Mitochondrial damage by the "pro-oxidant" peroxisomal proliferator clofibrate

Optimization of clofibrate by carvacrol results in a new hypolipidemic compound with low hepatic injury

The hepatic injury caused by clofibrate (CF) is strongly associated with oxidative stress and inflammation. This study aims to develop a hypolipidemic compound that possesses antioxidant, anti-inflammatory properties and reduces liver injury. Carvacrol-clofibrate (CF-Carvacrol) was synthesized by optimizing the structure of CF by carvacrol. CF-Carvacrol showed significant lipid-lowering effects in hyperlipidemic mouse models induced by Triton WR 1339. The molecular docking results showed that CF-Carvacrol has a good affinity for PPAR-α. The liver injury study showed that CF-Carvacrol had significantly lower liver injury compared to CF. Manifested as a significant decrease in liver weight and liver coefficient (P < 0.01), as well as a significant decrease in aspartate aminotransferase (AST), alanine aminotransferase (ALT), and alkaline phosphatase (ALP) (P < 0.01). Histopathology of liver tissue showed that the necrosis of liver cells, cytoplasmic looseness, nuclear degeneration, and infiltration of inflammatory cells were significantly reduced in the CF-Carvacrol group. CF-Carvacrol can significantly up-regulate the expression of Nrf2 and HO-1 in liver (P < 0.05, P < 0.01). The expression of inflammatory factors TNF-α and IL-6 in the liver was significantly down-regulation (P < 0.05, P < 0.01). The levels of superoxide dismutase (SOD) and glutathione (GSH) significantly increased (P < 0.05, P < 0.01), while the content of malondialdehyde (MDA) in lipid peroxidation significantly decreased (P < 0.01). These results revealed that CF-Carvacrol has significant hypolipidemic activity and mild liver injury, and may exert antioxidant and anti-inflammatory activity by activating the Nrf2/HO-1 signaling pathway to reduce liver injury.

The combination of sesamol and clofibric acid moieties leads to a novel potent hypolipidemic agent with antioxidant, anti-inflammatory and hepatoprotective activity

Oxidative stress and inflammation have been considered the main factors in the liver injury of clofibrate (CF). To obtain a novel antihyperlipidemic agent with antioxidant, anti-inflammation and hepatoprotection, the combination of sesamol and clofibric acid moieties was performed and achieved sesamol-clofibrate (CF-Sesamol). CF-Sesamol showed significant hypolipidemia effects in hyperlipidemia mice induced by Triton WR 1339, reducing TG by 38.8% (P < 0.01) and TC by 35.1% (P < 0.01). CF-Sesamol also displayed an alleviating effect on hepatotoxicity. The hepatic weight and hepatic coefficient were decreased. The amelioration of liver function was observed, such as aspartate and lactate transaminases (AST and ALT), alkaline phosphatase (ALP) and total proteins (TP) levels. Liver histopathological examination showed that hepatocyte necrosis, cytoplasmic loosening, nuclear degeneration and inflammatory cell infiltration reduced obviously by treatment with CF-Sesamol. Related molecular mechanisms on hepatoprotection showed that CF-Sesamol up-regulated Nrf2 and HO-1 expression and down-regulated p-NF-κB p65 expression in hepatic tissues. CF-Sesamol has significant antioxidant and anti-inflammatory effects. Plasma antioxidant enzymes such as SOD and CAT increased, anti-lipid peroxidation product MDA decreased. The expression of TNF-α and IL-6 inflammatory cytokines in liver was significantly lower than that in the CF group. The results indicated that CF-Sesamol exerted more potent antihyperlipidemic effects and definite hepatoprotective activity partly through the Nrf2/NF-κB-mediated signaling pathway.

Peroxisome proliferator-activated receptor alpha-mediated drug toxicity in the liver

INTRODUCTION

Drug-induced hepatic injury is the most common cause of acute liver failure in the United States. Peroxisome proliferator-activated receptor alpha (PPARα)-mediated drugs are included among the approximately 900 natural and synthetic substances, which have shown hepatotoxicity. Areas covered: This review will focus on fibrates - PPARα agonists and their implication in causing liver injury. Expert opinion: Compelling evidence for fibrate-induced hepatotoxicity is not available. Results have been varying because several large randomized clinical trials have reported similar elevations of plasma transaminase levels in fibrate or placebo treated groups. On the other hand, one meta-analysis has reported an increased risk of hepatotoxicity when fibrates are combined with statins. Fibrate induced clinically apparent liver damage has been demonstrated in case reports. However, there is a wide spectrum of clinical phenotypic presentations of these cases (onset of injury, pattern of enzyme elevation and resolution of the symptoms), which reduces the ability to identify specific cause and effect of any putative fibrate-induced hepatotoxicity. Thus, the current recommendations for using fibrates include monitoring of aminotransferase levels especially if combined with statins and discontinuation of the treatment only if the levels persist above three times the upper limit of normal.

Microplasma Induced Cell Morphological Changes and Apoptosis of Ex Vivo Cultured Human Anterior Lens Epithelial Cells - Relevance to Capsular Opacification

Inducing selective or targeted cell apoptosis without affecting large number of neighbouring cells remains a challenge. A plausible method for treatment of posterior capsular opacification (PCO) due to remaining lens epithelial cells (LECs) by reactive chemistry induced by localized single electrode microplasma discharge at top of a needle-like glass electrode with spot size ~3 μm is hereby presented. The focused and highly-localized atmospheric pressure microplasma jet with electrode discharge could induce a dose-dependent apoptosis in selected and targeted individual LECs, which could be confirmed by real-time monitoring of the morphological and structural changes at cellular level. Direct cell treatment with microplasma inside the medium appeared more effective in inducing apoptosis (caspase 8 positivity and DNA fragmentation) at a highly targeted cell level compared to treatment on top of the medium (indirect treatment). Our results show that single cell specific micropipette plasma can be used to selectively induce demise in LECs which remain in the capsular bag after cataract surgery and thus prevent their migration (CXCR4 positivity) to the posterior lens capsule and PCO formation.

Lipid peroxides as endogenous oxidants forming 8-oxo-guanosine and lipid-soluble antioxidants as suppressing agents

The oxidation of guanosine to 8-oxo-2'-deoxyguanosine (8-oxo-dG) in DNA is closely associated with induction of various diseases, but the endogenous oxidant species involved remains unclear. Hydrogen peroxides (H2O2) have been considered to be the oxidant, while lipid peroxides are another possible oxidant because generated easily in bio-membranes surrounding DNA. The oxidant potency was compared between lipid peroxides and H2O2. Linoleic acid hydroperoxides (LOOH) formed 8-oxo-dG at a higher level than H2O2 in guanosine or double-stranded DNA. In the presence of a physiological concentration of Fe(2+) to produce hydroxyl radicals, LOOH was also a stronger oxidant. In a lipid micelle, LOOH markedly produced 8-oxo-dG at a concentration one-tenth of that of H2O2. Upon adding to rat hepatic mitochondria, phosphatidylcholine hydroperoxides produced 8-oxo-dG abundantly. Employing HepG2 cells after pretreated with glutathione peroxidase inhibitor, LOOH formed 8-oxo-dG more abundantly than H2O2. Then, antioxidants to suppress the 8-oxo-dG formation were examined, when the nuclei of pre-incubated HepG2 with antioxidants were exposed to LOOH. Water-soluble ascorbic acid, trolox, and N-acetyl cysteine showed no or weak antioxidant potency, while lipid-soluble 2,6-dipalmitoyl ascorbic acid, α-tocopherol, and lipid-soluble phytochemicals exhibited stronger potency. The present study shows preferential formation of 8-oxo-dG upon LOOH and the inhibition by lipid-soluble antioxidants.

Live-cell imaging approaches for the investigation of xenobiotic-induced oxidant stress

BACKGROUND

Oxidant stress is arguably a universal feature in toxicology. Research studies on the role of oxidant stress induced by xenobiotic exposures have typically relied on the identification of damaged biomolecules using a variety of conventional biochemical and molecular techniques. However, there is increasing evidence that low-level exposure to a variety of toxicants dysregulates cellular physiology by interfering with redox-dependent processes.

SCOPE OF REVIEW

The study of events involved in redox toxicology requires methodology capable of detecting transient modifications at relatively low signal strength. This article reviews the advantages of live-cell imaging for redox toxicology studies.

MAJOR CONCLUSIONS

Toxicological studies with xenobiotics of supra-physiological reactivity require careful consideration when using fluorogenic sensors in order to avoid potential artifacts and false negatives. Fortunately, experiments conducted for the purpose of validating the use of these sensors in toxicological applications often yield unexpected insights into the mechanisms through which xenobiotic exposure induces oxidant stress.

GENERAL SIGNIFICANCE

Live-cell imaging using a new generation of small molecule and genetically encoded fluorophores with excellent sensitivity and specificity affords unprecedented spatiotemporal resolution that is optimal for redox toxicology studies. This article is part of a Special Issue entitled Air Pollution, edited by Wenjun Ding, Andrew J. Ghio and Weidong Wu.

Reduction in reactive oxygen species production by mitochondria from elderly subjects with normal and impaired glucose tolerance

OBJECTIVE

Aging increases the risk of developing impaired glucose tolerance (IGT) and type 2 diabetes. It has been proposed that increased reactive oxygen species (ROS) generation by dysfunctional mitochondria could play a role in the pathogenesis of these metabolic abnormalities. We examined whether aging per se (in subjects with normal glucose tolerance [NGT]) impairs mitochondrial function and how this relates to ROS generation, whether older subjects with IGT have a further worsening of mitochondrial function (lower ATP production and elevated ROS generation), and whether exercise reverses age-related changes in mitochondrial function.

RESEARCH DESIGN AND METHODS

Mitochondrial ATP and ROS production were measured in muscle from younger individuals with NGT, older individuals with NGT, and older individuals with IGT. Measurements were performed before and after 16 weeks of aerobic exercise.

RESULTS

ATP synthesis was lower in older subjects with NGT and older subjects with IGT versus younger subjects. Notably, mitochondria from older subjects (with NGT and IGT) displayed reduced ROS production versus the younger group. ATP and ROS production were similar between older groups. Exercise increased ATP synthesis in the three groups. Mitochondrial ROS production also increased after training. Proteomic analysis revealed downregulation of several electron transport chain proteins with aging, and this was reversed by exercise.

CONCLUSIONS

Old mitochondria from subjects with NGT and IGT display mitochondrial dysfunction as manifested by reduced ATP production but not with respect to increased ROS production. When adjusted to age, the development of IGT in elderly individuals does not involve changes in mitochondrial ATP and ROS production. Lastly, exercise reverses the mitochondrial phenotype (proteome and function) of old mitochondria.

Peroxisomes are oxidative organelles

Peroxisomes are multifunctional organelles with an important role in the generation and decomposition of reactive oxygen species (ROS). In this review, the ROS-producing enzymes, as well as the antioxidative defense system in mammalian peroxisomes, are described. In addition, various conditions leading to disturbances in peroxisomal ROS metabolism, such as abnormal peroxisomal biogenesis, hypocatalasemia, and proliferation of peroxisomes are discussed. We also review the role of mammalian peroxisomes in some physiological and pathological processes involving ROS that lead to mitochondrial abnormalities, defects in cell proliferation, and alterations in the central nervous system, alcoholic cardiomyopathy, and aging. Antioxid.

Behaviour and biomarkers of oxidative stress in Gambusia holbrooki after acute exposure to widely used pharmaceuticals and a detergent

Pharmaceuticals are continuously dispersed into the environment, as a result of human and veterinary use, and have become a relevant environmental concern. In the present study, the acute toxicity of three therapeutic agents (diazepam, clofibrate, and clofibric acid) and a detergent, sodium dodecylsulphate (SDS), to the euryhaline fish Gambusia holbrooki was evaluated. Special attention was devoted to oxidative stress parameters. G. holbrooki males, captured in the estuary of the Minho River (NW Portugal), were exposed for 96 h to the selected compounds. The following oxidative stress biomarkers were evaluated in gills and liver tissues: reduced and oxidised glutathione, lipid peroxidation, and several antioxidant enzymes, namely (1) total and selenium-dependent glutathione peroxidase (GPx), (2) glutathione reductase (GRed), (3) copper-zinc superoxide dismutase (Cu-ZnSOD) and manganese superoxide dismutase (MnSOD), and (4) glutathione-S-transferases (GSTs). In the particular case of diazepam, swimming behaviour was also evaluated. The obtained results indicate an overall diminished oxidative stress response caused by SDS and diazepam. Oxidative-based alterations were observed after exposure to clofibrate and clofibric acid, with modifications of several enzymatic activities. Diazepam caused evident behavioural changes: animals showed dark pigmentation and also abnormal postures, namely lethargy and anomalous movement.

Strategies to reduce late-stage drug attrition due to mitochondrial toxicity

Mitochondrial dysfunction is increasingly implicated in the etiology of drug-induced toxicities and negative side-effect profiles. Early identification of mitochondrial liabilities for new chemical entities is therefore crucial for avoiding late-stage attrition during drug development. Limitations of traditional methods for assessing mitochondrial dysfunction have discouraged routine evaluation of mitochondrial liabilities. To circumvent this bottleneck, a high-throughput screen has been developed that measures oxygen consumption; one of the most informative parameters for the assessment of mitochondrial status. This technique has revealed that some, but not all, members of many major drug classes have mitochondrial liabilities. This dichotomy encourages optimism that efficacy can be disassociated from mitochondrial toxicity, resulting in safer drugs in the future.

Bioavailable flavonoids to suppress the formation of 8-OHdG in HepG2 cells

Antioxidative flavonoids, ubiquitously included in vegetables, fruits and teas, are expected to prevent degenerative diseases. It is unclear, however, whether flavonoids can enter the cellular nuclei and suppress the oxidative damage of DNA. Here, several flavonoids at the physiological concentration of 10 microM were dosed to 2.5x10(7) HepG2 cells. The nuclei were isolated and determined in the incorporated flavonoid levels, and simultaneously exposed to reactive oxygen generated from 25 mM of 2,2'-azobis(2-amidinopropane) dihydrochloride. Most of the tested flavonoids were incorporated into the cells in the range between 1000 and 1600 pmol/10(7) cells, and were in the nuclei at 250-450 pmol/10(7) cells at the maximum incorporation after 30min of cell incubation. In the cells, 23% of quercetin (3,5,7,3',4'-OH) and 8% of luteolin (5,7,3',4'-OH) were the original aglycone forms and the others were the methylated and gulucuronide/sulfate conjugates, while 72% of kaempferol (3,5,7,4'-OH) and 85% of apigenin (5,7,4'-OH) were aglycones and located in the nuclei at the similar ratio of metabolites. Quercetin and luteolin significantly suppressed the formation of 8-oxo-7,8-dihydrodeoxyguanosine by 25% and 15%, respectively, compared to those in 0-time incubated cells with the flavonoids. Under such conditions of low level and hydroxyl-masked in the nuclei, the limited flavonoids were bioavailable antioxidants to prevent genetic damage and they were B-ring catechols such as quercetin and luteolin.

Co-induction of methyltransferase Rv0560c by naphthoquinones and fibric acids suggests attenuation of isoprenoid quinone action in Mycobacterium tuberculosis

The superoxide generator menadione was previously demonstrated as an inducer of growth stage dependent protein patterns in Mycobacterium tuberculosis. The present study refines this observation by characterizing a novel 27-kDa protein that had not been observed in previous studies relying on younger cultures. A very similar response, based on two-dimensional gel electrophoretic analyses, was induced by the closely related naphthoquinone plumbagin. The 27-kDa protein was also induced by the pro-oxidant peroxisome proliferator gemfibrozil and to a lesser extent by the structurally related compounds fenofibrate and clofibrate. N-terminal sequence data of proteolytic fragments from the 27-kDa protein demonstrated its identity with protein Rv0560c, previously demonstrated to be inducible by salicylate, which also possesses peroxisome proliferating properties. Protein Rv0560c bears three conserved motifs characteristic of S-adenosylmethionine-dependent methyltransferases. Further sequence similarities suggest a function in the bio syn thesis of isoprenoid compounds, e.g., tocopherol, ubiquinone, and sterols. Such involvement is supported by the recognized yet unexplained widespread interference of menadione, salicylate, and fibrates with the isoprenoid quinones ubiquinone, menaquinone, and vitamin K. Induction of Rv0560c by fibrates, salicylate, and naphthoquinones is thus suggested to be caused by action on the plasma membrane, reminiscent of cytochrome P450BM-3 induction by fibrates in Bacillus megaterium, which catalyzes the hydroxylation of fatty acids and thus modulates membrane properties.

Mutagenicity of the peroxisome proliferators clofibrate, Wyeth 14,643 and di-2-ethylhexyl phthalate in the lacZ plasmid-based transgenic mouse mutation assay

Background

Peroxisome proliferators are considered rodent carcinogens that are putative human non-carcinogens based on the presumed absence of direct genetic toxicity in rodent and human cells and the resistance of human cells to the induction of peroxisomes by peroxisome proliferators. The highly sensitive lacZ plasmid-based transgenic mouse mutation assay was employed to investigate the mutagenicity of several peroxisome proliferators based on several lines of evidence suggesting that these agents may in fact exert a genotoxic effect.

Methods

Male and female lacZ-plasmid based transgenic mice were treated at 4 months of age with 6 doses of 2,333 mg di-2-ethylhexyl phthalate (DHEP), 200 mg Wyeth-14,643, or 90 mg clofibrate per kg of bodyweight, respectively, over a two-week period. Control animals were treated with the respective vehicles only (35% propyl glycol for DEHP and Wyeth-14,643 treatment controls and sterile water for clofibrate treatment controls).

The mutant frequency in liver, kidney and spleen DNA was determined as the proportion of retrieved mutant and wild-type lacZ plasmids expressed in Escherichia Coli C host cells employing a positive selection system for mutant plasmids.

Results

Exposure to DEHP or Wyeth-14,643 significantly increased the mutant frequency in liver, but not in kidney or spleen, of both female and male mice. Treatment with clofibrate did not lead to an increased mutant frequency in any of the organs studied.

Conclusion

The results indicate that some peroxisome proliferators display an organ-specific mutagenicity in lacZ plasmid-based transgenic mice consistent with historical observations of organ- and compound-specific carcinogenicity.

Effects of human pharmaceuticals on cytotoxicity, EROD activity and ROS production in fish hepatocytes

Pharmaceuticals are found in the aquatic environment but their potential effects on non-target species like fish remain unknown. This in vitro study is a first approach in the toxicity assessment of human drugs on fish. Nine pharmaceuticals were tested on two fish hepatocyte models: primary cultures of rainbow trout hepatocytes (PRTH) and PLHC-1 fish cell line. Cell viability, interaction with cytochrome P450 1A (CYP1A) enzyme and oxidative stress were assessed by using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrasodium bromide tetrazolium (MTT), 7-ethoxyresorufin-o-deethylase (EROD) and dichlorofluorescein (DCFH-DA) assays, respectively. The tested drugs were clofibrate (CF), fenofibrate (FF), carbamazepine (CBZ), fluoxetine (FX), diclofenac (DiCF), propranolol (POH), sulfamethoxazole (SFX), amoxicillin (AMX) and gadolinium chloride (GdCl(3)). All substances were cytotoxic, except AMX at concentration up to 500 microM. The calculated MTT EC(50) values ranged from 2 microM (CF) to 651 microM (CBZ) in PLHC-1, and from 53 microM (FF) to 962 microM (GdCl(3)) in PRTH. CF, FF, and FX were the most cytotoxic drugs and induced oxidative stress before being cytotoxic. Compared to hepatocytes from human and dog, fish hepatocytes seemed to be more susceptible to the peroxisome proliferators (PPs) CF and FF. In PLHC-1 cells none of the tested drugs induced the EROD activity whereas POH appeared as a weak EROD inducer in PRTH. Moreover, in PRTH, SFX, DiCF, CBZ and to a lesser extend, FF and CF inhibited the basal EROD activity at clearly sublethal concentrations which may be of concern at the biological and chemical levels in a multipollution context.

Transcription profiling distinguishes dose-dependent effects in the livers of rats treated with clofibrate

Peroxisome proliferators such as the fibrates act via the peroxisome proliferator activated receptor (PPAR)-alpha as hypolipidemic agents. Many peroxisome proliferators are also nongenotoxic hepatic carcinogens and hepatotoxicants in rodents. We performed transcription profiling using cDNA microarrays on livers of rats treated for 5 days with 3 doses of the peroxisome proliferator clofibrate. All 3 doses had hepatic effects as assessed by liver to body weight ratio, alanine aminotransferase (ALT) increases and histopathology examination. Analysis of the transcription profiling data identified changes in the expression of many genes within several mechanistic pathways that support existing hypotheses regarding peroxisome proliferator mediated carcinogenicity. Additionally, the transcription profiling, histopathology, and clinical chemistry results suggested a biphasic response to clofibrate. These findings provide insight into the pathogenesis of toxic and carcinogenic effects of clofibrate in rodents and demonstrate the ability of cDNA microarrays to provide information regarding mechanisms of toxicity identified during the drug development process.

Relative molecular similarity in selected chemical carcinogens and the nucleoside triphosphate chain

Several markers of cell toxicity are useful as screening tests for epigenetic carcinogens. The direct effects of chemicals on ATPase and GTPase function are pertinent to the early stages of carcinogenesis. Interference with triphosphate-diphosphate exchange mechanisms may result from the interaction of carcinogens with the substrate triphosphate chain. To investigate this hypothesis, a computational chemistry programme is used in this study to investigate molecular similarity in ATPase inhibitors, carcinogens and tumour promoters, in relation to the nucleoside triphosphate chain. The results show that atoms in the investigated molecular structures superimpose on sets of oxygen atoms in the triphosphate chain with interatomic distances < 0.3A. Relative molecular similarity to the substrate triphosphate chain is discussed in terms of the established inhibitory properties of carcinogens/tumour promoters on ATPase function, the carcinogen/ tumour promoting properties of ATPase inhibitors and the prediction of carcinogenic activity from chemical structure.

Hepatic expression of polymerase beta, Ref-1, PCNA, and Bax in WY 14,643-exposed rats and hamsters

The hepatic levels of three protein markers of oxidative stress, polymerase beta, Ref-1, and PCNA, and of the pro-apoptotic protein, Bax, were quantitated after exposure to WY 14,643 (500 ppm in the feed) for 6 or 34 days in a rodent that is susceptible peroxisome proliferator (PP)-induced liver tumors (the Sprague Dawley rat) and in a rodent that is relatively resistant PP-induced liver tumors (the Syrian hamster). The analysis of detergent-extracted whole liver homogenates by immunoblotting showed a marked increase in the abundance of a 45-kDa variant of polymerase beta immunoreactivity and significant increases in the expression of Ref-1 and PCNA in WY 14,643-exposed rats. In contrast. WY 14,643-exposed hamsters expressed only trace levels of the polymerase beta variant and showed significant decreases in the expression of Ref-1 and PCNA. Long-term WY 14,643 exposure was associated with marked decreases in Bax expression in both species. Dose-response studies in the rat showed that the hepatic expression of the polymerase beta and Ref-1 were significantly increased after 6 days of exposure to WY 14,643 at levels of 5 and 50 ppm, respectively. The analysis of subcellular fractions of rat liver showed that the pathological increases in the levels of polymerase beta, Ref-1, and PCNA were especially prominent in mitochondria-enriched particulate liver subfractions. These results indicate that WY 14,643 exposure is associated with an increase in oxidative stress to the liver and that liver mitochondria are a major target of WY 14,643-associated liver damage. Our data are consistent with the hypothesis that the chronic overexpression of mutagenic or oncogenic effectors like polymerase beta and Ref-1 in a setting of increased hepatocyte proliferation and decreased apoptosis may facilitate peroxisome proliferator-induced hepatocellular carcinoma in the rat.

Effect of diet on cancer development: is oxidative DNA damage a biomarker?

Free radicals and other reactive species are generated in vivo and many of them can cause oxidative damage to DNA. Although there are methodological uncertainties about accurate quantitation of oxidative DNA damage, the levels of such damage that escape immediate repair and persist in DNA appear to be in the range that could contribute significantly to mutation rates in vivo. The observation that diets rich in fruits and vegetables can decrease both oxidative DNA damage and cancer incidence is consistent with this. By contrast, agents increasing oxidative DNA damage usually increase risk of cancer development. Such agents include cigarette smoke, several other carcinogens, and chronic inflammation. Rheumatoid arthritis and diabetes are accompanied by increased oxidative DNA damage but the pattern of increased cancer risk seems unusual. Other uncertainties are the location of oxidative DNA damage within the genome and the variation in rate and level of oxidative damage between different body tissues. In well-nourished human volunteers, fruits and vegetables have been shown to decrease oxidative DNA damage in several studies, but data from short-term human intervention studies suggest that the protective agents are not vitamin C, vitamin E, beta-carotene, or flavonoids.

Sensitive and real-time determination of H2O2 release from intact peroxisomes

Peroxisomes are essential and ubiquitous cell organelles having a key role in mammalian lipid and oxygen metabolism. The presence of flavine oxidases makes them an important intracellular source of H(2)O(2): an obligate product of peroxisomal redox reactions and a key reactive oxygen species. Peroxisomes proliferate in response to external signals triggered by peroxisome-proliferator-activated receptor signalling pathways. Peroxisome-derived oxidative stress as a consequence of this proliferation is increasingly recognized to participate in pathologies ranging from carcinogenesis in rodents to alcoholic and non-alcoholic steatosis hepatitis in humans. To date, no sensitive approach exists to record H(2)O(2) turnover of peroxisomes in real time. Here, we introduce a sensitive chemiluminescence method that allows the monitoring of H(2)O(2) generation and degradation in real time in suspensions of intact peroxisomes. Importantly, removal, as well as release of, H(2)O(2) can be assessed at nanomolar, non-toxic concentrations in the same sample. Owing to the kinetic properties of catalase and oxidases, H(2)O(2) forms fast steady-state concentrations in the presence of various peroxisomal substrates. Substrate screening suggests that urate, glycolate and activated fatty acids are the most important sources for H(2)O(2) in rodents. Kinetic studies imply further that peroxisomes contribute significantly to the beta-oxidation of medium-chain fatty acids, in addition to their essential role in the breakdown of long and very long ones. These observations establish a direct quantitative release of H(2)O(2) from intact peroxisomes. The experimental approach offers new possibilities for functionally studying H(2)O(2) metabolism, substrate transport and turnover in peroxisomes of eukaryotic cells.

Clofibrate-induced relocation of phosphatidylcholine transfer protein to mitochondria in endothelial cells

The phosphatidylcholine transfer protein (PC-TP) is a specific transporter of phosphatidylcholine (PC) between membranes. To get more insight into its physiological function, we have studied the localization of PC-TP by microinjection of fluorescently labeled PC-TP in foetal bovine heart endothelial (FBHE) cells and by expression of an enhanced yellow fluorescent protein-PC-TP fusion protein in FBHE cells, human umbilical vein endothelial cells, and HepG2 cells. Analysis by confocal laser scanning microscopy showed that PC-TP was evenly distributed throughout the cytosol with an apparently elevated level in nuclei. By measuring the fluorescence recovery after bleaching it was established that PC-TP is highly mobile throughout the cell, with its transport into the nucleus being hindered by the nuclear envelope. Given the proposed function of PC-TP in lipid metabolism, we have tested a number of compounds (phorbol ester, bombesin, A23187, thrombin, dibutyryl cyclic AMP, oleate, clofibrate, platelet-derived growth factor, epidermal growth factor, and hydrogen peroxide) for their ability to affect intracellular PC-TP distribution. Only clofibrate (100 microM) was found to have an effect, with PC-TP moving to mitochondria within 5 min of stimulation. This relocation did not occur with PC-TP(S110A), lacking the putative protein kinase C (PKC)-dependent phosphorylation site, and was restricted to the primary endothelial cells. Relocation did not occur in HepG2 cells, possibly due to the fact that clofibrate does not induce PKC activation in these cells.

Mechanism of clofibrate hepatotoxicity: mitochondrial damage and oxidative stress in hepatocytes

Peroxisome proliferators have been found to induce hepatocarcinogenesis in rodents, and may cause mitochondrial damage. Consistent with this, clofibrate increased hepatic mitochondrial oxidative DNA and protein damage in mice. The present investigation aimed to study the mechanism by which this might occur by examining the effect of clofibrate on freshly isolated mouse liver mitochondria and a cultured hepatocyte cell line, AML-12. Mitochondrial membrane potential (Delta Psi(m)) was determined by using the fluorescent dye 5,5',6,6'-tetrachloro-1,1', 3,3'-tetraethyl-benzimidazolylcarbocyanine iodide (JC-1) and tetramethylrhodamine methyl ester (TMRM). Application of clofibrate at concentrations greater than 0.3 mM rapidly collapsed the Delta Psi(m) both in liver cells and in isolated mitochondria. The loss of Delta Psi(m) occurred prior to cell death and appeared to involve the mitochondrial permeability transition (MPT), as revealed by calcein fluorescence studies and the protective effect of cyclosporin A (CsA) on the decrease in Delta Psi(m). Levels of reactive oxygen species (ROS) were measured with the fluorescent probes 5-(and-6)-carboxy-2',7'-dichlorofluorescein diacetate (DCFDA) and dihydrorhodamine 123 (DHR123). Treatment of the hepatocytes with clofibrate caused a significant increase in intracellular and mitochondrial ROS. Antioxidants such as vitamin C, deferoxamine, and catalase were able to protect the cells against the clofibrate-induced loss of viability, as was CsA, but to a lesser extent. These results suggest that one action of clofibrate might be to impair mitochondrial function, so stimulating formation of ROS, which eventually contribute to cell death.

Mitochondrial superoxide production in kainate-induced hippocampal damage

The objective of this study was to determine the role of mitochondrial superoxide radical-mediated oxidative damage in seizure-induced neuronal death. Using aconitase inactivation as an index of superoxide production, we found that systemic administration of kainate in rats increased mitochondrial superoxide production in the hippocampus at times preceding neuronal death. 8-Hydroxy-2-deoxyguanosine, an oxidative lesion of DNA, was also increased in the rat hippocampus following kainate administration. Manganese(III) tetrakis(4-benzoic acid)porphyrin, a catalytic antioxidant, inhibited kainate-induced mitochondrial superoxide production, 8-hydroxy-2-deoxyguanosine formation and neuronal loss in the rat hippocampus. Kainate-induced increases of mitochondrial superoxide production and hippocampal neuronal loss were attenuated in transgenic mice overexpressing mitochondrial superoxide dismutase-2. We propose that these results demonstrate a role for mitochondrial superoxide production in hippocampal pathology produced by kainate seizures.

Induction of the mitochondrial permeability transition in vitro by short-chain carboxylic acids

We recently reported that acrylic acid (AA) induces the MPT in vitro, which we suggested might be a critical event in the acute inflammatory and hyperplastic response of the olfactory epithelium. The purpose of the present investigation was to determine if induction of the MPT is a general response to short-chain carboxylic acids or if there are critical physical chemical parameters for this response. Freshly isolated rat liver mitochondria were incubated in the presence of varying concentrations of selected carboxylic acids. All of the acids that we tested caused a concentration-dependent induction of the MPT, which was blocked by cyclosporine A. Although the C4 carboxylic acids were slightly more potent than the C5 acids, there was no correlation with the degree of saturation, the octanol/water coefficient (log P), or the dissociation constant (pK(a)) of the acids that we tested. We conclude that induction of the MPT in vitro is a general response to short-chain carboxylic acids having a pK(a) of 4 to 5.

Caloric restriction prevents oxidative damage induced by the carcinogen clofibrate in mouse liver

Long-term caloric restriction in rodents is known to decrease levels of oxidative damage, which may contribute to an 'anti-ageing' effect. We show here that a shorter period (10 months) of caloric restriction had only small effects on levels of oxidative DNA and protein damage in the livers of mice, but completely attenuated increased oxidative damage caused by the carcinogen clofibrate. Since clofibrate is thought to exert its actions by increasing oxidative damage, our data suggest that 10 months of caloric restriction can increase the resistance of tissues to agents inducing oxidative stress. This may be an important factor in explaining how caloric restriction decreases cancer incidence.

Novel role of oxidants in the molecular mechanism of action of peroxisome proliferators

Peroxisome proliferators are nongenotoxic rodent carcinogens that act as tumor promoters by increasing cell proliferation; however, their precise mechanism of action is not well understood. Oxidative DNA damage caused by leakage of hydrogen peroxide (H2O2) from peroxisomes was hypothesized initially as the mechanism by which these compounds cause liver tumors. It seems unlikely that oxidants of peroxisomal origin explain the mechanism of action of peroxisome proliferators because treatment with these compounds in vivo does not lead to increased H2O2 production. On the other hand, Kupffer cell-derived oxidants, such as superoxide, may play a role in initiating tumor nerosis factor-alpha (TNF-alpha) production that leads to hepatocyte proliferation. Peroxisome proliferators have been shown to activate Kupffer cells both in vitro and in vivo, and the use of Kupffer cell inhibitors such as methyl palmitate and dietary glycine have demonstrated that Kupffer cells are responsible for hepatocyte proliferation by mechanisms involve TNF-alpha. Moreover, peroxisome proliferators activate the transcription factor NF-kappaB, one of the major regulators of TNF-alpha expression, in Kupffer cells. Importantly, activation of NF-kappaB by peroxisome proliferators was shown to be oxidant-dependent, leading to the hypothesis that oxidants of Kupffer cell origin are involved in the mechanism of action. Many of the effects of peroxisome proliferators, including peroxisome induction and hepatomegaly, involve the peroxisome proliferator-activated receptor-alpha (PPARalpha). Recently, it was shown that peroxisome proliferator-induced cell proliferation and tumors require the PPARalpha. However, PPARalpha is not involved in TNF-alpha production by Kupffer cells because it is not expressed in this cell type. How it is involved in liver tumor remains unclear and one possible explanation is that both Kupffer cell TNF-alpha and parenchymal cell PPARalpha are required. Collectively, recent data are consistent with the hypothesis that oxidants play a role in signaling hepatocellular proliferation due to peroxisome proliferators via activation of NF-kappaB and incrase in mitogenic cytokines such as TNF-alpha.