Transcription regulation of peroxisomal fatty acyl-CoA oxidase and enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase in rat liver by peroxisome proliferators

Targeted disruption of the alpha isoform of the peroxisome proliferator-activated receptor gene in mice results in abolishment of the pleiotropic effects of peroxisome proliferators

To gain insight into the function of peroxisome proliferator-activated receptor (PPAR) isoforms in rodents, we disrupted the ligand-binding domain of the alpha isoform of mouse PPAR (mPPAR alpha) by homologous recombination. Mice homozygous for the mutation lack expression of mPPAR alpha protein and yet are viable and fertile and exhibit no detectable gross phenotypic defects. Remarkably, these animals do not display the peroxisome proliferator pleiotropic response when challenged with the classical peroxisome proliferators, clofibrate and Wy-14,643. Following exposure to these chemicals, hepatomegaly, peroxisome proliferation, and transcriptional-activation of target genes were not observed. These results clearly demonstrate that mPPAR alpha is the major isoform required for mediating the pleiotropic response resulting from the actions of peroxisome proliferators. mPPAR alpha-deficient animals should prove useful to further investigate the role of this receptor in hepatocarcinogenesis, fatty acid metabolism, and cell cycle regulation.

Peroxisome proliferators differentially regulate long-chain acyl-CoA thioesterases in rat liver

We have investigated the effects of peroxisome proliferators on rat liver long-chain acyl-CoA thioesterase activities. Subcellular fractionations of liver homogenates from control, clofibrate- and di(2-ethylhexyl)phthalate-treated rats confirmed earlier studies which demonstrated that peroxisome-proliferating drugs induce long-chain acyl-CoA thioesterase activity mainly in the mitochondrial and cytosolic fractions. The aim of the present study was to investigate whether the induced activities were due to increases in normally expressed enzymes, or due to induction of novel enzymes. To investigate whether structurally different peroxisome proliferators differentially induced thioesterase activities, we tested the effects of di(2-ethylhexyl)phthalate (a plastisizer) and the hypolipidemic drug clofibrate. For this purpose, we established an analytical size exclusion chromatography method. Chromatography of solubilised mitochondrial matrix proteins showed that the activity in control mitochondria was mainly due to enzymes with molecular masses of about 50 kDa and 35 kDa. The activity in samples prepared from clofibrate- and di(2-ethylhexyl)phthalate-treated rats eluted as proteins of about 40 kDa and 110 kDa. Highly purified peroxisomes contained two peaks of activity, which were not induced, that corresponded to molecular masses of 40 kDa and 80 kDa. The 80-kDa peak was shown to be due to dimerization by addition of glycerol. Chromatography of cytosolic fractions from control rat livers indicated the presence of long-chain acyl-CoA thioesterases with molecular masses of approximately 35 kDa and 125 kDa and a broad peak corresponding to a high-molecular-mass protein. The activity in cytosolic fractions from peroxisome-proliferator-treated rats eluted mainly as peaks corresponding to 40, 110 and 150 kDa. In addition, in the 110-kDa peak, a different degree of induction and different chain-length specificities were caused by clofibrate and di(2-ethylhexyl)phthalate, suggesting that these peroxisome proliferators differentially regulate the cytosolic acyl-CoA thioesterase activities. Western blot analysis showed that enzymes in the 40-kDa peak of the peroxisomal and cytosolic fractions were structurally related, but not identical, to a 40-kDa mitochondrial very-long-chain acyl-CoA thioesterase. Our data show that the increased acyl-CoA thioesterase activities in mitochondria and cytosol were mainly due to induction of acyl-CoA thioesterases which are not, or only weakly, expressed under normal conditions.

On the mechanism of stimulation of peroxisomal beta-oxidation in rat heart by partially hydrogenated fish oil

By feeding rats a diet containing 20% (w/w) partially hydrogenated fish oil (PHFO), an apparent 6.3-fold increase in the cyanide insensitive palmitoyl-CoA-dependent NAD+ reduction was observed for the heart peroxisomal fractions. This finding was confirmed by a 7.6-fold and 7.9-fold increase in the specific activity of fatty acyl-CoA oxidase, with palmitoyl-CoA and erucoyl-CoA as the substrates, respectively. Immunoblots after SDS-PAGE of rat heart peroxisomal fractions revealed a 12-fold increase in the 52 kDa fatty acyl-CoA oxidase (FAO) subunit for PHFO-fed rats, whereas the 72 kDa subunit of FAO and several other peroxisomal proteins (including the trifunctional enzyme delta 3,delta 2-enoyl-CoA isomerase, 2-enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase) increased only 2- to 3-fold. The increase in the 52 kDa subunit was markedly higher than the increase in the steady-state mRNA level of FAO (2.0-fold), and is most likely caused by a rather selective stabilization of the 52 kDa FAO subunit. Interestingly, PHFO feeding caused a larger increase in fatty acyl-CoA oxidase and catalase activities than did clofibrate in the heart. The opposite was the case in the liver, especially for fatty acyl-CoA oxidase. Rats fed a semisynthetic diet containing 6% (w/w) erucic acid (C22:1(n - 9), cis) or brassidic acid (C22:1(n - 9), trans) revealed a 5-fold and 3-fold increase vs. the control (pellet fed) rats in heart FAO activity, respectively, as well as a proportional and selective increase in the specific content of 52 kDa FAO subunit. Thus, the relatively high content of C22 monoene fatty acids appears to be one of the main factors responsible for the increase in rat heart peroxisomal FAO activity during PHFO feeding. However, the PHFO diet increased the heart peroxisomal FAO activity more than diets containing a similar amount of C22:1 in the form of erucic or brassidic acid, and additional compounds of lipid or a more xenobiotic nature may also play a role. SDS-PAGE electrophoresis of highly purified rat liver peroxisomes revealed that the specific content of polypeptides with mobilities corresponding to that of the beta-oxidation enzyme system, increased by a factor of < 2 as a result of feeding the PHFO diet. The 3.1-fold increase in cyanide insensitive palmitoyl-CoA-dependent NAD+ reduction was comparable to the increase (4.1-fold) in the acyl-CoA oxidase activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification and characterization of DNA elements implicated in the regulation of CYP4A1 transcription

We have identified a peroxisome proliferator response element (PPRE) approx. 4300 nucleotide upstream of the rat cytochrome P-450 CYP4A1 gene. Two members of the steroid-hormone-receptor superfamily, the peroxisome proliferator-activated receptor-alpha (PPAR alpha) and the retinoid X receptor-alpha (RXR alpha), bind specifically to this element as a heterodimer, and this element confers responsiveness to the peroxisome proliferator Wyeth-14,643 when tested in co-transfection assays. A second element, located 35 nucleotides further upstream, fails to bind PPAR alpha/RXR alpha heterodimers and is unresponsive to Wy-14,643 in co-transfection assays. Both elements are, however, responsive to 9-cis-retinoic acid in the presence of RXR alpha, when tested in the co-transfection assay. As RXR alpha fails to bind to either element as a homodimer, we suggest that RXR alpha interacts with PPAR alpha to regulate transcription via the proximal element, and interacts with some other cellular factor to regulate transcription via the more distal element. This is consistent with previous reports that a number of peroxisome proliferator-regulated genes contain PPRE-like elements as part of their regulatory sequences, which may be recognized by several receptor combinations. This provides further evidence that PPARs and their co-factors are important in mediating the pleiotropic action of peroxisome proliferators.

Fibrates downregulate apolipoprotein C-III expression independent of induction of peroxisomal acyl coenzyme A oxidase. A potential mechanism for the hypolipidemic action of fibrates

Epidemiological and transgenic animal studies have implicated apo C-III as a major determinant of plasma triglyceride metabolism. Since fibrates are very efficient in lowering triglycerides, it was investigated whether fibrates regulate apo C-III gene expression. Different fibrates lowered rat liver apo C-III mRNA levels up to 90% in a dose- and time-dependent manner, whereas intestinal apo C-III mRNA remained constant. This decrease in liver apo C-III mRNA was rapid (1 d) and reversible, since it was restored to control levels within 1 wk after cessation of treatment. In addition, fenofibrate treatment abolished the developmental rise of hepatic apo C-III mRNA observed during the suckling-weaning period. Administration of fibrates to rats induced liver and intestinal expression of the acyl CoA oxidase gene, the rate-limiting enzyme for peroxisomal beta-oxidation of fatty acids. In primary cultures of rat and human hepatocytes, fenofibric acid lowered apo C-III mRNA in a time- and dose-dependent manner. This reduction in apo C-III mRNA levels was accompanied by a decreased secretion of apo C-III in the culture medium of human hepatocytes. In rat hepatocytes fenofibric acid induced acyl CoA oxidase gene expression, whereas acyl CoA oxidase mRNA remained unchanged in human hepatocytes. Nuclear run-on and transient transfection experiments of a reporter construct driven by the human apo C-III gene promoter indicated that fibrates downregulate apo C-III gene expression at the transcriptional level. In conclusion, these studies demonstrate that fibrates decrease rat and human liver apo C-III gene expression. In humans the mechanisms appears to be independent of the induction of peroxisomal enzymes. This downregulation of liver apo C-III gene expression by fibrates may contribute to the hypotriglyceridemic action of these drugs.

Evidence for transcriptional induction of the liver fatty-acid-binding-protein gene by bezafibrate in the small intestine

The effect of bezafibrate on cytosolic fatty-acid-binding-protein (FABPc) production along the small intestine has been investigated in mice. This drug increased the intestinal fatty-acid-binding-protein (I-FABPc) and liver fatty-acid-binding-protein (L-FABPc) mRNA levels in the duodenum. The extents of induction in the duodenum and in the liver are similar. However, the degree of stimulation gradually decreases along the length of the gut, no effect being found in the ileum. An efficient absorption of this drug as early as the proximal part of the small intestine may explain this phenomenon. The L-FABPc gene is silent in terminal ileum of mice, but a direct infusion of bezafibrate into the ileum switches it on. We used this original model to follow the time course of induction of the L-FABPc gene by bezafibrate. L-FABPc mRNA was first detected 4 h after fibrate infusion, reached a maximum level at 16 h and subsequently decreased at 24 h. This induction was totally blocked by cycloheximide. Sunflower oil also caused small increases in the L-FABPc mRNA levels. The transcriptional origin of the induction triggered both by bezafibrate and sunflower oil was demonstrated by run-on assays. These data indicate that (a) the transcription of the L-FABPc gene is induced by bezafibrate via de novo protein synthesis and (b) components of sunflower oil can transcriptionally activate the L-FABPc gene. Our results also demonstrate that the mouse terminal ileum is a useful system for studying the regulation of L-FABPc gene expression both in vivo and in vitro.

Mechanistically-based human hazard assessment of peroxisome proliferator-induced hepatocarcinogenesis

In this review we have evaluated the relationship between peroxisome proliferation and hepatocarcinogenesis. To do so, we identified all chemicals known to produce peroxisome proliferation and selected those for which there are data (on peroxisome proliferation and hepatocarcinogenesis) which meet certain criteria chosen to facilitate comparison of these phenomena. The summarised data and definition of the methodology used has been collected in appendices. These comparisons enabled us to evaluate the relationship between these phenomena using reliable data. As there is a good correlation between them, we further explored the mechanisms of action that have been proposed (direct genotoxic activity, production of hydrogen peroxide, cell proliferation and receptor activation). The relationship between these events in other species, including humans, was also reviewed and finally an overview of the assessment of human hazard is presented in section IX. Some of the first chemicals which were shown to produce peroxisome proliferation were also hepatocarcinogens whose carcinogenicity could not be readily explained by genotoxic activity. This raised the suggestion that the unusual phenomenon of peroxisome proliferation was intricately linked to the carcinogenic activity of these agents. Three questions have exercised the attention of regulatory, industrial and academic toxicology since then; are chemicals which elicit peroxisome proliferation in the liver actually a coherent class of chemical carcinogens?; does the early biological phenomenon of peroxisome proliferation have real predictive value for and mechanistic association with rodent carcinogenesis?; and what hazard/risk do these agents pose to humans that may be exposed to them? Whether peroxisome proliferators are indeed a discrete class of rodent carcinogens would appear to be the single, most important question. If so, then the assumptions and procedures relevant to human hazard and risk assessment should be applied to the class and should be essentially generic; if not, each chemical should be considered independently. Our critical analysis of the published data for over 70 agents which have been shown to possess intrinsic ability to induce peroxisome proliferation in the livers of rodents has led to the conclusion that there exists a strong correlation between peroxisome proliferation as n early effect in the liver and hepatocarcinogenicity in chronic exposure studies. An almost perfect correlation was observed between the induction of peroxisomes in the rodent liver and the eventual appearance of tumours following chronic exposure The few exceptions to this were largely explainable (section II).(ABSTRACT TRUNCATED AT 400 WORDS)

The choice of resuspension medium for isolated rat liver nuclei: effects on nuclear morphology and in vitro transcription

Standard protocols for in vitro transcription assay (nuclear run-off) include 10-40% (v/v) glycerol (of various ionic strength) in the medium used for resuspension/storage of the isolated nuclei. In the present work the morphological and functional properties of nuclei isolated from rat liver have been studied as a function of the content of glycerol, sucrose and inorganic ions (K+ and Mg2+) in the resuspension medium. In contrast to earlier reports, glycerol was found not to be essential to maintain morphological integrity and RNA polymerase activity in frozen/stored nuclei. Nuclear pellets, resuspended and stored in isoosmotic sucrose media, were found to give morphologically intact and transcriptionally active nuclei. Furthermore, these nuclei displayed a higher specific hybridization signal for the differentially expressed genes encoding peroxisomal beta-oxidation enzymes, relative to the total RNA synthesis, than nuclei resuspended and stored in a hyperosmotic glycerol-containing medium. The concentrations of inorganic ions were also found to affect nuclear morphology. Flow cytometry indicated DNA leakage from nuclei at insufficient concentrations of K+ and Mg2+, and high ionic strength favoured aggregation and disintegration of nuclei. Our findings indicate that quantitative results from nuclear run-off experiments should be interpreted with caution until the process of transcription in isolated nuclei is better understood.

Coordinate induction of hepatic fatty acyl-CoA oxidase and P4504A1 in rat after activation of the peroxisome proliferator-activated receptor (PPAR) by sulphur-substituted fatty acid analogues

1. In the liver of rat fed a single dose of 3-thia fatty acids, 3-dithiahexadecanedioic acid (3-thiadicarboxylic acid) and tetradecylthioacetic acid, steady-state levels of P4504A1 and fatty acyl-CoA oxidase mRNAs increased in parallel. The increases were significant 8 h after administration, reaching a maximum after 12 h and decreased from 12 to 24 h after administration. 2. The corresponding enzyme activities of P4504A1 and fatty acyl-CoA oxidase were also induced in a parallel manner by the 3-thia fatty acids. The enzyme activities were significantly increased 12 h after administration and increased further after 24 h. This may reflect a possible effect of the 3-thia fatty acids not only on mRNA levels, but also on the translation and degradation rate of the two enzymes. 3. Repeated administration of 3-thia fatty acids resulted in an increase of the specific P4504A1 protein accompanied with an increased lauric acid hydroxylase activity. The correlation between induction of P4504A1 and fatty acyl-CoA oxidase mRNAs and their enzyme activities may reflect a coordinated rather than a causative induction mechanism, and that these genes respond to a common signal. This suggests that the increased P450 activity may not be responsible or be a prerequisite for fatty acyl-CoA oxidase induction. 4. Since the peroxisome proliferator-activated receptor (PPAR) plays a role in mediating the induction of fatty acyl-CoA oxidase, we analysed the activation of PPAR by fatty acids and sulphur-substituted analogues utilizing a chimera between the N-terminal and DNA-binding domain of the glucocorticoid receptor and the putative ligand-binding domain of PPAR. Arachidonic acid activated this chimeric receptor in Chinese hamster ovary cells. Inhibitors of P450 did not affect the activation of PPAR by arachidonic acid. Furthermore, dicarboxylic acids including 1,12-dodecanedioic acid or 1,16-hexadecanedioic acid only weakly activated the chimera. 3-Thidicarboxylic acid, however, was a much more effective activator than the non-sulphur-substituted analogues. In conclusion, the data suggest that the most likely mechanism of the induction process is fatty acid-induced activation of PPAR, which then leads to a coordinated induction of P4504A1 and fatty acyl-CoA oxidase.

Dehydroepiandrosterone 3 beta-sulphate is an endogenous activator of the peroxisome-proliferation pathway: induction of cytochrome P-450 4A and acyl-CoA oxidase mRNAs in primary rat hepatocyte culture and inhibitory effects of Ca(2+)-channel blockers

The role of steroids related to the adrenal androgen dehydroepiandrosterone (5-androstene-3 beta-ol-17-one; DHEA) in regulating the expression of peroxisomal and cytochrome P-450 4A (CYP4A) enzymes active in fatty acid metabolism was assessed using a primary rat hepatocyte culture system. Exposure of hepatocytes to the peroxisome proliferator, clofibric acid (10-250 microM), for 48-96 h led to substantial increases in CYP4A protein, CYP4A1, CYP4A2 and CYP4A3 mRNAs, and the mRNAs encoding both forms of peroxisomal acyl-CoA oxidase (ACOX-I and ACOX-II), as judged by Northern-blot analysis using gene-specific oligonucleotide probes. Although DHEA treatment in vivo is effective in inducing these mRNAs in rat liver, it had no effect in the cultured hepatocytes. In contrast, treatment of the cells with DHEA 3 beta-sulphate (DHEA-S; 10-250 microM) stimulated major increases in CYP4A and ACOX mRNA levels. Examination of several analogues indicated a preference for 3 beta-sulphate over 17 beta-sulphated steroids and the inactivity of a 3 alpha-hydroxy-17 beta-sulphate derivative (DHEA-S > 5-androstene-3 beta,17 beta-diol 3-sulphate approximately 5 alpha-androstene-3 beta-ol-17-one 3-sulphate > 5-androstene-3 beta, 17 beta,17 beta-diol 17-sulphate approximately 5 beta-androstane-3 alpha-ol-17-one 3-sulphate >> 5 alpha-androstane-3 alpha, 17 beta-diol 17-sulphate). Induction of CYP4A mRNAs by either DHEA-S or clofibric acid was partially blocked by structurally diverse Ca(2+)-channel antagonists (nicardipine, nifedipine and diltiazem; 50 microM), suggesting that both the steroidal and fibrate classes of CYP4A inducers stimulate peroxisomal-proliferative responses via a Ca(2+)-dependent pathway. Retinoic acid alone slightly induced CYP4A mRNAs but did not enhance the induction by clofibrate or DHEA-S. As DHEA-S corresponds to a physiologically important major circulating androgen, these findings suggest that it may serve as an endogenous regulator of hepatic peroxisome enzyme levels. They further suggest that Ca(2+)-channel blockers may be useful pharmacological tools for the further study of the underlying cellular mechanism whereby endogenous steroids and fibrate drugs induce peroxisome proliferation, and the relationship of these events to activation of the peroxisome proliferator-activated receptor.

Relationship between plasma lipids and palmitoyl-CoA hydrolase and synthetase activities with peroxisomal proliferation in rats treated with fibrates

1. The time-course of the effect of clofibrate (CFB), bezafibrate (BFB) and gemfibrozil (GFB) on lipid plasma levels and palmitoyl-CoA hydrolase and synthetase activities, as well as the correlations with the peroxisomal proliferation phenomenon have been studied in male Sprague-Dawley rats. 2. The administration of the three drugs caused a significant reduction in body weight gain, accompanied with a paradoxical increase in food intake in groups treated with BFB and GFB. 3. Drug treatment produced gross hepatomegaly and increase in peroxisomal beta-oxidation, and these parameters were strongly correlated. The order of potency was BFB > CFB > or = GFB. 4. Both plasma cholesterol (BFB approximately CFB > GFB) and triglyceride (BFB approximately GFB > CFB) levels were reduced in treated animals. There was an inverse correlation between these parameters and peroxisomal beta-oxidation, although the peroxisomal proliferation seemed to explain only a small part of the hypolipidemic effect observed. 5. Cytosolic and microsomal (but not mitochondrial) palmitoyl-CoA hydrolase activities were increased by the three drugs (BFB > CFB > GFB), probably by inducing the hydrolase I isoform, which is insensitive to inhibition by fibrates in vitro. The increased hydrolase activities were directly and strongly correlated with peroxisomal beta-oxidation. 6. Palmitoyl-CoA synthetase activity was also increased by the treatment with fibrates (BFB > CFB > GFB), probably as a consequence of the enhancement of hydrolase activities. 7. Some of the effects of fibrate treatment can be explained, at least in part, in terms of peroxisomal induction and caution should be exercised in the extrapolation of these results to species, such as man,that are insensitive to peroxisomal proliferation.

Isolation of the human peroxisomal acyl-CoA oxidase gene: organization, promoter analysis, and chromosomal localization

Peroxisomal acyl-CoA oxidase (ACOX; EC 1.3.3.6) is the first enzyme of the fatty acid beta-oxidation pathway, which catalyzes the desaturation of acyl-CoAs to 2-trans-enoyl-CoAs, and it donates electrons directly to molecular oxygen, thereby producing H2O2. The discovery of carcinogenic peroxisome proliferators, which markedly increase the levels of this H2O2-producing ACOX in rat and mouse liver, generated interest in peroxisomal beta-oxidation system genes. The present study deals with the structural organization of human ACOX gene. This gene spans approximately 33 kb and consists of 14 exons and 13 introns. Primer-extension analysis revealed three principal cap sites, which were mapped at 50, 52, and 53 nt upstream of the initiator methionine codon. The 5' flanking region of the ACOX gene was sequenced up to 500 bp upstream of the cap sites. This promoter region is G + C-rich and contains three copies of the "GC box" hexanucleotides. Multiple GC boxes are a characteristic feature of the rat ACOX and bifunctional protein genes of the beta-oxidation system. A + T-rich TATA-boxlike sequences, TTTATTT and TTATT, have also been identified in this human ACOX gene, but typical CCAAT motifs are absent. This ACOX gene has been mapped to chromosome 17q25 by in situ hybridization, using a biotinlabeled probe.

Transcriptional activation by amphipathic carboxylic peroxisomal proliferators is induced by the free acid rather than the acyl-CoA derivative

Most peroxisomal proliferators consist of a carboxylic group attached to a hydrophobic backbone yielding an amphipathic carboxylate molecule. The respective CoA derivatives of peroxisomal proliferators, formed by ATP-dependent CoA thioesterification catalyzed by long-chain-acyl-CoA synthase, have been repeatedly considered as the immediate inducers of peroxisome and other genes. In this study, the putative requirement for prior CoA thioesterification of peroxisomal proliferators was evaluated by analyzing the induced expression of a reporter plasmid promoted by the peroxisomal acyl-CoA-oxidase promoter in cells transiently cotransfected with expression vectors for the peroxisome-proliferator-activated receptor and the long-chain-acyl-CoA synthase. Transcriptional activation of peroxisomal acyl-CoA oxidase by peroxisomal proliferators was inhibited in the presence of transfected functional acyl-CoA synthase. The inhibitory effect was negatively correlated with the capacity of the acyl-CoA synthase to catalyze CoA thioesterification of the respective proliferator. Hence, the immediate inducer is the peroxisomal proliferator free acid rather than the respective CoA derivative or a metabolite derived from the peroxisomal-proliferator-CoA intermediate.

Transactivation of the peroxisome proliferator-activated receptor is differentially modulated by hepatocyte nuclear factor-4

Peroxisome proliferator-activated receptors (PPARs) stimulate the expression of several genes involved in lipid metabolism by binding to specific cis-acting peroxisome proliferator-responsive elements (PPREs) via cooper-ativity with retinoid X receptors. We demonstrate here that hepatocyte nuclear factor-4 (HNF-4), another member of the nuclear hormone receptor superfamily, bound with differing affinities to the PPREs from the genes encoding rat acyl-CoA oxidase and hydratase-dehydrogenase, the first two enzymes of the peroxisomal beta-oxidation pathway. In cotransfection assays, HNF-4 repressed rat PPAR-dependent activation of a reporter gene linked to the acyl-CoA oxidase PPRE, either in the absence or presence of the peroxisome proliferator, Wy-14,643. Rat PPAR-dependent activation of a reporter gene linked to the hydratase-dehydrogenase PPRE was less efficiently repressed by HNF-4 in the absence of Wy-14,643 than was activation from the acyl-CoA oxidase PPRE. However, in the presence of Wy-14,643, HNF-4 functioned cooperatively with PPAR to significantly enhanced induction from the hydratase-dehydrogenase PPRE. These results suggest that the genes encoding the first two enzymes of the peroxisomal beta-oxidation pathway are subject to differential regulation by the interplay of multiple members of the steroid/nuclear hormone receptor superfamily, mitigated in part by the structures of the PPREs and by the presence of activators of PPARs.

Acyl-CoA synthetase mRNA expression is controlled by fibric-acid derivatives, feeding and liver proliferation

Several enzymes of the beta-oxidation pathway have been shown to be induced after stimulation with peroxisomal proliferators, including several hypolipidemic drugs. We investigated the regulation of the long-chain-acyl-CoA synthetase (ACS) gene in the liver. Fenofibrate, a hypolipidemic drug and potent peroxisomal proliferator, induced ACS gene expression in several tissues. In liver, large increases in ACS mRNA levels and ACS activity were observed after fenofibrate administration. Adipose tissue ACS mRNA levels and ACS activity were also stimulated upon fibrate treatment but to a lesser extent in comparison with liver ACS mRNA. Kidney ACS mRNA was only weakly induced, except for the highest dose and the longest treatment period, where a strong induction was observed. In contrast to these tissues, heart ACS mRNA and ACS activity remained almost unchanged after fenofibrate treatment. These effects of fenofibrate could be reproduced by other fibrates such as clofibrate. In addition, it is demonstrated that both nutritional composition and liver proliferation trigger ACS gene expression in liver. Consequently, these data suggest that ACS is a highly regulated enzyme with a potentially important control function in lipid metabolism.

Transcriptional induction of the fatty acid binding protein gene in mouse liver by bezafibrate

The mechanism by which hypolipidemic peroxisome proliferators of the fibrate family induce the liver fatty acid binding protein in liver of rodents is unknown. In order to delineate the level at which this protein is induced, the transcriptional activity of the specific gene encoding for liver fatty acid binding protein was measured in isolated hepatocyte nuclei obtained from male Swiss mice daily force-fed during 7 days with 400 mg/kg body weight bezafibrate. This treatment induced a 4-fold increase in the liver fatty acid binding protein transcription rate. Liver fatty acid binding protein mRNA level, measured by Northern blot analysis and cytosolic content of this protein, analyzed by immunoblotting, increased concurrently. From these results we conclude that the increase in the cytosolic liver fatty acid binding protein level by bezafibrate is due to an enhancement of the transcription rate of the liver fatty acid binding protein gene. Whether the transcriptional effect is mediated by peroxisome proliferator-receptor remains to be elucidated.

Diverse peroxisome proliferator-activated receptors bind to the peroxisome proliferator-responsive elements of the rat hydratase/dehydrogenase and fatty acyl-CoA oxidase genes but differentially induce expression

The ability of peroxisome proliferator-activated receptors (PPARs) to induce expression of a reporter gene linked to a peroxisome proliferator-responsive element (PPRE) from either the rat enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase gene or acyl-CoA oxidase [acyl-CoA:oxygen 2-oxidoreductase, EC 1.3.3.6] gene was examined by transient transfection assays in COS cells. Mouse and rat PPARs, as well as Xenopus PPAR alpha (xPPAR alpha) could induce expression of a reporter gene linked to the hydratase/dehydrogenase PPRE in the presence of the peroxisome proliferators ciprofibrate or Wy-14,643, whereas xPPAR beta and xPPAR gamma were ineffective. A similar induction of expression of a reporter gene linked to the acyl-CoA oxidase PPRE was observed with all PPARs except xPPAR beta. Extracts from cells transfected with PPAR-encoding genes contained factors that bound to both PPREs. In vitro synthesized PPARs could interact weakly with both PPREs; however, binding of each PPAR to both PPREs was significantly increased by the addition of COS cell nuclear extracts, demonstrating that efficient PPAR/DNA binding requires auxiliary cofactors. One cofactor was identified as the 9-cis-retinoic acid receptor, RXR alpha (retinoid X receptor alpha). Cooperative DNA binding and heteromerization between RXR alpha and each of the PPARs could be seen with both PPREs. Our results demonstrate that PPAR/PPRE binding and cooperativity with RXR alpha (and other cofactors) are obligatory but not necessarily sufficient for peroxisome proliferator-dependent transcription induction and that distinct PPREs can selectively mediate induction by particular PPARs.

Peroxisome proliferator-activated receptors and lipid metabolism

PPARs are nuclear hormone receptors which, like the retinoid, thyroid hormone, vitamin D, and steroid hormone receptors, are ligand-activated transcription factors mediating the hormonal control of gene expression. Two lines of evidence indicate that PPARs have an important function in fatty acid metabolism. First, PPARs are activated by hypolipidemic drugs and physiological concentrations of fatty acids, and second, PPARs control the peroxisomal beta-oxidation pathway of fatty acids through transcriptional induction of the gene encoding the acyl-CoA oxidase (ACO), which is the rate-limiting enzyme of the pathway. Furthermore, the PPAR signaling pathway appears to converge with the 9-cis retinoic acid receptor (RXR) signaling pathway in the regulation of the ACO gene because heterodimerization between PPAR and RXR is essential for in vitro binding to the PPRE and because the strongest stimulation of this gene is observed when both receptors are exposed simultaneously to their activators. Thus, it appears that PPARs are involved in the 9-cis retinoic acid signaling pathway and that they play a pivotal role in the hormonal control of lipid metabolism.

Genome organization and expression of the rat ACBP gene family

Acyl-CoA-Binding Protein (ACBP)/Diazepam-Binding Inhibitor (DBI) is a 10 kD protein which has been implicated in a surprisingly large number of biochemical functions. We have unambiguously demonstrated that ACBP binds acyl-CoA esters with high affinity and in vivo functions as an acyl-CoA ester pool former. We have molecularly cloned and characterized the rat ACBP gene family which comprises one expressed and four processed pseudogenes. One of these was shown to exist in two allelic forms. A comprehensive computer-aided analysis of the promoter region of the expressed ACBP gene revealed that it exhibits all the hallmarks of typical housekeeping genes. In addition, the promoter region harbors a number of potential tissue specific cis-acting elements that may in part regulate the level of ACBP expression in specialized cells.

Mechanisms of regulation of liver fatty acid-binding protein

Liver fatty acid-binding protein (L-FABP) expression is modulated by developmental, hormonal, dietary, and pharmacological factors. The most pronounced induction is seen after treatment with peroxisome proliferators, which induce L-FABP coordinately with microsomal cytochrome P-450 4A1 and the enzymes of peroxisomal fatty acid beta-oxidation. These effects of peroxisome proliferators may be mediated by a receptor which has been shown to be activated by peroxisome proliferators in mammalian cell transfection studies. However, the peroxisome proliferators tested thus far do not bind to this receptor, known as the peroxisome proliferator-activated receptor (PPAR), and its endogenous ligand(s) also remain unknown. Peroxisome proliferators inhibit mitochondrial beta-oxidation, and one hypothesis is that the dicarboxylic fatty acid metabolites of accumulated LCFA, formed via the P-450 4A1 omega-oxidation pathway, serve as primary inducers of L-FABP and peroxisomal beta-oxidation. We have tested this hypothesis in primary hepatocyte cultures exposed to clofibrate (CF). Inhibition of P-450 4A1 markedly diminished, via a pre-translational mechanism, the CF induction of L-FABP and peroxisomal beta-oxidation. In further experiments, long-chain dicarboxylic acids, the final products of the P-450 4A1 omega-oxidation pathway, but not LCFA, induced L-FABP and peroxisomal beta-oxidation pre-translationally. These results suggest a role, in part, for long-chain dicarboxylic acids in mediating the peroxisome proliferator induction of L-FABP and peroxisomal beta-oxidation. We also found that LCFA, which undergo rapid hepatocellular metabolism, could become inducers of L-FABP and peroxisomal beta-oxidation under conditions where their metabolism was inhibited.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of a peroxisome proliferator-responsive element upstream of the gene encoding rat peroxisomal enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase

Ciprofibrate, a hypolipidemic drug that acts as a peroxisome proliferator, induces the transcription of genes encoding peroxisomal beta-oxidation enzymes. To identify cis-acting promoter elements involved in this induction, 5.8 kilobase pairs of promoter sequence from the gene encoding rat peroxisomal enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase (EC 4.2.1.17/EC 1.1.1.35) was inserted upstream of a luciferase reporter gene. Transfection of this expression vector into rat hepatoma H4IIEC3 cells in the presence of ciprofibrate resulted in a 5- to 10-fold, cell type-specific increase in luciferase activity as compared to cells transfected in the absence of drug. A peroxisome proliferator-responsive element (PPRE) was localized to a 196-nucleotide region centered at position -2943 from the transcription start site. This PPRE conferred ciprofibrate responsiveness on a heterologous promoter and functioned independently of orientation or position. Gel retardation analysis with nuclear extracts demonstrated that ciprofibrate-treated or untreated H4IIEC3 cells, but not HeLa cells or monkey kidney cells, contained sequence-specific DNA binding factors that interact with the PPRE. These results have implications for understanding the mechanisms of coordinated transcriptional induction of genes encoding peroxisomal proteins by hypolipidemic agents and other peroxisome proliferators.

A negative regulating element controlling transcription of the gene encoding acyl-CoA oxidase in Saccharomyces cerevisiae

Peroxisomes are induced in Saccharomyces cerevisiae when this yeast is grown in the presence of oleate, and are repressed when glucose is supplied as the carbon source. Concomitant with this is an induction/repression of peroxisomal beta-oxidation enzymes. We are investigating the transcriptional control of acyl-CoA oxidase, the first and rate-limiting enzyme in the peroxisomal beta-oxidation cycle. The promoter region of POX1 from S. cerevisiae has been analyzed in POX1/lacZ fusions. Expression of the POX1/lacZ fusion protein underwent glucose repression and oleate induction. By deletion, DNA band shift and DNase I footprinting analyses we have identified a region that is involved in transcriptional repression of POX1. Elimination of this DNA sequence results in constitutive expression of POX1 when S. cerevisiae is grown on a fermentable carbon source or glycerol.

Differential tissue-specific expression and induction of cytochrome P450IVA1 and acyl-CoA oxidase

We have examined the tissue-specific expression and inducibility of acyl-CoA oxidase and cytochrome P450IVA1 (P450IVA1) RNA in rats. Groups of three rats were dosed daily by gavage with methylclofenapate at 25 mg/kg in 5 ml/kg corn oil for nine weeks, or were administered a vehicle control. P450IVA1 and acyl-CoA oxidase RNA were detected using an RNase protection assay. Similar levels of acyl-CoA oxidase RNA were present in control liver and kidney, but the level of this RNA in lung, muscle and testis was 6-11%, and in pancreas was 0.13%, of that in liver. Treatment of rats with methylclofenapate led to an 11-fold induction of acyl-CoA oxidase RNA in liver and also produced a significant induction of this RNA in kidney, lung, muscle and testis of 1.7-fold, 1.3-fold, 2-fold and 1.7-fold, respectively. Acyl-CoA oxidase RNA was not induced in pancreas. P450IVA1 RNA was present in control liver and also in kidney of control rats at 28% of the level in liver. In contrast to acyl-CoA oxidase RNA, P450IVA1 RNA was not detected in lung, pancreas or testis. Methylclofenapate treatment of rats led to an 18-fold induction of P450IVA1 RNA in liver, and a sevenfold induction in kidney. Induction of P450IVA1 was not detected in any of the other tissues examined. Quantification of the relative amounts of acyl-CoA oxidase and P450IVA1 RNA in control liver revealed that acyl-CoA oxidase RNA was present in a 17.5-fold molar excess over P450IVA1 RNA. Western blotting with an anti-P450IVA IgG revealed two bands of similar apparent molecular mass in liver and kidney microsomes, but not in microsomes from the testis of control rats. Methylclofenapate treatment of rats caused an increase in the intensity of these bands in microsomes from liver, but no induction was obvious in kidney. Immunocytochemical staining for both the microsomal P450IVA and peroxisomal acyl-CoA oxidase proteins was restricted to the proximal convoluted tubule in the kidney cortex, with staining being most intense in the S3 region.

The ontogeny of peroxisome-proliferator-activated receptor gene expression in the mouse and rat

The expression of the gene coding for peroxisome-proliferator-activated receptor (PPAR), a novel transacting factor belonging to the steroid superfamily, has been determined in the mouse and rat throughout development using hybridization histochemistry. Messenger RNA is demonstrable in the liver and brown fat from the fetal period onwards and, additionally, in the heart, kidney and gut post-natally. It is proposed that the upregulation of transcription of peroxisomal beta oxidation genes in specific tissues follows binding of the receptor to its natural ligand. Thus PPAR may have an important role in cold adaptation and non-shivering thermogenesis as well as in detoxification.

Differences in the response of Sprague-Dawley and Lewis rats to bezafibrate: the hypolipidemic effect and the induction of peroxisomal enzymes

The effects of bezafibrate administered at 10 and 50 mg/kg/day for 7 days to male Sprague-Dawley (SD) and Lewis rats were investigated in order to determine the interrelation between the changes in serum and hepatic lipid contents and activities of selected peroxisomal, microsomal and mitochondrial enzymes in the two rat strains. In both strains, bezafibrate effectively reduced serum and hepatic lipids, increased the liver weight, induced a proliferation of peroxisomes, and selectively elevated the activities of carnitine acetyltransferase and of the enzymes of the peroxisomal beta-oxidation system. Moreover, immunoblotting revealed that the drug specifically enhanced the concentration of only those peroxisomal enzymes involved in fatty acid beta-oxidation. The data obtained demonstrate that although the responses initiated by bezafibrate are qualitatively similar in both strains, they differ in their magnitude in a dose-dependent manner, with the Lewis strain exhibiting a more pronounced response than the SD rats. These results show that dose-dependent strain differences as well as the generally known species differences should be taken into account in pharmacological and toxicological evaluations of fibrates in rodents. Furthermore, generalization and extrapolation from rodent studies should be treated with great caution.

Induction of the major integral membrane protein of mouse liver peroxisomes by peroxisome proliferators

Peroxisome proliferators are known to increase the volume of the peroxisomal compartment in rodent liver. We have examined the induction of the major integral membrane protein of mouse liver peroxisomes (PMP68) by a number of these agents, and compared this with their effect on the peroxisomal bifunctional protein (PBP), an enzyme of the beta-oxidation pathway which is located in the peroxisome matrix. Dietary clofibrate, di-2-(ethylhexyl)phthalate and Wy-14,643, three structurally unrelated proliferators, all increased the mRNA and protein content of PMP68 approx. 2-fold, whereas PBP was induced 8-13-fold. The kinetics and sequence of induction of PMP68 and PBP following a single dose of Wy-14,643 were compared and shown to be similar, and the effects were reversible. Another proliferator, BM 15766, caused maximal induction of PMP68 but only a low induction of PBP; further PBP induction was achieved by the administration of BM 15766 in combination with Wy-14,643. Similarly, BM 15766 and Wy-14,643 increased transcription of the PMP68 gene in vitro, whereas PBP gene transcription was increased by Wy-14,643 but not by BM 15766. Thus peroxisome proliferators enhance the expression of the genes for both the membrane protein PMP68 and the matrix protein PBP, but the regulation of this expression appears to be mediated by different mechanisms.

Fibrates influence the expression of genes involved in lipoprotein metabolism in a tissue-selective manner in the rat

The influence of different fibrates on apolipoprotein metabolism was investigated. Administration of fenofibrate provoked a dose-dependent decrease in plasma cholesterol concentration that was already evident after 1 day. Intestinal apolipoprotein (apo) A-I and apo A-IV mRNA levels remained fairly constant. In contrast, liver apo A-I, apo A-II, and apo A-IV mRNA levels decreased in a dose-dependent fashion, which was associated with a lower transcription rate of the apo A-I but not the apo A-II gene. The decline in hepatic apo A-I, apo A-II, and apo A-IV mRNA had already started after 1 day and was associated with a drop in plasma apo A-I and apo A-IV concentrations. Plasma apo E had already decreased after 1 day of fenofibrate, whereas apo B initially remained constant and increased only after 14 days of fenofibrate at the highest dose. Hepatic and intestinal apo B mRNA contents and liver, heart, kidney, and testis apo E mRNA contents were only marginally affected after treatment with fenofibrate. Liver low density lipoprotein receptor mRNA levels rose slightly after a 3-day administration of the highest dose of fenofibrate. Both clofibrate and gemfibrozil had effects comparable to those of fenofibrate on liver and intestinal apolipoprotein mRNA levels except for liver apo A-II mRNA, which decreased only marginally. Compared with fenofibrate, clofibrate caused similar changes in plasma cholesterol, apo A-I, apo A-IV, and apo E concentrations, whereas gemfibrozil increased plasma cholesterol and apo E without changing apo A-I and apo A-IV concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of peroxisomal beta-oxidation genes by retinoic acid in cultured rat hepatocytes

Retinoic acid is reported here to induce peroxisomal beta-oxidation activities in cultured rat hepatocytes, with a concomitant increase in respective peroxisomal mRNAs. The concentrations of retinoic acid required for inducing liver peroxisomal acyl-CoA oxidase were similar to those required for inducing liver transglutaminase. A putative 5'-flanking response element for retinoic acid may be found within the enhancer region involved in the induction of peroxisomal genes by xenobiotic amphipathic carboxylates.

The mouse peroxisome proliferator activated receptor recognizes a response element in the 5' flanking sequence of the rat acyl CoA oxidase gene

Peroxisome proliferators are a diverse group of chemicals, including several hypolipidaemic drugs, that activate a nuclear hormone receptor termed the peroxisome proliferator activated receptor (PPAR). The peroxisomal enzyme acyl CoA oxidase (ACO) is the most widely used marker of peroxisome proliferator action. We have examined the 5' flanking region of the rat ACO gene for sequences that mediate the transcriptional effect of peroxisome proliferators and have identified an element located 570 bp upstream of the ACO gene that confers responsiveness to the hypolipidaemic peroxisome proliferator Wy-14,643. This peroxisome proliferator response element (PPRE) contains a direct repeat of the sequence motifs TGACCT and TGTCCT and binds PPAR. These data therefore indicate an important role of PPAR in mediating the action of peroxisome proliferators including the induction of ACO.

Induction of acyl-CoA oxidase and cytochrome P450IVA1 RNA in rat primary hepatocyte culture by peroxisome proliferators

We have characterized the induction of acyl-CoA oxidase and cytochrome P450IVA1 RNAs in a primary hepatocyte culture system in vitro, using a sensitive and specific RNAse protection assay. Hepatocytes were cultured with a maximal inducing dose of the peroxisome proliferator clofibric acid (1 mM), or vehicle control, for 4 days, and the level of RNAs compared with the level in rats which had been treated with corn oil or clofibric acid (300 mg/kg) for 4 days. The level of acyl-CoA oxidase and P450IVA1 RNAs in 4-day-old control hepatocytes was less than 2% of that in control liver. However, the level of these RNAs in RNA from treated hepatocytes was 61% of that in liver RNA from treated rats. Hepatocytes were treated with the potent peroxisome proliferator methylclofenapate (100 microM), and the induction of RNAs determined at various times after exposure. P450IVA1 RNA was significantly induced 1 h after dosing, rising to 34-fold above control after 8 h, whereas acyl-CoA oxidase RNA was not significantly induced until 4 h, increasing to 5.2-fold above control after 8 h. A similar time course of induction was seen after treatment of hepatocytes with 100 microM-nafenopin, 100 microM-methylclofenapate, 1 mM-clofibric acid or 1 mM-mono(ethylhexyl) phthalate, suggesting that the differential time course of induction of P450IVA1 and acyl-CoA oxidase RNAs is not related to the esterification, structure or potency of the peroxisome proliferator, but is intrinsic to the process of peroxisome proliferation. Hepatocytes were treated with methylclofenapate in the presence and absence of cycloheximide. P450IVA1 RNA was significantly induced by methylclofenapate in the presence of cycloheximide, rising to 17-fold above control after 8 h. However, no induction of acyl-CoA oxidase RNA was detected in the presence of cycloheximide. Therefore we characterize the induction of acyl-CoA oxidase and P450IVA1 RNAs in primary hepatocyte culture in vitro as a faithful model of the induction response in rat liver, and suggest that induction of P450IVA1 RNA is a primary event in the process of peroxisome proliferation.

Peroxisome induction potential and lipid-regulating activity in rats. Quantitative microscopy and chemical structure-activity relationships

Structurally diverse lipid-regulating agents induce hepatomegaly, hepatic peroxisome proliferation, and hepatocarcinoma in rats by mechanisms not fully understood. Nevertheless the initial hepatic response is a prompt, florid proliferation of peroxisomes. In investigations reported here, changes in the rat hepatic peroxisome compartment were measured by quantitative microscopy to determine chemical structure requirements that relate to peroxisome proliferation and lipid regulation. Aryloxyalkanoic acids plus amide analogs, and thio, benzimidazole, phenylpiperazine, and oxazole derivatives induced peroxisome proliferation and generally decreased plasma triglyceride and total cholesterol levels. These compounds contain an acidic function or are readily metabolized to a chemical with an acidic function. Substitution of the acidic function with an adamantyloxy eliminated peroxisome proliferation and induced contrasting effects on lipid profile, increasing triglycerides and decreasing total cholesterol. A previously unreported, direct correlation emerged between peroxisome proliferation and plasma high-density lipoprotein-cholesterol levels. These effects could not be elicited separately, negating identification of functional groups that could be associated with either activity. Chemical structure and resulting peroxisome proliferation with changes in plasma lipoproteins are therefore closely interrelated in rats.

An overview of peroxisome proliferator-induced hepatocarcinogenesis

Peroxisome proliferators are hepatocarcinogens in rats and mice. Chronic administration of these compounds results in the development of altered areas and neoplastic nodules followed by hepatocellular carcinomas. All three types of hepatic lesions do not express gamma-glutamyltranspeptidase, glutathione 8-transferase-P, and alpha-fetoprotein and are resistant to iron accumulation after overload. The mechanism by which nongenotoxic peroxisome proliferators induce hepatic tumors is not well understood. It has been proposed that with continuous administration of peroxisome proliferators, liver cells are subjected to persistent oxidative stress resulting from marked proliferation of peroxisomes and a differential increase in the levels of H2O2 producing (20- to 30-fold) and degrading (2-fold) enzymes. Free oxygen radicals lead to DNA damage (both directly and through lipid peroxidation) and thus may cause initiation and promotion of the carcinogenic process.

Localization and differential induction of cytochrome P450IVA and acyl-CoA oxidase in rat liver

The peroxisome proliferators are structurally diverse chemicals which induce hyperplasia, hypertrophy and the proliferation of peroxisomes in the rodent liver. Cytochrome P450IVA1 and peroxisomal enzymes, such as acyl-CoA oxidase, are induced and are early markers of treatment with peroxisome proliferators. In this study, rats were dosed intraperitoneally with the potent peroxisome proliferator methylclofenapate and the hepatic induction response was studied. There was no significant change in the enzyme activities of laurate hydroxylase (cytochrome P450IVA1) or acyl-CoA oxidase in the first 8 h after treatment, but the activities had doubled at 24 h, suggesting that these enzymes are not involved in the mediation of early events in peroxisome proliferation. Hepatic cytochrome P450IVA1 mRNA was significantly increased at 6 and 8 h after treatment, rising to 15-fold above control values at 30 h. In contrast, acyl-CoA oxidase mRNA showed no significant change in the first 8 h, but increased to 13-fold above control values at 24 and 30 h, thereby demonstrating different kinetics of induction of the two mRNAs. In order to determine whether cytochrome P450IVA1 and peroxisomal enzymes were included in the same cells, rats were treated daily with sub-maximal (2 or 5 mg/kg) and maximal (25 mg/kg) inducing doses of methylclofenapate for 4 days. The lobular distribution of induced proteins was determined immunocytochemically with antibodies raised against P450IVA1 and acyl-CoA oxidase. Livers from control animals showed minimal staining for both proteins. However, in the livers of animals treated with 2 or 5 mg of methylclofenapate/kg, both acyl-CoA and P450IVA immunostaining was increased, mainly in the centrilobular area. Immunostaining of serial sections revealed that these proteins were induced in the same region of the lobule. A maximal inducing dose of methylclofenapate (25 mg/kg) caused panlobular induction of both proteins. The results demonstrate that these proteins are induced in a dose-dependent manner in the same, spatially distinct, sensitive region of the liver lobule.

Thyromimetic effect of peroxisomal proliferators in rat liver

Amphipathic carboxylates, of varying hydrophobic backbones, which act as peroxisomal proliferators (aryloxyalkanoic acids, methyl-substituted dicarboxylic acid) induce in euthyroid or thyroidectomized rats, as well as in rat hepatocytes cultured in 3,5,3'-tri-iodo-L-thyronine (T3)-free media, liver enzyme activities that are classically considered to be thyroid-hormone-dependent (malic enzyme, mitochondrial alpha-glycerophosphate dehydrogenase, glucose-6-phosphate dehydrogenase and S14). The dose required in vivo for the thyromimetic effect of peroxisomal proliferators was 10(3)-fold higher than the dose of T3 required. Similarly, peroxisomal proliferators were active in culture in the range 1-100 microM compared with 1 nM for T3. Their maximal inductive capacities were, however, similar to or greater than that of T3. The thyromimetic effect of peroxisomal proliferators was only partially correlated with their capacities as inducers of liver peroxisomal enzymes. The thyromimetic effect with respect to liver malate dehydrogenase and S14 resulted from an increase in their mRNA contents. The increase in liver S14 mRNA was accounted for by transcriptional activation of the S14 gene. T3 binding to isolated liver nuclei or nuclear extract was competitively displaced by some but not all of the non-thyroidal inducers of the above liver activities. In contrast with the thyromimetic effect induced in liver cells, no increase in growth hormone mRNA was observed in cultured GH1 pituitary cells incubated in the presence of non-thyroidal amphipathic carboxylates. The characteristics of the thyromimetic effect of amphipathic carboxylic peroxisomal proliferators indicate that these agents may act as transcriptional activators of thyroid-hormone-dependent genes in the rat liver.

Activation of a member of the steroid hormone receptor superfamily by peroxisome proliferators

We have cloned a member of the steroid hormone receptor superfamily of ligand-activated transcription factors. The receptor homologue is activated by a diverse class of rodent hepatocarcinogens that causes proliferation of peroxisomes. Identification of a peroxisome proliferator-activated receptor should help elucidate the mechanism of the hypolipidaemic effect of these hepatocarcinogens and aid evaluation of their potential carcinogenic risk to man.

Isolation of peroxisomal enoyl-CoA hydratase in rainbow trout and immunochemical identification with the bifunctional enzyme

Peroxisomes are the sites for β-oxidation of long-chain fatty acids. The peroxisomal bifunctional enzyme (PBE) enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase catalyzes the second and third reactions of the β-oxidation system. Originally termed PPA-80 for peroxisome-proliferation associated 80,000 MW polypeptide, PBE levels are monitored to measure peroxisome proliferation in rodents and other species. The quantity of a 79,000 MW polypeptide in the light mitochondrial fraction of the liver, as analyzed by SDS-PAGE, increases when rainbow trout are exposed to peroxisome proliferating agents. This correlates with increases in acyl-CoA oxidase activity and peroxisome volume density. In the present study, peroxisomal enoyl-CoA hydratase was purified from trout liver and analyzed by immunoblotting with anti-PBE. A positive reaction with the 79,000 MW polypeptide band was observed providing strong evidence that this is the bifunctional enzyme.

Turnover of fatty acyl-CoA oxidase in the liver of rats fed on a partially hydrogenated marine oil

The change in turnover of fatty acyl-CoA oxidase (FAO), the rate-limiting enzyme of the peroxisomal beta-oxidation system, was investigated in rats fed a 30% (w/w) partially hydrogenated marine oil (PHMO) diet. The FAO activity increased five-fold after two weeks of PHMO feeding, and decreased after withdrawal of the diet. Based on in vivo experiments using L-[4,5-3H]leucine and an immunoprecipitation technique, the increase in the activity of FAO could be accounted for by a 1.6-fold higher rate of FAO synthesis and a 3.4-fold slower rate of FAO degradation as compared to controls. In the same PHMO-fed rats, the rates of synthesis and degradation of carnitine palmitoyltransferase were 1.8-fold higher and 2.0-fold slower, respectively, as compared to controls. The results indicate that the observed increase in the activity of the enzymes of peroxisomal beta-oxidation is mainly due to a reduced rate of FAO degradation in the liver of rats fed the PHMO diet.

The rat clofibrate-inducible CYP4A gene subfamily. I. Complete intron and exon sequence of the CYP4A1 and CYP4A2 genes, unique exon organization, and identification of a conserved 19-bp upstream element

The P450 CYP4A1 and CYP4A2 genes were isolated from a rat genomic library constructed in the vector lambda EMBL3 and their complete sequences were determined. The CYP4A1 and CYP4A2 genes spanned 14,144 and 10,576 bp and contained 13 and 12 exons, respectively. The CYP4A1 gene contained an additional intron that splits the exon corresponding to exon 12 of the CYP4A2 gene, resulting in a noncoding 13th exon in CYP4A1. The exon numbers of these genes were distinct among known P450 genes, and yet several intron-exon junctions along the P450 amino acid coding region were conserved with P450 genes in the CYP2, CYP11, and CYP21 gene families. On the basis of these data, the number of exons in the putative ancestral P450 gene was estimated. The evolutionary implications of this finding are discussed. No consensus TATA sequence was found upstream of either gene's transcription start site. Comparison of the CYP4A1 and CYP4A2 promoters with other genes that lack TATA boxes did not reveal any strong consensus sequence in their immediate upstream regions. However, a conserved 19-bp sequence was located at the positions of 42 and 48 bp upstream from the CYP4A1 and CYP4A2 genes' start sites, respectively. The CYP4A2 gene also contained two 378-bp direct repeats upstream from the start site; these repeats are derived from portions of the long interspersed middle repetitive element present in high copy numbers in the rat genome.