Mitochondrial 2,4-dienoyl-CoA reductases in the rat: differential responses to clofibrate treatment

Peroxisome proliferator-activated receptor alpha target genes

The peroxisome proliferator-activated receptor alpha (PPARα) is a ligand-activated transcription factor involved in the regulation of a variety of processes, ranging from inflammation and immunity to nutrient metabolism and energy homeostasis. PPARα serves as a molecular target for hypolipidemic fibrates drugs which bind the receptor with high affinity. Furthermore, PPARα binds and is activated by numerous fatty acids and fatty acid-derived compounds. PPARα governs biological processes by altering the expression of a large number of target genes. Accordingly, the specific role of PPARα is directly related to the biological function of its target genes. Here, we present an overview of the involvement of PPARα in lipid metabolism and other pathways through a detailed analysis of the different known or putative PPARα target genes. The emphasis is on gene regulation by PPARα in liver although many of the results likely apply to other organs and tissues as well.

Mitochondrial 2,4-dienoyl-CoA reductase deficiency in mice results in severe hypoglycemia with stress intolerance and unimpaired ketogenesis

The mitochondrial β-oxidation system is one of the central metabolic pathways of energy metabolism in mammals. Enzyme defects in this pathway cause fatty acid oxidation disorders. To elucidate the role of 2,4-dienoyl-CoA reductase (DECR) as an auxiliary enzyme in the mitochondrial β-oxidation of unsaturated fatty acids, we created a DECR–deficient mouse line. In Decr^−/− mice, the mitochondrial β-oxidation of unsaturated fatty acids with double bonds is expected to halt at the level of trans-2, cis/trans-4-dienoyl-CoA intermediates. In line with this expectation, fasted Decr^−/− mice displayed increased serum acylcarnitines, especially decadienoylcarnitine, a product of the incomplete oxidation of linoleic acid (C_18:2), urinary excretion of unsaturated dicarboxylic acids, and hepatic steatosis, wherein unsaturated fatty acids accumulate in liver triacylglycerols. Metabolically challenged Decr^−/− mice turned on ketogenesis, but unexpectedly developed hypoglycemia. Induced expression of peroxisomal β-oxidation and microsomal ω-oxidation enzymes reflect the increased lipid load, whereas reduced mRNA levels of PGC-1α and CREB, as well as enzymes in the gluconeogenetic pathway, can contribute to stress-induced hypoglycemia. Furthermore, the thermogenic response was perturbed, as demonstrated by intolerance to acute cold exposure. This study highlights the necessity of DECR and the breakdown of unsaturated fatty acids in the transition of intermediary metabolism from the fed to the fasted state.

Fatty acids released from triacylglycerols or obtained from the diet serve as a main energy provider to the heart and skeletal muscle, and when carbohydrates are scarce, fatty acids provide energy for the whole organism. Inherited disorders of mitochondrial β-oxidation are among the most common inborn errors of metabolism affecting infants and children. Under normal conditions, patients are usually asymptomatic; but when challenged with metabolic stress, severe phenotypes arise. Here we describe the generation of a mouse model in which the total degradation of unsaturated fatty acids is prevented by disruption of an auxiliary enzyme of β-oxidation. Although degradation of saturated fatty acids proceeds normally, the phenotype presented here is in many ways similar to mouse models of the disrupted classical β-oxidation pathway, but with additional unique features. The null mutant mice are asymptomatic until exposed to fasting, during which they switch on ketogenesis, but simultaneously develop hypoglycemia. A number of human patients suffer from idiopathic hypoglycemia (hypoglycemia of unknown cause). Our mouse model links this disease state to a specific defect in the breakdown of polyunsaturated fatty acids. Furthermore, it shows that degradation of unsaturated fatty acids is essential for balanced fatty acid and energy metabolism, as well as adaptation to metabolic stress.

Transcriptome and proteome analysis identifies the pathways that increase hepatic lipid accumulation in zinc-deficient rats

For identification of the underlying molecular changes in hepatic lipid metabolism in zinc deficiency, rats were force-fed a zinc-deficient diet. Subsequently DNA-microarray and proteome profiling was performed in combination with hepatic lipid analysis. Of 6200 target sequences analyzed, 268 transcripts showed altered expression levels in livers of zinc-deficient rats, with 43 genes thereof related to hepatic lipid metabolism. Northern blot analysis and quantitative real-time RT-PCR were employed to confirm changes in mRNA levels. Proteins involved in lipid metabolism were identified by proteome analysis. Functional gene clusters with uniform changes in transcript levels suggested that the pathways required for lipolysis and mitochondrial as well as peroxisomal fatty acid degradation were downregulated, whereas those needed for de novo fatty acid synthesis and triglyceride assembly were increased. Subsequent enzymatic analysis of liver tissues confirmed an almost 40% greater triacylglycerol concentration in zinc-depleted rats, as well as an altered fatty acid composition of the lipid fraction as determined by gas chromatography. Liver lipids of zinc-deficient rats had significantly greater proportions of cis-9-oleic acid, cis-11-vaccenic acid, caprylic acid, myristic acid, alpha-linolenic acid, and eicosapentaenoic acid, and significantly less stearic and arachidonic acids. These alterations in hepatic metabolism are discussed in the context of changes in mRNA and protein levels of enzymes and transporters responsible for fatty acid metabolism, sequestration, and their transcriptional control.

Inverse gene expression patterns for macrophage activating hepatotoxicants and peroxisome proliferators in rat liver

Macrophage activation contributes to adverse effects produced by a number of hepatotoxic compounds. Transcriptional profiles elicited by two macrophage activators, LPS and zymosan A, were compared to those produced by 100 paradigm compounds (mostly hepatotoxicants) using cDNA microarrays. Several hepatotoxicants previously reported to activate liver macrophages produced transcriptional responses similar to LPS and zymosan, and these were used to construct a gene signature profile for macrophage activators in the liver. Measurement of cytokine mRNAs in the same liver samples by RT-PCR independently confirmed that these compounds are associated with macrophage activation. In addition to expected effects on acute phase proteins and metabolic pathways that are regulated by LPS and inflammation, a strong induction was observed for many endoplasmic reticulum-associated stress/chaperone proteins. Additionally, many genes in our macrophage activator signature profile were well-characterized PPARalpha-induced genes which were repressed by macrophage activators. A shared gene signature profile for peroxisome proliferators was determined using a training set of clofibrate, WY 14643, diethylhexylphthalate, diisononylphthalate, perfluorodecanoic acid, perfluoroheptanoic acid, and perfluorooctanoic acid. The signature profile included macrophage activator-induced genes that were repressed by peroxisome proliferators. NSAIDs comprised an interesting pharmacological class in that some compounds, notably diflunisal, co-clustered with peroxisome proliferators whereas several others co-clustered with macrophage activators, possibly due to endotoxin exposure secondary to their adverse effects on the gastrointestinal system. While much of these data confirmed findings from the literature, the transcriptional patterns detected using this toxicogenomics approach showed relationships between genes and biological pathways requiring complex analysis to be discerned.

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

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

The biochemistry of hypo- and hyperlipidemic fatty acid derivatives: metabolism and metabolic effects

A selection of amphipatic hyper- and hypolipidemic fatty acid derivatives (fibrates, thia- and branched chain fatty acids) are reviewed. They are probably all ligands for the peroxisome proliferation activation receptor (PPARalpha) which has a low selectivity for its ligands. These compounds give hyper- or hypolipidemic responses depending on their ability to inhibit or stimulate mitochondrial fatty acid oxidation in the liver. The hypolipidemic response is explained by the following metabolic effects: Lipoprotein lipase is induced in liver where it is normally not expressed. Apolipoprotein CIII is downregulated. These two effects in liver lead to a facilitated (re)uptake of chylomicrons and VLDL, thus creating a direct transport of fatty acids from the gut to the liver. Fatty acid metabolizing enzymes in the liver (CPT-I and II, peroxisomal and mitochondrial beta-oxidation enzymes, enzymes of ketogenesis, and omega-oxidation enzymes) are induced and create an increased capacity for fatty acid oxidation. The increased oxidation of fatty acids "drains" fatty acids from the body, reduces VLDL formation, and ultimately explains the antiadiposity and improved insulin sensitivity observed after administration of peroxisome proliferators.

Microarray analysis of the in vivo effects of hypophysectomy and growth hormone treatment on gene expression in the rat

Complementary DNA microarrays containing 3000 different rat genes were used to study the consequences of severe hormonal deficiency (hypophysectomy) on the gene expression patterns in heart, liver, and kidney. Hybridization signals were seen from a majority of the arrayed complementary DNAs; nonetheless, tissue-specific expression patterns could be delineated. Hypophysectomy affected the expression of genes involved in a variety of cellular functions. Between 16-29% of the detected transcripts from each tissue changed expression level as a reaction to this condition. Chronic treatment of hypophysectomized animals with human GH also caused significant changes in gene expression patterns. The study confirms previous knowledge concerning certain gene expression changes in the above-mentioned situations and provides new information regarding hypophysectomy and chronic human GH effects in the rat. Furthermore, we have identified several new genes that respond to GH treatment. Our results represent a first step toward a more global understanding of gene expression changes in states of hormonal deficiency.

Sesamin, a sesame lignan, is a potent inducer of hepatic fatty acid oxidation in the rat

The effects of sesamin, one of the most abundant lignans in sesame seed, on hepatic fatty acid oxidation were examined in rats that were fed experimental diets containing various amounts (0%, 0.1%, 0.2%, and 0.5%) of sesamin (a 1:1 mixture of sesamin and episesamin) for 15 days. Dietary sesamin dose-dependently increased both mitochondrial and peroxisomal palmitoyl-coenzyme A (CoA) oxidation rates. Mitochondrial activity almost doubled in rats on the 0.5% sesamin diet. Peroxisomal activity increased more than 10-fold in rats fed a 0.5% sesamin diet in relation to rats on the sesamin-free diet. Dietary sesamin greatly increased the hepatic activity of fatty acid oxidation enzymes, including carnitine palmitoyltransferase, acyl-CoA dehydrogenase, acyl-CoA oxidase, 3-hydroxyacyl-CoA dehydrogenase, enoyl-CoA hydratase, and 3-ketoacyl-CoA thiolase. Dietary sesamin also increased the activity of 2,4-dienoyl-CoA reductase and delta3,delta2-enoyl-CoA isomerase, enzymes involved in the auxiliary pathway for beta-oxidation of unsaturated fatty acids dose-dependently. Examination of hepatic mRNA levels using specific cDNA probes showed a sesamin-induced increase in the gene expression of mitochondrial and peroxisomal fatty acid oxidation enzymes. Among these various enzymes, peroxisomal acyl-CoA oxidase and bifunctional enzyme gene expression were affected most by dietary sesamin (15- and 50-fold increase by the 0.5% dietary level). Sesamin-induced alterations in the activity and gene expression of carnitine palmitoyltransferase I and acyl-CoA oxidase were in parallel with changes in the mitochondrial and peroxisomal palmitoyl-CoA oxidation rate, respectively. In contrast, dietary sesamin decreased the hepatic activity and mRNA abundance of fatty acid synthase and pyruvate kinase, the lipogenic enzymes. However, this lignan increased the activity and gene expression of malic enzyme, another lipogenic enzyme. An alteration in hepatic fatty acid metabolism may therefore account for the serum lipid-lowering effect of sesamin in the rat.

Rat peroxisome proliferator-activated receptors and brown adipose tissue function during cold acclimatization

Brown adipose tissue (BAT) hyperplasia is a fundamental physiological response to cold; it involves an acute phase of mitotic cell growth followed by a prolonged differentiation phase. Peroxisome proliferator-activated receptors (PPARs) are key regulators of fatty acid metabolism and adipocyte differentiation and may therefore mediate important metabolic changes during non-shivering thermogenesis. In the present study we have investigated PPAR mRNA expression in relation to peroxisome proliferation in rat BAT during cold acclimatization. By immunoelectron microscopy we show that the number of peroxisomes per cytoplasmic volume and acyl-CoA oxidase immunolabeling density remained constant (thus increasing in parallel with tissue mass and cell number) during the initial proliferative phase and the acute thermogenic response but increased after 14 days of cold exposure, correlating with terminal differentiation of BAT. A pronounced decrease in BAT PPARalpha and PPARgamma mRNA levels was found within hours of exposure to cold, which was reversed after 14 days, suggesting a role for either or both of these subtypes in the proliferation and induction of peroxisomes and peroxisomal beta-oxidation enzymes. In contrast, PPARdelta mRNA levels increased progressively during cold exposure. Transactivation assays in HIB 1B and HEK-293 cells demonstrated an adrenergic stimulation of peroxisome proliferator response element reporter activity via PPAR, establishing a role for these nuclear receptors in hormonal regulation of gene transcription in BAT.

Early modulation of genes encoding peroxisomal and mitochondrial beta-oxidation enzymes by 3-thia fatty acids

The aim of the present study was to elucidate the effects of a single dose of 3-thia fatty acids (tetradecylthioacetic acid and 3-thiadicarboxylic acid) over a 24-hr study period on the expression of genes related to peroxisomal and mitochondrial beta-oxidation in liver of rats. The plasma triglyceride level decreased at 2-4 hr, 4-8 hr, and 8-24 hr, respectively, after a single dose of 150, 300, or 500 mg of 3-thia fatty acids/kg body weight. Four to eight hours after administration of 3-thia fatty acids, a several-fold-induced gene expression of peroxisomal multifunctional protein, fatty acyl-CoA oxidase (EC 1.3.3.6), fatty acid binding protein, and 2,4-dienoyl-CoA reductase (EC 1.3.1.43) resulted, concomitant with increased activity of 2,4-dienoyl-CoA reductase and fatty acyl-CoA oxidase. The expression of carnitine palmitoyltransferase-I and carnitine palmitoyltransferase-II increased at 2 and 4 hr, respectively, although at a smaller scale. In cultured hepatocytes, 3-thia fatty acids stimulated fatty acid oxidation after 4 hr, and this was both L-carnitine- and L-aminocarnitine-sensitive. The hepatic content of eicosapentaenoic acid and docosahexaenoic acid decreased throughout the study period. In contrast, the hepatic content of oleic acid tended to increase after 24 hr and was significantly increased after repeated administration of 3-thia fatty acids. Similarly, the expression of delta9-desaturase was unchanged during the 24-hr study, but increased after feeding for 5 days. To conclude, carnitine palmitoyltransferase-I expression seemed to be induced earlier than 2,4-dienoyl-CoA reductase and fatty acid binding protein, and not later than the peroxisomal fatty acyl-CoA oxidase. The expression of delta9-desaturase showed a more delayed response.

Extraperoxisomal targets of peroxisome proliferators: mitochondrial, microsomal, and cytosolic effects. Implications for health and disease

Peroxisome proliferators are a structurally diverse group of compounds that include the fibrate hypolipidemic drugs, the phthalate ester industrial plasticizers, the phenoxy acid herbicides, and the anti-wetting corrosion inhibitors perfluorinated straight-chain monocarboxylic fatty acids. Administration of these chemicals to rodents results in a number of effects, the most prominent being hepatomegaly and induction of peroxisomal enzyme activities. Several of these compounds have also been associated with the production of liver tumors in rodents and are classified as nongenotoxic hepatocarcinogens. Experimental evidence suggests that humans are not susceptible to these effects following exposure to peroxisome-proliferating compounds. This has led to the proposal that an "actual threat to humans" from exposure to one of these compounds seems "rather unlikely". Indeed, recent reports suggest that peroxisome proliferators may prove valuable as antitumor agents in humans. However, this assessment is preliminary given that peroxisome proliferators also produce a myriad of extraperoxisomal effects in livers and other tissues of experimental animals. Such effects include both stimulation and inhibition of mitochondrial and microsomal metabolism and alteration of the activities of various cytosolic enzymes. These responses may be directly or indirectly related to the effects on peroxisomes or may be totally independent of these events. Whether the extraperoxisomal effects of these compounds occur in humans is not known and their potential impact on human health remains to be investigated.

Molecular cloning and characterization of the human mitochondrial 2,4-dienoyl-CoA reductase gene (DECR)

2,4-Dienoyl-CoA reductase (EC 1.3.1.34) is an auxiliary enzyme of beta-oxidation, and it participates in the metabolism of unsaturated fatty enoyl-CoA esters having double bonds in both even- and odd-numbered positions. In this article we describe the molecular cloning of the human gene for the 120-kDa isoform of mitochondrial 2,4-dienoyl-CoA reductase (DECR). The gene is approximately 30 kb and comprises 10 exons varying in size from 79 to 203 bp and 9 introns whose sizes vary from 95 bp to about 10 kb. The 5' UTR and 3' UTR are included in exons 1 and 10, respectively. The promoter region contains putative binding sites for several transcription factors, e.g., Sp1, AP-2, AP-4, and C/EBP, but no TATA box was found. Primer extension analysis and 5' RACE-PCR revealed variability in the length of the 5'-UTR, the longest being 72 bp. Through the use of FISH analysis on metaphase chromosomes with a genomic fragment of 2,4-dienoyl-CoA reductase, the gene was assigned to the chromosomal band 8q21.3.

Reciprocal responses to dietary diacylglycerol of hepatic enzymes of fatty acid synthesis and oxidation in the rat

The activities of hepatic enzymes of fatty acid synthesis and oxidation were compared in rats fed on diacylglycerol and triacylglycerol. In the first trial, rats were fed on diacylglycerol or triacylglycerol (rapeseed oil) for 14 d. The diacylglycerol preparation contained 65.2 g and 32.6 g fatty acids/100 g total fatty acids as 1,3-species and 1,2-species respectively. Fatty acid compositions of these dietary lipids were similar. Dietary acylglycerols were added to experimental diets to provide the same amounts of fatty acids (93.9 g/kg diet). Dietary diacylglycerol compared with triacylglycerol significantly reduced the concentrations of serum and liver triacylglycerol. The activities of enzymes of fatty acid synthesis (fatty acid synthetase, glucose 6-phosphate dehydrogenase (EC 1.1.1.49) and malic enzyme (EC 1.1.1.40)) were significantly lower in rats fed on diacylglycerol than in those fed on triacylglycerol. In contrast, the rates of mitochondrial and peroxisomal oxidation of palmitoyl-CoA in liver homogenates were higher in rats fed on diacylglycerol than in those fed on triacylglycerol. In the second trial, varying amounts of dietary triacylglycerol were replaced by diacylglycerol while the dietary fatty acid content was maintained (93.9 g/kg diet). After 21 d of the feeding period the significant reductions in serum and liver triacylglycerol levels were confirmed in groups of rats fed on the diets in which diacylglycerol supplied more than 65.8 g fatty acids/kg diet (65.8 and 93.9 g/kg). Reductions in the activities of enzymes of fatty acid synthesis and increases in palmitoyl-CoA oxidation rates by both mitochondrial and peroxisomal pathways were also apparent when diacylglycerol replaced triacylglycerol in diets to supply more than 65.8 g fatty acid/kg. Increasing dietary levels of diacylglycerol also progressively increased the activities of enzymes involved in the beta-oxidation pathway (carnitine palmitoyltransferase (EC 2.3.1.21), acyl-CoA dehydrogenase (EC 1.3.99.3), acyl-CoA oxidase (EC 1.3.3.6), enoyl-CoA hydratase (EC 4.2.1.17), 3-hydroxyacyl-CoA dehydrogenase (EC 1.1.1.35), 2,4-dienoyl-CoA reductase (EC 1.3.1.34) and delta 3, delta 2-enoyl-CoA isomerase (EC 5.3.3.8)) in the liver. These results suggest that alteration of fatty acid metabolism in the liver is a factor responsible for the serum triacylglycerol-lowering effect of dietary diacylglycerol.

Peroxisomal beta-oxidation and polyunsaturated fatty acids

Peroxisomes are capable of oxidizing a variety of substrates including (poly)unsaturated enoyl-CoA esters. The beta-oxidation of unsaturated enoyl-CoA esters in peroxisomes, and also in mitochondria, is not just chain-shortening but also involves the metabolizing of pre-existing carbon-to-carbon double bonds. In addition to the enzymes of the beta-oxidation spiral itself, this metabolism requires the participation of auxiliary enzymes: delta 3, delta 2-enoyl-CoA isomerase; 2,4-dienoyl-CoA reductase; 2-enoyl-CoA hydratase 2 or 3-hydroxyacyl-CoA epimerase; and delta 3,5 delta 2,4-dienoyl-CoA isomerase. Many of these enzymes are present as isoforms, and can be found located in multiple subcellular compartments, for example, peroxisomes, mitochondria or the endoplasmic reticulum, while some of the activities are integral parts of multifunctional enzymes of beta-oxidation systems.