Interactions between Human Liver Fatty Acid Binding Protein and Peroxisome Proliferator Activated Receptor Selective Drugs

Disclosing Environmental Ligands of L-FABP and PPARγ: Should We Re-evaluate the Chemical Safety of Hydrocarbon Surfactants?

Chemical contaminants can cause adverse effects by binding to the liver-fatty acid binding protein (L-FABP) and peroxisome proliferator-activated nuclear receptor γ (PPARγ), which are vital in lipid metabolism. However, the presence of numerous compounds in the environment has hindered the identification of their ligands, and thus only a small portion have been discovered to date. In this study, protein Affinity Purification with Nontargeted Analysis (APNA) was employed to identify the ligands of L-FABP and PPARγ in indoor dust and sewage sludge. A total of 83 nonredundant features were pulled-out by His-tagged L-FABP as putative ligands, among which 13 were assigned as fatty acids and hydrocarbon surfactants. In contrast, only six features were isolated when His-tagged PPARγ LBD was used as the protein bait. The binding of hydrocarbon surfactants to L-FABP and PPARγ was confirmed using both recombinant proteins and reporter cells. These hydrocarbon surfactants, along with >50 homologues and isomers, were detected in dust and sludge at high concentrations. Fatty acids and hydrocarbon surfactants explained the majority of L-FABP (57.7 ± 32.9%) and PPARγ (66.0 ± 27.1%) activities in the sludge. This study revealed hydrocarbon surfactants as the predominant synthetic ligands of L-FABP and PPARγ, highlighting the importance of re-evaluating their chemical safety.

Impact of Intracellular Lipid Binding Proteins on Endogenous and Xenobiotic Ligand Metabolism and Disposition

The family of intracellular lipid binding proteins (iLBPs) is comprised of 16 members of structurally related binding proteins that have ubiquitous tissue expression in humans. iLBPs collectively bind diverse essential endogenous lipids and xenobiotics. iLBPs solubilize and traffic lipophilic ligands through the aqueous milieu of the cell. Their expression is correlated with increased rates of ligand uptake into tissues and altered ligand metabolism. The importance of iLBPs in maintaining lipid homeostasis is well established. Fatty acid binding proteins (FABPs) make up the majority of iLBPs and are expressed in major organs relevant to xenobiotic absorption, distribution, and metabolism. FABPs bind a variety of xenobiotics including nonsteroidal anti-inflammatory drugs, psychoactive cannabinoids, benzodiazepines, antinociceptives, and peroxisome proliferators. FABP function is also associated with metabolic disease, making FABPs currently a target for drug development. Yet the potential contribution of FABP binding to distribution of xenobiotics into tissues and the mechanistic impact iLBPs may have on xenobiotic metabolism are largely undefined. This review examines the tissue-specific expression and functions of iLBPs, the ligand binding characteristics of iLBPs, their known endogenous and xenobiotic ligands, methods for measuring ligand binding, and mechanisms of ligand delivery from iLBPs to membranes and enzymes. Current knowledge of the importance of iLBPs in affecting disposition of xenobiotics is collectively described. SIGNIFICANCE STATEMENT: The data reviewed here show that FABPs bind many drugs and suggest that binding of drugs to FABPs in various tissues will affect drug distribution into tissues. The extensive work and findings with endogenous ligands suggest that FABPs may also alter the metabolism and transport of drugs. This review illustrates the potential significance of this understudied area.

Regulation of Autophagy via Carbohydrate and Lipid Metabolism in Cancer

Metabolic changes are an important component of tumor cell progression. Tumor cells adapt to environmental stresses via changes to carbohydrate and lipid metabolism. Autophagy, a physiological process in mammalian cells that digests damaged organelles and misfolded proteins via lysosomal degradation, is closely associated with metabolism in mammalian cells, acting as a meter of cellular ATP levels. In this review, we discuss the changes in glycolytic and lipid biosynthetic pathways in mammalian cells and their impact on carcinogenesis via the autophagy pathway. In addition, we discuss the impact of these metabolic pathways on autophagy in lung cancer.

Current, emerging, and potential therapies for non-alcoholic steatohepatitis

Non-alcoholic fatty liver disease (NAFLD) has been identified as the most common chronic liver disease worldwide, with a growing incidence. NAFLD is considered the hepatic manifestation of a metabolic syndrome that emerges from multiple factors (e.g., oxidative stress, metabolic disorders, endoplasmic reticulum stress, cell death, and inflammation). Non-alcoholic steatohepatitis (NASH), an advanced form of NAFLD, has been reported to be a leading cause of cirrhosis and hepatic carcinoma, and it is progressing rapidly. Since there is no approved pharmacotherapy for NASH, a considerable number of therapeutic targets have emerged with the deepening of the research on NASH pathogenesis. In this study, the therapeutic potential and properties of regulating metabolism, the gut microbiome, antioxidant, microRNA, inhibiting apoptosis, targeting ferroptosis, and stem cell-based therapy in NASH are reviewed and evaluated. Since the single-drug treatment of NASH is affected by individual heterogeneous responses and side effects, it is imperative to precisely carry out targeted therapy with low toxicity. Lastly, targeted therapeutic agent delivery based on exosomes is proposed in this study, such that drugs with different mechanisms can be incorporated to generate high-efficiency and low-toxicity individualized medicine.

Dihydroartemisinin engages liver fatty acid binding protein and suppresses metastatic hepatocellular carcinoma growth

Dihydroartemisinin non-covalently binds liver fatty acid binding protein (FABP1) with micromolar affinity, acts as a FABP1-dependent peroxisome proliferator-activated receptor alpha agonist and inhibits metastatic hepatocellular carcinoma growth.

Plasma levels and tissue expression of liver-type fatty acid-binding protein in patients with breast cancer

BACKGROUND

Liver-type fatty acid-binding protein (L-FABP) is widely expressed in hepatocytes and plays a role in lipid metabolism. It has been demonstrated to be overexpressed in different types of cancer; however, few studies have investigated the association between L-FABP and breast cancer. The aim of this study was to assess the association between plasma concentrations of L-FABP in breast cancer patients and the expression of L-FABP in breast cancer tissue.

METHOD

A total of 196 patients with breast cancer and 57 age-matched control subjects were studied. Plasma L-FABP concentrations were measured using ELISA in both groups. The expression of L-FABP in breast cancer tissue was examined using immunohistochemistry.

RESULT

The patients had higher plasma L-FABP levels than the controls (7.6 ng/mL (interquartile range 5.2-12.1) vs. 6.3 ng/mL (interquartile range 5.3-8.5), p = 0.008). Multiple logistic regression analysis showed an independent association between L-FABP and breast cancer, even after adjusting for known biomarkers. Moreover, the rates of pathologic stage T2+T3+T4, clinical stage III, positive HER-2 receptor status, and negative estrogen receptor status were significantly higher in the patients with an L-FABP level greater than the median. Furthermore, the L-FABP level gradually increased with the increasing stage. In addition, L-FABP was detected in the cytoplasm, nuclear, or both cytoplasm and nuclear of all breast cancer tissue examined, not in the normal tissue.

CONCLUSIONS

Plasma L-FABP levels were significantly higher in the patients with breast cancer than in the controls. In addition, L-FABP was expressed in breast cancer tissue, which suggests that L-FABP may be involved in the pathogenesis of breast cancer.

Fatty acid binding protein is required for chitin biosynthesis in the wing of Drosophila melanogaster

Chitin, the major structural polysaccharide in arthropods such as insects and mites, is a linear polymer of N-acetylglucosamine units. The growth and development of insects are intimately coupled with chitin biosynthesis. The membrane-bound β-glycosyltransferase chitin synthase is known to catalyze the key polymerization step of N-acetylglucosamine. However, the additional proteins that might assist chitin synthase during chitin biosynthesis are not well understood. Recently, fatty acid binding protein (Fabp) has been suggested as a candidate that interacts with the chitin synthase Krotzkopf verkehrt (Kkv) in Drosophila melanogaster. Here, using split-ubiquitin membrane yeast two-hybrid and pull-down assays, we have demonstrated that the Fabp-B splice variant physically interacts with Kkv in vitro. The global knockdown of Fabp in D. melanogaster using RNA interference (RNAi) induced lethality at the larval stage. Moreover, in tissue-specific RNAi experiments, silenced Fabp expression in the epidermis and tracheal system caused a lethal larval phenotype. Fabp knockdown in the wings resulted in an abnormal wing development and uneven cuticular surface. In addition to reducing the chitin content in the first longitudinal vein of wings, Fabp silencing also caused the loss of procuticle laminate structures. This study revealed that Fabp plays an important role in chitin synthesis and contributes to a comprehensive understanding of the complex insect chitin biosynthesis.

Cellular retinoid-binding proteins transfer retinoids to human cytochrome P450 27C1 for desaturation

Cytochrome P450 27C1 (P450 27C1) is a retinoid desaturase expressed in the skin that catalyzes the formation of 3,4-dehydroretinoids from all-trans retinoids. Within the skin, retinoids are important regulators of proliferation and differentiation. In vivo, retinoids are bound to cellular retinol-binding proteins (CRBPs) and cellular retinoic acid–binding proteins (CRABPs). Interaction with these binding proteins is a defining characteristic of physiologically relevant enzymes in retinoid metabolism. Previous studies that characterized the catalytic activity of human P450 27C1 utilized a reconstituted in vitro system with free retinoids. However, it was unknown whether P450 27C1 could directly interact with holo-retinoid-binding proteins to receive all-trans retinoid substrates. To assess this, steady-state kinetic assays were conducted with free all-trans retinoids and holo-CRBP-1, holo-CRABP-1, and holo-CRABP-2. For holo-CRBP-1 and holo-CRABP-2, the k_cat/K_m values either decreased 5-fold or were equal to the respective free retinoid values. The k_cat/K_m value for holo-CRABP-1, however, decreased ∼65-fold in comparison with reactions with free all-trans retinoic acid. These results suggest that P450 27C1 directly accepts all-trans retinol and retinaldehyde from CRBP-1 and all-trans retinoic acid from CRABP-2, but not from CRABP-1. A difference in substrate channeling between CRABP-1 and CRABP-2 was also supported by isotope dilution experiments. Analysis of retinoid transfer from holo-CRABPs to P450 27C1 suggests that the decrease in k_cat observed in steady-state kinetic assays is due to retinoid transfer becoming rate-limiting in the P450 27C1 catalytic cycle. Overall, these results illustrate that, like the CYP26 enzymes involved in retinoic acid metabolism, P450 27C1 interacts with cellular retinoid-binding proteins.

FABP3 Deficiency Exacerbates Metabolic Derangement in Cardiac Hypertrophy and Heart Failure via PPARα Pathway

Background: Cardiac hypertrophy was accompanied by various cardiovascular diseases (CVDs), and due to the high global incidence and mortality of CVDs, it has become increasingly critical to characterize the pathogenesis of cardiac hypertrophy. We aimed to determine the metabolic roles of fatty acid binding protein 3 (FABP3) on transverse aortic constriction (TAC)-induced cardiac hypertrophy. Methods and Results: Transverse aortic constriction or Ang II treatment markedly upregulated Fabp3 expression. Notably, Fabp3 ablation aggravated TAC-induced cardiac hypertrophy and cardiac dysfunction. Multi-omics analysis revealed that Fabp3-deficient hearts exhibited disrupted metabolic signatures characterized by increased glycolysis, toxic lipid accumulation, and compromised fatty acid oxidation and ATP production under hypertrophic stimuli. Mechanistically, FABP3 mediated metabolic reprogramming by directly interacting with PPARα, which prevented its degradation and synergistically modulated its transcriptional activity on Mlycd and Gck. Finally, treatment with the PPARα agonist, fenofibrate, rescued the pro-hypertrophic effects of Fabp3 deficiency. Conclusions: Collectively, these findings reveal the indispensable roles of the FABP3-PPARα axis on metabolic homeostasis and the development of hypertrophy, which sheds new light on the treatment of hypertrophy.

Protective effect of surfactin on copper sulfate-induced inflammation, oxidative stress, and hepatic injury in zebrafish

Surfactin, an antibacterial peptide, produced by various Bacillus subtilis strains, have broad-spectrum antibacterial and immune-enhancing functions. In this study, we investigated the anti-inflammatory, antioxidant, and hepatoprotective effect of surfactin on zebrafish (Danio rerio) larvae following their exposure to copper sulfate (CuSO₄ ). The mature AB wild-type and a transgenic line of zebrafish larvae that expressed enhanced GFP (EGFP) named Tg (Lyz:EGFP) were exposed to 0, 20, 40, and 60 μg/mL surfactin after incubation with 3.2 μg/mL CuSO₄ for 2 h from 72 h postfertilization (hpf). Different endpoints, such as migration of GFP-labeled neutrophils, analysis of inflammatory cytokines and transaminases, markers of oxidation, expression of certain genes, and histological changes of liver, were studied to evaluate the function of surfactin. The protein expression levels of NF-κBp65, TNF-α, cyclooxygenase-2 (COX-2), and iNOS were determined in murine macrophage RAW 264.7 cells by western blotting. Our results show that surfactin reduced migration of neutrophils and relieved hepatic injury. In addition, surfactin reduced the index levels of inflammatory factors, oxidative stress response, and improved hepatic function. Surfactin also significantly inhibited the expression of IL-1β, IL-8, TNF-α, nitric oxide, NF-κBp65, COX-2, and iNOS, and increased the expression of IL-10. Thus, our results demonstrate that surfactin has anti-inflammatory, antioxidant, and hepatoprotective activities. Surfactin has potential as a novel inflammation and immune adjustment.

Characterization of a fatty acid-binding protein from the Pacific oyster (Crassostrea gigas): pharmaceutical and toxicological implications

Pharmaceuticals and their metabolites constitute a class of xenobiotics commonly found in aquatic environments which may cause toxic effects in aquatic organisms. Several different lipophilic molecules, including some pharmaceuticals, can bind to fatty acid-binding proteins (FABPs), a group of evolutionarily related cytoplasmic proteins that belong to the intracellular lipid-binding protein (iLBP) family. An oyster FABP genome-wide investigation was not available until a recent study on gene organization, protein structure, and phylogeny of Crassostrea gigas iLBPs. Higher transcript levels of the C. gigas FABP2 gene were found after exposure to sewage and pharmaceuticals. Because of its relevance as a potential biomarker of aquatic contamination, in this study, recombinant FABP2 from C. gigas (CgFABP2) was successfully cloned, expressed, and purified, and in vitro and in silico assays were performed using lipids and pharmaceuticals. This is the first characterization of a protein from the iLBP family in C. gigas. Homology modeling and molecular docking were used to evaluate the binding affinities of natural ligands (palmitic, oleic, and arachidonic acids) and pharmaceuticals (ibuprofen, sodium diclofenac, and acetaminophen). Among the tested fatty acids, CgFABP2 showed preference for palmitic acid. The selected pharmaceuticals presented a biphasic-binding mode, suggesting a different binding affinity with a preference for diclofenac. Therefore, the approach using circular dichroism and in silico data might be useful for ligand-binding screening in an invertebrate model organism.

Utilizing systems biology to reveal cellular responses to peroxisome proliferator-activated receptor γ ligand exposure

•

Human (HepG2) cells were exposed to PPARγ ligands to induce systems-level effects.

•

Ciglitazone decreases HepG2 cell viability while GW 9662 had no effect.

•

Ciglitazone and GW 9662 increase neutral lipids as a function of concentration.

•

Cholesterol biosynthesis transcripts are affected by ciglitazone and GW 9662.

•

Ciglitazone alters lipid profiles but GW 9662 was similar to vehicle-exposed cells.

Peroxisome proliferator-activated receptor γ (PPARγ) is a nuclear receptor that, upon activation by ligands, heterodimerizes with retinoid X receptor (RXR), binds to PPAR response elements (PPREs), and activates transcription of downstream genes. As PPARγ plays a central role in adipogenesis, fatty acid storage, and glucose metabolism, PPARγ-specific pharmaceuticals (e.g., thiazolidinediones) have been developed to treat Type II diabetes and obesity within human populations. However, to our knowledge, no prior studies have concurrently assessed the effects of PPARγ ligand exposure on genome-wide PPARγ binding as well as effects on the transcriptome and lipidome within human cells at biologically active, non-cytotoxic concentrations. In addition to quantifying concentration-dependent effects of ciglitazone (a reference PPARγ agonist) and GW 9662 (a reference PPARγ antagonist) on human hepatocarcinoma (HepG2) cell viability, PPARγ abundance in situ, and neutral lipids, HepG2 cells were exposed to either vehicle (0.1% DMSO), ciglitazone, or GW 9662 for up to 24 h, and then harvested for 1) chromatin immunoprecipitation-sequencing (ChIP-seq) to identify PPARγ-bound regions across the entire genome, 2) mRNA-sequencing (mRNA-seq) to identify potential impacts on the transcriptome, and 3) lipidomics to identify potential alterations in lipid profiles. Following exposure to ciglitazone and GW 9662, we found that PPARγ levels were not significantly different after 2–8 h of exposure. While ciglitazone and GW 9662 resulted in a concentration-dependent increase in neutral lipids, the magnitude and localization of PPARγ-bound regions across the genome (as identified by ChIP-seq) did not vary by treatment. However, mRNA-seq and lipidomics revealed that exposure of HepG2 cells to ciglitazone and GW 9662 resulted in significant, treatment-specific effects on the transcriptome and lipidome. Overall, our findings suggest that exposure of human cells to PPARγ ligands at biologically active, non-cytotoxic concentrations results in toxicity that may be driven by a combination of both PPARγ-dependent and PPARγ-independent mechanisms.

From Congenital Disorders of Fat Malabsorption to Understanding Intra-Enterocyte Mechanisms Behind Chylomicron Assembly and Secretion

During the last two decades, a large body of information on the events responsible for intestinal fat digestion and absorption has been accumulated. In particular, many groups have extensively focused on the absorptive phase in order to highlight the critical "players" and the main mechanisms orchestrating the assembly and secretion of chylomicrons (CM) as essential vehicles of alimentary lipids. The major aim of this article is to review understanding derived from basic science and clinical conditions associated with impaired packaging and export of CM. We have particularly insisted on inborn metabolic pathways in humans as well as on genetically modified animal models (recapitulating pathological features). The ultimate goal of this approach is that "experiments of nature" and in vivo model strategy collectively allow gaining novel mechanistic insight and filling the gap between the underlying genetic defect and the apparent clinical phenotype. Thus, uncovering the cause of disease contributes not only to understanding normal physiologic pathway, but also to capturing disorder onset, progression, treatment and prognosis.

Current treatment paradigms and emerging therapies for NAFLD/NASH

Non-alcoholic fatty liver disease (NAFLD) is one the fastest emerging manifestations of the metabolic syndrome worldwide. Non-alcoholic steatohepatitis (NASH), the progressive form of NAFLD, may culminate into cirrhosis and hepatocellular cancer (HCC) and is presently a leading cause of liver transplant. Although a steady progress is seen in understanding of the disease epidemiology, pathogenesis and identifying therapeutic targets, the slowest advancement is seen in the therapeutic field. Currently, there is no FDA approved therapy for this disease and appropriate therapeutic targets are urgently warranted. In this review we discuss the role of lifestyle intervention, pharmacological agents, surgical approaches, and gut microbiome, with regard to therapy for NASH. In particular, we focus the role of insulin sensitizers, thyroid hormone mimetics, antioxidants, cholesterol lowering drugs, incretins and cytokines as therapeutic targets for NASH. We highlight these targets aiming to optimize the future for NASH therapy.

Small-Molecule Modulation of PPARs for the Treatment of Prevalent Vascular Retinal Diseases

Vascular-related retinal diseases dramatically impact quality of life and create a substantial burden on the healthcare system. Age-related macular degeneration, diabetic retinopathy, and retinopathy of prematurity are leading causes of irreversible blindness. In recent years, the scientific community has made great progress in understanding the pathology of these diseases and recent discoveries have identified promising new treatment strategies. Specifically, compelling biochemical and clinical evidence is arising that small-molecule modulation of peroxisome proliferator-activated receptors (PPARs) represents a promising approach to simultaneously address many of the pathological drivers of these vascular-related retinal diseases. This has excited academic and pharmaceutical researchers towards developing new and potent PPAR ligands. This review highlights recent developments in PPAR ligand discovery and discusses the downstream effects of targeting PPARs as a therapeutic approach to treating retinal vascular diseases.

Identification of a non-classical three-dimensional nuclear localization signal in the intestinal fatty acid binding protein

The intestinal fatty acid binding protein (FABP) is a small protein expressed along the small intestine that bind long-chain fatty acids and other hydrophobic ligands. Several lines of evidence suggest that, once in the nucleus, it interacts with nuclear receptors, activating them and thus transferring the bound ligand into the nucleus. Previous work by our group suggests that FABP2 would participate in the cytoplasm-nucleus translocation of fatty acids. Because the consensus NLS is absent in the sequence of FABP2, we propose that a 3D signal could be responsible for its nuclear translocation. The results obtained by transfection assays of recombinant wild type and mutated forms of Danio rerio Fabp2 in Caco-2 cell cultures, showed that lysine 17, arginine 29 and lysine 30 residues, which are located in the helix-turn-helix region, would constitute a functional non-classical three-dimensional NLS.

Signaling and other functions of lipids in autophagy: a review

The process of autophagy is integral to cellular function. In this process, proteins, organelles, and metabolites are engulfed in a lipid vesicle and trafficked to a lysosome for degradation. Its central role in protein and organelle homeostasis has piqued interest for autophagy dysfunction as a driver of pathology for a number of diseases including cancer, muscular disorders, neurological disorders, and non-alcoholic fatty liver disease. For much of its history, the study of autophagy has centered around proteins, however, due to advances in mass spectrometry and refined methodologies, the role of lipids in this essential cellular process has become more apparent. This review discusses the diverse endogenous lipid compounds shown to mediate autophagy. Downstream lipid signaling pathways are also reviewed in the context of autophagy regulation. Specific focus is placed upon the Mammalian Target of Rapamycin (mTOR) and Peroxisome Proliferator-Activated Receptor (PPAR) signaling pathways as integration hubs for lipid regulation of autophagy.

Nontarget Screening of Per- and Polyfluoroalkyl Substances Binding to Human Liver Fatty Acid Binding Protein

More than 1000 per- and polyfluoroalkyl substances (PFASs) have been discovered by nontarget analysis (NTA), but their prioritization for health concerns is challenging. We developed a method by incorporating size-exclusion column co-elution (SECC) and NTA, to screen PFASs binding to human liver fatty acid binding protein (hL-FABP). Of 74 PFASs assessed, 20 were identified as hL-FABP ligands in which eight of them have high binding affinities. Increased PFAS binding affinities correlate with stronger responses in electrospray ionization (ESI^-) and longer retention times on a C18 column. This is well explained by a mechanistic model, which revealed that both polar and hydrophobic interactions are crucial for binding affinities. Encouraged by this, we then developed an SECC method to identify hL-FABP ligands, and all eight high-affinity ligands were selectively captured from 74 PFASs. The method was further applied to an aqueous film-forming foam (AFFF) product in which 31 new hL-FABP ligands were identified. Suspect and nontargeted screening revealed these ligands as analogues of perfluorosulfonic acids and homologues of alkyl ether sulfates (C₈- and C₁₀/EO_n, C₈H₁₇(C₂H₄O)_nSO₄^-, and C₁₀H₂₁(C₂H₄O)_nSO₄^-). The SECC method was then applied to AFFF-contaminated surface waters. In addition to perfluorooctanesulfonic acid and perfluorohexanesulfonic acid, eight other AFFF chemicals were discovered as novel ligands, including four C₁₄- and C₁₅/EO_n. This study implemented a high-throughput method to prioritize PFASs and revealed the existence of many previously unknown hL-FABP ligands.

Two fatty acid-binding proteins expressed in the intestine interact differently with endocannabinoids

Two different members of the fatty acid-binding protein (FABP) family are found in enterocyte cells of the gastrointestinal system, namely liver-type and intestinal fatty acid-binding proteins (LFABP and IFABP, also called FABP1 and FABP2, respectively). Striking phenotypic differences have been observed in knockout mice for either protein, for example, high fat-fed IFABP-null mice remained lean, whereas LFABP-null mice were obese, correlating with differences in food intake. This finding prompted us to investigate the role each protein plays in directing the specificity of binding to ligands involved in appetite regulation, such as fatty acid ethanolamides and related endocannabinoids. We determined the binding affinities for nine structurally related ligands using a fluorescence competition assay, revealing tighter binding to IFABP than LFABP for all ligands tested. We found that the head group of the ligand had more impact on binding affinity than the alkyl chain, with the strongest binding observed for the carboxyl group, followed by the amide, and then the glycerol ester. These trends were confirmed using two-dimensional ¹ H-¹⁵ N nuclear magnetic resonance (NMR) to monitor chemical shift perturbation of the protein backbone resonances upon titration with ligand. Interestingly, the NMR data revealed that different residues of IFABP were involved in the coordination of endocannabinoids than those implicated for fatty acids, whereas the same residues of LFABP were involved for both classes of ligand. In addition, we identified residues that are uniquely affected by binding of all types of ligand to IFABP, suggesting a rationale for its tighter binding affinity compared with LFABP.

High Glucose and Liver Fatty Acid Binding Protein Gene Ablation Differentially Impact Whole Body and Liver Phenotype in High-Fat Pair-Fed Mice

Ad libitum-fed diets high in fat and carbohydrate (especially fructose) induce weight gain, obesity, and nonalcoholic fatty liver disease (NAFLD) in humans and animal models. However, interpretation is complicated since ad libitum feeding of such diets induces hyperphagia and upregulates expression of liver fatty acid binding protein (L-FABP)-a protein intimately involved in fatty acid and glucose regulation of lipid metabolism. Wild-type (WT) and L-fabp gene ablated (LKO) mice were pair-fed either high-fat diet (HFD) or high-fat/high-glucose diet (HFGD) wherein total carbohydrate was maintained constant but the proportion of glucose was increased at the expense of fructose. In LKO mice, the pair-fed HFD increased body weight and lean tissue mass (LTM) but had no effect on fat tissue mass (FTM) or hepatic fatty vacuolation as compared to pair-fed WT counterparts. These LKO mice exhibited upregulation of hepatic proteins in fatty acid uptake and cytosolic transport (caveolin and sterol carrier protein-2), but lower hepatic fatty acid oxidation (decreased serum β-hydroxybutyrate). LKO mice pair-fed HFGD also exhibited increased body weight; however, these mice had increased FTM, not LTM, and increased hepatic fatty vacuolation as compared to pair-fed WT counterparts. These LKO mice also exhibited upregulation of hepatic proteins in fatty acid uptake and cytosolic transport (caveolin and acyl-CoA binding protein, but not sterol carrier protein-2), but there was no change in hepatic fatty acid oxidation (serum β-hydroxybutyrate) as compared to pair-fed WT counterparts.

Evaluation of the Developmental Toxicity Induced by E804 in Zebrafish Embryos

E804, a derivative of indirubin, have multi-biological activities such as anticancer and anti-inflammatory activities, but little is known about its developmental toxicity. In this study, we investigated the toxicity of E804 on the developments of zebrafish embryos. Our results showed that E804 treatment caused a significant increase of the malformation rate compared with the control groups. Pericardial edema and curved body shape were the most morphological abnormalities observed in E804-treated group. The hatching rates and body length of the zebrafish larvae was significantly decreased in E804-treated groups. E804 also affect the development of heart, liver, phagocytes and vascular formation. Further studies showed that the level of reactive oxygen species was significantly increased. The activity of total superoxide dismutase decreased and the concentration of malondialdehyde were increased. Much more apoptotic cells were detected in E804-treated group, compared with the control. In addition, gene-expression results showed that the pathways of oxidative stress and apoptosis were provoked in E804 treated groups. Taken together, our findings will be helpful to understanding E804-induced developmental toxicity and the underlying mechanism.

Effect of liver fatty acid binding protein (L-FABP) gene ablation on lipid metabolism in high glucose diet (HGD) pair-fed mice

Liver fatty acid binding protein (L-FABP) is the major fatty acid binding/"chaperone" protein in hepatic cytosol. Although fatty acids can be derived from the breakdown of dietary fat and glucose, relatively little is known regarding the impact of L-FABP on phenotype in the context of high dietary glucose. Potential impact was examined in wild-type (WT) and Lfabp gene ablated (LKO) female mice fed either a control or pair-fed high glucose diet (HGD). WT mice fed HGD alone exhibited decreased whole body weight gain and weight gain/kcal food consumed-both as reduced lean tissue mass (LTM) and fat tissue mass (FTM). Conversely, LKO alone increased weight gain, lean tissue mass, and fat tissue mass while decreasing serum β-hydroxybutyrate (indicative of hepatic fatty acid oxidation)-regardless of diet. Both LKO alone and HGD alone significantly altered the serum lipoprotein profile and increased triacylglycerol (TG), but in HGD mice the LKO did not further exacerbate serum TG content. HGD had little effect on hepatic lipid composition in WT mice, but prevented the LKO-induced selective increase in hepatic phospholipid, free-cholesterol and cholesteryl-ester. Taken together, these findings suggest that high glucose diet diminished the effects of LKO on the whole body and lipid phenotype of these mice.

Human Liver Fatty Acid Binding Protein-1 T94A Variant, Nonalcohol Fatty Liver Disease, and Hepatic Endocannabinoid System

Hepatic endocannabinoids (EC) and their major binding/"chaperone" protein (i.e., liver fatty acid binding protein-1 [FABP1]) are associated with development of nonalcoholic fatty liver (NAFLD) in animal models and humans. Since expression of the highly prevalent human FABP1 T94A variant induces serum lipid accumulation, it is important to determine its impact on hepatic lipid accumulation and the EC system. This issue was addressed in livers from human subjects expressing only wild-type (WT) FABP1 T94T (TT genotype) or T94A variant (TC or CC genotype). WT FABP1 males had lower total lipids (both neutral cholesteryl esters, triacylglycerols) and phospholipids than females. WT FABP1 males' lower lipids correlated with lower levels of the N-acylethanolamide DHEA and 2-monoacylglycerols (2-MAG) (2-OG, 2-PG). T94A expression in males increased the hepatic total lipids (triacylglycerol, cholesteryl ester), which is consistent with their higher level of CB1-potentiating 2-OG and lower antagonistic EPEA. In contrast, in females, T94A expression did not alter the total lipids, neutral lipids, or phospholipids, which is attributable to the higher cannabinoid receptor-1 (CB1) agonist arachidonoylethanolamide (AEA) and its CB1-potentiator OEA being largely offset by reduced potentiating 2-OG and increased antagonistic EPEA. Taken together, these findings indicate that T94A-induced alterations in the hepatic EC system contribute at least in part to the hepatic accumulation of lipids associated with NAFLD, especially in males.

Structural and Functional Interaction of Δ9-Tetrahydrocannabinol with Liver Fatty Acid Binding Protein (FABP1)

Although serum Δ⁹-tetrahydrocannabinol (Δ⁹-THC) undergoes rapid hepatic clearance and metabolism, almost nothing is known regarding the mechanism(s) whereby this highly lipophilic phytocannabinoid is transported for metabolism/excretion. A novel NBD-arachidonoylethanolamide (NBD-AEA) fluorescence displacement assay showed that liver fatty acid binding protein (FABP1), the major hepatic endocannabinoid (EC) binding protein, binds the first major metabolite of Δ⁹-THC (Δ⁹-THC-OH) as well as Δ⁹-THC itself. Circular dichroism (CD) confirmed that not only Δ⁹-THC and Δ⁹-THC-OH but also downstream metabolites Δ⁹-THC-COOH and Δ⁹-THC-CO-glucuronide directly interact with FABP1. Δ⁹-THC and metabolite interaction differentially altered the FABP1 secondary structure, increasing total α-helix (all), decreasing total β-sheet (Δ⁹-THC-COOH, Δ⁹-THC-CO-glucuronide), increasing turns (Δ⁹-THC-OH, Δ⁹-THC-COOH, Δ⁹-THC-CO-glucuronide), and decreasing unordered structure (Δ⁹-THC, Δ⁹-THC-OH). Cultured primary hepatocytes from wild-type (WT) mice took up and converted Δ⁹-THC to the above metabolites. Fabp1 gene ablation (LKO) dramatically increased hepatocyte accumulation of Δ⁹-THC and even more so its primary metabolites Δ⁹-THC-OH and Δ⁹-THC-COOH. Concomitantly, rtPCR and Western blotting indicated that LKO significantly increased Δ⁹-THC's ability to regulate downstream nuclear receptor transcription of genes important in both EC ( Napepld > Daglb > Dagla, Naaa, Cnr1) and lipid ( Cpt1A > Fasn, FATP4) metabolism. Taken together, the data indicated that FABP1 may play important roles in Δ⁹-THC uptake and elimination as well as Δ⁹-THC induction of genes regulating hepatic EC levels and downstream targets in lipid metabolism.

Impact of Fabp1 Gene Ablation on Uptake and Degradation of Endocannabinoids in Mouse Hepatocytes

Liver fatty-acid-binding protein (FABP1, L-FABP) is the major cytosolic binding/chaperone protein for both precursor arachidonic acid (ARA) and the endocannabinoid (EC) products N-arachidonoylethanolamine (AEA) and 2-arachidonoylglycerol (2-AG). Although FABP1 regulates hepatic uptake and metabolism of ARA, almost nothing is known regarding FABP1's impact on AEA and 2-AG uptake, intracellular distribution, and targeting of AEA and 2-AG to degradative hepatic enzymes. In vitro assays revealed that FABP1 considerably enhanced monoacylglycerol lipase hydrolysis of 2-AG but only modestly enhanced AEA hydrolysis by fatty-acid amide hydrolase. Conversely, liquid chromatography-mass spectrometry of lipids from Fabp1 gene-ablated (LKO) hepatocytes confirmed that loss of FABP1 markedly diminished hydrolysis of 2-AG. Furthermore, the real-time imaging of novel fluorescent NBD-labeled probes (NBD-AEA, NBD-2-AG, and NBD-ARA) resolved FABP1's impact on uptake vs intracellular targeting/hydrolysis. FABP1 bound NBD-ARA with 2:1 stoichiometry analogous to ARA, but bound NBD-2-AG and NBD-AEA with 1:1 stoichiometry-apparently at different sites in FABP1's binding cavity. All three probes were taken up, but NBD-2-AG and NBD-AEA were targeted to lipid droplets. LKO reduced the uptake of NBD-ARA as expected, significantly enhanced that of NBD-AEA, but had little effect on NBD-2-AG. These data indicated that FABP1 impacts hepatocyte EC levels by binding EC and differentially impacts their intracellular hydrolysis (2-AG) and uptake (AEA).

Scp-2/Scp-x ablation in Fabp1 null mice differentially impacts hepatic endocannabinoid level depending on dietary fat

Dysregulation of the hepatic endocannabinoid (EC) system and high fat diet (HFD) are associated with non-alcoholic fatty liver disease. Liver cytosol contains high levels of two novel endocannabinoid binding proteins-liver fatty acid binding protein (FABP1) and sterol carrier protein-2 (SCP-2). While Fabp1 gene ablation significantly increases hepatic levels of arachidonic acid (ARA)-containing EC and sex-dependent response to pair-fed high fat diet (HFD), the presence of SCP-2 complicates interpretation. These issues were addressed by ablating Scp-2/Scp-x in Fabp1 null mice (TKO). In control-fed mice, TKO increased hepatic levels of arachidonoylethanolamide (AEA) in both sexes. HFD impacted hepatic EC levels by decreasing AEA in TKO females and decreasing 2-arachidonoyl glycerol (2-AG) in WT of both sexes. Only TKO males on HFD had increased hepatic 2-AG levels. Hepatic ARA levels were decreased in control-fed TKO of both sexes. Changes in hepatic AEA/2-AG levels were not associated with altered amounts of hepatic proteins involved in AEA/2-AG synthesis or degradation. These findings suggested that ablation of the Scp-2/Scp-x gene in Fabp1 null mice exacerbated hepatic EC accumulation and antagonized the impact of HFD on hepatic EC levels-suggesting both proteins play important roles in regulating the hepatic EC system.

Ablating both Fabp1 and Scp2/Scpx (TKO) induces hepatic phospholipid and cholesterol accumulation in high fat-fed mice

Although singly ablating Fabp1 or Scp2/Scpx genes may exacerbate the impact of high fat diet (HFD) on whole body phenotype and non-alcoholic fatty liver disease (NAFLD), concomitant upregulation of the non-ablated gene, preference for ad libitum fed HFD, and sex differences complicate interpretation. Therefore, these issues were addressed in male and female mice ablated in both genes (Fabp1/Scp2/Scpx null or TKO) and pair-fed HFD. Wild-type (WT) males gained more body weight as fat tissue mass (FTM) and exhibited higher hepatic lipid accumulation than WT females. The greater hepatic lipid accumulation in WT males was associated with higher hepatic expression of enzymes in glyceride synthesis, higher hepatic bile acids, and upregulation of transporters involved in hepatic reuptake of serum bile acids. While TKO had little effect on whole body phenotype and hepatic bile acid accumulation in either sex, TKO increased hepatic accumulation of lipids in both, specifically phospholipid and cholesteryl esters in males and females and free cholesterol in females. TKO-induced increases in glycerides were attributed not only to complete loss of FABP1, SCP2 and SCPx, but also in part to sex-dependent upregulation of hepatic lipogenic enzymes. These data with WT and TKO mice pair-fed HFD indicate that: i) Sex significantly impacted the ability of HFD to increase body weight, induce hepatic lipid accumulation and increase hepatic bile acids; and ii) TKO exacerbated the HFD ability to induce hepatic lipid accumulation, regardless of sex, but did not significantly alter whole body phenotype in either sex.

Effect of Fabp1/Scp-2/Scp-x Ablation on Whole Body and Hepatic Phenotype of Phytol-Fed Male Mice

Liver fatty acid binding protein (Fabp1) and sterol carrier protein-2/sterol carrier protein-x (SCP-2/SCP-x) genes encode proteins that enhance hepatic uptake, cytosolic transport, and peroxisomal oxidation of toxic branched-chain fatty acids derived from dietary phytol. Since male wild-type (WT) mice express markedly higher levels of these proteins than females, the impact of ablating both genes (TKO) was examined in phytol-fed males. In WT males, high phytol diet alone had little impact on whole body weight and did not alter the proportion of lean tissue mass (LTM) versus fat tissue mass (FTM). TKO conferred on dietary phytol the ability to induce weight loss as well as reduce liver weight, FTM, and even more so LTM. Concomitantly TKO induced hepatic lipid accumulation, preferentially threefold increased phospholipid (PL) at the expense of decreased triacylglycerol (TG) and total cholesterol. Increased PL was associated with upregulation of membrane fatty acid transport/translocase proteins (FATP 2,4), cytosolic fatty acid/fatty acyl-CoA binding proteins (FABP2, ACBP), and the rate limiting enzyme in PL synthesis (Gpam). Decreased TG and cholesterol levels were not attributable to altered levels in respective synthetic enzymes or nuclear receptors. These data suggest that the higher level of Fabp1 and Scp2/Scpx gene products in WT males was protective against deleterious effects of dietary phytol, but TKO significantly exacerbated phytol effects in males.

Impact of Fabp1/Scp-2/Scp-x gene ablation (TKO) on hepatic phytol metabolism in mice

Studies in vitro have suggested that both sterol carrier protein-2/sterol carrier protein-x (Scp-2/Scp-x) and liver fatty acid binding protein [Fabp1 (L-FABP)] gene products facilitate hepatic uptake and metabolism of lipotoxic dietary phytol. However, interpretation of physiological function in mice singly gene ablated in the Scp-2/Scp-x has been complicated by concomitant upregulation of FABP1. The work presented herein provides several novel insights: i) An 8-anilino-1-naphthalenesulfonic acid displacement assay showed that neither SCP-2 nor L-FABP bound phytol, but both had high affinity for its metabolite, phytanic acid; ii) GC-MS studies with phytol-fed WT and Fabp1/Scp-2/SCP-x gene ablated [triple KO (TKO)] mice showed that TKO exacerbated hepatic accumulation of phytol metabolites in vivo in females and less so in males. Concomitantly, dietary phytol increased hepatic levels of total long-chain fatty acids (LCFAs) in both male and female WT and TKO mice. Moreover, in both WT and TKO female mice, dietary phytol increased hepatic ratios of saturated/unsaturated and polyunsaturated/monounsaturated LCFAs, while decreasing the peroxidizability index. However, in male mice, dietary phytol selectively increased the saturated/unsaturated ratio only in TKO mice, while decreasing the peroxidizability index in both WT and TKO mice. These findings suggested that: 1) SCP-2 and FABP1 both facilitated phytol metabolism after its conversion to phytanic acid; and 2) SCP-2/SCP-x had a greater impact on hepatic phytol metabolism than FABP1.

Impact of dietary phytol on lipid metabolism in SCP2/SCPX/L-FABP null mice

In vitro studies suggest that liver fatty acid binding protein (L-FABP) and sterol carrier protein-2/sterol carrier protein-x (SCP2/SCPx) gene products facilitate uptake and metabolism and detoxification of dietary-derived phytol in mammals. However, concomitant upregulation of L-FABP in SCP2/SCPx null mice complicates interpretation of their physiological phenotype. Therefore, the impact of ablating both the L-FABP gene and SCP2/SCPx gene (L-FABP/SCP2/SCPx null or TKO) was examined in phytol-fed female wild-type (WT) and TKO mice. TKO increased hepatic total lipid accumulation, primarily phospholipid, by mechanisms involving increased hepatic levels of proteins in the phospholipid synthetic pathway. Concomitantly, TKO reduced expression of proteins in targeting fatty acids towards the triacylglycerol synthetic pathway. Increased hepatic lipid accumulation was not associated with any concomitant upregulation of membrane fatty acid transport/translocase proteins involved in fatty acid uptake (FATP2, FATP4, FATP5 or GOT) or cytosolic proteins involved in fatty acid intracellular targeting (ACBP). In addition, TKO exacerbated dietary phytol-induced whole body weight loss, especially lean tissue mass. Since individually ablating SCPx or SCP2/SCPx elicited concomitant upregulation of L-FABP, these findings with TKO mice help to resolve the contributions of SCP2/SCPx gene ablation on dietary phytol-induced whole body and hepatic lipid phenotype independent of concomitant upregulation of L-FABP.

Fatty acid-binding protein 1 is preferentially lost in microsatellite instable colorectal carcinomas and is immune modulated via the interferon γ pathway

Fatty acid-binding protein 1 (FABP1) is an intracellular protein responsible for the transportation of long chain fatty acids. Aside from its functions in lipid metabolism and cellular differentiation, FABP1 also plays a role in inflammation through its interaction with peroxisome proliferator-activated receptors (PPARs). Previously, we compared expression of colonic epithelium genes in a subset of microsatellite instable (MSI) colorectal carcinomas (medullary carcinomas) to normal colonic mucosa and found that FABP1 expression was markedly decreased in the tumors. Further analysis of RNA expression in the colorectal subtypes and The Cancer Genome Atlas data set found that FABP1 expression is decreased in the CMS1 subset of colorectal carcinomas, which is characterized by microsatellite instability. As MSI colorectal carcinomas are known for their robust immune response, we then aimed to link FABP1 to the immune microenvironment of MSI carcinomas. To confirm the gene expression results, we performed immunohistochemical analysis of a cohort of colorectal carcinomas. FABP1 was preferentially lost in MSI carcinomas (123/133, 93%) compared with microsatellite stable carcinomas (240/562, 43%, P<0.0001). In addition, higher numbers of tumor-infiltrating lymphocytes were present in tumors with loss of FABP1 (P<0.0001). Decreased expression of the fatty acid storage and glucose regulator, PPARγ, was associated with the loss of FABP1 (P<0.0001). Colorectal cancer cell lines treated with interferon γ exhibited decreased expression of FABP1. FABP1 expression was partially recovered with the treatment of the cell lines with rosiglitazone, a PPARγ agonist. This study demonstrated that the loss of FABP1 expression is associated with MSI carcinomas and that interferon γ stimulation plays a role in this process via its interaction with PPARγ.

Liver Fatty Acid Binding Protein Deficiency Provokes Oxidative Stress, Inflammation, and Apoptosis-Mediated Hepatotoxicity Induced by Pyrazinamide in Zebrafish Larvae

Pyrazinamide (PZA) is an essential antitubercular drug, but little is still known about its hepatotoxicity potential. This study examined the effects of PZA exposure on zebrafish (Danio rerio) larvae and the mechanisms underlying its hepatotoxicity. A transgenic line of zebrafish larvae that expressed enhanced green fluorescent protein (EGFP) in the liver was incubated with 1, 2.5, and 5 mM PZA from 72 h postfertilization (hpf). Different endpoints such as mortality, morphology changes in the size and shape of the liver, histological changes, transaminase analysis and apoptosis, markers of oxidative and genetic damage, as well as the expression of certain genes were selected to evaluate PZA-induced hepatotoxicity. Our results confirm the manner of PZA dose-dependent hepatotoxicity. PZA was found to induce marked injury in zebrafish larvae, such as liver atrophy, elevations of transaminase levels, oxidative stress, and hepatocyte apoptosis. To further understand the mechanism behind PZA-induced hepatotoxicity, changes in gene expression levels in zebrafish larvae exposed to PZA for 72 h postexposure (hpe) were determined. The results of this study demonstrated that PZA decreased the expression levels of liver fatty acid binding protein (L-FABP) and its target gene, peroxisome proliferator-activated receptor α (PPAR-α), and provoked more severe oxidative stress and hepatitis via the upregulation of inflammatory cytokines such as tumor necrosis factor alpha (TNF-α) and transforming growth factor β (TGF-β). These findings suggest that L-FABP-mediated PPAR-α downregulation appears to be a hepatotoxic response resulting from zebrafish larva liver cell apoptosis, and L-FABP can be used as a biomarker for the early detection of PZA-induced liver damage in zebrafish larvae.

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

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

Non-alcoholic steatohepatitis: emerging molecular targets and therapeutic strategies

Non-alcoholic fatty liver disease - the most common chronic liver disease - encompasses a histological spectrum ranging from simple steatosis to non-alcoholic steatohepatitis (NASH). Over the next decade, NASH is projected to be the most common indication for liver transplantation. The absence of an effective pharmacological therapy for NASH is a major incentive for research into novel therapeutic approaches for this condition. The current focus areas for research include the modulation of nuclear transcription factors; agents that target lipotoxicity and oxidative stress; and the modulation of cellular energy homeostasis, metabolism and the inflammatory response. Strategies to enhance resolution of inflammation and fibrosis also show promise to reverse the advanced stages of liver disease.

Fatty acid binding proteins have the potential to channel dietary fatty acids into enterocyte nuclei

Intracellular lipid binding proteins, including fatty acid binding proteins (FABPs) 1 and 2, are highly expressed in tissues involved in the active lipid metabolism. A zebrafish model was used to demonstrate differential expression levels of fabp1b.1, fabp1b.2, and fabp2 transcripts in liver, anterior intestine, and brain. Transcription levels of fabp1b.1 and fabp2 in the anterior intestine were upregulated after feeding and modulated according to diet formulation. Immunofluorescence and electron microscopy immunodetection with gold particles localized these FABPs in the microvilli, cytosol, and nuclei of most enterocytes in the anterior intestinal mucosa. Nuclear localization was mostly in the interchromatin space outside the condensed chromatin clusters. Native PAGE binding assay of BODIPY-FL-labeled FAs demonstrated binding of BODIPY-FLC(12) but not BODIPY-FLC(5) to recombinant Fabp1b.1 and Fabp2. The binding of BODIPY-FLC(12) to Fabp1b.1 was fully displaced by oleic acid. In vivo experiments demonstrated, for the first time, that intestinal absorption of dietary BODIPY-FLC(12) was followed by colocalization of the labeled FA with Fabp1b and Fabp2 in the nuclei. These data suggest that dietary FAs complexed with FABPs are able to reach the enterocyte nucleus with the potential to modulate nuclear activity.

Impact of SCP-2/SCP-x gene ablation and dietary cholesterol on hepatic lipid accumulation

While a high-cholesterol diet induces hepatic steatosis, the role of intracellular sterol carrier protein-2/sterol carrier protein-x (SCP-2/SCP-x) proteins is unknown. We hypothesized that ablating SCP-2/SCP-x [double knockout (DKO)] would impact hepatic lipids (cholesterol and cholesteryl ester), especially in high-cholesterol-fed mice. DKO did not alter food consumption, and body weight (BW) gain decreased especially in females, concomitant with hepatic steatosis in females and less so in males. DKO-induced steatosis in control-fed wild-type (WT) mice was associated with 1) loss of SCP-2; 2) upregulation of liver fatty acid binding protein (L-FABP); 3) increased mRNA and/or protein levels of sterol regulatory element binding proteins (SREBP1 and SREBP2) as well as increased expression of target genes of cholesterol synthesis (Hmgcs1 and Hmgcr) and fatty acid synthesis (Acc1 and Fas); and 4) cholesteryl ester accumulation was also associated with increased acyl-CoA cholesterol acyltransferase-2 (ACAT2) in males. DKO exacerbated the high-cholesterol diet-induced hepatic cholesterol and glyceride accumulation, without further increasing SREBP1, SREBP2, or target genes. This exacerbation was associated both with loss of SCP-2 and concomitant downregulation of Ceh/Hsl, apolipoprotein B (ApoB), MTP, and/or L-FABP protein expression. DKO diminished the ability to secrete excess cholesterol into bile and oxidize cholesterol to bile acid for biliary excretion, especially in females. This suggested that SCP-2/SCP-x affects cholesterol transport to particular intracellular compartments, with ablation resulting in less to the endoplasmic reticulum for SREBP regulation, making more available for cholesteryl ester synthesis, for cholesteryl-ester storage in lipid droplets, and for bile salt synthesis and/or secretion. These alterations are significant findings, since they affect key processes in regulation of sterol metabolism.

Fatty Acid-binding Proteins Interact with Comparative Gene Identification-58 Linking Lipolysis with Lipid Ligand Shuttling

The coordinated breakdown of intracellular triglyceride (TG) stores requires the exquisitely regulated interaction of lipolytic enzymes with regulatory, accessory, and scaffolding proteins. Together they form a dynamic multiprotein network designated as the “lipolysome.” Adipose triglyceride lipase (Atgl) catalyzes the initiating step of TG hydrolysis and requires comparative gene identification-58 (Cgi-58) as a potent activator of enzyme activity. Here, we identify adipocyte-type fatty acid-binding protein (A-Fabp) and other members of the fatty acid-binding protein (Fabp) family as interaction partners of Cgi-58. Co-immunoprecipitation, microscale thermophoresis, and solid phase assays proved direct protein/protein interaction between A-Fabp and Cgi-58. Using nuclear magnetic resonance titration experiments and site-directed mutagenesis, we located a potential contact region on A-Fabp. In functional terms, A-Fabp stimulates Atgl-catalyzed TG hydrolysis in a Cgi-58-dependent manner. Additionally, transcriptional transactivation assays with a luciferase reporter system revealed that Fabps enhance the ability of Atgl/Cgi-58-mediated lipolysis to induce the activity of peroxisome proliferator-activated receptors. Our studies identify Fabps as crucial structural and functional components of the lipolysome.

Effects of Diisodecyl Phthalate on PPAR:RXR-Dependent Gene Expression Pathways in Sea Bream Hepatocytes

Evidence that endocrine-disrupting chemicals (EDCs) may target metabolic disturbances, beyond interference with the functions of the endocrine systems has recently accumulated. Among EDCs, phthalate plasticizers like the diisodecyl phthalate (DiDP) are commonly found contaminants of aquatic environments and have been suggested to function as obesogens by activating peroxisome proliferator activated receptors (PPARs), a subset of nuclear receptors (NRs) that act as metabolic sensors, playing pivotal roles in lipid homeostasis. However, little is known about the modulation of PPAR signaling pathways by DiDP in fish. In this study, we have first investigated the ligand binding efficiency of DiDP to the ligand binding domains of PPARs and retinoid-X-receptor-α (RXRα) proteins in fish using a molecular docking approach. Furthermore, in silico predictions were integrated by in vitro experiments to show possible dose-relationship effects of DiDP on PPAR:RXR-dependent gene expression pathways using sea bream hepatocytes. We observed that DiDP shows high binding efficiency with piscine PPARs demonstrating a greater preference for RXRα. Our studies also demonstrated the coordinate increased expression of PPARs and RXRα, as well as their downstream target genes in vitro. Principal component analysis (PCA) showed the strength of relationship between transcription of most genes involved in fatty acid metabolism and PPAR mRNA levels. In particular, fatty acid binding protein (FABP) was highly correlated to all PPARs. The results of this study suggest that DiDP can be considered an environmental stressor that activates PPAR:RXR signaling to promote long-term changes in lipid homeostasis leading to potential deleterious physiological consequences in teleost fish.

Molecular mechanism of PPARα action and its impact on lipid metabolism, inflammation and fibrosis in non-alcoholic fatty liver disease

Peroxisome proliferator-activated receptor α (PPARα) is a ligand-activated transcription factor belonging, together with PPARγ and PPARβ/δ, to the NR1C nuclear receptor subfamily. Many PPARα target genes are involved in fatty acid metabolism in tissues with high oxidative rates such as muscle, heart and liver. PPARα activation, in combination with PPARβ/δ agonism, improves steatosis, inflammation and fibrosis in pre-clinical models of non-alcoholic fatty liver disease, identifying a new potential therapeutic area. In this review, we discuss the transcriptional activation and repression mechanisms by PPARα, the spectrum of target genes and chromatin-binding maps from recent genome-wide studies, paying particular attention to PPARα-regulation of hepatic fatty acid and plasma lipoprotein metabolism during nutritional transition, and of the inflammatory response. The role of PPARα, together with other PPARs, in non-alcoholic steatohepatitis will be discussed in light of available pre-clinical and clinical data.

Characterization of two distinct modes of drug binding to human intestinal fatty acid binding protein

The aqueous cytoplasm of cells poses a potentially significant barrier for many lipophilic drugs to reach their sites of action. Fatty acid binding proteins (FABPs) bind to poorly water-soluble fatty acids (FAs) and lipophilic compounds and facilitate their intracellular transport. Several structures of FA in complex with FABPs have been described, but data describing the binding sites of other lipophilic ligands including drugs are limited. Here the environmentally sensitive fluorophores, 1-anilinonapthalene 8-sulfonic acid (ANS), and 11-dansylamino undecanoic acid (DAUDA) were used to investigate drug binding to human intestinal FABP (hIFABP). Most drugs that bound hIFABP were able to displace both ANS and DAUDA. A notable exception was ketorolac, a non-steroidal anti-inflammatory drug that bound to hIFABP and displaced DAUDA but failed to displace ANS. Isothermal titration calorimetry revealed that for the majority of ligands including FA, ANS, and DAUDA, binding to hIFABP was exothermic. In contrast, ketorolac binding to hIFABP was endothermic and entropy-driven. The X-ray crystal structure of DAUDA-hIFABP revealed a FA-like binding mode where the carboxylate of DAUDA formed a network of hydrogen bonds with residues at the bottom of the binding cavity and the dansyl group interacted with residues in the portal region. In contrast, NMR chemical shift perturbation (CSP) data suggested that ANS bound only toward the bottom of the hIFABP cavity, whereas ketorolac occupied only the portal region. The CSP data further suggested that ANS and ketorolac were able to bind simultaneously to hIFABP, consistent with the lack of displacement of ANS observed by fluorescence and supported by a model of the ternary complex. The NMR solution structure of the ketorolac-hIFABP complex therefore describes a newly characterized, hydrophobic ligand binding site in the portal region of hIFABP.

Epidermal fatty acid-binding protein protects nerve growth factor-differentiated PC12 cells from lipotoxic injury

Epidermal fatty acid-binding protein (E-FABP/FABP5/DA11) binds and transport long-chain fatty acids in the cytoplasm and may play a protecting role during neuronal injury. We examined whether E-FABP protects nerve growth factor-differentiated PC12 cells (NGFDPC12 cells) from lipotoxic injury observed after palmitic acid (C16:0; PAM) overload. NGFDPC12 cells cultures treated with PAM/bovine serum albumin at 0.3 mM/0.15 mM show PAM-induced lipotoxicity (PAM-LTx) and apoptosis. The apoptosis was preceded by a cellular accumulation of reactive oxygen species (ROS) and higher levels of E-FABP. Antioxidants MCI-186 and N-acetyl cysteine prevented E-FABP's induction in expression by PAM-LTx, while tert-butyl hydroperoxide increased ROS and E-FABP expression. Non-metabolized methyl ester of PAM, methyl palmitic acid (mPAM), failed to increase cellular ROS, E-FABP gene expression, or trigger apoptosis. Treatment of NGFDPC12 cultures with siE-FABP showed reduced E-FABP levels correlating with higher accumulation of ROS and cell death after exposure to PAM. In contrast, increasing E-FABP cellular levels by pre-loading the cells with recombinant E-FABP diminished the PAM-induced ROS and cell death. Finally, agonists for PPARβ (GW0742) or PPARγ (GW1929) increased E-FABP expression and enhanced the resistance of NGFDPC12 cells to PAM-LTx. We conclude that E-FABP protects NGFDPC12 cells from lipotoxic injury through mechanisms that involve reduction of ROS.

Epidermal fatty acid-binding protein (E-FABP) may protect nerve cells from the damaging exposure to high levels of free fatty acids (FA). We show that E-FABP can neutralize the effects of reactive oxygen species (ROS) generated by the high levels of FA in the cell and protect PC12 cells from lipotoxic injuries common in Type 2 diabetes neuropathy. Potentially, E-FABP gene up-regulation may be mediated through the NFkB pathway and future studies are needed to further evaluate this proposition.

Human FABP1 T94A variant impacts fatty acid metabolism and PPAR-α activation in cultured human female hepatocytes

Although human liver fatty acid-binding protein (FABP1) T94A variant has been associated with nonalcoholic fatty liver disease and reduced ability of fenofibrate to lower serum triglycerides (TG) to target levels, molecular events leading to this phenotype are poorly understood. Cultured primary hepatocytes from female human subjects expressing the FABP1 T94A variant exhibited increased neutral lipid (TG, cholesteryl ester) accumulation associated with (1) upregulation of total FABP1, a key protein stimulating mitochondrial glycerol-3-phosphate acyltransferase (GPAM), the rate-limiting enzyme in lipogenesis; (2) increased mRNA expression of key enzymes in lipogenesis (GPAM, LPIN2) in heterozygotes; (3) decreased mRNA expression of microsomal triglyceride transfer protein; (4) increased secretion of ApoB100 but not TG; (5) decreased long-chain fatty acid (LCFA) β-oxidation. TG accumulation was not due to any increase in LCFA uptake, de novo lipogenesis, or the alternate monoacylglycerol O-acyltransferase pathway in lipogenesis. Despite increased expression of total FABP1 mRNA and protein, fenofibrate-mediated FABP1 redistribution to nuclei and ligand-induced peroxisome proliferator-activated receptor (PPAR-α) transcription of LCFA β-oxidative enzymes (carnitine palmitoyltransferase 1A, carnitine palmitoyltransferase 2, and acyl-coenzyme A oxidase 1, palmitoyl) were attenuated in FABP1 T94A hepatocytes. Although the phenotype of FABP1 T94A variant human hepatocytes exhibits some similarities to that of FABP1-null or PPAR-α-null hepatocytes and mice, expression of FABP1 T94A variant did not abolish or reduce ligand binding. Thus the FABP1 T94A variant represents an altered/reduced function mutation resulting in TG accumulation.

Swimming improves high-fat induced insulin resistance by regulating lipid and energy metabolism and the insulin pathway in rats

In this study, we aimed to determine the preventive and therapeutic effects of swimming on insulin resistance in high-fat-fed rats. Sprague-Dawley rats were divided into 4 groups and fed for 8 weeks as follows: i) the control (Con) group fed a control diet; ii) the high-fat (HF) group fed a high-fat diet; iii) the treatment (ST) group fed a high-fat diet and trained with swimming from the 4th week; and iv) the prevention (SP) group fed a high-fat diet and trained with swimming from the 1st week of the experiment. A hyperinsulinemic-euglycemic clamp was used to evaluate the insulin sensitivity of the rats. The ultrastructure of the liver cells was observed by electron microscopy. Hepatic lipid accumulation was observed by Oil Red O staining. Quantitative RT-PCR and western blot analysis were performed to detect the expression of proteins related to lipid metabolism, energy metabolism and insulin signaling transduction. After 8 weeks of feeding, compared with the Con group, the glucose infusion rate (GIR) was significantly decreased; a significant lipid accumulation was observed in the liver, while the ultrastructure of the liver cells was damaged in the HF group. Proteins related to lipid metabolism in the liver and skeletal muscle, including FAT and FABP were upregulated, while CPT1 and PPAR levels were downregulated in the HF group. The levels of the energy-metabolism-related molecules, AMPKα2, PGC1α, PGC1β and MFN2 were downregulated in skeletal muscle in the HF group. The expression levels of insulin signaling transduction molecules, INSR, IRS1, PI3K/p85, AKT2 and GLUT4, as well as the phosphorylation levels of INSR, IRS1, PI3K/p85 and AKT2 were lower in skeletal muscles in the HF rats. Compared with HF group, the GIR levels were significantly increased in the ST and SP groups. Lipid accumulation and damage to the ultrastructure of the liver cells were improved in both groups. The expression of molecules related to lipid metabolism in the liver and skeletal muscle, energy metabolism in skeletal muscle and insulin signaling transduction were all markedly upregulated. In conclusion, swimming can effectively improve insulin sensitivity and even prevent insulin resistance by affecting the expression of proteins related to lipid metabolism, energy metabolism and insulin signaling transduction in rats fed a high-fat diet.

Structural and functional interaction of fatty acids with human liver fatty acid-binding protein (L-FABP) T94A variant

The human liver fatty acid-binding protein (L-FABP) T94A variant, the most common in the FABP family, has been associated with elevated liver triglyceride levels. How this amino acid substitution elicits these effects is not known. This issue was addressed using human recombinant wild-type (WT) and T94A variant L-FABP proteins as well as cultured primary human hepatocytes expressing the respective proteins (genotyped as TT, TC and CC). The T94A substitution did not alter or only slightly altered L-FABP binding affinities for saturated, monounsaturated or polyunsaturated long chain fatty acids, nor did it change the affinity for intermediates of triglyceride synthesis. Nevertheless, the T94A substitution markedly altered the secondary structural response of L-FABP induced by binding long chain fatty acids or intermediates of triglyceride synthesis. Finally, the T94A substitution markedly decreased the levels of induction of peroxisome proliferator-activated receptor α-regulated proteins such as L-FABP, fatty acid transport protein 5 and peroxisome proliferator-activated receptor α itself meditated by the polyunsaturated fatty acids eicosapentaenoic acid and docosahexaenoic acid in cultured primary human hepatocytes. Thus, although the T94A substitution did not alter the affinity of human L-FABP for long chain fatty acids, it significantly altered human L-FABP structure and stability, as well as the conformational and functional response to these ligands.

Structural basis for ligand regulation of the fatty acid-binding protein 5, peroxisome proliferator-activated receptor β/δ (FABP5-PPARβ/δ) signaling pathway

Fatty acid-binding proteins (FABPs) are a widely expressed group of calycins that play a well established role in solubilizing cellular fatty acids. Recent studies, however, have recast FABPs as active participants in vital lipid-signaling pathways. FABP5, like its family members, displays a promiscuous ligand binding profile, capable of interacting with numerous long chain fatty acids of varying degrees of saturation. Certain "activating" fatty acids induce the protein's cytoplasmic to nuclear translocation, stimulating PPARβ/δ transactivation; however, the rules that govern this process remain unknown. Using a range of structural and biochemical techniques, we show that both linoleic and arachidonic acid elicit FABP5's translocation by permitting allosteric communication between the ligand-sensing β2 loop and a tertiary nuclear localization signal within the α-helical cap of the protein. Furthermore, we show that more saturated, nonactivating fatty acids inhibit nuclear localization signal formation by destabilizing this activation loop, thus implicating FABP5 specifically in cis-bonded, polyunsaturated fatty acid signaling.

Recent insights into the molecular pathophysiology of lipid droplet formation in hepatocytes

Triacyglycerols are a major energy reserve of the body and are normally stored in adipose tissue as lipid droplets (LDs). The liver, however, stores energy as glycogen and digested triglycerides in the form of fatty acids. In stressed condition such as obesity, imbalanced nutrition and drug induced liver injury hepatocytes accumulate excess lipids in the form of LDs whose prolonged storage leads to disease conditions most notably non-alcoholic fatty liver disease (NAFLD). Fatty liver disease has become a major health burden with more than 90% of obese, nearly 70% of overweight and about 25% of normal weight patients being affected. Notably, research in recent years has shown LD as highly dynamic organelles for maintaining lipid homeostasis through fat storage, protein sorting and other molecular events studied in adipocytes and other cells of living organisms. This review focuses on the molecular events of LD formation in hepatocytes and the importance of cross talk between different cell types and their signalling in NAFLD as to provide a perspective on molecular mechanisms as well as possibilities for different therapeutic intervention strategies.

Fatty acid binding proteins: tissue-specific functions in health and disease

PURPOSE OF REVIEW

The purpose of this study is to review recent evidence for the role of the cytosolic fatty acid binding proteins (FABPs) as central regulators of whole-body metabolic control.

RECENT FINDINGS

Dysregulated FABPs have been associated with a number of diseases, including obesity and nonalcoholic fatty liver disease (FABP1, FABP2, FABP4), cardiovascular risk (FABP3) and cancer (FABP5, FABP7). As underlying mechanisms become better understood, FABPs may represent novel biomarkers for therapeutic targets. In addition, the role of FABPs as important signalling molecules has also been highlighted in recent years; for example, FABP3 may act as a myokine, matching whole-body metabolism to muscular energy demands and FABP4 functions as an adipokine in regulating macrophage and adipocyte interactions during inflammation.

SUMMARY

In addition to their traditional role as fatty acid trafficking proteins, increasing evidence supports the role of FABPs as important controllers of global metabolism, with their dysregulation being linked to a host of metabolic diseases.