Isolation and partial characterization of plasma membrane fatty acid binding proteins from myocardium and adipose tissue and their relationship to analogous proteins in liver and gut

Cypate and Cypate-Glucosamine as Near-Infrared Fluorescent Probes for In Vivo Tumor Imaging

Near-infrared (NIR) imaging is a promising technique for use as a noninvasive and sensitive diagnostic tool. Although the NIR fluorescently labeled glucose analog glucosamine (cypate-glucosamine) has applications in preclinical imaging, the transport pathways and fate of this probe in tissues remain unaddressed. Here, we have synthesized and characterized cypate and cypate-glucosamine conjugate (cy-2-glu), and investigated the probable transport pathways of these probes in vitro and in vivo. We compared uptake of the probes in the presence and absence of excess d-glucose, "saturated cypate" and palmitic acid in two normal-cancer cell line pairs: lung cancer (A549)-normal (MRC9) and prostate cancer (DU145)-normal (BPH). Breast cancer (MDA-MB-231) and liver cancer (HepG2) cell lines were also examined. Results support use of the glucose transport pathway by cy-2-glu and fatty acid transport pathway by cypate. Mass spectrometry data on the in vitro extracts revealed deamidation of cy-2-glu in prostate and liver cells, suggesting release of glucosamine. In vivo biodistribution studies in mice engrafted with breast tumors showed a distinct accumulation of cy-2-glu in liver and tumors, and to a lesser extent in kidneys and spleen. A negligible accumulation of cypate alone in tumors was observed. Analysis of urine extracts revealed renal excretion of the cy-2-glu probe in the form of free cypate, indicating deamidation of cy-2-glu in tissues. Thus, investigation of the metabolic pathways used by NIR probes such as cy-2-glu advances their use in the detection and monitoring of tumor progression in preclinical animal studies.

Postprandial control of fatty acid transport proteins' subcellular location is not dependent on insulin

Fatty acid transport proteins rapidly translocate to the plasma membrane in response to various stimuli, including insulin, influencing lipid uptake into muscle. However, our understanding of the mechanisms regulating postprandial fatty acid transporter subcellular location remains limited. We demonstrate that the response of fatty acid transporters to insulin stimulation is extremely brief and not temporally matched in the postprandial state. We further show that high-fat diet-induced accumulation of fatty acid transporters on the plasma membrane can occur in the absence of insulin. Altogether, these data suggest that insulin is not the primary signal regulating fatty acid transporter relocation in vivo.

Activation of AMPKα2 Is Not Required for Mitochondrial FAT/CD36 Accumulation during Exercise

Exercise has been shown to induce the translocation of fatty acid translocase (FAT/CD36), a fatty acid transport protein, to both plasma and mitochondrial membranes. While previous studies have examined signals involved in the induction of FAT/CD36 translocation to sarcolemmal membranes, to date the signaling events responsible for FAT/CD36 accumulation on mitochondrial membranes have not been investigated. In the current study muscle contraction rapidly increased FAT/CD36 on plasma membranes (7.5 minutes), while in contrast, FAT/CD36 only increased on mitochondrial membranes after 22.5 minutes of muscle contraction, a response that was exercise-intensity dependent. Considering that previous research has shown that AMP activated protein kinase (AMPK) α2 is not required for FAT/CD36 translocation to the plasma membrane, we investigated whether AMPK α2 signaling is necessary for mitochondrial FAT/CD36 accumulation. Administration of 5-Aminoimidazole-4-carboxamide ribonucleotide (AICAR) induced AMPK phosphorylation, and resulted in FAT/CD36 accumulation on SS mitochondria, suggesting AMPK signaling may mediate this response. However, SS mitochondrial FAT/CD36 increased following acute treadmill running in both wild-type (WT) and AMPKα 2 kinase dead (KD) mice. These data suggest that AMPK signaling is not required for SS mitochondrial FAT/CD36 accumulation. The current data also implicates alternative signaling pathways that are exercise-intensity dependent, as IMF mitochondrial FAT/CD36 content only occurred at a higher power output. Taken altogether the current data suggests that activation of AMPK signaling is sufficient but not required for exercise-induced accumulation in mitochondrial FAT/CD36.

Pathways of polyunsaturated fatty acid utilization: implications for brain function in neuropsychiatric health and disease

Essential polyunsaturated fatty acids (PUFAs) have profound effects on brain development and function. Abnormalities of PUFA status have been implicated in neuropsychiatric diseases such as major depression, bipolar disorder, schizophrenia, Alzheimer's disease, and attention deficit hyperactivity disorder. Pathophysiologic mechanisms could involve not only suboptimal PUFA intake, but also metabolic and genetic abnormalities, defective hepatic metabolism, and problems with diffusion and transport. This article provides an overview of physiologic factors regulating PUFA utilization, highlighting their relevance to neuropsychiatric disease.

From fatty-acid sensing to chylomicron synthesis: role of intestinal lipid-binding proteins

Today, it is well established that the development of obesity and associated diseases results, in part, from excessive lipid intake associated with a qualitative imbalance. Among the organs involved in lipid homeostasis, the small intestine is the least studied even though it determines lipid bioavailability and largely contributes to the regulation of postprandial hyperlipemia (triacylglycerols (TG) and free fatty acids (FFA)). Several Lipid-Binding Proteins (LBP) are expressed in the small intestine. Their supposed intestinal functions were initially based on what was reported in other tissues, and took no account of the physiological specificity of the small intestine. Progressively, the identification of regulating factors of intestinal LBP and the description of the phenotype of their deletion have provided new insights into cellular and molecular mechanisms involved in fat absorption. This review will discuss the physiological contribution of each LBP in the main steps of intestinal absorption of long-chain fatty acids (LCFA): uptake, trafficking and reassembly into chylomicrons (CM). Moreover, current data indicate that the small intestine is able to adapt its lipid absorption capacity to the fat content of the diet, especially through the coordinated induction of LBP. This adaptation requires the existence of a mechanism of intestinal lipid sensing. Emerging data suggest that the membrane LBP CD36 may operate as a lipid receptor that triggers an intracellular signal leading to the modulation of the expression of LBP involved in CM formation. This event could be the starting point for the optimized synthesis of large CM, which are efficiently degraded in blood. Better understanding of this intestinal lipid sensing might provide new approaches to decrease the prevalence of postprandial hypertriglyceridemia, which is associated with cardiovascular diseases, insulin resistance and obesity.

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.

Intestinal absorption of long-chain fatty acids: evidence and uncertainties

Over the two last decades, cloning of proteins responsible for trafficking and metabolic fate of long-chain fatty acids (LCFA) in gut has provided new insights on cellular and molecular mechanisms involved in fat absorption. To this systematic cloning period, functional genomics has succeeded in providing a new set of surprises. Disruption of several genes, thought to play a crucial role in LCFA absorption, did not lead to clear phenotypes. This observation raises the question of the real physiological role of lipid-binding proteins and lipid-metabolizing enzymes expressed in enterocytes. The goal of this review is to analyze present knowledge concerning the main steps of intestinal fat absorption from LCFA uptake to lipoprotein release and to assess their impact on health.

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.

Fatty acid transport: the roads taken

Efficient uptake and channeling of long-chain fatty acids (LCFAs) are critical cellular functions. Although spontaneous flip-flop of nonionized LCFAs from one leaflet of a bilayer to the other is rapid, evidence is emerging that proteins are important mediators and/or regulators of trafficking of LCFAs into and within cells. Genetic screens have led to the identification of proteins that are required for fatty acid import and utilization in prokaryotic organisms. In addition, functional screens have elucidated proteins that facilitate fatty acid import into mammalian cells. Although the mechanisms by which these proteins mediate LCFA import are not well understood, studies in both prokaryotic and eukaryotic organisms provide compelling evidence that uptake of LCFAs across cellular membranes is coupled to esterification by acyl-CoA synthetases. This review will summarize results of studies of non-protein-mediated and protein-mediated LCFA transport and discuss how these different mechanisms may contribute to cellular metabolism.

Heart microvessels and aortic endothelial cells express the 15 kDa heart-type fatty acid-binding proteins

Due to their hydrophobic nature, free fatty acids require carriers for transport across and within the cells. The endothelial layer is the first barrier to be traversed by the fatty acids, from the plasma to the underlying cells and tissues. We tried to find out whether cytosolic fatty acid-binding proteins (FABPs) are present in the endothelium of large vessels (aortic endothelial cells) and small vessels (myocardial capillaries) using the following experimental approaches: (i) loading the delipidated aortic endothelial cell (EC) homogenate and the heart cytosolic proteins and membrane proteins with [14C]palmitate or [14C]oleate, respectively, followed by autoradiographic detection of electrophoretically separated bands; (ii) detection by immunoprecipitation of heart-type FABP (H-FABP) using an affinity-purified antibody raised against bovine H-FABP (anti-H-FABP), and (iii) localization of FABP by indirect immunofluorescence and gold-immunocytochemistry applied to cultured EC and to thick and thin frozen sections of mouse heart. The results showed that: (i) within the EC homogenate proteins that express affinity for [14C]palmitate have an apparent Mr of 15000, and 40000-45000, that correspond as molecular mass to cytosolic and membrane FABPs, respectively. Similar affinity was found by incubation with [14C]oleate, that binds to a protein of Mr 15000 in the heart cytosol, and to a 40-45 kDa protein in the membrane fraction; (ii) anti-H-FABP immunoprecipitated specifically a cytosolic 15 kDa peptide (H-FABP); (iii) by indirect immunofluorescence, cytosolic H-FABP was localized on heart microvessels and myocytes and also in cultured aortic EC where intense spotted fluorescence characteristic for cytosolic antigens was present; (iv) by immunocytochemistry, H-FABP was detected in the EC cytoplasm, and in close proximity to the cytoplasmic aspect of plasmalemma and vesicle membranes. Together the data attest the presence of the 15 kDa, heart-type FABP in the endothelium of aorta and heart microvessels.

Ethanol up-regulates fatty acid uptake and plasma membrane expression and export of mitochondrial aspartate aminotransferase in HepG2 cells

To explain the increased plasma mitochondrial aspartate aminotransferase (mAspAT) observed in alcoholics, we cultured HepG2 hepatoma cells in ethanol. Acute (24 hour) exposure to 0, 20, 40, or 80 mmol/L ethanol produced a dose-dependent (r = .98) increase in mAspAT messenger RNA (mRNA) of < or = thirteen-fold, with no significant change in the cellular content of mAspAT or of several other enzymes. The recovery of mAspAT in the medium over 24 hours of ethanol exposure correlated with both ethanol concentration and with mAspAT mRNA (r = .90), reaching 808% of cellular enzyme content/24 hours at 80 mmol/L. Recovery of all other enzymes studied was < or = 20% of cellular content and unaffected by ethanol. Plasma membrane mAspAT content also correlated with mAspAT mRNA (r = .96) and mitochondrial levels were unchanged. No mitochondrial morphologic abnormalities were observed at any ethanol concentration studied. In cells cultured chronically at 0 to 80 mmol/L ethanol, fatty acid uptake Vmax increased in parallel with plasma membrane expression of mAspAT (r = .98). Cellular triglyceride content was highly correlated with Vmax. Thus, the data suggest that: 1) the increased plasma mAspAT observed in alcoholics may reflect pharmacologic upregulation of mAspAT mRNA and of mAspAT synthesis by ethanol; and 2) increased mAspAT-mediated fatty acid uptake may contribute to alcoholic fatty liver.

Mechanisms and kinetics of alpha-linolenic acid uptake in Caco-2 clone TC7

The uptake kinetics of alpha-linolenic acid (18:3(n - 3)), an essential fatty acid, were investigated in the human intestinal cell line Caco-2. Four clones (PD10, PF11, PD7 and TC7) from the heterogeneous parental Caco-2 cells population were used. After a screening step using isolated cells, the TC7 clone was selected for the study of alpha-linolenic acid uptake. [1-(14)C]linolenic acid dissolved in 10 mM taurocholate was presented to the microvillus plasma membrane (apical side) of TC7 differentiated cells, grown on a semi-permeable polycarbonate membrane. The results show that the initial rate of uptake is not a linear function of the 18:3(n- 3) monomer concentration in the incubation medium. In the monomer concentration range studied (0.2 to 36 microM) apical uptake was saturable and followed Michaelis-Menten kinetics (V(max) = 15.4 +/- 0.6 nmol/mg protein per min, K(m) = 14.3 +/- 1.3 microM). In addition, it was temperature- and energy-dependent but was apparently unaffected by the sodium gradient and intracellular metabolic fate of 18:3(n - 3). Excess of unlabeled saturated or unsaturated long chain fatty acids (C16 to C22) led to a 27-68% reduction of [1-(14)C]linolenic acid uptake. Likewise basolateral uptake was saturable (V(max) = 4.9 +/- 0.7 nmol/mg protein per min, K(m) = 8.7 +/- 2.9 microM). These facts argue in favour of the existence in these human intestinal cells of a carrier-mediated transport system for alpha-linolenic acid and probably other long chain fatty acids as well.

The role of the placenta in fetal nutrition and growth

The placenta plays a key role in the nutrition of the fetus. It mediates the active transport of nutrients and metabolic wastes across the barrier separating maternal and fetal compartments, as well as modifying the composition of some nutrients through its own metabolic activity. The function of the placenta is essential to the growth of a healthy fetus; it is becoming apparent that the activities of the placenta are in turn modulated by signals originating from the fetus. Communication between placenta and fetus is especially critical in intrauterine growth retardation. The importance of the interaction of factors like insulin-like growth factor and epidermal growth factor with their receptors is becoming increasingly clear.

The effect of intracellular pH on long-chain fatty acid uptake in 3T3-L1 adipocytes: evidence that uptake involves the passive diffusion of protonated long-chain fatty acids across the plasma membrane

To understand the mechanism of long-chain fatty acid permeation of the plasma membrane in mammalian cells, the effects of changes in the cytoplasmic pH on the internalization of physiologically relevant, submicromolar concentrations of uncomplexed long-chain fatty acids were investigated in 3T3-L1 adipocytes. The acidification of the cytoplasm upon NH4Cl prepulsing of intact cells was accompanied by a rapid reduction of cellular long-chain fatty acid uptake (measured as the total accumulation of [9,10-3H]oleate). This was followed by a slow recovery to normal levels of uptake as the cytoplasmic pH recovered. Conventional filtration assays do not distinguish between fatty acid movement across the plasma membrane and intracellular steps, such as binding to cytoplasmic fatty acid-binding proteins or metabolism. While the in vitro binding of a photoreactive fatty acid, 11-m-diazirinophenoxy[11-3H]undecanoate, to a cytoplasmic fatty acid-binding protein was insensitive to changes in pH from pH 7.5 to 5.5, the in vitro conversion of oleate into oleoyl-CoA by cellular acyl-CoA synthetase decreased dramatically. Therefore, the labelling of the 15 kDa cytoplasmic fatty acid-binding protein in intact cells by the photoreactive fatty acid was used as a more direct measure of the permeation of the probe across the plasma membrane. Acidification of the cytoplasm resulted in an immediate reduction in the labelling of this protein in intact adipocytes. Its photolabelling recovered, however, upon the recovery of the cytoplasmic pH to normal levels. This was due to effects of the cytoplasmic pH on the permeation of the photoreactive fatty acid across the plasma membrane rather than its binding to the 15 kDa protein or metabolism in vivo. This is the first demonstration that the movement of physiologically relevant, submicromolar concentrations of uncomplexed long-chain fatty acids across the plasma membrane of intact cells is coupled to the cytoplasmic pH and suggests that it occurs by the diffusion of the protonated long-chain fatty acid through the lipid bilayer.

3T3 fibroblasts transfected with a cDNA for mitochondrial aspartate aminotransferase express plasma membrane fatty acid-binding protein and saturable fatty acid uptake

To explore the relationship between mitochondrial aspartate aminotransferase (mAspAT; EC 2.6.1.1) and plasma membrane fatty acid-binding protein (FABPpm) and their role in cellular fatty acid uptake, 3T3 fibroblasts were cotransfected with plasmid pMAAT2, containing a full-length mAspAT cDNA downstream of a Zn(2+)-inducible metallothionein promoter, and pFR400, which conveys methotrexate resistance. Transfectants were selected in methotrexate, cloned, and exposed to increasing methotrexate concentrations to induce gene amplification. Stably transfected clones were characterized by Southern blotting; those with highest copy numbers of pFR400 alone (pFR400) or pFR400 and pMAAT2 (pFR400/pMAAT2) were expanded for further study. [3H]Oleate uptake was measured in medium containing 500 microM bovine serum albumin and 125-1000 microM total oleate (unbound oleate, 18-420 nM) and consisted of saturable and nonsaturable components. pFR400/pMAAT2 cells exhibited no increase in the rate constant for nonsaturable oleate uptake or in the uptake rate of [14C]octanoate under any conditions. By contrast, Vmax (fmol/sec per 50,000 cells) of the saturable oleate uptake component increased 3.5-fold in pFR400/pMAAT2 cells compared to pFR400, with a further 3.2-fold increase in the presence of Zn2+. Zn2+ had no effect in pFR400 controls (P > 0.5). The overall increase in Vmax between pFR400 and pFR400/pMAAT2 in the presence of Zn2+ was 10.4-fold (P < 0.01) and was highly correlated (r = 0.99) with expression of FABPpm in plasma membranes as determined by Western blotting. Neither untransfected 3T3 nor pFR400 cells expressed cell surface FABPpm detectable by immunofluorescence. By contrast, plasma membrane immunofluorescence was detected in pFR400/pMAAT2 cells, especially if cultured in 100 microM Zn2+. The data support the dual hypotheses that mAspAT and FABPpm are identical and mediate saturable long-chain free fatty acid uptake.

Membrane permeation and intracellular trafficking of long chain fatty acids: insights from Escherichia coli and 3T3-L1 adipocytes

Long chain fatty acids are important substrates for energy production and lipid synthesis in prokaryotes and eukaryotes. Their cellular uptake represents an important first step leading to metabolism. This step is induced in Escherichia coli by growth in medium containing long chain fatty acids and in murine 3T3-L1 cells during differentiation to adipocytes. Consequently, these have been used extensively as model systems to study the cellular uptake of long chain fatty acids. Here, we present an overview of our current understanding of long chain fatty acid uptake in these cells. It consists of several distinct steps, mediated by a combination of biochemical and physico-chemical processes, and is driven by conversion of long chain fatty acids to acyl-CoA by acyl-CoA synthetase. An understanding of long chain fatty acid uptake may provide valuable insights into the roles of fatty acids in the regulation of cell signalling cascades, in the regulation of a variety of metabolic and transport processes, and in a variety of mammalian pathogenic conditions such as obesity and diabetes.

Arachidonic acid and free fatty acids as second messengers and the role of protein kinase C

In addition to serving as the precursor to a plethora of eicosanoids and other bioactive molecules, arachidonate may function as a bona fide second messenger. A number of studies have documented the ability of arachidonate to regulate the function of multiple targets in vitro systems. This has been particularly well established and studied with the activation of protein kinase C by arachidonate in a mechanism distinct from activation by diacylglycerol. In cells, arachidonate induces a number of activities, many of which may be independent of further metabolism to eicosanoids; suggesting possible direct action of arachidonate. This review summarizes the current state of knowledge on the possible second messenger function of arachidonate with specific emphasis on the regulation of protein kinase C.

Plasma membrane fatty acid-binding protein (FABPpm) of the sheep placenta

Fatty acid-binding protein (FABPpm) has been identified and characterised from sheep placental membranes. Binding of [14C]oleate to placental membranes was found to be time- and temperature-dependent. Addition of a 20-fold excess unlabelled oleic, palmitic, or linoleic acid reduced the binding of [14C]oleate to the membranes to around 50% of total binding, whereas D-alpha-tocopherol at similar concentrations did not affect [14C]oleate binding. This indicates that the binding sites are specific to fatty acids. Specific binding of [14C]oleate was reduced by heat denaturation or trypsin digestion of the membranes, suggesting that the fatty acid-binding sites are protein in nature. FABPpm was then solubilised from sheep placental membranes, and subsequently purified to electrophoretic homogeneity using an oleate-agarose affinity column. The purified FABPpm had an apparent molecular mass of 40 kDa, as determined by SDS-PAGE and by gel permeation chromatography. The [14C]oleate-binding activity of the purified protein was also confirmed by PAGE followed by autoradioblotting. The specific binding for oleate was around 1.5 nmol per mg of membrane protein. Our data indicate the presence of FABPpm in sheep placental membranes.

The capillary transport system for free fatty acids in the heart

The nature of the process by which free fatty acids, which are tightly bound to albumin, traverse the endothelium of cardiac capillaries to reach the cardiac muscle cells, so that they are extracted to a net extent of approximately 40%, needs clarification. Previous studies have indicated that a membrane fatty acid-binding protein provides for carrier-mediated uptake of free fatty acids by isolated hepatocytes, cardiomyocytes, and jejunal mucosal cells. A monoclonal monospecific antibody was prepared against purified membrane fatty acid-binding protein from rat liver. Multiple-indicator dilution experiments were carried out in the isolated rat heart with labeled albumin, sucrose, and palmitate in the presence of control perfusate or perfusate containing either specific antibody or comparable nonspecific myeloma cell supernatant (each of the latter containing additional albumin, in identical concentrations). Analysis of the labeled-sucrose curves provided a permeability-surface area product for sucrose to which that for palmitate could be compared. In comparison with control supernatants, myeloma supernatant produced a minor inhibition of palmitate uptake, as a result of the increase in albumin concentration. The specific antibody, which contained identical albumin concentrations, produced a major inhibition of palmitate uptake, significantly greater than with the myeloma supernatant. The data indicate that the membrane fatty acid-binding protein mediates the transfer of free fatty acid across the endothelial cells of cardiac capillaries for presentation to heart muscle. Passive intramembrane lateral diffusion of palmitate could not provide an explanation for the findings.

Constitutive expression of a saturable transport system for non-esterified fatty acids in Xenopus laevis oocytes

In the presence of 150 microM BSA, uptake of [3H]oleate by Xenopus laevis oocytes was a saturable function of the unbound oleate concentration (Vmax. 110 +/- 4 pmol/h per oocyte; Km 193 +/- 11 nM unbound oleate). Oleate uptake was three orders of magnitude faster than that of another test substance, [35S]bromosulphophthalein, and was competitively inhibited by 55 nM unbound palmitate (Vmax. 111 +/- 14 pmol/h per oocyte; Km 424 +/- 63 nM unbound oleate) (P < 0.01). Oleate uptake was also inhibited by antibodies to a 43 kDa rat liver plasma-membrane fatty acid-binding protein, a putative transporter of long-chain fatty acids in mammalian cells; uptake of the medium-chain fatty acid [14C]octanoate was unaffected. Immunofluorescence and immunoblotting demonstrated that the antiserum reacted with a single 43 kDa protein on the oocyte surface. Hence a protein related to the mammalian plasma-membrane fatty acid-binding protein may play a role in saturable uptake of long-chain fatty acids by Xenopus oocytes.

Evaluation of a nitroxyl fatty acid as liver contrast agent for magnetic resonance imaging

In this study, we report the synthesis and the evaluation as MRI contrast agent of a new compound (nitroxyl fatty acid, NFA), where a pyrrolidinoxyl radical (3-carboxy-proxyl, PCA) is linked to a fatty acid moiety. Fatty acids were selected as vector because they present a high affinity for the liver, their efficient cellular uptake being the result of a specific interaction with a transmembrane transporter (liver plasma membrane-fatty acid binding protein). The uptake of 3H-oleic acid is inhibited after the injection of 1 mmol/kg of NFA, suggesting that NFA recognizes the same transmembrane transporter as the natural fatty acids. The higher relaxivity R1 of NFA in albumin solutions, compared with PCA, was explained by the immobilization of the nitroxyl radical in the protein. MR imaging was performed using T1-weighted images on mice in order to compare the contrast effect obtained after the injection of 1 mmol/kg of radical. The % signal enhancement in the liver 5 min after intravenous injection was 49 +/- 4 and 14 +/- 5 for NFA and PCA, respectively. NFA allowed a better delimitation of some necrotic tumors (Novikoff hepatocarcinoma) due to its preferential uptake by the nontumorous tissue.

Fatty acid-binding protein in bovine longissimus dorsi muscle

Experiments were conducted to purify and obtain amino acid sequence data for fatty acid-binding protein from bovine longissimus dorsi muscle. 1. Separation of sarcoplasmic proteins by fractionation on Sephadex and DEAE cellulose columns indicated that a low abundance fatty acid-binding protein exists in bovine l. dorsi muscle. 2. Fatty acid-binding protein in bovine l. dorsi muscle cannot be separated from myoglobin by standard protein purification procedures. However, partial sequencing of a partially purified protein fraction indicated the presence of an NH2-terminally blocked protein. 3. Myoglobin bound palmitate under the in vitro conditions of these experiments.

Characteristics of oleate binding to liver plasma membranes and its uptake by isolated hepatocytes

To clarify mechanisms of hepatic free fatty acid uptake, [3H]oleate uptake by isolated rat hepatocytes was studied, using solutions of 150 microM bovine serum albumin at oleate:albumin molar ratios of 0.033-6.7:1. Oleate partitioning between liver plasma membranes and albumin was also studied, and used to ascertain the membrane binding function for oleate. The experimental uptake curve was complex, but could be resolved by computer fitting into a sum of two components, one a saturable and the second a linear function of the unbound oleate concentration. The saturable component comprises > 90% of total oleate uptake when the oleate:albumin molar ratio is < 2.5, but < 50% when this ratio is > 5. Membrane binding also consisted of a sum of a saturable and a linear component. By comparison of the computer-fitted uptake and binding functions, separate rate constants for the transfer into the cell of the saturably and non-saturably bound oleate were estimated to be 0.7 s-1 and 0.05 s-1, respectively. The former is compatible with a specific, protein-mediated process. It is 15-times greater than the corresponding rate constant for transfer of non-saturably bound oleate into the cell, which in turn is similar to reported rates of non-specific 'flip-flop' of fatty acids across lipid bilayers. The observed kinetics are not consistent with models in which uptake occurs principally from the albumin-bound pool of oleate, or solely from the oleate which has partitioned passively into the lipid bilayer of the plasma membrane.

Presence of functionally active cytochrome P-450IIE1 in the plasma membrane of rat hepatocytes

Recent experiments have described the presence of cytochrome P-450 and certain P-450 isozymes in the plasma membrane of rat liver. Experiments were carried out to evaluate whether cytochrome P-450IIE1 was present in the plasma membrane fraction of livers from control rats and rats treated with 4-methylpyrazole, which induces this isozyme. Using immunofluorescence, fluorescence was detected at the surface of intact hepatocytes that were initially incubated with anti-P-450IIE1 IgG, but not preimmune IgG, followed by incubating with goat antirabbit IgG conjugated with either fluorescein or rhodamine. The fluorescence appeared to be uniformly distributed across the entire surface. Intense intracellular staining could be observed when the hepatocytes were permeabilized by acetone treatment. Similar results were obtained with control hepatocytes; however, the fluorescence intensity was considerably less than that shown by the induced hepatocytes. Hepatocytes isolated from the pericentral zone of the liver acinus displayed more intense fluorescence at the surface than did hepatocytes from the periportal zone. Purified plasma membranes oxidized dimethylnitrosamine to formaldehyde at rates that were 14% to 30% that of the microsomes, which exceeds the 3% contamination of the plasma membranes by microsomes as assessed by glucose-6-phosphatase activity. Immunoblots of the plasma membranes revealed the presence of a single band, whose intensity of staining was 14% to 26% that of the microsomes. Oxidation of dimethylnitrosamine and immunoblot intensity were about twofold greater with plasma membrane fractions from 4-methylpyrazole-treated rats than controls. These results suggest the presence of inducible, functionally active P-450IIE1 in the plasma membrane, which may be of toxicological significance in view of the preferential metabolism of a variety of hepatotoxins and carcinogens and the elevated production of reactive oxygen intermediates by this isozyme.

Purification of rat liver mitochondrial aspartate aminotransferase and separation of its isoforms utilizing high-performance liquid chromatography

A rapid method for purification of mitochondrial aspartate aminotransferase from rat liver employing high-performance liquid chromatography is reported. The product is purified 80-fold with a recovery greater than or equal to 50% in a single day. The amino acid composition, N-terminal amino acid sequence, specific activity, and spectral characteristics of the isolated enzyme are similar to those previously reported for this protein. The protein is homogeneous by standard electrophoretic and chromatographic criteria, but can be resolved into at least five isoforms by a carboxymethylated resin column using high-performance liquid chromatography. The principal isoform initially isolated is converted into two additional isoforms with lower specific activity upon storage at 4 degrees C.

Plasma membrane fatty acid-binding protein and mitochondrial glutamic-oxaloacetic transaminase of rat liver are related

The hepatic plasma membrane fatty acid-binding protein (h-FABPPM) and the mitochondrial isoenzyme of glutamic-oxaloacetic transaminase (mGOT) of rat liver have similar amino acid compositions and identical amino acid sequences for residues 3-24. Both proteins migrate with an apparent molecular mass of 43 kDa on SDS/polyacrylamide gel electrophoresis, have a similar pattern of basic charge isomers on isoelectric focusing, are eluted similarly from four different high-performance liquid chromatographic columns, have absorption maxima at 435 nm under acid conditions and 354 nm at pH 8.3, and bind oleate with a Ka approximately 1.2-1.4 x 10(7) M-1. Sinusoidally enriched liver plasma membranes and purified h-FABPPM have GOT enzymatic activity; the relative specific activities (units/mg) of the membranes and purified protein suggest that h-FABPPM constitutes 1-2% of plasma membrane protein in the rat hepatocyte. Monospecific rabbit antiserum against h-FABPPM reacts on Western blotting with mGOT, and vice versa. Antisera against both proteins produce plasma membrane immunofluorescence in rat hepatocytes and selectively inhibit the hepatocellular uptake of [3H]oleate but not that of [35S]sulfobromophthalein or [14C]taurocholate. The inhibition of oleate uptake produced by anti-h-FABPPM can be eliminated by preincubation of the antiserum with mGOT; similarly, the plasma membrane immunofluorescence produced by either antiserum can be eliminated by preincubation with the other antigen. These data suggest that h-FABPPM and mGOT are closely related.

Isolation and characterization of a fatty acid binding protein in adipose tissue of Gallus domesticus

1. Fatty acid binding protein (A-FABP) was isolated from chicken adipose cytosol. 2. Relative mol. wt of chicken A-FABP was determined to be 14,400 from SDS-polyacrylamide electrophoresis; the pI was 5.1; and amino acid composition data indicated structural homology with mammalian heart and adipose FABPs. 3. Polyclonal antisera prepared against A-FABP exhibited monospecificity for chicken A-FABP and no cross-reactivity with chicken liver proteins was observed. 4. Determination of relative ligand binding characteristics indicated A-FABP exhibited greatest binding activity in response to linoleate, followed by oleate, palmityl CoA and palmitate; no binding affinity for cholesterol was detected.

Translocation of oleic acid across the erythrocyte membrane. Evidence for a fast process

To clarify divergent views concerning the mechanism of fatty acid translocation across biomembranes this issue was now investigated in human erythrocytes. Translocation rates of exogenously inserted radioactive oleic acid across the membrane of native cells were derived from the time-dependent increase of the fraction of radioactivity becoming non-extractable by albumin. No accumulation of non-extractable unesterified oleic acid occurred. The rate of transfer was markedly suppressed by SH-reagents and by ATP-depletion. The suppression, however, resulted from a mere decrease of incorporation of oleic acid into phospholipids and was not accompanied by an increase of non-extractable unesterified oleic acid. These findings were reconcilable with the concept of a slow, possibly carrier-mediated fatty acid transfer as well as a very fast presumably, diffusional process not resolvable by the albumin extraction procedure. This ambiguity was resolved by using resealed ghosts, which are unable to incorporate oleic acid into phospholipids. In such ghosts all of the oleic acid inserted into the membrane remains extractable by albumin even after prolonged incubation. On the other hand, ghosts containing albumin accumulated non-extractable oleic acid. The rate of accumulation was beyond the time resolution of the albumin extraction procedure at 4 degrees C. Oleic acid uptake into albumin-containing ghosts became kinetically resolvable when the fatty acid was added as a complex with albumin. Correspondingly, time-resolvable release of oleic acid, originally complexed to internal albumin, into an albumin-containing medium was demonstrated at 4 degrees C. Rate and extent of these redistributions of oleic acid were dependent on the concentrations of internal and external albumin. This indicates limitation by the dissociation of oleic acid from albumin and not its translocation across the membrane. Translocation of oleic acid, which is probably a simple diffusive flip-flop process, must therefore occur with a half-time of less than 15 s. These findings raise doubts on the physiological role of presently discussed concepts of a carrier-mediated translocation of fatty acids across plasma membranes.

At physiologic albumin/oleate concentrations oleate uptake by isolated hepatocytes, cardiac myocytes, and adipocytes is a saturable function of the unbound oleate concentration. Uptake kinetics are consistent with the conventional theory

To reexamine the role of albumin in cellular uptake of long chain fatty acids, we measured [3H]oleate uptake by isolated hepatocytes, adipocytes, and cardiac myocytes from incubations containing oleate/albumin complexes at molar ratios from 0.01:1 to 2:1. For each ratio the uptake was studied over a wide range of albumin concentrations. In all three cell types and at any given oleate/albumin ratio, the uptake appeared saturable with increasing concentrations of oleate:albumin complexes despite the fact that the unbound oleate concentration for each molar ratio is essentially constant. However, the "Km" but not the "Vmax" of these pseudosaturation curves was influenced by substrate availability. At low albumin concentrations, uptake velocities did not correlate with unbound oleate concentrations. However, observed and expected uptake velocities coincided at albumin concentrations approaching physiologic levels and were a saturable function of the oleate/albumin ratios and the consequent unbound oleate concentrations employed. Hence, under the experimental conditions employed in this study using a variety of suspended cell types, oleate uptake kinetics were consistent with the conventional theory at physiologic concentrations of albumin.

Lipoprotein lipase. A multifunctional enzyme relevant to common metabolic diseases

Lipoprotein lipase is an important regulator of lipid and lipoprotein metabolism. It also contributes to the lipid and energy metabolism of different tissues in varying ways. Although the synthesis, manner of secretion, and mechanism of endothelial binding of lipoprotein lipase appear similar in all tissues, the factors that control gene expression and posttranslational events related to processing vary from tissue to tissue. The actual molecular events that determine this tissue specificity are not yet understood. In the future, however, it may be possible to stimulate or inhibit the activity of lipoprotein lipase in specific tissues and to alter metabolic processes so as to improve the quality and length of life in patients with metabolic diseases such as hypertriglyceridemia, HDL2 deficiency, and obesity.

Hepatic oleate uptake. Electrochemical driving forces in intact rat liver

Recent observations suggest that the hepatic uptake of oleate may be sodium coupled. To assess the electrochemical forces driving fatty acid uptake, we used microelectrodes to monitor continuously the electrical potential difference across the plasma membrane in the perfused rat liver while simultaneously monitoring the rate of tracer [3H]oleate uptake from 1% albumin solutions. Isosmotic cation or anion substitution was used to vary the potential difference over the physiologic range. Depolarization of cells from -29 to -19 mV by substituting gluconate for chloride reduced steady-state oleate uptake by 34%. Conversely, hyperpolarization of cells to -52 mV by substituting nitrate for chloride increased uptake by 41%. Replacement of perfusate sodium with choline depolarized the cells to -18 mV and reduced uptake by 58%, an amount greater than expected from the degree of depolarization alone. Oleate in higher concentrations (1.5 mM in 2% albumin) depolarized cells by 3 mV in the presence of sodium, but had no effect in sodium-free buffer. These results suggest that a portion of oleate uptake in the intact liver occurs by electrogenic sodium cotransport. Uptake appears to be driven by both the electrical and sodium chemical gradients across the plasma membrane.

Oleate uptake by cardiac myocytes is carrier mediated and involves a 40-kD plasma membrane fatty acid binding protein similar to that in liver, adipose tissue, and gut

Uptake of [3H]oleate by canine or rat cardiac myocytes is saturable, displays the countertransport phenomenon, and is inhibited by phloretin and trypsin. Cardiac myocytes contain a basic (pI approximately 9.1) 40-kD plasma membrane fatty acid binding protein (FABPPM) analogous to those recently isolated from liver, adipose tissue, and gut, unrelated to the 12-14-kD cytosolic FABP in these same tissues. An antibody to rat liver FABPPM selectively inhibits specific uptake of [3H]oleate by rat heart myocytes at 37 degrees C, but has no influence on nonspecific [3H]oleate uptake at 4 degrees C or on specific uptake of [3H]glucose. Uptake of long-chain free fatty acids by cardiac muscle cells, liver, and adipose tissue and absorption by gut epithelial cells is a facilitated process mediated by identical or closely related plasma membrane FABPs.