Ebselen: a substrate for human thioredoxin reductase strongly stimulating its hydroperoxide reductase activity and a superfast thioredoxin oxidant

Synthesis and evaluation of multi-target-directed ligands against Alzheimer's disease based on the fusion of donepezil and ebselen

A novel series of compounds obtained by fusing the cholinesterase inhibitor donepezil and the antioxidant ebselen were designed as multi-target-directed ligands against Alzheimer's disease. An in vitro assay showed that some of these molecules did not exhibit highly potent cholinesterase inhibitory activity but did have various other ebselen-related pharmacological effects. Among the molecules, compound 7d, one of the most potent acetylcholinesterase inhibitors (IC50 values of 0.042 μM for Electrophorus electricus acetylcholinesterase and 0.097 μM for human acetylcholinesterase), was found to be a strong butyrylcholinesterase inhibitor (IC50 = 1.586 μM), to possess rapid H2O2 and peroxynitrite scavenging activity and glutathione peroxidase-like activity (ν0 = 123.5 μM min(-1)), and to be a substrate of mammalian TrxR. A toxicity test in mice showed no acute toxicity at doses of up to 2000 mg/kg. According to an in vitro blood-brain barrier model, 7d is able to penetrate the central nervous system.

Thioredoxin 1 is inactivated due to oxidation induced by peroxiredoxin under oxidative stress and reactivated by the glutaredoxin system

The mammalian cytosolic thioredoxin system, comprising thioredoxin (Trx), Trx reductase, and NADPH, is the major protein-disulfide reductase of the cell and has numerous functions. Besides the active site thiols, human Trx1 contains three non-active site cysteine residues at positions 62, 69, and 73. A two-disulfide form of Trx1, containing an active site disulfide between Cys-32 and Cys-35 and a non-active site disulfide between Cys-62 and Cys-69, is inactive either as a disulfide reductase or as a substrate for Trx reductase. This could possibly provide a structural switch affecting Trx1 function during oxidative stress and redox signaling. We found that two-disulfide Trx1 was generated in A549 cells under oxidative stress. In vitro data showed that two-disulfide Trx1 was generated from oxidation of Trx1 catalyzed by peroxiredoxin 1 in the presence of H2O2. The redox Western blot data indicated that the glutaredoxin system protected Trx1 in HeLa cells from oxidation caused by ebselen, a superfast oxidant for Trx1. Our results also showed that physiological concentrations of glutathione, NADPH, and glutathione reductase reduced the non-active site disulfide in vitro. This reaction was stimulated by glutaredoxin 1 via the so-called monothiol mechanism. In conclusion, reversible oxidation of the non-active site disulfide of Trx1 is suggested to play an important role in redox regulation and cell signaling via temporal inhibition of its protein-disulfide reductase activity for the transmission of oxidative signals under oxidative stress.

Ebsulfur is a benzisothiazolone cytocidal inhibitor targeting the trypanothione reductase of Trypanosoma brucei

Trypanosoma brucei is the causing agent of African trypanosomiasis. These parasites possess a unique thiol redox system required for DNA synthesis and defense against oxidative stress. It includes trypanothione and trypanothione reductase (TryR) instead of the thioredoxin and glutaredoxin systems of mammalian hosts. Here, we show that the benzisothiazolone compound ebsulfur (EbS), a sulfur analogue of ebselen, is a potent inhibitor of T. brucei growth with a favorable selectivity index over mammalian cells. EbS inhibited the TryR activity and decreased non-protein thiol levels in cultured parasites. The inhibition of TryR by EbS was irreversible and NADPH-dependent. EbS formed a complex with TryR and caused oxidation and inactivation of the enzyme. EbS was more toxic for T. brucei than for Trypanosoma cruzi, probably due to lower levels of TryR and trypanothione in T. brucei. Furthermore, inhibition of TryR produced high intracellular reactive oxygen species. Hydrogen peroxide, known to be constitutively high in T. brucei, enhanced the EbS inhibition of TryR. The elevation of reactive oxygen species production in parasites caused by EbS induced a programmed cell death. Soluble EbS analogues were synthesized and cured T. brucei brucei infection in mice when used together with nifurtimox. Altogether, EbS and EbS analogues disrupt the trypanothione system, hampering the defense against oxidative stress. Thus, EbS is a promising lead for development of drugs against African trypanosomiasis.

Selenium supplementation for sepsis: a meta-analysis of randomized controlled trials

BACKGROUND

Recently, several studies were conducted to investigate the effect of selenium supplementation in septic patients. However, no consistent conclusion was made. Thus, we aimed to systematically summarize the available randomized controlled trials (RCTs) to evaluate the effect of selenium supplementation on important clinical outcomes in septic patients.

METHODS

A systematic literature search of Pubmed, Embase, and the Cochrane Central Register of Controlled Trials was conducted (up to August 25, 2012). RCTs were included if they reported the effect of selenium supplementation on the treatment of septic patients. A fixed-effect model was used, and in the case of significant heterogeneity, a random-effects model was employed.

RESULTS

Five studies with a total of 530 patients were included. Pooled analysis showed that selenium supplementation did not reduce all-cause mortality (relative risk [RR] = 0.89, 95% confidence interval [CI]: 0.73-1.07, P = .21), hospital-acquired pneumonia (RR = 1.15, 95% CI: 0.73-1.82, P = .55), or length of intensive care unit stay (weighted mean differences = 2.32 days, 95% CI: -0.05 to 4.69; P = .05). In addition, no significant difference was observed regarding adverse events between groups (RR = 0.97, 95% CI: 0.72-1.33, P = .87).

CONCLUSIONS

The present meta-analysis showed no benefit of selenium supplementation in patients with sepsis. Due to the limited number of RCTs included, more prospective multicenter clinical trials on selenium therapy in septic patients are warranted in the future.

Organoselenium compounds modulate extracellular redox by induction of extracellular cysteine and cell surface thioredoxin reductase

The effect of selenium compounds on extracellular redox modulating capacity was studied in murine macrophage RAW 264.7 cells and differentiated human THP-1 monocytes. The arylselenium compounds benzeneselenol (PhSeH), dibenzyl diselenide (DBDSe), diphenyl diselenide (DPDSe), and ebselen were capable of inducing extracellular cysteine accumulation via a cystine- and glucose-dependent process. Extracellular cysteine production was dose-dependently inhibited by glutamate, an inhibitor of cystine/glutamate antiporter (Xc(-) transporter), supporting the involvement of Xc(-) transporter for cystine uptake in the above process. These arylselenium compounds also induced cellular thioredoxin reductase (TrxR) expression, particularly at the exofacial surface of cells. TrxR1 knockdown using small interfering RNA attenuated TrxR increases and cysteine efflux induced in cells by DPDSe. Sodium selenite (Na2SeO3), selenomethionine (SeMet), seleno-l-cystine (SeCySS), and Se-methylselenocysteine (MeSeCys) did not have these effects on macrophages under the same treatment conditions. The effects of organoselenium compounds on extracellular redox may contribute to the known, but inadequately understood, biological effects of selenium compounds.

Enhancement of auranofin-induced apoptosis in MCF-7 human breast cells by selenocystine, a synergistic inhibitor of thioredoxin reductase

Thioredoxin system plays an important role in regulation of intracellular redox balance and various signaling pathways. Thioredoxin reductase (TrxR) is overexpressed in many cancer cells and has been identified as a potential target of anticancer drugs. Auranofin (AF) is potent TrxR inhibitor with novel in vitro and in vivo anticancer activities. Selenocystine (SeC) is a nutritionally available selenoamino acid with selective anticancer effects through induction of apoptosis. In the present study, we demonstrated the synergistic effects and the underlying molecular mechanisms of SeC in combination with AF on MCF-7 human breast cancer cells. The results showed that SeC and AF synergistically inhibited the cancer cell growth through induction of ROS-dependent apoptosis with the involvement of mitochondrial dysfunction. DNA damage-mediated p53 phosphorylation and down-regulation of phosphorylated AKT and ERK also contributed to cell apoptosis. Moreover, we demonstrated the important role of TrxR activity in the synergistic action of SeC and AF. Taken together, our results suggest the strategy to use SeC and AF in combination could be a highly efficient way to achieve anticancer synergism by targeting TrxR.

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.

In vivo redox state of human thioredoxin and redox shift by the histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA)

The cytosolic thioredoxin (Trx1) system is essential for maintaining a reduced intracellular environment, via reduced Trx1 acting as a general protein disulfide reductase. Trx1 is implicated in cell signaling such as proliferation, DNA synthesis, enzyme activation, cell cycle regulation, transcription, gene activation, and prevention of apoptosis. Human Trx1 contains the active-site cysteines, Cys32 and Cys35, and three additional structural cysteines, Cys62, Cys69, and Cys73, that regulate Trx1 structure and activity via a second disulfide formation, S-glutathionylation or S-nitrosylation. The present study uses an electrophoretic redox Western blot method to analyze the oxidation state of Trx1 in vivo separating the protein-changed isoform following alkylation with iodoacetic acid in 8M urea. Treatment with the histone deacetylase inhibitor SAHA increased Trx1 inhibitor thioredoxin interacting protein (Txnip) levels, decreased Trx1 activity, and switched the Trx1 oxidation state toward a more oxidized one, as a result of complex formation with Trx1, and increased reactive oxygen species (ROS). SAHA is currently in clinical trials for cancer treatment, and one possible mechanism for its anticancer effect is via effects on the Trx1 system. Determining the exact oxidation state of human cytosolic Trx1 may be useful in developing and evaluating cancer drugs and antioxidant agents.

Pharmacology of iron transport

Elucidating the molecular basis for the regulation of iron uptake, storage, and distribution is necessary to understand iron homeostasis. Pharmacological tools are emerging to identify and distinguish among different iron transport pathways. Stimulatory or inhibitory small molecules with effects on iron uptake can help characterize the mechanistic elements of iron transport and the roles of the transporters involved in these processes. In particular, iron chelators can serve as potential pharmacological tools to alleviate diseases of iron overload. This review focuses on the pharmacology of iron transport, introducing iron transport membrane proteins and known inhibitors.

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.

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.

Small molecule inhibitors of mammalian thioredoxin reductase

Mammalian thioredoxin reductases (TrxRs) are a family of NADPH-dependent flavoproteins with a penultimate selenocysteine residue at the carboxy-terminus. Besides their native substrate thioredoxins (Trx), the enzymes show a broad substrate specificity, at least partially, because of the C-terminal redox-active site that is easily accessible in the reduced form. TrxRs are ubiquitous in all kinds of cells and have a critical role in regulating intracellular redox signaling. In recent years, a wealth of evidence has revealed that overactivation/dysfunction of TrxRs is closely related to various diseases, especially in tumor development, and thus the past decades have witnessed an expanding interest in finding TrxRs inhibitors, which might be promising agents for cancer chemotherapy. Herein we reviewed the small molecule inhibitors of mammalian TrxRs, with an emphasis on those that have potential anticancer activity. This review includes the nonpatent references up to 2010 that deal with mammalian TrxR inhibitors.

Oxidative stress in MeHg-induced neurotoxicity

Methylmercury (MeHg) is an environmental toxicant that leads to long-lasting neurological and developmental deficits in animals and humans. Although the molecular mechanisms mediating MeHg-induced neurotoxicity are not completely understood, several lines of evidence indicate that oxidative stress represents a critical event related to the neurotoxic effects elicited by this toxicant. The objective of this review is to summarize and discuss data from experimental and epidemiological studies that have been important in clarifying the molecular events which mediate MeHg-induced oxidative damage and, consequently, toxicity. Although unanswered questions remain, the electrophilic properties of MeHg and its ability to oxidize thiols have been reported to play decisive roles to the oxidative consequences observed after MeHg exposure. However, a close examination of the relationship between low levels of MeHg necessary to induce oxidative stress and the high amounts of sulfhydryl-containing antioxidants in mammalian cells (e.g., glutathione) have led to the hypothesis that nucleophilic groups with extremely high affinities for MeHg (e.g., selenols) might represent primary targets in MeHg-induced oxidative stress. Indeed, the inhibition of antioxidant selenoproteins during MeHg poisoning in experimental animals has corroborated this hypothesis. The levels of different reactive species (superoxide anion, hydrogen peroxide and nitric oxide) have been reported to be increased in MeHg-exposed systems, and the mechanisms concerning these increments seem to involve a complex sequence of cascading molecular events, such as mitochondrial dysfunction, excitotoxicity, intracellular calcium dyshomeostasis and decreased antioxidant capacity. This review also discusses potential therapeutic strategies to counteract MeHg-induced toxicity and oxidative stress, emphasizing the use of organic selenocompounds, which generally present higher affinity for MeHg when compared to the classically studied agents.

Redox pioneer: Professor Arne Holmgren

Dr. Arne Holmgren (Ph.D., 1968) is recognized here as a redox pioneer, because he has published at least one article on redox biology that has been cited over 1000 times and has published at least 10 articles, each cited over 100 times. He is widely known for his seminal discoveries and in-depth studies of thioredoxins, thioredoxin reductases, and glutaredoxins. Dr. Holmgren, active throughout his career at Karolinska Institutet, Sweden, has led the field of research about these classes of proteins for more than 45 years, continuously building upon his sequence determination of Escherichia coli thioredoxin in the late 1960s and discovery of the thioredoxin fold in the 1970s. He discovered and named glutaredoxin and he determined the structure and function of several members of these glutathione-dependent disulfide oxidoreductases. He still continues to broaden the frontiers of knowledge of thioredoxin and glutaredoxin systems. The thioredoxin fold is today recognized as one of the most common protein folds and the intriguing complexity of redox systems, redox signaling, and redox control of cellular function is constantly increasing. The legacy of Dr. Holmgren's research is therefore highly relevant and important also in the context of present science. In a tribute to his work, questions need to be addressed toward the physiological importance of redox signaling and the impact of glutaredoxin and thioredoxin systems on health and disease. Dr. Holmgren helped lay the foundation for the redox biology field and opened new vistas in the process. He is truly a redox pioneer.

Screening for dimethylarginine dimethylaminohydrolase inhibitors reveals ebselen as a bioavailable inactivator

Dimethylarginine dimethylaminohydrolase (DDAH) is an endogenous regulator of nitric oxide production and represents a potential therapeutic target. However, only a small number of biologically useful inhibitors have been reported, and many of these are substrate analogs. To seek more diverse scaffolds, we developed a high-throughput screening (HTS) assay and queried two small libraries totaling 2446 compounds. The HTS assay proved to be robust, reproducible and scalable, with Z' factors ≥ 0.78. One inhibitor, ebselen, is structurally divergent from substrate and was characterized in detail. This selenazole covalently inactivates DDAH in vitro and in cultured cells. The rate constant for inactivation of DDAH (44,000 ± 2,400 M(-1)s(-1)) is greater than those reported for any other target, suggesting this pathway is an important aspect of ebselen's total pharmacological effects.

Beneficial effects of N-acetylcysteine and ebselen on renal ischemia/reperfusion injury

INTRODUCTION

It has been demonstrated that peroxynitrite accompanies acute renal ischemia and contributes to the pathophysiology of renal damage. Therefore, we aimed to investigate the roles of N-acetylcysteine (NAC), a well-known powerful antioxidant, and ebselen (E), a scavenger of peroxynitrite, on renal injury induced by renal ischemia/reperfusion injury (IRI) of rat kidney.

MATERIALS AND METHODS

Forty male Sprague-Dawley rats were divided into five groups: sham, renal IRI, renal IRI+NAC, renal IRI+E, and renal IRI+NAC+E. IR injury was induced by 60 min of bilateral renal ischemia followed by 6 h of reperfusion. After reperfusion, kidneys and blood samples were obtained for histopathological and biochemical evaluations.

RESULTS

Renal IR resulted in increased malondialdehyde and nitrite/nitrate levels suggesting increased lipid peroxidation and peroxynitrite production and decreased superoxide dismutase and glutathione peroxidase activities. Both NAC and E alone significantly decreased malondialdehyde and nitrite/nitrate levels and increased superoxide dismutase and glutathione peroxidase activities. Additionally in the renal IRI+NAC+E group, all biochemical results were quite close to those of sham group. Histopathologically, the kidney injury in rats treated with combination of NAC and E was found significantly less than the other groups.

CONCLUSIONS

Both NAC and E are able to ameliorate IRI of the kidney by decreasing oxidative and nitrosative stresses and increasing free radical scavenger properties. Additionally, combination of NAC and E prevents kidney damage more than when each drug is used alone, suggesting that scavenging peroxynitrite nearby antioxidant activity is important in preventing renal IRI.

High-throughput screen identifies novel inhibitors of cancer biomarker α-methylacyl coenzyme A racemase (AMACR/P504S)

α-methylacyl coenzyme A racemase (AMACR) is a metabolic enzyme whose overexpression has been shown to be a diagnostic indicator of prostatic adenocarcinoma and other solid tumors. Here, we confirm that attenuation of AMACR expression diminishes the growth of prostate cancer cell lines by using stably expressed short-hairpin RNA constructs. This observation strongly suggests that the AMACR enzyme may be a target for therapeutic inhibition in prostate cancer. To this end, we report here a novel assay capable of screening libraries of diverse small molecules for inhibitors of AMACR activity. This assay facilitated the screening of approximately 5,000 unique compounds and the discovery of 7 distinct chemical entities capable of inhibiting AMACR at low micromolar concentrations. The most potent inhibitor discovered is the seleno-organic compound ebselen oxide [inhibitory concentration (IC(50)): 0.80 μmol/L]. The parent compound, ebselen (IC(50): 2.79 μmol/L), is a covalent inactivator of AMACR (K(I)((inact)): 24 μmol/L). Two of the AMACR inhibitors are selectively toxic to prostate cancer cell lines (LAPC4/LNCaP/PC3) that express AMACR compared to a normal prostate fibroblast cell line (WPMY1) that does not express the protein. This report shows the first high-throughput screen for the discovery of novel AMACR inhibitors, characterizes the first nonsubstrate-based inhibitors, and validates that AMACR is a viable chemotherapeutic target in vitro.

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.

Reduction of diphenyl diselenide and analogs by mammalian thioredoxin reductase is independent of their gluthathione peroxidase-like activity: a possible novel pathway for their antioxidant activity

Since the successful use of the organoselenium drug ebselen in clinical trials for the treatment of neuropathological conditions associated with oxidative stress, there have been concerted efforts geared towards understanding the precise mechanism of action of ebselen and other organoselenium compounds, especially the diorganyl diselenides such as diphenyl diselenide, and its analogs. Although the mechanism of action of ebselen and other organoselenium compounds has been shown to be related to their ability to generally mimic native glutathione peroxidase (GPx), only ebselen however has been shown to serve as a substrate for the mammalian thioredoxin reductase (TrxR), demonstrating another component of its pharmacological mechanisms. In fact, there is a dearth of information on the ability of other organoselenium compounds, especially diphenyl diselenide and its analogs, to serve as substrates for the mammalian enzyme thioredoxin reductase. Interestingly, diphenyl diselenide shares several antioxidant and neuroprotective properties with ebselen. Hence in the present study, we tested the hypothesis that diphenyl diselenide and some of its analogs (4,4'-bistrifluoromethyldiphenyl diselenide, 4,4'-bismethoxy-diphenyl diselenide, 4.4'-biscarboxydiphenyl diselenide, 4,4'-bischlorodiphenyl diselenide, 2,4,6,2',4',6'-hexamethyldiphenyl diselenide) could also be substrates for rat hepatic TrxR. Here we show for the first time that diselenides are good substrates for mammalian TrxR, but not necessarily good mimetics of GPx, and vice versa. For instance, bis-methoxydiphenyl diselenide had no GPx activity, whereas it was a good substrate for reduction by TrxR. Our experimental observations indicate a possible dissociation between the two pathways for peroxide degradation (either via substrate for TrxR or as a mimic of GPx). Consequently, the antioxidant activity of diphenyl diselenide and analogs can be attributed to their capacity to be substrates for mammalian TrxR and we therefore conclude that subtle changes in the aryl moiety of diselenides can be used as tool for dissociation of GPx or TrxR pathways as mechanism triggering their antioxidant activities.

Reduction of diphenyl diselenide and analogs by mammalian thioredoxin reductase is independent of their gluthathione peroxidase-like activity: a possible novel pathway for their antioxidant activity

Since the successful use of the organoselenium drug ebselen in clinical trials for the treatment of neuropathological conditions associated with oxidative stress, there have been concerted efforts geared towards understanding the precise mechanism of action of ebselen and other organoselenium compounds, especially the diorganyl diselenides such as diphenyl diselenide, and its analogs. Although the mechanism of action of ebselen and other organoselenium compounds has been shown to be related to their ability to generally mimic native glutathione peroxidase (GPx), only ebselen however has been shown to serve as a substrate for the mammalian thioredoxin reductase (TrxR), demonstrating another component of its pharmacological mechanisms. In fact, there is a dearth of information on the ability of other organoselenium compounds, especially diphenyl diselenide and its analogs, to serve as substrates for the mammalian enzyme thioredoxin reductase. Interestingly, diphenyl diselenide shares several antioxidant and neuroprotective properties with ebselen. Hence in the present study, we tested the hypothesis that diphenyl diselenide and some of its analogs (4,4'-bistrifluoromethyldiphenyl diselenide, 4,4'-bismethoxy-diphenyl diselenide, 4.4'-biscarboxydiphenyl diselenide, 4,4'-bischlorodiphenyl diselenide, 2,4,6,2',4',6'-hexamethyldiphenyl diselenide) could also be substrates for rat hepatic TrxR. Here we show for the first time that diselenides are good substrates for mammalian TrxR, but not necessarily good mimetics of GPx, and vice versa. For instance, bis-methoxydiphenyl diselenide had no GPx activity, whereas it was a good substrate for reduction by TrxR. Our experimental observations indicate a possible dissociation between the two pathways for peroxide degradation (either via substrate for TrxR or as a mimic of GPx). Consequently, the antioxidant activity of diphenyl diselenide and analogs can be attributed to their capacity to be substrates for mammalian TrxR and we therefore conclude that subtle changes in the aryl moiety of diselenides can be used as tool for dissociation of GPx or TrxR pathways as mechanism triggering their antioxidant activities.

Mitochondrial dysfunction induced by different organochalchogens is mediated by thiol oxidation and is not dependent of the classical mitochondrial permeability transition pore opening

Ebselen (Ebs) and diphenyl diselenide [(PhSe)(2)] readily oxidize thiol groups. Here we studied mitochondrial swelling changes in mitochondrial potential (Deltapsim), NAD(P)H oxidation, reactive oxygen species production, protein aggregate formation, and oxygen consumption as ending points of their in vitro toxicity. Specifically, we tested the hypothesis that organochalchogens toxicity could be associated with mitochondrial dysfunction via oxidation of vicinal thiol groups that are known to be involved in the regulation of mitochondrial permeability (Petronilli et al. J. Biol. Chem., 269; 16638; 1994). Furthermore, we investigated the possible mechanism(s) by which these organochalchogens could disrupt liver mitochondrial function. Ebs and (PhSe)(2) caused mitochondrial depolarization and swelling in a concentration-dependent manner. Furthermore, both organochalchogens caused rapid oxidation of the mitochondrial pyridine nucleotides (NAD(P)H) pool, likely reflecting the consequence and not the cause of increased mitochondrial permeability (Costantini, P., Chernyak, B. V., Petronilli, V., and Bernardi, P. (1996). Modulation of the mitochondrial permeability transition pore (PTP) by pyridine nucleotides and dithiol oxidation at two separate sites. J. Biol. Chem. 271, 6746-6751). The organochalchogens-induced mitochondrial dysfunction was prevented by the reducing agent dithiothreitol (DTT). Ebs- and (PhSe)(2)-induced mitochondrial depolarization and swelling were unchanged by ruthenium red (4microM), butylated hydroxytoluene (2.5microM), or cyclosporine A (1microM). N-ethylmaleimide enhanced the organochalchogens-induced mitochondrial depolarization, without affecting the magnitude of the swelling response. In contrast, iodoacetic acid did not modify the effects of Ebs or (PhSe)(2) on the mitochondria. Additionally, Ebs and (PhSe)(2) decreased the basal 2' 7' dichlorofluorescin diacetate (H(2)-DCFDA) oxidation and oxygen consumption rate in state 3 and increased it during the state 4 of oxidative phosphorylation and induced the formation of protein aggregates, which were prevented by DTT. However, DTT failed to reverse the formation of protein aggregates, when it was added after a preincubation of liver mitochondria with Ebs or (PhSe)(2). Similarly, DTT did not reverse the Ebs- or (PhSe)(2)-induced Deltapsim collapse or swelling, when it was added after a preincubation period of mitochondria with chalcogenides. These results show that Ebs and (PhSe)(2) can effectively induce mitochondrial dysfunction and suggest that effects of these compounds are associated with mitochondrial thiol groups oxidation. The inability of cyclosporine A to reverse the Ebs- and (PhSe)(2)-induced mitochondrial effects suggests that the redox-regulated mitochondrial permeability transition (MPT) pore was mechanistically regulated in a manner that is distinct from the classical MPT pore.

The selenium-independent inherent pro-oxidant NADPH oxidase activity of mammalian thioredoxin reductase and its selenium-dependent direct peroxidase activities

Mammalian thioredoxin reductase (TrxR) is an NADPH-dependent homodimer with three redox-active centers per subunit: a FAD, an N-terminal domain dithiol (Cys(59)/Cys(64)), and a C-terminal cysteine/selenocysteine motif (Cys(497)/Sec(498)). TrxR has multiple roles in antioxidant defense. Opposing these functions, it may also assume a pro-oxidant role under some conditions. In the absence of its main electron-accepting substrates (e.g. thioredoxin), wild-type TrxR generates superoxide (O ), which was here detected and quantified by ESR spin trapping with 5-diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide (DEPMPO). The peroxidase activity of wild-type TrxR efficiently converted the O adduct (DEPMPO/HOO(*)) to the hydroxyl radical adduct (DEPMPO/HO(*)). This peroxidase activity was Sec-dependent, although multiple mutants lacking Sec could still generate O . Variants of TrxR with C59S and/or C64S mutations displayed markedly reduced inherent NADPH oxidase activity, suggesting that the Cys(59)/Cys(64) dithiol is required for O generation and that O is not derived directly from the FAD. Mutations in the Cys(59)/Cys(64) dithiol also blocked the peroxidase and disulfide reductase activities presumably because of an inability to reduce the Cys(497)/Sec(498) active site. Although the bulk of the DEPMPO/HO(*) signal generated by wild-type TrxR was due to its combined NADPH oxidase and Sec-dependent peroxidase activities, additional experiments showed that some free HO(*) could be generated by the enzyme in an H(2)O(2)-dependent and Sec-independent manner. The direct NADPH oxidase and peroxidase activities of TrxR characterized here give insights into the full catalytic potential of this enzyme and may have biological consequences beyond those solely related to its reduction of thioredoxin.

Improved tricyclic inhibitors of trypanothione reductase by screening and chemical synthesis

Trypanothione reductase (TryR) is a key validated enzyme in the trypanothione-based redox metabolism of pathogenic trypanosomes and leishmania parasites. This system is absent in humans, being replaced with glutathione and glutathione reductase, and as such offers a target for selective inhibition. As part of a program to discover antiparasitic drugs, the LOPAC1280 library of 1266 compounds was screened against TryR and the top hits evaluated against glutathione reductase and T. brucei parasites. The top hits included a number of known tricyclic neuroleptic drugs along with other new scaffolds for TryR. Three novel druglike hits were identified and SAR studies on one of these using information from the tricyclic neuroleptic agents led to the discovery of a competitive inhibitor (K_i=330 nm) with an improved potency against T. brucei (EC₅₀=775 nm).

Thioredoxin reductase 1 ablation sensitizes colon cancer cells to methylseleninate-mediated cytotoxicity

The relationship between selenium and cancer is complex because individuals with low serum selenium levels benefit from selenium supplementation, but those with high serum selenium levels are at increased risk for other diseases. This suggests that the use of selenocompounds might be limited to particular circumstances, such as adjuvant therapy. A contributor to this dichotomy may be the activity of certain selenium containing enzymes like the cytosolic thioredoxin reductase (TR1). We evaluated the cellular response to select selenocompounds that have anticancer activity when TR1 was attenuated by siRNA in RKO colon cancer cells. Methylseleninic acid (MSA), which is a substrate for TR1, enhanced cytotoxicity to colon cancer cells when TR1 was attenuated. MSA induced stress in the endoplasmic reticulum, as measured by GRP78 protein levels. However, this pathway did not appear to account for the change in cytotoxicity when TR1 was attenuated. Instead, knockdown of the cytosolic TR plus incubation with MSA increased autophagy, as measured by LC3B cleavage, and apoptosis, as measured by Annexin V and mitochondrial dysfunction. Therefore, the use of selenocompounds with anticancer activity, like MSA, might be utilized most effectively with agents that targets TR1 in chemotherapeutic applications.

Metabolism of selenium compounds catalyzed by the mammalian selenoprotein thioredoxin reductase

The mammalian thioredoxin reductases (TrxR) are selenoproteins with a catalytic selenocysteine residue which in the oxidized enzyme forms a selenenylsulfide and in the reduced enzyme is present as a selenolthiol. Selenium compounds such as selenite, selenodiglutathione and selenocystine are substrates for the enzyme with low K(m)-values and the enzyme is implicated in reductive assimilation of selenium by generating selenide for selenoprotein synthesis. Redox cycling of reduced metabolites of these selenium compounds including selenide with oxygen via TrxR and reduced thioredoxin (Trx) will oxidize NADPH and produce reactive oxygen species inducing cell death at high concentrations explaining selenite toxicity. There is no free pool of selenocysteine since this would be toxic in an oxygen environment by redox cycling via thioredoxin systems. The importance of selenium compounds and TrxR in cancer and cardiovascular diseases both for prevention and treatment is discussed. A selenazol drug like ebselen is a direct substrate for mammalian TrxR and dithiol Trx and ebselen selenol is readily reoxidized by hydrogen peroxide and lipid hydroperoxides, acting as an anti-oxidant and anti-inflammatory drug.

The use of ebselen for radioprotection in cultured cells and mice

Ionizing radiation induces the production of reactive oxygen species (ROS), which play an important causative role in cell death. Therefore, compounds that control the level of ROS may confer radioprotective effects. Ebselen, a seleno-organic compound, has been shown to protect against cell injury caused by ROS. The objective of this study was to examine the effects of ebselen on radiation-dependent toxicity. We investigated the protective role of ebselen against ionizing radiation in U937 cells and mice. Upon exposure to 20 Gy of gamma-irradiation, there was a distinct difference between untreated cells and the cells pretreated with 5 microM ebselen for 2 h with respect to viability, cellular redox status, and oxidative damage to cells. When cells were exposed to 2 Gy of gamma-irradiation, there was a distinct difference between the untreated cells and the cells pretreated with ebselen with respect to apoptotic features and mitochondrial function. Ebselen administration for 14 days at a daily dosage of 10 mg/kg provided substantial protection against killing and oxidative damage to mice exposed to whole-body irradiation. These data indicate that ebselen may have great potential as a new class of in vivo, non-sulfur-containing radiation protector.

Focus on mammalian thioredoxin reductases--important selenoproteins with versatile functions

Thioredoxin systems, involving redox active thioredoxins and thioredoxin reductases, sustain a number of important thioredoxin-dependent pathways. These redox active proteins support several processes crucial for cell function, cell proliferation, antioxidant defense and redox-regulated signaling cascades. Mammalian thioredoxin reductases are selenium-containing flavoprotein oxidoreductases, dependent upon a selenocysteine residue for reduction of the active site disulfide in thioredoxins. Their activity is required for normal thioredoxin function. The mammalian thioredoxin reductases also display surprisingly multifaceted properties and functions beyond thioredoxin reduction. Expressed from three separate genes (in human named TXNRD1, TXNRD2 and TXNRD3), the thioredoxin reductases can each reduce a number of different types of substrates in different cellular compartments. Their expression patterns involve intriguingly complex transcriptional mechanisms resulting in several splice variants, encoding a number of protein variants likely to have specialized functions in a cell- and tissue-type restricted manner. The thioredoxin reductases are also targeted by a number of drugs and compounds having an impact on cell function and promoting oxidative stress, some of which are used in treatment of rheumatoid arthritis, cancer or other diseases. However, potential specific or essential roles for different forms of human or mouse thioredoxin reductases in health or disease are still rather unclear, although it is known that at least the murine Txnrd1 and Txnrd2 genes are essential for normal development during embryogenesis. This review is a survey of current knowledge of mammalian thioredoxin reductase function and expression, with a focus on human and mouse and a discussion of the striking complexity of these proteins. Several yet open questions regarding their regulation and roles in different cells or tissues are emphasized. It is concluded that the intriguingly complex regulation and function of mammalian thioredoxin reductases within the cellular context and in intact mammals strongly suggests that their functions are highly fi ne-tuned with the many pathways involving thioredoxins and thioredoxin-related proteins. These selenoproteins furthermore propagate many functions beyond a reduction of thioredoxins. Aberrant regulation of thioredoxin reductases, or a particular dependence upon these enzymes in diseased cells, may underlie their presumed therapeutic importance as enzymatic targets using electrophilic drugs. These reductases are also likely to mediate several of the effects on health and disease that are linked to different levels of nutritional selenium intake. The thioredoxin reductases and their splice variants may be pivotal components of diverse cellular signaling pathways, having importance in several redox-related aspects of health and disease. Clearly, a detailed understanding of mammalian thioredoxin reductases is necessary for a full comprehension of the thioredoxin system and of selenium dependent processes in mammals.

Guanosine and synthetic organoselenium compounds modulate methylmercury-induced oxidative stress in rat brain cortical slices: involvement of oxidative stress and glutamatergic system

Excessive formation of reactive oxygen species (ROS) and disruption of glutamate uptake have been pointed as two key mechanisms in methylmercury-toxicity. Thus, here we investigate the involvement of glutamatergic system in methylmercury (MeHg) neurotoxicity and whether diphenyl diselenide, ebselen and guanosine could protect cortical rat brain slices from MeHg-induced ROS generation. MeHg (100 and 200 microM) increased 2',7'-dichlorodihydrofluorescin (DCFH) oxidation after 2h of exposure. At 50 microM, MeHg increased DCFH oxidation only after 5h of exposure. Guanosine (1 and 5 microM) did not caused any effect per se; however, it blocked the increase in DCFH caused by 200 or 50 microM MeHg. Ebselen (5 and 10 microM) decreased significantly the DCFH oxidation after 2 and 5h of exposure to MeHg. Diphenyl diselenide (5 microM) did not change the basal DCFH oxidation, but abolished the pro-oxidant effect of MeHg. MK-801 also abolished the pro-oxidant effect of MeHg. These results demonstrate for the first time the potential antioxidant properties of organoseleniun compounds and guanosine against MeHg-induced ROS generation after short-term exposure in a simple in vitro model. In conclusion, endogenous purine (guanosine) and two synthetic organoselenium compounds can modulate the pro-oxidant effect of MeHg in cortical brain slices.

Thioredoxin reductase-1 negatively regulates HIV-1 transactivating protein Tat-dependent transcription in human macrophages

Epidemiological studies suggest a correlation between severity of acquired immunodeficiency syndrome (AIDS) and selenium deficiency, indicating a protective role for this anti-oxidant during HIV infection. Here we demonstrate that thioredoxin reductase-1 (TR1), a selenium-containing pyridine nucleotide-disulfide oxidoreductase that reduces protein disulfides to free thiols, negatively regulates the activity of the HIV-1 encoded transcriptional activator, Tat, in human macrophages. We used a small interfering RNA approach as well as a high affinity substrate of TR1, ebselen, to demonstrate that Tat-dependent transcription and HIV-1 replication were significantly increased in human macrophages when TR1 activity was reduced. The increase in HIV-1 replication in TR1 small interfering RNA-treated cells was independent of the redox-sensitive transcription factor, NF-kappaB. These studies indicate that TR-1 acts as a negative regulator of Tat-dependent transcription. Furthermore, in vitro biochemical assays with recombinant Tat protein confirmed that TR1 targets two disulfide bonds within the Cys-rich motif required for efficient HIV-1 transactivation. Increasing TR1 expression along with other selenoproteins by supplementing with selenium suggests a potential inexpensive adjuvant therapy for HIV/AIDS patients.

Peroxynitrite detoxification and its biologic implications

Peroxynitrite is a cytotoxic oxidant formed in vivo from the diffusional-controlled reaction between nitric oxide and superoxide radicals. Increased peroxynitrite formation has been related to the pathogenesis of multiple diseases, thus underlining the importance of understanding the mechanisms of its detoxification. In nature, different enzymatic routes for peroxynitrite decomposition have evolved. Among them, peroxiredoxins catalytically reduce peroxynitrite in vitro; modulation of their expression affects peroxynitrite-mediated cytotoxicity, and their content changes in pathologic conditions associated with increased peroxynitrite formation in vivo, thus indicating a physiologic role of these enzymes in peroxynitrite reduction. Selenium-containing glutathione peroxidase also catalyzes peroxynitrite reduction, but its role in vivo is still a matter of debate. In selected cellular systems, heme proteins also play a role in peroxynitrite detoxification, such as its isomerization by oxyhemoglobin in red blood cells. Moreover, different pharmacologic approaches have been used to decrease the toxicity related to peroxynitrite formation. Manganese or iron porphyrins catalyze peroxynitrite decomposition, and their protective role in vivo has been confirmed in biologic systems. Glutathione peroxidase mimetics also rapidly reduce peroxynitrite, but their biologic role is less well established. Flavonoids, nitroxides, and tyrosine-containing peptides decreased peroxynitrite-mediated toxicity under different conditions, but their mechanism of action is indirect.