Ligand specificity and conformational stability of human fatty acid-binding proteins

Fatty acid transport into the brain: of fatty acid fables and lipid tails

The blood-brain barrier formed by the brain capillary endothelial cells provides a protective barrier between the systemic blood and the extracellular environment of the central nervous system. Brain capillaries are a continuous layer of endothelial cells with highly developed tight junctional complexes and a lack of fenestrations. The presence of these tight junctions in the cerebral microvessel endothelial cells aids in the restriction of movement of molecules and solutes into the brain. Fatty acids are important components of biological membranes, are precursors for the biosynthesis of phospholipids and sphingolipids and are utilized for mitochondrial β-oxidation. The brain is capable of synthesizing only a few fatty acids. Hence, most fatty acids must enter into the brain from the blood. Here we review current mechanisms of transport of free fatty acids into cells and describe how free fatty acids move from the blood into the brain. We discuss both diffusional as well as protein-mediated movement of fatty acids across biological membranes.

The human fatty acid-binding protein family: evolutionary divergences and functions

Fatty acid-binding proteins (FABPs) are members of the intracellular lipid-binding protein (iLBP) family and are involved in reversibly binding intracellular hydrophobic ligands and trafficking them throughout cellular compartments, including the peroxisomes, mitochondria, endoplasmic reticulum and nucleus. FABPs are small, structurally conserved cytosolic proteins consisting of a water-filled, interior-binding pocket surrounded by ten anti-parallel beta sheets, forming a beta barrel. At the superior surface, two alpha-helices cap the pocket and are thought to regulate binding. FABPs have broad specificity, including the ability to bind long-chain (C16-C20) fatty acids, eicosanoids, bile salts and peroxisome proliferators. FABPs demonstrate strong evolutionary conservation and are present in a spectrum of species including Drosophila melanogaster, Caenorhabditis elegans, mouse and human. The human genome consists of nine putatively functional protein-coding FABP genes. The most recently identified family member, FABP12, has been less studied.

Overview of lipid peroxidation products and hepatic protein modification in alcoholic liver disease

OBJECTIVES

Oxidative stress is one component of alcoholic liver disease (ALD) that is manifested in the peroxidation of cellular lipids producing the electrophile, 4-hydroxynonenal (4-HNE). This electrophile is proposed to modify essential cellular proteins resulting in loss of protein function and cellular homeostasis. Studies were initiated to identify hepatic proteins that are targets of 4-HNE modification and determine their relationship with progression of the early stages of ALD.

METHODS

Rat and mouse models were developed using the Lieber-DeCarli diet to simulate early stages of ALD consisting of fatty liver (steatosis) and hepatocellular injury indicated by a 1.5-2-fold elevation of plasma ALT activity. Liver samples obtained from control and ethanol treated animals were subjected to two-dimensional electrophoresis and immunoblotting using polyclonal antibodies generated against 4-HNE epitopes for detection of proteins modified by 4-HNE. Following identification of 4-HNE adducted proteins, the respective recombinant proteins modified with physiologic concentrations of 4-HNE were evaluated to determine the functional consequences of 4-HNE modification.

RESULTS

One group of proteins identified included Hsp70, Hsp90 and protein disulfide isomerase (PDI), all of which are involved in protein folding or processing are targets of adduction. In vitro assays indicated significant impairment of the protein activities following modification with physiologically relevant concentrations of 4-HNE. Liver fatty acid binding protein, L-FABP, was also identified as a target and additional studies revealed that the levels of this protein were significantly decreased because of chronic ethanol ingestion. Erk1/2 was identified as a target for modification and subsequently determined to have impaired activity.

CONCLUSIONS

Inhibition of Hsp70, Hsp90 and PDI function could be involved in initiation of the early phases of ER stress contributing to stimulation and accumulation of hepatic lipids. Likewise, impairment of L-FABP activity could also disrupt lipid transport also contributing to steatosis. The modification and inhibition of Erk1/2 by 4-HNE may also contribute to the decreased hepatocellular proliferation associated with ALD. Collectively, these results provide new information concerning the mechanisms whereby the modification of hepatic proteins by 4-HNE contributes to ALD.

Identification of differential hepatic proteins in rare minnow (Gobiocypris rarus) exposed to pentachlorophenol (PCP) by proteomic analysis

Pentachlorophenol (PCP) is a ubiquitous contaminant that has been shown to lead to hepatoxicity and is implicated in the incidence of liver tumors in human. A number of previous studies have described the toxic effects of PCP based on conventional toxicological indices. However, little evidence on protein levels is available at present. For further understanding of mechanisms of action and identifying the potential protein biomarkers for PCP exposure, two-dimensional electrophoresis coupled with mass spectrometry has been used to identify proteins differentially expressed in the livers of rare minnow (Gobiocypris rarus) following PCP exposure of 0.5, 5, 50 μg/L. After comparison of the protein profiles from treated and control groups, 39 protein spots were found altered in abundance (>2-fold) from male and female PCP-treated groups. Matrix-assisted laser desorption/ionization (MALDI) tandem time-of-flight mass spectrometry (TOF/MS) analysis allowed the unambiguous identification, and 18 protein spots were identified successfully, 12 proteins in females and 6 proteins in males, respectively. These proteins were involved in transport, metabolism, response to oxidative stress and other biological processes. Of these proteins, four differentially expressed mRNA encoding proteins underwent quantitative analysis by quantitative real-time PCR (QRT-PCR). The consistent and discrepant results between mRNA and protein levels suggested that complicated regulatory mechanisms of gene expression were implicated in the response to PCP exposure. In addition, marked gender differences in response to PCP have been described from the comparison of the male and female liver protein profiles.

Experimental characterization of the mechanism of perfluorocarboxylic acids' liver protein bioaccumulation: the key role of the neutral species

Perfluorocarboxylic acids (PFCAs) of chain length greater than seven carbon atoms bioconcentrate in the livers of fish. However, a mechanistic cause for the empirically observed increase in the bioconcentration potential of PFCAs as a function of chain length has yet to be determined. To this end, recombinant rat liver fatty acid-binding protein (L-FABP) was purified, and its interaction with PFCAs was characterized in an aqueous system at pH 7.4. Relative binding affinities of L-FABP with PFCAs of carbon chain lengths of five to nine were established fluorimetrically. The energetics, mechanism, and stoichiometry of the interaction of perfluorooctanoic acid (PFOA) with L-FABP were examined further by isothermal titration calorimetry (ITC) and electrospray ionization combined with tandem mass spectrometry (ESI-MS/MS). Perfluorooctanoic acid was shown to bind to L-FABP with an affinity approximately an order of magnitude less than the natural ligand, oleic acid, and to have at least 3:1 PFOA:L-FABP stoichiometry. Two distinct modes of PFOA binding to L-FABP were observed by ESI-MS/MS analysis; in both cases, PFOA binds solely as the neutral species under typical physiological pH and aqueous concentrations of the anion. A comparison of their chemical and physical properties with other well-studied biologically relevant chemicals showed that accumulation of PFCAs in proteins as the neutral species is predictable. For example, the interaction of PFOA with L-FABP is almost identical to that of the acidic ionizing drugs ketolac, ibuprofen, and warfarin that show specificity to protein partitioning with a magnitude that is proportional to the K(OW) (octanol-water partitioning) of the neutral species. The experimental results suggest that routine pharmacochemical models may be applicable to predicting the protein-based bioaccumulation of long-chain PFCAs.

Genomic organization of Atlantic salmon (Salmo salar) fatty acid binding protein (fabp2) genes reveals independent loss of duplicate loci in teleosts

Gene and genome duplications are considered to be driving forces of evolution. The relatively recent genome duplication in the common ancestor of salmonids makes this group of fish an excellent system for studying the re-diploidization process and the fates of duplicate genes. We characterized the structure and genome organization of the intestinal fatty acid binding protein (fabp2) genes in Atlantic salmon as a means of understanding the evolutionary fates of members of this protein family in teleosts. A survey of EST databases identified three unique salmonid fabp2 transcripts (fabp2aI, fabp2aII and fabp2b) compared to one transcript in zebrafish. We screened the CHORI-214 Atlantic salmon BAC library and identified BACs containing each of the three fabp2 genes. Physical mapping, genetic mapping and fluorescence in situ hybridization of Atlantic salmon chromosomes revealed that Atlantic salmon fabp2aI, fabp2aII and fabp2b correspond to separate genetic loci that reside on different chromosomes. Comparative genomic analyses indicated that these genes are related to one another by two genome duplications and a gene loss. The first genome duplication occurred in the common ancestor of all teleosts, giving rise to fabp2a and fabp2b, and the second in the common ancestor of salmonids, producing fabp2aI, fabp2aII, fabp2bI and fabp2bII. A subsequent loss of fabp2bI or fabp2bII gave the complement of fabp2 genes seen in Atlantic salmon today. There is also evidence for independent losses of fabp2b genes in zebrafish and tetraodon. Although there is no evidence for partitioning of tissue expression of fabp2 genes (i.e., sub-functionalization) in Atlantic salmon, the pattern of amino acid substitutions in Atlantic salmon and rainbow trout fabp2aI and fabp2aII suggests that neo-functionalization is occurring.

Molecular biomarkers in stroke diagnosis and prognosis

Serum biomarkers related to the cascade of inflammatory, hemostatic, glial and neuronal perturbations have been identifed to diagnose and characterize intracerebral hemorrhage and cerebral ischemia. Interpretation of most markers is confounded by their latent rise, blood-brain barrier effects, the heterogeneity of etiologies and the wide range of normal values, limiting their application for early diagnosis, lesion size estimation and long-term outcome prediction. Certain hemostatic and inflammatory constituents have been found to predict response to thrombolysis and worsening due to infarct progression and secondary hemorrhage, offering a potential role for improved treatment selection and individualization of therapy. Biomarkers will become increasingly relevant for developing targets for neuroprotective therapies, monitoring response to treatment and as surrogate end points for treatment trials.

A global metabolite profiling approach to identify protein-metabolite interactions

Understanding the biochemical functions of proteins is an important factor in elucidating their cellular and physiological functions. Due to the predominance of biopolymer interactions in biology, many methods have been designed to interrogate and identify biologically relevant interactions that proteins make to DNA, RNA, and other proteins. Complementary approaches that can elucidate binding interactions between proteins and small molecule metabolites will impact the understanding of protein-metabolite interactions and fill a need that is outside the scope of current methods. Here, we demonstrate the ability to identify natural protein-metabolite interactions from complex metabolite mixtures by combining a protein-mediated small molecule enrichment step with a global metabolite profiling platform.

Expression of E-FABP in PC12 cells increases neurite extension during differentiation: involvement of n-3 and n-6 fatty acids

Epidermal fatty acid-binding protein (E-FABP), a member of the family of FABPs, exhibits a robust expression in neurons during axonal growth in development and in nerve regeneration following nerve injury. This study examines the impact of E-FABP expression in normal neurite extension in differentiating pheochromocytoma cell (PC12) cultures supplemented with selected long chain free fatty acids (LCFFA). We found that E-FABP binds to a broad range of saturated and unsaturated LCFFAs, including those with potential interest for neuronal differentiation and axonal growth such as C22:6n-3 docosahexaenoic acid (DHA), C20:5n-3 eicosapentaenoic acid (EPA), and C20:4n-6 arachidonic acid (ARA). PC12 cells exposed to nerve growth factor (NGFDPC12) exhibit high E-FABP expression that is blocked by mitogen-activated protein kinase kinase (MEK) inhibitor U0126. Nerve growth factor-differentiated pheochromocytoma cells (NGFDPC12) antisense clones (NGFDPC12-AS) which exhibit low E-FABP expression have fewer/shorter neurites than cells transfected with vector only or NGFDPC12 sense cells (NGFDPC12-S). Replenishing NGFDPC12-AS cells with biotinylated recombinant E-FABP (biotin-E-FABP) protein restores normal neurite outgrowth. Cellular localization of biotin-E-FABP in NGFDPC12 was detected mostly in the cytoplasm and in the nuclear region. Treatment of NGFDPC12 with DHA, EPA, or ARA further enhances neurite length but it does not trigger further induction of TrkA or MEK phosphorylation or E-FABP mRNA observed in differentiating PC12 cells without LCFFA supplementation. Significantly, DHA and EPA neurite stimulating effects are higher in NGFDPC12-S than in NGFDPC12-AS cells. These findings are consistent with the scenario that neurite extension of differentiating PC12 cells, including further stimulation by DHA and EPA, requires sufficient cellular levels of E-FABP.

Capturing proteins that bind polyunsaturated fatty acids: demonstration using arachidonic acid and eicosanoids

Polyunsaturated fatty acids (PUFA) and their biological derivatives, including the eicosanoids, have numerous roles in physiology and pathology. Although some eicosanoids are known to act through receptors, the molecular actions of many PUFA remain obscure. As the three-dimensional structure of eicosanoids allows them to specifically bind and activate their receptors, we hypothesized that the same structure would allow other proteins to associate with PUFA and eicosanoids. Here, we demonstrate that biotinylation of arachidonic acid and its oxygenated derivatives 5-hydroxyeicosatetraenoic acid (5-HETE) and leukotriene (LT) B(4) can be used to pull down associated proteins. Separation of proteins by two-dimensional gel electrophoresis indicated that a large number of proteins bound each lipid and that proteins could distinguish between two enantiomers of 5-HETE. Individual proteins, identified by matrix assisted laser desorption/ionization-time of flight mass spectrometry, included proteins that are known to bind lipids, including albumin and phosphatidylethanolamine-binding protein, as well as several novel proteins. These include cytoskeletal proteins, such as actin, moesin, stathmin and coactosin-like protein, and G protein signaling proteins, such as Rho GDP dissociation inhibitor 1 and nucleoside diphosphate kinase B. This method, then, represents a relatively simple and straightforward way to screen for proteins that directly associate with, and are potentially modulated by, PUFA and their derivatives.

Protein reconstitution and three-dimensional domain swapping: benefits and constraints of covalency

The phenomena of protein reconstitution and three-dimensional domain swapping reveal that highly similar structures can be obtained whether a protein is comprised of one or more polypeptide chains. In this review, we use protein reconstitution as a lens through which to examine the range of protein tolerance to chain interruptions and the roles of the primary structure in related features of protein structure and folding, including circular permutation, natively unfolded proteins, allostery, and amyloid fibril formation. The results imply that noncovalent interactions in a protein are sufficient to specify its structure under the constraints imposed by the covalent backbone.

Loss of intestinal fatty acid binding protein increases the susceptibility of male mice to high fat diet-induced fatty liver

Mice lacking I-FABP (encoded by the Fabp2 gene) exhibit a gender dimorphic response to a high fat/cholesterol diet challenge characterized by hepatomegaly in male I-FABP-deficient mice. In this study, we determined if this gender-specific modification of liver mass in mice lacking I-FABP is attributable to the high fat content of the diet alone and whether hepatic Fabp1 gene (encodes L-FABP) expression contributes to this difference. Wild-type and Fabp2-/- mice of both genders were fed a diet enriched with either polyunsaturated or saturated fatty acids (PUFA or SFA, respectively) in the absence of cholesterol. Male Fabp2-/- mice, but not female Fabp2-/- mice, exhibited increased liver mass and hepatic triacylglycerol (TG) deposition as compared to corresponding wild-type mice. In wild-type mice that were fed the standard chow diet, there was no difference in the concentration of hepatic L-FABP protein between males and females although the loss of I-FABP did cause a slight reduction of hepatic L-FABP abundance in both genders. The hepatic L-FABP mRNA abundance in both male and female wild-type and Fabp2-/- mice was higher in the PUFA-fed group than in the SFA-fed group, and was correlated with L-FABP protein abundance. No correlation between hepatic L-FABP protein abundance and hepatic TG concentration was found. The results obtained demonstrate that loss of I-FABP renders male mice sensitive to high fat diet-induced fatty liver, and this effect is independent of hepatic L-FABP.

Modeling fatty acid delivery from intestinal fatty acid binding protein to a membrane

Intestinal fatty acid binding protein (IFABP) interacts with biological membranes and delivers fatty acid (FA) into them via a collisional mechanism. However, the membrane-bound structure of the protein and the pathway of FA transfer are not precisely known. We used molecular dynamics (MD) simulations with an implicit membrane model to determine the optimal orientation of apo- and holo-IFABP (bound with palmitate) on an anionic membrane. In this orientation, the helical portal region, delimited by the alphaII helix and the betaC-betaD and betaE-betaF turns, is oriented toward the membrane whereas the putative beta-strand portal, delimited by the betaB-betaC, betaF-betaG, betaH-betaI turns and the N terminus, is exposed to solvent. Starting from the MD structure of holo-IFABP in the optimal orientation relative to the membrane, we examined the release of palmitate via both pathways. Although the domains can widen enough to allow the passage of palmitate, fatty acid release through the helical portal region incurs smaller conformational changes and a lower energetic cost.

Evolutionary coupling of structural and functional sequence information in the intracellular lipid-binding protein family

We have mined the evolutionary record for the large family of intracellular lipid-binding proteins (iLBPs) by calculating the statistical coupling of residue variations in a multiple sequence alignment using methods developed by Ranganathan and coworkers (Lockless and Ranganathan, Science 1999:286;295-299). The 213 sequences analyzed have a wide range of ligand-binding functions as well as highly divergent phylogenetic origins, assuring broad sampling of sequence space. Emerging from this analysis were two major clusters of coupled residues, which when mapped onto the structure of a representative iLBP under study in our laboratory, cellular retinoic-acid binding protein I, are largely contiguous and provide useful points of comparison to available data for the folding of this protein. One cluster comprises a predominantly hydrophobic core away from the ligand-binding site and likely represents key structural information for the iLBP fold. The other cluster includes the portal region where ligand enters its binding site, regions of the ligand-binding cavity, and the region where the 10-stranded beta-barrel characteristic of this family closes (between strands 1' and 10). Linkages between these two clusters suggest that evolutionary pressures on this family constrain structural and functional sequence information in an interdependent fashion. The necessity of the structure to wrap around a hydrophobic ligand confounds the typical sequestration of hydrophobic side chains. Additionally, ligand entry and exit require these structures to have a capacity for specific conformational change during binding and release. We conclude that an essential and structurally apparent separation of local and global sequence information is conserved throughout the iLBP family.

FABP3 and FABP10 in Atlantic salmon (Salmo salar L.)--general effects of dietary fatty acid composition and life cycle variations

The increased use of dietary plant oil supplementation combined with high dietary lipid loads challenges the lipid transport systems of cultivated fish species. Fatty acid binding proteins (FABPs) have been thoroughly studied as intracellular fatty acid transporters in vertebrates, but no data have been reported in Atlantic salmon. In the present study, comparative characterizations were performed, and dietary influence of plant oil supplementation on FABP3 and FABP10 expression was studied for several tissues in two separate dietary trials. In trial I, groups (6 fish each) were fed diets for 42 weeks (body mass 142+/-1 to 1463+/-83 g) (mean+/-S.D.), containing graded levels of rapeseed oil substituting for fish oil using a linear regression design. In trial II, groups (3 fish each) were fed 100% fish oil or 100% plant oil for 22 months (0.160+/-0.052 to 2523+/-590 g) (mean+/-S.D.) and sampled at regular intervals. Liver and muscle tissues appeared to express several FABPs possibly linked to different metabolic functions. FABPs mRNA expression did not change with dietary inclusion of 75% rapeseed oil, whereas FABP3 protein expression seemed to be affected by dietary rapeseed oil inclusion. Significant changes in red muscle FABP3 mRNA expression correlate to significant changes in total beta-oxidation capacity during the energy consuming process of smoltification.

Nuclear magnetic resonance structure-based epitope mapping and modulation of dust mite group 13 allergen as a hypoallergen

IgE-mediated allergic response involves cross-linking of IgE bound on mast cells by specific surface epitopes of allergens. Structural studies on IgE epitopes of allergens are essential in understanding the characteristics of an allergen and for development of specific allergen immunotherapy. We have determined the structure of a group 13 dust mite allergen from Dermatophagoides farinae, Der f 13, using nuclear magnetic resonance. Sequence comparison of Der f 13 with homologous human fatty acid-binding proteins revealed unique surface charged residues on Der f 13 that may be involved in IgE binding and allergenicity. Site-directed mutagenesis and IgE binding assays have confirmed four surface charged residues on opposite sides of the protein that are involved in IgE binding. A triple mutant of Der f 13 (E41A_K63A_K91A) has been generated and found to have significantly reduced IgE binding and histamine release in skin prick tests on patients allergenic to group 13 dust mite allergens. The triple mutant is also able to induce PBMC proliferation in allergic patients with indices similar to those of wild-type Der f 13 and shift the secretion of cytokines from a Th2 to a Th1 pattern. Mouse IgG serum raised using the triple mutant is capable to block the binding of IgE from allergic patients to wild-type Der f 13, indicating potential for the triple mutant as a hypoallergen for specific immunotherapy. Findings in this study imply the importance of surface charged residues on IgE binding and allergenicity of an allergen, as was also demonstrated in other major allergens studied.

Candesartan Reduces Urinary Fatty Acid-Binding Protein Excretion in Patients with Autosomal Dominant Polycystic Kidney Disease

BACKGROUND

Free fatty acids (FFAs) bound to albumin are overloaded in renal proximal tubules and exacerbate tubulointerstitial damage. Liver-type fatty acid-binding protein (L-FABP) is an intracellular carrier protein of FFAs that is expressed in renal proximal tubules in humans. Urinary L-FABP reflects the clinical prognosis of chronic glomerulonephritis. The aim of the present study was to determine whether urinary L-FABP excretion is altered in patients with autosomal dominant polycystic kidney disease (ADPKD) and whether candesartan cilexetil, an angiotensin II receptor antagonist, affects these levels.

METHODS

Subjects comprised 20 normotensive ADPKD patients (8 men and 12 women, mean age 42.6 years) and 20 age-matched healthy volunteers (8 men and 12 women, mean age 44.0 years). The 20 ADPKD patients participated in a randomized double-blind placebo-controlled study of candesartan cilexetil for 6 months. Urinary L-FABP levels were measured by a newly established ELISA method.

RESULTS

Urinary L-FABP levels were significantly higher in ADPKD patients (154.5 +/- 110.6 microg/g Cr) than in healthy subjects (5.5 +/- 3.8 microg/g Cr) (P < 0.001). Candesartan cilexetil reduced urinary L-FABP levels from 168.5 +/- 104.5 microg/g Cr to 98.5 +/- 68.5 microg/g Cr after 3 months (P < 0.01) and to 44.6 +/- 30.8 microg/g Cr after 6 months (P < 0.001). Placebo had no effect on L-FABP levels (before, 140.5 +/- 100.5 microg/g Cr; at 3 months, 148.5 +/- 108.5 microg/g Cr; at 6 months, 150.5 +/- 110.8 microg/g Cr). During the 6 months, serum creatinine, blood urea nitrogen, 24-hour creatinine clearance and blood pressure showed little change in either group.

CONCLUSIONS

Increased urinary L-FABP levels may be associated with the development of ADPKD, and candesartan cilexetil has a beneficial effect on reducing these levels.

Ligand-binding specificity of an invertebrate (Manduca sexta) putative cellular retinoic acid binding protein

Intracellular lipid-binding proteins (iLBPs) are small cytoplasmic proteins that specifically interact with hydrophobic ligands. Fatty acid-binding proteins (FABPs), cellular retinoic acid-binding proteins (CRABPs) and cellular retinol-binding proteins (CRBPs) belong to the iLBP family. A recently identified insect (Manduca sexta) iLBP has been reported to possibly represent an invertebrate CRABP mimicking the role of CRABPs in vertebrate organisms. The presence in this protein of the characteristic binding triad residues involved in the interaction with ligand carboxylate head groups, a feature pertaining to several FABPs and to CRABPs, and the close phylogenetic relationships with both groups of vertebrate heart-type FABPs and CRBPs/CRABPs, makes it difficult to assign it to either FABPs or CRABPs. However, its negligible interaction with retinoic acid and high affinity (K(d) values in the 10(-8) M range) for fatty acids have been established by means of direct and competitive binding assays. As shown by phylogenetic analysis, the M. sexta iLBP belongs to a wide group of invertebrate iLBPs, which, besides being closely related phylogenetically, share distinctive features, such as the conservation of chemically distinct residues in their amino acid sequences and the ability to bind fatty acids. Our results are in keeping with the lack of cellular retinoid-binding proteins in invertebrates and with their later appearance during the course of chordate evolution.

[Is the ileal bile acid-binding protein (I-BABP) gene involved in cholesterol homeostasis?]

In the body, cholesterol balance results from an equilibrium between supplies (diet and cellular de novo synthesis), and losses (cellular use and elimination in feces, essentially as bile acids). Bile acids are synthesized from cholesterol in the liver. After conjugation to glycine or taurine, bile acids are secreted with bile in the intestinal lumen where they actively participate to the digestion and absorption of dietary fat and lipid-soluble vitamins. In healthy subjects, more than 95% of bile acids are reabsorbed throughout the small intestine and returned by the portal vein to the liver, where they are secreted again into bile. This enterohepatic circulation is essential for maintenance of bile acids balance, and hence, for cholesterol homeostasis. Indeed, the bile acids not reclaimed by intestinal absorption constitute the main physiological way to eliminate a cholesterol excess. Little is known about the molecular mechanisms controlling bile acids reabsorption by the small intestine. The intestinal bile acids uptake mainly takes place through an active transport located in the distal part of the small intestine. To date, four unrelated proteins exhibiting a high affinity for bile acids have been identified in the ileum, and only one, the ileal bile acid-binding protein (I-BABP) is a soluble protein. Therefore, it is thought to be essential for efficient bile acids desorption from the apical plasma membrane, as well as for bile acids intracellular trafficking and targeting towards the basolateral membrane. If this assumption is correct, the I-BABP expression level might be rate limiting for the enterohepatic bile acids circulation, and hence, for cholesterol homeostasis. It was found that both bile acids and cholesterol, probably via oxysterols, are able to up-regulate the transcription rate of I-BABP gene. The fact that intracellular sterol sensors (FXR, LXR, and SREBP1c) are involved in the control of the I-BABP gene expression strongly suggests that I-BABP exerts an important role in maintenance of cholesterol balance.

Fatty acid binding proteins stabilize leukotriene A4: competition with arachidonic acid but not other lipoxygenase products

Leukotriene A(4) (LTA(4)) is a chemically reactive conjugated triene epoxide product derived from 5-lipoxygenase oxygenation of arachidonic acid. At physiological pH, this reactive compound has a half-life of less than 3 s at 37 degrees C and approximately 40 s at 4 degrees C. Regardless of this aqueous instability, LTA(4) is an intermediate in the formation of biologically active leukotrienes, which can be formed through either intracellular or transcellular biosynthesis. Previously, epithelial fatty acid binding protein (E-FABP) present in RBL-1 cells was shown to increase the half-life of LTA(4) to approximately 20 min at 4 degrees C. Five FABPs (adipocyte FABP, intestinal FABP, E-FABP, heart/muscle FABP, and liver FABP) have now been examined and also found to increase the half-life of LTA(4) at 4 degrees C to approximately 20 min with protein present. Stabilization of LTA(4) was examined when arachidonic acid was present to compete with LTA(4) for the binding site on E-FABP. Arachidonate has an apparent higher affinity for E-FABP than LTA(4) and was able to completely block stabilization of the latter. When E-FABP is not saturated with arachidonate, FABP can still stabilize LTA(4). Several lipoxygenase products, including 5-hydroxyeicosatetraenoic acid, 5,6-dihydroxyeicosatetraenoic acid, and leukotriene B(4), were found to have no effect on the stability of LTA(4) induced by E-FABP even when present at concentrations 3-fold higher than LTA(4).

Expression of liver fatty acid binding protein in rat nonalcoholic fatty liver

AIM: To study the effect of liver fatty acid binding protein(L-FABP) in a model of rat nonalcoholic fatty liver.

METHODS: Expression of L-FABP gene was examined in fatty liver rats by reverse transcription and polymerase chain reaction amplification and Western blot.

RESULTS: In high-fat diet group, mRNA and protein expression of L-FABP increased at 2 wk, and elevated remarkably at 12 wk (1.42±0.034 vs 0.90±0.04; 13 372.00±23.86 vs 6857.33±32.96 637; P < 0.05).

CONCLUSION: Enhanced expressin of L-FABP at early staage is adapative reaction. With the advanced expression, disturbance of fatty acid metabolism may lead to nonalcoholic fatty liver.

Regulation of gene expression in inflammatory bowel disease and correlation with IBD drugs: screening by DNA microarrays

Potential biomarkers for Crohn's disease (CD) and ulcerative colitis (UC) were identified from two sets of full thickness pathologic samples utilizing DermArray and PharmArray DNA microarrays relative to uninvolved (Un) colon or normal colon. Seven of the over-expressed genes were verified using quantitative RT-PCR (i.e., TMPT, FABP1, IFI27, LCN2, COL11A2, HXB, and metallothionein). By correlating gene expression profiles between inflammatory bowel disease (IBD) tissue samples and IBD drug-treated cell cultures it might be possible to identify new candidate molecular target genes for IBD therapy and drug discovery. Potential biomarkers for CaCo2 cell cultures, which are routinely used as a GI tract surrogate model for in vitro pharmacokinetic studies, treated with azathioprine, 5-aminosalicylic acid, metronidazole, and prednisone were also identified from another experiment. Metallothionein mRNA expression was found to be down-regulated in azathioprine-treated CaCo2 cells, and was coincidentally up-regulated in the CD sample, thus resulting in an anti-correlation. These results suggest that this new screening methodology is feasible, that metallothioneins might be biomarkers for azathioprine therapy in vivo in CD, and that azathioprine might mechanistically down-regulate metallothionein gene expression. Correlations were also observed between IBD samples and either metronidazole- or 5-aminosalicylic acid-treated CaCo2 cells. Similar comparisons of disease tissue samples in vivo vs drug-treated cell cultures in vitro might reveal new mechanistic insights concerning established or experimental drug therapies. This affordable in vitro methodology is promising for expanded studies of IBD and other diseases.

Stabilization of leukotriene A4 by epithelial fatty acid-binding protein in the rat basophilic leukemia cell

Leukotriene A(4) (LTA(4)) is a chemically unstable triene epoxide product of 5-lipoxygenase metabolism of arachidonic acid. Despite this chemical reactivity and its synthesis at the perinuclear membrane, LTA(4) is enzymatically converted into the cysteinyl leukotrienes and leukotriene B(4). Furthermore, LTA(4) participates in transcellular biosynthesis and is thus transferred between cells as an intact molecule. A cytosolic fatty acid-binding protein present in the rat basophilic leukemia cells was identified using mass spectrometry. This protein was determined to be the stabilizing factor present in the cell cytosol responsible for increasing the effective chemical half-life of LTA(4). Rat epithelial fatty acid-binding protein (E-FABP) was isolated using partial protein purification and immunoprecipitation. In-gel digestion with trypsin followed by peptide fingerprint analysis using matrix-assisted laser desorption ionization mass spectrometry and sequencing the major tryptic peptide obtained from liquid chromatography/mass spectrometry/mass spectrometry analysis identified E-FABP in the active fraction. Semi-quantitative Western blot analysis indicated that E-FABP in the cytosolic fraction of RBL-1 cells was present at approximately 1-3 pmol/10(6) cells. E-FABP (9 microm) was tested for its ability to stabilize LTA(4), and at 37 degrees C E-FABP was able to increase the half-life of LTA(4) from the previously reported half-life less than 3 s to a half-life of approximately 7 min. These results present a novel function for the well studied fatty acid-binding protein as a participant in leukotriene biosynthesis that permits LTA(4) to be available for further enzymatic processing in various cellular regions.

Survey of binding properties of fatty acid-binding proteins

Fatty acid-binding proteins (FABPs) are members of a super family of lipid-binding proteins, and occur intracellularly in vertebrates and invertebrates. This review briefly addresses the structural and molecular properties of the fatty acid binding proteins, together with their potential physiological role. Special attention is paid to the methods used to study the binding characteristics of FABPs. An overview of the conventional (Lipidex, the ADIFAB and ITC) and innovative separation-based techniques (chromatographic and electrophoretic methods) for the study of ligand-protein interactions is presented along with a discussion of their strengths, weak points and potential applications. The best conventional approaches with natural fatty acids have generally revealed only limited information about the interactions of fatty acid proteins. In contrast, high-performance affinity chromatography (HPAC) studies of several proteins provide full information on the binding characteristics. The review uses, as an example, the application of immobilized liver basic FABP as a probe for the study of ligand-protein binding by high-performance affinity chromatography. The FABP from chicken liver has been immobilized on aminopropyl silica and the developed stationary phase was used to examine the enantioselective properties of this protein and to study the binding of drugs to FABP. In order to clarify the retention mechanism, competitive displacement studies were also carried out by adding short chain fatty acids to the mobile phase as displacing agents and preliminary quantitative structure-retention relationship (QSRRs) correlations were developed to describe the nature of the interactions between the chemical structures of the analytes and the observed chromatographic results. The results of these studies may shed light on the proposed roles of these proteins in biological systems and may find applications in medicine and medicinal chemistry. This knowledge will yield a deeper insight into the mechanism of fatty acid binding in order to indisputably show the central role played by FABPs in cellular FA transport and utilization for a proper lipid metabolism.

Water dynamics in the large cavity of three lipid-binding proteins monitored by (17)O magnetic relaxation dispersion

Intracellular lipid-binding proteins contain a large binding cavity filled with water molecules. The role played by these water molecules in ligand binding is not well understood, but their energetic and dynamic properties must be important for protein function. Here, we use the magnetic relaxation dispersion (MRD) of the water 17O resonance to investigate the water molecules in the binding cavity of three different lipid-binding proteins: heart fatty acid-binding protein (H-FABP), ileal lipid-binding protein (I-LBP) and intestinal fatty acid-binding protein (I-FABP). Whereas about half of the crystallographically visible water molecules appear to be expelled by the ligand, we find that ligand binding actually increases the number of water molecules within the cavity. At 300 K, the water molecules in the cavity exchange positions on a time-scale of about 1ns and exchange with external water on longer time-scales (0.01-1 micros). Exchange of water molecules among hydration sites within the cavity should be strongly coupled to ligand motion. Whereas a recent MD simulation indicates that the structure of the cavity water resembles a bulk water droplet, the present MRD results show that its dynamics is more than two orders of magnitude slower than in the bulk. These findings may have significant implications for the strength, specificity and kinetics of lipid binding.

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Localization of brain-type fatty acid-binding protein in Kupffer cells of mice and its transient decrease in response to lipopolysaccharide

Brain-type fatty acid-binding protein (B-FABP) was localized in Kupffer cells of liver of postnatal day 10 (P10) and older mice in immunolight and electron microscopy as well as by in situ hybridization histochemistry. The immunoreaction products were localized in the cytoplasmic matrix but not within the nucleus. After peritoneal injection of lipopolysaccharide (LPS), the immunoreaction for B-FABP decreased markedly in Kupffer cells at 1 h postinjection and thereafter gradually recovered to the preinjection level by 24 h postinjection, although no decrease in the mRNA expression was detected in Northern blotting throughout the course after the injection. The specific localization of B-FABP, but not the other FABPs, in Kupffer cells, and its rapid decrease after LPS injection suggest the intimate involvement of B-FABP in Kupffer cells in the inflammatory reaction, probably through mediation of n-3 polyunsaturated fatty acids, which are strong binders of B-FABP.

Automated nano-electrospray mass spectrometry for protein-ligand screening by noncovalent interaction applied to human H-FABP and A-FABP

A method for ligand screening by automated nano-electrospray ionization mass spectrometry (nano-ESI/MS) is described. The core of the system consisted of a chip-based platform for automated sample delivery from a 96-well plate and subsequent analysis based on noncovalent interactions. Human fatty acid binding protein, H-FABP (heart) and A-FABP (adipose), with small potential ligands was analyzed. The technique has been compared with a previously reported method based on nuclear magnetic resonance (NMR), and excellent correlation with the found hits was obtained. In the current MS screening method, the cycle time per sample was 1.1 min, which is approximately 50 times faster than NMR for single compounds and approximately 5 times faster for compound mixtures. High reproducibility was achieved, and the protein consumption was in the range of 88 to 100 picomoles per sample. Futhermore, a novel protocol for preparation of A-FABP without the natural ligand is presented. The described screening approach is suitable for ligand screening very early in the drug discovery process before conventional high-throughput screens (HTS) are developed and/or used as a secondary screening for ligands identified by HTS.

Gene structure and M20T polymorphism of the Schistosoma mansoni Sm14 fatty acid-binding protein. Molecular, functioanl, and immunoprotection analysis

The Schistosoma mansoni Sm14 antigen belongs to the fatty acid-binding protein family and is considered a vaccine candidate against at least two parasite worms, Fasciola hepatica and S. mansoni. Here the genomic sequence and the polymorphism of Sm14 have been characterized for the first time. We found that the conserved methionine at position 20 is polymorphic, being exchangeable with threonine (M20T). To evaluate the function of the amino acid residue at this position, we have also constructed the mutant Sm14-A20 besides the two native isoforms (Sm14-M20 and Sm14-T20). The three purified recombinant His(6)-tagged Sm14 proteins (rSm14-M20, rSm14-T20, and rSm14-A20) present a predominant beta-barrel structure as shown by CD spectroscopy. Thermal and urea unfolding studies evidenced a higher structural stability of rSm14-M20 over the other forms (rSm14-M20>rSm14-T20>rSm14-A20). All of the Sm14 proteins were able to bind 11-(dansylamino)undecanoic acid (DAUDA) without substantial difference in the binding affinity. However, rSm14-M20 exhibited a higher affinity for natural fatty acids than the rSm14-T20 and rSm14-A20 proteins as judged by competitive experiments against DAUDA (rSm14-M20>rSm14-T20>rSm14-A20). The rSm14-M20 or rSm14-T20 isoforms but not the rSm14-A20 mutant was able to induce significant protection against S. mansoni cercariae challenge in immunized mice. The level of protection efficacy correlates with the extent of structure stability of the recombinant Sm14 isoforms and mutant.

New insights into intracellular lipid binding proteins: The role of buried water

The crystal structures of most intracellular lipid binding proteins (LBPs) show between 5 and 20 internally bound water molecules, depending on the presence or the absence of ligand inside the protein cavity. The structural and functional significance of these waters has been discussed for several LBPs based on studies that used various biophysical techniques. The present work focuses on two very different LBPs, heart-type fatty acid binding protein (H-FABP) and ileal lipid binding protein (ILBP). Using high-resolution nuclear magnetic resonance spectroscopy, certain resonances belonging to side-chain protons that are located inside the water-filled lipid binding cavity were observed. In the case of H-FABP, the pH- and temperature-dependent behavior of selected side-chain resonances (Ser82 OgH and the imidazole ring protons of His93) indicated an unusually slow exchange with the solvent, implying that the intricate hydrogen-bonding network of amino-acid side-chains and water molecules in the protein interior is very rigid. In addition, holo H-FABP appeared to display a reversible self-aggregation at physiological pH. For ILBP, on the other hand, a more solvent-accessible protein cavity was deduced based on the pH titration behavior of its histidine residues. Comparison with data from other LBPs implies that the evolutionary specialization of LBPs for certain ligand types was not only because of mutations of residues directly involved in ligand binding but also to a refinement of the internal water scaffold.

Strategies for the diagnosis of mitochondrial fatty acid beta-oxidation disorders

Mitochondrial fatty acid beta-oxidation disorders (FAOD) are a group of clinically and biochemically heterogeneous inherited metabolic defects. The spectrum of phenotypes has expanded from hepatic encephalopathy to encompass myopathy, cardiomyopathy, peripheral neuropathy, sudden death and pregnancy complicated by fetal FAOD. Pre-symptomatic diagnosis is important to prevent morbidity and this is now achievable through newborn screening using tandem mass spectrometry (MS/MS). Moreover, most of the diagnosed defects are treatable and the prognosis is generally favourable. This article reviews the features of FAOD, critically evaluates methods of investigation including metabolite analyses in body fluids, in vitro oxidation rates and acylcarnitine profiling studies, enzymatic and mutational tests, and discusses genotype-phenotype correlation, treatment and monitoring options. Based on this knowledge, strategies for the biochemical investigation and differential diagnosis of patients presenting clinically, asymptomatic neonates detected by newborn screening, infants born after complications during late pregnancy, and cases of sudden death with suspected FAOD are presented. Laboratory investigation commonly begins with a search for diagnostic metabolites in physiological fluids, followed by in vitro functional studies if the initial findings are inconclusive, and confirmation by enzymology and molecular analyses. Occasionally a stress test in vivo may be required. At other times there may be no firm diagnosis achieved.

Solution structure and backbone dynamics of human epidermal-type fatty acid-binding protein (E-FABP)

Human epidermal-type fatty acid-binding protein (E-FABP) belongs to a family of intracellular 14-15 kDa lipid-binding proteins, whose functions have been associated with fatty acid signalling, cell growth, regulation and differentiation. As a contribution to understanding the structure-function relationship, we report in the present study features of its solution structure and backbone dynamics determined by NMR spectroscopy. Applying multi-dimensional high-resolution NMR techniques on unlabelled and 15N-enriched recombinant human E-FABP, the 1H and 15N resonance assignments were completed. On the basis of 2008 distance restraints, the three-dimensional solution structure of human E-FABP was subsequently obtained (backbone atom root-mean-square deviation of 0.92+/-0.11 A; where 1 A=0.1 nm), consisting mainly of 10 anti-parallel beta-strands that form a beta-barrel structure. 15N relaxation experiments (T1, T2 and heteronuclear nuclear Overhauser effects) at 500, 600 and 800 MHz provided information on the internal dynamics of the protein backbone. Nearly all non-terminal backbone amide groups showed order parameters S(2)>0.8, with an average value of 0.88+/-0.04, suggesting a uniformly low backbone mobility in the nanosecond-to-picosecond time range. Moreover, hydrogen/deuterium exchange experiments indicated a direct correlation between the stability of the hydrogen-bonding network in the beta-sheet structure and the conformational exchange in the millisecond-to-microsecond time range. The features of E-FABP backbone dynamics elaborated in the present study differ markedly from those of the phylogenetically closely related heart-type FABP and the more distantly related ileal lipid-binding protein, implying a strong interdependence with the overall protein stability and possibly also with the ligand-binding affinity for members of the lipid-binding protein family.

Long-chain polyunsaturated fatty acid accretion in brain

Brain is highly enriched in long-chain polyunsaturated fatty acids (PUFAs), particularly arachidonic acid and docosahexaenoic acid, which play important roles in brain structural and biologic functions. Plasma transport, in the form of free fatty acids or esterified FAs in lysophosphatidylcholine and lipoproteins, and de-novo synthesis contribute to brain accretion of long-chain PUFAs. Transport of long-chain PUFAs from plasma may play important roles because of the limited ability of brain to synthesize long-chain PUFAs, in the face of high demand for them. Although several proteins involved in facilitated fatty acid transport (e.g. fatty acid transport protein, fatty acid binding protein and very-long-chain acyl-coenzyme A synthetase) have been found in brain, their roles in fatty acid accumulation in brain are poorly defined. The primary pathways that are involved in long-chain PUFA accumulation in brain may vary according to brain region and developmental stage.

Fatty-acid-binding proteins do not protect against induced cytotoxicity in a kidney cell model

Intracellular accumulation of fatty acids (FAs) is a well-described consequence of renal ischaemia and may lead to lethal cell injury. Fatty-acid-binding proteins (FABPs) are small cytosolic proteins with high affinity for FAs. They may protect vital cellular functions by binding to and promoting the metabolism of FAs, thereby reducing their intracellular concentration. In this study we investigated the putative cytoprotective role of FABPs in a Madin-Darby canine kidney (MDCK) cell model for renal damage. We studied the effects of transfection with cDNA encoding heart FABP, adipocyte FABP or liver FABP on cytotoxicity induced by chemical anoxia or FAs. Transfection of MDCK type II cells with these cDNA types caused a 5-20-fold increase in FABP content, but did not change the rate or extent of palmitate uptake. After 1 h of incubation with KCN, all cell types showed reduced viability and cellular ATP content and an intracellular accumulation of non-esterified FAs. High extracellular concentrations of oleate, but not palmitate, caused a markedly decreased cell viability and cellular ATP content. Oleate accumulated in non-esterified form in these cells. Simultaneous addition of glucose ameliorated the damaging effects of KCN or oleate, indicating that glycolytic ATP could substitute for uncoupled oxidative phosphorylation. No significant differences in the effects of chemical anoxia or oleate were observed between non-transfected, mock-transfected and FABP-cDNA-transfected cells. Non-esterified FA accumulation was not reduced in any of the FABP-cDNA-transfected cell lines. In conclusion, our data do not provide evidence for a cytoprotective role of FABP in this kidney cell model.