROS-induced apoptosis, we investigated hydrogen peroxide-induced cell death and signaling in aortic VSMCs from wild-type and LAR(-/-) mice. Histone-associated DNA fragmentation and caspase-3/7 activity were significantly enhanced, mitochondrial membrane integrity was compromised, and cell viability was significantly decreased following H(2)O(2) treatment in LAR(-/-) VSMCs compared with wild-type cells. Stronger and sustained increase in autophosphorylation and activity of Fyn, an Src family tyrosine kinase, was observed in LAR(-/-) cells compared with wild-type cells following H(2)O(2) treatment. LAR binds to activated Fyn in H(2)O(2)-treated VSMCs, and recombinant LAR dephosphorylates phosphorylated-Fyn in vitro. In addition, LAR deficiency enhanced H(2)O(2)-induced phosphorylation of Janus kinase 2 (JAK2), signal transducer and activator of transcription 3 (STAT3), and p38 mitogen-activated protein kinase (MAPK). PP2, a Fyn-specific inhibitor, blocked JAK2, STAT3, and p38 MAPK activation and significantly attenuated apoptosis induced by H(2)O(2). AG490, a JAK2-specific inhibitor, significantly attenuated H(2)O(2)-induced apoptosis, and blocked H(2)O(2)-induced activation of STAT3, but not p38 MAPK in both wild-type and LAR(-/-) VSMCs. Attenuation of Fyn expression by short hairpin RNA significantly decreased H(2)O(2)-induced downstream signaling and apoptosis in VSMCs. Together, these data indicate that LAR regulates Fyn/JAK2/STAT3 and Fyn/p38 MAPK pathways involved in ROS-induced apoptosis.

Redox control of endothelial function and dysfunction: molecular mechanisms and therapeutic opportunities

The endothelium is essential for the maintenance of vascular homeostasis. Central to this role is the production of endothelium-derived nitric oxide (EDNO), synthesized by the endothelial isoform of nitric oxide synthase (eNOS). Endothelial dysfunction, manifested as impaired EDNO bioactivity, is an important early event in the development of various vascular diseases, including hypertension, diabetes, and atherosclerosis. The degree of impairment of EDNO bioactivity is a determinant of future vascular complications. Accordingly, growing interest exists in defining the pathologic mechanisms involved. Considerable evidence supports a causal role for the enhanced production of reactive oxygen species (ROS) by vascular cells. ROS directly inactivate EDNO, act as cell-signaling molecules, and promote protein dysfunction, events that contribute to the initiation and progression of endothelial dysfunction. Increasing data indicate that strategies designed to limit vascular ROS production can restore endothelial function in humans with vascular complications. The purpose of this review is to outline the various ways in which ROS can influence endothelial function and dysfunction, describe the redox mechanisms involved, and discuss approaches for preventing endothelial dysfunction that may highlight future therapeutic opportunities in the treatment of cardiovascular disease.

The chemistry of cell signaling by reactive oxygen and nitrogen species and 4-hydroxynonenal

During the past several years, major advances have been made in understanding how reactive oxygen species (ROS) and nitrogen species (RNS) participate in signal transduction. Identification of the specific targets and the chemical reactions involved still remains to be resolved with many of the signaling pathways in which the involvement of reactive species has been determined. Our understanding is that ROS and RNS have second messenger roles. While cysteine residues in the thiolate (ionized) form found in several classes of signaling proteins can be specific targets for reaction with H(2)O(2) and RNS, better understanding of the chemistry, particularly kinetics, suggests that for many signaling events in which ROS and RNS participate, enzymatic catalysis is more likely to be involved than non-enzymatic reaction. Due to increased interest in how oxidation products, particularly lipid peroxidation products, also are involved with signaling, a review of signaling by 4-hydroxy-2-nonenal (HNE) is included. This article focuses on the chemistry of signaling by ROS, RNS, and HNE and will describe reactions with selected target proteins as representatives of the mechanisms rather attempt to comprehensively review the many signaling pathways in which the reactive species are involved.

Thiol chemistry and specificity in redox signaling

Exposure of cells to sublethal oxidative stress results in the modulation of various signaling pathways. Oxidants can activate and inactivate transcription factors, membrane channels, and metabolic enzymes, and regulate calcium-dependent and phosphorylation signaling pathways. Oxidation and reduction of thiol proteins are thought to be the major mechanisms by which reactive oxidants integrate into cellular signal transduction pathways. This review focuses on mechanisms for sensing and transmitting redox signals, from the perspective of their chemical reactivity with specific oxidants. We discuss substrate preferences for different oxidants and how the kinetics of these reactions determines how each oxidant will react in a cell. This kinetic approach helps to identify initial oxidant-sensitive targets and elucidate mechanisms involved in transmission of redox signals. It indicates that only those proteins with very high reactivity, such as peroxiredoxins, are likely to be direct targets for hydrogen peroxide. Other more modestly reactive thiol proteins such as protein tyrosine phosphatases are more likely to become oxidized by an indirect mechanism. The review also examines oxidative changes observed during receptor-mediated signaling, the strengths and limitations of detection methods for reactive oxidant production, and the evidence for hydrogen peroxide acting as the second messenger. We discuss areas where observations in cell systems can be rationalized with the reactivity of specific oxidants and where further work is needed to understand the mechanisms involved.

Spatially distinct production of reactive oxygen species regulates platelet activation

Platelets play a key role in hemostasis and changes in redox balance are known to alter platelet activation and aggregation. Interestingly, activation of platelets leads to production of reactive oxygen species (ROS), but the role(s) of these ROS remain unclear. Using flow cytometry and chemiluminescence, agonist-induced ROS generation was found to be spatially distinct with stimulation through the major collagen receptor GPVI inducing only intraplatelet ROS while thrombin induced production of extracellular ROS. Platelet activation by either the GPVI-selective agonist convulxin or thrombin was differentially regulated by ROS generation. Thus, surface expression of CD62P, CD40L, or activated integrin alphaIIbbeta3 was abrogated by pharmacologic antioxidants but externalization of phosphatidylserine was not inhibited. Furthermore, extracellular antioxidants SOD/catalase markedly inhibited thrombin-, but not convulxin-, induced CD62P expression and alphaIIbbeta3 activation. The data suggest that ROS selectively regulate biochemical steps in platelet activation and that distinct source(s) of ROS and discrete redox-sensitive pathway(s) may control platelet activation in response to GPVI or thrombin stimulation. Thus, targeting ROS with site-specific antioxidants may differentially regulate platelet activation via thrombin or collagen.

NADPH oxidase 1 plays a critical mediating role in oncogenic Ras-induced vascular endothelial growth factor expression

Reactive oxygen species (ROS)-generating enzyme Nox1 is important in the induction of oncogenic Ras transformation phenotypes, but it is not defined whether Nox1 is involved in Ras-induced upregulation of vascular endothelial growth factor (VEGF), a potent stimulator of tumor angiogenesis. Here we describe that ablation of the Nox1 activity by Nox1 small-interference RNAs (siRNAs) or diphenylene iodonium (DPI) inhibited synthesis of both VEGF proteins and VEGF mRNAs in K-Ras transformed normal rat kidney (KNRK) cells. Nox1siRNAs and DPI suppressed extracellular signal-regulated kinase (ERK)-dependent phosphorylation of a transcription factor Sp1 and Sp1 binding to a VEGF promoter. Furthermore, tumors derived from Nox1siRNA-transfected KNRK cells markedly decreased neovascularization. The Nox1 activity was required for VEGF production in human colon cancer CaCO-2 cells, as in the case of KNRK cells. However, since overexpression of Nox1 in normal rat kidney cells failed to induce VEGF, the Nox1 activity alone was not sufficient to upregulate VEGF expression, which suggests that unlike the previously proposed model, Nox1 may act in concert with other effectors integrated into the Ras network. We propose that Nox1 mediates oncogenic Ras-induced upregulation of VEGF and angiogenesis by activating Sp1 through Ras-ERK-dependent phosphorylation of Sp1.

Protein tyrosine phosphatases: structure-function relationships

Structural analysis of protein tyrosine phosphatases (PTPs) has expanded considerably in the last several years, producing more than 200 structures in this class of enzymes (from 35 different proteins and their complexes with ligands). The small-medium size of the catalytic domain of approximately 280 residues plus a very compact fold makes it amenable to cloning and overexpression in bacterial systems thus facilitating crystallographic analysis. The low molecular weight PTPs being even smaller, approximately 150 residues, are also perfect targets for NMR analysis. The availability of different structures and complexes of PTPs with substrates and inhibitors has provided a wealth of information with profound effects in the way we understand their biological functions. Developments in mammalian expression technology recently led to the first crystal structure of a receptor-like PTP extracellular region. Altogether, the PTP structural work significantly advanced our knowledge regarding the architecture, regulation and substrate specificity of these enzymes. In this review, we compile the most prominent structural traits that characterize PTPs and their complexes with ligands. We discuss how the data can be used to design further functional experiments and as a basis for drug design given that many PTPs are now considered strategic therapeutic targets for human diseases such as diabetes and cancer.

NADPH oxidase promotes pancreatic cancer cell survival via inhibiting JAK2 dephosphorylation by tyrosine phosphatases

BACKGROUND & AIMS

Growth factors, such as insulin-like growth factor-1 (IGF-I), protect pancreatic cancer (PaCa) cells from death. We recently showed that reactive oxygen species (ROS) produced by nicotinamide adenine dinucleotide phosphate (NADPH) oxidase Nox4 mediate the antiapoptotic effect of growth factors. Here, we examine the mechanisms of the antiapoptotic role of NADPH oxidase. We hypothesized that ROSs produced by NADPH oxidase inhibit key protein tyrosine phosphatases (PTPs) and thus sustain the activation of kinases mediating antiapoptotic pathways in PaCa cells.

METHODS

Transfections and pharmacologic inhibition were used to assess the effects of NADPH oxidase on Janus kinase 2 (JAK2) kinase, the low molecular weight-protein tyrosine phosphatase (LMW-PTP), and apoptosis.

RESULTS

We found that 1 target of ROSs is JAK2, an important antiapoptotic kinase in PaCa cells. Both serum-induced and IGF-I biphasic JAK2 phosphorylation, with a rapid (minutes) and transient first phase, and a slow and sustained (24-72 hours) second phase. Nox4 mediated the sustained phase of JAK2 phosphorylation, which was required for the antiapoptotic effects of IGF-I and serum. Transfection experiments identified the LMW-PTP as a negative regulator of sustained JAK2 phosphorylation. Growth factors inhibited LMW-PTP through its oxidation by NADPH oxidase. LMW-PTP colocalizes with Nox4 both in PaCa cells and in human pancreatic adenocarcinoma.

CONCLUSIONS

The results suggest a novel signaling pathway, in which NADPH oxidase activation results in inhibition of PTPs, such as LMW-PTP, leading, in turn, to enhanced and sustained phosphorylation of kinases, such as JAK2, and suppression of apoptosis. This pathway mediates the prosurvival effect of ROSs and suggests new targets for pancreatic cancer treatment.

Shear-induced disulfide bond formation regulates adhesion activity of von Willebrand factor

von Willebrand factor (VWF) is the largest multimeric adhesion ligand circulating in blood. Its adhesion activity is related to multimer size, with the ultra-large forms freshly released from the activated endothelial cells being most active, capable of spontaneously binding to platelets. In comparison, smaller plasma forms circulating in blood bind platelets only under high fluid shear stress or induced by modulators. The structure-function relationships that distinguish the two types of VWF multimers are not known. In this study, we demonstrate that some of the plasma VWF multimers contain surface-exposed free thiols. Physiological and pathological levels of shear stresses (50 and 100 dynes/cm(2)) promote the formation of disulfide bonds utilizing these free thiols. The shear-induced thiol-disulfide exchange increases VWF binding to platelets. The thiol-disulfide exchange involves some or all of nine cysteine residues (Cys(889), Cys(898), Cys(2448), Cys(2451), Cys(2490), Cys(2491), Cys(2453), Cys(2528), and Cys(2533)) in the D3 and C domains as determined by mass spectrometry of the tryptic VWF peptides. These results suggest that the thiol-disulfide state may serve as an important structural determinant of VWF adhesion activity and can be modified by fluid shear stress.

The low-molecular-weight phosphotyrosine phosphatase is a negative regulator of FcγRIIA-mediated cell activation

Activation of human platelets by cross-linking of the low-affinity receptor for immunoglobulin G (FcγRIIA) is initiated by Src kinase–mediated phosphorylation of the immunoreceptor tyrosine–based activation motif (ITAM) within the receptor, but the identity of the enzyme responsible for its dephosphorylation and inactivation is unknown. Here we report that the 18-kDa low-molecular-weight phosphotyrosine phosphatase (LMW-PTP) is expressed in human platelets and undergoes subcellular redistribution upon FcγRIIA cross-linking. In vitro, LMW-PTP was found to efficiently dephosphorylate activated FcγRIIA and LAT, but not Syk or phospholipase Cγ2. In the megakaryocytic cell line DAMI, antibody-induced phosphorylation of FcγRIIA was rapid and transient. The late dephosphorylation of FcγRIIA was dramatically delayed upon reduction of LMW-PTP expression by siRNA. Strikingly, overexpression of LMW-PTP resulted in the inhibition of antibody-induced phosphorylation of FcγRIIA, and caused a more rapid dephosphorylation. In addition, overexpression of LMW-PTP inhibited activation of Syk downstream of FcγRIIA and reduced intracellular Ca2+ mobilization. These results demonstrate that LMW-PTP is responsible for FcγRIIA dephosphorylation, and is implicated in the down-regulation of cell activation mediated by this ITAM-bearing immunoreceptor.

Redox-dependent and ligand-independent trans-activation of insulin receptor by globular adiponectin

Adiponectin/ACRP30 is an adipose tissue-derived hormone with antiatherogenic, antidiabetic, and insulin-sensitizing properties. Although the metabolic effects of adiponectin on glucose and lipid metabolism are well known, the signaling pathways triggered by adiponectin receptors remain to be elucidated. We report evidence that in hepatic cells, adiponectin stimulation produces a transient burst of reactive oxygen species (ROS) through activation of the small GTPase Rac1 and 5-lypoxigenase. Furthermore, adiponectin-induced oxidants cause the oxidation/inhibition of protein-tyrosine phosphatase (PTP) 1B, one of the major phosphotyrosine phosphatases involved in the control of insulin receptor phosphorylation. Adiponectin causes increased association of PTP1B to insulin receptor and the oxidation/inhibition of the phosphatase, ultimately provoking the ligand-independent trans-phosphorylation of insulin receptor. We also report evidence that redox signaling plays a key role in both mitogen-activated protein kinase activation and hepatic glucose consumption induced by adiponectin.

CONCLUSION

These results point to ROS as critical regulators of the cross-talk between adiponectin and insulin pathways and provide a redox-based molecular mechanism for the insulin-sensitizing function of adiponectin.

Oxidative stress-induced expression and modulation of Phosphatase of Regenerating Liver-1 (PRL-1) in mammalian retina

The phosphatase of regenerating liver-1, PRL-1, gene was detected in a screen for foveal cone photoreceptor-associated genes. It encodes a small protein tyrosine phosphatase that was previously immunolocalized to the photoreceptors in primate retina. Here we report that in cones and cone-derived cultured cells both PRL-1 activity and PRL-1 gene expression are modulated under oxidative stress. Oxidation reversibly inhibited the phosphatase activity of PRL-1 due to the formation of an intramolecular disulfide bridge between Cys104 within the active site and another conserved Cys, Cys49. This modulation was observed in vitro, in cell culture and in isolated retinas exposed to hydrogen peroxide. The same treatment caused a rapid increase in PRL-1 expression levels in cultured cells which could be blocked by the protein translation inhibitor, cycloheximide. Increased PRL-1 expression was also observed in living rats subjected to constant light exposure inducing photooxidative stress. We further demonstrated that both oxidation and overexpression of PRL-1 upon oxidative stress are greatly enhanced by inhibition of the glutathione system responsible for cellular redox regulation. These findings suggest that PRL-1 is a molecular component of the photoreceptor's response to oxidative stress acting upstream of the glutathione system.

Redox regulation of protein tyrosine phosphatase 1B (PTP1B): a biomimetic study on the unexpected formation of a sulfenyl amide intermediate

The effect of steric and electronic environments around the sulfur and nitrogen atoms and the role of nonbonded S...O/N interactions on the cyclization reactions of amide substituted benzene sulfenic acids are described. The reaction profiles and the role of different substituents on the cyclization are investigated in detail by theoretical calculations. It is shown that the synthetic thiols having ortho-amide substituents may serve as good models for the enforced proximity of the amide and cysteine thiol groups at the active site of protein tyrosine phosphatase 1B (PTP1B). However, some of the sulfenic acids derived from such models do not effectively mimic the cyclization of protein sulfenic acids. This is mainly due to the requirement of very high energy for breaking the S-O bond to form a planar five-membered ring of isothiazolidinone. It is shown that the sulfenic acid having two substituents-an amide moiety and a heterocyclic group-in the ortho-positions undergoes a rapid cyclization reaction to produce the corresponding sulfenyl amide species. These studies reveal that the introduction of a substituent at the 6-position of the benzene ring enhances the cyclization process not only by facilitating a closer approach of the -OH group and the backbone -NH moiety but also by increasing the electrophilicity of the sulfur atom in the sulfenic acid.

Hydrogen peroxide sensing and signaling

It is well established that oxidative stress is an important cause of cell damage associated with the initiation and progression of many diseases. Consequently, all air-living organisms contain antioxidant enzymes that limit oxidative stress by detoxifying reactive oxygen species, including hydrogen peroxide. However, in eukaryotes, hydrogen peroxide also has important roles as a signaling molecule in the regulation of a variety of biological processes. Here, we will discuss the molecular mechanisms by which hydrogen peroxide is sensed and the increasing evidence that antioxidant enzymes play multiple, key roles as sensors and regulators of signal transduction in response to hydrogen peroxide.

Low molecular weight protein tyrosine phosphatase (LMW-PTP) and its possible physiological functions of redox signaling in the eye lens

Low molecular weight protein tyrosine phosphatase (LMW-PTP) was cloned from human lens epithelial B3 cells (HLE B3) and the recombinant enzyme was purified to homogeneity. The pure enzyme reacted positively with anti-LMW-PTP antibody, displayed tyrosine-specific phosphatase activity and was extremely sensitive to H(2)O(2). The inactivated LMW-PTP could be regenerated by thioltransferase (TTase)/GSH system as demonstrated by both activity assay and by mass spectrometry (MS). The MS study also showed that an intramolecular disulfide bond was formed between C13 and C18 at the active site, and was reduced by the TTase/GSH system. The putative role of LMW-PTP in regulating platelet derived growth factor (PDGF)-stimulated cell signaling was demonstrated in wild type mouse lens epithelial cells (LEC) in which LMW-PTP was transiently inactivated, corroborated with the transient phosphorylation of Tyr857 at the active site of PDGF receptor and the downstream signaling components of Akt and ERK1/2. In contrast, LMW-PTP activity in PDGF-stimulated LEC from TTase(-/-) mice was progressively lost, concomitant with the high basal and sustained high phosphorylation levels at Tyr857, Akt and ERK1/2. We conclude that the reversible LMW-PTP activity regulated by ROS-mediated oxidation and TTase/GSH reduction is the likely mechanism of redox signaling in lens epithelial cells.

Low Molecular Weight Protein Tyrosine Phosphatase and Caveolin-1: Interaction and Isoenzyme-Dependent Regulation

Low molecular weight protein tyrosine phosphatases (LMW-PTPs) are small enzymes that are ubiquitous in many organisms. They are important in biological processes such as cell proliferation, adhesion, migration, and invasiveness. LMW-PTP is expressed in mammalian cells as two isoforms (IF1 and IF2) originating through alternative splicing. We have previously shown that IF2 targets lipid rafts called caveolae and interacts with caveolin-1, their major structural protein. Caveolae are cholesterol- and sphingolipid-rich membrane microdomains that have been implicated in a variety of cellular functions, including signal transduction events. Caveolin-1 contains a scaffolding region that contributes to the binding of the protein to the plasma membrane and mediates protein omo- and etero-oligomerization. Interaction of many signaling molecules with the scaffolding domain sequesters them into caveolae and inhibits or suppresses their activities. Caveolin-interacting proteins usually have a typical sequence motif, also present in all the LMW-PTPs, which is characterized by aromatic or large hydrophobic residues in specific positions. We have examined here the interaction of the LMW-PTP isoforms with caveolin-1 and its molecular mechanism, together with the consequences for their tyrosine phosphatase activities. We found that IF1 and IF2 are both capable of interacting with defined regions of caveolin-1 and that their putative caveolin binding sequence motif is not responsible for the association. The formation of LMW-PTP/caveolin-1 complexes is accompanied by modulation of the enzyme activities, and the inhibitory effect elicited against IF1 is stronger than that against IF2. The caveolin scaffolding domain is directly involved in the observed phenomena.

Nox1 redox signaling mediates oncogenic Ras-induced disruption of stress fibers and focal adhesions by down-regulating Rho

Generation of reactive oxygen species (ROS) by Ras oncogene-induced NADPH oxidase (Nox) 1 is required for Ras transformation phenotypes including anchorage-independent growth, morphological transformation, and tumorigenesity, but the signaling mechanism downstream of Nox1 remains elusive. Rho is known to be a critical regulator of actin stress fiber formation. Nonetheless, Rho was reported to no longer couple to loss of actin stress fibers in Ras-transformed Swiss3T3 cells despite the elevation of Rho activity. In this study, however, we demonstrate that Rho is inactivated in K-Ras-transformed normal rat kidney cells, and that abrogation of Nox1-generated ROS by Nox1 small interference RNAs or diphenyleneiodonium restores Rho activation, suggesting that Nox1-generated oxidants mediate down-regulation of the Rho activity. This down-regulation involves oxidative inactivation of the low molecular weight protein-tyrosine phosphatase by Nox1-generated ROS and a subsequent elevation in the tyrosine-phosphorylated active form of p190RhoGAP, the direct target of the phosphatase. Furthermore, the decreased Rho activity leads to disruption of both actin stress fibers and focal adhesions in Ras-transformed cells. As for Rac1, Rac1 also appears to participate in the down-regulation of Rho via Nox1. Our discovery defines a mediating role of Nox1-redox signaling for Ras oncogene-induced actin cytoskeletal changes.

Redox signalling in anchorage-dependent cell growth

Current data have provided new perspectives concerning the regulation of non-transformed cell proliferation in response to both soluble growth factors and to adhesive cues. Non-transformed cells are anchorage dependent for the execution of the mitotic program and cannot avoid the concomitant signals starting from mitogenic molecules, as growth factors, and adhesive agents belonging to extracellular matrix. Reactive oxygen species play a key role during both growth factor and integrin receptor signalling and these second messengers are recognised to have a synergistic function for anchorage-dependent growth signalling. Redox regulated proteins include protein tyrosine phosphatases and protein tyrosine kinases, although with opposite regulation of their enzymatic activity, and cytoskeletal proteins as beta-actin. In this review we support a role of ROS as key second messengers granting a proper executed mitosis for anchorage-dependent cells, through redox regulation of several downstream targets. Deregulation of these redox pathways may help to guide transformed cells to elude the native apoptotic response to abolishment of signals started by cell/ECM contact, sustaining ectopic anchorage-independent cancer cell growth.

Use and abuse of exogenous H2O2 in studies of signal transduction

The goal of this review is to present a rationale for the use of exogenous H(2)O(2), which has been demonstrated to have both toxicological and physiological signaling roles. Reasons for the use of exogenous application of nontoxic concentrations of H(2)O(2) in model systems and caveats for interpretation of the data obtained will both be presented. Briefly, an argument for the cautious use of the addition of exogenous H(2)O(2) is that, because of the permeability of cell membranes to this neutral small molecule, a concentration that is produced locally and that is necessary for the physiological action can be mimicked. On the other hand, it must be recognized that the addition of an agent or its enzymatic generation in the medium may produce reactions that may not normally occur because the total dose of H(2)O(2) and the concentration of H(2)O(2) in some cellular locations will exceed what is normally achieved even under a pathophysiological state. For this reason, this review will try to provide an unbiased balanced pros- and -cons analysis of this issue.

Integrin-mediated cell adhesion and spreading engage different sources of reactive oxygen species

The tightly regulated production of intracellular reactive oxygen species (ROS) participates in several biologic processes such as cellular growth, programmed cell death, senescence, and adhesion. It is increasingly evident that the same enzymatic processes that were originally linked to ROS generation during host defence or apoptosis execution are also involved in redox-mediated signal transduction. We investigated in murine NIH3T3 fibroblasts the contribution of a variety of redox-dependent events during signal transduction initiated by integrin engagement due to fibronectin stimulation and report that a mitochondrial ROS release occurs, strictly confined to the early phase of extracellular matrix (ECM) contact (10 min). Besides, 5-lipoxygenase (5-LOX) is engaged by integrin receptor ligation as another ROS source, contributing to the more-intense, second ROS burst (45 min), possibly orchestrating the spreading of cells in response to ECM contact. To define a potential mechanism for ROS signaling, we demonstrate that on integrin recruitment, the Src homology-2 domain-containing phosphatase 2 (SHP-2) undergoes a reversible oxidization/inactivation to which mitochondrial and 5-lipoxygenase ROS contribute differentially. In keeping with a key role of oxidants during integrin signaling, the inactivation of SHP-2 prevents the dephosphorylation and inactivation of SHP-2 substrates (p125FAK and SHPS-1), thus enabling the continued propagation of the signal arising by integrin engagement.

Protein tyrosine phosphorylation and reversible oxidation: two cross-talking posttranslation modifications

In addition to protein phosphorylation, redox-dependent posttranslational modification of proteins is emerging as a key signaling system, conserved throughout evolution, and influencing many aspects of cellular homeostasis. Recent data have provided new insight about the interplay between phosphorylation- and redox-dependent signaling, and reactive oxygen species have been included among intracellular signal transducers of growth factor and extracellular matrix receptors. Both tyrosine phosphorylation and thiol oxidation are reversible and dynamic, and this review will particularly focus on the cross-talk between these posttranslational protein regulatory means. Although these modifications share their reversibility, their effects on enzymatic activity of protein tyrosine phosphatases (PTPs) and protein tyrosine kinases (PTKs) may be even opposite. Indeed, while tyrosine phosphorylation is frequently correlated to enzyme activation, thiol oxidation leads to inactivation of PTPs and to superactivation of PTKs. Several papers describe that both these modifications occur during the same input, (i.e., cell proliferation and motility induced by numerous growth factors and cytokines). The review will discuss several aspects of phosphorylation\oxidation interplay, describing both convergent and divergent features of the integrated and coordinated function of PTPs and PTKs during signaling.

Protein tyrosine phosphatases: from genes, to function, to disease

The protein tyrosine phosphatase (PTP) superfamily of enzymes functions in a coordinated manner with protein tyrosine kinases to control signalling pathways that underlie a broad spectrum of fundamental physiological processes. In this review, I describe recent breakthroughs in our understanding of the role of the PTPs in the regulation of signal transduction and the aetiology of human disease.

A novel redox-based switch: LMW-PTP oxidation enhances Grb2 binding and leads to ERK activation

Low molecular weight-PTP has been reported as a redox-sensitive protein during both platelet-derived growth factor and integrin signalling. In response to oxidation the phosphatase undergoes a reversible inactivation, which in turn leads to the increase in tyrosine phosphorylation of its substrates and the properly executed anchorage-dependent proliferation program. Here, we report that an exogenous oxidative stress enhances LMW-PTP tyrosine phosphorylation, through oxidation/inactivation of the enzyme, thus preventing its auto-dephosphorylation activity. In particular, we observed a selective hyper-phosphorylation of Tyr132, that acts as a docking site for the adaptor protein Grb2. The redox-dependent enhancement of Grb2 recruitment to LMW-PTP ultimately leads to an improvement of ERK activation, likely triggering a prosurvival signal against the oxidant environment.

Lipopolysaccharide Transiently Activates THP-1 Cell Adhesion

Lipopolysaccharide stimulation of adherent THP-1 cells induces morphological changes that are associated with the reorganization of the actin cytoskeleton. We hypothesized that LPS would also increase THP-1 cell adhesion and sought to determine the signaling mechanisms regulating this response. We show that LPS significantly increases THP-1 cell attachment after 1 h, supporting the idea that LPS can stimulate integrin function. By 4 h however, the number of adherent cells returned to control levels. Importantly, detached cells were determined to be viable by propidium iodide staining, indicating that the increase in cell adhesion was transient. LPS-induced adhesion to fibrinogen- but not fibronectin-coated wells was also transient, suggesting that adhesion reflected beta2 integrin activation. This idea was supported by the fact that LPS-induced adhesion could be blocked by a function-blocking anti-beta2 integrin antibody. Interestingly, the protein tyrosine phosphatase (PTP) inhibitor, phenylarsine oxide, prevented cell detachment. Taken together, these data suggest that LPS-mediated integrin activation is transient and can be regulated by PTP-mediated signaling events.

Specificity in reactive oxidant signaling: think globally, act locally

Although reactive oxidants have long been stigmatized as unwanted metabolic byproducts, the expression of oxidases specifically functioning to produce these same molecules in a regulated fashion is surprisingly pervasive throughout metazoan and plant evolution. Although the involvement of oxidants in many signaling pathways is well documented, the cellular strategies for conferring pathway specificity to such reactive molecules have remained more recondite. Recent studies now suggest that cells may spatially restrict oxidant production to allow microdomain-specific signaling.