Fatty acid oxidation by human liver peroxisomes

Carnitine O-octanoyltransferase (CROT) deficiency in mice leads to an increase of omega-3 fatty acids

Introduction: Carnitine O-octanoyltransferase (CROT) is a well-established peroxisomal enzyme involved in liver fatty acid oxidation, but less is known about its recently discovered role in promoting vascular calcification, and whether CROT-dependent liver metabolism contributes to the latter. To date, CROT function in the context of calcification potential has been conducted in the dyslipidemic low-density lipoprotein receptor-deficient (Ldlr-/-) mice. Objectives: To differentiate peroxisome and CROT-dependent lipid biology from that of lipoprotein-mediated lipid biology, we therefore conducted a metabolomic analysis of the liver and plasma of normolipidemic CROT-deficient (Crot-/-) mice. Methods: We performed LC-MS-based metabolomics on liver and plasma derived from Crot-/- and Crot +/- mice and sibling Crot+/+ mice, using a dual-phase metabolite extraction protocol, and multiple LC-MS acquisition strategies. Results: We identified between 79 to 453 annotated metabolites from annotated metabolites from liver samples, and 117 to 424 annotated metabolites from plasma samples. Through differential abundance analysis, we determined that omega-3 fatty acids such as EPA, DPA, and DHA were higher in the liver of Crot-/- and Crot +/- mice than Crot+/+ mice. EPA were higher in plasma of Crot-/- mice than Crot+/+ mice. We also determined that the anti-inflammatory dicarboxylic acids, tetradecanedioic acid and azelaic acid, were higher in the plasma of CROT-deficient mice. Conclusion: Our study associated genetic CROT deletion with increased levels of anti-inflammatory molecules in mouse liver and plasma. These results suggest a potential mechanism for anti-calcification effects of CROT suppression and the potential use of omega-3 fatty acids as biomarkers for future CROT inhibition therapies.

Tyrosine Residues 232 and 401 Play a Critical Role in the Binding of the Cofactor FAD of Acyl-coA Oxidase

Acyl-coA oxidase (ACO) is an important flavoenzyme responsible for the first step of peroxisomal fatty acid β-oxidation. In this study, the roles of Tyr232 and Tyr401 in flavin adenine dinucleotide (FAD) binding and enzyme catalysis of ACO were explored using site-directed mutagenesis. For mutant proteins, different levels of activity loss were observed. Wavelength scanning of Y232 and Y401 mutant proteins indicated that there is no FAD binding in Y401S and Y401G mutant ACO. Structure analysis indicated that the phenolic hydroxyl and benzene ring of the side chain could stabilize FAD binding through hydrogen bonds network and hydrophobic pocket formation. These results indicated that these two tyrosine residues play a critical role in the FAD binding of ACO.

The Craft of Peroxisome Purification-A Technical Survey Through the Decades

Purification technologies are one of the working horses in organelle proteomics studies as they guarantee the separation of organelle-specific proteins from the background contamination by other subcellular compartments. The development of methods for the separation of organelles was a major prerequisite for the initial detection and characterization of peroxisome as a discrete entity of the cell. Since then, isolated peroxisomes fractions have been used in numerous studies in order to characterize organelle-specific enzyme functions, to allocate the peroxisome-specific proteome or to unravel the organellar membrane composition. This review will give an overview of the fractionation methods used for the isolation of peroxisomes from animals, plants and fungi. In addition to "classic" centrifugation-based isolation methods, relying on the different densities of individual organelles, the review will also summarize work on alternative technologies like free-flow-electrophoresis or flow field fractionation which are based on distinct physicochemical parameters. A final chapter will further describe how different separation methods and quantitative mass spectrometry have been used in proteomics studies to assign the proteome of PO.

Induction of cytochrome P450-dependent mixed function oxidase activities and peroxisome proliferation by chloramine-T in male rat liver

Chloramine-T is an antimicrobial agent recognized for its disinfectant properties widely used in food industry. As an N-chloro-compound, chloramine-T contains electrophilic chlorine and in water it hydrolyses to hypochlorite. Chlorine as hypochlorous acid or hypochlorite is a very reactive chemical which can function as both an oxidant and halogenating agent. Because chloramine-T could be involved in the metabolic activation of drugs, in the present study the effects of chloramine-T on the activities of some drug metabolizing enzymes in rat liver microsomes and peroxisome proliferation were determined in vivo. Rats were treated orally with chloramine-T at doses of 1.25, 2.50, 5 and 10 mg/kg body weight (bw)/day for 6 days. The activities of CYP2E1, CYP1A1/2 CYP2B1/2, CYP3A4 and CYP4A1/2 enzymes significantly increased after treatment with 2.50, 5 and 10 mg/kg bw/day, in a dose-dependent manner as compared to control. This effect was not observed after chloramine-T treatment at dose of 1.25 mg/kg bw/day. Our results suggest that chloramine-T may potentiate the toxicity of many xenobiotics via metabolic activation and/or accumulation of reactive metabolites.

Peroxisomes (Microbodies) in Human Liver: Cytochemical and Quantitative Studies of 85 Biopsies

The number, intracellular distribution, and staining characteristics of human hepatocellular peroxisomes that had been made visible by cytochemical staining for catalase were evaluated in biopsies from 75 patients with hepatic, inflammatory, or malignant disease and ten normal individuals. Intensity of staining was found to be proportional to enzymatic activity by microspectrophotometry. Scanning transmission electron microscopy (STEM) image analysis demonstrated an inverse relationship between peroxisomal size and contrast. Peroxisomes were more abundant, and often concentrated in a perinuclear configuration in cholestatic and cirrhotic livers. Decreased peroxisomal staining was common in cholestasis, cirrhosis, hepatitis, and in almost all patients with malignancies, both with and without hepatic metastases.

PPARα Signaling is Activated by Cocoa in Mouse Liver

In this study we evaluated in mouse liver the effects of cocoa on PPARα signaling. To this aim, mouse diet was supplemented with 10%, w/w, cocoa for one and two weeks. We quantified the expression of PPARα target genes and PPARα gene level and some parameters related to PPARα activation (hepato-somatic index, peroxisomal β-oxidation system and catalase activity). Moreover, we evaluated antioxidant capacity of cocoa by detecting the expression of CAT and SOD1 genes (known to be involved in oxidative balance) and hypolipidemic properties on serum triglycerides. We made a parallel treatment with 0.025%, w/w, ciprofibrate, a well-known PPARα activator, to quantify signal modulation by cocoa. It is known that PPARα activation by ciprofibrate is mediated by direct binding to the receptor and strongly induces expression of target genes. Our results show that cocoa weakly up-regulates PPARα target genes as a consequence of the modulation of the PPARα gene level and does not improve the triglyceride profile in blood. Finally, cocoa increased SOD1 gene expression suggesting an antioxidant effect.

A role for the peroxisomal 3-ketoacyl-CoA thiolase B enzyme in the control of PPARα-mediated upregulation of SREBP-2 target genes in the liver

Peroxisomal 3-ketoacyl-CoA thiolase B (Thb) catalyzes the final step in the peroxisomal β-oxidation of straight-chain acyl-CoAs and is under the transcription control of the nuclear hormone receptor PPARα. PPARα binds to and is activated by the synthetic compound Wy14,643 (Wy). Here, we show that the magnitude of Wy-mediated induction of peroxisomal β-oxidation of radiolabeled (1-(14)C) palmitate was significantly reduced in mice deficient for Thb. In contrast, mitochondrial β-oxidation was unaltered in Thb(-/-) mice. Given that Wy-treatment induced Acox1 and MFP-1/-2 activity at a similar level in both genotypes, we concluded that the thiolase step alone was responsible for the reduced peroxisomal β-oxidation of fatty acids. Electron microscopic analysis and cytochemical localization of catalase indicated that peroxisome proliferation in the liver after Wy-treatment was normal in Thb(-/-) mice. Intriguingly, micro-array analysis revealed that mRNA levels of genes encoding cholesterol biosynthesis enzymes were upregulated by Wy in Wild-Type (WT) mice but not in Thb(-/-) mice, which was confirmed at the protein level for the selected genes. The non-induction of genes encoding cholesterol biosynthesis enzymes by Wy in Thb(-/-) mice appeared to be unrelated to defective SREBP-2 or PPARα signaling. No difference was observed in the plasma lathosterol/cholesterol ratio (a marker for de novo cholesterol biosynthesis) between Wy-treated WT and Thb(-/-) mice, suggesting functional compensation. Overall, we conclude that ThA and SCPx/SCP2 thiolases cannot fully compensate for the absence of ThB. In addition, our data indicate that ThB is involved in the regulation of genes encoding cholesterol biosynthesis enzymes in the liver, suggesting that the peroxisome could be a promising candidate for the correction of cholesterol imbalance in dyslipidemia.

Biochemistry and genetics of inherited disorders of peroxisomal fatty acid metabolism

In humans, peroxisomes harbor a complex set of enzymes acting on various lipophilic carboxylic acids, organized in two basic pathways, alpha-oxidation and beta-oxidation; the latter pathway can also handle omega-oxidized compounds. Some oxidation products are crucial to human health (primary bile acids and polyunsaturated FAs), whereas other substrates have to be degraded in order to avoid neuropathology at a later age (very long-chain FAs and xenobiotic phytanic acid and pristanic acid). Whereas total absence of peroxisomes is lethal, single peroxisomal protein deficiencies can present with a mild or severe phenotype and are more informative to understand the pathogenic factors. The currently known single protein deficiencies equal about one-fourth of the number of proteins involved in peroxisomal FA metabolism. The biochemical properties of these proteins are highlighted, followed by an overview of the known diseases.

Dissimilar properties of vaccenic versus elaidic acid in beta-oxidation activities and gene regulation in rat liver cells

Vaccenic acid (trans-11-C(18:1)) chemically resembles elaidic acid (trans-9-C(18:1)) which is assumed to increase the risk of cardiovascular diseases, and thus could exert similar effects. Possible different oxidation rates of vaccenic versus elaidic acid were checked in muscles and liver, and through related gene expression in normal rat liver cells. In hepatic mitochondria, carnitine palmitoyltransferase (CPT) I exhibited comparable activity rates with both trans-isomers. CPT II activity was 30% greater (P < 0.05) with vaccenic than with elaidic acid as nonesterified fatty acids (NEFAs) or acyl-CoAs. Activity of the first beta-oxidation step was similar between the isomers in all the tissue slices and liver extracts assayed. Respiration rates were comparable with both trans-isomers as NEFAs in various liver extracts, but were 30% greater (P < 0.05) with vaccenoyl-CoA than with elaidoyl-CoA in liver mitochondria. Vaccenic acid was oxidised 25% more (P < 0.05) by liver peroxisomes than elaidic acid. In hepatocytes cultured with trans- and corresponding cis-C(18:1) isomers, gene expression of CPT I, hydroxyacyl-CoA dehydrogenase and hydroxymethylglutaryl-CoA synthase was at least 100% increased (P < 0.05), but was unchanged with vaccenic acid, relative to controls. In conclusion, the position and geometry of the double bonds in acyl chains are suggested to confer on vaccenic and elaidic acid specific biochemical properties that might differently affect their fates in tissues.

beta-oxidation modulates metabolic competition between eicosapentaenoic acid and arachidonic acid regulating prostaglandin E(2) synthesis in rat hepatocytes-Kupffer cells

The ability of n-3 PUFA to competitively inhibit the use of arachidonic acid (AA) for membrane phospholipid synthesis and prostaglandin E(2) (PGE(2)) production has been well demonstrated in single cell models. In the present study, we investigated the metabolic competition between AA and eicosapentaenoic acid (EPA) for PGE(2) synthesis in a rat hepatocyte-Kupffer cell (HPC/KC) co-culture system when the cellular oxidation capacity was enhanced by exogenous l-carnitine. We demonstrate that in the absence of l-carnitine, 1) beta-oxidation rates of EPA and AA were comparable in HPCs and in KCs; 2) AA and not EPA was preferentially incorporated into glycerolipids; and 3) addition of EPA significantly decreased AA-dependent PGE(2) synthesis in HPCs and cyclooxygenase-2 (COX-2) expression in co-cultured HPCs/KCs. However, enhancing the cellular oxidation capacity by the addition of l-carnitine 1) significantly increased beta-oxidation of EPA in HPCs, but only marginally elevated the oxidation of AA in HPCs and the oxidation of both fatty acids in KCs; 2) decreased the esterification, but did not alter the preferential incorporation of AA into glycerolipids; and 3) alleviated the significant competitive inhibition of AA-dependent PGE(2) synthesis and COX-2 expression by EPA. Taken together, the results strongly suggest that l-carnitine affects competition between AA and EPA in PG synthesis in liver cells by enhancing oxidation of EPA in HPCs. This implies that the beneficial effects of n-3 PUFA, especially EPA, are affected by the cellular oxidation capacity.

Early dissimilar fates of liver eicosapentaenoic acid in rats fed liposomes or fish oil and gene expression related to lipid metabolism

The study was undertaken to determine whether eicosapentaenoic acid (EPA, 20:5 n-3) and docosahexaenoic acid (DHA, 22:6 n-3), esterified in phospholipids (PL) as liposomes or in triglycerides (TG) as oil, exhibited comparable fates in liver lipids and whether these fates were associated with gene expressions related to fatty acid (FA) metabolism. PL and TG mixtures with close contents in EPA and DHA were administered to rats over 2 weeks. Most relevant events occurred after 3 days for both treatments. At that time, liposomes, compared with oil, increased the liver content in PL with a FA composition enriched in n-6 FA, comparable in DHA and much lower in EPA. Moreover, liposomes increased the activity and mRNA levels of carnitine palmitoyltransferase (CPT) I. In contrast, fish oil exerted opposite effects on CPT I and increased the genic expression of lipogenic enzymes. Liposomes, unlike fish oil, apparently increased the mRNA levels of acyl-CoA oxidase and the activity of the peroxisomal FA-oxidising system. Concomitantly, mRNA levels of hepatic lipoprotein receptors were increased with both diets, but intracellular proteins involved in free FA uptake and lipid synthesis were up-regulated only with liposome-treated rats. The quasi absence of EPA in hepatic PL of liposome-treated rats on the short term could result from increased beta-oxidation activities through metabolic regulations induced by more available free EPA and other PUFA.

Hypolipidaemic effects of fenofibrate and fasting in the herbivorous grass carp ( Ctenopharyngodon idella) fed a high-fat diet

We investigated whether the hypolipidaemic effect of fenofibrate and fasting observed in most omnivorous mammals may also apply to herbivorous fish. Grass carp (Ctenopharyngodon idella) fed a high-fat (8 %) diet exhibited a marked increase in blood lipids and body fat after 6 weeks. They were then treated with fenofibrate (100 mg/kg body weight) in the same high-fat diet for 2 weeks, followed by fasting for 1 week. Plasma lipid concentration, body fat amount, fatty acid composition, plasma thiobarbituric acid-reactive substances and some parameters related to hepatic fatty acid oxidation were measured, and liver samples were stained for histological examination. Fenofibrate treatment decreased TAG and cholesterol concentrations in plasma, total lipids of the whole body and liver, and EPA and DHA contents in tissues. Further, a mobilisation of mesenteric fat concomitant with an increase in hepatic peroxisomal fatty acid oxidation and lipid peroxidation was observed. Compared with fenofibrate treatment, fasting decreased body weight and plasma TAG, but not plasma cholesterol. It also reduced the fat content of the whole body and increased the EPA and DHA contents in the liver and other tissues. Fatty acid oxidation was stimulated by fasting in mitochondria, but not in peroxisomes. These data suggest that fenofibrate and fasting regulate the lipid metabolism in grass carp through different metabolic pathways. The grass carp is moderately responsive to a fibrate derivative in comparison with the well-known excess responsiveness of the rat model, and so it could be used for the study of lipid abnormalities as a herbivorous model.

Administration of ciprofibrate to lactating mothers induces PPARalpha-signaling pathway in the liver and kidney of suckling rats

It is well known that the hypolipidemic drug ciprofibrate induces peroxisome proliferation in rodent liver, which in turn leads to the oxidative stress, and modifies some parameters related to cell proliferation and apoptosis. The administration of ciprofibrate to rats during the lactating period determined in their pups significant modifications in hepatic peroxisome enzyme activities, induction of the PPARalpha-target gene, Cyp4a10, and perturbation in cell proliferation and apoptosis, which affected the size of the liver. Moreover, this modification was associated to about two-fold induction of mRNA-PPARalpha. On the contrary, in the kidney, although a similar two-fold up-regulation of PPARalpha was detected, the induction of both peroxisomal enzyme activities and Cyp4a10 were weak, and no alterations were detected, neither in cell cycle nor in the size of the tissue. Our results indicate that the response to ciprofibrate is stronger in the liver than in the kidney of newborn rats.

Regulation of Lipid Flux between Liver and Adipose Tissue during Transient Hepatic Steatosis in Carnitine-depleted Rats

Rats with carnitine deficiency due to trimethylhydrazinium propionate (mildronate) administered at 80 mg/100 g body weight per day for 10 days developed liver steatosis only upon fasting. This study aimed to determine whether the transient steatosis resulted from triglyceride accumulation due to the amount of fatty acids preserved through impaired fatty acid oxidation and/or from up-regulation of lipid exchange between liver and adipose tissue. In liver, mildronate decreased the carnitine content by approximately 13-fold and, in fasted rats, lowered the palmitate oxidation rate by 50% in the perfused organ, increased 9-fold the triglyceride content, and doubled the hepatic very low density lipoprotein secretion rate. Concomitantly, triglyceridemia was 13-fold greater than in controls. Hepatic carnitine palmitoyltransferase I activity and palmitate oxidation capacities measured in vitro were increased after treatment. Gene expression of hepatic proteins involved in fatty acid oxidation, triglyceride formation, and lipid uptake were all increased and were associated with increased hepatic free fatty acid content in treated rats. In periepididymal adipose tissue, mildronate markedly increased lipoprotein lipase and hormone-sensitive lipase activities in fed and fasted rats, respectively. On refeeding, carnitine-depleted rats exhibited a rapid decrease in blood triglycerides and free fatty acids, then after approximately 2 h, a marked drop of liver triglycerides and a progressive decrease in liver free fatty acids. Data show that up-regulation of liver activities, peripheral lipolysis, and lipoprotein lipase activity were likely essential factors for excess fat deposit and release alternately occurring in liver and adipose tissue of carnitine-depleted rats during the fed/fasted transition.

Alteration of 20:5n-3 and 22:6n-3 fat contents and liver peroxisomal activities in fenofibrate-treated rainbow trout

Fish easily accumulate n-3 PUFA of exogenous origin, but the underlying mechanisms are not well established in the whole animal. This study was undertaken to investigate whether this feature was physiologically associated with mitochondrial and peroxisomal capacities that differentially affect FA oxidation. For this purpose, peroxisomal FA oxidation was increased by treating rainbow trout with fenofibrate, which strongly stimulates the peroxisome proliferator-activated receptor-a in rodents. Diets containing EPA and DHA, with or without fenofibrate added, were administered to male trout for 12 d. After treatment, neither liver hypertrophy nor accumulation of fat was apparent within the liver and muscle cells. However, fenofibrate treatment decreased the contents of EPA and DHA in the liver, white muscle, and intraperitoneal fat tissue, which represented (per whole body) at least 280 mg less than in controls. Carnitine-dependent palmitate oxidation rates, expressed per gram of liver, were slightly increased by fenofibrate when measured from tissue homogenates and were unchanged when calculated from isolated mitochondria, relative to control fish. The treatment altered neither carnitine palmitoyltransferase I activity rates, expressed per gram of liver, nor the sensitivity of the enzyme to malonyl-CoA inhibition, but did increase the malonyl-CoA content (+45%). Meanwhile, fenofibrate increased (by about 30%) the peroxisome-related activities, i.e., catalase, carnitine-independent palmitate oxidation, acyl-CoA oxidase, and the peroxisomal FA-oxidizing system, relative to the control group. The data strongly suggest that the induction of peroxisomal activities, some of which being able to oxidize very long chain FA, was responsible for the lower contents of EPA and DHA in the body lipids of fenofibrate-treated trout.

Effects of dioxane on cytochrome P450 enzymes in liver, kidney, lung and nasal mucosa of rat

The effect of acute and chronic dioxane administration on hepatic, renal, pulmonary and nasal mucosa P450 enzymes and liver toxicity were investigated in male rats. The acute treatment consisted of two doses (2 g/kg) of dioxane given for 2 days by gavage, whereas the chronic treatment consisted of 1.5% of dioxane in drinking water for 10 days. Both the acute and chronic dioxane treatments induced cytochrome P450 2B1/2- and P450 2E1-dependent microsomal monooxygenase activities (pentoxyresorufin O-depentylase and p-nitrophenol hydroxylase) in the liver, whereas in the kidney and nasal mucosa, only the 2E1 marker activities were enhanced. In addition in the liver, an induction of 2alpha-testosterone hydroxylase (associated with the constitutive and hormone-dependent P450 2C11) was also revealed, whereas the hepatic P450 4A-dependent omega-lauric acid hydroxylase was not enhanced by any dioxane treatment. These inductions were mostly confirmed by western blot analysis of liver, kidney and nasal mucosa microsomes. In the lung, no alteration of P450 activities was observed. To assess the mechanism of 2E1 induction, the hepatic, renal and nasal mucosa 2E1 mRNA levels were also examined. Following two kinds of dioxane administration, in the liver the 2E1 induction was not accompanied by a significant alteration of 2E1 mRNA levels, while both in the kidney and nasal mucosa the 2E1 mRNA increased about 2- to 3-fold, indicating an organ-specific regulation of this P450 isoform. Furthermore, dioxane was unable to alter the plasma alanine aminotransferase activity and hepatic glutathione (GSH) content, examined as an index of toxicity, when it was administered into rats with P450 2B1/2 and 2E1 preinduced by phenobarbital or fasting pretreatment. These results support the lack of or a poor formation of reactive and toxic intermediates during the biotrasformation of this solvent, even when its metabolism was enhanced by P450 inducers. The chronic administration of dioxane was also unable to induce the palmitoyl CoA oxidase, a marker of peroxisome proliferation, excluding this as a way to explain its toxicity. Thus, although the mechanism of dioxane carcinogenicity remains unclear, the present results suggest that the induction of 2E1 following a prolonged administration of dioxane might provide oxygen radical species, and thereby contribute to its organ-specific toxicity.

Hepatic steatosis is not due to impaired fatty acid oxidation capacities in C57BL/6J mice fed the conjugated trans-10,cis-12-isomer of linoleic acid

Decreased body fat mass and liver steatosis have been reported in mice fed diets containing the conjugated linoleic acid trans-10,cis-12-C18:2 (CLA2), but not in those fed diets containing cis-9,trans-11-C18:2 (CLA1). Because the decrease in fatty acid (FA) oxidation may cause fat accumulation, we questioned whether the effects of both CLAs on enzyme activities and mRNA expression were related to liver FA oxidation. To address this question, 7-wk-old male C57BL/6J mice were fed for 4 wk a diet supplemented with 1% CLA1, CLA2, or cis-9-C18:1 (control) esterified as triacylglycerols. In CLA2-fed mice, the proportions of CLA2 in the total FA of liver lipids were substantially lower than those of CLA1 in mice fed CLA1. The mitochondrial protein content per total liver was about 56% greater in CLA2-fed mice than in CLA1-fed mice and controls. Mitochondrial carnitine palmitoyltransferase I (CPT I) and carnitine-dependent palmitate oxidation activities were also significantly greater in CLA2-fed mice than in the two other groups. The amounts of malonyl-CoA per gram of liver and the sensitivity of CPT I to malonyl-CoA inhibition were greater in both groups of CLA-fed mice than in the controls. L-CPT I mRNA expression doubled in CLA2-fed mice and was 3 and 2 times greater for M-CPT I in the CLA1 and CLA2 groups, respectively, compared with controls. Peroxisomal FA oxidation-related activities and acyl-CoA oxidase mRNA expression were increased in CLA1-fed mice, and to a larger extent in CLA2-fed mice, relative to controls. These data indicate that FA oxidation capacities were increased in mice fed CLA2, but were likely depressed in vivo through malonyl-CoA inhibition.

Toxicokinetics of chloral hydrate in ad libitum-fed, dietary-controlled, and calorically restricted male B6C3F1 mice following short-term exposure

Chloral hydrate is widely used as a sedative in pediatric medicine and is a by-product of water chlorination and a metabolic intermediate in the biotransformation of trichloroethylene. Chloral hydrate and its major metabolite, trichloroacetic acid, induce liver tumors in B6C3F1 mice, a strain that can exhibit high rates of background liver tumor incidence, which is associated with increased body weight. This report describes the influence of diet and body weight on the acute toxicity, hepatic enzyme response, and toxickinetics of chloral hydrate as part of a larger study investigating the carcinogenicity of chloral hydrate in ad libitum-fed and dietary controlled mice. Dietary control involves moderate food restriction to maintain the test animals at an idealized body weight. Mice were dosed with chloral hydrate at 0, 50, 100, 250, 500, and 1000 mg/kg daily, 5 days/week, by aqueous gavage for 2 weekly dosing cycles. Three diet groups were used: ad libitum, dietary control, and 40% caloric restriction. Both dietary control and caloric restriction slightly reduced acute toxicity of high doses of chloral hydrate and potentiated the induction of hepatic enzymes associated with peroxisome proliferation. Chloral hydrate toxicokinetics were investigated using blood samples obtained by sequential tail clipping and a microscale gas chromatography technique. It was rapidly cleared from serum within 3 h of dosing. Trichloroacetate was the major metabolite in serum in all three diet groups. Although the area under the curve values for serum trichloroacetate were slightly greater in the dietary controlled and calorically restricted groups than in the ad libitum-fed groups, this increase did not appear to completely account for the potentiation of hepatic enzyme induction by dietary restriction.

Chromatin sphingomyelin changes in cell proliferation and/or apoptosis induced by ciprofibrate

It has been shown that neutral-sphingomyelinase and sphingomyelin-synthase activities are present in chromatin and they modify the sphingomyelin (SM) content. The activity of the first enzyme is stimulated and the second inhibited, when the hepatocytes enter into the S-phase after partial hepatectomy, thus suggesting that ceramide may have a pivotal role in cell proliferation. An opposite function was attributed to ceramide in hepatocytes which undergo apoptosis after lobular ligature. In order to clarify this point, a model was developed in which the same liver cells undergo proliferation followed by induced apoptosis. To this purpose, the rats were treated for 7 days with ciprofibrate and then left without treatment for 4 days. During the treatment, the peroxisome enzyme markers increase their activity and the number of proliferating cells increases, reaching a maximum after 3 days of treatment, as shown by the number of cells positive for the proliferating cell nuclear antigen. At the same time, the chromatin sphingomyelinase activity reaches the maximum, while a similar increase is not found in the cytoplasm or in the isolated nuclei. On the contrary, SM-synthase activity is depressed in chromatin, but not in the nuclei in which a peak is shown after 3 days of ciprofibrate treatment. After drug withdrawal, the hepatocytes undergo apoptosis as confirmed by the increase of Bax and tissue transglutaminase (tTGase) expression; the chromatin SM increases as a consequence of an increase of SM-synthase activity. It can be hypothesised that chromatin SM may have a role in cell duplication by influencing the chromatin structure stability.

Sub-chronic dietary toxicity of potassium perfluorooctanesulfonate in rats

Perfluorooctanesulfonate (PFOS) is a widely disseminated persistent compound found at low (part-per-billion) concentrations in serum and liver samples from humans and fish-eating wildlife. This study investigated the hypotheses that early hepatocellular peroxisomal proliferation and hepatic cellular proliferation are factors in chronic liver response to dietary dosing, that lowering of serum total cholesterol is an early clinical measure of response to treatment, and that liver and serum PFOS concentrations are proportional to dose and cumulative dose after sub-chronic treatment. PFOS was administered in diet as the potassium salt at 0, 0.5, 2.0, 5.0, and 20 parts per million (ppm) to Sprague Dawley rats for 4 or 14 weeks. At 4 weeks, effects included decreased serum glucose and an equivocal (<twofold) increase in hepatic palmitoyl CoA oxidase (PCoAO) activity in 20 ppm dose-group males in one of two assay systems [corrected]. At 14 weeks, the 20 ppm males had increased liver weight, decreased serum cholesterol, increased non-segmented neutrophils, and increased ALT. Relative liver weights and urea nitrogen were increased in both sexes at 14 weeks. Hepatocytic hypertrophy and cytoplasmic vacuolation were observed in the 5 or 20 ppm male and the 20 ppm female dose groups. An increase in hepatic PCoAO activity was not observed at 14 weeks, and the average hepatocyte proliferation index was not increased, although, individual animals had mild increases. Serum and liver PFOS concentrations were proportional to dose and cumulative dose. Serum concentrations were generally higher in females than in males. The liver-to-serum PFOS ratios ranged from approximately 3:1 to 12:1. After 14 weeks, the no-observed-adverse effect level (NOAEL) in males and females was 5 ppm. The NOAEL corresponded to mean serum PFOS concentrations of 44 ppm (microg/ml) in males and 64 ppm in females and mean liver PFOS concentrations of 358 ppm in males and 370 ppm in females. Results for this study: (1) did not provide strong evidence for hepatocellular peroxisomal or cellular proliferation at the doses tested; (2) suggested that lowering of serum total cholesterol may not be the earliest clinically-measurable response to treatment in the rat; and (3) confirmed that serum and liver PFOS concentrations on repeated dosing are proportional to dose and cumulative dose.

Cytochemical and biochemical demonstration of an ATPase in membranes of human peroxisomes

We demonstrated a neutral Mg-ATPase activity in human peroxisomal membranes. To establish the precise experimental conditions for detection of this ATPase, both cytochemical and biochemical characterizations were first carried out in liver peroxisomes from control and cipofibrate-treated rats. The results demonstrated an Mg-ATPase reaction in both normal and proliferated peroxisomes. The nucleotidase activity, with marked preference for ATP, was sensitive to the inhibitors N-ethylmaleimide and 7-chloro-4-nitro-benzo-2-oxadiazole (NBDCl). An ultrastructural cytochemical analysis was developed to evaluate the peroxisomal localization, which localized the reaction product to the peroxisomal membrane. These characteristics can help to differentiate the peroxisomal ATPase from the activity found in mitochondria and endoplasmic reticulum. The conditions established for detecting the rat peroxisomal ATPase were then applied to human peroxisomes isolated from liver and skin fibroblasts in culture. A similar Mg-ATPase activity was readily shown, both cytochemically and biochemically, in the membranes of human peroxisomes. These results, together with previous evidence, strongly support the presence of a specific ATPase in the human peroxisomal membrane. This ATPase may play a crucial role in peroxisome biogenesis.

Clofibrate pretreatment in mice confers resistance against hepatic lipid peroxidation

Pretreatment with peroxisome proliferators protects mice against various hepatotoxicants. Since our previous work suggested that the hepatoprotection may involve an increased ability to cope with oxidative stress, the present work directly addressed this possibility. Several observations indicated a heightened defense against oxidative stress accompanies the hepatoprotection produced by clofibrate. Firstly, the carbonyl content of hepatic proteins from clofibrate-pretreated mice was 40% lower than those from vehicle-treated controls. Secondly, liver homogenates from clofibrate-pretreated mice produced less thiobarbituric acid reactive substances upon incubation under aerobic conditions or exposure to ferrous sulfate. This effect was not due to lower levels of peroxidation-prone polyunsaturated fatty acids in clofibrate-treated livers. Thirdly, in vitro experiments indicated that the antioxidant factor in liver homogenates from clofibrate-pretreated mice was not glutathione. Rather, since it was inactivated by proteases and heat treatment, we concluded that a protein is involved. Collectively, our results suggest that a resistance to lipid peroxidation develops in mouse liver during exposure to clofibrate. The identity of the putative antioxidant protein and its contribution to the protection against liver toxicity observed in this and other laboratories awaits future investigation.

Effects on male rats of di-(2-ethylhexyl) phthalate and di-n-hexylphthalate administered alone or in combination

The effects of phthalate esters of branched chain alcohols, typified by di-(2-ethylhexyl)phthalate (DEHP) differ from those of esters of straight chain alcohols typified by di-n-hexyl phthalate (DnHP). The former induce liver enlargement and proliferation of hepatic peroxisomes, while the latter cause no peroxisome proliferation but cause fat accumulation in the liver. Both classes of phthalate esters are hypolipidaemic and cause thyroid changes associated with an increased rate of thyroglobulin turnover. As phthalate esters are used as mixtures, we have examined the effect of mixtures of the compounds. Groups of five male Wistar albino rats were administered either control diet or diets containing either 10000 ppm of DEHP, 10000 ppm of DnHP or 10000 ppm DEHP plus 10000 ppm DnHP for 14 days. Rats receiving diets containing DEHP showed the expected increase in relative liver weight, in "peroxisomal" fatty acid oxidation and in CYP4A1. Serum triglyceride and serum cholesterol were also reduced, and the thyroid showed the histological changes mentioned above. Rats consuming diets containing DnHP showed no increase in relative liver weight and no induction of peroxisomal fatty acid oxidation or CYP4A1. However, there was a marked accumulation of fat in the liver. The fall in serum cholesterol was similar to that in rats treated with DEHP, but the fall of serum triglyceride was more pronounced. Thyroidal changes were again observed. In general, changes in rats treated with a mixture of DEHP and DnHP were very similar to those found with rats treated with DEHP alone. The liver was enlarged, and peroxisomal fatty acid oxidation and CYP4A1 were both induced. The amount of fat in the liver was much less than in rats receiving DnHP alone. Thyroid changes were similar to those in rats receiving the individual compounds. The effect on serum cholesterol seemed additive, but the levels of serum triglyceride were intermediate between the groups receiving the single compounds.

Oxidative stress, microsomal and peroxisomal fatty acid oxidation in the liver of rats treated with acetone

Parameters of oxidative stress, microsomal cytochrome P450 activity and peroxisomal fatty acid oxidation were studied in liver of rats following acetone (1% v/v) consumption for 7 days. Acetone treatment increased the activity of catalase and decreased the activities of superoxide dismutase (SOD) and glutathione peroxidase (GTPx), but did not significantly modify the liver content of malondialdehyde (MDA) and reduced glutathione. Also, acetone increased the total content of cytochrome P450, the microsomal lauric acid hydroxylation, aminopyrine N-demethylation and the peroxisomal beta-oxidation of palmitoyl CoA. These effects were similar to those found previously in starved and ethanol-treated rats, supporting the hypothesis that ketone bodies would be the common inducer of microsomal and peroxisomal fatty acid oxidation in these metabolic states.

Suppression of mouse hepatocyte apoptosis by peroxisome proliferators: role of PPARalpha and TNFalpha

Peroxisome proliferators (PPs) are a diverse group of nongenotoxic chemicals that in rodents cause hepatic peroxisome proliferation, liver enlargement, increased replicative DNA synthesis and suppression of apoptosis. The effects of PPs in vivo can be reproduced in vitro where PPs can induce mouse hepatocyte DNA synthesis and suppress both spontaneous apoptosis and that induced by transforming growth factor beta (TGFbeta). In vitro, high concentrations (>500 U/ml) of exogenous tumour necrosis factor (TNFalpha) [M. Rolfe, N.H. James, R.A. Roberts, TNF suppresses apoptosis and induces S-phase in rodent hepatocytes: a mediator of the hepatocarcinogenicity of peroxisome proliferators?, Carcinogenesis 18 (1997) 2277-2280] are also able to stimulate hepatocyte DNA synthesis and suppress apoptosis, implicating TNFalpha in mediating or permitting the liver growth response to PPs. Here, using cultured mouse hepatocytes isolated from PPARalpha null mice, we have examined the role of the peroxisome proliferator activated receptor alpha (PPARalpha) in mediating the suppression of apoptosis caused by PPs. In addition we have investigated further the role of TNFalpha in mediating the rodent response to PPs. The PP nafenopin (50 microM) was unable to stimulate DNA synthesis measured by bromodeoxyuridine incorporation in these PPARalpha null mouse hepatocytes (96% of control), unlike epidermal growth factor, a growth factor used as a positive control. In assays of apoptosis using H33258 staining of chromatin condensation, nafenopin was unable to suppress either spontaneous or TGFbeta1-induced apoptosis. In contrast, high concentrations of TNFalpha (>500 U/ml) were able to both stimulate DNA synthesis (204% of control) and suppress apoptosis in PPARalpha null hepatocytes (40% and 38% of control for spontaneous and TGFbeta1-induced apoptosis respectively). However, TNFalpha could not stimulate beta-oxidation of palmitoyl CoA in either PPARalpha null mouse or B6C3F1 (PPARalpha wild type) mouse hepatocytes. These data confirm the dependence of the response to PPs on PPARalpha by demonstrating that PPARalpha mediates the suppression of hepatocyte apoptosis in response to PPs. In addition, the data provide evidence that high concentrations of TNFalpha can modulate DNA synthesis and apoptosis in the absence of PPs and PPARalpha. Thus, in vivo, physiological levels of TNFalpha may be permissive for a PPARalpha-dependent growth response to PPs.

Proliferation of hepatic peroxisomes in rats following the intake of green or black tea

Rats maintained on green, black or decaffeinated black tea (2.5%, w/v) as their sole drinking fluid displayed higher hepatic CN- insensitive palmitoyl CoA oxidase activity than controls; the extent of increase was similar with the three types of tea. Morphological examination of the liver using electron microscopy revealed an increase in the number of peroxisomes in the tea-treated animals. The same treatment of the animals with green and black tea resulted in a similar rise in hepatic microsomal lauric acid hydroxylation. Analysis by HPLC of the aqueous tea extracts employed in the current study showed that the total flavanol content of the green variety was much higher than the black varieties, and confirmed the absence of caffeine in the decaffeinated black tea. It may be concluded from the present studies that neither caffeine nor flavanoids are likely to be responsible for the proliferation of peroxisomes observed in rats treated with tea.

Effect of clofibrate, a peroxisome proliferator, in sea bass (Dicentrarchus labrax), a marine fish

The effects of 35 or 70 mg/kg clofibrate on some peroxisomal, mitochondrial, and microsomal enzymes (markers of peroxisome proliferation) in liver, gill, and kidney of sea bass following 2 weeks i.p. treatment have been studied. Induction of lauric acid hydroxylase, UDP-glucuronyl transferase, palmitoyl-CoA oxidase, carnitine-palmitoyl-transferase, p-nitrophenylacetate hydrolase, and benzaldehyde and propionaldehyde dehydrogenase activities was not observed in any case. The clofibrate administration at the dose of 70 mg/kg induced in the liver the glutathione S-tranferase and reduced epoxide hydrolase activities. These results demonstrate that a marine fish species, such as sea bass, is refractory to peroxisome proliferation.

Modulation of peroxisomal and microsomal fatty acid oxidation by acetone. A comparative study between liver and kidney

The effect of acetone consumption on some microsomal and peroxisomal activities was studied in rat kidney and these results were compared with data from former investigations in liver. Acetone increased the microsomal lauric acid hydroxylation, the aminopyrine N-demethylation catalyzed by cytochrome P450 and the microsomal UDP-glucuronyltransferase activity. Also, acetone increased the peroxisomal beta-oxidation of palmitoyl CoA and catalase activities in kidney. These studies suggest that acetone is a common inducer of the microsomal and peroxisomal fatty acid oxidation, as previously shown in both starved and ethanol treated rats. Our results support the hypothesis that microsomal fatty acid omega-hydroxylation results in the generation of substrates being supplied for peroxisomal beta-oxidation. We propose that the final purpose of these linked fatty acid oxidations could be the catabolism of fatty acids or the generation of a substrate for the synthesis of glucose from fatty acids. This pathway would be triggered by acetone treatment in a similar way in liver and kidney.

Hypolipidaemic effects of fenofibrate are not altered by mildronate-mediated normalization of carnitine concentration in rat liver

The five-fold higher carnitine content in the liver of fenofibrate-treated rats addresses the question about the possible role of this enhancement in the hypolipidaemic effect of the drug and the underlying mechanisms. When fenofibrate was administered with mildronate (a gamma-butyrobetaine hydroxylase inhibitor) in suitable amount, the content in carnitine was found to be normalized in liver. However, triglyceride contents of liver and serum were then at least as low as in rats treated by fenofibrate only. When carnitine concentration was lowered by mildronate to the third of the normal value, a marked increase in triglycerides occurred both in liver and serum, while the five-fold increase in carnitine due to fenofibrate enhanced blood ketone body concentration with no effect on liver and serum triglycerides. Data suggest that the normal carnitine concentration is largely sufficient to meet the usual requirement for carnitine palmitoyltransferase I activity (CPT I). In rat liver, increase in mitochondrial CPT I activity and in peroxisomal fatty acid oxidation may constitute part of the hypolipidaemic effect of fenofibrate.

Peroxisomal and microsomal fatty acid oxidation in liver of rats after chronic ethanol consumption

1. Microsomal P450 and peroxisomal fatty acid oxidation activities were studied in liver of rats after long-term ethanol consumption. 2. Ethanol increased the microsomal lauric acid omega-hydroxylation and the aminopyrine N-demethylation catalyzed by cytochrome P450. 3. Ethanol increased peroxisomal beta-oxidation of palmitoyl CoA and catalase activity in liver. 4. Both microsomal and peroxisomal activities behaved in a coordinate way in the liver of rats with long-term ethanol consumption. 5. These results would support a role of microsomal omega-hydroxylation and peroxisomal beta-oxidation of fatty acids in an extramitochondrial pathway of lipid oxidation in the liver.

Effects of chronic ethanol consumption on extramitochondrial fatty acid oxidation and ethanol metabolism by rat kidney

1. We evaluated the effects of chronic ethanol consumption on microsomal and peroxisomal fatty acid oxidation and on ethanol oxidation by the kidney. 2. When mature rats were fed 20% ethanol for 10 weeks, an increase in alcohol dehydrogenase and catalase activities were observed in the kidney. 3. Renal microsomal and peroxisomal oxidation of fatty acids also increased by the treatment, but total cytochrome P450 content did not. 4. We concluded that chronic ethanol consumption results in an increased extramitochondrial disposition of fatty acids and ethanol oxidation by the kidney.

Normal sexual development of rats exposed to butyl benzyl phthalate from conception to weaning

Butyl benzyl phthalate (BBP) has been administered in drinking water (1000 micrograms/liter) to pregnant AP rats during gestation and lactation. The sexual development of the pups was then monitored until their termination at postnatal day 90 (pnd 90). Pups derived from animals exposed to diethylstilbestrol (DES) in drinking water (50 micrograms/liter) acted as a positive control group. Glass drinking bottles were employed following demonstration of absorption of BBP into plastic drinking bottles. Drinking water intake was monitored and the estimated average exposures of the dams was 182.6 micrograms/kg/day BBP and 8.6 micrograms/kg/ day DES. A total of 240 control pups, 204 BBP pups, and 64 DES pups were obtained for study. The sexual development of both genders of pups was monitored. The body weights of the DES pups were significantly reduced at birth, an effect that persisted until pnd 90. The body weights of the BBP pups were marginally increased at birth, but this difference resolved by pnd 90. DES affected the sexual development of the pups for all endopoints monitored-anogenital (AG) distance on pnd 2; average day of vaginal opening and prepuce separation; uterus, testes, and accessory sex gland weight; and cauda epididymis sperm count and homogenization resistant testicular sperm count at pnd 90. BBP failed to affect any of these parameters, with the exception of a 1.1-day advance in the average day of vaginal opening and a small increase in male AG distance on pnd 2. These last two effects are related to the increased weight of the BBP pups. The incidence of FSH-containing cells in the pituitary gland of animals from each group was unaffected at pnd 90. The effects observed for the DES pups are consistent with the results of earlier studies by Sharpe et al. (Environ. Health Perspect. 103, 1136-1143, 1995). However, the absence of an effect of BBP administration on pup testis weight and testicular sperm count at pnd 90 is in contrast to reductions in these measurements reported earlier by Sharpe et al. (Environ. Health Perspect. 103, 1136-1143, 1995) for similarly derived BBP pups.

Overexpression of mdr2 gene by peroxisome proliferators in the mouse liver

BACKGROUND

In mice, fibrates induce mdr2 gene expression, and its encoded P-glycoprotein in the canalicular domain of hepatocytes, as well as increasing biliary phospholipid output. It is not known whether this effect is restricted to fibrates or is a common property of peroxisome proliferators.

AIMS

To test the effect of structurally unrelated peroxisome proliferators on mdr2 gene expression and biliary phospholipid output, and to explore the molecular mechanism(s) of mdr2 gene induction.

METHODS

Male CFI mice were fed on a diet supplemented with several peroxisome proliferators: phenoxyacetic acid herbicides, plasticizers, acetylsalicylic acid and partially hydrogenated fish oil.

RESULTS

Increased levels of mdr2 mRNAs, assessed by Northern blot analysis, were observed in the liver of mice treated with phenoxyacetic acid herbicides: 2,4,5-trichlorophenoxyacetic acid 570+/-133%, 2,4-dichlorophenoxyacetic acid 233+/-54% (p<0.005); plasticizers: di-(2-ethylhexyl)phthalate 282+/-78%, di-(isoheptyl)phthalate 163+/-40%, phthalic acid dinonyl ester 225+/-48% (p<0.01); and partially hydrogenated fish oil 372+/-138% (p<0.005). P-glycoprotein traffic ATPase content increased in the canalicular domain of hepatocyte of mice treated with the herbicide 2,4,5-trichlorophenoxyacetic acid and with partially hydrogenated fish oil (108% and 87%, respectively, p<0.05) as well as biliary phospholipid output (106% and 74%, respectively, p<0.05). In 2,4,5-trichlorophenoxyacetic acid-fed mice we found five-fold increase on mdr2 transcription rate, assessed by nuclear run-off assay.

CONCLUSIONS

Peroxisome proliferators induce mdr2 gene, its encoded P-gp in the canalicular domain of hepatocytes and increase biliary phospholipid output. The modulation of mdr2 gene might be part of the pleiotrophic response of peroxisome proliferation in mice liver and seems to be regulated mainly at a transcriptional level.

Microsomal and peroxisomal fatty acid oxidation in bile duct ligated rats: a comparative study between liver and kidney

1. Microsomal cytochrome P-450 and peroxisomal fatty acid oxidation was studied in the kidney of rats 7 days after bile duct ligation (BDL) and a comparative study between kidney and liver was done. 2. Only in the liver did cholestasis decrease the cytocrome P-450 content and the peroxisomal fatty acid beta-oxidation, the catalase activity, and the microsomal metabolism of lauric acid and aminopyrine. 3. In contrast, cholestasis did not influence these activities in the kidney. The microsomal and peroxisomal activities studied responded in a coordinate way to cholestasis. 4. These results could suggest the possibility of a cause-and-effect relationship between microsomal cytochrome P-450 and peroxisomal activity.

Microsomal and peroxisomal fatty acid oxidation in streptozotocin diabetic rat liver

Microsomal lauric acid hydroxylation and fatty acid peroxisomal beta-oxidation were studied in hepatic subcellulant preparations from streptozotocin-induced diabetic and diabetic insulin-treated rats. 2. The liver microsomes of the streptozotocin diabetic rats displayed a similar activity to hydroxylate lauric acid as the control microsomes. 3. Diabetic insulin-treated rats showed lower (omega 1) and omega-lauric acid hydroxylase activities than diabetic and control rats. 4. Streptozotocin-induced diabetes and diabetic insulin-treated rats exhibited no significant changes on peroxisomal palmitoyl CoA beta-oxidation compared to the control rats. 5. Both microsomal and peroxisomal fatty acid oxidation responded in a similar way in this model of experimental diabetes.

Microsomal and peroxisomal fatty acid oxidation in liver of rats with bile duct ligation and two-thirds hepatectomy

Microsomal cytochrome P450 and peroxisomal activity were studied in liver of rats 7 days after two-thirds hepatectomy or bile duct ligation (BDL). Both surgical models decreased the hepatic microsomal cytochrome P450 content, but only cholestasis, produced by BDL, decrease the microsomal metabolism of lauric acid and aminopyrine, peroxisomal fatty acid beta-oxidation and catalase activity. The microsomal and peroxisomal activities responded in a coordinate way to cholestasis and two-thirds hepatectomy. These results suggest a cause-effect relationship between the microsomal cytochrome P450 and peroxisomal activity.

Spontaneous development of fatty liver in ferrets in a toxicology study

Ferrets were maintained for 12 months on different diets (A, meat and biscuit; B, all meat; C, meat and fish; D, high fibre) to ascertain the cause of spontaneous development of fatty liver. High hepatic triglyceride contents resulted on diets B = C > D; whereas ferrets on diet A (control) showed no accumulation of lipid in liver. Serum triglyceride and total cholesterol were unchanged by diet. These ferrets (F0 generation) were mated with ferrets on the same diet and the offspring (F1 generation), maintained on the same diets as the parents, were killed at 12 months and the livers studied similarly. Histology showed that hepatic lipid accumulation in the F1 generation was identical with that in the same dietary groups of the F0 generation; liver glutathione (GSH) reductase and thiobarbituric acid-reacting substances (an index of lipid peroxidation) were increased in ferrets maintained on diets B, C and D, liver GSH concentration and GSH peroxidase activities were unchanged. Other ferrets fed a high-fat diet (diet A plus 20% w/w beef suet) for 18 days exhibited hepatic lipid accumulation and decreased hepatic cyanide-insensitive palmitoyl CoA oxidation (-30%), but hepatic lauric acid hydroxylation and carnitine acyl transferase activities were unchanged. These data indicate that ferrets on high-fat diets show no increased rates of liver fatty acid oxidation, as seen in rats, but instead accumulate triglyceride in the liver with some degree of lipid peroxidation.

Comparative hypolipidaemic and peroxisomal effects of ciprofibrate, clofibric acid, and their respective difluorocyclopropyl and 4-fluoro-substituted analogues in rat

1. We investigated the biological activity of the difluoro analogue (WIN 36117) of ciprofibrate, a potent peroxisome proliferator, and re-examined the relative activity of clofibric acid and its 4-fluoro analogue (fluorofibric acid) in the rat. 2. Twenty-four hours after a single dose, ciprofibrate and WIN 36117 produced dosage-related reductions in plasma cholesterol (16-42 and 9-34% respectively) and triglycerides (14-32 and 9-22% respectively). However, a single dose of clofibric acid or fluorofibric acid produced hypocholesterolaemia only (32-58 and 9-29% reductions respectively). 3. After treatment for 7 days reductions in cholesterol were similar at all dosages of ciprofibrate (45% reduction, mean across groups) whereas the effects of WIN 36117, clofibric acid and fluorofibric acid were still dosage related (reductions of 21-44, 37-43 and 2-28% respectively). Hypotriglyceridaemia was produced by all compounds (ciprofibrate 36-50%, WIN 36117 14-36%, clofibric acid 18-48%, fluorofibric acid 6-28%). 4. After treatment for 14 days all compounds produced dosage-related decreases in plasma fibrinogen (ciprofibrate 18-33%, WIN 36117 7-11%, clofibric acid 13-26%, fluorofibric acid 7-15%). 5. Peroxisomal beta-oxidation activity was increased by WIN 36117 (4.8-fold) and fluorofibric acid (4.2-fold) although these increases were less than those produced by ciprofibrate (13.6-fold) and clofibric acid (7.0-fold). WIN 36117 and fluorofibric acid also produced smaller increases in peroxisome numbers, liver weight, and carnitine acetyl transferase activity and smaller decreases in glutathione S-transferase and glutathione peroxidase activities. 6. Maximal increases in peroxisomal beta-oxidation activity produced in cultured rat hepatocytes by WIN 36117 and fluorofibric acid were 58 and 72% of those produced by ciprofibrate and clofibric acid respectively. 7. These results indicate the difluoro and 4-fluoro analogues of ciprofibrate and clofibric acid are hypolipidaemic agents and peroxisome proliferators but with reduced potencies relative to the parent molecules.

Hepatocarcinogenic potential of di(2-ethylhexyl)phthalate in rodents and its implications on human risk

The plasticizer di(2-ethylhexyl) phthalate (DEHP), to which humans are extensively exposed, was found to be hepatocarcinogenic in rats and mice. DEHP is potentially set free from objects made of synthetic materials (e.g., those used in medicine). Chronically, the greatest amounts are transferred to persons undergoing hemodialysis (up to 3.1 mg/kg b.w. per day) who would thus be considered the individuals most endangered by tumorigenesis. Although toxicokinetics seem to play a certain unclear role in the course of DEHP-related toxicity, toxicodynamic factors appear more decisive. DEHP is a representative of "peroxisome proliferators" (PP), a distinct group of substances that, in rodents, do not only induce peroxisomes but also specific enzymes in other organelles, organ growth, and DNA synthesis. The cluster of the characteristic effects of PP is generally, although perhaps not quite appropriately summarized as "peroxisome proliferation," and is strongest in the liver. The lowest observed effect level (LOEL) and the no observed effect level (NOEL) of peroxisome proliferation in the rat, as determined by the induction of specific enzymes (peroxisomal beta-oxidation, carnitine-acetyl-transferase, cytochrome P-452), DNA synthesis, and hepatomegaly, may be assumed as 50 and 25 mg/kg b.w. per day, respectively. DEHP and other carcinogenic PP are neither genotoxic nor tumor initiators, but they appear to be tumor promoters, also implicating a threshold level for the carcinogenic effect. Although a causal relationship between a particular effect of peroxisome proliferation and hepatocarcinogenesis is as yet unknown, peroxisome proliferation as a whole phenomenon appears to be associated with the potential of tumor induction, as shown by comparison of the relative strength of individual PP and by comparison of species and organ specificities. Likewise, LOEL and NOEL of rodent carcinogenesis, that is, 300 and 50 to 100 mg/kg b.w. per day, respectively, are above but not too far from the corresponding values for the investigated parameters of peroxisome proliferation. Thus, with respect to dose alone, worst-case exposure in hemodialysis patients is at least 16-fold below the LOEL of any characterized PP-specific effect of DEHP and approximately 100-fold below that of DEHP-related tumorigenesis. Also, primates are less responsive to PP than rats with respect to the investigated biochemical and morphological parameters. If this lower primate responsiveness is extrapolated to estimate carcinogenicity in humans, we might thus arrive at an even larger safety margin than when based on exposure alone. Doses of PP hypolipidemics that had clearly induced several indicators of peroxisome proliferation in rats did not cause any clear-cut enhancements in the peroxisomes of patients, even though most of these hypolipidemics were considerably stronger PP than DEHP. Thus, an actual threat to humans by DEHP seems rather unlikely. Accordingly, hepatocarcinogenesis was neither enhanced in workers exposed to DEHP nor in patients treated with hypolipidemics.