In situ labeling of the adipocyte lipid binding protein with 3-[125I]iodo-4-azido-N-hexadecylsalicylamide. Evidence for a role of fatty acid binding proteins in lipid uptake

ALS-associated P56S-VAPB mutation restrains 3T3-L1 preadipocyte differentiation

Amyotrophic lateral sclerosis (ALS), which is the most common motor neuron disease in adults, is a neurodegenerative disease that involves the selective and systematic death of upper and lower motor neurons. In addition to the motor neuron death, altered metabolic functions, such as dyslipidemia, have also been reported for ALS patients; however, the underlying mechanism remains unknown. In the present study, we investigated the effects of ALS-associated P56S-vesicle-associated membrane proteinassociated protein B (VAPB), P56S-VAPB on 3T3-L1 preadipocyte differentiation and on the expression of differentiation-associated genes and unfolded protein response (UPR)-related genes. Experiments with 3T3-L1 cells transfected with wild-type (Wt)-VAPB and P56S-VAPB expression vectors showed that the size of lipid droplets was markedly smaller in P56S-VAPB-expressing cells, although fat accumulated intracellularly. In P56S-VAPB-expressing cells, increased the expression of PPARγ2, aP2, and C/EBPα, the genes deeply involved in adipocyte differentiation, was not observed. Furthermore, the expression levels of the UPR-related ATF4 and CHOP genes were found to be enhanced in the P56S-VAPB-expressing cells. From these results, P56S-VAPB was found to suppress adipocyte differentiation by promoting the activation of the ATF4-CHOP pathway. Given previous reports showing increased ATF4 and CHOP expression levels in neurons of ALS patients, results from the present study suggest that dyslipidemia is caused by enhanced ATF4-CHOP pathway in the adipose tissue of ALS patients.

Quantification of PERF 15 mRNA in tissue sections from rat testes

We previously conducted basic research to quantify in situ hybridization (ISH) signals in rat testes. In this experimental model, we selected ribosomal RNA (rRNA) as the hybridizable RNA in paraffin sections, since it allowed us to easily analyze ISH signals expressed with digoxygenin (DIG)-labeled probes quantitatively through “posterization” of the images. We applied this method to analyze the quantification of transcript, PERF 15 mRNA. PERF 15 is expressed specifically in the testes and localized in the rigid cytoskeletal structure of the sperm head, and has been considered to be involved in the apoptotic process of spermatogenic cells. Quantification of the signals may help to clarify the detailed function of PERF 15. We further analyzed the signals concomitant with a confocal laser scanning microscope. The peak of PERF 15 mRNA expression was found in diplotene spermatocytes, and the amount of PERF 15 mRNA was greatest in late pachytene and diplotene spermatocytes and early spermatids, followed by early pachytene spermatocytes, and then late spermatids. PERF 15 may be involved in the events leading to meiotic division, in which apoptosis is also involved. The present study may help to determine the concentration of mRNA in tissue sections.

Wide-angle X-ray solution scattering as a probe of ligand-induced conformational changes in proteins

A chemical genetics approach to functional analysis of gene products utilizes high-throughput target-based screens of compound libraries to identify ligands that modulate the activity of proteins of interest. Candidates are further screened using functional assays designed specifically for the protein--and function--of interest, suffering from the need to customize the assay to each protein. An alternative strategy is to utilize a probe to detect the structural changes that usually accompany binding of a functional ligand. Wide-angle X-ray scattering from proteins provides a means to identify a broad range of ligand-induced changes in secondary, tertiary, and quaternary structure. The speed and accuracy of data acquisition, combined with the label-free targets and binding conditions achievable, indicate that WAXS is well suited as a moderate-throughput assay in the detection and analysis of protein-ligand interactions.

Role of surface lysine residues of adipocyte fatty acid-binding protein in fatty acid transfer to phospholipid vesicles

The tertiary structure of murine adipocyte fatty acid-binding protein (AFABP) is a flattened 10-stranded beta-barrel capped by a helix-turn-helix segment. This helical domain is hypothesized to behave as a "lid" or portal for ligand entry into and exit from the binding cavity. Previously, we demonstrated that anthroyloxy-labeled fatty acid (AOFA) transfer from AFABP to phospholipid membranes occurs by a collisional process, in which ionic interactions between positively charged lysine residues on the protein surface and negatively charged phospholipid headgroups are involved. In the present study, the role of specific lysine residues located in the portal and other regions of AFABP was directly examined using site-directed mutagenesis. The results showed that isoleucine replacement for lysine in the portal region, including the alphaI- and alphaII-helices and the beta C-D turn, resulted in much slower 2-(9-anthroyloxy)palmitate (2AP) transfer rates to acidic membranes than those of native AFABP. An additive effect was found for mutant K22,59I, displaying the slowest rates of FA transfer. Rates of 2AP transfer from "nonportal" mutants on the beta-G and I strands were affected only moderately; however, a lysine --> isoleucine mutation in the nonportal beta-A strand decreased the 2AP transfer rate. These studies suggest that lysines in the helical cap domain are important for governing ionic interactions between AFABP and membranes. Furthermore, it appears that more than one distinct region, including the alphaI-helix, alphaII-helix, beta C-D turn, and the beta-A strand, is involved in these charge-charge interactions.

Effect of fatty acid-binding proteins on intermembrane fatty acid transport studies on different types and mutant proteins

Liposomes of different charge fixed to nitrocellulose filters were used to study the transfer of fatty acids to rat heart or liver mitochondria in the presence of fatty acid-binding protein (FABP) or albumin. [14C]Palmitate oxidation was used as a parameter. Different FABP types and heart FABP mutants were tested. The charge of the liposomes did not influence the solubilization and mitochondrial oxidation of palmitate without FABP and the amount of solubilized palmitate in the presence of FABP. Mitochondria did not show a preference for oxidation of FABP-bound palmitate over their tissue-specific FABP type. All FABP types increased palmitate oxidation by heart and liver mitochondria with neutral, positive and negative liposomes by 2.5-fold, 3.2-fold and twofold, respectively. Ileal lipid-binding protein and H-FABP mutants that do not bind fatty acid had no effect. Other H-FABP mutants had different effects, dependent on the site of mutation. The effect of albumin was similar to, but not dependent on, liposome charge. The ionic strength had only a slight effect. In conclusion, the transfer of palmitate from liposomal membranes to mitochondria was increased by all FABP types to a similar extent. The membrane charge had a large effect in contrast to the origin of the mitochondria.

The fatty acid transport function of fatty acid-binding proteins

The intracellular fatty acid-binding proteins (FABPs) comprise a family of 14-15 kDa proteins which bind long-chain fatty acids. A role for FABPs in fatty acid transport has been hypothesized for several decades, and the accumulated indirect and correlative evidence is largely supportive of this proposed function. In recent years, a number of experimental approaches which more directly examine the transport function of FABPs have been taken. These include molecular level in vitro modeling of fatty acid transfer mechanisms, whole cell studies of fatty acid uptake and intracellular transfer following genetic manipulation of FABP type and amount, and an examination of cells and tissues from animals engineered to lack expression of specific FABPs. Collectively, data from these studies have provided strong support for defining the FABPs as fatty acid transport proteins. Further studies are necessary to elucidate the fundamental mechanisms by which cellular fatty acid trafficking is modulated by the FABPs.

Cellular fatty acid transport in heart and skeletal muscle as facilitated by proteins

Despite the importance of long-chain fatty acids (FA) as fuels for heart and skeletal muscles, the mechanism of their cellular uptake has not yet been clarified. There is dispute as to whether FA are taken up by the muscle cells via passive diffusion and/or carrier-mediated transport. Kinetic studies of FA uptake by cardiac myocytes and the use of membrane protein-modifying agents have suggested the bulk of FA uptake is due to a protein component. Three membrane-associated FA-binding proteins were proposed to play a role in FA uptake, a 40-kDa plasma membrane FA-binding protein (FABPpm), an 88-kDa FA translocase (FAT/CD36), and a 60-kDa FA transport protein (FATP). In cardiac and skeletal myocytes the intracellular carrier for FA is cytoplasmic heart-type FA-binding protein (H-FABP), which likely transports FA from the sarcolemma to their intracellular sites of metabolism. A scenario is discussed in which FABPpm, FAT/CD36, and H-FABP, probably assisted by an albumin-binding protein, cooperate in the translocation of FA across the sarcolemma.

Identification of a functional peroxisome proliferator-responsive element in the murine fatty acid transport protein gene

Fatty acid transport protein (FATP), a plasma membrane protein implicated in controlling adipocyte transmembrane fatty acid flux, is up-regulated as a consequence of adipocyte differentiation and down-regulated by insulin. Based upon the sequence of the FATP gene upstream region (Hui, T. Y., Frohnert, B. I., Smith, A. J., Schaffer, J. A., and Bernlohr, D. A. (1998) J. Biol. Chem. 273, 27420-27429) a putative peroxisome proliferator-activated receptor response element (PPRE) is present from -458 to -474. To determine whether the FATP PPRE was functional, and responded to lipid activators, transient transfection of FATP-luciferase reporter constructs into CV-1 and 3T3-L1 cells was carried out. In CV-1 cells, FATP-luciferase activity was up-regulated 4- and 5.5-fold, respectively, by PPARalpha and PPARgamma in the presence of their respective activators in a PPRE-dependent mechanism. PPARdelta, however, was unable to mediate transcriptional activation under any condition. In 3T3-L1 cells, the PPRE conferred a small but significant increase in expression in preadipocytes, as well as a more robust up-regulation of FATP expression in adipocytes. Furthermore, the PPRE conferred the ability for luciferase expression to be up-regulated by activators of both PPARgamma and retinoid X receptor alpha (RXRalpha) in a synergistic manner. PPARalpha and PPARdelta activators did not up-regulate FATP expression in 3T3-L1 adipocytes, however, suggesting that these two subtypes do not play a significant role in differentiation-dependent activation in fat cells. Electromobility shift assays showed that all three PPAR subtypes were able to bind specifically to the PPRE as heterodimers with RXRalpha. Nuclear extracts from 3T3-L1 adipocytes also showed a specific gel-shift complex with the FATP PPRE. To correlate the expression of FATP to its physiological function, treatment of 3T3-L1 adipocytes with PPARgamma and RXRalpha activators resulted in an increased uptake of oleate. Moreover, linoleic acid, a physiological ligand, up-regulated FATP expression 2-fold in a PPRE-dependent manner. These results demonstrate that the FATP gene possesses a functional PPRE and is up-regulated by activators of PPARalpha and PPARgamma, thereby linking the activity of the protein to the expression of its gene. Moreover, these results have implications for the mechanism by which certain PPARgamma activators such as the antidiabetic thiazolidinedione drugs affect adipose lipid metabolism.

Intracellular transport of fatty acids in muscle. Role of cytoplasmic fatty acid-binding protein

Long-chain fatty acids represent a major substrate for energy production in striated muscles, especially in those muscles which have a high oxidative enzymatic capacity. Following their uptake from the extracellular compartment the fatty acids have to translocate through the aqueous cytoplasm of the myocytes to reach the mitochondria where they undergo oxidative degradation. This intracellular transport is assisted by cytoplasmic fatty acid-binding protein (FABPc), a small (15 kD) protein which shows a high affinity for the non-covalent binding of long-chain fatty acids, and of which several types occur. So-called heart-type or muscle-type FABPc is found in muscle cells, and is abundant especially in oxidative fibers. The muscular FABPc content appears to relate to the rate of fatty acid utilization, and also changes in concert to modulations in fatty acid utilization induced by (patho)physiological stimuli (e.g. endurance training, diabetes). The facilitation of intracellular fatty acid transport by FABPc is accomplished by increasing the concentration of the diffusing fatty acids in the aqueous cytoplasm and, most likely, also by interacting directly with membranes to promote transfer of fatty acids to and from the cytosolic binding protein.

Cellular differentiation and I-FABP protein expression modulate fatty acid uptake and diffusion

The effect of cellular differentiation on fatty acid uptake and intracellular diffusion was examined in transfected pluripotent mouse embryonic stem (ES) cells stably expressing intestinal fatty acid binding protein (I-FABP). Control ES cells, whether differentiated or undifferentiated, did not express I-FABP. The initial rate and maximal uptake of the fluorescent fatty acid, 12-(N-methyl)-N-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-octadec anoic acid (NBD-stearic acid), was measured in single cells by kinetic digital fluorescence imaging. I-FABP expression in undifferentiated ES cells increased the initial rate and maximal uptake of NBD-stearic acid 1.7- and 1.6-fold, respectively, as well as increased its effective intracellular diffusion constant (Deff) 1.8-fold as measured by the fluorescence recovery after photobleaching technique. In contrast, ES cell differentiation decreased I-FABP expression up to 3-fold and decreased the NBD-stearic acid initial rate of uptake, maximal uptake, and Deff by 10-, 4.7-, and 2-fold, respectively. There were no significant differences in these parameters between the differentiated control and differentiated I-FABP-expressing ES cell lines. In summary, differentiation and expression of I-FABP oppositely modulated NBD-stearic acid uptake parameters and intracellular diffusion in ES cells.

Role of membrane-associated and cytoplasmic fatty acid-binding proteins in cellular fatty acid metabolism

A number of membrane-associated and cytoplasmic fatty acid-binding proteins (FABPs) are now being implicated in the cellular uptake and intracellular transport of long-chain fatty acids (FA). These proteins each have the capacity of non-covalent binding of FA, are present in tissues actively involved in FA metabolism, and are upregulated in conditions of increased cellular FA metabolism. To date, five distinct membrane FABPs have been described, ranging in mass from 22 to 88 kDa and each showing a characteristic tissue distribution. Evidence for involvement in cellular fatty acid uptake has been provided for several of them, because it was recently found that isolated cell lines transfected with 88-kDa putative fatty acid translocase (FAT; homologous to CD36) or with 63-kDa fatty acid-transport protein show an increased rate of FA uptake. The (at least nine) FABPs of cytoplasmic origin belong to a family of small (14-15 kDa) lipid binding proteins, all having a similar tertiairy structure but differing in binding properties and in tissue occurrence. The biological functions of the various FABPs, possibly exerted in a concerted action among them, comprise solubilization and compartmentalization of FA, facilitation of the cellular uptake and intracellular trafficking of FA, and modulation of mitosis, cell growth, and cell differentiation. In addition, the FABPs have been suggested to participate in and/or modulate FA-mediated signal transduction pathways and FA regulation of gene expression, and to prevent local high FA concentrations thereby contributing to the protection of cells against the toxic effects of FA. In conclusion, long-chain fatty acids are subject to continuous interaction with multiple proteins, which interplay influences their cellular metabolism.

Molecular mechanism of cellular uptake and intracellular translocation of fatty acids

The molecular mechanism of the transport of long-chain fatty acids across cellular membranes and the necessity and precise functioning of specific proteins in this process are still unclear. Various alternative mechanisms have been proposed. Studies with artificial phospholipid bilayers support the concept that fatty acids may enter and traverse the plasma membrane without the involvement of proteins. On the other hand, a number of membrane-associated fatty acid-binding proteins (FABPs) have been described which putatively function as acceptors for fatty acids released from albumin or from lipoproteins. Albumin binding proteins located at the outer cell surface could play an additional role in the delivery of fatty acids. The subsequent transmembrane translocation of fatty acids could take place by a membrane protein acting as a translocase, or by simple diffusion of fatty acids through either the phospholipid bilayer or a pore or channel formed by one or more membrane fatty acid transporters. At the inner side of the plasma membrane, the fatty acid is bound to a cytoplasmic FABP, which serves to buffer the intracellular aqueous fatty acid concentration. The direction of fatty acid migration through the plasma membrane most likely is governed by the transmembrane gradient of fatty acid concentration, assisted to some extent and in selected tissues by co-transport of sodium ions. The intracellular transport of fatty acids from the plasma membrane to the sites of metabolic conversion (oxidation, esterification) or subcellular target (signal transduction) is greatly facilitated by cytoplasmic FABPs. In conclusion, cellular uptake and intracellular translocation of long-chain fatty acids is a multi-step process that is facilitated by various membrane-associated and soluble proteins. The mechanism of cellular uptake of fatty acids probably involves both a passive and carrier-mediated transmembrane translocation.

Modelling intracellular fatty acid transport: possible mechanistic role of cytoplasmic fatty acid-binding protein

A computer model is presented in which the role of cytoplasmic fatty acid-binding protein (FABP) in the intracellular translocation of fatty acids (FA) from one membrane to an opposite membrane is studied. The model consists of a cubical space, in which FABP and FA are allowed to diffuse at random. The amount of FA released from the donor membrane and reaching an opposite acceptor membrane is calculated in a variety of conditions. The data provided by the various simulations suggest that FABP can play a significant role in intracellular FA transport only if FABP is able to take up FA directly from FA containing membranes and to directly deliver FA to an acceptor membrane, thus preventing the unfavourable thermodynamical situation in which FA must solubilize in an aqueous environment prior to binding to FABP.

Intracellular lipid-binding proteins and their genes

Intracellular lipid-binding proteins are a family of low-molecular-weight single-chain polypeptides that form 1:1 complexes with fatty acids, retinoids, or other hydrophobic ligands. These proteins are products of a large multigene family of unlinked loci distributed throughout the genome. Each lipid-binding protein exhibits a distinctive pattern of tissue distribution. Transcriptional control, regulated by a combination of peroxisome proliferator activated receptors and CCAAT/enhancer-binding proteins, allows for a variety of both cell and tissue-specific expression patterns. In some cells, fatty acids increase the expression of the lipid-binding protein genes. Fatty acids, or their metabolites, are activators of the peroxisome proliferator-activated receptor family of transcription factors. Therefore, as the concentration of lipid in the diet increases, the expression of lipid-binding proteins coordinately increases. As revealed by X-ray crystallography, the lipid-binding proteins fold into beta-barrels, forming a large internal water-filled cavity. Fatty acid ligands are bound within the cavity, occupying only about one-third of the accessible volume. The bound fatty acid is stabilized via a combination of enthalpic and entropic forces that govern ligand affinity and selectivity. Cytoplasmic lipid-binding proteins are the intracellular receptors for hydrophobic ligands, delivering them to the appropriate site for use as metabolic fuels and regulatory agents.

Membrane-associated and cytoplasmic fatty acid-binding proteins

A number of cellular fatty acid-binding proteins are being implicated in the uptake and intracellular transport of long-chain fatty acids by parenchymal cells. Having been a topic of research for more than 20 years, cytoplasmic fatty acid-binding proteins now are assigned various pivotal functions in intracellular fatty acid transport and metabolism. More recently several membrane-associated fatty acid-binding proteins have been identified and these proteins are thought to function in the transmembrane transport of fatty acids. In this review, a short summary is provided of the latest developments in this research area.

Localization of mammary-derived growth inhibitor in capillary endothelial cells of the bovine mammary gland

Mammary-derived growth inhibitor (MDGI) has previously been localized in the mammary parenchyma, dependent on the stage of differentiation of the mammary gland. Here, we have elucidated the distribution of MDGI in the mammary stroma by a combined immunohisto- and cytochemical analysis with antibodies raised against MDGI. Distinct staining of capillary endothelial cells has been revealed. Although its subcellular distribution resembles former observations in secretory epithelial cells, the expression of MDGI in capillary endothelial cells clearly precedes that in secretory epithelial cells. On the other hand, no endothelial MDGI staining has been detected in bovine heart, which contains a fatty acid-binding protein almost identical to MDGI. The localization of MDGI in the mammary capillary endothelium is discussed in terms of its possible involvement in the intracellular transport of hydrophobic ligands or in the regulation of endothelial cell proliferation.

Release of fatty acid-binding protein and long chain fatty acids from isolated rat heart after ischemia and subsequent calcium paradox

To obtain insight into the relation between the release of heart-type fatty acid-binding protein (H-FABPc) and of long-chain fatty acids (FA) from injured cardiac tissue, rat hearts were Langendorff perfused according to the following scheme: 30 min normoxia, 60 min ischemia, 30 min reperfusion, 10 min Ca2+ free perfusion and finally 10 min Ca2+ repletion. During this protocol right ventricular (Qrv) and interstitial effluent samples (Qi) were collected at regular intervals. During reperfusion a total of 0.8 +/- 0.1 nmol H-FABPc but no FA were detected in the effluents. However, during Ca2+ readmission, 45 +/- 4 nmol H-FABPc (80-90% of total tissue content) was released with an initial (first 3 min) simultaneous release of FA (FA/H-FABPc ratio 0.90 +/- 0.07 mol/mol). Thereafter, FA release continued at 10-15 nmol per min mainly in Qrv while the rate of H-FABPc release decreased. During Ca2+ repletion, tissue FA content raised rapidly from 168 +/- 20 to 1918 +/- 107 nmol/g dry weight. These findings suggest that after severe cardiac damage initially FA is released bound to H-FABPc, whereas further FA release occurs in a non-protein bound manner.

Cellular binding proteins for fatty acids and retinoids: similar or specialized functions?

The cellular fatty acid-binding proteins (FABP) and cellular retinoid (retinol, retinoic acid)-binding proteins (CRtBP) are structurally and functionally-defined groups within an evolutionarily conserved gene family. CRtBP are expressed in both fully differentiated and developing tissues in a manner that supports a relationship to the action of retinoic acid in morphogenesis and cellular differentiation. The FABP are, by contrast, expressed only in fully differentiated tissues in a manner compatible with a major function in the metabolism of long-chain fatty acids (LCFA) for energy production or storage. The precise function(s) of FABP and CRtBP remain imperfectly understood, while subspecialization of function(s) within the two groups is suggested by the complex diversity in both of structurally distinct members that display striking tissue and temporal specificity of expression in addition to ligand specificity. Notwithstanding this considerable apparent functional diversity among the FABP and CRtBP, available evidence supports a dual set of generic functions for both protein groups in a) promoting cellular flux of poorly water-soluble ligands and their subsequent metabolic utilization or transformation, and b) sequestration of ligands in a manner that limits their association with alternative binding sites within the cell, of which members of the steroid hormone nuclear receptor superfamily (HNR) are a potentially important category. Theoretical as well as experimental models probing diffusional fluxes of LCFA in vitro and in living cells have provided support for a function for FABP in intracellular LCFA transport. Protein-bound ligand also appears to provide the substrate for metabolic transformation of retinoids bound to CRtBP, but convincing evidence is lacking for an analogous mechanism in the direct facilitation of fatty acid utilization by FABP. An emerging relationship between FABP and CRtBP function centers on their binding of, and induction by, ligands which activate or transform specific HNR-the retinoic acid receptors and the peroxisome proliferator activated receptor in the case of CRtBP and FABP, respectively. Evidence consistent with both a 'promotive' role (provision of ligands for HNR) and a 'protective' role (limiting availability of free ligand for HNR association) has been advanced for CRtBP. Available data supports a 'protective' function for cellular retinoic acid-binding proteins (CRABP) and liver FABP (L-FABP) and points to the existence of ligand-defined, lipid-binding-protein-HNR relationships in which CRABP serve to attenuate the induction of gene expression by retinoic acid, and in which L-FABP may modulate a cellular adaptive multigene response to increased LCFA flux or compromised LCFA utilization.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of rat L-FABP in mouse fibroblasts: role in fat absorption

Fatty acid-binding proteins (FABP) are abundant cytosolic proteins whose levels is responsive to nutritional, endocrine, and a variety of pathological states. Although FABPs have been investigated in vitro for several decades, little is known of their physiological function. Liver L-FABP binds both fatty acids and cholesterol. Competitive binding analysis and molecular modeling studies of L-FABP indicate the presence of two ligand binding pockets that accommodate one fatty acid each. One fatty acid binding site is identical to the cholesterol binding site. To test whether these observations obtained in vitro were physiologically relevant, the cDNA encoding L-FABP was transfected into L-cells, a cell line with very low endogenous FABP and sterol carrier proteins. Uptake of both ligands did not differ between control cells and low expression clones. In contrast, both fatty acid uptake and cholesterol uptake were stimulated in the high expression cells. In high expression cells, uptake of fluorescent cis-parinaric acid was enhanced more than that of trans-parinaric acid. This is consistent with the preferential binding of cis-fatty acids to L-FABP but in contrast to the preferential binding of trans-parinaric acid to the L-cell plasma membrane fatty acid transporter (PMFABP). These data show that the level of cytosolic fatty acids in intact cells can regulate both the extent and specificity of fatty acid uptake. Last, sphingomyelinase treatment of L-cells released cholesterol from the plasma membrane to the cytoplasm and stimulated microsomal acyl-CoA: cholesteryl acyl transferase (ACAT). This process was accelerated in high expression cells. These observations show for the first time in intact cells that L-FABP, a protein most prevalent in liver and intestine where much fat absorption takes place, may have a role in fatty acid and cholesterol absorption.

Fatty acid-binding protein and its relation to fatty acid oxidation

A relation between fatty acid oxidation capacity and cytosolic FABP content was found in heart and various muscles of the rat. Other tissues do not show such a relation, since they are involved in more or other pathways of fatty acid metabolism. At postnatal development FABP content and fatty acid oxidation capacity rise concomitantly in heart and quadriceps muscle in contrast to in liver and kidney. A dietary fat content of 40 en.% increased only the FABP content of liver and adipose tissue. Peroxisomal proliferators increased fatty acid oxidation in both liver and kidney, but only the FABP content of liver, and had no effect on heart and skeletal muscle. The FABP content of muscle did not show adaptation to various conditions. Only it increased in fast-twitch muscles upon chronic electrostimulation and endurance training.

Photoreactive fatty acid analogues that bind to the rat liver fatty-acid binding protein: 11-(5'-azido-salicylamido)-undecanoic acid derivatives

Photoreactive probes for the hydrophobic pocket of the liver fatty acid-binding protein, 11-(5'-azido-salicylamido)-undecanoic acid (5' ASU) and its acetyl ester (Ac5' ASU), were synthesized and their interaction with the protein was assessed. Fatty acid-binding proteins are closely related proteins which are abundantly expressed in tissues with active lipid metabolism. A simple model that assumes that the protein possesses a single kind of sites fitted the binding of radioiodinated 5' ASU to L-FABP satisfactorily. The apparent dissociation constant, 1.34 x 10(-7) M, evidenced a slightly higher affinity than that reported for C16-C20 fatty acids. Consistent with the binding curve, 5' ASU effectively competed with palmitic acid for the hydrophobic sites and the effect was nearly complete for concentrations of 1 microM; oleic acid, in turn, displaced the radiolabelled probe. Irradiation at 366 nm of 125I-5' ASU bound to L-FABP caused the covalent cross-linking of the reagent. The amount of radioactivity covalently bound reached a maximum after 2 min thus agreeing with the photo-activation kinetics of the unlabelled compound that evidenced a t1/2 of 31.1 sec. The yield with which probes bound to L-FABP became covalently linked to the protein, appraised after SDS-PAGE of irradiated samples, was estimated as 23 and 26 per cent for 5' ASU and Ac5' ASU respectively. In turn, irradiation of L-FABP incubated with 5' ASU or Ac5' ASU resulted in the irreversible loss of about one fourth its ability to bind palmitic acid.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of the adipocyte acid phosphatase as a PAO-sensitive tyrosyl phosphatase

We have partially purified an 18-kDa cytoplasmic protein from 3T3-L1 cells, which dephosphorylates pNPP and the phosphorylated adipocyte lipid binding protein (ALBP), and have identified it by virtue of kinetic and immunological criteria as an acid phosphatase (EC 3.1.3.2). The cytoplasmic acid phosphatase was inactivated by phenylarsine oxide (PAO) (Kinact = 10 microM), and the inactivation could be reversed by the dithiol, 2,3-dimercaptopropanol (Kreact = 23 microM), but not the monothiol, 2-mercaptoethanol. Cloning of the human adipocyte acid phosphatase revealed that two isoforms exist, termed HAAP alpha and HAAP beta (human adipocyte acid phosphatase), which are distinguished by a 34-amino acid isoform-specific domain. Sequence analysis shows HAAP alpha and HAAP beta share 74% and 90% identity with the bovine liver acid phosphatase, respectively, and 99% identity with both isoenzymes of the human red cell acid phosphatase but no sequence similarity to the protein tyrosine phosphatases (EC 3.1.3.48). HAAP beta has been cloned into Escherichia coli, expressed, and purified as a glutathione S-transferase fusion protein. Recombinant HAAP beta was shown to dephosphorylate pNPP and phosphoALBP and to be inactivated by PAO and inhibited by vanadate (Ki = 17 microM). These results describe the adipocyte acid phosphatase as a cytoplasmic enzyme containing conformationally vicinal cysteine residues with properties that suggest it may dephosphorylate tyrosyl phosphorylated cellular proteins.

The mouse intestinal fatty acid binding protein gene: nucleotide sequence, pattern of developmental and regional expression, and proposed structure of its protein product

The rat intestinal fatty acid binding protein (I-FABP) gene has been used as a model to study temporal and spatial differentiation of the gut epithelium while its protein product has been used as a model for examining the atomic details of noncovalent fatty acid-protein interactions. We have isolated the mouse I-FABP gene (Fabpi) and determined its nucleotide sequence. Comparisons of the orthologous mouse, rat, and human I-FABP genes revealed three conserved domains in their otherwise divergent 5' nontranscribed sequences. RNA blot hybridization and multilabel immunocytochemical methods were used to compare the developmental stage-specific patterns of activation of the rat and mouse genes. In addition, Fabpi expression in enterocytes was examined as a function of their differentiation along the crypto-to-villus and duodenal-to-colonic axes of the small intestine. Based on the similar temporal and geographic patterns of mouse and rat I-FABP expression described here and the results of our earlier studies of transgenic mice containing rat Fabpi/human growth hormone fusion genes, we propose that one of the conserved domains, spanning nucleotides -500 to -419 in mouse Fabpi, and/or a 14-bp element, are necessary for establishing and maintaining its region-specific expression along the duodenal-to-colonic axis of the perpetually renewing gut epithelium. Finally, predictions of the structure of mouse I-FABP using the refined 2.0 A model of rat I-FABP, suggest that a proline found at position 69 of the mouse, but not rat, protein may affect its ligand binding properties.

In situ binding of fatty acids to the liver fatty acid binding protein: analysis using 3-[125I]iodo-4-azido-N-hexadecylsalicylamide

A photoactivatable radioiodinated fatty acid analogue, 3-[125I]iodo-4-azido-N-hexadecylsalicylamide (125I-AHS) has been synthesized and used to investigate the involvement of cellular lipid carriers in hepatic fatty acid utilization. Photoactivation of Hep G2 internalized 125I-AHS revealed that several cellular proteins were crosslinked with the radiolabeled fatty acid analogue. Three predominant proteins in the membrane fraction of the cell with molecular masses 17, 50 and 127 kDa were crosslinked with the lipid analogue, as determined using autoradiography after SDS-PAGE. Three other proteins in the soluble fraction of the cell, with molecular masses 14, 24 and 35 kDa, were also labeled in situ. In contrast to the other labeled proteins, the fatty acid analogue accumulated on the cytoplasmic 14 kDa protein in a time and temperature dependent fashion. The in situ-labeled 14 kDa protein was identified from primary rat hepatocytes as the liver fatty acid binding protein by partial purification and its ability to be immunoprecipitated with immunospecific L-FABP antiserum. Collectively the results indicate that fatty acids traverse the plasma membrane and are bound cytoplasmically by the liver fatty acid binding protein, as well as other proteins in the cell. This represents the first demonstration in intact hepatocytes that the liver fatty acid binding protein participates in the process of intracellular fatty acid trafficking, and supports a model in which cytoplasmic lipid carriers solubilize fatty acids as a step in their metabolic utilization.