Dietary Vitamin E Decreases Doxorubicin- Induced Oxidative Stress Without Preventing Mitochondrial Dysfunction

Doxorubicin (DOX) is a widely prescribed antineoplastic and although the precise mechanism(s) have yet to be identified, DOX-induced oxidative stress to mitochondrial membranes is implicated in the pathogenic process. Previous attempts to protect against DOX-induced cardiotoxicity with alpha-tocopherol (vitamin E) have met with limited success, possibly as a result of inadequate delivery to relevant subcellular targets such as mitochondrial membranes. The present investigation was designed to assess whether enrichment of cardiac membranes with alpha-ocopherol is sufficient to protect against DOX-induced mitochondrial cardiotoxicity. Adult male Sprague-Dawley rats received seven weekly subcutaneous injections of 2 mg/kg DOX and fed either standard diet or diet supplemented with alpha-tocopherol succinate. Treatment with a cumulative dose of 14 mg/kg DOX caused mitochondrial cardiomyopathy as evidenced by histology, accumulation of oxidized cardiac proteins, and a significant decrease in mitochondrial calcium loading capacity. Maintaining rats on the alpha-tocopherol supplemented diet resulted in a significant (two- to four-fold) enrichment of cardiac mitochondrial membranes with alpha-tocopherol and diminished the content of oxidized cardiac proteins associated with DOX treatment. However, dietary alpha-tocopherol succinate failed to protect against mitochondrial dysfunction and cardiac histopathology. From this we conclude that although dietary vitamin E supplementation enriches cardiac mitochondrial membranes with alpha-tocopherol, either (1) this tocopherol enrichment is not sufficient to protect cardiac mitochondrial membranes from DOX toxicity or (2) oxidative stress alone is not responsible for the persistent mitochondrial cardiomyopathy caused by long-term DOX therapy.

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.

Mitochondrial electron transport inhibitors cause lipid peroxidation-dependent and -independent cell death: protective role of antioxidants

Mitochondrial electron transport inhibitors induced two distinct pathways for acute cell death: lipid peroxidation-dependent and -independent in isolated rat hepatocytes. The toxic effects of mitochondrial complex I and II inhibitors, rotenone (ROT) and thenoyltrifluoroacetone (TTFA), respectively, were dependent on oxidative stress and lipid peroxidation, while cell death induced by inhibitors of complexes III and IV, antimycin A (AA) and cyanide (CN), respectively, was caused by MMP collapse and loss of cellular ATP. Accordingly, cellular and mitochondrial antioxidant depletion or supplementation, in general, resulted in a dramatic potentiation or prevention, respectively, of toxic injury induced by complex I and II inhibitors, with little or no effect on complex III and IV inhibitor-induced toxicity. ROT-induced oxidative stress was prevented by the addition of d-alpha-tocopheryl succinate (TS) but surprisingly TS did not afford hepatocytes protection against TTFA-induced oxidative damage. TS treatment prevented ROT-induced mitochondrial lipid hydroperoxide formation but had no effect on the loss of mitochondrial GSH or cellular ATP, suggesting a mitochondrial lipid peroxidation-mediated mechanism for ROT-induced acute cell death. In contrast, only fructose treatment provided excellent cytoprotection against AA- and CN-induced toxicity. Our findings indicate that complex III and IV inhibitors cause a rapid and severe depletion of cellular ATP content resulting in acute cell death that is dependent on cellular energy impairment but not lipid peroxidation. In contrast, inhibitors of mitochondrial complex I or II moderately deplete cellular ATP levels and thus cause acute cell death via a lipid peroxidation pathway.

Enhanced antioxidant and cytoprotective abilities of vitamin E succinate is associated with a rapid uptake advantage in rat hepatocytes and mitochondria

Numerous in vitro studies attest to the enhanced ability of vitamin E succinate (TS), as compared with conventional vitamin E compounds such as unesterified d-alpha-tocopherol (T) and d-alpha-tocopheryl acetate (TA), to protect hepatocytes from toxic oxidative stress. In the present study we tested the hypothesis that this unique protective ability is related to an enhanced cellular accumulation of TS. The results of this study indicate, using both in vitro and in vivo model systems, that acute TS administration results in a rapid increase in T and TS content and antioxidant protection of hepatocytes and mitochondria. In contrast, conventional vitamin E compounds such as T and TA lack these same protective properties. We suggest that TS acts as a unique delivery system for T, rapidly accumulating in cellular and mitochondrial membranes and gradually releasing active T to prevent membrane oxidative damage. We propose that TS administration may prove useful for the prevention and treatment of oxidative stress-mediated diseases, especially those of mitochondrial origin.

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.

Antiproliferative and apoptotic effects of tocopherols and tocotrienols on normal mouse mammary epithelial cells

Studies were conducted to determine the comparative effects of tocopherols and tocotrienols on normal mammary epithelial cell growth and viability. Cells isolated from midpregnant BALB/c mice were grown within collagen gels and maintained on serum-free media. Treatment with 0-120 microM alpha- and gamma-tocopherol had no effect, whereas 12.5-100m microM tocotrienol-rich fraction of palm oil (TRF), 100-120 microM delta-tocopherol, 50-60 microM alpha-tocotrienol, and 8-14 microM gamma- or delta-tocotrienol significantly inhibited cell growth in a dose-responsive manner. In acute studies, 24-h exposure to 0-250 microM alpha-, gamma-, and delta-tocopherol had no effect, whereas similar treatment with 100-250 microM TRF, 140-250 microM alpha-, 25-100 microM gamma- or delta-tocotrienol significantly reduced cell viability. Growth-inhibitory doses of TRF, delta-tocopherol, and alpha-, gamma-, and delta-tocotrienol were shown to induce apoptosis in these cells, as indicated by DNA fragmentation. Results also showed that mammary epithelial cells more easily or preferentially took up tocotrienols as compared to tocopherols, suggesting that at least part of the reason tocotrienols display greater biopotency than tocopherols is because of greater cellular accumulation. In summary, these findings suggest that the highly biopotent gamma- and delta-tocotrienol isoforms may play a physiological role in modulating normal mammary gland growth, function, and remodeling.

Glutathione-dependent regulation of nitric oxide production in isolated rat hepatocyte suspensions

Freshly isolated suspensions of rat parenchymal liver cells (hepatocytes) spontaneously produce large amounts of nitrite following collagenase isolation. Our previous studies indicate that nitrite production is associated with the expression of inducible nitric oxide synthase (iNOS) and reflects NO production. Depletion of glutathione (GSH) with diethylmaleate (DEM) inhibited nitrite production, and this inhibition was time-dependent. DEM was more effective in blocking nitrite production if it was added within the first 1 hr of the start of the incubation. The reducing agent dithiothreitol (DTT) and the alkylating agent ethyl methanesulfonate (EMS) also inhibited hepatocyte nitrite production, and this inhibition was also greatest if they were added within 1 hr of initiating the incubation. However, EMS added at 3 hr still reduced 6-hr nitrite production by about 70%. This reduction in nitrite production by EMS added at 3 hr may be due to the direct modification of thiol groups on the iNOS protein because we have determined that iNOS activity is inhibited by the sulfhydryl modifying reagent N-ethylmaleimide (NEM). Western blots also indicate that the iNOS protein is expressed when EMS is added at 3 hr. The addition of DEM, DTT, or EMS at 0 time greatly reduced the levels of cellular iNOS mRNA relative to controls as determined by quantitative RT-PCR. Based on our results with mRNA levels, both DTT and depletion of cellular GSH appear to inhibit the early signaling events leading to iNOS expression and suggest that the control of iNOS induction in hepatocytes is sensitive to the thiol redox status of the cell.

Characterization of nitric oxide production following isolation of rat hepatocytes

Freshly isolated suspensions of rat parenchymal liver cells (hepatocytes) produce large amounts of nitrite following isolation. Nitrite production was inhibited by the inducible nitric oxide synthase (iNOS) inhibitor aminoguanidine, as well as the transcription inhibitor actinomycin D. Increases in iNOS mRNA, protein, and activity levels correlated with the formation of nitrite. iNOS mRNA was first detectable 2 h after the onset of hepatocyte incubations and peaked at 4 h. These results indicate that nitrite formation is a result of the large scale production of nitric oxide (NO) by hepatocytes in response to the time-dependent induction of iNOS. NO production by hepatocytes was attenuated by pretreatment of rats with the Kupffer cell inhibitor, gadolinium chloride. Also, the addition of the endotoxin neutralizing agent, polymyxin B; the protein kinase inhibitor, staurosporine, and antioxidants to perfusion buffers and hepatocyte suspensions also decreased nitrite formation. Collectively, our results suggest that iNOS is induced in hepatocytes in response to the stresses generated during collagenase isolation procedures. The response appears to be triggered by a complex interaction between several different factors including Kupffer cell activation, reactive oxygen species generation, and endotoxin contamination of collagenase preparations.

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.

Administration of the tris salt of alpha-tocopheryl hemisuccinate inactivates CYP2E1, enhances microsomal alpha-tocopherol levels and protects against carbon tetrachloride-induced hepatotoxicity

A series of tocopherol compounds were examined for their capacity to protect against carbon tetrachloride (CCl4)-induced hepatotoxicity in rats. Of the tocopherol compounds tested in our study, only the tris salt of d-alpha-tocopheryl hemisuccinate (TS-tris) protected against CCl4-induced hepatotoxicity. The administration of d-alpha-tocopherol (alpha-T) and the nonhydrolyzable tocopherol ether, d-alpha-tocopheryloxybutyrate tris salt (TSE-tris), failed to protect against CCl4-induced hepatotoxicity. TS-tris was the only tocopherol which significantly decreased CYP2E1 activity after 18 h. This decrease in CYP2E1 activity is likely to limit the activation of CCl4 and protect against CCl4-induced hepatotoxicity. Our results also suggest that TS-tris protection against CCl4-induced hepatotoxicity correlates with the enhanced capacity of TS-tris to deliver alpha-T and increase the antioxidant status of hepatocytes. TSE-tris did not increase cellular alpha-T levels, while administration of TS-tris produced large increases in alpha-T levels in liver homogenates as well as in liver nuclei, microsomes, mitochondria and plasma membranes. This enhanced ability to deliver tocopherol equivalents to parenchymal liver cells may be related in part to the ability of TS-tris to form liposomes in aqueous solutions. TS-tris administration protected against CCl4-induced microsomal lipid peroxide formation and inactivation of the microsomal enzyme glucose-6-phosphatase (G6Pase). Supplementation of animals with alpha-T protected against microsomal lipid peroxide formation but not against the inactivation of G6Pase. Based on our findings, we propose that high cellular levels of alpha-T protect against CCl4-induced hepatotoxicity by scavenging CCl4 radicals as well as protecting against lipid peroxidation. Our results do not support the importance of microsomal lipid peroxidation as an early event in acute CCl4-induced hepatic necrosis.

A fluorescence plate reader assay for monitoring the susceptibility of biological samples to lipid peroxidation

The susceptibility of biological samples to lipid peroxidation can be determined by exposing samples to a lipid peroxidation initiator and measuring the length of time prior to the onset of lipid peroxidation. Previous studies have shown that aldehydes generated by lipid peroxidation can react with amines to produce fluorescent products. We have utilized this principle to develop a fluorescence plate reader assay for measuring susceptibility to lipid peroxidation. In this assay, samples are placed in glycine/phosphate buffer and loaded into a 96-well plate. Lipid peroxidation initiators are added, and fluorescence is monitored over time. Samples were assayed for susceptibility to lipid peroxidation by both the thiobarbituric acid reactive substances assay and the fluorescence plate reader assay. We found good agreement between these two methods in assessing relative susceptibility to lipid peroxidation in liver microsomes and mitochondria. The fluorescence assay was also used to monitor lipid peroxidation in liposomes and rat liver homogenates. Fluorescence was stable over an extended time period and could be induced by a variety of lipid peroxidation initiators. The fluorescence plate reader assay offers a rapid method for monitoring lipid peroxidation in a large number of samples.

Sensitive method for measuring tissue alpha-tocopherol and alpha-tocopheryloxybutyric acid by high-performance liquid chromatography with fluorometric detection

The nonhydrolysable tocopherol ether analog, d-alpha-tocopheryloxybutyric acid (TSE), and its tocopherol ester counterpart, d-alpha-tocopheryl hemisuccinate (TS), have been shown to possess anti-tumor activity. In the present study, a sensitive high-performance liquid chromatography (HPLC) method using fluorometric detection is described for the simultaneous determination of TSE and alpha-T in biological specimens. Maximal sensitivity for the measurement of TSE and alpha-T was observed with the wavelengths, 210 nm excitation and 300 nm emission. Using an internal standard (I.S.) method, the amount of these tocopherol compounds was determined in standards, liver homogenates isolated from rats administered TSE-tris salt or vehicle (saline) and in HL-60 human leukaemia cells incubated with TSE-tris salt or saline. Treatment with TSE resulted in the significant accumulation of TSE, but not alpha-T, in the liver and HL-60 cells.

alpha-Tocopheryl hemisuccinate administration increases rat liver subcellular alpha-tocopherol levels and protects against carbon tetrachloride-induced hepatotoxicity

Rats were administered a series of tocopherol analogs 18 h prior to a hepatotoxic dose of carbon tetrachloride (CCl4). Of the compounds tested, only d-alpha-tocopheryl hemisuccinate (TS) provided significant protection against CCl4-induced hepatotoxicity. No protection was observed with either d-alpha-tocopherol (alpha-T) or a tocopherol succinate ether derivative, d-alpha-tocopheryloxybutyric acid (TSE). None of the tocopherol analogs significantly inhibited CYP2E1 activity as measured by oxidation of p-nitrophenol. Liver homogenates and subcellular fractions (cytosol, nuclei, plasma membranes, mitochondria and microsomes) were collected 18 h after tocopherol analog administration in the absence of CCl4. Homogenate and subcellular alpha-T levels were not significantly increased following TSE administration but were increased 2-3 fold following TS and alpha-T administration. Total tocopherol levels (alpha-T+ TS + TSE) in liver homogenates and subcellular fractions were highest in rats supplemented with TS. In these animals, TS was detected in all subcellular fractions and total tocopherol levels were increased from 6-23 fold over those seen in controls and 2-9 fold over alpha-T treated rats. In vitro studies in which liver homogenates and subcellular fractions were peroxidized with ascorbate and ADP/Fe suggest that increasing levels of alpha-T but not TS correlates with increased protection against lipid peroxidation. These results suggest that the ability of TS to protect against CCl4-induced hepatotoxicity relates to its enhanced hepatic accumulation and subsequent hydrolysis to alpha-T.

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

Suspensions of rat hepatocytes treated with the alkylating agent ethyl methanesulfonate (EMS) exhibited extensive lipid peroxidation as well as rapid and near complete depletion of cellular reduced glutathione (GSH) levels prior to cell death. Pretreatment of hepatocytes with medium deficient in sulfur amino acids accelerated cell death induced by EMS, confirming the previously reported cytoprotective role for GSH in this toxic event. Nearly all of the cellular GSH lost following 50 mM EMS treatment was accounted for as S-ethyl glutathione (GS-Et). No significant formation of glutathione disulfide was observed. The GS-Et formed was not exported from the cell but remained at high intracellular concentrations throughout the course of the experiment. In addition, EMS treatment inhibited the efflux of intracellular GSH and inhibited the cellular accumulation of glutamate (Glu). Supplementation of hepatocytes with 25 microM d-alpha-tocopheryl hemisuccinate (TS) protected these cells against EMS-induced lipid peroxidation and cell death. Cytoprotection with TS had no effect on EMS-induced depletion of intracellular GSH or intracellular levels of GS-Et or Glu. However, TS supplementation did prevent EMS-induced depletion of cellular protein thiols. Interestingly, the pretreatment of hepatocytes with 1 mM dithiothreitol promoted EMS toxicity. The results of this study suggest that the cytoprotective abilities of TS are related to the prevention of both EMS-induced lipid peroxidation and protein thiol depletion. Thus, the onset of lipid peroxidation and the loss of protein thiols in hepatocytes appear to be critical cellular events leading to EMS-induced cell death.

Cholesteryl hemisuccinate treatment protects rodents from the toxic effects of acetaminophen, adriamycin, carbon tetrachloride, chloroform and galactosamine

In addition to its use as a stabilizer/rigidifier of membranes, cholesteryl hemisuccinate, tris salt (CS) administration has also been shown to protect rats from the hepatotoxic effects of carbon tetrachloride (CCl4). To further our understanding of the mechanism of CS cytoprotection, we examined in rats and mice the protective abilities of CS and the non-hydrolyzable ether form of CS, gamma-cholesteryloxybutyric acid, tris salt (CSE) against acetaminophen-, adriamycin-, carbon tetrachloride-, chloroform- and galactosamine-induced toxicity. The results of these studies demonstrated that CS-mediated protection is not selective for a particular species, organ system or toxic chemical. A 24-h pretreatment of both rats and mice with a single dose of CS (100mg/kg, i.p.), resulted in significant protection against the hepatotoxic effects of CCl4, CHCl3, acetaminophen and galactosamine and against the lethal (and presumably cardiotoxic) effect of adriamycin administration. Maximal CS-mediated protection was observed in experimental animals pretreated 24 h prior to the toxic insult. These data suggest that CS intervenes in a critical cellular event that is an important common pathway to toxic cell death. The mechanism of CS protection does not appear to be dependent on the inhibition of chemical bioactivation to a toxic reactive intermediate (in light of the protection observed against galactosamine hepatotoxicity). However, based on the data presented, we can not exclude the possibility that CS administration inhibits chemical bioactivation. Our findings do suggest that CS-mediated protection is dependent on the action of the intact anionic CS molecule (non-hydrolyzable CSE was as protective as CS), whose mechanism has yet to be defined.

Growth inhibition of MCF-7 and MCF-10A human breast cells by alpha-tocopheryl hemisuccinate, cholesteryl hemisuccinate and their ether analogs

The growth inhibitory properties of alpha-tocopheryl hemisuccinate (vitamin E succinate) and related compounds were examined in MCF-7 human breast tumor cells and MCF-10A normal-like human breast cells since they have been suggested to be an effective antitumor compound. The data showed that both alpha-tocopherol hemisuccinate and a structurally-similar compound, cholesteryl hemisuccinate, inhibited the growth of MCF-7 and MCF-10A cells, while alpha-tocopherol, cholesterol, cholesteryl sulfate and Tris succinate had little effect on cell growth. The ether analogs of the succinate esters, alpha-tocopheryloxybutyric acid and cholesteryloxybutyric acid, also inhibited growth of MCF-7 and MCF-10A cells, indicating that hydrolysis of the succinate esters by esterases is not required for the antiproliferative effects. The antiproliferative effects of these succinate esters and ethers may be related to their physiochemical properties that allow incorporation into cell membranes.

Protection of acetaminophen-induced hepatocellular apoptosis and necrosis by cholesteryl hemisuccinate pretreatment

This study of acetaminophen (AAP) hepatotoxicity examined whether some aspects of the highly integrated process of drug-induced toxicity involves apoptosis, in addition to necrosis in vivo; and if so, whether cholesteryl hemisuccinate (CS) pretreatment would selectively interfere with apoptotic or necrotic liver cell death. We have previously demonstrated that CS preexposure in vivo, protects hepatocellular necrosis and necrosis-related events induced by carbon tetrachloride (CCl4) administration. Our study demonstrates that administration of hepatotoxic doses of AAP (350-500 mg/kg, i.p.) to ICR mice (CD-1) results in severe liver injury leading to cell death both by necrosis and apoptosis. AAP-induced cell death was preceded by massive elevation in serum alanine aminotransferase coupled with rapid loss of large genomic DNA (2-24 hr), fragmentation of DNA in the form of a ladder (2-24 hr), apoptotic nuclear condensation at early hours (2-6 hr) followed by massive fragmentation and margination of heterochromatin at later (6-24) hours and a near total loss of glycogen in pericentral areas. Although CS (100 mg/kg, i.p.) alone had no noticeable biochemical or morphological effects, its administration before AAP (350-500 mg/kg, i.p.) abrogated histological and biochemical diagnostics of both apoptosis and necrosis. These include near total absence of loss of large genomic DNA and glycogen, and dramatic protection from escalating levels of liver injury. CS pretreatment also arrested AAP-induced ultrastructural changes typical of both apoptosis and necrosis. Histopathological examination of periodic acid-Schiff stained liver sections mirrored the biochemical and ultrastructural findings. In conclusion, this study for the first time establishes that apoptosis, in addition to necrosis, significantly contributes to AAP hepatotoxicity in vivo, and preexposure of mice to CS prevents AAP-induced hepatic apoptosis and necrosis.

Tetrahydroaminoacridine-induced ribosomal changes and inhibition of protein synthesis in rat hepatocyte suspensions

Tacrine (tetrahydroaminoacridine) is currently the only drug approved for the treatment of Alzheimer's disease. Unfortunately, tetrahydroaminoacridine therapy is often limited by this drug's propensity to induce reversible hepatotoxicity. Using suspensions of freshly isolated rat hepatocytes, we investigated the mechanism of tetrahydroaminoacridine cytotoxicity by examining the effect of tetrahydroaminoacridine on hepatocyte viability, protein synthesis, protein, DNA and RNA levels and ultrastructure. Our experimental findings support the explanation that tetrahydroaminoacridine-induced hepatotoxicity results from tetrahydroaminoacridine's adverse effect on protein synthesis and ribosomal structure and function. We found that viable, tetrahydroaminoacridine-treated hepatocytes (1.0 to 2.0 mmol/L or 118 to 235 micrograms/10(6) cells) demonstrated a dose-dependent and dramatic aggregation of ribosomes on endoplasmic reticulum as well as the aggregation of other nucleic acids found in the nucleus (chromatin) and in mitochondria. These electron microscopy data suggest that tetrahydroaminoacridine treatment results in severe ribosomal dysfunction. This was confirmed by the observed rapid loss of cellular RNA content (but not DNA or protein) and the rapid and complete inhibition of protein synthesis in tetrahydroaminoacridine-treated cells (lowest concentration tested was 0.5 mmol/L or 58 micrograms/10(6) cells). Thus tetrahydroaminoacridine treatment appears to aggregate hepatocellular nucleic acids, and in doing so adversely affects ribosomal function and protein synthesis. We propose that these adverse effects of exposure to tetrahydroaminoacridine are responsible for tetrahydroaminoacridine-induced hepatotoxicity.

The selective antiproliferative effects of alpha-tocopheryl hemisuccinate and cholesteryl hemisuccinate on murine leukemia cells result from the action of the intact compounds

In the present study we have established that the antitumor activity of alpha-tocopheryl succinate (TS, vitamin E succinate) and cholesteryl succinate (CS) result from the action of the intact TS and CS compounds and not from the release of alpha-tocopherol, cholesterol, or succinate. We report that treatment of murine leukemia cell lines C1498 (myeloid) and L1210 (lymphocytic), with the tris salts of TS or CS, but not alpha-tocopherol and tris succinate or cholesterol and tris succinate, significantly inhibit the growth of these tumor cells and significantly enhance doxorubicin-induced tumor cell kill in a similar fashion. In contrast, the treatments mentioned above did not adversely affect the growth of murine normal bone marrow cells (colony-forming unit-granulocyte-macrophage). In fact, colony-forming unit granulocyte-macrophage cell growth was stimulated by exposure to CS and TS (as well as their ether analogues) at concentrations above 100 microM. Furthermore, pretreatment of colony-forming unit granulocyte-macrophage cells with TS or CS appears to protect these normal cells from the lethal effect of doxorubicin exposure. Selective inhibition of leukemia cell proliferation (identical to that noted for CS and TS) was also observed following the treatment of cells with the nonhydrolyzable ether forms of CS (cholesteryloxybutyric acid) and TS (alpha-tocopheryloxybutyric acid). These findings suggest that TS, alpha-tocopheryloxybutyric acid, CS, and cholesteryloxybutyric acid may prove clinically useful as selective antitumor agents when administered alone or in combination with doxorubicin by a route that ensures tissue accumulation of the intact compound.

Inhibition of cholinesterase activity by tetrahydroaminoacridine and the hemisuccinate esters of tocopherol and cholesterol

The anticholinesterase properties of tetrahydroaminoacridine (THA, Tacrine), alpha-tocopheryl hemisuccinate (TS), and cholesteryl hemisuccinate (CS), given alone and in combination, were examined in vitro. Results from these studies indicate that: [1] THA is a potent inhibitor of acetylcholinesterase (AChE, IC50 of 0.40 microM) and butyrylcholinesterase (BChE, IC50 of 0.10 microM) with greatest inhibitory activity towards BChE; [2] TS and CS are weak inhibitors of BChE (IC50 of 100 microM and 168 microM, respectively) but potent inhibitors of ACHE (IC50 of 1.73 microM and 0.79 microM, respectively); [3] both TS and CS treatment in combination with THA significantly increased THA's anticholinesterase activity. The percentage AChE inhibition observed with this combination was often significantly greater than the sum of the individual values (synergistic). The addition of 0.5 microM CS or TS to an ACHE preparation reduced THA's IC50 value from 0.40 microM or 0.18 microM, respectively [4]; inhibition of AChE by THA, TS and CS are mixed non-competitive while THA inhibition of BChE is mixed non-competitive and TS and CS inhibition of BChE are simple non-competitive; and [5] inhibition of cholinesterases by TS and CS occurs immediately (50 to 75%), during the first 30 min of incubation (25 to 50%) and is dependent on the anionic charged portion of the molecule. In conclusion, our experimental data indicate that TS and CS are potent inhibitors of AChE activity and significantly potentiate the anticholinesterase activity of THA. Such potent and synergistic inhibition of AChE suggest that TS or CS, alone and in combination with THA, may prove beneficial in the treatment of organophosphate poisoning and Alzheimer's disease.

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

Previous studies from our laboratory have demonstrated that the administration of alpha-tocopheryl hemisuccinate (TS), but not unesterified alpha-tocopherol (T), protects hepatocytes from a variety of toxic insults including chemicals, drugs, metals, and oxidative stress. One possible mechanism for this unique cytoprotection is that succinate released from cellular TS is used as a supplemental energy source during a toxic challenge. To test this hypothesis, we examined the effect of TS (25 microM) administration on cell viability, lipid peroxidation, and several cellular energy-related processes such as mitochondrial membrane potential (MMP, psi delta), lactate formation, and ATP and K+ concentrations in isolated hepatocyte suspensions during a toxic challenge with the alkylating agent, ethyl methanesulfonate (EMS). Data from these studies demonstrate that EMS treatment results in rapid cell death and lipid peroxidation following 2 h of incubation. Preceding EMS-induced cell death was a rapid loss of MMP, intracellular ATP and K+ levels, and mitochondrial ultrastructure as well as a transient increase in cellular lactate production. Pretreatment of hepatocytes with TS prior to EMS exposure prevented the loss of MMP and mitochondrial ultrastructural changes as well as lipid peroxidation and cell death. Cellular ATP levels and lactate production did not reflect the protection afforded to TS-treated hepatocytes. Protection against EMS-induced toxicity was not observed when hepatocytes were: (i) pretreated with TS and esterase inhibitors (preventing T and succinate release from TS); (ii) pretreated with other lipophilic succinate derivatives (cholesteryl hemisuccinate, monomethyl and dimethyl succinate); or (iii) pretreated with T and sodium succinate. Unlike monomethyl succinate, cytoprotective TS pretreatment did not stimulate gluconeogenesis or glycolysis. Hepatocytes isolated from rats pretreated for 24 h with T were not protected from the toxic effects of EMS, unlike TS-pretreated rats. In conclusion, TS cytoprotection against the mitochondrial toxicant EMS appears to be related to the hepatocellular accumulation of TS and the maintenance of mitochondrial function (MMP). Based on our earlier findings and the present observations, we propose that a unique subcellular disposition for TS and the subsequent release of T and succinate at a critical mitochondrial site is responsible for the observed cytoprotection.

Protection against carbon tetrachloride-induced hepatotoxicity by pretreating rats with the hemisuccinate esters of tocopherol and cholesterol

Previous studies have demonstrated that alpha-tocopheryl hemisuccinate (TS) protects hepatocyte suspensions from chemical-induced toxicity. It has been suggested that TS cytoprotection is related to unique properties of the TS molecule or is dependent on the cellular release and activity of unesterified alpha-tocopherol (T). To test the unique cytoprotective nature of TS in vivo, the protective ability of T and tocopherol esters against carbon tetrachloride (CCl4)-induced hepatotoxicity in rats was examined. Hepatoprotection [determined by serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels and histopathology] was not observed after T (or tocopheryl acetate and tocopheryl nicotinate) administration, even though this treatment resulted in a fivefold elevation in hepatic T content. Only pretreatment with TS (100 mg/kg, intraperitoneally) resulted in partial hepatoprotection against CCl4 (2.9 g/kg, orally) toxicity. These findings suggest that hepatoprotection results not from the cellular accumulation of T but rather from the intact TS molecule. To test this hypothesis, the hepatoprotective capacity of cholesteryl hemisuccinate (CS), unesterified cholesterol, and cholesteryl acetate (CA) was examined against CCl4 toxicity. As observed with the tocopherol derivatives, pretreatment with unesterified cholesterol or CA demonstrated no protective ability. However, when rats were pretreated with CS (100 mg/kg), the hepatotoxic effects of CCl4 (elevated serum AST and ALT levels and centrilobular necrosis) were completely prevented. The prevention of CCl4-induced hepatotoxicity by CS and TS do not appear to result from an alteration in hepatic drug metabolism. These data clearly demonstrate that CS and TS are unique and powerful cytoprotective agents against CCl4 hepatotoxicity in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

Cadmium toxicity: unique cytoprotective properties of alpha tocopheryl succinate in hepatocytes

Rat hepatocyte suspensions were exposed to toxic concentrations of cadmium (Cd) in the presence and absence of unesterified alpha-tocopherol (T) or alpha-tocopheryl succinate (TS). The exogenous administration of TS completely protected hepatocytes from Cd-induced injury and lipid peroxidation. However, hepatocytes exposed to T were not protected from the toxic manifestations of cadmium even though this treatment resulted in a rapid marked accumulation of cellular T. The rate of cadmium uptake by hepatocytes was not significantly altered by exogenous TS or T treatment. These studies indicate that TS cytoprotection against Cd toxicity results not from alterations in Cd uptake or the accumulation of T but rather from the cellular presence of the intact TS molecule. The data also indicate that the depletion of cellular T is not the critical cellular event that is responsible for Cd-induced injury. Instead it appears that TS possess unique cytoprotective properties that intervene in the critical cellular events that lead to Cd toxicity. Thus, TS administration represents a promising new strategy for the mechanistic study and prevention of tissue damage resulting from Cd exposure.

Oxygen toxicity: unique cytoprotective properties of vitamin E succinate in hepatocytes

Freshly isolated rat hepatocytes suspensions were incubated under an atmosphere of 95% O2/5% CO2 or 95% air/5% CO2 for 10 h. Cell injury and death were observed between the 6th and 10th hour of incubation, only in 95% O2-treated hepatocytes. Oxygen-induced injury was preceded by marked lipid peroxidation and rapid depletion of cellular alpha tocopherol content. The exogenous administration of unesterified alpha tocopherol (T, 25 microM) resulted in a 20-fold increase in cellular T levels (4.2 nmol/10(6) cells) but failed to protect these hepatocytes from the toxic effects of oxygen. In contrast, hepatocytes incubated with 25 microM of the succinate ester of alpha tocopherol (TS) contained both TS (3.0 nmol/10(6) cells) and T (1.4 nmol/10(6) cells) and were completely protected from the toxic effects of oxygen, including the induction of lipid peroxidation. These findings suggest that TS cytoprotection results not from the cellular accumulation of T but rather, from cellular TS accumulation. The data also indicate that the depletion of cellular T is not the critical cellular event that is responsible for hyperoxia (reactive oxygen intermediate)-induced injury. Instead, it appears that TS possesses unique cytoprotective properties that intervene in the critical cellular events that lead to oxygen toxicity. Thus, vitamin E succinate and our hyperoxic hepatocyte preparation provide a promising new model system for the study and prevention of tissue damage resulting from the toxic effects of hyperoxia and reactive oxygen intermediates.

Alpha-tocopheryl succinate protects hepatocytes from chemical-induced toxicity under physiological calcium conditions

Rat and canine hepatocyte suspensions were exposed to toxic concentrations of ethyl methanesulfonate (EMS) and ionophore A-23187 in the presence and absence of extracellular calcium (Ca2+) and alpha-tocopheryl succinate (alpha-TS). The exogenous administration of alpha-TS (25 microM) completely protected hepatocytes from chemically-induced toxicity when exposed to 'physiological' free extracellular calcium concentrations (0.8-1.5 mM). Under these protective conditions the cellular accumulation of both alpha-TS (2.8 nmol/10(6) cells) and alpha-T (0.91 nmol/10(6) cells) were observed. Hepatocytes exposed to unesterified alpha-tocopherol (alpha-T, 25 microM) or alpha-tocopheryl acetate (alpha-TA, 25 microM), however, were not protected from the toxic effect of chemicals even though these treatments resulted in the marked accumulation of cellular alpha-T (2.65 nmol/10(6) cells) and alpha-TA (2.3 nmol/10(6) cells), respectively. Our findings suggest that the supplementation of endogenous stores of alpha-T or alpha-TA does not promote protection against chemical toxicity and that alpha-TS cytoprotection results not from the accumulation of alpha-T but rather from the cellular presence of the intact alpha-TS molecule. Thus alpha-TS appears to possess cytoprotective properties that differ from other vitamin E congeners.

A role of vitamin E in protection against cell injury. Maintenance of intracellular glutathione precursors and biosynthesis

The depletion of cell calcium from isolated rat hepatocytes results in stimulated lipid peroxidation, loss of intracellular and mitochondrial GSH (reduced glutathione), and enhancement of both efflux and oxidation of GSH. These events are followed by cell injury and enhance the susceptibility of the cells to toxic chemicals. It is shown herein that an initial event in the generation of such injury is the depletion of cellular alpha-tocopherol. alpha-Tocopheryl succinate addition (25 microM) to the calcium-depleted cells markedly elevated the alpha-tocopherol content of the cells, inhibited the associated lipid peroxidation, and maintained intracellular GSH levels without affecting its efflux or redox status. This resulted in an enhanced formation of total glutathione after a 5-h incubation, which correlated with the alpha-tocopherol content of the cells, and was greater than that expected by a direct sparing action of vitamin E. Inhibition of hepatocyte glutathione biosynthesis by buthionine sulfoximine (0.5 mM) eliminated the enhancement of GSH formation by vitamin E. Analysis of endogenous and 35S-labelled precursors of glutathione biosynthesis by high-performance liquid chromatography demonstrated that the depletion of cellular alpha-tocopherol resulted in the efflux of glutathione precursors. It is concluded that cell injury associated with alpha-tocopherol depletion is partly the result of the efflux of glutathione precursors, and hence diminished biosynthesis and intracellular levels of GSH. These losses and resultant cell injury are preventable by maintenance of cellular alpha-tocopherol levels.

Mechanism of chemical-induced toxicity. I. Use of a rapid centrifugation technique for the separation of viable and nonviable hepatocytes

A major obstacle in defining the mechanism of chemical-induced toxicity has been the inability to distinguish between events that cause cell death and those that result from cell death. This problem results from measuring biochemical parameters in tissues or cell pellets containing both viable and nonviable cells. In the present study, we described a method for the rapid separation of viable hepatocytes from nonviable cells and medium prior to biochemical analysis. Separation of viable hepatocytes was accomplished in a microcentrifuge tube by layering a sample of isolated hepatocyte suspension over a dibutyl phthalate oil layer and centrifuging for several seconds. As a result, greater than 90% of the hepatocytes centrifuged through dibutyl phthalate were viable while greater than 90% of the cells recovered above the oil layer were nonviable. The separation of viable hepatocytes by the dibutyl phthalate method was not affected by the presence of the hepatotoxins, adriamycin (ADR) in combination with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) or ethyl methanesulfonate (EMS), though the ratio of viable to nonviable cells in the suspension was drastically reduced. The metabolic and morphological integrity of hepatocytes centrifuged through dibutyl phthalate was altered after cell suspensions were treated with the ADR-BCNU or EMS. These chemically treated viable hepatocytes showed degenerative ultrastructural changes and a greater than 80% reduction in intracellular K+ and glutathione concentrations. Because centrifugation through dibutyl phthalate does not significantly alter the concentration of intracellular constituents nor the ultrastructure of control hepatocytes, the signs of reversible injury observed in hepatocytes centrifuged through oil resulted from the chemical treatment. These data indicate that the dibutyl phthalate separation technique offers the advantage of monitoring only viable hepatocytes for changes in membrane integrity or metabolic performance during a toxic chemical insult.

Mechanism of chemical-induced toxicity. II. Role of extracellular calcium

Previous studies disagree as to if chemical-induced cell death is caused by the influx and accumulation of extracellular Ca2+. To determine the role of extracellular Ca2+ in toxic cell death, the viability (leakage of intracellular K+ and lactate dehydrogenase) and total Ca2+ content of isolated hepatocytes incubated in the presence or absence of extracellular Ca2+ were determined during a toxic insult with bromobenzene, ethyl methanesulfonate (EMS), Ca2+ ionophore A23187, and adriamycin (ADR) in combination with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). The present study utilized the dibutyl phthalate separation technique which enabled the analysis of only viable hepatocytes for changes in intracellular Ca2+ and K+ content during toxic cell injury. The three chemical treatments, bromobenzene, EMS, and ADR-BCNU, each caused an accelerated loss of viability in hepatocytes incubated without extracellular Ca2+ as compared to cells incubated with Ca2+. Furthermore, the total Ca2+ content of viable hepatocytes incubated in the presence of extracellular Ca2+ did not increase during chemically induced cell injury as compared to control cells. In fact, a significant decline in total cellular Ca2+ was observed in viable hepatocytes incubated in Ca2+-free medium during toxic cell injury. Treatment with Ca2+ ionophore A23187 was also toxic to hepatocytes incubated in the presence or absence of extracellular Ca2+. At high concentrations of ionophore (20 microM or 4 micrograms/10(6) cells), cell death was accelerated in hepatocytes incubated with Ca2+ as compared to cells incubated in Ca2+-free medium. In contrast, after treatment with lower concentrations of ionophore (10 microM or 2 micrograms/10(6) cells), the rate of cell death was reversed with hepatocytes incubated without extracellular Ca2+ dying first. Thus, depending on the concentration of A23187 and the time of exposure, the presence of extracellular Ca2+ can be shown either to accelerate or protect against cell death. Surprisingly, reversible and irreversible cell injury were not observed in hepatocytes incubated with extracellular Ca2+ and 2 microM A23187 though this treatment resulted in an 800% increase in total intracellular Ca2+ content. We conclude that chemical-induced hepatic cell death is not caused by an increase in total cellular Ca2+ resulting from the influx of extracellular Ca2+.

Vitamin E reversal of the effect of extracellular calcium on chemically induced toxicity in hepatocytes

Isolated rat hepatocytes were incubated in the presence or absence of extracellular calcium and alpha-tocopherol succinate with three different toxic chemicals; namely, adriamycin in combination with 1,3-bis(2-chloroethyl)-1-nitrosourea, ethyl methanesulfonate, and the calcium ionophore A23187. In the absence of extracellular calcium these three compounds were far more toxic to the cells than in its presence. The addition of vitamin E to calcium-free medium, however, protected hepatocytes against toxic injury, whereas cells incubated in medium containing calcium were not protected. Hepatocyte viability during each toxic insult correlated well with the cellular alpha-tocopherol content but not with the presence or absence of extracellular calcium. These results suggest that cellular alpha-tocopherol maintains the viability of the cell during a toxic insult and that the presence or absence of vitamin E in the incubation medium probably explains the conflicting reports on the role of extracellular calcium in toxic cell death.

Extracellular calcium protects isolated rat hepatocytes from injury

The incubation of isolated rat hepatocytes in calcium-free medium resulted in a pronounced increase in lipid peroxidation, mitochondrial and cytoplasmic glutathione depletion, glutathione disulfide formation and efflux of reduced glutathione as compared with hepatocytes incubated in calcium containing medium. These data suggest that extracellular calcium ions serve a protective role in isolated rat hepatocytes against cell injury.

Demonstration of major metabolic pathways for chlordecone (kepone) in humans

The hypothesis that liver is the site of the previously demonstrated chlordecone alcohol formation in man was tested. Human bile obtained from chlordecone-poisoned factory workers contained substantial amounts of free chlordecone, but little free chlordecone alcohol. However, when the same bile specimens were pretreated with beta-glucuronidase before analysis by gas-liquid chromatography, large amounts of chlordecone alcohol appeared, accounting for 75% of total organochlorine compounds. Confirmation of the identity of chlordecone and chlordecone alcohol was made by using gas liquid chromatography-mass spectrometry. Whereas biliary chlordecone alcohol was present predominantly as its glucuronide conjugate (93%), chlordecone was excreted primarily as the unaltered compound (72%) with only a small portion conjugated with glucuronic acid (9%). The remaining fraction of the total chlordecone measured in bile appeared to be a stable polar metabolite resistant to beta-glucuronidase. This unidentified metabolite could be converted to free chlordecone only by acid hydrolysis under harsh conditions. In contrast to human bile, rat bile contained only trace amounts of chlordecone alcohol (less than 0.5% of total chlordecone), thus indicating that hepatic metabolism of chlordecone is species-specific. We conclude that in man, the major metabolic route for chlordecone is its reduction in the liver followed by glucuronidation.

Treatment of chlordecone (Kepone) toxicity with cholestyramine. Results of a controlled clinical trial

Industrial workers exposed to the organochlorine pesticide, chlordecone (Kepone), had signs of toxicity in several organs. The extent of toxicity was proportional to the levels of this chemical in the tissues. In 22 patients, chlordecone was eliminated slowly from blood (half time of 165 +/- 27 days--mean +/- S.E.M.) and fat (half time of 125 days, with a range of 97 to 177), chiefly in the stool. Output of chlordecone in bile was 10 to 20 times greater than in stool, suggesting that chlordecone is reabsorbed in the "ntestine. Cholestyramine, an anion-exchange resin that binds chlordecone, increased its fecal excretion by seven times. In a five-month trial, cholestyramine significantly accelerated elimination of chlordecone from blood, with a half life of 80 +/- 4 days (S.E.M.) (P less than 0.005) and fat (half life of 64 days, with a range of 52 to 85) (P less than 0.05). Cholestyramine offers a practical means for detoxification of persons exposed to chlordecone and possibly to other lipophilic toxins.