Glutathione regulation of tumor necrosis factor-alpha-induced NF-kappa B activation in skeletal muscle-derived L6 cells

Tumor necrosis factor-alpha inhibits myogenesis through redox-dependent and -independent pathways

Muscle wasting accompanies diseases that are associated with chronic elevated levels of circulating inflammatory cytokines and oxidative stress. We previously demonstrated that tumor necrosis factor-alpha (TNF-alpha) inhibits myogenic differentiation via the activation of nuclear factor-kappaB (NF-kappaB). The goal of the present study was to determine whether this process depends on the induction of oxidative stress. We demonstrate here that TNF-alpha causes a decrease in reduced glutathione (GSH) during myogenic differentiation of C(2)C(12) cells, which coincides with an elevated generation of reactive oxygen species. Supplementation of cellular GSH with N-acetyl-l-cysteine (NAC) did not reverse the inhibitory effects of TNF-alpha on troponin I promoter activation and only partially restored creatine kinase activity in TNF-alpha-treated cells. In contrast, the administration of NAC before treatment with TNF-alpha almost completely restored the formation of multinucleated myotubes. NAC decreased TNF-alpha-induced activation of NF-kappaB only marginally, indicating that the redox-sensitive component of the inhibition of myogenic differentiation by TNF-alpha occurred independently, or downstream of NF-kappaB. Our observations suggest that the inhibitory effects of TNF-alpha on myogenesis can be uncoupled in a redox-sensitive component affecting myotube formation and a redox independent component affecting myogenic protein expression.

Mechanisms for reduction of endothelial activation by oleate: inhibition of nuclear factor-kappaB through antioxidant effects

In a model of early atherogenesis based on cultured endothelial cells, we observed that the incorporation of oleic acid in cellular lipids decreases the stimulated expression of several endothelial adhesion molecules and soluble products typically expressed during endothelial activation and involved in monocyte recruitment. We investigated possible mechanisms for this effect assessing the stimulated induction of nuclear factor-kappaB. In parallel, we also measured glutathione (GSH) content and the activity of antioxidant enzymes after oleate treatment and cytokine stimulation. Oleate prevented the stimulated depletion of GSH without any change in the activity of antioxidant enzymes. These results suggest an antioxidant mechanism by which oleate may exert direct vascular atheroprotective effects.

Ethyl pyruvate modulates inflammatory gene expression in mice subjected to hemorrhagic shock

Administration of pyruvate, an effective scavenger of reactive oxygen species, has been shown to be salutary in numerous models of redox-mediated tissue or organ injury. Pyruvate, however, is unstable in solution and, hence, is not attractive for development as a therapeutic agent. Herein, ethyl pyruvate, which is thought to be more stable than the parent compound, was formulated in a calcium-containing balanced salt solution [Ringer ethyl pyruvate solution (REPS)] and evaluated in a murine model of hemorrhagic shock and resuscitation (HS/R). Resuscitation with REPS instead of Ringer lactate solution (RLS) significantly improved survival at 24 h and abrogated bacterial translocation to mesenteric lymph nodes and the development of increased ileal mucosal permeability to FITC-labeled dextran (4,000 Da) at 4 h. Mice treated with REPS instead of RLS also had lower circulating levels of alanine aminotransferase at 4 h. Treatment with REPS instead of RLS decreased activation of nuclear factor-kappaB in liver and colonic mucosa after HS/R and also decreased the expression of inducible nitric oxide synthase, tumor necrosis factor, cyclooxygenase-2, and interleukin-6 mRNA in liver, ileal mucosa, and/or colonic mucosa. These data support the view that resuscitation with REPS modulates the inflammatory response and decreases hepatocellular and gut mucosal injury in mice subjected to HS/R.

Update on thiol status and supplements in physical exercise

Strenuous physical exercise represents a condition that is often associated with increased production of reactive oxygen species in various tissues. One of the most reliable indices of exercise-induced oxidant production is tissue glutathione oxidation. In humans, exercise-induced blood glutathione oxidation is rapid and subject to control by antioxidant supplementation. The objective of this brief review is to provide an update of our current understanding of cellular thiols and thiol antioxidants. Cellular thiols are critically important in maintaining the cellular antioxidant defense network. In addition, thiols play a key role in regulating redox-sensitive signal transduction process. Lipoic acid is a highly promising thiol antioxidant supplement. Recent studies have clarified that while higher levels of oxidants may indeed inflict oxidative damage, oxidants are not necessarily deleterious. Under certain conditions oxidants may function as cellular messengers that regulate a multitude of signal transduction pathways. In light of this, the significance of oxidants in various aspects of biology needs to be revisited.

Reactive oxygen species as mediators of organ dysfunction caused by sepsis, acute respiratory distress syndrome, or hemorrhagic shock: potential benefits of resuscitation with Ringer's ethyl pyruvate solution

Reactive oxygen species are reactive, partly reduced derivatives of molecular oxygen. Important reactive oxygen species in biological systems include superoxide radical anion, hydrogen peroxide, and hydroxyl radical. Peroxynitrite, is another important species in biological systems. A variety of enzymatic and non-enzymatic processes can generate reactive oxygen species in mammalian cells. An extensive body of experimental evidence from studies using animal models supports the view that reactive oxygen species are important in the pathogenesis of ischemia-reperfusion syndromes, sepsis, acute respiratory distress syndrome, and multiple organ dysfunction syndrome. This view is further supported by data from clinical studies that correlate biochemical evidence of reactive oxygen species-mediated stress with the development of acute respiratory distress syndrome or sepsis in patients. Ethyl pyruvate, a simple derivative of pyruvic acid, has been shown to be efficacious in several animal models of critical illness, and warrants further evaluation in this regard.

Tumor necrosis factor-alpha and muscle wasting: a cellular perspective

Tumor necrosis factor-alpha (TNF-alpha) is a polypeptide cytokine that has been associated with muscle wasting and weakness in inflammatory disease. Despite its potential importance in muscle pathology, the direct effects of TNF-alpha on skeletal muscle have remained undefined until recently. Studies of cultured muscle cells indicate that TNF-alpha disrupts the differentiation process and can promote catabolism in mature cells. The latter response appears to be mediated by reactive oxygen species and nuclear factor-kappaB which upregulate ubiquitin/proteasome activity. This commentary outlines our current understanding of TNF-alpha effects on skeletal muscle and the mechanism of TNF-alpha action.

tert-Butyl hydroperoxide-induced lipid signaling in hepatocytes: involvement of glutathione and free radicals

tert-Butyl hydroperoxide (TBHP) mobilizes arachidonic acid (AA) from membrane phospholipids in rat hepatocytes under cytotoxic conditions, thus leading to an increase in intracellular AA, which precedes cell death. In the present work, the involvement of lipid peroxidation, thiol status, and reactive oxygen species (ROS) in the intracellular AA accumulation induced by 0.5 mM TBHP was studied in rat hepatocytes. Cells treated with TBHP maintained viability and energy status at 10 min. However, TBHP depleted GSH, as well as inducing lipid peroxidation and ROS formation, detected by dichlorofluorescein (DCF) fluorescence. TBHP also significantly increased (32.5%) the intracellular [14C]-AA from [14C]-AA-labelled hepatocytes. The phospholipase A(2) (PLA(2)) inhibitor, mepacrine, completely inhibited the [14C]-AA response. The addition of antioxidants to the cell suspensions affected the TBHP-induced lipid response differently. The [14C]-AA accumulation correlated directly with ROS and negatively with endogenous GSH. No correlation between [14C]-AA and lipid peroxidation was found. Promethazine prevented lipid peroxidation and did not affect the [14C]-AA increase. We conclude that TBHP stimulates the release of [14C]-AA from membrane phospholipids through a PLA(2)-mediated mechanism. Endogenous GSH and ROS play a major role in this effect, while lipid peroxidation-related events are unlikely to be involved. Results suggest that specific ROS generated in iron-dependent reactions, different from lipid peroxyl radicals, are involved in PLA(2) activation, this process being important in TBHP-induced hepatocyte injury.

Rethinking cystic fibrosis pathology: the critical role of abnormal reduced glutathione (GSH) transport caused by CFTR mutation

Though the cause of cystic fibrosis (CF) pathology is understood to be the mutation of the CFTR protein, it has been difficult to trace the exact mechanisms by which the pathology arises and progresses from the mutation. Recent research findings have noted that the CFTR channel is not only permeant to chloride anions, but other, larger organic anions, including reduced glutathione (GSH). This explains the longstanding finding of extracellular GSH deficit and dramatically reduced extracellular GSH:GSSG (glutathione disulfide) ratio found to be chronic and progressive in CF patients. Given the vital role of GSH as an antioxidant, a mucolytic, and a regulator of inflammation, immune response, and cell viability via its redox status in the human body, it is reasonable to hypothesize that this condition plays some role in the pathogenesis of CF. This hypothesis is advanced by comparing the literature on pathological phenomena associated with GSH deficiency to the literature documenting CF pathology, with striking similarities noted. Several puzzling hallmarks of CF pathology, including reduced exhaled NO, exaggerated inflammation with decreased immunocompetence, increased mucus viscoelasticity, and lack of appropriate apoptosis by infected epithelial cells, are better understood when abnormal GSH transport from epithelia (those without anion channels redundant to the CFTR at the apical surface) is added as an additional explanatory factor. Such epithelia should have normal levels of total glutathione (though perhaps with diminished GSH:GSSG ratio in the cytosol), but impaired GSH transport due to CFTR mutation should lead to progressive extracellular deficit of both total glutathione and GSH, and, hypothetically, GSH:GSSG ratio alteration or even total glutathione deficit in cells with redundant anion channels, such as leukocytes, lymphocytes, erythrocytes, and hepatocytes. Therapeutic implications, including alternative methods of GSH augmentation, are discussed.

Activation of nuclear factor kappaB at the onset of ossification of the spinal ligaments

We examined the correlation between the activation of nuclear factor kappaB (NFkappaB), stimulated by environmental factors involving cytokines and growth factors in ligament cells, and the onset of ossification of the spinal ligaments (OSL) or diffuse idiopathic skeletal hyperostosis (DISH). Aseptic samples were taken carefully from non-ossified sites during surgery (75 patients). We carried out preliminary hematoxylin and eosin and toluidine blue staining, using five portions of each specimen, and excluded samples containing chondrocytic, osteoblastic, or inflammatory cells (n = 25). We used specimens from the remaining 50 patients (35 men and 15 women, ranging in age from 45-81 years); average age, 59.5 years (18 nuchal ligament specimens, and 32 yellow ligament specimens). OSL or DISH had occurred in 25 patients, 20 patients were in the non-OSL group (8 with cervical spondylotic myelopathy, and 12 with lumbar canal stenosis), and the remaining 5 samples were collected from patients with injury. For culture study, we used portions of the 14 largest samples from the above 50 patients. We extracted nuclear proteins and cytoplasmic proteins from non-ossified spinal ligaments in 50 patients and detected p65RelA/NFkappaB by Western blotting. Tumor necrosis factor-alpha (TNF alpha), interleukin 1beta (IL-1beta), platelet-derived growth factor BB (PDGF-BB) and transforming growth factor-beta1 (TGF-beta1) in cytoplasm were quantified by enzyme-linked immunosorbent assays (ELISA). Cultured cells from the 14 samples were then stimulated with 10, 100, 250, or 500 ng/ml of recombinant human (rh)PDGF-B or TGFbeta1. A control experiment was performed without rhPDGF-BB or TGFbeta1 stimulation. Alkaline phosphatase (ALP) activity was standardized by the DNA content of the cells. The number of NFkappaB-positive samples was significantly higher in patients with OSL or DISH than in non-OSL patients. This tendency was obvious in the case of OSL or DISH with non-insulin-dependent diabetes mellitus (NIDDM). In OSL and in DISH patients, significantly greater amounts of PDGF-BB and TGFbeta1 were seen in ligament cells than in non-OSL patients (P < 0.05). There was a positive correlation between the detection of p65RelA/NFkappaB band and the content of PDGF-BB and TGFbeta1 in ligament cells (P < 0.05). ALP activity tended to be higher in cells in the OSL group not receiving any other treatment. Our results indicate the possibility that NFkappaB, stimulated by environmental factors involving PDGF-BB and TGFbeta1 in ligament cells, influences the osteoblastic differentiation of undifferentiated mesenchymal cells.

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.

Capacity for recovery and possible mechanisms in immobilization atrophy of young and old animals

The effect of limb immobilization on muscle wasting and recovery of young and old rats was studied. Limb immobilization caused rapid and pronounced muscle weight loss, which was overcome efficiently in the muscles of young animals. However, muscles of old animals did not recover as well, indicating that muscle turnover (degradation and synthesis of proteins) is slower in old muscles than in young ones. The mechanisms of muscle wasting due to immobilization may involve two stages, the fast phase employing calcium-dependent proteolysis and the slower phase recruiting the lysosomal and ubiquitin-proteosome systems. The slow phase most probably involves the penetration of white cells between the muscle fibers and involves the secretion of cytokines that mediate a cascade of intracellular events, which culminates in muscle protein degradation. Thus, it was shown in our study and in other similar reports that through the influence of TNF-alpha and an increase in oxidative stress, there is marked activation of transcription factor NF-kappaB, which in turn induces many proteins to carry the signals that eventually result in protein breakdown. Because protein turnover was shown to slow down with age, it will be of great interest to study these events in aging muscles and to try to ascertain the specific events that make protein breakdown in aged muscles different from that in young ones.

Growth inhibition of subcutaneously transplanted hepatomas without cachexia by alteration of the dietary arginine-methionine balance

Previous studies have shown that alteration of the dietary arginine-methionine balance by use of synthetic L-amino acids inhibits tumor growth of a subcutaneously transplanted Morris hepatoma at the expense of maintaining body weight. However, L-methionine is susceptible to degradation and, therefore, may contribute to a deficiency state. The present studies were performed to determine whether growth of subcutaneous hepatoma transplants is inhibited, and body growth maintained, when rats are fed diets containing L-methionine in replacement of N-acetyl-L-methionine (NALM) for 28 days. Tumor-free and tumor-bearing rats fed a control diet, with amino acids replacing protein, had gains in body weight: 31.3 +/- 1.0 and 19.1 +/- 0.5 g (12% and 7%), respectively. Rats fed six experimental diets, with varying L-arginine-NALM balances, had body weight gains ranging from 18.4 +/- 0.3 to 26.7 +/- 0.9 g (7-10%). Tumor weight of control rats was 10.65 +/- 0.24% of body weight. Diets supplemented with L-arginine in combination with normal and deficient NALM decreased tumor weights by 35% and 38%, respectively, It is concluded that dietary replacement of L-methionine with NALM and supplementation with L-arginine inhibits growth of a subcutaneously transplanted Morris hepatoma in the absence of cachexia.

Cytokines and oxidative signalling in skeletal muscle

A growing body of literature indicates that cytokines regulate skeletal muscle function, including gene expression and adaptive responses. Tumour necrosis factor-alpha (TNF-alpha) is the cytokine most prominently linked to muscle pathophysiology and, therefore, has been studied most extensively in muscle-based systems. TNF-alpha is associated with muscle catabolism and loss of muscle function in human diseases that range from cancer to heart failure, from arthritis to AIDS. Recent advances have established that TNF-alpha causes muscle weakness via at least two mechanisms, accelerated protein loss and contractile dysfunction. Protein loss is a chronic response that occurs over days to weeks. Changes in gene expression required for TNF-alpha induced catabolism are regulated by the transcription factor nuclear factor-kappaB which is essential for the net loss of muscle protein caused by chronic TNF-alpha exposure. Contractile dysfunction is an acute response to TNF-alpha stimulation, developing over hours and resulting in decreased force production. Both actions of TNF-alpha involve a rapid rise in endogenous oxidants as an essential step in post-receptor signal transduction. These oxidants appear to include reactive oxygen species derived from mitochondrial electron transport. Such information provides insight into the cellular and molecular mechanisms of TNF-alpha action in skeletal muscle and establishes a scientific basis for continued research into cytokine signalling.

The transcription factors NF-kappab and AP-1 are differentially regulated in skeletal muscle during sepsis

Sepsis is associated with increased muscle proteolysis and upregulated transcription of several genes in the ubiquitin-proteasome proteolytic pathway. Glucocorticoids are the most important mediator of sepsis-induced muscle cachexia. Here, we examined the influence of sepsis in rats on the transcription factors NF-kappaB and AP-1 in skeletal muscle and the potential role of glucocorticoids in the regulation of these transcription factors. Sepsis was induced by cecal ligation and puncture (CLP). Control rats were sham-operated. NF-kappaB and AP-1 DNA binding activity was determined by electrophoretic mobility shift assay (EMSA) in extensor digitorum longus muscles at different time points up to 16 h after sham-operation or CLP. Sepsis resulted in an early (4 h) upregulation of NF-kappaB activity followed by inhibited NF-kappaB activity at 16 h. AP-1 binding activity was increased at all time points studied during the septic course. When rats were treated with the glucocorticoid receptor antagonist RU38486, NF-kappaB activity increased, whereas AP-1 activity was not influenced by RU38486. The results suggest that NF-kappaB and AP-1 are differentially regulated in skeletal muscle during sepsis and that glucocorticoids may regulate some but not all transcription factors in septic muscle.

X-linked vacuolated myopathy : TNF-alpha and IFN-gamma expression in muscle fibers with MHC class I on sarcolemma

The presence and the distribution of tumor necrosis factor-alpha, interferon-gamma, and p65 subunit of nuclear factor-kappaB, molecules known to induce synergistically and to mediate major histocompatibility complex (MHC) class I expression, were determined in muscle sections from control and X-linked vacuolated myopathy patients. MHC class I colocalized with tumor necrosis factor-alpha and interferon-gamma, as well as with p65, in most of the membrane attack complex- and/or calcium-positive muscle fibers in X-linked vacuolated myopathy. These results suggest that the expression of MHC class I in X-linked vacuolated myopathy could be induced by tumor necrosis factor-alpha and interferon-gamma and partly mediated by nuclear factor-kappaB.

Muscle cachexia: current concepts of intracellular mechanisms and molecular regulation

OBJECTIVE

To review present knowledge of intracellular mechanisms and molecular regulation of muscle cachexia.

SUMMARY BACKGROUND DATA

Muscle cachexia, mainly reflecting degradation of myofibrillar proteins, is an important clinical feature in patients with severe injury, sepsis, and cancer. The catabolic response in skeletal muscle may result in muscle wasting and weakness, delaying or preventing ambulation and rehabilitation in these patients and increasing the risk for pulmonary complications.

RESULTS

Muscle cachexia, induced by severe injury, sepsis, and cancer, is associated with increased gene expression and activity of the calcium/calpain- and ubiquitin/proteasome-proteolytic pathways. Calcium/calpain-regulated release of myofilaments from the sarcomere is an early, and perhaps rate-limiting, component of the catabolic response in muscle. Released myofilaments are ubiquitinated in the N-end rule pathway, regulated by the ubiquitin-conjugating enzyme E2(14k) and the ubiquitin ligase E3 alpha, and degraded by the 26S proteasome.

CONCLUSIONS

An understanding of the mechanisms regulating muscle protein breakdown is important for the development of therapeutic strategies aimed at treating or preventing muscle cachexia in patients with severe injury, sepsis, cancer, and perhaps other catabolic conditions as well.

NF kappa b signaling in posthypoxic endothelial cells: relevance to E-selectin expression and neutrophil adhesion

Our previous studies have implicated the nuclear transcription factor kappa B (NF kappa B) in the regulation of adhesion molecule expression in endothelial cells exposed to anoxia-reoxygenation (A/R) or a redox imbalance. The objectives of this study were (1) to define the kinetics of NF kappa B activation by examining I kappa B alpha degradation and the nuclear translocation of p65 in response to A/R or redox imbalance (induced by treatment of cells with diamide and buthionine sulfoximine) and (2) to determine whether the signal for I kappa B alpha degradation, nuclear translocation of p65, and E-selectin-mediated neutrophil adhesion is related to the activity of protein tyrosine kinase (PTK), protein tyrosine phosphatase (PTPase) and/or protein kinase C (PKC). The results demonstrate that both A/R and redox imbalance led to I kappa B alpha degradation within 30 min and the concomitant appearance of p65 in the nucleus, consistent with rapid cytosolic activation of NF kappa B and subsequent nuclear translocation of the activated p65 subunit. Inhibition of PKC blocked I kappa B alpha degradation and p65 translocation in A/R-challenged, but not redox-altered, endothelial cells. However, both A/R- and redox-induced NF kappa B activation was blocked by inhibition of PTK. Similarly, A/R-induced E-selectin expression and neutrophil-endothelial cell adhesion were blocked by inhibition of PKC or PTK, while only PTK inhibited the redox-induced adhesion response. Pretreatment of cells with N-acetyl cysteine effectively blocked A/R- or redox-induced I kappa B degradation and significantly attenuated the respective neutrophil adhesion responses. Collectively, these findings indicate that A/R-induced E-selectin expression and neutrophil-endothelial cell adhesion are mediated by both PKC and PTK, which signal rapid activation of NF kappa B. This A/R-induced NF kappa B signaling response appears to be mediated, at least in part, by intracellular redox imbalance.

N-acetylcysteine and endothelial cell injury by sulfur mustard

Understanding the underlying mechanisms of cell injury and death induced by the chemical warfare vesicant sulfur mustard (HD) will be extremely helpful in the development of effective countermeasures to this weapon of terror. We have found recently that HD induces both apoptosis and necrosis in endothelial cells (Toxicol. Appl. Pharmacol. 1996; 141: 568-583). Pretreatment of the endothelial cells for 20 h with the redox-active agent N-acetyl-L-cysteine (NAC) selectively prevented apoptotic death induced by HD. In this study, we tested the hypotheses that pretreatment with NAC acts through two different pathways to minimize endothelial injury by HD: NAC pretreatment acts via a glutathione (GSH)-dependent pathway; and NAC pretreatment acts to suppress HD-induced activation of the nuclear transcription factor NFkappaB. We used a fluorescence microscopic assay of apoptotic nuclear features to assess viability and electrophoretic mobility shift assays (EMSAs) to assess the activity of NFkappaB following exposure to HD. The cells were treated with 0-10 mM GSH for 1 h prior to and during exposure to 0 or 500 microM HD for 5-6 h. Cells were also treated with 50 mM NAC or 200 microM buthionine sulfoximine (BSO), an inhibitor of GSH synthesis, alone or in combination overnight prior to exposure to 0 or 500 microM HD for 5-6 h. Externally applied GSH up to a concentration of 5 mM had no toxic effect on the cells. Mild toxicity was associated with 10 mM GSH alone. There was a dose-related enhancement of viability when 2.5 and 5 mM GSH were present during the HD exposure. Pretreatment with BSO alone had no discernible toxicity. However, pretreatment with this inhibitor of GSH synthesis potentiated the toxicity of HD. Pretreatment with 50 mM NAC, as previously reported, provided substantial protection. Combining pretreatment with both BSO and NAC eliminated the protective effect of NAC pretreatment alone on HD injury. These observations are highly suggestive that NAC enhances endothelial survival via GSH-dependent effects and confirms and extends the work of others with different models that externally supplied GSH alone may be a fairly effective countermeasure against HD injury of endothelium. We next examined the hypothesis that HD may activate the nuclear transcription factor NFkappaB by performing EMSAs with nuclear extracts of endothelial cells following exposure to 0, 250 or 500 microM HD. This demonstrated an up to 2.5-fold increase (scanning densitometry) in activation of NFkappaB binding to its consensus sequence induced by 500 microM HD after 5 h of HD exposure. Paradoxically, treatment of the endothelial cells alone with 50 mM NAC activated NFkappaB, although HD-induced activation of NFkappaB was partially suppressed by NAC at 5 h. Factor NFkappaB is an important transcription factor for a number of cytokine genes (e.g. tumor necrosis factor, TNF), which can be activated following stress in endothelial cells. Taken together, these observations suggest that the protective effects of NAC may be mediated by enhanced GSH synthesis. The increased GSH may act to scavenge HD and also prevent oxidative activation of NFkappaB. Under some conditions, NAC may act as an oxidizing agent and thus increase NFkappaB activity. The NFkappaB-dependent gene expression may be important in inducing endothelial cell death as well as in generating a local inflammatory reaction associated with the release of endothelial-derived cytokines.

Reactive oxygen species in cell signaling

Reactive oxygen species (ROS) are generated as by-products of cellular metabolism, primarily in the mitochondria. When cellular production of ROS overwhelms its antioxidant capacity, damage to cellular macromolecules such as lipids, protein, and DNA may ensue. Such a state of "oxidative stress" is thought to contribute to the pathogenesis of a number of human diseases including those of the lung. Recent studies have also implicated ROS that are generated by specialized plasma membrane oxidases in normal physiological signaling by growth factors and cytokines. In this review, we examine the evidence for ligand-induced generation of ROS, its cellular sources, and the signaling pathways that are activated. Emerging concepts on the mechanisms of signal transduction by ROS that involve alterations in cellular redox state and oxidative modifications of proteins are also discussed.

Tunicamycin treatment reduces intracellular glutathione levels: effect on the metastatic potential of the rhabdomyosarcoma cell line S4MH

Highly metastatic cells are known to overexpress certain Asn-linked oligosaccharides in the plasmatic membrane. Another phenotypic characteristic of malignant cells consists in the expression of high levels of intracellular glutathione (GSH). The aim of the present work was to demonstrate that the inhibition of N-glycosylation induces changes in intracellular GSH levels, and in turn participates in the inhibition of the metastatic potential of tumor cells by tunicamycin treatment. Firstly, we demonstrated that in comparison to the poorly metastatic cell line F21, the highly metastatic cells S4MH express a higher number of Asn-linked beta1-6 branched oligosaccharides and sialic acid (SA) and/or chitobiose oligosaccharides in glycoproteins involved in the regulation of the adhesion efficiency of tumor cells on endothelial cells and extracellular matrix. Our results showed that the decrease in S4MH cell adhesion efficiency on endothelial cells and extracellular matrix after the inhibition of N-glycan processing by tunicamycin treatment was caused by: (1) inhibition of the expression of N-glycan structures recognized by endothelial endogenous lectins, including beta1-6 branched oligosaccharides and SA and/or chitobiose oligosaccharides, and (2) redistribution of cell surface glycoproteins with beta1-6 branched oligosaccharides and/or SA and/or chitobiose oligosaccharides in their structures, caused by the depletion of intracellular GSH levels. The latter condition prevents the organization of these glycoproteins in the plasmatic membrane of S4MH cells necessary for anchoring to the substratum.

NF-kappaB mediates the protein loss induced by TNF-alpha in differentiated skeletal muscle myotubes

Nuclear factor-kappaB (NF-kappaB) regulates the transcription of a variety of genes involved in immune responses, cell growth, and cell death. However, the role of NF-kappaB in muscle biology is poorly understood. We recently reported that tumor necrosis factor-alpha (TNF-alpha) rapidly activates NF-kappaB in differentiated skeletal muscle myotubes and that TNF-alpha acts directly on the muscle cell to induce protein degradation. In the present study, we ask whether NF-kappaB mediates the protein loss induced by TNF-alpha. We addressed this problem by creating stable, transdominant negative muscle cell lines. C2C12 myoblasts were transfected with viral plasmid constructs that induce overexpression of mutant I-kappaBalpha proteins that are insensitive to degradation via the ubiquitin-proteasome pathway. These mutant proteins selectively inhibit NF-kappaB activation. We found that differentiated myotubes transfected with the empty viral vector (controls) underwent a drop in total protein content and in fast-type myosin heavy-chain content during 72 h of exposure to TNF-alpha. In contrast, total protein and fast-type myosin heavy-chain levels were unaltered by TNF-alpha in the transdominant negative cell lines. TNF-alpha did not induce apoptosis in any cell line, as assessed by DNA ladder and annexin V assays. These data indicate that NF-kappaB is an essential mediator of TNF-alpha-induced catabolism in differentiated muscle cells.

Intra- and extracellular measurement of reactive oxygen species produced during heat stress in diaphragm muscle

Skeletal muscles are exposed to increased temperatures during intense exercise, particularly in high environmental temperatures. We hypothesized that heat may directly stimulate the reactive oxygen species (ROS) formation in diaphragm (one kind of skeletal muscle) and thus potentially play a role in contractile and metabolic activity. Laser scan confocal microscopy was used to study the conversion of hydroethidine (a probe for intracellular ROS) to ethidium (ET) in mouse diaphragm. During a 30-min period, heat (42 degrees C) increased ET fluorescence by 24 +/- 4%, whereas in control (37 degrees C), fluorescence decreased by 8 +/- 1% compared with baseline (P < 0.001). The superoxide scavenger Tiron (10 mM) abolished the rise in intracellular fluorescence, whereas extracellular superoxide dismutase (SOD; 5,000 U/ml) had no significant effect. Reduction of oxidized cytochrome c was used to detect extracellular ROS in rat diaphragm. After 45 min, 53 +/- 7 nmol cytochrome c. g dry wt(-1). ml(-1) were reduced in heat compared with 22 +/- 13 nmol. g(-1). ml(-1) in controls (P < 0.001). SOD decreased cytochrome c reduction in heat to control levels. The results suggest that heat stress stimulates intracellular and extracellular superoxide production, which may contribute to the physiological responses to severe exercise or the pathology of heat shock.

Cellular thiols and redox-regulated signal transduction

In contrast to the conventional notion that reactive oxygen is mostly a trigger for oxidative damage of biological structures, now we know that low physiologically relevant concentrations of ROS can regulate a variety of key molecular mechanisms that may be linked with important cell functions (Fig. 4). Redox-based regulation of gene expression has emerged as a fundamental regulatory mechanism in cell biology. Several proteins, with apparent redox-sensing activity, have been described. Electron flow through side-chain functional CH2-SH groups of conserved cysteinyl residues in these proteins account for the redox-sensing properties. Protein thiol groups with high thiol-disulfide oxidation potentials are likely to be redox-sensitive. The ubiquitous endogenous thiols thioredoxin and glutathione are of central importance in redox signaling. Signals are transduced from the cell surface to the nucleus through phosphorylation and dephosphorylation chain reactions of cellular proteins at tyrosine and serine/threonine. Protein phosphorylation, one of the most fundamental mediators of cell signaling, is redox-sensitive. DNA-binding proteins are involved in the regulation of cellular processes such as replication, recombination, viral integration and transcription. Several studies show that the interaction of certain transcription regulatory proteins with their respective cognate DNA sites is also redox-regulated. Changes in the concentration of Ca2+i control a wide variety of cellular functions, including transcription and gene expression; Ca(2+)-driven protein phosphorylation and proteolytic processing of proteins are two major intracellular events that are implicated in signal transduction from the cell surface to the nucleus. Intracellular calcium homeostasis is regulated by the redox state of cellular thiols, and it is evident that cell calcium may play a critical role in the activation of the redox-sensitive transcription factor NF-kappa B. Among the several thiol agents tested for their efficacy in modulating cellular redox status, N-acetyl-L-cysteine and alpha-lipoic acid hold most promise for human use. A strong therapeutic potential of strategies that would modulate the cellular thioredoxin system has been also evident.

Thiol homeostasis and supplements in physical exercise

Thiols are a class of organic sulfur derivatives (mercaptans) characterized by the presence of sulfhydryl residues. In biological systems, thiols have numerous functions, including a central role in coordinating the antioxidant defense network. Physical exercise may induce oxidative stress. In humans, a consistent marker of exercise-induced oxidative stress is blood glutathione oxidation. Physical training programs have specific effects on tissue glutathione metabolism that depend on the work program and the type of tissue. Experimental studies show that glutathione metabolism in several tissues sensitively responds to an exhaustive bout of exercise. Study of glutathione-deficient animals clearly indicates the central importance of having adequate tissue glutathione to protect against exercise-induced oxidative stress. Among the various thiol supplements studied, N-acetyl-L-cysteine and alpha-lipoic acid hold the most promise. These agents may have antioxidant effects at the biochemical level but are also known to influence redox-sensitive cell signaling.

Glucocorticoids induce proteasome C3 subunit expression in L6 muscle cells by opposing the suppression of its transcription by NF-kappa B

Muscle wasting in catabolic conditions results from activation of the ubiquitin-proteasome proteolytic pathway by a process that requires glucocorticoids and is generally associated with increased levels of mRNAs encoding components of this proteolytic system. In L6 muscle cells, dexamethasone stimulates proteolysis and increases the amount of the proteasome C3 subunit protein by augmenting its transcription. Transfection studies with human C3 promoter-luciferase reporter genes and electrophoretic mobility shift assays revealed that a NF-kappaB.protein complex containing Rel A is abundant in L6 muscle cell nuclei. Glucocorticoids stimulate C3 subunit expression by antagonizing the interaction of this NF-kappaB protein with an NF-kappaB response element in the C3 subunit promoter region. Dexamethasone also increased the cytosolic amounts of the NF-kappaB p65 subunit and the IkappaBalpha inhibitor proteins in L6 cells. Incubation of L6 cells with a cytokine mixture not only increased the amount of activated NF-kappaB but also decreased C3 promoter activity and lowered endogenous C3 subunit mRNA. Thus, NF-kappaB is a repressor of C3 proteasome subunit transcription in muscle cells, and glucocorticoids stimulate C3 subunit expression by opposing this suppressor action.

Induction of oxidative stress and TNF-alpha secretion by dichloroacetonitrile, a water disinfectant by-product, as possible mediators of apoptosis or necrosis in a murine macrophage cell line (RAW)

The water disinfectant by-product dichloroacetonitrile (DCAN) is a direct-acting mutagen and induces DNA strand breaks in cultured human lymphoblastic cells. Cellular activation by environmental agents may exert detrimental effects to the cells. Activated macrophages produce reactive oxygen intermediates such as H(2)O(2), (-)OH and O(2). Therefore, the effect of various concentrations of DCAN (100-400 microM) on the activity macrophage cells (RAW 264.7) was studied. In these cells, DCAN-induced oxidative stress was characterized by the production of reactive oxygen intermediates (ROI). Also, the ratios of intracellular GSH/GSSG was assessed and used as a biomarker for oxidative stress. The secretion of TNF-alpha was assessed since macrophages are known to secrete TNF-alpha as a result of cellular oxidative stress. Electrophoretic detection of DNA degradation and light microscopy was utilized for the characterization of DCAN-induced apoptosis. Lactate dehydrogenase (LDH) leakage and trypan blue exclusion were used as markers of cellular necrosis. Following exposure to DCAN (200 microM and 400 microM), intracellular GSSG was increased (2.5-fold of control, P<0. 05). DCAN activation of RAW cells was detected by elevated levels of intracellular ROI (1.9-2.5-fold than control, P<0.05) and increased secretion of TNF-alpha (4.5 fold-than control, P <0.05). Elecrophoresis of genomic DNA of treated cells indicated a dose-dependent increase in degradation of genomic DNA. Morphological studies also indicated that exposure of RAW cells to 100 microM or 200 microM DCAN incites apoptotic cell death. At higher concentrations (400 microM), however, significant (P<0.05) increase in LDH leakage and decrease in cell viability (55% of control) indicative of cellular necrosis, were observed. These studies indicate that DCAN induces dose-dependent apoptosis or necrosis in RAW cells that could be due to the disturbance in intracellular redox status and initiation of ROI-mediated oxidative mechanisms of cellular damage.

Regulation of antioxidant enzyme gene expression in response to oxidative stress and during differentiation of mouse skeletal muscle

Various properties of skeletal muscle, including high metabolic activity and high levels of heme-containing proteins, render it particularly susceptible to free radical injury. Indeed, cellular injury from reactive oxygen species (ROS) has been implicated in many muscle disorders. Thus muscle cell survival is critically dependent on the ability of the cell to respond to periods of oxidative stress. To investigate this important homeostatic response, we studied the effect of oxidative challenges on the expression of genes encoding the antioxidant enzymes Cu,Zn-superoxide dismutase (CuZnSOD), Mn-superoxide dismutase (MnSOD), glutathione peroxidase (GPx), and catalase (CAT) in myotube cultures. Using Northern blot analysis, we found that treatment with the pro-oxidant paraquat resulted in time- and dose-dependent increases of transcript levels that were greatest for GPx and CAT (approximately 4-5 fold). CuZnSOD and MnSOD transcripts were also increased, albeit more modestly (approximately 2-3 fold). Transcript levels were also induced by treatment of the cells with two other pro-oxidants, menadione and H2O2, and correlated with the level of oxidative injury to the cells, measured as protein carbonyl group formation. Activities of all of the enzymes increased in response to the oxidative challenges, although the magnitudes of the increases were less robust than the increases of the respective transcript levels. In studying the effect of cellular differentiation on antioxidant gene expression and susceptibility to oxidative stress, we found that pro-oxidant treatment resulted in greater oxidative injury to differentiated myotubes than to undifferentiated myoblasts. Furthermore, the increased susceptibility of myotubes correlated with decreased antioxidant defenses-as muscle cells differentiated, both transcript and activity levels of antioxidant enzymes decreased. These data suggest that muscle cells regulate antioxidant defenses in response to oxidative stress and cellular differentiation.

Overexpression of gamma-glutamylcysteine synthetase suppresses tumor necrosis factor-induced apoptosis and activation of nuclear transcription factor-kappa B and activator protein-1

Tumor necrosis factor (TNF) is a highly pleiotropic cytokine whose activity is at least partially regulated by the redox status of the cell. The cellular redox status is controlled primarily by glutathione, a major cellular antioxidant, whose synthesis is regulated by the rate-limiting enzyme gamma-glutamylcysteine synthetase (gamma-GCS). In the present report we investigated the effect of gamma-GCS overexpression on the TNF-induced activation of nuclear transcription factors NF-kappa B and AP-1, stress-activated protein kinase/c-Jun amino-terminal kinase (JNK) and apoptosis. Transfection of cells with gamma-GCS cDNA blocked TNF-induced NF-kappa B activation, cytoplasmic I kappa B alpha degradation, nuclear translocation of p65, and NF-kappa B-dependent gene transcription. gamma-GCS overexpression also completely suppressed NF-kappa B activation induced by phorbol ester and okadaic acid, whereas that induced by H2O2, ceramide, and lipopolysaccharide was minimally affected. gamma-GCS also abolished the activation of AP-1 induced by TNF and inhibited TNF-induced activation of JNK and mitogen-activated protein kinase kinase. TNF-mediated cytotoxicity and activation of caspase-3 were both abrogated in gamma-GCS-overexpressing cells. Overall, our results indicate that most of the pleiotropic actions of TNF are regulated by the glutathione-controlled redox status of the cell.

The effect of exercise training on oxidative damage of lipids, proteins, and DNA in rat skeletal muscle: evidence for beneficial outcomes

Moderate daily exercise is known to be beneficial to health, reducing risks of a number of age-related disorders. Molecular mechanisms that bring about these effects are not clear. In contrast, it has been claimed that some types of prolonged physical exertion are detrimental to health because active oxygen species are generated excessively by enhanced oxygen consumption. Using two age groups of rats, young (4 week) and middle aged (14 months), we investigated the effects of long-term swimming training on the oxidative status of phospholipids, proteins, and DNA. The concentration of thiobarbituric acid reactive substances and 4-hydroxynonenal protein adducts did not differ in the gastrocnemius muscle between exercised and nonexercised animals in the two age groups. The extent of carbonylation in a protein of molecular weight around 29 KDa and the amount of 8-hydroxydeoxyguanosine in nuclear DNA were smaller (p<.05) in the exercised rats than in the sedentary animals. Activities of DT-diaphorase (C1: 29.3+/-1.9; C2: 36.1+/-2.6; E1: 27.2+/-1.3; C2: 33.4+/-2.9 nmol/mg protein) and proteasome, a major proteolytic enzyme for oxidatively modified proteins were significantly higher in the exercised animals of both age groups (p<.05). The adaptive response against oxidative stress induced by moderate endurance exercise constitutes a beneficial effect of exercise.

Cytokine-induced glucose uptake in skeletal muscle: redox regulation and the role of alpha-lipoic acid

In L6 myotubes, glucose uptake stimulated by interferon (IFN)-gamma or lipopolysaccharides (LPS) and a combination of LPS, IFN-gamma, and tumor necrosis factor (TNF)-alpha was inhibited by the antioxidant pyrrolidinedithiocarbamate and potentiated in reduced glutathione (GSH)-deficient cells. Also, the stimulatory effect of LPS and IFN-gamma individually, and of a combination of LPS, IFN-gamma, and TNF-alpha, on glucose uptake was associated with an increased level of intracellular oxidants (dichlorofluorescein assay) and loss of intracellular GSH. Study of the individual effects of LPS, IFN-gamma, and TNF-alpha as well as of a combination of the three activators provided evidence against a role of nitric oxide in mediating the stimulatory effect of the above-mentioned agents on glucose uptake. We also observed that the insulin-mimetic nutrient alpha-lipoic acid (LA; R-enantiomer) is able to stimulate glucose uptake in cytokine-treated cells that are insulin resistant. This study shows that cytokine-induced glucose uptake in skeletal muscle cells is redox sensitive and that, under conditions of acute infection that is accompanied with insulin resistance, LA may have therapeutic implications in restoring glucose availability in tissues such as the skeletal muscle.

Molecular mechanisms of neutrophil-endothelial cell adhesion induced by redox imbalance

Previous studies have implicated a role for intracellular thiols in the activation of nuclear factor-kappaB and transcriptional regulation of endothelial cell adhesion molecules. This study was designed to determine whether changes in endothelial cell glutathione (GSH) or oxidized glutathione (GSSG) can alter neutrophil adhesivity and to define the molecular mechanism that underlies this GSSG/GSH-induced adhesion response. Treatment of human umbilical vein endothelial cell (HUVEC) monolayers for 6 hours with 0.2 mmol/L diamide and 1 mmol/L buthionine sulfoximine (BSO) decreased GSH levels and increased the ratio of GSSG to GSH without cell toxicity. These redox changes are similar to those observed with anoxia/reoxygenation. Diamide plus BSO-induced thiol/disulfide imbalance was associated with a biphasic increase in neutrophil adhesion to HUVECs with peak responses observed at 15 minutes (phase 1) and 240 minutes (phase 2). N-Acetylcysteine treatment attenuated neutrophil adhesion in both phases, which indicated a role for GSH in the adhesion responses. Interestingly, phase 1 adhesion was inversely correlated with GSH levels but not with the GSSG/GSH ratio, whereas phase 2 neutrophil adhesion was positively correlated with GSSG/GSH ratio but not with GSH levels. Intercellular adhesion molecule-1 and P-selectin-specific monoclonal antibodies attenuated the increased neutrophil adhesion during both phases, whereas an anti-E-selectin monoclonal antibody also attenuated the phase 2 response. Pretreatment with actinomycin D and cycloheximide or with competing ds-oligonucleotides that contained nuclear factor-kappaB or activator protein-1 cognate DNA sequences significantly attenuated the phase 2 response, which implicated a role for de novo protein synthesis. Surface expression of intercellular adhesion molecule-1, P-selectin, and E-selectin on HUVECs correlated with the phase 1 and 2 neutrophil adhesion responses. This study demonstrates that changes in endothelial cell GSSG/GSH cause transcription-independent and transcription-dependent surface expression of different endothelial cell adhesion molecules, which leads to a 2-phase neutrophil-endothelial adhesion response.

Nuclear factor kappa B activity in response to oxidants and antioxidants

The goal of this chapter was to review the current protocols that are available to measure the activation of NF-kappa B. The methods discussed all have their pitfalls when used in isolation. To obtain meaningful information, nuclear translocation and transcriptional activation should be studied in conjunction. Study of NF-kappa B regulated protein expression is the most physiologically relevant approach to monitoring the transcription regulatory effect of NF-kappa B. Because of the limitations of transcriptional analysis in primary cell cultures or tissues, incorporation of multiple approaches is recommended when the involvement of NF-kappa B in a disease process is evaluated.

Mitochondria mediate tumor necrosis factor-alpha/NF-kappaB signaling in skeletal muscle myotubes

Tumor necrosis factor-alpha (TNF-alpha) is implicated in muscle atrophy and weakness associated with a variety of chronic diseases. Recently, we reported that TNF-alpha directly induces muscle protein degradation in differentiated skeletal muscle myotubes, where it rapidly activates nuclear factor kappaB (NF-kappaB). We also have found that protein loss induced by TNF-alpha is NF-kappaB dependent. In the present study, we analyzed the signaling pathway by which TNF-alpha activates NF-kappaB in myotubes differentiated from C2C12 and rat primary myoblasts. We found that activation of NF-kappaB by TNF-alpha was blocked by rotenone or amytal, inhibitors of complex I of the mitochondrial respiratory chain. On the other hand, antimycin A, an inhibitor of complex III, enhanced TNF-alpha activation of NK-kappaB. These results suggest a key role of mitochondria-derived reactive oxygen species (ROS) in mediating NF-kappaB activation in muscle. In addition, we found that TNF-alpha stimulated protein kinase C (PKC) activity. However, other signal transduction mediators including ceramide, Ca2+, phospholipase A2 (PLA2), and nitric oxide (NO) do not appear to be involved in the activation of NF-kappaB.

Skeletal muscle myocytes undergo protein loss and reactive oxygen-mediated NF-kappaB activation in response to tumor necrosis factor alpha

Skeletal muscle atrophy and weakness are thought to be stimulated by tumor necrosis factor alpha (TNF-alpha) in a variety of chronic diseases. However, little is known about the direct effects of TNF-alpha on differentiated skeletal muscle cells or the signaling mechanisms involved. We have tested the effects of TNF-alpha on the mouse-derived C2C12 muscle cell line and on primary cultures from rat skeletal muscle. TNF-alpha treatment of differentiated myotubes stimulated time- and concentration-dependent reductions in total protein content and loss of adult myosin heavy chain (MHCf) content; these changes were evident at low TNF-alpha concentrations (1-3 ng/ml) that did not alter muscle DNA content and were not associated with a decrease in MHCf synthesis. TNF-alpha activated binding of nuclear factor kappaB (NF-kappaB) to its targeted DNA sequence and stimulated degradation of I-kappaBalpha, an NF-kappaB inhibitory protein. TNF-alpha stimulated total ubiquitin conjugation whereas a 26S proteasome inhibitor (MG132 10-40 microM) blocked TNF-alpha activation of NF-kappaB. Catalase 1 kU/ml inhibited NF-kappaB activation by TNF-alpha; exogenous hydrogen peroxide 200 microM activated NF-kappaB and stimulated I-kappaBalpha degradation. These data demonstrate that TNF-alpha directly induces skeletal muscle protein loss, that NF-kappaB is rapidly activated by TNF-alpha in differentiated skeletal muscle cells, and that TNF-alpha/NF-kappaB signaling in skeletal muscle is regulated by endogenous reactive oxygen species.

A positively charged alpha-lipoic acid analogue with increased cellular uptake and more potent immunomodulatory activity

alpha-Lipoic acid (LA) is taken up by cells and reduced to its potent dithiol form, dihydrolipoate(DHLA), much of which is rapidly effluxed out from cells. To improve retention in cells, the LA molecule was modified to confer a positive charge at physiological pH. N,N-dimethyl,N'-2-amidoethyl-lipoate was synthesized. The protonated form of the new molecule is referred to as LA-Plus. The uptake of LA-Plus by human Wurzburg T cells was higher compared to that of LA. Several-fold higher amounts of DHLA-Plus, the corresponding reduced form of LA-Plus, were detected in LA-Plus treated cells compared to the amount of DHLA found in cells treated with LA. At 100 microM, LA did not but LA-Plus inhibited H2O2 induced NF-kappaB activation and NF-kappaB directed IL-2 receptor expression. Both LA and LA-Plus synergised with selenium in inhibiting H2O2 induced NF-kappaB activation. At 150 microM LA-Plus, but not LA, inhibited TNFalpha induced NF-kappaB activation. At 5 microM LA-Plus, but not LA, protected against both spontaneous and etoposide induced apoptosis in rat thymocytes. LA-Plus is thus an improved form of LA with increased therapeutic potential.

Redox signaling and the emerging therapeutic potential of thiol antioxidants

Oxidation-reduction (redox) based regulation of signal transduction and gene expression is emerging as a fundamental regulatory mechanism in cell biology. Electron flow through side chain functional CH2-SH groups of conserved cysteinyl residues in proteins account for their redox-sensing properties. Because in most intracellular proteins thiol groups are strongly "buffered" against oxidation by the highly reduced environment inside the cell, only accessible protein thiol groups with high thiol-disulfide oxidation potentials are likely to be redox sensitive. The list of redox-sensitive signal transduction pathways is steadily growing, and current information suggests that manipulation of the cell redox state may prove to be an important strategy for the management of AIDS and some forms of cancer. The endogenous thioredoxin and glutathione systems are of central importance in redox signaling. Among the thiol agents tested for their efficacy to modulate cellular redox status, N-acetyl-L-cysteine (NAC) and alpha-lipoic acid hold promise for clinical use. A unique advantage of lipoate is that it is able to utilize cellular reducing equivalents, and thus it harnesses the metabolic power of the cell to continuously regenerate its reductive vicinal dithiol form. Because lipoate can be readily recycled in the cell, it has an advantage over N-acetyl-L-cysteine on a concentration:effect basis. Our current knowledge of redox regulated signal transduction has led to the unfolding of the remarkable therapeutic potential of cellular thiol modulating agents.