Acyl-CoA esters antagonize the effects of ligands on peroxisome proliferator-activated receptor alpha conformation, DNA binding, and interaction with Co-factors

A FRET sensor for the real-time detection of long chain acyl-CoAs and synthetic ABHD5 ligands

Intracellular long-chain acyl-coenzyme As (LC-acyl-CoAs) are thought to be under tight spatial and temporal controls, yet the ability to image LC-acyl-CoAs in live cells is lacking. Here, we developed a fluorescence resonance energy transfer (FRET) sensor for LC-acyl-CoAs based on the allosterically regulated interaction between α/β hydrolase domain-containing 5 (ABHD5) and Perilipin 5. The genetically encoded sensor rapidly detects intracellular LC-acyl-CoAs generated from exogenous and endogenous fatty acids (FAs), as well as synthetic ABHD5 ligands. Stimulation of lipolysis in brown adipocytes elevated intracellular LC-acyl-CoAs in a cyclic fashion, which was eliminated by inhibiting PNPLA2 (ATGL), the major triglyceride lipase. Interestingly, inhibition of LC-acyl-CoA transport into mitochondria elevated intracellular LC-acyl-CoAs and dampened their cycling. Together, these observations reveal an intimate feedback control between LC-acyl-CoA generation from lipolysis and utilization in mitochondria. We anticipate that this sensor will be an important tool to dissect intracellular LC-acyl-CoA dynamics as well to discover novel synthetic ABHD5 ligands.

Mechanisms Mediating the Regulation of Peroxisomal Fatty Acid Beta-Oxidation by PPARα

In mammalian cells, two cellular organelles, mitochondria and peroxisomes, share the ability to degrade fatty acid chains. Although each organelle harbors its own fatty acid β-oxidation pathway, a distinct mitochondrial system feeds the oxidative phosphorylation pathway for ATP synthesis. At the same time, the peroxisomal β-oxidation pathway participates in cellular thermogenesis. A scientific milestone in 1965 helped discover the hepatomegaly effect in rat liver by clofibrate, subsequently identified as a peroxisome proliferator in rodents and an activator of the peroxisomal fatty acid β-oxidation pathway. These peroxisome proliferators were later identified as activating ligands of Peroxisome Proliferator-Activated Receptor α (PPARα), cloned in 1990. The ligand-activated heterodimer PPARα/RXRα recognizes a DNA sequence, called PPRE (Peroxisome Proliferator Response Element), corresponding to two half-consensus hexanucleotide motifs, AGGTCA, separated by one nucleotide. Accordingly, the assembled complex containing PPRE/PPARα/RXRα/ligands/Coregulators controls the expression of the genes involved in liver peroxisomal fatty acid β-oxidation. This review mobilizes a considerable number of findings that discuss miscellaneous axes, covering the detailed expression pattern of PPARα in species and tissues, the lessons from several PPARα KO mouse models and the modulation of PPARα function by dietary micronutrients.

Physiological relevance of ACOT8-Nef interaction in HIV infection

During human immunodeficiency virus (HIV) infection, Nef viral protein plays a crucial role in viral pathogenesis and progression of acquired immunodeficiency syndrome. Nef is expressed in the early stages of infection and alters the cellular environment increasing infectivity, viral replication, and the evasion of host immune response through several mechanisms. Nef has numerous functional domains that allow it to interact with a number of proteins, interfering with intracellular traffic. Among these proteins, human peroxisomal thioesterase 8, ACOT8, has been shown to be an important cellular partner of Nef. It has been suggested that this interaction may be involved in Nef-dependent endocytosis and also in the modulation of lipid composition in membrane rafts. However, the actual role of this interaction, as well as the mechanisms involved, has not yet been fully elucidated. In this review, we focused on the interplay between Nef and ACOT8 proteins, highlighting the possible physiological relevance in HIV infection.

Fatty acid activation in thermogenic adipose tissue

Channeling carbohydrates and fatty acids to thermogenic tissues, including brown and beige adipocytes, have garnered interest as an approach for the management of obesity-related metabolic disorders. Mitochondrial fatty acid oxidation (β-oxidation) is crucial for the maintenance of thermogenesis. Upon cellular fatty acid uptake or following lipolysis from triglycerides (TG), fatty acids are esterified to coenzyme A (CoA) to form active acyl-CoA molecules. This enzymatic reaction is essential for their utilization in β-oxidation and thermogenesis. The activation and deactivation of fatty acids are regulated by two sets of enzymes called acyl-CoA synthetases (ACS) and acyl-CoA thioesterases (ACOT), respectively. The expression levels of ACS and ACOT family members in thermogenic tissues will determine the substrate availability for β-oxidation, and consequently the thermogenic capacity. Although the role of the majority of ACS and ACOT family members in thermogenesis remains unclear, recent proceedings link the enzymatic activities of ACS and ACOT family members to metabolic disorders and thermogenesis. Elucidating the contributions of specific ACS and ACOT family members to trafficking of fatty acids towards thermogenesis may reveal novel targets for modulating thermogenic capacity and treating metabolic disorders.

Hepatic lipid accumulation: cause and consequence of dysregulated glucoregulatory hormones

Fatty liver can be diet, endocrine, drug, virus or genetically induced. Independent of cause, hepatic lipid accumulation promotes systemic metabolic dysfunction. By acting as peroxisome proliferator-activated receptor (PPAR) ligands, hepatic non-esterified fatty acids upregulate expression of gluconeogenic, beta-oxidative, lipogenic and ketogenic genes, promoting hyperglycemia, hyperlipidemia and ketosis. The typical hormonal environment in fatty liver disease consists of hyperinsulinemia, hyperglucagonemia, hypercortisolemia, growth hormone deficiency and elevated sympathetic tone. These endocrine and metabolic changes further encourage hepatic steatosis by regulating adipose tissue lipolysis, liver lipid uptake, de novo lipogenesis (DNL), beta-oxidation, ketogenesis and lipid export. Hepatic lipid accumulation may be induced by 4 separate mechanisms: (1) increased hepatic uptake of circulating fatty acids, (2) increased hepatic de novo fatty acid synthesis, (3) decreased hepatic beta-oxidation and (4) decreased hepatic lipid export. This review will discuss the hormonal regulation of each mechanism comparing multiple physiological models of hepatic lipid accumulation. Nonalcoholic fatty liver disease (NAFLD) is typified by increased hepatic lipid uptake, synthesis, oxidation and export. Chronic hepatic lipid signaling through PPARgamma results in gene expression changes that allow concurrent activity of DNL and beta-oxidation. The importance of hepatic steatosis in driving systemic metabolic dysfunction is highlighted by the common endocrine and metabolic disturbances across many conditions that result in fatty liver. Understanding the mechanisms underlying the metabolic dysfunction that develops as a consequence of hepatic lipid accumulation is critical to identifying points of intervention in this increasingly prevalent disease state.

Long-chain acyl-CoA esters in metabolism and signaling: Role of acyl-CoA binding proteins

Long-chain fatty acyl-CoA esters are key intermediates in numerous lipid metabolic pathways, and recognized as important cellular signaling molecules. The intracellular flux and regulatory properties of acyl-CoA esters have been proposed to be coordinated by acyl-CoA-binding domain containing proteins (ACBDs). The ACBDs, which comprise a highly conserved multigene family of intracellular lipid-binding proteins, are found in all eukaryotes and ubiquitously expressed in all metazoan tissues, with distinct expression patterns for individual ACBDs. The ACBDs are involved in numerous intracellular processes including fatty acid-, glycerolipid- and glycerophospholipid biosynthesis, β-oxidation, cellular differentiation and proliferation as well as in the regulation of numerous enzyme activities. Little is known about the specific roles of the ACBDs in the regulation of these processes, however, recent studies have gained further insights into their in vivo functions and provided further evidence for ACBD-specific functions in cellular signaling and lipid metabolic pathways. This review summarizes the structural and functional properties of the various ACBDs, with special emphasis on the function of ACBD1, commonly known as ACBP.

Integrated physiology and systems biology of PPARα

The Peroxisome Proliferator Activated Receptor alpha (PPARα) is a transcription factor that plays a major role in metabolic regulation. This review addresses the functional role of PPARα in intermediary metabolism and provides a detailed overview of metabolic genes targeted by PPARα, with a focus on liver. A distinction is made between the impact of PPARα on metabolism upon physiological, pharmacological, and nutritional activation. Low and high throughput gene expression analyses have allowed the creation of a comprehensive map illustrating the role of PPARα as master regulator of lipid metabolism via regulation of numerous genes. The map puts PPARα at the center of a regulatory hub impacting fatty acid uptake, fatty acid activation, intracellular fatty acid binding, mitochondrial and peroxisomal fatty acid oxidation, ketogenesis, triglyceride turnover, lipid droplet biology, gluconeogenesis, and bile synthesis/secretion. In addition, PPARα governs the expression of several secreted proteins that exert local and endocrine functions.

Proline- and acidic amino acid-rich basic leucine zipper proteins modulate peroxisome proliferator-activated receptor alpha (PPARalpha) activity

In mammals, many aspects of metabolism are under circadian control. At least in part, this regulation is achieved by core-clock or clock-controlled transcription factors whose abundance and/or activity oscillate during the day. The clock-controlled proline- and acidic amino acid-rich domain basic leucine zipper proteins D-site-binding protein, thyrotroph embryonic factor, and hepatic leukemia factor have previously been shown to participate in the circadian control of xenobiotic detoxification in liver and other peripheral organs. Here we present genetic and biochemical evidence that the three proline- and acidic amino acid-rich basic leucine zipper proteins also play a key role in circadian lipid metabolism by influencing the rhythmic expression and activity of the nuclear receptor peroxisome proliferator-activated receptor α (PPARα). Our results suggest that, in liver, D-site-binding protein, hepatic leukemia factor, and thyrotroph embryonic factor contribute to the circadian transcription of genes specifying acyl-CoA thioesterases, leading to a cyclic release of fatty acids from thioesters. In turn, the fatty acids act as ligands for PPARα, and the activated PPARα receptor then stimulates the transcription of genes encoding proteins involved in the uptake and/or metabolism of lipids, cholesterol, and glucose metabolism.

Acyl-CoA binding protein gene ablation induces pre-implantation embryonic lethality in mice

Unique among the intracellular lipid binding proteins, acyl-CoA binding protein (ACBP) exclusively binds long-chain fatty acyl-CoAs (LCFA-CoAs). To test if ACBP is an essential protein in mammals, the ACBP gene was ablated by homologous recombination in mice. While ACBP heterozygotes appeared phenotypically normal, intercrossing of the heterozygotes did not produce any live homozygous deficient (null) ACBP((-/-)) pups. Heterozygous and wild type embryos were detected at all post-implantation stages, but no homozygous ACBP-null embryos were obtained-suggesting that an embryonic lethality occurred at a pre-implantation stage of development, or that embryos never formed. While ACBP-null embryos were not detected at any blastocyst stage, ACBP-null embryos were detected at the morula (8-cell), cleavage (2-cell), and zygote (1-cell) pre-implantation stages. Two other LCFA-CoA binding proteins, sterol carrier protein-2 (SCP-2) and sterol carrier protein-x (SCP-x) were significantly upregulated at these stages. These findings demonstrate for the first time that ACBP is an essential protein required for embryonic development and its loss of function may be initially compensated by concomitant upregulation of two other LCFA-CoA binding proteins, but only at the earliest pre-implantation stages. The fact that ACBP is the first known intracellular lipid binding protein whose deletion results in embryonic lethality suggests its vital importance in mammals.

Natural ligand binding and transfer from liver fatty acid binding protein (LFABP) to membranes

Liver fatty acid-binding protein (LFABP) is distinctive among fatty acid-binding proteins because it binds more than one molecule of long-chain fatty acid and a variety of diverse ligands. Also, the transfer of fluorescent fatty acid analogues to model membranes under physiological ionic strength follows a different mechanism compared to most of the members of this family of intracellular lipid binding proteins. Tryptophan insertion mutants sensitive to ligand binding have allowed us to directly measure the binding affinity, ligand partitioning and transfer to model membranes of natural ligands. Binding of fatty acids shows a cooperative mechanism, while acyl-CoAs binding presents a hyperbolic behavior. Saturated fatty acids seem to have a stronger partition to protein vs. membranes, compared to unsaturated fatty acids. Natural ligand transfer rates are more than 200-fold higher compared to fluorescently-labeled analogues. Interestingly, oleoyl-CoA presents a markedly different transfer behavior compared to the rest of the ligands tested, probably indicating the possibility of specific targeting of ligands to different metabolic fates.

n-3 PUFA as regulators of cardiac gene transcription: a new link between PPAR activation and fatty acid composition

The fatty acids regulate gene expression directly binding to nuclear receptors or affecting the protein content of transcription factors. In this work, supplementing primary cultures of neonatal rat cardiomyocytes with 60 microM EPA or DHA, we demonstrated by an ELISA assay an increased PPAR beta/delta binding to DNA. n-3 PUFA supplementation deeply changed the acyl composition of both cytosolic and nuclear fractions. The high content of total fatty acids, particularly EPA and DHA, and its increase following supplementation suggested a selective accumulation of n-3 PUFAs in the nucleus, supporting the direct interaction of n-3 PUFA with PPAR. The activity of acyl-CoA thioesterase (ACOT), catalyzing the reaction leading to NEFA from acyl-CoA, increased in n-3 PUFA supplemented cells. The NEFA/acyl-CoA ratio is an important regulator of the fatty acid transport to the nucleus and consequent modulation of gene transcription, and although ACOT activity is not the only parameter of this ratio, it is important for the control of the NEFA pool composition. Our data further clarify what happens in cardiomyocytes following n-3 PUFA supplementation, linking the modification of acyl composition to ACOT activity and PPAR activation.

Arabidopsis CER8 encodes LONG-CHAIN ACYL-COA SYNTHETASE 1 (LACS1) that has overlapping functions with LACS2 in plant wax and cutin synthesis

Plant cuticle is an extracellular lipid-based matrix of cutin and waxes, which covers aerial organs and protects them from many forms of environmental stress. We report here the characterization of CER8/LACS1, one of nine Arabidopsis long-chain acyl-CoA synthetases thought to activate acyl chains. Mutations in LACS1 reduced the amount of wax in all chemical classes on the stem and leaf, except in the very long-chain fatty acid (VLCFA) class wherein acids longer than 24 carbons (C(24)) were elevated more than 155%. The C(16) cutin monomers on lacs1 were reduced by 37% and 22%, whereas the C(18) monomers were increased by 28% and 20% on stem and leaf, respectively. Amounts of wax and cutin on a lacs1-1 lacs2-3 double mutant were much lower than on either parent, and lacs1-1 lacs2-3 had much higher cuticular permeability than either parent. These additive effects indicate that LACS1 and LACS2 have overlapping functions in both wax and cutin synthesis. We demonstrated that LACS1 has synthetase activity for VLCFAs C(20)-C(30), with highest activity for C(30) acids. LACS1 thus appears to function as a very long-chain acyl-CoA synthetase in wax metabolism. Since C(16) but not C(18) cutin monomers are reduced in lacs1, and C(16) acids are the next most preferred acid (behind C(30)) by LACS1 in our assays, LACS1 also appears to be important for the incorporation of C(16) monomers into cutin polyester. As such, LACS1 defines a functionally novel acyl-CoA synthetase that preferentially modifies both VLCFAs for wax synthesis and long-chain (C(16)) fatty acids for cutin synthesis.

L-FABP directly interacts with PPARalpha in cultured primary hepatocytes

Although studies with liver type fatty acid binding protein (L-FABP) gene ablated mice demonstrate a physiological role for L-FABP in hepatic fatty acid metabolism, little is known about the mechanisms whereby L-FABP elicits these effects. Studies indicate that L-FABP may function to shuttle lipids to the nucleus, thereby increasing the availability of ligands of nuclear receptors, such as peroxisome proliferator-activated receptor-alpha (PPARalpha). The data herein suggest that such mechanisms involve direct interaction of L-FABP with PPARalpha. L-FABP was shown to directly interact with PPARalpha in vitro through co-immunoprecipitation (co-IP) of pure proteins, altered circular dichroic (CD) spectra, and altered fluorescence spectra. In vitro fluorescence resonance energy transfer (FRET) between Cy3-labeled PPARalpha and Cy5-labeled L-FABP proteins showed that these proteins bound with high affinity (Kd approximately 156 nM) and in close proximity (intermolecular distance of 52A). This interaction was further substantiated by co-IP of both proteins from liver homogenates of wild-type mice. Moreover, double immunogold electron microscopy and FRET confocal microscopy of cultured primary hepatocytes showed that L-FABP was in close proximity to PPARalpha (intermolecular distance 40-49A) in vivo. Taken together, these studies were consistent with L-FABP regulating PPARalpha transcriptional activity in hepatocytes through direct interaction with PPARalpha. Our in vitro and imaging experiments demonstrate high affinity, structural molecular interaction of L-FABP with PPARalpha and suggest a functional role for L-FABP interaction with PPARalpha in long chain fatty acid (LCFA) metabolism.

Liver fatty acid binding protein gene ablation enhances age-dependent weight gain in male mice

Although studies performed in vitro and with transfected cells in culture suggest a role for liver fatty acid binding protein (L-FABP) in regulating fatty acid oxidation and fat deposition, the physiological significance of this possibility is not completely clear. To begin to address this question, the effect of L-FABP gene ablation on phenotype of standard rodent chow-fed male mice was examined with increasing age up to 18 months. While young (2-3 months old) L-FABP null mice displayed no visually obvious phenotype, with increasing age >9 months the L-FABP null mice were visibly larger, exhibiting increased body weight due to increased fat and lean tissue mass. Liver lipid concentrations were unaffected by L-FABP gene ablation with the exception of triacylglycerol, which was decreased by 74% in the livers of 3-month-old mice. Likewise, serum lipid levels were not altered in L-FABP null mice with the exception of triacylglycerol, which was increased in the serum of 18-month-old mice. Increased body weight, fat tissue mass, and lean tissue mass in 18-month-old L-FABP null mice were accompanied by increased hepatic levels of low-density lipoprotein (LDL) receptor, peroxisome proliferator-activated receptor (PPAR) alpha, and PPARalpha-regulated proteins such as fatty acid transport protein (FATP), fatty acid translocase (FAT/CD36), carnitine palmitoyl transferase I (CPT I), and lipoprotein lipase (LPL). A key enzyme in cholesterol biosynthesis, 3-hydroxy-3-methylglutaryl Coenzyme A (HMG-CoA) reductase, was down-regulated in L-FABP null mice. These findings were consistent with a proposed role for L-FABP as an important physiological regulator of PPARalpha.

Glucose directly links to lipid metabolism through high affinity interaction with peroxisome proliferator-activated receptor alpha

The pathophysiology of diabetes is characterized not only by elevated glucose but also elevated long chain fatty acid levels. We show for the first time that the peroxisome proliferator-activated receptor-alpha (PPARalpha) binds glucose and glucose metabolites with high affinity, resulting in significantly altered PPARalpha secondary structure. Glucose decreased PPARalpha interaction with fatty acid metabolites and steroid receptor coactivator-1 while increasing PPARalpha interaction with DNA. Concomitantly, glucose increased PPARalpha interaction with steroid receptor coactivator-1, DNA binding, and transactivation of beta-oxidation pathways in the presence of activating ligands. Heterodimerization of PPARalpha to the retinoid X receptor-alpha resulted in even larger increases in transactivation with the addition of glucose. These data suggest that PPARalpha is responsible for maintaining energy homeostasis through a concentration-dependent regulation of both lipids and sugars and that hyperglycemic injury mediated by PPARalpha occurs not only indirectly through elevated long chain fatty acid levels but also through direct action of glucose on PPARalpha.

Role of fatty acid binding proteins and long chain fatty acids in modulating nuclear receptors and gene transcription

Abnormal energy regulation may significantly contribute to the pathogenesis of obesity, diabetes mellitus, cardiovascular disease, and cancer. For rapid control of energy homeostasis, allosteric and posttranslational events activate or alter activity of key metabolic enzymes. For longer impact, transcriptional regulation is more effective, especially in response to nutrients such as long chain fatty acids (LCFA). Recent advances provide insights into how poorly water-soluble lipid nutrients [LCFA; retinoic acid (RA)] and their metabolites (long chain fatty acyl Coenzyme A, LCFA-CoA) reach nuclei, bind their cognate ligand-activated receptors, and regulate transcription for signaling lipid and glucose catabolism or storage: (i) while serum and cytoplasmic LCFA levels are in the 200 mircroM-mM range, real-time imaging recently revealed that LCFA and LCFA-CoA are also located within nuclei (nM range); (ii) sensitive fluorescence binding assays show that LCFA-activated nuclear receptors [peroxisome proliferator-activated receptor-alpha (PPARalpha) and hepatocyte nuclear factor 4alpha (HNF4alpha)] exhibit high affinity (low nM KdS) for LCFA (PPARalpha) and/or LCFA-CoA (PPARalpha, HNF4alpha)-in the same range as nuclear levels of these ligands; (iii) live and fixed cell immunolabeling and imaging revealed that some cytoplasmic lipid binding proteins [liver fatty acid binding protein (L-FABP), acyl CoA binding protein (ACBP), cellular retinoic acid binding protein-2 (CRABP-2)] enter nuclei, bind nuclear receptors (PPARalpha, HNF4alpha, CRABP-2), and activate transcription of genes in fatty acid and glucose metabolism; and (iv) studies with gene ablated mice provided physiological relevance of LCFA and LCFA-CoA binding proteins in nuclear signaling. This led to the hypothesis that cytoplasmic lipid binding proteins transfer and channel lipidic ligands into nuclei for initiating nuclear receptor transcriptional activity to provide new lipid nutrient signaling pathways that affect lipid and glucose catabolism and storage.

Peroxisome proliferator-activated receptor subtype- and cell-type-specific activation of genomic target genes upon adenoviral transgene delivery

Investigations of the molecular events involved in activation of genomic target genes by peroxisome proliferator-activated receptors (PPARs) have been hampered by the inability to establish a clean on/off state of the receptor in living cells. Here we show that the combination of adenoviral delivery and chromatin immunoprecipitation (ChIP) is ideal for dissecting these mechanisms. Adenoviral delivery of PPARs leads to a rapid and synchronous expression of the PPAR subtypes, establishment of transcriptional active complexes at genomic loci, and immediate activation of even silent target genes. We demonstrate that PPARgamma2 possesses considerable ligand-dependent as well as independent transactivation potential and that agonists increase the occupancy of PPARgamma2/retinoid X receptor at PPAR response elements. Intriguingly, by direct comparison of the PPARs (alpha, gamma, and beta/delta), we show that the subtypes have very different abilities to gain access to target sites and that in general the genomic occupancy correlates with the ability to activate the corresponding target gene. In addition, the specificity and potency of activation by PPAR subtypes are highly dependent on the cell type. Thus, PPAR subtype-specific activation of genomic target genes involves an intricate interplay between the properties of the subtype- and cell-type-specific settings at the individual target loci.

Stability of fatty acyl-coenzyme A thioester ligands of hepatocyte nuclear factor-4alpha and peroxisome proliferator-activated receptor-alpha

Although long-chain fatty acyl-coenzyme A (LCFA-CoA) thioesters are specific high-affinity ligands for hepatocyte nuclear factor-4alpha (HNF-4alpha) and peroxisome proliferator-activated receptor-alpha (PPARalpha), X-ray crystals of the respective purified recombinant ligand-binding domains (LBD) do not contain LCFA-CoA, but instead exhibit bound LCFA or have lost all ligands during the purification process, respectively. As shown herein: (i) The acyl chain composition of LCFA bound to recombinant HNF-4alpha reflected that of the bacterial LCFA-CoA pool, rather than the bacterial LCFA pool. (ii) Bacteria used to produce the respective HNF-4alpha and PPARalpha contained nearly 100-fold less LCFA-CoA than LCFA. (iii) Under conditions used to crystallize LBD (at least 3 wk at room temperature in aqueous buffer), 16:1-CoA was very unstable in buffer alone. (iv) In the presence of the respective nuclear receptor (i.e., HNF-4alpha and PPARalpha), LBD 70-75% of 16:1-CoA was degraded after 1 d at room temperature in the crystallization buffer, whereas as much as 94-97% of 16:1-CoA was degraded by 3 wk. (v) Cytoplasmic LCFA-CoA binding proteins such as acyl-CoA binding protein, sterol carrier protein-2, and liver-FA binding protein slowed the process of 16:1-CoA degradation proportional to their respective affinities for this ligand. Taken together, these data for the first time indicated that the absence of LCFA-CoA in the crystallized HNF-4alpha and PPARalpha was due to the paucity of LCFA-CoA in bacteria as well as to the instability of LCFA-CoA in aqueous buffers and the conditions used for LBD crystallization. Furthermore, instead of protecting bound LCFA-CoA from autohydrolysis like several cytoplasmic LCFA-CoA binding proteins, these nuclear receptors facilitated LCFA-CoA degradation.

Peroxisome proliferator-activated receptor alpha interacts with high affinity and is conformationally responsive to endogenous ligands

Although the peroxisome proliferator-activated receptor (PPAR alpha) binds and is activated by a variety of synthetic xenobiotics, the identity of the high affinity endogenous ligand(s) is incompletely resolved. Likewise, it is not known how putative endogenous ligands alter PPAR alpha conformation in order to affect transcriptional regulation. Direct fluorescence binding and fluorescence displacement assays showed for the first time that PPAR alpha exhibits high affinity (1-14 nM K(d) values) for unsaturated long chain fatty acyl-CoAs as well as unsaturated long chain fatty acids commonly found in mammalian cells. Fluorescence resonance energy transfer between PPAR alpha aromatic amino acids and bound corresponding naturally occurring fluorescent ligands (i.e. cis-parinaroyl-CoA, trans-parinaric acid) yielded intermolecular distances of 25-29 angstroms, confirming close molecular interaction. Interestingly, although PPAR alpha also exhibited high affinity for saturated long chain fatty acyl-CoAs, regardless of chain length (1-13 nM K(d) values), saturated long chain fatty acids were not significantly bound. In contrast to the similar affinities of PPAR alpha for fatty acyl-CoAs and unsaturated fatty acids, CoA thioesters of peroxisome proliferator drugs were bound with 5-6-fold higher affinities than their free acid forms. Circular dichroism demonstrated that high affinity ligands (long chain fatty acyl-CoAs, unsaturated fatty acids), but not weak affinity ligands (saturated fatty acids), elicited conformational changes in PPAR alpha structure, a hallmark of ligand-activated nuclear receptors. Finally, these ligand specificities and induced conformational changes correlated functionally with co-activator binding. In summary, since nuclear concentrations of these ligands are in the nanomolar range, long chain fatty acyl-CoAs and unsaturated fatty acids may both represent endogenous PPAR alpha ligands. Furthermore, the finding that saturated fatty acyl-CoAs, rather than saturated fatty acids, are high affinity PPAR alpha ligands provides a mechanism accounting for saturated fatty acid transactivation in cell-based assays.

Identification of fatty acid oxidation disorder patients with lowered acyl-CoA thioesterase activity in human skin fibroblasts

BACKGROUND

Acyl-CoA thioesterases are enzymes that hydrolyze acyl-CoAs to the free fatty acid and coenzyme A (CoASH). These enzymes have been identified in several cellular compartments and are thought to regulate intracellular levels of acyl-CoAs, free fatty acids and CoASH. However, to date no patients deficient in acyl-CoA thioesterases have been identified.

DESIGN

Acyl-CoA thioesterase activity was measured in human skin fibroblasts. Western-blot analysis was used to determine Type-II acyl-CoA thioesterase protein levels in patients.

RESULTS

Acyl-CoA thioesterase activity was found in human fibroblasts with all saturated acyl-CoAs from C4-CoA to C18-CoA, with highest activity detected with lauroyl-CoA and myristoyl-CoA (C12-CoA and C14-CoA). An antibody that recognizes the major isoforms of Type-II acyl-CoA thioesterases precipitated the majority of acyl-CoA thioesterase activity in fibroblasts, showing that the main thioesterase activity detected in fibroblasts is catalyzed by Type-II thioesterases. Measurement of acyl-CoA thioesterase activity from fibroblasts of 34 patients with putative fatty acid oxidation disorders resulted in the identification of three patients with lowered Type-II acyl-CoA thioesterase activity in fibroblasts. These patients also had lowered expression of Type-II acyl-CoA thioesterase protein in fibroblasts as judged by Western-blot analysis. However, mutation analysis failed to identify any mutation in the coding sequences for the mitochondrial acyl-CoA thioesterase II (MTE-II) or the cytosolic acyl-CoA thioesterase II (CTE-II).

CONCLUSIONS

We have described three patients with lowered Type-II acyl-CoA thioesterase protein and activity in human skin fibroblasts, which is the first description of patients with a putative defect in acyl-CoA thioesterases.

Liver fatty acid-binding protein colocalizes with peroxisome proliferator activated receptor alpha and enhances ligand distribution to nuclei of living cells

Although it is hypothesized that long-chain fatty acyl CoAs (LCFA-CoAs) and long-chain fatty acids (LCFAs) regulate transcription in the nucleus, little is known regarding factors that determine the distribution of these ligands to nuclei of living cells. Immunofluorescence colocalization showed that liver fatty acid-binding protein (L-FABP; binds LCFA-CoA as well as LCFA) significantly colocalized with PPARalpha in nuclei of transfected L-cell fibroblasts. Colocalization with a DNA binding dye (SYTO59) revealed that, within the nucleus of control L-cells, the nonhydrolyzable fluorescent LCFA-CoA (BODIPY-C16-S-S-CoA) was distributed primarily in a punctate pattern throughout the nucleoplasm, while nonmetabolizable fluorescent LCFAs (BODIPY-C16 and BODIPY-C12) were localized primarily near the nuclear envelope membranes. L-FABP overexpression selectively increased the targeting of BODIPY-C16-S-S-CoA by 1.9- and 2.7-fold into the nuclear membrane and nucleoplasm, respectively. L-FABP also increased the targeting of fluorescent LCFAs (especially long-chain-length BODIPY-C16) by 1.7-fold to the nuclear membrane and 7.4-fold into the nucleoplasm. A cis-parinaric acid displacement assay showed that L-FABP bound BODIPY-C12 and BODIPY-C16 with K(i)s of 10.1 +/- 2.5 and 20.7 +/- 1.5 nM, respectively, in the same range as naturally occurring LCFAs. Finally, solid-phase extraction and HPLC analysis revealed that, depending on the fatty acid content of the culture medium, L-FABP expression also increased the cellular LCFA-CoA pool size and altered the LCFA-CoA acyl chain composition. Thus, L-FABP may function as a carrier for selectively enhancing the distribution of LCFA-CoA, as well as LCFA, to nuclei for potential interaction with nuclear receptors.

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.

The role of corepressors in transcriptional regulation by nuclear hormone receptors

Nuclear receptors (also known as nuclear hormone receptors) are hormone-regulated transcription factors that control many important physiological and developmental processes in animals and humans. Defects in receptor function result in disease. The diverse biological roles of these receptors reflect their surprisingly versatile transcriptional properties, with many receptors possessing the ability to both repress and activate target gene expression. These bipolar transcriptional properties are mediated through the interactions of the receptors with two distinct classes of auxiliary proteins: corepressors and coactivators. This review focuses on how corepressors work together with nuclear receptors to repress gene transcription in the normal organism and on the aberrations in this process that lead to neoplasia and endocrine disorders. The actions of coactivators and the contributions of the same corepressors to the functions of nonreceptor transcription factors are also touched on.

Physical and Functional Interaction of Acyl-CoA-binding Protein with Hepatocyte Nuclear Factor-4α

Although acyl-CoA-binding protein (ACBP) has been detected in the nucleus, the physiological significance of this observation is unknown. As shown herein for the first time, ACBP in the nucleus physically and functionally interacted with hepatocyte nuclear factor-4 alpha (HNF-4 alpha), a nuclear binding protein that regulates transcription of genes involved in both lipid and glucose metabolism. Five lines of evidence showed that ACBP bound HNF-4 alpha in vitro and in the nucleus of intact cells. (i) ACBP interaction with HNF-4 alpha elicited significant changes in secondary structure. (ii) ACBP and HNF-4 alpha were coimmunoprecipitated by antibodies to each protein. (iii) Double immunolabeling and laser scanning confocal microscopy (LSCM) of rat hepatoma cells and transfected COS-7 cells significantly colocalized ACBP and HNF-4 alpha within the nucleus and in the perinuclear region close to the nuclear membrane. (iv) LSCM fluorescence resonance energy transfer determined an intermolecular distance of 53 A between ACBP and HNF-4 alpha in rat hepatoma cell nuclei. (v) Immunogold electron microscopy detected ACBP within 43 A of HNF-4 alpha. These interactions were specific since ACBP did not interact with Sp1 or glucocorticoid receptor in these assays. The functional significance of ACBP interaction with HNF-4 alpha was evidenced by mammalian two-hybrid and transactivation assays. ACBP overexpression in COS-7 or rat hepatoma cells enhanced transactivation of an HNF-4 alpha-dependent luciferase reporter plasmid by 3.2- and 1.6-fold, respectively. In contrast, cotransfection with antisense ACBP expression vector inhibited transactivation. LSCM of the individual triple fluorescent-labeled (HNF-4 alpha, ACBP, and luciferase) rat hepatoma cells showed a high correlation (r2, 0.936) between the level of luciferase and the level of ACBP expression. In summary, ACBP physically interacted with HNF-4 alpha in vitro and in intact cells, although ACBP expression level directly correlated with HNF-4 alpha-mediated transactivation in individual cells.

Ablation of the liver fatty acid binding protein gene decreases fatty acyl CoA binding capacity and alters fatty acyl CoA pool distribution in mouse liver

Although liver fatty acid binding protein (L-FABP) is known to bind not only long chain fatty acid (LCFA) but also long chain fatty acyl CoA (LCFA-CoA), the physiological significance of LCFA-CoA binding has been questioned and remains to be resolved. To address this issue, the effect of L-FABP gene ablation on liver cytosolic LCFA-CoA binding, LCFA-CoA pool size, LCFA-CoA esterification, and potential compensation by other intracellular LCFA-CoA binding proteins was examined. L-FABP gene ablation resulted not only in loss of L-FABP but also in concomitant upregulation of two other intracellular LCFA-CoA binding proteins, acyl CoA binding protein (ACBP) and sterol carrier protein-2 (SCP-2), by 45 and 80%, respectively. Nevertheless, the soluble fraction from livers of L-FABP (-/-) mice bound 95% less radioactive oleoyl-CoA than wild-type L-FABP (+/+) mice. The intracellular LCFA-CoA binding protein fraction (Fraction III) from wild-type L-FABP (+/+) mice, isolated by gel permeation chromatography of liver soluble proteins, exhibited one high-affinity binding and several low-affinity binding sites for cis-parinaroyl-CoA, a naturally occurring fluorescent LCFA-CoA. In contrast, high-affinity LCFA-CoA binding was absent from Fraction III of L-FABP (-/-) mice. While L-FABP gene ablation did not alter liver LCFA-CoA pool size, LCFA-CoA acyl chains of L-FABP (-/-) mouse livers were enriched 2.1-fold in C16:1 and decreased 1.9-fold in C20:0 fatty acids. Finally, L-FABP gene ablation selectively increased the amount of LCFAs esterified into liver phospholipid > cholesteryl ester, while concomitantly decreasing the amount of fatty acids esterified into triglycerides by 40%. In summary, these data with L-FABP (-/-) mice demonstrated for the first time that L-FABP is a physiologically significant contributor to determining liver cytosolic LCFA-CoA binding capacity, LCFA-CoA acyl chain distribution, and esterified fatty acid distribution.

Peroxisome proliferator-activated receptors (PPARS): regulators of gene expression in heart and skeletal muscle

The peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily. The three isoforms (PPARalpha, beta/delta and gamma) have been implicated in the regulation of the expression of genes involved in lipid metabolism. Although their prominent role in lipid homeostasis is well established, the way in which the activity of each of the PPAR isoforms is regulated under physiological and pathological conditions is still subject of intensive research. In skeletal as well as cardiac muscle cells it has been demonstrated that the expression of a large panel of proteins involved in the transport and metabolic conversion of fatty acids is under control of PPARs. The pivotal role of the PPARalpha isoform in cardiac fatty acid metabolism has been confirmed in PPARalpha-null mice. The exact role of PPARbeta/delta in the regulation of muscle metabolism is still a matter of debate. Whereas several studies provided evidence to support the notion that PPARalpha and PPARbeta/delta have redundant roles, other studies suggest that PPARalpha activity is counteracted by PPARbeta/delta. Marked effects of bona fide PPARgamma ligands (the anti-diabetic thiazolidinediones) on skeletal and cardiac muscle function and phenotype, have also been reported. However, next to activating PPARgamma, the thiazolidinediones do affect other cellular processes as well. To date it is being realized that the control of the trans-activating capacity of each of the PPAR isoforms is multi-factorial and, in addition to ligand availability, depends on such factors as isoform-specific phosphorylation and selective interaction with various proteins acting either as co-activator or co-repressor.

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.

The role of transcription factors in allergic inflammation

The induction of allergic inflammation and the expression of allergic disorders are dependent on the coordinated regulation of numerous genes. The products of these genes determine lymphocyte phenotype, immunologic responsiveness, eosinophil and mast cell development, activation, migration and life span, adhesion molecule expression, cytokine synthesis, cell-surface receptor display, and processes governing fibrosis and tissue repair. Although the expression of gene products involved in these processes is regulated at multiple levels (eg, transcription, mRNA processing, translation, phosphorylation, and degradation), transcription represents an essential and often the most important determinant of their contribution to cellular function. Signal-dependent and cell type-specific regulation of gene expression is generally achieved by means of combinatorial interactions between sequence-specific transcription factors that recruit chromatin remodeling machinery and general transcription factors to promoter and enhancer regions of RNA polymerase II-dependent genes. As targets of signal-transduction pathways, transcription factors integrate the response of the cell to the myriad of inputs it receives. This integration can be accomplished by the effect of signaling cascades on the activation status or subcellular locus of transcription factors or by transcription factor dimerization induced by means of ligand binding. This review will identify the major families of transcription factors important in allergic mechanisms and discuss their interactions, their mechanisms of action, and their interrelated and competitive actions, as well as implications for therapy of allergic disorders.

The gene encoding the Acyl-CoA-binding protein is activated by peroxisome proliferator-activated receptor gamma through an intronic response element functionally conserved between humans and rodents

The acyl-CoA-binding protein (ACBP) is a 10-kDa intracellular protein that specifically binds acyl-CoA esters with high affinity and is structurally and functionally conserved from yeast to mammals. In vitro studies indicate that ACBP may regulate the availability of acyl-CoA esters for various metabolic and regulatory purposes. The protein is particularly abundant in cells with a high level of lipogenesis and de novo fatty acid synthesis and is significantly induced during adipocyte differentiation. However, the molecular mechanisms underlying the regulation of ACBP expression in mammalian cells have remained largely unknown. Here we report that ACBP is a novel peroxisome proliferator-activated receptor (PPAR)gamma target gene. The rat ACBP gene is directly activated by PPARgamma/retinoid X receptor alpha (RXRalpha) and PPARalpha/RXRalpha, but not by PPARdelta/RXRalpha, through a PPAR-response element in intron 1, which is functionally conserved in the human ACBP gene. The intronic PPAR-response element (PPRE) mediates induction by endogenous PPARgamma in murine adipocytes and confers responsiveness to the PPARgamma-selective ligand BRL49653. Finally, we have used chromatin immunoprecipitation to demonstrate that the intronic PPRE efficiently binds PPARgamma/RXR in its natural chromatin context in adipocytes. Thus, the PPRE in intron 1 of the ACBP gene is a bona fide PPARgamma-response element.

Modulation by nutrients and drugs of liver acyl-CoAs analyzed by mass spectrometry

The profile of liver acyl-CoAs induced by dietary fats of variable compositions or by xenobiotic hypolipidemic amphipathic carboxylates was evaluated in vivo using a novel electrospray ionization tandem mass spectrometry methodology of high resolution, sensitivity, and reliability. The composition of liver fatty acyl-CoAs was found to reflect the composition of dietary fat. Treatment with hypolipidemic carboxylates resulted in liver dominant abundance of their respective acyl-CoAs accompanied by an increase in liver fatty acyl-CoAs. Cellular effects exerted by dietary fatty acids and/or xenobiotic carboxylic drugs may be transduced in vivo by their respective acyl-CoAs.

Opposing effects of fatty acids and acyl-CoA esters on conformation and cofactor recruitment of peroxisome proliferator-activated receptors

The peroxisome proliferator-activated receptors (PPARs) bind and are activated by a variety of fatty acids and derivatives thereof. Agonist binding enhances PPAR-mediated transactivation via release of corepressors and recruitment of coactivator complexes. Recently, we and others have reported that acyl-CoA esters act as PPAR antagonists in vitro. Here, we show that the binding of the nonhydrolyzable acyl-CoA analogue, S-hexadecyl-CoA, differentially affected conformation and coactivator recruitment of the individual PPAR subtypes. In protease protection assays, S-hexadecyl CoA increased the sensitivity of PPARalpha and PPARdelta towards chymotrypsin, whereas the action of chymotrypsin on PPARgamma was only marginally affected, suggesting distinct subtype-dependent differences in the effects of S-hexadecyl-CoA on conformation of the PPARs. In keeping with these findings, S-hexadecyl-CoA abrogated ligand-induced recruitment of coactivators to PPARalpha and PPARdelta, whereas coactivator recruitment to PPARgamma was unaffected by S-hexadecyl-CoA.

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.