Modification of cysteine residues by cyclopentenone prostaglandins: interplay with redox regulation of protein function

Post-Translational Modifications to Cysteine Residues in Plant Proteins and Their Impact on the Regulation of Metabolism and Signal Transduction

Cys is one of the least abundant amino acids in proteins. However, it is often highly conserved and is usually found in important structural and functional regions of proteins. Its unique chemical properties allow it to undergo several post-translational modifications, many of which are mediated by reactive oxygen, nitrogen, sulfur, or carbonyl species. Thus, in addition to their role in catalysis, protein stability, and metal binding, Cys residues are crucial for the redox regulation of metabolism and signal transduction. In this review, we discuss Cys post-translational modifications (PTMs) and their role in plant metabolism and signal transduction. These modifications include the oxidation of the thiol group (S-sulfenylation, S-sulfinylation and S-sulfonylation), the formation of disulfide bridges, S-glutathionylation, persulfidation, S-cyanylation S-nitrosation, S-carbonylation, S-acylation, prenylation, CoAlation, and the formation of thiohemiacetal. For each of these PTMs, we discuss the origin of the modifier, the mechanisms involved in PTM, and their reversibility. Examples of the involvement of Cys PTMs in the modulation of protein structure, function, stability, and localization are presented to highlight their importance in the regulation of plant metabolic and signaling pathways.

Oxidative stress elicits the remodeling of vimentin filaments into biomolecular condensates

The intermediate filament protein vimentin performs an essential role in cytoskeletal interplay and dynamics, mechanosensing and cellular stress responses. In pathology, vimentin is a key player in tumorigenesis, fibrosis and infection. Vimentin filaments undergo distinct and versatile reorganizations, and behave as redox sensors. The vimentin monomer possesses a central α-helical rod domain flanked by N- and C-terminal low complexity domains. Interactions between this type of domains play an important function in the formation of phase-separated biomolecular condensates, which in turn are critical for the organization of cellular components. Here we show that several oxidants, including hydrogen peroxide and diamide, elicit the remodeling of vimentin filaments into small particles. Oxidative stress elicited by diamide induces a fast dissociation of filaments into circular, motile dots, which requires the presence of the single vimentin cysteine residue, C328. This effect is reversible, and filament reassembly can occur within minutes of oxidant removal. Diamide-elicited vimentin droplets recover fluorescence after photobleaching. Moreover, fusion of cells expressing differentially tagged vimentin allows the detection of dots positive for both tags, indicating that vimentin dots merge upon cell fusion. The aliphatic alcohol 1,6-hexanediol, known to alter interactions between low complexity domains, readily dissolves diamide-elicited vimentin dots at low concentrations, in a C328 dependent manner, and hampers reassembly. Taken together, these results indicate that vimentin oxidation promotes a fast and reversible filament remodeling into biomolecular condensate-like structures, and provide primary evidence of its regulated phase separation. Moreover, we hypothesize that filament to droplet transition could play a protective role against irreversible damage of the vimentin network by oxidative stress.

Desmin Reorganization by Stimuli Inducing Oxidative Stress and Electrophiles: Role of Its Single Cysteine Residue

The type III intermediate filament proteins vimentin and GFAP are modulated by oxidants and electrophiles, mainly through perturbation of their single cysteine residues. Desmin, the type III intermediate filament protein specific to muscle cells, is critical for muscle homeostasis, playing a key role in sarcomere organization and mitochondrial function. Here, we have studied the impact of oxidants and cysteine-reactive agents on desmin behavior. Our results show that several reactive species and drugs induce covalent modifications of desmin in vitro, of which its single cysteine residue, C333, is an important target. Moreover, stimuli eliciting oxidative stress or lipoxidation, including H2O2, 15-deoxy-prostaglandin J2, and CoCl2-elicited chemical hypoxia, provoke desmin disorganization in H9c2 rat cardiomyoblasts transfected with wild-type desmin, which is partially attenuated in cells expressing a C333S mutant. Notably, in cells lacking other cytoplasmic intermediate filaments, network formation by desmin C333S appears less efficient than that of desmin wt, especially when these proteins are expressed as fluorescent fusion constructs. Nevertheless, in these cells, the desmin C333S organization is also protected from disruption by oxidants. Taken together, our results indicate that desmin is a target for oxidative and electrophilic stress, which elicit desmin remodeling conditioned by the presence of its single cysteine residue.

Vimentin single cysteine residue acts as a tunable sensor for network organization and as a key for actin remodeling in response to oxidants and electrophiles

Cysteine residues can undergo multiple posttranslational modifications with diverse functional consequences, potentially behaving as tunable sensors. The intermediate filament protein vimentin has important implications in pathophysiology, including cancer progression, infection, and fibrosis, and maintains a close interplay with other cytoskeletal structures, such as actin filaments and microtubules. We previously showed that the single vimentin cysteine, C328, is a key target for oxidants and electrophiles. Here, we demonstrate that structurally diverse cysteine-reactive agents, including electrophilic mediators, oxidants and drug-related compounds, disrupt the vimentin network eliciting morphologically distinct reorganizations. As most of these agents display broad reactivity, we pinpointed the importance of C328 by confirming that local perturbations introduced through mutagenesis provoke structure-dependent vimentin rearrangements. Thus, GFP-vimentin wild type (wt) forms squiggles and short filaments in vimentin-deficient cells, the C328F, C328W, and C328H mutants generate diverse filamentous assemblies, and the C328A and C328D constructs fail to elongate yielding dots. Remarkably, vimentin C328H structures resemble the wt, but are strongly resistant to electrophile-elicited disruption. Therefore, the C328H mutant allows elucidating whether cysteine-dependent vimentin reorganization influences other cellular responses to reactive agents. Electrophiles such as 1,4-dinitro-1H-imidazole and 4-hydroxynonenal induce robust actin stress fibers in cells expressing vimentin wt. Strikingly, under these conditions, vimentin C328H expression blunts electrophile-elicited stress fiber formation, apparently acting upstream of RhoA. Analysis of additional vimentin C328 mutants shows that electrophile-sensitive and assembly-defective vimentin variants permit induction of stress fibers by reactive species, whereas electrophile-resistant filamentous vimentin structures prevent it. Together, our results suggest that vimentin acts as a break for actin stress fibers formation, which would be released by C328-aided disruption, thus allowing full actin remodeling in response to oxidants and electrophiles. These observations postulate C328 as a "sensor" transducing structurally diverse modifications into fine-tuned vimentin network rearrangements, and a gatekeeper for certain electrophiles in the interplay with actin.

Microsomal prostaglandin E synthase-1 inhibition promotes shunting in arachidonic acid metabolism during inflammatory responses in vitro

Microsomal Prostaglandin E Synthase 1 (mPGES-1) is the key enzyme for the generation of the pro-inflammatory lipid mediator prostaglandin E₂ (PGE₂), which contributes to several pathological features of many diseases. Inhibition of mPGES-1 has been shown to be a safe and effective therapeutic strategy in various pre-clinical studies. In addition to reduced PGE₂ formation, it is also suggested that the potential shunting into other protective and pro-resolving prostanoids may play an important role in resolution of inflammation. In the present study, we analysed the eicosanoid profiles in four in vitro inflammation models and compared the effects of mPGES-1 inhibition with those of cyclooxygenase-2 (Cox-2) inhibition. Our results showed a marked shift to the PGD₂ pathway under mPGES-1 inhibition in A549 cells, RAW264.7 cells and mouse bone marrow-derived macrophages (BMDMs), whereas enhanced prostacyclin production was observed in rheumatoid arthritis synovial fibroblasts (RASFs) treated with an mPGES-1 inhibitor. As expected, Cox-2 inhibition completely suppressed all prostanoids. This study suggests that the therapeutic effects of mPGES-1 inhibition may be mediated by modulation of other prostanoids in addition to PGE₂ reduction.

Protective effects of an electrophilic metabolite of docosahexaenoic acid on UVB-induced oxidative cell death, dermatitis, and carcinogenesis

Docosahexaenoic acid (DHA), a representative omega-3 (ω-3) polyunsaturated fatty acids, undergoes metabolism to produce biologically active electrophilic species. 17-Oxo-DHA is one such reactive metabolite generated from DHA by cyclooxygenase-2 and dehydrogenase in activated macrophages. The present study was aimed to investigate the effects of 17-oxo-DHA on ultraviolet B (UVB)-induced oxidative stress, inflammation, and carcinogenesis in mouse skin. UVB-induced epidermal cell death was ameliorated by topically applied 17-oxo-DHA. Topical application of 17-oxo-DHA onto hairless mouse skin inhibited UVB-induced phosphorylation of the proinflammatory transcription factor, STAT3 on tyrosine 705 (Tyr705). The 17-oxo-DHA treatment also reduced the levels of oxidative stress markers, 4-hydroxynonenal-modified protein, malondialdehyde, and 8-oxo-2'-deoxyguanosine. The protective effects of 17-oxo-DHA against oxidative damage in UVB-irradiated mouse skin were associated with activation of Nrf2. 17-Oxo-DHA enhanced the engulfment of apoptotic JB6 cells by macrophages, which was related to the increased expression of the scavenger receptor CD36. The 17-oxo-DHA-mediated potentiation of efferocytic activity of macrophages was attenuated by the pharmacologic inhibition or knockout of Nrf2. The pretreatment with 17-oxo-DHA reduced the UVB-induced skin carcinogenesis and tumor angiogenesis. It was also confirmed that 17-oxo-DHA treatment significantly inhibited the phosphorylation of the Tyr705 residue of STAT3 and decreased the expression of its target proteins in cutaneous papilloma. In conclusion, 17-oxo-DHA protects against UVB-induced oxidative cell death, dermatitis, and carcinogenesis. These effects were associated with inhibition of STAT3-mediated proinflammatory signaling and also activation of Nrf2 with subsequent upregulation of antioxidant and anti-inflammatory gene expression.

Biosynthesis of prostaglandin 15dPGJ2 -glutathione and 15dPGJ2-cysteine conjugates in macrophages and mast cells via MGST3

Inhibition of microsomal prostaglandin E synthase-1 (mPGES-1) results in decreased production of proinflammatory PGE₂ and can lead to shunting of PGH₂ into the prostaglandin D₂ (PGD₂)/15-deoxy-Δ^12,14-prostaglandin J₂ (15dPGJ₂) pathway. 15dPGJ₂ forms Michael adducts with thiol-containing biomolecules such as GSH or cysteine residues on target proteins and is thought to promote resolution of inflammation. We aimed to elucidate the biosynthesis and metabolism of 15dPGJ₂ via conjugation with GSH, to form 15dPGJ₂-glutathione (15dPGJ₂-GS) and 15dPGJ₂-cysteine (15dPGJ₂-Cys) conjugates and to characterize the effects of mPGES-1 inhibition on the PGD₂/15dPGJ₂ pathway in mouse and human immune cells. Our results demonstrate the formation of PGD₂, 15dPGJ₂, 15dPGJ₂-GS, and 15dPGJ₂-Cys in RAW264.7 cells after lipopolysaccharide stimulation. Moreover, 15dPGJ₂-Cys was found in lipopolysaccharide-activated primary murine macrophages as well as in human mast cells following stimulation of the IgE-receptor. Our results also suggest that the microsomal glutathione S-transferase 3 is essential for the formation of 15dPGJ₂ conjugates. In contrast to inhibition of cyclooxygenase, which leads to blockage of the PGD₂/15dPGJ₂ pathway, we found that inhibition of mPGES-1 preserves PGD₂ and its metabolites. Collectively, this study highlights the formation of 15dPGJ₂-GS and 15dPGJ₂-Cys in mouse and human immune cells, the involvement of microsomal glutathione S-transferase 3 in their biosynthesis, and their unchanged formation following inhibition of mPGES-1. The results encourage further research on their roles as bioactive lipid mediators.

Alexander disease GFAP R239C mutant shows increased susceptibility to lipoxidation and elicits mitochondrial dysfunction and oxidative stress

Alexander disease is a fatal neurological disorder caused by mutations in the intermediate filament protein Glial Fibrillary Acidic Protein (GFAP), which is key for astrocyte homeostasis. These mutations cause GFAP aggregation, astrocyte dysfunction and neurodegeneration. Remarkably, most of the known GFAP mutations imply a change by more nucleophilic amino acids, mainly cysteine or histidine, which are more susceptible to oxidation and lipoxidation. Therefore, we hypothesized that a higher susceptibility of Alexander disease GFAP mutants to oxidative or electrophilic damage, which frequently occurs during neurodegeneration, could contribute to disease pathogenesis. To address this point, we have expressed GFP-GFAP wild type or the harmful Alexander disease GFP-GFAP R239C mutant in astrocytic cells. Interestingly, GFAP R239C appears more oxidized than the wild type under control conditions, as indicated both by its lower cysteine residue accessibility and increased presence of disulfide-bonded oligomers. Moreover, GFP-GFAP R239C undergoes lipoxidation to a higher extent than GFAP wild type upon treatment with the electrophilic mediator 15-deoxy-Δ12,14-prostaglandin J2 (15d-PGJ2). Importantly, GFAP R239C filament organization is altered in untreated cells and is earlier and more severely disrupted than GFAP wild type upon exposure to oxidants (diamide, H2O2) or electrophiles (4-hydroxynonenal, 15d-PGJ2), which exacerbate GFAP R239C aggregation. Furthermore, H2O2 causes reversible alterations in GFAP wild type, but irreversible damage in GFAP R239C expressing cells. Finally, we show that GFAP R239C expression induces a more oxidized cellular status, with decreased free thiol content and increased mitochondrial superoxide generation. In addition, mitochondria show decreased mass, increased colocalization with GFAP and altered morphology. Notably, a GFP-GFAP R239H mutant recapitulates R239C-elicited alterations whereas an R239G mutant induces a milder phenotype. Together, our results outline a deleterious cycle involving altered GFAP R239C organization, mitochondrial dysfunction, oxidative stress, and further GFAP R239C protein damage and network disruption, which could contribute to astrocyte derangement in Alexander disease.

Formation of a bioactive cyclopentenone and its adducts with amino acids in sterilized-fruits and - vegetables baby foods

The formation of the molecule 4,5-dihydroxy-2-cyclopenten-1-one (DHCP) from the thermal treatment of pectin-containing foods was investigated in small-scale laboratory preparation of sterilized vegetable puree (carrot, zucchini and tomato) and fruit puree (peach and mixture of pear and apple) and in commercial baby foods. DHCP attracts attention due to its cytotoxicity as well as potential antiviral and anti-inflammatory effects. However, its effects and the difficulty of its identification in food are mediated in part by the formation of Michael adducts of DHCP with amino acids. The results revealed that DHCP reacted efficiently with cysteine and glutathione, and to a lesser extent with histidine. Mass spectrometry analysis confirmed the formation of adducts of DHCP with amino acids in a model system, being in a real food system difficult to investigate. However, these formed adducts are of potential interest, although it is not known whether they are safe, bioactive or reversible.

The innate immune system and fever under redox control: A Narrative Review

ABSTRACT:

In living cells, redox potential is vitally important for normal physiological processes that are closely regulated by antioxidants, free amino acids and proteins that either have reactive oxygen and nitrogen species capture capability or can be compartmentalized. Although hundreds of experiments support the regulatory role of free radicals and their derivatives, several authors continue to claim that these perform only harmful and non-regulatory functions. In this paper we show that countless intracellular and extracellular signal pathways are directly or indirectly linked to regulated redox processes. We also briefly discuss how artificial oxidative stress can have important therapeutic potential and the possible negative effects of popular antioxidant supplements.

Dynamic posttranslational modifications of cytoskeletal proteins unveil hot spots under nitroxidative stress

The cytoskeleton is a supramolecular structure consisting of interacting protein networks that support cell dynamics in essential processes such as migration and division, as well as in responses to stress. Fast cytoskeletal remodeling is achieved with the participation of regulatory proteins and posttranslational modifications (PTMs). Redox-related PTMs are emerging as critical players in cytoskeletal regulation. Here we used a cellular model of mild nitroxidative stress in which a peroxynitrite donor induced transient changes in the organization of three key cytoskeletal proteins, i.e., vimentin, actin and tubulin. Nitroxidative stress-induced reconfiguration of intermediate filaments, microtubules and actin structures were further correlated with their PTM profiles and dynamics of the PTM landscape. Using high-resolution mass spectrometry, 62 different PTMs were identified and relatively quantified in vimentin, actin and tubulin, including 12 enzymatic, 13 oxidative and 2 nitric oxide-derived modifications as well as 35 modifications by carbonylated lipid peroxidation products, thus evidencing the occurrence of a chain reaction with formation of numerous reactive species and activation of multiple signaling pathways. Our results unveil the presence of certain modifications under basal conditions and their modulation in response to stress in a target-, residue- and reactive species-dependent manner. Thus, some modifications accumulated during the experiment whereas others varied transiently. Moreover, we identified protein PTM "hot spots", such as the single cysteine residue of vimentin, which was detected in seven modified forms, thus, supporting its role in PTM crosstalk and redox sensing. Finally, identification of novel PTMs in these proteins paves the way for unveiling new cytoskeleton regulatory mechanisms.

Nitric Oxide and Electrophilic Cyclopentenone Prostaglandins in Redox signaling, Regulation of Cytoskeleton Dynamics and Intercellular Communication

Nitric oxide (NO) and electrophilic cyclopentenone prostaglandins (CyPG) are local mediators that modulate cellular response to oxidative stress in different pathophysiological processes. In particular, there is increasing evidence about their functional role during inflammation and immune responses. Although the mechanistic details about their relationship and functional interactions are still far from resolved, NO and CyPG share the ability to promote redox-based post-translational modification (PTM) of proteins that play key roles in cellular homeostasis, signal transduction and transcription. NO-induced S-nitrosylation and S-glutathionylation as well as cyclopentenone-mediated adduct formation, are a few of the main PTMs by which intra- and inter-cellular signaling are regulated. There is a growing body of evidence indicating that actin and actin-binding proteins are susceptible to covalent PTM by these agents. It is well known that the actin cytoskeleton is key for the establishment of interactions among leukocytes, endothelial and muscle cells, enabling cellular activation and migration. In this review we analyze the current knowledge about the actions exerted by NO and CyPG electrophilic lipids on the regulation of actin dynamics and cytoskeleton organization, and discuss some open questions regarding their functional relevance in the regulation of intercellular communication.

Protein Lipoxidation: Basic Concepts and Emerging Roles

Protein lipoxidation is a non-enzymatic post-translational modification that consists of the covalent addition of reactive lipid species to proteins. This occurs under basal conditions but increases in situations associated with oxidative stress. Protein targets for lipoxidation include metabolic and signalling enzymes, cytoskeletal proteins, and transcription factors, among others. There is strong evidence for the involvement of protein lipoxidation in disease, including atherosclerosis, neurodegeneration, and cancer. Nevertheless, the involvement of lipoxidation in cellular regulatory mechanisms is less understood. Here we review basic aspects of protein lipoxidation and discuss several features that could support its role in cell signalling, including its selectivity, reversibility, and possibilities for regulation at the levels of the generation and/or detoxification of reactive lipids. Moreover, given the great structural variety of electrophilic lipid species, protein lipoxidation can contribute to the generation of multiple structurally and functionally diverse protein species. Finally, the nature of the lipoxidised proteins and residues provides a frameshift for a complex interplay with other post-translational modifications, including redox and redox-regulated modifications, such as oxidative modifications and phosphorylation, thus strengthening the importance of detailed knowledge of this process.

Activation of phospholipase A2 by prostaglandin in vitro

Prostaglandins are a diverse family of biological active molecules that are synthesized after liberation of arachnidonic or linolenic acid from the plasma membrane by phospholipase A2 (PLA2). Specific prostaglandins may be pro-inflammatory or anti-inflammatory due to a poorly understood biochemical equilibrium. Some of the anti-inflammatory prostaglandins namely, prostaglandin A1 (PGA1) and prostaglandin E1 (PGE1) have a cyclopentenone moiety that can react and modify a protein's activity. These two prostaglandins are electrophilic reactive lipid species and are formed as a result of the reaction cascade initiated by PLA2. It was of interest to study the interaction with these prostaglandins as they could either amplify or block this enzyme's activity. We found that the former is true initially as there is a shorter time to activate the protein on the lipid membrane and an overall increase in hydrolysis was observed when PGA1 and PGE1 prostaglandin was added with PLA2 and liposomes. The interfacial activation model was further explored in which there is a modification of the enzyme rather than a modifcation of the substrate. However, after a time the protein was shown to form amyloid like fibrils thereby blocking further hydrolysis. The fibrillization kinetics in the presence of the one of the prostaglandins was monitored using thioflavin T (ThT) and the resulting fibrils were characterized using transmission electron microscopy (TEM) and atomic force microscopy (AFM). Modification of PLA2 by these prostaglandins leading to amyloid like fibrils gives an additional perspective of control of the interfacial activation mechanism of this enzyme.

A novel near-infrared fluorescent probe based on isophorone for the bioassay of endogenous cysteine

A dicyanoisophorone/acrylate-combined probe (DDP) was synthesized and designed as a near-infrared (NIR) fluorescent sensor for the rapid identification of Cys over Hcy and GSH in aqueous solution with a large Stokes shift (143 nm). The detection limit of Cys was 1.23 μM, which was lower than that of the intracellular Cys concentration. DDP was cell membrane-permeable and had been successfully applied to the detection of intracellular Cys in HeLa cells. The detection mechanism was determined by ¹H NMR titration, MS and DFT calculations.

A Comprehensive Review on Source, Types, Effects, Nanotechnology, Detection, and Therapeutic Management of Reactive Carbonyl Species Associated with Various Chronic Diseases

Continuous oxidation of carbohydrates, lipids, and amino acids generate extremely reactive carbonyl species (RCS). Human body comprises some important RCS namely hexanal, acrolein, 4-hydroxy-2-nonenal, methylglyoxal, malondialdehyde, isolevuglandins, and 4-oxo-2- nonenal etc. These RCS damage important cellular components including proteins, nucleic acids, and lipids, which manifests cytotoxicity, mutagenicity, multitude of adducts and crosslinks that are connected to ageing and various chronic diseases like inflammatory disease, atherosclerosis, cerebral ischemia, diabetes, cancer, neurodegenerative diseases and cardiovascular disease. The constant prevalence of RCS in living cells suggests their importance in signal transduction and gene expression. Extensive knowledge of RCS properties, metabolism and relation with metabolic diseases would assist in development of effective approach to prevent numerous chronic diseases. Treatment approaches for RCS associated diseases involve endogenous RCS metabolizers, carbonyl metabolizing enzyme inducers, and RCS scavengers. Limited bioavailability and bio efficacy of RCS sequesters suggest importance of nanoparticles and nanocarriers. Identification of RCS and screening of compounds ability to sequester RCS employ several bioassays and analytical techniques. Present review describes in-depth study of RCS sources, types, properties, identification techniques, therapeutic approaches, nanocarriers, and their role in various diseases. This study will give an idea for therapeutic development to combat the RCS associated chronic diseases.

Prostaglandin A1 Decreases the Phosphorylation of Tau by Activating Protein Phosphatase 2A via a Michael Addition Mechanism at Cysteine 377

Prostaglandin (PG) A1 is a metabolic product of cyclooxygenase 2 (COX-2) that is potentially involved in regulating the development and progression of Alzheimer's disease (AD). PGA1 is a cyclopentenone (cy) PG characterized by the presence of a chemically reactive α,β-unsaturated carbonyl. PGA1 is potentially involved in the regulation of multiple biological processes via Michael addition; however, the specific roles of PGA1 in AD remain unclear. Tau^P301S transgenic (Tg) mice were used as in vivo AD models, and neuroblastoma (N) 2a cells were used as an in vitro neuronal model. The PGA1-binding proteins were identified by HPLC-MS-MS after intracerebroventricular injection (i.c.v) of PGA1. Western blotting was used to determine tau phosphorylation in PGA1-treated Tg mice in the absence or in the presence of okadaic acid (OA), an inhibitor of protein phosphatase (PP) 2A. A combination of pull-down assay, immunoprecipitation, western blotting, and HPLC-MS-MS was used to determine that the PP2A scaffold subunit A alpha (PPP2R1A) is activated by the direct binding of PGA1 to cysteine 377. The effect of inhibiting tau hyperphosphorylation was tested in the Morris maze to determine the inhibitory effects of PGA1 on cognitive decline in tau^P301S Tg mice. Incubation with N2a cells, pull-down assay, and mass spectrometry (MS) analysis revealed and indicated that PGA1 binds to more than 1000 proteins; some of these proteins are associated with AD and especially with tauopathies. Moreover, short-term administration of PGA1 in tau^P301S Tg mice significantly decreased tau phosphorylation at Thr181, Ser202, and Ser404 in a dose-dependent manner. This effect was caused by the activation of PPP2R1A in tau^P301S Tg mice. Importantly, PGA1 can form a Michael adduct with cysteine 377 of PPP2R1A, which is critical for the enzymatic activity of PP2A. Long-term treatment of tau^P301S Tg mice with PGA1 activated PP2A and significantly reduced tau phosphorylation resulting in improvements in cognitive decline in tau^P301S Tg mice. Our data provided new insight into the mechanisms of the ameliorating effects of PGA1 on cognitive decline in tau^P301S Tg mice by activating PP2A via a mechanism involving the formation of a Michael adduct with cysteine 377 of PPP2R1A.

Oxidative stress mitigation by antioxidants - An overview on their chemistry and influences on health status

The present review paper focuses on the chemistry of oxidative stress mitigation by antioxidants. Oxidative stress is understood as a lack of balance between the pro-oxidant and the antioxidant species. Reactive oxygen species in limited amounts are necessary for cell homeostasis and redox signaling. Excessive reactive oxygenated/nitrogenated species production, which counteracts the organism's defense systems, is known as oxidative stress. Sustained attack of endogenous and exogenous ROS results in conformational and oxidative alterations in key biomolecules. Chronic oxidative stress is associated with oxidative modifications occurring in key biomolecules: lipid peroxidation, protein carbonylation, carbonyl (aldehyde/ketone) adduct formation, nitration, sulfoxidation, DNA impairment such strand breaks or nucleobase oxidation. Oxidative stress is tightly linked to the development of cancer, diabetes, neurodegeneration, cardiovascular diseases, rheumatoid arthritis, kidney disease, eye disease. The deleterious action of reactive oxygenated species and their role in the onset and progression of pathologies are discussed. The results of oxidative attack become themselves sources of oxidative stress, becoming part of a vicious cycle that amplifies oxidative impairment. The term antioxidant refers to a compound that is able to impede or retard oxidation, acting at a lower concentration compared to that of the protected substrate. Antioxidant intervention against the radicalic lipid peroxidation can involve different mechanisms. Chain breaking antioxidants are called primary antioxidants, acting by scavenging radical species, converting them into more stable radicals or non-radical species. Secondary antioxidants quench singlet oxygen, decompose peroxides, chelate prooxidative metal ions, inhibit oxidative enzymes. Moreover, four reactivity-based lines of defense have been identified: preventative antioxidants, radical scavengers, repair antioxidants, and those relying on adaptation mechanisms. The specific mechanism of a series of endogenous and exogenous antioxidants in particular aspects of oxidative stress, is detailed. The final section resumes critical conclusions regarding antioxidant supplementation.

Type III intermediate filaments as targets and effectors of electrophiles and oxidants

Intermediate filaments (IFs) play key roles in cell mechanics, signaling and homeostasis. Their assembly and dynamics are finely regulated by posttranslational modifications. The type III IFs, vimentin, desmin, peripherin and glial fibrillary acidic protein (GFAP), are targets for diverse modifications by oxidants and electrophiles, for which their conserved cysteine residue emerges as a hot spot. Pathophysiological examples of these modifications include lipoxidation in cell senescence and rheumatoid arthritis, disulfide formation in cataracts and nitrosation in endothelial shear stress, although some oxidative modifications can also be detected under basal conditions. We previously proposed that cysteine residues of vimentin and GFAP act as sensors for oxidative and electrophilic stress, and as hinges influencing filament assembly. Accumulating evidence indicates that the structurally diverse cysteine modifications, either per se or in combination with other posttranslational modifications, elicit specific functional outcomes inducing distinct assemblies or network rearrangements, including filament stabilization, bundling or fragmentation. Cysteine-deficient mutants are protected from these alterations but show compromised cellular performance in network assembly and expansion, organelle positioning and aggresome formation, revealing the importance of this residue. Therefore, the high susceptibility to modification of the conserved cysteine of type III IFs and its cornerstone position in filament architecture sustains their role in redox sensing and integration of cellular responses. This has deep pathophysiological implications and supports the potential of this residue as a drug target.

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Type III intermediate filaments can be modified by many oxidants and electrophiles.

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Oxidative modifications of type III IFs occur in normal and pathological conditions.

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The conserved cysteine residue acts as a hub for redox/electrophilic modifications.

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Cysteine modifications elicit structure-dependent type III IF rearrangements.

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Type III intermediate filaments act as sensors for oxidative and electrophilic stress.

The chemistry, biology and pharmacology of the cyclopentenone prostaglandins

The cyclopentenone prostaglandins (CyPGs) are a small group compounds that are a subset of the eicosanoid superfamily, which are metabolites of arachidonic acid as well as other polyunsaturated fatty acids. The CyPGs are defined by a structural feature, namely, a five-membered carbocyclic ring containing an alfa-beta unsaturated keto group. The two most studied members are PGA₂ and 15d-PGJ₂ (15-deoxy-Δ12,14-prostaglandin J₂); other less studied members are PGA₁, Δ¹²-PGJ₂, and PGJ₂. They are involved in a number of biological activities including the ability to resolve chronic inflammation and the growth and survival of cells, particularly those of cancerous or neurological origin. Also, they can activate the prostaglandin DP2 receptor as well as the ligand-dependent transcription factor PPAR-gamma. Their ability to promote the resolution of chronic inflammation makes it of particular interest to have a good understanding of their actions. Since their discovery, the literature on the CyPGs has greatly expanded both in size and in scope; these reports are covered in the current review.

Insight into the cellular effects of nitrated phospholipids: Evidence for pleiotropic mechanisms of action

Nitrated phospholipids have been recently identified in biological systems and showed to display anti-oxidant and anti-inflammatory potential in models of inflammation in vitro. Here, we have explored the effects of nitrated 1-palmitoyl-2-oleyl-phosphatidyl choline (NO2-POPC) in cellular models. We have observed that NO2-POPC, but not POPC, induces cellular changes consisting in cytoskeletal rearrangement and cell shrinking, and ultimately, loss of cell adhesion or impaired cell attachment. NO2-POPC releases NO in vitro and induces accumulation of NO in cells. Nevertheless, the effects of NO2-POPC are not superimposable with those of NO donors, which points to distinctive mechanisms of action. Notably, they show a stronger parallelism, although not complete overlap, with the effects of nitrated fatty acids. Interestingly, redistribution of vimentin by NO2-POPC is attenuated in a C328S mutant, thus indicating that this residue may be a target for direct or indirect modification in NO2-POPC-treated cells. Additionally, NO2-POPC interacts with several typical lipoxidation targets in vitro, including vimentin and PPARγ constructs, likely through cysteine residues. Therefore, nitrated phospholipids emerge as potential novel electrophilic lipid mediators with selective actions.

Integrated approaches to unravel the impact of protein lipoxidation on macromolecular interactions

Protein modification by lipid derived reactive species, or lipoxidation, is increased during oxidative stress, a common feature observed in many pathological conditions. Biochemical and functional consequences of lipoxidation include changes in the conformation and assembly of the target proteins, altered recognition of ligands and/or cofactors, changes in the interactions with DNA or in protein-protein interactions, modifications in membrane partitioning and binding and/or subcellular localization. These changes may impact, directly or indirectly, signaling pathways involved in the activation of cell defense mechanisms, but when these are overwhelmed they may lead to pathological outcomes. Mass spectrometry provides state of the art approaches for the identification and characterization of lipoxidized proteins/residues and the modifying species. Nevertheless, understanding the complexity of the functional effects of protein lipoxidation requires the use of additional methodologies. Herein, biochemical and biophysical methods used to detect and measure functional effects of protein lipoxidation at different levels of complexity, from in vitro and reconstituted cell-like systems to cells, are reviewed, focusing especially on macromolecular interactions. Knowledge generated through innovative and complementary technologies will contribute to comprehend the role of lipoxidation in pathophysiology and, ultimately, its potential as target for therapeutic intervention.

Regulation of stress signaling pathways by protein lipoxidation

Enzymatic and non-enzymatic oxidation of unsaturated fatty acids gives rise to reactive species that covalently modify nucleophilic residues within redox sensitive protein sensors in a process called lipoxidation. This triggers adaptive signaling pathways that ultimately lead to increased resistance to stress. In this graphical review, we will provide an overview of pathways affected by protein lipoxidation and the key signaling proteins being altered, focusing on the KEAP1-NRF2 and heat shock response pathways. We review the mechanisms by which lipid peroxidation products can serve as second messengers and evoke cellular responses via covalent modification of key sensors of altered cellular environment, ultimately leading to adaptation to stress.

Fluorescent Imaging for Cysteine Detection In Vivo with High Selectivity

As an essential amino acid, cysteine is involved in various biosynthetic and metabolic processes, such as protein synthesis, hormone synthesis, and redox homeostatic maintenance. Inordinate cysteine levels are often associated with serious diseases. Thus, designing and synthesizing a novel fluorescent probe for determining the concentration of cellular cysteine, which could indirectly monitor the prevalence of these diseases, is essential. We developed a florescence probe P-Cy with good sensitivity for cysteine detection in vivo. P-Cy only exhibited good response toward cysteine but did not show response toward other biothiols, such as homocysteine (Hcy) and glutathione (GSH). In this study, we used P-Cy by successfully imaging cellular endogenous and exogenous cysteine levels. Furthermore, P-Cy was also performed in mice to detect cysteine level, indicating that P-Cy is a powerful tool for cysteine detection in situ.

A concise appraisal of lipid oxidation and lipoxidation in higher plants

Polyunsaturated fatty acids present in plant membranes react with reactive oxygen species through so-called lipid oxidation events. They generate great diversity of highly-reactive lipid-derived chemical species, which may be further degraded enzymatically or non-enzymatically originating new components like Reactive Carbonyl Species (RCS). Such RCS are able to selectively react with proteins frequently producing loss of function through lipoxidation reactions. Although a basal concentration of lipoxidation products exists in plants (likely involved in signaling), their concentration and variability growth exponentially when plants are subjected to biotic/abiotic stresses. Such conditions typically increase the presence of ROS and the expression of antioxidant enzymes, together with RCS and also metabolites resulting from their reaction with proteins (advanced lipoxidation endproducts, ALE), in those plants susceptible to stress. On the contrary, plants designed as resistant may or may not display enhanced levels of ROS and antioxidant enzymes, whereas levels of lipid oxidation markers as malondialdehyde (MDA) are typically reduced. Great efforts have been made in order to develop methods to identify and quantify RCS, ALE, and other adducts with high sensitivity. Many of these methods are applied to the analysis of plant physiology and stress resistance, although their use has been extended to the control of the processing and conservation parameters of foodstuffs derived from plants. These foods may accumulate either lipid oxidation/lipoxidation products, or antioxidants like polyphenols, which are sometimes critical for their organoleptic properties, nutritional value, and health-promoting or detrimental characteristics. Future directions of research on different topics involving these chemical changes are also discussed.

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Lipid (per)oxidation occurs in plants as a signaling mechanism and after stress.

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Electrophylic mediators are widely used to assess plant physiology.

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Few lypoxidation targets have been identified in plants, mainly related to stress.

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Lipoxidation frequently inactivates or highly affects enzyme activity in plants.

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Lipid oxidation/lipoxidation affect the quality and healthy properties of plant foods.

Lipids and lipoxidation in human brain aging. Mitochondrial ATP-synthase as a key lipoxidation target

The human brain is a target of the aging process like other cell systems of the human body. Specific regions of the human brain exhibit differential vulnerabilities to the aging process. Yet the underlying mechanisms that sustain the preservation or deterioration of neurons and cerebral functions are unknown. In this review, we focus attention on the role of lipids and the importance of the cross-regionally different vulnerabilities in human brain aging. In particular, we first consider a brief approach to the lipidomics of human brain, the relationship between lipids and lipoxidative damage, the role of lipids in human brain aging, and the specific targets of lipoxidative damage in human brain and during aging. It is proposed that the restricted set of modified proteins and the functional categories involved may be considered putative collaborative factors contributing to neuronal aging, and that mitochondrial ATP synthase is a key lipoxidative target in human brain aging.

The function of the oxylipin 12-oxophytodienoic acid in cell signaling, stress acclimation, and development

Forty years ago, 12-oxophytodienoic acid (12-OPDA) was reported as a prostaglandin (PG)-like metabolite of linolenic acid found in extracts of flaxseed. Since then, numerous studies have determined the role of 12-OPDA in regulating plant immunity, seed dormancy, and germination. This review summarizes our current knowledge of the regulation of 12-OPDA synthesis in the chloroplast and 12-OPDA-dependent signaling in gene expression and targeting protein functions. We describe the properties of OPDA that are linked to the activities of PGs, which are derived from arachidonic acid and act as tissue hormones in animals, including humans. The similarity of OPDA with bioactive PGs is particularly evident for the most-studied cyclopentenone, PG 15-dPGJ2. In addition to chemical approaches towards 12-OPDA synthesis, bio-organic synthesis strategies for 12-OPDA and analogous substances have recently been established. The resulting availability of OPDA will aid the identification of additional effector proteins, help in elucidating the mechanisms of OPDA sensing and transmission, and will foster the analysis of the physiological responses to OPDA in plants. There is a need to determine the compartmentation and transport of 12-OPDA and its conjugates, over long distances as well as short. It will be important to further study OPDA in animal and human cells, for example with respect to beneficial anti-inflammatory and anti-cancer activities.

The cysteine residue of glial fibrillary acidic protein is a critical target for lipoxidation and required for efficient network organization

The type III intermediate filament protein glial fibrillary acidic protein (GFAP) contributes to the homeostasis of astrocytes, where it co-polymerizes with vimentin. Conversely, alterations in GFAP assembly or degradation cause intracellular aggregates linked to astrocyte dysfunction and neurological disease. Moreover, injury and inflammation elicit extensive GFAP organization and expression changes, which underline reactive gliosis. Here we have studied GFAP as a target for modification by electrophilic inflammatory mediators. We show that the GFAP cysteine, C294, is targeted by lipoxidation by cyclopentenone prostaglandins (cyPG) in vitro and in cells. Electrophilic modification of GFAP in cells leads to a striking filament rearrangement, with retraction from the cell periphery and juxtanuclear condensation in thick bundles. Importantly, the C294S mutant is resistant to cyPG addition and filament disruption, thus highlighting the critical role of this residue as a sensor of oxidative damage. However, GFAP C294S shows defective or delayed network formation in GFAP-deficient cells, including SW13/cl.2 cells and GFAP- and vimentin-deficient primary astrocytes. Moreover, GFAP C294S does not effectively integrate with and even disrupts vimentin filaments in the short-term. Interestingly, short-spacer bifunctional cysteine crosslinking produces GFAP-vimentin heterodimers, suggesting that a certain proportion of cysteine residues from both proteins are spatially close. Collectively, these results support that the conserved cysteine residue in type III intermediate filament proteins serves as an electrophilic stress sensor and structural element. Therefore, oxidative modifications of this cysteine could contribute to GFAP disruption or aggregation in pathological situations associated with oxidative or electrophilic stress.

Senescent cells expose and secrete an oxidized form of membrane-bound vimentin as revealed by a natural polyreactive antibody

Studying the phenomenon of cellular senescence has been hindered by the lack of senescence-specific markers. As such, detection of proteins informally associated with senescence accompanies the use of senescence-associated β-galactosidase as a collection of semiselective markers to monitor the presence of senescent cells. To identify novel biomarkers of senescence, we immunized BALB/c mice with senescent mouse lung fibroblasts and screened for antibodies that recognized senescence-associated cell-surface antigens by FACS analysis and a newly developed cell-based ELISA. The majority of antibodies that we isolated, cloned, and sequenced belonged to the IgM isotype of the innate immune system. In-depth characterization of one of these monoclonal, polyreactive natural antibodies, the IgM clone 9H4, revealed its ability to recognize the intermediate filament vimentin. By using 9H4, we observed that senescent primary human fibroblasts express vimentin on their cell surface, and MS analysis revealed a posttranslational modification on cysteine 328 (C328) by the oxidative adduct malondialdehyde (MDA). Moreover, elevated levels of secreted MDA-modified vimentin were detected in the plasma of aged senescence-accelerated mouse prone 8 mice, which are known to have deregulated reactive oxygen species metabolism and accelerated aging. Based on these findings, we hypothesize that humoral innate immunity may recognize senescent cells by the presence of membrane-bound MDA-vimentin, presumably as part of a senescence eradication mechanism that may become impaired with age and result in senescent cell accumulation.

Prostaglandin J2: a potential target for halting inflammation-induced neurodegeneration

Prostaglandins (PGs) are produced via cyclooxygenases, which are enzymes that play a major role in neuroinflammation. Epidemiological studies show that chronic treatment with low levels of cyclooxygenase inhibitors (nonsteroidal anti-inflammatory drugs (NSAIDs)) lowers the risk for Alzheimer's disease (AD) and Parkinson's disease (PD) by as much as 50%. Unfortunately, inhibiting cyclooxygenases with NSAIDs blocks the synthesis of downstream neuroprotective and neurotoxic PGs, thus producing adverse side effects. We focus on prostaglandin J2 (PGJ2) because it is highly neurotoxic compared to PGA1, D2, and E2. Unlike other PGs, PGJ2 and its metabolites have a cyclopentenone ring with reactive α,β-unsaturated carbonyl groups that form covalent Michael adducts with key cysteines in proteins and GSH. Cysteine-binding electrophiles such as PGJ2 are considered to play an important role in determining whether neurons will live or die. We discuss in vitro and in vivo studies showing that PGJ2 induces pathological processes relevant to neurodegenerative disorders such as AD and PD. Further, we discuss our work showing that increasing intracellular cAMP with the lipophilic peptide PACAP27 counteracts some of the PGJ2-induced detrimental effects. New therapeutic strategies that neutralize the effects of specific neurotoxic PGs downstream from cyclooxygenases could have a significant impact on the treatment of chronic neurodegenerative disorders with fewer adverse side effects.

Vimentin filament organization and stress sensing depend on its single cysteine residue and zinc binding

The vimentin filament network plays a key role in cell architecture and signalling, as well as in epithelial-mesenchymal transition. Vimentin C328 is targeted by various oxidative modifications, but its role in vimentin organization is not known. Here we show that C328 is essential for vimentin network reorganization in response to oxidants and electrophiles, and is required for optimal vimentin performance in network expansion, lysosomal distribution and aggresome formation. C328 may fulfil these roles through interaction with zinc. In vitro, micromolar zinc protects vimentin from iodoacetamide modification and elicits vimentin polymerization into optically detectable structures; in cells, zinc closely associates with vimentin and its depletion causes reversible filament disassembly. Finally, zinc transport-deficient human fibroblasts show increased vimentin solubility and susceptibility to disruption, which are restored by zinc supplementation. These results unveil a critical role of C328 in vimentin organization and open new perspectives for the regulation of intermediate filaments by zinc.

Protein lipoxidation: Detection strategies and challenges

Enzymatic and non-enzymatic lipid metabolism can give rise to reactive species that may covalently modify cellular or plasma proteins through a process known as lipoxidation. Under basal conditions, protein lipoxidation can contribute to normal cell homeostasis and participate in signaling or adaptive mechanisms, as exemplified by lipoxidation of Ras proteins or of the cytoskeletal protein vimentin, both of which behave as sensors of electrophilic species. Nevertheless, increased lipoxidation under pathological conditions may lead to deleterious effects on protein structure or aggregation. This can result in impaired degradation and accumulation of abnormally folded proteins contributing to pathophysiology, as may occur in neurodegenerative diseases. Identification of the protein targets of lipoxidation and its functional consequences under pathophysiological situations can unveil the modification patterns associated with the various outcomes, as well as preventive strategies or potential therapeutic targets. Given the wide structural variability of lipid moieties involved in lipoxidation, highly sensitive and specific methods for its detection are required. Derivatization of reactive carbonyl species is instrumental in the detection of adducts retaining carbonyl groups. In addition, use of tagged derivatives of electrophilic lipids enables enrichment of lipoxidized proteins or peptides. Ultimate confirmation of lipoxidation requires high resolution mass spectrometry approaches to unequivocally identify the adduct and the targeted residue. Moreover, rigorous validation of the targets identified and assessment of the functional consequences of these modifications are essential. Here we present an update on methods to approach the complex field of lipoxidation along with validation strategies and functional assays illustrated with well-studied lipoxidation targets.

Protein disulfide isomerase as a novel target for cyclopentenone prostaglandins: implications for hypoxic ischemic injury

Cyclooxygenase-2 (COX-2) is an important contributor to ischemic brain injury. Identification of the downstream mediators of COX-2 toxicity may allow the development of targeted therapies. Of particular interest is the cyclopentenone family of prostaglandin metabolites. Cyclopentenone prostaglandins (CyPGs) are highly reactive molecules that form covalent bonds with cellular thiols. Protein disulfide isomerase (PDI) is an important molecule for the restoration of denatured proteins following ischemia. Because PDI has several thiols, including thiols within the active thioredoxin-like domain, we hypothesized that PDI is a target of CyPGs and that CyPG binding of PDI is detrimental. CyPG-PDI binding was detected in vitro via immunoprecipitation and MS. CyPG-PDI binding decreased PDI enzymatic activity in recombinant PDI treated with CyPG, and PDI immunoprecipitated from neuronal culture treated with CyPG or anoxia. Toxic effects of binding were demonstrated in experiments showing that: (a) pharmacologic inhibition of PDI increased cell death in anoxic neurons, (b) PDI overexpression protected neurons exposed to anoxia and SH-SY5Y cells exposed to CyPG, and (c) PDI overexpression in SH-SY5Y cells attenuated ubiquitination of proteins and decreased activation of pro-apoptotic caspases. In conclusion, CyPG production and subsequent binding of PDI is a novel and potentially important mechanism of ischemic brain injury. We show that CyPGs bind to PDI, cyclopentenones inhibit PDI activity, and CyPG-PDI binding is associated with increased neuronal susceptibility to anoxia. Additional studies are necessary to determine the relative role of CyPG-dependent inhibition of PDI activity in ischemia and other neurodegenerative disorders.

Neuroinflammation and J2 prostaglandins: linking impairment of the ubiquitin-proteasome pathway and mitochondria to neurodegeneration

The immune response of the CNS is a defense mechanism activated upon injury to initiate repair mechanisms while chronic over-activation of the CNS immune system (termed neuroinflammation) may exacerbate injury. The latter is implicated in a variety of neurological and neurodegenerative disorders such as Alzheimer and Parkinson diseases, amyotrophic lateral sclerosis, multiple sclerosis, traumatic brain injury, HIV dementia, and prion diseases. Cyclooxygenases (COX-1 and COX-2), which are key enzymes in the conversion of arachidonic acid into bioactive prostanoids, play a central role in the inflammatory cascade. J2 prostaglandins are endogenous toxic products of cyclooxygenases, and because their levels are significantly increased upon brain injury, they are actively involved in neuronal dysfunction induced by pro-inflammatory stimuli. In this review, we highlight the mechanisms by which J2 prostaglandins (1) exert their actions, (2) potentially contribute to the transition from acute to chronic inflammation and to the spreading of neuropathology, (3) disturb the ubiquitin-proteasome pathway and mitochondrial function, and (4) contribute to neurodegenerative disorders such as Alzheimer and Parkinson diseases, and amyotrophic lateral sclerosis, as well as stroke, traumatic brain injury (TBI), and demyelination in Krabbe disease. We conclude by discussing the therapeutic potential of targeting the J2 prostaglandin pathway to prevent/delay neurodegeneration associated with neuroinflammation. In this context, we suggest a shift from the traditional view that cyclooxygenases are the most appropriate targets to treat neuroinflammation, to the notion that J2 prostaglandin pathways and other neurotoxic prostaglandins downstream from cyclooxygenases, would offer significant benefits as more effective therapeutic targets to treat chronic neurodegenerative diseases, while minimizing adverse side effects.

Discovery of a highly potent anti-inflammatory epoxyisoprostane-derived lactone

Epoxyisoprostanes EI (1) and EC (2) are effective inhibitors of the secretion of proinflammatory cytokines IL-6 and IL-12. In detailed studies toward the investigation of the molecular mode of action of these structures, a highly potent lactone (3) derived from 1 was identified. The known isoprostanoids 1 and 2 are most likely precursors of 3, the product of facile intramolecular reaction between the epoxide with the carboxylic acid in 2.

Electrophilic lipid mediator 15-deoxy-Δ12,14-prostaglandin j2 modifies glucocorticoid signaling via receptor SUMOylation

Cortisol, the central stress hormone in humans, activates the glucocorticoid receptor (GR). Anti-inflammatory effects are the most important pharmaceutical effects mediated by the GR. Inasmuch as electrophilic cyclopentenone prostaglandin 15-deoxy-Δ(12,14)-prostaglandin J2 (15d-PGJ2) has potent anti-inflammatory properties and activates the SUMOylation pathway, we have investigated the effect of 15d-PGJ2 on glucocorticoid signaling and receptor SUMOylation. To this end, we studied isogenic HEK293 cells expressing either wild-type GR or SUMOylation-defective GR. Interestingly, 15d-PGJ2 triggered SUMO-2 and -3 (SUMO-2/3) modification in the primary SUMOylation sites of the GR. Gene expression profiling and pathway analyses indicate that 15d-PGJ2 inhibits GR signaling in a genome-wide fashion that is significantly dependent on the GR SUMOylation sites. Chromatin immunoprecipitation assays showed that the repressive effect of 15d-PGJ2 on GR target gene expression occurs in parallel with the inhibition of receptor binding to the target gene chromatin. Furthermore, depletion of UBC9, the sole SUMO E2 conjugase, from HEK293 cells confirmed the involvement of active SUMOylation in the regulatory process. Taken together, our data indicate that GR SUMOylation modulates the glucocorticoid signaling during acute cell stress. Our data also suggest that GR SUMOylation modulates cross talk of the glucocorticoid signaling with other transcription factors that are responsive to cell stress.