Mechanism of arylating quinone toxicity involving Michael adduct formation and induction of endoplasmic reticulum stress

A novel hydroxyfuroic acid compound as an insulin receptor activator. Structure and activity relationship of a prenylindole moiety to insulin receptor activation

Background

Diabetes Mellitus is a chronic disease and many patients of which require frequent subcutaneous insulin injection to maintain proper blood glucose levels. Due to the inconvenience of insulin administration, an orally active insulin replacement has long been a prime target for many pharmaceutical companies. Demethylasterriquinone (DMAQ) B1, extracted from tropical fungus, Pseudomassaria sp., has been reported to be an orally effective agent at lowering circulating glucose levels in diabetic (db/db) mice; however, the cytotoxicity associated with the quinone moiety has not been addressed thus far.

Methods

A series of hydroxyfuroic acid compounds were synthesized and tested for their efficacies at activating human insulin receptor. Cytotoxicity to Chinese hamster ovary cells, selectivities over insulin-like growth factor-1 (IGF-1), epidermal growth factor (EGF), and fibroblast growth factor (FGF) receptors were examined in this study.

Result and Conclusion

This study reports a new non-quinone DMAQ B1 derivative, a hydroxyfuroic acid compound (D-410639), which is 128 fold less cytotoxic as DMAQ B1 and as potent as compound 2, a DMAQ B1 synthetic derivative from Merck, at activating human insulin receptor. D-410639 has little activation potential on IGF-1 receptor but is a moderate inhibitor to EGF receptor. Structure and activity relationship of the prenylindole moiety to insulin receptor activation is discussed.

Dequalinium, a new inhibitor of Mycobacterium tuberculosis mycothiol ligase identified by high-throughput screening

Mycothiol ligase (MshC) is a key enzyme in the biosynthesis of mycothiol, a small molecular weight thiol that is unique to actinomycetes and whose primary role is to maintain intracellular redox balance and remove toxins. MshC catalyzes the adenosine triphosphate (ATP)-dependent condensation of cysteine and glucosamine-inositol (GI) to produce cysteine-glucosamine-inositol (CGI). MshC is essential to Mycobacterium tuberculosis and therefore represents an attractive target for chemotherapeutic intervention. A screening protocol was developed to identify MshC inhibitors based on quantification of residual ATP using a coupled luminescent assay. The protocol was used to screen a library of 3100 compounds in a 384-well plate format (Z'>or=0.65). Fifteen hits (0.48%) were identified from the screen, and 2 hits were confirmed in a secondary assay that measures production of CGI. The structures of both hits contain N-substituted quinolinium moieties, and the more potent of the 2-namely, dequalinium chloride-inhibits MshC with an IC50 value of 24+/-1 microM. Further studies showed dequalinium to be an ATP-competitive inhibitor of MshC, to bind MshC with a KD of 0.22 microM, and to inhibit the growth of M. tuberculosis under aerobic and anaerobic conditions with minimum inhibitory and anaerobic bactericidal concentrations of 1.2 and 0.3 microg/mL, respectively. The screening protocol described is robust and has enabled the identification of new MshC inhibitors.

Increased age reduces DAF-16 and SKN-1 signaling and the hormetic response of Caenorhabditis elegans to the xenobiotic juglone

Cells adapt to stressors by activating mechanisms that repair damage and protect them from further injury. Stress-induced damage accumulates with age and contributes to age associated diseases. Increased age attenuates the ability to mount a stress response, but little is known about the mechanisms by which this occurs. To begin addressing this problem, we studied hormesis in the nematode Caenorhabditis elegans. When exposed to a low concentration of the xenobiotic juglone, young worms mount a robust hormetic stress response and survive a subsequent exposure to a higher concentration of juglone that is normally lethal to naïve animals. Old worms are unable to mount this adaptive response. Microarray and RNAi analyses demonstrate that an altered transcriptional response to juglone is responsible in part for the reduced adaptation of old worms. Many genes differentially regulated in young versus old animals are known or postulated to be regulated by the FOXO homologue DAF-16 and the Nrf2 homologue SKN-1. Activation of these pathways is greatly reduced in juglone stressed old worms. DAF-16- and SKN-1-like transcription factors play highly conserved roles in regulating stress resistance and longevity genes. Our studies provide a foundation for developing a molecular understanding of how age affects cytoprotective transcriptional pathways.

The WD40 repeat protein WDR-23 functions with the CUL4/DDB1 ubiquitin ligase to regulate nuclear abundance and activity of SKN-1 in Caenorhabditis elegans

The transcription factor SKN-1 protects Caenorhabditis elegans from stress and promotes longevity. SKN-1 is regulated by diverse signals that control metabolism, development, and stress responses, but the mechanisms of regulation and signal integration are unknown. We screened the C. elegans genome for regulators of cytoprotective gene expression and identified a new SKN-1 regulatory pathway. SKN-1 protein levels, nuclear accumulation, and activity are repressed by the WD40 repeat protein WDR-23, which interacts with the CUL-4/DDB-1 ubiquitin ligase to presumably target the transcription factor for proteasomal degradation. WDR-23 regulates SKN-1 target genes downstream from p38 mitogen-activated protein kinase, glycogen synthase kinase 3, and insulin-like receptor pathways, suggesting that phosphorylation of SKN-1 may function to modify its interaction with WDR-23 and/or CUL-4/DDB-1. These findings define the mechanism of SKN-1 accumulation in the cell nucleus and provide a new mechanistic framework for understanding how phosphorylation signals are integrated to regulate stress resistance and longevity.

Vitamin E: the shrew waiting to be tamed

Vitamin E is the last of all vitamins whose essentiality is not yet understood. Its widely accepted role as a lipophilic antioxidant has been questioned, since proof of its in vivo relevance remained scarce. The influence of vitamin E on biomarkers of oxidative stress in vivo is inconsistent and metabolites of vitamin E having reacted as an antioxidant are hardly detectable. Novel functions of vitamin E include the regulation of enzymes, most of which are membrane bound or activated by membrane recruitment. Also, expression of genes responds to vitamin E. The search for a transcription factor common to all regulated genes failed so far and a receptor that specifically binds vitamin E has not yet been identified. According to microarray data, pathways preferentially affected by the vitamin E status are the inflammatory response and cellular traffic. A role of vitamin E in cellular trafficking could best explain the neurological symptoms seen in vitamin E deficiency. Emerging knowledge on vitamin E is compiled here with the perspective to unravel the molecular mechanisms that could more likely explain the essentiality of the vitamin than its ability to scavenge free radicals.

Leishmanicidal metabolites from Cochliobolus sp., an endophytic fungus isolated from Piptadenia adiantoides (Fabaceae)

Protozoan parasites belonging to genera Leishmania and Trypanosoma are the etiological agents of severe neglected tropical diseases (NTDs) that cause enormous social and economic impact in many countries of tropical and sub-tropical areas of the world. In our screening program for new drug leads from natural sources, we found that the crude extract of the endophytic fungus Cochliobolus sp. (UFMGCB-555) could kill 90% of the amastigote-like forms of Leishmania amazonensis and inhibit by 100% Ellman's reagent reduction in the trypanothione reductase (TryR) assay, when tested at 20 microg mL(-1). UFMGCB-555 was isolated from the plant Piptadenia adiantoides J.F. Macbr (Fabaceae) and identified based on the sequence of the internally transcribed spacer (ITS) regions of its ribosomal DNA. The chromatographic fractionation of the extract was guided by the TryR assay and resulted in the isolation of cochlioquinone A and isocochlioquinone A. Both compounds were active in the assay with L. amazonensis, disclosing EC(50) values (effective concentrations required to kill 50% of the parasite) of 1.7 microM (95% confidence interval = 1.6 to 1.9 microM) and 4.1 microM (95% confidence interval = 3.6 to 4.7 microM), respectively. These compounds were not active against three human cancer cell lines (MCF-7, TK-10, and UACC-62), indicating some degree of selectivity towards the parasites. These results suggest that cochlioquinones are attractive lead compounds that deserve further investigation aiming at developing new drugs to treat leishmaniasis. The findings also reinforce the role of endophytic fungi as an important source of compounds with potential to enter the pipeline for drug development against NTDs.

Cellular and molecular mechanisms of cigarette smoke-induced lung damage and prevention by vitamin C

BACKGROUND

Cigarette smoke-induced cellular and molecular mechanisms of lung injury are not clear. Cigarette smoke is a complex mixture containing long-lived radicals, including p-benzosemiquinone that causes oxidative damage. Earlier we had reported that oxidative protein damage is an initial event in smoke-induced lung injury. Considering that p-benzosemiquinone may be a causative factor of lung injury, we have isolated p-benzosemiquinone and compared its pathophysiological effects with cigarette smoke. Since vitamin C is a strong antioxidant, we have also determined the modulatory effect of vitamin C for preventing the pathophysiological events.

METHODS

Vitamin C-restricted guinea pigs were exposed to cigarette smoke (5 cigarettes/day; 2 puffs/cigarette) for 21 days with and without supplementation of 15 mg vitamin C/guinea pig/day. Oxidative damage, apoptosis and lung injury were assessed in vitro, ex vivo in A549 cells as well as in vivo in guinea pigs. Inflammation was measured by neutrophilia in BALF. p-Benzosemiquinone was isolated from freshly prepared aqueous extract of cigarette smoke and characterized by various physico-chemical methods, including mass, NMR and ESR spectroscopy. p-Benzosemiquinone-induced lung damage was examined by intratracheal instillation in guinea pigs. Lung damage was measured by increased air spaces, as evidenced by histology and morphometric analysis. Oxidative protein damage, MMPs, VEGF and VEGFR2 were measured by western blot analysis, and formation of Michael adducts using MALDI-TOF-MS. Apoptosis was evidenced by TUNEL assay, activation of caspase 3, degradation of PARP and increased Bax/Bcl-2 ratio using immunoblot analysis and confocal microscopy.

RESULTS

Exposure of guinea pigs to cigarette smoke resulted in progressive protein damage, inflammation, apoptosis and lung injury up to 21 days of the experimental period. Administration of 15 mg of vitamin C/guinea pig/day prevented all these pathophysiological effects. p-Benzosemiquinone mimicked cigarette smoke in causing protein modification and apoptosis in vitro and in A549 cells ex vivo as well as apoptosis and lung damage in vivo. All these pathophysiological events were also prevented by vitamin C.

CONCLUSION

p-Benzosemiquinone appears to be a major causative factor of cigarette smoke-induced oxidative protein damage that leads to apoptosis and lung injury. The pathophysiological events are prevented by a moderately large dose of vitamin C.

Novel Pathways Associated with Quinone-Induced Stress in Breast Cancer Cells

Hormone-dependent breast cancers that overexpress the ligand-binding nuclear transcription factor, estrogen receptor (ER), represent the most common form of breast epithelial malignancy. Exposure of breast epithelial cells to a redox-cycling and arylating quinone induces mitogen-activated protein kinase phosphorylation of the cytoskeletal filament protein, cytokeratin-8, along with thiol arylation of H3 nuclear histones. Exogenous or endogenous quinones can also induce ligand-independent nuclear translocation and phosphorylation of ER; with excess exposure, these quinones can arylate ER zinc fingers, impairing ER DNA-binding and altering ER-inducible gene expression. Immunoaffinity enrichment for low abundance proteins such as ER, coupled with modern mass spectrometry techniques, promises to improve understanding of the protein-modifications produced by endogenous and exogenous quinone exposure and their role in the development or progression of epithelial malignancies such as breast cancer.

Nutritional benefit of olive oil: the biological effects of hydroxytyrosol and its arylating quinone adducts

Olive oil is the essential component of the Mediterranean diet, a nutritional regimen gaining ever-increasing renown for its beneficial effects on inflammation, cardiovascular disease, and cancer. A unique characteristic of olive oil is its enrichment in oleuropein, a member of the secoiridoid family, which hydrolyzes to the catechol hydroxytyrosol and functions as a hydrophilic phenolic antioxidant that is oxidized to its catechol quinone during redox cycling. Little effort has been spent on exploring the biological properties of the catechol hydroxytyrosol quinone, a strong arylating electrophile that forms Michael adducts with thiol nucleophiles in glutathione and proteins. This study compares the chemical and biological characteristics of hydroxytyrosol with those of the tocopherol family in which Michael adducts of arylating desmethyltocopherol quinones have been identified and correlated with biologic properties including cytotoxicity and induction of endoplasmic reticulum stress. It is noted that hydroxytyrosol and desmethyltocopherols share many similarities, suggesting that Michael adduct formation by an arylating quinone electrophile may contribute to the biological properties of both families, including the unique nutritional benefit of olive oil.

Gamma-tocopheryl quinone, not alpha-tocopheryl quinone, induces adaptive response through up-regulation of cellular glutathione and cysteine availability via activation of ATF4

alpha-Tocopheryl quinone (alpha-TQ) and gamma-TQ are oxidized metabolites of the corresponding tocopherol (T) isoforms, which are vitamin E homologues. Unlike alpha-TQ, gamma-TQ functions as an arylating agent that reacts with nucleophiles such as reduced sulphydryl groups and it has unique biological properties such as high toxicity. Increasing evidence indicates that reactive oxygen species and other physiologically existing oxidative stimuli upregulate the antioxidant system, thereby triggering the adaptive response. The present study used PC12 cells and immature primary cortical cells to examine the possible adaptive cytoprotective effects of gamma-TQ against oxidative stress. Pre-treatment with gamma-TQ at sub-lethal concentrations resulted in cytoprotective effects against oxidative stress. gamma-TQ induced a significant increase in the cellular glutathione (GSH) levels while alpha-TQ did not. gamma-TQ did not induce any considerable change in the activity of glutamate-cysteine ligase (GCL), the rate-limiting enzyme in GSH synthesis, whereas it increased the cellular GSH levels by facilitating the availability of cysteine through the induction of xCT, which is the core sub-unit of the x(c)(-) high-affinity cystine transporter system. An activating transcription factor 4 (ATF4)-small interfering RNA effectively attenuated the xCT mRNA level as well as the increase in cellular cysteine levels induced by gamma-TQ, while the NF-E2-related factor (Nrf2)-small interfering RNA treatment did not. Collectively, these findings indicate that gamma-TQ acts as a signal messenger to induce adaptive response through the upregulation of intracellular GSH synthesis via transcriptional activation of ATF4 in order to cope with the forthcoming oxidative insult.

Tocopherol metabolism, oxidation and recycling under high light stress in Arabidopsis

Tocopherols are synthesized and accumulated by all plants and many cyanobacteria. The quenching and scavenging of reactive oxygen species and lipid peroxy radicals by tocopherols can result in the formation of various tocopherol oxidation compounds. A targeted GC/MS profiling method was developed to quantify all tocopherols and pathway intermediates, and 23 potential alpha- and gamma-tocopherol oxidation products. This method was used to study the response of wild-type Arabidopsis (Col) and the tocopherol biosynthetic mutants vte1, vte2 and vte4 during 12 h low- and high-light treatments (LL and HL, 90 and 1500 mumol photon m(-2) sec(-1), respectively) and a subsequent 12 h dark recovery period. All tocopherols and pathway intermediates exhibited HL-dependent increases except 2,3-dimethyl-6-phytyl-1,4-benzoquinone (DMPBQ) in vte1 and beta-tocopherol in Col. Profiling of potential tocopherol oxidation products during HL treatment indicated the presence of only alpha-tocopherolquinol (alpha-TQH(2)) in Col and only gamma-tocopherolquinol (gamma-TQH(2)) in vte4, both of which accumulated to similar levels and with similar kinetics the two genotypes. However, during dark recovery, the level of alpha-TQH(2) in Col decreased several times faster than that of gamma-TQH(2) in vte4, suggesting the presence of biochemical processes with higher specificity for alpha-TQH(2). (14)C-labeled alpha-tocopherolquinone (alpha-TQ) applied to isolated Col chloroplasts was converted to (14)C-alpha-tocopherol, demonstrating the existence of a plastid-based system for recycling oxidized alpha-tocopherol. The accumulation of (14)C-trimethylphytylbenzoquinone (TMPBQ) by isolated vte1 plastids treated with (14)C-labeled alpha-TQ is consistent with the tocopherolquinone-recycling pathway utilizing a yet to be identified plastid-localized dehydratase that converts tocopherolquinone to TMPBQ.

In vivo gamma-tocopherol supplementation decreases systemic oxidative stress and cytokine responses of human monocytes in normal and asthmatic subjects

We have recently reported that gamma-tocopherol (gammaT) reduces allergen- and zymosan-induced inflammation using rodent models. As an initial step in extending these observations to humans, we conducted an open-label, Phase I dosing study of two doses (one or two capsules daily for 1 week) of a gamma-tocopherol-rich preparation containing 623 mg of gamma-tocopherol, 61.1 mg of d-alpha-tocopherol, 11.1 mg of d-beta-tocopherol (11.1 mg), and 231 mg of d-sigma-tocopherol per capsule. Endpoints for this study include serum levels of 5-nitro-gamma-tocopherol, as a marker of oxidative stress, and changes in serum gamma-, alpha-, and delta-tocopherol and gamma-2'-carboxyethyl-6-hydroxychroman (CEHC) 6 and 24 h after the first dose and after 1 week of treatment. To assess the biological activity of this treatment, we obtained peripheral blood mononuclear cells at baseline and after 1 week of treatment with two capsules of a gamma-tocopherol-rich preparation/day and examined the inflammatory cytokine response of these cells in culture to ex vivo endotoxin/LPS (0.01 ng/ml) challenge. We also monitored a number of safety endpoints to examine how well this preparation is tolerated in eight normal volunteers (four allergic and four nonallergic) and eight allergic asthmatics. We further obtained human monocytes from a subset of these volunteers and treated them ex vivo with gammaT, alphaT, gamma-CEHC, and alpha-CEHC and assessed their actions on LPS-induced degradation of IkappaBalpha and JNK signaling and ROS generation. As detailed herein, this open-label study demonstrates that gamma-tocopherol-enriched supplementation decreased systemic oxidative stress, increased serum levels of gamma-tocopherol, and inhibited monocyte responses to LPS without any adverse health effects. Further, in vitro treatment of human monocytes with gamma-CEHC and alpha-CEHC inhibits ROS generation and LPS-induced degradation of IkappaB and JNK activation.

Thiols oxidation and covalent binding of BSA by cyclolignanic quinones are enhanced by the magnesium cation

A novel cyclolignanic quinone, 7-acetyl-3',4'-didemethoxy-3',4'-dioxopodophyllotoxin (CLQ), inhibits topoisomerase II (TOPO II) activity. The extent of this inhibition was greater than that produced by the etoposide quinone (EQ) or etoposide. Glutathione (GSH) reduces EQ and CLQ to their corresponding semiquinones under anaerobic conditions. The latter were detected by EPR spectroscopy in the presence of MgCl(2) but not in its absence. Semiquinone EPR spectra change with quinone/GSH mol ratio, suggesting covalent binding of GSH to the quinones. Quinone-GSH covalent adducts were isolated and identified by ESI-MS. These orthoquinones also react with nucleophilic groups from BSA to bind covalently under anaerobic conditions. BSA thiol consumption and covalent binding by these quinones are enhanced by MgCl(2). Complex formation between the parent quinones and Mg(+2) was also observed. Density functional calculations predict the observed blue-shifts in the absorption spectra peaks and large decreases in the partial negative charge of electrophilic carbons at the quinone ring when the quinones are complexed to Mg(+2). These observations suggest a possible role of Mg(+2) chelation by these quinones in increasing TOPO II thiol and/or amino/imino reactivity with these orthoquinones.

Endoplasmic reticulum stress induced by aqueous extracts of cigarette smoke in 3T3 cells activates the unfolded-protein-response-dependent PERK pathway of cell survival

Cigarette smoke (CS) generally places severe stress on cells, as reflected by gene expression profiling and pathway analysis, which, among other effects, also suggested activation of the unfolded protein response pathway triggered by the stressed endoplasmic reticulum (ER stress). Here, we present data indicating that noncytotoxic concentrations of aqueous extracts of CS induce a distinct ER stress response in immortalized nontransformed Swiss 3T3 cells, primarily by activating the PERK pathway of global protein synthesis inhibition. Activation of PERK and PERK-dependent signaling by aqueous extracts of CS was demonstrated by (i) the inhibition of protein synthesis, (ii) the phosphorylation of PERK and its substrate eIF2alpha, (iii) the activation of ATF4, and (iv) the expression of ATF4-dependent target genes chop, gadd34, BiP, and atf3. Within the dose range tested, all effects appeared to be transient in nature, while the periods of recovery from ER stress were clearly concentration dependent. In contrast to these data and to the effects seen with thapsigargin (used as positive control), only minor effects were observed for the activation of xbp-1, a common target of the other two canonical sensors of ER stress, i.e., ATF6 and IRE1. In mechanistic terms, neither the disruption of energy levels nor a contribution of arylating quinones played a major role under the experimental conditions tested. Notably however, the effects of aqueous extracts of CS on the ER could be mimicked in the presence of acrolein at CS-relevant concentrations, indicating that CS interferes with proper ER function, presumably due mainly to changes in cellular redox homeostasis. Since ER stress has been linked to diseases that are also related to CS exposure, these data are relevant in the discussion of a general molecular mechanism of CS-induced disease.

Cell stress induced by the parkinsonian mimetic, 6-hydroxydopamine, is concurrent with oxidation of the chaperone, ERp57, and aggresome formation

Parkinson's disease (PD) involves an irreversible degeneration of the nigrostriatal pathway. As most cases of PD are sporadic, environmental risk factors may underlie neurodegeneration in dopaminergic neurons. One such factor is 6-hydroxydopamine (6-OHDA), which is widely used as a parkinsonian mimetic. Studies have shown that 6-OHDA generates reactive oxygen species and induces cell stress, the unfolded protein response, and apoptosis. Present findings show that 6-OHDA, but not hydrogen peroxide, MPP+, or rotenone, leads to the rapid formation of high-molecular-weight species of protein disulfide isomerase-associated protein 3 (ERp57) in a dose- and time-dependent fashion. Moreover, ERp57 conjugates are blocked by N-acetylcysteine and glutathione, suggesting that they represent oxidized forms of protein. Surprisingly, conjugates are complexed with DNA, because treatment with DNase reduces their appearance. Subcellular fractionation indicates that both nuclear and mitochondrial DNA are associated with the protein. Finally, toxin-treated ERp57 rapidly forms juxtanuclear aggresome-like structures in dopaminergic cells, suggesting that ERp57 plays an early adaptive response in toxin-mediated stress. Understanding the signaling mechanisms associated with parkinsonian mimetics, as well as their temporal induction, may aid in designing better interventions in models of PD.

Toxicological evaluation of thiol-reactive compounds identified using a la assay to detect reactive molecules by nuclear magnetic resonance

We have recently reported on the development of a La assay to detect reactive molecules by nuclear magnetic resonance (ALARM NMR) to detect reactive false positive hits from high-throughput screening, in which we observed a surprisingly large number of compounds that can oxidize or form covalent adducts with protein thiols groups. In the vast majority of these cases, the covalent interactions are largely nonspecific (e.g., affect many protein targets) and therefore unsuitable for drug development. However, certain thiol-reactive species do appear to inhibit the target of interest in a specific manner. The question then arises as to the potential toxicology risks of developing a drug that can react with protein thiol groups. Here, we report on the evaluation of a large set of ALARM-reactive and -nonreactive compounds against a panel of additional proteins (aldehyde dehydrogenase, superoxide dismutase, and three cytochrome P450 enzymes). It was observed that ALARM-reactive compounds have significantly increased risks of interacting with one or more of these enzymes in vitro. Thus, ALARM NMR seems to be a sensitive tool to rapidly identify compounds with an enhanced risk of producing side effects in humans, including alcohol intolerance, the formation of reactive oxygen species, and drug-drug interactions. In conjunction with other toxicology assays, ALARM NMR should be a valuable tool for prioritizing compounds for lead optimization and animal testing.

Endoplasmic reticulum stress during the embryonic development of the central nervous system in the mouse

In the present study, we have found evidence for ER stress occurring during development of the central nervous system in the mouse. Several ER-resident stress-regulated chaperones, such as calreticulin, glucose regulated protein 78, glucose regulated protein 94, ER protein 57 and protein disulfide isomerase, were expressed at higher levels in embryonic brain and retina, compared with adult tissues. In contrast, calnexin, a chaperone that is not regulated by stress was equally abundant in embryonic and adult tissues. We also detected unfolded protein response during embryonic development. Both eukaryotic translation initiation factor 2 alpha and its phosphorylated form were more abundant in embryonic brain and retina than in adult tissues. Spliced X-box binding protein-1 mRNA was detected in embryonic brain and retina, while it was absent in adult counterparts. Partially glycosylated form of activating transcription factor 6 alpha, another ER stress indicator, was detected predominantly in embryonic brain. Finally, apoptotic pathway components, caspase-7 and -12, were more abundant in embryonic brain than in adult. The pattern of expression of chaperones together with activation of the unfolded protein response factors suggests the presence of ER stress during development of brain and retina. Furthermore, our data suggest that ER stress-like mechanism may induce apoptosis via activation of the caspases during embryonic development of the central nervous system.

Reactivity of molecular oxygen with ethoxycarbonyl derivatives of tetrathiatriarylmethyl radicals

Tetrathiatriarylmethyl (TAM) radicals are commonly used as oximetry probes for electron paramagnetic resonance imaging applications. In this study, the electronic properties and the thermodynamic preferences for O2 addition to various TAM-type triarylmethyl (trityl) radicals were theoretically investigated. The radicals' stability in the presence of O2 and biological milieu was also experimentally assessed using EPR spectroscopy. Results show that H substitution on the aromatic ring affects the trityl radical's stability (tricarboxylate salt 1-CO2Na > triester 1-CO2Et > diester 2-CO2Et > monoester 3-CO2Et) and may lead to substitution reactions in cellular systems. We propose that this degradation process involves an arylperoxyl radical that can further decompose to alcohol or quinone products. This study demonstrates how computational chemistry can be used as a tool to rationalize radical stability in the redox environment of biological systems and aid in the future design of more biostable trityl radicals.

Molecular mechanism of phase I and phase II drug-metabolizing enzymes: implications for detoxification

Enzymes that catalyze the biotransformation of drugs and xenobiotics are generally referred to as drug-metabolizing enzymes (DMEs). DMEs can be classified into two main groups: oxidative or conjugative. The NADPH-cytochrome P450 reductase (P450R)/cytochrome P450 (P450) electron transfer systems are oxidative enzymes that mediate phase I reactions, whereas the UDP-glucuronosyltransferases (UGTs) are conjugative enzymes that mediate phase II enzymes. Both enzyme systems are localized to the endoplasmic reticulum (ER) where a number of drugs are sequentially metabolized. DMEs, including P450s and UGTs, generally have a highly plastic active site that can accommodate a wide variety of substrates. The P450 and UGT genes constitute a supergene family, in which UGT proteins are encoded by distinct genes and a complex gene. Both the P450 and UGT genes have evolved to diversify their functions. This chapter reviews advances in understanding the structure and function of the P450R/P450 and UGT enzyme systems. In particular, the coordinate biotransformation of xenobiotics by phase I and II enzymes in the ER membrane is examined.

Vitamin E, antioxidant and nothing more

All of the naturally occurring vitamin E forms, as well as those of synthetic all-rac-alpha-tocopherol, have relatively similar antioxidant properties, so why does the body prefer alpha-tocopherol as its unique form of vitamin E? We propose the hypothesis that all of the observations concerning the in vivo mechanism of action of alpha-tocopherol result from its role as a potent lipid-soluble antioxidant. The purpose of this review then is to describe the evidence for alpha-tocopherol's in vivo function and to make the claim that alpha-tocopherol's major vitamin function, if not only function, is that of a peroxyl radical scavenger. The importance of this function is to maintain the integrity of long-chain polyunsaturated fatty acids in the membranes of cells and thus maintain their bioactivity. That is to say that these bioactive lipids are important signaling molecules and that changes in their amounts, or in their loss due to oxidation, are the key cellular events that are responded to by cells. The various signaling pathways that have been described by others to be under alpha-tocopherol regulation appear rather to be dependent on the oxidative stress of the cell or tissue under question. Moreover, it seems unlikely that these pathways are specifically under the control of alpha-tocopherol given that various antioxidants other than alpha-tocopherol and various oxidative stressors can manipulate their responses. Thus, virtually all of the variation and scope of vitamin E's biological activity can be seen and understood in the light of protection of polyunsaturated fatty acids and the membrane qualities (fluidity, phase separation, and lipid domains) that polyunsaturated fatty acids bring about.

Molecular mechanisms of 6-hydroxydopamine-induced cytotoxicity in PC12 cells: involvement of hydrogen peroxide-dependent and -independent action

The neurotoxin 6-hydroxydopamine (6-OHDA) has been widely used to generate an experimental model of Parkinson's disease. It has been reported that reactive oxygen species (ROS), such as the superoxide anion and hydrogen peroxide (H2O2), generated from 6-OHDA are involved in its cytotoxicity; however, the contribution and role of ROS in 6-OHDA-induced cell death have not been fully elucidated. In the present study using PC12 cells, we observed the generation of 50 microM H2O2 from a lethal concentration of 100 microM 6-OHDA within a few minutes, and compared the sole effect of H2O2 with 6-OHDA. Catalase, an H2O2-removing enzyme, completely abolished the cytotoxic effect of H2O2, while a significant but partial protective effect was observed against 6-OHDA. 6-OHDA induced peroxiredoxin oxidation, cytochrome c release, and caspase-3 activation. Catalase exhibited a strong inhibitory effect against the peroxiredoxin oxidation, and cytochrome c release induced by 6-OHDA; however, caspase-3 activation was not effectively inhibited by catalase. On the other hand, 6-OHDA-induced caspase-3 activation was inhibited in the presence of caspase-8, caspase-9, and calpain inhibitors. These results suggest that the H2O2 generated from 6-OHDA plays a pivotal role in 6-OHDA-induced peroxiredoxin oxidation, and cytochrome c release, while H2O2- and cytochrome c-independent caspase activation pathways are involved in 6-OHDA-induced neurotoxicity. These findings may contribute to explain the importance of generated H2O2 and secondary products as a second messenger of 6-OHDA-induced cell death signal linked to Parkinson's disease.

Antituberculotic and antiprotozoal activities of primin, a natural benzoquinone: in vitro and in vivo studies

Primin (=2-methoxy-6-pentylcyclohexa-2,5-diene-1,4-dione), a natural benzoquinone synthesized in our laboratory, was investigated for its in vitro antiprotozoal, antimycobacterial, and cytotoxic potential. Primin showed very potent activity against Trypanosoma brucei rhodesiense (IC50 0.144 microM) and Leishmania donovani (IC50 0.711 microM), and revealed low cytotoxicity (IC50 15.4 microM) on mammalian cells. Only moderate inhibitory activity was observed against Mycobacterium tuberculosis, Trypanosoma cruzi, and Plasmodium falciparum. When tested for in vivo efficacy in a Trypanosoma b. brucei rodent model, primin failed to cure the infection at 20 mg/kg given intraperitoneally. Primin was too toxic in vivo at a higher concentration (30 mg/kg, injected i.p. route) in mice infected with L. donovani. Taken together, primin can serve as a lead compound for the rational design of more potent and less toxic antiprotozoal agents.

Curcumin induces pro-apoptotic endoplasmic reticulum stress in human leukemia HL-60 cells

Curcumin has been shown to induce apoptosis in many cancer cells. However, the molecular mechanism(s) responsible for curcumin-induced apoptosis is not well understood and most probably involves several pathways. In HL-60 cells, curcumin induced apoptosis and endoplasmic reticulum (ER) stress as evidenced by the survival molecules such as phosphorylated protein kinase-like ER-resident kinase, phosphorylated eukaryotic initiation factor-2alpha, glucose-regulated protein-78, and the apoptotic molecules such as caspase-4 and CAAT/enhancer binding protein homologous protein (CHOP). Inhibition of caspase-4 activity by z-LEVD-FMK, blockage of CHOP expression by small interfering RNA, and treatment with salubrinal, an ER inhibitor, significantly reduced curcumin-induced apoptosis. Removing two double bonds in curcumin, which was speculated to form Michael adducts with thiols in secretory proteins, resulted in a loss of the ability of curcumin to induce apoptosis as well as ER stress. Thus, the present study shows that curcumin-induced apoptosis is associated with its ability to cause ER stress.

Vitamin E metabolism

The aim of this paper is to provide an overview of vitamin E metabolism. The topics covered include: major classes of vitamin E metabolites; their production pathways and route of excretion; possible biological activities of vitamin E metabolites; and use of vitamin E metabolites as markers of oxidant generation. Recent investigations into vitamin E metabolism have also highlighted important new areas of research, such as the potential for high dose vitamin E supplementation to interfere with drug metabolism, as well as alternative methods to alter vitamin E bioavailability in vivo. These issues will also be discussed in the review.

Studies in Vitamin E: Biochemistry and Molecular Biology of Tocopherol Quinones

Tocopherols and tocotrienols, parent congeners in the vitamin E family, function as phenolic antioxidants. However, there has been little interest in their quinone electrophiles formed as a consequence of oxidation reactions, even though unique biological properties were suggested by early studies conducted immediately after the discovery of vitamin E. Oxidation of tocopherols and tocotrienols produces para- and ortho-quinones, and quinone methides, while oxidation of their carboxyethyl hydroxychroman derivatives produces quinone lactones. These quinone electrophiles are grouped in two subclasses, the nonarylating fully methylated alpha-family and the arylating desmethyl beta-, gamma-, and delta-family. Arylating quinone electrophiles form Michael adducts with thiol nucleophiles, provided by cysteinyl proteins or peptides, which can be identified and quantified by tetramethylammonium hydroxide thermochemolysis. They have striking biological properties which differ significantly from their nonarylating congeners. They are highly cytotoxic, inducing characteristic apoptotic changes in cultured cells. Cytotoxicity is intimately associated with the induction of endoplasmic reticulum stress and a consequent unfolded protein response involving the pancreatic ER kinase (PERK) signaling pathway that commits overstressed cells to apoptosis. The step-function difference between arylating and nonarylating tocopherol quinones is conceivably the basis for distinct biological properties of parent tocopherols, including the epigenetic modification of a histone thiol, the ceramide pathway, natriuresis, and the activity of COX-2, NF-kappaB, PPARgamma, and cyclin. The role of alpha-tocopherol in the origin and evolution of the western hominin diet, the so-called "Mediterranean" diet, and the prominence of alpha-tocopherol in colostrum, mother's milk, and infant nutrition are considered. Finally, the discordance introduced into the diet by arylating tocopherol quinone precursors through the wide use of vegetable oils in deep-frying is recognized.

Acetaminophen induces ER dependent signaling in mouse liver

Role of endoplasmic reticulum (ER) in liver injury by acetaminophen (AAP) was studied in vivo in mice. Sublethal dose of AAP resulted in a decrease in microsomal total glutathione and in the reduced-to-total glutathione ratio; redox state of thiols of ER resident oxidoreductases ERp72, PDI was shifted towards the oxidized form; ER stress-responsive transcription factor ATF6 was activated. Transcriptional activation and elevated expression of GADD153/CHOP, an ER stress-responsive proapoptotic transcription factor, was observed upon AAP addition. Transient activation of the ER-resident caspase-12 was shown followed by an elevation in procaspase-12 level. Caspase-3 and caspase-8 activation could not be detected. AAP treatment resulted in an increased apoptosis of hepatocytes. Buthionine-sulfoximine treatment was unable to mimic the effects by AAP indicating that glutathione depletion itself is insufficient to provoke apoptosis. The results show that intraluminal redox imbalance of the ER and consequential activation of signaling processes and proapoptotic events are involved in hepatocellular damage caused by AAP overdose.