Oligomerization of the murine fatty acid transport protein 1

Fatty acid metabolism in aggressive B-cell lymphoma is inhibited by tetraspanin CD37

The importance of fatty acid (FA) metabolism in cancer is well-established, yet the mechanisms underlying metabolic reprogramming remain elusive. Here, we identify tetraspanin CD37, a prognostic marker for aggressive B-cell lymphoma, as essential membrane-localized inhibitor of FA metabolism. Deletion of CD37 on lymphoma cells results in increased FA oxidation shown by functional assays and metabolomics. Furthermore, CD37-negative lymphomas selectively deplete palmitate from serum in mouse studies. Mechanistically, CD37 inhibits the FA transporter FATP1 through molecular interaction. Consequently, deletion of CD37 induces uptake and processing of exogenous palmitate into energy and essential building blocks for proliferation, and inhibition of FATP1 reverses this phenotype. Large lipid deposits and intracellular lipid droplets are observed in CD37-negative lymphoma tissues of patients. Moreover, inhibition of carnitine palmitoyl transferase 1 A significantly compromises viability and proliferation of CD37-deficient lymphomas. Collectively, our results identify CD37 as a direct gatekeeper of the FA metabolic switch in aggressive B-cell lymphoma.

Tetraspanin CD37 deficiency has been reported as a prognostic marker for aggressive B-cell lymphoma. Here, the authors show that CD37 interacts with the fatty acid transporter 1 to inhibit palmitate uptake and its deficiency leads to increased fatty acid metabolism which promotes tumorigenesis in B-cell lymphoma.

Intestinal Saturated Long-Chain Fatty Acid, Glucose and Fructose Transporters and Their Inhibition by Natural Plant Extracts in Caco-2 Cells

The intestinal absorption of fatty acids, glucose and fructose is part of the basic requirements for the provision of energy in the body. High access of saturated long-chain fatty acids (LCFA), glucose and fructose can facilitate the development of metabolic diseases, particularly the metabolic syndrome and type-2 diabetes mellitus (T2DM). Research has been done to find substances which decelerate or inhibit intestinal resorption of these specific food components. Promising targets are the inhibition of intestinal long-chain fatty acid (FATP2, FATP4), glucose (SGLT1, GLUT2) and fructose (GLUT2, GLUT5) transporters by plant extracts and by pure substances. The largest part of active components in plant extracts belongs to the group of polyphenols. This review summarizes the knowledge about binding sites of named transporters and lists the plant extracts which were tested in Caco-2 cells regarding uptake inhibition.

The cold-induced lipokine 12,13-diHOME promotes fatty acid transport into brown adipose tissue

Brown adipose tissue (BAT) and beige adipose tissue combust fuels for heat production in adult humans, and so constitute an appealing target for the treatment of metabolic disorders such as obesity, diabetes and hyperlipidemia. Cold exposure can enhance energy expenditure by activating BAT, and it has been shown to improve nutrient metabolism. These therapies, however, are time consuming and uncomfortable, demonstrating the need for pharmacological interventions. Recently, lipids have been identified that are released from tissues and act locally or systemically to promote insulin sensitivity and glucose tolerance; as a class, these lipids are referred to as 'lipokines'. Because BAT is a specialized metabolic tissue that takes up and burns lipids and is linked to systemic metabolic homeostasis, we hypothesized that there might be thermogenic lipokines that activate BAT in response to cold. Here we show that the lipid 12,13-dihydroxy-9Z-octadecenoic acid (12,13-diHOME) is a stimulator of BAT activity, and that its levels are negatively correlated with body-mass index and insulin sensitivity. Using a global lipidomic analysis, we found that 12,13-diHOME was increased in the circulation of humans and mice exposed to cold. Furthermore, we found that the enzymes that produce 12,13-diHOME were uniquely induced in BAT by cold stimulation. The injection of 12,13-diHOME acutely activated BAT fuel uptake and enhanced cold tolerance, which resulted in decreased levels of serum triglycerides. Mechanistically, 12,13-diHOME increased fatty acid (FA) uptake into brown adipocytes by promoting the translocation of the FA transporters FATP1 and CD36 to the cell membrane. These data suggest that 12,13-diHOME, or a functional analog, could be developed as a treatment for metabolic disorders.

Investigation of the fatty acid transporter-encoding genes SLC27A3 and SLC27A4 in autism

The solute carrier 27A (SLC27A) gene family encodes fatty acid transport proteins (FATPs) and includes 6 members. During fetal and postnatal periods of development, the growing brain requires a reliable supply of fatty acids. Because autism spectrum disorders (ASD) are now recognized as disorders caused by impaired early brain development, it is possible that functional abnormalities of SLC27A genes may contribute to the pathogenesis of ASD. Here, we confirmed the expression of SLC27A3 and SLC27A4 in human neural stem cells derived from human induced pluripotent stem cells, which suggested their involvement in the developmental stage of the central nervous system. Additionally, we resequenced the SLC27A3 and SLC27A4 genes using 267 ASD patient and 1140 control samples and detected 47 (44 novel and 29 nonsynonymous) and 30 (17 novel and 14 nonsynonymous) variants for the SLC27A3 and SLC27A4, respectively, revealing that they are highly polymorphic with multiple rare variants. The SLC27A4 Ser209 allele was more frequently represented in ASD samples. Furthermore, we showed that a SLC27A4 Ser209 mutant resulted in significantly higher fluorescently-labeled fatty acid uptake into bEnd3 cells, a mouse brain capillary-derived endothelial cell line, compared with SLC27A4 Gly209, suggesting that the functional change may contribute to ASD pathophysiology.

Structural characterization and subcellular localization of Drosophila organic solute carrier partner 1

Background

Organic solute carrier partner 1 (OSCP1) is known to facilitate the transport of various organic solutes into cells and reported to play a role in cell growth and cell differentiation. Moreover, OSCP1 is known as a tumor suppressor gene that is frequently down-expressed in nasopharyngeal carcinomas and acute myeloid leukemia. However, the underlying mechanisms of action remain unclear and the subcellular localization of OSCP1 has yet to be determined in detail.

Results

Drosophila contains a single orthologue of OSCP1 (dOSCP1) that shares 58% homology with its human counterpart. To study the expression pattern and subcellular localization of dOSCP1, we prepared a specific antibody. Subcellular localization analyses of dOSCP1 with these revealed localization in the plasma membrane, endoplasmic reticulum, Golgi apparatus and mitochondria, but no detection in cytosol. dOSCP1 signals were also detected in the nucleus, although at weaker intensity than in plasma membranes and subcellular organelles. In addition, native polyacrylamide gel electrophoresis analysis with and without β-mercaptoethanol treatment revealed that recombinant dOSCP1 forms dimers and trimers in solution. The dimer form of dOSCP1 could also be detected by Western immunoblot analyses in third instar larval extracts.

Conclusions

The data revealed that dOSCP1 localizes not only in the plasma membrane but also in the nucleus, ER, Golgi apparatus and mitochondria. It is therefore conceivable that this protein may interact with various partners or form multimeric complexes with other proteins to play multiple roles in cells, providing clues to understanding the functions of dOSCP1 during Drosophila development.

Endothelial fatty acid transport: role of vascular endothelial growth factor B

Dietary lipids present in the circulation have to be transported through the vascular endothelium to be utilized by tissue cells, a vital mechanism that is still poorly understood. Vascular endothelial growth factor B (VEGF-B) regulates this process by controlling the expression of endothelial fatty acid transporter proteins (FATPs). Here, we summarize research on the role of the vascular endothelium in nutrient transport, with emphasis on VEGF-B signaling.

SLC27 fatty acid transport proteins

The uptake and metabolism of long chain fatty acids (LCFA) are critical to many physiological and cellular processes. Aberrant accumulation or depletion of LCFA underlie the pathology of numerous metabolic diseases. Protein-mediated transport of LCFA has been proposed as the major mode of LCFA uptake and activation. Several proteins have been identified to be involved in LCFA uptake. This review focuses on the SLC27 family of fatty acid transport proteins, also known as FATPs, with an emphasis on the gain- and loss-of-function animal models that elucidate the functions of FATPs in vivo and how these transport proteins play a role in physiological and pathological situations.

The FATP1-DGAT2 complex facilitates lipid droplet expansion at the ER-lipid droplet interface

A complex between the ER resident protein FATP1 and the lipid droplet–localized DGAT2 protein facilitates lipid droplet expansion in C. elegans and mammalian cells.

At the subcellular level, fat storage is confined to the evolutionarily conserved compartments termed lipid droplets (LDs), which are closely associated with the endoplasmic reticulum (ER). However, the molecular mechanisms that enable ER–LD interaction and facilitate neutral lipid loading into LDs are poorly understood. In this paper, we present evidence that FATP1/acyl-CoA synthetase and DGAT2/diacylglycerol acyltransferase are components of a triglyceride synthesis complex that facilitates LD expansion. A loss of FATP1 or DGAT2 function blocked LD expansion in Caenorhabditis elegans. FATP1 preferentially associated with DGAT2, and they acted synergistically to promote LD expansion in mammalian cells. Live imaging indicated that FATP1 and DGAT2 are ER and LD resident proteins, respectively, and electron microscopy revealed FATP1 and DGAT2 foci close to the LD surface. Furthermore, DGAT2 that was retained in the ER failed to support LD expansion. We propose that the evolutionarily conserved FATP1–DGAT2 complex acts at the ER–LD interface and couples the synthesis and deposition of triglycerides into LDs both physically and functionally.

Fatty acid transport proteins, implications in physiology and disease

Uptake of long-chain fatty acids plays pivotal roles in metabolic homeostasis and human physiology. Uptake rates must be controlled in an organ-specific fashion to balance storage with metabolic needs during transitions between fasted and fed states. Many obesity-associated diseases, such as insulin resistance in skeletal muscle, cardiac lipotoxicity, and hepatic steatosis, are thought to be driven by the overflow of fatty acids from adipose stores and the subsequent ectopic accumulation of lipids resulting in apoptosis, ER stress, and inactivation of the insulin receptor signaling cascade. Thus, it is of critical importance to understand the components that regulate the flux of fatty acid between the different organ systems. Cellular uptake of fatty acids by key metabolic organs, including the intestine, adipose tissue, muscle, heart, and liver, has been shown to be protein mediated and various unique combinations of fatty acid transport proteins (FATPs/SLC27A1-6) are expressed by all of these tissues. Here we review our current understanding of how FATPs can contribute to normal physiology and how FATP mutations as well as hypo- and hypermorphic changes contribute to disorders ranging from cardiac lipotoxicity to hepatosteatosis and ichthyosis. Ultimately, our increasing knowledge of FATP biology has the potential to lead to the development of new diagnostic tools and treatment options for some of the most pervasive chronic human disorders. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

Fatty acid flux in adipocytes: the in's and out's of fat cell lipid trafficking

The trafficking of fatty acids into and out of adipocytes is regulated by a complex series of proteins and enzymes and is under control by a variety of hormonal and metabolic factors. The biochemical basis of fatty acid influx, despite its widespread appreciation, remains enigmatic with regard to the biophysical and biochemical properties that facilitate long-chain fatty acid uptake. Fatty acid efflux is initiated by hormonally controlled lipolysis of the droplet stores and produces fatty acids that must transit from their site of production to the plasma membrane and subsequently out of the cells. This review will focus on the "in's and out's" of fatty acid trafficking and summarize the current concepts in the field.

Membrane Fatty Acid Transporters as Regulators of Lipid Metabolism: Implications for Metabolic Disease

Long-chain fatty acids and lipids serve a wide variety of functions in mammalian homeostasis, particularly in the formation and dynamic properties of biological membranes and as fuels for energy production in tissues such as heart and skeletal muscle. On the other hand, long-chain fatty acid metabolites may exert toxic effects on cellular functions and cause cell injury. Therefore, fatty acid uptake into the cell and intracellular handling need to be carefully controlled. In the last few years, our knowledge of the regulation of cellular fatty acid uptake has dramatically increased. Notably, fatty acid uptake was found to occur by a mechanism that resembles that of cellular glucose uptake. Thus, following an acute stimulus, particularly insulin or muscle contraction, specific fatty acid transporters translocate from intracellular stores to the plasma membrane to facilitate fatty acid uptake, just as these same stimuli recruit glucose transporters to increase glucose uptake. This regulatory mechanism is important to clear lipids from the circulation postprandially and to rapidly facilitate substrate provision when the metabolic demands of heart and muscle are increased by contractile activity. Studies in both humans and animal models have implicated fatty acid transporters in the pathogenesis of diseases such as the progression of obesity to insulin resistance and type 2 diabetes. As a result, membrane fatty acid transporters are now being regarded as a promising therapeutic target to redirect lipid fluxes in the body in an organ-specific fashion.

Functional domains of the fatty acid transport proteins: studies using protein chimeras

Fatty acid transport proteins (FATP) function in fatty acid trafficking pathways, several of which have been shown to participate in the transport of exogenous fatty acids into the cell. Members of this protein family also function as acyl CoA synthetases with specificity towards very long chain fatty acids or bile acids. These proteins have two identifying sequence motifs: The ATP/AMP motif, an approximately 100 amino acid segment required for ATP binding and common to members of the adenylate-forming super family of proteins, and the FATP/VLACS motif that consists of approximately 50 amino acid residues and is restricted to members of the FATP family. This latter motif has been implicated in fatty acid transport in the yeast FATP orthologue Fat1p. In the present studies using a yeast strain containing deletions in FAT1 (encoding Fat1p) and FAA1 (encoding the major acyl CoA synthetase (Acsl) Faa1p) as an experimental platform, the phenotypic and functional properties of specific murine FATP1-FATP4 and FATP6-FATP4 protein chimeras were evaluated in order to define elements within these proteins that further distinguish the fatty acid transport and activation functions. As expected from previous work FATP1 and FATP4 were functional in the fatty acid transport pathway, while and FATP6 was not. All three isoforms were able to activate the very long chain fatty acids arachidonate (C(20:4)) and lignocerate (C(24:0)), but with distinguishing activities between saturated and highly unsaturated ligands. A 73 amino acid segment common to FATP1 and FATP4 and between the ATP/AMP and FATP/VLACS motifs was identified by studying the chimeras, which is hypothesized to contribute to the transport function.

Structural and functional characterization of the bovine solute carrier family 27 member 1 (SLC27A1) gene

The Solute Carrier Family 27 Member 1 (SLC27A1) is an evolutionarily conserved protein involved in regulating the long chain fatty acid uptake into cells. It has been shown to be expressed in tissues undergoing rapid fatty acid metabolism such as heart, skeletal muscle and adipose tissues, but no expression is detected in liver. Here we report the molecular characterization of the bovine SLC27A1 gene and draw a comparison with orthologous genes of some monogastric species. The bovine SLC27A1 gene is organized in 13 exons and extends over more than 40 kb of genomic DNA. It codes for a protein of 646 amino acids with a predicted molecular weight of 71 kDa which has 92%, 88% and 88% similarity with the human, mouse and rat SLC27A1 proteins respectively. The bovine SLC27A1 RNA expression was high in heart, testis, nervous tissue and muscle and very low in liver. Surprisingly, adipose tissues showed very low RNA expression levels contrary to the results described for both human and mouse genes. On the other hand, discordances observed between the bovine SLC27A1 RNA and protein expression patterns suggest that complex regulation mechanisms may be involved in determining the final SLC27A1 protein levels in each tissue. Finally, we have identified an alternative transcript generated by exon skipping of exon 3 to 7 which could encode a cytosolic SLC27A1 isoform of approximately 37 kDa.

Protein-mediated fatty acid uptake: novel insights from in vivo models

Long-chain fatty acids are both important metabolites as well as signaling molecules. Fatty acid transport proteins are key mediators of cellular fatty acid uptake and recent transgenic and knockout animal models have provided new insights into their contribution to energy homeostasis and to pathological processes, including obesity and insulin desensitization.

Scavenger receptor class B Type I (SR-BI) assembles into detergent-sensitive dimers and tetramers

High density lipoproteins (HDL) are protective against cardiovascular disease due to their important role in the reverse cholesterol transport (RCT) pathway. The selective transfer of cholesteryl ester (CE) from the HDL core to cells, the last step in RCT, is mediated by scavenger receptor class B type I (SR-BI). SR-BI is a heavily glycosylated cell surface receptor that is highly expressed in the liver, ovaries, testes and adrenal glands, where selective uptake of HDL-CE is most prevalent. Previous studies have shown that SR-BI oligomerizes with itself in steroidogenic tissues as well as in diverse cell lines. In the present study, we provide further evidence for the homo-oligomerization of SR-BI. We show by FPLC and blue native PAGE that SR-BI forms complexes whose sizes suggest the formation of monomers, dimers, and tetramers. Interestingly, homo-oligomerization occurs even with the absence of SR-BI's C-terminal cytoplasmic domain. Finally, we report that an inhibitor of SR-BI-mediated cholesterol transport, BLT-1, and mutations in the putative leucine zipper region of SR-BI have profound effects on SR-BI function, however, they do not affect receptor self-association. These observations indicate that SR-BI homo-oligomerization occurs even when the receptor is non-functional.

Deletion and alanine mutation analyses for the formation of active homo- or hetero-dimer complexes of mouse choline kinase-α and -β

Choline kinase (CK) is the first-step regulatory enzyme for the biosynthesis of phosphatidylcholine in all mammalian cells. It exists as at least three isoforms (alpha1, alpha2 and beta) that are encoded by two separate genes termed ck-alpha and ck-beta. The active enzyme has been proposed to consist of either their homo- or hetero-dimeric forms. Here, we report on the identification of several essential domains and amino acid residues involved in their active dimer formation. Full-length cDNAs or their truncated or alanine-mutated versions for mouse CK-alpha1 and CK-beta tagged with either HA or Myc at their N-termini were expressed in COS-7 cells. Each dimer formation was analyzed by immuno-precipitation followed by Western blotting. Kinetic analysis for CK reaction was performed with different expression products. Both the N-terminal domain-1 and C-terminal portions (E424-K430 for CK-alpha1 and Q379-K385 for CK-beta) were shown to be critical for the formation of active homo- or hetero-dimer complex. Interestingly, D320 in the CK-motif of CK-alpha1 was found to be essential for alpha1/alpha1 homo-dimerization but not for alpha1/beta hetero-dimerization. A mutation of the corresponding D276 of CK-beta to A276 did not show any effect on either its homo- or hetero-dimerization but it caused a strong inhibition of CK activity in either case.

Fatty acid transport protein 1 and long-chain acyl coenzyme A synthetase 1 interact in adipocytes

The fatty acid transport proteins (FATP) and long-chain acyl coenzyme A synthetase (ACSL) proteins have been shown to play a role in facilitating long-chain fatty acid (LCFA) transport in mammalian cells under physiologic conditions. The involvement of both FATP and ACSL proteins is consistent with the model of vectorial acylation, in which fatty acid transport is coupled to esterification. This study was undertaken to determine whether the functions of these proteins are coordinated through a protein-protein interaction that might serve as a point of regulation for cellular fatty acid transport. We demonstrate for the first time that FATP1 and ACSL1 coimmunoprecipitate in 3T3-L1 adipocytes, indicating that these proteins form an oligomeric complex. The efficiency of FATP1 and ACSL1 coimmunoprecipitation is unaltered by acute insulin treatment, which stimulates fatty acid uptake, or by treatment with isoproterenol, which decreases fatty acid uptake and stimulates lipolysis. Moreover, inhibition of ACSL1 activity in adipocytes impairs fatty acid uptake, suggesting that esterification is essential for fatty acid transport. Together, our findings suggest that a constitutive interaction between FATP1 and ACSL1 contributes to the efficient cellular uptake of LCFAs in adipocytes through vectorial acylation.

Fatty acid transport proteins and insulin resistance

PURPOSE OF REVIEW

Disturbed fatty acid metabolism and homeostasis is associated with insulin resistance. The aim of this review, therefore, is to summarize recent developments relating to the relevance and importance of the fatty acid transport proteins (FATPs) in the aetiology of insulin resistance. In particular, the potential differences between the six members of the FATP family will be considered.

RECENT FINDINGS

FATP1 knockout mice failed to develop insulin resistance associated with lipid infusion or a high-fat diet, as wild-type mice did. FATP1-mediated fatty acid uptake may cause intramuscular lipid accumulation leading to insulin resistance in muscle if the fatty acids are not oxidized. While mouse models demonstrated an absolute requirement for FATP4 for survival, they provided no direct evidence for a role of FATP4 in insulin resistance. However, expression of FATP4 in human adipose tissue was increased in obesity (independent of genetic factors). While other members of the FATP family have important roles in fatty acid metabolism, they have not been clearly linked to insulin resistance. FATP-mediated fatty acid uptake may be driven by intrinsic acyl-CoA synthase activity.

SUMMARY

Any role in the development of insulin resistance is likely to be different for each member of the FATP family. So far, both FATP1 and FATP4 have been associated with parameters related to insulin resistance. Whether increased FATP-mediated fatty acid uptake is beneficial or detrimental may be dependent on the tissue in question and on the subsequent fate of the fatty acids. These issues remain to be resolved.

Comparative Biochemical Studies of the Murine Fatty Acid Transport Proteins (FATP) Expressed in Yeast

The fatty acid transport protein (FATP) family is a group of proteins that are predicted to be components of specific fatty acid trafficking pathways. In mammalian systems, six different isoforms have been identified, which function in the import of exogenous fatty acids or in the activation of very long-chain fatty acids. This has led to controversy as to whether these proteins function as membrane-bound fatty acid transporters or as acyl-CoA synthetases, which activate long-chain fatty acids concomitant with transport. The yeast FATP orthologue, Fat1p, is a dual functional protein and is required for both the import of long-chain fatty acids and the activation of very long-chain fatty acids; these activities intrinsic to Fat1p are separable functions. To more precisely define the roles of the different mammalian isoforms in fatty acid trafficking, the six murine proteins (mmFATP1-6) were expressed and characterized in a genetically defined yeast strain, which cannot transport long-chain fatty acids and has reduced long-chain acyl-CoA synthetase activity (fat1Delta faa1Delta). Each isoform was evaluated for fatty acid transport, fatty acid activation (using C18:1, C20:4, and C24:0 as substrates), and accumulation of very long-chain fatty acids. Murine FATP1, -2, and -4 complemented the defects in fatty acid transport and very long-chain fatty acid activation associated with a deletion of the yeast FAT1 gene; mmFATP3, -5, and -6 did not complement the transport function even though each was localized to the yeast plasma membrane. Both mmFATP3 and -6 activated C20:4 and C20:4, while the expression of mmFATP5 did not substantially increase acyl-CoA synthetases activities using the substrates tested. These data support the conclusion that the different mmFATP isoforms play unique roles in fatty acid trafficking, including the transport of exogenous long-chain fatty acids.

Role of FATP in parenchymal cell fatty acid uptake

Long-chain fatty acids (LCFAs) represent key metabolites for energy generation and storage. Transport and metabolism of LCFA are believed to be regulated by membrane-associated proteins that bind and transport LCFA. Identifying the postulated fatty acid transporters is of considerable interest since altered fatty acid uptake has been implicated in disease such as insulin resistance and obesity. Recently, a family of membrane associated proteins, termed fatty acid transport proteins (FATPs), have been described that enhance uptake of LCFAs. Until today, six members of this family, designated FATP1-6, have been characterized. This review will focus on FATP structure, expression patterns, regulation, mechanism of transport and clinical implications.

Transgenic expression of fatty acid transport protein 1 in the heart causes lipotoxic cardiomyopathy

Evidence is emerging that systemic metabolic disturbances contribute to cardiac myocyte dysfunction and clinically apparent heart failure, independent of associated coronary artery disease. To test the hypothesis that perturbation of lipid homeostasis in cardiomyocytes contributes to cardiac dysfunction, we engineered transgenic mice with cardiac-specific overexpression of fatty acid transport protein 1 (FATP1) using the alpha-myosin heavy chain gene promoter. Two independent transgenic lines demonstrate 4-fold increased myocardial free fatty acid (FFA) uptake that is consistent with the known function of FATP1. Increased FFA uptake in this model likely contributes to early cardiomyocyte FFA accumulation (2-fold increased) and subsequent increased cardiac FFA metabolism (2-fold). By 3 months of age, transgenic mice have echocardiographic evidence of impaired left ventricular filling and biatrial enlargement, but preserved systolic function. Doppler tissue imaging and hemodynamic studies confirm that these mice have predominantly diastolic dysfunction. Furthermore, ambulatory ECG monitoring reveals prolonged QT(c) intervals, reflecting reductions in the densities of repolarizing, voltage-gated K+ currents in ventricular myocytes. Our results show that in the absence of systemic metabolic disturbances, such as diabetes or hyperlipidemia, perturbation of cardiomyocyte lipid homeostasis leads to cardiac dysfunction with pathophysiological findings similar to those in diabetic cardiomyopathy. Moreover, the MHC-FATP model supports a role for FATPs in FFA import into the heart in vivo.

Conjugated linoleic acid isomers and trans fatty acids inhibit fatty acid transport in hepatoma 7288CTC and inguinal fat pads in Buffalo rats

Conjugated linoleic acid (CLA) and some trans fatty acids (FA) decrease tumor growth and alter tumor and host lipid uptake and storage. The goal of this study was to test the hypothesis that the acute inhibitory effects of CLA isomers and trans FAs on FA transport in tumors and white adipose tissue are mediated via an inhibitory G-protein coupled (GPC), FFA receptor (FFAR). Experiments were performed in hepatoma 7288CTC and inguinal fat pads in Buffalo rats during perfusion in situ. CLA isomers and trans FAs (0.03-0.4 mmol/L, in plasma) were added to the arterial blood, and FA uptake or release was measured by arterial minus venous difference. In hepatoma 7288CTC, the CLA isomers, t10,c12-CLA > (+/-)-9-HODE [13-(S)-hydroxyoctadecadienoic acid] > t9,t11-CLA, and the trans FAs, linolelaidic = vaccenic > elaidic, decreased cAMP content and inhibited FA uptake, 13(S)-HODE release, extracellular signal-regulated kinase p44/p42 phosphorylation, and [(3)H]thymidine incorporation. Other CLA isomers, c9,t11-CLA, 13-(S)-HODE, c9,c11-CLA, and c11,t13-CLA, had no effect. In inguinal fat pads, FA transport was inhibited by t10,c12-CLA = linolelaidic acid > trans vaccenic acid, whereas c9,t11-CLA had no effect. In both hepatoma 7288CTC and inguinal fat pad, addition of either pertussis toxin or 8-Br-cAMP to the arterial blood reversed the inhibitions of FA transport. These results support the idea that an inhibitory GPC FFAR reduces cAMP and controls FA transport by CLA isomers and trans FAs. Ligand activity is conferred by the presence of a trans double bond proximal to the carboxyl group.

Regulation of fatty acid transport

PURPOSE OF REVIEW

The aim of the present review is to summarize recent developments in the area of regulation of fatty acid transport.

RECENT FINDINGS

While controversy still exists regarding the contribution of passive diffusion versus protein-mediated fatty acid transport, both processes are now widely accepted. With the recent identification of an increasing number of putative fatty acid transporters, emphasis has been placed on regulation including fatty acid transport function of the protein, and also possible associated functions (acylCoA synthase activity and vectorial channelling to intracellular processing). Deciphering these issues has been facilitated through the use of loss-of-function (such as knockout) and gain-of-function (cell transfectants and transgenic mice) models.

SUMMARY

It is likely that our concept of fatty acid transport will continue to converge, incorporating the individual functions of the wide variety of fatty acid transporters into an integrated physiologic framework with relevance to a number of diseases.

Different mechanisms can alter fatty acid transport when muscle contractile activity is chronically altered

We examined whether skeletal muscle transport rates of long-chain fatty acids (LCFAs) were altered when muscle activity was eliminated (denervation) or increased (chronic stimulation). After 7 days of chronically stimulating the hindlimb muscles of female Sprague-Dawley rats, the LCFA transporter proteins fatty acid translocase (FAT)/CD36 (+43%) and plasma membrane-associated fatty acid-binding protein (FABPpm; +30%) were increased (P < 0.05), which resulted in the increased plasmalemmal content of these proteins (FAT/CD36, +42%; FABPpm +13%, P < 0.05) and a concomitant increase in the LCFA transport rate into giant sarcolemmal vesicles (+44%, P < 0.05). Although the total muscle contents of FAT/CD36 and FABPpm were not altered (P > 0.05) after 7 days of denervation, the LCFA transport rate was markedly decreased (-39%). This was associated with reductions in plasmalemmal FAT/CD36 (-24%) and FABPpm (-28%; P < 0.05). These data suggest that these LCFA transporters were resequestered to their intracellular depot(s) within the muscle. Combining the results from these experiments indicated that changes in rates of LCFA transport were correlated with concomitant changes in plasmalemmal FAT/CD36 and FABPpm, but not necessarily with their total muscle content. Thus chronic alterations in muscle activity can alter the rates of LCFA transport via different mechanisms, either 1) by increasing the total muscle content of FAT/CD36 and FABPpm, resulting in a concomitant increase at the sarcolemma, or 2) by reducing the plasma membrane content of these proteins in the absence of any changes in their total muscle content.

Targeted Deletion of Fatty Acid Transport Protein-4 Results in Early Embryonic Lethality

Fatty acid transport protein-4 (FATP4) is the major FATP in the small intestine. We previously demonstrated, using in vitro antisense experiments, that FATP4 is required for fatty acid uptake into intestinal epithelial cells. To further examine the physiological role of FATP4, mice carrying a targeted deletion of FATP4 were generated. Deletion of one allele of FATP4 resulted in 48% reduction of FATP4 protein levels and a 40% reduction of fatty acid uptake by isolated enterocytes. However, loss of one FATP4 allele did not cause any detectable effects on fat absorption on either a normal or a high fat diet. Deletion of both FATP4 alleles resulted in embryonic lethality as crosses between heterozygous FATP4 parents resulted in no homozygous offspring; furthermore, no homozygous embryos were detected as early as day 9.5 of gestation. Early embryonic lethality has been observed with deletion of other genes involved in lipid absorption in the small intestine, namely microsomal triglyceride transfer protein and apolipoprotein B, and has been attributed to a requirement for fat absorption early in embryonic development across the visceral endoderm. In mice, the extraembryonic endoderm supplies nutrients to the embryo prior to development of a chorioallantoic placenta. In wild-type mice we found that FATP4 protein is highly expressed by the epithelial cells of the visceral endoderm and localized to the brush-border membrane of extraembryonic endodermal cells. This localization is consistent with a role for FATP4 in fat absorption in early embryogenesis and suggests a novel requirement for FATP4 function during development.

Myocardial recovery from ischemia is impaired in CD36-null mice and restored by myocyte CD36 expression or medium-chain fatty acids

Long-chain fatty acid uptake, which provides a large part of myocardial energy, is impaired in human and murine hearts deficient in the membrane fatty acid translocase, FAT/CD36. We examined myocardial function in CD36-null mice using the working heart. Fatty acid oxidation and stores of glycogen, triglycerides, and ATP were reduced in CD36-deficient hearts and were restored to WT levels by rescue of myocyte CD36. Under normal perfusion conditions, CD36-null hearts had similar cardiac outputs and end-diastolic pressures as WT or transgenic hearts. After 6 min of ischemia, cardiac output decreased by 41% and end diastolic pressure tripled for CD36-null hearts, with no significant changes in WT or transgenic hearts. Null hearts also failed more frequently after ischemia as compared with WT or transgenics. To dissect out contribution of fatty acid uptake, a perfusate-lacking fatty acids was used. This decreased cardiac output after ischemia by 30% in WT hearts as compared with 50% for CD36-deficient hearts. End diastolic pressure, a negative index of myocardial performance, increased after ischemia in all heart types. Addition to the perfusate of a medium-chain fatty acid (caprylic acid) that does not require CD36 for uptake alleviated poor ischemic tolerance of CD36-null hearts. In summary, recovery from ischemia is compromised in CD36-deficient hearts and can be restored by CD36 rescue or by supplying medium-chain fatty acids. It would be important to determine whether the findings apply to the human situation where polymorphisms of the CD36 gene are relatively common.