Glutathione-mediated formation of oxygen free radicals by the major metabolite of oltipraz

Efficacy of oltipraz in preventing acetaminophen-induced liver injury in mice

Oltipraz (OPZ) is a synthetic dithiolethione with potential as a cancer chemopreventive agent, which can work by inducing detoxification enzymes. OPZ is an activator of nuclear factor erythroid 2-related factor 2 (Nrf2), suggesting its involvement in enzyme induction and possible protection against drug-induced liver injury. In this study, we present OPZ-mediated protection of mice against acetaminophen (APAP)-induced liver injury and discuss its possible contributing factors. Overnight-fasted male CD-1 mice were administered APAP intraperitoneally, and some mice were administered OPZ 16 h before APAP. Hepatotoxicity was assessed by measuring serum alanine aminotransferase leakage and histopathological evaluation. The hepatic mRNA expressions of CYP2E1, glutamate cysteine ligase (GCL), and NAD(P)H:quinone oxidoreductase (NQO1) were measured by real-time reverse-transcription polymerase chain reaction. OPZ protected mice from APAP-induced liver injury in a dose-dependent manner, but did not alter hepatic glutathione (GSH) content or GCL expression in control mice, indicating that its hepatoprotective effect is not due to changes in basal GSH levels. OPZ did not affect CYP2E1 expression or APAP-induced early GSH depletion, suggesting it does not inhibit the metabolic activation of APAP to produce N-acetyl-p-benzoquinone imine. In contrast, after GSH depletion, OPZ accelerated hepatic GSH recovery. APAP significantly increased GCL expression during liver injury, but OPZ treatment only led to additional NQO1 expression. This suggests that NQO1 is responsible for the enhanced GSH recovery and protection against APAP-induced liver injury seen in OPZ-treated mice. In summary, OPZ protects against APAP-induced liver injury by inducing NQO1 expression and resulting in improved GSH recovery.

Oxygen-mediated Polymerization Initiated by Oltipraz-derived Thiones

A pyrrolopyrazine-thione derived from oltipraz, a compound that has been investigated as a chemopreventive agent, affords radicals in the presence of thiols and oxygen via a redox cycle, an attribute that suggests its suitability as an initiator for oxygen-mediated polymerization. Here, we explore the utilization of this pyrrolopyrazine-thione, generated in situ from a precursor, as an initiator for the radical-mediated thiol-ene polymerization. While the pyrrolopyrazine-thione was shown to be capable of generating radicals in the presence of atmospheric oxygen and thiol groups, the reaction extents achievable were lower than desired owing to the presence of unwanted side reactions that would quench radical production and, subsequently, suppress polymerization. Moreover, we found that complex interactions between the pyrrolopyrazine-thione, its precursor, oxygen, and thiol groups determine whether or not the quenching reaction dominates over those favorable to polymerization.

Further structure-activity relationships study of substituted dithiolethiones as glutathione-inducing neuroprotective agents

Background

Parkinson’s disease is a neurodegenerative disorder associated with oxidative stress and glutathione depletion. The induction of cellular glutathione levels by exogenous molecules is a promising neuroprotective approach to limit the oxidative damage that characterizes Parkinson’s disease pathophysiology. Dithiolethiones, a class of sulfur-containing heterocyclic molecules, are known to increase cellular levels of glutathione; however, limited information is available regarding the influence of dithiolethione structure on activity. Herein, we report the design, synthesis, and pharmacological evaluation of a further series of dithiolethiones in the SH-SY5Y neuroblastoma cell line.

Results

Our structure–activity relationships data show that dithiolethione electronic properties, given as Hammett σ_p constants, influence glutathione induction activity and compound toxicity. The most active glutathione inducer identified, 6a, dose-dependently protected cells from 6-hydroxydopamine toxicity. Furthermore, the protective effects of 6a were abrogated by the inhibitor of glutathione synthesis, buthionine sulfoximine, confirming the importance of glutathione in the protective activities of 6a.

Conclusions

The results of this study further delineate the relationship between dithiolethione chemical structure and glutathione induction. The neuroprotective properties of analog 6a suggest a role for dithiolethiones as potential antiparkinsonian agents.

The complexity of the Nrf2 pathway: beyond the antioxidant response

The NF-E2-related factor 2 (Nrf2)-mediated signalling pathway provides living organisms an efficient and pivotal line of defensive to counteract environmental insults and endogenous stressors. Nrf2 coordinates the basal and inducible expression of antioxidant and Phase II detoxification enzymes to adapt to different stress conditions. The stability and cellular distribution of Nrf2 is tightly controlled by its inhibitory binding protein Kelch-like ECH-associated protein 1. Nrf2 signalling is also regulated by posttranslational, transcriptional, translational and epigenetic mechanisms, as well as by other protein partners, including p62, p21 and IQ motif-containing GTPase activating protein 1. Many studies have demonstrated that Nrf2 is a promising target for preventing carcinogenesis and other chronic diseases, including cardiovascular diseases, neurodegenerative diseases and pulmonary injury. However, constitutive activation of Nrf2 in advanced cancer cells may confer drug resistance. Here, we review the molecular mechanisms of Nrf2 signalling, the diverse classes of Nrf2 activators, including bioactive nutrients and other chemicals, and the cellular functions and disease relevance of Nrf2 and discuss the dual role of Nrf2 in different contexts.

Synthesis and structure-activity relationships study of dithiolethiones as inducers of glutathione in the SH-SY5Y neuroblastoma cell line

Parkinson's disease is a neurodegenerative disorder that involves the degeneration of nigrostriatal dopaminergic neurons. Elevated levels of reactive oxygen species have been shown to deplete cellular levels of the ubiquitous antioxidant glutathione, leading to oxidative stress and eventual neuronal cell death. Dithiolethiones, a class of sulfur-containing heterocyclic molecules, have been shown to induce cellular production of glutathione in a variety of tissues, but have not been extensively evaluated in neurons. Herein, we report the synthesis and preliminary structure-activity relationships study of several substituted dithiolethiones. Three molecules were identified (D3T, CPDT, and 2d) that potently induced cellular glutathione in the SH-SY5Y neuroblastoma cell line. Furthermore, these compounds were found to provide neuroprotection in the 6-hydroxydopamine model of neurotoxicity. This study suggests that dithiolethione-mediated neuroprotection may have potential as a disease-modifying antiparkinsonian therapy.

Hydrogen peroxide is a second messenger in phase 2 enzyme induction by cancer chemopreventive dithiolethiones

The ability of three dithiolethione cancer chemopreventives, oltipraz 1, anetholedithione (ADT) 2, 1,2-dithiole-3-thione (D3T) 3, and the major metabolite, 4, of 1, to induce the cytoprotective enzyme NQO1 in Hepa 1c1c7 cells and the inhibition of this induction by catalase are demonstrated. The ability of 1, 3, and 4 to form O(2)(*) has been reported, and it is here demonstrated that 2 decomposes in the presence of GSH to form, upon addition of the nitrone spin trap DMPO, the DMPO-OH adduct that is detectable by EPR. Decomposition of 2 in the presence of GSH elicits, upon the addition of hydroethidine and excitation at 510 nm, fluorescence at 580 nm that is diminished by the addition of superoxide dismutase. The compound 4, is a product of the reduction of 1, and it is demonstrated that 2 and 3 decompose in the presence of reductants such as thiolates and NaBH(4), followed by addition of CH(3)I, to form the dimethylated products of reductive cleavage of the S(1)-S(2) bond. The same products are isolated subsequent to lysis in buffer containing CH(3)I of Hepa 1c1c7 cells treated with 2 or 3. Reductive cleavage of 2 and 3 in aqueous ethanol by NaBH(4) in an argon atmosphere, followed by acidic destruction of remaining borohydride and neutralization and introduction of O(2) results in the reformation of 2 and 3 to the extent of 80 and 33%, respectively. The data in toto are consistent with a model in which dithiolethiones, generally, undergo reductive cleavage in Hepa 1c1c7 cells, thereby resulting in the generation of O(2)(*) that dismutates to H(2)O(2), that subsequently, by direct or indirect means, effects the nuclear translocation of transcription factor Nrf2, that upregulates phase 2 enzyme expression.

Oxidized metabolites of oltipraz exert cytoprotective effects against arachidonic acid through AMP-activated protein kinase-dependent cellular antioxidant effect and mitochondrial protection

Oltipraz protects cells from chemical-induced carcinogenesis partly because of phase 2 enzyme induction. Certain oltipraz metabolites also induce phase 2 enzymes. This study investigated the cytoprotective effects of the oxidized metabolites of oltipraz against arachidonic acid (AA), a proinflammatory fatty acid that causes cellular reactive oxygen species (ROS) production and mitochondrial impairment, and the mechanistic basis of their action in HepG2 cells. Treatment with 4-methyl-5-(pyrazin-2-yl)-3H-1,2-dithiol-3-one (M1) or 7-methyl-6,8-bis(methylthio)H-pyrrolo[1,2-a]-pyrazine (M2), but not 7-methyl-8-(methylsulfinyl)-6-(methylthio)H-pyrrolo[1,2-a]pyrazine (M3) or 7-methyl-6,8-bis(methylsulfinyl)H-pyrrolo[1,2-a]pyrazine (M4), enabled cells to protect against AA-induced apoptosis. M1 and M2 treatment protected cells from ROS produced by AA and inhibited AA-induced glutathione depletion. Moreover, both M1 and M2 effectively inhibited mitochondrial dysfunction induced by AA, although M2 alone slightly elicited it at a relatively high concentration. M1 and M2 activated AMP-activated protein kinase (AMPK), but M3 and M4 failed to do so. AMPK activation by M1 and M2 contributed to cell survival against AA through a decrease in cellular ROS production and prevention of mitochondrial dysfunction, as shown by the reversal of the metabolites' restoration of mitochondrial membrane potential by compound C treatment or overexpression of a dominant-negative mutant AMPK. Consistently, 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside, an AMPK activator, also had a cytoprotective and antioxidant effect against AA. Our results demonstrate that, of the major metabolites of oltipraz, M1 and M2 are capable of protecting cells from AA-induced ROS production and mitochondrial dysfunction, which may be associated with AMPK activation.

Dithiolethiones for cancer chemoprevention: where do we stand?

Dithiolethiones are a well-known class of cancer chemopreventive agents; the key mechanism of action of dithiolethiones involves activation of Nrf2 signaling and induction of phase II enzymes. In the past, attention has been focused mainly on 4-methyl-5-pyrazinyl-3H-1,2-dithiole-3-thione (oltipraz), which showed ability as a wide-spectrum inhibitor of chemical carcinogenesis in preclinical models. However, clinical trials of oltipraz have shown questionable efficacy, and at the high doses employed in such studies, significant side effects were observed. Dithiolethiones that are markedly more effective and potent than oltipraz in both induction of phase II enzymes and inhibition of chemical carcinogenesis in preclinical studies have been identified, and these compounds have shown pronounced organ specificity in vivo. Further investigation of these compounds may lead to development of effective and safe agents for cancer prevention in humans.

Assessment of wound-site redox environment and the significance of Rac2 in cutaneous healing

We have previously reported that H(2)O(2) is actively generated by cells at the wound site and that H(2)O(2)-driven redox signaling supports wound angiogenesis and healing. In this study, we have standardized a novel and effective electron paramagnetic resonance spectroscopy-based approach to assess the redox environment of the dermal wound site in vivo. Rac2 regulates inducible NADPH oxidase activation and other functional responses in neutrophils. Using Rac2-deficient mice we sought to investigate the significance of Rac2 in the wound-site redox environment and healing responses. Noninvasive measurements of metabolism of topically applied nitroxide (15)N-perdeuterated tempone in murine excisional dermal wounds demonstrated that the wound site is rich in oxidants, the levels of which peak 2 days postwounding in the inflammatory phase. Rac2-deficient mice had threefold lower production of superoxide compared to controls with similar wounds. In these mice, a lower wound-site superoxide level was associated with compromised wound closure. Immunostaining of wound edges harvested during the inflammatory phase showed that the numbers of phagocytic cells recruited to the wound site in Rac2-deficient and control mice were similar, but the amount of lipid peroxidation was significantly lower in Rac2-deficient mice, indicating compromised NADPH oxidase activity. Taken together, the findings of this study support that the wound site is rich in oxidants. Rac2 significantly contributes to oxidant production at the wound site and supports the healing process.

Interactions of the major metabolite of the cancer chemopreventive drug oltipraz with cytochrome c: a novel pathway for cancer chemoprevention

The major metabolite of the cancer chemopreventive agent oltipraz, a pyrrolopyrazine thione (PPD), has been shown to be a phase 2 enzyme inducer, an activity thought to be key to the cancer chemopreventive action of the parent compound. In cells, mitochondria are the major source of reactive oxygen species (ROS) and cytochrome c (cyt c) is known to participate in mitochondrial electron transport and confer antioxidant and peroxidase activities. To understand possible mechanisms by which PPD acts as a phase 2 enzyme inducer, a study of its interaction with cyt c was undertaken. UV-visible spectroscopic results demonstrate that PPD is capable of reducing oxidized cyt c. The reduced cyt c is stable for a long period of time in the absence of an oxidizing agent. In the presence of ferricyanide, the reduced cyt c is rapidly oxidized back to its oxidized form. Further, UV-visible spectroscopic studies show that during the reduction process the coordination environment and redox state of iron in cyt c are changed. Low-temperature EPR studies show that during the reduction process, the heme iron changes from a low-spin state of s = 1/2 to a low-spin state of s = 0. Room-temperature EPR studies demonstrate that PPD inhibits the peroxidase activity of cyt c. EPR spin trapping experiments using DMPO show that PPD inhibits the superoxide radical scavenging activity of oxidized cyt c. From these results, we propose that PPD interacts with cyt c, binding to and then reducing the heme, and this may enhance ROS levels in mitochondria. This in turn could contribute to the mechanism by which the parent compound, oltipraz, might trigger the cancer chemopreventive increase in transcription of phase 2 enzymes. The modifications of cyt c function by the oltipraz metabolite may have implications for the regulation of apoptotic cell death.

Generation of superoxide from reaction of 3H-1,2-dithiole-3-thione with thiols: implications for dithiolethione chemoprotection

3H-1,2-Dithiole-3-thione (D3T), a potent member of dithiolethiones, induces phase 2 enzymes by activating an Nrf2/Keap1-dependent signaling pathway. It was proposed that interaction between D3T and two adjacent sulfhydryl groups of Keap1 might cause dissociation of Keap1 from Nrf2, leading to Nrf2 activation. This study was undertaken to investigate the reactions between D3T and thiols, including the dithiol compound, dithiothreitol (DTT), and the monothiol, glutathione (GSH). We reported here that under physiologically relevant conditions incubation of D3T with DTT caused remarkable oxygen consumption, indicating a redox reaction between D3T and the dithiol molecule. Incubation of D3T with GSH also led to oxygen consumption, but to a less extent. Electron paramagnetic resonance (EPR) studies showed that the redox reaction between D3T and DTT generated superoxide. Superoxide was also formed from the redox reaction of D3T with GSH. These findings demonstrate that D3T reacts with thiols, particularly a dithiol, generating superoxide, which may provide a mechanistic explanation for induction of Nrf2-dependent phase 2 enzymes by D3T.

Thioredoxin glutathione reductase from Schistosoma mansoni: an essential parasite enzyme and a key drug target

BACKGROUND

Schistosomiasis--infection with helminth parasites in the genus Schistosoma, including S. mansoni--is a widespread, devastating tropical disease affecting more than 200 million people. No vaccine is available, and praziquantel, the only drug extensively utilized, is currently administered more than 100 million people yearly. Because praziquantel resistance may develop it is essential to identify novel drug targets. Our goal was to investigate the potential of a unique, selenium-containing parasite enzyme thioredoxin glutathione reductase (TGR) as a drug target.

METHODS AND FINDINGS

Using RNA interference we found that TGR is essential for parasite survival; after silencing of TGR expression, in vitro parasites died within 4 d. We also found that auranofin is an efficient inhibitor of pure TGR (Ki = 10 nM), able to kill parasites rapidly in culture at physiological concentrations (5 microM), and able to partially cure infected mice (worm burden reductions of ~60%). Furthermore, two previously used antischistosomal compounds inhibited TGR activity, suggesting that TGR is a key target during therapy with those compounds.

CONCLUSIONS

Collectively, our results indicate that parasite TGR meets all the major criteria to be a key target for antischistosomal chemotherapy. To our knowledge this is the first validation of a Schistosoma drug target using a convergence of both genetic and biochemical approaches.

Thioamido Coordination in a Thioxo-1,2,4-triazole Copper(II) Complex Enhances Nonapoptotic Programmed Cell Death Associated with Copper Accumulation and Oxidative Stress in Human Cancer Cells

The thioamido function of [CuCl2(1H)]Cl (2) (1=4-amino-1,4-dihydro-3-(2-pyridyl)-5-thioxo-1,2,4-triazole), a cytotoxic copper complex, was converted into thioether moieties, leading to the synthesis of [CuCl2(3)]2 (4) and [CuCl2(5)] (6) (3=6-methyl-3-pyridin-2-yl-7H-[1,2,4]triazolo[3,4-b][1,3,4]thiadiazine; 5=4-amino-5-ethylthio-3-(2-pyridyl)-1,2,4-triazole). These complexes were structurally characterized, and their stability constants, along with their biological activity, were determined. 4 and 6 were slightly less stable and significantly less active than 2. However, as 2, both complexes induced nonapoptotic vacuolar cell death. Copper uptake, investigated in both 2-sensitive and -insensitive cell types, was markedly higher in sensitive cells where it was associated with an increase in oxidized glutathione. These data suggest that the thioamido function enhances the cytotoxicity of copper complexes in cancer cells promoting the accumulation of the metal and its interaction with cell thiols.

Nrf2 regulates an adaptive response protecting against oxidative damage following diquat-mediated formation of superoxide anion

Mouse embryonic fibroblasts derived from Nrf2-/- mice (N0) and Nrf2+/+ mice (WT) have been used to characterize both basal and diquat (DQ)-induced oxidative stress levels and to examine Nrf2 activation during exposure to DQ-generated superoxide anion. Microarray analysis revealed that N0 cells have similar constitutive mRNA expression of genes responsible for the direct metabolism of reactive oxygen species but decreased expression of genes responsible for the production of reducing equivalents, repair of oxidized proteins and defense against lipid peroxidation, compared to WT cells. Nonetheless, the basal levels of ROS flux and oxidative damage biomarkers in WT and N0 cells were not different. Diquat dibromide (DQ), a non-electrophilic redox cycling bipyridylium herbicide, was used to generate intracellular superoxide anion. Isolated mitochondria from both cell lines exposed to DQ produced equivalent amounts of ROS, indicating a similar cellular capacity to generate ROS. However, N0 cells exposed to DQ for 24-h exhibited markedly decreased cell viability and aconitase activity as well as increased lipid peroxidation and glutathione oxidation, relative to WT cells. 2',7'-Dichlorofluorescein fluorescence was not increased in WT and N0 cells after 30-min of DQ exposure. However, increased levels of ROS were detected in N0 cells but not WT cells after 13-h of DQ treatment. Additionally, total glutathione concentrations increased in WT, but not N0 cells following a 24-h exposure to DQ. DQ exposure resulted in activation of an antioxidant response element-luciferase reporter gene, as well as induction of Nrf2-regulated genes in WT, but not N0 cells. Thus the enhanced sensitivity of N0 cells does not reflect basal differences in antioxidative capacity, but rather an impaired ability to mount an adaptive response to sustained oxidative stress.

Oltipraz-induced phase 2 enzyme response conserved in cells lacking mitochondrial DNA

Oltipraz, a member of a class of 1,2-dithiolethiones, is a potent phase 2 enzyme inducing agent used as a cancer chemopreventive. In this study, we investigated regulation of the phase 2 enzyme response and protection against endogenous oxidative stress in lymphoblastic leukemic parental CEM cells and cells lacking mitochondrial DNA (mtDNA) (rho0) by oltipraz. Glutathione (GSH) levels (total and mitochondrial) and glutathione S-transferase (GST) activity were significantly increased after pretreatment with oltipraz in both parental (rho+) and rho0 cells, and both cell lines were resistant to mitochondrial oxidation, loss of mitochondrial membrane potential, and cell death in response to the GSH depleting agent diethylmaleate. These results show that the phase 2 enzyme response, by enhancing GSH-dependent systems involved in xenobiotic metabolism, blocks endogenous oxidative stress and cell death, and that this response is intact in cells lacking mtDNA.