Membrane topology of the murine fatty acid transport protein 1

Identification and Functional Characterization of the FATP1 Gene from Mud Crab, Scylla paramamosain

In mammals, fatty acid transport protein 1 (FATP1) plays important roles in cellular uptake and activation of long-chain fatty acid (LCFA), especially in processes of transportation, oxidation and triacylglycerol synthesis. However, the role of FATP1 in invertebrates, especially decapod crustaceans, is still poorly understood. In this study, the cDNA of a FATP1 gene from a decapod crustacean, mud crab Scylla paramamosain, was cloned and functionally characterized. The FATP1 gene encoded a polypeptide consisting of 643 amino acids that exhibits all the typical features of the FATP family and shares high homology with the other FATP orthologs of crustaceans. The relative mRNA expression levels of FATP1 were observed to be higher in metabolically active tissues such as hepatopancreas, stomach and gill than in other crab parts. Knockdown of the FATP1 mRNA in vivo significantly reduced triacylglycerols and total lipid levels in the hepatopancreas, accompanied by an increase in the expression of genes related to fatty acid transportation, allocation and hydrolysis, including long-chain acyl-CoA synthetase 3/4 (ACSL3/4) and carnitine palmitoyl transferase 1 (CPT1), and a decrease in the expression of genes related to fatty acid synthesis such as acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS) in the hepatopancreas. Furthermore, increased dietary n-3 long-chain polyunsaturated fatty acid (LC-PUFA) levels resulted in the up-regulation of the FATP1 expression in the hepatopancreas, accompanied by an increase in LC-PUFA content, especially eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3), in both polar (PLs) and neutral lipids (NLs) in the hepatopancreas and muscles of crabs. These findings suggested that the FATP1 gene identified in S. paramamosain might play important roles in regulating long-chain fatty acid metabolism and deposition in crustaceans.

The role of candidate transport proteins in β-cell long-chain fatty acid uptake: Where are we now?

Type 2 diabetes (T2D) in humans is typically preceded by elevated levels of circulatory long-chain free fatty acids (LC-FFA). These excess LC-FFA are widely thought to be taken up by pancreatic β-cells, contributing to their dysfunction and death during the development of T2D; a process that has been termed lipotoxicity. Depending on their degree of saturation and carbon chain length, LC-FFA can exert different effects on pancreatic β-cells viability and function in vitro. Long-chain saturated fatty acids (LC-SFA) are thought to be toxic, whereas monounsaturated fatty acids are not and may even offer protection against the toxic effects of LC-SFAs. However, the mechanism of LC-FFA uptake into pancreatic β-cells is poorly understood, partly because it has been an understudied area of research. Determining how LC-FFA are taken up into β-cells is crucial for later formulation of therapies to prevent potential cellular overload of LC-FFA, thereby slowing the onset of T2D. In this work, we detail more than 40 years of literature investigating the role of membrane-associated transport proteins in LC-FFA uptake. By focussing on what is known in other cell types, we highlight where we can extrapolate our current understanding of protein-mediated transport to β-cells and uncover where further understanding is required.

HBx induced upregulation of FATP2 promotes the development of hepatic lipid accumulation

The hepatitis B Virus X (HBx) protein plays a crucial role in the HBV-induced hepatic steatosis. Fatty acid transport protein 2 (FATP2) is a key protein that is involved in hepatic lipogenesis, and it was found to be highly expressed in various metabolic diseases. However, Whether FATP2 is a key factor in the pathogenesis of HBx-induced hepatic steatosis remains unclear. In this study, we found that FATP2 was up-regulated by HBx in vitro and in vivo and participated in HBx-induced hepatic lipid accumulation. Treatment of HBx-expressing cell lines and mice with FATP2 inhibitor (FATP2i) lipofermata ameliorated HBx-induced lipid accumulation and reduced oxidative stress and inflammation caused by lipid accumulation. Moreover, the liver injury of mouse was restored after FATP2i treatment. In summary, our results reveal that FATP2 is a key driver factor for HBx-induced hepatic lipid accumulation, and inhibition of FATP2 can ameliorates lipid accumulation caused by HBx. This study provides new insights into the mechanism of HBV-induced hepatic steatosis.

The oleaginous yeast Starmerella bombicola reveals limitations of Saccharomyces cerevisiae as a model for fatty acid transport studies

Saccharomyces cerevisiae is the model organism to most yeast researchers, and information obtained from its physiology is generally extrapolated to other yeasts. Studies on fatty acid transport in S. cerevisiae are based on the expression of both native fatty acid export genes as well as heterologous proteins. Starmerella bombicola, on the other hand, is an oleaginous yeast of industrial relevance but its fatty acid transport mechanisms are unknown. In this study, we attempt to use existing knowledge from S. cerevisiae to study fatty acid transport in S. bombicola, but the obtained results differ from those observed in S. cerevisiae. First, we observed that deletion of SbPRY1 in S. bombicola leads to higher fatty acid export, the opposite effect to the one previously observed for the Pry homologues in S. cerevisiae. Second, following reports that human FATP1 could export fatty acids and alcohols in S. cerevisiae, we expressed FATP1 in a fatty acid-accumulating S. bombicola strain. However, FATP1 reduced fatty acid export in S. bombicola, most likely due to its acyl-CoA synthetase activity. These results not only advance knowledge on fatty acid physiology of S. bombicola, but also improve our understanding of S. cerevisiae and its limitations as a model organism.

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.

Solute Carrier Family 27 Member 6 (SLC27A6) Possibly Promotes the Proliferation of Papillary Thyroid Cancer by Regulating c-MYC

To investigate the expression and mechanism of LSC27A6 in papillary thyroid cancer (PTC). We analyzed the differential expression of SLC27A6 in PTC tissues and normal tissues based on the TCGA database and validated it using immunohistochemistry. Wilcoxon rank sum, chi-square test, or Fisher exact exam were used to analyze the relationship between the expression of SLC27A6 and clinicopathological information. Samples were divided into two groups according to whether BRAF was mutated or not, and Wilcoxon rank sum was used to determine whether the expression of SLC27A6 was related to BRAF mutation. The effects of SLC27A6 on the proliferation, migration, and apoptosis of PTC cells were detected by cell counting kit-8 (CCK8), colony formation assay, transwell assay, and flow cytometry. Spearman correlation analysis was used to evaluate the relationship between SLC27A6 and c-MYC. Protein expression was detected by Western blot. The expression of SLC27A6 was higher in PTC and positively correlated with N stage. SLC27A6 expression was higher in samples with BRAF mutations. Down-regulation of SLC27A6 inhibited cell proliferation, migration, and invasion and induced apoptosis. Spearman correlation analysis showed that SLC27A6 was positively correlated with c-MYC. Knockdown of SLC27A6 inhibited c-MYC expression. Our results suggest that SLC27A6 is overexpressed in PTC tissues and affects the progression of PTC by regulating c-MYC.

Lipid Transport in Brown Adipocyte Thermogenesis

Non-shivering thermogenesis is an energy demanding process that primarily occurs in brown and beige adipose tissue. Beyond regulating body temperature, these thermogenic adipocytes regulate systemic glucose and lipid homeostasis. Historically, research on thermogenic adipocytes has focused on glycolytic metabolism due to the discovery of active brown adipose tissue in adult humans through glucose uptake imaging. The importance of lipids in non-shivering thermogenesis has more recently been appreciated. Uptake of circulating lipids into thermogenic adipocytes is necessary for body temperature regulation and whole-body lipid homeostasis. A wide array of circulating lipids contribute to thermogenic potential including free fatty acids, triglycerides, and acylcarnitines. This review will summarize the mechanisms and regulation of lipid uptake into brown adipose tissue including protein-mediated uptake, lipoprotein lipase activity, endocytosis, vesicle packaging, and lipid chaperones. We will also address existing gaps in knowledge for cold induced lipid uptake into thermogenic adipose tissue.

Fatty acid metabolism and acyl-CoA synthetases in the liver-gut axis

Fatty acids are energy substrates and cell components which participate in regulating signal transduction, transcription factor activity and secretion of bioactive lipid mediators. The acyl-CoA synthetases (ACSs) family containing 26 family members exhibits tissue-specific distribution, distinct fatty acid substrate preferences and diverse biological functions. Increasing evidence indicates that dysregulation of fatty acid metabolism in the liver-gut axis, designated as the bidirectional relationship between the gut, microbiome and liver, is closely associated with a range of human diseases including metabolic disorders, inflammatory disease and carcinoma in the gastrointestinal tract and liver. In this review, we depict the role of ACSs in fatty acid metabolism, possible molecular mechanisms through which they exert functions, and their involvement in hepatocellular and colorectal carcinoma, with particular attention paid to long-chain fatty acids and small-chain fatty acids. Additionally, the liver-gut communication and the liver and gut intersection with the microbiome as well as diseases related to microbiota imbalance in the liver-gut axis are addressed. Moreover, the development of potentially therapeutic small molecules, proteins and compounds targeting ACSs in cancer treatment is summarized.

Topology of phosphatidylserine synthase 1 in the endoplasmic reticulum membrane

Phosphatidylserine (PS) synthase 1 (PSS1) of mammalian cells is a multiple membrane-spanning protein of the endoplasmic reticulum (ER) and regulated by inhibition with the product PS. Alanine-scanning mutagenesis of PSS1 has revealed eight amino acid residues as those crucial for its activity and six as those important for its regulation. Furthermore, three missense mutations in the human PSS1 gene, which lead to regulatory dysfunctions of PSS1 and are causative of Lenz-Majewski syndrome, have been identified. In this study, we investigated the membrane topology of PSS1 by means of epitope insertion and immunofluorescence. According to a 10-transmembrane segment model supported by topology analysis of PSS1, all the 8 amino acid residues crucial for the enzyme activity were localized to the luminal side of the lipid bilayer or the lumen of the ER, whereas all the 9 amino acid residues involved in the enzyme regulation were localized to the cytosol or the cytoplasmic side of the lipid bilayer of the ER. This localization of the functional amino acid residues suggests that PSS1 is regulated by inhibition with PS in the cytoplasmic leaflet of the ER membrane and synthesizes PS at the luminal leaflet.

Local Mitochondrial ATP Production Regulates Endothelial Fatty Acid Uptake and Transport

Most organs use fatty acids (FAs) as a key nutrient, but little is known of how blood-borne FAs traverse the endothelium to reach underlying tissues. We conducted a small-molecule screen and identified niclosamide as a suppressor of endothelial FA uptake and transport. Structure/activity relationship studies demonstrated that niclosamide acts through mitochondrial uncoupling. Inhibitors of oxidative phosphorylation and the ATP/ADP translocase also suppressed FA uptake, pointing principally to ATP production. Decreasing total cellular ATP by blocking glycolysis did not decrease uptake, indicating that specifically mitochondrial ATP is required. Endothelial FA uptake is promoted by fatty acid transport protein 4 (FATP4) via its ATP-dependent acyl-CoA synthetase activity. Confocal microscopy revealed that FATP4 resides in the endoplasmic reticulum (ER), and that endothelial ER is intimately juxtaposed with mitochondria. Together, these data indicate that mitochondrial ATP production, but not total ATP levels, drives endothelial FA uptake and transport via acyl-CoA formation in mitochondrial/ER microdomains.

Bile acids mediated potential functional interaction between FXR and FATP5 in the regulation of Lipid Metabolism

Perturbation in lipid homeostasis is one of the major bottlenecks in metabolic diseases, especially Non-alcoholic Fatty Liver Disease (NAFLD), which has emerged as a leading global cause of chronic liver disease. The bile acids (BAs) and their derivatives exert a variety of metabolic effects through complex and intertwined pathways, thus becoming the attractive target for metabolic syndrome treatment. To modulate the lipid homeostasis, the role of BAs, turn out to be paramount as it is essential for the absorption, transport of dietary lipids, regulation of metabolic enzymes and transporters that are essential for lipid modulation, flux, and excretion. The synthesis and transport of BAs (conjugated and unconjugated) is chiefly controlled by nuclear receptors and the uptake of long-chain fatty acids (LCFA) and BA conjugation via transporters. Among them, from in-vivo studies, farnesoid X receptor (FXR) and liver-specific fatty acid transport protein 5 (FATP5) have shown convincing evidence for their key roles in lipid homeostasis and reversal of fatty liver disease substantially. BAs have a wider range of biological effects as they are identified as modulators for FXR and FATP5 both and therefore hold a significant promise for altering the lipid content in the treatment of a metabolic disorder. BAs also have received noteworthy interest in drug delivery research due to its peculiar physicochemical properties and biocompatibility. Here, we are highlighting the connecting possibility of BAs as an agonist for FXR and antagonist for FATP5, paving an avenue to target them for designing synthetic small molecules for lipid homeostasis.

Chronic effects of high-intensity interval training on postprandial lipemia in healthy men

The aim of this study was to determine the chronic (≥72 h postexercise) effects of high-intensity interval training (HIIT) on postprandial lipemia and metabolic markers in healthy volunteers. Eight physically active young men (mean ± SD: age 22 ± 3 yr, height 1.77 ± 0.07 m, body mass 67.7 ± 6.2 kg) underwent two 6-h mixed-meal tolerance tests and resting vastus lateralis muscle biopsies before the first session and ≥72 h after the final session of 4 wk of HIIT [16 sessions in total; 10 × 60-s bouts of cycling at 90% maximal oxygen uptake (V̇o_2max), interspersed with 60-s intervals at 45% V̇o_2max]. Arterialized and deep venous blood samples from across the forearm, brachial artery blood flow measurements, and whole-body indirect calorimetry data were obtained before, and at regular intervals for 6 h after, consumption of a standardized mixed meal. The main findings revealed that, when assessed ≥72 h postexercise, postprandial free fatty acid (FFA) uptake across the forearm was increased in response to exercise training (P = 0.025). However, 4 wk of HIIT did not alter fasting or postprandial circulating triglyceride concentrations or their tissue uptake, despite a 10.2% ± 7.7% improvement in V̇o_2max (P = 0.004). Protein content of adipose triglyceride lipase in the vastus lateralis at rest was reduced by 25% ± 21% (P = 0.01). Collectively, these findings suggest that 4 wk of HIIT enhances postprandial clearance of FFA when assessed ≥72 h postexercise but does not confer persisting (training) adaptations in postprandial triglyceridemia.NEW & NOTEWORTHY When assessed ≥72 h after the last exercise session, 4 wk of high-intensity interval training (HIIT) did not improve triglyceridemia but enhanced free fatty acid uptake into muscle with a concurrent reduction in skeletal muscle adipose triglyceride lipase protein content. This suggests that previously reported acute reductions in postprandial triglyceridemia following a single bout of HIIT do not translate to sustained improvements after 4 wk of HIIT, supporting the concept of frequent exercise for the maintenance of lipemic control.

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.

Fatty acid transporting proteins: Roles in brain development, aging, and stroke

Polyunsaturated fatty acids are required for the brain development and significantly impact aging and stroke. Due to the hydrophobicity of fatty acids, fatty acids transportation related proteins that include fatty acid binding proteins (FABPs), long chain acyl-coA synthase (ACS), fatty acid transportation proteins (FATPs), fatty acid translocase (FAT/CD36) and newly reported major facilitator superfamily domain-containing protein (Mfsd2a) play critical roles in the uptake of various fatty acids, especially polyunsaturated fatty acids. They are not only involved in neurodevelopment, but also have great impact on neurological disease, such as aging related dementia and stroke.

Substrate preferences of long-chain acyl-CoA synthetase and diacylglycerol acyltransferase contribute to enrichment of flax seed oil with α-linolenic acid

Seed oil from flax (Linum usitatissimum) is enriched in α-linolenic acid (ALA; 18:3Δ^{9cis,12cis,15cis} ), but the biochemical processes underlying the enrichment of flax seed oil with this polyunsaturated fatty acid are not fully elucidated. Here, a potential process involving the catalytic actions of long-chain acyl-CoA synthetase (LACS) and diacylglycerol acyltransferase (DGAT) is proposed for ALA enrichment in triacylglycerol (TAG). LACS catalyzes the ATP-dependent activation of free fatty acid to form acyl-CoA, which in turn may serve as an acyl-donor in the DGAT-catalyzed reaction leading to TAG. To test this hypothesis, flax LACS and DGAT cDNAs were functionally expressed in Saccharomyces cerevisiae strains to probe their possible involvement in the enrichment of TAG with ALA. Among the identified flax LACSs, LuLACS8A exhibited significantly enhanced specificity for ALA over oleic acid (18:1Δ^9cis ) or linoleic acid (18:2Δ^9cis,12cis ). Enhanced α-linolenoyl-CoA specificity was also observed in the enzymatic assay of flax DGAT2 (LuDGAT2-3), which displayed ∼20 times increased preference toward α-linolenoyl-CoA over oleoyl-CoA. Moreover, when LuLACS8A and LuDGAT2-3 were co-expressed in yeast, both in vitro and in vivo experiments indicated that the ALA-containing TAG enrichment process was operative between LuLACS8A- and LuDGAT2-3-catalyzed reactions. Overall, the results support the hypothesis that the cooperation between the reactions catalyzed by LACS8 and DGAT2 may represent a route to enrich ALA production in the flax seed oil.

Kidney Proximal Tubule Lipoapoptosis Is Regulated by Fatty Acid Transporter-2 (FATP2)

Albuminuria and tubular atrophy are among the highest risks for CKD progression to ESRD. A parsimonious mechanism involves leakage of albumin-bound nonesterified fatty acids (NEFAs) across the damaged glomerular filtration barrier and subsequent reabsorption by the downstream proximal tubule, causing lipoapoptosis. We sought to identify the apical proximal tubule transporter that mediates NEFA uptake and cytotoxicity. We observed transporter-mediated uptake of fluorescently labeled NEFA in cultured proximal tubule cells and microperfused rat proximal tubules, with greater uptake from the apical surface than from the basolateral surface. Protein and mRNA expression analyses revealed that kidney proximal tubules express transmembrane fatty acid transporter-2 (FATP2), encoded by Slc27a2, but not the other candidate transporters CD36 and free fatty acid receptor 1. Kidney FATP2 localized exclusively to proximal tubule epithelial cells along the apical but not the basolateral membrane. Treatment of mice with lipidated albumin to induce proteinuria caused a decrease in the proportion of tubular epithelial cells and an increase in the proportion of interstitial space in kidneys from wild-type but not Slc27a2^-/- mice. Ex vivo microperfusion and in vitro experiments with NEFA-bound albumin at concentrations that mimic apical proximal tubule exposure during glomerular injury revealed significantly reduced NEFA uptake and palmitate-induced apoptosis in microperfused Slc27a2^-/- proximal tubules and Slc27a2^-/- or FATP2 shRNA-treated proximal tubule cell lines compared with wild-type or scrambled oligonucleotide-treated cells, respectively. We conclude that FATP2 is a major apical proximal tubule NEFA transporter that regulates lipoapoptosis and may be an amenable target for the prevention of CKD progression.

Fatty acid transport protein 1 (FATP1) localizes in mitochondria in mouse skeletal muscle and regulates lipid and ketone body disposal

FATP1 mediates skeletal muscle cell fatty acid import, yet its intracellular localization and metabolic control role are not completely defined. Here, we examine FATP1 localization and metabolic effects of its overexpression in mouse skeletal muscle. The FATP1 protein was detected in mitochondrial and plasma membrane fractions, obtained by differential centrifugation, of mouse gastrocnemius muscle. FATP1 was most abundant in purified mitochondria, and in the outer membrane and soluble intermembrane, but not in the inner membrane plus matrix, enriched subfractions of purified mitochondria. Immunogold electron microscopy localized FATP1-GFP in mitochondria of transfected C2C12 myotubes. FATP1 was overexpressed in gastrocnemius mouse muscle, by adenovirus-mediated delivery of the gene into hindlimb muscles of newborn mice, fed after weaning a chow or high-fat diet. Compared to GFP delivery, FATP1 did not alter body weight, serum fed glucose, insulin and triglyceride levels, and whole-body glucose tolerance, in either diet. However, fatty acid levels were lower and β-hydroxybutyrate levels were higher in FATP1- than GFP-mice, irrespective of diet. Moreover, intramuscular triglyceride content was lower in FATP1- versus GFP-mice regardless of diet, and β-hydroxybutyrate content was unchanged in high-fat-fed mice. Electroporation-mediated FATP1 overexpression enhanced palmitate oxidation to CO2, but not to acid-soluble intermediate metabolites, while CO2 production from β-hydroxybutyrate was inhibited and that from glucose unchanged, in isolated mouse gastrocnemius strips. In summary, FATP1 was localized in mitochondria, in the outer membrane and intermembrane parts, of mouse skeletal muscle, what may be crucial for its metabolic effects. Overexpressed FATP1 enhanced disposal of both systemic fatty acids and intramuscular triglycerides. Consistently, it did not contribute to the high-fat diet-induced metabolic dysregulation. However, FATP1 lead to hyperketonemia, likely secondary to the sparing of ketone body oxidation by the enhanced oxidation of fatty acids.

The Concise Guide to PHARMACOLOGY 2013/14: transporters

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. Transporters are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Box C/D small nucleolar RNA (snoRNA) U60 regulates intracellular cholesterol trafficking

Mobilization of plasma membrane (PM) cholesterol to the endoplasmic reticulum is essential for cellular cholesterol homeostasis. The mechanisms regulating this retrograde, intermembrane cholesterol transfer are not well understood. Because mutant cells with defects in PM to endoplasmic reticulum cholesterol trafficking can be isolated on the basis of resistance to amphotericin B, we conducted an amphotericin B loss-of-function screen in Chinese hamster ovary (CHO) cells using insertional mutagenesis to identify genes that regulate this trafficking mechanism. Mutant line A1 displayed reduced cholesteryl ester formation from PM-derived cholesterol and increased de novo cholesterol synthesis, indicating a deficiency in retrograde cholesterol transport. Genotypic analysis revealed that the A1 cell line contained one disrupted allele of the U60 small nucleolar RNA (snoRNA) host gene, resulting in haploinsufficiency of the box C/D snoRNA U60. Complementation and mutational studies revealed the U60 snoRNA to be the essential feature from this locus that affects cholesterol trafficking. Lack of alteration in predicted U60-mediated site-directed methylation of 28 S rRNA in the A1 mutant suggests that the U60 snoRNA modulates cholesterol trafficking by a mechanism that is independent of this canonical function. Our study adds to a growing body of evidence for participation of small noncoding RNAs in cholesterol homeostasis and is the first to implicate a snoRNA in this cellular function.

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.

Comprehensive gene expression changes associated with mouse postnatal kidney development

PURPOSE

To provide a portrait of the molecular alterations in renal growth that occur in mice postnatally, we performed gene expression profiling at discrete time points during the first 5 weeks of life.

MATERIALS AND METHODS

Kidneys were harvested from C57BL/6 mice at embryonic day 19.5, and postnatal days 1, 3, 5, 7, 10, 14, 21, 28 and 35. Total RNA was extracted and gene expression profiling was done using microarrays (Agilent Technologies, Santa Clara, California). Transcripts whose expression levels changed during the study course were identified using StepMiner software (http://chicory.stanford.edu/sahoo/public/StepMiner/). Biological functions of the modulated genes were identified using IPA® software.

RESULTS

Postnatal kidney growth and development are associated with widespread changes in gene expression with 6,949 transcripts significantly up-regulated and 6,696 down-regulated during the first 5 weeks of life. Pathway analysis showed progressive down-regulation of pathways associated with cell growth and embryonic development (postnatal days 5 to 7). This was followed by increased expression of transcripts associated with lipid/energy metabolism and molecular transport (postnatal days 10 to 14), and down-regulation of genes related to DNA replication, cell cycle, tissue development, protein trafficking and cell morphology (postnatal days 14 to 21).

CONCLUSIONS

To our knowledge we report the most comprehensive temporal survey of postnatal kidney development to date. This data set provides a framework for interpreting nephropathy, such as that induced by congenital obstruction.

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.

Enhanced fatty acid oxidation and FATP4 protein expression after endurance exercise training in human skeletal muscle

FATP1 and FATP4 appear to be important for the cellular uptake and handling of long chain fatty acids (LCFA). These findings were obtained from loss- or gain of function models. However, reports on FATP1 and FATP4 in human skeletal muscle are limited. Aerobic training enhances lipid oxidation; however, it is not known whether this involves up-regulation of FATP1 and FATP4 protein. Therefore, the aim of this project was to investigate FATP1 and FATP4 protein expression in the vastus lateralis muscle from healthy human individuals and to what extent FATP1 and FATP4 protein expression were affected by an increased fuel demand induced by exercise training. Eight young healthy males were recruited to the study. All subjects were non smokers and did not participate in regular physical activity (<1 time per week for the past 6 months, VO_2peak 3.4±0.1 l O₂ min⁻¹). Subjects underwent an 8 week supervised aerobic training program. Training induced an increase in VO_2peak from 3.4±0.1 to 3.9±0.1 l min⁻¹ and citrate synthase activity was increased from 53.7±2.5 to 80.8±3.7 µmol g⁻¹ min⁻¹. The protein content of FATP4 was increased by 33%, whereas FATP1 protein content was reduced by 20%. Interestingly, at the end of the training intervention a significant association (r² = 0.74) between the observed increase in skeletal muscle FATP4 protein expression and lipid oxidation during a 120 min endurance exercise test was observed. In conclusion, based on the present findings it is suggested that FATP1 and FATP4 proteins perform different functional roles in handling LCFA in skeletal muscle with FATP4 apparently more important as a lipid transport protein directing lipids for lipid oxidation.

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

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

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.

FATP1 inhibits 11-cis retinol formation via interaction with the visual cycle retinoid isomerase RPE65 and lecithin:retinol acyltransferase

The isomerization of all-trans retinol (vitamin A) to 11-cis retinol in the retinal pigment epithelium (RPE) is a key step in the visual process for the regeneration of the visual pigment chromophore, 11-cis retinal. LRAT and RPE65 are recognized as the minimal isomerase catalytic components. However, regulators of this rate-limiting step are not fully identified and could account for the phenotypic variability associated with inherited retinal degeneration (RD) caused by mutations in the RPE65 gene. To identify new RPE65 partners, we screened a porcine RPE mRNA library using a yeast two-hybrid assay with full-length human RPE65. One identified clone (here named FATP1c), containing the cytosolic C-terminal sequence from the fatty acid transport protein 1 (FATP1 or SLC27A1, solute carrier family 27 member 1), was demonstrated to interact dose-dependently with the native RPE65 and with LRAT. Furthermore, these interacting proteins colocalize in the RPE. Cellular reconstitution of human interacting proteins shows that FATP1 markedly inhibits 11-cis retinol production by acting on the production of all-trans retinyl esters and the isomerase activity of RPE65. The identification of this new visual cycle inhibitory component in RPE may contribute to further understanding of retinal pathogenesis.

Unraveling the pheromone biosynthesis activating neuropeptide (PBAN) signal transduction cascade that regulates sex pheromone production in moths

Studies over the past three decades have demonstrated that female moths usually produce sex pheromones as multicomponent blends in which the ratios of the individual components are precisely controlled, making it possible to generate species-specific pheromone blends. Most moth pheromone components are de novo synthesized from acetyl-CoA in the pheromone gland (PG) through modifications of fatty acid biosynthetic pathways. Pheromone biosynthesis activating neuropeptide (PBAN), a neurohormone produced by a cephalic organ (subesophageal ganglion) stimulates sex pheromone biosynthesis in the PG via an influx of extracellular Ca(2+). In recent years, we have expanded our knowledge of the precise mechanisms underlying silkmoth (Bombyx mori) sex pheromone production by characterizing a number of key molecules. In this review, we want to highlight our efforts in elucidating these mechanisms in B. mori and to understand how they relate more broadly to lepidopteran sex pheromone production in general.

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.

A novel role for fatty acid transport protein 1 in the regulation of tricarboxylic acid cycle and mitochondrial function in 3T3-L1 adipocytes

Fatty acid transport proteins (FATPs) are integral membrane acyl-CoA synthetases implicated in adipocyte fatty acid influx and esterification. Whereas some FATP1 translocates to the plasma membrane in response to insulin, the majority of FATP1 remains within intracellular structures and bioinformatic and immunofluorescence analysis of FATP1 suggests the protein primarily resides in the mitochondrion. To evaluate potential roles for FATP1 in mitochondrial metabolism, we used a proteomic approach following immunoprecipitation of endogenous FATP1 from 3T3-L1 adipocytes and identified mitochondrial 2-oxoglutarate dehydrogenase. To assess the functional consequence of the interaction, purified FATP1 was reconstituted into phospholipid-containing vesicles and its effect on 2-oxoglutarate dehydrogenase activity evaluated. FATP1 enhanced the activity of 2-oxoglutarate dehydrogenase independently of its acyl-CoA synthetase activity whereas silencing of FATP1 in 3T3-L1 adipocytes resulted in decreased activity of 2-oxoglutarate dehydrogenase. FATP1 silenced 3T3-L1 adipocytes exhibited decreased tricarboxylic acid cycle activity, increased cellular NAD(+)/NADH, increased fatty acid oxidation, and increased lactate production indicative of altered mitochondrial energy metabolism. These results reveal a novel role for FATP1 as a regulator of tricarboxylic acid cycle activity and mitochondrial function.

Novel role of FATP1 in mitochondrial fatty acid oxidation in skeletal muscle cells

Carnitine palmitoyltransferase 1 (CPT1) catalyzes the first step in long-chain fatty acid import into mitochondria, and it is believed to be rate limiting for β-oxidation of fatty acids. However, in muscle, other proteins may collaborate with CPT1. Fatty acid translocase/CD36 (FAT/CD36) may interact with CPT1 and contribute to fatty acid import into mitochondria in muscle. Here, we demonstrate that another membrane-bound fatty acid binding protein, fatty acid transport protein 1 (FATP1), collaborates with CPT1 for fatty acid import into mitochondria. Overexpression of FATP1 using adenovirus in L6E9 myotubes increased both fatty acid oxidation and palmitate esterification into triacylglycerides. Moreover, immunocytochemistry assays in transfected L6E9 myotubes showed that FATP1 was present in mitochondria and coimmunoprecipitated with CPT1 in L6E9 myotubes and rat skeletal muscle in vivo. The cooverexpression of FATP1 and CPT1 also enhanced mitochondrial fatty acid oxidation, similar to the cooverexpression of FAT/CD36 and CPT1. However, etomoxir, an irreversible inhibitor of CPT1, blocked all these effects. These data reveal that FATP1, like FAT/CD36, is associated with mitochondria and has a role in mitochondrial oxidation of fatty acids.

FATP1 localizes to mitochondria and enhances pyruvate dehydrogenase activity in skeletal myotubes

Fatty acid transport protein 1 (FATP1) has been previously immunolocalized in intracellular compartments. Here we show that FATP1 localizes to the mitochondria in cultured myotubes, by immunoblots of subcellular fractions and immunocytology of the fusion protein FATP1-GFP. FATP1 strongly stimulates CO(2) production from glucose whereas nonmitochondrial metabolism of glucose is only slightly enhanced. FATP1 raises the activity and activates the pyruvate dehydrogenase (PDH) complex and the pyruvate decarboxylase PDH-E1 catalytic subunit, without changing E2, E3BP or E1alpha and increasing E1beta protein content. These data reveals the localization and points to a regulatory function of FATP1 in myotube mitochondria.

Functional characterization of the Bombyx mori fatty acid transport protein (BmFATP) within the silkmoth pheromone gland

Fatty acid transport protein (FATP) is an evolutionarily conserved membrane-bound protein that facilitates the uptake of extracellular long chain fatty acids. In humans and mice, six FATP isoforms have been identified and their tissue-specific distributions suggest that each plays a discrete role in lipid metabolism in association with fatty acid uptake. While the presence of FATP homologs in insects has been demonstrated, their functional role remains to be characterized. Pheromonogenesis is defined as the dynamic period in which all machinery required for sex pheromone biosynthesis is generated and organized within the pheromone gland (PG) cells. By exploiting this unique system in the PG of the silkmoth, Bombyx mori, we found that BmFATP is predominantly expressed in the PG and undergoes up-regulation 1 day prior to eclosion. Before eclosion, B. mori PG cells accumulate cytoplasmic lipid droplets (LDs), which play a role in storing the pheromone (bombykol) precursor fatty acid in the form of triacylglycerol. RNAi-mediated gene silencing of BmFATP in vivo significantly suppressed LD accumulation by preventing the synthesis of triacylglycerols and resulted in a significant reduction in bombykol production. These results, in conjunction with the findings that BmFATP stimulates the uptake of extracellular long-chain fatty acids and BmFATP knockdown reduces cellular long-chain acyl-CoA synthetase activity, suggest that BmFATP plays an essential role in bombykol biosynthesis by stimulating both LD accumulation and triacylglycerol synthesis via a process called vectorial acylation that couples the uptake of extracellular fatty acids with activation to CoA thioesters during pheromonogenesis.

Mammalian long-chain acyl-CoA synthetases

Acyl-CoA synthetase enzymes are essential for de novo lipid synthesis, fatty acid catabolism, and remodeling of membranes. Activation of fatty acids requires a two-step reaction catalyzed by these enzymes. In the first step, an acyl-AMP intermediate is formed from ATP. AMP is then exchanged with CoA to produce the activated acyl-CoA. The release of AMP in this reaction defines the superfamily of AMP-forming enzymes. The length of the carbon chain of the fatty acid species defines the substrate specificity for the different acyl-CoA synthetases (ACS). On this basis, five sub-families of ACS have been characterized. The purpose of this review is to report on the large family of mammalian long-chain acyl-CoA synthetases (ACSL), which activate fatty acids with chain lengths of 12 to 20 carbon atoms. Five genes and several isoforms generated by alternative splicing have been identified and limited information is available on their localization. The structure of these membrane proteins has not been solved for the mammalian ACSLs but homology to a bacterial form, whose structure has been determined, points at specific structural features that are important for these enzymes across species. The bacterial form acts as a dimer and has a conserved short motif, called the fatty acid Gate domain, that seems to determine substrate specificity. We will discuss the characterization and identification of the different spliced isoforms, draw attention to the inconsistencies and errors in their annotations, and their cellular localizations. These membrane proteins act on membrane-bound substrates probably as homo- and as heterodimer complexes but have often been expressed as single recombinant isoforms, apparently purified as monomers and tested in Triton X-100 micelles. We will argue that such studies have failed to provide an accurate assessment of the activity and of the distinct function of these enzymes in mammalian cells.

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.

Topology of the yeast fatty acid transport protein Fat1p: mechanistic implications for functional domains on the cytosolic surface of the plasma membrane

The fatty acid transport protein (FATP) Fat1p in the yeast Saccharomyces cerevisiae functions in concert with acyl-coenzyme A synthetase (ACSL; either Faa1p or Faa4p) in vectorial acylation, which couples the transport of exogenous fatty acids with activation to CoA thioesters. To further define the role of Fat1p in the transport of exogenous fatty acids, the topological orientation of two highly conserved motifs [ATP/AMP and FATP/very long chain acyl CoA synthetase (VLACS)], the carboxyl 124 amino acid residues, which bind the ACSL Faa1p, and the amino and carboxyl termini within the plasma membrane were defined. T7 or hemagglutinin epitope tags were engineered at both amino and carboxyl termini, as well as at multiple nonconserved, predicted random coil segments within the protein. Six different epitope-tagged chimeras of Fat1p were generated and expressed in yeast; the sidedness of the tags was tested using indirect immunofluorescence and protease protection by Western blotting. Plasma membrane localization of the tagged proteins was assessed by immunofluorescence. Fat1p appears to have at least two transmembrane domains resulting in a N(in)-C(in) topology. We propose that Fat1p has a third region, which binds to the membrane and separates the highly conserved residues comprising the two halves of the ATP/AMP motif. The N(in)-C(in) topology results in the placement of the ATP/AMP and FATP/VLACS domains of Fat1p on the inner face of the plasma membrane. The carboxyl-terminal region of Fat1p, which interacts with ACSL, is likewise positioned on the inner face of the plasma membrane. This topological orientation is consistent with the mechanistic roles of both Fat1p and Faa1p or Faa4p in the coupled transport/activation of exogenous fatty acids by vectorial acylation.

A Model for Fatty Acid Transport into the Brain

A key function of fatty acid (FA) transport into the brain is to supply polyunsaturated fatty acids (PUFA) that are not synthesized in brain cells but are essential signaling molecules and components of the phospholipid membrane. In addition, common dietary FAs such as palmitic acid are also rapidly taken up by the brain and esterified to phospholipids or oxidized to provide cellular energy. Most evidence shows that FA crossing the blood brain barrier (BBB) is derived mainly from FA/albumin complexes and, to a lesser extent, from circulating lipoproteins. Our model proposes that FA diffuse across the lipid bilayer of the BBB without specific transporters to reach brain cells. They cross the luminal and transluminal leaflets of the endothelial cells and the plasma membrane of neural cells by reversible flip-flop. Acyl-CoA synthetases trap FA by forming acyl-CoA, which cannot diffuse out of the cell. Selection of FA is controlled largely by enzymes in the pathways of intracellular metabolism, beginning with the acyl-CoA synthetase.

Keratinocyte-specific expression of fatty acid transport protein 4 rescues the wrinkle-free phenotype in Slc27a4/Fatp4 mutant mice

FATP4 (fatty acid transport protein 4; also known as SLC27A4) is the most widely expressed member of a family of six long chain fatty acid transporters. FATP4 is highly expressed in enterocytes and has therefore been proposed to be a major importer of dietary fatty acids. Two independent mutations in Fatp4 cause mice to be born with thick, tight, shiny, "wrinkle-free" skin and a defective skin barrier; they die within hours of birth from dehydration and restricted movements. In contrast, induced keratinocyte-specific deficiency of FATP4 in adult mice causes only mild skin abnormalities. Therefore, whether the loss of FATP4 from skin or a systemic gestational metabolic defect causes the severe skin defects and neonatal lethality remain important unanswered questions. To investigate the basis for the phenotype, we first generated wild-type tetraploid/mutant diploid aggregates that should lead to rescue of any abnormalities caused by loss of FATP4 from the placenta. However, the skin phenotype was not ameliorated. We then generated transgenic mice expressing exogenous FATP4 either widely or specifically in suprabasal keratinocytes, and we bred the transgenes onto the Fatp4(-/-) background. Both modes of FATP4 expression led to rescue of the neonatally lethal skin defects, and the resulting mice were viable and fertile. Keratinocyte expression of an FATP4 variant with mutations in the acyl-CoA synthetase domain did not provide any degree of rescue. We conclude that expression of FATP4 with an intact acyl-CoA synthetase domain in suprabasal keratinocytes is necessary for normal skin development and that FATP4 functions in establishing the cornified envelope.

New concepts of cellular fatty acid uptake: role of fatty acid transport proteins and of caveolae

Efficient uptake and channelling of long-chain fatty acids (LCFA) are critical cell functions. Evidence is emerging that proteins are important mediators of LCFA-trafficking into cells and various proteins have been suggested to be involved in this process. Amongst these proteins is a family of membrane-associated proteins termed fatty acid transport proteins (FATP). So far six members of this family, designated FATP 1–6, have been characterized. FATP 1, 2 and 6 show a highly-conserved AMP-binding region that participates in the activation of very-long-chain fatty acids (VLCFA) to form their acyl-CoA derivatives. The mechanisms by which FATP mediate LCFA uptake are not well understood, but several studies provide evidence that uptake of LCFA across cellular membranes is closely linked to acyl-CoA synthetase activity. It is proposed that FATP indirectly enhance LCFA uptake by activating VLCFA to their CoA esters, which are required to maintain the typical structure of lipid rafts in cellular membranes. Recent work has shown that the structural integrity of lipid rafts is essential for cellular LCFA uptake. This effect might be exerted by proteins, e.g. caveolin-1 and FAT/CD36, that use lipid rafts as platforms and bind or transport LCFA. The proposed molecular mechanisms await further experimental investigation.

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.

Cellular uptake of fatty acids driven by the ER-localized acyl-CoA synthetase FATP4

Long-chain fatty acids are important metabolites for the generation of energy and the biosynthesis of lipids. The molecular mechanism of their cellular uptake has remained controversial. The fatty acid transport protein (FATP) family has been named according to its proposed function in mediating this process at the plasma membrane. Here, we show that FATP4 is in fact localized to the endoplasmic reticulum and not the plasma membrane as reported previously. Quantitative analysis confirms the positive correlation between expression of FATP4 and uptake of fatty acids. However, this is dependent on the enzymatic activity of FATP4, catalyzing the esterification of fatty acids with CoA. Monitoring fatty acid uptake at the single-cell level demonstrates that the ER localization of FATP4 is sufficient to drive transport of fatty acids. Expression of a mitochondrial acyl-CoA synthetase also enhances fatty acid uptake, suggesting a general relevance for this mechanism. Our results imply that cellular uptake of fatty acids can be regulated by intracellular acyl-CoA synthetases. We propose that the enzyme FATP4 drives fatty acid uptake indirectly by esterification. It is not a transporter protein involved in fatty acid translocation at the plasma membrane.

Docosahexaenoic acid supply in pregnancy affects placental expression of fatty acid transport proteins

BACKGROUND

Better understanding of the mechanisms involved in docosahexaenoic acid (DHA) transfer to the neonate may contribute to improve dietary support for infants born prematurely to mothers with placental lipid transport disorders.

OBJECTIVE

We studied whether DHA supplements modify the messenger RNA (mRNA) expression of placental lipid transport proteins to allow a selective transfer of DHA to the fetus.

DESIGN

Healthy pregnant women (n = 136) received, in a double-blind randomized trial, 500 mg DHA + 150 mg eicosapentaenoic acid, 400 microg 5-methyl-tetrahydrofolic acid, 500 mg DHA + 400 microg 5-methyl-tetrahydrofolic acid, or placebo during the second half of gestation. We analyzed the fatty acid composition of maternal and cord blood phospholipids and of placenta; we quantified placental mRNA expression of fatty acid-transport protein 1 (FATP-1), FATP-4, FATP-6, fatty acid translocase, fatty acid-binding protein (FABP) plasma membrane, heart-FABP, adipocyte-FABP, and brain-FABP.

RESULTS

The mRNA expression of the lipid carriers assayed did not differ significantly between the 4 groups. However, the mRNA expression of FATP-1 and FATP-4 in placenta was correlated with DHA in both maternal plasma and placental phospholipids, although only FATP-4 expression was significantly correlated with DHA in cord blood phospholipids. Additionally, the mRNA expression of several membrane lipid carriers was correlated with EPA and DHA in placental triacylglycerols and with EPA in placental free fatty acids.

CONCLUSIONS

Correlation of the mRNA expression of the membrane placental proteins FATP-1 and especially of FATP-4 with maternal and cord DHA leads us to conclude that these lipid carriers are involved in placental transfer of long-chain polyunsaturated fatty acids.

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.

Translocation of long chain fatty acids across the plasma membrane--lipid rafts and fatty acid transport proteins

Translocation of long chain fatty acids across the plasma membrane is achieved by a concert of co-existing mechanisms. These lipids can passively diffuse, but transport can also be accelerated by certain membrane proteins as well as lipid rafts. Lipid rafts are dynamic assemblies of proteins and lipids, that float freely within the two dimensional matrix of the membrane bilayer. They are receiving increasing attention as devices that regulate membrane function in vivo and play an important role in membrane trafficking and signal transduction. In this review we will discuss how lipid rafts might be involved in the uptake process and how the candidate proteins for fatty acid uptake FAT/CD36 and the FATP proteins interact with these domains. We will also discuss the functional role of FATPs in general. To our understanding FATPs are indirectly involved in the translocation process across the plasma membrane by providing long chain fatty acid synthetase activity.

Differential expression of fatty acid transport proteins in epidermis and skin appendages

Epidermis and sebocyte-derived lipids are derived both from de novo synthesis and through uptake of fatty acids from the circulation. Plasma membrane proteins can significantly contribute to the latter process. In particular, fatty acid transport proteins (FATP/solute carrier family 27) are integral transmembrane proteins that enhance the uptake of long-chain fatty acids into cells. Using specific antisera against all six mammalian FATP, we found that both human and mouse skin express FATP1, -3, -4, and -6. In adult skin, FATP1 and -3 are expressed predominantly by keratinocytes, whereas FATP4 is strongly expressed by sebaceous glands and FATP6 by hair follicle epithelia. Sustained barrier disruption leads to increases in FATP1 and -6 levels as well as a robust increase in CD36 protein. Notably, expression of FATP1 by embryonic keratinocytes at day 18.5 was lower, and FATP4 increased in comparison with adult epidermis. Together, these findings indicate that FATP are not only expressed by different cell types within the skin, but also that their localization is dynamically regulated during development.

Docosahexaenoic acid, fatty acid-interacting proteins, and neuronal function: breastmilk and fish are good for you

In contrast to other tissues, the nervous system is enriched in the polyunsaturated fatty acids (PUFAs): arachidonic acid (AA, 20:4 n-6) and docosahexaenoic acid (DHA, 22:6 n-3). Despite their abundance in the nervous system, AA and DHA cannot be synthesized de novo by mammals; they, or their precursors, must be ingested from dietary sources and transported to the brain. During late gestation and the early postnatal period, neurodevelopment is exceptionally rapid, and substantial amounts of PUFAs, especially DHA, are critical to ensure neurite outgrowth as well as proper brain and retina development. Here, we review the various functions of DHA in the nervous system, the proteins involved in its internalization and metabolism into phospholipids, and its relationship to several neurological disorders, including Alzheimer's disease and depression.