Physiological properties and functions of intracellular fatty acid-binding proteins

The X-ray structure of zebrafish (Danio rerio) ileal bile acid-binding protein reveals the presence of binding sites on the surface of the protein molecule

The ileal bile acid-binding proteins (I-BABPs), also called ileal lipid-binding proteins or gastrotropins, belong to the family of the fatty acid-binding proteins and play an important role in the solubilization and transport of bile acids in the enterocyte. This article describes the expression, purification, crystallization, and three-dimensional structure determination of zebrafish (Danio rerio) I-BABP both in its apo form and bound to cholic acid. This is the first X-ray structure of an I-BABP. The structure of the apoprotein was determined to a resolution of 1.6 A, and two different monoclinic crystal forms of the holoprotein were solved and refined to 2.2 A resolution. Three protein molecules are present in the asymmetric unit of one of the co-crystal forms and two in the other, and therefore, the results of this study refer to observations made on five different protein molecules in the crystalline state. In every case, two cholate ligands were found bound in approximately the same position in the internal cavity of the protein molecules, but an unexpected result is the presence of clear and unambiguous electron density for several cholate molecules bound on hydrophobic patches on the surface of all the five independent protein molecules examined. Isothermal titration calorimetry was used for the thermodynamic characterization of the binding mechanism and has yielded results that are consistent with the X-ray data. Ligand binding is described in detail, and the conformational changes undergone by the protein molecule in the apo-to-holo transition are examined by superposition of the apo- and holoprotein models. The structure of the holoprotein is also compared with that of the liver BABP from the same species and those of other I-BABPs determined by NMR.

Fatty acid-binding protein regulates LPS-induced TNF-alpha production in mast cells

There has been increasing evidence for the involvement of fatty acid-binding proteins (FABPs) in the cytokine production of macrophages and dendritic cells probably through the control of cellular lipid metabolism and signal transduction. Since mast cells (MCs) are recently shown to be involved in immune response through modification of cytokine production, it is possible that some FABPs could also be involved in the immune response of MCs. In this study, we found that epidermal-type FABP (E-FABP) was expressed in murine bone marrow-derived MCs (BMMCs). Using BMMCs from genetically E-FABP-null mutated mice, we demonstrated that E-FABP in BMMCs plays a key role in the production of TNF-alpha following lipopolysaccharide (LPS) stimulation. In the in vivo septic peritonitis model (cecal ligation and puncture model), E-FABP-null mice showed a significantly increased mortality compared to wild-type mice. However, no significant difference in antigen-induced cytokine production was observed between wild-type and E-FABP-null BMMCs, and systemic anaphylaxis was equally induced in vivo in both wild-type and E-FABP-null mice. These results suggest that E-FABP is specifically involved in the LPS-induced cytokine production of MCs, and could play a role in the host-defense against bacterial infection, possibly through regulation of TNF-alpha production.

Fatty acid-binding proteins: role in metabolic diseases and potential as drug targets

Lipids are vital components of many biological processes and crucial in the pathogenesis of numerous common diseases, but the specific mechanisms coupling intracellular lipids to biological targets and signalling pathways are not well understood. This is particularly the case for cells burdened with high lipid storage, trafficking and signalling capacity such as adipocytes and macrophages. Here, we discuss the central role of lipid chaperones--the fatty acid-binding proteins (FABPs)--in lipid-mediated biological processes and systemic metabolic homeostasis through the regulation of diverse lipid signals, and highlight their therapeutic significance. Pharmacological agents that modify FABP function may provide tissue-specific or cell-type-specific control of lipid signalling pathways, inflammatory responses and metabolic regulation, potentially providing a new class of drugs for diseases such as obesity, diabetes and atherosclerosis.

A protocol for the combined sub-fractionation and delipidation of lipid binding proteins using hydrophobic interaction chromatography

Cellular lipids frequently co-purify with lipid binding proteins isolated from tissue extracts or heterologous host systems and as such hinder in vitro ligand binding approaches for which the apo-protein is a prerequisite. Here we present a technique for the complete removal of unesterified fatty acids, phospholipids, steroids and other lipophilic ligands bound to soluble proteins, without protein denaturation. Peroxisome proliferator activated receptor gamma ligand binding domain and intracellular fatty acid binding proteins were expressed in an Escherichia coli host and completely delipidated by hydrophobic interaction chromatography using phenyl sepharose. The delipidation procedure operates at room temperature with complete removal of bound lipids in a single step, as ascertained by mass spectrometry analysis of organic solvent extracts from purified protein samples. The speed and capacity of this method makes it amenable to scale-up and high-throughput applications. The method can also easily be adapted for other lipid binding proteins that require delipidation under native conditions.

Fatty acid binding protein: localization and functional significance in the brain

Long chain fatty acids are important nutrients for brain development and function. However, the molecular basis of their actions in the brain is still to be clarified. Fatty acid-binding proteins (FABPs) belong to the multigene family of the intracellular lipid-binding protein. FABPs bind to long chain fatty acids, being involved in the promotion of cellular uptake and transport of fatty acids, the targeting of fatty acids to specific metabolic pathways, and the regulation of gene expression. FABPs are widely expressed in mammalian tissues, with distinct expression patterns for the individual protein. Although FABPs have been implicated to serve as regulators in systemic cellular metabolic pathways, recent studies have demonstrated the ability of FABPs to regulate functions of the brain, one of the most fat-enriched tissues in the body. This review summarizes the localization of FABPs in the brain, and recent progress in elucidating the function of FABPs in the brain.

The interaction of liver fatty-acid-binding protein (FABP) with anionic phospholipid vesicles: is there extended phospholipid anchorage under these conditions?

Liver FABP (fatty-acid-binding protein) binds a variety of non-polar anionic ligands including fatty acids, fatty acyl CoAs, lysophospholipids and bile acids. Liver FABP is also able to bind to anionic phospholipid vesicles under conditions of low ionic strength, and membrane binding results in the release of bound ligand. However, the molecular interactions involved in binding to the phospholipid interface and the mechanism of ligand release are not known. Ligand release could be due to a significant conformational change in the protein at the interface or interaction of a phospholipid molecule with the ligand-binding cavity of the protein resulting in ligand displacement. Two portal mutant proteins of liver FABP, L28W and M74W, have now been used to investigate the binding of liver FABP to anionic phospholipid vesicles, monitoring changes in fluorescence and also fluorescence quenching in the presence of brominated lipids. There is a large increase in fluorescence intensity when the L28W mutant protein binds to vesicles prepared from DOPG (dioleoyl-sn-phosphatidylglycerol), but a large decrease in fluorescence intensity when the M74W mutant binds to these vesicles. The Br(4)-phospholipid prepared by bromination of DOPG dramatically quenches both L28W and M74W, consistent with the close proximity of a fatty acyl chain to the tryptophan residues. The binding of liver FABP to DOPG vesicles is accompanied by only a minimal change in the CD spectrum. Overall, the results are consistent with a molecule of anionic phospholipid interacting with the central cavity of the liver FABP, possibly involving the phospholipid molecule in an extended conformation.

Serum adipocyte fatty acid binding protein as a new biomarker predicting the development of type 2 diabetes: a 10-year prospective study in a Chinese cohort

OBJECTIVE

Adipocyte fatty acid-binding protein (A-FABP) is abundantly expressed in adipocytes and plays a role in glucose homeostasis in experimental animals. We have previously shown that circulating A-FABP levels are associated with the metabolic syndrome, which confers an increased risk of type 2 diabetes. Here we investigated whether serum A-FABP levels could predict the development of diabetes in a 10-year prospective study.

RESEARCH DESIGN AND METHODS

Baseline serum A-FABP levels were measured with an enzyme-linked immunosorbent assay in 544 nondiabetic subjects, recruited from the Hong Kong Cardiovascular Risk Factor Prevalence Study cohort, who were followed prospectively to assess the development of type 2 diabetes. The role of A-FABP in predicting the development of type 2 diabetes over 10 years was investigated using Cox regression analysis.

RESULTS

At baseline, serum sex-adjusted A-FABP levels were higher in subjects with impaired glucose tolerance (IGT) or impaired fasting glucose (IFG) (P < 0.00001 versus normal glucose tolerance) and correlated positively with adverse cardiometabolic risk factors. Over 10 years, 96 subjects had developed type 2 diabetes. High baseline A-FABP was predictive of type 2 diabetes, independent of obesity, insulin resistance, or glycemic indexes (relative risk [RR] 2.25 [95% CI 1.40-3.65]; P = 0.001; above versus below sex-specific median). High A-FABP levels remained an independent predictor of type 2 diabetes in the high-risk IGT/IFG subgroup (adjusted RR 1.87 [1.12-3.15]; P = 0.018).

CONCLUSIONS

Serum A-FABP was associated with glucose dysregulation and predicted the development of type 2 diabetes in a Chinese cohort.

Adipocyte- and heart-type fatty acid binding proteins are both expressed in subcutaneous and intramuscular porcine ( Sus scrofa ) adipocytes

Adipocyte- (A) and heart- (H) type fatty acid binding proteins (FABP) contribute to efficient fat storage and utilization, respectively. To understand regional-differences in lipid metabolism between tissues, A- and H-FABP transcript and protein levels were studied in adipocytes isolated from subcutaneous adipose tissue or skeletal muscle in growing pigs (Sus scrofa). Interestingly, H-FABP was expressed in adipocytes isolated from both sites. We also showed that A-FABP and H-FABP were expressed at a lower level in intramuscular adipocytes than in subcutaneous adipocytes. A discrepancy was observed between age-related changes in A-FABP content in isolated adipocytes and cell diameter or lipid content variations in tissues during growth.

Structural basis for activation of fatty acid-binding protein 4

Fatty acid-binding protein 4 (FABP4) delivers ligands from the cytosol to the nuclear receptor PPARgamma in the nucleus, thereby enhancing the transcriptional activity of the receptor. Notably, FABP4 binds multiple ligands with a similar affinity but its nuclear translocation is activated only by specific compounds. To gain insight into the structural features that underlie the ligand-specificity in activation of the nuclear import of FABP4, we solved the crystal structures of the protein complexed with two compounds that induce its nuclear translocation, and compared these to the apo-protein and to FABP4 structures bound to non-activating ligands. Examination of these structures indicates that activation coincides with closure of a portal loop phenylalanine side-chain, contraction of the binding pocket, a subtle shift in a helical domain containing the nuclear localization signal of the protein, and a resultant change in oligomeric state that exposes the nuclear localization signal to the solution. Comparisons of backbone displacements induced by activating ligands with a measure of mobility derived from translation, libration, screw (TLS) refinement, and with a composite of slowest normal modes of the apo state suggest that the helical motion associated with the activation of the protein is part of the repertoire of the equilibrium motions of the apo-protein, i.e. that ligand binding does not induce the activated configuration but serves to stabilize it. Nuclear import of FABP4 can thus be understood in terms of the pre-existing equilibrium hypothesis of ligand binding.

A single amino acid mutation in zebrafish (Danio rerio) liver bile acid-binding protein can change the stoichiometry of ligand binding

In all of the liver bile acid-binding proteins (L-BABPs) studied so far, it has been found that the stoichiometry of binding is of two cholate molecules per internal binding site. In this paper, we describe the expression, purification, crystallization, and three-dimensional structure determination of zebrafish (Danio rerio) L-BABP to 1.5A resolution, which is currently the highest available for a protein of this family. Since we have found that in zebrafish, the stoichiometry of binding in the protein cavity is of only one cholate molecule per wild type L-BABP, we examined the role of two crucial amino acids present in the binding site. Using site-directed mutagenesis, we have prepared, crystallized, and determined the three-dimensional structure of co-crystals of two mutants. The mutant G55R has the same stoichiometry of binding as the wild type protein, whereas the C91T mutant changes the stoichiometry of binding from one to two ligand molecules in the cavity and therefore appears to be more similar to the other members of the L-BABP family. Based on the presence or absence of a single disulfide bridge, it can be postulated that fish should bind a single cholate molecule, whereas amphibians and higher vertebrates should bind two. Isothermal titration calorimetry has also revealed the presence in the wild type protein and the G55R mutant of an additional binding site, different from the first and probably located on the surface of the molecule.

The effect of supplementation with n-3 fatty acids on the physical performance in subjects with spinal cord injury

A global physical evaluation was performed in 21 males with spinal cord injury (SCI), at the beginning and at three and six months of omega-3 fatty acid (FA) supplementation. A significant increase in the proportion of eicosapentaenoic acid and docosahexanoic acid in plasma was observed in response to the supplementation (p<0.05). After six months of FA supplementation, strength endurance time increased from 127.7+/-19.0 s to 215.2+/-45.6 s in the right arm, and from 139+/-27.6 s to 237.7+/-48.7 s, in the left arm. The time to perform 20 repetitions of 70% maximum workload showed a reduction of 41% between the first and the third test. The time taken to cover a 90 meter long track, with a 6% slope, was reduced from 66.9+/-8.0 s to 59.3+/-6.7 s, at the end of the study (p<0.05). In conclusion, omega-3 FA supplementation could contribute to improve the functional capabilities in SCI subjects.

Fat tissue: views on reconstruction and exploitation

Transplantation of autologous fat as pedicle or transposition flaps has been a classical method in plastic surgery for tissue reconstruction. The injection of fat for soft tissue reconstruction is also an old innovation. This approach has some significant drawbacks such as resorption of the fat transplant. To regenerate additional and self-regenerating adipose tissue for reconstructive purposes, a thorough understanding of adipose tissue (mesodermal stem cells, adipoblasts, pre-adipocytes, mature, lipid-synthesizing, and lipid-storing white or brown adipocytes) on cellular and molecular levels is required. Several transcription factors that play a central role in the control of adipogenesis have been identified. Among these are the CCAAT/enhancer binding protein gene family and peroxisome proliferator-activated receptor-gamma. Hormones and growth factors, such as insulin and insulin-like growth factor (IGF), transfer external signals to differentiating adipocytes. In an attempt to improve the quality of tissue-engineered fat by culture-expanded adipocytes, various pre-adipocyte and stem cell culture conditions and expansion methods have been developed. In the presence of fetal calf serum, spontaneous differentiation of pre-adipocytes into fat cell clusters occurs to some degree. This in vitro differentiation can be enhanced by addition of inducing agents such as dexamethasone, isobutylmethylxantine, and insulin into the culture medium. Recent work has shown the multipotency of pre-adipocytes, which are fibroblast-like precursors of adipocytes. With use of specific culture conditions, human adipose tissue-derived stem cells can be induced to express markers of adipocyte, osteoblast, and myocyte cell lineages. The multipotent characteristics of adipose tissue-derived stem cells, as well as their abundance and accessibility in the human body, make them a potential cell source for tissue engineering applications.

Adipogenic potential can be activated during muscle regeneration

Fatty degeneration is observed in various neuromuscular diseases, but the mechanism(s) of its initiation remains unclear. To gain insight into the regulation of fatty degeneration, we employed a freeze-induced model of muscle degeneration/regeneration. Using this model, we examined the distribution of adipocyte-like cells with Oil Red-O staining and the expression pattern of adipogenic transcriptional factors, an adipocyte-terminal differentiation marker, and Wnt10b signaling molecules during muscle regeneration. Mice were subjected to freeze injury, and the gastrocnemius muscles were isolated 1, 3, 5, 7, 10, 14 and 28 days after surgery. Adipocyte-like cells with nuclei were readily observed, but not in normal muscle. Large amount of lipid accumulation was also observed in regenerating muscle. The area of Oil Red-O staining was significantly increased from 3 to 5 days after muscle injury and then rapidly decreased to almost control levels by day 10. Adipogenic transcriptional factors, sterol regulatory element binding protein-1c, CCAAT/enhancer-binding proteins alpha, beta and delta, peroxisome-proliferator activated receptors gamma1 and gamma2, and the terminal differentiation marker, leptin were significantly up-regulated in the early stage of muscle regeneration, suggesting activation of the adipogenic potential. Secreted Frizzled-related protein-2, a Wnt pathway inhibitory protein, was strongly up-regulated 3 days after muscle injury, suggesting active repression of the Wnt10b pathway. In regenerating muscle, expression of CCAAT/enhancer-binding protein alpha and peroxisome-proliferator activated receptor gamma2 proteins were increased 3 days after muscle injury. Taken together, our results suggest that adipogenic potential can be activated during muscle regeneration through increased adipogenic signaling in conjunction with decreased Wnt10b signaling.

Identification of a functional polymorphism at the adipose fatty acid binding protein gene (FABP4) and demonstration of its association with cardiovascular disease: a path to follow

Fatty acid binding proteins (FABPs) are proteins that reversibly bind fatty acids and other lipids. So far, nine tissue-specific cytoplasmic FABPs have been identified. Adipose tissue FABP (FABP4) has been suggested to be a bridge between inflammation and other pathways related to the metabolic syndrome. In this regard, genetic variability at the FABP4 locus has been shown to be associated with plasma lipid levels, type 2 diabetes, and coronary heart disease risk.

Examination of the Role of Intestinal Fatty Acid-Binding Protein in Drug Absorption Using a Parallel Artificial Membrane Permeability Assay

Transcellular diffusion across the absorptive epithelial cells (enterocytes) of the small intestine is the main route of absorption for most orally administered drugs. The process by which lipophilic compounds transverse the aqueous environment of the cytoplasm, however, remains poorly defined. In the present study, we have identified a structurally diverse group of lipophilic drugs that display low micromolar binding affinities for a cytosolic lipid-binding protein - intestinal fatty acid-binding protein (I-FABP). Binding to I-FABP significantly enhanced the transport of lipophilic drug molecules across a model membrane, and the degree of transport enhancement was related to both drug lipophilicity and I-FABP binding affinity. These data suggest that intracellular lipid-binding proteins such as I-FABP may enhance the membrane transport of lipophilic xenobiotics and facilitate drug access to the enterocyte cytoplasm and cytoplasmic organelles.

Adipocyte fatty acid-binding protein (aP2), a newly identified LXR target gene, is induced by LXR agonists in human THP-1 cells

The liver X receptors (LXRalpha and LXRbeta), ligand-activated transcription factors, belong to the superfamily of nuclear hormone receptors and have been shown to play a major role in atherosclerosis by modulating cholesterol and triglyceride metabolism. In this report, we describe a novel LXR target, the adipocyte fatty acid binding protein (aP2), which plays an important role in fatty acid metabolism, adipocyte differentiation and atherosclerosis. While LXR agonists induce aP2 mRNA expression in human monocytes (THP-1 cells) and macrophages in a time- and concentration-dependent manner, they have no effect on aP2 expression in human adipocytes. The increase in aP2 mRNA level was additive when THP-1 cells were treated with LXR and PPARgamma agonists. Also, an RXR agonist induced aP2 expression in these cells. While no additive effect was observed with LXR and RXR agonists, additive effects were observed with RXR and PPARgamma agonists. GW9662, a potent PPARgamma antagonist, inhibited PPARgamma-induced aP2 expression without affecting LXR-mediated aP2 expression indicating the induction is mediated directly through LXR activation. Analysis of human aP2 promoter revealed a potential LXR response element (LXRE). Gel shift data showed that the LXRalpha/RXRalpha heterodimer bound to the LXRE motif in aP2 promoter in vitro in a sequence-specific manner. Deletion and mutation analyses of the proximal aP2 promoter confirm that this is a functional LXRE. These data indicate for the first time that human macrophage aP2 promoter is a direct target for the regulation by LXR/RXR heterodimers.

Effects of oxidized low- and high-density lipoproteins on gene expression of human macrophages

OBJECTIVE

Oxidized low-density lipoprotein (ox-LDL) is a major factor in foam cell formation, whereas the role of oxidized high-density lipoprotein (ox-HDL) in this process is not known. The objective of the present study was to examine the effects of ox-LDL and ox-HDL on the gene expression of cultured human macrophages.

MATERIAL AND METHODS

Gene expression of human macrophages was studied after incubation for 1 day and 3 days with native and oxidized LDL and HDL using cDNA expression array. Expression of granulocyte-macrophage colony-stimulating factor 1, which was constantly up-regulated by ox-LDL and down-regulated by ox-HDL after 1- and 3 days of incubation in cDNA microarray experiments, was verified by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR).

RESULTS

Genes that showed altered expression were divided into six groups; 1) lipid metabolism, 2) inflammation, growth and hemostasis, 3) matrix metalloproteinases and tissue inhibitors of matrix metalloproteinases, 4) enzymes, 5) structural and binding proteins and 6) annexins.

CONCLUSIONS

The microarray method was found to be applicable in analyzing changes in gene expression induced by oxidized lipoproteins in cultured human macrophages. Our results reflect different functional roles of ox-LDL and ox-HDL in foam cell formation.

Altered emotional behavioral responses in mice lacking brain-type fatty acid-binding protein gene

Brain-type fatty acid-binding protein (B-FABP) belongs to a family of intracellular lipid-binding proteins. B-FABP exhibits a binding affinity to long-chain fatty acids (FAs) whose effects on brain functions including development, emotion, learning and memory have been proposed. B-FABP is localized in the ventricular germinal cells in embryonic brain and astrocytes in developing and mature brain of rodents. In the present study we generated the mouse harboring a null mutation in the B-FABP gene and studied its phenotype. B-FABP mutant mice exhibited the enhanced anxiety and increased fear memory as well as the decreased content of docosahexaenoic acid (DHA) in their brain during the neonatal period without detection of any histological changes in the brain. In the adult brain, B-FABP was localized more numerously to the astrocytes in the amygdala and septal area than to those in the hippocampal area. Analysis of FA content in the amygdala of adult brain revealed that arachidonic and palmitic acids increased significantly in the mutant mice compared with wild-type. Furthermore, the response of N-methyl-d-aspartate receptor-mediated current to DHA in isolated neurons from B-FABP mutant brain was significantly decreased compared with that of wild-type, while no significant differences were detected in behavioral responses related to the spatial learning/memory or in the hippocampal long-term potentiation. These data indicate that B-FABP is crucially involved in the fear memory and anxiety through its binding with FAs and/or its own direct effects on pertinent metabolism/signaling of FAs.

Crystal structure of axolotl (Ambystoma mexicanum) liver bile acid-binding protein bound to cholic and oleic acid

The family of the liver bile acid-binding proteins (L-BABPs), formerly called liver basic fatty acid-binding proteins (Lb-FABPs) shares fold and sequence similarity with the paralogous liver fatty acid-binding proteins (L-FABPs) but has a different stoichiometry and specificity of ligand binding. This article describes the first X-ray structure of a member of the L-BABP family, axolotl (Ambystoma mexicanum) L-BABP, bound to two different ligands: cholic and oleic acid. The protein binds one molecule of oleic acid in a position that is significantly different from that of either of the two molecules that bind to rat liver FABP. The stoichiometry of binding of cholate is of two ligands per protein molecule, as observed in chicken L-BABP. The cholate molecule that binds buried most deeply into the internal cavity overlaps well with the analogous bound to chicken L-BABP, whereas the second molecule, which interacts with the first only through hydrophobic contacts, is more external and exposed to the solvent.

Hierarchical subfunctionalization of fabp1a, fabp1b and fabp10 tissue-specific expression may account for retention of these duplicated genes in the zebrafish (Danio rerio) genome

Fatty acid-binding protein type 1 (FABP1), commonly termed liver-type fatty acid-binding protein (L-FABP), is encoded by a single gene in mammals. We cloned and sequenced cDNAs for two distinct FABP1s in zebrafish coded by genes designated fabp1a and fabp1b. The zebrafish proteins, FABP1a and FABP1b, show highest sequence identity and similarity to the human protein FABP1. Zebrafish fabp1a and fabp1b genes were assigned to linkage groups 5 and 8, respectively. Both linkage groups show conserved syntenies to a segment of mouse chromosome 6, rat chromosome 4 and human chromosome 2 harboring the FABP1 locus. Phylogenetic analysis further suggests that zebrafish fabp1a and fabp1b genes are orthologs of mammalian FABP1 and most likely arose by a whole-genome duplication event in the ray-finned fish lineage, estimated to have occurred 200-450 million years ago. The paralogous fabp10 gene encoding basic L-FABP, found to date in only nonmammalian vertebrates, was assigned to zebrafish linkage group 16. RT-PCR amplification of mRNA in adults, and in situ hybridization to whole-mount embryos to fabp1a, fabp1b and fapb10 mRNAs, revealed a distinct and differential pattern of expression for the fabp1a, fabp1b and fabp10 genes in zebrafish, suggesting a division of function for these orthogolous and paralogous gene products following their duplication in the vertebrate genome. The differential and complementary expression patterns of the zebrafish fabp1a, fapb1b and fabp10 genes imply a hierarchical subfunctionalization that may account for the retention of both the duplicated fabp1a and fabp1b genes, and the fabp10 gene in the zebrafish genome.

Fatty acid handling protein expression in adipose tissue, fatty acid composition of adipose tissue and serum, and markers of insulin resistance

OBJECTIVE

Proteins involved in cellular fatty acid (FA) uptake and metabolism may be of relevance in the context of disturbed FA metabolism associated with insulin resistance. Therefore this study investigated relationships between FA handling protein mRNA expression in adipose tissue, FA composition of adipose tissue and serum, and markers of insulin resistance.

SUBJECTS

75 subjects with a range of insulin sensitivities recruited from a cohort of 294 healthy 63-year-old Swedish men.

MEASUREMENTS

Anthropometric and biochemical variables (e.g. waist-hip-ratio (WHR) and homeostasis model assessment (HOMA) index of insulin sensitivity), FA composition of the subcutaneous (s.c.) gluteal adipose tissue, serum nonesterified FA (NEFA) and serum phospholipid compartments (by gas-liquid chromatography; n = 294), and mRNA levels of FA handling proteins (adipocyte and keratinocyte lipid binding proteins, fatty acid transport protein (FATP) -1 and -4, CD36/fatty acid translocase, plasma membrane fatty acid binding protein, and acyl-CoA synthase-1 (ACS1)) in s.c. gluteal adipose tissue (by quantitative real-time polymerase chain reaction; n = 75).

RESULTS

ACS1 expression was negatively correlated with measures of insulin resistance and central obesity (ACS1 versus HOMA: r = -0.28, P<0.05; ACS1 versus WHR: r = -0.23, P<0.05), with an opposite trend for FATP4. Further analysis of ACS1 expression levels revealed correlations with adipose tissue 16:0 (r = -0.27, P<0.05) and NEFA 16:1 (r = 0.29, P<0.05), FA composition variables which in turn correlated with HOMA index (r = 0.39, P<0.001 and r = -0.23, P<0.05, respectively, n = 75). Moreover, NEFA 16:1 predicted ACS1 expression independently of HOMA, WHR and adipose tissue 16:0 in multiple regression analysis (standardized coefficient = 0.27, P<0.05).

CONCLUSION

Significant associations were found between measures of insulin sensitivity, adipose tissue FA handling protein expression, and specific FA composition variables. Although causal relationships could not be identified these findings suggest a role of FA handling proteins in relation to insulin sensitivity, via their involvement in FA trafficking and metabolism. In particular they indicate links between ACS1 activity, the distribution of 16:0 and 16:1, and insulin sensitivity, which may be of physiological relevance.

PPARγ agonist induced cardiac enlargement is associated with reduced fatty acid and increased glucose utilization in myocardium of Wistar rats

In toxicological studies, high doses of peroxisome proliferator-activated receptor-gamma (PPARgamma) agonists cause cardiac enlargement. To investigate whether this could be explained by a large shift from free fatty acid to glucose utilization by the heart, Wistar rats were treated for 2-3 weeks with a potent, selective PPARgamma agonist (X334, 3 micromol/kg/d), or vehicle. X334 treatment increased body-weight gain and ventricular mass. Treatment lowered plasma triglycerides by 61%, free fatty acid levels by 72%, insulin levels by 45%, and reduced total plasma protein concentration by 7% (indicating plasma volume expansion) compared to vehicle animals. Fasting plasma glucose levels were unaltered. To assess cardiac free fatty acid and glucose utilization in vivo we used simultaneous infusions of non-beta-oxidizable free fatty acid analogue, [9,10-(3)H](R)-2-bromopalmitate and [U-(14)C]2-deoxy-d-glucose tracers, which yield indices of local free fatty acid and glucose utilization. In anesthetized, 7 h fasted animals, left ventricular glucose utilization was increased to 182% while free fatty acid utilization was reduced by 28% (P<0.05) compared to vehicle. In separate studies we attempted to prevent the X334-induced hypolipidemia. Various dietary fat supplements were unsuccessful. By contrast, restricting the time during which the treated animals had access to food (promoting endogenous lipolysis), restored plasma free fatty acid from 27% to 72% of vehicle control levels and prevented the cardiac enlargement. Body-weight gain in these treated-food restricted rats was not different from vehicle controls. In conclusion, the cardiac enlargement caused by intense PPARgamma activation in normal animals is associated with marked changes in free fatty acid/glucose utilization and the enlargement can be prevented by restoring free fatty acid availability.

Lipid metabolism and adipokine levels in fatty acid-binding protein null and transgenic mice

Fatty acid-binding proteins (FABPs) facilitate the diffusion of fatty acids within cellular cytoplasm. Compared with C57Bl/6J mice maintained on a high-fat diet, adipose-FABP (A-FABP) null mice exhibit increased fat mass, decreased lipolysis, increased muscle glucose oxidation, and attenuated insulin resistance, whereas overexpression of epithelial-FABP (E-FABP) in adipose tissue results in decreased fat mass, increased lipolysis, and potentiated insulin resistance. To identify the mechanisms that underlie these processes, real-time PCR analyses indicate that the expression of hormone-sensitive lipase is reduced, while perilipin A is increased in A-FABP/aP2 null mice relative to E-FABP overexpressing mice. In contrast, de novo lipogenesis and expression of genes encoding lipoprotein lipase, CD36, long-chain acyl-CoA synthetase 5, and diacylglycerol acyltransferase are increased in A-FABP/aP2 null mice relative to E-FABP transgenic animals. Consistent with an increase in de novo lipogenesis, there was an increase in adipose C16:0 and C16:1 acyl-CoA pools. There were no changes in serum free fatty acids between genotypes. Serum levels of resistin were decreased in the E-FABP transgenic mice, whereas serum and tissue adiponectin were increased in A-FABP/aP2 null mice and decreased in E-FABP transgenic animals; leptin expression was unaffected. These results suggest that the balance between lipolysis and lipogenesis in adipocytes is remodeled in the FABP null and transgenic mice and is accompanied by the reprogramming of adipokine expression in fat cells and overall changes in plasma adipokines.

Low- and high-carbohydrate diets: body composition differences in rats

OBJECTIVE

To determine differences in adipose tissue mass, cell size, and lipid metabolism transcripts and differences in composition of body weight loss and energy expenditure (EE) after isoenergetic, energy-restricted intake of low-carbohydrate/high-fat/high-protein (LC) and high-carbohydrate/low-fat/moderate-protein (HC) diets.

RESEARCH METHODS AND PROCEDURES

Ten-week old female Sprague-Dawley rats were fed a high-fat diet ad libitum for 8 weeks to induce weight gain and fat deposition. Weight-matched rats were then assigned to isoenergetic LC (Atkins) and HC (American Dietary Association Exchange) diets for 10 weeks at 65% of ad libitum energy intake.

RESULTS

There was no significant difference in the serum lipid profiles or amount of body weight lost between the HC and LC groups, whereas a higher insulin sensitivity index (p < 0.01) resulted from the HC compared with the LC diet. Compared with the post-restriction LC group, the HC group demonstrated (p < 0.05) higher EE during active hours, lower mRNA levels for the lipogenic genes peroxisome proliferator-activated receptor gamma2, lipoprotein lipase, and, adipocyte fatty acid binding protein, decreased adipocyte cell volume, and decreased fat mass.

DISCUSSION

Results indicated down-regulation of lipogenic genes, decreased fat mass, and, therefore, improved body composition in the post-restriction HC compared with the LC group. The small mean differences between the two diet groups (p = 0.11) in 24-hour EE over the 10 weeks of diet intervention would account for the majority of the lower mean body weights in the post-restriction HC group. These data suggest that macronutrient composition of the diet influences body composition and indicate a distinction between HC and LC diets.

Decreased keratinocyte motility in skin wound on mice lacking the epidermal fatty acid binding protein gene

Fatty acids are shown to be important in various skin functions. Fatty acid binding protein (FABP) is postulated to serve as a lipid shuttle, solubilizing hydrophobic fatty acids and delivering them to the appropriate metabolic system. Among the FABP family proteins, epidermal-type FABP (E-FABP) is solely expressed in keratinocyte but its specific role in skin is not yet fully established. We found an elevated expression of E-FABP in regenerative keratinocytes of healing wounds. However, E-FABP null mice showed no marked differences compared to wild type mice in the process of wound closure, in vivo. On the other hand, in keratinocyte culture, E-FABP gene disruption decreased the cell motility, but did not affect the cell proliferation. E-FABP deletion may be compensated for in vivo by the microenvironment comprised of various cells such as fibroblasts and endothelial cells around the wound. Our analyses suggest that the E-FABP elevation may be necessary for the activation of cell motility within regenerative epidermis during wound healing.

Principles of lysosomal membrane digestion: stimulation of sphingolipid degradation by sphingolipid activator proteins and anionic lysosomal lipids

Sphingolipids and glycosphingolipids are membrane components of eukaryotic cell surfaces. Their constitutive degradation takes place on the surface of intra-endosomal and intra-lysosomal membrane structures. During endocytosis, these intra-lysosomal membranes are formed and prepared for digestion by a lipid-sorting process during which their cholesterol content decreases and the concentration of the negatively charged bis(monoacylglycero)phosphate (BMP)--erroneously also called lysobisphosphatidic acid (LBPA)--increases. Glycosphingolipid degradation requires the presence of water-soluble acid exohydrolases, sphingolipid activator proteins, and anionic phospholipids like BMP. The lysosomal degradation of sphingolipids with short hydrophilic head groups requires the presence of sphingolipid activator proteins (SAPs). These are the saposins (Saps) and the GM2 activator protein. Sphingolipid activator proteins are membrane-perturbing and lipid-binding proteins with different specificities for the bound lipid and the activated enzyme-catalyzed reaction. Their inherited deficiency leads to sphingolipid- and membrane-storage diseases. Sphingolipid activator proteins not only facilitate glycolipid digestion but also act as glycolipid transfer proteins facilitating the association of lipid antigens with immunoreceptors of the CD1 family.

Localization of epidermal-type fatty acid binding protein in macrophages in advanced atretic follicles of adult mice

The localization of epidermal-type fatty acid binding protein (E-FABP) in the mature mouse ovary was examined by immuno-light and electron microscopy. Numerous macrophages immunopositive for both anti-E-FABP and F4/80 antibodies, together with immunonegative cells, were found in advanced atretic follicles that had eccentric lumens containing deformed ova. While some E-FABP-immunopositive macrophages were spider in shape and appeared singly, others, especially close to the lumen, were round and voluminous and tended to be aggregated. The voluminous macrophages contained phagosomes of various sizes and they were regarded as those actively involved in the phagocytosis of apoptotic granulosa cells. E-FABP-immunopositive macrophages and their processes were often apposed to adjacent immunonegative cells, and some of them lined the lumen containing deformed ova. On the other hand, E-FABP-immunonegative cells in the atretic follicles were classified into two types: the one, a minority, was characterized by small mitochondria containing non-tubular cristae and presumably represented residual granulosa cells, while the other dominant type was characterized by large mitochondria containing tubular cristae and presumably represented theca cells originally surrounding the follicles to be atretic. The present detection of E-FABP-immunopositivity selectively in macrophages of the atretic follicles suggests possible involvement of E-FABP and/or its ligand fatty acids in the process of follicular atresia, and it makes more reliable the identification of the advanced atretic follicles with the antral spaces obliterated, which could provide further details on the histology of the follicular atresia than before.

Skeletal Muscle Lipid Metabolism in Exercise and Insulin Resistance

Lipids as fuel for energy provision originate from different sources: albumin-bound long-chain fatty acids (LCFA) in the blood plasma, circulating very-low-density lipoproteins-triacylglycerols (VLDL-TG), fatty acids from triacylglycerol located in the muscle cell (IMTG), and possibly fatty acids liberated from adipose tissue adhering to the muscle cells. The regulation of utilization of the different lipid sources in skeletal muscle during exercise is reviewed, and the influence of diet, training, and gender is discussed. Major points deliberated are the methods utilized to measure uptake and oxidation of LCFA during exercise in humans. The role of the various lipid-binding proteins in transmembrane and cytosolic transport of lipids is considered as well as regulation of lipid entry into the mitochondria, focusing on the putative role of AMP-activated protein kinase (AMPK), acetyl CoA carboxylase (ACC), and carnitine during exercise. The possible contribution to fuel provision during exercise of circulating VLDL-TG as well as the role of IMTG is discussed from a methodological point of view. The contribution of IMTG for energy provision may not be large, covering ∼10% of total energy provision during fasting exercise in male subjects, whereas in females, IMTG may cover a larger proportion of energy delivery. Molecular mechanisms involved in breakdown of IMTG during exercise are also considered focusing on hormone-sensitive lipase (HSL). Finally, the role of lipids in development of insulin resistance in skeletal muscle, including possible molecular mechanisms involved, is discussed.

Sirolimus upregulates aP2 expression in human monocytes and macrophages

Rapamycin (Sirolimus) is a potent immunosuppressive drug that reduces renal transplant rejection. Hyperlipidemia is a significant side effect of rapamycin treatment, and frequently leads to cardiovascular disease. Adipocyte fatty acid binding protein (aP2) is a member of the cytoplasmic fatty acid binding protein (FABP) family. aP2 has been shown to affect insulin sensitivity, lipid metabolism, lipolysis, and has recently been shown to play an important role in atherosclerosis. We found that aP2 messenger RNA (mRNA) was increased in human THP-1 cells after rapamycin treatment. Exposure of human differentiated THP-1 cells to rapamycin led to a time- and dose-dependent induction of aP2 mRNA expression. While aP2 expression was undetectable in undifferentiated THP-1 cells, aP2 was induced in these cells by rapamycin. These data suggest that rapamycin-induced aP2 may play a role in increased triglyceride accumulation.

Studies of plasma membrane fatty acid-binding protein and other lipid-binding proteins in human skeletal muscle

The first putative fatty acid transporter identified was plasma membrane fatty acid-binding protein (FABPpm). Later it was demonstrated that this protein is identical to the mitochondrial isoform of the enzyme aspartate aminotransferase. In recent years data from several cell types have emerged, indicating that FABPpm plays a role in the transport of long-chain saturated and unsaturated fatty acids. In the limited number of studies in human skeletal muscle it has been demonstrated that dietary composition and exercise training can influence the content of FABPpm. Ingestion of a fat-rich diet induces an increase in FABPpm protein content in human skeletal muscle in contrast to the decrease seen during consumption of a carbohydrate-rich diet. A similar effect of a fat-rich diet is also observed for cytosolic fatty acid-binding protein and fatty acid translocase/CD36 protein expression. Exercise training up regulates FABPpm protein content in skeletal muscle, but only in male subjects; no significant differences were observed in muscle FABPpm content in a cross-sectional study of female volunteers of varying training status, even though muscle FABPpm content did not depend on gender in the untrained state. A higher utilization of plasma long-chain fatty acids during exercise in males compared with females could explain the gender-dependent influence of exercise training on FABPpm. The mechanisms involved in the regulation of the function and expression of FABPpm protein remain to be clarified.

Triacilglicerol intramuscular: um importante substrato energético para o exercício de endurance

Os ácidos graxos são uma importante fonte de energia para exercício de endurance. Os ácidos graxos plasmáticos encontram-se disponíveis para as fibras musculares sob a forma de ácidos graxos associados à albumina ou agregados à molécula de triacilglicerol (TAG) encontrada nas lipoproteínas. Entretanto, além dessas fontes plasmáticas, a hidrólise do TAG encontrado no músculo também pode contribuir com a oferta de ácidos graxos durante o exercício de endurance. O objetivo do presente trabalho foi realizar uma extensa revisão da literatura sobre a importância do TAG intramuscular como substrato energético. A revisão da literatura sugere que a contribuição dos estoques endógenos de TAG durante a realização do exercício de endurance é bastante relevante. Além disso, pode-se concluir que uma adaptação induzida pelo treinamento de endurance é o aumento dos estoques intramusculares de TAG. Após o treinamento de endurance, também é observado aumento na capacidade de utilização desses estoques. Apesar de parecer importante, a contribuição do TAG intramuscular ainda é motivo de controvérsia na literatura. Essa discrepância de resultados está relacionada às metodologias empregadas para estimar a sua oxidação no exercício. A fim de esgotar este assunto de maneira apropriada, mais pesquisas, com novos métodos (ex.: utilização de isótopos, ressonância magnética nuclear e microscopia eletrônica), precisam ser conduzidas.

Functional analysis of peroxisome-proliferator-responsive element motifs in genes of fatty acid-binding proteins

Retinoic acids and long-chain fatty acids are lipophilic agonists of nuclear receptors such as RXRs (retinoic X receptors) and PPARs (peroxisome-proliferator-activated receptors) respectively. These agonists are also ligands of intracellular lipid-binding proteins, which include FABPs (fatty acid-binding proteins). We reported previously that L (liver-type)-FABP targets fatty acids to the nucleus of hepatocytes and affects PPARalpha activation, which binds together with an RXR subtype to a PPRE (peroxisome-proliferator-responsive element). In the present study, we first determined the optimal combination of murine PPAR/RXR subtypes for binding to known murine FABP-PPREs and to those found by computer search and then tested their in vitro functionality. We show that all PPARs bind to L-FABP-PPRE, PPARalpha, PPARgamma1 and PPARgamma2 to A (adipocyte-type)-FABP-PPRE. All PPAR/RXR heterodimers transactivate L-FABP-PPRE, best are combinations of PPARalpha with RXRalpha or RXRgamma. In contrast, PPARalpha heterodimers do not transactivate A-FABP-PPRE, best combinations are of PPARgamma1 with RXRalpha and RXRgamma, and of PPARgamma2 with all RXR subtypes. We found that the predicted E (epidermal-type)- and H (heart-type)-FABP-PPREs are not activated by any PPAR/RXR combination without or with the PPAR pan-agonist bezafibrate. In the same way, C2C12 myoblasts transfected with promoter fragments of E-FABP and H-FABP genes containing putative PPREs are also not activated through stimulation of PPARs with bezafibrate applied to the cells. These results demonstrate that only PPREs of L- and A-FABP promoters are functional, and that binding of PPAR/RXR heterodimers to a PPRE in vitro does not necessarily predict transactivation.

Investigation of in vivo fatty acid metabolism in AFABP/aP2(-/-) mice

The metabolic impact of the murine adipocyte fatty acid-binding protein (AFABP/aP2) on lipid metabolism was investigated in the AFABP/aP2(-/-) mouse and compared with wild-type C57BL/6J littermates. Mice were weaned on a high-fat diet (59% of energy from fat) and acclimated to meal feeding. Stable isotopes were administered, and indirect calorimetry was performed to quantitate fatty acid flux, dietary fatty acid utilization, and substrate oxidation. Consistent with previous in situ and in vitro studies, fasting serum nonesterified fatty acid (NEFA) release was significantly reduced in AFABP/aP2(-/-) (17.1 +/- 9.0 vs. 51.9 +/- 22.9 mg.kg(-1).min(-1)). AFABP/aP2(-/-) exhibited higher serum NEFA (1.4 +/- 0.6 vs. 0.8 +/- 0.4 mmol/l, AFABP/aP2(-/-) vs. C57BL/6J, respectively) and triacylglycerol (TAG; 0.23 +/- 0.09 vs. 0.13 +/- 0.10 mmol/l) and accumulated more TAG in liver tissue (2.9 +/- 2.3 vs. 1.1 +/- 0.8% wet wt) in the fasted state. For the liver-TAG pool, 16.4 +/- 7.3% of TAG-fatty acids were derived from serum NEFA in AFABP/aP2(-/-). In contrast, a significantly greater portion of C57BL/6J liver-TAG was derived from serum NEFA (42.3 +/- 25.5%) during tracer infusion. For adipose-TAG stores, only 0.29 +/- 0.04% was derived from serum NEFA in AFABP/aP2(-/-), and, in C57BL/6J, 1.85 +/- 0.97% of adipose-TAG was derived from NEFA. In addition, AFABP/aP2(-/-) preferentially oxidized glucose relative to fatty acids in the fed state. These data demonstrate that in vivo disruption of AFABP/aP2(-/-) leads to changes in the following two major metabolic processes: 1) decreased adipose NEFA efflux and 2) preferential utilization of glucose relative to fatty acids.

Tryptophan insertion mutagenesis of liver fatty acid-binding protein: L28W mutant provides important insights into ligand binding and physiological function

Liver fatty acid-binding protein (FABP) binds a variety of non-polar anionic ligands including fatty acids, fatty acyl CoAs, and bile acids. Previously we prepared charge reversal mutants and demonstrated the importance of lysine residues within the portal region in ligand and membrane binding. We have now prepared several tryptophan-containing mutants within the portal region, and one tryptophan at position 28 (L28W) has proved remarkably effective as an intrinsic probe to further study ligand binding. The fluorescence of the L28W mutant was very sensitive to fatty acid and bile acid binding where a large (up to 4-fold) fluorescence enhancement was obtained. In contrast, the binding of oleoyl CoA reduced tryptophan fluorescence. Positive cooperativity for fatty acid binding was observed while detailed information on the orientation of binding of bile acid derivatives was obtained. The ability of bound oleoyl CoA to reduce the fluorescence of L28W provided an opportunity to demonstrate that fatty acyl CoAs can compete with fatty acids for binding to liver FABP under physiological conditions, further highlighting the role of fatty acyl CoAs in modulating FABP function in the cell.

Fatty acid-binding proteins--insights from genetic manipulations

Fatty acid-binding proteins (FABPs) belong to the conserved multigene family of the intracellular lipid-binding proteins (iLBPs). These proteins are ubiquitously expressed in vertebrate tissues, with distinct expression patterns for the individual FABPs. Various functions have been proposed for these proteins, including the promotion of cellular uptake and transport of fatty acids, the targeting of fatty acids to specific metabolic pathways, and the participation in the regulation of gene expression and cell growth. Novel genetic tools that have become available in recent years, such as transgenic cell lines, animals, and knock-out mice, have provided the opportunity to test these concepts in physiological settings. Such studies have helped to define essential cellular functions of individual FABP-types or of combinations of several different FABPs. The deletion of particular FABP genes, however, has not led to gross phenotypical changes, most likely because of compensatory overexpression of other members of the iLBP gene family, or even of unrelated fatty acid transport proteins. This review summarizes the properties of the various FABPs expressed in mammalian tissues, and discusses the transgenic and ablation studies carried out to date in a functional context.

Fatty acid binding proteins--the evolutionary crossroads of inflammatory and metabolic responses

Fatty acid binding proteins (FABPs) are members of a highly conserved family of proteins with the task of protecting a cell's delicate lipid balance. Yet they fail when faced with metabolic or inflammatory stress, turning the cytosol into an inhospitable environment with less than ideal outcomes. This review will focus on how FABPs direct lipid traffic and simultaneously control inflammatory and metabolic pathways under the pressures of the Metabolic Syndrome.

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).

Sequence and structural analysis of cellular retinoic acid-binding proteins reveals a network of conserved hydrophobic interactions

Proteins in the intracellular lipid-binding protein (iLBP) family show remarkably high structural conservation despite their low-sequence identity. A multiple-sequence alignment using 52 sequences of iLBP family members revealed 15 fully conserved positions, with a disproportionately high number of these (n=7) located in the relatively small helical region. The conserved positions displayed high structural conservation based on comparisons of known iLBP crystal structures. It is striking that the beta-sheet domain had few conserved positions, despite its high structural conservation. This observation prompted us to analyze pair-wise interactions within the beta-sheet region to ask whether structural information was encoded in interacting amino acid pairs. We conducted this analysis on the iLBP family member, cellular retinoic acid-binding protein I (CRABP I), whose folding mechanism is under study in our laboratory. Indeed, an analysis based on a simple classification of hydrophobic and polar amino acids revealed a network of conserved interactions in CRABP I that cluster spatially, suggesting a possible nucleation site for folding. Significantly, a small number of residues participated in multiple conserved interactions, suggesting a key role for these sites in the structure and folding of CRABP I. The results presented here correlate well with available experimental evidence on folding of CRABPs and their family members and suggest future experiments. The analysis also shows the usefulness of considering pair-wise conservation based on a simple classification of amino acids, in analyzing sequences and structures to find common core regions among homologues.

Specific localization of epidermal-type fatty acid binding protein in dendritic cells of splenic white pulp

Dendritic cells in the splenic white pulp of mice were intensely immunoreactive for epidermal-type fatty acid binding protein (E-FABP). This specific immunostaining revealed a clear difference in morphology between the dendritic cells in the periarterial lymphoid sheath (PALS) and follicular dendritic cells in the follicles in terms of cell sizes and process branching. No immunoreactivity was detected in dendritic cells in the marginal zones and the red pulp, although endothelial cells of almost all capillaries in the red pulp were immunoreactive for E-FABP. After peritoneal injection of lipopolysaccharide, the immunoreactive cells in PALS progressively enlarged and became rounded in shape with a peak in size at 24 h postinjection and they eventually resumed the dendritic form at 48 h postinjection. Within each of the enlarged immunoreactive cell perikarya were included small immunonegative apoptotic cells, presumptive lymphocytes. Taken together, E-FABP is useful as a marker for dendritic cells in the splenic white pulp, and may be involved through combination with fatty acids in antigen presentation and retention as well as in cytokine production.

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.

Adipogenic differentiating agents regulate expression of fatty acid binding protein and CD36 in the J744 macrophage cell line

Adipocyte fatty acid binding protein (aP2) is a key mediator of intracellular transport and metabolism of fatty acids. Its expression during adipocyte differentiation is regulated through the actions of peroxisome proliferator-activated receptor gamma (PPARgamma) and CCAAT/enhancer binding protein alpha (C/EBPalpha). Macrophages also express aP2, and the lack of macrophage aP2 significantly reduces atherosclerotic lesion size in hypercholesterolemic mice. We investigated the regulation of expression of macrophage aP2 and CD36, a fatty acid membrane binding protein and scavenger receptor, in response to the adipogenic agents isobutylmethylxanthine (IBMX), insulin, and dexamethasone, a combination of agents shown to induce fibroblast-to-adipocyte differentiation. Treatment of J774 macrophages with adipogenic agents significantly induced aP2 mRNA expression, while CD36 expression was inhibited. Dexamethasone was essential and sufficient to induce aP2 expression, and insulin had a synergistic effect. However, IBMX antagonized induced-aP2 expression. aP2 protein expression and [14C]oleic acid uptake by macrophages were also increased by dexamethasone. Unlike what occurs in adipocytes, adipogenic agents had mixed effects on the expression of PPARgamma and C/EBPalpha in macrophages. Our data demonstrate differences in the regulation of aP2 in adipocytes and macrophages and show that macrophage aP2 expression by adipogenic agents is independent of the PPARgamma and/or C/EBPalpha signaling pathway.

The polyprotein and FAR lipid binding proteins of nematodes: shape and monomer/dimer states in ligand-free and bound forms

Nematodes produce two classes of small, helix-rich fatty acid- and retinol-binding proteins whose structures and in vivo functions remain to be elucidated. These are the polyprotein allergens (NPA) and the FAR proteins. The solution properties of recombinant forms of these proteins from parasitic [Ascaris suum (As) and Onchocerca volvulus (Ov)] and free-living [Caenorhabditis elegans (Ce)] nematodes have been examined. Analytical ultracentrifugation (AUC) showed that, contrary to previous findings, the rAs-NPA-1A polyprotein unit (approximately 15 kDa) is a monomer, and this stoichiometry is unaltered by ligand (oleic acid) binding. The rOv-FAR-1 and rCe-FAR-5 proteins differ in that the former forms a tight dimer and the latter a monomer, and these oligomeric states are also unaffected by ligand binding or protein concentration. Sedimentation equilibrium experiments showed that the partial specific volume v of the unliganded proteins agree well with the value calculated from amino acid composition extrapolated to experimental temperature, and was unaffected upon ligand binding. Data from small-angle X-ray scattering (SAXS) indicated that both of the monomeric proteins rAs-NPA-1A and rCe-FAR-5 are globular, although slightly elongated and flattened. These data are in good agreement with shapes predicted from sedimentation velocity experiments and hydrodynamic bead modelling. On the basis of functional and secondary structural homology with the ligand-binding domain of the retinoic acid receptor RXRalpha, de novo atomic resolution structures for rAs-NPA-1A and rCe-FAR-5 have been constructed which are consistent with the SAXS and hydrodynamic data.

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.

A common polymorphism in the fatty acid transport protein-1 gene associated with elevated post-prandial lipaemia and alterations in LDL particle size distribution

The fatty acid transport proteins (FATPs) have been implicated in facilitated cellular uptake of non-esterified fatty acids (NEFAs), thus having the potential to regulate local and systemic NEFA concentrations and metabolism. Hypothesising that genetic variation within the FATP genes may affect lipid metabolism, we investigated a G/A substitution at position 48 in intron 8 of the fatty acid transport-1 (FATP1) gene with respect to associations with fasting and post-prandial plasma lipid and lipoprotein variables in 628 healthy 50-year-old Swedish men and 426 Swedish women, aged 37-65 years. A subset of 105 men with the apoE3/E3 genotype underwent an oral fat tolerance test. Although fasting plasma TG concentrations were not different, male A/A individuals had significantly higher post-prandial TG concentrations and VLDL(1) (S(f) 60-400 apoB100)-to-VLDL(2) (S(f) 20-60 apoB100) ratio compared to male G/A and G/G individuals. A/A individuals apparently failed to suppress plasma NEFA concentrations during the oral fat tolerance test. Furthermore, fasting plasma concentrations of the largest, most buoyant LDL subfraction (LDL-I) were significantly lower in carriers of the A allele in the male cohort. Electromobility shift assays and reporter gene studies indicated that binding of nuclear factors and effect on transcriptional activity differ between the intron 8 alleles. These findings suggest that through regulation of NEFA trafficking, FATP1 might play a role in post-prandial lipid metabolism and development of cardiovascular disease.

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

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

The FAR protein family of the nematode Caenorhabditis elegans. Differential lipid binding properties, structural characteristics, and developmental regulation

Parasitic nematodes of humans and plants secrete a structurally novel type of fatty acid- and retinol-binding protein, FAR, into the tissues they occupy. These proteins may interfere with intercellular lipid signaling to manipulate the defense reactions of the host or acquire essential lipids for the parasites. The genome of the nematode Caenorhabditis elegans encodes eight FAR-like proteins (Ce-FAR-1 to -8). These fall into three discrete groups as indicated by phylogenetic sequence comparisons and intron positions, the proteins from parasitic nematodes falling into group A. Recombinant Ce-FAR-1 to -7 were produced in Escherichia coli and tested for lipid binding in fluorescence-based assays. Ce-FAR-1 to -6 bound DAUDA (11-((5-dimethylaminonaphthalene-1-sulfonyl)amino)undecanoic acid), cis-parinaric acid, and retinol with dissociation constants in the micromolar range, whereas Ce-FAR-7 bound the latter two lipids relatively poorly. Each protein produced a characteristic shift in peak fluorescence emission of DAUDA, and one (Ce-FAR-5) produced a shift greater than has been observed previously for any lipid-binding protein. Selected Ce-FAR proteins were analyzed by circular dichroism (CD) and differential scanning calorimetry, were found to be helix-rich, and exhibited high thermal stability (transition midpoint, 82.7 degrees C). CD and secondary structure predictions, however, both indicated that Ce-FAR-7 possesses substantially less helix than the other FAR proteins. The genes encoding the Ce-FAR proteins were found to be transcribed differentially through the life cycle of C. elegans, such that Ce-far-4 was transcribed at highest levels in the fourth larval stage, and Ce-far-3 and -7 predominated in males.

Structure, mRNA expression and linkage mapping of the brain-type fatty acid-binding protein gene (FABP7) from zebrafish (Danio rerio)

The brain fatty acid-binding protein (B-FABP) is involved in brain development and adult neurogenesis. We have determined the sequence of the gene encoding the B-FABP in zebrafish. The zebrafish B-FABP gene spans 2370 bp and contains four exons interrupted by three introns. The coding sequence of zebrafish B-FABP gene is identical to its cDNA sequence and the coding capacity of each exon is the same as that for the human and mouse B-FABP genes. A 1249 bp sequence 5' upstream of exon 1 of the zebrafish B-FABP gene was cloned and sequenced. Several brain development/growth-associated transcription factor binding elements, including POU-domain binding elements and the proposed lipogenic-associated transcription factor NF-Y elements, were found within the 5' region of the B-FABP gene. RT-PCR analysis using mRNA extracted from different tissues of adult zebrafish demonstrated that the zebrafish B-FABP mRNA was predominant in brain with lower levels in liver, testis and intestine, but not in ovary, skin, heart, kidney and muscle. Quantitative RT-PCR revealed a similar tissue-specific distribution for zebrafish B-FABP mRNA except that very low levels of B-FABP mRNA, normalized to beta-actin mRNA, were detected in the heart and muscle RNA, but not in liver RNA. Zebrafish B-FABP mRNA was detected by RT-PCR in embryos beyond 12 h postfertilization, suggesting a correlation of zebrafish B-FABP mRNA expression with early brain development. Radiation hybrid mapping assigned the zebrafish B-FABP gene to linkage group 17. Conserved syntenies of the zebrafish B-FABP gene and the human and mouse orthologous B-FABP genes were observed by comparative genomic analysis.