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

Cytoplasmic fatty acid-binding proteins in metabolic diseases and cancers

Cytoplasmic fatty acid-binding proteins (FABPs) are multipurpose proteins that can modulate lipid fluxes, trafficking, signaling, and metabolism. FABPs regulate metabolic and inflammatory pathways, its inhibition can improve type 2 diabetes mellitus and atherosclerosis. In addition, FABPs are involved in obesity, metabolic disease, cardiac dysfunction, and cancers. FABPs are promising tissue biomarkers in solid tumors for diagnostic and/or prognostic targets for novel therapeutic strategies. The signaling responsive elements of FABPs and determinants of FABP-mediated functions may be exploited in preventing or treating these diseases.

Intra-cardiac transfer of fatty acids from capillary to cardiomyocyte

Blood-borne fatty acids (Fa) are important substrates for energy conversion in the mammalian heart. After release from plasma albumin, Fa traverse the endothelium and the interstitial compartment to cross the sarcolemma prior to oxidation in the cardiomyocytal mitochondria. The aims of the present study were to elucidate the site with lowest Fa permeability (i.e., highest Fa resistance) in the overall Fa trajectory from capillary to cardiomyocyte and the relative contribution of unbound Fa (detach pathway, characterized by the dissociation time constant τ_AlbFa) and albumin-bound Fa (contact pathway, characterized by the membrane reaction rate parameter d_Alb) in delivering Fa to the cellular membranes. In this study, an extensive set of 34 multiple indicator dilution experiments with radiolabeled albumin and palmitate on isolated rabbit hearts was analysed by means of a previously developed mathematical model of Fa transfer dynamics. In these experiments, the ratio of the concentration of palmitate to albumin was set at 0.91. The analysis shows that total cardiac Fa permeability, P_tot, is indeed related to the albumin concentration in the extracellular compartment as predicted by the mathematical model. The analysis also reveals that the lowest permeability may reside in the boundary zones containing albumin in the microvascular and interstitial compartment. However, the permeability of the endothelial cytoplasm, P_ec, may influence overall Fa permeability, P_tot, as well. The model analysis predicts that the most likely value of τ_AlbFa ranges from about 200 to 400 ms. In case τ_AlbFa is fast, i.e., about 200 ms, the extracellular contact pathway appears to be of minor importance in delivering Fa to the cell membrane. If Fa dissociation from albumin is slower, e.g. τ_AlbFa equals 400 ms, the contribution of the contact pathway may vary from minimal (d_Alb≤5 nm) to substantial (d_Alb about 100 nm). In the latter case, the permeability of the endothelial cytoplasm varies from infinite (no hindrance) to low (substantial hindrance) to keep the overall Fa flux at a fixed level. Definitive estimation of the impact of endothelial permeability on P_tot and the precise contribution of the contact pathway to overall transfer of Fa in boundary zones containing albumin requires adequate physicochemical experimentation to delineate the true value of, among others, τ_AlbFa, under physiologically relevant circumstances. Our analysis also implies that concentration differences of unbound Fa are the driving force of intra-cardiac Fa transfer; an active, energy requiring transport mechanism is not necessarily involved. Membrane-associated proteins may facilitate Fa transfer in the boundary zones containing albumin by modulating the membrane reaction rate parameter, d_Alb, and, hence, the contribution of the contact pathway to intra-cardiac Fa transfer.

Modulation of endothelium function by fatty acids

The endothelium acts as the barrier that prevents circulating lipids such as lipoproteins and fatty acids into the arterial wall; it also regulates normal functioning in the circulatory system by balancing vasodilation and vasoconstriction, modulating the several responses and signals. Plasma lipids can interact with endothelium via different mechanisms and produce different phenotypes. Increased plasma-free fatty acids (FFAs) levels are associated with the pathogenesis of atherosclerosis and cardiovascular diseases (CVD). Because of the multi-dimensional roles of plasma FFAs in mediating endothelial dysfunction, increased FFA level is now considered an essential link in the onset of endothelial dysfunction in CVD. FFA-mediated endothelial dysfunction involves several mechanisms, including dysregulated production of nitric oxide and cytokines, metaflammation, oxidative stress, inflammation, activation of the renin-angiotensin system, and apoptosis. Therefore, modulation of FFA-mediated pathways involved in endothelial dysfunction may prevent the complications associated with CVD risk. This review presents details as to how endothelium is affected by FFAs involving several metabolic pathways.

Fatty acids and evolving roles of their proteins in neurological, cardiovascular disorders and cancers

The dysregulation of fat metabolism is involved in various disorders, including neurodegenerative, cardiovascular, and cancers. The uptake of long-chain fatty acids (LCFAs) with 14 or more carbons plays a pivotal role in cellular metabolic homeostasis. Therefore, the uptake and metabolism of LCFAs must constantly be in tune with the cellular, metabolic, and structural requirements of cells. Many metabolic diseases are thought to be driven by the abnormal flow of fatty acids either from the dietary origin and/or released from adipose stores. Cellular uptake and intracellular trafficking of fatty acids are facilitated ubiquitously with unique combinations of fatty acid transport proteins and cytoplasmic fatty acid-binding proteins in every tissue. Extensive data are emerging on the defective transporters and metabolism of LCFAs and their clinical implications. Uptake and metabolism of LCFAs are crucial for the brain's functional development and cardiovascular health and maintenance. In addition, data suggest fatty acid metabolic transporter can normalize activated inflammatory response by reprogramming lipid metabolism in cancers. Here we review the current understanding of how LCFAs and their proteins contribute to the pathophysiology of three crucial diseases and the mechanisms involved in the processes.

Role of the Fatty Acid Binding Proteins in Cardiovascular Diseases: A Systematic Review

Cardiovascular diseases (CVD) remain a global pandemic and leading cause of deaths worldwide. While several guidelines have been developed to control the development of CVDs, its prevalence keeps on increasing until this day. Cardiovascular risk factors, such as reduced exercises and high fat or glucose diets, culminate in the development of the metabolic syndrome and eventually atherosclerosis, which is driven by high blood lipid and cholesterol levels, and by endothelial dysfunction. Late complications of atherosclerosis give rise to serious clinical cardiovascular manifestations such as myocardial infarction and hypertension. Therefore, endothelial functions and the lipid metabolism play critical roles in the pathogenesis of CVDs. Fatty acid-binding proteins are a family of intracellular proteins expressed in many cell types known mainly for their interaction with and trafficking of cellular lipids. The roles of a number of isoforms in this family have been implicated in lipid metabolic homeostasis, but their influence on endothelial function and vascular homeostasis remain largely unknown. This review's purpose is to update fundamentals about the connection between cardiovascular disease, metabolism, endothelial function, and mainly the roles of fatty acid-binding proteins.

Emerging Roles of Vascular Endothelium in Metabolic Homeostasis

Our understanding of the role of the vascular endothelium has evolved over the past 2 decades, with the recognition that it is a dynamically regulated organ and that it plays a nodal role in a variety of physiological and pathological processes. Endothelial cells (ECs) are not only a barrier between the circulation and peripheral tissues, but also actively regulate vascular tone, blood flow, and platelet function. Dysregulation of ECs contributes to pathological conditions such as vascular inflammation, atherosclerosis, hypertension, cardiomyopathy, retinopathy, neuropathy, and cancer. The close anatomic relationship between vascular endothelium and highly vascularized metabolic organs/tissues suggests that the crosstalk between ECs and these organs is vital for both vascular and metabolic homeostasis. Numerous reports support that hyperlipidemia, hyperglycemia, and other metabolic stresses result in endothelial dysfunction and vascular complications. However, how ECs may regulate metabolic homeostasis remains poorly understood. Emerging data suggest that the vascular endothelium plays an unexpected role in the regulation of metabolic homeostasis and that endothelial dysregulation directly contributes to the development of metabolic disorders. Here, we review recent studies about the pivotal role of ECs in glucose and lipid homeostasis. In particular, we introduce the concept that the endothelium adjusts its barrier function to control the transendothelial transport of fatty acids, lipoproteins, LPLs (lipoprotein lipases), glucose, and insulin. In addition, we summarize reports that ECs communicate with metabolic cells through EC-secreted factors and we discuss how endothelial dysregulation contributes directly to the development of obesity, insulin resistance, dyslipidemia, diabetes mellitus, cognitive defects, and fatty liver disease.

Serum heart-type fatty acid-binding protein decreases and soluble isoform of suppression of tumorigenicity 2 increases significantly by long-term physical activity

The aim of this prospective study was to investigate the influence of long-term physical activity on biomarkers for myocyte ischemia (heart-type fatty acid-binding protein, H-FABP), matrix remodelling/vascular stress (soluble isoform of suppression of tumorigenicity 2, sST2) and inflammation (soluble urokinase-type plasminogen activator receptor, suPAR). In this prospective observational study 109 subjects were recruited, 98 completed the study. Subjects were asked to perform exercise within the calculated training pulse for 8 months. The performance gain was measured/quantified by bicycle stress tests at the beginning and end of the observation period. Twenty-seven subjects with a performance gain <2.9% were excluded. suPAR, H-FABP and sST2 were measured in serum at baseline and after 2, 4 and 8 months by ELISA. We found a significant decrease in H-FABP (1.86 (0.86) to 1.29 (0.98) ng/mL; p<0.01) and a significant increase in sST2 levels (6126 (2759) to 6919 (3720) pg/mL; p=0.045) during the observation period of 8 months while there was no remarkable change in suPAR levels. We interpret the activity-induced decrease in H-FABP as sign of lower subclinical myocardial ischemia and better perfusion, probably due to a more economic metabolization and electrolyte balance. The increase in sST2 might reflect physiological sports-induced vascular stress. As H-FABP and sST2 play an important role in the pathomechanism of ischemic cardiomyopathy (iCMP) further studies should investigate the influence of regular physical activity on these biomarkers in a population of patients with iCMP. TRIAL REGISTRATION NUMBER: NCT02097199.

Visualizing Fatty Acid Flux

In a recent issue of Cell Metabolism, He et al. (2018) describes a novel technique to visualize cardiac intravascular lipoprotein lipase-mediated processing of triglyceride-rich lipoproteins and then follow the flux of released fatty acids across the endothelium to the underlying cardiomyocytes at high spatial resolution. This allows for detailed analyses of this clearly complex process.

Endothelium as a gatekeeper of fatty acid transport

The endothelium transcends all clinical disciplines and is crucial to the function of every organ system. A critical, but poorly understood, role of the endothelium is its ability to control the transport of energy supply according to organ needs. Fatty acids (FAs) in particular represent a key energy source that is utilized by a number of tissues, but utilization must be tightly regulated to avoid potentially deleterious consequences of excess accumulation, including insulin resistance. Recent studies have identified important endothelial signaling mechanisms, involving vascular endothelial growth factor-B, peroxisome proliferator-activated receptor-γ, and apelin, that mediate endothelial regulation of FA transport. In this review, we discuss the mechanisms by which these signaling pathways regulate this key endothelial function.

Endothelial fatty acid transport: role of vascular endothelial growth factor B

Dietary lipids present in the circulation have to be transported through the vascular endothelium to be utilized by tissue cells, a vital mechanism that is still poorly understood. Vascular endothelial growth factor B (VEGF-B) regulates this process by controlling the expression of endothelial fatty acid transporter proteins (FATPs). Here, we summarize research on the role of the vascular endothelium in nutrient transport, with emphasis on VEGF-B signaling.

Fatty acid-binding protein 4 (FABP4) and FABP5 modulate cytokine production in the mouse thymic epithelial cells

Thymic stromal cells, including cortical thymic epithelial cells (cTEC) produce many humoral factors, such as cytokines and eicosanoids to modulate thymocyte homeostasis, thereby regulating the peripheral immune responses. In this study, we identified fatty acid-binding protein (FABP4), an intracellular fatty acid chaperone, in the mouse thymus, and examined its role in the control of cytokine production in comparison with FABP5. By immunofluorescent staining, FABP4(+) cells enclosing the thymocytes were scattered throughout the thymic cortex with a spatial difference from the FABP5(+) cell that were distributed widely throughout the cTEC. The FABP4(+) cells were immunopositive for MHC class II, NLDC145 and cytokeratin 8, and were identified as part of cTEC. The FABP4(+) cells were identified as thymic nurse cells (TNC), a subpopulation of cTEC, by their active phagocytosis of apoptotic thymocytes. Furthermore, FABP4 expression was confirmed in the isolated TNC at the gene and protein levels. To explore the function of FABP in TNC, TSt-4/DLL1 cells stably expressing either FABP4 or FABP5 were established and the gene expressions of various cytokines were examined. The gene expression of interleukin (IL)-7 and IL-18 was increased both in FABP4 and FABP5 over-expressing cells compared with controls, and moreover, the increase in their expressions by adding of stearic acids was significantly enhanced in the FABP4 over-expressing cells. These data suggest that both FABPs are involved in the maintenance of T lymphocyte homeostasis through the modulation of cytokine production, which is possibly regulated by cellular fatty acid-mediated signaling in TEC, including TNC.

Arachidonic acid cascade in endothelial pathobiology

Arachidonic acid (AA) and its metabolites (eicosanoids) represent powerful mediators, used by organisms to induce and suppress inflammation as a part of the innate response to disturbances. Several cell types participate in the synthesis and release of AA metabolites, while many cell types represent the targets for eicosanoid action. Endothelial cells (EC), forming a semi-permeable barrier between the interior space of blood vessels and underlying tissues, are of particular importance for the development of inflammation, since endothelium controls such diverse processes as vascular tone, homeostasis, adhesion of platelets and leukocytes to the vascular wall, and permeability of the vascular wall for cells and fluids. Proliferation and migration of endothelial cells contribute significantly to new vessel development (angiogenesis). This review discusses endothelial-specific synthesis and action of arachidonic acid derivatives with a particular focus on the mechanisms of signal transduction and associated intracellular protein targets.

Serum Heart-Type Fatty Acid-Binding Protein Concentration Positively Correlates With the Length of Aortic Dissection

BACKGROUND

Patients with aortic dissection (AD) often demonstrate positive heart-type fatty acid-binding protein (H-FABP), but its significance is unclear.

METHODS AND RESULTS

In 63 of 64 consecutive AD patients, the serum H-FABP concentration was measured and the H-FABP positive rate calculated (cutoff value: 6.2 ng/ml) for each of following factors: (1) with or without dissection of the ascending aorta; and (2) a thrombosed false lumen; (3) length score; (4) presence of shock; (5) malperfusion of limbs; (6) ST elevation and/or depression on electrocardiogram; and (7) renal dysfunction. In total 36 AD patients had a positive H-FABP test. Statistically significant differences in the H-FABP positive rate were observed between those with and without ascending AD (76.7% vs 39.4%, p = 0.003), and in the length score (p < 0.001). Multivariate logistic regression test demonstrated that the most significant factor was the length score (odds ratio: 2.239 (95% confidence interval: 1.119-4.481), p = 0.023). Moreover, a significant correlation was observed between length score and absolute H-FABP value (r = 0.420, p = 0.001).

CONCLUSIONS

In patients with AD, an increased serum H-FABP concentration is caused by the protein being released not only from the cardiac muscle but also from skeletal muscle, or possibly the aortic wall. Physicians using H-FABP for detection of myocardial injury need to be aware that patients with a long or ascending AD will show an elevation of H-FABP.

Fatty acid binding proteins stabilize leukotriene A4: competition with arachidonic acid but not other lipoxygenase products

Leukotriene A(4) (LTA(4)) is a chemically reactive conjugated triene epoxide product derived from 5-lipoxygenase oxygenation of arachidonic acid. At physiological pH, this reactive compound has a half-life of less than 3 s at 37 degrees C and approximately 40 s at 4 degrees C. Regardless of this aqueous instability, LTA(4) is an intermediate in the formation of biologically active leukotrienes, which can be formed through either intracellular or transcellular biosynthesis. Previously, epithelial fatty acid binding protein (E-FABP) present in RBL-1 cells was shown to increase the half-life of LTA(4) to approximately 20 min at 4 degrees C. Five FABPs (adipocyte FABP, intestinal FABP, E-FABP, heart/muscle FABP, and liver FABP) have now been examined and also found to increase the half-life of LTA(4) at 4 degrees C to approximately 20 min with protein present. Stabilization of LTA(4) was examined when arachidonic acid was present to compete with LTA(4) for the binding site on E-FABP. Arachidonate has an apparent higher affinity for E-FABP than LTA(4) and was able to completely block stabilization of the latter. When E-FABP is not saturated with arachidonate, FABP can still stabilize LTA(4). Several lipoxygenase products, including 5-hydroxyeicosatetraenoic acid, 5,6-dihydroxyeicosatetraenoic acid, and leukotriene B(4), were found to have no effect on the stability of LTA(4) induced by E-FABP even when present at concentrations 3-fold higher than LTA(4).

Fatty acid-binding proteins inhibit hydration of epoxyeicosatrienoic acids by soluble epoxide hydrolase

Epoxyeicosatrienoic acids (EETs) are potent regulators of vascular homeostasis and are bound by cytosolic fatty acid-binding proteins (FABPs) with K(d) values of approximately 0.4 microM. To determine whether FABP binding modulates EET metabolism, we examined the effect of FABPs on the soluble epoxide hydrolase (sEH)-mediated conversion of EETs to dihydroxyeicosatrienoic acids (DHETs). Kinetic analysis of sEH conversion of racemic [(3)H]11,12-EET yielded K(m) = 0.45 +/- 0.08 microM and V(max) = 9.2 +/- 1.4 micromol min(-1) mg(-)(1). Rat heart FABP (H-FABP) and rat liver FABP were potent inhibitors of 11,12-EET and 14,15-EET conversion to DHET. The resultant inhibition curves were best described by a substrate depletion model, with K(d) = 0.17 +/- 0.01 microM for H-FABP binding to 11,12-EET, suggesting that FABP acts by reducing EET availability to sEH. The EET depletion by FABP was antagonized by the co-addition of arachidonic acid, oleic acid, linoleic acid, or 20-hydroxyeicosatetraenoic acid, presumably due to competitive displacement of FABP-bound EET. Collectively, these findings imply that FABP might potentiate the actions of EETs by limiting their conversion to DHET. However, the effectiveness of this process may depend on metabolic conditions that regulate the levels of competing FABP ligands.

Branching out: a molecular fingerprint of endothelial differentiation into tube-like structures generated by Affymetrix oligonucleotide arrays

The process of endothelial differentiation into a network of tube-like structures with patent lumens requires an integrated program of gene expression. To identify genes upregulated in endothelial cells during the process of tube formation, RNA was prepared from several different time points (0, 4, 8, 24, 40, and 48 hours) and from three different experimental models of human endothelial tube formation: in collagen gels and fibrin gels driven by the combination of PMA (80), bFGF (40 ng/ml) and bFGF (40 ng/ml) or in collagen gels driven by the combination of HGF (40 ng/ml) and VEGF (40 ng/ml). Gene expression was evaluated using Affymetrix Gene Chip oligonucleotide arrays. Over 1000 common genes were upregulated greater than twofold over baseline at one or more time points in the three different models. In the present study, we discuss the identified genes that could be assigned to major functional classes: apoptosis, cytoskeleton, proteases, matrix, and matrix turnover, pumps and transporters, membrane lipid turnover, and junctional molecules or adhesion proteins.

Binding of cytochrome P450 monooxygenase and lipoxygenase pathway products by heart fatty acid-binding protein

Arachidonic acid metabolism by lipoxygenases and cytochrome P450 monooxygenases produces regioisomeric hydroperoxyeicosatetraenoic acids (HPETEs), hydroxyeicosatetraenoic acids (HETEs), epoxyeicosatrienoic acids (EETs), and dihydroxyeicosatrienoic acids (DHETs), which serve as components of cell signaling cascades. Intracellular fatty acid-binding proteins (FABPs) may differentially bind these nonprostanoid oxygenated fatty acids, thus modulating their metabolism and activities. Vascular cells, which express heart FABP (H-FABP), utilize oxygenated fatty acids for regulation of vascular tone. Therefore, the relative affinities of H-FABP for several isomeric series of these compounds were measured by fluorescent displacement of 1-anilinonaphthalene-8-sulfonic acid (ANS). In general, H-FABP rank order affinities (arachidonic acid > EETs > HETEs > DHETs) paralleled reversed-phase high-performance liquid chromatography retention times, indicating that the differences in H-FABP affinity were determined largely by polarity. H-FABP displayed a similar rank order of affinity for compounds derived from linoleic acid. H-FABP affinity for 20-HETE [apparent dissociation constant (K(d)') of 0.44 microM] was much greater than expected from its polarity, indicating unique binding interactions for this HETE. H-FABP affinity for 5,6-EET and 11,12-EET (K(d)' of approximately 0.4 microM) was approximately 20-fold greater than for DHETs (K(d)' of approximately 8 microM). The homologous proteins, liver FABP and intestinal FABP, also displayed selective affinity for EET versus DHET. Thus, FABP binding of EETs may facilitate their intracellular retention whereas the lack of FABP affinity for DHETs may partially explain their release from cells. The affinity of H-FABP for EETs suggests that this family of intracellular proteins may modulate the metabolism, activities, and targeting of these potent eicosanoid biomediators.