Regulation of mitochondrial and microsomal phospholipid synthesis by liver fatty acid-binding protein

Role of lysophosphatidic acid and its receptors in health and disease: novel therapeutic strategies

Lysophosphatidic acid (LPA) is an abundant bioactive phospholipid, with multiple functions both in development and in pathological conditions. Here, we review the literature about the differential signaling of LPA through its specific receptors, which makes this lipid a versatile signaling molecule. This differential signaling is important for understanding how this molecule can have such diverse effects during central nervous system development and angiogenesis; and also, how it can act as a powerful mediator of pathological conditions, such as neuropathic pain, neurodegenerative diseases, and cancer progression. Ultimately, we review the preclinical and clinical uses of Autotaxin, LPA, and its receptors as therapeutic targets, approaching the most recent data of promising molecules modulating both LPA production and signaling. This review aims to summarize the most update knowledge about the mechanisms of LPA production and signaling in order to understand its biological functions in the central nervous system both in health and disease.

Hepatic AQP9 expression in male rats is reduced in response to PPARα agonist treatment

The peroxisome proliferator receptor α (PPARα) is a key regulator of the hepatic response to fasting with effects on both lipid and carbohydrate metabolism. A role in hepatic glycerol metabolism has also been found; however, the results are somewhat contradictive. Aquaporin 9 (AQP9) is a pore-forming transmembrane protein that facilitates hepatic uptake of glycerol. Its expression is inversely regulated by insulin in male rodents, with increased expression during fasting. Previous results indicate that PPARα plays a crucial role in the induction of AQP9 mRNA during fasting. In the present study, we use PPARα agonists to explore the effect of PPARα activation on hepatic AQP9 expression and on the abundance of enzymes involved in glycerol metabolism using both in vivo and in vitro systems. In male rats with free access to food, treatment with the PPARα agonist WY 14643 (3 mg·kg(-1)·day(-1)) caused a 50% reduction in hepatic AQP9 abundance with the effect being restricted to AQP9 expressed in periportal hepatocytes. The pharmacological activation of PPARα had no effect on the abundance of GlyK, whereas it caused an increased expression of hepatic GPD1, GPAT1, and L-FABP protein. In WIF-B9 and HepG2 hepatocytes, both WY 14643 and another PPARα agonist GW 7647 reduced the abundance of AQP9 protein. In conclusion, pharmacological PPARα activation results in a marked reduction in the abundance of AQP9 in periportal hepatocytes. Together with the effect on the enzymatic apparatus for glycerol metabolism, our results suggest that PPARα activation in the fed state directs glycerol into glycerolipid synthesis rather than into de novo synthesis of glucose.

Role of Phospholipids in Endocytosis, Phagocytosis, and Macropinocytosis

Endocytosis, phagocytosis, and macropinocytosis are fundamental processes that enable cells to sample their environment, eliminate pathogens and apoptotic bodies, and regulate the expression of surface components. While a great deal of effort has been devoted over many years to understanding the proteins involved in these processes, the important contribution of phospholipids has only recently been appreciated. This review is an attempt to collate and analyze the rapidly emerging evidence documenting the role of phospholipids in clathrin-mediated endocytosis, phagocytosis, and macropinocytosis. A primer on phospholipid biosynthesis, catabolism, subcellular distribution, and transport is presented initially, for reference, together with general considerations of the effects of phospholipids on membrane curvature and charge. This is followed by a detailed analysis of the critical functions of phospholipids in the internalization processes and in the maturation of the resulting vesicles and vacuoles as they progress along the endo-lysosomal pathway.

Human liver fatty acid binding protein (hFABP1) gene is regulated by liver-enriched transcription factors HNF3β and C/EBPα

The human liver fatty acid binding protein (hFABP1) participates in cellular long-chain fatty acid trafficking and regulation of lipid metabolism and changes in hFABP1 are associated with an increased risk for type 2 diabetes, cardiovascular disease (CVD), and metabolic syndromes. Gene regulation of hFABP1 is not fully understood. Therefore, in the present study, the full length hFABP1 promoter (nucleotides -2125 to +51) and a series of truncated promoter regions were cloned. A luciferase reporter assay revealed that nucleotides -255 to +50 in the promoter region contained full of maximum hFABP1 promoter activity compared with the full length promoter. Furthermore high activity was shown when the plasmid was transfected into liver-derived cells such as the human hepatoblastoma cell line HepG2 and the hepatoma cell line Huh7. TFSEARCH and TESS programs were used to predict potential transcription factor binding sites. Two putative binding sites for the liver-enriched transcription factors hepatocyte nuclear factor 3β (HNF3β) and CCAAT/enhancer binding protein α (C/EBPα) were identified in the -255 nt to -155 nt hFABP1 promoter region. Site-directed mutagenesis of these two sites reduced dramatically hFABP1 promoter activity. In addition, the electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation assay (ChIP) revealed that these binding sites were recognized by HNF3β and C/EBPα respectively. Overexpression of HNF3β and C/EBPα enhanced the transcription of hFABP1 and consequently improved the protein level of hFABP1 in HepG2 cells, while knockdown of HNF3β and C/EBPα showed the inverse effects. Taken together, the hFABP1 gene is highly transcribed in liver-derived cells, and regulated predominantly by liver-enriched transcription factors HNF3β and C/EBPα.

Human lysophosphatidylcholine acyltransferases 1 and 2 are located in lipid droplets where they catalyze the formation of phosphatidylcholine

Phosphatidylcholine (PC) is synthesized by two different pathways, the Lands cycle and the Kennedy pathway. The recently identified key enzymes of the Lands cycle, lysophosphatidylcholine acyltransferase 1 and 2 (LPCAT1 and -2), were reported to localize to the endoplasmic reticulum and to function in lung surfactant production and in inflammation response. Here, we show in various mammalian cell lines that both enzymes additionally localize to lipid droplets (LDs), which consist of a core of neutral lipids surrounded by a monolayer of phospholipid, mainly PC. This dual localization is enabled by the monotopic topology of these enzymes demonstrated in this study. Furthermore, we show that LDs have the ability to locally synthesize PC and that this activity correlates with the LPCAT1 and -2 expression level. This suggests that LPCAT1 and -2 have, in addition to their known function in specialized cells, a ubiquitous role in LD-associated lipid metabolism.

A role of liver fatty acid-binding protein in cisplatin-induced acute renal failure

Previous studies from our laboratory showed that increased fatty acid oxidation by the kidney is cytoprotective during cisplatin (CP)-mediated nephrotoxicity. In this study, we determined the effects of CP and fibrates on peroxisome proliferation and the expression of liver fatty acid-binding protein (L-FABP) in normal mice, and in mice transgenically overexpressing human L-FABP (h-L-FABP). Labeling of peroxisomes demonstrated reduced peroxisomal staining in the proximal tubule of CP-treated mice compared with control mice. There was increased peroxisomal labeling in the proximal tubules of both control and CP-treated mice when either was treated with fibrate; a known peroxisome proliferator-activated receptor-alpha ligand. L-FABP protein expression, not detected in control or CP-treated mice, was significantly increased in the proximal tubules of fibrate-treated mice of either group. In the transgenic mice, CP increased the shedding of h-L-FABP in the urine, which was decreased by fibrate as was the acute renal failure. A cytosolic pattern of h-L-FABP expression was found in the proximal tubules of untreated transgenic mice with a nuclear presence in CP-treated mice. Fibrate pretreatment restored the cytosolic expression pattern in CP-treated mice. Our study shows that fibrate may improve CP-induced acute renal failure due to both peroxisome proliferation and increased L-FABP in the cytosol of the proximal tubule.

Lysophosphatidic acid affinity chromatography reveals pyruvate kinase as a specific LPA-binding protein

Lysophosphatidic acid is a pleiotropic lipid signaling molecule that evokes a broad array of cellular responses including proliferation, tumor cell invasion, neurite retraction, cytoskeletal rearrangements and smooth muscle contraction. Generally, lysophosphatidic acid triggers physiological responses through interaction with specific plasma membrane receptors called LPA 1-4. There is, however, increasing evidence in support of intracellular proteins that interact with LPA. We employed Affigel-immobilized LPA to isolate cytoplasmic proteins that interact with this lysophospholipid. Among the proteins retained by this affinity matrix, pyruvate kinase, clathrin heavy chain and heat shock protein 70 (Hsp70) were identified by mass spectrometry. Isothermal titration calorimetry showed that pyruvate kinase contains one binding site for LPA (Ka approx. 10(6) M(-1)). Furthermore, LPA dissociates enzymatically active pyruvate-kinase tetramers into less active dimers, and is maximally active at concentrations close to its critical micelle concentration. These effects were not mimicked by other lysophospholipids. Co-immunoprecipitation experiments showed that pyruvate kinase interacts with clathrin, and confocal imaging revealed co-localization between clathrin and pyruvate kinase in the perinuclear region of cells. Our data suggest that pyruvate kinase partly exists in complex with clathrin in subcellular membranous areas, and that locally increased LPA levels can trigger inactivation of the metabolic enzyme.

Critical role of acylglycerol kinase in epidermal growth factor-induced mitogenesis of prostate cancer cells

The bioactive phospholipids, LPA (lysophosphatidic acid) and PA (phosphatidic acid), regulate pivotal processes related to the pathogenesis of cancer. Recently, we cloned a novel type of lipid kinase that phosphorylates monoacylglycerols (such as 2-arachidonoylglycerol, an endogenous cannabinoid receptor ligand) and diacylglycerols, to form LPA and PA, respectively. This AGK (acylglycerol kinase) is highly expressed in prostate cancer cell lines and the results reviewed here suggest that AGK might be a critical player in the initiation and progression of prostate cancer. Intriguingly, down-regulation of endogenous AGK inhibited EGF (epidermal growth factor), but not LPA-induced ERK1/2 (extracellular-signal-regulated kinase 1/2) activation and progression through the S-phase of the cell cycle. In this review, we will summarize the evidence demonstrating that AGK amplifies EGF growth signalling pathways that play an important role in the pathophysiology of prostate cancer. Because LPA has long been implicated as an autocrine and paracrine growth stimulatory factor for prostate cancer cells, the identification of this novel lipid kinase that regulates its production could provide new and useful targets for preventive or therapeutic measures.

Phosphorylation of Rat Liver Mitochondrial Glycerol-3-phosphate Acyltransferase by Casein Kinase 2

We have previously shown rat liver mitochondrial glycerol-3-phosphate acyltransferase (mtGAT), which catalyzes the first step in de novo glycerolipid biosynthesis, is stimulated by casein kinase 2 (CK2) and that a phosphorylated protein of approximately 85 kDa is present in CK2-treated mitochondria. In this paper, we have identified the (32)P-labeled 85-kDa protein as mtGAT. We have also investigated whether the phosphorylation of mtGAT is because of CK2. Mitochondria were treated with CK2 and [gamma-(32)P]GTP as the phosphate donor. Autoradiography, Western blot, and immunoprecipitation results showed mtGAT was phosphorylated by CK2. Next, we incubated mitochondria with CK2 and either ATP or GTP, in the presence of heparin, a known inhibitor of CK2. Heparin inhibited CK2-induced stimulation of mtGAT activity; this inhibition resulted in decreased (32)P-labeling of mtGAT. Additionally, mitochondria were treated with CK2 and [gamma-(32)P]ATP in the presence of staurosporine (a serine/threonine protein kinase inhibitor), genistein (a tyrosine kinase inhibitor), and 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB, a CK2 inhibitor). Only DRB, the CK2 inhibitor, greatly reduced the amount of (32)P-incorporation into mtGAT by CK2. Finally, isolated mitochondrial outer membrane was incubated with cytosol in the presence of [gamma-(32)P]GTP; (32)P-labeled mtGAT was detected. Collectively, these data suggest that CK2 phosphorylates mtGAT. The impact of our results in the regulation of mtGAT and other anabolic processes is discussed.

Evidence for megalin-mediated proximal tubular uptake of L-FABP, a carrier of potentially nephrotoxic molecules

Liver-type fatty acid binding protein (L-FABP) binds with high affinity to hydrophobic molecules including free fatty acid, bile acid and bilirubin, which are potentially nephrotoxic, and is involved in their metabolism mainly in hepatocytes. L-FABP is released into the circulation, and patients with liver damage have an elevated plasma L-FABP level. L-FABP is also present in renal tubules; however, the precise localization of L-FABP and its potential role in the renal tubules are not known. In this study, we examined the cellular and subcellular localization of L-FABP in the rat kidney and tried to determine from where the L-FABP in kidney tissues had originated. Immunohistochemical studies of kidney sections localized L-FABP in the lysosomes of proximal tubule cells (PTC). In rats with carbon tetrachloride (CCl4)-induced acute liver injury, we detected high levels of L-FABP in the circulation and in the kidney compared with those in the control rat by immunoblotting, while reverse transcription-polymerase chain reaction showed that the level of L-FABP mRNA expression in the kidney of CCl4-treated rats was low and did not differ from that in the control rat. When 35S-L-FABP was intravenously administered to rats, the kidneys took up 35S-L-FABP more preferentially than the liver and heart, and histoautoradiography of kidney sections revealed that 35S-L-FABP was internalized via the apical domains of PTC. Quartz-crystal microbalance analysis revealed that L-FABP bound to megalin, a multiligand endocytotic receptor on PTC, in a Ca2+-dependent manner. Degradation assays using megalin-expressing rat yolk sac tumor-derived L2 cells demonstrated that megalin mediated the cellular uptake and catabolism of 125I-L-FABP. In conclusion, circulatory L-FABP was found to be filtered by glomeruli and internalized by PTC probably via megalin-mediated endocytosis. These results suggest a novel renal uptake pathway for L-FABP, a carrier of hydrophobic molecules, some of which may exert nephrotoxic effects.

Membrane lipids and cell death: an overview

In this article we overview major aspects of membrane lipids in the complex area of cell death, comprising apoptosis and various forms of programmed cell death. We have focused here on glycerophospholipids, the major components of cellular membranes. In particular, we present a detailed appraisal of mitochondrial lipids that attract increasing interest in the field of cell death, while the knowledge of their re-modelling and traffic remains limited. It is hoped that this review will stimulate further studies by lipid experts to fully elucidate various aspects of membrane lipid homeostasis that are discussed here. These studies will undoubtedly reveal new and important connections with the established players of cell death and their action in promoting or blocking membrane alteration of mitochondria and other organelles. We conclude that the new dynamic era of cell death research will pave the way for a better understanding of the 'chemistry of apoptosis'.

Stimulation of rat liver mitochondrial sn-glycerol-3-phosphate acyltrasferase by polymyxin B via enhanced extraction of lysophosphatidic acid

The purpose of this investigation was to determine how polymyxin B stimulates the activity of mitochondrial glycerophosphate acyltransferase. Polymyxin B did not change the integrity of the mitochondrial outer membrane as judged by testing the latency (>80%) of cytochrome oxidase activity. The stimulation totally disappeared when polymyxin B-treated mitochondria were washed. The FA side chain in polymyxin B was unnecessary for stimulation, as the nonapeptide was as effective as the whole antibiotic. The stimulation by polymyxin B or the nonapeptide was observed only in the presence of BSA. Cytochrome c, when added to the incubation medium instead of albumin, did not stimulate the mitochondrial enzyme, but did produce a stimulatory effect of polymyxin B on the mitochondrial acyltransferase. As reported earlier for the bacterial and microsomal acyltransferase, other polycationic compounds such as spermine and spermidine stimulated mitochondrial glycerophosphate acyltransferase. The stimulation of the mitochondrial acyltransferase by spermine and spermidine also occurred only in the presence of BSA. The analysis of the products of esterification demonstrated the presence of more lysophosphatidic acid (LPA) in the polymyxin B- and polyamine-stimulated assays in comparison to their respective control. Furthermore, in comparison to the albumin-treated control, there was 60% more LPA present in the assay supernatant fractions of polymyxin B-treated samples. Our results suggest that polymyxin B stimulates the mitochondrial glycerophosphate acyltransferase activity by enhancing the extraction of more LPA from the mitochondria to the supernatant fraction.

Casein kinase II stimulates rat liver mitochondrial glycerophosphate acyltransferase activity

Rat liver mitochondrial glycerophosphate acyltransferase (mtGAT) possesses 14 consensus sites for casein kinase II (CKII) phosphorylation. To study the functional relevance of phosphorylation to the activity of mtGAT, we treated isolated rat liver mitochondria with CKII and found that CKII stimulated mtGAT activity approximately 2-fold. Protein phosphatase-lambda treatment reversed the stimulation of mtGAT by CKII. Labeling of both solubilized and non-solubilized mitochondria with CKII and [gamma-32P]ATP resulted in a 32P-labeled protein of 85kDa, the molecular weight of mtGAT. Our findings suggest that CKII stimulates mtGAT activity by phosphorylation of the acyltransferase. The significance of this observation with respect to hormonal control of the enzyme is discussed.

Lysophosphatidic acid (LPA) receptors are activated differentially by biological fluids: possible role of LPA-binding proteins in activation of LPA receptors

Lysophosphatidic acid (LPA) exerts multiple biological functions through G protein-coupled receptors (EDG2/LPA(1), EDG4/LPA(2), and EDG7/LPA(3)) and is present in serum where it is associated with albumin. In this study we examined LPA activity in various biological fluids by measuring the LPA-induced increase in the intracellular concentration of calcium ion in three types of Sf9 insect cells, each expressing one of the LPA receptors. Using this system, we found that EDG2 and EDG4, but not EDG7, were activated strongly by addition of incubated plasma. By contrast, LPA detected in seminal plasma, which contains a low concentration of albumin, selectively activated EDG7. After LPA in these samples was extracted and reconstituted, it activated all three receptors. We also found that serum albumin readily inhibits the activation of EDG7 but not the activation of EDG2 or EDG4. In addition, plasma from Nagase analbuminemic rats but not plasma from control Sprague-Dawley rats was found to strongly activate EDG7, although the plasma of these two types of rats contained equal amounts of LPA and activated both EDG2 and EDG4. The present study shows that serum albumin can negatively regulate EDG7 but not EDG2 or EDG4, and suggests that protein factors are present in seminal plasma and deliver LPA efficiently to EDG7 but not to EDG2 or EDG4.

Mitochondrial glycerol phosphate acyltransferase directs the incorporation of exogenous fatty acids into triacylglycerol

The mitochondrial isoform of glycerol-3-phosphate acyltransferase (GPAT), the first step in glycerolipid synthesis, is up-regulated by insulin and by high carbohydrate feeding via SREBP-1c, suggesting that it plays a role in triacylglycerol synthesis. To test this hypothesis, we overexpressed mitochondrial GPAT in Chinese hamster ovary (CHO) cells. When GPAT was overexpressed 3.8-fold, triacylglycerol mass was 2.7-fold higher than in control cells. After incubation with trace [(14)C]oleate ( approximately 3 microm), control cells incorporated 4.7-fold more label into phospholipid than triacylglycerol, but GPAT-overexpressing cells incorporated equal amounts of label into phospholipid and triacylglycerol. In GPAT-overexpressing cells, the incorporation of label into phospholipid, particularly phosphatidylcholine, decreased 30%, despite normal growth rate and phospholipid content, suggesting that exogenous oleate was directed primarily toward triacylglycerol synthesis. Transiently transfected HEK293 cells that expressed a 4.4-fold increase in GPAT activity incorporated 9.7-fold more [(14)C]oleate into triacylglycerol compared with control cells, showing that the effect of GPAT overexpression was similar in two different cell types that had been transfected by different methods. When the stable, GPAT-overexpressing CHO cells were incubated with 100 microm oleate to stimulate triacylglycerol synthesis, they incorporated 1.9-fold more fatty acid into triacylglycerol than did the control cells. Confocal microscopy of CHO and HEK293 cells transfected with the GPAT-FLAG construct showed that GPAT was located correctly in mitochondria and was not present elsewhere in the cell. These studies indicate that overexpressed mitochondrial GPAT directs incorporation of exogenous fatty acid into triacylglycerol rather than phospholipid and imply that (a) mitochondrial GPAT and lysophosphatidic acid acyltransferase produce a separate pool of lysophosphatidic acid and phosphatidic acid that must be transported to the endoplasmic reticulum where the terminal enzymes of triacylglycerol synthesis are located, and (b) this pool remains relatively separate from the pool of lysophosphatidic acid and phosphatidic acid that contributes to the synthesis of the major phospholipid species.

Lysophosphatidic acid synthesis and release

Lysophosphatidic acid (LPA) is a bioactive phospholipid controlling numerous cellular responses through the activation of specific G-protein coupled transmembrane receptors. LPA is present in several biological fluids (serum, plasma, aqueous humor) and can be secreted by several cell types (platelets, fibroblasts, adipocytes, cancer cells). Whereas, multiple pathways of synthesis and degradation of LPA have been described, their relative contribution in extracellular secretion and biodisponibility is still a matter of debate. The first part of the present review is devoted to the description of the different enzymes involved in LPA synthesis (acyltransferases, phospholipases, kinases) and degradation (lysophospholipases, lipid-phosphatases), as well as to the molecules involved in LPA transport (albumin, fatty acid binding proteins, gelsolin, lipoproteins). In a second part, the different physio-pathological situations (aggregation, cancer, injuries) associated with LPA production, as well as the potential role played by LPA in genesis of certain diseases (cancer, obesity, arteriosclerosis) are listed and analyzed.

Physiological and nutritional regulation of enzymes of triacylglycerol synthesis

Although triacylglycerol stores play the critical role in an organism's ability to withstand fuel deprivation and are strongly associated with such disorders as diabetes, obesity, and atherosclerotic heart disease, information concerning the enzymes of triacylglycerol synthesis, their regulation by hormones, nutrients, and physiological conditions, their mechanisms of action, and the roles of specific isoforms has been limited by a lack of cloned cDNAs and purified proteins. Fortunately, molecular tools for several key enzymes in the synthetic pathway are becoming available. This review summarizes recent studies of these enzymes, their regulation under varying physiological conditions, their purported roles in synthesis of triacylglycerol and related glycerolipids, the possible functions of different isoenzymes, and the evidence for specialized cellular pools of triacylglycerol and glycerolipid intermediates.

Identification of two transmembrane regions and a cytosolic domain of rat mitochondrial glycerophosphate acyltransferase

The topography of rat glycerophosphate acyltransferase (GAT) in the transverse plane of the mitochondrial outer membrane (MOM) was investigated. Computer analysis of the amino acid (aa) sequence derived from rat mitochondrial GAT cDNA (GenBanktrade mark accession nos. and ) predicts the presence of two possible transmembrane domains (aa 473-493 and 574-594) separated by an 80-aa stretch (aa 494-573). To determine the actual orientation of the native protein, we prepared anti-peptide antibodies to three regions: one in between (aa 543-559) and the other two (aa 420-435 and 726-740) flanking the two putative transmembrane regions. Both immunoreaction and immunoprecipitation experiments employing intact and solubilized mitochondria indicate that regions on the N- and C-terminal sides of the transmembrane regions are sequestered on the inner surface of the MOM, while the region between the transmembrane domains is present on the cytosolic face of the MOM. Additionally, two green fluorescent protein (GFP) fusion proteins consisting of full-length GAT fused to GFP at either the C terminus or inserted 115 amino acids from the N terminus were also constructed to determine the orientation of the N and C termini. COS-1 cells expressing these fusion proteins were fractionated to obtain mitochondria. Protease digestion of intact and solubilized COS-1 cell mitochondria revealed that the GFP domains of these fusion proteins are sequestered on the inner side of the MOM. The present findings indicate that GAT is a dual-spanning, transmembrane protein adopting an inverted "U" conformation in the transverse plane of the MOM, where the N and C termini are sequestered on the inner surface of the MOM, while aa 494-573 are exposed on the cytosolic surface of the MOM.

Liver and intestinal fatty acid-binding protein expression increases phospholipid content and alters phospholipid fatty acid composition in L-cell fibroblasts

Although fatty acid-binding proteins (FABP) differentially affect fatty acid uptake, nothing is known regarding their role(s) in determining cellular phospholipid levels and phospholipid fatty acid composition. The effects of liver (L)- and intestinal (I)-FABP expression on these parameters were determined using stably transfected L-cells. Expression of L- and I-FABP increased cellular total phospholipid mass (nmol/mg protein) 1.7- and 1.3-fold relative to controls, respectively. L-FABP expression increased the masses of choline glycerophospholipids (ChoGpl) 1.5-fold, phosphatidylserine (PtdSer) 5.6-fold, ethanolamine glycerophospholipids 1.4-fold, sphingomyelin 1.7-fold, and phosphatidylinositol 2.6-fold. In contrast, I-FABP expression only increased the masses of ChoGpl and PtdSer, 1.2- and 3.1-fold, respectively. Surprisingly, both L- and I-FABP expression increased ethanolamine plasmalogen mass 1.6- and 1.1-fold, respectively, while choline plasmalogen mass was increased 2.3- and 1.7-fold, respectively. The increase in phospholipid levels resulted in dramatic 48 and 33% decreases in the cholesterol-to-phospholipid ratio in L- and I-FABP expressing cells, respectively. L-FABP expression generally increased polyunsaturated fatty acids, primarily by increasing 20:4n-6 and 22:6n-3, while decreasing 18:1n-9 and 16:1n-7. I-FABP expression generally increased only 20:4n-6 proportions. Hence, expression of both I- and L-FABP differentially affected phospholipid mass, class composition, and acyl chain composition. Although both proteins enhanced phospholipid synthesis, the effect of L-FABP was much greater, consistent with previous work suggesting that these two FABP differentially affect lipid metabolism.

Binding of fatty acids and peroxisome proliferators to orthologous fatty acid binding proteins from human, murine, and bovine liver

Liver-type fatty acid binding protein (L-FABP) has been proposed to be involved in the transport of fatty acids and peroxisome proliferators from the cytosol into the nucleus for interaction with the peroxisome proliferator-activated receptors (PPARs). On the basis of this premise, we investigated by isothermal titration calorimetry the binding of myristic, stearic, oleic, and docosahexaenoic acids to three orthologous L-FABPs and compared these results to those obtained for several xenobiotics [Wy14,643, bezafibrate, 5,8,11,14-eicosatetraynoic acid (ETYA), and BRL48,482] known for their peroxisome proliferating activity in rodents. Recombinant human, murine, and bovine L-FABPs were analyzed and the thermodynamic data were obtained. Our studies showed that fatty acids bound with a stoichiometry of 2:1, fatty acid to protein, with dissociation constants for the first binding site in the nanomolar range. With dissociation constants above 1 microM the drug peroxisome proliferators showed weaker binding, with the exception of arachidonate analogue ETYA, which bound with a similar affinity as the natural fatty acid. Some of the thermodynamic data obtained for fatty acid binding could be explained by differences in protein structure. Moreover, our results revealed that binding affinities were not determined by ligand solubility in the aqueous phase.

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.

Phosphatidic acid, a key intermediate in lipid metabolism

Phosphatidic acid (PtdOH) is a key intermediate in glycerolipid biosynthesis. Two different pathways are known for de novo formation of this compound, namely (a) the Gro3P (glycerol 3-phosphate) pathway, and (b) the GrnP (dihydroxyacetone phosphate) pathway. Whereas the former route of PtdOH synthesis is present in bacteria and all types of eukaryotes, the GrnP pathway is restricted to yeast and mammalian cells. In this review article, we describe the enzymes catalyzing de novo formation of PtdOH, their properties and their occurrence in different cell types and organelles. Much attention has recently been paid to the subcellular localization of enzymes involved in the biosynthesis of PtdOH. In all eukaryotic cells, microsomes (ER) harbour the complete set of enzymes catalyzing these pathways and are thus the usual organelle for PtdOH formation. In contrast, the contribution of mitochondria to PtdOH synthesis is restricted to certain enzymes and depends on the cell type. In addition, chloroplasts of plants, lipid particles of the yeast, and peroxisomes of mammalian cells are significantly involved in PtdOH biosynthesis. Redundant systems of acyltransferases, the interplay of organelles, regulation of the pathway on the compartmental level, and finally the contribution of alternative pathways (phosphorylation of diacylglycerol and cleavage of phospholipids by phospholipases) to PtdOH biosynthesis appear to be required for the balanced formation of this important lipid intermediate. Dysfunction of enzymes involved in PtdOH synthesis can result in severe defects of various cellular processes. In this context, the possible physiological role(s) of PtdOH and its related metabolites, lysophosphatidic acid and diacylglycerol, will be discussed.

Phosphatidic acid synthesis in mitochondria. Topography of formation and transmembrane migration

The topography of formation and migration of phosphatidic acid (PA) in the transverse plane of rat liver mitochondrial outer membrane (MOM) were investigated. Isolated mitochondria and microsomes, incubated with sn-glycerol 3-phosphate and an immobilized substrate palmitoyl-CoA-agarose, synthesized both lyso-PA and PA. The mitochondrial and microsomal acylation of glycerophosphate with palmitoyl-CoA-agarose was 80-100% of the values obtained in the presence of free palmitoyl-CoA. In another series of experiments, both free polymyxin B and polymyxin B-agarose stimulated mitochondrial glycerophosphate acyltransferase activity approximately 2-fold. When PA loaded mitochondria were treated with liver fatty acid binding protein, a fifth of the phospholipid left the mitochondria. The amount of exportable PA reduced with the increase in the time of incubation. In another approach, PA-loaded mitochondria were treated with phospholipase A(2). The amount of phospholipase A(2)-sensitive PA reduced when the incubation time was increased. Taken together, the results suggest that lysophosphatidic acid (LPA) and PA are synthesized on the outer surface of the MOM and that PA moves to the inner membrane presumably for cardiolipin formation.

Down-regulation of liver and heart specific fatty acid binding proteins by endotoxin and cytokines in vivo

Fatty acid binding proteins (FABPs) are abundantly present in tissues that actively metabolize fatty acids (FA). While their precise physiological function is not known, FABPs have been shown to play a role in the uptake and/or utilization of FA within the cell. FA metabolism is markedly altered during the host response to infection and inflammation. Previous studies have demonstrated that endotoxin or bacterial lipopolysaccharide (LPS) enhances hepatic FA synthesis and re-esterification while inhibiting FA oxidation in liver, heart and muscle. Now, we have examined the in vivo effects of LPS and cytokines on FABPs in liver (L-FABP), heart and muscle (H-FABP). Syrian hamsters were injected with LPS, tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) and the mRNA and protein content for L-FABP and H-FABP were analyzed. 16 h after administration, LPS (100 microg/100 g body weight) produced a 72% decrease in L-FABP mRNA levels in liver and this effect was sustained for 24 h. LPS also produced a 41% decrease in the protein content of L-FABP in liver after 24 h of treatment. TNF-alpha and IL-1beta decreased L-FABP mRNA levels in liver by 30 and 45%, respectively. LPS decreased H-FABP mRNA levels in skeletal muscle by 60% and in heart by 65%. LPS also produced a 49% decrease in H-FABP protein content in muscle. Neither TNF-alpha nor IL-1beta had any significant effect on H-FABP mRNA expression in heart and muscle. Taken together, these results indicate that LPS decreases FABP mRNA and protein levels in liver, heart and muscle, tissues that normally utilize FA as their primary fuel, whereas the inhibitory effect of cytokines is limited to the liver. The LPS-induced decrease in L-FABP and H-FABP may be an additional mechanism contributing to the decrease in FA oxidation that is associated with the host response to infection and inflammation.

Isolation and characterization of two distinct forms of liver fatty acid binding protein from the rat

Liver fatty acid binding protein (L-FABP) appears to contain several different forms that may result from post-translational modification or bound ligand. To further assess this possibility, L-FABP was purified from rat liver homogenate and two putative isoforms separated using a sulfonyl column, a strong cation exchange resin. Fraction I eluted at 0.2 M NaCl, had a pI of 7.59, and following a final size exclusion step contained > 98% L-FABP. Fraction II eluted at 1.0 M NaCl, had a pI of 7.59, and following a final size exclusion step contained > 99% L-FABP. Both fractions contained approx. 0.15 moles of endogenous bound fatty acid per mole of protein, while L-FABP not subjected to the cation exchange step contained 0.75 moles of fatty acid per mole of protein. Fractions I and II had a greater proportion of saturated and monounsaturated fatty acids with a large reduction in polyunsaturated fatty acids compared to L-FABP not fractionated by cation exchange. Mass spectral analysis indicated the molecular mass of Fraction I was 14,315.02 +/- 0.35 Da and Fraction II was 14,315.86 +/- 0.34 Da. The peptide map for each fraction was determined by limited digestion of each fraction with either trypsin, Asp-N, or chymotrypsin to yield overlapping peptide fragments. Mass spectral analysis of these digests indicated the two proteins had identical amino acid fragments and that Cys69 was reduced and there were no Asn to Asp exchanges. Hence, these two forms of L-FABP were not isoforms and were not the result of differences in bound fatty acid. It is proposed that these two distinct forms of rat L-FABP were structural conformers based on two alternative folding pathways.

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.

Cyclosporin A inhibits oxidant and calcium stimulated phospholipase D activity in the rat intestinal mitochondria

Mitochondrial swelling and calcium cycling occurs during oxidative stress and can be prevented by cyclosporin A (CysA). Our earlier work has shown that enterocyte mitochondria contains a phospholipase D (PLD) which can be activated by superoxide or calcium. In this study, we have shown that enterocyte mitochondrial PLD activated by these agents can be inhibited by cyclosporin A. This was clearly shown by the absence of phosphatidic acid (PA) formation and phosphatidylethanolamine (PE) degradation. Since this PLD specifically utilizes PE as substrate, PLD activity was also assessed by ethanolamine formation which was inhibited by CysA. CysA also inhibited the cabbage PLD activity as judged by phosphatidylethanol formation. These results suggest that cyclosporin A is an inhibitor of PLD and this may be one of the mechanism by which CysA protects enterocyte mitochondria from oxidative stress.

Characterisation and properties of an intracellular lipid-binding protein from the tapeworm Moniezia expansa

The tapeworm Moniezia expansa contains an extremely abundant cytoplasmic lipid-binding protein (LBP). It is a small protein consisting of 66 amino acids with a molecular mass of 7943 +/- 1.5 Da. The amino acid sequence has been established by Edman degradation and confirmed by PCR analysis. The Moniezia LBP shows no sequence similarity with any previously described binding protein, but does show similarity with antigen B from Echinococcus glanulosus and Echinococcus multilocularis and with Taenia crassiceps antigen. The predicted structure for Moniezia LBP shows four helices and a putative tyrosine kinase site on the loop between helix 1 and 2. Each of the four helices has a well defined hydrophobic face. Studies with fluorescent probes suggest a single hydrophobic binding site. Results indicate that the single tryptophan residue in the molecule (Trp41) is involved in ligand binding, and calculation of the Stern-Volmer quenching constant shows that Trp41 is in a relatively hydrophobic environment.

Activation of intestinal mitochondrial phospholipase D by polyamines and monoamines

Intestinal mitochondria have a phospholipase D (PLD) activity which was stimulated by polyamines and monoamines resulting in the formation of phosphatidic acid (PA) from endogenous phospholipids. When stimulated by polyamines, mitochondrial PLD utilized endogenous phosphatidylethanolamine (PE) as substrate whereas stimulated by monoamines, both PE and phosphatidylcholine (PC) were hydrolysed. Stimulation of PA formation by spermine was enhanced by the presence of calcium. Since polyamines are known to alter the calcium transport by mitochondria and PA is known to possess an ionophore effect, stimulation of PA formation in mitochondria by polyamines suggests that polyamine-induced alteration in calcium homeostasis might involve a PA related mechanism.

Mammalian mitochondrial glycerol-3-phosphate acyltransferase

Glycerol-3-phosphate acyltransferase (GPAT) is the first committed, and presumed to be a rate-limiting, step in glycerophospholipid biosynthesis. There are two isoforms of GPAT, a mitochondrial and a microsomal form. Mitochondrial GPAT has recently been purified and its gene has been cloned and expressed in baculovirus-infected cells. The GPAT activity was reconstituted using the purified enzyme and various phospholipids. Mitochondrial GPAT prefers saturated fatty acyl-CoA as a substrate. This preference may contribute to the observed asymmetric distribution of saturated and unsaturated fatty acids at the sn-1 and sn-2 positions of cellular glycerophospholipids. A region of homology to various acyltransferases that may be important for catalysis or fatty acyl-CoA binding is present in mitochondrial GPAT. Mitochondrial GPAT is upregulated at the transcriptional level by refeeding a high carbohydrate, fat-free diet to previously fasted mice and by insulin administration to diabetic animals, whereas microsomal GPAT activity is largely unaffected by these treatments.

Lysophosphatidic acid as a phospholipid mediator: pathways of synthesis

From very recent studies, including molecular cloning of cDNA coding for membrane receptors, lysophosphatidic acid (LPA) reached the status of a novel phospholipid mediator with various biological activities. Another strong argument supporting this view was the discovery that LPA is secreted from activated platelets, resulting in its appearance in serum upon blood coagulation. The metabolic pathways as well as the enzymes responsible for LPA production are poorly characterized. However, a survey of literature data indicates some interesting issues which might be used as the basis for further molecular characterization of phospholipases A able to degrade phosphatidic acid.

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.

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.

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.

Analysis of the ligand binding properties of recombinant bovine liver-type fatty acid binding protein

The coding part of the cDNA for bovine liver-type fatty acid binding protein (L-FABP) has been amplified by RT-PCR, cloned and used for the construction of an Escherichia coli (E. coli) expression system. The recombinant protein made up to 25% of the soluble E. coli proteins and could be isolated by a simple two step protocol combining ion exchange chromatography and gel filtration. Dissociation constants for binding of oleic acid, arachidonic acid, oleoyl-CoA, lysophosphatidic acid and the peroxisomal proliferator bezafibrate to L-FABP have been determined by titration calorimetry. All ligands were bound in a 2:1 stoichiometry, the dissociation constants for the first ligand bound were all in the micro molar range. Oleic acid was bound with the highest affinity and a Kd of 0.26 microM. Furthermore, binding of cholesterol to L-FABP was investigated with the Lipidex assay, a liposome binding assay and a fluorescence displacement assay. In none of the assays binding of cholesterol to L-FABP was observed.

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.

Carnitine palmitoyltransferase activities: effects of serum albumin, acyl-CoA binding protein and fatty acid binding protein

The carnitine palmitoyltransferase activity of various subcellular preparations measured with octanoyl-CoA as substrate was markedly increased by bovine serum albumin at low microM concentrations of octanoyl-CoA. However, even a large excess (500 microM) of this acyl-CoA did not inhibit the activity of the mitochondrial outer carnitine palmitoyltransferase, a carnitine palmitoyltransferase isoform that is particularly sensitive to inhibition by low microM concentrations of palmitoyl-CoA. This bovine serum albumin stimulation was independent of the salt activation of the carnitine palmitoyltransferase activity. The effects of acyl-CoA binding protein (ACBP) and the fatty acid binding protein were also examined with palmitoyl-CoA as substrate. The results were in line with the findings of stronger binding of acyl-CoA to ACBP but showed that fatty acid binding protein also binds acyl-CoA esters. Although the effects of these proteins on the outer mitochondrial carnitine palmitoyltransferase activity and its malonyl-CoA inhibition varied with the experimental conditions, they showed that the various carnitine palmitoyltransferase preparations are effectively able to use palmitoyl-CoA bound to ACBP in a near physiological molar ratio of 1:1 as well as that bound to the fatty acid binding protein. It is suggested that the three proteins mentioned above affect the carnitine palmitoyltransferase activities not only by binding of acyl-CoAs, preventing acyl-CoA inhibition, but also by facilitating the removal of the acylcarnitine product from carnitine palmitoyltransferase. These results support the possibility that the acyl-CoA binding ability of acyl-CoA binding protein and of fatty acid binding protein have a role in acyl-CoA metabolism in vivo.

Growth promotion of transfected hepatoma cells by liver fatty acid binding protein

Former studies have linked hepatocyte growth with liver fatty acid binding protein (L-FABP) of rat liver cytosol. In search for the roles of L-FABP in hepatocytes, we previously stably transfected rat L-FABP sense and antisense cDNAs into rat hepatoma HTC cells that do not contain L-FABP RNA or protein, thereby providing a zero-background, homologous cell model of L-FABP-expression suitable for controlled studies of its intracellular functions in hepatocyte-derived cells. The present study demonstrates the abilities of L-FABP to promote DNA synthesis and cell growth, preserve cell morphology, extend survival, and act cooperatively with unsaturated fatty acids in the transfected hepatoma cells in the absence of serum. Following removal of serum, the three control L-FABP-nonexpressing cell lines increased in cell lines increased in cell number for 24 hr and thereafter declined, whereas the three L-FABP-expressing cell lines exhibited a 39% higher rate of DNA synthesis per cell at 24 hr and grew in cell number for 48 hr. As a result, at 72 hr there were 2.5-fold (avg.) as many L-FABP-expressing cells than L-FABP-nonexpressing cells. In addition, the L-FABP-expressing cells retained their original polygonal morphology at 48 hr, when in contrast most of the control nonexpressing cells were spherical in shape with membrane blebs. In an effort to identify the agonists that collaborate with L-FABP in the growth promotion and preservation of cell morphology, various free fatty acids were examined at 48 hr for their ability to eliminate the differences in behavior of the two cell types in the serum-free medium. The unsaturated fatty acids, oleic acid (18:1 omega 9), linoleic acid (18:2 omega 6), alpha-linolenic acid (18:3 omega 3), and arachidonic acid (20:4 omega 6), at 1 microM markedly elevated the level of DNA synthesis in the more depressed control L-FABP-nonexpressing cells and moderately raised it in the less depressed L-FABP-expressing cells. In accord, the control L-FABP-nonexpressing cells needed 10(-6)-10(-5) M linoleic acid to achieve the extent of DNA synthesis attained by the expressing cells in the absence of added fatty acid. At 10 microM linoleic acid, their levels of DNA synthesis were equal. In contrast, five saturated fatty acids had no detectable effect on DNA synthesis.(ABSTRACT TRUNCATED AT 400 WORDS)