Redox regulation of signal transduction: tyrosine phosphorylation and calcium influx

Regulation of Tyrosine Phosphorylation in Isolated T Cell Membrane by Inhibition of Protein Tyrosine Phosphatases

Jurkat T cells activated by the phosphotyrosine phosphatase inhibitors H2O2 or vanadate were found to have a similar pattern of tyrosine phosphorylation when compared with T cells stimulated by anti-CD3 Ab cross-linking, suggesting that protein tyrosine phosphatase (PTP) inhibitors affect the early steps of TCR signaling. To study the role of PTPs in the most proximal membrane events of tyrosine phosphorylation, subcellular fractions of T cells were treated with the PTP inhibitors in the presence of ATP. In the membrane fraction, tyrosine phosphorylation of Lck, Fyn, and CD3ζ can be induced by PTP inhibitors, but not by anti-CD3. Detailed characterization of this cell-free system showed that the pattern and the order of induced tyrosine phosphorylation is similar to that induced in intact cells. Upon removal of the PTP inhibitor, the tyrosine-phosphorylated proteins, including Lck, Fyn, Syk, Zap70, and CD3ζ, are rapidly dephosphorylated. Preliminary characterizations indicate that a PTP distinct from CD45, SHP1, and SHP2 is present in T cell membranes and the inhibition of this yet unidentified PTP is most likely responsible for the Lck-dependent tyrosine phosphorylation triggered by PTP inhibitors.

Serum factors and the cellular redox status regulate cellular responsiveness to MHC class II-triggered homotypic B cell adhesion

Cross-linking the major histocompatibility complex (MHC) class II (MHC-II) by their cognate ligands including mAb induces homotypic cell adhesion. It has been shown that merely surface MHC-II expression is insufficient to induce the response. We found that MHC-II-triggered cell adhesion in human B cell lines was absent when serum was removed from medium. Even in the presence of serum, the response was prevented when cells were treated with the glutathione synthesis inhibitor, buthionine sulfoximine, an irreversible catalase inhibitor, aminotriazole, and H2O2. Flow cytometrical analysis showed that these changes in cellular responsiveness were unlikely to be merely the result of altered surface Ag expression. In addition, the response was independent of the two major lymphocyte adhesion receptors, LFA-1 and intercellular adhesion molecule-1 (ICAM-1). These findings suggest that serum- and redox-sensitive intracellular events regulate cellular responsiveness to MHC-II-triggered LFA-1/ICAM-1-independent cell adhesion independently of differentiation.

The regulation of reactive oxygen species production during programmed cell death

Reactive oxygen species (ROS) are thought to be involved in many forms of programmed cell death. The role of ROS in cell death caused by oxidative glutamate toxicity was studied in an immortalized mouse hippocampal cell line (HT22). The causal relationship between ROS production and glutathione (GSH) levels, gene expression, caspase activity, and cytosolic Ca2+ concentration was examined. An initial 5-10-fold increase in ROS after glutamate addition is temporally correlated with GSH depletion. This early increase is followed by an explosive burst of ROS production to 200-400-fold above control values. The source of this burst is the mitochondrial electron transport chain, while only 5-10% of the maximum ROS production is caused by GSH depletion. Macromolecular synthesis inhibitors as well as Ac-YVAD-cmk, an interleukin 1beta-converting enzyme protease inhibitor, block the late burst of ROS production and protect HT22 cells from glutamate toxicity when added early in the death program. Inhibition of intracellular Ca2+ cycling and the influx of extracellular Ca2+ also blocks maximum ROS production and protects the cells. The conclusion is that GSH depletion is not sufficient to cause the maximal mitochondrial ROS production, and that there is an early requirement for protease activation, changes in gene expression, and a late requirement for Ca2+ mobilization.

Reactive oxygen species and intracellular Ca2+, common signals for apoptosis induced by gallic acid

Gallic acid (3,4,5-trihydroxybenzoic acid), a naturally occurring plant phenol, induces cell death in apparently different manners, depending on cell lines. Flow cytometric analysis and agarose gel electrophoresis indicated that internucleosomal breakdown of chromatin DNA was observed in HL-60RG cells but not in dRLh-84, HeLa, and PLC/PRF/5 cells, and that the action of gallic acid was independent of cell cycle. A detailed study of signal transduction revealed that the gallic acid-induced cell death of all cells tested in this study was prevented by treatment with the intracellular thiol antioxidant N-acetyl-L-cysteine, catalase, and the intracellular calcium chelator bis-(o-aminophenoxy)-N,N,N,N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM). However, the effects of ascorbic acid, superoxide dismutase, EGTA, the endonuclease inhibitor zinc sulfate, the calmodulin inhibitor N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), and the NADPH oxidase inhibitor diphenyleneiodonium chloride on cell death were different depending on the cell type, suggesting that the death signal induced by gallic acid was diverse among different cell types, although the production of reactive oxygen species, such as H2O2, and the elevation of intracellular calcium concentration were required as common signals.

Hydrogen Peroxide Induces Up-Regulation of Fas in Human Endothelial Cells

Hydrogen peroxide (H2O2), an oxidant generated by inflammatory cells, is an important mediator of injury of endothelial cells (ECs). Here we show that H2O2 induces up-regulation of the expression of Fas, a death signal, in human ECs in culture. Flow cytometric analysis with a mAb against human Fas showed that incubation for 24 h with H2O2 induced a dose-dependent increase in the level of Fas in ECs. Coincubation with catalase, which rapidly degrades H2O2, inhibited H2O2-induced up-regulation of Fas. H2O2 also induced a dose-dependent increase in Fas mRNA level. A significant increase in Fas mRNA levels was observed from 6 h after stimulation with H2O2. Vanadate, a protein phosphatase inhibitor, significantly enhanced Fas mRNA and protein levels in H2O2-treated ECs. On the other hand, genistein, a tyrosine kinase inhibitor, inhibited H2O2-induced Fas mRNA expression. Furthermore, a flow cytometric method with propidium iodide staining and electron microscopic analysis showed that incubation with an agonistic Ab against Fas (anti-Fas IgM) induced apoptosis in H2O2-treated cells. These findings suggest that H2O2 induces up-regulation of Fas in ECs and that activation of protein tyrosine kinase may be involved in the mechanism of H2O2-induced Fas expression. Therefore, Fas-mediated apoptosis may have a pathologic role in H2O2-induced EC injury and thereby provide a new therapeutic target.

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.

Alterations in human B cell calcium homeostasis by polycyclic aromatic hydrocarbons: possible associations with cytochrome P450 metabolism and increased protein tyrosine phosphorylation

Previous studies performed in this laboratory have shown that certain benzo(a)pyrene (BaP) metabolites, such as benzo(a)pyrene-7,8-dihydrodiol (BaP-7,8-diol) and benzo(a)pyrene-7,8-dihydrodiol-9,10-epoxide (BPDE), were more effective in elevating intracellular Ca2+ in normal human peripheral blood mononuclear cell (HPBMC) T and B cells than was BaP. Additionally, it has been shown that the suppression of human T cell mitogenesis produced by polycyclic aromatic hydrocarbons (PAHs) and certain BaP metabolites is reversed by treatment with alpha-naphthoflavone (ANF), a cytochrome P450 1A and 1B inhibitor. ANF also diminishes the elevation in intracellular calcium (Ca2+) produced by BaP in HPBMC. In the present studies, we further defined the relationships between intracellular Ca2+ elevation produced by BaP and two immunotoxic P450-derived metabolites, BaP-7,8-diol and BPDE in the Daudi human B cell line. At 1, 4, and 18 h, both BaP-7,8-diol and BPDE produced a significant rise in intracellular Ca2+. This effect, however, was not observed with BaP or benzo(e)pyrene (BeP), a nonimmunotoxic PAH. To evaluate the potential role of cytochrome P450 metabolism in PAH-induced Ca2+ elevation, Daudi cells were pretreated with ANF for 4 h, followed by treatment with BaP metabolites for 18 h. ANF completely reversed the rise in Ca2+ produced by BaP-7,8-diol, but had no effect on the Ca2+ elevation produced by BPDE. These results suggest that BPDE may be the ultimate P450 metabolite responsible for Ca2+ elevation in human B cells. BaP-7,8-diol and BPDE were found to increase tyrosine phosphorylation in Daudi whole cell lysates and to increase tyrosine phosphorylation of two important Src-related protein tyrosine kinases (PTKs), Lyn and Syk. Inhibition of tyrosine phosphorylation by herbimycin A was found to largely prevent the increase in intracellular Ca2+ produced by BaP-7,8-diol and BPDE, suggesting that Ca2+ elevation is coupled to increased tyrosine phosphorylation in Daudi. BPDE was found to produce a statistically significant increase in tyrosine phosphorylation of Lyn and Syk within 10 min of exposure. Collectively, these data demonstrate that certain P450-derived metabolites of BaP may be responsible for PTK activation and an increase intracellular Ca2+, which may alter antigen receptor signaling in human B cells.

Glutathione and HIV infection: reduced reduced, or increased oxidized?

Glutathione is the main intracellular defence against oxidative stress and regulates the cellular redox potential. HIV infection is accompanied by severe metabolic and immune dysfunctions. Several laboratories have demonstrated that the intracellular redox balance is disturbed in CD4+ T cells from HIV-seropositive subjects, which may potentiate HIV replication and partly explain the immunological abnormalities associated with HIV disease. The importance of glutathione for immune function, regulation of gene expression, as well as therapeutic interventions with redox-active drugs are discussed in this commentary.

Suppression of intracellular hydrogen peroxide generation and catalase levels in CD8+ T-lymphocytes from HIV+ individuals

CD8+ T-lymphocytes from HIV+ individuals contain short telomeres, a sign of cell senescence. To test our hypothesis that the cell type is functionally defective in the biochemical indices related to cell proliferation, we investigated the profiles of intracellularly generated H2O2 levels with or without PMA as well as immunoreactive catalase levels using flow cytometric method. We observed that, in HIV+ but not in HIV- individuals, the constitutively generated H2O2 level was significantly lower in CD8+ T-cells compared with CD4+ T-cells. Importantly, activated effector CD8+CD28- cells showed remarkably low H2O2 levels compared with CD8+CD28+ cells, and the latter in HIV+ individuals also showed low levels. A similar defect of CD8+ cells of HIV+ individuals was also seen with H2O2 levels stimulated with PMA in the presence of a catalase inhibitor. Furthermore, the immunoreactive catalase content was lower in CD8+ cells compared with CD4+ cells only in HIV+ individuals. These results suggest that CD8+ T-lymphocytes are functionally defective with the constitutively generated and PMA-elicited levels of H2O2 and the corresponding scavenger. Diminished immunocompetence of HIV+ individuals may be caused, in part, by the functional defect of CD8+ T-cells.

Redox regulation of genes that protect against carcinogens

Most carcinogens require activation to electrophilic metabolites or species that generate reactive oxygen in order to initiate the tumorigenic process. These reactive intermediates can, in turn, be detoxified by endogenous enzyme systems that and in the protection of cells from either toxic or mutagenic product formation. The levels of many of these enzymes are elevated by numerous compounds found in the diet, or by antioxidants. Recent evidence describes the mechanism for this induction of carcinogen detoxication enzymes to be regulated at the transcriptional level. Nuclear transcription factors bound to sites common among these carcinogen detoxication genes are activated by as yet unknown signal transduction pathways. The activity of these nuclear transcription factors are modulated by pro- and antioxidant reagents, suggesting that a redox-sensitive component governs the induction of enzymes involved in carcinogen metabolism. In this review, evidence for the redox regulation of the genes encoding carcinogen detoxication enzymes is presented. Evidence is also presented suggesting the participation of nuclear factor kappa B (NF-kappa B), mitogen-activated protein (MAP) kinase, and basic leucine zipper (bZIP) proteins and their activation pathways in this induction.

Selenium supplementation suppresses tumor necrosis factor alpha-induced human immunodeficiency virus type 1 replication in vitro

Selenium is a nutritionally essential trace element that is important for optimal function of the immune system. It is incorporated into selenoproteins as the amino acid selenocysteine and it is known to inhibit the expression of some viruses. In this study, we show that selenium supplementation for 3 days prior to exposure to tumor necrosis factor alpha (TNF-alpha) partially suppresses the induction of human immunodeficiency virus type 1 (HIV-1) replication in both chronically infected T lymphocytic and monocytic cell lines. In acute HIV-1 infection of T lymphocytes and monocytes in the absence of exogenous TNF-alpha, the suppressive effect of selenium supplementation was not observed. However, selenium supplementation did suppress the enhancing effect of TNF-alpha on HIV-1 replication in vitro in acutely infected human monocytes, but not in T lymphocytes. Selenium supplementation also increased the activities of the selenoproteins, glutathione peroxidase (GPx) and thioredoxin reductase (TR), which serve as cellular antioxidants. Taken together, these results suggest that selenium supplementation may prove beneficial as an adjuvant therapy for AIDS through reinforcement of endogenous antioxidative systems.

Cell signaling pathways elicited by asbestos

In recent years, it has become apparent that minerals can trigger alterations in gene expression by initiating signaling events upstream of gene transactivation. These cascades may be initiated at the cell surface after interaction of minerals with the plasma membrane either through receptorlike mechanisms or integrins. Alternatively, signaling pathways may be stimulated by active oxygen species generated both during phagocytosis of minerals and by redox reactions on the mineral surface. At least two signaling cascades linked to activation of transcription factors, i.e., DNA-binding proteins involved in modulating gene expression and DNA replication, are stimulated after exposure of lung cells to asbestos fibers in vitro. These include nuclear factor kappa B (NF kappa B) and the mitogen-activated protein kinase (MAPK) cascade important in regulation of the transcription factor, activator protein-1 (AP-1). Both NF kappa B and AP-1 bind to specific DNA sequences within the regulatory or promoter regions of genes that are critical to cell proliferation and inflammation. Unraveling the cell signaling cascades initiated by mineral dusts and pharmacologic inhibition of these events may be important for the control and treatment of mineral-associated occupational diseases.

Reactive oxygen species generation by human spermatozoa is induced by exogenous NADPH and inhibited by the flavoprotein inhibitors diphenylene iodonium and quinacrine

Human spermatozoa possess a specialized capacity to generate reactive oxygen species (ROS) that is thought to be of significance in the redox regulation of sperm capacitation (De Lamirande and Gagnon, 1993; Aitken et al., 1995). However, the mechanisms by which ROS are generated by these cells are not understood. In this study we have examined the possible significance of NADPH as a substrate for ROS production by human spermatozoa. Addition of NADPH to viable populations of motile spermatozoa induced a sudden dose-dependent increase in the rate of superoxide generation via mechanisms that could not be disrupted by inhibitors of the mitochondrial electron transport chain (antimycin A, rotenone, carbonyl cyanide m-chlorophenylhydrazone [CCCP], and sodium azide), diaphorase (dicoumarol) xanthine oxidase (allopurinol), or lactic acid dehydrogenase (sodium oxamate). However, NADPH-induced ROS generation could be stimulated by permeabilization and was negatively correlated with sperm function. Both NADH and NADPH were active electron donors in this system, while NAD+ and NADP+ exhibited little activity. Stereo-specificity was evident in the response in that only the beta-isomer of NADPH supported superoxide production. The involvement of a flavoprotein in the electron transfer process was indicated by the high sensitivity of the oxidase to inhibition by diphenylene iodonium and quinacrine. These results indicate that NAD(P)H can serve as an electron donor for superoxide generation by human spermatozoa and present a simple strategy for the production of motile populations of free radical generating cells with which to study the significance of these molecules in the control of normal and pathological sperm function.

Activation of the receptor for advanced glycation end products triggers a p21(ras)-dependent mitogen-activated protein kinase pathway regulated by oxidant stress

Advanced glycation end products (AGEs) exert their cellular effects on cells by interacting with specific cellular receptors, the best characterized of which is the receptor for AGE (RAGE). The transductional processes by which RAGE ligation transmits signals to the nuclei of cells is unknown and was investigated. AGE-albumin, a prototypic ligand, activated p21(ras) in rat pulmonary artery smooth muscle cells that express RAGE, whereas nonglycated albumin was without effect. MAP kinase activity was enhanced at concentrations of AGE-albumin, which activated p21(ras) and NF-kappaB. Depletion of intracellular glutathione rendered cells more sensitive to AGE-mediated activation of this signaling pathway. In contrast, signaling was blocked by preventing p21(ras) from associating with the plasma membrane or mutating Cys118 on p21(ras) to Ser. Signaling was receptor-dependent, because it was prevented by blocking access to RAGE with either anti-RAGE IgG or by excess soluble RAGE. These data suggest that RAGE-mediated induction of cellular oxidant stress triggers a cascade of intracellular signals involving p21(ras) and MAP kinase, culminating in transcription factor activation. The molecular mechanism that triggers this pathway likely involves oxidant modification and activation of p21(ras).

Redox-regulated signaling by lactosylceramide in the proliferation of human aortic smooth muscle cells

Previously, our laboratory reported that lactosylceramide (LacCer) stimulated human aortic smooth muscle cell proliferation via specific activation of p44 mitogen-activated protein kinase (MAPK) in the p21(ras)/Raf-1/MEK2 pathway and induced expression of the transcription factor c-fos downstream to the p44 MAPK signaling cascade (Bhunia A. K., Han, H., Snowden, A., and Chatterjee S. (1996) J. Biol. Chem. 271, 10660-10666). In the present study, we explored the role of free oxygen radicals in LacCer-mediated induction of cell proliferation. Superoxide levels were measured by the lucigenin chemiluminescence method, MAPK activity was measured by immunocomplex kinase assays, and Western blot analysis and c-fos expression were measured by Northern blot assay. We found that LacCer (10 microM) stimulates endogenous superoxide production (7-fold compared with control) in human aortic smooth muscle cells specifically by activating membrane-associated NADPH oxidase, but not NADH or xanthine oxidase. This process was inhibited by an inhibitor of NADPH oxidase, diphenylene iodonium (DPI), and by antioxidants, N-acetyl-L-cysteine (NAC) or pyrrolidine dithiocarbamate. NAC and DPI both abrogated individual steps in the signaling pathway leading to cell proliferation. For example, the p21(ras).GTP loading, p44 MAPK activity, and induction of transcription factor c-fos all were inhibited by NAC and DPI as well as an antioxidant pyrrolidine dithiocarbamate or reduced glutathione (GSH). In contrast, depletion of GSH by L-buthionine (S, R)-sulfoximine up-regulated the above described signaling cascade. In sum, LacCer, by virtue of activating NADPH oxidase, produces superoxide (a redox stress signaling molecule), which mediates cell proliferation via activation of the kinase cascade. Our findings may explain the potential role of LacCer in the pathogenesis of atherosclerosis involving the proliferation of aortic smooth muscle cells.

Control of Cell Cycle Progression in Human Natural Killer Cells Through Redox Regulation of Expression and Phosphorylation of Retinoblastoma Gene Product Protein

Using thiol deprivation, we have previously shown that the response of natural killer (NK) cells to interleukin-2 (IL-2) is subject to redox regulation downstream of IL-2 binding and internalization. We have now used the IL-2–dependent cell line, NK3.3 to study redox regulation of NK cells further, and found that NK3.3 cells neither incorporated [3H]-thymidine nor completed the G1-S phase transition in medium lacking the thiol-related compounds, L-cystine, and glutathione, despite the presence of sufficient IL-2. Thiol deprivation did not alter the induction of DNA interferon-γ activated sequence (GAS)-binding activity in response to IL-2. However, the retinoblastoma gene product (RB), a cyclin-dependent kinase (CDK) substrate, was phosphorylated within 24 hours after IL-2 stimulation in standard medium, but its expression and phosphorylation were reduced in thiol-depleted medium in both NK3.3 cells and freshly isolated NK cells. These reductions were not associated with an increased level of p27Kip1, an inhibitor of CDKs CDK6/2 in association with G1 cyclins. Reducing agents, N-acetylcysteine, reduced glutathione or 2-ME restored both RB phosphorylation and DNA synthesis in thiol-deprived NK3.3 cells. The in vitro kinase activities of CDK6 and CDK2 were prematurely increased by thiol deprivation. This enhancement was associated with CDK hyperphosphorylation and prolonged phosphorylation, and could be observed before and beyond IL-2 stimulation. The data suggest the possibility that the premature and prolonged enhancement of CDK activity in thiol-deprived NK cells is associated with, and therefore may contribute to, the reduced expression and phosphorylation of RB, and the associated cell cycle arrest.

Depletion of glutathione by benzo(a)pyrene metabolites, ionomycin, thapsigargin, and phorbol myristate in human peripheral blood mononuclear cells

Previous studies in this laboratory have shown that polycyclic aromatic hydrocarbons (PAHs) alter Ca2+ homeostasis and inhibit activation of both B and T lymphocytes obtained from rodents and humans. In the present studies, we demonstrate that alpha-naphthoflavone (ANF), an inhibitor of cytochrome P4501A activity, reduced the Ca2+ elevation produced by BaP in human peripheral blood mononuclear cell (HPBMC) lymphocytes. These results suggested that BaP metabolites may play a role in intracellular Ca2+ homeostasis in human lymphocytes. Reactive oxidative intermediates of BaP produced in HPMBC are known to be highly carcinogenic and have also been shown to be immunosuppressive. We examined the effects of benzo(a)pyrene (BaP), 7,12-dimethylbenz(a)anthracene (DMBA), benzo(e)pyrene (BeP), and anthracene, as well as certain BaP metabolites, on the levels of intracellular Ca2+ and glutathione in HPBMC. While BaP, DMBA, BeP, and anthracene did not cause a statistically significant decrease in GSH in HPBMC at concentrations of 1 or 10 microM following a 6-, 48-, or 72-hr exposure, reactive BaP metabolites including 4,5-epoxide BaP and 7,8-diol-9,10-epoxide BaP consistently produced a 20-30% depletion of glutathione in HPBMC following a 6-hr treatment period. These BaP metabolites also elevated intracellular Ca2+ in HPBMC during a 6-hr incubation. Results of these experiments suggest that metabolism of BaP to certain epoxide metabolites may be responsible for sulfhydryl damage leading to transient GSH depletion and Ca2+ elevation. These results are consistent with the hypothesis that sulfhydryl damage by certain PAH metabolites may lead to altered Ca2+ homeostasis, leading to inhibition of cell activation and proliferation in HPBMC.

Glutathione deficiency is associated with impaired survival in HIV disease

Glutathione (GSH), a cysteine-containing tripeptide, is essential for the viability and function of virtually all cells. In vitro studies showing that low GSH levels both promote HIV expression and impair T cell function suggested a link between GSH depletion and HIV disease progression. Clinical studies presented here directly demonstrate that low GSH levels predict poor survival in otherwise indistinguishable HIV-infected subjects. Specifically, we show that GSH deficiency in CD4 T cells from such subjects is associated with markedly decreased survival 2-3 years after baseline data collection (Kaplan-Meier and logistic regression analyses, P < 0.0001 for both analyses). This finding, supported by evidence demonstrating that oral administration of the GSH prodrug N-acetylcysteine replenishes GSH in these subjects and suggesting that N-acetylcysteine administration can improve their survival, establishes GSH deficiency as a key determinant of survival in HIV disease. Further, it argues strongly that the unnecessary or excessive use of acetaminophen, alcohol, or other drugs known to deplete GSH should be avoided by HIV-infected individuals.

Study of gene regulation by NF-kappa B and AP-1 in response to reactive oxygen intermediates

Reactive oxygen intermediates (ROIs), such as hydrogen peroxide or superoxide, are an evolutionarily ancient threat to all organisms. Exposure of bacteria to ROIs initiates a genetic program that coordinates the production of novel proteins with protective functions. This genetic response is mediated by regulatory proteins that have the potential to initiate transcription of genes when the levels of the ROIs increase. In plant cells, a variety of viral pathogens increase hydrogen peroxide production, which is required to mount a defensive genetic response. It was suggested that in this case H2O2 is used as a secondary messenger and an immediate-early pathogen signal. In higher vertebrates, two transcription factors, nuclear factor kappa B and activator protein 1, were found to respond to ROIs. Both are well studied: they are induced by a great variety of seemingly unrelated conditions and serve important roles in immune, inflammatory, and other pathogen-related genetic responses. In this article we discuss how the ROI responsiveness of transcription factors can be experimentally studied and summarize evidence to suggest that ROIs have been conserved during evolution as messengers of a general pathogen response.

Redox regulation of the mitogen-activated protein kinase pathway during lymphocyte activation

We have previously demonstrated an obligatory requirement for intracellular reactive oxygen species generation during T lymphocyte activation, and have proposed that intracellular reactive oxygen species may act as signalling agents in the regulation of certain cellular processes, for example, during cell cycle entry. To test this hypothesis, we have been interested to determine which, if any, cell cycle entry events are affected by oxidative signalling. In earlier studies, we have identified the transcription factors NF-kappa B and AP-1 as molecular targets for oxidative signalling processes during cell cycle entry, and have shown that oxidative signalling is involved in the regulation of early changes in gene expression during the G0 to G1 phase transition. To extend these initial observations, we have examined the effect of antioxidant treatment on the activity of the mitogen-activated protein kinases erk1 and erk2, as members of a signal transduction pathway known to directly regulate transcription factor function. Using as a probe cysteamine, an aminothiol compound with both antioxidant and antiproliferative activity, we have identified erk2, a key element of the MAP kinase pathway, as being responsive to oxidative signalling during lymphocyte activation. These observations provide further evidence to suggest a role for intracellular oxidant generation as a regulatory mechanism during cell cycle entry, and establish a link between oxidative signalling and other aspects of the intracellular signalling network that is activated in response to mitogenic stimulation.

Activation of signal transduction kinases by tamoxifen

PURPOSE

To study the signal transduction mechanisms of tamoxifen via the activation of MAPKs, JNK and ERK in order to understand its regulation of gene expression.

METHODS

The effects of tamoxifen (TAM) on the activation of serine/threonine mitogen-activated protein kinase (MAPK, p42/ERK2) and the stress-activated protein kinases (p46 SAPK or c-Jun N-terminal kinase, JNK1) were evaluated using a human cervical epitheloid carcinoma HeLa cell line.

RESULTS

TAM activated both JNK1 and ERK2 activities in a time- and dose-dependent manner in HeLa cells. The activation of JNK1 was enhanced when the cells were pretreated with prooxidant H2O2.

CONCLUSIONS

These studies show that TAM activates the signal transduction kinases, JNK1 and ERK2, which may play important roles in the regulation of gene expression by TAM.

Superoxide: a two-edged sword

Superoxide (O2-) is the compound obtained when oxygen is reduced by one electron. For a molecule with an unpaired electron, O2- is surprisingly inert, its chief reaction being a dismutation in which it reacts with itself to form H2O2 and oxygen. The involvement of O2- in biological systems was first revealed by the discovery in 1969 of superoxide dismutase, an enzyme that catalyzes the dismutation of O2-. Since then it has been found that biological systems produce a bewildering variety of reactive oxidants, all but a few arising ultimately from O2-. These oxidants include O2- itself, H2O2 and alkyl peroxides, hydroxyl radical and other reactive oxidizing radicals, oxidized halogens and halamines, singlet oxygen, and peroxynitrite. These various oxidants are able to damage molecules in their environment, and are therefore very dangerous. They are thought to participate in the pathogenesis of a number of common diseases, including among others malignancy, by their ability to mutate the genome, and atherosclerosis, by their capacity for oxidizing lipoproteins. Their properties are put to good use, however, in host defense, where they serve as microbicidal and parasiticidal agents, and in biological signalling, where their liberation in small quantities results in redox-mediated changes in the functions of enzymes and other proteins.

N-acetyl-cysteine and L-2-oxothiazolidine-4-carboxylic acid enhance contact-dependent growth of HIV in resting peripheral blood mononuclear cells (PBMC) in vitro and increase recovery of HIV from human-PBMC SCID mice

OBJECTIVES

To ascertain the effects of N-acetyl-cysteine (NAC) and L-2-oxothiazolidine-4-carboxylic acid (OTC) on HIV replication in resting T lymphocytes mixed with chronically infected U1 promonocytic cells; examine the phenotypes of NAC- and OTC-treated cells; and monitor HIV recovery from hu-PBMC SCID mice (SCID mice infected with HIV-1BaL reconstituted with human peripheral blood mononuclear cells) treated with oral OTC.

DESIGN AND METHODS

Unstimulated PBMC from uninfected donors preincubated for 2 days with pH-adjusted NAC or OTC were cultured at a concentration of 1 x 10(6) cells/ml with 100 U1 cells that were chronically infected with HIV-1IIIB. HI-1 production in the presence or absence of zidovudine was measured by p24 assay at 1-3 weeks, and results were compared with values from the same cell cultures maintained without NAC or OTC exposure. In some experiments U1 cells were separated from PBMC by a 0.4 micron membrane. NAC-treated and -untreated cells were subjected to FACS analysis of multiple-cell-surface adhesion and activation molecules and the results were compared. Hu-PBMC SCID mice were fed OTC for 3 days prior to infection with HIV-1BaL and for the next 3 weeks. Mice were then sacrificed and peritoneal lavage cells were cultured for virus analysis.

RESULTS

Unstimulated, non-dividing PBMC supported high levels of HIV replication when in direct contact with U1 cells in the presence of NAC or OTC; CD2 and CD54 (I-CAM1) were down-regulated on NAC-treated PBMC; and OTC-treated mice produced significantly higher yields of HIV-1 from peritoneal cells than did untreated mice.

CONCLUSIONS

At concentrations < or = 5 mM, NAC and OTC potentiate HIV growth in unstimulated PBMC in vitro and in SCID mice. Caution in the use of these agents as antiviral monotherapies is advisable.

Oxidants as stimulators of signal transduction

Redox (oxidation-reduction) reactions regulate signal transduction. Oxidants such as superoxide, hydrogen peroxide, hydroxyl radicals, and lipid hydroperoxides (i.e., reactive oxygen species) are now realized as signaling molecules under subtoxic conditions. Nitric oxide is also an example of a redox mediator. Reactive oxygen species induce various biological processes such as gene expression by stimulating signal transduction components such as Ca(2+)-signaling and protein phosphorylation. Various oxidants increase cytosolic Ca2+; however, the exact origin of Ca2+ is controversial. Ca2+ may be released from the endoplasmic reticulum, extracellular space, or mitochondria in response to oxidant-influence on Ca2+ pumps, channels, and transporters. Alternatively, oxidants may release Ca2+ from Ca2+ binding proteins. Various oxidants stimulate tyrosine as well as serine/threonine phosphorylation, and direct stimulation of protein kinases and inhibition of protein phosphatases by oxidants have been proposed as mechanisms. The oxidant-stimulation of the effector molecules such as phospholipase A2 as well as the activation of oxidative stress-responsive transcription factors may also depend on the oxidant-mediated activation of Ca(2+)-signaling and/or protein phosphorylation. In addition to the stimulation of signal transduction by oxidants, the observations that ligand-receptor interactions produce reactive oxygen species and that antioxidants block receptor-mediated signal transduction led to a proposal that reactive oxygen species may be second messengers for transcription factor activation, apoptosis, bone resorption, cell growth, and chemotaxis. Physiological significance of the role of biological oxidants in the regulation of signal transduction as well as the mechanisms of the oxidant-stimulation of signal transduction are discussed.

HIV infection--induced posttranslational modification of T cell signaling molecules associated with disease progression

In attempt to elucidate the mechanism of the HIV infection induced T cell unresponsiveness, we studied signal-transducing molecules proximal to the T cell receptor (TCR) in T lymphocytes of HIV-infected individuals. Total amounts of protein tyrosine kinases (PTKs) Lck, Fyn, and ZAP-70 and the zeta chain of the TCR were found significantly decreased in T cells of symptomatic/AIDS patients as well as in T cells of individuals in acute and early asymptomatic stages of HIV infection. Unexpectedly, the detection of Lck, Fyn, and ZAP-70 was reversed after the treatment of cell lysates with dithiothreitol. This suggests that PTKs Lck, Fyn, and ZAP-70 were modified by a mechanism altering the status of sulfhydryl groups. Moreover, this mechanism seems to affect selectively T cells of HIV infected patients since B cell PTKs Syk and Lyn were detected structurally and functionally intact. Interestingly, similar alterations of signaling molecules were not detected in T cells of HIV-infected long-term asymptomatic individuals. Modification of T cell PTKs may thus underlie the HIV-induced impairment of lymphocyte function and may potentially predict disease progression.

Fas-mediated apoptosis is modulated by intracellular glutathione in human T cells

Fas antigen is a member of the tumor necrosis factor receptor family that transduces a lethal signal to the Fas-sensitive cells. We previously established the Fas-resistant variant cell lines LAC2D1R and JKT2D1R from the parental Fas-sensitive cell lines, SUPT13 and Jurkat, respectively. Recently, we isolated the Fas-resistant variant CEM2D1R from CCRF-CEM. All of the variants were Fas+ but resistant to Fas-mediated apoptosis. Further biochemical analysis revealed that the intracellular glutathione (GSH) content of the Fas-resistant variants was higher than in the original cells. When the Fas-resistant variants were incubated with buthionine sulfoximine (BSO) or in GSH-free/cysteine-free medium to deplete GSH, Fas resistance was reversed. Incubation of the cells with cycloheximide also decreased intracellular GSH and reversed the Fas resistance. Furthermore, incubation of activated peripheral blood lymphocytes with BSO enhanced Fas-mediated apoptosis. When the Fas-sensitive cells were incubated with N-acetylcysteine (NAC), intracellular GSH was increased and Fas-mediated apoptosis was blocked. In contrast, Fas-resistant variants, as well as Fas-sensitive cells pre-treated with NAC remained susceptible to allogeneic lymphokine-activated killer cells, most likely due to perforin-dependent killing. The results suggest that Fas-mediated apoptosis, but not perforin-dependent killing, is modulated by intracellular GSH in human T lymphocytes.

Oxidative stress triggers tyrosine phosphorylation in B cells through a redox- and inflammatory cytokine-sensitive mechanism

Exposure to oxidants such as hydrogen peroxide (H2O2) and gamma-ray irradiation has been recently shown to trigger tyrosine phosphorylation in B cells as does cross-linking surface immunoglobulin (sIg) by antigens or anti-immunoglobulins. We studied the mechanism by which H2O2 induced tyrosine phosphorylation in B cells and compared it with the mechanism utilized by sIg. Both anti-immunoglobulin M (anti-IgM) and H2O2 induced tyrosine phosphorylation through protein tyrosine kinase (PTK) activation. However, the tyrosine phosphorylation caused by H2O2 but not that induced by anti-IgM, was modulated by agents affecting cellular thiols and glutathione contents including dithiothreitol, 2-mercaptoethanol, and buthionine sulphoximine. Moreover, the tyrosine phosphorylation caused by the oxidant but not that induced by anti-IgM was markedly augmented by two inflammatory cytokines, tumour necrosis factor-alpha and interleukin-1 alpha, although these agents by themselves did not stimulate PTK activity nor induce tyrosine phosphorylation. These findings demonstrate that oxidative stress but not surface IgM (sIgM) ligation triggers tyrosine phosphorylation through a mechanism that is sensitive to cellular thiols and these inflammatory cytokines.

Activation of mitogen-activated protein kinase by H2O2. Role in cell survival following oxidant injury

The mitogen-activated protein kinase (MAPK) family is comprised of key regulatory proteins that control the cellular response to both proliferation and stress signals. In this study we investigated the factors controlling MAPK activation by H2O2 and explored the impact of altering the pathways to kinase activation on cell survival following H2O2 exposure. Potent activation (10-20-fold) of extracellular signal-regulated protein kinase (ERK2) occurred within 10 min of H2O2 treatment, whereupon rapid inactivation ensued. H2O2 activated ERK2 in several cell types and also moderately activated (3-5-fold) both c-Jun N-terminal kinase and p38/RK/CSBP. Additionally, H2O2 increased the mRNA expression of MAPK-dependent genes c-jun, c-fos, and MAPK phosphatase-1. Suramin pretreatment completely inhibited H2O2 stimulation of ERK2, highlighting a role for growth factor receptors in this activation. Further, ERK2 activation by H2O2 was blocked by pretreatment with either N-acetyl-cysteine, o-phenanthroline, or mannitol, indicating that metal-catalyzed free radical formation mediates the initiation of signal transduction by H2O2. H2O2-stimulated activation of ERK2 was abolished in PC12 cells by inducible or constitutive expression of the dominant negative Ras-N-17 allele. Interestingly, PC12/Ras-N-17 cells were more sensitive than wild-type PC12 cells to H2O2 toxicity. Moreover, NIH 3T3 cells expressing constitutively active MAPK kinase (MEK, the immediate upstream regulator of ERK) were more resistant to H2O2 toxicity, while those expressing kinase-defective MEK were more sensitive, than cells expressing wild-type MEK. Taken together, these studies provide insight into mechanisms of MAPK regulation by H2O2 and suggest that ERK plays a critical role in cell survival following oxidant injury.

Requirement for generation of H2O2 for platelet-derived growth factor signal transduction

Stimulation of rat vascular smooth muscle cells (VSMCs) by platelet-derived growth factor (PDGF) transiently increased the intracellular concentration of hydrogen peroxide (H2O2). This increase could be blunted by increasing the intracellular concentration of the scavenging enzyme catalase or by the chemical antioxidant N-acetylcysteine. The response of VSMCs to PDGF, which includes tyrosine phosphorylation, mitogen-activated protein kinase stimulation, DNA synthesis, and chemotaxis, was inhibited when the growth factor-stimulated rise in H2O2 concentration was blocked. These results suggest that H2O2 may act as a signal-transducing molecule, and they suggest a potential mechanism for the cardioprotective effects of antioxidants.

p21ras as a common signaling target of reactive free radicals and cellular redox stress

Reactive free radicals have been implicated in mediating signal transduction by a variety of stimuli. We have investigated the role of p21ras in mediating free radical signaling. Our studies revealed that signaling by oxidative agents which modulate cellular redox status, such as H2O2, hemin, Hg2+, and nitric oxide was prevented in cells in which p21ras activity was blocked either through expression of a dominant negative mutant or by treating with a farnesyltransferase inhibitor, as assessed by NF-kappa B binding activity. Furthermore, the NF-kappa B response to these oxidative stress stimuli was found to be enhanced when cells from the human T cell line, Jurkat, were pretreated with L-buthionine-(S,R)-sulfoximine, an inhibitor of glutathione synthesis. We directly assayed p21ras and mitogen-activated protein kinase activities in Jurkat cells and found both of these signaling molecules to be activated in cells treated with the redox modulating agents. Blocking glutathione synthesis made cells 10- to 100-fold more sensitive to these agents. Finally, using recombinant p21ras in vitro, we found that redox modulators directly promoted guanine nucleotide exchange on p21ras. This study suggests that direct activation of p21ras may be a central mechanism by which a variety of redox stress stimuli transmit their signal to the nucleus.

Iron chelation decreases NF-kappa B and HIV type 1 activation due to oxidative stress

An important aspect of human immunodeficiency virus (HIV-1) infection is the regulation of its expression by nuclear factor kappa B (NF-kappa B) through redox-controlled signal transduction pathways. In this study, we demonstrate that iron chelation by deferoxamine (DFO) protects against the cytotoxic and reactivating effects of hydrogen peroxide (H2O2). These protective effects were observed both in lymphocytic (ACH-2) and promonocytic (U1) cells latently infected by HIV-1. Concomitantly, NF-kappa B activation by H2O2, when followed by gel retardation assay, was decreased in the DFO-treated U1 and ACH-2 cells. This latter DFO-mediated effect was specific, as DFO did not clearly affect AP-1 DNA-binding activity when studied after H2O2-induced stress. More importantly, DFO protected against the H2O2-induced activation of HIV-1 as evidenced by reverse transcriptase activity in the supernatant. DFO also protected against PMA-induced NF-kappa B activation as well as TNF-alpha-induced HIV-1 activation. Furthermore, DFO attenuated the p24 response in PBMC infected with HIV-1 and stimulated with IL-2. These different effects of DFO were obtained at DFO concentrations lower than 5 microM. Other chemically unrelated iron chelators also provided protection against cytotoxicity, NF-kappa B activation, and HIV-1 activation in U1 cells challenged with H2O2.

Apoptosis: molecular mechanisms and implications for human disease

Apoptosis is a highly regulated process of cell death with characteristic morphological changes that are distinct from necrosis. The biochemical machinery responsible for apoptotic cell death appears to be constitutively expressed in most, if not all, cells and can be triggered by a variety of signals, including sustained increases in the intracellular Ca2+ level. Apoptosis is the main mechanism of cell deletion during development, normal cell turnover, hormone-induced tissue atrophy, and pathological processes such as T-cell depletion in HIV/AIDS and neurodegenerative disease. The aim of this review is to briefly summarize current knowledge of the molecular mechanisms of apoptosis and its role in human disease.

Redox regulation of programmed cell death in lymphocytes

A redox imbalance caused by an over-production of prooxidants or a decrease in antioxidants seems to play a role in the programmed cell death that occurs in various developmental programs. Such a physiological function for oxidative stress is particularly applicable to the immune system, wherein individual lymphocytes undergo continuous scrutiny to determine if they should be preserved or programmed to die. Following activation, lymphocytes produced increased levels of reactive oxygen species (ROS) which may serve as intracellular signaling molecules. The ultimate outcome of this increased ROS formation, i.e., lymphocyte proliferation versus programmed cell death, may be dictated by macrophage-derived costimulatory molecules that bolster or diminish lymphocyte antioxidant defenses. HIV-1-infected individuals display multiple symptoms of redox imbalance consistent with their being in oxidative stress, and lymphocytes from such individuals are more prone to undergo apoptosis in vitro. It is suggested that oxidative stress, and lymphocytes from such individuals are more prone to undergo apoptosis in vitro. It is suggested that oxidative stress is a physiological mediator of programmed cell death in lymphoid cells, and that HIV disease represents an extreme case of what can happen when regulatory safeguards are compromised.

Separation of oxidant-initiated and redox-regulated steps in the NF-kappa B signal transduction pathway

Studies presented here show that overall NF-kappa B signal transduction begins with a parallel series of stimuli-specific pathways through which cytokines (tumor necrosis factor alpha), oxidants (hydrogen peroxide and mitomycin C), and phorbol ester (phorbol 12-myristate 13-acetate) individually initiate signaling. These initial pathways culminate in a common pathway through which all of the stimulating agents ultimately signal NF-kappa B activation. We distinguish the stimuli-specific pathways by showing that the oxidative stimuli trigger NF-kappa B activation in only one of two human T-cell lines (Wurzburg but not Jurkat), whereas tumor necrosis factor alpha and phorbol 12-myristate 13-acetate readily stimulate in both lines. We propose the common pathway as the simplest way of accounting for the common requirements and properties of the signaling pathway. We include a redox-regulatory mechanism(s) in this common pathway to account for the previously demonstrated redox regulation of NF-kappa B activation in Jurkat cells (in which oxidants don't activate NF-kappa B); we put tyrosine phosphorylation in the common pathway by showing that kinase activity (inhibitable by herbimycin A and tyrphostin 47) is required for NF-kappa B activation by all stimuli tested in both cell lines. Since internal sites of oxidant production have been shown to play a key role in the cytokine-stimulated activation of NF-kappa B, and since tyrosine kinase and phosphatase activities are known to be altered by oxidants, these findings suggest that intracellular redox status controls NF-kappa B activation by regulating tyrosine phosphorylation event(s) within the common step of the NF-kappa B signal transduction pathway.

Glutathione precursor and antioxidant activities of N-acetylcysteine and oxothiazolidine carboxylate compared in in vitro studies of HIV replication

N-Acetyl-L-cysteine (NAC) and L-2-oxothiazolidine 4-carboxylate (OTC) are pro-GSH drugs that been proposed for AIDS therapy. In this article we compare the antiviral activities of these compounds in various in vitro HIV infection models. Although both compounds blocked cytokine induction of HIV in acute and chronic infection models, and in HIV-LTR reporter cell systems, NAC was far more effective than OTC, even at suboptimal doses. To test whether this difference is due to GSH conversion efficacies of these compounds, we measured GSH restoration by NAC or OTC in GSH-depleted peripheral blood mononuclear cells (PBMCs), using flow cytometry. In isolated PBMCs, NAC fully replenishes depleted intracellular GSH whereas OTC only minimally replenishes GSH. This ability to replenish GSH in vitro and its ability to scavenge free radicals directly explain why NAC has more potent antiviral activities in vitro.