Peroxisome proliferator-induced long chain acyl-CoA thioesterases comprise a highly conserved novel multi-gene family involved in lipid metabolism

Evidence for mitochondrial thioesterase 1 as a peroxisome proliferator-activated receptor-alpha-regulated gene in cardiac and skeletal muscle

The physiological role of mitochondrial thioesterase 1 (MTE1) is unknown. It was proposed that MTE1 promotes fatty acid (FA) oxidation (FAO) by acting in concert with uncoupling protein (UCP)3. We previously showed that ucp3 is a peroxisome proliferator-activated receptor-alpha (PPAR alpha)-regulated gene, allowing induction when FA availability increases. On the assumption that UCP3 and MTE1 act in partnership to increase FAO, we hypothesized that mte1 is also a PPAR alpha-regulated gene in cardiac and skeletal muscle. Using real-time RT-PCR, we characterized mte1 gene expression in rat heart and soleus muscles. Messenger RNA encoding for mte1 was 3.2-fold higher in heart than in soleus muscle. Cardiac mte1 mRNA exhibited modest diurnal variation, with 1.4-fold higher levels during dark phase. In contrast, skeletal muscle mte1 mRNA remained relatively constant over the course of the day. High-fat feeding, fasting, and streptozotocin-induced diabetes, interventions that increase FA availability, muscle PPAR alpha activity, and muscle FAO rates, increased mte1 mRNA in heart and soleus muscle. Conversely, pressure overload and hypoxia, interventions that decrease cardiac PPAR alpha activity and FAO rates, repressed cardiac mte1 expression. Specific activation of PPAR alpha in vivo through WY-14643 administration rapidly induced mte1 mRNA in cardiac and skeletal muscle. WY-14643 also induced mte1 mRNA in isolated adult rat cardiomyocytes dose dependently. Expression of mte1 was markedly lower in hearts and soleus muscles isolated from PPAR alpha-null mice. Alterations in cardiac and skeletal muscle ucp3 expression mirrored that of mte1 in all models investigated. In conclusion, mte1, like ucp3, is a PPAR alpha-regulated gene in cardiac and skeletal muscle.

The expression of cytosolic and mitochondrial type II acyl-CoA thioesterases is upregulated in the porcine corpus luteum during pregnancy

Acyl-CoA thioesterases hydrolyze acyl-CoAs to free fatty acids and CoASH, thereby regulating fatty acid metabolism. This activity is catalyzed by numerous structurally related and unrelated enzymes, of which several acyl-CoA thioesterases have been shown to be regulated via the peroxisome proliferator-activated receptor alpha, strongly linking them to fatty acid metabolism. Two protein families have recently been characterized, the type I acyl-CoA thioesterase gene family and the type II protein family, which are expressed in cytosol, mitochondria and peroxisomes. Still, only little is known about regulation of their expression and precise functions in vivo. In the present study, we have investigated the activity and expression of acyl-CoA thioesterase in the porcine ovary during different phases of the estrus cycle. The activity was low in homogenates obtained during the immature and follicular phases, increasing nearly 4-fold during the luteal phase, with the highest activity being found in the pregnant corpus luteum (about 7-fold higher than in immature follicles). The increase in homogenate activity in corpus luteum from pregnant pigs was due to a moderate increase in the cytosolic activity, and an approximately 20-25-fold increase in the mitochondrial fraction. Western blot analysis showed no detectable expression of the type I acyl-CoA thioesterases (CTE-I and MTE-I) and revealed that the increased activity in cytosol and mitochondria is due to increased expression of the type II acyl-CoA thioesterases (CTE-II and MTE-II). This apparent hormonal regulation of expression of the type II acyl-CoA thioesterase may provide new insights into the functions of these enzymes in the mammalian ovary.

Overlapping transcriptional programs regulated by the nuclear receptors peroxisome proliferator-activated receptor alpha, retinoid X receptor, and liver X receptor in mouse liver

Lipid homeostasis is controlled in part by the nuclear receptors peroxisome proliferator (PP)-activated receptor alpha (PPARalpha) and liver X receptor (LXR) through regulation of genes involved in fatty acid and cholesterol metabolism. Exposure to agonists of retinoid X receptor (RXR), the obligate heterodimer partner of PPARalpha, and LXR results in responses that partially overlap with those of PP. To better understand the gene networks regulated by these nuclear receptors, transcript profiles were generated from the livers of wild-type and PPARalpha-null mice exposed to the RXR pan-agonist 3,7-dimethyl-6S,7S-methano, 7-[1,1,4,4-tetramethyl-1,2,3,4-tetrahydronaphth-7-yl]-2E,4E-heptadienoic acid (AGN194,204) or the PPAR pan-agonist WY-14,643 (WY; pirinixic acid) and compared with the profiles from the livers of wild-type and LXRalpha/LXRbeta-null mice after exposure to the LXR agonist N-(2,2,2-trifluoroethyl)-N-[4-(2,2,2-trifluoro-1-hydroxy-1-trifluoromethylethyl)phenyl] sulfonamide (T0901317). All 218 WY-regulated genes altered in wild-type mice required PPARalpha. Remarkably, approximately 80% of genes regulated by AGN194,204 required PPARalpha including cell-cycle genes, consistent with AGN-induced hepatocyte proliferation having both PPARalpha-dependent and -independent components. Overlaps of approximately 31 to 62% in the transcript profiles of WY, AGN194,204, and T0901317 required PPARalpha and LXRalpha/LXRbeta for statistical significance. Ofthe 50 overlapping genes regulated by T0901317 and WY, all but one were regulated in a similar direction. These results 1) identify new transcriptional targets of PPARalpha and RXR important in regulating lipid metabolism and liver homeostasis, 2) illustrate the importance of PPARalpha in regulation of gene expression by a prototypical PP and by an RXR agonist, and 3) provide support for an axis of PPARalpha-RXR-LXR in which agonists for each nuclear receptor regulate an overlapping set of genes in the mouse liver.

Overexpression of human acyl-CoA thioesterase upregulates peroxisome biogenesis

The biological functions of human acyl-CoA thioesterase III (ACTEIII/PTE-1), initially identified as an HIV-1 Nef binding protein, have remained unclear. We report herein that the stable overexpression of ACTEIII/PTE-1 in human and murine T-cell lines resulted in an increase in both peroxisome number and lipid droplet formation in a manner dependent on the amount of the protein. Peroxisome proliferation was evidenced by immunofluorescence staining for catalase, a peroxisome marker protein, as well as by direct peroxisome enumeration on electron micrographs. Consistently, the amount of catalase was elevated as the amount of ACTEIII/PTE-1 was increased. ACTEIII/PTE-1 mutants with reduced enzymatic activity or with the defect in peroxisome localization did not induce peroxisome proliferation, indicating that peroxisome proliferation was mediated by metabolites generated by ACTEIII/PTE-1 within peroxisomes. Finally, thymocytes isolated from a T-cell-specific ACTEIII/PTE-1 transgenic mouse as well as human and murine cell lines of lymphoid and non-lymphoid origins exhibited a similar proliferation of peroxisomes. Thus, ACTEIII/PTE-1 may be involved in the metabolic regulation of peroxisome proliferation.

Differential regulation of cytosolic and peroxisomal bile acid amidation by PPARα activation favors the formation of unconjugated bile acids

In human liver, unconjugated bile acids can be formed by the action of bile acid-CoA thioesterases (BACTEs), whereas bile acid conjugation with taurine or glycine (amidation) is catalyzed by bile acid-CoA:amino acid N-acyltransferases (BACATs). Both pathways exist in peroxisomes and cytosol. Bile acid amidation facilitates biliary excretion, whereas the accumulation of unconjugated bile acids may become hepatotoxic. We hypothesized that the formation of unconjugated and conjugated bile acids from their common substrate bile acid-CoA thioesters by BACTE and BACAT is regulated via the peroxisome proliferator-activated receptor alpha (PPARalpha). Livers from wild-type and PPARalpha-null mice either untreated or treated with the PPARalpha activator WY-14,643 were analyzed for BACTE and BACAT expression. The total liver capacity of taurochenodeoxycholate and taurocholate formation was decreased in WY-14,643-treated wild-type mice by 60% and 40%, respectively, but not in PPARalpha-null mice. Suppression of the peroxisomal BACAT activity was responsible for the decrease in liver capacity, whereas cytosolic BACAT activity was essentially unchanged by the treatment. In both cytosol and peroxisomes, the BACTE activities and protein levels were upregulated 5- to 10-fold by the treatment. These effects caused by WY-14,643 treatment were abolished in PPARalpha-null mice. The results from this study suggest that an increased formation of unconjugated bile acids occurs during PPARalpha activation.

Molecular Cloning and Characterization of Two Mouse Peroxisome Proliferator-activated Receptor α (PPARα)-regulated Peroxisomal Acyl-CoA Thioesterases

Peroxisomes are organelles that function in the beta-oxidation of long- and very long-chain acyl-CoAs, bile acid-CoA intermediates, prostaglandins, leukotrienes, thromboxanes, dicarboxylic fatty acids, pristanic acid, and xenobiotic carboxylic acids. The very long- and long-chain acyl-CoAs are mainly chain-shortened and then transported to mitochondria for further metabolism. We have now identified and characterized two peroxisomal acyl-CoA thioesterases, named PTE-Ia and PTE-Ic, that hydrolyze acyl-CoAs to the free fatty acid and coenzyme A. PTE-Ia and PTE-Ic show 82% sequence identity at the amino acid level, and a putative peroxisomal type 1 targeting signal of -AKL was identified at the carboxyl-terminal end of both proteins. Localization experiments using green fluorescent fusion protein showed PTE-Ia and PTE-Ic to be localized in peroxisomes. Despite their high level of sequence identity, we show that PTE-Ia is mainly active on long-chain acyl-CoAs, whereas PTE-Ic is mainly active on medium-chain acyl-CoAs. Lack of regulation of enzyme activity by free CoASH suggests that PTE-Ia and PTE-Ic regulate intraperoxisomal levels of acyl-CoA, and they may have a function in termination of beta-oxidation of fatty acids of different chain lengths. Tissue expression studies revealed that PTE-Ia is highly expressed in kidney, whereas PTE-Ic is most highly expressed in spleen, brain, testis, and proximal and distal intestine. Both PTE-Ia and PTE-Ic were highly up-regulated in mouse liver by treatment with the peroxisome proliferator WY-14,643 and by fasting in a peroxisome proliferator-activated receptor alpha-dependent manner. These data show that PTE-Ia and PTE-Ic have different functions based on different substrate specificities and tissue expression.

Biochemical and Molecular Characterization of ACH2, an Acyl-CoA Thioesterase from Arabidopsis thaliana

By using computer-based homology searches of the Arabidopsis genome, we identified the gene for ACH2, a putative acyl-CoA thioesterase. With the exception of a unique 129-amino acid N-terminal extension, the ACH2 protein is 17-36% identical to members of a family of acyl-CoA thioesterases that are found in both prokaryotes and eukaryotes. The eukaryotic homologs of ACH2 are peroxisomal acyl-CoA thioesterases that are up-regulated during times of increased fatty acid oxidation, suggesting potential roles in peroxisomal beta-oxidation. We investigated ACH2 to determine whether it has a similar role in the plant cell. Like its eukaryotic homologs, ACH2 carries a putative type 1 peroxisomal targeting sequence (-SKL(COOH)), and maintains all the catalytic residues typical of this family of acyl-CoA thioesterases. Analytical ultracentrifugation of recombinant ACH2-6His shows that it associates as a 196-kDa homotetramer in vitro, a result that is significant in light of the cooperative kinetics demonstrated by ACH2-6His in vitro. The cooperative effects are most pronounced with medium chain acyl-CoAs, where the Hill coefficient is 3.8 for lauroyl-CoA, but decrease for long chain acyl-CoAs, where the Hill coefficient is only 1.9 for oleoyl-CoA. ACH2-6His hydrolyzes both medium and long chain fatty acyl-CoAs but has highest activity toward the long chain unsaturated fatty acyl-CoAs. Maximum rates were found with palmitoleoyl-CoA, which is hydrolyzed at 21 micromol/min/mg protein. Additionally, ACH2-6His is insensitive to feedback inhibition by free CoASH levels as high as 100 microm. ACH2 is most highly expressed in mature tissues such as young leaves and flowers rather than in germinating seedlings where beta-oxidation is rapidly proceeding. Taken together, these results suggest that ACH2 activity is not linked to fatty acid oxidation as has been suggested for its eukaryotic homologs, but rather has a unique role in the plant cell.

Combined cDNA array/RT‐PCR analysis of gene expression profile in rat gastrocnemius muscle: relation to its adaptive function in energy metabolism during fasting

We evaluated the effects of fasting on the gene expression profile in rat gastrocnemius muscle using a combined cDNA array and RT-PCR approach. Of the 1176 distinct rat genes analyzed on the cDNA array, 114 were up-regulated more than twofold in response to fasting, including all 17 genes related to lipid metabolism present on the membranes and all 10 analyzed components of the proteasome machinery. Only 7 genes were down-regulated more than twofold. On the basis of our analysis of genes on the cDNA array plus the data from our RT-PCR assays, the metabolic adaptations shown by rat gastrocnemius muscle during fasting are reflected by i) increased transcription both of myosin heavy chain (MHC) Ib (associated with type I fibers) and of at least three factors involved in the shift toward type I fibers [p27kip1, muscle LIM protein (MLP), cystein rich protein-2], of which one (MLP) has been shown to enhance the activity of MyoD, which would explain the known increase in the expression of skeletal muscle uncoupling protein-3 (UCP3); ii) increased lipoprotein lipase (LPL) expression, known to trigger UCP3 transcription, which tends, together with the first point, to underline the suggested role of UCP3 in mitochondrial lipid handling (the variations under the first point and this one have not been observed in mice, indicating a species-specific regulation of these mechanisms); iii) reduced expression of the muscle-specific coenzyme Q (CoQ)7 gene, which is necessary for mitochondrial CoQ synthesis, together with an increased expression of mitochondrial adenylate kinase 3, which inactivates the resident key enzyme for CoQ synthesis, 3-hydroxy-3-methylglutaryl CoA reductase (HMGR), the mRNA level for which fell during fasting; and iv) increased transcription of components of the proteasomal pathways involved in protein degradation/turnover.

DNA microarray profiling of developing PS1-deficient mouse brain reveals complex and coregulated expression changes

Presenilin 1 (PS1) plays a critical role in the nervous system development and PS1 mutations have been associated with familial Alzheimer's disease. PS1-deficient mice exhibit alterations in neural and vascular development and die in late embryogenesis. The present study was aimed at uncovering transcript networks that depend on intact PS1 function in the developing brain. To achieve this, we analyzed the brains of PS1-deficient and control animals at embryonic ages E12.5 and E14.5 using MG_U74Av2 oligonucleotide microarrays by Affymetrix. Based on the microarray data, overall molecular brain development appeared to be comparable between the E12.5 and E14.5 PS1-deficient and control embryos. However, in brains of PS1-deficient mice, we observed significant differences in the expression of genes encoding molecules that are associated with neural differentiation, extracellular matrix, vascular development, Notch-related signaling and lipid metabolism. Many of the expression differences between wild-type and PS1-deficient animals were present at both E12.5 and E14.5, whereas other transcript alterations were characteristic of only one developmental stage. The results suggest that the role of PS1 in development includes influences on a highly co-regulated transcript network; some of the genes participating in this expression network may contribute to the pathophysiology of Alzheimer's disease.

The human bile acid-CoA:amino acid N-acyltransferase functions in the conjugation of fatty acids to glycine

Bile acid-CoA:amino acid N-acyltransferase (BACAT) catalyzes the conjugation of bile acids to glycine and taurine for excretion into bile. By use of site-directed mutagenesis and sequence comparisons, we have identified Cys-235, Asp-328, and His-362 as constituting a catalytic triad in human BACAT (hBACAT) and identifying BACAT as a member of the type I acyl-CoA thioesterase gene family. We therefore hypothesized that hBACAT may also hydrolyze fatty acyl-CoAs and/or conjugate fatty acids to glycine. We show here that recombinant hBACAT also can hydrolyze long- and very long-chain saturated acyl-CoAs (mainly C16:0-C26:0) and by mass spectrometry verified that hBACAT also conjugates fatty acids to glycine. Tissue expression studies showed strong expression of BACAT in liver, gallbladder, and the proximal and distal intestine. However, BACAT is also expressed in a variety of tissues unrelated to bile acid formation and transport, suggesting important functions also in the regulation of intracellular levels of very long-chain fatty acids. Green fluorescent protein localization experiments in human skin fibroblasts showed that the hBACAT enzyme is mainly cytosolic. Therefore, the cytosolic BACAT enzyme may play important roles in protection against toxicity by accumulation of unconjugated bile acids and non-esterified very long-chain fatty acids.

Increased fatty acid oxidation in transgenic mice overexpressing UCP3 in skeletal muscle

AIM

To determine the rates of substrate oxidation by skeletal muscle in vitro as well as tissue-specific glucose uptake in vivo in transgenic mice overexpressing uncoupling protein-3 (UCP3) in skeletal muscle.

METHODS

Soleus muscle was isolated from transgenic mice overexpressing UCP3 in skeletal muscle and wild-type mice. Rates of [1-14C]-palmitate oxidation and [2-14C]-pyruvate oxidation were determined by in vitro incubation of the soleus muscle. Tissue glucose uptake rates were characterized during a glucose tolerance test using 2-deoxy-[1-3H]-glucose as a tracer.

RESULTS

Oxidation of [1-14C]-palmitate to CO2 by isolated soleus muscle was increased in UCP3 transgenic mice (0.45 +/- 0.03 vs. 0.24 +/- 0.02 micro mol/h/g). [2-14C]-pyruvate oxidation, which is a measure of the activity of pyruvate carboxylase in introducing pyruvate carbon into the tricarboxylic acid cycle, was increased 1.4-fold in the presence of fatty acid in the UCP3 transgenic mice (3.84 +/- 0.28 vs. 5.36 +/- 0.29 micro mol/h/g). The plasma glucose concentration after an overnight fast was significantly lower in the UCP3 transgenic mice (3.56 +/- 0.37 vs. 5.11 +/- 0.33 m/mol). Only brown adipose tissue from the UCP3 transgenic mice showed increased tissue glucose uptake rates compared with the wild-type mice. Skeletal muscle uptake rates of 2-deoxyglucose were either unchanged (soleus and gastrocnemius) or reduced (diaphragm) in the UCP3 transgenic mice.

CONCLUSIONS

The improved glucose tolerance in the UCP3 transgenic mice does not appear to be the result of increased uptake into peripheral tissues. The increased fatty acid oxidation in skeletal muscle of UCP3 transgenic mice supports the proposed role of UCP3 in the export of fatty acid anions from mitochondria during fatty acid oxidation.

Effects of clofibric acid on mRNA expression profiles in primary cultures of rat, mouse and human hepatocytes

The mRNA expression profile in control and clofibric acid (CLO)-treated mouse, rat, and human hepatocytes was analyzed using species-specific oligonucleotide DNA microarrays (Affymetrix). A statistical empirical Bayes procedure was applied in order to select the significantly differentially expressed genes. Treatment with the peroxisome proliferator CLO induced up-regulation of genes involved in peroxisome proliferation and in cell proliferation as well as down-regulation of genes involved in apoptosis in hepatocytes of rodent but not of human origin. CLO treatment induced up-regulation of microsomal cytochrome P450 4a genes in rodent hepatocytes and in two of six human hepatocyte cultures. In addition, genes encoding phenobarbital-inducible cytochrome P450s were also up-regulated by CLO in rodent and human hepatocyte cultures. Up-regulation of phenobarbital-inducible UDP-glucuronosyl-transferase genes by CLO was observed in both rat and human but not in mouse hepatocytes. CLO treatment induced up-regulation of L-fatty acid binding protein (L-FABP) gene in hepatocytes of both rodent and human origin. However, while genes of the cytosolic, microsomal, and mitochondrial pathways involved in fatty acid transport and metabolism were up-regulated by CLO in both rodent and human hepatocyte cultures, genes of the peroxisomal pathway of lipid metabolism were up-regulated in rodents only. An up-regulation of hepatocyte nuclear factor 1alpha (HNF1alpha) by CLO was observed only in human hepatocyte cultures, suggesting that this trans-activating factor may play a key role in the regulation of fatty acid metabolism in human liver as well as in the nonresponsiveness of human liver to CLO-induced regulation of cell proliferation and apoptosis.

The biochemistry of peroxisomal beta-oxidation in the yeast Saccharomyces cerevisiae

Peroxisomal fatty acid degradation in the yeast Saccharomyces cerevisiae requires an array of beta-oxidation enzyme activities as well as a set of auxiliary activities to provide the beta-oxidation machinery with the proper substrates. The corresponding classical and auxiliary enzymes of beta-oxidation have been completely characterized, many at the structural level with the identification of catalytic residues. Import of fatty acids from the growth medium involves passive diffusion in combination with an active, protein-mediated component that includes acyl-CoA ligases, illustrating the intimate linkage between fatty acid import and activation. The main factors involved in protein import into peroxisomes are also known, but only one peroxisomal metabolite transporter has been characterized in detail, Ant1p, which exchanges intraperoxisomal AMP with cytosolic ATP. The other known transporter is Pxa1p-Pxa2p, which bears similarity to the human adrenoleukodystrophy protein ALDP. The major players in the regulation of fatty acid-induced gene expression are Pip2p and Oaf1p, which unite to form a transcription factor that binds to oleate response elements in the promoter regions of genes encoding peroxisomal proteins. Adr1p, a transcription factor, binding upstream activating sequence 1, also regulates key genes involved in beta-oxidation. The development of new, postgenomic-era tools allows for the characterization of the entire transcriptome involved in beta-oxidation and will facilitate the identification of novel proteins as well as the characterization of protein families involved in this process.

Transcription profiling in rat liver in response to dietary docosahexaenoic acid implicates stearoyl-coenzyme a desaturase as a nutritional target for lipid lowering

The gene expression profile in response to dietary docosahexaenoic acid rich oil for 6 wk was analyzed in the livers of male Sprague-Dawley rats to identify genes whose expression was regulated by dietary modification and correlated with serum lipid changes. Such genes may represent targets for intervention into cardiovascular health using nutraceuticals. High density glass microarrays containing approximately 7800 cloned expressed sequences from rat were used to identify those genes that responded to dietary long chain (n-3) fatty acids. In general, dietary long chain (n-3) fatty acids exhibited statistically significant lipid-lowering effects similar to a pharmaceutical alternative, fenofibrate, but showed narrower effects on the transcription of most of the genes assayed. The transcription patterns confirmed that the expression of several key genes involved in cholesterol metabolism, fatty acid beta-oxidation and lipogenesis was affected. These analyses indicated that stearoyl-coenzyme A (Delta9) desaturase, a key enzyme involved in the regulation of triglyceride biosynthesis and secretion, is a potential target for nutritional intervention for hyperlipidemia and cardiovascular health. In addition these results suggested that regulation of the farnesoid X receptor may be a key nutritionally regulated mediator of serum lipid changes. A nutritional product concept based on a convenient dietary aid demonstrated comparable efficacy with less spurious gene regulation than a pharmaceutical alternative.

Acyl-CoA thioesterase activity in human placental choriocarcinoma (BeWo), cells: effects of fatty acids

The effects of fatty acids on acyl-CoA thioesterase activity and peroxisome proliferator-activated receptor gamma (PPARgamma), a regulator of lipid metabolism, were investigated in placental choriocarcinoma (BeWo) cells. Substrate preference for acyl-CoA thioesterase was in the following order; gamma-linolenoyol-CoA>/=arachidonoyol-CoAz.Gt;palmitoyl-CoA>/=linoleyol-CoA. However, when these cells were incubated with fatty acids, acyl-CoA thioesterase activity was increased by both conjugated linoleic and gamma linolenic acids, but not by docosahexaenoic and eicosapentaenoic acids. In addition, these fatty acids also increased expression of PPARgamma in these cells, suggesting a putative relationship between free fatty acid generated by acyl-CoA thioesterase and expression of PPARgamma. Since expression of PPARgamma is critical for feto-placental growth, these fatty acids may be important during pregnancy.

beta-Adrenergic stimulation controls the expression of a thioesterase specific for very-long-chain fatty acids in perfused hearts

Arachidonic acid is not freely stored in the cells. A number of different pathways for the mobilization of this compound have been proposed, including a novel mechanism that involves the release of arachidonic acid from arachidonoyl-CoA by a thioesterase with substrate specificity for very-long-chain fatty acids. In rat heart, the acyl-CoA thioesterase activity can be regulated by a mechanism that involves beta-adrenoceptors. In this paper we demonstrate that beta-adrenergic agonists also regulate the acyl-CoA thioesterase mRNA levels. Isoproterenol (10(-7)M)-a concentration known to exert physiological responses-increases in a time-dependent manner the acyl-CoA thioesterase mRNA levels, an effect blocked by a specific beta-adrenoceptor antagonist. In addition, our results show that cAMP is involved in this process. The acyl-CoA thioesterase mRNA levels are also increased by fasting, but not by di(2-ethylhexyl)phthalate, a peroxisome proliferator. These results may suggest the existence of a beta-adrenoceptor-activated regulatory pathway for arachidonic acid release in cardiac tissue.

Human brain acyl-CoA hydrolase isoforms encoded by a single gene

Acyl-CoA hydrolases are a group of enzymes that catalyze the hydrolysis of acyl-CoA thioesters to free fatty acids and CoA-SH. The human brain acyl-CoA hydrolase (BACH) gene comprises 13 exons, generating several isoforms through the alternative use of exons. Four first exons (1a-1d) can be used, and three patterns of splicing occur at exon X located between exons 7 and 8 that contains an internal 3(')-splice acceptor site and creates premature stop codons. When examined with green fluorescent protein-fusion constructs expressed in Neuro-2a cells, the nuclear localization signal encoded by exon 9 was functional by itself, whereas the whole structure was cytosolic, suggesting nuclear translocation of the enzyme. This was consistent with dual staining of the cytosol and nucleus in certain neurons by immunohistochemistry using anti-BACH antibody. The mitochondrial targeting signals encoded by exons 1b and 1c were also functional and directed mitochondrial localization of BACH isoforms with the signals. Although BACH mRNA containing the sequence derived from exon 1a, but not exon X, was exclusively expressed in human brain, these results suggest that the human BACH gene can express long-chain acyl-CoA hydrolase activity in multiple intracellular compartments by generating BACH isoforms with differential localization signals to affect various cellular functions that involve acyl-CoAs.

Concerted regulation of free arachidonic acid and hormone-induced steroid synthesis by acyl-CoA thioesterases and acyl-CoA synthetases in adrenal cells

Although the role of arachidonic acid (AA) in the regulation of steroidogenesis is well documented, the mechanism for AA release is not clear. Therefore, the aim of this study was to characterize the role of an acyl-CoA thioesterase (ARTISt) and an acyl-CoA synthetase as members of an alternative pathway in the regulation of the intracellular levels of AA in steroidogenesis. Purified recombinant ARTISt releases AA from arachidonoyl-CoA (AA-CoA) with a Km of 2 micro m. Antibodies raised against recombinant acyl-CoA thioesterase recognize the endogenous protein in both adrenal tissue and Y1 adrenal tumor cells by immunohistochemistry and immunocytochemistry and Western blot. Stimulation of Y1 cells with ACTH significantly stimulated endogenous mitochondrial thioesterases activity (1.8-fold). Nordihydroguaiaretic acid (NDGA), an inhibitor of AA release known to affect steroidogenesis, affects the in vitro activity of recombinant ARTISt and also the endogenous mitochondrial acyl-CoA thioesterases. ACTH-stimulated steroid synthesis in Y1 cells was significantly inhibited by a synergistic effect of NDGA and triacsin C an inhibitor of the AA-CoA synthetase. The apparent IC50 for NDGA was reduced from 50 micro m to 25, 7.5 and 4.5 micro m in the presence of 0.1, 0.5 and 2 micro m triacsin C, respectively. Our results strongly support the existence of a new pathway of AA release that operates in the regulation of steroid synthesis in adrenal cells.

Do long-chain acyl-CoA synthetases regulate fatty acid entry into synthetic versus degradative pathways?

Recent studies suggest that the long-chain acyl-CoA synthetases (ACS) may play a role in channeling fatty acids either toward complex lipid synthesis and storage or toward oxidation. Each of the five members of the ACS family that has been cloned has a distinct tissue distribution and subcellular location, and is regulated independently during cellular differentiation and by diverse hormones and nuclear transcription factors including adrenocorticotropic hormone (ACTH), peroxisomal proliferator-activated receptor-alpha (PPARalpha) and sterol regulatory element binding protein. Taken as a whole, these features suggest that in liver, ACS1 and ACS5 may provide acyl-CoA destined primarily for triacylglycerol synthesis or for mitochondrial oxidation, respectively. ACS4 may provide acyl-CoA for both synthesis and peroxisomal oxidation, depending on whether the enzyme is associated with the mitochondrial-associated membrane or with peroxisomes. It should be emphasized that although the data for acyl-CoA channeling are strong, they are indirect. Rigorous testing of these predictions will be required.

The role Acyl-CoA thioesterases play in mediating intracellular lipid metabolism

Acyl-CoA thioesterases are a group of enzymes that catalyze the hydrolysis of acyl-CoAs to the free fatty acid and coenzyme A (CoASH), providing the potential to regulate intracellular levels of acyl-CoAs, free fatty acids and CoASH. These enzymes are localized in almost all cellular compartments such as endoplasmic reticulum, cytosol, mitochondria and peroxisomes. Acyl-CoA thioesterases are highly regulated by peroxisome proliferator-activated receptors (PPARs), and other nutritional factors, which has led to the conclusion that they are involved in lipid metabolism. Although the physiological functions for these enzymes are not yet fully understood, recent cloning and more in-depth characterization of acyl-CoA thioesterases has assisted in discussion of putative functions for specific enzymes. Here we review the acyl-CoA thioesterases characterized to date and also address the diverse putative functions for these enzymes, such as in ligand supply for nuclear receptors, and regulation and termination of fatty acid oxidation in mitochondria and peroxisomes.

Identification of a type 1 peroxisomal targeting signal in a viral protein and demonstration of its targeting to the organelle

Peroxisomes are unimembrane, respiratory organelles of the cell. Transport of cellular proteins to the peroxisomal matrix requires a type 1 peroxisomal targeting signal (PTS1) which essentially constitutes a tripeptide from the consensus sequence S/T/A/G/C/N-K/R/H-L/I/V/M/A/F/Y. Although PTS-containing proteins have been identified in eukaryotes, prokaryotes, and parasites, viral proteins with such signals have not been identified so far. We report here the first instance of a virus, the rotavirus, which causes infantile diarrhea worldwide, containing a functional C-terminal PTS1 in one of its proteins (VP4). Analysis of 153 rotavirus VP4-deduced amino acid sequences identified five groups of conserved C-terminal PTS1 tripeptide sequences (SKL, CKL, GKL, CRL, and CRI), of which CRL is represented in approximately 62% of the sequences. Infection of cells by a CRL-containing representative rotavirus (SA11 strain) and confocal immunofluorescence analysis revealed colocalization of VP4 with peroxisomal markers and morphological changes of peroxisomes. Further, transient cellular expression of green fluorescent protein (GFP)-fused VP4CRL resulted in transport of VP4 to peroxisomes, whereas the chimera lacking the PTS1 signal, GFP-VP4DeltaCRL, resulted in diffuse cytoplasmic staining, suggesting a CRL-dependent targeting of the protein. The present study therefore demonstrates hitherto unreported organelle involvement, specifically of the peroxisomes, in rotaviral infections as demonstrated by using the SA11 strain of rotavirus and opens a new line of investigation toward understanding viral pathogenesis and disease mechanisms.

The peroxisome proliferator-induced cytosolic type I acyl-CoA thioesterase (CTE-I) is a serine-histidine-aspartic acid alpha /beta hydrolase

Long-chain acyl-CoA thioesterases hydrolyze long-chain acyl-CoAs to the corresponding free fatty acid and CoASH and may therefore play important roles in regulation of lipid metabolism. We have recently cloned four members of a highly conserved acyl-CoA thioesterase multigene family expressed in cytosol (CTE-I), mitochondria (MTE-I), and peroxisomes (PTE-Ia and -Ib), all of which are regulated via the peroxisome proliferator-activated receptor alpha (Hunt, M. C., Nousiainen, S. E. B., Huttunen, M. K., Orii, K. E., Svensson, L. T., and Alexson, S. E. H. (1999) J. Biol. Chem. 274, 34317-34326). Sequence comparison revealed the presence of putative active-site serine motifs (GXSXG) in all four acyl-CoA thioesterases. In the present study we have expressed CTE-I in Escherichia coli and characterized the recombinant protein with respect to sensitivity to various amino acid reactive compounds. The recombinant CTE-I was inhibited by phenylmethylsulfonyl fluoride and diethyl pyrocarbonate, suggesting the involvement of serine and histidine residues for the activity. Extensive sequence analysis pinpointed Ser(232), Asp(324), and His(358) as the likely components of a catalytic triad, and site-directed mutagenesis verified the importance of these residues for the catalytic activity. A S232C mutant retained about 2% of the wild type activity and incubation with (14)C-palmitoyl-CoA strongly labeled this mutant protein, in contrast to wild-type enzyme, indicating that deacylation of the acyl-enzyme intermediate becomes rate-limiting in this mutant protein. These data are discussed in relation to the structure/function of acyl-CoA thioesterases versus acyltransferases. Furthermore, kinetic characterization of recombinant CTE-I showed that this enzyme appears to be a true acyl-CoA thioesterase being highly specific for C(12)-C(20) acyl-CoAs.

Characterization of an acyl-coA thioesterase that functions as a major regulator of peroxisomal lipid metabolism

Peroxisomes function in beta-oxidation of very long and long-chain fatty acids, dicarboxylic fatty acids, bile acid intermediates, prostaglandins, leukotrienes, thromboxanes, pristanic acid, and xenobiotic carboxylic acids. These lipids are mainly chain-shortened for excretion as the carboxylic acids or transported to mitochondria for further metabolism. Several of these carboxylic acids are slowly oxidized and may therefore sequester coenzyme A (CoASH). To prevent CoASH sequestration and to facilitate excretion of chain-shortened carboxylic acids, acyl-CoA thioesterases, which catalyze the hydrolysis of acyl-CoAs to the free acid and CoASH, may play important roles. Here we have cloned and characterized a peroxisomal acyl-CoA thioesterase from mouse, named PTE-2 (peroxisomal acyl-CoA thioesterase 2). PTE-2 is ubiquitously expressed and induced at mRNA level by treatment with the peroxisome proliferator WY-14,643 and fasting. Induction seen by these treatments was dependent on the peroxisome proliferator-activated receptor alpha. Recombinant PTE-2 showed a broad chain length specificity with acyl-CoAs from short- and medium-, to long-chain acyl-CoAs, and other substrates including trihydroxycoprostanoyl-CoA, hydroxymethylglutaryl-CoA, and branched chain acyl-CoAs, all of which are present in peroxisomes. Highest activities were found with the CoA esters of primary bile acids choloyl-CoA and chenodeoxycholoyl-CoA as substrates. PTE-2 activity is inhibited by free CoASH, suggesting that intraperoxisomal free CoASH levels regulate the activity of this enzyme. The acyl-CoA specificity of recombinant PTE-2 closely resembles that of purified mouse liver peroxisomes, suggesting that PTE-2 is the major acyl-CoA thioesterase in peroxisomes. Addition of recombinant PTE-2 to incubations containing isolated mouse liver peroxisomes strongly inhibited bile acid-CoA:amino acid N-acyltransferase activity, suggesting that this thioesterase can interfere with CoASH-dependent pathways. We propose that PTE-2 functions as a key regulator of peroxisomal lipid metabolism.

Concordant mRNA expression of UCP-3, but not UCP-2, with mitochondrial thioesterase-1 in brown adipose tissue and skeletal muscle in db/db diabetic mice

A recent hypothesis concerning the function of uncoupling protein-3 (UCP-3) depends upon a positive relationship with mitochondrial thioesterase (MTE-1) in situations where fatty acid beta-oxidation is increased. MTE-1 mRNA levels are raised in transgenic mice overexpressing UCP-3 in skeletal muscle and we sought to extend these findings by quantifying in vivo expression of endogenous MTE-1, UCP-1, UCP-2, and UCP-3 mRNA levels in white adipose tissue, interscapular brown adipose tissue, and skeletal muscle in db/db mice. In this study we show that changes in MTE-1 mRNA levels as a result of differences between db/db vs db/+ mice or following long-term treatment of db/db mice with rosiglitazone or Wy-14,643 were more closely correlated with changes in UCP-3 than either UCP-1 or UCP-2 mRNA levels in the tissues examined. The present data contribute to the argument that UCP-3 and MTE-1 are linked within the same metabolic pathway either in response to, or as regulators of, fatty acid beta-oxidation.

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.

Overexpression of UCP-3 in Skeletal Muscle of Mice Results in Increased Expression of Mitochondrial Thioesterase mRNA

Mice overexpressing human UCP-3 in skeletal muscle (UCP-3tg) are lean despite overeating, have increased metabolic rate, and their skeletal muscle mitochondria show increased proton conductance. The true function of UCP-3 however, has yet to be determined. It is assumed that UCP-3tg mice have increased fatty acid beta-oxidation to fuel their increased metabolic rate. In this study we have quantified skeletal muscle mRNA levels of a number of genes involved in fatty acid metabolism. mRNA levels of uncoupling protein-2, carnitine palmitoyl transferase-1beta and fatty acid binding proteins, and transporters were unchanged when compared to wild-type mice. Lipoprotein lipase mRNA was slightly, but significantly, increased by 50%. The most notable change in gene expression was a threefold increase in mitochondrial thioesterase (MTE-1) expression. In the face of a chronic increase in mitochondrial uncoupling these changes suggest that increased flux of fatty acids through the beta-oxidation pathway does not necessarily require marked changes in expression of genes involved in fatty acid metabolism. The large increase in MTE-1 both confirms the importance of this gene in situations where mitochondrial beta-oxidation is increased and supports the hypothesis that UCP-3 exports fatty acids generated by MTE-1 in the mitochondrion.

Physiological role of UCP3 may be export of fatty acids from mitochondria when fatty acid oxidation predominates: an hypothesis

This hypothesis proposes a physiological role for uncoupling protein-3 (UCP3) in the export of fatty acid anions from muscle and brown adipose tissue (BAT) mitochondria when fatty acids are the predominant substrate being used. It proposes that excess acyl CoA within the mitochondria is hydrolyzed by a mitochondrial acyl CoA thioesterase, yielding fatty acid anion and CoASH. The fatty acid anion is exported to the cytosol by being carried across the inner mitochondrial membrane by UCP3. The CoASH is conserved within the mitochondrion to participate in other reactions for which it is needed during fatty acid oxidation in the beta-oxidation cycle and in the tricarboxylic acid cycle. The export of the fatty acid anion thus permits continued rapid fatty acid oxidation in the face of an oversupply. The hypothesis provides a logical explanation for the observed up-regulation of gene expression for UCP3 in muscle when there is a switch to fatty acid oxidation, as during fasting, and in BAT when fatty acid oxidation is stimulated, as during exposure to cold. It provides a plausible physiological role for UCP3 as a transporter protein, not as an uncoupling protein.

Identification of novel peroxisome proliferator-activated receptor alpha (PPARalpha) target genes in mouse liver using cDNA microarray analysis

Peroxisome proliferators, which function as peroxisome proliferator-activated receptor-alpha (PPARalpha) agonists, are a group of structurally diverse nongenotoxic hepatocarcinogens including the fibrate class of hypolipidemic drugs that induce peroxisome proliferation in liver parenchymal cells. Sustained activation of PPARalpha by these agents leads to the development of liver tumors in rats and mice. To understand the molecular mechanisms responsible for the pleiotropic effects of these agents, we have utilized the cDNA microarray to generate a molecular portrait of gene expression in the liver of mice treated for 2 weeks with Wy-14,643, a potent peroxisome proliferator. PPARalpha activation resulted in the stimulation of expression (fourfold or greater) of 36 genes and decreased the expression (fourfold or more decrease) of 671 genes. Enhanced expression of several genes involved in lipid and glucose metabolism and many other genes associated with peroxisome biogenesis, cell surface function, transcription, cell cycle, and apoptosis has been observed. These include: CYP2B9, CYP2B10, monoglyceride lipase, pyruvate dehydrogenase-kinase-4, cell death-inducing DNA-fragmentation factor-alpha, peroxisomal biogenesis factor 11beta, as well as several cell recognition surface proteins including annexin A2, CD24, CD39, lymphocyte antigen 6, and retinoic acid early transcript-gamma, among others. Northern blotting of total RNA extracted from the livers of PPARalpha-/- mice and from mice lacking both PPARalpha and peroxisomal fatty acyl-CoA oxidase (AOX), that were fed control and Wy-14,643-containing diets for 2 weeks, as well as time course of induction following a single dose of Wy-14,643, revealed that upregulation of genes identified by microarray procedure is dependent upon peroxisome proliferation vis-à-vis PPARalpha. However, cell death-inducing DNA-fragmentation factor-alpha mRNA, which is increased in the livers of wild-type mice treated with peroxisome proliferators, was not enhanced in AOX-/- mice with spontaneous peroxisome proliferation. These observations indicate that the activation of PPARalpha leads to increased and decreased expression of many genes not associated with peroxisomes, and that delayed onset of enhanced expression of some genes may be the result of metabolic events occurring secondary to PPARalpha activation and alterations in lipid metabolism.

Regulation of arachidonic acid release in steroidogenesis: role of a new acyl-CoA thioestrase (ARTISt)

It has been well established that arachidonic acid (AA) and its metabolism to leukotrienes plays an obligatory role in steroid production. The release of AA is regulated by hormone stimulation and protein phosphorylation. We have cloned a cDNA of a phosphoprotein with a molecular mass of 43 kDa (p43), purified from the cytosol of stimulated adrenal glands. This protein acts as intermediary in the stimulation of steroid synthesis through AA release, and has been found to be a member of a recently described acyl-CoA thioesterase family. In view of the mandatory role of this protein in the activation of AA-mediated steroidogenesis, the term Arachidonic acid-Related Thioesterase Involved in Steroidogenesis (ARTISt), is proposed for p43. The present study describes the production of the recombinant protein by cDNA expression in Escherichia coli and its functional characterization. Recombinant acyl-CoA thioesterase was capable to release AA from the respective acyl-CoA, and this activity was affected by well-recognized inhibitors of AA release and metabolism: 4-bromophenacyl bromide (BPB) and nordihydroguariaretic acid (NDGA). In addition, the inhibition of acyl-CoA thioesterase activity by NDGA correlates with the inhibition of steroid synthesis produced by this compound in adrenal cortex cells. Moreover, the recombinant protein was phosphorylated in vitro by PKA. These results provide the first evidence linking acyl-CoA thioesterases with the regulation of steroidogenesis, and support a regulatory role for acyl-CoA thioesterases in steroidogenic tissues, suggesting an alternative pathway for AA release in signal transduction.