Intracellular fatty-acid-binding proteins. Characteristics and function

FABP1 controls hepatic transport and biotransformation of Δ9-THC

The increasing use of medical marijuana highlights the importance of developing a better understanding of cannabinoid metabolism. Phytocannabinoids, including ∆⁹-tetrahydrocannabinol (THC), are metabolized and inactivated by cytochrome P450 enzymes primarily within the liver. The lipophilic nature of cannabinoids necessitates mechanism(s) to facilitate their intracellular transport to metabolic enzymes. Here, we test the central hypothesis that liver-type fatty acid binding protein (FABP1) mediates phytocannabinoid transport and subsequent inactivation. Using X-ray crystallography, molecular modeling, and in vitro binding approaches we demonstrate that FABP1 accommodates one molecule of THC within its ligand binding pocket. Consistent with its role as a THC carrier, biotransformation of THC was reduced in primary hepatocytes obtained from FABP1-knockout (FABP1-KO) mice. Compared to their wild-type littermates, administration of THC to male and female FABP1-KO mice potentiated the physiological and behavioral effects of THC. The stark pharmacodynamic differences were confirmed upon pharmacokinetic analyses which revealed that FABP1-KO mice exhibit reduced rates of THC biotransformation. Collectively, these data position FABP1 as a hepatic THC transport protein and a critical mediator of cannabinoid inactivation. Since commonly used medications bind to FABP1 with comparable affinities to THC, our results further suggest that FABP1 could serve a previously unrecognized site of drug-drug interactions.

Fatty Acid Binding Protein-1 (FABP1) and the Human FABP1 T94A Variant: Roles in the Endocannabinoid System and Dyslipidemias

The first discovered member of the mammalian FABP family, liver fatty acid binding protein (FABP1, L-FABP), occurs at high cytosolic concentration in liver, intestine, and in the case of humans also in kidney. While the rat FABP1 is well studied, the extent these findings translate to human FABP1 is not clear-especially in view of recent studies showing that endocannabinoids and cannabinoids represent novel rat FABP1 ligands and FABP1 gene ablation impacts the hepatic endocannabinoid system, known to be involved in non-alcoholic fatty liver (NAFLD) development. Although not detectable in brain, FABP1 ablation nevertheless also impacts brain endocannabinoids. Despite overall tertiary structure similarity, human FABP1 differs significantly from rat FABP1 in secondary structure, much larger ligand binding cavity, and affinities/specificities for some ligands. Moreover, while both mouse and human FABP1 mediate ligand induction of peroxisome proliferator activated receptor-α (PPARα), they differ markedly in pattern of genes induced. This is critically important because a highly prevalent human single nucleotide polymorphism (SNP) (26-38 % minor allele frequency and 8.3 ± 1.9 % homozygous) results in a FABP1 T94A substitution that further accentuates these species differences. The human FABP1 T94A variant is associated with altered body mass index (BMI), clinical dyslipidemias (elevated plasma triglycerides and LDL cholesterol), atherothrombotic cerebral infarction, and non-alcoholic fatty liver disease (NAFLD). Resolving human FABP1 and the T94A variant's impact on the endocannabinoid and cannabinoid system is an exciting challenge due to the importance of this system in hepatic lipid accumulation as well as behavior, pain, inflammation, and satiety.

Chemotherapy-Induced Intestinal Mucosal Barrier Damage: a Cause of Falsely Elevated Serum 1,3-Beta-d-Glucan Levels?

Blood citrulline and intestinal fatty acid binding protein were determined as biomarkers for intestinal mucositis. Biomarker levels were correlated with corresponding serum 1,3-beta-D-glucan levels in 56 samples obtained from 33 cases with underlying hematological malignancies receiving induction chemotherapy. No correlation between biomarkers of intestinal mucositis and BDG levels was observed. (This study has been registered at ClinicalTrials.gov under registration no. NCT01576653.).

The human liver fatty acid binding protein T94A variant alters the structure, stability, and interaction with fibrates

Although the human liver fatty acid binding protein (L-FABP) T94A variant arises from the most commonly occurring single-nucleotide polymorphism in the entire FABP family, there is a complete lack of understanding regarding the role of this polymorphism in human disease. It has been hypothesized that the T94A substitution results in the complete loss of ligand binding ability and function analogous to that seen with L-FABP gene ablation. This possibility was addressed using the recombinant human wild-type (WT) T94T and T94A variant L-FABP and cultured primary human hepatocytes. Nonconservative replacement of the medium-sized, polar, uncharged T residue with a smaller, nonpolar, aliphatic A residue at position 94 of the human L-FABP significantly increased the L-FABP α-helical structure content at the expense of β-sheet content and concomitantly decreased the thermal stability. T94A did not alter the binding affinities for peroxisome proliferator-activated receptor α (PPARα) agonist ligands (phytanic acid, fenofibrate, and fenofibric acid). While T94A did not alter the impact of phytanic acid and only slightly altered that of fenofibrate on the human L-FABP secondary structure, the active metabolite fenofibric acid altered the T94A secondary structure much more than that of the WT T94T L-FABP. Finally, in cultured primary human hepatocytes, the T94A variant exhibited a significantly reduced extent of fibrate-mediated induction of PPARα-regulated proteins such as L-FABP, FATP5, and PPARα itself. Thus, while the T94A substitution did not alter the affinity of the human L-FABP for PPARα agonist ligands, it significantly altered the human L-FABP structure, stability, and conformational and functional response to fibrate.

Inhibitors of Fatty Acid Synthesis Induce PPAR α -Regulated Fatty Acid β -Oxidative Genes: Synergistic Roles of L-FABP and Glucose

While TOFA (acetyl CoA carboxylase inhibitor) and C75 (fatty acid synthase inhibitor) prevent lipid accumulation by inhibiting fatty acid synthesis, the mechanism of action is not simply accounted for by inhibition of the enzymes alone. Liver fatty acid binding protein (L-FABP), a mediator of long chain fatty acid signaling to peroxisome proliferator-activated receptor-α (PPARα) in the nucleus, was found to bind TOFA and its activated CoA thioester, TOFyl-CoA, with high affinity while binding C75 and C75-CoA with lower affinity. Binding of TOFA and C75-CoA significantly altered L-FABP secondary structure. High (20 mM) but not physiological (6 mM) glucose conferred on both TOFA and C75 the ability to induce PPARα transcription of the fatty acid β-oxidative enzymes CPT1A, CPT2, and ACOX1 in cultured primary hepatocytes from wild-type (WT) mice. However, L-FABP gene ablation abolished the effects of TOFA and C75 in the context of high glucose. These effects were not associated with an increased cellular level of unesterified fatty acids but rather by increased intracellular glucose. These findings suggested that L-FABP may function as an intracellular fatty acid synthesis inhibitor binding protein facilitating TOFA and C75-mediated induction of PPARα in the context of high glucose at levels similar to those in uncontrolled diabetes.

Loss of liver FA binding protein significantly alters hepatocyte plasma membrane microdomains

Although lipid-rich microdomains of hepatocyte plasma membranes serve as the major scaffolding regions for cholesterol transport proteins important in cholesterol disposition, little is known regarding intracellular factors regulating cholesterol distribution therein. On the basis of its ability to bind cholesterol and alter hepatic cholesterol accumulation, the cytosolic liver type FA binding protein (L-FABP) was hypothesized to be a candidate protein regulating these microdomains. Compared with wild-type hepatocyte plasma membranes, L-FABP gene ablation significantly increased the proportion of cholesterol-rich microdomains. Lack of L-FABP selectively increased cholesterol, phospholipid (especially phosphatidylcholine), and branched-chain FA accumulation in the cholesterol-rich microdomains. These cholesterol-rich microdomains are important, owing to enrichment therein of significant amounts of key transport proteins involved in uptake of cholesterol [SR-B1, ABCA-1, P-glycoprotein (P-gp), sterol carrier binding protein (SCP-2)], FA transport protein (FATP), and glucose transporters 1 and 2 (GLUT1, GLUT2) insulin receptor. L-FABP gene ablation enhanced the concentration of SCP-2, SR-B1, FATP4, and GLUT1 in the cholesterol-poor microdomains, with functional implications in HDL-mediated uptake and efflux of cholesterol. Thus L-FABP gene ablation significantly impacted the proportion of cholesterol-rich versus -poor microdomains in the hepatocyte plasma membrane and altered the distribution of lipids and proteins involved in cholesterol uptake therein.

Liver fatty acid-binding protein and obesity

While low levels of unesterified long chain fatty acids (LCFAs) are normal metabolic intermediates of dietary and endogenous fat, LCFAs are also potent regulators of key receptors/enzymes and at high levels become toxic detergents within the cell. Elevated levels of LCFAs are associated with diabetes, obesity and metabolic syndrome. Consequently, mammals evolved fatty acid-binding proteins (FABPs) that bind/sequester these potentially toxic free fatty acids in the cytosol and present them for rapid removal in oxidative (mitochondria, peroxisomes) or storage (endoplasmic reticulum, lipid droplets) organelles. Mammals have a large (15-member) family of FABPs with multiple members occurring within a single cell type. The first described FABP, liver-FABP (L-FABP or FABP1), is expressed in very high levels (2-5% of cytosolic protein) in liver as well as in intestine and kidney. Since L-FABP facilitates uptake and metabolism of LCFAs in vitro and in cultured cells, it was expected that abnormal function or loss of L-FABP would reduce hepatic LCFA uptake/oxidation and thereby increase LCFAs available for oxidation in muscle and/or storage in adipose. This prediction was confirmed in vitro with isolated liver slices and cultured primary hepatocytes from L-FABP gene-ablated mice. Despite unaltered food consumption when fed a control diet ad libitum, the L-FABP null mice exhibited age- and sex-dependent weight gain and increased fat tissue mass. The obese phenotype was exacerbated in L-FABP null mice pair fed a high-fat diet. Taken together with other findings, these data suggest that L-FABP could have an important role in preventing age- or diet-induced obesity.

Fluorescent n-3 and n-6 very long chain polyunsaturated fatty acids: three-photon imaging in living cells expressing liver fatty acid-binding protein

Despite the considerable beneficial effects of n-3 and n-6 very long chain polyunsaturated fatty acids (VLC-PUFAs), very little is known about the factors that regulate their uptake and intracellular distribution in living cells. This issue was addressed in cells expressing liver-type fatty acid-binding protein (L-FABP) by real time multiphoton laser scanning microscopy of novel fluorescent VLC-PUFAs containing a conjugated tetraene fluorophore near the carboxyl group and natural methylene-interrupted n-3 or n-6 grouping. The fluorescent VLC-PUFAs mimicked many properties of their native nonfluorescent counterparts, including uptake, distribution, and metabolism in living cells. The unesterified fluorescent VLC-PUFAs distributed either equally in nuclei versus cytoplasm (22-carbon n-3 VLC-PUFA) or preferentially to cytoplasm (20-carbon n-3 and n-6 VLC-PUFAs). L-FABP bound fluorescent VLC-PUFA with affinity and specificity similar to their nonfluorescent natural counterparts. Regarding n-3 and n-6 VLC-PUFA, L-FABP expression enhanced uptake into the cell and cytoplasm, selectively altered the pattern of fluorescent n-6 and n-3 VLC-PUFA distribution in cytoplasm versus nuclei, and preferentially distributed fluorescent VLC-PUFA into nucleoplasm versus nuclear envelope, especially for the 22-carbon n-3 VLC-PUFA, correlating with its high binding by L-FABP. Multiphoton laser scanning microscopy data showed for the first time VLC-PUFA in nuclei of living cells and suggested a model, whereby L-FABP facilitated VLC-PUFA targeting to nuclei by enhancing VLC-PUFA uptake and distribution into the cytoplasm and nucleoplasm.

L-FABP directly interacts with PPARalpha in cultured primary hepatocytes

Although studies with liver type fatty acid binding protein (L-FABP) gene ablated mice demonstrate a physiological role for L-FABP in hepatic fatty acid metabolism, little is known about the mechanisms whereby L-FABP elicits these effects. Studies indicate that L-FABP may function to shuttle lipids to the nucleus, thereby increasing the availability of ligands of nuclear receptors, such as peroxisome proliferator-activated receptor-alpha (PPARalpha). The data herein suggest that such mechanisms involve direct interaction of L-FABP with PPARalpha. L-FABP was shown to directly interact with PPARalpha in vitro through co-immunoprecipitation (co-IP) of pure proteins, altered circular dichroic (CD) spectra, and altered fluorescence spectra. In vitro fluorescence resonance energy transfer (FRET) between Cy3-labeled PPARalpha and Cy5-labeled L-FABP proteins showed that these proteins bound with high affinity (Kd approximately 156 nM) and in close proximity (intermolecular distance of 52A). This interaction was further substantiated by co-IP of both proteins from liver homogenates of wild-type mice. Moreover, double immunogold electron microscopy and FRET confocal microscopy of cultured primary hepatocytes showed that L-FABP was in close proximity to PPARalpha (intermolecular distance 40-49A) in vivo. Taken together, these studies were consistent with L-FABP regulating PPARalpha transcriptional activity in hepatocytes through direct interaction with PPARalpha. Our in vitro and imaging experiments demonstrate high affinity, structural molecular interaction of L-FABP with PPARalpha and suggest a functional role for L-FABP interaction with PPARalpha in long chain fatty acid (LCFA) metabolism.

Sterol carrier protein-2: new roles in regulating lipid rafts and signaling

Sterol carrier protein-2 (SCP-2) was independently discovered as a soluble protein that binds and transfers cholesterol as well as phospholipids (nonspecific lipid transfer protein, nsLTP) in vitro. Physiological functions of this protein are only now beginning to be resolved. The gene encoding SCP-2 also encodes sterol carrier protein-x (SCP-x) arising from an alternate transcription site. In vitro and in vivo SCP-x serves as a peroxisomal 3-ketoacyl-CoA thiolase in oxidation of branched-chain lipids (cholesterol to form bile acids; branched-chain fatty acid for detoxification). While peroxisomal SCP-2 facilitates branched-chain lipid oxidation, the role(s) of extraperoxisomal (up to 50% of total) are less clear. Studies using transfected fibroblasts overexpressing SCP-2 and hepatocytes from SCP-2/SCP-x gene-ablated mice reveal that SCP-2 selectively remodels the lipid composition, structure, and function of lipid rafts/caveolae. Studies of purified SCP-2 and in cells show that SCP-2 has high affinity for and selectively transfers many lipid species involved in intracellular signaling: fatty acids, fatty acyl CoAs, lysophosphatidic acid, phosphatidylinositols, and sphingolipids (sphingomyelin, ceramide, mono-di-and multi-hexosylceramides, gangliosides). SCP-2 selectively redistributes these signaling lipids between lipid rafts/caveolae and intracellular sites. These findings suggest SCP-2 serves not only in cholesterol and phospholipid transfer, but also in regulating multiple lipid signaling pathways in lipid raft/caveolae microdomains of the plasma membrane.

Membrane binding proteins are the major determinants for the hepatocellular transmembrane flux of long-chain fatty acids bound to albumin

PURPOSE

The hepatic transmembrane flux of long-chain fatty acids (LCFA) occurs through passive and fatty acid transport protein facilitated processes from blood. The extent that these transport processes can be related to the unbound and protein-bound fractions of LCFA in blood is not clear.

METHODS

We used hepatocyte suspensions, hepatoma monolayers, and perfused rat livers to quantitate the transport of purified [(3)H]palmitate ([(3)H]PA) and 12-(N-methyl)-N-[(7-nitrobenz-2oxa-1,3-diazol-4yl-)amino]octadecanoicacid (12-NBDS) from solutions with a constant unbound LCFA concentration with varying bovine serum albumin (BSA) concentrations and in the presence and absence of antisera raised against cytosolic liver fatty acid binding protein (L-FABP).

RESULTS

In the absence of L-FABP antisera, using an unbound ligand concentration that was adjusted to remain constant at each BSA concentration, hepatocyte [(3)H]PA and 12-NBDS uptake rates increased linearly with an increase in BSA concentration (p < 0.0001). In the presence of L-FABP antisera, [(3)H]PA uptake showed a greater reduction in the presence of 100 muM BSA than 5 muM BSA. The calculated permeability surface area product (PS) confirmed that both unbound and bound fractions of LCFA contributed to the overall flux, but only the PS for the protein-bound fraction was reduced in the presence of L-FABP antisera. In situ rat liver perfusion studies showed that the only rate process for the disposition of [(3)H]PA in the liver inhibited by L-FABP antisera was that for influx, as defined by PS, and that it reduced PS in the perfused liver by 42%.

CONCLUSION

These results suggest that, at physiological albumin concentrations, most of the LCFA uptake is mediated from that bound to albumin by a hepatocyte basolateral membrane transport protein, and uptake of unbound LCFA occurring by passive diffusion contributes a minor component.

Effect of branched-chain fatty acid on lipid dynamics in mice lacking liver fatty acid binding protein gene

Although a role for liver fatty acid protein (L-FABP) in the metabolism of branched-chain fatty acids has been suggested based on data obtained with cultured cells, the physiological significance of this observation remains to be demonstrated. To address this issue, the lipid phenotype and metabolism of phytanic acid, a branched-chain fatty acid, were determined in L-FABP gene-ablated mice fed a diet with and without 1% phytol (a metabolic precursor to phytanic acid). In response to dietary phytol, L-FABP gene ablation exhibited a gender-dependent lipid phenotype. Livers of phytol-fed female L-FABP-/- mice had significantly more fatty lipid droplets than male L-FABP-/- mice, whereas in phytol-fed wild-type L-FABP+/+ mice differences between males and females were not significant. Thus L-FABP gene ablation exacerbated the accumulation of lipid droplets in phytol-fed female, but not male, mice. These results were reflected in the lipid profile, where hepatic levels of triacylglycerides in phytol-fed female L-FABP-/- mice were significantly higher than in male L-FABP-/- mice. Furthermore, livers of phytol-fed female L-FABP-/- mice exhibited more necrosis than their male counterparts, consistent with the accumulation of higher levels of phytol metabolites (phytanic acid, pristanic acid) in liver and serum, in addition to increased hepatic levels of sterol carrier protein (SCP)-x, the only known peroxisomal enzyme specifically required for branched-chain fatty acid oxidation. In summary, L-FABP gene ablation exerted a significant role, especially in female mice, in branched-chain fatty acid metabolism. These effects were only partially compensated by concomitant upregulation of SCP-x in response to L-FABP gene ablation and dietary phytol.

Effect of dexamethasone, 2-bromopalmitate and clofibrate on L-FABP mediated hepatoma proliferation

Cytosolic liver fatty acid binding protein (L-FABP) is involved in many intracellular functions including cellular mitogenesis. We investigated the role of L-FABP and the plasma membrane liver fatty acid binding proteins (L-FABP(pm)) in the modulation of hepatoma growth and proliferation, hypothesizing that agents that affect either the content of, or ligand binding to, L-FABP would affect hepatocellular mitogenesis. L-FABP expressing 1548-rat hepatoma cells were treated with 0.5 microM dexamethasone or 500 microM clofibrate for 4 days to downregulate and upregulate L-FABP expression, respectively. The competitive inhibitor 2-bromopalmitate (BrPA, 600 microM) was used to inhibit ligand binding to L-FABP. The peripherally present plasma membrane fatty acid transporter was inactivated by treating cells with 1:50 rabbit antisera (FABP-Ab) raised against L-FABP. Western blot analysis was used to monitor L-FABP levels while [(3)H]-thymidine incorporation and growth curves were used to monitor hepatocellular proliferation. [(3)H]-Palmitate clearance studies were performed using monolayer cultures. Palmitate clearance in dexamethasone-, BrPA- and FABP-Ab-treated cells was significantly reduced when compared with control (P < 0.05), while clofibrate treatment moderately increased the rate. [(3)H]-Thymidine incorporation by dexamethasone- and BrPA-treated cells was significantly lower than control (P < 0.05), suggesting that hepatocellular proliferation was inhibited. Clofibrate treatment did not statistically affect growth rate. Lowering L-FABP using dexamethasone or interfering with its activity using BrPA significantly affected hepatocellular proliferation. This may be due to the non-availability of long-chain fatty acids or other intracellular mediators that are transported by L-FABP to the nucleus.

Liver fatty acid-binding protein gene ablation inhibits branched-chain fatty acid metabolism in cultured primary hepatocytes

Whereas the role of liver fatty acid-binding protein (L-FABP) in the uptake, transport, mitochondrial oxidation, and esterification of normal straight-chain fatty acids has been studied extensively, almost nothing is known regarding the function of L-FABP in peroxisomal oxidation and metabolism of branched-chain fatty acids. Therefore, phytanic acid (most common dietary branched-chain fatty acid) was chosen to address these issues in cultured primary hepatocytes isolated from livers of L-FABP gene-ablated (-/-) and wild type (+/+) mice. These studies provided three new insights: First, L-FABP gene ablation reduced maximal, but not initial, uptake of phytanic acid 3.2-fold. Initial uptake of phytanic acid uptake was unaltered apparently due to concomitant 5.3-, 1.6-, and 1.4-fold up-regulation of plasma membrane fatty acid transporter/translocase proteins (glutamic-oxaloacetic transaminase, fatty acid transport protein, and fatty acid translocase, respectively). Second, L-FABP gene ablation inhibited phytanic acid peroxisomal oxidation and microsomal esterification. These effects were consistent with reduced cytoplasmic fatty acid transport as evidenced by multiphoton fluorescence photobleaching recovery, where L-FABP gene ablation reduced the cytoplasmic, but not membrane, diffusional component of NBD-stearic acid movement 2-fold. Third, lipid analysis of the L-FABP gene-ablated hepatocytes revealed an altered fatty acid phenotype. Free fatty acid and triglyceride levels were decreased 1.9- and 1.6-fold, respectively. In summary, results with cultured primary hepatocytes isolated from L-FABP (+/+) and L-FABP (-/-) mice demonstrated for the first time a physiological role of L-FABP in the uptake and metabolism of branched-chain fatty acids.

Overexpression of acyl-coA binding protein and its effects on the flux of free fatty acids in McA-RH 7777 cells

Overexpression of acyl-CoA binding protein (ACBP) was induced in a rat hepatoma cell line (McA-RH 7777) by stable integration of rat ACBP cDNA. The transfected cells (ACBP-27) had 3.5-fold higher concentrations of ACBP than control cells (14 vs. 4 ng/microg DNA). Both ACBP-27 and control cells were cultured in the presence of various concentrations of radiolabeled palmitic acid; and the effects of ACBP on lipogenesis and beta-oxidation were studied. Incubation of the cells with 100 microM palmitic acid resulted in 42% greater incorporation of the fatty acid in ACBP-27 cells as compared to that in the control cells. This increased incorporation of the fatty acid was observed predominantly in the triglyceride fraction. Higher concentrations of palmitic acid (200 to 400 microM) were associated with a significant decrease in the production of 14CO2 in the ACBP-27 cell line than in the control cells, while lower concentrations had no effect. Our data suggest a role for ACBP in the partitioning of fatty acids between esterification reactions leading to the formation of neutral lipids and beta-oxidation. ACBP may play a regulatory role by influencing this important branch point in intermediary lipid metabolism.

Lipid carrier proteins and ethanol

Ethanol has a pronounced effect on lipid homeostasis. It is our overall hypothesis that certain lipid carrier proteins are targets of acute and chronic ethanol exposure and that perturbation of these proteins induces lipid dysfunction leading to cellular pathophysiology. These proteins include both intracellular proteins and lipoproteins. This paper examines recent data on the interaction of ethanol with these proteins. In addition, new data are presented on the stimulatory effects of ethanol on low-density-lipoprotein (LDL)-mediated cholesterol uptake into fibroblasts and direct perturbation of the LDL apolipoprotein, apolipoprotein B. A cell model is presented that outlines potential mechanisms thought to be involved in ethanol perturbation of cholesterol transport and distribution.

Long chain fatty acids inhibit and medium chain fatty acids activate mammalian cardiac hexokinase

We investigated the effect of non-esterified fatty acids (FAs) on bovine heart hexokinase (type I: ATP: D-hexose 6-phosphotransferase, EC 2.7.1.1). Long chain FAs (C14 to C20) inhibited the enzyme in a way that correlated positively with both the chain length and the degree of unsaturation. Medium chain FA with 12 or less carbons activated hexokinase in a chain length dependent manner with the greater activation shown by laurate. The activation constant of laurate was 91.5 microM with a maximal activation of 60.3%. Oleate caused a maximal decrease in specific activity of 25% with an inhibition constant of 79 microM. Using the fluorescent probe cis-parinarate, we found a saturable binding site with K(d) of 3.5 microM. Oleate competed the fluorescent probe from the protein with a K(d) of 1.4 microM. Medium chain FAs did not compete the probe from HK. The binding of fatty acid to the protein appears to be entropically driven as indicated by an Arrhenius analysis (DeltaS=+231.6 J mol(-1) deg(-1)). The presence of oleate significantly increased the K(ATP)(m) from 0.47 mM to 0.89 mM while the K(glucose)(m) in the presence of the FA (0.026+/-0.003 mM) was not significantly different from the control (0.014+/-0.004 mM). A decrease in V(max) values in the presence of oleate indicated that a mixed allosteric inhibition was operating.

The muscle fatty acid binding protein of spadefoot toad (Scaphiopus couchii)

Fatty acid binding protein was purified from skeletal muscle of the spadefoot toad (Scaphiopus couchii), an estivating species. While estivating, this animal relies on the fatty acid oxidation for energy. Hence we were interested in the behaviour of fatty acid binding protein under conditions of elevated urea (up to 200 mM) and potassium chloride such as exist during estivation. Also we examined whether there were interactions between glycolytic intermediates and the binding ability of the protein. The amount of bound fatty acid (a fluorescence assay using cis-parinarate) was not affected (P < 0.05) by glucose, fructose 6-phosphate or phosphoenolpyruvate at physiological concentrations. By contrast, glucose 6-phosphate increased the amount of bound cis-parinarate but the apparent dissociation constant was not different from the control. Fructose 1,6-bisphosphate but not fructose 2,6-phosphate decreased cis-parinarate binding by 40%, commensurate with doubling the apparent dissociation constant (1.15-2.62 microM). Urea, guanidinium and trimethylamine N-oxide at 200 mM increased cis-parinarate binding 60% over controls. Urea (1 M) and KCl (200 mM) did not affect cis-parinarate binding compared to controls. The interaction of this fatty acid transporter with fructose 1,6-bisphosphate is discussed in terms of reciprocal interaction with phosphofructokinase since fatty acid is also an inhibitor of phosphofructokinase.

Microsomal fatty acyl-CoA transacylation and hydrolysis: fatty acyl-CoA species dependent modulation by liver fatty acyl-CoA binding proteins

arachidonoyl-CoA. In summary, the data established for the first time a role for both L-FABP and ACBP in microsomal phosphatidic acid biosynthesis. By preferentially stimulating microsomal transacylation of unsaturated long chain fatty acyl-CoAs while concomitantly exerting their differential protection from microsomal acyl-CoA hydrolase, L-FABP and ACBP can uniquely function in modulating the pattern of fatty acids esterified to phosphatidic acid, the de novo precursor of phospholipids and triacylglycerols. This may explain in part the simultaneous presence of these proteins in cell types involved in fatty acid absorption and lipoprotein secretion.

Holo-sterol carrier protein-2. (13)C NMR investigation of cholesterol and fatty acid binding sites

Although sterol carrier protein-2 (SCP-2) stimulates sterol transfer in vitro, almost nothing is known regarding the identity of the putative cholesterol binding site. Furthermore, the interrelationship(s) between this SCP-2 ligand binding site and the recently reported SCP-2 long chain fatty acid (LCFA) and long chain fatty acyl-CoA (LCFA-CoA) binding site(s) remains to be established. In the present work, two SCP-2 ligand binding sites were identified. First, both [4-(13)C]cholesterol and 22-(N-(7-nitrobenz-2-oxa-1, 3-diazol-4-yl)amino)-23,24-bisnor-5-cholen-3beta-ol (NBD-cholesterol) binding assays were consistent with a single cholesterol binding site in SCP-2. This ligand binding site had high affinity for NBD-cholesterol, K(d) = 4.15 +/- 0.71 nM. (13)C NMR-labeled ligand competition studies demonstrated that the SCP-2 high affinity cholesterol binding site also bound LCFA or LCFA-CoA. However, only the LCFA-CoA was able to effectively displace the SCP-2-bound [4-(13)C]cholesterol. Thus, the ligand affinities at this SCP-2 binding site were in the relative order cholesterol = LCFA-CoA > LCFA. Second, (13)C NMR studies demonstrated the presence of another ligand binding site on SCP-2 that bound either LCFA or LCFA-CoA but not cholesterol. Photon correlation spectroscopy was consistent with SCP-2 being monomeric in both liganded and unliganded states. In summary, both (13)C NMR and fluorescence techniques demonstrated for the first time that SCP-2 had a single high affinity binding site that bound cholesterol, LCFA, or LCFA-CoA. Furthermore, results with (13)C NMR supported the presence of a second SCP-2 ligand binding site that bound either LCFA or LCFA-CoA but not cholesterol. These data contribute to our understanding of a role for SCP-2 in both cellular cholesterol and LCFA metabolism.

Differential absorption and esterification of dietary long-chain fatty acids by larvae of the dragonfly, Aeshna cyanea

In order to evaluate whether dietary long-chain fatty acids were differentially absorbed, Aeshna cyanea larvae received 5 microliters oral doses containing combinations of two radiolabeled fatty acids at nearly equal radioactive and nmolar concentrations: (1) 3H-oleic and 14C-palmitic acids; (2) 3H-oleic and 14C-stearic acids; and (3) 3H-palmitic and 14C-stearic acids. After 3 h or 1 day, hemolymph samples, midgut tissue, midgut contents and fat body tissue were collected and assayed for labeled fatty acids. The 3H/14C ratios indicated that there was a preference for absorption of the monounsaturated oleic acid over both saturated palmitic and stearic acids and that the shorter palmitic acid was absorbed at a higher rate than the longer stearic acid. There were also differences in the 3H/14C ratios of the various lipid classes of the midgut wall, hemolymph, and fat body that reflected differential esterifications and transport of these fatty acids.

Fatty acid binding to rat liver fatty acid-binding protein is modulated by early glycolytic intermediates

Fatty acid binding to rat liver fatty acid binding protein in the presence of glycolytic metabolites and at different pH (optimal 7.2) and ionic strength was studied. Binding decreased logarithmically with ionic strength. Glucose and glucose-6-phosphate increased fatty acid binding significantly with K0.5 within physiological ranges while glucose-1-phosphate and phosphate ion caused no effect.

Fatty acid binding protein isoforms: structure and function

Although structural aspects of cytosolic fatty acid binding proteins (FABPs) in mammalian tissues are now well understood, significant advances regarding the physiological function(s) of these proteins have been slow in forthcoming. Part of the difficulty lies in the complexity of the multigene FABP family with nearly twenty identified members. Furthermore, isoelectric focusing and ion exchange chromatography operationally resolve many of the mammalian native FABPs into putative isoforms. However, a more classical biochemical definition of an isoform, i.e. proteins differing by a single amino acid, suggests that the operational definition is too broad. Because at least one putative heart H-FABP isoform, the mammary derived growth inhibitor, was an artifact (Specht et al. (1996) J. Biol. Chem. 271: 1943-49), the ensuing skepticism and confusion cast doubt on the existence of FABP isoforms in general. Yet, increasing data suggest that several FABPs, e.g. human intestinal I-FABP, bovine and mouse heart H-FABP, rabbit myelin P2 protein and bovine liver L-FABP may exist as true isoforms. In contrast, the rat liver L-FABP putative isoforms may actually be due either to bound ligand, post-translational S-thiolation and/or structural conformers. In any case, almost nothing is known regarding possible functions of either the true or putative isoforms in vitro or in vivo. The objective of this article is to critically evaluate which FABPs form biochemically defined or true isoforms versus FABPs that form additional forms, operationally defined as isoforms. In addition, recent developments in the molecular basis for FABP true isoform formation, the processes leading to additional operationally defined putative isoforms and insights into potential function(s) of this unusual aspect of FABP heterogeneity will be examined.

Differential influence of rat liver fatty acid binding protein isoforms on phospholipid fatty acid composition: phosphatidic acid biosynthesis and phospholipid fatty acid remodeling

The ability of two rat liver fatty acid binding protein (L-FABP) isoforms to influence microsomal phosphatidic acid biosynthesis, a key intermediate in glycerolipid formation, and phospholipid fatty acid remodeling was examined in vitro. Isoform I enhanced microsomal incorporation of [1-14C]-oleoyl-CoA into phosphatidic acid 7-fold while isoform II had no effect relative to basal. In contrast, isoform II enhanced microsomal incorporation of [1-14C]-palmitoyl-CoA into phosphatidic acid 4-fold while isoform I had no effect. These results suggest that each L-FABP isoform selectively utilized different acyl-CoAs for glycerol-3-phosphate esterification. Both isoforms stimulated phosphatidic acid formation by increasing glycerol-3-phosphate acyltransferase activity, not by increasing lysophosphatidic acid acyltransferase activity. Furthermore, the effects of L-FABP on phosphatidic acid biosynthesis could not be correlated with protection from acyl-CoA hydrolysis. L-FABP isoforms also influenced phospholipid fatty acid remodeling in a phospholipid-dependent manner. Isoform I preferentially enhanced oleate and palmitate esterification into phosphatidylethanol-amine, while isoform II stimulated esterification into phosphatidylcholine, phosphatidylserine and sphingomyelin. Taken together, these data demonstrated a unique role of each L-FABP isoform in modulating microsomally derived phospholipid fatty acid composition. (c) 1998 Elsevier Science B.V.

Lipid specificity and location of the sterol carrier protein-2 fatty acid-binding site: a fluorescence displacement and energy transfer study

Although it was recently recognized that sterol carrier protein-2 (SCP-2) interacts with fatty acids, little is known regarding the specificity of SCP-2 for long-chain fatty acids or branched-chain fatty-acid-like molecules. Likewise the location of the fatty-acid binding site within SCP-2 is unresolved. A fluorescent cis-parinaric acid displacement assay was used to show that SCP-2 optimally interacted with 14-22 carbon chain lipidic molecules: polyunsaturated fatty acids > monounsaturated, saturated > branched-chain isoprenoids > branched-chain phytol-derived fatty acids. In contrast, the other major fatty-acid binding protein in liver, fatty-acid binding protein (L-FABP), displayed a much narrower carbon chain preference in general: polyunsaturated fatty acids > branched-chain phytol-derived fatty acids > 14- and 16-carbon saturated > branched-chain isoprenoids. However, both SCP-2 and L-FABP displayed a very similar unsaturated fatty-acid specificity profile. The presence and location of the SCP-2 lipid binding site were investigated by fluorescence energy transfer. The distance between the SCP-2 Trp50 and bound cis-parinaric acid was determined to be 40 A. Thus, the SCP-2 fatty-acid binding site appeared to be located on the opposite side of the SCP-2 Trp50. These findings not only contribute to our understanding of the SCP-2 ligand binding site but also provide evidence suggesting a potential role for SCP-2 and/or L-FABP in metabolism of branched-chain fatty acids and isoprenoids.

Expression of fatty acyl-CoA binding proteins in colon cells: response to butyrate and transformation

Fatty acyl-CoA affect many cellular functions as well as serving as cellular building blocks. Several families of cytosolic fatty acyl-CoA binding proteins may modulate the activities of fatty acyl-CoA. Intestinal enterocytes contain at least three unique families of cytosolic proteins that bind fatty acyl-CoA: acyl-CoA binding protein (ACBP), fatty acid binding proteins (including the liver, L-FABP and intestinal, I-FABP), and sterol carrier protein-2 (SCP-2). Immortalized rat colon epithelial cell lines expressed only ACBP and SCP-2 at levels of 0.75 +/- 0.13 and 0.42 +/- 0.02 ng/microgram protein. Ras and src transformation increased colon cell density and differentially altered ACBP and SCP-2 expression without affecting I-FABP or L-FABP levels. ACBP levels were 1.8-fold and 1.5-fold increased in ras- and src-transformed cells, respectively. In contrast, SCP-2 expression was significantly decreased 55 and 67% in ras- and src-transformed cells, respectively. Butyrate treatment of ras- and src-transformed cells decreased cell proliferation up to 60-85% as compared to 25-30% in control cells. Butyrate treatment decreased ACBP expression in all cell lines but had no effect on the levels of SCP-2, I-FABP, or L-FABP. These studies suggest that the differential expression of ACBP and SCP-2 in rat colonic cell lines, as well as their modulation by butyrate, may be altered by cell transformation.

Fatty acid binding protein: stimulation of microsomal phosphatidic acid formation

The effect of fatty acid binding proteins (FABPs) on two key steps of microsomal phosphatidic acid formation was examined. Rat liver microsomes were purified by size-exclusion chromatography to remove endogenous cytosolic fatty acid and fatty acyl-CoA binding proteins while recombinant FABPs were used to avoid cross-contamination with such proteins from native tissue. Neither rat liver (L-FABP) nor rat intestinal fatty acid binding protein (I-FABP) stimulated liver microsomal fatty acyl-CoA synthase. In contrast, L-FABP and I-FABP enhanced microsomal conversion of [14C]oleoyl-CoA and glycerol 3-phosphate to [14C]phosphatidic acid by 18- and 7-fold, respectively. The mechanism for this stimulation, especially by I-FABP, is not known. However, several observations presented here suggest that, like L-FABP, I-FABP may interact with fatty acyl-CoA and thereby stimulate enzyme activity. First, I-FABP decreased microsomal membrane-bound oleoyl-CoA. Second, oleoyl-CoA displaced I-FABP bound fluorescent fatty acid, cis-parinaric acid, with Ki of 5.3 microM and 1.1 sites. Third, oleoyl-CoA decreased I-FABP tryptophan fluorescence with a Kd of 4.2 microM. Fourth, oleoyl-CoA red shifted emission spectra of acrylodated I-FABP, a sensitive marker of I-FABP interactions with ligands. In summary, the results demonstrate for the first time that both L-FABP and I-FABP stimulate liver microsomal phosphatidic acid formation by enhancing synthesis of phosphatidate from fatty acyl-CoA and glycerol 3-phosphate.

Role of long-chain fatty acyl-CoA esters in the regulation of metabolism and in cell signalling

The intracellular concentration of free unbound acyl-CoA esters is tightly controlled by feedback inhibition of the acyl-CoA synthetase and is buffered by specific acyl-CoA binding proteins. Excessive increases in the concentration are expected to be prevented by conversion into acylcarnitines or by hydrolysis by acyl-CoA hydrolases. Under normal physiological conditions the free cytosolic concentration of acyl-CoA esters will be in the low nanomolar range, and it is unlikely to exceed 200 nM under the most extreme conditions. The fact that acetyl-CoA carboxylase is active during fatty acid synthesis (Ki for acyl-CoA is 5 nM) indicates strongly that the free cytosolic acyl-CoA concentration is below 5 nM under these conditions. Only a limited number of the reported experiments on the effects of acyl-CoA on cellular functions and enzymes have been carried out at low physiological concentrations in the presence of the appropriate acyl-CoA-buffering binding proteins. Re-evaluation of many of the reported effects is therefore urgently required. However, the observations that the ryanodine-senstitive Ca2+-release channel is regulated by long-chain acyl-CoA esters in the presence of a molar excess of acyl-CoA binding protein and that acetyl-CoA carboxylase, the AMP kinase kinase and the Escherichia coli transcription factor FadR are affected by low nanomolar concentrations of acyl-CoA indicate that long-chain acyl-CoA esters can act as regulatory molecules in vivo. This view is further supported by the observation that fatty acids do not repress expression of acetyl-CoA carboxylase or Delta9-desaturase in yeast deficient in acyl-CoA synthetase.

Metallothionein-IIA promoter induction alters rat intestinal fatty acid binding protein expression, fatty acid uptake, and lipid metabolism in transfected L-cells

Mouse L-cell fibroblasts, transfected with the cDNA encoding for rat intestinal fatty acid-binding protein (I-FABP) under the control of the human metallothionein-IIA promoter, were tested for their protein inducibility by the heavy metals cadmium (Cd2+) and zinc (Zn2+). I-FABP levels were quantitated by Western immunoblotting. Expression of I-FABP in all transfected cell lines tested was induced several-fold by optimized levels of Cd2+ and Zn2+. Induction conditions had no effect on cell growth rates or cell densities for any of the cell lines. Induction of high I-FABP-expressing cells (H141) decreased the initial rate and extent of uptake of cis-parinaric acid, a nonmetabolizable fatty acid, and of [3H]oleic acid, an esterifiable fatty acid. These effects of induction were specific for I-FABP-expressing cells since they were not observed in control cells or cells expressing a high level of liver (L-) FABP. Induction of H141 cells also significantly altered the esterification and distribution of exogenous [3H]oleic acid, especially among triglycerides and phosphatidylcholine, but less so among other glycero-phospholipids, cholesteryl esters, and phosphatidylethanolamine. Induction of H141 cells normalized [3H]oleic acid esterification into cholesteryl esters, phosphatidylcholine, total neutral lipids, and total phospholipids such that they no longer differed from control levels. In contrast, induction did not normalize [3H]oleic acid esterification into triacylglycerols and phosphatidylethanolamine to control levels in H141 cells; both remained significantly increased over control cells. Therefore, promoter induction levels of Cd2+ and Zn2+ enhanced I-FABP expression in H141 cells, thereby modulating both fatty acid uptake and intracellular esterification into neutral and phospholipids.

Human intestinal fatty acid binding protein: report of an assay with studies in normal volunteers and intestinal ischemia

BACKGROUND

Human intestinal fatty acid binding protein (hIFABP) is a cytoplasmic protein of mature small intestinal epithelium. Work with the rat demonstrated that serum levels of IFABP correlated with early phases of intestinal mucosal injury. The aim of this study was to develop an assay for hIFABP and assess its usefulness as a marker for intestinal mucosal injury in human beings.

METHODS

Recombinant hIFABP (r-hIFABP) was used to produce rabbit anti-hIFABP. Specificity and avidity of binding were tested with immunoprecipitation and Scatchard analysis. r-hIFABP was labeled with 125I, and a competitive assay was developed. Urine and serum from normal volunteers and from patients with necrotizing enterocolitis (NEC), acute thromboembolic related intestinal ischemia, and systemic inflammatory response syndrome were tested for hIFABP.

RESULTS

Molecular weight was 10(-12) kd, limit of detection was 1.87 ng/ml, and no cross-reactivity occurred when tested against rat IFABP or human heart FABP. Mean levels of hIFABP (ng/ml) were controls (serum less than 1.87, urine less than 1.87), NEC (serum 14.7 ng/ml), intestinal ischemia (serum 50 ng/ml, urine 52.3 ng/ml), systemic inflammatory response syndrome (serum 5.3 ng/ml, urine 13.2 ng/ml).

CONCLUSIONS

This assay is quantitative for hIFABP in serum and urine. Results from both normal persons and those with various causes of intestinal ischemia parallel our previous findings in the rat. Preliminary findings suggest that hIFABP may serve as a diagnostic marker for early intestinal mucosal compromise and, in addition, that it should prove useful as a tool in developing rationale therapeutic regimens to treat these complex clinical problems.

The sterol carrier protein-2 fatty acid binding site: an NMR, circular dichroic, and fluorescence spectroscopic determination

The interaction and orientation of fatty acids with recombinant human sterol carrier protein-2 (SCP-2) were examined by nuclear magnetic resonance (NMR), circular dichroism (CD), and fluorescence techniques. 13C-NMR spectroscopy of stearic acid and oleic acid as well as fluorescence spectroscopy of cis-parinaric acid demonstrated that SCP-2 bound naturally occurring fatty acids with near 1:1 stoichiometry. Several findings indicated that the fatty acid was oriented in the binding site with its methyl end buried in the protein interior and its carboxylate exposed at the surface: the chemical shift of bound [18-13C]-stearate; dicarboxylic/monocarboxylic acid cis-parinaric acid displacement; complete ionization of the carboxylate group of SCP-2 bound [1-13C]stearate at neutral pH; lack of electrostatic interactions between 13C-fatty acids with SCP-2 cationic residues: pH titratability of the SCP-2 bound [1-13C]stearate carboxylate group. SCP-2 did not undergo global structural changes upon ligand binding or pH decrease as indicated by the absence of significant changes in NMR and only small alterations in time resolved fluorescence parameters. However, SCP-2 did undergo secondary structural changes detected by CD in the pH range 5-6. While these changes in secondary structure did not alter the fatty acid:SCP-2 binding stoichiometry, the affinity for fatty acid was increased severalfold at lower pH. In summary, 13C-NMR, CD, and fluorescence spectroscopy provided a detailed understanding of the interaction of fatty acids with SCP-2 and further showed for the first time the orientation of the fatty acid within the binding site. The pH-induced changes in SCP-2 secondary structure and ligand binding activity may be important to the mechanism whereby this protein interacts with membrane surfaces to enhance lipid binding/transfer.

Time-resolved fluorescence of intestinal and liver fatty acid binding proteins: role of fatty acyl CoA and fatty acid

The effect of fatty acyl CoA and fatty acid on the solution structure and dynamics of two intestinal enterocyte fatty acid binding proteins, intestinal (I-FABP) and liver (L-FABP), was examined by time-resolved fluorescence of FABP aromatic amino acid residues. I-FABP Trp displayed two rotational correlation times, 6.6 and 0.4 ns. reflecting motion of the protein as a whole and segmental mobility of Trp. Neither oleoyl CoA, oleic acid, nor CoASH altered overall I-FABP rotational correlation time. However, oleic acid and CoASH increased I-FABP Trp segmental mobility, while oleoyl CoA and CoASH decreased I-FABP Trp limiting anisotropy (order). The angle of I-FABP Trp "wobbling in a cone" was increased by ligands in the order oleoyl CoA > CoASH > oleic acid. L-FABP Trp segmental mobility. L-FABP overall rotational motion, in contrast to that of I-FABP, was significantly increased by ligands in the order oleoyl CoA > oleic acid > CoASH. cis-Parinaric acid and cis-parinaroyl CoA bound to L-FABP also reflected overall L-FABP motion but yielded longer rotational correlation times, 8.2 and 10.7 ns, than the respective apo-FABPs. Such effects were not observed with I-FABP. Finally, both cis-parinaric acid and cis-parinaroyl CoA were much less ordered in the I-FABP ligand binding site than with L-FABP. These observations suggest that the rotational dynamics of L-FABP and its conformation are more sensitive to ligands than I-FABP. Further, ligands such as fatty acids, fatty acyl CoAs, and/or CoASH differentially modulate the I-FABP and L-FABP dynamics, and the ligand binding sites of these proteins differ in their ability to order the ligands.

Intestinal fatty acid-binding protein expression stimulates fibroblast fatty acid esterification

The effect of intestinal fatty acid binding protein (I-FABP) expression on cell growth and cell lipid content is not known. Therefore, mouse L-cell fibroblasts were transfected with the cDNA encoding for I-FABP. The high expression clones expressed 0.35% of the total cytosolic proteins as I-FABP. Mock transfected L-cells did not differ from control L-cells in any properties tested. Neither the growth rate, maximal cell density, nor [3H]oleic acid uptake differed in I-FABP expressing as compared to control cells. In contrast, I-FABP expression increased triacylglycerol and cholesteryl ester mass (nmol/mg protein) by 63% and 25%, respectively. Phospholipid mass was unchanged in I-FABP expressing cells. The initial [3H]oleic acid esterification into triacylglycerols and cholesteryl esters was increased 3.9- and 2.5-fold in I-FABP expressing cells. Although, the initial [3H]oleic acid esterification into total phospholipids was unchanged, within the phospholipid fraction the initial [3H]oleic acid esterification into phosphatidylethanolamine was increased 70% and decreased 50% in phosphatidylcholine in I-FABP expressing cells. These observed differences suggest a distinct role for I-FABP in stimulating net formation, and not just turnover, of triacylglycerides and cholesteryl esters in transfected L-cell fibroblasts.

Acyl-CoA binding proteins: multiplicity and function

The physiological role of long-chain fatty acyl-CoA is thought to be primarily in intermediary metabolism of fatty acids. However, recent data show that nM to microM levels of these lipophilic molecules are potent regulators of cell functions in vitro. Although long-chain fatty acyl-CoA are present at several hundred microM concentration in the cell, very little long-chain fatty acyl-CoA actually exists as free or unbound molecules, but rather is bound with high affinity to membrane lipids and/or proteins. Recently, there is growing awareness that cytosol contains nonenzymatic proteins also capable of binding long-chain fatty acyl-CoA with high affinity. Although the identity of the cytosolic long-chain fatty acyl-CoA binding protein(s) has been the subject of some controversy, there is growing evidence that several diverse nonenzymatic cytosolic proteins will bind long-chain fatty acyl-CoA. Not only does acyl-CoA binding protein specifically bind medium and long-chain fatty acyl-CoA (LCFA-CoA), but ubiquitous proteins with multiple ligand specificities such as the fatty acid binding proteins and sterol carrier protein-2 also bind LCFA-CoA with high affinity. The potential of these acyl-CoA binding proteins to influence the level of free LCFA-CoA and thereby the amount of LCFA-CoA bound to regulatory sites in proteins and enzymes is only now being examined in detail. The purpose of this article is to explore the identity, nature, function, and pathobiology of these fascinating newly discovered long-chain fatty acyl-CoA binding proteins. The relative contributions of these three different protein families to LCFA-CoA utilization and/or regulation of cellular activities are the focus of new directions in this field.

Spontaneous and protein-mediated sterol transfer between intracellular membranes

Relatively little is known regarding intracellular cholesterol trafficking pathways. To resolve some of these potential pathways, spontaneous and protein-mediated sterol transfer was examined between different donor-acceptor membrane pairs in vitro using L-cell fibroblast plasma membrane (PM) and microsomal (MICRO) and mitochondrial (MITO) membranes. Several new exciting insights were provided. First, the initial rate of spontaneous molecular sterol transfer was more dependent on the type of acceptor than donor membrane, i.e. spontaneous intracellular sterol trafficking was vectorial. Therefore, the rate of sterol desorption from the donor membrane was not necessarily the rate-limiting step in molecular sterol transfer. Second, the rate of molecular sterol transfer was not obligatorily correlated with the direction of the cholesterol gradient. For example, although PM had a 3.2-fold higher cholesterol/phospholipid ratio than MITO, spontaneous sterol transfer was 4-5-fold faster up (MITO to PM) rather than down (PM to MITO) the concentration gradient. Third, sterol carrier protein-2 differentially stimulated the initial rate of sterol transfer for all donor-acceptor combinations, being most effective with PM donors: PM-MICRO, 27-fold; and PM-MITO, 12-fold. Sterol carrier protein-2 was less effective in enhancing sterol transfer in the reverse direction, i.e. MICRO-PM and MITO-PM (5- and 4-fold, respectively). Fourth, liver fatty acid-binding protein was limited in stimulating the initial rate of sterol transfer from PM to PM (1.5-fold), from PM to MITO (3-fold), and from MICRO to MITO (3-fold). In summary, these observations present important insights into potential sterol trafficking pathways between the major membrane components of the cell.

Liver and intestinal fatty acid binding proteins in control and TGF beta 1 gene targeted deficient mice

The effect of transforming growth factor beta-1 (TGF beta 1) expression on fatty acid binding proteins was examined in control and two strains of gene targeted TGF beta 1-deficient mice. Homozygous TGF beta 1-deficient 129 x CF-1, expressing multifocal inflammatory syndrome, had 25% less liver fatty acid binding protein (L-FABP) when compared to control mice. The decrease in L-FABP expression was not due to multifocal inflammatory syndrome since homozygous TGF beta 1-deficient/immunodeficient C3H mice on a SCID background had 36% lower liver L-FABP than controls. This effect was developmentally related and specific to liver, but not the proximal intestine, where L-FABP is also expressed. Finally, the proximal intestine also expresses intestinal-FABP (I-FABP) which decreased 3-fold in the TGF beta 1-deficient/immunodeficient C3H mice only. Thus, TGF beta 1 appears to regulate the expression of L-FABP and I-FABP in the liver and the proximal intestine, respectively.

Liver fatty acid-binding protein expression in transfected fibroblasts stimulates fatty acid uptake and metabolism

The role of cytosolic liver fatty acid binding protein (L-FABP) in fatty acid uptake and metabolism was examined using cultured L-cell fibroblasts transfected with the cDNA encoding for L-FABP. [3H]Oleic acid was used to determine the effects of intracellular esterification on fatty acid uptake and to determine esterified fatty acid localization to specific lipid classes. cis-Parinaric acid, a poorly esterified fatty acid, was used to determine uptake in the absence of any appreciable esterification. High-expression L-cells had a 80% and 50% greater initial uptake rate for both [3H]oleic acid and cis-parinaric acid, respectively compared to low-expression L-cells. Maximal uptake of [3H]oleic acid did not plateau because of intracellular esterification. In high-expressing cells, maximal cis-parinaric acid uptake rapidly plateaued at a level 34% higher than in low-expression cells. After 1 min of incubation, the majority of cellular [3H]oleic acid was unesterified, with the bulk of the esterified portion preferentially localized to phospholipids. After 5 and 30 min, cells expressing L-FABP esterified a significantly greater amount of [3H]oleic acid into both the neutral lipid and phospholipid fractions than did low-expression cells. L-FABP expression also selectively stimulated [3H]oleic acid incorporation into choline glycerophospholipids. Thus, L-FABP expression not only stimulated fatty acid uptake at all time points, but also stimulated intracellular esterification into specific lipid pools. These results show in detail for the first time using an intact cell culture system that L-FABP expression not only stimulated fatty acid uptake, but also increased intracellular esterification of exogenously supplied fatty acids.

Liver fatty acid binding protein enhances sterol transfer by membrane interaction

Among the large family of fatty acid binding proteins, the liver L-FABP is unique in that it not only binds fatty acids but also interacts with sterols to enhance sterol transfer between membranes. Nevertheless, the mechanism whereby L-FABP potentiates intermembrane sterol transfer is unknown. Both fluorescence and dialysis data indicate L-FABP mediated sterol transfer between L-cell fibroblast plasma membranes occurs by a direct membrane effect: First, dansylated-L-FABP (DNS-L-FABP) is bound to L-cell fibroblast plasma membranes as indicated by increased DNS-L-FABP steady state polarization and phase resolved limiting anisotropy. Second, coumarin-L-FABP (CPM-L-FABP) fluorescence lifetimes were significantly increased upon interaction with plasma membranes. Third, dialysis studies with 3H-cholesterol loaded plasma membranes showed that L-FABP added to the donor compartment of the dialysis cell stimulated 3H-cholesterol transfer whether or not the dialysis membrane was permeable to L-FABP. However, L-FABP mediated intermembrane sterol transfer did require a sterol binding site on L-FABP. Chemically blocking the ligand binding site also inhibited L-FABP activity in intermembrane sterol transfer. Finally, L-FABP did not act either as an aqueous carrier or in membrane fusion. The fact that L-FABP interacted with plasma membrane vesicles and required a sterol binding site was consistent with a mode of action whereby L-FABP binds to the membrane prior to releasing sterol from the bilayer.

Developmental role of fatty acid-binding proteins in mouse brain

While the functions of the cytoplasmic fatty acid-binding proteins (FABPs) are not well defined, one possibility in neural tissue is in establishing and maintaining the high levels of polyunsaturated fatty acids in membrane lipids characteristic of this tissue and thought essential for normal function. We investigated the reactivity of a protein in developing mouse brain to antiserum prepared against rat heart (H)-FABP. By immunoblot analysis, levels of H-FABP in brain were nearly undetectable until fetal day 17-19, after which levels increased until at least postnatal day 14. Levels of H-FABP were lower in the adult mouse brain, suggesting a function for the protein during differentiation of neural tissue. In immunohistochemical studies with postnatal day 14 mouse brain, the most intensely stained area was the choroid plexus. H-FABP also localized to regions of the somatosensory cortex and to the spinal trigeminal nucleus. In addition, H-FABP was present in the thalamus, entorhinal and piriform cortex, and throughout the pontine and medullary nuclei. Tracts related to the auditory system, including ventral cochlear nucleus and lateral lemniscus, also were H-FABP-positive. In cerebellum, the molecular layer was heavily labeled in cells and processes; in the granule cell layer, there was punctate staining suggestive of mossy fiber terminals. Small cells adjacent to Purkinje cells were intensely stained, while the Purkinje cells were negative. We conclude that H-FABP in brain participates in neurite formation and synapse maturation, and may be related to the similar pattern of expression of GABA related markers.

Intestinal and liver fatty acid binding proteins differentially affect fatty acid uptake and esterification in L-cells

Differential effects of intestinal (I-FABP) or liver (L-FABP) fatty acid binding proteins on fatty acid uptake and esterification were examined using transfected mouse L-cell fibroblasts. L-FABP, but not I-FABP, expression increased the initial rate and extent of cis-parinaric acid uptake by 50 and 29%, respectively, compared to control cells. I-FABP and L-FABP expression preferentially increased [3H]-oleic acid incorporation into triacylglycerols by 5.5-fold and 3.8-fold, respectively. While both L-FABP and I-FABP increased esterification of [3H]-oleic acid into ethanolamine glycerophospholipids, these proteins had opposite effect on esterification into choline glycerophospholipids. These data show for the first time that distinct FABP differentially affect both fatty acid uptake and intracellular esterification.

A novel lipid-binding protein from the cestode Moniezia expansa

A lipid-binding protein (LBP) has been purified from the cytosol of the cestode Moniezia expansa. The native LBP was found to be an oligomer of approx. 250 kDa, consisting of 11 kDa monomers. The LBP bound saturated and unsaturated fatty acids, but not their CoA derivatives, with KD values in the range 0.68-7.8 microM. Cholesterol, dihydroergosterol, bilirubin and retinoids were also bound, but alpha-tocopherol, bile acids, alk-2-enals and alka-2,4-dienals were not. Evidence suggests that there are two binding sites per subunit, each with different specificities. The fluorescent fatty acid 11-[(5-dimethylaminonaphthalene-1-sulphonyl)amino]undecanoic acid (DAUDA) and retinol both showed an additional high-affinity binding site with a density of approximately 0.1 per subunit, suggesting specific binding to the oligomer. The amino acid composition of Moniezia LBP was distinct from that of previously characterized fatty acid-binding proteins (FABPs). The protein was not N-terminally blocked and yielded a unique amino acid sequence, unrelated to that of any known FABP; there was also evidence of microheterogeneity. Polyclonal antibodies raised to the Moniezia protein did not cross-react with mammalian, nematode or digenean FABP. The Gibbs free energy for protein folding (13.02 kJ/mol; 3.1 kcal/mol), determined by urea denaturation, was identical for both the native and ligand-bound Moniezia LBP. CD spectra showed that the Moniezia protein contained 36% alpha-helix and that the secondary structure underwent only minor changes on ligand binding. Moniezia LBP binds a range of anthelmintics, with KD values again in the range 0.66-7.3 microM. It is possible that, in helminths, binding proteins may play a role in determining the specificity and site of action of anthelmintics.

Modulation of mitogenesis by liver fatty acid binding protein

Liver fatty acid binding protein (L-FABP), a cytoplasmic 14 kDa protein previously termed Z protein, is conventionally considered to be an intracellular carrier of fatty acids in rat hepatocytes. The following evidence now indicates that L-FABP is also a specific mediator of mitogenesis of rat hepatocytes: a. the synergy between the action of L-FABP and unsaturated fatty acids, especially linoleic acid, in the promotion of cell proliferation; b. the specific requirement for L-FABP in induction of mitogenesis by two classes of nongenotoxic hepatocarcinogenic peroxisome proliferators (amphipathic carboxylates and tetrazole-substituted acetophenones); c. the direct correlation between the binding avidities of different prostaglandins for L-FABP and their relative growth inhibitory activities toward cultured rat hepatocytes; d. the temporal coincidences between the covalent binding to L-FABP by chemically reactive metabolites of the genotoxic carcinogens, 2-acetylaminofluorene and aminoazo dyes, and their growth inhibitions of hepatocytes during liver carcinogenesis in rats; e. and f. the marked elevations of L-FABP in rat liver during mitosis in normal and regenerating hepatocytes, and during the entire cell cycle in the hyperplastic and malignant hepatocytes that are produced by the genotoxic carcinogens, 2-acetylaminofluorene and aminoazo dyes. These actions of L-FABP are consistent with those of a protein involved in regulation of hepatocyte multiplication. Discovery that L-FABP, the target protein of the two types of genotoxic carcinogens, is required for the mitogenesis induced by two classes of nongenotoxic carcinogens points to a common process by which both groups of carcinogens promote hepatocyte multiplication. The implication is that during tumor promotion of liver carcinogenesis, these genotoxic and nongenotoxic carcinogens modify the normal process by which L-FABP, functioning as a specific receptor of unsaturated fatty acids or their metabolites, promotes the multiplication of hepatocytes.