Redox modifications of protein-thiols: emerging roles in cell signaling

Detection of redox-based modification in two-dimensional electrophoresis proteomic separations

Oxidative stress arises when cellular defenses against molecular oxygen and its by-products (reactive oxygen species; ROS) are overcome leading to covalent modification of lipids, DNA, and protein. Redox-based modification of proteins can be conveniently studied by proteomic analysis using two-dimensional electrophoresis (2D SDS-PAGE). Despite some technical shortcomings, this technique allows rapid and quantitative analysis of paired samples, the visualization of discrete protein spots, and provides a robust platform for subsequent analysis and identification of specific proteins. Exposure to oxidative stress introduces a wide range of reversible or irreversible alterations to amino acid side chains. These include carbonylation, glutathionylation, formation of mixed disulphides, effects on disulphide bridge patterns, ubiquitinylation, and racemization. Identification of proteins targeted for specific modification adds a deeper dimension to the dissection of effects of oxidative stress on the proteome with potentially far-reaching implications. This article describes key methodologies now available for identification of redox-based modifications in proteins separated by 2D SDS-PAGE.

A biphasic U-shape effect of cellular oxidative stress on the macrophage anti-oxidant paraoxonase 2 (PON2) enzymatic activity

Expression of macrophage paraoxonase 2 (PON2), a cellular lactonase with anti-oxidant and anti-atherogenic properties, was shown to be upregulated under high oxidative stress. The aim of the present study was to analyze the relationship between the extent of cellular oxidative stress in J774A.1 macrophage and PON2 lactonase activity under various levels of oxidation, obtained by cell incubation with either anti-oxidants or oxidants. PON2 activity exhibited a U-shape response curve. In the oxidative stress range below that of control untreated cells, PON2 activity decreased upon increasing macrophage oxidative state, whereas in the range over that of control untreated cells, PON2 activity increased. The biphasic effect of oxidative stress on macrophage PON2 activity could be related to PON2 inactivation (decreased enzymatic activity) under oxidative stress induction at its low range, whereas at high range of oxidative stress, macrophage anti-oxidant compensatory mechanism up-regulates PON2 (increased protein expression), in order to cope with oxidative burden.

Transcriptional regulation via cysteine thiol modification: a novel molecular strategy for chemoprevention and cytoprotection

Chemoprevention refers to the use of defined nontoxic chemical regimens to inhibit, reverse, or retard the process of multistage carcinogenesis that involves multiple signal transduction events. Identification of signaling molecules associated with carcinogenesis as prime targets of chemopreventive agents has become an area of great interest. Recent studies have implicated cysteine thiols present in various transcription factors, such as NF-kappaB, AP-1, and p53 as redox sensors in transcriptional regulation of many genes essential for maintaining cellular homeostasis. Some chemopreventive and cytoprotective agents have been found to target cysteine thiols present in key transcription factors or their regulators, thereby suppressing aberrant over-activation of carcinogenic signal transduction or restoring/normalizing or even potentiating cellular defense signaling. The focus of this review is the oxidation or covalent modification of thiol groups present in key representative redox-sensitive transcription factors and their regulating molecules as a unique strategy for molecular target-based chemoprevention and cytoprotection.

Copper-dependent interaction of glutaredoxin with the N termini of the copper-ATPases (ATP7A and ATP7B) defective in Menkes and Wilson diseases

The P-type ATPases affected in Menkes and Wilson diseases, ATP7A and ATP7B, respectively, are key copper transporters that regulate copper homeostasis. The N termini of these proteins are critical in regulating their function and activity, and contain six copper-binding motifs MxCxxC. In this study, we describe the identification of glutaredoxin (GRX1) as an interacting partner of both ATP7A and ATP7B, confirmed by yeast two-hybrid technology and by co-immunoprecipitation from mammalian cells. The interaction required the presence of copper and intact metal-binding motifs. In addition, the interaction was related to the number of metal-binding domains available. GRX1 catalyses the reduction of disulphide bridges and reverses the glutathionylation of proteins to regulate and/or protect protein activity. We propose that GRX1 is essential for ATPase function and catalyses either the reduction of intramolecular disulphide bonds or the deglutathionylation of the cysteine residues within the CxxC motifs to facilitate copper-binding for subsequent transport.

Protein S-thiolation and depletion of intracellular glutathione in skin fibroblasts exposed to various sources of oxidative stress

It is well established that oxidative stress plays a central role in the onset and progression of over a 100 different diseases. Recently, a growing body of evidence has shown that chemicals/agents as diverse as aromatic compounds, UV radiation and redox-active metals also generate oxy-radicals in vivo and lead to cellular oxidative stress. In this study we have used mouse skin fibroblasts to study the effects of oxidative stress caused by organic aromatic compounds (xylene), UV radiation and redox-active metals. Specifically, we tested the effect of these treatments on intracellular GSH levels, as well as on protein S-thiolation. We show that acute exposure of these diverse set of conditions cause dramatic depletion of the intracellular GSH pool in mouse skin fibroblast cells. We also found evidence of synergistic effects of combined exposure to different sources of oxidative stress. Furthermore, we also found that these treatments also caused significant S-thiolation (protein mixed-disulfide formation) of a 70kDa cytosolic protein.

The NADPH oxidase NOX4 drives cardiac differentiation: Role in regulating cardiac transcription factors and MAP kinase activation

Reactive oxygen species (ROS) generated by the NOX family of NADPH oxidases have been described to act as second messengers regulating cell growth and differentiation. However, such a function has hitherto not been convincingly demonstrated. We investigated the role of NOX-derived ROS in cardiac differentiation using mouse embryonic stem cells. ROS scavengers prevented the appearance of spontaneously beating cardiac cells within embryoid bodies. Down-regulation of NOX4, the major NOX isoform present during early stages of differentiation, suppressed cardiogenesis. This was rescued by a pulse of low concentrations of hydrogen peroxide 4 d before spontaneous beating appears. Mechanisms of ROS-dependent signaling included p38 mitogen-activated protein kinase (MAPK) activation and nuclear translocation of the cardiac transcription factor myocyte enhancer factor 2C (MEF2C). Our results provide first molecular evidence that the NOX family of NADPH oxidases regulate vertebrate developmental processes.

Differential regulation of NF-kappaB activation and function by topoisomerase II inhibitors

Background

While many common chemotherapeutic drugs and other inducers of DNA-damage result in both NF-κB nuclear translocation and DNA-binding, we have previously observed that, depending on the precise stimulus, there is great diversity of the function of NF-κB. In particular, we found that treatment of U-2 OS osteosarcoma cells with the anthracycine daunorubicin or with ultraviolet (UV-C) light resulted in a form of NF-κB that repressed rather than induced NF-κB reporter plasmids and the expression of specific anti-apoptotic genes. Anthracyclines such as daunorubicin can induce DNA-damage though inhibiting topoisomerase II, intercalating with DNA and undergoing redox cycling to produce oxygen free radicals. In this study we have investigated other anthracyclines, doxorubicin and aclarubicin, as well as the anthracenedione mitoxantrone together with the topoisomerase II inhibitor ICRF-193, which all possess differing characteristics, to determine which of these features is specifically required to induce both NF-κB DNA-binding and transcriptional repression in U-2 OS cells.

Results

The use of mitoxantrone, which does not undergo redox cycling, and the reducing agent epigallocatechingallate (EGCG) demonstrated that oxygen free radical production is not required for induction of NF-κB DNA-binding and transcriptional repression by these agents and UV-C. In addition, the use of aclarubicin, which does not directly inhibit topoisomerase II and ICRF-193, which inhibits topoisomerase II but does not intercalate into DNA, demonstrated that topoisomerase II inhibition is not sufficient to induce the repressor form of NF-κB.

Conclusion

Induction of NF-κB DNA-binding and transcriptional repression by topoisomerase II inhibitors was found to correlate with an ability to intercalate into DNA. Although data from our and other laboratories indicates that topoisomerase II inhibition and oxygen free radicals do regulate NF-κB, they are not required for the particular ability of NF-κB to repress rather than activate transcription. Together with our previous data, these results demonstrate that the nature of the NF-κB response is context dependent. In a clinical setting such effects could profoundly influence the response to chemotherapy and suggest that new methods of analyzing NF-κB function could have both diagnostic and prognostic value.

Nitric oxide and posttranslational modification of the vascular proteome: S-nitrosation of reactive thiols

Nitric oxide (NO*) is known to exert its effects via guanylyl cyclase and cyclic GMP-dependent pathways and by cyclic GMP-independent pathways, including the posttranslational modification of proteins. Much ongoing research is focused on defining the mechanisms of NO*-mediated protein modification, the identity and function of the modified proteins, and the significance of these changes in health and disease. S-nitrosation or thionitrite formation has only been found on a limited number of residues in a subset of proteins in in vitro and in vivo studies. Protein S-nitrosation also appears to be reversible. There are several theories about the in vivo S-nitrosating agent, and most suggest a role for oxidation products of NO* in this process. Flux in cellular S-nitrosoprotein pools appears to be regulated by NO* availability and is redox-sensitive. An analysis of S-nitrosation in candidate proteins has clarified the mechanism by which NO* regulates enzymatic and cellular functions. These findings suggest the utility of using proteomic methods to identify unique targets for protein S-nitrosation to understand further the molecular mechanisms of the effects of NO*.

A scent of therapy: pharmacological implications of natural products containing redox-active sulfur atoms

A range of sulfur-containing natural products from plants, fungi, bacteria and animals have recently been investigated to determine their therapeutic potential. Preliminary in vitro and in vivo studies of compounds such as ergothioneine, ovothiols, allicin, leinamycin, varacin, lenthionine and diallyltetrasulfide have provided evidence for antioxidant, antibacterial, antimicrobial, antifungal and anticancer properties. The biological activity of these compounds is the result of specific chemical properties which converge in chemotypes such as thiols, disulfides, sulfenic and sulfinic acids,thiosulfinates, sulfoxides, sulfones and polysulfides. Redox-activity, catalysis, metal binding, enzyme inhibition and radical generation allow reactive sulfur species to interact with oxidative stressors, to affect the function of redox-sensitive cysteine proteins and to disrupt the integrity of DNA and cellular membranes. In some cases, the biological activity of sulfur-containing plant products depends on initial enzymatic activation, which allows thiosulfinates and isothiocyanates to be generated with high target selectivity. Not surprisingly, research into the biochemical and pharmacological properties of the lesser known sulfur chemotypes is rapidly gathering momentum.