Enzymatic changes in rat liver associated with low and high doses of a peroxisome proliferator

Peroxisome proliferator-activated receptors, estrogenic responses and biotransformation system in the liver of salmon exposed to tributyltin and second messenger activator

The mechanisms by which organotin compounds produce modulations of the endocrine systems and other biological responses are not fully understood. In this study, juvenile salmon were force-fed diet containing TBT (0: solvent control, 0.1, 1 and 10mg/kg fish) for 72 h. Subsequently, fish exposed to solvent control and 10mg TBT were exposed to waterborne concentration (200 microg/l) of the adenylate cyclase (AC) stimulator, forskolin for 2 and 4h. The overall aim of the study was to explore whether TBT endocrine disruptive effects involve second messenger activation. Liver was sampled from individual fish (n=8) at the end of the exposures. The transcription patterns of peroxisome proliferator-activated receptor (PPAR) isotype and acyl-coenzyme A oxidase 1 (ACOX1), aromatase isoform, estrogen receptor-alpha (ER alpha), pregnane X receptor (PXR), CYP3A and glutathione S-transferase (GST) genes were measured by quantitative polymerase chain reaction (qPCR). Our data showed a consistent increase in PPAR alpha, PPAR beta and PPAR gamma mRNA and protein expression after TBT exposure that were inversely correlated with ACOX1 mRNA levels. Forskolin produced PPAR isotype-specific mRNA and protein effects that were modulated by TBT. ACOX1 expression was decreased (at 2h) and increased (at 4h) by forskolin and the presence of TBT potentiated these effects. TBT apparently increased mRNA and protein levels of cyp19a, compared to the solvent control, whereas cyp19b mRNA levels were unaffected by TBT treatment. Combined TBT and forskolin exposure produced respective decrease and increase of mRNA levels of cyp19a and cyp19b, compared with control. TBT decreased ER alpha mRNA at low dose (1mg/kg) and forskolin exposure alone produced a consistent decrease of ER alpha mRNA levels that were not affected by the presence of TBT. Interestingly, PXR and CYP3A mRNA levels were differentially affected, either decreased or increased, after exposure to TBT and forskolin, singly and also in combination. GST mRNA was increased by TBT exposure. Exposure to forskolin alone increased GST expression with time, and combined exposure with TBT potentiated these respective effects. Overall, the present study demonstrates multiple biological effects of TBT given singly or in combination with cAMP activator. There are no studies known to us that have evaluated the endocrine disruptive effects of TBT in the presence of a second messenger activator, and our data suggest that TBT may exert endocrine, biotransformation and lipid peroxidative effects through modulation of cAMP/PKA second messenger signaling with overt physiological consequences.

Modulation of acute steroidogenesis, peroxisome proliferator-activated receptors and CYP3A/PXR in salmon interrenal tissues by tributyltin and the second messenger activator, forskolin

There are uncertainties regarding the role of sex steroids in sexual development and reproduction of gastropods, leading to the recent doubts as to whether organotin compounds do inhibit steroidogenic enzymes in these species. These doubts have led us to suspect that organotin compounds may affect other target molecules, particularly signal transduction molecules or secondary mediators of steroid hormone and lipid synthesis/metabolism. Therefore, we have studied the effects of TBT exposure through food on acute steroidogenesis, PPARs and CYP3A responses in the presence and absence of a cyclic AMP (cAMP) activator, forskolin. Two experiments were performed. Firstly, juvenile salmon were force-fed once with diet containing TBT doses (0.1, 1 and 10mg/kg fish) dissolved in ethanol and sampled after 72h. Secondly, fish exposed to solvent control and 10mg/kg TBT for 72h were transferred to new tanks and exposed to waterborne forskolin (200microg/L) for 2 and 4h. Our data show that juvenile salmon force-fed TBT showed modulations of multiple biological responses in interrenal tissues that include, steroidogenesis (cAMP/PKA activities; StAR and P450scc mRNA, and plasma cortisol), and mRNA for peroxisome proliferator-activated receptor (PPAR) isoforms (alpha, beta, gamma), acyl-CoA oxidase-1 (ACOX1) and CYP3A/PXR (pregnan X receptor). In addition, forskolin produced differential effects on these responses both singly and also in combination with TBT. Overall, combined forskolin and TBT exposure produced higher effects compared with TBT exposure alone, for most of the responses (cortisol, PPARbeta, ACOX1 and CYP3A). Interestingly, forskolin produced PPAR isoform-specific effects when given singly or in combination with TBT. Several TBT mediated toxicity in fish that includes thymus reduction, decrease in numbers of lymphocytes, inhibition of gonad development and masculinization, including the imposex phenomenon have been reported. When these effects are considered with the present findings, it suggests that studies on mechanisms of action or field studies may reveal endocrine, reproductive or other effects of TBT at lower concentrations than those reported to date from subchronic tests of fishes. Since the metabolic fate of organotin compounds may contribute to the toxicity of these chemicals, the present findings may represent some new aspects of TBT toxicity not previously reported.

Inhibition of peroxisome proliferator-activated receptor α signaling by vitamin D receptor

Peroxisome proliferator-activated receptors (PPARs) are nuclear fatty acid receptors that have been implicated to play an important role in lipid and glucose homeostasis. PPARalpha potentiates fatty acid catabolism in the liver and is activated by the lipid-lowering fibrates, whereas PPARgamma is essential for adipocyte differentiation. Here we report that nuclear vitamin D(3) receptor (VDR) represses the transcriptional activity of PPARalpha but not PPARgamma in a 1,25(OH)(2)D(3)-dependent manner. The analysis using chimeric receptors revealed that ligand binding domain of PPARalpha and VDR was involved in the molecular basis of this functional interaction and that the DNA binding domain of VDR was not required for the suppression, suggesting a novel mechanism that might involve protein-protein interactions rather than a direct DNA binding. Furthermore, the treatment of rat hepatoma H4IIE cells with 1,25(OH)(2)D(3) diminishes the induction of AOX mRNA by PPARalpha ligands, Wy14,643. VDR signaling might be considered as a factor regulating lipid metabolism via PPARalpha pathway. We report here the novel action of VDR in controlling gene expression through PPARalpha signaling.

Role of acyl-CoA binding protein in acyl-CoA metabolism and acyl-CoA-mediated cell signaling

Long-chain acyl-CoA esters (LCA) act both as substrates and intermediates in metabolism and as regulators of various intracellular functions. Acyl-CoA binding protein (ACBP) binds LCA with high affinity and is believed to play an important role in intracellular acyl-CoA transport and pool formation and therefore also for the function of LCA as metabolites and regulators of cellular functions . The free concentration of cytosolic LCA is efficiently buffered to low nanomole concentration by ACBP and fatty acid binding protein (FABP). An additional important factor is the activity of acyl-CoA hydrolases. The estimated cellular free LCA concentration is two to four orders of magnitude lower than the concentrations reported to be necessary to regulate most LCA-affected cellular functions. Preliminary evidence indicates that the regulatory effect of LCA might be mediated by the LCA/ACBP complex.

Role of acylCoA binding protein in acylCoA transport, metabolism and cell signaling

Long chain acylCoA esters (LCAs) act both as substrates and intermediates in intermediary metabolism and as regulators in various intracellular functions. AcylCoA binding protein (ACBP) binds LCAs with high affinity and is believed to play an important role in intracellular acylCoA transport and pool formation and therefore also for the function of LCAs as metabolites and regulators of cellular functions [1]. The major factors controlling the free concentration of cytosol long chain acylCoA ester (LCA) include ACBP [2], sterol carrier protein 2 (SCP2) [3] and fatty acid binding protein (FABP) [4]. Additional factors affecting the concentration of free LCA include feed back inhibition of the acylCoA synthetase [5], binding to acylCoA receptors (LCA-regulated molecules and enzymes), binding to membranes and the activity of acylCoA hydrolases [6].

Role of long-chain fatty acyl-CoA esters in the regulation of metabolism and in cell signalling

The intracellular concentration of free unbound acyl-CoA esters is tightly controlled by feedback inhibition of the acyl-CoA synthetase and is buffered by specific acyl-CoA binding proteins. Excessive increases in the concentration are expected to be prevented by conversion into acylcarnitines or by hydrolysis by acyl-CoA hydrolases. Under normal physiological conditions the free cytosolic concentration of acyl-CoA esters will be in the low nanomolar range, and it is unlikely to exceed 200 nM under the most extreme conditions. The fact that acetyl-CoA carboxylase is active during fatty acid synthesis (Ki for acyl-CoA is 5 nM) indicates strongly that the free cytosolic acyl-CoA concentration is below 5 nM under these conditions. Only a limited number of the reported experiments on the effects of acyl-CoA on cellular functions and enzymes have been carried out at low physiological concentrations in the presence of the appropriate acyl-CoA-buffering binding proteins. Re-evaluation of many of the reported effects is therefore urgently required. However, the observations that the ryanodine-senstitive Ca2+-release channel is regulated by long-chain acyl-CoA esters in the presence of a molar excess of acyl-CoA binding protein and that acetyl-CoA carboxylase, the AMP kinase kinase and the Escherichia coli transcription factor FadR are affected by low nanomolar concentrations of acyl-CoA indicate that long-chain acyl-CoA esters can act as regulatory molecules in vivo. This view is further supported by the observation that fatty acids do not repress expression of acetyl-CoA carboxylase or Delta9-desaturase in yeast deficient in acyl-CoA synthetase.

Inhibition of peroxisome proliferator signaling pathways by thyroid hormone receptor. Competitive binding to the response element

Peroxisome proliferators (e.g. clofibric acid) and thyroid hormone play an important role in the metabolism of lipids. These effectors display their action through their own nuclear receptors, peroxisome proliferator-activated receptor (PPAR) and thyroid hormone receptor (TR). PPAR and TR are ligand-dependent, DNA binding, trans-acting transcriptional factors belonging to the erbA-related nuclear receptor superfamily. The present study focused on the convergence of the effectors on the peroxisome proliferator response element (PPRE). Transcriptional activation induced by PPAR through a PPRE was significantly suppressed by cotransfection of TR in transient transfection assays. The inhibition, however, was not affected by adding 3,5,3'-triiodo-L-thyronine (T3). Furthermore, the inhibition was not observed in cells cotransfected with retinoic acid receptor or vitamin D3 receptor. The inhibitory action by TR was lost by introducing a mutation in the DNA binding domain of TR, indicating that competition for DNA binding is involved in the molecular basis of this functional interaction. Gel shift assays revealed that TRs, expressed in insect cells, specifically bound to the 32P-labeled PPRE as heterodimers with the retinoid X receptor (RXR). Both PPAR and TR bind to PPRE, although only PPAR mediates transcriptional activation via PPRE. TR.RXR heterodimers are potential competitors with PPAR.RXR for binding to PPREs. It is concluded that PPAR-mediated gene expression is negatively controlled by TR at the level of PPAR binding to PPRE. We report here the novel action of thyroid hormone receptor in controlling gene expression through PPREs.

Impact of cytochrome P450 system on lipoprotein metabolism. Effect of abnormal fatty acids (3-thia fatty acids)

Fatty acid omega-hydroxylation, peroxisomal and mitochondrial fatty acid oxidation and related lipid-metabolizing enzymes are constitutive activities of mammalian cells. The past 5 years have witnessed an increased interest in the modulation of these pathways and functions by a new group of abnormal fatty acids (sulfur-substituted fatty acid analogs), due to the metabolic and nutritional aspects related to human health and disease, and possible treatment of certain inherited peroxisomal and mitochondrial disorders. The purpose of this review is to present an overview of current knowledge in the field and to provide an account of recent developments, particularly with respect to the chemical nature of the biologically active factors and their possible mechanism of action.

Hepatic peroxisome proliferation in rodents and its significance for humans

Peroxisomes are subcellular organelles found in all eukaryotic cells. In the liver they are usually round and measure about 0.5-1.0 microns; in rodents they contain a prominent crystalloid core, but this may be absent in newly formed rodent peroxisomes as well as in human peroxisomes. A major role of the peroxisomes is the breakdown of long-chain fatty acids, thereby complementing mitochondrial fatty-acid metabolism. Many chemicals are known to increase the number of peroxisomes in rat and mouse hepatocytes. This peroxisome proliferation is accompanied by replicative DNA synthesis and liver growth. No clear structure-activity relationships are apparent. Many of these peroxisome proliferators contain acid functions that can modulate fatty acid metabolism. Two mechanisms have been proposed for the induction of peroxisome proliferation. One is based on the existence of one or several specific cytosolic receptors that bind the peroxisome proliferator, facilitating its translocation to the cell nucleus and the activation of the expression of specific genes. The second, perhaps more general, hypothesis involves chemically mediated perturbation of lipid metabolism. These two hypotheses are not mutually exclusive. Many peroxisome proliferators have been shown to induce hepatocellular tumours, despite being uniformly non-genotoxic, when administered at high dose levels to rats and mice for long periods. Three mechanisms have been proposed to explain the induction of tumours. One is based on increased production of active oxygen species due to imbalanced production of peroxisomal enzymes; it has been proposed that these reactive oxygen species cause indirect DNA damage with subsequent tumour formation. In rodents, an alternative mechanism is the promotion of endogenous lesions by sustained DNA synthesis and hyperplasia. Thirdly, it is conceivable that sustained growth stimulation may be sufficient for tumour formation. Marked species differences are apparent in response to peroxisome proliferations. Rats and mice are extremely sensitive, and hamsters show an intermediate response while guinea pigs, monkeys and humans appear to be relatively insensitive or non-responsive at dose levels that produce a marked response in rodents. These species differences may be reproduced in vitro using primary culture hepatocytes isolated from a variety of species including humans. The available experimental evidence suggests a strong association and a probable casual link between peroxisome-proliferator-elicited liver growth and the subsequent development of liver tumours in rats and mice. Since humans are insensitive or unresponsive, at therapeutic dose levels, to peroxisome-proliferator-induced hepatic effects, it is reasonable to conclude that the encountered levels of exposure to these non-genotoxic agents do not present a hepatocarcinogenic hazard to humans.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of the peroxisome proliferator mono(2-ethylhexyl)phthalate in primary hepatocyte cultures derived from rat, guinea pig, rabbit and monkey. Relationship between interspecies differences in biotransformation and peroxisome proliferating potencies

Primary hepatocyte cultures derived from rat, rabbit, guinea pig and monkey have been treated in vitro with metabolites of di(2-ethylhexyl)phthalate, i.e. mono(2-ethylhexyl)phthalate (MEHP), mono(5-carboxy-2-ethylpentyl)phthalate (metabolite V) and mono(2-ethyl-5-oxohexyl)phthalate (metabolite VI). In rat hepatocyte cultures MEHP and metabolite VI were equally potent in inducing peroxisome proliferation, while metabolite V was much less potent. In rat hepatocytes a 50% increase in both peroxisomal palmitoyl-CoA oxidase activity and microsomal lauric acid omega-hydroxylation activity was found after treatment with 5-15 microM MEHP. In guinea pig, rabbit and monkey hepatocyte cultures, a 50% increase in peroxisomal palmitoyl-CoA oxidase activity was found after treatment with 408-485 microM MEHP. No induction of lauric acid omega-hydroxylation activity was found. These results indicate that peroxisome proliferation can be induced by MEHP in rabbit, guinea pig and monkey hepatocytes, but that these species are at least 30-fold less sensitive to peroxisome proliferation induction than rats. The proposed mechanistic inter-relationship between induction of lauric acid omega-hydroxylation activity and peroxisome proliferation is found in rat hepatocytes, but not in hepatocytes of the other three species. Treatment of guinea pig hepatocyte cultures with MEHP resulted in an increase in triglyceride concentrations in the hepatocytes. In rat and rabbit hepatocyte cultures, triglyceride concentrations were much less altered by MEHP. In monkey hepatocytes a decrease in hepatic triglyceride concentration was found after treatment with MEHP. These effects are in agreement with in vivo effects observed before. After treatment of primary hepatocyte cultures with MEHP, high concentrations of omega- and (omega-1)-hydroxylated metabolites of MEHP were found in media from rat, rabbit and guinea pig cultures. The formation of these metabolites did not decline in time. During treatment the metabolite profile in media from rat hepatocyte cultures moved towards omega-hydroxy metabolites of MEHP. In media from monkey hepatocyte cultures the lowest concentrations of hydroxylated metabolites were determined. No major species differences were found in the potency to form oxidized MEHP metabolites, and thus no unique metabolite differences were found, which could explain the species differences in sensitivity for peroxisome proliferation.

Peroxisome proliferating sulphur- and oxy-substituted fatty acid analogues are activated to acyl coenzyme A thioesters

In liver homogenates from untreated rats the sulphur-substituted fatty acid analogues tetradecylthioacetic acid (CMTTD) was activated to its acyl-coenzyme A thioester. The activation was found to take place in the mitochondrial, microsomal and peroxisomal fractions. The activity of CMTTD-CoA synthetase was 50% compared to palmitoyl-CoA synthetase in all cellular fractions. When rats were treated with the peroxisome proliferating sulphur-substituted fatty acid analogues CMTTD and 3-dithiahexadecanedioic acid (BCMTD), the CMTTD-CoA synthetase activity was induced in mitochondrial, peroxisomal and microsomal fractions. Palmitoyl-CoA synthetase was induced proportionally. In rats treated with tetradecylthiopropionic acid (CETTD) of low peroxisome proliferating potency, the activities of CMTTD-CoA synthetase and palmitoyl-CoA synthetase were inhibited in mitochondrial and microsomal fractions. In contrast, all three sulphur-substituted acids induced the activity of palmitoyl-CoA synthetase and CMTTD-CoA synthetase in peroxisomes. Both the CMTTD-CoA and palmitoyl-CoA synthetase activities were induced by CMTTD and BCMTD, in close correlation to the induction of peroxisomal beta-oxidation. During the three treatment regimes, CMTTD-CoA synthetase activity ran parallel to the palmitoyl-CoA synthetase activity at a rate of 50% in all cellular fractions. Thus, CMTTD is assumed to be activated by the long-chain acyl-CoA synthetase enzyme. Rats were treated for 5 days with sulphur- and oxy-substituted fatty acid analogues, clofibric acid and fenofibric acid. All compounds which induced peroxisomal beta-oxidation activity in vivo could be activated to their respective CoA thioesters in liver homogenate. CETTD which induced peroxisomal beta-oxidation only two-fold, was activated at a rate of 50% compared to palmitate. Fenofibric acid induced peroxisomal beta-oxidation 9.6-fold, while it was activated at a rate of only 10% compared to palmitate. Thus, no correlation was found between rate of activation in vitro and induction of peroxisomal activity in vivo. On the other hand, tetradecylsulfoxyacetic acid (TSOA) and tetradecylsulfonacetic acid (TSA) (sulphuroxygenated metabolites of CMTTD) with no inductive effects, were not activated to their respective CoA derivatives. Altogether the data suggest that the enzymatic activation of the peroxisome proliferating compounds is essential for their proliferating activity, but the rate of activation does not determine the potency of the proliferators. The role of the xenobiotic-CoA pool in relation to the whole coenzyme A profile during peroxisome proliferation is discussed.

Induction of peroxisome proliferation in rainbow trout exposed to ciprofibrate

Rainbow trout (Salmo gairdneri), average body weight of 450 g, were treated with 15, 25, or 35 mg/kg of ciprofibrate via intraperitoneal injection every other day for 2 to 3 weeks. The effects on hepatic peroxisomal acyl-CoA oxidase, polypeptide PPA-80, catalase, and liver weight were measured. The treatment of trout with ciprofibrate showed significant dose-related increases in peroxisomal acyl-CoA activity, polypeptide PPA-80, and catalase after 3 weeks of exposure. Peroxisomal oxidase activity showed a significant (p = 0.0008) increase (78%) at 35 mg/kg and a marginal (p = 0.1) increase (27%) at 25 mg/kg after 3 weeks of exposure. Densitometric analysis of polypeptide PPA-80 and catalase showed increases up to 48 and 236% at 35 mg/kg, respectively. Morphometric analysis on livers of trout administered 35 mg/kg for 3 weeks showed a 2.3-fold increase of peroxisomal volume density, as compared to control. This study demonstrates the induction of peroxisome proliferation in rainbow trout administered ciprofibrate, a known peroxisome proliferator in rodents.

Effects of the tumor promoter 12-O-tetradecanoylphorbol 13-acetate on peroxisomal activities and enzyme activities involved in lipid metabolism in rat liver

The effects of 12-O-tetradecanoylphorbol 13-acetate (TPA) on hepatic lipids and key enzymes involved in esterification, hydrolysis and oxidation of long-chain fatty acids at increasing doses were investigated in rats. TPA administration tended to decrease the mitochondrial activities of palmitoyl-CoA synthetase and carnitine palmitoyltransferase. The microsomal palmitoyl-CoA synthetase activity was increased. TPA administration was also associated with a dose-dependent increase of glycerophosphate acyltransferase activity both in the mitochondrial and microsomal fractions in particular. The data are consistent with a decreased catabolism of long-chain fatty acids at the mitochondrial level, and an increased capacity for esterification of fatty acids in the microsomal fraction. Peroxisomal beta-oxidation was increased about 2-fold in the peroxisome-enriched fraction of TPA-treated rats while the catalase and urate oxidase activities were only marginally affected. TPA administration revealed elevated capacity for hydrolysis of palmitoyl-CoA and palmitoyl-L-carnitine in the microsomal fraction. Neither increased cytosolic palmitoyl-CoA hydrolase activity nor increased hydroxylation of lauric acid nor changes of the hepatic content of cytochrome P-450 isoenzymic forms were observed in the TPA-treated animals. There was no induction of the protein content of the bifunctional enoyl-CoA hydratase. Thus, TPA behaves more like choline-deficient diet and ethionine treatment than well-known peroxisome proliferators. It seems possible that TPA selectively stimulated the peroxisomal activities, i.e., peroxisomal beta-oxidation rather than evoking a peroxisome proliferation capacity.

Chemically induced proliferation of peroxisomes: implications for risk assessment

An increasing number of beneficial and economically important drugs, industrial chemicals, and agrichemicals are being found to cause a dose-related hepatomegaly in rodent species which is associated with the proliferation of the subcellular organelle, the peroxisome. The prolonged proliferation of hepatocellular peroxisomes and the enhanced production of the normal peroxisomal metabolic byproduct, hydrogen peroxide, in these animals during chronic bioassays has been hypothesized to account for the tumorigenicity of several of these compounds, most of which lack any measurable genotoxicity in in vitro and in vivo assays. This paper briefly reviews the basic morphology and enzymology of the peroxisome and its relationship to specific pathologic changes in animals. The potential impact of the mechanism of action of peroxisome proliferators upon the design of toxicity studies and, in conjunction with interspecies sensitivity data, upon risk assessment is discussed.

Carboxylic metabolites of tiadenol as "proximate" inducers of hepatic peroxisomal beta-oxidation activity

Chronic exposure of rats to the hypolipidemic agent tiadenol causes a dramatic dose-dependent increase of peroxisomal beta-oxidation activity. To elucidate which metabolite of the drug is the "proximate" inducer (tiadenol is eliminated completely in metabolized form after acute administration) we investigated the qualitative and quantitative metabolic profile of the drug at different doses (50, 150, 300 mg/Kg in two-weeks chronically treated rats, in parallel to that of a model compound, tiadenol-disulfoxide, a weak inducer of palmitoyl-CoA oxidation activity. No changes in the biodisposition of tiadenol (and tiadenol-disulfoxide) were found following chronic treatment for all the doses tested. For both the compounds a strict correlation was evidenced between the extent of formation of carboxylic metabolites and their inductive potencies on peroxisomal beta-oxidation activity. This indicates that tiadenol carboxylic metabolites act as the enzymatic effectors.

Studies by the National Toxicology Program on di(2-ethylhexyl)phthalate

In a 2-year feed study previously reported by the National Toxicology Program (NTP), the plasticizer di(2-ethylhexyl)phthalate (DEHP) was found to produce increased incidences of hepatocellular neoplasms in both sexes of Fischer 344 (F344) rats and B6C3F1 mice. Further studies by the NTP on this chemical have investigated its genotoxicity, dermal absorption, reproductive and developmental toxicity, and biochemical mechanism of action. DEHP was not mutagenic in Salmonella typhimurium (strains TA98, TA100, TA1535 or TA1537), in L5178Y mouse lymphoma cells, or in Drosophila melanogaster. DEHP did not induce chromosomal aberrations, but did cause a marginal dose-related increase in sister chromatid exchanges in CHO cells. In a dermal absorption study, DEHP was not absorbed well through the skin of F344 rats. In a fertility assessment study, DEHP was shown to be a reproductive toxicant in both male and female CD-1 mice. The teratogenic potential of DEHP was evaluated in F344 rats and CD-1 mice. In the rat study, there were no significant differences in percent fetuses malformed between control and treatment groups, even at dose levels (1.0, 1.5 and 2.0%) which produced significant maternal and fetal toxicity. In the mouse study, the incidence of fetuses with malformations was significantly increased at dose levels which produced maternal and/or fetal toxicity (0.10 and 0.15%), and at a dose level (0.05%) which did not cause maternal or fetal toxicity. The no-observed effect level for developmental toxicity in mice was 0.025% DEHP. Kinetic data on the rates of formation of H2O2 by peroxisomal palmitoyl CoA oxidase, and of degradation of H2O2 by catalase, was used to estimate in vitro steady-state H2O2 concentrations during peroxisomal oxidation of palmitoyl CoA. Increases in steady-state H2O2 in liver homogenates of rats treated with DEHP, di(2-ethylhexyl)adipate, or nafenopin, a hypolipidemic drug, correlated well with the carcinogenic potential of these chemicals determined in previous carcinogenicity studies, and are consistent with but not definitive evidence for the involvement of peroxisome proliferation in the hepatocarcinogenesis of these compounds.

Induction of cytosolic clofibroyl-CoA hydrolase activity in liver of rats treated with clofibrate

Among subcellular fractions of liver homogenates of rats, the clofibroyl-CoA hydrolase activity is found mainly in the cytosolic fraction. It is here shown that the subcellular distribution of clofibroyl-CoA hydrolase appears to be different from the distribution of palmitoyl-CoA hydrolase activity. Thus, in contrast to the case with palmitoyl-CoA, no hydrolysis of clofibroyl-CoA was catalysed by the microsomal fraction. Furthermore, the hydrolysis of palmitoyl-CoA and clofibroyl-CoA in the cytosolic fraction seemed to be catalyzed by two different enzymes. Rats treated with clofibrate (0.3%, w/w) showed a significant increased clofibroyl-CoA hydrolase activity where the cytosolic hydrolase was increased 3.5-fold. Clofibrate administration also elevated the specific clofibroyl-CoA hydrolase activity by factors of 1.7 and 1.5 in the mitochondrial and the light-mitochondrial fractions, respectively. Thus, it is possible that clofibroyl-CoA hydrolase has also a multiorganelle localization.

Concomitant induction of cytosolic but not microsomal epoxide hydrolase with peroxisomal beta-oxidation by various hypolipidemic compounds

The effects of two cholesterol-lowering (probucol and 1-benzyl-imidazole), three triglyceride- and cholesterol-lowering (clofibrate, tiadenol and fenofibrate) and one triglyceride-lowering (acetylsalicylic acid) compounds on the specific activities of two lipid-metabolizing enzymes (cyanide-insensitive peroxisomal beta-oxidation and palmitoyl-CoA hydrolase) and two xenobiotic metabolizing enzymes (cytosolic (cEH) and microsomal epoxide hydrolase (mEHb] from the livers of male Fischer F-344 rats were investigated. With the exception of probucol and acetylsalicylic acid, all compounds tested caused a dose-dependent hepatomegaly. Taken on a weight basis fenofibrate was the most effective inducer, causing a 20-fold induction of peroxisomal beta-oxidation, a 13-fold induction of cEH activity and a 16-fold induction of palmitoyl-CoA hydrolase activity. The other compounds with triglyceride-lowering activity also induced cEH as well as peroxisomal beta-oxidation and palmitoyl-CoA hydrolase activity. The potency of each individual drug was similar for induction of cEH activity as compared with that of peroxisomal beta-oxidation and palmitoyl-CoA hydrolase activity, but very dissimilar for mEHb, which upon treatment with any of the triglyceride-lowering compounds was either not or only minimally (less than 1.5-fold) induced. 1-Benzylimidazole possessing exclusively cholesterol-lowering activity increased mEHb much more than either cEH or peroxisomal beta-oxidation. The absence of an enhancement of cEH activity in in vitro studies confirmed that the increase in enzyme activity by the test compounds is not caused by activation. cEH activity was also induced in the kidney but only about 2-fold by fenofibrate, tiadenol and acetylsalicylic acid.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of a high-fat diet with partially hydrogenated fish oil on long-chain fatty acid metabolizing enzymes in subcellular fractions of rat liver

Hepatic metabolism of long-chain fatty acids were studied in young male rats fed a semisynthetic diet containing 20% (w/w) partially hydrogenated fish oil (PHFO)2, with or without 2% (w/w) linoleic acid. The enzymic activities involved in the formation and breakdown of long-chain acyl-CoA were both increased in the animals fed the semisynthetic diet, compared to pellet-fed control animals. Thus, the specific palmitoyl-CoA synthetase activity increased slightly in both the mitochondrial (1.4-fold) and the microsomal (1.6-fold) fractions. In the peroxisome-enriched fraction the activity was increased (about 2.6-fold) only on addition of linoleic acid to the diet. The data are consistent with an increased catabolism of long-chain fatty acids by a peroxisomal and a mitochondrial pathway. Thus, the total carnitine palmitoyltransferase activity increased 2-fold in the mitochondrial fraction, and was partly prevented by added linoleic acid. Peroxisomal beta-oxidation activity was also increased (about 7-fold) in livers of PHFO-fed rats, but did not change when linoleic acid was added. The PHFO-fed rats also revealed elevated capacity for hydrolysis of palmitoyl-CoA in both the mitochondrial (2.4-fold) and the cytosolic (2.0-fold) fractions and the latter was almost completely and selectively prevented by added linoleic acid. The s values of mitochondria and peroxisomes varied with the dietary regime, and some of the observed changes in the specific activities of the fatty acid metabolizing enzymes with multiple subcellular localization can be explained as an effect of changes in the s values of the organelles. Thus, the s value of mitochondria increased 1.8-fold as a result of PHFO feeding, but was fully prevented by linoleic acid in the diet. On the other hand, the s values of peroxisomes decreased by about 50% on feeding a PHFO diet, and by about 25% with added linoleic acid.

Separation and measurement of clofibroyl coenzyme A and clofibric acid in rat liver after clofibrate adminstration by reversed-phase high-performance liquid chromatography with photodiode array detection

A method to identify and quantitate clofibric acid and clofibroyl coenzyme A (CoA) products in rat liver was developed using reversed-phase high-performance liquid chromatography. The system was developed with baseline separation of clofibroyl-CoA from clofibric acid using isocratic elution, with a mobile phase consisting of 52% methanol and 28 mM potassium phosphate buffer (pH 4.2). With this high methanol concentration, the large amount of UV-absorbing materials present in the liver extracts were eluted earlier than the investigated compounds. Clofibroyl-CoA has a characteristic absorbance spectrum with distinct peaks at 260 and 230 nm, while clofibric acid showed only a distinct peak at 230 nm. Using an on-line photodiode array detector, the spectra could be recorded during analysis without interrupting the flow of the mobile phase. This spectral analysis identification possibilities and evaluation of the purity of the chromatographic peaks. In a perchloric extract of rat liver, the recovery of clofibric acid and clofibroyl-CoA added to the liver extract ranged from 70 to 80%. A linear relationship was observed between clofibric acid and clofibroyl-CoA concentration and the area of their peaks in the chromatogram. The detection limit of the method was lower than 5 pmol for both compounds when the absorbance was recorded at 230 nm. The method could be used without modification for the estimation of clofibroyl-CoA and clofibric acid in biological extracts.

Different induction of microsomal carboxylesterases, palmitoyl-CoA hydrolase and acyl-L-carnitine hydrolase in rat liver after treatment with clofibrate

The levels of hepatic carboxylesterases, including palmitoyl-CoA hydrolase and decanoyl-D,L-carnitine hydrolase, were studied in total homogenates and subcellular fractions prepared from the livers of male rats fed diets containing 0.3% clofibrate. The microsomal carboxylesterase as well as the fatty acyl-thioesterase are differently induced by clofibrate feeding. The specific activities of acetanilide carboxylesterase and decanoyl-D,L-carnitine hydrolase increased more than 3-fold in the microsomal fraction, compared to pellet-fed control animals. The microsomal activities of palmitoyl-CoA hydrolase and propanidid hydrolase were decreased by about 20 to 40% in clofibrate-treated rats. The specific clofibrate hydrolase activity remained unchanged after clofibrate administration, indicating that this microsomal carboxylesterase is not induced by its own substrate. The data suggest a different distribution of the differing carboxylesterase along the endoplasmic reticulum.

Peroxisome proliferators show tumor-promoting but no direct transforming activity in vitro

The chemically unrelated hypolipidemic drugs, tiadenol, niadenate, and clofibrate have been tested for carcinogenic and tumor-promoting potential in the C3H/10TI/2 C18 cell test system. None of these chemicals were carcinogenic, while both niadenate and clofibrate were active tumor promoters at micromolar concentrations. All 3 drugs induced the differentiation of C3H/10T1/2 C18 cells to adipocytes. This latter finding confirms previously observed effects of the tumor promoter TPA.

The tumour promoter 12-O-tetradecanoylphorbol-13-acetate increases the activities of some peroxisome-associated enzymes in in vitro cell culture

A study was conducted on the effects of 12-O-tetradecanoyl-phorbol-13-acetate (TPA) on peroxisomal enzyme activities in mouse embryo fibroblasts C3H/10T1/2 C18 cells and chemically transformed C3H/10T1/2 MCA16 cells. TPA is a potent tumour promoter and treatment with this compound of the two cell lines induced peroxisomal fatty acid beta-oxidation, carnitine acetyltransferase, palmitoyl-CoA hydrolase, and catalase activities after 240 h of treatment. Stimulation of the corresponding enzyme activities was dose-related and cycloheximide inhibited the TPA-induced enzyme activities, except that of carnitine acetyltransferase. The MCA16 cells appeared to be more sensitive than the C18 cells in inducing peroxisome-associated enzyme activities after TPA treatment. The activities of the microsomal marker, NADPH-cytochrome c reductase and the mitochondrial marker, glutamate dehydrogenase were not enhanced by TPA treatment. The results indicate that TPA has peroxisomal effects and may be classified as a peroxisome proliferator.

Correlation between the cellular level of long-chain acyl-CoA, peroxisomal beta-oxidation, and palmitoyl-CoA hydrolase activity in rat liver. Are the two enzyme systems regulated by a substrate-induced mechanism?

Data obtained in earlier studies with rats fed diets containing high doses of peroxisome proliferators (niadenate, tiadenol, clofibrate, or nitotinic acid) are used to look for a quantitative relationship between peroxisomal beta-oxidation, palmitoyl-CoA hydrolase, palmitoyl-CoA synthetase and carnitine palmitoyltransferase activities, and the cellular concentration of their substrate and reaction products. The order of the hyperlipidemic drugs with regard to their effect on CoA derivatives and enzyme activities was niadenate greater than tiadenol greater than clofibrate greater than nicotinic acid. Linear regression analysis of long-chain acyl-CoA content versus palmitoyl-CoA hydrolase and peroxisomal beta-oxidation activity showed highly significant linear correlations both in the total liver homogenate and in the peroxisome-enriched fractions. A dose-response curve of tiadenol showed that carnitine palmitoyltransferase and palmitoyl-CoA synthetase activities and the ratio of long-chain acyl-CoA to free CoASH in total homogenate rose at low doses before detectable changes occurred in the peroxisomal beta-oxidation and palmitoyl-CoA hydrolase activity. A plot of this ratio parallelled the palmitoyl-CoA synthetase activity. The specific activity of microsomally localized carnitine palmitoyl-transferase was low and unchanged up to a dose where no enhanced peroxisomal beta-oxidation was observed, but over this dose the activity increased considerably so that the specific of the enzyme in the mitochondrial and microsomal fractions became comparable. The mitochondrial palmitoyl-CoA synthetase activity decreased gradually. The correlations may be interpreted as reflecting a common regulation mechanism for palmitoyl-CoA hydrolase and peroxisomal beta-oxidation enzymes, i.e., the cellular level of long-chain acyl-CoA acting as the metabolic message for peroxisomal proliferation resulting in induction of peroxisomal beta-oxidation and palmitoyl-CoA hydrolase activity. The findings are discussed with regard to their possible consequences for mitochondrial fatty acid oxidation and the conversion of long-chain acyl-L-carnitine to acyl-CoA derivatives.

Induction of fatty acid beta-oxidation and peroxisome proliferation in the liver of rhesus monkeys by DL-040, a new hypolipidemic agent

Many structurally unrelated hypolipidemic agents and certain phthalate-ester plasticizers induce hepatomegaly and proliferation of peroxisomes in liver parenchymal cells of rodents, but there is relatively limited evidence regarding the ability of such compounds to induce peroxisome proliferation in the livers of nonrodent species including man. The present study was designed to determine if DL-040 (4-(((1,3-benzodioxol)-5-yl)methyl)amino-benzoic acid), a newly developed hypolipidemic agent, induces peroxisome proliferation in the liver of adult rhesus monkeys. Feeding of DL-040 (300 mg/kg body wt for 1 week; and 400 mg/kg body wt for 10 weeks) caused a significant increase in peroxisome population as determined by ultrastructural and morphometric analyses. The DL-040-induced peroxisome proliferation was accompanied by increases in the levels of catalase, carnitine acetyltransferase and the peroxisomal fatty acid beta-oxidation system. As expected, DL-040 caused a significant reduction of serum cholesterol and low density lipoprotein content. These data suggest that hepatic peroxisome proliferation is inducible in nonhuman primates at dose levels that exceed therapeutic levels.

Induction of peroxisomal enzymes and palmitoyl-CoA hydrolase in rats treated with cholestyramine and nicotinic acid

Male Wistar rats were given 200 mg/kg/day nicotinic acid or 1000 mg/kg/day cholestyramine by stomach tube for ten days. Peroxisomal palmitoyl-CoA oxidation (cyanide-insensitive) and the activities of palmitoyl-CoA hydrolase and urate oxidase were significantly increased in the total liver homogenate. Subcellular fractionation showed enhanced enzyme activities after drug treatment mainly in the peroxisome-containing fractions. The increase in urate oxidase activity and its subcellular distribution suggest that the tested drugs induce core-containing peroxisomes. The findings are similar to those previously reported with low doses of peroxisome-proliferating hypolipidemic drugs and with acetylsalicylic acid, a drug which is structurally similar to nicotinic acid. Since cholestyramine is not absorbed, its influence on hepatic enzymes probably occurs indirectly as a consequence of enhanced catabolism of cholesterol.