Decreased Hepatic Triglyceride Accumulation and Altered Fatty Acid Uptake in Mice with Deletion of the Liver Fatty Acid-binding Protein Gene

A novel multiprotein complex is required to generate the prechylomicron transport vesicle from intestinal ER

Dietary lipid absorption is dependent on chylomicron production whose rate-limiting step across the intestinal absorptive cell is the exit of chylomicrons from the endoplasmic reticulum (ER) in its ER-to-Golgi transport vesicle, the prechylomicron transport vesicle (PCTV). This study addresses the composition of the budding complex for PCTV. Immunoprecipitation (IP) studies from rat intestinal ER solubilized in Triton X-100 suggested that vesicle-associated membrane protein 7 (VAMP7), apolipoprotein B48 (apoB48), liver fatty acid-binding protein (L-FABP), CD36, and the COPII proteins were associated on incubation of the ER with cytosol and ATP. This association was confirmed by chromatography of the solubilized ER over Sephacryl S400-HR in which these constituents cochromatographed with an apparent kDa of 630. No multiprotein complex was detected when the ER was chromatographed in the absence of PCTV budding activity (resting ER or PKCzeta depletion of ER and cytosol). Treatment of the ER with anti-apoB48 or anti-VAMP7 antibodies or using gene disrupted L-FABP or CD36 mice all significantly inhibited PCTV generation. A smaller complex (no COPII proteins) was formed when only rL-FABP was used to bud PCTV. The data support the conclusion that the PCTV budding complex in intestinal ER is composed of VAMP7, apoB48, CD36, and L-FABP, plus the COPII proteins.

PPAR/RXR Regulation of Fatty Acid Metabolism and Fatty Acid omega-Hydroxylase (CYP4) Isozymes: Implications for Prevention of Lipotoxicity in Fatty Liver Disease

Fatty liver disease is a common lipid metabolism disorder influenced by the combination of individual genetic makeup, drug exposure, and life-style choices that are frequently associated with metabolic syndrome, which encompasses obesity, dyslipidemia, hypertension, hypertriglyceridemia, and insulin resistant diabetes. Common to obesity related dyslipidemia is the excessive storage of hepatic fatty acids (steatosis), due to a decrease in mitochondria β-oxidation with an increase in both peroxisomal β-oxidation, and microsomal ω-oxidation of fatty acids through peroxisome proliferator activated receptors (PPARs). How steatosis increases PPARα activated gene expression of fatty acid transport proteins, peroxisomal and mitochondrial fatty acid β-oxidation and ω-oxidation of fatty acids genes regardless of whether dietary fatty acids are polyunsaturated (PUFA), monounsaturated (MUFA), or saturated (SFA) may be determined by the interplay of PPARs and HNF4α with the fatty acid transport proteins L-FABP and ACBP. In hepatic steatosis and steatohepatitis, the ω-oxidation cytochrome P450 CYP4A gene expression is increased even with reduced hepatic levels of PPARα. Although numerous studies have suggested the role ethanol-inducible CYP2E1 in contributing to increased oxidative stress, Cyp2e1-null mice still develop steatohepatitis with a dramatic increase in CYP4A gene expression. This strongly implies that CYP4A fatty acid ω-hydroxylase P450s may play an important role in the development of steatohepatitis. In this review and tutorial, we briefly describe how fatty acids are partitioned by fatty acid transport proteins to either anabolic or catabolic pathways regulated by PPARs, and we explore how medium-chain fatty acid (MCFA) CYP4A and long-chain fatty acid (LCFA) CYP4Fω-hydroxylase genes are regulated in fatty liver. We finally propose a hypothesis that increased CYP4A expression with a decrease in CYP4F genes may promote the progression of steatosis to steatohepatitis.

Mechanisms and clinical implications of hepatocyte lipoapoptosis

Nonalcoholic fatty liver disease (NAFLD) is characterized by insulin resistance, elevated serum levels of free fatty acids (FFAs) and fatty infiltration of the liver. Accumulation of triglycerides in the hepatocyte results from the uptake and esterification of circulating FFAs by the liver. Contrary to current theory, hepatic steatosis appears to be a detoxification process, as FFAs are directly cytotoxic for the hepatocyte and inhibition of triglyceride formation enhances FFAs toxicity. Hepatocyte apoptosis is a key feature of NAFLD and correlates with disease severity. Since FFA-induced toxicity, or lipoapoptosis, represents a mechanism for the pathogenesis of NAFLD, this article will highlight the cellular pathways contributing to hepatocyte lipoapoptosis. To date, there is no proven effective therapy for patients with NAFLD and insights into the molecular mediators of lipoapoptosis should help promote effective therapeutic strategies for this disease.

Caveolin, sterol carrier protein-2, membrane cholesterol-rich microdomains and intracellular cholesterol trafficking

While the existence of membrane lateral microdomains has been known for over 30 years, interest in these structures accelerated in the past decade due to the discovery that cholesterol-rich microdomains serve important biological functions. It is increasingly appreciated that cholesterol-rich microdomains in the plasma membranes of eukaryotic cells represent an organizing nexus for multiple cellular proteins involved in transmembrane nutrient uptake (cholesterol, fatty acid, glucose, etc.), cell-signaling, immune recognition, pathogen entry, and many other roles. Despite these advances, however, relatively little is known regarding the organization of cholesterol itself in these plasma membrane microdomains. Although a variety of non-sterol markers indicate the presence of microdomains in the plasma membranes of living cells, none of these studies have demonstrated that cholesterol is enriched in these microdomains in living cells. Further, the role of cholesterol-rich membrane microdomains as targets for intracellular cholesterol trafficking proteins such as sterol carrier protein-2 (SCP-2) that facilitate cholesterol uptake and transcellular transport for targeting storage (cholesterol esters) or efflux is only beginning to be understood. Herein, we summarize the background as well as recent progress in this field that has advanced our understanding of these issues.

High dietary fat exacerbates weight gain and obesity in female liver fatty acid binding protein gene-ablated mice

Since liver fatty acid binding protein (L-FABP) facilitates uptake/oxidation of long-chain fatty acids in cultured transfected cells and primary hepatocytes, loss of L-FABP was expected to exacerbate weight gain and/or obesity in response to high dietary fat. Male and female wild-type (WT) and L-FABP gene-ablated mice, pair-fed a defined isocaloric control or high fat diet for 12 weeks, consumed equal amounts of food by weight and kcal. Male WT mice gained weight faster than their female WT counterparts regardless of diet. L-FABP gene ablation enhanced weight gain more in female than male mice-an effect exacerbated by high fat diet. Dual emission X-ray absorptiometry revealed high-fat fed male and female WT mice gained mostly fat tissue mass (FTM). L-FABP gene ablation increased FTM in female, but not male, mice-an effect also exacerbated by high fat diet. Concomitantly, L-FABP gene ablation decreased serum beta-hydroxybutyrate in male and female mice fed the control diet and, even more so, on the high-fat diet. Thus, L-FABP gene ablation decreased fat oxidation and sensitized all mice to weight gain as whole body FTM and LTM-with the most gain observed in FTM of control vs high-fat fed female L-FABP null mice. Taken together, these results indicate loss of L-FABP exacerbates weight gain and/or obesity in response to high dietary fat.

Hepatic phenotype of liver fatty acid binding protein gene-ablated mice

Although the function of liver fatty acid binding protein in hepatic fatty acid metabolism has been extensively studied, its potential role in hepatic cholesterol homeostasis is less clear. Although hepatic cholesterol accumulation was initially reported in L-FABP-null female mice, that study was performed with early N2 backcross generation mice. To resolve whether the hepatic cholesterol phenotype in these L-FABP(-/-) mice was attributable to genetic inhomogeneity, these L-FABP(-/-) mice were further backcrossed to C57Bl/6 mice up to the N10 (99.9% homogeneity) generation. Hepatic total cholesterol accumulation was observed in female, but not male, L-FABP(-/-) mice at all (N2, N4, N6, N10) backcross generations examined. The greater total cholesterol was due to increased hepatic levels of both unesterified (free) cholesterol and esterified cholesterol. Altered hepatic cholesterol accumulation correlated directly with L-FABP's ability to bind cholesterol with high affinity as shown by direct L-FABP binding of fluorescent cholesterol analogs (NBD-cholesterol, dansyl-cholesterol), a photoactivatable cholesterol analog [free cholesterol benzophenone (FCBP)], and free cholesterol (circular dichroism, isothermal titration microcalorimetry). One mole of fluorescent sterol was bound per mole of L-FABP. This was confirmed by photo-cross-linking studies with the photoactivatable cholesterol analog FCBP and by isothermal titration calorimetry with free cholesterol, which showed that L-FABP bound only one sterol molecule per L-FABP molecule. In contrast, the hepatic phenotype of male, but not female, L-FABP(-/-) mice was characterized by decreased hepatic triacylglycerol levels at all backcross generations examined. Taken together, these data support the hypothesis that L-FABP plays a role in physiological regulation of not only hepatic fatty acid metabolism, but also that of hepatic cholesterol.

Proteomic profiling of the prechylomicron transport vesicle involved in the assembly and secretion of apoB-48-containing chylomicrons in the intestinal enterocytes

Intracellular assembly of chylomicrons (CM) occurs in intestinal enterocytes through a series of complex vesicular interactions. CM are transported from the ER to the Golgi using a specialized vesicular compartment called the prechylomicron transport vesicle (PCTV). In this study, PCTVs were isolated from the enteric ER of the Syrian Golden hamster, and characterized using 2-DE and MS. Proteomic profiles of PCTV-associated proteins were developed with the intention of identifying proteins involved in the formation, transport, lipidation, and assembly of CM particles. Positively identified proteins included those involved in lipoprotein assembly, namely microsomal triglyceride transfer protein and apolipoprotein B-48, as well as proteins involved in vesicular transport, such as Sar1 and vesicle-associated membrane protein 7. Other groups of proteins found were chaperones, intracellular vesicular trafficking proteins, fatty acid-binding proteins, and lipid-related proteins. These findings have increased our understanding of the transport vesicle involved in the intracellular assembly and transport of CM and can provide insight into potential cellular factors responsible for dysregulation of intestinal CM production.

Identification of a physiologically relevant endogenous ligand for PPARalpha in liver

The nuclear receptor PPARalpha is activated by drugs to treat human disorders of lipid metabolism. Its endogenous ligand is unknown. PPARalpha-dependent gene expression is impaired with inactivation of fatty acid synthase (FAS), suggesting that FAS is involved in generation of a PPARalpha ligand. Here we demonstrate the FAS-dependent presence of a phospholipid bound to PPARalpha isolated from mouse liver. Binding was increased under conditions that induce FAS activity and displaced by systemic injection of a PPARalpha agonist. Mass spectrometry identified the species as 1-palmitoyl-2-oleoyl-sn-glycerol-3-phosphocholine (16:0/18:1-GPC). Knockdown of Cept1, required for phosphatidylcholine synthesis, suppressed PPARalpha-dependent gene expression. Interaction of 16:0/18:1-GPC with the PPARalpha ligand-binding domain and coactivator peptide motifs was comparable to PPARalpha agonists, but interactions with PPARdelta were weak and none were detected with PPARgamma. Portal vein infusion of 16:0/18:1-GPC induced PPARalpha-dependent gene expression and decreased hepatic steatosis. These data suggest that 16:0/18:1-GPC is a physiologically relevant endogenous PPARalpha ligand.

Polymorphisms of microsomal triglyceride transfer protein gene and phosphatidylethanolamine N-methyltransferase gene in alcoholic and nonalcoholic fatty liver disease in Koreans

BACKGROUND

The pathogenesis of fatty liver is likely to depend on a complex interaction of environmental and genetic factors. We investigated a large-scale analysis of the association between microsomal triglyceride transfer protein (MTTP) and phosphatidylethanolamine N-methyltransferase (PEMT) polymorphism in alcoholic and nonalcoholic fatty liver disease.

METHODS

Five hundred and eighty-eight patients who visited the health promotion center were enrolled. To elucidate the possible role of genetic variation affecting triglyceride metabolism in fatty liver disease, the MTTP-I128T and PEMT-V175M polymorphisms were studied.

RESULTS

The I/I genotype and I allele frequency of MTTP polymorphism with alcoholic fatty liver was significantly higher than that of the normal control group (P=0.026 vs. 0.005). Genotype and allele frequency of PEMT, however, did not show a significant difference between control and fatty liver. I/I genotype of MTTP gene frequency in the drinkers with fatty livers was 85.4%, which was significantly higher than that in the drinkers without fatty liver, which was 68.4% (P=0.013). With regard to biochemical indicators, the alanine aminotransferase value of the I/I group was significantly higher than that of the I/T and T/T groups (P=0.04). Asparate aminotransferase, gamma-glutamyl transpeptidase, triglyceride, apolipoprotein B, and glucose concentration tended to be lower in the I/T and T/T groups than in the I/I group, but no statistically significant difference was found.

CONCLUSION

In this study, MTTP-I128T polymorphism is associated with central obesity, elevated liver enzymes, and alcoholic fatty liver disease.

Similarities and differences in the pathogenesis of alcoholic and nonalcoholic steatohepatitis

Subpopulations of individuals with alcohol-induced fatty livers and nonalcoholic steatosis develop steatohepatitis. Steatohepatitis is defined histologically: increased numbers of injured and dying hepatocytes distinguish this condition from simple steatosis. The increased hepatocyte death is generally accompanied by hepatic accumulation of inflammatory cells and sometimes increases in myofibroblastic cells, leading to hepatic fibrosis and eventually, cirrhosis. The purpose of this review is to summarize similarities and differences in the pathogenesis of steatohepatitis in alcoholic fatty liver disease and nonalcoholic fatty liver disease.

Pxmp2 is a channel-forming protein in Mammalian peroxisomal membrane

Background

Peroxisomal metabolic machinery requires a continuous flow of organic and inorganic solutes across peroxisomal membrane. Concerning small solutes, the molecular nature of their traffic has remained an enigma.

Methods/Principal Findings

In this study, we show that disruption in mice of the Pxmp2 gene encoding Pxmp2, which belongs to a family of integral membrane proteins with unknown function, leads to partial restriction of peroxisomal membrane permeability to solutes in vitro and in vivo. Multiple-channel recording of liver peroxisomal preparations reveals that the channel-forming components with a conductance of 1.3 nS in 1.0 M KCl were lost in Pxmp2^−/− mice. The channel-forming properties of Pxmp2 were confirmed with recombinant protein expressed in insect cells and with native Pxmp2 purified from mouse liver. The Pxmp2 channel, with an estimated diameter of 1.4 nm, shows weak cation selectivity and no voltage dependence. The long-lasting open states of the channel indicate its functional role as a protein forming a general diffusion pore in the membrane.

Conclusions/Significance

Pxmp2 is the first peroxisomal channel identified, and its existence leads to prediction that the mammalian peroxisomal membrane is permeable to small solutes while transfer of “bulky” metabolites, e.g., cofactors (NAD/H, NADP/H, and CoA) and ATP, requires specific transporters.

Differential expression of lumican and fatty acid binding protein-1: new insights into the histologic spectrum of nonalcoholic fatty liver disease

The basis of hepatocellular injury and progressive fibrosis in a subset of patients with nonalcoholic fatty liver disease (NAFLD) is poorly understood. We sought to identify hepatic proteins that are differentially abundant across the histologic spectrum of NAFLD. Hepatic protein abundance was measured in liver samples from four groups (n = 10 each) of obese (body mass index >30 kg/m(2)) patients: (1) obese normal group (normal liver histology), (2) simple steatosis (SS), (3) nonalcoholic steatohepatitis (NASH)-mild (steatohepatitis with fibrosis stage 0-1), and (4) NASH-progressive (steatohepatitis with fibrosis stage 2-4). Hepatic peptides were analyzed on an API Qstar XL quadrupole time-of-flight mass spectrometer using Analyst QS software. Linear trends tests were performed and used to screen for differential abundance. Nine known proteins were expressed with differential abundance between study groups. For seven proteins differential abundance is likely to have been on the basis increased hepatic lipid content and/or inflammation. Lumican, a 40-kDa keratin sulfate proteoglycan that regulates collagen fibril assembly and activates transforming growth factor-beta and smooth muscle actin, was expressed similarly in obese normal and SS but was overexpressed in a progressive manner in NASH-mild versus SS (124%, P < 0.001), NASH-progressive versus NASH-mild (156%, P < 0.001) and NASH-progressive versus obese normal (178%, P < 0.001). Fatty acid binding protein-1 (FABP-1), which is protective against the detergent effects of excess free fatty acids, facilitates intracellular free fatty acid transport and is an important ligand for peroxisome proliferator-activated receptor-mediated transcription, was overexpressed in SS when compared to the obese normal group (128%, P < 0.001), but was paradoxically underexpressed in NASH-mild versus SS (73%, P < 0.001), NASH-progressive versus NASH-mild (81%, P < 0.001), and NASH-progressive versus obese normal (59%, P < 0.001).

CONCLUSION

Histologically progressive NAFLD is associated with overexpression of lumican, an important mediator of fibrosis in nonhepatic tissues, whereas FABP-1 is paradoxically underexpressed in NASH, suggesting a new potential mechanism of lipotoxicity in NAFLD. Further studies are needed to determine the biologic basis of lumican and/or FABP-1 dysregulation in NAFLD.

Proteome analysis of fatty liver in feed-deprived dairy cows reveals interaction of fuel sensing, calcium, fatty acid, and glycogen metabolism

The liver of dairy cows is involved in signaling the current hepatic metabolic state to the brain via metabolites and nerval afferents to control and adjust feed intake. Feed deprivation may result in mobilization of body reserves favoring hepatic steatosis. While the overall metabolic changes are well characterized, specific regulatory mechanisms are not readily understood. To identify molecular events associated with metabolic adaptation and the control of energy homeostasis, liver specimens from six ad libitum-fed and six feed-deprived cows were analyzed for selected metabolites, for the activation of AMP kinase, and for regulatory/regulated proteins using two-dimensional gel electrophoresis and MALDI-TOF-MS. Feed deprivation increased total liver fat and the calcium content, as well as augmented AMPK phosphorylation, while it decreased the contents of protein, glucose, glycogen, and cholesterol when expressed as a percentage of dry matter. Among 34 differentially expressed proteins identified, we found downregulation of proteins associated with fatty acid oxidation, glycolysis, electron transfer, protein degradation, and antigen processing, as well as cytoskeletal rearrangement. Proteins upregulated after feed deprivation included enzymes of the urea cycle, fatty acid or cholesterol transport proteins, an inhibitor of glycolysis, and previously unknown changes in calcium signaling network. Direct correlation was found between expression of glycolytic enzymes and glucose/glycogen content, whereas inverse correlation exists between expression of β-oxidative enzymes and total liver fat content. In conclusion, the regulatory response of identified proteins may help to explain development and consequences of hepatic lipidosis but also offers novel candidates potentially involved in signaling for maintaining energy homeostasis.

Cideb, an ER- and lipid droplet-associated protein, mediates VLDL lipidation and maturation by interacting with apolipoprotein B

Secretion of triacylglycerol-enriched very-low-density lipoproteins (VLDLs) from the liver is vital for maintaining plasma lipid homeostasis. However, the process of VLDL assembly and lipidation is not well characterized. Here, we observed that liver of Cideb null mice had higher levels of triacylglycerols accompanied by low level of VLDL secretion. Furthermore, VLDL particles secreted from hepatocytes of Cideb null mice have low levels of triacylglycerols but normal levels of apoB. We also observed that Cideb is localized to endoplasmic reticulum and lipid droplets. Importantly, we have identified apoB as a Cideb-interacting protein. By infecting adenoviruses expressing various Cideb truncations into hepatocytes of Cideb null mice, we found that Cideb requires both its apoB-binding and lipid droplet association domains to restore the secretion of triacylglycerol-enriched VLDL particles. Our data suggest that Cideb promotes the formation of triacylglycerol-enriched VLDL particles and provides a molecular insight into VLDL lipidation and maturation in hepatocytes.

Increased susceptibility to diet-induced gallstones in liver fatty acid binding protein knockout mice

Quantitative trait mapping identified a locus colocalizing with L-Fabp, encoding liver fatty acid binding protein, as a positional candidate for murine gallstone susceptibility. When fed a lithogenic diet (LD) for 2 weeks, L-Fabp(-/-) mice became hypercholesterolemic with increased hepatic VLDL cholesterol secretion. Seventy-five percent of L-Fabp(-/-) mice developed solid gallstones compared with 6% of wild-type mice with an increased gallstone score (3.29 versus 0.62, respectively; P < 0.01). Hepatic free cholesterol content, biliary cholesterol secretion, and the cholesterol saturation index of hepatic bile were increased in LD-fed L-Fabp(-/-) mice. Chow-fed L-Fabp(-/-) mice demonstrated increased fecal bile acid (BA) excretion accompanied by decreased ileal Asbt expression. By contrast, there was an increased BA pool and decreased fecal BA excretion in LD-fed L-Fabp(-/-) mice, associated with increased proximal intestinal Asbt mRNA expression, suggesting that intestinal BA absorption was enhanced in LD-fed L-Fabp(-/-) mice. The increase in biliary BA secretion and enterohepatic pool size in LD-fed L-Fabp(-/-) mice was accompanied by downregulation of Cyp7a1 mRNA and increased intestinal mRNA abundance of Fgf-15, Fxr, and Fabp6. These findings suggest that changes in hepatic cholesterol metabolism and biliary lipid secretion as well as changes in enterohepatic BA metabolism increase gallstone susceptibility in LD fed L-Fabp(-/-) mice.

Diet-induced alterations in intestinal and extrahepatic lipid metabolism in liver fatty acid binding protein knockout mice

Liver fatty acid binding protein (L-FABP) is highly expressed in both enterocytes and hepatocytes and binds multiple ligands, including saturated (SFA), unsaturated fatty acids (PUFA), and cholesterol. L-fabp (-/-) mice were protected against obesity and hepatic steatosis on a high saturated fat (SF), high cholesterol "Western" diet and manifested a similar phenotype when fed with a high SF, low cholesterol diet. There were no significant differences in fecal fat content or food consumption between the genotypes, and fatty acid (FA) oxidation was reduced, rather than increased, in SF-fed L-fabp (-/-) mice as evidenced by decreased heat production and serum ketones. In contrast to mice fed with a SF diet, L-fabp (-/-) mice fed with a high PUFA diet were not protected against obesity and hepatic steatosis. These observations together suggest that L-fabp (-/-) mice exhibit a specific defect in the metabolism of SFA, possibly reflecting altered kinetics of FA utilization. In support of this possibility, microarray analysis of muscle from Western diet-fed mice revealed alterations in genes regulating glucose uptake and FA synthesis. In addition, intestinal cholesterol absorption was decreased in L-fabp (-/-) mice. On the other hand, and in striking contrast to other reports, female L-fabp (-/-) mice fed with low fat, high cholesterol diets gained slightly less weight than control mice, with minor reductions in hepatic triglyceride content. Together these data indicate a role for L-FABP in intestinal trafficking of both SFA and cholesterol.

Liver fatty acid binding protein gene ablation enhances age-dependent weight gain in male mice

Although studies performed in vitro and with transfected cells in culture suggest a role for liver fatty acid binding protein (L-FABP) in regulating fatty acid oxidation and fat deposition, the physiological significance of this possibility is not completely clear. To begin to address this question, the effect of L-FABP gene ablation on phenotype of standard rodent chow-fed male mice was examined with increasing age up to 18 months. While young (2-3 months old) L-FABP null mice displayed no visually obvious phenotype, with increasing age >9 months the L-FABP null mice were visibly larger, exhibiting increased body weight due to increased fat and lean tissue mass. Liver lipid concentrations were unaffected by L-FABP gene ablation with the exception of triacylglycerol, which was decreased by 74% in the livers of 3-month-old mice. Likewise, serum lipid levels were not altered in L-FABP null mice with the exception of triacylglycerol, which was increased in the serum of 18-month-old mice. Increased body weight, fat tissue mass, and lean tissue mass in 18-month-old L-FABP null mice were accompanied by increased hepatic levels of low-density lipoprotein (LDL) receptor, peroxisome proliferator-activated receptor (PPAR) alpha, and PPARalpha-regulated proteins such as fatty acid transport protein (FATP), fatty acid translocase (FAT/CD36), carnitine palmitoyl transferase I (CPT I), and lipoprotein lipase (LPL). A key enzyme in cholesterol biosynthesis, 3-hydroxy-3-methylglutaryl Coenzyme A (HMG-CoA) reductase, was down-regulated in L-FABP null mice. These findings were consistent with a proposed role for L-FABP as an important physiological regulator of PPARalpha.

Gene therapy for diabetes: metabolic effects of helper-dependent adenoviral exendin 4 expression in a diet-induced obesity mouse model

Exendin 4 (Ex4) is a glucagon-like peptide-1 receptor (GLP- 1R) agonist which is available as a short-acting injectable treatment for type 2 diabetes. Our aim was to characterize the long-term effects of elevated steady-state levels of Ex4 provided by in vivo gene therapy. We constructed a helper-dependent adenoviral (HDAd) vector for long-term expression of Ex4 in vivo. A high-fat diet (HFD)-induced obesity (DIO) mouse model was chosen to approximate the metabolic derangements seen in obese patients. Mice were treated with a single injection of HDAd-Ex4 and were monitored for 15 weeks. Both hepatic Ex4 RNA and plasma Ex4 were detectable at the end of the study. HDAd-Ex4 treatment improved glucose homeostasis without increasing insulin levels. However, there was evidence of enhanced insulin action and decreased gluconeogenic enzyme expression. HDAd-Ex4 caused decreased weight gain without detectable changes in food intake, in part, due to increases in energy expenditure (EE). HDAd-Ex4 DIO mice also had reduced hepatic fat and an improved adipokine profile. In the liver, there was decreased expression of genes that were involved in de novo fatty acid synthesis. These observations are important in considering the development of longer acting GLP-1R agonists for the treatment of type 2 diabetes.

Molecular mechanisms of lipotoxicity in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is characterized by insulin resistance, which results in elevated serum concentration of free fatty acids (FFAs). Circulating FFAs provide the substrate for triacylglycerol formation in the liver, and may also be directly cytotoxic. Hepatocyte apoptosis is a key histologic feature of NAFLD, and correlates with progressive inflammation and fibrosis. The molecular pathways leading to hepatocyte apoptosis are not fully defined; however, recent studies suggest that FFA-induced apoptosis contributes to the pathogenesis of nonalcoholic steatohepatitis. FFAs directly engage the core apoptotic machinery by activating the proapoptotic protein Bax, in a c-jun N-terminal kinase-dependent manner. FFAs also activate the lysosomal pathway of cell death and regulate death receptor gene expression. The role of ER stress and oxidative stress in the pathogenesis of nonalcoholic steatohepatitis has also been described. Understanding the molecular mediators of liver injury should promote development of mechanism-based therapeutic interventions.

The emerging functions and mechanisms of mammalian fatty acid-binding proteins

Fatty acid-binding proteins (FABPs) are abundant intracellular proteins that bind long-chain fatty acids with high affinity. Nine separate mammalian FABPs have been identified, and their tertiary structures are highly conserved. The FABPs have unique tissue-specific distributions that have long suggested functional differences among them. In the last decade, considerable progress has been made in understanding the specific functions of the FABPs and, in some cases, their mechanisms of action at the molecular level. The FABPs appear to be involved in the extranuclear compartments of the cell by trafficking their ligands within the cytosol via interactions with organelle membranes and specific proteins. Several members of the FABP family have been shown to function directly in the regulation of cognate nuclear transcription factor activity via ligand-dependent translocation to the nucleus. This review will focus on these emerging functions and mechanisms of the FABPs, highlighting the unique functional properties of each as well as the similarities among them.

Fatty acid transport protein 4 is dispensable for intestinal lipid absorption in mice

FA transport protein 4 (FATP4), one member of a multigene family of FA transporters, was proposed as a major FA transporter in intestinal lipid absorption. Due to the fact that Fatp4(-/-) mice die because of a perinatal skin defect, we rescued the skin phenotype using an FATP4 transgene driven by a keratinocyte-specific promoter (Fatp4(-/-);Ivl-Fatp4(tg/+) mice) to elucidate the role of intestinal FATP4 in dietary lipid absorption. Fatp4(-/-);Ivl-Fatp4(tg/+) mice and wild-type littermates displayed indistinguishable food consumption, growth, and weight gain on either low or high fat (Western) diets, with no differences in intestinal triglyceride (TG) absorption or fecal fat losses. Cholesterol absorption and intestinal TG absorption kinetics were indistinguishable between the genotypes, although Western diet fed Fatp4(-/-);Ivl-Fatp4(tg/+) mice showed a significant increase in enterocyte TG and FA content. There was no compensatory upregulation of other FATP family members or any other FA or cholesterol transporters in Fatp4(-/-);Ivl-Fatp4(tg/+) mice. Furthermore, although serum cholesterol levels were lower in Fatp4(-/-);Ivl-Fatp4(tg/+) mice, there was no difference in hepatic VLDL secretion in-vivo or in hepatic lipid content on either a chow or Western diet. Taken together, our studies find no evidence for a physiological role of intestinal FATP4 in dietary lipid absorption in mice.

Altered hepatic triglyceride content after partial hepatectomy without impaired liver regeneration in multiple murine genetic models

Liver regeneration is impaired following partial hepatectomy (PH) in mice with genetic obesity and hepatic steatosis and also in wild-type mice fed a high-fat diet. These findings contrast with other data showing that liver regeneration is impaired in mice in which hepatic lipid accumulation is suppressed by either pharmacologic leptin administration or by disrupted glucocorticoid signaling. These latter findings suggest that hepatic steatosis may actually be required for normal liver regeneration. We have reexamined this relationship using several murine models of altered hepatic lipid metabolism. Liver fatty acid (FA) binding protein knockout mice manifested reduced hepatic triglyceride (TG) content compared to controls, with no effect on liver regeneration or hepatocyte proliferation. Examination of early adipogenic messenger RNAs revealed comparable induction in liver from both genotypes despite reduced hepatic steatosis. Following PH, hepatic TG was reduced in intestine-specific microsomal TG transfer protein deleter mice, which fail to absorb dietary fat, increased in peroxisome proliferator activated receptor alpha knockout mice, which exhibit defective FA oxidation, and unchanged (from wild-type mice) in liver-specific FA synthase knockout mice in which endogenous hepatic FA synthesis is impaired. Hepatic TG increased in the regenerating liver in all models, even in animals in which lipid accumulation is genetically constrained. However, in no model -- and over a >90-fold range of hepatic TG content -- was liver regeneration significantly impaired following PH.

CONCLUSION

Although hepatic TG content is widely variable and increases during liver regeneration, alterations in neither exogenous or endogenous lipid metabolic pathways, demonstrated to promote or diminish hepatic steatosis, influence hepatocyte proliferation.

Liver-type fatty acid-binding protein and lipid transport

Liver-type fatty acid-binding protein (L-FABP), a member of the genetically related cytosolic fatty acid binding protein (FABP) family, has both similar and different function and conformation compared with other family members. L-FABP, which is mainly found in liver and small intestine, participates in transporting fatty acids and combining a variety of hydrophobic group and is associated with many diseases. In recent years, the transfer mechanism of L-FABP was attracting great attention, and the research methods were developed from in vitro to in vivo, and from cell molecular level to gene ablation animals. Although a part of the mechanism has been revealed, the research should be continued to demonstrate it in deepth and resolve some new questions. This article aims to review the characteristics, conformation and in vivo study status of L-FABP.

Angiotensin II-induced non-alcoholic fatty liver disease is mediated by oxidative stress in transgenic TG(mRen2)27(Ren2) rats

BACKGROUND/AIMS

Non-alcoholic fatty liver disease (NAFLD) is a common health problem and includes a spectrum of hepatic steatosis, steatohepatitis and fibrosis. The renin-angiotensin system (RAS) plays a vital role in blood pressure regulation and appears to promote hepatic fibrogenesis. We hypothesized that increased RAS activity causes NAFLD due to increased hepatic oxidative stress.

METHODS

We employed the transgenic TG(mRen2)27(Ren2) hypertensive rat, harboring the mouse renin gene with elevated tissue Angiotensin II (Ang II).

RESULTS

Compared with normotensive Sprague-Dawley (SD) control rats, Ren2 developed significant hepatic steatosis by 9 weeks of age that progressed to marked steatohepatitis and fibrosis by 12 weeks. These changes were associated with increased levels of hepatic reactive oxygen species (ROS) and lipid peroxidation. Accordingly, 9-week-old Ren2 rats were treated for 3 weeks with valsartan, an angiotensin type 1 receptor blocker, or tempol, a superoxide dismutase/catalase mimetic. Hepatic indices for oxidative stress, steatosis, inflammation and fibrosis were markedly attenuated by both valsartan and tempol treatment.

CONCLUSIONS

This study suggests that Ang II causes development and progression of NAFLD in the transgenic Ren2 rat model by increasing hepatic ROS. Our findings also support a potential role of RAS in prevention and treatment of NAFLD.

Estimated liver weight is directly related to hepatic very low-density lipoprotein-triglyceride secretion rate in men

BACKGROUND

Animal studies suggest that liver weight is directly related to hepatic very low-density lipoprotein-triglyceride (VLDL-TG) secretion, independently of body size. This relationship has never been examined in humans.

MATERIALS AND METHODS

We measured VLDL-TG secretion rate by using stable isotope-labelled tracers in 21 healthy, non-obese men (age: 25 +/- 3 years; body mass index: 24.8 +/- 1.6 kg m(-2)), and evaluated the relationship between VLDL-TG secretion and indices of total and regional adiposity (body mass index, total body fat, trunk fat), metabolic parameters (free fatty acid, glucose, and insulin concentrations, homeostasis model assessment index of insulin resistance, resting energy expenditure), and estimated liver weight.

RESULTS

Correlation analysis showed that estimated liver weight was positively associated with total VLDL-TG secretion rate (r = 0.722, P < 0.001), VLDL-TG secretion rate per liter of plasma (r = 0.562, P = 0.008), VLDL-TG secretion rate per kilogram of body weight (r = 0.555, P = 0.009), and VLDL-TG secretion rate per kilogram of liver weight (r = 0.620, P = 0.003). In multiple regression analysis, estimated liver weight was the only significant predictor of VLDL-TG secretion rate regardless of units of expression, explaining 31-52% of total variance; none of the metabolic parameters and indices of body fatness entered the regression models.

CONCLUSIONS

We conclude that estimated liver weight is directly related to hepatic VLDL-TG secretion rate in healthy non-obese men; this relationship is likely not mediated by interindividual variation in body size.

Fatty acid-binding proteins: role in metabolic diseases and potential as drug targets

Lipids are vital components of many biological processes and crucial in the pathogenesis of numerous common diseases, but the specific mechanisms coupling intracellular lipids to biological targets and signalling pathways are not well understood. This is particularly the case for cells burdened with high lipid storage, trafficking and signalling capacity such as adipocytes and macrophages. Here, we discuss the central role of lipid chaperones--the fatty acid-binding proteins (FABPs)--in lipid-mediated biological processes and systemic metabolic homeostasis through the regulation of diverse lipid signals, and highlight their therapeutic significance. Pharmacological agents that modify FABP function may provide tissue-specific or cell-type-specific control of lipid signalling pathways, inflammatory responses and metabolic regulation, potentially providing a new class of drugs for diseases such as obesity, diabetes and atherosclerosis.

Regulation of energy substrate utilization and hepatic insulin sensitivity by phosphatidylcholine transfer protein/StarD2

Phosphatidylcholine transfer protein (PC-TP, also known as StarD2) is a highly specific intracellular lipid binding protein with accentuated expression in oxidative tissues. Here we show that decreased plasma concentrations of glucose and free fatty acids in fasting PC-TP-deficient (Pctp(-/-)) mice are attributable to increased hepatic insulin sensitivity. In hyperinsulinemic-euglycemic clamp studies, Pctp(-/-) mice exhibited profound reductions in hepatic glucose production, gluconeogenesis, glycogenolysis, and glucose cycling. These changes were explained in part by the lack of PC-TP expression in liver per se and in part by marked alterations in body fat composition. Reduced respiratory quotients in Pctp(-/-) mice were indicative of preferential fatty acid utilization for energy production in oxidative tissues. In the setting of decreased hepatic fatty acid synthesis, increased clearance rates of dietary triglycerides and increased hepatic triglyceride production rates reflected higher turnover in Pctp(-/-) mice. Collectively, these data support a key biological role for PC-TP in the regulation of energy substrate utilization.

Intestinal fatty acid binding protein regulates mitochondrion beta-oxidation and cholesterol uptake

The role of intestinal fatty acid binding protein (I-FABP) in lipid metabolism remains elusive. To address this issue, normal human intestinal epithelial cells (HIEC-6) were transfected with cDNA to overexpress I-FABP and compared with cells treated with empty pQCXIP vector. I-FABP overexpression stimulated mitochondrial [U-14C]oleate oxidation to CO2 and acid-soluble metabolites via mechanisms including the upregulation of protein expression and the activity of carnitine palmitoyltransferase 1, a critical enzyme controlling the entry of fatty acid (FA) into mitochondria, and increased activity of 3-hydroxyacyl-CoA dehydrogenase, a mitochondrial beta-oxidation enzyme. On the other hand, the gene and protein expression of the key enzymes FA synthase and acetyl-coenzyme A carboxylase 2 was decreased, suggesting diminished lipogenesis. Furthermore, I-FABP overexpression caused a decline in [14C]free cholesterol (CHOL) incorporation. Accordingly, a significant lessening was observed in the gene expression of Niemann Pick C1-Like 1, a mediator of CHOL uptake, along with an increase in the transcripts and protein content of ABCA1 and ABCG5/ABCG8, acting as CHOL efflux pumps. Furthermore, I-FABP overexpression resulted in increased levels of mRNA, protein mass, and activity of HMG-CoA reductase, the rate-limiting step in CHOL synthesis. Scrutiny of the nuclear receptors revealed augmented peroxisome proliferator-activated receptor alpha,gamma and reduced liver X receptor-alpha in HIEC-6 overexpressing I-FABP. Finally, I-FABP overexpression did not influence acyl-coenzyme A oxidase 1, which catalyzes the first rate-limiting step in peroxisomal FA beta-oxidation. Overall, our data suggest that I-FABP may influence mitochondrial FA oxidation and CHOL transport by regulating gene expression and interaction with nuclear receptors.

Diet-induced obesity and hepatic steatosis in L-Fabp / mice is abrogated with SF, but not PUFA, feeding and attenuated after cholesterol supplementation

Liver fatty acid (FA)-binding protein (L-Fabp), a cytoplasmic protein expressed in liver and small intestine, regulates FA trafficking in vitro and plays an important role in diet-induced obesity. We observed that L-Fabp(-/-) mice are protected against Western diet-induced obesity and hepatic steatosis. These findings are in conflict, however, with another report of exaggerated obesity and increased hepatic steatosis in female L-Fabp(-/-) mice fed a cholesterol-supplemented diet. To resolve this apparent paradox, we fed female L-Fabp(-/-) mice two different cholesterol-supplemented low-fat diets and discovered (on both diets) lower body weight in L-Fabp(-/-) mice than in congenic wild-type C57BL/6J controls and similar or reduced hepatic triglyceride content. We extended these comparisons to mice fed low-cholesterol, high-fat diets. Female L-Fabp(-/-) mice fed a high-saturated fat (SF) diet were dramatically protected against obesity and hepatic steatosis, whereas weight gain and hepatic lipid content were indistinguishable between mice fed a high-polyunsaturated FA (PUFA) diet and control mice. These findings demonstrate that L-Fabp functions as a metabolic sensor with a distinct hierarchy of FA sensitivity. We further conclude that cholesterol supplementation does not induce an obesity phenotype in L-Fabp(-/-) mice, nor does it play a significant role in the protection against Western diet-induced obesity in this background.

A new model for nonalcoholic steatohepatitis in the rat utilizing total enteral nutrition to overfeed a high-polyunsaturated fat diet

We have used total enteral nutrition (TEN) to moderately overfeed rats high-polyunsaturated fat diets to develop a model for nonalcoholic steatohepatitis (NASH). Male Sprague-Dawley rats were fed by TEN a 187 kcal.kg(-3/4).day(-1) diet containing 5% (total calories) corn oil or a 220 kcal.kg(-3/4).day(-1) diet in which corn oil constituted 5, 10, 25, 35, 40, or 70% of total calories for 21 or 65 days. Rats fed the 5% corn oil, 220 kcal.kg(-3/4).day(-1)diet had greater body weight gain (P < or = 0.05), fat mass (P < or = 0.05), and serum leptin and glucose levels (P < or = 0.05), but no liver pathology. A dose-dependent increase in hepatic triglyceride deposition occurred with increase in percent corn oil in the 220 kcal.kg(-3/4).day(-1) groups (P < or = 0.05). Steatosis, macrophage infiltration, apoptosis, and focal necrosis were present in the 70% corn oil group, accompanied by elevated serum alanine aminotransferase (ALT) levels (P < or = 0.05). An increase in oxidative stress (thiobarbituric acid-reactive substances) and TNF-alpha expression (P < or = 0.05) was observed in the 70% corn oil group, as well as an increase in hepatic CYP2E1 and CYP4A1 expression (P < or = 0.05). Significant positive correlations were observed between the level of dietary corn oil and the degree of pathology, ALTs, oxidative stress, and inflammation. Liver pathology was progressive with increased necrosis, accompanied by fibrosis, observed after 65 days of TEN. Increased expression of CD36 and l-fabp mRNA suggested development of steatosis was associated with increased fatty acid transport. These data suggest that intragastric infusion of a high-polyunsaturated fat diet at a caloric level of 17% excess total calories results in pathology similar to clinical NASH.

Solution-state molecular structure of apo and oleate-liganded liver fatty acid-binding protein

Rat liver fatty acid-binding protein (LFABP) is distinctive among intracellular lipid-binding proteins (iLBPs): more than one molecule of long-chain fatty acid and a variety of diverse ligands can be bound within its large cavity, and in vitro lipid transfer to model membranes follows a mechanism that is diffusion-controlled rather than mediated by protein-membrane collisions. Because the apoprotein has proven resistant to crystallization, nuclear magnetic resonance spectroscopy offers a unique route to functionally informative comparisons of molecular structure and dynamics for LFABP in free (apo) and liganded (holo) forms. We report herein the solution-state structures determined for apo-LFABP at pH 6.0 and for holoprotein liganded to two oleates at pH 7.0, as well as the structure of the complex including locations of the ligands. 1H, 13C, and 15N resonance assignments revealed very similar types and locations of secondary structural elements for apo- and holo-LFABP as judged from chemical shift indices. The solution-state tertiary structures of the proteins were derived with the CNS/ARIA computational protocol, using distance and angular restraints based on 1H-1H nuclear Overhauser effects (NOEs), hydrogen-bonding networks, 3J(HNHA) coupling constants, intermolecular NOEs, and residual dipolar (NH) couplings. The holo-LFABP solution-state conformation is in substantial agreement with a previously reported X-ray structure [Thompson, J., Winter, N., Terwey, D., Bratt, J., and Banaszak, L. (1997) The crystal structure of the liver fatty acid-binding protein. A complex with two bound oleates, J. Biol. Chem. 272, 7140-7150], including the typical beta-barrel capped by a helix-turn-helix portal. In the solution state, the internally bound oleate has the expected U-shaped conformation and is tethered electrostatically, but the extended portal ligand can adopt a range of conformations based on the computationally refined structures, in contrast to the single conformation observed in the crystal structure. The apo-LFABP also has a well-defined beta-barrel structural motif typical of other members of the iLBP protein family, but the portal region that is thought to facilitate ligand entry and exit exhibits conformational variability and an unusual "open cap" orientation with respect to the barrel. These structural results allow us to propose a model in which ligand binding to LFABP occurs through conformational fluctuations that adjust the helix-turn-helix motif to open or close the top of the beta-barrel, and solvent accessibility to the protein cavity favors diffusion-controlled ligand transport.

Loss of intestinal fatty acid binding protein increases the susceptibility of male mice to high fat diet-induced fatty liver

Mice lacking I-FABP (encoded by the Fabp2 gene) exhibit a gender dimorphic response to a high fat/cholesterol diet challenge characterized by hepatomegaly in male I-FABP-deficient mice. In this study, we determined if this gender-specific modification of liver mass in mice lacking I-FABP is attributable to the high fat content of the diet alone and whether hepatic Fabp1 gene (encodes L-FABP) expression contributes to this difference. Wild-type and Fabp2-/- mice of both genders were fed a diet enriched with either polyunsaturated or saturated fatty acids (PUFA or SFA, respectively) in the absence of cholesterol. Male Fabp2-/- mice, but not female Fabp2-/- mice, exhibited increased liver mass and hepatic triacylglycerol (TG) deposition as compared to corresponding wild-type mice. In wild-type mice that were fed the standard chow diet, there was no difference in the concentration of hepatic L-FABP protein between males and females although the loss of I-FABP did cause a slight reduction of hepatic L-FABP abundance in both genders. The hepatic L-FABP mRNA abundance in both male and female wild-type and Fabp2-/- mice was higher in the PUFA-fed group than in the SFA-fed group, and was correlated with L-FABP protein abundance. No correlation between hepatic L-FABP protein abundance and hepatic TG concentration was found. The results obtained demonstrate that loss of I-FABP renders male mice sensitive to high fat diet-induced fatty liver, and this effect is independent of hepatic L-FABP.

A Thr94Ala mutation in human liver fatty acid-binding protein contributes to reduced hepatic glycogenolysis and blunted elevation of plasma glucose levels in lipid-exposed subjects

Liver fatty acid-binding protein (L-FABP) is a highly conserved key factor in lipid metabolism. Amino acid replacements in L-FABP might alter its function and thereby affect glucose metabolism in lipid-exposed subjects, as indicated by studies in L-FABP knockout mice. Amino acid replacements in L-FABP were investigated in a cohort of 1,453 Caucasian subjects. Endogenous glucose production (EGP), gluconeogenesis, and glycogenolysis were measured in healthy carriers of the only common Thr(94)-to-Ala amino acid replacement (Ala/Ala(94)) vs. age-, sex-, and BMI-matched wild-type (Thr/Thr(94)) controls at baseline and after 320-min lipid/heparin-somatostatin-insulin-glucagon clamps (n = 18). Whole body glucose disposal was further investigated (subset; n = 13) using euglycemic-hyperinsulinemic clamps without and with lipid/heparin infusion. In the entire cohort, the only common Ala/Ala(94) mutation was significantly associated with reduced body weight, which is in agreement with a previous report. In lipid-exposed, individually matched subjects there was a genotype vs. lipid-treatment interaction for EGP (P = 0.009) driven mainly by reduced glycogenolysis in Ala/Ala(94) carriers (0.46 +/- 0.05 vs. 0.59 +/- 0.05 mgxkg(-1)xmin(-1), P = 0.013). The lipid-induced elevation of plasma glucose levels was smaller in Ala/Ala(94) carriers compared with wild types (P < 0.0001). Whole body glucose disposal was not different between lipid-exposed L-FABP genotypes. In summary, the Ala/Ala(94)-mutation contributed significantly to reduced glycogenolysis and less severe hyperglycemia in lipid-exposed humans and was further associated with reduced body weight in a large cohort. Data clearly show that investigation of L-FABP phenotypes in the basal overnight-fasted state yielded incomplete information, and a challenge test was essential to detect phenotypical differences in glucose metabolism between L-FABP genotypes.

Role of fatty acid binding proteins and long chain fatty acids in modulating nuclear receptors and gene transcription

Abnormal energy regulation may significantly contribute to the pathogenesis of obesity, diabetes mellitus, cardiovascular disease, and cancer. For rapid control of energy homeostasis, allosteric and posttranslational events activate or alter activity of key metabolic enzymes. For longer impact, transcriptional regulation is more effective, especially in response to nutrients such as long chain fatty acids (LCFA). Recent advances provide insights into how poorly water-soluble lipid nutrients [LCFA; retinoic acid (RA)] and their metabolites (long chain fatty acyl Coenzyme A, LCFA-CoA) reach nuclei, bind their cognate ligand-activated receptors, and regulate transcription for signaling lipid and glucose catabolism or storage: (i) while serum and cytoplasmic LCFA levels are in the 200 mircroM-mM range, real-time imaging recently revealed that LCFA and LCFA-CoA are also located within nuclei (nM range); (ii) sensitive fluorescence binding assays show that LCFA-activated nuclear receptors [peroxisome proliferator-activated receptor-alpha (PPARalpha) and hepatocyte nuclear factor 4alpha (HNF4alpha)] exhibit high affinity (low nM KdS) for LCFA (PPARalpha) and/or LCFA-CoA (PPARalpha, HNF4alpha)-in the same range as nuclear levels of these ligands; (iii) live and fixed cell immunolabeling and imaging revealed that some cytoplasmic lipid binding proteins [liver fatty acid binding protein (L-FABP), acyl CoA binding protein (ACBP), cellular retinoic acid binding protein-2 (CRABP-2)] enter nuclei, bind nuclear receptors (PPARalpha, HNF4alpha, CRABP-2), and activate transcription of genes in fatty acid and glucose metabolism; and (iv) studies with gene ablated mice provided physiological relevance of LCFA and LCFA-CoA binding proteins in nuclear signaling. This led to the hypothesis that cytoplasmic lipid binding proteins transfer and channel lipidic ligands into nuclei for initiating nuclear receptor transcriptional activity to provide new lipid nutrient signaling pathways that affect lipid and glucose catabolism and storage.

FABPs as determinants of myocellular and hepatic fuel metabolism

In vitro experiments and expression patterns have long suggested important roles for the genetically related cytosolic fatty acid binding proteins (FABPs) in lipid metabolism. However, evidence for such roles in vivo has become available only recently from genetic manipulation of FABP expression in mice. Here, we summarize the fuel-metabolic phenotypes of mice lacking the genes encoding heart-type FABP (H-/- mice) or liver-type FABP (L-/- mice). Cytosolic extracts from H-/- heart and skeletal muscle and from L-/- liver showed massively reduced binding of long chain fatty acids (LCFA) and, in case of L-/- liver, also of LCFA-CoA. Uptake, oxidation, and esterification LCFA, when measured in vivo and/or ex vivo, were markedly reduced in H-/- heart and muscle and in L-/- liver. The reduced LCFA oxidation in H-/- heart and L-/- liver was not due to reduced activity of PPARa, a fatty acid-sensitive transcription factor that determines the lipid-oxidative capacity in these organs. In H-/- mice, mechanisms of compensation were partially studied and included a redistribution of muscle mitochondria as well as increases of cardiac and skeletal muscle glucose uptakes and of hepatic ketogenesis. In skeletal muscle, the altered glucose uptake included decreased basal but increased insulin-dependent components. Metabolic compensation was only partial, however, since the H-/- mice showed decreased exercise tolerance. In conclusion, the recent studies established H- and L-FABP as major determinants of regional LCFA utilization; therefore the H-/- and L-/- mice are attractive models for studying principles of fuel selection and metabolic homeostasis.

The physiological and molecular regulation of lipoprotein assembly and secretion

Triglycerides are insoluble in water and yet are transported at milligram per millilitre concentrations in the bloodstream. This is made possible by the ability of the liver and intestine to assemble lipid-protein emulsions (i.e. lipoproteins), which transport hydrophobic molecules. The assembly of triglyceride-rich lipoproteins requires the coordination of protein and lipid synthesis, which occurs on the cytoplasmic surface of the endoplasmic reticulum (ER), and their concerted assembly and translocation into the luminal ER secretory pathway as nascent lipoprotein particles. The availability of lipid substrate for triglyceride production and the machinery for lipoprotein assembly are highly sensitive to nutritional, hormonal, and genetic modulation. Disorders in lipid metabolism or an imbalance between lipogenesis and lipoprotein assembly can lead to hyperlipidemia and/or hepatic steatosis. We selectively review recently-identified machinery, such as transcription factors and nuclear hormone receptors, which provide new clues to the regulation of lipoprotein secretion.

L-FABP T94A is associated with fasting triglycerides and LDL-cholesterol in women

To determine the possible role of the common FABP1 T94A polymorphism in modulating susceptibility to traits of the metabolic syndrome, we analysed a random sample of 826 subjects from the European Prospective Investigation into Cancer and Nutrition (EPIC)-Potsdam cohort. Multivariate adjusted linear trend regression analysis of metabolic, anthropometric and blood pressure variables in FABP1 T94A genotypes were performed in both genders. In women, a significant trend of higher plasma triglyceride (P=0.01) and LDL-cholesterol (P=0.02) concentrations were seen for A-allele carriers after adjustment for age, menopausal status, hormone intake and Apo E genotype. Because elevated triglyceride and cholesterol levels are important risk factors of cardiovascular disease (CVD) and type 2 diabetes mellitus (T2DM), we additionally analysed the association of the T94A variant and disease risks in two studies enrolling 220 incident CVD and 192 incident T2DM patients of the EPIC-Potsdam cohort. After adjusting for age, sex, BMI and other covariates, we found no association between FABP1 T94A and CVD or T2DM. In conclusion, our study provides evidence for an association of the FABP1 T94A polymorphism and fasting triglycerides and LDL-cholesterol levels in females. These results support previous findings in fenofibrate-treated individuals and thereby provide some additional indication of the functional relevance of the FABP1 T94A SNP in hepatic fatty acid and lipid metabolism in humans.

Decreased expression of Intestinal I- and L-FABP levels in rare human genetic lipid malabsorption syndromes

We investigated, for the first time, the expression of I- and L-FABP in two very rare hereditary lipid malabsorption syndromes as compared with normal subjects. Abetalipoproteinemia (ABL) and Anderson's disease (AD) are characterized by an inability to export alimentary lipids as chylomicrons that result in fat loading of enterocytes. Duodeno-jejunal biopsies were obtained from 14 fasted normal subjects, and from four patients with ABL and from six with AD. Intestinal FABP expression was investigated by immuno-histochemistry, western blot, ELISA and Northern blot analysis. In contrast to normal subjects, the cellular immunostaining for both FABPs was clearly decreased in patients, as the enterocytes became fat-laden. In patients with ABL, the intestinal contents of I- (60.7 +/- 13.38 ng/mg protein) and L-FABP (750.3 +/- 121.3 ng/mg protein) are significantly reduced (50 and 35%, P < 0.05, respectively) as compared to normal subjects (I-135.3 +/- 11.1 ng, L-1211 +/- 110 ng/mg protein). In AD, the patients also exhibited decreased expression (50%, P < 0.05; I-59 +/- 11.88 ng, L-618.2 +/- 104.6 ng/mg protein). Decreased FABP expression was not associated with decreased mRNA levels. The results suggest that enterocytes might regulate intracellular FABP content in response to intracellular fatty acids, which we speculate may act as lipid sensors to prevent their intracellular transport.

Regulation of Triglyceride Metabolism. II. Function of mitochondrial GPAT1 in the regulation of triacylglycerol biosynthesis and insulin action

GPAT1, one of four known glycerol-3-phosphate acyltransferase isoforms, is located on the mitochondrial outer membrane, allowing reciprocal regulation with carnitine palmitoyltransferase-1. GPAT1 is upregulated transcriptionally by insulin and SREBP-1c and downregulated acutely by AMP-activated protein kinase, consistent with a role in triacylglycerol synthesis. Knockout and overexpression studies suggest that GPAT1 is critical for the development of hepatic steatosis and that steatosis initiated by overexpression of GPAT1 causes hepatic, and perhaps also peripheral, insulin resistance. Future questions include the function of GPAT1 in relation to the other GPAT isoforms and whether the lipid intermediates synthesized by GPAT and downstream enzymes in the pathway of glycerolipid biosynthesis participate in intracellular signaling pathways.

Liver fatty acid-binding protein initiates budding of pre-chylomicron transport vesicles from intestinal endoplasmic reticulum

The rate-limiting step in the transit of absorbed dietary fat across the enterocyte is the generation of the pre-chylomicron transport vesicle (PCTV) from the endoplasmic reticulum (ER). This vesicle does not require coatomer-II (COPII) proteins for budding from the ER membrane and contains vesicle-associated membrane protein 7, found in intestinal ER, which is a unique intracellular location for this SNARE protein. We wished to identify the protein(s) responsible for budding this vesicle from ER membranes in the absence of the requirement for COPII proteins. We chromatographed rat intestinal cytosol on Sephacryl S-100 and found that PCTV budding activity appeared in the low molecular weight fractions. Additional chromatographic steps produced a single major and several minor bands on SDS-PAGE. By tandem mass spectroscopy, the bands contained both liver and intestinal fatty acid-binding proteins (L- and I-FABP) as well as four other proteins. Recombinant proteins for each of the six proteins identified were tested for PCTV budding activity; only L-FABP and I-FABP (23% the activity of L-FABP) were active. The vesicles generated by L-FABP were sealed, contained apolipoproteins B48 and AIV, were of the same size as PCTV on Sepharose CL-6B, and by electron microscopy, excluded calnexin and calreticulin but did not fuse with cis-Golgi nor did L-FABP generate COPII-dependent vesicles. Gene-disrupted L-FABP mouse cytosol had 60% the activity of wild type mouse cytosol. We conclude that L-FABP can select cargo for and bud PCTV from intestinal ER membranes.

Keratinocyte-specific expression of fatty acid transport protein 4 rescues the wrinkle-free phenotype in Slc27a4/Fatp4 mutant mice

FATP4 (fatty acid transport protein 4; also known as SLC27A4) is the most widely expressed member of a family of six long chain fatty acid transporters. FATP4 is highly expressed in enterocytes and has therefore been proposed to be a major importer of dietary fatty acids. Two independent mutations in Fatp4 cause mice to be born with thick, tight, shiny, "wrinkle-free" skin and a defective skin barrier; they die within hours of birth from dehydration and restricted movements. In contrast, induced keratinocyte-specific deficiency of FATP4 in adult mice causes only mild skin abnormalities. Therefore, whether the loss of FATP4 from skin or a systemic gestational metabolic defect causes the severe skin defects and neonatal lethality remain important unanswered questions. To investigate the basis for the phenotype, we first generated wild-type tetraploid/mutant diploid aggregates that should lead to rescue of any abnormalities caused by loss of FATP4 from the placenta. However, the skin phenotype was not ameliorated. We then generated transgenic mice expressing exogenous FATP4 either widely or specifically in suprabasal keratinocytes, and we bred the transgenes onto the Fatp4(-/-) background. Both modes of FATP4 expression led to rescue of the neonatally lethal skin defects, and the resulting mice were viable and fertile. Keratinocyte expression of an FATP4 variant with mutations in the acyl-CoA synthetase domain did not provide any degree of rescue. We conclude that expression of FATP4 with an intact acyl-CoA synthetase domain in suprabasal keratinocytes is necessary for normal skin development and that FATP4 functions in establishing the cornified envelope.

Long-chain fatty acids induce lipid droplet formation in a cultured human hepatocyte in a manner dependent of Acyl-CoA synthetase

Lipid droplets (LDs) are intracellular storage sites of neutral lipids, which accumulate in fatty liver disease. Here, we investigated the effects of fatty acids and glucose on LD formation in a cultured human hepatocyte, HuH7, by adding them to culture media. Fatty acids with carbohydrate chains C12-C18 efficiently induced LDs, but those of C8 and C10 were ineffective. Glucose did not induce LD formation even in the presence of insulin. Oleic acid induced significant increases in cellular neutral lipids, and cell fractionation revealed that most of the newly synthesized neutral lipids were concentrated in LDs together with LD proteins. The LD formation was not abrogated by removal of medium glucose but was significantly inhibited by an ACSL inhibitor, triacsin C. These results demonstrate that long-chain fatty acids contribute to LD formation to a greater extent than glucose, possibly by being taken up into the cells, activated by ACSL, reconstituted into neutral lipids and then stored in LDs. Pregnenolone and lithium did not suppress oleic acid-dependent LD formation, despite previous reports of their ability to inhibit LD formation in macrophages and adipocytes suggesting differences among LD formations in these cells.

Protection against Western diet-induced obesity and hepatic steatosis in liver fatty acid-binding protein knockout mice

Liver fatty acid-binding protein (L-Fabp) regulates murine hepatic fatty acid trafficking in response to fasting. In this study, we show that L-Fabp(-/-) mice fed a high-fat Western diet for up to 18 weeks are less obese and accumulate less hepatic triglyceride than C57BL/6J controls. Paradoxically, both control and L-Fabp(-/-) mice manifested comparable glucose intolerance and insulin resistance when fed a Western diet. Protection against obesity in Western diet-fed L-Fabp(-/-) mice was not due to discernable changes in food intake, fat malabsorption, or heat production, although intestinal lipid secretion kinetics were significantly slower in both chow-fed and Western diet-fed L-Fabp(-/-) mice. By contrast, there was a significant increase in the respiratory exchange ratio in L-Fabp(-/-) mice, suggesting a shift in energy substrate use from fat to carbohydrate, findings supported by an approximately threefold increase in serum lactate. Microarray analysis revealed increased expression of genes involved in lipid synthesis (fatty acid synthase, squalene epoxidase, hydroxy-methylglutaryl coenzyme A reductase), while genes involved in glycolysis (glucokinase and glycerol kinase) were decreased in L-Fabp(-/-) mice. Fatty acid synthase expression was also increased in the skeletal muscle of L-Fabp(-/-) mice. In conclusion, L-Fabp may function as a metabolic sensor in regulating lipid homeostasis. We suggest that L-Fabp(-/-) mice are protected against Western diet-induced obesity and hepatic steatosis through a series of adaptations in both hepatic and extrahepatic energy substrate use. (HEPATOLOGY 2006;44:1191-1205.).

Regulation of hepatic cholesterol synthesis by a novel protein (SPF) that accelerates cholesterol biosynthesis

Supernatant protein factor (SPF) is a novel cholesterol biosynthesis-accelerating protein expressed in liver and small intestine. Here, we report on the physiological role of SPF by using Spf-deficient mice. Although plasma cholesterol levels were similar in chow-fed Spf-/- and wild-type (WT) mice, fasting significantly decreased plasma cholesterol levels in Spf-/- mice but not in WT mice. While fasting reduced hepatic cholesterol synthesis rate in WT mice, a more pronounced reduction was observed in Spf-/- mice. The expression of cholesterogenic enzymes was dramatically suppressed by fasting both in WT and Spf-/- mice. In contrast, hepatic SPF expression of WT mice was up-regulated by fasting in peroxisome proliferator-activated receptor alpha (PPAR-alpha)-dependent manner. These results indicate that in WT mice, the decrease of hepatic cholesterol synthesis under fasting conditions is at least in part compensated by SPF up-regulation. Fibrates, which function as a PPAR-alpha agonist and are widely used as hypotriglycemic drugs, reduced hepatic cholesterol synthesis and plasma cholesterol levels by approximately one-half in Spf-/- mice but not in WT mice. These findings suggest that co-administration of fibrates and an SPF inhibitor may reduce not only plasma triglyceride but also cholesterol levels, indicating that SPF is a promising hypocholesterolemic drug target.

Coordinate transcriptional repression of liver fatty acid-binding protein and microsomal triglyceride transfer protein blocks hepatic very low density lipoprotein secretion without hepatosteatosis

Unlike the livers of humans and mice, and most hepatoma cells, which accumulate triglycerides when treated with microsomal triglyceride transfer protein (MTP) inhibitors, L35 rat hepatoma cells do not express MTP and cannot secrete very low density lipoprotein (VLDL), yet they do not accumulate triglyceride. In these studies we show that transcriptional co-repression of the two lipid transfer proteins, liver fatty acid-binding protein (L-FABP) and MTP, which cooperatively shunt fatty acids into de novo synthesized glycerolipids and the transfer of lipids into VLDL, respectively, act together to maintain hepatic lipid homeostasis. FAO rat hepatoma cells express L-FABP and MTP and demonstrate the ability to assemble and secrete VLDL. In contrast, L35 cells, derived as a single cell clone from FAO cells, do not express L-FABP or MTP nor do they assemble and secrete VLDL. We used these hepatoma cells to elucidate how a conserved DR1 promoter element present in the promoters of L-FABP and MTP affects transcription, expression, and VLDL production. In FAO cells, the DR1 elements of both L-FABP and MTP promoters are occupied by peroxisome proliferator-activated receptor alpha-retinoid X receptor alpha (RXRalpha), with which PGC-1beta activates transcription. In contrast, in L35 cells the DR1 elements of both L-FABP and MTP promoters are occupied by chicken ovalbumin upstream promoter transcription factor II, and transcription is diminished. The combined findings indicate that peroxisome proliferator-activated receptor alpha-RXRalpha and PGC-1beta coordinately up-regulate L-FABP and MTP expression, by competing with chicken ovalbumin upstream promoter transcription factor II for the DR1 sites in the proximal promoters of each gene. Additional studies show that ablation of L-FABP prevents hepatic steatosis caused by treating mice with an MTP inhibitor. Our findings show that reducing both L-FABP and MTP is an effective means to reduce VLDL secretion without causing hepatic steatosis.

Microarray analysis indicates an important role for FABP5 and putative novel FABPs on a Western-type diet

Liver parenchymal cells play a dominant role in hepatic metabolism and thereby total body cholesterol homeostasis. To gain insight into the specific pathways and genes involved in the response of liver parenchymal cells to increased dietary lipid levels under atherogenic conditions, changes in parenchymal cell gene expression upon feeding a Western-type diet for 0, 2, 4, and 6 weeks were determined using microarray analysis in LDL receptor-deficient mice, an established atherosclerotic animal model. Using ABI Mouse Genome Survey Arrays, we were able to detect 7,507 genes (28% of the total number on an array) that were expressed in parenchymal cells isolated from livers of LDL receptor-deficient mice at every time point investigated. Time-dependent gene expression profiling identified fatty acid binding protein 5 (FABP5) and four novel FABP5-like transcripts located on chromosomes 2, 8, and 18 as important proteins in the primary response of liver parenchymal cells to Western-type diet feeding, because their expression was 16- to 22-fold increased within the first 2 weeks on the Western-type diet. The rapid substantial increase in gene expression suggests that these FABPs may play an important role in the primary protection against the cellular toxicity of cholesterol, free fatty acids, and/or lipid oxidants. Furthermore, as a secondary response to the Western-type diet, liver parenchymal cells of LDL receptor-deficient mice stimulated glycolysis and lipogenesis pathways, resulting in a steady, more atherogenic serum lipoprotein profile (increased VLDL/LDL).

Adaptations to the loss of intestinal fatty acid binding protein in mice

It was shown previously that the intestinal fatty acid binding protein (I-FABP) is not essential for the absorption of dietary fat. One notable feature of I-FABP deficiency was the enhancement of body weight gain in male mice but not in female mice. To explore a possible cause for this gender dimorphic effect, we examined the changes in expression of genes that encode liver fatty acid binding protein (L-FABP) and ileal lipid binding protein in the small intestine resulting from I-FABP deficiency. The results indicate that both L-FABP and ilbp levels are modestly increased in the small intestine of chow-fed mice lacking I-FABP. There was no discernible alteration of overall morphology or histology in the small intestine but changes in liver histology were evident in I-FABP deficient male mice. Glucose tolerance was also investigated in aged mice. I-FABP deficiency had no effect on glucose tolerance in male mice but it appeared to be improved in female mice. Thus, male and female mice clearly respond differently to the loss of I-FABP from the small intestine but the observed changes in the abundance of L-FABP and ilbp protein do not readily account for this phenomenon.