Decreased body weight and hepatic steatosis with altered fatty acid ethanolamide metabolism in aged L-Fabp -/- mice

Design, synthesis, and biological evaluation of novel highly selective non-carboxylic acid FABP1 inhibitors

Histologic spectrum studies in patients revealed fatty acid binding proteins 1 (FABP1) as a potential new target for the treatment of metabolic associated fatty liver disease. However, there is no FABP1 inhibitor has been reported except the first-in-class FABP1 inhibitor bearing acid moiety reported by our laboratory. Herein, we firstly report the structure-activity relationship of novel non-carboxylic acid FABP1 inhibitors, which resulted in the identification of the potent and selective FABP1 inhibitor 30. The IC50 value of compound 30 for subtype FABP4 in the same family was greater than 80 μM. Moreover, compound 30 significantly alleviated the hepatic steatosis in DIO mice, which is equivalent to that of clinical drug obeticholic acid. This study might be provided a promising probe for the development of FABP1 inhibitors and thus can help to further elucidate the pharmacology of FABP1.

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.

Intestinal peroxisome proliferator-activated receptor α-fatty acid-binding protein 1 axis modulates nonalcoholic steatohepatitis

BACKGROUND AND AIMS

Peroxisome proliferator-activated receptor α (PPARα) regulates fatty acid transport and catabolism in liver. However, the role of intestinal PPARα in lipid homeostasis is largely unknown. Here, intestinal PPARα was examined for its modulation of obesity and NASH.

APPROACH AND RESULTS

Intestinal PPARα was activated and fatty acid-binding protein 1 (FABP1) up-regulated in humans with obesity and high-fat diet (HFD)-fed mice as revealed by using human intestine specimens or HFD/high-fat, high-cholesterol, and high-fructose diet (HFCFD)-fed C57BL/6N mice and PPARA -humanized, peroxisome proliferator response element-luciferase mice. Intestine-specific Ppara or Fabp1 disruption in mice fed a HFD or HFCFD decreased obesity-associated metabolic disorders and NASH. Molecular analyses by luciferase reporter assays and chromatin immunoprecipitation assays in combination with fatty acid uptake assays in primary intestinal organoids revealed that intestinal PPARα induced the expression of FABP1 that in turn mediated the effects of intestinal PPARα in modulating fatty acid uptake. The PPARα antagonist GW6471 improved obesity and NASH, dependent on intestinal PPARα or FABP1. Double-knockout ( Ppara/Fabp1ΔIE ) mice demonstrated that intestinal Ppara disruption failed to further decrease obesity and NASH in the absence of intestinal FABP1. Translationally, GW6471 reduced human PPARA-driven intestinal fatty acid uptake and improved obesity-related metabolic dysfunctions in PPARA -humanized, but not Ppara -null, mice.

CONCLUSIONS

Intestinal PPARα signaling promotes NASH progression through regulating dietary fatty acid uptake through modulation of FABP1, which provides a compelling therapeutic target for NASH treatment.

Molecular basis for the interaction of cellular retinol binding protein 2 (CRBP2) with nonretinoid ligands

Present in the small intestine, cellular retinol binding protein 2 (CRBP2) plays an important role in the uptake, transport, and metabolism of dietary retinoids. However, the recent discovery of the interactions of CRBP2 with 2-arachidonoylglycerol and other monoacylglycerols (MAGs) suggests the broader involvement of this protein in lipid metabolism and signaling. To better understand the physiological role of CRBP2, we determined its protein-lipid interactome using a fluorescence-based retinol replacement assay adapted for a high-throughput screening format. By examining chemical libraries of bioactive lipids, we provided evidence for the selective interaction of CRBP2 with a subset of nonretinoid ligands with the highest affinity for sn-1 and sn-2 MAGs that contain polyunsaturated C18-C20 acyl chains. We also elucidated the structure-affinity relationship for nonretinoid ligands of this protein. We further dissect the molecular basis for this ligand's specificity by analyzing high-resolution crystal structures of CRBP2 in complex with selected derivatives of MAGs. Finally, we identify T51 and V62 as key amino acids that enable the broadening of ligand selectivity to MAGs in CRBP2 as compared with retinoid-specific CRBP1. Thus, our study provides the molecular framework for understanding the lipid selectivity and diverse functions of CRBPs in controlling lipid homeostasis.

The Role of Elevated Liver-Type Fatty Acid-Binding Proteins in Liver Diseases

Liver-type fatty acid-binding protein (L-FABP) is mainly expressed in the liver as well as the proximal tubular epithelial cells in the kidney. In general, the proteins and enzymes existing within the hepatocytes have the potential to become biomarkers, for instance alanine aminotransferase, which reflects hepatocellular damage. However, due to reduced hepatocellular function in late stage of chronic liver diseases (e.g. cirrhosis), proteins and enzymes relating to hepatocellular damage are not always accurate measures of disease progression. Recently, several publications have demonstrated elevated serum L-FABP levels during the progression of human liver diseases, including hepatocellular carcinoma (HCC), and were a prognostic factor for survival in acute and chronic liver disease patients. However, the study regarding serum L-FABP levels and hepatic L-FABP expression in liver diseases is not sufficient to understand the molecular mechanism of L-FABP during the progression of these disease states. In this review, we focus on the use of serum and/or hepatic L-FABP expression as a biomarker in human liver diseases, including mechanistic potential in HCC.

Retinol-binding protein 2 (RBP2): biology and pathobiology

Retinol-binding protein 2 (RBP2; originally cellular retinol-binding protein, type II (CRBPII)) is a 16 kDa cytosolic protein that in the adult is localized predominantly to absorptive cells of the proximal small intestine. It is well established that RBP2 plays a central role in facilitating uptake of dietary retinoid, retinoid metabolism in enterocytes, and retinoid actions locally within the intestine. Studies of mice lacking Rbp2 establish that Rbp2 is not required in times of dietary retinoid-sufficiency. However, in times of dietary retinoid-insufficiency, the complete lack of Rbp2 gives rise to perinatal lethality owing to RBP2 absence in both placental (maternal) and neonatal tissues. Moreover, when maintained on a high-fat diet, Rbp2-knockout mice develop obesity, glucose intolerance and a fatty liver. Unexpectedly, recent investigations have demonstrated that RBP2 binds long-chain 2-monoacylglycerols (2-MAGs), including the canonical endocannabinoid 2-arachidonoylglycerol, with very high affinity, equivalent to that of retinol binding. Crystallographic studies establish that 2-MAGs bind to a site within RBP2 that fully overlaps with the retinol binding site. When challenged orally with fat, mucosal levels of 2-MAGs in Rbp2 null mice are significantly greater than those of matched controls establishing that RBP2 is a physiologically relevant MAG-binding protein. The rise in MAG levels is accompanied by elevations in circulating levels of the hormone glucose-dependent insulinotropic polypeptide (GIP). It is not understood how retinoid and/or MAG binding to RBP2 affects the functions of this protein, nor is it presently understood how these contribute to the metabolic and hormonal phenotypes observed for Rbp2-deficient mice.

Retinol-binding protein 2 (RBP2) binds monoacylglycerols and modulates gut endocrine signaling and body weight

Expressed in the small intestine, retinol-binding protein 2 (RBP2) facilitates dietary retinoid absorption. Rbp2-deficient (Rbp2-/- ) mice fed a chow diet exhibit by 6-7 months-of-age higher body weights, impaired glucose metabolism, and greater hepatic triglyceride levels compared to controls. These phenotypes are also observed when young Rbp2-/- mice are fed a high fat diet. Retinoids do not account for the phenotypes. Rather, RBP2 is a previously unidentified monoacylglycerol (MAG)-binding protein, interacting with the endocannabinoid 2-arachidonoylglycerol (2-AG) and other MAGs with affinities comparable to retinol. X-ray crystallographic studies show that MAGs bind in the retinol binding pocket. When challenged with an oil gavage, Rbp2-/- mice show elevated mucosal levels of 2-MAGs. This is accompanied by significantly elevated blood levels of the gut hormone GIP (glucose-dependent insulinotropic polypeptide). Thus, RBP2, in addition to facilitating dietary retinoid absorption, modulates MAG metabolism and likely signaling, playing a heretofore unknown role in systemic energy balance.

Mechanisms underlying reduced weight gain in intestinal fatty acid-binding protein (IFABP) null mice

Intestinal-fatty acid binding protein (IFABP; FABP2) is a 15-kDa intracellular protein abundantly present in the cytosol of the small intestinal (SI) enterocyte. High-fat (HF) feeding of IFABP-/- mice resulted in reduced weight gain and fat mass relative to wild-type (WT) mice. Here, we examined intestinal properties that may underlie the observed lean phenotype of high fat-fed IFABP-/- mice. No alterations in fecal lipid content were found, suggesting that the IFABP-/- mice are not malabsorbing dietary fat. However, the total excreted fecal mass, normalized to food intake, was increased for the IFABP-/- mice relative to WT mice. Moreover, intestinal transit time was more rapid in the IFABP-/- mice. IFABP-/- mice displayed a shortened average villus length, a thinner muscularis layer, reduced goblet cell density, and reduced Paneth cell abundance. The number of proliferating cells in the crypts of IFABP-/- mice did not differ from that of WT mice, suggesting that the blunt villi phenotype is not due to alterations in proliferation. IFABP-/- mice were observed to have altered expression of genes and proteins related to intestinal structure, while immunohistochemical analyses revealed increased staining for markers of inflammation. Taken together, these studies indicate that the ablation of IFABP, coupled with high-fat feeding, leads to changes in gut motility and morphology, which likely contribute to the relatively leaner phenotype occurring at the whole-body level. Thus, IFABP is likely involved in dietary lipid sensing and signaling, influencing intestinal motility, intestinal structure, and nutrient absorption, thereby impacting systemic energy metabolism.NEW & NOTEWORTHY Intestinal fatty acid binding protein (IFABP) is thought to be essential for the efficient uptake and trafficking of dietary fatty acids. In this study, we demonstrate that high-fat-fed IFABP-/- mice have an increased fecal output and are likely malabsorbing other nutrients in addition to lipid. Furthermore, we observe that the ablation of IFABP leads to marked alterations in intestinal morphology and secretory cell abundance.

The temporal-spatial patterns, polymorphisms and association analysis with meat quality traits of FABP1 gene in domestic pigeons (Columba livia)

1. Fatty acid-binding proteins (FABP) are members of lipid-binding proteins, which participate in the metabolism and intracellular transportation of lipids. This study was designed to investigate the expression patterns, polymorphisms and associations with meat quality traits of the FABP1 gene in pigeons. 2. The temporal-spatial expression patterns showed FABP1 was widely expressed in all eleven tissues from 0-4 weeks of age, the expression level in the liver was the highest, followed by the small intestine and subcutaneous fat. 3. Five novel SNPs were found; all of them were synonymous and in Hardy-Weinberg equilibrium. Association analysis revealed that for the SNP of G161C, the AB and BB genotypes had higher (P ≤ 0.01) inosinic acid concentrations in breast muscle than the AA genotype. The BB genotype showed the highest (P < 0.01) intramuscular fat among the three genotypes, and significantly greater FABP1 mRNA levels were observed in the breast muscle of the BB genotype than in the AA and AB genotypes (P < 0.01). In the SNP C1376T, the AB and BB genotypes showed higher (P < 0.01) intramuscular fat than the AA genotype, and the relative mRNA expression level of the BB (P < 0.01) and AB (P < 0.05) genotypes was higher than that of the AA genotype in breast muscle. Correlation analysis implied that the FABP1 mRNA expression level was closely related to the inosinic acid (P < 0.05) and intramuscular fat content (P < 0.01). Oil red O staining of frozen sections of breast muscle on d 28 for SNPs G161C and C1376T also indicated that the BB genotype had the highest intramuscular fat content in both SNPs. In addition, correlation analysis implied the FABP1 mRNA expression level was closely related to inosinic acid (P < 0.05) and intramuscular fat content (P < 0.01). 4. The results suggested that FABP1 could be a potential candidate gene in marker-assisted selection for breeding pigeons with high-quality meat.

Serum Liver-Type Fatty Acid-Binding Protein Is a Possible Prognostic Factor in Human Chronic Liver Diseases From Chronic Hepatitis to Liver Cirrhosis and Hepatocellular Carcinoma

Liver‐type fatty acid–binding protein (L‐FABP) is a key regulator of fatty acid metabolism, but serum L‐FABP levels are not well investigated in chronic liver diseases. We aimed to elucidate the prognostic ability of serum L‐FABP in human chronic liver diseases and compare it with the albumin‐bilirubin (ALBI) score. In 242 chronic liver disease patients, including chronic hepatitis (CH, n = 100), liver cirrhosis (LC, n = 142), and presence of hepatocellular carcinoma (HCC, n = 144), serum L‐FABP levels were correlated with liver function (P < 0.0001), increased in LC compared with CH (P < 0.01), and correlated to ALBI score (P < 0.0001). Serum L‐FABP levels were increased in the presence of HCC (P < 0.0001), correlating to des‐gamma‐carboxy prothrombin (P < 0.0001), alpha‐fetoprotein (P = 0.009), and Barcelona‐Clinic Liver Cancer stage. In the average follow‐up period of 1,054 days, serum L‐FABP levels were elevated (P < 0.0001) in patients who eventually died. The area under the curve (AUC) of serum L‐FABP (0.764) was higher than that of ALB (0.709), and the patients with serum L‐FABP ≤ 6.8 ng/mL had significantly longer rates of survival (P < 0.0001). Serum L‐FABP (hazard ratio [HR] 4.0; P < 0.001), HCC (HR 3.7; P = 0.001), ALBI score (HR 2.7; P < 0.001), and age (HR 1.0; P = 0.049) were independent predictors of survival. In the subgroup who maintained liver function, the AUC of serum L‐FABP (0.751) was higher than that of ALB (0.643). In this subgroup, serum L‐FABP (HR 4.4; P = 0.002) and HCC (HR 13.9; P < 0.001) were independent predictors of survival. Conclusion: Serum L‐FABP is a possible predictor of survival in chronic liver diseases from CH to LC and HCC, including any subgroup that maintains liver function.

Hepatocyte and stellate cell deletion of liver fatty acid binding protein reveals distinct roles in fibrogenic injury

Liver fatty acid binding protein (L-Fabp) modulates lipid trafficking in enterocytes, hepatocytes, and hepatic stellate cells (HSCs). We examined hepatocyte vs. HSC L-Fabp deletion in hepatic metabolic adaptation and fibrotic injury. Floxed L-Fabp mice were bred to different transgenic Cre mice or injected with adeno-associated virus type 8 (AAV8) Cre and fed diets to promote steatosis and fibrosis or were subjected to either bile duct ligation or CCl4 injury. Albumin-Cre-mediated L-Fabp deletion revealed recombination in hepatocytes and HSCs; these findings were confirmed with 2 other floxed alleles. Glial fibrillary acid protein-Cre and platelet-derived growth factor receptor β-Cre-mediated L-Fabp deletion demonstrated recombination only in HSCs. Mice with albumin promoter-driven Cre recombinase (Alb-Cre)-mediated or AAV8-mediated L-Fabp deletion were protected against food withdrawal-induced steatosis. Mice with Alb-Cre-mediated L-Fabp deletion were protected against high saturated fat-induced steatosis and fibrosis, phenocopying germline L-Fabp-/- mice. Mice with HSC-specific L-Fabp deletion exhibited retinyl ester depletion yet demonstrated no alterations in fibrosis. On the other hand, fibrogenic resolution after CCl4 administration was impaired in mice with Alb-Cre-mediated L-Fabp deletion. These findings suggest cell type-specific roles for L-Fabp in mitigating hepatic steatosis and in modulating fibrogenic injury and reversal.-Newberry, E. P., Xie, Y., Lodeiro, C., Solis, R., Moritz, W., Kennedy, S., Barron, L., Onufer, E., Alpini, G., Zhou, T., Blaner, W. S., Chen, A., Davidson, N. O. Hepatocyte and stellate cell deletion of liver fatty acid binding protein reveal distinct roles in fibrogenic injury.

Oleoylethanolamide differentially regulates glycerolipid synthesis and lipoprotein secretion in intestine and liver

Dietary fat absorption takes place in the intestine, and the liver mobilizes endogenous fat to other tissues by synthesizing lipoproteins that require apoB and microsomal triglyceride transfer protein (MTP). Dietary fat triggers the synthesis of oleoylethanolamide (OEA), a regulatory fatty acid that signals satiety to reduce food intake mainly by enhancing neural PPARα activity, in enterocytes. We explored OEA's roles in the assembly of lipoproteins in WT and Ppara ^-/- mouse enterocytes and hepatocytes, Caco-2 cells, and human liver-derived cells. In differentiated Caco-2 cells, OEA increased synthesis and secretion of triacylglycerols, apoB secretion in chylomicrons, and MTP expression in a dose-dependent manner. OEA also increased MTP activity and triacylglycerol secretion in WT and knockout primary enterocytes. In contrast to its intestinal cell effects, OEA reduced synthesis and secretion of triacylglycerols, apoB secretion, and MTP expression and activity in human hepatoma Huh-7 and HepG2 cells. Also, OEA reduced MTP expression and triacylglycerol secretion in WT, but not knockout, primary hepatocytes. These studies indicate differential effects of OEA on lipid synthesis and lipoprotein assembly: in enterocytes, OEA augments glycerolipid synthesis and lipoprotein assembly independent of PPARα. Conversely, in hepatocytes, OEA reduces MTP expression, glycerolipid synthesis, and lipoprotein secretion through PPARα-dependent mechanisms.

Hepatic lipid metabolism and non-alcoholic fatty liver disease in aging

Aging is associated with dysregulation of glucose and lipid metabolism. Various factors that contribute to the dysregulation include both modifiable (e.g. obesity, insulin resistance) and non-modifiable risk factors (age-associated physiologic changes). Although there is no linear relationship between aging and prevalence of non-alcoholic fatty liver disease, current data strongly suggests that advanced age leads to more severe histological changes and poorer clinical outcomes. Hepatic lipid accumulation could lead to significant hepatic and systemic consequences including steatohepatitis, cirrhosis, impairment of systemic glucose metabolism and metabolic syndrome, thereby contributing to age-related diseases. Insulin, leptin and adiponectin are key regulators of the various physiologic processes that regulate hepatic lipid metabolism. Recent advances have expanded our understanding in this field, highlighting the role of novel mediators such as FGF 21, and mitochondria derived peptides. In this review, we will summarize the mediators of hepatic lipid metabolism and how they are altered in aging.

Tumor suppressor RIZ1 in obesity and the PI3K/AKT/mTOR pathway

OBJECTIVE

The aim of this study was to investigate the shared molecular pathways of obesity and cancer by exploring the role of RIZ1 in obesity and the phospatidylinositol 3-kinase (PI3K)/V-Akt murine thymoma viral oncogene homolog (PKB) (AKT)/mechanistic target of rapamycin (mTOR) pathway.

METHODS

Male wild type (WT) and Riz1(-/-) mice (KO) were fed a standard diet (STD) or a high-fat (HF) diet for up to 8 months. These mice were studied for phenotypic and molecular changes.

RESULTS

Riz1(-/-) mice gained more weight on a HF diet compared to WT mice, with higher free fatty acid and increased visceral fat. Metabolic cage analysis of Riz1(-/-) mice showed lower oxygen consumption but no changes in food intake and ambulatory activity. Riz1(-/-) mice showed impaired glucose regulation but no change in insulin sensitivity. RNA-seq and quantitative RT-PCR analysis found altered expression in certain glycolysis and ATP production genes such as Ubiad1, Atp5g2, and Cyp4a12. The PI3K/AKT/mTOR pathway was activated in the Riz1(-/-) mice fed a HF diet with higher Akt3 mRNA levels and increased phosphorylation of AKT (Ser473), mTOR, and S6.

CONCLUSIONS

The results identify RIZ1 as an important regulator of both Akt3 transcription and AKT phosphorylation and suggest a role for RIZ1 in HF-induced obesity and the AKT pathway.

Phenotypic divergence in two lines of L-Fabp-/- mice reflects substrain differences and environmental modifiers

Phenotypic divergence in diet-induced obesity (DIO) and hepatic steatosis has been reported in two independently generated lines of L-Fabp(-/-) mice [New Jersey (NJ) L-Fabp(-/-) vs. Washington University (WU) L-Fabp(-/-) mice]. We performed side-by-side studies to examine differences between the lines and investigate the role of genetic background, intestinal microbiota, sex, and diet in the divergent phenotypes. Fasting-induced steatosis was attenuated in both L-Fabp(-/-) lines compared with C57BL/6J controls, with restoration of hepatic triglyceride levels following adenoviral L-Fabp rescue. Both lines were protected against DIO after high-saturated-fat diet feeding. Hepatic steatosis was attenuated in WU but not NJ L-Fabp(-/-) mice, although this difference between the lines disappeared upon antibiotic treatment and cohousing. In contrast, there was phenotypic divergence in L-Fabp(-/-) mice fed a high cocoa butter fat diet, with WU L-Fabp(-/-) mice, but not NJ L-Fabp(-/-) mice, showing protection against both DIO and hepatic steatosis, with some sex-dependent (female > male) differences. Dense mapping revealed no evidence of unintended targeting, duplications, or deletions surrounding the Fabp1 locus in either line and only minor differences in mRNA expression of genes located near the targeted allele. However, a C57BL/6 substrain screen showed that the NJ L-Fabp(-/-) line contains ∼40% C57BL/6N genomic DNA, despite reports that these mice were backcrossed six generations. Overall, these findings suggest that some of the phenotypic divergence between the two L-Fabp(-/-) lines may reflect unanticipated differences in genetic background, underscoring the importance of genetic background in phenotypic characterization.

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).

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.

[Role of metabolic lipases and lipotoxicity in the development of non-alcoholic steatosis and non-alcoholic steatohepatitis]

Non-alcoholic fatty liver disease (NAFLD) has become the most common liver disease in developed countries, covering a spectrum of pathological conditions ranging from single steatosis to non-alcoholic steatohepatitis, cirrhosis and hepatocellular carcinoma. Its pathogenesis has been often interpreted by the "double-hit" hypothesis, where the lipid accumulation in the liver is followed by proinflammatory mediators inducing inflammation, hepatocellular injury and fibrosis. Nowadays, a more complex model suggests that free fatty acids and their metabolites could be the true lipotoxic agents that contribute to the development of NAFLD and hepatic insulin resistance, suggesting a central role for metabolic lipases in that process.

Changes in FABP1 and gastrin receptor expression in the testes of rats that have undergone electrical injury

Testicular trauma may occur due to accidental electrical injury. The aim of this study was to investigate alterations in the levels of fatty acid-binding protein 1 (FABP1) and gastrin receptor (gastrin R) in the testes following electrical injury. Sprague-Dawley rats were divided into control, fatal electrocution (220 V, 50 Hz, 60 sec) and electrical injury (220 V, 50 Hz, 60 sec) groups (n=8 per group). The animals in the fatal electrocution and electrical injury groups were deeply anesthetized with sodium pentobarbital prior to each treatment, in which the current was delivered via an anode connected to the left foreleg and a cathode to the right hindleg. The rats that survived were subsequently sacrificed by cervical dislocation. Control animals received cervical dislocation alone. Immunohistochemical analysis was performed to evaluate the protein expression of FABP1 and gastrin R in the testes. Sections were evaluated by digital image analysis. The expression levels of FABP1 and gastrin R were significantly increased following electrical injury, supported by an increase in the integrated optical density (IOD) when compared with that in the control group (P<0.05). However, no significant difference was found in FABP1 and gastrin R expression levels between the fatal electrocution and control groups. In summary, the protein expression levels of FABP1 and gastrin R were found to be significantly altered by electrical injury, suggesting that these two proteins may be important in underlying mechanisms of testicular injury during electrical injury. The findings indicate that such alterations would be reflected in abnormal testicular function.

Enterocyte fatty acid-binding proteins (FABPs): different functions of liver and intestinal FABPs in the intestine

Fatty acid-binding proteins (FABP) are highly abundant cytosolic proteins that are expressed in most mammalian tissues. In the intestinal enterocyte, both liver- (LFABP; FABP1) and intestinal FABPs (IFABP; FABP2) are expressed. These proteins display high-affinity binding for long-chain fatty acids (FA) and other hydrophobic ligands; thus, they are believed to be involved with uptake and trafficking of lipids in the intestine. In vitro studies have identified differences in ligand-binding stoichiometry and specificity, and in mechanisms of FA transfer to membranes, and it has been hypothesized that LFABP and IFABP have different functions in the enterocyte. Studies directly comparing LFABP- and IFABP-null mice have revealed markedly different phenotypes, indicating that these proteins indeed have different functions in intestinal lipid metabolism and whole body energy homeostasis. In this review, we discuss the evolving knowledge of the functions of LFABP and IFABP in the intestinal enterocyte.

Molecular characterization and different expression patterns of the FABP gene family during goat skeletal muscle development

The FABP (adipocyte fatty acid-binding protein) genes play an important role in intracellular fatty acid transport and considered to be candidate genes for fatness traits in domestic animal. In this study, we cloned the cDNA sequences of goat FABP family genes and their expression patterns were detected by semi-quantitative RT-PCR and quantitative real time RT-PCR. Expression analysis showed that goat FABP1 gene was predominantly expressed in liver, kidney and large intestine. While FABP4 was widely expressed in many tissues with a high expression level was observed in the fat, skeletal muscle, stomach and lung. Notably, FABP2 gene was expressed specifically in small intestine. Moreover, goat FABP3 was expressed at 60 day with the highest level, then significantly (p < 0.01) decreased at the 90 day. No significant expression differences were observed in longissimus dorsi muscles among 3 day, 30 day and 60 day. Goat FABP4 was expressed at 3 day with the lowest level, then significantly (p < 0.01) increased to a peak at the 60 day. In addition, a significant relationship between FABP3 mRNA expression levels and intramuscular fat (IMF) content was observed. These results suggest that the FABP3 and FABP4 may be important genes for meat quality and provides useful information for further studies on their roles in skeletal muscle IMF deposit.

High dietary fat intake influences the activation of specific hindbrain and hypothalamic nuclei by the satiety factor oleoylethanolamide

Chronic exposure to a diet rich in fats changes the gastrointestinal milieu and alters responses to several signals involved in the control of food intake. Oleoylethanolamide (OEA) is a gut-derived satiety signal released from enterocytes upon the ingestion of dietary fats. The anorexigenic effect of OEA, which requires intestinal PPAR-alpha receptors and is supposedly mediated by vagal afferents, is associated with the induction of c-fos in several brain areas involved in the control of food intake, such as the nucleus of the solitary tract (NST) and the hypothalamic paraventricular (PVN) and supraoptic nuclei (SON). In the present study we investigated whether the exposure to a high fat diet (HFD) alters the hindbrain and hypothalamic responses to OEA. To this purpose we evaluated the effects of OEA at a dose that reliably inhibits eating (10mg/kg i.p.) on the induction of c-fos in the NST, area postrema (AP), PVN and SON in rats maintained either on standard chow or a HFD. We performed a detailed analysis of the different NST subnuclei activated by i.p. OEA and found that peripheral OEA strongly activates c-fos expression in the AP, NST and in the hypothalamus of both chow and HFD fed rats. The extent of c-fos expression was, however, markedly different between the two groups of rats, with a weaker activation of selected NST subnuclei and stronger activation of the PVN in HFD-fed than in chow-fed rats. HFD-fed rats were also more sensitive to the immediate hypophagic action of OEA than chow-fed rats. These effects may be due to a decreased sensitivity of vagal afferent fibers that might mediate OEA's actions on the brain and/or an altered sensitivity of brain structures to OEA.

Oleoylethanolamide: a novel potential pharmacological alternative to cannabinoid antagonists for the control of appetite

The initial pharmaceutical interest for the endocannabinoid system as a target for antiobesity therapies has been restricted by the severe adverse effects of the CB1 antagonist rimonabant. This study points at oleoylethanolamide (OEA), a monounsaturated analogue, and functional antagonist of anandamide, as a potential and safer antiobesity alternative to CB1 antagonism. Mice treated with equal doses (5 or 10 mg/kg, i.p.) of OEA or rimonabant were analyzed for the progressive expression of spontaneous behaviors (eating, grooming, rearing, locomotion, and resting) occurring during the development of satiety, according to the paradigm called behavioral satiety sequence (BSS). Both drugs reduced food (wet mash) intake to a similar extent. OEA treatment decreased eating activity within the first 30 min and caused a temporary increase of resting time that was not accompanied by any decline of horizontal, vertical and total motor activity. Besides decreasing eating activity, rimonabant caused a marked increase of the time spent grooming and decreased horizontal motor activity, alterations that might be indicative of aversive nonmotivational effects on feeding. These results support the idea that OEA suppresses appetite by stimulating satiety and that its profile of action might be predictive of safer effects in humans as a novel antiobesity treatment.

Hepatic ATGL mediates PPAR-α signaling and fatty acid channeling through an L-FABP independent mechanism

Adipose TG lipase (ATGL) catalyzes the rate-limiting step in TG hydrolysis in most tissues. We have shown that hepatic ATGL preferentially channels hydrolyzed FAs to β-oxidation and induces PPAR-α signaling. Previous studies have suggested that liver FA binding protein (L-FABP) transports FAs from lipid droplets to the nucleus for ligand delivery and to the mitochondria for β-oxidation. To determine if L-FABP is involved in ATGL-mediated FA channeling, we used adenovirus-mediated suppression or overexpression of hepatic ATGL in either WT or L-FABP KO mice. Hepatic ATGL knockdown increased liver weight and TG content of overnight fasted mice regardless of genotype. L-FABP deletion did not impair the effects of ATGL overexpression on the oxidation of hydrolyzed FAs in primary hepatocyte cultures or on serum β-hydroxybutyrate concentrations in vivo. Moreover, L-FABP deletion did not influence the effects of ATGL knockdown or overexpression on PPAR-α target gene expression. Taken together, we conclude that L-FABP is not required to channel ATGL-hydrolyzed FAs to mitochondria for β-oxidation or the nucleus for PPAR-α regulation.

Nonalcoholic fatty liver disease, hepatic insulin resistance, and type 2 diabetes

Nonalcoholic fatty liver disease (NAFLD), hepatic insulin resistance, and type 2 diabetes are all strongly associated and are all reaching epidemic proportions. Whether there is a causal link between NAFLD and hepatic insulin resistance is controversial. This review will discuss recent studies in both humans and animal models of NAFLD that have implicated increases in hepatic diacylglycerol (DAG) content leading to activation of novel protein kinase Cϵ (PKCϵ) resulting in decreased insulin signaling in the pathogenesis of NAFLD-associated hepatic insulin resistance and type 2 diabetes. The DAG-PKCϵ hypothesis can explain the occurrence of hepatic insulin resistance observed in most cases of NAFLD associated with obesity, lipodystrophy, and type 2 diabetes.

Direct comparison of mice null for liver or intestinal fatty acid-binding proteins reveals highly divergent phenotypic responses to high fat feeding

The enterocyte expresses two fatty acid-binding proteins (FABP), intestinal FABP (IFABP; FABP2) and liver FABP (LFABP; FABP1). LFABP is also expressed in liver. Despite ligand transport and binding differences, it has remained uncertain whether these intestinally coexpressed proteins, which both bind long chain fatty acids (FA), are functionally distinct. Here, we directly compared IFABP(-/-) and LFABP(-/-) mice fed high fat diets containing long chain saturated or unsaturated fatty acids, reasoning that providing an abundance of dietary lipid would reveal unique functional properties. The results showed that mucosal lipid metabolism was indeed differentially modified, with significant decreases in FA incorporation into triacylglycerol (TG) relative to phospholipid (PL) in IFABP(-/-) mice, whereas LFABP(-/-) mice had reduced monoacylglycerol incorporation in TG relative to PL, as well as reduced FA oxidation. Interestingly, striking differences were found in whole body energy homeostasis; LFABP(-/-) mice fed high fat diets became obese relative to WT, whereas IFABP(-/-) mice displayed an opposite, lean phenotype. Fuel utilization followed adiposity, with LFABP(-/-) mice preferentially utilizing lipids, and IFABP(-/-) mice preferentially metabolizing carbohydrate for energy production. Changes in body weight and fat may arise, in part, from altered food intake; mucosal levels of the endocannabinoids 2-arachidonoylglycerol and arachidonoylethanolamine were elevated in LFABP(-/-), perhaps contributing to increased energy intake. This direct comparison provides evidence that LFABP and IFABP have distinct roles in intestinal lipid metabolism; differential intracellular functions in intestine and in liver, for LFABP(-/-) mice, result in divergent downstream effects at the systemic level.

Liver fatty acid-binding protein (L-Fabp) modifies intestinal fatty acid composition and adenoma formation in ApcMin/+ mice

Evidence suggests a relationship between dietary fat intake, obesity, and colorectal cancer, implying a role for fatty acid metabolism in intestinal tumorigenesis that is incompletely understood. Liver fatty acid-binding protein (L-Fabp), a dominant intestinal fatty acid-binding protein, regulates intestinal fatty acid trafficking and metabolism, and L-Fabp deletion attenuates diet-induced obesity. Here, we examined whether changes in intestinal fatty acid metabolism following L-Fabp deletion modify adenoma development in Apc(Min)(/+) mice. Compound L-Fabp(-/-)Apc(Min)(/+) mice were generated and fed a 10% fat diet balanced equally between saturated, monounsaturated, and polyunsaturated fat. L-Fabp(-/-)Apc(Min)(/+) mice displayed significant reductions in adenoma number and total polyp area compared with Apc(Min)(/+)controls, reflecting a significant shift in distribution toward smaller polyps. Adenomas from L-Fabp(-/-)Apc(Min)(/+) mice exhibited reductions in cellular proliferation, high-grade dysplasia, and nuclear β-catenin translocation. Intestinal fatty acid content was increased in L-Fabp(-/-)Apc(Min)(/+) mice, and lipidomic profiling of intestinal mucosa revealed significant shifts to polyunsaturated fatty acid species with reduced saturated fatty acid species. L-Fabp(-/-)Apc(Min)(/+) mice also showed corresponding changes in mRNA expression of enzymes involved in fatty acid elongation and desaturation. Furthermore, adenomas from L-Fabp(-/-)Apc(Min)(/+) mice displayed significant reductions in mRNA abundance of nuclear hormone receptors involved in cellular proliferation and in enzymes involved in lipogenesis. These findings collectively implicate L-Fabp as an important genetic modifier of intestinal tumorigenesis, and identify fatty acid trafficking and metabolic compartmentalization as an important pathway linking dietary fat intake, obesity, and intestinal tumor formation.

Liver fatty acid binding protein (L-Fabp) modulates murine stellate cell activation and diet-induced nonalcoholic fatty liver disease

Activation of hepatic stellate cells (HSCs) is crucial to the development of fibrosis in nonalcoholic fatty liver disease. Quiescent HSCs contain lipid droplets (LDs), whose depletion upon activation induces a fibrogenic gene program. Here we show that liver fatty acid-binding protein (L-Fabp), an abundant cytosolic protein that modulates fatty acid (FA) metabolism in enterocytes and hepatocytes, also modulates HSC FA utilization and in turn regulates the fibrogenic program. L-Fabp expression decreased 10-fold following HSC activation, concomitant with depletion of LDs. Primary HSCs isolated from L-FABP(-/-) mice contain fewer LDs than wild-type (WT) HSCs, and exhibit up-regulated expression of genes involved in HSC activation. Adenoviral L-Fabp transduction inhibited activation of passaged WT HSCs and increased both the expression of prolipogenic genes and also augmented intracellular lipid accumulation, including triglyceride and FA, predominantly palmitate. Freshly isolated HSCs from L-FABP(-/-) mice correspondingly exhibited decreased palmitate in the free FA pool. To investigate whether L-FABP deletion promotes HSC activation in vivo, we fed L-FABP(-/-) and WT mice a high-fat diet supplemented with trans-fatty acids and fructose (TFF). TFF-fed L-FABP(-/-) mice exhibited reduced hepatic steatosis along with decreased LD abundance and size compared to WT mice. In addition, TFF-fed L-FABP(-/-) mice exhibited decreased hepatic fibrosis, with reduced expression of fibrogenic genes, compared to WT mice.

CONCLUSION

L-FABP deletion attenuates both diet-induced hepatic steatosis and fibrogenesis, despite the observation that L-Fabp paradoxically promotes FA and LD accumulation and inhibits HSC activation in vitro. These findings highlight the importance of cell-specific modulation of hepatic lipid metabolism in promoting fibrogenesis in nonalcoholic fatty liver disease. (Hepatology 2013).

Liver fatty acid binding protein gene-ablation exacerbates weight gain in high-fat fed female mice

Loss of liver fatty acid binding protein (L-FABP) decreases long chain fatty acid uptake and oxidation in primary hepatocytes and in vivo. On this basis, L-FABP gene ablation would potentiate high-fat diet-induced weight gain and weight gain/energy intake. While this was indeed the case when L-FABP null (-/-) mice on the C57BL/6NCr background were pair-fed a high-fat diet, whether this would also be observed under high-fat diet fed ad libitum was not known. Therefore, this possibility was examined in female L-FABP (-/-) mice on the same background. L-FABP (-/-) mice consumed equal amounts of defined high-fat or isocaloric control diets fed ad libitum. However, on the ad libitum-fed high-fat diet the L-FABP (-/-) mice exhibited: (1) decreased hepatic long chain fatty acid (LCFA) β-oxidation as indicated by lower serum β-hydroxybutyrate level; (2) decreased hepatic protein levels of key enzymes mitochondrial (rate limiting carnitine palmitoyl acyltransferase A1, CPT1A; HMG-CoA synthase) and peroxisomal (acyl CoA oxidase 1, ACOX1) LCFA β-oxidation; (3) increased fat tissue mass (FTM) and FTM/energy intake to the greatest extent; and (4) exacerbated body weight gain, weight gain/energy intake, liver weight, and liver weight/body weight to the greatest extent. Taken together, these findings showed that L-FABP gene-ablation exacerbated diet-induced weight gain and fat tissue mass gain in mice fed high-fat diet ad libitum--consistent with the known biochemistry and cell biology of L-FABP.

The human liver fatty acid binding protein (FABP1) gene is activated by FOXA1 and PPARα; and repressed by C/EBPα: Implications in FABP1 down-regulation in nonalcoholic fatty liver disease

Liver fatty acid binding protein (FABP1) prevents lipotoxicity of free fatty acids and regulates fatty acid trafficking and partition. Our objective is to investigate the transcription factors controlling the human FABP1 gene and their regulation in nonalcoholic fatty liver disease (NAFLD). Adenovirus-mediated expression of multiple transcription factors in HepG2 cells and cultured human hepatocytes demonstrated that FOXA1 and PPARα are among the most effective activators of human FABP1, whereas C/EBPα is a major dominant repressor. Moreover, FOXA1 and PPARα induced re-distribution of FABP1 protein and increased cytoplasmic expression. Reporter assays demonstrated that the major basal activity of the human FABP1 promoter locates between -96 and -229bp, where C/EBPα binds to a composite DR1-C/EBP element. Mutation of this element at -123bp diminished basal reporter activity, abolished repression by C/EBPα and reduced transactivation by HNF4α. Moreover, HNF4α gene silencing by shRNA in HepG2 cells caused a significant down-regulation of FABP1 mRNA expression. FOXA1 activated the FABP1 promoter through binding to a cluster of elements between -229 and -592bp, whereas PPARα operated through a conserved proximal element at -59bp. Finally, FABP1, FOXA1 and PPARα were concomitantly repressed in animal models of NAFLD and in human nonalcoholic fatty livers, whereas C/EBPα was induced or did not change. We conclude that human FABP1 has a complex mechanism of regulation where C/EBPα displaces HNF4α and hampers activation by FOXA1 and PPARα. Alteration of expression of these transcription factors in NAFLD leads to FABP1 gen repression and could exacerbate lipotoxicity and disease progression.