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

Design, synthesis, and biological evaluation of first-in-class FABP1 inhibitors for the treatment of NASH

The fatty acid-binding protein 1 (FABP1) is a fatty acid transporter protein that is considered as an emerging target for metabolic diseases. Despite forceful evidence that the inhibition of FABP1 is essential for ameliorating NASH, pharmacological control and validation of FABP1 are hindered by a lack of relevant inhibitors as pharmacological tool. Therefore, the development of effective FABP1 inhibitors is a current focus of research. Herein, we firstly reported the comprehensive structure-activity relationship (SAR) study of novel FABP1 inhibitors derived from high throughput screening of our in-house library, which resulting in the identification of the optimal compound 44 (IC50 = 4.46 ± 0.54 μM). Molecular docking studies revealed that 44 forms stable hydrogen bonds with amino acids around the active pocket of FABP1. Moreover, 44 alleviated the typical histological features of fatty liver in NASH mice, including steatosis, lobular inflammation, ballooning and fibrosis. Additionally, 44 has been demonstrated to have lipid metabolism regulating, anti-oxidative stress and hepatoprotective properties. This study might be provided a promising insight into the field of NASH and inspiration for the development of FABP1 inhibitors.

Derlin-1 ameliorates nonalcoholic hepatic steatosis by promoting ubiquitylation and degradation of FABP1

BACKGROUND AND AIMS

The functions of liver fatty acid binding protein 1 (FABP1) in the regulation of nonalcoholic fatty liver disease (NAFLD) have been previously established. However, how FABP1 expression is dynamically regulated in metabolic disorders is unclear. Previous studies have reported that ubiquitin proteasome-mediated degradation of FABP1 is involved, but the mechanism remains unknown.

METHODS

Dysregulated expression of hepatic FABP1 and Derlin-1 was observed in NAFLD patients. We performed mice hepatic tissue coimmunoprecipitation based mass spectrum assays. Interaction between Derlin-1 and FABP1, and its impact on FABP1 ubiquitination status was evaluated by coimmunoprecipitation. The role of Derlin-1 in lipid deposition was tested using adenovirus-mediated overexpression in C57BL/6 mice, as well as by Derlin-1 overexpression or knockdown in HepG2 cells.

RESULTS

As a subunit of the endoplasmic reticulum-associated degradation complex, Derlin-1 was negatively associated with NAFLD patients, interacted with and ubiquitinated FABP1. Derlin-1 suppressed FABP1 levels and inhibited lipid deposition through a FABP1-dependent pathway. Additionally, Trim25, an E3 ubiquitin ligase present in the endoplasmic reticulum, was recruited to promote Derlin-1-related polyubiquitylation of FABP1, thereby creating a ubiquitin-associated network for FABP1 regulation. Derlin-1 overexpression ameliorated hepatic steatosis in both C57BL/6 mice and HepG2 cells, and contributed to attenuated weight gain, lower liver weight, and visceral fat mass.

CONCLUSIONS

FABP1 was degraded by Derlin-1 through ubiquitin modification. Negative regulation of FABP1 by Derlin-1 overexpression, suppressed lipid metabolism and alleviated lipid deposition in vivo and in vitro. Hence, Derlin-1 activation in hepatocytes may represent a potential therapeutic strategy for NAFLD.

Polymorphism in genes encoding two fatty acid binding proteins increases risk of ischemic stroke in a Chinese Han population

Background: Dyslipidemia is an independent predictor of ischemic stroke (IS). Genetic variations in lipid-metabolism related genes may increase the risk of IS. Fatty acid-binding protein 1 (FABP1) and fatty acid-binding protein 2 (FABP2) are lipid chaperones responsible for lipid transport and metabolism. The present study aimed to determine the association between FABP1 or FABP2 and ischemic stroke.

Methods: A total of 251 participants were recruited composed of 138 patients with ischemic stroke and 113 healthy subjects. DNA was extracted from peripheral blood samples. The rs2241883 polymorphism in FABP1 and rs1799883 polymorphism in FABP2 were genotyped using polymerase chain reaction-restriction fragment length polymorphism. Generalized multifactor dimensionality reduction (GMDR) was used to find out the interaction combinations between two SNPs and environmental factors.

Results: The GA genotype of FABP2 rs1799883 increased susceptibility to ischemic stroke under overdominant inheritance model (p = 0.042). After adjusting for the risk factors of IS, it was associated with a significantly higher risk of IS in the codominant inheritance model (adjust OR = 3.431, 95%CI = 1.060–11.103, p = 0.04). The interactions of FABP1 rs2241883 and FABP2 rs1799883 were not associated with IS risk (p = 0.172). Moreover, interaction analysis of two genes (rs1799883 and rs2241883) and two environmental factors (smoking and alcohol consumption) was associated with an increased risk of IS (p = 0.011).

Conclusion: The GA genotype of FABP2 rs1799883, interactions between rs1799883, rs2241883 and smoking and alcohol consumption were associated with IS risk in Chinese Han populations.

The association between a Fatty Acid Binding Protein 1 (FABP1) gene polymorphism and serum lipid abnormalities in the MASHAD cohort study

INTRODUCTION

Dyslipidemia is a known risk factor for cardiovascular disease and is partially determined by genetic variations in the genes involved in lipoprotein metabolism. Therefore, we aimed to assess the association between a polymorphism of the Fatty Acid Binding Protein1 (rs2241883) gene locus and dyslipidemia in an Iranian cohort.

MATERIALS AND METHODS

This is a case-control study 2737 individuals were recruited (2203 subjects with dyslipidemia and 534 controls). Dyslipidemia was defined as total cholesterol≥200 mg/dl, or TG≥150 mg/dl, or LDL-C≥130 mg/dl, or HDL-C<40 mg/dl in males and <50 mg/dl in females. Serum lipid profile was determined using a Alcyon Abbott biochemical auto analyzer, USA. Genotyping was made through double amplification refractory mutation system polymerase chain reaction (ARMs PCR).

RESULT

The frequency of TT, CT, CC genotypes of rs2241883 polymorphism of FABP1 gene were 65.5, 33.4, 5.1 in subjects with dyslipidemia and 56.9%, 40.4%, 2.6% in subjects without dyslipidemia, respectively. Using a dominant genetic model, subjects carrying C allele (CC&CT genotypes) had a 22% lower risk of dyslipidemia (OR: 0.78, CI 95%: 0.62-0.98 P, 0.03). Individuals with CT vs. TT genotypes had a significantly lower risk of a high serum TC and LDL level. Further analysis showed that there was a positive association between FABP1 genotype (CT) and isolated HTG as well as combined dyslipidemia. The change of a polar amino acid (threonine) in position T94A to a hydrophobic amino acid (alanine) can cause transformation protein.

CONCLUSIONS

A CC genotype of the rs2241883 polymorphism of the FABP1 gene appears to confer a higher risk of dyslipidemia in our representative cohort of Iranian individuals.

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.

Sterol Carrier Protein-2/Sterol Carrier Protein-x/Fatty Acid Binding Protein-1 Ablation Impacts Response of Brain Endocannabinoid to High-Fat Diet

Brain endocannabinoids (EC) such as arachidonoylethanolamine (AEA) and 2-arachidonoylglycerol (2-AG) primarily originate from serum arachidonic acid (ARA), whose level is regulated in part by a cytosolic ARA-binding protein, that is, liver fatty acid binding protein-1 (FABP1), not expressed in the brain. Ablation of the Fabp1 gene (LKO) increases brain AEA and 2-AG by decreasing hepatic uptake of ARA to increase serum ARA, thereby increasing ARA availability for uptake by the brain. The brain also expresses sterol carrier protein-2 (SCP-2), which is also a cytosolic ARA-binding protein. To further resolve the role of SCP-2 independent of FABP1, mice ablated in the Scp-2/Scp-x gene (DKO) were crossed with mice ablated in the Fabp1 gene (LKO) mice to generate triple knock out (TKO) mice. TKO impaired the ability of LKO to increase brain AEA and 2-AG. While a high-fat diet (HFD) alone increased brain AEA, TKO impaired this effect. Overall, these TKO-induced blocks were not attributable to altered expression of brain proteins in ARA uptake, AEA/2-AG synthesis, or AEA/2-AG degrading enzymes. Instead, TKO reduced serum levels of free ARA and/or total ARA and thereby decreased ARA availability for uptake to the brain and downstream synthesis of AEA and 2-AG therein. In summary, Scp-2/Scp-x gene ablation in Fabp1 null (LKO) mice antagonized the impact of LKO and HFD on brain ARA and, subsequently, EC levels. Thus, both FABP1 and SCP-2 participate in regulating the EC system in the brain.

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.

Multiple endocannabinoid-mediated mechanisms in the regulation of energy homeostasis in brain and peripheral tissues

The endocannabinoid (eCB) system is widely expressed in many central and peripheral tissues, and is involved in a plethora of physiological processes. Among these, activity of the eCB system promotes energy intake and storage, which, however, under pathophysiological conditions, can favour the development of obesity and obesity-related disorders. It is proposed that eCB signalling is evolutionary beneficial for survival under periods of scarce food resources. Remarkably, eCB signalling is increased both in hunger and in overnutrition conditions, such as obesity and type-2 diabetes. This apparent paradox suggests a role of the eCB system both at initiation and at clinical endpoint of obesity. This review will focus on recent findings about the role of the eCB system controlling whole-body metabolism in mice that are genetically modified selectively in different cell types. The current data in fact support the notion that eCB signalling is not only engaged in the development but also in the maintenance of obesity, whereby specific cell types in central and peripheral tissues are key sites in regulating the entire body's energy homeostasis.

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.

The aryl hydrocarbon receptor-cytochrome P450 1A1 pathway controls lipid accumulation and enhances the permissiveness for hepatitis C virus assembly

Viruses hijack and modify host cell functions to maximize viral proliferation. Hepatitis C virus (HCV) reorganizes host cell metabolism to produce specialized membrane structures and to modify organelles such as double-membrane vesicles and enlarged lipid droplets (LDs), thereby enabling virus replication and assembly. However, the molecular bases of these host-HCV interactions are largely unknown. Here, using a chemical screen, we demonstrate that the benzamide derivative flutamide reduces the host capacity to produce infectious HCV. Flutamide disrupted the formation of enlarged LDs in HCV-infected cells, thereby abolishing HCV assembly. We also report that aryl hydrocarbon receptor (AhR), a known flutamide target, plays a key role in mediating LD accumulation and HCV production. This AhR function in lipid production was also observed in HCV-uninfected Huh-7 cells and primary human hepatocytes, suggesting that AhR signaling regulates lipid accumulation independently of HCV infection. We further observed that a downstream activity, that of cytochrome P450 1A1 (CYP1A1), was the primary regulator of AhR-mediated lipid production. Specifically, blockade of AhR-induced CYP1A1 up-regulation counteracted LD overproduction, and overproduction of CYP1A1, but not of CYP1B1, in AhR-inactivated cells restored lipid accumulation. Of note, HCV infection up-regulated the AhR-CYP1A1 pathway, resulting in the accumulation of enlarged LDs. In conclusion, we demonstrate that the AhR-CYP1A1 pathway has a significant role in lipid accumulation, a hallmark of HCV infection that maximizes progeny virus production. Our chemical-genetic analysis reveals a new strategy and lead compounds to control hepatic lipid accumulation as well as HCV infection.

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.

Δ9-Tetrahydrocannabinol induces endocannabinoid accumulation in mouse hepatocytes: antagonism by Fabp1 gene ablation

Phytocannabinoids, such as Δ9-tetrahydrocannabinol (THC), bind and activate cannabinoid (CB) receptors, thereby "piggy-backing" on the same pathway's endogenous endocannabinoids (ECs). The recent discovery that liver fatty acid binding protein-1 (FABP1) is the major cytosolic "chaperone" protein with high affinity for both Δ9-THC and ECs suggests that Δ9-THC may alter hepatic EC levels. Therefore, the impact of Δ9-THC or EC treatment on the levels of endogenous ECs, such as N-arachidonoylethanolamide (AEA) and 2-arachidonoylglycerol (2-AG), was examined in cultured primary mouse hepatocytes from WT and Fabp1 gene-ablated (LKO) mice. Δ9-THC alone or 2-AG alone significantly increased AEA and especially 2-AG levels in WT hepatocytes. LKO alone markedly increased AEA and 2-AG levels. However, LKO blocked/diminished the ability of Δ9-THC to further increase both AEA and 2-AG. In contrast, LKO potentiated the ability of exogenous 2-AG to increase the hepatocyte level of AEA and 2-AG. These and other data suggest that Δ9-THC increases hepatocyte EC levels, at least in part, by upregulating endogenous AEA and 2-AG levels. This may arise from Δ9-THC competing with AEA and 2-AG binding to FABP1, thereby decreasing targeting of bound AEA and 2-AG to the degradative enzymes, fatty acid amide hydrolase and monoacylglyceride lipase, to decrease hydrolysis within hepatocytes.

Chinese olive extract ameliorates hepatic lipid accumulation in vitro and in vivo by regulating lipid metabolism

Chinese olive contains plenty of polyphenols, which possess a wide range of biological actions. In this study, we aimed to investigate the role of the ethyl acetate fraction of Chinese olive fruit extract (CO-EtOAc) in the modulation of lipid accumulation in vitro and in vivo. In cellular studies, CO-EtOAc attenuated oleic acid-induced lipid accumulation; we then elucidated the molecular mechanisms of CO-EtOAc in FL83B mouse hepatocytes. CO-EtOAc suppressed the mRNA levels of fatty acid transporter genes (CD36 and FABP) and lipogenesis genes (SREBP-1c, FAS, and ACC1), but upregulated genes that govern lipolysis (HSL) and lipid oxidation (PPARα, CPT-1, and ACOX). Moreover, CO-EtOAc increased the protein expression of phosphorylated AMPK, ACC1, CPT-1, and PPARα, but downregulated the expression of mature SREBP-1c and FAS. AMPK plays an essential role in CO-EtOAc-mediated amelioration of lipid accumulation. Furthermore, we confirmed that CO-EtOAc significantly inhibited body weight gain, epididymal adipose tissue weight, and hepatic lipid accumulation via regulation of the expression of fatty acid transporter, lipogenesis, and fatty acid oxidation genes and proteins in C57BL/6 mice fed a 60% high-fat diet. Therefore, Chinese olive fruits may have the potential to improve the metabolic abnormalities associated with fatty liver under high fat challenge.

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.

Association of IL-1β, IL-1Ra and FABP1 gene polymorphisms with the metabolic features of polycystic ovary syndrome

BACKGROUND

Polycystic ovary syndrome (PCOS), a highly prevalent endocrinopathy is currently being designated as chronic low grade inflammatory state. IL-1β, IL-1Ra and FABP1 are critical mediators of inflammatory processes and are speculated to play a role in the pathogenesis of PCOS. The aim of this study was to study the association of IL-β, IL-1Ra and FABP1 gene polymorphisms with PCOS and related metabolic features.

SUBJECTS

95 PCOS and 45 age matched healthy control subjects were enrolled in this study.

METHODS

Polymorphism in genes IL-1β, IL-1Ra and FABP1 was studied by PCR, PCR-RFLP and sequencing methods, respectively. Hormonal and lipid profiles were evaluated for all the subjects.

RESULTS

Hormonal and lipid profiles showed significant differences between PCOS and control subjects. Allele and genotype frequencies of IL-1β, IL-1Ra and FABP1 gene polymorphisms did not vary between the control and PCOS group. However, T allele of C[-511]T variant of IL-1β, allele II in intron 2 of IL-1Ra and A allele of A/G variant of FABP1 (rs2197076) showed significant association with many metabolic features associated with PCOS.

CONCLUSIONS

Polymorphism in genes encoding cytokines and proteins involved in lipid metabolism can provide insights into the genetics of the disease and may contribute to assess the associated risk of cardiovascular diseases (CVD), dyslipidemia and metabolic syndrome (MetS) associated with PCOS.

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.

Fabp1 gene ablation inhibits high-fat diet-induced increase in brain endocannabinoids

The endocannabinoid system shifts energy balance toward storage and fat accumulation, especially in the context of diet-induced obesity. Relatively little is known about factors outside the central nervous system that may mediate the effect of high-fat diet (HFD) on brain endocannabinoid levels. One candidate is the liver fatty acid binding protein (FABP1), a cytosolic protein highly prevalent in liver, but not detected in brain, which facilitates hepatic clearance of fatty acids. The impact of Fabp1 gene ablation (LKO) on the effect of high-fat diet (HFD) on brain and plasma endocannabinoid levels was examined and data expressed for each parameter as the ratio of high-fat diet/control diet. In male wild-type mice, HFD markedly increased brain N-acylethanolamides, but not 2-monoacylglycerols. LKO blocked these effects of HFD in male mice. In female wild-type mice, HFD slightly decreased or did not alter these endocannabinoids as compared with male wild type. LKO did not block the HFD effects in female mice. The HFD-induced increase in brain arachidonic acid-derived arachidonoylethanolamide in males correlated with increased brain-free and total arachidonic acid. The ability of LKO to block the HFD-induced increase in brain arachidonoylethanolamide correlated with reduced ability of HFD to increase brain-free and total arachidonic acid in males. In females, brain-free and total arachidonic acid levels were much less affected by either HFD or LKO in the context of HFD. These data showed that LKO markedly diminished the impact of HFD on brain endocannabinoid levels, especially in male mice.

Female Mice are Resistant to Fabp1 Gene Ablation-Induced Alterations in Brain Endocannabinoid Levels

Although liver fatty acid binding protein (FABP1, L-FABP) is not detectable in the brain, Fabp1 gene ablation (LKO) markedly increases endocannabinoids (EC) in brains of male mice. Since the brain EC system of females differs significantly from that of males, it was important to determine if LKO differently impacted the brain EC system. LKO did not alter brain levels of arachidonic acid (ARA)-containing EC, i.e. arachidonoylethanolamide (AEA) and 2-arachidonoylglycerol (2-AG), but decreased non-ARA-containing N-acylethanolamides (OEA, PEA) and 2-oleoylglycerol (2-OG) that potentiate the actions of AEA and 2-AG. These changes in brain potentiating EC levels were not associated with: (1) a net decrease in levels of brain membrane proteins associated with fatty acid uptake and EC synthesis; (2) a net increase in brain protein levels of cytosolic EC chaperones and enzymes in EC degradation; or (3) increased brain protein levels of EC receptors (CB1, TRVP1). Instead, the reduced or opposite responsiveness of female brain EC levels to loss of FABP1 (LKO) correlated with intrinsically lower FABP1 level in livers of WT females than males. These data show that female mouse brain endocannabinoid levels were unchanged (AEA, 2-AG) or decreased (OEA, PEA, 2-OG) by complete loss of FABP1 (LKO).

FABP-1 gene ablation impacts brain endocannabinoid system in male mice

Liver fatty acid-binding protein (FABP1, L-FABP) has high affinity for and enhances uptake of arachidonic acid (ARA, C20:4, n-6) which, when esterified to phospholipids, is the requisite precursor for synthesis of endocannabinoids (EC) such as arachidonoylethanolamide (AEA) and 2-arachidonoylglycerol (2-AG). The brain derives most of its ARA from plasma, taking up ARA and transporting it intracellularly via cytosolic fatty acid-binding proteins (FABPs 3,5, and 7) localized within the brain. In contrast, the much more prevalent cytosolic FABP1 is not detectable in the brain but is instead highly expressed in the liver. Therefore, the possibility that FABP1 outside the central nervous system may regulate brain AEA and 2-AG was examined in wild-type (WT) and FABP1 null (LKO) male mice. LKO increased brain levels of AA-containing EC (AEA, 2-AG), correlating with increased free and total ARA in brain and serum. LKO also increased brain levels of non-ARA that contain potentiating endocannabinoids (EC*) such as oleoyl ethanolamide (OEA), PEA, 2-OG, and 2-PG. Concomitantly, LKO decreased serum total ARA-containing EC, but not non-ARA endocannabinoids. LKO did not elicit these changes in the brain EC and EC* as a result of compensatory up-regulation of brain protein levels of enzymes in EC synthesis (NAPEPLD, DAGLα) or cytosolic EC chaperone proteins (FABPs 3, 5, 7, SCP-2, HSP70), or cannabinoid receptors (CB1, TRVP1). These data show for the first time that the non-CNS fatty acid-binding protein FABP1 markedly affected brain levels of both ARA-containing endocannabinoids (AEA, 2-AG) as well as their non-ARA potentiating endocannabinoids. Fatty acid-binding protein-1 (FABP-1) is not detectable in brain but instead is highly expressed in liver. The possibility that FABP1 outside the central nervous system may regulate brain endocannabinoids arachidonoylethanolamide (AEA) and 2-arachidonoylglycerol (2-AG) was examined in wild-type (WT) and FABP-1 null (LKO) male mice. LKO increased brain levels of arachidonic acid-containing endocannabinoids (AEA, 2-AG), correlating with increased free and total arachidonic acid in brain and serum. Read the Editorial Highlight for this article on page 371.

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.

Recent insights into the biological functions of liver fatty acid binding protein 1

Over four decades have passed since liver fatty acid binding protein (FABP)1 was first isolated. There are few protein families for which most of the complete tertiary structures, binding properties, and tissue occurrences are described in such detail and yet new functions are being uncovered for this protein. FABP1 is known to be critical for fatty acid uptake and intracellular transport and also has an important role in regulating lipid metabolism and cellular signaling pathways. FABP1 is an important endogenous cytoprotectant, minimizing hepatocyte oxidative damage and interfering with ischemia-reperfusion and other hepatic injuries. The protein may be targeted for metabolic activation through the cross-talk among many transcriptional factors and their activating ligands. Deficiency or malfunction of FABP1 has been reported in several diseases. FABP1 also influences cell proliferation during liver regeneration and may be considered as a prognostic factor for hepatic surgery. FABP1 binds and modulates the action of many molecules such as fatty acids, heme, and other metalloporphyrins. The ability to bind heme is another cytoprotective property and one that deserves closer investigation. The role of FABP1 in substrate availability and in protection from oxidative stress suggests that FABP1 plays a pivotal role during intracellular bacterial/viral infections by reducing inflammation and the adverse effects of starvation (energy deficiency).

Association of a Human FABP1 Gene Promoter Region Polymorphism with Altered Serum Triglyceride Levels

Liver fatty acid-binding protein (L-FABP), also known as fatty acid-binding protein 1 (FABP1), is a key regulator of hepatic lipid metabolism. Elevated FABP1 levels are associated with an increased risk of cardiovascular disease (CVD) and metabolic syndromes. In this study, we examine the association of FABP1 gene promoter variants with serum FABP1 and lipid levels in a Chinese population. Four promoter single-nucleotide polymorphisms (SNPs) of FABP1 gene were genotyped in a cross-sectional survey of healthy volunteers (n = 1,182) from Fuzhou city of China. Results showed that only the rs2919872 G>A variant was significantly associated with serum TG concentration(P = 0.032).Compared with the rs2919872 G allele, rs2919872 A allele contributed significantly to reduced serum TG concentration, and this allele dramatically decreased the FABP1 promoter activity(P < 0.05). The rs2919872 A allele carriers had considerably lower serum FABP1 levels than G allele carriers (P < 0.01). In the multivariable linear regression analysis, the rs2919872 A allele was negatively associated with serum FABP1 levels (β = —0.320, P = 0.003), while serum TG levels were positively associated with serum FABP1 levels (β = 0.487, P = 0.014). Our data suggest that compared with the rs2919872 G allele, the rs2919872 A allele reduces the transcriptional activity of FABP1 promoter, and thereby may link FABP1 gene variation to TG level in humans.

Loss of L-FABP, SCP-2/SCP-x, or both induces hepatic lipid accumulation in female mice

Although roles for both sterol carrier protein-2/sterol carrier protein-x (SCP-2/SCP-x) and liver fatty acid binding protein (L-FABP) have been proposed in hepatic lipid accumulation, individually ablating these genes has been complicated by concomitant alterations in the other gene product(s). For example, ablating SCP2/SCP-x induces upregulation of L-FABP in female mice. Therefore, the impact of ablating SCP-2/SCP-x (DKO) or L-FABP (LKO) individually or both together (TKO) was examined in female mice. Loss of SCP-2/SCP-x (DKO, TKO) more so than loss of L-FABP alone (LKO) increased hepatic total lipid and total cholesterol content, especially cholesteryl ester. Hepatic accumulation of nonesterified long chain fatty acids (LCFA) and phospholipids occurred only in DKO and TKO mice. Loss of SCP-2/SCP-x (DKO, TKO) increased serum total lipid primarily by increasing triglycerides. Altered hepatic level of proteins involved in cholesterol uptake, efflux, and/or secretion was observed, but did not compensate for the loss of L-FABP, SCP-2/SCP-x or both. However, synergistic responses were not seen with the combinatorial knock out animals-suggesting that inhibiting SCP-2/SCP-x is more correlative with hepatic dysfunction than L-FABP. The DKO- and TKO-induced hepatic accumulation of cholesterol and long chain fatty acids shared significant phenotypic similarities with non-alcoholic fatty liver disease (NAFLD).

Human FABP1 T94A variant enhances cholesterol uptake

Although expression of the human liver fatty acid binding protein (FABP1) T94A variant alters serum lipoprotein cholesterol levels in human subjects, nothing is known whereby the variant elicits these effects. This issue was addressed by in vitro cholesterol binding assays using purified recombinant wild-type (WT) FABP1 T94T and T94A variant proteins and in cultured primary human hepatocytes expressing the FABP1 T94T (genotyped as TT) or T94A (genotyped as CC) proteins. The human FABP1 T94A variant protein had 3-fold higher cholesterol-binding affinity than the WT FABP1 T94T as shown by NBD-cholesterol fluorescence binding assays and by cholesterol isothermal titration microcalorimetry (ITC) binding assays. CC variant hepatocytes also exhibited 30% higher total FABP1 protein. HDL- and LDL-mediated NBD-cholesterol uptake was faster in CC variant than TT WT human hepatocytes. VLDL-mediated uptake of NBD-cholesterol did not differ between CC and TT human hepatocytes. The increased HDL- and LDL-mediated NBD-cholesterol uptake was not associated with any significant change in mRNA levels of SCARB1, LDLR, CETP, and LCAT encoding the key proteins in lipoprotein cholesterol uptake. Thus, the increased HDL- and LDL-mediated NBD-cholesterol uptake by CC hepatocytes may be associated with higher affinity of T94A protein for cholesterol and/or increased total T94A protein level.

Association of L-FABP T94A and MTP I128T polymorphisms with hyperlipidemia in Chinese subjects

The purpose of this study was to evaluate the relation between the L-FABP T94A and MTP I128T polymorphisms and hyperlipidemia in Chinese subjects. We recruited 390 volunteers: 201 hyperlipidemic and 189 healthy volunteers. The L-FABP T94A and MTP I128T polymorphisms were genotyped using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). Anthropometry, lipid profile, and liver function of the subjects were determined. We observed that male carriers of the L-FABP A94 allele had significantly higher body weight (P = 0.012), higher body mass index (BMI) (P = 0.014), and higher plasma triacylglycerol levels (TAG) (P = 0.033) and lower ratios of high-density lipoprotein cholesterol (HDL-C) to total cholesterol (TC) (P = 0.008) than T94 homozygotes. The MTP T128 allele was associated with significantly lower serum TC (P < 0.001) and low-density lipoprotein cholesterol (LDL-C) (P < 0.001) levels in males. There was a direct correlation between the MTP T128 allele and a decreased risk of hyperlipidemia after adjusting for body mass index (OR = 0.327, 95 % CI: 0.178-0.600, P < 0.001). In conclusion, both the MTP I128T and the L-FABP T94A polymorphisms can affect serum lipid levels in the Chinese population. The MTP T128 allele offers protection against hyperlipidemia in the Chinese population.