DNA binding activity of the glucocorticoid receptor is sensitive to redox changes in intact cells

Glucocorticoid Receptor Signaling in Diabetes

Stress and depression increase the risk of Type 2 Diabetes (T2D) development. Evidence demonstrates that the Glucocorticoid (GC) negative feedback is impaired (GC resistance) in T2D patients resulting in Hypothalamic-Pituitary-Adrenal (HPA) axis hyperactivity and hypercortisolism. High GCs, in turn, activate multiple aspects of glucose homeostasis in peripheral tissues leading to hyperglycemia. Elucidation of the underlying molecular mechanisms revealed that Glucocorticoid Receptor (GR) mediates the GC-induced dysregulation of glucose production, uptake and insulin signaling in GC-sensitive peripheral tissues, such as liver, skeletal muscle, adipose tissue, and pancreas. In contrast to increased GR peripheral sensitivity, an impaired GR signaling in Peripheral Blood Mononuclear Cells (PBMCs) of T2D patients, associated with hyperglycemia, hyperlipidemia, and increased inflammation, has been shown. Given that GR changes in immune cells parallel those in brain, the above data implicate that a reduced brain GR function may be the biological link among stress, HPA hyperactivity, hypercortisolism and hyperglycemia. GR polymorphisms have also been associated with metabolic disturbances in T2D while dysregulation of micro-RNAs—known to target GR mRNA—has been described. Collectively, GR has a crucial role in T2D, acting in a cell-type and context-specific manner, leading to either GC sensitivity or GC resistance. Selective modulation of GR signaling in T2D therapy warrants further investigation.

Generalized and tissue specific glucocorticoid resistance

Glucocorticoids (GCs) are steroid hormones that influence several physiologic functions and are among the most frequently prescribed drugs worldwide. Resistance to GCs has been observed in the context of the familial generalized GC resistance (Chrousos' syndrome) or tissue specific GC resistance in chronic inflammatory states. In this review, we have summarized the major factors that influence individual glucocorticoid sensitivity/resistance. The fine-tuning of GC action is determined in a tissue-specific fashion that includes the combination of different GC receptor promoters, translation initiation sites, splice isoforms, interacting proteins, post-translational modifications, and alternative mechanisms of signal transduction.

Cysteine oxidation impairs systemic glucocorticoid responsiveness in children with difficult-to-treat asthma

BACKGROUND

The mechanisms underlying glucocorticoid responsiveness are largely unknown. Although redox regulation of the glucocorticoid receptor (GR) has been reported, it has not been studied in asthmatic patients.

OBJECTIVE

We characterized systemic cysteine oxidation and its association with inflammatory and clinical features in healthy children and children with difficult-to-treat asthma. We hypothesized that cysteine oxidation would be associated with increased markers of oxidative stress and inflammation, increased features of asthma severity, decreased clinically defined glucocorticoid responsiveness, and impaired GR function.

METHODS

PBMCs were collected from healthy children (n = 16) and children with asthma (n = 118) aged 6 to 17 years. Children with difficult-to-treat asthma underwent glucocorticoid responsiveness testing with intramuscular triamcinolone. Cysteine, cystine, and inflammatory chemokines and reactive oxygen species generation were quantified, and expression and activity of the GR were assessed.

RESULTS

Cysteine oxidation was present in children with difficult-to-treat asthma and accompanied by increased reactive oxygen species generation and increased CCL3 and CXCL1 mRNA expression. Children with the greatest extent of cysteine oxidation had more features of asthma severity, including poorer symptom control, greater medication use, and less glucocorticoid responsiveness despite inhaled glucocorticoid therapy. Cysteine oxidation also modified the GR protein by decreasing available sulfhydryl groups and decreasing nuclear GR expression and activity.

CONCLUSIONS

A highly oxidized cysteine redox state promotes a posttranslational modification of the GR that might inhibit its function. Given that cysteine oxidation is prevalent in children with difficult-to-treat asthma, the cysteine redox state might represent a potential therapeutic target for restoration of glucocorticoid responsiveness in this population.

Modulation of nuclear receptor function by cellular redox poise

Nuclear receptors (NRs) are ligand-responsive transcription factors involved in diverse cellular processes ranging from metabolism to circadian rhythms. This review focuses on NRs that contain redox-active thiol groups, a common feature within the superfamily. We will begin by describing NRs, how they regulate various cellular processes and how binding ligands, corepressors and/or coactivators modulate their activity. We will then describe the general area of redox regulation, especially as it pertains to thiol-disulfide interconversion and the cellular systems that respond to and govern this redox equilibrium. Lastly, we will discuss specific examples of NRs whose activities are regulated by redox-active thiols. Glucocorticoid, estrogen, and the heme-responsive receptor, Rev-erb, will be described in the most detail as they exhibit archetypal redox regulatory mechanisms.

ROS-mediated regulation of CXCR4 in cancer

Oxidative stress and the accumulation of reactive oxygen species (ROS) play a role in cancer cells developing an advanced, phenotypic signature that associates with metastasis and progression. Increased ROS concentrations are involved in promoting cancer development and metastasis by inducing expression of oncogenes, suppressing activity of anti-survival molecules and by activating various cell survival and proliferation signaling pathways. Oxidative stress is higher in the epithelium of cancer patients than patients without the disease, and antioxidant trials are currently being explored as a therapeutic option. However, studies have shown that ROS increases expression of CXCR4 in cancer and immune cells. CXCR4 expression in tumors strongly correlates to metastasis and poor prognosis. Herein, we discuss an emerging relationship between ROS and CXCR4 in cancer cells.

Modulation of oxidative stress responses in the human retinal pigment epithelium following treatment with vitamin C

Oxidative stress (OS) in the retina plays an important role in the development and progression of age-related macular degeneration (AMD). Our previous work has shown that OS can quantitatively regulate the expression of AP-1 family genes in the retinal pigment epithelium (RPE). In this study, we sought to determine whether AP-1 genes can be used as cellular biomarkers of OS to evaluate the efficacy of ascorbate, the major aqueous-phase antioxidant in the blood, in reducing OS in RPE cells in vitro. Human ARPE19 cells were pretreated with increasing levels of ascorbate (0-500 µM) for 3 days which was then removed from the medium. OS was induced 24 h later by the addition of hydrogen peroxide for 1-4 h, to bring the final media concentration of H(2)O(2) to 500 µM. FosB, c-Fos, and ATF3 gene expression was examined from 0 to 24 h after OS. Pretreatment with 200 µM ascorbate maximally reduced the transcriptional OS response of AP-1 genes by up to 87% after 1 and 4 h, compared to controls. One hundred micromolar of ascorbate provided a statistically significant, but far more modest effect. Ascorbate supplementation of 100-200 µM appears to strongly inhibit OS-induced activation of AP-1 in vitro, but pretreatment with higher levels of ascorbate conferred no additional advantage. These studies suggest that there are optimal levels of antioxidant supplementation to the RPE in vitro. Laboratory assays based upon transcription factor biomarkers may be useful to define beneficial molecular responses to new antioxidants, alternative dosing regimens, and to explore therapeutic efficacy in OS models in vitro.

Protection against glucocorticoid-induced damage in human tenocytes by modulation of ERK, Akt, and forkhead signaling

Antiinflammatory glucocorticoid (GC) injections are extensively used to treat painful tendons. However, GC cause severe tissue wasting in other collagen-producing tissues such as skin and bone. The objective of this study was to determine the effects of GC on tenocytes and to explore strategies to protect against unwanted side effects of GC treatment. Cell survival, collagen production, and the induction of signaling pathways in primary human tenocytes treated with dexamethasone (Dex) were assessed. Antioxidant and growth factor approaches to protection were tested. Dex treatment resulted in reduced viable cell number, cell proliferation, and collagen production. Dex induced reactive oxygen species generation in tenocytes and strongly up-regulated the stress-response transcription factors FOXO1 and FOXO3A. Phosphorylation of ERK and protein kinase B/Akt, which regulate cell proliferation and also inhibit forkhead activity, was decreased. Chemical inhibition of ERK or Akt activity significantly reduced tenocyte cell number. Ameliorating the Dex-induced reduction in ERK or Akt activity by cotreatment with vitamin C or insulin protected against the Dex-induced reduction in cell number. Silencing FOXO1 prevented the Dex-induced reduction in collagen 1α1 expression. Cotreatment with vitamin C or insulin protected against the Dex-induced increase in FOXO and the Dex-induced inhibition of collagen 1α1 expression. Reduced ERK and Akt activation and increased forkhead signaling contribute to the negative effects of GC on tenocytes. Cotreatment therapies that target these signaling pathways are protective. Vitamin C in particular may be a clinically useable co-therapy to reduce connective tissue side effects associated with GC therapy.

T cells from burn-injured mice demonstrate a loss of sensitivity to glucocorticoids

Glucocorticoids (GC) are steroid hormones that modulate T cell functions and restrain their hyperresponsiveness following stimulation. Naive T lymphocytes are sensitive to GC but become more resistant when they are activated. A balance between activation and inhibition signals is important for a targeted and effective T cell response. Thermal injury is characterized by an immune dysfunction and hyperactive T cells visible at day 10 postburn. In this study, our objective was to evaluate T cell sensitivity to GC following thermal injury and to identify mechanisms that could modulate their sensitivity. One mechanism that we hypothesized was increased p38 mitogen-activated protein kinase (MAPK) activity that could lead to GC resistance. Male C57BL/6 mice underwent a full-thickness 20% total body surface area. At 10 days postinjury, splenic T cells were isolated. Glucocorticoid receptor (GR) expression was higher in T cells from burn-injured mice. Interestingly, these cells were also less sensitive to GC-induced apoptosis prior to and poststimulation. Furthermore, anti-CD3-activated T cells from burn-injured mice showed increased proliferation and CD25 expression, which resisted corticosterone's (CORT) suppressive effect. Anti-CD3-activated CD4(+)CD44(+) memory cells from burn-injured mice expressed the highest level of CD25 and were resistant to CORT. Increased phosphorylation of p38 MAPK was also noted in activated T cells from burn-injured mice. Pharmacological inhibition of p38 MAPK decreased cell proliferation and normalized interferon-gamma (IFNgamma) production. In conclusion, we demonstrate that a unique event like burn injury induces a loss of sensitivity to GC in splenic T cells and have identified p38 MAPK as a key modulator for this resistance.

Nuclear redox signaling

Reactive oxygen species have been described to modulate proteins within the cell, a process called redox regulation. However, the importance of compartment-specific redox regulation has been neglected for a long time. In the early 1980s and 1990s, many in vitro studies introduced the possibility that nuclear redox signaling exists. However, the functional relevance for that has been greatly disregarded. Recently, it has become evident that nuclear redox signaling is indeed one important signaling mechanism regulating a variety of cellular functions. Transcription factors, and even kinases and phosphatases, have been described to be redox regulated in the nucleus. This review describes several of these proteins in closer detail and explains their functions resulting from nuclear localization and redox regulation. Moreover, the redox state of the nucleus and several important nuclear redox regulators [Thioredoxin-1 (Trx-1), Glutaredoxins (Grxs), Peroxiredoxins (Prxs), and APEX nuclease (multifunctional DNA-repair enzyme) 1 (APEX1)] are introduced more precisely, and their necessity for regulation of transcription factors is emphasized.

Quantitative AP-1 gene regulation by oxidative stress in the human retinal pigment epithelium

The purpose of this study was to characterize the early molecular responses to quantified levels of oxidative stress (OS) in the human retinal pigment epithelium (RPE). Confluent ARPE-19 cells were cultured for 3 days in defined medium to stabilize gene expression. The cells were exposed to varying levels of OS (0-500 microM H(2)O(2)) for 1-8 h and gene expression was followed for up to 24-h after OS. Using real-time qPCR, we quantified the expression of immediate early genes from the AP-1 transcription factor family and other genes involved in regulating the redox status of the cells. Significant and quantitative changes were seen in the expression of six AP-1 transcription factor genes, FosB, c-Fos, Fra-1, c-Jun, JunB, and ATF3 from 1-8 h following OS. The peak level of induced transcription from OS varied from 2- to 128-fold over the first 4 h, depending on the gene and magnitude of OS. Increased transcription at higher levels of OS was also seen for up to 8-h for some of these genes. Protein translation was examined for 24-h following OS using Western blotting methods, and compared to the qPCR responses. We identified six AP-1 family genes that demonstrate quantitative upregulation of expression in response to OS. Two distinct types of quantifiable OS-specific responses were observed; dose-dependent responses, and threshold responses. Our studies show that different levels of OS can regulate the expression of AP-1 transcription factors quantitatively in the human RPE in vitro.

Adaptation to oxidative stress, chemoresistance, and cell survival

The discovery of some additional properties and functions of reactive oxygen species (ROS), beyond their toxic effects, provides a novel scenario for the molecular basis and cell regulation of several pathophysiologic processes. ROS are generated by redox-sensitive, prosurvival signaling pathways and function as second messengers in the transduction of several extracellular signals. A complex intracellular redox buffering network has developed to adapt and protect cells against the dangerous effects of oxidative stress. However, pathways involved in ROS-adaptive response may also play a critical role in protecting cells against cytotoxic effects of anticancer agents, thus supporting the hypothesis of a correlation between adaptation/resistance to oxidative stress and resistance to anticancer drugs. This review summarizes the main systems involved in the adaptive responses: an overview on the pathophysiologic relevance of mitochondria on redox-sensitive transcription factors and genes and main antioxidant networks in tumor cells is provided. One of the major aims is to highlight the adaptive mechanisms and their interplay in the intricate connection between oncogenic signaling, oxidative stress, and chemoresistance. Clarification of these mechanisms has tremendous application potential, in terms of developing novel molecular-targeted anticancer therapies and innovative strategies for rational combination of these agents with chemotherapeutic or tumor-specific biologic drugs.

Nuclear translocation of haeme oxygenase-1 is associated to prostate cancer

The role of oxidative stress in prostate cancer has been increasingly recognised. Acute and chronic inflammations generate reactive oxygen species that result in damage to cellular structures. Haeme oxygenase-1 (HO-1) has cytoprotective effects against oxidative damage. We hypothesise that modulation of HO-1 expression may be involved in the process of prostate carcinogenesis and prostate cancer progression. We thus studied HO-1 expression and localisation in 85 samples of organ-confined primary prostate cancer obtained via radical prostatectomy (Gleason grades 4–9) and in 39 specimens of benign prostatic hyperplasia (BPH). We assessed HO-1 expression by immunohistochemical staining. No significant difference was observed in the cytoplasmic positive reactivity among tumours (84%), non-neoplastic surrounding parenchyma (89%), or BPH samples (87%) (P=0.53). Haeme oxygenase-1 immunostaining was detected in the nuclei of prostate cancer cells in 55 of 85 (65%) patients but less often in non-neoplastic surrounding parenchyma (30 of 85, 35%) or in BPH (9 of 39, 23%) (P<0.0001). Immunocytochemical and western blot analysis showed HO-1 only in the cytoplasmic compartment of PC3 and LNCaP prostate cancer cell lines. Treatment with hemin, a well-known specific inducer of HO-1, led to clear nuclear localisation of HO-1 in both cell lines and highly induced HO-1 expression in both cellular compartments. These findings have demonstrated, for the first time, that HO-1 expression and nuclear localisation can define a new subgroup of prostate cancer primary tumours and that the modulation of HO-1 expression and its nuclear translocation could represent new avenues for therapy.

The dietary antioxidant resveratrol affects redox changes of PPARalpha activity

BACKGROUND AND AIMS

Gene-environment interaction is behind the pathogenesis of most widespread diseases, and nutrition is among the environmental factors with the highest impact on human health. The mechanisms involved in the interaction between nutritional factors and the genetic background of individuals are still unclear. The aim of this study was to investigate whether resveratrol (RES), an antioxidant polyphenol of red wine, can influence the activity of PPARalpha in the rat hepatoma cell line McArdle-RH7777. PPARalpha is a transcriptional factor that regulates gene expression when activated by endogenous or exogenous long-chain fatty acids. Its activation results in significant protection from cardiovascular diseases in humans.

METHODS AND RESULTS

By means of the electromobility shift assay (EMSA), we observed that PPARalpha is redox-sensitive as it displays reduced DNA-binding activity following in vivo treatment of the cells with 1mmol/L diethylmaleate (DEM), a glutathione-depleting agent. This finding could be relevant considering the important role of redox balance in pathological and physiological processes. We also observed a dual effect of 100mumol/L RES on PPARalpha activity: it was able to prevent, to a large extent, the DEM-induced reduction of DNA-binding activity at earlier time points, when the effect of DEM was stronger, but it depressed PPARalpha activity at later time points, when the effect of DEM was greatly reduced.

CONCLUSION

A nutritional substance, such as RES, is able to influence the activity of gene-regulating factors, but the net effect is difficult to predict when the compound involved has multiple biological properties. Caution is therefore warranted before drawing conclusions about the potential benefits of RES for human health.

Genetic and molecular mechanisms of chemical atherogenesis

Injury to the cellular components of the vascular wall and blood by endogenous and exogenous chemicals has been associated with atherosclerosis in humans and experimental systems. The genetic and molecular mechanisms responsible for initiation and promotion of atherosclerotic changes include modulation of extracellular matrix-integrin axis, genes involved in the regulation of growth and differentiation and possibly, genomic stability. This review summarizes seminal studies over the past 20 years that shed light on critical gene-gene and gene-environment interactions mediating the atherogenic response to chemical injury.

Effect of antioxidant diets on mitochondrial gene expression in rat brain during aging

Age-related increase of reactive oxygen species (ROS) is particularly detrimental in postmitotic tissues. Calorie restriction (CR) has been shown to exert beneficial effects, consistent with reduced ROS generation by mitochondria. Many antioxidant compounds also mimic such effects. N-acetyl cysteine (NAC) provides thiol groups to glutathione and to mitochondrial respiratory chain proteins; thus, it may counteract both ROS generation and effects. In the present study we investigated, in different rat brain areas during aging (6, 12, and 28 months), the effect of 1-year treatment with CR and dietary supplementation with NAC on the expression of subunit 39 kDa and ND-1 (mitochondrial respiratory complex I), subunit IV (complex IV), subunit alpha of F0F1-ATP synthase (complex V) and of adenine nucleotide translocator, isoform 1 (ANT-1). The observed age-related changes of expression were prevented by the dietary treatments. The present study provides further evidence for the critical role of mitochondria in the aging process.

Effect of acrolein and glutathione depleting agents on thioredoxin

Acrolein is a widespread environmental pollutant that reacts rapidly with nucleophiles, especially cellular thiols. In addition to glutathione (GSH), thioredoxin (Trx) and thioredoxin reductase (TR) contain thiol groups and may react with electrophiles. In the present study, A549 cells treated with 5-25 microM acrolein for 30 min lost cellular Trx activity in a dose-dependent fashion. Over 90% of Trx activity was lost at concentrations of 25 microM or greater. In contrast, Trx protein content, as assessed by western blotting, was not altered immediately after the 30 min acrolein treatment. Both Trx activity and protein levels increased 4h after the acrolein treatment. However, Trx activity remained below control levels at 24h. A similar dose-response relationship was seen with TR in A549 cells exposed to acrolein. There was, however, a rapid recovery of TR activity such that it attained normal levels by 4h after doses < or = 75 microM acrolein. Diethyl maleate (DEM), a common but not highly specific, agent used to deplete GSH, also inactivated Trx. A 2 h exposure of A549 cells to 1 mM DEM depleted cellular GSH by ~50% and diminished Trx activity by over 67%. Lower DEM doses (0.125 mM and 0.25 mM) for 1h had no significant effect on GSH but significantly decreased Trx activity 12 and 23%, respectively. Similar to immediately after acrolein exposure, DEM did not affect Trx protein levels. A Trx-1-GFP fusion protein was transfected into A549 cells. While the fusion protein was expressed, the Trx component was inactive by the insulin reducing assay. In summary, Trx and TR are inactivated by acrolein. In addition, the GSH depleting agent DEM inactivates Trx somewhat more effectively than it depletes GSH. The Trx-1-GFP fusion protein, while readily expressed, appears to have little or no activity, perhaps because the small size of Trx-1 (12 kDa) is affected by the larger GFP.

Comparing nitrosative versus oxidative stress toward zinc finger-dependent transcription. Unique role for NO

During inflammatory reactions, cells are under nitrosative and/or oxidative stress. The zinc finger transcription factors vitamin D receptor (VDR) and retinoid X receptor (RXR) were used as a model system to characterize effects of NO. and/or reactive oxygen species on zinc finger-dependent gene expression. Nitric oxide (NO.) as well as H(2)O(2), singlet oxygen ((1)O(2)), peroxyl radicals (ROO.) and peroxynitrite (ONOO-), respectively, were shown to inhibit VDR/RXR-DNA complex formation in vitro in a dose-dependent manner. While NO-induced inhibition of VDR/RXR-DNA complex formation could be restored nearly completely by subsequent treatment with dithiothreitol, inhibition by H(2)O(2) proved to be only partially reversible, and inhibition by (1)O(2), ROO. or ONOO- was found to be irreversible. In cells transiently transfected with VDR and RXR, subtoxic concentrations of NO. or hydroperoxides and intracellular generation of superoxide anion radicals inhibited VDR/RXR-dependent reporter gene activity in a dose-dependent manner. Interestingly, cells can repair the zinc fingers of VDR and RXR after nitrosative stress but not after oxidative stress. The results indicate that, among the reactive species investigated, only NO. may act sufficiently gentle to be considered as a regulator and not only as an inhibitor of gene expression via zinc finger transcription factors.

Oxidation of zinc finger transcription factors: physiological consequences

Redox-sensitive cysteine residues are present in the interaction domains of many protein complexes. There are examples in all of the major categories of transcription factors, including basic region, leucine zipper, helix-loop-helix, and zinc finger. Zinc finger structures require at least two zinc-coordinated cysteine sulfhydryl groups, and oxidation or alkylation of these can eliminate DNA-binding and transcriptional functions. We review here the evidence for oxidation of zinc finger cysteines, the pathways and reactive oxygen intermediates involved, and the functional and physiological consequences of these reactions. Despite skepticism that the strongly reducing intracellular environment would permit significant oxidation of cysteine residues within zinc finger transcription factors, there is compelling evidence that oxidation occurs both in vitro and in vivo. Early reports demonstrating reversible oxidation of zinc-coordinated cysteines with loss of binding function in vitro were shown to reflect accurately the changes in intact cells, and these in turn have been shown to correlate with physiological changes. In particular, the accumulation of oxidized Spl zinc fingers during aging, and estrogen receptors in tamoxifen-resistant breast cancers are dramatic examples of what may be a general sensitivity of zinc finger factors to changes in the redox state of the cell.

Nitrosation and oxidation in the regulation of gene expression

A growing body of evidence suggests that the cellular response to oxidative and nitrosative stress is primarily regulated at the level of transcription. Posttranslational modification of transcription factors may provide a mechanism by which cells sense these redox changes. In bacteria, for example, OxyR senses redox-related changes via oxidation or nitrosylation of a free thiol in the DNA binding region. This mode of regulation may serve as a paradigm for redox-sensing by eukaryotic transcription factors as most-including NF-kappaB, AP-1, and p53-contain reactive thiols in their DNA binding regions, the modification of which alters binding in vitro. Several of these transcription factors have been found to be sensitive to both reactive oxygen species and nitric oxide-related species in vivo. It remains entirely unclear, however, if oxidation or nitrosylation of eukaryotic transcription factors is an important mode of regulation, or whether transcriptional activating pathways are principally controlled at other redox-sensitive levels.-Marshall, H. E., Merchant, K., Stamler, J. S. Nitrosation and oxidation in the regulation of gene expression.

Oxidative stress and gene regulation

Reactive oxygen species are produced by all aerobic cells and are widely believed to play a pivotal role in aging as well as a number of degenerative diseases. The consequences of the generation of oxidants in cells does not appear to be limited to promotion of deleterious effects. Alterations in oxidative metabolism have long been known to occur during differentiation and development. Experimental perturbations in cellular redox state have been shown to exert a strong impact on these processes. The discovery of specific genes and pathways affected by oxidants led to the hypothesis that reactive oxygen species serve as subcellular messengers in gene regulatory and signal transduction pathways. Additionally, antioxidants can activate numerous genes and pathways. The burgeoning growth in the number of pathways shown to be dependent on oxidation or antioxidation has accelerated during the last decade. In the discussion presented here, we provide a tabular summary of many of the redox effects on gene expression and signaling pathways that are currently known to exist.

Redox regulation of the glucocorticoid receptor

Redox regulation is currently considered as a mode of signal transduction for coordinated regulation of a variety of cellular processes. The transcriptional regulation of gene expression is also influenced by cellular redox state, most possibly through the oxido-reductive modification of transcription factors. The glucocorticoid receptor belongs to a nuclear receptor superfamily and acts as a ligand-dependent transcription factor. We demonstrate that the glucocorticoid receptor function is regulated via redox-dependent mechanisms at multiple levels. Moreover, it is suggested that redox regulation of the receptor function is one of dynamic cellular responses to environmental stimuli and plays an important role in orchestrated crosstalk between central and peripheral stress responses.

Redox regulation of cellular signalling

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

Effect of redox conditions on the DNA-binding efficiency of the retinoic acid receptor zinc-finger

Retinoic acid and its derivatives are involved in many important biological processes. In the present study, we have shown that the DNA binding domain of the retinoic acid receptor, which contains two zinc fingers with the Zn(II) tetrahedrally coordinated by four Cys, is susceptible to intracellularly relevant oxidizing agents. In the presence of hydrogen peroxide or hypochlorite, the zinc-finger DNA binding activity was abolished in a concentration dependent manner. The loss of DNA binding activity was correlated with the release of Zn(II) from the zinc-finger motif as a consequence of Zn(II)-thiolate bond oxidation. A combination of glutathione and Zn(II) was able to restore the activity, suggesting that oxidation of the zinc-finger by hydrogen peroxide or hypochlorite resulted in the formation of disulfide bonds between the Cys present in the Zn(II)-binding motif. Our results indicate that in situations of oxidative-stress zinc-finger containing transcription factors may be particularly susceptible to oxidation, resulting in the disruption of control and regulation of gene expression.

Suppression of CYP1A1 transcription by H2O2 is mediated by xenobiotic-response element

We have previously demonstrated that H2O2 downregulates CYP1A1 and CYP1A2 transcription in isolated rat hepatocytes (C. W. Barker, et al., 1994, J. Biol. Chem. 269, 3985-3990). In the present study, induction of chloramphenicol acetyltransferase (CAT) expression driven by 3.1 kb of rat CYP1A1 upstream regulatory sequences was suppressed by 56% in Hepa-1 cells treated with H2O2. Similarly, H2O2 inhibited CAT expression from vectors containing two copies of either xenobiotic-response element (XRE) 1 or XRE2. H2O2 did not inhibit basal CAT expression in cells that were not treated with the inducer beta-napthoflavone. Electrophoretic mobility shift assays demonstrated that the suppression of XRE-dependent transcription by H2O2 was not due to changes in nuclear aryl hydrocarbon (Ah) receptor DNA binding activity. Several types of experiments indicated that modulation of XRE enhancer strength by various means could modify H2O2-dependent suppression of CAT expression. Conditions that increased the transactivation potential of the Ah receptor (increase in XRE copy number or shortening of the distance between XREs and the minimal CYP1A1 promoter) attenuated the action of H2O2, while conditions that reduced XRE-mediated transactivation potential (decrease in XRE copy number, increase of the distance between the XRE and the promoter, or reduction of the number of bound Ah receptors by lowering the concentration of inducer) potentiated the inhibitory action of H2O2.

Glucocorticoids may alter antioxidant enzyme capacity in the brain: kainic acid studies

Glucocorticoids (GCs) predispose hippocampal neurons to damage during metabolic stressors. One component of hippocampal GC-endangerment may be changes in neuronal defenses against oxidative challenge. Previous experiments showed a decrease in basal levels of copper/zinc superoxide dismutase (Cu/Zn SOD) and glutathione peroxidase (GSPx) in the brain of rats treated with GCs [L. McIntosh, K. Hong, R. Sapolsky, Glucocorticoids may alter antioxidant enzyme capacity in the brain: baseline studies, 1997.]. In this study we administered the excitotoxin kainic acid (KA) to generate reactive oxygen species (ROS) in the brain, and monitored the activity of four antioxidant enzymes over 24 h in GC-free and GC-supplemented rats. We tested the response pattern in three regions of the brain (hippocampus, cortex, cerebellum) and the liver as a peripheral control. In the hippocampus, KA induced Cu/Zn SOD and catalase, but GCs prevented the induction of catalase and maintained the lowered GSPx activity seen previously in the baseline studies. In the cortex, KA induced Cu/Zn SOD, Mn SOD and catalase activity, but there was no significant GC effect. There was no response to KA in the cerebellum, but GCs decreased GSPx activity. In the liver, KA produced a rise in Cu/Zn SOD and catalase activity, and GC-treated animals showed a slower return to baseline. These experiments indicate that the impairment of antioxidant enzyme defenses, particularly the hippocampal peroxidases, could be a component of GC-mediated neuroendangerment.

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.

Redox regulation of cellular activation

Growing evidence has indicated that cellular reduction/oxidation (redox) status regulates various aspects of cellular function. Oxidative stress can elicit positive responses such as cellular proliferation or activation, as well as negative responses such as growth inhibition or cell death. Cellular redox status is maintained by intracellular redox-regulating molecules, including thioredoxin (TRX). TRX is a small multifunctional protein that has a redox-active disulfide/dithiol within the conserved active site sequence: Cys-Gly-Pro-Cys. Adult T cell leukemia-derived factor (ADF), which we originally defined as an IL-2 receptor alpha-chain/Tac inducer produced by human T cell lymphotrophic virus-I (HTLV-I)-transformed T cells, has been identified as human TRX. TRX/ADF is a stress-inducible protein secreted from cells. TRX/ADF has both intracellular and extracellular functions as one of the key regulators of signaling in the cellular responses against various stresses. Extracellularly, TRX/ADF shows a cytoprotective activity against oxidative stress-induced apoptosis and a growth-promoting effect as an autocrine growth factor. Intracellularly, TRX/ADF is involved in the regulation of protein-protein or protein-nucleic acid interactions through the reduction/oxidation of protein cysteine residues. For example, TRX/ADF translocates from the cytosol into the nucleus by a variety of cellular stresses, to regulate the expression of various genes through the redox factor-1 (Ref-1)/APEX. Further studies to clarify the regulatory roles of TRX/ADF and its target molecules may elucidate the intracellular signaling pathways in the responses against various stresses. The concept of "redox regulation" is emerging as an understanding of the novel mechanisms in the pathogenesis of several disorders, including viral infections, immunodeficiency, malignant transformation, and degenerative disease.

The dual role of S-nitrosoglutathione (GSNO) during thymocyte apoptosis

Nitric oxide (NO) is known to regulate redox-sensitive signalling pathways in physiology and pathophysiology. Depending on its concentration, the NO-releasing compound S-nitrosoglutathione (GSNO) causes negative and positive regulation of thymocyte apoptosis. At levels below 0.6 mM, GSNO produces deoxyribonucleic acid (DNA) laddering, which is inhibited by activation of protein kinase C (PKC), cycloheximide treatment, and calcium chelation. Higher concentrations of the NO donor (1-2 mM) suppress thymocyte apoptosis initiated by the classical agonist dexamethasone. Inhibition of apoptosis by NO is analogous to the action of the thiol-blocking compound N-ethylmaleimide (NEM) and the glutathione-S-transferase substrate 1-chloro-2,4-dinitrobenzene (CDNB). Inhibition of apoptosis results from thiol modification of critical proteins in response to NO treatment. Depending on the concentration, GSNO can be involved either in toxic or in protective signalling in thymocyte biology.

N-acetylcysteine: pharmacological considerations and experimental and clinical applications

The diversity of application of the thiol drug NAC in both the experimental setting, as a tool for the study of the mechanisms and consequences of oxidative stress, and the clinical setting, as a therapeutic agent, clearly reflects the central role played by the redox chemistries of the group XVI elements, oxygen and sulfur, in biology. As our understanding of such redox processes increases, particularly their roles in specific pathophysiological processes, new avenues will open for the use of NAC in the clinical setting. As a drug, NAC represents perhaps the ideal xenobiotic, capable of directly entering endogenous biochemical processes as a result of its own metabolism. Thus, it is hoped that the experience gained with this unique agent will help in future efforts to design antioxidants and chemoprotective principles which are able to more accurately utilize endogenous biochemical processes for cell- or tissue-specific therapy.

Redox regulation of transcriptional activators

Transcription factors/activators are a group of proteins that bind to specific consensus sequences (cis elements) in the promoter regions of downstream target/effector genes and transactivate or repress effector gene expression. The up- or downregulation of effector genes will ultimately lead to many biological changes such as proliferation, growth suppression, differentiation, or senescence. Transcription factors are subject to transcriptional and posttranslational regulation. This review will focus on the redox (reduction/oxidation) regulation of transcription factors/activators with emphasis on p53, AP-1, and NF-kappa B. The redox regulation of transcriptional activators occurs through highly conserved cysteine residues in the DNA binding domains of these proteins. In vitro studies have shown that reducing environments increase, while oxidizing conditions inhibit sequence-specific DNA binding of these transcriptional activators. When intact cells have been used for study, a more complex regulation has been observed. Reduction/oxidation can either up- or downregulate DNA binding and/or transactivation activities in transcriptional activator-dependent as well as cell type-dependent manners. In general, reductants decrease p53 and NF-kappa B activities but dramatically activate AP-1 activity. Oxidants, on the other hand, greatly activate NF-kappa B activity. Furthermore, redox-induced biochemical alterations sometimes lead to change in the biological functions of these proteins. Therefore, differential regulation of these transcriptional activators, which in turn, regulate many target/effector genes, may provide an additional mechanism by which small antioxidant molecules play protective roles in anticancer and antiaging processes. Better understanding of the mechanism of redox regulation, particularly in vivo, will have an important impact on drug discovery for chemoprevention and therapy of human disease such as cancer.

A p53-independent pathway for activation of WAF1/CIP1 expression following oxidative stress

Incubating human cells in diethylmaleate (DEM) depletes the intracellular pool of reduced glutathione (GSH) and increases the concentration of oxidative free radicals. We found that DEM-induced oxidative stress reduced the ability of p53 to bind its consensus recognition sequence and to activate transcription of a p53-specific reporter gene. Nevertheless, DEM treatment induced expression of WAF1/CIP1 but not GADD45 mRNA. The fact that N-acetylcysteine, a precursor of GSH that blocks oxidative stress, prevented WAF1/CIP1 induction by DEM suggests that WAF1/CIP1 induction probably was a consequence of the ability of DEM to reduce intracellular GSH levels. DEM induced WAF1/CIP1 expression in Saos-2 and T98G cells, both of which lack functional p53 protein. DEM treatment did not produce an increase in membrane-associated protein kinase C, but ERK2, a mitogen-activated protein kinase, was phosphorylated in a manner consistent with ERK2 activation. DEM treatment also produced a dose-dependent delay in cell cycle progression, which at low concentrations (0.25 mM) consisted of a G2/M arrest and at higher concentrations (1 mM) also involved G1 and S phase delays. Our results indicate that oxidative stress induces WAF1/CIP1 expression and arrests cell cycle progression through a mechanism that is independent of p53. This mechanism may provide for cell cycle checkpoint control under conditions that inactivate p53.

Differentially expressed mRNAs as a consequence of oxidative stress in intact cells

Intracellular redox conditions influence the activity of several transcription factors leading to a modulation of the expression of the genes controlled by these factors. We examined the changes in cell transcription patterns after oxidative stress induced by diethylmaleate (DEM). Using the differential display technique we identified several differentially expressed sequence tags, four of which are identical or highly homologous to sequences contained in the human cDNAs encoding vimentin, c-fos, cytochrome oxidase IV and ribosomal protein L4; another one corresponds to a transcript of the mitochondrial genome of unknown function. The remaining five cDNAs are not recorded in any sequence data bank. One of these, named Rox3, lights up two mRNA species of approximately 3400 and 3600 bp, significantly increased after treatment with DEM or with other oxidizing agents. This increase appears precociously after exposure to DEM and it is completely prevented by pretreatment with N-acetylcysteine. The Rox3 fragment was used to screen a cDNA library; one fully sequenced clone showed 100% homology with the putative human guanine nucleotide regulatory protein nep1.

Redox signalling and the control of cell growth and death

Cells maintain a reduced intracellular state in the face of a highly oxidizing extracellular environment. Redox signalling pathways provide a link between external stimuli, through the flavoenzyme-mediated NADPH-dependent reduction of intracellular peptide thiols, such as glutathione, thioredoxin, glutaredoxin, and redox factor-1, to the posttranslational redox modification of certain intracellular proteins. This can affect the proteins' correct folding, assembly into multimeric complexes, enzymatic activity, and their binding as transcription factors to specific DNA sequences. Such changes have been linked to altered cell growth and death.