Role of cellular thiol status in tocopheryl hemisuccinate cytoprotection against ethyl methanesulfonate-induced toxicity

Power provides protection: Genetic robustness in yeast depends on the capacity to generate energy

The functional basis of genetic robustness, the ability of organisms to suppress the effects of mutations, remains incompletely understood. We exposed a set of 15 strains of Saccharomyces cerevisiae form diverse environments to increasing doses of the chemical mutagen EMS. The number of the resulting random mutations was similar for all tested strains. However, there were differences in immediate mortality after the mutagenic treatment and in defective growth of survivors. An analysis of gene expression revealed that immediate mortality was lowest in strains with lowest expression of transmembrane proteins, which are rich in thiol groups and thus vulnerable to EMS. A signal of genuine genetic robustness was detected for the other trait, the ability to grow well despite bearing non-lethal mutations. Increased tolerance of such mutations correlated with high expression of genes responsible for the oxidative energy metabolism, suggesting that the negative effect of mutations can be buffered if enough energy is available. We confirmed this finding in three additional tests of the ability to grow on (i) fermentable or non-fermentable sources of carbon, (ii) under chemical inhibition of the electron transport chain and (iii) during overexpression of its key component, cytochrome c. Our results add the capacity to generate energy as a general mechanism of genetic robustness.

The ability to suppress phenotypic effects of mutations is termed genetic robustness. Its functional basis and evolutionary origin remain insufficiently understood despite decades of research. In fact, it is still largely untested whether genetic robustness is a trait of substantial, within-species variation. We used a model organism, Saccharomyces cerevisiae, to study both phenotypic signs and functional underpinnings of genetic robustness. We introduced random mutations into a set of well-characterized yeast strain. There was considerable variation in the growth rate among clones recovered after mutagenesis, which is an indication of genetic robustness. Using available data on gene expression for our strains, we found that genetic robustness was strongest among strains with enhanced expression of genes related to the energy metabolism. We reasoned that, regardless of the specific mutations, the capacity to generate metabolic energy may be a general underlying mechanism for buffering the effects of random mutations across the genome. We confirmed this hypothesis in further experiments in which we showed that genetic robustness decreases when the energy metabolism is compromised and increases when it is boosted.

Genetic damage, but limited evidence of oxidative stress markers in diethyl maleate-induced glutathione depleted mouse lymphoma L5178Y (TK(+/-)) cell cultures

Depletion of glutathione (GSH) in cells exposed to certain xenobiotics has been proposed to result in oxidative stress, which could lead to damage of cellular macromolecules such as proteins, lipids, and DNA. Diethyl maleate (DEM) is known to conjugate with GSH and rapidly lower cellular GSH levels. The objective of this study was to investigate the influence of DEM-induced GSH depletion on various genotoxicity and gene expression end points in mouse lymphoma L5178Y (TK(+/-)) cell cultures. Cells were exposed to DEM for 4 h at concentrations of 0, 6.7, 13.5, 26.9, 53.8, 107.6, 215.3, and 430.6 µg/mL (0.039-2.5 mM). Genotoxicity was evaluated by examining the induction of in vitro micronuclei (20 h post-treatment) and DNA strand breaks as measured by comet (immediately following treatment), and correlating these observations to cellular GSH levels. In the current study, GSH was decreased more than 50% at the lowest test concentration (6.7 µg/mL) and more than 95% at ≥ 107.6 µg/mL. A significant increase in micronuclei and DNA strand breaks was observed at concentrations of ≥ 26.9 µg/mL. Gene expression of seven apoptosis and oxidative-stress related genes showed significant alterations in only three genes only at the highest test concentration. Quantifiable levels of 8-OH-dG (≥ 2 adducts per 1 × 10(8) NT) were not detected at any treatment concentration. These results demonstrate an association between DEM-induced genotoxicity and GSH depletion in mouse lymphoma L5178Y (TK(+/-)) cells, but not with other oxidative markers.

DietaryRRR-α-tocopherol succinate attenuates lipopolysaccharide-induced inflammatory cytokines secretion in broiler chicks

The anti-inflammatory effects of two esters of α-tocopherol (α-TOH),all-rac-α-TOH acetate (dl-α-TOA) andRRR-α-TOH succinate (d-α-TOS), on broilers repeatedly challenged with lipopolysaccharide (LPS) were investigated. Three hundred and twenty 1-d-old broiler chicks were allotted into four treatment groups and fed on a control diet (30 mg/kgdl-α-TOA) or diets containing 10, 30, 50 mg/kgd-α-TOS. Half of the birds from each treatment group were challenged with 0·9 % NaCl solution or LPS (250 μg/kg body weight) at 16, 18 and 20 d of age. The results indicated that the pretreatment of birds with 50 mg/kgd-α-TOS markedly reduced serum PGE2secretion and increased the concentrations of serum or hepatic α-TOH. When LPS-challenged birds were pretreated with 30 or 50 mg/kgd-α-TOS, the increases of plasma and splenic concentrations of interferon-γ, IL-1β, IL-2, IL-6, IL-4 and IL-10 were dramatically attenuated. Also, a significant decrease of hepatic reactive oxygen species (ROS) and hepatic or splenic phosphokinase C (PKC) activities was found in birds pretreated with 30 or 50 mg/kgd-α-TOS. Furthermore,d-α-TOS inhibited the activation of NF-κB by preventing the degradation of inhibitory-κBα. In conclusion, D-α-TOS is able to prevent LPS-induced inflammation responsein vivo.The beneficial effect may depend on suppressing the secretion of various plasma and splenic inflammatory mediators through inhibiting NF-κB activation and by blocking ROS signalling, in which PKC may play an assistant role.

Effect of RRR-alpha-tocopherol succinate on the growth and immunity in broilers

The objective of this study was to compare the effect of 2 esters of alpha-tocopherol, all-rac-alpha-tocopherol acetate and RRR-alpha-tocopherol succinate (d-alpha-TOS) on growth and immunity in broiler chicks. Three hundred twenty 1-d-old commercial Arbor Acres broilers were randomly distributed to 4 treatments, each of which had 8 pens of 10 chicks per pen. Birds in the control group were fed with the diets supplemented with 30 mg/kg of all-rac-alpha-tocopherol acetate or the basal diet with d-alpha-TOS supplementation at 10 mg/kg (TOS1 group), 30 mg/kg (TOS2 group), and 50 mg/ kg (TOS3 group), respectively, for 42 d. The results showed that there was no significant difference (P > 0.05) in BW gain, feed intake, or G:F among the treatments. Significant positive correlations existed between dietary supplemental alpha-TOS levels and plasma (R(2) = 0.9831, P < 0.01) or hepatic (R(2) = 0.9336, P < 0.05) alpha-tocopherol concentrations and a negative correlation with plasma (R(2) = 0.9487, P < 0.05) or hepatic (R(2) = -0.9901, P = 0.0518) malondialdehyde levels. The concentrations of serum glutathione (GSH) were highest at 50 mg/kg at 42 d of age (P < 0.05), and hepatic GSH was significantly higher at 30 and 50 mg/kg compared with the other groups. Marked enhancement of splenic T- and B-lymphocyte proliferation occurred in group TOS3 as compared with the other groups. The study suggests that the immunoenhancement effect observed in broilers fed additional d-alpha-TOS between 30 and 50 mg/kg might result from increased retention of alpha-tocopherol and reduction in lipid peroxidation, as evidenced by the decrease in malondialdehyde and the increase in GSH.

Modification of actions of heat shock on development and apoptosis of cultured preimplantation bovine embryos by oxygen concentration and dithiothreitol

Preimplantation embryos exposed to elevated temperatures have reduced developmental competence. The involvement of reactive oxygen species in these effects has been controversial. Here we tested hypotheses that (1) heat shock effects on development and apoptosis would be greater when embryos were cultured in a high oxygen environment (air; oxygen concentration = approximately 20.95%, v/v) than in a low oxygen environment (5% oxygen) and (2) that these effects would be reversed by addition of the antioxidant dithiothreitol (DTT). Heat shock of 41 degrees C for 9 hr reduced development of two-cell embryos and Day 5 embryos to the blastocyst stage embryos when in high oxygen. There was no effect of heat shock on development when embryos were in low oxygen. Furthermore, induction of TUNEL-positive cells in Day 5 embryos by heat shock only occurred when embryos were in high oxygen. Addition of DTT to two-cell embryos either did not reduce effects of a heat shock of 41 degrees C for 15 hr on development or caused slight protection only. In contrast, treatment of Day 5 embryos with DTT reduced effects of heat shock on development and apoptosis. In summary, oxygen tension was shown to be a major determinant of the effects of heat shock on development and apoptosis in preimplantation bovine embryos. Protective effects of the antioxidant DTT were stage specific and more pronounced at later stages of development.

Vitamin E succinate protects hepatocytes against the toxic effect of reactive oxygen species generated at mitochondrial complexes I and III by alkylating agents

The mechanism of alpha-tocopheryl succinate (TS) cytoprotection against mitochondria-derived oxidative stress was investigated. Incubation of isolated rat hepatocytes with ethyl methanesulfonate (EMS), a mitochondrial alkylating toxicant caused mitochondrial dysfunction and necrotic cell death that was dependent on the production of reactive oxygen species (ROS) and lipid peroxidation. Mitochondria isolated from these cells showed a 3-fold increase in lipid hydroperoxides and a selective depletion of alpha-tocopherol (T), which preceded cell death. The pretreatment of hepatocytes with TS dramatically enriched cells and mitochondria with alpha-tocopherol and provided these membranes with complete protection against EMS-induced oxidative damage. TS pretreatment suppressed EMS-induced cellular ROS production, generated from mitochondrial complex I and III sites. In addition, the treatment with either rotenone (ROT, a complex I inhibitor) or antimycin A (AA, a complex III inhibitor) potentiated EMS-induced lipid peroxidation and necrotic cell death which were again completely prevented by TS treatment. Surprisingly, TS did not protect hepatocytes against thenoyltrifluoroacetone (TTFA), a complex II inhibitor-induced enhancement of EMS-induced toxic oxidative damage. We conclude that the inhibition of mitochondrial ROS production and lipid peroxidation by T released from TS, are the critical events responsible for TS-mediated cytoprotection against toxic oxidative stress derived from both mitochondrial complexes I and III. Our findings suggest that TS treatment may prove useful in combating diseases associated with mitochondrial-derived oxidative stress.

Glutathione depletion and the production of reactive oxygen species in isolated hepatocyte suspensions

Diethyl maleate (DEM) (5 mM) and ethyl methanesulfonate (EMS) (35 mM) treatments rapidly depleted cellular reduced glutathione (GSH) below detectable levels (1 nmol/10(6) cells), and induced lipid peroxidation and necrotic cell death in freshly isolated rat hepatocytes. In hepatocytes incubated with 2.5 mM DEM and 10 mM EMS, however, the complete depletion of cellular GSH observed was not sufficient to induce lipid peroxidation or cell death. Instead, DEM- and EMS-induced lipid peroxidation and cell death were dependent on increased reactive oxygen species (ROS) production as measured by increases in dichlorofluorescein fluorescence. The addition of antioxidants (vitamin E succinate and deferoxamine) prevented lipid peroxidation and cell death, suggesting that lipid peroxidation is involved in the sequence of events leading to necrotic cell death induced by DEM and EMS. To investigate the subcellular site of ROS generation, the cytochrome P450 inhibitor, SKF525A, was found to reduce EMS-induced lipid peroxidation but did not protect against the loss of cell viability, suggesting a mitochondrial origin for the toxic lipid peroxidation event. In agreement with this conclusion, mitochondrial electron transport inhibitors (rotenone, thenoyltrifluoroacetone and antimycin A) increased EMS-induced lipid peroxidation and cell death, while the mitochondrial uncoupler, carbonyl cyanide m-chlorophenylhydrazone, blocked EMS- and DEM-mediated ROS production and lipid peroxidation. Furthermore, EMS treatment resulted in the significant loss of mitochondrial alpha-tocopherol shortly after its addition, and this loss preceded losses in cellular alpha-tocopherol levels. Treatment of hepatocytes with cyclosporin A, a mitochondrial permeability transition inhibitor, oxypurinol, a xanthine oxidase inhibitor, or BAPTA-AM, a calcium chelator, provided no protection against EMS-induced cell death or lipid peroxidation. Our results indicate that DEM and EMS induce cell death by a similar mechanism, which is dependent on the induction of ROS production and lipid peroxidation, and mitochondria are the major source for this toxic ROS generation. Cellular GSH depletion in itself does not appear to be responsible for the large increases in ROS production and lipid peroxidation observed.

Lipoprotein-associated alpha-tocopheryl-succinate inhibits cell growth and induces apoptosis in human MCF-7 and HBL-100 breast cancer cells

alpha-Tocopheryl succinate (alpha-TS) is a potent inhibitor of tumor cell proliferation. The goal of the present study was to investigate whether and to what extent alpha-TS associates with plasma lipoproteins and if alpha-TS-enriched lipoproteins inhibit breast cancer cell growth in a manner comparable to the free drug. In vitro enrichment of human plasma revealed that alpha-TS readily associated with the main lipoprotein classes, findings confirmed in vivo in mice. At the highest alpha-TS concentrations, lipoproteins carrying 50000 (VLDL), 5000 (LDL) and 700 (HDL) alpha-TS molecules per lipoprotein particle were generated. alpha-TS enrichment generated lipoprotein particles with slightly decreased density and increased particle radius. To study whether the level of LDL-receptor (LDL-R) expression affects alpha-TS uptake from apoB/E containing lipoprotein particles human breast cancer cells with low (MCF-7) and normal (HBL-100) LDL-R expression were used. The uptake of free, VLDL- and (apoE-free) HDL(3)-associated alpha-TS was nearly identical for both cell lines. In contrast, uptake of LDL-associated alpha-TS by HBL-100 cells (normal LDL-R expression) was about twice as high as compared to MCF-7 cells (low LDL-R expression). VLDL and LDL-associated alpha-TS inhibited proliferation most effectively at the highest concentration of alpha-TS used (100% inhibition of MCF-7 growth with 20 microg/ml of lipoprotein-associated alpha-TS). However, also alpha-TS-free VLDL and LDL inhibited HBL-100 cell proliferation up to 55%. In both cell lines, alpha-TS-enriched HDL(3) inhibited cell growth by 40-60%. Incubation of both cell lines in the presence of free or lipoprotein-associated alpha-TS resulted in DNA fragmentation indicative of apoptosis. Collectively, the present findings demonstrate that: (1) alpha-TS readily associates with lipoproteins in vitro and in vivo; (2) the lipoprotein-enrichment efficacy was dependent on the particle size and/or the triglyceride content of the lipoprotein; (3) uptake of LDL-associated alpha-TS was apparently dependent on the level of LDL-R expression; and (4) lipoproteins were efficient alpha-TS carriers inducing reduced cell proliferation rates and apoptosis in human breast cancer cells as observed for the free drug.

Role of glutathione and oxidative stress in phalloidin-induced cholestasis

BACKGROUND/AIMS

Biliary glutathione is an important generator of the bile-salt independent flow, and is known to be regulated by the hepatic glutathione availability. We investigated, in an acute model of phalloidin-induced cholestasis, biliary glutathione secretion and the role of hepatic glutathione, oxidative stress, and protein kinase c activation, which have been implicated in many hepatotoxin-induced hepatic dysfunctions.

METHODS

Rats were given a single dose of 80 microg/100 g body weight of phalloidin and the hepatic thiols and glutathione content, redox state and vesicular activity were evaluated during both development of and recovery from cholestasis. The prophylactic effect of N-acetylcysteine (a precursor of glutathione synthesis and an antioxidant) was also examined. In addition, in the isolated perfused rat liver, we studied the prophylactic effect of the PKc inhibitor H7 on phalloidin-induced cholestasis.

RESULTS

In the early stages of cholestasis, phalloidin induced a decline in bile flow, mainly related to a disruption of biliary glutathione secretion. The decline in biliary glutathione content was not associated with increased glutathione degradation, indicated by a parallel decline in biliary non-protein thiols and by the lack of an increase in biliary gamma-glutamyltranspeptidase. There was also no evidence of hepatic depletion of glutathione or of oxidative stress, as measured by the oxidized-to-reduced glutathione ratio. Moreover, phalloidin resulted in inhibition of vascular transcytosis as assessed by horseradish peroxidase labeling. Pre-treatment with N-acetylcysteine did not counteract the decline in biliary glutathione secretion and bile flow produced by phalloidin, supporting the view that the hepatic availability of glutathione and oxidative stress injury are not implicated in the early stages of cholestatic injury. Moreover, treatment with H-7 did not alter the biliary glutathione output, or the decline in bile flow induced by the toxin.

CONCLUSIONS

This study suggests that the phalloidin-induced inhibition of bile formation may be attributed to rapid disruption of the hepatocanalicular transport of glutathione.

Time-dependent production of nitric oxide by rat hepatocyte suspensions

Isolated hepatocyte suspensions prepared by collagenase perfusion released high levels of nitrite into the extracellular medium during an 8-hr incubation. The release was time dependent, with increases first occurring by 4 hr and continuing throughout the remainder of the incubation period. Nitrite production was inhibited by the nitric oxide synthase (NOS) inhibitors aminoguanidine and N(G)-nitro-L-arginine methyl ester (L-NAME), indicating that the nitrite is derived from nitric oxide (NO) production from NOS activity. Nitrite production was not related to bacterial or Kupffer cell contamination. The protein synthesis inhibitor cycloheximide and the transcription inhibitor actinomycin D also prevented nitrite production by parenchymal hepatocytes. Calcium-independent NOS enzyme activity increased with incubation times, and this increase coincided with the observed increases in nitrite production. Our results suggest that NOS is induced following the isolation of hepatocytes, and this induction results in the formation of high levels of NO.