Inactivation of a redox-sensitive protein phosphatase during the early events of tumor necrosis factor/interleukin-1 signal transduction

Redox Balance in Type 2 Diabetes: Therapeutic Potential and the Challenge of Antioxidant-Based Therapy

Oxidative stress is an important factor in the development of type 2 diabetes (T2D) and associated complications. Unfortunately, most clinical studies have failed to provide sufficient evidence regarding the benefits of antioxidants (AOXs) in treating this disease. Based on the known complexity of reactive oxygen species (ROS) functions in both the physiology and pathophysiology of glucose homeostasis, it is suggested that inappropriate dosing leads to the failure of AOXs in T2D treatment. To support this hypothesis, the role of oxidative stress in the pathophysiology of T2D is described, together with a summary of the evidence for the failure of AOXs in the management of diabetes. A comparison of preclinical and clinical studies indicates that suboptimal dosing of AOXs might explain the lack of benefits of AOXs. Conversely, the possibility that glycemic control might be adversely affected by excess AOXs is also considered, based on the role of ROS in insulin signaling. We suggest that AOX therapy should be given in a personalized manner according to the need, which is the presence and severity of oxidative stress. With the development of gold-standard biomarkers for oxidative stress, optimization of AOX therapy may be achieved to maximize the therapeutic potential of these agents.

Redox regulation of the insulin signalling pathway

The peptide hormone insulin is a key regulator of energy metabolism, proliferation and survival. Binding of insulin to its receptor activates the PI3K/AKT signalling pathway, which mediates fundamental cellular responses. Oxidants, in particular H₂O₂, have been recognised as insulin-mimetics. Treatment of cells with insulin leads to increased intracellular H₂O₂ levels affecting the activity of downstream signalling components, thereby amplifying insulin-mediated signal transduction. Specific molecular targets of insulin-stimulated H₂O₂ include phosphatases and kinases, whose activity can be altered via redox modifications of critical cysteine residues. Over the past decades, several of these redox-sensitive cysteines have been identified and their impact on insulin signalling evaluated. The aim of this review is to summarise the current knowledge on the redox regulation of the insulin signalling pathway.

Prolonged insulin stimulation down-regulates GLUT4 through oxidative stress-mediated retromer inhibition by a protein kinase CK2-dependent mechanism in 3T3-L1 adipocytes

Although insulin acutely stimulates glucose uptake by promotion of GLUT4 translocation from intracellular compartments to the plasma membrane in adipocytes and muscles, long term insulin stimulation causes GLUT4 depletion that is particularly prominent in the insulin-responsive GLUT4 storage compartment. This effect is caused mainly by accelerated lysosomal degradation of GLUT4, although the mechanism is not fully defined. Here we show that insulin acutely induced dissociation of retromer components from the low density microsomal membranes of 3T3-L1 adipocytes that was accompanied by disruption of the interaction of Vps35 with sortilin. This insulin effect was dependent on the activity of protein kinase CK2 but not phosphatidylinositol 3-kinase or extracellular signal-regulated kinase 1/2. Knockdown of Vps26 decreased GLUT4 to a level comparable with that with insulin stimulation for 4 h. Vps35 with a mutation in the CK2 phosphorylation motif (Vps35-S7A) was resistant to insulin-induced dissociation from the low density microsomal membrane, and its overexpression attenuated GLUT4 down-regulation with insulin. Furthermore, insulin-generated hydrogen peroxide was an upstream mediator of the insulin action on retromer and GLUT4. These results suggested that insulin-generated oxidative stress switches the GLUT4 sorting direction to lysosomes through inhibition of the retromer function in a CK2-dependent manner.

Do reciprocal interactions between cell stress proteins and cytokines create a new intra-/extra-cellular signalling nexus?

Cytokine biology began in the 1950s, and by 1988, a large number of cytokines, with a myriad of biological actions, had been discovered. In 1988, the basis of the protein chaperoning function of the heat shock, or cell stress, proteins was identified, and it was assumed that this was their major activity. However, since this time, evidence has accumulated to show that cell stress proteins are secreted by cells and can stimulate cellular cytokine synthesis with the generation of pro- and/or anti-inflammatory cytokine networks. Cell stress can also control cytokine synthesis, and cytokines are able to induce, or even inhibit, the synthesis of selected cell stress proteins and may also promote their release. How cell stress proteins control the formation of cytokines is not understood and how cytokines control cell stress protein synthesis depends on the cellular compartment experiencing stress, with cytoplasmic heat shock factor 1 (HSF1) having a variety of actions on cytokine gene transcription. The endoplasmic reticulum unfolded protein response also exhibits a complex set of behaviours in terms of control of cytokine synthesis. In addition, individual intracellular cell stress proteins, such as Hsp27 and Hsp90, have major roles in controlling cellular responses to cytokines and in controlling cytokine synthesis in response to exogenous factors. While still confusing, the literature supports the hypothesis that cell stress proteins and cytokines may generate complex intra- and extra-cellular networks, which function in the control of cells to external and internal stressors and suggests the cell stress response as a key parameter in cytokine network generation and, as a consequence, in control of immunity.

Oxidative stress induces inactivation of protein phosphatase 2A, promoting proinflammatory NF-κB in aged rat kidney

The molecular inflammation hypothesis of aging proposes that redox dysregulation causes an age-related activation of NF-κB and its signaling to upregulate various proinflammatory genes. In the present study, we focused on the inactive form of the protein phosphastase 2A (PP2A). More specifically, we aimed to define the correlation between PP2A inactivation and NF-κB activation by age-related oxidative stress. Experimentations were designed to determine the effect of oxidative stress-induced PP2A inactivation on NF-κB activity, utilizing prooxidants t-BHP and AAPH, the PTP inhibitor Na3VO4, and the PP2A inhibitor Calyculin A and PP2A siRNA, in HEK293T cells. We also assessed the phosphorylation of PP2A catalytic subunit (PP2Ac) and the activities of PP2A and NF-κB in aged rat kidney, utilizing aging-retarding 40% calorie restriction (CR) -60% of food intake and inflammation-triggering LPS paradigms. Results revealed that an oxidative stress-induced PTK/PTP imbalance led to phosphorylation of PP2Ac, following exposures to t-BHP, AAPH, and Na3VO4 in HEK293T cells. Subsequently, we found that Calyculin A and PP2A siRNA activates NIK/IKK and MAPKs, leading to upregulation of NF-κB and its dependent oxidative stress. Also, the contrasting relation between PP2A inactivation and NF-κB activation was confirmed by AAPH-induced oxidative status in mice, and non-induced normal status or LPS-induced inflammatory status in aged rats while the antioxidative, anti-inflammatory, anti-aging effects of CR significantly blunted these actions. Thus, we present evidence that PP2A inactivation via PTK/PTP imbalance provoked by oxidative stress causes NF-κB activation, which contributes to the accumulation of oxidative stress in aged rat kidney.

Acute exercise induce endothelial nitric oxide synthase phosphorylation via Akt and AMP-activated protein kinase in aorta of rats: Role of reactive oxygen species

BACKGROUND

Acute exercise increases reactive oxygen species (ROS) levels, including hydrogen peroxide (H2O2). H2O2 promotes endothelial nitric oxide synthase (eNOS) activation and phosphorylation in endothelial cells. With this in mind, the present study was designed to evaluate ex vivo eNOS phosphorylation in rat aortas incubated with H2O2 and to test this hypothesis in vivo in the aortas of rats submitted to acute exercise.

METHODS

For ex vivo studies, six groups of aortic tissue were formed: control, H2O2, N-acetylcysteine (NAC), LY294002, compound C, and LY294002 plus compound C. While incubation with H2O2 increased Akt, AMPK and eNOS phosphorylation, pre-incubation with NAC strongly reduced the phosphorylation of these enzymes. For in vivo studies, male Wistar rats were divided into four groups: control, cont+NAC, exercise, and exer+NAC. After a 3h swimming session, animals were decapitated and aortas were excised for biochemical and immunoblotting analysis.

RESULTS

Acute exercise increased superoxide levels and dichlorofluorescein (DCF) concentrations, and this increase was related to phosphorylation of Akt, AMPK and eNOS. On the other hand, use of NAC reduced superoxide levels and DCF concentration. Reduced superoxide levels and DCF in the exer+NAC group were associated with decreased Akt, AMPK and eNOS phosphorylation. These results appear to be connected with vascular function because VASP phosphorylation increased in acute exercise and decreased in exer+NAC.

CONCLUSION

Our results indicate that ROS induced by acute exercise play the important role of activating eNOS, a process apparently mediated by Akt and AMPK.

Dynamic processes that reflect anti-apoptotic strategies set up by HspB1 (Hsp27)

Human HspB1 (also denoted Hsp27) is an oligomeric anti-apoptotic protein that has tumorigenic and metastatic roles. To approach the structural organizations of HspB1 that are active in response to apoptosis inducers acting through different pathways, we have analyzed the relative protective efficiency induced by this protein as well its localization, oligomerization and phosphorylation. HeLa cells, that constitutively express high levels of HspB1 were treated with either etoposide, Fas agonist antibody, staurosporine or cytochalasin D. Variability in HspB1 efficiency to interfere with the different apoptotic transduction pathways induced by these agents were detected. Moreover, inducer-specific dynamic changes in HspB1 localization, native size and phosphorylation were observed, that differed from those observed after heat shock. Etoposide and Fas treatments gradually shifted HspB1 towards large but differently phosphorylated oligomeric structures. In contrast, staurosporine and cytochalasin D induced the rapid but transient formation of small oligomers before large structures were formed. These events correlated with inducer-specific phosphorylations of HspB1. Of interest, the formation of small oligomers in response to staurosporine and cytochalasin D was time correlated with the rapid disruption of F-actin. The subsequent, or gradual in the case of etoposide and Fas, formation of large oligomeric structures was a later event concomitant with the early phase of caspase activation. These observations support the hypothesis that HspB1 has the ability, through specific changes in its structural organization, to adapt and interfere at several levels with challenges triggered by different signal transduction pathways upstream of the execution phase of apoptosis.

Positive and negative regulation of insulin signaling by reactive oxygen and nitrogen species

Regulated production of reactive oxygen species (ROS)/reactive nitrogen species (RNS) adequately balanced by antioxidant systems is a prerequisite for the participation of these active substances in physiological processes, including insulin action. Yet, increasing evidence implicates ROS and RNS as negative regulators of insulin signaling, rendering them putative mediators in the development of insulin resistance, a common endocrine abnormality that accompanies obesity and is a risk factor of type 2 diabetes. This review deals with this dual, seemingly contradictory, function of ROS and RNS in regulating insulin action: the major processes for ROS and RNS generation and detoxification are presented, and a critical review of the evidence that they participate in the positive and negative regulation of insulin action is provided. The cellular and molecular mechanisms by which ROS and RNS are thought to participate in normal insulin action and in the induction of insulin resistance are then described. Finally, we explore the potential usefulness and the challenges in modulating the oxidant-antioxidant balance as a potentially promising, but currently disappointing, means of improving insulin action in insulin resistance-associated conditions, leading causes of human morbidity and mortality of our era.

Reactive oxygen and nitrogen species as signaling molecules regulating neutrophil function

As a cornerstone of the innate immune response, neutrophils are the archetypical phagocytic cell; they actively seek out, ingest, and destroy pathogenic microorganisms. To achieve this essential role in host defense, neutrophils deploy a potent antimicrobial arsenal that includes oxidants, proteinases, and antimicrobial peptides. Importantly, oxidants produced by neutrophils, referred to in this article as reactive oxygen (ROS) and reactive nitrogen (RNS) species, have a dual function. On one hand they function as potent antimicrobial agents by virtue of their ability to kill microbial pathogens directly. On the other hand, they participate as signaling molecules that regulate diverse physiological signaling pathways in neutrophils. In the latter role, ROS and RNS serve as modulators of protein and lipid kinases and phosphatases, membrane receptors, ion channels, and transcription factors, including NF-kappaB. The latter regulates expression of key cytokines and chemokines that further modulate the inflammatory response. During the inflammatory response, ROS and RNS modulate phagocytosis, secretion, gene expression, and apoptosis. Under pathological circumstances such as acute lung injury and sepsis, excess production of ROS may influence vicinal cells such as endothelium or epithelium, contributing to inflammatory tissue injury. A better understanding of these pathways will help identify novel targets for amelioration of the untoward effects of inflammation.

Can the different heat shock response thresholds found in fermenting and respiring yeast cells be attributed to their differential redox states?

In this study we used a heat-shock (HS) reporter gene to demonstrate that respiring cells are intrinsically less sensitive (by 5 degrees C) than their fermenting counterparts to a sublethal heat shock. We also used an oxidant-sensitive fluorescent probe to demonstrate that this correlates with lower levels of sublethal reactive oxygen species (ROS) accumulation in heat-stressed respiring cells. Moreover, this relationship between HS induction of the reporter gene and ROS accumulation extends to respiring cells that have had their ROS levels modified by treatment with the anti-oxidant ascorbic acid and the pro-oxidant H(2)O(2). Thus, by demonstrating that the ROS/HSR correlation previously demonstrated in fermenting cells also holds for respiring cells (despite their greater HS insensitivity and higher level of intrinsic thermotolerance), we provide evidence that the intracellular redox state may influence both the sensitivity of the heat-shock response (HSR) and stress tolerance in the yeast Saccharomyces cerevisiae.

Heme oxygenase-1/carbon monoxide: from basic science to therapeutic applications

The heme oxygenases, which consist of constitutive and inducible isozymes (HO-1, HO-2), catalyze the rate-limiting step in the metabolic conversion of heme to the bile pigments (i.e., biliverdin and bilirubin) and thus constitute a major intracellular source of iron and carbon monoxide (CO). In recent years, endogenously produced CO has been shown to possess intriguing signaling properties affecting numerous critical cellular functions including but not limited to inflammation, cellular proliferation, and apoptotic cell death. The era of gaseous molecules in biomedical research and human diseases initiated with the discovery that the endothelial cell-derived relaxing factor was identical to the gaseous molecule nitric oxide (NO). The discovery that endogenously produced gaseous molecules such as NO and now CO can impart potent physiological and biological effector functions truly represented a paradigm shift and unraveled new avenues of intense investigations. This review covers the molecular and biochemical characterization of HOs, with a discussion on the mechanisms of signal transduction and gene regulation that mediate the induction of HO-1 by environmental stress. Furthermore, the current understanding of the functional significance of HO shall be discussed from the perspective of each of the metabolic by-products, with a special emphasis on CO. Finally, this presentation aspires to lay a foundation for potential future clinical applications of these systems.

Cell proliferation, reactive oxygen and cellular glutathione

A variety of cellular activities, including metabolism, growth, and death, are regulated and modulated by the redox status of the environment. A biphasic effect has been demonstrated on cellular proliferation with reactive oxygen species (ROS)-especially hydrogen peroxide and superoxide-in which low levels (usually submicromolar concentrations) induce growth but higher concentrations (usually >10-30 micromolar) induce apoptosis or necrosis. This phenomenon has been demonstrated for primary, immortalized and transformed cell types. However, the mechanism of the proliferative response to low levels of ROS is not well understood. Much of the work examining the signal transduction by ROS, including H(2)O(2), has been performed using doses in the lethal range. Although use of higher ROS doses have allowed the identification of important signal transduction pathways, these pathways may be activated by cells only in association with ROS-induced apoptosis and necrosis, and may not utilize the same pathways activated by lower doses of ROS associated with increased cell growth. Recent data has shown that low levels of exogenous H(2)O(2) up-regulate intracellular glutathione and activate the DNA binding activity toward antioxidant response element. The modulation of the cellular redox environment, through the regulation of cellular glutathione levels, may be a part of the hormetic effect shown by ROS on cell growth.

Synergistic roles of platelet-derived growth factor-BB and interleukin-1beta in phenotypic modulation of human aortic smooth muscle cells

The phenotype of smooth muscle cells (SMCs) plays an important role in vascular function in health and disease. We investigated the mechanism of modulation of SMC phenotype (from contractile to synthetic) induced by the synergistic action of a growth factor (platelet-derived growth factor, PDGF-BB) and a cytokine (interleukin, IL-1beta). Human aortic SMCs grown on polymerized collagen showed high expression levels of contractile markers (smooth muscle alpha-actin, myosin heavy chain, and calponin). These levels were not significantly affected by PDGF-BB and IL-1beta individually, but decreased markedly after the combined usage of PDGF-BB and IL-1beta. PDGF/IL-1beta costimulation also induced a sustained phosphorylation of Akt and p70 ribosomal S6 kinase (p70S6K). The effects of PDGF/IL-1beta costimulation on contractile marker expression and Akt and p70S6K phosphorylation were blocked by the phosphatidylinositol 3-kinase inhibitors wortmannin and LY294002 and by adenovirus expressing a dominant-negative Akt, and they were mimicked by constitutively active Akt. PDGF-BB/IL-1beta induced a sustained phosphorylation of PDGF receptor (PDGFR)-beta and its association with IL-1 receptor (IL-1R1). Such activation and association of receptors were blocked by a PDGFR-beta neutralizing antibody (AF385), an IL-1R1 antagonist (IL-1ra), as well as a specific inhibitor of PDGFR-beta phosphorylation (AG1295); these agents also eliminated the PDGF-BB/IL-1beta-induced signaling and phenotypic modulation. PDGF-BB/IL-1beta inhibited the polymerized collagen-induced serum response factor DNA binding activity in the nucleus, and this effect was mediated by the PDGFR-beta/IL-1R1 association and phosphatidylinositol 3-kinase/Akt/p70S6K pathway. Our findings provide insights into the mechanism of SMC phenotypic modulation from contractile to synthetic, e.g., in atherosclerosis.

Role of insulin-induced reactive oxygen species in the insulin signaling pathway

Oxidants, including hydrogen peroxide (H2O2), have been recognized for years to mimic insulin action on glucose transport in adipose cells. Early studies also demonstrated the complementary finding that H2O2 was elaborated during treatment of cells with insulin, suggesting that cellular H2O2 generation was integral to insulin signaling. Recently, reactive oxygen species elicited by various hormones and growth factors have been shown to affect signal transduction pathways in various cell types. We recently reported that insulin-stimulated H2O2 modulates proximal and distal insulin signaling, at least in part through the oxidative inhibition of protein tyrosine phosphatases (PTPases) that negatively regulate the insulin action pathway. Nox4, a homologue in the family of NADPH oxidase catalytic subunits, was found to be prominently expressed in insulin-sensitive cells. By various molecular approaches, Nox4 was shown to mediate insulin-stimulated H2O2 generation and impact the insulin signaling cascade. Overexpression of Nox4 also significantly reversed the inhibition of insulin-stimulated receptor tyrosine phosphorylation by PTP1B, a widely expressed PTPase implicated in the negative regulation of insulin signaling, by inhibiting its catalytic activity. These recent studies have provided insight into Nox4 as a novel molecular link between insulin-stimulated reactive oxygen species and mechanisms involved in their modulation of insulin signal transduction.

Hsp27 consolidates intracellular redox homeostasis by upholding glutathione in its reduced form and by decreasing iron intracellular levels

Small stress proteins [small heat shock proteins (sHsps)] are molecular chaperones that modulate the ability of cells to respond to oxidative stress. The current knowledge concerning the protective mechanism generated by the expression of mammalian heat shock protein-27 (Hsp27) that allows cells to increase their resistance to oxidative stress is presented. We describe the effects mediated by Hsp27 expression toward crucial enzymes such as glucose-6-phosphate dehydrogenase and glutathione reductase that uphold glutathione in its reduced form. New data are presented showing that the expression of sHsps correlates with a drastic decrease in the intracellular level of iron, a catalyzer of hydroxyl radical (OH( . )) generation. A decreased ability of sHsps expressing cells to concentrate iron will therefore end up in a decreased level of oxidized proteins. In addition, we propose a role of Hsp27 in the presentation of oxidized proteins to the proteasome degradation machinery. We also present an analysis of several Hsp27 mutants that suggests that the C-terminal part of this stress protein is essential for its protective activity against oxidative stress.

Redox paradox: insulin action is facilitated by insulin-stimulated reactive oxygen species with multiple potential signaling targets

Propelled by the identification of a small family of NADPH oxidase (Nox) enzyme homologs that produce superoxide in response to cellular stimulation with various growth factors, renewed interest has been generated in characterizing the signaling effects of reactive oxygen species (ROS) in relation to insulin action. Two key observations made >30 years ago-that oxidants can facilitate or mimic insulin action and that H(2)O(2) is generated in response to insulin stimulation of its target cells-have led to the hypothesis that ROS may serve as second messengers in the insulin action cascade. Specific molecular targets of insulin-induced ROS include enzymes whose signaling activity is modified via oxidative biochemical reactions, leading to enhanced insulin signal transduction. These positive responses to cellular ROS may seem "paradoxical" because chronic exposure to relatively high levels of ROS have also been associated with functional beta-cell impairment and the chronic complications of diabetes. The best-characterized molecular targets of ROS are the protein-tyrosine phosphatases (PTPs) because these important signaling enzymes require a reduced form of a critical cysteine residue for catalytic activity. PTPs normally serve as negative regulators of insulin action via the dephosphorylation of the insulin receptor and its tyrosine-phosphorylated cellular substrates. However, ROS can rapidly oxidize the catalytic cysteine of target PTPs, effectively blocking their enzyme activity and reversing their inhibitory effect on insulin signaling. Among the cloned Nox homologs, we have recently provided evidence that Nox4 may mediate the insulin-stimulated generation of cellular ROS and is coupled to insulin action via the oxidative inhibition of PTP1B, a PTP known to be a major regulator of the insulin signaling cascade. Further characterization of the molecular components of this novel signaling cascade, including the mechanism of ROS generated by insulin and the identification of various oxidation-sensitive signaling targets in insulin-sensitive cells, may provide a novel means of facilitating insulin action in states of insulin resistance.

Heme oxygenase-1: redox regulation of a stress protein in lung and cell culture models

Reactive oxygen species (ROS) may contribute to tissue damage in many pathophysiological conditions and participate in physiological signaling processes. The mechanisms by which cells sense prooxidant states, and activate signaling pathways leading to adaptive responses, remain incompletely understood. Bacteria contain several transcriptional regulators (e.g., OxyR) and a low-molecular-weight heat shock protein (HSP33), whose activity increases upon oxidation of critical sulfhydryl residues. These proteins participate in cellular adaptation to oxidative stress. In higher organisms, heme oxygenase-1 (HO-1) has been widely studied as a model for redox-regulated gene expression. Expression of HO-1 responds to chemical and physical agents that directly or indirectly generate ROS. Depletion of cellular reduced glutathione may act as a signal for HO-1 transcriptional activation. Furthermore, antioxidants and metal-chelating compounds can modulate HO-1 expression. Several signaling molecules (e.g., mitogen-activated protein kinases), transcriptional regulators (activator protein-1, NF-E2-related factor-2, hypoxia-inducible factor-1, Bach-1), as well as two enhancer regions in the ho-1 5' regulatory region, participate in the regulation of the ho-1 gene. HO-1 protein expression can occur in the lung in response to oxidative stress associated with infection, altered oxygen tension, and inflammatory diseases. HO-1 remains widely regarded as a protective mechanism against oxidative tissue injury.

TNF-Induced β2 Integrin Activation Involves Src Kinases and a Redox-Regulated Activation of p38 MAPK

We previously demonstrated that the TNF-alpha-induced inside-out signaling leading to beta(2) integrin activation is redox regulated. To identify kinases involved in this pathway, the effects of kinase inhibitors on the expression of beta(2) integrin activation neoepitope (clone 24) were investigated. We show that both p38 MAPK (inhibited by SB203580) and Src kinases (inhibited by PP2) are involved in beta(2) integrin activation by TNF and oxidants in human neutrophils. Src kinases appeared constitutively active in resting neutrophils and not further activated by TNF or oxidants in nonadherent conditions. However, PP2 blocked both TNF-induced expression of the 24 epitope and cell adhesion promoted by the integrin activating anti-CD18 KIM185 mAb, showing that both the inside-out and the outside-in signaling involve Src kinases. p38 MAPK was activated by TNF and oxidants in nonadherent conditions i.e., with 10 mM EDTA. This activation in EDTA resulted in CD11b, CD35 and CD66 up-regulation and in an oxidative response, all blocked by SB203580 and PP2. p38 MAPK was not activated upon direct integrin activation by KIM185 mAb. Thus, p38 activation allows the study to distinguish the initial transduction pathway leading to beta(2) integrin activation from the signaling resulting from integrin engagement. Finally, p38 MAPK activation by TNF was blocked by diphenylene iodonium, an inhibitor of flavoprotein oxidoreductase, and by the free radical scavenger N-acetylcystein. Taken together, these results demonstrate, for the first time, that constitutively activated Src tyrosine kinases and a redox-regulated activation of p38 MAPK are involved in TNF inside-out signaling leading to beta(2) integrin activation.

Transplasma membrane electron transport: enzymes involved and biological function

The notion of transmembrane electron transport is usually associated with mitochondria and chloroplasts. However, since the early 1970s, it has been known that this phenomenon also occurs at the level of the plasma membrane. Ever since, evidence has accumulated for the existence of a plethora of transplasma membrane electron transport enzymes. In this review, we discuss the various enzymes known, their molecular characteristics and their biological functions.

A role for protein phosphatase-2A in p38 mitogen-activated protein kinase-mediated regulation of the c-Jun NH(2)-terminal kinase pathway in human neutrophils

Human neutrophil accumulation in inflammatory foci is essential for the effective control of microbial infections. Although exposure of neutrophils to cytokines such as tumor necrosis factor-alpha (TNFalpha), generated at sites of inflammation, leads to activation of MAPK pathways, mechanisms responsible for the fine regulation of specific MAPK modules remain unknown. We have previously demonstrated activation of a TNFalpha-mediated JNK pathway module, leading to apoptosis in adherent human neutrophils (Avdi, N. J., Nick, J. A., Whitlock, B. B., Billstrom, M. A., Henson, P. M., Johnson, G. L., and Worthen, G. S. (2001) J. Biol. Chem. 276, 2189-2199). Herein, evidence is presented linking regulation of the JNK pathway to p38 MAPK and the Ser/Thr protein phosphatase-2A (PP2A). Inhibition of p38 MAPK by SB 203580 and M 39 resulted in significant augmentation of TNFalpha-induced JNK and MKK4 (but not MKK7 or MEKK1) activation, whereas prior exposure to a p38-activating agent (platelet-activating factor) diminished the TNFalpha-induced JNK response. TNFalpha-induced apoptosis was also greatly enhanced upon p38 inhibition. Studies with a reconstituted cell-free system indicated the absence of a direct inhibitory effect of p38 MAPK on the JNK module. Neutrophil exposure to the Ser/Thr phosphatase inhibitors okadaic acid and calyculin A induced JNK activation. Increased phosphatase activity following TNFalpha stimulation was shown to be PP2A-associated and p38-dependent. Furthermore, PP2A-induced dephosphorylation of MKK4 resulted in its inactivation. Thus, in neutrophils, p38 MAPK, through a PP2A-mediated mechanism, regulates the JNK pathway, thus determining the extent and nature of subsequent responses such as apoptosis.

[Regulation of gene expression by active oxygen species]

Reactive oxygen species (ROS) are generated from cells stimulated by various cytokines, hormones, and stresses, and regulate cellular functions such as gene expression and cell growth. They affect activities of many types of molecular targets, including signaling molecules and transcription factors. Early-response genes (c-fos, egr-I and JE) that encode transcription factors are induced by ROS, and activities of their products are modulated by ROS through redox-based mechanisms. We isolated a novel gene, hic-5, that was induced by hydrogen peroxide and encodes a focal adhesion protein. hic-5 was found to translocate to the nucleus in cells treated with ROS and regulates several cellular genes. We propose that hic-5 is a key element in the transduction of signals from the cell surface to the nucleus under oxidative stress.

Involvement of IL-1beta and IL-10 in IFN-alpha-mediated antiviral gene induction in human hepatoma cells

Crosstalk between interferons (IFNs) and several cytokines is likely to play an important role in viral clearance in chronic hepatitides B and C. We investigated the influence of this phenomenon on IFN-inducible antiviral gene expression in HuH-7 human hepatoma cells. HuH-7 cells were treated with IFN-alpha in the absence or presence of interleukin-1beta (IL-1beta) or IL-10 and the expression of antiviral genes such as 2(')5(')-oligoadenylate synthetase (2(')5(')-OAS) and double-stranded RNA-dependent protein kinase (PKR), as well as activation of signal transducer and activator of transcription 1 (STAT1), a key step for relaying the IFN-alpha signals, was analyzed by Northern blotting, Western blotting, and the reporter gene transfection assay. IL-1beta potentiated IFN-alpha-induced 2(')5(')-OAS and PKR gene expression, similar to expression of the transfected reporter genes containing the IFN-stimulated regulatory elements, while IL-10 suppressed IFN-alpha-stimulated gene expression. With regard to IFN-alpha signaling, IL-1beta enhanced both tyrosine and serine phosphorylation of STAT1 through p38 mitogen-activated protein kinase activation. In contrast, IL-10 inhibited IFN-alpha-mediated tyrosine phosphorylation of STAT1 by induction of a Janus kinase inhibitor, JAB. IL-1beta and IL-10 interact with IFN-alpha to up- and down-regulate antiviral gene expression, respectively, by modulating STAT1 activation induced by IFN-alpha.

Hydrogen peroxide generated during cellular insulin stimulation is integral to activation of the distal insulin signaling cascade in 3T3-L1 adipocytes

In a variety of cell types, insulin stimulation elicits the rapid production of H(2)O(2), which causes the oxidative inhibition of protein-tyrosine phosphatases and enhances the tyrosine phosphorylation of proteins in the early insulin action cascade (Mahadev, K., Zilbering, A., Zhu, L., and Goldstein, B. J. (2001) J. Biol. Chem. 276, 21938-21942). In the present work, we explored the potential role of insulin-induced H(2)O(2) generation on downstream insulin signaling using diphenyleneiodonium (DPI), an inhibitor of cellular NADPH oxidase that blocks insulin-stimulated cellular H(2)O(2) production. DPI completely inhibited the activation of phosphatidylinositol (PI) 3'-kinase activity by insulin and reduced the insulin-induced activation of the serine kinase Akt by up to 49%; these activities were restored when H(2)O(2) was added back to cells that had been pretreated with DPI. Interestingly, the H(2)O(2)-induced activation of Akt was entirely mediated by upstream stimulation of PI 3'-kinase activity, since treatment of 3T3-L1 adipocytes with the PI 3'-kinase inhibitors wortmannin or LY294002 completely blocked the subsequent activation of Akt by exogenous H(2)O(2). Preventing oxidant generation with DPI also blocked insulin-stimulated glucose uptake and GLUT4 translocation to the plasma membrane, providing further evidence for an oxidant signal in the regulation of the distal insulin-signaling cascade. Finally, in contrast to the cellular mechanism of H(2)O(2) generation by other growth factors, such as platelet-derived growth factor, we also found that insulin-stimulated cellular production of H(2)O(2) may occur through a unique pathway, independent of cellular PI 3'-kinase activity. Overall, these data provide insight into the physiological role of insulin-dependent H(2)O(2) generation, which is not only involved in the regulation of tyrosine phosphorylation events in the early insulin signaling cascade but also has important effects on the regulation of downstream insulin signaling, involving the activation of PI 3'-kinase, Akt, and ultimately cellular glucose transport in response to insulin.

Synergism between nitric oxide and hydrogen peroxide in the inhibition of platelet function: the roles of soluble guanylyl cyclase and vasodilator-stimulated phosphoprotein

In previous studies, a strong synergism between low concentrations of hydrogen peroxide and nitric oxide in the inhibition of agonist-induced platelet aggregation has been established and may be due to enhanced formation of cyclic GMP. In this investigation, hydrogen peroxide and NO had no effect on the activity of pure soluble guanylyl cyclase or its activity in platelet lysates and cytosol. H(2)O(2) was found to increase the phosphorylation of vasodilator-stimulated phosphoprotein (VASP), increasing the amount of the 50-kDa form that results from phosphorylation at serine(157). This occurs both in the presence and in the absence of low concentrations of NO, even at submicromolar concentrations of the peroxide, which alone was not inhibitory to platelets. These actions of H(2)O(2) were inhibited to a large extent by an inhibitor of cyclic AMP-dependent protein kinase, even though H(2)O(2) did not increase cyclic AMP. This inhibitor reversed the inhibition of platelets induced by combinations of NO and H(2)O(2) at low concentrations. The results suggest that the action on VASP may be one site of action of H(2)O(2) but that this event alone does not lead to inhibition of platelets; another unspecified action of NO is required to complete the events required for inhibition.

Is the effect of interleukin-1 on glutathione oxidation in cultured human fibroblasts involved in nuclear factor-kappaB activation?

Our understanding of the interleukin-1 (IL-1) signaling molecular mechanisms has recently made considerable progress, with the discovery of the IL-1 receptor-associated kinase and the downstream enzymatic cascade that leads to the activation of nuclear factor-kappaB (NF-kappaB). IL-1 signaling and especially NF-kappaB activation are thought to be redox-sensitive, even though the precise nature and the molecular targets of the oxidants/antioxidants involved remain largely unknown. Here, we investigated the possible role of cellular oxidized/reduced glutathione (GSSG/GSH) balance in IL-1 signaling. We describe a quantitative method based on capillary electrophoresis designed to assay both intracellular GSH and GSSG in adhering fibroblasts. This method allows the GSSG/GSH balance to be followed during IL-1 stimulation. Our data show that IL-1 induces rapid and transient oxidation of intracellular glutathione in human fibroblasts. Using various antioxidants, including pyrrolidine dithiocarbamate and curcumin, we were unable to show a direct relationship between this IL-1-induced glutathione oxidation and NF-kappaB activation. Of the five antioxidants tested, only curcumin was able to inhibit IkappaBalpha degradation upstream and, hence, NF-kappaB DNA-binding activity and NF-kappaB-dependent expression of IL-6 downstream.

Selenium: a key element that controls NF-kappa B activation and I kappa B alpha half life

Treatment of mammalian cells with hydrogen peroxide induces the nuclear translocation of the transcription factor NF-kappa B and its binding to kappa B DNA sequences present in the promoter region of numerous genes. The role of selenium in NF-kappa B activation was analyzed in human T47D cells overexpressing the seleno-dependent detoxifiant enzyme glutathione peroxidase. Following exposure to H(2)O(2), these cells showed a seleno-dependent decreased accumulation of intracellular ROS and NF-kappa B activation. This phenomenon was correlated with an inhibition of the nuclear translocation of NF-kappa B (p50 subunit) and with an absence of I kappa B alpha degradation. We also report that the half-life of I kappa B alpha in untreated cells was increased two-fold by the overexpression of active glutathione peroxidase. These results suggest that selenium is a key element that through its modulation of glutathione peroxidase activity can inhibit NF-kappa B activation and can up-regulate I kappa B alpha normal half life.

Modulation of protein kinase activity and gene expression by reactive oxygen species and their role in vascular physiology and pathophysiology

Emerging evidence indicates that reactive oxygen species, especially superoxide and hydrogen peroxide, are important signaling molecules in cardiovascular cells. Their production is regulated by hormone-sensitive enzymes such as the vascular NAD(P)H oxidases, and their metabolism is coordinated by antioxidant enzymes such as superoxide dismutase, catalase, and glutathione peroxidase. Both of these reactive oxygen species serve as second messengers to activate multiple intracellular proteins and enzymes, including the epidermal growth factor receptor, c-Src, p38 mitogen-activated protein kinase, Ras, and Akt/protein kinase B. Activation of these signaling cascades and redox-sensitive transcription factors leads to induction of many genes with important functional roles in the physiology and pathophysiology of vascular cells. Thus, reactive oxygen species participate in vascular smooth muscle cell growth and migration; modulation of endothelial function, including endothelium-dependent relaxation and expression of a proinflammatory phenotype; and modification of the extracellular matrix. All of these events play important roles in vascular diseases such as hypertension and atherosclerosis, suggesting that the sources of reactive oxygen species and the signaling pathways that they modify may represent important therapeutic targets.

Naturally occurring polyphenolic antioxidants modulate IgE-mediated mast cell activation

Reactive oxygen species (ROS) are known to modulate activities of a host of kinases, phosphatases and transcription factors. Rutin and chlorogenic acid (CGA) are the major polyphenolic antioxidants present in the small molecular fraction of smokeless tobacco leaf extracts, as ascertained by reverse-phase high-pressure liquid chromatography (HPLC) and mass spectrometry. Levels of intracellular ROS in resting versus antigen-immunoglobulin E (IgE)-challenged murine mast cells were measured at 510 nm by fluorescence-activated cell sorting (FACS) using carboxy-dichlorofluorescein (DCFH-DA). Enhanced ROS production was observed in IgE-sensitized mast cells following antigenic challenge. Rutin and CGA reduced ROS levels in antigen-IgE-activated mast cells. Concomitantly, they also profoundly inhibited histamine release by these activated mast cells. In contrast, rutin and CGA augmented the inducible cytokine messages, i.e. interleukin (IL)-10, IL-13, interferon-gamma (IFN-gamma), IL-6 and tumour necrosis factor-alpha (TNF-alpha) in IgE-sensitized mast cells following antigen challenge. This study indicates that tobacco polyphenolic antioxidants that quench intracellular ROS, differentially affect two effector functions of antigen-IgE-activated mast cells. This model system may be employed to determine the molecular target of polyphenols. The potential role of these polyphenolic antioxidants on IgE-mediated allergy in vivo depends on a balance of their differential effects on mast cell activation.

Reactive oxygen species as mediators of angiotensin II signaling

Angiotensin II stimulates a plethora of signaling pathways leading to cell growth and contraction. Recent work has shown that reactive oxygen species are involved in transducing many of the effects of angiotensin II, and are in fact produced in response to agonist-receptor binding. Angiotensin II stimulates a NAD(P)H oxidase to produce superoxide and hydrogen peroxide, both of which may act on intracellular growth-related proteins and enzymes to mediate the final physiological response. Of particular importance is hydrogen peroxide, which mediates angiotensin II stimulation of such important intracellular signals as EGF-receptor transactivation, p38 mitogen activated protein kinase, and Akt. Future work will be directed towards identifying other important redox-sensitive signaling pathways and their relationship to the physiology and pathophysiology of the renin-angiotensin system.

Tin-protoporphyrin potentiates arsenite-induced DNA strand breaks, chromatid breaks and kinetochore-negative micronuclei in human fibroblasts

Numerous reports have shown that oxidative stress is involved in arsenite-induced genetic damage. Arsenite is also a potent inducer of heme oxygenase (HO)-1. To understand whether HO-1 could function as a cellular antioxidant and protect cells from arsenite injury, the effects of tin-protoporphyrin (SnPP), a competitive inhibitor of HO-1, on arsenite-induced genetic damage were examined in human skin fibroblasts (HFW). In the present study, we found that SnPP at 100 microM significantly potentiated arsenite-induced cytotoxicity, DNA strand breaks (assayed by alkaline single cell gel electrophoresis(SCGE)), and chromatid breaks. Although arsenite alone mainly induced kinetochore-plus micronuclei (K(+)-MN), SnPP only synergistically enhanced kinetochore-negative micronuclei (K(-)-MN). The increase in K(-)-MN by SnPP cotreatment was consistent with the increase in DNA strand breaks and chromatid breaks caused by SnPP. However, at higher arsenite doses, K(+)-MN was significantly reduced by SnPP. Pretreatment of HFW cells with hemin, an inducer of HO-1, significantly attenuated the cytotoxicity of arsenite. Therefore, the present results suggest that HO-1 induction by arsenite plays certain roles in protecting cells from arsenite-induced injury.

Interleukin-1 promotes expression and phosphorylation of neurofilament and tau proteins in vivo

Slow-release, IL-1-impregnated pellets implanted in rat cerebral cortex to simulate chronic overexpression of IL-1 significantly increased relative tissue levels of tau mRNA and of tau immunoreactivity in neuronal cell bodies and in swollen dystrophic neurites that also overexpressed phosphorylated and nonphosphorylated neurofilament epitopes. In addition, rats with IL-1-impregnated pellets, but not control rats or those with sham pellets, showed focal immunoreactivity for hyperphosphorylated tau epitopes present in paired helical filaments. Our results are consistent with an important driving role for IL-1 in the pathogenesis of Alzheimer-type neuronal and neuritic changes.

Phosphorylation of 130- and 95-kDa substrates associated with tumor necrosis factor-alpha receptor CD120a (p55)

Cross-linking of CD120a (p55), a receptor for tumor necrosis factor alpha (TNFalpha), initiates downstream events, including the activation of protein Ser/Thr kinases. In this report, we have characterized two protein Ser/Thr kinase substrates that are intrinsically associated with CD120a (p55) in mouse macrophages, and we have investigated the mechanism involved in their phosphorylation. pp130 and pp95 were detected by co-immunoprecipitation with CD120a (p55) from lysates of mouse bone marrow-derived macrophages and were phosphorylated on Ser and Thr residues during in vitro kinase assays in the presence of [gamma-(32)P]ATP. The level of phosphorylation of pp130 and pp95 was rapidly and transiently increased in response to TNFalpha in [(32)P]orthophosphate-labeled macrophages, although the level of pp130 protein associated with CD120a (p55) remained unchanged as detected by [(35)S]methionine labeling. In contrast, pp130 and pp95 were efficiently phosphorylated in in vitro kinase assays of CD120a (p55) immunoprecipitates from unstimulated cells, and the level of phosphorylation was rapidly and transiently reduced in response to TNFalpha. Both pp130 and pp95 were sensitive to dephosphorylation with purified protein phosphatase 2A, and okadaic acid, a PP1/PP2A inhibitor, mimicked the ability of TNFalpha to stimulate the phosphorylation of pp130 and pp95 in intact (32)P-labeled macrophages. Collectively, these findings suggest that pp130 and pp95 are constitutively associated with CD120a (p55) and become inducibly phosphorylated in macrophages in response to TNFalpha. We propose that the underlying mechanism of their phosphorylation may involve the inactivation of a cytoplasmic pp130/pp95 Ser/Thr phosphatase.

Phosphorylation of tumor necrosis factor receptor CD120a (p55) by p42(mapk/erk2) induces changes in its subcellular localization

The interaction of tumor necrosis factor-alpha (TNFalpha) with its receptor sets in motion downstream signaling events including the activation of members of the mitogen-activated protein kinase (MAPK) family. In this study, we show that p42(mapk/erk2) phosphorylates sequences present within the cytoplasmic domain of CD120a (p55). By using a GST-CD120a-(207-425) fusion protein as substrate, phosphorylation was induced following stimulation of mouse macrophages with TNFalpha, granulocyte-macrophage colony-stimulating factor, macrophage colony-stimulating factor, and zymosan particles and was blocked by immunodepletion of p42(mapk/erk2) and by specific inhibition of p42(mapk/erk2) activation with PD098059. Transfection of COS-7 cells with CD120a (p55), wild-type p42(mapk/erk2), and constitutively active MEK-1 followed by metabolic labeling with [(32)P]orthophosphate indicated that p42(mapk/erk2) phosphorylated the cytoplasmic domain of CD120a (p55) in intact cells. As a consequence of phosphorylation, CD120a (p55) expression at the plasma membrane and Golgi apparatus was lost and the receptor accumulated in intracellular tubular structures associated with the endoplasmic reticulum. Mutation of the four Ser and Thr ERK consensus phosphorylation sites to Ala residues inhibited the ability of the receptor to redistribute to intracellular tubules in a p42(mapk/erk2)-dependent fashion; whereas mutation of the phosphorylation sites to Asp and Glu residues mimicked the effect of receptor phosphorylation. These findings thus indicate that the phosphorylation of CD120a (p55) alters the subcellular localization of the receptor and may thereby result in changes in its signaling properties.

Structure-based functional motif identifies a potential disulfide oxidoreductase active site in the serine/threonine protein phosphatase-1 subfamily

In previous work, 3-dimensional descriptors of protein function ('fuzzy functional forms') were used to identify disulfide oxidoreductase active sites in high-resolution protein structures. During this analysis, a potential disulfide oxidoreductase active site in the serine/threonine protein phosphatase-1 (PP1) crystal structure was discovered. In PP1, the potential redox active site is located in close proximity to the phosphatase active site. This result is interesting in view of literature suggesting that serine/threonine phosphatases could be subject to redox control mechanisms within the cell; however, the actual source of this control is unknown. Additional analysis presented here shows that the putative oxidoreductase active site is highly conserved in the serine/threonine phosphatase-1 subfamily, but not in the serine/threonine phosphatase-2A or -2B subfamilies. These results demonstrate the significant advantages of using structure-based motifs for protein functional site identification. First, a putative disulfide oxidoreductase active site has been identified in serine-threonine phosphatases using a descriptor built from the glutaredoxin/thioredoxin family, proteins that have no apparent evolutionary relationship whatsoever to the PP1 proteins. Second, the proximity of the putative disulfide oxidoreductase active site to the phosphatase active site provides evidence toward a regulatory control mechanism. No sequence-based method could provide either piece of information.

Tyrosine phosphatases as targets in metal-induced signaling in human airway epithelial cells

We previously showed that exposure to metal-laden combustion particles disregulates protein tyrosine phosphate homeostasis in human airway epithelial cells (HAEC). More recently, we reported that exposure to certain metal ions activates mitogen-activated protein kinases in HAEC. To study the mechanism responsible, we examined the effects of arsenic (As), vanadium (V), and zinc (Zn) on tyrosine phosphate catabolism in BEAS S6 cells or cultured human bronchial epithelial cells. Western blots and immunocytochemical analyses showed that exposure to noncytotoxic levels of As, V, or Zn resulted in increased levels of protein phosphotyrosines in HAEC. Tyrosine phosphatase activity, measured against [(32)P]-labeled PolyGlu:Tyr, was markedly inhibited in cells treated with V or Zn but was unaffected by exposure to As. Fast performance liquid chromatography fractionation and subsequent in-gel phosphatase activity assay of HAEC protein extracts revealed the presence of numerous tyrosine phosphatases, of varying molecular weights, that were effectively inhibited by exposure to V or Zn ions. As had no discernible effect on these enzymes. The protein tyrosine phosphatase PTP1B, immunoprecipitated from HAEC, was similarly inhibited by V and Zn but not by As ions. These data show that V and Zn may induce tyrosine phosphate accumulation by inhibiting dephosphorylation and implicate kinase activation as the mechanism in HAEC exposed to As. These findings suggest that metal exposure can activate signaling pathways through multiple mechanisms.

TNF-alpha and IL-1alpha induce heme oxygenase-1 via protein kinase C, Ca2+, and phospholipase A2 in endothelial cells

Heme oxygenase-1 (HO-1), an enzyme important in protection against oxidant stress, is induced in human vascular endothelial cells by the cytokines tumor necrosis factor-alpha (TNF-alpha) and interleukin-1alpha (IL-1alpha). However, the signaling mediators that regulate the induction are not known. This study examined the involvement of protein kinase C (PKC), phospholipase A2 (PLA2), calcium, and oxidants in cytokine induction of HO-1. Acute exposure to the PKC activator phorbol 12-myristate 13-acetate (PMA) stimulated HO-1 mRNA. However, prolonged exposure, which downregulates most PKC isoforms, blocked induction of HO-1 mRNA by IL-1alpha and TNF-alpha. Additionally, the phosphatase inhibitors okadaic acid and calyculin enhanced cytokine induction of HO-1. Mepacrine, a PLA2 inhibitor, prevented HO-1 induction by cytokine, suggesting a role for arachidonate, the product of PLA2 hydrolysis of phospholipids, in HO-1 expression. The intracellular calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM) blocked cytokine induction of HO-1. Paradoxically, the calcium ionophore A-23187 prevented HO-1 induction by cytokine but not by PMA. Finally, the oxidant scavenger N-acetylcysteine inhibited HO-1 induction by cytokines. These results demonstrate that TNF-alpha and IL-1alpha induction of HO-1 requires PKC-mediated phosphorylation and PLA2 activation as well as oxidant generation.

Myocardial adaptation to ischaemia--the preconditioning phenomenon

The phenomenon of ischaemic preconditioning, highlights a new and endogenous route to myocardial protection, which we believe could be exploited in our search for new therapeutic ways to protect the infarcting myocardium. Ischaemic preconditioning has been shown to be associated with both an early, or acute phase of protection lasting approximately 1-2 hours, as well as a delayed phase or "second window of protection" seen at least 24 hours following the initial sublethal ischaemic insult, and lasting up to 72 hours. We believe that both responses are triggered by similar receptor mediated events in addition to using the similar signalling pathways involving kinase cascades. However it is thought that the ultimate target or end-effector through which the protection is manifest may be different for the early vs. late effects. Some evidence exists that the end-effector involved in early preconditioning may be via the ATP-sensitive potassium channel (K(ATP)). With respect to the second window of protection, the cellular mechanisms underlying this are not fully understood at present, however we believe that they may be dependent upon a similar signalling transduction pathway with upregulation of cytoprotective proteins such as the heat stress proteins, and/or anti-oxidant proteins. Evidence demonstrating that preconditioning can occur in the human myocardium is also accumulating. In this respect cultured human ventricular myocytes as well as human atrial muscle have been shown to be preconditioned with brief episodes of simulated ischemia. These human preparations also respond to the known triggers and possible end-effectors of preconditioning, (e.g. adenosine receptor stimulation and K(ATP) channel opening) as well as being able to elicit their responses through the PKC signalling pathway. Further support for this phenomenon, in man, comes from PTCA studies demonstrating that this invasive procedure can put patients into a "preconditioned state"; this effect being associated with reduced ischaemic symptoms as well as the involvement of the adenosine receptor and K(ATP) channel. Of further interest is the observation that patients with a previous history of angina, prior to a MI, sustain smaller infarcts and have an improved survival. However the most direct evidence that preconditioning occurs in man comes from studies in patients undergoing coronary artery bypass surgery. The above evidence that preconditioning can occur in man makes it now possible to begin to design clinical studies investigating cardioprotective properties of drugs that can specifically mimic this phenomenon.

Protein-damaging stresses activate c-Jun N-terminal kinase via inhibition of its dephosphorylation: a novel pathway controlled by HSP72

Various stresses activate the c-Jun N-terminal kinase (JNK), which is involved in the regulation of many aspects of cellular physiology, including apoptosis. Here we demonstrate that in contrast to UV irradiation, heat shock causes little or no stimulation of the JNK-activating kinase SEK1, while knocking out the SEK1 gene completely blocks heat-induced JNK activation. Therefore, we tested whether heat shock activates JNK via inhibition of JNK dephosphorylation. The rate of JNK dephosphorylation in unstimulated cells was high, and exposure to UV irradiation, osmotic shock, interleukin-1, or anisomycin did not affect this process. Conversely, exposure of cells to heat shock and other protein-damaging conditions, including ethanol, arsenite, and oxidative stress, strongly reduced the rate of JNK dephosphorylation. Under these conditions, we did not observe any effects on dephosphorylation of the homologous p38 kinase, suggesting that suppression of dephosphorylation is specific to JNK. Together, these data indicate that activation of JNK by protein-damaging treatments is mediated primarily by inhibition of a JNK phosphatase(s). Elevation of cellular levels of the major heat shock protein Hsp72 inhibited a repression of JNK dephosphorylation by these stressful treatments, which explains recent reports of the suppression of JNK activation by Hsp72.

Transformation is associated with an increase in sensitivity to TNF-mediated lysis as a result of an increase in TNF-induced protein tyrosine phosphatase activity

Using the tumor necrosis factor (TNF)-resistant cell line B/CN and an anchorage-independent variant, 10ME, we have shown a relationship between transformation and sensitivity to TNF. Here, we report a role for protein tyrosine phosphorylation in expression of these phenotypes. Several studies have demonstrated the involvement of protein phosphorylation in the TNF signaling pathways that leads to cell death. We show that TNF treatment of the TNF-sensitive, transformed cells results in a marked increase in protein tyrosine phosphatase (PTP) activity and a decrease in protein tyrosine kinase (PTK) activity. In contrast, TNF treatment of the TNF-resistant, non-transformed parental cells results in a marked increase in PTK activity. Also, the PTP inhibitors vanadate and bromotetramisole decrease TNF lytic activity, indicating that the PTP activity observed is an integral part of the lytic process. Treatment of targets with vanadate prior to TNF exposure had no effect on TNF-mediated lysis. In contrast, the addition of vanadate up to 4 hr after TNF-treatment resulted in a decrease in TNF-mediated lysis. Our findings indicate that the phosphatase activity is induced after TNF binds its receptor. Our data also indicate that the decrease in TNF-mediated lysis caused by PTP inhibitors is not due to the inhibition of the TNF lytic mechanism. Instead, vanadate increases a TNF resistance mechanism; it does so by blocking the PTP-mediated inhibition of the TNF resistance mechanism. Further, the lineage relationship of these cell lines suggests that there is a biochemical relationship between anchorage-independence, tumorigenicity and the protein tyrosine phosphorylation that governs sensitivity to TNF.

Redox regulation of cellular signalling

Extracellular stimuli elicit a variety of responses, such as cell proliferation and differentiation, through the cellular signalling system. Binding of growth factors to the respective receptor leads to the activation of receptor tyrosine kinases, which in turn stimulate downstream signalling systems such as mitogen-activated protein (MAP) kinases, phospholipase Cgamma (PLCgamma) and phosphatidylinositol 3-kinase. These biochemical reactions finally reach the nucleus, resulting in gene expression mediated by the activation of several transcription factors. Recent studies have revealed that cellular signalling pathways are regulated by the intracellular redox state. Generation of reactive oxygen species (ROS), such as H2O2, leads to the activation of protein tyrosine kinases followed by the stimulation of downstream signalling systems including MAP kinase and PLCgamma. The activation of PLCgamma by oxidative radical stress elevates the cellular Ca2+ levels by flux from the intracellular Ca2+ pool and from the extracellular space. Such reactions in the upstream signalling cascade, in concert, result in the activation of several transcription factors. On the other hand, reductants generally suppress the upstream signalling cascade resulting in the suppression of transcription factors. However, it is well known that cysteine residues in a reduced state are essential for the activity of many transcription factors. In fact, in vitro, oxidation of NFkappaB results in its activation, whereas reductants promote its activity. Thus, cellular signalling pathways are generally subjected to dual redox regulation in which redox has opposite effects on upstream signalling systems and downstream transcription factors. Not only are the cellular signalling pathways subjected to redox regulation, but also the signalling systems regulate the cellular redox state. When cells are activated by extracellular stimuli, the cells produce ROS, which in turn stimulate other cellular signalling pathways, indicating that ROS act as second messengers. It is thus evident that there is cross talk between the cellular signalling system and the cellular redox state. Cell death and life also are subjected to such dual redox regulation and cross talk. Death signals induce apoptosis through the activation of caspases in the cells. Oxidative radical stress induces the activation of caspases, whereas the oxidation of caspases results in their inactivation. Furthermore, some cell-death signals induce the production of ROS in the cells, and the ROS produced in turn stimulate the cell-death machinery. All this evidence shows that the cell's fate is determined by cross talk between the cellular signalling pathways and the cellular redox state through a complicated regulation mechanism.

Inhibition of tumor necrosis factor-alpha- and interleukin-1-induced endothelial E-selectin expression by thiol-modifying agents

The expression of endothelial-leukocyte adhesion molecules has been postulated to be regulated by redox-sensitive events. Tumor necrosis factor-alpha (TNF-alpha)- and interleukin-1 (IL-1)-induced E-selectin expression was analyzed after pretreating human umbilical vein endothelial cells with different thiol-modifying agents, ie, diamide, phenylarsine oxide, N-ethylmaleimide, and diethyl maleate. E-selectin protein expression was quantified by indirect immunofluorescence. All compounds suppressed the cytokine-induced E-selectin expression in a concentration-dependent manner, whereas the antioxidant N-acetylcysteine showed no effect. The inhibitory effect of diamide (100 micromol/L, 1 hour) was reversible within 6 hours when the cells were allowed to recover before application of cytokines. Reversibility was strongly delayed when cells were deprived of glutathione by buthionine sulfoximine pretreatment. Glutathione depletion alone did not influence cytokine-induced E-selectin expression. Analysis of cellular glutathione status showed a 3-fold increase in oxidized glutathione after diamide treatment. Monochlorobimane labeling also revealed a decrease in total cellular thiols. During recovery, the glutathione status was restored within 1 hour, whereas total thiol content and E-selectin expression needed at least 6 hours to return to baseline. Complete inhibition of E-selectin expression by the vicinal thiol blocker phenylarsine oxide (0.5 micromol/L) was reversed by dithiols like dithiothreitol or dimercaptopropanol, but not by the monothiol 2-mercaptoethanol. These data suggest that proteins with essential thiols, most probably vicinal thiols. are involved in the IL-1- and TNF-alpha-mediated induction of E-selectin. These thiols must be in the reduced state; oxidation or other modification thereof attenuates or abolishes the cells' response to the cytokines.

H2O2 acts on cellular membranes to generate ceramide signaling and initiate apoptosis in tracheobronchial epithelial cells

Hydrogen peroxide (H2O2) is an inflammatory oxidant which contributes to the pathogenesis of chronic diseases such as lung injury of the respiratory tract, atherosclerosis and cancer. The mechanisms and target sites of this reactive oxidant are mainly unknown. So far there are opposing reports as to whether reactive oxidants inhibit or promote apoptosis. We activated the death pathway in primary tracheobronchial epithelial (TBE) cells with H2O2 (20–200 microM) and observed the morphological changes, DNA laddering patterns, and DNA fragmentation associated with apoptosis. Elevation of ceramide with exogenous ceramide analogs was sufficient for apoptosis induction with the same characteristics and in the same time frame. H2O2 induced rapid sphingomyelin hydrolysis to ceramide, the elevation of which paralleled the induction of apoptosis. Furthermore, H2O2 acted directly on TBE cells membrane preparations devoid of nuclei, stimulating sphingomyelin hydrolysis through a neutral Mg2+ dependent sphingomyelinase (SMase). These data suggest that the formation of ceramide from sphingomyelin in the plasma membrane is a key event in H2O2-induced apoptosis in tracheobronchial epithelial cells.

Arsenic toxicity is enzyme specific and its affects on ligation are not caused by the direct inhibition of DNA repair enzymes

The molecular mechanism of arsenic toxicity is believed to be due to the ability of arsenite [As(III)] to bind protein thiols. Numerous studies have shown that arsenic is cytotoxic at micromolar concentrations. Micromolar As can also induce chromosomal damage and inhibit DNA repair. The mechanism of arsenic-induced genotoxicity is very important because arsenic is a human carcinogen, but not a mutagen, and there is a need to establish recommendations for safe levels of As in the environment. We have measured the dose-response for arsenic inhibition of several purified human DNA repair enzymes, including DNA polymerase beta, DNA ligase I and DNA ligase III and have found that most enzymes, even those with critical SH groups, are very insensitive to As. Many repair enzymes are activated by millimolar concentrations of As(III) and/or As(V). Only pyruvate dehydrogenase, one of eight purified enzymes examined so far, is inhibited by micromolar arsenic. In contrast to the purified enzymes, treatment of human cells in culture with micromolar arsenic produces a significant dose-dependent decrease in DNA ligase activity in nuclear extracts from the treated cells. However, the ligase activity in extracts from untreated cells is no more sensitive to arsenic than the purified enzymes. Our results show that direct enzyme inhibition is not a common toxic effect of As and that only a few sensitive enzymes are responsible for arsenic-induced cellular toxicity. Thus, arsenic-induced co-mutagenesis and inhibition of DNA repair is probably not the result of direct enzyme inhibition, but may be an indirect effect caused by As-induced changes in cellular redox levels or alterations in signal transduction pathways and consequent changes in gene expression.

Modulation of antioxidant responses by arsenic in maize

The effects of arsenic on the expression of the antioxidant genes encoding superoxide dismutase, catalase, and glutathione S-transferase, as well as the activity of SOD and CAT enzymes, were examined at different developmental stages and in different tissues. Both CAT and SOD activities increased in response to low concentrations (0.01-0.1 mM) of arsenic in developing maize embryos. In germinating embryos both CAT and SOD activities increased in response to a wide range of arsenic concentrations (0.01-10 mM). Cat1 transcript increased in response to arsenic in developing and germinating embryos and in young leaves. Conversely, Cat2 increased at low concentrations of arsenic only in germinating embryos. Cat3 transcript levels increased in response to low concentrations of arsenic only in developing embryos. Sod3 transcript increased at low concentrations of arsenic in developing, germinating embryos and in leaves. The cytosolic Sod4 and Sod4A increased in response to arsenic in germinating embryos, while only Sod4 transcript increased in response to arsenic in leaves. Expression of Gst1 was similar to that of Cat1 in all tissues examined. These results indicate that arsenic triggers tissue and developmental stage specific defense responses of antioxidant and detoxification related genes in maize.

The pharmacology and toxicology of polyphenolic-glutathione conjugates

Polyphenolic-glutathione (GSH) conjugates and their metabolites retain the electrophilic and redox properties of the parent polyphenol. Indeed, the reactivity of the thioether metabolites frequently exceeds that of the parent polyphenol. Although the active transport of polyphenolic-GSH conjugates out of the cell in which they are formed will limit their potential toxicity to those cells, once within the circulation they can be transported to tissues that are capable of accumulating these metabolites. There are interesting physiological similarities between the organs that are known to be susceptible to polyphenolic-GSH conjugate-mediated toxicity. In addition, the frequent localization of gamma-glutamyl transpeptidase to cells separating the circulation from a second fluid-filled compartment coincides with tissues that are susceptible either to polyphenolic-GSH conjugate-induced toxicity or to quinone and reactive oxygen species-induced toxicity. Polyphenolic-GSH conjugates therefore contribute to the nephrotoxicity, nephrocarcinogenicity, and neurotoxicity of a variety of polyphenols.

Differential regulation of extracellular signal-regulated kinase and nuclear factor-kappa B signal transduction pathways by hydrogen peroxide and tumor necrosis factor

Reactive oxygen metabolites are increasingly recognized for their ability to stimulate signal transduction pathways. This is important because these oxidants are frequently generated at sites of inflammation. However, little is known about the manner in which reactive oxygen species may selectively stimulate distinct signaling pathways. We have examined this question by stimulating mesothelial cells with hydrogen peroxide (H2O2) as a model oxidant stimulus. The response to H2O2 was examined by measuring the activation of the extracellular signal-regulated kinase (ERK1/2) and the nuclear factor-kappa B (NF-kappa B) signal transduction pathways. We found that H2O2 stimulated activity of the ERK1/2 pathway in a dose- and time-dependent manner. The ability of H2O2 to activate ERK1/2 was similar to that found with tumor necrosis factor (TNF) stimulation. The oxidant effect was inhibited by various reactive oxygen scavengers. An inhibitor of mitogen-activated protein kinase/extracellular signal-regulated kinase, the upstream kinase that activates ERK1/2, inhibited the oxidant effect. The superoxide anion (O2-) also stimulated ERK1/2 activity. In contrast, H2O2 did not stimulate proteolysis of I kappa B-alpha and induced only a small degree of NF-kappa B nuclear translocation. Stimulation of the cells with O2- also induced a minimal degree of NF-kappa B activation. TNF was a potent stimulus for I kappa B-alpha proteolysis and NF-kappa B activation, demonstrating that the cells did have a functional NF-kappa B pathway. These results suggest that oxidants may selectively stimulate certain pathways, thereby preserving some specificity of the signaling process. Furthermore, different cell types and distinct signaling pathways within cells may demonstrate unique profiles in the manner in which they respond to oxidant stimulation.

Thiol redox modulation of tumor necrosis factor-alpha responsiveness in cultured AIDS-related Kaposi's sarcoma cells

Both clinical and experimental evidence indicates that AIDS-related Kaposi's sarcoma (AIDS-KS) has a multifactorial pathogenesis with factors such as HIV viral load, latent virus induction, and opportunistic infections contributing to disease progression. However, a consistent feature that unites these apparently diverse putative etiologic agents is sustained serum elevations of pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-alpha). While virtually every cell responds to TNF-alpha with gene activation, the extent of TNF-alpha-mediated cellular signaling is regulated by a delicate balance between signal activation and signal arresting events. Reactive oxygen intermediates (ROI), which are generated as a consequence of TNF-alpha membrane interaction, are part of this TNF-alpha-initiated cellular activation cascade. Previous studies in our laboratory have shown that AIDS-KS cells possess impaired oxygen intermediate scavenging capacities, thereby establishing conditions permissive for the intracellular retention of ROI. In this study, we used cellular capacity to upregulate the cytoprotective enzyme superoxide dismutase (SOD) to address the extent of cellular response to TNF-alpha. Concurrent with the SOD analyses, nucleotide profiles were obtained to assess cellular bioenergetic responses during TNF-alpha challenge. Proliferative growth levels of mitochondrial (Mn)SOD activities showed an activity spectrum ranging from lowest activity in AIDS-KS cells, to intermediate levels in matched, nonlesional cells from the AIDS-KS donors, to highest activities in HIV normal fibroblasts. In contrast, following TNF-alpha challenge, the AIDS-KS and KS donor nonlesional cells showed a 11.89- and 5.86-fold respective increase in MnSOD activity, while the normal fibroblasts demonstrated a 1.35-fold decrease. Subsequent thiol redox modulation studies showed that only the normal fibroblast cultures showed a potentiation of TNF-alpha-mediated MnSOD upregulation following GSH depletion. In addition, provision of the GSH precursor, N-acetylcysteine during TNF-alpha challenge only diminished MnSOD activity and mitochondrial compartmentalization in the AIDS-KS cells, a finding that likely reflects the lower levels of reduced thiols in this cellular population. Our data, which show that a perturbation in their cellular thiol redox status accentuates AIDS-KS cellular responsiveness to TNF-alpha, suggest a biochemical rationale for the recognized TNF-alpha AIDS-KS clinical correlation.