Ebselen, a selenoorganic compound as glutathione peroxidase mimic

Ebselen and congeners inhibit NADPH oxidase 2-dependent superoxide generation by interrupting the binding of regulatory subunits

NADPH oxidases (Nox) are a primary source of reactive oxygen species (ROS), which function in normal physiology and, when overproduced, in pathophysiology. Recent studies using mice deficient in Nox2 identify this isoform as a novel target against Nox2-implicated inflammatory diseases. Nox2 activation depends on the binding of the proline-rich domain of its heterodimeric partner p22phox to p47phox. A high-throughput screen that monitored this interaction via fluorescence polarization identified ebselen and several of its analogs as inhibitors. Medicinal chemistry was performed to explore structure-activity relationships and to optimize potency. Ebselen and analogs potently inhibited Nox1 and Nox2 activity but were less effective against other isoforms. Ebselen also blocked translocation of p47phox to neutrophil membranes. Thus, ebselen and its analogs represent a class of compounds that inhibit ROS generation by interrupting the assembly of Nox2-activating regulatory subunits.

The acute inhibitory effect of iodide excess on sodium/iodide symporter expression and activity involves the PI3K/Akt signaling pathway

Iodide (I(-)) is an irreplaceable constituent of thyroid hormones and an important regulator of thyroid function, because high concentrations of I(-) down-regulate sodium/iodide symporter (NIS) expression and function. In thyrocytes, activation of phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) cascade also inhibits NIS expression and function. Because I(-) excess and PI3K/Akt signaling pathway induce similar inhibitory effects on NIS expression, we aimed to study whether the PI3K/Akt cascade mediates the acute and rapid inhibitory effect of I(-) excess on NIS expression/activity. Here, we reported that the treatment of PCCl3 cells with I(-) excess increased Akt phosphorylation under normal or TSH/insulin-starving conditions. I(-) stimulated Akt phosphorylation in a PI3K-dependent manner, because the use of PI3K inhibitors (wortmannin or 2-(4-Morpholinyl)-8-phenyl-4H-1-benzopyran-4-one) abrogated the induction of I(-) effect. Moreover, I(-) inhibitory effect on NIS expression and function were abolished when the cells were previously treated with specific inhibitors of PI3K or Akt (Akt1/2 kinase inhibitor). Importantly, we also found that the effect of I(-) on NIS expression involved the generation of reactive oxygen species (ROS). Using the fluorogenic probes dihydroethidium and mitochondrial superoxide indicator (MitoSOX Red), we observed that I(-) excess increased ROS production in thyrocytes and determined that mitochondria were the source of anion superoxide. Furthermore, the ROS scavengers N-acetyl cysteine and 2-phenyl-1,2-benzisoselenazol-3-(2H)-one blocked the effect of I(-) on Akt phosphorylation. Overall, our data demonstrated the involvement of the PI3K/Akt signaling pathway as a novel mediator of the I(-)-induced thyroid autoregulation, linking the role of thyroid oxidative state to the Wolff-Chaikoff effect.

Thiol peroxidases ameliorate LRRK2 mutant-induced mitochondrial and dopaminergic neuronal degeneration in Drosophila

Mutations in leucine-rich repeat kinase 2 (LRRK2) are common causes of familial Parkinson's disease (PD). LRRK2 has been shown to bind peroxiredoxin-3 (PRDX3), the most important scavenger of hydrogen peroxide in the mitochondria, in vitro. Here, we examined the interactions of LRRK2 and PRDX3 in Drosophila models by crossing transgenic LRRK2 and PRDX3 flies. As proof of principle experiments, we subsequently challenged LRRK2 and LRRK2/PRDX3 flies with a peroxidase mimic, Ebselen. We demonstrated that co-expression of PRDX3 with the LRRK2 kinase mutant G2019S in bigenic Drosophila ameliorated the G2019S mutant-induced reduction in peroxidase capacity, loss of dopaminergic neurons, shortened lifespan and mitochondrial defects of flight muscles in monogenic flies expressing the G2019S alone. Challenges with Ebselen recapitulated similar rescue of these phenotypic features in mutant-expressing Drosophila. The peroxidase mimic preserved neuronal and mitochondrial and neuronal integrity and improved mobility and survival in mutant-expressing Drosophila. Taken together, our study provides the first in vivo evidence to suggest that phosphoinhibition of endogenous peroxidases could be a mechanism in LRRK2-induced oxidant-mediated neurotoxicity. Our therapeutic experiments also highlight the potential of thiol peroxidases as neuroprotective agents in PD patients carrying LRRK2 mutations.

Glutathione peroxidase mimic ebselen improves glucose-stimulated insulin secretion in murine islets

AIMS

Glutathione peroxidase (GPX) mimic ebselen and superoxide dismutase (SOD) mimic copper diisopropylsalicylate (CuDIPs) were used to rescue impaired glucose-stimulated insulin secretion (GSIS) in islets of GPX1 and(or) SOD1-knockout mice.

RESULTS

Ebselen improved GSIS in islets of all four tested genotypes. The rescue in the GPX1 knockout resulted from a coordinated transcriptional regulation of four key GSIS regulators and was mediated by the peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α)-mediated signaling pathways. In contrast, CuDIPs improved GSIS only in the SOD1 knockout and suppressed gene expression of the PGC-1α pathway.

INNOVATION

Islets from the GPX1 and(or) SOD1 knockout mice provided metabolically controlled intracellular hydrogen peroxide (H2O2) and superoxide conditions for the present study to avoid confounding effects. Bioinformatics analyses of gene promoters and expression profiles guided the search for upstream signaling pathways to link the ebselen-initiated H2O2 scavenging to downstream key events of GSIS. The RNA interference was applied to prove PGC-1α as the main mediator for that link.

CONCLUSION

Our study revealed a novel metabolic use and clinical potential of ebselen in rescuing GSIS in the GPX1-deficient islets and mice, along with distinct differences between the GPX and SOD mimics in this regard. These findings highlight the necessities and opportunities of discretional applications of various antioxidant enzyme mimics in treating insulin secretion disorders. REBOUND TRACK: This work was rejected during standard peer review and rescued by Rebound Peer Review (Antioxid Redox Signal 16: 293-296, 2012) with the following serving as open reviewers: Regina Brigelius-Flohe, Vadim Gladyshev, Dexing Hou, and Holger Steinbrenner.

An ebselen like catalyst with enhanced GPx activity via a selenol intermediate

Benzamide ring-substituted, quinine-derived ebselen analogue is synthesized which exists in selenol form upon addition of PhSH. It catalyses oxidation of PhSH with H₂O₂ faster (10³-fold) than ebselen.

Cytotoxicity and genotoxicity evaluation of organochalcogens in human leucocytes: a comparative study between ebselen, diphenyl diselenide, and diphenyl ditelluride

Organochalcogens, particularly ebselen, have been used in experimental and clinical trials with borderline efficacy. (PhSe)₂ and (PhTe)₂ are the simplest of the diaryl dichalcogenides and share with ebselen pharmacological properties. In view of the concerns with the use of mammals in studies and the great number of new organochalcogens with potential pharmacological properties that have been synthesized, it becomes important to develop screening protocols to select compounds that are worth to be tested in vivo. This study investigated the possible use of isolated human white cells as a preliminary model to test organochalcogen toxicity. Human leucocytes were exposed to 5–50 μM of ebselen, (PhSe)₂, or (PhTe)₂. All compounds were cytotoxic (Trypan's Blue exclusion) at the highest concentration tested, and Ebselen was the most toxic. Ebselen and (PhSe)₂ were genotoxic (Comet Assay) only at 50 μM, and (PhTe)₂ at 5–50 μM. Here, the acute cytotoxicity did not correspond with in vivo toxicity of the compounds. But the genotoxicity was in the same order of the in vivo toxicity to mice. These results indicate that in vitro genotoxicity in white blood cells should be considered as an early step in the investigation of potential toxicity of organochalcogens.

Neuroprotective effect of ebselen against intracerebroventricular streptozotocin-induced neuronal apoptosis and oxidative stress in rats

The goal of this study was to examine the neuroprotective effect of ebselen against intracerebroventricular streptozotocin (ICV-STZ)-induced oxidative stress and neuronal apoptosis in rat brain. A total of 30 adult male Sprague-Dawley rats were randomly divided into 3 groups of 10 animals each: control, ICV-STZ, and ICV-STZ treated with ebselen. The ICV-STZ group rats were injected bilaterally with ICV-STZ (3 mg/kg) on days 1 and 3, and ebselen (10 mg/kg/day) was administered for 14 days starting from 1st day of ICV-STZ injection to day 14. Rats were killed at the end of the study and brain tissues were removed for biochemical and histopathological investigation. Our results demonstrated, for the first time, the neuroprotective effect of ebselen on Alzheimer's disease (AD) model in rats. Our present study, in ICV-STZ group, showed significant increase in tissue malondialdehyde levels and significant decrease in enzymatic antioxidants superoxide dismutase and glutathione peroxidase in the frontal cortex tissue. The histopathological studies in the brain of rats also supported that ebselen markedly reduced the ICV-STZ-induced histopathological changes and well preserved the normal histological architecture of the frontal cortex tissue. The number of apoptotic neurons was increased in frontal cortex tissue after ICV-STZ administration. Treatment of ebselen markedly reduced the number of degenerating apoptotic neurons. The study demonstrates the effectiveness of ebselen, as a powerful antioxidant, in preventing the oxidative damage and morphological changes caused by ICV-STZ in rats. Thus, ebselen may have a therapeutic value for the treatment of AD.

Glutathione peroxidase activity is neuroprotective in models of Huntington's disease

Huntington's disease is a fatal neurodegenerative disorder caused by a CAG repeat expansion encoding a polyglutamine tract in the huntingtin (Htt) protein. Here we report a genome-wide overexpression suppressor screen in which we identified 317 ORFs that ameliorate the toxicity of a mutant Htt fragment in yeast and that have roles in diverse cellular processes, including mitochondrial import and copper metabolism. Two of these suppressors encode glutathione peroxidases (GPxs), which are conserved antioxidant enzymes that catalyze the reduction of hydrogen peroxide and lipid hydroperoxides. Using genetic and pharmacological approaches in yeast, mammalian cells and Drosophila, we found that GPx activity robustly ameliorates Huntington's disease-relevant metrics and is more protective than other antioxidant approaches tested here. Notably, we found that GPx activity, unlike many antioxidant treatments, does not inhibit autophagy, which is an important mechanism for clearing mutant Htt. Because previous clinical trials have indicated that GPx mimetics are well tolerated in humans, this study may have important implications for treating Huntington's disease.

A dual enzyme microgel with high antioxidant ability based on engineered seleno-ferritin and artificial superoxide dismutase

An antioxidant microgel with both glutathione peroxidase (GPx) and superoxide dismutase (SOD) activities is reported. Using computational design and genetic engineering methods, the main catalytic components of GPx are fabricated onto the surface of ferritin. The resulting seleno-ferritin (Se-Fn) monomers can self-assemble into nanocomposites that exhibit remarkable GPx activity due to the well organized multi-GPx catalytic centers. Subsequently, a porphyrin derivative is synthesized as an SOD mimic, and is employed to construct a synergistic dual enzyme system by crosslinking Se-Fn nanocomposites into a microgel. Significantly, this dual enzyme microgel is demonstrated to display better antioxidant ability than single GPx or SOD mimics in protecting cells from oxidative damage.

Low Po₂ conditions induce reactive oxygen species formation during contractions in single skeletal muscle fibers

Contractions in whole skeletal muscle during hypoxia are known to generate reactive oxygen species (ROS); however, identification of real-time ROS formation within isolated single skeletal muscle fibers has been challenging. Consequently, there is no convincing evidence showing increased ROS production in intact contracting fibers under low Po₂ conditions. Therefore, we hypothesized that intracellular ROS generation in single contracting skeletal myofibers increases during low Po₂ compared with a value approximating normal resting Po₂. Dihydrofluorescein was loaded into single frog (Xenopus) fibers, and fluorescence was used to monitor ROS using confocal microscopy. Myofibers were exposed to two maximal tetanic contractile periods (1 contraction/3 s for 2 min, separated by a 60-min rest period), each consisting of one of the following treatments: high Po₂ (30 Torr), low Po₂ (3-5 Torr), high Po₂ with ebselen (antioxidant), or low Po₂ with ebselen. Ebselen (10 μM) was administered before the designated contractile period. ROS formation during low Po₂ treatment was greater than during high Po₂ treatment, and ebselen decreased ROS generation in both low- and high-Po₂ conditions (P < 0.05). ROS accumulated at a faster rate in low vs. high Po₂. Force was reduced >30% for each condition except low Po₂ with ebselen, which only decreased ~15%. We concluded that single myofibers under low Po₂ conditions develop accelerated and more oxidative stress than at Po₂ = 30 Torr (normal human resting Po₂). Ebselen decreases ROS formation in both low and high Po₂, but only mitigates skeletal muscle fatigue during reduced Po₂ conditions.

Ebselen alters mitochondrial physiology and reduces viability of rat hippocampal astrocytes

The seleno-organic compound and radical scavenger ebselen (2-phenyl-1,2-benzisoselenazol-3(2H)-one) have been extensively employed as an anti-inflammatory and neuroprotective compound. However, its glutathione peroxidase activity at the expense of cellular thiols groups could underlie certain deleterious actions of the compound on cell physiology. In this study, we have analyzed the effect of ebselen on rat hippocampal astrocytes in culture. Cellular viability, the intracellular free-Ca(2+) concentration ([Ca(2+)]c), the mitochondrial free-Ca(2+) concentration ([Ca(2+)]m), and mitochondrial membrane potential (ψm) were analyzed. The caspase-3 activity was also assayed. Our results show that cell viability was reduced by treatment of cells with ebselen, depending on the concentration employed. In the presence of ebselen, we observed an initial transient increase in [Ca(2+)]c that was then followed by a progressive increase to an elevated plateau. We also observed a transient increase in [Ca(2+)]m in the presence of ebselen that returned toward a value over the prestimulation level. The compound induced depolarization of ψm and altered the permeability of the mitochondrial membrane. Additionally, a disruption of the mitochondrial network was observed. Finally, we did not detect changes in caspase-3 activation in response to ebselen treatment. Collectively, these data support the likelihood of ebselen, depending on the concentration employed, reduces viability of rat hippocampal astrocytes via its action on the mitochondrial activity. These may be early effects that do not involve caspase-3 activation. We conclude that, depending on the concentration used, ebselen might exert deleterious actions on astrocyte physiology that could compromise cell function.

Organochalcogens inhibit mitochondrial complexes I and II in rat brain: possible implications for neurotoxicity

Organochalcogens, such as organoselenium and organotellurium compounds, can be neurotoxic to rodents. Since mitochondrial dysfunction plays a pivotal role in neurological disorders, the present study was designed to test the hypothesis that rat brain mitochondrial complexes (I, II, I-III, II-III and IV) could be molecular targets of organochalcogens. The results show that organochalcogens caused statistically significant inhibition of mitochondrial complex I activity, which was prevented by preincubation with NADH and fully blunted by reduced glutathione (GSH). Mitochondrial complex II activity remained unchanged in response to (PhSe)₂ treatment. Ebs and (PhTe)₂ caused a significant concentration-dependent inhibition of complex II that was also blunted by GSH. Mitochondrial complex IV activity was not modified by organochalcogens. Collectively, Ebs, (PhSe)₂ and (PhTe)₂ were more effective inhibitors of brain mitochondrial complex I than of complex II, whereas they did not affect complex IV. These observations are consistent with organochalcogens inducing mitochondrial complex I and II inhibition via their thiol-oxidase-like activity, with Ebs, (PhSe)₂ and (PhTe)₂ effectively oxidising critical thiol groups of these complexes.

The protective effects of 6-CySeCD with GPx activity against UVB-induced injury in HaCaT cells

BACKGROUND

The generation of harmful reactive oxygen species (ROS) induced by UVB irradiation could induce cell apoptosis and change the cell cycle. 6A,6A'-dicyclohexylamine-6B,6B'-diselenide-bis-β-cyclodextrin (6-CySeCD) is a novel glutathione peroxidase (GPx; EC 1.11.1.9) mimic. The aim of this study was to investigate the anti-oxidative effects of 6-CySeCD in cultured immortalised human keratinocyte cells (HaCaT).

METHODS

HaCaT cells were treated with 30 mJ/cm(2) UVB to establish a damage model. The cultured HaCaT cells were randomly assigned to the control, UVB and treatment groups. The treatment group was incubated with 20 μmol/L of GPx mimics before UVB irradiation. Cell viability was detected by (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, the level of lipid peroxidation was determined by the formation of malondialdehyde (MDA), DNA fragmentation was observed using agarose gel electrophoresis and the levels of intracellular ROS and cell cycle progression were measured by flow cytometry.

RESULTS

The levels of cytotoxicity, intracellular ROS, lipid peroxidation and oxidative DNA damage significantly increased after UVB irradiation in the HaCaT cells. UVB irradiation caused pre-G1 -phase arrest in HaCaT cells and significantly reduced the number of HaCaT cells in the S phase. The GPx mimics 6-CySeCD and 2-phenyl-l,2-benzisoselenazol-3(2H)-one (ebselen) significantly blocked UVB-induced apoptosis and changed the cell cycle of the HaCaT cells. The blocked effect of pretreatment 6-CySeCD in UVB-irradiated HaCaT cells was better than that of pretreatment with ebselen.

CONCLUSION

6-CySeCD can relieve the damage induced by UVB irradiation in HaCaT cells.

Photo-oxidation-induced inactivation of the selenium-containing protective enzymes thioredoxin reductase and glutathione peroxidase

Singlet oxygen ((1)O(2)) is a reactive oxygen species generated during photo-oxidation, inflammation, and via peroxidase-catalyzed reactions (e.g., myeloperoxidase and eosinophil peroxidase). (1)O(2) oxidizes the free amino acids Trp, Tyr, His, Cys, and Met, and those species present on peptides/proteins, with this resulting in modulation of protein structure and function. Impairment of the activity of antioxidant enzymes may be of relevance to the oxidative stress observed in a number of pathologies involving either light exposure or inflammation. In this study, the effects of (1)O(2) on glutathione peroxidase (GPx) and thioredoxin reductase (TrxR) activity, including the mechanisms of their inactivation, were investigated. Exposure of GPx or TrxR, either as purified proteins or in cell lysates, to Rose Bengal and visible light (an established source of (1)O(2)) resulted in significant, photolysis time-dependent reductions in enzyme activity (10-40%, P<0.05). More extensive inhibition (ca. 2-fold) was detected when the reactions were carried out in D(2)O, consistent with the intermediacy of (1)O(2). No additional inhibition was detected after the cessation of photolysis, eliminating a role for photo-products. Methionine, which reacts rapidly with (1)O(2) (k~10(7)M(-1) s(-1))(,) significantly reduced photo-inactivation at large molar excesses, presumably by acting as an alternative target. Reductants (NaBH(4), DTT, GSH, or NADPH) added after the cessation of (1)O(2) formation were unable to reverse enzyme inactivation, consistent with irreversible enzyme oxidation. Formation of nonreducible protein aggregates and/or fragments was detected for both photo-oxidized GPx and TrxR by SDS-PAGE. An oxidant concentration-dependent increase in protein carbonyls was detected with TrxR but not GPx. These studies thus demonstrate that the antioxidant enzymes GPx and TrxR can be irreversibly inactivated by (1)O(2).

Glutathione and glutaredoxin act as a backup of human thioredoxin reductase 1 to reduce thioredoxin 1 preventing cell death by aurothioglucose

Thioredoxin reductase 1 (TrxR1) in cytosol is the only known reductant of oxidized thioredoxin 1 (Trx1) in vivo so far. We and others found that aurothioglucose (ATG), a well known active-site inhibitor of TrxR1, inhibited TrxR1 activity in HeLa cell cytosol but had no effect on the viability of the cells. Using a redox Western blot analysis, no change was observed in redox state of Trx1, which was mainly fully reduced with five sulfhydryl groups. In contrast, auranofin killed cells and oxidized Trx1, also targeting mitochondrial TrxR2 and Trx2. Combining ATG with ebselen gave a strong synergistic effect, leading to Trx1 oxidation, reactive oxygen species accumulation, and cell death. We hypothesized that there should exist a backup system to reduce Trx1 when only TrxR1 activity was lost. Our results showed that physiological concentrations of glutathione, NADPH, and glutathione reductase reduced Trx1 in vitro and that the reaction was strongly stimulated by glutaredoxin1. Simultaneous depletion of TrxR activity by ATG and glutathione by buthionine sulfoximine led to overoxidation of Trx1 and loss of HeLa cell viability. In conclusion, the glutaredoxin system and glutathione have a backup role to keep Trx1 reduced in cells with loss of TrxR1 activity. Monitoring the redox state of Trx1 shows that cell death occurs when Trx1 is oxidized, followed by general protein oxidation catalyzed by the disulfide form of thioredoxin.

Novel pathways and therapies in experimental diabetic atherosclerosis

Diabetic subjects are at a greater risk of developing major vascular complications due to abnormalities pertinent to the diabetic milieu. Current treatment options achieve significant improvements in glucose levels and blood pressure control, but do not necessarily prevent or retard diabetes-mediated macrovascular disease. In this review, we highlight several pathways that are increasingly being appreciated as playing a significant role in diabetic vascular injury. We focus particularly on the advanced glycation end product/receptor for advanced glycation end product (AGE/RAGE) axis and its interplay with the nuclear protein HMGB1. We discuss evidence implicating a significant role for the renin-angiotensin system, urotensin II and PPAR, as well as the importance of proinflammatory mediators and oxidative stress in cardiovascular complications. The specific targeting of these pathways may lead to novel therapies to reduce the burden of diabetic vascular complications.

Determination of ebselen-sensitive reactive oxygen metabolites (ebROM) in human serum based upon N,N'-diethyl-1,4-phenylenediamine oxidation

BACKGROUND

Oxidative stress occurs through free radical- and non-radical-mediated oxidative mechanisms, but these are poorly discriminated by most assays. A convenient assay for oxidants in human serum is based upon the Fe(2+)-dependent decomposition of peroxides to oxidize N,N'-diethyl-1,4-phenylenediamine (DEPPD) to a stable radical cation which can be measured spectrophotometrically.

METHODS

We investigated modification of the DEPPD oxidation assay to discriminate color formation due to non-radical oxidants, including hydroperoxides and endoperoxides, which are sensitive to ebselen.

RESULTS

Use of serum, which has been pretreated with ebselen as a reference, provides a quantitative assay for non-radical, reactive oxidant species in serum, including hydroperoxides, endoperoxides and epoxides. In a set of 35 human serum samples, non-radical oxidants largely accounted for DEPPD oxidation in 86% of the samples while the remaining 14% had considerable contribution from other redox-active chemicals.

CONCLUSIONS

The simple modification in which ebselen-pretreated sample is used as a reference provides means to quantify non-radical oxidants in human serum. Application of this approach could enhance understanding of the contribution of different types of oxidative stress to disease.

Effects of ebselen on radiocontrast media–induced hepatotoxicity in rats

Oxidative stress is accepted as a potential responsible mechanism in the pathogenesis of radiocontrast media (RCM)-induced hepatotoxicity. Therefore, we aimed to investigate the protective effects of ebselen against RCM-induced hepatotoxicity by measuring tissue oxidant/antioxidant parameters and histological changes in rats. Wistar albino rats were randomly separated into four groups consisting of eight rats per group. Normal saline was given to the rats in control group (group 1). RCM was given to the rats in group 2, and both RCM and ebselen were given to the rats in group 3. Only ebselen was given to the rats in group 4. Liver sections of the killed animals were analyzed to measure the levels of malondialdehyde (MDA) and activities of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px), as well as histopathological changes. In RCM group, SOD and CAT levels were found increased. In RCM-ebselen group, MDA, SOD and CAT levels were found decreased. In RCM-ebselen group, however, GSH-Px activities of liver tissue increased. All these results indicated that ebselen produced a protective mechanism against RCM-induced hepatotoxicity and took part in oxidative stress.

Selenium-containing amino acids are targets for myeloperoxidase-derived hypothiocyanous acid: determination of absolute rate constants and implications for biological damage

Elevated MPO (myeloperoxidase) levels are associated with multiple human inflammatory pathologies. MPO catalyses the oxidation of Cl⁻, Br⁻ and SCN⁻ by H₂O₂ to generate the powerful oxidants hypochlorous acid (HOCl), hypobromous acid (HOBr) and hypothiocyanous acid (HOSCN) respectively. These species are antibacterial agents, but misplaced or excessive production is implicated in tissue damage at sites of inflammation. Unlike HOCl and HOBr, which react with multiple targets, HOSCN targets cysteine residues with considerable selectivity. In the light of this reactivity, we hypothesized that Sec (selenocysteine) residues should also be rapidly oxidized by HOSCN, as selenium atoms are better nucleophiles than sulfur. Such oxidation might inactivate critical Sec-containing cellular protective enzymes such as GPx (glutathione peroxidase) and TrxR (thioredoxin reductase). Stopped-flow kinetic studies indicate that seleno-compounds react rapidly with HOSCN with rate constants, k, in the range 2.8×10³–5.8×10⁶ M⁻¹·s⁻¹ (for selenomethionine and selenocystamine respectively). These values are ~6000-fold higher than the corresponding values for H₂O₂, and are also considerably larger than for the reaction of HOSCN with thiols (16-fold for cysteine and 80-fold for selenocystamine). Enzyme studies indicate that GPx and TrxR, but not glutathione reductase, are inactivated by HOSCN in a concentration-dependent manner; k for GPx has been determined as ~5×10⁵ M⁻¹·s⁻¹. Decomposed HOSCN did not induce inactivation. These data indicate that selenocysteine residues are oxidized rapidly by HOSCN, with this resulting in the inhibition of the critical intracellular Sec-dependent protective enzymes GPx and TrxR.

Evaluation of the neurotoxic/neuroprotective role of organoselenides using differentiated human neuroblastoma SH-SY5Y cell line challenged with 6-hydroxydopamine

It is well established that oxidative stress plays a major role in several neurodegenerative conditions, like Parkinson disease (PD). Hence, there is an enormous effort for the development of new antioxidants compounds with therapeutic potential for the management of PD, such as synthetic organoselenides molecules. In this study, we selected between nine different synthetic organoselenides the most eligible ones for further neuroprotection assays, using the differentiated human neuroblastoma SH-SY5Y cell line as in vitro model. Neuronal differentiation of exponentially growing human neuroblastoma SH-SY5Y cells was triggered by cultivating cells with DMEM/F12 medium with 1% of fetal bovine serum (FBS) with the combination of 10 μM retinoic acid for 7 days. Differentiated cells were further incubated with different concentrations of nine organoselenides (0.1, 0.3, 3, 10, and 30 μM) for 24 h and cell viability, neurites densities and the immunocontent of neuronal markers were evaluated. Peroxyl radical scavenging potential of each compound was determined with TRAP assay. Three organoselenides tested presented low cytotoxicity and high antioxidant properties. Pre-treatment of cells with those compounds for 24 h lead to a significantly neuroprotection against 6-hydroxydopamine (6-OHDA) toxicity, which were directly related to their antioxidant properties. Neuroprotective activity of all three organoselenides was compared to diphenyl diselenide (PhSe)₂, the simplest of the diaryl diselenides tested. Our results demonstrate that differentiated human SH-SY5Y cells are suitable cellular model to evaluate neuroprotective/neurotoxic role of compounds, and support further evaluation of selected organoselenium molecules as potential pharmacological and therapeutic drugs in the treatment of PD.

Strategies for reducing or preventing the generation of oxidative stress

The reduction of oxidative stress could be achieved in three levels: by lowering exposure to environmental pollutants with oxidizing properties, by increasing levels of endogenous and exogenous antioxidants, or by lowering the generation of oxidative stress by stabilizing mitochondrial energy production and efficiency. Endogenous oxidative stress could be influenced in two ways: by prevention of ROS formation or by quenching of ROS with antioxidants. However, the results of epidemiological studies where people were treated with synthetic antioxidants are inconclusive and contradictory. Recent evidence suggests that antioxidant supplements (although highly recommended by the pharmaceutical industry and taken by many individuals) do not offer sufficient protection against oxidative stress, oxidative damage or increase the lifespan. The key to the future success of decreasing oxidative-stress-induced damage should thus be the suppression of oxidative damage without disrupting the wellintegrated antioxidant defense network. Approach to neutralize free radicals with antioxidants should be changed into prevention of free radical formation. Thus, this paper addresses oxidative stress and strategies to reduce it with the focus on nutritional and psychosocial interventions of oxidative stress prevention, that is, methods to stabilize mitochondria structure and energy efficiency, or approaches which would increase endogenous antioxidative protection and repair systems.

RETRACTED: Seleno-cyclodextrin sensitises human breast cancer cells to TRAIL-induced apoptosis through DR5 induction and NF-κB suppression

This article has been retracted: please see Elsevier Policy on Article Withdrawal (https://www.elsevier.com/about/policies/article-withdrawal). This article has been retracted at the request of This article has been retracted at the request of the authors and the Editor-in-Chief. The authors contacted the Editors of the European Journal of Cancer regarding oversights or labelling errors introduced in Fig. 2 and Fig.7 during data processing. The authors provided the raw data, however the Editors conclude that the raw data management cannot be relied upon and the article needs to be retracted.

Antiatherosclerotic and renoprotective effects of ebselen in the diabetic apolipoprotein E/GPx1-double knockout mouse

OBJECTIVE

To investigate the effect of the GPx1-mimetic ebselen on diabetes-associated atherosclerosis and renal injury in a model of increased oxidative stress.

RESEARCH DESIGN AND METHODS

The study was performed using diabetic apolipoprotein E/GPx1 (ApoE(-/-)GPx1(-/-))-double knockout (dKO) mice, a model combining hyperlipidemia and hyperglycemia with increased oxidative stress. Mice were randomized into two groups, one injected with streptozotocin, the other with vehicle, at 8 weeks of age. Groups were further randomized to receive either ebselen or no treatment for 20 weeks.

RESULTS

Ebselen reduced diabetes-associated atherosclerosis in most aortic regions, with the exception of the aortic sinus, and protected dKO mice from renal structural and functional injury. The protective effects of ebselen were associated with a reduction in oxidative stress (hydroperoxides in plasma, 8-isoprostane in urine, nitrotyrosine in the kidney, and 4-hydroxynonenal in the aorta) as well as a reduction in VEGF, CTGF, VCAM-1, MCP-1, and Nox2 after 10 weeks of diabetes in the dKO aorta. Ebselen also significantly reduced the expression of proteins implicated in fibrosis and inflammation in the kidney as well as reducing related key intracellular signaling pathways.

CONCLUSIONS

Ebselen has an antiatherosclerotic and renoprotective effect in a model of accelerated diabetic complications in the setting of enhanced oxidative stress. Our data suggest that ebselen effectively repletes the lack of GPx1, and indicate that ebselen may be an effective therapeutic for the treatment of diabetes-related atherosclerosis and nephropathy. Furthermore, this study highlights the feasibility of addressing two diabetic complications with one treatment regimen through the unifying approach of targeted antioxidant therapy.

Post-transcriptional defects of antioxidant selenoenzymes cause oxidative stress under methylmercury exposure

Methylmercury (MeHg) toxicity is a continuous environmental problem to human health. The critical role of oxidative stress in the pathogenesis of MeHg cytotoxicity has been clarified, but the molecular mechanisms underlying MeHg-mediated oxidative stress remain to be elucidated. Here we demonstrate a post-transcriptional effect of MeHg on antioxidant selenoenzymes by using a MeHg-susceptible cell line. MeHg-induced selenium deficiency leads to failure of the recoding of a UGA codon for selenocysteine and results in degradation of the major antioxidant selenoenzyme glutathione peroxidase 1 (GPx1) mRNA by nonsense-mediated mRNA decay (NMD), a cellular mechanism that detects the premature termination codon (PTC) located 5'-upstream of the last exon-exon junction and degrades PTC-containing mRNAs. In contrast, thioredoxin reductase 1 (TrxR1), another antioxidant selenoenzyme of the thioredoxin system, was likely skipped by NMD because of a UGA codon in the last exon. However, TrxR1 activity was decreased despite mRNA up-regulation, which was probably due to the synthesis of aberrant TrxR1 protein without selenocysteine. Changes in selenoenzyme GPx1 and TrxR1 mRNAs were observed earlier than was the incidence of oxidative stress and up-regulation of other antioxidant enzyme mRNAs. Results indicated that the MeHg-induced relative selenium-deficient condition affects the major antioxidant selenoenzymes GPx1 and TrxR1 through a post-transcriptional effect, resulting in the disturbance of cellular redox systems and the incidence of oxidative stress. Treatment with ebselen, a seleno-organic compound, effectively suppressed oxidative stress and protected cells against MeHg-induced relative selenium deficiency and cytotoxicity.

2-Tellurium-bridged β-cyclodextrin, a thioredoxin reductase inhibitor, sensitizes human breast cancer cells to TRAIL-induced apoptosis through DR5 induction and NF-κB suppression

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) exhibits potent antitumor activity via membrane receptors on cancer cells without deleterious side effects for normal tissue. Unfortunately, breast cancer cells, as many other cancer types, develop resistance to TRAIL; therefore, TRAIL sensitizing agents are currently being explored. 2-Tellurium-bridged β-cyclodextrin (2-TeCD) is a synthetic organotellurium compound, with both glutathione peroxidase-like catalytic ability and thioredoxin reductase inhibitor activity. In the present study, we reported that 2-TeCD sensitized TRAIL-resistant human breast cancer cells and xenograft tumors to undergo apoptosis. In vitro, 2-TeCD efficiently sensitized MDA-MB-468 and T47D cells, but not untransformed human mammary epithelial cells, to TRAIL-mediated apoptosis, as evidenced by enhanced caspase activity and poly (adenosine diphosphate-ribose) polymerase cleavage. From a mechanistic standpoint, we showed that 2-TeCD treatment of breast cancer cells significantly upregulated the messenger RNA and protein levels of TRAIL receptor, death receptor (DR) 5, in a transcription factor Sp1-dependent manner. 2-TeCD treatment also suppressed TRAIL-induced nuclear factor-κB (NF-κB) prosurvival pathways by preventing cytosolic IκBα degradation, as well as p65 nuclear translocation. Consequently, the combined administration suppressed anti-apoptotic molecules that are transcriptionally regulated by NF-κB. In vivo, 2-TeCD and TRAIL were well tolerated in mice and their combination significantly inhibited growth of MDA-MB-468 xenografts and promoted apoptosis. Upregulation of DR5 and downregulation of NF-κB by the dual treatment were also observed in tumor tissues. Overall, 2-TeCD sensitizes resistant breast cancer cells to TRAIL-based apoptosis in vitro and in vivo. These findings provide strong evidence for the therapeutic potential of this combination against breast cancers.

Functional mimics of glutathione peroxidase: bioinspired synthetic antioxidants

Oxidative stress is caused by an imbalance between the production of reactive oxygen species (ROS) and the biological system's ability to detoxify these reactive intermediates. Mammalian cells have elaborate antioxidant defense mechanisms to control the damaging effects of ROS. Glutathione peroxidase (GPx), a selenoenzyme, plays a key role in protecting the organism from oxidative damage by catalyzing the reduction of harmful hydroperoxides with thiol cofactors. The selenocysteine residue at the active site forms a "catalytic triad" with tryptophan and glutamine, which activates the selenium moiety for an efficient reduction of peroxides. After the discovery that ebselen, a synthetic organoselenium compound, mimics the catalytic activity of GPx both in vitro and in vivo, several research groups developed a number of small-molecule selenium compounds as functional mimics of GPx, either by modifying the basic structure of ebselen or by incorporating some structural features of the native enzyme. The synthetic mimics reported in the literature can be classified in three major categories: (i) cyclic selenenyl amides having a Se-N bond, (ii) diaryl diselenides, and (iii) aromatic or aliphatic monoselenides. Recent studies show that ebselen exhibits very poor GPx activity when aryl or benzylic thiols such as PhSH or BnSH are used as cosubstrates. Because the catalytic activity of each GPx mimic largely depends on the thiol cosubstrates used, the difference in the thiols causes the discrepancies observed in different studies. In this Account, we demonstrate the effect of amide and amine substituents on the GPx activity of various organoselenium compounds. The existence of strong Se···O/N interactions in the selenenyl sulfide intermediates significantly reduces the GPx activity. These interactions facilitate an attack of thiol at selenium rather than at sulfur, leading to thiol exchange reactions that hamper the formation of catalytically active selenol. Therefore, any substituent capable of enhancing the nucleophilic attack of thiol at sulfur in the selenenyl sulfide state would enhance the antioxidant potency of organoselenium compounds. Interestingly, replacement of the sec-amide substituent by a tert-amide group leads to a weakening of Se···O interactions in the selenenyl sulfide intermediates. This modification results in 10- to 20-fold enhancements in the catalytic activities. Another strategy involving the replacement of tert-amide moieties by tert-amino substituents further increases the activity by 3- to 4-fold. The most effective modification so far in benzylamine-based GPx mimics appears to be either the replacement of a tert-amino substituent by a sec-amino group or the introduction of an additional 6-methoxy group in the phenyl ring. These strategies can contribute to a remarkable enhancement in the GPx activity. In addition to enhancing catalytic activity, a change in the substituents near the selenium moiety alters the catalytic mechanisms. The mechanistic investigations of functional mimics are useful not only for understanding the complex chemistry at the active site of GPx but also for designing and synthesizing novel antioxidants and anti-inflammatory agents.

Identification and characterization of novel classes of macrophage migration inhibitory factor (MIF) inhibitors with distinct mechanisms of action

Macrophage migration inhibitory factor (MIF), a proinflammatory cytokine, is considered an attractive therapeutic target in multiple inflammatory and autoimmune disorders. In addition to its known biologic activities, MIF can also function as a tautomerase. Several small molecules have been reported to be effective inhibitors of MIF tautomerase activity in vitro. Herein we employed a robust activity-based assay to identify different classes of novel inhibitors of the catalytic and biological activities of MIF. Several novel chemical classes of inhibitors of the catalytic activity of MIF with IC(50) values in the range of 0.2-15.5 microm were identified and validated. The interaction site and mechanism of action of these inhibitors were defined using structure-activity studies and a battery of biochemical and biophysical methods. MIF inhibitors emerging from these studies could be divided into three categories based on their mechanism of action: 1) molecules that covalently modify the catalytic site at the N-terminal proline residue, Pro(1); 2) a novel class of catalytic site inhibitors; and finally 3) molecules that disrupt the trimeric structure of MIF. Importantly, all inhibitors demonstrated total inhibition of MIF-mediated glucocorticoid overriding and AKT phosphorylation, whereas ebselen, a trimer-disrupting inhibitor, additionally acted as a potent hyperagonist in MIF-mediated chemotactic migration. The identification of biologically active compounds with known toxicity, pharmacokinetic properties, and biological activities in vivo should accelerate the development of clinically relevant MIF inhibitors. Furthermore, the diversity of chemical structures and mechanisms of action of our inhibitors makes them ideal mechanistic probes for elucidating the structure-function relationships of MIF and to further determine the role of the oligomerization state and catalytic activity of MIF in regulating the function(s) of MIF in health and disease.

Curcumin-induced apoptosis in PC3 prostate carcinoma cells is caspase-independent and involves cellular ceramide accumulation and damage to mitochondria

Curcumin, the principal curcuminoid of tumeric, has potent anticancer activity. To determine the mechanism of curcumin-induced cytotoxicity in prostate cancer cells, we exposed PC3 prostate carcinoma cells to 25 to 100 microM curcumin for 24 to 72 h. Curcumin treatment of PC3 cells caused time- and dose-dependent induction of apoptosis and depletion of cellular reduced glutathione (GSH). Exogenous GSH and its precursor N-acetyl-cysteine, but not ascorbic acid (AA) or ebselen, decreased curcumin accumulation in PC3 cells and also prevented curcumin-induced DNA fragmentation. The failure of AA and ebselen to protect PC3 cells from curcumin-induced apoptosis argued against the involvement of reactive oxygen species; rather, GSH-mediated inhibition of curcumin-induced cytotoxicity was due to reduced curcumin accumulation in PC3 cells. Curcumin-treated PC3 cells showed apoptosis-inducing cellular ceramide accumulation and activation of p38 mitogen-activated protein kinase (MAPK) and c-jun N-terminal kinase (JNK). Caspase-3, caspase-8, and caspase-9 were activated, and cytochrome c and apoptosis-inducing factor (AIF) were released from mitochondria following curcumin treatment. Interestingly, curcumin-induced apoptosis was not prevented by p38 MAPK, JNK, or caspase inhibition. We conclude that curcumin-induced cytotoxicity was due to cellular ceramide accumulation and damage to mitochondria that resulted in apoptosis mediated by AIF and other caspase-independent processes.