Chemoprotective 3H-1,2-dithiole-3-thione induces antioxidant genes in vivo

3H-1,2-dithiole-3-thione suppresses LPS-induced proinflammatory responses in macrophages: potential involvement of antioxidant induction, NF-κB, and Nrf2

Previously, we reported that 3H-1,2-dithiole-3-thione (D3T), an Nrf2 activator, acted as a potential chemoprotectant against lipopolysaccharide (LPS)-induced mortality in mice. In view of the critical involvement of macrophages in the pathogenesis of LPS-induced endotoxemia, in the present study, we investigated the protective effects of D3T on LPS-induced proinflammatory responses in cultured murine RAW 264.7 macrophage cell line and primary peritoneal macrophages and the potential involvement of antioxidant induction, NF-κB, and Nrf2. We showed that treatment with D3T resulted in increased levels of a series of antioxidants in RAW 264.7 cells in a concentration-dependent manner. These included the reduced form of glutathione, glutathione peroxidase, glutathione reductase, glutathione S-transferase, and NADPH:quinone oxidoreductase 1. Catalase was also potently induced by D3T which, however, did not show a concentration dependency. Concurrent with the ability to induce the above cellular antioxidants, D3T pretreatment of RAW 264.7 cells also led to a concentration-dependent suppression of LPS-induced interleukin-1beta (IL-1β) production and nitric oxide release. LPS-stimulated tumor necrosis factor-alpha (TNF-α) production was also suppressed by D3T, but to a much lesser extent. Using NF-κB reporter gene-expressing RAW 264.7 cells, we further showed that D3T pretreatment also suppressed LPS-induced NF-κB activation. To investigate the potential involvement of Nrf2, a chief regulator of cellular antioxidant genes, we used peritoneal macrophages isolated from Nrf2^+/+ and Nrf2^-/- mice. Our results showed that D3T pretreatment suppressed LPS-induced proinflammatory responses in Nrf2^+/+ macrophages, and this inhibitory effect of D3T was completely lost in Nrf2^-/- macrophages. Collectively, the results of the present study demonstrated that D3T acted as a potent suppressor of LPS-induced proinflammatory responses in macrophages. Antioxidant induction, NF-κB suppression, and Nrf2 activation appeared to contribute to the anti-proinflammatory activity of D3T in macrophages.

NRF2 activation inhibits valproic acid-induced neural tube defects in mice

Valproic acid (VPA) is a widely prescribed medication that has traditionally been used to treat epilepsy, yet embryonic exposure to VPA increases the risk of the fetus developing neural tube defects (NTDs). While the mechanism by which VPA causes NTDs is unknown, we hypothesize that VPA causes dysmorphogenesis through the disruption of redox-sensitive signaling pathways that are critical for proper embryonic development, and that protection from the redox disruption may decrease the prevalence of NTDs. Time-bred CD-1 mice were treated with 3H-1,2-dithiole-3-thione (D3T), an inducer of nuclear factor erythroid 2-related factor 2 (NRF2)-a transcription factor that activates the intracellular antioxidant response to prevent redox disruptions. Embryos were then collected for whole embryo culture and subsequently treated with VPA in vitro. The glutathione (GSH)/glutathione disulfide (GSSG) redox potential (E_h), a measure of the intracellular redox environment, was measured in the developing mouse embryos. Embryos treated with VPA exhibited a transiently oxidizing GSH/GSSG E_h, while those that received D3T pretreatment prior to VPA exposure showed no differences compared to controls. Moving to an in utero mouse model, time-bred C57BL/6 J dams were pretreated with or without D3T and then exposed to VPA, after which all embryos were collected for morphological analyses. The prevalence of open neural tubes in embryos treated with VPA significantly decreased with D3T pretreatment, as did the severity of the observed defects evaluated by a morphological assessment. These data show that NRF2 induction via D3T pretreatment protects against VPA-induced redox dysregulation and decreases the prevalence of NTDs in developing mouse embryos.

Synthesis and Reactivity of 3H-1,2-dithiole-3-thiones

3H-1,2-Dithiole-3-thiones are among the best studied classes of polysulfur-containing heterocycles due to the almost explosive recent interest in these compounds as sources of hydrogen sulfide as an endogenously produced gaseous signaling molecule. This review covers the recent developments in the synthesis of these heterocycles, including both well-known procedures and important novel transformations for building the 1,2-dithiole-3-thione ring. Diverse ring transformations of 3H-1,2-dithiole-3-thiones into various heterocyclic systems through 1,3-dipolar cycloaddition, replacement of one or two sulfur atoms to form carbon- and carbon-nitrogen containing moieties, and other unexpected reactions are considered.

Copper-Catalyzed Tandem Sulfuration/Annulation of Propargylamines with Sulfur via C-N Bond Cleavage

Copper-catalyzed aerobic oxidative sulfuration and annulation of propargylamines with elemental sulfur is described. The tandem reaction involves C-N bond cleavage and the formation of multiple C-S bonds, affording 1,2-dithiole-3-thiones in good to excellent yields with good functional group tolerance.

Switching Selectivity of α-Enolic Dithioesters: One Pot Access to Functionalized 1,2- and 1,3-Dithioles

An operationally simple cascade protocol has been developed for the construction of 1,2- and 1,3-dithiole derivatives from α-enolic dithioesters. 1,2-Dithioles are achieved by the reaction of dithioesters with elemental sulfur in the presence of InCl₃ under solvent-free conditions. 1,3-Dithioles have been constructed via DABCO mediated self-coupling of dithioesters in open air enabling the formation of two new C-S bonds and one ring in a single operation in contiguous fashion. The reactions proceeded smoothly affording the desired sulfur-rich heterocycles in good to excellent yields, exhibiting gram-scale ability and broad functional group tolerance utilizing easy to handle cheap and easily available reagents. The probable mechanisms for the formation of 1,2- and 1,3-dithioles from α-enolic dithioesters have been suggested.

Advances in the development of novel antioxidant therapies as an approach for fetal alcohol syndrome prevention

Ethanol is the most common human teratogen, and its consumption during pregnancy can produce a wide range of abnormalities in infants known as fetal alcohol spectrum disorder (FASD). The major characteristics of FASD can be divided into: (i) growth retardation, (ii) craniofacial abnormalities, and (iii) central nervous system (CNS) dysfunction. FASD is the most common cause of nongenetic mental retardation in Western countries. Although the underlying molecular mechanisms of ethanol neurotoxicity are not completely determined, the induction of oxidative stress is believed to be one central process linked to the development of the disease. Currently, there is no known effective strategy for prevention (other than alcohol avoidance) or treatment. In the present review we will provide the state of art in the evidence for the use of antioxidants as a potential therapeutic strategy for the treatment using whole-embryo and culture cells models of FASD. We conclude that the imbalance of the intracellular redox state contributes to the pathogenesis observed in FASD models, and we suggest that antioxidant therapy can be considered a new efficient strategy to mitigate the effects of prenatal ethanol exposure.

Up-regulation of human prostaglandin reductase 1 improves the efficacy of hydroxymethylacylfulvene, an antitumor chemotherapeutic agent

Prostaglandin reductase 1 (PTGR1) is a highly inducible enzyme with enone reductase activity. Previous studies demonstrated the role of rat PTGR1 in the activation of acylfulvene analogs, a class of antitumor natural product derivatives. Of these, hydroxymethylacylfulvene (HMAF) was in advanced clinical development for the treatment of advanced solid tumors, including prostate, ovarian, and pancreatic cancers. However, the efficiency of human PTGR1 in activating acylfulvenes and its potential to enhance therapeutic efficacy have remained uncharacterized. In this study, human PTGR1 was polymerase chain reaction-cloned and purified. Conversion of HMAF to its cellular metabolite by the purified enzyme proceeded at a 20-fold higher rate than with the rat variant of the enzyme. The Km was 4.9 μM, which was 40-fold lower than for the rat variant and similar to the therapeutic dose. Human cell lines, including colon cancer lines, were transfected with a vector containing rat PTGR1 or human PTGR1, and cell viability was examined after dosing with HMAF. New data obtained in this study suggest that transfection with human PTGR1, or its induction in colon and liver cancer cell lines with 1,2-dithiol-3-thione, enhances susceptibility to the cytotoxic influences of HMAF by 2- to 10-fold. Furthermore, similar or enhanced enzyme induction and HMAF toxicity results from preconditioning cancer cells with the bioactive food components curcumin and resveratrol. The functional impact of PTGR1 induction in human cells and chemical-based strategies for its activation can provide important knowledge for the design of clinical strategies involving reductively activated cytotoxic chemotherapeutics.

The antioxidant 3H-1,2-dithiole-3-thione potentiates advanced glycation end-product-induced oxidative stress in SH-SY5Y cells

Oxidative stress is implicated as a major factor in the development of diabetes complications and is caused in part by advanced glycation end products (AGEs). AGEs ligate to the receptor for AGEs (RAGE), promoting protein kinase C (PKC)-dependent activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and superoxide radical generation. While scavenging antioxidants are protective against AGEs, it is unknown if induction of endogenous antioxidant defenses has the same effect. In this study, we confirmed that the compound 3H-1,2-dithiole-3-thione (D3T) increases reduced-state glutathione (GSH) concentrations and NADPH:quinone oxidoreductase 1 (NQO1) activity in SH-SY5Y cells and provides protection against H₂O₂. Surprisingly, D3T potentiated oxidative damage caused by AGEs. In comparison to vehicle controls, D3T caused greater AGE-induced cytotoxicity and depletion of intracellular GSH levels while offering no protection against neurite degeneration or protein carbonylation. D3T potentiated AGE-induced reactive oxygen species (ROS) formation, an effect abrogated by inhibitors of PKC and NADPH oxidase. This study suggests that chemical induction of endogenous antioxidant defenses requires further examination in models of diabetes.

Effects of potential dietary inhibitors of endogenous DNA damage on mutagenesis and lipid peroxidation in lacZ mice

The effects of a nine month administration of dietary: (1) 3H-1,2-dithiole-3-thione (D3T), (2) N-acetylcysteine (NAC), (3) antioxidant vitamin mix, (vitamin C+E), (4) free radical scavenger, amifostine, and (5) calorie restriction, (CR), on mutagenesis and lipid peroxidation in lung, kidney, spleen and liver of lacZ transgenic mice were examined. These agents/diets were chosen because they might inhibit certain proposed mechanisms of endogenous damage to DNA. The agents were added to a high fat, reduced antioxidant AIN-76 diet, to better approximate a Western style diet than the conventional AIN-76 diet. As the lacZ gene is not expressed, mutations in that gene are neutral, and simply accumulate over time. The mutant fractions in control mice increased about 50-100%. Most of the agents inhibited to various extents the age-related increase in mutagenesis in lung, kidney, and/or spleen, but no inhibition was observed in liver. There was no significant effect of age on lipid peroxidation levels in controls, possibly reflecting steady state turnover of lipid peroxidation products. Almost all of the treatments except D3T inhibited lipid peroxidation in most organs to different degrees. The vitamin C+E mix was the most effective at inhibiting lipid peroxidation, but a single most effective inhibitor of mutagenesis could not be discerned. Some associations were observed between the reduction in lipid peroxidation and the inhibition of mutagenesis. The results are consistent with a partial role for oxidative stress in the age-related increase in mutagenesis. These observations may have implications for chemoprevention of carcinogenesis.

Stabilization of Nrf2 protein by D3T provides protection against ethanol-induced apoptosis in PC12 cells

Previous studies have demonstrated that maternal ethanol exposure induces a moderate increase in Nrf2 protein expression in mouse embryos. Pretreatment with the Nrf2 inducer, 3H-1, 2-dithiole-3-thione (D3T), significantly increases the Nrf2 protein levels and prevents apoptosis in ethanol-exposed embryos. The present study, using PC12 cells, was designed to determine whether increased Nrf2 stability is a mechanism by which D3T enhances Nrf2 activation and subsequent antioxidant protection. Ethanol and D3T treatment resulted in a significant accumulation of Nrf2 protein in PC 12 cells. CHX chase analysis has shown that ethanol treatment delayed the degradation of Nrf2 protein in PC12 cells. A significantly greater decrease in Nrf2 protein degradation was observed in the cells treated with D3T alone or with both ethanol and D3T. In addition, D3T treatment significantly reduced ethanol-induced apoptosis. These results demonstrate that the stabilization of Nrf2 protein by D3T confers protection against ethanol-induced apoptosis.

Upregulation of endogenous glutathione system by 3H-1,2-dithiole-3-thione in pancreatic RINm5F beta-cells as a novel strategy for protecting against oxidative beta-cell injury

This study was undertaken to investigate the inducibility of glutathione (GSH), glutathione reductase (GR) and glutathione peroxidase (GPx) by 3H-1,2-dithiole-3-thione (D3T) in beta-cells, and the resultant cytoprotection against oxidant injury. Incubation of the insulin-secreting RINm5F cells with D3T led to significant induction of GSH, GR and GPx. D3T-mediated induction of GSH was abolished by buthionine sulfoximine (BSO), suggesting a critical involvement of gamma-glutamylcysteine ligase (gammaGCL). Consistently, incubation of RINm5F cells with D3T resulted in increased expression of gammaGCL protein and mRNA. Pretreatment of RINm5F cells with D3T provided remarkable protection against oxidant-elicited cytotoxicity. On the other hand, depletion of cellular GSH by BSO sensitized RINm5F cells to oxidant injury. Furthermore, cotreatment of RINm5F cells with BSO to reverse D3T-mediated GSH induction abolished the cytoprotective effects of D3T on oxidant injury. Taken together, this study demonstrates that upregulation of glutathione system by D3T is effective for protecting against oxidative beta-cell injury.

Nrf2-mediated transcriptional induction of antioxidant response in mouse embryos exposed to ethanol in vivo: implications for the prevention of fetal alcohol spectrum disorders

Nuclear factor erythroid 2-related factor 2 (Nrf2) is a transcription factor that is important in protection against oxidative stress. This study was designed to determine the role of Nrf2 signaling in transcriptional activation of detoxifying and antioxidant genes in an in vivo mouse fetal alcohol syndrome model. Maternal ethanol treatment was found to increase both Nrf2 protein levels and Nrf2-ARE binding in mouse embryos. It also resulted in a moderate increase in the mRNA expression of Nrf2 downstream target detoxifying and antioxidant genes as well as an increase in the expression of antioxidant proteins. Pretreatment with the Nrf2 inducer, 3H-1,2 dithiole-3-thione (D3T), significantly increased Nrf2 protein levels and Nrf2-ARE binding, and strongly induced the mRNA expression of Nrf2 downstream target genes. It also increased the expression of antioxidant proteins and the activities of the antioxidant enzymes. Additionally, D3T pretreatment resulted in a significant decrease in ethanol-induced reactive oxygen species generation and apoptosis in mouse embryos. These results demonstrate that Nrf2 signaling is involved in the induction of antioxidant response in ethanol-exposed embryos. In addition, the potency of D3T in inducing antioxidants as well as in diminishing ethanol-induced apoptosis suggests that further exploration of the antiteratogenic effect of this compound will be fruitful.

Oltipraz chemoprevention trial in Qidong, People's Republic of China: unaltered oxidative biomarkers

Aflatoxin, which leads to formation of carcinogen-DNA adducts as well as oxidized DNA, is a well-known risk factor for development of hepatocellular carcinoma. The aim of the present study was to investigate if the chemopreventive agent oltipraz had an effect on DNA oxidation measured as oxidized guanine derivatives in urine among healthy individuals living in a region of China at high risk of exposure to aflatoxin and development of hepatocellular carcinoma. Two hundred thirty-three healthy residents of Qidong, PRC, were randomized to 8 weeks treatment with placebo, oltipraz 125 mg daily, or oltipraz 500 mg weekly, with a subsequent 8-week follow-up period. Urine samples were collected as overnight voids. Samples collected 4 weeks into the treatment period and 6 weeks into the follow-up period were analyzed for oxidized guanine derivatives with a HPLC-MS/MS method. A repeated-measures analysis of variance showed no significant differences between the randomization groups regarding changes in oxidized guanine derivatives. In the present double-blind, randomized, placebo-controlled trial performed among healthy individuals, oltipraz had no major effect on oxidative DNA damage. Mechanisms other than prevention of oxidative DNA damage may be of higher importance when oltipraz is used as a chemopreventive agent in humans.

Detoxication enzyme inducers modify cytokine production in rat mixed glial cells

Pro-inflammatory cytokines, e.g. interleukin-1beta (IL-1beta), interleukin-6 (IL-6), tumor necrosis factor-alpha (TNFalpha) as well as neurotoxic molecules such as nitric oxide (NO), that are produced and released by activated glial cells, play an important role in inflammation and oxidative stress occurring during Multiple Sclerosis (MS). Reduction of these processes could therefore be of therapeutic interest. Dimethylfumarate (DMF) and sulforaphane (SP) are well known for their detoxicating properties. Furthermore, they have anti-inflammatory effects as shown clinically by the treatment of inflammatory skin diseases. However, their detoxication and anti-inflammatory action on brain-derived cells is unknown. In the present study we have studied, within the same concentration range, the anti-inflammatory and detoxicating effects of DMF and SP on the production and release of mediators of inflammation and detoxication from lipopolysaccharide (LPS) activated primary co-cultures of rat microglial and astroglial cells. DMF and SP attenuated the LPS-induced production and release of TNFalpha, IL-1beta, IL-6 and NO. In addition, DMF and SP increase both mRNA level and activity of NAD(P)H:quinone reductase (NQO-1), a detoxication enzyme, as well as the cellular glutathione content. We conclude that DMF or SP simultaneously can (1) reduce mediators of inflammation and (2) enhance detoxication enzymes in LPS stimulated co-cultures of astroglial and microglial cells. This double-sided effect could potentially be of therapeutic interest.

Cancer chemopreventive oltipraz generates superoxide anion radical

The cancer chemopreventive actions of oltipraz, a member of a class of 1,2-dithiolethiones, have been primarily associated with the induction of phase 2 enzymes mediated by a 41bp enhancer element known as the anti-oxidant response element in the promoter regions of many phase 2 genes. It has been suggested that oxygen radical formation by oltipraz may be a critical mechanism by which it exerts chemoprevention. Therefore, in the present work, studies were performed to directly determine if oltipraz generates oxygen free radicals. Electron paramagnetic resonance (EPR) spin trapping demonstrated that oltipraz slowly reacts in the presence of oxygen to generate the superoxide anion radical. This formation of superoxide by oltipraz was concentration- and time-dependent. EPR oximetry studies showed that oxygen was also slowly consumed paralleling the process of superoxide formation. Thus, oltipraz induced superoxide formation occurs and could be involved in the mechanism by which it exerts chemoprotection.

The chemical inducibility of mouse cardiac antioxidants and phase 2 enzymes in vivo

The recognition of the critical involvement of oxidative and electrophilic stress in cardiac disorders has led to extensive investigation of the protective effects of exogenous antioxidants on cardiac injury. On the other hand, another strategy for protecting against oxidative/electrophilic cardiac injury may be through induction of the endogenous antioxidants and phase 2 enzymes in myocardium by chemical inducers. However, our understanding of the chemical inducibility of cardiac antioxidants/phase 2 enzymes in vivo is very limited. In addition, careful studies on the basal levels of a scope of endogenous antioxidants/phase 2 enzymes in myocardium as compared with other tissues, such as liver, are lacking. Accordingly, this study was undertaken to determine the basal levels of endogenous antioxidants/phase 2 enzymes, including superoxide dismutase (SOD), catalase, reduced glutathione (GSH), GSH peroxidase (GPx), glutathione reductase (GR), GSH S-transferase (GST), and NAD(P)H:quinone oxidoreductase 1 (NQO1), and investigate the inducibility of the above antioxidants/phase 2 enzymes by the chemoprotectant, 1,2-dithiole-3-thione (D3T), in cardiac as well as hepatic tissues in C57BL/6 mice. Our results demonstrated that in C57BL/6 mice, the levels of catalase, GSH, GPx, GR, and GST were significantly lower in cardiac tissue than in hepatic tissue. The level of total SOD did not differ significantly between mouse heart and liver. Notably, heart contained a much higher NQO1 activity than liver. Immunoblotting and RT-PCR analyses further demonstrated the high expression of NQO1 protein and mRNA in myocardium. Oral administration of D3T at 0.25 and 0.5 mmol/kg body weight for 3 consecutive days resulted in a significant induction of cardiac SOD, catalase, GR, GST, and NQO1. No significant induction of cardiac GSH and GPx was observed with the above D3T treatment. Only GR, GST, and NQO1 in mouse liver were induced by the D3T treatment. Unexpectedly, we observed a significant D3T dose-dependent decrease in hepatic GPx activity. Taken together, this study demonstrates for the first time that: (1) the expression of NQO1 is remarkably high in mouse myocardium though other cardiac antioxidants/phase 2 enzymes are relatively lower as compared with liver; (2) a number of endogenous antioxidants/phase 2 enzymes in mouse cardiac tissue can be significantly induced by D3T following oral administration; and (3) the inducibility of endogenous antioxidants/phase 2 enzymes by D3T differs between mouse cardiac and hepatic tissues. This study provides a basis for future investigation of the cardioprotection of chemically induced endogenous antioxidants and phase 2 enzymes in myocardium in animal models of oxidative/electrophilic cardiac disorders.

Effects of 3H-1,2-dithiole-3-thione, 1,4-phenylenebis(methylene)selenocyanate, and selenium-enriched yeast individually and in combination on benzo[a]pyrene-induced mutagenesis in oral tissue and esophagus in lacZ mice

We have studied the effects of three chemopreventive agents alone or in binary combinations on benzo[a]pyrene (BaP)-induced mutagenesis in the oral cavity and esophagus of lacZ mice using galE(-) selection. The mice were fed diets supplemented with 1,4-phenylenebis(methylene)selenocyanate (p-XSC) at 2.5 and 10 ppm Se, selenium-enriched yeast (SeY) at 2.5 and 10 ppm Se, and 3H-1,2-dithiole-3-thione (D3T) at 65 and 250 ppm, for 6 weeks. Two weeks after the start of the dietary regimen, mice were gavaged with five doses of 125 mg/kg BaP over 2 weeks, and the experiment was terminated 2 weeks later. Mutagenesis was measured in tongue, other pooled oral tissues (OTs), and esophagus. In mice treated with BaP alone, mutagenesis in the above tissues was in the range of 21-32 mutants/10(5)pfu (ca. 6-10 background levels for the corresponding tissues). p-XSC modestly inhibited mutagenesis (10-33% inhibition) in all tissues, but statistical significance was only observed at the low dose in esophagus, and pooled OT. SeY was not inhibitory alone. Greater inhibitory effects were observed with D3T, and inhibition was statistically significant at the high dose in tongue and esophagus (ca. 33%). Two combinations of low doses of the inhibitors were tested, and the D3T + SeY mix was most effective, leading to statistically significant inhibition in all three tissues (ca. 30-40% inhibition). The mixture D3T + p-XSC was of similar effectiveness as the low dose of D3T alone. This study combined with those previously done in our laboratory demonstrates effectiveness of D3T and to a lesser extent, p-XSC in the inhibition of mutagenesis, and provides support for the use of certain combinations of inhibitors as a means to increase effectiveness and reduce the dose of chemopreventive agents.

Oxathiolene oxides: a novel family of compounds that induce ferritin, glutathione S-transferase, and other proteins of the phase II response

Compounds that induce the synthesis of cytoprotective phase II enzymes have shown promise as cancer chemopreventive agents. Although chemically diverse, phase II enzyme inducers are capable of participating in Michael reaction chemistry. We have synthesized a novel class of organosulfur compounds, termed oxathiolene oxides (OTEOs). Based on their chemical properties, we hypothesized that these compounds could function as phase II enzyme inducers. Northern blot analysis showed that oxathiolene oxides induce the phase II enzymes glutathione S-transferase (GST), NAD(P)H:quinone oxidoreductase 1 (NQO1), and ferritin H and L mRNA in a concentration-dependent fashion in a normal embryonic mouse liver cell line, BNLCL.2. OTEO-562 (3-cyclohexenyl-4-methyl-1,2-oxathiol-3-ene-2-oxide) was the strongest inducer. Western blot analysis demonstrated that GST-alpha and ferritin H protein levels were also induced in cells treated with OTEO-562, as was total GST and NQO1 enzyme activity. Further, induction of NQO1 activity by OTEO-562 was equivalent in aromatic hydrocarbon (Ah) receptor wild-type and Ah receptor mutant cell lines, suggesting that oxathiolene oxides activate phase II enzymes by an Ah receptor-independent mechanism. Consistent with this observation, OTEO-562 failed to induce cytochrome P450 1A1 mRNA. These results suggest that oxathiolene oxides may merit further investigation as candidate chemopreventive agents.

Antioxidant enzyme expression in health and disease: effects of exercise and hypertension

Antioxidant enzymes (superoxide dismutases, catalase and glutathione peroxidase) are components of an organism's mechanisms for combating oxidative stress which is generated in normal metabolism and which may also be a reaction in response to external stimuli. This review identifies the general significance of antioxidant enzymes in health and disease, and some of the diseases that are now believed to have oxidative stress as a component. A discussion is then presented of the molecular mechanisms by which antioxidant enzyme expression is controlled at the transcriptional and post-transcriptional levels. The final sections of the review highlight the effects of exercise and hypertension on antioxidant enzyme expression in a number of different tissues, and the possibilities for future studies in these areas are discussed.

Induction of cellular glutathione-linked enzymes and catalase by the unique chemoprotective agent, 3H-1,2-dithiole-3-thione in rat cardiomyocytes affords protection against oxidative cell injury

Considerable evidence suggests that reactive oxygen species (ROS) are crucially involved in the pathogenesis of cardiovascular diseases, such as myocardial ischemia-reperfusion injury. Consistent with this notion, administration of exogenous antioxidative compounds has been shown to provide protection against oxidative cardiac injury. However, whether induction of endogenous cellular antioxidants by chemicals (drugs) also offers protection against oxidative cardiac injury has not been extensively investigated. In the present study, with rat cardiomyocyte H9C2 cells as an in vitro model, we have investigated the induction of cellular antioxidants by the unique chemoprotective agent, 3 H -1,2-dithiole-3-thione (D3T) and the protective effects of the D3T-induced cellular antioxidants against ROS-mediated injury in cardiac cells. Incubation of H9C2 cells with micromolar concentrations of D3T for 24 h resulted in a significant induction of a battery of cellular antioxidants, including reduced glutathione (GSH), GSH peroxidase, GSSG reductase, GSH S-transferase and catalase. To further examine the protective effects of the induced endogenous antioxidants against oxidative cell injury, H9C2 cells were pre-treated with D3T and then incubated with xanthine oxidase (XO) plus xanthine, a system that generates ROS. We observed that D3T pre-treatment of H9C2 cells led to significant protection against XO/xanthine-induced cytotoxicity as determined by 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) reduction and morphological changes. Taken together, this study demonstrates for the first time that a number of endogenous antioxidants in cardiomyocytes can be induced by exposure to D3T, and that this chemical (drug) induction of cellular antioxidants is accompanied by markedly increased resistance to ROS-mediated cardiac cell injury.

Chemical induction of cellular antioxidants affords marked protection against oxidative injury in vascular smooth muscle cells

Extensive evidence suggests that reactive oxygen species are critically involved in the pathogenesis of cardiovascular diseases, such as atherosclerosis and myocardial ischemia-reperfusion injury. Consistent with this concept, administration of exogenous antioxidants has been shown to be protective against oxidative cardiovascular injury. However, whether induction of endogenous antioxidants by chemical inducers in vasculature also affords protection against oxidative vascular cell injury has not been extensively investigated. In this study, using rat aortic smooth muscle A10 cells as an in vitro system, we have studied the induction of cellular antioxidants by the unique chemoprotector, 3H-1,2-dithiole-3-thione [corrected] (D3T) and the protective effects of the D3T-induced cellular antioxidants against oxidative cell injury. Incubation of A10 cells with micromolar concentrations of D3T for 24 h resulted in a significant induction of a battery of cellular antioxidants in a concentration-dependent manner. These included reduced glutathione (GSH), GSH peroxidase, GSSG reductase, GSH S-transferase, superoxide dismutase, and catalase. To further examine the protective effects of the induced endogenous antioxidants against oxidative cell injury, A10 cells were pretreated with D3T and then exposed to either xanthine oxidase (XO)/xanthine, 4-hydroxynonenal, or cadmium. We observed that D3T pretreatment of A10 cells led to significant protection against the cytotoxicity induced by XO/xanthine, 4-hydroxynonenal or cadmium, as determined by 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium reduction assay. Taken together, this study demonstrates for the first time that a number of endogenous antioxidants in vascular smooth muscle cells can be induced by exposure to D3T, and that this chemical induction of cellular antioxidants is accompanied by markedly increased resistance to oxidative vascular cell injury.

Neuroprotection for Parkinson's disease: a new approach for a new millennium

Parkinson's disease (PD) is the only neurodegenerative disorder in which pharmacological intervention has resulted in a marked decrease in morbidity and a significant delay in mortality. However, the medium to long-term efficacy of this pharmacotherapy, mainly consisting of dopaminomimetics like L -dopa and dopamine receptor agonists, suffers greatly from the unrelenting progression of the disease process underlying PD, i.e., the degeneration of neuromelanin-containing, dopaminergic neurones in the substantia nigra. Efforts concentrated on understanding the mechanisms of dopaminergic cell death in Parkinson's disease have led to identification of a large variety of pathogenetic factors, including excessive release of oxygen free radicals during enzymatic dopamine breakdown, impairment of mitochondrial function, production of inflammatory mediators, loss of trophic support, and apoptosis. Therapeutic approaches aimed at correcting these abnormalities are currently being evaluated on their efficacy as neuroprotectants for PD. Here, we focus on the process of dopamine auto-oxidation, the chain of reactions leading to the formation of neuromelanin, as an often overlooked, yet obvious pathogenetic factor. In particular, we discuss the option of drug-mediated stimulation of endogenous mechanisms responsible for the detoxification of dopamine auto-oxidation products as a novel means of neuroprotection in Parkinson's disease.

Role of phase 2 enzyme induction in chemoprotection by dithiolethiones

One of the major mechanisms of protection against carcinogenesis, mutagenesis, and other forms of toxicity mediated by carcinogens is the induction of enzymes involved in their metabolism, particularly phase 2 enzymes such as glutathione S-transferases (GSTs), UDP-glucuronosyl transferases, and quinone reductases. Animal studies indicate that induction of phase 2 enzymes is a sufficient condition for obtaining chemoprevention and can be achieved by administering any of a diverse array of naturally-occurring and synthetic chemopreventive agents. Indeed, monitoring of enzyme induction has led to the recognition or isolation of novel, potent chemopreventive agents such as 1,2-dithiole-3-thiones, terpenoids and the isothiocyanate sulforaphane. For example, oltipraz, a substituted 1,2-dithiole-3-thione originally developed as an antischistosomal agent, possesses chemopreventive activity against different classes of carcinogens targeting multiple organs. Mechanistic studies in rodent models for chemoprevention of aflatoxin B(1) (AFB(1))-induced hepatocarcinogenesis by oltipraz indicates that increased expression of phase 2 genes is of central importance, although inhibition of phase 1 activation of AFB(1) can also contribute to protection. Exposure of rodents to 1,2-dithiole-3-thiones triggers nuclear accumulation of the transcription factor Nrf2 and its enhanced binding to the "antioxidant response element" (ARE), leading to transcriptional activation of a score of genes involved in carcinogen detoxication and attenuation of oxidative stress. Nrf2-deficient mice fail to induce many of these genes in response to dithiolethiones; moreover, basal expression of these genes is typically repressed. To test the hypothesis that enzyme induction is a useful strategy for chemoprevention in humans, three key elements are necessary: a candidate agent, an at-risk population and modulatable intermediate endpoints. Towards this end, a placebo-controlled, double blind clinical trial of oltipraz was conducted in residents of Qidong, PR China who are exposed to dietary aflatoxins and who are at high risk for the development of liver cancer. Oltipraz significantly enhanced excretion of a phase 2 product, aflatoxin-mercapturic acid, a derivative of the aflatoxin-glutathione conjugate, in the urine of study participants administered 125 mg oltipraz by mouth daily. Administration of 500 mg oltipraz once a week led to a significant reduction in the excretion of the primary oxidative metabolite of AFB(1), AFM(1), when measured shortly after drug administration. While this study highlighted the general feasibility of inducing phase 2 enzymes in humans, a longer term intervention is addressing whether protective alterations in aflatoxin metabolism can be sustained for extended periods of time in this high-risk population.

Potency of Michael reaction acceptors as inducers of enzymes that protect against carcinogenesis depends on their reactivity with sulfhydryl groups

Induction of phase 2 enzymes and elevations of glutathione are major and sufficient strategies for protecting mammals and their cells against the toxic and carcinogenic effects of electrophiles and reactive forms of oxygen. Inducers belong to nine chemical classes and have few common properties except for their ability to modify sulfhydryl groups by oxidation, reduction, or alkylation. Much evidence suggests that the cellular "sensor" molecule that recognizes the inducers and signals the enhanced transcription of phase 2 genes does so by virtue of unique and highly reactive sulfhydryl functions that recognize and covalently react with the inducers. Benzylidene-alkanones and -cycloalkanones are Michael reaction acceptors whose inducer potency is profoundly increased by the presence of ortho- (but not other) hydroxyl substituent(s) on the aromatic ring(s). This enhancement correlates with more rapid reactivity of the ortho-hydroxylated derivatives with model sulfhydryl compounds. Proton NMR spectroscopy provides no evidence for increased electrophilicity of the beta-vinyl carbons (the presumed site of nucleophilic attack) on the hydroxylated inducers. Surprisingly, these ortho-hydroxyl groups display a propensity for extensive intermolecular hydrogen bond formation, which may raise the reactivity and facilitate addition of mercaptans, thereby raising inducer potencies.

Chemoprotection by organosulfur inducers of phase 2 enzymes: dithiolethiones and dithiins

One of the major mechanisms of protection against carcinogenesis, mutagenesis, and other forms of toxicity mediated by carcinogens is the induction of enzymes involved in their metabolism, particularly phase 2 enzymes such as glutathione S-transferases, UDP-glucuronosyl transferases, and quinone reductases. Animal studies indicate that induction of phase 2 enzymes is a sufficient condition for obtaining chemoprevention and can be achieved by administering any of a diverse array of naturally-occurring and synthetic chemopreventive agents. Alliaceous and cruciferous plants are rich in organosulfur compounds with inducer activity. Indeed, monitoring of enzyme induction has led to the recognition or isolation of novel, potent chemopreventive agents such as 1,2-dithiole-3-thiones, dithiins and the isothiocyanate sulforaphane. For example, oltipraz, a substituted 1,2-dithiole-3-thione originally developed as an antischistosomal agent, possesses chemopreventive activity against different classes of carcinogens targeting multiple organs. Mechanistic studies in rodent models for chemoprevention of aflatoxin B1 (AFB1)-induced hepatocarcinogenesis by oltipraz indicates that increased expression of phase 2 genes is of central importance, although inhibition of phase 1 activation of aflatoxin B1 can also contribute to protection. Exposure of rodents to 1,2-dithiole-3-thiones triggers nuclear accumulation of the transcription factor Nrf2 and its enhanced binding to the Antioxidant Response Element, leading to transcriptional activation of a score of genes involved in carcinogen detoxification and attenuation of oxidative stress. Nrf2-deficient mice fail to induce many of these genes in response to oltipraz and the impact of this genotype on the chemopreventive efficacy of dithiolethiones is currently under investigation. To test the hypothesis that enzyme induction is a useful strategy for chemoprevention in humans, three key elements are necessary: a candidate agent, an at-risk population and modulatable intermediate endpoints. Towards this end, a placebo-controlled, double blind clinical trial of oltipraz was conducted in residents of Qidong, P.R. China who are exposed to dietary aflatoxins and who are at high risk for the development of liver cancer. Oltipraz significantly enhanced excretion of a phase 2 product, aflatoxin-mercapturic acid, a derivative of the aflatoxin-glutathione conjugate, in the urine of study participants administered 125 mg oltipraz by mouth daily. Administration of 500 mg oltipraz once a week led to a significant reduction in the excretion of the primary oxidative metabolite of AFB1, aflatoxin M1, when measured shortly after drug administration. While this study highlighted the general feasibility of inducing phase 2 enzymes in humans, a longer term intervention is addressing whether protective alterations in aflatoxin metabolism can be sustained for extended periods of time in this high-risk population. Food-based approaches to chemoprotection, targeted both to the general population and high-risk individuals, offer many practical advantages compared to the use of pharmaceutical agents. Thus, identification and utilization of naturally-occurring organosulfur chemoprotectors including dithiins should be a high priority.